Studies in Surface Science and Catalysis 75
NEW FRONTIERS IN CATALYSIS PART A
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J. T. Yates
Vol. 75
NEW FRONTIERS IN CATALYSIS Proceedings of the 10th International Congress on Catalysis, Budapest, July 19-24,1992 PART A Editors
L. GUCZl Institute of Isotopes of the Hungarian Academy of Sciences P. 0. Box 77, H - 1525 Budapest, Hungary
F. SOLYMOSI Institute of Solid State and Radiochemistry, Jdzsef Attila University P. 0. Box 168, H-6701 Szeged, Hungary
P. T ~ T E N Y I Institute of lsoropes of the Hungarian Academy of Sciences P. 0. Box 77, H- 1525 Budapest, Hungary
ELSEVIER
Amsterdam-London-New York-Tokyo 1993
Joint edition published by Elsevier Science Publishers B. V., Amsterdam, The Netherlands and Akadbmiai Kiad6, Budapest, Hungary Exclusive sales rights in the East European countries, Democratic People's Republic of Korea, Republic of Cuba, Socialist Republic of Vietnam and People's Republic of Mongolia Akadbmiai Kiad6, P. 0. Box 245, H-1519 Budapest, Hungary
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PREFACE The Tenth International Congress on Catalysis was held in Hungary, Budapest, July 19-24, 1992 under the auspices of the Council of the Congress on Catalysis, the Hungarian Academy of Sciences, the Chemical Society of Hungary and the International Union of Pure and Applied Chemistry. Z. G. Szab6 was the Honorary Chairman and P. TMnyi and F. Nagy served as the Chairman and Co-Chairman, respectively. The Organizing Committee included L. Guczi (Chair), A. SBrksny (Secretary) M. Bart6k, P. Fejes, Gy. GBti, D. Ka116, J. Margitfalvi, L. Mark6, Z. PaBl, J. Petr6 and F. Solymosi. More than 1000 delegates (including local organizers) attended the Congress which was headquartered in the Budapest Convention Centre located in the pleasant hills of the Buda part of the Hungarian capital. As has been the tradition in previous Congresses, this Congress was divided into two parallel sessions with 105 oral presentations and two further sessions in which 360 posters were displayed. All these presentations and posters were chosen by the Paper Selection Committee from the approximately 800 abstracts that were submitted. The theme of the Congress was "New Frontiers in Catalysis". Judging from the lively discussions (on the average 9 per oral presentation) this theme was of considerable interest to the participants. Both the papers selected for oral presentation (together with discussions) and those in the poster sessions are contained in these Proceedings. We are grateful to the outstanding scientists who accepted our invitation to present the 6 plenary lectures that introduced the various topics covered in the Congress. Generous financial contributions from the Council of the European Communities and many institutions and corporations made it possible for about 80 scientists from Central and Eastern Europe, the EC countries, China, USA and India to participate in the Congress. Several students were also able to attend because they were allowed to pay a reduced registration fee. Social events including receptions, a Congress banquet, a concert, sightseeing tours and excursions were organized to make the stay of our foreign guests more eqioyable. The Organizers are deeply indebted to the scientists from 42 countries who accepted our invitation to come to Hungary and make the Congress yet another outstanding success in the 40 year tradition of such events. We hope you enjoyed the meeting and will find these Proceedings a valuable addition to your catalysis library. Our best wishes go to the Organizers of the Eleventh International Congress on Catalysis which will be held during July 1996 in Baltimore, Maryland in the United states. Budapest, December 1992 The Editors
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CONTENTS
PART A Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysis: Past, Present and Future J.A.Ra bo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interfacial Coordination Chemistry: Concepts and Relevance to Catalysis Phenomena M.Che . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . New Catalytic Aspects of Heteropolyacids and Related Compounds - To the Molecular Design of Practical Catalysts M.Misono . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Catalytic Conversion of Methane to Oxygenates and Higher Hydrocarbons J.H. Lunsford . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . New Reactions in Various Fields and Production of Specialty Chemicals W.F. Hola'erich . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure-Function Relationships in Heterogeneous Catalysis: The Embedded Surface Molecule Approach and its Applications P. Johnston andR. W. Joyner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotopic Tracer Studies of Chain Propagation and Termination during FischerTropsch Synthesis over RuriO, K. R. KrishnaandA. T. Bell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liquid Phase Oxidation of Glyoxal to Glyoxylic Acid by Air on Platinum Catalysts P. Gallezot, F. Fache, R. de Mesanstourne, Y. Christidis, G. Mattioda and A. Schouteeten. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Effect of Oxygen Binding Energy on the Selective Oxidation of Butane over V/y-AI,O, P. J. Andcrsen and H. H. Kung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Secondary Oxygen Exchange Reactions during the Partial Oxidation of Methane M. M. Koranne, J . G. Goodwin, Jr. and G. Marcelin . . . . . . . . . . . . . . . . . . . . Role of Free Radicals in Heterogeneous Complete Oxidation of Organic Compounds over IV Period Transition Metal Oxides Z. R. tsmagilov, S. N. Pak, L. G. Krishtopa and V. K. Yermolaev . . . . . . . . . . . . . 1H Broad-Line NMR at 4 K for Studying the Acidity of Solids: Application to Zeolites P. Batamack, C. Doremieux-Morin andJ. Fraissard. . . . . . . . . . . . . . . . . . . . Preparation of Bifunctional Catalysts by Solid-state Ion Exchange in Zeolites and Catalytic Tests H. G. Karge, Y. Zhang and H.K. Beyer . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transition Metalfiolite Catalysts by Design: Nucleation and Growth of Mono- and Bimetallic Particles in Zeolite Y W. M. H. Sachtler, Z. Zhang, A. Yu.Stakheev and J. S. Feeley . . . . . . . . . . . . . . Promotion of H-ZSM-5 by Alumina J . Volter, H. D. Lanh, B. Parlitz, E. Schreier and K Ulbricht . . . . . . . . . . . . . . . The Effect of Preparation Method on Metal-Support Interaction in PdL-Zeolite Catalysts G. Larsen and G.L. Haller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . On the Nature of Superactive Centers in H-FeZSM-5 Zeolites. Quantum-Chemical Calculations M. J. Filatov, A. G. Pelmenschikov and G. M. Zhiabmirov . . . . . . . . . . . . . . . . .
V 1
31
69 103 12 7
165
181
195
20s 2 19
231 '
243
257
271 203 '
291
31 1
Vlll CO Oxidation on Pd(l10): A Model System for Chemical Oscillations in Heterogeneous Catalysis M.Ehsasi, M.Bcrdau, A. Karpowicz, K. Christmann and J. H. Block . . . . . . . . . . Nature of Metal-Mctal Bonding in Mixed Metal Catalysls R. A. Campbell, J. A. Rodriguez and D. W. Goodman . . . . . . . . . . . . . . . . . . . The Reduction of Nitric Oxide by Hydrogen over Pt, Rh and Pt-Rh Single Crystal Surfaces H. Hirano, T. Yamada, K. I. Tanaka, J. Siera and B. E. Nieuwenhuys . . . . . . . . . . Spectroscopic Studies on the Reaction Pathways of Methanol Dissociation on Pd Catalyst A. Berkb, J. Raskb and F. Solytnosi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyclotrimcrisation of Acetylene to Benzene over Single Crystal Palladium and Gold/Palladium Surfaces and over Supported Palladium Catalysts C. J. Baddeley, R. M. Ormerod and R. M.Lambert . . . . . . . . . . . . . . . . . . . . . Surface Chemistry for Automotive Emissions Control: Interactions of Nitric Oxide on a (1 11) PI-Rh Alloy Surface G. B. Fisher, C. L. DiMaggio and D. D. Beck. . . . . . . . . . . . . . . . . . . . . . . . Shape Selective Alkylation of Benzene with Long Chain Alkenes over Zeolites S. Sivasanker, A. Thangaraj, R. A. Abdullu and P. Ratnasamy . . . . . . . . . . . . . . Comparison o f SAPO-37 with Faujasites in Cracking Reactions M. Briend, M. Dcreninski, A. Lamy and D. Bari.homeuf. . . . . . . . . . . . . . . . . . Catalytic Activity of Modified ZSM-5 Zeolites in the Dehydrogenation and Aromatization Reactions of Propane and n-Butane P. Fejes, J. Halasz, 1. Kiricsi, Z. Kele, Gy. Tasi, 1. Hannus, C. Fernandez, J . B. Nagy, A. Rockenbauer and Gy. Schiihel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Coke Formation during the Transformation of Propene, Toluene and Propene-Toluenc Mixture on HZSM-5 P. Magnoux, F. Muchudo andM. Guisnct . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrocarbon Formation from Melhanol/Dimethyl Ether ovcr Protonated Zeolites and Molecular Sieves. Ncw Insights from Recent Expcrimcnts S. Kolboc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-D Exchangc betwecn Zeolites and Alkanes. Evidence for Formation and Rearrangement of Pentacoordinated Carbonium Ions C. J. A. MOIU,L. Nogucira, S. C. Menezes, V. Alekstich, R. C. L. Pereira and W.B.Kover.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . New Surface Nb-Dimers Chemically Interacted with SO2: Regulation of the Catalysis by Molecular Design of Reaction Sites N. Ichilatni and Y. Iuasawa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Expert Systems Approach to Catalyst Design - Application and Experimental Verification T. Hattori, H. Niwa, A. Saisuma, S. Kit0 and Y. Murakami . . . . . . . . . . . . . . . . Metal Oxide Vapour Synthesis (MOVS): A New Preparative Method for Heterogeneous Metal Oxide Catalytic Systems E. C. Alyea, K. F. Brown, K. J. Fisher and K. D. L. Smith . . . . . . . . . . . . . . . . . Designing of New Catalysts for Olefin Metathesis on the Base of Photoreduced Si lica-Molybdena V. B. Kazansky, B. N. Sheliniov and K. A. Vikulov . . . . . . . . . . . . . . . . . . . . . . SiOz-Grafted Dinuclear Molybdenum Catalyst Derived from Mo2(0Ac)4 Highly Active for Olefin Metathesis Reaction M.Ichikawa, Q. Zhuang, G.J. Li, K Tanaka, T. Fujimoto andA. Fuhoka . . . . . . . Molccular Design of Supported Metal Oxide Catalysts I. E. Wachs, G. Deo, D. S. Kim, M.A. Vuurman and H. Hu . . . . . . . . . . . . . . . .
32 1 333 345 359 37 1 383 397 409
42 1
435 449
463 477 489 503
515 529 543
IX Structural Characteristics of Alumina-Supported Activated Hydrodcsulfurization Catalysts. An XPS, NO Adsorption and Sulphydryl Group Study L. Portcla, P. Grangc and B. Dcltnon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559 Structure-Function Relations in Layered Transition Metal Sulfide Catalysts M. Daagc, R. R. Chianclli and A. F. Ruppert . . . . . . . . . . . . . . . . . . . . . . . . . 57 1 Elementary Steps of Hydrogenative Sulfur-, Nitrogen- and Oxygen-Removal from Organic Compounds on Sulfided Catalysts H. Schulz andN.M. Rahtnan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585 Sites Characterization on Model Ruthenium Sulphide M. Lacroir, C. Mirodatos, M. Breysse, T.Decamp and S. Yuan . . . . . . . . . . . . . . 597 High Resolution Electron Microscopy Characterization of the Poorly Crystalline Structure of Molybdenum Disulfide-Based Catalysts S. Fuenres, M. Avalos-Borja, D. Acosra, F. Pedraza and J. Cruz . . . . . . . . . . . . . 6 1 1 Deutcrium Solid State NMR Study of Molecular Mobility and Catalytic Dehydration of tert.-Butyl Alcohol on H-ZSM-5 Zcolite A. G. Stcpanov, A. G. Maryasor: V. N. Romannikovand K. I. Zatnaraev. . . . . . . . 621 Stationary Liquid-Phase Homogeneous Transition Metal Catalysis I. T. H o r v d h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635 An in situ Radioactive Tracer Technique for Studying Adsorption-Desorption Dynamics on a Working Catalyst U. Schriidcr, L. Cidcr and N. .H. Schoon . . . . . . . . . . . . . . . . . . . . . . . . . . 643 Application of Scanning Tunncling Microscopy/Spectroscopy (STWSTS) to Catalyst Research: Pt/Si02 M. Komiyama andM. Kirino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659 Characterization and Catalytic Properties of Pt-lr Small Bimetallic Cluster in NaY 0. B. Yang and S. I. w o o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671 Laser Raman Charactcrization of Surface Phase Precious Metal Oxides Formed on G O , Micro Domains Gcncrated within an Alumina Host by Sol Synthesis L. L. Murrcll, S. J . Tausrcr and D. R. Andcrson . . . . . . . . . . . . . . . . . . . . . . . 681 Direct Propane Animoxidation to Acrylonitrile: Kinetics and Nature of the Active Phase A. Andcrsson, S. L. T. Andcrsson, G. Cenri, R. K. Grasselli, M. Sanati and F. TriJro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 691 Catalytic Oxidation of Fluorene to 9-Fluorenone - Development and Characterization of Catalysts F. Majunke, H. Borchcrt and M. Bacrns . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 Ammoximation of Cyclohexanone on Titanium Silicalite: Investigation of the Reaction Mechanism A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. Petrini, G. Leofanti, M. Padovan, M. Mantegazza and P. Roffia . . . . . . . . . . . . . . . . . . . . . . . . . . 719 On the Catalytic Oxidation of Methanol with Vanadium (IV) in Sulphuric Acid Solution R. Larsson and B. Folksson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 731 Role of Chromium Introduced into 12-Molybdophosphates as Catalysts for Oxidation of Hydrocarbons K. Bruckman andJ..Haher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741 Correlation between Catalytic and Structural Properties of Modified Molybdenum and Vanadium Oxides in the Oxidation of Ethane in Acetic Acid or Ethylene M. Mcrzouki, B. Taouk, L. Tessier, E. Bordes and P. Courtine . . . . . . . . . . . . . . 753 The Promoting Effect of La203on the CO Hydrogenation over Rh/Si02 A. L. Borer and R. Prins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 765 Effect of Chlorine on the Rhenium-Alumina Interaction in Low-Loaded Re/A203 and PtRe/A1203 Industrial Catalysts G. Munucra, P. Malci andA. Caballero. . . . . . . . . . . . . . . . . . . . . . . . . . . 781
X A New Ap roach to Loss of Alkali Promoter from Industrial Catalysts: Importance of Excited tates of Alkali L. Holmlid K. Engvall, C. Aman and P. G. Menon . . . . . . . . . . . . . . . . . . . . . The Relation betwccn Catalytic and Electronic Properties of Supported Platinum Catalysts: The Local Density of States as Determined by X-Ray Absorption Spectroscopy M.Vaarkamp, J. T.Miller, F. S. Modica, G. S. Lane and D. C. Koningsberger . Direct MAS/MES Evidence for Electronic Metal-Support Interactions in Dilute sib and 57Fe Carbon and Alumina-Supported Catalysts C. H. Barrholomew, L. R. Neuhauer and P.A. Smith . . . . . . . . . . . . . . . . . . . . Formation and Properties of Dispersed Pd Particles over Graphite and Diamond 0. S. Alckseev, L. V. Nosova and Yu. A. Ryndin . . . . . . . . . . . . . . . . . . . . . . . Kinetics of Alkane Hydrogcnolysis on Clean and Coked Platinum and PlatinumRhenium Catalysts G. C. Bond R. H. Cunnirighanr and E. L. Short . . . . . . . . . . . . . . . . . . . . . . . Toluene Hydrogenation over Supported Platinum Catalysts S.-D. Lin and M. A. Vannice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Catalytic Activity of Mo0,/ZrO2 in thc Hydrogenation and Metathesis of Propcnc V. Indovina, A. Cimirio, D. Cordischi, S. Della Bclla, S. De Rossi, G. Ferraris, D. Gauoli, M. Occhiuzzi and M. Valigi . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pt-Sn-Alumina Catalysts: Rclating Characterization and Alkane Dehydrocyclization Data B. H. Davis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bimetallic Pt-Sn/AI,O, and Pt-Au/SiO, Catalysts: A Comparison of Reactivity, Adsorption behavior and Microstructure J . Schwank, K. Balakrishnan and A. Sachdev . . . . . . . . . . . . . . . . . . . . . . . . Hydroformylation of 1-Hcxcne by Soluble and Zeolite-Supported Iridium Species J.-Z. Zhang, Z. Li and C.-Y. Wang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E
PART B Bulk Tungsten Carbide as Catalyst in Hydrocarbon Reactions: Association of Selectivity Differences with Surface Composition as Compared to the Selectivity of Pt Series Metals A. Frennet, G. Leclercq, L. Leclercq, G.Maire, R. Ducros, M. Jardinier-Offergel4 F. Bouillon, J-M. Bastin, A. Ldperg, P. Blehcn, M. Dufour, M.Kamal, L. Feigenbaum, J-M. Giraudon, V. Keller, P. Wehrer, M. Cheval, F. Garin, P. Kons, P. Delcamhe and L. BinJt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface and Catalytic Properties of Molybdenum Nitrides L. T. Thompson, C. W. Colling, D. Choi, B. G. Demczyk and J.-G. Choi . . . . . . . . n-Hcxane Isomcrization on High Specific Surfacc Mo2C Activated by an Oxidative Treatment M . J . Ledoux, C. Pham-Huu, H . Dunlop andJ. Guille . . . . . . . . . . . . . . . . . . . Highly Dispersed Metal Colloids: Spectroscopy and Surface Chemistry in Solution J. S. Bradley, J. M. Millar, E. W. Hill, C. Klein, B. Chaudret and A. Duteuil . . . . . . Preparation of Amorphous Cu-Ti and Cu-Zr Alloys of High Surface Area by Chemical Modification S.Yoshida, T.Kakchi, S. Matsumoto, T. Tanaky H. Kanai and T. Funabiki . . . . . . A Mechanistic Proposal for Alkane Dchydrocyclization Rates on PtL-Zeolite. Inhibited Deactivation of Pt Sites within Zeolite Channels E. Iglesia andJ. E. Baurngartner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroconversion of n-Alkanes and Decalin over Bifunctional Ptmazzite Catalysts F. Fajula, M. Boulet, B. Coq, V. Rajaofanova, F. Figueras and T. Des Courieres . . .
795
809
82 1 a37
849 861
875
889
905 919
927 94 1
955 969
981
993
1007
XI Effect of Sulfur on thc Performance of Pt/KL Hexane Aromatization Catalyst J. L. Kao, G. B. McVicker, M. M. J. Treacy, S. B. Rice, J. L. Robbins, W. E. Gales, 10 19 J. J. Ziemiak, V. R. Cross and T. H. Vanderspurt . . . . . . . . . . . . . . . . . . . . . . Aromatization of n-Hcxane by Aluminium-Stabilized Magnesium Oxide-Supported Noble Metal Catalysts E. G. Derouane, V. Jullien-Lardot, R. J . Davis, N. Blom and P. E. Hojlund-Nielsen . ,1031 Effect of the Alkali Cation on Hcptane Aromatization in L Zeolite R.F. Hicks, W . J . Hun andA. B. Kooh . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,1043 Reduction and Aromatization Activity of MoOJA1203 Catalysts: The Identification of the Active Mo Oxidation State on the Basis of Reinterpreted Mo 3d XPS Spectra W. Griinert, A. Yu. Stakheev, R. Feldhaus, K. Anders, E. S. Shpiro and Kh. M . Minachev. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053 Reaction Routes for Methane Conversion on Transition Metals at Low Temperature T. Koerts and R. A. Van Santcn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .I065 The Mechanism of Alkane Oxidative Dehydrogenation on Chloride and Oxychloride Catalysts R. Burch, S. Chalker and P. Loader. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1079 Transient botopic Studies of the Role of Lattice Oxygen during Oxidative Coupling of Methane on S r L 0, and Ca/ThO, Catalysts Z. Kalenik and E. E. ................................... 1093 The Role of the Proton in Oxidation Processes on Metal-Oxygen Clustcr Compounds S. Kasztelan, G. E. McCarwy and J . B. Moflat . . . . . . . . . . . . . . . . . . . . . . . 1 1 05 Correlations between p-Type Semiconductivity and C Selectivity for Oxidative Coupling of Methane (OCM) over Acceptor Doped SrTid, C. Yu, W. Li, W. Feng, A. Qi and Y. Chen . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 19 Mechanistic Aspects of the Selective Oxidation of Methane to C1-Oxygenates over Mo03/Si02 Catalysts in a Single Catalytic Step M.A. Banares, I. Rodriguez-Rutnos, A. Guerrero-Rub andJ. L. G. Fierro . . . . . . . 1 131 On the Mechanism of Xylenc Isomerization and its Limitations as Reaction Test for Solid Acid Catalysts A. Corma, F. Llopis arid J . B. Monfon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 45 Aluminum Coordination and Lewis Acidity in Aluminas and Steamed Zeolites H. Yong, D. Costcr, F. R. Chen, J. G. Davis andJ. J. Fripiat . . . . . . . . . . . . . . . 1 1 59 Characterization of Basic Sites on Fine Particles of Alkali and Alkaline Earth Metal Oxides in Zeolites H. Tsuji, F. Yagi, H. Hattori and H. Kita . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 17 1 Zr02-S0,2- Catalysts. Nature and Stability of Acid Sites Responsible for n-Butane Isomerization P. Nascimento, C. Akratopoulou, M. Oszagyan, G. Couahrier, C. Travers, J.F. Joly 1185 andJ. C. Vedrine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Activity for Vapor-Phase Aldol Condensation and Acid-Base Properties of Metal-Oxide Catalysts M.Ai . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1199 Reactions of Multifunctional Organic Compounds - Hydrogenation of Acrolein on Modified Pt-Catalysts 121 1 T. B. L. W. Marinelli, J . €I. Vleerning and V. Ponec . . . . . . . . . . . . . . . . . . . . . Acetonitrile Synthesis from CO, H and NH, over Fe/C and K,Fe/C M. V. Baduni, L. M. Eshcltnan and%, N. Delgass . . . . . . . . . . . . . . . . . . . . . . 1223 Reductive Amination of Diethylene Glycol to Morpholine on Supported Nickel Catalysts - Its Activity, Selectivity, Stability and Possibility of Reactivation K. Jiratova, 0. Solcova, H. Snajduufova, L. Moravkova and H. Zahradnikova . . . . . 1235 An Improved Asymmetric Oxidation of Sulfides to Sulfoxides by Titanium Pillared Montmorillonite - The First Example in Heterogeneous Catalysis B. M. Choudury, S. Shohha Rani and Y. V. Suhba Rao . . . . . . . . . . . . . . . . . . . 1247
kolf.
XI I
Hydrogenation of CO, over Copper, Silver and GoldEirconia Catalysts: Comparative Study of Catalyst Properties and Reaction Pathways A. Baiker, M. Kilo, M, Maciejewski S. Menzi and A. Wokaun . . . . . . . . . . . . . . . 1 2 5 7 Shape-Selective Reactions for Methy lamine Synthesis from Methanol and Ammonia K. Segawa and H. Tuchibana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 7 3 Selective Catalytic Reduction of NO by Hydrocarbon in Oxidizing Atmosphere M. Iwamoto, N. Mizuno and H. Yahiro . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 85 The Making of Catalysts by Controlled Oxidative Degradation of Planar Metal Complexes on Alumosilicate Su ports: Exhaust Gas Purification Catalysts for Power Plants, Automobiles and Small utfits F. Steinbach, A, Brunner, H. Miiller, A. Drechsler, S. Fromming, W. Strehlau and U.Stan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1299 Oxidation of CO on Pd Particles on a-Al,O,: Reverse Spillover L. Kieken and M. Boudart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1 3 Microkinetic Analysis of the Selective Catalytic Reduction (SCR) of Nitric Oxide over Vanadianitania-Bascd Catalysts J . A, Dumesic, N.-Y. Topsoe, T. Slabiak, P. Morsing, B. S. Clausen, E. Tiirnqvist and H. Topsoe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1325 An Infrared Study of an Active NO Decomposition Catalyst J. Vulyon and W. K. Hall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1339 Acidity of WO /Ti0 Catalysts for Selective Catalytic Reduction (SCR) F. Hilbrig, H. &hme?z and H. Knijzinger . . . . . . . . . . . . . . . . . . . . . . . . . . . 1351 Mcmbrane Catalysis over Palladium and its Alloys J. N. Armor and T. S.Farris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1363 Fixed Bed Catalytic Reactors Based on Sintcred Metals F. vun Looij, A. Mulder, A. Q. M. Boon, J. F. Scheepens and J. W. Geus . . . . . . . . 1 37 7 Mixed Spinels with Cerium--SO, Emission Control from Fluid Catalytic Cracking (FCC) Regenerator J. S. Yoo,A. A. Bhattacharyya, C. A. Radlowski and J . A. Karch . . . . . . . . . . . . . 139 1 Sintering, Poisoning and Regcncration of Pt/MgO J. Adamiec, J. A. Szyriiura and S. E. Wanke . . . . . . . . . . . . . . . . . . . . . . . . . 1405 Development of a Micro Hydroprocessing Test for Rapid Evaluation of Catalysts C. Sudhakar, L. T. Mtshali, P. 0. Fritz and M. S. Patel . . . . . . . . . . . . . . . . . . . 1 4 1 9 Alkali Promoter Synergism in Selective Oxidation M. M. Bhasin and C. D. Hendrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 1 C-C Bond Formation via P-Addition with Oxygen Retention Reversal in Oxygenate Synthesis K. Klier, R. G. Herman, P. B. Himeljiarb, C.-W. Young, S. Hou andJ. A. Marcos . . . 1 4 4 1 Selective Gasoline Synthesis from CO, on a Highly Active Methanol Synthesis Catalyst and an H-Fe-Silicate of MFI Structure T. h i , T. Takeguchi, A. Kohama and K. Kitagawa . . . . . . . . . . . . . . . . . . . . . 1453 The Selective Synthesis of C2t Oxygenates from Syngas Related Reactions over Ni- and Rh-Based Catalysts M. W. Balakos, S. S. C. Chuang, R. Krishnamurthy and G. Srinivas . . . . . . . . . . . 1 4 6 7 Development of New Catalysts Formulations for Higher Alcohols Synthesis. Characterisation, Reactivity, Mechanistic Studies and Predictive Correlations A. Kiennemann, S. Boujana, C. Diagne and P. Chautnette . . . . . . . . . . . . . . . . . 1479 Characterization of MoS,-K+/SiO, Catalysts for Synthesis of Mixed Alcohols from Syngas H.-B. Zhang, Y.-Q. Yang, H. P. Huang, G. D. Lin and K. R. Tsui . . . . . . . . . . . . . 1 4 9 3 Catalytic Activity of Reduced Cu,Zn(l-,)O and CuO/Cu,Zn(l-,10 in C02/H2 Reactions D. Stirling, F. S. Stone andM. S. Spencer. . . . . . . . . . . . . . . . . . . . . . . . . . 1507
8
Xlll
Reactivities of Surface Intermediates on an Sm203 Catalyst Studied by in situ Infrared spectroscopy Y. Sakata, M. Yoshino, T. Fuhda, H. Yamaguchi, H. Imamura and S.Tsuchiya . . . . 1 519 In situ Investigation of the Water-Gas Shift Reaction over Magnetite by Mossbauer Spectroscopy A. Andreev, I. Mitov, V. Idakiev, T. Tomov and S. Asenov . . . . . . . . . . . . . . . . . 152 3 In situ FT-IR Study of 02,CO, CO,, CH, and C2H4 Adsorption or Reaction on the La O,/MgO Catalyst S. Shen, R. Hou,, W. Ji, Z. Yan andX. Ding. . . . . . . . . . . . . . . . . . . . . . . . . 1527 Study by in situ Laser Raman Spectroscopy of a VPO Catalyst in the Course of n-Butane Oxidation to Maleic Anhydride J. C. Volta, R. Olier, M. Roullet, F. B. Ahdelouahab and K. Bere . . . . . . . . . . . . . 1531 Investigation of Water-Gas Shift Reaction Under Dynamic Conditions P. Capek and K. Klusacek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1535 Application of the Transient Response Method to the Study of the Catalytic System NOt02tCO/CuO 1539 D. Panayorov and D. Mehandjicv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization and Activity of Vanadium Oxide Catalysts in Selective Catalytic Reduction of Nitric Oxide U. S. Ozkan, Y. Cai, M. W. Kumthekar and L. Zhang . . . . . . . . . . . . . . . . . . . . 1543 Theoretical Study of CO Chcmisorption on Rh and Pd Clusters A. Goursot, I. Pupai aridD. R. Salahub . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1547 New Dynamic Mcthod Approach to the Roles of Reversible and Irreversible Adsorption in Heterogeneous Catalysis G.Lu,S.ChenandS.Peng. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1551 Discrimination and Regulation of Multi-Reaction Pathways in Heterogeneous Catalysis M.Kobayashi, T. Kanno and M.Hakozaki . . . . . . . . . . . . . . . . . . . . . . . . . . 1555 Adsorption Study by Transient Tracing Methods. Theory and Modeling P. Szedlacsek, A. Efytathiou, C.O. Bennett and S. L. Suib . . . . . . . . . . . . . . . . . 1559 In situ Determination of Surface Carbon Species Formed on Rh/AI203 during CON2 Reaction by Using Various Transient and Isotopic Methods A. M. Efstathiou, T. Chafik, D. Bianchi and C. 0.Bennett . . . . . . . . . . . . . . . . . 1563 The Calculation of Surface Orbital Energies for Specific Types of Active Sites on Dispersed Metal Catalysts 1567 R. L. Augustinc, K, M. Lahanas and F. Cole. . . . . . . . . . . . . . . . . . . . . . . . . Oxidation and Removal of Chlorinated Hydrocarbons J. M. Berry, H. G. Stengcr, Jr., G. E. Buzan and K. Hu . . . . . . . . . . . . . . . . . . . 157 1 Structure and Reactivity of Carbidic Intermediates for the Methanation Reaction on Ni(100), Ni(ll1) and Ni(ll0) Surfaces 1 575 H. Hirano and K. Tanaka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CI Adsorption on Ag(Il1) and its Promoter Action D. Wang, K. Wu, Y. Cao, X. U’ei andX. Guo . . . . . . . . . . . . . . . . . . . . . . . . . 1579 The NOtH Reaction on Pt(100): Steady State and Oscillatory Kinetics M. Slinko, j.Fink, T. Liihcr, H. H. Madden, S, J. Lombardo, R. Imbihl and G. Err1 . . 1583 HREELSFDS Identification of Intermediates in the Low-Temperature H2+02, NOtH,, NH3t02 Reactions on Pt(ll1) Surface V. V. Gorodetskii, M. Yu. Smirnov and A. R. Cholach . . . . . . . . . . . . . . . . . . . 1587 Surface Science Studies on the Mechanism for H-D Exchange of Methane over Pt(111) Surfaces Under Vacuum F.Zaera.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1591 Catalysis at Experimentally Designed Surfaces: n-Butane Hydrogenolysis at Sn/Group VLII Surface Alloys 1595 A. D. Logan and M. T. Paflirt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XIV Surface Science and Kinetic Studies on Model Cu/Rh(lW) Catalysts J. Szanyi and D. W. Goodman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1599 Angular Distribution of Desorbing Reaction Products and Dynamics of Some Catalytic Processes on the Surfaces of Pt and Ir M. U. Kisliuk, V. V. Savkin, T. N. Bakuleva, A. G. Vlasenko, V. V. Migulin, I. I. Tretiakov and A. V. Sklyarov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 60 3 Ship-in-Bottle Synthesis of NaY Zeolite-Included Pt, and Pt 12 Carbonyl Clusters: Structures and Catalysis in COtNO Reaction G . J . Li, T. Fujimoto, A. Fukuoka andM. Ichikawa . . . . . . . . . . . . . . . . . . . . . 1607 The Preparation and Characterization of High-Silica Y Zeolite Prepared by Combined Chemical and Hydrothermal Dealumination X. Liu, Z. Pci, L. She, X.-W. Li, J . Shao, S. Lin, R. Tang and X. Ma0 . . . . . . . . . . . 161 1 Active Sites of Novel Iron Supported Y-Type Zeolite R. Iwamoto, I. Nakamura and A. Iino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1 5 A Novel Application of XRD Technique for the Characterization of Secondary Pore Structure in Modified Y-Zeolites S. D. Phatak, R. P. Mehrotra, S. M. Dhir and T. S. R. Prasada Rao . . . . . . . . . . . 16 1 g IR-Spectroscopic Evidence for Acetonitrile Interaction with Carbenium Ions in Zeolites D. S. Bystrov, A. A. Tsygancnko and H. Forstcr . . . . . . . . . . . . . . . . . . . . . . . 162 3 Acid-Base Properties of Zeolites: An XPS Approach Using Pyridinc and Pyrrole Probe Molecule R. B. Borade, M. Huang, A. Adnot, A. Sayari and S. Kaliaguine . . . . . . . . . . . . . 162 5 Sulfate as Promotor of Acidity of High Microporous and Thermostable Titanium Pi I lared Montmoril loni te F. Admaiai, A. Bernicr and P. Grange . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1629 Platinum Cluster Supported on Zeolite A by Ion Exchange of Pt(NH,)42+ R. Ryoo andS. J . Cho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1633 Preparation of Thermalstable Pillared Clays S. Mcndioroz, F. Gonzalez, C. Pesqucra, I. Benito, C. Blanco and G. Poncclet . . . . 1637 In situ X-Ray Analysis of CO- and CH30H-Induced Growth of Pd Particles Encaged in Zeolite Y w. Vogel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 4 1 Metal-Support Interactions on Pd-Containing Zeolite Catalysts M. F. Savchirs, Eh. Ya. Ustilovskaya, V. Z. Veshtort, L. A. Agabekova and 1 645 Yu. G. Egiazarov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transfer of Metal Ions between Metal Oxides and Zeolites. Preparation of Highly Active Cu-Zeolite Based Catalysts for Reduction of NO, at Low Temperature B. Wichterlovy Z. Sobalik, M. Petras, I. Jirka and V. Bosacek . . . . . . . . . . . . . . 1649 Microkinetic Analysis of Lsobutane Reactions Catalyzed by Y Zeolite J. E. Rekoske, R. J . Madon, L. M. Aparicio and J . A. Dumesic . . . . . . . . . . . . . . 1653 Peculiarities of Ethylene Conversion on Zeolites and Phosphoric Acid A. G. Anshits, S. N. Vereshchagin and N. N. Shishkina . . . . . . . . . . . . . . . . . . . 166 1 Benzene Alkylation in Vapour-Phase with Ethene on a Zeolite Catalyst G. Maria, G. Pop, G. Musca andR. Boeru . . . . . . . . . . . . . . . . . . . . . . . . . . 1665 On the Nature of Zeolite Catalyst Effect on the Selectivity of Toluene Nitration by Acyl Nitrates S. M. Nagy, K. A. Yarovoy, L. A. Vostrikova, K. G. lone and V. G. Shuhin . . . . . . . 1669 Interactions in Monoalkylbcnzenes Disproportionation Among Zeolite Characteristics and Reaction Mechanisms I. Wang, T.-C. Tsai and C.-L. Ay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1673 New Support Materials for Rhodium Catalysts: Characterization of Rh/AIP04-31 and Rh/MnAPO-31 A. Trunschke, H. Zubowa, B. Parlitz, R. Frickc and H. Miessner . . . . . . . . . . . . . I 677
xv Transformation of Thiols and Organic Sulfides over Zeolites M. Ziolek and P. Decyk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 1 Para-Selectivity of ZSM-5 Type Metallosilicates for Alkylation of Toluene with Methanol S. Namba, H. Ohta, J.-H. Kim and T. Yashima . . . . . . . . . . . . . . . . . . . . . . . . 1685 Hydroxylation of Toluene with Hydrogen Peroxide on HY Zeolites with Various Si/AI Ratios T. Yashima, Y. Kobayashi, T.Komatsu and S. Namba . . . . . . . . . . . . . . . . . . . i 689 Toluene Alkylation over Aluminophosphate-Based Molecular Sieves S.H. Ohand W. Y. L e e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1693 The Cracking Reaction Path in 1-Hexene Isomerization on SAF'O-11 and Pd/SAPO- 11 S.-Y. Lim and S . J . Choung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1697 Epoxidation of Alkcnes Catalyzed by Decatungstate as Pillars in Layered Double Hydroxides T. Tatsumi, H. Tajima, K. Yamamoto and H. Tominaga . . . . . . . . . . . . . . . . . . 1703 Conversion of Ethane into Aromatic Compounds on ZSM-5 Zeolites Modified by Zinc F. Roessner, A. Hagen, U. Mroczek, H. G.Karge and K.-H. Steinberg . . . . . . . . . 1707 In Situ FTlR and GC Kinetic Studies: Complementary Methods in the Mechanistic Study of Butanol Dehydration on Zeolite H-ZSM-5 M. A. Makarova, E. A. Paukditis, J . M. Thomas, C. Williams and K. I. Zamaraev . . . 1 7 11 Catalytic Properties of Fcrrisilicate Analogs of Somc Medium Pore Zeolites in C, and C, Aromatic Hydrocarbon Reations A.Raj, K . R . R e d d y , J . S . R e d d y a n d R . K u m a r . .. . . . . . . . . . . . . . . . . . . . . 171s The Influence of the Catalyst Preparation on the Catalytic Properties of ZeoliteSupported Catalysts Y. W. Chen and W. J. Wang. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1719 Reactions of Acetone, Methanol and Ammonia on ZSM-5 Zeolites J . Novakova. L. Bosacek Z. Doleisek and L. Kubelkova . . . . . . . . . . . . . . . . . . 1. 7. _2 _3 Rcgioselective Hydroicenation *Using Platinum-Support Zeolite Modified by CVD-Method H. Kuno, M. Shihagaki, K. Takahashi, I. Hona'a andH. Matsushita. . . . . . . . . . . 1727 Nickel, Cobalt and Zinc Substituted Synthetic Mica-Montmorillonite: Synthesis, Characterization and Propene Oligomerization Activity J. C. Q. Fletcher, A. P. Vogel and C. T.O'Connor . . . . . . . . . . . . . . . . . . . . . 1731 Isobutane/l-Butcne Alkylation on Pentad-Type Zeolite Catalysts J . Weitkamp and P. A. Jacobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173s Active Sites in HZSM-5 and S A P 0 Molecular Sieves for Alcohol Conversion C. Bezouhanova, Yu. KalrBachcv and H. Lechert . . . . . . . . . . . . . . . . . . . . . . . 1739 The Synthesis of Cobalt Supported Catalysts by Electroless Plating Techniques N. J. Coville, S. E. Colley, J. A. Beetge and S. W. Orchard. . . . . . . . . . . . . . . . 1743 A New Precursor for the Preparation of Novel Copper Chromite Catalysts R. Prasad. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1747 Pd,Fe : Pd Surface Segregation and Catalytic Activity J. C. Ertolini, Y. Dehauge, P. Delichere, J. Massardier, J.L. Rousset, P. Ruiz and B. Tardy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1751 A Study of Spreading of Vanadia on Titania Polymorphs Using Mechanical Mixtures M. Sanati, A. Andersson and L. R. Wallenberg. . . . . . . . . . . . . . . . . . . . . . . . 1755 Structure and Activity of Copper Catalysts Prepared from Amorphous Cu-Zr and Cu-Ti Alloy Precursors: A Comparative Study A. Molndr, T. Katona, Cs.Kopasz and Z. Hegediis . . . . . . . . . . . . . . . . . . . . . 1759 Preparation of W/AI 0 by Chemisorption of WOCI, to Surface Saturation M.Lindblad and L. Zindfors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1763
is.
xv I Highly Active V 0 Thin Films Prepared by Chemical Vapor Deposition on Silica for Oxidative Dciydrogenation of Alcohols T. Okuhara, K. Inumaru, M. Misono and N. Matsubayashi . . . . . . . . . . . . . . . . 1767 Influence of the Chemical Composition on the Preparation of Cu-Co-Zn-Al Mixed Oxide Catalysts with a High Metal Dispersion A. J. Marchi, J. I. Di Cositno and C. R. Apesteguia . . . . . . . . . . . . . . . . . . . . . 177 1 Sol-Gel Derived Heterogeneous Catalysts T. Walton and P. A. Sermon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,1775 Forming of Pyrogcnic SiO, and TiO, and their Applications as New Types of High Surface Area Catalyst Supports M. Bankmann, B. Despreyrou, H. Krause, J. Ohmer and R. Brand. . . . . . . . . . . 1781 Ncw Preparation Method of Small Particles in Ni/SiO, Catalysts Involving Chelate Ligands Z. X. Cheng, C. Louis andM. Chc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1785 Stepwise Monitoring of Mixed Oxide Catalyst Preparations by XAS Spectroscopy 0. ClauseandM. C h c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1789 Improving SO2 Resistence of Base Metal Perovskite Type Oxidation Catalyst W. Li, H. Dai and Y. Lieu. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1793 Carbide Catalysts: Laser Pyrolysis Synthesis and Catalytic Activity J . M. Slencel, P. C. Ekluiid, X.-X. Bi, B. H. Davis, G.T. Hager and F J . Derbyshirc . . 1 7 9 7 Structural Support Effccts in thc Systematic Preparation of Pd/SiO2 Catalyst for Mcthanol Synthesis by Ion Exchangc Techniques A. L. Bonivardi, M. A. Bulranas and D. L. Chiavassa . . . . . . . . . . . . . . . . . . . . 1801 The Advantagcous Usc of Microwave Radiation in the Preparation of Supported Nickel Catalysts G. Bond, R. B. Moycs, S. D. Pollington and D. A. Whan . . . . . . . . . . . . . . . . . . 1805 New Preparation Methods for Active Superfine Catalysts by Spray Reaction T. Uematsu, S. Shitnazu, T. Kanieyatna and K. Fukuda . . . . . . . . . . . . . . . . . . . 1809 Preparation of Supported Cu-Ni Bimetallic Catalysts by Alkoxide Method with High Activities for Hydrogenation or Dehydrogenation T. Sodesawa, S. Sam and F. Nozaki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 81 3 Dy-Cu Alloy Films: Catalytic Activity, Cornposition, Structure K. N . Zhavoronkova and 0.A. Boeva . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1 7 Adsorption and Catalytic Properties of Highly Disperse Silver Catalysb N. E. Bogdanchikova, D. A. Bulushev, Yu. D. Pankratiev and A. V. Khasin . . . . . . . 1823 New Insight into the Changing Catalyst/Polymer Morphology during Olefin Polymerization: The Application of Tomography W. C. Conner, M. Ferrero, S. Webb, R. Sommer, M. Chiovetta, K. Jones and P. Spannc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1827 Catalytical and High Resolution Electron Microscopy Studies of the System Pt/ZnAI 0, with Scvcral Platinum Contents G. Aguifar-Rim, M. A. Valenzuela, D. R. Acosta and I. Schifter . . . . . . . . . . . . . 1 83 1 The Application of Dilatometry for Investigation of Heterogeneous Catalysis 1835 L. A. Rudnitsky andA. M. Alekseev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Artificial Control of Catalytic Activity of Pd by a Shear Horizontal Surface Acoustic Wave Y. Inoue, H. Kawaguchi, M. Mutsukawa and K. Sat0 . . . . . . . . . . . . . . . . . . . . 1839 Characterization of Silica-Supported Palladium-Cobalt Alloys W. Juszczyk, Z. Karpinski, Z. Paul, J. Pielaszek . . . . . . . . . . . . . . . . . . . . . . . 1843 Temperature-Programmed Reduction in Catalysis: A Critical Evaluation of the Method G. Fierro, M.Lo Jacono, M. Inversi, G. Moretti, P. Porla and R. Lavecchia . . . . . . 1 847
XVI I Scanning Tunneling Microscopy Study of Pd/Graphite: Microstructure and Reactivity K . L . YeungandE.E. Wolf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1851 IR Study of Adsorption and Dcuteration of d6-Acetone on PtEnO Catalysts: Effects of the Sample Pretreatments F. Boccuzzi, G. Chiotti andA. Chiorino . . . . . . . . . . . . . . . . . . . . . . . . . . . la55 Surface Energetic Characterization of Supported Metal Catalysts by Gas/Solid Titration Microcalorimctry J . M.Guil, A. P. Masiu, A. R. Paniego and J . M. T. Menayo. . . . . . . . . . . . . . . . 1 a59 Study of the Effects of Annealing on thc Morphology of Platinum Cluster Size on Highly Oriented Pyrolytic Graphite by Scanning Tunneling Microscopy S. Lee, H. Permuna and K. Y. S. Ng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1863 Distribution of Mo Oxidation States in Reduced Mo/A1203 Catalysts. Correlation with Catalytic Activity J . Yusumuru, M. Yatnada, M. Houalla and D. M.Hercules . . . . . . . . . . . . . . . . 1867 Effect of Catalyst Prcparation on the Performance of Supported Ru-Cu Bimetallic Systems R. Muggiore, C. Crisalulli, S. Scire and S.Galvagno . . . . . . . . . . . . . . . . . . . . 1 a7 1 Revisiting Diffuse Rcflectancc Spectroscopy for the Characterization of Metal and Scmiconducting Oxide Catalysts A. R a h i , A. Bcn.salctn, J . C. Muller, D. Tessier and F. Bozon-Verduraz . . . . . . . . . 1 a75 Thc Thermal Stability of the Adsorbcd/Latticc Oxygen Spccics on the Oxide Catalysts Surfaccs M. Caldaruru and N . I. loncscu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 a79 Chemical Anchoring of Noble Metal Aminc Precursors to Silica: An in situ UV Diffuse Reflcctance Study W. Zou and R. D. Gonzulez . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 a83 Reaction Sites on the AI,03 Support of Pd/AI 0 J . L. Fulconcr, B. Chcn, S. A. Larson and E. dsiao . . . . . . . . . . . . . . . . . . . 1887 Thc External Magnetic Field Effect on the H2-O2 Reaction on the Sn02 Surface H . Ohnishi, H. Susaki mid M. Ippotntnarsu . . . . . . . . . . . . . . . . . . . . . . . . . . 1 a9 1 Characterization of Different Surface Mo Species in Mo/A1203 Catalysts by Time Differential Perturbed Angular Correlation S. Guida, Y. Tingyun, Y. Fushan, R. Liguo and N. Xinbo . . . . . . . . . . . . . . . . . . 1 a95
2
PART C New Catalytic Phases for the HDN and MHC Reactions in CoMoP-Alumina Catalysts A. Morales, R. Prada-Silvy and V. Leon . . . . . . . . . . . . . . . . . . . . . . . . . . . Multi-Purposc Ni-Mo/Al,O, Catalysts for Gas Oil Hydrotreating A. F. Sotnogy\jari, M. C. Ohulla and P. S.Herrera . . . . . . . . . . . . . . . . . . . . . characterization of Phosphorus Containing Ni-W/A120, Catalysts P. AtanasoLia and T. Haluchcv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concerted Mechanism of Thiophcne Hydrogenolysis by Sulfide HDS Catalysts A. N . Siartsev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Highly Active Ni-W/AI,O, Catalyst for Upgrading Unconventional Feedstocks H. Shimudu, T. Kumcoka, H. Yanase, M. Waranabe, A. Kinoshita, T. Saro, Y. Yoshitnura, N . Mustuhayashi and A. Nishijima . . . . . . . . . . . . . . . . . . . . . . Adsorption and Aclivation of Thiophenc on MoS2, Co$, and RuS2 C. Rong and A'. Qin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correlation of HDS Activity with Heat of Adsorption over Carbon Supported CoMo Catalysts S.-K. Ihtn, Y.-H. Moon and C.-D. lhtti . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 a99 1903 1907 191 1 191 5 19 1 9 1923
XVlll
Effect of Fluoride on the Surface Structure of WOJAIz03 Hydrotreating Catalysts R. L. Cordcro, J . R. Solis, J . V. G. Ramos, A. B. Patricio and A. L. Agudo . . . . . . . 1 9 27 Hydrodcsulfurization Activity of Zeolite Supported Nickel- and Cobalt Sulfide Catalysts W. J . J . Welters, T. I.Korunyi, V. H. J. de Beer and R. A. van Santen . . . . . . . . . . . 1931 Enhanced HDS Activity via Multiple Impregnation of Sulfided Mo/AI,O, Catalysts C.-S. Kim, F. E. Massoth, C. Geantet and M. Breysse . . . . . . . . . . . . . . . . . . . 1935 Surface Structure of Molybdenum Nitride and its Activity for Hydrodesulfurization and Hydrodcnitrogenation M. Nagai, T. Miyao, T. Tsirboi and T. Kusagaya . . . . . . . . . . . . . . . . . . . . . . . 1939 Hydrocracking Gas Oils from Synthetic Crude with Mixed Pillared Clay-Alumina Supportcd Catalysts J . Monnicr, J.-P.Charland, J . R. Brown and M. F. Wilson . . . . . . . . . . . . . . . . 1 9 4 3 Transformations of Thiophene, Tetrahydrothiophene and Butancthiol ovcr Co-Black, Co/AI 0,. Mo/AI 0 and Co-Mo/A1203 during Temperature-Programnied Reaction 1947 V. V. k0;onoi; Y. und 0. V. Krylov . . . . . . . . . . . . . . . . . . . . . . . . . . . Partial Oxidation Reaction of Methane with Oxygen or Carbon Dioxide by Transition Metal Catalysts Supportcd on Ultrafine Single-Crystal Magnesium Oxide 0. Takayam, I. Mutsuura, K. Nitiu und Y. Yoshidu . . . . . . . . . . . . . . . . . . . . . 1951 Surface Oxygen Species and their Reactivitics in thc Oxidation of CH4, C2H6 and C2H4 ovcr Cerium Oxide at Mild Temperatures C. Li, Q. Xin, X . Guoand T. Onishi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1955 Oxidative Catalytic Conversions of Tctrahydrofuran Derivatives R. Skolrncisicrc, L. Lciiis, M. Flcishcr and M. Shymansku . . . . . . . . . . . . . . . . . 1959 Role of Mo and Sb in Oxide Catalysts for Selective Oxidation of Propylene B. Zhou, X. Guo and K. T. Chucing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1963 Partial Oxidation of Methane at Low Prcssurc ovcr Silica Gel and Silica-Supported Sn, Zr and Gc Oxides T. Ono, K. lkliiu and Y. Shigcmura . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1967 The Influence of the Support on the Performance of Heterogeneous Catalysts for the Wacker Oxidation of Alkcncs A. W. Stobbc-Krccmcrs, J . J . F. Scholten, M. Soedc and J . W.Vecnman . . . . . . . . . 1971 Optimization of NiO/MoOflcO, Catalytic System for Direct Oxidation of Propcne to Acrylic Acid C. Muzzocchia, R. Anouchinsky, A. Kaddouri and E. Tempcsti . . . . . . . . . . . . . . 1 9 7 5 Role of Amorphous Phase and its Modification in V-P-0 Catalysts for Maleic Anhydride Synthesis from Butane N. Yamazoe, H . Morishigc, J . Tamuki andN. Miura . . . . . . . . . . . . . . . . . . . . 1979 Selective Oxidation and Amnioxidation of Propane t o form Acrolcin and Acrylonitrile Y. Moro-oh, N . Miura, N . Fujikunw, Y.-C. Kim and W.Uedcr . . . . . . . . . . . . . . 1983 Oxidation of Propcnc on Alkaline Metal-Doped MoOfliO, Catalysts: A FT-IR Study C. Martin, I. Martin, C. Mcndizahal and V. Rives . . . . . . . . . . . . . . . . . . . . . . 1987 Oxidation of Methanol to Formaldehyde over Antimony-Molybdenum Oxide Catalyst R. S. Mann andR. A. Diaz-Real. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1991 A Comparison between Epoxidation and Degradation of Ethylene and Propylcne ovcr Silver C. Hcnriqucs, M. F. Portclu, C. Muzzocchia and E. Guglielniinotti . . . . . . . . . . . 1 9 9 5 Liquid-Phase Oxidation of Benzene with Molecular Oxygen Catalyzcd by Cu-Zcol i tes T. Ohiani, S. Nishiyama, S. Tsuruya and M . Masai . . . . . . . . . . . . . . . . . . . . . 1999
XIX Formation of Formaldehyde from Methanol over Supported Titanium Oxide H. Imai, Y. Murakami and H. Irikawa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2003 A Study on P-Mo-As Heteropoly Compounds as Catalyst for Selective Oxidation of Methacrolcin B. Zhong, W. Zheng, R. He, G. Huang andX. Li . . . . . . . . . . . . . . . . . . . . . . . 2007 Silica as an Ammoximation Catalyst for the Production of Cyclohexanone Oxime D. P. Dreoni, D. Pinelli, F. Trifro, Z. Tvaruzkova, K. Habersberger and P. Jiru . . . 201 1 Kinetics of the Redox Reactions of the 02:Propylene:y-Bismuth Molybdate System: A TAP Reactor Study D. R. Coulson, P. L. Mills, K. Kourrakis, P. W. J. G. Wij,en, J . J. Lerou and L. E. Manzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 015 Partial Oxidation of Propene in the Presence of Steam Y. A. Saleh-Alhamed, R. R. Hudgins and P. L. Silvcslon . . . . . . . . . . . . . . . . . . 201 9 The Kinetics of Activation of Industrial and Model Iron Catalysts for Ammonia Synthesis in Dried and Wet Atmosphere A. Baranski, A. Kotarha, J . M. Lagan, A. PattckJanczyk, E. Pyrczak andA. Reizer . . 2023 Thc Modifying Role of Ru, Mo and Rare-Earth Elements (REE) in the Creation of New Generation of Catalysts Based on Iron Hydroxides R. V. Chesnokotq L. M. Dniytricnko, I. G. Brodskaja, A. M. Alekseev, A. A. Vasilevirch, N. A. Duhyaga and 1. I. Bondclrrsova . . . . . . . . . . . . . . . . . . .2027 The Role of Various Modes of Adsorbcd CO in Synthesis Gas Conversion on Lanthanidc Ions Promoted by Supported Pd Catalysts Yu. N. Nogin, N. V. ChesnokoL, and V. I. Kovalchuk . . . . . . . . . . . . . . . . . . . . 2031 Influence of thc Intcraction of Support with Active Species on Sintering and Stability of Alumina Supported Oxide Catalysts 0. A. Kirichenko, M. P. Vorohkva and V. A. Ushakov . . . . . . . . . . . . . . . . . . . 2035 Structural Transformation and Catalytic Bchaviors of Rhodium Ternary Oxides during Calcination and Reduction Treatments K. Kunintori, H. Oyanagi, H. Shindo, T. Ishigakiand T. Uchijima . . . . . . . . . . . . 2039 Markcd Support Effcct of Disperscd ZrO, Catalysts in Propenc-Deuterium Addition and Exchangc Rcaction S. Naito, M. Tunirnoto, M. SomaandY. Udagawa. . . . . . . . . . . . . . . . . . . . . . 2043 Acidity Generation of Binary Mctal Oxide Catalysts A. Gervasini, G. Bcllussi, J . Fenyvesi andA. Aurorcx . . . . . . . . . . . . . . . . . . . . 2047 Effects of Potassium Addition on the Performance of a Nickelhlagnesia Catalyst for Steam Reforming of Methane S. Kitahayashi, Y. Ogino, T. Yamazaki andS. Ozawa . . . . . . . . . . . . . . . . . . . . 2051 Weakening of Hydrogen Poisoning by Sm,O, Promoter in Activation of Dinitrogen on Ru/AI,O? Catalyst Y. Kadonaki, S. Muratu and K.-I. Aiku . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2055 XANES, EXAFS and Reaction Studies of Some Well-Dispersed Ferric Oxide Catalysts W. Ji, Y. Kuo, S. Shen, S. Li and H. Wang . . . . . . . . . . . . . . . . . . . . . . . . . . 2059 C 0 2 Derivatives Adsorbcd on Promoted Surfaces J . A. K. Paul, Y. Shao and 0.Axelsson . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2063 The Effcct of Mctallic Promoters on Supported Cobalt Catalysts A. Hofi E. A. Blekkari, A. Holmen and D. Schanke . . . . . . . . . . . . . . . . . . . . . 2067 The Structure-Activity Relationship of Re,O, Metathesis Catalysts Supported on Phosphatcd Alumina and Silica-Alumina R. SpronkandJ. C. M o l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2071 Modification of the Site Structure of MoS,/y-Al,O, Catalysts by Addition of P, F and Alkaline Elements 0. Poulet, R. Huhaut, S. Kaszrelan and J . Grimblor . . . . . . . . . . . . . . . . . . . . . 2075
xx Cerium Oxides Supported on Alumina-Crystallite Structures and Catalytic Activity M. Hancda, T. Miki, T. Mizushitna, N. Kakuta andA. Ueno . . . . . . . . . . . . . . . . 2079 The Role of Zirconium Dioxide in the Activation of Water and as the Catalytic Site for Low-Temperature Stcam Reforming over RhErO, A. Igarashi, T. Ohtaka, T. Honnma and C. Fukuhara . . . . . . . . . . . . . . . . . . . . 2083 Surfacc Structure and Reactivity of Magnesia-Supported Nickel Catalysts: A Model System A. Parmaliana, F. Arena, F. Frusteri, N. Mondello and N. Giordano . . . . . . . . . . 2087 Effect of Tin and Iron Deposition on the Catalytic Properties of Platinum Supported on Graphite E. Latny-Pitara, L. El Ouauani-Benhima andJ. Barbier. . . . . . . . . . . . . . . . . 2091 Propertics of a Latcrite Iron Mineral: Characterization, Catalytic Behavior and Promoter Effect M. R. Goldwasser, M. L. Cubeiro, M. J. Perez Zurila and C. Franco . . . . . . . . . . 2095 Platinum Catalysts Supported on High Surfacc Area Molybdenum or Tungsten Trioxides for Hydrogenation Reactions C. Hoang-Van, 0. Zegaoui and Y. Arnaud. . . . . . . . . . . . . . . . . . . . . . . . . . 2099 Pt-C Interaction in Catalyst Supported on a Carbon Black Subjected to Different Heat Treatments F. Coloma, C. Prado-Burgucre and F. Rodriguez-Reinoso . . . . . . . . . . . . . . . . . 2 1 03 Mechanism of thc Effect of Additives on Catalytic Properties of Palladium L. N. Edygcnova, N. V. Atiisimova, A. V. Korolev and D. M. Doroshkevich . . . . . . . 2 107 Pt-Ge/AI 0, Catalysts: Influence of the Thermal Treatments and the Redox Cycles J . A. M. Eorrea, S. R. de Miguel, G. T. Baronetii A. A. Castro and 0.A. Scelza . . . . 2 1 1 1 Effect of the Support on thc Copper State in Copper-Titanium Oxidation Catalysts T. S. Perkevich, L. Ya. Mostovaya, Yu. G. Egiazarov and N. A. Kovalenko . . . . . . . 2 1 15 Effect of Addition of Pd, Co and Pd-Co on CeO,. Syngas Conversion and Acetaldehyde Reaction H. Idriss, C. Diagne, J. P. Hindertnann, A. Kinnemann and M.A. Barteau . . . . . . . 2 1 19 The Role of Electric Ficld Acting on the characteristic of Adsorption of Solid Surface, the Oxygen Adsorption on Tin Oxide Film in an External Electric Field R.Zhou . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2123 Catalytic Bchaviour of LiFcO Anode for Solid Oxide Fuel Cells R. T. Baker, I. S. Mercalfe, P. Middleton, P. Petrolekas and B. C. H. Steele . . . . . 2 1 27 Elcctrochemical Modification of the Activity and Selectivity of Metal Catalysts M. Stoukides, D. Eng, P.-H. Chiang and H. Alqahrany . . . . . . . . . . . . . . . . . . . 2 131 The Selective Hydrogcnation of Acetylene by the Electrochemically Pumped Hydrogen over Cu in the Presence of Abundant Ethylene K. Otsuka, T. Yagi and M. Harano. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 135 Solid Electrolytes for in situ Promotion of Catalyst Surfaces: The NEMCA Effect C. G. Vaycnas, S. Bebelis, I. V. Yenlekakis, P. Tsiakaras, H. Karasali and 2 1 39 Ch. Karavasilis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theory and Experiment of Photo-Activation of Catalytic Sites and Active SiteSupport Interactions 0. Novnro and J. Garcia-Prieto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 143 Photocatalytic and Physicochemical Studies on Metallised Titania Systems B. Viswanarhan, U. D. Mary and R. P. Viswanath . . . . . . . . . . . . . . . . . . . . . . 2 1 47 Heterogeneous Photocatalysis: Mechanistic Considerations of Photocatalytic Reductions and Photocatalytic Oxidations on Semiconductor Oxidc Surfaces R.I. Bickley, L. Paltnisano, M. Schiavello and A. Sclafani . . . . . . . . . . . . . . . . . 2 1 5 1 De-NOF-ing Photocatalysis - Excited States of Copper Ions Anchored onto Zeolite and their Role in Photocatalytic Decomposition of NO at 275 K M. Anpo, T. Nomura, Y. Shioya, M. Che, D. Murphy and E. Giamello . . . . . . . . . . 2 155
k.
xx I A Novel Series of Photocatalysts with an Ion-Exchangeable Layered Structure of Niobate
K . Domen, J. Yoshimura, T. Sekine, J. K o n h , A. Tanaka, K. Maruya and T. Onishi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2159 Photocatalysis by Coordinatively Unsaturated Rhodium Complex Stabilized on Porous Glass for Alkane Dehydrogenation Y. Wady C. Nakano, Y. Yamauchi and A. Morikuwa . . . . . . . . . . . . . . . . . . . . Hetcrogcneous Photocatalysis as a Method of Water Decontamination: Degradation of 2-, 3- and 4-Chlorobenzoic Acids over Illuminated TiO, at Room Temperature J.-C. D'Oliveira, W. D. W. Jayatilak, K. Tennakone, J.-M. Herrmann and P. Pichat. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Investigation of thc Electron Transfer Mechanism between Methyl Viologen Radicals and Protons Via a Noble Metal Catalyst R. Bauer and H. A. F. Werner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Fibre - A Novel Catalyst Material for Selective Dehydrogenation of Alcohols B. Zhang, L. Lu, Y. Xiao, D. Jin, J. Ai and Z. Zhou . . . . . . . . . . . . . . . . . . . . . Use of Electron Spectroscopy Methods in the Study of the Structure of Surface Layers of Hydride Catalysts R. Kh. lbrasheva, R. G. Baisheva, Z. Kaizi, T. A. Solomina, G. 1. Leonova and K. A. Zhubanov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shape-Selectivity of Alkali Metals Graphite Intercalation Compounds for Catalytic Isomerization, Hydrogenation and Alkylation of Aliphatic and Aromatic Hydrocarbons S. Tsuchiya, S. Sakai, M. Kikugawa, T. Mitsuno and Y. Sakata . . . . . . . . . . . . . . Activation and Reactivity of Titanium Oxynitrides in Ammonia Decomposition C. H. Shin, G. Bugli and G. Djega-Mariadassou . . . . . . . . . . . . . . . . . . . . . . A Study on the Preparation and Characterization of a NIP Catalysts J . Shen, Z. Li, Q. Zhang, Y. Chen, Q. Bao and Z. Li . . . . . . . . . . . . . . . . . . . . . Investigations of Hydrodenitrogenation of Quinoline over Molybdenum Nitride K. S. Lee, J. A. Rcimer andA. T. Bell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . CO Oxidation, NO Decomposition and NO Reduction by CO on Superconducting and Related Cuprates 1. Halasz, A. Brenner, M. Shelef and K Y. S. Ng . . . . . . . . . . . . . . . . . . . . . . . The Active Oxygen on the Li/La203 Catalyst Surface and its Catalytic Behavior in the Oxidative Coupling of Methane L. Wang, J. Wang, S. Yuan and Y. Wu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidative Coupling of Methane over SrO Promoted La20&aO: A Comparative Study of the Kinetics and Mechanism Y.-D.Xu, L. Yu, J.-S. HuangandZ.-Y. L i n . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidative Coupling of Methane over Sol-Gel Magnesium Oxide Catalysts: Effect on Selectivity to Olefin Formation R. Gomez, T. Lopez, L. Herrera, A. A. Castro, 0. Scelza, G. Baronetti E. Lazzari, A. Cuan, M. Campos, E. Poulain, A. Ramirez-Solis and 0.Novaro . . . . . . . . . . . Influence of the Ion Charge and Coordination State on Catalytic Properties of Barium Ferroniobate for Methane Oxidation D. Filkova, 1. Mitov, L. Petrov, V. Bychkov, M. Sinev, Yu. Tulenin and P. Shiryaev . . Reaction Performances of Methane Oxidative Coupling Along Catalyst Bed with Sm-CaO Catalyst C. Tang, L. Lin, Z. Xu and J. Zang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Pressure on the Methane Oxidative Dimerization h.P. Tulenin, A. A. Kadushin, V. A. Seleznev andA. F. Shestakov . . . . . . . . . . .
2 163
2167
2 i 73 2 177
21 81
2 185 2 189 2 193 2197 2201 2205 2209
22 1 3 22 17 222 1 2225
XXI I
Methane Oxidative Coupling over Complex Metal Oxides Possessing K2NiF4 and Related Structure Q. Yan, Y. Jin, Y. Wang, Y. Chen andX. Fu . . . . . . . . . . . . . . . . . . . . . . . . . 2229 Oxidative Coupling of Methane over PbO/PbAI2O4 Catalysts S.-E. ParkandJ.-S. Chang. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2233 Characterization of TiLaNa Catalysts in the Oxidative Coupling of Methane S. Rossini, S.-T.Brandao, 0. Forlani, L. Lietti, A. Santucci, D. Sanfilippo and 2237 P.Villa.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Properties of Ca-Doped La203Catalysts for Coupling of Methane X. Yang, Y. Bi, K. Zhrn and Y. Wu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2241 Synergy between High Temperature Stable Carbonates and Irreducible Oxides: Destruction of Non-Selective Surface Oxygen on Oxidative Coupling of Methane Catalysts J.-L. Duhois andC. J . Cameron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2245 Laser Stimulated Oxidative Coupling of Methane to Ethene on LiCIOflb,(PO,), S-H. Zhong and H-Q. M a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 49 Temperature-Programmed Studies of Surface Oxygen Species in the Oxidative Coupling of Mcthane G. W. Keulks, N. Liao, W. An and D. Li . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2253 Oxidative Dimcrization of Methane on Alkali Chloride Promoted Co304 M. Gratzcl, D. Klissurski, J. Kiwi and K. R. Thunipi . . . . . . . . . . . . . . . . . . . . 2257 Oxidative Coupling of CH4tCD4Mixture ovcr Manganese Oxide Catalysts Y. G. Borodko and L. M. Iojp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2261 A Comparison of thc behavior of Catalysts for Methane Coupling by Transient Analysis R. Spinicci andA. Toj-anari . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2265 Oxidative Coupling of Methane to C Hydrocarbons over Doped Titania Catalysts D. Papageorgiou, D. Varn\nuka a n d k . E. Verykios . . . . . . . . . . . . . . . . . . . . 2269 The Importance of Carbon Dioxide in Oxidative Coupling of Methane A. Machocki. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 7 3 Kinetic Effects in Conversion of Propane, Isobutanc and Propane-Isobutanc Mixtures on K-Pt/y-AI20, Catalysts, Modificd by Sn and In L. C. Loc, H. S. Thoang, N. A. Gaidai and S. L. Kiperrnan . . . . . . . . . . . . . . . . . 2277 The Reaction of Reduction Catalyzed by Homogeneous and Immobilized Binuclear Rh(I1) Complexes with Rh-Rh Bond V. Z. Sharf and V. I. Isacva . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2281 A Highly Active, Heterogeneous Hydroformylation Catalyst: Rh(CO)(acac)L,L= =poly-TRIM Bound Phosphine J. Hjortkjaer, B. Heinrich, C. Andersson andA. Nikiridis . . . . . . . . . . . . . . . . . 2285 Selective Hydrogcnation of C, - Acetlylenes over an Ion-Exchanged Copper on Silica Catalyst J. T. Wehrli, D. J. Thomas, M . S. Wainwright, D. L. Trimrn andN. W. C a n t . . . . . . 2289 Chiral Metal Transition Complexes in Zeolites: Enantioselective Hydrogenation of Dehydrophenylalanine Derivatives A. Corma, M. Iglesias, C. dcl Pino and F. Sanchez. . . . . . . . . . . . . . . . . . . . . 2293 Selective Vapor Phase Hydroformylation of Olefins over Cluster-Derived Cobalt Catalysts Promoted by Alkaline Earth Oxides K. Takcuchi, T. Hanaoku, T. Maisuzaki, Y. Sugi, M. Reinikainen and M. Huuska . . . 2297 Design of Platinum Based Metallic Catalysts for Selective Hydrogenation of Crotonaldehyde A. Jentys, C. G. Raah and J. A. Lercher . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 30 1 1,3-Butadicne Hydrogenation in 1-Butene over Alumina Supported Pd-Ag Catalysts J. W. Hightower, B. Furlong, A. Sarkciny andL. Guczi . . . . . . . . . . . . . . . . . . . 2 3 0 5
XXlll Oxidative Dehydrogenation of Propane in Presence of Rare Earth Vanadates J . Casriglioni, P. Poir and R. Kiefl’er . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2309 Reactions of Cyclopropane and 1-Butene over Reduced Molybdena-Alumina Catalysts 1, Oliveros, C. Bolivar, P. Marcano, C. Scott, M J . Perez Zurira and J. Goldwasser . . 231 3 Effects of Mixtures of Modifiers on Optical Yield in Enantioselective Hydrogenation: a Test of the Template Model K. E. Simons, P. A. Meheux, A. lbbotson and P. B. Wells . . . . . . . . . . . . . . . . . 231 7 Ni/M Type Bimetallic Reducing Systems Zs. Bodnrir, T. Mallat andJ. Pefr6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2321 New Aspccts on the Mechanism of Olcfin Polymerization with Reduced Philipps Catalysts H. L. Krauss, H . A. Schmidt, B. Siebenhaar, P. Wolff and Q. X i n g . . . . . . . . . . . . 2325 The Role of Porosity in Ethylene Polymerization on Cr/SiO, Catalysts I. G. Dalla Lana, J. A. Szytnura and P. A. Zielinski . . . . . . . . . . . . . . . . . . . . . 2329 Partial Hydrogenation of Alkynes and Dienes on Highly Selective Fe-Cu/SiO, Catalysts Y. Nitta, Y, Hirarnatsu, Y. Okarnoto and T.lmanaka . . . . . . . . . . . . . . . . . . . . 2333 Supported Dehydrogenation Catalysts Based on Iron Oxide D. E. Stohhe, F. R. van Buren, A. J. van Dillen and J. W. Geuss . . . . . . . . . . . . . 2337 Sulfur Resistance of Nickel Catalysts Supported on K-Clinoptilolite Containing Iron in Ethylbenzene Hydrogenation A. Arcoyu, X. L. Seoane andJ. Soria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2341 Heterogeneous Ethylene Hydroformylation Catalyzed by Oxide-Supported [ R h 2(CO)30]2-Anion: Influence of the Nature of the Support C. dossi, A. Fusi, L. Garlaschelli, R. Psaro and R. Ugo . . . . . . . . . . . . . . . . . . 2345 Isolated and Compctitive Hydrogenation to Characterize Ni-B Catalysts G. Jannes, P. Kercla, B. Lenoble, P. Vanderwegen, C. Verlinden and J.P. Puttcmans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2349 The Role of Surface Fugacities and of Hydrogen Desorption Sites in Catalytic Reactions of Alkanes E. Iglesia, J. E. Baumgartner and G. D. Meilzner . . . . . . . . . . . . . . . . . . . . . . 2353 Heterogeneous Catalysis of a-Octene Hydroformylation. the Catalytic Behaviors of Non-Homodisperse EGG-Shell Catalysts W. Huang, L.-H. Yin and C.-Y. Wang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2359 Hydroformylation of Ethylene over Silica-Supported Pt/Sn Catalysts P. Ramirez de la Piscina, J. L. G. Fierro, G. Muller, J. Sales and N. Homs . . . . . . . 2363 Kinetics of 2,2,3,3-TetramethylbutaneHydrogenolysis over Rh/Al203 Catalysts B. Coq, T. Tazi, R. Dutarfre and F. Figueras . . . . . . . . . . . . . . . . . . . . . . . . . 2367 Surface Organometallic Chemistry on Metals: Role of a Surface Organometallic Fragment Sn(n-C4b)x on the Selective Hydrogenation of Citral with a Rh/SiO, Catalyst B. Didiflon, A. El Mansour, J. P. Candy, J . M. Bassef, F. Le Peletier and J. P. Boitiaux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2371 Hydrogenolysis of Methylcyclobutane on Supported Pt Model Catalysts C. Zimmermann and K. Hayek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2375 Preparation of Egg-Shell Type Pd-Ag and Pd-Au Catalysts by Selective Deposition and Hydrogenation of 1,3-Butadiene H. Miura, M. Terasaka, K. Oki and T. Matsuah . . . . . . . . . . . . . . . . . . . . . . . 2379 Hydrogenation and Deuteration of Butadiene and Cyclohexadiene over Reduced and Sulfided Molibdena-Alumina A. Ridey, D. Smrz and W. K. Hall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2383 Hydrocracking Boscan Heavy Oil with Unimodal and Bimodal Catalysts M. Ternan andJ. Menashi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2387
XXlV A Kinetic Model for Aromatization Processes over ZSM-5 Catalysis. Arornatization of Short Chain Hydrocarbons over HZSM-5 D. B. Luk’yanov, V. I. Shtral, V. I. Timoshenko andS. N. Khadzhiev . . . . . . . . . . . 2 3 9 1 On the Role of Reversible and Irreversible Adsorption Hydrogen in the Dehydrogenation and Reforming Reactions Y. Sun, S. Chen and S. Peng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 9 5 Catalyst Design for the Upgrading of Australian Coal-Derived Liquids A. T. TownsendandF. P. Larkins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2399 The Use of Intermetallic Hydrides on Basis b n t h a n with Nickel and Cobalt for Hydrogenation of hphaltenes Concentratc N. M. Parfenova, I. M. Halpcrin, S. R. Sergienko, V. A. Pherapontov and E. V. Starodubtzeva. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2403 n-Hexane bornerization and Aromatization on the Catalysts Derived from AluminaSupported Pt-Sn Clusters X . Li, Y. Wei, J . ChcngandR. Li . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2407 The Effect of Rhodium Particle Size on n-Butane Hydrogenolysis Activity and Sclcctivity D. Kulakkad, S. L. Anderson andA. K. Darye . . . . . . . . . . . . . . . . . . . . . . . . 2411 Pretreatment Effccts on Active State and Aromatization Activity of GaiZSM-S Catalysts K. M. Dooley, C. Chang, V. Kanazircv and G. L. Price . . . . . . . . . . . . . . . . . . . 2 4 15 New Modification Method of Pt/L Zcolitc Catalyst for Hcxanes Aromatization H. Kursuno) T. Fukirnaga and M . Sugimoto . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 19 High Teniperaturc Scnsitivity of Paraffin Hydrocracking R. T. Hanlon, C. R. Kcnticdy, R. A. Ware andS. S. Wong . . . . . . . . . . . . . . . . . 2 4 2 3 Effect of Modification of the Alumo-Platinum Rcforming Catalyst with Dy, Cr, Ba and Nytrogcn on its Catalytic and Physico-Chemical Propertics G. M. Scn‘kov, E. A. Skrigan, E. A. Pairkshris, M. F. Gorbutsevich, A. M. Nikitina and E. N. Ermolcwko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 2 7 Transformation of Propane over GamZSM-S Catalyst: On the Nature of the Active Sites for the Dehydrogenation Reaction P. Mcriaudeau and C. Naccachc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 1 Study of the Selective Scmi-Hydrogenation of Various Carbon-Carbon Triple Bonds over a Pd/Sepiolite Catalyst M. A. Aratncndia, V. Boruu, C. Jimcnez, J . M. Marinas) M. E. Semperc, F. J . Urbano and L. Villar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 3 5 n-Arylhydroxylamines Transformation in the Presense of Heterogeneous Catalysts I.A. Makuryan and V. I. Sawhcnko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2439 Highly Efficient Enantio-Differentiating Hydrogcnation Catalyst Prepared from Ultrasonicated Rancy Nickcl by Asymmetric Modification A. Tail T. Kikukunq T. Sugiinuru, Y. Inoue, S. Abc, T. Osawa and T. Haradu . . . . . 2 4 4 3 New Vapor Phasc Process for Synthesis of Ethyleniminc by Catalytic Intramolecular Dehydration of Monoethannlaminc M. Ueshimy Y. Shimasaki, K. Ariyoshi, H. YanoandH. Tsuneki . . . . . . . . . . . . . 2 4 4 7 Catalyst for Vinyl Chloride Synthesis N. A. Prolardirta, V. V. Chcsnokor; B. P. Zolotolwkii, L. N. Yelesina, V. G. Yenakaeva and V. F. Tarasov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2451 A New Process for Production of 1,3-Diniethyladamantanc: K. Takagi and Y. Naruse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 5 5 Palladium Bascd Catalysts for Hydrogenation of Nitrobcnzcne E. Brazi, G. Cordicr and G. N. Sauvion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2459 Modification of Pd and Pt by Thiophene and Carbon Tetrachloride during Hydrogcnation and 1somcr;irationof (+)-Apopinenc G. V. Smilh, F. Norheisz, A. G. Zsigmond andM. Barr6k . . . . . . . . . . . . . . . . . . 2 4 6 3
xxv Selective Hydrogenation of Carbonyl Groups by Means of Hectorite-Intercalated Rhodium Complexes S. Shimazu, T. Chiaki and T. Uetnarsu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enantioselectivc Hydrogenation of a-Keto Esters over Pt/AI 03 Catalyst: Kinetic Aspects of the Rate Acceleration Effect Induced by Addition ofCinchonidinc J. L. Margitjialvi, B. Mindcr, E. Tulas, L. Botz andA. Baiker . . . . . . . . . . . . . . . Crotonaldehydc Hydrogenation over PtniO2 Catalysts. Influence of the Catalysts Pretreatments R. Makouangou, A. Dauschcr and R. Touroude . . . . . . . . . . . . . . . . . . . . . . . Selective Hydrogenation of a,fbUnsaturated Aldehydes over Supported Ru A. Waghray, R. Oukaci andD. G. Blackmond . . . . . . . . . . . . . . . . . . . . . . . . Hydrogenolysis of Tctrahydrofurane on Platinum K. Kreuzcr and R. Kranicr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of Polymer Support Morphology on Ion-Exchanger Catalysts Activity in tert.-Alkyl-Mcthyl Ethers Synthcsis K. Jerabck, T. Hochniunri and Z.Prokop . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Catalytic Organic Solid-Gas Reactions R. Lamarline, F. Sabra andA. Sclarnia . . . . . . . . . . . . . . . . . . . . . . . . . . . . Infrared Study of Coke Deposition on Alumina J. Darka andR. P. Eischcns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen Surface Concentration Effect on the Temperature-Programmed Hydrogemtion of Adsorbed CdrbOnaCeOUS Species on an Iron/Alumina Catalyst H. Halafi, E. Borgsicdr, A. M. Efsrarhiou, S. L. Suib and D. Bianchi . . . . . . . . . . . R I R and Catalytic Studies of the Effects of Sulphur Poisons on Cu/A1203 Catalyst Selectivity M. B. Padlcy, C. H. Rochester, G. J. Hurchings, P. I. Okoye and F. King . . . . . . . . Enhancement of the Stability of PtSn Catalysis in Regeneration Cycles by A1203 Doping with Rare-Earth Oxides Y. Fan, L. Lin, J. Zung andZ. X u . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Development of a New Catalyst for the Hydroconversion of Heavy Oils M . M. Rarnirez dc Agudelo and C. Galarraga . . . . . . . . . . . . . . . . . . . . . . . . Molecular Poisoning of Ni/Si02 Catalyst. A Magnetic and Catalytic Study of the Effect of Thiophenc, Carbonyl Sulfide and Carbon Disulfide Adsorption J. B. Baumgartner, R. Frety and M. Guenin . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Sintering in 0, Atmosphere on the Surface Properties of PtRh/Al,O, Catalysts S. Kacimi, C. Kappcnsrein and D. Duprez . . . . . . . . . . . . . . . . . . . . . . . . . . Coke Deposits on Pt/A120, Catalysts: R I R and HRTEM Studies L. Murchesc, E. Borello, S. Coluccia, G. Martra andA. Zecchina . . . . . . . . . . . . Molecular Orbital Study of the Chemisorption of Small Molecules on MgO Surfaces H. Kobayashi, A. SI. Amant, D. R. Salahub and T. 110. . . . . . . . . . . . . . . . . . . Applications of a New Isothermal Single Pellet Diffusion Reactor S. S. Au, J. B. Burr andJ. S. Dranofl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coke Elimination from Pt-Re/AI O3 by Ozone Containing Mixtures C. L. Pieck, E. L. Jahlonski and.? M. Parera . . . . . . . . . . . . . . . . . . . . . . . . New Model of Deactivation of Iron Catalysts for Ammonia Synthesis W. Arabczyk and K. Kalucki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pore Restriction in Resid Hydrotrcating Catalysts P.-S. E. Dai and B. H. Barrlcy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of the Internal Mass Transfcr Resistance on the Ni/A1203 Deactivation by Thiophene S. Zrncevic and V. Totilasic, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Study of Coke Formation in Rcsid Catalytic Cracking T.C.Ho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2467 2471 2475 2479 2483 2487 2491 2495 2499 2503 2507 25 1 1 25 1 5 251 9 2523 2527 2531 2535 2539 2543 2547 2551
XXVl Selective Hydrogenation of Adiponitrile (ADN) to Hexamethylenediamine (HMD) in the Liquid Phase on Raney Catalysts: Synergetic Effect between the Base in the Solution and the Iron Dope on the Catalyst J. F. Spindler, G. Cordier, J. Jenck and P. Fouilloux . . . . . . . . . . . . . . . . . . . . Acid Resistant Copper Chromium Oxide Catalysts Used in the Hydrogenolysis of Fatty Acids K. Kochloefl, G. Maletz, G. Hausinger and M.Schneider . . . . . . . . . . . . . . . . . Synthesis of Anthraquinones by Cyclization of Ortho-Benzoylbenzoic Acids in the Presence of Solid Acid Catalysts S. A. Amitina and V. G. Shubin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acid-Base Properties of Sulfided Ni-Mo-Y Zeolite Catalysts for Water-Gas Shift Reaction M.Laniecki and W. Zmierczak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Investigation on Adsorption and Acid-Base Properties of Solid Catalysts by Infrared Thermography S. Marengo, G. Rainiondini and P. Comotti . . . . . . . . . . . . . . . . . . . . . . . . . Nature of the Extra-Framework Aluminum in MFI Type Zeolites Synthesized in Fluoride Medium. Influence on the Catalytic Properties of Zeolites Q. Chen, J. L. Guth andJ. Fraissard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron - an Acidity and Texture Modifier for Alumina Supported Catalysts S. Engels, E. Herold, H. Lausch, H. Mayr, H.- W. Meiners and M. Wilde . . . . . . . . Sulfate-Modified Superacid ZrO, Catalysts: Study of the Surface Acidity and of the Activity towards Alcohols C. Morterra, G. Cerrato, C. Emunuel and V. Bolis . . . . . . . . . . . . . . . . . . . . . Highly-Dispersed Heteropoly Anions on Metal Oxide Carriers Modified with Silane Coupling Agents and their Catalytic Properties Y. Kera, H . Nishizima andM. Kamaah . . . . . . . . . . . . . . . . . . . . . . . . . . . . New Catalytic Systems: Heteropolyanions Doped Conjugated Polymers as the Catalysts for Ethyl Alcohol Conversion J. Pozniczek, I. Kulszewicz-Bajer, M. Zagorska, M. Hasik, A. Bielanski and A . Pron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Thermal Treatment on Surface Properties and Catalytic Activities of a Prepared Ti0 ZrO Catalyst J. A. Navio, d M a A a s , F. J. Marchena, J. M. Campelo andJ. M. Marinas . . . . . . Gas Phase Synthesis of MTBE over Acid Zeolites A. Nikolopoulos, T. P. Palucka, P. V. Shertukak, R. Oukaci, J. G. Goodwin, Jr. and G. Marcelin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acidity-Tunable Pillared Mica Catalyst Derived from Talc K. Urabe, I. Kenmoku, K. Kawabe and Y. lzumi . . . . . . . . . . . . . . . . . . . . . . . Beckmann Rearrangement over Solid Acid Catalysts T. Curtin, J . B. McMonagle and B. K. Hodnett . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Solid Superacids of Tungsten Oxide Supported on Tin Oxide, Titanium Oxide and Iron Oxide and their Catalytic Action K. Arata and M.Hino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of the Method of Platinum Inclusion into Spherical Promoter of Catalytic Cracking Termofor on its Activity in CO Combustion M. I. Levinbuk, V. B. Mclnikov, H. K. Shapieva, V. I. Vershinin, V. A. Kuzmin and V. Je. Varshaver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cooxidation CO and H, on Palladium Catalyst N. A. Boldyreva and V. K. Yatsimirsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Structure of Supported Pd, Kinetics and Mechanism of the Low-Temperature Oxidation of Carbon Monoxide S. N. Pavlova, V. A. Sadykov, D. I. Kochubei, B. N. Novgorodov, G. N. Kryukova and V. A. Razabbarov. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2555 2559 2565 2569 2 573 2577 2 58 1 2585 2589
2593 2597 260 1 2605 2609 26 13
26 1 7 262 1
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XXVl I Transition Metal Compound Oxide Catalysts for Lowering the Light Off-Tempcraturc of Particlcs from Diesel Exhaust C. Setzer, W. Schiitz and F. Schuth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FTIR Study of Reduction Mcchanism of NO by CzH, and C3Hg on Vanadium Oxides Layered on ZrO T. Ohno, F. Hatayama, Maruoka and H. Miyala . . . . . . . . . . . . . . . . . . . . . The Selcctivc Catalytic Reduction of Nitrogen Oxides with Ammonia in a Catalytically Active Ljungstroem Heat Exchanger H.-G. Lintz and T.Turek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evaluation of Nitrous Oxidc Formation over Rhone-Poulenc's DN 110 SCR Catalyst E. Garcin and F. L u c k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytically Accclcratcd Solid-Gas Reaction between NO and Ba-Cu-0 for Efficicnt NO Removal M. Machih, S. Ogata, K. Yasuoka, K. Eguchi and H. Arai . . . . . . . . . . . . . . . . Catalytic Propcrty of Perovskite-Type Oxides for the Direct Decomposition of Nitric Oxide Y, Teraoku, T. Huradu, H. Furukuwa andS. Kagawa . . . . . . . . . . . . . . . . . . . . TPD-Analysis of Mctallic Three-Way Catalysts. Effect of Zr, Si, La and Ba on Thermal Aging M. Huuska and T. Muunula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mcchanistic Studics of CO Oxidation on Highly Dispersed Gold Catalysts for Use in Room-Tempcraturc Air Purification M, Haruta, S. Tsubota, T. Kobuyashi, A. Ueah, H. Sakurai and M. A n d o . . . . . . . . Low-Tempcraturc Oxidation of Light Paraffins and Olefins at Solid Surfaces: FT-IR Studies G. Busca, V. Lorcnzelli, G. Raniis and V. S.Escribano . . . . . . . . . . . . . . . . . . . Catalytic Diesel Engine Emission Control. - Studies on Model Reactions over a EUROPT-1 (Pt/SiO ) Catalyst E. Xue, K . Seshun, J! G. van Otntnen andJ. R. H. Ross . . . . . . . . . . . . . . . . . . . Carbon-Oxygen Rcaction on Cu/V/K Catalyst for Soot Oxidation P. Ciambelli, V. PaltnaandS. Vaccaro. . . . . . . . . . . . . . . . . . . . . . . . . . . . Kinetics of the Selective Catalytic Reduction of NO by NH3 over a Commercial Catalyst W. R. A. M. Robinson, J. G. van Ommen, A. Woldhuis and J. R. H. Ross . . . . . . . . Selective Catalytic Reduction of NO on Copperan-Alumina in the Cleanup of High Sulfur Content Flue Gas: Catalyst Development and Design G. Centi, N . Passurini, S. Perathoner, A. Riva . . . . . . . . . . . . . . . . . . . . . . . . The Role of NO? in thc Selective Catalytic Reduction of NO, J. Blanco, P. Avrla andJ. L. G. Fierro . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalysts for the Elimination of Sulphur Dioxide from Streams by the Claus Reaction at Low Temperature E. Alvarez, A. Mendioroz andJ. M. Palacios . . . . . . . . . . . . . . . . . . . . . . . . . Promoter Effects on Platinum Catalysts for Automotive Exhaust Control J. R. Gonzalez-Velasco, J. Entrena, J. A. Gonzalez-Marcos, J . I. Gutierrez-Ortiz and M. A. Gutierrez-Ortiz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correlation of Bulk and Surface Thermodynamics of some Transition Metal Oxides; Application to Exhaust Gas Catalysts A. D. van Langeveli, A. C. T. van Duin, J. W. Bijsterbosch, F. Kapteijn and J. A.Moulijn. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coadsorption and Reaction of NO and CO on Cu/A1203 Catalysts R. Hierl, H.-P. Urbach andH. Knozinger . . . . . . . . . . . . . . . . . . . . . . . . . . . Activity and Electronic Properties of Automotive Emission Control Catalysts B. H. Engler, D. Lindner, E. S. Lox and P. Albers . . . . . . . . . . . . . . . . . . . . . .
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XXVl I I Alumina Supported Manganese Catalysts for Low Temperature Sclcctive Catalytic Reduction of NO with NH, L. Singorcdjo and F. Kupteijn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduction of Nitric Oxide by Carbon Monoxide on Palladium Based Binictallic Catalysts J. Massardicr, A. El Hamdaoui, G. Bergeret and A. Renouprez . . . . . . . . . . . . . . Activity and Characterization of Palladium Catalysts for Nitric Oxide Dccomposition A. Ogary A. Obuchi, K. Mizuno, A. Ohi and H. Ohuchi . . . . . . . . . . . . . . . . . . Effect of Calcination on V-0-Ti-P Catalysts J. Soria, J. C. Conesa, M. Lopez-Granndos, J. L. G. Fierro, J. F. Garcia dc la Ban& and H. Hcincmann . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methanol Synthesis over Copper Based Catalysts: Comparison of Co-Precipitated, Rancy-Type and Catalysts Derived from Amorphous Alloy Precursors A. C. Sofianos, J . Hewling, M. S. Scurrell and A. Armbrusler . . . . . . . . . . . . . . . Activity and Selcctivity of Ruthenium-Cobalt Bimetallic Catalysts in Carbon Monoxide Hydrogcnation S. A. Korili and G. P. Sakclluropoulos . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surfacc Kinetics and Periodic Operation of Higher Alcohol Synthesis J.-L. Li, Q.-M. Zhu, J.-L. Hu and N.J. Yuan . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Selective CO Hydrogenation to Isobutcne over Oxide Catalyst K. Muriiya, A. Tukasan.a, T. Haraoka, M. Aikuwu, T. Arai K. Dottien and T.Onishi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conversion of C,-C, Alcohols into Aromatics on the Modified ZSM-5 Zeolites. Active Centres and Reaction Pathways D.-Z. WUlig, J.-Y. WUIIRatld X.-D. LU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Activities and Selectivities of Supported Co-Ru, Co-Pd and Co-Pt Bimetallic Catalysts in Fischer-Tropsch Synthesis M. P. Kapoor, A. L. Lapidus and A. Yu.Krylova . . . . . . . . . . . . . . . . . . . . . . Ncw Ternary Cu-V-Zn Catalyts for Conversion of CO, by H into Methanol N. Kunoun, M. P. Asticr, F. Lecomte, B. Pommier and C. M. %'ajonk . . . . . . . . . . Alkali-Promoted MoSZ Catalysts for Alcohol Synthesis: The Effect of Alkali Promotion and Preparation Condition on Activity and Selectivity H. C. Woo, T. Y. Park, Y. G. Kim, In-S.Nam, J . S. Lee and J. S. Chung . . . . . . . . . New Alkcnc Oligomerization Catalyst NiSOdy-A1203 and its Characteristics T. Cai, D. CaoandL. L i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Properties of Partially Reduced Fe/Si02 in CO Hydrogenation S. H. Moon, C. W. Park and H. K. Shin . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogenation o f Carbon Monoxide over Rh/ZrO2 Catalysts Promoted by Molybdenum Oxide E. Guglielniinotti, E. Cianiello, F. Pinna, G. Strukul, S. Marlinengo and L. Zanderighi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Identifying the Reaction Network of the Higher Alcohol Synthesis over AlkaliPromoted ZnCrO Catalysts L. Lielti, E. Tronconi and P. Forzarti . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects o f Formic Acid and CO, in CO Hydrogenation to Methanol over CopperBased Catalysts and Nature of Active Sites J . Cai, Y. Liao, H. Chcn and K. R. Tsai . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of ZnO-Supported Cu, Cu-Mn, Cu-Fe, Cu-Co and Cu-Ni Catalysts in CO Hydrogenation P. A. Sermon, M. A. M. Luengo ( Y a m ) and Y. Wung . . . . . . . . . . . . . . . . . . . . Methanol Synthesis from CO, and H over Supported Copper-Zinc Oxide Catalyst. Significant Influence of Support on d t h a n o l Formation H. Arakuwa and K. Sayama. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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XXlX C02 Hydrogenation over Platinum Group Metals Supported on (30,: Evidence for a Transient Mctal-Support Interaction A. Trovarelli, G. Dolcetri, C. de Leitenburg and J . Kaspar . . . . . . . . . . . . . . . . . Potassium Promotion of Cu-ZnO-A1203 Catalysts for Higher Alcohol Synthesis I. Boz, D. Chadwick, I. S. Metcalfe and K. Zheng . . . . . . . . . . . . . . . . . . . . . . Conversion of Syngas to Aromatic Hydrocarbons on Cobalt-Manganese-Zeolite Catalysts G. Baurle, K. Guse, M. Lohrengel and H. Papp . . . . . . . . . . . . . . . . . . . . . . . Light Olefins Formation from Syngas over Zr02-ZnO Catalysts W.Zhang, R. Gao, G. Su and Y. Yin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Selectivity of Diesel Fraction in Fischer-Tropsch Synthesis with Co/Nb,O, A. Fryahan, R. R. Soares and M. Schmal . . . . . . . . . . . . . . . . . . . . . . . . . . . The Catalytic Behavior of Some Zr-Ni-Co-Cu-Ru Intermetallic Compounds in Fischer Tropsch Reactions I. R. Harris, I. T. Caga, A. Y. Tala andJ. M. Winterbollom . . . . . . . . . . . . . . . . X-Ray Powder Diffraction In Situ Characterisation of the (Cu, Zn, Al)-Hydrotalcite Phase in Cu-ZnO-A1203-Catalysts Highly Active in Methanol Synthesis K. Richrer, W. Kraus, G. Nolze and B. Peplinski . . . . . . . . . . . . . . . . . . . . . . . Catalyst Component Interactions in Ag-Cu/ZnO/A1203 Catalyst for Carbon Oxygenate Synthesis E. Kis, G. Lotnic, G. Boskovic, R. Neducin and P. Putanov . . . . . . . . . . . . . . . . Cobalt Reducibility and Magnesium Promotion in Silica-Supported Fischer-Tropsch Catalysts I. Puskris, T. H. Fleisch, J . B. Hall, B. L. Meyers and R. T. Roginski . . . . . . . . . . . Transient Response Study of the Oxidation and Hydrogenation of Carbon Monoxide Adsorbed on Pd/A1203 G. Kadinov, S. Todorova andA. Palazov . . . . . . . . . . . . . . . . . . . . . . . . . . . Higher Alcohol Synthesis over Functionalized Solid-Base Catalysts from Methanol as a Building Block W. Uea'a, T. Ohshia'a, T. Kuwabara and Y. Morikawa . . . . . . . . . . . . . . . . . . . In Situ 1H NMR Study of the Adsorption of Hydrogen and Formic Acid on Copper Based Methanol Synthesis Catalysts A. Bena'ada, J . B. C. Cobb, B. T. Heaton and J. A. lggo . . . . . . . . . . . . . . . . . . Isomerisation of Long-Chain n-Alkanes on Pt/H-ZSM-22 and Pt/H-Y Zeolite Catalysts and on their Intimate Mixtures J. A. Martens, L. Uyrrerhoeven, P. A. Jacobs and G. F. Froment . . . . . . . . . . . . . Kinetics of Bimodal Grain Size Distribution of a Ni Catalyst During Hydrogenation of co M.Kolb, M. Agnelli and C. Miroa'atos . . . . . . . . . . . . . . . . . . . . . . . . . . . . Critical Point Properties of the Surface Structure During CO Oxidation M.Kolb.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selectivity Effects Related with Reaction Mechanism and Diffusion Limitations over Deactivating Catalysts K Kumbilieva, S. L. Kiperman andL. Petrov. . . . . . . . . . . . . . . . . . . . . . . . Author index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Studies in Surface Science and Catalysis (other volumes in the series). . . . . . .
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Guczi, L. et ul. (Editors), NewFrontiers in Cufalysk Proceedings of the 10th International Congress on Calalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights resewed
CATALYSIS: PAST, PRESENT AND FUTURE J . A. Rabo UOP, Tarrytown Technical Center, 777 Old Saw Mill River Road, Tarrytown, New York 10591, USA
1. FOUR DECADES OF CATALYSIS
Has catalysis fulfilled its expectations in the last 40 years? How does progress in catalysis compare with other areas of science and technology? Furthermore, what is the impact of catalysis on human society and environment today? The 10th International Congress on Catalysis (ICC) has assembled to consider these important questions. During these 40 years, electronics has emerged from fundamental science to a consumer industry, whose diverse products touch many aspect of daily life. Biochemistry and related biosciences grew from intractable complexity to a systematic unveiling of life’s chemical mechanism, and technologies have revolutionized the pharmaceutical industry. In these 40 years, catalysis grew to become the critical technological underpinning of our chemicals, fuels, and pharmaceutical industries. Catalysis has also become the principal challenge for future progress in these technologies. Future catalysis will be aimed at reducing the cost of raw materials and creating new, exciting products that use catalysts of higher catalytic specificity. How important is catalysis to industry and, more broadly, to humanity? A recent report [ 11 set the value of the worldwide catalyst market at $5 billion, with $1.9 billion alone in
the United States. The cost of catalyst is estimated to be about 0.1 percent of the value of the fuels produced and about 0.22 percent of the petrochemicals produced [2]. This cost translates to worldwide product values for fuels and chemicals of $2.4 trillion. This amount is more than the gross national product (GNP) of some major industrial nations and is about half of the U.S. GNP. Clearly, the impact of catalysis must be measured by the impact of its products, not by the cost of the catalyst itself [3]. In the last decade, catalysis has become the critical technology for the protection of our environment in addition to its role in the fuel and petrochemical industries. Part of the challenge is to reduce pollutants emitted in the processing of fuels and chemicals, and another is to help produce new, environmentally friendlier fuels and Chemicals. An additional and ever-growing role is to reduce undesirable emissions from automobiles and power plants. In this field alone, the yearly catalyst market in the United States was about $650 million in 1989. In Europe, the total catalyst market was $900 million in the same period, and growth was projected to $1.8 billion by 1993, with autoemission catalysts being the fastest growing component. The societal impact of these industries, fuels and petrochemicals, in which catalysis is a critical component also needs to be considered. According to a recent study of U.S. industry, the chemicals and allied products industry was the largest of all U.S. industries, with yearly sales of $210 billion compared, for example, with $127 billion for automobiles [4]. Employment in chemicals in the United States was 805,000 compared to 1.04 million in automobiles. Another excellent study reports that of the 63 major product and 34
2
process innovations between 1930 and 1980 in the chemicals and fuels industries, more than 60% of the products and 90% of the processes were based on catalysis [5]. This information clearly points to the critical role of catalysis not only to the chemical and fuel industries but also to the economic progress of society as a whole [6]. In the four decades since the first ICC, catalysis technology has blossomed in large measure as a result of the enthusiasm of participating scientists. The industrial challenge and competition to rapidly develop new catalytic processes substantially contributed to progress in catalysis. Over these years, the ICC has provided an important forum to report on new discoveries in catalysis science and technology. The major events reported at past ICC's have been reviewed in several articles [7,8].
2. COMPLEXITY OF CATALYSIS The amazing concept of catalysis, that a foreign material can greatly accelerate chemical reactions without itself changing (ultimately) in the process, has inspired many theoreticians and experimentally inclined chemists and chemical engineers. This field offers opportunities of great scientific and enormous economic significance. As a result, catalysis, which began as a one-time experimental endeavor in chemistry, substantially broadened in the last 40 years to include among its researchers representatives of a wide array of disciplines, such as chemistry, engineering, and materials and surface science. The totality of these disciplines reflects the multidisciplinary character of the field of catalysis. The amazing chemical concept of catalytic phenomena inspired the birth of many early "theories," many of which required modifications or did not survive a careful check of their general applicability. Problems usually involved lack of adequate limitations given for the applicability of the proposed hypotheses. To conceive the best future strategy for catalysis as a whole, scientists must first to recognize how far they still have to go before they see the "light at the end of the tunnel." The important questions are whether the whole scientific basis of catalytic phenomena has been uncovered and whether catalysis phenomena have been adequately described to render solutions that are tractable using present science. The answers to these questions can be found in the following brief review of progress in two important areas: metal catalysis and acid catalysis with zeolites. 2.1 Metal Catalysis
Science in merul curulysis has been greatly aided by two factors. First, studies of single metal crystals provided a model for practical, finely dispersed, supported metal catalysts. Second, the evolution of an amazing array of surface science techniques provided in situ investigative tools, such as surface crystallography using low-energy electron diffraction (LEED),for the study of metal surfaces. In addition, the structure of ligands, such as hydrogen, carbon monoxide, and olefins, has been defined together with the metal atom or cluster with which they are associated. In this field, I3C nuclear magnetic resonance
3 (NMR) spectroscopy, electron energy loss spectroscopy (EELS),and other spectroscopic techniques have been particularly effective [9]. Because optimized industrial metal catalysts usually contain one or two metals and an oxide in addition to a variety of additives, much attention has been given to alloy chemistry, particularly on the metal surface, on metal-oxide interaction, and on adsorbate-adsorbent interaction. The complexity raised by these phenomena induced attempts to generalize the chemistry by taking into account concepts from both organometallic chemistry and solid-state chemistry. The significance of electronic versus structure effects has been debated at length. In spite of the enormous growth in information on the surface Chemistry of noble metals, some of the most important aspects of metal catalysis remain unexplained, and the apparent complexity of the phenomena seems to have grown. For example, recent surfaces studies [ 101 show that chemisorbed ligands significantly change the structural position of the adjacent metal atom by stretching their bonds relative to their adjacent metal atoms. This suggests a balancing act between the organometallic complex formed with the reactant molecule and the metal crystal itself, based on the thermodynamic preference of each species. Quantitative evaluation of the energy balance between the short-range effects in the organometallic "molecule" and the long-range effects in the metal crystal is difficult, and this evaluation can be handled only by using approximations, particularly regarding the long-range effect. In hydrocarbon catalysis on noble metals, a carbonaceous overlayer covers a significant amount of the metal, affecting or even templating the surface metal layer, with important involvement in surface chemistry and in catalytic mechanism [ll]. The specific role of this carbon overlayer is unknown. Most recently, researchers found that under certain conditions carbon monoxide and oxygen on platinum produce a dynamic cycle, which forms a carbon-monoxide-rich overlayer followed by an oxygen-rich overlayer in a cyclic manner [ 121. Furthermore, both the carbon-monoxide-rich overlayer and the oxygen-rich overlayer display a long-range order, which suggests a long-range directing effect of unknown source. Again, the problem of reconciliation between short-range and long-range effects emerges. Clearly, even when using the whole m y of surface science techniques available today, the study of noble metal single crystals still reveals new, unexplained phenomena. Recently interest has grown in the modeling of catalytic phenomena on noble metals using both ab initio methods as well as a variety of approximations in the computations for the long-range effects that cannot be handled today by rigorous, ab initio methods even with the use of supercomputers.
By comparison with single crystals, industrial metal catalysts are vastly more complex. Instead of having well-grown crystals, they usually contain a variety of supported metal clusters or alloy particles. Industrial catalysts are usually multifunctional, that is, they serve more than one role in chemistry and technology. The result is a complex catalyst composition and chemistry with chemical interaction between the various ingredients. For these reasons, chemical intuition and experience play as large a role as science, used as predictive tool, in developing new and better industrial catalysts. Thus, in spite of great
4
enlargement of knowledge, researchers are still searching for the "light at the end of the metal catalysis tunnel." 2.2 Acid Catalysis with Zeolites
In catalysis with metal oxides, the elucidation of catalytic phenomena has been aggravated by a lack of techniques available to define the structure of the surface with the precision required to construct useful theoretical models for reaction intermediates. This troublesome uncertainty about surface structure was to be absent in acid catalysis wirh zeolites. Researchers hoped that X-ray crystallography would provide precise information on all atoms in the zeolite crystal, and thus catalytic sites would be well defined. Now, 35 years after the first experiment with crystalline H-Y zeolite and after three decades of great success in industrial applications, researchers may finally be close to explaining, with some precision, the structure and chemistry of protonic catalytic sites in zeolites. In this case, the problem of characterizing protonic sites rests on the fact that these sites are usually introduced by a proton attack on an oxide ion via thermolysis of the cation. The proton, with its large electron affinity and minuscule size, affects not only the lone electron pair of the adjacent oxide ion, but also the bonds surrounding the newly formed acidic hydroxyl group. The result is a dislocation in the Si-OH-A1 linkage that evaded precise characterization until recently.
m+
The experiments with H-X and H-Y zeolites 35 years ago showed that with aluminosilicates, crystallinity is critical for strong acid catalytic action [13]. Early hopes to define the protonic catalytic sites were hampered by the chemical instability of the primary protonic sites in zeolite crystals. This instability caused various degrees of framework aluminum hydrolysis. Whether the partially hydrolyzed aluminum, or possibly its combination with the nonhydrolyzed protonic site, is the strong acid catalytic site has been a recurring question all through the years. The hundreds of IR spectroscopic investigations involving protonic zeolites provided little more than fingerprint-type characterization. 'H MAS NMR spectroscopy also olfered hope for fundamental information on protonic acidity [14]; however, the small range of the proton chemical shift limits the observation of differences between acidic hydrogens in solid acids to a small range. Furthermore, the acid catalytic action in catalysis should correspond to the deprotonation energy of the solid, that is, the energy needed to transfer the proton from the catalyst to the reactant, more than to the mere polarity of the 0-H linkage, which has been traced by static spectroscopic observations. The actual transfer of the proton involves substantial changes in the bond structure all around the protonic site. These changes depend on many material factors, including the polarity of the 0-Hlinkage and the polarizability of the surrounding bonds, as well as some long-range effects. Theoretical modeling has been applied to elucidate the case of zeolite acidity by using R,Al-OH-SIR, type molecules and, more recently, by using larger model segments of the zeolite crystal. The most important result of the theoretical modeling work is the conclusion that the bond structure of the Al-OH-Si group controls the ionic character, or
5 the acidity, of the 0-Hlinkage. According to this model, the deprotonation energy of zeolites decreases with increasing Si-OH-A1 bond angle [ 151. Thus, the intrinsic strength of acid sites must also increase with increasing Si-OH-A1 bond angles. This conclusion, drawn from theory, finds good corroboration with the relationship between AI-0-Si bond angles of zeolites such as Y, mordenite, and ZSM-5 and their acid catalytic activity. The precision of theoretical modeling becomes questianable when it has to reconcile the results between short- and long-range effects in zeolite structures. The problem is centered on a recently proposed interpretation regarding the cause of the great difference in acid catalytic activity and acid strength between H-zeolite crystals and amorphous silica-alumina gels [16]. The interpretation rests on the idea that without the influence of outside factors, the protonated Al-OH-Si group prefers to form strongly covalent 0-H bonds by forming relatively low (sp'-type) Al-0-Si bond angles. In zeolites, and particularly in silicon-rich zeolites, the Al-0-Si bond angles range between 135 and 170"; thus, they are higher than sp3-typebond angles. It was suggested that lowering these high angles for the Al-OH-Si units, without affecting the nonprotonated Si-0-Si linkages, is prevented or at least minimized by the zeolite crystal to maintain long-range order. Thus, if long-range order in the crystal prevails over the short-range bonding preference of the AI-OH-Si group, then the original, large bond angles will be retained. In this case, the zeolite retains its crystallinity and displays strong acidity. In cases where the local bonding preference prevails (adjacent to OH group), then the crystal may collapse, and the OH groups will be of much weaker acidity. The H-zeolites with high aluminum and consequently high H-content (%/A1 5 2) indeed lose crystallinity, and crystalline H-zeolites all require the presence of stable Si-0-Si linkages. The observed structural loss is probably the result of the reduction of the bond angle in the A1-OH-Si groups. The maintenance of crystallinity and the consequent strong acidity found in silicon-rich zeolites (Si/Al > 2) depend on an energy balance between the favored bond formation in the AI-OH-Si group and the bond formation favored by the crystal. The latter favors retention of long-range order and symmetry. Here again, the issue is the balance and reconciliation between the short- and the long-range effects. The arguments, outlined previously, attribute the lower acidity of amorphous silica-alumina gel to lower A1-OH-Si bond angles relative to those found in crystalline H-zeolites. However, the following most recent experimental evidence suggests a new and unanticipated chemical step, which is a new addition to the complexity of this catalytic phenomenon. [17] 'H MAS NMR evidence taken following dehydration of amorphous silica-alumina gel and H-Y zeolite indicates that upon removal of the last H20molecule, the amorphous material responds by actually breaking the Si-0 linkage in the A1-OH-Si unit while the dehydrated H-Y zeolite maintains its original AI-OH-Si linkages. These distinctly different chemical steps presumably occur because when the stabilizing H20 ligand is removed, the AI-OH-Si unit prefers to form a strong 0-H bond in the silica-alumina gel by cleaving the Si-0 bond and forming a strongly covalent Al-OH linkage. This step is prevented in silicon-rich zeolites, probably because of the stabilizing effect of the long-range atomic order and related lattice energy of the crystal.
6
2.3 Summary These two short and sketchy excursions into metal and acid catalysis certainly do not portray the depth of science in these fields. They were only intended to exemplify the point that catalysis researchers are still "digging deep in the tunnel." Although some light is filtering through, the basic problem remains the same as before: to search for the definition of the catalytic site and the steps of the catalytic reaction mechanism. This brief review suggests that the best strategy for future catalysis is to continue fundamental research, with the goal of understanding these phenomena. Until such understanding is achieved, the discovery of major new catalysts may evolve more from chemical intuition and experience in the field than from the use of scientific knowledge applied as a predictive tool of research.
3. NEW 0PPOR"ITIES
IN CATALYSIS TECHNOLOGY'
3.1 Chemicals 3.1.1 Lower Cost Feedstocks
For typical commodity chemical processes, feedstock constitutes about 60 to 70% of the total manufacturing costs. Thus, a great financial impact can result from moving to a lower cost feedstock. For example, the Monsanto process for making acetic acid by methanol wbonylation involved a feedstock change: a shift from ethylene, which was used in the previously dominant Wacker process, to methanol. Since its launching in 1970, the Monsanto process has captured most of the world's new capacity for making acetic acid. Feedstock price changes in recent years have further magnified the cost advantage of the methanol carbonylation route. By far the strongest current thrust toward lower cost feedstocks is the effort to substitute alkanes (ethane, propane, and butane) for the corresponding olefins and to convert methane to olefins or aromatics. The difference in price between these alkanes and their olefin counterparts can frequently be as much as IOC per pound, which is substantial. An excellent example is the production of maleic anhydride, a monomer for specialty plastics. Over the last 40 years, advances in catalytic technology have enabled the industry to switch from high-priced, toxic benzene to butenes and, more recently, to the lower cost butane. This latter development was possible only as a result of the discovery of the vanadyl phosphate catalyst by Chevron Research and Technology Company. A new process for producing maleic anhydride by using novel catalyst and reactor technologies is currently under development by Du Pont and is scheduled for commercialization in the mid- 1990s.
* Section 3 has been adapted with minor changes from Reference 18.
7 An extensive worldwide effort is now under way to develop the catalytic oxidative coupling of methane to petrochemicals as well as liquid fuels. This effort encompasses oxidative coupling of methane to ethylene or aromatics, oxidative methylation of propylene to C, olefins, and dehydrogenative coupling of methane to aromatics. This area of methane conversion captured exceptional interest and attention all over the world in the mid-1980s. The relative abundance of LPG (liquefied petroleum gas, containing mostly propane and butane) and the strong demand for aromatics have prompted British Petroleum (BP) and UOP to develop a process for the catalytic conversion of LPG to aromatics. [19] The new process employs a zeolite-based catalyst developed by BP in conjunction with UOP’s continuous catalyst regeneration system. The LPG is converted to a mixture of aromatics, 95 % of which are benzene, toluene, and xylenes. The aromatics yield is about 65 % . In another alkane utilization project, BP Chemicals is developing a process for the direct one-step ammoxidation of propane to acrylonitrile. The key to the process is a proprietary catalyst. Now at the pilot-plant stage, the process is targeted for commercialization in the mid-1990s. A new commercial development in catalytic alkane dehydrogenation relates to the production of isobutylene and propylene. The isobutylene requirement is for the production of gasoline octane enhancers (for example, methyl tertiary butyl ether, or MTBE), and the propylene need is driven by changes in the feedstock used to produce ethylene. These changes have resulted in less by-product propylene production. Several companies have recently installed or are currently installing new plants for the production of isobutylene and propylene via new dehydrogenation technology (UOP). [20] This remarkable development may provide opportunities for new catalysts that would be capable of promoting oxidative dehydrogenation of lower alkanes (for example, ethane, propane, and isobutane) to their corresponding olefins. Direct functionalization of hydrocarbons remains a significant approach (for example, ethane to ethanol, propane to acrylonitrile, and butane to maleic anhydride or 1,4,butanediol).
The catalytic dehydrogenation of paraffins is also widely practiced commercially for the production of linear olefins in the CIO-Cl7range. These olefins are used in the manufacture of biodegradable detergent intermediates. Typically, olefins in the C,o-C,, range are used in the production of linear alkylbenzene (LAB), whereas heavier olefins in the C14-C1,range are used in the preparation of detergent alcohols via hydroformylation. The preparation of yet heavier olefins by catalytic dehydrogenation is also possible for specialized applications, including the manufacture of synthetic lubricating oils and oil additives. Worth noting at this point is the recent introduction of solid (heterogeneous) acid catalysts by UOP for the alkylation of benzene with heavy olefins in the production of LAB. [21] These acid catalysts can replace traditional catalysts, such as hydrogen fluoride (HF) or aluminum chloride (AlCl,) that are used for this purpose. With these solid acid catalysts, the operational hazards associated with the handling and processing of corrosive catalysts can be avoided, and the environmental consequences of this alkylation process can be ameliorated.
The C, chemistry (that is, chemical processes with carbon monoxide, carbon dioxide, or methanol as the starting material) now provides another interesting arena for feedstock-driven innovations in industrial catalysis. The aftermath of the oil embargo of 1973 was an extensive worldwide effort to pursue C, chemistry for the production of chemicals as well as fuels. This effort eventually subsided when the cost of carbon from CIsources such as coal and natural gas appeared unable to compete effectively with the cost of C, from petroleum-based sources, even at the much inflated prices of the latter. However, some significant changes have occurred in the last decade, and so the opportunities for making chemicals via C, chemistry should be revisited. In particular, methanol should be considered as a feedstock because of the substantial downward trend in the cost of methanol relative to the cost of ethylene. Recent reports indicate that rhodium-based homogeneous catalysts promote the reductive carbonylation of methanol to acetaldehyde at selectivities approaching 90% and at much lower pressure than required for prior-art catalysts. With the addition of ruthenium as cocatalyst, the in situ reduction of acetaldehyde to ethanol may possibly be achieved, thus providing a new catalyst system for the homologation of methanol to ethanol. One great challenge for catalysis has been the possibility of producing ethylene glycol via the oxidative coupling of methanol rather than the standard process based on ethylene as feedstock. Significant progress has been made recently in the catalytic oxidative dimerization of dimethyl ether to dimethoxyethane. Dimethoxyethane, in turn, should be hydrolyzable to ethylene glycol. Unlike methane coupling, which requires a temperature in excess of 60O0C,the oxidative coupling of dimethyl ether proceeds at about 200°C with a mixed magnesium-tin oxide catalyst. By using the ether rather than methanol, protection against side reactions has been achieved. These results are an extremely interesting lead that, coupled with the favorable trends in methanol pricing, could pave the way to another major advance in industrial catalysts: the production of ethylene glycol from dimethyl ether. In addition to acetaldehyde, ethanol, and ethylene glycol, other large-volume chemicals currently made from ethylene or propylene may become attractive candidates for manufacture via C, chemistry.
3.1.2 New Catalytic Oxidation Processes Of the different classes of catalytic reactions, hydrocarbon oxidation (that is, reaction with oxygen) generally has the lowest selectivity. In addition to the desired partial oxidation products, significant quantities of carbon monoxide, carbon dioxide, and water are often obtained. The result is complex and costly separations that, in turn, lead to processes with unusually high capital intensity. The annual capital expenditure for oxidation processes, per annual pound of product, is usually several times that for nonoxidative catalytic processes. Well-recognized examples of major opportunities include the one-step oxidation of methane to methanol, of higher alkanes to alcohols, of propylene to propylene oxide, and of benzene to phenol. Recent reports of success in these areas are tantalizing and suggest
9
that catalytic oxidation should be one of the most important and fruitful areas for innovation in industrial catalysis. 3.1.3 New Products
New developments in catalysis can and will be the enabling technology that gives rise to new products in many sectors of the chemical industry. The'potential impact of catalysis on new products is described in the following sections on polymers, pharmaceuticals, and biologically derived products. 3.1.3.1 Polymers
The production of raw polymers (for example, pellets) in the United States approached
60 billion pounds per year in 1990, which corresponds to a $30 billion business. In terms of fabrication into end-use articles, such as synthetic fibers, films, containers, and structural parts, the polymer business is significantly larger. Catalysis contributes to both monomer and polymer synthesis for a major part of this industry. Every polymer scientist involved with synthesis or structure-property studies has a wish list for new monomers or polymers that have not yet been synthesized via clear-cut economic routes for commercial practice. Almost every family of polymeric materials can utilize advances in catalysis in monomer production or in the polymerization process. Changes in material requirements, environmental issues, feedstock availability and economics, and worldwide competitive pressures make future catalytic advances extremely important. The primary area of intense catalytic activity involves the synthesis of new or improved polyolefins. This industry, which evolved out of the original Ziegler-Natta catalyst discovery in the 1950s, produces tens of billions of pounds per year of polyolefins worldwide. New catalyst breakthroughs can lead to new markets of significant volume, including diversified products such as syndiotactic polypropylene, true thermoplastic elastomers, for example, propylene-elastomeric polypropylene resin (EPR) and propylene block copolymers; and the incorporation of polar monomers into various polyolefin classes. For example, if acrylates, vinyl esters, acrylonitrile, and the like can be incorporated into the present low-pressure polyolefin synthesis, the result would be a new family of olefin-based polymers that are likely to have major commercial significance. Of course, improvements in the present catalytic systems will have a pronounced effect on the polyolefins that are commercially available. Improvements in molecular weight distribution control (for example, narrow molecular weight distribution), the ability to synthesize EPRs in gas-phase reactors, and the control of catalyst decay (for example, improved efficiency) are advances that will surely occur. Novel metathesis catalysts for the synthesis of cyclic olefins have resulted in a host of new polymeric structures. Several of these, such as trans-poly(octenamer), have reached commercial status. The few catalysts that are effective are often expensive and require
10
unattractive precautions during industrial scaleup. Further advances could make this area a promising one for industrial exploitation. In the field of functionalized monomers, the trend will likely revolve around new or existing monomers via biomass, coal, or C, conversion as the carbon source availability changes together with economics. Improved catalysts for creating monomers from agricultural commodities or waste materials offer increasingly important opportunities. This field can benefit from the genetic engineering of specific enzyme catalysts. Oxidative coupling of methanol to ethylene glycol and ethanol to 1,4-butanediolcan open new routes to these important monomers. A new process (noted earlier) involving the one-step ammoxidation of propane to acrylonitrile could change the commercial position. Other possibilities, for example, using shape-selective catalysts such as zeolites, could yield lower cost routes to 4,4-diphenol via phenol coupling. Such a molecule is of interest for liquid crystalline and engineering polymers. Improved nonphosgene routes to diisocyanates are desired. Functionalized oligomers such as hydroxyl (OH)-, amino (NHJ-, and wboxyl (CO0H)-terminated polyolefins can yield important blocks for step-growth polymers, such as urethanes and polyesters. Functionalized fluoroolefin oligomers for inclusion in step-growth polymers can likewise offer a new variety of polymers. A nonchlorine route to siloxane polymer precursors is also desired. In the field of polymers, a number of prior failures primarily resulting from the unavailability of appropriate catalysts have been well documented in the literature. The reaction of acetaldehyde to yield polyvinyl alcohol is one such example. Because phosgene is highly toxic, a nonphosgene route to polycarbonate is desirable. The polymerization of phenol to a highly linear unsubstituted poly(l,4-phenylene oxide) of high molecular weight would be of interest as would the polymerization of polyphenylene by benzene coupling. Many other novel polymers are waiting to be polymerized from available monomers. Cationic polymerization continues to be an area of increasing research made possible by improved Lewis or Brmsted acid catalysis. Continued improvement can yield higher molecular weights and industrially acceptable process conditions. Extension to other monomers, such as vinyl acetate, would be of future interest. Anionic polymerintion is of interest primarily for unique block copolymers. However, the extension to additional monomers and the resultant block structures deserve more attention.
3.1.3.2 Pharmaceuticals The 1990s will definitely be labeled the "decade of chirality." Many of the opportunities and challenges in this explosively evolving field stem from the pharmaceutical area and the growing recognition that the "wrong" enantiomer of a racemic drug represents a medical pollutant whose toxic side effects can far outweigh the therapeutic value of the pharmaceutically active enantiomer. The classic example in this area is thalidomide. The R-isomer of thalidomide is an effective sedative; tragically, the drug was sold as the racemate, and it was subsequently discovered that the S-isomer is a powerful teratogen. More recently, Eli Lilly was forced to withdraw its Oraflex anti-inflammatory drug
11
because of liver damage caused by the "inactive" R-isomer. Although recent regulatory changes by the U.S. Food and Drug Administration stopped short of requiring that all drugs be sold as a single enantiomer, the trend in this direction by drug companies is obvious. Among the available strategies for the manufacture of optically pure substances is asymmetric catalysis, which has powerful and unique advantages. Perhaps the foremost is the multiplication of chirality, the stereoselective production of a large quantity of chiral product by using a catalytic amount of a chiral source. Unlike fermentation, asymmetric catalysis is characterized by generality; processes are not limited to "biological"-type substrates, and the R- and S-isomers are made with equal ease. Asymmetric catalysis also circumvents the disposal of large amounts of spent nutrient media that are generated during fermentation. By comparison, optical resolution (or diastereomeric crystallization) is extremely labor intensive and necessarily prcr uces ;3% of the "wrong" isomer, which must be destroyed or racemized in a seps aw .tep.
3.1.3.3 Biologically Derived Products The rapidly growing field of biotechnology brings with it opportunities in the field of enzyme-catalyzed reactions. The role of genetically engineered microorganisms in synthesizing rare and valuable peptides used in human therapeutics is now well established. The same techniques of molecular biology can also be used to enhance the properties of enzymes as catalysts for industrial processes that are similar to classic catalytic technology. This approach can potentially revolutionize the applications of biological systems in catalysis. Enzymes and other biological systems work well in dilute aqueous solution at moderate temperature, pressure, and pH. The reactions catalyzed by these systems are typically environmentally friendly in that few by-products or waste products are generated. The catalysts and the materials that they synthesize are, as a rule, biodegradable and therefore do not persist in the environment. The reactions are typically selective with extremely high yields; and enzymes can be used to catalyze a whole sequence of reactions in a single reactor, resulting in vastly improved overall yields with high positional specificity and 100% chiral synthesis in most cases. The improved use of enzyme catalyst technology with whole-cell catalysis, reactions catalyzed by single enzymes, and mixed enzymatic and chemical syntheses are all important for the development of new catalyst technology. Whole cells of various microorganisms are being used more frequently in the catalytic synthesis of complex molecules from simple starting materials. The use of whole microbial cells as biosynthetic catalysts takes advantage of one of the unique properties of enzymes: they were designed by nature to function together in complex synthetic or degradative pathways. Because of this property, whole cells and microorganisms can be used as catalytic entities that cany out multiple reactions for the complete synthesis of complex chiral molecules. A patent was recently issued for a genetically engineered Eschen'chia coli that synthesizes the molecule D-biotin directly from glucose. Biotin has three c h i d
12
centers, and the current chemical synthesis requires 13 to 14 steps with low yields. Similarly, researchers are constructing a microorganism that directly catalyzes the synthesis of a vitamin C precursor from glucose. Combining genes from various organisms results in a process that uses a microbially synthesized intermediate with a final chemical conversion to vitamin C. Whole cells of microorganisms are also used in the synthesis of antibiotics from carbohydrate starting materials, and whole cells are used in the biocatalysis of certain steroids. A number of specialty chemicals with complex synthetic schemes can be produced most efficiently by intact microorganisms using a series of enzyme-catalyzed reactions designed by nature to work together. The biotechnology field also has a growing number of examples of reactions of industrial significance catalyzed by isolated enzymes. The conversion of corn starch into corn syrup by the enzymes alpha- and gluco-amylase and glucose isomerase is a large industrial process for generating corn sweetener for soft drinks and other uses. The enzymatic conversion of acrylonitrile to acrylamide has recently been commercialized in Japan, Japanese companies and researchers have been diligent in developing enzymatic processes for the synthesis of fine chemicals. Enzyme-catalyzed reactions are used by the Japanese for the synthesis of monosodium glutamate, L-tryptophan, and phenylalanine. The stereospecificity of enzyme-catalyzed reactions has been used to advantage in polymer synthesis as well. Workers at ICI have developed a combined enzymatic and chemical process for the synthesis of polyphenylene from benzene. Benzene is oxidized to a cis-dihydrodiol by an enzyme-catalyzed oxygenation. The diol is derivatized, polymerized, and rearomatized to give polyphenylene in a reaction that cannot be carried out by classical chemical methods because of solubility problems. This new route to polyphenylene is an excellent example of combined enzyme and classic chemical synthesis to make a product that is otherwise too expensive for practical use. Other biological polymers are also finding their way into the field of catalysis in various applications. Microorganisms are used to synthesize materials such as poly@eta-hydroxybutyrate), a biodegradable plastic, and researchers are exploring a series of synthetic silklike materials that may have use5 in applications requiring high tensile strength. 3.2 Production of Fuels 3.2.1 Summary of Challenges for Fuel Processing Catalysis in the 1990s
Some of the challenges for fuel processing catalysis in the 1990s are: 0 Changing feedstocks - Heavier feeds - Higher sulfur and metals content 0 Alternative fuels, CH,, Syngas 0 Changing fuel product slate - More motor fuel - Less fuel oil
13 0
Changing motor fuel specifications and environmental issues
- Gasoline reformulation
0
Desulfurization of motor fuels including diesel oil Reduction of SO, and NO, emissions Use of noncorrosive, inert catalysts Provide economic raw materials for petrochemicals
Some of these challenges and opportunities in petroleum catalysis are described in the following sections.
3.2.2 Existing Fuels Although the current understanding of how the individual components of gasoline affect various environmental issues is limited, several components in gasoline are now considered harmful to the environment if released to the atmosphere in high concentration as either spills, vaporization losses, or the result of incomplete combustion. They include aromatics, notably benzene, which is a carcinogenic reagent; high-vapor-pressure hydrocarbons such as butane; and reactive hydrocarbons such as olefins and sulfur compounds, which could promote the formation of smog and acids. The petroleum industry is responding to these concerns by directing its research toward gasoline reformulation, which will require a number of advances in catalytic technology. Innovations in catalytic cracking catalysts over the last 30 years have improved the conversion of the heavier components of crude oil to gasoline and diesel oil, allowing a reduction in crude imports to the United States of more than 400 million barrels a year. In the future, new types of cracking catalysts and additional process steps will be required to produce motor fuels that are environmentally more acceptable. To reduce the aromatics content of gasoline and at the same time provide higher octane, paraffinic components will require that fluid catalytic cracking (FCC) catalysts produce more olefins. The highly reactive olefins can be isomenzed, oligomerized, or alkylated with paraffins to produce gasoline components. They can also be reacted with methanol to produce a variety of high-octane ethers. For example, the zeolite ZSM-5 has been shown to be active in producing olefins when mixed with conventional faujasite-type cracking catalysts. Molecular sieve catalysts have also been proved active in isomenzing and oligomenzing olefins to a variety of liquid fuels. Cracking catalysts will also have to become more rugged to endure the higher temperatures required to produce more olefins for subsequent processing into high-octane gasoline components. The catalytic cracking process has been the workhorse of modem refineries for 30 years and, with improved catalysts, will continue to be the main process for converting the heavier end of crude oil into more environmentally acceptable components for gasoline and diesel fuels.
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3.2.3 Oxygenates for Octane Boosting The need to reduce the amount of aromatics in gasoline has created a need for organic oxygenates as replacement octane enhancers. Today the two predominant oxygenates used as octane enhancers are methyl tertiary butyl ether (MTBE) and ethanol (EtOH). However, other oxygenates, such as alcohols, ethers, acetates, and carbonates, are known Octane boosters. Some of these compounds have been evaluated only partially for their performance characteristics and may represent major growth opportunities in oxygenated fuels. Currently, approximately 150,000 barrels per day of oxygenated compounds (MTBE and EtOH) are added to gasoline in the United States. By the year 2000, projections indicate that 750,000 barrels per day (the equivalent of roughly 10% of current U.S. production of petroleum) of oxygenates will be required in the United States for the gasoline pool. In the United States, the 1990 MTBE production capacity was 117,200 barrels per day. By 1993, it is targeted at 256,400 barrels per day. Iso-olefins (for example, isobutylene) are now produced as a by-product of fluidized catalytic crackers in petroleum refineries. These quantities will support the production of ether at a rate of only 200,000 to 300,000 barrels per day. The isomerization of nC, to C, olefins produced in the catalytic cracking process could substantially boost the production potential of gasoline oxygenates without substantial added cost to the refiner. The catalyst challenge here is to achieve high skeletal isomerization selectivity and particularly to attain adequate catalyst life, that is durability, and still carry out the isomerization reaction. The process technology leading to the majority of oxygenates now in use has been made efficient through catalyst modifications and engineering design. However, new processes leading to the same oxygenates may have cost advantages if different building blocks and feedstock sources (crude oil versus coal versus natural gas) are used. The precursors of established octane enhancers, alcohols and ethers, rely heavily on natural gas and crude oil as their feedstock supply. Industry estimates suggest that when crude oil prices pass approximately $30 per barrel, coal gasification to syngas and the conversion of syngas to hydrocarbons and oxygenates may become cost competitive. Therefore, indirect liquefaction of coal may be a way to produce oxygenated fuels for the transportation industry.
3.3 Environmental Protection Public interest in protecting the environment has increased and expanded greatly. The challenge is to preserve the benefits of modem technology without seriously contaminating the natural world. Three strategies are available for reducing the impact of chemicals on the environment: waste minimization, emission abatement, and remediation. Waste minimization calls for the design and development of products and processes that are inherently low polluting or nonpolluting. The abatement of emissions can often be achieved by trapping harmful effluents or converting them to harmless substances, for example, conversion of nitric
15
oxide to nitrogen. Where an environmental insult has occurred, effective means of remediation are needed to restore the environment to its "green" state. As shown by the examples in the following sections, catalysis can contribute to these three approaches.
3.3.1 Alkylation Catalysts The alkylation of paraffins with olefins is one of the major refinery operations. The process reacts isobutane with olefins, generally propylene and butenes, to produce highly branched C, and CBparaffins, respectively. They constitute a premium high-octane gasoline component (95 to 98 research octane number for C, alkylate). The alkylation process is assuming increasing importance with increased olefin production from modem FCC units and with the recent emphasis on clean fuels of lower aromatics content. Both of the currently used liquid acid catalysts, sulfuric acid and hydrogen fluoride, are corrosive. Acid waste disposal in the process catalyzed by sulfuric acid is of increasing environmental concern because liquid hydrogen fluoride is a potential health hazard. As a result of increasing concerns and possible legislative action addressing environmental and safety issues, current alkylation processes may face critical scrutiny. Exploratory studies have shown that new catalysts can be developed that are cleaner and safer than those presently used. However, powerful acid catalysts are still required for alkylation. The formidable challenge to the industry is the production of a novel catalyst system that makes a new process economically feasible: a new process must have high yield of alkylate, selectivity to produce high-octane gasoline, long life cycle, regenerability, and greatly reduced environmental and safety risks. This challenge would benefit from broad-based fundamental studies of acid catalysis and from increased exploratory research. At the present time, no satisfactory solid motor fuel alkylation catalyst exists. An alternative, if temporary, solution in this field may be a combination of modifications both in the chemistry of the currently used halide catalyst and the process design that can substantially reduce undesirable emissions, and even the hazard of accidental emissions, in the alkylation process.
3.3.2 Replacements for Chlorofluorocarbons Chlorofluorocarbons (CFCs) are now believed to contribute to the seasonal ozone depletion over the Antarctic continent. However, because they are crucial to many aspects of modem society and have no available replacements, ceasing their production immediately is not practical. By 1988, the total CFC consumption worldwide had grown to 2.5 billion pounds per year for use as refrigerants, foam-blowing agents for polystyrene and polyurethane, and industrial solvents and cleaning agents. Ironically, the high stability and inertness of CFCs, which make them so valuable, have led to their downfall. Once released at ground level into the atmosphere, they rise slowly into the stratosphere, where they are degraded by high-energy radiation from the sun and release chlorine-containing free radicals that trigger a catalytic ozone depletion
16
cycle. Following detailed, though not definitive studies, agreement was reached on a major global environmental treaty, the Montreal Protocol, to phase out CFC production by the turn of the century. A race by all CFC producers then began to find suitable and environmentally acceptable substitutes. The strategy is to reduce its atmospheric lifetime by introducing hydrogen into the molecule so that it is removed from the atmosphere by reaction with hydroxyl radicals in the stratosphere. The commercially viable synthesis of these new compounds is a major challenge for catalysis because catalysts used for the production of CFCs lack the required selectivity and activity to be acceptable for the production of hydrogen-containing substitutes. The projected costs for these molecules, hydrogenated chlorofluorocarbons (HCFCs), are approximately two to five times those of the CFCs they are replacing because of the complexity of the new manufacturing processes. Although rapid progress is being made toward the production of HCFCs, they are not entirely satisfactory and may have to be phased out in turn. Consequently, major advances in catalytic science and technology will be required to develop more acceptable substitutes before the turn of the century.
3.3.3 Emission Abatement Catalytic technology is playing an ever-increasing role in environmental protection. In 1989, for the first time in the United States, the market for emission control catalysts (largely for automotive emissions) exceeded the market for petroleum refining catalysts. However, the area of stationary emission control (for example, from power plants) has been flagged as one that will experience large (20%per year) growth in the years ahead. Many chemical production plants also need to reduce emissions. Thus, novel catalysis will in many cases be the critical technology that enables most of the benefits created by the chemical and petroleum industries to be maintained, but with improved preservation of the environment. Catalysts for automotive emission control are now well developed in the United States and, in general, meet mandated standards for removal of hydrocarbons, CO, and NO,. Recent Clean Air Act revisions will require significantly greater reductions in 1993 in the emissions of hydrocarbons, CO, and NO, than those that are now mandated. In addition, stringent local automobile emission standards, as in California and Vermont, will require up to a tenfold reduction in emissions by the late 1990s. These reductions will require more active catalysts, new catalyst supports (for example, metallic supports), and new reactor designs that enhance low-temperature performance. A first step in this direction is the electrically heated converter, which offers a severalfold reduction in emissions over currently available technology. Because of the rapidly escalating price of rhodium, which promotes NO, reduction, the industry needs to develop lower cost NO, reduction catalysts or practical catalysts for NO, decomposition. Cleaner gasolines may provide opportunities to develop lower cost catalysts that contain a smaller amount of the expensive noble metals. Finally, an improved technology needs to be developed for recovering rhodium because of its high cost.
17
The control of power plant emissions is another major area of opportunity for catalysis. In particular, NO, emissions need to be removed either via selective catalytic reduction (SCR) or, if achievable, via the preferable NO, dissociation. The abatement of NO, from power plants is important in efforts to control acid rain and photochemical smog, the latter being linked with harmful ozone production. Selective catalytic reduction removes NO, in flue gas by reacting oxides of nitrogen with ammonia to form nitrogen and water. First commercialized in Japan, SCR is now being used extensively there. It has also been commercialized in Germany. The catalyst is the heart of SCR technology, and it must provide both high activity and high selectivity (toward nitrogen formation). Major improvements in SCR catalyst performance can be achieved through the strategic design of the catalyst pore structure. 3.3.4 Biodegradation of Organic Waste Enzymes, like man-made catalysts, accelerate the rate of reactions. Reactions catalyzed by enzymes include the oxidation and hydrolysis of natural and synthetic organic chemicals regarded as pollutants of soil and groundwater. Enzymes have some natural advantages over other catalysts in the degradation of environmental pollutants. Enzymes are most active against materials at low concentration (micromolar to millimolar range) in the presence of water and can be simple and inexpensive to manufacture because they are grown along with microorganisms. Enzymes themselves are biodegradable. Enzymes can exhibit either narrow or broad substrate specificity. The latter characteristic is desirable for enzymes that attack and degrade organic contaminants. For example, methane monooxygenase has an amazingly broad substrate specificity and can catalyze the oxidation of alkenes; ethers; and alicyclic, aromatic, or heterocyclic molecules. This enzyme system can also degrade synthetic organics, such as the chlorinated solvents chloroform, dichloroethylene, trichloroethylene, and l,l,l-trichloroethane. The ability of enzymes to degrade natural organics, such as the components of gasoline, crude oil, and most solvents, as well as synthetic organics, such as trichloroethylene or polychlorinated biphenyls, means that most, if not all, organic contaminants can be degraded in reactions catalyzed by enzymes. Chlorinated organics, such as dichlorodiphenyl and trichloroethane and its by-product dichlorodiphenyl ethylene, pentachlorophenol, chlorocatechols, and other chlorinated aromatics used as preservatives and pesticides, are degraded in oxidation reactions catalyzed by enzymes from bacteria and other microorganisms. Even the most complex halogenated organics, such as polychlorinated biphenyls (PCBs) and chlorinated solvents, are subject to catalytic attack by certain microorganisms. The PCBs were developed earlier in the century as oils for use in electrical equipment and as lubricating fluids in industrial applications because they gave good insulating and lubricating properties without being explosive or flammable. However, PCBs were later discovered to bioaccumulate and are now classed as environmental hazards. Recently, enzymes have been found in microbes that will reductively dechlorinate PCBs and oxidize
18
them in the presence of molecular sieves. Even though these enzymes had not evolved sinply to degrade PCBs, they have a broad-enough substrate specificity to catalyze the initial degradation of PCB molecules in the environment. The use of oxygenates with broad substrate specificity in bacteria may be the only practical method of treating PCB-contaminated soil and water because of the low cost and adaptability of oxygenates in the environment. Similarly, chlorinated solvents have been widely adopted because of their excellent solvent properties and lack of flammability. Trichloroethylene (TCE), perchloroethylene, and trichloroethane have been used widely in the past as dry cleaning solvents and chemical degreasers for metal finishing and electronic applications. Surprising numbers of different oxidative enzyme systems that will attack TCE or other chlorinated aliphatics have been identified. The technique of enzyme recruitment offers the prospect of producing single organisms that contain a spectrum of genetically engineered enzymes capable of degrading hazardous waste in the environment. Enzyme recruitment permits microorganisms to degrade new molecules and broadens the ability of microorganisms to attack synthetic organic chemicals. The idea of using enzymes that can reproduce themselves, can be made cheaply, and can work under conditions that are often found in the environment may be one of the most effective means modem science can devise for treating and degrading hazardous waste, including organic chemicals synthesized to be stable under harsh conditions. The broad substrate specificity of certain enzymes offers the opportunity to use enzyme catalysis for improving and protecting the environment.
4. NEW TRENDS
IN CATALYSIS
4.1 New Catalytic Materials
In the last four decades, the largest impact on heterogeneous catalysis technology was the introduction of new catalytic materials. The catalyst base expanded to include a host of new oxides, zeolites, immobilized enzymes, membranes, and clays. The discovery of synrheric zeolires grew out of an interest in providing the industry with adsorbents that had superior separation selectivity, based on polarity and molecular sieve effects. Because the zeolite crystal has uniform pores of the size of small to medium molecules throughout its entire length, diffusion was expected to be a potential problem. The utility of zeolites as catalysts was based on the discovery that large-pore zeolites can be transformed to strong acid catalysts by introducing acidic 0 - H groups, either by applying cation exchange with multivalent cations or by thermalyzing ammonium cations. Slow diffusion did not become a practical problem, even though the reactant molecules in the first major applications, catalytic cracking and hydrocracking, were close in size to the pores of the Y zeolite. Later, the molecular sieve effect as well as the limiting diffusion rates have been successfully applied to benefit catalytic selectivity in several commercial processes.
19
The successful use of protonic zeolites, with their strong acidity, in a large number of commercial applications inspired the discovery of a much wider range of microporous crystals with well-defined intracrystalline pores and cavities. Through creative research strategies, such as the use of templates in synthesis, the family of molecular sieve crystals has grown beyond aluminosilicates to include pure silica, aluminophosphates, and a wide variety of additional, diverse compositions with up to six or even more framework elements [22]. Most of these materials are capable of transforming to a protonic form and thus can act as acid catalysts. The pore sizes vary from about 3 A to about 20 A. However, the pore size that may be achievable in the future with creative synthesis strategy has no theoretical limit. The chemical composition originally limited to aluminosilicates and silica has expanded to include more than a dozen elements without any emerging rule that would exclude the incorporation of additional elements as framework ions [23]. At present, the field of new microporous crystals is growing rapidly, is freed by the original limitations in composition, and is fueled by an amazing inflow of' creative ideas in material synthesis. Important catalyst applications of microporous crystals have protonic acidity as the principal catalytic function. So far, materials containing silicon and aluminum display the strongest acidity, which benefits cracking, hydrocracking, and small-paraffin isomerization reactions. With other chemical compositions, the intrinsic acid strength tends to become weaker, and thus, it can be controlled with the choice of framework elements. The underlying cause of changes in protonic acid strength in microporous crystals is complex. Part of the problem is that with several compositions, the site of protonic acidity has not been established with precision. For aluminosilicates and for most other microporous crystals, the underlying cause of strong acidity has been emerging only recently (see Section 2). The intrinsic acid strength of a H-zeolite hydrogen is controlled both by chemical arid structural factors. With the wide choice of available chemical compositions, crystal structures, pore shapes and sizes, the design of catalytic sites that meet desired acid strength and steric requirements has come close to reality. Acid strength and a shzpe-selective reaction environment can be "engineered" into unique combinations by using appropriate synthesis and postsynthesis treatment strategies to achieve improved catalytic seiectiviiy for both old and new catalytic reactions (see Section 3). Although catalysis with microporous crystals rests mainly on acid catalysis, these materials are also often used as bifunctional catalysts, which usually contain a noble metal that is finely dispersed in the microprous surface in the crystals. The proximity between small noble metal clusters and acid sites within the intracrystalline pores is the reason for effective catalytic action with these materials. The study of these metal dispersions has resulted in a substantial body of scientific literature during the last 30 years. Ammg the elements used in the formulation of new microporous crystals are several transition metals and other elements with variable valence. Molecular sieves containing these elements provide the opportunity to introduce transition metal catalysis or redox catalysis by framework ions contained in the crystal structure. These new compositions
20
offer new opportunities in catalysis because they have transition metal ions in low, tetrahedral coordination. Depending on the crystal structure and chemical composition, these ions are all in one or in a small number of crystallographically well-defined sites. By comparison with traditional, supported transition metal oxides, these new crystals offer unique advantages based on the potential uniformity of the transition metal ion sites and the high concentration of catalytic sites throughout the crystal. Recent NMR experiments indicate that tetrahedral aluminum ions in aluminophosphate molecular sieves readily accept water molecules by assuming octahedral coordination in a reversible process. Such evidence suggests that framework-transition-metal ions may also be capable of interacting directly with reactant molecules. These materials offer new opportunities to catalysis in the future. Aluminophosphate-based molecular sieves have been reported to show high selectivity and stability in several new or improved industrial applications, including catalytic dewaxing, olefin isomerization, CBaromatics processing, and production of the C, to C, olefins from methanol. The realization of new, successful catalytic applications will also be helpful in providing new incentives for the rapidly expanding synthesis research activities for new microporous crystals.
Clays belong to the original category of catalytic materials used in old-time catalysis. With their ion exchange capacity used to generate acid sites, clays have provided cheap catalysts for nonregenerable catalyst applications in the petroleum industry. The rebirth of interest in clay products rests on the high and uniform surface area obtainable by delamination and by the conceptual adaptability of postsynthesis treatments used successfully with zeolites in the past. Recently, the highest interest has been associated with the ability to vary the gallery space between clay layers by the introduction of uniform inorganic or organic cationic pillars, which provide large, accessible surface area and molecular sieve action that is defined by the size of the pillar. By comparison with zeolites and other three-dimensional microporous crystals, the geometry and the chemistry of the gallery space is less uniform for pillared clays because of the somewhat ill-defined position of the pillars in the gallery space. This limits expectations for shape-selective action similar to those known for zeolites. Recent advances in pillared clay synthesis resulted in the development of large-surface materials with about 18 A uniform gallery spacing and with hydrothermal stability matching that of steam-stabilized zeolites. Such properties render clay products as good catalyst candidates in catalytic cracking and hydrocracking for treatment of heavy petroleurn fractions. Delaminated clays remain attractive catalyst candidates in applications where low catalyst cost is critical. Clearly, the future of clay-based materials rests on the adaptation of synthesis strategies applied for the modification of the clay layers and on better control of pillar ordering.
Homogeneous carulysis benefits from its close relationship with organometallic chemistry. This branch of chemistry, with many research incentives of its own, provides the basic chemistry as well as most of the synthetic methods needed for the "construction"
21
of advanced homogeneous catalysts. Recently, one of the prominent aspects of homogeneous catalysis relates to a desire to use paraffins as feedstocks in organic synthesis and the synthesis of chiral compounds. Taking ideas from the catalytic function and structure of enzymes, they also adopt ligands to achieve the specific steric hinderance required for stereospecific synthesis steps. Of course, the predictability of the catalytic function is limited here also by the influence of factors both within and beyond the transition metal complex, such as the solvent, additive functions, and redox phenomena. However, strategic manipulation of relevant parameters can result in success, as may be illustrated by the synthesis of the drug naproxen. Currently, the noncatalytic chemical synthesis produces a mixture of the two optical monomers from which mixture the S-naproxen must be separated because the R-naproxen is a toxin. Recently, the selective synthesis of the desired S-isomer has been achieved with high selectivity by asymmetric hydrogenation of cr-(6-methoxy-2-napthyl) acrylic acid with a ruthenium complex containing a chiral ligand. Similar success was achieved in synthesizing Ldopa, which is used in the treatment of Parkinson's disease. The stereospecific synthesis of chiral compounds at a level of selectivity that permits the direct use of the product without the currently required expensive separation and purification is an important future objective in catalysis. Beyond the obviously important applications in the production of pharmaceuticals, chiral compounds also offer new material properties for both monomer and polymer molecules. New properties for chiral compounds produced at moderate cost by highly selective and efficient catalysts would create new applications and new markets well beyond what can be envisioned today. An interesting marriage between enzymes and microporous crystals is the occlusion of small, enzymelike molecules in the intracrystalline cavities of zeolites [23]. If successful, this approach in catalysis will result in higher catalyst stability, lower cost, and easy use of enzyme action in a traditional, and cheap, continuous flow operation. 4.2 Catalyst Characterization and Theoretical Modeling
The last four decades have seen the 'wolution of many new catalyst characterization techniques. Many of them were adapted from surface science, a distinct discipline in science that has its own theories and rapidly developing techniques. In addition, techniques previously used in material science have also been widely adapted to catalysis. A critical discussion of the methods would require a long review of its own (see list of prominent techniques in Table 1). This section discusses only the trends that are particularly relevant to future progress in catalysis. Catalysts and catalytic phenomena are characterized for several reasons. First of all, catalyst characterization is needed to define a catalyst beyond its chemical composition and its crystalline components so that its catalytic properties can be reproduced. Furthermore, in the development of new catalysts, new chemical concepts regarding composition or structure are conceived. Following synthesis, the match between the chemical concept and the actual catdyst product must be confirmed. Catalysts also undergo changes in use. Industrial catalysts are usually used for several months or years before they are replaced,
22
regenerated, or rejuvenated. In this period, the catalyst degrades in performance to a certain extent, and so developing a characterization that indicates the change in the property most relevant to catalytic performance is important. Successful industrial catalysts are often multifunctional, that is, containing several catalyst components; and their inanufacture requires meticulous characterization following each significant catalyst synthesis step. Regeneration, or rejuvenation, of used catalysts also requires a proof of restoration of critical catalyst properties, including both physical and chemical characteristics. Therefore, characterization routinely involves a long list of physical and chemical methods and usually also includes an actual catalysis test. The pracrical need to monitor the evolution and the replication of catalysts resulted in the adoption of many of the techniques listed in Table 1. Clearly, catalyst characterization serves many needs from research to manufacture. The Characterization may relate to the catalyst support, to the precursor phases of the catalyst, to the catalytic site, or to the catalytic reaction itself. In most cases, the information gained is used to judge successful reproduction of structural or chemical characteristics of the bulk or the surface of the catalys: but is insufficient to reveal the structure or chemistry of the catalytic site itself. Although these techniques serve a well-justified purpose, additional methods that are capable of unveiling the cheniistry of catalytic reaction steps at the catalytic site and revealing the structure and chemistry of the catalytic site itself at an atomic scale are important for future progress. Rapid progress in catalysis depends on the predictive value of these experiments. Therefore, the precision used to define the catalyst and its reaction mechanism will affect :.he effec!ivencss of the specific steps chosen i n the evolution of new superior catalysts. The most effective approach in characterization requires the use of methods simulating the x x t i u n environment or at least conditions compatible with the reaction conditions. Altlmgh experiments of this type require complex manipulation of the catalyst sample and the rexrion environment, they have been successfully adopted in many research labtmtories. Aiming at direct, in situ observation of catalytic phenomena must be an importmt future goal in catalysis restzrch. The problem is considerable:. Information on the identity of reaction intermediates is of great value because it gives ‘he clearest focus in the design of further catalyst improvement steps. A description of the structure at the catalytic site can be used as a basis to model catalytic phenomena. Here, an important question is the degree of precision needed to build useful theoretical models for the catalyst, Ultimately, a useful operatiorial lin’age should be established between characterization and theoretical modeling, both of which are important research tools in the design of advanced catalysts. Today, for many cases, such conceptualization of catalyst development is not realistic. However, recent progress in computer techniques has been dramatic, bringing improvements in speed and in cost reduction. Methods of surface chemistry have already been successfully adapted in many cases to catalytic studies. Various analytic methods, such as NMR, LEED, EELS, and F T I R spectroscopy (Table I ) , have been used to identify the chemistry and structure of small
23
chemisorbed molecules, such as CO and ethylene. Nuclear magnetic resonance has become a particularly powerful and diverse tool in the hands of leading NMR scientists to probe catalysis at the molecular level. Some of the sophisticated NMR methods applied to catalytic surfaces suggest great potential for this technique in identifying reaction intermediates for both metal and acid catalysts. For example, "C N M R evidence recently helped to identify the reaction-intermediate alkoxy complex formed between propylene and H-Yzeolite. This observation showed that in acid catalysis at low reaction temperatures, an alkoxy reaction intermediate prevails; but at higher temperatures, where this complex becomes unstable, a "true" carbocation type mechanism sets in. However, more that just these two options for reaction intermediates need to be considered because a recent mechanistic study has shown convincing evidence for the parallel Occurrence of both bimolecular H- shift type and superacid type monomolecular mechanisms in the cracking of hydrocarbons.
5. CATALYSIS OF THE FUTURE At its best, progress in science and technology is discontinuous; it is based on creative, unanticipated discoveries. Therefore, any projection of the future in catalysis will surely miss some of the most important elements. Thus, the future can be projected only on the basis of present knowledge, identified needs, and anticipated improvements. The easier part of the projection is based on identified needs for new products, environmental control, and desired economic improvements, such as the use of cheaper feedstocks. These topics with anticipated future directions have been discussed in Section 3.
This section focuses on some of the basic principles and directions that seem worthy of contemplation in the formulation of a research strategy for new catalysts and related processes. This discussion covers catalyst characterization, catalyst synthesis, industrial catalytic process development, and the discipline of future catalysis. 5.1 Characterization of Catalysis Phenomena
Any experimental plan developed for the creation of a better catalyst requires a description of the catalytic site and its chemical and catalytic function that is as accurate as possible. Superior knowledge of the active site obviously translates to a better plan for catalyst improvement. This consideration leads to the most important future objective in catalysis science: the characterization of caralytic sites af the atomic and molecular level, with sufficient detail in chemistry and structure to allow the planning of modifications that will lead to desired changes in chemical and catalytic characteristics. This objective requires a depth of knowledge similar to that in available homogenous catalysis. Beyond the atomic and molecular description of the catalytic site itself, scientists must also seek to understand the relationship between the chemistry of the active site and
long-range effects exerted by the crystalline base or other long-range phenomena such as ordered overlayers. As was discussed in Section 2, the long-range effects are important, and in some cases they are the key to catalytic phenomena. The formation of a catalytic
24 site or the formation of reaction intermediates tends to distort the crystal and cause disorder. The direction of the long-range effect is generally the maintenance or restoration of lattice order to minimize lattice energy and thus to increase thermodynamic stability of the whole crystal. The quantitative assessment of the preferred bond structure at the molecular catalytic site (short-range effect) and the effect of the distortion caused by the active site on the whole crystal (long-range effect) is difficult because of the large size of the computational terms related to the long-range effect. Therefore, scientists need to search for some traceable spectroscopic evidence to monitor long-range stress in catalyst crystals. Characterization of catalytic active sites at the atomic scale received a great boost from the development of electron tunneling and electron force microscopes. These methods have the potential to describe the surface structure of both metals and oxides and to reveal even small atomic dislocations. They can even map atoms and molecules in the chemisorbed overlayer. More important, these techniques also have some future potential for indicating the bonding characteristics of surface atoms by applying perturbations. Such measurements may ultimately be developed to characterize the chemistry of surface atoms. Today, the electron tunneling and atomic force microscopes are designed to monitor the surface of single crystals. To render these revolutionary, atomic-level characterization techniques useful to catalysis, scientists have to redesign the delicate mechanical control system to allow monitoring of microcrystalline material. Such redesign would involve the development of improved mechanics and computer software to allow the mapping of small crystal surfaces and the automatic reorientation of the sample to find the next crystal surface. The development of electron tunneling and atomic force spectroscopic techniques for microcrystalline catalysts would be an important step in achieving catalytic site characterization of practical catalysts at the atomic level. Future progress in catalysis science also needs improved knowledge of the reaction chemistry so that scientists can contemplate modifications for improvement. With this long-range objective in mind, the determination of the catalytic reaction steps and reaction intermediates is essential to future progress. Recent years have seen excellent progress in this field from a variety of sophisticated techniques, including 13CNMR, EELS,and FTIR, for the identification of chemisorbed molecules and catalytic reaction intermediates on noble metals [9].The next step in this field is to extend the application of these techniques to industrial catalysts, including supported metal catalysts and acidic zeolites, under practical reaction conditions. In this field, methods with adequate response time are needed to identify relatively unstable reaction intermediates as well. The ultimate purpose of gaining detailed knowledge about catalytic sites and catalytic reaction intermediates is to bring catalysis, as a branch of chemistry, to the level where the experimental plan of a catalyst chemist compares in predictive scientific quality to that experienced in inorganic or organic synthesis. To render the conceptual planning in catalysis manageable, to reduce available choices, and to rank the experimental options based on relevant scientific considerations, the
25
modeling of the catalytic site and catalytic reaction intermediates is desirable. If scientists are to prepare theoretical models that can be used as predictive tools in the design of better catalysts, they need to be well aware of the chemistry and physics of bulk solids and surfaces as well as molecules. Quantum-mechanical modeling should be applied with a critical attitude in the application of any assumption used to'abbreviate the computation for the short-range and, especially, the long-range effects. The development of scientifically reliable theoretical modeling is a challenging and difficult task. Today, ab initio calculations are still limited to small atom clusters, or they require the use of generalized or simplified expressions for long-range effects. The future direction in theoretical modeling is almost obvious. If current trends in computer and software development are considered, scientists cannot escape the conclusion that in the future, computational modeling will probably grow faster than any other area in catalysis science. For this reason, theoretical modeling will no doubt have an important place in future catalysis. 5.2 Catalyst Synthesis
Catalyst synthesis, a critical subdiscipline of catalysis, has a methodology of its own. Since the great success of zeolite catalysts in industry, catalyst synthesis adopted several synthesis topics and characterization methods from material science. Today the objectives of catalyst synthesis are far-reaching. They include the whole variety of industrial oxide catalysts, supported metals, biocatalysts, and zeolites as well as microporous or layered materials with wide ranges of chemical compositions, pore sizes, and crystal structures. With zeolites, large pore size seems to be of premium value. Large-pore zeolites with pore sizes of about 8 A are yearly bested by new-generation, microporous crystals, with pore sizes of 12 and most recently with 20 A. The main incentive for catalysis in this field is that these large pores, acting as molecular sieves, may benefit catalytic selectivity in the treatment of heavy petroleum products. They may also act as receptors for immobilized, enzymelike molecules. At present, this area of synthesis research is achieving a high rate of productivity worldwide. As scientists look for fundamental principles in the strategy for future catalysts, they must recognize the differences between biocatalysts and man-made catalysts. Biocatalysts have uniform catalytic sites, and consequently they have high catalytic specificity to the product they catalyze. Man-made heterogeneous catalysts, such as oxides, supported metals, and even acidic zeolites, contain a range of active sites. Consequently, catalytic specificity to the desired product is lower. This simplistic comparison between biocatalysts and heterogeneous catalysts points to catalyst-site uniformity as a desirable feature for future heterogeneous catalysts.
Several approaches have recently been used to develop solid catalysts with uniform catalytic sites. The "fixing" of enzymes on supports and, more recently, the immobilization of enzymelike molecules in zeolites are good examples. In the latter case, catalytic molecules, such as porphyrine, phtalocyanine, or d e n , that may contain a variety of
26
transition metals have been immobilized in zeolite crystal cavities using ship-in-bottle synthesis. Recent data indicate synergistic phenomena between the zeolite structure and the occluded complex. These phenomena give rise to substantial stabilization of the complex, and the result is much superior catalytic turnover numbers relative to the solution of the same complex. In addition, catalytic specificity is strongly influenced not only by the active complex itself, but also by the uniform spatial constraint around the active site. This spatial constraint is provided by the intracrystalline environment (regio selectivity) of the zeolite support. Microporous crystals may also offer several new strategies for providing uniform catalytic sites. Recent developments in the synthesis of microporous crystals, particularly in aluminophosphates, permit the synthesis of compositions containing a variety of transition metal ions, or other elements of variable valence, in the crystal framework. These tetrahedral ions, which occupy well-defined positions on the intracrystalline surface, can occupy identical crystallographic sites by the proper choice of crystal type, transition metal, and its concentration. Some tetrahedral cations have been observed to be capable of reversible change in coordination, from tetrahedral to octahedral, upon hydration-dehydration cycles, vide infra. Therefore, these transition metal cations are probably accessible, in varying degrees, to catalytic reactants as well. Uniform catalytic sites combined with shape selectivity offer new opportunities in catalysis. The substitution of aluminum or other small framework cations with the much larger transition metal cations must result in a distortion of the substituted crystal site that causes stretching and bending of adjacent bonds. This crystal site distortion must result in a distortion of the local site symmetry and may lead to stereospecificity or even chirality. Such changes may form the basis for stereospecific catalysis. In other fields of catalyst synthesis, a particularly important breakthrough in catalysis would be the synthesis of thin, continuous layers of microporous crystals that are usable as catalytic membranes. This development would probably revolutionize not only catalyst synthesis but also catalytic reactor design because of its large potential applications in combining molecular sieve separation with catalysis. In spite of its enormous potential impact, no new, inspiring approaches that are comparable in novelty and creativity to those used in the synthesis of new molecular sieves have been reported in this field. Aiming at the formation of uniform surfaces and uniform catalytic sites, scientists may imagine that the atomic force microscope or a mechanical system of similar design may be used not only to monitor the surface layer of crystals but also to modify them. Ultimately scientists may devise a mechanism to imprint or replicate uniform surface structures on metals or oxides so that they become uniform catalytic sites. In the interest of maintaining high catalytic specificity, several interesting strategies have been already described in Sections 2.1.3.2 and 3.1.3.3. with regard to biologically derived cafalytic systems.
27 5.3 hdustrial Catalytic Process Development
The objective of industrial catalysis is to develop economically superior processes based on innovations that lead to the use of cheaper feedstocks, to higher product yield, or to other process efficiencies that translate to superior process konomics. Industrial research organizations depend on their experience, which is based on both successful and unsuccessful experiments in catalyst synthesis or on the process side, to develop their favored research strategy. Recent success in the use of bio-derived catalysts in a variety of pharmaceutical syntheses and specialty chemicals syntheses substantially broadened the spectrum of the knowledge base on which the research strategy for new catalytic processes should be conceived. Because of the differences in applicable catalyst types, such as enzymes, zeolites, superacids, and supported metal systems, both the catalysts and the processes of reasonably conceived competing approaches may be profoundly different. They may even require researchers of different disciplines for efficient, creative pursuit. For this reason, future industrial success for new catalysis technology will probably favor those scientists who can consider all potential options from the point of view of catalyst type and process approaches to develop the best conceptual research strategy toward a new process objective. Of course, ultimate success in all cases depends on one or more creative researchers who make the key discoveries needed for the catalyst or process. Developing new competitive catalytic process technology is indeed a complex job that requires a broad-minded research organization and people of excellence in a whole spectrum of disciplines, including chemical synthesis, chemical engineering, and material science. Looking toward the needs of future catalysis technology in the 1990s and beyond, the following fundamental guidelines come to mind: 0
0
Learn from biocatalysis and seek catalyst synthesis approaches that develop catalysts with uniform catalytic sites. Look for approaches for uniform catalytic sites not only in the use of enzymelike species but also in seeking novel approaches of "surface synthesis" using both metals and oxides. Industrial catalysts are often multifunctional. Sometimes the functions relate to two interacting catalytic steps (bifunctional catalysts), However, in many cases, one or more catalyst additives are only present to help correct a certain catalyst deficiency, such as rapid metal aggregation, or to aid catalyst burn-off. Based on past experience, if a new catalyst does not have sufficiently high catalytic selectivity or activity, fixing these initial problems requires using additives, dopants, or expensive catalyst regenerative or rejuvenation methods often requires large cost and long time. For these reasons, a strategy aiming at uniform catalytic sites may be a rewarding principle. Acid catalysis is the most common and important catalytic function in industrial catalysis. All ranges of acidity, including mild, strong, and "super" acidity, are important for distinct areas of industrial applications. Environmental considerations place high priority on the development of chemically inert, solid superacids. This fact focuses on the theory of solid acids.
28
Theory is needed to elucidate the specific role and contributions of structural and chemical factors affecting strong and super acidity as well as on the interaction between protonic and Lewis acid sites. The goal is to gain maximum acid strength. A basic understanding here would provide a rich basis for new, superior synthesis approaches to solid acid catalysts in the whole range of acidity. Catalytic process technology with solid catalysts rests in most cases on the use of the traditional fixed-bed reactor, often with relatively unsophisticated flow control for feed and product. Imaginative catalyst flow control, such as fluid- or moving-bed applications, revolutionized catalytic cracking, gasoline reforming, and ethylene polymerization, can release the processes from reactor-size constraints and provide superior operational control and stability with added benefits in the optimization of operating conditions. In several areas, new creative engineering may benefit future catalysis technology. New imaginative process solutions may be similar to those successfully applied recently in sorption-separation processes with respect to countercurrent reactant-flow control, use of desorbents, and so forth. These ideas may find applicability in cases of multistep catalytic reactions conducted on a single catalyst, particularly in cases using multiple feeds. 5.4
T h e Discipline of Catnlysis
The makeup of the discipline in catalysis is continuously evolving. In the last 40 years, the addition of scientists from surface science, material science, inorganic synthesis and theory has given catalysis enormous scientific strength. The most important aspect of this change is the broadening of scientific topics and the focus toward more quantitative scientific methods. This trend will, no doubt, stay with catalysis in the future as well. The field will broaden to include biocatalysis and new areas of inorganic synthesis for catalytic materials. Surface science will blossom, together with theory, particularly when theory can play a role as a predictive tool in an operational linkage between conceptual planning and catalyst synthesis. Changes in the future makeup of the discipline of catalysis must be reflected in the institutional training of future scientists. Therefore, efforts should be made to develop broad as well as scientifically sound curriculums at selected universities where catalysis programs are on the teaching agenda. REFERENCES 1. Creek, B. F.: Chem. Eng. News 1989, 67(22)29. 2. Chem. Week, 1979, 124(14)47. 3. Roth, J. F.: Chemtech, June 1991, 357. Chemtech 1990, 20(4)224. 5 . Bennett, M. J.; Boccone, A. A . and Vline, C. H.: Chemtech 18, 162 (1988). 6. Cusumano, J. A.: Chemistry for the 21st Century; Thomas, J. M.; Zamaraev, K. I. ; Blackwell Scientific Publication, 1992. 7. Heinemann, H.: Proc 9th Int. Conf. on Catalysis, Vol. 5 , pg 1-10, 1988. 4.
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8. Heinemann, H.: Catalysis Science and Technology, Vol 1, pg. 1-41, Springer Verlag 1981. 9. Slichter, C. P.: Ann. Rev. Phys. Chem. 1986, 37, 25-51. 10. Somorjai, G. A.: ACS Symposium series, Vol. 428, pg. 218, 1990. 11. Somorjai, G. A,: 12. (a) Somorjai, G. A.: Catalysis Letters, Vol. 12, pg. 17, 1992. (b) Erte, S.: Oscillatory, Adv. Catalysis, in print. 13. Rabo, J. A.; Pickert, P. E.; Stamires, D. N.; Boyle, J. E.: Proc. 2nd Intern. Congr. Catalysis, Edition Tech., Paris, 1960, 2055. 14. Pfeiffer, H.; Freude, D. and Hunger, M.: Zeolites, 5 , (1985) 274-286. 15. (a) Dwyer, J. and O’Malley, P. J.: Stud. Surf. Sci. Cat., 35, (1988) 5 . @) Carson, R.; Cooke, E. M.; Dwyer, J.; Hinchliffe, A. and O’Malley, P. J.: Zeolites as Catalysts Sorbents and Detergent Builders, Extended Abstract, 1988, 151-152. 16. Rabo, J. A. and Gajda, G.: Catal. Rev. Sci. Eng. 31(4), (1989-90) 385-430. 17. Doremieux-Morin, Batanak, Martin, Brkpeaultaud, and Fraissand: Catalysis Letters 9, (1991) 403-410. 18. Bell, T. B.; Boudard, M.; Ensley, B. D.; Estell, D.; Grubbs, R. H.; Hegedus, L. L.; Manzer, L. E.; Rabo, J. A,; Rebek, Jr., J.; Roth, J. F.; Somorjai, G. A.; Weekman, V. W.: Catalysis Look to the Future, National Academy Press, USA, 1992. 19. Gosling, C. D.; Wilcher, F. P.; Sullivan, L.; and Mountford, R. A.: Process LPG to BTX Products, Hydrocarbon Processing, (1991). 20. Gregor, J. H.; Bakas, S. T.; and Olowetz, M. A.: Converting Field Butanes into MTBE, presented at the Gas Processors Association, Anaheim, CA, Mar. 16-18, 1992. 21. Vora, B. V.; Pujado, P. R.; Imai, T.; and Fritsch, T. R.: Recent Advances in the Production of Detergent Olefins and Linear Alkylbenzenes, presented at the Society of Chemical Industry, University of Cambridge, England, Mar. 26-28, 1990. 22. Flanigen, E. M.; Patton, R. L.; Wilson, S. T.: Stud. Surf. Sci. Catal., 37(Innovation Zeolite Mater. Sci.) (1990) 13-27. 23. (a) Schulz-Ekloff, et al.: Zeolites, 4 (1984) 30-34. @) Jacobs, P. et al.: Stud. Surf. Sci. Catal, 59 (1991) 395-403. (c) -ova, et al., Vestn. Mosk. Univ., Ser. 2c Khim. 18(G) (1977), 660-663.
Guczi, L. er al. (Editors), New Fronriers in Cafalysb Proccedings of the 10th International Congress on Catalysis, 19-24July, 1992, Budapest, Hungary 0 1993 Elsevicr Scicnce Publishers B.V. All rights reserved
INTERFACIAL COOKDINATION CHEMISTRY: CONCEPTS AND RELEVANCE TO CATALYSIS PHENOMENA
M. Che Laboratoire de Reactivite de Surface et Structure, URA 1106, CNRS, Universite P. et M. Curie, 4, Place Jussieu, 75252 Paris Cedex 05, France
"Seen from the bottom of a well, the sky appears very small" (Han Yu, 768-824)
Abstract The concepts of the so-called interfacial coordination chemistry (ICC) are established for complexes interacting with oxide surfaces and involving transition metal ions (TMIs). The latter can function either as probes of their own interaction with the oxide surface, as adsorption and/or catalytic sites or as precursors of oxide-supported metal particles in mono or polymetallic systems. The concepts of ICC are compared with those of solution and solid state coordination chemistries and a general classification of the different coordination chemistry types is proposed. The analogies and differences are stressed and concern the mobility, accessibility and reactivity of the transition metal complexes. Those parameters, which are largely dependent on the bonding strength of the TMI with the oxide support and in turn on the role of the oxide support and choice of the preparation method, determine the catalytic properties. Examples of catalytic reactions will be given which involve the oligomerisation of olefins and the oxidation of methanol and can be described a t a molecular level. Finally, the importance of the bonding strength of TMIs with the oxide support will be stressed in determining the preparation and properties of small supported metal particles which exhibit a strong memory effect with respect to their oxidized precursor state. The present molecular approach suggests that both the preparation and catalytic phenomena can be seen in a more predictive manner and that more effort should be devoted in the future to fundamental research in this area.
32 1. INTRODUCTION
In all their compounds, cations are surrounded by anions or neutral molecules. The groups immediately surrounding a cation are called ligands and the branch of chemistry which is concerned with the combined behaviour of cations and their ligands is called coordination chemistry (CC) [ll. The principles have been established from results obtained essentially by J ~ r g e n s e n and Werner at the turn of the century on solution complexes containing only one transition metal ion (TMI) surrounded by ligands [l]. The main goals were to understand the properties of such complexes, i.e., their reactivity towards simple reagents such as silver nitrate, their structures, the nature of the different chemical bonds involved, and the presence and number of geometric as well as optical complexes. Later, attention was focused on their optical and magnetic properties along with their chemical behaviour particularly in redox and ligand substitution reactions. Successive theories were proposed (valence bond, crystal field, molecular orbital, angular overlap..) which have greatly helped t o improve our understanding of transition metal complexes in solution and inorganic transition metal compounds in the solid state. Attempts have been made in the past to apply such theories t o catalytic systems. It must be recognized that they were by and large more successful for homogeneous than for heterogeneous systems, which involve complex gassolid or liquid-solid interfaces. The present review is aimed a t showing that substantial progress has now been made for heterogeneous systems so that a general picture of a field which we shall call interfwial coordination chemistry (ICC) can be presented. I t concerns the preparation of catalysts composed essentially of TMIs and oxide supports, their catalytic properties in the oxidized state and finally the preparation of supported transition metals.
2. EARLY STUDIES OF INTERFACIAL COORDINATION CHEMISTRY
2.1. Transition metal ions supported on oxides The preparation of supported catalysts via the deposition of transition metal complexes on metal oxides has been known for a long time and can be considered a s an example of coordination chemistry a t the liquid-solid interface. For instance, the ability of silica to adsorb Na+, Ag+, Cu2+ and Fe3+ ions had been noted by several authors as early as 1925 [2-41. In 1932, using chemical analysis, Berton [51 showed that C U ( N H ~ )and ~ ~ Ni(NH3)c2+ + could adsorb on silica with loss of 2 NH3 molecules. Later, Smith and Jacobson [61 observed the same type of effect with Ag+, Zn2+, Cu2+, Ni2+ and Co2+ ions complexed with ammonia, ethylenediamine and diethylenetriamine ligands. In 1961, Kozawa [7] assumed that the adsorption of complexes leads to bonds between the metal cation and the silica surface. The next two papers are important contributions to our understanding of coordination chemistry a t interfaces. Burwell et al. [BI presented evidence that the adsorption of transCo(en)~C12+on silica occurs by substitution of a C1- ligand by a surface SiO-
33 group and gave some insight into the bonding of labile and inert transition metal complexes with silica. Using W - v i s . spectroscopy also, in addition t o magnetic measurements, Hathaway and Lewis showed in 1969 [9] that the surface of silica could act both as a bi- or terdentate ligand towards the Ni(I1) ion. In 1972, Anderson [lo] showed that the exchange technique, to which we shall return later, leads t o an octahedral complex much more effectively bonded to the surface than via the impregnation method. The first successful attempt to rationalize the adsorption of transition metal complexes on inorganic oxides was made by Brunelle [113 and concerned cationic complexes of Pt [12-161, Pd [16-191, Co, Ni, or Cu [9,161 on silica or alumina or anionic complexes of Pt [16,20-241, Pd [18,19] or other precious metals such as Au, Rh, Ru and Ir [16,241 on alumina. The results were explained in terms of surface polarization of oxides as a function of pH and adsorption of counterions by electrostatic attraction. The previous results show that transition metal complexes can be bonded to the support surface either by electrostatic adsorption or through ligand substitution. There was, however, no systematic study of the changes occurring to the complex during the successive preparation steps. 22. zeolitic and layered materials Zeolitic materials have well defined crystalline structures and exhibit, inside cavities of known dimensions, cationic sites where cations can be sited [25,26]. Transition metal complexes may either be synthesized within the zeolitic cavities (ship in a bottle synthesis) or exchanged in as complex cations [27]. This has the advantage that the site symmetry is better defined than on conventional oxides. There are unfortunately complicating factors due to synthesis and/or postsynthesis treatments which lead to locations other than cation exchange sites: lattice framework positions, surface defect sites, oxide agregates inside or outside pores, bulk oxides on the external zeolite surface [28]. However, if care is taken, it is possible t o study TMIs essentially in cationic exchange positions. The coordination of the TMIs in zeolites, mostly investigated after ion exchange and drying andor calcination, has been reviewed by Kazanskii and coworkers [29,30]. One of the most important aspects which emerged from the latter studies is that TMIs can change cationic sites upon adsorption [31,321. For example, it was shown by spectroscopic studies that TMIs can reversibly migrate upon water ad(de)sorption between hidden sites where they are bonded t o framework oxygen ions (SI in hexagonal prisms and/or SIIin sodalite cages with octahedral and CgV symmetries respectively) and the large cavities of faujasite-type zeolites where they can freely tumble in the form of hexaaqua complexes [33,34]. This is an interesting case where the TMI-zeolite bonding can reversibly change from iono-covalent to electrostatic. Similar studies have been conducted in layered materials which indicate that the adsorbed complexes can adopt distorted symmetries [35,361.
23. Simple and mixed oxides One of the first rationalizations of catalysis was the electronic theory of catalysis [37] which attempted to find a relationship between the catalytic properties of solids and their electronic properties estimated in terms of the
34
band theory. Much experinicntal work was stimulated by these ideas but the results often did not support the prccppts of the theory [%I. A new approach was then taken and catalysis no longer studied from a delocalized a nd physical standpoint but from a localized and chemical one thanks to Ilowden and Wells [39] who, a t the 2nd International Congress on Catalysis in Paris, proposed t o apply the crystul field theory to catalytic phenomena. Their paper can be regarded a s one of the major contributions to the field together with that of Taylor [40] who was the first to describe active surface sites. If simple concepts of electron transfer a r e augmented by the application of crystal field theory in the investigation of the coordination changes in distorted surface complexes, then i t emerges t h a t a n y ratecontrolling step which involves the adsorption of a species so polarised a s to restore near octahedral symrnc>try about a surface cation (in a n ionic solid) contributes to higher enerbies of activation and lower activities a t the cation electron configurations do, d5 and d l o , The same authors were also able to suggest t ha t the theory provides a reasonable basis for the exploration of the relationship between homogeneous and heterogeneous catalysis and stressed the importance of lattice defects. Point defects were shown to play a n important role not only in adsorption [41) but also in catalysis 142,431 with, in some cases, mechanisms derived from coordination chemistry principles (441. Dislocations were also showr: l o be important in catalysis [45]. A s pointed out by Rurwell e t al. [4G,47], oxide surfaces will contain coordinativeiy unsaturated anions ;is wt.11 a s cations and chemisorption will frequently involve both. They introduced the cus (coordinatively unsaturated) notation as a subscript attached tc anions or cations to stress the coordination aspect [48]. IIaber an d Stone [491 considered chemisorption a s a process by which the coordination characteristic of the bulk :is partially ur coinpletel:; restored (Fig. 1) and calculated the crystal field stabilization (CFS) afforded by the chemisorption of oxygen on the various crystallographic faces of a h c k c l oxide crystal. They found t h a t for low overall coverage of' NiO by oxygen, the adsorption o n (110) planes is particularly strong so that, those planes a r e covered a t the expenses of the ( 100) and (111) planes, thus giving a theoretical foundation for expecting crystallographic inequality. 'The early sixties period turned out t o be very important since the work of Dowden and Wells [39] triggered n number of major contnbutions. I n this context of new interest in localized electronic defects, the concept of' using oxide solid solutions was largely investigateci h y Sc>iwood, Cimina, Stone arid coworkers [50-531. The principle, a s applied to oxide catalysis, i s to select ;i diamagnetic oxide of' high crystal symmetry as the matrix (solvent) a n d to dissolve in i t a second oxide which contains a n ion or ions of part,icular catalytic interest 1541. In many cases the solute oxide is a transit,ion nwtal appreciated since oxide. The potentialities of solid solutions can now beEin t,o one can investigate not only the catalytic activity of H selected TMI as the catalyst changes from insulating and paramagnetic behaviour in very dilute solutions t o semiconducting an d magnetic hehaviour for increasing TMI concentrations, but also how adsorbed molecules a r e bonded to the TMI, depending on its location (face, edge or corner) and coordination at the swface.
35
Figure 1. Changes in nickel ion coordination during the chemisorption of oxygen on NiO. (a), (100) plane; (b), (110) plane; (c),(111) plane. Diagrams in the middle column refer to a bare surface a t the instant of cleavage. Left column, bare surface, relaxed position; right column, after adsorption of oxygen. black circles : nickel ion; open circles : lattice oxygen ions; shaded circles : adsorbed oxygen ions [49].
3. THE PRESENT TRENDS IN INTERFACIAL COORDINATIONCHEMISIRY As summarized above, early work that often did not distinguish electrostatic adsorption (Fig. 2, model 111) [551 from grafting [561 (Fig. 2, model IV) [551 was mainly concerned with the gas-solid interface, since the catalysts were studied afier preparation rather than during preparation.
36
Figure 2. Representation of the outer sphere of solvation and coordination sphere of TMIs a t various positions relative to the oxide-fluid (solution or gas) interface. The left-hand side represents the solid oxide. The shaded part on the right-hand side represents the solution while the lower white part on the same side represents the gas [55].The meaning of the arrow is given in 8 4.4.
37 Now it is known, particularly from zeolite preparation chemistry, that the different types of zeolites which are finally obtained have their origin in the very early stages of the hydrothermal process, i.e., during the aqueous chemistry part of their genesis. The same should be true for the preparation of oxide systems, all the more as often amorphous rather than crystalline materials are obtained. This has been a strong motivation particularly in our laboratory to follow the ion-support interactions (ISI) which develop through the successive stages of catalyst preparation. This generally implies the study of the liquid-solid interface before that of the gas-solid interface. This approach raises a number of questions: Are there any concepts which can help our understanding of catalyst preparation? What is the role of the support? Which are the parameters which are at hand t o control ISI? How many different types of surface species are obtained? What does it mean to select one preparation method rather than another? Finally, having answered those questions, can we then hope to move from catalyst preparation to catalyst design? In order to answer those questions, we need to have a suitable probe. It is particularly fortunate that TMIs act most often as catalytic sites on an oxide support for they turn out to be also the most appropriate probes to follow their own irteraction with the oxide support and see the evolution of the catalytic system all along its preparation. Because of their partly filled d orbitals, any change in their first coordination sphere immediately affects their optical and magnetic properties and can thus be detected by spectroscopy. In short, TMIs are both actors and spies watching their own acting. Furthermore, in order to broaden the interest of our approach, we have tried as much as possible to select catalyst preparation methods which only require rather simple chemicals such as transition metal mononuclear complexes, essentially aqueous solutions and simple oxides, on the one hand, and are similar t o those used in the industry, on the other hand. This differs from the supported homogeneous catalysis approach [571 where more involved inorganic or organometallic compounds o r clusters are anchored [561 onto oxide supports, often via organic solvents. This aspect, which is well documented elsewhere [58-611, is not dealt with in this review. Neither will we cover the case of polyoxometalates or polyoxoacids which will be discussed by Misono in this Congress [621.
4. ICC CONCEII'S AND TRANSITION METAL IONSAS PROBES In order .to know how the properties of transition metal complexes are modified a t fluid-solid interfaces during the different catalyst preparation steps, we need to have a reference state. Because of the wealth of data and theories available on solution CC, the latter appears t o be the most convenient one. 41. The reference state: solution coordination chemistty In aqueous solution, TMIs are complexed (Fig. 2, model I) by water and/or other ligands. This defines the inner sphere usually called coordination
38 sphere. One defines also an outer sphere of solvation formed by a layer of water molecules, with which the complex can move (i.e., rotate and translate) within the liquid medium. The concepts of such complexes, which a r e well established and described in textbooks [ll, need not be repeated here. 4 2 The role of the support and the competitive ion exchange method In order to understand the role of the support, the strategy adopted is to use a transition metal complex and investigate how the coordination sphere is modified during the preparation method and further post-synthesis (deposition) treatments. 4.2.1. The fluid-solidinterface during competitive ionexchange Because of its pedagogical interest, the competitive ion exchange method has been selected to show that, if the adequate parameters of each preparation step are controlled, it becomes possible to obtain the TMI successively (Fig. 2) in solution (model I), then p11 (models II-V) and in (model VI) the surface, t o finally reach the bulk of the oxide (model VII). In the following, we will often refer to the Ni/Si02 system, which has been studied in a more systematic way. 4.2.1.1. The physical nature of the oxide surface The mobility of a TMI is strongly modified in the pore of an oxide. The perturbation produced by the liquid-solid interface, important for the first two layers of water which are strongly immobilized [631, decreases as one moves away from the surface. This anomalous water which has a thickness of 8 or 10 water layers freezes a t temperatures lower than 0°C but does not crystallize. Cations can be trapped within this glassy matrix . The previous results have been obtained on a set of copper impregnated silicas with a mean pore diameter ranging from 4 to 100 nm [641. The EPR signal of Cu2+ ions (3d9, S = 1/2, I = 1/2) was found to be characteristic of their location. For pores smaller than 6 nm and containing only a few layers of water, the signal, below -4OoC,is axially symmetric due to Cu2+ in the glassy matrix. In large pores, a second EPR signal, isotropic and typical of Cu2+ in bulk water, is superimposed on the first one. A t room temperature, the rotation or reorientation rate in the first layers of water was found to be 1.5 times slower than in bulk water. Both signals are assigned to the [Cu(H20)6I2+ complex and it appears that there is no specific adsorption a t the surface. The perturbation can be more important in faujasite-type zeolites with smaller pores. During the exchange procedure, copper ions enter into the supercages through the large windows (0.8-0.9nm). With a capacity of 28 water molecules and 1.14nm internal diameter [651, those cages can accomodate the LCu(H20)6I2+ complex with its outer sphere of solvation. An EPR study [661 shows that the rotation rate of the complex is 10 times slower than in bulk water, due t o the close vicinity of the cavity walls. In all the examples cited above, the coordination chemistry of the TMIs is basically not changed and model I (Fig. 2) is convenient. Only the mobility of TMIs is affected by the solid, which can be considered as a microcontainer, all the more a s the pores are smaller.
39 The interface appears t o drastically affect the dielectric constant of water. The associated dipoles in the first water layer in contact with the surface are aligned. As a consequence, the dielectric constant E of water drastically decreases from its bulk value 78.5 t o 3 2 in the second layer and 6 in the first layer [671. This is discussed further in the next section. A direct physical interaction is likely to occur between TMIs and the oxide for pHs that do not lead to dissociation of surface OH groups. Through its OH groups, the oxide is able to enter the outer sphere of solvation of the complex in the same way as the solvent molecules (Fig. 2, model I) and thus acts as a solid solvent (Fig. 2, model 11). This aspect, which is little documented, is different from that encountered in zeolites which can act as solid ionic solvents [681 or in oxides where TMIs come to rest after high temperature diffusion to form doped or mixed oxides (8 2.3) [691. Copper ions deposited a t the surface of alumina or magnesia by impregnation (see 8 4.3.1) are characterized by EPR as species immobilized a t room temperature [64]. This specific interaction refers now to the chemical nature of the oxide surface and this is considered in the next paragraphs. 42.16.The oxide surf8ce as a supramolecular counterion or capacitor plate Oxides in aqueous suspension are generally electrically charged as shown directly by electrophoresis experiments [ll]. This is attributed to the amphoteric dissociation of the surface hydroxyl groups following the reactions:
SOH2+
CJ
SOH
+
H+
K1
(1)
SOH
e
SO'
+
H+
K2
(2)
where K1 and K2 are the equilibrium constants of processes (1) and (2) respectively. There is a characteristic pH = (pK1 + pK2)/2 called the isoelectric point (IEP) or zero point charge (ZPC) a t which a given immersed oxide surface has a zero net charge 1701, i.e., [SOH2+] = [SO' I. At a pH above its IEP, the oxide surface is negatively charged so that cations may be adsorbed. By contrast, a t a pH below its IEP, the oxide surface is positively charged leading t o an adsorption of anions [ll].The IEP can be used to predict molecular structures of surface metal oxide species on oxide supports [11,711. It is important to stress that the primary role of the pH, on the solid side, is to act as a surface charge selection switch 172,731. In practice, the ion exchange method is often used in its competitive form, where two types of ion compete for the same adsorption sites. The equilibria can be represented by the following equations: SOH2+A' +B'
SO'
C+
+ D+
CJ
SOH2+B- + A -
CJ
SO-
D+ + C+
competitive anion exchange
(3)
competitive cation exchange
(4)
In contrast to impregnation where both the cations and their counter-anions (or vice-versa) are deposited, the ion exchange method leads to the adsorption of only one type of ions. Another very interesting feature of the ion exchange
40
method is that the counterion (see equation 5 below) can be easily removed by washing since it is repelled from the surface which possesses charges of the same sign. This is a n important advantage over the impregnation method where washing cannot be used owing t o the weak interaction between the precursor and the surface. The secondary role of the pH, now on the solution side, i s to act as a transition metal complex selection switch [731. In many complexes, the nuclearity and coordination number remain constant and only the nature of the ligands changes (Fig. 3a) [741. In some others, the nuclearity and coordination number vary, as in polyoxo- molybdates and tungstates [751. Cation exchange of nickel on silica using nickel nitrate in water-ammonia solution has been studied in detail by UV-vis. spectroscopy [761. The NO3counterion, monitored by a band a t 33000 cm-l assigned to the (n+n*) transition, is removed upon successive washings (Fig. 4). This indicates that the exchange has occurred with the oxide surface which acts a s a supramolecular counterion. Here, the TMI is bonded to the oxide surface by ligand-screened electrostatic adsorption (Fig. 2, model 111).The reaction can be written as follows: (Ni2+, 2NO3’)
+ 2 (=SO’,
NH4+) CJ 2 (NH4+, NO3-) + (2 SiO’,Ni2+)
(5)
where Ni2+ stands for the hexaammine [Ni(NH3)6]2+ complex. In parallel, a set of 3 bands characteristic of octahedral Ni(I1) (3d8) is also observed. A t a pH above 9, the [Ni(NH3)612+ complex is adsorbed without modification of its W-visible spectrum even after the 2nd washing step, indicating a pure electrostatic adsorption with formation of surface ion pairs, (2 =SiO- , [Ni(NH3)6]2+), where the whole surface is considered as a n ion. Adsorbed complexes exhibit reactivities, such as ligand exchange, closely similar to those of their solution analogues [81. The decrease of the dielectric constant E of water mentioned earlier (8 4.2.1.1) greatly favors the stability of ion pairs a t the surface. The forces maintaining the TMIs on a negatively charged surface can be described in the double layer electrostatic model [77] and are similar to those involved between the plates of a capacitor so that the oxide surface can be envisaged as a capacitorplate. Ammonium ions also present in the solution can compete with the nickel hexaammine complexes to occupy the exchange sites of silica as follows:
The competition level between NH4+ and Ni2+ ions, expressed as the ratio of their concentrations, allows to better control the amount and dispersion of nickel adsorbed.
41
Figure 3. (a) Distribution of the Ni(I1) ammine complexes a s a function of pH for (NH4+) = 1M; (b) Amounts of Ni(I1) adsorbed on SiOz as a function of pH after exchange with 0.01 and 0.05M initial Ni(I1) ammoniacal solutions P41.
.
Figure 4. W - v i s . spectra of a 2.7 wt% Ni/SiO2 sample: (a) after exchange with a 4 M N H 3 solution, (b) after a first washing with a 1.33 M N H 3 solution, (c) after a second washing with a 0.44 M N H 3 solution and (d) after a third washing with a 0.148 M N H 3 solution. All the spectra a r e recorded after centrifugation t761.
T/KC r n - l
50
25
15
’
I
NO;
125
NI**
x
8
I
I I
z o o 300
I ,
I
1
L O O 500
1 1
I
600 700
aoo
42
4.2.1.3. The oxide surface as a supramolecular ligand a. From electrcwtatic adsorption to grafting During the different preparation steps, the Ni(I1) ions remain in octahedral coordination as shown by the presence of 3 spin-allowed transitions in the UVvis-NIR spectra of the Ni/Si02 system (Fig. 5 ) [78]. Because it is essentially free from interference with other transitions, the v3 band has been used to measure the crystal field strength experienced by the Ni(I1) ions [761. The near IR (NIR) bands can help to identify the presence of NH3 and H 2 0 ligands (Fig. 5) [79] and it is seen, in particular, that the Ni(I1) complex gradually looses its ammonia ligands.
I
3
~
5
A/100nm
h
7
8
9
11
13
15
A/
1O O n m
17
19
21
23
25
Figure 5. UV-vis-NIR spectra of a 1.7 wt% Ni/Si02 sample after different steps of preparation: (a) wet after ion exchange in a 1.5 M NH3 solution and centrifugation, (b) filtered and dried at 20°C in air for 15 h, (c) filtered and dried a t 80°C in an oven during 15h, (d) calcined a t 500°C in oxygen and rehydrated for 1 year and (e) unexchanged ammoniated silica dried a t 80°C in an oven for 15h [76]. The results show that, after electrostatic adsorption, g r a f t i n g occurs via substitution of 2 NH3 by 2 surface =SiO- ligands according to the process:
Since grafting occurs during the third washing step (Fig. 4) in conditions of pH and NH3 concentration corresponding t o the stability range of the complex
43
[Ni(NH3)4(H20)2l2+, it is most likely that the grafting reaction actually proceeds via the intermediate tetraammine complex a8 follows:
It appears that the ligand exchange NH3/H2O in the initial hexaammine complex weakens the coordination sphere making grafting possible via two surface bonds (Fig. 2, model IV),with formation of a neutral cis-octahedral complex. The latter still contains ammonia ligands evidenced by their NIR bands (Fig. 5 ) . It is important t o note that the oxide, entering into the coordination sphere of Ni2+, becomes a supramolecular bidentate ligand via vicinal =SiO' groups. The latter have been found to be the most probable grafting sites, on the basis of geometrical considerations [761 and by analogy with the silicate structure [801. This has been recently confirmed by EXAFS studies [81]. The chelating effect of the surface appears to be the driving force for the formation of the surface cis-octahedral complex. Equation (7), which is the sum of equations (8) and (91, is accompanied by an entropy increase with the release of water molecules and the disappearance of charged species leading to a subsequent disordering of nearby solvent molecules [82]. This aspect, already mentioned in earlier thermodynamical studies on ion exchange with silica gel 1831 is well documented in solution coordination chemistry [ll. In certain cases, the oxide surface can act also as a monodentate ligand a t the fluid-solid interface and lead to surface trans-octahedral complexes. This occurs, for instance, when the TMI in the precursor complex is protected by strong field ligands, fiuch as ethylenediamine in trans-Co(en)2C12+, so as to offer only one possible grafting site by a Cl'/rSiO' ligand exchange [81. The last point worth t o mention is that grafting always leads to m i x e d complexes with two types of ligands : rigid ligands provided by the surface and mobile ligands provided by the fluid phase (liquid or gas, as discussed later).
b. The oxide surface and the ligand spectmchemical series The inclusion of the oxide surface within the spectrochemical series of ligands is a difficult problem since we are dealing with mixed complexes with different types of ligands, which each contributes differently t o the overall crystal field experienced by the TMI. Attempts have been made in the past but failed to give conclusions in agreement with theoretical expectations [361. The problem can however be solved, providing first the surface complexes along the different preparation steps be prepared and identified and second the rule of average environment [84] be applied [76]. The v3 transition energies of the [Ni(NH3)6I2+(28 200 cm-l) and [Ni(H20)6l2+ (25 300 cm-l) complexes are taken as references [85-873. The decrease of the crystal field strength upon substitution of NH3 by H 2 0 in the [Ni(NH3)6I2+ complex is assumed t o be a
44
linear function of the number of NH3 ligands. A bathochromic Av3 shift of -480 cm-1 is thus readily obtained for each NH3 substituted by H2O. Since the v 3 transition of [Ni(&i0)6I4- in solution is not available, we have to apply the rule using [Ni(H2O)6I2+(25 300 cm-l) and [Ni(=SiO)~(H20)4] (24 700 cm-1) as references. The composition of the latter complex derives from the fact that ammonia is no longer detected in its spectrum (Fig. 5d) and that more than two bonds between Ni2+ and the surface would lead to a very distorted symmetry and hence to more than the three bands observed. The application of the rule of average environment gives a shift of -300 cm-l for each H 2 0 molecule replaced by a =SiO' ligand in the [Ni(=Si0)2(H20)4]complex. The substitution first of NH3 by H 2 0 (Av3 = -480 cm-1) and then of H 2 0 by =SO' (Av3 = -300 cm-1) gives a total bathochromic shift of -780 cm-1 when NH3 is replaced by zSi0'. The values derived above allow to insert the S i O within the spectrochemid aeries as follows:
- ligand
This order is consistent with theoretical expectations. As a matter of fact, the three ligands are all o-donors. However, it is known from molecular orbital theory that the crystal (ligand) field they can produce is all the more important as their Ir-donor character decreases [ll. This is in good agreement with the presence of two, one and zero lone pairs on the donor atom for the ESiO-, H20 and NH3 ligands respectively. The oxalate CzO42- ligand, with two terminal 0' ions, appears to be a very close analogue of the surface bidentate ligand which is 9 adonor n donor weak ligand. Similar results are obtained with the C d S i 0 2 system prepared by ion exchange from [Cu(en)2]2+ precursors [881. The same approach has been taken for other systems such as Pd/Si02, PdAl2O3 and Pdfaujasite-type zeolites using square planar Pd(I1) precursor complexes and led to a spectrochemical series of supports [89].
-
4.2.1.4. The oxide surface as a reactant A number of papers have reported the presence of intermediate compounds (silicates in particular) after preparation involving the liquid-solid interface, which suggests that the support can act as a reactant. Although there has been no systematic study on this aspect, the contact time between the solution and the support seems to play a n important role. For instance, after an exchange process of 600 hours, nickel is found to be essentially as a nickel silicate deposited on silica [90,91]. There are some favourable conditions for the oxide surface to become a reactant. One of the main parameters to achieve this goal is again the pH which, in its third role (see 8 4.2.1.2), controls the solubility o f the oxide depending on ite acid-base character [ l l l . For instance, the solubility of an
45
acidic oxide such as silica increases rapidly in a basic medium [921. An amphoteric oxide such as alumina dissolves either in basic or acidic medium, while basic oxides such as NiO dissolve only in acidic medium [lll. The solubility of the oxide can be regarded as a depolymerisation process leading to support monomers, which can then copolymerise with the transition metal complexes regarded as solution monomers. This reaction, particularly for long contact times of the oxide support with the exchange solution, can lead to new phases such as nickel silicates for the NUSiO2 system. Since the pH conditions required for the oxide dissolution cotncide with those required for ion exchange, it is important to look for parameters which can put the two latter phenomena out of phase, i.e., inhibit the copolymerisation process. It is first necessary to understand how bond formation occurs between molecular solution species. Sol-gel processes indicate that there are some preferred pathways such as : M-OH M-OH
+ +
M-H20 M-OH
(j
U
M-OH-M M-O-M
+ +
H20 H20
(11)
(12)
where M represents the support ion and/or any TMI. Olation (reaction 11)and oxolation (reaction 12) are condensation processes in which a hydroxy or a n 0x0 bridge between two metal centres are formed respectively [93]. I t becomes clear now that if H20, OH or 0, which all are bridging ligands, are substituted by non bridging ones such as NH3 around the transition metal ion, bond formation between the two metal centres will be inhibited. This is illustrated by recent EXAFS experiments performed with the Ni/SiO2 system [811. At pH around 9, in a domain where the silica surface can dissolve, the coordination sphere of Ni2+ ions, which is protected by ammonia or ethylenediamine (en), is not modified by electrostatic adsorption on the support. This shows that M-O-M bond formation does not occur in presence of such strong non bridging ligands. Below pH around 9, ammonia in smaller concentration can be substituted by the weaker water ligand t o form penta- and tetraammine complexes (Fig. 3a). Under these conditions, the coordination sphere is weakened and grafting of the Ni2+ ions occurs with formation on silica of a surface silicate, where a large proportion of nickel ions are engaged. This demonstrates that the support, although less soluble than at the pH of the preceding example, can nevertheless become a reactant, because olation (reaction 11) between Si-OH and the Ni-OH2 end of molecular [Ni(NH3)6-n(H2O)nI2+ solution monomers (n=1,2) can now occur. The stability of the species involved and the kinetics of the processes (oxide dissolution and bond formation by copolymerisation), which all are temperature dependent, seem t o be important factors. As a matter of fact, drying experiments performed at higher temperatures (353K) always lead to silicate, whatever the pH of the exchange solution. By contrast, the Ni(en)gZ+ complex, which is very stable above pH 7 and involves non bridging and strong chelating ligands, does not lead to silicate formation but to isolated Ni2+ ions stable up to calcination temperatures of 600°C [811. We can now better understand [94], at least from a qualitative point of view, the curves of Figure 3b. If the ion exchange were the only process involved, the
46
amount of Ni deposited onto silica as a function of the pH of the exchange solution would qualitatively follow the exchange capacity derived from the pH dependence of the zeta potential of silica (Fig. 6) [731. Note here that the fourth role of the pH, on the solid side again, is to control quantitatively the exchange site concentration.
0
2
L
6
8
10pH
0
2
6
6
8
10pH
Figure 6 . The variation of the exchange capacity expressed by the pH dependence of the zeta potential of a ) S i 0 2 and b) Ti02 obtained by electrophoresis [73]. The latter shows that the cationic exchange capacity of silica is negligible in the so-called inert zone and increases significantly only above pH 7 (the negative sign just indicates that the surface charge is negative as expected from equation 2). The experimental results, however, show (Fig. 3b) that the ion exchange is not the only process involved. Below pH 6, the absence of adsorption is explained by the vicinity of the isoelectric point of silica: the surface charge density is very low and the amount of Ni(I1) complexes adsorbed electrostatically is therefore negligible. On the other hand, the dissolution of silica is also negligible, hence nickel silicates cannot form. While the amount of adsorbed nickel is expected t o increase after the inert zone above pH 7, the presence of the maximum around 8.6 is against expectation. The latter can be explained by reference to Figure 3a which exhibits also maxima for tetra- and pentaammine Ni(I1) concentrations precisely around this pH. The spectroscopy results (8 4.2.1.3. a) have shown that the tetraammine complex is indeed important in the grafting process. Between pH 8 and 10, it is likely that the ion exchange of Ni(I1) ions onto silica gives rise to a mixture of nickel silicates and Ni(I1) electrostatically adsorbed whose relative concentrations probably depend on the contact time between the exchange solution and the support. Above pH 9 . 5 , i t is clear t h a t the complexation of Ni(I1) as hexaammine (Fig. 3a) with no bridging ligand inhibits the silicate formation and favours pure electrostatic adsorption, which is quantitatively limited, the competition with NH4+ ions being important. The adsorption decrease between pH 8.6 and 10 is due to the increase of hexaammine Ni(I1) concentration. The sharp adsorption increase when pH exceeds 10 can be attributed to the dissolution of silica which becomes significant above pH 8. The mechanism of Iler [921 (Fig. 7) explains how the latter leads to a significant increase of the silanol groups density and hence of the exchange capacity of silica.
47 0 si-0-
1
)Si$OH O
SI-0-
0
Si-0-
I
>I+H I
+OH
\ ,SiSOH O t
Figure 7. The solubility of Si02 and the formation of new surface OH groups
WI.
As shown above, the two domains of interfacial coordination chemistry ($2.1.2.3.) and surface solid state chemistry (this 8) have distinct characteristics with the oxide surface acting essentially as supramolecular ligand and reactant respectively. The previous results suggest that parameters in the frontier region between the two domains are not completely known as yet. It is felt, in particular, that the contact time between the support and the exchange solution is an important parameter but this has not been much studied. The passage from one t o the other domain may be reversible. For instance, Clause et al. [95] have shown that the nickel silicate phase formed in samples prepared by ion exchange from ammoniacal solutions could decompose and that thermal treatment under reduced pressure favoured the process.
43.2. The fluid-solidinterface during oxidizing thermal treatment In 8 4.2.1.3., the experimental conditions were such (oxidizing conditions, short contact times between the solid and the liquid and ligand excess) that the TMIs remained in the same oxidation state and coordination number a s those of their solution precursors. If we now, say by thermal treatment in oxidizing atmosphere, slowly remove the ligands, we can still keep the oxidation state constant while decreasing the coordination number at the gas-solid interface. The TMIs grafted on the surface by reversible ion exchange reactions are expected to be highly dispersed on the surface. Under calcination in pure oxygen, the exchanged ions behave as isolated species and never lead to NiO up to 700°C. From the number and intensity of the bands in the UV-vis-NIR spectra, it is possible t o conclude that the symmetry of the Ni2+ ions decreases when the calcination temperature increases from 25 to 700°C leading to a change of the coordination number from 6 to 3 (Fig. 8). In the latter case, the oxide surface becomes a terdentate ligand. In addition t o the 2 first ionocovalent bonds created during grafting (reaction 71, a dative bond is formed with a bridging oxygen of a siloxan surface group (5320) leading basically to a
48
C3, symmetry 1963. Adopting the nomenclature proposed originally for surface oxide ions [97,98] to metal cations [99,1001, the oxidizing thermal treatment changes hexacoordinated (6c) Niec2+ ions into tricoordinated (312)ones Ni,,2+, i.e., into TMIs coordinated with a single type of ligands, the surface rigid one (Fig. 2, model V). Meanwhile the number of vacant coordination sites increases fiom zero to three. It is those vacant sites that constitute the driving force for adsorption and catalysis. There is however, a critical (usually called Tamman [ l o l l ) temperature of the support, above which this trend will be reversed (unless volatile oxides are formed). It corresponds to the introduction of the TMI into the support [69,102,103] and thus to a n increase of the coordination number up to the one imposed by the host matrix [102,1031. The temperature appears also as a n important parameter t o switch from interfacial coordination chemistry to bulk solid state coordination chemistry (Fig. 2, model VII).
a) Niec2+
b) Ni5c2+
c) Ni4c2+
d) N$+
Figure 8. The changes of coordination experienced by grafted complexes upon oxidizing thermal treatment of the Ni/Si02 system. 45. The role of the support as a function of the preparation method
As described for the competitive ion exchange method, the support oxide fullfils a different role a t each step of the preparation. There exists no systematic study for other catalyst preparation methods. There are however enough results t o be able to draw some conclusions for the most common preparation methods [73,1041. Whenever possible, we will insist on the role of the support a t each preparation step and also on coordination chemistry aspects. 4.3.1. Impregnation In this method, a preformed support is impregnated with a n aqueous solution of a metal salt. Here both the TMI and its counter ion are deposited on the support which acts only as a mere physical surfiace. It should be noted that there is no washing step after impregnation, since the metal salt would be eliminated, owing t o the weakness of interaction with the support. The subsequent drying step, usually performed at around 100°C, is important since the metal salt is reformed from the individual solvated cations and anions and redistributed a t the support surface in a way which depends both on the drying
49
speed and the support porosity [104]. During the calcination step performed in air or oxygen, two phenomena generally occur. First, the decomposition of the impregnated salt leads to the corresponding oxide according to a reaction such as :
and second, some chemical bonding is established upon dehydroxylation between the precursor oxide and the support. A typical example for the Mo/Al2O3 system can be written as follows [ 1051:
O*& O%fO
qH0 - AlVH- 0 - + HO' - 0 - Al-
\
9' P'
OH + -0-Al-0-Al-0+ 2H20
(14)
During the latter step, it can be seen that the support through its hydroxyl groups acts as a reactant.
4.32. Deposition-@pitation Deposition-precipitation[106,107] consists in precipitating a metal salt on an oxide surface by varying the pH of the solution homogeneously. Geus et al[l061081 proposed the thermal decomposition of urea to produce OH' ions, which becomes significant above 60°C. It is important the pH be raised slowly, so as to involve the OH groups in copolymerisation processes deshbed by reactions (11) and (12) between solution monomers and the support as a solid and no longer as dissolved monomers. The following scheme illustrates the fact that the surface is a nucleation initiator because of the favorable arcid-base reaction between the surface hydroxyl groups and the metal hydroxy complex :
The hydroxy complex can be formed by UOH' ligand eubstftution (L= H20, NH3.J or a dissociation reaction [lo91 such as :
The subsequent step of growth can then occur via olation (see reaction 111, i.e., elimination of water from the p(H3O2) ligands [1101 leading to a p-(OH) bridging ligand, considered to be the first step of precipitation [111,1121. The surface plays the role of a reactant with its hydroxyl groups as shown below. The difference with the growth mechanism occurring in solution is that TMI's have to diffuse into the pores of the oxide and reach the support surface.
50
Ligand removal in a TMI complex can also lead to precipitate the active phase. For instance, by thermal decomposition of nickelhexaammine at 90°C with a C02 bubbling a nickel aluminium hydroxycarbonate is precipitated on alumina. This however occurs with 'I-or e-Al203 but not observed with a-Al203 11131. This example shows that the surface is not inert and because of the dissolution of the oxide to form molecular species, it can act as a reactant 11141, thus changing the nature of the precipitate [1131. Clause et al. [1151 reached the same conclusion for the Ni/SiO2 system and presented evidence by E M S that nickel silicates likely of nepouite structure, (Ni3Si205(OH)~), were formed.
4.3.3. Coprecipitation The coprecipitation in a precursor form (hydroxides, carbonates..) of both the support and the active phase from a solution has the advantage to produce an intimate mixing. The coprecipitate leads on calcination to a support with the active component dispersed throughout the bulk as well as at the surface. The support can thus be considered as a solid solvent. The ideal case is obtained for systems such as NiO-MgO or COO-MgO,where the MgO host matrix of rock-salt structure acts also as a sterically demanding polydentate ligand [1161. Depending on the location of the TMI, the oxide can act as either a penta-, tetra- or terdentate sterically demanding ligand (Fig. 9). The TMI, now in surface framework position (Fig. 2, model VI), is all the less likely t o change i t s coordination with the rigid surface ligand a s its coordination number is important. The same notation has been adopted for 0 2 [97,981 and TMIs [99,1001 in positions of low coordination and can be easily extended to other types of ions, providing the coordination number be known. This problem, easy for TMIs, becomes major for s and p group elements, because they lack partly filled d orbitals. 4.3.4. Grafting This process consists in performing a chemical reaction between a metal chloride (or oxochloride) and the hydroxyl groups of a n oxide as follows [117]: MCln + m S-OH
-+
MCln-m (0S)m
+ m HCl
(18)
51
e
o
e
o .
e 0
o .
e 0
0
o
. .
e
0 0
. .
o 0
0
e .
.
o
e
0 0
--Figure 9. Representation of a surface (100) plane of MgO showing surface imperfections such as steps, kinks, corners, etc.. which provide sites for Mic"+ of low coordination (lc = 3c, 4c, 5c). M"+ ions can be either Mg2+ and 0 1 ~ 2 ions or TMIs such as Co2+,Ni2+ etc. where M stands for the metal and S for the support while m=1-3 depends on the reaction conditions and content of surface hydroxyl groups [118,1191. The metal (0xo)chloride is either gaseous or dissolved in an organic solvent [120,1211. This approach. The advantage with metal (oxo)chloride vapour is to deal with isolated molecules. Grafting leads directly to model IV of Figure 2 with the oxide, via the functional hydroxyl groups, acting as a molecular reactant. Oxide ions in position of the lowest coordination can be obtained. Thus, Anpo et ions, in the form of terminal vanadyl bonds, by reacting al. [1221 prepared 0lc2VOC13 with hydroxyl groups of porous Vycor glass according to the reaction :
O ' A0' V = OC1,
+
3 SiOH
Ji
J i $i
+ 3HC1
(19)
4.4. The nature of ion-supportinteractions
Table 1 summarizes the main points discussed in the previous sections. I t also gives the nature of ion-support interactions (IS111731. The strength of IS1 increaaes along the sequence of models (I -+ VII) given by the arrow in Figure
52 2. Weak ion-support interactions (WISI) a r e expected for ions in
extraframework positions (models I1 + V) and strong ion-support interactions (SISI) for ions in surface framework positions (model VI). For the latter ions, the IS1 increases with the coordination number. The IS1 is a very important factor for the preparation of supported metals ( 0 7), since it largely contributes to the reducibility of the ions.
1
IAOAOAO
loAoAoAl OBOAOBOAOB BOAOBOAOBO
b
a
C
o B o A O B o AW BOAOBOAOBO OBOAOBOBOA AOBOAOBOAO d
e
Figure 10. The various possible interactions between TMIs or oxides and a support oxide, where A stands for TMIs, B for support cations and 0 for 02oxide ions [73]. Figure 10 extends the different possible interactions [73], first considered by Roozeboom et al. [1231, between the support and the active phase with formation of a) a 3-dimensional (3D) metal oxide on the support, b) a 2D metal oxide (monolayer or islands) on the support, c) an intermediate phase on the support or inbetween the two components oxides, d) a solid solution or a doped oxide (for small TMI concentrations) and e) isolated TMIs. Apart from case el, the interaction increases between the component oxides on going from a) t o d). The obtention of only one of the preceding cases depends on the nature of the component oxides or precursor oxides and on the preparation parameters. Case e) with isolated ions can be obtained by ion exchange or grafting a t low metal chloride loading (otherwise the monolayer-case b) can be involved). Case b) may be produced by the so-called solid-solid adsorption which consists in heating a mechanical mixture of the active precursor and the support [1241. This leads to a spontaneous dispersion of the active precursor on the support. Despite the clear-cut distinction between the systems of Figure 10, the real situation is very often a combination of the different cases.
53
Preparation method
Impregnation
Deposition-Precipitation
Role of the support
Nature of the TMIsupport interaction
Microcontainer Mere physical surface Solid solvent
Van der Waals forces Van der Waals forces Hydrogen bonding
Acid-base reactant
Iono-covalent bonding
(Nucleation initiator) ~~~
Coprecipitation
Iono-covalent bonding
Solid solvent (Sterically demanding polydentate ligand)
Ion pair (Electrostatic adsorption 1 Iono-covalent bonding (Grafting)
Capacitor plate
Ion exchange
Reactant (monoor polydentate ligand) Grafting
Iono-covalent bonding
Reactant (monoor polydentate ligand) I
I
6. TRANSITION METAL IONS As ADSORPTION AND CATALYTIC SITJ3S Many adsorption and catalysis studies have been performed in the past. We will again restrict our attention to studies related to TMIs either in their oxidized or reduced state and select examples which emphasize the coordination chemistry and molecular aspects.
54
6.1.Adsorption sites 6.1.1. Surface ions in framework positions
A well known example of adsorption in solution coordination chemistry is the reversible fixation of molecular oxygen by cobalt complexes, referred t o a s reversible oxygen carriers [ 1251 following the reaction :
I
h
I
h
(20)
where cobalt ions are coordinated t o a Lewis base B and square planar sterically demanding ligands such as Schiff bases, porphyrins etc.. Recently, the same type of reversible adsorption has been observed on purely inorganic systems [ 1261. A good example of synthetic heterogeneous inorganic oxygen carrier i s represented by dilute CoO-MgO solid solutions. Even in a high surface area state, the latter are mainly in the form of microcrystals of prevalent cubic shape. On the faces of such cubes (Fig. 91, the CosC2+ions are penta-coordinated, i.e., in a coordination similar t o that of their solution analogues. It can be seen that, in the solid system, the equatorial square planar ligand is constituted by 4 05c2- ions and the Lewis base by an axial 0 ~ ~ 2 ion. The reversibility of oxygen adsorption has been studied by IR and ESR [127] and the end-on structure of the oxygen superoxide adduct confirmed with 1 7 0 by ESR [128]. Both were found t o be again similar to those of their solution analogues [126,129]. The coordination of oxide anions has been successfully determined in the case of alkaline-earth oxides by diffuse reflectance and photoluminescence techniques, which both led to similar results [971. In spite of the large variety of possible configurations t o be expected in a high surface-area oxide, the reflectance spectra [130,1311 show only a few bands for the alkaline-earth oxides investigated. Each band can be considered as representing a particular local surface coordination of the oxide ions. The bands in the spectra are changed by the adsorption of gases : those of lowest frequency corresponding to ions in lowest coordination a r e most affected. The adsorbing gases convert low-coordination surface ions to ions of higher coordination and the higher Madelung potential arising from this increase in coordination displaces the surface absorption toward higher frequency. The absorption bands measured in the reflectance spectra and the corresponding bands in the excitation spectra 11321 match well for all the alkaline-earth oxides 1973. These results demonstrate that a real surface anion coordination chemistry is made possible by such techniques and this can be regarded a s a major advance. Recent results indicate that static and dynamic photoluminescence techniques can now be applied t o follow catalyst behaviour
55 during preparation and catalysis [ 133,1341 and photochemistry on solid surfaces 11351. On alkaline-earth oxides, adsorption of CO at 300K involves a very small fraction of surface sites (below 0.5%) and occurs on the most basic sites, i.e., tricoordinated 03,2- ions (Fig. 9) [136]. The situation is rather complex because the probe molecule on those sites is not only a ligand but also a reactant [1371. At 77K, the CO coverage is very high and adsorption produces bands due to CO adsorbed on a c i d i c Mg5,2+, Mg4,2+ and Mg3,2+ sites, whose relative intensities also depend on the morphology of the particles [1371. Similar conclusions a r e obtained on MgO-Coo and MgO-NiO solid solutions. The main conclusion to be drawn is that the reactivity of the surface ions in framework positions, which is essentially a function of their location at the oxide surface, is all the more important a s their coordination is low. Similar conclusions were reached earlier by Kibblewhite and Tench [138] i n adsorption studies of halogens on MgO. 6.1.2. Ions in extraframework positions By contrast with the previous systems, i t i s possible to maximize the concentration of M3,n+ ions, i.e., in positions of low coordination, using ion exchange or grafting methods. Thus, it was found (4 4.2.2.) that the calcination of the Ni/Si02 system, prepared by competitive ion exchange, produces Ni3,2+ ions. By slight reduction, i t is possible to obtain Ni3,+ ions, isoelectronic with Cu2+ (3d9) and thus observable by ESR. Upon adsorption of 13C (1=1/2) labeled CO, ESR signals with hyperfine structures a r e observed which reversibly depend upon the pressure. The magnetic parameters can be analyzed to give the following models [1391 :
1
2
3
4
(21)
where a stands for axial and e for equatorial. Some of the models have been confirmed by IR using 12CO/13CO isotopic mixtures [1401. The important conclusion is that now the TMI coordination is able to change and reversibly depends on the ligand gas phase pressure. In the sequence 1to 4 above, the surface ligand changes from ter- to monodentate. The change i n coordination i s such that the TMI forms bonds with gas phase monodentate ligands so a s to restore its preferred coordinations, i.e., those observed i n solutions. This can be illustrated by Figure 11 which gives a plot of the gl ESR component a s a function of gll, so that each point represents a particular complex [141]. The dashed regions have been drawn from ESR data of solution
56 complexes [142]. It can be seen that, in the sequence 1 to 4, the coordination of Ni+ ions gradually moves toward that of their solution analogues. In other words, due to its rigidity, the surface ligand imposes a distorted coordination t o Ni+ ions. As soon as gas phase monodentate (and thus sterically non demanding) ligands become available, Ni+ ions impose their coordination and therefore take up their preferred geometry. The same trend is observed for the Ni2+ state. Upon water adsorption, the low coordinated Ni3c2+ ions restore their octahedral coordination moving backward from model d) to a) (Fig. 8).
g// =2.7
17777777/7/71 2.2
I
I
I
2.3
2.4
ZJ
gll
Figure 11. The gl ESR component of grafted and solution Ni+ complexes plotted as a function of gl/ (adapted from ref. 141). The structure of the grafted complexes is given by the models 1 to 4 drawn in (21) Thermal reduction of Mo/SiOB samples, prepared by the grafting method using gas phase MoCl5, induces the formation of three Mo5+ species in hexa-, penta-, and tetra-coordination (respectively M06~5+,Mosc5+ and M O ~ ~ ~ + ) [120,143]. They all possess a molybdenyl bond. During adsorption of water, the Mo5+ EPR spectra recorded a t 77K, undergo a stepwise transformation : in the first step, for low water pressure ( 1 Torr), the signal of M04c5+ disappears, while that of M0sc5+ increases, then in the second step, for higher pressures (18 Torr), the signal of M0sc5+ disappears while that of M06~5+increases. The spin concentration remains constant during these changes and water only acts as a l i g a n d . While adsorption can change the number of bonds with the support surface for Ni/Si02, this is not observed with the Mo/SiOz system, probably because the iono-covalent bonding with the oxide surface involves higher oxidation state for molybdenum than for nickel ions. 6.2. Catalytic sites
The reactivity of a TMI depends on its ability to be coordinatively unsaturated in order t o i) bind one or several reactive ligand(s1, ii) catalyse their
57
transformation into (a) given product(s), iii) and finally release the product(s1 recovering i t s original unsaturated state. The generality of the ligand dissociation-association process has led to formulate the 18-electron stability rule and the 16- and 18-electron rule [1,57,1441for reactivity of transition metal complexes, both valid essentially for o donor - R acceptor strong ligands such as CO, CN - and organic unsaturated molecules. The latter rule has been very useful in discernment of preferred reaction pathways in homogeneous catalysis [1,57,144]. This rule does not apply to weak Q donor - 7c donor ligands such as those involved in the preparation of catalysts (=SO', H20, NH3, etc..). This rule will not be obeyed strictly by surfaces complexes which concern mixed ligands, i.e., weak and strong ligands. Using the electron counting adopted in coordination chemistry [57], with two electrons for the surface rigid oxygen ligand [139,141,145], complexes 1 and 2 are both 17-electron species, while 3 and 4 are 19-electron species which are known to be very reactive [146,147]. This is considered next. 62.1. Dimerisation of olefins As Ni+ ions are the active sites in the homogeneous olefin dimerisation reaction [1481, the coordination of ethene onto supported Ni+ ions has also been investigated [ 1391. The 17-electron dicarbonyl species 2 progressively reacts with ethene giving rise to a complex EPR spectrum assigned, by employing 1% enriched CO, t o the 19-electron [Ni(CO)2(C2H4)]+species, 5, with two inequivalent CO ligands. The similarity between the EPR g tensors of species 3 and 5 suggests that the structure of the latter is probably a trigonal bipyramid with one axial COa ligand and one equatorial COe ligand. Thus the ethene molecule is in an equatorial position and species 5 is the result of the addition of one ethene molecule to species 2. Increasing the ethene pressure up to 600 torr caused a further change in the EPR spectrum. Using 13C enriched CO, a new species, 6, can be identified with again a trigonal bipyramid structure with the CO molecule in axial position and the two ethene molecules in equatorial positions. The 19-electron species 6, [Ni(CO)(C2H4)2]+,is obtained from species 5 by a ligand exchange reaction with C2H4, replacing COe in the complex.
5
b
58
Catalytic activity tests were performed on systems containing the two species 5 and 6. With species 5, no appreciable dimerisation reaction is observed after 24 hours whereas with species 6, i t does occur. The product distribution is very close to that observed in the catalytic tests performed without carbon monoxide. It is therefore necessary that two ethene molecules be bonded to the same Ni+ ion for dimerisation to take place. It is important to note that there are f o u r types of ligands around the Ni+ ion : 1) the reactants which are transformed into the 2)nd type of ligands, i.e., the products, 3) the CO spectator ligand, which all are mobile ligands, and finally 4) the rigid surface ligand. When alkylphosphines rather than CO are used as spectator ligands, two effects are observed [149]. The first, of electronic nature, is proportional to the number and basicity of the alkylphosphines introduced and leads t o a n increase of the electronic density at the metal centre. This can be followed by IR using CO as a probe molecule [96,141]. The consequence is a weakening of the affinity for soft Lewis bases such as the reaction products (for instance, butenes for ethene dimerisation) and an increased stability of the catalyst by a weakening of the Ni+-products bond. To this electronic effect, is superposed a conjugated steric effect due to both the surface and the spectator ligand, characterized by a T o l m a n a n g l e [571. For instance, with bulky alkylphosphines the selectivity can be drastically increased in 1-butene [1491 or 2-3 dimethyl- 1 butene [1501 for ethene and propene dimerisation respectively.
6.23. Oxidation of methanol Methanol oxidation via the selectivity of the products obtained appears to be one of the most suitable molecular probe. Known t o be structure sensitive [151,152], this reaction can also distinguish acid-base from redox catalysis [1531. In addition, it was found to be very sensitive to molybdenum dispersion, by comparing catalysts prepared by impregnation and grafting, with low and high dispersion respectively [154]. On the former catalysts, formaldehyde is the main product. On grafted Mo/SiOz samples, the relative amounts of formaldehyde and methyl formate, which are the main products, depend on molybdenum dispersion, the latter product becoming more important for increasing dispersions. On the basis of kinetic data, the following mechanism was proposed : on Mo sites, methanol leads to formaldehyde which spills over to silica, where it further reacts with methoxy groups to form methyl formate via a hemiacetal intermediate. In order to confirm this mechanism, model reactions between methanol and 0- generated in different ways, both adsorbed on the surface of grafted catalysts, have been studied by ESR. The mechanism was found to occur within the coordination sphere of the active M05+sites and different elementary steps such as ligand to ligand hydrogen or proton transfer and ligand to metal electron transfer could be evidenced [1551.
6. THE CLASSIFICATION OF COORDINATION CHEMISTRIES On the basis of the models given in Figure 2 and of the results given in 8 4 and 5, it is possible to propose a simple classification of the different types of CC
59 related to systems involving TMIs and oxide supports. This classification [55] can be easily extended to other supports (sulfides in particular). ICC is clearly intermediate between solution (Fig. 2, model I) and solid state (Fig. 2, model VII) CC. I t can be subdivided into two CC referring to ions i n extraframework positions (Fig. 2, model 11-V), on the one hand and to surface ions in framework positions (Fig. 2, model VI and Fig. 9), on the other hand. Whereas the reactivity of the former ions depend on the gas phase pressure in the former case, i t essentially depends for the latter on the location of the ions a t the surface. In following the arrow in Figure 2, the reactivity, mobility and accessibility of the TMIs continuously decrease by contrast with ISIs, which continuously increase. Concerning the mobility and as pointed out earlier 1721, the zeolites present such a structure t h a t ions can move from hidden sites to large cavities on adsorption of polar molecules. The interface can now be defined, from the standpoint of the probe, as the intermediate region where there is a transition from the properties of TMIs in solution to those in the solid state: it extends from 8 to 10 water layers, on the liquid side (see 4 4.2.1.11, to a depth, on the solid side, equivalent to the radius of the TMI solvation sphere (see Fig. 2). The importance of the interface has already been pointed out by Haber [1561.
7. TRANSITION METAL IONS AS PRECURSORS OF SUPPORTED METAW 7.1. The present knowledge
Although the preparation of supported metal particles in real systems has been the subject of numerous studies [104,157], the concepts which can be used t o control a priori their preparation are still ill-defined. This is in contrast to model systems where the concepts a r e better established, particularly a s regards to the mechanisms of nucleation and growth [1581 and the control of the size of supported metal particles [1591. In model systems, the latter are obtained for instance by condensation of metal vapours on flat substrates 11601. The common concern, however, to both types of systems is to identify t h e processes which lead to metal particles and the nature o f t h e i r bonding with the support. Perhaps, the best understood systems are those involving metal particles i n zeolitic matrices, which were extensively studied more than a decade ago. Several reviews have appeared [ 161-1631but the subject is witnessing a renewed interest. The emphasis, clearly molecular, is now on the elementary steps during the calcination and reduction stages which influence the final metallic s t a t e [164]. For instance, starting from [Pt(NH3)4I2+ in Y zeolite, a pretreatment in H2 leads to [Pt(NH3)2(H)2Io.This intermediate bears no charge and is highly mobile, leading to Pt agglomeration [165]. The same is thought to occur with [Pd(NH3)4]2+ on silica and a pretreatment in H2 likewise gives rise to a poor dispesion [166]. By contrast, a pretreatment in He or Ar leads to very highly dispersed Pd particles, while a pretreatment i n oxygen gives intermediate dispersions. In the case of Pd exchanged Y zeolite, it is possible to regenerate the isolated Pd2+ ions by contacting the metal particles with nitric oxide [ 1671.
60
Returning to metals supported on conventional oxides, a literature survey indicates that small metal particles appear as wetting the support. They are very flat, forming one-dimensional structures and irregular agglomerates of two to threee monolayers thick 11683. They are usually not completely reduced and the unreduced or partly reduced remaining ions are located a t the metalsupport interface [169,1701. Those ions appear to act as anchoring sites for the metal particles and constitute a chemical glue. Although this has not been realized, two types of chemical glue can be distinguished. In the first, the anchoring sites are exposed framework cations due to 02-vacancies and direct metal atom-cation bonding becomes possible, as suggested by EXAFS results on the Rh/TiO2 system [171]. The second type of chemical glue is likely to be composed of extraframework cations as in the case of metals in zeolites. In model systems such as Pt/SiOa [1601 or NdSiOz [1721, prepared by metal vapour condensation on oxides kept a t low temperatures, so as t o avoid surface atom diffusion, the cations, extraframework by preparation, are most probably generated by reoxidation of the impinging atoms by surface hydroxyl groups giving off molecular hydrogen. However as discussed earlier [173], the concentrations of the order of 3.1012 cm-2 are too low to detect the ca tions. 7 9 . The challenge It is generally accepted that the stronger the IS1 in supported systems, the smaller the metal particle size generated by thermal reduction of the precursor system. This statement is however ambiguous, since the stronger the ISI, the higher the reduction temperature and the more important the surface diffusion of the atoms obtained. This should lead eventually to larger particles. It is also accepted, other things being equal, that the mean particle size obtained for a given system increases with the metal content. Finally, the presence of several types of ISIs is thought to be detrimental t o the obtention of monodisperse metal particles. With this picture in mind, how then can we control a priori the size and the physico-chemical properties of metal particles supported on oxides? It is important t o answer those questions. The lack of control of those characteristics is the most probable explanation for the discrepancies observed between the results reported by different authors for the same system and reaction when turnover frequencies are plotted against particle size [1041. 7.3. A possible new approach
Marcilly gave a very thoughtful account of the preparation of supported metal catalysts and insisted on the importance of the nucleation and growth processes and of the saturation parameter [1571. The approach we have taken is to separate the nucleation and growth processes and this is illustrated for the Ni/SiO2 system [174]. A two-step procedure is adopted with: i) creation of Ni nuclei ; the hypothesis is that the latter are nickel ions in strong interaction with silica and thus difficult t o be reduced, ii) formation of a "reservoir" of nickel atoms for the particle growth on the ionic nuclei ; the hypothesis is that the nickel ions, precursors of the atom reservoir, must be in weak interaction with the support, so as to be reduced at low temperatures. This should lead to atoms, with limited diffusion ability on the silica surface.
61
From results of previous works performed on the Ni/SiO2 system, the blank samples were prepared by incipient wetness impregnation from a solution of nickel nitrate on silica. After temperature programmed reduction (TPR), the average diameter of nickel particles measured by transmission electron microscopy (TEM) is about 608, for samples containing from 1.6 to 3.5 wt% Ni. The nickel nuclei were prepared using two different methods leading to Ni ions in strong interaction with silica : - by cation exchange with nickel ethylenediamine, followed by calcination in air in order t o decompose the ethylenediamine. After TPR, the average Ni particle diameter for samples containing less than 2.4 wt% Ni is smaller than 208,. - by impregnation of high amounts of nickel, followed by a thorough washing in water in order t o eliminate nickel interacting weakly with silica. The average particle diameter after TPR is less than 25A for samples containing from 0.64 to 5.1 wt% Ni. Nickel was then impregnated on samples containing the "nuclei". After reduction, the average particle diameter is smaller and the size distribution is more homogeneous than for the blank samples, even if the total metal loading largely exceeds that of the latter. The striking result is that the average particle size can decrease when the overall amount of metal increaees, providing the number of nickel nuclei be increased for a given amount of impregnated nickel and whatever the preparation method of the nuclei. It can also decrease, when the amount of impregnated nickel decreases for a fixed number of Ni nuclei. These results suggest the possibility of controlling the mechanisms of nucleation and growth during the formation of nickel metal particles supported on silica. It appears possible to control not only the particle size by the number of nuclei and the amount of metal in the "reservoir" but also the electronic properties (by the atoms t o cations ratio), which both are crucial points for the preparation of supported metal catalysts. It is hoped that this approach can be easily extended to bimetallic supported catalysts, and may lead the way t o control also the composition and the electronic structure of bimetallic systems, in the same way as for model systems [1751. The implications of ICC in the preparation of supported metal particles are obvious since they can help to produce the adequate ISIs in the oxidized precursors. The examples above show that the final metallic state exhibits a strong memory effect with respect to its oxidized precursor. As the reduction proceeds, hydroxyls groups are formed which can be used in further cation deposition by ion exchange and this opens up new possibilities for catalyst design [1761.
8. CONCLUSIONS AND POSSIBLE DIRECTIONS FOR THE FUTURE In this review, an attempt has been made to establish the emerging concepts of interfacial coordination chemistry (ICC) and their relevance t o catalysis phenomena. This led t o raise two important questions : what is the role of the oxide support? and what is the nature of ion-support interaction (ISI)?. The adopted approach has been to take the transition metal ion as its own probe and to study its interaction with the oxide support during the successive steps of
62
catalyst preparation, involving both the liquid-solid and'gas-solid interfaces. In contrast to expectation, the obtention of different types of IS1 can be turned into an advantage to control the preparation of supported metal particles. We have mainly concentrated our attention to cationic transition metal complexes but the approach i s the same for anionic complexes. It is our hope that, though probably not definitive, the answers to the two previous questions may help to consider both the preparation and catalytic phenomena in a more predictive manner, i.e., t o move from catalyst preparation t o catalyst design, as mentioned earlier [177-1793. They may also form a reasonable basis to understand catalysis a t a molecular level particularly during the reaction. Future progress will certainly come from a concerted effort to i) adapt the currently available techniques t o in-situ studies, ii) investigate kinetic phenomena involving labile and inert complexes in catalyst preparation, iii) model the elementary steps which describe catalyst preparation and catalytic reactions a t a molecular level, iv) perform theoretical calculations to provide a more solid foundation to existing models and evidence new directions for future experimental work, and v) transfer and adapt the concepts of relevant fields to that of catalysis and further enlarge and/or improve the existing basic concepts, such as those presented here. This can be achieved particularly with model systems, such as zeolites with well defined structures. Those solids were shown to present a rich coordination chemistry and t o possess properties similar to those of cryptands [721. For the latter reason, they have been recently named zeolates [180]. They are the subject of intensive research and are likely to lead to new approaches and new materials. It is fair to recognize that the recent advances we have witnessed in the last years in ICC owe much to modern characterization techniques [1811. The present molecular approach suggests that more effort should be devoted in the future t o fundamental research in this area.
Acknowlegments It is a pleasure to thank most warmly my collaborators, whose dedication, skill and enthusiasm led t o the work reported in this paper. They all have contributed to the common goal. I also wish to acknowledge the collaboration with a number of foreign laboratories on various aspects of coordination chemistry, photochemistry, spectroscopy and catalysis. I am indebted to Pr M. Anpo and Dr T. Setoyama (Japan), Prs K. Dyrek and J . Haber and Drs K. Bruckman and Z. Sojka (Poland), Drs B. Canosa and A.R. Gonzalez-Elipe (Spain), P r s E. Giamello, S. Coluccia and A. Zecchina (Italy), Dr K. Hadjiivanov (Bulgaria), Pr. J.M. Thomas (UK), and Pr. C.O. Bennett (USA), for their valuable collaborations. I am grateful to UniversitB P. et M. Curie, CNRS, Institut Francais du PBtrole (Drs E. Freund, C. Marcilly, J.P. Boitiaux and 0. Clause), Rhbne Poulenc (Drs J.P. Brunelle, G. Blanchard and G.N. Sauvion) and Gaz de France (Dr J. Saint-Just) for providing the intellectual environment and the financial support. Finally, I wish to dedicate this contribution to Pr J. Turkevich, Drs C. Naccache and B. Imelik and the late Dr A.J. Tench. I owe them so much.
63 9. REFERENCES
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Guczi, L. er al. (Editors), New Fronriers in Caralysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
NEW CATALYTIC ASPECTS OF HETEROPOLYACIDS AND RELATED COMPOUNDS TO THE MOLECULAR DESIGN OF PRACTICAL CATALYSTS
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M.Misono Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Abstract Recent progress in basic and application studies of heteropoly compound catalysts oriented to "catalyst design" is described, placing stress on the merits of their molecularly well-defined structure. Based on this, future opportunities of heteropoly compound catalysts are discussed, with examples of promising basic researches as well a s existing industrial processes. 1. INTRODUCTION
1.1. Catalyst design of practical catalystsbased on crystalline mixedoxides There are many demands in our society for new efficient catalysts in the various fields of energyhesources, synthesis of materials, and environment Ell. But the requirements imposed on the new catalysts are very severe and can not be satisfied by the simple extrapolation of the existing catalytic technology. Novel concepts for catalyst design are desirable for the development of new catalysts that meet those requirements. Catalyst design at atomic level utilizing advanced surface technology is one of the possibilities. It has been demonstrated in laboratory that atomically controlled surfaces are sometimes very active. However, if one considers the necessary conditions of industrial catalysts; high volume density of active sites (for this, porous materials are needed), stability, economy, etc., the synthesis of industrial catalysts by this method turns out to be unrealistic a t least in a mid-term perspective. As an alternative way, we have attempted molecular design of mixed oxide catalysts by using "crystalline compounds", of which the bulk structure is known and which have potentially high activity and selectivity so as t o be applicable to practical uses. Heteropoly compounds, perovskites and zeolites are the examples that we chose as starting materials [2]. We assume that the following relationships, Scheme 1, are essential for the design of catalysts and that the molecular design of catalysts would become possible if the relationships are established a t the atomidmolecular level.
12.Heteropoly compounds as catalysts Heteropolyanions and isopolyanions are polymeric oxoanions (polyoxometalates) formed by condensation of oxoanions [31. The term heteropoly
70 Scheme 1. Useful relationships for catalyst design
=[-]I[
Performance
physical
compounds (HPA) is used for the acid forms and the salts. HPA-related compounds are organic and metallo-organic complexes of polyanions. HPA catalysts are used as acid as well as oxidation catalysts both in solid state and in solution. Various types of catalysis exist for HPA: Three different types for solid HPA catalysts (surface, bulk-type (I) (=pseudoliquid)and bulktype (11)catalysis, both in gas-solid and in liquid-solid systems) and two types for solution systems (homogeneous and phase-transfer catalysis), as described in the later section. Advantageous characteristics of HPA catalysts are summarized in Table 1. Attempts to utilize HPA for catalysts have a long history as compiled in the reviews on HPA in 1952 and 1978 [4]. A bench scale test was reported for alkylation of aromatics in 1971. First industrial process with a HPA catalyst was launched in 1972 for the hydration of propene in liquid phase. The essential role of Keggin structure for oxidation of methacrolein was clearly indicated in a patent in 1975. Systematic basic researches of heterogeneous catalysis started in the mid 70's mostly in Japan to elucidate the quantitative correlations between the acid-redox properties and catalytic performances of HPA [ 2 , 4 - 71. Pseudoliquid phase (bulk-type (I) catalysis) was reported in 1979 and the bulk-type (11) behavior in 1983. Similar efforts for homogeneous systems were carried out in the same period actively in Russia and Japan. In the 80'6, several new industrial processes were initiated in Japan by using Table 1 Advantages of HPA catalysts 1. Catalystdesign
1-1. By the control of acidic and redox properties. HPA having wide range of the two properties can be prepared. 1-2. By multi-functionality. Acid + redox, multielectron transfer, etc. 2. Molecularity metal oxide cluster 2-1. Molecular design of solid catalysts. 2-2. Cluster models of mixed oxide catalysts and of relationships between solid and solution catalysts. 2-3. Description of catalytic processes a t atomidmolecular level. Spectroscopic study and model compounds of reaction intermediates. 3. Unique reaction field 3-1. Bulk-type catalysis. "Pseudoliquid" and bulk-type (11) behavior provide unique reaction fields. 3-2. Pseudoliquid makes spectroscopic and stoichiometric studies feasible and realistic. 3-3. Phase-transfer catalysis. 4. Unique basicity of polyanion 4-1. Selective coordination and stabilization of reaction intermediates. 4-2. Ligands for metals and organometallics.
-
71
HPA catalysts, as listed in Table 2 [ll. In addition, there are a few small-scale processes. The research activity of HPA catalysis is now very high and seems to still grow, if a huge number of papers published every year and a wide variety of reactions to which HPA catalysts are applied are considered. We reviewed thoroughly the heterogeneous catalysis of HPA in 1987 [4]. Catalysis in solution has been reviewed in 1982 [61, 1983 [8] and 1992 [9]. The latter [91 also describes catalytic reactions on solid HPA. Two typical industrial processes have been sketched in our recent review 111. To avoid overlapping with those reviews, I shall not try in this article to cover the whole literature, but describe only the essential characteristics of HPA catalysts with discussion on the future opportunities, according to the items given in Table 1. Stress will be placed on the progress in the last five years mainly of heteroeeneous *. So early works will not be referred individually, but only referred to previous reviews, unless the citation is indispensable. Usage of HPA for a precursor of oxide catalysts as well as the cases in which polyanions decompose by some reasons are not included in this article, although sometimes good performances are observed. We believe that, if one wishes to design catalysts taking advantage of the molecular nature of HPA, one must be very careful about the structure and stoichiometrv. Table 2 Large-scale industrial processes using heteropoly compounds Reaction Phase Capacity Start Remarks (103 t / year) liq (aq) 50 1972 first HPA process Hydration of propene (dilute HPA) Oxidation of gas - soli d 220 1982 Mo-V-P methacrolein Hydration of liq (aq) 3 3 1994 C4 separation isobutene two phases (conc. HPA) liq (aq) 40 1935 toMEK Hydration of n-butene Polymerizaliq (THF-aq) 2-3? 1987 phase-transfer catalysis tion of THF 2. SI'RU-,
ACIDITY, AND REDOX PROPERTIES, etc.
21.Structure of heternplyanion- primary structure Several heteropolyanion structures are known; Keggin (XM12040), Silverton (xM@42), Anderson (xM60241, Lindqvist (xM60241, Waugh ( m g o 3 2 ) , Dawson (X2M18062) structures, etc. [31. We called the polyanions the primary structure differentiating from the secondary structure as described below 171. They are oxide cluster ions with high molecular weight. As a n example, the structure of PW12O40 (a Keggin-type heteropolyanion) is shown in Figure la. In t h s polyanion, twelve WO6 octahedra surround a PO4 tetrahedron. W is called polyatom or addenda atom and P heteroatom or central atom. There are four different oxygen atoms in this structure (one terminal, two bridging and one inner oxygen). Hereafter, PMol2, etc. will be used for PMo12040, etc.,
72 PolYa polyanion
b
@ : H+(H20)",Cs', pyH+, etc.
(a)
(b)
-
~808,
-
(C)
Figure 1. Primary, secondary and tertiary structure of HPA. (a) Primary structure (Keggin structure, PW12O40), (b) Secondary structure (H3PW12040.6H20), (c) Tertiary structure (Cs2.5 salt, cubic structure as (b)). unless the use is misleading. Similarly HPMol2, Cs2.5PW12, etc. denote H3PMo12040, cS2.5HO.5Pw12040 etc. In aqueous solution, they tend to be hydrolyzed a t high pH. The stability and the dynamic equilibrium of dissociation-association of polyanions are dependent on several factors such as the constituent elements, the degree of reduction and the kind of solvents (and coexisting molecules; oxygen-bases like alcohols and ethers usually stabilize Keggin structure so as to be used for purification-extraction). Lacunary polyanions formed by partial hydrolysis are important intermediates for the synthesis of mixed-polyatom polyanions and related complexes [31. Complexes of polyanions with organic molecules are good model compounds of reaction intermediates. Acidity and redox properties also vary widely, as reviewed before in relation to catalysis [4,8, 91. These are the most important features of HPA catalysts. The Keggin heteropolyanions are very strong acids in general and release all the protons in aqueous solution, while the extent of dissociation much differs in organic solvents. Quantum mechanical studies on the electronic structure of polyanions are being continued [lo]. I
22.S
-
in the -lid state secondary and tertiary structum HPA in the solid state is an ionic crystal (sometimes amorphous) consisting of large polyanion, cation, water of crystallization and some additional molecules. As we pointed out previously 171, it is important for the understanding of the catalysis of solid HPA to distinguish between the primary structure and the secondary structure (Figures l a and b). Recently, it was realized that, in addition to these structures, the tertiary and higher order structures (the structures of HPA as assembly) were very influential on the catalytic function (Figure lc). The preparation process is very important in this respect. Behavior of the salts of small cations such as Na and Cu (group A salts) is similar to that of the acid forms, but is very different in several respects from the group B salts (salts of large cations like Cs, K and NH4) [4]. The secondary
73 structure of the former is flexible and, due to this nature, polar molecules are readily absorbed in interstitial positions of the solid bulk to cause pseudoliquid behavior. The group A salts are very soluble in water and other polar solvents, while the g r ~ u pB salts are not. Absorption property and solubility are closely related with each other, since both depend on the coordination of absorbed molecule or solvent with polyanion and cation. Thermal stability of group B salts is high. This is presumably due to the absence of proton and water in the lattice of B salts, as the decomposition starts by the water formation from oxygen of polyanion and proton (or water) in the lattice. The thermal stability of HPA catalysts particularly of the mixtures (or solid solutions) is of great concern in heterogeneous catalysis. The states of proton and water of crystallization in the acid form were studied by high resolution solid-state P-NMR and IR 1111. Higher basicity of bridging oxygen was indicated by preferential bonding of proton. The structure and dynamics of water were examined by H-NMR and it was indicated that water molecules tended to be structured a t higher water contents [121. Baba et al. observed for Ag-HPA reversible changes of line width of H-NMR and catalytic activity by the introduction of hydrogen and suggested a correlation between proton mobility and catalytic activity [131. Interesting behaviors of HPA in the solid state have been reported by Siedle and coworkers C141. For example, the following hydrogenation takes place in the solid state (Eq (1)).
-
[(PPh~)~Ir(c~H~~ (solid) ) ~ ~+P9H2 w~~O~o
[(PPh3)2IrH2]3PW12040(solid) + 3CgH16
(1)
Photo-polymerization found by Kudo and coworkers was developed to synthesize inorganic photoresists, which have high resolution as well as high resistance against oxygen plasma etching [ 151. 23. Acid and redox properties in the solid state (1) Acidity. As summarized previously [4], the acidity of acid forms generally reflects the acidity in solution. The acidic properties of salts are much more complicated. Five mechanisms have been proposed for the generation of acidity in metal salts [4]; (i) protons in acidic salts, (ii) partial hydrolysis to form weak protons, (iii) acidic dissociation of water coordinated with multivalent metal ion, (iv) Lewis acidity of multivalent metal ion, and (v) protons formed by the reduction of metal ion with H2. In addition, surface and bulk acidity must be considered in order to understand the acid catalysis of solid HPA. Many efforts are still being devoted for the characterization of acidic properties of HPA, by means of adsorption and thermal desorption of basic molecules [17,181 as well as by NMR [ l l , 13, 161. (2) Reduction-oxidation properties. Two-step reduction of PMol2 by H2 and corresponding two redox cycles were demonstrated by TPR, IR, and ESR (Eq (2)) [41. The oxygen involved in the second step is presumed to be the bridging oxygen of Keggin anion. However, studies of the redox chemistry of polyanions
74
have been devoted mostly to electron transfer reactions by electrochemical methods and few basic studies exist for the oxygen transfer reactions. Recently, Kawafune and Matsubayashi reported the foimation of oxygendeficient polyanion by an oxygen transfer reaction (Eq (3))1191. They assigned this species to a polyanion lacking bridging oxygen on the basis of IR spectral changes during the course of the reduction. PM012040
+
PPh3
-
PM01203g
+
Ph3P=O
(3)
Structure and reactivity of lacunary polyanions (lacking terminal oxygen) as well a8 oxygen transfer reactions of mixed-polyatom polyanions and of reduced polyanions have been reported, as well 1201. The progress of those studies would be very useful for the elucidation of oxidation catalysis. The oxidizing ability of catalyst that changes with the constituent elements of polyanion and countercations is often represented by the rate of reduction of HPA catalysts (reducibility of HPA). Here, the distinction between the surfaceand bulk-reducibility is necessary (see later section) [211. Recently redox potential was measured in the solid state for a single crystal of HqSiW12040. 31H20 1221. The extent of reversibility of the redox cycle was studied as functions of temperature and the extent of reduction [23]. The redox cycle tends to be reversible if the extent of reduction is low or the reduction is limited to near the surface. The exclusion of V from the polyanion framework upon reduction was indicated for PVMoll by V-NMR [871. 24. Absorption pmperties and pseudoliquid
The ease of absorption depends on the polarity (or basicity) and the size of molecules to be absorbed and the rigidness of the secondary structure of solid HPA [4]. The rigidness or the reactivity of HPA depends on the countercations (size, charge, etc.) and water content [4, 241. Alcohols readily move into and out of the bulk of free acids. Pyridine is absorbed easily, but its desorption needs a high temperature. Under controlled conditions uniform pyridinium salts are formed (pyridine:proton = 1:l and 2:l) and after desorption a cubic secondary structure is maintained 141. This behavior is utilized to control the tertiary structure, e. g., in the case of the preparation of catalysts for methacrolein oxidation [l]. The quantity of absorbed molecules tends to be integral multiples of the number of protons, forming stable secondary structures. The results of ethanol absorption a t 301 K are shown in Figure 2 [25]. A t a higher temperature and pressure, the processes are rapid and reversible. Upto the ratio of 2:1, the lattice constant increases only slightly, and above that stoichiometry the lattice starts to expand significantly. The rate of absorption was measured and analyzed quantitatively, after a controlled pretreatment of HPW12. The rate was higher for more polar (or more basic) and smaller molecules. As reported before such nonpolar molecules as hydrocarbons are adsorbed only on the surface. Group B salts like C~,H3-~PW12040 (x>2) adsorb even polar molecules only on the surface. Diffusion coefficients in the lattice of HPWl2 are ca. 103 times lower than those in the micropores of zeolites [251. Niiyama and his coworkers measured the
75
L
a
a
Z 6 3
0
G
a 5 0
w
Figure 2. Absorption of ethanol in H3PW12040 a t 301 K [251. Ethanol pressurekPa; (a) 0.47, (b) 1.7, (c) 6.0, (d) 0. effective diffusion constant by means of both transient response method and quartz crystal microbalance. They obtained values of the order of 10 -11 - 10-10 m s-1, which are much lower than those in the gas phase but are close to those in liquid phase [26]. The presence of pseudoliquid phase and its roles in catalysis (bulk-type (I)) have been demonstrated for several heterogeneous acid catalysis [4]. High activity and unique selectivity are often obtained due to this behavior, since nearly all protons in the solid bulk can take part in catalysis and reaction intermediates like carbocations are stabilized by coordination with polyanion in this phase. Spectroscopic and stoichiometric approaches become feasible and realistic owing to the uniform reaction phase and the amplification of surface phenomena into the whole bulk. Phase transition of pseudoliquid phase and characteristic pressure dependencies intrinsic to the phase have already been reported [4,271. 26. SupportedHPA
Supported HPA catalysts are important for applications. The structure (primary to tertiary), acid, redox and other properties are very much dependent on the support materials, the level of loading and the method of preparation, as early studies demonstrated [4, 9, 281. In addition to the ordinary metal oxide supports such as Si02, A1203 and Ti02, active carbons, ion-exchange resins, and high-surface area salts of HPA have been used. A silane coupling reagent was tested for binding HPA on oxide support [29]. Insolubility in aqueous or organic solvents is also an important property when HPA is applied to liquidphase reactions. Basic solids like A1203 and MgO tend t o decompose HPA [4,30,911, although significant activities were reported in some cases. On the other hand, SiO2 is relatively inert. In the cases of HPWl2 and HPMol2 on Si02, the XRD line widths and intensities as well as the catalytic activities showed that, upto a
76
certain quantity of HPA loaded (20 wt%), HPA molecules are dispersed as thin layers on the support and above that quantity they form thick layers or separate particlee [31]. Recently, research groups in Paris studied in detail HSiMol2 eupported on SiO2 [32 - 341. Spectroscopic studies with IR, Raman and NMR as well as catalytic tests with dehydration and dehydrogenation of methanol [32, 341 indicated that the Keggin structure was maintained and catalytic properties corresponding to parent HPA were revealed a t high loading levels. But, a t a very low level of loading, strong interactions between the heteropolyacid and eurface eilanol groups suppressed the acidity and redox catalysis predominated. They also indicated that the state in solution is influential on the dispereion after supported on SiO2 [331. The thermal stability of HPA on Si02 seeme to be comparable or slightly lower than the parent HPA [34, 351, whereas Kaeztelan et al. reported HPMol2 became stable upto 853 - 873 K when supported on Si02 [361. Besides decomposition, formation and reformation of Keggin structure take place under certain (wet) conditions [4, 37, 381. The decomposition results in a dramatic change in the catalytic function as shown for example in Figure 3 [341. K and Cs salts which have high surface areas are useful supports for HPA. Increased stability as well as improved yield in selective oxidation of acrolein wae observed when H3+xPVxMo12-x040'swere supported on K3PMo12 [39,40]. They claim that free acids covered epitaxially the surface of the support. We suspected, on the other hand, the formation of a solid solution for acidic Cs salts of HPWl2 [411 and HPMol2 [421. A certain kind of active carbon was an excellent support of HPA to make HPA insoluble [431. It was recently reported that proton transfer from HPA to
P-
; I !
"
523 553573 623 Temp. / K
673
Figure 3. Selectivitiee of the reaction of methanol over HqSiMo12040.14H20 (uneupported) as a function of pretreatment temperature [34]. ( 0 ) ;CH20, ( 0 ) ;HCOOH, ( 0 ) ;(CH30)2CH2, (@; (CH3)2O.
77 carbon support brought about tight binding [441. High activity was reported for HPA supported on ion-exchange resin [451 and doped in polyacetylene [46].
26.Heterogeneous catalysis There are three types of heterogeneous catalysis with respect to the contribution of the catalyst bulk as shown in Table 3. These three are prototypes and the actual reactions would exhibit intermediate behavior depending on the relative rates of diffusion and reaction. Surface-type is the ordinary heterogeneous catalysis. Bulk-type (I) is the reaction in the pseudoliquid observed in acid catalysis at relative low temperatures for acid form and group A salts. The bulk-type (11) catalysis was demonstrated for several catalytic oxidations at high temperatures and the mechanism was proven for the oxidation of H2 over HPMol2 [471 and HPWl2 1481 (see later section). Redox (or Mars-van Krevelen) mechanism has been quantitatively shown for the oxidation of H2 and CO over alkali salts of HPMo12 by the coincidence of the three rates a t the stationary oxidation state of catalyst, that is, the rates of catalytic oxidation, reduction and reoxidation of catalyst as summarized in Table 4 121,231. Table 3 Three types of heterogeneous catalysis of HPA Types Remarks Surface-type Ordinary type. Reactions take place on the surface. Rate = surface area Bulk-type (I) "Pseudoliquid phase". Reactants are absorbed in solid bulk and react. Rate = volume(weight) Bulk-type (11) Main reactions occur on the surface, but by diffusion of redox carriers, whole bulk takes part. Rate = volume (weight)
Examples Oxidations of aldehydes and CO Dehydration of alcohols a t low temperatures Oxidative dehydrogenation, oxidation of H2
Table 4 Comparison of the rates of reactions at stationary state of HPMol2 at 623 K co Reaction H2 2.4 130 Catalytic oxidation 110 3.1 Reduction of catalyst 110 2.6 Reoxidation of catalyst by 0 2 0.30 0.038 Degree of reduction of catalyst Rates are in the unit of 10-3 electron anion-1 min-1, and degree of reduction in electron anion-1.
78 3.CONTROL OF ACID AND REMlX PROPERTIES FOR THE DESIGN OF SOLID HPA CATALYSTS (discussion will be limited to HPA with Keggin anion) 3.1. Control of the acidity of solid HPA
The acidity of acid forms in solid generally follows the acidity in solution that is intrinsic nature of the polyanions. The acidity can be further controlled by salt formation and subsequent heat treatment. Five mechanisms as described above must be taken into account for the control of the acidity of salts. In addition, considerations on the difference between the bulk and surface acidity are necessary for acid forms and group A salts. The hulk protonic acidity of group A salts decreases monotonically with the extent of neutralization. But the hydrolysis during the neutralization produces new weak acid sites (mechanism (ii)). The protonic acidity of group B salts exhibits peculiar changes upon neutralization, due to the significant change in the particle size (hence, the specific surface area) as in the case of Cs salts described below (Figure 5). Variation of the acidity as well as the absorption properties with the extent of neutralization is shown in Figure 4 for typical group A and B salts, that is, Na and Cs salts. It has been reported that the protonic sites created by reduction of counter cations such as Ag (mechanism (v)) show extraordinarily high catalytic activity 191. Surface protonic acidity of a given bulk composition is sensitive to the method of preparation and pretreatment (due to changes of the surface composition or the tertiary structure). It must be noted that some discrepancies exist in the acidities reported. For example, Serwicka et al. [171 reported that the acid strength of H3+xPVxM012-~040strongly depended on the water content, whereas in our experience with HPWl2 and HPMol2, water molecules were easily replaced by pyridine to form quantitatively pyridinium salts, and the acid strength did not change with water content [41, water content affecting the rate of absorption but not the equilibrium. They also reported that the acid quantity increased significantly when free acids were supported on K3PMo12 and ascribed the increase to the formation of thin film of the acid on the support. However, as described below, in our experiments, although the surface acidity increased, nearly uniform solid solution was formed by migration of cations. The discrepancies may be in part due to the difference in the HPA used. But, it is evident that cautious and quantitative examination is necessary to elucidate the acidity a t molecular level.
3.2. Control of&x pmperties The redox properties can be controlled in a similar way as the acidity. Mocontaining polyanions are stronger oxidants and hence better oxidation catalysts in general than W-polyanions, reflecting the redox potential in solution. The oxidizing abilities measured by the reduction of HPA catalysts were inconsistent in early studies [41, and rational explanations of the differences between group A and B salts, and of the effects of the methods of reduction and preparation were not possible. We were able to solve most of these problems by the concept of surface- and bulk-type catalysis [21,231. The reduction by H2 of free acid and group A salts proceeds both on surface and in bulk, but the
79
" 0
1
2
3
x in MxH3.xPW1204~
1
0
3
2
x in MxH3-xPW1204~
0.5
r(
'M c
0
x in MxH3.xPW1204~
373
473
513
2a
673
Evacuation Temp. / K
Figure 4. Variations of (A) surface area, (B) acidity (bulk) and (C)absorption property with alkali salt formation, and (D)acid strength distribution measured by thermal desorption of pyridine. (a) H3PW12040, (b) H3PMo120q0, (4NaH2PWi2040, (4Na3PW12040, (el Cs3PW12040, (0 SiO2-A1203.
reduction by CO only near the surface. Hence, the rate of reduction by H2 normalized to catalyst weight can be regarded to represent the oxidizing ability to the first approximation. On the other hand, the rates of of the catalysts reduction by CO divided by the surface area express the oxidizing ability of the Surface. In this way, it was shown that the oxidizing ability decreased monotonically for both cases with the increase in the extent of neutralization with alkali, except for the reduction by H2 of Cs2.5PMo12, for which a higher value than the monotonous change was observed. This high value is due to its very high surface area and rigid secondary structure (this implies that the reaction is not ideal bulk-type). The reason for the decrease of oxidizing ability with alkali content is not fully understood, although the electronegativity of cation and a role of proton in reduction process are suggested for the reason. Transition metals are believed to play additional roles in the redox processes; that is, a reservoir for electrons
80 and active sites for the activation of reductants and molecular oxygen [4]. Recently, Lee and coworkers extended the idea of the reservoir effect and showed that the reduction and reoxidation steps had fair correlations with the electronegativity for several metal ions [491. Reoxidation is usually a slow step in redox catalysis and the slow reoxidation not only retards the rate of catalytic reactions but also accelerates the degradation of polyanion due to overreduction. The effect of mixed polyatom is an important factor for the control of redox properties. Mixed-polyatom HPA catalysts are actually utilized in industrial catalysts and many studies have been carried out on those HPA, but the effect is not well elucidated. For example, although the redox potential in solution shows that PMol2-xVx and PMol2-xWx are stronger oxidants than PMol2 and PW12, the rate of reduction by H2 is slower and less reversible for solid HPMol2-xVx than for solid PMol2 [50,511. The effects of substitution by V for Mo on the catalytic activity are also controversial [4, 17,40, 50,871. Differences between redox processes for solutions and solids as well as the effects of countercations for solids are the possible reasons for the discrepancies. ! I . & colrelationewithcatalytic activity Examples of good correlations for surface-type and bulk-type (I) catalysis are given before [41. Here new results that are understood by concepts recently advanced will be described. (1)Acid Catalysis. Rates of bulk-type reactions show close correlations with the bulk-type acidity measured by thermal desorption of absorbed pyridine, and also with the electronegativity of metal ion (a bulk property) [4]. For the latter, the correlation is not direct and rather complex, but good correlations are often observed by choosing the reaction system and parameter appropriately. On the other hand, the catalytic activities for surface-type reactions are sensitive to the surface composition and change rather irregularly, unless for some reasons the homogenization of the catalyst composition takes place 141. This is particularly so in the case of group B salts which have more rigid secondary structure. By taking advantage of this nature of group B salts we synthesized a highly active solid acid catalyst, that is, Cs2.5H0.5PW12040. We prepared carefully several acidic Cs salts, CsXH3-,PW12040 (Csx sa't) ix = 0, 1, 2, 2.5, 2.75, and 3), and measured their physical, acidic, absorption aild catalytic properties. The properties were in several respects in contrast to what we obsented for Na salts (see Figure 4). In the gas-solid systems [41, the catalytic activities decreased once t o nearly zero, then sharply rose to give a maximum a t x=2.5 and fell to zero again a t x=3. It was remarkable that the activity at x=2.5 was higher than that of the free acid (x=O). Recently we found that the activity a t x=2.5 was even more enhanced for liquid-solid systems [52]. In Figure 5 , the rate of alkylation and the surface acidity are shown as fbnctions of x. Here the surface acidity was estimated by multiplying the formal concentration of proton on the surface by the specifi * surface area. The estimation of the number of surface protons is based on the assumption that the distribution of proton is uniform in the solid and surface he surface acidity changes as in Figure 5 are understandable, because Ca2.F qnd Cs3 salt have very high surface areas (100 - 180 m2 g-11, due t o their very sn all particle size ( 6 - 10 nm), and the density of proton decreases monotonically with x. In
81
0
1 2 x in CsXH3.,PW12040
3
Figure 5. Catalytic activity for alkylation and surface acidity of CsxPWl2 [41al.
general, the two lines in Figure 5 agreed. Hence, the high activity of Cs2.S salts is primarily due to its high surface acidity. The assumption of uniform composition was confirmed by the highresolution P-NMR of the Cs2.5 salt [41al. Figure 6 is the P-NMR spectrum obtained for dried Cs2.5 salt that had been heated at 300 "C. The four lines are assigned to the Keggin anions having 0, 1, 2, and 3 protons coordinated with the anion, respectively. When the samples absorbed water, the signals at lower field shifted reversibly to higher field and overlapped on the highest-field peak, so it was necessary to dry the sample carefully [41a]. The interesting P-NMR spectra reported for acidic salts of potassium are obscured in part by the presence of absorbed water, as the authors indicated [39]. The relative peak intensities in Figure 6 approximately agreed with those expected from the random distribution of protons, indicating uniform acidic salts (i. e., solid solution of C s salt and free acid). The same P-NMR spectrum was observed after heat treatment of a sample that was prepared by impregnating HPWl2 on Cs3 salt to form the same average composition. Hence, it is certain that homogenization proceeded by the migration of Cs ion and proton as suggested before [41. The results in Figures 5 and 6 demonstrate the importance of the tertiary structure of HPA in heterogeneous catalysis. The activity of (382.5 salt was much greater than conventional solid acid catalysts a s shown in Figure 7. The much greater activities (per proton) of Csx salts than Si02-Al203 and S042-/Zr02 cannot be explained simply by the surface acidity. This might be due to additional effects like acid-base bifunctional acceleration by proton (acid) and polyanion (base) and/or a special state of proton on the surface. High activities of HPA catalysts sensitive to pretreatment have been reported for alkylation and acylation of aromatics in liquid-solid system [53]. (2) Oxidation catalysis. Figure 8a shows the correlation of the rates of oxidation of acetaldehyde (surface-type) with the surface oxidizing ability measured by the reduction of catalyst by CO (r[COl, surface-type). In Figure 8b a similar relationship for oxidative dehydrogenation of cyclohexene (bulk-type)
82
is presented. In the latter case, the oxidizing ability was measured by the reduction of catalyst with H2 (r[H21, bulk-type). Good correlations are obtained for both cases. However, only a poor correlation exists if the former reaction (surface) is plotted against the latter oxidizing ability (bulk). This may be
A
A
l ” ” I ’ -15 ” ’ 1
-10
.n
-20 PPm
Figure 6. P-NMR of Cs2.5PW12 (calcined a t 573 K) [41al. Dotted line; expected for statistical distribution.
Catalyst
-1 -1
Activity / mmol g h D 20 40 60
CS2.5H0.5PW1 2040 H3PW12040
SO~~-/Z~O~ Amberlyst 15 Nafion-H Si02-AI203
HZSM-5 H2S04 Figure 7. Comparison of the catalytic activities (rates) of several solid acids [52]. 1,3,5-Trimethylbenzene + cyclohexene (343 K).
a3 evident; for example, the activity for surface-type reaction (acetaldehyde) and r(C0) of Na2 salts varied fiom one lot to another (Na2-1 to Na2-4, Figure 8a), due to the difference in the specific surface area, while those Na2 salts had almost the same values for the bulk-type reactions (Figure 8b). These results demonstrate that, although poor correlations and discrepancies were reported in earlier studies, good correlations exist in principle between the oxidizing ability of catalyst and the catalytic activity for oxidation, and that the concept of bulk and surface catalysis ought to be properly considered. For the oxidation of acetaldehyde over C~,H3-~PMo12040 (x = 0 - 3) and HPMol2 impregnated on Cs3 salt, activity patterns similar to that in Figure 5 were observed [42].
r(Hz)/ l o 5 rnol min-' g-' Figure 8. Correlations between catalytic activity and oxidizing ability for (a) oxidation of acetaldehyde (surface-type) and (b) oxidative dehydrogenation of cyclohexene (bulk-type) [31,421. (3) Combination of acidity and redox properties in catalysis. The acidity and oxidizing ability work cooperatively for oxidation of methacrolein, while they function competitively for oxidative dehydrogenation of isobutyric acid [41. These two reactions are contrastive from the standpoint of the bulk vs. surface concept, as well. The former is surface-type and the latter bulk-type. Therefore, the catalyst design of the two reactions should be considerably different, although both reactions form methacrylic acid with HPA catalysts in good yields (Eq (4)). The former is already industrialized (Table 2) and the latter on a development stage [54 - 561. CH3 CH,=&CHO p
CH3 -CH~=&COOH
CH3
3
CH,-CH-COOH
(4a)
-CH,=~:-COOH
(4b)
84
Scheme 2. Oxidation of acetaldehyde over HPA catalysts
COX
acid
CH3COOCH3
Roles of acidity and oxidizing ability were studied in detail for the oxidation of acetaldehyde [57] using various salts of HPMol2, on the basis of the reaction scheme deduced (Scheme 2). The rate of each reaction step of the scheme was estimated from the oxidations of acetaldehyde and acetic acid, and was compared with the acidity and oxidizing ability of the surface of the catalysts. The comparisons revealed that the oxidizing ability functions mainly in the CHQCOOHand CH3CHO COX,and the acidity steps of CH3CHO CH30H + CO and CHQOH+ CH3COOH accelerates CH3COOH CH3COOCH3, although the controlling factor of the selectivity of the first step was not made clear. HPA catalysts with mixed polyatoms (Mo and V) are used for oxidation of methacrolein in the industrial process [l,41, probably because redox and acid properties of these polyanions are suitable.
--
-
-
4. DEVELOPMENT OF THE CONCEPT OF BULK-TYPE CATALYSIS
4.1. Pseudoliquid phase in liquid-solidsystem The pseudoliquid is revealed when the reacting molecules are more stable in this phase than in the gas or adsorbed phase. So the behavior must exist for liquid-solid systems, as well, if the combination of reactants, catalysts and solvents is adequate. Bulk-type reactions have actually been suggested for liquid-solid systems [53,901. Figure 9 shows the relative activities of the Cs2.5 salt and the acid form of PW12 for three organic reactions C581. Both catalysts are much more active than the conventional solid acids, as in Figure 7. In
Activity / mmol g” Catalyst
(b) Phenol
(a) TMB 0
40
h”
0
300 600 900
(c) Benzopinacol 40
0 I
cs2.5~.5pw12040 H3PW12040
Figure 9. Comparison of the catalytic activities of solid Cs2.5 salt and HPW12 for three reactions in liquid phase [581. (a) 1,3,5-Trimethylbenzene + cyclohexene (343 K), (b) phenol + l-dodecene (373 K), (c) rearrangement of benzopinacol(298 K).
85
addition, it was noted that, as the polarity of reactant increases from the left to the right of the figure, the activity of the free acid increased relatively and became much greater than the Cs2.5 salt for pinacol rearrangement. For the latter reaction, the amount of reacting molecules held by HPWl2 was measured during the reaction by analyzing the amount left in solution. As expected, the amount was significant and corresponded t o about 20 layers if allotted to surface. This clearly indicates the presence of pseudoliquid behavior, and explains the high activity of HPW12. 4.2. Buut-type (II ~atalysis )
In addition to the catalysis in the bulk of the pseudoliquid, there is another class of bulk-type catalysis which was found for oxidation reactions over some HPA catalysts a t relatively high temperatures (Table 3). Independence of the rate on the surface area, prolonged reactivity upon repeated pulses of reductants, good correlations with the oxidizing ability of catalyst bulk, smaller support effect, etc. are the indications of the bulk-type behavior. The acid forms and group A salts, for which bulk-type (I) (pseudoliquid) catalysis is observed, tend to exhibit this type of catalysis, too. But the mechanisms are very different between the two types of bulk catalysis. Reactions to form protons such as oxidative dehydrogenation and oxidation of H2 belong to the bulk-type (II), regardless of the polarity or the size of reactant molecules. Due to the rapid diffusion of protons and electrons, whole bulk participates in the redox cycle, while main reactions proceed on the surface. Recently we proved the bulk-type catalysis, by analyzing quantitatively the reactions of H2-D2 [47,481. The reaction was very similar in the presence of 0 2 , so that the conclusion can be applied to the catalytic oxidation of H2. First, the data for HPWl2 were analyzed. As shown in Figure 10a, the isotopic equilibration in the gas phase was very rapid, and, to our surprise, the H content in the gas phase also increased rapidly. Hydrogen in the whole system (gas and solid phase) was equilibrated in a short period, while the uptake of H + D by the solid and the formation of water were slow. From
I..-.
I
20 Time/min
40
0
20 40 Time/min
Figure 10. H2-D2 reaction over (a) HPWlZ and (b) HPMol2 at 573 K 147,481.
86
detailed kinetic analysis, Scheme 3 and rate data shown in Table 5 were obtained [48]. First and last steps of the scheme are very rapid and the second step is the slow step, the equilibrium strongly favoring the starting system (left hand side of the first step). Rapid appearance of protons of catalyst into the gas phase is well understood by this scheme. Thus, the reason why the reduction of catalyst by H2 becomes bulk-type is that the formation of water from proton and oxygen of polyanion takes place in the whole solid bulk and is rate-determining. It can be shown for HPA catalyst on the basis of a redox mechanism that the rate of catalvtic oxidation of H2 becomes bulk-type, if the rate of catalyst reduction is bulk-type even though the reoxidation is surface-type [21,471. In the case of PMol2, R1 and R3 were comparable and R2 was small [471, so that D atoms flow from D2 in the gas phase into water molecule via solid bulk. Small dependency of the rate on specific surface area was well reproduced also for PMo12 by a simulation using the data in Table 5. Scheme 3. Isotopic equilibration and exchange of hydrogen isotopes H2 (D2) [gas]
rapid - H+ (D') [whole bulk] R1 (forward) R2 (reverse)
* slow
H(D),O [whole bulk]
R3
rapid
H(D),O [gas]
Table 5 Rate data of H2-D2 reaction over HPWl2 and HPMol2 a t 573 K X R1 R2 R3 HPW 12 52 51 0.6 0.8 HPMol2 3.2 0.5 1.8 0.5 R1 and R2; rates of forward and reverse reactions of the first step, R3; rate of the second step, in 10-6 mol gl min-1. x; the volume fraction of the rapid diffusion zone to the whole particle.
TO DESCRIBE THE REACTION PROCESSES OF HETEXOGENEOUS CATALYSISAT AIYIMIC/MOLECULAR LEwEL 5. REACTION MECHANISM A'IT-
6.1. Dehydration ofethanol in pseudoliquid [591
As stated in the earlier section, the uniform three-dimensional pseudoliquid is suitable for the spectroscopic studies. It was attempted to elucidate the reaction mechanism of dehydration of ethanol by means of solid-state highresolution NMR combined with IR, thermal desorption and transient response methods. The transient response method by using ethanol-do and -dg confirmed that a large number of ethanol molecules are absorbed in the
87 interstitial position of the catalyst bulk under the reaction conditions and the reaction proceeds in this bulk phase. Plausible reaction intermediates such as protonated ethanol dimer, ( C Z H ~ O H ) ~ Hmonomer, +, C2H5OH2+, and ethoxy group coordinated with polyanion were directly detected by NMR. These assignments were supported by IR and fully consistent with stoichiometry of ethanol absorption shown in Figure 2. The chemical shift of proton of the dimer species, 9.5 ppm, suggested that the pseudoliquid is a superacid.
e
a
I 16.8 72.0 i
f
b 100
60
20
-20 P W
100
60
20
-20 PPm
-
100
60
20
-20
PPn’
Figure 11. Transformation of protonated ethanol in HPWl2 by heat treatment as measured by C-NMR [591. (a) Dimer, (b) 333 K, (c) 343 K, (d) 363 K, (e) 373 K, (0 423 K. Figure 11 shows the changes of C-NMR of ethanol in pseudoliquid upon heat treatment. Signals a t 61.9 and 17.2 ppm in spectrum a are assigned to CH2 and CH3 of the protonated dimer, (C2H50H)2H+,respectively. Signals at 65.0 and 16.8 ppm originate from the protonated monomer, and signal at 72.0 p p x from protonated dimethyl ether. The spectrum f is purely due to ethoxy group. Spectra b e are of mixtures of those four species. The analysis of the spectral changes combined with the quantitative analysis of the products of thermal desorption and the pressure dependency of the stoichiometry of ethanol absorption lead to the following reaction scheme (Scheme 4). Pressure dependencies of the rate and selectivity were semi-quantitatively reproduced by assuming proper values for rate and equilibrium constants of the reactions in Scheme 4. Gradual variations of pressure dependency and selectivity of ethanol dehydration with x for Csx salts were similarly explained [60]. An interesting NMR study of methanol dehydration on the surface of K salt has been reported [82], but it may be obvious that pseudoliquid behavior makes the quantitative study much more feasible.
-
88 Scheme 4. Reaction scheme for ethanol dehydration in pseudoliquid phase
inactive 62. Oxidation of methacrolein
The direct spectroscopic detection of reaction intermediates of this reaction has not been very successful, probably because this is a surface-type reaction. However, molecular nature of HPA catalysts led us to propose the following reaction scheme (Eq (5)) based on several kinds of circumstantial evidence [41. A similar mechanism was proposed for oxidation of acrolein [611. RCHO 4RCH(0M)Z +* [M=Mo, HI (11
RCOOM -RCOOH
(5)
(21
The first step is reversible and rapid, being catalyzed by acidity of catalyst as discussed before [4]. Presence of direct exchange of isotopic oxygen between polyanion and methacrolein was recently confirmed by means of an elaborate pulse-mass experiment [62]. This supports the presence of the intermediate, (11,in Eq (5). The second step is rate-determining and controlled by the oxidizing ability of catalyst, as a fair correlation was observed [7, 631. Oxygen atoms of polyanion are involved in this step 171. It is to be pointed out that organic complexes of polyanion that contain essential substructures in common with the intermediates assumed in Eq (51, (1)and (21, have been reported [41.
--
6. CATALYSIS IN SOLUTION UNIQUE BASICITY OF POLYANION
Many interesting reactions are catalyzed by HPA in solution. Strong protonic acidity as well as unique basicity of polyanions, e. g., coordination with euch organic molecules as oxygen-bases (indicating oxophilicity through hydrogen bonding), are the reasons of the efficient catalysis. Solid HPA catalysts in general exhibit good performance for reactions of molecules containing oxygen and nitrogen atoms [41. This is also related to the oxophilic nature of polyanions and the tendency to form pseudoliquid.
89 Kozhevnikov and Matveev [8] studied various reactions in aqueous and nonaqueous media catalyzed by HPA, and they correlated the rate of acid catalysis with the acid strength they determined [64]. Izumi's group and Aoshima and coworkers of Asahi Chemical studied further and broadened the possibility of HPA catalysts. High catalytic activities are observed in aqueous solutions as well as in organic media; particularly in the latter case, very high activities are oRen obtained. Typical examples are listed in Table 6. Izumi et al. [65] observed for hydration of isobutene in aqueous HPA solution a reaction order higher than one and activation energy lower than that for mineral acids and proposed that polyanions accelerate the reaction not only by their strong acidity but also by stabilizing a protonated intermediate by Table 6 Organic reactions catalyzed by HPA in liquid phase Reaction Ratio of rate (HPNmineral acid or TsOH) Styrene+HCHO ca. 300 THF + acetic acid ca. 103 Decomposition of ca. 103 PhC(CH31200H 1.5- 8 Dehydration of butanediol 2 - 10 Hydration of olefins
Reaction temp. solvent 338K dioxane 368K neat 300K acetone 430 K water 350 K water
coordination. They extended this hypothesis to several reactions [661. It was reported recently that the higher activity could be explained solely by stronger acidity, solubility and salt effects, as the rate constant followed Eq (6) including log k = 1.04H0 - 3.46
(6)
both EPA and mineral acids [671. However, other studies indicate that the role of polyanion as soft base is one of the essential feature of HPA catalysis. Aoshima and coworkers [681 studied hydration of isobutene in aqueous phase (from a mixture of isobutene and n-butenes) and found a very high activity and selectivity in a highly concentrated solution, as shown in Figure 12. Rate was expressed by Eq (7). On the basis of detailed examination, they concluded that the high activity (ca. ten times that of mineral acids) was due to the combination of three factors of similar order (Eq (8)); rate = k ~+l[polyanionl[isobuteneJ 10 times
=
2 (increase in acid strength) x 2 (solubility increase) x 2 (acceleration by coordination)
(7)
(8)
High selectivity (predominance of hydration of isobutene over hydration of nbutenes, and absence of dimerization of isobutene) was assumed to be brought about by the competitive coordination of reactants and water with polyanion and this coordination strongly depends on the concentrations as indicated by
I
14 9
0 7
X
Q
m
t 2 Q
m
v)
1 0.0 1 0.1
0.1
1
1
[tHPA],[tlN03] / mo1.1~
Concentration of H 3 P M ~ l ~ O ~ d m o l . d m - 3
Figure 12. Rates and selectivities of hydration of isobutene catalyzed by HPA in liquid phase at 353 K. SBA; sec-Butyl alcohol, TBA; tert-butyl alcohol, DIB; dimers of isobutene. the results in Figure 12. The optimization of the concentration and the stabilization of HPA by controlling the oxidation state and adding oxygen-bases were the reasons of the successful commercialization [l]. These remarkable coordination effects were utilized in the industrial process of polymerization of tetrahydrofuran under the conditions of phasetransfer catalysis [691. Here, the presence of two types of coordination, via proton and water, was indicated by IR and the variation of the former was in parallel with the reaction rate. HPA is efficient for other polymerization reactions like polycondensation of benzyl derivatives 1701. Selective synthesis of glycoside derivatives from saccharides was also possible with HPA [72]. Baba and Ono [711 compared dehydration of 1,4-butanediol catalyzed by HSiW12 in dioxane and in aqueous phase. The rate equation (Eq (9)) in aqueous solution indicated the presence of an additional reaction path (k2)
which was accelerated in proportion to the concentration of polyanion, thus implying the complex formation between protonated intermediate and polyanion. The rate was 70 times greater in organic media. This was explained by the retardation effect of water coordination in aqueous phase. Interesting applications of HPA as phase-transfer catalysts for oxidation reactions have been reported by Venture110 et al. [73J and Ishii et al. [74J. This is in part related to the unique basicity of polyanion. Peroxopolyanion formed by the reaction with H202 in aqueous phase becomes slightly soluble into organic phase, where epoxidation of olefin proceeds. Since water is absent in organic phase, secondary reaction to produce diol from epoxide is suppressed, resulting in a very high selectivity of epoxidation. Recently, the structures of probable peroxo intermediates have been reported [75, 76J. HPA catalysts sometimes form a separate liquid phase complexing with solvents. By this phenomena, separation of HPA catalysts from products
91
becomes easier [77]. Similar two phase formation is utilized in the industrial process of hydration of isobutene 11, 681. H202 is used with HPA in uniform solution systems, as well. As listed in Table C (Appendix), variety of reactions have been tested. The effect of mixedpolyatom polyanions is remarkable in the oxidation of cycloolefins to &aldehydes [78 - 801, as shown in Figure 13 for oxidation of cyclopentene in tributyl phosphate. Highest yield was obtained for HPMoGW6 which is a mixture of HPMo12-xWx. The mixing of polyatoms takes place during the preparation in aqueous solution. The mixing does not occur in tributyl phosphate, as shown by the low yield observed for a mixture of HPMol2 and HPW12. The formation of diol, the main by-product, increases with increase in the acidity (or the W content) of HPA. Theoretical explanation of the synergistic effect has been attempted [Sl], although there is a possibility that the formation of active peroxo species is accompanied by the degradation of Keggin anion. GA
I
I
n L U
0
3
6
9 1 2
x in H ~ P M O ~ ~ - ~ W ~ O ~ ~
I 0
CPD CPO
I
I
I
1
I
20
40
60
80
100
Selectivity (%I
Figure 13. Effects of mixed polyatom on the catalytic oxidation of cyclopentene by H202 (303 K, solvent: PO(OBu)3) [79]. *; HPMol2 + HPW12, GA; glutaraldehyde, CPD; cyclopentanediol, CPO; cyclopentene oxide. 7. FUTURE0PPOR"ITIES
In Tables A - D of Appendix, various reactions catalyzed by HPA are collected mainly from recent publications (typical examples in early studies are included) [98]. Even though the examples are limited to acid and oxidation catalysis, the high potentiality of HPA catalysts may be obvious. HPA catalysts are expected to develop in the following directions. (1)Precise control of acid and redox as well as absorption properties As described above, the precise control of acidity and redox properties has recently become much more realistic for the molecular design of catalysts. The
92
concept of bulk- and surface-type properties are important for this purpose. The control of the secondary and tertiary structures are necessary and useful for HPA catalysts in the solid state. Effects of mixed-polyatom polyanions in catalysis are the urgent subjects to be elucidated a t molecular level. (2) Beneficial uee of bulk-type catalysis
Although it was shown that, owing to the pseudoliquid behavior, high activity is oftsn obtained and strong dependency of selectivity on the behavior in certain cases, successful applications of the behavior to practically useful reactions have not been reported. Further efforts are desirable t o improve existing catalysts, e. g., those for methacrylic acid synthesis, and to create novel catalysts, taking account of the bulk-type (11)catalysis.
(3) Cluster models Description of elementary processes at atomidmolecular level by using spectroscopy and stoichiometry has been successful to considerable extents, thanks to pseudoliquid behavior. However, dynamics of the intermediate detected spectroscopically remains t o be studied in relation to molecular properties of polyanions. Synthesis of model compounds of reaction intermediates, together with the studies of their structure and reactivity, are important subjects for the future. (4) Novel polyanions - mixed-polyatom polyanions and adducts t o
organometallics I t is interesting and promising t o design molecular catalysts by substitution or addition of certain elements on the framework of heteropolyanions, a s illustrated for example for polyanions of Keggin-type in Figure 14 (tsken from [3cl and modified). Activation of molecular oxygen, oxidation of alkanes, application a s "inorganic enzyme", etc. have already been attempted by using transition metals and organometallics attached'to polyanions in solution systems [831. As for alkane oxidation in heterogeneous systems, oxidation of n-butane and pentane to maleic anhydride [841 and isobutane to methacrylic acid have been attempted [851, although further improvements are needed.
Figure 14. Lacunary polyanions as starting units of molecular design [3c].
93 Besides Keggin-type polyanions, there are many known polyanions which have been seldom tested of the catalytic function. By choosing the reaction systems properly these polyanions would be stabilized and become promising catalysts. Crystalline molybdophosphates with large cavity [861 and intercalated HPA [881 are potential candidates for catalysts. ( 5 ) Utilization of unique basicity
HPA catalysts exhibit efficient performances when the reaction environment (concentration, solvent, water content, two-phase, etc.) is carefully chosen. This indicates that proper selection of the reaction field for each HPA catalyst leads to invention of novel catalytic systems. (6) Combination with metals Complexes of polyanions with organometallics and transition metalsubstituted polyanions are described above. In addition, there are HPA catalysts that show high performance in combination with noble metals. Wacker-type oxidation in solution (HPA + Pd salts) have been studied for many years [8, 93. Recently, high performances were reported for reductive carbonylation of nitrobenzene with Pd [89] and selective hydrogenation of acetylenic compounds with Rh [921. The authors claim that HPA functions as a macroligand of noble metal ions. The hydrogen-enhanced acid catalyses observed for Ag-HPA as well as HPA + supported Pd, Pt are other examples [9]. (7) Other possibilities Photocatalytic reactions [93] as well as electrocatalytic reactions [94] and modification of electrodes [95] with HPA have been studied extensively. Selective absorption into pseudoliquid is potentially usable for separation, e. g., membrane reactor. Applications of biological activities of HPA to medicines [96] and applications of HPA to electronic materials such as photoresists [ E l , electrochromic device, humidity sensor [97] and solid electrolytes have been attempted.
Acknowledgements. The author is very much indebted to Prof. T. Okuhara and other members of our laboratory not only for the collaboration in the research on HPA catalysis but also for the literature search and the preparation of manuscript.
94
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98 9.APPENDM Table A: Homogeneous Acid-Catalyzed Reactions Reaction
? + c=c-c CIC-C-C
=( +
H20
-
+ H,O
CH,OH
F:
C-7-C
n
Reparks
Ref.
H3PW12040
T=313-353 K
H3PW12040
T=473 K, 200 atm
a
H3PW12040
T=363-393 K
C
H3PMo12040 T=298 K, Y=lOO% (Y: yield)
d
68
OH
-C-v-C-C
-
Catalyst
OH
0
Cyclotrimerization of propionaldehyde
T=298 K, Y=85%
H3PW12040* T=333 K, nH20 MW-3000 (n=0-6)
(PTMG)
OAc
H3PW12040
n 69
H3PW12040
T=368 K, H3PW 1204o:BF3*EbO = 50 : l(ratio of activity)
66
H3PW12040
T=298 K, Y=60-98% (2 h)
72
OAc
a) K Yamada, Petrotech, 13 (1990) 627, b) G. Maksimov, et al., React. Kinet. Catal. Lett., 39 (1989) 317, c) M. A. Schwegler, Appl. Catal., 74 (1991) 191, d) M. V. Joshi, e t al., J. Catal., 120 (1989) 282, e) J P 199045,439.
99 Table B: Heterogeneous Acid-Catalyzed Reactions Reaction
Remarks
Ref.
H3PW 1 2 0 4 d S i 0 2
T=348 K
91
H3PW12O40
T=403 K
b
--*
Q+-
T=308 K, 53 sensitive to pretreatment
-
CH,COOH
+
C,H,OH CH,CUOC,H,
+
+ CH,COOH
H3PW12O40
58
T=423 K, S=91% (9O%-conv.)
52 Cs2.5H0.5PW 12O40 T=373 K, CS~,~:SO~~-IZ~O~
t o 1 1
H4SiW12040 /Amberlyst 15
T=343 K, S=98.6% (11%-conv.)
T=411 K
C H OH ( CH OC €I:))
-C,-C,
C
=43 : 1 (activity/g)
-
H20
52
H3PW 12O40
-0
+
Cs2.5H0.5PW12O40 T=343 K, C~2,cj:H2SO4=3000: 1 (activity/H+)
H,O
~ O C O C € I ,
/J(
Catalyst
IIydrocarbons
T=573 K, H3PW12O40 Cs2.5H0.5PW 12O40 T=563 K, S=74% (C2-C4 olefins)
d
53
e f g
T=263 K, S=94,7% (30%-conv.)
h
T=413 K
1
a ) Y. Ono, e t al., Stud. Surf. Sci. Catal., 20 (1985) 167, b) S. M. Kulikov, et al., Kinet. Katal., 27 (1985) 750, c) Y. Izumi, e t al., J. Catal., 84 (1983) 402, d) T. Baba, e t al., Appl. Catal., 22 (1986) 321, e) Y. Ono, et al., 71CC, Tokyo (19801, p 1414, f) T. Okuhara, et al., J. Chem. SOC.Chem. Commun., (1984) 697, g) H. Hayashi and J. B. Moffat, J. Catal., 77 (1982) 437, h ) A. Igarashi, eta]., J. Japan Petrol. Inst., 22 (1979) 331, i) J P 1991-300,150.
100
Table C: Homogeneous Oxidation Reactions
-
Reaction PhCOCH,Ph t 0, PhCOOH + PhCHO
LtX
n+ 0,-
at-0 0
Catalyst
Remarks
H5PMo10V2040
T=333 K, 02=1 atm, Y(PhCHO)=93%
a
H7PWgFegNi037
T=423 K, TON=9730 (3 h)
b
H5PMo10V2040
T=333 K, 02=1 atm, Y=80%
c
H5PMo1Ov2O40
T=343 K, Y=98% (6 h )
d
H5PMo10V2040
T=293 K
e
H3PMo6W6040
T=303 K, S=60-70%
0,
Ref.
0
6 t
Gt OH
HBr
0'
Hzo2
t
1/20, +
H,O
H+rW042-/P043-/QX T=343 K, Y=82% T=333K, Y=76% (CP)3PW 1 2 0 4 0 CP=cetylpyridinium ion
-
COOH CCOOH
OH
(CP13PW 1 2 0 4 0
-0
t-BuOHreflux, Y=70%
7880 73 74
74
0
+
H,O,
T=303 K, S=78% (conv.=100%)
f
H3PMo12040
reflux, S=100%
6
Pwllcoo3g5-
T=298 K
h
0
r+ -0c0CH3 H,O,
a) B. El Ali, e t al., J. Chem. SOC.Chem. Commun., (1989) 825, b) J. E. Lyons, et al., Stud. Surf. Sci. Catal., 67 (1991) 99, c) M. Lissel, e t al., Tetrahedron Lett., 33 (1992) 1795, d) R. Neumann, et al., J. Org. Chem., 5 4 (1989) 4607, e) R. Neumann, e t al., J. Chem. SOC.Chem. Commun., (1988) 1285, 0 M. Shimizu, e t al., Tetrahedron Lett., 30 (1989) 471, g) D. Attanasio, e t al., J. Mol. Catal., 57 (1989) L1, h) C. L. Hill, J. Am. Chem. SOC.,108 (1986) 536.
101
Table D: Heterogeneous Redox Reactions
--
Reaction CH,=C(CHS)CHO+ 0 2 CH,=C(CH,)COOH
Catalyst
Remarks
Ref.
CSH~PVMO 11040
T=553 K, S=80-86%
a
CH3CHO + 0 2
CsxH3-xPMo12040
T=573 K, S=40-90%
57
H5PV2M010040
T=573 K, b, 56 S=72% (52%-conv.)
PbFeBiPMolpO,
T=673 K, S=80% (97%-conv.)
CHSCOOH
C Cd-CHO
C
Cs2,5H0,5PMo12040+V02+ T=563 K, S=31%
d
BiPMo 12OX+VO2+
T=633 K, S=30-40%
e
H3PMo12040
T=623 K, S d 5 %
85
H5PV2M010040
T=583 K, S=55%
f
Electrocatalytic Reduction NO + H * + a- -NH3
H3PW12O40
T=298 K
94
Photocatalytic Reaction H,S + 1/20,+h v -S+H,O
W2M0100405-
T=298 K
93
& +o,-
0
/\/+0,-
OQO
,(+o,-
0
0
5:
C=C-COOH
M+ 0,-
o
e
o
a) M. Ueshima, et al., Hyoumen, 24 (1986) 582; S. Nakamura, et al., 7ICC, Tokyo (19801, p 755,
b) M. Akimoto, et al., J. Catal., 89 (1984) 196, c) T. Ohara, Shokubai, 19 (1977) 157, d) M. Ai, J. Catal., 85 (1984) 324, e) M. Ai,8ICC, Berlin (19841, Vol. 5, p 475,O G. Centi, et al., Appl. Catal., 46 (1989) 197.
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Guczi, L.a 01. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
THE CATALYTIC CONVERSION OF METHANE TO OXYGENATES AND HIGHER HYDROCARBONS
J.H. Lunsford Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA
ABSTRACT The catalytic conversion of methane has been achieved both in oxidative and nonoxidative processes, although the yields of useful products are only marginal for commercial utilization. Significant advances have been made in the oxidative coupling of methane to ethane and ethylene. The mechanism generally involves a complex set of coupled heterogeneous and homogeneous reactions. Less progress has been made in the conversion of methane to methanol and formaldehyde; however, it appears that the production of methanol also involves a heterogeneous-homogeneous pathway. By contrast, nonoxidative processes, which occur mainly on metals, are dominated by surface reactions. The conversion of methane alone to higher hydrocarbons has been demonstrated in a cyclic mode, but thermodynamic considerations limit the possibilities for a continuous process. A number of alkylation reactions utilizing methane are thermodynamically feasible, and in a few cases the reactions occur with the aid of a catalyst.
INTRODUCTION Methane, which is the principal component of most natural gas, is an abundant hydrocarbon resource. In fact, the known reserves of natural gas, in BTU equivalents, are nearly equal to the reserves of liquid petroleum (ie.,crude oil) [l]. Natural gas, however, is often produced in regions of the world that are far removed from major industrial centers, and its transportation is relatively expensive. Therefore, over the past decade considerable effort has been devoted to the development of processes for the conversion of methane to more valuable and easily transportable chemicals and fuels. These processes, which include oxidative coupling and formation of oxygenates, are known as direct processes, in contrast to indirect processes, which involve the formation of synthesis gas (CO and H2) as the initial step. An economic comparison of several
104
direct and indirect processes is given in a paper by Renesme et al. [2] and references therein. A number of different processes involving the direct conversion of CH, are thermodynamically allowed. These include the oxidative coupling reaction, which in its simplest form may be written as: 1 -0,
-
C2H4 + H 2 0 2 and the conversion of CH, to C H 3 0 H : 2CH4
+
1
CH, + -0, 2
+
CH30H
These processes, which may occur both homogeneously and over a catalyst, require elevated temperatures. The products generally are more reactive than the methane; therefore, it is difficult to obtain both high conversion and high selectivity for the desired product. This is a classic case of a kinetically controlled reaction, and to date a catalyst has not been discovered that will circumvent these kinetic limitations. A number of nonoxidative reactions also are thermodynamically allowed [3,4]. For example, the reaction of methane with propylene to form isobutane
CH, + C,H4
- i-C4Hlo
(3)
has a AG of -0.21 kcal/mol at 500K. This reaction is particularly attractive as a source of isobutylene for the production of methyl t-butyl ether (MTBE). Limited success has been achieved in promoting these reactions, mainly because of the difficulty in activating methane, which has a C-H bond strength of 104 kcal/mole. As with the oxidative processes, undesirable side reactions often prevail, including the formation of coke. Several reviews have recently appeared on the catalytic conversion of methane, with emphasis on the oxidative coupling reaction {1,3,5,6].A recent book edited by Wolf [7] and a collection of papers published in Catalysis Today [8] give accounts of recent progress in this area of research. The purpose of this paper is not to provide another comprehensive review, but rather to introduce the reader to this field of research and to identify specific areas where progress has been made in understanding the fundamentals of the catalytic process. Because of the large amount of effort that has been devoted to oxidative coupling and the level of understanding that has been achieved, this subject will be treated more extensively than the formation of oxygenates and nonoxidative reactions. OXIDATIVE COUPLING Equation 1 represents an oversimplification of the complex set of reactions that occur during the oxidative coupling process. The mechanistic details will be outlined below; however, the overall reaction can be better described as follows:
105
CH,
+
-
0, C,H,, C,H,, CO,, CO, H,O, H,,higher HCk
(4)
The desired product is ethylene, but the technology is already available for converting ethane to ethylene. Therefore, for purposes of describing selectivity or productivity, C2H4 and C2H6 are often lumped together and defined as C, products. If the higher hydrocarbons are included, the group is designated as C,, products. Similarly, the undesirable side products CO and CO, are represented by COX. Classes of Catalysts. Catalysts which are both active and selective for the oxidative coupling of methane may be described as strongly basic metal oxides, although caution must be exercised in attempting to define or measure basicity. Moreover, the most effective coupling catalysts are not simple basic oxides or even monophasic oxides; therefore, the surface and the bulk compositions may be quite different. Dubois and Cameron [9] have pointed out that most of the active and selective catalysts are composed of basic, irreducible surface oxides which form stable carbonate species. They also noted that at high temperatures, the effective catalysts are p-type conductors under a normal oxygen atmosphere. A representative list of the effective oxidative coupling catalysts is given in Table 1. Comparisons with respect to activity are difficult because the optimum performance is usually achieved under oxygen-limited conditions. All of the results listed in this Table were obtained under co-feed conditions, (i.e., CH, and 0, were simultaneously fed into the reactor). It will be noted that the LiCl/MnO, and BaPb0, catalysts contain reducible metal ions, but these probably are covered with a group IA or IIA oxide so that the reagents, and particularly the methyl radical intermediate, do not directly interact with the reducible metal oxide. Previously, it was thought that BaPbO,, an n-type oxide conductor, was an example of a monophasic coupling catalyst; however, more recent work has revealed that the surface rapidly becomes enriched in barium and that a surface BaO/Ba02 layer is responsible for the coupling reaction [17]. With the exception of the C a 4 N i b . 1 0 xcatalyst, all of those listed in the Table exhibit favorable selectivities only at T I 650°C. The optimum temperature is unique to each catalyst. At these high temperatures, catalyst stability is a serious problem because of sintering or loss of an important component such as Li or C1. Moreover, gas phase reactions at these elevated temperatures may decrease the selectivity. The C a 4 N i b , 1 0 , catalyst of Somorjai, Heinemann, and co-workers [ 18,191 is of interest because of the high selectivity that has been reported at 600 C. However, at this stage of development, the catalyst is relatively inactive, and the high C,, selectivity is maintained only for about 5 h. The hydrocarbon formation reaction may involve a different mechanism from that described in the next section.
TABLE 1. REPRESENTATIVE OXIDATIVE COUPLING CATALYSTS
Catalyst
Temp. “C
Flow-rate mL/(rnin.g)
Ratio c0:xa
CH4 Conv.
Selectivity,%
%
c2+
cox
Ref
7 wt ’3% LiRMgO
720
12
8:4:88
37.8
50.3
49.7
10
La203
750
10,Ooo
8:2:90
12.2
67.4
32.6
11
4 wt % Na/La203
775
168
50:9:41
21.8
56.8
43.2
12
1 wt ’3% Sr/La203
880
3667
91:9:0
15.8
80.8
19.2
13
15 mol % CaOhlgO
750
439
13:1.4:86
16.5
67.1
32.9
14
20 rnol % LiC1/Mn02
750
100
5:2.6:92
47.2
64.7
35.3
15
LiCa2Bi304C16
720
25
20:1070
41.7
46.5
53.5
16
BaPb03
800
100
32:860
21.3
66.0
34.0
17
7
18
Ca4NiKg.10,
600
5
“C:O:X refers to mole percentages of CH4 02.and inert gas. ’H20 was used instead of an inert gas.
29:9:62b
10.0
93
107
Mechanism. At the most superficial level, the oxidative coupling reaction may be described by the reaction scheme
Scheme 1
in which ethane, the initial product, is converted both heterogeneously and homogeneously to ethylene. All three hydrocarbons are potentially a direct source of COX. Ideally, one would like to have a system wherein C2H4 is the only source of COX, and the relative reactivities are C2H6 > CH, > C2H4. Normally, the sequence of hydrocarbon reactivity over coupling catalysts is C2H6 > C2H4 > CH,, although over LiC1/MnOx, Burch and coworkers [20] have reported that CH, is more reactive than C2H4. Unfortunately, even over this catalyst, the C, yield was only 12%. At a more detailed level, the oxidative coupling reaction involves a network of heterogeneous and homogeneous reactions which may be partially described by Scheme 2 [21]. The heterogeneous portion of this cycle results in the production of CH,. radicals. These radicals enter the gas phase where they may either couple (the desired reaction) or enter into a chain-branching oxidation process which results in the formation of CO and CO,. Obviously, CH, can be activated in a purely homogeneous manner, and the coupling reaction will occur, but this reaction is strongly quenched by the presence of a surface (even fused-quartz) [22]. At the temperatures and residence times of most catalytic reactions, the purely homogeneous activation of CH, is negligible, and the role of the surface is to generate CH,. radicals. Evidence to support the significance of surface-generated radicals in the coupling reaction was provided by Campbell et al. [23],who employed a matrix-isolation electron spin resonance technique to demonstrate that over Li/MgO, > 40% of the C2H6 was derived from gas-phase CH,. radicals. Recently, Cutnian and coworkers [24], using a mass spectrometer, have shown that over Sr/La,O,, > 75% of the C2H6 resulted from the coupling of CH,. radicals which emanated from the surface. On these closed-shell metal oxides, the radicals are not strongly bound to the surface; therefore, at the high temperature of the reaction, they rapidly desorb. Their lifetimes on the surface may be so short that their formation could be described as an example of an Eley-Rideal type mechanism. It has been proposed that a reactive form of oxygen (see below) promotes the homolytic cleavage of the C-H bond to produce the gas phase CH,. radicals and a surface OH- ion.
108
OH -
CH;
+
CH; + M
CH3*+ 0,
-
-
C,H, + M
CH,O + OH.
Scheme 2
Other groups, however, have noted the strong basicity of the oxidative coupling catalysts and have suggested that the cleavage is heterolytic, ie.,
Lapszewicz and Jing [25]have examined four metal oxides of differing basicity for their abilities to promote the oxidative coupling reaction and the isotope exchange reaction between CH, and D,. No direct correlation was observed between the rates of conversion and the isotopic exchange. This observation is evidence against heterolytic cleavage in the coupling reaction. Reaction 5 (forward and reverse) would result in the exchange reaction, which apparently is independent of the coupling reaction. Experiments utilizing CH, and CD, have confirmed that ethane is produced by the coupling of methyl radicals and that the coupling probably occurs in the gas phase [26]. The data of Nelson et al. [26] and Mims et u1. [27] showed that the main product of the coupling reaction was CH3CD3. Only the coupling of methyl groups could give this result. Moreover, if the methyl groups spent a significant amount of time on the surface, isotopic exchange via reaction 5 would have resulted in a scrambling of H / D in the product molecules. The kinetic isotope effects (KIE) derived from the coupling reaction with CH,/CD, have also provided valuable information concerning the rate-limiting step in the catalytic cycle. Following the proposal by I t o et ul. [ 101, the cycle of Scheme 1 may be written as
109
1 ?02+ 0
k8 +
0;- 9 20; k-8
Here, 0; is depicted as the active form of oxygen, but it may, in fact, be a peroxide ion, O;-; 0 symbolizes an oxygen vacancy. Originally, it was suggested that reaction 6 was not rate limiting, based upon the high reactivity of 0,- ions in the gas phase and on surfaces [lo]. The results of Cant and co-workers [28,29] and others [30,31], however, have clearly shown that there exists a KIE when CH, is replaced by CD,, or when the two are added simultaneously and the KIE is determined from the isotopic distribution in the product. No isotope effect was detected when D 2 0 was co-fed with CH,, indicating that the breaking of an OH bond (reaction 7) was not rate limiting. Based on the observed ME, it was concluded that reaction 6 was rate limiting [28]. Subsequent results from our laboratory have shown that the KIE is a function of oxygen partial pressure [32]. Over Li/MgO at 7OO0C,and at a CH, pressure of 190 Torr, the KIE varied from 1.30 to 1.45 as the 0, partial pressure increased from 19 to 190 Torr. These data suggest that there is no single rate-limiting step. Rather, at high partial pressures of O, reaction 6 becomes rate-limiting, while at low partial pressures of O, reaction 8 occurs at a rate comparable to that of reaction 6. A kinetic model has been developed which adequately accounts for the changes that occur in the KIE as a function of 0, pressure [32]. The model includes not only reactions 6-8, but also 156 gas phase reactions and the reaction of CH3. radicals with the surface. One of the most significant results of the model is its ability to reproduce, in a functional sense, kinetic data obtained by Roos et al. [33] over Li/MgO. They found that the CH, conversion rate followed a first-order dependence with respect to 0, and a complex dependence with respect to CH,, as shown in Fig. 1. The results of the model, which includes comparable rates for reactions 6 and 8, are depicted by the dashed lines which go through the data points in Fig. 1. When kg and k-, were made much smaller, the reaction became zero-order with respect to CH, pressure over most of the pressure range. The transition from first-order to nearly zero-order at 150 Torr no longer occurred. Roos el af. interpreted their results in terms of LangmuirHinshelwood kinetics, but it is now evident that the results can be explained equally well by a model in which reactions 6 and 8 occur at comparable rates. The question of CH, adsorption under reaction conditions, as required by Langmuir-Hinshelwood kinetics, has been addressed in several transient studies. Ekstrom and Lapszewicz [34] reported that there is a pool of strongly adsorbed methane onn the surface of Sm2O3 and Li/Sm203 at 7OO0Cc.Methane appeared to desorb for
110
periods in excess of 100 s. The reagent that led to these results was CD,, which has a molecular weight of 20 amu, the same as that of D,O. When CH, was used instead of CD,, a much shorter transient was observed [35]. Cameron [36] has pointed out that D,O, rather than CD,, could have been the long-lived species, even though a trap was used to remove water vapor. 0.8
6
2
0.6
-----I
12
0.6
- 6
-I*
- 4
z
0.4
3
0
0
Y
a
0.2
0.2
- 2 ,
0 0
20
40
.
I
60
P(Oz), Torr
,
,
80
.
0 100
0
200
400
600
P(CH4), Torr
Figure 1. A comparison of the rates of CH, conversion with respect to (a) variation 0, partial pressure and (b) variation in CH, partial pressure. The circles represent the experimental data of ref. [33], and the dashed lines were obtained from the model. The experimental data was adjusted for a reaction temperature of 7OO0C,usin an activation cm3 molecule-'s-l was used. energy of 55 kcal/mole. A value of k-8 = 1.02 X Peil et al. [37] have found that methane was held up for a short time on Sm20, at 600'C; however, they did not detect any holdup of CH, on Li/MgO at 645°C [38]. Over Sr/La,O3, Kalenik and Wolf [39] found that CH, was not present as a long-lived species on the surface at 750 C. van der Wiele and co-workers [40] likewise observed that no measurable amount of CH, or C2H4 was adsorbed on Li/MgO at 8000C. Stratman et nf. [41] reported that CH, is not adsorbed, or is only weakly absorbed, on a Ba/Sr/Sin203 catalyst at temperatures between 300 and 6000C. Thus, it appears that S m 2 0 3 is anomalous in its ability to adsorb CH, under reaction conditions.
The Origin and Role o f C 0 2 . Since the utilization of oxidative coupling as a practical process is largely limited by the formation of CO,, it is important to understand the origin of this undesirable product. The presence of CO, not only represents a loss in selectivity, but also a large amount of heat is associated with its formation. The removal of this heat is a major engineering problem associated with the coupling reaction.
111
As indicated in Scheme 1, CO, may be formed from any of the three hydrocarbons, and it may be produced either heterogeneously or homogeneously. Actually, the homogeneous reactions yield mainly CO; however, most of the good coupling catalysts also are effective for the conversion of CO to CO,. Studies concerning the origin of CO, are complicated by the fact that each of the three hydrocarbons competes for the same set of active centers; therefore, results with individual hydrocarbons may not reflect the situation when all three hydrocarbons are resent simultaneously. The most definitive experiments have been carried out using P3C-labelled hydrocarbons. Ekstrom and co-workers [42] have carried out a series of experiments in which l3C2H4 or I3C2H6 were added to the feed gas over an Sm203 catalyst at 700'C and the amount of the I3C in the COX was measured. Samarium oxide happens to be one of the oxidative coupling catalysts that gives a relatively large amount of CO. The COX products contained a much larger amount of I3C than was present in the feed as. For example, when the feed gas contained 15% I3C, the COX contained cu. 50% I C. These results establish that the COXis largely derived from the C2 products rather than from CH, under these conditions.
5
*
Nelson and Cant [43] have carried out similar studies over Li/MgO. From data obtained at T > 740°C, they conclude that C, oxidation is responsible for the formation of 30-80% of the COX,but at T c 700"C, more than 90% of the COX is derived from methane. These results suggest that the undesirable secondary oxidation of the C2 products could be minimized by operating the catalyst at T < 700°C. Nevertheless, one still has to deal with the direct pathways for the total oxidation of CH,. The mechanism for the total direct oxidation of CH, is not clear, although it is evident that molecular oxygen plays an important role. When N 2 0 was substituted for 0, as the oxidant, the selectivity at 3% conversion over Li/MgO at 710' C increased from 50% to 88% [44]. Earlier, we speculated that methylperoxy radicals formed via the equilibrium reaction
CH,. + 0,
-
CH,O,*
(9)
were responsible for the total oxidation of CH, [45]. Nelson and Cant [43] have argued that this is an unlikely source of CO, on the surface. Our model likewise indicates that reaction 9 in the gas phase does not contribute significantly to the formation of CO, [321. Another pathway for the formation of CO, involves the reaction of CH3. radicals with a metal oxide. Methyl radicals will collide with a surface many times (ca 10') before they react by coupling with another methyl radical. Each of these collisions with the surface is a potential opportunity for a nonselective reaction. Recently, Tong and Lunsford [46] carried out both qualitative and quantitative studies to determine the extent to which CH3*radicals react and the oxides which are most reactive. The results
112
show that the most reactive oxides are those which have multiple cationic oxidation states. The radicals react principally by a reductive addition process, such as
-
CH,* + M("")+ 02- M"'(OCH,)-
(10)
to form a surface methoxide ion. On transition metal oxides, the methoxide ions are readily oxidized to carboxylates and, ultimately, to C 0 2 . Quantitative data for the reaction of CH,. radicals with several metal oxides are given in Table 2, from which it and on MgO is The is evident that the reactive sticking coefficient on ZnO is These values are consistent with the sticking coefficient on Li/MgO is also observation that ZnO is a nonselective oxidation catalyst, whereas MgO and Li/MgO are moderately selective. When the rate constants listed in the Table were incorporated into our model, it was found that ca. 5% of the C 0 2 was derived from these surface reactions at 700" C over Li/MgO. The contribution of the CH,. radicals reacting with the surface at 650"C, for example, may be significantly greater, since the activation energy for the radical reaction is much less than for the overall catalytic reaction. TABLE 2. RATE CONSTANTS, ACTIVATION ENERGIES, AND STICKING COEFFICIENTS ~~~
'
oxides
ka,s- rn-2
MgO CeO, NiO MOO, NnO
3.0 x 3.1 2.1 4.3 4.4
10, 10,
~
E,,kcal mol-'
Sa
5.7
1.2 2.1 x 10-6
2.6
1.8
104
lo4 104
10-5
The transient study of Peil et al. [38] on Li/MgO revealed that a C2H6 precursor remained on the surface for times of ca. 3 s at 645°C. They speculated that the precursor may be adsorbed CH,. radicals; however, the reversible adsorption of CH, . radicals on a closed-shell oxide such as Li/MgO seems unlikely, except at a few defect sites. On reducible metal oxides, reaction 10 may be preceded by the reversible reaction which is consistent with the low activation energies reported in Table 2 [46]. CH,.
+
M(n+l)+OZ-, H,Cb+M(n+l-b)* 02-
(11)
In almost all transient studies, it has been observed that a significant delay occurs in the desorption of CO,, but after accounting for the re-adsorption phenomena, the residence time on a selective form of La203 may be as short as 0.6 s at 6200Cor 0.1 s at 750'C according to Lacombe el af. [47]. van der Wiele and co-workers [40] have
113
found that CO, produced during reaction is hardly adsorbed at all on Li/MgO at 800"C, although it could poison catalytic sites. Lithium carbonate formation and decomposition is not a part of the catalytic cycle.
Ross and co-workers [33] pointed out that CO, strongly poisons active centers on Li/MgO, and the apparent kinetic order with respect to 0, is affected by the formation of CO,. Xu el a/. [48] have recently shown that over Li/MgO the apparent activation energy both for the generation of CH,. radicals and for the overall consumption of CH, is strongly influenced by the presence of CO,. The intrinsic activation energy for the formation of CH,. radicals is 26 2 2 kcal/mole, but upon the addition of only 0.7 Torr of C02, the E, approaches a limiting value of ca. 50 kcal/mole. Under conventional catalytic conditions, sufficient CO, is produced to attain this limiting E,. The effect of 0, may be described as a typical case in which the catalyst is poisoned by a reaction product
-d -q dt
.
W([C41, QI 1 1+K[CO*]
(12)
If K[C02] > > 1, then
k,
=
k -, and E, K
=
E
+
1
where I is the heat of adsorption for CO,. Temperature-programmed desorption experiments have shown that CO, is strongly held on Li/MgO as surface carbonates [48]; therefore, E, is dominated by A.
Nature of the A c h e Surface. The diversity of the catalysts used in oxidative coupling makes it difficult to generalize about the nature of the active surface, but there is evidence that certain types of oxygen species are involved. These include 0-,O:-, Oj, and oxide ions in low coordination centers on the surface. In a limited number of cases, spectroscopic evidence is available. Following the earlier studies of Abraham and co-workers [49] on Li-doped MgO single crystals, Lunsford and co-workers 1501 suggested that [Li'O-] centers were responsible for the activation of CH, over Li/MgO catalysts. Although most of the [Li'O-] centers observed by ESR are in the bulk of the catalyst, it is proposed that these are in equilibrium with surface 0- centers via hole transport. The [Li'O-] centers have also been observed in Li/ZnO catalysts, and the analogous [Na'O-] centers have been found in Na/CaO [45,51]. Although the presence of these centers is generally correlated with CH,. radical formation and coupling activity, their concentrations are small and the reactivity of 0- should be so great that reaction 6 would never be rate-limiting, an observation which is in conflict with the observed KIE. The latter inconsistency has not been reconciled, although it has been suggested that the active centers may be more correctly described as 0'- ions. Two groups [52,53] have carried out theoretical calculations on the reactivity of Li'O- centers with CH, (reaction 6 ) , and the most refined calculation indicates a barrier height of only 6 kcal/mole for the hydrogen abstraction reaction, which is considerably less than the experimental value of 25 kcal/mole for CH,. radical formation [54].
114
Goodman and coworkers [SS] have used high resolution electron energy loss spectroscopy (HREELS) to study centers that are present in a thin film of Li/MgO on a Mo (100) surface. Upon annealing the sample at T 2 727'C, a loss feature was observed at - 1.6 eV, which is attributed to [I,itO-] centers. After annealing at T 1 873"C, additional loss features were found at 3.6 and 5.3 eV. These two features are assigned to F-aggregates and F-centers (oxygen vacancies containing two electrons) in MgO, respectively. The authors found that the concentration of F-centers correlated well with the formation of ethane, and they concluded that these F-centers were responsible for the generation of CH3. radicals. Dubois el af. [9] have described the redox mechanism during coupling by the following equilibria:
where VA represents an oxygen vacancy without an electron; V& an oxygen vacancy with one electron, and h+ a positive charge carrier. In this nomenclature, a V, center is equivalent to an F-center. This set of reactions points out that 05,, O:-, and 0- may be related through equilibria at the surface, which makes it difficult to establish the role of a particular type of oxygen under reaction conditions. Nevertheless, it is interesting to note that under ex siru conditions, 0, ions have been detected by ESR on La203 [S6,57],and 0;- ions have been observed by XPS on Ba/La203 and RaPbO, catalysts [S8,59]. Both Na and Ba are known to form stable peroxides, and Sinev et al. [60], as well as Otsuka et al. [61] have shown that these peroxides are capable of converting CH, to C2H6 at relatively low temperatures, although the selectivities were small, and the process was not catalytic. Peroxide ions have been detected in situ at ca. 740°C on Ba/MgO catalysts containing 0.5 and 15 mole '30Ba, using laser Raman spectroscopy 62 . As shown in Fig. 2, the 0;- ions are characterized by a Raman band at 844 cm-I. !They are stable in the presence of O,, but they react over a period of several minutes in CH,. On the 0.5 mole 940 Ba/MgO sample, the ions were stable in He for a period of several minutes.
Tong et al. [12] observed that CeO,, upon the addition of Na2C0,, was transformed from a total oxidation catalyst into one which was reasonably selective for the conversion of CH, to C2H4 and C2H6. Ion scattering spectroscopy revealed that the surface was essentially covered by sodium oxide/carbonate. Similar results were observed over Na/La203 and Na/Yb,O,, which led the authors to conclude that a sodium oxide phase, probably Na202, was responsible for the activation of CH,. A similar model had previously been proposed by Gaffney er al. [63] to explain the behavior of a Na/Pr60,, catalyst, which is effective in the redox mode. In the latter
115
case, the lanthanide oxide serves not only as a support for the active Na phase, but also as a solid that is capable of storing oxygen. On a Li/MgO catalyst containing 4.9 wt % Li after reaction at 7 5 0 ° C Mirodatos et al. [64] observed by XPS that the surface was mainly covered by a lithium oxide/carbonate phase; only 8% of the surface was magnesia. Even at a much lower coverage of 0.023 wt 96 Li, Martin and Mirodatos [65] point out that there is ample Li2C03 to provide monolayer coverage. They note that pure Li2C03 has a specific activity that is about one-fourth that of a Li/MgO catalyst, and they question whether MgO may be simply a support for the active Li phase. This seems unlikely, however, as both Peng et uf. [66] and Hutchings et uf. [67] have recently reported that Li/MgO catalysts containing as little as 0.2 wt % Li give conversions and selectivities comparable to those found in catalysts having much more Li. Moreover, we have observed that a used Li/MgO catalyst containing 0.5 wt % Li had a surface composed mainly of MgO, not a Li phase [68]. Finally, molten Li2C03 is a relatively poor oxidative coupling catalyst [69]. (It should be noted that the melting point of Li2C03 is 723"C, and a Li/MgO catalyst is active and selective well below this temperature. In general, this catalyst should not be described as a liquid film of Li2C03 on MgO.) As will be discussed below, the Li phase inhibits the catalytic reaction, much as Na2C03 inhibits the activity of La203 [12], except that in this case, Na202 appears to become the new active phase. Lithium does not form stable peroxides; hence, it probably does not contribute to the activity of Li/MgO, except through the formation of specific centers.
700
Figure 2. Rarnan spectra of peroxide ions on 0.5 mole % Ba/MgO at 745'C: spectrum obtained in O,, (b) spectrum obtained after brief exposure to CH,.
(a)
116
It has been observed by two groups that the incorporation of CaO into MgO greatly improves the activity and C2 selectivity of the catalyst [14,70]. As shown in Fig. 3, the C2 formation rate correlates well with the lattice strain and with the basicity, as determined from the infrared spectrum of a surface carbonate. Phillip et al. [14] take this as evidence that segregation and surface reconstruction are incomplete on the highindex planes that form the surface. Cunningham and co-workers [70] had previously concluded that Ca2+ causes a "rumpling" of the surface which results in a more active catalyst. The enhanced basicity is associated with the morphological changes in the surface layer.
11.
9-
1
0
1
I
50
I
I
I
100
MgO mol% in CaO
Figure 3. Correlation of the reciprocal value of the wavenumber of difference ( A V ) of the asymmetric and symmetric CO stretching band, of the strain in the MgO lattice, and of the C2 formation rate versus MgO content. ( A V ) - ' values were obtained at room temperature (0)and after heating at GOO'C (0)(Ref 14). The role of morphology in oxidative coupling is perhaps better demonstrated with simple metal oxides. Morphological aspects of catalysts for oxidative coupling have been recently reviewed by Martin and Mirodatos [ 6 5 ] ; therefore, only a few examples will be given here. The effect of bulk structure is found in the work of Lo et al. [71], who studied the catalytic properties of Sm,O, in both the cubic and the monoclinic form. The cubic phase was more active and more selective toward C2 hydrocarbons than was
117
the monoclinic phase. A catalyst containing both phases was even more selective for the formation of C2 hydrocarbons. The relationship between crystalline morphology and catalytic behavior has recently been demonstrated by LeVan ef a/. [11,72], who studied LazO3. The decomposition of lanthanum nitrate in air at either 650°C (LT) or 800'C (HT) results in a pure hexagonal structure, but the morphologies of the two materials, as determined by electron microscopy, are very different, The low-temperature treatment results in thin plates, whereas the high-temperature treatment results in smpller, tri-dimensional particles. Both the CH, conversion and the C2 selectivity were greater over the La203LT catalyst than over the La203-HT catalyst. The surface area of the former was greater than that of the latter, which makes it difficult to establish the specific activities. The authors suggest that the greater selectivity over the La,O,-LT platelets is related to the more stable oxycarbonates in this material. These oxycarbonates are assumed to be intermediates in the formation of CO,; therefore, they argue that the more selective catalyst would have the most stable carbonate. In a subsequent publication, LeVan et al. [73] note that a strontium-doped L a 2 0 3 retains a platelet structure at temperatures > 8OO0C, and they attribute the high C2+ selectivity of the Sr/La203 catalyst at 850-900 C to the stabilization of this platelet structure. The addition of chlorine, either via the gas phase or the catalyst, has a remarkable effect on the product selectivity in oxidative coupling [ 16,74-781. The most notable result is the exceptionally large IZ2H,/C2H, ratios that have been achieved. As chlorine atoms are capable of initiating the dehydrogenation of C2H6, it has been proposed that large C2H,/C,H, ratios result primarily from gas phase reactions. For example, Warren [78] studied the effect of adding ethyl chloride to a catalyst composed of Ba or Sr on a non-basic metal oxide support. The effect of adding 500 ppm of ethyl chloride to the feed gas was to increase the conversion and selectivity, as well as the C2H4/C2H6 ratio. It is proposed that chlorine radicals are released from the surface into the gas phase, where they initiate free radical reactions that result in the large C Z H ~ / C ~ratios. H~ Burch er af. [79] have suggested that with alkali chloride-MnC1, catalysts, the improved C2 selectivity results from the formation of a manganese oxychloride surface, but they also believe that C2H4 may be formed from C2H4 in the gas voids within the catalyst bed. HCl is first produced on the chlorine-containing catalysts by a hydrolysis reaction. The HCl then reacts with CH3. or OH. radicals to form C1 atoms, which in turn react with C2H6 in the gas phase [75]. Recent work by Lambert and co-workers [80] on modified Mn3O4 catalysts has confirmed the positive effects of chlorine introduction via the gas phase and the presence of KC1. The greater C, selectivity was attributed either to site modification or the formation of active chlorine radicals on the surface. We have found that the incorporation of chloride ions into Li-modified MgO results in a stable and active catalyst that is capable of converting CH, to C2H4 [81].
118
Ethylene-to-ethane ratios of > 5 have been achieved for periods of 180 h at 640°C, with C2 yields of ca. 19%. Consistent with the large C2H4/C2H6 ratios, the catalysts are also effective in the oxidative dehydrogenation of ethane [82]. At 620 C, C2H6 conversions of 75% have been attained, at 78% C2H4 selectivity. As shown in Fig. 4, the Cl/Li ratio is a critical factor in obtaining high C2H4 productivity; the ratio must be I0.9 in order to have a good catalyst [68]. The absolute amount of Li' (or CI-) in the catalyst is of secondary importance. Characterization of the catalyst by TPD, XKD, and XPS methods indicates that a principal role of the chlorine is to bind lithium as LiCI, and thus prevent the formation of Li2C03. The carbonate spreads over the surface and prevents the conversion of C2H. 6 to C2H4. Since the activation energy is no longer determined by carbonate formation, E, decreases from 50 kcaljmole on a Li/MgO catalyst to 35 kcal/rnole on a Li-MgO-C1 catalyst. For this system, the role of chlorine-containing molecules in the gas phase is believed to be relatively unimportant.
-
80
bQ
Y
Q)
c 60
5 w
I
.c
0
1
t-
-
t
ti
-
020 ' Plcfi,l/Plo+1 PlC&l,1-291 Torr
-
-
41
10
I
I
I
15
20
25
30
Content of Chlorine in the Catalyst (wt%) Figure 4. The effect of chloride ions on the oxidative dehydrogenation of ethane over Li-MgO-Cl catalysts. Thomas et al. [ 161 have shown that a number of monophasic bismuth oxychloride catalysts exhibit exceptionally large C2H4/C2€16ratios during the coupling reaction, but their lifetimes were rather brief. Recently, Khan and Ruckenstein [83] have demonstrated that BiOCI-Li2C03-Mg0catalysts can be operated for times greater than
119
5 h at 750'C. Their best catalyst gave 83% C2 selectivity at 18% CH, conversion, with a C2H4/C2H6 ratio of 2.9. From XPS measurements, it was evident that the Li stabilized the chlorine on the surface. The role of Bi may be to stabilize Li on the catalyst. As noted previously by Ross and co-workers [84], the addition of a third metal to a Li/MgO catalyst greatly improves the lifetime at elevated temperatures by increasing the retention of lithium. Cross Coupling Reactions. Although most of the research on oxidative coupling has been carried out using only methane as the hydrocarbon, at least two groups have studied the cross coupling between methane and toluene [85,86]. A lithium-promoted Y203-CaO catalyst was shown to be particularly effective for this reaction [85]. When ~ of 20 and a hydrocarbon: 0, ratio of 10, the operating at a CH4 : C ~ H S C Hratio conversion of toluene was cu. 32% at 700" C. The ethylbenzene and styrene selectivities were 41% and 23%, respectively, based on the moles of toluene converted. At the same conditions, the C2+selectivity was less than the C,, selectivity. No evidence was found for the production of bibenzyl or stilbene; thus, the cross coupling of benzyl radicals was favored over the homo-coupling. Experiments carried out with C6D5CD3 and CH, confirm the cross coupling reaction. For example, the ethylbenzene fraction was composed of mainly C6D5-CD,-CH3, with some C6DS-CDH-CH3. The authors believe that the cross coupling reaction occurs on the surface, while the C2 formation probably occurs in the gas phase. FORMATION OF OXYGENATES
The direct production of methanol and formaldehyde from methane, both heterogeneously and homogeneously, has recently been reviewed by Brown and Parkyns [87]. They point out that yields of oxygenate are only a few percent, at most, for catalyzed reactions, and they are considerably greater for homogeneous or heterogeneously-initiated reactions. To put matters in perspective, however, it should be pointed out that the conversions and selectivities required for the commercially feasible direct conversion of methane to oxygenates are considerably less than those required for the oxidative coupling reaction. Two recent developments are noteworthy in the catalytic formation of oxygenates. Lyons et al. [88] have reported that iron sodalites, [FeISOD, having iron both in the framework and in ion exchange sites are capable of catalyzing the oxidation of CH, to CH30H with a conversion of 6% and a CH30H selectivity of 64%. The reaction was carried out at 442'C, with a 3:l methane-to-air ratio and a total pressure of 800 psig. The reactor provided an "open reaction zone" for reaction intermediates, which suggests that both heterogeneous and homogeneous reactions are involved. The second study involves the conversion of CH, and 0, to HCHO over silica catalysts. Although the activity of silica was known for this reaction, Parrnaliana et al. [89] have shown that quite good formaldehyde selectivities can be achieved over a W.R. Grace Grade 250 MP silica which had been prepared by a sol-gel method. For example,
120
a selectivity of 77% HCHO was observed at a space time yield of 3.5 gHcHo/kg,,,h. These results were obtained at 1.7 bar and 5 2 0 ° C in a "specifically designed" batch reactor provided with an external recycle. Both the activities and selectivities of the several silicas that were examined varied greatly. The highest purity silica, Cab-O-Sil M5,had a comparable selectivity of 6496, but the activity was about a factor of 10 less. NONOXIDATIVE AND RELATED REACTIONS
The catalytic methylation of olefinic and aromatic hydrocarbons, within thermodynamic limitations, offers an interesting opportunity for the utilization of methane. Solid super acids catalyze the addition of CH4 to C2H4, but at high CH4/C2H4 ratios so as to minimize oligomerization [go]. Earlier work by Rieche er af. [91] and Lijffler er af. [4] indicated that such methylation reactions were possible on metals, and more recent studies demonstrate the surface chemistry that is involved. Using '3C-labeled methane, Koerts and van Santen [92] have conclusively shown that ethylene and propylene may be methylated to the corresponding alkanes over Ru/Si02 and Co/Si02 catalysts. For example, durin the methylation of C2H4 over Ru/Si02, 60% of the propane product contained one 'C atom. These reactions involve a series of steps in which methane is first dissociatively adsorbed on the reduced metal surface at 450'C, which is quickly cooled so as to stabilize CH, species. Then the alkene is adsorbed on this surface at 50' C. Finally, the catalyst is exposed to flowing H2 a t 50 C, and hydrocarbons are desorbed. A model depicting the reaction cycle is shown in Fig. 5 . The principal hydrocarbon product following the reaction of methane and propylene is propane and butane, with smaller amounts of pentane.
Figure 5. Reaction cycle for the conversion of methane with an alkene to a homologue on ruthenium (Ref 92).
121
Prior to the work of Koerts and van Santen, Belgued et al. [93] had studied the production of higher hydrocarbons when CH, was reacted with a supported Pt, Ru and Co catalyst in a similar manner. In this case, CH, was added to t h e reduced metal under a high flow rate so as to remove H,, and C2H6 appeared in the gas phase in a transient manner. Upon substituting flowing H, for CH,, the surface CH, species were converted into saturated hydrocarbons. The highest yields of C,, products were observed over Ru at 160°C. In this case, 37% of the CH, was converted and recovered as higher hydrocarbons. This yield, however, is based on the amount of CH, reacted on the metal, not on the total amount of CH, that was passed over the catalyst. In an earlier section it was pointed out that the CaNiK material of Somorjai and Heinemann [18,19] may catalyze the formation of higher hydrocarbons by a mechanism other than through CH,. radicals. The authors speculate that CH, forms CH, intermediates and H atoms on the surface. The former may react to produce C H, intermediates and then final products by a mechanism similar to that in the FiscKerTropsch reaction. If such is the case, this catalyst has more in common with the supported metals described above than with the usual oxidative coupling catalysts, but with the distinction that it is able to convert methane to higher hydrocarbons in a catalytic manner. Similarly, Abasov et al. [94] have demonstrated that a preoxidized Ni/A1203 catalyst at 650°C is capable of converting methane to benzene with up to 51% selectivity, at an initial conversion level of 18%. The reactant gas contained CH, and 0, in a 9:l ratio. The catalyst deactivated over a period of 20 min, but it could be briefly reactivated by heating in air. Clearly, CH, groups must be formed on the surface and these combine to produce C6H6. The selectivity with which this process occurs is remarkable. SUMMARY AND PROSPECTS
Since the early work of Keller and Bhasin [95] and Hinsen et al. [Y6] considerable progress has been made in our understanding of the oxidative coupling reaction. Many of the materials that are effective are unique among oxidation catalysts in that they do not contain metal ions that are capable of going through a redox cycle. Instead, the surface oxygen ions appear to change oxidation state. the mechanism of the reaction is also unusual because of the importance of a homogeneous component which may, in fact, limit the C,, yields that can be achieved. The empirical evidence suggests that the maximum yield is about 30% in a conventional reactor. If this is indeed the case, the utilization of oxidative coupling as a commercial process will require either improved separation methods (eg., in the removal of ethylene from diluted gas streams) or special applications (e.g., the use of oxidative coupling in combination with a gas-fired electrical power plant.
122
The direct catalytic conversion of methane to oxygenates ia at a11 even more rudimentary stage with respect to fundamental understanding; however, as noted above, the requirements in terms of yield are less demanding. Methanol is a relatively stable molecule in the gas phase, but i t reacts rapidly at elevated temperatures on many catalysts. Initial results indicate that the catalytic conversion of methane to methanol also involves both heterogeneous and homogeneous components, with higher pressures favoring the latter. Although relatively little research has been devoted to the nonoxidative conversion of methane, one can imagine a catalytic cycle in which CH, fragments are formed from methane o n a surface, and these then react, perhaps with another molecule, to yield more useful products. There are analogies between these surface reactions and Fischer-Tropsch chemistry, h i t to date i t has not been possible to rnaintain activity. With methane reacting alone, graphite is the only tht.rmodynarni~~iily stable product at moderate temperatures. One might hope to remove the surface hydrogen selectively, without attacking the CH, fragment, and there is evidence that this has been achieved to a limited extent. Clearly, the conversion of methane by direct processes remains ;I challenging but elusive goal for those working in the field of catalysis. I n a sense, the selective activation of methane is one of the "last frontiers" of catalytic science and technology. One can identify with those in the last century who faced the seemingly impossible task of transforming dinitrogen and dihydrogen into ammonia. Hopefully, o u r sticcess will be equally a s reinarkable ;is theirs. ACKNOWLEDG M ENI'
The author is indebted to ;I large group of gr:iduate students and research associates who contributed significantly to the field of methane oxidation during their time at Texas A&M University. These individuals include K.D. Campbell, S.J. Conway, D.Dissanayake, D.J. Driscoll, M.Hatano, P.G. Hinson, 'I. Ito, K.C.C. Kharas, C.-H. Lin, W. Martir, E. Morales, C. Shi, Y. Tong, D. Wang, J.-X Wang, M. Xu. X. Wang and H.-S. Zhang. I n addition, M.P. Rosynek was involved with niany aspects of the research and was most helpful i n the preparation of this manuscript. l'he laser Raman studies were carried out with G. Mestl and 13. Knijzinger at the University of Munich. Financial support was provided by the National Science Foundation and the G:is Research 1nst i tu te. REFERENCES 1. 2.
3. 4.
J.H. Lunsford, Catal. Today 6, 23s (1090). G. Renesme, J. Saint-Just and Y. Muller, Catal. Today 13,371 (1992). R. Pitchai and K. Klier, Catnl. Rev.-Sci. Eng. 28, 13 (1986). I.D. Liiffler, W.1. Maier, J.G. Aiiclraclc, I. 'lliies arid P. voii I
inf(x,y) H20-*.HOZ + / x - y / ZOH or H 2 0 (Equ. 2)
H20.**HOZ = H3O++ 20-.
(Equ. 3)
For the D HY zeolite, N(H3O+) is the same as for ND when N(H20) = 1; when the water content is increased, it remains constant to N(H20) = 2 and then increases to 0.4 for 4.4 H 2 0 per site (Fig. 2A). For the DED HY sample, containing A1 atoms only as extra-framework amorphous silica-aluminas, we showed that there is no ZOH bridge in the "anhydrous" zeolite but that adsorption of water can (re)create such bridges (14). There is no detectable hydroxonium up to N(H20) = 1.6, but for 2.2 H 2 0 per site, which is the maximum value able to enter this hydrophillic compound, N(H3O+) is about 0.2, the number of Bronsted acid sites being known from the total balance of the hydrogen atoms (21).
248
O
A
0
1
2..3
4
5
6
7
1s'
-.- 0
0.0
0
MFI; H Olas
1
. ,
. ,
2
3
. , . , . , . 4
5
6
7
Figure 2. Number of hydroxonium ions formed per initial Bronsted acidic site (N(H3O+)), versus the number of adsorbed water molecules per initial Bronsted acidic site (N(H20)) for samples: A: HY family: 0 :
ND; A: D;
:DED. B: HMFI family: 0 :De; A:TPA; 0: Bu;
.
: Ga-MFI.
For the HZSM-5 samples (8,21) the situation is as follows (Fig. 2B): when N(H20) increases from 0 to 1, N(H30C) reaches 0.2 only for the De sample. It remains constant up to N(H20) equals 1.5 and 3 for the De and TPA samples, respectively; then it increases strongly to about 0.6 for N(H20) = 6. The plateau about N(H20) = 1 is replaced by a small increase for the Bu sample, followed by a stronger one from N(H2O) = 1.5; N(H3O+) reaches 0.6 for N(H2O) = 3.7. For the Ga-MFI sample, there is no hydroxonium ion for N(H20) less than 0.7. Then N(H3O+) increases continuously, reaching 0.7 for N ( H 2 0 ) equal to 5.5.
3.2. MAS NMR Some of the MAS spectra of the "anhydrous" species have been already published (8,12,14). Those of the D HY, Bu HZSM-5 and Ga-MFI samples are shown in Fig. 3 with the simulated spectra and the signals which make them up, The spectra of "anhydrous" ND and D HY samples (12) (Fig. 3A) contain the Gaussian signals at 4.3 and 5.4 ppm, characteristic of the protons of
249
ZOH bridges in HY zeolite (1,2). There are also small numbers of silanol groups (very weak for ND). After adsorption of a small amount of water on D, (N(H20) = 0.1 to 0.25) its spectrum contains a signal at 7 ppm c o r r e s p o n d i n g t o w a t e r bonded to Lewis acid sites which are not extraframework A1 atoms ( 7 ) . T h e s p e c t r u m of the "anhydrous" DED (14) contains a signal due to AlOH groups (1,2). This signal is wide, probably because of a lowering of the real symmetry of the A1 atom sites (22,23). The spectra of the "anhydrous" samples of HZSM-5 all show silanol and ZOH groups (2.3 and 4.3 ppm, respectively) (8) (Fig.3 B and C). However, in some spectra, especially that of Bu (Fig 3B), the ZOH signal is partly broadened, leading us to assume that part of the A1 atom sites h a v e a l o w symmetry (22,23). The De sample spectrum (8) contains a normal width (1 ppm) signal of AlOH at 3 ppm. For "anhydrous" Ga-MFI, in addition to normal width expected signals Figure 3. 1 H M A S - N M R for silanol and ZOH, the spectrum spectra (experimental, (Fig. 3C) includes a wide Gaussian simulated and components) signal with strong side-bands, obtained on "anhydrous" located at the same position (4.5 samples ( *: spinning sideppm) as ZOH. This signal may also bands): A: D HY; B: Bu be attributed to GaOH groups as HzSlbf-5; C: Ga-HMFI. the chemical shift is not known for these species. 4. DISCUSSION
For abscissa values less than unity, N(H3 O + ) increases continuously with N(H20) for all samples (Fig. 2A and 2B) with the exception of DED
250 HY and Ga-MFI (Fig. 2). In these cases, hydroxonium ions are only detectable for large values of N(H20) (>1.6 for DED and > 0.7 for Ga-MFI) (Fig. 2 A and B). As already mentionned, there are no initial ZOH in the "anhydrous" DED sample (14), but AlOH groups of the extraframework amorphous debris. The Gaussian 1H MAS signal of these AlOH groups,, with strong spinning side bands, is then wide (4 ppm) as is one of the 4.5 ppm signal of "anhydrous" Ga-MFI. Because of the analogies between DED and Ga-MFI, we assume that the widest l H MAS signal at 4.5 pprn in the spectrum of Ga-MFI is attributable to GaOH species rather than to SiO(H)Ga groups N(H3O+) is approximately constant for N(H20) greater than 1 for the ND HY (Fig. 2A). This is in agreement with Equ. 2 and 3 (12), which describe the results when the framework has no defect and especially no dealumination whatsoever. For all the other samples, N(H3 0 + ) increases markedly when N(H2O) is larger than 1 (Fig. 2). We have already shown in the case of D HY that this increase is to relate to defects of the framework (Lewis acid sites detected through water molecules interacting with them) (13). We assume that the Lewis centres are then aluminium atoms still bonded to the framework by less than the initial four A1-0- bonds (25). This is one type of framework defects ; many other types may happen giving rise to an increase i n the hydroxonium concentration in presence of water. For example, as already mentionned, we showed that, in amorphous silica-alumina, Bronsted sites can be formed by the interaction between AlOH groups and water molecules (14); of course the presence of an unsaturated neighbour Si atom is then needed. Our present results suggest the possibility of a lowering of the symmetry of some ZOH groups for the Bu HZSM-5 sample (wide 4.3 ppm signal); this has to be confirmed. Moreover, Staudte et al. proved that extraframework aluminium atoms are able to dissociate water molecules and create Bronsted acid sites (26). 5. CONCLUSION
Comparison of some acid zeolites using broad-line NMR at 4 K shows that the number of hydroxonium ions per initial SiO(H)T bridge of the samples, i.e. the dissociation coefficient, increases with the number of adsorbed water molecules. When the number of water molecules is greater than that of initial bridges, the hydroxonium concentration remains constant for the only sample which has no initial defect, ND HY. It increases again strongly for the other samples, whose defects have been characterized by MAS NMR. The defects that we assume to be responsible for the increase in the number of Bronsted acid sites in the
251
presence of water are the following: extraframework silico-aluminate debris, AlOH (and perhaps GaOH) species still bonded to the framework and Lewis acid sites which can be A1 atoms still bonded to the framework. We cannot exclude the presence of some other active centres, but silanol groups have never shown any activity.
To summarize, though l H MAS NMR does n o t allow to determine the strength of acid sites i n zeolites directly, i t is very helpful through the analysis of the O H groups in "anhydrous" samples and the characterization of the oxygen-protonated species bonded to the T atoms in partly hydrated samples. Broad-line 1H NMR at 4 K can be used to measure the intrinsic Bronsted acidity of the initial SiO(H)T bridges. It shows also that an extra Bronsted acidity is developed depending on zeolite characteristics, especially on the dealurnination (its degree and the nature of the species formed during the dealumination -some of them still bonded to the framework-). This development is related to the hydration degree. Clearly, both 1 H NMR methods will be useful for the future understanding of acidity and dealumination processes.
6. ACKNOWLEDGMENT We thank Dr Marie GRUIA for preparing the Ga-MFI zeolite and Prof. Dieter FREUDE for his contribution to HZSM-5 study (8).
7. REFERENCES 1 2 3
4 5 6
7
H. Pfeifer, J. Chem. SOC.,Faraday Trans. I , 84 (1988) 3777. D. Freude, Stud. Surf. Sci. Catal., 52 (1989) 169. H. Pfeifer, D. Freude and M. Hunger, Zeolites, 5 (1985) 274. G. Engelhardt, H.-G. Jerschkewitz, U . Lohse, P. Sarv, A . Samoson and E. Lippmaa, Zeolites, 7 (1987) 289. D. Freude, J. Klinowski and H. Hamdan, Chem. Phys. Letters, 149 (1988) 355. E. Brunner, H. Ernst, D. Freude, M. Hunger, C.B. Krause, D. Prager, W. Reschetilowski, W. Schwieger and K.-H. Bergk, Zeolites, 9 (1989) 282. M. Hunger, D. Freude and H. Pfeifer, J. Chem. SOC., Faraday Trans., 87 (1991) 657.
252 8 9 10 11 12 13
P. Batamack, C. DorCmieux-Morin, J. Fraissard and D. Freude, J. Phys. Chem., 95 (1991) 3790. C. DorCmieux-Morin, J. Magn. Res., 21 (1976) 419. C. DorCmieux-Morin, J. Magn. Res., 33 (1979) 505. P. Batamack, C. DorCmieux-Morin, R. Vincent and J. Fraissard, Chem. Phys. Letters, 180 (1991) 545. P. Batamack, C. DorCmieux-Morin and J. Fraissard, J. Chim. Phys. 89 (1992) 423. P. Batamack, C. Dortmieux-Morin and J. Fraissard, Cat. Letters, 11 (1991) 119.
14
P. Batamack, C. DorCmieux-Morin and J. Fraissard, Cat. Letters, 9 (1991) 403.
15 16 17 18 19
G.E. Pake, J. Chem. Phys., 16 (1948) 327. E.R. Andrew and R.J. Bersohn, J. Chem. Phys., 18 (1950) 159. R.E. Richards and J.A.S. Smith, Trans. Faraday SOC., 48 (1952) 675. E.R. Andrew and N.D. Finch, Proc. Phys. SOC.,70B (1957) 980. A.L. Porte, H.S. Gutowsky and J.E. Boggs, J. Chem. Phys., 36 (1962) 1695.
20 21
R. Vincent, unpublished results. P. Batamack, Doctorat Thesis, P. and M. Curie University, Paris (1991).
22 23 24
25
26
J. Bohm, D. Fenzke and H. Pfeifer, J. Magn. Res., 55 (1983) 197. E. Brunner, J. Chem. SOC., Faraday Trans., 86 (1990) 3957. W. Schwieger, K.-H. Bergk, D. Freude, M. Hunger and H. Pfeifer, ACS Symposium Series, 398 (1989) 274. P. Batamack, C. DorCmieux-Morin and J. Fraissard, to be published. B. Staudte, M. Hunger and M. Nimz, Zeolites, 11 (1991) 837. Symposium Series, 398 (1989) 274.
253 DISCUSSIONS
Q: H. G. Kar e (Germany) 1) The lfI broad-line N M R spectra exhibit three stron 1 overla ing sections with three maxima to which the various protonated species (ZOH, ZO *--OH O--H 0+,"free" H20) contribute to a varyin extent. How accurate is the determination o? the num&r of H@+ ions [N(H O+)]derived rom these spectra by simulation taking into account the "weighted contrhtions of these oxygen-protonated species", i.e. what are the limits of error ? 2) In this study single water molecules hydrogen-bonded to bridgin Si*-OH-AI, i.e. ZOH*-OH2, were observed whereas these species were not detected by fil [l]. Could you please comment on this ? 3) It seems to be desirable to re are one series of homologous acidic zeolite samples, e.g. H-ZSMJ with decreasing A l / l l + k and minimum defects, in order to check whether or not the novel 1H NMR technique provides the expected systematic change in the strength of the acidic sites. It would then be of great interest to compare the results with those of other experimental methods which are believed to be suitable for measuring and ranking the acidity strength of solid acids such as zeolites. Could you, please, comment ?' A. Jentys, G. Warecka, M. Derewinski and J. A. Lercher, J. Phys. Chem., 93,4837 [l] (1989)
f
Y
Y
A: J. Fraissard 1) From the weighted contribution of the characteristic protonated s ecies we estimate the limits of error on the determination of the number of H O+ ions to tl %. The number of adsorbed water molecules on the "anhydrous" zeolite samdes is determined with an error of 25% at present. These limits are given for HY zeolite in the Figures 2A and 2B [2]. The accuracy on the number of adsorbed water molecules will be improved if we can use the same thin lass ampoule for the pretreatment and the various experiments. It is not ossible for us to comment on the results of J. Lercher et al. Concerning our study, (i) the di ferences between the theoretical spectra of the various protonated species are characteristic as long as the distances between hydrogen atoms belonging to distinct species are sufficiently greater than internal H-H distances; that this criterion is satisfied can be checked by examining the distance parameters displayed for the simulation (the displayed values for the inter-species H-H distances are lower than the true ones because of spectral enlargement due to other nuclei with non zero spin); (ii) the assumption that there is no hydrogen-bond between SiO(H)AI and H20, i.e. no ZOH--OH2, leads to no acceptable quantitative simulation of the experimental spectra. 3) We agree that new experiments on series of homolo ous acid zeolite samples are needed. We are looking forward to doing some on a series of - Z S M J zeolites with various Si/AI ratios; one very important point is that the samples must contain few defects. It would be very interesting to study other sample series: some with only one type of defect. As pointed out in the remark, we must compare the results obtained on the acid strength with those from other methods. P. Batamack, C. Doremieux-Morin, J. Fraissard, Catal. Lett., 11, 119 (1991) [2]
s
I)
P
a
Q: J. J. Fripiat (USA) Are you not afraid that adding water to a zeolite prepared at elevated temperature will hydrolyze some M-O-M bridges (M = Al, Si,etc,) and reconstruct a surface that will be different from that on which acid catalysis will create ? A: J. Fraissard We know that h drolysis of the zeolitic framework of acidic zeolites may happen after water adsorption. have to find a compromise to ensure that the samples have been homogenized before the NMR experiment and to minimize hydrolysis. Usually, we maintain the samples at 375 K for 2 or 3 hours. Under such conditions, for the ND HY zeolite which
d
254 has no initial defect, we see no sign of hydrolysis except when the number of adsorbed water molecules is larger than the number of acid hydroxyl groups. Then the 'H MAS NMR spectra of the sam les show a visible but very small signal corresponding to water molecules of the species.
m2~)6~R
Q: J. Lercher (Austria) Our IR spectra of water adsorbed on HZSM-5 (1:l stoichiometry between water and SiOHAl groups) suggest that the hydroxonium ion is formed and that it is hydrogen bonded to the surface. The difference between these results measured at 300 K and your results might be raised by the shift in the equilibrium caused by a slightly positive enthalpy of the reaction (AH 0.5 kJ/mol)
The explanation agrees also very well with recent calculations of Sauer et al. A: J. Fraissard The aim of our study is primarily to measure the hydroxonium concentration in the samples. We did not plane to determine the actual symmetry of these ions. nevertheless, we d o not claim that the hydroxonium ions d o not form hydrogen-bonds to the framework. The optimum geometry proposed by Sauer for these groups (with two hydrogen-bonds to framework oxygens, as you noted) corresponds to a theoretical model with the three H atoms at the apices of any triangle. We have not such a theoretical model programmed now. However, after simulations using an equilateral triangle, w e were able to enhance the quality of the simulations for most of the samples by using a theoretical configuration where the H30+ H atoms are assumed to be at the apices of an isosceles triangle. The two equal sides of this triangle are in agreement with the length corresponding to an average dipolar interaction for the two longest sides of Suer's model. The base of the configuration is also in agreement with Sauer's model.
Q: R. Kumar (India) 1) Does your Ga-MFI contain AI ? Table 1 of your paper shows that Si/AI ratio in GaMFI is 32. Is it Si/Ga; otherwise what is Si/Ga ? 2) Fig. 2 of your paper shows no difference between MFI and Ga-MFI while in Fig. 3 Ga-MFI exhibits two peaks or splitting in 'H-NMR at 2.3 and 4.3 ppm. However pure MFI exhibits comparatively a much less intense shoulder at 2.3 ppm apart from the peak at 4.3 ppm. Please comment.
A: J. Fraissard 1) We apologize for the error in Table 1 and in the text: in the case of Ga-MFI the given value (32) is the Si/Ga ratio and not Si/AI. Nevertheless, there is a small amount of A1 in the sample due to seeding with h4FI crystals during the synthesis. 2) There is indeed a difference between the curves obtained for Ga-MFI and AI-MFI in Figure 2B, especially for values of H20/as x'
2
Cat.
: PdCI,, CaCI, / H - ZSM - 5; Pd : 1.0 wt. - %
m,,,, act. red. feed
: 0.2509 : 675 K. 2h. HV : 575 K, 3h. H, (60 ml min") : € 6 (1.5 Vol. %), H, (30 Vol. %)in He : 15ml.min"
0
-
.
-
30
0
v, 20 w W
>
2'0 0 V
0 TIME
O N STREAM [h]
Figure 5. Hydroconversion of ethylbenzene over PdC12, CaClfl-ZSM-5 after successive solid-state ion exchange and reduction (see legend of Fig. 2) 3.4 Bifunctional lBd,H-ZSM-5catalysts with modified acidity
hexane conversion
- ethylcyclo-
After reduction, H-ZSM-5 catalysts loaded with Pd via solid-state reaction were also active in dehydrogenation and dehydroisomerisation of ethylcyclohexane. Similar to the findings with ethylbenzene conversion as a test reaction, the catalytic performance of the reduced Pd,H-ZSM-5 samples improved after modification of the acidic properties by solid-state incorporation of Ca2+. Again, this effect was much more pronounced when the solid-state ion exchange with both metals was conducted successively, i.e., first with CaC12 and subsequently with PdC12. An example of the performance of a reduced Pd,Ca,H-ZSM-5 catalyst ( W A I F =0.2, Pd: 1.0 wt.-%) in ethylcyclohexane conversion is demonstrated in Fig. 6. The ECHx conversion was three times higher than in the case of non-modified reduced Pd,H-ZSM-5, and the yield of the main dehydrogenation product (EB) was higher by a factor of about 20. Again, the conversion of the feed (ECHx) was almost constant with increasing Ca content but the selectivity toward the main product (EB) increased, provided the solid-state ion exchange was carried out in a two-step procedure.
264 I
1
I
I Cat. m,,,,
-
-
-
. I
: PdCI.. CaCI, I H ZSM 5: Pd : 1.O wt. % : 0.250~
.
red. : 575 K, 3h. H, (60 ml min") feed : ECHx (1.3 Vol. K), H, (30 Vol. - %) in He : 15ml.min"
l m
II
I
I
-
m
Tr..ct.
=
-
[v(Ei
-
2 >
1
I
I
I
: 495K
=
--
*
m , =-
-
0
Y(C,-Ca)J
U
0
1
2 3 4 TIME O N STREAM [h]
5
Figure 6. Dehydrogenation and hydroisomerisation of ethylcyclohexane over PdC12, CaCldH-ZSM-5 after successive solid-state ion exchange and reduction (see legend of Fig. 2) 3.5 Effect of modification via Ca2+ incorporation o n t h e particle size
distribution of Pd" As was demonstrated in the previous paragraphs, incorporation of Ca2+ lower-
ed the acidity (number of acidic Brmsted sites) of the Pd-loaded H-ZSM-5 samples and improved their catalytic properties. However, the effect on the acidity was independent on the procedure of Ca2+ exchange into the zeolite. For example, the conversion of ethylbenzene over Pd,Ca,H-ZSM-5 (Ca/AlF= 0.2, Pd = 1.0 wt.-8) upon selective disproportionation into benzene and diethylbenzenes (i.e. in the absence of hydrogen) was essentially the same irrespective as to whether Ca2+ and Pd2 + were introduced simultaneously or successively. This result suggested that, besides the influence on acidity, the incorporation of Ca2+ must have at least one additional effect on the properties of the Pd-loaded H-ZSM-5 samples prepared by solid-state reaction. As a possible explanation i t was assumed t h a t the procedure of solid-state ion exchange of Ca2 + and Pd2 + affects the distribution of Pd2+ over the cation sites in Pd,Ca,H-ZSM-5. It appeared likely that this distribution, in turn, influences the particle sizes and dispersion of the Pd" aggregates formed upon subsequent reduction of Pd,Ca,H-ZSM-5. In order to check this assumption, two series of reduced Pd,Ca,H-ZSM-5 samples obtained via (A) simultaneous and (B) successive introduction of Ca2+ and Pd2+ were investigated by transmission electron microscopy. The results of evaluation of the electron micrographs are shown in Fig. 7. It is obvious from the representation of case A that simultaneous exchange of Ca2 +
265 I
I
I
I
I
I
I
Pd : 1.Owt-X; Ca(0.2)/W.ZSM-5
40
-_
? .-
-~
I
I
I
I .
_I I
I I
30
I
I
1
I
P d : l.Owt-X;Ca(0.4)/H-ZSM-S 0 c 0
I
Pd: 1 . 0 ~ - X Ca(O.Z)/H-ZSM-5 ;
0 0 r
z
I
-
I
I
I
I
Pd : 1.O wt-X; Ca (0.4) / H-ZSM-5
__
-
t .z
_-
_-
40
I
I
I
I
I I
Pd : 1.Owt-X; Ca(0.6)/H.ZSM.S
-
__
30
-
: 20
-
_~ _-
z
-
_-
0
.-
10 0
I
I
D I A M E T E R [nm]
I
I
I
I
I
I I
I I
Pd : 1.Owt-X; Ca(0.6)/H(-ZSM-5
I
I
I
I
D I A M E T E R [nml
Figure 7. Size distribution of Pd-particles from electronmicroscopic micrographs of PdC12, CaCln-ZSM-5 after simultaneous (A) and successive (B) introduction of Ca2+ and Pd2+ via solid-state ion exchange followed by reduction and Pd2+ into H-ZSM-5 results in Pd" particle size distributions which become broader and shift to bigger particles with increasing Ca2 + content. As was pointed out previously (vide supra, see Table 31, the catalytic behaviour of these reduced Pd,Ca,H-ZSM-5 deteriorated in the same sequence, i.e. from Ca/AlF = 0.2 through Ca/AlF = 0.6. In contrast, successive exchange of Ca2+ brings about particle size distributions which significantly narrow and shift to smaller Pd" aggregates when the Ca2+ content is increased. This is in parallel with the continuous improvement of the catalytic performance of these samples with Ca2+ contents increasing from W A I F = 0.2 to Ca/AlF = 0.6 (vide supra, see Table 3). A tentative explanation of this finding is as follows. In the case of successive ion exchange, the first step leads to a relative uniform population of the cation sites by Ca2+ and to a correspondingly homogeneous distribution of the remaining protons. This in turn gives rise to a uniform distribution of the subsequently introduced Pd2+ cations. Reduction of these homogeneously distributed noble metal cations generates a high number of small-sized Pd" particles. Simultaneous introduction of Ca2+ and Pd2+ into the H-ZSM-5 structure by solid-state reaction might form more frequently "islands" with either adjacent Ca2+ or Pd2+ populations. The reduction of such samples is likely to give a smaller number of bigger particles in particular when, by high loadings with Ca2+,
266 only a smaller number of cation sites are available for accommodating the noble metal. I t is worth noting that the favourable effect of successive introduction of Ca2+ and Pd2+ on Pd" size distribution and catalytic behaviour is more pronounced in the case of higher Pd and/or Ca loadings. A similar effect of pre-coverage with non-reducible metal cations (such a s Mg2+) on the properties of bifunctional catalysts has been observed in producing noble-metal-containing catalysts with other supports than zeolites [ 171. Also, Homeyer e t al. [181and Bai et al. [19] reported in more recent papers on promoting effects when preparing Pd-containing faujasite catalysts (via conventional ion exchange) after modification of the zeolite by Ca2+. However, in these cases a different mechanism for the promoting effect of Ca2+ seems to predominate.
3.6 Test on t h e location of t h e I'd" particles via olefin hydrogenation The reduced Pd,Ca,H-ZSM-5 catalysts prepared by successive solid-state ion exchange of first Ca2+ and secondly Pd2+ (Ca/AlP=0.6, Pd: 1.0 wt.-%) were used for hydrogenation of n-octene-(1)and 2.4.4-trimethylpentene41).This is a suitable test to check as to whether the reaction occurs mainly on the external or internal surface of the ZSM-5 crystallites [201 (see also Refs. 121-2211.The intrinsic rates of the conversions of both olefins are comparable. The conditions for the test experiments were in our case as follows: 0.05 g catalyst; activation: 675 K, 10-5 Pa, 3 h; reduction: 575 K, H2, 60 ml min-1, 3 h; feed: 3.5 kPa octene, 48.3 kPa H2, 48.3 kPa He, 40 ml min-1, Treact.:315 K. The results of the test experiments clearly showed that the linear n-octene-(1) was hydrogenated to about 93% whereas the conversion of the branched 2.4.4-trimethylpentene-(11, which had essentially no access to the interior of the reduced Pd,Ca,H-ZSM-5 catalysts, amounted to only about 3%. The straightforward conclusion from this finding is that the finely dispersed Pd" particles produced through the above described procedure are indeed localized inside the porous ZSM5 structure where they to a very high degree could catalyse the hydrogenation of the slim n-octene-(1)molecules.
-
-
4. CONCLUSlONS
Solid-state ion exchange of Pd into H-ZSM-5 is an advantageous method to prepare bifunctional catalysts, possessing both acidic and hydrogenation functions, being capable, for instance, to hydrogenate ethylbenzene and to dehydrogenate and dehydroisomerise ethylcyclohexane. The catalytic performance with respect to activity and selectivity (e.g. for the desired main products ethylcyclohexane or ethylbenzene) can be much improved by modification of the acidity through solid-state incorporation of Ca2+. Successive introduction of first Ca2+ and subsequently Pd2+ is more effective than simultaneous exchange. This is due to the fact that successive solid-state ion exchange provides, after reduction, more homogeneously and finely dispersed Pd" aggregates than simultaneous solid-state reaction of CaC12 and PdC12 with H-ZSM-5. This effect is probably due to a more uniform distribution of Pd2+ over the cation sites achieved by successive solidstate ion exchange prior to reduction.
267 5. ACKNOWLEDGEMENT
Financial support by DEGUSSA (Wolfgang, F.R.G.) and helpful discussions with Prof. Dr. P. Kleinschmit and Dr. A. Kiss (DEGUSSA) and Prof. Dr. J. Weitkamp (University of Stuttgart) as well as financial support by the Federal Ministry of Research and Technology (BMFI’, Project 03C 252 A7) i s gratefully acknowledged. Also, the authors thank Mrs. U. Klengler for the preparation of the excellent micrographs and Mrs. E. Popovik for valuable technical assistance. 6, REFERENCES 1 J.A. Rabo, M.L. Poutsma and G.W. Skeels, in J.W. Hi htower (Editor), Proc. 5th Int. Congress on Catalysis, Miami Beach, Flo., US%, August 20-26,1972, North-Holland Publishing Co., New York, 1973, p .1353-1361. 2 J.A. Rabo and P.H. Kasai, Progress in Solid State ehemistr 9 (1975) 1-19. 3 A. Clearfield, C.H. Saldarriaga and R.C. Buckley, in J.l 3 . Uytterhoeven (Editor), Proc. 3rd Int. Conference on Molecular Sieves; Recent Progress Reports, Zurich, Switzerland, Sept. 3-7, 1973; University of Leuwen Press, 1973, Leuwen, Be1 ‘urn, Paper No. 130, pp. 241-245. 4 A.V. Kucherov anfA.A. Slinkin, Zeolites 6 (1986) 175-180. 5 A.V. Kucherov and A.A. Slinkin, Zeolites 7 (1987) 38- 42. 6 B. Wichterlovh, S. Beran, L. Kubelkovh, J. Novhkovh, A. Smie&ovh and R. Sebik, in H.G. Karge and J. Weitkamp (Editors), Proc. Int. Symp. ”Zeolites as Catalysts, Sorbents and Detergent Builders - Applications and Innovations”, Wurzburg, FRG, Sept. 4-8,1988, Elsevier, Amsterdam, 1989; Studies Surf. Sci. Catalysis 46 (1989)347-353. 7 S. Beran, B. Wichterlovh and H.G. Karge. Faraday Trans. I86 - J. Chem. SOC. (1990) 3033-3037. 8 H.K. Beyer, H.G. Karge and G. Borbely, Zeolites 8 (1988) 79-82. 9 H.G. Karge, H.K. Beyer and G. Borbely, Catalysis Today 3 (1988) 41-52. 10 H.G. Karge, G. BorbBly, H.K. Beyer and G. Onyestyhk, in M.J. Philips and M. Ternan (Editors), Proc. 9th Int. Con ess on Catal sis,Cal ary, Ottawa, Chemica Institute of d n a d a , 8ttawa. 1988, Canada. June 26-July - 1.1988. . .396-403. 11 V G . Karge, V. Mavrodinova, Z. Zheng and H.K. Beyer, Appl. Catal. 75 (1991) 343-358. 12 H.G. Karge and H.K. Beyer, in P.A. Jacobs, N.I.Jaeger, L. Kubelkovh and B. Wichterlovh (Editors), Proc. Int. Symp. ”Zeolite Chemistry and Catalysis”, Prague, Czechoslovakia, Sept. 8-13, 1991, Elsevier, Amsterdam, 1991; Studies Surf. Sci. Catal sis 69 (1991) 43-64. 13 H.G. Karge, J. Ladebec ,Z. Sarbak and K. Hatada, Zeolites 2 (1982) 94-102. 14 H.G. Karge and V. Dondur, J. Phys. Chem. 94 (1990) 765-772. 15 H.G. Karge, V. Dondur and J. Weitkamp, J. Phys. Chem. 95 (1991) 283-288. 16 H.G. Karge and L. Jozefowicz, publication in preparation 17 A. Kiss, private communication. 18 S.T. Homeyer, Z. Karpinski and W.M.H. Sachtler, J. Catal. 123 (1990) 60-73. 19 X. Bai, Z. Zhang and W.M.H. Sachtler, Appl. Catal. 72 (1991) 165-178. 20 J. Weitkamp and S. Ernst, private communication. 21 R.M. Dessau, J. Catal. 77 (19821304-306. 22 R.M. Dessau. J. Catal. 89 (1984) 520-526.
Y
l
268 DISCUSSION
Q: A. 0. I. Krause (Finland) You indicated that the acidity plays an important role in your system. You used chlorides in the catalyst prcparation. What are the amounts of chlorides left on the catalysts and do they have any influence '? What would the situation be if some other salts - like nitrates were used instead of chlorides '? A: H. G. Karge The amount of chlorides left after solid-state reaction in the zeolite structure depends, inter alias, on the composition of the chloride/zeolite mixture (under-stoichiometric, stoichiometric or excess amount of chloride), reaction conditions, type of cation (mono-, bi-, tri-valent), type of zeolite structure) etc. This was systematically studied in some cases, for instance with LaCl3/NH4Y and LaClflaY. We did not observe any negative effect of the residual chloridc. Most of the rcmaining chlorides could be removed by short contact of solid-state ion-exchanged material with water [ l , 21. We also employed other salts such as nitrates, carbonates, sulfatcs and acetates. In gcncral, performance of chlorides was better, the reaction of sulfates, acetates and carbonates was negatively affected by dccomposition. Only in the case of Pd(NO& which decomposed in a complicated manner under formation of N2, N20, NO a relatively high and easy exchange with HY was achieved. H. G. Karge, G. BorbCly, H. K. Beyer, G. Onyestyak, Proc. 9rh Inr. Congress on [I] Carafysis, (Eds: M. J. Philips, M. Ternan), Chemical Institute of Canada, Ottawa, Vol. 1 , 3 9 6 (1988) DGMK-Bereichle-Tagungsbericht 9101, DGMK-Fachbereichtstagung "C -Chemie[2] Angcwandte Heterogene Katalyse-Cq-Chemie", Leipzig, ISBN No. 3-92d164-07-4, ISSN NO. 0938-068X, pp. 191, (1991) Q: J. Weitkamp (Germany) 1) 1 am wondering whether the particle size distribution in the final catalyst cannot be influenced by the reduction conditions of Pd(II) introduced via solid-state ion exchange. 2) Your IR data seem to indicate that the weakly acidic sites represented by the 3647 cm-l line are taking part in the solid-state ion exchange in addition to the strongly acidic sites. Can you comment on this '?
A: H. G. Karge 1) We did not find i! significant effect of reduction conditions (e.g. pre-oxidation, reduction temperature) on the catalyst performance. However, systematic studies on the influence of those conditions on the particle size distribution were not carried out. 2) Indeed, the weakly acidic so-called silanol groups (indicated by an IR stretch band around 3740 em-') do react via solid-state exchange with salts as well, but to a lesser extent than the more strongly acidic bridging OH groups (Bronsted acid sites). However, the cations are only looscly held at the sites of those silanol groups [3]. H. K. Beyer, H. G. Karge and G. BorbCly, Zeolires, 8, 79 (1988) [3]
Q: J. Fraissard (France) When there is a solid-state reaction between zeolites and oxides, there is generally diffusion of thc oxidc into the channels, lost of crystallinity etc. With chlorines wc may have residual chlorine in the zeolite. Do you have some comments on thesc problems '? A: H. G. Karge In almost all cases of solid-state reaction between salts or oxides and zeolites studied so far, we did not observe any loss of crystallinity of the zeolite. This was checked by XRD and
269 measurements of the conventional ion exchange capacity (i.e. in a ueous sus ension) of the materials before and after solid-state reaction. Only in the case of uCIflZS -5 we found, by 27Al MAS NMR, a sli ht dealumination even though the XRD pattern did not indicate any structural change [4]. f chlorides are used as reactants residual chlorine may indeed be left in the zeolite structure after solid-state reaction. This chloride usually remains in the structure in the form of occluded metal chloride molecules. We did not observe a negative effect of those occluded species, for instance in catalysis of acid catalyzed hydrocarbon reaction [l]. By contrast, according to the pioneering studies by J. Rabo on salt occlusion one would expect an improved thermal stability of the structure which contains some additional salt [ 5 ] . [4] H. G. Karge, B. Wichterlova, H. K. Beyer, J. Chem. SOC.Faraday Trans., 88, 1345 (1 992) [5] J. A. Rabo, Salt Occlusion in Zeolite Crystals, in: Zeolite Chemistry and Catalysis (Ed.: J. A. Rabo) ACS monograph 171, American Chemical Society, Washington D. C., p. 332 (1976)
J
B
R
Q: R. Kumar (India) First of all let me appreciate the work you resented. My question is related to simultaneous vs successive solid state ion exchange o Ca and Pd. Successive method is more effective than simultaneous one. Does competition between Ca(I1) and Pd(1I) ions, during simultaneous ion-exchange play any role, in addition to the explanation you have given in your paper '? Please comment.
P
A: H. G. Karge We feel that in successive ion-exchange the cations introduced first (e.n. Ca(1I)) are more homogeneously distributed through thezeolite structure than in the case &f compeiitive simultaneous incorporation with other cations (Pd(I1)). The more homogeneous distribution of the Ca(I1) cations will, in turn, provide via the "anchoring" effect a more homogeneous distribution and higher dispersion of the noble metal aggregates formed upon subsequent reduction.
Q: W. M. H. Sachtler (USA) We have a plied solid state ion exchange to PdCIB+HY and PdC12tNaC1, also to GaCIQ+HZSM-l We have also used a related technicwe to reiuvenate aged Pd/HNaY catal$ts and obtained high dispersion of Pd after reductibn. My qkstion is:\hat are your ideas on the mechanism of these processes: are the chlorides sublimed and then physisorbed in the zeolite, or is surface diffusion the prevailing transport mechanism ? A: H. G. Karge The mechanism of the solid-state reaction between compounds (salts, oxides) of the cations to be introduced and the zeolite (hydrogen forms, alkaline metal forms, etc.) is still not clarified. Currently we are conducting in situ experiments to solve this problem. It might well be that either of the mechanisms you mentioned will be o erative depending on the system. In the case of relatively volatile chlorides such as idCl2, sorption of PdC12 molecules into the porous zeolite structure followed by reaction at the OH groups could be predominant whereas surface diffusion prevails in the case of less volatile salts (NaCI) or oxides (Mn304). Comparison between the reaction in the systems CsCI/HZSM-5 and Cs4[PW1~040]/HZSM-5showed a significantly less degree of exchange in the case with the big Keggin ions which are unable to penetrate into the zeolite channels. This seems to indicate that for exchange with alkaline metal salts the diffusion of the salt molecules into the pores is a prerequisite for the solid-state ion exchange to occur.
Q: J. M. Thomas (United Kingdom) My question is prompted by your electron micrograph which shows that there is quite a
270 a few (2 to 5 ) atoms they are invisible in the electron microscope (unless special techniques using Rutherford Backscattering for example). What one would like to know is whether, by careful preparation, you can achieve so high a dispersion of Pd that the individual particles are individual particles are invisible in the electron microscope ?
A: H. G . Karge We are quite aware of the fact that part of the Pd might have smaller sizes than indicated in our histograms derived from electron micrographs. The only thing we claim and could experimentally demonstrate is the increase of Pd(0) dispersion in the case of successive solidstate reaction of, e.g., CaCI2 and PdC12 with H Z S M J . We did not yet attempt to achieve such a high dispersion that the individual Pd(0) particles become invisible in the electron microscope.
Q: W. 0. Haag (USA) 1) The solid state exchange method that you have pioneered presents an important method for preparing dual-functional catalysts. In your catalytic testing, you showed that Ca incorporation can be used to reduce the acidity. I would like to ask if you have examined your bifunctional catalysts in reaction requiring a high acidity such as paraffin isomerization? 2) In your abstract you mention a value of about 100 kJ/mol for the desorption activation energy of the most frequent type of ammonia sorption. This implies a non-homogeneous set of acid sites in your sample we have found that a carefully prepared sample of HZSM-5 shows an NH, TPD peak which represents a homo energetic site. The E, for desorption is 148 kJ/mol, in excellent agreement with your value of 146 kJ/mol obtained by microcalorimetry. This agreement can be taken as evidence that the rate of adsorption of NH, on HZSM-5 occurs with essentially no activation energy.
A: H. G. Karge 1) We did not yet employ the bifunctional catalysts prepared via solid-state ion-exchange for isomerization of n-paraffins. However, we reacted ethy l-cyclohexane in the presence of H over reduced (Pd(0)-containing) Ca,HZSM-5 and obtained, besides the main product etf y lbenzene, dimethylcyclohexanes. 2) Indeed, the HZSM-5 sample used for preparation of our bifunctional catalysts was a commercial product with a relatively high content of extra-framework aluminium as shown by IR (pyridine adsorption), EPR (NO adsorption) and MAS NMR. The acidity strength was not homogeneous as was also substantiated by deconvolution of the TPD peaks employing a kinetics model described [6]. H. G. Karge and V. Dondur, J . Phys. Chem., 94,765 (1990) [6]
Guczi, L. el al. (Editors), New Frontiers in Caralysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
TRANSITION METAL/ZEOLlTE CATALYSTS BY DESIGN: NUCLEATION AND GROWTH OF MONO- AND BIMETALLIC PARTICLES IN ZEOLITE Y
W.M.H. Sachtlefl, Z. Zhanf, A. Yu.Stakheevb and J. S. Feeleye aV.N. Ipatieff Laboratory, Center for Catalysis and Surface Science, Northwestern University, Evanston, IL 60208, USA bOn leave from the N.D. Zelinskii Institute, Russian Academy of Sciences, Moscow, Russia CEngelhardCorporation, Menlo Park, Edison, NJ 08818, USA
Abstract A highly dynamic model emerges for mono- and bimetallic particles inside a Y zeolite under conditions of catalytic CO hydrogenation. Nickel tetracarbonyl and Pd carbonyl clusters are the most probable vehicles in a rather dramatic relocation of Ni and Pd atoms between cages, but also between the outer mantle and the inner core of the zeolite. Zeolite protons interfere by acting as anchors and as nucleation sites. Growing metal particles appear able to destroy their zeolite cages. 1. INTRODUCTION
Our research in recent years has identified a number of mechanisms by which mono- or bimetallic particles are formed in zeolite cages, following ion exchange. Of particular importance are: (1)pH of the washing liquid prior t o calcination, (2) gas flow and heating program during calcination, (3)reduction temperature, (4) exposure of reduced clusters to CO, and ( 5 ) their interaction with zeolite protons. For Ni a high pH during washing promotes hydrolysis, which prevents the escape of Ni2+ions to hidden sites during subsequent calcination, as hydroxide particles in supercages are transformed to easily reducible oxide particles. For Pd very small primary clusters react with CO to form Pd carbonyl clusters which swiftly coalesce to larger clusters; at low temperature this process is controlled by the aperture of the cage windows: in Y sieves it stops at the stage of Pd,,(CO),, in the sieve 5A it stops at Pd&CO),. Mono- and bimetallic Pd/Y, Ni/Y and (PdNi)/Ycatalysts have been tested for the hydrogenation of CO at 10 bar; this work revealed activity and selectivity changes with time on stream that are, in part, ascribed to changes in the size of the metal and alloy particles during the catalytic reaction. In the present work these changes under catalytic conditions will be studied using X-
272
ray Photoelectron Spectroscopy OLPS) and Extended X-ray Absorption Fine Structure (EXAFS).The former technique permits the analysis of changes in metal concentration in the zone near the external surface of the catalyst which is probed by escaping photoelectrons; the latter technique detects the average coordination number of metal atoms with other heavy atoms, from which the nuclearity of metal particles can be derived. In isolation XPS data can still be ambiguous; for instance an observed decrease in the Ni/Si ratio could mean either that Ni has left the surface zone or that the Ni particles have coalesced to very big clusters. To decide between these alternatives X-ray Diffraction (XRD) has been combined with XPS. In the present work XPS and XRD have been applied to catalysts before and aRer certain times on stream of CO hydrogenation; EXAFS has been used to monitor effects on Pd nuclearity of CO exposure at various temperatures. 2. EXPERIMENTAL
Preparation and pretreatment conditions and results of catalytic tests of CO hydrogenation over zeolite Y supported Ni, Pd and Pd+Ni were described elsewhere (1).The following nomenclature will be used to refer to samples: Me(A or B) where Me = Ni, Pd, or PdNi; A or B refers to wash at pH = 6 or 10.5, respectively, subsequent to ion exchange with Nay. Equipment and procedure to obtain XPS data were previously published (2). For EXAFS analysis an ion-exchanged Pd/NaY sample was calcined at 500°C and pelletized in air. After mounting it in the sample holder it was dried in He at 250°C in a cell described previously (3). Following reduction at 200°C and cooling to -195°C the sample was warmed to a specified temperature under 1atm of CO for 20 min. After cooling to -195°C it was sealed for E M S data collection. The temperatures are precise within * 5°C. Data were collected at station A3 of CHESS. Typical beam energies and currents were 5.0 GeV and 70 mA, respectively. X-ray diffraction (XRD) data were measured using nickel filtered Cu-K = radiation with a Rigaku Geigerflex Diffractometer. The average metal particle sizes were calculated with the Scherrer equation. The Ni(ll1) peak at 20 = 44.6270 and the Pd(ll1) peak at 2 0 = 40.2280 were used for Ni and Pd particles. 3. RESULTS
The EXAFS data show that exposure to CO at or below 0°C does not induce observable changes of the Pd cluster size. The Pd clusters do increase
273
at 20°C and a major increase occurs at 50°C. In this paper attention is focused on changes of Pd nuclearity; no mention will be made on changes of coordination by CO. Quantitative curve fitting with a Pd foil as the reference gives a nearest Pd-Pd bond length of 2.72i0.02 A. The first Pd-Pd shell coordination numbers (CN) and Debye-Waller factors (a)of the samples are given in Table 1. Absorption of CO was large at -40°C and even larger at 80"C, presumably due to capillary condensation.
-
Table 1 EXAFS Parameters Sample
pretreatment reduced
EXAFS-A
3.9
CO, T 5 0°C
EXAFS-B I
At? x 100
CN
0.98
4.0
0.98 I
1
EMS-C
c o , 20°C
4.9
0.98
EXAFS-D
CO, 50°C
6.9
0.69
I
MetaYsilicon atomic ratios, binding energies, BE, and the Auger parameter, a',all derived from XPS analysis of the monometallic samples, are compiled in Table 2. The Ni samples of the A and B series differ significantly. The Ni/Si atomic ratio of Ni(A) after calcination is close to the bulk value (0.0651, and the Ni 2p, binding energy (857.0 eV) corresponds to Nil' ions in zeolite (4). The NVSi ratio of Ni(B) exceeds the bulk value by more than an order of magnitude. After reduction at 760°C the Ni2p, signal completely disappears from the Ni(B) spectrum, but about 10% of the Ni remains unreduced in Ni(A). After reduction the Ni/Si ratio decreases by approximately 60% for both catalysts. Carbon monoxide hydrogenation does not cause changes in the Ni2p, BE. No Ni carbide formation is observed in these samples which had been re-reduced at 500°C before the XPS measurements. During CO hydrogenation the Ni/Si ratio decreases by almost 50% for Ni(A), but by more than an order of magnitude for Ni(B) to a value significantly below the bulk ratio.
274
Table 2
Atomic ratios, Binding Energies, and atpa(ev) from XPS for Monometallic Samples
No significant differences are detected between the calcined Pd(A) and Pd(B). The BE of Pd 3d, is 337.5 eV [i.e. the value of the Pd2+ion (5)l for both samples. The PdSi ratios indicate a slight surface Pd depletion with respect to the bulk (0.075). Reduction in flowing H, is complete at 500°C. Both samples have higher BE values of Pd 3d,, than bulk Pd (335.0 eV); a’pd is significantly lower than for bulk Pd (663.7 eV). ARer reaction both the Pd 3d,,, BE and are shifted towards the values of bulk Pd. The change in Pd 3d,, BE is smaller for Pd(A) than for Pd(B). For bimetallic (PdNi)/NaYsamples after calcination the values of the Pd 3d, and Ni 2p, BE and metaVsilicon ratios are quite similar t o those found for monometallic samples. However, these values change significantly after reduction. With PdNi(A) the BE’s for Ni2p, (851.2eV) and Pd 3d,, (334.7eV) are markedly higher and (663.6eV) is lower than the values for the bulk metals. The NUSi and PdSi ratios decrease slightly after reduction (Fig.1). Reaction conditions cause shifts of the Binding Energy of Ni and Pd and of dPd towards the values for the bulk metal. For PdNi(B) the BE’s of Ni 2p, and Pd
275 3& after reduction are equal to those of the bulk metal or even lower. Reduction also results in a twofold decrease in Ni/Si ratio, but a three-fold increase in Pd/Si ratio. After reaction an approximately five-fold decrease in both the Pd/Si and the NUSi ratio is observed. The same trend is observed for the PdNi(B) sample treated in CO at 60°C.
Ni.Pd/NaY Catalysts 0rrldmat. a14
a1 1
-
o m a
\
rrdwt.
B$s -tar.
C)
at M
Fig. 1 MetaYSi ratios for PdNi/NaY samples different histories including different pH of wash after ion exchange. --overall MetaVSi ratio
QOJ
am
No loss in zeolite crystallinity upon ion exchange, washing or calcination is detected by XRD (6).However, after reduction at 760°C Ni(A) shows a 30% loss in crystallinity. In the course of the catalytic reaction the average Ni particle size decreases, remarkably, from 23.3 nm to 12.6 nm. No loss of crystallinity is observed. For Ni(B)reduction produces
catalvst ( w t % > from n - h e x a n e from m e s i t y l e n e _______-____________------------------------------------100 A1203 75 A1203, 25 ZSM-5 50 A1203, 50 ZSM-5 30 A 1 ~ 0 3 , 70 ZSM-5 100 ZSM-5
0.4 1.7 3.5 4.5
1.5 3.8 10.4 13.0
7.5
24.8
1.4 7.6 25.0 27.5 41.2
-
1.4
2.5 4.4 4.4
1.7
3.9
-
4.3
4.4 8.6 6.9 6.0 4.6
The observed coking rates f o r n-hexane, 5-15 x lo-” g/gz-,,,lAkm, are s l i g h t l y h i g h e r than the rate of l x l O - ” g/gzrrrrrm, reported by G i c l e n and P a l e k a r C71. The d a t a , which a r e d i r e c t l y o b t a i n e d a f t e r t h e c a t a l y t i c e x p e r i m e n t s , shown i n F i g u r e 1 and 2, a r e d i s p l a y e d s e p a r a t e l y i n F i g u r e 5. The coke, produced f r o m n-hexane w i t h i n 4 h, i n c r e a s e s n e a r l y l i n e a r l y w i t h i n c r e a s i n g c o n t e n t o f H-ZSM-5 i n t h e m i x t u r e . However t h e amount o f coke f r o m m e s i t y l e n e i s n e a r l y c o n s t a n t for all the mixtures and for H-ZSM-5 and t h e d i f f e r e n c e and H-ZSM-5 i s r e l a t i v e l y s m a l l . T h i s means between A l = O = t h e coke production from n-hexane i s v e r y d i f f e r e n t and on H-ZSM-5 on FIllOB w h i l e the coking o f mesit y l e n e reveals o n l y small d i f f e r e n c e s between the two c a t a l y s t s . The low v a l u e s o f m e s i t y 71 l e n e coke i n comparison t o 0 t h o s e o f n-hexane coke a r e m a i n l y due to t h e space v e l o c i t y l o w e r by a f a c t o r o f 10’” i n t h e e x p e r i m e n t s w i t h mesitylene. Further 6tudies of the coke were made u s i n g t h e EPR t e c h n i q u e . The o b s e r ved spectrum c o n s i s t e d o f o n l y one s i n g l e l i n e . T h i s i s i n accordance w i t h literature data for coke 0 PO 40 00 80 100 formed at temperatures above 300-C [ B , 9 ] . F i g u r e 6 displays the spin intensities of t h i s signal v e r s u s t h e coke c o n t e n t o f F i g u r e 5. Coke from n-hexane t h e samples, d e s c r i b e d in and m e s i t y l e n e i n t h e dependence T a b l e 1. on H-ZSM-5 i n H-ZSM-5.A130s. Each o f t h e samples containing different amounts o f z e o l i t e . i s coked d u r i n a 2 o r 3 d i f f e r e n t t i m e s on stream. I n t h e case o f coked nhexane t h e s p i n c o n c e n t r a t i o n s o f a l l t h e samples can be app r o x i m a t e d by a n e a r l y s t r a i g h t l i n e i n dependence on t h e amount o f coke. T h i s i n d i c a t e s t h a t a t l e a s t i n t h i s case t h e radical density i s p r o p o r t i o n a l t o t h e coke c o n t e n t . S i m i l a r r e l a t i o n s h i p s a r e known f o r e t h e n e c o k i n g [ B ] and f o r methanol c o k i n g [ S ] . The s p i n d e n s i t y , c a l c u l a t e d from t h e s l o p e o f t h e L i t e r a t u r e data are ranging curve, i s about 3 x 10-4N/C,t,,. from lo-* t o lo-= N/Cltem [8,91.
-
:": 6
E
loo/
Q
mrrltylrnr coking
4-
2
4t
F i g u r e 6. EPR s p i n d e n s i t y v s . amount o f c o k e on v a r i o u s m i x t u r e s o f H-ZSM-5.AlTOx.
7j I
F i g u r e 7. EPR l i n e w i d t h v s . amount o f c o k e on v a r i o r m i x t u r e s o f H-ZSM-5-Als03.
However t h i s r e l a t i o n i s more c o m p l i c a t e d w i t h c o k e f r o m mesitylene. The coke on samples c o n t a i n i n g u p t o 50% H-ZSM-5 d i s play an e x p e c t e d l i n e a r r e l a t i o n s h i p w i t h t h e s p i n i n t e n s i t y . But t h e samples w i t h 70 and 100% H-ZSM-5 e x h i b i t a l r e a d y a t very low c o k e c o n t e n t s a much h i g h e r r a d i c a l d e n s i t y o f a b o u t 6 x lo-" N/Cac,,. T h i s i s a d i r e c t h i n t t h a t coke f r o m m e s i t y l e n e on H-ZSM-5 has d i f f e r e n t c h e m i c a l p r o p e r t i e s . Moreover i n F i g u r e 7 t h e l i n e w i d t h o f t h e EPR s i g n a l s i s p l o t ted i n dependence on t h e coke c o n t e n t , The l i n e w i d t h h a s been proposed t o i n d i c a t e t h e H / C r a t i o o f t h e c o k e [lo]. T h e r e f o r e a decreasing l i n e w i d t h should indicate a transition of olefinic t o aromatic coke. U s i n g t h i s e x p l a n a t i o n , t h e d a t a shown i n F i g u r e 7 r e v e a l t h e f o l l o w i n g . I n t h e case o f coke f r o m n-hexane t h e d a t a f r o m a l l samples can be a p p r o x i m a t e d i n dependence on t h e c o k e c o n t e n t by one d e c r e a s i n g c u r v e . T h i s means t h e i n i t i a l , t h e s m a l l amounts o f c o k e a r e o l e f i n i c , t h e final, the l a r g e amounts a r e a r o m a t i c . T h i s i s i n a c c o r d a n c e with the proposed g e n e r a l mode o f c o k i n g p r o c e e d i n g v i a t h e s t e p s p a r a f f i n s , o l e f i n s , o l i g o m e r s t o mono- and p o l y a r o m a t i c e
c111.
The curves for the linewidth from mesitylene coke are different from those curves obtained with n-hexane coke. Samples c o n t a i n i n g 75 and 100% H-ZSM-5 show a v e r y d r a m a t i c decay o f t h e l i n e w i d t h a l r e a d y a t v e r y low coke c o n t e n t s . T h i s change corresponds w i t h the observed change in the spin density o f t h e s e samples. T h i s means t h a t t h e t r a n s i t i o n f r o m olefinic t o aromatic coke occurs much e a s i e r or earlier firstly i n cases when t h e c a t a l y s t c o n s i s t s o f p u r e o r n e a r l y pure H-ZSM-5 and secondly when the coke i s formed f r o m m e s i t y l e n e and n o t f r o m n-hexane. The reason c o u l d be t h a t m e s i t y l e n e i n c o n t r a s t t o n-hcxane i s able t o s u p p l y d i r e c t l y C. r i n g s u s a b l e f o r t h e p r o d u c t i o n o f a r o m a t i c coke.
3.4.
R e l a t i o n s between c a t a l y s i s , a c i d i t y and c o k i n g The c a t a l y t i c r e s u l t s have demonstrated t h a t t h e a c t i v i t y of H-ZSM-5 i n t h e c o n v e r s i o n o f n-hexane can be promoted by a suited addition o f Alz03. T h i s s y n e r g e t i c e f f e c t i s n o t accompanied by an enhanced coking, as t o be seen by comparing F i g u r e 1 and 5. T h i s means Alr03 can be used f o r a p a r t i a l substitution of the z e o l i t e without a s i g n i f i c a n t d e c l i n e i n activity b u t w i t h decreased c o k i n g . These f i n d i n g s a r e n o t only of t h e o r e t i c a l b u t a l s o o f t e c h n i c a l i n t e r e s t . C1lrO3 i s o f t e n used as a binder in the production o f technically applied z e o l i t e s . The p r e s e n t r e s u l t s i n d i c a t e t h a t A1703 can be used i n a d o u b l e f u n c t i o n as a promotor and as a b i n d e r . The reason o f t h e p r o m o t i o n c o u l d be t h e v a r i e d a c i d i t y . The observed d e c l i n e o f B r o n s t e d s i t e s by A1203 would n o t e x p l a i n an i n c r e a s e d a c t i v i t y . B u t Al=03 i n t r o d u c e s L e w i s s i t e s i n t o t h e m i x t u r e . A c r a c k i n g o f n-hexane a c c o r d i n g t o d u a l f u n c t i o n mechanism i n c l u d i n g B r o n s t e d as w e l l as L e w i s s i t e s would e x p l a i n t h e p r e s e n t r e s u l t s . There a r e h i n t s i n t h e l i t e r a t u r e that L e w i s s i t e s can p l a y an a c t i v e r o l e i n t h e a b s t r a c t i o n and a d s o r p t i o n o f hydrogen C121. The promotion o f the m e s i t y l e n e c o n v e r s i o n by A1303 s h o u l d have t h e same reason as i n t h e case o f n-hexane. The d r a m a t i c s h i f t o f t h e s e l e c t i v i t y towards an i s o m e r i z a t i o n i s suggested t o be a consequence o f t h e s i z e o f t h e m o l e c u l e and i t s s p e c i fic c o k i n g b e h a v i o u r . The d i a m e t e r o f t h e m e s i t y l e n e m o l e c u l e i s l a r g e r t h a n t h e p o r e w i d t h o f t h e H-ZSM-5. Hence t h e isomer i z a t i o n should be restricted to the outer surface. In previous experiments i t could be shown that coke from mesitylene i s d e p o s i t e d on t h e o u t e r s u r f a c e o f t h e c r y s t a l 5 whereas coke from n-hexane f i l l s t h e p o r e s C131. The p r e s e n t r e s u l t s i n d i c a t e t h a t m e s i t y l e n e e a s i l y forms a r o matic coke. Behrsing et al. [14] observed t h a t s h e e t s o f aromatic coke are poisoning whereas even thick layers of amorphous coke are not. Altogether these r e s u l t s provide direct h i n t s t h a t m e s i t y l e n e forms p o i s o n i n g a r o m a t i c coke on the outher surface. This supports the conclusion t h a t the observed low s e l e c t i v i t y f o r i s o m e r i z a t i o n on p u r e H-ZSM-5 i s
291 due to a selfpoisoning by s u r f a c e coke. The o b s e r v e d h i g h selectivity for c r a c k i n g s h o u l d t h e n be due t o a r e a c t i o n o f at least partially demethylated mesitylene within the channels. This i s supported by t h e r e s u l t s o f T s i a o e t a l . C151 t h a t s u r f a c e c o k i n g s t i l l p e r m i t s a c c e s s t o most o f t h e i n t e r n a l volume. The observed shift o f s e l e c t i v t y f o r i s o m e r i z a t i o n by F11=O3 addition consequently should be due to a decrease o f t h e poising s u r f a c e l a y e r o f a r o m a t i c coke. T h i s i s i n accordance with two observations: The added A l 8 O 4 i s a c t i v e i n c o k i n g , t o o . T h e r e f o r e t h e t o t a l amount o f c o k e i s d i s t r i b u t e d between A1203 and H-ZSM-5, t h u s l o w e r i n g t h e p o r t i o n o f c o k e on t h e z e o l i t e . The p o i s o n i n g a r o m a t i c c o k e was o n l y o b s e r v e d i n samThe h i g h e s t s e l e c t i v i t y was obp l e s w i t h 70 and 100% H-ZSM-5. served with samples containing only 25 and 50% H-ZSM-5, respectively. These samples were c o v e r e d w i t h o l e f i n i c coke. This coke should be less poisoning and therefore the i s o m e r i z a t i o n s h o u l d be l e s s i n h i b i t e d .
4.
CONCLUSION
A l 2 0 3 , added as A l O ( 0 H ) p r o m o t e s t h e a c t i v i t y o f H-ZSM-5 i n the conversions of n-hexane and o f m e s i t y l e n e . Moreover t h e selectivity in the conversion of mesitylene i s strongly shifted towards i s o m e r i z a t i o n a t t h e expense o f c r a c k i n g . I n addition t h e Bronsted a c i d i t y i s decreased, t h e L e w i s a c i d i t y i s increased. A c o o p e r a t i o n o f B and L s i t e s i s s u g g e s t e d t o be r e s p o n s i b l e for the observed enhanced c o n v e r s i o n . The amount of coke, produced f r o m n-hexane i n c r e a s e s w i t h t h e portion of H-ZSM-5 i n the m i x t u r e s . The o b s e r v e d EPR s p i n density of t h e coke i n c r e a s e s n e a r l y p r o p o r t i o n a l l y w i t h t h e amount o f coke. I n t h e same sequence t h e l i n e w i d t h o f t h e EPR signal decreases, indicating a t r a n s i t i o n from o l e f i n i c t o aromatic c o k e . The f o r m a t i o n o f a r o m a t i c c o k e f r o m m e s i t y l e n e occurs much e a s i e r on pure H-ZSM-5 than on the mixed catalysts. It i s suggested t h a t a s u r f a c e l a y e r o f a r o m a t i c coke inhibits t h e i s o m e r i z a t i o n o f m e s i t y l e n e and t h a t added AlZO3 diminishes this l a y e r , thus enhancing the s e l e c t i v i t y for isomerization. eltogether the r e s u l t s demonstrate t h a t Al2O3, well known a s a b i n d e r , can be e f f e c t i v e l y u s e d a s a p r o m o t o r o f H-ZSM-5 c a t a l y s t s , too.
292 5.
1
REFERENCES
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2 O.W. Bragin, W.I. J a k e r s o n , T.B. V a s i n a , S.H. Isajew, L.I. Lafer, W.D. Nissenbaum, I z v . Acad. Nauk, SSSR, S e r . Khim., 1989, 254. 3 W.I. J a k e r s o n , W.D. Nissenbaum, T.B. V a s i h a , L . I . L a f e r , S.A. I s a j e w , E.E. Denisowa, J.L. Duch, O.W. Bragin, i b i d . 1990, 1244. 4 P.B. Weisz, I n d . Eng. Chem. Fundam., 25 ( 1 9 8 6 ) 53. 5 U. Kurschner, B. P a r l i t z , E. S c h r e i e r , G. ehlmann, J. V o l t e r , A p p l . C a t a l . , 30 ( 1 9 8 7 ) 159. 6 L.M. K u s t o v , V.B. Kazansky, S. Beran, L. K u b e l k o v a , P . J i r u , J. Phys. Chem., 9 1 ( 1 9 8 7 ) 5247. 7 B. G i e l e n , M.G. P a l e k a r , Z e o l i t e s , 9 (19E9) 208. 8 H.G. Karge, J.-P. Lange, A . G u t s z e , M. L a n i e c k i , J. C a t a l . , 114 (1988) 144. 9 R.H. M e i n h o l d , D.M. B i b b y , Z e o l i t e s , 10 ( 1 9 9 0 ) 121. 10 H.L. R e t c o f s k y , G.P. Thompson, R. Raymound, R . A . F r i e d e l , f u e l , 54 ( 1 9 7 5 ) 126. 11 M. G u i s n e t , P. Magnoux, A p p l . C a t a l . , 54 ( 1 9 8 9 ) 1. 12 L.M. K u s t o v , V.B. Kazansky, J. chem. SOC. f a r a d a y T r a n s . , 87 ( 1 9 9 1 ) 2675. 13 J. K a r g e r , H. P f e i f e r , J . Caro, H. Bulow, H. S c h l o d d e r , R. M o s t o w i c z , J. V o l t e r , A p p l . C a t a l . , 29 ( 1 9 8 7 ) 21. 1 4 T.H. B e r h s i n g , H. J a e g e r , J . V . S a n d e r s , A p p l . C a t a l . 54 ( 1 9 8 9 ) 289. 1 5 C. T s i a o , C . Dybowski, A.M. G a f f n e y , J.64. S o f r a n k o , J. C a t a l . , 128 ( 1 9 9 1 ) 520.
DISCUSSION Q: J. J. Fripiat (USA) Have you tried other aluminas than boehmite? A: J. Volter Yes, we have also tried mixtures with y-Al 0,. The same thermal treatment at 500 (%2 was used with these mixtures. But no catalytic was observed, as to be seen in Figure 1.
e8ct
Q: J. Datka (Poland) How do you explain the effect of S O 2 addition the HZSM-5:' Is it the modification of activity or pore blocking ? A: J. Volter Up to now we have not characterized the mixtures with SiOp We assume a transport inhibition by diffusion through the pores of the SO,, which envelopes the zeolite crystals.
Q: T. S. R. Prasada Rao (India) Will addition of A1203 to HZSM-5 change the shape selective properties of HZSM-5 !' A: J. Volter A shape selectivity in the isornerization of mesitylene could not be observed because the equilibrium concentration of 1.2.3-trimethylbenzene is extremely low. However in the
293 isomerization of m-xylene an enrichment of p-xylene has been found in additional experiments. This indicates para-selectivity due to added alumina.
Q: D. Ka116 (Hungary) 1) What was the crystal size of Z S M J '? 2) What was the temperature of contacting or embedding ZSM-5 into alumina; was there an intimate contact between these two constituents '? 3) Did you observe any steric hindrance within the reactive pores during reaction of pseudocumene ? A: J. Volter The crystal size was about 5-7 p. H Z S M J powder was mixed at room temperature with peptsized pseudo boehmite. Formed spheres were calcined at 500 No steric hindrance was observed. The equilibrium concentration of 1.2.3-trimethylbenzene is extremely low.
oc.
Q: V. Haensel (USA) The term promotion of HZSM-5 by dilution with alumina requires clarification. The retardation of the formation of aromatic coke is more indicative of a dilution of the very active HZSM-5 rather than a promotion. The best way of comparing catalyst activities is at equal and preferably low conversions (by design different contact times). Under such conditions the ratio of olefinic coke and aromatic coke from mesitylene can be compared. A: J. Volter We agree that the term promotion of HZSM-5 by alumina requires further clarifying experiments. Nevertheless, in the case of coking of mesitylene there are several distinct hints on an exceptional behavior: (i) coking of mesitylene on A1203 is only slightly slower than coking on HZSM-5 whereas coking of n-hexane occurs much more rapidly on H Z S M J than on A1203 (see Figure 5); (ii) previous experiments (ref. 13 in the pa er) indicated that coke from mesitylene mainly is deposited on the outer surface of the H M-5 crystals whereas coke from n-hexane is deposited within the channels; (iii) the concentration of free radicals, the spin densitylg of coke is represented by the slope of the curves in Figure 6. In the case of n-hexane coking this density is independent of the composition of the mixtures and of the coke content. However in the case of coke from mesitylene the spin density is low with samples containin 50 or more % A1 0 but is considerably higher with samples containing 70 or 100% $M-5. We conch& &at this difference in the densit of free radicals indicates that the binding state of the C atoms in the coke differs between d M - 5 rich samples and A120 rich samples. (iv) a shift in the binding state in the coke can Jirectly be deduced from the linewidth of the EPR signal (ref. 10 in our paper). As to be seen in Figure 7, the coke from n-hexane displays the expected smooth transition from olefinic to aromatic coke with increasing coke content. However coke from mesitylene displays an exception. Comparing the same small coke contents, the coke on the A+OQ rich samples exhibits a broad signal samples the linewidth is distinctly narrower, This indicates a rapid transition to aromatic coke on the zeolite sample, only. To sum up all these findings are consistent with the following model. Coking of nhexane starts in the channels of HZSM-5 with olefinic coke which is then transformed to aromatic coke. Coke from mesitylene also displays a relatively slow transition from olefinic to aromatic coke on A120, rich samples, However on HZSM-5 coke is formed on the outer surface and this enables the very rapid transformation to spacious aromatic coke.
2
Q: S. Csicsery (USA)
1) Were conversions shown for unit H Z S M J ?
294 2) Selectivities in your study were measured at different conversion levels. In consecutive reactions the product mixture could contain a larger absolute amount of the primary product at low conversion than at higher conversion levels. Is it possible that the selectivity effects shown for mesithylene are really effects of the conversion level ?
A: J. Volter The conversions were shown per weight unit of the mixtures. The dependcncc of the selectivity for isomerization on the conversion was checked in additional experiments. Different conversion levels were obtained by applying different amounts of catalysts. Typical results are shown in Table 1: Table 1. Conversion of mesitylene on HZSM-5 and on a HZSM Amount of catalyst
%
+ A1203 mixture
Content of HZSM-5 50%
100%
1.0 g
Conversion Selectivity
76.0 58.0
87.0 35.0
0.5 g
Conversion Selectivity
45.9 73.2
72.5 38.2
The sample containing 50% A1203 displays a selectivity for isomerization of 58% at a conversion of 76%. The selectivity of the pure HZSM-5 at an even lower conversion of 72.5% was only 38.2%. This indicates that the added A1203 causes a strong increase of selectivity whereas the lowered conversion has a comparatively small effect.
Q: R. van Nordstrand (USA) 1) Was your boehmite really crystalline bochmite or pscudo boehmite ? 2) Was any acid used with the boehmite for peptization ? 3) What was the source of the boehmite ):
A: J. Volter We used amorphous pseudo boehmite. It was peptisized with diluted H N O 3 The source of the boehmite was a technical product from the Leuna-Werke AG, Leuna, Germany.
Q: J. Fraissard (France) During the solid-state reaction there is a migrations of A1203 into the pores; which causes the formation of Lewis acid sites which are analogous to the non-framework species due to dealuminations. Since you detect a synergy effect, Bronsted and Lewis acid site should be adjacent. What is the influence of then non-framework species in comparisons with the alumina around the zeolite crystallites?
A: J. Volter 27AI-NMR spectra indicated that the content of framework Al remained unchanged. Up to now we have made no detailed studies concerning the non-framework Al species. An effect of the alumina around the zeolite crystals is to be seen in the coking behavior (Figures 5). Mesitylene is coked on pure HZSM-5 as well as on pure A1203 n-Hexane is coked almost exclusively on HZSM-5, not on A1203. Q: T. Inui (Japan) It is thought that through mixing or some solid acidity is produced by tight mechanochemical contact. Therefore, a more effective promotion would be expected when
295 you use an alumina source which has a larger surface area, such as hydrogels of aluminium hydroxide, or more complete mixing of ZSM-5 with some alumina. What d o you think about these points of view ?
A: J. Volter Many thanks for your kind suggestions. We completely agree and will include your proposals in our further program.
Q: W. 0. Haag (USA) Your data on NH3 TPD show a strong reduction of the Bronsted acid peak occurring at 450-500 OC upon addition of alumina. I am surprised that you d o not see a new peak at 550600 OC resulting from ammonia desorption from the added alumina. Can you tell us how the samples were prepared and pretreated with NH3 prior to the TPD experiment ? A: J. Volter Dr. H. Berndt from our Center for Heterogeneous Catalysis has reinvestigated the desorption of NH3 using a method of stepped thermal desorption. He could identify a small high temperature peak from alumina between 360 and 500 q,which is the same region where the high temperature peaks from HZSM-5 and from the mixtures were observed. Therefore the added boehmitc should increase the acidity, but the opposite was observed. This is a further support for our conclusion that boehmite interacts with HZSM-5 and decreases the acidity of the mixtures. The relatively low temperature of the high temperature peak could be due to specific experimental arrangements. The conditions of preparation and pretreatment were the same as described.
Q: H. G. Karge (Germany) Did you observe a hyperfine splitting of the ESR signal in the case of formation of "nonaromatic (olefinic) coke" which would distinguish it from so-called aromatic coke? I think the density of spins gives information about the number of radicals associated with coke species but does not provide any knowledge about their chemical nature. Did you complement your investigations of the nature of the coke by, e.g., M A S NMR, H/C measurements, chemical analysis?
1v
A: J. Volter We have only studied the coking at 530 OC and this "high temperature coke" gives only a single line spectrum which is in accordance with your findings (ref. 8 in our paper). We agree that the spin density gives no information about the chemical nature of the coke. We only conclude that a distinct strift in the spin density is indicative of a shift in the chemical nature. A more detailed discussion is given in on answer to professor Haensel. Up to now we have no further results concerning the nature of the coke.
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L ef al. (Editors), New Frontiers h Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved G&,
THE EFFECT OF PREPARATION METHOD ON METALSUPPORT INTERACTION IN Pa-ZEOLITE CATALYSTS G. Larsen and G. L. Haller
Department of Chemical Engineering, Yale University, New Haven, CT 06520, USA
Almtract The method of preparation, ion exchange (IE)versus impregnation (IP), effects the final dispersion, acidity a n d sintering stability of L-zeolite supported catalysts. The origin of the dispersion effect appears to be in the shape and size of the PdO precursor formed upon calcination which is more elongated with respect to the zeolite channel and has a lower nearest neighbor oxide coordination number when IP is used. While acidity introduced into the L-zeolite before preparation does not appear to effect the initial dispersion, acidity introduced after reduction of the IE preparation stabilizes the Pd particles toward sintering in H2.
1. INTRODUCTION The discovery that Pt/L-zeolite is a non-acidic reforming catalyst that has high selectivity for the conversion of n-hexane to benzene has stimulated much interest in this catalyst [ 11. There is evidence of both geometric (channelling of reactant) [21 and electronic effects I31 on reactions in Pt/L-zeolite. The latter is most evident In the relative adsorption equilibrium c o n s t a n t s extracted from competitive hydrogenation of benzene and toluene, a sensitive chemical probe of the electronic structure of the metal 141. These results suggest that L-zeolite acts as a base toward Pt particles. The degree of interaction also appears to be a function of the preparation method s u c h that impregnation to incipient wetness results in a higher dispersion than ion exchange [ 5 ] . The density of states a t the Fermi level is about four times greater for Pd t h a n for Pt [61 and correlates with the adsorption equilibrium constants extracted from competitive hydrogenation of benzene and toluene [4]. This suggested that Pd will interact less strongly with Lzeolite channel walls than does Pt b u t might be stabilized by acid sites. The objective of this study was both to confirm this hypothesis and to investigate the effect of preparation method on the dispersion/interaction to ascertain whether the higher dispersion of the impregnation preparation is a property of the L-zeolite more-or-less independent of the metal.
298
2. EXPERIMENTAL 2.1. CATALYST PREPARATION The catalysts were prepared by impregnation to incipient wetness (IP) or ion exchange (IE) using Pd(NH&(N03)2 precursor a n d KL-zeolite (Tosoh Corp.) to produce approdmately 1 wt % Pd. Under an 0 2 flow of 36 l / h / g cat, the temperature was ramped from ambient to 623K in 5 h and held this temperature another hour. The temperature was ramped down to 493K in 0.5 h while purging with He. The catalysts were then reduced at 493K for 1 h under pure H2. A Pd/Si02 catalyst was produced by ion exchange using Davison 923 silica following the method of Baltanas et al. [7]except for the Pd precursor, which in our case was once again Pd(NH3)4(N03)2. This reference catalyst was calcined and reduced exactly in the same fashion as the Pd/L preparations. In an attempt to remove acidity introduced by the reduction of the Pd/KL ion exchange pre aration [ 5 ] , the product was back exchanged by K+ (or Mg2+, Ca2+ or Ba +). For the impregnation reparation, catalysts were also back exchanged with Mg2+, Ca2+ or Ba + following reduction.
i
!f
OF ADSORBED H2 BY OZ The relative stability of the resulting catalyst dispersion was determined by sintering in one atm of H2 at different temperatures and measurement of the change in dispersion by 0 2 titration of adsorbed H2. This experiment is carried out in a conventional Pyrex vacuum system. After reduction the glass cell is sealed off and subsequently outgassed a t room temperature for 10 min in order to remove the beta-hydride phase [81. A 3:4 02:Pds global stoichiometry is derived by assuming that all hydrogen is titrated to H20 and that the a final PdS:O stoichiometry is one. The most significant advantage of this method over the hydrogen chemisorption technique is that it does not require the use of fairly high temperatures necessary for the desorption of surface hydrogen that results after the reduction step. We found this particularly advantageous since our Pd/L catalysts exhibited low sintering resistance. 2.2. TI"I0N
2.3. EXAFSMEASUREMENTS The EXAFS coordination number of Pd was determined in a manner similar to that described for Pt 191. Details of the EXAFS analysis package may also be found in Ref. [lo]. The calcined and reduced precursors were characterized by in-situ EXAFS studies performed at the C2 station of the Cornell High Energy Synchrotron Source (CHESS). The experimental setup and data analysis is described elsewhere 191.
2.4. COMPETITIVE HYDROGENATION OF BENZENE AND TOLUENE The ratio of adsorption constants (toluene/benzene) at 323K were measured by competitive toluene /benzene hydrogenation. Pyrex 0.8 mm ID U-tubes were used as flow reactors for the kinetic measurements at atmospheric pressure. Ultra high purity (uhp) H2 and He gases were further purified using H2O and 0 2 traps from Analabs. Pre-blended
299
mixtures of uhp 2.0% benzene/He and 0.6% toluene/He were purchased from Matheson. Typically, a 0.5 g catalytic bed was reactivated under H2 flow at the reduction temperature for 1 h. The ratio of toluene to benzene adsorption coefficients was kinetically determined a t 3 5 3 K on four selected samples using the method of Tri et al. [ 111. The reactor effluent was analyzed by an on-line 5880 Hewlett-Packard gas chromatograph equipped with a 100 m capillary column coated with methylsilicone.
3. RESULTS The two preparations, ion exchange (IE) and impregnation (IP), and the reference Pd/Si02, were characterized by chemisorption ( 0 2 titration of chemisorbed H2), catalysis (benzene hydrogenation) and physical analysis of particle size (extended X-ray absorption fine structure, EXAFS). Because we were particularly interested in the anchoring of Pd as it is affected by method of preparation and acidity of the support, the degree of sintering in H2 was also investigated.
3.1. EXAFS AND DISPERSION The percent dispersion measured by 0 2 titration of chemisorbed H2 and coordination number from EXAFS analysis of the catalysts reduced a t 493K (in situ in the EXAFS cell) are exhibited in Table 1. Table 1 Percent dispersion by 0 2 titration of chemisorbed H2 and coordination number (CN) from EXAFS analysis of the reduced catalsyts catalysts Percent Dispersion EXAFS CN 7.3d 26 0.77 wt% Pd/K(H)L-IEa 28 Pd/KL-IE,BEb 27 Pd/ MgKL-IE,BE Pd / CaKL-IE,BE 27 27 Pd/ BaKL-IE,BE 5.6e 26 1.08 wt% Pd/KL-IPC 25 Pd/ MgKL-IP,BE 24 Pd/ CaKL-IP,BE 24 Pd/ BaKL-IP,BE 2.36 wt% Pd/Si02 44 4.2 a IE = ion exchange. bBE = back exchanged following reduction. CIP = impregnation. dThe dispersion after calcination a t 623K and reduction a t 493K in situ in the EXAFS cell was 20 percent. eThe dispersion after calcination a t 623K and reduction at 493K in situ was 32 percent.
The EXAFS magnitude functions of the calcined precursors are shown - 3 for Pd/Si02, Pd/K(H)L-IE and Pd/KL-IP, respectively. In each case, these are compared to the magnitude function for bulk PdO.
in Figures 1
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0.014
3
B q
d
0.012 0.010 0.008
0.000 0
4 6 Distance, A
2
8
lo
Figure 1 . The EXAFS of PdO (dashed line) and of the precursor to Pd/SiOz calcined at 623K in situ in the E M S cell (solid line).
0 014
3 0.012
B
d 9
d
1
E
0.010 0.008
0.006 0.004 0.002 0 000 0
I
0
2
,
,
,
I
4
,
,
,
I
,
6
,
,
1
I
8
10
,
Distance, A Figure 2. The EXAFS of PdO (dashed line) and of the precursor to Pd/K(H)L-IEcalcined at 623K in situ in the EXAFS cell (solid line).
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0.020
Y 0.015
0.000 0
2
4
6
8
10
Distance, A Figure 3. The EXAFS of PdO (dashed line) and of the precursor to Pd/KLIP calcined at 623K in situ in the EXAFS cell (solid line).
3.2.CATALYTIC STUDIES The rate of benzene hydrogenation a t 323K is given in Table 2. Also, tabulated In Table 2 are the ratios Ktoluene/Kbenzene = K t / b measured kinetically by the competitive hydrogenation of benzene in the presence of toluene.
3.3. SINTERING STUDIES In Figure 4, the relative dispersion (see Table 1 for Initial dispersion) Is plotted agalnst the temperature at which the catalysts were sintered for one h in 1 atm of H2. Four catalysts, two from each preparation method with acidity of each varied by K+ exchange by Mg+2, were investigated by following the dispersion by H2 titration by 02. In the critlciu temperarure range between 450K and 6 0 0 K , the absolute particle size determined by EXAFS was followed under the same conditions in situ in the EXAFS cell. Unfortunately, the different H2 flow conditions in the cell resulted in somewhat different dispersions (see footnotes to Table 1).
302 Table 2 Benzene/ toluene competitive hydrogenation on Pd Catalysts Kt/b Benzene Catalysts hydrogenation rated 0.77 wt% Pd/K(H)L-IEa 1.7 9.7 Pd/KL-IE,BE” 1.6 6.1 Pd/MgKL-IE,BE 1.6 6.9 Pd / CaKL-IE,BE 1.9 6.8 Pd/BaKL-IE,BE 1.7 8.3 1.08 wt% Pd/KL-IPC 1.o 0.4 1.7 0.6 Pd/ MgKL-IP,BE Pd/CaKL-IP,BE 1.4 0.7 1.3 Pd/ BaKL-IP,BE 1.7 2.36 wt% Pd/SiO2 2.0 4.2 aIE = ion exchange. bBE = back exchanged following reduction. CIP = impregnation. dThe turnover frequency x lO4,at T= 323K, P H =~ 9.21 kPa, Pbz = 0.18 Wa, Ptol = 0.02-0.20 kPa with the site density based on the 0 2 titration of chemisorbed Ha values given in Table 1.
a 1.0 0
.CI
E
& 0.8 b WJ
z
H
4 0.4 B
b 0.2 400
500
600
700
000
Temperature, K
Flgure 4. Relative dispersion as a function of sintering temperature for Pd/K(H)L-IE (01,Pd/MgLIE,BE (HI, and Pd/KL-IP (01,and Pd/MgL-IP (0).
W
400
500
600
Temperature, K
Flgure 5. EXAFS C. N. as a function of sinterlng temperature for Pd/K(H)L-IE (0)and Pd/KL-IP (0).
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4. DISCUSSION Based o n the 0 2 titration of H2 (see Table l), one would have to conclude that there is not a measurable effect of the preparation method on the dispersion. However, there is an apparent effect when the reduction is carried out in situ in the EXAFS cell (where the H2 penetration into the catalyst is mostly by diffusion because the cell behaves more like a stirred tank than a plug flow reactor). As observed for Pt 151, the IP preparation results in a lower average coordination number, 5.6, than does IE, 7.3. It should also be noted that the dispersion measured by 0 2 titration of chemisorbed H2 is in qualitative agreement with the EXAFS, i.e., Pd,/Pdtohl = 0.32 and 0.20, for IP and IE, respectively. However, the dispersions that would be estimated from the EXAFS coordination number, i.e., -1 and 0.9, for IP and IE, respectively, are very much higher than obtained from titration. While either the EXAFS or titration might be in error, we believe it is likely that it is the stoichoimetry of the titration that does not apply and note that If we assume a stoichiometxy of 2PdH t 1 / 2 0 2 ---> 2Pd t H2O (instead of 2Pd0), the EXAFS and titration dispersion estimates would be brought into agreement. One could even argue that the basicity of the L-zeolite would decrease the the 0 2 chemisorption in this direction, but s u c h an argument cannot be sustained for the case of Pd/Si02 where the difference in the EXAFS and titration dispersion estimates is comparable. The effect of preparation method and varied acidity (cation exchange of K + with Mg2+, Ca2+ and B a 2 + ) was examined by t h e effect o n competitive adsorption of toluene and benzene (Kt/b) and benzene hydrogenation rate, see Table 2. The dominant effect is clearly preparation method a n d is seen in the benzene hydrogenation rate. Because hydrogenations are generally agreed not to be particle size dependent and because the apparent dispersion is similar (see Table 11, the order of magnitude greater rates on the IE prepared Pd/L catalysts relative to the IP preparation m u s t be attributed to the method of preparation. We will return to a possible interpretation of t h i s observation below after discussion of the observed differences in the precursors observed by EXAFS. The fact that Kt/b is neither affected greatly by method of preparation or acidity of the support is perhaps easier to understand. Both theoretical [13]and experimental values for P d / S i 0 2 indicate the there is little selectivity between toluene and benzene (Kt/b 1) s u c h that only large perturbations of the electronic structure could effect this. One of the motivations for this work was to see if the acidity of the support affected the dispersion of Pd. That it does not in the preparations shown in Table 1 is perhaps not surprising since the acidity was varied by ion exchange of K+ after the Pd particles had already formed. However, in a parallel set of experiments, K+ was exchanged with NH4+ and the zeolite calcined a t 773K to form R(H)L-zeolite (the degree of exchange was 3, 6 and 15% of the 9 K+ per unit cell) before impregnation with Pd(NH3)4(N03)2. Measurement of the surface area
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304
after decationation indicated little loss in crystallinity (surface area constant at about 300 mz/g), b u t the resulting dispersion was nearly constant also. Examination by EXAFS of the PdO precursor to Pd particles formed by calcination at 623K,see Figures 1-3 and Table 3, show clear differences based on both the kind of support (Si02 vs L-zeolite) and method of preparation (IE vs IP o n L-zeolite). The bulk phase that might be expected if the Pd(NH3)4(N03)2 were completely decomposed would be PdO. On Si02, there is no evidence for PdO particles, but an isolated surface species with about 3.6 oxide anions in the coordination shell of Pd would be consistent with the EXAFS magnitude function shown in Figure 1 where no second shell scattering is observed. The second shell Pd-Pd scattering (through Pd-O-Pd bonding) of both the L-zeolite preparations are qualitatively similar to bulk PdO, but there are distinct differences between the IE and IP preparations. In the case of the IE, the EXAFS magnitude function shown in Figure 2 is consistent with small PdO particles with a nearest neighbor o d d e coordination number of 3.5 (relative to 4 in the bulk) and 2.9 at 3.0881 and 3.8 a t 3.3781 (6.7total) Pd cations in the next-nearest neighbor shell (relative 8 at 3.05A and 4 at 3.43A, 12 total in the bulk). The IP preparation has significantly smaller nearest neighbor CN, 3.0. In the next nearest shell, the distances and total CN, 6.5,is almost the same as for the IE preparation but distribution is quite dmerent. In the IP preparation, the CN of the shorter distance is lower, 1.7, and the CN of the longer distance is greater, 4.8,than in the IE preparation. Qualitatively, it can be said that the PdO like particles are more spherical in the IE preparation and those in the IP preparation are oblong. Table 3. EXAFS analysis of first three coordination shells (subscripted 1, 2 and 3) of precursors calcined at 623K; N is the CN, DWf the Debye Waller term relative to the PdO reference and R the interatomic distance Catalysts
N1 DWf1
xi05
Pd/ Si02 Pd/K(H)L-IE Pd/KL-IP PdO(mode1)
3.6 3.5 3.0 4.0
5 9 1 -
R1
A
N 2 DWf2 x 105
2.00 2.03 2.9 2.02 1.7 2.02 4.0
15 11
-
R2
81
N3 DWf3
3.08 3.8 3.07 4.8 3.05 8.0
R3
x 105
81
12 10
3.37 3.39
-
3.43
There are two other important (and somewhat strange) observations concerning this PdO like structure of the precursor in the IP reparation. Firstly, there is apparent structure out to distances of 101, structure which is not observed in bulk PdO, see Figure 3. Secondly, there is apparently a second scatterer in the nearest shell that is not an oxide ion. This is seen in the magnitude function as peak almost double the size of
the Pd-0 peak of bulk PdO. This must be a heavy 2 element since the isolated and back transformed EXAFS function can be truncated at l d - 1 and very well fitted by only oxide back scattering to obtain t h e coordination number of 3.0 given in Table 3, i.e., the other scatterer has intensity only at wavevectors greater than 12A-1. It is diaacult to imagine what this species might be, but aside from the oxide ions needed to balance the Pd2+ charge, only the oxide ions of the zeolite lattice and K+ ions should reside in the zeolite channel after calcination. Thus, one may postulate a Pd-K interaction, but it is not very satisfyfngsince both the Pd and K should be cations at this stage. The long range structure may be more plausibly suggested to be a one dimensional PdO stretched along the zeolite channel. This picture is also consistent with the shapes of the PdO like particles deduced abovc from the CN. The oblong or stretched PdO particle shape would be likely since the channels would constrain growth in three dimensions and more probable for the IP preparation than the IE preparation since the Pd2+ cations would be anchored to the zeolite framework before calcination in the latter case. While the Kt/b is not a very useful parameter to sense the acidity of the support in the case of Pd, the rate of benzene hydrogenation probably is. We have previously observed that there is a correlation between the increase in the acidity of L-zeolite and an increase in the rate of benzene hydrogenation on Pt/L catalysts [3]. Perhaps more to the point, Chou and Vannice have reported a similar correlation for Pd using C, Si02, Al203, SiO2-Al203 and Ti02 supports [ 141. Chou and Vannice proposed that the benzene adsorbed on acidic sites on the oxide surface near Pd particles in the adlineation region and that this adsorbed benzene then reacts with H2 activated by Pd, i.e., that there is an effective increase in site density. This would not involve any change in electronic properties of the metal particles ( a s implied in our interpretation 131) and this is reasonable since the particles on normal supports are both much larger and not surrounded by the support as in the case of L-zeolite. Whatever might be the correct mechanistic interpretation of the rate-acidity correlation, the empirical observation from Table 2 is that the IE preparation results in greater acidity than IP. This is to be expected 151, but the fact that the back exchanged (BE) catalysts are not substantially less acidic (see Table 2) implies that the exchange is not very effective in removing the acidity developed during reduction of the IE preparation. Divalent cations were effective for removing acidty of Pt/L-zeolite but K+ was not [151. Because both K+ and divalent alkane earth cations are ineffective with regard to the exchange of H+ from Pd/L-zeolite, the protons must be more thermodynamically stable in this case and may be directly associated with the Pd as has been suggested for Pd/Y-zeolite [ 161. While introduction of acidity into L-zeolite did not provide anchoring sites to increase dispersion (see above), increased acidity does appear to inhibit sintering to a degree. This is most obvious in Figure 4 when a sintering temperature around 550K is used. The IE preparations sinter less rapidly than the IP preparations. It is interesting also that the BE catalysts (K+exchanged by Mg2+) sinter slightly less rapidly than the initial Pd/KL catalysts in dependent of preparation method. In the case of the IP catalysts, the BE presumably does not involve any removal of
306 protons, but is the result of the replacement of K+ by Mg2+ in the Lzeolite channels. The effect on sintering may be greater t h a n on the benzene hydrogenation rate (compare Pd/KL-IP to Pd/MgKL-IP,BE in Table 2 and Figure 4).
5. SUMMARY AND CONCLUSIONS The property of higher dispersion resulting from a n IP preparation than from an IE preparation is apparently a property of the L-zeolite and occurs for both Pt [5] and Pd, While the interaction between L-zeolite and either Pt or Pd is qualitatively similar, the high density of states of Pd appears to cause the interaction to be weaker in the Pd case. This is reflected in the stability with respect to sintering in H2. Between 493 and 753K, the temperature of reduction has a small effect on dispersion in Pt/L-zeolite I121 while Pd/L-zeolite is very unstable to even modest thermal treatments in H2. The dispersion decreases to about one fourth of that after the initial 493K reduction following reduction a t 753K. Further evidence that this is the result of the high density of states on Pd is indicated by the fact that the relative loss of dispersion by Pd/K(H)Lzeolite (the protons resulting from the reduction of the ion exchange preparation) is slower than Pd/KL-zeolite (impregnation preparation) a t 553K.
6. ACKNOWLEDGMENTS This research was supported by the DOE Office of Basic Energy Sciences. Partial support from S u n Company is also acknowledged. We wish to thank the Cornell High Energy Synchrotron Source for beam time. We also wish to thank Daniel Resasco for measurement of some surface areas.
7. REFERENCES 1
J. R. Bernard, Proc. 5th Int. Conf. Zeolites, p. 686 Heyden, London,
1980. 2 W. E. Alvarez and D. E. Resasco, Catal. Lett, 8 (1991) 53. 3 G . Larsen and G. L. Haller, Catal. Lett., 3 (1989) 103. 4 T. T. Phuong, J. Massardler and P. Gallezot, J. Catal. 102 (1986) 456. 5 L. M. Kustov, D. Ostgard and W. M. H. Sachtler, Catal. Lett., 9 (1991) 121. 6 C. Kittel, Elementary Solid State Physics, p. 162, New York. J. Wfley &Sons, 1962. 7 M. A. Baltanas and A. L. Bonivardl, J. Catal. 125 (1990) 243. 8 J. E. Benson, H. G. Wang and M. Boudart, J. Catal., 30 (1973) 146. 9 B. J. McHugh, G. Larsen, and G. L. Haller, J. Phys. Chem., 9 4 (1990) 8621.
307 10 B. J. McHugh, Ph. D. Thesis, Yale Unlversity (1991). 1 1 T. M. Tri, J. Massardier, P. Gallezot, and B. Imelik, Metal-Support and Metal-Additive Effects in Catalysis (B. Imelik et al., eds.), p. 1 4 1 . Elsevier, Amsterdam, 1982. 1 2 T. R. Hughes, W. C. Buss, P. W. Tamm and R. L. Jacobson, Proc. 7th Int. Congr. Zeolites, p. 725 Tokyo,1986. 13 C. Minot and P. Gallezot, 123 (1990)341. 14 P. Chou and M. A. Vannice, J. Catal., 107 (1987) 129. 15 G. Larsen and G. L. Haller, Cataysis Today, in press. 16 S. T. Homeyer, Z. Karpinski and W. M. H. Sachtler, J. Catal. 123 (1990) 60. DISCUSSION Q: H. Topsoe (Denmark) In your work you find that the dispersion measured by EXAFS coordination number analysis is higher than that measured by chemisorption. You favor the dispersion measured by EXAFS for your system, this method may give you the best value of the dispersion but I would like to point out that your true dispersion is most likely lover than that measured by your EXAFS analysis. As we have recently shown [l, 21, this is due to the fact that the EXAFS analysis does not take properly into account the anachronic motion of atoms on small particles and resulting contributions to the low k part of the spectrum. To what extent do you think that the lower dispersions may have an effect on your assignment of the location of your metal particles. [l] Hansen et al., Phys. Rev. Letters, 64, 3155 (1990) Clausen el al., J . Catal., submitted [2] A: G. L. Haller Catal. and am aware of this analysis. I I have, of course, read the submission to the .I. suspect that it is generally true that the EXAFS coordination analysis may under estimated the true particle size. Unfortunately, we do not have an independent particle size analysis by, for example, electron microscopy in the case of Pd in L-zeolite. However, in the case of Pt/L-zeolite which we have investigated more extensively, there is both a good linear correlation between EXAFS coordination number determined size and H/Pt estimation of average particle size and this particle size is in reasonable agreement with what we see by electron microscopy [3] for the EXAFS-H/Pt correlation. In the case of Pd/L-zeolite we estimate dispersions of c0.3 by 0, titration of chemisorbed H2 and dispersion -1 from EXAFS. The former would imply that most of the particles were outside the L-zeolite pores; the latter that most of the particles are inside. It is probable that the true dispersion is somewhere in between these extremes and that we have only a portion of the Pd particles in the L-zeolite pores. G. Larsen, G. L. Haller, Cafalysis Today, in press [3]
Q: E. Derouane (Belgium) The low Pd dispersions that you measure by the titration technique can indicate either that the Pd particles are too big to be in the zeolite L cages, in which case it would explain the low Pd/L-zeolite interaction, or that the stoichiometry you use for the titration is not correct, as just suggested by Sachtler. Should we 100 % agree that the second possibility is the correct one ? 3
A: G. L. Haller We are of the opinion that the assumed stoichiometry is in error based on our EXAFS results. However, the EXAFS coordination numbers also are somewhat uncertain (see remark
and reply to the question of H. Topsoe for the reason). Thus, we must agree that the wrong assumed stoichiometry may not be the only reason for the apparent low dispersion measured by titration.
0:W. P. Hettinger, Jr. (USA) There are many ionic forms of palladium. The question is: Did you try other ionic species for ion exchange to determine the variation in results which could occur ?'
A: G. L. Haller This was not a variable that we investigated. The exchange precursor was always Pd(NH3)4(N03)2. Q: W. M. H. Sachtler (USA) We have studied the Pd/L system by various methods including TPR. We found that the H2 consumption for Pd atom was H/Pd = 2 for reduced and reoxidized samples, but for freshly calcined samples it was significantly different from 2. EPR showed a resonance of Pd(II1) ions. We assume that Pd(II1) in the main channels consume 3 H atoms per Pd, but Pd(II1) in cancrinite cages consumes only one H atom per Pd, if the reduction is limited to Pd(II1) 4 Pd(I1). How does the existence of Pd(II1) ions in L-zeolites affect the interpretation of your EXAFS results ?
A: G. L. Haller We have not performed TPR and have no direct evidence for Pd(II1). With respect to our EXAFS results of the precursors to the Pd metal particles, we would have to observe that the 623 K calcincd ion exchanged preparation results in a very good fit to the PdO model both in Pd-0 coordination numbers and distances and in both nearest neighbor and second nearest neighbor spheres. However, the impregnated sample (after 623 K calcination) exhibits a Fourier magnitude at the Pd-0 distance that exceeds that of bulk PdO, see Figure 3. It is clear that this is mostly coming from intensity in the high k portion of the E M S magnitude function. It is possible that this is the result of Pd(II1) in the cancrinite cages. This would predict that the TPR results would be distinctly different for ion exchange and impregnation preparations and that there should be more Pd(II1) in impregnation preparations. Q: M. Ichikawa (Japan) 1) How do you eliminate the diffusion control of reactant molecules inside your Pd/L-zeolite ? 2) How do you envision the location of Pd particles attached to oxygen atoms inside L-zeolite in terms of thc metal-support interaction? How may 0 atoms be involved in the anchoring of Pd particles as deduced from the EXAFS data '?
A: G. L. Haller 1) The TOFs are very low so one would not expect diffusion control for this reason. On the other hand, the low reaction temperature almost assured that the pores are filled with liquid reactant so that some effect of transport is a reasonable expectation for all particles not near the surface of the zeolite particles. All we can say is, assuming that most of the Pd is in the zeolite pores for both preparations, the order of magnitude change in rate suggests that the effect is mostly chemical. 2) We have not attempted to analyze our EXAFS at low k where Pd-0 contribution would occur after the reduction to metal. There is reason to believe that there may a small component of unreduced Pd (see comment of Sachtler) so it would then be necessar to sort out the contributions from anchoring of Pt metal particles and the interaction of Pdrf in the cancrinite cages. In short, we cannot answer your question.
309 Q: D. Ostgard (USA) 1) Since the PdKL-IP catalyst forms oblong PdO like particles and the Pd/KL-IE forms spherical PdO like particles, is it possible that after reduction the Pd/KL-IP catalyst has more encapsulated Pd in comparison to the Pd/KL-IE analog ? Could this higher lever of "benzene inaccessible" Pd contribute to the lower turnover frequency observed for the Pd/KL-IP catalyst'? 2) Why does the back exchange of protons for the Pd/KL-IP catalyst increase the benzene turnover frequency, while the opposite trend is observed for the Pd W- I E catalysts? 3) What is the pH of your back exchange solution? Have your ever considered using a basic solution'?
A: G. L. Haller 1) The different shapes we have deduced are for the oxide precursors of the ion exchange and impregnation reparations. We have no evidence that the Pd metal shapes are titration of chemisorbed H the average particle sizes are not different and, based on the different for the two kinds of preparations (although the E h S dispersion are, but they were reduced under different conditions from those used in benzene hydrogenation). Note that the Pd/SiO2 reference catalysts should be expected to have a remarkabl larger benzene TOF if transport or steric hindrance were the only cause of variation in T Fs but its value lies in between the two kinds of preparations. 2) In the case of the ion exchange preparation, the back exchange is mostly partly between protons and the cation used for back exchange and this reduces acidity. In the case of the impregnation preparation, the back exchange is between K O and doubly charged alkaline earth cations. The latter can cause some hydrolysis of H@. Thus, is both cases the rate is being increased when the acidity is increased. 3) The pH was that of distilled water. Although it may appear reasonable to expect that some protons would be inco rated by the zeolite in this situation, the observed K are consistent with net removal o BrBnsted acidity. It is likely that a basic solution woullkave been more effective as you suggest.
82
Y
Q: C. Apesteguia (Argentina) Benzene hydrogenation is a structure insensitive reaction on most transition metals, the activity per surface atom being independent of the metal particle size or of support influence. Thus, benzeneholuene competitive hydrogenation reaction is preferentially employed for detecting possible electronic changes of the metal surface. Your results show the contrary effect, benzene hydrogenation rate relating better than benzene/toluene hydrogenation to electronic modification of Pd. What is your interpretation of these results ?
A: G. L. Haller It has been demonstrated (see our ref. 4) that the K v is~ a very sensitive function of the metal density of states at the Fermi level to the K decreases Ru > Rh > Pd in the second row of group VIII and 0 s > Ir > PI and that #byb is -1 for Pd on silica support. The exceptionally high density of states of.Pd makes KVb very insensitive to support electronic effects. However, as is true in all of these systems (see ref. 3) the benzene hydrogenation rate is more affected by support effects than Kt/b One normally does not try to use this information because 1) there is no internal reference like we have in K v b it is a kinetic rather than a thermodynamic property so can be compromised by transport as well as chemical effects (see re ly to Ostgard) and it is not possible to give the correlation between support (electronic) e ect and benzene hydrogenation rate a physical interpretation as we believe we can for Kt/b However, in the case of Pd, we have little choice but to fall back the empirical observation that systems where K is discriminating, e.g., PtL, the rate of benzene hydrogenation decreases as the metaYkecomes more electron rich. Thus, the results in Table 2 su est that Pd in Lzeolite prepared by ion exchange is more electron deficient that Pd/S& while the impregnation preparation appears to be more electron rich.
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Gwzi, L. er 01. (Editors), New Fronriers in Curalysis Proceedings of the 10lh International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
ON THE NATURE OF SUPERACTIVE CENTERS IN H-FeZSM-5 ZEOLITES. QUANTUM-CHEMICAL CALCULATIONS M. J. Filatov, A. G. Pelmenschikov and G. M. Zhidomirov Institute of Catalysis, Russian Academy of Sciences, Siberian Division, Lavrentieva Str. 5, 630090 Novosibirsk, Russia
Abstract A model of the super active Fe-containing catalytic centers
detected in the thermally activated H-FeZSM-5 zeolites C11 is proposed. With the help of the NDDO/MC quantum chemical calculations the extraordinary catalytic properties of these centers in the reactions of oxidation by the N 2 0 molecules [ 1 ,31 are explained. The calculations f o r some intermediates of the N20 decomposition reaction have been performed and results are compared with the experimental data [I,21.
1. INTRODUCTION
The results of the experiments [ I - 3 1 point to existence of the unusual Fe-containing catalytic centers in the H-FeZSM-5 zeolites (the iron being introduced at the stage of zeolite synthesis in concentration w I wt. %! of Fe203). In the reaction ( 1 ) N20 t Z
-
N2 + OZ
(1)
these centers, following refereed as 2 centers, are 2 - 3 orders of magnitude more active than the iron atoms of Fe203 surface. A form of oxygen appearing in the reaction ( 1 ) exhibit an anomalously high activity in the reactions of O2 isotopic exchange and oxidation of CO and CH4 molecules [ 1 I. In the present paper a model of such super active centers is proposed. This model is based on the following reasons. According to the data of e.s.r., i.r. and Mossbauer
31 2
spectroscopy [5-101 the Fe atoms in the H-FeZSM-5 Specimens exist in two main chemical forms: a) the lattice atoms in tetrahedral coordination - Fef + ) (signal at g=4.3 in the e.s.r. spectra); b) the atom in the super f y y nuclei of the ferric hydroxide (signals at g=2.0, 2.3, 5.3 and of the type nFe2O3emH20 - Fe 7.9 deynding on the method of preparation). The Fe atoms are inert towards the adsorptioh of CO and H2 molecules at T=77 K and don’t react with these molecules up the temperature T=1073 K [51. In the same conditions, the Fe atoms exhibit chemical activity towards CO and H2 molecules [51. This acJivity is accepted to be due to the low atoms which are formed by dehydroxilation of coordinated Fe the nFe203 mH20 nuclei during thermal activation [5,101. A state of such Fenf atoms i n the sufficiently small clusters could differ appreciably from that in a bulk Fe203. The ferric hydroxide nuclei could be divided into two types depending on their genesis. Firstly, these are the impurities n the e.s.r. which are formed during synthesis and appeared i spectra of as-synthesized specimens as a signal at g=2.0 [6,9,101,Secondly, these are the nuclei which are formed by the Fe atoms leaving a lattice during thermal activation [6,9,101. In the e.s.r. spectra this process results in decreasing of the intensity of the signal at g=4.3 and simultaneous increasing of the intensities of the signals at g=2.0 and g=5.3 [9,101.According to the data of [6Ifand [31 the dependencies of the relative intensity of Fe e.s.r. signals for the as-synthesized specimens and conversion degree of benzene selective oxidation on the Fe content behave similarly. When the iron content being greater than rn 2 wt. % of Fe203, these dependences run into plateau. It is quite naturally to assume that the Z centers are formed by the Fef atoms leaving a lattice during thermal activation of a zeolite The chemical behavior of A 1 and Fe atoms in oxides submits to the rule of structural similarity. It is displayed for example in isodimorphism of the next compounds (T = Fe and Al): K20*11T203,SrO*6T203,lvlgT203, K2T203, a-TOeOH, r-TO*OH, T(0HI3 etc. [ill. Recently a validity of this rule was confirmed also for zeolite lattices of the type TP04 and TZSM-5 [81. This structural similarity is a consequence of the ion radii and proximity of chemical properties
.
*)
f and nf
- abbreviation of the words framework and nonframework respectively.
313
ilectronegativity - of Fe and A 1 atoms and of the same valences of Fe and A 1 atoms in this compounds. In accordance with that, the mechanism of extraction of A 1 atoms from zeolite lattices suggested in [12,131 will be used f o r the case of T=Fe in the present wort. According to this mechanism [I41 extraction of the Fe atoms from zeolite lattice occurs through the formation of super fine OH HO\ / O OH hydroxide clusters of the type HO\ /OF or Hduring the initial stages of zeolite thermal decomposition. Comparison of the catalytic properties of Fe-Al-Si and Fe-Si zeolites [ I 1 shows that active sites in both cases are identical [II. Hence, we set up the hypothesis that the Z centers are formed from the binary ferric hydroxide complex of HC)\ 0 OH ,Fe1.3 ML, the binding energy decreases until a value of 335.60 eV is reached at -20 ML. Pd coverages of W(110) > Mo(ll0) > Re(0001) = Cu > Ru(0001) > Rh(100) > Ni > Pd, decreasing from left to right in the transition series. This general trend is exactly the opposite from that observed for bulk alloys. The origin of these modified electronegativities is not clear; however, these results indicate that charge transfer at the surface cannot be accurately predicted using bulk electronegativities. In addition to the perturbations observed for the electronic properties of model bimetallic systems, the chemical properties of the overlayer have also been found to be perturbed. For example, the desorption temperature of CO from the Pd,,o/Ta(llO) system [15] is found to be 235K below that observed for Pd(100). In Fig. 5 the shift in the CO desorption temperature for Cu, Ni and Pd monolayers is shown relative to the temperature measured for CO desorption from the corresponding (100) metal surfaces along with the shift in the core-level binding energies of the clean overlayers. It is noticed that for these systems there is a good qualitative correlation between the two sets
339 of data. This correlation is explained through the use of the Blyholder bonding model for CO to transition metals [28,29]. In this model we assume that the CO-metal bond is dominated by the donation of electron density from the occupied electronic states of the metal into the unoccupied CO 2n' molecular orbitals ( A backdonation). Recent theoretical [30-321 and inverse photoemmission [33] studies have shown that n backdonation is more important than bonding between the CO 5a molecular orbitals and the unoccupied metal electronic states. In the systems in whic'h there is charge transfer from the overlayer (Pd, Ni) to the substrate, there is an increase in the separation between the occupied valence levels of the overlayer, E* below the vacuum, and the empty 2n' orbitals of CO, E, above the vacuum level. Recent studies have shown that shifts in core-level binding energies are parallelled by similar shifts in the levels measured by ultraviolet photoemmission (UPS) [34,35] and work function measurements [36].
Pd/W(l 10) 4 Pd/Re(0001)-
Ni/W(110)
+0.80 eVI -180 K
Figure 5. Correlation between the shift in surface core-level binding energy and the shift in CO TPD maximum. The properties of the Pd, N i and Cu monolayers are compared with the corresponding values for the (100) face of the pure metals.
Ka
+ 0.80 eV -l,o
+0.35 eV -50K a
K q -
L
Ni/Mo(l10)
+ 0.25 eV
Ni/Ru(0001)
-30
+ 0.05 eV
+50K
+ 0.02 eV
Cu/Re(0001)L
+25 K
Cu/Ru(0001)
-0.13eV + 30 K
Cu/Rh(lOO)
0 Shift in XPS Surface Core Binding Energy Shift in CO TPD maximum
340 According to first order perturbation theory an increase in the E,,-E, separation should lead to a decrease in the CO-metal bond strength. The data for Ni and Pd overlayers indicate that the largest perturbations in the core-level binding energies also show the largest decrease in the CO desorption temperatures. For the Cu overlayer cases in which the substrate donates charge to the Cu, the E,,-E, separation is decreased, resulting in a stronger CO-Cu bond strength. These results indicate that the density of states near the Fermi level of the metal overlayer is, in general an excellent indicator of the ability to chemisorb CO. In Fig. 5 the Ni,,o/Ru(OOO1) and Cu,,,JRe(0001) systems do not conform to the general trends. A reduction in the core-level binding energy is not accompanied by a reduction in the CO desorption temperature. Instead an increase in the CO desorption temperature is observed. These systems have approximately the same occupation levels in the valence band and thus the admetal-substrate interaction can affect the relative position and symmetry of the admetal valence levels in the absence of charge transfer and without significantly altering the core-level binding energies. For systems involving similar valence band occupancies, the CO chemisorption properties cannot be predicted simply from the observed core-level shifts. 5. CONCLUSIONS
1) The electronic and chemical properties of Cu, Ni and Pd monolayers supported on transition metal substrates are significantly different from those found for Pd( loo), Ni(100) and Cu(100). 2) Charge transfer is an important component of the factors that contribute to the ability of the metal overlayers to form a bond with the substrate, thus affecting the metalsubstrate bond strength. 3) Surface electronegativities have been determined and are found to be different from the results observed for bulk electronegativities. 4) The electronic state of the metal overlayer is an excellent indicator of its ability to chemisorb CO. 6. REFERENCES
1 2
3
4 5
6 7
J. H. Sinfelt, Bimetallic Catalysts, Wiley, New York, 1983. J. A. Rodriguez and D. W Goodman, J. Phys. Chem., 95 (1991) 4196; and references therein. J. A. Rodriguez and D. W. Goodman, Surf. Sci. Rept., 14 (1991) 1; and references therein. C. T. Campbell, Ann. Rev. Phys. Chem., 41 (1990) 775; and references therein. R. A. Campbell and D. W. Goodman, Rev. Sci. Instrum., in press. R. A. Campbell and D. W. Goodman, to be published. J. A. Rodriguez, R. A. Campbell, and D. W. Goodman, J. Vac. Sci. Technol. A, in press.
341
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
K. Christman, G. Ertl, and H. Shimuzu, J. Catal., 61 (1980) 397. J. T. Yates, C. H. F. Peden, and D. W. Goodman, J. Catal., 94 (1985) 576. X.Jiang and D. W. Goodman, Surf. Sci., 255 (1991) 1. W. F. Egelhoff, Phys. Rev. B, 29 (1984) 4769. D. E. Eastman, F. J. Himpsel, and J. F. van der Veen., J. Vac. Sci. Technol., 20 (1982) 609. W. F. Egelhoff, Surf. Sci. Rept., 6 (1987) 253. J. A. Rodriguez, R. A. Campbell, and D. W. Goodman, J. Phys. Chem., 95 (1991) 2477. B. E. Koel, R. J. Smith and P. J. Berlowitz, Surf. Sci., 231 (1990) 325. R. A. Campbell, W. K. Kuhn, and D. W. Goodman, to be published. R. A. Campbell, J. A. Rodriguez, and D. W. Goodman, Surf. Sci., 240 (1990) 71. J. A. Rodriguez, R. A. Campbell, and D. W. Goodman, J. Phys. Chem., 95 (1991) 5716, R. A. Campbell, J. A. Rodriguez, and D. W. Goodman, J. Chem. Phys., submitted for publication. P. J. Berlowitz and D. W. Goodman, Langmuir, 4 (1988) 1091. W. Schlenk and E. Bauer, Surf. Sci., 93 (1980) 9. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithica, NY, 1960. R. E. Watson and L. H. Bennett, Phys. Rev. B, 18 (1978) 6439. R. E. Watson, L. J. Swartzendruber, and L. H. Bennett, Phys. Rev. B, 24 (1981) 6211. R. E. Watson and L. H. Bennett, Phys. Rev. Lett., 43 (1979) 1130. R. E. Watson, J. W. Davenport, and M. Weinert, Phys. Rev. B, 35 (1987) 508. R. E. Watson, J. W. Davenport, and M. Weinert, Phys. Rev. B, 36 (1987) 6396. G. Blyholder, J. Phys. Chem., 68 (1964) 2722. G. Blyholder, J. Phys. Chem., 79 (1975) 756. P. S. Bagus, K. Herman, and C. W. Bauschlicher, J. Chem. Phys., 81 (1984) 1966. W. Muller and P. S. Bagus, J. Vac. Sci. Technol. A, 3 (1985) 1623. K. Herman, P. S. Bagus, and C. J. Nelin, Phys, Rev. B, 35 (1987) 9467. G. Rangelov, N. Memmel, E. Bertel, and V. Dose, Surf. Sci., 251 (1991) 965. M. W. Ruckman, V. Murgai, and M. Strongin, Phys. Rev. B, 34 (1986) 6759. G. W. Graham, J. Vac. Sci. Technol. A, 4 (1986) 760. D. R. Baer, C. W. Hubbard, and R. L. Gordon, J. Vac. Sci. Technol. A, in press.
342
DISCUSSION Q: V. Poncc (The Netherlands) Scvcral very good thcorctical papers calculated elcctron density contours for several monolayers, like Fe on W (or other similar combination). These maps do not indicate any charge transfer of importance. Even monolayers of alkali metals (and that is an extreme case) do not show an extended charge transfcr. Why do you insist that you B.E. shifts (XPS data) prove an clectron transfer'! The shifts can be explained alternatively, for example, by other initial state effects, than charge transfer.
A: D. W. Goodman Our theory is based on general [rends observed for Cu, Ni and Pd films supported on several metal substratcs. It explains in a simple and clcar way all the existing experimental data. Reliable quantum-mechanical calculations that deal in a systematic way with the bimetallic surfaces in Figures 3, 4 and 5 have not been published. A detailed explanation for the correlations in Figures 3 and 4 will require the use of a theoretical method that is able to predict cohesive energies and charge distributions at a quantilative level. It is wcll known that shift5 in core-level binding energies can depend not only on chargetransfer processes but also on other phenomena. However, for the bimetallic systems investigated in this study, the magnitude and direction of the core-level shift arc dominated by charge transfer effects. For example, in the case of Pd/Ta(llO) the direction of charge transfer predictcd by XPS agrecs with the results of work function measurements, UPS, CO-FTIR, CO-HREELS and CO-TPD. In a similar way, XPS and all the other techniques indicate that the bond of Ni on W(110) is less ionic than that of Pd [ 11, and that Pd transfers more charge to W(110) than to Ru(0001) [2]. J. A. Rodriguez and D. W. Goodman, Surf: Sci., 257,897 (1992) [I] R. A. Campbell et al., fhys. Rev. B, in press (1992) [2] 0:D. A. King (United Kingdom) I am going to attempt to come between Vladiniir and Wayne. To Vladimir, I would point out that recent intcrprctations of theoretical calculations for Cs on W surfaces at low coverages, suggesting that there is no charge transfer, are incorrect. The charge is transferred to the image plane between the ion and its image, according to the classical model: this happen to Iic between Cs and W atoms, but this does not indicate covalency, as pointed out recently [3]. And to Wayne, I would point out that in the same paper we showed that you cannot interpret core levcl shift data in terms of charge transfer alone: final state (relaxation) and environmcntal contributions can be at least as big, and in some cases these various efforts are additive, but in others they can cancel each other out. Benesh and King, Chem. Phys. L e f f .(1992) [3]
A: D. W. Goodman We cannot rule out that final-state effects contribute to the core-level shifts of the bimetallic surfaces. Ncverthelcss, the correlations in Figures 1, 3, 4 and 5 indicate that the core-lcvel shifts are dominated by initial-state effccts. Particularly interesting are the data for Cu/Rh(100) and CuDa(ll0) in Figure 3a. The change in the direction of the core-level shift cannot bc explained on the basis of final-state effects.
Q: H. Niemantsverdrict and R. van Santen (Thc Netherlands) 1) Binding cncrgy shifts caused by alloying are relatively small, therefore we wonder if strain effects in the overlaycr have an effect as well. If we take a surface layer of Pd on bulk Pd, as referenced, and attribute, for simplicity, the surface core level shift to the narrowed d-band and thc resulting negative chargc on the surface atoms, then stretching of the Pd surface laycr narrows the d-band furthcr and increases the negative charge on the surface atoms. Compression goes the other way. Do you have an idea about the magnitude of this "horizontal" contribution to the binding energy shifts of your metal layers.
343 2) Although we do not disagree that charge transfer is involved, to some extent, we are not sure that it is the only reason for the altered chemisorption of CO. It is well documented that interaction energies of adsorbed molecules with surfaces change with the geometry and the coordination of metal atoms in the surface. Strain effects alter the local geometry of the overlayer atoms from substrate to substrate. It seems to us that your interpretation in terms of changes in electron donation properties is too simple, as structural and electronic effects are inseparable. A: D. W. Goodman 1) According to your hypothesis stretching of the Pd surface layer should increase the negative charge on the surface atoms, moving their core-levels to lower binding energy. The experimental results show a different trend. A pseudomorphic monolayer of Pd on Ta(ll0) has a lower atomic density and a higher Pd 3dq2 binding energy than the surface layer of Pd(ll1) or Pd(100). The "strain effect" also cannot explain the data in Figure 3a. for Cu/Rh(100) and Cuna(l10). The surface atomic densities of Cu/Rh(100) and Cuna(l10) are both much smaller than that of Cu(lOO), but the direction of the core-level shift is different in each case. The "strain effect" plays a role in the properties of a metal overlayer, but in many bimetallic systems, the importance of this effect is secondary when compared to that of charge transfer [ 11. 2) The "strain effect" cannot explain the CO chemisorption properties of Pd and Ni overlayers [4]. For monometallic surfaces, it is well established that the strength of the metalCO bond increases when the atomic density of the surface decreases. Thus, strained overlayers should show an increase in the strength of the metal-CO bond. However, a monolayer of Pd on Ta(ll0) or W(110) has a smaller surface-atomic density and co desorption temperature than those of Pd(ll1). The effects of stretching the Pd atoms are overcome by those of charge transfer from Pd to the Ta or W substrate. The Pd-Pd separation in Pd/Re(0001) is almost identical to that of Pd(lll), nevertheless, the supported Pd monolayer shows a decrease of -100 K in the CO desorption temperature. This change in the chemical properties cannot be attributed to structural differences. R. A. Campbell et al., Surf: Sci., 240, 71 (1992) [2] Q: W. Griinert (Germany) 1) The electron transfer reported by you should be also reflected in the structure of the conductive bands of your systems. Have you performed UPS with your systems ? 2) What is the reason for the intermediate increase of the Pd FWHM when you build up a system with many monolayers of Pd on you system ?
A: D. W. Goodman 1) UPS results indicate that the shifts in the core-levels track the shifts in the valence levels [ 11 2) For systems with medium Pd coverages, the convolution of electron emissions from the metal-metal interface and top layers is responsible for the increase in the FWMH. Q: D. Chadwick (United Kingdom) Many of the metals used in your study have loss features associated with their photoelectron peaks in a metal-metal system with significant metal-metal bonding, one might expect the loss processes associated with on component to be weakly excited in the photoelectron spectrum of the other component. Have you attempted to study the loss features in the photoelectron spectra ? A: D. W. Goodman This is an interesting point and indeed may well be the case; however, we have not carried out these kinds of experiments.
344
Q: J. C. Bertolini (France) You show a nice correlation between the upward shift of Pd core levels (for Pd deposited on metals) and the CO bond strength. But, with respect to particle size effect it is exactly the contrary (i.e. for very small sizes the Pd 3d levels shift upwards while the CO bond strength is increased). Can you comment on that '!
A: D. W. Goodman We are not familiar with the details of the experiments to which you refer for very small Pd particles. The comparison of photoemission results for a solid and a small particle is not a trivial task d u e to changes in the screening of the core-hole. In a solid metal, the electron screening of the core-hole is much more effective than in a small metal particle, and as a consequence the core levels of a solid should appear at a lower binding energy than those of a particle. Thus, it is not surprising that uncoordinated atoms of very small Pd particles have a higher Pd 3d binding energy and a stronger metal-CO bond than the atoms of solid Pd.
0:D. Wang (China) What is the "degree of applicability" of your developed model of metal-metal interaction to the case of supported bimetallics of very small particle size? In particular, please comment with reference to the difference between your models of mixed metals where the individual components are in separate phases and bimetallics in the form of small particles where the individual components are well-mixed [ 5 ] . [S] J. H. Sinfelt, Bimetallic Catalysts, Wiley, New York, (1983) A: D. W. Goodman After comparing our results for bimetallic surfaces with those reported in the literature for bulk three-dimensional alloys, we have found important differences in the direction of the charge transfer between metals [I]. The nature of a heteronuclear metal-metal bond depends strongly on the geometry of the system. Bimetallic systems involve species with similar electron donor-eleclron acceptor properties, and the subtle balance that determines the flow of charge transfer between elements can be easily affected by changes in the coordination number or in the geometrical arrangement of the atoms. In principle, data for bimetallic surfaces or bulk alloys should be extrapolated with caution when predicting the behavior of small particles.
Guczi, L ef al. (Editors),New Frontiers in Carofysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
THE REDUCTION OF NITRIC OXIDE BY AYDROGEN OVER Pt,Rh AND Pt-Rh SINGLE CRYSTAL SURFACES
H. Hiran&, T. Yamadaa, K I. Tanah-&, J. Sierab and B. E. Nieuwenhuysb aThe Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan bGorlaeus Laboratories, Leiden University, P.O.Box 9502, 2300 RA Leiden, The Netherlands
ABSTRACT The reduction of NO by hydrogen has been studied over Pb., - Rb.,5 (1001, (1111, (410), Rh(100) and Pt(100) single crystal surfaces in the 10 mbar range. The surfaces were analysed using AES and LEED. Both the activity expressed as conversion after a constant reaction time and selectivity depend strongly on the surface structure and composition. The activity for the (100) surfaces decreases in the order Pt(100) 2 Pt-Rh(100) > Rh(100). The activity of pure Rh is drastically enhanced by alloying with 25% Pt. The selectivity towards N, for the (100) surfaces decreases in the order Rh(100) > PtRh(100) > Pt(100) a t a temperature of 575K and Rh(100) > Pt(100) > PtRh(100) at 520K . The activity for the alloy surfaces decreases in the order Pt-Rh(100) > Pt-Rh(410) > Pt-Rh(lll1, and the selectivity towards N, formation decreases in the order Pt-Rh(410) > Pt-Rh(100) > Pt-Rh(lll1, a t 520K and 575K.The differences in selectivity and activity can be understood on the basis of the relative concentrations of N, NO and H on the various surfaces.
INTRODUCTION The catalytic reduction of NO by hydrogen is of great importance for the emission control of automotive exhaust gases [ll. Reduction of NO with H, may produce several N-containing products such as N,, NH, and N,O. The formation of NH, and N,O is undesired and catalysts favouring N, as a major reaction product are required. The noble metals Rh and Ru are very efficient for this purpose [l]. Metals like Pt and Pd, on the other hand, favour under certain conditions the reaction pathway which leads to the formation of NH, in large quantities [l]. More information is needed on the factors determining the large differences in the selectivity of the NO-H, reaction over different metals and surfaces structures. Previously, it was
346 found that the selectivity may be determined by the relative concentrations of NO, N and H distributed over the catalyst surface [2,31. Nitrogen adatoms formed on a Pt-Rh(100) [3,41 or a Rh(100) [5J surface are easily hydrogenated a t 400-45OK, resulting in the formation of NH,,. Dinitrogen can be formed in a large temperature range provided that sufficient N atoms are available. Below 600K, the main contribution to the formation of N, over the PtRh(100) surface occurs via the reaction NO,, + N,, + N, + Oeh. In this work, the reduction of NO by hydrogen was studied over the (1111, (100) and (410) surfaces of a Pt,,-Rh,,,, single crystal and the pure Pt(100) and Rh(100) single crystal surfaces. The purpose of this paper is to establish the effects of the surface structure and alloying on the kinetics and selectivity of the NOH, reaction. A comparison between pure Pt(100) and Rh(100) on the one hand, and Pt-Rh(100) on the other, provides the information needed to understand the effect of alloying.
Kinetic results I n fig.1 the formation rates of NH,, N, and N,O over the Pt(100) surface are shown versus reaction time a t two selected temperatures, 520K and 575K. P I I1001
520K
575 K
P t I1001
0
time ( m i d Figure 1 Formation of N,, NH, and N,O over Pt(100) (1x1). The reaction was performed in a batch reactor at a total pressure of 6 mbar with a NO/H, ratio of 1 to 5. For details, concerning the experimental equipment we refer to previous papers [2-41. The NO consumption indicates that the reaction of NO with H, proceeds relatively easy a t both temperatures. Nitric oxide is converted into N, and NH, in comparable amounts a t 520K. On the other hand, more NH, is formed a t 575K. The formation of N,O was below the limit of detection a t both temperatures. The formation rates of N,, NH, and N,O over Rh(100), under identical conditions as described for Pt(100), are shown in fig.2. The conversion of nitric oxide over the Rh(100) surface proceeds much slower than over the Pt(100) surface. The reaction is very slow a t 520K and the detection of products is difficult. Although formed in minor concentrations N, was the only reaction product detected. At 575K, the conversion of nitric oxide has increased drastically and detection of products is easily accomplished. The main product is still N,, although the contribution of N,O is significant. The formation of NH, was below the limit of detection, even at high conversions of nitric oxide. At 575K, total NO conversion is reached after 20 min. of reaction time.
347
The results obtained on Pt-Rh(100) are shown in fig.3. The reduction of nitric oxide by hydrogen proceeds relatively easy a t 520K and 575K. The activity of this surface is only slightly smaller than that of the Pt(100) RhllOOl
520K
R h l 1001
575K
time (min) Figure 2 Formation of N,, NH, and N,O over Rh(100).
PlRhllOOl
PlRh11001
520K
575K
_.
N2
05
0
A
N20
0
NO
0
0 2
time (min) Figure 3 Formation of N,, NH, and N,O over Pt-Rh(100). surface. N, and NH, are the products which are formed in detectable concentrations. A remarkable observation is that that the selectivity towards N, a t 520K is higher over the Pt(100) surface than over the Pt-Rh(100) surface. Although Rh( 100) has the higher selectivity towards N2, alloying Pt(100) with Rh, lowers the selectivity towards N,. These differences in selectivity towards N, are small but reversed a t 575K. The results obtained on Pt-Rh(ll1) are shown in fig.4. The only reaction product formed over this surface is NH,, and the formation rates of N, and N,O are below the limit of detection a t 520K and 575K, even after long periods of reaction time. The conversion of NO is relatively slow compared with the Pt(100) and Pt-Rh(100) surfaces. The reaction rate increases rapidly when the temperature is raised to 575K. Total NO conversion a t this temperature is reached after 5 min. Fig.5 shows the results obtained over the Pt-Rh(410) surface. The reaction products formed are N, and NH,. The NO conversion is slower than over the Pt(100) and Pt-Rh(100) surfaces but
348
higher than over the Pt-Rh(ll1) surface. The selectivity towards N, is relatively high, a higher selectivity towards N, is only observed over the Rh(100) surface.
I
PtRhllll I
520K
1o ,L: r
PtRhl111 I
575K
; O
0.5
0
0
0
0
0
0 0
2
0
6
2
6
L
time (min) Figure 4 Formation of N,,
NH, and N,O over Pt-Rh(ll1).
I
I
PtRhlLlO
I
SZOK
PtRhlLlOI
575K
1 .o Pt-Rh(l11). However, it is well established that a number of free metal sites is needed before NO dissociation occurs [9]. The adsorption of the H, molecule (step 3) proceeds with a relatively low sticking probablity on Pt and Rh surfaces a t 400-500K.The sticking probability of hydrogen depends on the surface structure. On the Pt(100) surface a higher sticking probability is measured than on the Pt(ll1) surface. The dissociation of the H, molecule proceeds very fast on group MI1 metals a t 400-500K [lo]. On the Pt-Rh(100) [3,4] and Rh(100) [51 surfaces it was found that hydrogenation of Na, (step 4) occurs at 400-45OK. NH,, was found as a major surface intermediate when N,, was exposed to hydrogen. The formation of NH, via N,, + 3H,, was found a t a temperature of 450K. The decomposition of NH, over Pt surfaces increases in the order (111)~(100)c(210) [ll]. The NH, surface intermediate may decompose before total hydrogenation has occurred. In fact, it was found that the formation of NH,* on Pt-Rh(100) and Rh(100) in hydrogen is completely reversible and NH,, --f N,, + 0.5 H, occurs when hydrogen is pumped off [41. The formation of H,O (step 5 ) occurs through OH,, intermediates and is a very fast process [lo]. The combination of two N,, adatoms (step 6) occurs in a wide temperature range [3]. This step also strongly depends on the surface structure and the metal being used. On Rh surfaces the metal-N bond is stronger than on the corresponding Pt surfaces. The metal-N bond strength increases in the order (111) c (100). In contrast to the Pt(100) surface where this step occurs almost instantaneously a t the reaction temperatures considered, build-up of Nndatakes place on the Rh(1001, Pt-Rh(100) and PtRh(410) surfaces. The NO,, + N,, reaction can give N2@)+ Opd.or N,O,,, via steps 7 and 8, respectively. On Pt-Rh(100) it was found that step 7 is a main route through which N,,, is formed in the temperature range below 600K 131. At higher temperatures step 6 is the dominant mechanism for N, formation. Little is known about the specific contributions of steps 7 or 8 over Pt and Rh surfaces. Based on the kinetic and spectroscopic results described in this paper a
352
model is proposed for the plane to plane variation of the selectivity and activity of the NO-H, reaction over the Pt,Rh and Pt-Rh alloy surfaces. As shown in fig.6 the selectivity of the NO-H, reaction for N, varies between 0 and 100% after 10% NO conversion. The NO conversion after 3 min. of reaction time as displayed in fig 7 varies between 5 and 90%. A low conversion over Pt-Rh(ll1) is observed in combination with a high selectivity towards NH, (-100%). It is known that dissociation of NO is slow over the Pt-(lll) surface, and that the behaviour of Pt-Rh alloy surfaces towards NO dissociation displays a Pt-like behaviour at high NO coverages. The high normalised N, AES signal intensity on Pt-Rh(ll1) observed by AES is ,therefore, assigned to molecularly adsorbed NO. The Pt-Rh(ll1) surface will be covered mainly by NO under reaction conditions and the coverages of N, and O,,,,,are small. This is also supported by the low normalised O,,,, signal intensity. Only NH, is formed as a product. The NH, molecule may be formed through process 4 as over Pt-Rh(100) or via an [NOHI intermediate. Gorodetskii [121, reported the formation of [NOH] on Pt. However, the [NOH] intermediate was found to be unstable at temperatures higher than 400K. The formation of NH, is favoured over the formation of N,O or N,. No ordered LEED structures were observed on PtRh(lll), indicating that the NO molecules do not form large ordered islands. Thus, for the Pt-Rh(ll1) surface it is assumed that step (2) occurs relatively slow, and the O,,,,and N,,,, react to H,O and NH, through steps ( 5 ) and (41, respectively. N-N combination is unlikely due to the low concentration of N,,,,. The formation of N, and N,O is below the limit of detection, indicating that steps (7) and (8) do not take place at a detectable rate. The conversion of NO over the Pt-Rh(100) surface after 3 min. reaction time at 620K is almost 70% and the selectivity towards N, is 32%. The normalised N, AES signal intensity is 0.038 and for the O,,,, normalised signal intensity a value of 0.055 has been found. By EELS it was found that the N, signal intensity is caused by nitrogen atoms 141. This surface may be covered by a small amount of NO,,,,, and a larger amount of O.,,, and N,,,,.Also H,, which is not detectable by means of AES may be present on the surface. A ~(2x2)surface structure is observed by means of LEED. Both Oh and N,,,, form a ~(2x2)surface structure on the Pt-Rh(100) surface. However, the LEED pattern as observed after the NO-H, reaction resembles the pattern observed for the Pt-Rh 42x2)-N surface structure. Islands of N adatoms may facilitate the formation of N, via step 6. The large O,,,, signal could be partly due to the formation of subsurface oxygen. Thus, for this surface NO dissociation occurs easily and the formation of N, through step 6 has increased. The high activity indicates that free sites remain available during the reaction. These sites probably consists of Pt atoms since the Rh atoms may be covered by N,,,,. Over Pt(100) the highest conversion was observed and the selectivity towards N, is 60% a t 520K. The dissociation of NO over Pt(100) proceeds easily provided that vacancies are available. Since steps 4,5,6, and most likely also step 7 are fast these free sites remain available during the and O,,,, signal course of the reaction. This is supported by the low N,, peak. The intensities. The N,, signal intensity is partly due to the Pt,,, Pt(100) surface exhibits a higher selectivity towards N,, than the Pt-Rh(100) alloy surface a t 520K. Pure Rh, however, has a higher selectivity towards N,
353 than pure Pt. This may be explained by an effect of strongly bound nitrogen atoms on the rhodium atoms surrounding the platinum atoms. At 520K, the nitrogen atoms adsorbed on Rh sites are strongly bound to the surface and the formation of N, is slow. If an NO molecule dissociates on Pt sites, the nitrogen atoms may be surrounded by a matrix of strongly bound nitrogen atoms on Rh sites. The formation of N, will be slow at this temperature and the subsequent hydrogenation of this N-atom on the Pt-site can occur. On Pt(100), the nitrogen atom may recombine with NO, or N on other neighbouring Pt-sites forming N, or N,O and N, respectively. The formation of NH, through Nad. + Ha&, over "isolated' Pt-sites is reduced. This explains the enhanced NH, production over the Pt-Rh(100) alloy surface a t 520K in comparison with pure Pt(100). The high conversion of NO over the PtRh(100) and Pt(100) surfaces is explained by the intrinsic high NO dissociation activity combined with the relatively low M-N bond strength on platinum which prevents that the reaction is inhibited by blocking of active sites by Nab. At 575K3,the selectivity towards N, over the Pt-Rh(100) surface is higher than over the Pt(100) surface. A t this temperature the enhancement of the N, production over Pt-Rh(100) may be ascribed to N-N combination from sites containing few or more Rh atoms. The N, production over the Rh(100) surface is observed a t 575K. The selectivity towards N, and the activity over Pt-Rh(4lO) a t 520K are 100% and 20%, respectively. The value for the selectivity towards N, is 40% at 575K. It is known that the dissociation of NO proceeds relatively easy on this surface and a high concentration of nitrogen is found by means of Na TDS. The nitrogen atoms adsorbed a t the steps of the single crystal surface have an enhanced adsorption energy. The activity will decrease due to the higher number of metal sites which are occupied by strongly bound nitrogen. This is illustrated by the relatively low conversion of NO over the PtRh(410) surface. The value for the normalised N,,,, signal intensity is 0.065, the largest found in this study. These nitrogen atoms are strongly bound to the step sites of the surface, A t these sites an enhanced NH, decomposition may also take place before total hydrogenation to NH, has occurred. The oxygen concentration on or near the surface layer is quite large as indicated by the normalised O ,,, signal intensity of 0.035. The LEED pattern after the reaction points to a (2x1) surface structure. The single crystal surface may facet under the reaction conditions. However, if this would be the case desorption of the adsorbates restores the structure characteristic of the f.c.c. (410) surface. The selectivity towards N, has the highest value for the Rh(100) surface. However, the conversion of NO is low a t temperatures below 600K, due to the high number of strongly bound nitrogen atoms. This is also reflected by the high value of the N, AES signal intensity, 0.045. By EELS it was found that this signal is mainly caused by adsorbed nitrogen atoms. The accumulation of N adatoms during the NO-H, reaction at a total pressure of 2 x 10-7 mbar over a polycrystalline Rh surface was reported by Obuchi et a1 1131. The high selectivity towards N, may be explained by the high number of nitrogen atoms on the surface. A ~(2 x 2 )surface structure was found by means of LEED. The relatively low ,O ,,, AES signal indicates that this structure consists mainly of Nab. Therefore, it is believed that large islands of NsdBare formed during the reaction. This facilitates the
354 formation of N, via step 6. By EELS it was found that NH,, can actually be formed on the Rh(100) surface [51. However, NH, is not formed under the described reaction conditions. The low rate of NH, formation may be caused by a low Hmb concentration in the Ns, islands and the formation of NH, species a t the boundaries of these islands only. Also, a n enhanced decomposition of NH, could take place on Rh. The low activity of the NO/H, reaction at 520K is ascribed to the low number of free active sites available for reaction. Most of the metal atoms are blocked by nitrogen atoms, or NO molecules. At 575K3,N,O formation is observed, this indicates that even at this temperature the number of free sites must be relatively small. The N,O intermediate will decompose into N, and Oadaif a sufficient number of vacancies is available, If these vacancies are absent, N,O will desorb from the surface.
CONCLUSIONS The activity and selectivity of the NO-H, reaction over Pt, Rh and Pt-Rh surfaces depends strongly on the metal and surfaces structure. The activity a t 520K decreases in the order Pt(100) 2 Pt-Rh(100) > Pt-Rh(410) > PtR h ( l l 1 ) > Rh(100). The selectivity towards N, on the other hand decreases in the order Rh(100) > Pt-Rh(410) > Pt(100) > Pt-Rh(ll1). The selectivity towards N, ranges from 100% to 0%. The selectivity is determined by the relative concentrations of NO, N and N adsorbed on the various surfaces. A high selectivity towards N, is observed with relatively strong adsorbed nitrogen atoms on Rh(100) and Pt-Rh(410). However, the high selectivity towards N, is a t the expense of the activity for the reaction.
REFERENCES 1 2
3
4
5 6 7 8 9 10 11 12 13
K.C.Taylor in "Automotive Catalytic Converters (Springer, Berlin, 1984) J.Siera, B.E.Nieuwenhuys, H.Hirano, T.Yamada and K.I.Tanaka, Catal.Lett. 3 (1989) 179 H.Hirano, T.Yamada, K.I.Tanaka, J.Siera, P.Cobden and B.E.Nieuwenhuys, Surf.Sci. 262 (1992) 97 T.Yamada, H.Hirano, K.I.Tanaka, J.Siera and B.E.Nieuwenhuys, SurfSci. 226 (1990) 1 to be published R.F.van Slooten and B.E.Nieuwenhuys, J.Cata1. 122 (1990) 429 and refs. therein R.M.Wolf, J S i e r a , F.C.M.J.M.van Delft and B.E.Nieuwenhuys, Faraday Disc. ChemSoc. 87 (1989) 275 R.I.Mase1, Catal.Rev.Sci.Eng. 28 (1986) 335 R.M.Wolf, J.W.Bakker and B.E.Nieuwenhuys, SurfSci. 246 (1991) 135 B.E.Nieuwenhuys, SurfSci. 126 (1983) 307 D.G.LoMer and L.D.Schmidt, SurfSci. 59 (1976) 195 V.V.Gorodetskii, private communication A.Obuchi, S.Naitom, T.Onishi and K.Tamaru, SurfSci. 130 (1983) 29
355 DISCUSSION
Q: Z. %hay (Hungary) 1)Have you considered the dissociation of NO assisted by adsorbed hydrogen ? 2) If Ha removes only oxygen, at low coverage NO adsorption and dissociation should occur and on hydrogen adsorption (following the NO adsorption) only water and no N2 should be. formed. Is this true ? A: B. Nieuwenhuys and K. Tanaka 1) From a thermodynamic point of view hydrogen could facilitate the dissociation of NO. However, we have not found any direct evidence for such an effect. All our experiments including TDS, XPS and FEM studies show that dissociation of NO starts at temperatures at which some of the adsorbed NO desorbs, creating vacant metal sites required for N O dissociation. In this stage hydrogen can also be adsorbed on the surface and it reacts with oxygen resulting in more vacancies and, hence, in faster dissociation and reaction. The observed reaction temperatures of hydrogen with NO adsorbed at low coverages d o not differ significantly from the NO dissociation temperatures [ 1,2]. Therefore, hydrogen assisted N O dissociation does not seem to play an important role. However, at relatively high hydrogen ressures hydrogen seems to be able to displace some of the more weakly bound adsorbed LO and reaction starts at lower temperature [l]. This effect may also called "hydrogen assisted NO dissociation". 2) Starting with a low NO precoverage, dissociation starts at a much lower temperature than on surfaces initially covered with a monolayer NO. On some Rh surfaces dissociation can already start below room temperature [l NO dissociation and interaction of NO with hydrogen have been studied in detail on a Pt h(100) surface using LEED, AES and EELS [2]. In the presence of NO at T > 350 K the (1x1) surface structure is slowly converted to an oxygen (3x1) surface structure due to NO dissociation and Rh surface segregation. In the presence of a NO + H2 mixture the surface structure formed is the nitrogen ~ ( 2 x 2 )showing again that hydrogen reacts more easily with oxygen than with nitrogen. R. M. Wolf, J. W. Bakker, B. E. Nieuwenhuys, Surf.Sci.246, 135 (1991) H. Hirano, T. Yamada, K. I. Tanaka, J. Sera, B. E. Nieuwenhuys, Surf.Sci.,222, L804 (1989); ibid 262,97 (1992)
k.
[i]
Q: R. W. Joyner (United Kingdom) Your conclusion that the selectivities are proportional to surface coverage are at first sight not surprising, since presumably you are dealing with Langmuir-Hinshelwood kinetics. But it suggests that the activations energies observed an those for desorption and not reaction. Do you agree and d o you make different observations at higher temperatures ? A: B. Nieuwenhuys and K. Tanaka Yes. The activities and the selectivities shown in the figure have been measured at relatively low temperatures (500-600 K), where for some of the surfaces the reaction rate may be controlled by desorption. An example is the Rh(100) surface. The intrinsic activity of the Rh(100) surface for NO dissociation and reaction is high. However in this low temperature range the activity found for this surface is very low due to nitrogen inhibition. As a result the order in activity and selectivity of the various surfaces examined are dependent on the temperature. An example can be found in Figure 6 showing that at 520 K the Pt-Rh(410) surface is more selective to N2 formation than the Pt-Rh(100) whereas at 575 K the Pt-Rh(100) surface shows a better selectivity.
Q: J. H. Block (Germany) My question concerns the surface composition of the Pt-Rh alloy. Frequently chemisorption processes are combined with surface segregation of one of the alloy compounds. Is such a phenomenon observed ?
356 A: B. Nieuwenhuys and K. Tanaka Yes. Pt-Rh alloy surfaces show a very dynamic behavior in adsorption and reaction. Under reducing conditions Pt surface segregation is observed. Under oxidizing conditions, Rh segregates to the surface. The 0 induced (0 from 02 or NO) Rh surface segregation can eventually lead to the separation of Rh-oxides. Under reducing conditions alloy formation takes again place. Q: R. A. van Santen (The Netherlands) The presentation focuses an the role of nitrogen. However it is well known that 0 has a large negative effect on the rate of NO reduction. What information d o you have on t?l e role of oxygen in the reaction ?
A: B. Nieuwenhuys and K. Tanaka It is right that Rh, Pt and Pt-Rh alloy catalysts are not selective towards the reduction of NO in the presence of oxygen. On the other hand our paper (see Table I and the text) su ests that there is no direct relation between the amount of oxygen (from NO) as found by Al% and the activity/selectivity, while we did find a correlation between activity/selectivity and the N concentration. In our opinion this result is caused by the fact that the oxygen measured by AES is situated beneath the surface (subsurface 0 ) and is not adsorbed on the surface. It should also be kept in mind that hydrogen-rich conditions were used in our experiments (H 0 ratio of 5/1), because it was the purpose of the study to examine the selectivity to N e N and NO formation. Q: D. Chadwick (United Kingdom) Your activities and selectivities are quoted after 3 minutes reaction time. For those cases with high N coverage, can you comment on the products observed from zero to 3 minutes reaction time ?
A: B. Nieuwenhuys and K. Tanaka Due to space limitation we could not show the variation of the rates of formation of the various products with increasing reaction time. For the Pt-Rh(100) surface we have published these data in an earlier paper [3]. For Rh(100), the surface with a high N coverage, the only reaction product observed at 500-600 K from zero to 3 minutes is N p The order in conversion/selectivity for the various surfaces studied does not change going from zero to three minutes. H. Hirano, T. Yamada, K. I. Tanaka, J. Siera, P. Cobden and B. E. Nieuwenhuys, [3] Surf.Sci.,262, 97 (1992) Q: G. B. Fisher (USA) 1) In actual automotive exhaust, among the gases present on the rich side of stoichiometry are NO, C O and Hp. Do ou have any experiments which indicate how the NO-H reaction completes with the NO-!( 0 reaction on the Pt-Rh alloys ? 2)%0u have shown the trade-off between selectivity to N formation and activity for the NO-Hp reaction on several surfaces. Does it seem reasonable %at this is most directly related to an inhibition of the H2 dissociation step in the reaction in the presence of high N atom surface coverage which lead to good N selectivity? I would seem reasonable from one point of view, since in 1988 [4] we found &at high N atom coverage on Rh(ll1) dramatically inhibited dissociative 0 2 adsorption. It would be very interesting to know the rate constant of H2 dissociative chemisorption (and NO dissociation) as a function of N atom coverage on
thrir a'l??B. Fisher, et al., Proc. offhe 91h Inr. Cong. on Cufulysis (Eds.: M. J. Phillips and M. Ternan), The Chemical Institute, Ottawa, p. 1355, (1988)
357
A: B. Nieuwenhuys and K. Tanaka 1) No. We have measured the NO + H2 and NO + CO reactions over Pt-Rh alloys. However, we did not study the NO reduction in the presence of both CO and H NO an reduction with h dro en is a much h t e r reaction than NO reduction with CO. and H2 the CO adsorption will compete with NO and hydrogen atmosphere of C6, adsorption. Hence, we may expect CO inhibition of the reduction of NO by hydrogen at lower temperatures (T c 500 K). 2) The model that you propose: inhibition of hydrogen adsorption/dissociation by a high concentration of N adatoms seems reasonable to us. It is well documented in the literature that for dissociation of CO, NO and 02 several adjacent free metal atoms are required. For h drogen the available information is not that clear. In many papers it has been reported that dissociation requires an ensemble of free metal atoms. However, other papers suggest that hydrogen can also dissociate an isolated Pd or Pt atoms in the surface. We found that en can adsorb on Pt-Rh(100) or Rh(100) covered with an overlayer of N,ds to form Hence, we do not have a direct answer on the question to which extent H2 is inhibited by a high concentration of N . We agree that it would be interesting to know the rate constants of H2 dissociative a&%ption and NO dissociation as a function of N atom covera e. K. I. Tanaka, T. amada, B. E. Nieuwenhuys, Surj Sci., 242,503 (1991) [5]
80
42
#
Q: G. A. Sormorjai (USA) Would it be advantageous to study alloy effects by depositing one monolayer of both alloy components on a third, neutral metal instead of one monolayer of one component on the single crystal of the other metal ? A: B. Nieuwenhuys and K. Tanaka In the studies described in our papers bulk Pt-Rh alloys have been used. In general, three different types of alloy surfaces can be prepared: 1) two dimensional alloys prepared by depositing component A on a substrate of component B: by depositing of both components A and B on an inert substrate: by using a bulk alloy A-B. ch approach has its specific advantages and limitations. In 3) the bulk composition is well-defined and it is easy to study the large effect of the surface structure on adsor tion and catalysis. For Pt-Rh alloys most of the work has been done with 3). Some limited in ormation is available for surfaces prepared by 1) and 2) [6]. These results are in line with those re rted for bulk single crystal surfaces of Pt-Rh. L. D. Schmidt et al. and K. I. Tanaka et a]., to be published
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Guczi, L.d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
SPECTROSCOPIC STUDIES ON THE REACTION PATHWAYS OF METHANOL DISSOCIATION ON Pd CATALYST A. Berkb, J . Raskb and F. Solymosi
Institute of Solid State and Radiochemistry of J6zsef Attila University, P.O.Box 168, 6701 Szeged, Hungary
Abstract The adsorption and dissociation of methanol on Pd(100) and supported Pd catalysts were investigated in the temperature range 90-300 K. By means of photoelectron, infrared and thermal desorption spectroscopies, condensed layer, chemisorbed and dissociated methanol were distinguished. Methanol decomposes through the formation of a methoxy species. No adsorbed CH3 was detected either on Pd(100) o r on supported Pd. In the latter case the transient formation of acetaldehyde was identified at 200 K. Potassium additives significantly increased the extent of dissociation, markedly stabilized the methoxy species formed, and induced a small extent of cleavage of the methanolic C-0 bond. 1. INTRODUCTION
Study of the adsorption and dissociation of methanol on metal surfaces
is considered to provide an important insight into the mechanism of methanol synthesis. Spectroscopic studies on single-crystal surfaces of Pt metals unequivocally showed that the primary step in the dissociation of chemisorbed methanol is cleavage of the 0-H bond, and formation of the methoxy species [1,2]. However, Winograd et al. [31, recently reported that below a monolayer of adsorbed methanol on Pd(ll1) surface the rupture of both 0-H and C - 0 bonds occurs: the primary decomposition products were adsorbed methyl, methoxy and water. An interesting finding of this study was that the methyl group formed on Pd(ll1) exhibited an unusually high thermal stability: it was detected even above 400 K. The work of Winograd et al. initiated several studies on this subject and led to a controversy [4-71. The 0-H bond breaking of methanol dissociation has been questioned by Yates et al. [41. They found no ' oto ic exchan e ilathe desorbing CO and methanol products from coadsorbed lf %H3 OH and %H3 OH. They pointed out that, if > 1% of adsorbed methanol dissociate b C - 0 bond fission, this would have been detected via the production of ?C'B y0 o r an isotopically mixed methanol species. The low thermal stability of CH3 group produced by the dissociation of CH3I on Pd(100) surface was also not in accord with the results of Winograd et al. [ 3 1 . In this case, however, coadsorbed iodine was also present, which could influence the stability of CH3 groups [71. In the present comparative study we examine the reaction pathways of methanol dissociation both on a Pd(100) single-crystal at low pressure and on supported Pd at high pressure. Attention is also paid t o the effect of an alkali additive on this process.
The experiments with the Pd single-crystal were perform?qo in an ultrahlgh vacuum system with a background pressure of 5x10 mbar, produced with an ion-getter and titanium sublimation pumps. The system was equipped wlth a quadrupole mass analyzer, an electrostatic hemlspherlcal analyzer (Leybold-Hereaus LHS-lo), a dlfferentlally pumped W photon (He I, 1 1 ) source for UPS, an A1 Kcr X-ray anode for XPS and an electron gun for AES measurements. All binding energies, Eb, are reported relatlve to the Pd Ferml level which places the Pd 3 d W level at 335.3 eV. The cleanlng procedure, deposltlon of potassium and other expbimental details are described in our prevlous paper [7,81. Infrared spectra were recorded with a Blorad (Dlgllab Div. 1 Fourl-qr transform infrared spectrometer with a wavenumber accuracy of f 2 cm . Pd/SiO2 samples were prepared by lnclpient wetting of slllca (Cabosll) with an aqueous solution of palladium chlorlde wlth or without potassium nitrite. The self supporting wafers were oxidized and reduced at 573 K for 60 min. 3. RESULTS 3.1. Clean Pd(100) Xps studies. The featureless baseline in the C(ls) region for clean Pd
reveals that no carbon impurity is present. The low exposure of the clean Pd(100) surface to methanol at 90 K produced a feature at 286.4 eV In the C(ls) spectrum. Thls binding energy corresponds well to the previously values reported for methanollc C(ls) [ 3 , and references therelnl. The C(ls) peak shape and position remalned constant up to 2.0 L exposure. With further lncrease of the methanol exposure, the peak shifted to higher binding energy and became broader, whlch 1s regarded as an indication that two closely related carbon atoms are present. It 1s very likely that thls change In the C(ls) region of the XPS spectrum results from the formation of a multllayer. Thls 1s supported by the TPD measurements; the first monolayer of adsorbed methanol is completed at about 1.8-2.0 L; above this exposure, formatlon of a multllayer begins. Figure 1A deplcts C(ls) spectra at low methanol exposure as a function of temperature. The C(ls) intensity decays between 170 and 200 K as a result of the desorption of methanol. It 1s an lmportant feature that no splittlng of the C(ls) peak is observed at 90-170 K, although the fwhm value (-2.1 eV) of the peaks for 150 and 170 K suggests the contrlbutlon of two C-contalnlng compounds. Upon heatlng to 200 K, a new weak feature developed at 285.7 eV. Thls could be either adsorbed methoxy or CO, or both, as XPS can not differentiate between them. It is an important observation that thls peak underwent very llttle change up to 400 K before the desorptlon of CO. In subsequent experiments, methanol was adsorbed at 300 K. In thls case we obtained one lntense signal at 285.9 eV in the C(ls) region which lntensifled with gradual lncrease of the methanol exposure. This peak disappeared completely by 500 K. No remaining carbon was detected by sensitive XPS measurements above thls temperature.
.
Ups measurement%. the intensity of
The adsorption of methanol on Pd(100) markedly suppressed the Pd d-band and produced photoemisslon peaks at 5.2,
361
B1ND:fiG E N f R G Y
'
Figure 1. XPS of the C(ls) level ( A ) and He I 1 UPS ( B ) of adsorbed methanol
on clean Pd(100) at different temperatures.
I-----x)O
Figure
200
300.
&
'
SbO - 600 - i / K
-
--
2. TPD spectra following methanol adsorption on clean ( A ) and K-dosed Pd(100) ( B ) .
6.7, 9.2, 11.8 and 16.3 eV , which are characteristic for molecularly adsorbed methanol 111. However, at low methanol exposure we also succeded to identify the primary product of the dissociation of adsorbed methanol: the two-peak structure at 5.0 and 9.1 eV can be attributed to adsorbed methoxy species [1,9-111 (Fig. lB).These signals are assigned to the 2e and the le-5a orbitals, respectively [1,9-111.When the adsorbed layer (at or below the monolayer) was heated, a radical change occurred at 200 K: photoemission signals appeared at 8.5 and 10.7 eV, which were eliminated only above 500 K.
TPD studies. Low exposures of methanol (up to 1.5 L) produced only
a single peak ( B ) at 201 K, which shifted to lower temperature (183 K) on increase of the exposure. This peak became saturated at about 2.0 L.'A new state ( a ) started to develop at this exposure: its saturation could not be attained even at very high exposure (Fig. 2). Its peak temperature (138 K) showed little variance with the exposure. At higher temperature the evolution of H2 ( T p = 365 K) and CO (Tp = 506 K) was observed. Their amounts increased only up to the formation of a methanol monolayer (2.0 L ) , but even at this stage only a fraction of the values corresponding to saturation for adsorbed CO and H was attained. Neither methane nor water evolution was observed.
3.2. Potassium-dosed Pd(100) The adsorption and reaction of CHmH on K-dosed Pd was studied at different K coverages, but only results obtained at monolayer K are reported here. Xps studies. The deposition of potassium on Pd(100) produced a double structure at 296.3 and 293.7 eV in the XPS spectrum due to emission from the K(2p312) and K(2pi12) levels, respectively. Methanol adsorption caused the K(2p) peaks to shift by approximately 0.3 eV toward lower binding energies and to intensify by about 20%. In the C(ls) region one peak appeared at 287.7 eV (Fig. 3A), its shape and intensity remaining practically the same up to 150 K. Above this temperature the position of the peak shifted to 286.9 eV. The next changes occurred above 300 K, when a broad peak was obtained at 285.3 eV, which is composed at least from two features characterized by binding energies of 286.2 and 284.4 eV. This broad peak has been transformed into a peak at 286.2 eV, which vanished around 650 K. UPS studies. Adsorption of CH30H at low exposure produced two weak peaks in the He11 UPS spectrum, at somewhat different energies than for the clean surface, at 4.6 and at 8.5 eV. On increase of the exposure, all the emissions due to adsorbed methanol appeared (Fig. 3B). When the adsorbed layer was heated, however, the two-peak structure at 4.6 and 8.5 eV again became the dominant spectral feature. Both peaks were seen up to about 450 K. Above this temperature, another pair of peaks developed at 8.6 and 11.9 eV and were eliminated above 600 K.
BINDING ENERGY ( e V 1
Figure 3. Effects of potassium on XPS ( A ) and UPS (B) spectra of adsorbed methanol on Pd(100) surface
TPD studies. In the presence of preadsorbed potassium, the amount of reversibly adsorbed methanol increased by a factor of 2-3, and Tp shifted to 242-258 K (Fig. 2).The significantly larger amounts of CO and H2 formed suggest that the surface concentration of irreversibly adsorbed methanol is also enhanced. As a result of the stabilization of CO and H by potassium [ 12,131, both species desorbed at significantly higher temperatures as compared t o the clean surface. In addition, as was observed in other cases [13], potassium promoted the dissolution of hydrogen into Pd. The back-diffusion of dissolved hydrogen and its release proceeded only above 600 K. A new feature is the formation of a small amount of methane above 300 K (not shown in the figure).
3.3. Supported palladium
In this case the adsorption and dissociation of methanol were studied by means of infrared spectroscopy. Spectra following methanol adsorption and evacuation at 203 K are presented in Fig. 4. The absorption bands at can be assigned to the asymmetric and symmetric 2954 and c848 cm stretching vibrations of the methyl gr3up [21. The symmetric deformational motion o(methy1) is observed at1450 cm ._The rocking vibration, methyl), which should give a band at 1170-1120 cm was not detected due to the low transmittance of the Pd/SiOz sample in this frequency range. However, it
',
364
3100 3000 2900 2800 2200
1800
1600
I400
3100 3000 2900 2800 wavenumbers (cm-1)
Figure 4. IR spectra of adsorbed CHU3H on 10% Pd/SiO2 ( A ) and 2,5% K + 10% Pd/SiOZ (B): ( 1 ) 1 Torr CHU3H at 213 K and evacuation at ( 2 ) 213 K, (3) 253 K, ( 4 ) 293 K.
was easlly detected on alurnlna- supported Pd. These spectral features are considered to be indicative of formation of the rnethoxy specles [2,14,15]. When the adsorbed layer was heated, the high-frequency doublet was present up to 273 K, and,the ow frequency band up to 263 K. A peak at 1714 was also observed at 203 K, which is tentatively aldehyde1 [16]. attributed to the CO vlbratlon of acetaldehyde [uCO-< Thls band was also reglstered following acetaldehyde-fdsorptlon on Pd/SlOz at 213 K. Other weak absorbances at 1786 and 1886 crn were identified at 203 K. These underwent significant lntenslflcatlon at hlgher ternperatures,wltha shift to higher frequencles. The positions ?$ the peaks at 293 K were 1838 and 1926 crn-'. A very weak band at PO80 cm developed above 203 K. The absorbances at 2080, 1926 and 1838 cm' are attributed to linear, twofold- and threefold-brldge attachment of CO to Pd [l?]. On Pd-free slllca we observed only the weak absorption bands due to adsorbed methanol at 203 K which disapeared during subsequent heatlng around 263 K. When 2.5% potasslum wasladded to Pd/SiO2, the high-frequency dcyblet appeared at 2954 and 2845 cm , and the low-frequency band at 1450 crn In this case the band charac_4erlstlc for the CO vlbratlon of acetaldehyde has been detected at 1717 cm . In contrast wlth the K-free sample, all these bands exhlblted relatlvely high stablllty; they vanlshed only at and above room temperature. Anqther interestlng feature 1s the appearance of absorption band at 2925 cm whlch became qulte intense above 253 K. It 1s worth rnentlonlng that only v e q weak bands were identifled in the CO stretdhlng reglon of 1800-2100 cm at 200-300 K (Flg. 4B).
.,
ern-'
.
4. DISCUSSION
4.1. Clem Pd(lOO1
Methanol readlly adsorbs on a clean Pd(100) surface, wlth a high sticking probabllity, whlch 1s almost constant up to rnonolayer. By means of TPD measurements, we can dlstlngulsh a chemisorbed (0) and a condensed layer (a), with desorptlon energies of 46.0 and 35.0 kJ/mol respectlvely. It may be assumed that the methanol lnitlally adsorbed on Pd(100) dissociates, the 0-H bond undergolng cleavage to give methoxy and H. However, XPS measurements at 90 K dld not reveal the formation of any dlssociatlon products of methanol. The C(ls) slgnal at 286.4 eV In the XPS spectrum clearly belongs to chemisorbed methanol. Thls 1s supported by the more molecular sensitive UPS spectra, whlch showed the characterlstic photoemission signals of the adsorbed methanol molecule at and above mono layer [ 1,111 . However,ln harmony wlth the results of Chrlstmann and Demuth [ll, the analysis of the magnlfled dlfference (Ups) spectra revealed that the
dissociation of adsorbed methanol also occurs to a small extent and methoxy species is formed (Fig. 1B). A larger fraction of it is hydrogenated to methanol and desorbs with Tp = 183 K. A smaller fraction of adsorbed methoxy further dissociates to CO(a) and H(a).This latter process Is characterized by a shift of the C(ls) binding energy to 285.7 eV, and by the appearance of new signals in the UPS spectrum at 8.5 and 10.7 eV. This is supported by the OKLL Auger spectra of the adsorbed species, which are practically the same as those observed following CO adsorption. (The kinetic energy of the CO feature is almost 2 eV lower than that of the CH3OH feature and its intensity is considerably higher. 1 Further evidence for CO was provided by the elimination temperature of all these peaks in XPS and UPS ( - 450-500 K), which corresponds to the desorption of CO from Pd( 100). It is important that there is no spectroscopic evidence of rupture of the methanolic C-0 bond or formation of the methyl(a1 species on a clean Pd(100) surface. The C(ls) binding energy of methyl(a1 produced by the thermal dissociation of methyl iodide on clean Pd(100) appeared at 284.2 eV, and a single photoemission signal in the UPS at 8.5 eV [7,181. The adsorbed methyl group produced in this way was not stable, as its characteristic signals vanished at or below 250 K, leaving behind a small amount of carbon. The above feature suggests that the behavior of the Pd(100) surface, as regards reactivity towards methanol, differs basically from that of the Pd(ll1) surface [ 3 1 .
4.2. K-promoted PdlOO)
Preadsorbed potassium dramatically altered the adsorption and reactions of methanol on the Pd(100) surface, as evidenced by the changes in the TDS. XPS and UPS spectra. The most important features were as follows: ( i ) preadsorbed potassium only slightly altered the sticking coefficient of methanol, ( i i ) it increased the binding energy of chemisorbed methanol, and (iii) it resulted in an increase in the surface concentration of irreversibly bonded methoxy species and caused its significant stabilization. The basic question is the effect of potassium on the pathway of methanol dissociation. In this case it was more easy to detect the two-peak structure at 4.6 and 8.5 eV in the UPS associated with methoxy species. As potassium at monolayer on Pd(100) exhibits mainly a metallic character [191, we may assume the occurrence of a direct chemical interaction between methanol and metallic potassium even at 90 K, in which the surface species KKH3 is formed: K + CH30H = CH30K + H(a) The photoemission signals attributed to the methoxy species were present in the spectra even after the coadsorbed layer had been heated to 450 K, indicating the high thermal stability of KOCH3 on Pd(100). Although, U-H bond cleavage is the major pathway of methanol dissociation on K-dosed Pd(100), the appearance of a new C(ls) feature at 284.4 eV at 300 K and the desorption of methane suggests, that the formation of CH3 species may also occur in the presence of potassium. Following the dissociation of CH3I on K-promoted Pd(100), the C(ls) binding
energy due to the adsorbed CH3 group was identified at the same kinetic energy [181. Other spectral changes occurred above 450-500 K. A new C(ls) signal appeared in the XPS spectrum at 286.2 eV, and new photoemissions at 8 . 3 and 11.9 in the UPS, which can be attributed to CO(a) formed in dissociation of the methoxy species. This assignment is in harmony with the high stability of this new peak and the high-temperature release of adsorbed CO (Fig. 2B). There is no doubt that potassium also interacted with the CO formed, and increased its binding energy to Pd. The peak temperature for desorption of CO agrees well with the value obtained following CO adsorption on potassium-dosed Pd(100) [121.
4.3. Supported Pd
The features observed on supported Pd are in harmony with those found for the Pd(100) surface. In order to establish whether methanol dissociation involves cleavage of methanolic C-0 bond and formation of the CH3 species, the adsorption of CH3I was examined on the same Pd samples. Methyl group formed in the dissociation on Pd/SiOz is characterized by absorption bands at 2930 (C-H stretching) and 1390 cm-’ (C-H asymmetric bending). Nearly identical features were observed on single-crystal surfaces by means of HREELS [ 2 0 ] . Analysis of the spectra of methanol adsorbed at different temperatures led to the conclusion that the methyl group was not formed to a detectable extent on clean Pd/Si02. In the pres nce of potassium, however, there was a clear absorption band at 2925 cm-’ above 253 K, which may indicate the formation of CH3 or CHx species. This should be confirmed by more detailed measurements. In conclusion, it may be stated that the primary route of dissociation of methanol on both Pd(100) and Pd/SiOZ surfaces is 0-H cleavage. Potassium promotes the dissociation and greatly stabilizes the methoxy specles in the cleavage of the 0-C bond also occurs to a form of K-OCH3. It appears that small extent in the presence of a potassium additive. References 1 K. Christmann and J.E. Demuth, J. Chem. Phys., 76 (1982) 6308, 6318. 2 J . L . Davis and M.A. Barteau, Surf. S c i . , 187 (1987) 387. 3 R.J. Levis, J . Zhicheng and N. Winograd, J. Am. Chem. SOC.,l l l ( 1 9 8 9 ) 4605. 4 X. Guo, L. Hanley and J.T. Yates, Jr., J. Am. Chem. SOC.,111 (1989) 3155. 5 N. Kruse, M. Rebholz, V. Matalin, C.K. Chuah and J.H. Block, Surf. Sci., 238 (1990) L457. 6 M. Rebholz, V. Matalin, R. Prins and N. Kruse, Surf. S c i . , 2511252 (1991) 1117. 7 F. Solymosi and K. R~vBsz,J. Am. Chem. Soc., 113 (3991) 9145. 8 F. Solymosi and K. RevBsz, Surf. S c i . , 259 (1991) 95. 9 J. Paul, L. Wallden and A. RosBn, Surf. S c i . , 146 (1984) 43. 10 B. A. Sexton, A.E. Hughes and N.R. Avery, Surf. Sci., 155 (1985) 366. 11 F. Solymosi, A. Berk6 and T.I. Tarnbczy, J. Chem. Phys., 87 (1987) 6745.
12 A. Berkd and F. Solymosi, J . Chem. Phys., 90 (1989) 2492. 13 F. Solymosi and I . Kovhcs, Surf. S c l . 259 (1991) 95., F. Solymosi and I . Kovhcs, Surf. S c l . 260 (1992) 139. 14 C . Schlld, A. Wokaun and A . Balker, J . Mol. Catal. 63 (1990) 223. 15 J . C . Lavalley, J . Saussey, J . Larnotte, R. Breault, J . P . Hindermann and A. Klennemann, J . Phys. Chem. 94 (1990) 5941. 16 M . A . Henderson, Y. Zhou and J.M. Whlte, J . Am. Chem. Soc., 1 1 1 (1989) 1185. J.L. Davls and M . A . Barteau, J. Am. Chem. Soc., 111 (1989) 1782. 17 A.M. Bradshaw and F. Hoffmann, Surf. Sci. 72 (1978) 513. 18 K. MvBsz and F. SolymoSi, Surf. S c l . In press 19 A. Berkd and F. Solymosi, Surf. S c l . , 187 (1987) 359. 20 Y . Zhou, M . A . Henderson, W . M . Feng and J.M. White, Surf. S c l . , 224 (1989) 376.
DISCUSSION
Q: G. A. Somorjai (USA) 1) It is interesting that you detect methyl adsorbed on Pd(ll1) and methoxy on Pd(100). What is our experimental evidence for this? 2) O/H2 produce methanol over Pd on La203 perhaps because of the presence of different oxidation states of Pd. Have you considered the study of oxygen coadsorption with CHsOH?
e
A: A. Berkd 1) Both UPS and U S measurements show that the ener y positions of the peaks following CH OH adsor tion on Pd(100) are characteristic or rnethoxy s ecies. The positions of 1s signa s for CH30 and CH3 are separated by 1.5-2 e and well distin uishable. CH3 on Pd(ll1) was detected in ref. 3 in the paper. 27 We performed some experiments for coadsorption of oxygen and methanol, too. We concluded that in the presence of oxygen the amount of the irreversible adsorbed methanol increases and the C 1s signal characteristic for (CHaO), is more intensive.
B
P
f:
Q: J. H. Block (German ) Considering the in uence of a potassium overlayer it would be interestin to know the roperties of the layer with lowest work function (0= 0.1). In a aper by G. Chuak et al. rl] it was shown that the CH3OH decomposition on Rh coul be retarded by a positive electrostatic field. This field effect should be highest at lowest work function. [l] G. K. Chuak et al., Journal & Physique, 47, 59 (1986) and J. Card., 119, 342 (1989)
K
a
d
A: A. Berkd We found both on Pd(100) (present study) and on Rh(ll1) [2]that the stabilizing effect of potassium at OK 0.10, when potassium is almost completely ionized, is much less than at or above monolayer, when potassium is mainly in metallic form. [2] F. Solymosi, A. Berkd and T. I. Tarndczi,J. Chem. Phys, 87,6745 (1987)
-
Q: D. Chadwick (United Kingdom) In the case of the potassium promoted alladium surfaces, the photoelectron peaks of carbon and potassium are close together. arbon has a low cross section compared to
e
potassium and the satellite lines of potassium occur in the C 1s range. In these circumstances It can be difficult to unambiguously identify minority carbon species.
A: A. Berk6 The XPS spectra of Pd(100) covered by monolayer potassium really show some features in the C 1s range which can be certainly ordered to potassium caused satellites. We believe, however, that the photoelectron peaks in the difference spectra entirely due to adsorbed species formed during CH30H adsorption.
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Guczi, L d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th Inlemational Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights resewed
CYCLOTRIMERISATION OF ACETYLENE TO BENZENE OVER SINGLE CRYSTAL PALLADIUM AND GOLD/PALLADIUM SURFACES AND OVER SUPPORTED PALLADIUM CATALYSTS C. J. Baddeley, R. M. Ormerod and R. M.Lambert Department of Chemistry, Unviesity of Cambridge, Lensfield Road, Cambridge, CB2 lEW, United Kingdom
Abstract Kinetic and spectroscopic studies of ethyne cyclisation to benzene over Pd and Pd/Au yield detailed information about the reaction mechanism. The existence of a critical ethyne coverage threshold for reaction is dramatically demonstrated by experiments using coadsorbed NO which compresses the reactant surface phase without itself participating in the reaction. Results obtained from both single crystal model systems and supported Pd catalysts clearly show that C4H4 is the crucial surface intermediate in the cyclisation reaction. Data obtained with well characterised PdAu single crystal systems reveal the sensitivity of this reaction to the electronic structure, atomic composition and morphology of the metal surface. 1. INTRODUCTION
In favourable cases [ 1,21, electron spectroscopic measurements combined with kinetic studies, carried out on model single crystal surfaces can yield fundamental and unambiguous information about catalytic reaction mechanisms and the role of promoters and poisons in catalysis. One such example is the low temperature cyclotrimerisation of acetylene to benzene on P d ( l l l ) , an efficient and unusual reaction, reported almost simultaneously by Tysoe et a1 [31 and by Sesselmann et a1 [41. A remarkable feature of the reaction is the evolution of benzene at very low temperatures (-200 K). Another striking observation is that the benzene is evolved in two very different temperature regimes; clearly resolved desorption maxima occur at -230 K and -530 K. The mechanism of this interesting reaction has been studied by a variety of methods [5-81. Deuterium isotope labelling was used to establish that the molecular pathway from acetylene to benzene proceeds via a n associative mechanism, involving no cleavage of C-C or C-H bonds [61. An earlier molecular beam study suggested that the reaction proceeds via a C4 intermediate El. It was also demonstrated that dissociative chemisorption of cis-3,4-dichlorocyclobutene(C4H4C12 : DCB) could be used to seed the surface with a C4H4 species [7]. This C4H4 species reacted with coadsorbed C2D2 to form benzene (CeH4D2) with identical kinetics to those exhibited by the overall acetylene trimerisation reaction (CsD6). CsH4D2 (from C4H4 and C2D2) and C6D6 (from 3C2D2), thus strongly suggesting that the molecular formula of the C4 intermediate in acetylene cyclisation is indeed
372 C4H4. Further experiments confirmed that benzene desorption is the rate determining step for the evolution of benzene into the gas phase 161. The chemical identity and chemisorption geometry of the C4H4 species has been investigated using HREELS, NEXAFS and ARUPS 18-lo,], which together indicate that the C4 species is a tilted metallocycle. In this paper we present recent work aimed at further elucidating the mechanistic chemistry of acetylene cyclotrimerisation, addressing in particular the mode of low temperature benzene formation. Both single crystal model systems and practical catalysts have been used. We have investigated the reactive properties of thin Pd overlayers on an Au(ll1) substrate and those of AdPd surface alloy phases derived from such overlayer systems, with respect to their activity towards benzene formation, in an attempt to establish the role of "geometric" and "electronic" effects on the catalytic chemistry. As this is a synthesis reaction which occurs both under UHV conditions and at atmospheric pressure, it represents an excellent system for the study of promoter activity and catalyst poisoning and the role of electronic and structural effects in catalysis.
2.ExpERIMENTAL The single crystal work was carried out in two different UHV chambers, described previously [NO, ADESI. The Pd(ll1) and Au(ll1) specimens were cleaned by standard methods desribed previously [11,121. Palladium was deposited using a resistively heated collimated source which was outgassed for prolonged periods before use. Research grade acetylene purified by bulb-tobulb distillation was admitted t o the sample using calibrated quartz capillary arrays. Catalysts containing between 2 and 10 % wlw Pd dispersed on silica, alumina, titania and charcoal supports were used in a single-pass quartz tube microreactor with mass spectroscopic detection. Following pretreatment in flowing oxygen at 473 K, samples were reduced in hydrogen at 470 K or 770 K, before purging in helium at 770 K and cooling in helium to the reaction temperature. Palladium surface areas were determined by CO chemisorption followed by temperature programmed desorption.
3.RESUL'IS AND DISCUSSION A characteristic feature of acetylene cyclotrimerisation on Pd(ll1) is that there is a distinct acetylene coverage threshold below which benzene formation does not occur; instead only self-hydrogenation, fragmentation and decomposition of the reactant occur. This coverage threshold coincides with the point at which the (d3xd3)R3OoLEED structure of the acetylene overlayer reaches its maximum degree of perfection [51. Deuterium labelling shows that although benzene formation does not occur until the 43 structure is complete the reactively formed benzene molecules do contain acetylene molecules that were initially chemisorbed into the 43 sites [61, i.e. it is not a second more weakly bound acetylene species that is responsible for benzene formation. These results can be explained by the requirement of sufllciently close approach of three chemisorbed acetylene molecules for reaction to occur: a
373 high molecular packing density in the reacting adsorbed layer then leads to the formation of weakly bound tilted benzene molecules, which undergo facile desorption at -200 K giving rise to the low temperature desorption peak. As the surface becomes depleted, the remaining benzene molecules can adopt a flatlying strongly bound adsorption geometry, and do not desorb from the surface until the temperature exceeds 500 K. This hypothesis is very strongly supported by the results of experiments involving two-dimensional compression of the acetylene overlayer on Pd(l11) caused by the addition of a chemically inert 'spectator' molecule, nitric oxide. Using LEED and TDS we have shown that the coadsorption of nitric oxide and acetylene on Pd(ll1) leads to islanding behaviour; acetylene adsorption leads to compression of the coadsorbed NO into separate domains. We have exploited this to test the compressiodtilting hypothesis referred to above. BENZENE DESORPTION
BENZENE
C2H2 dose
_____
clean surface
I
-.J
L L 4 . 2 2L
-4.45
L-.------
'
--.'\---.---.15
-
-.-------o.6
0.6 L
L--0.65
-'
2
-----
clean surface -
------4.12
L L
200 300 400500 600 700
TEMPEAATUREIK
Figure 1. Benzene formation induced by coadsorbed NO. Surface predosed with 0.7 L NO a t 300 K followed by varying doses of C2H2 at 170 K. Top spectrum shows 0.6 L C2H2 on clean Pd(ll1) at 170 K.
Figure 2. Benzene formation induced by coadsorbed NO. Varying precoverages of C2H2 dosed at 140 K followed by 1.0 L
NO.
In the case of the acetylene component the effects of compression are most dramatically illustrated by the very large change observed in the threshold for benzene formation. When the NO pretreated surface is cooled to 170 K and exposed to acetylene, the onset of benzene formation occurs at much lower acetylene doses than on the clean Pd(ll1) surface (fig.1). It can be seen that very substantial benzene production occurs in the acetylene exposure range
374 0.15-0.60 L, whereas on the clean Pd(ll1) surface benzene is only detected for acetylene exposures 2 0.6 L. The known coverage dependence of the acetylene sticking probability [5] can be used to express these exposures as actual acetylene coverages: for the clean surface the threshold for benzene formation occurs at 8 = 0.33, whereas in the presence of preadsorbed NO it occurs at an apparent coverage of 8 I0.08. Thus, the presence of preadsorbed NO results in phase separation and compression leading to local densities of acetylene which are a t least a factor of four greater than they would be on the clean Pd(ll1) surface. If the order of adsorption is reversed, the same phenomenon is obsewed, as one might expect if separate adsorbate islands are being formed. However, in this case the promoting effect of NO is even more dramatic (fig. 2), benzene formation is observed for acetylene exposures as low as 0.06 L, ten times lower than for the clean Pd(ll1) surface; although the total number of chemisorbed acetylene molecules is very small, they are strongly compressed into islands in which the local density Figure 3. Comparison of benzene exceeds the threshold for benzene yield as a function of acetylene formation and highly selective precoverage; (A) after 1.0 L NO exposure and (B) on clean Pd(ll1). cyclotrimerisation of acetylene to benzene occurs for acetylene loadings which would not yield any benzene at all in the absence of NO. Figure 3 shows the benzene yield as a function of acetylene precoverage on P d ( l l l ) , A) after 1.0 L NO exposure and B) on clean Pd(ll1). The dramatic reduction in the coverage threshold for benzene formation in the presence of coadsorbed NO is clearly observed. A t sufficiently high NO exposures and moderate acetylene coverages, e.g. 8(C2H2) = 0.16, there is effectively 100% conversion of acetylene to benzene. In the absence of nitric oxide the highest efficiency observed is -30% [131. From Figure 1,i t is also evident that the presence of coadsorbed NO leads to a dramatic change in the kinetics of benzene desorption; all the benzene is evolved in a single low temperature desorption peak at -240 K, in contrast t o the clean surface where reactively formed benzene desorbs in two peaks [5,6]. The present results strongly support the proposed mechanism for this process whereby conditions of surface crowding lead to formation of a tilted, weakly bound benzene species. On the unpromoted surface, benzene desorption and the transformation of unreacted acetylene to vinylidene reduces the surface coverage to such an extent that the benzene molecules can adopt a more strongly bound flat-lying geometry, giving rise to the high temperature benzene desorption peak. However, in the presence of NO, strong compression of the acetylene domains is maintained throughout the reaction. This results
375 in all the benzene being evolved from a high density system and hence only the low temperature peak is observed. The tilted adsorption geometry of benzene a t high surface coverages on Pd(ll1) has been confirmed by NEXAFS, XPS and UPS [14]. In these experiments a dense benzene overlayer was chemisorbed on Pd(ll1) at low temperature. The variation in intensity of the C ( ~ s ) + R *resonance as a function of polarisation vector angle corresponds to a tilt angle of about 33" between the molecular plane and the metal surface, i.e. at high coverages benzene is indeed significantly tilted with respect to the Pd(ll1) surface - in this case the necessary surface pressure is provided by the benzene molecules themselves. ARUPS measurements for benzene adsorbed a t room temperature indicate a parallel adsorption geometry [ 151 in apparent disagreement with the above result. However, our XPS measurements show that there is a -35% reduction in adsorbate coverage when a low temperature chemisorbed layer is warmed to 300 K [10,14Dl, whilst thermal desorption data show that significant benzene desorption occurs below room temperature [61. Thus the results are consistent: at low temperatures and high surface coverages, steric effects result in the adsorption of a weakly bound tilted species, whilst reduced surface coverages lead to a flat-lying geometry [ 151 with a corresponding increase in adsorption enthalpy [5,6]. The presence of a reactive C4H4 intermediate under tricyclisation reaction conditions may be convincingly demonstrated by inducing this species to participate in alternative reaction channels. This has been achieved with both single crystal model catalysts and with high area supported catalysts, as follow8. When a Pd(ll1) surface is precovered with atomic oxygen prior to acetylene adsorption, a small quantity of furan (C4H40) is detected in subsequent desorption, along with benzene and H2 (from acetylene cyclotrimerisation and decomposition, respectively) and the products of total oxidation ((302 + H2O). The formation of furan by partial oxidation of C4H4 provides direct evidence for the presence of a C4H4 species on the metal surface during acetylene cyclotrimerisation. It is most unlikely that this partial oxidation of acetylene to furan would have been predicted on the basis of previous knowledge; coadsorbed oxygen and hydrocarbon generally react t o form H2O and CO2, and acetylene would be expected to be the most susceptible of all hydrocarbons to combustion. However, given the apparently unique ability of Pd(ll1) to generate substantial surface concentrations of the C4H4 metallocycle from C2H2, the scavenging of the C4H4 by coadsorbed oxygen to yield C4H40 is not unexpected. When the C4H4 species is generated on the surface by the dissociative chemisorption of C4H4C12, furan is also formed but in much larger yield, confirming that the C4H4C12 can indeed be used as a reagent for dosing the surface with the relevant C4 intermediate. Furan formation is found to be favoured at lower oxygen coverages (as can be seen from figure 4); at higher oxygen coverages, total oxidation to C02, CO and H20 is favoured, and a lower coverage of C4H4 is attained due to the decreased availability of chemisorption sites. A t low oxygen precoverages the overall selectivity towards furan formation is as high as 80%.
376
oxyg-.1 precoverage
540 K
-m
clliync
_---.
C
0-l
./------.'--'-
%
buta-l,3-diene 200
300 400 500 600 700 TEMPERATURE/K
Figure 4. Partial oxidation of C4H4 intermediate t o furan showing effect of oxygen precoverage.
0
1200 Timels
2400
Figure 5 . Time variation in benzene, buta-1,3-diene and acetylene production on switching gas feed from He to acetylene at 540 K; PdlAl2O3 catalyst.
In order to establish whether the studies performed on well characterised single crystal surfaces under UHV conditions yield information which is relevant to an understanding of the properties of practical high area supported catalysts, we investigated the behaviour of practical supported Pd catalysts under high pressure conditions. For temperatures above 480 K, switching the gas feed from helium to acetylene immediately resulted in a high level of benzene synthesis which fell off with time to a substantially lower steady state value. This behaviour occurred for all the samples employed, including those supported on charcoal and TiO2, and a typical benzene reaction profile for a PdAl2O3 sample is shown in figure 5. In addition to benzene, butadiene and butene are detected as products; both exhibiting the same time profile as benzene. No C3 or C5 products were detected. The fall from the initial high level of benzene production t o its steady state level correlates with the increase in the amount of unreacted acetylene to its maximum level. The synthesis of the C4 species, butadiene and butene, and the complete absence of any C3 or C5 products, provides direct evidencefor the presence of a C4 species on the surface under steady state reaction conditions. We propose that this C4 species is precisely the C4H4 metallocycle identified by single crystal work [7-9,111; butadiene and butene are exactly what one would predict
377 as primary and secondary hydrogenation products, respectively, of this key C4H4 intermediate. These observations demonstrate that in this case there is a direct correspondence between structure and reactivity data obtained with model systems under idealised conditions of low pressure and the behaviour of a practical catalyst under working conditions (>lo12 times higher pressure). BENZENE DESORPTION: UNANNEALEDPd FLMS
--_
BENZENE DESORPTION. ANNEALED 1ML I’d FILMS
Pd coverage / monolayers
-
-5
0 200
400
600
TEMPERATURE /K
Figure 6. Benzene desorption from unannealed Pd films of increasing thickness deposited on Au(ll1) at c 170 K, following a saturation exposure of acetylene (6 U170 K).
200
400
600
TEMPERATURE /K
Figure 7. Benzene desorption following saturation acetylene exposure (6LA70 K).
The available evidence indicates that acetylene cyclotrimerisation is sensitive to both the atomic geometry and the electronic structure of the surface. This may be investigated further by the application of alloy systems. We have demonstrated that gold is totally inert towards acetylene trimerisation and have therefore used Pd/Au(lll) thin films of varying thickness and morphology and Au/Pd surface alloys of different compositions in order t o examine “geometric“ and “electronic” effects in the cyclisation of acetylene to benzene. Figure 6 shows the desorption of reactively formed benzene from unannealed Pd films deposited on Au(ll1) at < 170 K, as a function of Pd overlayer film thickness. The bottom spectrum confirms the inertness of the Au(ll1) surface towards cyclotrimerisation, while the second spectrum demonstrates that a 1 monolayer (ML) unannealed film of Pd is active towards benzene formation, though the yield is quite small and all desorption occurs from the high temperature desorption state, with no low temperature desorption occurring, indicating an appreciable effect due to the underlying
378 gold. 2 ML and 3 ML Pd films give rise to increased benzene formation and a new desorption state appears at -410 K, intermediate in temperature between the two desorption maxima associated with the clean Pd(l1l) surface [5,61.We attribute this feature to the effects of surface roughness of the Pd film arising from the low deposition temperature; this is in line with LEED data which shows the Pd overlayer t o be highly disordered a t this thickness. It can be seen that for 5 ML unannealed Pd films benzene formation is almost entirely suppressed, even though electronic perturbation of the surface Pd atoms due to the underlying Au(ll1) must be negligible. Therefore we conclude that this effect is morphological, reflecting the rough surface produced by low temperature deposition (confirmed by LEED observations which show a complete absence of beams due to the underlying substrate). It thus demonstrates the extreme structure sensitivity of cyclotrimerisation to the presence of (111)-like ensembles of Pd atoms. If the Pd films are annealed prior t o acetylene adsorption, the results are dramatically different. Figure 7 shows our findings for 1 ML annealed Pd films as a function of increasing pre-annealing temperature. Pretreatment at 300 K leads to a very substantial increase in the subsequent benzene yield, and the appearance of the low temperature desorption state. This annealing procedure also transforms the initially diffuse LEED pattern into a sharp (111) pattern characteristic of a well ordered pseudomorphic (111)-oriented Pd film [12]. This behaviour may be understood in terms of increased perfection of the palladium monolayer resulting in Pd(ll1)-like behaviour. Annealing to 400 K leads to essentially all benzene desorption occurring a t low temperature, a process which is complete for films annealed to 475 K, i.e. all the benzene desorbs from the weakly bound tilted state. For Pd overlayers on Au(ll1) the onset of Au and Pd intermixing is detectable between 200 K and 300 K, with significant intermixing occurring by 475 K. These effects may be attributed to the progressive breaking up of Pd ensembles a t the surface resulting from increased interdiffusion of Au and Pd with increasing preannealing temperature. In some respects this is analogous to the NO-induced behaviour described earlier. For temperatures above 525 K metal interdiffusion is rapid such t h a t for films preannealed t o 550 K, benzene formation is almost completely suppressed. 4. CONCLUSIONS
1. Coadsorbed nitric oxide exerts a very large promoting effect on the conversion of acetylene to benzene on Pd(ll1). This effect is attributable to the formation of separate adsorbate domains of high local coverage as a result of strong mutual compression. Under appropriate conditions benzene synthesis occurs a t a n apparent acetylene coverage which is ten times less than the critical coverage threshold observed on clean Pd(ll1); -100% conversion to benzene occurs.
2. Steric crowding leads to reactively formed benzene being generated in a tilted geometry followed by subsequent low temperature evolution into the gas phase. As the surface becomes depleted of reactant and product species, the remaining benzene molecules adopt a more strongly bound, flat-lying geometry, which desorbs in the high temperature peak.
379 3. The presence of a reactive C4H4 intermediate under tricyclisation reaction conditions has been demonstrated with both single crystal model catalysts and high area supported catalysts, using coadsorbed oxygen and hydrogen, respectively. These reagents provide competing reaction channels, leading to the synthesis of furan from preoxygenated Pd (l l 1 ) and butadiene and butene under steady state turnover conditions over supported Pd catalysts. In the former case the overall selectivity towards partial oxidation can be a s high a s -80%, whilst the latter demonstrates there is a direct correspondence between single crystal data obtained under idealised conditions of low pressure and the behaviour of practical catalysts at pressures > 1012 times higher. 4. Pd/Au(lll) films and Pd/Au surface alloys have been used to show the sensitivity of acetylene cyclotrimerisation to electronic structure, morphology and atomic composition. Well ordered monolayer Pd films do catalyse benzene formation. Rough Pd films are less catalytically efficient than smooth ones, and appear to give rise to a new binding site for the reactively formed benzene. Sufficiently high degrees of surface roughness strongly quench benzene formation demonstrating the extreme structure sensitvity of this reaction t o the availability of (111)-like ensembles of Pd atoms. A d P d alloy formation has a major effect on the yield and desorption kinetics of reactively formed benzene. 6. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
14. 15.
R.B. Grant and R.M. Lambert, J. Catalysis, 1985,92,364 R.B. Grant and R.M. Lambert, Langmuir, 1985,1, 29. W.T. Tysoe, G.L. Nyberg and R.M. Lambert, J. Chem. SOC.,C h e m . Commun., 1983,623. W. Sesselman, B. Woratschek, G. Ertl, J. Kuppers, H. Haberland, Surface Science, 1983, 130,245. W.T. Tysoe, G.L. Nyberg and R.M. Lambert, Surface Science, 1983, 136, 128. C.H. Patterson and R.M. Lambert, J. Phys. Chem., 1988,92, 1266. C.H. Patterson and R.M. Lambert, J. Am. Chem. Soc., 1988,110,6871. C.H. Patterson, J.M. Mundenar, P.Y. Timbrell, A.J. Gellman and R.M. Lambert, Surface Science, 1989,208,93. H. Hofmann, F. Zaera, R.M. Ormerod, R.M. Lambert, D.K. Saldin, J.M. Yao, L.P. Wang, D.W. Bennet and W.T. Tysoe, in preparation. R.M. Ormerod, Ph.D. Thesis, Cambridge, 1989. R.M. Ormerod and R.M. Lambert, Catalysis Letters, 1990,6, 121. R.M. Ormerod, C.J. Baddeley and R.M. Lambert, Surface Science, 1991, 259, L709. H. Hofmann, F. Zaera, R.M. Ormerod, R.M. Lambert, D.K. Saldin, J.M. Yao, L.P. Wang, D.W. Bennet and W.T. Tysoe, Surface Science, in press. H. Hoffmann, F. Zaera, R.M. Ormerod, R.M. Lambert, L.P. Wang and W.T. Tysoe, Surface Science, 1990,232,259. F.P. Netzer and J.U. Mack, J. Chem. Phys., 1983,79, 1017.
DISCUSSION Q: M.M. Bhasin (USA) It is a very nice piece of work. I am quite intrigued by the role of NO in acetylene trimerisation to benzene. You mentioned that NO does not react but merely plays a "compression" role to bring acetylene molecules together for benzene formation. If this is so, NO should not be unique and other oxygenated compounds including 0 2 should also work. Have you studied such molecules for assisting in acetylene trimerisation?
A: R. M. Ormerod This is a good question. We have in fact studied the coadsorption of 0 2 and acetylene on Pd(ll1) ion in considerable detail [ l ] and find that the activity of benzene formation and kinetics of benzene desorption are very different from those observed for the NO/acetylcne system. Coadsorbed atomic oxygen generally suppresses benzene formation, with dcsorption being relatively favoured from the high temperature desorption state. These observations, which contrast strongly with those for the NO/C2H2 system, can however be rationalized in terms of our understanding of the reaction pathway. In the case of nitric oxide, C H2 coadsorption results in the formation of separate domains of high local coverage $1, which is facilitated by the essentially continuous compressibility of the unit cell of NO on Pd(ll1). In the case of preadsorbed oxygen, a rigid lattice is adopted, i.e. the unit cell cannot be compressed, and acetylene molecules can adsorb within the ~ ( 2 x 2 )structure formed by 0 atoms [l]. However, at very low oxygen precoverages an effect similar to the "NO effect" is observed, as 0 atoms form islands and the acetylene molecules preferentially chemisorb outside these islands. This mcans that acetylene preferentially adsorbs into a restricted region of the surface, which results in a lowering of the threshold coverage for benzene formation [ 11. In summary, we agree that NO should not be unique in this rcspcct and this supported by our observation of a similar effect in the 02/C2H2 system at low oxygen coverages; thc requirements being that the coadsorbate has a unit cell which can be compressed, and a mutual repulsion exists between it and acetylene such that separate domains of the two species are formed. Thus we predict that such an effect, though not as dramatic, would be observed following coadsorption of CO and acetylene on Pd(ll1). R.M. Ormerod, R. M. Lambert,J. Phys. Chem. (1992) in press R. M. Ormerod, R. M. Lambert, Surface Sci., 225, L20 (1990)
Q: F. S. Stone (United Kingdom) You explain the very interesting result of the effect of NO in facilitating bcnzenc formation from adsorbed acetylene as a two-dimensional compression, NO being described as a chcmically inert spectator molecule. However, NO may in fact bc exerting a chemical effect, ils electron donor adsorption on Pd serving to weaken the n-bonding between the coadsorbed acetylene and the metal, i.e. to decrease the heat of adsorption of acctylcnc. Acetylene molecules thereby become more mobile and able to react more readily. The heat of adsorption of the product benzene may be similarly reduced by co-adsorbed NO, rcndcring its dcsorption more facile. A: R. M. Ormerod We explain the effect of NO in facilitating acetylene cyclotrimcrisaion to benzene as a twodimensional compression, on the basis of a combined LEEDhhermal desorption investigation which we have reported in more detail elsewhere [ 2 ] .Briefly, LEED and TDS indicate that nitric oxide exists in islands of high local coverage, despite having been initially adsorbed to only a low coverage on the Pd(ll1) surface, as a result of the strong mutual cornprcssion between the NO and C H;! molecules. This leads to thc formation of separate domains of adsorbed NO and C H2 o? high local coverage (above the %?' I coverage threshold for bcnzcnc formation) despite &e actual exposure being considerably less than that required to form benzene on the clean Pd(l11) surface. This effect occurs as a consequcnce of the
381 continuous compressibility of the unit cell of NO on Pd(ll1) and the mutual repulsion which exists between the NO and C2Ha molecules, In the above we have considered NO as a chemically inert spectator molecule, though we acknowledge that there may also be an additional effect associated with NO. However, such an effect IS likely to be small since LEED and TDS clearly demonstrate that adsorption occurs into separate domains and therefore a chemical effects only likely to occur at the domain boundaries. Thus, in summary, all our observations can be rationalized in terms of our understanding of this reaction and a two-dimensional compression, without the need to invoke a chemical effect, though the latter cannot be completely excluded as an additional effect. 0:S. Siege1 (USA) Recently, Professor Taube (Stanford University, USA) described $-benzene complexes of certain transition metals. These complexes may serve as useful models for the structure and Properties of your weakly adsorbed benzene. Alkyl substitution lowers the stability of these complex?, a characteristic of mono-alkene complexes in which electron transfer from metal to the TI (anti-bonding) alkene orbital can be important. In contrast, q6-benzene complexes are stabilized b alkyl substitution. Is it reasonable to suppose, therefore, that the co-adsorbed NO changes t e relative stability of the two forms of adsorbed benzene through an electronic effect?
i
A: R. M. Ormerod The work of Professor Taube on q2- and q6-benzene complexes is very interesting and certainly may be of some relevance to the cyclotrimerisation of acetylene to benzene on extended Pd surfaces. However, as in our answer to professor Stone's questionkomment, the crucial factor is the strong mutual compression of the nitric oxide and acetylene into se arate domains of high local coverage, and thus any electronic interaction between the N 8 and acetylene is only likely to occur at the domain boundaries. In our proposed reaction pathway all the benzene is synthesized at low temperature in a weakly bound tilted conformation. On the clean Pd(ll1) surface, desorption of some of the benzene and the transformation of unreacted acetylene to vinylidene increases the amount of free surface to such an extent that the remaining benzene molecules can adopt a more strongly bound flat-lying geometry, which gives rise to the high temperature benzene desorption peak. However, in the presence of nitric oxide, strong compression of the acetylenebenzene domains is maintained throughout the temperature range of the low temperature benzene desorption state. Thus, all the benzene is evolved from a high density system and hence only the low temperature peak is observed.
0:C. J. Cameron (France)
What was the reasoning behind the addition of Ail ta Pd ? Was a combustion carried out after benzene desorption in order to determine whether other higher hydrocarbons such as polyacetylene are on the catalyst surface?
A: R. M.Ormerod Previous work by ourselves and other research groups indicated that the efficiency of acetylene cyclotrimerisation is sensitive to both the atomic geometry and the electronic structure of the metal surface. Bimetallic alloys have been used for a variety of catalytic reactions in an attempt to elucidate effects of electronic structure and surface geometry (ligand and ensemble effects). Having redicted and then demonstrated that gold is totally inert towards acetylene cyclisation, the Ru(1ll)/Pd system was chosen since it enables one to readily investigate the effect of variations in both the thickness and roughness of evaporated Pd films upon the trimerisation reaction, whilst thermal treatment can lead to surface alloy formation providing another way of altering the surface properties in a systematic and controlled manner. We have found that the trimerisation reaction is indeed strongly influenced by such factors and this has increased our understanding of this interesting and unusual reaction. Classical electronic dopants (e.g. K and Cl) were avoided as these
immediately introduce effects associated with site-blocking because the trimerisation reaction is very sensitive to geometric effects since it relies on the formation of an ensemble of three acetylene molecules [ l ] and thus it is difficult to separate electronic and structural effects using such additives. In previous work we have demonstrated that the surface is free of carbon aftcr a desorption sweep to 900 K, [3] and thus no higher hydrocarbon such as polyacetylene remain on the catalyst surface. The reaction can in fact be cycled many times in this manner without a decrease in benzene yield [4]. However, this is not a result of complete conversion to benzene; on the clean Pd(l1 I ) surface the maximum conversion to benzene is about 30 % [3], the remaining -70% acetylene undergoing dehydrogenation, decomposition and subsequent dissolution into the Pd bulk, an effect which is relatively unique to Pd. Subsequent oxygen chemisorption and tenipcrature programmed dcsorption leads to high temperature CO desorption indicating that this carbon can be segregated back to the surface by thermal oxygen treatment. In the case of NO/C,H, coadsorption almost complete conversion to benzene occurs such that none of the acetylene undergoes thermal decomposition, and so both the surface and bulk remain free of carbon. H. Hoffmann, F. Zacra, R. M. Ormerod, R. M. Lambert, M. Yao, D. K. Saldin, L. P. [3] Wang, D. W. Bennett, W. T. Tysoe, Surface Sci., 268, 1 (1992) [4] W. T. Tyso, G. L. Nyberg, R. M. Lambcrt, Surjhce Sci., 135, 128 (1983) Q: V. Ponec (The Netherlands) Do you think that in your Pd on Au systems absolutely no Au atoms penctrated through and into the Pd layer'! Is it really excluded ? Bearing in mind the surface sensitivity of Auger
and XPS methods, I have some doubts. A: R. M. Ormerod l n referring to a monolayer of Pd we do not necessarily imply that there is no Au in the top layer. However, a combination of angle-resolved Auger and XPS, and ion-scattering measurementS suggest that the extent of intermixing is only vcry small at 170 K and only slightly greater at 300 K, significant intermixing not occurring until the temperature exceeds 450 K. This accounts for the bcnzcnc desorption behavior observed in Figure 7 of the paper. One of the reasons why gold was chosen for this study (aside from it being inert for the trimerisation reaction) was the knowledge that intermixing was likely to readily occur to give a range of surface compositions electronically modified with respect to clean Pd(l1 l), without sufferlng from the problems associated with site-blocking electron donating and accepting additives such as potassium and chlorine. Q: M. Ichikawa (Japan) It seems to me, that the reaction of acetylene trimerised to benzene require the same size of metal ensemblc. Basically you suggest that the benxnc formation proceed by a single Pd
atom isolated on the Pd Au alloy system or how do you appreciate thc role of Au in alloying with Pd in terms of electronic effects or ensemble-breaking effects to explain the effect in acetylene trimcrisation reaction !' A: R. M. Ormerod We are not proposing that a singlc Pd atom is capable of catalyzing the cyclotrimerisation of acetylene, though we are suggesting that an Au atom can substitute for a eiven Pd atom, and the ensemble that Pd atom was associated with remains active for bezzene formation, as neither the structural nor electronic properties arc strongly perturbed. In another paper we have used coadsorbcd oxygen to identify the critical ensemble necessary for trimerisation to occur [l]; this being the availability of three three-fold hollow sites around a single Pd atom, one acetylene molecule adsorbing into each of these. In the Pd/Au system, if a single Pd atom is surrounded by 6 Au atoms we would consider this to be too electronically modified for benzene formation to occur. However, if it is surrounded by say 3 Au atoms and 3 Pd atoms then acetylene adsorption may be able to adsorb sufficiently strongly in the three three-fold hollow sites to enable cyclotrimerisation to occur.
Guczi, I^ er al. (Editors), New Frontiers in Carolysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary (B 1993 Elsevier Science Publishers B.V. All rights reserved
SURFACE CHEMISTRY FOR AUTOMOTIVE EMISSIONS CONTROL: INTERACTIONSOF NITRIC OXIDE ON A (111) Pt-Rh ALLOY SURFACE G. B. Fisher, C. L. DiMaggio and D.D. Beck Physical Chemistry Department, General Motors Research and Environmental Staff, Warren, MI 48090-9055, USA
ABSTRACT As part of a study of catalytic materials important for automotive emissions control, the adsorption and dissociation of nitric oxide on a Ptl0Rh,,(lll) crystal has been studied using high resolution electron energy loss spectroscopy (EELS), t e m p e r a t u r e p r o g r a m m e d d e s o r p t i o n ( T P D ) , A u g e r e l e c t r o n spectroscopy (AES), ion scattering spectroscopy (ISS), and low energy electron diffraction (LEED). The composition of the surface region according t o AES and ISS was enriched in P t to be about 30% Pt-70% Rh. Low coverages of NO ( 99.9 96 pure) was dried over molecular sieves (4A) prior to use. A number of zeolites and molecular sieves were used in these studies. Their source and properties are presented in Table 1.
2.2 Methods
The alkylation reaction was carried out either at atmospheric pressure using a glass (10 mm i.d) reactor or in a stainless steel (SS 316; 19 mm i.d) reactor (Catatest, Model BL-2, GComecanique, France) at high pressures. Both the glass and SS reactors were fixed bed down-flow types.
399 The catalyst was always dried at 480" C for 6 h in flowing dry air, prior to the start of the run. The dry catalyst was then cooled to the reaction temperature in a flow of dry N2. Benzene was injected first for a one hour period at the reaction temperature prior to injection of the actual alkylation mixture Olefins + Benzene). The exact reaction arameters used in different experiments wil be presented along with the data in the Lter sections. The analysis of the reactants and products was carried out using a capillary column (HP 1,50 m X 0.5 mm) in a gas chromatograph (FID; HP model 5880 A).
\
3. RESULTS AND DISCUSSION
3.1 Alkylation with l-hexene Table 2 presents the results of the alkylation of benzene over a number of zeolites. Two different types of zeolites have been used in these studies, viz., Zeolites possessin pores with 12 membered ring (12 MR) openings (wide pore zeolites) and those with 1 membered ring (10 MR) openings (medium pore zeolites). The wide pore zeolites used are H-Y H+, H-M, H-L and H-ZSM-12. The medium pore zeolites are EU-1 and ZSM-5. For comparison purposes an amorphous SiO2-Al20 was also used. Zeolite Y possess a three dimensional pore system (0. 4 nm dia.) with cages (1.3 nm dia.). This has the most open pore system among the large-pore (12 MR) zeolites. Zeolite has a three dimensional pore system consisting of two types of pores (0.73 and 0.55 nm dia.). Mordenite, L and ZSM-12 possess non-connecting unidimensional pores with cross sections of 0.67 x 0.7 nm, 0.71 nm and 0.57 x 0.60 nm, respectively. The medium pore zeolite ZSM-5 has a two dimensional pore system with two types of pores along two perpendicular axes. The pore dimensions for the two sets of ores are 0.54 x 0.56 nm and 0.51 x 0.55 nm. EU-1 has a unidimensional pore system (g58 x 0.41 nm) with side pockets of dimensions (0.68 x 0.58 x 0.81 nm). When alkylation was carried out with 1-hexene, the 2 4 and 3 4 isomers were found to be produced (Table 2). Similar results have also been obtained Venuto et al. [15] during their limited study on benzene alkylation with l-hexene over RE-X. The 14 isomer (wherein the phenyl group is attached to the terminal carbon of hexene) was not detected due to the lower stability of the primary carbenium ion compared to the secondary carbenium ions [16]. The 34 isomer is produced by the isomerization of the (2-carbenium ion to the C3-carbenium ion which occurs rapidly even at relatively low temperatures over even weak acid sites. As the stability of (C2) and (C3) ions are nearly the same, one should obtain 2 4 and 3 4 isomers in near equal amounts, i.e., the ratio of the 29hexane to 3+hexane (24/34) should be L 1. However, Table 2 shows that the 2 4 / 3 4 ratio is always > 1 for all the zeolites and even silica-alumina. The reason for this is the competitive nature of the alkylation and olefin-isomerization reactions and the lack of any skeletal isomerization at the conditions of the reaction. Even before the equilibration of the hexene isomers (or carbenium ions) is achieved, alkylation of the reactant l-hexene to the 2 9 isomer takes place. The values of the 2 4 / 3 4 ratios on the large pore zeolites are mostly close to 1. The one exception is ZSM-12 which has a value of 2.66. This could be due to its highly puckered ring system leading to a smaller pore-opening (0.57 x 0.60 nm). It is surprising to note that even in the case of silica-alumina which possesses an average pore-diameter much larger (- 2.7 nm) than those of the zeolites, the 2 4 content is large (24/3+ = 1.91). The reason is probably related to its lower acidity (acid strength) when compared to the zeolites and the consequent slower isomerization of the C2 carbenium ion.
8
3
400
Table 2 Alkvlation of benzene with 1-hexene over different catalvsts C6= utilization (wt. %) Conversion, (wt.%) Zeolite Col.No.: DAl-Y RE-Y H-(3 HWa) H-M (1) H-M (1)' H-M (2) H-K-L ZSM-12 H-EU-1 ZSMJ Si-Al
TOS 1-hexene C6< C6 C6-satu- OthersC C 6 4 (h). olefinsa fraction ratesb 1 2 3 4 5 6 7 4 88.21 17.76 13.91 5.10 63.23 93.55 2.72 62.91 8.01 26.36 15 65.01 27.18 4 92.20 0.31 15.54 3.11 81.04 84.37 39.74 1.21 59.05 17 54.33 20.71 2.10 34.81 3 100.00 100.00 12.56 50.53 0.60 8.72 1.27 89.41 9 100.00 94.68 2 0.30 24.07 2.32 73.31 94.51 55.06 - 49.15 50.85 95.68 10 32.55 4 100.00 10.06 62.80 5.90 21.24 88.35 0.48 57.93 18 96.94 35.58 1.80 39.79 17.06 43.23 18.24 21.47 3 100.00 99.52 18 97.99 82.32 0.60 6.04 8.20 85.16 4 66.36 65.13 0.61 1.15 8.95 89.29 19 61.51 58.01 0.54 5.69 3.50 90.27 19.14 3 0.16 72.99 1.83 25.02 83.44 15.19 19 86.11 0.33 13.56 70.78 4 75.07 0.13 11.17 4.76 83.94 98.39 19 67.66 92.90 0.12 12.28 2.80 84.82 2 32.18 2.46 46.68 4.01 46.85 95.75 14 31.81 96.19 0.94 48.55 3.63 46.68 4 27.35 12.29 48.56 18.20 20.95 94.39 13 3.77 58.05 16.28 21.89 23.34 94.58 4 26.58 94.66 20.13 62.27 5.68 11.81 -8 81.29 10.84 1.11 61.62 12.27 24.35 ~
24/34
Ae
8 9 1.26 1.36 0.9 1.20 0.5 1.26 1.50 2.08 9.7 2.11 2.85 9.3 1.30 3.10 12.9 2.14 4.27 14.2 3.37 4.27 6.0 1.84 2.16 2.0 2.66 2.84 1.2 5.70 7.65 18.3 12.00 8.13 -41.1 1.91 2.39 11.3
~~
Conditions : Temp. (K) =,413; Press. (MPa) = 0.1; WHSV (h-1) = 4.0; Benzene : 1-hexene (mole) ratio = 10; : 433 K also in the case of H-M (1). a: Excludin double bond isomerization to i-Cg=. b: 2-Me anc! 3-Me pentanes and n-hexane. c: Mostly < Cg-alkyl benzenes and heavier material. d: n- and i-products; n 4 > > i4; mostly monoalkyl benzenes. e: A = Change in 24/34 ratio per hour x 100.
401
In the case of the two medium pore zeolites, EU-1 and ZSMJ, the 2 4 / 3 4 ratios are much larger (5.7 and 12.0, respectively) indicative of the shape-selectivity of these zeolites. It is, however, interesting to note that EU-1 which has a much smaller pore opening than ZSMd leads to a lower 24/34 ratio. Apparently, product shapeselectivity (which is determined by the size of the pore opening) is not the only important factor in determining the 24/34 ratios. The presence of large side pockets in EU-1 (Table 1) provides enough space for the bimolecular alkylation reaction, which leads to the formation of the bulkier 3 4 isomer. In ZSMJ, apparently bimolecular reactions are sterically more hindered [17]. Thus, the above results indicate that both product and transition state selectivities may be playing a role in determining the 2 4 / 3 4 ratios in the case of zeolites. The change of the 24/34 ratio during run is presented in column 9, Table 2. In the case of DAl-Y,the change is negligible (0.9 units). The rate of change is highest (12.9) for mordenite, while P possesses an intermediate value of 9.7. The magnitude of the above changes are related to the size and type of the pore systems. The 3-dimensional pore systems (Y and p) produce smaller changes. Mordenite with a unidimensional pore system leads to a larger change in the 24/34 ratio with increasing duration of run. However, the change of the ratio in both L, possessin a larger pore diameter (0.71 nm) and ZSM-12, possessing smaller pore opemngs (0.5 x 0.6 nm) is similar (2.0 and 1.2, respectively). The selectivity for the 2 4 isomer increases in the case of EU-1, while it decreases rapidly in the case of ZSMJ. The decrease in the formation of the bulkier 3 4 isomer over EU-1 is due to probably the blockage (or non-accessibility) of the side-pockets wherein such isomers may be formed preferentially. The decrease in the 2 4 isomer formation over ZSM-5 is probably due to the progressive blockage of the pore-openings and the increased contribution of the external surface to the reaction as the reaction progresses. The initial activity and deactivation rate (decrease in conversion with time) of the different zeolites is found to be a ain different (column 3, Table 2). The large pore zeolites, Y and Mordenite have igher initial activities and also deactivate rapidly. While the rapid deactivation of Y is attributed to its lar er acid site density, deactivation medium pore zeolites, EU-1 of mordenite is due to its unidimensional pore system. and ZSMJ, both possess low initial activities and deactivation rates. Similarly, they also ossess low selectivities for he 1-benzenes (column 7, Table 2). Again, the selectivities !or hexyl-benzene production o not change significantly in the case of the above two zeolites. Zeolite Y and ZSM-12 exhibit good selectivities for hexyl-benzene formation both initially and after a TOS of 15-19 h, while p and H-M (1) exhibit high selectivities only after many hours on stream. The dealuminated mordenite H-M (2), on the other hand, has good selectivities both when TOS = 4 and 19 h. The sha e-selectivity of zeolites is accentuated by the presence of extraneous materials insi e the pores or at the pore-openings. The presence of extraneous materials inside the pores is often detected by the adsorption of hydrocarbons and NMRmethods [ 181. The amorphous content of zeolite samples was quantitatively estimated by the adsorption of Argon using the t-plot method [19]. However, how much of this amorphous material affects the pore openings or the pore-dimensions is not easy to determine. Zeolite+ and its Ga-isomorph (3 (Ga) contain substantial amounts of amorphous materials (Table 1). We also find that their 24/34 ratios are higher than even that of mordenite. Similarly, the dealuminated mordenite, H-M (2), also contains
?
f
he
7
B
402
amorphous materials and has a larger 2 4 / 3 4 ratio than H-M (1). However, eventhough RE-Y is more amorphous than DAl-Y, the 2 4 / 3 4 ratios are similar in the two zeolites due to the larger and more open pores in the FAU-system. Increasin the temperature of the reaction in the case of H-M (1) increases olefin conversion wit out increasing the selectivity for alkyl benzenes formation (Table 2). Similar results were also observed in the case of Y and p.
fl
3.2 Alkylation with l-octene The alkylation of 1-octene was carried out over the two wide ore zeolites Y and mordenite which possess significantly different pore structures. e results are presented in Table 3.
TR
Table 3 Alkylation of benzene with I-octene Cg-ole fin Product distribution, (wt. %) Zeolite TOS conv. Alkyl-benzenes (wt.%’.) (h) c8 c8 H-Y H-M (1)
6 12 3 5
Others* 24/44+
i-24 2 4 i-34 3 4 i-44 4 4
-- 100 100
20.85 16.49 17.30 3.13
95.8 95.9
0.23 0.39 0.46 1.32
6.12 11.17 27.01 24.33
0.14 0.31 2.75 1.29
4.77 8.70 16.16 11.59
0.16 0.28 3.13 1.26
4.28 7.77 2.66 1.39
63.45 54.89 30.53 55.69
1.43 1.44 4.74 9.68
Conditions : Temp. (K = 413; Press (MPa) = 0.1; WHSV (h-1) = 1.0; Benzene : Olefin (mole) ratio = 2d. a .. Includes cracked and isomerization products. t : Includes iso-phenyl compounds also.
As expected, Y does not appear to possess any shape-selectivive effect, while mordenite possesses very large shape-selective characteristics. The 2 4 / 3 4 ratio (measure of shape-selectivity) increases rapidly with increasing time on stream in the case of mordenite, but does not increase in the case of Y (last column, Table 3). Large amounts of iso-compounds are found to be produced over both zeolites the iQformation being more in the case of mordenite. As the mordenite pores are narrower than Y, this observation is surprising. This suggests that the contribution of the external surface of the crystallites to the reaction is probably significant in the case of mordenite. 3.3 Alkylation with 1-dodecene The results of the alkylation studies carried out with 1-dodecene are presented in Table 4. The studies were carried out at a pressure of 0.6 MPa and a temperature of 408 K at which conditio s benzene remains in the liquid phase. Also a low space velocity (WHSV = 0.8 h- ) was used. Under the above conditions, catalyst deactivation was much smaller, 100 % conversion being observed for upto about 20 h for most of the catalysts. The lower deactivation noted at 0.6 MPd is probably due to the solvent action
?
403
of liquid benzene dissolving away the deactivating components from the catalyst surface. Under the above conditions, the yields of the C12-alkylated products (+dodecanes) were also high, the selectivitiesbeing > 80 % in the case of RE-Y, SiO2-Al2O3and p.
Table 4 Alkylation of benzene with 1-dodecene C12-alkyl benzene distribution, (wt. %) Catalyst
24
36
44
54
64
26/64 24/34 ratio ratio Si-AI 33.4 21.9 14.8 15.6 14.2 2.4 1.5 RE-Y 17.6 19.1 19.8 22.0 21.5 0.8 0.9 39.1 26.4 18.9 10.8 4.8 8.2 1.5 H* tr 70.7. * 1.8 563.7 35.4 0.9 65.8 34.2 tr 1.9 HF* 16.7 16.4 17.5 24.1 25.3 0.7 1.0 Conditions : Temp. (K) = 408; Press. (MPa) = 0.6; WHSV (h-1) = 0.8 ; Benzene : Olefin (mole) ratio = 10; Conversion of olefin 100 %; Data obtained at 10 h on stream) + : Includes iso-phenyl compounds. + t :Conversion incomplete. * : C al 1 benzene fraction from commercial LAB: liquid phase reaction; &S?indet erminate. ** : 24/44 ratio.
-
As discussed in an earlier section, based on the relative stabilities of the different secondary carbenium ions, one would expect the isomer content to increase with the C-number. This is found to be so in the case of both HF and RE-Y. Apparently, RE-Y is non-shape selective even during alkylation with a large molecule like dodecene. I3 and mordenite, which were both non-shape selective or weakly shape-selective durin alkylation with 1-hexene, now exhibit marked shape-selectivities wth dodecene. 0 son has also observed differences in isomer distribution of dodecyl benzenes during studies using HF, H SO4 and AICl3 [20]. ‘he shape-selectivity effect, as expected, is much larger for the unidimensional mordenite system than for p; in fact, mordenite is so shape-selective that the 6 4 isomer is not detected at all. Further, both H-M (2) and H-M(3) deactivated more rapidly than the other zeolites, the selectivity for the C12 a1 1 benzenes being less than 10 % at the above conditions. Interestingly, silica-aluminais ound to produce relatively more of the 2 4 and 3 4 isomers when compared to HF or RE-Y. Apparently, as discussed earlier, acidity differences are probably responsible for this behaviour.
f
?
404 3.4 Alkylation with mixture of olefins (Cio-Ci3)
The results of alkylation with a commercial mixed olefin feed (C1 -Q) are presented in Table 5. The composition of the feed according to the C- ractions is also resented in the table. Due to the large number of isomeric olefins in each carbon Fraction, and the difficulty in separating and identifying them, estimation of the individual components has not been carried out.
P
Table 5 Alkylation of benzene with a mixture of (Cio-Ci3) olefins+ Product distribution, (wt. %) Product Cata1yst Cn
n4
H-T$
H+
H-Y
RE-Y
Si-Al
HF**
8.0 3.5 2.9 2.5 16.1 10.1 7.8 2.1 3.1 10.5 7.1 5.2 2.9 1.3 6.7 4.3 2.2 1.9 0.8 1.0 41.3
4.7 2.8 2.6 2.6 11.4 7.3 6.4 6.2 3.1 10.9 7.3 6.2 6.4 6.4 4.0 2.8 2.4 2.6 2.6 1.3 31.0
2.9 2.8 3.5 5.0 7.9 7.4 7.9 8.8 3.9 6.1 6.6 7.0 8.4 7.0 2.3 2.3 2.8 3.2 3.0 1.2 19.2
4.9 3.4 2.5 3.0 12.0 8.0 5.7 5.9 2.7 11.8 8.0 5.4 5.7 5.2 4.5 4.5 2.0 2.0 2.0 0.8 33.2
5.0 3.5 3.0 3.1 5.9 5.8 6.4 8.4 4.3 5.7 5.7 6.1 8.4 8.8 2.9 2.9 3.2 4.3 4.7 2.0 19.6
(1) \
c10
2 3 4
c11
c12
5 2 3 4 5 6 2 3
I
22.5 7.2 6.3 13.1 38.5 8.8 tr 3.6 tr
4
c13
5 6 2 3 4
5 6 7
2 4 (%)
64.6
Conditions : Temp. (K) 408; Press. (MPa) = 0.6; WHSV (h-l) = 0.8; Benzene : Olefin (mole) ratio = 10. + : Composition of feed (wt. %) : Olefins total = 9.7- C1 = 1.9; C11 = 3.1; C12 = 2.9; C13 = 1.8. Paraifins )Clo, dll, 812 and C13) constitute the rest. *8 8 :. Conversion incomplete. Com osition of parent olefin mixture not known; Commercial LAB product; liqui! phase reaction; WHSV indeterminate.
405
The results are found to be similar to those reported earlier. SiO2-AI203, HQ, H-Y and H-M produce more 241 isomers than RE-Y and HF. As expected, shapeselectivity in the case of H-M and H+ increases with increase in the C-number of the fraction. It is to be noted that the end- henyl isomers of the C11 and C13-alkyl benzenes are present in smaller quantities ue to the lower alkylation probability (half) at the central carbon atoms. At the conditions of the study, the conversions of the olefins over the samples (except H-M (1)) were 100 %. The selectivity for alkyl benzenes were also very high, being > 90 % for all the catalysts (except H-M (1)). The influence of process arameters on the life of H-Y in the alkylation with mixed olefins is resented in Take 6. Increasin the tern erature beyond 403 K does % ole& is found in the product) of not increase the ife defined as time at which > the catalyst. Similar y, operating at low benzene to olefin ratios lowers the life of the catalyst. Increasing the pressure increases the life of the catalyst, though beyond 0.6 MPa the effect is not significant. The reason for increased life at higher pressures has already been explained as being due to the solvent action of liquid benzene present in the system.
2
PI
l.1
Table 6 Alkylation*of benzene with a mixture of olefins: Influence of process parameters on life of catalyst Temp. Press.** Life of catalyst + Benzene : Olefin (mole) ratio (K) (MW (h) 10 4 13 0.1 24 76 10 413 0.6 10 413 3.0 80 10 373 0.6 Incomplete tonversion 70 10 403 0.6 10 423 0.6 72+ + 5 4 13 3.0 55 10 413 3.0 80 15 413 3.0 76 Conditions :Vertical flow SS reactor; WHSV (h-1) = 0.8. * : Catalyst, H-Y. * * : Pressurized with N2 t : Life for c 0.1 % olefin content in product; 2 4 h. + t : Product was pale yellow. 4. CONCLUSIONS
Zeolite-Y does not exhibit shape-selectivity effects in the alkylation of benzene with longchain alkanes with C-number as large as 13. Mordenite and f3 exhibit shapeselectivities, their shape-selectivitiesbeing more ronounced with increasing C-number of the alkane. The shape-selectivities observe1 are attributed to both product and transition-state types.
406
5. ACKNOWLEDGEMENTS We thank Dr. H.S. Soni, Anuj Ra' and J.S.Reddy for their help during the above study. This work was partly funded by NDP.
J
6. REFERENCES
1. P.R. Pujado, in " Handbook of Petroleum refining process" (R.A. Meyers, ed.), Mc Graw-Hill, 1986. 2. R.T. Sebulsky and A.M. Henke, Ind. Eng. Chem., Process Dev., 10 (1971) 272. 3. He Ming-Yuan, L. Zhonghui and and M. Enze, Catal. Today, 2 (1988) 321. 4. L.B. Young, US Pat. 4,301,317 (1981). 5. T. Berna, L. Jose, D. Moreno and Alfonso, EP Appl. 0 353,813 (1991). 6. B.V. Vora, P.R. Pujado, T. Imai and T.R. Fritsch, Paper presented in "Recent advances in the detergent industry", Society of Chemical Industry, Univ. of Cambridge, England, 26-28 March 1990. 7. J.J. Wise, US Pat. 3,251, 897 (1966). 8. K.A. Becker, H.G. Kar e and W.D. Streubel, J.Catal., 28 (1973) 403. 9. W.W. Kaeding and R.#Holland, J.Catal., 109 (1988) 212. 10. D.W. Breck and G.W. Skeels, in "Proc. 6th Intern. Zeol. Conf." (D.H. Olson and A. Bisio, eds.), Butterworths, 1984, p.87. 11. J. Bandiera, C. Hamon and C. Naccache, Proc. 6th Intern. Zeol. Conf. (D.H. Olson and A. Bisio, eds.), Butterworths, 1984, p.337. 12. R.N. Bhat and R. Kumar, J.Chem.Biotechnol., 48 (1990) 453. 13. J.L. Casci, B.M. Lowe and T.V. Whittam, EP Ap 1.42 226 (1981). 14. A.R. Pradhan, Thesis, Univ. Poona (1991); S. rnst, P.A. Jacobs, J.A. Martens and J. Weitkamp, Zeolites, 7 (1986) 25. 15. P.B. Venuto, L.A. Hamilton, P.S. Landis and J.J. Wise, J.Catal., 5 (1966) 81. 16. A. Corma, A. Lopez A do, I. Nebot and F. Tomas, J.Catal., 77 (1982) 159. 17. D.H. Olson and W.O. aag, Am. Chem. SOC.Symp. Ser. 248 (1984) 275. 18. A.Corma and J.Nieman, Proc. Akzo Catal. Symp. 1991, Fluid Catalytic Cracking, Scheveningen, The Netherlands, p.217. 19. M.F.L. Johnson, J.Catal., 52 (1978) 425. 20. A.C. Olson, Ind.Eng.Chem., 52 (1960) 833.
E
f?
407
DISCUSSION Q: H. G. Karge (Germany) You mentioned the rapid deactivation of mordenite catalysts used for benzene alkylation by long chain alkenes. During our early work on benzene alkylation by ethene or propene, we observed the same phenomenon. However, the life time of the mordenite catalysts was much improved via dealumination. Possibly, dealuminated mordenites may also exhibit a better performance in the case of alkylation of benzene by long alkenes.
A. P. Ratnasamy Our experiments on alkylation with long chain olefins over mordenites dealuminated to different levels upto a SiOdA1203 ratio of about 100 have suggested the existence of an optimum Si02/AI203 ratio of about 45.
Q: W. 0. Haag (USA) As you pointed out in your paper, there are two possibilities to account for high selectivity for 2-phenylalkane: (1) a rapid alkylation relative to isomerisation or (2) shape selective preference via transition state or product diffusion control. We have also examined benzene alkylation with linear olefins. We used, for example, mordenite catalyst and either 1octene or 4-octene. If alkylations were fast relative to isomerisation, the predominant product in the latter case would be 3-phenyloctane. However, both olefins give the same product distribution which was predominantly 2-phenyloctane. Thus, shape selective control is the operating mechanism. A: P. Ratnasamy We agree that shape selective control is probably the primary operating mechanism in mordenite catalysts.
Q: D. Ka116 (Hungary) Did you determine the primary products of alkylation at low conversions '?
A: P. Ratnasamy No, we did not.
Q: D. Barthomeuf (France) With regard to fast deactivation, do you think that it arises from the acidity of the zeolite, from the microporosity or from other parameters. Do all the zeolites you study give the same deactivation rate ?
A: P. Ratnasamy Different zeolites deactivate at different rates. Both acidity (especially acid site density) and microporosity probably influence the rate of deactivation.
Q: H. Schulz (Germany) As a procedure for reactivation of the catalyst, a simple thermal treatment could be successful (as we have observed in the regime of reamination) during methanol conversion on HZSM-5 (International Zeolite Conference in Montreal, July, 1992). Have you tried this procedure in your experiments for the various zeolites ? Could you also give some numbers in deposit selectivity, because these would characterize the task of reactivation further '?
A: P. Ratnasamy We have also observed that a limited reactivation takes place (especially in the case of some high silica zeolites) with thermal treatments under certain conditions. The shape selectivity change (Column 9 in Table 2) is a measure of "DEPOSIT" effects. Real numbers in % deposits are not available.
408
Q: K. P. de Jong (T'he Netherlands) 1) How do you characterize the heavy deposits present on the spent catalyst in order to discriminate between what you called "heavy alkylate" and "oligomeric species"? 2) You mentioned that solvent extraction suffices to rejuvenate the catalysts. Which solvents are appropriate? Which procedures (e.g. temperature, time) for extraction did you apply7
A: P. Ratnasamy 1) Heavy alkylates are poly-alkylation products of benzene, while oligomeric species are dimers, trimers etc. of the long chain olefin. 2) Passing benzene at reaction conditions for 10 hours was found to remove a significant fraction of the heavy deposits.
Guczi, L et al. (Editors), New Frontiers in Caralysis Proceedings of the 10th International Congnss on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
COMPARISON OF SAP037 WITH FAUJASITES IN CRACKING REACTIONS
M. Brienci, M. Derewinski, A. Lamy and D. Barthomeuf Laboratoire de Reactivite de Surface et Structure, URA 1106 CNRS, Universite P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France
Abstract The n-octane cracking has been studied with a series of H-SAPO-37 of increasing Si content and for HY, dealuminated HY and LZY-82. The influence of framework Si content in SAPO-37 strongly depends on the pretreatment temperature which very likely modifies the distribution of atoms in the structure. Silica impurities are not involved in this process. The catalytic activity of H-SAPO-37 increases up to a pretreatment of 1173 K while that of the Si-A1 faujasites decreases. In this high pretreatment temperature range SAPO-37 is more active than the faujasites. The selectivity indicates a higher olefidparaffin ratio for the Si rich SAPO-37 samples i.e. a lower hydrogen transfer activity. The ratio iso-olefidiso-paraffin is higher in SAPO-37 than in LZY-82 while it is the opposite for the ratio branchednon branched products. 1. INTRODUCTION
SAPO-37 molecular sieve which has the faujasite structure (1)exhibits acidic (2-5) and catalytic properties (3,513). It was shown that in the n-decane hydrocracking (8) or in the o-xylene isomerization (10) H-SAPO-37 is as active as HY. It can also be compared to USY in the cracking of gas oil (6,9). By contrast it is 10 to 15 times less active than USY in the cracking of n-heptane (9) and n-octane (12). The question arises as to which parameters may affect the catalytic properties upon modifications due to pretreatment conditions (temperature, atmosphere) or due to the presence of additional reactants or phases in the solid. 2.EXpERIMENTAL
A series of SAPO-37 materials with increasing Si contents are prepared with tetramethyl-(TMA) and tetrapropyl (TPA) ammonium hydroxides as templates (1,2,4,12). HNaY (referred to as HY) containing 10 Na+/u.c. and LZY-82 (framework Si/Al= 4.5) from Union Carbide are used. A Y zeolite dealuminated with (NH4)2 SiF6 (14) gives HYD sample (frameworkSi/Al= 7). The table 1
* On leave from the Institute of Catalysis and Surface Chemistry , Krakow, Poland.
41 0
Table 1 Characterization of SAF'O-37 materials Si-0.12
Si-0.13
Si x(a) 0.12 0.13 MY 0.50 0.49 Pz 0.38 0.38 Z+/u.c.(b) 22.3 20.7 a,(C) 24.76 24.76 Thermal stability 0 2 (K) nd(d) 1270f 30 02+H20(K) nd 1275 f 30
Si-0.14
Si-0.16
Si-0.20
Si-0.22
0.14 0.48 0.38 21.7 24.75
0.16 0.48 0.36 21.9 24.74
0.20 0.50 0.30 19.7 24.70
0.22 0.46 0.32 18.5 24.71
nd nd
nd nd
nd nd
1320 f30 1240 f 30
(a) x, y, z : atomic fraction (b) number of charges per unit cell (from template content) (c)
unit cell parameter for as synthetized samples
(d) not determined
summarizes the chemical composition of the SAPO's. They are referred to by the atomic fraction of silicon. The crystallinity of the zeolites was checked to be very good in the as synthetized form or after the catalytic test. The n-octane cracking was carried out a t 723 K in a flow of hydrogen (flow rate 13.2 mumin.) 0.06. SAPO-37 materials were activated from 873 K to 1223 K with P,,-&PH~= under flowing oxygen for 6 h in the standard pretreatment. It was checked that TPA and TMA were decomposed after heating at 873 K (4,151. The temperature was then decreased under oxygen t o 723 K. A He flow was passed for a few minutes through the catalyst to purge the system and hydrogen was introduced. Specific pretreatments will be described further. The accuracy on per cent conversion is 10 %. Each catalytic test was performed on a fresh sample.
The highly crystalline samples described in the paper consist of only SAPO-37 as a pure phase. Attempts to introduce Si contents higher than an atomic fraction of 0.22 lead to silica as an extraphase which could be removed by ultrasonic treatment finally giving a pure SAPO-37 phase with a framework Si content of 0.19. For an ideal SAF'O-37 structure where all the atoms Si, Al, P have the same environment, the Si content is x = 0.125 (2). Table 1shows higher values of x which are interpreted as due to the presence of islands rich in Si (7,12,16). For the present case, these islands consist of only silica as seen by NMR (12) in the as synthetized materials. No Si-A1 faujasite phase comparable to that described in (7,161 is observed. The existence of these islands decreases, for a given number of Si atoms, the number of Si-0-A1 negatively charged species, i.e. of the protonic centers. This is confirmed by the decrease in the
41 1
number of charges Z per unit cell as the Si content increases (table 1).The catalytic activity should reflects these changes. In addition it is well known that the catalytic properties of zeolites also depend on the pretreatment and on test conditions. The various parameters are considered in what follows. In the experimental conditions used, the SAPO-37 materials loose 20 to 40 % of their activity between 5 and 35 minutes after the beginning of a run. The n-octane % conversion and the selectivities remain then constant. After up to 15 h on stream the catalyst is white showing the absence of coke. In the same experimental conditions HY zeolite becomes black. 3.1. Standard pretreatment at 873K
For the same pretreatment as SAPO-37, HY o r LZY-82 give conversions between 20 to 25 % compared to 1 to 3 % for SAPO-37. Experiments conducted with HY at a higher flow rate (56.2 ml/min) decrease the conversion to a value close to that of SAPO's.This flow is used for comparison of the samples in table 2. The higher activity for the Si-Al faujasites is explained by Corma e t al. (9) by the need for stronger acid sites in the cracking of n-paraffins compared to that of gas oil. The n-octane cracking results are reported in table 2 for five SAPO-37 samples typical of low (x = 0.12-0.13) or high (x = 0.16 to 0.22) Si contents. The two first materials give very similar results different from the ones of Si-0.16, -0.20 and -0.22. They are characterized by a higher conversion. The products range up to C5 hydrocarbons. For the C4-C5 domain for which the accuracy is good, the table 2 gives the molar fraction in the products (alkanes, alkenes, branched or not). Samples Si-0.12 and Si-0.13 are again similar and different from the ones with high Si content. They give a lower formation of n-alkanes Table 2 n-Octane cracking a t 723 K after activation a t 873 K (flow rate 13.2 mVmin for SAPO's materials)
Conversion % n-alkanedb) iso-alkanes n-alkenes iso-alkenes isoln OJP c2(c)
c3 c4
c5
CdC4 (a)
Si-0.12
Si-0.13
1.9 22 21 42 l5 0.6 1.3 3 21 47
2.0 22 21 42 I5 0.6 1.3 3 21 48 21 0.44
21 0.45
Si-0.16 1.3 25
9 39 27 0.6 1.9 9 25 38 21 0.66
Si-0.20
Si-0.22
1.3 26 14 40 a0
1.6 24
2.7 24
15 35
18 33 25 0.75 1.4 5 25
0.5
1.5 6
23 43 22 0.54
26 0.7 1.6 7
25 41 22 0.61
flow rate (56.2 mllmin)
(b) mole per cent for C4 and C5 only
(4For C2 t o C5 the difference to 100 % is due to thermal cracking
ma)
43 22 0.58
41 2
and !so-alkenes with a higher production of iso-alkanes and n-alkenes. The situation for the two classes of materials (low or high Si) is then reverse for isoalkanes and iso-alkenes. The selectivity expressed as the ratios of is0 t o non branched hydrocarbons (isoh) and of olefixdparaffin (o/p) shows for the first ratio no mejor change with the Si content due to a compensation effect in the changes in linear and branched products. The values are close to 0.75 obtained for HY a t a similar % conversion. The o/p ratio higher for the Si rich materials reflects the important production of iso-alkenes for Si-0.16 to Si-0.22 catalysts. It is higher than for HY. This suggests that the hydrogen transfer decreases in the order : HY > Si-0.20 > Si-0.22 > Si-0.16. The three last materials pretreated a t 873 K appear the most interesting with a low hydrogen transfer activity and a high iso-alkenes production. The distribution of the products with the C number shows also in table 2 very similar behavior of Si-0.12 and Si-0.13 compared to Si-0.16 t o Si-0.22. The two Sipoor catalysts give less light hydrocarbons C2 and C3 which is in line with higher hydrogen transfer (17) as seen above, The ratios C3/C4 which may express the influence on catalysis of the field gradient in the cages (18) is lower in Si-0.12 and Si-0.13 suggesting an homogeneous field in the cages. This is in line with a regular distribution of isolated Si atoms as seen by NMR in these two samples (12) which generates mainly isolated Si-OH-A1 acidic sites homogeneously distributed in the framework. The higher values obtained for the other samples including HY are in agreement with an heterogeneous field related to a n irregular distribution of charges in the framework i.e. t o the existence of Si islands in SAPO's.
33. Intluence of pretreatment temperature The per cent conversion as a function of pretreatment temperature is reported in figure 1A for four SAPO-37 typical of the two classes (low or high silica contents) described above. It shows that the order of catalytic activity observed after the 873 K pretreatment (table 2) is no longer valid. For 923 K and higher heating temperatures the two silicon poor materials are the less active due t o a sharp decrease in conversion between pretreatments at 873 and 923 K. A second feature of figure 1A is the increase in n-octane cracking up to 1173 K for all the samples. This is usually not observed in catalysis with Si-A1 zeolites, the catalytic properties decreasing in the 1000-1200 K range. This is described in figure 1B which compares a SAPO-37 sample (Si-0.22) with HY, HYD, LZY82 pretreated a t increasing temperatures and tested in the same experimental conditions (i.e. a flow rate of 13.2 ml/min). For the three faujasite-type catalysts the % conversion decreased regularly for temperatures of pretreatment ranging from 873 K to 1223 K. The SAPO-37 catalysts behaviour is then unique. It will be considered further with regards t o simultaneous results of selectivity. Between 1173 and 1223 K the SAPO-37 materials loose their catalytic properties. This is related to the beginning of the structure collapse (15). At around 1173 K, the SAPO's particularly the Si rich become more active than HYD or LZY-82. The selectivity changes for all the reaction products have been checked for all the samples and all the pretreatment temperatures. The main results for C4and C5 hydrocarbons are summarized in the figure 2 for the products classified like in table 2 as n- or iso-alkanes and n- or iso-alkenes. The fig. 2A
41 3
6
I I
A
!
01'
9io
1
'
1100
I
I
V,
1
90 0
(K)
1100
(KI
Figure 1. Changes in % conversion in n-octane cracking at 723 K as a function of pretreatment temperature for (A), Si-0.12 (a),Si-0.13 (b), Si-0-16(c), Si-0.22 (d) and (B),Si-0.16 (a'), HY (b'), HYD (c'), LZY-82 (d') (flow rate 13.2 ml/min).
I S
El
I
2 -
O 900
1000
1100
( K ) 900
1000
1100
(K)
Figure 2. Changes in the products distribution as a function of pretreatment temperature for Si-0.12 (A) and Si-0.22 (B).(a) : n-alkenes, b : iso-alkenes, c : n-alkanes, d : iso-alkanes. represents the results for Si-0.12 which are identical to those of Si-0.13. The silicon rich samples are exemplified in figure 2B by the sample Si-0.22. The behaviour of the two classes of SAPO's are somewhat different. For the silicon poor catalysts big changes occur between pretreatment temperatures of 873 and 923 K for the iso-paraffins and iso-olefins. The iso-paraffins decrease by around 75 % due to the drop of the formation of isopentane and isobutane. Simultaneously a rise in iso-olefins production by almost 50 % is due to an increase in the formation of isobutene. For pretreatment temperatures higher than 923 K a slow increase in iso-paraffins and a small decrease in
41 4
....
1-4
&-.4
“10,
d
Figure 3. Changes in the ratio olefirdparafin as a function of pretreatment temperature for Si-0.12 and Si-0.13 (a), Si-0.16 (b), Si-0.22 (c), LZY-82 (d).
n-olefins are observed. The Si-0.22 catalyst is not very sensitive to the pretreatment temperature. Small changes are observed between 1073 and 1173 K. The o/p ratios of all the catalysts are reported in figure 3. The Si-0.12 and Si-0.13 materials show a maximum a t 923 K. For the silicon rich samples the o/p ratios decrease between 1073 and 1173 K. It follows that after the pretreatment at 1173 K, the Si-0.12 and Si-0.13 catalysts have the highest o/p ratios, quite opposite t o the 873 K case. The LZY-82 zeolite gives the lowest o/p ratios, i.e. the highest hydrogen transfer. The isoln ratios reported in figure 4A are very close for all the SAPO’s, being not very sensitive to the pretreatment temperature. This behaviour results for a large part from a compensating influence of iso-paraffins and iso-olefins as seen in figure 2. LZY-82 gives the highest isoh ratios. Large differences are observed for the formation of iso-olefins or isoparaffins. They are described in figure 4B by the ratio of the two products. The Si-0.12 and Si-0.13 samples give the largest changes with a sharp maximum a t 923K. Above this temperature all the SAPO’s show a decrease in the ratio isolefirdiso-paraffin. All of them give higher ratios than LZY-82. Table 2 showed that the C3/C4 ratio of the catalysts Si-0.12 and Si-0.13 pretreated at 873 K is lower t h a n €or the other materials. For higher pretreatment temperatures of all the SAPO’s this ratio is 0.66 2 0.02, value very similar to that of LZY-82 (0.66) pretreated between 1073 and 1173 K. The increase of this ratio for the two silicon poor materials strongly suggests that the field distribution in their cages (14,18) is changed between 873 and 923 K and becomes similar t o the one in the other catalysts. This may be explained by a change in the distribution of charges, some isolated Si atoms forming islands. In the same temperature range a minimum in the oxidizing and reducing properties is observed for the samples Si-0.13 reflecting similarly strong modifications of the properties as the dehydroxylation proceeds (5). This may also explain the minimum observed in percent conversion for the two silicon poor samples in figure 1A and the maxima seen in figures 3 and 4B for
41 5
the ratios o/p and iso-ohso-p. After their pretreatment a t T > 923 K similar values for all the samples tested suggest that the field gradient is heteregeneous in a similar way for all of them. The rise in catalytic conversion (figure 1)after pretreatments above 923 K can be discussed considering also the selectivity results. The ratios described in figures 3 and 4 show some changes mainly between 1073 and 1173 K but the increase in the n-octane cracking starts already after the pretreatment at 973 K (or 923 K).This suggests that there is no major modification in the reaction mechanism. The rise would rather be related to an increase in the number of sites active for the reaction. It is well known that the hydrocarbon cracking requires strong acid sites. It has been shown that the number of such protonic sites increases upon heating up to 1173 K (19). Two reasons might explain this rise, either an increase in the number of Lewis sites (5,201 which may increase the protonic strength of existing weaker sites, or the change in the environment of Si which transforms isolated Si atoms in islands. The strength of sites at the border of the islands is higher than for isolated Si atoms (7). This second hypothesis would explain the increase in oxidizing and reducing power of sites in the same temperature range. This point is still under study.
Figure 4. Changes in the ratios total isohotal n (A) and iso-alkenehso-alkane (B) as a function of pretreatment temperature for Si-0.12 and Si-0.13 (a), Si-0.16 (b), Si-0.22 (c), LZY-82(d).
33.Influence of pretreatment axnosphere In order to better understand the changes occuring in the SAPO-37 materials upon heating, different procedures have been followed involving 0 2 (necessary to burn the template), He, H2 or N2. In the standard pretreatment (A) after the oxygen treatment at each temperature T, the temperature is decreased from T to 723 K in an oxygen flow. After a few minutes under He in order to purge the
41 6
system the hydrogen flow is established and the reaction starts immediately. Three other ways have been checked after the pretreatment at T (K) under oxygen for 8 hours. They are : B) A t the temperature T change of the gas from 0 2 to H2 (after a He purge) standing two hours at T(K) and decrease of the temperature to 723 K at which the reaction is-started. C) At the temperature T change of the gas from 0 2 t o He and keeping the temperature at T(K) for 2h more. Decrease of the temperature to 723 K. Introduction of H2 and starting of the reaction. D)At the temperature T keeping of the 0 2 flow and decrease of the temperature to 723 K. After a purge with He, keeping of a H2 flow for 2h before starting the reaction. E) An influence of nitrogen has been observed on the properties of SAPO-37 (9,211. A pretreatment has been conducted under a N2 flow (SO ml/min) at 823 K for 4 h. It is followed by a treatment under oxygen a t 1073 K. The procedure is then carried out as in A. The figure 51 reports for sample Si-0.14 and for the various cases the n-octane conversion as a function of pretreatment temperature T. Some remarks can be drawn. The trend for an increase in conversion already seen in figure 1A for all the samples pretreated according to A is observed for the procedures B, C and D up to around 1100 K. In this temperature range no major influence of the atmosphere is seen after the template decomposition
1
"
900
1000
1100
( K ) 900
1000
1100
(
0
Figure 5 . Changes for sample Si-0.14 in the % conversion as a function of pretreatment temperature (I)for various pretreatments noted A, B, C,D, E (see text) and (11) for various teet modifications after the standard pretreatment A, Si-0.14 (a),Si-0.14 with feed n-octane + toluene (b), Si-0.14 + Si02 (c), LZY-82 (a') LZY-82 with feed n-octane + toluene (b') (For LZY-82 the flow rate is 56.2 mVmin instead of 13.2). in oxygen. The template decomposition in nitrogen (E)followed by the burning of coke in oxygen does not modify significantly the results either. The main differences occur for the pretreatment at 1173 K where the procedures A and C give high conversions while B and D lead to 30 to 40 % lower conversions. For
41 7
the two last treatments hydrogen is contacted with the samples for 2 h either at T or at 723 K before the test. In the cases of A and C no reducing atmosphere is involved before starting the catalytic test. This fact seems to be the directing feature which improves the performances. This occurs at a temperature where a large increase in Lewis acidity (4,6) and reducing properties (6)are observed. This might correspond to some reorganization of the materials.
3.4. lntluence of toluene The .gas oil cracking was reported to give similar activities for SAPO-37 and USY while SAPO-37 was ten times less active than USY in n-hexane cracking (9). The lower activity of SAPO-37 in the present study compared to HY, LZY-82 and HYD (figure 1) a t temperatures of activation less than around 1160 K suggested to look at the influence of non-paraffinic hydrocarbons present in gas oil. A mixture of toluene and n-octane (Ptoi/Pn.oct=0.1) was used as the feed. The ratio Phydrocarbon$PH2was as previously. The curve b figure 611 shows a lower activity in the presence of toluene compared t o the measurements in the absence of the aromatic (curve a) for the same pretreatment A. The influence of toluene was checked on LZY-82 tested at a flow rate of 66.2 mVmin (instead of 13.2 mumin) in order to have conversion rates close to those of SAPO-37. The tests performed after pretreatment8 at 973 and 1073 K (figure 611) give similar results in the presence (b') or not (a') of toluene. It can be concluded that the differences in gas oil and n-paraffin cracking between SAPO-37 and the Y zeolites can not arise from the aromatics in gas oil.
a.Influence of Si@ and IZY-82 Some silica impurities were found by electron microscopy in the silicon rich samples (12). It might be supposed that upon heating, a reaction occurs between SAPO-37 and this silica generating new acidic Si-OH-A1 species. In order to check such a possibility a mixture of the sample Si-0.14with 16 96 pure silica was tested after pretreatment in conditions A from 973 to 1173 K. The curve c figure 511 expressed for the same weight of SAPO-37as curve a shows no synergistic effect due to the presence of silica. The presence of a faujasite phase was observed by some authors in their as synthetized SAPO-37 materials (7,8,16) but not in the present case (12). The influence of such a very active catalytic phase was checked by mixing the Si0.14 sample with 10 % by weight of LZY-82. The pure LZY-82 tested after pretreatment A at 1073 K at a flow rate of 13.2 mVmin give a 9 % conversion. After mixing with SAPO-37 the point d shows only a slight increase in conversion of the pure SAP0 phase (curve a). This indicates, no synergistic effect due t o LZY-82. In conclusion SAPO-37 materials exhibit properties which change upon heating in a way not observed previously for Si-A1 zeolites. The presence of phosphorous very likely induce modifications of the Si location which then involves changes in acidic and catalytic behaviour.
41 8
Acknowledgments We thank D. Delafosse and M.J. Peltre for very interesting discussions. BREFERENCES 1 2 3 4 5
6 7 8 9 10 11
l2
I3 14
15 l6 17
18 19 20
21
B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent NO 4 440 871 (1984). L. Sierra de Saldarriaga, C. Saldarriaga, M.E. Davis, J. Am. Chem. SOC., 109 (1987) 2686. E.M. Flanigen, R.L. Patton, S.T. Wilson, in "Innovation i n Zeolite Materials Science" (Eds P.J. Grobet et al.), Stud. Surf. Sci. Catal., 37 (1988) 13. S. Dzwigaj, M. Briend, A. Shikholeslami, M.J. Peltre, D. Barthomeuf, Zeolites, 10 (1990) 157. B.L. Su, A. Lamy, S. Dzwigaj, M. Briend, D. Barthomeuf, App. Catal., 75 (1991)311. G.C. Edwards, J.P. Gilson, V. Mc Daniel, US Patent no 4 681 864 (1987). J.A. Martens, C. Janssens, P.J. Grobet, H.K. Beyer, P.A. Jacobs in "Zeolites, Facts, Figures, Future", (P.A. Jacobs and R.A. Van Santen, eds), Stud. Surf. Sci. Catal., 49A (1989) 215. J.A. Martens, P.J. Grobet, P.A. Jacobs, J. Catal., 126 (1990) 299. A. Corma, V. Fornes, M.J. Franco, F.A. Mocholi, J. Perez-Pariente, in "Fluid Catalytic cracking 11" (M.L. Occelli, ed) Am. Chem. SOC. Washington D.C. 452 (1991) 79. J. Dwyer, K. Karim, A.F. Ojo, J. Chem. SOC. Far. Trans., 87 (1991) 783. A.F. Ojo, J. Dwyer, J. Dewing, K. Karim, J. Chem. SOC. Far. Trans., 87 (1991) 2679. P.P. Man, . M. Briend, M.J. Peltre, A. Lamy, P. Beaunier, D. Barthomeuf, Zeolites, 11 (1991) 563. J.M. Lopes, F. Lemos, F. Ramoa Ribeiro, E.G. Derouane in "Zeolite Chemistry and Catalysis" (P.A. Jacobs et al., eds), Stud. Surf. Sci. Catal., 69 (1991)365. C. Mirodatos, D. Barthomeuf, J. Catal., 114 (1988) 121. M. Briend, A. Shikholeslami, M.J. Peltre, D. Delafosse, D. Barthomeuf, J. Chem. SOC. Dalton Trans., (1989) 1361. L. Maistriau, N. Dumont, J.B. Nagy, 2.Gabelica, E.G. Derouane, Zeolites, 10 (1990)243. P.B. Venuto, E.T. Habib, Fluid Catalytic Cracking with Zeolites Catalysts, Marcel Dekker, New York, (1979) 98-117. C. Mirodatos, D. Barthomeuf, J. Catal., 93 (1985) 246. Bao Lian Su, D. Barthomeuf, submitted. M. Briend, M.J. Peltre, A. Lamy, P. Man, D. Barthomeuf, J. Catal., submitted. J. Pires, M. Brotas de Carvalho, F.R. Ribeiro, E.G. Derouane, React. Kin. C a d . Lett., 43 (1991) 313.
41 9
DISCUSSION
Q: W. P. Hettingcr, Jr. (USA) Catalytic cracking activity and selectivity of a catalyst are quite sensitive to contact time and temperature. Further, in commercial use the catalyst temperature drops from regeneration temperature to exit temperature of the reactor, due to endothermic reaction. All of this short period of time, of 1-4 seconds, the catalyst is deactivating also due to coke formation. Have you studied conversion performance as a function of time and temperature so as to better characterize the performance of a SAPO-31 crystal versus a faujasite ciystal ?
A: D. Barthomeuf With regard to deactivation it was checked that with time on stream, SAPO-37 deactivates less than faujasites and after 15 h it keeps a light color while faujasites become black. The effect of pretreatment temperature described in this paper suggests in SAPO-37 a redistribution of atoms in the solid upon heating. Such a change is expected to occur in the regeneration step at the first regeneration. Other studies on the effect of temperature or contact time are in progress. By comparison with the large amount of work published on faujasites, the small number of studies carried out on SAPO-37 do not allow yet to answer all questions. Q: B. W. Wojciechowski (USA) We have recently become aware of the fact that various hydrocarbon cracking processes proceed in various measure via two processes. Initiation involving the protolysis reaction and chain propagation involving hydrogen transfer and isomerization. In all cases where the propagation reaction is significant dilution of the reactant by an "inert" leads to a change in the olefin/parafin ratio and in isomerization selectivity. In view of this, using different flow rates (of carrier I presume) for HY and SAP0 catalysts distorts any interpretations based on the measurement of these quantities.
A: D. Barthomeuf Only in the part 3.1 of the paper which compares samples after pretreatment at 873 K, different flow rates were used for, on the one side SAPO-37 and on the other side HY.In this part it is shown that conversion and selectivities of SAPO-37 (i.e. measured at the same flow rate) depends on the Si content. In the part 3.2 where all the samples, pretreated at high temperature, are compared at a same flow rate (13.2 rnl/min) LZY-82 give selectivities different from SAPO's. The study of the catalytic activity of SAPO's is at its beginning compared to all what has been done on Si-AI faujasites and it is difficult to speculate in detail on what mechanism is occurring on these new materials. Q: T. S. R. Prasada Rao (India) It is a very interesting piece of work and results are encouraging when we discuss about FCC catalysts, the question of hydrothermal stability is very important. How far SAPO's are hydrothermally stable ? Do they compare with Y zeolite in this aspect ?
A: D. Barthomeuf We studied the thermal and hydrothermal stability of SAPO-37 [I]. SAPO-37 is very stable at high temperature even in the presence of water. By contrast it is not stable below 373 K - 353 K in atmospheric conditions. In this temperature range it is very stable in dry conditions. [l] J . Chem. Soc., Dalton Trans., 1989, 1361
Q: J. B. Nagy (Belgium) Did you also determine the number and strength of the acid sites in both SAPO-37 and
420 faujasite ? Did you find a relationship between the acidity and the catalytic activity ? In SAPO-37 there arc acid sites of different nature, indeed.
A: D. Barthomeuf We compared the acidity of SAPO-37 and faujasites in previous apers [2-41. The main conclusions are as follows: i) above a Si content of 0.12 the number o acid sites in SAPO-37 decreases due to thc formation of Si islands at higher Si levels; ii) the acid strength of "isolatcd" Si atoms is of medium strcngth; iii) the protons linked to Si04 tetrahedra at the border of Si islands are very strong, even stronger than in ultrastable Y. S . Dzwigaj, M. Briend, A. Shikolcslami, M. J. Peltre and D. Barthomew, Zeolites, 121 10, 157 (1990); ibid., 11, 563 (1991) Bao Lian Su, A. Lamy, S . Dzwidgaj, M. Briend and D. Barthomeuf, Appl. Catalysis, [3] 75,31 1 (1991) Srud. SurF Sci. Catal., 69, 313 (1991) [4]
P
Ouczi, L et al. (FAitors), New Frontiers in Catalysis
Procccdinga of thc 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elscvicr Science Publishers B.V.All righta nscrvcd
CATALYTICACI'IVITY OF MODIFIED ZSMJ ZEOLITES IN THE DEHYDROGENATION AND AROMATIZATION REACTIONS OF PROPANE AND n-BUTANE
P. Fejesa, J. H a l h 9 , I. Kiricsia, 2.K e l 8 , Gy. Tasia, I. Hannufl; C.Fernan&@, J. B. Nagc, A. Rockenbauerd and Gy.Sch&kP aApplied Chemistry Department,J6zsef Attila University, Rerrich t6r 1,6720 Szeged, Hungary bLaboratoire de Dynamique et Structure des Materiaux Moleculaires, 59655 Villeneuve d'Ascq Cedex, France CLaboratoirede Catalyse, Facultes Universitaires Notre-Dame de la Paix, 5000 Namur, Belgium dCentral Research Institute of Chemistry, Hungarian Academy of Sciences, Pusztaszeri bt 59,1025 Budapest, Hungary
Abstract In order to enhance catalytic activity in the dehydrogenation and aromatization of light paraffins, a set of potential catalysts: (i) V(f)-ZSM-5 (originating from direct synthesis), (ii) V(ex)-ZSM-5 ("ex": partial exchange of Ht in H-ZSMd by V@' ions), and (iii) solid exchange of H in H-ZSMd for Ga, In, and V by baking the mixtures of ZSM-5 with the respective oxides were investigated by physical (XRD, IR, ESR, MAS-NMR spectroscopy, and adsorption) and chemical methods. Inthe "as synthesized V(f)ZSM-S sample fl' ions reside in framework positions according ESR and %MAS-NMR spectra. Coincidence between experimental and theoretical spectra could be attained only by supposingthe existence of two different framework surroundings. The structure of V(f)-ZSM-B is strained: heating (especially in the presence of water vapour) causes release of Vfrom the framework. Severe conditions cause clustering of @' in the voids with the annihilation of hyperfine splitting in the ESR spectra. Catalytic results demonstrate that in the presence of modifyers propane and n-butaneconversion turns from cracking to dehydrogenation and aromatization pathways.
1. INTRODUCTION
Increasing attention has been directed recently toward zeolitic materials, especially metallo-silicalites which show enhanced activity in the dehydrogenation and aromatiza-
422
tion of light paraffins [ 1-41, conversion of methanol to hydrocarbons [5],propane ammoxidation to acrylonitrile [6] and even in liquid phase oxidation of phenol to a mixture of hydroquinone and catechol [7]. Trivalent ions (like Fe3', Ga3' and €I3+) used for the substitution of Ap' in the structures are suitable for tuning the Bronsted acidity of the zeolite. Tetravalent ions like T f ' , Ge4+ do not endoa the framework with net electric charge or acidic property, therefore, the catalytic behaviour, if any, is due to the properties of the heteroatoms. Most promising are those substitutions where the extraneous ions are transition metals possessing several (but at least two) oxidation states. In accordance with Pauling's minimum radius ratio these ions can be introduced into the zeolitic lattice with the greatest probability if their ionic radii are close to that of Si4', while the framework itself remains uncharged in the substitution process. It means in other words that from this class of metals those in the fourth oxidation state are the most favoured substituents for $'. The more or less stable "as synthesized" form can then be brought into an "out of balance" condition by artificially changing the oxidation state of the (mostly transition-) metallic component. Lower oxidation states than four generate net negative electric charge in the framework with cation exchange (and acidic) properties, higher oxidation state of the substituent ion leads, at least in principle, to net positive charge of the framework and anion exchange. As concerns the experimental methods, either direct hydrothermal synthesis and solid phase ion exchange may result in isomorphous substitution, nevertheless, their extent can be very different. This paper deals with the modificationof ZSM-5 zeolites with Ga,O,, ln203,VO ,, and VC13 applying solid exchange, and by introducing p' in hydrothermal synthesis. The catalysts were characterized by XRD, IR-spectroscopy, physical adsorption and for the \P' and @' containing samples by ESR- and 5'V-MAS-NMR spectroscopy, respectively. Catalytic activity was tested in the (oxidative/non-oxidative) dehydrogenation of propane and n-butane. 2. EXPERIMENTAL
Catalyst reparation (0-ZSM-5 (f = "framework")sample was synthesizedwith a SO, to VO, ratio equal to 42 by X-ray fluorescence analysis. This value was very close to that ( S i B = 40) reported by Miyamoto et a/. [6]. The synthesis was carried out (cf. [8]) at pH = 9.1, outgoing from the batch composition: 1.O SO, 0.2 VO, 39.2 H20, 0.3 TPA-Br. It is worth to note here that ZSM-5 seeds (Si/Al = 40.0) were added to the slurry to facilitate crystallization. The product was fully crystalline by X-ray diffraction and had ZSM-5 structure in both "as synthesized" (A.S.) and "burned off (B.O. forms. ' (VClJ as The catalyst samples containing Ga3', I&',p' (V205,and 9 modifying additives were prepared by baking the physical mixtures of Ga20/H-ZSM-5, or In203/H-ZSM-5,V20JH-ZSM-5 and VCliH-ZSM-5 at 873 K. The H-ZSM-5 was a standard sample with Si/N = 40.0. The same ZSM-5 sample was used in order to obtain pf(ex)-ZSM-5 (ex = "exchanged") by ion exchange in 0.1 molar vanadyl oxalate solution.
$'
423
Catalyst characterization The catalysts were characterized by XRD, IR-spectroscopy and for the vanadium containing samples by ESR- and MAS-NMR-spectroscopy.ESR spectra were recorded on a JEOL-FE3X type spectrometer in the X-band using Mn2+ in MgO matrix for field calibration. 57V- and ,'Si-NMR spectra were taken on a Bruker MSL 400 spectrometer using MAS conditions. The frequenc , the pulse width and waiting time were for 57V: 105.2 MHz, 1.0 ps and 0.2 s and for Si: 79 MHz, 4.0 ps (goo), 10.0 s, respectively. Change in acidQ of the samples as the result of the modificationwas followed by IR-spectroscopy; N2 adsorption at 77 K, oxygen and hydrogen uptake of catalysts were measured in a volumetric equipment.
A
Catalytic reactions Conversionof propane and n-butane was studied in a fixed-bed flow reactor in the presence and in the absence of 0, under atmospheric pressure. 0.5 g solid sample was filled in a quartz tubular reactor, and a pretreatment in nitrogen flow at 500 OC was utilized for 3 hours to standardize the catalyst surface. For catalytic test the inlet gas composition of 20%alkane t 80% nitrogen or 100/0 alkane t 10% oxygen t 80% nitrogen was used. Product analysis was performed by GC using an on-line Porapack-Q column for the analysis of gaseous compounds, and a 30 m long SPB-1 capillary column for the analysis of the collected liquid phase. I
3. RESULTS AND DISCUSSION 3.1.
Characterization of catalysts
3.1. I . V-containing samples
Characterization by ESR-spectroscopy The p'(ex)
and p'(r)
I
ESR spectra (which were regis-
tered at ambient temperature) Fig. 1: Line diagram of ESR spectra consist of sixteen slightly unevenlv spaced lines arising from the flipping of the 3d' unpaired electron of p + - i o n and its interaction with the nuclear spin of the 100% abundant nucleus (I =7/2) which gives rise to an eightfold hyperfine splitting of all anisotropic components. The structure of the spectra is depicted in Fig. 1. The difference between the spectra of p'(ex) and p+(r) is slight; at first sight the spectrum of pp'(r) is shifted to higher fields which means 52 gauss for the first arallel lines diminishing to 16 for the last (ex) spectrum. ones due to the greater spectral width of the Table 1 summarizes the main diagonal elements of the g-and the hyperfine coupling tensors @). Notable differences show up only in the parallel components (9,) of
J+
424 Table 1 ESR parameters of V-containing materials Compound
91
9,
Al (gauss)
A, (gauss)
---
V 0 2 + (ex)-ZSM8
1.963
2.007
190.8
75.4
1.992 113.9
p' Q-ZSM-5 (A.S.)
1.949
1.990
185.8
72.5
1.976 110.3
V02' lexI-NaY
1.963
2.007
190.8
75.4
1.992 113.9
P
the (s) and in the perpendicular components (AL) of the hyperfine coupling tensors . The reduced values of the g and tensors main diagonal elements for the fl (9 specimen can be interpreted by a lesser separation of the vanadium d-orbitals or by the weakening of the covalent character of V-0 bonds. Though the reduced covalency of planar a-bond should increase the anisotropic angular contribution to the hyperfine interaction, the observed hyperfine constants for the @' (r) specimen are actually reduced which indicates the overcompensating role of the Fermi interaction if the symmetry is distorted. The values of the coupling tensors (by the reduction of the Fermi contact term) substantiate this view. The A.S. @'(r)-ZSM-5 samples have reddish-yellowcolour which turns to white after burning off the template. The spin concentration in the A S . specimen is estimated to be about 2.3 * 1Om g-' which is reduced to practically nil in the B.O. samples. On reducing the B.O. sample with H2 at 670 K or above, the characteristic spectrum of p' reappears, indicatingthe reversibilify of the fl r. Ip + e- transition (as adsorption measurements reveal: for at least 50% of p' present). Upon dehydration of hydrated B.O. fl'(9-ZSM-5 zeolites (colour: yellow) at 570 K or above in vacuum the COlOur ofthe samples turns to grey, a Fig. 2: ESR spectrum of sample exhibiting clear indication for formation of a grey colour l ! 'and pt) oxidation state of mixed ( vanadium ions. As ESR spectra reveal, a complex set of events takes place during the heating in the presence of water vapour, the colour change being only one visible consequence out of many. Fig. 2 shows an ESR spectrum of a sample exhibiting grey colour. The magnified parts (at low and high fields) disclose the presence of two P+-species. The very complex spectrum (consisting of 32 lines t 6 Mn2' calibration lines; here shown only
'
'
425
one) looks like the superposition of two spectra, one for @' (ex) (weaker lines) and one for p'(f),the only difference being that at high fields the sequence of lines [caused by the greater spectral width of p' (ex)] is reversed here. Prolonged steaming at 570 K makes the p'(ex) lines more intense, eventually the whole spectrum coalesces into a broad signal lackin hyperfine structure on which only the Mn" calibration lines can be recognized (Fig. 3). The spectrum is caused in all probability by hydrated clusters of Voxides in the zeolitic voids. All these observations clearly indicate the presence of p ' in special framework positions in the A.S. samples the number of which might be limited, setting an upper limit for the V02/Sit3? ratio at about 0.025. At least 50% of @ ions are situated on chanFig. 3: ESR spectra of steamed nel walls capable to participate in fast redox sample reaction steps. Another part is more or less hidden, nevertheless, it is still accessible to 0, (or H2). The interaction of p'(f) building units with water vapour seems to be very specific (colour changes and other strange observations): water is able to disrupt the strained V - 0 - 9 linkages and causes release of V which is not a very att1 ractive feature when these materials are applied under arduous conditions as heterogeneous catalysts. The P'-ZSMd specimens obtained by solid exchange with VC13 exhibit similar ESR spectra as shown in Fi 2, however, the intensities for $'(f) are smaller than for p+(ex). Whether the nests of the zeolitic structure are filled up with VO, units, or real "framework exchange" occurs is open for further investigations. AS concerns location of P'(0 ions in the zeolitic framework, models Fig. 4: Experimental (A) and theoretical (6) can be imagined and depicted, never%-MAS-NMR spectra of VQ-ZSM-5 theless, they are and remain speculative for a while without solid substantiation. As a closing remark of these ESR studies it's worthwhile to mention that incompletely oxidized (r) specimens still in contact with 0, at ambient temperature
p'
426
show the characteristic ESR spectrum of 0; radical ion (< g > = 2.0027; g, set partly resolved, the gXxand gw lines unresolved). It is believed that this radical ion is quite unstable, therefore, only its degradation products (eventually 0'- ions) can be made responsible for oxygen transfer at high temperatures. Characterization by MAS-NMR spectroscopy The 5'V-MAS-NMRspectrum of the A.S. V(f)-ZSM-5 specimen can tentatively be decomposed in two components (contributions about 10% and 90%, respectively).The respective isotropic chemical shifts, quadrupolar coupling constants and the chemical shift anisotropies with respect of V205 are:
- 672 ppm, CA = 3.9 MHz
61,
=
6:,,
= -
and A ' 6 = 510 ppm
680 ppm, C i = 3.7 MHz and A26 = 430 ppm
The spectrum simulated with these constants resulted in a good fit (see Fig. 4) with that observed experimentally. Work is in progress to arrive to a better refinement using spectra from different frequencies. The VC/.@ZSM-5 specimen obtained with solid exchange resulted in a spectrum (with respect of VOCI,) which could be decomposed also in two components (contributions 88% and 12%, respectively) with the following parameters:
- 612 ppm, CA 6 MHz and A ' 6
= 480 ppm
6j,,
=
6:,,
= - 607 ppm, C; = 6 MHz and A26 = 800 ppm
A similar spectrum was obtained for Nay (Si/A/ = 2.2) exchanged with VO(COO),. Based essentially on the different chemical shifts, quadrupolar coupling constants and not at last on the great similarity of the VCIJH-ZSM-5 and v"(ex) Nay spectra these last parameters were attributed to extra-frameworkV species. The greatest difference between V(f) and V(ex) "V-NMR spectra is manifested in the quadrupolar coupling constants. Even when it is believed that irrespective of notation V(r) is a framework species, the value 6:,, = - 680 ppm and especially the chemical shift anisotropy: A26 = 430 ppm is too high for regular tetrahedrally coordinated V ions (for regular tetrahedral environment: Gis0 = - 605 ppm, A 6 < 100 ppm [9]). The most acceptable interpretation seems to suppose the presence of isolated V ions in strongly distorted tetrahedral surroundings as part of the framework near to the channel walls. On the other hand adsorption studies with H, and 0, do not substantiate such a high (90%) contribution to the NMR spectrum by this species (see later). The MAS "Si-NMR spectrum of the V(f) sample (Fig. 5.b) is shifted to lower fields by about 2 ppm as compared with the reference ZSM-5 (Fig. 5.a). The origin of this shift is not clear yet; probably due to the presence of paramagnetic V species modifying the magnetic interaction of Si nuclei. Work is in progress to decompose the 29Si-NMRspectrum quantitatively in order to determine the contribution from the Si(7V) configuration.
427 Characterization by 1. R. spectroscopy
The absorption of silanol groups at 3740 cm-' could clearly be observed on V(f)-ZSM-5 samples which were activated in vacuum or in 0, at 673 K. After hydrating the samples thus pretreated and evacuated at 723 K the I.R. spectrum shows a broad OH band centered at 3600 cm-' in addition to the Si-OH band. Acidity tests with pyridine as a probe molecule showed only Lewis acidity, therefore, it can be concluded that the concentration of Brtinsted acidic sites is negligible independently of the pretreatment conditions. Adsorption studies No H, or 0, uptake could be observed Fig. 5: MAS ?3i-NMR spectra below 570 K. At 670 K the hydrogen adsorption of of the a: reference ZSM-5 the (at the same temperature) preoxidized V(0sample b: V-ZSM-5 sample ZSM-5 sample in repeated cycles was nearly twice (2.21 cm3/g, S.T.P.) that of oxygen (1.I0cm3/g, S.T.P.) indicating a real redox behaviour. As the spin concentration: 2.3 * 1O2' spin/g might be overestimated, the previous values reveal that at least 50% of the V ions are involved in this redox reaction. 3.1.2. Other samples
Each modified catalyst sample preserved the crystal structure characteristic for zeolite ZSM-5. As the XRD patterns taken on the wide/H-ZSM-li samples showed, these specimens contained part of the oxides as a separate phase. No indication of the presence of any foreign phase could be detected in the VC/dH-ZSM-5sample by XRD. The Bronsted acidities of the oxide/H-ZSM-5 samples were roughly one order of magnitude lower than those of the parent H-ZSM-5. This reduction is caused by the "neutralization" reaction taking place between the "modifying oxide" and the Bronsted acidic sites of the zeolite during "solid exchange". 3.2. Catalytic investigations
The results of the catalytic measurements in the reactions of propane (t 0,)' and n-butane (+ 0,) are shown in Figures 6-9 (the reaction temperature: 723 K, space velocity: 1500 h-', time on stream: 3 h). In the absence of dioxygen the conversions of the paraffins were low in comparisonto those where 0, was present. The differences in the conversions over the different catalysts are not considerable (except for H-ZSMd), the lesser activity of the modified ZSMQ samples can be
428
0c r a c k 60
products
hydrocarbons
40
a dehydrogenates
30
El oxygenates
20
m aromatics
10
-
conversion
0
H-ZSM-6
VCb-ZSM-5 O+03-ZSM-5
Vfl-ZSM-5
ln2%-ZSM-5
Fig. 6: Product distribution of propane transformation in the absence of 0, over different catalysts
Selectivity(%) 60
R
crack products
50 hydrocarbons
40
0dehydrogenates 30
EBB
=
20
10
oxygenates aromatics
- Conversion
0
H-ZSM-5
VCg-ZSM-5
V(f)-ZSM-5
In,03-ZSM-5
G+03-ZSM-5
Fig. 7: Product distribution of propane transformation in the presence of 0, over different catalysts
accounted for by the decreasing Bronsted acidity. For the product distribution (the selectivities are computed on common C basis) over H-ZSM-5 the high cracking activity is characteristic, the amounts of dehydrogenates (propene or n-butenes) and aromatics are below 10%. The VCI3/H-ZSM-5 behaves similarly as H-ZSM-5 in the absence of 0,. A significant increase in aromatics production
429
Selectivity(%) 60 60
orrok produotr
40
hydroorrbonr
30
@Zi
20
drhydrOQin.te8
oxygenrtea
10
0 H-ZSM-6
VCb-ZSM-6 Q+03-ZSM-6 ln2G-ZSM-6
V(r)-ZSM-6
-
womrtlor oonverrlon
Fig. 8: Product distribution of n-butane transformation in the absence of 0, over different catalysts
Selectivity (%) 60
crack products
hydrocarbone
0 dehydrogenate8 oxygenate8 aromettce
H-Z8M-6
VCg-ZSM-6
VZO~-ZSM-5Qcl~OgZSM-dIn203-ZSM-5V(fb2SM-6
- oonverelon
Fig. 9: Product distribution of n-butane transformation in the presence of 0, over different catalysts
with concurrent decrease of crack products can be obsewed when Ga 0 and 1n2,03are added to zeolite H-ZSM-5. The catalytic effect of V(r)-ZSM-5is similar; ?n&e reactions of n-butanethe highest aromatic selectivity was found over this catalyst both in the absence and in the presence of dioxygen. The formation of oxygenates is characteristic for catalysts containing vanadium in extra-framework positions.
430 In the absence of 0, a relatively fast deactivationof catalysts could be observed by coke formation over Lewis acid sites [lo].When present, 0, reacts with carbenaceous deposits to form carbon oxides, at the same time new pathways open up for the formation of oxygenates. The lifetime of the catalysts increases considerably as well. Over acidic H-ZSMd zeolite the activation of the starting paraffin is assumed to be the protonation of the alkane forming carbonium ion followed eventually by the scission of the C-C bond in Q position. The modified zeolites (including the A.S. V(f)-ZSM-5 as well) have no or only very low Bronsted acidity in comparison to the parent zeolite sample. The literature extensively deals with Ga,O, and h,O3 claiming that both oxides are active in the oxidative dimerization and cyclization of olefins [ll]. The modified samples contain Ga3', /n3+ and p' @+)ions mainly in exchange (in the case of Valso in framework) position in a highly dispersed state, therefore, it is believed that the dominant step in these transformations is dehydrogenation (and/or oxidative dehydrogenation) resulting in carbenium ions. Whether these steps are of unimolecular or bimolecular character is hard to answer yet. It is also open for further tests if the dehydrogenates (formed in consecutive reaction steps) form aromatics in the chemisorbed phase or the cyclization takes place as a homogeneous gas phase reaction. These results demonstrate that by suppressing the Bronsted acidity via addition of Ga203and h203to zeolite H-ZSM-5or introducing tetravalent ions, like p', capable to participate in reversible redox steps, propane and n-butane conversion turns from cracking to the arornatization pathway. In order to achieve this effect the presence of oxidative or dehydrogenating Lewis sites are necessary. 4. REFERENCES
1 2 3
4 5 6 7 8
9 10 11
G.L. PRICE and V. KANAZIREV, J. Catal., 126 (1990)132. G. CENT1 and G. GOLINELLI, J. Catal., 115 (1989)452. M.F.M. POST, T. HUIZINGA, C.A. EMEIS, J.M. NANNE and W.H.J. STORK, Stud. Surf. Sci. Catal. 46 (1989)365. I.E. MAXWELLand W.H.J. STORK, Stud. Surf. Sci. Catal., 58 (1991)571. A. MIYAMOTO, D. MEDHANAVYN and T. INUI, Appl. Catal., 28 (1986)89. A. MIYAMOTO, Y. IWAMOTO, H.MATSUDA and T. INUI, Stud. Surf. Sci. Catal. 49 (1989)1233 P.R.H. PRASAD RAO, A.V. RAMASWAMY and P. RATNASAMY, private commun. P. FEJES, I. MARSI, I. KIRICSI, J. H A ~ S ZI., HANNUS, A. ROCKENBAUER, G. TASI, L. KORECZ and G. SCHOBEL, Stud. Surf. Sci. Catal., 69 (1991)173. O.B. LAPINA, V.M. MASTIKHIN, A.V. NOSOV, T. BEUTEL and H. KNOZINGER, Catal. Lett., 13 (1992)203. C.R. BAYENSE, A.J.H.P. VAN DER POL and J.H.C. VAN HOOFF, Appl. Catal., 72 (1991)81. J. H A i S Z , K. VARGA and P. FEJES, J. Mol. Catal., 51 (1989)303.
Acknowledgement The grants (OTKA No. 693/86,1182/90 and 1614/90)of the Hungarian Academy of Sciences is gratefully acknowledged.
431
DISCUSSION
Q: H. Karge (Germany) In your study you made use of the solid-state reaction between VCI3 or Ga 0 and
9
HZSM-5. Did you in those experiments also monitor the stoichiometry of the reactign Did you, for instance, determine the amount of HCI or H20 released during the reaction ? Such measurements should provide some information about the nature of the incorporated species, i.e. they should answer the question whether or not complex cations such as VCI2+ or GaO+ are accommodated on cation sites rather than V(I1I) or Ga(I1I).
A: P. Fejes Your remarks are absolutely right. The amount of HCI (in the case of VCI ZSM-5 exchange) or H 2 0 (in the other cases) evolved during solid exchange is indeed-"some" measure of the extent of reaction taking place during heating. On the other hand there are so many disturbing effects (see e. g. the structural water of the zeolite or oxides which may react with the anhydrous VCI3 setting free HCI without exchange) that we did not aim to determine the exact degree attained at solid exchange. The most disturbing beside this was the presence of unreacted additive which could be identified by X-ray as a separate crystalline phase. I am afraid the same difficulties are general in solid exchange which we tried to circumvent about 8 years ago by separate volatilization of the active ingredient. This approach is feasible when you deal e.g. with VC13 However, what to d o if you want to introduce V in exchange position using V20, ?
Q: B. Wichterlova (Czechoslovakia) We have performed a detailed ESR study of the vanadia introduced into HZSM-5 by an ion exchange of vanadyl cations, by an induction of V205 with HZSM-5 in the solid state and by direct synthesis leading to the "framework" vanadia. We have concluded that the framework V(IV) ions are in the form of vanadyl square planar complexes strongly bonded i d t o the zeolite framework and possessing extraframework oxygen ligand.
A: P. Fejes Of course, we have the copy and we know the paper by Petras and Wichterlova [l]. However, on the basis of our ESR and 51V-MAS-NMR studies we arrived at another conclusion, i.e. we think that in the V(Q-ZSM-S samples the V ions reside in strongly distorted tetrahedral framework surroundings near to the channel walls. In all probability the four V - 0 linkages are not equivalent: one V - 0 bond is shorter and this picture is not far away from that you mentioned here. M. Petras and B. Wichterlova,J. Phys. Chem., 96, 1805 (1992) [l]
Q: E. Derouane (Belgium) 1) For a reaction as complex as the aromatization of propane and butane, which requires both acidic and dehydrogenation sites, one should not conclude about selectivity changes without information about the absolute or relative amounts of these sites. Even more, when comparisons are made at different conversion levels. Finally, it is difficult to conclude about differences between impregnated and synthesized V-ZSM-5 materials without information on the Vcontent in the different phases. 2) Early work from Mobil, in the early seventies, demonstrated clearly that addition of 0, to an alkane/alkene feed increases dehydrogenation and aromatization over zeolites containing redox species. That is indeed what you report for ropane and you also observe a reduction in cracking. For butane, however, using V(f)-Z M-5 you report an increased catalytic activity and no change in dehydro enation and aromatization. How d o you explain the difference between propane and butane .
8
432
A:
of all, for aromatization to be effective dehydrogenation sites are absolutely sufficient. Ring closure (e.g. in the case of hexatrienes having appropriate conformation) may take lace as,a homogeneous gas phase reaction. Llectivity is an experimental information which you obtained from analytical data. It is another question how to explain it ? Re arding the extreme complexity of catalytic steps involved (even when we are workin on t e elementary steps of cracking since 30 years) we were aiming to separate groups o similar reactions (cracking, formation of oxygenates, aromatics, etc.) and to see how they change in dependence of catalyst compositions in a narrow range of conversion (exception Figure 9). Flow reactor is unsatisfactory to arrive to exactly the same conversion. The Vcontent in the A. S . samples was exactly measured by X-ray fluorescence. The other samples obtained with solid exchange necessarily contained unreacted oxide prohibiting exact analysis for the zeolitic phase. 2) We do not report about increased catalytic activity for V(f)-ZSM-5 in general. We just state that for aromatization V(f)-ZSMJ is the second and third best catalyst with propane feed and the best if the feed is butane. The explanation is to be found in the complex set of elementar transformations the unrevealing of which might be the task of heterogeneous catalysis or the next decade.
B a
r
Q: K. Dooley (USA) Your paper states "Reversible oxygen and hydrogen uptake was observed for Vcontaining samples only", you also report a ve low ratio or cracking products to aromatics for Ga 0 -ZSM-5 in propane aromatization. owever, the patent literature indicates that your dLa-&M-5 should be the best of the materials which you tested. I sus ect that your Ga203-ZSM-5 sample was insufficiently reduced. We have shown [2, 37 that Gafl3 reduction, ultimately to Ga+Z- (Z- is an anionic lattice site of the zeolite) is absolutely necessary in preparin active GaZSM-5 catalysts from mechanical mixtures, and in some cases impregnates. e have also seen that, subsequent to this initial reduction, redox behavior for GaZSM-5 can be observed. Did you determine the dispersion or Ga in your sam la? G. L. Price, V. Kanazirev, J. Cutaf., 176,767 (1990) K. M.Dooley, C. Chang, G. L. Price, Appl. Caul. A , , 84,17 (1992)
%
J
I:/
A: P. Fejes You are ri ht. Oxygen and hydrogen u take was measured for the AS. V(f)-ZSM-5 samples only. ' h e Ga and In containing ZSh-5 specimens which were prepared by solid exchange have not been reduced at all. In the light of your remarks this might be the reason that the observed selectivity for aromatics (between about 20-30%) was markedly less than that characteristic for the best of this sort of catalysts. Nevertheless, I would like to stress that these ex eriments were not aiming to arrive to the best catalyst formulations. As ar as dispersion of Ga in the samples prepared by solid exchange is concerned, may I mention that the ultimate catalyst was a very complex mechanical mixture consisting of exchanged ZSM-5 zeolite and unreacted oxide prohibiting any attempt to determine dispersion of Ga in the zeolitic phase.
P
Q: J. N. Armor (USA) 1) What is the level of exchange of vanadium into ZSM-5 and Y type zeolites and what is the wt% of vanadium in these materials for the ion-exhanged preparations '? 2) For the solid state exchange did you see a difference in performance for vanadium oxide and catalysts derived from VCIg derived products '? It is vev important to determine the level of vanadium exchange and distinguish between precipitated or exchanged forms of vanadium for ion exchanged preparations.
-
433 A. P. Fejes 1) It is impossible to obtain com lete exchange of (Na)-ZSM-5 (after B.O.acidic sites were neutralized in 0.01 molar N a O d or NaY in 0.1 molar vanadyl oxalate solution. In the case of (Na)-ZSM-5 the degree of ekchange was about 52%, for NaY it was about 64%. These samples were utilized only in the FSR and 51V-MAS-NMR studies. Though we omitted lo mention it in the paper the 23Na-MAS-NMR spectra were also registered the analysis of which we have to skip here. 2) For VC13-ZSM-5 and V205-ZSM-5 we have comparative data only for the conditions of Figure 9. The two specimens behave similarly in spite of the differences in the degree of exchange and even in the quality of active sites (VCI3-ZSMJ contains V mainly in the V(III) oxidation state which is dominantly V(V)in the case of V205-ZSM-5).
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Guczi, L. er d.(Editors),New Frontiers in Carolysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
MECHANISM OF COKE FORMATION DURING THE TRANSFORMATION OF PROPENE, TOLUENE AND PROPENE-TOLUENE MIXTURE ON HZSMJ P. M a g n o d , F.Machaabb and M. Guisnep aURA CNRS 350, Labratoire de Catalyse en Chimie Organique, UFR Sciences, 40, avenue du Recteur Pineau, 86022 Poitiers, France bunhersidad Central de Venezuela, Escuela de Quimica, Facultad de Ciencias, Apartado 47102, Caracas, Venezuela
Abstract The formation of coke was investigated during the transformation of propene, of toluene and of a mixture propene-toluene on a HZSMS zeolite. At 120°C while no products are desorbed from the zeolite, coking occurs very rapidly from propene and from the mixture, slowly from toluene. The coke molecules result mainly from condensation reactions : from propene the main "coke" components are c1&35 aliphatic hydrocarbons, from toluene, methyldiphenyl and triphenylmethane compounds, from the mixture, diisopropyltoluenes. These coke molecules are located inside the pores, their retention being due to their low volatility and also, for diisopropyltoluenes, to a steric blockage at channel intersections. At 450°C very little toluene is transformed. Propene and the mixture are rapidly converted into CpC7 olefins and C & 3aromatics. The rate of coking is not very dependent on the reactant. Whatever the reactant, methylpyrenes are the main coke components. However these coke molecules are probably formed through different reaction paths from toluene or from propene and propene-toluene mixture. They are sterically blocked at channel intersections. Mechanisms are proposed to explain the formation of the various coke molecules. INTROD UCTlON
The main cause of deactivation of zeolites is the formation and the retention of secondary products heavier than the reactants and the desired products inside the pores or on the outer surface of the crystallites. Although these compounds are not always polyaromatic most of the authors call them coke. The major difficulty in the determination of the modes of coking and of deactivation is to establish the composition of coke [l]. In situspectroscopicmethods (IR, 1% NMR, UV-VIS, EPR) can give information concerning the coke content, the chemical identity of coke components during the reaction, the nature of the active sites and the mode of deactivation [2]. Unfortunately none of these methods is able to give the coke composition [31. To determine the complete distribution of the coke components, the totality of the coke must be recovered from the coked zeolite (e.g. by dissolving the zeolite in
436
acid or basic solutions) then analyzed through adequate techniques (GC, HPLC, HNMR, MS, electron microscopy...). Thanks to this method, often associated with the characterization of the coked zeolites by adsorption, the effect of the pore structure on the modes of coking and deactivation of zeolites during hydrocarbon transformations is now well-understood [ 11. Coking and deactivation however depend also on the characteristics of the active sites and on the operating conditions : temperature, reactants... While the effect of the reaction temperature was extensively investigated (4-111the influence of the nature of the reactant (12151 has been little studied. The reactant is often olefinic or paraffinic, more rarely aromatic and only three papers [ 13,14,16] concern hydrocarbon mixtures which, however, are frequently used as reactant or are formed as products in industrial processes. The aim of this work is to specify at low (120°C) and at high (450°C) temperatures the rate of coking the composition, the location and the mode of formation of coke during the transformation of propene, of toluene and of a mixture propene-toluene on a HZSM5 zeolite. EXPERIMENTAL
HZSM5 (Nao.oolH2.1AI2.1Sig3.90192, spherulite) was synthesized by a method proposed by Guth and Caullet [17]. The transformation of propene (a), toluene (b) and of the mixture propene-toluene (c) was carried out in a flow reactor at 120°C and 45OoC, under the following conditions : a) Ppropene = 0.1 bar, Pnitrogen = 0.9 bar ; b) Ptoluene = 0.15 bar, Pnitrogen = 0.85bar ; C) Ppropene = 0.1 bar, Ptoluene = 0.15 bar, Pnitrogen = 0.75 bar. Reaction products were analyzed on line by GC with a 50 m fused silica capillary column Plot A1203/KOH for the transformation of propene, a 30 m fused silica capillary column DB Wax for that of toluene and a 50 m fused silica capillary column CP Sil 5 for that of the mixture. For the kinetic study of coking the WWH (grams of reactant injected per gram of zeolite and per hour) was equal to 9.7 (propene), 32.2 (toluene) and to 9.7+ 32.2 (propene + toluene mixture). The coked samples used for coke analysis were obtained by operating with a WWH value of about 1.O (propene), 3.2 (toluene) and to 1.0 + 3.2 (propene + toluene mixture). The experimental methods used to recover and to analyze the coke components have already been described (181. RESULTS 1. Reaction products The reaction products were analyzed on line during the kinetic study of coking. At 120"C, there were no products observed during the transformation of propene and of toluene ; traces of isopropyltoluene (0.1 O/O conversion) were found in the transformation of the mixture propene-toluene. Moreover a small amount of oligomers (C124222) was found by condensing the effluents of transformation of propene during the preparation of the coked samples for coke analysis. - At 450°C very little toluene was converted (0.3 O/O), benzene being the main product with traces of xylenes and light alkenes and alkanes. Propene was converted at 65 O/O into a mixture of aliphatic compounds (10 O/O C2, 5 O/O C3. 25 O/O
-
437
C4, 10 O/o C5, 10 O/O C&7) mainly olefinic and of Q-Ce aromatics (6 %). The same products with practically the same distribution were observed from the mixture propene-toluene. Most of them resulted therefore from transformation of propene. The conversion calculated on propene (55 O/O) was smaller than that obtained with pure propene (65 O/O). Whatever the reactant the deactivation of the zeolite was slow.
2. Rate of coke formation At 120"C, the formation of coke was much faster from propene than from toluene (Table 1). At short time-on-stream the percentage of coke formed from the mixture propene-toluene was practically the same as from propene while at long time-onstream it was slightly greater. At 450°C the coking rate was not very dependent on the reactant. The formation of coke from propene and from the mixture was much slower than at 120°C while slightly slower from toluene. Table 1 Percentages of coke (wt P : propene : T : toluene
P T P+T
O/O)
formed on HZSM5 for various time-on-stream t (mn).
120°C 60
2
5
7.5
7.3 0.5 8.0
7.0
8.0 1.1 8.4
360
15
450°C 60
360
8.9 1.9 9.4
0.7 0.6 0.7
0.9 0.9 1.0
1.3 1.2 2.4
3. Coke composition Table 2 gives the color and the percentage of coke of the samples prepared for coke analysis. Whatever the operating conditions all the coke components were soluble in methylene chloride. Most of them were located inside the pores for they were not dissolved by soxhlet extraction (maximal yield 5 O/O for 24 hrs).
Table 2 Characteristics of the coked samples t : time-on-stream ; wt coke. P : propene ; T : toluene
120
P T P+T
6, 20 6, 20 6, 20
450
P T P+T
6 6 6
beige yellow clear yellow clear grey grey grey
O/O
C : percentage of
9.0, 9.5 0.5, 1.0 9.0,9.7 3.0 1.6 2.6
438
3.1. At 120°C. the composition of coke depended very much on the reactant but very little on time-on-stream. This is clearly shown by GC, MS and HNMR analysis. The main families of the coke components determined by GC-MS analysis are given in table 3. From toluene three families were found, the components resulting from the association of 2 or 3 toluene molecules. The coke formed from propene contains C12 to C35 aliphatic hydrocarbons with a number of unsaturations + cycles equal to 0 (alkanes), to 1 (olefins or cyclanes) or to 2 (diolefins or cyclenes). The main components of the coke formed from the propene-toluene mixture have the same general formula (CnH2n-6). They are mono, bi or triisopropyltoluenes resulting from the alkylation of toluene by propene. nPropyltoluenes and diphenylmethane compounds were also found but in small amounts. On the other hand the aliphatic hydrocarbonsformed from pure propene were not observed. Table 3 Coke formed on HZSMS from various reactants (P : propene ; T : toluene) at 120°C. Main families of components and main components in each family. Formula
0
alkanes
C12H26
CnH2n
1
alkenes andor
CnH2n-2
2
naphtenes
CnH2n-6
4
General formula
P
CnH2n+2
(9 wt '10)
P+T
In + Cy
Developed formula
Reactant (%coke)
bp 760
P-3
Size
(A)
I I
to
175 to
10-35 :4
C35H72
488
C13H20'
210 (6x95)
x=2 (9 wt 010 )
~=1-3
I n t C y : Number of unsaturations + cydes ; : Main families
3.2. At 450°C. GC, MS and HNMR analysis show that the composition of coke depends little on the reactant. The main components belonged to two families
439 whose general formulae were CnH2n- 16 and CnH2n-22, which corresponds to methylpyrenes and methylfluorenes. In the case of toluene, traces of CnH2n-18 compounds (methylphenanthrene or anthracene) were also observed (Table 4). Table 4 Coke formed on HZSMS from various reactants (P : propene ; T : toluene) at 450°C. Main families of components and main components in each family Readant ( % coke)
P+T
General formula
CnH2n-16
In + Cy
9
Developed formula
~
C
H
~
Formula
)
.
~=2-4
(2.6 wt '10) cn~2n-22.
12
@cH3'x
x=o-5
C16H16 ,
bp760 ("Cl
Size
295
(6~9.5)
(A)
x=3 ClgH16
400
(8.5x8.5)
x=3
IneCy : Number of unsaturations + cycles ; Main families.
DISCUSSION
Whatever the temperature and the reactant, heavy secondary reaction products are formed and remain blocked in or on the ZSMS zeolite. For simplification these products will be called coke, even if their components are not always polyaromatic. The mode of formation of the coke components, the cause of their retention and their location will be examined.
440
1. Mode of coke formation 1.1. At 120°C At this temperature and whatever the reactant there are practically no desorbed products. However from propene and from the mixture propene-toluene the initial rate of coke formation is relatively high. Thus in the first two minutes the conversion of propene into coke is above 20 O/O and that of the mixture above 5 O/O while that of toluene is only 0.2 %. This difference in reactivity can be explained from the differences in the difficulty of the reactions involved in the formation of the different cokes.
1.1.1. From propene Coke is constituted of alkanes, alkenes and cyclanes having 12 to 35 carbon atoms. Obviously, these compounds cannot result from a simple oligomerization of propene but involve other reactions i.e. rearrangement and cracking of oligomers, condensation of olefinic cracking products with propene, cyclization, hydrogen transfer between oligomers. The coke components are different from those found on a USHY zeolite (28 to 34 carbon atoms, presence of compounds with 3 or 4 insaturations + cycles, more pronounced branching [ 181). However the same reactions are probably involved in the formation of coke with both zeolites, the lower density of the acid sites of HZSM5 and the steric constraints at their vicinity limiting hydrogen transfer reactions and oligomerization of branched olefins. 1.1.2. From toluene The coke components result from the condensation of 2 or 3 toluene molecules. Methyldiphenyl and triphenylmethane are formed through the following reactions.
Methyldiphenylmethane compounds were proposed as intermediates in toluene disproportionation [19]. Their formation and that of triphenylmethane compounds is slow for i) they require strong acid sites and ii) these bimolecular reactions are very limited in the narrow space available near the acid sites of HZSMS (channel intersections). With USHY, diphenyl and triphenylmethane compounds were also observed but they led rapidly through cyclization and hydrogen transfer to anthracenic and phenanthrenic hydrocarbons [20].Here again the lower density of the acid sites of HZSM5 and steric constraints explain the differences in coke composition.
441 1.1.3. From the mixture propene-toluene The main coke components result from the alkylation of toluene by propene :
Toluene inhibits the reactions of propene condensation. This could be due to the greater basicity of toluene with consequently a stronger adsorption on the acid sites (in the form of benzenium ions) and above all a preferential attack of isopropyl carbocations by toluene. The formation of diphenylmethane compounds is very limited, which seems to show that propene affects either the adsorption of toluene in the form of benzylic cations or the attack of these cations by toluene molecules. 1.2. At 450°C Whatever the reactant the formation of coke is very slow. From propene and from the mixture propene-toluene it is much slower than the conversion into desorbed products : thus, in the first 15 minutes, propene is converted at 0.3 O/O into coke against 65 O/O into desorbed products, the mixture propene-toluene at 0.07 O/O against 15.7 O/O (conversionscalculated on the mixture). On the other hand 0.08 O/O toluene is converted into coke but only 0.3 O/O into desorbed products. The reactant has very little effect on the coke composition. The main coke components are always alkylpyrenes and among alkylpyrenes, the trimethylpyrenes are always predominant. The other coke components are fluorenic compounds. Two hypotheses can be advanced to explain the similarity of the coke components : 1. Alkylpyrenes and fluorenes result from the same reactions probably between aromatic and olefinic compounds (see reactions 1 in figure 1). This is quite possible from the mixture but also from propene for propene is rapidly transformed into C2-C7 alkenes and benzenic compounds. This is more improbable from toluene for only traces of alkenes are observed in the reaction products.
alk
: a l k y l a t i m , c y c l : c y c l i s a t i o n . HT : Hydrogen Transfer
Figure 1. Formation of pyrene from alkenes + benzenic compounds
442
2. Coke components result from different reaction paths : e.g. path 1 from the mixture propene-toluene and from propene (Figure 1) and path 2 from toluene (Figure 2) and their formation involves different intermediates. The kinetic diameter of these intermediates being smaller than the pore apertures they can desorb from the zeolite. However the-desorption of certain intermediate molecules is slower than their transformation into bulky compounds, larger than the pore apertures. Therefore these bulky compounds remain sterically Mocked at channel intersections and constitute the coke components. Obviously their size and their shape are determined by the size and the shape of the channel intersections.
cycl: cyclisation,tr-: HT: Hydrogen Transfer
transalkylatim,ml: e n l a r p e n t of cycle
Figure 2. Formation of pyrene from toluene.
2. Location of the coke molecules and cause of their retention Whatever the temperature and the reactant all the coke components are soluble in methylene chloride after dissolution of the zeolite in a hydrofluoric solution. However more than 95 OO/ of coke cannot be recovered by a 24 hour soxhlet treatment of the coked zeolite by methylene chloride. This shows that the coke components are blocked inside the pores. However the nature of the coke molecules and therefore their physical properties : size, shape, volatility (Tables 3 and 4) are not identical. Their location in the pores and the cause of their retention can consequently be different. 2.1. At 120°C 2.1.1. From propene The coke molecules formed from propene are aliphatic hydrocarbons with 12 to 35 carbon atoms with a limited degree of branching. The kinetic diameter of the molecules is less than 5 A, the length being from 10 to 35 A. They are therefore located in the channels ; the distance between two channel intersections being equal to 10 A, the coke molecules occupy a volume comprising 2 to 4 channel intersections. The maximum number of coke molecules is equal to about 2.1020 per gram of zeolite, i.e. about half of the channel intersections and 1.5 times less than the theoretical number of acid sites. The volume occupied by the coke
443
molecules estimated from the density of Ci2-C35 alkanes is equal to 0.1 15 cm3/g which corresponds to 70 % of the pore volume of the zeolite. The boiling point of these coke molecules is between 175 and 490°C i.e. higher than the reaction temperature. This low volatility is probably responsible for the retention of the coke molecules. However diffusion limitations in the narrow channels of HZSM5 can also play a role. In particular they explain why no products even dimers and trimers cannot be observed in the effluents, their desorption from the zeolite being slower than their transformation into bulkier compounds. 2.1.2. From toluene The kinetic diameter of the coke molecules is close to that the channels and their boiling point much higher than the reaction temperature. Given their shape and size these molecules are located at channel intersections for their migration in the narrow channels of HZSMS is certainly slow. Another cause of their retention in the zeolite pores is their low volatility. 2.1.3. From the mixture propene-toluene Diisopropyltoluenes are the main coke components. Their kinetic diameter is greater than the apertures of the channels and their size and shape compatible with the size and shape of the channel intersections. Therefore these molecules are sterically blocked at channel intersections.This steric blockage does not occur in the case of para-isopropyltoluene which was also observed in the coke components. This coke molecule which is smaller than the aperture of the channels is then retained in the zeolite pores only because of its low volatility. The maximum number of coke molecules (about 3.8.1020 per gram of zeolite) is close to the number of channel intersections and slightly greater than the number of acid sites. It can therefore be concluded that each coke molecule occupies one channel intersection. The volume occupied by the coke molecules estimated from the density of isopropyltoluenes,is of about 0.10 cm3 per gram of zeolite i.e. close to the volume of the channel intersections (0.13 cm3g-l). 2.2. At 450°C The same coke components are formed from propene, from toluene or from the mixture. Their boiling point being lower than the reaction temperature, their retention in the zeolite pores is necessarily due to a steric blockage at channel intersections. In agreement with this proposal their size is greater than the size of the pore apertures and close to the size of the channel intersections. Moreover their shape is compatible with the shape of the channel intersections. CONCLUSIONS
Whatever the reaction temperature, products of the transformation of propene, of toluene or of the mixture propene-toluene are retained inside the pores of the HZSM5 zeolite (coke). The rate of formation of these products, their nature and the cause of their retention inside the pores depend mainly on the temperature. At low temperature coke molecules result principally from condensation reactions while at high temperature, alkylation, cyclization and hydrogen transfer reactions are also involved. At low temperature the retention in the pores of the coke molecules
444
is mainly due to their low volatility, at high temperature to their steric blockage at channel intersections. Because of the easier retention of the coke molecules in the zeolite pores, the rate of coke formation is greater at low than at high temperature. ACKNOWLEDGMENTS F.M. acknowledges a fellowship within the frame of the postgraduated programme of cooperation CEFI-CONICIT. REFERENCES M. Guisnet and P. Magnoux, Appl. Catal., 54 (1989) 1. H.G. Karge, in "Introduction to zeolite science and practice", (H. van Bekkum, E.M. Flanigen, J.C. Jansen, eds.), Studies in Surface Science and Catalysis, Vol. 58, 531, Elsevier, Amsterdam 1991. 3 M. Guisnet and P. Magnoux, in "Zeolite Microporous Solids : Synthesis, Structure and Reactivity" (E.G. Derouane, ed.), NATO AS1 Series, to be published. 4 D. Eisenbach and G Gallei, J. Catai., 56 (1979) 377. 5 B.E. Langner, Ind. Eng. Chem. Processs. Des. Dev., 20 (1981) 326. 6 D.G. Blackmond, J.G. Goadwin and J.E. Lester, J. Catal., 78 (1982) 247. 7 A.K. Ghosh and R.A. Kydd, J. Catal., 100 (1986) 185. 8 J.P. Lange, A. Gutsze and H.G. Karge, J. Catal., 114 (1988) 136. 9 H.G. Karge, J.P. Lange, A. Gutsze and M. Laniecki, J. Catal., 114 (1988) 144. 10 P. Magnoux, V. Fouch6 and M. Guisnet, Bull. Soc. Chim., (1987) 696. 11 J.R. Anderson, Y.F. Chang and R.J. Western, J. Catal., 118 (1989) 466. 12 S. Maixner, C.Y. Chen, P.J. Grobet, P.A. Jacobs and J. Weitkamp, in "New Developments in Zeolite Science and Technology", Proc. 7th Int. Zeolite Conference (Y. Murakami et al, eds.), Elsevier, Amsterdam (1986) 693. 13 D.E. Walsh and L.D. Rollmann, J. Catal., 49 (1977) 369. 14 D.E. Waish and L.D. Rollmann, J. Catal., 56 (1979) 195. 15 J. Kiirger, H. Pfeifer, J. Caro, M. Bulow, H. Schlodder, R. Mostowicz and J. Volter, Appl. Catal., 29 (1987) 21. 16 L.D. Rollmann and D.E. Walsh, in "Progress in Catalyst Deactivation" (J.L. Figuereido, ed.), NATO AS1 Series, Voi. 54, 81, Martinus Nijhoff Publishers, I The Hague, 1982. 17 J.L. Guth and Ph. Cauilet, J. Chim. Phys., 83 (1986) 155. 18 P. Magnoux, P. Roger, C. Canaff, V. Fouchb, N.S. Gnep and M. Guisnet, in "Catalyst Deactivation" (B. Delmon and G.F. Froment, eds.), Studies in Surface Science and Catalysis, 34, 317, Elsevier 1987. 19 M. Guisnet and N.S. Gnep, in "Zeolites : Science and Technology" (F.R.Ribeiro et al, eds.), NATO AS1 Series E80, 571, Martinus Nijhoff Publishers, The Hague (1984). 20 P. Magnoux, C. Canaff, F. Machado and M. Guisnet, J. Catal., to be published. 1 2
445
DISCUSSION
Q: Wm. C. Conner (USA) Do you have any information as to the decrease in total pore volume measured by adsorption (Nz or other) with the formation of coke in your samples ? Is the decrease in adsorption volume greater or equal to the volume of coke being formed'?In a similar manner does the effective diffusivity decrease linearly or more rapidly with 'koke'l retention by the ZSM-5 '?
A: M. Guisnet In this study the coked samples were not characterized by adsorption. However adsorption experiments with nitrogen and n-hexane were previously carried out on ZSM5 samples coked during n-heptane cracking at 450 OC [l]. For these samples the composition of coke was similar to that found during propene transformation at the same temperature. At low 'coke content (08
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Figure 2. Catalytic performance of unpromoted Sn02 catalyst. of styrene; A , yield of CO+CO2.
0,selectivity; 0 , conversion; 0 ,yield
The results on promoted Sn02 catalysts are summarized in Figure 3. SnO Si02 and SnO P O5 catalysts exhibits the highest selectivity among t% examined catafysts. We have reported that the Sn02-P205 catalyst [8] and the supported Sn02 catalyst on Si02 [9] exhibit very high selectivity
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F i g u r e 3. Performances o f Sn02 c a t a l y s t s promoted by v a r i o u s oxides. For t h e symbols, see F i g u r e 1.
496
toward the present reaction. The figure indicates that the S i 0 2 used as a promoter also remarkably improves the selectivity of Sn02. The increase in the selectivity by the addition of SiO and P2O5 may be caused not only by activating the styrene formation but aiso by suppresing the complete oxidation: the yield of styrene is remarkably increased, while the yield of carbon oxides i s drastically decreased. It should be added that these catalysts produced a small amount of benzaldehyde which was not formed on the unpromoted Sn02 catalyst. The selectivity of benzaldehyde at 723 K was 0.9% on Sn02-SiO and 2.3% on Sn02-P205. W03, Nb205, gbi03, BfOg. Ge02, V 05,Moo3, Ti02 and Ce02. also improve the selectivi y mo erate y. Among tiese, promoting effects of W03 and Ge02 are similar to those of Si02 and P2O5, that i s , these promoters increase the rate of styrene formation and decrease the rate of complete oxidation. However, the effect on benzaldehyde formation was quite different. The selectivity of benzaldehyde was 5.5% on Sn02-W03 and 0.4% on Sn02-GeOq. On the other hand, MOO , Nb20+. SbiOg. V205, and Ce02 seem to have another kind of promoting efject. he a d tion of these oxides increases i n both rates of styrene formation and complete oxidation. These promoters also accelerated the formation of benzaldehyde: the selectivity was 7.5% on Sn02-Nb20 and 3.5% on Sn02-V205. B O3 and Ti02 improve the selectivity through tge suppression of complete oxfdation. Benzaldehyde formation also was not significant: the selectivity was less than 0 . 4 % on both catalysts. Although the addition of ZrO only slightly improves the catalytic property of Sn02, the differences In the selectivities of styrene and benzaldehyde were noticeable, and, further, the difference in CO2/CO ratio was significant. The additions of Ma,ZnO, PbO, BaO, CuO and Bi20 increase the yield of carbon oxides, leading to the decrease in the selectfvity of styrene. The selectivity of benzaldehyde was moderately high on Sn02-Zn0 ( 1 . 2 % ) and Sn02-Cu0 (1.8%), but negligible on the other promoted Sn02 catalysts.
ONb
oMo V
0
I
I
I
Y
-5 0 5 10 Difference i n reaction rate / mmol g h-1 Figure 4 . Comparison of overall catalytic performance predicted by INCAP with the difference in reaction rate measured experimentally at 698 K.
497
COMPARISON WITn INCAP The INCAP estimates the overall performance of catalyst component by integrating the API (Activity Pattern index) on the basis of catalytic functions f o r the target reaction and f o r the plausible side reactions, as mentioned above. The integration procedure i s , roughly speaking, similar to subtraction o f the catalytic activity f o r side reactions from that for the target reaction. Thus, the integrated API, i.e., the overall catalytic performance estimated by the INCAP, corresponds to the difference between the rate of styrene formation and that of side reactions. In Figure 4 . the integrated API was plotted against the difference in the reaction rate experimentally obtained at 698K. As shown, a good agreement was obtained between the estimated performance and the experimental one except Bi, Nb, Mo and V. The deviation i s especially significant on Vand Mo-promoted Sn02 catalysts, which are very active for both target and side reactions. Probably, slight errors in API's may result in the large deviation in the plot shown in Figure 4 . Thus, the integrated API, which i s a sophisticated function of both activity and selectivity, does not seem to be an adequate index to the overall performance of catalyst, especially when API's are not necessary reliable. This problem may be overcome by introducing new indexes to the catalytic activity and selectivity which are defined as follows: Activity
=
Selectivity
=
XmAPI(target reaction)
+
XmAPI(side reactions)
XmAPI(target reaction) CmAPI(target reaction) + CmAPI(side reactions)
When the activity index thus calculated i s plotted against the logarithm of reaction rate experimentally measured at 698 K, as shown in Figure 5 ,
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h
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0,
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0
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Reaction Rate / mnol g-'
h-'
Figure 5 . Comparison of activity index estimated by the INCAP with experimentally measured activity.
0
20
40
60
80
100
Selectivity / %
Figure 6. Comparison of selectivity index estimated by the INCAP with experimentally measured selectivity.
498
a fairly good correlation i s obtained with exceptions of Ti- and Cepromoted Sn02 catalysts. The selectivity index also agrees fairly well with the experimentally measured selectivity at the conversion of around 20%, as shown in Figure 6 . In this case, Zr-, B i - and V-promoted Sn02 catalysts form the exceptions. These correlation and agreement strongly demonstrate the feasibility of expert systems approabh to the catalyst design. CONCLUSION AND FUTURE PROSPECT
A good agreement between the catalytic performance estimated by the INCAP with experimentally measured performance demonstrates the feasibility of expert systems approach to the design of catalysts. However, the exceptions such as Ti and Ce in Figure 5 and Zr, Bi, and V in Figure 6 would suggest that still some room f o r improvement of the INCAP is left. Further experimental verification of the INCAP which i s in progress suggests that the disagreement is partly due to the lack of API data f o r some catalyst components and the poor knowledge about the synergistic effect especially on the generation of acid sites over mixed oxide catalysts. The latter problem is going to be overcome by using a neural network [ l o ] . Another problem of the INCAP i s that the reaction mechanism of the target reaction or a similar reaction has to be encoded in the knowledge base in advance. For this problem, an expert system for the creation of reaction mechanism is being developed 1111. These subsystems will be included in an advanced catalyst design system, ARCADE (Btificial Intelligence System for Catalyst Design) 161. REFERENCES
R. Banares-Alcantara, A.W. Westerberg, E.I. KO and M.D. Rychener, Comput. Chem. Eng.. 11 (1987) 265; 12 (1988) 923. X.D. Hu, H.C. Foley and A.B. Stiles, Ind. Eng. Chem. Res., 30 (1991) 1419.
T. Hattori, S. Kito and Y. Murakami, Chem. Lett., (1988) 1269; S. Kito, T. Hattori and Y. Murakami, Appl. Catal., 48 (1989) 107. S . Kito, T. Hattori and Y. Murakami. Chem. Eng. Sci.. 45 (1990) 2661: S. Kito, T. Hattori and Y. Murakami, Catalytic Science and Technology, Kodansha, Tokyo/VCH, Weinheim, 1991, p.285. 5 T. Hattori, H. Niwa. A. Satsuma, S . Kito and Y. Murakami, Appl. Catal., 50 (1989) L11.
6 7 8 9 10 11
T. Hattori and S. Kito. Catal. Today,
10 (1991) 165. T . Tagawa, T. Hattori and Y. Murakami, J. Catal.. 75 (1982) 5 6 , 6 6 . Y . Murakami, K. Iwayama, H. Uchida, T. Hattori and T. Tagawa, J. Catal., 71 (1981) 257; Appl. Catal., 2 (1982) 67. T. Tagawa, S . Kataoka, T. Hattori and Y. Murakami, Appl. Catal., 4 (1982) 1 ; T. Hattori, S. Itoh. T. Tagawa and Y. Murakami. "Preparation of Catalysts IV." Elsevier. Amsterdam, ( 1 9 8 7 ) , p.113. S. Kito, T . Hattori and Y. Murakami, Anal. Sci., in press: S. Kito, T.
Hattori and Y. Murakami, Ind. Eng. Chcm. Res., in press. T. Hattori, S . Kito, Y. Murakami and S . Yoneda. to be presented in 2nd International Symposium on Computer Applications to Materials and Molecular Science and Technology, Sept. 1992, Yokohama, Japan.
499
DISCUSSION
Q: Y. Sun (China) 1) How do you deal with the conflicts between the intermediate results from the different knowledge bases ? 2) Have you taken into consideration the activity, selectivity and deactivation?
A: T. Hattori 1) In the INCAP, the catalyst design problem is decomposed into five subproblems, and the INCAP consists of five subsystems corresponding to individual subproblems. However, since the subsystems have different tasks and characteristics from each other, it seems impossible that the intermediate conclusions from the different subsystems come into conflict with each other. In each of subsystems, the following conflict resolution strategies, which are frequently used in the conventional knowledge-based systems, are used: the first-matched rule is preferred and the most recently used rule is preferred. 2) We have adapted the integrated API as a measure of catalytic performance including the activity and the selectivity. But, as shown in the text, it does not seem to be an adequate index, when the API contains some errors. Then, we have introduced new indexes, shown in Figures 5 and 6, representing the activity and the selectivity separately. In an advanced system, overall performance of catalyst will be presented by integrating both indexes, but at present we d o not intend to include the deactivation.
Q: A. T. Bell (USA)
A critical input to your algorithm is the MI. This index requires knowledge of the relative activities of a series of catalysts for some test reactions. Since the activation energies of different catalysts, for the same reaction, can be different, it is possible that the API for a given catalyst depends on the temperature. How does your program take this into account'?
A: T. Hattori It is highly recommended to develop or to start with a small expert system possessing a realistic target, but not a large system applicable for general purpose. So,we have developed the first version of INCAP presented in this paper by taking the oxidative dehydrogenation of ethylbenzene as an example. The activity data for the API have been taken from the results of reactions in the temperature range of 600-800 K which is close to that of the present reaction. The second version for the selection of multicomponent catalysts is also based on the assumption of the same temperature range. Therefore, it is unnecessary in these versions to consider the question you raised. However, account by using an evaluation factor (Ed which expresses the adaptability of AF'I to the problem reaction. The Ef , which takes numerical values from ten to one representing the adaptability, should be modified according to the temperature difference.
Q: M. M. Bhasin (USA) I commend you on very good correlations and predictions of new compositions. My question relates to the input activity data you used. Would you tell us for what reactions the activity data was used and was it based on reaction rates per gram, per surface area, i.e., turnover number or some other basis ?
A: T. Hattori The MI in the first version of INCAP includes the activities for the complete oxidations and partial oxidations of propene, butene and toluene, and thereafter some activity data for the oxidations of ethene, methane, hydrogen, and so on are added. The heat of formation and the electroconductivity of oxide are also included as a measure of the activity for complete oxidation, though small values are allocated to the evaluation factors (Ed for these mentally
500 as well as the partial charge of oxygen in oxide and the electronegativity of cation as a measure of the acid and base properties with small q. The activity data used for the API were, in most cases, based on the rate per surface area, but some activity data per weight are also used, because we prefer to include more API data. It should be noted that the API is not quantitative parameter, but only semi-quantitative one. Q: V. Ponec (The Nctherlands) Would you specify from which reactions a rather high activity index for SiO, has been derived, as well as the high selectivity of this catalyst ?
A: T. Hattori The API includes the acid and base properties of oxides as well as the activity data for some catalytic reactions. The INCAP reasons that the promoter should possess or synergistically generate the acidic property of medium strength based on acid-base mechanism for the target reaction (7) but not stron redox activity, strong acidic property and partial oxidation activity. The high activity of nOp-SiOp catalyst is derived from the acidic property of SiOa and the high selectivity from the poor redox property and the poor partial oxidation activity of S i O F
l
Q: J. M. Thomas (United Kingdom) In multicomponent catalysts, it is known that the overall activity is greater than the sum of the activities of the individual components: in other words there is synergy. How does your expert system approach cope with this synergy ? I thought I saw in your final slide reference to neural networks.
A: T. Hattori In the oxidative dehydrogenation of ethylbenzene on promoted SnO, catalysts which is taken in the present study as an example, the most important synergy is considered to be the acid strength of mixed oxide catalysts. The direct approach to this problem has not been taken in the INCAP, but the problem is dealt with indirectly: The electronegativity of promoter cation is used as a measure of the acid strength of synergistically generated acid sites, because the acid strength of mixed oxides is reported to increase with the averaged electronegativity of cations of constituent oxides [l]. We have found that the acid strength of binary oxides can be estimated by using a neural network. Or, in other words, the acid strength can be represented as a complicated function, which is given as a network pattern in a computer, of several physical/chemical properties of constituent oxides. In one of the test-versions of the INCAP, the network pattern thus obtained is used as a subsystem for this problem. K. Shibata et al., Bull. Chem. Soc., Japan, 46, 2895 (1973) [l] [2] IEC. Research, 31,979 (1991)
Q: F. Kapteijn (The Netherlands) It is a great challenge to design catalysts on the basis of an expert system. What are your expectations in this respect? How far d o you think you can come to predict a catalyst composition for a given reaction by this approach.
A: T. Hattori The catalyst design is a big and complicated problem which requires a lot of experimental works for catalyst preparation, characterization, testing and so on. The present study may suggest that the expert system will be useful to reduce the number of repetitions in the trial-and-error processes of experimental works. We expect that in near future the catalysts can be designed more efficiently by the aid of computer system. As for the prediction of catalyst composition, it is difficult to say how far it is, because it differs from case to case. The most essential knowledge for the prediction of the composition
501
is that of the synergistic effect. Then the synergistic effect is less significant or less complicated, the present approach may be useful to some extent. However, another approach will be necessary for the prediction of catalyst composition for general cases. A neural network will be one of the candidates for such purpose. Q: K. Zheng (United Kingdom) It seems to me that your expert system only considers the bulk composition of certain elements and compounds. How could you take into account the fact that a small amount of surface segregation could have a drastic effect on the activity and selectivity of catalysts ?
A: T. Hattori The performance of catalyst depends on many factors such as the catalyst component, the composition, the microscopic and macroscopic structure, and so on. In other words, the total problem of catalyst design consists of many subproblems each of which seems difficult to solve. The first subproblem to be solved is obviously the selection of catalyst components. The present study aims to solve the first subproblem by the expert systems approach, and this aim has been achieved without taking the other subproblems into account. The scatters in the correlations shown in Figures 5 and 6 may be due to the other factors, possibly including the surface segregation, and these subproblems might be the subjects for future studies.
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Guni, L et al. (Editors), New Frontiers in CadysiE Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights resewed
METAL OXIDE VAPOUR SYNTHESIS (MOVS):A NEW PREPARATIVE METHOD FOR HETEROGENEOUS METAL OXIDE CATALYTIC SYSTEMS E. C. Alyea, R F. Brown, R J. Fisher and K D. L. Smith
Guelph-Waterloo Centre for Graduate Work in Chemistry (GWC)2, Guelph Campus, Department of Chemistry and Biochemistry, University of Guelph, N1G 2W1 Guelph, Ontario, Canada
Abstract Metal Oxide Vapour Synthesis (MOVS) has been developed as a new methodology for the preparation of unsupported and supported heterogeneous metal oxide systems. The production of single and mixed metal oxide systems by the cocondensation of volatile metal oxides and a suitable solvent, and if desired, subsequent deposition on a support, is described. Other metal oxides have been incorporated into the preparative procedure by the addition of alkoxides and other soluble metal salts to the condensate, with subsequent conversion to metal oxide forms by calcination. To illustrate the potential of the MOVS methodology for catalyst design, examples are cited of the unique nature and high efficiency in catalytic conversions of molybdenum oxide systems derived from a-Mo03 by evaporation and cocondensation with methanol.
Introduction Perusal of the vast literature describing preparative methods for heterogeneous metal oxide catalytic systems indicates the dominance of traditional methods such as impregnation and precipitation [l]. Deposition of soluble metal salts on a support, with subsequent drying and calcination (usually above 5OO0C),gives the heterogeneous metal oxide catalyst. The ease of incorporating mixed metal oxide and promoter species is a distinct advantage of such classical methods. Modification of the traditional procedures to include organometallic and carbonyl precursors can give highly active catalysts for many organic transformations. An elegant example of such catalyst design for specific activity, as well as spectroscopic characterization of the active sites, is the anchoring of tetra-n-ally1 dimolybdenum on silica and alumina by Iwasawa and co-workers [2]. The continuing need for metal oxide catalytic systems of improved dispersion, higher activity and greater selectivity has lead to our development of a new methodology of wide aplicability, Metal Oxide Vapour Synthesis (MOVS).
504 Our MOVS preparative technique utilizes and extends Metal Vapour Synthesis (MVS) technology, largely developed in the 1970’s for the synthesis of novel organometallic compounds [3]. Indeed, MVS itself can be used to obtain zerovalent metal atom slurries which can be dispersed on solid supports and into zeolite frameworks in one .step procedures [4, 5, 61. The enormous potential of MVS for the preparation of new materials and catalytic systems has already been suggested [7]. In initiating studies of heterogeneous catalysts, we reported earlier that molybdenum atoms dispersed on y-alumina by MVS could be oxidized to a supported oxide form [S]; the generality of the method for converting MVS-deposited metal atoms into heterogeneous metal oxide catalytic systems on any suitable support is outlined in a patent [9]. The present paper describes the general MOVS methodology for the cocondensation of volatile metal oxides with an organic solvent and the subsequent deposition of unsupported and supported metal oxide catalytic systems [9]. A preliminary communication outlining the method highlighted the high efficiency of molybdenum oxide catalysts obtained by MOVS for the partial oxidation of methanol [lo].
Experimental ‘TORROVAP’ R e a c t a- purchased from Torrovap Industries Inc., Markham, Ontario. Figure 1 illustrates the 5L rotary reaction vessel of the Torrovap system which encases all the major components involved with a description of the synthetic MOVS method. The reaction vessel encloses a dual resistive evaporation source, heated solvent inlet with shower head, and a tube for product extraction to a Schlenk flask. A rotary pump is used for initial pumpdow and system roughing to torr. An oil diffusion pump increases the vacuum to 10- torr and is protected by a nitrogen trap (also called a cryostatic pump) which prevents the backstreaming of pump oil as well as prevents its contamination by ligand and solvent chemicals used in the synthesis process. In MVS, the vacuum (lom6torr) is necessary for operation of the electron guns as well as preventing metal atom agglomeration, oxide formation and sometimes even product decomposition. Furthermore, metal atoms can be pyrophoric in the presence of oxygen. Thus, vacuum is a crucial part of the MVS methodology. However, it should be stressed that in the MOVS procedure, the primary role of a vacuum is to reduce the boiling or sublimation point of the metal oxides used. Thus, a vacuum increases the efficiency of the resistive furnace, which has a practical limit of 1500°C, to generate vapours from metal oxides of low volatility, such as Nb205 (mp 1485’C, bp 2927OC) or WO (mp 1473OC, bp lS50°C)[ll]. More volatile metal oxides, such as Moo3 (mp 795dC, bp
7
1257OC) or V205 (mp. 69OoC, bp 2052°C)[9], do not require high vacuum. Our evaporation source generally consists of a GTE Products C o p . integral tungsten/alumina crucible, of which several sizes are available.
505
A- Rotary Vacuum Seal D- Quartz Microbalance
G- Liquid N2 Bath
B- Solvent Inlet Head E- Evaporation Source H- 5L Rotary Reaction Vessel
C- Button Heater F- Schlenk Tube
Figure 1. The rotary reaction vessel of the Torrovap system.
-1vst
svntheses
A) Unsupported MOVS Catalysts Vaporization is usually performed under a vacuum of ca. 3 ~ 1 0 -torr. ~ The filament current of the resistive furnace is slowly increased to provide the temperature necessary for vaporization. Under typical operating conditions of the Torrovap, MOO requires 18-20 amps, V205 requires 20-25 amps, while Nb205 or W 0 3 require fu I scale of about 30 amps. It should be remembered that filament current is a function of the composition, length and thickness of the resistive filament, thus the maximum current is variable. Throughout vaporization, a continual flow of solvent is cocondensed with the metal oxide vapours on the sides of the rotating reactor vessel, which is immersed in
?
liquid nitrogen. Exact solvent volumes are experimentally determined, and vary widely with the choice of solvent and metal oxide. For example, 1 gram of Moo3 requires ca. 100 mL methanol to prevent spontaneous precipitation upon warming. Although less methanol can be used for the production of unsupported catalysts, it usually results in diminished purity of the final product. Purity can also be affected if the rate of metal oxide vaporization is too great (if the mean free path of generated vapour molecules is less than the distance from the furnace to the cocondensation surface then moleculemolecule interactions lead to the formation of bulk metal oxide). For the MOVS cocondensation of V 0 with methanol, volumes can be greatly reduced as the final 2 5 product still remains in solution. After the vaporization is complete, the cocondensate is warmed, resulting in the formation of a product solution which is easily removed from the Torrovap system by means of a Schlenk tube. After removal from the system, the volume of the cocondensate solution is typically reduced by vacuum evaporation before collecting the product, which can be converted to final oxide form by either thermal or photochemical activation procedures.
Example: Unsupported Molybdenum Oxide Catalyst, Photoactivated Form
Moo3 vapours were cocondensed with methanol in a ratio of 1 gram Mo03:100 mL of methanol. The cocondensate product solution was allowed to warm to room
temperature and precipitation of M O ~ O ~ ( O C H ~ ) ~ ’ ~(I)C occurred H ~ O H over 24 hours; the white product was recovered after vacuum evaporation to insure complete formation of (I). BET surface area of (I) was found to be 7.0 m2/g. The recovered product was lightly ground to produce a uniform powder and then exposed to air and fluorescent light for up to 3 months. During this time (I) turned a light medium blue in colour. This procedure could be accomplished within 24 hours by exposure of (I) to ultraviolet radiation with a Gaussian distribution of less than 350 nm. The photochemically altered molybdenum oxide matrix was converted to its final activated form (a blue amorphous powder) by heating to 280°C in the reactor system under a flow of air.
B) Supported MOVS Catalysts Supported MOVS catalysts can be readily obtained by introducing a desired support to the MOVS cocondensate product solution extracted from the Torrovap system. In general, once the support has been added to the MOVS cocondensate product solution, the temperature of this solution can be maintained at some fixed value or can be gradually allowed to increase to room temperature at a desired rate. Adjustment of the temperature of the MOVS cocondensate product solution can effect the dispersion properties of the final supported catalysts and therefore, should be experimentally optimized for each case. For Mo03/CH30H cocondensate product
507 solutions, a gradual increase from 200K to room temperature over a period of three to four hours was found to lead to satisfactory results. The concentration of the product solution can also be a very important factor. For Mo03/CH30H cocondensate product solutions, a concentration of greater than 1 g Mo03/100 mL CH30H results in the rapid formation of (I) upon warming, which drastically hinders or prevents the deposition of (I) to supports. For V205/CH30H cocondensate product solutions, introduction of support leads to rapid decolorization of the yellow solution, indicating that deposition is rapid. Thus, variables such as equivalent V205 concentration of the solution, the temperature of the solution during the deposition, the desired V205 loading level of the final catalyst, and the amount of support to be added, all play an important role in determining the surface properties of the final catalyst. Example: V205 supported on
y-A1203,
Ti02 (anatase), MgO and H-ZSMJ
V205 vapours were cocondensed with methanol in a ratio of lg of V205 5 0 mL of methanol. After the cocondensate product solution has been warmed and extracted from the Torrovap system, the desired amount of support is added. (i.e. for 10% loading, log of support would be added to the cocondensate product solution in which lg of V205 had been cocondensed). This mixture was stirred by a magnetic stirrer. If powders are desirable, a magnetic stirrer works well, however, if the mesh size of the support is to be maintained, then rotation of the flask on a rotary evaporator without vacuum works well. Typically, these solutions are stirred for 24 hours, then the impregnated supports are recovered after concentration by vacuum evaporation, and converted to their final activated oxide form by thermal calcination for four hours at 4OOOC.
C) Modification of MOVS Catalysts Incorporation of a second metal oxide component can be readily accomplished by co-evaporation and cocondensation as the Torrovap system is equipped with a dual resistive evaporation source. Thus, mixed metal oxide systems, such as Mo03/W03/CH30H or Mo03/V205/CH30H are readily obtained by placing the appropriate ratio of metal oxides into the two crucibles. These mixed metal oxide cocondensations work well as long as both components have appreciable vapour chemistry. Syntheses of this type of cocondensation follow the methods as outlined in A) and B). Metal oxides which do not have appreciable vapour chemistry can be incorporated by the addition of soluble metal alkoxides and other soluble metal salts to the MOVS cocondensate product solution. For Mo03/CH30H systems, the addition of these soluble complexes are best performed before the precipitation of (I) occurs.
508 Example: Addition of Ti(OC3H7), to Mo03/CH30H cocondensate Moo3 was cocondensed with methanol as described in A). After the cocondensate product solution was extracted from the Torrovap system, and placed in an ethanovdry ice cold bath, titanium isopropoxide was added dropwise to the product solution via a syringe. During this slow dropwise addition, the product solution was stirred resulting in the slow precipitation of a pale yellow solid. After 24 hours, the pale yellow powder was recovered after removal of the solvent by vacuum evaporation. The powder was thermally activated at 4OO0C for 4 hours.
Results and Discussion To our knowledge there are no previous reports of the preparation of heterogeneous metal oxide catalytic systems by the cocondensation of a volatile metal oxide with a suitable solvent, followed by deposition on a solid support. Indeed, there are few reports of metal oxide-solvent cocondensations [3]; a pioneering study by Timms and Cook [12] involved MOO and W03 and a few common solvents such as acetone and methanol. Following Mc arron and co-worker's formulation of a product derived from MOO '2H 0 and methanol as M o ~ O ~ ( O C H ~ ) ~ ' ~ C(1)[13], H ~ O DeKock H and 3 2 McAfee [ 141 assigned their Mo03-methanol cocondensate product as (I). While all of these earlier workers realized, through thermal and photochemical decomposition studies, that (I) was a potential model for the partial oxidation of methanol to formaldehyde, no attempt to support (I) and test its catalytic activity was reported. The theoretical prediction by Allison and Goddard [ 151 that "dual dioxo dimolybdenum sites", assumed to exist in (I) [ 151, should greatly enhance catalytic activity for oxidative dehydrogenation, together with the practical demonstration by Iwasawa and co-workers of the high activity of their dimolybdenum catalysts [2] prompted us to begin our MOVS studies by supporting (I) on 7-alumina and testing its catalytic performance for the partial oxidation of methanol. The thermal activation at 55OoC of this deposited precursor gave a supported catalyst with an improved performance compared to a commercial 10%Mo03/7-A1203 catalyst [8]. Subsequently we found that either unsupported (I) or 7-alumina supported (I), if first photoactivated, yielded excellent catalysts with 100% conversion and 96% selectivity to formaldehyde at temperatures B. 15OoC lower than a traditional standard, Fe2(Mo04)3/Mo03 [lo]. This success prompted us to prepare other supported catalysts by the MOVS methodology. Synthetic details for the unsupported, photoactivated MOVS molybdenum oxide catalyst are provided in the Experimental. The use of a large excess of methanol ensures a slow deposition of (I) over several hours and consequently higher purity. Our recent characterization studies [ 161, which complement the earlier results of DeKock and McAfee [15] and McCarron and co-workers [14], confirms the presence of dual dioxo dimolybdenum sites and methoxy bridges in (I). After photoactivation and heating
2
509
to 280°C, the product is a light blue powder which has been shown by XRD to contain a-Mo03 and an amorphous component. Particle sizes were estimated from SEM micrographs of 200K magnification to be less than 50 nm [lo, 161. Afte catalytic testing for methanol conversion the BET surface area was found to be 25 m /g and the pore radii range was 16 to 62 angstroms. Retarding the precipitation of (I) by employing a 200-250 fold excess of methanol in the cocondensation of Moo3 gives a solution upon warming to room temperature. In addition to the deposition of (I) on alumina as already described [8, 161, we have used a variety of supports for (I), such as H-ZSMJ and other metal oxides. Preliminary testing results for some of these MOVS molybdenum oxide catalysts on A1203, Ti0 (anatase) and Sn02, activated photochemically and followed by heating to 280'6 indicate high efficiency for the partial oxidation of ethanol to acetaldehyde [17]. The dispersion obtained for a 10% loading (equivalent Moo3 by AA) on Sn02 is shown in the micrograph presented in Figure 2a). In anticipation that other highly dispersed heterogeneous metal oxide catalysts could be obtained by the MOVS methodology, we have cocondensed volatile metal oxides such as WO , Nb205, V20 FeO, etc. [ 111 with suitable solvents for subsequent 3 deposition on various supports. &e specific example of V205 is described in the Experimental, and its dispersion on T i 0 (anatase) is shown in the micrograph 2 presented in Figure 2b). Furthermore, r u e d cocondensations involving two metal oxides are readily performed in the Torrovap apparatus. The micrograph of a Mo03/W03 catalyst obtained by deposition from methanol is shown in Figure 2c). Obviously there are a multitude of possibilties to explore utilizing the MOVS methodology, using characterization and testing results as guidelines of which are most worthwhile. Since many metal oxides are not volatile enough (in practice, if the m.p. is greater than a.1500°C) or decompose upon heating [ l l ] their incorporation into a multicomponent catalyst cannot be accomplished by a direct MOVS procedure. However, adding a soluble salt or organometallic compound of the desired metal oxide to the MOVS cocondensate of another metal oxide can give, after deposition and calcination, a mixed metal oxide catalytic systen. Alternatively, a promoter or other metal oxide can be added to a catalyst derived by MOVS by the usual wet impregnation method. The specific example outlined in the Experimental section is the preparation of a Mo03/Ti0 catalyst by the addition of Ti(OPri)4 to the cocondensate of Moo3 and methanol. "he micrograph of the derived catalyst is shown in Figure 2d). The amorphous nature of the product, and/or the small particle sizes produced by the MOVS method, is indicated by XRD. Indeed, XRD generally shows similar results for all the MOVS catalytic systems, with only d-spacings due to the supports being observed. Further details of characterization studies (BET, XRD, FI'IR, AA, SEM) will be presented at the conference. Examples of the high efficiency of the MOVS catalytic systems for some selective oxidations will also be described as illustrations of the potential of the MOVS methodology for the preparation of heterogeneous metal oxide systems with excellent catalytic activity.
i
51 0
F?gure 2. Scanning electron micrographs of MOVS catalysts (clockwise from upper left) a) 10% equivalent Moo3 loading on Sn02; 30K, dotted scale = 1.0 pm b) V205 on TiOZ (anatase); 20K, dotted scale= 1.5 )Lm c) unsupported muted MOO /W03; 20K, dotted scale= 1.5 ,.rn d) Mo03D'i02 made by TitOP?)4 addition; 20K, dotted scale= 1.5 pm
Acknowledgements Initial studies were funded by an Imperial Oil Ltd. University Research Grant and a NSERC Canada operating grant. Current investigations are funded by a NSERC Canada strategic grant.
51 1
References 1. B. Delmon, P. Grange, P.A. Jacobs and G. Poncelot (eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1987. Vol IV, Studies in Surface Science and Catalysis, 2.(a) Y. Iwasawa, H. Kuba, M. Yamagishi, and S. Ogasawara, Chem. Lett., (1980) 1165. (b) Y. Iwasawa, S. Ogasawara, Y. Sat0 and H. Kuroda in Proceedings of the Climax Fourth International Conference on the Chemistry and Uses of Molybdenum, H.F. Barry and P.C.H. Mitchell (eds.), Climax Molybdenum Company, Ann Arbor, Michigan, 1982, p. 283. 3. J.R. Blackborow and D. Young, "Metal Vapour Synthesis in Organometallic Chemistry", Springer-Verlag, New York, 1979. 4. G.A. Ozin, M.P. Andrews, L.F. Nazar, H.X.Huber and C.G. Francis, Coord. Chem. Rev., (1983) 203. 5. M.P. Andrews in "Experimental Organometallic Chemistry", A.L. Wayda and M.Y. Darensbourg, M.Y. (eds), ACS Symposium Series, Chicago, 1985, ACS Press, Washington D.C., 1987. 6. K.J. Klabunde, J. Mol. Catal., 21, (1983) 57. 7. G.A. Ozin, Chem. Tech., 15,(1985) 488. 8. E.C. Alyea, K.F. Brown and K.J. Fisher, Proceedings of the 11th Canadian Symposium on Catalysis, Halifax, July 15-18, 1990, p.364. 9. E.C. Alyea, K.F. Brown and K.J. Fisher, U.S. Patent 5 047 379 (1991). (1990) L11. 10. E.C. Alyea, K.F. Brown and K.J. Fisher, J. Mol. Catal., 11. G.V. Samsonov, (ed),'The Oxide Handbook", 2nd edition, Plenum Press, New York, 1982, p. 51-57. 12. P.L. Timms and N.D. Cook, J. Chem. SOC.Dalton Trans., (1983) 239. 13. E.M. McCarron, R.H. Staley and A.W. Sleight, Inorg. Chem., (1984) 1043. 14. C.W. DeKock and L.V. McAfee, Inorg. Chem., 24, (1985) 4293. 15. J.N. Allison and W.A. Goddard 111, J. Catal., e2, (1985) 127. 16. E.C. Alyea and KF. Brown, "Proceedings of the 12th Canadian Symposium on Catalysis", Banff, May 1992, Elsevier, Amsterdam, 1992. 17. E.C. Alyea, K.F. Brown, L. Durham and I. Svazic, Proceedings of the 12th Canadian Symposium on Catalysis, May 1992, Elsevier, Amsterdam, 1992.
a,
a,
u,
a,
51 2
DISCUSSION Q: J. M. Thomas (United Kingdom) Molybdenum oxide is very stable in an electron beam, and a reat deal is known about the structure of the stoichiometric Moo3 and the sheer phases (LfOnO~n-l, Mon03n-a.-, n=10-20) of this oxide. Even material that appears X-ray amorphous frequently turns out, not unexpectedly, to possess an ordered structure at the local level. High-resolution electron microscopy ought to be used to characterize your fascinating new catalyst. You would, 1 imagine, soon find but if, locally, there is a-Moo3 or any other comer showing (or edgeshowing) Moo6 present.
A: E. C. Alyea We have initially examined the surface morphology of our MOVS catalysts using scanning electron microscopy, which was more readily available. Now that SEM shows that homogeneous dispersions of fine particles are commonly achieved for the various types of supported catalysts (the original 10 division, 18 mm dotted scales in Figure 2 were omitted during the 50% photoreduction; i.e. the dotted scales are 9 mm long in the published SEM Figure), we certainly plan to use "EM for higher resolution studies. Q: G. B. Fisher (USA) What is the range of article sizes you can make for MOO, for other oxides, and for W the particle size typically depend on parameters such as base metals, if possible? ~ O does evaporation rates of (i.e. oxygen) pressure or vacuum, solvent choice, time in solvent, etc. ? A: E. C. Alyea The particle sizes as estimated by SEM are reproducibly found to be in the 100 nm ran e for all of the MOVS catalysts. An unsu ported photoactivated sample derived from a-Mo 3 had particles in the 15-50 nm range. A e optimization of experimental conditions for each catalyst type, as the solvent choice, concentration and deposition rate, is undergoing further development.
8
Q: J. W. Geus (The Netherlands) My question is concerned with the application of the oxide from the methanol solution/suspension onto suitable catalyst supports. The question therefore is dealing with the solution/suspension resulting from the vapor deposition usually after concentration by evaporation of part of the liquid. To prevent deposition of the active precursor mostly at the external edge of the bodies of the support, pore-volume impregnation is to be preferred. To achieve a significant loading of the support, a fairly high concentration of the active precursor in the solution/suspension is usually required with the pore volumes of most supports. During concentration by evaporation of methanol formation of colloidal particles of the active precursor generally will result. In view of the low diffusitivies of colloidal particles and of filtration effects a non-uniform distribution of the active precursor throughout the support may result. Have you data about the penetration of active precursors from concentrated solutions/suspensions into layers of usual support, such as, silica or alumina? A: E. C. Alyea As described in the experimental section, many variables are involved and must be controlled in order to achieve a uniform dispersion of the active precursor throughout the support. In the case of a-Mo03 cocondensation with MeOH, an excess of the solvent allows a solution to be maintained for 24 hours or more; the presence of MoOg.3MeOH is suggested by the +66 ppm resonance observed by g5M0 NMR spectroscopy. It ap ears that the insoluble polymeric precursor, Mo205(0CH3) .2CHqOH (I may ),deposit irectly on the supports rather than by prior formation of colloicfal particles.
B
51 3 Although our preliminary characterization studies (see ref. 16, now published in Vol. 73 of Studies in Surface Science and Catalysis, p. 279), for further details of the catalysts derived from a-MOO, indicate that good distributions of amorphous metal oxide layers with high surface areas may be achieved by MOVS, other surface studies (as "EM and PES) are planned to gain more information on the dispersion and penetration properties of these new catalysts.
Q: Y.Iwasawa (Japan) You showed a nice crystallographic picture of a molybdenum dimer having methoxybridging. Do you suggest that an active intermediate at the steady state of methanol oxidation is also the bridging-methoxy species ? A: E. C. Alyea X-ray analysis of the crystalline compound obtained during the epoxidation of cyclohexene by the precursor (I)verified the presence of two methoxy-bridges. This result, coupled with the high efficiency of methanol oxidation (ref. lo), does indeed suggest the presence of "dual dioxo-molybdenum active sites", whose importance was theoretically predicted (ref. 15).
Q: J. S. Yo0 (USA) 1) How does the catalytic activity differ between your MOVS catalyst and the classical vapor deposited counter part prepared from volatile precursor such as MoO&!12 and MoCl3 for the oxidation reactions under normal pressure to produces aldehydc and epoxyde ? 2) Please tell me what was the main chemical cause for your observed remarkable catalytic activity beside the well dispersed particle size effect ? A: E. C. Alyea We have not yet made a direct comparison of catalytic activity between our MOVS catalyst derived from a-MOO, and catalysts obtained from more volatile molybdenum precursors. Our initial comparison studies (8, 10) showed that either unsupported (I) or yalumina supported (I)yielded excellent catalysts with 100% conversion and 96% selectivity for methanol conversion to formaldehyde at temperatures ca 150 % lower than the commercial catalyst, Fe.#o04)3/Mo0 The unique dimeric active sites, w%ch are maintained under catalytic conditions (ref. 16) undoubtedly augment the activity due to the high dispersion and sufface areas produced by MOVS.
Q: M. M. Bhasin (USA) Would you clarify if the much higher activity you observed for MOVS synthesized materials is based on measured surface Mo concentration or per gm or surface area bases! Second question is regarding the stability of these materials under reaction conditions or heat alone since they may be metastable or unstable. A: E. C. Alyea Our activity calculations for methanol conversion were based on per gram of catalyst. Since typical loadings were in the 5 1 0 % range for the MOVS catalysts derived from aMoO3, the latter catalysts displayed activities that were 100-fold or greater than that observed for the standard catalyst. These catalysts are also showing high efficiencies for ethanol oxidation to acetaldehyde (ref. 17), olefin metathesis and oxidation, and syn gas conversions. The MOVS catalysts we have tested for alcohol oxidation are stable, after activation to 280 OC,during repeated cycles to 400 OC over several days.
514
0:M. A. Baltanas (Argentina) Could you please comment on: 1) the long-term stability of your highly dispersed supported oxides, and 2) the feasibility of scaling-up these preparates for later industrial application '1 A: E. C. Alyea As mentioned above, the supported MOVS catalysts generally have good stability during repeated testing. One exception was (I) deposited on SiOa which reverted to a-MOO upon heating to 400 OC; XRD also showed the presence of a-MOO, when unsupportcd was heated to 280 OC. Since a-MOO, is both inexpensive and readily volatile (even without a high vacuum), scalc-up to an industrial scale is both feasible and anticipated (ref. 9).
8)
Q: J. C. Volta (France) Can you control the orientation of your MOO, on the different support (TiOp y-AlgO3, etc,) by your MOVS technique? You do not give any electron diffraction studies on your materials '1 A: E. C. Alyea The MOVS methodology leads to the deposition of amorphous mctal oxide layers on the various supports. Whether any short-range order exists will be detcnnincd in future surface studics using TEM.
Q: H. L. Krauss (Germany) How did you count/characterize the active sites/coordinatively unsaturated sites in your catalysts? How many vacant sites do you claim to exist at the dimeric surface species espccially ? A: E. C. Alyea This paper, and our work to date on MOVS catalysts, has focused on the novel synthetic procedures for fabricating several varicties of hetcrogcneous metal oxide catalytic systems. Since initial catalytic testing for selective oxidations have proven to be very promising, further surface characterization studies are therefore warranted. Our first use of TPD/R cxpcriments for surface site identification and characterization has been reported [ 11 and wc plan to answer many "characterization" questions before the next Catalysis Congress. E. C. Alyea and V. Moravek, in Studies in Surface Science an Catalysis, Vol. 73, [l] p. 315,(1992)
Guczi, L. el ul. (Editors), New Frontiers in Caralysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 19 2 3 Elsevier Science Publishers B.V.All rights resewed
DESIGNING OF NEW CATALYSTS FOR OLEFIN METATHESIS ON THE BASE OF PHOTOREDUCED SILICA-MOLYBDENA V. B. Kazansky, B. N. Shelimov and K A. Vikulov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy.of Sciences, Moscow 117 913, Russia
Abstract
Cyclopropane (CP), methylcyclopropane (MCP) and cycloheptatriene (CHT) chemisorption on Mo/SiO catalysts photoreduced in CO was found to result in 2 a sharp increase of their specific activities in olefin metathesis (propene, 1-hexene, ethyl oleate). IR and UV-VIS spectroscopic studies revealed that promoting effect of CP, MCP and CHT treatment was associated with the formation of thermally stable carbene complexes Mo=CH2 and Mo=CH-CH3, arising from CP, MCP and CHT interaction with low coordinated Mo4+ ions in photoreduced Mo/Si02. IR data showed fast reversible transformation of
Mo=CH to Mo=CH-CH3, thus providing the first direct spectroscopic confir2 mation of a chain carbene mechanism of propene metathesis. Mo=C bond energy in Mo=CH2 (-435f25 kJ/mole) was determined from calorimetric measurements. Introduction
At present i t is generally accepted that catalytic olefin metathesis reaction proceeds via a chain mechanism, involving as key intermediates metal-carbenes and metallacyclobutane complexes (mainly, those of Mo, W, and Re) 1 1 1 . Most convincingly, participation of these intermediates in the metathesis has been demonstrated for homogeneous catalytic systems. Later this concept was extended on heterogeneous metathesis, and, in particular, on supported molybdena oxide catalysts 121. Perhaps, the most intriguing step in mechanism of the catalytic metathesis is the initiation, that is generation of carbene species further participating in the chain processes. It is generally believed that in real catalytic systems carbene species are most likely derived from a decomposition or transformations of organo-metallic compounds, formed by alkylation of transition metal salts with co-catalysts or promoters ( s u c h as alkyl tin o r alkyl aluminum compounds) [ l l , for example, LnM-CH2R --- >
51 6 L HM=CHR. However contribution of such a transformation to the n process of metallocomplex decomposition is very low (1-3 % ) .
-->
overall
Another possible way to produce metal-carbenes is a direct interaction of olefin molecules with coordinatively unsaturated transition metal ions, for instance, via an intramolecular 1,2-H atom transfer o r olefin n-complex 3 conversion to 9 -ally1 complex, followed by its transformation to a metallacyclobutane intermediate. However, these reactions should be regarded only as a hypothesis, because corresponding carbene complexes have been never isolated and characterized by any physico-chemical techniques. In the present work a new approach to designing of metathesis catalysts was employed, based on the combination of low-temperature selective photoreduction of silica-molybdena in carbon monoxide along with subsequent treatment with carbene-generating compounds (cyclopropane, methylcyclopropane, or cycloheptatriene). This way enables to obtain highly active catalysts and produce high concentrations of the surface carbene complexes, sufficient for their spectroscopic identification. Experimental Catalyst samples were prepared by routine impregnation of silica with an aqueous solution of ammonium paramolybdate, followed by calcination in air at 773 K and vacuum treatment at 1073 K. Mo-loadings were varied from 0.1 up to 2.5 wt %. Photoreduction was carried out by illumination with unfiltered light of a Hg-lamp in CO atmosphere at room temperature, followed by evacuation at 473 K. The average oxidation state of Mo ions was calculated from the amount of carbon dioxide formed during photoreduction and/or from the amount of oxygen consumed during reoxidation of photoreduced samples at 773 K. The next step of the activation procedure involved chemisorption of cyclopropane, methylcyclopropane or 1,3,5-cycloheptatriene at 293 K in order to convert photoreduced ions into active surface carbene complexes. To UV-VIS diffuse detect them two spectroscopic techniques were used. reflectance spectra were recorded using a Hitachi M 340 spectrophotometer and diffuse reflectance IR spectra were taken with a Perkin-Elmer 580B spectrophotometer, supplied with a home made diffuse reflectance attachment. The catalytic testing in propene metathesis at 293 K was done in a gas-circulating evacuable system with mass-spectroscopic analysis of the products. Liquid phase metathesis of 1-hexene and ethyl oleate was carried chromatography. out at 293-343 K with the product analysis by gas Calorimetric measurements were performed with a Calvet type DAK-1-1 automatic calorimeter. Results and discussion Mechanism of Mo
4+
/SiOz
photoreduction.
The first step in catalyst preparation procedure was photoreduction of
Mo6+/Si02 in CO a t 293 K. As discussed earlier ( 3 1 , photoreduction of ions can be described by the following scheme:
Mo
6+
51 7 0
*
0
0 +
co >
0
0 I
I
1:
0 I
II 4+ Mo + / \ 0 0 I I
co2
Mo4+ yields reached 70-80 % of the total molybdenum content in the sample 6+ 4+ photoreduction to Mo was (for 1 wt % Mo/Si02), and selectivity of Mo 96-97 X
.
W-VIS diffuse reflectance spectrum of a representative Mo
4+ /Si02 sample
is shown in Fig. la. Three absorption bands at 350, 630 and 780 nm in the transition region were assigned to Mo4+ ions in a distorted tetrahedral propene were strongly or trlgonal coordination [31. Ethylene and 4+ chemlsorbed on photoreduced Mo /SiOz at room temperatqe, apparently d-d
forming n-complexes with the Mo4+ ions, characterized by a broad intense absorption band at 580-600 nm (Fig. lb). Olefin desorption from the n-complexes was found to occur only at 573-623 K, Mo4+ ionz+in the original coordination state being regenerated. On the contrary, Mo /Si02 thermally reduced in H2 exhibited no distinct absorption bands in the d-d
transition
region and no noticeable changes were found in the spectra after olefin adsorption. Apparently, this can be explained by lower concentrations of 4+ ions in the thermally reduced catalysts coordinatively unsaturated Mo and/or by their coordinative nonhomogeneity.
UV-VIS diffuse Fig. 1 reflectance spectra of Mo/S102: (a) photoreduced in CO and outgassed at 423 K; (b) after C3H6 adsorption on sample (a); (c) after CP adsorption on sample (a) and evacuation (d) after at 623 K; oxidation of sample (c) by O2 at 293 K.
200
340 480 620 760 900
Wavelength, nm
51 8 Cyclopropane chemisorption
on
photoreduced
Mo
4+
/SiOz.
4+
Cyclopropane (CP) chemisorption on Mo ions occurred already at 293 K and resulted in opening the CP ring and in formation of Mo-cyclobutane complexes (Scheme 1). Their major part then decomposed to Mo=CH2 complexes and ethylene molecules, the latter being chemisorbed on the neighbor molybdenum ions to form n-complexes. Upon heating in vacuum at 623 K ethylene desorbed into the gas phase, while Mo=CH carbenes remained on the 2 surface, their concentration being almost unchanged. The minor part of Mo-cyclobutane complexes isomerized to propene n-complexes, which at 623 K decomposed to evolve propene. The proposed scheme is supported by analysis of the thermodesorption products, which mainly consisted of ethylene (-95%). The rest were unreacted CP and propene (4-5 X I . Scheme 1. 0 Mo /
0 I
0
0
I1 +
\'
293K>
Mo
\
/
0 I
\
0 I
0
CH2
0
MO / 0 I
0 I
1
293K
II
293K>
/
0
CH2
+
0 1
CII2
0
\
0
I
\' Mo &HZ / \ 0 0 I I
+ C3H6
\
0 I
I
0 II
\\ //
Mo 0 I
Mo
\
\\ // /
623K> -
CH-CH3
-> 623K
+
Mo /
0 I
Mo + /
\
C2H4
\
0
0
0
1
I
I
Scheme 1 was confirmed by spectroscopic studies. Fig.2 demonstrates diffuse reflectance IR spectra in the C-H stretching vibration region of a photoreduced silica-molybdena catalyst after CP chemisorption. A t 293 K a rather complicated spectrum (b) arose, which is a superposition of the spectra of several kinds of surface species (carbenes, Mo-cyclobutanes, n-complexes). However after vacuum heat treatment of the sample at elevated temperatures its IR spectrum was considerably simplified. Thus, after hegiing at 623 K it consisted of only two absorption bands at 3080 and 2945 cm (Fig. 2c), which could be ascribed to asymmetric and symmetric stretching C-H vibrations in Mo=CH2 carbene complex, respectively. Identification of
Mo=CH2 carbenes was
further
supported
by
the
interaction with ethylene-d After admission of heavy ethylene at 293 K the 4' absorption bands of Mo=CH2 complexes disappeared and in the C-D stretching region two new absorption bands
Mo=CD2 carbenes (Fig. 2d). This is explained by nonproductive ethylene metathesis reaction: Mo=CH + C D -> Mo=CD + CH =CD After removing heavy 2 2 4 [MO-~MO]~
_____________
a2dt162
U2d@
532
A (3500C)
B (4500C)
Fig. 2. ESR spectra of Mm(0AchbiOz by Thermal Activation at 3 5 P C (A) and 4 5 F C (B) The symmetrical signal centered at g=1.928 reached the maximum intensities by the 3500C-activation, but deceased sharply by heating at the exceeding temperatures above 400oC. The resulting ESR signal (Fig.Z(B)) after the thermal activation at 45WC was totally changed to the asymmetric signal similar to that of Mo(V) oxide species which has been reported on some Mo catalysts derived from other MO precursors such as MoO-j[lS] and Mo(NMe&[5] on Si@. After activation at 350oC the catalyst showed the highest spin concentration, the ratio of spin number to the number of paired molybdenum(Mo2) is 0.83. This result suggests that 83 mole % of the M%(OAc)4 precursor was uniformaly converted to the one-electron oxidised s p e c i e s ( M o ~ M ografted ]~ onto Sic. The Mo-Mo bond order in [Mo=MoIV is chaged from 4 to 3.5, which is reflected in the UV-vis band of d ---> d* transition being shifted from 435 nm to 471 nm by thermal activation of the M q ( O A c w i 0 2 .
3.2. Ligand interaction of Mqt(0Ac)q with SiOz The surface reaction of acetato ligands in the dry-mixed Moz(OAc)4 /SiOz was studied by FTIR in varying the activation temperatures in vacuo. As shown in Fig.5, the fresh sample of Mm(OAc)4/Si02 gave a main pair absorption bands at 1523(asyn) and 1447(syn) with side bands at 1497(asyn) and 1416(syn) cm-' which are assigned to bidentate carboxyls of Mm(0Ach. A small sharp peak at 1353 cm'l is likely to be the deformation of C-H bond i n acetate ligand. These carboxyl IR bands are identical with those of Moz(0Ach in crystal. Additional medium band at 1709 cm'l and a weak band at 1754 cm-l may be due to carbonyl stretching mode of monodentate acetate attached to Moz center and silica surface[ 131, respectively. During the dry-mixing of M@(OAcb with the partialy dehydrated Si& the sharp intense peak at 3710 cm'* assigned to the isolated SiOH was strongly reduced. These results suggest that bidentate acetato-ligands attached to the precursor Mm(0Ac) are partially removed in the reaction with the support SiOH by dry-mixing at 2510OoC, being intact to 9 0 2 . By raising the activation temepratures above 2OOOC the main pair bands at 1523 and 1447 cm-l were suppressed in their intensities and broaden and relatively shifted to lower frequencies. The intensities of bands at 1520 and 1440 cnil after 350°C-activation were decreased to ca 114 of those of the original sample. The acctato absorption bands completely disappeared by heating the sample at 456C.
533 Fig.3. FTIR Spectra of Mm(OAc)4/Si02 by Thermal Activation at 25OC (a), lOOOC (b), 14OOC (c), 21OOC (d), 27CPC (e), 340°C (g), and 45OOC (h).
3.3. Mo-K edge XANES and EXAFS characterization of SiOz-supported Moz(OAc)4 The XAh'ES(X-ray near-edge absorption spectrum) can provide crucial information about the site symmetry of Mo atoms and bonding properties of the substrates. The fresh Mn(OAc)l/SiOz prepared by dry-mixing at 25-100°C gave the XANES idential with that of the reference Mn(OAc)4 crystal diluted with BN(boron nitride), as shown in Fig. 4. Further- more, the thermal activation of dry-mixed sample at lower than 35OOC resulted in a minor difference of the Mo-K edge XANES, compared with that of the fresh sample. This result reveals that the local structure and symmetry of dinuclear Mo skeleton is still retained even by thermal activation at below 35OoC, likely as the original Mm(OAc)4, regardless the partial removal of acetato ligands and one-electron oxidation. By contrary, after the exceeding activation at above 4OO0C, the Mo-K edge XANES showed a substantial change of the spectra (Fig,4 e-d). The resulting XANES at 450°C-activation resembles those of Mo03(Td symmetry) and a M n 0 4 ( 0 h symmetry) having a clear shoulder peak due to the pre-edge absorption of 1s-5p transition of Mo(V) or Mo(V1) oxides. Fig.5 represents a set of the Fourier transforms of the k3 weighted Mo-K edge EXAFS for the Mo2(OAc)@iO2 after thermal treatments at 25, 250, 350 and 45OOC. The curve-fitting data are summerized in Table 1. The EXAFS data suggested that after activation at 35U'C the catalyst has a Mo-Mo d i s t a n c e ( R ~ ~ ~ 0 = 2 , 1and f i )C.N.(coordination number)=0.6 similar to that of the precursor(RM~M0=2.10~ C.N.=1.0) within the experimental errors, implying the multiple bonding of Mo-Mo is retained after the thermal activation at below 350OC. The C.N. of Mo-0 bond(R~*0=2.0d) considerably decreased from 3.7 to 1.8 by heating from 25 to 350"C, reflected in a progressing removal of bidentate acetate 1igands.of Mm(OAc)4 being grafted on Si02. The Mo-Mo distance is slightly enlonged after the 350°C-thermal activation
534 Fig.4. Mo-K edge XANES of Mm(OAc)4/Si02 by Thcrmal Activation at Diffcrcnt Temperature in Vacuo
Mofresh f
K)
I
1
19972 xx)o4 20036 20068 2
Distonce R
(1)
0
Distance R
(%I
Fig.5. Fourier Transform of EXAFS Data for Moz(0Ac)l in crystal(A), Ma(OAck/SiO:! after Thermal Activation at 2S'C(B), 3500C(C),and 4500C(D)
of Mm(OAc)4/Si02, possibly due to the one- electron oxidation to give [MOgMo]", whcrc the Mo-Mo bond order is changed from 4 to 3.5. In contrast to these the catalyst activated at 45OoC h a d R M ~ M ~ = ~ . S w O h&i c h is f a i r l y l e n g t h e n e d c o m p a r e d w i t h t h a t of Mo2(0Ae~(Rhb~0=2.1Oii), indicative of cleavage of the Mo-Mo bonding. The resulting
535 sampleoafter 4500C-activation consisted with two kinds of M o - 0 bondings, one is R M ~ o=l.73A (C.N.=0.5) and the other is R~e0=2.01&C.N.=1.8), which are likely associated with Mo=O and M o - 0 bonds, respectively. Along with those evidences in EXAFS evaluation in Table 1, the XANES of the catalyst activated 45@C(Fig. 4) propose a dimeric structure with a briclgcd oxygen alike a highly dispersed Moo3 or Mo04" ensemble, as the analogy of XANES spectra of the references. Table 1 EXAFS Parameters for Mo2(0Ac)dSi02 by Thermal Activation in Vacuo at Difercnt Temepraturcs. (4.lwt% Mo loading) Moz(OAc)r/SiOz Activation Temp
Bond
C.N.
K(&
AEo(ev)
.(A)
F(%)
25oc
Mo-Mo Mo-0
1.o 3.7
2.10 2.11
8.92 -7.78
0.049 0.089
1.2
350°C
Mo-Mo Mo-0
0.6 1.8
2.11 2.06
12.78 -6.58
0.048 0.078
1.6
450°C
Mo-Mo Mo-O(l) Mo-0(2)
0.7 0.5 1.8
2.80 1.73 2.01
-4.64 44.28 -2.55
0.074 0.044 0.074
3.4 6.6
C.N.= coordination number, R(&=interatomic distance, AEo=inner potential correction, u ( ~ ) = D e b y e - ~ a l lfactor, er F(%): correction factor: Experimcntal errors: 0.2 for C.N. and 0.02 for R(A) Accordingly it is suggested from the data of FTIR, UV-vis, ESR and XANES-EXAFS that a nobel dinuclear Mo species "[MoCMo]" I' is homogeneously grafted on Si02 by the following succcssivc reactions between Moz(0Ac)j precursor and Si02 under the thermal activation:
M o(0 ~A c k
+
Si02
350°C
+M ~ ~ ( o A c ) ~ / s i O ~
CH3 I
o/c \o 1
I
MoSMO I I
450-C
?\
0 '
Mo' I
'Mo
3.4. Actvity of propene metathesis and ethene homologation The reaction of propene and ethene(l0-16 tom) were carried out on the resulting catalyst by using a closed circulating reactor with a 150 ml volume. The amount of catalysts were 0.005-0.3 g. The dry-mixed Mo2(OAc)d!3Oa was completely inactive for propene metathesis(2 CH3CH=CH2 - - - - - > CH2=CH? + 2-butenes) and ethene homologation( 3CH?=CHz ------> 2 CH3CH=CH2) until activated at 2000C, and a quite low activity even after heating at below 300OC. It was of interest that the Mm(OAc)4/Si02 activated at above 300°C exhibited remarkable actvitics for propene metathesis at the reaction temeperature of 0-2OOC. Fig.6 shows the spccific activities for propene metathesis to give an equal mixture of ethene and but-2-ene(Z:E=ca 0.5) as a function of the activation temeprature
536 for Mo(OAc)4/SiO2,The catalyst activated at 350°C provided the highest activity for propene metathesis, whereas it was inactive for the ethene homologation reation even at 110OC. After being activated at 45@C, the specific activity of the catalyst for propene metathesis markedly decreased. In contrast, the 4500C-activated catalyst showed appreciable activity for ethene homologation, as with conventional rholybdenum catalysts[8]. The trend of catalyst performance in propene metathesis on the thermally activated Ma(OAc)llSiO;! parallelled the change in ESR signal intensities(g=1.928) owing the formation of SiQ2-grafted [ M O E M O ]as~ depicted in Fig.6. This implys that the Si02grafted [MoEMo]" species is specifically resposible for olefin metathesis. The specific activities of propene metathesis on some supported molybdenum catalysts prepared from various Mo precursors, its activation(pretreatment) and supporting materials, as summerized in Table 2. The 35O0C-activated Mm(0Ach /Si02 gave the highest activity for propene metathesis, which were 102-16 times more active per unit of Mo sites than those on the catalysts prepared from other Mo precursors and supporting materials under the similar reaction conditions, e.g., reaction temperature and propene pressure.
-
2.8 -
3.0
F1
N -
P X
I
0 X
P
?
3 2.0 2E
2.0 6,
:g1.0
1.0
\
.-c
-
4-
t
e
c
B
0 0
!i
e .ii
Y
0
0
.s n
v)
300
350
400
Activation temperature
450
v)
(OC)
Fig. 6 Dependency of specific Activities for Propene Metathesis(2O0C,Pa~6=l0torr) and Spin Concentrations of Moz(OAc)6iOz upon Thermal Activation at Different Temepratures
3.5. Poisoning of NO for propene metathesis on 350°C-activated Mo~(OAc)q/Si02. It has been reported that NO and 0 2 considerably suppress the propene metathesis on the conventional Mo catalysts[ 161. NO having an unpaired electron acts as one-electron donating molecule for the paramagnetic species. N0(99.9% Purity) was admitted at 25OC onto the catalyst activated at 35OOC, followed with the propene metathesis after evacuation of It is of noteworthy that NO efficiently surpress the metathesis the gas-phase NO at 2%. activity upon exposure to 35OoC-activated Moz(OAc)4/SiOz. T h e r a t e s of p r o p e n e metathesis at 20°C decreased in the linear-relatioship with the amount of chemisorbed NO. The number of chemisorbed NO to completely poison the propene metathesis approximately corresponds to the number of a M m pair a s the SiOz-grafted [MozMo]", which was
537 equivalent to total spin conccntration of ESR signal. It was also found that the ESR signal intensity (g=1.928) greatly decreased upon exposure of NO at 2@C, resulting in the new ESR signal associated with an adduct of [M~ZMO]"with NO.
3.6. Activity of Mm(0Ac)q-derived catalysts supported on A1203 and Nay-zeolite. In stead of S O ? , A h 0 3 and NaY zelite were used to impregnate with Mm(0Ac)t. The Mo?(OAc)t/A1203 and Moz(OAch/NaY-zeolite have been prepared by the similar drymixing technique with grinding under the Nz atmosphere at 25OC. The XRD measurements suggcst that Moz(0Ac)t is highly dispersed on A1203 and inside Nay-zeolite by heating in vacuo at 50°C and 250°C, respectively, where any Mo2(0Ac)? crystal does not exist in the dry-mixting s a m p l e s . T h e catalysts were prepared by thermal activation of Mo2(0Ac)t/NaY-zeolite in vacuo at 300 and 3500C, which showed a intense but broden ESR signals, relatively different from that of Me(OAc)4/SiOz activated at 35oOC. The in-situ F'UR study on the surface reaction betwen Moz(0Ach / N a y zeolite demonstrated that Moz(0Ac)t is mainly converted to mono-dentate Ma-acetate interacted with the Lewis sites such as A13+on the internal walls of zeolites, in formying "Mo-0-C(=O --AI3+)C&" The intense band at 1578 cm-l appeared by thermal activation of Mn(0Ac)dNaY at 250-35oOC. Nevertheless, the catalysts derived from Mrn(OAc)r/Al203 exhibited the catalytic activity for propene metathesis at 25°C with a lower than Mo2(0Ac)4/SiOz, whereas negligibly on Mm(OAc)j/Si02 after the similar thermal activation at 300 and 3500C, as shown in Table 4. The rcsults suggest that the surface aciditiy is affected to reduce the stability of Mo2(0Ach, resulting in the cleavage of Mo-Mo bond. Table 2. Relative Activities for Propene Metathesis on supported Molybdenum Catalysts Prepared from Defferent Precursors Impregnated on S i a , A1203 and Nay-zeolite Precursor/Support (activation) M a ( OAc)4/Si 02 evac. 3500C Mm(OAc)4/Alz03 evac, 3500C Ma(OAc)4/NaY zeolite evac, 3500C Ma(NMa&/Si02 evac, 2800C Mo(n-ally)dA1203 Ma04(Cz04)2-/A1203 CO red, SOOOC MoOx(x=2.9-2.3)KiQ H2,55ooc S n M a added
Temp (OC)
Specific Activity (mole/Mo atom/sec)
reference
(tom)
10
20
4.8 X 1(r2
this work
50
20
0.15
this work
15
20
7.2 x 1(r4
this work
15
20
4.2 X 1(r6
this work
18 21
25 0
9.8 X 1(r6 8.2 x l(r5
5 18
120
25
7.2 x 1 ~ 3
19
25 25
25 25
2.7 x i(r7 5.4 x 10-4
17 17
c3H6
4. CONCLUSION
(1) Mm(0Ac)l is an effective precursor to homogeneously graft a dinuclear MO species on SiO2 by the thermal activation, where bidentate acetato ligandz react with OH groups on Si@ in keeping the multiple Mo-Mo bonding. ( 2 ) The XANES-EXAFS data suggest that the Me(0Ac)l impregnated on SiO. retain its quadruple Mo-Mo bond after thermal activation at below 350OC in vacuo, giving the Si@bound Mo species having a multiple Mo-Mo bond ( R M @ M O = ~C.N.=O.6), ,~~, which is identical with those of the precursor Mm(OAcb (RM13440=2.10f$ C.N.=1.0). (3) The resulting Si02-grafted dinuclear species exhibited the remarkable activity for propene metathesis, whereas inactive for ethene homologation. (4) The surface-bound Mo species prepared from 3500C-activated Mm(OAc)r/SiO? showed a intense and sharp ESR signals centered with g//=gl=1.928, which is nssigncd to [ M o d ~ l o ] ~By . the exceeding thermal activation at abovc 40(r’C, the Mo-Mo bond was cleaved to give the highy dispersed Mov oxide, which is not active for propene metathesis. (5) NO specifically poison the propene mctathesis on the 350°C-activated catalyst. ( 6 ) Thc catalysts preparared from Mo2(0Ac)l/Ah@ and Moo,(OAcmaY-zcolite are not active for propene metathesis.
+On leave form Deartmcnt of Chemical Engineering, Tianjin Univcrsity, Tianjin, 300072, China; #On leave from Dalian Institute of Chemical Physics, Dalian, 116012, China Acknowledgement A part of this work has been financially supported by a Grant-in-Aid for Science Research (02640330) from the Ministry of Education, Science and Culture of Japan. 5. References
1. M. Ichikawa, Tailored Metal Catalysis, ed. Iwasawa, Reidel, Dordrecht, 184, pp184-263: Metal Clusters in Catalysis, ed. B.C. Gates, L. Guczi and H. Knozinger, Elsevier, Amsterdam, 1986 2. M. Ichikawa, Polyhedron, 1,2351(1988) 3. A. Fukuoka, T. Kimura, N. Kosugi, H. Kuroda, Y. Minai, Y. Sakai, T. Tominaga, and M. Ichikawa, J. Catal. , 434(1990) 4. Y. Sato, Y. Iwasawa, and H. Kuroda, J. C.S., Chem. Commun., llOl(1982 5. Q . Zhuang, K. Tanaka, and M. Ichikawa, J.C.S., Chem. Commun., 1477(1990) 6. K. Asakura, K. Kitamura-Bando, Y. Iwasawa, H. Asakawa, and K. Isobe, J. Am. Chem. S o c . U , 9 0 9 6 ( 1990) 7. K.A. Vikulov, I.V. Elev, B.N. Shelimov, and V. A. Kazansky, J. Mol. C a t a l . 3 , 126, (1989) 8. K. Tanaka, K. Tanaka, H. Takeo, and C. Matsumura, J. Am. Chem. S O C . , ~ , 2422(1987)18: M. Kazuta and K. Tanaka, J. Catal., 164(1990) 229 (1988) 9. B. Zhang, Y. Li, Q. Lin, and D. Jin, J. Mol. Catal., 10. I.V. Elev, B.N. Shelimov, and V.A. Kazansky, J. C a t a l . , U , 229(1988) 11. N. Kosugi and H. Kuroda, EXAFS2W03, Research Center for Spectrochemistry, Univ. Tokyo, Japan( 1987) 12. F.A. Cotton, J.G. Jr., Norman, J. Coord. ChemJ, 161(1971) 13. T. Fukushima, H. Arakawa and M. Ichikawa, J. Phys. Chem., 4440(1985) 14. F.A. Cotton and E. Pedersen, Inorg. Chem., 14,399(1975) 15. R.F. Howe and I.R. Leith, J.C.S., Faraday Trans, I, 1967 (1973) 16. A.A. Olsthoon and J.A. Moulijn, J. Mol. Catal., S, 147 (1980) 17. K. Tanaka and K-I. Tanaka, J. Chem. Soc.,Faraday Trans.Ia,601(1988) 18. Y. Iwasawa, H. Ichinose, and S. Ogasawara, J. C S., Faraday Trans. I , Z , 1763(1981) 19. A.N. Startsev, O.V. Limov, and S.A. ShKuropat, React. Kinet. Catal. Let.t., 41, 121(1990)
m,
m, a,
a
a,
539 DISCUSSION
Q: A. A. Tsyganenko (Russia) You have used IR spectroscopy to follow the process of catalyst preparation. Have you not tried to study coordination and oxidation states of molybdenum in the catalyst using CO as a test molecule ?
A: M. Ichikawa We have performed IR observation on the interaction of CO and NO with the resulting catalysts after the thermal activation of Mo&OAc)4/SiO A weak CO band was detected at 2130 cm-l in Co admission on 350 OC-activated Mo&Ac)4/Si02, which is reflected in a weak CO interaction with the Si02-grafted [Mo=Mo]" species.
Q: J. C. Mol (The Ncthcrlands) One of your conclusions is that catalysts prepared from MoOAOAc)dAl,O and M002(0Ac)~/NaY-zcoliteare not active for propcne metathesis. It can be seen in yourtable 2, however, that the alumina-supported MoOAOAc), catalyst exhibits an activity for propene metathesis which is comparable to that of SnMe4-,trcatcd MoO,/TiO? For the latter catalyst this was reported [ I ] to be a high activity. Why did your say that this system is not active ? H. Tanaka, K. Tanaka, J . Chem. Soc. Chern. Commun., 748,1984 [I]
A: M. Ichikawa Compared with the Si02-grafted Mo dimer species, we found that the relative activities of the catalysts prepared from Mo (OAc) impregnated on A1203 and NaY zeolite were 102104 times lower oc the basis of OAc), as the precursor, although their activities are comparable for those on SnMe4-modi ied MoRiO, catalyst. This may be associated with the inhomogencity of the active Mo sites in the conventional MoniOp catalysts, the limited part (less than 1%) of which are catalytically active for the olefin metathesis (ref. 17 in our paper).
bo ?(
Q: W. Grunert (Germany) You have highly active metathesis catalysts, but they are unusual in many respects. In homogeneous catalysis, there was little success with dimolecular precursors possessing metal-metal-bonds the most active catalysts are monomolecular. The oxidation state of your Mo precursor is different from that usually considered to be active for carbene formation (+4). Now, in addition to your written papcr you propose that in your case a pairwise carbene mechanism is operative instead of the widely accepted non-pairwise mechanism. Now, a conclusive decision of the latter problem could be obtained from experiments with labelled olefins. Have you evidence of this kind to support your hypothesis that a pairwise carbene mechanism operates :j
A: M. Ichikawa We propose the airwise mechanism for the olefin metathesis to explain the marked activities of [Mo=Mo] species grafted on Si02, which is by contrary completely inactive for the homologation reaction. This is the crucial evidence to take account in the difference from the carbene mechanism proposed for the conventional Mo catalysts which are active for both reactions. We are performing the '%Xabelling tracer experiment on the Si02-grafted MO dimer catalyst to justify the metathesis mechanism.
e '
Q: W. K. Hall (USA) We showed over a decade ago that NO can be used as a selective poison to estimate the density of catalytic sites on conventional molybdena-alumina catalysts. You have done likevise and observe that the activity is "killed" by the adsorption of one NO per two molybdenum:
540 a) does that mean that one NO poisons one dimolecular site, and b) have you observed (NO), species present by IR '? We observed these were the "poison" species in our work. A: M. Ichikawa The ropene metathesis was completely retarded by NO chemisorbed on SiO2-grafted [Mo=MoF in ca. 1:l molar ratio, which implies that one NO poisons each site of the paramagnetic Mo=MoIV grafted on $30, which is active for the olefin metathesis. We observed the bands at 1740 and 1820 cm-1 characteristic of dinitrosyl "(NO)," on the 350 OC-activated Mo OAc),lsiO2 in NO chemisor tion at 25-70 OC, possibly formed in the disproportionation o mono-dentate NO bound with fMo2Iv species.
6
P
Q: J. C. Conesa (Spain) I should like to ask about the fitting of the EXAFS data in the sample activated at 450 OC.The fit of the short Mo-O(1) distance needed for the AEo parameter in a rather large value, 44 eV, quite outside the ran e normally accepted for this technique. Could you comment on confirmations of the relia ility of this result ? I wish also to have a confirmation of the ESR g value in the work by Cotton quoted by you: is it g = 2.417 as written in the preprint book, or close to 1.9 as mentioned during your oral presentation ?
Q
A: M. Ichikawa 1) We performed the curve-fittin analysis for the Mo Kedge EXAFS data of Mo-0 contribution on the sample of 450 &-activated Mo OAc)dSiO, using the reference of Moo3 and K2MoOa The CN and R parameters were o tained with a good fitting in terms of Debye-Waller factor, but they provided larger values of inner potential correction (Ed and correction factor (F) for shorter Mo=O bond. This is possibly due to the difficulty to extract the smaller contribution of Mo=O bond from the relatively noisy background in the region of R = 0.1-0.2 nm. We have reevaluated the EXAFS data for Mo-0 bonding to improve the curve-fitting, giving the final parameters: c.f. CN (Mo=O(l)) = 0.6, R=0.182 nm in Eo -10.2. 2) Cotton et al., reported the symmetric sharp ESR band centered at g(paralle1) = gberpendicular) = 1.941 for the oneelectron oxidized species of Mo$C,HFOO), with eleven hyperfine structure, which is quite similar to that of M o d 0 c)q/Si02 atter the 350 OC-activation.
z!
Q: D. C. Koningsberger (The Netherlands) What is the Mo-Mo coordination distance in your reference compound: MoAOAc), (RzCH3) ? How did you separate in your analysis Mo-0 and Mo-Mo at approximately the same coordination distance ? Did you perform a statistical anal sis of your data ? Is the two shell model: Mo-O(l) and Mo-O(2) of the sample reduced at 450 statistically significant 7 A AEo value of 44 eV is impossible and much too large: what is your reference for the Mo-O( 1) coordinates ? A: M. Ichikawa The EXAFS analysis for Mo (OAc), in crystal provided the Mo-Mo coordination distance of 0.21 nm with CN = 1.8. (R o - ~= 0.21 nm in crystal XRD), which is good agreement with that of the high1 disperseha on%iO, at room temperature. We extract first the Mo- o contribution in k-l space = 4-12 1/A using the reference values of Mo-Mo distance as reported in the crystal XRD. The Mo-0 bond contribution was
d
541
se arated by subtraction of the Mo-Mo bond in FT data, using the FT parameters for the 4 0 OC activated sample having no Mo-Mo bond contribution. We performed the statistical analysis of our data of 450 OC activated sample for the two shell model of Mo=O(l) and Mo-0 (2) using the references, 0 Moog and TdK2Mo04 crystals. The unusually large value of Eo for Mo=O(l) in the 4 5 0 k activated sample has been re-examined to improve the cuwe-fittin analysis, giving the smaller value of Eo = -10.2, as we discussed in the answer to Dr.d n e s a .
P
Q: J. Vedrine (France) You have shown that ESR spectrum intensity (Figure 6) follows the activity of propene metathesis as function of activation temperature. However, the ESR si nal shown corresponds to a d1 ion interacting with two Mo ions (I1 hff lines) as Mo( )Mo(VI) or Mo(V)Mo(IV). For active dimer you should see (I=1 a spectrum at much lower magnetic field (so called fine structure). The question is: what 2sR spectrum did you record between 0 and 3000 G ?
f
A: M. Ichikawa The electronic structure of [Mo=MoIVspecies is denoted as a%c46'to be consisted of mixed valent Mo(II)-Mo(III). The ESR si nals for the [Mo=Mo]VX4 species are reasonabl observed at g = 1.02-1.94 varing X=CI&,C2H5,C H7 and SO4 (ref. 12). No other ES signals except the sharp band centered at = 1.92i were observed in the record between 0-3000 G on our samples of Mo2(0Ac)4/Si 2 after 350 OC-activation.
R
%
Q: J. J. Rooney (Ireland) M. A. McCann (his results has been published) has recently shown that MOAOAC)~ is a catalyst by itself in solvent at 25 OC for metathesis polymerization of norbornene the high molecular weight linear polymer. This rules out a pairwise mechanism for the M02 complex. A metallocyclobutane /CH2 [Mol
\
\
/CH2 CH2
will give metathesis or homologation depending on the oxidation state of Mo.
A. M. Ichikawa Thank ou very much for your suggestive comment (c.f. Mc Cann) to show the catalytic activity of bo2(0Ac), as the precursor in the methathesis polymerization of norbornene, although it is not clear what a Mo species is generated in solution and responsible for the olymerization. We propose a painvise mechanism for olefin metathesis on our SiO grafted L o dimer species, but not yet ruled out other possible mechanisms incluzng the metaloc clobutane or carbene intermediate. Recently, we have found in TPD study that Mo2(Oic)4/Si02 gave an equimolar mixture of CO and CH4 with a small amount of ethene and ethane by the thermal activation at 300-350 OC. These suggest that acetate ligand of recursoc M02(0Ac)~may provide carbene (CH2:) in the thermal activation to bind with L o species grafted on silica which may be incorporated in olefin metathesis. We are stufying now by FTIR using l k - and 2H-labelled olefins and acetate to characterize the in situ intermediate species derived from the SiO2-grafted MoA0Ac)q active for olefin metathesis.
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Guczi, L.el nl. (Editors), New Fronriers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
MOLECULAR DESIGN OF SUPPORTED METAL OXIDE CATALYSTS
I. E. Wachs, G. Deo, D.S. Kim,M.A. Vuurman and H. Hu Zettlemoyer Center for Surface Studies, Departments of Chemistry and Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA
Abstract This study demonstrates that molecular design of supported metal oxide catalysts is possible from molecular level information obtained from combined Raman spectroscopy and the methanol oxidation reaction. The important factors that influence the molecular design of the supported metal oxide catalysts are the specific oxide support (factor of -10,) and the specific sur-face metal oxide (factor of - l o 1 ) . The synthesis method is not critical since it does not influence the surface metal oxide structure or reactivity. Calcination temperature is not important as long as moderate temperatures (350-5OO'C) are used.
1. INTRODUCTION Many recent studies have demonstrated that two-dimensional transition metal oxide overlayers are formed when one metal Nb,O,, oxide component (i.e., Re,O,, CrO,, Moo3, WO,, V20,, etc.) is deposited on a second metal oxide substrate (i.e., A120,, TiO,, SiO,, etc.) [ l ] . The molecular structures and reactivity of these surface metal oxide species have been intensively investigated over the past decade because of the importance of these supported metal oxide materials in numerous catalytic applications [1,2]. The major structural information about these surface metal oxide species has been derived from Raman spectroscopy studies because of the molecular nature of this characterization method and its ability to discriminate between different metal oxide species that may simultaneously be present in the catalyst. Corresponding reactivity studies have demonstrated that these surface metal oxide species possess the active sites for many catalytic reactions. The fundamental information currently available about these oxide catalysts is beginning t o allow us to molecularly design supported metal oxide catalysts. The molecular design of supported metal oxide catalysts requires that we specify the synthesis method,' oxide support, catalyst composition, calcination temperature, location and structure of surface metal oxide species, as well as its reactivity. Consequently, the influence of each of the above parameters
544 upon the catalytic properties of supported metal oxide catalysts needs t o be examined. T h e present study primarily focuses on t h e molecular design aspects of supported vanadium oxide catalysts because these catalysts constitute a very important class of heterogeneous oxide catalysts. However, comparison with other supported metal oxide systems (MOO,, Re207 ,and CrO,) wi 1 1 also be made.
2. EXPERIMENTAL The oxide supports employed in t h e present study were: TiO, (Degussa, -55 m2/g), A1,0, (Harshaw, -180 m2/g), SiO, (Cabot, -300 m 2 / g ) , Zr02 (Degussa, -39 m2/g) and Nb,O, (Niobium Products Co., -50 m2/g). Many different synthesis methods have been uoed t o prepare supported metal oxide catalysts. In t h e case of supported vanadium oxide catalysts, t h e catalysts were prepared by vapor phase grafting with VOCl,, nonaqueous impregnation (vanadium alkoxidee), aqueous impregnation (vanadium oxalate), as well dry impregnation with crystalline V 2 0 s (spontaneous dispersion) Supported chromium oxide (1% wt.), rhenium oxide (1% wt.) and molybdenum oxide (1% wt.) catalysts were prepared by t h e incipient-wetness impregnation method using an aqueous solution of chromium nitrate, HReOq, and ammonium heptamolybdate, respectively. T h e molecular structures of t h e supported metal oxide catalysts were characterized by laser Raman spectroscopy under in silu as well as ambient conditions. The laser Raman spectroscope coneiet of a. Spectra Physics Ar' laser producing 1-100 mW of power measured at t h e sample. T h e scattered radiation was focussed into a Spex Triplemate spectrometer coupled t o a Princeton Applied Research O M A I 1 1 optical multichannel analyzer. About 100-200 mg of t h e pure catalysts were pelletized and used f o r obtaining the Raman spectra in the in silu mode. For ambient spectra 5-20 mg of catalysts was placed on a K B r backing. T h e supported metal oxide catalysts were examined f o r their reactivity in t h e methanol oxidation reaction. T h e reactor was operated in t h e differential mode by keeping conversions below 8%. A methanol/oxygen/helium mixture of -6/13/81 at 1 atm pressure was used a s the reactant gas f o r all t h e data presented. The analyeis was performed on a n online gas chromatograph (GC) (HP 5840A) containing t w o columns (Poropak R and Carbosieve SIX) and t w o detectors (FID and TCD). Reaction d a t a at 230 'C are presented in the form of turnover number (TON) defined a s t h e number of moles o f methanol converted per mole of vanadium atom per second. T h e reaction data f o r some catalysts were also obtained at 200, 240, 260 'C t o calculate t h e activation energy and check f o r diffusional limitations in the reactor. No maes and heat transfer limitations were observed.
.
-
3. RESULTS AND DISCUSSION It is well known that supported metal oxide catalystr possess surface metal oxide phases (see Figure 1) that are formed by t h e reaction of the deposited metal oxide. with t h e surface hydroxyls of t h e high surface a r e a oxide supports. Direct evidence for t h e titration of t h e surface hydroxyls (OH) by t h e deposited metal oxides is obtained from in rifr infrared studies which reveal t h e sequential disappearance of the support OH bands during t h i s procemm [3]. T h e coneurnption of surface hydroxyls can also be monitored by CO, chemisorption studies since CO, titratee t h e basic surface hydroxyls [3] , Both measurements, infrared spectroecopy of surface hydroxyls and chemisorption of CO, on t h e surface hydroxyls, provide a quantitative measure of t h e surface coverage of the metal oxide overlayer. Consequently, a necessary condition f o r t h e formation of surface metal oxide overlayers is t h e presence of reactive surface hydroxyls on t h e oxide support. Oxide supports such am Al,O,, TiO,, ZrO,, and Nb,O, have a high surface density of reactive surface hydroxyls and tend t o form a closed packed monolayer of t h e surface metal oxide phase, whereas, oxide supports such as SiO, which have a lower density of reactive surface hydroxyls d o not form a closed packed monolayer o f t h e surface metal oxide phase (see Table 1). Recent studies employing electrochemical methods [4] and non aqueoun a1 lyl preparation [5] have demonstrated that somewhat higher surface density on SiOz can be achieved with the special preparations, but still resulted in only a small fraction of a close-packed monolayer.
Table 1 Monolayer loading and surface density of some supported metal oxides as a function of oxide support Oxide Surface support a r e a ( m 2 / g >
Monolayer loading (wt .%) V,O, MOO, CrO, Re,O,
Surface density (wt . s , / m 2 ) V205 MOO, CrO, Re20,
A1,0,
20 1 8 1 3 6 6 -4 3 -6 4 4 3 3 2
.11 .11 .lo .11 .01
TiO, ZrO, Nb,O, SiO,
180 55 39 55 300
17
5 5 -
6.5
.lo .07 -09 .11 -s o 9 .08 -.13 .07 .07 .01 , 0 1 .02
--
Thus, the surface hydroxyl chemistry controls t h e formation and location of the surface metal oxide species present in supported metal oxide catalysts. The molecular structures of t h e surface metal oxide species present in the metal oxide overlayer are not well understood
546 and the influence of surface coverage (catalyst composition). specific oxide support. preparation method, calcination temperature, and promoters need t o be establ ished. The Haman spectra of titania supported vanadia catalysts reveal the of three different vanadia species O A t h e TiO, presence support undei m sdu conditions [6]. At low loadings, a single is due to an sharp band i s present at -1030 cm-' which isolated tetrahedral coordinated surface vanadium oxide species containing one terminal V=O bond and thiee bridging V0-Ti bonds [7]. A t intcrmediate loadings, a second band is present at -930 cm-' which has been assigned to a polymerizcd, tetrahedral coordinated surface vanadium oxide species [ 7 ] . At high loadings, a third sharp band is present at 994 cm-l d u e to crystal 1 ine V,O, which indicates that t h e close-packed surface vanadium oxide monolayer has been formed and all t h e reactive surface hydroxyls consumed. T h u s , t h e catalyst composition is a critical parameter since it influences t h e formation o f different vanadium oxide structures. T h e reactivity of t h e titania supported vanadium oxide catalysts was probed by thc methanol oxidation reaction. Thc methanol oxidation reaction is very sensitive to the nature o € surface sites present in oxide catalysts. Surface redox sites (sites that are capable of being reduced arid oxidized) form prirnari ly formaldehyde as we1 1 as methyl formate arid dimethyl methane as the reaction products. Surface acid sites, Lewis as well a s Bronsted, result in the formation of dimethyl ether. Surface basic sites yield CO/CO, as the reaction products [ S ] . The oxidation o f methanol over the titania supported vanadia catalysts yielded formaldchyde exclusively, 98%+, as t h e reaction product. 'I'he titania support in the absence of surface vanndia yielded dirnethyl ether and trace amounts o f CO,. The. almost coiiiplete format ion of formaldehyde demonstrates that t h e reactivity of t h e titania supported vanadia catalysts i s due t o the surface vanadia redox sites. The reactivity is measured a s the turnovci. number (TON), the number of moles of methanol reacted per surface moie o f vanadium atom per second, and is presented in Table 2 as a function of the vanadia loading. The TON increases somewhat with initial surface vanadium oxide coverage, and decreases at surface coverages approaching and exceeding monolayeris t w o ordcrs of coverage. Note that t h e TON of bulk V,O, magnitude less than thc titania supported vanadia catalysts indicating that crystal1 ine V205 is significantly l e s s active than surface vanadia species. T h e slight increase in TON with the increase in surface coverage appears to be related to t h e presence o f the polymerized surface vanadia species. Similar coverage effects were also observed f o r supported rhenium oxide [Y], molybdenum oxide [lo] and chromium oxide catalysts [ll]. T h u s , the reactivity of surface metal oxide species has a slight dependence on surface coverage (less than a factor of 10').
547
Table 2 The TON of V,O,/TiO,
catalysts as a function of loading
wt .% V205/Ti02
0.5 1 .o 1.8 2.5 4.5 5.0 bulk V20,
T.O.N. (sec-l) 1 .o 1.9 4.4 8.4 2.2 1.7 0.022
Many research groups claim that the synthesis method is critical for forming metal oxide monolayers that possess specific structures and reactivities. To investigate this issue, a series of V20,/Ti02 catalysts were synthesized by different methods: equilibrium adsorption, vanadium oxalate, vanadium alkoxides and vanadium oxychloride grafting. The in siiu Raman spectra of all these catalysts exhibit a sharp band at -1030 cm-' characteristic of the isolated surface vanadium oxide species. Thus, the synthesis method does not affect the final structure of the surface vanadium oxide species on titania. Similar conclusions were also found f o r molybdenum oxide supported on titania [12], silica [5], and alumina [5]. Reactivity studies with methanol oxidation exhibited essentially identical turnover numbers. Consequently, the preparation method is not a critical parameter since it does not influence the structure of the surface metal oxide species or its reactivity. The nature of the supported metal oxide phase is influenced by the calcination temperature. Moderate calcination temperatures, 350-500 'C, are required to decompose the metal oxide precursors (oxalates, alkoxides, oxychlorides, etc.) to form the surface metal oxide species [l3]. Insufficient calcination temperatures do not completely decompose the precursors and, consequently, do not react with the surface hydroxyls to form the surface metal oxide species. However, high calcination temperatures, greater than 600 'C, can result in shrinking of the surface area of the oxide support and decreasing the available surface area for the surface metal oxide species. Consequently, high calcination temperatures increase the surface coverage of the metal oxide species and, in severe cases, destroy the surface metal oxide phase and form crystal 1 ine V20, and eventually sol id state solutions [13,14]. Thus, calcination temperature is an important parameter that controls the activation and deactivation of supported metal oxide catalysts. However, supported metal oxide catalysts are typically prepared by calcining at 400-500 'C which would eliminate problems with activation and deactivation. A series of -1% V,05 catalysts were prepared in order to
investigate the influence of different oxide supports upon the molecular structure and reactivity of the surface vanadium oxide species. The low loading catalysts were selected because at these surface coverages only the isolated surface vanadium oxide species is predomina,ntly present on the different oxide supports and potential complication due to additional surface vanadium oxide species could be eliminated. The in aiiu Raman band due to the V=O bond was found to vary from 1015-1038 cm-' as a function of the different oxide supports (see Table 3). The slight difference in band position is due to slightly different V=O bond lengths of the isolated surface vanadium oxide species on different oxide supports. The important point is that the Raman spectra reveal that the same surface vanadium oxide species are present on all the different ox'de supports. The same conclusion is reached from solid state 'lV NMR studies of these catalysts [S]. Similar observations have also been made for supported molybdenum oxide [lo], rhenium oxide [9,11], and chromium oxide [9,11]. Thus, the in sifu surface metal oxide molecular structures are independent of the specific oxide support. Table 3
In sifu Raman band posit,ion for M=O terminal stretching vibration, as a function of oxide support for 1% supported catalysts metal oxide (MxO,) Oxide Support
Raman Band for M=O terminal bond Mo Cr Re
V
SiO, Nb205
TiO, ZrO,
A1203
1038 1031 1027 1024 1015
978 990 993 986 992
985 997 1009 1010 1002
1015 1008 1005 990 1000
The reactivity of the surface vanadium oxide species on the different oxide supports was probed by the methanol oxidation reaction. F o r all the supported vanadium oxide catalysts, with the exception of alumina which possesses a high concentration of acid sites, the surface vanadia redox sites produced formaldehyde almost exclusively. On alumina, only a trace of formaldehyde was formed because the surface acid sites formed V205/A1203 system the dimethyl ether. Thus, for the formaldehyde produced was taken as representative of the reactivity of the surface vanadia redox sites. The reactivity of the surface vanadia species on different oxide supports was found to dramatically depend on the specific oxide support (a factor of 10'). Similar trends were also observed for supported molybdenum oxide [lo], rhenium oxide [ll], and chromium oxide [ll]. The specific oxide support is a critical effect on the parameter since it has such a profound reactivity of the surface metal oxide species. is either due to The origin of this support effect
differences in the terminal M=O bond o r the bridging M-OSupport bond. Many publications have proposed that the terminal bond is responsible for catalysis g.nd its activity is directly related t o the M=O bond strength. T o investigate thie, the reactivity wae plotted against the Raman position of (shorter bond corresponds t o higher Raman the M=O bond position). The plot of TON versus Raman M=O position for supported vanadium oxide, molybdenum oxide, rhenium oxide, and chromium oxide is presented in Figure 2. There does not appear t o be any relationship between the catalyst reactivity and the terminal M=O bond strength as was previously proposed. A more plausible conclusion is that the reactivity is related t o the bridging M-0-Support bond since the oxide support has a very significant effect on the reactivity. This suggests that the reactivity should alao be a function of the specific metal oxide epecies. Indeed, supported molybdenum oxide catalysts are about one order of magditude less reactive than supported vanadia catalysts. The trend in reactivity with specific oxide support appears to be related to the surface reducibility of the oxide supports. The more reducible oxide supports (TiO,, ZrO,, and Nb,O,) always exhibit very high TON while the irreducible oxide supports (A1,0, and SiO,) always exhibit very low TON [15]. Additional information about the reactivity was obtained by determining the kinetic parameters during methanol oxidation for vanadia, molybdena, rhenia, and chromia on different oxide supports. For all these systems the activation energy is approximately the same, 18-22 kcal/mol. The activation energy corresponds t o that expected for the breaking of the C-H bond of a surface methoxide intermediate, CH30Btas, and should be e pre-exponential independent of the specific catalyst [lS] factore, however, vary by orders of magnitude as the oxide support is varied. The difference in pre-exponential factors suggest that the number of active sites are responsible f o r the different TON. In calculating the TON we assume that all the surface vanadia species are participating in the reaction all the time. However, the different pre-exponential factor suggests that on less reducible oxide supports such as A1,0, and SiO, only a small fraction of the surface vanadia species participate at a given time in the reaction, and that for reducible oxide supports such as TiO,, ZrO,, and Nb205 a significantly larger fraction of the surface vanadia specie6 participate at a given time in the reaction. The different reactivities may also be due t o different activity per active site. Experiments are currently in progress t o quantify the number of active sitee participating in the reaction at given time and their reactivity. Thus, the oxide supports control the reactivity of the surface metal oxide species.
.
550 Table 4 The TON for methanol oxidation supported metal oxide catalysts
Ox i de Support
SiO, *l2'3 Nb205
TiO,
ZrOl
*
reaction
various
1%
TON (sec-l)
V,05
Moo3
2 .o* 1 o - ~ 2 .011 0-2 7.0*lo-' 1 .8*10° 2.3*10°
3.9*10-,
---
for
*
3.2*10-, 3.1wlO-l 9 .2*10-,
Cr03
Re207
1.6*10-' 1.6*10-3 5 .8*10-, 3.OelO-l 1 .3*10°
2.0*10-,1
---
1 .2*10-,t 1.2*100 1 .7*10"
metal oxide waa found to be volatile and left deposits on tube no redox products observed due to activity of alumina support
Many publications have also claimed that the modification or structure of the oxide support is critical for obtaining a good catalyst [17]. To investigate this issue a series of V,O,/TiO, catalysts were prepared on different TiO, support structures (anatase, rutile, anatase+rutile, anatase+brookite, and B ) . In sifu Raman characterization revealed that the same surface vanadia species were present on all the different titania supports with a band at 1024-1031 cm-' for 1% V,O,/TiO,. The reactivity of these different titania supported vanadia catalysts was probed with methanol oxidation and found to be identical with a TON of 2.1f0.5 sec-', Thus, contrary to previous conclusions the structure or modification of the oxide support does not affect the surface metal oxide structure or the catalyst reactivity.
4 . CONCLUSIONS
The above discussion demonstrates that it is possible to molecularly design supported metal oxide catalysts with the assistance of molecular characterization methods such as Raman spectroscopy. The formation and location of the surface metal oxide species are controlled by the surface hydroxyl chemistry, and the surface metal oxide species are located in the outermost layer of the catalysts as an overlayer. The catalyst composition is a critical parameter since it affects the presence of different metal oxide species (isolated surface species, polymerized surface species, and crystalline phases), and the reactivity, TON, also varies somewhat with surfacc metal oxide coverage. The preparation method is not a critical parameter since it does not influence the structure or reactivity of the surface metal oxide species. Calcination temperature is a n important parameter that controls activation
and deactivation of supported metal oxide catalysts, but calcination temperature is not critical if moderate temperatures, 350-450 'C, are used. The specific oxide support is a critical parameter since it dramatically affects the reactivity of the surface metal oxide species, but the structure of the oxide support has no effect on the surface metal oxide structure and reactivity. In summary, the critical parameters that affect the catalytic propertiee are the specific oxide support (factor of lo3) and catalyst composition or surface metal oxide coverage (factgr of 10').
5. ACKNOWLEDGEMENT
G. Deo and H. Hu acknowledge the support by NSF grant # CTS9006258.
6. REFERENCES (a) L. Dixit, D.L. Gerrard, H. Bowley, Appl. Spectrosc. Rev., 22 (1986) 189; (b) J . R . Bartlett, R.P. Cooney, In Spectroscopy ofznorganic-based Materials, R. J .H. Clark, R.E. Hester, Eds; Wiley: New York, 1987; p 187; (c) F.D. Hardcastle and I.E. Wachs, In Proc. 9th Intern. Congr. Cafal., M . S . Phi 1 1 ips and M . Ternan, Eds., Vol. 4 , Chemical Institute of Canada, Ontario, 1988, p 1449. 2. I.E. Wachs, Chem. Eng. Sc. Vol., 45(8) (1990) 2561. 3 . K. Segawa and W.K. Hall, J. Catal., 77 (1982) 221; D.S. Kim, Y. Kurusu, I .E. Wachs, In Proc. 9th Inter. Congr. Catal., M . S . Phillips and M . Ternan, Eds., Vol. 4, Chemical Institute Canada, Ontario, 1988, p 1460; A.M. Turek, J.E. Wachs, and E. DeCanio, J. Phys. Chem, submitted. 4. M . de Boer, A.J. van Dillen, D.C. Koningsberger, M.A. Vuurman, I.E. Wachs, and J.W. Geus, submitted to Catal. Lett. 5. C.C. Williams, J . G . Ekerdt, J . M . Jehng, F.D. Hardcastle, A.M. Turek, and I.E. Wachs, J . Phys. Chem., 95 (1991) 8781; C.C. Williams, J.G. Ekerdt, J.M. Jehng, F.D. Hardcastle, and I.E. Wachs, J . Phys. Chem., 95 (1991) 8791. 6. M.A. Vuurman, A.M. Hirt, I.E. Wachs, J. Phys. Chem., 95 (1991) 9928. 7. I.E. Wachs, J. Catal., 124 (1990) 570. 8. H. Eckert, G . Deo , and I.E. Wachs, unpublished results. 9. M.A. Vuurman, I.E. Wachs, D.J. Stufkens, and A. Oskam, J. Mol. Catal., submitted. 10.H. Hu and I.E. Wachs, in preparation. ll.D.S. Kim and I.E. Wachs, J. Catal., submitted 12.T. Machej, J . Haber, A.M. Turek, and I.E. Wachs, Appl. Catal , 70 (1991) 115. 1 3 . R . Y . Saleh, I.E. Wachs, S . S . Chan, and C.C. Chersich, J. Catal , 98 (1986) 102. 14.G. Deo and I.E. Wachs, J. Phys. Chem., 95 (1991) 5889. 1
.
.
552 1 5 . G . D e o a n d I . E . Wachs, J. C a t a l . , 129 (1991) 307. 1 6 . T . S . Yang a n d J . H . L u n s f o r d , J . C a t a l . , 103 (1987) 55; W.E. F a r e n e t h , F . O h u c h i , R.H. S t a l e y , U . C h o w d h r y a n d A . W. S l e i g h t , J. P h y s . Chem., 89 (1985) 2493. 17.A. V e j u x a n d P.J. C o u r t i n e , J . S o l i d S t a t e C h e m . , 23 (1978) -93.
H
Figure 1
0
H 0
H 0
I
I
I
S u r f a c e Metal O x i d e (MO,) of S u r f a c e Hydroxyls (OH)
are f o r m e d b y T i t r a t i o n o n Oxide S u p p o r t s
10.00 I
1%V24
+ 1% Moog A 1% Cr03 X
1% Re20,
x
SiOp
Nb205
0.00
Figure 2
A*
%$si02-
T h e p l o t o f TON of t h e v a r i o u s s u p p o r t e d m e t a l o x i d e s v e r s u s t h e i n s i i u Raman b a n d p o s i t i o n of t h e t e r m i n a l M=O s t r e t c h . T h e Raman b a n d p o s i t i o n i s r e l a t e d t o t h e M=O b o n d s t r e n g t h .
553 DISCUSSION
Q: U. S. Ozkan (USA) I have a comment and a question. My comment is that we have compared the two phases of the titania support in selective catalytic reduction of NO over vanadia catalysts. We were able to identify polymeric vanadate species on both phases of the titania support. We did not see more differences between the catalytic activity of the vanadia species found on the two different support materials. My question is: How did you calculate the turnover frequency (or turnover number, TON) for bulk V2O5 ? Could you describe it in detail '? A: I. E. Wachs Thank you for your comment regarding the similar properties of anatase and rutile supported vanadia catalysts for the selective catalytic reduction of NO,. To calculate the TON for V205 the surface of crystalline, V2O5 was assumed to have the same structure as the bulk, i.e., the area covered by a VO2.5 unit was 1.03 nm. In addition, with the two up-two down arrangement of V205 bulk structure, only half of the area of the bulk of V2O5 was considered as active. Based on this assumption the number of active vanadium units could be calculated and, consequently, the TON could be obtained.
Q: F. Solymosi (Hungary) Your paper contained many interesting data. I have a little problem with your last conclusion, namely that the catalytic behavior of V2O5 was completely independent of the structure, preparation, and pretreatment of the Ti02 support. We know that the properties and reactivity of T i 0 2 can be strongly influenced by those factors. Therefore, I would rather prefer to say that in the oxidation of methanol the properties of Ti02 support d o not play an important role. This could be different for other reactions. A: I. E. Wachs The reason that many researchers have claimed that the type of Ti0 support is critical is because they have been using commercial titania pigments that are usualfy contaminated with K, P, A,and Si on the surface. We have carefully compared different titania phases that were synthesized in the laboratory and had relatively clean titania surfaces (as determined by XF'S). Such clean titania samples show no difference between the type of titania support. These findings are not limited to methanol oxidation since they have been observed for much more complex reactions (see comments of Professor Trifiro for o-xylene oxidation and Professor Ozkan for selective catalytic reduction of NO&. Q: A. K. Datye (USA Your results show that changing the support can increase the reactivity of the dispersed oxide by 3 orders of magnitude. It is indeed remarkable that the structure of the dispersed oxide as seen by Raman spectroscopy is not affected by the support. For comparison, it would help if you provide data on the reactivity of the corresponding bulk oxides, also as a turnover number.
A: I. E. Wachs We have also measured the turnover number for bulk 2.2~10-2s-1, which is two orders of magnitude less magnitude more than V2O@iO2. It is not possible phases because they are not thermally stable and of the oxidation reaction. We have recently measured the TON for bulk MOOS,and found it to be -10-2 s-1.
554 Q: F. Trifiro (Italy) For some years now we have investigated the catalytic behavior of V-Ti02 (either rutile or anatase) for oxidation of o-xylene to phthalic anhydride and ammoxidation of xylenes and toluenes. We have found very slight differences between the two types of support. I confirm the results of Professor Wachs concerning the Ti02 as support for V 2 0 5 A: I. E. Wachs Thank you for your comments regarding the similar catalytic properties of vanadia supported on anatase and rutile for the oxidation/ammoxidation of o-xylene and butenes. Q: Y.Iwasawa (Japan) I agree with the question of Professor Solymosi. Different surface structures have been reported from different re aration methods (see pp.5 0 3,5 1x5 2 9.My question is concerned with an increase in the 0 for methanol oxidation as a function of Mo content on support. Could you make some comment on the meaning of this ?
!L
A I. E. Wachs The papers that typically claim differcnt surface metal oxide structures usually d o not extensively characterize their samples and d o not compare differcnt preparations side by side (especially important in such studies is to maintain the same metal oxide loading). I would be happy to characterize samples from other laboratories by Raman spectroscopy to confirm this conclusion. Our recent data show that the TON for the MoOfliO2 catalysts vary from 2x1O-l to 4x10-1 s-l, and the apparent increase in the TON is minor. Q: J. G. van Ommen (The Netherlands) The fact that you find the same behavior for V205 on T i 0 2 rutilc and anatase is very surprising. Why is the type of support so important and not the structure ? My question is: can you exclude that the anatase you use is not covered with a surface layer of rutile ? If this is the case (or anatase coverin rutile, which is not to be expected) you should find the same results for V2O5 activity in Me H oxidation and V205 reduction. An other remark: old results also from our group show that careful washing of K and P containing sample gives clean anatase and/or rutile surfaces,which show different behavior in MeOH oxidation, reduction (TPR) and especially in toluene oxidation. This cannot be explained by the presence of surface contaminations !’
%
A: I. E. Wachs The reason that different support types (A1 O,, Ti02, SiOp etc.) affect the catalytic properties of vanadiuni oxide monolayers is that &c support is a ligand that controls the case of oxygen removal 6 o m the V-0-S (S=support) band. However, this is a local phenomenon that does not de end on long range order (anatase or rutile). Consequently, only the support type (A1203, Ti 2, SO,, etc.) is critical. This is a very interesting question. Raman spectroscopy is much more sensitive to anatase compared to rutile and, thus, we should be able to detect an anatase overlayer on rutile (which is probable at mild calcination temperatures when rutile is not formed). The revcrsc situation is more difficult since a rutile overlayer will not be readily dctectcd by Raman spectroscopy. We plan to attempt such a synthcsis to address this question, but I fccl that the titania overlayers will not adopt the bulk structures because of epitaxial growth. The surface of the extracted Ti02 [ l ] still did contain impurities of P205 and K@ as determined by X-ray fluorescence. In addition, the surface concentration is important, not the bulk concentration of these impuritics as shown by X-ray fluorescence. For this reason it appears that the surface of these vanadia-titania catalysts still contain the impuritics. B. Claudel, M. Nueilati and J. Andricu, Appl. Cat& 11, 317 (1984) [l]
8
Q: A. Andersson (Sweden) 1) In the case of vanadia on anatase, you have observed two Raman bands at 1030 and 930 cm-I which you assign to isolated and polymerized vanadia species, respectively. Have you considered the possibility that the two bands can be from differing species on different surface planes of titania ? 2) You have found the same vanadia species to be present on the various polymorphs of titania. In our laboratory we have found similar spectral features as you have for vanadia on anatase. However, for vanadia on TiOfl) we observed a Raman band at 970 cm-l. Also, w e found the monolayer on anatase to give 50% selectivity for benzaldehyde formation in toluene oxidation. The monolayer our Ti0 ) gave only combustion. Even though vanadia the V - 0 bond lenghts angles most can be in tetrahedral coordination on differs. Can you comment on this ? A: I. E. Wachs 1) I do not think these two Raman bands are due to diffcrent surface planes of titania because they are observed on other oxide supports as well. The same surface species are essentially formed on all oxide supports. 2) We have observed the 1030 cm-l Raman band shift to 970 cm-l upon addition of is made from a potassium titanate small amounts of K to i02 Since Ti0 precursor, it may be some residual is present which would alter the spectral and catalytic properties of your TiOfl) catalysts.
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Q: W. K. Jozwiak (Poland) I like very much your generalizing approach trying to put some order in so complicated matter of the heterogeneous catalysis. Even with the most unfavorable starting materials (such as mechanical mixture of a-Cr O3 and Si02 (also Al 0,or MgO) w e have been able to obtain the effective ethylene po6merization catalyst. h u s , the process of chromium redispersion (confirmed by XRD and TPR methods) takes place in the oxidative atmosphere above 500 OC. Probably the crucial role is played by the support hydroxyl groups, governing the sort of equilibrium sintering +redispersion between surface and bulk oxide phases. DO you suppose that such behavior may be common among your oxidehpport systems ? A: I. E. Wachs The spreading of metal oxides onto supports have been demonstrated in the literature for many oxide systems (Moo3, V20 , W03, etc.). These oxides are all mobile at elevated temperatures and spontaneously diffuse over oxide surfaces of supports (silica is somewhat of an exception because of the low concentration of reactive surface hydroxyls). Thus, oxides with low melting temperature will have sufficient mobility to spontaneously disperse over oxide supports. Q: D. Wang (China) Your work demonstrated the importance of surface hydroxyl groups in the preparation of supported metal oxides. Is there any difference in hydroxyl groups (1) of any one support, and (2) of different supports ? A: I. E. Wachs The hydroxyl groups of oxide supports depend on the support type (A1203 T i 0 SiO,, etc.) and usually several different hydroxyls are present on the same support. -4 very 2etailed article on surface hydroxyls has recently been written [2] H.-P. Boehm and H. Knozinger in “Catalysis Science and Technology”,(Eds.: J. R. [2] Anderson and M. Boudart), 1983, Vol. 4, Chap. 2
556 Q: J. M. Thomas (United Kingdom) In describinb the pre aration of your surface overlayers you emphasized, not unnaturally, that it is the 0I ! concentration of the exterior surface of the support that is all important. One wonders, therefore, whether you have considered taking several of the minerals that have constitutional OH in their structures as supports. Perhaps kaolinite, with nice flat surfaces such in OH groups, is not stable enough. But what of the oxy-hydroxides (e.g. CrOOH) which would give you the added advantage of being able to graft one functional group in a justaposed manner to another (transition metal) functional group. This would then enable you to produce bifunctional, surface-modified catalysts.
A: I. E. Wachs I agree with you that it should be possible to react the surface hydroxyls of minerals with metal oxide overlayers. I am currently not aware of any such studies. Q: H. L. Krauss (Germany) I wonder how you got good catal tic properties in the Cr/SiO2 system at such moderate activation temperatures as 350-500 O(!. Many of our results, in fact all, show that activation temperatures of >600 OC,preferentially 800 OC, are to be applied to get good catalysLs in cases when a reduction of the chromium species is a crucial step in the preparation of the active catalytic species.
A: I. E. Wachs The methanol oxidation reaction is a unimolecular reaction involving only the active surface site (surface chromate species). However, the types of reactions that you are referring to are bimolecular reactions, which apparently are inhibited by surface hydroxyls adjacent to the active surface site, and elevated calcination temperatures are required to remove these hydroxyls from the silica surface. A detailed discussion of the comparison between methanol oxidation and ethylene olymerization over CrO SiO catalysts can be found recently [3]. D. S. Kim, K. k g a w a , T. S . Ya and I. EfWa&s,J. Catuf.,136,539 (1992). [3]
Q: E. Bordes (France) You mentioned that by Raman spectroscopy on V-oxides you see a slight diffcrencc in V=O band position, corresponding to slight difference in bond length. These diffcrcnces should be reflected also in catalytic reactivities. I wonder if the reaction with methanol is not too much simple. V-based catalysts have the capability to cyclize molecules. If you used butane, for example, do not you think that the reactivities would be more differentiated, and particularly the selectivities in various products.
A: I. E. Wachs It has been reported in the literature by many authors that the V=O bond length controls the catalytic activity. However, there was not direct proof of this assumption. We have, for the first time, measured the V=O bond lengths by Raman spectroscopy and find that it does not correlate with catalytic activity. This is not surprising since the V-0-S (S=support) bond is critical in determining the catalytic activity. The results are not unique to methanol oxidation since we have also found a similar activity trend for butene oxidation. An examination of the literature data for o-xylene oxidation, a significantly more complex molecule, also shows similar activity trend. Thus, the reactivity results for methanol oxidation appear to be quite general.
Q: H. H. Kung (USA) This was a very nice paper. My question is to ask for you comment as to the possibility that while spectroscopy sees the majority species, the reaction is catalyzed by some minority species. For example, a 1 wt% and a 2 wt% V,O$Kl2 may be indistinguishable with Raman spectroscopy, yet they are somewhat different catalytically in butane oxidation.
557 Also what is your view on the possibility that in hydrocarbon oxidation reactions, the water that is produced results in a constant partial pressure of water that causes migration of the surface species ? Thus the catalyst is in a different state than the one characterized structurally.
A I. E. Wachs Thank you for your nice comment. It is true that not all the surface species may simultaneous1 be participating catalytically, but that they may be taking turns performin the catalysis. {urthermore, we find that the same surface metal oxide species are found on a I oxide supports because they have a preferred coordination to oxide support surfaces. Thus, it is very unlikely that there is some unusual coordination that is not detected spectroscopically and is responsible for the catalytic behavior. In addition, doping the supported vanadium oxide catalysts with potassium results in change in the Raman spectra and a corresponding change in the methanol oxidation TON suggesting that the active sites are indeed being probed by Raman spectroscopy. Your results with butane oxidation reflect the complexity of this reactant which ma simultaneously be interacting with the surface vanadia s ecies and surface hydroxyls. In SUC a situation, increasing the vanadia coverage does not a fect the vanadia structure (as long as all the vanadia is dispersed), but decreases the surface hydroxyl concentration which is beneficial.
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Q: L. Dixit (India) Your Raman s ectral observations on titania supported vanadia catalysts reveal that there exist three di ferent vanadia species on a Ti0 sup ort. The observation of a single sharp band (at low loading) that is present at 1 0 3 8 ~ m is- ~due to isolated tetrahedrally coordinated surface vanadium oxide species containing one terminal V=O bond and three bridging V-0-Ti bonds. This is true, However, the assignment of bands at moderate and high loadings of vanadia, which appear at 930 cm-l and 994 cm-l, respectively to polymerised tetrahedrally coordinated surface vanadium oxide species and crystalline V f l 5 may be mixed. I would like to know whether structure-dependent position of bands of different vanadium oxide precursors compatible with V=O bond length variations are also compatible in terms of their intensities’? Do you have any intensity data of bands in terms of their structures ?
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A: I. E. Wachs The Raman bands at 930 and 994 cm-l are not related since the 930 cm-l band precedes the appearance of the 994 cm-1 band due to crystalline V f l s Furthermore, crystalline V f l 5 does not possess a band in this region. The different Raman band positions are directly related to the vanadium-oxygen bond lengths [4]. We have no relative intensity data at present, but the bands at higher wavenumbers should be slightly stronger due to the increased electron density of the vanadium-oxygen bonds. [4] Hardcastle and I. E. Wachs,J. Phys. Chem., 95, 5031 (1991)
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Guczi, L. el al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
STRUCTURAL CHARACTERISTICS OF ALUMINA-SUPPORTEDACTIVATED HYDRODESULFURIZATION CATALYSTS. AN XPS, NO ADSORPTION AND SULPHYDRYL GROUP STUDY L. Portelab, P. Grangea and B. Delmona aUnite de Catalyse et Chimie des Materiaux Divises - Universite Catholique de Louvain, 2/17 PI. Croix du Sud, 1348 Louvain-la-Neuve,Belgium bOn leave from: GRECAT - Instituto Superior Tecnico - Depto. Eng. Quimica Universidade Tecnica de Lisboa, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal
Abstract In the present work we analysed the surface properties of alumina-supported Co-Mo hydrodesulfurization catalysts, submitted to five different activation procedures. We measured the dispersion of the active elements (by XPS), the coordinative unsaturation state (infrared study of NO adsorption) and the sulfhydryl group (chemical titration with AgN03) concentration. The results confirmed the deep influence of the activation conditions on the parameters analysed. We also showed that hydrogen sulfide tended to adsorb on the coordinatively unsaturated sites (CUS). The structure of the catalysts was shown to change, from the monometallic samples to the bimetallic, leading to a behaviour different from that of its individual components. Namely, the NO adsorption capability, the SH concentration and, sometimes, the surface sulfur concentration, decreased. The results obtained allowed us to speculate that one of the roles of cobalt, in CoMo catalysts, would be that of improving the efficiency of surface sulfur removal.
1. INTRODUCTION To be effective, the optimization of catalytic formulations must take into account the structure of the active phase. Since it is known that its structure may change during activation and catalytic work, it is also important to know its final state, after these treatments. This is particularly the case of alumina-supported cobalt-molybdenum catalysts used in hydrodesulfurization (HDS). These catalysts are prepared in their oxide form, although the stable state of the active phase during catalytic work is the sulfided one. Industrially, the activation may be carried-out using the actual feedstock as the sulfiding agent, in the presence of hydrogen. An alternate procedure is to treat first the oxide catalyst with a highly reactive sulfur-containing molecule, to form the sulfided phases, and only then submit it to the reaction mixture. This is the method most frequently used in laboratories. A mixture of hydrogenhydrogen sulfide is often employed. In previous investigation works performed in our laboratory, it was found that the type of activation procedure deeply influenced the activity and selectivity of an aluminasupported Co-Mo HDS catalyst (1,2). Other literature results suggest that some of the important features of the active phase structure of these catalysts are CUS, associated with either Mo (3,4,5) or Co (6), and sulfhydryl (SH) groups (7,8). The possible location for this
560
type of structures in the Mo phase is, according to the literature, the edges of MoS2 crystallites. Whatever the explanation, the dispersion of the active phase in supported catalysts must be a very important parameter. In the present work we carried out an IR study of NO adsorption, quantitative SH analysis and X-ray Photoelectron Spectroscopy (XPS),on alumina-supported catqlysts (two monometallic, Co and Mo,and two bimetallic, Co-Mo). Then, we analysed the modifications of the active phase of those catalysts, brought by the different activation procedures, in terms of the above mentionned features.
2. EXPERIMENTAL 2.1. Preparation of catalysts A detailed description of the preparation of the catalysts used in this study was given elsewhere (9). Briefly, the Mo/Al203 catalyst was prepared by the pore-filling impregnation of a y-Al203 (Rhbne-Poulenc, SBET 200 m2/g) with a solution of ammonium heptamolybdate, in two steps, with intermediate drying (373K) and drying and calcination (773K) at the end. The final Mo loading was 15.1 wt% MoO3, determined by atomic absorption spectroscopy. The Co/A12O3 catalyst was prepared by wet impregnation of the same support with an aqueous solution of cobalt nitrate. Water was evaporated under vacuum and agitation. The catalyst was dried and calcined as above, The Co loading was 2.9 wt% COO. The CoMo/A12Og catalyst (CoMo (lab.)) was prepared by wet impregnation (as above) of the Mo/Al2O3 catalyst with a solution of cobalt nitrate in ethanol (77 ~01%)and water. The latter was used to avoid the redissolution of Moo3 during the Co impregnation (10). The catalyst was also dried and calcined as already described. The final composition was 3.1 w t l COO and 14.9 wt% MoO3. The second Co-Mo catalyst used was the HR306 from Procatalyse (CoMo (ind.)) with a composition of 2.7 wt% COOand 13.5 wt% MoO3.
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2.2. Activation of catalysts All catalysts were activated according to a standard, simultaneous reductiodsulfidation, procedure (RS), under a flow of 15 vol% H2S in H2. The catalysts were first dried under Ar at 423K for 30 mn. Then, the activation gases (100cc/mn) were introduced and the temperature raised at 10Wmn up to 673K (60 mn at steady temperature), with an intermediate step at 573K (30 mn.). Four modifications of this procedure were also used, namely pre- or post-sulfidation (resp. S-RS and RS-S) with a mixture of 15 vol% H2S/Ar, and pre- or post-reduction (resp. R-RS and RS-R) with H2. In these two-part activations, after the first 60 mn period at the final temperature, the gases were switched to the mixture corresponding to the second part of the activation. Then, the activation continued for another 60 mn period.
2.3. XPS analysis The experimental conditions of the XPS analyses will be published elsewhere (1 1). However, it is important to mention that the activated catalysts were never allowed to contact air, from the moment they were activated. This was done by means of an iso-octane meniscus, removed later before the analysis.
561 2.4. Adsorption of NO
The adsorption of NO was camed out inside the cell where the IR measurements took place. The activated catalysts were ground and wafers of about 5 to 8 mg were made. They were reactivated inside the cell, according to the procedures described above. No argon drying was performed, the activation gases were directly introduced in the cell and the temperature raised as above. In two-part activations, e.g., S-RS, only the second part was performed inside the cell. Afterwards the IR cell was allowed to cool down to room temperature and was evacuated for lh (final pressure c.a. 10-5 tom). At this point reference spectra were taken and the catalyst was evacuated for 30 mn at 473K. After cooling again to room temperature, 40 mbar of NO (purified by the freeze-thaw technique) were admitted in the cell and the adsorption was carried out for 30 mn at ambient temperature. At the end, the excess of NO was removed via condensation in a cold trap (liquid N2 temperature) and new spectra were taken. We considered the intensity of the band of NO adsorbed on cobalt, as the height of the peak corresponding to rhe NO dimer symmetric stretching vibration (around 1850 cm-1) (2). For molybdenum, we took the height of the peak corresponding to the NO dimcr asymmetric stretching vibration (around 1700 cm-1) (2). These are the two bands that do not coincide in the bimetallic catalysts. All intensities were normalized to a constant wafer weight of 10 mg. 2.5. Determination of SH groups The surface SH group concentration was determined according to a method developed by Maternova (12), with the additional feature that, after the activation, the reactor was
evacuated at 423K to remove any trace of H2S that would interfere with the subsequent titrations. A blank run was performed before every measurement. About 750 mg of catalyst were used in each case.
3. RESULTS 3.1. X-ray Photoelectron Spectroscopy In TABLE 1 we present the results of the XPS analysis of the catalysts. The values represent the ratio of the normalized XPS peak intensities and correspond to atomic ratios. TABLE 1 XPS peak intensity ratios Mo co ACT. MolA1 SJMo 'co/AT MoJAI OXIDE 0.077 0.016 0.075 S-RS 0.031 3.9 0.01 0.067 R-RS 0.071 1.6 0.011 0.078 0.067 1.9 0.01 0.076 RS RS-S 0.041 3.6 0.009 0.072 RS-R 0.055 1.8 0.01 0.075
* . ColAl SJMo
:% 0.02 0.026 0.025 0.023
2.2 1.8 1.9 2.6 2.0
0- o in MolA1 ColAl SJM0
:0.072 % %% 0.023 0.058 0.018 0.072 0.024 0.065 0.019
2.2 1.8 1.9 2.2 1.7
After mathematical decompositiorf.of the Mo3d peak envelope, in the sulfided samples, it was possible to conclude that more than 82% of the original Mo(V1) was converted to lower oxidation states (more than 87% in the Mo catalyst), of which the Mo(IV) majoritary. It was not necessary to assume the presence of oxidation states lower to obtain a good fit.
562
TABLE 1 confirms that the activation procedures always influence the dispersion of Co and Mo on the surface of the catalysts. For a given activation procedure it is observed that the two Co-Mo catalysts present Mo/Al ratios usually higher than those of the Mo catalyst. Nevertheless, the Mo/Al relative variations as a function of the activttion procedures, remain similar for all of them. The highest Mo/Al ratio is always achieved with the R-RStreatment. The influence of the activation procedure is less important for the Co catalyst. For the latter, the Co/Al ratio remains nearly constant. However, when dispersed over Mo, as with the two Co-Mo catalysts, two effects can be observed. First, there is a significant increase in the amount of Co present at the surface; second, the dispersion of Co in the latter case becomes more sensitive to the activation procedures. The amount of sulfur present at the surface, expressed as the S/Mo ratio, is also sensitive to the activation procedure. It is generally situated around the stoichiomemc value (S/M0=2). This ratio is very high in the cases where H2S was used in the absence of hydrogen, in a certain stage of the activation (S-RSand RS-S).For the Mo catalyst these two procedures yield S/Mo ratios well above the stoichiometric ratio (S/Mo 4), contrary to what happens with the Co-Mo samples.
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3.2. NO adsorption on the catalysts In fig1 we present the IR spectra obtained after NO adsorption on the RS (simultaneous reductionhulfidation) activated catalysts.
fig.1 NO adsorption on catalysts. A) Moly-AI203; B) Co/y-A1203; C) CoMoly-AI203 (lab.); D) CoMoly-Al203 (ind.) The position of these bands (1785 cm-l and 1700 cm-l for the Mo; 1857 cm-l and 1796 cm-l for the Co; 1846 cm-l, 1788 cm'l and 1694 cm-l for the two Co-Mo catalysts) agrees with the literature results (2,6). For the rest of the activations the spectra were similar to these ones, within each type of catalysts, except the intensity of the bands. These are reported in TABLE 2.
The results presented in TABLE 2 show that, for monometallic catalysts, the dependence of the intensity of the bands of adsorbed NO on the activation is different from that of the bimetallic ones. Furthermore, the intensities of the bands of NO on the monometallic catalysts are usually higher than the corresponding ones in the bimetallic catalysts. The most evident exception to the previous observation comes from the two Co-Mo catalysts activated by RS-R, where the intensities are higher than those of the corresponding Mo and Co catalysts. Finally, the activation procedure RS-S inhibits almost any NO adsorption on any of the catalysts. TABLE 2 Normalized intensities of the IR bands of NO adsorbed on Co and Mo Mo co Co-Mo (lab) Co-Mo (ind) I NO(Co) I NO(Mo) I NO(Co) I NO(Mo) I NO(Co) ACTIV. I NO(Mo) S-RS 1.85 3.76 0.84 1.11 1.69 RTRS 3.28 5.67 0.75 0.86 0.25 RS 2.96 2.61 1.42 2.28 1.54 3.22 RS-S 0.13 0.4 0.08 0.09 0.04 0.04 RS-R 1.52 1.3 3.17 6.45 3.7 1 8.85
::::
3.3. SH group determination In TABLE 3 we present the results concerning the quantitative determination of the surface SH groups of the catalysts. This concentration is also very sensitive to the activation procedures. For the three Mo-containing catalysts the variation of this concentration as a function of the activation procedure is approximately the same. For a given activation. the values presented by the Mo/Al203 are always higher than those of the two Co-Mo samples. TABLE 3 Concentration of surface SH groups (mol swg)/10-4 ACTIVATION Mo co Co-Mo (lab) S-RS 5.3 1.o 4.9 8.2 0.6 7.4 R-RS 7.7 0.4 6.4 RS 6.4 3.3 6.4 RS-S 7.5 2.0 5.8 RS-R
Co-Mo (ind) 4.6 7.3 6.2 5.8 5.8
The Co catalyst presents the lowest SH concentration, and the influence of the activation procedure is not similar to that of the other catalysts.
4. DISCUSSION
From the results presented above, and considering that the metal loading was almost constant, it is evident that the modification of the activation procedure brings about deep changes in the texture and surface structure of the active phase. 4.1. XPS
In alumina-supported Mo-containing catalysts it is known that, upon sulfidation, the structure of the Mo phase changes from a layer (ordered or patch-like) of molybdenum trioxide to small plate-like crystallites of molybdenum disulfide, distributed all over the surface (13,14). The MoS2 thus formed, can be viewed as a stacking of slabs, each of which consists of a layer of Mo atoms between two layers of sulfur atoms. They form a hexagonal
564
structure, where each Mo atom is bonded to six sulfur atoms (three in each layer) and each sulfur atom is bonded to three Mo atoms. At the edges of the slabs, the S atoms are bonded to two Mo atoms at the most (18). Due to their lower coordination state, these edge S atoms should logically undergo reactions in an easier way than those present in the basal planes. The direct consequence of the structural modification due to the sulfidation, is that a larger proportion of A1 atoms, that were initially covered by M o o 3 become free from this overlayer. The A1 X P S signal increases, in this situation. On the other hand, a loss of Mo signal can also occur due to the "agglomeration" of Mo atoms into MoS2 crystallites. This explains the general decrease of the Mo/Al ratio we observe upon sulfidation, and it is in close agreement with what was reported by Okamoto et al. (4). For the three Mo-containing catalysts, the similar response of the Mo/Al ratio to the activation procedures suggests that the same type of transformations is at stake, irrespective of the presence or absence of cobalt. The higher Mo/AI ratio found in the Co-Mo catalysts, despite the possibility that the presence of Co may hinder Mo, could be due to a lower tendency of molybdenum to form larger crystallites, either with larger slabs or with a higher number of slabs per crystallite. The deposition of cobalt over molybdenum greatly enhances the surface concentration and dispersion of the former. Probably, this occurs via Co-Mo interactions, which may depress the tendency of Co to combine with the support. The higher sensitivity of Co dispersion to the activation procedures, that our results show, would then be explained. In the literature, this Co-Mo interaction is supposed to be the result of the formation of a Co layer between the alumina support and the Mo oxide layer (15, 16). The result we obtained further confirms that cobalt and molybdenum, when present together, have a mutual beneficial influence on the dispersion of each other (21). This is still true, even after the activation. When H2S is used in the absence of H2 (activations S-RS and RS-S),it is possible that a larger amount of sulfur may remain attached to the surface of the catalyst. The higher S/Mo ratio we obtained may be a consequence of it, as was also observed in (17). The much larger values obtained for these activations in the Mo catalyst than in the corresponding CoMo ones, strongly suggests that cobalt plays a role in the removal of these sulfur species. 4.2.
NO adsorption
The formation of CUS should be favoured at the edges of MoS2 crystallites, since it is there that the S atoms can be found in a lower coordination state, as developed in the geometrical model proposed by Wambeke et al. (18). The formation of this type of structures could be the result of the full hydrogenation of SH groups or S anions, with the concomitant desorption of H2S. Another possibility would be the partial hydrogenation of bridging S atoms, with the simultaneous breaking of a Mo-S bond, yielding an SH group and a CUS. The tricoordinated basal plane S atoms would be less prone to undergo similar processes. The almost complete inhibition of the NO adsorption on all catalysts activated according to RS-S indicates that, most probably, H2S adsorbs (dissociatively or not) on the same sites as NO, as was suggested by Okamoto and co-workers (4). If we admit that the CUS are the adsorption sites of the reactant molecules, this fact may explain the inhibitive effect of H2S on the HDS activity of these Catalysts, as reported in many references (see, for instance (20)),. Another possibility for explaining this effect is that a higher H2S partial pressure would render more difficult the desorption, as H2S, of the S atom left on the CUS after the desulfurization reaction. In the post-reduction treatment, RS-R,the increased intensity of the band of NO adsorbed on cobalt in the bimetallic catalysts, as compared to what happens with the monometallic samples, could be due to an enhanced reducibility of Co because of its higher dispersion state. On the other hand, since the dispersion state of Mo did not change as much, from the monometallic to the bimetallic catalysts, the increased NO adsorption on this metal should be attributed to other factors. The lower amount of sulfur found in CoMo catalysts,
565
when compared to the Mo sample, seems to indicate that the enhanced NO adsorption capability of the former could be due to a higher unsaturation state of the Mo phase. The presence of H2S in the final stages of the other activation procedures, is probably the cause for the fact that such an increased NO adsorption is not verified then. The decrease of the NO adsorption on Mo could also be due to an interference of cobalt with the Mo sites capable of forming CUS,as'was observed by Tops@ et al. (6). Although the total number of sites available for NO adsorption decreases from the monometallic to the bimetallic catalysts, the unsaturation degree of the remaining sites seems to have increased, as the shift towards lower wavenumbers of the bands of adsorbed NO indicate. This fact is true for both Co and Mo atoms. For Co, this effect could be attributed to its enhanced dispersion in the Co-Mo catalysts. However, for Mo it is most probable that a direct effect of the presence of Co, through an enhanced sulfur removal, be the reason for this effect, since its dispersion state!did not change as much. The fact that, logically, the formation of CUS is easier at the edges of the MoS2 crystallites allows us to give a physical meaning to the variation of the dispersion indicator, the XPS Mo/Al ratio.
' 9 1
0,08 0,07
/
A/ -
0
3 0806
A-
' i3
0,05
-1
0,04 0 03 -,--
0
l
b
)
5
1
2
I NO(Mo)
3
4
0
1
2
3
4
I NO(Mo)
fig.2 a) Variation of the MoIAI XPS ratio with the intensity of the band of NO adsorbed on Mo in the Moly-Al2O3 catalyst; b) Variation of the SH concentration with the intensity of the band of NO adsorbed on Mo (Moly-Al2O3 catalyst). In fig.2a it is possible to see that, with an increase in the intensity of the bands of NO adsorbed on Mo in the MoIAl203 catalyst, there is a simultaneous increase of the MoIA1 ratio. This would mean that a high Mo/Al ratio corresponds to a high crystallite edge area. Since the active phase loading is constant, this means that the crystallite size must be changing. More precisely, once a higher edge area is achieved with smaller crystallites, the size should be decreasing with the increase of the Mo/Al ratio. Evidently, no conclusions can be drawn concerning the number of slabs per crystallite, for the total edge area may not be affected by a variation of that parameter. The insertion of Co in the Mo catalyst eliminates the relationship above shown, although there is still a general increase of the ratio, for high intensities of the adsorbed NO band. This is possibly because Co interacts directly with the sites capable of forming CUS.
4 3 . Sulfhydryl groups The above mentionned structural factors, leading to an easier formation of CUS on the edges of MoS2 crystallites, also favor the formation of SH groups in that place. During activation, S H groups may be formed by the hydrogenation of terminal or bridging sulfur anions. Alternatively, and according to the literature, H2S may adsorb dissociatively on the CUS forming SR groups (19). These possibilities show that the activation conditions may influence the SH concentration of a catalyst. This is what we observed in the present work. The similarity of behaviour, with regard to the activation. of the three Mocontaining catalysts, strongly suggests that the changes observed are essentially due to the SH groups associated with the M o phase. The higher SH concentration presented by the Mo catalyst may seem surprising if one thinks that the corresponding concentration in the Co catalyst is not negligible. As the surface concentration of Co in the Co-Mo catalysts is twice as high as in the Co one, the S H groups associated with Co should be proportionally more important, leading to a much higher overall SH concentration. One may infer. therefore, that the structure of the bimetallic catalyst changed in such a way as to prevent the formation of as many SH groups as if Mo and Co behaved independently. Simultaneously, the cause for the improved sulfur removal on Co-Mo catalysts mentionned above could also lead to the decrease of their SH group's concentration. It is interesting to remark that, in an IR study of high temperature pyridine adsorption, an increase of the Bronsted acidity was remarked when passing from a Co-Mo to an Mo catalyst (19). If we plot now the SH concentration as a function of the intensity of the band of NO adsorbed on Mo, see fig.2b. it is possible to see that there is an increase of the former's concentration as the latter increases. This is an indication, as was suggested at the begginning of this section, that in the Mo sulfide pha$e the SH groups should be formed on the same zone of the crystallites as the CUS. i.e., its edges. In fact, from the graphs presented i n fig.2 it issues that there is a proportionality between the St1 concentration and the Mo dispersion indicator. Again, as happened with the CUS, the insertion of Co in the Mo catalyst eliminates this direct proportionality, although a general increase of the SH concentration with the Mo/AI ratio is still observed for the two Co-Mo catalysts. This is an evidence for the interaction of Co with these special Mo atoms. capable of bearing these groups. in freshly activated catalysts. With the method used to determine quantitatively the SH concentration, it is not possible to distinguish between different types of SH groups (if more than one exists): linear or bridging ones for instance (18.22). The existence of polysulfide structures (S,) at the surface of the catalysts has been mentionned in the literature (23). Under the conditions of the activations it could be possible that these species become hydrogenated, and then detected by the SH titration procedure we used. We think, however. that if any polysulfides exist on the surface of the catalysts, it is unlikely that any significant formation of SH groups on them occurs. This is a consequence of the fact that it i s precisely in the two situations where the SlMo ratio largely exceeds the stoichiometric value, that the SH concentration is lower.
5. CONCLUSION In the present work we confirmed that although the composition of the active phase of tIDS catalysts remains constant, it is possible to alter its structure significantly upon submitting them to different activation procedures. Particularly. it was shown that hydrogen sulfide tended to adsorb on the CUS of the catalyst's active phase. The main changes observed in the analysed parameters when passing from the Mo to any of the CoMo catalysts can be summarized as follows: i) the SH concentration and the intensity of the bands of adsorbed NO decrease, (the latter is always true, except when the
567 catalysts are post-reduced (RS-R), where a large increase is observed); ii) for the two activations where H2S is used in the absence of hydrogen, the S/Mo ratio passes from a value that largely exceeds the stoichiometric one, to another close to it; iii) there is a generalized shift of the position of the adsorbed NO bands towards lower wavenumbers. The introduction of cobalt in the Mo catalyst seems to have two main consequences: the first is that Mo atoms that usually bear special type of structures (CUS and SH groups) loose part of their capability for that function; and second, the general reduction state of the active phase elements is increased. The role of cobalt is, then, consistent with the idea of enhancement of the coordinative unsaturation degree of the Mo atoms and more efficient removal of adsorbed sulfur.
6. REFERENCES B.Delmon, Proc.Climax 3rd Int.Conf. Chem.Uses Molybd. (H.F.Barry,P.C.H.Mitchell eds.), Climax Molybdenum Co.,1979, p.73 R.Prada Silvy, J.L.G.Fierro, P.Grange, B.Delmon, Prep.Catal.IV (B.Delmon, P.Grange, P.Jacobs, G.Poncelet, eds.), Elsevier, Amsterdam, 1987, p.605 Y.Okamoto, A.Maezawa, T.Imanaka, J.Catal., 120 (1989), 29 Y.Okamoto, H.Tomioka, Y.Katoh, TJmanaka, S.Teranishi, J.Phys.Chem., 84 (1980), 1833 H.J.Jung, J.L.Schmitt, H.Ando, Prw.Climax 4th 1nt.Conf.Chem.Uses Molybd., (H.F.Barry, P.C.H.Mitchel1eds.), Climax Molybdenum Co., 1982, p.246 N.-Y. TODS0e. H.To~s0e.J.Catal.. 84 (19831.386 (1988), 473 B.Delm340K t h e d e h y d r a t i o n r e a c t i o n is completed w i t h i n 10 minutes [ 7 a ] . T h e r e f o r e , s p e c t r a , o b t a i n e d f o r a d s o r - bed t - B ~ O H [ 2 - ~ t l which ~] was h e a t e d a t 373K f o r an hour ( F i g . 3 ) , c o n t a i n no c o n t r i b u t i o n from t h e u n r e a c t e d a l c o h o l and a r i s e o n l y from o l i g o m e r i c prod u c t s and w a t e r . F i g . 3 shows t h a t *H NMR spectrum of d e h y d r a t i o n p r o d u c t s i s a s u p e r p o s i t i o n of two l i n e s w i t h quadrupole s p l i t t i n g s ( 3 / 4 ) Q 1 , ( 3 / 4 ) Q 0 which belong t o CD3 and CD2(CD) groups of t h e o l i g o m e r s formed, p l u s one more l i n e , l o c a t e d a t t h e c e n t e r of t h e spectrum. T h i s l i n e m a y be t e n t a t i v e l y a s c r i b e d t o t h e s i g n a l of d e u t e r a t e d w a t e r , i . e . t o D20 o r DHO molecuI e s t h a t can be formed i n H-D i s o t o p e exchange r e a c t i o n between and H2O m o l e c u l e s , i n i t i a l l y formed upon d e h y d r a t i o n of Inversion-r-ecovery 2H NMR s p e c t r a . I n o r d e r t o r e s o l v e b e t t e r v a r i o u s s i g n a l s t h a t a r e p r e s e n t i n t h e s p e c t r a of F i g . 3 , we have c a r r i e d o u t i n v e r s i o n - r e c o v e r y e x p e r i m e n t s . One can e x p e c t t h a t v a r i o u s CD3, CD2 and CD grou p s of adsorbed o l i g o m e r s and adsorbed deuterated water w i l l d e m o n s t r a t e d i f f e r e n t molecular m o b i l i t i e s and t h e r e f o r e d i f f e r e n t r e l a x a i o n times T i . T h i s means t h a t v a r i o u s components of t h e observed o v e r a l l H NMR s i g n a l w i l l recover a f t e r t h e i n v e r t i n g 180" p u l s e w i t h d i f f e r e n t r e l a x a t i o n r a t e s t o t h e i n i t i a l e q u i l i b r i u m i n t e n s i t i e s . I t a l l o w s one t o d i s t i n g u i s h more r e l i a b l y between v a r i o u s l i n e s t h a t c o n t r i b u t e t o t h e o v e r a l l s p e c t r u m , a n d even sometimes t o r e v e a l some a d d i t i o n a l l i n e s t h a t a r e n o t observed i n t h e o r d i n a r y 2 H NMR spectrum. The f o l l o w i n g f o u r l i n e s a r e c l e a r l y d i s t i n g u i s h e d i n t h e i n v e r s i o n - r e c o v e r y s p e c t r a , r e c o r d e d a t 2433 f o r t h e p r o d u c t s of t h e s l c o h o l d e h y d r a t i o n ( F i g . 4 ) . (1) The l i n e w i t h t h e quadrupole s p l i t t i n g
5
626
I . . . . , . . . . I . . . . , . . . . I . . . ,
100000
0 HERTZ
I . . . . , _ . . . I . . . . ,
100000
0 HERTZ
-100000
. . . .
I . . . .
-100000
I....,....I....,....I...'
100000
0 HERTZ
-100000
I . . . . . . . . . I . . . . , . . . . I . . . ,
100000
0 HERTZ
-100000
F i g u r e 3. 'H NMR l i n e s h a p e s f o r t h e p r o d u c t s of ~ - B u O H [ ~ - ~ Hd e~h] y d r a t i o n on H-ZSM-5 ( S i / A 1 = 2 9 ) . T e m p e r a t u r e s o f t h e s p e c t r a r e g i s t r a t i o n a r e g i v e n above each spectrum.
(3/4)Q -11.6 kHz. T h e time To, when t h e i n t e n s i t y o f t h i s NMR l i n e t u r n s from i n v e r t e d n e g a t i v e p o s i t i o n t o t h e normal p o s i t i v e one, i . e . p a s s e s t h r o u g h z e r o , is a p p r o x i m a t e l y 10 m s . ( 2 ) The l i n e w i t h (3/4)Q1=37.6 kHz a n d C0C15 m s . (3) T h e l i n e w i t h (3/4)Q0=123 kHz a n d ToL.10 m s . ( 4 ) T h e l i n e w i t h (3/4) Q3'1.9k0.1 kHz. F o r t h i s l i n e To 55 m s . T h u s , i n v e r s i o n - r e L o v e r y e x p e r i m e n t a l l o w s u s t o c l e a r l y r e v e a l t w o more l i n e s w i t h q u a d r u p o l e s p l i t t i n g s (3/4)Q2 a n d (3/4)Q3 i n a d d i t i o n t o t h e l i n e s w i t h (3/4)Q1 a n d (3/4)Q0. T h e f i r s t o f t h e two new l i n e s h a s t h e same q u a d r u p o l e s p l i t t i n g (3/4)Q2 a s t h a t f o r (CD3l3C g r o u p i n t-BuOH[ 2-2H9J, a n d thus should be a t t r i b u t e d t o tert-butyl groups i n t h e r e a c t i o n products. S i n c e water i s o n e o f d e h y d r a t i o n p r o d u c r s , w e a s s i g n t h e s i g n a l w i t h (3/4)Q3 t o D20 ( o r DHO). To p r o v e t h i s a s s i g n m e n t w e a d s o r b e d D20 o n H-ZSM5 i n t h e a m o u n t e q u a l t o t h e a m o u n t o f water t h a t s h o u l d b e e v o l v e d i n t h e d e h y d r a t i o n r e a c t i o n . D20 a d s o r b e d on H-ZSM-5 z e o l i t e i n t h i s s e p a r a t e e x p e r i m e n t e x h i b i t s 2H NMR s p e c t r u m of t h e t y p e , g i v e n i n F i g . 5 A i n t h e t e m p e r a t u r e r a n g e 290-35OK. I t i s s e e n t h a t t h e o b s e r v e d l i n e s h a p e f o r t h e s i g n a l from a d s o r b e d D 0 (Fig.5A) c o i n c i d e s w i t h t h a t f o r t h e s i g n a l s w i t h t h e s p l i t t i n g (3/4)$3, t h a t were o b s e r v e d i n b o t h i n v e r s i o n - r e c o v e r y s p e c t r a ( F i g . 4 ) and o r d i n a r y s p e c t r a (Figs.3A-C). Thus, w e c o n c l u d e t h a t t h e l i n e w i t h (3/4)Q3 i n d e e d b e l o n g t o D20.
tie
627
l
.
.
.
.
,
.
.
.
.
I
0
100000
.
HERTZ
_
.
.
,
.
,
I
.
.
.
-100000
F i g u r e 4 . 2H NMR i n v e s i o n - r e c o v e r y s p e c t r a r e c o r d e d a t 243K r t h eD e l a y p r o d u c t s of t-BuOH[2- Hg] d e h y d r a t i o n on H-ZSM-5 (Si/A1=29) a ft o 373K. time t v is g i v e n above e a c h s p e c t r u m .
5
When i n c r e a s i n g t h e t e m p e r a t u r e of t h e s p e c t r a r e g i s t r a t i o n from 173K t o 373K, t h e l i n e i n t e n s i t y a t J o grows w h i l e t h e r e l a t i v e i n t e n s i t i e s of t h e s o l i d - l i k e s i g n a l s w i t h t h e s p l l t t i n g s ( 3/4)Q1 a n d ( 3 / 4 ) Q , d e c r e a s e s ( F i g . 3 ) . T h i s r e d i s t r i b u t i o n of t h e l i n e i n t e n s i t i e s can b e e x p l a i n e d i n terms of e x i s t e n c e of t h e two o l i g o m e r i c s p e c i e s t h a t d i f f e r i n t h e i r molec u l a r m o b i l i t i e s . I n t h e i n v e r s i o n - r e c o v e r y spectra ( F i g . 6 ) , r e c o r d e d a t 373K f o r t h e same sample a s i n f i g . 4 , t h e f o l l o w i n g t h r e e l i q u i d - l i k e l i n e s , b e s i d e s two s o l i d - l i k e l i n e s , a r e c l e a r l y d i s t i n g u i s h e d . (1) The l i n e a t Jo w i t h t h e w i d t h 7.8 kHz a n d Z "30 m s . T h i s l i n e is most c l e a r l y o b s e r v e d i n t h e s p e c t r u m f o r tv=120ms. The narrow l i n e a t u o w i t h r h e w i d t h of 2 . 5 kHz and ~ o ~ 1 2 0 m s .which , r e p r e s e n t s t h e s i g n a l of D 0 (DHO). The s h a p e and t h e w i d t h of t h i s l i n e c o i n c i d e w i t h Lhose f o r t h e l i n e o f D20 on H-ZSM-5
(8)
628
373K
- 100000
I . . . . I . . . . l . . . . , . . . . I . I .
0
100000
HERTZ
l.........I....,....I....
0
100000
-100000
HERTZ
F i g u r e 5. 2H NMR s p e c t r a o f D20, a d s o r b e d o n H-Z91-5 amounL o f a d s o r b e d water is 214 p m o l / g , 0 . 5 2 w t % 1.
z e o l i t e ( S i / A l =29,
a t 373K ( F i g . 5 R ) . ( 3 ) T h e l i n e w i t h t h e w i d t h 27 kHz a n d ' L o e 7 0 m s . T h i s l i n e is most c l e a r l y o b s e r v e d i n t h e s p e c t r u m w i t h tv=30 m s . T h e s i g n a l s w i t h t h e l i n e w i d t h s 7 . 8 a n d 27 kHz s h o u l d b e a t t r i b u t e d r e s p e c t i v e l y t o CD3 a n d CD2 (CD) g r o u p s o f o l i g o m e r i c s p e c i e s , whose m o b i l i t i e s a r e s u f f i c i e n t t o a v e r a g e o u t t h e q u a d r u p o l e s p l i t t i n g s ( 3 / 4 ) 4 1 a n d ( 3 / 4 ) Q 0 [ 2, && (22nQ1)-' , (?NQ0)-' 1 , i . e . s p e c i e s w i t h ' C c 4 1 ~ 1 0 -s. ~ The predominance o f t h e l i q u i d l i k e l i n e s h a p e a t 373K s u g g e s t s t h a t most of CD3 a n d CD2(CD) g r o u p s , g i v i n g r i s e t o t h e s p l i t t i n g s ( 3 / 4 ) Q a n d ( 3 / 4 ) Q , a t lower t e m p e r a t u r e s , a t 373K become i n v o l v e d i n i s o t . r o p i c a y n a m i c p r o c e s s . Note, t h a t v a r i a t i o n s i n t h e *Id NMR s p e c t r a w i t h t h e t e m p e r a t u r e a r e r e v e r s i b l e , i . e . t h e s p e c t r u m of F i g . 3A is a g a i n o b s e r v e d when t h e t e m p e r a t u r e of s p e c t r u m r e g i s t r a t i o n -is d e c r e a s e d f r o m 373K b a c k t o 173K.
4 . 3 . On the d i f f u s i o n c o e f f i c i e n t s of t e r t - B u t y l alcohol a n d oligomeric p r o d u c t s of i t s d e h y d r a t i o n i n H-ZSM-5 z e o l i t e . C o e f f i c i e n t s of i n t r a c r y s t a l l i n e d i f f u s i o n o f t h e m o l e c u l e s i n z e o l i t e a t low l o a d i n g s c a n b e m t i m a t e d u s i n g t h e E i n s t e i n e q u a t i o n 1 4 , 5 1 :
D =
(5)
6Th
>
. t h e mean s q u a r e d i s p l a c e m e n t o f t h e m o l e c u l e i n s i d e t h e c r y s w h e r e ,can be neglected, since the accumulation on the catalyst surface is much greater. The relationship between the bulk concentration and the concentration at the external surface of the catalyst disc is given by
N = k,(Cb -
c,)
( 41
where N is the molar flux, k, the external mass transfer coefficient and C, = C(t,O) the concentration at the external surface. The molar flux can also be written
Combining eqs (3)-(5) and neglecting the gas phase accumulation give the boundary condition at the external surface of the disc (x=O)
649
The boundary condition in the center of the catalyst is due to the symmetry
ace, u2) T
=
O
(7)
where L is the thickness of the catalyst disc. The initial conditions for a step from unlabelled to labelled compound are C(0. x) = 0
qo, x)
=
o
The eight equations of the mathematical model given above make it possible to calculate the local fractional coverage of the radioactive species as a function of time and position in the catalyst disc. The next step is to express the observed radioactivity as a function of the total fractional coverage of the labelled compound. The local radioactivity in the catalyst disc is proportional to the local fractional coverage of this compound. However, in order to express the part of the observed radioactivity from this particular local fractional coverage of labelled compound, the decrease in radioactivity due to the absorption of P'-particles in the catalyst must be accounted for. The contribution of the emitted P--particles from the disc element dx, at the position x, to the observed total radioactivity can be written
dl = k B(x.r)e-'%
(9)
where p is the linear absorption coefficient and k is a constant. The local fractional coverage is symmetric about the center line x = W of the catalyst disc, since the gas is flowing on both sides of the disc. This means that it is sufficient to integrate from the external surface to the middle of the disc. The total observed radioactivity will thus be
When the catalyst disc is in isotopic equilibrium with the fresh labelled gas mixture the observed radioactivity is I,.,= and the fractional coverage of labelled CO is 8 4 . This makes it possible to calculate k from eq. (10) giving
650
and
The fractional coverage of the labelled compound has to be defined in an appropriate way when determining the constant k in eq. (10). At the partial pressure corresponding to 1000 ppm CO at atmospheric pressure in the present experiment, the palladium part of the catalyst is considered to be completely covered with CO. The fractional coverage of CO is here normalized to unity despite the fact that the fractional coverage according to geornetrical constraints may correspond to a value less than unity. Since isotopic effects can be neglected. the fresh mixture with labelled CO is considered as an indivisible unity which during the isotopic exchange process is diluted with unlabelled CO of the same partial pressure. When this fresh labelled mixture is in isotopic equilibrium with the catalyst surface the fractional coverage of labelled CO is given the value 8=1. Whereas, when this labelled mixture is diluted with unlabelled CO the fractional coverage will be 8 < 1 and will reach the value 8 = 0 when all of the labelled mixture is washed out from the system. Correspondingly the concentration of labelled CO is C=Cco,totin the fresh labeIled gas mixture.
4.
RESULTS AND DISCUSSION
4.1
Desorption of CO
Fig. 3 shows the rapid decrease of radioactivity from the catalyst disc when the inflow is switched over to unlabelled CO. A similar effect is also demonstrated when the inflow suddenly contains a step of C2H2 or NO at low concennation. It is a well-known fact that CO desorbs very slowly in an inert gas atmosphere or in vacuum at room temperature, but the rate of a desorption is known to increase dramatically in the presence of CO in the gas phase [6-211. This enhanced desorption is not preceded by any dissociation and recombination of CO on the surface which can proceed on certain catalysts, but occurs via the displacement of an adsorbed C O molecule as a whole. As can be seen from Fig. 3 the isotopic exchange proceeded until the radioactivity completely disappeared.
651 14
CO
Relative 1 Count Rate
A
0.5
14
NO
CO
I
Relative Count Rate
0
500
1000
1 G Time I s
Time I s
Fig. 3 Count rate from a 14C0 covered 5% Pd/AI,O, catalyst. T = 25OC P = 1 atm. a. Flushed with Ar, 100 ml mid'. b. Flushed with 1000 ppm CO in Ar, 100 ml mid' c. Flushed with 1000 ppm NO in Ar, 100 ml mid' d. Flushed with 1% C2H2 in Ar, 100 ml min-' The desorption of CO in the presence of C2H2 is not as rapid as the isotopic exchange of CO. The effect of C2H2 is an example of the so-called cross-desorption, which was recently demonstrated by Cidex and Schoon [22-251. As can be seen from Fig. 3 this cross-desorption does not completely remove CO from the catalyst surface. The cross-desorption of CO in the presence of NO, which hitherto has been studied very little, removed on the other hand CO more completely from the catalyst surface. 4.2
Quantitative evaluation of the isotopic exchange
The mathematical model has to be completed by assuming the form of the rate equations for the adsorption and desorption of CO, when the isotopic exchange of CO is to be quantitatively described. In a first attempt these equations were chosen to be
652
which means that only the parameter k,,, has to be determined when fitting the calculated radioactivity to the observed one. The values of the mass transfer coefficient k,, the effective diffusivity D,, and the linear absorption coefficient p necessary for this calculation are given in Table 1. The value kexc = 30 s-l at 25°C gave a very good agreement between calculated and observed radioactivity. This indicates that the mathematical model is a good description of the rate of radioactivity change during the isotopic exchange process. The corresponding quantitative evaluation of the cross-desorption kinetics will be given elsewhere [26]. 4.3
Comments on the tracer method
The present tracer technique is a good complement to the FTIR method since the measuring cell can be used for both methods. By only removing the photomultiplicator and the light conductor, the cell is ready for transmission FTIR experiments. The well-known advantage of the radioactive tracer technique is that a step in a labelled compound can be introduced at the inflow without changing the chemical composition in the reactor and thus without disturbing the chemical equilibria between the gas and the catalyst surface. This is important in the study of surface processes, including adsorption and desorption, since it is often difficult to find a kinetic model that is valid over a wide range of chemical compositions. The maximum allowable temperature with the present set up for radioactivity measurements depends on the cooling of the glass rod and the photomultiplier. Water was used as a coolant medium, but this can be changed to liquid nitrogen to achieve better cooling. The highest temperature used here was 110°C. The upper temperature limit is set by the O-rings, that can withstand 320°C.However, other cells for in situ IR studies have been developed that permit even higher temperatures [27]. The O-rings in these cells are replaced by Cu and Graphoil gaskets. It would also be possible to replace a window by a solid scintillator if even higher temperatures are required. The maximum pressure for the cell, in the present work was 20 bar and was determined by the maximum allowable pressure for the windows [28]. Since the average residence time in the reactor cell is very short compared to the time constants of most processes studied, the reactor was considered to behave like a continuous stirred tank reactor from a mixing point of view. However, just after a step change this approximation is not valid for a rapid process. During this short part of the step change, the response data cannot be used for determination of rate constants.
653
5.
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support of the Swedish Research Council for Engineering Sciences. We also thank the people at the Department of Reactor Physics at Chalmers University of Technology for technical assistance and helpful discussions.
6.
NOMENCLATURE
A = CC0,tot =
c c,
=
=
Cf
=
Deff
= =
Inlax
=
kexc
=
L
=
c,
d I = I k
kc
=
=
N
=
N,
=
9
--= -
‘a rd
t
v X
-=
=
-
area of the catalyst disc, m2 total concentration of CO in the gas phase, mol mm3 concentration of labelled CO in the gas phase in the catalyst, mol n f 3 concentration of labelled CO in the bulk gas phase, mol m-3 concentration of labelled co in the feed, mol m-3 concentration of labelled CO at the external surface of the catalyst disc, mol m-3 effective diffusivity, m2 s-l local radioactivity contribution to the measured radioactivity, cps measured radioactivity, cps maximum measured radioactivity, cps proportionality constant between the local fractional coverage of labelled CO and the local radioactivity rate constant of isotopic exchange, sml mass-transfer coefficient, m s-l thickness of the catalyst disc, m molar flux of labelled CO to the catalyst disc, mol s-l m-2 site density, mol (kg catalyst)-’ volumetric flow rate through the cell, m3 s-l adsorption rate of labelled CO, mol s-’m-3 desorption rate of labelled CO, mol s”m” time, s volume of reactor cell, m3 length coordinate perpendicular to the external surface, m
Greek Symbols e 8 p p
= = = =
void fraction fractional coverage of labelled CO linear absorption coefficient density of the catalyst disc
654 7.
REFERENCES
1 2 3
K.C. Campbell and S.J. Thomson, Progr. Surf. Membr. Sci., 9 (1975) 163. G.F. Bemdt, Catalysis, 6 (1983) 144, The Royal Society of Chemistry, London 1983. S.J. Thomson in Characterisation of Catalysts, (J.M. Thomas and R.M. Lambert, eds.), John Wiley & Sons, Chichester, 1980. R.F. Hicks, C.S. Kellner, B.J. Savatsky, W.C. Hecker, and A.T. Bell, J. Catal., 71 (1981) 216. E. Wicke and R. Kallenbach, Kolloid Z., 97 (1941) 135. K. Klier, A.C. Zettlemoyer, and H. Leidheiser, Jr., J. Chem. Phys., 52 (1970) 589. J.T. Yates, Jr., T.M. Duncan, S.D. Worley, and R.W. Vaughan, J. Chem. Phys., 70 (1979) 1219. J.T. Yates, Jr., T.M. Duncan, and R.W. Vaughan. J. Chem. Phys., 71 (1979) 3908. J.T. Yates, Jr., D.W. Goodman, J. Chem. Phys., 73 (1980) 5371. T. Matsushima, Surface Sci., 79 (1979) 63. T. Matsushima, Surface Sci., 89 (1979) 665. T. Matsushima, J. Catal., 64 (1980) 38. T. Yamada, T. Onishi, and K. Tamaru, Surface Sci., 133 (1983) 533. T. Yamada and K. Tamaru, Surface Sci., 138 (1984) L155. T. Yamada and K. Tamaru, Surface Sci., 146 (1984) 341. V.P. Zhdanov. Surface Sci., 157 (1985) L384. T. Yamada, T. Onishi, and K. Tamaru, Surface Sci., 157 (1985) L389. K. Tamaru and T. Yamada, Shokubai, 27 (1985) 350. K. Tamaru, T. Yamada, R. Zhai, and Y. Iwasawa, Proc. 9th Int. Congr. Catal., Calgary 1988, The Chemical Institute of Canada, Ottawa, 1988, p. 1006. K. Tamaru, Colloids and Surfaces, 38 (1989) 125. T. Yamada, R. Zhai, Y. Iwasawa, and K. Tamaru, Bull. Chem. SOC.Jpn., 62 (1989) 2387. L. Cider and N.-H. Schoon, Appl. Catal., 68 (1991) 191. L. Cider and N.-H. Schoon, Appl. Catal., 68 (1991) 207. L. Cider and N.-H. Schoon, Ind. Eng. Chem. Res., 30 (1991) 1437. L. Cider, U. Schriider, N.-H. Schoon, and B. Albinsson, I. Mol. Catal.. 67 (1991) 323. U. Schroder and N.-H. Schoon, to be published in 1992.> B.J. Savatsky and A.T. Bell in Catalysis Under Transient Conditions, (A.T. Bell and L.L. Hegedus, eds.) ACS Symposium Ser. 178, American Chemical Society, Washington, D.C., 1982. A.T. Bell in Chemistry and Physics of Solid Surfaces, Springer Ser. Chem Phys., (Chem. Phys. Solid Surf. 5). 35 (1984) 23.
4 5 6 7
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
655
DISCUSSION
Q: M. A. Baltanas (Argentina) Just a few remarks about your paper, which I enjoyed very much: 1) Concerning the "similar information" you could have obtained via IR, certainly your technique does not require a preformed, transparent pellet. You can as well use a powder sample ! 2) Two possible improvements are: (i) the use of a metal holder heated from the exterior with an inductive coil, and (ii) the use of flow-thru set-up to avoid channelling and mixed signals proper of cross flow.
A: U. Schroder Thank yor for your valuable comments.
Q: D. G. Blackmond (USA) 1) Can you comment on how your technique compares with other steady-state switching techniques which utilize non-radioactive isotopes, detecting the gas-phase exiting the reactor by mass spectroscopy (for example, see p.18iand p. 219 ) ? In particular, your experiment contains information about the changing surface concentration of an isotope together with its changing gas phase concentration. How can these contributions be deconvoluted? 2) The comparison of CO desorption rates in the presence and absence of gas-phase CO appears to be misleading since in the first case the system is under adsorptiondesorption equilibrium (CO exchan e might be a better term than desorption) and in the second system is far from equilibrium. an you comment ?
E
A: U. Schroder 1) The main advantage with our technique is that we measure the adsorption directly on the working catalyst. This means that we, for instance, can follow a slow adsorption process during reaction. This may be difficult with techniques where the adsorption rate is obtained indirectly by measuring the concentration in the effluent from the reactor. Another advantage is that the technique is very cheap and simple. Since the gas volume of the cell is very small, the gas-phase contribution to the measured radioactivity is also very small. Furthermore, in experiments where we make a step from labeled to unlabeled CO, the radioactivity from the gas-phase is continuously flushed away. The contribution from the radioactivity in the gas-phase can, moreover, be compensated for by making an experiment where you replace the catalyst disc by a disc of only the support material. 2) We completely agree with you that ''exchange rate" might be a better term than "desorption rate" since it is possible that the exchange with the gas-phase only occurs from certain sites and that CO on the other sites are displaced because of the mobility of CO on the surface. Q: A. K. Datye (USA) The information obtained with this technique can also be obtained, in principle, by IR spectroscopy. Furthermore, IR transparent discs are not necessary since diffuse reflectance can be performed on powders. How wold you compare your technique with other techniques such as IR that provide in situ probes of adsorbed species'?
A: U. Schroder With our technique we actually measure the amount of adsorbed species regardless of the adsorption mode. Using IR measurements for the same purpose, you must correlate the IR absorption with the amount of adsorbed species. This calibration can be very tricky, especially when you have a coadsorption of several species. The IR absorption band is not only dependent on how much of a certain compound that is adsorbed, but also on the
656
adsorption mode and on the interaction with other adsorbed molecules. IR is a very powerful method to study the adsorption mode but not as a tool for quantitative measurements for complicated mixtures. The evaluations of diffuse reflectance IR is moreover known to be more complicated than for transmission IR. Finally, our technique is cheaper, and in our opinion easier to apply, than IR.
Q: Wm. C. Conner (USA) Could you envision employing your technique to image the adsorption as contrast with integratin the adsorption over the whole surface. This would complement the studies such as I . Schmidts that thermally image the surface. those by ! A: U. Schrtjder Yes,one possibility would be to have an aluminium plate with a small hole between the catalyst disc and the scintillator. This plate would screen off all the emitted p-particles except from a small point of the catalyst disc. The only problem one would have to solve, would be how to move the hole over the catalyst disc in situ.
Q: J. W. Hightower (USA) In the case of 14C0 removal by NO, how can ou distinguish between 14C0 displacement and reactive removal, e.g. NO t CO 4 1/2 2 + C02 '? Did you measure for CO2, N2, N20, etc. in the effluent from the reactor system '?
IJ
A: U. Schrtjder Yes, we measured the concentration of CO, NO and C02 in the effluent from the reactor. Under the conditions in the experiments reported here, we did not find any COP At higher temperatures, however, we have found some C 0 2
Q: X. X.Guo (China) If the thermal desorption is negligibly small, then the only rate process to cause a change in isotopic concentration of the catalyst (O),is the exchange process. However, in your paper, eqn. (13) and eqn. (14) are used as two independent rate equations. I can not understand the physical meaning of eqn. (13). It seems to be the diffusion rate equation. But in your experiment, the diffusion process is only to change the intra-particle concentration of isotope molecules in gas (or precursor state) phase. It has no direct relation with 0,because 0, only changes with exchange process. A: U. SchrBder In eqn. (13) we have assumed that the adsorption rate of labeled CO, rar is proportional to the concentration of labeled CO, denoted C, in the gas phase and to the fractional coverage of unlabeled CO on the catalyst, QtOt- 0,. The reason for this assumption is that some authors have found that the desorption rate of CO is proportional to the concentration of CO in the gas-phase and to the coverage of CO. Under our conditions the covera e is at maximum and therefore the on1 way that we can get a net adsorption of labeled 0 is by dis lacement of an unlabeled C In the same way, the only way we can et a net desorption of abeled CO is to replace the adsorbed labeled CO by an unlabeled CO. %he desorption rate of labeled CO must therefore be equal to the adsorption rate of unlabeled CO since we have steady state conditions. In the same way as in eqn. (13) this rate is proportional to the pressure of unlabeled CO in the gas hase and to the fraction of labeled CO, 0, on the catalyst. The concentration of unlabele CO can be written as Cco,tot-C.
P
8
B.
B
Q: D. Wang (China) In the work from our laboratory (X. X. Guo et al. Surf. Sci. in press, 1992), we found that desorption from Re(001) under a CO ambience follows a rate law for desor tion that is not well-fitted by a 1st order nor a 2nd order rate equation. Instead a (1st + 2n ) order rate
a
657 law gives a good fit. Have you tested your data for a desorption (under a gas ambience) rate law and, if you have, how was the test done and what is your deduced rate law'?
A: U. Schroder We have done these experiments at different CO pressures and we have found that the desorption rate is very well fitted by a rate law, which is first order w.r.t. CO pressure. It is always easier to get a better fit if you combine two rate equations instead of only using one of them.
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Guczi, L. et d.(Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
APPLICATION OF SCANNING TUNNELING MICROSCOPY/SPECTROSCOPY (STM/STS) TO CATALYST RESEARCH: Pt/SiOt
M.Komiyama and M. Kirino Department of Chemistry, Yamanashi University, Takeda, Kofu 400, Japan
Abstract Scanning tunneling microscopy/spectroscopy (STM/STS) was applied to the system of platinum supported on silica for the first time. This was accomplished by employing a model Pt/SiO, catalyst which consists of platinum metal particles vacuum-deposited on thin (ca. 0.7 nm) native silica film present on silicon wafer surface. Choosing optimal tunneling conditions was critical in STM/STS observation of this system. With the optimum tunneling conditions platinum metal particles of f e w nanometer diameters were routinely observable. Topographic and barrier-height images of the surface were obtained and their interpretations are given. These results open the possibility of applying STM/STS to the samples which have been considered not practical for STM/STS study due to their low conductivities. 1. INTRODUCTION
Scanning tunneling microscopy (STM), with its very high spatial resolution that for some samples reaches atomic level, is a powerful tool for t h e studies of solid surfaces 111. STM can observe not only the topographic images of solid surfaces but also surface barrier-height images, electronic density of states near the Fermi edge and other spectroscopic information with similar spatial resolution. Such features make STM a promising tool for the surface study of heterogeneous catalysts, in which local topographic as well as electronic characteristics is of major concern. Despite such expectations, so far t h e application of STM/STS to catalyst research has been limited. This may partly due to the fact that STM is considered to be applicable only to conductors and semiconductors, whereas t h e large body of practical catalysts are non-conductors. The other problem that may arise in studying practical catalysts by STM is that STM by itself is not possible to distinguish elemental species it is observing, and thus unless one know, a priori, surface elemental compositions in relation to surface topograph, it is rather difficult to interpret the STM images obtained. In our attempt to apply STM/STS to the study of heterogeneous catalysts, we chose supported metal catalysts, in which several interesting topographic and electronic questions to be answered are present. In doing so we tried to circumvent t h e difficulties stated above by constructing model catalyst systems which simulate the actual supported metal catalysts. Thus the first STM observations of supported metal catalysts concerned platinum ultrafine particles (ca. 2-4 nm in diameter) vacuum-deposited on thin carbon film and on thin titanium oxide film [2,3]. These papers were followed by others,
660 including palladium I41 and platinum [5] ultrafine particles, both vacuumdeposited on cleaved graphite crystal, platinum ion-exchanged on oxidized graphite surface 161 and platinum vacuum-deposited on gallium oxide film 171. As may be obvious from these works, STM studies of supported metal catalysts have been limited t o t h e systems with conductor o r semiconductor support materials. Few of those who wanted t o study t h e surface topograph of supported metal catalysts on non-conductive materials used atomic force microscope (AFM): AFM images of palladium vacuum-deposited on mica IS] and on silica [91 have been reported. One shortcoming of AFM, however, is t h a t it provides only topograph, and no spectroscopic information is available. In t h e present paper we report our challenge t o apply STM/STS t o metal catalysts supported on non-conductive surfaces. Platinum supported on silica was chosen as a sample system. Very low conductivity of silica was compensated by employing very thin (ca. 0.7 nm) silicon oxide present on a commercial silicon wafer surface as a support. Tunneling conditions were found t o be critical in successful STM/STS observation of this system. The optimal tunneling conditions and t h e topographic and barrier height images of the sample a r e presented, and their interpretations a r e given. 2. EXPERIMENTAL 2.1. Equipment A home-built STM/STS equipment was used in t h e present study. The STM unit employed here is made of stainless steel and machinable glass. The scanner is a tripod type, and the probe tip was a mechanically formed Pt-lr wire. This configuration made t h e scanner operatable under ambient, in situ and ultrahigh-vacuum conditions, although t h e present work was performed o n l y under ambient conditions. Vibration isolation of t h e scanner was e f f e c t e d by an air-suspended table and a stack of stainless steel slabs each isolated by pieces of silicone rubber. When operating under ambient conditions t h e scanner was covered by a lead box in order to isolate it from sound and electronic noises. The above STM unit is driven by a feedback circuit system. The topographic images were taken either by constant current mode through negative feedback and reading t h e voltage Vz applied to t h e z-piezo, or by constant height mode under slow feedback and reading the tunnel current variation. Barrier-height images were obtained by modulating Vz and detecting t h e tunnel current modulation by a lock-in amplifier. Other spectroscopic information such a s It-Vt characteristics and its derivatives were also obtainable using similar modulation-lockin detection procedure. The topographic, barrier-height and other signals obtained were digitized by 12-bit A/D converter and stored in a microcomputer for d a t a processing. The d a t a handling software constructed include d a t a filtering, base plane subtraction, image rotation, zooming and three- and two-dimensional graphic present a t ions. The above STM/STS equipment exhibited atomic resolution for graphite crystal and resolved monoatomic s t e p s of metal surfaces. Barrier-height and other spectroscopic images also showed similar spatial resolutions, 2.2. Sample preparation As a model silica support a native oxide film present on t h e surface of a commercial p-type Si(100) wafer was employed. The thickness of t h e native
661 oxide was determined by X-ray photoelectron spectroscopy (XPS), and calculated to be close to 0.7 nm from the 2p electron emission intensities originating from 0- and IV-valent Si, taking into account t h e electron escape depth at each electron energy. This native oxide-covered Si wafer was ultrasonically cleaned in ethanol, dried and then Pt was vacuum-deposited on its surface. The amount of Pt deposited was determined by XPS. STM/STS observation of this Pt/SiO, model catalyst surface was performed under ambient conditions.
3. RESULTS AND DISCUSSION 3. I. Identification of optimal tunneling conditions STM operates on the principles of electron tunneling. In order for an electron to tunnel from the tip to the sample surface (or vice versa), wave functions of both sides have to meet spatially as well as energetically, The former requirement is satisfied by placing the tip within 1 nm distance from the sample surface. The latter may be satisfied if applied tunneling voltage Vt places the tunneling electron energy at the energy level of non-zero density of states of the material to be tunneled into. For a metal tip and a metal sample this rquirement is satisfied for V t anywhere around Fermi enegy to few eV above or below it. In the case of a metal tip and a semiconductor sample one may want to place Vt above the energy band gap of the semiconductor surface, unless there exist enough surface density of states in the band gap to accomodate the tunneling current It. Compared to these systems, the system to be examined here, vis., silicon, a semiconductor, covered with t h i n silicon oxide, a non-conductor, is problematic. In this particular case one has to find the condition for an electron to tunnel into the silica surface, not to the underlying silicon interface. In the latter case electrons would be tunneling through the silica layer, and then we may be losing information on t h e silica surface, on which we want to place our catalyst metal. In search of t h e optimum electron tunneling conditions for this system, we first tried to obtain the energy distribution of its surface density of states (SDOS). With the SDOS of the sample known w e may be able to locate the optimum tunneling conditions. In principle SDOS can be obtained by STM as dzlt/dVtz versus Vt [I]. Our attempt to obtain this relationship on our SiOz/Si system, however, was unsuccessful, probably due to the charge buildup at the silica interfaces with the sweeping of Vt, which washed out any details of the SDOS of the sample that may have been present in t h e above relationship. Instead of measuring SDOS, therefore, we measured tip-sample distance as a function of Vt at constant It. SDOS is convoluted in the It-Vt relation at constant tip-sample distance, and since t h e tip-sample distance is exponentially dependent on It, the tip-sample distance as a function of V t at constant It should also reflect the energy distribution of SDOS. Figure 1 shows this tip-sample distance variation, represented as the variation of the voltage Vz applied to the z-piezo, w i t h Vt at 0.1 nA It. In the figure Vz is found to increase, i.e., tip retracts from the sample surface, stepwise with increasing Vt. Each step increase of Vz reflects the opening of a new tunneling channel for electrons. The long plateau located in between ca. 0.2 V and ca. 2.5 V is interpreted as the region where electrons tunnel into the bulk conduction band of silicon below the surface oxide: below 0.2 V electrons may be tunneling into SDOS in the band gap of silicon.
662
100 V I
vz
0
1
3
2
4
Vt
Figure 1. Voltage applied t o t h e z-piezo Vz as a function of tunneling v o l t a g e Vt a t constant It of 0.1 nA. Increase in Vz m e a n s t h e i n c r e a s e in t h e tip-sample distance. Sample: n a t i v e oxide-covered Si. Tunneling current: 0.1 nA, bias: t i p negative. At ca. 2.5 V a n opening of a n o t h e r tunneling channel is indicated by t h e presence of a s t e p increase in t h e tip-sample distance. W e a t t r i b u t e this t o t h e opening of a tunneling channel t o t h e s u r f a c e silica layer. Above this bias voltage e l e c t r o n s m a y tunnel i n t o t h e silica l a y e r and below t h a t rhey tunriel through t h e oxide layer i n t o underlying silicon i n t e r f a c e directly. T h e above discussion suggests t h e possibility of probing i n t e r f a c e b e t w e e n t h e s u r f a c e silicon oxide and t h e silicon s u b s t r a t e if o n e chooses V t below t h e s t e p energy of ca. 2.5 V. This a p p e a r s to b e t r u e , and t h e d e t a i l e d s t u d y on t h i s point will b e published e l s e w h e r e [lo]. On t h e o t h e r hand if w e would like t o study t h e s u r f a c e of silica l a y e r and anything on it, Figure I tells us t h a t we h a v e t o k e e p Vt a t t h e p l a t e a u l o c a t e d above 2.5 V. C a r e must b e taken, however, t h a t w e also h a v e t o k e e p Vt below t h e field emission range, which is l o c a t e d above ca. 5 eV for e i t h e r c o m p o n e n t of t h e t i p employed here, vis., P t and Pd. It is also noted t h a t t h e relationship exemplified in F i g u r e 1 is also dependent on tunneling c u r r e n t : increasing It tends t o s h i f t t h e s t e p s t o higher Vt. In t h e present study w e employed low It value of 0.1 nA, i n o r d e r t o e n s u r e wide Vt r a n g e for t h e tunneling t o t h e oxide surface. Tunneling bias employed h e r e i s 4 V, leaving m o r e t h a n 1 V margin on higher and lower sides. These conditions kept us f r o m losing tunneling into t h e oxide s u r f a c e due t o t h e small SDOS variation t h a t m a y b e p r e s e n t locally on t h e s u r f a c e , and a l s o kept us within t h e tunneling region and not in t h e field emission region. T h e small It employed m e a n t l a r g e r tip-sample d i s t a n c e for a given Vt value , and this k e p t o u r probe tip from bumping i n t o sharply varying s u r f a c e s t r u c t u r e , although l a r g e d i s t a n c e m e a n t losing s o m e of t h e topographic details.
663
3 nm
Figure 2. Topographic image of native oxide-covered silicon surface. Image taken under ambient conditions at 4.0 V Vt and 0.1 nA It. Small height variations less than 0.5 nm are observed, and the surface may be considered fairly flat. 3.2. STM image of the native oxide-covered silicon surface With t h e optimum tunneling conditions thus identified, t h e topographic images of the surface of t h e native oxide on t h e silicon wafer was obtained. A typical scan, of an area of 47 nm by 35 nm, is shown i n Figure 2. Although small height variations of less than 0.5 nm are apparent in the figure, the surface may be considered fairly smooth and flat. This is in accordance with t h e expectation for this particular surface, which is a single crystal silicon cut and polished to expose (100) surface. It is noted that the structure at the silica-silicon interface, probed using low Vt, is somewhat different compared to the one shown in Figure 2: one or two clear monoatomic or diatomic s t e p lines are often observed in areas of similar dimensions [lo]. The reason that w e do not observe such clear structure in Figure 2 may be that the step structure is blurred when t h e surface is oxidized and/or t h e large tunneling distance employed here kept us from observing such detailed structures. It should be emphasized here that Figure 2 is the first relaiable STM image of silicon oxide surface reported. Although its thickness is very thin, the oxide entirely consists of IV-valent Si as found by XPS. This means that the surface of this thin silica film could be used as a model surface to the bulk silica. The low crystallinity expected for this thin silica film would simulate t h e non-crystalline, high surface area silica. Thus the successful observation of the surface of t h i n silicon oxide film existing on silicon wafer presents the possibility of applying STM to the studies on silica as well as silica-containing materials, including metal catalysts supported on silica.
3.3. Topographic and barrier-height images of Pt/SiO, With the successful STM observation of the surface of thin silica film, we extended our attempts to t h e examination of platinum ultrafine particles deposited on this silica surface. The platinum was vacuum-deposited on the above native oxide-coverd silicon wafer. The Pt/Si atomic ratio of the prepared sample was 0.4, as determined by XPS.
664
3 nm
Figure 3. Topographic ( a ) and barrier-height (b) images of P t vacuum-deposited on the surface of thin silica film. Scan conditions: Vt 4.0 V, It 0.1 nA, tip negative. The concurrently obtained barrier-height image (b) was taken with Vz modulation amplitude of 160 mV peak-to-peak, which translates t o the tip-sample distance modulation of 0.16 nm peak-to-peak. The modulation frequency was 700 Hz. The z-axis of t h e barrier-height image is arbitrary.
A topographic scan of ca. 20 nm by 20 nm of this sample surface is shown in Figure 3 (a) along with its barrier-height image (b). Figure 3 (c) and (d) on the next page a r e t h e two-thirds of t h e lower left portion of t h e images ( a ) and (b), respectively, enlarged and rotated 90 degrees t o t h e left. In t h e topographic image ( a ) and i t s enlargement (c), particles of varying diameters with t h e height of ca. 1 t o 2 nm a r e apparent. The topographic image ( a ) is presented a s is taken, without any d a t a filtering. The scans were highly stable, as may be apparent in the figure, resulting in the image quality comparable t o that of highly conductive P t / C sample [3]. The scans shown in Figure 3 (a) and (c), and other topographic scans on this sample not shown here, indicate that t h e size distribution of t h e Pt particles deposited is very sharp: observed P t particles w e r e mostly in t h e size range of 2.5 t o 5 nm in diameter. This observation is quite similar t o that made on P t / C samples [2,3], i n which vacuum-deposited P t particles a r e mostly in t h e size range of 2-4 nm. As for t h e absolute value of t h e P t particles observed by STM, w e always have t o take into account t h e possibility
665
Figure 3 (cont.). Topographic (c) and barrier-height (d) images of twothirds of the lower left portion of (a) and (b), respectively, enlarged and rotated 90 degrees to the left.
of overestimation (21. Since no other particle size measurements have been performed on the current sample, u n l i k e the previous Pt/C [Z], the extent of size overestimation in the present sample by STM is not accuratly known. However, the main source of overestimation is considered to be in the tipsample separation distance (21, i t is expected not to exceed f e w tenths of a nanometer. The barrier-height images shown i n Figure 3 (b) and (d), taken concurrently with the topographic images shown in ( a ) and (c), are slightly noisy, but shows apparent barrier-height variations on t h e surface. The images show that where Pt particles exist barrier-height is higher compared to the places in between the particles. This barrier-height variation resembles that of the Pt/TiO, system [3], in which Pt particles show heigher barrier-height than titania substrate. Compared to these systems it is noted that in Pt/C sample Pt particles exhibited lower barrier-height compared to the substrate carbon [Z]. Barrier-height images reflect the surface work function variation [ 11, and as seen above work function variations with spatial resolution less than a nanometer is obtainable by STM, the resolution of which has not been possible by any other method. It should be noted, however, that because the barrierheight is determined by not only the work function of the sample but also that
666 of t h e tip, t h e image shown in Figure 3 (b) and (d) does not i n d i c a t e t h e absolute value of t h e s a m p l e work function. Nevertheless, t h e information on t h e s u r f a c e work function variation would provide us variable insights on t h e c a t a l y s t surfaces. 4. CONCLUSIONS
I t h a s been d e m o n s t r a t e d t h a t STM observation of non-conductive m a t e r i a l s is possible, provided t h a t a p p r o p r i a t e s a m p l e w a s chosen a n d o p t i m u m tunneling conditions used. H e r e as a n e x a m p l e STM/STS observations of silica s u r f a c e and m e t a l particles dispersed o n i t w e r e a t t e m p t e d : c l e a r topographic and barrier-height images w e r e obtained. T h e observed s a m p l e r e p r e s e n t s o n e of t h e most widely used c a t a l y s t s y s t e m , a n d this o p e n s t h e possibility of applying STM/STS t o t h e c a t a l y s t s consisting of non-conductive materials.
I
R.J. Behm, N. G a r c i a a n d H. R o h r e r (eds.), Scanning Tunneling Microscopy and R e l a t e d Methods, KIuwer A c a d e m i c Publishers, Dordrecht, 1990. 2 M, Komiyama, S. Morita and N. Mikoshiba, J. Microsc., 152 (1988) 197. 3 M. Komiyama, J. Kobayashi and S. Morita, J. Vac. Sci. Technol. A, 8 (1990) 608. 4 A. Humbert, M. Dayez, S . Granjeaud, P. Ricci, C. Chapon and R. Henry, J. Vac. Sci. Tecnol. 8, 9 (1991) 804. 5 J. Xhie, K. S a t t l e r , U. Muller, N. Venkateswarn and G. Raina, J. Vac. Sci. Technol. 8, 9 (1991) 833. 6 K. Yeung and E.E. Wolf, J. Vac. Sci. Technol. 8, 9 (1991) 798. 7 W. Hanrieder, A. Scholz and H. Meixner, Surf. Sci., 243 (1991) 794. 8 J. Colchero, 0. Marti, J. Mlynek, A. H u m b e r t , C.R. Henry and C. Chapon, J. Vac. Sci. Tecnol. B, 9 (1991) 794. 9 R. Eerlandsson, M. Eriksson, L. Olsson, U. Helmersson, 1. Lundstrom a n d L.-G. Petersson, J. Vac. Sci. Tecnol. 8, 9 (1991) 825. 10 Wl. Komiyama, to b e published.
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DISCUSSION
Q: A. T. Bell (USA) Many people who have attempted to use STM for characterization of metal particles have failed to obtain atomic resolution. Could this be due to the fact that as the tip approaches the particle channeling occurs from different partions of the probe tip. What is being done to overcome this limitation of STM ?
A: M.Komiyama It is true that most of the published STM observations of supported metal particles fail to resolve metal atom images. There may be two possibilities for this: (i) It is known that the atomic corrugation of metal surfaces is an order of magnitude smaller than that of graphite, and therefore it is much more difficult to observe metal atoms by STM. An equipment that have a resolving power of graphite atoms and experimental conditions that routinely give graphite atomic resolution does not necessarily guarantee the observation of metal atoms. If this is the case, improvement of the equipment performance and of experimental conditions would solve the problem. (ii) Another possibility is that there exist essential limitations such as electron delocalization that prohibit STM observation of metal atoms for such ultrafine metal particle systems. Whether this is true or not must await further investigations. However, there exist one work on Pd/C system that appears to be reaching atomic resolution for the supported Pd particles of ca. 1.5 nm size (text, ref. 4). This seems to indicate that the lack of atomic resolution on the dispersed metal particles in the published papers is not due to some essential factors but rather dependent on the equipment performance and experimental conditions.
Q: J. M. Thomas (United Kingdom) If, as Professor Bell suggests, the reason for the relatively poor resolution that STM has so far achieved with supported metal particles stems from the curvature of the small particles - resulting in "delocalized" tunneling - then it might-be useful to employ facetted Pt and Pd surfaces. You may recall the electron micrograph (shown by Professor King in his plenary lecture) published by my two former colleagues Peter Harris and David Jefferson. When small Pt articles on silica are exposed to H the surfaces become very flat (and develop an ordered P'S" overlayer). These sulphided an?I facetted Pt particles would be worth investigating to test the power of STM. Its spatial resolution may not be any better than that of HREM (high-resolution, electronmicroscopy). But its great attraction is that it may be used (in situ) or in air.
A: M. Komiyama I thank you for the useful comments.
Q: M. M. Bhasin (USA) In the STM images of Pt/SiO,, the silica background appeared a lot rougher than in the silica sample. How do you explain that observation? Could that be due to the Pt deposition procedure used? Also would you comment on the feasibility of obtaining atomic resolution with such supported metal catalysts.
A: M.Komiyama We believe that the structures found on the Pt-deposited silica surface are all due to the deposited Pt. Platinum was deposited on the surface by vacuum deposition, in which Pt wire was heated on a W rod. The energy of the Pt atoms arriving at the silica surface is expected to be. too small to cause any modification on the surface. As for the feasibility of obtaining atomic resolution on the supported metal, please see the answer to Professor Bell.
Q: Wm. C. Conner (USA) What do the subsequent imager look like '? Is there any evidence for the STM modifying the surface ?
A: M. Komiyama Within the experimental conditions employed here, the images are reproducible. Surface modifications are observed when much higher tunneling voltages are employed, or when the tip is intentionally or unintentionally driven to push or scratch the surface. Q: P. Sermon (United Kin dom) With regard to the a vanta es of STM versus AFM in the study of catalysts with an insulator component like silica, fwanted merely to draw your attention to the AFM image of a sol- el derived silica-based catalyst in this Proceeding (see p.1775). For this delicate assem I of colloidal particles of silica STM was impossible. However, the advantages of STM-SJS are clear from your paper. In other words they complement each other well.
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A: M. Komiyama I fully agree with your o inion that AFM possesses advantages over STM/STS for certain samples, and that STM/ TS and AFM compliment each other.
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Q: A. Datye (USA) One of the fundamental problems in the application of STM/AFM to small metal particles is that the diameter of the tip is much greater than the diameter of the metal particles. The image obtained represents a convolution of the tip with the sample. What are the prospects of interpreting the image to determine the true morphology of the particles ? Also, d o you characterize your tips after obtaining the image'?
A: M. Komiyama What is convoluted in the observed image is not the tip diameter (i.e. apex curvature) but the distance between the surface and the tip apex (text, ref. 2), and the tip diamctcr would impose the limitations only through the distance it can physically approach to the surface (for instance if the curvature is very large compared to the particle-particle distance then the tip cannot trace the region in between the particles). Therefore it is important (to keep in mind that the STM-observed particle size is always larger by the tip-surface distance, which is commonly a fraction of a nanometer). Tip characterization after measurements is not normally done, but we routinely check the tip by using a graphite sample, making it certain that the tip can resolve graphite atoms.
Q:J. W. Niemantsverdriet (The Netherlands) The SiO2 layer you are using has a thickness of less than a nanometer, and leaves few opportunities to d o something with you model catalyst in reaction or adsorption studies with oxidizing s ecies. 1) Is t is thin SiO, layer sufficiently characteristic of a real silica, in the sense that all silicon atoms are in the 4t oxidation state ? 2) Have you tried taking STM imaging from thicker SiO, layers, and if so, have you an idea of the maximum thickness permitted for STM ?
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A: M. Komiyama The system we examined is a "model" system, with the advantages and disadvantages commonly associated with any model systems. We believe that at this moment the present s stem comes closest to the actual silica-supported catalyst, as far as STWSTS is concerned. 'ken what we should d o is to obtain as much information as possible with this system to the extent the system allows. The limitations of the current system must be complimented by other sample systems and/or other experimental techniques.
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For the specific question: (1) As far as XPS is concerned, the oxidic silicon are almost entire1 in the 4+ oxidation state. (2) The investigation on the effect of silica thickness on STMdTS observation is in progress. Q: W. Ghnert (Germany) Is it possible to perform STM at elevated temperatures ? What are the limits of the measurement temperature imposed, for instance, by the piezo ? A: M. Komiyama The Curie point of piezo elements do impose a limitation on an upper limit of the operation temperature. This, however, could be able to be circumvented by equipment design that keep temperature rise away from the piezo elements. more severe problem, as it seems, is a temperature drift during STM measurements, which increases with temperature and distorts the tip-sample configuration thus distorting STM images. This should also be able to be circumvented by equipment design, to a certain extent, such as making it highly symmetrical so as to cancel the uneven expansion or contraction of the STM parts caused by the temperature drift.
Q: L. A. Rudnitski (Russia) You can receive high resolution potential surfaces of barrier-height tunneling for metal ultrafine particles on flat surfaces. Next one needs interpretation of the investigation results. It is very difficult to do because barrier height is determined not only by the nature of the investigated surface and tip surface, but also by their mutual arrangement in space. Work function of complicated (rough) surfaces is a mean value formed by contributions of all surface atoms orbital electronegativity but with essential differences in their weight coefficient. The weight coefficient value for a surface atom depends on its size, its coordination, and its distance from the oint of electronegativity, meaning that the electrone ativity values of surface atoms dif ering in their coordination can differ by one eV or more. dependence at small distances from the surface: the value of the work function is stabilized at a distance exceeding 1.5-2 nanometers only. All this ap lies to tip and surface in investigating work functions. The space variation of the tip work unction can be observed when the tip comes down from the metal particle apex to their bases. So at atomic size order of distance from tip to rough surface of particles one can expect the effect of space chan es of contact potential difference value to be displayed. The value of the contact potential dif erence between the end of the tip and other surface gives on essential contribution to tunneling barrier-height. For rough metal surfaces on which surface atom arrangement is known one can approximately calculste the work function space distribution and then compare results of the calculation with experimental date.
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A: M. Komiyama Thank you for your comments.
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Guni, L a al. (Editors), New Frontiers in Caralysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
CHARACTERIZATION AND CATALYTIC PROPERTIES OF Pt-Ir SMALL BIMETALLIC CLUSTER IN NaY
0.B. Yang and S.I. Woo Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, P.O.Box 150, Chengryang, Seoul, Korea
ABSTRACT The formation of small Pt-Ir bimetallic clusters (about 1 nm) by the reducton of Pt and Ir co-ion-exchanged NaY with Hz at 300OC after calcined with oxygen at 300OC was evidenced by temperature programmed oxidation/reduction (TPOIR), 1 2 9 Xe NMR, hydrogen chemisorption and catalytic probe reaction. These small Pt-Ir bimetallic clusters inside the supercage were more stable than Ir monometallic cluster and remained inside the supercage after re-oxidation treatment below 400OC. Temperature programmed oxidationlreduction (TPOIR) results suggested that the interaction between Pt and Ir takes place in the calcination step prior to hydrogen reduction. The Pt-Ir/NaY bimetallic catalyst showed a higher activity and better activity maintenance than those of PtlNaY catalyst in n-heptane reforming reaction.
1, INTRODUCTION Pt-lr bimetallic catalyst exhibits a higher activity and better activity maintenance than Pt catalyst in the reforming reaction [ I ] . Sinfelt and co-workers reported that the bimetallic clusters of Pt and Ir were formed in silica [2] and alumina [3]. It has been understood that the specia! (character of Pt-Ir bimetallic catalyst is related to the interaction hetween Pt and Ir. Interaction hetween Pt and Ir has been assumed to occur during the reduction of ionic precursors. However, a recent laser Raman study of Pt-lr/A1203 catalyst hy Chan et al. [4] indicated that interaction between the two metals may in fact take place upon drying the freshly impregnated samples.
672 The catalyst deactivated in the reforming reaction must be regenerated to be used commercially again. Catalyst regeneration is achieved hy burning the coke deposit in an oxygen-containing atmosphere around 500OC. The combustion process of coke, however, causes an increase in the size of the metal crystal size. Therefore, the stability of bimetallic clusters under the oxidation condition is very important in the Pt-lr reforming catalyst. Not any works have been known to us for characterization of Pt-Ir bimetallic clusters in N a y . However, our recent result showed that very small (about 1 nm) Pt-Ir himetallic clusters of uniform size were formed mostly inside the zeolite supercage after calcining and reducing with Hz at 3OOOC [ S ] . The physicochemical characteristics and catalytic properties of small Pt-Ir bimetallic clusters should he different from those of large metallic clusters. The oxidation and reduction properties and catalytic properties of small Pt-Ir bimetallic clusters were studied by temperature programmed oxidation/reduction (TPO/TPR) and n-heptane reaction. 12QXe NMR and hydrogen chemisorption were also used to characterize small Pt-Ir bimetallic clusters in N a y .
2, EXPERIMENTAL Monometallic Pt and Ir clusters in NaY (abbreviated as Pt(x)/NaY and Ir(x)/NaY %) were prepared by calcining and reducing and x indicates weight Pt(NH3)42+-exchanged NaY (PtNaY) and Ir(NH3)5C12*-exchanged NaY (IrNaY) with 0 2 and HP at 300OC, respectively. Pt-lr bimetallic clusters in NaY (Pt(x)Ir(x)/NaY) were prepared by calcining and reducing co-ion-exchanged NaY (PtIrNaY) with 02 and Hz at 300OC, respectively [ 5 ] . The experimental details of 12QXe NMR spectroscopy and hydrogen chemisorption were described in previous work [6]. A conventional apparatus of temperature programmed study was used to determine the TPO/R profiles. TPO spectra were obtained using a high through-put pressure reducing system combined with a computer interfaced mass spectrometer (VG Quadrupoles SX 300). TPO was carried out with 100 mg of ion-exchanged NaY samples by raising temperature from 25OC to 500OC at a rate of 8oC/min. in the 5 % 02/N2 flow of 60 mllmin.. The decrease in oxygen partial pressure was recorded every 10 s. The mass numbers monitored during the TPO of the uncalcined ion-exchanged sample were 15 (NH), 17 (NH3) and 32 (02). Hydrogen consumption during the TPR was monitered by a Hewlett-Packard 5890A thermal conductivity detector (TCD). A molecular sieve trap, kept at -8OOC, was placed after the TPR cell in order to remove water. After the sample was calcined at 300OC, TPR was performed with 100 mg of sample by raising temperature from 25OC to 500OC at a rate of 80C/min. in the 5 % Hz/Nz flow of 40 mllmin.. n-Heptane reforming reaction was carried out in a differential fixed bed reactor with 100 mg of catalyst at atmospheric pressure and 400OC for 4 h . Catalysts calcined at
673 300OC with 0 2 and reduced at 400OC with HZ for 2 h were used in the reaction. The n-heptane (Aldrich, 99 + %) was fed by a calibrated micro-syringe pump (Sage Instrument Model 3413) into a preheated zone where heptane was mixed with hydrogen and vaporized. The total flow rate (Hz + n-heptane) was 67 mllmin. with the Hz/n-heptane mole ratio of 8 . 5 . Products were analyzed by an on-line HP 5890A gas chromatograph equipped with a 50 m crosslinked methyl silicon fused silica capillary column and a FID detector.
3, RESULTS AND DISCUSSION No response was observed for mass number 15 (NH) in all TPO spectra. The loss of ammonia ligand began at IOOOC and almost was completed at about 300OC in all samples. The spectra of 0 2 consumption during TPO are shown in Figure 1. The TPO spectrum of Pt(4)NaY and Ir(4)NaY showed a sharp peak of 02 consumption at 308OC and 330OC, respectively. Pt(3)Ir( I)NaY and Pt(Z)Ir(Z)NaY bimetallic clusters exhibited a TPO peak at 297OC and 298OC, respectively. The TPO peak temperatures of PI-Ir ion coexchanged Nays are significantly lower than those of PtNaY and IrNaY indicating that the oxidation of co-ion-exchanged Nays was facilitated due to the possible interaction between the cationic species of Pt and Ir. Figure 2 shows the TPR spectra of PtNaY, IrNaY, and PtIrNaY samples calcined at 300OC. The TPR peak temperatures of Pt(4)NaY and Ir(4)NaY were observed at 112oC and 142OC, respectively. The Pt-Ir co-ion-exchanged samples exhibited a TPR peak hetween 112OC and 1420C. The TPR peak of Pt(3)Ir(l)NaY, Pt(2)1r(Z)NaY and Pt(3)lr(l)NaY samples appeared at 124OC with shoulders at 112OC and 142OC. The TPR spectra of these Pt-Ir co-ion-exchanged samples can not be obtained by the weight average of corresponding PtNaY and IrNaY. The TPR spectra of these samples calcined at 500OC are also shown i n Figure 3. The Pt(4)NaY sample exhibited a pair of peaks at I12OC and 320OC, whereas Ir(4)NaY showed a sharp peak at 21OoC. The PtIrNaY samples exhibited a sharp and strong peak at 210OC and weak peaks at 112oC and 320OC. The area of TPR peak of PtIrNaY at 210OC increased with increasing the Ir content, while the area of the peak at 320OC decreased. From the previous works on the TPR of PtNaY [7,8], the peak at 112OC can be assigned to highly dispersed Pt2' inside the supercage of Pt(4)NaY calcined at 300OC. The peak at 320OC is likely due to Pt4' in sodalite cage which requires a reduction temperature higher than Ptz' ions in the supercages of Pt(4)NaY calcined. This peak was not observed when the samples were calcined at 300OC. About 80 % of Pt4' ions are in sodalite cage calculated from TPR peak area. Huang et al. [9] reported that the TPR peak of Ir/AIz03 catalyst at 210oC is due to the reduction of large Iron crystallites. The existence of the agglomerated lrOz particles on the exterior surface of NaY can be easily identified by the sharp peak at 210OC in the TPR spectrum of IrNaY precalcined at 500OC (Figure 3), while these agglomerated IrOz particles were not present after calcination of IrNaY at 300OC indicated by the absence of TPR peak at 210OC. Our previous I2eXe NMR study
674
Figure I . The spectra of 0 2 consumption during T P O of ionexchanged N a y s ; Pt(4)NaY (a), Pt(3)Ir( I)NaY (b). Pt(2)Ir(2)NaY ( c ) , Pt( 1)Ir(3)NaY (d) and Ir(4)NaY (e).
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Figure 2 . The TPR spectra of PtNaY, IrNaY and PtlrNaY calcined at 300OC; Pt(4)NaY (a), Pt(3)lr( I)NaY (h), Pt(2)Ir(2)NaY (c), Pt( I)Ir(3)NaY (d) and Ir(4)NaY (e).
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Figure 3. The TPR spectra of PtNaY, IrNaY and PtlrNaY calcined at 500OC; Pt(4)P.iaY x 1.5 (a). Pt(3)lr( I)NaY (h), Pt(2)lr(Z)NaY x 0.75 ( c ) , P t ( l ) l r ( 3 ) N a Y x 0.5 ( d ) , and Ir(4)NaY x 0.5 (e).
100 300 500 Temperature/"c
675 indicated that highly dispersed Pt-Ir bimetallic clusters were formed inside the supercage of NaY by calcining and reducing PtIrNaY samples at 300OC [5]. Hence, the peak of TPR spectrum of IrNaY (Figure 2) at 142OC is arising from highly-dispersed Ir clusters inside the supercage of N a y . Therefore, the peak of TPR spectra of PtIrNaYs calcined at 300OC ohserved at 124oC is ascrihed to the reduction of oxygen containg bimetallic Pt-lr complex located in the zeolite supercage. This bimetallic oxygen containing complex is seemed to he already formed during the calcination step as indicated by the T P O spectra of Figure I . However, the TPR spectra of PtIrNaYs calcined at 500OC do not have any peaks at 124OC as shown in Figure 3 . I t can be concluded from these results and further evidences by 129Xe NMR, XRD and Hz. chemisorption results that most of Ir is present on the exterior surface as agglomerated IrO2 clusters and PI is present as Pt2' i n the supercage and Pt4' in sodalite cage after calcination of PtIrNaY at 500OC. The chemical shift of 129Xe NMR spectrum is very sensitive to the change in the chemical state and the number .)f metallic species inside the supercage of Y zeolites. The NMR shifts of Pt/NaY, Ir/NaY and Pt-Ir/NaY after reoxidation at various temperatures were measured to determine the stahility of the corresponding metal clusters in N a y . PtiNaY, Ir/NaY and Pt-IrlNaY calcined and reduced at 300OC were characterized with '29Xe NMR in our previous work [S] were used as a reference catalyst. The Iz9Xe NMR chemical shifts of the reference catalyst reoxidized at 300, 400 and 500OC followed hy the reduction at 400OC are shown in Figure 4. The dispersions (HIM: adsorbed hydrogen atom / metal atom) of metal clusters after reoxidation and reduction are shown in Figure 5. The chemical shifts of Pt(4)/NaY, Ir(4)/NaY and Pt(l)lr(S)/NaY decreased as re-oxidation temperature increased, while the chemical shifts of Pt(3)lr( l ) / N a Y and Pt(Z)Ir(2)/NaY increased below reoxidation temperature of 400OC and
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Figure 4. Effect of reoxidation temperature on the 129Xe NMR chemical shift; Pt(4)/NaY (A),Pt(3)Ir( I)/NaY (+), Pt(2)Ir(2)/NaY (+I, Pt(l)Ir(3) and Ir(4)/NaY ( X ) . / N a y (0)
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Figure 5. Effect of reoxidation temperature on the metal dispersion; Pt(4)/NaY (&, Pt(3)Ir(l)/NaY (+), Pt(2)lr(Z)/NaY (*), Pt( 1)Ir(3)/NaY (0) and Ir(4)/NaY (X).
676 decreased at the reoxidation temperature of 5OOOC a s shown in Figure 4 . The chemical shift of Ir(4)/NaY is equal to that of NaY zeolite after reoxidation at 400OC. This indicates that most o f I r clusters are migrated to external zeolite surface to form large IrOz particles after reoxidation at 400OC, which was confirmed by the small value of I r dispersion (see Figure 5 ) and X-ray diffraction 151. Migration of Pt clusters to external zeolite surface was insignificant helow the reoxidation temperature of 400OC. The chemical shifts of Pt(3)Ir( I ) / N a Y and Pt(’,)lr(Z)/NaY increased with the increase in the reoxidation temperature helow 400OC might he due to the surface enrichment of Pt of Pt-lr himetallic clusters during the reoxidation helow 400OC, because the chemical shift caused hy Pt atom is mu,:h larger than hy Ir atom. Another explanation might he due to the formation of highly dispersed Pt and Ir clusters inside the supercage, which will result in the increase of chemical shift due to the formation of more clusters. This possihility can he ruled out on the hasis of metal dispersion a s shown in Figure 5 . If highly dispersed Ir clusters were formed, the metal dispersion should not decrease. The dispersions of Pt(3)lr( 1 ) / N a y and Pt(Z)lr(Z)/NaY decreased helow reoxidation temperature of 400OC. At the reoxidation temperature of 500°C, Ir is totally segregated from the Pt-lr bimetallic clusters due to the lack of stahilization ohtained hy forming himetallic clusters and migrated t o the cxternirl surface, resulting in the significant decreases in both chemical shift and dispersion. The segregation of Ir clusters from Pt( I)Ir(3)/NaY occured helow reoxidation temperature of 400OC due to the higher content of Ir in the Pt-lr himetallic cluster. Figure 6 shows the total conversion of n-C7 as a function of reaction time. In all catalysts steady state appears to he achieved at 4 h of reaction. IrlNaY catalyst showed a conversion higher than Pt/NaY and Pt-lr/NaY himetallic catalysts. The activity of lr/NaY catalyst was maintained during the reaction hecause of high activity o f hytlrogenolysis of C7 resuiting i n the less amount of coke on the catalyst surface. After 4 h of reaction Pt/NaY catalyst was the least active. Pt-lr/NaY bimetallic catalysts had a hetter activity maintenance than Pt/NaY monometallic catalyst. The improved activity maintenance of Pt-lr himetallic catalysts (as shown in Figure 6) is due to the less formation of coke and the nature of more graphitic coke [ l o ] . Figure 7 shows the product distributions a s a tunc:ion of Ir content in the catalyst containing 4 wt % total metal loading of Pt and Ir at the reaction time of 4 h . Cs and C7 isomers a r e main products in all catalysts. A:< the I r content increased the selectivity of toluene significantly decreased and that of C I - Cs hydrocarhons increased. This is due to the high activity of I r for the hydr,)grnolysis reaction.
4, CONCLUSIONS The TPO and TPR results suggested that the interaction between Pt and Ir takes place in the calcination step prior to hydrogen reduction. The small (about 1 nm) Pt-lr himetallic clusters formed after calcination and reduction at 30OOC. a r e stable and remain
677
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Figure 6. Total conversion of n-heptane as a function of reaction time at 400OC; Pt(4)/NaY (A), Pt(3)Ir( l)/NaY (+),Pt(S)Ir(2YNaY (+I, Pt(l)lr(3) / N a y (0) and Ir(4)INaY ( X ) .
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0.4 0.6 0.8 Ir/(Pt+ I r )
Figure 7. Product distributions of Pt(4) / N a y , Ir(4)/NaY and Pt-Ir/NaY catalysts in n-heptane reaction; C i (A), C Z - n-Cs (+),CS and C7 isomers (*), cyclic compound (0) and toluene (X).
mostly inside the zeolite supercage after re-oxidation below 400OC. However, these Pt-Ir bimetallic clusters are migrated and agglomerated to large metallic crystallites on the exterior surface of NaY after re-oxidation above 400OC. The Pt-Ir/NaY bimetallic clusters showed a higher activity and better activity maintenance than Pt/NaY monometallic clusters in n-heptane reforming reaction.
5, ACKNOWLEDGMENT This research was funded by Korea Science and Engineering Fund (1990 Discussion of 129Xe NMR data with Professor R. Ryoo is greatly appreciated.
6, REFERENCES 1 J . H . Sinfelt, US Patent No. 3 953 368 (1976). 2 J.H. Sinfelt, G.H. Via and F . W . Lytle, J. Chem. Phys. 76 (1982) 2779. 3 R.L. Garten and J.H. Sinfelt, J . Catal. 62 (1980) 127.
- 1993).
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678 S . C . C h a n , S.C. Fung a n d J . H . Sinfelt, J . C.atal. 113 (1988) 164. O . B . Yang, S . I . W o o and R . Ryoo, revised for publication i n J . Catal. (1991) O . B . Yang, S . I . W o o and R . R y o o , J . Catal. 123 (1990) 375. M . S . Tzou, B . K . Teo and W . M . H . Sachtler, J . Catal. 113 (1988) 220. 8 N . Wagstaff and R . Prins, J . Catal. 59 (1979) 434. 9 Y . T . H u a n g , S.C. Fung, W . E . Gates and G.B.McVicker, J . Catal. 1 I8 (1989) 192. 10 O.B. Yang and S.I. Woo, suhrnitted to Catalysis Letters (1992). 4 5 6 7
DISCUSSION
Q: M. Ichikawa (Japan) Zeolite frameworks accommodate the alloy particles of Pt-Zr to prevent sintering similar to the Re-Ir cluster in NaY reported by us. You mcntioncd metal aggregation in a large crystallites after the calcination at 1100 %. 1) What kind of chemical reaction do you expect for the methyl migration from inside to outside of the zeolite, in particle for Pt and Ir particles ? Generally, PtO easily sublimes under the oxygen atmosphere even at the lowcr temperature, which might aid the methyl segregation and aggregntion in the PtIr alloy systems. 2) Have you observed any evidence for phase-segregation of PtIr particles in NaY by meas of Xe NMR and TPR before and after the calcinations '!
A: S. 1. Woo Minachev and Isakov reported (ACS Monograph, 171 (1976)) that the stability of Pt clusters entrapped inside NaY supercagcs. Our result (Figure 4) also indicated that Pt clusters in NaY are stable and not migrated to the exterior surface of NaY below the tem eraturc of migrated calcination and reduction 01 300 OC. However, thc calcination above 400 platinum clusters into the exterior surface. hence, it can bc explained by your suggcstion that volatile or more mobile PtO spccies easily formed above 400 OC. The formation of Ir oxide spccics is more favorable than that of platinum oxide species, resulting in the complcte migration o f Ir clusters to the exterior surface of NaY 111. It was reported that sintering the lr/alumina was occurred via atomic or molecular species of volatilc Ir oxide species. Hence, the phase scparation of Ir and Pt must be occurred whcn the Pt-Ir bimetallic clusters are calcined above 400 OC due to the different rate of formation of volatile oxide phases. Actually, the increase in the chemical shift of Pt(2)Ir(2) and Pt(3)Ir( 1) as shown in Figure 4, indicate the phase separation due to the surfacc enrichment of Pt, arising from the more removal of iridium oxide species during calcination. Y. F. Chu and E. Ruckenstcin,J. Calal., 55, 281 (1978) [l]
$
Q: V. Ragaini (Italy) One of the prohlcms in bimctallic catalysts is to find separately the dispcrsion of each metal ( i t . the number of surface metal atoms of a type over the total number of metal atoms of the same type). Are your Xc-NMR plus TPR methods able to solve this problem ?
A: S . I. Woo Yes, it is possible to calculate the surface composition of Pt and Ir nietals in the Pt-Ir bimetallic cluster with the assumption that the intrinsic chemical shift caused by one Pt or Ir metal atom is not changed with the formation of' Pt-Ir bimetallic clusters. This might bc reasonable in the case of Pt-Ir bimetallic clusters, because the electron band structure of these two metals is nearly same. Then the chemical shift of bimetallic clusters can be expressed as bR+ = H p t . 6 ~+ Hlr.blr, where H is the surface mole fraction.
679 Q: J. Fraissard (France) I d o not agree with your first conclusion deduced from isotherms. Xenon adsorbed on Pt is highly polarized and increases the subsequent adsorptions - 'so the difference you have detected can be greater than the number of atoms covering the metal particles. I assume that you have worked at constant metal atoms. Did you work at constant Xe concentrations, or at least did you verify that the isotherms are roughly the same, that is independent of nature and relative concentrations of Ir and Pt ? Is there a large difference of the curves 6 = f(Xe) for Pt and Ir ? Note: It is surprising to have no reference about the authors we have found the Xe technique !
A: S. I. Woo You raised a very important question about the validity of the measurement of xenon isotherms due to the polarization of xenon atom next to the larger of xenon atom adsorbed on the surface of metal clusters. This will overestimate the amount of xenon adsorbed on the metal cluster. We performed a variable-temperature xenon adsorption experiment similar to your recent papers [2] with our samples, which will be published after complete analysis. This result indicated that the change in the adsorption temperature resulted in the different amount of xenon adsorbed on metal cluster obtained by the extrapolation of xenon pressure to zero. The higher the adsorption temperature, the higher the amount of xenon adsorbed is. The higher temperature of adsorption is, the mean free path and the dipole moment is the smaller, which indicates that the degree of polarization of xenon gas next to xenon atoms adsorbed on metal clusters decreased. However, the difference is small. The question about whether the xenon isotherms are independent of the nature and relative concentrations of Ir and Pt is also a good one. Our previous result (to be published in J. Catal. (1992)) indicated that if the number and size of clusters are the same, the xenon isotherms are all identical in the case of Pt-Ir bimetallic clusters. The final question about whether the large difference in the chemical shift of Pt and Ir clusters was observed as a function of xenon pressure, the answer is yes. The chemical shift is strong function of the xenon pressure and increased rapidly as the xenon pressure decreased. J. Chen and J. Fraissard,J. Phys. Chem., 96, 1809, 1814 (1992) [2] Q: Wm. C. Conner (USA) As Fraissard has shown on numerous occasion, the chemical shift of 129Xe depends on both morphological w e - X e and Xe-wall) interaction as well as prober of the chemical environment. How did you deconvolute the chemical and morphological factors in your 129Xe data, i.e., did you complete 6 vs l2%eadSstudies ? How would your interpretation compare to those of Fraissard '? A: S. I. Woo Yes, you are quite right. The chemical shifts of Pt, Ir and Pt-Ir bimetallic clusters in NaY are composed of the morphological interactions (Xe-Xe and Xe-wall) and the interaction with metal clusters. Our samples have the same number of ion-exchange of cations and the same number of metal clusters. The numbers of H+ ions after reduction of Pt and Ir cations are almost identical. Hence the difference in the chemical shift is mainly due to the interaction of xenon with metal clusters. We measured 6 at the various amounts of xenon adsorbed. This data can be interpreted according to the interpretation of Professor Fraissard's group.
Q: W. M. H. Sachtler (USA) Reduction of ion-exchanged Pt ions by H:, creates protons. In n-C7 reforming therefore two types of coke are usually found, one associated with the metal and one linked to the protons. Why did your TPO data fail to show the coke on acid sites ?
680 From results of Barbier et al. it is clear that the position of the TPO peak is an indicative of the kinetics of coke combustion not necessary the nature of the coke. If the activation energy of coke combustion catalyzed by Ir is higher than that catalyzed by Pt this would cause a shift in TPO peak even if the nature of the coke were identical.
A: s. I. woo The coke formed on the proton site reacted with oxygen to give CO2 around 450 OC as reported [3]. The TPO spectra of coke formed on Pt, Ir, and Pt-Ir clusters in NaY shows on1 one peak which can be assigned to the coke formed on the metal cluster. Pt(4)/Na$ Pt(3)Ir(l)/NaY shows a very weak band around 450 OC, which might suggest the coke formed on proton site. We speculate that the protons located near metal clusters in the same supercage may not be actke to form coke or thermodynamically the formation of coke on the metal cluster might be more feasible. Your commenis on the assignment of TPO peak is quite reasonable. We are now trying ESCA study of coke to study its nature. J. Barbier et al., Catalyst Deactivation, (Ed.: D. L. Trimm et al.), p 53, (1987) [3]
Guczi, L. el al. (Editors), New Fronriers in Caralysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
LASER RAMAN CHARACTERIZATION OF SURFACE PHASE PRECIOUS METAL OXIDES FORMED ON Ce02 MICRO DOMAINS GENERATED WITHIN AN ALUMINA HOST BY SOL SYNTHESIS L. L. Murrell, S. J. Tauster and D. R. Anderson
Engelhard Corporation, 101 Wood Avenue, Iselin, NJ 08830-0770,USA
ABsrRAff Laser Raman Spectroscop (LRS) has shown in other work that Rh, Ir, Pd, and Pt form a surface p ase precious metal oxide structure which interacts strongly with the surface of bulk Ce02. This interaction leads to a M - 0 structure interacting with the bulk Ce02 surface. Recent work from our laboratory has shown that mixed sols of alumina and ceria lead to hiphly-stable structures of Ce02 micro domains, or small crystals, within the alumina host structure. By the use of LRS we have been able to observe the formation of the same precious metal surface phase oxides bonded to the surface of the Ce02 domains, present within the alumina host, as those established to be present on the surface of bulk Ce02. In addition, we have direct evidence from LRS for the stabilization of very high dispersions of 5% Pt even after calcination at 900OC. This unusual stability is due to the Pt-0 surface phase bonded to the Ce02 micro domain surface.
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1. INTRODUCTION Previous work from our laboratory employing LRS has shown that Rh, Ir, Pd, and Pt form a surface phase precious metal oxide structure, M - 0 , which interacts strongly with the surface of bulk Ce02[l]. This Strong Oxide Support Interaction (SOSI) leads to a M - 0 structure with each group separated by ca. 1 nm from each other for ca. 5 wt% Pt metalequivalent loading levels on bulk Ce02 of 130 m2/g surface area. The M0 surface complex on Ce02 stabilizes the Ce02 phase to sintering and surface area collapse, while being stabilized in turn, in a highlyhase by the Ce02 support. Reduction of the M - 0 structure on CeO in owing hydrogen results in formation of a highly-dispersed dispersed meta phase which was characterized by conventional pulsed CO chemisorption techniques[l]. In this work it was also shown that calcination a >900OC results in significant sintering of the C e 0 2 phase to ca. 10 m /g surface area, with the conversion of the M - 0 structure to a highly-agglomerated phase which is for the most part detached from the reduced Ce02 surface area.
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602 In other work from our laboratory[2] mixed sol slurries of alumina and ceria lead to stable thin or thick, rod-shaped Ce02 micro domains, or crystals, within the alumina host structure, depending on the size of the Ce02 sol used. By the use of LRS we have been able to observe the formation of the same precious metal surface phase oxides, M - 0 , as those established to be present on the surface of bulk CeO . In addition, we have direct evidence from LRS for the stabilization o high dispersions of 5 wt % Pt even after calcination at 900°C. This unusual stability of a P t - 0 surface phase structure is due to bonding to the Ce02 micro domain surface. Unlike bulk Ce02 the micro domains of Ce02 within the alumina host are stable to high temperature conditions. Cyclic redox aging conditions result in very severe sintering of all four of the Group VIII metals investi ated for the micro domain Ce-A1 system. Despite the observation o precious metal sintering after cyclic aging treatment, probably due to metal sintering under the reducing conditions in the cyclic aging cycle, the Ce02 domains remain stable within the alumina host structure. The implication of these studies to automotive catalysts sug ests that dispersed and stabilized micro domains do not im rove per ormance by improving precious metal dispersion compared to bu k Ce02 systems.
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2. EXPERIMENTAL
The details of the preparation of Rh, Ir, Pd, and Pt samples o n the mixed ceria-alumina supports in this paper have been described elsewhere[ 11, and all samples were prepared by the standard incipient wetness procedure followed hy drying and calcination at 500OC. The details of the preparation of mixed Ce-AI systems from mixed slurries of Nyacol sols have also been described elsewhere[2]. The alumina sol employed was a ca. 20 nm size product made by Nyacol Corporation, and the ceria sol was also of 20 nm size obtained from Nyacol. The Raman spectrometer em loyed in this work has been described in detail elsewhere[3]. 1 of the normalized Raman intensities for the M - 0 bands at 600-700 cm-I were obtained by dividing the intensity of these Mbands by the intensity of the Raman bands at 560 cm-1 or 447 cm- of Ce02 depending entirely on which was appropriate. The 560 cm-1 band, for example, was too close to overlapping with the Raman bands of Rh and Ir at 590 and 614 cm-l, respectively, to be used for the case of the Rh and Ir samples, and the 447 cm-l band was of use only for the case of the 500oC calcined samples where the intensity was similar in size t o that of the M - 0 bands.
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3. RESULTS AND DISCUSSION
There are very few references in the literature where mixed sols have been used to prepare novel materials for application in heterogeneous catalysis[4-h]. Nothing has been published to the best of our knowledge where isolated micro domains, or sinall crystals, of one oxide are stabilized within a porous alumina host by mixed sol synthesis, except for recent work from our laboratory[l]. This is somewhat surprising as transitional aluminas[7], and their surface-stabilized counterparts[8,9]
683 constitute the most useful supports for hi$h temperature ap lication. The critical role of the alumina sol size in maintaining hig pore volume in the gelled alumina has been demonstrated[2], and all of the mixed oxides described in this work used an alumina sol of ca. 20 nm size. The size of the ceria sol has a marked influence on ceria domain size and shape, especially at low ceria contents[2], but the review of these details goes beyond the scope of this paper. In this work, directed at studying hi h temperature stability of M - 0 structures on ceria micro domains, t e ceria sol size of 20 nm was chosen for all preparations because earlier studies have shown that ceria domain, if smaller than those obtained from a 20 nm ceria sol, convert at high temperatures to a size approximately that obtained from for the 20 nm ceria sol precursor. Two compositions were chosen for study: 20% ceria - 80% alumina and 50% ceria - 50% alumina, hereafter referred to as 20 Ce-80 A1 and 50 Ce-50 Al.
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a
The ability of these two mixed oxide compositions to stabilize Pt at high temperature was first investigated using ethane hydrogenolysis as a probe reaction of relative metal dispersions after high temperature calcination treatment. After calcination at 900oC in air for 2 hours, 0.5 wt% Pt on 20 Ce-80 Al and 0.5 wt% Pt on 50 Ce-50 Al had a conversion of ethane to methane at 4100C of 34% and 74%, respectively. If a 0.5 wt% Pt on A1203 reference sample was calcined at 9OOoC for 2 hours the ethane conversion was only 3% at 410OC consistent with the formation of >30 nm size Pt particles observed by x-ray diffraction for this calcined sample, see Figure 1. The percent ethane conversion for a 500oC calcined 0.5 wt% Pt on A1203 catalyst as a reference was 83%. These results show clearly that Pt on micro domain Ce-AI systems maintain very good Pt dispersion when calcined at 900oC in air. These results will be useful to compare to the results obtained from laser Raman spectroscopy for related systems, vida infra.
0.5 W I N PI on A1202 Cap.& lo 0 6 WM Fl on 20 Ccao A1 and 50 C c 5 0 A1 C.1sIn.d W0.C for 2 H n
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Figure 1. Ethane hydrogenolysis of 0.5 wt % Pt on alumina and on two ceria-alumina compositions.
Figure 2. Raman spectrum of 2.5 wt % Pt on 50% CeO 50% Al2O3, calcined at 500 C, 2 hrs.
8-
684
The ability of Ce-A1 micro domain systems to maintain a dispersed Pt phase for low Pt levels of ca. 0.5 wt% when calcined at high temperature is entirely consistent with analogous results for bulk CeO seen in earlier workll]. The bulk Ce02 system when calcined at 900 C, however, loses significant surface area as documented elsewhere[ 11 and, as a consequence, cannot maintain hi h levels of Pt in a dispersed phase, The key question which can %e asked is, do ceria micro domain s stems stabilize high Pt contents in a dispersed phase compared to bulk ZeO, when calcined at high temperature? The answer to this question is one that laser Raman spectroscopy is well suited to address. In revious work[l] it wa shown that the intensity of the strong Raman &and at ca. 660 cm- was directly related to the concentration of the Pt-0 surface structure interacting with the bulk ceria surface. In an exactly analogous wa the Raman spectra for 2.5 and 5 wt% Pt on 50 Ce-50 A1 calcined at 50008 shows an intense Raman band ' t ca. 660 cm-1; see Figures 2 and 3. The Pt-0 Raman band at 654 cm- can be normalized by comparison to the Raman bands at 564 and 452 cm-1 assigned to bands of Note that the Raman band of CeO2 at ca. 450 cm-1 in crystalline CeO Figures 2 and are substantially reduced in intensity compared to those reviously for bulk Ce02. This remarkable low intensity is possibly repo9ed ue to disorder within the Ce02 micro domains. The Raman spectra for 2.5 and 5 wt% Pt on 50 Ce-50 Al calcined at 900oC are shown in Figures 4 and 5. Note that intense Raman bands at ca. 660 cm-1 are maintained for both of these samples. This is compelling evidence that the micro domain Ce02 structures within the alumina host have maintained a high capacity to maintain the Pt-0 surface structure at high Pt concentrations in marked contrast to bulk Ce02[1]. The normalized M-O Raman intensities for Rh, Ir, and Pt on 50 Ce-50 Al calcined at 500oC are shown in Figure 6.
2
d
4
3
B
41000
16000 40000
38000
16000~ 24000
-
1200 1100 1000
DO0
800
7W
600
500
400
300
ZOO
F4mrn.n Shln (cm - I )
Figure 3. Raman spectrum of 5 wt % Pt on 50% Ce02 - 50% Al2O3, calcined at 500OC, 2 hrs.
Figure 4. Raman spectrum of 2.5% Pt on 50% C e 0 2 - 50% 4 2 0 3 , calcined at 9000C, 2 hrs.
685
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OWW
io,
woo0
Mow -
t
i
81ow
-
loow blow
-
OW0
-
1 0 . 1 OO 8
1100 1100 low
ow
100 I W 100 I W Rmmm lhln (om - V
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Figure 5. Raman spectrum of 5 wt % Pt on 50% Ce02 - 50% Al2O3, calcined at 9OOOC, 2 hrs.
Nomat!xad lo Wl. W Pl tqulvalant Yat.1
Figure 6. Normalized raman intensities versus normalized metal content for Rh, Ir, and Pt on 50% CeO 50% Al2O3, calcined at 58$C, 2 hrs.
The ability to use the normalized Raman intensity of the Pt-0 surface structure as a direct measure of the M - 0 dispersion on bulk CeO has been discussed previous1 [l]. It can be noted from Figure 6 that ?lh and Pt show a normalized aman band increasing proportionately to precious metal content, whereas Ir shows significant deviation from a linear relationship. This result for Ir is probably related to a marked tendency or Ir to sinter when calcined at 500OC in air compared to Rh and Pt[lO]. In any case, a substantial percent of Ir is interacting with the micro domai Ce02 surface as judged by the uite intense Raman bands at ca. 620 cm-f for both the 2.4 and the 4.9 wt% Ir on 50 Ce-50 Al samples, see Figures 7 and 8.
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Figure 7. Raman spectrum of 2.4 wt % Ir on 50% CeO - 50% Al2O3, hrs. calcined SOOOC,
'i
Figure 8. Raman spectrum of 4.9 wt % Ir on 50% CeO - 50% Al2O3, calcined at 50006, 2 hrs.
686 The relative Raman intensities for 2.5 and 5 wt% Pt on 50 Ce-50 A1 after calcination at 9OOOC are shown in Figure 9. The fact that the normalized Raman intensities for the two Pt on 50 Ce-50 Al samples calcined at 900OC are no longer directly related to Pt content is not a surprise, There is considerable loss of supp rt surface area for the 50 Ce-50 Al sample from ca. 200 to ca. 100 m /g surface area when calcined at 9OOoC. Apparently, there is some re-structuring of the Ce02 domains for the 50 Ce-50 A1 support such that the capacity of the system to form the Pt-0 surface structure is approximately equivalent to ca. 2.5 wt% Pt. In the case of 5 wt% Pt on 50 Ce-50 At, about half of the Pt sinters and/or becomes encapsulated when the sample is calcined at high temperature. The availability of sufficient CeO domain surface area to bond to all of the Pt present in the 5% samp e appears to not be a factor, because the normalized Raman intensity for 2.5 wt% Pt on 20 Ce80 Al calcined at 500 and at YOO°C is 2.6, very similar to that for the 2.5 wt% Pt on 50 Ce-50 A1 sample, see Figure 6. The main conclusion from these investigations is that despite significant loss in surface area for the Ce-At systems when calcined at YOOoC that ca. 2.5 wt% Pt remains interacting with the ceria micro domain surface as sintering of the support occurs. Apparently, a quite significant amount of ceria surface capable of formin the Pt-0 surface structure is maintained after high temperature ca cination. Based on comparisons to the studies of bulk Ce02 systems[l], one can conclude that in the case of 2.5 wt% Pt on either 20 Ce-80 A1 or 50 Ce-50 A1 that all of the Pt is present as a surface Pt-0 phase after calcination at YOOOC. In the case of 5 wt% Pt, half of the Pt either sinters or is encapsulated when calcined at YOO°C, leaving the remaining half of the Pt highly-dispersed as the Pt-0 surface structure.
3
i!
k
Figure Y. Normalized Rarnan intensity of Pt on 50% Ce02 - 50% AI203, calcined at YOOoC, 2 hrs.
4. CONCLUSIONS Micro domains of CeO;? formed within an alumina host structure are stable to calcination conditions of 9OOOC despite a substantial decrease in the composite Ce-AI support surface area. These ceria domains have the structure of "rice- rain" cylinders after calcination treatment, with much higher sur ace area than would be present for bulk Ce02. The
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687 micro domain ceria systems in an alumina host form surface interactive oxide phases with Rh, Ir, Pd, and Pt as found earlier for bulk Ce02. Investigation of the normalized Raman intensities indicate that for 5 wt% Pt equivalent metal contents that the relative dispersions of the metals are in the following order after a 500OC calcination: Rh 1 Pt > > Ir. The normalized Raman intensities of 2.5 wt% Pt on two different CeAl compositions argues that the Pt-0 surface structure is maintained as a discrete molecular entity on the Ce02 micro domain surface despite being calcined at 900OC, and where the Pt-0 structure is resent during the high temperature calcination treatment. The Pt-0 sur ace structure is apparently maintained as a discrete structure bound to the stabilized Ce02 domains which are, in turn, stabilized by the alumina host structure. At still high Pt contents some of the Pt is lost from the ceria domain surface to form a sintered and/or encapsulated structure during high temperature calcination. The Raman bands for the M - 0 structure were at 654 and 643 cm-1 for Pt and Pd, respectively, and at 618 and 590 cm-l for Ir and Rh respectively, on Ce02 micro domains.
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5. REFERENCES 1 2
3 4 5 6 7 8 9 10
L. L. Murrell, S. J. Tauster, and D. R. Anderson, Proceedings, 2nd Inter. Congress on Catal. and Automotive Pollution Control, CAPoC, Brussels, Belgium (1990) 275. L. L. Murrell, S. J. Tauster, Proceedings, 2nd Inter. Congress on Catal. and Automotive Pollution Control, CAPoC, Brussels, Belgium (1990) 547. I. E. Wachs, F. D. Hardcastle, S. S. Chan, Spectrosc. 1 (1986) 30. A. Baiher, P. Dollenmeier, M. Glinski, A. Reller, and V. K. Sharma, J. Catal. 111 (1988) 273. T. H. Vanderspurt and M. A. Richard, Mater. Res. SOC. Symp. Proc., 111 (1988) 341. R. Roy, S. Komarneni, and D. M. Roy, Mater. Res. SOC. Symp. Proc., 32 (1984) 347. D. L. Trimm and A. Stanislaus, Applied Catalysis, 21 (1986) 215. L. L. Murrell and N. C. Dispenziere, Jr., J. Catal. 111 (1988) 450. L. L. Murrell, N. C. Dispenziere, Jr., and K. S. Kim, Catalysis Letters, 2 (1989) 263. G. B. McVicker, R. T. K. Baker, R. L. Garten, and E. L. Kugler, J. Catal. 65 (1980) ,207.
ACKNOWLEDGEMENTS The authors are indebted to Bill Larsen and Cesar Tolentino for their fine experimental expertise in the preparation of these systems, and to many fruitful discussions with Nyacol personnel during the course of this work, especially Tom Swank and Bob Lurie.
688
DISCUSSION Q: F. Bozon-Verduraz (France) 1) Could you comment about the nature of the interaction of the precious metal with the Ce02 microdomains after the calcination pretreatment and in reducing conditions (ethane hydrogenolysis) '? 2) Ceria is known to chemisorb significant quantities of CO. Did you take it into account on your platinum dispersion measurements ? A: L. L. Murrell 1) In our work we used both pulsed CO chemisorption and static CO chemisorption to determine the metal dis ersion following reduction in flowing H2 at 500 for CeO, of ca. 130 m2/g surface area. For our high purity ceria support there was not a large uptake of C o by either the pulsed or the static method. Professor J. Kaspar has reported very significant CO uptake on (30, prepared by precipitation from Ce(N0 solutions. More work will be required to explain the apparent differences seen from dfkerent C e 0 2 supports used in various laboratories. 2) All of the Group VIII metals, Rh, Ir, Pd and Pt, form the same interactive M - 0 group with the microdomain ceria surface similar to that obtained for high purity bulk CeO When the samples are investigated for ethane hydrogenolysis activity they are reduced in at 400 OC which converts the M - 0 surface structures to zero valent Group VIIf metal atoms/clusters.
8
Q: S. I. Woo (Korea) Bulk PtO decom osed above 500 OC and Pt oxide species supported on alumina also decomposed a%ove55foC as reported in the literature. How can you assign the Pt-0 relating band in the Raman spectra of Pt oxide is stable due to the stabilization of CeO what is the stabilization mechanism ? Is there any differences in the stretching vibration of Pt-0 between Pt oxides calcined at 500 OC and Pt oxides calcined at 900 OC '?
A: L. L. Murrell The intcraction between Pt and the surface of ceria results in the valence stabilization of Pt in a 2+ oxidation state. Since this is also true for Rh, Ir and Pd, there appears to be a specific density of such M - 0 interactive sites which, in the case of Pt, prevent the transformation of the surface phase Pt-0 complex to Pt. The Raman spectra show that PtO on ceria micro domains have exactly the same Pt-0 frequency calcined at either 500 or 900 %. This frequency invariance is due to separation of the M - 0 groups on the ceria micro domain surface by ca. 1 nm, similar to that reported previously for bulk Ce02 The stabilization observed at 900 OC for PtO on ceria micro domains is simply a reflection that the ceria domains are stabilized within the alumina host structure so that a high ceria surface area is maintained at 900 OC. This is quite different than bulk ceria which does not have a stable surface area when calcined at 900 OC. Q: Wm. C. Conner (USA) What are the differences in pore structure for CeO A1203 compared to the pure A1203 support ? How do you explain the resultant pore morp ology based on the physical picture you draw of the Ce02 in a A1203 framework '?
x'
A: L. L. Murrell
It is recognized that this is an important and fundamental question related to any oxide prepared by mixed sol synthesis procedures. The mixed ceria-alumina compositcs made by sol synthesis fall into two extremes of ceria morphology within the alumina acting as the host structure. For both extremes of ceria morphology, stocky cylinders or thin rods, the pore size and the pore volume of the ceria-alumina composites are essentially identical to that of the
689 alumina itself. This is even true at a 50% ceriaJO% alumina composition. The reason for this in the case of high ceria compositions(> 10% ceria) is simply that the crystalline Ce02 domains within the alumina host are very close to the same size as that of the alumina primary particles, i.e., 20 nm diameter. Therefore, the pore size is produced by primary particles of alumina or ceria of about equal size so that the pore size distribution and pore volume is not altered by substituting ceria for alumina. Thin rod shapes are observed by transmission electron microscopy only for ceria contents below 5wt% when a 1 nm size ceria sol is used as a precursor. At these low a r i a contents the thin rods can not alter the pore volume of the alumina host structure in a significant way.
Q: J. C. Conesa (Spain) I would say that with a such highly dispersed system, showing strong Pt-Ce interaction, EXAFS would be a technique of choice to characterize the system. Have you been able to get data of this kind on your samples ? A: L. L. Murrell I would agree that EXAFS of the surface phase oxide structures of the platinum group metals interacting with CeO, surface sites would be very informative. Such studies could help us understand the structures responsible for the "redoxcenter-stabilization"between the M-0 functions (seen by Raman spectroscopy) and the specific sites on Ce02 which form these very high temperature stable surface groups. Such studies were beyond the expertise in our laboratory at the time the work was done.
Q: D. G . Blackmond (USA) The ethane hydrogenolysis results for the Pt-Ce-Al systems demonstrate that not only does the Ce help to maintain the small particle size of the Pt, but (3.0, is also prevented from "decorating" these crystallites as has been observed to happen with combinations of noble metals and reducible oxides. Is this a function of the preparation method ? A: L. L. Murrell The ethane hydrogenolysis (EH) results are for samples calcined at high temperature followed by reduction in H, at 400 OC. The EH activity of such samples is not greatly different compared to Pt on alumina as a reference reduced at 400 % also. At least for a 400 OC reduction temperature reducedceria-species do not seem to be coverin the active zero valent metal clusters. A. Datye has observed evidence that for 500 reduction temperatures, some decrease in EH activity occurs compared to a 400 % reduced sample. Therefore, it is possible that ceria is capable of typical-SMSI decoration behavior, and/or interaction with surface and sub-surface reduced Ce centers, but at directionally higher reduction temperatures than observed for T i 0 2
o&:
Q: M. Ichikawa (Japan) I would touch on the function of CeOJ to stabilize Pt particles in keeping high dispersion. Seems that some PtO is volatile under the strong calcination and hiehtemperature oxidation reaction. Ceb, would suppress the volatility of PtO, in forming Pt-O-Ce bonding '? Otherwise, CeO, is overlayered on Pt particles or Ce(lll)/Ce(IV) ion acts as anchor to fix the Pt particle ? Do you have any evidence to discuss the chemistry of Ceo, to promote the stabilization of Pt particle'? We have observed the similar promotion of ao, on Pd-CeO catalysts for CO, + toluene conversion, where we discuss on the Go, promotion &e to its oxygen-reversion and redox-cycle of Ce(III)/Ce(IV). A: L. L. Murrell , or on There is no evidence of the PtO surface phase oxide groups on either bulk micro domains of CeO within an alumina host being volatile even at 900 q.&e MO groups (M = Rh, Ir, Pcf Pt) all form the same surface density of oxide groups, probably
690 interacting to give a Md+ - Ce(4-d/2)+interaction. Perhaps the best model is valencc stabilization of the platinum group mctals similar to Fe(Ir), interacting with the surface Lewis acid centers of Ti02 Once the platinum group metal oxides are reduced in flowing H a then the Ce02 support does not have a high stabilization capacity toward the zero valcnt metal at high temperatures. The stabilization at 900 OC in oxygen of 2.5 wt% Pt on ceria micro domain is dircctly related to interaction of PtO with a micro domain ceria phasc with an effcctive Ce02 surface area of ca. 60 m2/g for the 50 Cc-50 Al composite described in thc paper. Q: C. H. Bartholomew (USA)
I have two related questions: 1 ) Did I hear you say that Pt-0 structiires are only stable one the CeO microdomains? ? Several Why is not Pt-0 stable on a G O 2 coating that intcracts strongly with patents havc claimed that Ce02 thermally stabilized Pt and Pd on aluniina. 2) Pd/Ce/alumina systcms are reported to havc great thermal stability in oxidizing atmosphcrc to very high tempcraturc\. Is the some phcnornenon operating from the Pd-ceriaalumina system ?
A: L. L. Murrcll All of the platinum group mctals, Rh, Ir, Pd, and Pt, form thc surface phase M - 0 groups on both bulk ceria, and on micro domain ceria. Careful studies of CcO2coated aluminas where no CeO crystals were present showed no high temperature stabilization capacity towards Pt, a n f n o evidcncc that a Pt-0 structure was present on the surfacc of the ceriadoped alumina. On some aluminas, for unknown rcasons, the surface ccria-doped phase will partially transform to micro crystals upon calcination at > 600 @2of ca. 7 nm size based on X-ray diffraction. If such CcO, crystals arc present in the patents mentioned, thc stabilization could be provided by cithcr X-ray detectable G O 2 particlcs or by X-ray undctectable Ce02 particles. In the case of Pd/Ce/aluniina systems stablc to high temperatures, 1 would expect that in the absence of CeO microcrystals the stability of Pd/Ce/alumina would be analogous to Pd/La/alumina or P&a/alumina. The Cc, La, and Ba components act as stabilizers of the alumina phase which is itself a strongly interactivc phasc towards PdO in high temperature oxidizing environrncnts. Pd does form prcdoniinantly a Pd-0 surface phase complex with the ceria micro domain surface within an alumina host structure rathcr than form a PdO phase on the alumina surface.
Guni, L er al. (Editors), New Frontiers in CoralysC Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
DIRECT PROPANE AMMOXIDATION TO ACRYLONITRILE: KINETICS AND NATURE OF THE ACI'IVE PHASE A. Anderssona, S. L. T. Anderssona, G. Centic, R. K Grassellib, M..Sanatia and F. Trifiroc aDept. of Chem. Techn., Chemical Center, Univ. of Lund, P.O.Box 124,221 00 Lund, Sweden bDept. of Chem., Georgetown University, Washington, C D 20057, USA (Presently Central Research Lab., Mobil, Princeton, New Jersey, USA) %pt. of Ind. Chem. & Materials, V. le Risorgimento 4, 40136 Bologna, Italy
The kinetics of the direct synthesis of acrylonitrile from propane on V-Sb-Al-(W) mixed oxides
indicate that acrylonitrile (ACN) forms by two parallel pathways, one directly from propane and the second, which is the prevalent path, through the intermediate formation of propylene (C3=). The limiting factor in the formation of ACN is the relative slowness of the step of aLlylic oxidation to ACN of the intermediate C3=, and the higher rate of C3= oxidation to carbon oxides as compared to that of ACN to CO,. The step of C3= oxidation to ACN is controlled by the surface availability of hW3 which, in tum, depends considerably on the side reaction of NH3 oxidation to N2. The catalytic behavior of different modified V-Sb(Al)-0 systems and their characterization by X-ray diffraction analysis and Raman, Infrared and X-ray Photoelectron spectroscopies indicate that i ) a reduction of both V and Sb occurs during the catalytic reaction. ii) the presence of vanadium not stabilized in the rutile-like phase is responsible for the side reaction of NH3 oxidation and lowering of the selectivity, iii) alumina reacts with antimony forming an AlSb04 rutile phase which could be epitaxially intergrown or in solid solution with the VSb04/Sb04 system, which, in turn, limits the presence of not stabilized (unselective) vanadium species, and iv) antimony oxide supported on alumina is also selective in propane ammoxidation, but forming acetonitrile as the main product. The doping with vanadium of this sample increases slightly the activity, but especially gives rise to the formation of acrylonitrile instead of acetonitnle.
Introduction There is increasing interest in the development of a process for direct acrylonitile synthesis as an alternative to the conventional method based on the olefinic feedstock [ 1,2], and recently it was announced that commercialization of the process would be possible in a few years. Multicomponent metal oxide catalysts based on V, Sb, W, M o and other elements have been patented for this reaction [3-7, and references therein], but an analysis of the patent data indicates that catalysts based on V-antimonate, in particular, seem to be the most promising [46]. In this paper we report the results of extensive research on V-antimonate based catalysts directed towards the development of a more rational approach to the design of catalytic systems for direct propane ammoxidation to acrylonitrile. The approach involved (i) a kinetic study of the reaction in order to identify the role of the competitive pathways of transformation and (ii) combined catalytic and physico-chemical characterization of modified systems in order to establish correlations between catalytic behavior and surface or structural features.
692
Experimental The catalysts were prepared, according to patent indications [1,2], using a sluny procedure (over 16 hours under stimng and reflux) with SkO3, NH4VO3, ammonium-tungstate and alumina or Al(OH)3 as the starting compounds. The solid was dried and then calcined up to 600-900°C (as specified in the text) with a heating rate of about 5O'Chours. The surface area 2 of the vanadium-antimonate samples without alumina was in the 10-15 m /g range, and about 10 times higher for the samples containing alumina. Further details on the characterization of the samples and on the method of preparation have been reported previously [3-71. The catalytic tests were made in a fixed-bed quartz tubular flow reactor (with a catalyst charge of about 2-7 grams diluted with quartz spheres) [6] working at atmospheric pressure, with on-line gas chromatographic analyses of reagents and products. HCN and NH3 conversions were determined by titration after adsorption in 0.1 N NaOH solution, as described elsewhere [6]. Preliminary tests as well as calculations indicate that mass and heat transfer limitations may be considered negligible in our experimental conditions. Homogeneous gasphase reactions were also checked and found to be negligible in our experimental conditions. The samples were characterized by X-ray diffraction analysis (XRD), Raman and infrared spectroscopies, and X-ray photoelectron spectroscopy (XPS). In particular, XPS analyses were camed out using a Kratos XSAM 800 instrument with Mg Ka x-ray radiation and using the Au 4f712 peak (84.0 eV) to correct for the charge effect. Quantitative determinations were made by correcting the ratios of the peak areas (Ahp, Vzp3/2, Sb3d3/2 and W4d) of the analyzed elements for the instrumental sensitivities for the various core lines measured for the pure oxides. The V2p3/2 intensity was calculated after subtraction of 01, and Sb3d512 lines generated by the Mg Ka3,4 satellites. Raman spectra were recorded on a Bruker IFS66FRA106 fourier-transformed instrument. Infrared spectra were recorded using a Perkin-Elmer 1700 Fourier-transform instrument.
Results and Discussion Kinetics and Reactlon Network
In order to obtain reliable information on the kinetics and reaction network in propane animoxidation on V-Sb-A1 based mixed oxides, a catalyst formulation and method of preparation analogous to those reported for the best patented catalysts [1,2] was adopted. We shall therefore refer in this section to a V-Sb-A1 mixed oxide doped with tungsten; the V:Sb:W atomic ratio is 1:5:1 and the composition is 30% wt of active phase (assumed to be VSb04+2Sb204+W03) and 70% wt of alumina. The preparation of this sample involves i) a two electron redox reaction in aqueous ammonia solution between Sb"' and Vv starting from S b O 3 and NHqVO3, ii) the addition of ammonium tungstate and formation of the mixed hydroxide containing V V formed in the previous step is oxidized by 0 2 dissolved in solution), Sb"', SbV and lVGI'l' W by H2O/N€I3 removal, and iii) drying and calcination up to a final calcination temperature of 650°C [4,6]. The surface areas of the resulting samples are about 110 m2/g and do not change after catalytic tests. Reported i n Fig. 1 are the results of tubular-flow-reactor tests carried out at 520°C in differential reaction conditions (conversion of the three reagents below about 5%). In these condi-
693 tions a highly selective transformation of propane to propylene Selectlvlty.% rate propane depletlon, m o m g (C3=) takes place. Acrylonitrile 100 (ACN) forms with a selectivity of about 8-10% and other reac75 tion products (HCN, ace0.005 tonitrile, carbon oxides and * scox C1-C2 hydiocarbons) form with 50 + SHCN selectivities in the 0-4% range, 0.002 -4 r a t . It should also be noted that the 25 0 rompty rate of propane transformation increases with increasing alkane concentration following a Lang0 0 0 2.5 5 75 10 12.5 15 muir-Hinshelwood model with Propane concentration,% saturation of the active sites, but the rate of alkane transformation Flg, 1 Rate (I) of propane conversion at 520'C and selectivity to propylene (Cp), acrylonitrile (ACN) and COXas a function of propane concen- due to homogeneous reactions tration for reagent conversions below 5%. €xp : 0.5 g of catalyst (as (empty reactor) is much lower described in the text), 15.1% 02,7.3% NH3.The dotted line shows the in all ranges investigated. rate of propane depletion in the empty reactor measured to lest the conReported in Fig. 2 are the retribution of homogeneous reactions. sults of tests carried out in integral reaction conditions with varying amounts of catalyst at fixed reaction temperature. The composition of the reagent mixture corresponds to that reported in various patents as optimal [1,2]. The selectivity to C3= decreases continuously with increasing contact time and propane conversion, whereas the ACN selectivity passes through a maximum for a propane conversion of around 40-50%. For higher propane, but also higher ammonia, conversions the selectivity decreases with a parallel increase in the formation of carbon oxides. The maximum in the yield to ACN is obtained for propane conversions around 70-80%. Figures 1 and 2 thus show that propylene is the primary product of propane transformation, whereas ACN forms mainly by consecutive transformation of the C3= intermediate. The formation of carbon oxides competes with the reaction for the formation of ACN, especially at the higher propane and ammonia conversions. These data, in combination with a more extensive analysis of the dependence of product formation on reagent concentration, space-velocity and reaction temperature [6] allows the development of a kinetic model of the reaction. The reaction network can be described by six parallel reactions of formation (C3=, ACN, carbon oxides, acetonitrile, HCN and C2 hydrocarbons), three reactions of consecutive transformation of the intermediate C3= to ACN, acetonitrile and HCN, five consecutive reactions of consecutive decomposition to carbon oxides of the other products, and one side reaction of NH3 oxidation to N2. Based on a Langmuir-Hinshelwood approach for the reaction rates, it is possible to derive rate and adsorption constants that fit the experimental data over a wide range of reagent compositions (0-20% for propane, oxygen and ammonia) and reaction temperatures (410-53o'C) [6].The lines in Fig. 2 were calculated from the kinetic model and the symbols represent experimental data.
694 An especially important aspect that emerges from Yo Yield, % ~ ~ _ _ _ _ _ ~ Selectivity, the kinetic analysis is the 1 ' 0 0 presence of a side reaction of ammonia conversion to N2, that has a negatively effect on the selectivity to ACN. The rate of this side reaction of NH3 critically depends on i) the specific nature of the catalyst, as also discussed below, and ii) the reaction conditions and type of reactor [7]. The data reported in Table 0 20 40 60 80 Conversion C3,Yo 1 illustrate how the maximum selectivity to A C P Flg. 2 Yield (Y) and selectivity (S) lo propylene (&=) and acrylonitrile (ACN) as obtained (and relative proa function of propane conversion. EN^.: temp., 520'C; C3, 5.62%: 02,11.8%; pane and NH3 converNH3, 11.8%; catalyst as described in the text, total flow rate 3.6 Uh. Symbols. sions) depends on the NH3 experimentalvalues. Llnes:calculated values from the kinetic model IS]. to propane inlet ratio. NH3 to C3 ratios higher than the stoichioniemc value of 1.0 are necessary to increase the selectivity and especially the propane conversion corresponding to the maximum in the selectivity. It is also shown that the maximum in ACN selectivity is obtained for relatively low NH3 conversions. In conclusion, the kinetics and reaction network analysis show that i) ACN forms mainly from the C3= intermediate, ii) conv. for max.ACN Max. ACN selcctivity,% NH3/C3 the limiting factor in the sclcctivity,96 formation of ACN is the N H3 c3 relative slowness of the 0.5 41 43 23 C3= to ACN step com0.9 49 42 30 pared to competitive un1.3 53 36 36 selective pathways, and iii) 1.8 55 29 40 the presence of a side reac2.2 56 27 42 tion of NH3 oxidation limits the effectively available surface ammonia for ACN synthesis. This also explains why apparently in the V-Sb-W-A1 mixed oxide system it is more critical to achieve the C3= to ACN step as compared to the propane to C3= step, notwithstanding the different reactivities of olefinic and paraffinic hydrocarbons. It is worth noting also that i n all cases (see Fig. 2 ) not converted C3= is found together with ACN; this aspect agrees with patented results [1,2].
Table I Maximum selectivity to acrylonitrile (ACN) and ammonia and propane conversions corresponding to the maximum in ACN selectivity as a function of the inlet ammonia to propane ratio. Exp.: temp., 500'C; propane, 5.9%; 02 to propane, 2.1.
695
Kinetics and reaction network analysis also suggest future work in the area of co-catalysts to enhance the C3= to ACN conversion. Also of critical importance is suitable doping in order to limit as much as possible the rate of side ammonia conversion and to achieve an increase in the surface ammonia available for ACN synthesis [7]. The use of high, propane/(@+NH3) ratios with propane recycling and 0 2 rather than air as the source of feed oxygen is also suggested on the basis of kinetic results [6]. Characterization of the Active Phase
Unsupported V-Antimonate Samples. The preparation procedure for V-antimonate based catalysts reported in the patents [ 1,2] (hereinafter called slur method) involves a two electron 7 redox reaction in an aqueous medium between Vv and Sb1 (added as NH4VO3 and Sb2O3) to form V"' and SbV [8]; V"', however, is oxidized to V" by the @ dissolved in the solution. The mixed hydroxide obtained by evaporation of the solution and drying of the solid transforms to a crystalline VSb04 rutile phase [9] at temperatures in the 500-7WC range, as shown by X-ray diffraction (XRD) analysis. XRD monophasic samples with m i l e structure can be obtained using this procedure for stoichiometric Sb:V=1 .O samples, but infrared (IR) and Raman spectra suggest that these samples are not monophasic. In fact, IR spectra (Fig. 3) show the presence of a band at 1020 cm-l and two strong shoulders at around 920 and 850 cm-I that cannot be attributed to the V-antimonate m i l e phase [lO,lll, characterized from bands at about 740, 660 and 550 cm-', due to vSb-0 vibrations. The former bands suggest the presence of an XRD amorphous, partially reduced, vanadiumoxide that has not reacted to form the m i l e phase. In addition, it is reasonable to assume that unreacted antimony oxide is also present, even though not detected. As shown in Fig. 3 the relative intensities of these bands due to unreacted vanadium-oxide with respect to those of the m i l e phase depend on the preparation method. For example, compared in Fig. 3 are the IR spectra of two samples with a Sb:V ratio of 1.0, but prepared by solid state reaction between V2O5 and S k O 3 (a) and by i / , , --.A the sluny method (b). Both 1200 800 crri' 800 4 3 samples were calcined at 600°C for 6 hours. In agreeFig. 3 Infrared spectra (KBr lechnique) of a fresh Sb:V =1.0 sample pre ment, Raman spectra of these pared by the solid slate (a) or slurry melhod (b), of used Sb:V=1.0 sample samples show the presence of (c) (sample b after propane ammoxidalion), of fresh and used Sb:V=5.0 very broad and not well resample (Preparation by slurry method) (d and e, respectively). I
I
696 Table II X-ray photoelectronspectra for unsupported Sb-V-0 catalysts. The indicative Sb3*/Sb5' ratio was obtained by fitting components for St& and Sk013to Sb3d5/2 spectra and correcting fora mean valence 01 4.33 for Sb6013
Sb:V bulk
Preparation
1 .O
slurry -fresh SIUKY - used solid state - used slurry -fresh slurry - used
1.0 1.0
5.0 5.0
Binding energies and peak width at half height (in the brackets) Sbd3/2 540.1(1.6) 540.0(1.6) 540.4(2.1) 540.3(2.2) 540.1(2.1)
Surface
indicative
Sbl V
Sb3+lSb5+
0.66 0.56 1.2 2.8
8911 1 1 oo/o 66/34 68/32 91/09
v2~3R
517.1(1.7) 516.4(2.6) 516.6 (2.1) 516.8(2.0) 516.2(2.1)
3.8
solved bands centred at approximately 860 and 400 cm.' in sample (a) and additional weak, but sharper, bands at 995, 700,284 and 145 cm-' in sample (b)due to V2O5. No bands due to antimony oxides were detected in these samples. After catal tic tests in propane ammoxidation, the IR bands at frequencies higher than around 800 cm-7 decrease considerably in intensity (Fig. 3) and similar IR spectra are found independently of the preparation. Raman spectra also indicate the disappearence of the band at 860 cm-' as well as V2O5 bands, when present. Thus, apparently, during the catalytic reaction an in-situ evolution occurs leading to the further reaction of V-oxide and Sb- oxide (unreacted after the calcination stage) to form the Vantimonate phase. However, X-ray photoelectron (XPS) data (Table 11) give a different indication. The following aspects derive from XPS characterization: a) a reduction of surface vanadium species occurs (forming p ' ), as indicated by the shift in the binding energy and broadening of the VzP3/2 peak, and b) a reduction of antimony also occurs, as tentatively suggested by the deconvolution of the Sb3d5/2 peak. The reported Sb3+/Sb5+ratio is only indicative, because of the small shifts between S b O 3 (540.0 eV), Sb2O4 (540.2 eV) and Sb6013 (540.7 eV); nevertheless the change suggests a surface enrichment in Sb111 after catalytic reaction as found also in Sb-Sn oxides [12]. It is worth noting the different S b N surface atomic ratio between the samples prepared by the slurry method and solid state methods. Berry et al. [13] in their XPS characterization of stoichiometric V-antimonate prepared by solid state reaction also an enrichment of antimony at the surface in agreement with observations in other antimonate systems with rutile structure [14]. The S b N ratio in sample (b) does not change greatly after reaction (Table II), notwithstanding the change in IR and Raman spectra (Fig. 3). These results can be interpreted as follows. In samples with Sb:V=1.0 the formation of the rutile phase is not complete, but an amorphous layer of reduced vanadium oxide is spread on the VSb04 particles, and probably on the unreacted amorphous Sb-oxide. This explains the surface enrichment with vanadium. The amount of the unreacted vanadium-oxide depends on the method used to prepare the samples. During catalytic tests vanadium and antimony species are reduced, as shown by XPS, but do not form the vanadium antimonate phase (the S b N surface ratio does not change very much); probably, the contemporaneous presence of both reduced vanadium (V3+ or V4+) and oxidized antimony (Sb") is required [ 5 ] . It is reasonable to assume that the latter aspect is inhibited during reaction. The presence of unreacted surface vanadium-oxide has a considerable influence on the cata-
697 Table 111 Catalytic tests in propane ammoxidation of unsupported Sb-V-O catalysts. Exp. cond.: 2 g of catalyst, reaction temp., 510'C, propane, 6.62 ; 02, 16.9%; NH3,9.6%. [AcCN indicates acetonitrile,other symbols as in text].
Sb:V bulk
Preparation
1.0 1.0 1.0 5.0 5.0
slurry-fresh slurry-used solidstate-used slurry-fresh slurry-used
I
conversion, % propane NH3 toN2I C3=
ACN
8.3 30.8 36.7 16.9 33.4
1.2 10.5 18.9 9.8 16.9
34.8 20.3 22.4 33.4 21.9
90.3 43.2 31.5 79.1 36.6
selectiviry, % AcCN CO 0.9 3.5 2.8 3.7 3.1
37.8 17.4 16.1 33.4 16.8
C02
HCN
38.9 15.9 10.4 19.1 14.3
11.7 18.9 15.1 16.9 14.7
lytic behavior in propane ammoxidation, as shown in Table III. The fresh sample is not selective and forms mainly carbon oxides, due to the very high rate of side conversion of NH3 to N2. The in-siru evolution, as discussed above, leads to an increase in the selectivity to both C3= and ACN, and a parallel decrease in propane conversion, but especially a decrease in the NH3 side conversion to N2. It should also be noted that the sample with a higher surface Sb/V atomic ratio, but equal bulk stoichiometric ratio, shows a lower rate of NH3 oxidation to N2 and an enhanced selectivity, even though lower than that shown by alumina supported samples. In the unsupported V-Sb samples with an Sb:V atomic ratio higher than stoichiometric for VSb04, similar results are obtained. The IR spectrum (Fig. 3) of the fresh sample also shows the additional bands for frequencies higher than around 800 cm-l attributed to unreacted vanadium-oxide. These bands disappear after catalytic tests in propane ammoxidation. The difference in the IR spectrum below 800 ern-' as compared to that of the V:Sb=1.0 sample is due to the presence of S b O 4 characterized by bands at 765,750,660 and 615 cm-'. The presence of Sb2O4 is confirmed by XRD and Raman data. The XPS indications are similar to those previously discussed. Also in this case, the S b N atomic ratio is lower than the bulk value of 5.0, however, in this case, due to the presence of S b O 4 crystallites, the attribution of the lower ratio to the presence of a surface layer with unreacted vanadium-oxide is more questionable. The presence of excess antimony as compared to the stoichiometric amount for VSbO4 thus may enhance the formation of the rutile phase, as suggested also by the decrease in the relative intensities of the IR bands for frequencies higher than 800 cm-'. Even so, formation of the m i l e phase still does not appear to be complete. Catalytic data (Table III), in agreement, indicate a still relatively high rate of NH3 conversion to N2 and relatively low selectivities to C3= and ACN. V-Antimonate Supported on Alumina. Alumina-supported V-antimonate catalysts generally have a surface area in the 80-150 m2Ig, depending on calcination temperature and method of preparation [4,5], which is about one order of magnitude higher than that of catalysts for propylene oxidation to ACN. However, the role of alumina is not just that of a simple support to enhance the specific area of the V-antimonate active phase due to the lower reactivity of propane as compared to propylene, but rather the alumina interacts directly with both antimony and vanadium modifying the nature of the active component. In fact, S b O 4 reacts with A1203 forming the m i l e AISbO4 phase, isostructural with VSb04. The reaction between Sb2O4 and A1203 occurs in the 800-900°C temperature range of
698 calcination, but in the presence of V the reaction occurs at lower temperatures, in the 700-800'C range. Reported in Figure 4 is the XRD pattern of AlSb04 formed by solid state reaction between .-,x S b O 4 and alumina. Both t 400 0 the method of preparation -C and the amount of antimony oxide employed are equivalent to that used in the preparation of patented V- antimonate on 0 alumina catalysts [ 1,2] 2rtheta (see also previous secFlg. 4 XRD patterns for Sb-oxide [30% wt] on alumina and calcined at 9OO'C (a) tion). The cell parameters and subsequently doped with 1% wt (as V2O5) of vanadium and calcined again at of the AISb04 phase 9OO'C (b). correspond well to those reported in the literature [lo]. When this sample is doped with 1% wt of vanadium-oxide (corresponding to a Sb:V ratio of 10) a shift is observed in the diffraction line (Fig. 4) towards the position found for VSb04. The cell parameters, accordingly, are intermediate between those of AlSbO4 (a=b=4.51 and c=2.96 A [lo]) and those reported for VSbO4 (a=b=4.61 and c=3.06 A [9,15]). The extent of modification is proportional to the amount of vanadium introduced. This indicates that V reacts with AlSbO4 to form a solid solution or intergrown phase. No XRD lines due to vanadium- or antimony-oxide are detected. In samples prepared directly with Sb,V and alumina similar observations can be made upon changing the relative amount of alumina with respect to the amount of active phase. Also in this case the cell parameters of the alumina-supported V-antimonate are found to be intermediate between those of Al- and V-antimonate. For these samples, a crystalline rutile phase is detected for calcination temperatures in the 700-800°Crange, slightly higher than those used for the catalytic tests (around 650°C),but lower than those required to form the AISb04 phase in the absence of vanadium. Reported in Fig. 5 is the comparison of the catalytic behavior of the sample of antimony oxide on alumina, calcined at 900°C (sample a of Fig. 4) with that of the same sample, but doped with 1% vanadium (sample b of Fig. 4). Tests were carried out at low conversion in order to evidence the initial selectivity of the catalysts more clearly. Doping of the AlSb04/A1203 matrix with vanadium leads to a relatively small modification in the activity, but a drastic change in the selectivity. Sb-supported on alumina or AlSb04 catalysts are capable of selective activation of propane, but mainly towards acetonitrile. On the contrary, when V is introduced in the Al-antimonate rutile matrix, as indicated from XRD data (Fig. 4), ACN forms with relatively high selectivity. It should be noted that 1% wt of vanadium corresponds to a Sb:V = 10 ratio and thus to about half the vanadium present in the samples prepared according to patent indications [1,2]. The N H 3 side conversion is relatively low in both I
a.
Y
699 cases, but decreases after doping. The XPS characteriza?& tion of these samples is re50 __ ported in Table IV. The formation of AlSbO4 induces an enhanced pre40 sence of Sb5+ as compared to that found on 30 unsupported antimonate samples. Doping with V 20 does not modify the indicative Sb3+/Sb5+ ratio, 10 but increases the SbIA1 -alum doped V surface ratio, indicating 0 umina C3= ACN AcCN COX HCN C3 NH3 that reaction with V inSelectivity conv. creases surface spreading. The surface Sbff ratio is FIg. 5 Comparison of the catalytic behavior of samples (a) and (b) of Fig. 4 (Sbin good agreement with alumina and Sb-alum. doped V, respectively). Exp.: reaction temp., 500'C; prothe bulk value. pane, 10.8%; ammonia, 15.1; oxygen, 29,4 %. Also reported in Table IV are X P S data characterizing both fresh and used catalysts prepared according to patent indications. The results indicate that the presence of alumina stabilizes the antimony and vanadium with respect to reduction in comparison to unsupported V- antimonate. After reaction a probable spreading of the phases containing Sb and V occurs, as suggested from the increase in both the surface Sb/V and Sb/Al ratios. The W/V ratio is lower than the bulk value and different preparations gave values between zero and 0.80. Analogous observations of incorporation of W in the antimonate lattice of GaSb04 based catalysts for propane ammoxidation have been reported [ 161.
/I
Conclusions The analysis of the reaction network and the reaction kinetics indicate that propane is transformed to ACN mainly through the intermediate formation of propylene. The limiting factor in ACN selectivity is the rate of this second step of propylene transformation to ACN, which is influenced considerably by the availability of surface ammonia. The latter depends on the rate of the side conversion of NH3 to N2. This side conversion of ammonia is tentatively related to the presence of unreacted vanadium oxide, not stabilized by the formation of the rutile antimonate phase, even in the presence of excess antimony. The use of alumina as the support not only induces a spreading of the active component, but due to the formation of isostructural AlSbO4 i ) induces site isolation, ii) stabilizes Sb and V with respect to reduction and iii) probably avoids or limits the presence of not stabilized vanadium oxide, thus reducing the rate of the side reaction of ammonia and increasing the selectivity in propane ammoxidation.
700 Table IV X-ray photoelectron spectra for Sb-V-0catalysts supported on alumina. The indicative Sb3'/Sb5' ratio was obtalned by fitting components for Sb203 and Sb60i3 to S h d w spectra and correcting a mean valence of 4.33 for Sb6013
I ~
Sample
fresh Sb-alumina170% A12031 calclned 9OO'C fresh Sb-aluminadoped V calcined 9OO'C .Sb:V 10 lresh SWlw ( 5 1 :l)-alumina 170% A12031 - CalC. 650'C usedSWlw (5:l:l)-alumina 170% A12031 - calc. 650%
Binding energies and peak width at halfheight (in the bracket) v2p3/2 Sb3d3/2
Surface Surface SblV Sb/AI
540.7 (2.2)
Surface WIV
Indicative Sb3'lSbs'
0.22
33/67 37/63
540.8 (2.1)
517.0 (2.2)
12
0.49
540.6 (2.4)
517.5 (2.4)
2.8
0.086
0.48
51/49
540.2 (2.3)
517.2 (2.0)
4.0
0.17
0
83/17
Furthermore, the catalytic behavior of antimony oxide supported on alumina compared with that for the same sample doped with vanadium, indicates that vanadium induces a slightly increase in the activity in propane conversion, but especially creates sites to form selectively acrylonitrile instead of acetonitrile, as occur on Sb-oxide supported on alumina. Additional studies are necessary for a further clarification of these preliminary indications, but tentatively may be suggested that on a Sb-alumina system operates a different mechanism like an acid-catalyzed mechanism, whereas a metallo-radical mechanism predominates in the presence of vanadium sites. This may explain the change observed in the products of propane ammoxidation when Sb-alumina matrix is doped with vanadium.
References [I] L.C. Glaeser, J.F. Brazdil, D.D. Suresh, D.A. Omdoff, R.K. Grasselli, US.Patent4,788,173 (1988). [21 A.T. Gutrnann, R.K.Grasselli, J.F. Brazdil, US Patent, 4,746,641 (1988). [31 G. Centi, D. Pesheva, F. Trifirb, Appf. Cat& 33 (1987) 343. [4] G. Centi, R.K. Grasselli, E. Patank, F. Trifirb, in New Developmentsin Selective Oxidation, G. Centi and F. Trifirb Eds.; Elsevier Science Pub.: Amsterdam 1990; 635. [51 G. Centi, F. Trifirb, R.K. Grasselli, Chim. Ind. (Milan), 72 (1990)617. [6] R. Catani, G. Centi, F. Trifirb, R.K. Grassclli, Ind. Eng. Chem. Research, in press (1992). [71 G. Centi, R.K. Grasselli, F. Trifirb, Catal. Today, in press (1992). [8] B.B. Pal, K.K.Sen Gupta, Inorg. Chem., 14 (1975) 2268. [9] F.J. Berry, M.E. Brett, W.R. Patterson,J. Chem SOC.Dalton Trans.. (1983) 9. [ 101 C. Rocchiccioli-Deltcheff, T. Dupuis, R. Frank, M. Harmelin, J . Chimie Phys. Biol., 67 (1970) 2037. [11] E. Husson, Y.Repclin, H. Brusset, A. Cerez, Spectrochim.Acta, 35A (1979) 1177. [I21 P.A. Cox, R.G. Egdell, C. Hardings, W.R. Patterson, P.J. Taverner, Surf. Sci., 123 (1982) 179. [I31 F. Berry, M.E. Brett,R.A. Marbrow, W.R. Patterson, J . Chem. SOC.Dalton Trans., (1984) 985. [I41 G.Centi, F. Trifirb, Cataf.Rev.-Sci.Eng.,28 (1986) 165. [15] J. Birchall, A.W. Sleight,Inorg. Chem., 15 (1976) 868. [I61 S.Yu. Burylin, Z.G. Osipova, V.D. Sokolovskii, I.P. Olen'kova,Kinet. Card., 30 (1989) 1095.
701 DISCUSSION
Q: M.Sinev (Russia) A strong influence of ammonia on the rate and selectivity can be explained by different effects: i) a "formal-kinetic effect" (higher reaction order with res ect to NH3 of ACN production as compared with NH oxidation to N2), ii) influence of h&3 on the reactivity of intermediates, iii) influence of on the state of catalyst (or its components) and iv) something else. What is your explanation of the strong influence on the reaction 7
h3
A: F. Trifiro All these factors contribute in determining the effect of ammonia on the formation of acrylonitrile and their relative im ortance depends also on the specific characteristics of the V-antimonate itself. The rates o ammonia oxidation to Np and of acrylonitrile synthesis from the intermediate propylene show a different order as respect to ammonia concentration, and, due to the higher rate of propylene conversion to carbon oxides than the rate of acrylonitrile consecutive oxidation to carbon oxides, a decrease in the pro ylene to acrylonitrile step strongly influences the selectivity to the sum of propy ene plus acrylonitrile. However, additional factor is the partial reduction of the surface induced from ammonia which thus induces also a modification of the reactivity of the catalyst itself. It should also be mentioned that the presence of a competitive reaction of ammonia oxidation, which is a function of the amount of V(V) ions on the surface, leads to a decrease of the amount of ammonium ions on the surface necessary for the selective transformation of the reaction intermediates to acrylonitrile instead of carbon oxides. On the other hand, the amount of V(V) ions on the surface depends on the characteristics of the sample during the catalytic reaction influenced both from the preparation method and from the reaction conditions, Various factors therefore contribute in determining the effect of ammonia on the selectivity to acrylonitrile, but the key problem is to limit the side reaction of ammonia oxidation in order to increase the selectivity to acrylonitrile.
P
P
Q:J. Haber (Poland) In your catalytic system you have two redox pairs (V3+/V4+/V5+and Sb3+/Sb4+/Sb5+). Could you find which of them is responsible for the activation of the hydrocarbon and which for interaction with oxygen '? A: F. Trifiro Present knowledges on the mechanism of reaction do not allow an unequivocal answer to this question, because is not clear what is the real role of each of these two components in the reaction mechanism. After reaction we noted by XPS a reduction of both vanadium and antimony which suggests the possible role of both coniponents in the mechanism of reaction. On the other hand, our data indicate that the catalytic behavior of the rutile V-antimonate phase in propane ammoxidation is markedly different from the behavior in the same reaction of various other M-antimonates with mile structure and the doping of these samples with even small amounts of vanadium can increase considerably the formation of acrylonitrile. This suggests a specific role of vanadium in the mechanism of both activation of propane and selective transformation to acrylonitrile, but vanadium must be stabilized in a valence state lower than five (V(II1) and V(IV) from the rutile antimonate structure to be selective. However, these evidences do not allow to clearly assip the catalytic role of each components and our future research programme, in fact, is to study more in detail the mechanism of this reaction and the nature of the active sites for each of the various steps of the reaction. In conclusion, the V(IV)/V(III)redox couple is tentatively responsible of the oxidative deh dro enation of propane to propylene, whereas our preliminary indications su V(I ) (III) redox couple responsible for the N insertion step. The Sb(II1) and S&ytiAE have mainly a structural role of stabilization of the various valence states of vanadium in the
$2
702 mtile antimonate structure, but we cannot exclude also a role of Sb(II1) ions in the activation step of oxygen and of Sb(V) ions in the N insertion step, even though V-antimonate behaves in a different manner, from the catalytic point of view, from the other Me-antimonates active in propylene ammoxidation. Q: D. D. Suresh (USA) You have carried out good investi ation in your laboratory. This is a two part question: first, is the surface concentration of V, b and Al in the catalyst same as in the bulk ? Second, what is the mechanism of propylene formation from propane, the a - H abstraction and oxygen insertion ?
A: F. Trifiro The surface concentration is effectively different from the bulk and, in general, we found surface enrichments in antimony as typical of other M-antimonate systems as well as the possibility of surface segregation of antimony oxide particles. However, the correlation between surface characteristics and catalytic behavior is much less clear than, for example, in the case of propylene ammoxidation on other M-antimonate (for example, iron-antimonate). As regards to the second question, tentatively we believe that V(IV) ions in the antimonate matrix are responsible of the activation step of propane to form propylene, whereas some preliminary data indicate that the mechanism of acrylonitrile synthesis differs from that proposed for acrylonitrile synthesis from propylene on other M-antimonates and that vanadium ions are also involved in this step. However, we d o not have clear evidences on the mechanism of reaction. A: R. K. Grasselli As a general comment, and in partial response also to Dr. Suresh's question, I should like to offer a mechanistic view of propane ammoxidation over Sb-Al-oxide based catalysts and V-Sb-Al-oxide based catalysts, consistent with the experimental data given in our paper. The propane ammoxidation pathway (attached Scheme 1) over Sb-Al-oxide is predominantly an acid catalyzed reaction which leads via the first formed, chemisorbed propylene intermediate primarily to acetonitrile, HCN and CO,. It is catalyzed by [AI-O-Sb]+ acid sites. However, there are also [Sb-0-Sb] selective oxidation sites in this catalyst from whence a chemisorbed propylene, once formed, can yield an allylic intermediate which results in dcsorbcd acrylonitrile (i.e., the desired product) when there is sufficient NH3 present to convert the [Sb-0-Sb] site to an [Sb-NH-Sb] site. Conversely, if there is insufficient NH3 present, the so formed acrolein at the [Sb-0-Sb] site gets further cracked to acetonitrile, HCN, and COXas shown in Scheme 1. The calculated yields agree well with the experimentally observed yields. On V-Sb-Al-oxide, the propane ammoxidation pathway is metalloradical initiated by [V=O] sites (Scheme 2). The primary intermediate is again propylene which now encounters many more [Sb-0-Sb] and [Sb-NH-Sb] surface sites than acid sites, with the concomitant result of yielding acrylonilrile as the major useful product. It is hypothesized that the presence of vanadium not only provides the [V=O ] paraffin activation sites but also helps in the reoxidation mechanism of the [Sb-0-Sb] and [Sb-NH-Sb] sites. The calculated product yields agree well with the cxperimentally observed yields. Q: 0. V. Krylov (Russia) The temperature of propane ammoxidation is rather high (500 OC). It is known that at such temperatures alkanes give alkyl radicals upon interaction with metal oxides. Vanadium oxides are catalysts of such radicals formation. It is possible that the first step of reaction is the radical formation with reduction of V-atoms and initiation of gaseous reaction. The second step is surface redox reaction with Sb participation. What d o you think ):
703
A
CHlCN HCN t 2COx 3COx
PRODUCTS 2'
ACN
AcCN
HCN
cox
C-Ykld,bs.
14
17
47
8
12
C-YLeld,,lc.
14
17
42
11
16
CH&N HCN
t COX t
2COx
3COx
PRODUCTS C-YleldobS, C-YieldcalC.
ACN
AcCN
33
44
2
4
17
33
45
3
3
16
&*
I
HCN
COX
[ ] = Sudace Species
Scheme 2. Propane Amrnoxidation Pathways (Metalloradical Catalyzed) over V-Sb-AI-oxide
704 A: F. Trifiro We cannot exclude that a radical mechanism initiated from vanadium may occurs which imply the presence of the V(lll)/V(lV) redox couple as effectively found in the V-antimonate structure. On the other hand, V-antimonate, differently from other systems for ammoxidation, are active also at lower reaction temperatures (around 430-450 though with lower productivity, which suggests that a gas-phase mechanism initiated from a alkyl radical generation on the surface is not the predominant reaction pathway. We believe, on the contrar , that the generation of gas-phase alkyl radicals at temperatures of reaction of around 500 0 ; is a pathway for the formation of carbon oxides and that therefore must be reduced to increase the selectivity to acrylonitrile. About the possible role of Sb in the mechanism of reaction was my replay in the previous two questions.
8:::;
Q: E. Bordes (France) Do you observe the three oxidation states of vanadium (v(lll),v(Iv), v(v))'1 What could be their role? How vanadyl groups (if exist) would be displayed on the surface '?
A: F. Trifiro The rutile structure is a quite flexible structure which can accommodate various elements. In particular, along the c axis large channels are present which allow fast ionic diffusion and easily accommodation of foreign elements (such as also V(V)). In addition, our recent data indicate that the V-antimonate is a mixed valence system with vanadium in the (III) and (IV) valence states in the bulk and a consequent presence of also Sb(lll)/Sb(V) ions in the structure and this mixed valence situation probably reflects also in some surface modification. In the bulk of the rutile antimonate, vanadyl groups cannot be present due to the ionic radius, but on the surface the situation is different (see above) and by infrared analysis we see, in fact, the presence of V=O double bonds. About the role of the different valence states of vanadium, we believe that the catalytic behavior of V-antimonate can be associated to the presence of a larger amount of V(IV) ions stabilized in the rutile matrix, but with the presence of both the V(III>/V(IV) and V(lV)/V(V) redox couples responsible for the propane oxidative dehydrogenation and N insertion steps, respectively. However, V(V) ions are also responsible of the side reaction of ammonia oxidation to N2 and therefore their amount must be limited. Q: M. Farinha Portela (Portugal) Was it observed a change of the Sb/V ratio on the catalyst surface after reaction '1
A: F. Trifiro V-antimonate samples are selective in ropane ammoxidation to acrylonitrile when a large excess of antimony is present ( S b J r a t i o s from 2 to 6 ) and therefore a-Sbflq crystallites are present on the surface of the V-antimonate phase. After reaction, a partial change of the dimensions of these crystallites occurs. The XPS analysis, in the presence of a biphasic system with possibility of changes in the relative dimensions of the two components, do not provide enough accurate data to clarify if the Sb/V ratio remains constant or not after reaction, but indicatively suggests an increase. Q: P. L. Villa (Italy) In a work to be published on Fe-Sb oxides by Mossbauer, XRD and IR on a sample with Sb/Fe atomic ratio of 2, we found that all Sb was present as Sb(V) in the fresh catalyst and the system was found to be monophasic with a rutile structure. So rutile seems to be a structure able to accommodate unusual amounts of ions in a hi h valence state, in excess to what produced by stoichiometry (FeSb04 has Sb/Fe = 1). e also found that the used catalyst contained some Sb in a lower oxidation state. You found only one phase by XRD in your binary V-Sb-0 system and V(V) from IR characterization. I would suggest that V(V) is located inside the rutile structure and is not present as an amorphous phase. I would ascribe
a
705 the lower V=O signal seen at the IR simply to a phenomenon of reduction of vanadium in the rutile, because this structure would be able to accommodate, up to a certain extent, ions with different oxidation states. Along this line the role of A1 in the ternary system could well be to allow all the V and Sb to remain more easily in the 5t state while reaction is occurring. A: F. Trifiro The V-antimonate shows some analogies with Fe-antimonate, but a main difference about the antimony which is not present only as Sb(V) but both as Sb(1II) and Sb(V). W e agree on your indications about Fe-antimonate which are in agreement with previous data published from some of us about the same system, but the results cannot be simply applied also to the V-antimonate system which shows some different characteristics. About the possibility of V=03+ groups inside the rutile phase, for structural reasons this is not possible, but on the surface cavities created from the channels present in the futile structure can be readily accommodate these ions. In this sense we discussed on surface amorphous V-oxide, not as a real separate phase overlying the V-antimonate. Finally, we perfectly agree about the role of A1 which has a structural role of stabilization of the valence states of the Vantimonate elements (especially as regards to the decomposition of the phase when the catalytic reaction is carried out in too strong reducing conditions) and not only of a support for V-antimonate.
This Page Intentionally Left Blank
Guczi, L. et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
CATALYTIC OXIDATION OF FLUORENE TO 9-FLUORENONE DEVELOPMENT AND CHARACTERIZATION OF CATALYSTS
-
F. Majunke, H. Borchert and M. Baerns Ruhr-University Bochum, P.O.Box 102148, D-W 4630 Bochum, Germany
Abstract The title reaction was studied using unsupported V205-Fq03 catalysts of different atomic ratios of V to Fe (1 : 0.13 to 1 : 1.4); some of the catalysts were doped with Cs2SO4 (atomic ratio of V to Cs = 1 : 0.06). Reaction temperature, fluorene partial pressure and partly calcination temperature were varied within the following limits: 572 S TR/K 5 673, 0.2 IpF/kPa 5 0.7, Tcalc = 623 K and 773 K. Optimum catalyst composition for obtaining a maximum in 9-fluorenone selectivity required the presence of cesium sulfate. When adding Cs2S04 selectivity was increased from 70 to 98%. The beneficial effect of cesium sulfate doping on catalytic performance is explained by changes of the acid-base properties of the catalyst surface; this was also manifested by different strengths of adsorption of the reactants as derived from pseudo-adsorption constants of Hougen-Watson-type kinetics.
1. INTRODUCTION
‘The catalytic gas-phase oxidation of fluorene to 9-fluorenone has received only little attention in the past. Dashevskii et al. investigated the reaction on a pumice-supported V2O5Fq03-K2S04 catalyst (atom ratios of V : Fe : K = 1 : 0.3 : 0.5); no information on the reaction scheme and the effect of the composition of the catalyst on its performance were given [ l , 21. From anthracene oxidation, however, it is known that the addition of K2SO4 to a vanadium-iron-oxide catalyst leads to an increase of the yield of 9,IO-anthraquinone up to 90% while without potassium sulfate doping the yield was significant lower [3]. By ir spectroscopy it was found that alkali addition to V2O5 weakens the V-0-V-bond strength as derived from a shift of the absorption bands to lower wave numbers [4, 51. Also, the oxygen-exchange rate between the catalyst and the gas phase increased with alkali compound doping as was found by applying the l*O-tracer technique [6, 71; this increase was enhanced when an alkali metal of higher molecular mass was used. Recently the effect of different alkali metal sulfate promoters (M = Li, K, Cs) on activity and selectivity in the catalytic gas-phase oxidation of fluorene, anthracene and phenanthrene using VzOs-based catalysts was studied [8]; it was observed that alkali addition favors the selectivity of inner-ring
708
oxidation products. This latter finding was ascribed to a decrease‘of Lewis-acid sites and to an increase of basic oxygen-anion sites which favor both, selective adsorption of the weakly acidic inner-ring positions of the polycyclic aromatics (i.e., the CH2-position in the case of fluorene) and probably also insertion of nucleophilic oxygen 02-into the hydrocarbon molecule. Also the increase of the exchange rate of oxygen between the catalytic solid and the gas phase as well as the weakening of the V-0-V-bond may be a further reason for the sharp increase in selectivity; these phenomena may be assumed to contribute to a fast transformation of weakly-bound electrophilic surface oxygen into nucleophilic 02-species suitable for insertion into the molecule. Against the above background the following objectives were pursued in the present study: (1) investigating the effects of varying the proportions of V 2 0 5 and F%03 in the catalyst composition, of cesium sulfate doping and of the calcination temperature on the product distribution of the fluorene oxidation (for the reaction scheme see Figure 1); (2) relating the catalytic results to certain properties of the catalyst i.e., acid-base properties and alkali content, which had been reported earlier [8]; (3) elucidating the effect of alkali doping on the basis of adsorption data of fluorene and fluorenone as derived from Langmuir-Hinshelwood-type kinetics.
Figure 1. Reaction scheme of the catalytic oxidation of fluorene [9] (since ‘6s r; all COXformation via step j = 6 was lumped in r3)
709 2. EXPERIMENTAL
2.1 Catalyst preparation Vanadium oxide being the base compound of the catalysts was modified by ferric oxide and cesium sulfate. The catalysts were prepared by coprecipitation of the hydroxides of iron and vanadium by ammonia from an aqueous solution of NH4VO3 and FeC13. Water was then evaporated. The precursor was finally dried at 120°C (for further details see [3, 10, 111). For doping, CszS04 was added to the solution prior to coprecipitation. Catalyst compositions, catalyst pretreatment i.e., temperatures and duration of calcination as well as BET-surface areas of the catalytic solids are listed in Table 1. 2.2 Equipment Oxidation experiments were performed in an electrically heated fixed-bed quartz reactor (d = 0.8 cm, Itoh, = 30 cm, lcat = ca. 5 cm); the equipment was described in detail previously [12]. For isothermal operation the mass of catalyst of approximately 1 g was diluted by 1 to 2 g of quartz. The products were analyzed by GC and HPLC. Most of the condensable substances (fluorene, 9-fluorenone, phthalic anhydride, 2,3-indenedicarboxylic anhydride and maleic anhydride) were separated on an OV-1 capillary column for GC analysis (Sichromat 2, Siemens, FID); for double check these compounds were also analyzed by HPLC (WATERS 712, UV array detector); for details see [13]. CO and CO;! were quantitatively determined by GC analysis (Delsi 11, TCD, two columns packed with molecular sieve 5-A and porapack Q).
2.3 Catalyst characterization The specific surface area was determined by applying the l-point BET method and using low-temperature (77 K) nitrogen adsorption after the fresh catalyst samples had been calcinated in air for 12 hours at the temperatures given in Table 1. Cesium concentration on the surface as presented in the discussion, was obtained by XPS-analysis using A1 cathode (Leybold-Heraeus AG, LHS 10 spectometer). For identification of acid sites by in-situ IRspectroscopy a DRIFT-equipment was used (for details see [ S ] ) .
2.4 Reaction Conditions The conditions of catalytic testing i.e., reaction temperature and reactant concentration are given in Table 1. Different degrees of conversion at constant reaction temperature were obtained by varying the contact time from 0.07 to 1 g s ml-1 to determine the maximum selectivity for 9-fluorenone.
71 0
Table 1 Bulk composition (atomic ratio), BET-surface area after calcination of the catalytic mate. rial, calcination temperature TCalcand reaction temperature TR (initial concentration of fluorene CF = 0.18 mol / m3STp and oxygen Cb2 = 8.8 mol / m3STp) Catalyst composition V : Fe : Cs 1 : 0.13 : 0 1 1 1 1 1 1
: 0.77 : 0.06
:1 :1 :1 : 1.4 : 1.4 1 : 1.4 1 : 1.4
:o
: 0.06 : 0.06 :0 :0 : 0.06 : 0.06
6
723 623 623 623 773 623 773 623 773
572; 588; 598 606; 615; 625 623; 653; 673 623; 653; 673 623; 653; 673 623; 653; 673 623; 653; 673 653 623; 653; 673 653; 673
3. RESULTS 3.1 Catalytic performance As main products of the catalytic oxidation of fluorene, 9-fluorenone, phthalic anhydride and the carbon oxides were observed. Only traces of maleic anhydride and 2,3-indenedicarboxylic anhydride were found for all catalysts. The performance of the various catalysts is described by their activity and product distribution, especially with respect to 9-fluorenone selectivity.
Effect of catalyst composition. Activity was characterized by the initial rates of fluorene consumption ro for the various catalysts as derived from kinetics [9]. It decreased by increasing the vanadium-to-iron ratio (see Table 2; run nos. 10/16, 11/17 and 12/18); when the V205-Fq03 catalysts were doped with cesium sulfate activity markedly increased as illustrated by comparing run nos. 7/10, 811 1 and 9/12. Selectivity to 9-fluorenone and phthalic anhydride stayed nearly constant up to fluorene conversions of 95% for all V2O5-FqO3 catalysts; when increasing conversion beyond this value 9-fluorenone selcctivity decreased in favor of phthalic anhydride and carbon oxides due to consecutive reaction. When cesium was added, selectivities increased and were nearly constant over the whole range of conversion. Maximum 9-fluorenone selectivities obtained with the various catalysts at different temperatures are presented in the bar chart shown in Figure 2. Adding F q 0 3 to pure V205 resulted in an increase of 9-fluorenone selectivity when a certain proportion of Fe203 was exceeded. A vanadium-to-iron ratio of 1 : 0.13 did not show any significant change within experimental accuracy while selectivity increased by about 10 % at ratios of 1 : 1 and 1 : 1.4 respectively.
71 1
Table 2 Effect of catalyst-precursor calcination temperature Tcalc on specific surface area SBET (fresh catalyst) and catalytic performance (initial reaction rate ro) for different catalysts (C; = 0.18 m 0 l / r n 3 ~ C& ~ ~ ; = 8.8 1 n o l / m 3 ~ ~ ~ ) run no.
Catalyst composition V : Fe : Cs atomic ratio
TCdC
SBET
K
m2
1 2 3 4 5 6
1 : 0.13 : 0
723
7 8 9
1 :1
:o
10 11 12 13 14 15
1 :1
: 0.06
16 17 18
1 : 1.4 : 0.06
1)
TR
ro 1)
K
pmol m-2 s-1
6
572 588 598 606 615 625
0.021 0.037 0.061 0.085 0.121 0.179
623
4
623 653 673
0.053 0.118 0.193
623
2
773
1
623 653 673 623 653 673
0.364 0.450 0.514 0.239 0.3 19 0.382
623
1
623 653 673
0.180 0.237 0.288
g-1
ro was derived from kinetics [9] and is refered to SBET
The addition of cesium sulfate markedly increased the 9-fluorenone selectivity for V ~ 0 5 Fez03 catalysts of different vanadium-to-iron ratios applied (1 : 0.77; 1 : 1; 1 : 1.4). The absolute value of the 9-fluorenone selectivity was, however, affected by the vanadium-toiron ratio in a similar manner as outlined above; the 9-fluorenone selectivity amounted to ca. 95% for V : Fe : Cs = 1 : 0.77 : 0.06, to 97% for V : Fe : C S = 1 : 1 : 0.06 and to 99% for V : Fe : Cs = 1 : 1.4 : 0.06.
Effect of calcination temperature. By increasing the calcination temperature from 623 K to 773 K there was an decrease of the activity i.e., ro for the catalyst with V : Fe : Cs = 1 : 1 : 0.06 given in Table 2 (compare run nos. 10/13, 11/14 and 12/15) decreased. For the Vz05-F%03-catalyst having a ratio of vanadium to iron of 1 : 1.4 as well as for the cesium-doped catalyst with V : Fe : Cs = 1 : 1 : 0.06 a slight increase of 9-fluorenone
71 2
selectivity was observed from 68% to 72% and 96% to 98% (TR = 653 K) respectively when the calcination temperature was raised. This relationship could not be validated for the catalysts with V : Fe : Cs = 1 : 1.4 : 0.06 since any changes in selectivity above 99% were beyond experimental accuracy.
'""PI
3 653 673
623 663 613
80
I
572 58E 590 606 615 625
I013 0
100
1,077 006
1 1 0
1:l:O.Ob
V 123
723
623
623
1:l.C:O
006
11006 1140 1 1 4 o w
Fe : Cs 623 aic
623
623
773
713 173
fK
Figure 2. Maximum 9-fluorenone selectivity in air oxidation of fluorene as function of catalyst composition (Vo: Fe : Cs atomic ratio) and its calcination temperature at various reaction temperature (CF = 0.4 Vol. %)
3.2 Adsorption of reactants For characterizing the adsorption properties of the solids the catalytic surface pseudo-adsorption constants and heats of adsorptions were derived from kinetic data on the fluorene oxidation which have been evaluated according to Hougen-Watson-type rate equation (eq. 1) based on reaction scheme presented in Figure 1 (for details see [9]). kj K, C, Co2
r. = J
(1
+ C Ki Ci) (1 + K02 c02)
kj : reaction rate constant for reaction step j (= 1-5)
K, : pseudo-adsorption constant for the organic compound i
Various V205-Fq03 catalysts partly doped with cesium sulfate were included in the kinetic evaluation (see Table 2). For undoped V205-Fe203 catalysts the pseudo-adsorption constants for fluorene and 9-fluorenone at a reference temperature of 623 K as well as the respective heats of adsorption are given in Table 3; adsorption of phthalic anhydride was
713
negligible. From these data it may derived that adsorption capacity for both, fluorene and 9fluorenone increases with increasing iron content but that the strength of fluorene adsorption is markedly reduced while the strength of 9-fluorenone adsorption is weak for both catalysts. Equation (1) was, however, only applicable as long as there was no cesium sulfate present. If cesium sulfate was added the adsorption terms (K, C,) in the denominator of eq. (1) were significantly smaller than 1 and could be therefore neglected. Under these circumstances and when oxygen partial pressure was assumed constant the rate equation (1) could be simplified to
From these results it may be derived that cesium sulfate doping reduces the propensity of the catalytic surface for keeping the reactants and the products adsorbed; whereby their chance to be totally oxidized is markedly decreased which goes along with an increase in selectivity. Table 3 Adsorption constant Ki for a reference temperature of 623 K and heat of adsorptionAHi for V205-Fq03-catalysts (F: fluorene, NON: 9-fluorenone; data from [9]) Catalyst composition V : Fe atomic ratio
1 : 0.13 1 :1
KF
KNON
m3 mol-1
m3
1398 & 5 3149 _+14
-111.8 f0.3 -29.8 f2.6
mol-1
342 & 14 884 f 41
AHNON
kT niol-1 -24.3 f0.4 -25.6 f2.5
4. DISCUSSION
Activity and selectivity for the various catalyst compositions are discussed in terms of the V-to-Fe ratio and of cesium sulfate doping. When increasing the iron content activity related to specific surface area is decreased for undoped as well as for the cesium sulfate doped V205-Fq03 catalysts. Selectivty to 9-fluorenone, however, is slightly increased which may be explained on the basis of surface acidity and basicity. Surface acidity of a pumice-supported V205-Fq03-catalyst is decreased by an increase of the iron content as was reported earlier [14]. A similar relationship between surface acidity and selectivity derived from in-situ IR-spectroscopy was found for the oxidation of anthracene to 9,lO-anthraquinone on supported V-Mo-P-oxide catalysts [8]. These results led to the conclusion that acidic centers favor the formation of strongly adsorbed intermediates and subsequently outer-ring oxidation products are formed which are finally converted to phthalic anhydride and/or to carbon oxides. This reasoning is supported by the conclusion derived from the adsorption of reactants as observed by the kinetics of
714
fluorene oxidation (see above). Thus, it appears that the increase in 9-fluorenone selectivity has to be related to a change of the acid-base surface properties. The increase in surface basicity andlor the decrease in acidity certainly favors the adsorption of the weak acidic CH2position of the inner ring leading to the selective formation of 9-fluorenone. In addition, the adsorption of the electron-rich aromatic rings on the acidic sites assisting total oxidation is suppresed; hereby the non-selective oxidation of fluorene and 9-fluorenone is reduced. Apart from the effect of the V-to-Fe ratio, selectivity is also markedly enhanced by adding cesium sulfate to the V205-Fq03 catalysts. The reasoning for this relationship goes along the same lines as indicated for the positive effect of Fq03. The'cesium sulfate addition leads also to a significant decrease in the adsorption of fluorene and 9-fluorenone as revealed by the kinetic evaluation (see above). This is partly supported by the observation that cesium is enriched on the surface compared to the bulk as observed by XPS [8]. Thus, the promoting effect of the basic alkali compound may be traced back to a decrease of the acidic sites. In addition, a weakening of the V-O-V-bond strength and an increase of the oxygen-exchange rate by alkali doping may also contribute to an increase in selectivity as outlined in the introductory paragraph. Since also activity was increased by cesium doping it may be assumed that the faster transformation of electrophilic surface oxygen into lattice oxygen not only favors the formation of selective nucleophilic oxygen species but also increases the total amount of active sites. Acknowledgement This work was supported by Deutsche Forschungsgemeinschaft. Finally, thanks are due to Q. Wei who performed some of the catalytic experiments. Literature 1 2 3 4 5
6 7 8 9 10
M.M. Dashevskii and G.M. Petrenko, Zhur. Prikland. Khim., 35 (1962) 693. G.M. Oetrenko and G.N. Terent'eva, Zhur. Prikland. Khim., 38 (1965) 1109. FIAT Final Rep. No. 1313 (Vol 1) 332. D.V. Fikis, K.W. Heckley, W.J. Murphy and R.A. Ross, Can. J. Chem., 56 (1978) 3078. T. Tamaka, R. Tsuchitami, M. Ooe, T. Funabiki and S. Yoshida, J. Phys. Chem., 90 (1986) 4905. K. Hirota and Y. Kera, J. Phys. Chem., 72 (1968) 3133. K. Hirota and Y. Kera, J. Phys. Chem., 73 (1969) 3973. M. Baerns, H. Borchert, R. Kalthoff, P. Kdner, F. Majunke, S. Trautmann and A. Zein, in: Stud. Surf. Sci. Cat., to be published M. Baerns and H. Borchert, to be published in 1992 N.T. Do, R. Kalthoff, J. Laacks, S. Trautmann and M. Baerns in: "New Develop ments in Selective Oxidation", Stud. Surf. Sci. Cat. (G. Centi and F. Trifiro, eds.), Elsevier Science Publisher B.V.. 55 (1990) 247 M. Baerns, R. Kalthoff, P. Kaner and A.'Zein, Erdol-Erdgas-Kohle, 106 (1990) 166. M. Baerns, R. Kalthoff, P. Kaner and A. Zein; Dechema Monographie Katalyse (ed.: H. Kral and D. Behrens) Verlag Chemie, Weinheim, 118 (1989) 231. A. Zein and M. Baerns, J. Chromat. Sci., 27 (1989) 249. M. Ai, J. Catal., 52 (1978) 16. I
11 12 13 14
.
71 5
DISCUSSION
Q: R. K. Grasselli (USA) First let me congratulate you on this excellent piece of work. 1) My question is if the enormous beneficial effect of Cs addition to your catalyst is dependent or independent of the preferred over &NO3, for example ? Is there any evidence 2) Since there is no preference o think, then let me comment that in multicomponent molybdenas for the selective oxidation of isobutene to methacrolein, or ammoxidation to methacrylnitrate there too is observed an enormous selectivity advantage to the useful products by Cs addition, it is independent of the Cs compound ( G $ O or CsN03) used. On the contrary, in the old naphthalene to phthalic anhydride catalyst, K & 0 addition appears critical for enhanced useful yields and K & 0 7 is observed in the catalyst. wonder, based on your data and data I quoted above, if the V-oxide based naphthalene to phthalic anhydride catalyst, as well as the V-oxide based catalyst for the all combustion of o-xylene to phthalic anhydride would be improved by using the addition of Cs$04 instead of K2S04 ’?
4
A: M. Baerns 1) We have not applied any other cesium-compounds than Cs$O, as promoter. It was our intention to compare the effect of different alkali sulfates on the product distribution. Alkali sulfates were described in literature as useful additives for hydrocarbon oxidation reactions as you yourself frequently mentioned. With respect to the presence of Cs f17we cannot give an answer. The low content of Cs did not allow any identification by X D. 2) The different strengths of the alkali effect when using different alkali sulfates (Lipso,, K2SO4, Cs2S04) has been studied in the oxidation of phenanthrene [l] One may assume that the strength of adsorption of the aromatic ring system on acidic sites is reduced more effectively when a more basic alkali cation is interacting with the catalytic surface. Thus, if one postulates the same kind of mechanism for na hthalene oxidation an improvement in PA selectivity ought to be expected when using 0$04 instead of K$O@ M. Baerns, H. Borchert, R. Kalthoff, K. Kassner, F. Majunke, S. Trautmann and [l] A. &in, Stud Surf. Sci. Cat., 12, 5 1 (1992)
P
Q: J. Haber (Poland) Very often the increase of acidity of the catalyst decreases the selectivity in oxidation of aromatic compounds because the organic molecule becomes more strongly bonded to the surface, its residence time on the surface increases and so does the probability of the attack by electrophilic oxygen. This effect should however show up in the values of the heat of adsorption. Did you observe the change of the heat of adsorption on adding cesium and was this change in line with the variation of selectivity ?
A: M. Baerns As we have shown by our kinetic study, the addition of &2SO leads to an elimination of the inhibiting term of rate equation which is due to the decrease of the adsorption constant (the term ci x Ki becomes negligible compared to 1 resulting in a pseudo-first order rate equation in the hydrocarbon (see equation 2). That is to say no heat of adsorption could be derived. Your assumption on the change of the heat of adsorption appears, however, valid, in another study [l] (see in Table 1) we have, in fact, shown that the initial heat of adsorption is decreased (61 kJ mol-1 for V : Fe = 1:1.4 and 13 kJ mol-1 for V : Fe : Cs = 1 : 1,4 : 0.06, respectively). Q: H.-g. Huang (Taiwan) It is difficult to imagine that the CHp-group in a hydrocarbon molecule is acidic and needs a basic site to activate it. On the other hand, for most olefinic and aromatic compound
71 6 oxidation, as alkali is added, the activity decreases rapidly. How would you compromise your results with the about facts ?
A: M.Baerns It is well known that CH2-groups differ with respedt to their acidity in aliphatic hydrocarbons. Also in fluorene a weak acidic function may be assumed since a proton can be easilv abstracted by a basic site due to the stabilization of the remaining negative charge by mesdmeric effects.In the present study also a decrease of activity could be observed if the reaction rate was related to the catalyst mass. If it is, however, referred to the specific surface area, the reaction rate is enhanced by adding the alkali compound. This is probably due to the weakening of the V-0-V bond. Q: S. L. Kiperman (Russia) The equation (1)
kj x Ki x ci x C
O ~
'j =
(1 t ZKi x ci)( 1 t
x~02)
given in this paper, is obviously based on the assumption that there are two kinds of active sites on the catalyst surface. Which evidence does exist for this assumption ? Therc are two possible cases: a) this kinetic equation can describe the results obtained; b) this equation has been selected as the best by discrimination among others. Both-of these cases are not a prove for the hypothesis of two active sites. Indeed, if the terms in brackets of the denominator are multiplied by each other, we obtain: ki x Ki x cj x ~ 0 2 rj =
1 t ZKI x C, t K&02
t Ko2 x
ZKi x ci x C
O ~
This equation does not rove the hypolhesis, rather it calls about the concentration of intermediate on the sur ace, this concentration is cj x c o p
P
A: M.Baerns We have performed two series of experiments; one was designed to study reactant and roduct inhibition and the other one to elucidate the effect of oxygen partial pressure. In the ormer case oxygen was in excess and could be considered to be constant while in the latter case oxygen was widely varied. From the experimental evidence, its kinetic evaluation and model discrimination our rate equation (1) was indeed the best. We agree completely with your statement that we cannot prove any mechanistic assumptions by this "kinetic" method; we do believe, however, that a dual-site mechanism in the rate-limiting step of the hydrocarbon oxidation is rather likely.
F
Q: M. Sinev (Russia) Do you have an experimental evidence for decreasing of the V-0-V binding energy after the addition of Cs,SO, '? What is the mechanism of this influence of additive on binding energy ?
A: M.Baerns It is known from literature that alkali addition to V20, leads to a shift of the V=O- and V-0-V-stretchin bonds to lower wavenumbers, which could be ascribed to a decrcasc in binding energy [ 1.
f
71 7 The mechanism of this influence may be explained by an extended distance between the V-atoms because the alkali compound is probably incorporated into the catalyst structure. Furthermore, it is known from literature, that alkali compounds act as electron donors for the neighbored V-atoms; as a consequence the partial positive electronic charge of V is reduced resulting in a less polarized and so less strong V-oxygen bond [3 D. V. Fikis, W. J. Morphy,. R.A. Ross, Can. J. Chem., 6, 2530 (1978) D. B. Dadyburjar, S. S.Jener, E. Ruckenstein, Cataf.Rev. Sci. Eng., 19, 293 (1979)
5.
I:]
Q: J. C. Volta (France) In your introduction, you did connect the study of the variation of the acid base properties of the catalyst with that of the doping by Cs. Why 1 Can you comment on the location and the role of Cs in the catalyst '? A: M. Baerns Adding a basic compound like Cs+ leads to a decrease of the surface acidity of the vanadia-iron catalyst used in this study. This was also shown earlier by ammonia adsorption experiments [ 11 The &(I) compounds plays a binfunctional role: it can be described as a compound which &creases the acidity of the surface and which also decreases the vanadium-oxygenbinding ener (cp. also our answer to M. Sinev). From X experiments it is known (see the above cited reference, Table 3) that alkali is enriched on the surface; only small amounts are obviously distributed in the bulk of the catal st. knic radius of the cations contained in the catalysts: &(I): 0.167 nm; Fe(1I): 0.074 nm; Fe(II1): 0.064 nm; V(IV): 0.063 nm; V(V): 0.059 nm.
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Guczi, L. et 01. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary (B 1993 Elsevier Science Publishers B.V.All rights reserved
AMMOXIMATION OF CYCLOHEXANONEON TITANIUM SILICALITE: INVESTIGATION OF THE REACTION MECHANISM A. Zecchinaa, G.Spotoa, S. Bordigaa, F. Geobaldoa, G.Petrinib, G.Leofantib, M. Padovanb, M. Mantegauab and P. Rojfiab
aDipartirnento di Chirnica Inorganica, Chirnica Fisica e Chirnica dei Materiali, dell’Universita di Torino, V.P. Giuria 7, 10125 Turin, Italy bENICHEM ANIC, Centro Ricerche di Bollate, V.S. Pietro 50, 20021 Bollate (MI), Italy
ABBTRACT
In a series of combined adsorption, reactivity and spectroscopic experiments in solution with ketones of different size and shape, the fundamental role of the hydroxylamine pathway in the cyclohexanone ammoximation is established. In presence of H202 and NH3, framework Ti(1V) forms octahedrally distorted aquohydroperoxo and ammonia-hydroperoxo mixed complexes: the latter one being the precursor of the hydroxylamine formation. This intermediate immediately reacts with cyclohexanone in the channels or diffuses to the external solution where it gives oximation reaction with larger ketones which could not penetrate the channels. 1. INTRODUCTION
Cyclohexanonoxime is the key intermediate in the manufacture of caprolactame, a raw material for the production of synthetic fibers. The main disadvantages of the current technology are: the numerous step involved, the use of hazardous and environmentally undesirable chemicals and the heavy coproduction of ammonium sulfate, a low valuable salt. Enichem Anic is developing a new simplified process for the synthesis of cyclohexanonoxime by ammoximation of cyclohexanone with ammonia and hydrogen peroxide on titanium silicalite catalyst TiS [l-31 (a very selective oxidation catalyst with H202 [4,5]). Two reaction pathways can be hypothesized i.e.: i) the hvdroxvlamine path, whereby through the action of Ti centres, NH3 and H202 give initially NH20H following the scheme: NH,
Ti(lV) H2°2
NH20H
which then reacts with cyclohexanone to give cyclohexanonoxime:
ii) the m e D a m , whereby in presence of gives primarily the imine:
o= 0
NH,-
NH3, cyclohexanone
0 NH
+
H20
c)
which then reacts with H202 through the agency of Ti centres to
The mechanism via imine intermediate has been proposed in recent contributions [6,7,8] on the basis of spectroscopic experiments. In this paper we report new results on ammoximation which are in favour of a reaction mechanism via hydroxylamine intermediate. 2. EXPERIMENTAL
Samples of TiS were synthesized according to patent literature [ 9 ] and characterized by the procedure described in ref. [lo]. The adsorption experiments of ketones in liquid phase were carried out at 70° C for 6 hours, by adding 10 ml of the ketone solution (about 1% wt) in t-butanollwater to 1 g of adsorbent. The amount of ketone adsorbed wa6 obtained from the final concentration in the liquid phase, as determined by gaschromatography. The ammoximation runs were performed under standard conditions as described in detail in ref. [1,2,3]. The oximation runs were performed at 70° C for 40 min. by adding an aqueous solution of hydroxylamine base (50% wt) to a solution of ketone in the same solvent used for the ammoximation reaction. The ammonia oxidation test was carried out as reported in ref [ll]. At the end the catalyst was filtered off and hydroxylamine was determined by polarography or by quantitative oximation of cyclohexanone. The IR and W - V i s spectra have been performed with Bruker IFS 4 8 and a Varian Cary 5 spectrometer respectively.
-
3. REBULTB AND DIBCUBBION
The results of cyclohexanone ammoximation tests with TiS and
721
other Ti containing catalysts are compared in Table 1, and clearly show that TiS is the only catalyst able to give oxime with yields higher than 90% based on hvD (at least in those specific experimental conditi6ns).
Table 1. Ammoximation in water/t-butanol by different lysts: catalyst concentration 2 . 0 % (wt.); temperature NH3/H202 2.4; reaction time 1.5 hours. Catrlyat
TI
s None 910, smorphour 0 Slllcrllt. 0
cata80°C;
H ,O,/Cy-Hoxrnonr Cyolohrxrnonr Oxlmr ylrld molar ratlo C o w . Oalrno ~ m l o o l . baaad on H,Ot s s
0.3
1.07
63.7
0.6
1.03
66.7
1.3
0.7
1.09
69.4
0.6
0.3
H-ZSU 5
0
1.08
63.9
0.9
0.4
TI&/Sl& TIo#/SIO,
1.5
1.04
49.3
9.3
4.4
9.8
1.08
88.8
86.9
64.0
TI-Slllcsl.
1.5
1.05
99.9
98.2
93.2
According to [2,3] we so conclude that this is the most significant parameter to discriminate properly and unproperly prepared TiS. There is also large evidence that TiS samples giving low ammoximation yields, based on hydrogen peroxide, contain little framework Ti [12]: this allows to hypothesize that the reactive center is a framework isolated Ti. Catalyst characterization: seneral consideratiom Ti-Silicalite is usually characterized by: I) X-Ray diffraction (expansion of lattice parameters Ti content) [ 9 ] ; 11) infrared spectroscopy: characteristic band at 960 cm”, attributed to a stretching mode of a tetrahedral [SiO,] unit perturbed by the presence of an adjacent framework titanium or to a stretching mode of a [Ti04] unit itself [10,14,15]; 111) reflectance spectroscopy in the W-Vis: characteristic band at 4 8 0 0 0 cm-’ attributed to a C-T transition of tetrahedral (framework) Ti; absence of any relevant absorption in the 40000-30000 cm-l range (extralattice Ti, in form of TiOZ microparticles) [10,13]; IV) absence of a Raman peak at 140 cm-’, due to extraframework Ti (anatase microparticles) [lo]. None of these techniques gives fully quantitative results. However, when all the previous conditions are fulfilled as in the present case (TS1 in ref [lo]), it can be stated with confidence that nearly all Ti is in framework.
722
Ketone reactivity in ammoximation In order to solve the mechanism dilemma, we have programmed a series of reactivity and adsorption tests in solution, involving ketones with different reactivity, molecular size and shape. Ammoximation is a general reaction for carbonylic compounds like ketones and aldehydes [l] (Table 2). Table 2 . Ammoximation of different ketones in t-butanoljwater: TiS concentration 2 % (wt.); temperature 8 0 O C; NH3/H202 molar ratio 2.4; reaction time 5 hours. etone
H *O, /Ketone molar ratlo 1.08
yclo p e n t anone
1.07
yclododecanone
1.02 1.02
Ketone Conv. s 92.4 99.9 93.6 63.0 60.8 3.8
Select.
s 99.9 90.4 91.0 99.9 98.1 14.4
H O Yl:d 8
93.8 84.8 82.8 62.0 48.9 0.5
From these results we conclude that there is a large scale of reactivity ranging from cyclohexanone (the most reactive) to 2-tert-butyl-cyclohexanone (almost non reactive). These results are consistent with both hydroxylamine and imine intermediates hypothesis as they can be explained: i) according to first hypothesis in terms of different reactivity of ketones with hydroxylamine; ii) according to the latter hypothesis in terms of different diffusion properties of ketone or the corresponding imines into the catalyst. Adsorption of ketones on Ti-Silicalite To further elucidate the mechanism, we have compared in detail the adsorptive behavior of cyclohexanone and other three ketones: 4-tert-butylcyclohexanone, 2-tert-butylcyclohexanone and cyclododecanone. The adsorption data of these ketones on TiS from an aqueous-alcoholic solution (the same used in the ammoximation test) are shown in Table 3. It can be seen that, near the reaction temperature, cyclohexanone is adsorbed on the catalyst, while the others are not. Because of their shape and size, the three ketones are unable to diffuse to the active sites located in the channels. These results immediately discard the hypothesis of an imine intermediate, because 4-tertbutylcyclohexanone and cyclododecanone, although not present in the channels, give the ammoximation reaction, the first with good yield. In other words, the imine observed by IR appears to
723 be more a systematic spectator (because of the presence of NH3), than an active participant in the ammoximation reaction. T a b l e 3 Adsorption data of ketones in t-butanol/water
Ketone
equilibrium concentration solution (mol/l)
C yc lo hexa no ne 4 t b u c yc Io h exa n o n e 2- t bu-cyclohexanone cyclodo decanone
- -
1
TiS (mmol/lOOg)
0.12 0.07 0.08 0.08
13 0 0 0
Ketone reactivitv in oximation with hvdroxvlamine Since hydroxylamine is the reaction intermediate, the different ketone reactivity in ammoximation reported in Table 2 must be explained by a different reactivity of ketones towards hydroxylamine to give oxime. So we made oximation experiments in the same experimental conditions of the ammoximation test but in absence of the catalyst. In Fig. 1 the results are compared with 100 cyclohexanone c those obtained in ammoximation: there is a strong 4-t-but ylcyclohexan~ evidence of a correlation between the ketone reac/ tivity in both reactions. Hence the oximation is the rate determining step of 0 / the overall reaction, at w2. 40least for the ketones but ,/' P Icyclohexanone.
/'
-1
s. X
P
20-
/ i;
2-1-butylcyclohexanone 0
'
0
10
I
"
'
'
'
30 40 50 60 7 0 80 90 OXMATION (KETONE CONVERSION) 20
100
Fig.1. Comparison of ketones reactivity in ammoximation and oximation reactions.
724
Ammonia oxidation catalvzed bv Ti-Silicalite To complete the investigation we studied the oxidation of ammonia with hydrogen peroxide on TiS. We found that TiS is a very selective catalyst for the oxidation of ammonia to hydroxylamine [ll]. From table 4 it can be seen that in the same conditions of ammoximation reaction (but in the absence of cyclohexanone), hydroxylamine can be synthesized with yields, based on hydrogen peroxide, higher than 60%. These results definitely substantiate an ammoximation mechanism via hydroxylamine intermediate. Table
4.
Oxidation of ammonia with H202 on TiS (temp. 70°
Reaction time (h)
Cat.
conc
( w t %)
0.8
NH,/HzOz molar ratio
C)
HZ 0, yield ( % moles)
0.8
1.7
30 11
0.5 48.0
0.5
1.7
31
63.2
The w - v i s DRS of the active Ti centres in Dresence of reactants: rea'ction mechanism. As the activity and the adsorption tests could not elucidate the intimate reaction mechanism and the structure of the active centres, we have programmed a series of spectroscopic experiments. W-Vis diffuse reflectance spectroscopy appears to be the most promising and selective tool for TiS characterization during reaction conditions. In fact, being the siliceous framework optically inactive in the 50000-15000 cm" range, any optical activity comes from Ti centres and so gives selective information about the structural and chemical situation of the Ti sites. Another merit of W - V i s spectroscopy is that it can operate also with the sample fully immersed in the liquid reaction medium, containing H20, NH-,, H202 and cyclohexanone. In Fig. 2 (spectrum 1) the characteristic peak at % 48000 cm-' typical of a well synthesized Ti-Silicalite, outgassed at RT, associated to tetrahedral Ti(1V) [14] is shown. The observed frequency is in agreement with that calculated by means of the Jorgensen equation [ 141. The shoulder at 34000 cm" corresponds to a very small amount of extraframework Ti. As it has been shown in previous papers [10,14,15] the +Si-o-Tic bonds connecting the [Ti04] unit to the siliceous framework and emerging in the channel, can be opened by the action of water with formation of +TiOH (and more unfrequently +Ti(OH)2)
725
structures.Immereion of TiS in water gives spectrum 2. The abrupt frequency shift of the maximum to lower wavenumbers indicates that a ligand (H20) addition reaction has taken place, which can be represented as mainly due to:
0'
0' H20
whereby tetrahedrally coordinated Ti(1V) complexes is transformed into distorted octahedrally coordinated aquo-hydroxo complexes. The observed frequency can be derived from the Jbrgensen equation [16]. When an ammonia solution is used instead of pure water (spectrum 3), H20 ligands are substituted by NHJ and an extra peak at = 37000 cm" due to a C-T transition from coordinated NH3 to Tioct(IV) in complexes
shows up. The observed frequency can be calculated as above [16]. Finally when H202 solution is dosed, spectrum 4 is obtained: in agreement with ref. 1161, the strong band at 25000 cm'l is assigned to a C-T transition from a hydroperoxo group to a octahedrally coordinated Ti(1V) in a complex formed following the reaction:
0' Hzo
0,H20
When NH3 is dosed on the preformed hydroperoxo species (but the same occurs when H202 is dosed on preadsorbed NH3), the band initially at 25000 cm" immediately undergoes an upward (spectrum 5) and then very slowly declines shift to 27500 cm" (spectrum 6). If the experiment is done at 6 0 ° C the second step becomes very fast and the hydroperoxo species is readily consumed. The upward shift can be explained on the basis of the formation of the mixed complex:
726 0 NH, O-)(-OOH
0 H,O
because the insertion of the basic NH3 ligand in the Ti(1V) sphere, depresses its propensity to accept one electron from the OOH ligand during a charge transfer transition, with subsequent increase of the C-T frequency. It is evident that the mixed complex can be the precursor of hydroxylamine formation in reaction a) (which is very slow at RT but becomes fast at 6 0 ° C).
t
Fig. 2. DRS of TiS in solution. 2a: spectrum 1: TiS in vacuo (reference spectrum); 2: in H20; 3 : in NH3 solution (NH3/Ti = 5:l); 4: in H202 solution (H202/Ti = 5:l); 2b: spectrum 4: in H202 solution (different sample; H202/Ti = 1:l); 5: the same after NH3 addition (NH3/H202 = 1:l); 6: after 3 0 min. Direct dosage of a NH20H solution on Ti-Silicalite gives a very similar spectrum. These results, together with the chemical proofs, support the hydroxylamine path. Dosage of cyclohexanone at this stage does not give the usual band at 34000 cm" of the C=O chromophore (observed when cyclohexanone is dosed first). A s oxime does not have any relevant absorption in the 45000-30000 cm-' range, this result is in favour of the reaction b)
.
727
T A similar set of experiments has been performed in an IR cell. In this case the dosage of the reactants was made from both the gas phase and from solution. In order to simulate as much as possible the working conditions, spectra were recorded in presence of physically adsorbed water. This had however severe spectroscopic consequences, because the strong H20 absorptions (especially in the 3800-3000 c m ' l range and at zs 1600 cm") tend to obscure the infrared manifestation of the other reactants (adsorbed NH3, H202, etc.). Despite these difficulties, we have found that: 1) dosage of NH3 on preadsorbed cyclohexanone gives the imine through the reaction c) as demonstrated by the decrement of the carbonyl absorption at 16801720 c m ' l and formation of a weak imine band at 1655 cm"; 2) dosage of NH3 at 60° C on preadsorbed H202 gives a very weak band at 1590 cm" indicative of NH20H formation (the same band is formed when NH20H is directly dosed on TiS); 3) when cyclohexanone is dosed on TiS previously contacted with NH3 and H202 at 60° C, we can observe the stretching and bending mode of -CH2- groups of the ring, with their full intensity, while the carbonyl band is weaker (which is consistent with both of the imine and oxime formation); 4) dosage of cyclohexanone on TiS containing NH20H leads to the immediate formation of oxime (essentially the same CH2 bands of cyclohexanone but no evidence of carbonyl band). The results shown in poits 1) and 4) indicate that IR spectroscopy alone is not capable to unambiguously discriminate between the two mechanisms (even if evidence of the hydroxylamine formation is obtained). This explains why a few authors, [6,7,8] on the basis of spectroscopic measurements, have suggested the imine pathway.
CONCLUBION Adsorption and reactivity experiments in solution with ketones of different shape and size, demonstrate that the ammoximation reaction procedes hydroxylamine intermediate. The role of the catalyst centres is to form mixed Ti(1V) ammoniahydroperoxide octahedral complexes which evolve to hydroxylamine. Hydroxylamine can then either immediately react with small ketones diffused in the channels (cyclohexanone) or migrate to the external surface and than react with larger carbonylic compounds which could not penetrate the channels. 4.
a
728 REFERENCES
P. Roffia, M. Padovan, G. Leofanti, M.A. Mantegazza, G. De Alberti, G.R. Tanszik, U.S. Pat. 4 , 7 9 4 , 1 9 8 ( 1 9 8 8 ) . P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti, P. Gervasutti, Stud. Surf. Sci. Catal. , 5 5 ( 1 9 9 0 ) 4 3 - 5 1 . P. Roffia, G. Leofanti, A. Cesana, M. Mantegazza, M. Padovan, G. Petrini, S. Tonti, V. Gervasutti, R. Varagnolo, Chim. Ind., (Milan) 7 2 ( 1 9 9 0 ) 5 9 8 - 6 0 3 . V. Romano, A. Esposito, F. Maspero, C. Neri, M.G. Clerici, Chim. Ind., (Milan) 7 2 ( 1 9 9 0 ) 6 1 0 - 6 1 6 . M.G. Clerici, G. Bellussi, V. Romano, -J. Catal., 1 2 9 (1 9 9 1 ) 159-167.
A.
Thangaraj, S. Silvanasanker, P. Ratnasamy, J.
Catal.,
1 3 1 (1 9 9 1 ) 394-400.
J. Sudhakar Reddy, S. Silvanasanker, P. Ratnasamy, J. Mol. Catal., 6 9 ( 1 9 9 1 ) 3 8 3 - 3 9 2 . Z. Tvaruzkova, K. Haberseberger, N. Zilkova, P. Jiru, Appl. Catal. A: General, 7 9 ( 1 9 9 1 ) 1 0 5 - 1 1 4 . M. Taramasso, G. Perego, B. Notari, U.S. Pat. 4 . 4 1 0 . 5 0 1 (1983). 10)
A. Zecchina, G. Spoto, S. Bordiga, A. Ferrero, G. Petrini, G. Leofanti, M. Padovan, Stud. Surf. Sci. Catal., 6 9
11)
M. A. Mantegazza, M. Padovan, G. Petrini, P. Roffia, It. Pat. Appl. MI 91A001915 ( 1 9 9 1 ) . G. Petrini, A. Cesana, G. De Alberti, F. Genoni, G. Leofanti, M. Padovan, G. Paparatto, P. Roffia, Stud. Surf. Sci. Catal., 6 8 ( 1 9 9 0 ) 7 6 1 - 7 6 6 . G. Bellussi, V. Fattore, Stud. Surf. Sci. Catal., 6 9
(1 9 9 1 ) 251-258. 12)
13)
(1 9 9 1 ) 79-92. 14)
15)
16)
M. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti, G. Petrinil in "Structure and Reactivity of Surfaces" C. Morterra et al. Eds. Elsevier, Amsterdam, ( 1 9 8 9 ) 1 3 3 . A. Zecchina, G. Spoto, S. Bordiga, M. Padovan, G. Leofanti, G. Petrini, in It Catalysis and Adsorption by Zeolitesf1 Ohlman et al. Eds. , Elsevier, Amsterdam, ( 1 9 9 1 ) 6 7 1 - 6 8 0 . C.K. Jtirgensen, Prog. Inorg. Chem. , S. J. Lippard Ed. Intersci. Pub., J. Wiley N.Y., 1 2 , ( 1 9 7 0 ) 101.
729 DISCUSSION
Q: S.Kalia uine (Canada)
8
By XP we have observed that upon adsorption of H 2 0 2 the coordination of Ti changes from tetrahedral to octahedral configuration, is in agreement with your UV-visible result. However, we d o not obtain octahedral Ti upon adsorption of water. From your UVvisible spectra would you exclude the possibility of a pentacoordinated Ti formed by the interaction with water. Such a pentacoordinated site has been proposed by the group of Professor Schulz-Ekloff to explain this XANES data and by our own groups in the interpretation of our XANES and EXAFS results.
A A. Zecchina Our UV-Vis spectra has been taken with the sample in full immersion in water (in order to approach as much as possible the reaction conditions). The XANES results you are mentioning are obtained on samples containing some adsorbed water and not fully immersed. The two different conclusions concerning the coordination state can so find a simple explanation on this basis. Q: H. L. Krauss (Germany) If the reaction
0
=O
+
NH2OH
-0
is the rate determining step, then the reaction
= NOH + H 2 0
NH,
t H202
+
should give (in presence of
TiS) NH20H more than used up in the first reaction. Can you use this procedure (without the cyclohexanone) for a synthesis of hydroxylamine ? It could be better in the sense of avoiding poisons in the waste (environmental danger) compared with old procedures.
A: A. Zecchina We can assert that the reaction between ketone and hydroxylamine to give oxime is the rate determining step in ammoximation only in the case of ketones which give ammoximation yields lower than cyclohexanone: in fact, as the first step is common, this difference is due to different reaction rates in the second step. About the catalytic oxidation of ammonia to hydroxylamine we agree with you: it is an interesting reaction from the synthetic point of view and we have patented it (ref. 11).
Q: S. Sivasanker (India) Peroxydicyclohexylamine is an important (major) byproduct of the ammoximation reaction. This compound is formed by the condensation of cyclohexylimine and cyclohexanone with H 2 0 . Your mechanism (hydroxylamine) does not explain the formation of this compound. In hct, the formation of this compound suggests the presence of cyclohexylimine species in the system.
A: A. Zecchina On the basis of our experimental results, peroxydicyclohexylimine is not a typical byproduct of the ammoximation reaction; it has been found only with catalysts of low activity in ammoximation. The synthesis of peroxydicyclohexylimine is well known: on the subject there are scientific papers [l] and patents: it is a noncatalyzed reaction between cyclohexanone, ammonia and hydrogen peroxide. [l] E. G. E. Hawkins,J. Chem. Soc., C, 2663,1969
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Guczi, L ef aL (Editors), New Frontiers in Caralysir Promdings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 6 1993 Elsevier Science Publishers B.V. All rights reserved
ON THE CATALYTIC OXIDATION OF METHANOL WITH VANADIUM (IV)IN SULPHURIC ACID SOLUTION R. Lmsson and B. Folkesson Group of Catalysis Research, Chemical Center, University of Lund, P.O.Box 124,221 00 Lund, Sweden
Abstract The oxidation of methanol by vanadium(IV)has been studied by measuringin a closed system the carbon dioxide formed by gas chromatography. The catalyst was a platinum sol. precipitated on titanium dioxide dispersed in the solution. The rate constant was found to change during the time of reaction between two distinct values, one valid at time zero and the other one valid for the later part of the reaction period.
1. INTRODUCTION The direct use of methanol in fuel cells has been severely hindered by the poisoning of the platinum catalysts by oxidation intermediates. Therefore the reduction of suitable redox systems might offer an alternativeway to utilize methanol in galvanic devices. We have studied the oxidation of methanol in sulphuric acid of various concentrations using vanadium(IV) as the oxidant and a Pt/ri02 system as the catalyst. The catalyst was prepared in the way prescribed by Mills and Porter [ 11. A Pt(1V) solution was reduced with citrate and the collodial sol was precipitated onto Ti02 (DegussaP25 ) by adding a solution of concentrated NaCl. The reaction was performed in a tennostated vessel with a reflux condenser. The solution of
VOSO4 and H2SO4 was brought to the desired temperature and freed from oxygen by bubbling nitrogen through the system. A cenain volume of methanol of the same temperature was added . The system was closed and the remaining nitrogen atmosphere was circulated through the system by means of a peristaltic pump. Samples of the gas was
732 taken for analysis in a gas chromatograph at chosen time intervals. The amount of carbon dioxide found in this way was transformed to a corresponding decrease in the amount of vanadium(IV)assuming the following reaction: CH30H + 6V02+
+ 6 H+ = C02 +
6 V3+
+
5 H20.
1 0.8
h
L \
0.6
0
F >
C
0.4
0.2 0 0
5
10 15 time ( h )
Figure 1. The experimental results for C H 2.
20
~ = 2s M. ~
~
RESULTS
2.1. Kinetics experiments The results were treated in the form In V(IV)@(IV) versus time. An example of such a plot is given in Figure 1. It is obvious that a simple first order reaction is not the case. The reaction seems to run fast in the starting period and to reach a state of more constant rate later on . In this it follows - almost exactly the observations made previously [2] from measurements of the concentration of V(II1). The conclusions reached at that time, however, must be somewhat modified as it is obviously possible to obtain the same results by measuring both C02 and V(III).
-
We make here the assumption that the catalyst originally has a certain activity but that the catalyst system is changed during the first period of the reaction so that its activity is somewhat decreased. If we designate the rate constant of this process of declination as k",it follows that at the time t the fraction e-k"t remains of the catalyst having the initial activity and the fraction 1 e-k"t has the lower activity. We further designate ko as the rate constant of
-
733 reaction (1) with the more active catalyst and kl as the corresponding rate constant of the same reaction with the less active catalyst.
k = (ko -k1) e-k" t
+ kl
(3)
d [V(IV)]/dt = - k [V(IV)]
(4)
This is transformed into
For very large values oft this expression is reduced to
This is the relation for the straight line defined by the points for very high t values in Fig 1. The intercept of this line makes it possible to determine k" if ko can be determined in other ways. ko can actually be obtained as the limit of eqn ( 6) for t approaching zero. ( For small x it holds that 1-e-x
-
x. )
lim In [V(IV)]o/ [V(IV)] = (k,-kl )/k" * k't
+ kit
= ko t
(8)
t -> 0
ko can thus be determined as the limiting slope of curves such as those of Fig 1.
In Table 1 we give the resulting values of ko, kl and k" for three different values of the sulphuric acid concentration and for three different temperatures. Figures 2 and 3 give representative examples of the results in the form of Arrhenius graphs. The activation energies that are derived from such graphs are given as a function of the sulphuric acid concentration in Figure 4. , The astonishing result appears that strongly negative activation energies are obtained for the supposed catalyst transformation process, described by k". This will be discussed in the following.
734 Table 1 Results of the oxidation of methanol to carbon dioxide.
0.5
60 70 80
1.92 4.10 6.03
1.44 1.72 2.03
0.00141 0.01256 0.0404
0.48 2.38 4.00
340 190 99
1.o
60 70 80
2.03 3.91 4.23
2.03 2.49 2.87
0 0.02528 0.035 1
0 1.42 1.36
56 39
60 70 80
4.49 7.82 13.5
4.05 5.15 6.45
0.00455 0.04797 0.10212
0.44 2.67 7.05
97 56 69
3.0
3.1
2.0
2.8
2.9 1000/T
( 1/K)
Figure 2. Arrhenius plot for kl and Q2s04= 0.5 M.
735
6.0
5.5
Iu -c
5.0 4.5 2.8
2.9
1000/T
3.0
3.1
(1/K )
Figure 3. Arrhenius plot fork" and C H ~ S O = 0.5 ~ M
20 10
-
0
.
Ea(k") Ea(k1) Ea(kO)
Figure 4. The acid dependence of the activation energies.
2.2 SIMS and XPS results The catalyst system was investigated by SIMS and XPS before and after each experiment. Some of these investigations have been described before [3]. The most obvious result is that the Pt/Ti ratio as measured by XPS decreases strongly following the reaction. The SIMS spectrum of an unused catalyst sample showed, besides the Ti and Ti0 signals also some signals of mass numbers 28,29 and 41 that can be attributed to decomposition products of the citrate used for preparing the platinum sol. After a short period of reaction, the latter signals were reduced and they dissappeared seemingly after a few hours.
736 3. DISCUSSION 3.1 Analysis of the kinetics results Suppose that there is an equilibrium reaction coupled to the rearrangement reaction of the catalyst , The catalyst in the original state is designated as KATo and in the later state as KATl KATo t X = KATo X -> KATl
(9)
If the equilibrium constant is designated as K and the rate constant for the transformation of KATo X to KATl is called k', we have [KATo X ] = K [KATo] [XI d[KATl]/dt = k' K [XI[KATol where k" = K
* k' [X]
(12)
In k" = In K t In k' + In [XI
(13)
So far we have not indicated what "X" means. Let us assume, however, that the concentration of X is independent of time and temperature. It then holds that d In k"/d( l/r) = d In K/ d( lfl)
+ d In k' / d( lfl)
(14)
This means that
where Q is the heat of reaction of the equilibrium (9).
If Q > E,' equation (15) results in negative activation energies, as observed.
If the trend given in Figure 4 shall be reproduced it follows that the heat of reaction must decrease with increasing acid concentration. Q = A - B [H+] .
(16)
737 3.2. Proposed explanation of the catalyst rearrangement The first criterion for deciding about the precise nature of the species called X in eqn (9) must be that the heat of reaction when [H+] = 0 must be strongly positive. On increasing the the hydrogen concentration [H+] some component of the equilibrium will be affected so that the
value of Q decreases, i.e., Ea" increases. Keeping these criteria in mind some reactions can be suggested may result in the transformation of the properties of the catalyst.
Pt + CH30H -> Pt(CH30H)
- in a speculative way - that
(18)
The first example implies the incorporation of vanadyl ions in the lattice of titanium dioxide, The vanadyl ions might in the subsequent step affect the properties of the catalyst, e.g., change the degree of metal support interaction. The second example implies the adsorption of methanol on the platinum surface w h m it can in the next reaction step form the wellknown [4] poisonous species found in methanol electrochemistry, such as CO and HCO. Of course other possibilities of a catalyst change are a recrystallisation of the Pt particles or the covering of the Pt particles with a layer of titanium dioxide. The latter reaction can be described in analogy with (17-18) as Pt + Ti02 = Ti02(Pt) (19) 4. CONCLUSIONS
The present investigation addresses the question, how one might characterize a rearrangement of a catalyst system. In the specific case of methanol oxidation we have measured the precursor activity ( ko ), the resultant catalyst activity (kl ) and the rate of transformation (k" ). By treating the activation energy of the latter in terms of the true activation constant of the transformation reaction ( E i ) and the heat of reaction of a pre-equilibrium one might be able to define criteria for ways in which the rearrangement can take place and rule out others. It should be remarked that very simple arguments can be used to explain the observed effects. It is not unreasonable that the platinum particles are covered by a T i 0 2 layer, through which the reaction takes place. In this way we have a close analogue to the so-called SMSI ( strong
738 metal support interaction) of gas phase catalysis involving Pt / Ti@ in a reducing atmosphere [5] .One of the explanations [6] of this effect is the one that the reaction takes place at the borderline between Pt and a TiO, layer creeping up on the catalyst surface. If this is the case it niay be quite reasonable that the W r i ratio decreases and that there is a well defined rearrangement of the catalyst and a change of its properties.
5. REFERENCES 1 A. Mills and G. Porter, J. Chem. Soc.,Faraday Trans. 78 (1982) 3659. 2 B. Folkesson, R. Larsson and J. Zander, J. Electroanal. Chem. 267 ( 1989) 149. 3 B. Folkesson, R. Larsson and B. Rebenstorf, J. Electroanal. Chem. 272 (1989) 231. 4 B.Beden, A.Bewick and C.Lamy, J. Electroanal. Chem. 148 (1983) 147. 5 S. J. Tausler, S. C. Fung and L. J. Garten, J. Am. Chem. SOC.100 ( 1978) 170. 6
R. Burch and A.R. Flambard, J. Catal. 78 (1982) 389.
739 DISCUSSION
Q: J. C. Conesa (Spain) Have you considered the possibility of TiO, being reduced to give Ti(ll1) modifying the catalyst propertics without having to invoke coverage of Pt particles'?
A: R. Larsson The standard potential (at 25 OC) for the reaction 2Ti02(h,s)
t
2H+ t 2e- + Ti*Og(h,s) t H20
is = -0.091 V; there h means hydrated form of the solid oxide system. This Ti(1V) might be reduced by methanol (Eo = t0.044 V) to Ti(II1) to a slight degree. On the other hand, as long as vanadium (IV) is present in excess (E, = t0.36 V), it will easily reoxidize Ti(II1). Such a redox sequence may perhaps trigger the recrystallisation of T i 0 2 to cover the platinum surface (formula 19), as suggested as one possible explanation for the observed effect. The suggested coverage is supported by SIMS indicating a decrease in platinum signal intensity after the reaction.
Q: J. Rask6 (Hungary) Once you had methanol, why do you wish to oxidize it into carbon dioxide ?
A: R. Larsson What we want to achieve is a reduction of V(IV) to V(II1) in order to use the couple together with V(V)/V(IV) as a galvanic device. Hence it is advantageous to reach the carbon(TV)oxidation state from methanol.
Q: J. W. Hightower (USA) The curvature in Figure 1 was explained by assuming two different rate constants k and k,., Can you absolutely rule out the possibility that the curvature may be caused simpfy by poisoning'? Have you run one experiment to completion, then carried out a second experiment and observed the same curvature without any "regeneration" between the two runs? A: R. Larsson We have observed that the catalyst is still active after having filtered it and washed it with watcr and dried it. As there is probably some loss of platinum in this procedure it is, however, not meaningful to present any absolute data on the rate. Some - if not all - of the reactions (formulae 17-19) suggested as the cause of change from rate constants k to k, can be described as poisoning of the catalyst. The main idea ofthe paper was that, given complete thermodynamic data for the various reactions proposed, one should be able to deduce from the acid dependence of the activation energies, which of the poisoning reactions that was at hand.
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Guczi, L. a al. (Editors), New Fronriers in Caralysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
ROLE OF CHROMIUM INTRODUCED INTO 12-MOLYBDOPHOSPHATES AS CATALYSTS FOR OXlDATION OF HYDROCARBONS
K Brilckman and J. Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30239 Krakow, Poland
A bst m c t Chromium 12-molybdophosphate Cr[PMol 20401 w a s o b t a i n e d w i t h C r 3 + i n c a t i o n i c p o s i t i o n s . On h e a t i n g i t t r a n s f o r m s r e v e r s i b l y around
650K w i t h t h e d e s t r u c t i o n o f Keggin anions i n t o an amorphous phase w h i c h i s s t a b l e t o about 750K, above w h i c h c r y s t a l l i n e Ma03 appears. When a f t e r c o o l i n g t h e amorphous phase i s exposed t o w a t e r vapour, r e c o n s t r u c t u r e o f t h e i n i t i a l phase t a k e s place. XPS s h o w s h i g h l y i o n i c bonding w h i c h r e s u l t s i n h y d r o x y l a t i o n o f t h e s u r f a c e and g e n e r a t i o n o f strong a c i d i c s i t e s . Presence o f c h r o m i u m i n c a t i o n i c p o s i t i o n s increases redox p r o p e r t i e s o f Keggin anions r e n d e r i n g CrIPMol a highly a c t i v e and s e l e c t i v e c a t a l y s t f o r o x i d a t i o n o f m e t h a n o l t o formaldehyde, propane t o propene and pentane t o m a l e i c anhydride. It i s suggested t h a t h i g h a c t i v i t y i n o x i d a t i v e dehydrogenation o f propane a s opposed t o h i g h a c t i v i t y i n i n s e r t i o n o f oxygen i n case o f pentane m a y be due t o t h e presence o f amorphous phase.
1. INTRODUCTION Heteropolyoxometalates w i t h Keggin s t r u c t u r e a r e w e l l k n o w n as o x i d a t i o n and a c i d c a t a l y s t s , i n w h i c h t h e redox and acid-base p r o p e r t i e s may be m o d i f i e d by choosing d i f f e r e n t heteroatoms, d i f f e r e n t a t o m s of t h e addenda o r s u b s t i t u t i n g t h e addenda w i t h a l t e r v a l e n t a t o m s [1,2]. One o f t h e m o s t s t u d i e d as s e l e c t i v e o x i d a t i o n c a t a l y s t i s t h e 12-molybdophqsphoric a c i d and i t s s a l t s . I t has been found t h a t t h e a c i d s h o w s h i g h e s t a c t i v i t y and s e l e c t i v i t y i n o x i d a t i o n of hydrocarbons b u t r a t h e r l o w t h e r m a l s t a b i l i t y , w h e r e a s s t a b l e a l k a l i
742
m e t a l s a l t s are c a t a l y t i c a l l y i n a c t i v e . ConsiIJErable a t t e n t i o n has been p a i d t o m o d i f i c a t i o n o f o x i d a t i v e propertit.).; iJI4 s u b s t i t u t i o n o f t h e Mo atoms by altervalerit atoms, the mixed addenda vanadomolybdoptiosphates H3+x[PMo 2-xVx0401 being found t o be the m o s t s e l e c t i v e i n o x i d a t i o n of alkanes [31. Very l i t t l e i s k n o w n about c a t a l y t i c p r o p e r t i e s o f c h r o m i u m containing heteropolyoxometalqtes. C h r o m i u m may e n t e r i n t o t h e hetei-opolymetalate s t r u c t u r e e i t h e r as t h e t e t r a h e d r a l l y c o o r d i n a t e d tieteroatotn o f t h e Keggin u n i t , o r as s u b s t i t u e n t of t h e addenda a t o m o r as t h e cation. One could a n t i c i p a t e t h a t depending on i t s p o s i t i o n i t w o u l d tnodify i n d i f f e r e n t w a y t h e c a t a l y t i c p r o p e r t i e s o f t h e heteropolyoxometalate. I t seemed t h e r e f o r e i n t e r e s t i n g t o u n d e r t a k e an a t t e m p t t o synthesize t h e 12-molybdophosphates i n t h e presence o f various chromium s a l t s in order t o introduce chromium in d i f f e r e n t s t r u c t u r a l p o s i t i o n s o f the Keggin s a l t . I n t h i s paper w e r e p o r t t h e r e s u l t s obtained when c h r o m i u m i s present i n f o r m o f t h e c a t i o n and w e compare c a t a l y t i c p r o p e r t i e s o f t h e r e s u l t i n g c a t a l y s t w i t h those o f t h e m o s t 6c t ive van ado m o 1y b do p ho sp h a ric and m o 1 y bdo p hos p h o r i c acids.
2. EXPERIMENTAL 2.1. Platerials C r ( I II) derived p r o p o r a t i o n w a s obtained by m i x i n g aqueous s o l u t i o n s o f C I - ( N O ~ ) ~ and H3[PMo 120401 (Fluke Gat-antie p u r i s s . p.a.1 in stoichiometr-ic amounts. The p r e c l p i t a t e d hydrate of chromium m o l y bdop ti0 s p ha t e w a s put-i f 1 e d by m u 1 t ip 1e c r y s t a1 1 i z a t 1 on. DT G a n a l y s i s revealed t h a t I t c o n t a i n s 25 H20 p e r c h e m i c a l f o r m u l a of s a l t . The p r e c i p i t a t e w a s d r i e d and c a l c i n e d f o r 24 h r a t 623K. I t s BET s u r f a c e area amounted t o 1.94 m2.g-l. I n o r d e r t o c o n f i r m t h a t c h r o m i u m i s p r e s e n t only i n t h e c a t i o n i c p o s i t i o n s t h e p r e p a r a t i o n w a s dissolved i n w a t e r and KC1 w a s added t o p r e c i p i t a t e ttie Keggin anions i n f o r m o f i n s o l u b l e K3 [ P M O , ~ ~XPS ~ ~ a] n. a l y s i s s h o w e d t h a t the p r e c i p i t a t e c h a r a c t e r i z e d by X-ray d i a g r a m o f
K3 [PMo 20401 contained
no t r a c e s o f chromium. The IR s p e c t r a of t h e p r e p a r a t i o n contained bands i d e n t i c a l t o those o f t t i e Keggin anion i n K3[PMo120401. It m a y be thus concluded t h a t the c a t o l y s t has t h e c o m p o s i t i o n C r [ P I - l 0 , ~ 0 ~I~t 1 .
w i l l be f u r t h e r r e f e r r e d t o as CrPMo
743 T h e H5 [PV2Mo ,o0401 (HPVMo) p r e p a r a l l . : ( w a s obtained according t o the method o f Tsigdinus and Hallada (41 and p u r i f i e d by m u l t i p l e c r y s t a l l i z a t i o n f r o m aqueous solution. Elemental analysis revealed t h a t the r e a l composition o f the preparation w a s H4.92 [PV Mo o. 30401. I t was d r i e d a t 363K and calcined f o r 2 tit- a t 623K. I t s BET s u r f a c e area amounted t o 3.84 m2.g- '. H3 [PMo 20401 (HPMo) w a s c o m m e r c i a l hydrated molybdophosphorlc acid (Fluka Chemie A.G.) dried a t 363K and calcined f o r 2 h r a t 623K. I t s BET surface area w a s 2.36 rn2.g-'. 2.2. Co to1 y t i c m e a s u r e m e n t s C a t a l y t i c p r o p e r t i e s w e r e determined i n an i s o t h e r m a l continuous f l o w m i c r o r e a c t o r (made o f glass f o r CH30H and C3H8 o x i d a t i o n and o f
m e t a l i n the case of pentane oyidation). On-line gas chromatographic analysis w a s used t o determine the c o m p o s i t i o n o f the feed b e f o r e and a f t e r the c a t a l y t i c reactor. In the case o f CH30H o x i d a t i o n t h e m i x t u r e of reagents was obtained by passing t h e f l o w through methanol s s t u r a t o r maintained a t 284.5 K while f o r C5H oxidation by passlng i t through pentane s a t u r a t o r kept a t 2 7 3 K. The gas f l o w r a t e and t h e c a t a l y s t w e i g h t w e r e a l w a y s a d j u s t e d i n a w a y securing the t o t a l conversion t o be s m a l l e r than 10%. 2 . 3 . U i f f c r c n t i a l Thermal A n a l y s i s DTG/DTA experiments w e r e c a r r i e d out i n a i r w i t h an analyzer F.Paulik L.Erdey. The wei h t of sample w a s about 500 m g and t h e heating r a t e w a s 5 K.min .
-9
2.4.lnfrored Spectroscopy I R spectra were recorded on the FT-IR s p e c t r o m e t e r D i g i l a b FTS- 14V a t r o o m temperature i n K B r powder. 2.5.XRD A n a l y s i s Powder X-ray d i f f r a c t i o n p a t t e r n s w e r e obtained w i t h a DRON-2 d i f f r a c t o m e ter using CuK, radiation. 2.6.ESCA SPS spectra w e r e recorded on a VG S c i e n t i f i c ESCA-3 s p e c t r o m e t e r using AIK, r a d i a t i o n (1486.6 eV). Binding energies Eb w e r e
referenced t o the Cls peak assumed t o have Eb value o f 285.0 eV.
744
3. RESULTS AND DISCUSSION T a b l e 1 p r e s e n t s t h e Eb values o f Cr2p3l2, P2p and 01s e l e c t r o n s r e g i s t e r e d w i t h Cr[PMo] c a t a l y s t , s t r l c t l y s t o i c h i o m e t r i c K3[PMo 20401 c o n t a i n i n g t h e same Keggln anion as t h e c a t a l y s t , and some reference compounds. The Eb values o f P2p and 01s e l e c t r o n s o f Cr[PMo] a r e i d e n t i c a l t o those of K[PMo] w h i c h i s c o n s i s t e n t w i t h t h e c o n c l u s i o n
Table 1 R e s u l t s o f XPS a n a l y s i s
Cornpou11 d
B i n d i n g energy (eV) r2P3 / 2 P2P
576.2
Cr203 C t-( OH )3
577.3
Cr03
580.1
K3 [PMo 20401
01s
53 1 .O
34.0 ~~
~~
~~
t h a t n o c h r o m i u m w a s i n c o r p o r a t e d i n t o t h e Keggin anion. The Eb v a l u e o f Cr2p e l e c t r o n s corresponds t o t h e range I n w h i c h values f o r C? i o n s are encountered although i t i s c o n s i d e r a b l y h i g h e r t h a n i n t h e oxide and hydroxide. T h i s i n d i c a t e s t h a t bonding b e t w e e n C?' c a t i o n s and Keggin anions i s m u c h m o r e i o n i c than i t Is i n o x i d e and hydroxide. T h i s i s i n l i n e w i t h t h e b e h a v i o u r o f Cr[PMol i n c a t a l y t i c r e a c t l o n s as discussed b e l o w . I n o r d e r t o d e t e r m i n e t h e t h e r m a l s t a b i l i t y o f t h e c a t a l y s t DTG/DTA e x p e r i m e n t s w e r e c a r r i e d o u t (Fig. 1). The broad e n d o t h e r m i c peak on t h e DTA curve w i t h m i n i m u m a t 418 K i s due t o t h e dehydration, w a t e r being released i n t w o s t e p s as m a n i f e s t e d by corresponding t w o peaks on t h e DTG curve. A f t e r d e h y d r a t i o n f u r t h e r t w o s m a l l e n d o t h e r m i c peaks are observed a t 663 and 703K, o f w h i c h t h e f i t - s t i s accompanied b y tl srnall loss o f w e i g h t , and f i n a l l y a l a r g e e x o t h e r m i c peak appears
745
w i t h m a x i m u m a t 768 K, c o r r e s p o n d i n g t o t h e d e c o m p o s i t i o n o f K e g g i n anions. I t should be e m p h a s i z e d t h a t i n case o f HIPMI and H[PVMol t h e
I
. .
DI
Fig.1. DTG/DTA a n a l y s i s o f Cr[PMo12040] S a m p l e w e i g h t
Fig.2. X-ray p o w d e r d i f f r a c t i o n d i a g r a m o f Cr[PMo 20401. F r e s h
0.583 g, h e a t i n g r a t e 5 K . m i n - l . Dotted line shows fragment of DTA c u r v e o f H5[PV2Mo 00401.
s a m p l e (a) and s a m p l e s c a l c i n e d f o r 2 hr a t 6 2 3 K (b), 7 2 3 K (c) 8 2 3 K (d).
e n d o t h e r m i c d e h y d r a t i o n peak I s i m m e d i a t e l y f o l l o w e d b y s t r o n g e x o t h e r m i c peak o f K e g g i n a n i o n decomposition, appearing s t 683 K i.e. a t t h e t e m p e r a t u r e - 1 0 0 K l o w e r . C o m p a r i s o n w i t h X-ray d i f f r a c t i o n p a t t e r n s r e g i s t e r e d a f t e r c a l c i n a t i o n a t d i f f e r e n t t e m p e r a t u r e s (Fig.2) i n d i c a t e s t h a t a f t e r h e a t i k g a t 6 2 3 K a t which d e h y d r a t i o n i s completed, b u t b e f o r e t h e n e x t e n d o t h e r m i c peak appears, a n amorphous m a t e r i a l i s formed, w h i c h l o s t i t s c r y s t a l s t r u c t u r e b u t K e g g i n anions r e m a i n e d u n p e r t u r b e d ( c f . l R s p e c t r u m a i n Fig.3). A f t e r h e a t i n g a t 7 2 3 K i.e. above t h e t w o e n d o t h e r m i c p e a k s t h e m a t e r i a l r e m a i n s amorphous b u t t h e Keggin anions w e r e d e s t r o y e d and f e w v e r y w e a k b r o a d l i n e s appear i n t h e X-ray p a t t e r n . Only a f t e r c a l c i n a t i o n a t 8 2 3 K above t h e e x o t h e r m i c peak t h e X-ray p a t t e r n r e v e a l s t h e p r e s e n c e o f c r y s t a l l i n e MOOS. C o n t r a r y t o t h a t i n t h e case of H[PMol and H[PVMol peaks o f
746
c r y s t a l l i n e Moo3 appear i n t h e X-ray diagrar-n a l r e a d y a f t e r h e a t i n g a t 683 i r n m e d i a t e l y a f t e r c o m p l e t i o n o f t h e d e h y d r a t i o n . F u r t h e r i n f o r m a t i o n on the s t r u c t u r e o f t h e c a t a l y s t w a s o b t a i n e d f r o m IR s p e c t r a o f t h e s e samples.Fig.3A i l l u s t r a t e s t h e 400- 1 2 0 0 c m - 1
Fig.3. FT-IR s p e c t r a o f C r [ P M o ~ ~ O ~A.~ Ia .f t e r d i f f e r e n t t h e r m a l t r e a t m e n t f o r 2 h r a t 623K (a), 723(b), c o o l e d t o r.t. and exposed t o w a t e r vapour (c). 8. F r e s h s a m p l e (a), a f t e r c a l c i n a t i o n f o r 2 hr a t 623K (b), 723K (c) and 823K (d)
r a n g e o f IR s p e c t r a o f t h e Cr[PMol p r e p a r a t i o n w h i c h h a s b e e n h e a t e d t o 723K and t h e n c o o l e d t o r o o m t e m p e r a t u r e and exposed t o w a t e r v a p o u r and F i g . 3 8 - t h e s a m e r a n g e o f IR s p e c t r u m f o r Cr[PMol w h i c h a f t e r h e a t i n g a t 723 h a s been f u r t h e r h e a t e d a t 823K. It m a y be seen t h a t a f t e r heating a t 723K t h e c h a r q c t e r i s t i c s p e c t r u m o f the Keggin anion w i t h bands a t 1062, 965, 871 and 790 cm-l 151 d i s a p p e a r s and a b r o a d f e a t u r e l e s s band b u i l d s up i n s t e a d . When a t t h i s p o i n t t h e s a m p l e is c o o l e d and exposed t o h u m i d i t y , t h e s p e c t r u m c h a r a c t e r i s t i c f o r t h e K e y g i n a n i o n r e a p p e a r s a l m o s t unchanged i n d i c a t i n g t h a t t h i s a n i o n h a s been r e c o n s t r u c t e d . When, h o w e v e r , t h e s a m p l e i s f u r t h e r h e a t e d t o &23K, i t s h o w s b r o a d bands a t 980, 867 and 618 c m indicative of
747
Moo3 for-t-riatioil. In t h e case o f HIPMol and I i ~ P V P l o I s a m p l e s t h e IR
, spectrur-ii o f Moo3 becomes v i s i b l e a l r e a d y a l t e r h e a t i n g t o 6 7 3 K and
does n o t change on c o o l i n g and exposure t o h u m i d a t m o s p h e r e . T h e DTG/DTA, XRD and IR d a t a d i s c u s s e d above l e a d t o t h e c o n c l u s i o n t h a t CrPMo t r a n s f o r m s on h e a t i n g i n t o an amorphous phase w i t h a broad t e m p e r a t u r e range o f s t a b i l i t y , w h i c h can be r e v e r s i b l y t r a n s f o r m e d back i n t o t h e i n i t i a l Cr[PMol phase. T h f s phase i s s t a b l e t o a b o u t 7 5 0 K i.e. t h e t e m p e r a t u r e a l m o s t 100 degrees h l g h e r t h a n t h a t a t which t h e molybdophosphoric a c i d and it s s u b s t i t u t e d d e r i v a t l v e s decompose i r r e v e r s i b l y w i t h t h e f o r m a t i o n o f Moo3. C a t a l y t i c p r o p e r t i e s . o f Cr[PMol i n o x i d a t i o n o f methanol, propane and pentane a r e s u m m a r i z e d i n T a b l e s 2, 3 and 4 r e s p e c t i v e l y and c o m p a r e d Table 2 A c t i v i t y and s e l e c t i v i t y o f molybdophosphates i n o x i d a t i o n o f CH30H (Reaction t e m p e r a t u r e 533K)
Catalyst
Activity mmol.h-lg-'
S e l e c t i v i t y (23 (CH3) 20 CH20 MF* ML** CO C02
63.1
46.0
38.0
3.4
10.5
1.9
0.2
162.7
21.8
56.0 16.8
2.1
2.9
0.3
H~ [ P m 20401
62.0
43.5
18.5
10.5
2.4 28.0 0.1
H g [PV2MO 100,401
56.3
49.9
33.0
0.3
C r [PMo 20401 C r [PMo 2040]/Si02
*/
MF = HCOOCH3,
+*/ ML = (CH30)2CH3,
9.0
6.0
0.1
He/02/CH30H = 77/16/7
w i t h t h e a c t i v i t y o f H[PMo] and H[PVMo] k n o w n t o b e t h e b e s t h e t e r o p o l y o x o m e t a l a t e c a t a l y s t f o r o x i d a t i o n o f a l k a n e s [3,6,7,81. A t o x i d e s u r f a c e s m e t h a n o l r e a c t s t o f o r m d i m e t h y l e t h e r on a c t i v e s i t e s o f acid-base p r o p e r t i e s o r t r a n s f o r m s i n t o f o r m a l d e h y d e w h e n a c t i v e s i t e s w i t h r e d o x p r o p e r t i e s a r e i n v o l v e d [41. A n a l y s i s o f the d a t a summarized in Table 2 shows that i n identfcal experimental conditions t h e amount o f d i m e t h y l e t h e r f o r m e d on CrPMo i s c o m p a r a b l e t o t h a t o b t a i n e d w i t h m o l y b d o p h o s p h o r i c a c i d a l t h o u g h CrPMo i s a n e u t r a l s a l t . Apparently, because bonding i n t h i s s a l t i s h i g h l y i o n i c , t h e S u r f a c e becotnes e a s i l y h y d r o x y l a t e d by h e t e r o l y t i c s p l i t t i n g o f bonds i n H20,
748 w i t h p r o t o n b e i n g l o c a l i z e d on t h e b u l k y K e g g i n a n i o n and t h e OH group on t h e Cr3+ c a t i o n . As t h e K e g g i n a n i o n i s l a r g e , t h e p r o t o n I s d e l o c a l i z e d o v e r i t s a l l oxygen a t o m s and s h o w s s t r o g l y a c i d i c p r o p e r t i e s . Indeed, IR s p e c t r u m o f CrPMo even a f t e r c a l c i n a t i o n a t 6 2 3 K shows a b r o a d band c e n t r e d around 3400 c m - l w h i c h m a y be a s s i g n e d t o hydrogen bonded OH'groups o r h y d r a t e d p r o t o n s . I t i s n o t e w o r t h y t h a t t h e IR s p e c t r u m o f amorphous m a t e r i a l o b t a i n e d a f t e r c a l c l n a t l o n a t 7 2 3 K i n which K e g g i n anions have a l r e a d y been destroyed, s t i l l r e v e a l s t h e presence o f this band, a l b e i t o f m u c h s m a l l e r i n t e n s l t y (Fig.4). S i m u l t a n e o u s l y , h o w e v e r , s e l e c t i v i t y t o f o r m a l d e h y d e on Cr[PMo] Is h i g h e r t h a n t h a t on H[PVMol w h i c h i s t h e b e s t k n o w n o x i d a t i o n c a t a l y s t A
El
rk
A1
Lo00
3200
16M3
2600
800
WAV ENUM BER
Fig.4. IR s p e c t r a o f C r [PMo 20401 c a l c i n e d a t 6 2 3 K ( A ) and 7 2 3 K (B). Table 3 A c t i v i t y and s e l e c t i v i t y o f m o l y b d o p h o s p h a t e s i n o x i d a t i o n o f propane (Reaction temperature 673K)
Catalyst
Activity mmo1.h-l.g-'
CI' PMO12040
0.41
H g P V 2 M 0 1 0 0 4 0 0.48
c 1 c2
S e 1 e c t i v i t y(W) C ~ CH3 H 4~0 HD*
0.02 0.2
0.02
55
5.7
50
2.0 .
E x p e r i m e n t a l c o n d i t i o n s : He/02/C3H8 = 89.5/3.5/7.0 6
/ HD = 1,s-hexadiene
1.0
-
co
C O ~
9.2
28.5
13.6 34.1
749
among t h e h e t e r o p o l y s c i d s and t h e i r compounds. T o t a l a c t i v i t y i n c r e a s e s c1i-ometical.ly and t h e r e l a t i v e r a t i o of t h e a c t i v i t y ’ a l o n g t h e o x i d a t i o n r o u t e t o t h a t along t h e acid-base r o u t e r i s e s w h e n CrPMo i s d e p o s i t i e d on S i 0 2 a s s u p p o r t . Table 4 Conversion and s e l e c t i v i t y o f molybdophosphate c a t a l y s t s i n pentane o x i d a t i o n ( R e a c t i o n t e m p e r a t u r e 5 13K) Conv.
Selectivity(%)
Ca to1 y s t
%
MA*
co
co2
C r [PMo 120401
19.1
65.7
16.4
17.9
H3 [PMo 120401
10.4
25.8
49.3
24.9
H g IPMO 1 oV2040I
15.1
44.0
29.3
26.6
E x p e r i m e n t a l c o n d i t i o n s : He/02/C5H
16.2/2.9/0.9
A n a l y s i s o f t h e c a t a l y t i c p r o p e r t i e s o f CrPMo s u m m a r i z e d i n T a b l e s 2,3 and 4 l e a d s t o t h e c o n c l u s i o n t h a t i t i s a n a c t i v e and s e l e c t i v e c a t a l y s t f o r t h e o x i d a t i o n o f hydrocarbons. In i d e n t i c a l e x p e r i m e n t a l c o n d i t i o n s i t s h o w s g r e a t e r a c t i v i t y i n o x i d a t i o n o f p e n t a n e and c o n s i d e r a b l y h i g h e r s e l e c t i v i t y t o m a l e i c a n h y d r i d e t h a t H[PVMol d e s c r i b e d i n l i t e r a t u r e and p a t e n t s as c a t a l y s t o f b e s t p e r f o r m a n c e I n t h e o x i d a t i o n o f pentane. Moreover, i t s g r e a t advantage o v e r t h e hetet-opolyacids i s a m u c h h i g h e r t h e r m a l s t a b i l i t y and hence p r o l o n g e d l i f e t i m e . I t i s n o t e w o r t h y t h a t s i m i l a r l y as i n t h e c a s e of t i e t e r o p o l y a c i d s m a l e i c a n h y d r i d e i s f o r m e d f r o m pentane as t h e s i n g l e oxygenated product, t h e o n l y o t h e r p r o d u c t s b e i n g c a r b o n oxides. A n i n t e r e s t i n g general c o n c l u s i o n e m e r g e s that t h e p r e s e n c e o f c h r o m i u m in the cationic s i t e s influences the c a t a l y t i c a c t i v i t y o f the Keggin anions. When propane i s oxidized, propene appeers as t h e p r e v a i l i n g product indicating t h a t oxidative dehydrogenation i s proceeding f a s t i n c o n i p a r i s o n w i t h t h e n e x t s t e p o f n u c l e o p h i l i c oxygen a d d i t i o n , i n w h i c h a c r o l e i n i s f o r m e d . O x i d a t i o n o f propane was c a r r i e d o u t a t 673K i.e. i n t h e t e m p e r a t u r e range, i n w h i c h t h e c a t a l y s t was r e v e r s i b l y t r a n s f o r m e d i n t o an arnorphqus phase and t h e s t r u c t u r e o f K e g g i n anions w a s destroyed, w h e r e a s o x i d a t i o n o f pentaneain w h i c h a d d i t i o n
7 50
K when the c,tJtolyst c o u l d s t i l l e x i s t i n 1I.s c r y s t a l l i n e f o r m . One c o u l d advance a h y p o t h e s i s t h a t a b s t r a c t i o n o f h y d r o g e n f r o m a l k a n e s t o formi a l k e n e s c a n p r o c e e d on amorphous phase w i t h s h o r t r a n g e o r d e r o n l y . T a k i n g i n t o a c c o u n t t h e i n s t a b i l i t y o f h e t e r o p o l y a c i d s one c o u l d s u s p e c t t h a t a l s o in t h e c a s e o f H[PVMol a n u n d e t e c t a b l e amorphous phase i s f o r m e d a t t h e s u r f a c e , w h i c h i s r e s p o n s i b l e f o r t h e o x i d a t i v e d e h y d r o g e n a t i o n o f propane.
o f o,:ygeti t.ook p l o c e was s t u d i e d a t 513
4. CONCLUSIONS
C h r o m i u m 12-molybdophosphate due t o h i g h l y i o n i c b o n d i n g and l a r g e s i z e o f th? K e g g i n a n i o n behaves i n c a t a l y t i c r e a c t i o n s a s c a t a l y s t exhibiting strong acidity, comparable w i t h t h a t o f molybdophosphoric acid. C h r o m i u m , a l t h o u g h i t i s p r e s e n t a s Cr3+ i n c a t i o n i c p o s i t i o n s , strongly m o d i f i e s t h e redox properties o f the Keggin anion rendering i t a v e r y a c t i v e and s e l e c t i v e o x i d a t i o n c a t a l y s t . On h e a t i n g i t t r a n s f o r m s r e v e r s i b l y w i t h t h e d e s t r u c t i o n o f t h e K e g g i n a n i o n i n t o an a m o r p h o u s phase w h i c h s h o w s Q b r o a d r a n g e o f t h e r m a l s t a b i l i t y b e t w e e n 650 a n d 750K. I n t h i s t e m p e r a t u r e r a n g e i t behaves a s s e l e c t i v e c a t a l y s t f o r o x i d a t i v e d e h y d r o g e n a t i o n of propane t o propene w h e r e a s a t l o w e r t e m p e r a t u r e s i t i s an e f f i c i e n t c a t a l y s t f o r n u c l e o p h i l i c a d d i t i o n o f oxygen i n o x i d a t i o n o f p e n t a n e t o m a l e i c anhydride. A h y p o t h e s i s m a y be advanced t h a t t h e p r e s e n c e of t h e a m o r p h o u s phase i s r e s p o n s i b l e f o r t h i s difference i n catalytic properties.
5. REFERENCES 1 . M.T.Pope, H e t e r o p o l y - and I s o p o l y o x o m e t a l a t e s , S p r i n g e r V e r l a g ,
B e r l i n 1983. 2. M.Misono, Catal.Rev.-Sci.Eng., 29 ( 1987) 269. 3. G.Centi, J.Lopez Nieto, C.lapalucci, K.Bruckman, E.M.Serwicka, A p p l . C a t a l . 46 ( 1 969) 197. 4. G.A.Tsigdinos, C.J.Hal\ada, Inorg.Chem. 7 (1968) 137. 5. C.Rocchiccioli-Del t c h e f f , E.Thouvenot, R.Frsnck, S p e c t r o c h i m . A c t a 3 2 A ( 1976) 34. 6. I 1 wt 9i ) used in these works (6,7,11) which make unlikely the incorporation of all the Rhenium within the surface layers of the alumina (12), and therefore its reduction to form a well dispersed metallic phase as seems to occur in the industrial PtRe catalysts studied in this work. ACKNOWLEDGEMENT We thanks the financial support from PR84/0330 and PB88/263 ) to this work.
the PGC/MEC Program
(Projects
792
BIBLIOGRAPHY 1. P. Malet, G. Munuera and A. Caballero. J. Catal. 115 (1989) 567 2. A. Caballero, A.R. Gonzalez-Elipe, P. Malet, G. Munuera, J. Garcia, J.C. Conesa and E. Burattini. Physica B 158 (1989) 158. 3. M. Kobayashi, Y. Inoue, N. Tajahashi, R.L. Bunvell Jr., J.B. Butt and J.B. Cohen. J. Catal. 50 (1980) 464 4. P. Malet and A. Caballero. JCS Faraday Trans 1,84 (1988) 2369. 5. M.G.V. Moderate and C.H. Rochester. JCS Faraday Trans 1,85 (1989) 3495. 6. L. Guczi A. Beck. Polyhedron 7 (1988) 2387. 7. L. Guczi, S. Dobos, A. Beck , A. Vizi-Orosz. Catal. Today 6 (1989) 97. 8. H.C. Yao, M. Shelef. J. Catal. 44 (1976) 392. 9. D.R. Sort, J.M. Khalid, J.R. Katzer. J. Catal. 72 (1981) 288. 10. F.A. Conon Acc Chem Res. 11 (1978) 225; ibid 11 (1978) 356. 11. A S . Fung, P.A. Tooley, M.J.Kelley, D.C. Koningsberger and B.C. Gates, J. Phys Chem. 95 (1991) 225 12. P. Amoldy, V.M. Van Oers, O.S.L. Bruinsma, V.H.J. DeBeer, J.A. Moujlin. J. Catal. 93 (1985) 231. DISCUSSION Q: H. Kluksdahl (USA)
Re(VII) oxide interacts strongly with the alumina support in reforming catalysts and, unlike the high volatibility of the (I) and (VI) oxides, there is no loss of rhenium in an oxidizing atmosphere below ca. 973 K and in reasonably dry atmospheres. We have observed a rather simple procedure for displacing Re(I) from oxidized catalysts by treating with an aqueous solution containing simple, non-oxidizing inorganic anions such as nitrate, chloride, carbonate, etc. (US Patent, H. E. Kluksdahl, J. R. Hopkins, early 1970s). This displacement is virtually quantitative with recoveries of 95% or more. We would not expect lower valent rhenium species to be so displaced. Some amount of moisture is commonly present in reforming catalyst treatments and usage. Would you expect moisture to hydro1 ze your Re(VI)/CI species '? How dry was your system at the conditions where you claimed e(IV)/CI ?
i
A: G. Munuera In our samples water va or is able to produce hydrolysis but not oxidation of the Re(IV)/Re(VI) up to Re(VI1). you can see in Figure 3, water vapor produce changes in the TPR profiles of the samples pretreated at 723 K giving a single-peak profile for Pt t Re reduction due to the hydrolysis of the strong Re(IV)-O-AI3+ interactions existing in these dried samples but without change in the oxidation state of the Re that remains as Re(IV)/Re(VI) (see Table 1). However, using samples with a higher Re loading (ca. 1-3 %) we know by XPS that though dry oxygen is only able to oxidize rhenium up to Re(IV)/Re(VI), in the presence of water vapor a partial oxidation up to Re(VI1) occurs the extent of which increases with the temperature and time of reaction (or just by exposure to the air). In the conditions you describe, using aqueous solutions, I can guess that oxygen dissolved in the water or from the air may be involved in the oxidation of all the rhenium up to Re(VI1). In our experimental conditions, though water vapor was present in some of the experiments oxygen was always absent what prevent the oxidation up to Re(VII).
K
Q: R. Prins (Switzerland)
The X-ray absorption white line intensity was influenced by chlorine. This may suggest that C1 is sitting on the Pt, but in that case one should be able to observe a Pt-Cl bond in EXAFS. Did you see such an EXAFS contribution ? One would not expect chlorine to adsorb
793 on the Pt, because CI is also strongly adsorbed by the alumina and there is so much more A1203 surface area. So,how is CI influencing the Pt white line intensity ?
A: G. Munuera We have not presented in our paper any EXAFS/XANES data for Pt, as you ask in your written question, but for Re. However, I must say that we have studied both Pt (ref. 2) and Re LI ledges. In our pre-calcined chlorinated prccuaon, once pretreated either at 423 or 723 K, chforine is not interacting with the metals as indicated by the fitting analysis of the E M S spectra, though XPS shows that it is present at the support. Moreover, the observed increase in the intensity of the LIII "white line" in the case of the Re after the pretreatment at 723 K occurs in all four samples used in our work, whatever they contain or not chlorine. Therefore, this fact can not be related to dircct interactions of the rhenium with the chlorine.
Q: G. L. Haller (USA) We have recently investigated PtRe/A1203 TPR using X-ray absorption spectroscopy. Using a variety of Re references compounds, it was observed that there docs not exist a linear relation between the Re LII white line and formal oxidation state. However, there is a linear relations between the edge s!hift and Re formal oxidation state, because these shifts are small, thcy require a simultaneous observation of a reference material, e.g., Re powder, but with case one can follow the shift in the edge during TPR of catalysts of commercial loading, e.g., 0.3 wt % Pt- 0.3 wt % Re/A1203 Since the white line intensities (your table 5) an likely affected by ligands, e.g., CI- and symmetry, I suggest that you also measure the edge shifts as well as the white line intensity of Re. A: G. Munuera I am glad to hear that you also found a non-linear relationship between the Re LIII white line intensity and the formal oxidation state, as we have reported in our paper. However, in the context of our work what we like to stress is the observed net increase in the intensity of this line in all the four samples containing Re after the non-oxidizing pretreatments at 723 K which correlates well with the changes observed in the TPR profiles though the oxidation state remains close to Re(IV) and with the same local symmetry (see Table 1 and 4). So, we have ascribed this fact to a deeper interaction of the rhenium with the more "ionic" environment what seems confirmed by our CO/FTIR data. In relation with the small shift you have observed in the LIIIedge for different oxidation states of the rhenium we have not made such a careful study with our samples and reference materials (Re02 R e 0 3 and Re04NH4) though it seems interesting to obtain the average oxidation state of the rhenium to compare with data from TPR. Q: B. Notari (Italy) Chlorine plays important roles both in the preparation of the reforming catalysts and in their utilization. It is well documented that under typical reforming conditions (753 K, H2.pressure 10-30 bar) Pt is reduced to Pt metal and it provides the (de)hydrogenation activity while the chlorine is bound to the Al 0 and provides the acid activity. These two functions are 2 2 necessary to give rise to the bifunctional mechanism" through which all the different reactions of naphtha reforming take place. Therefore the role of chlorine during utilization is very clear and is the very reason why industrial catalysts contains chlorine. On top of that, chlorine also has a role during the preparation of the catalyst, but this is a transient role. A: G. Munuera As you say, it is well known that the main role of chlorine in the reforming bifunctional catalysts is to provide the acidic function at the support. nevertheless, before their use bimetallics PtRe/Al,O, catalysts should be preconditioned, then reduced and finally sulfided
794 under well specified conditions. The aim of our work was to examine the effect of some of these steps in the final state of the metallic phase and, in particular, in the formation of a PtRe-alloyed phase. What our combined TPREXAFS-XANES/FTIR results, together with n-butane hydrogenolysis "tests" (studied in ref. l), seem to indicate is that chlorine has also a role in the development of such alloyed phase during the conditioning of the industrial catalyst before use what, in our view, is more than a transient effect. In short, what w e found is that when the chlorinated industrial catalysts are thermally pretreated at 723 K part of the rhenium is unable to form the PtRe alloy. This may affect the poisoning stability of the metallic phase. it is worth of mention that new formulations of the PtRe reforming catalysts use Pt:Re ratios of 1:2 instead of 1:l.
Q: J. Ryczkowski (Poland) Usually preparation of bimetallic catalysts in the way of impregnation can create mixed interaction in final catalyst. This preparation procedure should mainly produce metal(1)support and metal(2)-support interaction. Direct metal(1)-metal(2) interaction can be formed only by chance. How d o you think, what is the amount of really alloyed phase in your bimetallic catalysts ? If I am right, in your bimetallic systems you have mainly metal(1) and metal(2)-alumina interaction: metal( 1) Re
metal(2) Pt
A1203
Do not you think, that the observed effects came from se arate surface species and the presence of neighboring metal atoms can only influence them {And finally, how is the alloy phase, if it is present, changing during the experiments and during the course of the reaction :p A: G. Munuera Our results in this and in our previous paper (ref. 1) are difficult to explain, in our view, without considering the formation of a PtRe-alloyed phase during the reduction of the catalyst precursors. The way in which alloyed phases are formed in bimetallic catalysts is still unclear, particularly when metal loadings become so small as in our PtRe/A1203 samples. In principle it seems, as you say, that alloying can only occur by chance unless the two metallic precursors were originally close together at the support. However, we must assume, according to our TPR profiles, that during reduction of the catalyst precursors platinum is reduced first so that small clusters of this metal should be able to diffuse at the surface of the A1@3 support (particularly when it is highly hydroxylated) reaching in thcir way ReO, clusters that ma be reduced by the hydrogen spelt over from the Pt clusters to form a PtRealloyed phase. d f course, some of the Pt particles and some of the reduced Re (in particular when directly reduced at the higher temperatures) can remain as monometallic particles. During n-butane hydrogenolysis "tests" (ref. l), using the pulse technique, Re is readily poisoned so, physical mixtures of monometallic catalysts show steady state selcctivities (CJC,), after a few pulses, close to that for Pt/AI O3 while in the bimetallic catalysts, where Pt and PtRe particles would remain active, the v a k e depend on their relative amounts. In our mild reaction conditions the PtRe particles should remain unchanged as suggested by the reproducibility of test reaction.
Guni, L. d al. (Editors), New Fronriers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
A NEW APPROACH TO LOSS OF ALKALI PROMOTER FROM INDUSTRIAL CATALYSTS: IMPORTANCE OF EXCITED STATES OF ALKALI L. Holmlida, K Enpalla, C. Amana and P. G. Moronb aDepartment of Physical Chemistry, University of Goteborg and Chalmers University of Technology, 412 96 Goteborg, Sweden bDepartment of Engineering Chemistry I, Chalmers University of Technology, 412 96 Goteborg, Sweden
Abstract Using molecular beam and mass spectrometric techniques, the emission of highly excited states (so-called Rydberg states) of potassium from commercial iron-oxide catalysts for the styrene process was studied. The earlier results have been confirmed and extended using surface ionization detection and K molecular beam impact on plane catalyst surfaces. It is shown that clusters of K are very likely to be formed at the catalyst surface at normal operating temperatures in the experiments. The relevance of these results for the observed loss of potassium promoter under process conditions is discussed.
1. INTRODUCTION The importance and widely prevalent practice of alkali doping to promote various heterogeneous catalysts has been reviewed by Mross [l]. The promoter can influence the activity and selectivity of the catalyst. while the surface physics approach to the promoter effect is concerned mainly with the indirect geometric and/or electronic effects of the alkali promoter like modified bonding of ad-species to the surface, recent progress [2,3] on the other hand stresses the direct chemical interaction between the promoter and adsorbed molecules. In many cases, the promoter effect is still not understood very
796
well, despite the routine use of promoters in catalytic processes and the intense research in this field. The migration and loss of potassium from the iron-oxide catalyst for the styrene process continues to be.a serious production problem even 60 years after the commercialisation of this process. We have used molecular-beam and mass spectrometric techniques to etudy emission of K species from commercial styrene catalysts at typical process temperatures of 860-900 K (590-630 " C ) . Our work [4,5] is complementary to the recent work of Ertl and coworkers [6,7] at Berlin and Geus and coworkers [8,9] at Utrecht, using entirely different techniques. The techniques we have used [4,5] for detection of highly excited alkali atoms are mainly variations of weak field ionization, at field strengths less than 1000 V/cm. In this way, only highly excited so called Rydberg states are detected. Such states have a very large distance between the core ion and the excited electron or electrons. Another basic technique used in combination with field ionization is direct line-ofsight sampling from the catalyst into the field ionization detector or into a mass spectrometer. In the present study we apply still another technique to detect the alkali lost from the catalyst, namely surface ionization. This method provides absolute measurements of the losses, since it detects all alkali species with a similar efficiency. Clusters of alkali will probably also be dissociated and give ions in the detector, and the signal obtained is thus proportional to the total alkali flux. The present contribution will first describe the results concerning excited state emission from the sample, and after that focus on just one aspect of immediate interest for the loss of promoter, namely the evidence for loss in the form of clusters.
2. EXCITED STATE EMISSION The loss of alkali promoter from metal oxide catalyst surfaces has been studied with molecular beam methods in a few cases [4,5,10]. The results found for the iron oxide catalysts for the styrene process are summarized in the table, where A and B are the two catalyst types studied in [4,5]. From the results found in [4,5] an energy diagram for the K promoter in the iron oxide material has been obtained, as shown in Fig. 1. One of the measurement types of interest in the present context is the angular distributions of the emitted K species. In Fig. 2 an example of the results obtained with catalyst A is given. This result is almost cosine shaped, which is the form expected from statistical theory.
797
Table 1 Summary of results obtained for iron-oxide catalysts Cata1yst
Species detected
Detection Barrier technique (eV)
AnquRef. lar dep.
A
K
1.0f0.2
T
A
K2*
EB FI
1.7f0.3
4
B B B B
K
EB
3.1f0.2
5
K2*
FI SI
0.8420.07
VFI
1.0;1.5
all K*
NL NL NP
4
5, P 11, P 11
EB: alactron impact, FI: field lonlritlon, 91: aurface ionization, VFI: ionication in I: tharmil, NL: lobe in normal dlrectlon, NP: narrow paakm, P: praaant atudy
2*
-K
A
I I1
m
Surface
vary weak flalda,
K
I
Fig. 1. Energy diagram for the potassium promoter in the iron oxide catalyst material.
-
798
0
m/z 39
1200
Fig. 2. Angular distribution for R at m/z 39 at 7 4 3 K, from a sample of catalyst type A . The mass spectrometer signal was obtained with electron impact ionization. 150°
3. EXPERIMENTAL The apparatus used is shown schematically in Fig. 3 . It is a UHV vacuum chamber with a differentially pumped potassium molecular beam source, with two pump stages. The extruded commercial styrene catalyst samples are received in the form of small cylinders, with a length of about 6.5 mm and a diameter of 3.2 mm. Each sample is cut to expose a flat surface to the beam. It is mounted in a tube made from Ta foil, with an opening large enough to expose the flat part of the catalyst sample. A s shown in Fig. 3, the exposed angle in the detector plane is approximately 90°. The holder is heated by an AC current through it, at about 30 A. The sample is usually at ground potential. The catalyst is a commercial type of potas-
Sample
-
100 mrn
Y
SI
Y
lo-' mbEir
Fig. 3. Schematic drawing of apparatus.
799 sium promoted iron oxide for the styrene process. Catalyst B is the better one of the two catalysts studied in Ref. [5]. The molecular beam of potassium is a thermal one (effusive) of very low density. It is directed towards the catalyst surface at an angle of -45O towards the surface normal. The beam flux is turned off by closing a slide valve. The surface ionization detector can be rotated 360° around the sample. In a certain range it blocks the molecular beam. Measurements with the beam on can not thus be obtained in that angular range. The detector consists of a hot Re ribbon with exposed dimensions 0.7~10 mmz, which ionizes the impinging flux of potassium from the sample. The ribbon temperature is approximately 1450 K, and it is held at a voltage of 150 V. The ions formed on the ribbon surface are accelerated from the ribbon to the concentric collector at ground, where the ion current is measured.
4. RESULTS AND DISCUSSION The angular variation of the emitted potassium flux is studied as a function of the sample temperature and the electric field strength outside the surface. With no impinging K beam, the emission is slightly non-cosine, i.e. it is not purely thermal. A s seen in Fig. 4, the peak is flat-topped, having too large vlshouldersvl at f20-25O. This should be compared with the mass spectrometric results in Fig. 2, where the distribution has no such feature. Since in that experiment the detector had almost no sensitivity to clusters and detected only K atoms, the difference in the results may be due to loss of clusters of K from the sample.
-60"
60"
Fig. 4. Angular variation of K (all states) emission at 990 K, measured with surface ionization from a sample of catalyst type B.
0
In experiments with a K beam impinging on the sample surface, several different types of results are found depending on the
state of the catalyst (e.g. time of use in the experiments), the temperature and voltage of the sample, and the intensity of the beam. Also surface scattering phenomena are observed, but they fall outside the present theme, and will be described elsewhere. Instead, we will focus on the cases where a lobe in the normal direction is obtained. One example is shown in Fig. 5, with quite low sample temperature which gives a low direct emission from the sample. The full angular range is not covered in this figure, since the detector blocks the impinging beam, but the distribution should be symmetrical around the normal direction. Thus, a very strong peak in the normal direction is obtained, which indicates that some special processes act in the desorption. There exist a few possibilities:
30"
6 . 2 5 ~O1 -'. A/cm2
60"
\
Fig. 5. Angular d i s tribution of emission with a K beam and the sample at 80 V, measured with surface ionization from catalyst type B at 790 K. The inner curve is the signal with the beam o f f .
e
1) Emission from the bulk with kinetic energy in the normal direction could be a possible mechanism. However, the surface is only flat on a macroscopic scale, and it is quite porous and irregular on a microscopic scale. The surface experienced by the emitted atoms will thus be very rough, and energetic emission will thus give a broad distribution. 2) Emission of highly excited Rydberg atoms of K* or K2* is another possibility. Since the potential of interaction with the surface is very long range, the roughness of the surface will not influence the direction of emission very much. Thus, one can expect such a distribution for the excited states. In fact, similar distributions haveZPeen obtained in the experiments with field ionization of K presented in part in Refs. [5,12]. One example of a sharp distribution in the normal direction is shown in Fig. 6.
801
\
I
\
/~
-30" \ \
-60",
30" 3
1
/
\
/
-
0
\
60"
Fig. 6. A ular distribution of K" from catalyst type A measured by field ionization, at a temperature of 870 K.
c 4
3) Emission of clusters Kn. Such clusters are probably formed oVtsi.de the surface by condensation of excited Rydberg states K . The driving force for the condensation is,the huge interaction force between pairs of excited atoms K [13], and between the excited states and ground state atoms and molecules. The emitted atoms and excited states should have a cosine distribution, as expected from basic theory. The combination of many atoms into clusters will however average out the velocities parallel to the surface, and give the clusters a common direction close to the normal out from the surface. Clusters Kn will thus have the observed sharp angular distribution. Since we do not know if the states K2* are formed in the experiments from dissociation of clusters at some distance from the surface, due to finite life-time effects of the probably electronically excited clusters, we are not sure that the two last explanations above are independent. In fact, we may observe the common factor of a sharp angular distribution of the clusters formed outside the sample surface in both types of experiments. Thus, it is very likely that clusters are formed and contribute to the loss of K from the catalyst in the experiments. Some direct evidence has also been found for the interaction of alkali atoms on the surface. In Fig. 7, the signal (in this case in fact the ion signal) without a K beam is a few lobes and peaks, as commonly found for the ion emission [ll]. When the thermal K beam is allowed to strike the sample, the angular distribution changes strongly, and the lobe at 14O is removed completely. Instead, the flux from the surface is found in some type of flat-topped lobe at 6 0 ° . This means that the atoms from the beam, despite the extremely low density used, combines with the K emitted from the sample, removes it and
802 30" Fig. 7. Angular
60~ distribution of K
emission with a K beam and the sample at 80 V, measured with direct ion collection from catalyst type B at 970 K. The inner curve is the signal with the beam off.
1 . 2 5 ~O-7. 1 A/cm
\
*
0
forces it to leave the surface in another direction. This implies strong i2teraction forces, as can only be expected for excited states K , and probably also loss as clusters, in this special case as cluster ions.
5. ALKALI LOSS PROCESSES One must also inquire into the relevance of the present results for the loss of alkali promoter in real catalytic reactions. First of all, the conditions outside the surface at high surrounding gas pressure are of course very different from the ultra high vacuum experiments which we have performed. However, the mechanisms which give formation of excited states are related only to the actual desorption and emission processes of alkali, which should be quite independent of the coverage on the surface of other molecules. Thus, we expect the formation of excited states to be similar also at high pressure. However, the gas present on the surface and in the gas phase outside the surface will strongly influence the life-time of the excited states. Specifically, the surrounding gas will decrease the rate of cluster formation very much, by quenching and reacting with the excited states before they can combine to form clusters. The loss of alkali promoter from the styrene catalyst samples in our experiments seems to be much faster than from the catalyst samples used in the real styrene process. This may not be strictly correct since only desorption loss from the catalyst is considered here. The solid-state transformations under real process conditions are determined both by the temperature and also by the presence of superheated steam and the product hydrogen. According to Muhler et al. [7], the active form of the catalyst is a dynamic equilibrium between KFe02 phases. The attack of product hydrogen (from dehydrogena and K2Fe22034 ion of ethyl benzene) on these active phases reduces them to KOH
I
~
KFe02
!
'
K + /l V
j
A I
+K+
K5e22034
1
"' 1
J ACTIVE STATE
,",","
> I
3 4
DEACTIVATION
~
Fe304
1 CORE. ~~
> I I-
~~
~~~
+ segregated
promoter phases -
.
1 SHELL.
Fe304
~~ ~
~
-~
1 I
~~
SPATIAL DISINTEGRATION
Fig. 8 . The active phase, its deactivation, and the spatial disintegration of styrene catalyst without any promoter additives, according to Muhler et al. [7]. and Fe304, and causes phase segregations in a K-rich core and a K-poor shell within each catalyst extrudate (Fig. 8 ) . These phases are catalytically inactive and nonselective, and also mechanically weak. The ease of thermal desorption of potassium from the catalyst and its mobility under the styrene-process conditions are thus crucial for both the macro-scale migration and loss of K in the direction of process flow in the catalyst bed, and for the micro-scale migration of K from the outside to the core of every catalyst particle or extrudate. Hence we are extending our studies to follow the influence of the ambient pressure and of the nature of the gas (steam, H ) on the characteristics of potassium emission from the catafyst (subject to the limitations of the UHV system). The alkali loss which takes place could be related to loss as ground state atoms K, but several other possitilitie3,do exist. The detecand as clusters Kn tion of loss as excited states K and K shows that the behaviour of alkali loss from the catalyst is much more complicated than hitherto realized. The operational stability of the catalyst under styrene process conditions can perhaps be enhanced by the addition of other oxides (e.g oxides of Car La, Ce, V, W) as promoters, which can play a double role: a) retard the undesired solid state transformations [7], shown in Fig. 8 , and b) raise the temperature threshold for the thermal desorption of K from the catalyst [ 5 ] . Our comparative study [5] of two commercial styrene catalyst showed that the better catalyst had indeed a much higher temperajjyre threshold for the desorption of K both as K atoms and as K excited species. In this respect, the present molecular beam study is proving to be a valuable fingerprint and possibly even an accelerated life test for the styrene catalyst.
804
ACKNOWLEDGEMENTS We thank Jorgen Lundin for taking part in preliminary experiments. This project was supported by the National Swedish Board for Technical Development (STU).
REFERENCES 1 W.-D.
Mross, Catal. Rev.
-
Sci. Eng. 25 (1983) 637.
2 F.M. Hoffmann and M.D. Wiesel, Abstract 85, ECOSS-12 (european Conf. on Surface Science), Stockholm, 1991. T o appear in Surface Science. 3 J.B.C. Pettersson, L. Holmlid and K. Moller, Appl. Surface Sci. 40 (1989) 151.
4 J. Lundin, K. Engvall, L. Holmlid and P.G. Menon, Catal. Lett. 6 (1990) 85. 5 K. Engvall, L. Holmlid and P.G. Menon, Appl. Catal. 77 (1991) 235. 6 M. Muhler, R. Schlogl, A. Reller and G. Ertl, Catal. Lett. 2
(1989) 201. 7 M. Muhler, J. Schutze, M. Wesemann, T. Rayment, A. Dent, R. Schlogl and G. Ertl, J. Catal. 126 (1990) 339 and forthcoming publications. 8 D. E. Stobbe, F. R. van Buren, A. V. Stobbe-Kreemers, J. A. Schokker, A. J. van Dillen and J. W. Geus, J. Chem. SOC. Faraday Trans. 87 (1991) 1623, 1631, 1639 and forthcoming publications.
9 D. E. Stobbe, On the Development of Supported Dehydrogenation Catalysts Based on Iron oxide, Ph.D. Thesis, University of Utrecht, 1990. 10 K. Engvall, L. Holmlid, H. Prinz and H. Hofmann, Catal. Letters 11 (1991) 41. 11 C. h a n and L. Holmlid, to be published.
12 K. Engvall and L. Holmlid, Appl. Surface Sci., accepted. 13 C. h a n , J.B.C. (1990) 189.
Pettersson and L. Holmlid, Chem. Phys. 147
DISCUSSION
Q: G. L. Haller (USA)
1) Since you are detecting neutral and or excited K but potassium exists as KO) in the catalyst, can you tell us what is being oxidized in the catalyst and how its composition evolves in time 7 2) I realize that practical packed bed reactors would have complications of K readsorption, excited state quenching, etc., but can you make an order of magnitude estimate of
the predicted loss of K from an operating catalyst reactor to determine if the mechanisms you are investigating apply to commercial reactors ? A: L. Holmlid 1) The most likely mechanism for emission of the Rydberg states is that alkali ions diffuse through the uppermost surface layer, and that excited states are formed by recombination with thermal electrons (maybe even some kind of "convey" electrons) on the way out from the surface. Since neutral atoms are emitted, the sample does not even have to be a good conductor. There are several different possibilities for the state of the alkali in the catalyst: i) it is already in a "zero valent" state due to loss of oxygen from the catalyst, b) it is bound to O(II), and gives a free valence in the iron oxide, which is part of the destruction of the oxide structure, c) it is bound in K 0 leaving KO behind, which may react with H on the surface to form the final inactive formkOH. Since the material is a good conductor, there is always a good supply of electrons. 2) We have discussed and evaluated the efficiency of the loss processes in ref. 4. The rates of loss we observe are much larger than required to explain the loss in the real process, so there is certainly quenching processes etc. taking place. The quenching rates for the excited states are not known yet, so the estimate required is not easy to calculate, but the Rydberg research field is active so there is hope in a few years' time.
Q: K. Kochloefl (Germany) 1) As is well known, K migrates during the catalyst activation and during ethylbenzene dehydrogenation within the catalyst particles (e.g. extrusions). Did you measure K-losses in the case of activated and spent catalyst as well'? 2) According to my experience, K-losses depend first of all on reaction temperature and partial pressure of ethylbenzene in the steam. A: P. G. Menon 1) During the styrene process, K migrates not only on the microscale of the catalyst extrudate from its exterior to the core, but also on the macroscale of the reactor or catalyst bed in the direction of process flow. This was indeed the motivation for us to undertake this study using the molecular-beam technique. A comparative study of K loss from fresh and used catalysts is of course essential and this is in progress in our laboratory right now.
A: L. Holmlid 2) We agree to this remark. The loss as Rydberg states has activation energies of 0.8-1.7 eV, which means that the rate of loss increases with temperature. The dependence on ethyl benzene partial pressure (decrease of loss rate with increasing partial pressure) may mean that this molecule acts as a very efficient quencher (or even reaction partner) for the Rydberg states, which is expected.
Q: N. Pernicone (Italy) You know that basically two types of industrial styrene catalysts are currently used, one type is calcined at very high temperatures and contains K ferrites as the main phases. The other type is calcined at lower tern eratures, so that no bulk interaction occurs between Fe and K oxides, and contains a-Fe2 as the main phase. To which type do your A and B catalysts belong '? Moreover, what is your opinion about possible differences in the alkali loss mechanism between the two types '?
8
A: P. G. Menon In reply to Dr. Pernicone, let me point out that the active phase of the styrene catalyst is dynamic equilibrium between KFe02 and K 2 F e z 0 4, as has been shown by recent work of Hirano in Japan, Ertl and coworkers at Berlin, and Geus and coworkers at Utrecht. This equilibrium is established only under the actual process conditions, and not by the initial
calcination. The catalysts A and B, used in our work, are real commercial samples. In a comparative study of K emission from these two catalysts, we have shown recently that the better catalyst B has indeed a higher temperature threshold for the emission of K species (ref. 5).
Q: A. Baranski (Poland) The system and the technique that the paper has dealt with are suitable for checking the famous idea concerning "binding the pressure gap between UHV data and industrial practice". Let me remind you that a few years ago in Topsoe Laboratories the industrial yield of ammonia synthesis was successfully calculated [ l , 21 from UHV data published by Ertl. In this paper, similarly, UHV data were used to determine the loss of potassium. Once again, pressure is the most essential difference between laboratory and real conditions. The question is: would it be possible to develop a theory (and an algorithm) enabling one to introduce a pressure correction to laboratory results ? P. Stolze, I. K. Norskov, Phys. Rev. Left.,55, 2502 (1985) [l] P. Stolze, I. K. Norskov,J. Catal., 110, l(1988) [2]
A: P. G. Menon The idea of "bridging the pressure gap" is certainly in our minds too. We are already using industrial styrene catalysts directly in an UHV equipment. However, the styrene catalyst undergoes radical solid-state transformations under the actual process conditions with ethyl benzene and steam above 873 K. Simulating such conditions in UHV will not be so easy as was the case with ammonia synthesis. Hence we are concentrating more on comparative studies of K emissions from different commercial styrene catalysts of known process performance as also from fresh and used calalysts of the same type. It is too early to make extrapolations. Still, please see my reply to Dr. Pernicone.
Q: D. G. Blackmond (USA) What is the initial chemical state of the alkali species on the catalyst surface ? Compounds such as K 2 0 or KOH tend to be much more stable against loss of potassium component to zero-valent potassium on single metal crystals. A: L. Holmlid The various states of the alkali in the catalyst are described by our energy diagram in Figure 1 and the transformations of the catalyst in Figure 8. These figures are consistent with the recent model for the solid state chemistry of the catalyst by Muhler et al.(ref. 7). We imply that the excited states are not formed on the surface, but during emission from the bulk, as shown in that figure. The comparison with a metal surface in any form is not easy to do, since metal surfaces in general d o not emit Rydberg states.
Q: C. Cameron (France) From an inorganic chemistry point of view, it is difficult to believe that potassium metal could be lost from an oxide catalyst composed of potassium and iron oxides. The potential for converting K+ 4 KO is very high, and I am sure higher than that required for the reduction of iron oxide to a sub-oxide. (Your energy potential diagram is not convincing). 1) Is it possible that the KO that you detect is due to the analytical technique employed '? 2) If you really believe that K+ is reduced to KO, where does the electron come from (i.e. e - + K + 4 KO)?
A: L. Holmlid The neutral atoms are the result of the dissociation of even strongly ionic species like alkali salts is generally correct and observed experimentally in many cases. This is due to the avoided crossing of the ionic and covalent energy curves (surfaces), which means that an adiabatic (e.g. thermal) dissociation of the ionically bonded molecule gives neutral products. Further, an energy consideration shows that the ion pair K+ and M' is at a higher energy (and
807 thus the less probable state) than K and M: the ionization energy of K is 4.3 eV, while the electron affinity of M is considerably lower (1.5 eV for 0).Only in the case of a conductor with a surface work function comparable to or higher than 4.3 eV will a considerable fraction of the desorbing K be emitted through a so called surface ionization process as K+ ions. We have looked for ions from such a process with the present catalysts and at the temperatures studied, and we can not find any. So,the answer to your questions are as follows: 1) No, the emission of neutral K atoms is not due to the technique we use, but is expected as explained above. 2) The electron mentioned goes with the K atom: during dissociation of an ionic salt the two fragments respectively oxidize and reduce each other, as described above (due to the avoided crossing of the potential energy curves for the two states of the system).
Q: K. Aika (Japan) Have you measured oxygen element free from the catalyst'?During the loss of potassium are you leaving oxygen (counter element of potassium) on the catalyst ? A: L. Holmlid and P. G. Menon Oxygen is left on the catalyst sample, but part of it is also lost by desorption from the surface. In ref. 4 we show mass spectra with both m/e 16 and 32 lost from the catalyst. Recent AES studies in our group show considerable oxygen loss from the sample in the industrial process. Please remember here that, in the industrial process, it is the product hydrogen formed in the ethyl benzene dehydrogenation which starts reducing the catalyst, thereby causing the alkali migration and the deactivation and mechanical destruction of the catalyst, as shown in Fig. 8.
Q: G. B. Fisher (USA) la) Since this is a commercial catalyst, how is it that for single excited ion emission that loses of emission can be seen from presumably randomly oriented particles ?' lb) If for the emission of clusters of potassium in the normal direction you invoke a long range interaction, how long range does that need to be and does it vary with cluster size or excited K species type ): 2) Does the amount of K you see desorbing depend on temperature '? FS it related K diffusion energies or something else '? A: L. Holmlid la) This point has been specially investigated, and it is treated in detail in two manuscripts submitted recently. In short, the very long-range potential interaction between the surface of the catalyst and the Rydberg states means that the desorption barrier is at such a long distance from the catalyst surface that the surface structure becomes unimportant. lb) The form of the interaction with the surface is not important for the cluster formation. Any surface will give a cosine distribution for thermally desorbed species. The agglomeration of such species to clusters will give a lobe in the macroscopic normal direction, which is the average direction of the motion of the species from the surface, independent of the form of the interaction. 2) The Rydberg state signal depends on temperature: activation energies of 0.8-1.7 eV are given in Table 1 in the paper and have been published before. The relation to the energetics is shown in Figure 1.
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Guczi, L et al. (Editon), New Frontiers in Catalysis P m d i n g s of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 6 1993 Elsevier Science Publishers B.V. All rights reserved
THE RELATION BETWEEN CATALYTIC AND ELECfRONIC PROPERTIES OF SUPPORTED PLATINUM CATALYSTS: THE LOCAL DENSITY OF STATES AS DETERMINED BY X-RAY ABSORITION SPECTROSCOPY M. Vaarkampa, J. T.Millerb, F. S. Modicab, G. S. Laneb and D. C. KoningsbergerC aSchuit Catalysis Institute, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands bAmmOil Research Center, P.O.Box 3011, Naperville, IL 60566, USA Institute, Laboratory of Inorganic Chemistry and Catalysis, University of Utrecht, P.O.Box 80083,3508 TB Utrecht, The Netherlands
Abstract The intensity of the white line of the L,, and L,,, X-ray absorption edge spectra of small platinum particles increases with decreasing particle size. The combined white line intensity of the L,, and L,,, X-ray absorption spectra for platinum catalysts with comparable average particle size is higher when dispersed on acidic than on neutral supports. This indicates that platinum particles are more electron deficient on acidic than on neutral supports. The propane hydrogenolysis TOF for platinum supported on y-Al,O, or H-LTLis found to be more than a n order of magnitude higher than for platinum supported on a non-acidic K-LTL zeolite. The differences in catalytic behavior are related to differences in the d-band density of states. 1. INTRODUCTION
Electronic properties of highly dispersed metal particles are thought to be affected by support and/or promoter ions [l]. Charge transfer or polarization at the metal-support or metal-promoter interface may change the d-band density of states leading to different catalytic properties. Information about the d-band density of states in transition metal clusters can be obtained from the white line intensities of the L,,(transition from 2p,, to 5d,) and L,,, (transition from 2p,, to 5d,, and 5d,) X-ray absorption edges. A basic theory of white lines has been given by Mott [23, Brown et al. [31 and Mattheiss and Dietz 141. The theoretical calculations of the unoccupied Pt d-states show that the J=5/2 final state is predominant. Brown et al. [31 showed that the J=5/2 states contribute about 14 times more to the final d-states than the J=3/2 states. Mattheiss and Dietz [4] calculated that the ratio of the unoccupied states (h,,&,,)
81 0
ranges from 3.5 within 0.5 eV of the Fermi level to 2.9 over the entire unoccupied conduction band. These calculations explain why the intensity of the white line of the L,,, X-ray absorption edge is much higher than the intensity of the L,, edge in bulk platinum. Several authors have used the L,,, X-ray absorption edge spectra to characterize the chemical state of the absorbing atom in transition metal compounds. Lytle et al. [5,6] have shown for iridium, platinum, and gold th a t the intensity of the white line of the L,,, X-ray absorption edge is proportional to the d-electronvacancies. Also, changes in the L,,, white line intensity of the X-ray absorption spectrum for Pt/SiO, have been related to alterations in the d-band density of states resulting from a metal-support interaction a t high reduction temperatures [71. Gallezot e t al. [8,9] reported that platinum clusters in Ce-promoted NaY zeolite have a larger number of d-holes than platinum in NaY zeolite. While the support and promoter ions do affect the white line intensity, the metal particle size is also important. Gallezot et al. [8,91 reported th a t the white line intensity of the L,,, edge of 1-nm Pt clusters in PtlNaY is larger th a n 3-nm clusters. Mansour et al. [lo] carried out a more quantitative study including the white line of the L,, absorption edge for PtJSiO, with a n average particle size of 15 8, (72 atoms) and determined that there were 14% more d-holes than in bulk platinum metal. For a series ofPt/SiO, catalysts with increasing particle size (with first shell EXAFS coordination numbers of N=6.6 and larger), the combined white line intensity of the L,, and L,,, edge increased a s the platinum particle size decreased [ l l l . Here we present the results of a study of the white lines of the L,, and L,,, X-ray absorption edges for a series of platinum catalysts supported on y-Al,O,, H-LTL and K-LTL zeolites. It will be shown th a t also for very small platinum clusters (N=3.9 to 4.9) in the P a - L T L catalyst, the white lines of both the L,, and L,,, edges are strongly influenced by particle size. After accounting for the particle size, platinum on acidic supports (y-Al,O, and H-LTL) is more electron deficient than on non-acidic supports (K-LTL). In addition, propane hydrogenolysis h a s been used a s a catalytic reaction to investigate the influence of the changes in the d-band density of states due to support and/or promoter ions on the catalytic properties of the platinum particles. 2. EXPERIMENTAL 2.1. Catalyst Preparation A 1.0 wt% Pt/y-Al,O, catalyst (200 m2/g, 0.6 cm3/g) was prepared by impregnation with an aqueous solution of H,PtCl,. The catalyst was dried in air a t 120°C, and reduced (heating rate 5"C/min) a t 450°C for 4 hr, followed by passivation in air a t RT. K-LTL was obtained from Linde. Excess alkali was reduced by water wash until the pH of the wash solution was 9.5 to give a WA1 molar ratio of 1.05. H-LTL (WAl molar ratio of 0.34) was prepared by repeated NH,NO, exchange of K-LTL followed by calcination a t 500°C. The Pt was loaded (1.2 wt% Pt/K-LTL and 1.0 wt% Pt/H-LTL) by impregnation using tetraammine platinum (11) nitrate followed by drying at 120°C.
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2.2. Propane Hydrogenolysis Reactions The conversion of propane was conducted a t 400°C and atmospheric pressure in a fixed-bed, bench-scale reactor using 3.78 vol% propane in H, , The catalysts were prereduced at 450°C or 600"C, and the conversion was adjusted to between 2 to 10% by changing the propane space velocity. Turnover frequency (TOF) was determined using hydrogen chemisorption as a measure of the active platinum surface. 2.3. X-ray Absorption Experiments X-ray Absorption experiments were carried out a t the Synchrotron Radiation Source in Daresbury, U.K., Wiggler Station 9.2, using a Si (220) double crystal monochromator. At the Pt LllIedge (11564 eV), the estimated resolution was 3 eV. The monochromator was detuned to 50% intensity to avoid the effects of higher harmonics present in the X-ray beam. The measurements were done in the transmission mode. In order to obtain a n absolute energy calibration of the data a third ionchamber was used with a platinum metal foil (thickness 4 pm) placed between the 2nd and the 3rd ionchamber. The data of the foil were used to calibrate the energy axis of the x-ray absorption data of the catalysts. Selfsupporting wafers were reduced in a controlled-atmosphere cell [121. The X-ray absorption data were obtained a t liquid nitrogen temperature in the presence of H2.
The Ptly-Al,O, samples were rereduced a t 300 and 450°C and are designated as Al(300) and A1(450),respectively. Similarly, PtK-LTL samples were reduced at 300 or 450°C and are designated as K-LTL(3OO) and K-LTL(450). K-LTL(GOO), which had been prereduced a t 600°C, was rereduced in the cell at 450°C (the temperature limit of the EXAFS cell). PW-LTL was reduced at 450"C, and is designated H-LTL(450).
3. RESULTS 3.1. Structural Characterization Detailed structural characterizations of the catalysts by TPR, H, TPD, hydrogen chemisorption, and EXAFS are reported elsewhere for Pt/y-Al,O, [131, P W - L T L [14] and P a - L T L [ E l . The EXAFS first-shell coordination numbers, which are important for a n evaluation of the white line results are given in the Table 1. 3.2. Propane Hydrogenolysis For each of the catalysts, hydrogenolysis of propane yielded methane and ethane in equal molar amounts. The catalytic turnover frequencies are given in the Table. For the P a - L T L and Ptly-Al,O, catalysts, there was little deactivation. The Pt'H-LTL catalyst, however, deactivated more rapidly, and conversion is reported at 10 minutes on stream and extrapolated to zero time on stream. The catalytic activities fall into two groups, predominantly determined by the
81 2 Table 1
E M S coordination numbers and propane hydrogenolysis activity Samples
Coordination number
K-LTL(300 K-LTL(45O) K-LTL(600 H-LTL(450)
3.9 4.4 4.9 5.4
AL(300) AL(450
4.7 5.6
(')
(b)
Hydrogenolysis TOF'"'
0.063 0.022 0.53"' 0.40'" 0.62
400°C and 1 atm, TOF (molecules/s/surface Pt). Initial TOF extrapolated to zero time on stream. TOF at 10 min on stream.
acidity of the support. The TOFs for platinum on acidic supports, e.g. H-LTL(450) and A1(450), were more than a n order of magnitude higher than on the non-acidic support, e.g. K-LTL(450) and K-LTL(6OO). Of the acidic supports, platinum on chlorided alumina was more active.
3.3. White Line Spectra of Pt bIand L,,,X-ray Absorption Edges A preedge background was subtracted over the entire range of data for both the L,, and L,,, edges to remove the contribution of all other absorbers to the X-ray absorption spectrum [lo]. This isolates the partial cross section being studied. To ensure t h a t the X-ray absorption edge being studied was well separated from the background, the region used for fitting the preedge was extended from 200 to 80 eV below the X-ray absorption edge. The position of the absorption edge (Fermi energy) was defined as the inflection point of the spectrum. The energy scale for the absorption data was defined as the energy above this inflection point (E-Eedge). In order to obtain an atomic cross section, the data were normalized by dividing the absorption intensity by the step height of the absorption edge. To do this, the fit of the preedge was extrapolated into the post-edge region. The postedge (EXAFS)region was smoothed by a cubic spline fit of the background and the step height was defined as the intensity difference between this background in the postedge region and the extrapolated preedge, determined at the normalization energy. The L,,edge was normalized a t 58 eV and the L,,, edge at 50 eV. This procedure guarantees a systematic approach with a normalization which is independent of the shape of the white line [81.
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-
4 1
e
1.2 y
0.8
0.4
P o
-30
-20
-10
0
10
20
30
Figure 1. (a) L,,, en (b) L,, x-ray absorption edge of Pt-foil (---*j,K-LTL(3OO) (- - -4 and K-LTL(6OO)(- 4. (-)K-LTL(450) Figure 1 shows the X-ray absorption data of the L,,, (Figure la) and the L,, (Figure l b ) edge of platinum foil, K-LTL(SOO), K-LTL(450), and K-LTL(6OO).The white line intensities of both the L,, and the L,,, edge decrease with increasing platinum particle size (or increasing reduction temperature). Since the white line intensity is particle size dependent, the influence of the interaction of the platinum with the support and/or promoter ions on the white line intensity can be properly evaluated only when the particle size is taken into account. For example, despite the larger particle size for Al(300) (N=4.7), the white line intensity of both the L,, and the L,,, edge is higher than for K-LTL(450) (N=4.4) (Figure 2). This indicates that the platinum in Al(300) is more electron deficient than in K-LTL(450).
f ::: 1 0'4
P o
-30
-20
-10
0
10
20
30
L
Figure 2. (a) L,,,en (b) L,, x-ray absorption edge of Al(300) (-4en K-LTL(450) (- - - 4.
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The X-ray absorption spectra for Al(4501, H-LTL(450) and K-LTL(6OO) are shown in Figure 3. Since the Al(450) and H-LTL(450) have about the same particle size, the higher white line intensity of the Al(450) indicates greater electron deficiency than the H-LTL(450). On the other hand, in spite of the smaller platinum particle size in K-LTL(GOO),the white line intensity is still slightly larger then for H-LTL(450). Thus, after accounting for the effect of particle size, H-LTL(450) is shown to be more electron deficient than K-LTL(600).
30
-20
10
0
10
20
30
30
20
Figure 3. (a) L,,, en (b ) L,, x-ray absorption edge of Pt-foil (- -), H-LTL(450) (- - - -) and Al(450) (-).
10
(-*-*-),
0
10
20
30
K-LTL(6OO)
4. DISCUSSION
As pointed out by Pease [161 and Mansour [lo], the white line can be described as a superposition of a n arctangent function (representing the atomic cross section for X-ray absorption) and a Lorentzian distribution function (representing the 2p+5d electronic transitions). The absorption edge can be lowered below the Fermi energy, particularly if a white line is broadened by the resolution function of the spectrometer or due to lifetime effects of the excited states. The amount of lowering of the absorption edge increases with these broadening effects and the strength of the white line. As shown in Figure 1, the white lines of both the L,, and L,,, edges a re strongly dependent on the particle size. The width and the intensity (determined by the area) of both white lines increases with decreasing particle size. Since the degeneracy for the 2 ~ , ,state ~ is twice th a t for the 2 ~ , , ~ state, the intensity of the L,, edge should be counted twice in determining the total d-hole density. Comparison of Figure l a with l b clearly shows th a t the intensity and shape of the L,, edge is more strongly influenced by the particle size than the intensity of the L,,, edge, indicating th at the final states with J=3/2 are more sensitive to the particle size than the 5 5 1 2 states. Also apparent in Figure l a and l b, is t ha t the onset of the L,, and L,,, absorption edge of both catalysts originates at higher energy than for bulk platinum metal indicating t h a t the small platinum particles are electron deficient compared to bulk platinum metal.
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These results on Pt/K-LTL confirm the earlier results for PdSiO, t h a t the white line intensity increases with decreasing particle size [ll].This may lead to the suggestion that small transition metal particles are more electron deficient than bulk metal. However, Hartree-Fock-SlaterLCAO calculations on small copper particles [171 have clearly demonstrated that the Cu-Cu distance becomes shorter than in bulk metal. This is due to a decreased degree of delocalization (more electron density between the metal atoms) since metal atoms in small metal particles have, on average, fewer neighboring atoms than in bulk metal. An increase in the d-band density of states of surface atoms due to the same effect has been discussed by Cyrot-Lackman et al. [18] and Saillard et al. 1191. The decreased d-valence electron bandwidth for surface atoms leads to a n increase of the d-valence electron band filling. These studies predict that small transition metal particles have a higher electron density than bulk metal, which is in contradiction with the interpretation above for the results of the white line. The apparent contradiction has been solved by a recent theoretical study of Ravenek et al. on Ir, and Ir,, clusters [203. Hartree-Fock-Slater LCAO calculations, indeed, indicate a higher number of d-electrons for the Ir., cluster in comparison to Ir,, . However, introduction of a core hole, to mimic the X-ray absorption process, into the calculations results in a lower number of d-electrons for the Irq+compared to the IT1< cluster. The higher electron deficiency for the smaller cluster, as suggested by the larger white line intensity, for example, is actually due to a less efficient screening of the core hole induced by the absorption of the X-ray photon in smaller clusters. The electron density of the neutral metal particle, however, is higher for the smaller particle in contrast to the white line results. In order to determine the influence of the support and/or promoter ions on the d-band density of states, one must first account for the contribution to the white line resulting from differences in particle size. In Figure 2 the white lines are compared for Al(300) and K-LTL(4501, i.e., platinum dispersed on a n acidic and non-acidic support. In addition to the differences j n the support composition, the platinum particles are larger on the alumina catalyst. Inspection of Figure 2 clearly indicates that the d-band density of states does depend on the type of support. If the platinum particles had been of identical size, the differences in the white line intensity would have been even larger. Nevertheless, it can be concluded that the platinum particles supported on y-Al,O, are more electron deficient than those dispersed in K-LTL zeolite. For this comparison, however, it is not possible to separate the influence of the potassium ion, i.e., a base promoter, from the effect of the support composition on the d-band density of states. In Figure 3, the L,, and L,,, white line intensities are shown for Al(4501, H-LTL(450) and K-LTL(6OO). The total white line intensity (L,, plus L,,,) of the acidic A1(450), coordination number 5.6, is higher than for H-LTL(4501, coordination number 5.4. Although both supports are acidic, the platinum in the Al(450) is clearly more electron deficient than in H-LTL(450). Without accounting for the particle size, the white line intensity for H-LTL(450) and K-LTL(6OO) would suggest a slightly larger or similar electron deficiency. However, the platinum particles are smaller in K-LTL(6OO)than in H-LTL(4501, N = 4.9 and 5.4, respectively. Since the white line intensity decreases with increasing particle size,
81 6
a H-LTL(450) catalyst with N=5.4 would have a lower white line intensity than K-LTL(6OO).Therefore, after accounting for particle size effects, the platinum in H-LTL(450) is seen to be more electron deficient than K-LTL(6OO). The propane hydrogenolysis TOF was determined for K-LTL(450),K-LTL(GOO), H-LTL(450) and Al(450) and increased in the order K-LTL < H-LTL < Al. As previously observed for hydrogenolysis of neopentane 121,221and ethane [231, the propane TOF is higher for platinum on acidic supports, i.e., y-A1203 and H-LTL. In each of the previous studies, the increase in activity has been attributed to electron-deficient platinum. The electron deficient nature of the platinum was thought to be an intrinsic property of the small metal particles [23] or the result of donation of platinum electron density t o the support [21,22]. In a recent study, Samant and Boudart investigated the electron deficiency of platinum in a series of PtiY catalysts of similar platinum particle size by several techniques, including XANES [24]. They concluded that all of the observations ascribed to electron deficiency were the result of the intrinsic properties of the very small platinum particles which form on acidic supports. If the electron deficiency is only the result of the small particle size, then the smallest platinum particles would have the highest hydrogenolysis activity. In this study, the most active catalysts for hydrogenolysis of propane, e.g., the acidic supported catalysts, have the largest particle size. Furthermore, after accounting for the differences in particle size, we determine that the platinum on the acidic supports is more electron deficient as judged by the combined white line intensity. We conclude, therefore, that the support acidity andor promoter ions do affect the d-band density of states, and that these perturbations in the electronic structure affect the catalytic activity, at least, for hydrogenolysis. In [13] we show that the nature of the metal-support interface in Pt/K-LTL changes with increasing reduction temperature. Table 1 shows that the propane hydrogenolysis activity decreases by a factor 3 by changing the reduction temperature from 450'C at 600°C. A more systematic study of the white line intensity is needed before the influence of the nature of the metal-support interface can be distinguished from that of promotor ions. The presence of chemisorbed hydrogen also influences the intensity of the white line [71 and this influence should be evaluated as well. 6. CONCLUSIONS
The white line intensity of both the platinum L,, and L,,, X-ray absorption edges increase with decreasing particle size. The changes in the intensity are much larger for the L,, edge. This means that the intensity of the L,, edge whiteline must be taken into account before any conclusion can be drawn about changes in d-band density of states. After correcting for the differences in particle size, platinum on acidic supports is more electron deficient than on non-acidic supports. The electron deficiency of the platinum increased in the same order as the activity for propane hydrogenolysis. Changes in support acidity and/or promoter ions alters the platinum d-band density of states, affecting the reactivity for hydrogenolysis reactions.
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6. REFERENCES
1. M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogenous Catalytic Reactions, Princeton University Press, Princeton N.J., 1984. London, 62 (1949)416. 2. N.F.Mott, Proc. Phys. SOC. 3. M. Brown, R.E. Peierles and E.A. Stern, Phys. Rev. B, 15 (1977)738. 4. L.F. Mattheiss and R.E. Dietz, Phys. Rev. B, 22 (1980)1663. 5. F.W. Lytle, J . Catal., 43 (1976)376. 6. F.W. Lytle, P.S.P. Wei, R.B. Gregor, G.H. Via and J.H. Sinfelt, J. Chem. Phys., 70 (1979)4849. 7. F.W. Lytle, R.B. Gregor, E.C. Marques, D.R. Sandstrom, G.H. Via and J.H. Sinfelt, J. Catal., 95 (1985)546. 8. P. Gallezot, J. Datka, Y. Massardier, M. Primet and B. Imelik, Proc. 6th Int. Congress on Catal., (G.C. Bond, P.B. Wells and F.C. Tomkins Eds.), The Chemical Society, London, 1976,vol 2,p 696. 9. P. Gallezot, R. Weber, R.A. Dalla Betta and M. Boudart, Z. Naturforsch., 34a (1979)40. 10. A.N. Mansour, J.W. Cook and D.E. Sayers, J. Phys. Chem., 88 (1984)2330. 11. A.N. Mansour, D.E. Sayers, R.A. van Santen and D.C. Koningsberger, to be published. 12. F.W. Kampers, T.M.J. Maas, J . van Grondelle, P. Brinkgreve and D.C. Koningsberger, Rev. Sci. Instr., 60 (1989)2635. 13. M. Vaarkamp, J. van Grondelle, R.A. van Santen, J.T. Miller, B.L. Meyers, F.S. Modica, G.S. Lane and D.C. Koningsberger, Proc. 9th Int. Zeolite Conf., Montreal, 1992,accepted. 14. M. Vaarkamp, J.T. Miller, B.L. Meyers, F.S. Modica, G.S. Lane and D.C. Koningsberger, to be published. 15.M. Vaarkamp and D.C. Koningsberger, to be published. 16.D.M. Pease, Appl. Spec., 30 (1976)405. 17. B. Delley, D.E. Ellis, A.J. Freeman, E.J. Baerends and D. Post, Phys. Rev. B, 27 (1983)2132. 18. M.C. Desjonquhres and F. Cyrot-Lackman, J. Chem. Phys., 64 (1976)3707. 19. J.Y. Saillard and R. Hoffmann, J. Am. Chem. SOC.,106 (1984)2006. 20.W. Ravenek, A.P.J. Jansen and R.A. van Santen, J. Phys. Chem., 93 (1989) 6445. 21. R.A. Dalla Betta and M. Boudart, Proc. 5th Int. Cong. on Catal., 2 (1973)1329. 22. S.T. Homeyer, 2. Karpinski and W.M.H. Sachtler, J. Catal., 123 (1990)60. 23. C. Naccache, N. Kaufherr, M. Dufaux, J. Pandiera and B. Imelik, Mol. Sieves11, ACS Symp. Ser. 40 (1977)538. 24. M.G. Samant and M. Boudart, J . Phys. Chem., 95 (1991)4070.
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DISCUSSION
Q: T. Uematsu (Japan) Surface activity or reactivity is strongly dependent on the particle size. You have demonstrated the charge transfer and metal-support interaction by EXAFS. The interaction was controlled by the platinum particle size. However, such interaction would be also influenced by the activities of supporting oxides. In this view, the effects of particle sizes of the support must be taken into account, especially for metal clusters (superfine particles) supported on fine particle of supporting metal oxides. What is your opinion ? A: D. C. Koningsberger The total white-line intensity of supported small metal particles is in my opinion determined by metal (i) particle size/shape, (ii) type of adsorbate, (iii) type of support and (iv) structure of metal-support interface. The influence of the support is most probably due to a charge transfer from or to the metal particle. However, there is still no conclusive experimental evidence for this. 1 do not see how the particle size of the oxide support can influence the d-band density of states of the supported metal particles, as you suggest. There is may be an indirect effect, thc support particle size may influence the size/shape of the metal particles, which in turn influence the white-line intensity of the supported metal particles.
Q: G. L. Haller (USA) Because the effects you observed for both Pt LII and LIII edges as a function of support acidity and or Pt particle size are changes in line width rather than intensity at the edge, this suggests you may be measuring changes in the 5d bond shape rathcr than charge transfer. Charge transfer can be measured quantitatively by measuring shifts in the absorption edge (relations to the simultaneously measured edge of a Pt foil references). How did you look at your edgc shifts and are they consistent with the charge transfer interpretation ?
A: D.C. Koningsbcrger The shape of both the LII and the LIII white-line is indeed changing as a function of particle size and type of support. However, we have determined also the total white-linc area of both the LJI and the LIII edge and the total white-line intensity is showing the results which we discussed in the paper. We have not yet measured enough accurate edge position data to relate changes in white-line intensity to changes in edge position.
Q: V. Haensel (USA) Particle size depends to a large extent on treatment following impregnation. For exampb, with a y-alumina support the treatment with air following impregnation (at u p to 500 C) and subsequent reduction with H produces highly dispersed catalyst particles while H2 treatment only produces larger c a d y s t particles and lower catalyst activity.
A: D.C. Koningsberger I fully agree that particle size depends on the treatment following impregnation. We have found that also direct reduction in hydrogen using a very small temperature ramp (5 Wmin) leads to very stable highly active small metal particles.
Q: K. Klier (USA) Do you have data that could be quantitatively evaluated as to the amount of charge transferred upon chemisorption of "electron accepting" adsorbates, e.g. oxygen, hydrogen and chlorine'? Does the LII intensity change correlate with electronegativity of these adsorbates ?
81 9
A: D. C. Koningsberger We do not have yet enough data to correlate the electronegativity of different adsorbates with changes in white-line intensities. We d o have found that desorption of chemisorbed hydrogen leads to a decrease in white-line intensity, showing that the chemisorption of hydrogen withdraw electrons from platinum.
Q: A. Renouprez (France) The normalization, subtraction procedure is known to be a critical step in absorption edge data treatment. Did you try to interpret your results using the L - k x Ln method or LnI(foil)-L I(sample)? Do you not think that the larger LII mod#cations compared I ofserved in alloys) could be better interpreted in terms of spd-hybridization to L ~ (also or d-rearrangements rather than pure charge transfer ?
A: D. C. Koningsberger We d o have subtracted the white-line data of the samples from the corresponding white-line of the platinum foil to establish the change in white-line intensity. As demonstrated in the paper the spd rehybridization (as we believe to be the origin of the particle size effect on the white-line) is separated from the effect of the support by trying to compare catalysts with the same average metal particle size.
Q: R. W. Joyner (United Kingdom) In interpreting small changes in X-ray white lines it is important to recall the lessons from XPS studies of small particles. XPS binding energy shifts are observed, but are now considered largely to be caused by find state relaxation effects. You are careful to limit and define the ways your comparison are mode. However, even if two catalysts have the same average (nearest neighbor coordination number, then size distribution and shape may vary. Thus, even when carefully controlled, white line charges may reflect final state effects rather than electron transfer to or from the support. A: D. C. Koningsberger The white-line intensity is most probably determined by final state effects, which means that the white-line is sensitive to the averaged particle size and shape of the metal particles. We have not yet fully established how large this effect is. We have tried in the paper to correct for the particle size effect by trying to compare catalyst with the same averaged particle size. We d o not believe that the shape of the platinum particles in H-L and K-L for these small clusters is different. More research is needed before the influence of the shape of the metal particles on the white-line intensity is determined.
Q: R. Prins (Switzerland) The largest change in white line intensity is observed in the L edge. However, this + 5d3j2 transition, and since the 5 t 3 bond ~ should be line is due to the 2p completely filled, actuaig the intensity of the L line should be very small. Changes in the Ln intensity are then induced by mixing of fie d3/2 and d5/2 bands and this will be very sensitive to the Fermi surface, and thus to metal particle size, shape and metal atom packing. Since most of the change in white line intensity is seen in the LII edge, the total white line intensity change is very sensitive to several factors. If that is true, is there then any hope to use the white line intensity and say something about particle size and charge?
A: D. C. Koningsberger As already mentioned in my answer to the question made by T. Uematsu I agree that the total white-line intensity is determined by several factors: (i) particle size/shape, (ii) type of adsorbate, (iii) type of support and (iv) structure of metal-support interface. It must be possible to separate support effects from other effects by careful elimination; for
instance by studying metal particles with the same average size reduced at the same temperature (i.e. same metal-support interface) covered with (and without) the same adsorbate.
Q: E. Iglesia (USA) I concur fully with your experimental observation that the rates of propane reactions increase with increasing acidity of the support. We have observed similar increases in the bifunctional hydrocracking of higher alkanes with increasing support acidity. I disagree, however, with: 1) your description of the process as hydrogenolysis on Pt clusters and 2) your explanation of the phenomenon as one arising from differences in the electronic properties of Pt clusters. The process is likely to occur by a bifunctional mechanism where the rate-limiting step is the reaction of equilibrium concentrations of propene on acid sites. Therefore, your choice of Pt sites for turnover calculations is the predominant reason for the apparent turnover rate changes. Would you comment on my disagreement'?Have you measured the rates of propene cracking on your catalyst supports ? A: D. C. Koningsberger While bifunctional hydrocracking of higher alkanes is well known, we do not believe that the enhanced activity for propane conversion we observe for Pt on acidic supports is the result of a bifunctional reaction path. Monomolecular cracking of propene is not possible since neither a-hydrogen elimination from a secondary carbenium ion, nor p-hydrogen elimination can lead to the formation of stable products. Bimolecular rcaction of propene (i.e. oligomerization followed by cracking) does not yield methane and, additionally, yields C3+ products, which were not observed. Since the reaction products are methane and ethane in a molar ratio of 1:1, the bifunctional reaction pathway can be ruled out.
Guczi, L. et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 43 1993 Elsevier Science Publishers B.V.All rights reserved
DIRECT MAS/MES EVIDENCE FOR ELECTRONIC METAL-SUPPORT INTERACTIONS IN DILUTE 57C0 AND 57Fe CARBON AND ALUMINASUPPORTED CATALYSTS
C.H.Bartholomew, L. R. Neubauer and P. A. Smith B W Catalysis Laboratory, Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602, USA
Abstract Mossbauer absorption spectroscopy (MAS) and Mossbauer emissions spectroscopy (MES) studies of 1-3% 57Fe and 1% 57Co on carbon and alumina supports were conducted as a function of reduction temperature. Catalysts were prepared by nonaqueous evaporative deposition to maximize the reduction of cobalt and iron to the metal. Metal surface areas of the catalysts were determined by H2 adsorption, while extents of reduction to the metal were determined by both Mossbauer spectroscopy and by titration of reduced catalysts with oxygen at 673 K. MAS/MES data for 1 and 3% 57Fe/C and 1% 57Co/C catalysts reduced at 773 K indicate the presence of only one phase--superparamagnetic (SP) clusters of metal having diameters of about 1-2 nm. Room temperature isomer shifts for these carbon supported metal clusters of 0.10-0.15 mm/s indicate a decrease in electron density of the metal nuclei relative to the bulk metals. MES data for 577Co/A1203 suggest the existence of three phases: Cosp metal, Co(I1) oxide, and Co(1II) oxide, while MAS generally shows only Fesp metal clusters and Fe(I1) oxides to be present in 1-2% 57Fe/A1203, except for some ferromagnetic Fe metal in 2% 57Fe/A1203reduced at 873 K. Isomer shifts for the metal clusters in the Al2Oj-supported 57C0 and 57Fe catalysts are -0.05 to -0.15 mm/s indicating an increase in the electron density at metal nuclei. The presence of small metals clusters of 1-5 nm in these catalysts is confirmed by H2 adsorption. Moreover, Debye temperatures measured by Mossbauer are significantly lower than for bulk iron consistent with the lattice dynamics expected for small metal clusters having a large fraction of surface atoms. The verv significant isomer shifts observed for SP metal phases by Mossbauer are consistent with electronic modification of small metal clusters in supported Co or Fe. That the isomer shift is positive for metal/C catalysts and negative for rnetaVAl203 catalysts indicates this effect must be due to metal-support interactions. 1 . INTRODUCTION Most commercial catalysts incorporate a catalytic phase dispersed on a high surface area camer or "support" to facilitate the preparation of a well-dispersed, high surface area catalytic phase and stabilize the active phase against loss of surface area; in addition support-active phase interactions can dramatically influence the adsorption and activity/selectivity properties of the catalyst [ 1-41. "Metal-support interactions (MSI)" are a collection of phenomena which include ( I ) decoration/promotion of the metal surface with support moieties, ( 2 ) bifunctional/interfacial catalysis, and (3) electronic/geometric modifications of metal clusters by direct interaction with the support. These different MSI are extremely difficult to isolate and investigate; MSI of the third type, electronic effects, are particularly elusive as there are few in situ techniques which enable their direct measurement. Nevertheless, Mossbauer absorption spectroscopy (MAS) and Mossbauer emissions spectroscopy (MES) provide this capability for well-dispersed, supported cobalt and iron metal clusters [5-lo], since the isomer shift is a measure of the electron density at the 57Fe or 57Co nucleus.
822 Highly-dispersed, superparamagnetic (SP) clusters of Fe (Fe,,,) have been observed by MAS for Fe supported on M g 0 [ 5 ] ,carbon [6-9,l 1,121, alumina [7,13,141, and silica [14,15]. Topsoe et al. 151 first reported an isomer shift for supported FeSp,namely a value for Fe/MgO of -0.2 mm/s; positive isomer shifts of 0.15 and 0.10-0.15 were later reported for FeSpon carbon by Phillips and Dumesic 161 and Neubauer et al. [7,X] and negative shifts of -0.08 to -0.15 mm/s for Fesp/AI203 by Neubauer 171 and Smith and Bartholomew [lo]. Only recently was the significance of these isomer shifts addressed [7,8,10]. In this study MAS and MES of carbon- and A1203-supported 57Fe and 57Co provide evidence that the electronic properties of metal clusters are changed by the support environment.
2 . EXPEHIMENTAL 2.1. Materials. Catalysts were prepared by evaporative deposition of 57Fe or 57Co metal nitrate or chloride salts in either benzene or acetone: A1203 supports were previously dehydroxylated at 773-873 K. Catalysts containing 57C0 were doped to a 2-3 mCurie level. Catalysts were dried in vacuum at 373 K and reduced irr situ in H2 at 6734473 K. Details of the preparation and pretreatment are described elsewhere 17,X, 101. 2.2. Apparatus and Procedure. Mossbauer spectra of reduced catalysts were obtained in situ at 77 and 298 K using controlled-atmosphere cells and a spectrometer system previously described [7,X]. MAS experiments were conducted in the constant acceleration mode using a moving SO mCurie 57Fe in a Pd source; MES experiments were conducted with an 57Feenriched NqFe(CN)6.H20 absorber driven in the same constant acceleration mode but placed between the fi7Co-containingcatalyst (serving as gamma ray source) and a proportional counter tube. With the use of an absolute laser velocity calibrator it was possible to measure isomer shifts to within an absolute accuracy of kO.005 nun/s. Mossbauer spectra were corrected during the collection procedure to remove the curved background of instrumental origin. M spectra of reduced catalysts contained in a controlled-atmosphere cell [7,161 were measured at 29X and 77 K in 1 atm of hydrogen to ensure that no oxidation of the samples would occur. Mossbauer spectra were computer-fitted to Lorenzian lines with a least-square optimization procedure [ 161. Resonant absorption areas were found from integration of the background curvature-corrected spectra. H2 adsorption measurements were carried out in a vacuum adsorption apparatus described previously [ 171 or in the case of Fe/carbon catalysts in a flow system equipped with a thermal conductivity detector described elsewhere [ 181 according to a procedure [ 1 X] for measuring total chemisorbed hydrogen while minimizing spillover effects. The procedure in all cases involved cooling in H2 from about 650 K to ensure that a monolayer was adsorbed despite highly activated adsorption. Metal dispersion and crystallite size calculations were based on the assumptions that Co or Fe metal is present as spherical particles of uniform size and that unreduced iron is present in a separation dispersed oxide layer in intimate contact with the support [ 1 Y I. Thus, percentage dispersion was calculated for iron catalysts according to the relation %D = I.l17X/(Wf) (1) where X is the average H2 uptake in micromoles per gram of catalyst, W is the weight percentage of iron and f is the fraction of iron reduced to the metal as determined by oxygen titration 1201 or Mossbauer spectroscopy. Average crystallite diameters were calculated from %D assuming spherical metal crystallites of uniform diameter d with a site density of 17.3 atoms/nm2, calculated from a weighted average of the 3 most dense planes of bcc Fe 121: d(nm) = 122.5 / %D (2) Similar equations were used for cobalt catalysts for which the numerical coefficients in Equations 1 and 2 were 1.17Y and 96.2 assuming a fcc structure for supported Co 1211.
823 3 . RESULTS AND DISCUSSION 3.1.
Catalyst properties
H2 chemisorptive uptakes, metal dispersions, extents of reduction to the metallic state determined from oxygen titration and/or Mossbauer spectroscopy, and average metal crystallite diameters determined from H2 chemisorption of Fe/carbon, Fe/A1203, Co/carbon, and Co/AI2O3 catalysts are listed in Table 1. The relatively large values of H2 uptake are indicative of very significant amounts of reduced metal in these catalysts of mostly low metal concentration (1-3 wt.%). Extents of reduction determined by oxygen titration of 38-97% are much higher than reported previously for dilute Fe and Co catalysts prepared by conventional aqueous impregnation or precipitation methods; indeed, in earlier studies of dilute (1-5%) Fe on alumina [2,22-241 it was found that only a small fraction of the Fe could be reduced to the metal even at very high reduction temperatures. While high metal dispersions have been reported previously for Fe/C catalysts [6,8,12], the dispersions of 30-74% observed in this study for Fe/A1203 and Co/Al2O3 catalysts are much higher than those previously reported for similar catalysts prepared by conventional aqueous methods but are comparable with those reported for Fe/A1203 catalysts Table 1 H2 chemisorptive uptakes, dispersions, extents of reduction, and average metal crystallite diameters of Fekarbon, Fe/AI203, Co/carbon, and Co/Al2O3 catalysts Catalysta
1% Fe/C, Red. @ 623 K 3% Fe/C, Red. @ 623 K 10% Fe/C, Red. @ 623 K 1% Fe/A1203 (A), Red. @ 623 K 2% Fe/A1203 (A), Red. @ 623 K 0.5% Fe/A1203 (B) Reduced @ 773 K Reduced @ 873 K 2.0% Fe/A1203 (B) Reduced @ 773 K Reduced @ 873 K 1% Co/C, Red. @ 623 K 1% Co/Al203, Red. @ 673 K 2% Co/Al2O3, Red. @ 673 K
H2 Uptakeb (CLmol/g)
% Reductionc 0 2 Moss.
-
Dispersiond (% 1
Diam.e (nm)
36 41 12 24 23
143 76 14 24 57
300 for example. b i d you vary the pretreatment conditions and do these influence the kinetics ? 2) Are there any differences (even subtle) in the toluene ad/desorption on these supported systems and how does the amount of hydrogen compare to the toluene adsorption for these systems '?
v,
A: M. A. Vannice The reduction time at a given temperature was the same for all catalysts - 1 h except for Pt/TiO, (LTR) which was 2 h. Consequently, we do not know if reduction time affects the kinetics. Residual chloride would also have to be checked to clarify any observed influence. All of our adsorption measurements were conducted volumetrically; hence, toluene adsorption was not determined.
Q: S. L. Kiperman (Russia) The reasons for the changes of kinetic dependencies with the changes of temperature in this work are not clear. But in connection with it I would like to inform you on our results obtained in the study of kinetics and mechanisms of benzene and toluene hydrogenation over platinum-alumina catalysts. We studied these reactions with a detailed investigation of kinetics in a gradientless system and by a transient response method in isotopic variants. It was found that there are two kinds of hydrogen adsorbed on the platinum surface - weakly and tightly bonded. Surface hydrogenation proceeds via the interaction of the hydrocarbon only with weakly bonded hydrogen but isotopic exchange with tightly bonded hydrogen. A part of weakly bonded hydrogen changes with temperature and therefore changes the kinetics and mechanism of hydrogenation.
A: M. A. Vannice We agree with your proposed role of weakly adsorbed hydrogen and, in fact, we have stated in our earlier study of benzene hydrogenation over Pd that only the weakly bound hydrogen appeared to be involved (ref. 30). Based on our cunent studies of benzene hydrogenation on Pt and an assessment of the literature, we agree that your model is possible. However, we think the change in the kinetics with temperature reflects a greater rate inhibition due to an increase in Hdeficient surface species at higher temperatures. Q: W. K. Hall (USA) Is it not true that the hydrogenation of benzene to cyclohexadiene is thermodynamically uphill (forbidden) under the conditions of your reaction '? In this circumstance a stepwise hydrogenation is not possible. thus, hydrogen spillover may occur, but will not be efficient for benzene hydrogenation because multiple addition is difficult because of the low surface density of H atoms. Do you agree ? A: M. A. Vannice Using data from "TRC Thermodynamic Tables", Texas A and M University System, College Station, TX (1985) and calculational procedures used in the DIPPR Data Book,
873 we have calculated that the free energy change during the reaction H2 + c$&) + C&&) is about +12.4 kcal/mole, which is close to the activation energies we routinely see on either Pt or Pd and which have been reported for other Group VIII metals. Consequently, the activation barrier for the overall reaction to cyclohexane may well be the formation of a cyclohexadiene intermediate. Over small migration distances away from the metal the spillover hydrogen concentration could easily be high enough to allow two atoms to react with benzene to form cyclohexadiene, which along with cyclohexene is much more reactive than benzene. Furthermore, the simultaneous addition of 4 to 6 H atoms to avoid the formation of cyclohexadiene is statistically very improbable and, if this were the rate determining step, one could expect a 2nd order dependence or higher on hydrogen. Consequently, I do not agree with you.
Q: D. G. Blackmond (USA) You suggest that two types of sites, one on the metal and one on the support, are responsible for the hydrogenation activity on the acidic catalysts. However, there appears to be no trend in activation energies in comparing acidic and non-acidic supports. The observed activation energy could be a combination of that for the reaction occurring on each type of site. Do you believe that the two types of sites have similar activation energies for toluene hydrogenation ?
A: M. A. Vannice As mentioned above, activation energies for many metals are frequently reported around 10-14 kcal/mol for benzene and toluene hydrogenation, independent of the support, and this implies that the formation of a cyclohexadiene intermediate may well be the slow step. Our modelling studies show that the proposal of the addition of either the first or second H atom to the benzene ring as the rate determining step can explain the results for metal sites as well as sites on the oxide (to be published); even in the case with Pd where the support contribution is much greater (refs. 2, 30). Q: W. Griinert (Germany) My question concerns the use of irreversible hydrogen chemisorption for the assessment of Pt accessible from the gas phase. There was a long story about the origin of the SMSI effect. Certainly, there is migration of TiO, species onto the metal, but an electronic effect may be also considered. It might weaken the interaction between H2 and the metal, so that you under-estimate the Pt surface that may still supply weakly bound hydrogen for your reduction. Thus, I suggest that it would be useful to check the amount of Pt accessible to the gas phase by ISS.
A: M. A. Vannice Our earlier calorimetric measurements of H2 adsorption on PtRiO, (HTR) catalysts showed that the average heat of adsorption (13.4 kcal/mole) was close to the average value of 13.5 kcal/mole for all Pt catalysts including Pt powder, and the Qad values were also similar for irreversible adsorption (ref. 47). For the 0.09 % Pt/TiO catalyst both reversible and irreversible H uptakes were undetectable and for 0.95 &J PtRi02 the reversible uptake was about t k e e times larger than the irreversible uptake. The marked reduction in hydrogen chemisorption is clearly due to physical blockage of the Pt surface. However, the application of ISS could still be a useful characterization technique. Q: J. B. Butt (USA) 1) Were there significant differences in the amount of chloride contained in the catalysts investigated that might account for some of the differences in apparent acidity ? 2) The values of activation energy for the toluene hydrogenation (-12 kcal/mol) are similar to those reported for benzene hydrogenation over similar catalysts, therefore one might conclude that the methyl group does not significantly affect the energetics of the
874
hydrogenation reaction. Differences in the magnitude of turnover frequency would thus reside in some appropriate preexponential factor (i.e., changes in TOF are steric effects).
A: M. A. Vannice We did not analyze our reduced catalysts for residual chloride content but, based on your recent paper [l], we might anticipate little effect, at least for Pt/SiO and Pt/A1203 because our dispersions were high and our reduction temperature was 45b0 C. However the effect with SiO,-AI,O, and TiO, is not known, and chloride can alter the acidity 0; titania surfaces. The statement about steric effects could well be true, especially if the activation energy is primarily associated with the destruction of the resonance energy of the aromatic ring. However, on Pt TOF values for toluene are only 1/4 to 1/2 the values for benzene hydrogenation, and only a small change in activation energy is required to produce these differences. A. F. Flores, R. L. Burwell and J. B. Butt, J. Chetn. Soc., Furaduy Trans., 88, [l] 1191 (1992) W. Ranjani, J. Siriwardane and J. P. Wightman, J . Coll. Interface Sci., 94, 502 [2] (1983) Q: H. Schulz (Germany) In your excellent paper you have suggested an inhibiting unsaturated surface species in order to explain the observed kinetics of toluene hydrogenation. Could you pcrhaps speculate a little about the composition and structure of this species ):
A: M. A. Vannice During our subsequent studies of benzene and toluene hydrogenation we have found that our kinetic model is best fit by the assumption that a phenyl (or tolyl) species is the predominant Hdeficient species. Although no identification of such a species has been reported for Pt yet, three recent papers have reported phenyl groups on Ag(l11), Os(0001) and Ni(l1 l), thus we believe this is the most probable species [3-51. Zhou et al., Surt Sci., 238, 215 (1990) [3] Garen et al., Chem. Phys Leu., 165, 137 (1990) [4] H. P. Steinruck, W. Huber, T. Patche, D. Menzel, SurJ Sci., 218, 293 (1989) [5] Q: D. Wang (China) You suggest a correlation that hydrogenation activity is higher with the use o f acidic supports. I do not think of TiO, as very acidic, nor is A120, (although they are more acidic than SiO,), and that leaves only Si02-A1203 as an acidic support. Has work bcen done with more (clearly) acidic supports, e.g., Nb oxide and Ta oxide, to supportcd your correlation?
A: M.A. Vannice We have not used the oxides you mention. We do not consider this q-A1203to be very acidic - it certainly has few Bronsted sites. Titania can have acid sites and the presence of HCI (i.e., chloride) can increase acidity, primarily by forming weak Bronstcd sites [2].
Guczi, L
al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights resewed
THE CATALYTIC ACTIVITY OF MoO,/Zr02 IN THE HYDROGENATION AND METATHESIS OF PROPENE V. Indovina, A. Cimino, D. Cordischi, S. Della Bell4 S. De Rossi, G. Ferrark, D. Gazzoli, M. Occhiuzzi and M. Valigi
Centro di Studio del CNR su "Strutturaed Attivita catalitica di Sistemi di ossidi", Dipartimento di Chimica, Universita La Sapienza, P. le A. Moro 5, 00185 Roma, Italy
Abstract The catalytic activity of MoO,/ZrOn for propene hydrogenation has been studied in a flow apparatus at 195-298 K. At or above 273 K, propene metathesis is observed in addition to hydrogenation. The catalysts, prepared by equilibrium adsorption at pH=l, 2, 3 or 8,have a Mo surface concentration from 0.5 to 7.7 atoms nm-2. Before ESR, XPS, or catalysis measurements, the samples were heated in 0 2 at 773 K and reduced thereafter with H2 to a controlled extent, expressed by e/Mo (electrons acquired per Mo atom) in the range 448 K to 803 K. Both ESR and XPS analyses show that a large fraction of total Mo (30%) is already reduced to Mov at 473 K (e/Mo=0.3). The Mov content remains constant with further reduction up to e/M0=3. The ESR spectrum originates from a mononuclear Mov species in a square pyramidal configuration (ESR of 9sMoV). About 80% of Mov is destroyed upon 0 2 adsorption at 195 K and restored by evacuation at 773 K. The turnover frequency for the propene hydrogenation (Nh/molecules s-1 Mo atom-1) is very small up to e/Mo=l and increases markedly with e/Mo. The Nh value strongly increases with the Mo content up to about 5.6 atoms nm-2, and decreases at higher Mo loading (aggregation of Mo oxide). The turnover frequency for metathesis, Nm, is nearly constant in the range e/Mo=l to 3 and is almost independent of Mo content. The results show that the active sites for hydrogenation and metathesis are different. It is suggested that mononuclear Mov are the active sites for metathesis, whereas higher nuclearity species involving molybdenum in lower oxidation state are active for hydrogenation.
1. INTRODUCTION Supported molybdenum ions, mainly studied on silica, alumina and titania l1-41, show important catalytic properties. It is well known that the dispersion, oxidation state, and structural features of the supported ions may strongly depend on the support. It is therefore of interest to study the activity of molybdenum on other
876 supports; ZrO2 was chosen in view of its weak acid-base properties and because of its ability to maintain a high surface area. Within the same framework, we have investigated the activity of chromialzirconia [5-81,for H2- D2 equilibration [5], propene hydrogenation [9-111and propane dehydrogenation [12]. The activity for propene hydrogenation was substantially higher when zirconia instead of alumina or silica was used to support chromium. This paper reports the catalytic activity for the hydrogenation of propene on MoOx/Zr02 catalysts, previously reduced by H2 to a controlled extent. The concomitant metathesis of propene was also studied. The data illustrate the dependence of the activity on (i) the extent of reduction, (ii) concentration of molybdenum and (iii) poisoning with 0 2 at 195 K or NH3 at RT. A preliminary report of catalyst characterization by ESR and XPS spectroscopies, volumetric redox cycles, adsorption of 0 2 at 195 K and NH3 at RT is also given.
2. EXPERIMENTAL MoOx/Zr02 catalysts were prepared by contacting Zr02, prepared as previously described with a solution of ammonium heptamolybdate (AHM) at pH=l, 2, 3 or 8.The ZrO2 was calcined in air at 383, 823, or 923 K before contact with the AHM solution. The Mo surface concentration was in the range 0.5 to 7.7 atoms nm-2. Some samples were also prepared by adsorption from a solution of enriched 95Mo (96.8%,ESR experiments). The MoO,/ZrOp catalysts are designated as ZMox(T), where x specifies the analytical Mo content (w”/”) and T the calcination temperature (K) of the Zr02 used (Table 1). The catalyst (0.1 to 0.5 g) was placed in a silica reactor which could be connected to the flow section for catalytic experiments or to the gas adsorption section, as described elsewhere [9].In some experiments, a silica reactor was used equipped with a side ESR and/or XPS tube to which the powder could be transferred without exposure to air. After drying at 383 K, all samples were submitted to redox cycles consisting of (i) heating in oxygen at 773 K, (ii) reduction with hydrogen at 448 to 803 K, and (iii) reoxidation with oxygen at 773 K. The extent of reduction (e/Mo, electrons per Mo atom) was determined by monitoring the decrease of the H2 (step ii) and 0 2 pressure (step iii). After reduction to a controlled extent, 0 2 adsorption at 195 K or the catalytic activity for the hydrogenation and metathesis of propene were measured (flow apparatus, GC analysis). The reactant stream (95% H2, 5% C3H6) flowed through the catalyst (20 to 170 cm3 min-1) at a pressure of about 100 kPa. The reaction was generally followed for 1 h, with a first analysis after 5 min and then after every 15 min. At 195 K, propane was the only product. At or above 273 K, ethene, ethane, butene and butane were detected in addition to propane. The activity for the metathesis was stable (or moderately decreasing) with time on stream, whereas a marked deactivation was observed for hydrogenation, probably due to the formation of carbonaceous species strongly held on the surface of the catalyst. Therefore, the activity for hydrogenation (expressed as average tunover frequency, vide infra) was calculated from the initial rate at t=O [9].ESR measurements were made at RT or at 77 K on a Varian E-9 spectrometer operated at X-band frequencies. Absolute concentration of Mo species contributing to the ESR spectra were obtained from the integrated area by using a Varian strong pitch standard. The XPS analysis of
PI,
877
samples heated in oxygen at 773 K and reduced in situ by flowing H2 at 473 to 673 K was performed in a Leybold-Heraeus LHS 10 spectrometer. BE values are deduced by a best fitting procedure of the composite peak. Table 1 Catalysts and their main features Catalysts Z(383) ZMol.67(383) ZMo5.94(383) ZMo3.09(383) ZMo5.49(383) Z(823) ZMo0.48(823) ZMo2.49(823) ZMo5.64(823) ZMo0.99(823) Z(923) ZMo0.69(923) ZMo2.09(923) ZMol.36(923) ZMo0.41(923)
PH 1 1 3 8 2 2 2
8 1 1 2 8
Surface area (a) 0-11
118 64 102 64 103 40 40 37 46 41 25 21 24 23 25
Mo surface density (a) (atoms nm-2)
1.64 3.65 3.03 3.35 0.74 4.22 7.70 1.52 2.05 5.56 3.71 1.02
(a) Samples heated in 0 2 at 773 K
3. RESULTS 3.1. XPS characterlzatlon The analysis of MoOx/Zr02 samples after heating in 0 2 at 773 K shows the presence of Movl (M03d5/2 at a BE 232.8 f 0.2 eV, referred to the Zr(3d5~)=182.5eV peak). A linear correlation is found between the peak area intensity ratios, I(Mo3d)/l(Zr 3d), and the surface concentration of Mo, up to about 6 Mo atoms nm-2. The sample ZMo5.64(823), containing 7.7 atoms nm-2, shows evidence of aggregation of molybdenum species (Moo3 like). Experiments performed on the sample ZMo1.36(923) reduced in situ by flowing H2 at 473 to 673 K show the presence of Mov (BE 231.6 f 0.2 eV) in addition to unreduced Movl. The percentage of the Mov species is 30 f 10% at all reduction temperatures. After reduction at 623 or 673 K, the presence of Molv (BE 228.8 f 0.2 eV) is also detected.
3.2. ESR characterization After heating the samples with 02 at 773 K followed by evacuation at the same temperature, the formation of an extremely weak axial signal with g1=1.961 and
878
g,,=1.850 was observed at 77 K but not at RT. In view of the ESR parameters and relaxation features of this signal compared with those reported by Che et al. [13] and Shelimov et al. [14] for tetrahedral Mov on SiO2, the signal is assigned to a May, species. After reduction with H2 for 10 min at 523 K, a different signal (gl=l .955 and gl,=l .886), detectable at both RT and 77 K, is observed in all ZMo samples. The ESR analysis of spectra of dilute ZMo samples containing 95M0 allows assigning the signal to a mononuclear Mov species on the surface of Zr02 (Mo!~). On all samples, the Mov content increases with increasing extent of reduction, up to (e/Mo)~,=0.3, and remains almost constant thereafter. The fraction of total Mo detected by ESR as Mov is 0.3 in the most dilute samples and decreases markedly with increasing Mo content. 3.3. Redox properties of the system Samples were reduced with H2 at 333 K to 803 K to various extents in a random sequence and reoxidized thereafter with 0 2 at 773 K. From these experiments a linear correlation between the (e/Mo)~, values and the corresponding (e/Mo)02 values is found for all ZMo catalysts. Maximum reduction extents were (e/MO)~2=3.1,corresponding to an average oxidation number, k 2 . 9 . In more concentrated samples it was found that (e/MO)o2=(e/MO)~2,whereas in more dilute samples, (e/Mo)02 values were lower than (e/Mo)~, by no more than 15%.
I ” n
s
1
Q)
W
400
600
800 T/K
Figure 1. Extent of reduction, (e/M0)~2,as a function of reduction temperature. Sample ZMol.36(923). In some cases, the (e/Mo)H2 values were determined by reduction for 40 min at constant temperature, which was subsequently increased from 323 to 800 K in consecutive experiments. Before passing from a given reduction temperature to a higher one, the sample was heated in 0 2 at 773 K and evacuated at the same temperature. The results of a typical experiment on the sample ZMol.36(923) (Fig. 1) are: (i) upon reduction up to 525 K a value e/Mos0.3 (nS.7) is measured,
879 (ii) a very steep increase in the e/Mo value from 0.3 to 1.5 (?I decreases from 5.7 to 4.5) is subsequently observed when the reduction temperature is increased from 525 to 580 K, (iii) above 580 K the e/Mo value tends to level off until 680 K, and (iv) a second marked increase of e/Mo (from 1.5 to 3) is observed when the reduction temperature is increased from 680 to 800 K. In the e/Mo range from 0.3 to 3, the constancy of the surface concentration of Mov was check.ed by ESR. 3.4. Adsorption of oxygen at 195 K and ammonia at RT The irreversible uptake of oxygen at 195 K (double isotherm method) increases with increasing ( e / M o ) ~up ~ to about 0.5 and remains constant thereafter (Os/Mo= 0.2 to 0.3). The analogous pattern observed by ESR for the surface concentration of Mov as a function of e/Mo suggests that Mov is involved in the oxygen adsorption. ESR experiments show in fact that a large fraction of Mov (80%) is reversibly destroyed and superoxide species are formed upon adsorption of 02. Upon adsorption of NH3 at RT on ZMo samples reduced to e/Mo20.3 and therefore showing the ESR signal of M,o; at its maximum intensity, a different Mov species is detected by ESR (g,=1.948 and gll=1.893). The latter species, identified as Mo ,; by its similarity to a signal detected on SO2 [13], is stable upon evacuation
at RT, and is partially transformed in the Mo,;
species after evacuation at 473 K.
After a subsequent evacuation at 673 K the Mol, species is reversibly restored. 3.5. Hydrogenation of propene The catalytic activity for the hydrogenation of propene at 298 K, expressed as average turnover frequency, Nh (propane molecules per sec per total Mo atoms), is reported in Fig. 2 a for some ZMo catalysts as a function of e/Mo. If unreduced, all ZMo catalysts are inactive, whereas after reduction to e/Mo2l, a marked increase of activity with increasing extent of reduction is observed.
(e/Mo) H,
Mo ions nm-*
Figure 2. (a): Turnover frequency at 298 K for propene hydrogenation (Nh/molecules s-1 atom-1) as a function of the extent of reduction; (b): Nh at 298 K as a function of the Mo surface concentration. Crossed circle: Moo3 present.
880 When comparing samples of different Mo content reduced to about the same extent (e/Mo=3), the corresponding Nh values are found to increase strongly (by 2 orders of magnitude) with increasing surface concentration of Mo up to about 5.6 ions nm-2, and are found to decrease thereafter because of segregation of Moo3 like species (Fig. 2 b). 3.6. Metathesis of propene during hydrogenation The average turnover frequency for the metathesis of propene, Nm (molecules atom-1 s-1) as a function of e/Mo, is reported in Fig. 3. The catalysts show low activity for metathesis when reduced up to (e/Mo)~,=l. Above this value a marked increase in activity is observed, whereas metathesis is nearly constant for reduction in the region from e/Mo=l to 3. The Nm values change but little with increasing Mo surface concentration, up to about 7.7 nm-2. All Nm values are within a factor of 3, if samples are reduced to any extent in the range e/Mo=l to 3.
I"
3 -
0 0 0
0
0
0
1
2 3 (e/Mo) ti,
Figure 3. Turnover frequency at 298 K for propene metathesis (Nm/molecule s-1 atom-1) as a function of extent of reduction. Samples: (O)ZMo2.09(923)pH=l; ( 0 ) ZM01.36(923)pH=2.
3.7. Poisoning with oxygen or ammonia Poisoning experiments were performed by the step-wise addition of 0 2 at 195 K to ZMo samples previously reduced to ( e / M o ) ~ p 3and , which therefore showed a high activity for both hydrogenation and metathesis. The activity for hydrogenation is reduced to one-half and that for metathesis is unchanged at OdMo=O.l. At higher oxygen coverages (02/Mo=0.2), the activity for hydrogenation is suppressed, whereas that for metathesis is still unchanged. When hydrogenation is suppressed, the products of metathesis are ethene and butene only. Upon adsorption of ammonia at RT and evacuation at the same temperature, the activity for metathesis is suppressed, whereas that for hydrogenation is reduced by a factor of two.
88 1
3.8. A comparison with MoOx/A1203 catalysts The catalytic activity of MoOx/y-A1203for the hydrogenation and metathesis of propene has been extensively studied by Hall et al. [2]. In order to have directly comparable activity data, some new experiments were carried out by us on the same MoOx/A1203specimen (Mo 8 wt%) used in ref. 2 [15]. Representative results obtained on the MoOx/A1203 system in our or in other laboratories (as specified) are listed below and compared with those obtained on MoOx/Zr02. The formation of Mov in the MoOx/A1203 system is considerd first. The Mov concentration changes but little (by a factor of two) with increasing reduction with H2 up to 800 K [4]. The ESR signal of Mov is substantially broader on A1203 compared with Zr02, a feature due to superhyperfine interaction with 27AI nuclei [16] rather than to a higher heterogeneity of the MoV/AlzO3 system [4]. The Mov molar fraction is only 0.1 of total Mo in MoOx/A1203compared to 0.3 in MoOx/Zr02. Adsorption of 0 2 at 195 K on reduced MoOx/A1203yields 02/Mo=0.25 to 0.3, close to the values reported in ref. 17 (02/Mo=0.25 to 0.30) and in ref. 4 (02/Mo=0.33) for similar catalysts, and close to the values found on ZMo (02/M0=0.2 to 0.3). Concerning catalysis, the same dependence on e/Mo is observed for either Nh or Nm on MoOx/A1203 and MoOx/Zr02. Moreover, the Nh value at 293 K measured on MoOx/A1203 is in fair agreement with the value on ZMo2.09(923) at the same temperature ( N p l . 6 ~ 1 0 - 2s-1 and 13x10-2, respectively). The corresponding value relative to MoOx/A1203, calculated from the data of Fig. 1 of ref. 2, is somewhat lower (0.6~10-2),probably because a circulation technique was adopted. A good agreement is also found when the Nm values on various Mo-containing catalysts are compared: on MoOx/A1203, Nm=3.1~10-4s-1 [2], Nm=2.5~10-4[18] and Nm=3.0~104(our data). These values are close to those measured on MoOx/Zr02 (Nm=2 to 3x10-4 s-1, from Fig. 3). Finally, the same poisoning effect is found with 0 2 and NH3 on Zr02 and Al203-supported catalysts. Specifically, NH3 adsorption poisons the metathesis reaction and partially poisons hydrogenation, whereas 0 2 poisons hydrogenation but not metathesis.
4. DISCUSSION The interaction between a solute and a surface and the ensuing adsorption process have been both discussed by several authors, and summarized by Knozinger [l]. The role of the isoelectric point (IEP) of the surface, and the nature of the solute species have thus received attention. However, their relative importance, as for the case of MoOx/A1203[l], is still being studied. A distinction must be made between the "precursor state", before the treatment of the solid, and after treatment, a process involving dehydration and reduction processes. After heating the MoOx/Zr02 samples in 0 2 at 773 K, Movl-containing species (molybdates and polymolybdates) are well dispersed on the surface of ZrO2, provided that the surface concentration of molybdenum does not exceed about 5.6 atoms nm-2. Some aggregation of molybdenum species (Moo3 like) is observed by XPS in the most concentrated sample (7.7 atoms nm-2 sample). A suggestion concerning the nature of surface molybdenum species formed after reduction can be given on the basis of ESR, XPS and R data.
882 In the early stages of reduction up to 525 K (e/Mol0.3, R25.7), Mov is the only species detected by either ESR or XPS, in addition to unreduced Movl detected by XPS. The Mov species is strongly stabilized on the surface of zirconia, as shown by the constancy of its concentration (30%) upon reduction treatments up to 800 K. The apparent decrease of Mov detected by ESR with increasing Mo concentration is due to magnetic interaction, as has been discussed elsewhere [19]. It should be stressed that almost the same Mov concentration is found when comparing ZMo samples prepared from AHM solutions at different pH. Therefore, for all ZMo samples, irrespective of the pH of AHM solution, a nearly constant (molybdates)/(polymolybdates) ratio is inferred from the constant concentration of mononuclear Mov (ESR), which arises from the reduction of mononuclear molybdate species. A buffer effect of the ZrO2 surface seems to be operating. Specifically, a role of the Bronsted acid-base sites of ZrO2 can be recognized in the condensation-decondensation of molybdenum anions during heating in 0 2 and/or in subsequent reduction with H2. Thus, whereas the total Mo uptake is mainly controlled by the pH of the AHM solution, the nature of the molybdenum species is to large extent controlled by the nature of the adsorbing oxide; this is in substantial agreement with the case of Mo03/y-A1203. Indeed Butz el al. [20]warn against inferring the nature of surface molybdenum species from the study of the supernatant liquid in equilibrium adsorption studies of the MoO3ly-AI203 system. Upon reducing the catalysts at higher temperature ( ~ 5 2 5K), two steep increases of the e/Mo parameter are observed when the reduction temperature is increased (Fig. 1). The first increase (Ae/Mo-1) occurs when the temperature is brought from 525 to 600 K, and is attributed to the formation of Molv by reduction of polymolybdates. The assignment is supported by XPS results, which show the formation of Molv in the same temperature region, and by the close agreement between the experimental value of R at 650 K (R=6-1.5=4.5, from Fig. 1) and the corresponding calculated value, ri=4.3. The latter is obtained from R = ( X h V x 5)
+ (XU,lV
x
4)
taking x,.&v=o.3, as measured by both ESR and XPS, and xMotv=1-x,.,o~=0.7. The second steep increase (AelMo-1.5, temperature increasing from 650 to 800 K) is tentatively attributed to the formation of Moll that arises from the reduction of Molv. In this case, the experimental value of is after reduction at 800 K, R=6-3=3 (Fig. l), is also in reasonable agreement with the calculated value, ii=2.9. The calculation is performed by taking xM,,v=o.3, as above, and assuming complete reduction of Molv to Moll. Coming to catalysis, the nature of the active sites in the hydrogenation and metathesis of the ZMo system can be briefly discussed in light of molybdenum species. The marked differences in the behaviour of ZMo catalysts toward hydrogenation and metathesis, schematically illustrated in Table 2, show that distinct sites are operating for the two reactions. Metathesis is probably carried out on mononuclear Mov species. The statement is supported by (i) the fact that Mov (as detected by ESR) is constant in the same
883 reduction region where the activity for metathesis is also constant (Fig. 3) and (ii) the facile (structure-insensitive) nature of metathesis on ZMo catalysts. Hydrogenation takes place on more-reduced molybdenum species, possibly Moll, as suggested by the progressive and marked increase of Nh observed in the region in which the average oxidation number decreases from 5 to 3,namely in the region where Molv is progressively reduced to Moll. The demanding (structure-sensitive) nature of propene hydrogenation on ZMo catalysts and specifically the marked increase of activity with Mo content (Fig. 2 b ) suggests the presence of an active site consisting of a polynuclear molybdenum species. Table 2 Main differences in the behaviour of MoOX/ZrO2catalysts toward hydrogenation and metathesis of propene Feature observed decay of N with time on stream dependence of N on reduction dependence of N on Mo content poisoning with 0 2 poisoning with NH3
Hydrogenation
Metathesis
marked
moderate
Nh increases above e/Mo>l
Nm is constant above e/Mo> 1
Nh increases with Mo content
Nm is constant with Mo content
complete
absent
partial
complete
5. CONCLUSIONS Mononuclear Mov species in square pyramidal configuration are strongly stabilized on the surface of ZrO2. The Mov species probably arises from molybdate species following reduction with H2. By increasing the extent of reduction, first Molv, and then Moll are formed, but not from Mov whose concentration remains unchanged. It is suggested that the precursors of Molv and Moll are possibly polymolybdate species. Thus, two nearly independent redox couples can be Moll. The two envisaged on the surface of ZrO2: MOO:-/MOV and Mo,O!&/MolV, redox-couples are fully reversible in cycles with H2 and'O2. Active sites for the metathesis of propene are different from those for the hydrogenation. The dependence of activity on the extent of reduction and on Mo content, together with the poisoning experiments, suggest that Mov ions with special coordinative features are active in metathesis, whereas polynuclear molybdenum species, possibly containing Moll, are active in hydrogenation. The clear analogies in the catalytic behaviour of MoOx/A1203 and MoO,/Zr02 strongly suggest a common nature for the Mo species that are active in the two supports.
884
Acknowledgements This work has been financially supported by the Progetto Finalizzato Chimica Fine II of the Consiglio Nazionale delle Ricerche, Rome, Italy
6. REFERENCES 1 H. Knozinger, "Proceedings9 In?. Congr. Catal." (M.J. Phillips and M. Ternan Eds.), Chem. Inst. Canada, Ottawa, 1988, Vol. 5, p. 20 2 E. A. Lombardo, M. Lo Jacono and W. K. Hall, J. Catal., 64 (1980) 150 3 K. Segawa, D. S. Kim, Y. Kurusu and 1. Wachs, "Proceedings9 In?. Congr. Catal."(M. J. Phillips and M. Ternan Eds.), Chem. Inst. Canada, Ottawa, 1988, Vol. 4, p. 1960 4 C. V. Caceres, J. L. G. Fierro, J. Lazaro, A. Agudo Lopez and J. Soria, J. Catal. 722 (1990) 113. 5 A. Cimino, D. Cordischi, S. De Rossi, G. Ferraris, D. Gazzoli, V. Indovina, G. Minelli, M. Occhiuzzi and M. Valigi, "Proceedings9 In?. Congr. Catal." (M. J. Phillips and M. Ternan Eds.), Chem Inst. Canada, Ottawa, 1988, Vol. 3, p. 1465. 6 A. Cirnino, D. Cordischi, S. De Rossi, G. Ferraris, D. Gazzoli, V. Indovina, G. Minelli, M. Occhiuzzi and M. Valigi, J. Catal., 127(1991) 744. 7 A. Cimino, D. Cordischi, S. De Rossi, G. Ferraris, D. Gazzoli, V. Indovina, M. Occhiuzzi and M. Valigi, J. Catal., 727 (1991) 761. 8 A. Cimino, D. Cordischi, S. Febbraro, D. Gazzoli, V. Indovina, M. Occhiuzzi, M. Valigi, F. Boccuzzi, A. Chiorino and G. Ghiotti, J. Mol. Catal., 55 (1989) 23. 9 A. Cimino, D. Cordischi, S. De Rossi, G. Ferraris, D. Gazzoli, V. lndovina and M. Valigi, J. Catal., 727 (1991) 777. 10 V. Indovina, D. Cordischi, S. De Rossi, G. Ferraris, G. Ghiotti and A. Chiorino, J. Mol. Catal., 68 (1991 ) 53 11 V. Indovina, A. Cimino, S. De Rossi, G. Ferraris, G. Ghiotti and A. Chiorino, J. Mol. Catal.,to be published 12 S. De Rossi, G. Ferraris, S. Frerniotti, A. Cimino and V. Indovina, Appl. Catal., in press 13 M. Che, C. Louis and Z. Sojka, J. Chem. SOC.,Faraday Trans.,85 (1989) 3939 14 B. N. Shelimov, V. Pershin and V. B. Kazansky, J. Catal.,64 (1980) 426 15 The catalyst was kindly supplied by Professor M. Lo Jacono 16 D. Cordischi, V. lndovina and M. Occhiuzzi, 12th European Conference on Surface Science, Stockholm Sept. 9-12, 1991, Europhysics Conference Abstracts (H. Bernhoff, A. Nilsson and L. Wallden, Eds.), Vol. 15 F, Abstract W 432 17 E. Delgado, G. A. Fuentes, C. Hermann, G. Kunzmann and H. Knozinger, Bull. SOC.Chim. Belg., 93 (1984) 735 18 J. C. Mol and J. A. Moulijn, in "Catalysis"(J. R. Anderson and M.Boudart Eds.), Springer-Verlag, Berlin (1987), p. 70 19 D. Cordischi, V. lndovina and M. Occhiuzzi, J. Chem. SOC.,Faraday Trans.,87(1991) 3443 20 T. Butz, C. Vogdt, A. Led and H. Knozinger, J. Catal., 776(1989) 31
885
DISCUSSION Q: W.Griinert (Germany) I have some problems with your assignment of the metathesis activity to Mo(V). You reported that you have a constant fraction of Mo(V) between e/Mo = 0.3 to 3. However, in this range, the turnover number increases from 0.5~10-4to 3x104 s-l, with a pronounced step in between. So,there must be an additional effect with the active site, or even the assignment may be not valid. A V. Indovina The suggestion for Mo(V) as active site for metathesis is mainly based on the fact that Mo(V) is the only Mo species whose concentration remains constant (ESR and XPS evidence) in the same e/Mo region in which also the activity for metathesis is constant (eMo = 1 to 3). The pronounced step at e/Mo = 0.3, which you refer to, does in fact suggest that the active Mo(V) must in addition possess special coordinative features and this point is clearly stated in our conclusions. Namely, only a fraction of the total Mo(V) is active, 20 % at most relying on poisoning experiments with 0 , It is plausible that this special Mo(V) species are generated on the surface during the rduction from e/Mo = 0.3 to 1. Q: M. Ichikawa (Japan) One common argument to evaluate the promotion of metal oxide addition (ZrO2 etc.) is how to count the number of the active sites which are incorporated in the particular catalytic reaction. In your paper, oxygen specifically poisons the hydrogenation but not metathesis. Do you have any experimental evidence to evaluate the active sites by usin4 the selective poisoning molecules to differentiate the two catalytic reactivities modified by Zr02 on the oxide catalysts ’? A: V. Indovina Catalytic sites for metathesis have been differentiated from those active for hydrogenation by adsorption of ammonia (which poisons metathesis, nearly selectively) and oxygen (selective for hydrogenation). An assessment of the concentration of the active sites for the two reactions has not been yet performed on M o o f i r 0 An upper limit for the fraction of total molybdenum which is active in the metathesis % of total Mo) has been evaluated from the fraction of Mo(V) which remains after adsorption of 0 2 at RT (ESR measurement).
(g
Q: J. Goldwasser (Venezuela) Although the similarities between Mo/Zr02 and Mo/A1203 have been underlined, it is worth adding that on Mo/A1203 the EPR signal is dependent on the Mo loading; the results shown d o not indicate very clearly this dependence on Mo/Zr02
A V. Indovina The dependence of the Mo(V) ESR signal on the Mo loading and nature of the support has been reported in refs. 16 and 19 of the present paper. Q: Can Li (China) Mo(I1) and Mo(V) species in molybdenum oxide are very unstable. In your studies, these species may be stabilized by their special coordinative structures, e.g. as you suggested Mo(V) species in a square pyramidal configuration. What is mainly responsible for these species acting as active sites; the oxidation state of molybdenum, ial coordinative structure, or both ? Have you tried to characterize the Mo(I1) and Mo( ) species by their structural features ?
st=
A: V. Indovina Your question draw the attention to the point that the coordinative structure of Mo can play a role in addition to the oxidation state of Mo. We are well aware of this possibility, and in fact when discussing the related CrO,/ZrO, system we have stressed that the electronic structure alone does not guarantee a high catalytic activity. The coordinative unsaturation of the active site is an additional and essential factor (ref. 9 in the present paper). Relying on IR, ESR and catalytic results, we have inferred that active sites for the low temperature hydrogenation of propene on Cr0,/Zr02 catalysts are surface mononuclear Cr(II1) species with three coordinative vacancies [ 11. In an attempt to characterize the structural features of Mo(II) and Mo(V), an analogous investigation on the MOO /Zr02 system is now in rogress. De Rossi, G. Ferraris, G. Ghiotti, A. Chiorino, V. tndovina, A. Cimino, [l] J. Mol. Catal., 75, 305 (1992)
6
Q: J. C. Mol (The Netherlands) You mentioned that in your MoOx/Zr02 catalyst the metathesis reaction takes place on mononuclear Mo(V) species. However, from related systems it is often concluded that in supported molybdenum oxide catalysts Mo(1V) forms the active site for metathesis. In view of the fact that generally the number of active sites is only a small fraction of the molybdenum ions, I would like to ask if you really exclude Mo(1V) as the active site. With regard to your last conclusion about the analogy in the catalytic behavior of Mo0,/Zr02 and MoOx/A1203, does this mean that you also think that on alumina Mo(V) is the active site for metathesis ?
A: V. Indovina I am absolutely convinced by the evidence reported in the literature in favour of Mo(IV) as the active site for metathesis (see for instance p. 5 1 5 presented in this Proceedings). However, the Mo(1V) sites active for metathesis are produced by photo reduction of silica-molybdena catalysts in CO at 293 K. The catalysts obtained according to this procedure show a much higher activity compared to those obtained by thermal reduction. This marked difference suggests a different nature for the active sites in the thermally reduced samples. Passing to the last part of your question, the clear analogies in the catalytic behavior of MoOx/ZrOz and MoOx/A1203 strongly suggest a common nature for the Mo species that arc active in the two supports.
Q: I. E. Wachs (USA) Can you lease comment on the origin of the different effect of the support for MOO, and Cr8, during hydrogenation and metathesis ?
A: V. Indovina Your question refers to the oral presentation of the paper in which I have stressed that the catalytic activity of chromium for hydrogenation is substantially affected by the nature of the support (ZrO A1203, SO$, whereas that of molybdenum is not. A tentative explanation of this 8ifferent effect is the large dimension of the heptamolybdate precursor whose nature is little affected by the support. Q: W.K. Hall (USA) In our work (Proc. Conf. on Uses of Molybdenum, 1982), we found that tetrahedral Mo(VI) exists in small amounts on MoO$A1203 catalyst, this can be reduced to Mo(V), but no further. A second Mo(V) signal is developed as thc catalyst is reduced, this stems from reduction of the octahedral molybdenum present in the polymolybdate clusters. The latter dominates at intermediate values of e/Mo, but when e/Mo>3, the tetrahedral signal now remains. Have you seen this behavior in your system ? I noted that you said your Mo(V) was monomer.
887
A: V. Indovina In the case of MOO /Zr02, the tetrahedral Mo(V) is formed at very low concentration after heating t i e samples with 0 at 773 K followed by evacuation at the same temperature. After reduction with H2 at 200 to 800 K, a different Mo(V) signal is observed. The ESR analysis of spectra of samples containing 95Mo allows assigning the signal to a mononuclear Mo(V) species on the surface of ZrO, (Mo(V)~J
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Guczi, L el ul. (Editors), New Fronriers in Coralysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
Pt-Sn-ALUMINA CATALYSTS: RELATING CHARACTERIZATIONAND ALKANE DEHYDROCYCLIZATION DATA B. H.Davis Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, KY 405 11, USA
1.
INTRODUCTION
Bimetallic catalysts have been of interest in heterogeneous catalysis for many years. Schwab, in a classic paper ( l ) ,called attention to the role of alloy composition in determining catalytic activity. The electronic theory of catalysis (2,3)provided motivation for numerous studies with alloy catalysts during the 1950s. Introduction of the Pt-Re naphtha reforming catalyst by Chevron in the 1960s (4) provided additional motivation for the study of supported bimetallic catalysts. Many early investigators held the view that d-band electron concentration could be controlled by alloy formation so that catalytic selectivity and/or activity could be influenced through control of the d-band electron concentration (5). Many workers used concepts based upon alloy formation to explain the results from their studies of supported bimetallic catalysts (e.g., refs. 6,7). As more studies were completed with supported bimetallic catalysts, workers began to question the extent of alloy formation in these catalysts. Sinfelt (8) introduced the concept of metal clusters, without specifying a cluster composition, to account for the properties of supported bi- or poly-metallic catalysts. The Pt-Sn bimetallic catalyst was introduced shortly after the Pt-Re combination. While Pt-Sn received much attention, both for activity and characterization studies, it was not utilized as widely as the Pt-Re catalyst. However, the Pt-Re catalyst requires a complex pretreatment/induction period in order for the catalyst to exhibit its superior selectivity and aging properties. Continuous regeneration naphtha reforming processes have received considerable attention recently. Since it is not practical to use, in this process, catalysts which require elaborate pretreatments, the Pt-Sn bimetallic catalyst is receiving renewed attention. A considerable number of studies have been devoted to the characterization of supported and unsupported Pt-Sn catalysts. Likewise, a number of studies have been devoted to determining the selectivity and/or activity of these catalysts. Characterization data for supported catalysts can be found to promote the view that incomplete reduction of tin prohibits the formation of an alloy in the supported material; on the other hand, data can be found that supports alloy formation. Most
890 of the activity studies have been conducted at atmospheric pressures rather than the higher pressures normally encountered in naphtha reforming processes. We have therefore prepared a series of catalysts, using both acidic and nonacidic supports, with a range of Sn/R ratios. These catalysts have been utilized in a number of characterization studies and have been employed in activity measurements both at atmospheric and higher pressure alkane dehydrocyclization studies. A comparison of the characterizationand actiwty data is the subject of this paper.
2.
EXPERIMENTAL 2.1.
Catalysts.
Acidic porous transitional aluminas were prepared by precipitation from an aluminum salt solution or a nonporous Degussa Aluminum Oxide C was used. Nonacidicalumina was prepared by precipitationfrom a potassium aluminate solution by bubbling CO, into the solution. Surface areas of the supports were in the 110-210 m2/g range. A number of impregnation techniques were employed. Many of the preparations employed an organic solvent, acetone, to inhibit the hydrolysis of the tin chloride solution. In several instances, a specific bimetallic complex, e.g., [R,Sn,CI,]", was employed. To our knowledge, these early preparations were among the first, if not the first, to employ an organic solvent and a bimetallic complex for the preparationof bimetallic catalysts to ensure an initial uniform distribution of the two metals over the support surface (9). When a specific WSn complex was not utilized, an acetone solution of chloroplatinic acid and stannous chloride was used to impregnate the support. 2.2.
Characterization.
In-sit0 X-ray diffraction studies (XRD) were carried out wherein the reduction and/or oxidation procedures could be effected without either sample transfer or exposure to air following pretreatment. In other XRD studies the sample was pretreated in hydrogen and then passivated in a helium stream containing 1% oxygen. X-ray photoelectron spectroscopy (XPS) studies involved the pretreatment of the sample in a reaction chamber capable of high pressure (severalatmospheres) and temperature (to 500OC) followed by insertion of the sample holder directly into the XPS vacuum chamber without exposure of the sample to the atmosphere. MBssbauer studies were conducted using a reduced sample that had been transferred in a glove box to an argon-filled and sealed holder containing about 450 mg sample. Samples utilized for absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were prepared similarly to those employed for the Mdssbauer studies.
891
2.3. Reactions. For atmospheric conversion studies a glass reactor system, using glass beads as preheater, was utilized; reactant was fed using a syringe pump. For the higher pressure studies a stainless steel plug-flow reactor, utilizing quartz chips as preheater, was employed. Hydrogenfeed was controlled using a mass flow regulator and the hydrocarbon was fed using a positive displacement pump. The catalyst was pretreated at 500°C and the reaction was conducted at 482%. Product analysis was accomplished using gas chromatograph.
3.
RESULTS 3.1.
Characterization.
In-situ XRD studies of catalysts containing either 5 or 0.6% R and varying amounts of Sn on nonporous Degussa Aluminum Oxide C clearly showed the presence of PtSn alloy, and PtSn was the only alloy phase detected (Figure 1). This is surprising since a number of PtSn phases are stable (e.g., Pt,Sn, PtSn, RSn,, PtSnJ and alloys containing more Sn would be anticipated for the catalysts with higher Sn/Pt ratios. However, as the Sn/R ratio increases the relative area of XRD peaks corresponding to PtSn alloy increases (Figure 2.) Thus, the XRD data indicate
8 A
15-
10
-
A
A
5-
3 5
I
I
35
45
28
I
5
I
2
I
I
4
6
I
8
I
1
0
Sn/Pt
Figure 1. XRD profile for: (bottom) Degussa Aluminum Oxide C alumina support, (middle) 5 wt.% Pt (Pt:Sn = 3:8)on alumina after reduction at 40O0C and (top) 5 wt.% Pt (Pt:Sn = 3:8)on alumina after reduction at 500°C. Figure 2. integrated intensity of the 100% PtSn (102) alloy profile versus tin content for varying Sn/Pt ratios using United Catalysts, Inc. (I)and Degussa alumina (A)support.
892 that for the lower Sn/Pt ratios platinum Is present as both Pt" and PtSn alloy; the ratio of the two forms of platinum depends upon the Sn/Pt ratio. Early XPS studies did not provide evidence for the reduction of tin to the Sn" (10,l 1). However, later XPS studies using an instrument with a much better vacuum system resulted in the detection of Sn" and showed that as the Sn/Pt ratio increased so did the amount of tin in the RSn alloy. It must be emphasizedthat XPS data does not provide direct evidence for PtSn alloy formation. However, it is possible to use the XPS data to obtain a fraction of the tin that is present as Sno and this, together with the known amount of Pt and Sn present in the catalyst and the assumption, verified using both Rh/Sn catalysts and WSn on carbon or silica supports, that all platinum Is present as P t O , permits us to calculate the alloy composition for PtSn,. The data, plotted in Figure 3, indicate that the alloy apparently becomes richer in Sn as the Sn/Pt ratio increases (12). The number of Mbssbauer studies of PtSn-alumina catalysts is too large to describe here; however, the general results suggest that tin is present as Sn4+,Sn2+ and Sn'. Our results for two PtSn-alumina catalyst series are in agreement with this observation for the valence of tin (13). Moreover, as the ratio of Sn/Pt increases the amount of PtSn increases. In Figure 4 we have plotted the value of g for PtSn, as Sn/Pt increases; in making this plot we have assumed that all of the platinum is present as Pt'.
CX v)
E
L
0 w-
x
I
I
0
I
U
I 1 A
/
2
Q
1
0
I
I
2
1
4
1
6
Sn/Pt
I
a Sn/Pt Atomic Ratio
Flgure 3. Alloy composltlon, PtSn,, calculated from XPS data, assuming all SnO lo present as alloy, for varying Sn/Pt ratios using UCI (0) and Degussa (A)alumlna supports. Flgure 4. Composltlon of PtSn alloy calculated based upon Miissbauer data veraus Sn/Pt ratlo for Degussa alumina (0)and UCI alumina (A).
Structures for a series of Pt/Sn-alumina catalysts similar to those described above were obtained from NEXAFS and EXAFS data (14,15). Resolved near-edge
893
profiles indicate that the higher states of oxidation have larger gj band vacancies, presumably due to enhanced ionlcity of the platinum centers. Increasing the tin loading decreases the r! band vacancy. These results are in general agreement with a report by Lytle et al. (16). EXAFS data Indicate three types of Pt bonding in the nearest neighbor bonding: Pt-0,R-CI and Pt-Sn. These data are consistent with PtSn alloy formation but do not require it. In the preparation of the above catalysts chloride is incorporated since the metals were added as the chloride containing salt. Alumina Is particularly effective in retaining the halide added during catalyst preparation (10). The XPS study revealed a surprising fact concerning the surface chloride since the material pretreated in hydrogen at 4WoC had a significantly lower CI/AI peak area ratio than the same material following pretreatment in oxygen at 400OC or higher. Furthermore, it was observed that repeated pretreatments in hydrogen and then oxygen caused the lower and then the higher CI/AI ratios to also repeat. In summary, the characterization data indicate that PtSn alloy is formed following reduction of alumina supported catalysts. It appears that the dominant alloy is one with Pt:Sn = 1:1; other alloys, if present, constitute minor components of the bimetallic catalytic material. Furthermore, not all of the Pt is present as an alloy for catalysts with low Sn/Pt ratios and the fraction of alloy increases with increasing Sn/Pt ratios. Chloride is retained by the catalysts when alumina is the support; however, it appears that the fraction of chloride in the surface layers is lower in the reduced catalyst than in the oxidized material. Since there appears to be a gradual change to a higher fraction of Pt being present as PtSn alloy as the Sn/Pt ratio is increased, it appears that there should be a gradual change in catalyst properties if they follow the alloy composition.
3.2. Octane Dehydrocyclization. The n-octanedehydrocyclization activity of the nonacidic Pt-Sn-aluminacatalysts increase as the Sn/Pt ratio increases, then it attains its maximum activity and gradually declines as the Sn/Pt ratio increases further. For an alumina support with a surface area of about 200m2/gthe maximum activity is obtained for a Sn/Pt ratio of about 3-4 (Figure 5). The preceding acti\*ity results are obtained when the operating pressure is about one atmosphere. When operating at about 30 atmospheres the promoting effect of the tin is not apparent; in fact, it appears that tin is a catalyst poison for Sn/Pt = 6 to 10. The only C,-aromatics that can be formed from n-octane by a direct six-carbon ring closure are ethylbenzene and m - x y l e n e . The data in Table 1 show that equal amounts of ethylbenzene and &-xylene are formed for a catalyst containing only Pt; the unallowed C,-aromatics, and w x y l e n e , are present in minor amounts. There is a change in the distribution of the allowed C,-aromatics for a catalyst containing Sn/Pt ratio of about 3; for this catalyst w - x y l e n e is formed in about twice the amount of the ethylbenzene. Again, for this Sn/Pt catalyst only a minor fraction of the aromatics are those corresponding to those not allowed by a direct C, ring closure. Surprisingly,for the higher Sn/R ratios, where the total conversion to
894
C
.-0 E
s C
8
**I
\
\
\
1.o
\ \
SnIPt Figure 5. The variation of n-octane conversion at atmospheric pressure with increasing tin loading using a nonacldic alumina support (point at right is for Pt on SnO, only). Table 1 Aromatic Products from n-Octane Dehydrocyclizationwith Pt-Sn-NonacidicAlumina Catalysts ~
Aromatics Catalvst
Pressure
Pt-
Ethvlbenzene
o-Xvlene
m-Xvlene
p-Xvlene
1 atm.
48
50
1.2
1.o
Pt-Sn = 3:8
1 atm.
32
59
5.0
2.4
Pt
7.8 atm.
45
48
7.0
trace
Pt-Sn- = 1:3
7.8 atm.
38
62
trace
trace
aromatics is smaller, the fraction of unallowed C,-aromatics is higher than for Pt only or for low SnIPt ratio catalysts. The same aromatic selectivities are obtained at 1 and 7.8 atmospheres. The C,-aromatics produced using a Pt-alumina catalyst using an acidic alumina support, are nearly an equilibrium mixture of four possible isomers. Furthermore,the
895
conversion of n-octane to C,-aromatics is much slower than the isomerization of noctane to other isomeric C,-alkanes (Figure 6). Thus, with the acidic catalyst the isomerization of n-octane to a nearly equilibrium mixture of C,-alkanes is essentially completed before appreciable C,-aromatics are formed. in addition, the acidic catalyst produces aromatics more selectively than a corresponding Pt-nonacidic alumina catalyst. This means that there is a bifunctional cyclization pathway that is both more rapid and more selective for C,-aromatic production than for the monofunctionalcatalyst based upon a nonacidic support. The data in Figure 7 is for catalysts based upon an acidic support but with increasing Sn/Pt ratios. It is apparent that the addition of tin has a detrimental effect upon the conversion of n-octane. Thus, for the catalysts prepared by surface impregnatedthe presence of tin to give Sn/Pt of about 0.5 or higher, the tin acts as a catalyst poison.
8
-c2 0
al
>
5
0 al
c
Q c
6 C
% Conversion of n-Octane
SnIPt
Figure 6.
Aromatics formed from the conversion of Foctane using a nonacidlc (A)and an acidic (0) aiumins support (LHSV, varied; 7.8 atm; HJnoctane = 3:l; 482OC).
Figure 7.
n-Octane conversion with increasing Sn/Pt ratio using an acidic alumina support (482OC, 7.8 atm, LHSV = 1.8, and HJn-octane = 3/1).
4.
DISCUSSION
One of the proposed pathways for the metal catalyzed dehydrocyclization of alkanes involves the consecutive dehydrogenationto a conjugated triene followed by gas phase thermal cyclization. For dehydrocyclization of n-octane by this pathway with a nonacidic catalyst the following would apply:
896
+ + + c=c-c=c-c=c-c-cj+ c-c-c-c-c-c-c-c
+ + + c-c=c-c=c-c=c-cj+
ac-c (1)
a; (2)
For the catalyst containing only Pt, equal amounts of ethylbenzene and &-xylene are formed. Thus, reactions (1) and (2) occur to the same extent with Pt. The is aromatic products change when Sn is added to the catalyst so that --xylem favored over ethylbenzene 2 to 1; however, it is difficult to understand why the addition of Sn, with the formation of PtSn alloy, should alter the relative amounts of the octatrienes desorbed. On the other hand, if the initial adsorption determines the aromatic selectivity, the following applies:
(3)
@-c n
Cyciization by I involves a primary C-H and a secondary C-H bond whereas cyclization by I1 involves two secondary C-H bonds. Primary C-H bonds require more (at least 3 kcal/moie) energy for rupture than do secondary C-H bonds. Thus, reaction pathway 4, with cyclization through 11, Is favored by the addition of tin provided the adsorption potential of the metal surface is decreased by alloy formation. Since tin presumably donates electrons to decrease the number of dband holes (ens., EXAFS data), a product selectivity determined by adsorption appears more reasonable to us than the triene mechanism. For the bimetallic Pt-Sn on nonacidic alumina, increasing the Sn/Pt ratio increases both the extent of alloy formation and the catalytic activity. Thus, it is concluded that for the single-functionalmetal catalyzed pathway, the formation of PtSn alloy is beneficial since the alloy is more active than Pt alone.
897
For the acidic catalyst support, the situation is very different. The bifunctional pathway produces aromatics more rapidly and selectively than the metal catalyzed single-function pathway. Thus, it appears that the loss of acidic sites that is caused by the addition of tin by impregnation more than cancels any gain caused by alloy formation for the metal-only cyclization pathway. For the acidic catalyst it appears that the beneficial role for Sn is to primarily, or only, improve long-term aging by maintaining a better metal dispersion. REFERENCES 1. 2. 3.
4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14.
15. 16.
G. M. Schwab, Discuss. Faradav SOC.,4 (1950) 166. D. A. Dowden, J. Chem. SOC.,(1950) p. 242. D. D. Eley, Catalysis and the Chemical Bond, Univ. of Notre Dame Press, Notre Dame, 1954. R. L. Jacobson, H. E. Kluksdahl, C. S. McCoy and R. W. Davis, Proc. Amer. Petroleum Inst, Div. Ref., 49 (1969) 504. 0. Beeck, Discuss. Faradav SOC.,4 (1 950) 118. G. Reinacker and E. A. Bommer, Z. Anora. Chem., 242 (1939) 302. B. H. Davis, J. Catal.. 46 (1977) 348. J. H. Sinfelt, Bimetallic Catalysts: Discoveries, Concepts, and Applications, Wiley, New York, 1983. B. H. Davis, Bimetallic Catalyst Preparation, US Patent No. 3,840,475, (1 974). S. R. Adkins and B. H. Davis, J. Catal., 89 (1984) 371. B. A. Sexton, A. E. Hughes and K. Foger, J. Catal., 88 (1984) 466. Y.-X. Li, J. M. Stencel and B. H. Davis, ADDI. Catal., &4 (1990) 71. Y.-X. Li, K. J. Klabunde and B. H. Davis, J. Catal., 124 (1991) 1. Y.-X. Li, N.4. Chiu, W.-H. Lee, S. H. Bauer and B. H. Davis in Characterization and Catalyst Development (S. A. Bradley, M. J. Gattuso, and R. J. Bertolacini, Eds.), ACS Symp. Series 41 1 (1989) 328. N.4. Chiu, W.-H. Lee, Y.-X. Li, S. H. Bauer and B. H. Davis in Hydrotreating Catalysts. Preparation, Characterization and Performance (M. L. Occelli and R. G. Anthony, Eds.) Elsevier Sci. Pub., Amsterdam, (1989) 147. F. W. Lytle. P. S. P. Wei, R. B. Gregor, G. H. Via and J. H. Sinfelt, J. Chem. Phvs., 70 (1979) 4849.
DISCUSSION Q: A. Duncan (Norway) It was indicated during the presentation that for a co-precipitated catalyst a constant conversion is obtained with respect to the Sn/Pt ratio, in contact to the co-impregnated system. This, I assume, is due to the absence of PtSn alloying found. I am interested to know if (1) the quoted Sn/Pt ratios for the co-precipitated system relates to the total (bulk) Sn in the alumina support, and (2) if this is the case, is there any knowledge of the alumina surface composition (Sn/Pt) even though alloying in the Pt particles is not observed.
A: B. H. Davis 1) The Sn/Pt ratios for the co-precipitated tin-aluminum oxide refers to the bulk composition, and not to the surface com 2) The constant activity for the% t:S;?n) = 0, 1, and 3 are taken as an indication that the surface concentration of tin is sufficiently low so that acid sites are not eliminated to a significant extent for a given Sn/Pt ratio compared to a similar catalyst where the tin was added by co-impregnation with the Pt. In other words, for the same bulk Sn/Pt ratio, the co-impregnated sample will have a much higher surface concentration of tin. Q: S. Csicsery (USA) 1) Did you observe over the nonacidic Pt and Pt/Sn catalyst any meta- or paraxylenes, and, if yes, d o you have any hypothesis explaining their formation ? Could 7- or 8-membered ring intermediates be involved ? 2) Did you notice any difference of the CHdtotal hydrogenolysis products ratio between non-acidic Pt and Pt/Sn catalysts ? A: B. H. Davis 1) For the nonacidic Pt and lower Sn/Pt ratio catalysts at least 90 % or more of the aromatics are ethylbenzene and o-xylene. The minor amounts of m- and p- xylene are believed to be due to isomerization of the n-octane charge. rather than the formation of 7- or 8-membered rings. Pines and Chen [l] found only 17 % methyl label from the cyclization of n-[ l-I4C]-heptane with a nonacidic chromia catalyst, and explained this as being due to the formation of a cycloheptane intermediate. A$ you are well aware, Pines and coworkers then extended this concept to other catalysts and other reactants [2-41. Our I4C data for n-heptane dehydrocyclization, using a nonacidic chromia-alumina catalyst re ared according to the recipe provided by Pines and Chen, produced toluene with the 84C! in the positions expected for direct six-carbon ring formation (the position meta to the methyl group for n-[4-I4C]-heptane and the methyl plus ortho to the methyl group for n-[l-I4C]-heptane [5, 61. We have therefore not obtained data consistent with 7- or 8-membered ring intermediates with nonacidic chromia-alumina catalysts. In our studies [5, 61 we degraded the aromatic ring so that the relative activity of the methyl and each ring position could be calculated. For both [1-14C-] and [4-I4C] heptane dehydrocyclization, the 14C ring distribution eliminated cycloheptane as a significant reaction intermediate. 2) The low C1 - Cq gas make and the significant molar excess of hydrogen precluded us from making a definite conclusion concerning this point. H. Pines and C. T. Chen,J. Org. Chem., 26, 1057 (1961) [l] H. Pines and C. T. Goetsche1,J. Org. Chem., 30,3530 (1965) [2] H. Pines and S. M. Csicsery, J. Am. Chem. Soc., 84,292 (1962) H. Pines, C. T. Goetschel and S. M. Csicsery,J. Org. Chem, 28,2713 (1963) J. A. Feighan and B. H. Davis, J. Catal., 4,594 (1965) [5] B. H. Davis and P. B. Venuto,J. Org. Chem., 36,337 (1971) [6]
I:]
899
Q: G. Maire (France) For the bimetallic Pt-Sn on non-acidic alumina (see p.
) d o you exclude formation of both ethylbenzene and ortho-xylene via ring enlargement from the cyclopentanes formed '? It is known from the work of F. Gault (Advances in Catalysis, 1980) that such mechanism takes place on small particles of Pt or even on alloys.
A: B. H. Davis My view is that the formation of aromatics at both low (1 atm) or high (7-30 atm) occurs over a non-acidic Pt or PtSn catalyst by a direct six carbon ring formation. We are familiar with the work of the late Professor Gault. In fact, Professor Gault compared our earlier work to his data on page 56 in the reference cited [7], "The distribution determined by microwave spectrometr of the toluenes from n - h e p t a n e - l - l e is: methyl , meta-Y3C, 16 % 8 . These results can be compared to (13C), 50 %; ~ r t h o - ~ ~34C%; those obtained by Davis [9] with n-heptane-1- C. The formation of meta-labeled toluene can be explained neither by direct 1-6 carbon ring closure, nor by cyclic-type isomerization of n-heptane to 3-methyl hexane followed by 1-6 ring closure of the latter. "Alkylcyclopentanes undergo ring-opening hydrogenolysis to produce products that are very similar to the distribution expected for a statistical bond rupture mechanism [lo-121. Thus, for [1-14C]- or [l-13C]-heptane the following applies for cyclization to produce ethylcyclopentane and 1,2-dimethylcyclopentane followed by hydrogenolysis (only pathways leading to methyl hexanes are included; represents the position of the labeled atom):
u
b
b
900 Interchange of the label in the methyl groups of 1,2-dimethylcyclopentane leads to the same products except the bond rupture labels a through e will now be labeled clockwise rather than counter clockwise as above. Direct 1-6 ring closure of the two labeled methylhexanes will produce toluene labeled in the methyl and the meta-ring position, just as was observed by Professor Gault and coworkers with [l-lql-heptane. Professor Gault and coworkers also converted [ 1-1Ql-heptane with the same catalyst under the same conditions and wrote [13]: "A less accurate analysis of the toluene by microwave spectroscopy shows the presence of meta-labeled toluene only. an accurate mass spectrometric analysis of the methylcyclohexanes obtained by hydrogenation of toluenes shows that only 2.5 + 1 % of the label is located on the methyl group." Thus, the data for [ 1-l 3C]-heptane dehydrocyclization, based upon microwave analysis, does not agree with the data Professor Gault and coworkers obtained with [4-1*]-heptane using the same catalyst. Parayre et al. [14] converted 2-methyl [2-'3c] hexane with the same catalyst used for the dehydrocyclization of labeled n-heptane and concluded " ... that 1-6 and not 1-5 ring closure is the major mechanism of this reaction (dchydrocyclization to toluene). Amir-Ebrahimi and Gault [ 151 converted %Xabeled 3-methylhexane over the same catalyst and reported that "2-methyl hexane is aromatized almost exclusively by 1-6 ring closure ..." Pines and Nogueira [I61 found the methyl 14C labeled from the dehydrocyclization of n-[l-14C]-heptane that was in excellent agreement with the data we had reported earlier [9]. Pines and Nogueira chose to explain their data by a combination of mechanisms, including the formation of a cycloheptane intermediate. However, the data of Pines and Nogueira are consistent with more than 80 % of the toluene being formed by 1-6 ring closure. The experimental data of Professor Gault and coworkers, of Pines and Nogueira, and our data are all consistent with 80 % or more of the aromatics being derived from a 1-6 ring closure mechanism. It appears to us that the only disagreement concerns the formation of the products not consistent with 1-6 ring closure. We believe experimental error in measuring the location of the label is responsible for a significant amount of the "appearance" of these minor products. Finally, we do not want to even imply that alkylcyclopentanes are not formed, only that they do not make an important contribution to the formation of the primary aromatic products. They do, we believe, contribute to the appearance of the labeled carbon at positions not allowed by direct 1-6 ring formation. F. G. Gault, Adv. Cuful.,30, 1 (1981) [7] V. Amir-Ebrahimi, A. Choplin, P. Parayre, and F.G. Gault, Nouv. J . Chirn., 4, [8] 431 (1980) B. H. Davis, J. Cutul., 29,398 (1973) [9] S. M. Csicsery and R. L. Burnett, J. Cuful.,8, 75 (1967) [ 101 R. H. Hardy and B. H. Davis, Acfu. Chirn. Hung., 124, 269 (1987) [ll] C. S. Huang, D. E. Sparks, H. A. Dabbagh and B. H. Davis, J . Cutul., 134, 269 [12] (1991) F. G. Gault, private communication, 1977 [13] P. Parayre, V. Amir-Ebrahimi and F.G. Gault, J. Chern. Soc., Furuday I, 76, 1141 ~1723 (f980) V. Amir-Ebrahimi and F. G. Gault,J. Chem. SOC.,Furuuiiy I, 76, 1735 (1980) r151 H. Pines and L. Nogueira, J . Cutul., 70, 391 (1981) [16]
Q: W. P. Hettinger, Jr. (USA) With regard to the proposal from the floor that dehydrocyclization proceeds through a triene species, I have the following comments. Our work in dehydrocyclization in 1952
901 and reported in Ind. and Eng. Chem., 1955, and the Gordon Research Conference of 1953, clearly showed that dehydrocyclization, although sensitive to pressure, still proceeded quite well at high pressures where dehydrogenation to a triene is highly improbable. Our kinetics as presented, showed no evidence of an intermediate, therefore suggesting one step cyclization followed by high rate dehydrogenation. In later work, reported orally at the 2nd ICC in Paris, 1960, we showed that co-gelation of tetramethyl hexahydroxy platinum, coprecipitated with aluminum acetate produced a white catalyst, which stayed white on H reduction at 500 OC. This suggested isolation of single platinum atoms, which stilf showed dehydrocyclization, thereby suggesting cyclization takes place on a single atom, and does not require multiple platinum atoms to achieve cyclization. Therefore, I must support Dr. Davis' conclusion that in his studies direct cyclization to a cyclohexane ring followed by very high rate of dehydrogenation, or cyclopentane isomer formation followed by isomerization to a cyclohexane ring, is the mechanism and is related to the concentration of Pt-Sn content, undoubtedly a surface which highly utilizes the direct cyclization reaction.
A: B. H. Davis Thank you for your remark Q: J. Ryczkowski (Poland) As you have mentioned, not all of the Pt is present as an alloy for catalysts with low Sn/Pt ratios and the fraction of bimetallic phase increases with increasing S n P t ratios. My question is, have you any observation for forming other tin phases except Pt-Sn alloys ? What about tin oxide formation ?
A: B. H. Davis If one searches long enough in a heterogeneous system, one can find many structures. We have reported, for example, that PtSn2 particles are observed infrequently in a Pt-Sn-alumina catalyst prepared from coprecipitate tin-aluminum oxide [17]. However, in all catalysts the predominance of Pt/Sn alloy particles have the ratio of Pt:Sn = 1:l; this is true whether the support is alumina or silica. We have never obtained evidence for tin oxide (either SnO or SnOi) by XRD, even when the material contains Pt:Sn = 1:12 [18]. In fact, with a silica supported catalyst with Pt:Sn = 1:12, we observe XRD lines for metallic Sn for a sample following reduction at 482 OC and cooling to room temperature. However, even in this case we d o not observe metallic Sn XRD peaks at 482 OC when employing in-situ XRD measurements; presumably the reduced tin is spread in an amorphous layer upon the support surface at this high temperature. Even in the case when metallic tin is formed in the reduced sample, we d o not observe tin oxide in the calcined catalyst. In hundreds of catalyst preparation, we have never found evidence for bulk tin oxide formation. R. Srinivasan, L. A. Rice and B. H. Davis, J. Cafal.,129, 257 (1991) [17] R. Srinivasan and B. H. Davis, AppL Cafal.,87, 45 (1992) [ 181
Q: J. Volter (Germany) Our results confirm that in a Pt-Sn/A120g system an alloy is formed and is catalytically active. However, in your conclusion you refer only on differences in activity and stability in nonacidic and acidic catalysts without mentioning the selectivities. We have studied carrier free and therefore nonacidic Pt powder and the PtSn alloy and on the other hand acidic Pt/A120, and Pt-Sn/Al2Og samples in the conversion of n-heptane. In both systems Sn shifts the selectivity from hydrogenolytic cracking towards aromatization. This proves the effect of alloying. This effect can be explained by the ensemble theory. The large ensembles, necessary for hydrogenolytic splitting are predominantly blocked by the diluting tin and the selectivity is shifted towards the facile reaction, the dehydrocyclization. This concept implies that alloying is a sufficient but not
a necessary condition because a blockade can be caused by any inactive material. This could be tin as well as tin oxide, We have distinct hints that the bimetal effect on selectivity can be caused by reducible as well as by unreducible oxides of the added second metal. This concept could bridge the gap between the results of Dr. Davis, stressing the role of alloys and the results presented by Dr. Schwank et al., presented in the following paper [19]. They show that under their conditions Pt-Sn alloys are hardly formed. [19] J. Schwank, K. Balakrishnan and A. Sachdev, Proc. 10th h t . Congr. on Catal., Vol A, 9 0 5( 1992)
A: B. H. Davis Unfortunately, none of the characterization techniques that permit a direct measure of the crystal phase VEM, microdiffraction, XRD, etc.) provide a very reliable quantitative measure of the fraction of Pt that is present in an alloy form. Thus, it is not evident that there is a gap between the data we obtained and that of Schwank el al. Our conclusion is that alloy formation increases with an increasing S n P t ratio, and this was observed by Schwank et al. Based upon our XRD intensity data (Figure 2 of our paper) we would not expect more than 10 %, and perhaps less, of the Pt in the Sn:Pt = 1:l to be present as alloy. Schwank el al. [19] reported electron microscopy results for S n P t = 0.5 and 1.0. We d o not view their qualitative observations for these two catalysts to eliminate the formation of alloy. W e have prepared a series of catalyst using a Degussa alumina support material that is similar to the one used by Schwank et al. and have obtained both XRD and TEM microdiffraction data to show the presence of Pt:Sn = 1:l alloy (e.g., Figure 2 of our paper).
Q: Z. Pail (Hungary) The "gas phase cyclization of hexatrienes" has been mentioned in the paper. I do not believe that a molecules might lose several hydrogen atoms in catalytic steps and when it reaches the ultimate hexatriene stage, (when it may be attached to the surface by its three double bonds) it should desorb from the surface just in order to undergo thermal cyclization, then readsorb to form benzene. My views on the stepwise ("hexatriene") reaction route are illustrated by the scheme. A working Pt surface is likely covered by a partly dehydrogenated hydrocarbon pool even in excess hydrogen [20]; they may undergo further dehydrogenation, rehydrogenation, double bond- or cis-transisomerization and they may desorb in either stage. Surface unsaturated species are true intermediates of aromatization; those appearing in the gas phase are the products of surface dehydrogenation and desorption. Desorption should be less likely with increasing unsaturation (as indicated by shorter and shorter arrows). The surface pool "remembers" the original reactant structure, e.g., 1-hexene and 2-hexene have different reactivities [21]. Stepwise dehydrogenation produces either cis- or trans-isomers. The cis-isomer of hexatriene is aromatized likely rapidly and without desorption, the chance of its desorption being practically zero. It is possible that good aromatization catalysts have a special ability to form this precursor structure. The trans-isomer, on the other hand, has to isomerize and, during this process, it has also a minor chance to desorb to the gas phase. Hexatriene detected in the gas phase had always a trans structure [22]. Hydrogen is essential for surface transcis isomerization; as well as for the hdyrogenative desorption of unsaturated surface species since they tend to lose more H atoms than corresponding to exact hexene, hexadiene etc., stoichiometries. Trans- species unable to isomerize before their excessive dehydrogenation may be coke precursors [22]. Z. Pail, M.Rath, B. Brose, W. Gombler, React. Kin. Catal. Lett., 41, 43 (1992) Z. Pall, B. Brose, M. Rath, W. Gombler,J. Mol. Catal., 75, L13 (1992) [22] Z. Pail and P. Tktknyi,J. Catal., 30, 350 (1973)
I;?]
903
GAS:
A: B. H. Davis The results of kinetic tracer studies [23-261 were utilized to support a mechanism based upon gas phase hexatriene intermediates. We argue against the gas phase triene because (1) of the energy scheme the formation of a gas phase hexatriene is very unfavorable relative to the aromatic and (2) it does not account for change we obtain in the C8 aromatic distribution that is caused by the addition of tin: m - X y IcneEth y I benzene Pressure
Catalysts Pt -A1203 Pt-Sn-A120g Pt-SiO Pt -Sn 02
-ti
1 atm
13-26 atm
111
111 211
211 1/2 211
To obtain convincing evidence to support or refuse the operation of a mechanism involving an adsorbed triene is a demanding task. Cyclization of n-octane, for example, produces both Cg aromatics and alkyl substituted cyclopentanes. These substituted alkylcyclopentanes are formed and undergo hydrogenolysis to produce isooctanes more rapidly than the Cg-aromatics are formed [ll, 121. The scheme outlined by Pail does not provide a means of forming these substituted cyclopentanes. For simplicity, we prefer to view the cyclization to be the result of competitive cyclization pathways:
904
Our preference is for the simple mechanism where the CQ-and cyclization C5-pathways are similar, and a direct six carbon ring permits this. Obviously, our preference has no bearing on the validity of the triene mechanism advanced by Dr. Paal. Y.I. Derbentsev and G. V. Isagulyants, Usp. Khimii (Russian), 38, 1597 (1969) .,.I Guczi and P. Tkttnyi, Annals. N.Y.Acad Sci.,213, 173 (1973) Z. Pall and P. Tkttnyi, A m . Chim. Hung., 58, 105 (1968) B. A. Kazanskii, G. V. Isagulyants, M.I. Rozengart, Yu. G. Dubinsky and [26] L. I. Kovalenko, Proc. 5rh Intern. Cong. Cataf., (Ed.: J. W. Hightower), Vol. 2, p. 1277 (1973)
Guczi, L. el al. (Editors), New Frontiers in Curalysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
BIMETALLIC Pt-SdAI203 AND Pt-Au/SiO2 CATALYSTS: A COMPARISON OF REACTIVITY,ADSORPTION BEHAVIOR AND MICROSTRUCTURE J . Schwank, K Balakrishnan andA. Sachdev Department of Chemical Engineering, University of Michigan, 3074 H.H. Dow Building, 2300 Hayward Street, Ann Arbor, Michigan 48109-2136, USA
Abstract Two series of supported bimetallic catalysts containing 1 wt% platinum as the primary group VIII metal and varying contents of either gold or tin were prepared and characterized. The objective was to correlate the microstructure with the activity and selectivity trends in reactions of hydrogen with either nhexane or neopentane. In a multifaceted characterization protocol, special emphasis was placed on temperature programmed reduction, chemisorption of Ha,0 2 and CO, infrared spectroscopy of adsorbed CO, and analytical and high resolution electron microscopy. A major objective was to determine the state of the second metal and to assess the extent of solid solution or alloy formation in individual metal particles. The presence of gold manifested itself predominantly in terms of a geometric effect, breaking up larger ensembles of platinum. The behavior of the Pt-Sn catalysts was more complicated. Most of the tin was in a valence state of either Sn(I1) or Sn(IV), according to XPS results, and most likely complexed with the alumina support. Tin was always located within close proximity to platinum particles. In a few instances, a few platinum-tin alloy particles were observed. However, they did not appear to play a significant role in controlling the catalytic behavior. Most of the platinum was present in monometallic form, with some particles in intimate contact with patches of ionic tin. INTRODUCTION Supported bimetallic catalysts containing platinum as the primary metal are of great importance both from a n industrial as well as a fundamental research point of view. Adding tin t o Pt/Al2O3 reforming catalysts leads to improved activity maintenance and higher selectivity for cyclization and isomerization, thereby giving a higher octane number product [ll. Adding inert gold to platinum can give fundamental insight into geometric and electronic effects in bimetallic catalysis. Comparing the effect of gold and tin is instructive in view of the differences in miscibility with platinum. The Pt-Au phase diagram shows a miscibility gap between 18 and 98% platinum, and a n AugPt solid solution can be formed. In contrast, Sn is completely miscible with platinum in the bulk state and according t o the Pt-Sn phase diagram the
906 following phases exist: PtsSn, PtSn, PtaSng, PtSn2, and PtSw [21. However, the bulk phase diagram is not necessarily applicable t o the realm of nanometer scale metal particles in intimate contact with the alumina support. Sn and the alumina support can form a so called "Sn-aluminate" complex [3] stabilizing oxidation states of tin different from zero. The extent of metal-metal interactions in supported bimetallic systems is not always predictable based on the bulk miscibility of the two metals. Even if two metals are completely immiscible in the bulk state, they can still interact in the form of "bimetallic clusters" [41. In this work we have compared the effect gold and tin on the reactivity, adsorption behavior, and surface composition of supported platinum, keeping the nominal loading of platinum constant a t 1 wt % while systematically varying the amount of second metal. An effort has been made to correlate the surface characteristics of the catalysts with their bulk composition and structure, with the goal to better understand how the second metal component influences the catalytic behavior of platinum.
EXPERIMENTAL Two series of supported bimetallic catalysts, containing P t a s the primary group VIII metallic component, were prepared using impregnation techniques. The first series was the Pt-AdSiO2 catalyst series where the nominal Pt loading was kept constant a t 1 wt % and the Au nominal loading was 0, 0.3, and 0.7 wt %, respectively. In addition, a monometallic Au sample having a nominal Au loading of 2 wt % was prepared. Aerosil 200 silica with a BET surface area of 200 m2/g was used as support. The Pt-SdAlzOs catalyst series had a nominal Pt loading of 1 wt % and the Sn nominal loading was 0, 0.1, 0.5, and 1.0 wt %, respectively. A monometallic S n sample having a nominal Sn loading of 2 wt % was also prepared. Non-porous Aluminum Oxide C with a BET surface area of 90 m2/g was used as support. The alumina consisted of mainly the gamma phase and some of it was in the delta phase. There was less t h a n 0.5 % HC1 present in the alumina. Hydrogen hexachloroplatinate (IV)hydrate containing 39.7 wt% platinum, hydrogen tetrachloroaurate trihydrate containing 50 wt% gold, and tin (11) chloride having 61.7 wt% tin (all obtained from Aldrich Chemical Company) were used as precursors. The catalyst samples were prepared by impregnation or (for the bimetallics) co-impregnation of the support with a solution of the metallic precursors. The solvent used was distilled water in the case of the Pt-AdSiOz samples and acetone in the case of the Pt-Sn/Al203 samples. After impregnation the samples were allowed to dry initially a t room temperature and then overnight in a stream of flowing high-purity grade air at 393 K. The Pt-SdAl2Og samples were calcined in a stream of flowing high-purity grade air a t 773 K for 2 hrs. I n the case of the silica supported samples the calcination step was avoided. Aliquots of the catalyst samples were then reduced in flowing hydrogen (15 SCCWmin) a t 673 K for 5 hrs. Table 1 gives an overview of the catalysts, and lists both the nominal metal content based on the amount of precursor used for catalyst impregnation as well as the actual metal contents a s determined by neutron activation after reduction. Temperature programmed reduction experiments were carried out in a flow
907 reactor on air-dried, unreduced catalyst samples, with a heating rate of 10 Wmin in a stream of 3% H n 2 . Table 1 .. Determination of catalvst w o s i t i o n bv neutron activation analvsis Catalyst Code
Pt [wt%]
1.0 Pt/SiOn 1.0 Pt-0.3 AdSiO2 1.0 Pt-0.7 AdSiO2 2.0 AdSiO2 1.0 PVAl2O3 1.0 Pt-0.1 SdAl2O3 1.0 Pt-0.5 SdAl2O3 1.0 Pt-1.0 SdAl2O3 2.0 SdAl2O3
1.00 1.15 1.11
--0.99 0.96 1.00 0.89
Au [wt%l
rwt%i
Sn
---
---
0.24 0.47 1.25
--0.14 0.53 0.99 1.90
Numerical values in the catalyst codes indicate nominal metal loading (wt %). H2, 0 2 , and CO chemisorption experiments were performed in a staticvolumetric gas chemisorption system. Characterization by XPS was carried out using a Perkin Elmer PHI 5400 instrument with a n A1 anode of energy 1486.6 eV. The samples for XPS were reduced in a n in-situ reactor directly attached to the analysis chamber so that the samples were not exposed to air a t any time between the reduction and spectra collection. Fourier Transform Infrared spectroscopy of adsorbed CO was performed using a Digilab FTS 20/C instrument. The catalysts were pressed into a self-supporting wafer and mounted in a high-vacuum IR cell. Analytical microscopy was carried out in a JEOL 2000FX operated a t 200 kV. The elemental composition of small catalyst regions was determined by using Energy Dispersive Spectroscopy (EDX) with a stationary TEM probe. With this instrument, meaningful EDX results could only be obtained from regions larger than about 6 nm. To get better spatial resolution, a dedicated scanning transmission electron microscope (VG-HB501) was used. This instrument was equipped with a field emission gun which provided a very small electron probe and high current density making it possible t o extract EDX signals from regions as small as 1 nm in diameter. A collection period of a maximum of 200 seconds was allowed for each spectrum. Structural information of metallic regions was obtained from high resolution electron microscopy (HREM) on a JEM- 4000EX microscope operating a t 400 kV. To determine the accurate lattice spacings, optical diffractograms of the structure images were obtained. The distances on the diffractograms were calibrated using a S i c l l b specimen whose (220) lattice spacings measure 0.192 nm. In some cases it was necessary to compare the
908 experimental images with image simulations based on the multislice method
El.
Catalytic probe reactions of n-hexane and neopentane with hydrogen were carried out in a flow reactor under differential reaction conditions at pressures close t o 1atm (1 atm = 1.013 x 105 Pa). Product analysis was accomplished by means of gas chromatography. RESULTS AND DISCUSSION
Effect of gold and tinon temperaturepmgrammed reduction ofplatinum Temperature-programmed reduction experiments were conducted to evaluate the effect of gold and tin on the reducibility of platinum. Figure 1 illustrates the hydrogen consumption of freshly prepared catalysts as a function of reduction temperature, tracking the genesis of reduced metal species in the catalysts. The dashed line in the figures indicates the peak temperature for monometallic Pt reduction.
I
1 Pt-0.1% 1Pt
1 300
500 700 Temperature (K)
Figure 1 a. Temperatureprogrammed reduction profiles of Pt-AdSiOz catalyst series.
473
673
873
Temperature ( K) Figure 1b. Temperatureprogrammed reduction profiles of Pt-SdAlaOg catalyst series.
The monometallic PldSiOscatalyst had a single reduction peak a t 503 K corresponding t o the conversion of the precursor, chloroplatinic acid, into zero valent platinum. The P.OAulSiO2 catalyst showed the main reduction peak at 413 K and two minor peaks at 488 K and 528 K. The peak a t 488 K can be
909 attributed to the formation of gold particles with wide size distributions, and the 528 K peak is due t o the sublimation of chloroauric acid as AuC13. The TPR peaks in the bimetallic l.OPt-O.3AdSiO2 catalyst indicated a lack of significant interactions between Pt and Au. The large peak a t 503 K occurred exactly where the Pt peak in 1.OPt/SiO2 had appeared. The shoulder a t 433 K was probably due t o Au reduction. In the l.OPt-O.7AdSiOa bimetallic catalyst, on the other hand, the Pt reduction peak maximum was shifted t o a lower temperature of 493 K suggesting an interaction with the Au component. In the PVAl2O3 catalyst the Pt reduction peak maximum was located a t 513 K, 10 K higher than in the corresponding silica supported Pt sample. A shoulder a t higher temperatures was most likely due t o the reduction of a n oxychloridic surface species. The 2.OSdAl2O3 catalyst exhibited a broad reduction profile, stretching from 493 K to well beyond 800 K. The most striking feature in the TPR profiles of the bimetallic Pt-Sn samples was the almost complete absence of the broad Sn reduction profile stretching into the high temperature region. It seems that Pt facilitated the reduction of Sn, bringing its TPR peak down to a lower temperature where it was masked by the P t TPR peak. The Pt reduction peak gradually shifted t o higher temperatures with increasing tin content. The TPR results convincingly show that Pt and Sn must be in close interaction with each other, so that hydrogen which gets activated on Pt sites can reduce the neighboring S n W ) species.
Effect of gold and tin on chemisorptionbehavior of platinum Figure 2 compares the results of static volumetric chemisorption experiments of H2, CO, and 0 2 , on the Pt-AdSiO2 and Pt-SdAlaOs catalysts. The data represent the total gas uptake a t room temperature. Regardless of catalyst composition, approximately 35-45% of the hydrogen was weakly adsorbed and could be removed by evacuation for 1 hour a t room temperature. In the Pt-AdSiO2 catalyst series, about 10% of the CO was weakly adsorbed, compared to 16-17 % for the Pt-SdAl203 series. Oxygen, on the other hand, could not be removed by evacuation a t room temperature, except for the monometallic SdAl2O3 sample, where 30% of the oxygen was weakly held. As can be seen from Figure 2, adding Au t o Pt drastically lowered the WPt, COPt, and O P t ratios, suggesting that fewer Pt chemisorption sites were available on the surface of the bimetallic Pt-Au samples. The chemisorption results for the Sn series were quite different. Adding tin increased the gas uptake ratios, with a maximum for the lPt-O.lSdA1203 sample having a Sn/Pt atomic ratio of 0.235. There is another important difference between Au and Sn: in the PtAu/Si02 series, all three adsorbates showed the same trends of decreasing uptake with increasing AdSn atomic ratio. In the case of Pt-SdAlaOg, on the other hand, the oxygen uptake increased even a t higher tin loadings, where the WPt and C O P t ratios levelled off. It is important to note here that the monometallic Sn sample did not chemisorb significant amounts of oxygen at room temperature, most likely due to the fact that Sn by itself is not very effective in activating molecular oxygen a t room temperature. However, in presence of Pt, one could envision a scenario where activated oxygen species migrate from Pt sites to Sn sites, oxidizing them to a Sn(IV) state. A more
91 0
-
detailed discussion and interpretation of the chemisorption results has been given elsewhere [6, 71.
9
0.8
wpt
COPt A OPt
wpt
COPt A OPt 0
n /
0.6
Pt-Sn
0.4
0.2
0.0
! 0
I
Au/Pt or S d t atomic ratio
I 2
Figure 2. Ratio of WPt, CO/Pt, and O p t , as a function of overall metal composition. The gas uptake ratios are calculated from total gas uptake values a t room temperature and normalized on the basis of total Pt atoms in each catalyst. FTIR spectra of adsorbed CO indicated that the effects of Au and Sn on CO adsorption were mainly geometric in nature. Figure 3 summarizes the peak position for linearly adsorbed CO after exposure to 1 atm CO for 1 hour, followed by removal of the gas phase CO by evacuation at room temperature for 1 hour. Adding the second metal component led t o a shift t o lower wavenumbers, most likely due to decreased dipole-dipole coupling of adsorbed CO species. The total integrated intensity of the CO band decreased with increasing Au content, indicating fewer Pt sites exposed on the surface as compared to the monometallic Pt catalyst, in excellent agreement with the chemisorption results for CO. The integrated intensity for the bimetallic PtSn/A1203 catalysts, on the other hand, was significantly larger compared to monometallic PtJAl203, again in agreement with the CO chemisorption data.
91 1
0
1
2
Au/Pt or Sn/Pt atomic ratio Figure 3. IR peak positions for linearly adsorbed CO on Pt as a function of A u P t or S o t ratio. In the case of Pt-Sn, CO singleton vibration frequencies were determined by recording the CO vibration frequencies as a function of surface coverage and extrapolating to zero surface coverage. Regardless of Sn content, all catalysts had a singleton wavenumber of 2041 f 6 cm-1, supporting the notion that electronic effects are not of major importance in this catalyst system, in agreement with the work of Ponec et al. [81.
Characterizationby electmn microscopy A detailed electron microscopy study of the Pt-AdSiOz catalyst system has been published previously [9] and, therefore, only a brief summary will be given here. Monometallic PtJSiO:! catalysts were highly dispersed and had an average metal particle size of 3.7 f0.3 nm. Particles larger than 1 nm were found to be monocrystalline and maintained the bulk fcc lattice spacings of 0.227 nm and 0.196 nm for the P t ( l l 1 ) and Pt(200) planes, respectively. No evidence of lattice expansion or contraction was found in this size range. The average particle size in the monometallic AdSiOz catalyst was 12 nm, and all the Au particles examined had an fcc configuration. A few of the Au particles were multiply twinned. According t o the bulk phase diagram for Pt-Au the formation of the AusPt alloy is feasible for P t atomic ratios less than 18% and greater than 98%.
912
However one cannot predict directly from the phase diagram whether or not alloys do exist i n individual particles. A major complication in the identification of the tetragonal AusPt phase is the similarity of the lattice parameters with t h a t of elemental Pt. Therefore, it was impossible t o differentiate between these two components i n HREM , though it was possible to identify the presence of Au by using EDX. Adding Au resulted not only in differences as far as particle sizes were concerned, but also in specific microstructures. The l.OPt-0.3AdSiO2 catalyst possessed a bimodal particle size distribution with a n average particle size of 4.9 It0.5 nm. EDX and HREM results showed that metal particles smaller than 20 nm were monometallic Pt. There were a few large particles in the range of 20 to 35 nm, and EDX detected only Au in these large particles. The l.OPt-O.7Au/SiOa catalyst, on the other hand, had much smaller particles (< 10 nm). EDX analysis gave signals for both P t and Au within individual particles, with a A f l t signal ratio of approximately 3/1, suggesting the presence of the Au3Pt alloy. The microscopy data support the conclusions drawn from TPR, where an interaction between Pt and Au was observed only for the 1.OPt-0.7AdSiO2 catalyst. The majority of the metal particles in the Pt-Sn catalysts were less than 5 nm in diameter. The microstructures of individual particles were determined by lattice imaging. In cases where the interpretation of the lattice spacings observed in HRTEM was not straightforward, comparisons had to be made with computer-simulated images based on the multislice routine of known elements or alloys. However, this type of comparison could only be made for particles larger than 3 nm for which a bulk - type structure could be assumed [lo]. In the monometallic l.OPt/A1203 catalyst, the Pt particles were extremely small and from transmission electron micrographs, a number average particle size of 2.5 nm was determined, in good agreement with the surface average particle size of 2.9 nm obtained from hydrogen chemisorption results, The particle size distributions for all the bimetallic Pt-Sn catalysts were very similar, with the majority of the particles, like in the monometallic case, in the lOOOL (1x10-3 Torr x sec) readily oxidize ordered S n P t substitutional surface alloys. The oxidized Sn is pulled out of the surface and covers the metallic substrate. Furthermore, as was reported in 1991 (J. Phys. Chem., Surf. Sci.) it is difficult to force Sn into the bulk of the single crystal templates (e.g. alkaline ion scattering data indicate that for favorable Sn precoverages, Sn will only penetrate 1-2 atomic layers into the substrate lattice upon annealing to 1000 K).
H
91 8
A: J. Schwank Thank you for your comment. In our case, of course, the HRTEM samples were air exposed, likely causing surface oxidation. While HRTEM showed mainly the structure of monometallic Pt particles, our chemical probes gave indirect evidence that after reduction small amounts of Sn might be present on the Pt surface. However, we cannot tell at this point whether we are dealing with oxidic or zero-valent tin species on thc Pt surface.
Guczi, L el al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
HYDROFORMYLATIONOF 1-HEXENE BY SOLUBLE AND ZEOLITESUPPORTED IRIDIUM SPECIES J.-Z. Zhang, Z. Li and C.-Y. Wang Institute of Chemical Engineering of Coal, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
Abstract
The hydroformylation of 1-hexene at 150C and 40 atm CO : H,(1 : 1) in toluene using soluble and zeolite-supported iridium species is reported.The presence of excess phosphine during homogeneous and heterogeneous catalysis inhibits hydrogenation of 1-hexene and slightly lowers activity. The addition of formaldehyde to heterogeneous catalysis system increases the selectivity. The active species of immobilized catalyst formed in reaction environment is investigated.
1. INTRODUCTION Hydroformylation is the largest-volume process which employs a homogeneous catalyst[l]. However, homogeneous catalyst system shows many disadvantages (such as difficulties in the separation of catalyst from reaction mixture, instability of catalyst system, etc.) Consequently, a considerable effort has been expanded on the immobilization of homogeneous hydroformylation catalysts. Of interest to this study is the immobilization of transition metals by zeolite for use as heterogeneous olefin hydroformylation catalysts[2-41. It is based on the theory that the advantage of immobilization transition metals within the pore structure of zeolites is the possible enhancement reaction selectivity due to the molecular sieving effects of the zeolite. The active metal species are located within the narrow channel of zeolite where the formation of intermediate leading to the iso-isomer will be more retarded than that leading to the n-isomer. As a result, the high selectivity to n-aldehyde may be possible. Since rhodium-catalyst shows high activity and selectivity, the rhodium-catalyzed hydroformylation of olefin has been extensively investigated. Iridium lies in the same column in the Periodic Table as rhodium. Compounds such as HIr(CO)(PPh,),, (Bu,P),Ir(CO)CI and (Ph,P),Ir(CO)Cl, which are analogous to the rhodium catalyst with great catalytic activity and selectivity, are
920 suposed to have similar behavior with rhodium in catalyzing hydroformylation. However, to date there are few reports dealing with iridium-catalyzed hydroformylation homogeneously and no research shows the behavior of iridium-zeolite for use as heterogeneous olefin hydroformylation catalysts. In our initial report on 1-hexene hydroformylation by iridium-zeolite[5], it was observed that a great hydrogenation of olefin takes place. Mitsuo Yamaguchi[6] hydroformylated l-hexene using iridium catalyst modified by organic bases. The results showed that the hydrogenation of olefin was strongly inhibited by the addition of phosphine and high selectivity towards linear aldehydes was also obtained. However, Hydrogenation product hexane was still predominant (- 80%). The catalyst structure was not exactly clear. The purpose of this investigation is to compare the behavior of zeolite containing iridium with that of homogeneous iridium species as catalysts for liquid phase hydroformylation. Zeolite-supported iridium carbonyl phosphine complexes is included.
2. EXPERIMENTAL 2.1 Materials Iridium trichloride trihydrate was purchased from Kunming Noble Metal Research Institute. The zeolite ZSM-5 was obtained in powder on loan from NanKai University. Carbon monoxide and premixed hydrogen and carbon monoxide(1 : 1) were purchased from the Institute of Coal Chemistry Academia Sinica, and was used with further purification by 3093 deoxidant, TG-desulfurizing agent and 5A Zeolite. Toluene and phosphine PPh,(Ph = phenyl) were purchased from Beijng Chemical Plant and Shanghai First Chemical Plant respectively, and were used without further purification. 1-hexene with purity 99.5% was distilled before using. The other reagents were analytical grade and used without further purification.
2.2 Preparation of catalysts Homogeneous hydroformylation catalyst, HIr(CO)(PPh,),, was synthesized by modified literature methods[7], and characterizd by IR,HMR and elemental analysis. Cation exchange of Ir3+(from aqueous IrCl,) for Na' in NaZSM-5 was performed as follows. The zeolite was slurried in 0.1N NaCl at 95% . Then an aqueous solution of IrCI, was added dropwise over a period of 2h. The pH was maitained at 7.0 throughout the exchange by addition of O.1N NaOH or 0.1N HCI. The slurry was stirred at 9572 continuously for 5 h and cooled to room temperature with agitation overnight. The solid was recovered by filtering and washing with water and was dried to a free-flowing powder in air at 120%. The powder was pressed and crushed then sieved to 20-40 mesh size. Iridium content of IrZSM-5 was determined as follows. After filtering and
921
washing. the liquid was recovered, and was determined spectrophotometrically by a procedure similar to that of Liu [8]. Then the iridium content of the IrZSM-5 was calculated according to the amount added at the begining. IR spectra were obtained on solid using a Fourier Transform spectrometer (Digilab F.T.S.15).
2.3 Reactor system hydroformylations were performed in a lOOml stainless steel autoclave. The autoclave was linked with a set of gas purification system (to remove sulfur, oxygen and trace water). After charging with toluene as a solvent, 1-hexene and catalyst, it was flushed with the premixed(1 : 1) H,-CO and then compressed to the required pressure. The temperature was controlled to k 1 C by the heating mantle-temperature controller supplied with the autoclave. After the reaction, the autoclave was rapidly cooled to room temperature and the pressure was released, liquid products were obtained from filtering. Gas chromatographic analysis was achieved by Shimadzu GC-9A using a column with 10% PEG 20 M packed on chromsorb W.
3. RESULTS AND DISCUSSION 3.1 Homogeneous hydroformylationof l-hexene The results obtained from 1-hexene hydroformylation at 40 atm H, : CO(l : I) in 1.5 molar l-hexene in toluene at 150'c by homogeneous iridium Catalyst, HIr(CO)(PPh3),, are given in Table 1.
Table I Effect of PPh, on the homogeneous hydroformylation of 1-hexene P / Ir conversion selectivity n / i product contribution (mol%) hexane n-heptanal i-heptanal 1.o 0.993 0.720 2.53 27.8 51:2 20.3 5.0 0.999 0.645 1.83 35.4 41.7 22.8 12 0.994 0.595 1.62 40.2 36.6 22.6 24 0.960 0.650 2.19 33.5 42.9 19.6 98 0.918 0.752 2.49 22.7 49.3 19.8 Reaction conditions: pressure 40 atm, temperature 150'c, CO / H, (1 : l), time 22h The results indicate that addition of PPh, in small amount increases the activity. While the large amount of PPh, drastically lower the conversion of the 1-hexene. This is consistent with the results of literature concerning the rhodium
922
catalyst[l]. In addition, the presence of PPh, in reaction solution increaes the seletivity significantly. When the same molar PPh, as iridium (P / Ir = 1) was added, the selectivity reaches 72.0%, n / i ratio 2.53, while only 30% 1-hexene was converted to heptanal by using Ir(CO),as the catalyst with n / i ratio 1.2[6]. The addition of PPh, inhibits the hydrogenation of 1-hexene and favors the hydroformylation. First, in the reaction environment, excess PPh, inhibits the displacement of PPh, (in HIr(CO)(PPh,),) by CO, eg.
or further more to form hydrocarbonyl iridium complex, Ir(CO),, which shows low 'selectivity to aldehyde (as mentioned above). Next, PPh, makes H in the complex more negative which leads to the anti-Markownikoff addition to form normal aldehyde. Also, the steric influence of PPh, enhances the selectivity. 3.2 Heterogeneoushydroformylationof 1-bexene Heterogeneous 1-hexene hydroformylation results at 40 atm H, : CO(1 : 1) in 1.5M 1-hexene in toluene are shown in Table 2-4. Table 2 Heterogeneous hydroformylation of 1-hexene by IrZSM-5 at 40 atm (H, : CO 1 :1) 1.5M hexene in toluene, run time 22h. Ir conc. 0.5Ommol/ 1. cat. Ir loss conv. select. n/i product contribution (%) (mol%) hexane n-heptanal i-heptanal A 1.2 0.993 0.231 0.55 76.7 8.55 15.0 B 3.5 0.971 0.287 2.58 69.2 20.1 7.78 C 3.9 0.926 0.503 1.99 46.0 31.0 15.6 D 5.1 0.920 0.693 2.46 28.2 45.4 18.4 E 11.3 0.925 0.682 2.41 29.5 39.8 16.5 IrZSM-5 was prepared by exchange with IrCI, Catalyst A: Only IrZSM-5 was used as catalyst B: Formaldehyde was added in the concentration of 0.40mol/ 1 C: Excess PPh, was added at PPh, / Ir mol ratio of 24 D: Both formaldehyde and PPh, were added. E:HIr(CO)(PPh,), was loaded on ZSM-5 by impregnation Table 2 presents the results that show the behaviors of catalysts prepared from IrZSM-5. Catalyst A was cation-exchanged iridium zeolite ZSM-5 and was used as catalyst without adding any other agents. Compared the data with that of homogeneous catalyst, it shows serious hydrogenation of 1-hexene to hexane. With addition of formaldehyde to the reaction solution (in the situation of B),
923 slight improvement in selectivity is observed. When the excess PPh, is added (in the situation C), the product distribution signficantly changes. More than half 1-hexene is converted to heptanal. While formaldehyde and excess PPh, were added simutaniously in the reaction system, the selectivity to heptanal is increased greatly. Table 3 Effect of P / Ir ratio on the heterogeneous hydroformylation of 1-hexene product contribution P / Ir conversion selectivity n / i (moo (mol %) hexane n-heptanal i-heptanal 1.7 0.950 0.315 0.89 65.0 14.1 15.8 5.0 0.931 0.330 0.91 62.4 14.6 16.1 12 0.842 0.442 0.97 47.0 18.3 18.9 24 0.920 0.693 2.46 28.2 45.4 18.4 98 0.916 0.686 2.53 28.2 45.0 17.8 Reaction conditions are the same as table 1. Table 4 Effect of formaldehyde on the heterogeneous hydroformylation of l-hexene product contribution Conc.of conversion selectivity n / i formaldehyde (mol%) (mol / 1) hexane n-heptanal i-heptanal 0.04 0.945 0.594 2.00 38.4 37.4 18.7 0.20 0.930 0.727 2.57 25.4 48.7 18.9 0.40 0.920 0.693 2.46 28.2 45.4 18.4 0.80 0.887 0.685 2.50 27.9 43.4 17.4 2.00 0.924 0.625 1.97 34.7 38.3 19.4 Reaction conditions are the same as table 1. It is also noted from Table 2 that the n / i ratio is no more great than that of homogeneous hydroformylating system, Shape-selective effect of zeolite is not observed. In order to get an inside view of the system, the heterogeneous catalyst was recovered by filtration and was dried under vacuum. Its IR spectra was recorded. Two bands at 2020cm-Iand 1880cm-Iwere observed with small intensities. While the catalyst before reaction gave no absorbtion in this region. Therefore, there must be iridium carbonyl species formed in the reaction environment. The two bands are assigned to stretching vibrations of CO group coordinated with Ir. Since the conditions under which the catalyst D was prepared were the same as
924
that for the synthesis of HIr(CO)(PPh,),[7], it is well known that the homogeneous catalyst HIr(CO)(PPh,), gives two carbonyl stretching frequencies at 2070 cm-land 1930 cm-'[9]. It is likely that the active species for hydroformylation with catalyst D is the complex HIr(CO)(PPh,), on zeolite . The shifts of the frequencies probably is due to the effect of the zeolite. Table 2 also shows the iridium loss from catalyst. It indicates that the catalyst prepared by cation-exchange losses less iridium than by impregnation. Table 3 shows the results of hydroformylation of 1-hexene by PPh, / IrZSM-5 / Formaldehyde at the same condition as the homogeneous procedure. It is noted that striking similarity exists between the homogeneous hydroformylation catalyst, HIr(CO)(PPhJ,, and this type of heterogeneous catalyst. Both of these give high selectivty and high n / i ratios. The addition of PPh,dramatically inhibits the hydrogenation of 1-hexene although it slightly decreased the overall conversion. When 200 mol PPh, / mol Ir was used, the conversion of 1-hexene was still maintaned above 90%. Table 4 gives the results of hydroformylation of 1-hexene at 150'c with addition of formaldehyde in reaction solution. It appears that as the amount of formaldehyde is increased, the conversion of 1-hexene is slightly decreased. However, the selectivity and n / i ratio proceed through a maximum with the increasing concentration of formaldehyde. The proper concentration is 0.20mol/ 1. Up to now, the reason is not clear.
4. ACKNOWLEDGEMENT Financial support of this work was provided by National Natural Science Foundation of China.
5. REFERENCES 1 2 3 4 5 6 7 8 9
J. Falbe, Carbon Monoxide in Organic Synthesis, Springer, Berlin, 970 Nobuo Takahashi and Masayoshi Kobayashi, J. Catal., 85(1984)89 M.Primet, JCVedrine and C.Naccache, J. Mol. Catal., 4(1978)411 R.J.Devis, J.A.Rossin and M.E. Daris, J. Catal.', 98(1986)477 Changyou Wang, Junzhong Zhang, Fifth International Pittsburgh ( Conference 1197(1987) Mistso Yamaguchi, The Catalyst, 9(1967)161 US Patent No. 3,965,192 Wangyi Liu, Anal. Chem. 1(1977)12 I. Malatesta, G. Caglio and M. Angoletta, J.C.S. A(1965)6979.
925 DISCUSSION Q: D. Duprez (France) It is known that chlorine-metal ligands can affect the interaction of CO and of oxygenated compounds with the metal. In connection to this, did you verify the chlorine content of your Ir-zeolite catalyst and did you try to use a chlorine-free precursor of iridium '? A: D. Zhang The paper (presented on the Fifth Annual International Pittsburgh Coal Conference P 1097) discussed the effects of the chlorine (when IrZSM-5 was prepared, different amount of NaCl was added to the ion-exchange solution). It showed that high CIconcentration reduced the selectivity to aldehyde. We have not verified the chlorine content of the IrZSM-5. IrCI3 was used as the precursor of iridium only.
Q: P. Ratnasamy (India) Did you check the Ir content of your heterogeneous catalyst before and after the reaction ? How do you explain the lack of these selectivity ovcr IrZSM-5 '? A: D. Zhang After measuring spectrophotometrically the Ir content of liquid recovered from ionexchange slurry and the reaction mixture, the Ir content of IrZSM-5 before and after the reaction was calculated respectively. Proper pore size and structure of catalyst are thc basic condition for shape selectivity. It was assumed that Ir on the IrZSM-5 in the reaction condition is converted to the complex as HIr(CO)(PPh3)3, which showed high selectivity to aldehyde and it was because the bulky complex was not easily formed in the pore of the ZSM-5 that the catalyst showed little shapc selectivity.
Q: J. W. Hightower (USA) and G. Maire (France) Will you please explain the Table 2 and the effect of A-Z 1 A: D. Zhang Table 2 shows the results of hydroformylation of l-hexene by IrZSM-5 with adding other agents. Although adding PPh (C) or formaldehyde (€3) alone favors the selectivity to heptanal, adding both of them (Dj improves the selectivity much greatly. D and E have almost the same behavior, except that the Ir loss in E is greater than D.
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Studies in Surface Science and Catalysis 75
NEW FRONTIERS IN CATALYSIS PART B
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Studies in Surface Science and Catalysis Advisory Editors: 6.Delrnon and J. T. Yates
Vol. 75
NEW FRONTIERS IN CATALYSIS Proceedings of the 10th International Congress on Catalysis, Budapest, July 19-24,1992 PART B Editors
L. GUCZI Institute of Isotopes of the Hungarian Academy of Sciences P. 0. Box 77,H-1525 Budapest, Hungary
F. SOLYMOSI Institute of Solid State and Radiochemistry, Jozsef Attila University P. 0. Box 768, H-6701 Szeged, Hungary P. TETENYI Institute of Isotopes of the Hungarian Academy of Sciences P. 0. Box 77, H - 1525 Budapest, Hungary
E LSEVl E R
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CONTENTS PART A
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V Catalysis: Past, Present and Future J.A.Ra bo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Interfacial Coordination Chemistry: Concepts and Relevance to Catalysis Phenomena M.Che . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 New Catalytic Aspects of Heteropolyacids and Related Compounds - To the Molecular Design - of Practical Catalysts M.Misono . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 The Catalytic Conversion of Methane to Oxygenates and Higher Hydrocarbons J.H. Lunsford . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 New Reactions in Various Fields and Production of Specialty Chemicals W.F. Holderich . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Structure-Function Relationships in Heterogeneous Catalysis: The Embedded Surface Molecule Approach and its Applications P. Johnston and R. W. Joyner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Isotopic Tracer Studies of Chain Propagation and Termination during FischerTropsch Synthesis over Ru/Ti02 K. R. Krishna and A. T. Bell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Liquid Phase Oxidation of Glyoxal to Glyoxylic Acid by Air on Platinum Catalysts P. Gallezot, F. Fache, R. de Mesanstourne, Y . Christidis, G. Mattioda and A. Schouteeten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 The Effect of Oxygen Binding Energy on the Selective Oxidation of Butane over V/y-Al,O, P. J. Andersen and H. H. Kung . . . . . . . . . :. . . . . . . . . . . . . . . . . . . . . . . 205 Secondary Oxygen Exchange Reactions during the Partial Oxidation of Methane M. M. Koranne, J. G. Goodwin, Jr. and G. Marcelin . . . . . . . . . . . . . . . . . . . . ,219 Role o f Free Radicals in Heterogeneous Complete Oxidation of Organic Compounds over IV Period Transition Metal Oxides Z. R. Isrnagilov, S. N. Pak, L. G. Krishtopa and V. K Yermolaev . . . . . . . . . . . . . 231 IH Broad-Line NMR at 4 K for Studying the Acidity of Solids: Application to Zeolites P. Batamack, C. Doremieux-Morin and J. Fraissard . . . . . . . . . . . . . . . . . . . . 243 Preparation of Bifunctional Catalysts by Solid-state Ion Exchange in Zeolites and Catalytic Tests H. G. Karge, Y. Zhang and H.K. Beyer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Transition Metal/Zeolite Catalysts by Design: Nucleation and Growth of Mono- and Bimetallic Particles in Zeolite Y W. M.H. Sachtler, 2. Zhang, A. Yu. Stakheev and J. S. Feeley . . . . . . . . . . . . . . 271 Promotion of H-ZSM-5 by Alumina J. Volter, H. D. Lanh, B. Par& E. Schreier and K Ulbricht . . . . . . . . . . . . . . . 283 The Effect of Preparation Method on Metal-Support Interaction in PdL-Zeolite Catalysts 2 97 G. Larsen and G. L. Haller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . On the Nature of Superactive Centers in H-FeZSM-5 Zeolites. Quantum-Chemical Calculations M. J. Filatov, A. G. Pelmenschihmv and G. M.Zhiabmirov . . . . . . . . . . . . . . . . . 3 1 1
VI
CO Oxidation on Pd(ll0): A Model System for Chemical Oscillations in Heterogeneous Catalysis M. Ehsasi, M. Berdau, A. Karpowicz, K Christmann and J. H. Block . . . . . . . . . . Nature of Metal-Metal Bonding in Mixed Metal Catalysts R. A. Campbell,J. A. Rodriguez and D. W. Goodman . . . . . . . . . . . . . . . . . . . The Reduction of Nitric Oxide by Hydrogen over Pt, Rh and Pt-Rh Single Crystal Surfaces H. Hirano, T. Yamada, K I. Tanaka, J. Siera and B. E. Nieuwenhuys . . . . . . . . . . Spectroscopic Studies on the Reaction Pathways of Methanol Dissociation on Pd Catalyst A. Berkd, J. Raskb and F. Solymosi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyclotrimerisation o f Acetylene to Benzene over Single Crystal Palladium and Gold/Palladium Surfaces and over Supported Palladium Catalysts C. J. Baddeley, R. M. Ormerod and R. M. Lambert . . . . . . . . . . . . . . . . . . . . . Surface Chemistry for Automotive Emissions Control: Interactions of Nitric Oxide on a (111) Pt-Rh Alloy Surface G. B. Fisher, C. L. DiMaggio and D. D. Beck . . . . . . . . . . . . . . . . . . . . . . . . Shape Selective Alkylation of Benzene with Long Chain Alkenes over Zeolites S. Sivasanker, A. Thangaraj, R. A. Abdulla and P. Ratnasamy . . . . . . . . . . . . . . Comparison of SAPO-37 with Faujasites in Cracking Reactions M. Briend M. Derewinski, A. Lamy and D. Barthomeuf . . . . . . . . . . . . . . . . . . Catalytic Activity of Modified ZSM-5 Zeolites in the Dehydrogenation and Aromatization Reactions of Propane and n-Butane P. Fejes, J . Haldsz, I. Kiricsi, Z. Kele, Gy. Tasi, I. Hannus, C. Fernandez, J. B. Nagy, A. Rockenbauer and Gy. Schobel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Coke Formation during the Transformation of Propene, Toluene and Propene-Tolucnc Mixture on HZSM-5 P. Magnoux, F. Machaab and M. Guisnet . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrocarbon Formation from Methanol/Dimethyl Ether over Protonated Zeolites and Molecular Sieves. New Insights from Recent Experiments S.Kolboe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-D Exchange between Zeolites and Alkanes. Evidence for Formation and Rearrangement of Pentacoordinated Carbonium Ions C.J. A. Mota, L. Nogueira, S. C. Menezes, V. Alekstich, R. C. L. Pereira and W.B.Kover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . New Surface Nb-Dimcrs Chcrnically Interacted with SiO,: Regulation of the Catalysis by Molecular Design of Reaction Sites N. Ichikuni and Y. Iwasawa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Expert Systems Approach to Catalyst Design - Application and Experimental Verification T. Hattori, H. Niwa, A. Satsuma, S. Kit0 and Y. Murakami . . . . . . . . . . . . . . . . Metal Oxide Vapour Synthesis (MOVS): A New Preparative Method for Heterogeneous Metal Oxide Catalytic Systems
321 333 345 359 371 383 397 409
42 1 435 449
463 477 489
E.C.Alyea,K.F.Brown,K.J.FisherandKD.L.Smith. . . . . . . . . . . . . . . . . 503 Designing of New Catalysts for Olefin Metathesis on the Base of Photoreduced Silica-Molybdena V. B. Kazansky, B. N. Shelimov and K. A. Vikulov . . . . . . . . . . . . . . . . . . . . . . 51 5 SiO2-Grafted Dinuclear Molybdenum Catalyst Derived from M O ~ ( O A CHighly )~ Active for Olefin Metathesis Reaction M.Ichikawa, Q. Zhuang, G.-J. Li,K Tanaka, T. Fujimoto and A. Fukuoka . . . . . . . 529 Molecular Design of Supported Metal Oxide Catalysts I. E. Wachs, G. Deo) D. S. Kim, M. A. Vuurman and H. Hu . . . . . . . . . . . . . . . . 543
VI I Structural Characteristics of Alumina-Supported Activated Hydrodesulfurization Catalysts. An XPS, NO Adsorption and Sulphydryl Group Study L. Portela, P. Grange and B. Delmon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure-Function Relations in Layered Transition Metal Sulfide Catalysts M. Daage, R. R. Chianelli andA. F. Ruppert . . . . . . . . . . . . . . . . . . . . . . . . . Elementary Steps of Hydrogenative Sulfur-, Nitrogen- and Oxygen-Removal from Organic Compounds on Sulfided Catalysts H. Schulz andN.M. Rahman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sites Characterization on Model Ruthenium Sulphide M. Lacroix, C. Mirodatos, M. Breysse, T. Decamp and S. Yuan . . . . . . . . . . . . . . High Resolution Electron Microscopy Characterization of the Poorly Crystalline Structure of Molybdenum Disulfide-Based Catalysts S. Fuentes, M. Avalos-Borja, D. Acosta, F. Pedraza andJ. Cruz . . . . . . . . . . . . . Deuterium Solid State NMR Study of Molecular Mobility and Catalytic Dehydration of tert.Buty1 Alcohol on H-ZSMS Zeolite A. G. Stepanov, A. G. Maryasov, V. N.Romannikov and K I. Zamaraev . . . . . . . . Stationary Liquid-Phase Homogeneous Transition Metal Catalysis I.T.Hontcith . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An in situ Radioactive Tracer Technique for Studying Adsorption-Desorption Dynamics on a Working Catalyst U. Schroder, L. Cider and N. - H. Schoon . . . . . . . . . . . . . . . . . . . . . . . . . . Application of Scanning Tunneling Microscopy/Spectroscopy (STWSTS) to Catalyst Research: Pt/Si02 M. Komiyama andM. Kirino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization and Catalytic Properties of Pt-Ir Small Bimetallic Cluster in NaY 0. B. Yang and S. I. Woo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laser Raman Characterization of Surface Phase Precious Metal Oxides Formed on CeO, Micro Domains Generated within an Alumina Host by Sol Synthesis L. L. Murrell, S. J. Tauster and D. R. Anderson . . . . . . . . . . . . . . . . . . . . . . . Direct Propane Ammoxidation to Acrylonitrile: Kinetics and Nature of the Active Phase A. Andersson, S. L. T. Andersson, G. Centi, R. K Grasselli, M. Sanati and F.Trifiro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Oxidation of Fluorene to 9-Fluorenone - Development and Characterization of Catalysts F. Majunk, H. Borchert and M. Baerns . . . . . . . . . . . . . . . . . . . . . . . . . . . Ammoximation of Cyclohexanone on Titanium Silicalite: Investigation of the Reaction Mechanism A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. Petrinj G. Leofanti, M. Padovan, M.Mantegazza and P. Rofia . . . . . . . . . . . . . . . . . . . . . . . . . . On the Catalytic Oxidation of Methanol with Vanadium (IV) in Sulphuric Acid Solution R. Larsson and B. Folksson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of Chromium Introduced into 12-Molybdophosphates as Catalysts for Oxidation of Hydrocarbons K. Briickman andJ. Haber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correlation between Catalytic and Structural Properties of Modified Molybdenum and Vanadium Oxides in the Oxidation of Ethane in Acetic Acid or Ethylene M. Merzouki, B. Taouk, L. Tessier, E. Bordes and P. Courtine . . . . . . . . . . . . . . The Promoting Effect of La203on the CO Hydrogenation over Rh/Si02 A. L. Borer andR. Prins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Chlorine on the Rhenium-Alumina Interaction in Low-Loaded Re/AI2O3 and PtRe/AI2O3 Industrial Catalysts G. Munuera, P. Malet and A. Caballero . . . . . . . . . . . . . . . . . . . . . . . . . . .
559 571
585 597
611
62 1 635
643
659 671 6a i
691
707
7 19
731
741
753 765
781
Vlll
A New Ap roach to Loss of Alkali Promoter from Industrial Catalysts: Importance of Excited [tates of Alkali L. Holmlid, K Engvall, C. Aman and P. G. Menon . . . . . . . . . . . . . . . . . . . . . The Relation between Catalytic and Electronic Properties of Supported Platinum Catalysts: The Local Density of States as Determined by X-Ray Absorption Spectroscopy M. Vaarkamp,J. T.Miller, F. S. Modica, G. S. Lane and D. C. Koningsberger . Direct MAS/MES Evidence for Elcctronic Metal-Support Interactions in Dilute si& and 57Fe Carbon and Alumina-Supported Catalysts C. H. Bartholomew, L. R. Neubauer and P. A. Smith . . . . . . . . . . . . . . . . . . . . Formation and Properties of Dispersed Pd Particles over Graphite and Diamond 0.S. Alekxeev, L. V. Nosova and Yu. A. Ryndin . . . . . . . . . . . . . . . . . . . . . . . Kinetics of Alkane Hydrogenolysis on Clean and Coked Platinum and PlatinumRhenium Catalysts G. C. Bond R. H. Cunningham and E. L. Short. . . . . . . . . . . . . . . . . . . . . . . Toluene Hydrogenation over Supported Platinum Catalysts S.-D. Lin and M. A. Vannice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Catalytic Activity of MoOx/2r02 in the Hydrogenation and Metathesis of Propene V. Indovina, A. Cimino, D. Cordischi, S. Della Be114 S. De Rossi, G. Ferrark, D. Gazzoli, M. Occhiuzzi and M. Valigi . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pt-Sn-Alumina Catalysts: Relating Characterization and Alkane Dehydrocyclization Data B.H.Davis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bimetallic Pt-Sn/AI2O3 and Pt-Au/Si02 Catalysts: A Comparison of Reactivity, Adsorption behavior and Microstructure J. Schwank, K. Balakrishnan and A. Suchdev . . . . . . . . . . . . . . . . . . . . . . . . Hydroformylation of 1-Hexene by Soluble and Zeolite-Supported Iridium Species J.-Z. Zhang, Z. Li and C.-Y. Wang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
795
809
821 837
849 86 1
875
889
905 919
PART B Bulk Tungsten Carbide as Catalyst in Hydrocarbon Reactions: Association of Selectivity Differences with Surface Composition as Compared to the Selectivity of Pt Series Metals A. Frennet, G.Leclercq, L. Leclercq, G. Muire, R. Ducros, M. Jardinier-Offergeld F. Bouillon, J-M. Bastin, A. Lofberg, P. Blehen, M. Dufour, M. Kamal,
L. Feigenbaum, J-M. Giraudon, V. Keller, P. Wehrer, M. Cheval, F. Garin, P. Kons, P. Delcambe and L. Binst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
927
Surface and Catalytic Properties of Molybdenum Nitrides L. T. Thompson, C. W. Colling, D. Choi B. G. Demczyk and J.-G. Choi . . . . . . . . . 941 n-Hexane Isomerization on High Specific Surface Mo2C Activated by an Oxidative Trca tmen t
M. J. Ledoux, C, Pham-Huu, H. Dunlop and J. Guille . . . . . . . . . . . . . . . . . . . Highly Dispersed Metal Colloids: Spectroscopy and Surface Chemistry in Solution J . S. Bradley, J. M. Millar, E. W. Hill, C. Klein, B. Chaudret andA. Duteuil . . . . . . Preparation of Amorphous Cu-Ti and Cu-Zr Alloys of High Surface Area by Chemical Modification S. Yoshida, T.Kakehi, S. Mutsumoto, T. Tanaka, H. Kanai and T. Funabiki . . . . . . A Mechanistic Proposal for Alkane Dehydrocyclization Rates on PtL-Zeolite. Inhibited Deactivation of Pt Sites within Zeolite Channels
E. Iglesia and J. E. Baumgartner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroconversion o f n-Alkanes and Decalin over Bifunctional Ptmazzite Catalysts F. Fajula, M. Boulet, B. Cog, V. Rajaofanova, F. Figueras and T. Des Courieres . . .
955 969
981
993
1007
IX Effect of Sulfur on the Performance of PtKL Hexane Aromatization Catalyst J . L. Kao, G. B. McVicker, M. M. J. Treacy, S. B. Rice, J. L. Robbins, W. E. Gates, J. J. Ziemiak, V. R. Cross and T. H. Vanderspurt . . . . . . . . . . . . . . . . . . . . . . 101 9 Aromatization of n-Hexane by Aluminium-Stabilized Magnesium Oxide-Supported Noble Metal Catalysts E. G. Derouane, V. Jullien-Lardot, R. J. Davis, N. Blom and P. E. Hojlund-Nielsen . 1031 Effect of the Alkali Cation on Heptane Aromatization in L Zeolite R.F. Hicks, W.-J. Han andA. B. Kooh . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,1043 Reduction and Aromatization Activity of MoO$A$O3 Catalysts: The Identification of the Active Mo Oxidation State on the Basis of Reinterpreted Mo 3d X P S Spectra W. Griinert, A. Yu. Stakheev, R. Feldhaus, K. Anders, E. S. Shpiro and Mt. M.Minachev. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053 Reaction Routes for Methane Conversion on Transition Metals at Low Temperature T. Koerts and R. A. Van Santen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065 The Mechanism of Alkane Oxidative Dehydrogenation on Chloride and Oxychloride Catalysts R. Burch, S. Chalker and P. Loader. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1079 Transient Isotopic Studies of the Role of Lattice Oxygen during Oxidative Coupling of Methane on Sr/La 0 and Ca/Th02 Catalysts Z. Kalenik and E. E. ?V&. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093 The Role of the Proton in Oxidation Processes on Metal-Oxygen Cluster Compounds S. Kasztelan, G.B.McGarveyandJ. B.Moffat. . . . . . . . . . . . . . . . . . . . . . . 1105 Correlations between p-Type Semiconductivity and C Selectivity for Oxidative Coupling of Methane ( 0 0 over Acceptor Doped SrTid, C. Yu, W. Li, W. Feng, A. Qi and Y. Chen . . . . . . . . . . . . . . . . . . . . . . . . . . . 1119 Mechanistic Aspects of the Selective Oxidation of Methane to C1-Oxygenates over Mo03/Si02 Catalysts in a Single Catalytic Step M. A. Banares, I. Rodriguez-Ramos, A. Guerrero-Ruu and J. L. G. Fierro . . . . . . . 1 131 On the Mechanism of Xylene Isomerization and its Limitations as Reaction Test for Solid Acid Catalysts A. Corma, F. Llopis and J. B. Monton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 145 Aluminum Coordination and Lewis Acidity in Aluminas and Steamed Zeolites H. Yong, D. Coster, F. R. Chen, J. G. Davis and J. J. Fripiat . . . . . . . . . . . . . . . 1 159 Characterization of Basic Sites on Fine Particles of Alkali and Alkaline Earth Metal Oxides in Zeolites 117 1 H. Tsuji, F. Yagi, H. Hattori and H. Kita . . . . . . . . . . . . . . . . . . . . . . . . . . . Zr02-S042- Catalysts. Nature and Stability of Acid Sites Responsible for n-Butane bomerization P. Nascimento, C. Akratopoulou, M. Oszagyan, G. Coudurier, C. Travers, J.F. Joly 1 185 and J. C. Vedrine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Activity for Vapor-Phase Aldol Condensation and Acid-Base Properties of Metal-Oxide Catalysts M.Ai . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1199 Reactions of Multifunctional Organic Compounds - Hydrogenation of Acrolein on Modified Pt-Catalysts 1 2 11 T, B. L. W. Marinelli, J. H. Vleetning and V. Ponec . . . . . . . . . . . . . . . . . . . . . Acetonitrile Synthesis from CO, H and NH3 over Fe/C and K,Fe/C M. V. Badani, L. M. Eshelman and%. N . Delgass . . . . . . . . . . . . . . . . . . . . . . 1223 Reductive Amination of Diethylene Glycol to Morpholine on Supported Nickel Catalysts - Its Activity, Selectivity, Stability and Possibility of Reactivation K. Jiratova, 0. Solcova, H. Snajdaufova, L. Moravkova and H. Zahradnikova . . . . . 1 2 35 An Improved Asymmetric Oxidation of Sulfides to Sulfoxides by Titanium Pillared Montmorillonite - The First Example in Heterogeneous Catalysis B. M. Choudary, S. Shobha Rani and Y. V. Subba Rao . . . . . . . . . . . . . . . . . . . 1 2 47 ,
X
Hydrogenation of CO, over Copper, Silver and Gold/Zirconia Catalysts: Comparative Study of Catalyst Properties and Reaction Pathways A. Baiker, M. Kilo,M. Maciejewski, S. Menzi and A. Wokaun . . . . . . . . . . . . . . . 1 2 57 Shape-Selective Reactions for Methylamine Synthesis from Methanol and Ammonia K. Segawa and H. Tachibana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1273 Selective Catalytic Reduction of NO by Hydrocarbon in Oxidizing Atmosphere M. Iwamoto, N. Mizuno and H. Yahiro . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285 The Making of Catalysts by Controlled Oxidative Degradation of Planar Metal Complexes on Alumosilicate Supports: Exhaust Gas Purification Catalysts for Power Plants, Automobiles and Small Outfits F. Steinbach, A. Brunner, H. Miiller, A. Drechsler, S. Frotnming, W. Strehlau und U.Stan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1299 Oxidation of CO on Pd Particles on a-Al2O3:Reverse Spillover L. Kieken and M. Boudart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i 313 Microkinetic Analysis of the Selective Catalytic Reduction (SCR) of Nitric Oxide over VanadiaRitania-Based Catalysts J . A. Dumesic, N.-Y. Topsoe, T. Slabiak, P. Morsing, B. S. Clausen, E. Tornqvist and 1325 H. Topsoe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An Infrared Study of an Active NO Decomposition Catalyst J. Valyon and W. K Hall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1339 Acidity of W O /Ti0 Catalysts for Selective Catalytic Reduction (SCR) F. Hilbrig H. fchmefz arid H. Knuzinger . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 35 1 Mcmbranc Catalysis over Palladium and its Alloys J. N. ,Irrnor and T. S. Farris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1363 Fixed Bed Catalytic Rcactors Based on Sintcrcd Metals F. van Looij, A. Muldcr, A. Q. M. Boon, J. F. Scheepens and J. W. Geus . . . . . . . . 1 377 Mixed Spinels with Cerium--SO, Emission Control from Fluid Catalytic Cracking (FCC) Regenerator J . S. Yoo, A. A. Bhattacharyya, C. A. Radlowski and J. A. Karch . . . . . . . . . . . . . 1391 Sintcring, Poisoning and Regeneration of Pt/MgO J . Adamiec, J. A. Srymura andS. E. Wanke . . . . . . . . . . . . . . . . . . . . . . . . . 1405 Development of a Micro Hydroprocessing Test for Rapid Evaluation of Catalysts C. Sudhakar, L. T. Mtshali, P. 0. Fritz crnd M. S.Patel . . . . . . . . . . . . . . . . . . . 14 19 Alkali Promoter Synergism in Selective Oxidation M. M.Bhasin and C. D. Hendrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1431 C-C Bond Formation via @-Additionwith Oxygcn Rctcntion Reversal in Oxygenate Synthesis K.Klier, R. G. Herman, P. B. Himelfarb, C.- W. Young, S. Hou and J. A. Marcos . . . 1 44 1 Selective Gasoline Synthesis from CO, on a Highly Active Methanol Synthesis Catalyst and an H-Fe-Silicate of MFI Structure T. h i , T. Takeguchi, A. Kohamu and K. Kitagawa . . . . . . . . . . . . . . . . . . . . . 1453 The Selective Synthesis of C2+ Oxygenates from Syngas Related Reactions over Ni- and Rh-Based Catalysts M. W. Balakos, S. S. C. Chuang, R. Krishnamurthy and G. Srinivas . . . . . . . . . . . 1467 Dcvclopmcnt of New Catalysts Formulations Tor Higher Alcohols Synthesis. Characterisation, Reactivity, Mechanistic Studies and Predictive Correlations A. Kiennemann, S. Boujanq C. Diagne and P. Chaumette . . . . . . . . . . . . . . . . . 1479 Characterization of MoS,-K+/Si02 Catalysts for Synthcsis of Mixed Alcohols from Syngas H.-B. Zhang, Y.-Q. Yang, H. P. Huang, G. D. Lin and K. R. Tsai . . . . . . . . . . . . 1493 Catalytic Activity of Reduced Cu,Zn(,-,)O and C ~ O / C ~ , Z I I ( ~ ~ ,in~ OC02/H2 Reactions D. Stirling, F. S. Stone andM. S. Spencer. . . . . . . . . . . . . . . . . . . . . . . . . . 1507
XI Reactivities of Surface Intermediates on an Sm203 Catalyst Studied by in situ Infrared Spectroscopy Y. Sakata, M. Yoshino, T. Fukuda, H. Yamaguchi, H. Imamura and S. Tsuchiya . . . . 1 5 19 In situ Investigation of the Water-Gas Shift Reaction over Magnetite by Mossbauer Spectroscopy A. Andreev, I. Mitov, V. Idakiev, T. Tomov and S. Asenov . . . . . . . . . . . . . . . . . 1523 In situ FT-IR Study of O,, CO, CO,, CH, and C2H4 Adsorption or Reaction on the La 03/Mg0 Catalyst S. b e n , R. Hou, , W Ji, 2. Yan andX. Ding. . . . . . . . . . . . . . . . . . . . . . . . . 1527 Study by in situ Laser Raman Spectroscopy of a VPO Catalyst in the Course of nButane Oxidation to Maleic Anhydride J. C. Volta, R. Olier, M. Roullet, F. B. Abdelouahab and K. Bere . . . . . . . . . . . . . 1 53 1 Investigation of Water-Gas Shift Reaction Under Dynamic Conditions P. Capek and K Klusacek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1535 Application of the Transient Response Method to the Study of the Catalytic System N0+02+CO/CuO D. Panayotov and D. Mehana'jiev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3 9 Characterization and Activity of Vanadium Oxide Catalysts in Selective Catalytic Reduction of Nitric Oxide U.S. Ozkan, Y. Cai, M. W. Kumthekar andL. Zhang . . . . . . . . . . . . . . . . . . . . 1543 Theoretical Study of CO Chemisorption on Rh and Pd Clusters A. Goursot, I. Papai and D. R. Salahub . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1547 New Dynamic Method Approach to the Roles of Reversible and Irreversible Adsorption in Heterogeneous Catalysis G.Lu,S. ChenandS. Peng. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1551 Discrimination and Regulation of Multi-Reaction Pathways in Heterogeneous Catalysis M. Kobayashi, T. Kanno and M. Hakozaki . . . . . . . . . . . . . . . . . . . . . . . . . . 1555 Adsorption Study by Transient Tracing Methods. Theory and Modeling P. Szedlacsek, A. Efstathiou, C.O. Bennett and S. L. Suib . . . . . . . . . . . . . . . . . 1 5 5 9 In situ Determination of Surface Carbon Species Formed on Rh/A1203 during Cob€, Reaction by Using Various Transient and Isotopic Methods A. M. Efstathiou, T. Chafik, D. Bianchi and C. 0.Bennett . . . . . . . . . . . . . . . . . 1 5 6 3 The Calculation of Surface Orbital Energies for Specific Types of Active Sites on Dispersed Metal Catalysts R. L. Augustine, K. M. Lahanas and F. Cole . . . . . . . . . . . . . . . . . . . . . . . . . 1 567 Oxidation and Removal of Chlorinated Hydrocarbons J.M.Berty,H.G.Stenger,Jr.,G.E.BuzanandKHu.. . . . . . . . . . . . . . . . . . 1571 Structure and Reactivity of Carbidic Intermediates for the Methanation Reaction on Ni(100), Ni(ll1) and Ni(ll0) Surfaces H. Hirano and K Tanaka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 575 CI Adsorption on Ag(ll1) and its Promoter Action D. Wang, K. Wu, Y. Cao, X. Wei andX. Guo . . . . . . . . . . . . . . . . . . . . . . . . . 1579 The NO+H Reaction on Pt(100): Steady State and Oscillatory Kinetics M. Slinko, Fink, T. Loher, H. H. Madden, S. J . Lombardo, R. Imbihl and G. Ertl . . 1 5 8 3 HREELSnDS Identification of Intermediates in the Low-Temperature H,+O, NO+H,, NH3+02 Reactions on Pt(ll1) Surface V. V. Gorodetskii, M. Yu. Smirnov andA. R. Cholach . . . . . . . . . . . . . . . . . . . 1587 Surface Science Studies on the Mechanism for H-D Exchange of Methane over Pt(ll1) Surfaces Under Vacuum F.Zaera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1591 Catalysis at Experimentally Designed Surfaces: n-Butane Hydrogenolysis at Sn/Group VIII Surface Alloys 1595 A. D, Logan and M. T. Paffett . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Surface Science and Kinetic Studies on Model Cu/Rh(100) Catalysts J . Szanyi and D. W. Goodman. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1599 Angular Distribution of Desorbing Reaction Products and Dynamics of Some Catalytic Processes on the Surfaces of Pt and ir M. U. Kisliuk, V. V. Savkin, T. N. Bakuleva, A. G. Vlasenko, V. V. Migulin, I. I. Tretiakov and A. V. Sklyarov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 603 Ship-in-Bottle Synthesis of NaY Zeolite-Included Ptg and Pt,, Carbonyl Clusters: Structures and Catalysis in CO+NO Reaction G.-J. Li, T. Fujimoto, A. Fukuoka andM. Ichikawa . . . . . . . . . . . . . . . . . . . . . 1607 The Preparation and Characterization of High-Silica Y Zeolite Prepared by Combined Chemical and Hydrothermal Dealumination X. Liu, Z. Pei, L. She, X.-W. Li, J. Shao, S. Lih R. Tang and X Ma0 . . . . . . . . . . . 16 1 1 Active Sites of Novel Iron Supported Y-Type Zeolite R. Iwamoto, I. Nakamura and A. Iino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1 5 A Novel Application of XRD Technique for the Characterization of Secondary Pore Structure in Modified Y-Zeolites S. D. Phatak, R. P. Mehrotra, S. M.Dhir and T.S. R. Prasada Rao . . . . . . . . . . . 16 1 9 IR-Spectroscopic Evidence for Acetonitrile Interaction with Carbenium Ions in Zeolites D. S.Bystrov, A. A. Tsyganenko and H. Forster . . . . . . . . . . . . . . . . . . . . . . . 1 623 Acid-Base Properties of Zeolites: An WS Approach Using Pyridine and Pyrrole Probe Molecule R. B. Borade, M.Huang, A. Adnot, A. Sayari and S. Kaliaguine . . . . . . . . . . . . . 1 625 Sulfate as Promotor of Acidity of High Microporous and Thermostable Titanium Pillared Montmorillonite F. Admaiai, A. Bernier and P. Grange . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1629 Platinum Cluster Supportcd on Zeolite A by Ion Exchange of Pt(NH3)42+ R. RyooandS. J . Cho. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1633 Preparation of Thermalstable Pillared Clays S. Mendinroz, F. Goazulez, C. Pesquera, I. Benito, C. Blanco and G. Poncelet . . . . 1 637 In situ X-Ray Analysis of CO- and CH30H-Induced Growth of Pd Particles Encaged in Zeolite Y W. V o g e f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1641 Metal-Support Interactions on Pd-Containing Zeolite Catalysts M. F. Savchits, Eh. Ya. Ustilovskaya, V. Z. Veshtort, L. A. Agabekova and Yu. G.Egiazarov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1645 Transfer of Metal Ions between Metal Oxides and Zeolites. Preparation of Highly Active Cu-Zeolite Based Catalysts for Reduction of NO, at Low Temperature B. Wichterlova,Z. Sohalik, M.Petras, I. Jirka and V. Bosacek . . . . . . . . . . . . . . 1649 Microkinetic Analysis of lsobutane Reactions Catalyzed by Y Zeolite J. E. Rekoske, R. J. Madon, L. M. Aparicio and J. A. Dumesic . . . . . . . . . . . . . . 1653 Peculiarities of Ethylenc Convcrsion on Zeolites and Phosphoric Acid A. G. Anshits, S.N. Vereshchagin andN. N. Shishkina . . . . . . . . . . . . . . . . . . . 1661 Benzene Alkylation in Vapour-Phase with Ethene on a Zeolite Catalyst G. Maria, G. Pop, G.MuscaandR. Boeru. . . . . . . . . . . . . . . . . . . . . . . . . . 1665 On the Nature of Zeolite Catalyst Effect on the Selectivity of Toluene Nitration by Acyl Nitrates S. M.Nugy, K A. Yarovoy, L. A. Vostrikova, K. C. lone and V. G. Shubin . . . . . . . 1669 Intcractions in Monoalkyl benzenes Disproportionation Among Zeolite Characteristics and Reaction Mechanisms I. Wang, T.-C.TsaiandC-L.Ay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1673 New Support Materials for Rhodium Catalysts: Charactcrization of Rh/AIPO4-31 and RhMnAPO-31 A. Trunschke, H. Zuhowa, 8. Parlitz, R. Fricke and€€.Miessner . . . . . . . . . . . . . 1677
Transformation of Thiols and Organic Sulfides over Zeolites M. Ziolek and P. Decyk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 8 1 Para-Selectivity of ZSM-5 Type Metallosilicates for Alkylation of Toluene with Methanol S. Namba, H. Ohta, J.-H. Kim and T. Yashima . . . . . . . . . . . . . . . . . . . . . . . . 168s Hydroxylation of Toluene with Hydrogen Peroxide on HY Zeolites with Various Si/Al Ratios T. Yashima, Y. Kobayashi, T. Komatsu and S. Namba . . . . . . . . . . . . . . . . . . . 1 689 Toluene Alkylation over Aluminophosphate-Based Molecular Sieves S.H.OhandW.Y.Lee.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1693 The Cracking Reaction Path in 1-Hexene Isomerization on SAPO-11 and Pd/SAPO-11 X-Y. Lim andS.-J. Choung. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1697 Epoxidation of Alkenes Catalyzed by Decatungstate as Pillars in Layered Double Hydroxides T. Tatsumi, H. Tajima, K. Yamamoto and H. Tominaga . . . . . . . . . . . . . . . . . . 1703 Conversion of Ethane into Aromatic Compounds on Z S M J Zeolites Modified by Zinc F. Roessner, A. Hagen, U.Mroczek, H. G. Karge andkl-H. Steinberg . . . . . . . . . 1707 In Situ FTIR and G C Kinetic Studies: Complementary Methods in the Mechanistic Study of Butanol Dehydration on Zeolite H-ZSM-5 M. A. Makarova, E. A. Paukshtis, J. M. Thomas, C. Williams and K I. Zamaraev . . . 17 11 Catalytic Properties of Ferrisilicate Analogs of Some Medium Pore Zeolites in C, and C , Aromatic Hydrocarbon Reations A. Raj, R R. Reahy, J. S. Reddy and R. Kumar . . . . . . . . . . . . . . . . . . . . . . . 17 1 s The Influence of the Catalyst Preparation on the Catalytic Properties of ZeoliteSupported Catalysts Y. W.ChenandW.J. Wang. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1719 Reactions of Acetone, Methanol and Ammonia on Z S M J Zeolites J. Novakova, L. Bosacek, Z. Dolejsek and L. Kubelkova . . . . . . . . . . . . . . . . . . 1723 Regioselective Hydrogenation Using Platinum-Support Zeolite Modified by CVD-Method H. fino, M. Shibagaki, R Takahashi I. Honda andH. Matsushita . . . . . . . . . . . 1727 Nickel, Cobalt and Zinc Substituted Synthetic Mica-Montmorillonite: Synthesis, Characterization and Propene Oligomerization Activity J. C. Q. Fletcher, A. P. Vogel and C. T. O'Connor . . . . . . . . . . . . . . . . . . . . . I 731 bobutane/l-Butene Alkylation on Pentasil-Type Zeolite Catalysts J. Weitkamp and P. A. Jacobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1735 Active Sites in HZSM-5 and SAP0 Molecular Sieves for Alcohol Conversion C. Bezouhanova, Yu. Kalvachev and H. Lechert . . . . . . . . . . . . . . . . . . . . . . . 1739 The Synthesis of Cobalt Supported Catalysts by Electroless Plating Techniques N. J. Coville, S. E. Colley, J. A. Beetge andS. W. Orchard. . . . . . . . . . . . . . . . 1743 A New Precursor for the Preparation of Novel Copper Chromite Catalysts R.Prasad.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1747 Pd Fe : Pd Surface Segregation and Catalytic Activity J. zrtolini, Y.Debauge, P. Delichere, J. Massardier, J.L. Rousset, P. R u u and B. Tardy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1751 A Study of Spreading of Vanadia on Titania Polymorphs Using Mechanical Mixtures M. Sanati, A. Ana'ersson and L. R. Wallenberg. . . . . . . . . . . . . . . . . . . . . . . . 1 755 Structure and Activity of Copper Catalysts Prepared from Amorphous Cu-Zr and Cu-Ti Alloy Precursors: A Comparative Study A. Molnar, T. Katona, Cs.Kopasz and Z. Hegediis . . . . . . . . . . . . . . . . . . . . . 1759 Preparation of W/Al 0 by Chemisorption of W0Cl4 to Surface Saturation M. Lindblad and L. $.l?indfors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 3
b.
XIV
Highly Active V 0 Thin Films Prepared by Chemical Vapor Deposition on Silica for Oxidative DeiyJrogenation of Alcohols T. Okuhara, K. Inumaru, M. Misono and N. Matsubayashi . . . . . . . . . . . . . . . . 1767 Influence of the Chemical Composition on the Preparation of Cu-Co-Zn-Al Mixed Oxide Catalysts with a High Mctal Dispersion A. J. Marchi, J. 1. Di Cosimo and C. R. Apesteguia . . . . . . . . . . . . . . . . . . . . . 177 1 Sol-Gel Derived Heterogeneous Catalysts T. Walton and P. A. Sermon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1775 Forming of Pyrogenic SiO, and TiO, and their Applications as New Types of High Surface Area Catalyst Supports M. Bankmann, B. Despreyroux, H. Krause, J . Ohmer and R. Brand. . . . . . . . . . . 1781 New Preparation Method of Small Particles in Ni/SiO, Catalysts Involving Chelate Ligands Z. X.Cheng, C. Louis und M. Che . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1785 Stepwise Monitoring of Mixcd Oxide Catalyst Preparations by XAS Spectroscopy 0. Clause and M. Che . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 89 Improving SO, Resistencc of Base Metal Perovskite Type Oxidation Catalyst W. Li, H. DaiandY. Lieu. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1793 Carbide Catalysts: Laser Pyrolysis Synthesis and Catalytic Activity J. M. Stencel, P. C.Eklund, X.-X.Bi, B. H. Davis, G.T. Hager and F J . Derbyshire . . 1797 Structural Support Effects in the Systematic Preparation of Pd/SiO, Catalyst for Methanol Synthesis by Ion Exchange Techniques A. L. Bonivardi, M. A. Baltanas and D. L. Chiavassa . . . . . . . . . . . . . . . . . . . . 1801 The Advantageous Use of Microwave Radiation in the Preparation of Supported Nickcl Catalysts G. Bond R. B. Moyes, S. D. Pollington andD. A. W a n . . . . . . . . . . . . . . . . . . 1805 New Preparation Methods for Active Superfine Catalysts by Spray Reaction T. Uematsu, S. Shimazu, T. Kameyama andK. Fukuah . . . . . . . . . . . . . . . . . . . 1809 Preparation of Supported Cu-Ni Bimetallic Catalysts by Alkoxide Method with High Activities for Hydrogenation or Dehydrogenation T. Sodesawa, S. Sat0 and F. Nozaki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 13 Dy-Cu Alloy Films: Catalytic Activity, Composition, Structure I I
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6 (ppm) Figure 3. 13C NMR Spectrum (100 MHz) of CO (3 at.) in presence of PVP Stabilized Colloidal PdCu Alloys, ca, 2 wt% in 2-Ethoxyethanol (a) Pd65Cu35 (19000 scans, 3 s.pulse delay; (b) Pd55Cu45; (95000 scans, 3 s pulse delay) (c) Pd37Cu63 (19000 scans, 3 s pulse delay)
t sample of
ap roximately twenty individual particles. Careful EDAX analysis of three individual partic es of differing sizes shows identical compositions which correspond to the ratio of the acetates used.Carbn monoxide adsorbs readily on the bimetallic particles. Figure 2 shows the infrared absorption spectrum of CO on a Pd37Cu63 colloid, prepared under nitrogen in 2-ethoxyethanol as described above, showing bands at 2089,2045 and 1941 cm-1. Passage of a stream of nitrogen through the solution results in a total removal of the cu. 2089 cm-1 band, and a marked reduction in intensity in the other two absorptions.
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973
The 100 MHz 13C N M R spectrum of CO at 3 at. over ca. 2 wt% colloidal PdCu alloys (ca. 45A) at 3OoC is shown in Figure 3. Alloy particles with compositions Pd65Cu35, Pd55Cu5, and Pd37Cu63 were examined. Broad resonances at 177 ppm and 184 ppm were found for the Pd37Cu63 and Pd55Cu45 cases. For the P&5Cu35 case the resonance was too broad to locate. 4.DISCUSSION Infrared Spectroscopy of CO on Colloidal Palladium The infrared spectrum of CO adsorbed on 50A colloidal palladium nanocrystals is shown in Figure l(a). As we have observed previously for similar preparations, both bridged CO (1934 cm-1) and linear CO (2047 cm-1) are present. We take these spectra as corresponding to saturation coverage of CO. Coverage is an important parameter which influences the infrared spectra of adsorbed CO [4 and references therein] and comparisons with published data should be made at similar coverages. In the colloidal systems in this study however, the presence of the stabilizing polymer PVP at the surface of the colloidal metal particle complicates the assessment of the surface area available for CO adsorption, and thus also of coverage. For the purposes of this study we use the fact that the intensity of the absorption bands due to adsorbed CO reached a maximum after addition of a certain volume of CO, and we assume that this corresponds to saturation of the available surface sites. Intermediate levels of coverage are then simply reported as a percentage of the maximum. The effect of increasing CO coverage on the infrared absorptions of CO on a similar colloidal palladium preparation (ca. 50A, stabilized in dichloromethane solution by PVP with Pd:PVP = .06w/w) is shown in Figure l(b). The low frequency band at 1880 cm-1 at lowest coverage may be assigned to triply bridging CO in threefold sites on the palladium surface, the second band (ca. 1915 cm-1) at this coverage to the beginning of occupancy of twofold sites. As coverage increases, the doubly bridged CO absorption moves to higher frequency, reaching 1939 cm-l at maximum coverage. At intermediate coverages (>70%) a second, low intensity band at 2045 cm-1 appears, correspondingto the population of on-top sites by CO. This evolution of the IR spectrum of CO on colloidal palladium could be analyzed by comparison with similar spectra for CO on various low index faces of monocrystalline palladium [4] with which some similarities can be seen, but as we pointed out above, there is considerable uncertainty about the actual coverage levels in our experiments. The observation of coverage dependent phenomena such as intensity borrowing [4] between 12CO and 13CO vibrational bands for 12CO/13C0 mixtures on these particles would provide a more accurate measure of coverage, and these are planned. 4.1.
Bimetallic Palladium-Copper Colloids. In studies of CO surface chemistry on supported metal crystallites [5], the use of bimetallics has been particularly revealing, and in this context we prepared palladium-copper bimetallic colloids as a means of introducing an additional and potentially instructive complexity to the palladium nanocluster surfaces. Bimetallic PdCu colloids were recently reported by Esumi er af. [3] who prepared them in the absence of stabilizing polymers by the thermal decomposition of mixtures of copper acetate and palladium acetate in organic solvents above 115OC. The bimetallic particles produced by this method often contained CuO rather than Cu, depending on the solvent used, and particle sizes of the colloids were in the range 90-160A. We chose instead to reduce the acetates of palladium and copper by heating in a solvent (2ethoxyethanol)which, at its boiling point, would reduce Pd(I1). Palladium acetate is effectively reduced under these conditions, but copper acetate is not. However, the zerovalent Pd thus produced would be expected to reduce Cu(I1) at the surface of the growing Pd particle, thus providing a chemical means of assuring the formation of bimetallic particles. As a further 4.2.
974
modification of Esumi's method we used PVP as a stabilizing polymer, with a view to preparing small particles of narrow size distribution. TEM images of samples prepared in 2-ethoxyethanol consistently showed well formed and monodispersed particles, with diameters of ca. 45A. EDAX analysis of individual particles demonstrated that the solutions prepared from the mixed acetates were homogeneous in composition, each particle having the same composition, and that composition was the same as the ratio of acetates used. Thus a 5050 mixture of copper and palladium acetates gave particles with uniform composition Pd5oCu50. Analysis of the supporting polymer film confiied that the acetates of both metals had been completely reduced under the conditions of preparation. Over a series of preparations we observed electron diffraction rings consistent with an f.c.c. structure, although the crystallinity of the particles seemed to vary from preparation to preparation, Precise identification of the structures of these nanocrystalline clusters is the subject of further detailed study. We are especially intrigued by the prospect of preparing nonequilibrium structures at low temperatures and of observing the effect of particle size on structure.
Infrared Spectroscopy of CO on Pd-Cu Colloids Carbon monoxide added readily to the PdCu particles in dichloromethane, as shown by the infrared absorption spectra in Figure 2 for a sample with the composition Cu63Pd37, prepared in refluxing 2-ethoxyethanol. CO occupied both palladium and copper sites. Comparison with the IR spectrum of CO on co per containing bimetallic surfaces [5(e)] identifies the high frequency absorption (2093 cm- ) as CO on "on-top " copper sites - bound to single copper atoms in a linear fashion, and confirmed that the copper was in the zerovalent state since CO on oxidized Cu would absorb at >2100 cm-l. The bands at 2046 cm-l and 1936 cm-1 are in the ranges found by us for linear and bridged CO on Pd sites in similar PVP stabilized palladium colloids (see above and ref. Id) and similar in frequency to infrared absorptions assigned to on-top and bridged CO on single crystal palladium [4] and supported palladium crystallites [6] In other (ostensibly similar) preparations of the palladium-copper alloy colloid, we observed a shoulder at ca. 191Ocm-1, which we occasionally observed for CO on colloidal palladium, and can also be assigned to bridged CO on palladium. It is of interest to note that the frequencies of the [PdCO] absorptions were very close to those we reported for CO on pure palladium colloids of this approximate diameter, despite the fact that copper comprised 63% of the metal particle. This is consistent with a negligible electronic or "ligand effect of alloying, as has been observed in other alloy systems [5(e)l, but coverage dependent studies, which are planned, must be performed to confirm this. In comparing further these infrared absorptions with those for CO on the PVP stabilized monometallic palladium colloid of similar size (see above) it is interesting to note that whereas in the latter case the intensity if the bridged CO adsorptions is usually much greater than the linear CO absorption for the bimetallic surface the relative intensity of the terminal CO on palladium to the bridged CO was significantly higher. This is consistent with the geometric diluting effect of the copper atoms on the surface of the colloid particle, reducing the available number of adjacent pairs of palladium atoms and thus the abundance of bridged CO molecules. This dilution effect was also noted by Sachtler in the corresponding supported palladium silver system [5a]. The relative intensities of the [CuCO] and [PdCO] bands raise some questions. The [CuCO] band is certainly not almost twice the total [PdCO] intensity, which would have been be expected from the overall relative concentrations of the metals in this Cu63Pd37 colloid if the respective extinction coefficients are similar. In addition, it is known that the surface composition of binary alloys often differs from the bulk composition as a result of surface segregation caused by differing surface energies for the two component metals, and for the case of palladium-copper alloys an enrichment of copper at the surface would be expected [7]. Examination of the absorption intensities for the three types of CO seems thus seemed inconsistent with the composition of the alloy particles. However, it cannot be assumed that the extinction coefficient for CO on copper is the same as that for CO on palladium, and it is not
4.3.
P -
.
975
obvious that CO would adsorb on aff surface copper atoms, even though the colloid solution was exposed to 1 at. of CO for several minutes. CO is known to bind much less strongly to copper than to palladium [7], and in the presence of the stabilizing PVP CO would be expected to compete unfavorably for copper adsorption sites. We propose therefore that the polymer adsorbed to coordination sites on the copper more strongly than on palladium, and thus blocked possible copper binding sites for CO. Consistent with this we have noted in preparations of solutions of copper-rich alloy colloids stabilized by PVP that the viscosity of the solution increases with increasing copper content, implying that the interaction between surface copper atoms and the polymer, which contains nitrogen and oxygen donor atoms, is stronger than that with palladium and thus more effective in causing crosslinking between polymer chains and resulting in an increase in viscosity. A second phenomenon contributesto the apparently anomalous relative intensities of the [CuCO] and [PdCO] absorptions. When a stream of nitrogen was passed through the solution, CO was removed preferentially from copper sites, consistent with the lower adsorption strength of CO on copper compared to that of CO on palladium. It was also removed partially from the palladium surface atoms, and warming to 100°C under nitrogen removed all CO from the particle surface. If CO was then readded to the solution the three absorption bands were regenerated with only slight change in frequency, but the intensity of the [CuCO] absorption was significantly enhanced. More copper sites have been made accessible to CO on the surface of the alloy. If we make the reasonable assumption that the polymer interaction with the surface was similar to that before CO removal, then we must deduce a higher concentration of copper atoms at the surface than was originally the case. In the absence of CO, the surface energy difference between copper and palladium has become the dominant factor determining the surface composition, and an enrichment of copper is found. This implies that under carbon monoxide, when the surface of the alloy is covered with CO, there should be a surface enrichment in palladium. This would be expected from the fact that the binding energy for CO on palladium is greater than for CO on copper and thus the palladium atoms are stabilized in preference to copper atoms at the surface. Thus the very act of adsorbing CO on the alloy surface initiates an enrichment of the surface in favor of palladium. A similar adsorbate induced surface segregation of palladium over silver was observed some years ago by Sachtler in photoemission studies of the surface compositionof palladium silver alloys in the presence and absence of CO [8]. It is interesting to note that this sequential surface segregation of each metal depending on the adsorbate present takes place under mild conditions, implying considerable mobility of the metal atoms in the colloidal particles. Solution 13C N M R of CO in the presence of Palladium-Copper Alloy Colloids. We earlier reported [l(b)] the results of a high resolution liquid phase 13C NMR experiment on the adsorption of 13C0 on colloidal palladium, in which exchange between the adsorbed and free states of CO was exploited in a spin saturation transfer experiment to locate the resonance of the adsorbed CO. In that experiment the free CO resonance at 185ppm was exchange broadened in the presence of solutions of colloidal palladium, the line broadening reflecting the exchange rate. We observed that the linewidth of 13C0 (3 at.) in the presence of an approximately 2wt% solution of colloidal palladium (mean diameter 75A) stabilized with PVP was ca. 3 ppm at 30°C. As a preliminary to similar experiments on the bimetallic system, we have examined the lineshape of dissolved 13CO in the presence of the palladium-copper alloy colloids. The I3C resonances of CO (3 at) in the presence of 2-ethoxyethanol-DMF solutions (cu. 2 wt 96 metal) of three palladium-copper alloy colloids of differing copper content are shown in Figure 3(a-c). A sharp resonance at 175 ppm due to the carbonyl group in PVP was observed in each solution, and this served as an internal reference. For the Pd55Cu45 alloy, a broad resonance centered at 184.5 ppm and a width at half height of 4 ppm was observed, and for the Pd37Cu63 alloy the CO resonance was found at 177 ppm with a half4.4.
976
width of 2Sppm. For the Pd65Cu35 alloy, the CO resonance was not observed, and we assume that it was broadened into the baseline. Thus in the presence of each of the copper containing colloid solutions a broad CO resonance was found, but there are significant differences between these spectra and the previously reported pure palladium case. The fact that the colloid with the highest copper content (Figure 3(a)) resulted in an upfield shift for CO (from 185 ppm for free CO) suggests that the observed resonance is either the result of exchange coalescence of resonances for the free and adsorbed CO, if the latter has an upfield shift from free CO, or is the resonance of adsorbed CO itself possibly averaged over the whole particle. We previously noted a large downfield shift for CO adsorbed on pure palladium colloids [l(b),(d)], similar to that observed in the solid state by several authors [lo-121 and corresponding to the large Knight shift of palladium. This is a property of the metal itself, a property related to the presence of conduction electrons in the metallic particle. For alloys of palladium the magnitude of the Knight shift is known to vary with composition, and for palladium-copper alloys in the composition range of our colloids it is not possible to predict the expected shift for adsorbed CO. Thus a free CO resonance coalesced with a resonance due to adsorbed CO on a palladium-copper alloy might well give a single resonance upfield from that for free CO. The increasing linewidth at lower co per content (Figure 3(c) and (d))would then be interpretable as evidence for the alloy partic e becoming more palladium like, with the linewidth corresponding to an increase in exchange rate. These suggestions are tentative, and the testing of their relative merit awaits detailed studies including variable temperature and spin saturation transfer experiments to locate any as yet unobserved resonances. The possibility of resolving CO on copper from CO on palladium on a bimetallic surface is being pursued.
P
5.
CONCLUSIONS
Palladium in the form of nanoscale PVP stabilized colloidal crystallites displays in solution many of the characteristic properties of supported palladium crystallites. Carbon monoxide adsorbs in a manner similar to that which it adopts on supported palladium. Bimetallic palladium-copper nanoscale particles have been prepared as PVP stabilized colloids in organic solvents in monodispersed and compositionally homogeneous fonn. CO on copper sites on the alloy surface is identified by its vibrational spectrum, absorbing at higher frequency than that for CO on palladium, and a dilution effect of copper at the alloy surface is observed, The frequency of CO on palladium in the alloy system is unchanged from that for pure palladium particles, consistent with a negligible electronic or "ligand " effect of alloying. The thermodynamically preferred surface segregation of copper in palladium-copper alloys is reversed in the presence of adsorbed carbn monoxide, which binds more strongly to palladium atoms than to copper atoms, stabilizing surface palladium and thus enriching the surface in palladium. Mobility of metal atoms in the alloy particles is implied. The rate of the CO adsorption-desorption exchange reaction is markedly affected by the incorporation of copper in to palladium colloidal particles,
ACKNOWLEDGEMENTS The authors acknowledge with gratitude invaluable discussions with Prof. D.G. Blackmond (University of Pittsburgh) and Dr. F. M. Hoffmann (Exxon Corporate Research) on vibrational spectroscopy of adsorbed CO. A Summer Internship from Exxon Research and Engineering Company is gratefully acknowledged (A.D.).
977 REFERENCES 1 (a) J. S . Bradley, E. W. Hill, M. E. Leonowicz, H. Witzke, J. Mol. Catal., 59 (1987) 41; (b) J. S. Bradley, J. M. Millar, E. W. Hill, J. Amer. Chem. Soc., 113 (1991) 4016; (c) J. S. Bradley, J. Millar, E. W. Hill, M. Melchior, J. Chem. SOC.,Chem. Commun., (1990) 705; (d) J. S. Bradley, J. Millar, E. W. Hill, S. Behal, B. Chaudret, A. Duteil, Farad. Disc. Chem. SOC.,No 92 (1991) (in press); (e) J. S. Bradley, J. M. Millar, E. W. Hill; S . Behal. J. Catal, 129 (1991) 530. 2 Y.Takahashi, T. Ito, S . Sakai, Y. Ishii ; Chem. Commun., (1970) 1065. 3 K. Esumi, T. Tano, K. Torigoe, K. Meguro, Chem. Mater. 2 (1990) 564. 4 F. M. Hoffmann, Surface Science Reports, 3 (1983) 107. 5 (a) Y. Soma-Noto, W. M. H. Sachtler, J. Catal., 32 (1973) 315; (b) M. Primet, M. V. Matthieu, W. M. H. Sachtler, J. Catal., 44 (1976) 324; (c) E. L. Kugler, M. Boudart, J. Catal., 59 (1979) 201; (d) F. J. C. M. Toolenaar, D. Reinalda, V. Ponec, J. Catal., 64 (1980) 110; (e) H. A. C. M. Hendrickx, V. Ponec, Surf. Sci. 192 (1987) 234. 6 (a) H. A. C. M. Hendrickx, C. des Bouvrie, V. Ponec, J. Catal, 109 (1988) 120; (b) L-L. Shieu, Z. Karpinski, W. M. H. Sachtler, J. Phys. Chem., 93 (1989) 4890. 7 C. T. CamDbell. Ann. Rev. Phvs. Chem. 41 (1990'1775. 8 R. Bouwman, G. J. M. Lippiti, W. M. H. Sachtle;, J. Catal., 25 (1972) 350. 9 G. C. Carter, L. H. Bennett, D. J. Kahan, Progress in Material Science, 20 (1977) 1192 10 C. P. Slichter, Ann. Rev. Phys. Chem., 37 (1986) 25 11 K. W. Zilm, L. Bonneviot, D. M. Simonsen, G. G . Webb, G. L. Haller, J. Phys Chem. 1990.94.1463 12. S . E. Shore,'J. P. Ansermet, C. P. Slichter, J. H. Sinfelt, Phys. Rev. Lett., 58 (1987) 953
DISCUSSION Q: J. W. Geus (The Netherlands) In preparing colloidal solutions of ferromagnetic particles, you will have superparamagnetic properties as long as the particles are single domain (nickel smaller than about 100 nm, iron smaller than about 19 nm). Since the particles can readily rotate in the liquid, rotation of the magnetization within particles of a fixed orientation, which is generally more difficult, is not required to display superparamagnetic behavior. My question deals with the polymers you are using to stabilize the metal particles against clustering. Have you tried other polymers than the polyvinylbyrrolidone mentioned in your paper and could you specify the properties of the olymers required to achieve a sufficiently high colloidal stability ? In this connection t e interaction of the polymer with the metal particles is important. Is the interaction of a physical nature, where the number of atoms of a polymer chain contacting the metal surface determines the strength of the interaction or is it a chemical interaction '?
R
A: J. S. Bradley We find, as have many others before us, that polar polymers, or copolymers containing a polar monomer constituent, are necessary in order to stabilize colloidal metals of the type we report here. Among the polymers we have used successfully are sulfonated polystyrene, polymethacrylates, poly(pheny1ene oxide), cellulose nitrate, cellulose acetate. It is reasonable to assume that the metal polymer interaction takes place between the 0, N donor atoms of these polymers and the metal surface. Depending on the method of preparation it is possible that metal ions at the metal particle surface are the sites of attachment for the polymer, but this is only speculation. Some recent EXAFS results from our laboratory show metal-light atom contacts, imp1 ing ti ht complexation of the polymer, only after intentional oxidation of the metal sur&ce. d e assume that in the absence of a significant degree of surface oxidation that polymers of the type listed above are very loosely bound to the metal particles.
Q: R.W. Joyner (United Kingdom) I admire your optimism that the organic stabiliser will leave the metal particle when it is convenient. We know that many organic fragments are stongly attached to noble metal surfaces e.g. ethylidyne from ethene to >470 K, formate as formic acid often to >520 K, etc. Why should the poly-vinyl pyrrolidone be so obliging ? If your cannot make ethylidyne from ethene then either the surface is poisoned or no (111) planes are exposed. A: J. S. Bradley If we translate your terminology on the "obliging" nature of a polar polymer dissociating from a colloidal metal surface "when convenient" into the terminology of classical coordination chemistry the problem becomes trivial. W e are observing, for example in the adsorption of CO to PVP stabilized palladium, simply the displacement of a weakly bound ligand by a more strongly bound ligand, CO is a ligand which binds well to zerovalent metals, and would be expected to displace most of the pyrrolidone groups at the surface. To your supplementary comment I agree.
Q: P. Fouilloux (France) My question is about the protective layer of your colloids. here you show the CO adsorption IR spectra. In a catalytic reaction, the reactant molecules are bigger. What about the accessibility to the metallic surface for large molecules ? A: J. S. Bradley In simple catalytic screening of these colloids we have successfully hydrogenated acenaphthylene, a fairly large, easily hydrogenated olefin, which implies relatively easy access to the metal surface for incoming reactant molecules. Q: P. A. Sermon (United Kingdom) You clearly showed that your colloids are homogeneous and that surface composition in the Pd-Cu system changed with introduction and removal of CO in a rcversible way. Does this mean that the initial surface composition of the particles is not defined by differences in the rates of reduction of the Pd and Cu precursors ?
A: J. S. Bradley Yes, the surface composition is not defined by these relative rates, simply because the copper is in fact reduced at the palladium surface. That is the reason for the homogeneity of the particles. If the colloid preparation involved the simultaneous reduction of two salts using conditions under which both would reduce independently, we would expect to see a mixture of compositions.
Q: M. Ichikawa (Japan) 1) May we have your comments on the transition situation (chemistry and physics) between molecular cluster compound and metal colloid chemisorbed with CO ? What different features do you expect in bonding mode chemistry and morphological points of new for the colloidal metal species ? 2) Regarding the preferential formation of linear CO on smaller size Pd colloids, wonder whether the particle surface is oxidized or affected with the stabilizer PVP to prevent the bridging CO or not. How d o you explain the preferential trend of linear CO formation with decreasing size of Pd particles which is rather contradictory with the trend in CO chemisorption on the clean Pd particles or crystal surfaces ?
A: J. S . Bradley I think the state of affairs for a CO covered colloidal palladium particle is analogous to that for a CO covered supported metal particle, with the exception that for the
979 supported case there is the potential complication of reaction of the metal particle with the oxide surface-oxidation, corrosive chemisorption etc. See for example [l] in which infrared and NMR data for adsorbed CO are discussed. Comparison between CO covered colloidal metals and molecular carbonyl clusters become relevant and interesting in the size range c ca. 1.2 nm, that is to say in the size range for the larger carbonyl clusters. There are several reports of bulklike properties for molecular clusters of this size. Conversely very small colloidal particles might be expected to exhibit molecular properties. We reported for example [2] the adsorption of CO on very small (> 1,Pbenzodiphenylene sulfide > propanethiol> sulfolane. INTRODUCTION Since 1949 bifunctional reforming catalysts (e.g. PVCI-AI,O,) have been used by the petroleum industry to convert CG-C11 paraffins to highly aromatic blend stocks which are subsequently used for the production of high octane gasoline and a wide variety of chemicals (1). When hexane feedstocks are employed, conventional bifunctional catalysts exhibit poor benzene selectivity. In 1978 Bernard and Nury reported the unprecedented observation that PVKL is an exceptionally active and selective catalyst for directly aromatizing n-hexane to benzene (2). Subsequent studies by Bernard et. al. showed PVKL to be sensitive to sulfur poisoning (3). The sulfur sensitivity of a monofunctional PVKL hexane aromatization catalyst is qualitatively known to be substantially higher than that of conventional PVCI-AI,O, and Pt-RelCI-Al2O3 bifunctional reforming catalysts (4-5). Acceptable aromatization cycle lengths have, however, been achieved by lowering feed sulfur concentration to ultra-low levels (6). The present studies were undertaken to quantify the effects ppb sulfur concentration and various sulfur containing molecules have on the activity maintenance and selectivity patterns of PVKL catalysts. Such information was anticipated to be of use in clarifying the mechanism by which sulfur so markedly suppresses the performance of PVKL catalysts.
1020 EXPERIMENTAL
Loading Pt into bound 1/16 inch KL extrudate particles was accomplished by an established procedure (7). After drying, the freshly loaded catalyst was activated by air calcination at temperatures in the range of 423 to 783°K. PtlKL catalysts used in catalytic measurements were prereduced in flowing H, from 373 to 783°K. In general, catalytic tests were carried out in a continuous flow tubular reactor at conditions of 783*K, 6.2-6.6 WHSV, 788-836 kpa, H2/feed (mole ratio) 4-6. Reaction products were analyzed by an on-line G.C., and results are reported as wt% conversion and selectivity, which are defined as follows:
-
Conversion = 100 (n-CG t 2-MP t 3-MP t MCP t C6-olefins)f Selectivity = [Yield (Benzene ~roduced)/Conversion)x 100 Where n-C6, 2 4 P , 3-MP and MCP are n-hexane, 2-methylpentane, 3-methylpentane and methylcyclopentane, respectively; f is final product composition in wt%. 3-MP, n-C, and MCP were purchased from Phillips Chemical Company and dried over 3A sieve prior to catalytic measurements. The organo-sulfur compounds, thiophene, lI2-benzodiphenylene sulfide, propanethiol and sulfolane, used in this work were purchased from Aldrich and used as received. Doping liquid feeds with ppb organo-sulfur compounds was accomplished by multi-staged gravimetric dilution of standardized solutions.
To investigate the effect sulfur has on Pt dispersion and particle size distribution in zeolite crystals, both fresh and aged catalysts were examined using either the transmission electron microscope (TEM) andlor scanning transmission electron microscope (STEM). High resolution TEM images were obtained on a JEOL 200CX, with nominal point-to-point resolution of 2.3 A when operated at 200 kv. High resolution 2-contrast images Pt particles were obtained on a VG HB501A STEM, operated at 100 kv and equipped with a high-angle annular dark field detector. Details of the microscopic techniques and specimen preparation were published previously (8). RESULTS AND DISCUSSION of PVKl C-
We have extensively used both catalytic tests and HREM measurements to enhance our understanding of the main factors affecting PtlKL catalyst deactivation. Figure 1 shows benzene yields obtained from three replicate experiments of different run lengths on a PVKL catalyst under
1021
accelerated deactivation test conditions of 793"K, 788 kpa, 50 WHSV, H2/feed mole ratio of 6 and a nil sulfur feed (9) of n-C6 (70%) and MCP (30%). At run lengths of 1.5, 7.5 and 65 hr, the corresponding benzene yields were 56,52 and 2670, respectively. After 65 hr on feed, there was less than 2% carbonaceous material (i.e. coke) on the spent catalyst. Because of the small amount of coke on catalyst, it is assumed that coke plays a minor role in the overall deactivation of a PVKL catalyst.
-s
n
8
'0
.-* Q)
60 50
40 30
c
tt:c
20
:
10
0
10
20
30
40
50
60
i
Hour on Oil
Figure 1, Benzene yields of PtlKL from accelerated catalyst deactivation tests.
The above three aged catalysts recovered after 1.5, 7.5 and 65 hr on feed and a fresh catalyst were examined by STEM. The 2-contrast images obtained using STEM are sensitive to the atomic number Z, giving high contrast for high Z particles such as Pt when supported on low Z supports such as alumina, silica and zeolites (10). In general, the Z-contrast technique is more sensitive than conventional bright field TEM imaging methods for detecting small Pt particles on KL (8). Figure 2 shows the volume-weighted particle size distributions (11) of Pt clusters for the above samples, determined from the correspondingSTEM micrographs. The data in Figure 2 is corrected for image broadening due to the finite width of the electron beam, which is estimated to be 4A. An experimental calibration of this image broadening effect is based on the premise that upon sintering, Pt particle widths are limited by the zeolite L cage with a maximum diameter of -13A. Particle length along the channel is, however, unrestricted. In the Z contrast images the measured image widths of such agglomerated Pt clusters (measured from channel wall to channel wall) was typically 17A. This was assumed to translate into 13 A actual diameter.
1022
In the fresh catalyst, the majority of the Pt (>85%) is present as sub 7A clusters within the KL channels. With increasing exposure to feed, particle sizes coarsen with an accompanying build up of Pt clusters which exceed 13 A in diameter. In principle, if the zeolite structure were rigid, cluster widths could not exceed the channel width. However, clusters are free to grow along the channel directions forming, thereby, slightly elongated particles. After longer times on feed, Pt particles may begin to accumulate outside the channels either by diffusion of Pt atoms or by particle migration. Without the constraint of the KL cage, Pt can grow to much larger size. Based on the histogram in Figure 2d, an upper limit for the amount of external Pt clusters after 65 hr is about 15%0.
"L (a) Fresh
(b) 1.5 hra
20
5
10
15
20
Figure 2. Corrected volume-weighted particle size distribution. (a) fresh catalyst, (b)1.5 hr on oil, (c) 7.5 hr on oil and (d) 65 hr on oil. Under the above reaction conditions employed, Pt agglomeration appears to be the major cause of PtlKL catalyst deactivation. Coking is considered to play a minor role in PffKL deactivation. Poisonina PVKI -(&-&JJv Sulfur Four experiments were conducted to investigate the effect of ppb thiophenic sulfur concentrations have upon benzene yield of a PtlKL catalyst. Reaction conditions were 783"K, 6.6 WHSV, 823 kpa, H,/feed ratio of 6 and a mixed hexane (89%) and MCP (11%) feed. Results of these studies are shown in Figure 3. In the absence of added sulfur, benzene yields decreased from 67 to 367'0 over a 280 hr run length. At a given reaction time, benzene yields for the three sulfur added runs were
1023 inversely proportional to the feed sulfur levels. This behavior suggests that sulfur systematically lowers the number of accessible Pt sites within the KL channels. After extended reaction times, benzene yields for the three sulfur added runs converged to a limiting value of about 8%.This
-E
70
60 50
-* 40 a,
c
30 20
a,
10 0
Hour on Oil Figure 3. Benzene yields from the thiophene doped hexane runs. 80
NOS
70 60
\4
50
50 ppb S
40
30 20
i: - - . 0
1 *
40
.-
I =
80
.
* 1 =
-
-l-lOO ppb S * 1 .
*
.I
*
120 160 200 Hour on Oil
-
* I *
240
- 2 8 0. -.320 = I
Figure 4. Benzene selectivities from the thiophene doped hexane runs. residual, relatively stable benzene yield is most probably associated with Pt particles situated near the KL pore mouth or residing on the basal planes of the KL crystal. The initial rapid decline in benzene yield, followed by a low, yet flat benzene yield plateau indicates that Pt particles within the
1024
KL channels are more susceptible to sulfur poisoning than Pt particles on the outer surface of the KL zeolite. As shown in Figure 4, in the no sulfur added run, benzene selectivities decreased from 73 to 56% over a 280 hr reaction period. Limiting benzene selectivities of about 30% were observed in each of the three sulfur added runs. The low and constant benzene selectivities, when coupled with the observed stable benzene yields, further supports our suggestion that the residual performance patterns of sulfur poisoned PVKL catalysts result primarily from Pt particles outside the influence of the KL channels.
201 0
-
I
10
.
I
20
.
1
30
*
I
40
.
1
50
-
I
80
.
70
Benzene Yield (wt%)
Figure 5. Correlation of benzene selectivity with benzene yield in the thiophene doped runs. Benzene yield versus benzene selectivity Is presented in Figure 5. The common curve shared by the no sulfur and three sulfur added cases strongly suggests that sulfur deactivation occurs by the loss of active Pt sites and not by modifying the catalytic Pt sites. Further insight into the mechanism by which sulfur deactivates PVKL catalyst can be gained by considering the information summarized in Table 1. The times required for benzene yields to converge to the limiting value of 8% were estimated from benzene yield versus time curves as shown in Figure 3. By knowing the feed sulfur content, WHSV and the time required to reach the stable 8% benzene yield plateaus, the quantity of sulfur in terms of SiPt atom ratio responsible for leveling the benzene yields of a PVKL catalyst is easily calculated. At S/Pt atom ratio of about 0.1, the benzene yields of a PVKL catalyst are reduced to low, but stable steady-state values. Thus, only one Pt out of ten needs to be poisoned by sulfur for the performance of the catalyst to be drastically lowered.
1025 Table 1 Effect of Feed Sulfur Content on the Performance of a PVKL Catalyst Feed Sulfur
Time (hr) to Converge B8-neY ieldb 340 220 125
IDab)a 50 100
200
SIPt e,klLB& 0.08 0.10 0.10
a. sulfur added in the form of thiophene h run conditions: 783"K, 6.6 WHSV, 823 kpa, hexane feed and Hdfeed ratio of 6. The results of STEM measurements on several of the above catalysts are summarized in Table 2. As can be seen, the average Pt particle size increased with increasing feed sulfur concentration. Thus, feed sulfur concentration in the ppb range accelerates the rate of Pt agglomeration. As shown by TEM evidences presented in a later section of this paper, Pt agglomerates located inside the KL channels, near the channel openings and on the KL basal planes block access to the active Pt sites within the KL channels. Table 2 Extent of Pt Agglomeration of a Sulfur Poisoned PVKL Catalyst Feed Sulfur
oa None 50 100
Time on
Benzene Yield
Pt Particle Size
LliLUlL
A
Am!adi
285 235 190
37 13 10
13 17
20
a. sulfur added in the form of thiophene h Pt particle size determined by Zcontrast STEM measurements Effect of Sulfur Source on Poisonina PUKL C a t & t Because a large variety of sulfur compounds can be present in industrial light naphtha feedstocks, variations of the poisoning effect as a function of the sulfur compound type was investigated. Thiophene, 1,Pbenzodiphenylenesulfide, propanethioland sulfolane were chosen as the model sulfu compounds in this study. These compounds have different electronic, steric and oxidative properties.
1026
Figure 6 shows the poisoning effects of these compounds at a constant 280 ppb sulfur level on a WKL catalyst hexane aromatization activity. Thiophene is by far the most virulent poison studied. Over a period of 50 hr, benzene yield decreased from 46 to 1% with the thiophene added feed whereas benzene yield decreased to 33% with the nil sulfur feed. On the other hand, sulfolane is the least toxic compound studied. In this case, benzene yield decreased to 23% over the 50 hr run length. A bulky 1,2-benzodiphenylene sulfide showed only a slightly higher poisoning effect than the much smaller propanethiol.At 50 hr, the corresponding benzene yields were 18 and and 20%, respectively. 50
45
3 30 9)
9)
C
25
20
8 15
2 10 5
0 0
10
20
30
Hour on Oil
40
50
60
Figure 6. Effect of Molecular Sulfur Source on Poisoning a PtlKL Catalyst at 783OK, 6.2 WHSV, 836 kpa and Hdfeed ratio of 4.3. The above observations can be rationalized as follows: In general, the poisoning effect of a molecule on a catalyst is related to its ability to form a chemisorbed species with the catalyst surface (12). Organo-sulfur compounds which possess one or two pairs of electrons on sulfur often exhibit enhanced toxicities toward to noble metals (13-14). The highly conjugated x: electron systems present in thiophene and 1,241enzo-diphenylene sulfide would be expected to be more strongly chemisorbed than the aliphatic propanethiol. Differences in toxicity between thiophene and 1,Pbenzodiphenylene sulfide might reflect a steric effect, since a bulky molecule would be more difficult to diffuse into KL channels. In the case of sulfolane, a two-step reduction of a hexa-valent S to the di-valent sulfur is needed to poison Pt metal particles. As shown in Figure 7 by the bright field TEM of the fresh PtfKL catalyst, there are no visible large Pt clusters in the zeolite channels of the fresh catalyst. Based on the H/Pt atom ratio of 1 by H, Chemisorption, Pt must be highly dispersed in the fresh catalyst. In the above thiophene poisoned PtlKL catalyst shown in Figure 8, large Pt agglomerates in the range from 20 to 50 A are visible
1027
Figure 7. TEM micrograph of the fresh PVKL catalyst.
Figure 8. TEM micrograph of the thiophene poisoned PtlKL catalyst.
1028 inside the KL channels, near the pore mouth and on the KL basal planes. The observation that the Pt agglomerates tend to form at the KL channel entrance strongly supports the contention that sulfur deactivation, at least in the initial stages, of PtlKL occurs by pore mouth plugging of the KL channels by Pt or Pt-S particles. In conclusion, Pt agglomeration is a major cause of PVKL deactivation. Coking appears to play a minor role. Sulfur at ppb level accelerates Pt agglomeration. TEM measurements and SlPt ratio arguments suggest that large Pt agglomerates block access to the active Pt sites within the KL channels. Sulfur sensitivity of PtlKL catalyst is highly dependent upon the molecular structure of sulfur source. Our data suggest that the order of poisonous effect of organo-sulfur compounds is thiophene >> 1,Pbenzophenylene sulfide > propanethiol > sulfolane. REFERENCES 1. R. A. Flinn, 0. A. Larson and H. Beuther, Hydrocarbon Process Pet. Ref.,42,129 (1963). 2 J. R. Bernard and J. Nury, U.S. Patent 4,104,320 (1978). 3. C. Besoukhanova, C. M. Breysse, J, R. Bernard and D. Barthomeuf, Cataalyst Deactivation, 2pl (1980). 4. T. R. Hughes, W. C. Buss, P. W. Tamm and R. L. Jacobson, AlChE Meeting, April 8,1986, New Orleans, LA. 5 D. V. Law, P. W. Tamm and C. M. Deb, AlChE Meeting, March 29,1987, Houston, TX. 6. W. C. Buss, L. A. Field and R. C. Robinson, U. 5.Patent 4,456,527(1984). 7. K. R. Poeppelmeier, T. D. Trowbridge and J. L. Kao, U. 5.Patent 4,568,656 (1988). 8 S. B. Rice, J. Y. Koo, M. M. Disko and M. M. J. Treacy, UltramicroscopyLU, 108 (1990).
9 No sulfur was detected in this feed. 10. M. M. J. Treacy, ACS Symposium Series No. 248, p. 367 (1984). 11. R. S. Matyi, L. H.Schwartz and J. B. Butt, Catal. Rev-Sci. fng., 2pw, 41 (1987). 12 J. Barbier, E. Lamy-Pitara, P. Marecot, J. P. Boitiaux, J. Cosyns and F. Verna, Adv. Catal.,JZ, 279 (1990). 13. E. B. Mated, Adv. Catal., 9, 129 (1951). 14. C. H. Bartholomew, P. K. Agrawal and J. R. Katzer, Adv. Catal.,
a,135 (1982).
1029
DISCUSSION Q: Chyi-gang Huang (Taiwan) Your data show clearly that the catalyst deactivates rapidly even in the presence of trace amount of S. As we all know if is very difficult to remove S completely from oil cut. In this case, how could use catalyst industrially ?'
A: J. L. Kao There is a standard guard bed technology to protect reforming catalyst against sulfur poisoning. our process is proprietary, and I can not comment on it. Q: B. Notari (Italy) Your paper shows that even in the absence of sulfur there is a deactivation of Pt/KL, I received the impression that no while from the previous paper (see p. 9 9 3 ) . deactivaiton occurred under operating conditions. If deactivation takes place, the catalyst must be regenerated periodically. Normally this is done with chlorine compounds. Does this technology apply also to Pt/KL catalysts?
A: J. L. Kao We can regenerate our catalyst many times and we have had many patents covering this area. I can not comment on the regeneration of our catalyst. Q: M. M. Ramirez de Agudelo (Venezuela) Would you expect the same deactivation effect with other heteroatoms compounds present in the feedstock, namely nitrogen compounds ? A: J. L. Kao Yes. However, the organo-nitrogen compounds are less poisonous than the organosulfur compounds.
Q: B. H. Davis (USA) At one time it was my understanding that an active and stable Pt/KL catalyst would be very selective for hydrogenolysis of n-alkanes to produce methane relative to ethane, etc.; that is "terminal hydrogenolysis". Was the catalyst that was stable for thousands of hours selective for terminal hydrogenolysis ?
A: J. L. Kao We do not have enough data to show a clear correlation between the catalyst stability and the terminal cracking index in our bound Pt/KL extrudate catalyst. . Q: G. L. Hailer (USA) We have confirmed the effect of sulfur you report, i.e., destabilization and Pt particle growth outside of the pores of Lzeolite when the Pt/KL catalyst is exposed to sulfur at high temperature. Could you comment on the possible mechanism by which sulfur destabilizes small Pt particles in the L-zeolite pores toward migration and sintering ? In particular, could this be viewed as a competition between two kinds of electron donors, sulfur and the basic L-zeolite pore walls, and would this be consistent with stabilization of small Pt particles by electronic interaction with the L-zeolite walls ? A J. L. Kao J. Barbier et al., gave an excellent discussion on the roles of sulfur in the catalytic hydrogenation reactions [l]. As discussed on page 300 of this article, the interaction between Pt clusters and sulfur are very strong so that the Pt-Pt bond is modified, and "the
1030 mobility of the surface atoms can be increased and a new superficial structure can appear". We believe that these are the causes for sulfur sinterin Pt clusters in zeolitc L. I !Cosynes, F. Vema, Adv. [l] J. Barbier, E. Lamy-Pitara, P. Marecot, J. Boitiaux, . Curul., 37,279 (1990)
Q: D. C. Konin sberger (The Netherlands) Our EXAF experiments fulls support your conclusion. Pt particles grow if sulfur is
8
present in the feed (J. Catalysis accepted). The role of sulfur is to change the interactions of Pt with the support. Sulfur is most probably present is the interface between the platinum particles and the zeolite. A: J. L. Kao Thank you very much for your comments.
Q: T. S. R. Prasada Rao (India) You have indicated that sulfur at the level of ppb will affect the catalyst performance. In industry the present catalyst will tolerate upto 1 ppm, whereas, your catalyst will be affected even at 1 ppb. Therefore, it appears that the Pt/KL catalyst is more sulfur sensitive. If so how do you see the future of your catalyst for industrial application in this process ? A: J. L. Kao In the laboratory, we prefer to use a low sulfur feed in which the sulfur level is less than the detectable limit of 5 ppb.
Q: S. Sivasanker (India) Is the S-sensitivity of Pt-L catalyst influenced by the nature of the ions (exchanged) in the zeolite, just as activity is reported to be influenced.
A: J. L. Kao Regardless of the type of cations in LTL, all Pt/LTL catalysts arc generally sensitive to sulfur poisoning. Q: V. Fattore (Italy) Your catalyst can be regenerated many times. Is the regeneration carried out "in situ" or is necessary to take the catalyst out of the reactor and to go through a complex procedure ? Is the initial activity completely recovered '?
A: J. L. Kao We prefer to regenerate our catalyst in situ. I can not comment on the catalytic performance of the regenerated catalyst.
Guczi, L er ul. (Editors),New Frontiers in Caralysis Proceedingsof the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
AROMATIZATION OF n-HEXANE BY ALUMINIUM-STABILIZED MAGNESIUM OXIDE-SUPPORTED NOBLE METAL CATALYSTS
E. G. Derouanea, V.Jullien-LardoP, R. J. Davisb, N. BlomC and P. E. Hojlund-NielsenC aLaboratoirede Catalyse, Facultes Universitaires N.-D. de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium bDepartment of Chemical Engineering, Unviersity of Virginia, Charlottesville, Virginia 22903-2442, USA CHaldorTopsoe NS Research laboratories, Nymollevej 55, P.O.Box 213, 2800 Lyngby, Denmark
Abstract Several Pt on aluminum-stabilized magnesia (Mg(A1)O) catalysts with comparable metallic Pt content and dispersion have been prepared. They however differ by their Mg/Al ratio and their support specific surface area. A model is proposed to explain the control of the specific surface area of the support by the Mg/Al ratio. 27Al-MASNMR results indicate that Al(II1) species occupy both tetrahedral and octahedral sites in Mg(A1)O. The aromatization of n-hexane provides some information on the role played by the support. Additional data obtained for other catalysts differing by either the nature of the noble metal (Pd) or that of the support (Zn(Al)O, Zn(Cr)O) confirm the exceptional role of the basic Mg(A1)O support. An unusually high selectivity to aromatics is observed for Pd supported on Al-stabilized MgO. 1. INTRODUCTION
Pt clusters supported on high surface area aluminum-stabilized MgO (Mg(Al)O)have been observed to catalyze the aromatization of n-hexane with nearly the same activity and selectivity to aromatics as a reference Pt-zeolite L, thereby demonstrating that high surface area basic supports can be used advantageously in hydrocarbon reforming reactionslt2. It was indicated originally3, to explain the remarkable activity and selectivity of Pt-zeolite L, that the (basic zeolite) support could play two possible roles: either modify the atomic or electronic structure of the Pt clusters or participate directly in the reaction mechanism. Two other proposals have been advanced more recently to explain the high aromatization selectivity. Both consider the geometry of the zeolite support. One is the molecular die hypothesis4t5, the other is the confinement model6. Recent data are consistant
1032
with and favor an effect of confinement78. They support the idea that nonbinding interactions of n-hexane with the zeolite walls influence the selectivity (and activity) of the zeolite L catalyst. If the same mechanism holds for both Pt-zeolite L and Pt-Mg(Al)O catalysts, the unexpected and high aromatization activity and selectivity of the PtMg(A1)O catalyst suggest that geometric interpretations may not completely explain the catalysis observed over zeolite L materials2. I€is worth recalling in this respect that Pt-carbon catalysts have been observed t o aromatize hexadienes and hexatrienesg or n-hexanelOJl. It has also been proposed that Pt (111) hexagonal planes are responsible for n-hexane aromatization, when both carbonlOJ1 and silica12 supports are used. If a bifunctional mechanism operates for Pt-Mg(Al)O, in which the metal sites dehydrogenate hexane and the basic support sites participate to cyclization and aromatization, one expects noticeable changes in activity and selectivity when altering the nature of the basic support or that of the noble metal. The present work attempts to clarify this problem and to elucidate the role played by aluminum in the MgO support. Various Mg(Al)O basic supports were prepared with different Mg/Al ratio and other hydrotalcite-derived supports were obtained by changing the nature of the di- (by Zn) and trivalent (by Cr) elements. The noble metal was also varied: Pt was replaced by Pd. 2. EXPERIMENTAL 2.1. Catalysts
The hydrotalcite route was chosen to prepare high surface area mixed oxides M e ( I I ) x M e ( I I I ) ~ ~ ~with ( ~ ~Me(I1) x ~ O = Mg or Zn and Me(II1) = Al or Cr. The use of hydrotalcite anionic clays for the preparation of mixed oxides of diand trivalent elements has recently been reviewed extensivelyls. Typically, the preparation involves the coprecipitation, by KOH and K2CO3, of the Me(I1) and Me(II1) nitrates in a proportion suitable to achieve the desired Me(II)/Me(III) ratio in the final material. The precipitation of pure hydrotalcite-typeprecursors is achieved at pH = 8-10 (to avoid the precipitation of other products) and in the temperature range 293-363 K (the ideal temperature depends on the Me(II)/Me(III)ratio). In our case, the resulting precursor is calcined in air (usually for 12 h at 873 K unless stated otherwise) and the oxide product is subsequently impregnated with a solution of Pt or Pd tetramine salt, dried, calcined, and reduced as described earlier2. Some properties of the calcined hydrotalcite precursors and of the corresponding catalysts containing Pt or Pd are described in Table 1. The supports Me(II),Me( III)2/3(1-x)0 obtained by calcination of the hydrotalcite-structure precursors are denoted in the following sections by the codes Me(II)Me(III)X,with X the final atomic Me(IIYMe(II1) ratio. The final catalysts are designated M(Y)Me(II)Me(III)X,M being Pt or Pd and Y its loading (wt.-%). N designates MgAlX catalysts prepared in identical conditions, by only varying the Mg/Al ratio; HT identifies other materials prepared in different conditions (which need not be specified for the purpose of the present report); RD identifies the aluminum-stabilized Pt/MgO catalyst and catalytic runs described earlierlva.
1033 Table 1 Some properties of the noble metaumixed oxide catalystsa
1.7 3.3 5.5
173 271 73 70 168
0.75 0.76 0.74 0.62 0.81 0.80
a. See text for nomenclature. All precursors were calcined at 873 K prior to Pt or Pd impregnation, unless otherwise indicated by superscripts B, E and d. b. Calcined at 1173 K before impregnation by Pt. "S"in the code indicates the presence of MgA1204 evidenced by X-ray diffraction. c. Calcined at 723 K before impregnation by Pt. d. Calcined at 773 K before impregnation by Pt or Pd. 2.2. Characterization
The elemental composition of the catalysts and their precursors was determined by standard atomic absorption and ICP methods. 27Al NMR was also used to dose aluminum in the MgAl samples. The specific areas of the supports and/or catalysts were determined by N2 adsorption using the BET method. X-ray diffraction (XRDXPhilips PW-1730) using the Cu-&1 radiation (Ni filter) was used to study the decomposition of the hydrotalcite precursors and characterize the products. X-ray line broadening (Scherrer's formulation) was used t o evaluate the average particle size of the MgAlX-N supports. The decomposition of the MgAl hydrotalcite precursors was investigated by thermal analysis (Stanton Redcroft ST-780 thermal analyzer). The loss of water and C02 was found to be complete at 823 K. 27Al MASNMR spectra of the MgAl catalysts were obtained using a Bruker MSL-400spectrometer (static magnetic field = 9.4 T; magic angle spinning (MAS) rate = 10 kHz) in the free induction decay mode and using a small flip angle (about d12) corresponding to a pulse length of 1 ps. A recycling time of 0.1 s was found adequate for quantitative measurements.
1034 2.3. Catalyticteats
The catalysts were tested at atmospheric pressure with a feed gas consisting of n-hexane and hydrogen i n a molar ratio H2 to n-hexane of 6, without (HTruns) or with (RD and N-runs) He as a diluent (to adjust the H z p a r t i a l pressure to 4OkPa ). The amount of catalyst used was 1 g in the HT-runs and 0.3 g in the N-runs. All the other test conditions have been previously described2 or are indicated in Table 3. 3. RESULTSANDDISCUSSION
81. The structure of aluminum-stabilizedhigh surface area MgO
The thermal decomposition of MgAl-hydrotalcite precursors leads to high surface area and thermally stable MgO-type materialsl4-17. The surface area and the average crystallite size of the MgO products stay rather constant in the calcination temperature range 750-970 K14-16. Below 670 K, both MgO and hydrotalcite are presentl4~15whereas above 1173 K pure MgO, MgAl204 and traces of gamma-Al203 are found16, as evidenced by X-ray diffraction. Our results confirm these observations. It was also established that the substitution of Al into the MgO lattice inhibits the growth of the MgO c ~ ~ t a l s 1 3 . 1 If 6 . so, one should expect an important effect of the Mg/Al ratio on the MgO particle size determination and stabilization. On the basis of previous reportsl8-20, we describe the stabilization mechanism as follows. For decomposition temperatures in the range 750-970 K, the only XRD pattern observable for the calcination product of samples MgAlX-N and MgAl3RD, is that of MgO. However, the lattice parameter (ao = 0.421 nm for pure MgO) decreases uniformly as expected when the Al molar fraction f increases (Table 21, indicating that the smaller A P + cation is indeed replacing Mg2+in the rock-salt structure of MgO. Pauling's coordination rules predict, as neither Mg2+ or Al3+ have delectrons, that Al3+ may have a tetrahedral coordination whereas Mg2+ will prefer an octahedral coordination. In all three MgAlX-N hydrotalcites, only octahedral Al is observed at a chemical shift of about 9 ppm, in agreement with previous resultsls. After calcination i n air at 873 K, two resonances are observed at about 13 and 77 ppm. They correspond t o octahedrally (Alo)and tetrahedrally ( A ~ T ) coordinated aluminum species, respectively. This observation also agrees with the results of Reichlel5 (Al~/A10= 0.25 after calcination at 723 K),although in our case more tetrahedral A1 is present (Al~/Alo = 0.68-0.821,probably because of our higher calcination temperature. The results in Table 2 further indicate that the Al~/Aloratio decreases when f decreases and when the calcined sample is exposed to water. As no dense alumina or spinel phases are detected by XRD, all Al3+ should be incorporated in the rock-salt structure MgO lattice. A10 corresponds to the Al3+ species that occupy normal octahedral framework sites. We assign A ~ Tt o Al3+ ions that have migrated to a tetrahedral symmetry interstitial site, creating a Frenkel defect. This proposal is consistant with the fact that
1035 MgA1204, formed at high temperature, is an inverse (88 %) spinel (Al~/Alo= 0.79). Thus, as no bulk MgAl204 is observed by XRD, it is proposed that Mg(A1)O contains very small and local (Mg,Al) domains with inverse spinelrelated ordering. These domains distinguish themselves from the bulk spinel by their high reactivity towards water at room temperature. About 40% of the tetrahedral Al3+ i n the freshly calcined material returns to octahedral coordination when exposed to water vapor at 293 K (Table 2). Exposure to liquid water at room temperature converts completely Mg(A1)O (MgA13-N) to meixnerite, an hydrotalcite-like material of formula MgsAlz(OH)18.4H20. Table 2 Properties of the MgAlX-N/RD oxides calcined at 873 K
a. Molar fraction of Al(II1). b. From powder X-ray diffraction. c. Measured by 27Al MASNMR; the first value refers to the freshly calcined sample, the second to the same sample partially rehydrated by water vapor (over a saturated solution of NH4Cl at 293 K; 18 h). No important changes are observed in the XRD pattern. d. From XRD line broadening using Scherrer's formulation. e. As described in text. In the absence of evidence for phase segregation and because of its high reactivity towards water, it is felt that the nature of the Mg(Al)O material is adequately described by a model proposed earlier by two of us18-20. Considering a rock-salt structure, replacement of three Me(I1) in Me(I1)O by two Me(II1) creates one Me(I1) vacancy. The surface acts as a sink for the vacancy, i n order to optimize the lattice energy. Ultimately, one Me(I1) will be removed from the surface resulting in an additional -2e negative charge on the surface. Thus, a surplus of 0 2 - ions will apear on the surface18. Negatively charged (polar) (111) planes constituted of oxygen anions only, are necessary to accomodate this excess of 0 2 - ions, which explains the retention of an hexagonal morphology by Mg(Al)O (easily observed by transmission electron microscopy, provided the calcination temperature does not exceed about 950 K)and the "morphological
1036
memory" of Mg(A1)O proved by the aforementioned (hydrotalcite -(-H20,,)-> Mg(A1)O -(+H2Oaq)-> meixnerite) conversion. The stability of such an arrangement is explained by considering in addition to the Madelung energy, the attractive energy from the bulk-surface interaction and the repulsive energies from the negatively charged surface and from the positively charged bulkls. Note also that such anionic surface planes can bnly exist if the dipole moment in a direction perpendicular to their surface is allowed to vanishlg. We have used these considerations t o predict (a in Table 2) the specific surface areas of the above described MgAlX-N/RD oxides. An octahedral morphology, maximizing the (111)surface, was used for the sake of simplicity, As electroneutrality of the crystallites has to be maintained, it is demonstrated that for fully dehydrated Me(II)Me(III)Xoxides, the molar fraction of Me(II1) (f in Table 2) is related to a,the number of 0 2 - anions on the edge of the octahedron, by20:
f = (Wn) + (12/(2n2+ 1))
(1)
It shows clearly that an increase in f (more Me(II1)) forces n to decrease. This is immediatly confirmed by observing that the average particle sizes given in Table 2 decrease when f increases. Knowing the edge of the octahedron and the simple formulas which relate it to its surface and volume, as well as the density of MgO, one easily predicts the surface areas listed in Table 2. It is astonishing to note the good agreement between experimental and predicted values. Although the latter calculations suffer from several approximations, they should be sufEcient to convince the reader that Mg(A1)O particles, before bulk epinel formation at high temperature, are stabilized with a morphology favoring totally anionic (and thus highly basic) (111)surface planes and with a high surface area imposed by lattice energy considerations. It has been suggeeted that Mg(Al10 has strong basic sitesal. These statements hold when no phase segregation occurs. As the bulk spinel is progressively formed (at higher temperature), less AI(II1) remains in the MgO lattice (Mg/A1 = 0.5 for the Al-richer spinel), and surface area is lost, as observed experimentallyl6. The same considerations explain the change in basicity with calcination temperature21-24. Finally, it is clear that the high specific surface area stems from very small crystallite sizes and not from the presence of micropores. Mesopores may however be observed15. It is hence correct to state that such supports are nonmicroporous and not comparable to zeolites. 82. Catalytic aromatizetionselectivity ofnoble metal-mixedoxide cataly~ts
As shown in our earlier publicationslJ, plots of the selectivity to aromatics (defined as the percentage of aromatics in the feed-free products normalized to 100%)as a function of the conversion of n-hexane are surprisingly identical for Pt-Mg(Al)O and Pt-zeolite 1, catalysts. These catalysts do not only differ by the nature of the support but also by the size of the Pt-crystallites which are somewhat larger (about 2.0 nm) for Pt-Mg(A1)O than for Pt-zeolite L (about 1 nm). In addition, we also demonstrated that deactivation did not alter the
1037
intrinsic aromatics selectivity of the Pt-Mg(A1)O catalystz. A more detailed mechanistic study is necessary to understand the reason for such analogous plots. Because this manner of analyzing the data appears to be rather independent of metal loading and dispersion, and of the state of deactivation of the catalyst, such plots are most useful to relate and compare activity and selectivity for different catalysts without need for extensive metal dispersion studies and care to avoid deactivation. We have thus investigated several noble metal/mixed oxide catalysts in which the Mg(A1)O support was modified by changing its Mg/A ratio, other mixed oxide supports, i.e., ZnAl3 and ZnCr2, were considered, the amount of Pt was varied and Pt was also replaced by Pd. All the catalysts derive from hydrotalcite-type precurors. They are described in Table 1. As our discussion below will concentrate on aromatics selectivity, we will not discuss further in this paper possible changes in metal dispersion associated t o variations in metal loading, metal-support interaction, and catalyst activation (or deactivation). It suffices to say that the average Pt particle size measured on all catalysts by transmission electron microscopy is in the range 1.5-2.0 nm. Our experimental data, relevant for the present discussion, are summarized in Table 3 and Fig. 1.
Table 3 n-Hexane aromatization on Pt and Pd dispersed on mixed oxidesa support Metal Wt.-%
a. b. c. d. e. f.
MgA12 MgAl2Sb MgA15 Pt Pt Pt 0.75 0.76 0.74
MgAllO Pd 0.80
ZnAl3 Pt 0.62
ZnCr2
Pt
0.81
HT-mixed oxides and runs. Calcined in air at 1173 K for 6 h. Mixture of MgO and MgAl2O4 CXRD). Time on stream (h). WHSV for n-hexane (h-1). Product selectivities (Wt.-%)on a feed-free basis. PON = paraffins, olefins and naphthenes.
1038
The examination of Fig, 1 immediatly indicates that the same aromatics selectivity-n-hexaneconversion relationship holds for the three Pt(WMgA1X-N and the Pt(O.S)MgAI3-RD catalysts, confirming our previous observationl,2. Thus, although there may be a change in activity, neither Pt loading or Mg(A1)O surface area and composition alter the aromatics selectivity at identical conversion. Furthermore, the results obtained for the Pt(0.7)-MgA15HT and Pt(0.7)MgAl2S-HT catalysts also fit the same plot. The conclusion is that selectivity to aromatics is not affected by the partial pressures of H2 and nhexane (at constant Hfiydrocarbon ratio) in our conditions near atmospheric pressure. The fact that the Pt-catalyst supported on a mixture of MgO and MgA1204 (Pt(0.7)MgAl2S-HT)also aligns itself on this correlation will not be discussed further, but it indicates a resemblance between the MgA1204 spinelcontaining support and the other Mg(Al)O systems mentioned above. Three catalysts have a completely different behavior Pt(O.G)ZnA13-HT, Pt(0.8)ZnCr2-HT, and Pd(0.8)MgAllO-HT. It is obvious from Fig. 1 that Pt(O.G)ZnA13-HT and Pt(0.8)ZnCr2-HT have much lower aromatization selectivity and Table 3 indicates that this is related to a lower activity, combined to a higher isomerization-dehydrogenationand a lower cracking activity. The latter observation certainly results from the fact that the mixed oxides (ZnO)x. ( C ~ ~ Oand Q ) (Zn0),.(A1203)y ~ must be considered as acidic supports22. Catalysts involving Pt on such materials are thus closer in their behavior to more classical reforming compositions. ..
70
-
Pd(O.8)MgAIIO-HT
*'
Pt(O.7)MgAIS-HT
6050
V
4030
-
0 Pt(l.7)MgAIZ-N
Pt(l.8)MgAB-N 0 Pt(l.4)MnAIS-N
2010
~
0
~
10
~
20
30
40
50
60
70
Conversion of n-hexane (96)
Figure 1. Selectivity to aromatics formation during n-hexane reaction over Pt catalysts at 750 K (unless indicated otherwise in Table 3), atmospheric pressure, and a Hahydrocarbon molar ratio of six. N and RD runs use a Hediiuted feed, a pure Hhydrocarbon feed is used HT runs.
1039
The very high aromatics selectivity of the Pd(0.8)MgAllO-HTcatalyst is most interesting from both a practical and fundamental viewpoint. Table 3 indicates that it results from the strong suppression of isomerization-dehydrogenation reactions. Further work is necessary to clarify this finding, in particular to assess if i t is due to only a change in the metal nature or t o a (possibly combined effect of the) different metal-support interaction. Research in this direction is in progress. 4. CONCLUSIONS
The potential of basic supports for the selective aromatization of n-hexane to aromatics over noble metals (Pt, Pd) is confirmed. Pd clusters supported on Alstabilized high surface area MgO show a n unusually high aromatization selectivity. Hydrotalcite-structure precursors are particularly adequate for the preparation of high surface area mixed oxide supports. The use of such precursors is most convenient for the preparation of Mg(A1)O materials with surface area in the range 200-400m2.g1. The specific surface area of such supports can be controlled by varying the Mg/Al ratio. A model based on lattice energy calculations explains this finding. The high basicity of the rock-salt structure MdAl)O materials stems from their higG surface a r e a and from the preferenti51 exposure of their totally anionic (111)surface planes. 27NMR results indicate that part of the aluminum occupies interstitial sites with tetrahedral symmetry, a stituation also found in the inverse MgAl2O4 spinel. 6. ACKNOWLEDGEMENTS
VJL thanks Catalytica Inc. for a doctoral fellowship. RJD acknowledges an
ISIS postdoctoral fellowship for work a t FUNDP, Namur. Belgium. The
authors also thank Dr. C. Schramm (Catalytica Inc.) for performing the elemental analysis of some catalysts, Prof. Z. Paal for useful suggestions, and Ms. B. Delforge for her contribution to this work. This work was partially funded by the Belgian Program on Inter-University Research Projects in Interface Science (PAI-IUAP).
1. R.J. Davis and E.G. Derouane, Nature (London),349 (1991)313. 2. R.J. Davis and E.G. Derouane, J. Catal., 132 (1991)269. 3. C. Besoukhanova, J. Guidot, D. Barthomeuf, M. Breysse, and J.R. Bernard, J. Chem. SOC. Faraday Trans. I, 77 (1981)1595. Symp. Proc., 111 (1988)419. 4. S.J. Tauster and J.J. Steger, Mater. Res. SOC. 5. S.J. Tauster and J.J. Steger, J. Catal., 125 (1990)387. 6. E.G. Derouane and D. Vanderveken, Appl. Catal., 45 (1988)L15. 7. G.S. Lane, F.S.Modica, and J.T. Miller, J. Catal., 129 (1991)145. 8. I. Manninger, Z. Zhan, X.L. Xu, and Z. Paal, J. Mol. Cat., 66 (1991)223. 9. Z.Paal and P. TBthnyi, J. Catal., 30 (1973)350.
1040
10.S.R. Tennison, A.I. Foster, J.J. McCarroll, and R.W. Joyner, ACS Petroleum Division Reprints, Seattle Meeting, March 20-25,1983. 11. J.J. McCarroll, Surf Sci., 53 (1975)297. 12. Z.Pad, H. Groeneweg, and J. Paal-Lukacs, J. Chem. SOC. Faraday Trans., 86 (1990)3159. 13. F. Cavani, F. Trifiro, and A. Vaccari, Catal. Today, 11 (1991)173. 14.W.T. Reichle, Chemtech, 16 (1986)58. 16. W.T. Reichle, S.Y.Yang, and D.S.Everhardt, J. Catal., 101 (1986)362. 16. 5. Miyata, Clays and Clay Minerals, 28 (1980)60. 17. H.Schaper, J.J. Berg-Slot, and W.H.J. Stork,Appl. Catal., 64 (1979)79. 18. P.E. H~jlund-Nielsen,Nature (London),267 (1977)822. 19. J.G. Fripiat, A.A. Lucas, J.M. Andr6, and E.G. Derouane, Chem. Phys. Lett., 21 (1977)101. 20. E.G. Derouane, unpublished results. 21. T. Nakatsuka, H. Kawasaki, S. Yamashita, and S. Kohjiya, Bull, Chem. Soc. Jpn., 62 (1979)2449. 22. K.Tanabe, M. Misono, Y.Ono, and H. Hattori, "New solid acids and bases: their catalytic properties", Stud. Surf. Sci. Catal., 61 (1989)7,111,232,281. 23. S.Miyata, T. Kumura, H.Hattori, and K. Tanabe, Nippon Kagaku Zasshi, 92(1971)614. 24. W.T. Reichle, J. Catal., 94 (1985)547.
DISCUSSION Q: J. K. A. Clarke (Ireland) I would like to mention some experimental results, then offer a comment. We have found that magnesium oxide supporting platinum is a strong metalPupport interaction system. Moreover, it is one with electron transfer from the magnesia to the metal. We agree in this completely with Wanke's observations (see p.140 5 ). While the role of the aliminium in your catalysts, in its detail, still needs to be clarified, we believe that essentially your catalysts are the same as those I refer to. You have on this view, brought about the important modification of making the metal more electron-full. So to speak, the Pt becomes a little closer to being like Au electronically without however losing paraffin activating ability. Now (as Rooney has tought for some years) the ability of the metal to form carbenes is depressed, even suppressed a general trend as we go from the centre to the right-side of the transition-metal block. The result is less Cl carburization and less hydrocracking in the hydrocarbon reaction. By contrast the alkene and alkenic intermediates for dehydrocyclisation, defined over the last 10-20years by Tetenyi and Paal are formed preferentially. This viewpoint is useful too for understanding the increased hydrogenolysis on electron deficient platinum reported by Dr.Koningsbcrger (see p. 809 ) where acidic alumina was the support, and the older related finding of Della Betta and Boudart (1972). Finally, I report that we found residual chlorine from precursor platinum salt makes profound effects on Pt/MgO. You will be interested that we have prepared an absolutely
-
1041
chlorine-free "Derouane" Pt/Mg(AI)O catalyst and confirmed the very high aromatization selectivity for this catalyst. May I ask, are your catalysts prepared from chlorine-free platinum or palladium precursors ? A: E. G. Derouane Thank you for a very interesting comment and a possible explanation for the unusual behavior of our Pt/Mg(Al)O catalyst. Our communication did not concentrate on the mechanism of the aromatization reaction but well on the nature of the support and the unusual activity and selectivity of Pd. There may indeed be a support effect of the type you mention. Another possibility which we consider is that the hexagonal (1 11) surface of Mg(A1)O could favor by an epitaxial relationship the formation of hexagonal (111) surface planes of Pt. These have been suggested to be the active Pt surface planes for aromatization (see ref. 12 in our paper). In line with your remark, I also wish to add that we investigated the Au/Mg(Al)O catalyst and could not detect any aromatization activity.
Q: J. J. Rooney (Ireland) By the results reported earlier [l], 1 wish to demonstrate a remarkable MgO SMSI effect on Pt. The catalysts were prepared using H2PtC16, and were reduced in Hz at 723 K after drying at 393 K. They were finally treated overnight in 6.7 kPa D2 in situ at 711 K. The cyclopentane/D2 ratio was 0.56/6.7 kPa. The rates, ko, are expressed as % min-1g-1 Pt. As shown in Fig. 1 a significant shift in the distribution of D-isomer towards C5D1o appears which has never before recorded.
lo 60 50 40
30 20
Fig.1
Initial distributions for cyclopentane/Dp exchange at 372 K: (a) 2 % Pt/AI3+MgO (ko=2380); (b) 2 % Pt/MgO(I) (kp1478); (c) 0.05 % Pt/AIpOs (ko= 15443)
1042 This shows enhanced alkyl/olefin reversal and enhanced roll over of olefin. The Pt is electron-enriched in the direction of Au and the increased tendency to dehydrogenate via n-bonded species is exactly what would be good for selective aromatization of hexane. [l] T.Baird, E. J. Kelly, W. R. Patteson, J. J. Rooney, J. Chem. Soc. Chem. Comm., in press.
A E. G. Derouane Thank you for your comment. Your point is weil taken.
Q: K. P. de Jong (The Netherlands) The stabilized magnesia you have used has been covered by Shell patents. In case anyone wants to apply your catalyst, they will have to take this into account. Now my question. Your activity/selectivity data have been obtained after 1-4 hours on stream. How does the catalyst behave with time-an-stream, how sulfur sensitive is it, and finally, what is the coke content and the dispersion loss of the spent catalyst '?
A E. G. Derouane You certainly realized that we are perfectly aware of the work done by Wim Stork and his coworkers. Reference to it, with respect to the preparation of Mg(A1)O was made in our first two papers (see refs. 1 and 2). However, we found no indication that the Shell team supported noble metals on such supports and used these supported noble metal catalysts for useful catalytic reactions. I also wish to draw your attention to reference 18 in our paper, a communication to NATURE, in 1977, by P. Hojlund-Nielsen from Haldor Topsoe A/S. This communication predates the work published by Shell in both the patent and journal literatures and explains the basic rules for the stabilization of mixed oxidic phases such as Mg(A1)O.
Guczi, L ef al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
EFFECT OF THE ALKALI CATION ON FIEPTANE AROMATIZATION IN L ZEOLITE R.F. Hicks, W.-J.Han andA. B. Kooh Department of Chemical Engineering, University of California, Los Angeles, CA 90024-1592, USA
Abstract Platinum was deposited on a series of BaL zeolites containing different alkali cations. The composition, Pt dispersion, infrared spectrum of adsorbed CO, and the turnover frequency for heptane reforming on each catalyst was determined. The infrared spectrum of a saturation coverage of CO on Pt/BaL at 25OC consists of a broad infrared band which shifts from 2085 to 2025 cm" as the alkali element in the L is changed from Li to Cs. Heating the samples to 225OC causes new bands to appear at 2030, 1975, and 1940 cm-'. These bands are assigned to CO adsorbed on Pt in which the oxygen end of the molecule weakly bonds to cations in the L lattice. This and other evidence indicates that the Pt particles are primarily located inside the zeolite cavities. The specific activity of PmaL for heptane reforming at 440°C, 0.16 atm heptane and 0.95 atm €&is sensitive to the distribution of alkali cations. As the lighter elements, Li and Na, are exchanged for heavier ones, Rb and Cs, the turnover frequencies for formation of C,-C, hydrocarbons, ethylcyclopentane, benzene and toluene increase on average by 3.4, 3.3, 8.8, and 3.8 times, respectively. These data indicate that adsorption and catalysis on W a L is strongly influenced by metal-support interactions. 1. INTRODUCTION Bernard [ 11 discovered that platinum particles in non-acidic zeolites exhibit high selectivities for hexane aromatization. He suggested that the selectivity enhancement was due to an interaction between the metal particles and the non-acidic zeolite. Later, Barthomeuf and coworkers [2] found that the aromatization yield over Pt/L was sensitive to the alkali element exchanged into the zeolite. The conversion of hexane to benzene increased 24 times as the alkali element was progressively changed from Li to Cs. At the same time, the infrared band for adsorbed carbon monoxide on the platinum particles decreased by 20 cm-'. These authors concluded that the activity of the platinum particles was affected by an interaction with the alkali elements in the zeolite framework. Recently, Larsen and Haller [3] found additional evidence for metal-support interactions in the Pt/L system. They observed that the ratio of toluene to benzene adsorption equilibrium constants at 25OC fell by an order of magnitude as the zeolite composition was varied from acidic CeY to nonacidic BaL. However, the composition of the zeolite may not be the only factor which affects the catalytic properties of the platinum particles dispersed in the pores. Tauster and Steger [4]
1044
suggest that L zeolite orients the hexane molecule so that it adsorbs via the terminal carbon atom, favoring 1-6 ring closure and aromatization. This hypothesis is consistent with the observation that the aromatization activity correlates with the ratio of terminal to internal cracking of the hydrocarbon. Alternatively, the L zeolite channels may preorganize the hexane molecule into a pseudo-cycle resembling the transition state for aromatization, and in this way, promote the formation of benzene over isomers and cracked products [5,6]. The role of structure versus electronic effects in hydrocarbon reforming over ptn is a difficult issue to resolve, because no agreed-upon method exists for probing one of these properties independent of the other. Nevertheless, we thought that varying the cation dismbution in the L zeolite was one of the best ways to examine electronic effects with minimal perturbation of the structure. Taking the same approach as Barthomeuf and coworkers [2], we have prepared a series of non-acidic BaL zeolites with different alkali cations exchanged into the pores, then impregnated these zeolites with platinum, and determined the cation distribution, the Pt dispersion, the infrared spectrum of adsorbed CO on the Pt, and the turnover frequency for heptane reforming on each sample. These experiments indicate that metal-support interactions strongly influence adsorption and catalysis on the platinum particles dispersed in the L zeolite.
2. EXPERIMENTAL
Barium was exchanged into KL zeolite at 25OC by mixing 10 g of material with 500 ml of 0.3M Ba(NO,), for 106 h. The sample was filtered, washed, dried at 120°C for 48 h, and calcined at 590°C for 16 h to move the Ba” into the locked cation positions [7]. Different alkali elements were exchanged into the BaL zeolite by mixing 10 g of material with 500 ml of 0.3 M alkali-metal nitrate solution for 72 h. These samples were filtered, washed, and dried at 12OOC for 96 h. Platinum, 0.65fo.11 wt%, was added to the zeolites by incipientwetness impregnation of Pt(NH,).,CI,. After adding the Pt, the samples were heated in air from 25 to 250°C over 4 h, held at 25OOC for 1 h, heated in air from 250 to 350°C over 6 h, and held at 350°C for 3 h. The composition of the Pt/L catalysts was determined by inductively coupled plasma emission spectroscopy. The rates of heptane aromatization, isomerization, and hydrogenolysis were determined in a fixed-bed microreactor. Catalysts pellets (32-60 mesh), weighing 0.23kO.03 g, were charged to a stainless steel tube 6.35 mm in outside diameter, and this tube immersed in a clam-shell heater. The catalysts were reduced in 100 cm3/tnin H, for 1 h at 500°C. Then the temperature was lowered to 440°C and the reaction started by feeding to the reactor 0.16 a m GHlbr0.95 a m H,, and 6.7 atm He at 245 cm’/min. After 3 h of reaction, the catalyst activities stabilized, and the rates of heptane reforming were measured at 5 different flow rates. Intrinsic reaction rates were determined fiom the slopes of the straight-line plots of product concentration versus residence time. The products were analyzed by gas chromatography, using a J&W Megabore capillary column (DB-1 active phase, 30 m length) and FID. The metal dispersion was determined in situ before and after reaction by pulsed H2-02 titration. In addition, infrared spectra of adsorbed carbon monoxide were measured before and after reaction at saturation coverage and a range of temperatures between 21 and 35OOC.
1045
3. RESULTS The composition of the Pt/BaL catalysts are shown in Table 1. All the samples, except Pt/CsBaL, exhibit cation to aluminum mole ratios of 1.0, indicating that: the zeolites are nonacidic. The WCsBaL sample exhibits a cation to aluminum mole ratio of 0.96, suggesting that it contains a few Brqinsted acid sites. In the L zeolite framework there are 4 cation positions designated A, B, C, and D sites [8]. The C and D sites are located along the main channel walls and are fully exchangeable at 25OC. The A and B sites are located inside the erionite cages and are locked to exchange at 25OC. Hughes et al. [7] and Newell and Rees [9] found that barium can be transferred from the C and D sites to the A and B sites by calcining the sample at 590 to 65OOC after ion exchange. This treatment leaves 1.81 K+ and 1.27 Ba" per unit cell in the A and B sites. An additional 4.77 univalent cations per unit cell are located in the C and D sites. Assuming that our heat treatment caused the same amount of barium to move into the locked positions as observed in the earlier studies, then the remaining cations in the sample are distributed among the C and D sites as indicated in the last three columns of Table 1. Examination of these data reveals that the degree of exchange of the alkali cation with the potassium and barium increases with increasing size of the alkali element. For example, only 0.71 Li+ per unit cell exchanged into the L zeolite compared to 4.19 Rb+. Newell and Rees [9] also observed that the exchange capacity increased with cation size. Table 1 Composition of the WBaL catalysts Alkali (M+)
Platinum (wt a)
CatiodAl mole ratio
Cations in C and D sites per unit cell'
M+
Ba+'
K+
Li Na K Rb cs
0.66 0.76 0.64 0.57 0.54
1.01 1.00 1.01 1.00 0.96
0.7 1 1.41
0.46 0.36 0.10 0.09 0.00
3.14 2.64 4.56 0.40 1.15
4.19 3.26
'Assumes 1.81 K' and 1.27 Ba" in the A and B sites per unit cell. Shown in Fig. 1 are a series of infrared spectra of carbon monoxide adsorbed on 0.76% Pt/NaBaL. In this experiment, a thin wafer of the catalyst, 13 mm in diameter and weighing 0.14 g, was placed in a glass cell, reduced in 200 cm3/min H, at 300°C for 1 h, evacuated to 1 ~ 1 Ton 0 ~ at 300OC (1 TOIT= 133 N/mz), and cooled to 25OC. Then the sample was dosed with 76 pmol of CO, equivalent to 25 times the moles of Pt exposed, and the infrared spectrum shown in Fig. l a recorded. Two broad bands are observed in this spectrum: one at 2085 cm-' due to CO adsorbed on the Pt [lo], and one at 1800 cm-' due to a carbonate species bound to the support [ll]. The carbonate species is somehow associated with the platinum, because it is not detected upon dosing the BaL support with carbon monoxide.
1046 I
I
I
1785
-
uv -
2 0.4-
50
0.2 (b)
1800
I
I
A
Figure 1. Infrared spectra of CO adsorbed on 0.76% WNaBaL, (a) at 25OC,(b) at 225OC,(c) after cooling back down to 25OC.and (d) after exposure to 2 Torr 0,for 45 h and 2 Tom CO for 19 h at 65OC. Heating the 0.76% Pt/NaBaL catalyst to 225'C produces the spectrum shown in Fig. lb. This spectrum contains a large, broad band for adsorbed CO on Pt which is comprised of several overlapping peaks at 2065,2030,1975, and 1940 cm". The peaks below 2050 cm.' represent new adsorption sites for CO on the metal particles. Upon cooling down to 25OC, the peak at 2065 cm-', and to a lesser extent, the peaks at 2030 and 1975 cm", increase in intensity as shown in Fig. lc. These changes in the infrared spectra are not at all like the changes observed on heating and cooling a WSiO, or a WAl,O, catalyst [lo]. In this case, the infrared bands for adsorbed CO decrease in intensity and shift to lower wavenumbers on heating, then increase back to their original size on cooling. The unusual results obtained on 0.76% Pt/NaBaL indicate that carbon monoxide adsorption on the platinum particles is swngly affected by interactions with the surrounding zeolite walls. After recording spectrum (c), the sample was exposed to 2 TOITof 0,for 45 h and 2 Torr of CO for 19 h at 65°C. During the first few minutes of oxidation, the infrared bands rapidly decreased to about 1/3 their original size, and took on a shape similar to that of spectrum (a).
1047 Over the next 45 h of oxidation, the remaining bands slowly disappeared. When the carbon monoxide was reintroduced, the infrared bands for adsorbed CO grew in over several hours, ultimately producing the spectrum shown in Fig. Id. This spectrum contains a main peak at 2075 cm-' with shoulders at 2115,2035, and 1975 cm-I. Evidently, most of the sites giving rise to the bands at 2030 and 1975 cm-' are destroyed by oxidation at 65°C. Comparison of spectrum (d) to specmum (a) reveals that the amount of CO adsorbed by the Pt increases substantially following the cycle of heating to 225OC and oxidation at 65°C. suggesting that this treatment disperses some of the platinum inside the zeolite. If after this experiment, the sample is again heated to 225"C, cooled to mom temperature, oxidized at 65"C, and exposed to CO, the same sequence of spectra from (b) to (d) are observed. The experiment described above was repeated on all the catalyst samples with essentially the same results. Heating the samples in the presence of adsorbed CO to 225°C produces a broad band with maxima at 2065, 2030, 1975, and 1940 cm-'. These features remain on cooling the sample to 25"C, but are destroyed by a low temperature oxidation. The only discernable difference in the infrared spectra is that the frequency of the main infrared peak observed on first exposing the samples to CO at 25OC shifts from 2085 to 2025 cm-' as the alkali element in the BaL is progressively changed from Li to Cs. Shown in Table 2 are the activities and selectivities of the Pt/BaL catalysts for heptane reforming at 440°C, 0.16 atm heptane, 0.95 atm H,, 6.7 atm helium, 3 h of reaction, and conversions between 2 and 10%. The turnover frequencies are based on the moles of heptane converted into each product divided by the moles of Pt exposed at the end of the run. During 3 h of reaction, the Pt dispersions decrease by an average of 20% on the 5 samples. The activities of the empty reactor and the BaL supports have also been measured. The empty reactor produces C, to C, hydrocarbons at a rate which is 12 to 41 times slower than that of the Pt/BaL catalysts. Except for CsBaL, the BaL supports exhibit the same rates as the empty reactor, confirming that these zeolites are non-acidic. The CsBaL support, on the other hand, produces 3 times more C, to c, hydrocarbons than the empty reactor, which is probably due to the Brmsted acid sites on this sample. The turnover frequencies for the C,-C6 products listed in the Table are calculated based on the difference in the reaction rate between the catalyst and its support. Table 2 Activity and selectivity of Pt/BaL catalysts for heptane reforming Alkali
Li Na K Rb Cs
Platinum dispersion (%)
Turnover frequency' (xlO-' s-')
C,-C,
Ethylcyclopentane
Benzene Toluene
71 49 63
0.4 0.8 1.2 2.2 1.9
0.2 0.1 0.6 0.3 0.7
0.2 0.8 0.8 5.7 3.1
52 41
-
0.9 1.6 1.8 4.4 5.1
'Experimental uncertainty is +25% of the value. bSelectivity= (Benzene + TolueneMBenzene + Toluene + c&6)
Aromatics selectivityb(%)
73 75 68 82 81
1048
Examination of the data in Table 2 reveals that the alkali element has a significant effect on the turnover frequencies for heptane conversion into the various products. Changing the alkali element from Li to Rb increases the specific activities for forming C,-C, hydrocarbons, ethylcyclopentane, benzene, and toluene by 5.5, 1.5, 28.5, and 4.9 times, respectively. 4. DISCUSSION
The infrared spectra presented in Fig. 1 reveal a strong interaction between the platinum particles and the zeolite framework. This interaction generates new sites for CO adsorption on the Pt as evidenced by the bands appearing at 2030, 1975, and 1940 cm-'. These low frequency bands have been observed previously for CO adsorption on PtlL catalysts [2,12]. Barthomeuf first assigned these bands to platinum carbonyl clusters located in the L cavities [2], but later proposed that they were due to CO on the Pt interacting with the framework atoms of the zeolite [13]. We believe the latter assignment is correct for the following reasons. De Mallmann and Barthomeuf [14] have shown that platinum carbonyl clusters can be produced in basic zeolites by reaction of CO with PP2 ions. The resultant clusters are strongly colored, turning the samples bright red-purple, and they exhibit two intense infrared bands at 2026 and 1800 cm". These absorption features are characteristic of Chini complexes with the formula [15,16]. In order to make the clusters, must be present in the zeolite, presumably to balance their -2 charge. If the ammonium ion is removed by oxidation and reduction, the Pt carbonyl clusters do not form. Adsorption of carbon monoxide on our WBaL catalysts does not generate any features characteristic of Chini complexes. No color changes are observed, nor are intense infrared bands located at 2026 and 1800 cm-'. and after oxidation at 350°C and reduction at 500°C. no remains in the sample. An alternative assignment for the infrared peaks at 2030, 1975, and 1940 cm" is to carbon monoxide adsorbed on the platinum with the oxygen end of the molecule weakly bound to cations in the zeolite fmnework. This type of bond is an ion-dipole interaction, and is analogous to water attachment to the L zeolite lattice through its oxygen atom [17]. Oxygen bonding of carbon monoxide to the zeolite will decrease the C-0bond strength and increase the e-0'charge separation. A weaker C - 0 bond is consistent with the lower vibrational frequencies of these species compared to noninteracting CO on Pt. The added charge separation enhances the dipole moment of the molecule, and this is consistent with the greater intensity of the low frequency bands. Based on CO adsorption measurements at 225'C, we estimate that the molecular absorption coefficient of the broad band between 2050 and 1900 cm-' is 3.4 times greater than that of the band at 2085-2065 cm". Furthermore, heating the sample may be necessary so that the Pt atoms and CO molecules can move to positions which allow the oxygen atom to line up with the cations in the L lattice. Oxygen adsorption on the Pt could easily destroy this alignment The interaction of the CO molecule with the L framework can only occur if the platinum particles are located inside the zeolite cavities. The infrared spectra in Fig. 1 reveal that all the peaks for adsorbed CO on Pt are affected by the cycle of heating, cooling and oxidation at 65°C. The band at 2085 cm-' shifts to 2065 cm" after heating and cooling, and to 2075 cm-' after oxidation and a second exposure to carbon monoxide. This feature may be assigned to CO molecules on Pt particles inside the zeolite that do not f o m the ion-dipole bond. The infrared spectra do not contain a narrow peak between 2085 and 2110 cm-' which
w'
m'
1049
could be taken as evidence for large Pt particles outside the zeolite cavities [18]. The shoulder at 2115 cm-' which appears after oxidation at 65OC is most likely due to CO adsorbed next to oxygen on the Pt surface [19]. The procedure we used to prepare the 0.65% Pt/BaL catalysts is expected to yield small Pt particles inside the zeolite channels. After incipient-wetness impregnation of the Pt, our samples were oxidized at 35OOC and reduced at 500OC. This may be compared to the 0.8% Pt/BaL catalysts prepared by Hughes et al. [7] which were oxidized at 260 to 300OC and reduced at 500OC. Electron microscopy revealed that these latter catalysts contained Pt particles no larger than 10 A, and that these particles appeared to be located inside the channels. Barthomeuf and coworkers [2] have prepared PtKL catalysts containing large Pt particles outside the pores. However, in their case the metal loading was much higher, 5%, and the oxidation step was more severe, 48OOC. Hughes et al. [7] also showed that the metal particle size is cormsiderably overestimated from Pt dispersions based on chemisorption measurements. The Pt dispersion may be as low as 40%, and the particles still be no larger than 10 A. This discrepancy is probably due to the limited access of the adsorbates to the Pt atoms abutting the channel walls [7]. The data in Table 2 clearly show that the turnover frequencies for hydrogenolysis and aromatization depend on the distribution of cations in the zeolite lattice. These activities increase by about 5 and 10 times respectively as the lighter alkali elements are exchanged for the heavier ones. These results are qualitatively similar to the earlier study of Barthomeuf and coworkers [2], where they observed a large increase in the hexane conversion to benzene as the alkali cations are changed from Li to Cs. The increase in aromatization activity with alkali cation size correlates with a downward shift in the frequency of the main infrared band for adsorbed CO from 2085 to 2025 cm-'. This shift may be taken as evidence for an electronic effect of the support on the metal particles [12,20]. The results we have obtained may be contrasted to the study of hexane aromatization on Pt/KL and Pt/KY by Lane et al. [6]. These authors found that the turnover frequency for converting hexane to benzene is 10 times higher on the metal particles in the L zeolite compared to those in the Y zeolite. This difference in activity is very similar to what we observe for exchanging different cations into the L, and suggests that metal-support interactions may be an important factor contributing to the exceptional aromatization activity and selectivity of the Pt/BaL catalysts.
5. ACKNOWLEDGMENTS The authors thank Jeff Miller and Jerry Lane at Amoco Oil Company for determining the catalyst compositions. Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Additional funds were provided by a National Science Foundation Research Initiation Award.
6. REFERENCES 1 J.R. Bernard, in: Roc. 5th Intl. Congr. Zeolites, L.V.C. Rees (ed.), Heyden, London,
1980, p. 686.
2 C. Besoukhanova, J. Guidot, D. Barthomeuf, M. Breysse, and J.R. Bernard, J. Chem. Soc. Faraday Trans. 77 (1981) 1595.
1050 3 G. Larsen. and G.L.Haller, Catal Lett, 3 (1989) 103, 4 S.J. Tauster and J.J. Steger, J. Catal. 125 (1990) 387. 5
E.G. Derouane and D.J. Vanderveken, Appl. Catal. 45 (1988) L15.
6 G.S. Lane, F.S. Modica, and J.T. Miller, J. Catal. 129 (1991) 145. 7 T.R. Hughes, W.C. Buss, P.W. Tamm, and R.L. Jacobson, Stud. Surf. Sci. Catal. 28 (1986) 725. 8 D.W. Breck, Zeolite Molecular Sieves, Robert E. Krieger & Co., Malabar, EL, 1984, p. 114.
9 P.A. Newel1 and L.V.C. Rees, Zeolites 3 (1983) 22,28. 10 N. Sheppard and T.T. Nguyen, in: Advances in Infrared and Raman Spectroscopy, R.J.H. Clark and R.E. Hester (eds.), Heyden & Sons, London, 1978, p. 67.
11 L.H. Little, Infrared Spectra of Adsorbed Species, Academic Press, New York, 1966, p. 74. 12 G. Larsen and G.L. Haller, in: Catalytic Science and Technology, hot. First Tokyo Conf., Kodansha Ltd.,Tokyo, Japan, 1991, p. 135. 13 A. De Mallmann and D. Barthomeuf, in: Proc. Intern. Symp. on Zeolites as Catalysts, Sorbents and Detergent Builders, Elsevier, Amsterdam, 1989. 14 A. De Mallmann and D. Banhomeuf, Catal. Lett. 5 (1990) 293. 15 J.C. Calabrese, L.F.Dahl. P. Chini. G.Longoni, and S. Martinengo, J. Am. Chem. SOC. 96 (1974) 2614. 15 G. Longoni and P. Chini, J. Am. Chem. SOC. 98 (1976) 7225. 17 L.Bertsch, and H.W. Habgood, J. Phys. Chem. 67 (1963) 1621. 18 R.G. Greenler, K.D. Burch, K. Kretzschmar. A.M. Bradshaw, and B.E. Hayden, Surf. Sci. 152/153 (1985) 338. 19 M. %met, J. Catal. 88 (1984) 273. 20 P. Gallemt. Catal. Rev. Sci. Eng. 20 (1979) 121.
1051
DISCUSSION Q: E. Iglesia (USA) 1) How strong are these dipole-ion interactions (in kJ mol-l) and how likely are they to remain influential at >773 K during catalytic reactions of "non-polarizable" alkanes ? 2) The selectivity changes that occur as the identity of the cation varies are quite unremarkable, in spite of significant changes in the apparent turnover rates. Thus, in my view, the alkali polarizability affects the density o r accessibility of aclive Pt sites, not their electronic properties. Would you comment on whether your results support the presence of electronically modified Pt clusters during catalysis ?
A R. F. Hicks 1) If the adsorbed intermediates during the catalytic reaction d o not have a dipole moment like carbon monoxide, they will not form an ion-dipole interaction with the cations. In this case, this phenomenon should not influence catalysis. 2) The change in turnover frequency with alkali cation exchanged into the zeolite tends to point toward electronically modified Pt particles. As you point out, the selectivity is not much affected by the different cations. One can argue that high aromatization selectivity is the principal advantage provide by Pt/L zeolite. Evidently, this advantage does not directly result from electronic modification of the Pt particles. Q: F. Solymosi (Hungary) You found interesting spectral changes in the presence of CO at high temperature. I am sure that the explanation suggesting structural changes is the right one, as the increase of the adsorption capacity of Pt cannot be understood by the other ones. I point out that this structural change induced by CO adsorption is not unusual as it was observed for supported Rh, R, Ir and Re. However, it was not demonstrated so far for oxide-supported
Pt. A: R. F. Hicks No answer
Q: D. Barthomeuf (France) I would like just to comment on the remark of Professor Solymosi. Some years ago we published results on CO adsorptions on basic faujasites. It was shown already that the new CO band requires heating or a rather high CO pressure in order to be formed on some of these zeolites. My question is related to the role of Ba. Your results on the influence of alkaline cations show the same features as the ones we published in 1981. In our case there was no Ba in the zeolite. What is the role of Ba ? Why do you remove more and more Ba from the zeolite as you go from Li to Cs ?
A: R. F. Hicks 1) The role of barium is to remove all the acidity in the zeolite, as indicated by the work of Hughes and coworkers at Chevron, USA. 2) The Table on catalyst composition shows the distribution of cations in the A and B sites, i.e., those that can be exchanged the different alkali cations into the A and B sites. The lighter alkali elements have difficulty exchanging with all the barium in these sites. This has been observed by Newell and Rees as well. Q: R. van Nordstrand (USA) How did you measure Pt dispersion of Pt particles just fitting into pore of zeolite ? Did dispersion values measured by CO and by H, agree ?
A R. F. Hicks 1) The Pt dispersion should be taken as the ratio of adsorbed molecules to total Pt atoms in the sample normalized by the stoichiometry for adsorption, i.e. CO/Pt, = H/Pt = 1.0, where Pt, is a surface Pt atom. 2) Yes,C6 and H2dispersion values agreed. Q: M. Vaarkamp (The Netherlands) 1) Can you give information about the intensity of the vibrations associated with the acidit in your Sam les ? 2{ Is there a reition between the remaining acidity and the hydrogenolysis activity ?
A: R. F. Hicks 1) I did not tr to measure zeolite acidity from the intensity of 0-H stretching vibrations in the in rared. These bands did not correlate with any of the other catalytic roperties. Acidity was assessed by measurin the rate of heptane cracking on the zeolites at standard reaction conditions. Only L show a cracking activity higher than an empt tube. This is consistent with the cation/Al mole ratio of 0.96 for this sample. 2) h e crackin activity of zeolite supports is subtracted from the hydrogenolysis activity of the P t k catalysts. There is no relation between remaining acidity and hydrogenolysis activity.
E.
r
a
Q: Can Li (China) Is there any other evidence of the interaction between alkali cation with the oxygen end of adsorbed CO ? If the low frequency bands in Fig. l b are attributed to CO on PI interacting with the alkali cation, why do these bands change with temperature ? A: R. F. Hicks 1) This is the only system I know of where a Pt-CO-M+ ion-dipole interaction has been proposed. More experiments and theoretical studies are needed to decide whether this assi nment is correct. 2) heating is required to create the ion-dipole bond. heatin will also desorb CO: 2065 cm*l band disappears at 200-250 OC; 2050-1850 cm-9 bands disappear at 300-400 OC.
Guczi, L et al. (Editors), New Frontiers in Colalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All dghta reserved
REDUCTION AND AROMATIZATION ACTIVITY OF MoO3/AI203 CATALYSTS: THE IDENTIFICATIONOF THE ACTIVE Mo OXIDATION STATE ON THE BASIS OF REINTERPRETED Mo 3d XPS SPECTRA
W.Criinerp, A. Yu. Stakheevb, R. Feldhausa, K An&rsa, E. S.Shpirob and Mt. M. Minachevb aCentral Institute of Organic Chemistry, Permoserstrasse 15, D-07050 Leipzig, Germany bN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47,117913 Moscow, Russia
Abstract The active oxidation state of Mo in the aromatization of n-hexane over MoodA1203 was identified by XPS and pulse catalytic investigations. By application of a novel approach to the interpretation of the Mo 3d spectra of reduced supported Mo catalysts (linear relation between binding energies and oxidation states) it was shown that Mo(I1) is formed during the reduction of MoOdAl203. Mo(II) was identified as the active Mo state in the aromatization reaction by a correlation between the initial activity of reduced MO03/A203 catalysts (reaction temperature 723 K) and the amount of Mo(I1) in them. The product distribution in the cracking reaction that accompanies the aromatization is proposed as a chemical probe for the presence of Mo(0). 1. INTRODUCTION
The high versatility of molybdenum as a catalytic element is due to its ability to take different oxidation states. The elucidation of relations between the oxidation state of Mo and its catalytic properties is, therefore, a rewarding field of catalytic research. Unfortunately, progress in this area is impeded by uncertainties in the analysis of the Mo oxidation states. Thus, significant divergence has been reported to exist in the analysis of Mo(V) in supported catalysts between results obtained by XPS [l-41 and EPR [l, 2, 5-71. Another issue is the existence of Mo(I1) in reduced supported Mo catalysts. With alumina- and silica-supported Mo catalysts, Mo(II) formation has been indicated by volumetric reduction studies [8]and in IR investigations with the adsorption of probe molecules [9]. On the other hand, Mo(I1) has never been observed by XPS in these systems [3, 4, lo]. Recently, the present authors published a novel approach to the interpretation of Mo 3d XPS spectra of supported Mo catalysts [Ill. The method is based on a linear relation between binding energies (b.e.) and oxidation states, as earlier proposed by
1054
Haber et al. [12]. A detailed account on the application of this approach to the reduction of Mo03/ku203 catalysts and to the characterization of surface species capable of forming active sites for the metathesis reaction will be given elsewhere 113, 141. In the present contribution, information about the reduction of Mo03/A203 derived from X P S spectra on the basis of the above-mentioned linear approach will be used to identify the active Mo oxidation state in the dehydrocyclization reaction. It has been known since 1936 that MoS2 is active in the dehydrocyclization (DHC) of n-alkanes to aromatics [15], but little attention has been paid. to this facet of the catalytic capabilities of molybdenum in the literature. In the extensive research on the DHC reaction carried out by Soviet [16] and East German groups [17], attention was focused on supported chromia catalysts, with only a few studies dealing with molybdena (cf. [16]). Little is known about the active state of molybdenum in the dehydrocyclization (aromatization) reaction, which consists, in essence, of a series of dehydrogenation steps with intermediate cyclization [ 161. Maggiore et al. [ 181 attributed the activity of reduced M o o d d 2 0 3 catalysts in the dehydrogenation of cyclohexane to paired Mo species of low oxidation state. On the other hand, the hydrogenating properties of M003/A203 catalysts have been studied in great detail. The activity was shown to correlate with the quantity of coordinatively unsaturated sites counted by Hz and 0 2 chemisorption [19, 201. In catalysts derived from Mo ally1 complexes attached to the A1203 surface, Mo(I1) was identified as the active state in alkene hydrogenation [21]. However, in view of the large differences in the reaction temperatures (alkene hydrogenation: 5 300 K, DHC: 700 - 800 K), this conclusion cannot be automatically extended to dehydrogenation and DHC reactions. Very recently, the activity of M o O O i 0 2 catalysts in benzene hydrogenation has also been ascribed to Mo(I1) [22]. The basis adopted in [22] for the interpretation of the Mo XPS spectra is at variance with that applied in the present investigation. Other authors used the activity in benzene hydrogenation as a chemical probe for the presence of Mo(0) [23]. 2. EXPERIMENTAL
Catalysts, substances, and purification procedures employed in this study have been described elsewhere 13, 24 . The catalysts were obtained by pore volume impregnation l a solution of MOO3 in NH40H (pH = 8), with subsequent of y - A 2 0 3 (= 260 m4/g) with drying and calcination at 393 K and 823 K, respectively. The Mo contents were 7 and 13 wt.-% (7.4 and 12.9 wt.-%, by electron microprobe analysis, corresponding to 1.2 and 2.3 Mo/nm2, respectively [13]). These catalysts will be denoted as Mo7 and Mo13 in the present text. The experimental details of the XPS investigation have been given elsewhere [13]. In brief the spectra, on which the analysis presented below is based, were recorded with an XSAM 800 spectrometer of Kratos Ltd. It was equipped with a facility for the transfer of activated samples to the UHV system without contact to the ambient atmosphere. The procedure used to fit the Mo 3d spectra employed the above-mentioned
1055
approach based on a linear relation between binding energies and oxidation states [ 111. In this analysis, unambiguous results are obtained only for some of the Mo states involved (Mo(VI), Mo(II) (vide infra), and Mo(0)). For the remaining states (Mo(V), Mo(IV)), ranges of the possible contributions may be given (cf. Fig. la). In addition, an ambiguity exists with Mo(1I) because Mo(IV) which is engaged in structures with Mo-Mo bonds (e.g. Moo2 [12, 251) appears at the same b.e. This state will be, therefore, denoted as "Mo(I1)". For M003/Al203, it has been shown by independent analyses of average reduction degrees that this "Mo(II)" state is predominantly if not exclusively due to Mo(I1) [13]. The DHC of n-hexane was studied at 723 K in a pulse system (carrier gas - neon), which was an improved version of a scheme described earlier [24]. In these pulse systems, the product pulse may be analyzed either for their hydrocarbon composition (with separate analysis of H2) or for their total carbon and hydrogen content. Experiments with full analysis of the pulses (including C and H retained by the catalyst) may be approximated by comparing parallel runs with alternating choice of the mode of analysis. In the present scheme, a recirculation loop was included for the performance of slow reduction (oxidation) processes. From this cycle, the catalyst could be transferred to the pulse scheme by valve operation. area-%
area-% 75 -
75 b
2
50 -
50
25 -
25
0
0 -
673
773
873
973
673
773
873
973
Tred K K Figure 1. Distribution of Mo oxidation states in the reduction of a Mo03/Al203 catalyst (Mo13) in flowing H2, derived from Mo3d W S spectra under different preassumptions. Reduction time - 2 h. a) Linear approach: Binding-energy difference to Mo(V1): Mo(V) 0.8 eV, Mo(1V)is - 1.6 eV, "Mo(I1)" - 3.2 eV. b) Non-linear approach: Binding-energy difference to Mo(Vi): Mo(VI) - 1.6 eV, Mo(1V) - 3.2 eV.
Tred
3
9
-
1056
area-%
loo
75
%
A
Mo(VI)
50
25
0
1
1
I
0
30
60
I
90
120
1, t i i i t i
Figure 2. Distribution of Mo oxidation states in the reduction of a MoOa/A1203 catalyst (Mo13) at 823 K. Linear approach (cf. legend to Fig. la). This pulse system was also used for the determination of average reduction degrees e/Mo of reduced catalysts. For this purpose, the reduced samples were reoxidized at 823 K by oxygen pulses until the oxygen consumption stagnated at a level of = 95 %. The reoxidation was then completed in the circulation mode, where the residual oxygen consumption typically corresponded to 0.2-0.3 e/Mo. 3. RESULTS AND DISCUSSION Reduction oP M003/N203Catalysts
Fig. 1 demonstrates using Mo13 as an example how the assumption of different relations between binding energies and oxidation states determines what may be learned from a given set of Mo 3d spectra [U]about the course of the reduction. The interpretation on the basis of a non-linear relation (Fig. lb) is compared with the application of the novel linear approach (Fig. la). It is quite plausible that the change in the basis of interpretation is not reflected in the initial and final products of the reduction, Mo(VI) and Mo(0). Major differences, however, are observed with the intermediate states. The application of the linear approach leads to a significant decrease of the values obtained for Mo(V), which
1057
should give a better chance for the elimination of the discrepancies in the Mo(V) analysis by XPS and EPR (cf. "Introduction"). The behavior found for Mo(1V) depends most critically on the relation between b.e. and oxidation state applied. In Fig. la, Mo(IV) is defined as a species not engaged in a Mo-Mo bond ('IMO(1V)is''). The Mo(1V) curve of Fig. l b is now attributed to "Mo(I1)" (Fig. la), which was shown to predominantly consist of two-valent Mo by volumetric e/Mo analyses [ 131. The shaded regions in Fig. l a represent the intervals of possible contributions of Mo(V1) and Mo(IV)is (cf. "Introduction"), where low values of Mo(1V)is result from the assumption of a high Mo(V) level and vice versa. Fig. 1 shows that the metal appears already at a reduction temperature of 900 K. This is far below the temperatures given by other authors (1173 K, by XPS [26],by volumetric methods [27], by the catalytical effect in benzene hydrogenation [23]). On the other hand, the average reduction degrees obtained (reduction of Mo13 at 900 K: e/Mo = 3.6 ( X P S ) or 4.0 (reoxidation method)) agree well with data given by Chung et al. [27] for a catalyst of similar Mo loading (reduction at 923 K: e/Mo = 4.1). As there is no reason to assume the presence of only one oxidation state under these reduction conditions [13] the data of Chung et al. support the formation of Mo(0) already at reduction temperatures of 900-930 K. Fig. 2 presents the distribution of the Mo oxidation states in the reduction of Mo13 in flowing H2 at 823 K. It can be seen that after 30 min. the further progress of the reduction is mainly due to a transformation of Mo(IV)is into "Mo(I1)" while the remaining Mo states exhibit only marginal changes. The same trends, though less pronounced, were found with Mo7. Dehydrocyclization (DHC) of n-Hexane over Reduced MoO3/d203 Catalysts No dehydrocyclization activity was observed when Mo7 and Mo13 were employed in the oxidized state. Fig. 3 presents an example of the results obtained in the measurements of the DHC activity over a reduced Mo03/Al203 catalyst (Mo7, reduced at 823 K). The development of product yields in a sequence of 10 n-hexane pulses is displayed in Fig. 3a. The cracking yield is reported as the amount of initial product cracked. Minor yields of n-hexenes and the amount of H recovered in volatile products have been omitted from the figure. It can be seen that the DHC activity decreases considerably during the pulse sequence. This deactivation is obviously due to coke formation, in particular in the first pulses. Generally, the benzene yield was found to decrease smoothly. There were, however, cases where extensive coke formation or cracking led to anomalously low benzene yields in the first pulses (120 min. reduction at 823 K with Mo7, cf. Fig. 3b; reduction at temperatures 2 923 K, cf. Fig. 5). In these cases, a gross estimate of an "undisturbed" benzene yield has been derived by an extrapolation of the deactivation tendency. The development of the DHC activity with increasing time of reduction,in flowing H2 at 823 K is displayed in Fig. 3b. It is quite obvious that the activity increases with the length of the reduction period. The same trend is found with Mo13. Hence, the
1058
20
100 a mole-%
x, %
50
10
, i
i
*..._.' y ........?! ........
. I .
L
0
I
0
-
5
..............Ic, ~
~
10 pulse number
-
I
0 0
5
10 pulse number
Figure 3. DHC of n-hexane over reduced Mo03/Ab03 (MOT). Pulse reactor, T = 723 K, WHSV of carrier gas neon - 16 m3 * kg-' * h-', pulse size - = 1.5 p mole. a) Composition of products obtained from hexane pulses after 1 h reduction at 823 K (C - carbon recovered in volatile products). b) Influence of the length of the reduction at 823 K on the conversion (x) to benzene. Dotted lines are explained in the text.
d.40
Figure 4. Correlation of the DHC activity (first pulse) with e/Mo (a) and with the amount of "Mo(I1)" (b) in reduced MOO3/.4l203 catalysts. k - first-order rate constant of the reaction to benzene, in m3 * kg' * h-'.
1059
DHC activity exhibits a behaviour that has been observed to be typical of "Mo(I1)" under these reduction conditions in the XPS study (Fig. 2). On the other hand, due to the deactivation during the pulse sequence a clear cut quantity describing the catalytic activity is not easily derived. The dotted lines in Fig. 3b indicate three possibilities, among which the first one (conversion in pulse No. 1, i.e., disregard of catalyst deactivation) appears to be most plausible, but may be of minor relevance if one is interested in steady-state activities. The remaining two variants (conversion in pulse No. 10; linear back-extrapolation of the deactivation tendency) are intended to make allowance for features of the deactivation process. Attemps were made to find semiquantitative correlations between activities (conversions derived as described above and transformed into first-order rate constants) and properties reflecting the progress of the reduction (e/Mo; quantity of "Mo(I1)" as derived from the XPS measurements). Fig. 4 shows the results obtained with the activity in the first pulse. The correlation with e/Mo (Fig. 4a) yields separate straight lines for the two catalysts investigated, with almost identical gradients. On the other hand, Fig. 4b shows that the activity is proportional to the amount of "Mo(1I)". It is, therefore, quite clear that the active site is represented by the "Mo(I1)" XPS signal. Moreover, Fig. 4a implies that this active site is formed in a process that contributes to the increase of e/Mo. At reduction times exceeding 30 min. (cf. Fig. 2), this would be the case for the formation of Mo(I1) from Mo(IV), but not for a transformation of Mo(IV)is into Mo(IV) pairs. Hence, among the possible components represented by the "Mo(I1)" signal, Mo(I1) may be singled out as the state responsible for the DHC activity. Not unexpected, it turns out that the same oxidation state is active in the low-temperature hydrogenation and dehydrocyclization reactions. It should be noted, however, that all attempts to obtain similar correlations with activity data that include the influence of the deactivation process failed. Obviously, the deactivation involves quantities not accounted for in this analysis. Analogously, any correlation failed even for the first pulse when the total hexane conversion instead of the conversion to benzene was used to represent the activities. This may indicate that the desorption of benzene from the active sites is the rate-determining step under the conditions employed. Fig. 5 shows an interesting by-product of this investigation. In this figure, the influence of the reduction conditions on the product distribution in the concomitant cracking reaction is reported. The insert demonstrates that the variations in the product distribution were very smooth at a reduction temperature of 823 K When, however, the catalysts were reduced at temperatures 'r_ 923 K C2 (and Ci) products predominated among the cracking products of the first pulses while C4 was absent. These changes are, most likely, due to the presence of metallic molybdenum, which had already been found in the XPS investigation (cf. Fig. 1). In [23], Mo(0) formation was indicated by the benzene hydrogenation activity only after reduction at 1173 K. Hence, the present investigation suggests that cracking of hydrocarbon chains may be a more sensitive chemical probe for the presence of Mo(0) than the hydrogenation of benzene.
1060
cx CY
15
10
5
0
5
0
G , 973K(ml;
10
pulse number
823 K
( 0 )
c3
Figure 5. The influence of the reduction temperature on the distribution of the cracking products. Mo7, 2 h reduced in flowing H2 at the temperatures indicated, reaction conditions cf. Fig. 3. Insert: Averages of 10-pulse sequences are given. 4. CONCLUSIONS
‘The relation between oxidation state and dehydrocyclization (aromatization) activity of Mo in M003/M203 was studied by X P S and pulse catalytic techniques. Interpretation of the X P S spectra by use of a linear relation between binding energies and oxidation states supplied a basis for assuming Mo(I1) to be present in reduced Mo03/&03 catalysts. This assumption was supported by volumetric reduction studies. From a correlation between the (initial) activity of reduced Mo@/Al203 catalysts in the dehydrocyclization of n-hexane and their content of Mo(II), two-valent molybdenum was identified as the active Mo state. In the pulse dehydrocyclization of n-hexane, the product distribution in the concomitant cracking reaction is a sensitive probe for the presence of metallic molybdenum. 1 2 3 4
M.B. Ward, M.J. Lin, and J.H. Lunsford, J. Catal. 50 (1977) 306. L. Mendelovici and J.H. Lunsford, J. Catal. 94 (1985) 37. T.A. Patterson, J.C. Carver, D.E. Leyden, and D.M. Hercules, J. Phys. Chem. 80 (1976) 1700. D.S. Zing, L.E. Makovski, R.E. Tischer, F.R. Brown, and D.M. Hercules, J. Phys. Chem. 84 (1980) 2898.
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K.S. Seshadri and L. Petrakis, 3. Catal. 30 (1973) 195.
10 11 12
13 14 15 16
17
ia 19
20 21 22 23 24 25 26 27
S. Abdo, A. Kazusaka, and R. Howe, J. Phys. Chem. 85 (1981) 1380. S.R. Seyedmonir, and R. Howe, J. Chem. SOC., Faraday Trans.1 83 (1987) 3115. J. Valyon, and W.K. Hall, J. Catal. 84 (1983) 216. E. Guglielminotti, and F. Giamello, J. Chem. SOC.,Faraday Trans.1 81 (1985) 2307. A. Cimino, and B.A. de Angelis, J. Catal. 36 (1975) 11. W. Griinert, A.Yu. Stakheev, R. Feldhaus, K. Anders, E.S. Shpiro, Kh.M. Minachev, J. Phys. Chem. 95 (1991) 1323. J. Haber, W. Marczewski, J. Stoch, and I.+ Ungier, Ber. Bunsenges. Phys. Chern. 79 (1975) 970. W. Griinert, A.Yu. Stakheev, W. Morke, R. Feldhaus, K. Anders, E.S. Shpiro, Kh.M. Minachev, J. Catal., in press. W. Griinert, A.Yu. Stakheev, R. Feldhaus, K. Anders, E.S. Shpiro, Kh.M. Minachev, J. Catal., in press. B.L. Moldawski and G. D. Kamusher, Dokl. Akad. Nauk SSSR (Russ.) 1 (1936) 1862. M.I. Rozengart and B.A. Kazanski, Usp. Khim. (Russ.) 40 (1971) 1537; G.V. Isagulyants, Yu.G. Dubinski, and M.I. Rozengart, Usp. Khim. (Russ.) 50 (1981) 1541. H. Bremer, J. Muche, and M. Wilde, Z. anorg. allg. Chem. 405 (1974) 230; 407 (1974) 40, 51; H.-G. Vieweg, K Knapp, K. Kranke, D. Schonfeld, K Anders, and S. Nowak, Chem. Tech. (Leipzig) 34 (1981) 525; W. Saffert, W. Griinert, W. Schellong, K. Anders, and S. Nowak, Chem. Tech. (Leipzig) 38 (1986) 472. R. Maggiore, N. Giordano, C. Crisafulli, F. Castelli, L. Solarino, and J.C.J. Bart, J. Catal. 60 (1979) 193. W.K. Hall and W.S. Millman, Proc. 7'h Intern. Congr. Catal. 1980, Kodansha Ltd., Tokyo 1981, 1304. W.K. Hall, in 'The Chemistry and Physics of Solid Surfaces" (R. Vanselow and R. Howe, Eds.), Springer-Verlag, Berlin-New York 1986, Vol. VI, 73. Y. Iwasawa, N. Ito, T. Chiba, H. Ishii, and H. Kuroda, Chem Lett. 1141 (1985). R.B. Quincy, M. Houalla, A. Proctor, and D.M. Hercules, J. Phys. Chem. 94 (1990) 1520. A. Redey, J. Goldwasser, and W.K. Hall, J. Catal. 113 (1988) 82. W. Griinert, W. Saffert, R. Feldhaus, and K. Anders, J. Catal. 99 (1986) 149. B. Brox and I. Olefjord, Surf, Interface Anal. 13 (1988) 3. Y. Holl, R. Touroude, G. Maire, A. Muller, P.A. Engelhard, J. Grosmangin, J. Catal. 104 (1987) 202. J.4.Chung, J.P. Zhang, and R.L. Burwell Jr., J. Catal. 116 (1989) 506; J.-S. Chung and R.L. Burwell Jr., J. Catal. 116 (1989) 519.
1062
DISCUSSION
Q: W. M. H. Sachtler (USA) The coexistence of Mo(II), (IV), (V) and (VI) suggests that your samples are not in true thermodynamic equilibrium, as this result violates the Gibbs' phase rule. As the catalysts are unstable, why are those likely to change with reduction time and also with time on stream.
A: W. Griinert I would not consider the Mo species of identical oxidation states to form phases in the sense of Gibbs. Mo(VI), for instance, exists in the form of isolated and oligomeric surface complexes, with the strength of interaction with the alumina support and, hence, the stability against reduction differing bctwecn different typcs of Mo(VI) species. Of course, the samples are not in thermodynamic equilibrium after 2 h of reduction. This is quite clear from Fig. 2, and there is a poster of the group of Professor Hercules at this conference (see p. 18 6 7 ) that shows quite nicely that you may obtain similar reduction degrees as we had after 2 h at 823 K by a very long reduction at 50-100 K lower temperatures. The precautions we took for avoiding a change of the oxidation state during the activity measurements were a lower reaction temperature (100 K below the reduction temperature), the use of an inert carrier gas, and the use of the pulse technique. Q: H. L. Krauss (Germany) The simultaneous existence of Mo(VI) and Mo(I1) in your samples does not puzzle me at all: in our Mo catalysts on silicagel basis are found that the reduction of surface Mo(VI) (by COhv) is strongly dependent on the structure/coordination/clusteringof the molybdenum, (see paper of P. Morys and S . Schumerbeck, in Z. Naturforschung, 1987). Indeed there seems to exist no thermodynamic equilibrium - several structures are quite resistant or even inert vs. reduction.
Q: L. Murrcll (USA) Comments made by several of the questioners were not consistent with the nature of dispersed oxides on alumina supports. With most aluminas there is a density of Lewis acid centers separated by ca. 0.7 nm on thc surfacc that act as strong anchors for surface phase oxides such as MOO, and W03 WO, on the surface of alumina at a concentration which matches the number of Lewis acid sites cannot be reduced significantly even at the two hours at 1173 K in flowing H2. At higher WO, coverages of the alumina surface beyond 1.1, interaction of WO, with the Lewis site density reduction at 1023-1173 K produces a range of W(6-x)+centers just as the authors of this paper observe for MOO, on A120 . The surface MO, groups bound to the terminal hydroxyl groups have an inherent distrilhion of strengths of interaction due to their location at Lewis acid centers or near to strong Lewis acid centers and by the complex hydroxyl structure on the surface of aluminas. There is an inherent range of surface energy potentials which surface-bound groups experience which is clearly depicted in the range of oxidation states obtaincd under equilibrium reduction conditions for MO, groups reacted with the surface of alumina. A: W. Griinert Thanks for the comments of Professor Krauss and Dr. Murrell. In the case of MoO,/AI203, the anchoring stoichiometry between the MOO, groups and the sites of the support appears to be less stringent than in WOdAI20,. In the oligomeric structures generally considered to be present at higher Mo contents (but well below the monolayer capacity), there are probably very different types of interactions bctween Mo and the support. part of the Mo(VI) is engaged in direct interactions, another part via neighboring Mo or even more indirect. The latter part is destabilized rather that stabilized by the
1063 support. This is, probably, the reason why the temperature of beginning reduction of these samples is well below that of unsupported MOO,.
Q: R. W. Joyner (United Kingdom) Your assumptions of a linear relationship between binding energy and oxidation state for molybdenum may be reasonable for bulk, semi-infinite surfaces. For small particles it is complicated by charges in the final state relaxation, which increases the binding energy. Put simply, could the feature you assign to Mo(II) really arise from small particles I molybdenum metal ? A: W. Griinert In the case of isolating material, with which we are involved (the metal is not bound by the linear relation in the fit), I would not expect relaxation effects to cause significant binding energy shifts in small particles. Another point is M o o p which exhibits high electron mobility and a conductivity of the metallic type. Its binding-energy anomaly is, most likely, due to a final-state effect, and a particle-size effect with that part of "Mo(1I)" that possibly represents paired Mo(IV) might be considered. As to the metal, the uncertainty in the expectations for the binding energy (B.E.) of supported metal particles led us to release the condition of the linear relation with the b.e. for the zerovalent state. The B.E. of supported metal particles may deviate from the values of the bulk metal due to particle size effects as you inferred, but also due to interactions with residual cationic species, with reduced spots of the support, or just by differential charging effects. The presence of metal particles with a B.E. coinciding with ''Mo(II)'' cannot be ruled out with our spectroscopic material. However, the predominant contribution to "Mo(1I)" certainly comes from an oxidation state above zero. This state is formed at reduction temperatures as low as 673 K. Its exclusive assignment to a zerovalent s ecies would cause problems in the comparison of overall reduction degrees between XP and independent techniques as well as in the interpretation of line intensity data. The catalytic properties of the samples do not favour this view either. The problem with these spectra is that, due to the large line widths, they are very badly suited for the detection of any new species. This is the reason why we work with preassumptions about line positions and about the states to be included or not (e.g. Mo(III), which has never been found by EPR in samples of this type). Our aim is just to get a realistic first-order approximation to the real situation in this very important system. If there are minority species present with a b.e. near those of states already included in the analysis it would be impossible to detect them.
l
Q: V. B. Kazansky (Russia) What was the ty ical composition of your catalyst under reaction conditions according to your data
..p
A: W. Griinert Our XPS analysis for the sample with the highest activity yields 27-28 % Mo(VI), 55 % "Mo(II)", and Mo(V) ranging from 0 % (with Mo(1V)is = 17 %) to 7 % (with Mo(1V)is = 11 %). This is the catalyst with 13 wt % MoO3, reduced for 2 h at 823 K. I think that higher amounts of "Mo(1I)" and, hence, higher activities, may be obtained by prolonged reduction at this temperature. As mentioned in the lecture, the actual contribution of Mo(I1) to "Mo(1I))" cannot be exactly defined, although the reoxidation experiments imply that it is high. For the runs shown in Figure 4, the compositions were: Mo(VI) between 55 and 27 %,"Mo(II)" between 20 and 55 %, the Mo(V) ranges typically extended between 0 and 9 %, the ranges for Mo(1V)is decreased from 25-15 % at low reduction times (15 min) to 16-11% at high reduction times (120 min).
1064
Q: W. K. Hall (USA) Using the linear relation of binding energies with reduced states the distribution you obtain will differ from that obtained by the conventional method. Either result obtained by either method can be used to calculate the average extent of reduction, and this can be determined readily by measuring the amount of 0 2 required to return the reduced catalyst to its original Mo(VI) state, Which procedure gives the best fit of the data ?
A: W. Griinert We did these experiments, and we found that the average reduction degree, determined by reoxidation, was clearly above +2 under reduction conditions where a lication of the traditional approach confined the values drawn from the corresponding 8 s spectra to the region below +2. Hence, the evaluation by our approach supplied the better fit. I would like to stress, however, that this better coincidence is an argument that should not be overemphasized. Usually, comparisons of this type rest on work made with samples of different geometry (e. ., fixed bed or pressed powder), with different flow regimes (plug flow or badly de ined flow across the external surface of a sample preconditioned for X P S ) and, possibly, with different reactant purities. We found that the reduction degrees obtained depend sensibly on the residual artial pressure of O2 (and H 0),which may be influenced by the factors cited above. 81ustering tendencies in the phase upon reduction may complicate the picture as they give the signal areas of the individual states an unknown weight. I think the ultimate decision about the correctness of the different approaches to the interpretation of the spectra, including the recent attempts of Professor Hercules and his group, will rest on their success in the consistent explanation of catalytic activities and, perhaps, on future investi ations of model systems as those presented earlier by Professor Iwasawa and Professor B. Kazansky and their groups.
f
hk
4.
Guczi, L.et al. (Editors), New Fronriers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
REACTION ROUTES FOR METHANE CONVERSION ON TRANSITION METALS AT LOW TEMPERATURE
T.Koerts and R.A. Van Santen Schuit Institute of Catalysis, Department of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, P.O.Box 513,5600MB Eindhoven, The Netherlands
Abstract Two new metal catalysed routes for methane conversion are discussed: methane oligomerization and methane addition to olefins. These reactions are realized in a reaction cycle consisting of several steps. In both reaction sequences methane is first dissociatively adsorbed on a reduced transition metal catalyst between 500 and 800 K resulting in surface carbon and hydrogen. A particular highly reactive surface carbonaceous intermediate is found to produce C2+ hydrocarbons upon hydrogenation between 300 and 400 K. The maximum yield for higher hydrocarbons is 13 % obtained on a Ru/Si02 catalyst. When olefins are co-adsorbed together with surface carbon generated from methane, more C2+ hydrocarbons are produced. Experiments with 1% labelled C& and unlabelled olefins demonstrate that surface carbon from methane is incorporated into the co-adsorbed olefins. INTRODUCTION Methane conversion to higher hydrocarbons is of world wide interest. Up to now only indirect routes via synthesis gas have been commercialized. Hydrocarbons are formed from synthesis gas via methanol and the MTG process [l] or via the Fischer-Tropsch reaction [2,3]and the SMDS [4,51 process. Direct methane conversion using pyrolysis, resulting in acetylene and benzene, can only operate at temperatures above 1200 K [6] .Oxidative coupling of methane to ethylene has been proposed as a promising alternative route [7-121 and proceeds at temperatures between 850 and 1200 K.
1066
In the Fischer-Tropsch reaction it has been shown that carbidic C1 species are the reaction intermediates initiating chaingrowth upon hydrogenation [49,501. These C1 reaction intermediates can also be formed from other molecules than CO. Petit and Brady [13,14]used CH2N2 as a source for surface carbon, Cavalcanti et a1 [15,16]used CH3NO2, Van Barneveld and Ponec [171used CH(4_,)Clx and Williams et a1 [181 demonstrated that also surface carbon generated from methyliodide could be incorporated into C2+ hydrocarbons during CO hydrogenation. Recent experiments demonstrate that also hydrocarbon formation from surface carbon from methane is possible when methane is activated. Shelimov and Kazanski [19]activated methane by photo energy. Ceyer and co-workers [20,21]activated methane under UHV conditions by a krypton bombardment. During subsequent temperature desorption benzene appeared in the gas phase from the recombination of surface CH, fragments. Tanaka et a1 [221 generated CHI species from methane on cobalt and demonstrated their reactivity to ethene. Belgued et a1 [23]created CH, surface species on platinum and demonstrated C2+ hydrocarbons in a hydrogen flow. Here we present a cyclic reaction path for the formation of C2+ hydrocarbons on transition metal catalyst in which methane is thermally activated. In the first step methane is dissociatively adsorbed on a reduced group VIII metal between 500 and 800 K. Subsequently higher hydrocarbons are formed upon exposure to hydrogen at a lower temperature. The ability of the C1 intermediates to form carbon-carbon bonds can also be used to add these species to other co-adsorbed molecules. This is demonstrated for the alkylation of ethene, acetylene and propene using methane, according to a cyclic route. To distinguish methane addition from olefin disproportionation [24-261, experiments with 1 3 a 4 were performed.
Thermodynamic consideration The oligomerization of methane to alkanes is not thermodynamically allowed in one step. However, using two steps occurring at different conditions, this reaction can be made feasible. This is shown for the reaction of methane to ethane and hydrogen using bulk cobalt carbide as a reaction intermediate (see Fig 1).
1067
AG,-(lW 73
233
473
673
K)
= 71.O kJ/mol
873
Temperature [ K ]
Figure 1 . Gibbs free energies as a function of the temperature for the decomposition of methane on cobait ( I ) and the hydrogenation of cobalt carbide to ethane (2). Figure 1 shows that there is no common temperature where the two reactions steps are both negative. Tp perform the two reaction steps (AG> 650 OC since they fell below our accuracy of detection ( ~ 0 . 3sec). You have not been able to detect surface residency for the carbon in CH4 or products at 750 OC,which fits with all the previous work. This, however, leads your to conclude that an Eley-Rideal mechanism is operative at 750 k.However, this requires a change in mechanism between 650 OC and 750 OC. I suggest, barring other contrary results, that chemisorption of the carbon, i.e. the surface residence time, is just below the detectability of the method. At high temperatures the surface residence time and concentration may be small but still be important for reaction. A: E. E. Wolf The issue of methane adsorption on oxidative coupling catalysts at temperatures higher than 650 OC has been a subject of discussion between group from Pittsburgh and ours for quite some time. The authors agree that in order to resolve this issue, more precise experimental techniques are required. Thus, the assumption that methane adsorption plays an important role in the oxidative coupling process (T > 650 OC) is speculative. As mentioned above, in the response to the question by Dr. Rasko, the authors bclieve that CH4 can be activated in without adsorption of the methane molecules.
Q: K. Dooley (USA)
For your La203 - based catalysts, the overall rate of exchange for ' 6 0 ~ ~ 8 0 2 appeared to be raster than the rate of scrambling to '60180. However, when C02 isotopic transients were used, the overall rate of exchange for C16O2-C1b2 appeared to be slower than the rate or scrambling to C160i80. Do you think that you could conclude from this that most or the oxy en scrambling proceeds through a carbonate intermediate, while most of the 1602-w02 exchange proceeds through a different surface intermediate?
A: E. E. Wolf Although, a definite response to this question would require some additional experiments, the authors agree with the above interpretation. According to Klier and coworkers, the faster exchange rate of 1602/1802 species than the exchange rate for the scrambled oxygen, suggests an oxygen vacancy exchange mechanism. In the case of the isotopic exchange experiment involving carbon dioxide, surface lanthana carbonates start to play an important role. The presence of two intermediate mechanisms can also be inferred from the TPIE experiments with 0,and CO,.
Q: V. Ponec (The Netherlands) Novakova, Jiru and Klier showed many yeaJs ago (see in Cat. Revs. Sci. Eng.) that basically two mechanisms operate upon 02/02exchange: one by one atom exchange
1103 (R') and the other by damping of 0; accompanied by desorption of 0 2 @"-mechanism). I would guess that upon "Sr" promotion, the ratio R"/R' increases. Can you comment on this ? A: E. E. Wolf Since the presence of Sr promoter creates oxygen vacancies in the lattice of lanthanum oxide, it is expected that the rate of R'Lmechanism will increase. However, no experiments were conducted to determine exact values of the particular exchange rates for the above mechanisms. Q: J. C. Volta (France) If I understand you correctly, it appears that the presence of oxygen vacancies is an important parameter. O n your Sm20&a203 catalysts, is there any possibility of distinguishing the contribution of S m 2 0 3 wlth that of the doped (Sm)La2O3 material ? Do you compare pure S m 2 0 3 with the doped (Sm)La,O3 catalyst ?
A: E. E. Wolf No answer.
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Guczi, L a al. (Editors), New Frontiers in Catalysh
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights mewed
THE ROLE OF THE PROTON IN OXIDATION PROCESSESON METAL= OXYGEN CLUSTER COMPOUNDS S. hzte1ana, G. B. McGarveya and J. B. Moflalb
aKinetics and Catalysis Division, Institut Francais du Petrole, 1 i t 4 avenue de Bois-Preau, 92506 Rueil-Malmaison Cedex, France bDepartment of Chemistry and Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
Abstract Studies of the partial oxidation of methane and the oxidative dehydrogenation of isobutyric acid on metal-oxygen cluster compounds (MOCC) show that the anion of the MOCC is a prerequisite for the catalysis of these oxidation processes. However the results also clearly show the importance of the proton in both processes. The observations may be interpreted in terms of oxygen vacancies which may serve as sites for the dissociation of the gas phase oxidant, as adsorption sites for reactant molecules or as sites for the production of oxidizing species. Introduction The catalysis of oxidation processes has been studied for many years, although interest has intensified in the last decade and recent reviews encompass a wide variety of reactants and products (1,2). The mechanism of the catalysis of oxidation processes is in principle more complex than that of other processes such as those which are, for example, acid catalyzed] as a result of the presence of the oxidant, the possibility of a number of active species containing oxygen and the participation of a concurrent gas phase process. With acid catalyzed processes a number of catalysts have frequently been studied for their properties in a variety of such reactions. However with oxidation processes a comparison of the activity and selectivity of a given catalyst in a number of these reactions is less fiequently available. The present work reports the results of studies of the partial oxidation of methane t o formaldehyde and methanol and the oxidative dehydrogenation of isobutyric to methacrylic acid on metal-oxygen cluster compounds (MOCC), compares these observations and provides a common mechanism for the participation of the catalyst in the two oxidation processes.
1106
As is well known, the partial oxidation of methane requires the activation of one or at most two carbon-hydrogen bonds in the methane molecule. Space restrictions limit the discussion of previous work but a number of recent reviews have summarized the information presently available (3-10). The pioneering research in this area is due to Lunsford (11-12)and Somorjai (13-14))the former of whom worked with MgO and Mo/SiO, as catalysts with N20 as oxidant, the latter with V/SiO, and Mo/SiO,. In this laboratory the earliest studies of the conversion of methane were concerned with the partial oxidation to formaldehyde and methanol (15-29) on MOCC. Although polar compounds such as ammonia, pyridine and methanol have been shown, through the use of photoacoustic FTIR spectroscopy, t o penetrate into the bulk structures of MOCC there is no evidence that nonpolar compounds such as methane have similar capabilities (30-32). It is important t o note that although MOCC have discreet cage-like anions (Figure 1) these are packed structures and unlike the cages found in certain zeolites are unable t o occlude even the smallest molecules or atoms. Thus the penetration observed with polar molecules must involve the spatial regions between the cations and anions (Figure 2). Since the acidic forms of the MOCC have low surface areas (of the order of 10m2/g)and the methane molecule is unable t o diffuse into the bulk structure of these solids, optimization of the interaction between the former and the latter necessitated the introduction of a high area support. In this laboratory silica was the support of choice.
4
d
b
Figure 1. Metal-oxygen cluster anions (PW,,0,03-) of Keggin structure; large circles: central atom (PI and peripheral metal atoms (W); small circles: oxygen atoms
Figure 2. Anion-cation configurationin
H,PW,,O,, nH,O (33). HW and OW refer to hydrogen and oxygen in water molecules; OT, the terminal oxygen of the anion and HA the acidic proton.
1107
The production of methacrylic acid from the oxidative dehydrogenation of isobutyric acid is an important step in the process for obtaining methyl methacrylate from propylene (34). A number of catalysts, including various MOCC, have been shown to be active and selective in this reaction (35-52). In the present work the results obtained from studies of the partial oxidation of methane to formaldehyde and methanol and the oxidative dehydrogenation of isobutyric acid t o methacrylic acid on metal-oxygen cluster compounds are compared and contrasted and the implications of the effect of the exchange of cations on the reaction mechanism are examined. Experimental Continuous flow reactor systems were employed for studies of both the partial oxidation of methane and the oxidative dehydrogenation of isobutyric acid. Details of these reactors together with information on other aspects of the experiments have been provided elsewhere (15, 26,53-55). Results and Discussion The similarities between the processes for the partial oxidation of methane and the oxidative dehydrogenation of isobutyric acid are evident, at least in part, from the stoichiometricequations. The partial oxidation of methane to methanol and formaldehyde CH, CH,
+ 12 0, -+CH,OH + 0, + CH,O + H20
can be considered as processes involving the increase of the oxidation states of the carbon present in methane by two and four levels, respectively, requiring the extraction of two and four electrons, respectively, from the methane molecule. In the oxidative dehydrogenation of isobutyric acid to methacrylic acid (CH,),CHCOOH
+ 1 0, + CH,C(CH,)COOH + H,O 2
(3)
the reactant (IBA) suffers a net loss of only two electrons in the reaction although four carbon atoms are contained within the reactant and product molecules. It is also important to emphasize another obvious distinction between the two oxidation processes under discussion here. In the case of the partial oxidation of methane, the product molecules, either methanol or formaldehyde
1108
contain an additional oxygen atom as compared to the reactant, while with the oxidative dehydrogenation process the MAA has two fewer hydrogen atoms than IBA while retaining the original number of oxygen atoms. Thus there are both similarities and dissimilarities in the stoichiometry of these two processes. Silica-supported 12-molybdophosphoricacid is an effective catalyst for the partial oxidation of methane (conversion and selectivity to partial oxidation products 5 and 13%, respectively at 570°C). However, the effectiveness of the MOCC is evidently strongly dependent on the elemental composition of the anion. Substitution of tungsten for molybdenum as the peripheral metal element produces a catalyst with both reduced activity and selectivity (0.4% conversion and a trace of partial oxidation products). A less severe reduction is also observed when phosphorus is substituted by silicon as the central atom in the anion of the MOCC. These observations appear to be consistent with the results of calculations from this laboratory which showed that the strength of the bond between the peripheral metal atoms and the outer or terminal oxygen atoms of the MOCC anion is much higher with tungsten than with molybdenum, while the magnitude of the charge on the outer oxygen atoms in the latter was considerably higher than that in,the former (56). The terminal oxygen atoms in the molybdenum-containing anions are suMiciently labile to permit their participation in the oxidation process while those in the tungsten-containing anions are relatively more tightly bound and hence cannot easily participate in such a reaction. Further work (19) has provided evidence for the primary role of the anion in the activity and selectivity of the catalyst. As the loading of the catalyst on the support increases, the rate of formation of various products increases linearly, indicating that the active species are associated directly with the supported materials. In addition, at temperatures of approximately 923K, the activity and selectivity of the supported catalyst change abruptly to those expected for the support itself. Evidently the oxides formed at these relatively high temperatures are either inactive or barely active for methane oxidation. Apparently the active species are those anions possessing the Keggin structure. Stepwise exchange of the proton by cesium in the silica-supported 12molybdophosphoric acid produces significant effects on a number of features of the methane conversion process. As the protons are replaced by cesium ions the methane turnover rate decreases until at concentrations of cesium between 3 and 4 ions/MOCC anion the conversion has decreased t o values found for the silica support itself. In addition, for similar increases in cesiudanion the selectivity t o CO and formaldehyde decrease while that to CO, increases until again values corresponding to those for the support are obtained. Concomitantly the calculated activation energy decreases from 32 2 2 kcal mo1-l to 12 2 2 kcal mo1-l at approximately four Cs/anion, the latter value of activation energy being similar to that for the silica support itself.
1109
The oxidative dehydrogenation of isobutyric to methacrylic acid is also catalyzed by 12-molybdophosphoric acid and its monovalent salts (39-55). Studies of this process in this laboratory have employed the unsupported MOCC. As expected three products are formed: methacrylic acid (MAA),propene (P)and acetone (A). However the relative amounts of the three products as well as the conversion are found t o be dependent on both the anidn and cation employed (Table 1). Table 1 Conversion of Isobutyric Acid and Selectivities to Methacrylic Acid ( M U ) Propene (P) and Acetone (A) on 12-Tungstophosphoric Acid, 12Molybdophosphoric Acid and its Ammonium and Cesium Salts. MOCC
Conversiona
Selectivitya MAA
H3PMo12040 (NH4)3pw12040
(NH4)3PM012040 Cs3PM012040
a
69.7 29.6 62.5 22.3
40.1 0.9 49.3 39.3
Propene Acetone 23.2 99.1 13.2 9.9
36.7 0 .o 35.8 50.7
Mole percent Reaction Temperature 300°C W/F = 0.85 mg min mLml
It is evident from Table 1that the elemental composition of the anion is an important factor in the oxidative dehydrogenation of isobutyric acid. From a comparison of the results for ammonium 12-tungstophosphate, where tungsten occupies the peripheral metal atom positions, with those obtained with ammonium 12-molybdophosphate in which molybdenum is situated in the peripheral metal atom positions, it is clear that the latter catalyst is superior to the former for this process. It is interesting to recall the similar conclusion for the partial oxidation of methane. With the ammonium salt of 12-molybdophosphoricacid as prepared from a precipitation of stoichiometric amounts of the acid and ammonium carbonate the conversion of IBA is decreased somewhat, as compared to that obtained with the parent acid, the selectivities to MAA and propene increase and decease, respectively, while that to acetone remains relatively constant. It should be noted that earlier work has shown that residual protons, in quantities as large as 0.4O/anion (30) and a microporous structure (57) are found in this solid. In contrast with the ammonium salt, that with cesium as precipitated from the
1110
parent acid and cesium carbonate, yields a considerably smaller conversion and differing distributions of the products. Work in this laboratory has recently shown that cation exchange of the metal-oxygen cluster compounds with microporous structure can be achieved without loss of the porosity or inherent crystallinity (53, 54). For example ion exchange of cesium by a stepwise batch procedure into the ammonium salt of 12molybdophosphoric acid converts the surface area and pore structure of the former to that of the latter in continuous steps which are dependent on the extent of the exchange, and concomitantly the lattice parameter follows a similar pattern. Solids prepared by ion exchange of cesium into the cesium salt (to reduce the quantity of residual protons) and into the ammonium salt, as well as the ion exchange of the ammonium ion into the cesium salt have produced solids with a variety of concentrations of cesium iondanion. The selectivities to the various products from the oxidative dehydrogenation of IBA with the MOCC containing the various concentrations of cesium ions are shown in Table 2. Table 2 Selectivities of Isobutyric Acid to Methacrylic Acid (MAA), Acetone (A) and Propene (P) after Ion exchange of Protons by Cesium Ions with 12Molybdophosphoric Acid. Selectivity"
Cs'lAnion
MAA 1.15 1.53 2.44
45.2 46.3 48.8 41.5 39.3 10.4 4.0
2.59
2.78 2.82 3.08
a
Propene Acetone
18.6 14.0 17.8 14.8 9.9 10.0 9.0
35.3 37.1 32.5 42.9 50.7 79.6 87.0
Mole percent Reaction temperature 300°C W/F = 0.85 mg min mL-'
It is evident that with increase in the quantity of Cs' the selectivity t o methacrylic acid decreases drastically as the number of Cs+ per anion approaches three, the stoichiometric value. Concomitantly the selectivity to acetone increases in approximately inverse proportionality to that shown by MAA,
1111
suggesting that a common intermediate exists in the formation of these two species.
As noted above, cation exchange of the metal-oxygen cluster compounds can be achieved with semi-quantitative retention of pore structure. Thus, as the cations of ammonium 12-molybdophosphate are stepwise exchanged by cesium ions the surface area increases from 88.5 t o 144.1 m2/g, similar to values obtained previously for ammonium and cesium 12-molybdbphosphate (58). Concomitantly the average micropore radius increases from approximately 11t o 15& again in agreement with earlier results. It may be recalled that the parent MOCC, 12-molybdophosphoricacid, has a surface area of approximately 5 m2/g (54).It is evident from a comparison of these data with those in Table 1that the activity and selectivity of the present MOCC catalysts cannot be correlated with the surface area or pore structure. However, since polar molecules such as ammonia, pyridine and methanol are able to diffuse into the bulk of the metaloxygen cluster compounds (30-32)it is possible that isobutyric acid or a product therefrom may enter the bulk of the catalyst during the oxidative dehydrogenation process. Work in other laboratories has shown that the reduction rate of the MOCC in the oxidative dehydrogenation of isobutyric acid is proportional to the catalyst weight (46) and it is postulated that in this process electrons and protons that form on the surface of the catalyst are able t o migrate into the bulk thus leading to a total reduction of the solid. With both the conversion of methane and the oxidative dehydrogenation of isobutyric acid the process has been shown t o occur in the absence of an oxidant in the feedstream, although the conversions decrease rapidly with timeon-stream and the selectivities differ from those expected in the presence of an oxidant. Consequently, while not precluding the possibility of the participation of a gas phase process it is clear that the catalyst is playing an active role in these processes. Although the mechanism for either process on the metal-oxygen cluster compounds is as yet uncertain the similarity of the requirements for the two processes is intriguing. With each reactant it appears that the primary purpose of the catalyst is concerned with the activation of a C-H bond. Presumably a site of relatively high electron density is required for such purposes and the terminal (or possibly bridging) oxygen atoms of the anions of the MOCC appear suited t o such purposes. Although the existence of the anion of the metal-oxygen cluster compounds is undisputedly a necessary ingredient in the catalytic oxidation process, studies of the partial oxidation of methane and the oxidative dehydrogenation of isobutyric acid clearly show the importance of the proton. While a preliminary examination of the results appears to suggest the direct participation of the proton in both oxidation processes a more likely rationale may be found in earlier results from temperature programmed desorption experiments in this labo@tory (59).Of two peaks due to the desorption of water from 12-molybdophosphoric acid, that appearing at higher temperature was
1112
shown t o be attributable to water produced from the extraction of oxygen atoms from the anions by the acidic protons. Although this process would generate oxygen-deficient anions the cage-like structure is evidently retained (3). However, oxygen vacancies would of course be produced on these anions:
PMo1204i3
+ 2H+ + PMol,O,,O-l + H,O
(4)
The removal of protons through exchange with cesium would thus decrease the number of protons available for this extraction process and hence reduce the number of oxygen vacancies generated in the anions, The oxygen vacancies may serve as sites for the dissociation of the gas phase oxidant, as adsorption sites for reactant molecules or as sites for the generation of oxidizing species from adsorbed oxygen molecules. Additionally the effect of the exchange-deposited cesium ions may have an electronic basis with a strong dependence on the electronegativity of the ion and hence its electron-donating ability. Acknowledgement The financial assistance of the Natural Sciences and Engineering Research Council of Canada, Energy, Mines and Resources Canada and Esso Petroleum Canada is gratefully acknowledged. References 1
2
3 4 5 6
7 8
9 10 11
12 13 14
G. Centi and F. Trifiro (eds.), New Developments in Selective Oxidation: Proceedings of an International Symposium, Rimini, Italy, September 1822, Elsevier, Amsterdam, 1990. A. Bielanski and J. Haber, Oxygen in Catalysis, M. Dekker, New York, 1991. H.D. Gesser, N.R. Hunter and C.B. Prakash, Chem. Rev., 85 (1985) 235. N.R. Foster, Appl. Catal., 19 (1985) 1. R. Pitchai and K. Klier, Catal. Rev.-Sci. Eng., 28 (1986) 13. M.S. Scurrell,, Appl. Catal., 32 (1987)1. J.S.Lee and S.T. Oyama, Catal. Rev. Sci. Eng., 30 (1988) 24. G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse, Chem. SOC. Rev., 18 (1989) 261. Y.Amenomiya, V.I. Birss, M. Goledzinowski, J. Galuszka and A.R. Sanger, Catal. Rev. Sci. Eng., 32 (1990) 163. J.C. Mackie, Catal. Rev.-Sci. Eng., 33 (1991) 169. K.Aika and J.H. Lunsford, J. Phys. Chem., 81 (1977) 1393. M.B. Ward, M.J. Lin and J.H. Lunsford, J . Catal., 50 (1977) 306. M.M. Khan and G.A. Somojai, J. Catal., 91 (1985) 263. K.J. Zhen, M.M. Khan, C.H. Mak, K.B. Lewis and G.A. Somojai, J. Catal., 94 (1985) 501.
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15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
S. Kasztelan and J.B. Moffat, J. Catal., 106 (1987) 512. S. Kasztelanand J.B. Moffat, J. Chem. SOC.,Chem. Commun., (1987) 1663. J.B. Moffat, Keynotes in Energy-Related Catalysts, S. Kaliaguine (ed.), Elsevier, Amsterdam, 1988. J.B. Moffat, Methane Conversion, A Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 1987, D. Bibby (ed.), Elsevier, Amsterdam, 1988. S. Kasztelan and J.B. Moffat, J. Catal., 109 (1988) 206. S. Ahmed and J.B. Moffat, Appl. Catal., 40 (1988) 101. S. Ahmed and J.B. Moffat, Catal. Letters, 1(1988) 141. S. Kasztelan and J.B. Moffat, J . Catal., 112 (1988) 54. S. Kasztelan, E. Payen and J.B. Moffat, J . Catal., 112 (1988) 320. S. Kasztelan and J.B. Moffat, Proc. 9th Internat. Congr. on Catal., Calgary, 1988, M.J. Phillips and M. Ternan (eds.), Chemical Institute of Canada, Ottawa, 1988. S. Kasztelan and J.B. Moffat, J. Catal., 116 (1989) 82. S. Ahmed and J.B. Moffat, J. Phys. Chem., 93 (1989) 2542. S. Ahmed and J.B. Moffat, J. Catal., 118 (1989) 281. S. Kasztelan, E. Payen and J.B. Moffat, J. Catal., 125 11990)45. S. Kasztelan, E. Payen and J.B. Moffat, J. Catal., 128 (1991) 479. J.G. Highfield and J.B. Moffat, J. Catal., 88 (1984) 177. J.G. Highfield and J.B. Moffat, J. Catal., 89 (1984) 185. J.G. Highfield and J.B. Moffat, J. Catal., 95 (1985) 108. G.M. Brown, M.-R. Noe Spirlet, W.R. Busing and H.A. Levy, Acta Crystallogr. Sect. B, 33 (1977) 1038. R.K. Grasselli and J.D. Burrington, Adv. Catal., 30 (1981) 133. C. Virely, D. Fabregue and M. Forissier, Bull. SOC. Chim. Fr., 3 (1988) 458. J.C. Vedrine, J.M.M. Millett and J.C. Volta, Faraday Discuss. Chem. SOC., 87 (1989) 207.
37 38 39 40 41 42
X. Cui, M. Jin, Y. Ji and Y. Zheng, Cuihua Xuebao, 9 (1988) 241, CA 110 (17) 15359511. M. Ai,J. Catal., 98 (1986) 401. M. Otake and T. Onoda, in Proceedings, 7th International Congress on Catalysis, Tokyo 1980, T. Seiyama and K. Tanabe (eds.), Elsevier, Amsterdam, 1981. M. Otake and T. Onoda, J. Catal., 38 (1975) 494. M. Akimoto, Y. Tsuchida, K. Sat0 and E. Echigoya, J. Catal., 72 (1981)83. M. Akimoto, Y. Tsuchida, K. Sato and E. Echigoya, J. Catal., 86 (1984) 173.
43 44 45 46
M. Akimoto, H. Ikeda, A. Okabe and E. Echigoya, J. Catal, 89 (1984) 196. T. Komaya and M. Misono, Chem. Lett., (1983) 1177. M. Misono, N. Mizuno and T. Komaya in Proceedings 8th International Congress on Catalysis, Berlin, 1984, Vol. 5, p. 487, Dechema, Frankfurt--Main, 1984. M. Misono, Catal. Rev. Sci. Eng., 29 (1987) 269.
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47 48 49 50 51 52 53 54 55 56 57 58 59
K. Kurzinger, G. Emig and H. Hofmann, in Proceedings 8th International Congress on Catalysis, berlin, 1984, Dechema, Frankfurt-am-Main, 1984. G. Emig, A. Schraut and H. Siegert, Chem.-1ng.-Tech.,60 (1988) 1061. T. Haeberle and G. Emig, Chem. Eng. Technol., 11 (1988) 392. 0. Watzenberger, G. Emig and D.T.Lynch, J. Catal., 124 (1990) 247. V. Ernst, Y. Barbaux and P. Courtine, Catal. Today, l(1987) 167. W. Tonghao, L. Yuchun, Y. Hongmao, W. Guojia, Z. Hengbin, H. Shiying, and Z. Kaiji, J. Mol. Catal., 57 (1989) 193. G.B. McGarvey and J.B. Moffat, J. Catal., 128 (1991) 69. G.B. McGarvey and J.B. Moffat, J . Catal., 130 (1991) 483. G.B. McGarvey and J.B. Moffat, J. Catal., 132 (1991) 100. J.B. Moffat, J. Mol. Catal., 26 (1984) 385. J.B. Moffat, J . Mol. Catal., 52 (1989) 169. J.B. McMonagle and J.B. Moffat, J . Colloid Interface Sci., 101 (1984) 479. B.K. Hodnett and J.B. Moffat, J. Catal., 88 (1984) 253.
DISCUSSION
Q: J. Habet (Poland)
Partial oxidation of methane has been carried out at 570 %. It is well known that the molybdophosphoric and tungstophosphoric acids decompose at much lower temperatures. Do you have additional evidence that deposition of these acids on silica can stabilize them sufficiently so that the Keggin unit could exist under the conditions of the reaction ?
A: J. B. Moffat We have a variety of evidence to support our contention that silica-supported 12-molybdophosphoric acid (HPMo/SiOi) is more thermally stable than the unsupported material. In the partial oxidation of methane with nitrous oxide at 570 % on HPMo/Si02 a linear increase in the rate of formation of various products with increase in loading clearly indicates that the active species are associated directly with the supported materials, namely HPMo. This active species is thermally sensitive since, at temperatures of approximately 650 OC, the activity and selectivity of the supported catalyst abruptly change to those expected from the silica support itself. It is evident that the oxides formed at high calcination temperatures are either inactive or oorly active for the oxidation of methane. Thus the active species are those anions of e in structure which bulk HMPo evidently exist with temperatures up to approximately 650 OC. decomposes at approximately 350-400 OC, the silica support is evidently assisting in the production of an enhanced thermal stability for the supported HPMo. More direct evidence for the existence and stability of the Keggin anion on HPMo/SiO, at 570 OC has come from Raman, X-ray photoelectron and 31P NMR spectroscopy. XPS data show that the Keggin anion can be deposited uniformly on the silica surface in a highly dispersed form up to a coverage of 0.04 KY nm2. Both Raman and 31P spectroscopic results show that the highly dispersed HPMo is stable up to calcination or reaction temperatures of approximately 580-600 %. In contrast Raman spectroscopic results show that the particles of HPMo on the highly loaded catalysts (greater than 0.04 KU nm-2) decompose to form MOO,. Recent in situ Raman studies suggest that the stable species found on the silica surface after heating is the dehydrated Keggin anion formed on removal of an oxygen ion in the process of the desorption of water.
2%rice
1115
Q: M. Sinev (Russia) You have concluded that the presence of protons in "KU" leads to creation of oxygen vacancies. At reaction temperatures there is another possible way for vacancies to be created by reduction of the catalyst with the reactant. What is the relative efficiency for vacancy creation by dehydroxylation as compared with reduction ?
A J. B. Moffat The presence of protons is believed to be necessary for the generation of oxygen vacancies in the Keggin anions by extraction of anionic oxygen to form, water. However at reaction temperatures the reactant to be oxidized will also extract oxygen atoms. Work in our laboratory has shown that, in the absence of a gas phase oxidant, methane is initially oxidized on the metal-oxygen cluster compounds at a rate little different from that observed in the presence of an oxidant. This indicates that the catalyst itself is the primary source of oxygen. Since these observations are limited to a relatively short initial period of time, reduction of the catalyst is evidently occurring. Similar results have been obtained in our work on the conversion of isobutyric acid to methacrylic acid. These results also demonstrate that the reactant is participating in the reduction of the catalyst. The reduction of 12-molybdophosphoric acid/Si02 was also studied by experiments in which the samples were first reduced in hydrogen at 570 % for 1 hour followed by reoxidation in nitrous oxide. These experiments were carried out on aliquots of the catalyst variously exchanged with cesium. The quantity of oxygen required to reoxidize the samples of catalyst decreased linearly with increase in the amount of cesium but became constant at 3-4 cesium atoms per Keggin anion, again consistent with the results obtained in the partial oxidation of methane on 12-molybdophosphoric acid/SiOz variously exchanged with cesium cations.
Q: Y. Wu (China) The monovalent salts of HpA are mostly undissolved in H20. How do you carry out the exchange process between these monocations ? How do you control the exchange capacity ?
A: J. B. Moffat The ammonium and heavy alkali metal salt of 12-molybdophosphoric and 12-tungstophosphoric acid possess very low solubilities in aqueous solutions and hence can be used as inorganic ion exchangers. In our studies we have prepared various of the salts monovalent cations, in particular the ammonium, potassium and cesium salts of the aforementioned metal-oxygen cluster acids. These were contacted with aqueous solutions containing ammonium, potassium or cesium ions using a batch ion-exchange reactor. Although the ion exchange between the liquid and solid phases largely occurred in the first exchange six separate exchanges were carried out for each ion pair studied. Solid and liquid phase analyses to determine stoichiometries, solubilities and the extent of the ion-exchange reaction were made by application of ion chromatography.
Q: B. Viswanathan (India) Your EHMO calculations show that Mo-containing metal-oxygen cluster compounds possess oxidizing power. Can you predict a priori the oxidizing power (relative) of substituted systems (substitutions at MO position) ?
A: J. B. Moffat In principle it is certainly possible to predict a priori the relative oxidizing power of metal oxygen cluster compounds with partial substitution of the peripheral metal elements such as molybdenum. However it must be kept in mind that calculational techniques such as EXH produce quantities which have little or no value absolutely but my be useful on a relative basis. In applications of this latter method to the simulated
1116 12-tungstophosphate anion, it if found that the M o - 0 bond partitioned energy in the totally substituted anion, that is, the 12-molybdphosphate anion, is considerable less than that of the W - 0 bond, This suggests that the terminal oxygen atoms in the latter anion are much more labile, and consequently presumably better able to participate in oxidation processes than those in the former anion. However when substitution of four of the molybdenum atoms by tungsten atoms occurs the W - 0 and M o - 0 bond energies change relatively little. Thus any conclusions as to the relative oxidizing power of the partially substituted anions are probably unjustified.
Q: M. Tasi (The Netherlands) What is so particular about using NH4+ countercation, since no improvement in catalyst performance or surface area could be observed'? A: J. B. Moffat There are two aspects to this question and my reply: one relating to the physical, chemical and morphological properties, the other to the catalytic properties. We have shown that certain of the salts of the rnetal-oxygen cluster compounds, in particular those of the monovalent cations, may be prepared with high surface areas and micro oroushnesoporous structures. Thus, although, for example 1Ztungstophosphoric acid &,PW,,O,) has a surface area of less than 10 m2/g and no pore structure, the otassium ammonium, and cesium salts of this acid can be prepared with surface areas of $0,128 and 163 m2/g, respectively and average micropore radii of 0.9, 1.0 and 1.4 nm, respectively. In contrast the salts of the monovalent cations Na+ and Ag+ have negligible surface areas and no microporous structures. Similar results have been obtained for the solids containing P and Mo, Si and W, and As and W. In addition to the aforementioned morphological properties the ammonium salts of 12-molybdophos horic (HPMo) and 12-tungstophosphoric (HPW) acids are found to have enhanced t ermal stabilities as compared with the parent acids. Although HPMO and HPW decompose at approximately 350-400 OC,their ammonium salts show DTA peaks associated with decom osition at temperatures at least 150 higher. With res ect to the cata ytic properties of the ammonium salts, w e have found that (NH&PW 2 40 has a significantly higher activity for the conversion of mcthanol to higher hydrocarbons than its parent acid (HPW)and further, the former produces considerably larger quantities of saturated hydrocarbons than the latter. Evidences of shape selectivity have been found with the ammonium salt of 12-tungstophosphoric acid in the alkylation of toluene with methanol. Oxidation processes also reflect differences between the parent acid and the ammonium salt. In the oxidation of isobutyric acid to methacrylic acid (MAA) the ammonium salt of 12-molybdophosphoric acid produces a conversion of 49 % at 250 % and a selectivity to MAA of 58 % while the parent acid yields 13 and 50 %, respectively. Although the source of these differences is unclear at this time, it is nevertheless evident that the ammonium salts of the metal oxygen cluster compounds possess advantageous physical and chemical properties which distinguish them from their parent acids.
R
b
P
Q: T. S.R. Prasada Rao (India)
Have you tried the exchange of mixtures of cations like K+and Cs+ or Mo ? If you have, what is the activity and selectivity of these components for the reactions you tested in your study '?
A: J. B. Moffat The analogous effect which would be produced by exposing a solid sample of a metal-oxygen cluster compound to a solution containing two cations can be achieved by
1117 only partial exchange of the cation contained in the solid, thus producing a solid containing two types of cations. In brief, such solids show morphological properties such as surface area and pore structure falling between those expected for the salts of either cation. Similar observations have been made with these partially exchanged solids when employed as catalysts in the conversion of isobutyric acid, although we are continuing to produce data on this latter aspect. Somewhat in contrast, however, are the results from the conversion of methane, althoueh I did not have the time to speak about this in my lecture. Although the primary effect in the exchange of protons by other cations appears to be a reduction in oxygen vacancies, which effect does not depend on the nature or charge of the cation, there are two additional effects, of apparently less consequence. The second effect appears to have an electronic basis and is strongly dependent on the nature of the cation. A third effect is related to the intrinsic activity of the cations themselves.
Q: M. Baerns (Germany) The selectivity of the H isobutyric acid transformation to MAA, propene and acetone is markedly affected by the addition of Cs+ i.e., the exchange of protons, to the Kegginstructure-type metal oxygen cluster compounds. 1) Can you comment on the mechanism of this chemical reaction? 2) Could this reaction be used as a probe reaction for proton acidity (Bransted) and Cs+-inducedbasicity ? A: J. B. Moffat 1) In brief, our tentative mechanism for the isobutyric acid (IBA) process assumes that IBA chemisorbs on the Keggin anion with the double-bonded oxygen occupying an oxygen vacancy on the anion. Loss of a roton then produces the chemisorbed enol form of IBA. Transfer of a proton from one o the methyl groups to a terminal oxygen atom of the Keggin anion produces the chemisorbed methacrylic acid. 2) It is possible to imagine using iBA conversion as a probe reaction for proton acidity. However other reactions such as hydrocarbon cracking may be more suitable for such purposes.
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Guni, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July. 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
CORRELATIONS BETWEEN p-TYPE SEMICONDUCTIVITY AND C2 SELECTIVITY FOR OXIDATIVE COUPLING OF METHANE (OCM) OVER ACCEPTOR DOPED SrTiO3
C. Yu,W. Li, W. Feng, A. Qi and Y. Chen Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116012, China
Abstract In this work some acceptor doped SrTiOg were prepared and tested by OCM reaction, XRD, TPD-MS, electrical conductivity and XPS. C2 selectivity of the catalyst is improved effectively and the p-type semiconductivity is enhanced upon doping. Correlation between them is discussed. The best catalyst prepared is a SrTio.gLio.103-~ sample possessing C2 selectivity 57.3% and C2 yield 16.9%. 1. INTRODUCIION
Over hundred catalysts have been examined for the oxidative coupling of methane reaction (OCM) since the initial work of OCM by Keller and Bhasin in 1982. It has been found that most of the known selective OCM catalysts are composed of alkaline earth oxides, rare earth oxides or their mixture, usually promoted by alkali [1,2]. But the key requirements for good catalyst composition is little understood theoretically, so that the development of practically useful OCM catalyst is still a challenge at present. In early years it was known that OCM catalysts generally possess an intrinsic basic quality although the reason for this trend is not well defined. Recently it has also been suggested that many OCM catalysts are p-type semiconductor at working condition [3]. It has been reported that the original insulator MgO could be changed into a p-type semiconductor by doping with lithium, and consequently a significant increase in C2-production was observed [4]. Likewise, the doping of n-type semiconductor (such as Li-doped ZnO or Ba-doped Ce02) could also result in p-type semiconductivity, thus converting the very poor catalysts to the good ones [5,6]. Dubois and Cameron have recently examined a large number of OCM catalysts and came to the conclusion that most, if not all, of examined catalysts are high temperature p-type semiconductors [3,7]. There are several
1120
papers declaring that perovskites (ABO3) with or without cation substitution exhibit OCM activity, e.g. the C2 yield reported was around 12% for SrZr03 BaZr0.75Mgo.250~PI. It is also known that perovskite-type oxides are noticed for their special characteristics----theirflexibility in substitutional composition variation, their easily modified and sensitive semiconductivity and at the mean time good chemical and thermal stability [lo]. From our work we have found that the doping of SrTiO3 with some of low-valency-elements (such as Mg and Al), substituting partially for the matrix Ti, leads to promotion of the p-type OT: 0-1 and semiconductivity, creation of more mobile oxygen species (E, good 0 2 gas sensitivity [ 11,121, The main aim of this work intended to investigate experimentally the correlation between p-type semiconductivity and selectivity of OCM. Some acceptor (Li, Mg and A1 ) doped SrTiOg were prepared and tested by OCM reaction, XRD, TPD-MS, electrical conductivity and XPS. Li-doped sample SrTio,gLi0,103-i=, has been found to be a very good OCM catalyst. The relation between C2 selectivity and p-type conductivity is discussed. 2. EXPERIMENTAL
Catalysts used were prepared by calcining a powder mixture of SrC03, Ti02 and A1203 or MgO or lithium salts in the desired mole proportions at 120OOC in air for 2 hrs. The calcined material was crushed into granules of 0.9-2.1 mm before use. OCM reaction was carried out with 2 gm catalyst sample at 75OOC in a quartz fixed bed reactor of 13mm in diameter and 300 mm in length. The ratio of CHq:02:He in feed gas was 2.7:1:15.8. The flowrate of gas mixture was 50 mumin. The products were analyzed by gas chromatography using a thermal conductivity detector. Water was removed by a dry ice trap, then the reaction effluent passed through a packed column of carbonmolecular sieve 601 for separating H2, 02, CH4, C2H4 and C2H6 (heavy hydrocarbon negligible). During operation, the catalyst was preheated in He up to 75OoC and then the mixed reactants were introduced into the reactor. For comparison, all data cited were taken at the third hour. C2 represents the total amount of ethane and ethylene produced. For electrical conductivity measurement the calcined material was powdered and pressed into discs of 10 mm in diameter and 1 mm in thickness, followed by sintering at 12OO0C for 6 hrs. Both faces of the disc were covered with Pt film by sputtering. The measurement of sample conductivity was
1121
carried out in 02-N2 gas mixture of different 0 2 concentration. For more experimental details please refer to reference [ 131. TPD-MS experiments were carried out on instrument 5988A - GUMS. The granular sample in quartz tube was preheated in 0 2 stream at 800OC for 1 hr. After cooling to room temperature, the tube was purged by helium for 0.5 hr and then the TPD spectra of oxygen was recorded up to 8OOoC in He stream. The rate of temperature elevation was 16oC/min. The X-ray photoelectron spectra were obtained at room temperature from a Perkin-Elmer PHI - 550 spectrometer with A1 K a X-ray source. The binding energy of T i 2 (458.6eV) ~ ~ ~ was used as an internal standard.
3. RESULTS
3.1. Catalytic behavior SrTiO3 and doped S;Ti1-xMx03-tj (M=Li, Mg or Al) samples were prepared and their catalytic behavior for OCM were tested. In figure 1 the CH4 conversion and C2 selectivity are shown. It is found that the undoped SrTiO3 itself is quite active for deep oxidation of CH4 but the C2 production is very low. Doping with the acceptor impurities in SrTiO3 results in a significant improvement for C2 selectivity while the CH4 conversion is only little effected. It implies that the function of the catalytic site on SrTiO3 is altered upon doping. In the case of the same doping element, the selectivity improves as the doping content increases until it approaches a saturation level. In the case of different dopants, the lower the positive valence of the dopant, the greater is its effect shown (figure 2). With 10% doping from LiOH (SrTio.9Li0.103-tj ) the selectivity and yield of C2 are 50.9 and 13.6% respectively. They are much higher than the top values attained by the samples doped with Mg or Al. Other Li compounds have also been used to prepare SrTio,gLi0.103-r,. The catalytic behavior of those samples are listed in table 1. All of them are good OCM catalysts and the one prepared from Li2SO4 ,exhibits the best quality: the C2 selectivity is 57.3% and the C2 yield 16.9%. Clearly, Li doping is the most effective means to improve the C2 selectivity for OCM catalyst from SrTiO3 perovskite. The stability of a SrTi0.85Li0.1503-8 catalyst for OCM reaction has been studied in a primary test (figure. 3). Its activity and selectivity lower a little bit within the first few hours, then it becomes stable and keeps good until the end of the experiment after 50 hours.
1122
N V
Fig.:! Effect of different dopant in SrTi1-xMx03-6 on the catalytic behavior for OCM 0 CH4 conversion A C2 yield 0 C2 selectivity n
5
0.00 0.05 0.10 0.15 0.20 X in
SrTil-xM 0 x 3
24'20
0-60
f
.rl
-
c
lzE 30
J
20
,"
10
h
d
Fig.1 Effect of dopant content on the catalytic behavior for OCM a. SrTi 1-xAlx03-b b. SrTi 1 - ~ M g ~ 0 3 - 8 c. SrTi l-xLix03-6 0 CH4 conversion A C2 yield 0 C2 selectivity
4
g 0
EI
0
10
20
30
40
50
0
u
Reaction T i m e ( h )
Fig. 3 Catalytic behavior of SrTio.85Lio.1503-6 within 50 hours 0 CH4 conversion A C2 yield 0 C2 selectivity
1123
Table 1 Catalytic behavior of SrTi 0.9Li0.103-6 prepared from various Li compounds Catalytic behavior Li compound CH4 conv. (%) C2 sel.(%) C2 yield (%) Li2SO4 29.5 57.3 16.9 31.2 47.3 14.8 LiNO3 Li2CO3 30.1 46.3 13.9 LiOH.H20 26.7 50.9 13.6 45.9 13.1 LiC 1 28.4 ~~
3 2 Solid structure of SrTi i - x M x O ~ The solid structure of SrTiO3 and especially doped SrTii-xMx03-a is interesting for understanding their catalytic action. First €he crystalline structure of the samples was determined using XRD. The undoped SrTiO 3 is a perovskite of cubic structure. The SrTio.gA10.203-6, SrTio.gMg0.203-tj and SrTio.gLio.103-6 samples prepared are all of the same structure but with a very small amount of impurity phases, xSrO.(x-l)Ti02 and MgO, denoting that the main part of the added dopant incorporates into the perovskite matrix, partially substituting Ti. Figures 4 and 5 are the log-log plots of the electrical conductivity of SrTil-xMx03-g as a function of oxygen partial pressure, P02. It can be seen that in the region P02>1 Pa the conductivity increases with P02, indicating all samples are p-type semiconductors. In our OCM experiments, Po2=5x103 Pa in mixed reacting gas, therefore all catalysts worked under conditions in which they possess p-type semiconducting properties. At a constant P02, the electrical conductivity of sample varies with the dopant content and with the doping element. The data in figure 4 illustrate the effect of doping concentration of SrTi l-xMgx03-a, in which the conductivity increases with the Mg content. From figure 5, we see the log L increments of comparable dopant content are different for different doping elements. The order of the effect on electrical conductivity is Li>Mg>Al. The most effective component for promoting ptype semiconductivity of SrTiO 3 is lithium. For detecting oxygen species the samples were studied by XPS. In figure 6 the results of the SrTi1-xMgx03-6 samples are shown in comparison with the undoped SrTiO3. It is evident that over the undoped perovskite lattice oxygen (B.E. 529.9eV) appears absolutely predominant', whereas over the doped sample a second 0 1 s peak at B.E. 532.4eV contributed from the less negatively charged oxygen species becomes distinguished. The more the Mg content, the
1124
-
Fig. 4 Influence of oxygen partial pressure on the electrical conductivity of SrTi 1 - ~ M g ~ 0 3 - 6 x=o.1 A x=0.2 o x=0.3 o x=0.4
b
lo
-3
GI M 0
il
-4
-5
-1 0
0
5
Log Po2 (Pa)
700 "C h
c3
v
Fig. 5 Influence of oxygen partial pressure on the electrical conductivity of the acceptor doped SrTi 1-xMx03-6 -4 SrTiO3 0 SrTiO.8A10.203-8 1 SrTi0.8Mg0.203-8 -1 0 -5 0 5 A SrTiO.9LiO.103-6 Log Po2 (Pa)
Fig. 6 XPS spectra of 0 1 s for SrTi 1-xMgx03-8 1 . x=o 2. x=o.1 3. x=0.2 4. x=0.3 5. x=0.4 Binding energy (ev)
1125
higher is the second peak. Analogous plots were obtained for Li doped samples. By TPD-MS (figure 7), it has been found that the desorption curve of the structural oxygen species is consistently changed over the doped samples. Contrasting with the profile of the undoped SrTiO3, the major peak around 800OC is lowered and the shoulder peak around 700OC is raised. It seems that a part of lattice oxygen is turned into less strongly bounded species due to the presence of defect structure induced by doping.
Temperature ( 'C)
Fig. 7 @-TPD profiles obtained by MS Ion 32 SrTiO3 SrTi0.8Mg0.203-6 --* SrTio.gLio.103-8 Fig. 8 Reactivity of the oxygen species on catalysts towards CH4 a. SrTiO3 be SrTi0.8Mg0.203-6 c. SrTio.gLio.iO3-6 OC2H6 OC2H4 0 C02
-
--
+,
0
0.02 -c
1
a 0
k Pl
-
3.3. Reactivity of oxygen species on solid For investigating the relationship between the solid structure and its catalytic property further more, the following experiments were carried out. 2 gm fresh catalyst was degassed by purging in He at 5 W C for 30 minutes and at 7oOOC for 45 minutes. Then pure CH4 stream (16ml/min) passed over continuously at 75OOC and C2H6, C2H4 and Co;! were detected. From the results in figure 8, we see CH4 can be oxidized even without gaseous 02 supply though the conversion is low. Over the undoped SrTiO3 only C 0 2 is
1126
produced and the reaction stops after 5 minutes. Over SrTio.8Mgo.203-tj and SrTio.gLio.103-6, no (2% but C2H6 and C2H4 are formed and the activity lasts much longer. The ratio C2Hg / C2H4 is higher over SrTio.gLio.103-5than over SrTio.gMg0.203-6. Thus it is suggested: CH4 can be oxidized by oxygen essentially retained on the perovskite catalysts of both doped and undoped. But after doping with the acceptor impurities the function of the active oxygen species is evidently changed. Further by intermitant purging in CH4 and in He stream over a SrTio.gLi0.103-6 sample, we have found the activity lowered in continuous reaction can be recovered after a break. It verifies that the oxygen species in the bulk solid can actually diffuse to the catalyst surface and are available for the reaction. 4. DISCUSSION
The replacement of Ti by the acceptor impurities requires the presence of oxygen vacancies in order to attain electroneutrality. This is illustrated using Krtiger-Vink’s notations [ 141. A1203
(-2TiO 2) ----------,
2 A f ~ i + VO +300
The oxygen vacancy favours the accomodation of 0 2 and the generated h+, as an electron acceptor, promotes the transformation of lattice oxygen to non-fully reduced species, such as 0-, @--or Of. ( It is denoted as 0- for simplicity.) These two processes obey the mass action law [15].
Thus the presence of such dopants will increase the concentration of charge carrier and non-fully reduced oxygen and enhance the oxygen exchange rate and therefore the catalytic action of the catalyst.
1127
Fig. 9 Relation between the Cz selectivity and electrical conductivity of the catalysts a. SrTi I - ~ M ~ X O ~ ~ b. SrTil-,Mx034 0 x=o.1 0 x=0.2 0 M=O M=Alo.2 o x=0.3 o x=0.4 o M=Mg0.14.4 0 M=Lio.l
By correlating log L and selectivity of the catalysts, a good linear relation is obtained with SrTi1-xMgx03-a (x=O.l-0.4), as shown in figure 9a. When we examine the data of the samples with different dopant, Li, Mg or Al, together with the undoped SrTi03 we can also find that their catalytic selectivity improves generally with the increase of log L (figure 9b). But the data are dispersed to some degree. In this case it appears more complicate. It may be caused by the variation of K1 and K2 and also by the deviation of the oxygen exchange rate and charge carrier mobility, which in turn are composition dependent, effected by the valence and the radius of the doping ion. Anyhow, the correlation of C2 selectivity for OCM over the doped SrTilxMx03-a with its p-type semiconductivity probably suggests that certain charge transfer processes might be significant in OCM reactions. Further interpretation will be concerned in our future work. 5. CONCLUSION
1. The perovskite SrTiO3 is active for CH4 oxidation and acceptor dopants are effective in improving its Cz selectivity for OCM. The lower the valence of the added cation, the greater is its effect.
1128
2. The best catalyst prepared is a Li-doped sample, SrTio,9Lio.l03_tj, possessing C2 selectivity 57.3% and C2 yield 16.9%. 3. The presence of acceptor dopant in the perovskite matrix improves the ptype semiconductivity of SrTi 1-xMx03-5 and increases the concentration of the non-fully reduced oxygen on the catalyst. 4. The C2 selectivity for OCM over SrTil-xMxOg~tjcatalyst improves as the p-type semiconductivity is increased by doping.
6. A
C
K
"
T
This work was funded by National Natural Science Foundation of China and State Key Laboratory for Catalysis, Dalian Institute of Chemical Physics. The authors thank Z. Qu for measuring the electrical conductivity and also S. Sheng and H. Chen for XPS experiments.
7. REFERENCES [l] Y.Amenomiya, V.I. Birss, M. Goldedzinowski, J. Galuszka and A. R. Sanger, Catal.
Rev.-Sci. Eng. 32 (1990) 163. [2] J. S. Lee and S. T. Oyama, Catal. Rev.-Sci. Eng. 30 (1988) 249. [3] J. L. Dubois and C. J. Cameron,Appl. Catal. 67 (1990) 49. [4] T. It0 and J. H. Lunsford, Nature 314 (1985) 721. [5] I. Matsuura. Y.Utsumi, M. Nakai and T. Doi, Chem. Lett. (1986) 1981. [6] K. Otsuka, Y.Shimizu and T. Komatsu, Chem. Lett, (1987) 1835. [7]A. Kooh, J. L. Dubois, H. Mimoun and C. J. Cameron, International Chemical Congress of Pacific Basin Societies, Preprints of 3B Symposium, Honolulu, December 1989, paper 86. [8] H. Nagamoto, K. Amanuma, H. Nobutomo and H. Inoue, Chem. Lett. (1988) 237. [9] W. J. M. Vermeiren, I. D. M. L. Lenotte, J. A. martens and P. A. Jacobs, Studies in Surface Science and Catalysis vol. 61, Natural Gas conversion p. 33 (1991) elsevier. [lo] E. J. Baran, Catalysis Today 8 (1990) 133. [ l l ] Chunying Yu. Y. Shimizu and H. Arai, Chem. Lett. (1986) 563. [12] Chunying Yu, Y.Shimizu and H. Arai, J. Mater, Sci. Lett. 8 (1989) 765. [13] Chunying Yu, Y. Shimizu and H. Arai, Sensors and Actuators 14 (1988) 309. [14] F. A. Kridger and H. J. Vink, in "Solid State Physics," vol. 3, F. Seitz and D. Turnbull, Editors, Academic Press, New York (1956). [15] N. -H. Chan, R. K. Sharma and D. M. Smyth, J. Am. Ceram. SOC.65 (1982) 167.
DISCUSSION Q: M. Sinev (Russia) You have presented your data from the point of view of "collective" properties of the oxide. But the state of oxygen ions (or, in other words, local properties) is responsible for catalytic properties. Do you have any evidence for the importance of p-semiconductivity itself or there is just a secondary correlation between semiconductivity and catalytic performance ?
1129 A: Li Wenzhao You may notice that the creation of non-fully reduced oxygen species which is responsible for the C, selectivity was just caused by the p-type semiconductor character. Therefore, p-type semiconductivity should not be considered as a secondary factor affecting the catalytic performance. Author believes that over most of p-type semiconductor oxide catalysts its surface "localized" properties is to a certain extent influenced by the bulk properties, and there might exist a balance (or correlation) of oxygen species between the surface and the bulk. Q: C. Cameron (France) It is important to recognize that p-type conductivity is a bulk property and not a surface phenomenon. However this bulk property may have an important influence on surface chemistry. P-type conductors are, by definition, above-stoichiometric in oxygen (i.e, MntO~yt,) I n an oxygen containing atmosphere. This type of solid will therefore enable sign1 ican oxygen chemisorption, which is extremely important for an EleyRideal kinetic mechanism where gas phase methane reacts with activated surface oxygen ions. Oxygen ion mobility is also important, but for increased activity and not necessary for increased selectivity.
A: Li Wenzhao No answer.
Q: K. Aika (Japan) We may need several assumptions between the reactivity and bulk properties. 1) Have you measured the conductivity as a function of temperature and how does it relate with C selectivity as a function of temperature '? 2) How To you identify one of XPS peaks with 0-species ? Defects and 0-may be rich at high temperature reaction condition but may be poor at room temperature (XPS condition).
A: Li Wenzhao 1) Yes.The conductivity data measured at the temperature range from 25 to 800 @2 indicated that the conductivity increased with the temperature and a good linear relationship between logarithm of conductivity and l/T under 600-800 % was observed. We correlated the C, selectivity with the p-type conductivity only at reaction temperature (700-750 OC). 2. We defined the second detected 0 1s peak at B.E. 532.4 eV only with the term of "less negatively charged oxygen species (02-,0 2- or 0-)'I, which are difficult to be distinguished from each other by XPS alone. %mbining the XPS data with other obtained results in this work, it could be concluded that above mentioned oxygen species were mainly contributed from the acceptor doping.
Q: K. Dooley (USA) Your Li-doped SrTiO, was more selective than the other materials you prepared. What evidence do you have to exclude the possibility that the active phase in the Li-containing materials was not a surface layer of LiC03 or some other Li-containing carbonate ? Note that the carbonate may have been formed in situ, even though the doped titanium oxides were calcined at very high temperatures.
A: Li Wenzhao By XRD measurements we have not observed over the catalysts any detectable amount of the structure of lithium carbonates either before or after the OCM reaction. The fact that no loss of lithium has been found during a 50 hour test might also be a evidence that most of lithium has been incorporated into the perovskite structure.
1130
Q: F. Solymosi (Hungary) I would like to join to the discussion concerning the carbonate formation on Li-additive catalyst. As the author pointed out the doped samples have been sintered at 1200 OC.As a result of the pretreatment Li ions have been incorporated into the lattice, so only its very small fraction is in the surface layer. In the other words, there is no "free" Li', therefore carbonate formation in the presence of c02during the cooling of the sample should be very limited.
A: Li Wenzhao No answer. Q: P. G. Menon (Sweden) Are the selectivity values, reported b What was the methane conversion in your
at comparable levels of conversion '? (3M2ou,experiments ?
A: Li Wenzhao The methane conversion for OCM reaction over the undoped or d o ed catalysts selectivity tested in this work was generally maintained at 26-30 % level while the shifted within the range of 10-50 %.
8,
Q: I. S . Metcalfe (United Kingdom) You mentioned a linear correlation between selectivity and the logarithm of conductivity (see Figure 9a). However, the valuation in selectivity is quite modest. How much uncertainty is there in your measured values for the selectivity ?
A Li Wenzhao The relative experimental uncertainty in C2 selectivity measured values is estimated to be
IM$lSia
232.0 232.1 232.2 232.4
7.5 14.4 14.1 9.8
Corrected to account for the changes of surface area.
3.2. Activity data The reaction of CH4 with 0 2 on MoO3fSiO2 catalysts under the given conditions leads mainly to the formation of formaldehyde and COX,with minor amounts of dimerization products and methanol. The results obtained for a given set of Table 2 Influence of the temperature on activity and selectivity for the selective oxidation of CH4 on the catalyst with 0.8 Mo nm-2 T(K)
823 843 863 883 903
XCH4
0.1 0.5 1.3 3.2 5.0
Selectivity (%)
HCHO
co
co2
C2H6
CH30H
63.2 51.7 .34.4 22.6 12.5
36.6 42.9 59.8 69.2 76.0
5.3 5.7 8.0 10.5
t 0.9
0.1 0.1 0.1
1138
experimental parameters were reproducible in the sense that the conversion and selectivity were practically the same when the experiment was duplicated. The temperature dependence of the CH4 conversion and the HCHO selectivity for the 0.8 Mo nm-2 catalyst is summarized in Table 2. The increase in the reaction temperature resulted, as expected, in an increase of the methane and oxygen conversions but in a decrease of the selectivity towards selective oxidation products in favour of the deeper oxidation (COX)and dimerization products. A similar tendency has already been observed for the same catalyst when increasing the methane residence time while maintaining the reaction temperature (883 K) (Table 3). It can also be noted in Tables 2 and 3 that the disappearance of formaldehyde is accompanied by the formation of a much larger proportion of CO on the CH4 conversion and product distributions have been extended to the other on the CHq conversion and product distributions have been extended to the other catalysts compiled in Table 1. From the complete set of results, both the CHq conversion and HCHO selectivity were found to depend on the Mo-loading. Specifically, a volcano-shaped curve with a maximum for the catalyst 0.8 Mo nm-2 was obtained when the HCHO selectivity is plotted against the Moo3 content. Table 3 Influence of the methane residence time on activity and selectivity for the selective oxidation of CHq at 883 K on the catalyst 0.8 Mo nm-2
1 .o 3 .O 6.O
0.5 3.2 6.7
HCHO
co
c02
C2b
CH30H
58.5 22.6 8.6
36.5 69.0 72.6
5 .O 8.1 17.6
t
0.8
0.1 0.1
3.3. Oxygen isotopic analysis As already confirmed by flow microreactor tests, the Moo3 /SiOz catalysts were shown to promote the selective oxidation of methane with dioxygen. In order to analyze the different products resulting from the C& oxidation with 1602, a mass spectrometric analysis of the gas phase was performed. The concentration changes of the reactants CHq and 1 6 0 2 and of the products (2160, HCH160 and C1602 upon contacting a mixture CH4:1602 = 5 molar at 873 K over the catalyst 1.9 Mo nm-2 are shown in Fig. 3. Data in this figure illustrate the increases in C W , and in particular of C1602, concentration with the time of reaction while both CH4 and 1 6 0 2 are continuously consumed. Note also that the HCH160 concentration initially increases up to reach a plateau and then decreases for longer reaction times. These observations would indicate that formaldehyde becomes consumed and/or the catalyst is deactivated. It this respect, it is noteworthy to mention the absence of CH3160H (m/e = 32) which would mask the detection of HCH180 (m/e = 32) when using 1802 instead 1602 as oxidant.
1137
P(a.u .) 0.04
--'--
.,,,..,....
I.....).,
,
....
,,..,.."'
.... co ..., "*
0.21
0
10 20 30 40 Reaction time (min)
0
10 20 30 40 Reaction time (min)
Figure 3. Changes of the gas composition following introduction of 100 Tom CHq + 1602 (CH4:02 = 5 molar) to catalyst 1.9 Mo nm-2 at 873 K for reactants (a) and products (b). Figure 4 compiles the changes of concentration of the reactants and products when a CHq and 1802 reactant mixture is used, From these results it is clear that no HCH180 is observed in the products. This observation unambiguously indicates that oxygen incorporation into formaldehyde occurs via lattice 0 2 - ions of the supported Moo3 phase, and that only the gaseous 1802 will be involved in the reoxidation of catalyst surface. Accordingly, the isotopically labeled HCHlRO should only be expected to appear at very long times of reaction. It can also be noted that C160 (or C1602) concentration is much higher than that of CIS0 (or C1802).For complete oxidation, a superficial process which incorporates gaseous oxygen is expected, leading to formation of CIS0 and C1802 .Subsequently in this case, COXappears to be formed from primary partial oxidation products. In order to verify the above observations, CHq was also oxidized with 1 6 0 2 on the same catalyst, in which one third of the oxygen atoms had been replaced by a first HZreduction at 773 K followed by reoxidation at the same temperature with 1802. The changes of H C H W (nde = 29) and HCHl8O (m/e = 32) concentrations with the time of reaction are shown in Fig. 5 . It can be pointed out that formaldehyde will incorporate in the first instance lattice 1802- placed in those formaldehyde will incorporate, in the first instance, lattice 1802- placed in those regions of molybdena crystals close to the surface, followed by those located more inside. As it has been demonstrated that the Moo3 is almost quantitatively reduced to Moo2 at 773 K [22], then upon reoxidation with 1 8 0 2 about 1/3 of the lattice oxygens (M01602180) are incorporated in the molybdena crystals. Accordingly, one would expect that the
1138
P(a.u.) 0
..I..:,
P( a.u .)
---1
I
I
!
...... ...............
HCH160
I 0
----..
.......................
1 8 0 2 .. ..............,................ 10 20 30 40 Reaction time (min) L..
,
............
0
...........
__c__
10 20 30 40 Reaction time (min)
Figure 4. Changes of the gas composition following introduction of 100 Ton C& + 1802 mixture (CH4:02 = 5 molar) to the catalyst 1.9 Mo nm-2 at 873 K for reactants (a) and products (b).
HCH180/HCH160 ratio should be 1/3, but it is much lower. This fact can be explained assuming that the oxygen release from the molybdena crystals is essentially confined to the region of the Moo3 crystals close.to the surface, which might be oxygen -18 enriched during reoxidation of the H2-reduced sample. If a comparison is made between the data of Fig. 2 and 3, it is clear that the incorporation of oxygen atoms into formaldehyde molecule proceeds largely through oxygen release by Moo3 crystals. Therefore, this reaction follows a Mars-van Krevelen mechanism, as already proposed for the selective oxidation of olefins on bulk or supported Moo3 catalysts (23). As occurs for other metal oxides, the exchange reactions between the reactants and/or the products, i.e. the isotopic exchange of oxygen (IEO)and carbon dioxide (EC),would simultaneously be involved. These two possibilities have been analysed in separate experiments. In the first one, the masses corresponding to 1602 (m/e = 32), 1 8 0 1 6 0 (m/e = 34) and 1802 (m/e = 36) have been recorded once the 1802 gas phase was closed-recirculated through the catalyst bed at 873 K. In no case were the masses 32 and 34 detected, which excludes the participation of the IEO reaction. In the second one, the exchange of C1602 was studied on a Mo1801602/Si02 sample for which the starting 1 8 0 atoms in molybdena to the 160 atoms in carbon dioxide were close to 3.Figure 6shows the dependence of the masses 44 (C1602), 46 (C180160) and 48 (C1802) with the time of reaction. The observation of the mass 46 indicates clearly that the IOC takes place, however, as judging from the intensity of this mawits extent is very low. Consequently, the conclusions drawn from the experiments of the direct oxidation of methane are not altered.
1139
p(a.u.) 7 - 1 0 . 0 4
P(a.u.)
0.2C160180
Reaction time
(min)
Figure 5 . Changes of the HCH160 and H C H 1 8 0 compositions following introduction of 100 Torr CH4+02 (CH4:02 = 5 molar) mixture to the 1.9 Mo nm-2 catalyst at 873 K in which ca. 1/3 of 160-atoms have been replaced by 180-atoms.
0
L:
,
.c... ............ .
..,,_ ,.....
20 30 4 0 Reaction time (min) 10
Figure 6. Changes of the C1602, C 1 6 0 1 8 0 and C I S 0 2 compositions following introduction of 100 Torr C1602 to the same catalyst at 873 K.
4. CONCLUSIONS
The combined results of LRS, XPS and IR of NO chemisorbed on H2-reduced MoOs/SiO2 catalysts show that up to Mo-contents of 0.8 Mo nm-2 the catalysts consist of very small molybdenum oxide clusters weakly interacting with the carrier surface, whereas crystalline molybdenum (VI) oxide particles are developed at high Mo-contents. By comparing activity data and molybdena dispersion, as evaluated by the X P S Mo/Si intensity ratio and the absorbance of the infrared NO band at 1710 cm-1 (cf. Fig. 2), it can be established that there is a correlation between these two parameters. This means that the rate of reaction is determined by the number of sites which are located in the Moo3 clusters or particles. The isotopic tracer study used in this work was very conclusive in identifying the mechanistic pathway by which the oxygen is incorporated into the methane molecule to yield formaldehyde. The absence of HCH180 product in the oxidation of CHq with 1 x 0 2 and its observation in the oxidation of Cf14 with 1 6 0 2 over the 1.9 Mo nm-2 catalyst, i n which ca. 1/3 of the O-atoms were deliberately replaced by oxygen-18 (M01602 ‘go), demonstrate that incorporation of O-atoms into formaldehyde proceeds largely through oxygen release from Moo3 crystals. The reduced catalyst is further reoxidised by the gaseous oxygen, thus closing a redox cycle as the Mars-van Krevelen mechanism assumes.
1140
5. ACKNOWLEDGMENTS This research was supported by the CICYT (Spain) (Grant MAT 88-0239). One of us (M.A.B.) thanks to Ministry Of Education and Science for a fellowship.
6. REFERENCES 1 2 3 4
5 6 7 8 9 10
11 12 13 14 15
16 17 18 19
20
21 22 23
M.J. Brown and N.D. Parkins, Catal. Today, 8 (1991) 305. M. Yu. Sinev, V.N. Korshak and O.V. Krylov, Russ. Chem. Rev., 58 (1989) 22. K. Otsuka and M. Hatano, J. Catal.,l08 (1987) 252. H.R. Gerberich, A.K. Stauzenberger and W.C. Hopkins, in Concise Encyclopaedia of Chemical Technology, H.F. Mark et at. (eds.), Wiley, New York; 1990, p. 528. G. Kastanas, G. Tsigdinos and J. Schwank, 1989 Spring National AIChE Meeting, Houston, Texas, April 1989, Paper 52nd. N.D. Spencer, US Patent No. 4 607 127 (1986). E.Y. Garcia and D.G. Loffler, React. Kinet. Catal. Lett., 26 (1 984) 61. N.D. Spencer, J. Catal., 109 (1988) 187. S . Kasztelan and J.B. Moffat, J. Catal., 106 (1987) 51 2 . J.B. Moffat and S. Kasztelan, J. Catal., 109 (1988) 206. Y. Barbaux, A.R. Elmrani, E. Payen, L. Gengembre, J.P. Bonnelle and B. Grzybowska, Appl. Catal., 44 (1988) 117. J.C. Mackie, Catal. Rev.-Sci. Eng., 33 (1991) 169. P. Mars and D.W. van Krevelen, Chem. Eng. Sci., Suppl., 3 (1954) 41. T.J. Yang and J.H. Lunsford, J. Catal., 103 (1987) 5 5 . H.C. Yao and W.G. Rothschild, in Proceedings of the Climax Fourth International Congress on the Chemistry and Uses of Molybdenum, H.F. Barry and P.C.H. Mitchell (eds.), The Climax Molybdenum Co., Ann Arbor, Michigan, 1982, p. 3 1. J.B. Peri, J. Phys. Chem., 86 (1982) 1615. N.Y. Topsoe and H. Topsoe, J. Catal., 75 (1982) 354. J.L.G. Fierro, in Spectroscopic Characterization of Heterogeneous Catalysts, Part B: Chemisorption of Probe Molecules, J.L.G. Fierro (ed.), Elsevier, Amsterdam, 1990, Vol. 57B, p. B67. M. de Boer, J. van Dillen, D.C. Koningsberger, F.J.J.G. Janssen, T. Koerts and J.W. Geus, in Proceedings 3rd Workshop Meeting on Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, 1991, Vol. 1. C. Rocchiccioli-Deltcheff, M. Amirouche, M. Che, J.M. Tatibouet and M. Fournier, J. Catal., 125 (1990) 292. S. Kasztelan, E. Payen and J.B. Moffat, J. Catal., 112 (1988) 320. M.A. Baiiares and J.L.G. Fierro, An. Quim., 87 (1991) 223. A. Guerrero-Ruiz, J. Massardier, D. Duprez, M. Abon and J.C. Volta, in Proceedings 9th International Congress on Catalysis, M.J. Phillips and M. Ternan (eds.), The Chemical Institute of Canada, Ottawa, 1988, p. 1601.
1141
DISCUSSION Q: B. K. Hodnett (Ireland)
1) Please comment on the volatilization of molybdenum oxide in your reaction conditions. 2) Formaldehyde formation from methane involves formation of water according to: CH, t 2 0
-+
HCHO t H20
You report HCH160 only is formed when 1802 is used as the oxidizing agent. Do you observe the formation of H l6O only ? Do you envisage HCH160 formation during a single interaction between &I4 and the molybdenum or have considered an alternative such as:
-
CH, t 0 CH t OH. CH3. t 0 HC&O t H. OH. t H. 4 H2O -+
A: M. A. Banares Under the experimental conditions employed in this work the extent of molybdena volatilization ap ears to be very small. This is consistent with the extremely low vapor pressure of M o 8 at typical reaction temperatures of 873 K, and also confirmed by the almost constant ?PS intensity ratios IMlo/Isi for both fresh and used catalysts. Concerning the uestion of the formation of water, our isotopic studies indicate that both Hz160 and thus indicating a H21 0 molecules are formed when methane is oxidized with com lex reaction pathway for water molecules. However, the depenknce of Hz160 and H21B0 concentrations with time is quite different. The former is very small and increases steadily with time, the latter is much higher at very short times and then almost levels off. These trends can be rationalized assuming that methane combustion (with formation of water), via a suprafacial process with molecularly adsorbed l80 is operative at very short reaction times. Then, when the carbon dioxide is formed t 6 s process is strongly inhibited and simultaneously HCH160 and H l60are formed with participation of lattice 160. Only when the catalyst is reoxidized by 1802(g) after longer reaction times may the contribution of H l8O via lattice oxygen to the total water formed become significant. Concerning tie two suggested possibilities for the activation of methane molecules our results exclude participation of the radical mechanism. According to this mechanism concentration since one would expect higher HCHO selectivities of MoO@iO2 H-abstraction is the rate determining step. However, the radicals at the reaction catalyst with CaO, a well known catalyst we envisage HCHO temperature yielded essentially combustion formation during a single interaction between CH, and lattice oxygen.
9
Q: A. Parmaliana (Italy) 1)Have you tested the reactivity of the bare SiO you have used ? 2) Have you any evidence about the effect of the CH4/02 ratio on the product selectivity ? Did you operate in 0 2 deficient conditions ? 3) How can the reactivity of the bare SiOz be explained on the basis of the conventional two-step redox mechanism ?
A: M. A. Banares Blank experiments of the selective oxidation of methane under typical reaction conditions, e.g., atmospheric pressure, 873 K and C H d 0 2 = 10 molar ratio, either with empty reactor or with silica did not reveal the appearance of formaldehyde. These results are somewhat different from yours and other literature reports in which moderate
1142 methane conversion at reasonable formaldehyde selectivity were noted. The reasons for these discrepancies are not clear, although the presence of impurities (Na, Ca, Fe, Ti) and the surface texture of the silicas, and the carbon atoms deposited on the surface, when working under oxygen lean conditions, may play some role in the selective oxidation. It is true that the CH4/O2 ratio modifies the product distribution in partial oxidation of methane but only at very high C H 4 0 2 ratios. Under these circumstances the gas phase oxygen cannot resolve the appropriate level of catalyst oxidation since the oxygen diffusion is too small. This effect has already been noted and studied. However, in order to avoid these problems we decided to use CH,402 = 10 molar but always working at conversion levels of oxygen typically no higher than 60%.
P )
Q:K.Klier SA Your 0 experiments showing incorporation of surface lattice oxygen into the formaldehyde product are very interesting in view of several reports in the literature which indicate that formaldehyde may be formed by a gas phase free radical reaction. The importance of your experiment depends upon whether the '80 balances support your is not instantly converted to l6O2(g) conclusions. Are you sure that the injected l80 by a very rapid exchan e with MOO such tha?(@02(g) is in fact the oxidizing agent ? Can you present 1802, k0180and analyses during the course of methane oxidation to formaldehyde ?
16bz
A: M.A. Banares First of all, we appreciate your approval of our data supporting a redox mechanism for the selective oxidation of methane to formaldeh de. Concerning your questions on a rapid l80 exchange with lattice '60 when d * ( g ) is injected, we exclude this possibility. In reliminary experiments admixtures of l80 (s) pulses into the reactor containing Mo $30 did not show significant oxygen excgange neither at short nor at long exposure times. h i s behavior is quite different from many other semiconducting oxides (ZnO, TiO2, etc.) which usually show wide oxygen isotopic exchange even at moderate temperatures. Also, no oxygen exchange was observed when CH4 was present. On the contrary, carbon oxides, mainly CO d o exchan e with lattice oxygen of MOO,, as confirmed by monitoring C l 6 0 C166180 and El8O2 masses upon ex sing a to a molybdenum oxide catal st to whicgca. 1/3 of the 0-atoms were replaced by ga? phasc consisting of Therefore, we can conclude that under our experimental conditions carbon oxides instead of oxygen exchange with lattice oxygen.
8
%,
Q: U. S. Ozkan (USA) I have a comment and a short question. My comment is in response to a question iaised earlier. We have also been working on partial oxidation of methane to formaldehyde in our laboratory he same catalyst system. I d o not think the absence of methanol in the product system is a conclusive evidence which would rule out the possibility of formation of methanol as a reaction intermediate. We have also done methanol oxidation experiments over the same catalysts. Methanol is extremely reactive under the conditions used for methane oxidation. Therefore,it is not possible to exclude methanol as a reaction intermediate based on this evidence. My question is related to formation of CO and COZ: Can you comment on the possible mechanisms for the formation of CO and C02 using your experimental data '?
A: M. A. Banares Concerning the comment we would like to add only that we describe the appearance of trace amounts of methanol in the flow system or the absence in the recirculating apparatus as an experimental fact, however, the demonstration of this formation as intermediate prior, to transformation to formaldehyde is extremely difficult, if not impossible, with this set of data. Our main objective was to demonstrate that the selective
1143 oxidation of methane into formaldehyde occurs through a redox mechanism, the oxygen incorporated to formaldehyde comes from Moo, lattice. Concerning the question on the mechanism of COXformation, our kinetic results point to a reaction scheme:
As HCHO and CO selectivity trends are always opposite it appears that CO is a secondary product, however the C02 selectivity trend is quite complex. At very low CO selectivity, C 0 2 attains typical values of ca. 8 %, which suggests that CO, is a primary product (combustion). However, at higher methane conversions, when small amounts of HCHO are observed the CO selectivity tends to be constant or even decreases, while CO steadily increases. Therefore, another fraction of CO, may be formed via oxidation of CO (secondary product).
Q: 0. V. Krylov (Russia) I agree with your statement that MoO$%O2 is not a fully understood system. But it is rather selective compared with other systems. Activity is low. CH20 yield is not higher than 2-3 %. Such limit of yield can be explained by reactions in gas phase, that is by heterogeneous-homogeneous mechanism. You have explained your results by formation of olymolybdates at the surface. But polymolybdates can be formed also on Mo8-#AI2O . However, Mo03/A1203 is much less active than MoOfiiO,. Can you explain this .
4
A: M. A. Banares It is well established that molybdenum oxide develops a highly dispersed oxide phase, mainly bidimensional polymolybdates on the alumina surface, but conversely its dispersion is rather smaller on silica. Although the structure of the molybdenum oxide spread on both supports is similar, acidity differences exist between the two type of catalysts. Thus, while MoOdSi02 type catalyst is not acid, the MoO/A120 counterpart is rather acidic. As it is known, acidity should enhance the formation o? some sort of intermediates "methoxy-type" which may act as precursors in combustion reactions. This interpretation would explain the non-selective behavior of the latter catalysts.
Q: J. B. Moffat (Canada) 1) We have shown [ l ] that S i 0 2 has activity for the conversion of methane. 2) We have shown [2] that 12-molybdosilicic acid can form on MOO You have attributed the catalytic activity of M o 0 f i i 0 2 in the met process to a polymolybdate. Can you be certain that the aforementioned Si-Mo compound has not formed and is not responsible for the catalytic activity in this process ? S. Kasztelan, J. B. Moffat, J. Chem. SOC.,Chem. Commun., 1663, 1987 [l] S. Kasztelan, E. Payen, J. B. Moffat, J. Cqtul., 112, 320 (1988) [2] A: M. A. Banares A blank experiment using Si02 alone showed an extremely low activity, but the reaction products were only carbon oxides and water. Therefore, it appears that in our experimental conditions the silica carrier does not yield formaldehyde. Concerning the possibility of 12-molybdosilicide acid formation on catalyst surface during preparation and activation, our Raman data and infrared spectra of chemisorbed NO d o not support such a possibility. An explanation of this difference may be the high temperatures used in
1144 sample degassing or in catalytic reaction, typically 870 K, at which these heteropolymolybdates are not stable.
Q: Can Li (China) Have ou observed any change of the molybdena dispersion under reaction conditions i.y The so-called molybdenum oxide clusters may be more easily prepared on rts, such as Al,203. Have you tested the similar reaction on the molybdenum Other oxide wit other supports instead of SiO, ? A: M. A. Banares We have used X-ray photoelectron spectroscopy to monitor changes of the molybdenum oxide dispersion on the reaction conditions. We have recorded the XPS intensity of the Mo 3d peak relative to that of the Si 2p peak for all of the fresh and used catalysts, taking this ratio as an estimate of M o o 3 dispersion on the silica carrier. It was observed that the 1 ~ ratio8was essentially ~ unchanged for the fresh and used catalysts, which indicates the a sence of M o o 3 redistribution and/or evaporation phenomena. Alumina and zeolite-supported MOO catalysts have also been prepared and used in the selective oxidation of methane. Unzrtunatel y, alumina leads to methane combustion, while MoO,-zeolites provide small amounts of formaldehyde.
Q: I. E. Wachs (USA) I would just like to comment that Raman s ectroscopy reveals the same surface molybdenum oxide s ecies is present on both 4 0 2 and A120 supports at low Mo loading (see p. 5 4 3 Thus, the different catalytic properties of on Si02 and A1 0, must be due to the different environments around the Mo centers. The alumina surkce ossesses Lewis acid sites and surface hydroxyls, and the silica surface does not posses wis acid sites as well as a low concentration of surface hydroxyls.
p.
ko
Ee
A: M. A. Banares Thank you for your comment. In line with your reasoning we would like to add that that surface acid sites have a negative effect on the production of formaldehyde via the selective oxidation of methane. This tendency has been confirmed in our laboratory using a M003/HY zeolite catalysts, which showed much lower selectivity to formaldehyde than the Mo0@iO2 catalysts (Zeolites, in press).
Guczi, L . et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
ON THE MECHANISM OF XYLENE ISOMERIZATIONAND ITS LIMITATIONS AS REACTION TEST FOR SOLID ACID CATALYSTS A. Cormaa, F.Llopisb and J. B. Montonb
aInstituto de Tecnologia Quirnica, UPV-CSIC, Camino de Vera s/n, 46071 Valencia, Spain bDepartarnento de Ingenieria Quirnica, Universidad de Valencia, Burjassot, 46100 Valencia, Spain
The kinetics of the gas-phase isomerization of m-xylene, over a series of ultraestable Y zeolites, has been studied in a fixed-bed flow reactor under initial conditions. Using deuterated p-xylene as reactant it has been seen that more than 20% of the m- and o-xylene obtained, are formed via a bimolecular mechanism. The relative proportion of uni- to bimolecular mechanism depends on the zeolite composition. The relation between the turnover number, calculated from the kinetic rate constant for the unimolecular mechanism, and the framework Al content of the zeolite, was coincident with the topological zeolite model. Besides, the values of the adsorption constants indicate that dealumination has an influence on the adsorption capacity of the zeolite for m-xylene, specially visible at high Si/AI ratios, coincident with the result of direct adsorption experiments of m-xylene, which in turn could explain the observed changes in the ratio uni- to bimolecular xylene isomerization upon dealu minat ion.
INTRODUCTION The isomerization of meta-xylene to para- and ortho- isomers is an important petrochemical process [I]. Besides its technological interest, the isomerization of xylenes presents a clear advantage to characterize molecular sieve catalysts, since the kinetic diameter of the reactant and products are in the range of the pores of these catalysts. Then, the kinetic diameter of para-, meta- and ortho-xylene are 6.7, and 1,3,5-trimethylbenzene 7.1 and 7.4 8, respectively; while those for 1,2,4-; 1,2,3(TMB) are 7.4, 8.1 and 8.6 8, respectively. This reaction has also received considerable attention due to the fact the xylenes are used as model compounds in the elucidation of reaction mechanism for
1146 the positional isomerization of alkylaromatics. The mechanism generally accepted, involves the protonation of the benzene ring followed by a 1,2 methylshift (2-41. Others, even accepting the 1,2 methylshift mechanism, propose a triangular scheme for the, isomerization, which was explained on the basis of consecutive isomerization reactions occurring before the products came into the gas stream [5,6]. Finally, based on products distribution, a bimolecular mechanism has been suggested for isomerization of xylenes and diethylbenzenes [7,8]. Besides the disparity of the mechanisms proposed in the literature, it is now generally accepted that the isomerization occurs by a unimolecular 1,2 methylshift, while transalkylation, to give toluene and TMB, takes place via a bimolecular mechanism, i.e. two molecules of xylenes are involved in the transition state. It is under these assumptions that the xylene isomerization and the accompanying transalkylation are being used to characterize pore structure and cavity dimensions when the reaction takes place in confined spaces, as well as to characterize acid site density in any type of solid acid catalyst [9]. In the case of zeolites, where the number of acid sites can be directly related to the number of tetrahedral framework aluminums, changes for the isomerization to transalkylation ratio observed on samples with different framework Al contents, have been related to the density of acid sites [lo]. This was done on the assumption that isomerization occurs through one molecule adsorbed on one acid site, while transalkylation needs two molecules adsorbed on two adjacent sites [l 11.
If it appears clear that xylene isomerization occurs through a 1,2 methylshift mechanism in the case of liquid phase HB/BF3 catalyzed process [12], or on amorphous acid catalysts containing meso- and macropores [13], this is not true when the reaction is catalysed in confined spaces. In this case a high concentration of reactives and products are kept in close vicinity inside the pores, where they are allowed to multiple collisions, and therefore to secondary reactions, while diffusing through the pores into the gas stream. It becomes evident, that if under these conditions another isomerization mechanism involving a bimolecular process occurs, the actual conclusions on catalyst acid site density, and even catalyst geometrical characteristics, when based on the assumption that xylene isomerization is a unimolecular reaction, would need to be revised. In the present work, the mechanism of xylene isomerization on amorphous and crystalline alumino-silicates has been studied by using isotopically marked xylenes as well as, kinetics and adsorption studies. The influence of active site density, as measured by a topological model and solid 29Si MAS NMR, on the reaction mechanism is also discussed. EXPERIMENTAL
A series of ultraestable Faujasites (HYD) were obtained by dealumination of a NaY (SK-40, Union Carbide) using SiCI4 and following the procedure reported in the literature [14]. The degree of crystallinity of the samples was determined using the intensity of the [5,3,3] reflection. The framework aluminium content (AIF) was
1147 calculated from the unit cell parameter, by means of Fitchner-Schmittler’s equation [15]. The physicochemical characteristics of the samples are given in table 1. Table 1 . Physicochemical Characteristics of the Zeolite Samples Sample
Framework Si/AI ratio
A1F/(A1F+ siF)
HYD - 1 HYD - 2 HYD - 3 HYD - 4 HYD - 5
4.3 7.7 13.8 29.0 99.0
0.188 0.1 15 0.068 0.033 0.010
Crystallinity (YoSK-40) 100
100 85 85 85
a0 (nm)
2.455 2.443 2.435 2.429 2.425
Catalytic runs were performed in a continuous fixed bed reactor. A detailed description of the system and of the analytical procedure methods has been described elsewhere [i 61. Preliminary experiments were conducted in order to select the range of working conditions where inter and intraparticular diffusional limitations in the catalyst bed are avoided. On these bases, the average particle size of the catalyst used was 0.59 - 0.82 mm. The isomeritation was studied at 493, 513, 533, 553, 573 or 593 K reaction temperature, and the hydrocarbon partial pressure was varied between 0.08 to 0.8 atm. The contact time was changed by modifying weight of catalyst (25 - 267 mg) and flow rate (0.037 - 0.216 m m o k ) in order to keep the conversion low and constant in all cases (= 5 - 8 Yo). Additional runs were performed with a mixture of = 30% hexadeuterated (CGH4(CD3),) and = 70% normal (C6H4(CH3),) para-xylene. The reaction was studied at 0.2 atm of hydrocarbon partial pressure, and 493 K of temperature, and in all cases products accumulated between zero and 30 seconds were analysed by GC-MS. RESULTS AND DISCUSSION
The Br~rnstedacid sites in amorphous and crystalline alumino-silicates are associated to protonic hydrogens from bridged structures AI(0H)Si. In principle, the potential number of acid sites is equal to the number of framework Al. However not all of them are expected to show the same acid strength since this depends on the chemical environment. In this way it has been shown [17] that the density of positive charge increases when decreasing the number of Al in the second coordination sphere of a given Al. Then, if one takes a large pore zeolite, such as Faujasite, as a reference and builds a topological model considering a random distribution of Al, with the Lowenstein rule as the only limitation, up to 9 different types of Al
1148
environment could be found. The strongest acid sites will be associated to those Al with 0 Al In the next nearest neighbours (ONNN). The relative population of them will change with the number of Al per unit cell (uc). Then, if a random distribution occurs, one should found a maximum in activity for a given reaction at a certain framework Si/Al ratio. The particular ratio would depend on the acid strength needed for that reaction [16, 18). In the case of xylene isomerization which is catalyzed by medium-strong acid sites [19], a maximum in 15 Alluc, or what it is activity should be found for framework A1 content of equivalent for a framework Si/AI atomic ratio of = 10. The 29Si N M R results of the samples of Faujasite with different framework Al content 120, 211, show a good correlation with the structural model presented. Moreover, when the activity of those Samples for the isomerization of meta-xylene, measured as the initial rate, is plotted versus the zeolite Al content a maximum in activity is obtained at a framework Si/Al ratio of 7 - 10. If this is so, one could proceed further and to calculate the activity per acid site (turnover number, TON) by dividing the initial rate of isornerization, by the number of framework Al. In this case one should expect, taking into account the model, the turnover number to increase up to a maximum corresponding to a zeolite sample with = 7 Alluc, and after this to keep constant since for lower Al contents most of the sites are completely isolated and should be equivalent. 1
The results presented in figure l a show that the turnover number does not follow the predicted behaviour, but goes through a maximum. A reason for this unexpected behaviour could be related with the fact that the initial rate, which is the usual magnitude used in the literature to measure zeolite activity, includes not only the kinetic rate constant but also the adsorption parameters. Then, if the zeolite SIlAl ratio has an influence on both, rate and adsorption constants, the initial rate is not the parameter to be used to calculate turnover numbers, but the true kinetic rate constant. Then, a full kinetic study for the isomerization of meta-xylene was carried out on each one of the Faujasite zeolites by following the Hougen-WatsonLangmuir-Hlnshelwood methodology [22]. The initial rate for rneta- to para- and ortho-xylene were measured at 0,08,0,10, 0,14, 0,20,0,35,050,0,80atm partial pressures of meta-xylene and at different reaction temperatures, 513, 533, 553, 573, 593 K, and fitted to a kinetic equation derived for a first order gas-solid reaction, under differential conditions :
l,n where kg is Intrinsic rate constant, Ka is the adsorption constant for meta-xylene, and p is the partial pressure for meta-xylene. The kinetic and adsorption constants were obtained and the results are given in table 2. The catalytic activity measured by the true kinetic rate constant, for the different Al containing samples goes through a maximum as before. Meanwhile, the turnover number does not keep constant for samples with less than 7 Al/cu but decreases (figure 1b). At this point, two explanations could account for our observations: either the topological model was not correct, or the reaction was not occurring through the assumed unimolecular mechanism. Results from different authors [23,241 confirm
1149
Table 2. Kinetic Parameters and Statistical Parameter Values for the Best Fits to the Unimolecular Kinetic Model at 553 K IC @ r a m e m ko . lo5 (rnoVs.g cat) Ka ( atm-1) I
Sample HYD HYD HYD HYD HYD -
1 2 3 4 5
.
9.59 25.63 26.81 30.92 5.08
-
S
correl. coeff.
0.75 1.44 1.40 0.83 0.71
0.9978 0.9897 0.9944 0.9978 0.9938
MRD% 3.38 11.33 5.79 3.19 6.61
MRD = Mean Relative Deviation
r o. 1 o5 / AI
C.U.
ko. 1 O5
/
Al
C.U.
Figure 1 . Average turnover frequencies from : (a) initial rate of isomerization, (b) kinetic constant mechanism only unimolecular, at 553 K and 0.2 atm, as a function of the aluminium T-atom fraction. the adequacy of the topological model, as soon as the good correlation between the model and the catalytic results for other acid catalysed carbonium ion reactions [25]. A second explanation could be that the isomerization of xylenes in Faujasites can also occur through a mechanism other that the simple 1,2 methylshift. Indeed, it would be reasonable to suppose that inside of the zeolite cavities, the primary products formed can keep reacting before to come out into the gas stream. In the case of xylenes this could be explained assuming that transalkylation can also occur between xylenes and trimethylbenzenes, giving xylene isomers other than the initial one. In order to find if isomers of the fed xylene could be formed through bimolecular reactions, a mixture of deuterated and normal para-xylenes was fed
1150 into the reactor, If the isomerization takes only place through a unimolecular 1,2 methylshift reaction, only isomer products with molecular weight 112 and 106 will be found (scheme I). However, if the trimethylbenzenes, formed as primary products from the transalkylation of xylenes (scheme II), can again transalkylate with a xylene molecule before diffusing out of the pores into the gas phase, metaand ortho-xylene with a molecular weight of 109 will be found even at low levels of conversion (scheme 111) I
T3+0' CH3
CD3
1,2 methylshift
8" G3 +
CD3
CH3
When the isomerization reaction was carried out using the mixture of deuterated and undeuterated xylenes on Faujasite zeolites, it was found (table 3), that in all cases, a sensible amount of the 109 mass compound was present in the isomerized products. This indicate that some of the xylene isomers are formed via a bimolecular mechanism involving transalkylation of methyl groups. We have then to conclude that the isomerization of xylenes in the confined alfa cavities of the Faujasite occurs by both, a unimolecular 1,2 methylshift, and a bimolecular mechanism involving transalkylation of methyl groups. If this is so, the kinetic equation to be used, in order to calculate the kinetic rate constant, can not be that from equation 1 but :
in where the second term accounts for the bimolecular mechanism.
1151 Table 3.Conversion and ortho-xylene normalised distribution in deuterated experiments, at 493 K and 0.2atm partial pressure of para-xylene. molecular mass Sample HYD- 1 HYD - 2 HYD - 4
AIF/(AIF+SiF)
COnV (yo)
0.189 0.1 15 0.033
8.5 5.5
6.0
106
109
112
61.6 62.2 65.0
22.0 21 .o 2.5
16.4 16.7 32.5
The experimental results were fitted to this model, and the kinetic rate constants, for the uni- (k,) and bimolecular (k2) mechanism, as well as the corresponding adsorption constants (Ka1 and Ka2), were calculated and the results are given in table 4. When the turnover number calculated from the kinetic rate constant for the unimolecular mechanism was now plotted versus the framework Al content of the zeolite, this increases up to a framework SiiAl ratio of = 10, and it stays constant (figure 2)just as it was predicted by the topological zeolite model. However, and as it could be expected, the turnover number for the bimolecular mechanism goes through a maximum for a Si/AI ratio of = 10 and then decreases. Our results clearly show that the isomerization of xylenes can not be used as a test reaction to discuss catalyst characteristics as has been done up to now, unless it is proved that only a 1,2 methylshift mechanism is operating. If both uni- and bimolecular isomerization occurs the contribution of each one should be separated. Influence of Framework SilAl ratio
on the Isomeritation Mechanism
From the kinetic rate constants presented in table 4 for zeolites Y containing different amounts of framework Al, it is possible to calculate the influence of this parameter on the ratio of bimolecular to unimolecular mechanism. Figure 3,shows that this ratio decreases when decreasing the framework Al content, being the decrease much stronger for samples with Si/AI ratios higher than 10. When the isomerization reaction was carried out using the deuterated mixture on samples with different Al content, exactly the same conclusion was reached [26].From both, isotopic and kinetic results, it can be concluded that in highly aluminic Faujasites more than 20% of the total isomerization takes place by a bimolecular mechanism involving transalkylation of methyl groups. Moreover the ratio of bi- to unimolecular mechanism decreases when decreasing the density of framework aluminium. This observation correlates well with the fact that the selectivity for transalkylation of xylenes, to give toluene and trimethylbenzenes decreases when decreasing the framework Al content of Y zeolites [16]. In order to explain the influence of acid site density on the xylene isomerization mechanism,one can assume that the bimolecular type of isomerization occurs on two close acid sites.lt appears that such assumption can not involve the interaction of two close xylene molecules adsorbed as Wheland's
1152 Table 4. Kinetic Parameters and Statistical Parameter Values for the Best Fits to the Unimolecular and Bimolecular Kinetlc Model at 553 K Sample
-
HYD 1 HYD 2 HYD - 3 HYD - 4 HYD 5
-
-
3.38 27.54 23.38 13.31 3.69
2.38 16.73 17.28 11.35 0.69
2.01 2.00 2.16 1.92 0.79
Ka2
Exner
ARE%
3.32 3.26 2.29 1.79 1.65
0.30 0.26 0.25 0.1 1 0.37
11.6 10.8 9.8 4.7 21.5
k, kinetic constant in (m0Vs.g cat). Ka , adsorption constant in (atm-1).
k p . 1 O5
A1
C.U.
Figure 2. Average turnover frequencies from kinetic constant, at 553 K and 0.2 atm, (a) unimolecular fraction, (b) bimolecular fraction, as a function of the aluminium T-atom fraction.
complex, and should be better regarded as the interaction between one Wheland complex and another molecule in its close vicinity and "retained" by the strong electric fields present into the zeolite cavities. Then any zeolite change which produces a modification of the electric fields, will affect the adsorption properties of the zeolite, and therefore, the ratio of bi- to unimolecular reactions. The values of the adsorption constants calculated from equation (2) and given in table 4, indicate, that a sensible decrease in the value of the adsorption equilibrium constants occurs for catalyst samples with a framework Si/Al ratio below 10. This result indicates that dealumination has an influence on the adsorption capacity of the zeolite for meta-xylene adsorption, specially visible at
1163 high Si/AI ratios, which in turn could explain the observed changes in the ratio of bi- to unimolecular xylene isomeritation upon dealumination. When direct adsorption experiments of meta-xylene were carried out (figure 4), these clearly confirm the strong decrease in xylene adsorption in samples with SilAl ratios higher than 10. A comparison of figures 3 and ’4, shows a very good
agreement between the adsorpticn behaviour and the relative contribution of the bimolecular mechanism. Bimolec. Isom. (%)
30
/*
20
10
Y0
0
-
O
Figure 3. Influence of bimolecular mechanism, ( 0 ) according to kinetic model (eq. (2)), (0) according deuterated experiments (orto-Xi of weight log), as a function of the aluminium T-atom fraction.
Figure 4 . Variation of adsorption coverage per unit surface area as a function of the aluminium T-atom fraction.
CONCLUSIONS
The results clearly show that the isomerization of xylenes can not be used as a test reaction to discuss catalyst characteristics, as it has been done up to now, unless it is shown that only a 1,2 methylshift mechanism is operating. If both uniand birnolecular isomerization occurs the contribution of each one should be separated. Moreover, the relative proportion of these two mechanism depends on zeolite composition. The adsorption capacity, as well as, the kinetic adsorption constant strongly decrease on Faujasite zeolites with a framework Si/AI ratio below 10. The kinetic rate constants, instead of initial rate,should be used to calculate turnover numbers, if the effect of changes in adsorption, due to modify zeolite chemical composition, wants to be avoided.
1154
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
18 19 20 21
22 23 24 25 26
F.G. Dwyer, P.J. Lewis and F.M. Schneider, Chem. Eng. , 83 (1976) 90. A.P. Lien and D.A. Mc Cauley, J. Amer. Chem. SOC. , 74 (1952) 6246. S.U. Choi, W.C. Frith and H.C. Brown, J. Amer. Chem. SOC.,88 (1966) 4128. R.H. Allen and L.D. Yats, J. Amer. Chem. SOC. , 81 (1959) 5289. A.J. Silvestry and C.D. Prater, J. Phys. Chem. 68 (1964) 3268. D.J. Collins and R.P. Scharff, Appl. Catal. 8 (1983) 273. M.A. Lanewala and A.P. Bolton, J. Org. Chem. 34 (1969) 3107. A.P. Bolton, M.A. Lanewala and P.E. Pickert, J. Org. Chem. , 33 (1968) 1513. J. Dewing, J. Mol. Catal. , 27 (1984) 25. J.A. Martens, J. Perez-Pariente, E. Sastre, A. Corma and P.A. Jacobs, Appl. Catal., 45 (1988) 85. S. Bhatia, S. Chandra and T. Das, Ind. Eng. Chem. Res. , 28 (1989) 1185. H.C. Brown and H. Jungk, J. Amer. Chem. SOC., 77 (1955) 5579. A. Cortes and A. Corma, J. Catal. , 51 (1978) 338. H.K. Beyer and I.M. Belenykaja, Stud. Surf. Sci. Catal. 5 (1980) 203. H. Fitchner-Schmittler,V. Lohse, G.Engelhardt and V. Patzelova, Crystal Res. Tech. , 19 (1984) Kl - K3. A. Corma, V. Fornes, J. Perez-Pariente, E. Sastre, J.A. Martens and P.A. Jacobs, Perspectives in Molecular Sieve Science , ACS Symp. Ser. 368 (1988) 555. I.N. Senchenya, V.B. Kazansky and S. Beran, J. Phys. Chem. , 90 (1986) 4857. J.R. Sohn, S.J. De Canio, P. 0. Fritz and J.H. Lunsford, J. Phys. Chem. , 90 (1986) 4847. D. Barthomeuf, J. Phys. Chem., 83 (1979) 249. A. Corma, V. Fornes, F.V. Melo and J. Herrero , Zeolites , 7 (1987) 559. A.W. Peters in "Catal. Mater.: Relat. Struct. React." , ACS Symp. Ser. 248 (1984), 201 O.A. Hougen and K.H. Watson, Chemical Process Principles vol. 111 , Wiley, New York, 1947. D. Barthomeuf in "Zeolites: Science and Technology" , NATO ASI Ser. E , 80 (1984) 317. E.H. van Broekhoven, S. Daamen, R.G. Smeink, H. Wijngnards and J. Nieman in "Zeolites: Facts, Figures, Future", Elsevier Science Pub. B.V., Amsterdam (1988) 1291. A. Corma, V. Fornes, A. Martinez and A.V. Orchill& in "Perspectives in Molecular Sieve Science" , ACS Symp. Ser. 368 (1988) 544. A. Corma and E. Sastre , J. Catal. , 129 (1991) 177.
1155
DISCUSSION Q: W. Haag (USA) I definitely agree with you in regards to the importance of the sorption constants on activity and even the mechanism in zeolitecatalyzed reactions. My question is directed to an apparent change in the xylene sorption constant at very low A1 content, as obtained from your kinetic analysis. If is possible to verify this conclusion by working in the zeroorder kinetic regime, i.e., at higher pressure andfor lower temperature, which yields directly the intrinsic rate constant. Have you done such experiments ? A: A. Corma No. At the pressures we have worked the zero order kinetic regime has not been achieved.
Q: S. Csicsery (USA) This was an extremely interesting and enlightening lecture. if answered many questions I had left unanswered in my work with isomerization and transalkylation of dialkyl-benzenes. I have observed with methylethylbenzenes that the adivation energy of isomerization is much higher than that of transalkylations. (The latter was close to zero.) Therefore, by doing the reaction at higher temperature and at lower hydrocarbon partial pressure I minimized the contributions of the bimolecular reaction (I was able to estimate the extent of the bimolecular isomerizations from the amount of xylenes and diethylbenzenes made from methylethylbenzenes). However, over the very weakly active CaNaY, the bimolecular reaction predominated even at higher temperatures. Have you tested such weakly acidic zeolites, and if yes, did you observe a similar behaviour with the xylene system ? Q: J. Lercher (Austria) I would like to comment on Dr. Csicsery's question with respect to finding exclusively the bimolecular pathway of xylene isomerization with alkali earth-exchanged faujasites. We have measured the adsorption of several aromatic molecules including xylenes on alkali- and alkali earth-exchanged zeolites and observed significantly higher adsorption constants of these molecules in comparison with the adsorption constants found in H-zeolites. Thus, Dr. Csicsery finding could be explained (as suggested in the present paper) to be due to the higher concentration of xylene molecules under reaction conditions. A: A. Corma In the case of xylenes, transalkylation has a higher activation energy than 1,2 M-shift isomerization. When we have calculated the activation energy, E , for the bimolecular isomerization mechanism, this is higher than the Ea for the unimolecular (1,2 M-shift) isomerization process, and very close to the Ea for transalkylation to give toluene and trimethylbenzene. With respect to your observations with the weakly acid CaNaY, we think that besides possible effects of acidity, the adsorption properties of CaNaY should be different. We could expect a higher adsorption of xylenes on CaNaY than on USY or even H Y , and therefore a predominance of the bimolecular reaction in the former. Please, notice Professor J. Lercher's comment, who has confirmed the higher adsorption of xylenes on the CaNaY samples. Q: A. Vannice (USA) In your oral presentation ou represented your bimolecular Langmuir-Hinshelwod rate expression as kg[so]21 CHzsCH-CN + H20 The r e a c t i o n r e q u i r e s a r e l a t i v e l y h i g h temperature o f 360°C. Therefore, i t i s hard t o suppress completely t h e degradation o f HCHO t o methanol and
1206 Table 4 Reaction o f a c e t o n i t r i l e w i t h HCHO Zatalyst 3xide
Conv
Atomic ratio
(2)
Y i e l d (molX) methanol Acryloni
Si-Cs Si-Rb Si-K Si-Na Si-Li
100-4 100-4 100-6 100-4 100-4
43.5 51. 42.6 10.5 3.5
Si-Ba
Si-Ca Si-Mg
100-1 0 100-4 100-4
34.7 14. 6.
17.6 1.7 2.0
112. 150. 150.
49. 47. 50.
Si-Zn
100-4
16.
2.5
92.
68.
100-2.5 100-1 0 100-5
9. 16.5 43. 32.5
2.5 7.5 36. 1.7
180. 160. 83. 140.
51. 42. 66. 47.
100-5 100-1 2
35. 15.
0. 0.
7.5 130.
11. 24.
50-50 70-30 70-30 48-52
-
0. 0. 0. 0.
-
-
-
-
-
kl
Yg-K
Mg-Fe
A1-K
B i-Mo Sn-Mo Sn-W v-P
-
12.6 30. 19.4 3.0 49.
COP
36.0 40. 35.7 10.5 2.9
7.0 9.2 6.0 0.9 23.
Conv, conversion o f a c e t o n i t r i l e ; Y i e l d , y i e l d on a c e t o n i t r i l e basis: A c r y l o n i , a c r y l o n i t r i l e . C o n d i t i o n s a r e descibed i n Experimental.
C02. The r e s u l t s a r e l i s t e d i n Table 4. They a r e summarized as f o l l o w s . ( 1 ) Only a l k a l i n a n d a l k a l i n e e a r t h metal o x i d e s can promote t h e r e a c t i o n : a c i d i c and amphoteric oxides a r e i n a c t i v e f o r a c r y l o n i t r i l e p r o d u c t i o n . (2) The b e s t performances a r e o b t a i n e d w i t h s i l i c a - s u p p o r t e d Cs20, Rb20, and K20. (3) The a c t i v i t y f o r a c r y l o n i t r i l e f o r m a t i o n decreases i n t h e o r d e r o f Cs- Rb- K Na L i , and Ba Ca- Mg. i n d i c a t i n g t h a t t h e a c t i v i t y i s r e l a t e d t o t h e e l e c t r o n e g a € i v i t y ( b a s i c p r o p e r t y ) o f metal i o n s corresponding t o t h e o x i d e s supported on s i l i c a . (4) The degradation o f HCHO t o methanol and C02 i s enhanced as i n c r e a s i n g t h e e l e c t r o n e g a t i v i t y o f metal i o n s corresponding t o t h e metal oxides. r t i s concluded t h a t t h e r e a c t i o n o f a c e t o n i t r i l e w i t h HCHO i s n o t easy, t h e s i d e r e a c t i o n i s , t h e r e f o r e , enhanced,even o v e r Cs-, Rb-, o r K-based oxides, under t h e r e a c t i o n c o n d i t i o n s used. The performance o f t h e Si-Cs (100-4) c a t a l y s t was s t u d i e d i n more d e t a i l . F i g u r e 1 shows t h e r e s u l t s o b t a i n e d w i t h a acetonitrile/HCHO m o l a r r a t i o o f 0.83. The conversion o f a c e t o n i t r i l e reaches 45 % w i t h t h e a c r y l o n i t r i l e y i e l d o f 40 mol% on a c e t o n i t r i l e b a s i s [ t h e s e l e c t i v i t y o f a c e t o n i t r i l e t o
>
>
>
1207
acrylonitrile i s 83 mol% and the selectivity of HCHO to aGrylonitrile is about 30 mol%]. It should also be noted that even over a basic catalyst such as Cs20, main part of HCHO is consumed to form undetectable polymers.
Conv. acetonitrile conversion AN acrylonitrile MOH methanol Sa selectivity on acetonitrile basis Sf selectivity on HCHO bas i s acetonitri le/HCHO molar ratio = 0.83 T = 360°C
Contact time (s) Reaction o f acetonitrile with HCHO on S i - C s (100-4) catalyst
Flgure 1. DISCUSS ION
Vapor-phase aldol condensation may consist of the following three steps. by basic sites of catalyst; abstraction o f a proton from the methylene group adjacent to the electron-attractiog group ( X ) to form an intermediate carbon anionon; R-CH2-X + B 4 R-C-H-X t BH+ ( B = 0-- or OH-) The C-H bond strength may be related to the electron-attracting function of group X. (ii) activation of HCHO by acidic sites: protonation o f HCHO: (i) activation of reactant (R-CH2-X)
HCHO
+
(iii) reaction o f
hydration; H2$+ OH
t
H+
w
H2C'OH
the two activated molecules to form aldol followed by deR -$-X
R
4 H2$-y-X
+
R HzC=C-X
+
H20
H OH H The functions of catalyst for activity are considered as follows. (a) When the C-H bond is weak, the step (i) i s fast i r ~ d . therefore, the reaction is limited by the step (ii). Consequently, the reaction is promoted effectively by strongly acidic catalysts; e.g., X = -COOH. (b) When the C-H bond is relatively weak, the proton can be abstracted by
1208
weakly basic sites and the reaction is limited mainly by the step (ii). Therefore, the reaction is promoted effectively by strongly acidic catalysts with a certain extent of basic property: for example X = -COCH3, ( c ) When the C-H bond is strong, the rite is limited by the step (i). Strongly basic catalysts are required. This is the case of X = -CN. (d) The situation seems to be complicated in the case of X = -CHO: the reaction is promoted both by strongly basic oxides and by strongly acidic oxides with a certain extent o f basic property. Possibly, when the reactant (R-CH2-CHO) is activated enough, the activated compound can react with HCHO which is activated only slightly by very weakly acidic sites. On the other hand, when HCHO is activated strongly by strongly acidic sites, the activated HCHO species can react with the reactant (R-CH2-CHO) which is activated only slightly by weakly basic sites. In view of the selectivity to the aldol condensation products, the action of catalyst for promoting the side-reactions should be taken into account. ( 1 ) Degradation of HCHO to methanol and CO2 via methyl formate and formic acid is promoted by acid-base bifunctional action o f catalyst [15]. ( 2 ) Degradation o f HCHO by polymerization to undetectable polymers is promoted by strongly acidic and strongly basic catalysts, when the reaction conditions are severe. (3) Decarboxylation of carboxylic acid is promoted by basic sites. (4) Polymerization o f unsaturated aldehyde is promoted by strongly acidic sites. It can be concluded that the vapor-phase aldol condensation using HCHO i s favorable when the reactivity of reactant (R-CH2-X) is relatively high, because the degradation of HCHO is small and the selectivity on HCHO basis is high. This is the case of X = -COOH, -COCH3, and -CHO, where the reaction i s mainly promoted by action o f acidic sites. 5. REFERENCES 1 M. Ai, J. Synth. Org. Chem. Jpn., 35 (1977) 202. 2 M. Ai, Proceedings, 7th International Congress on Catalysis, Tokyo, 1980, Kodansha, Tokyo - Elsevier, Amsterdam, 1981, p. 1060. 3 M. Ai, Proceedings, 9th International Congress on Catalysis, Cargary, 1988, Chem. Inst. Canada,, Otawa, 1988, Vol. 4, p. 1562. 4 M. Ai, J. Catal., 107 (1987) 201. 5 M. Ai, J. Catal., 112 (1988) 194. 6 M. Ai, Bull. Chem. SOC. Jpn., 63 (1990) 199. 7 M. Ai, Appl. Catal., 59 (1990) 227. 8 M. Ai, Appl. Catal., 36 (1988) 221. 9 M. Ai, J. Catal., 124 (1990) 293. 10 M. Ai, Appl. Catal., 63 (1990) 363. 11 M. Ai, Proceedings, 12th Iberoamerican Symposium on Catalysis, Rio de Janeiro.1990. Inst. Brasil Petro.. D. 81. l 5 M Ai J. Catal., 50 (1977) 291 16 M. Ai, Bull. Chem. SOC. Jpn., 64 (1991) 1342 and 1346.
1209
DISCUSSION
Q: J. W.Geus (The Netherlands) My question concerns the reaction of basic compounds with silica. The surface of silica in contact with liquid water contains a high density of weakly acid -Si-OH groups. The Si-OH groups react with, e.g., KOH to SiOH groups and water. With more concentrated KOH solutions silica dissolves leading to a potassium water glass solution. In contact with water, the -Si-OK groups will dissociate to -SiO- and hydrated K+, thus, providing strongly electrophylic or basic - S O - roups. Exposed to a gas phase, however, SiOK group will be present at the surface. The i-OK groups have the character of a salt and do not exhibit basic properties. Since a gas flow is passed over the catalyst in your experiments, the question is whether the provided silica exhibits a neutral or a basic behavior.
i
A: M. Ai The surface of alkali metal oxides supported on silica may be completely covered with CO,, H20, or other compounds during the reactions. The presence of basic sites, therefore, cannot be measured by ordinary titration or adsorption from gas-phase. However, silica-supported alkali metal oxides exhibit a catalytic activity for many basecatalyzed reactions. We consider that the surface of alkali metal oxides is covered reversibly by C02, H20, or others and these compounds are easily replaced with the reactants such as carboxylic acids, aldehydes, or ketones during the reaction.
Q: F. Trifiro (Italy) 1) The acid catalysts you used present not only P but also V and Fe, which have redox property. Are their redox properties useful for the reactions you have investigated ? 2) It is possible to produce aldehyde in situ by oxidation of CH30H and or olefines. Do you think that there is any chance of having high selectivity and activity in the presence of oxygen, or are there some draw backs ?
A: M. Ai 1) The redox property itself is useless for catalyzing the aldol condensation, but it is useful for regeneration of deactivated catalysts. 2) In general, the number of function required as catalysts increases as the number of reaction combined. However, the combination of CH30H oxidation with aldol condensation is relatively easily, because HCHO is stable. In the case of olefin oxidation, the produced unsaturated compounds react hardly with HCHO. Q: D. Arntz (Germany) Compared with the homogeneously catalyzed liquid-phase aldol reaction in which specific reactions exist, d o you see an advantage for the gas-phase reaction in view of industrial realization economics ? A: M. Ai The gas-phase reactions were performed at a temperature much higher than the liquid-phase reactions. Therefore, the main disadvantage of the gas-phase reactions is the degradation of products. However, when the products are relatively stable, for example, carboxylic acids, the disadvantage of the gas-phase reactions is not fatal. Q: F. Figueras (France) A chemical approach in organic chemistry would be to study the influence of substituents on the rate. Have you investigated Hammett type relationships to check the acidic or basic character of the reaction ?
1210
A: M. Ai I did not yet investigate this check, but I think that it is a good idea. I would like to try this check when we will get several series of data obtained from reactants having different substituents.
Q: D. Barthomeuf (France) Your approach to give a general view of the properties of oxides is interesting. My question is related to your scale of acidity and basicity. How did you classify the various oxides ? We heard in this Congress that acidic and basic properties may depend on various parameters, one of them being the coordination number. As a results, for a given oxide, the acidity and basicity is not a unique property. A: M.Ai In the case of oxides of transition metal, the measurement by ordinary titration method is hard because of the color. Thus, we tried to measure the acid-base properties (i) by adsorption of basic or acidic molecules from gas phase and (ii) by catalytic activity for some test reactions of acid- or base-catal zed reactions. We measured for various kinds of single and mixed metal oxides. tried to classify various single-oxides according to the obtained results and the electronegativity of metal ions corresponding to the oxides. In many cases, acid-base properties very largely by a combination of foreign oxides. This is a very interesting aspect of solid acid-base.
d
Q: M. Shymanska (Latvia) You have presented a great deal of experimental material on the reactions of various functionalized organic compounds with formaldehyde in the presence of different metal oxides. The catalytic activity of oxides you explain from the viewpoint of C-H bond strength in the organic molecules and the ability of the oxides to influence it, but in all cases we have also the dehydration step in this reaction. What do you mean when you say that in this step in all cases less significant for the overall reaction rate ? A M. Ai I think that the rate of dehydration increases very rapidly as elevating the temperature and it is very fast above 200 OC. Indeed, we can get a good mass-balance between the reactants and the products. I considered that the dehydration step is catalyzed more easily than the other steps under the conditions used, that is, the reaction cannot stop at the step af aldols.
Guai, L et af. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest,Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
-
REACTIONS OF MULTIFUNCTIONALORGANIC COMPoUNpS HYDROGENATION OF ACROLEIN ON MODIFIED Pt-CATALYSTS
T.B. L. W.Marinelli, J. H. Vleeming and V.Ponec Gorlaeus Laboratories, Leiden University, P.O.Box 9502,2300 RA Leiden, The Netherlands
Abstract Pt-catalysts modified by alloying, poisoning and promoting have been investigated in the acrolein hydrogenation. Alloying with Cu and poisoning by thiophene does not increase the selectivity to dlylalcohol appreciably. If the Ptcatalysts are promoted with an alkali, a transition or a non-transition metal oxide, only the last class of promoters is found to cause a strong increase in the selectivity to allylalcohol. Pulses of CQ suppress the formation of propanal and improve the selectivity to allylalcohol more than that t o propanol. This suggests that CO inhibits the adsorption of acrolein through the C=C bond more than the adsorption through the C=O group. 1. INTRODUCTION
Problems of selective hydrogenation of a,R unsaturated aldehydes to unsaturated alcohols are frequently encountered in the pharmaceutical or food industry. For example, upon production of vitamin A all-trans-retinal has to be converted into retinol, with a high chemo- and stereo-selectivity 111, or in other processes - citral hydrogenated t o citronellol [21, hrfural to furfuryl alcohol [3], etc. The first mentioned reaction can be performed by a stoichiometric reduction with NaBH, or by a catalytic reduction, using Ru complexes [U. At the moment no explanation is available why for hydrogenation of a,Bunsaturated aldehydes homogeneous stoichiometric reductions or homogeneous catalytic reactions [2,41 appear to produce the desired products selectively whereas heterogeneous catalytic reactions fail. The fundamental knowledge concerning the factors governing the selective heterogeneous catalytic reductions is only very rudimental. The hope that a study with simple model-molecules could improve the situation, has lead to the investigations described below. Acrolein and hydrogen react in a network of reactions, which all are in Drinciple reversible: CH,-CH&H=O CH,=CH-CH=O CH2=CH2-CH,-OH
1212
One (or more) of the products shown induces reactions leading t o propane, ethylene and other undesired products. The most desirable is the formation of allylalcohol which is - at the same time - the product thermodynamically least favoured. With pure metals, and simple monofunctional molecules it has been established that hydrogenation of the C=C bond is always easier than that of the C - 0 bond. Thus, to reverse the relation, promoted metals have to be used as catalysts. 2. EXPERIMENTAL
Muteriuls. The support material used in this study was a very pure and inert silica (Aerosil 200 from Degussa (A200), surface area 200 m2/g).The precursors for the preparation of (I) the nickel, copper and nickellcopper catalysts were Cu(N0,);3H20 and Ni(NO,)$BH,O (J.T. Baker) and of (11) the (un)promoted platinum catalysts were H,Pt(OH)* (converted into H,PtCl, by HC1) (Johnson Matthey Chemicals), and Mn(NO,),GH,O (>95%, Aldrich), SnC1,2H,O (>95%), FeCl,, NH,VO, (99.5%), Bi(NO,);H,O (99%) (J.T. Baker), GeC1, (99.99%), Fe(N0,),9H10 (99%), LaC1;7HZO (99.9%) (Janssen Chimica), Ti(OC,H,), (Johnson Matthey Chemicals), GaCI,, Ce(NO,),’GH,O (98.5%),Pb(NO,), (99.5%), CdCl;H,O (98%), NaCl (>99.5%), KC1 (>99.5%) (Merck), Cr(NO,);SH,O (99%) (Riedel de Haen). Hydrogen (Hoekloos 3.0, >99.90% purity) used was further purified by passage through a BTS column and a column containing molecular sieve. Acrolein (Merck, >99.5%) was distilled under nitrogen and dried with MgSO, (Brocacef, >99.5%) for use. Catalyst preparation, The silica-supported nickel (5.0 wt%), copper (1.0 and 10 wt%), copper-platinum and (un)promoted platinum catalysts were prepared by the wet impregnation method, while the unsupported nickel-copper catalyst was prepared by the co-precipitation method [5]. The metal loading of the series promoted platinum catalysts was 5.0 wt%, and the promoter (MI was coimpregnated in a molar ratio of Pt:M = 4:l.In the series of the Cu-Pt catalysts a varying Cu/Pt molar ratio was used with a total metal loading of 5.0 wt%. All catalysts were prepared in ethanol or very pure water. The preparation of the EuroPt catalyst and its presulfurization, which was accomplished by using thiophene as a sulfur-donor molecule, is described by Dees et al. [61. Co-impregnation: a proper amount of both metal salts were solved in ethanol (or water) and mixed, subsequently an appropiate amount of support material Aerosil 200 is added, The solvent was evaporated under nitrogen atmosphere and the catalyst was then dried under vacuum overnight. Apparatus and procedure, The vapor-phase hydrogenation experiments were performed in a flow system. Hydrogen was passed through a saturator, filled with (liquid) acrolein (at about -25OC), and subsequently, from top t o bottom, through the reactor, a vertical glass tube of about 1.5 cm inner diameter. The reactor, containing the catalyst on a glass grid, was heated by a spiral oven. The gas flow rate was regulated by a calibrated mass flowmeter (INACOM INSTRUMENTS B.V.). Samples of acrolein and the products from hydrogenation propanal, allylalcohol, propanol and propane - were taken
-
1213
every 20 minutes with the help of an automatic sampling valve and analyzed by a gas chromatograph (Chrompack 437A) equipped with a FID detector, and connected to an integrator (Spectra Physics SP4270). Product separation was achieved using a 4-mx1/8” stainless steel column packed with 10% FFAP on Chromosorb WHP, 80/100 mesh. Each catalyst (5 to 50 mg) was reduced in situ at 573 K by a 14 mVmin hydrogen flow during 4 hours. Subsequently the reactor was cooled down to the reaction temperature under hydrogen. Acrolein was then added by passing a 14 ml/min hydrogen flow through the saturator. Partial pressures of hydrogen and acrolein were 978 and 33 mbar respectively. The hydrogenation reaction was monitored either as a function of time, with a constant reaction temperature of 348 K, or as a function of temperature. In the latter experiments the temperature was raised or lowered each hour by steps of 5 K. At each temperature three samples of the reaction mixture were analyzed by gas chromatography. All reactions were monitored at low conversion ( ~ 1 5 % ) .
3. RESULTS
100
10 1%
1
Fig.1: The influence of alloying Pt and Ni with Cu, and of pokoning Pt with thiophene on the selective hydrogenation of acrolein t o allylalcohol.
Influence of Alloying and Poisoning upon the selective hydrogenation of acrolein to allylalcohol: The nickel-copper catalyst and the silicaqsupported catalysts of nickel, copper, platinum and platinum-copper have been examined by X-ray diffraction. The results of the Ni-Cu and Cu-PtlA200 (Cu:Pt= 4:l and 1:l)
1214
catalysts indicate that alloys are formed indeed, only the Cu-Pt/A200 (1:4) catalyst did not show alloy formation. The selectivity t o allylalcohol in the hydrogenation of acrolein of the just mentioned catalysts is shown in Fig. 1. The nickel catalyst mainly forms the saturated aldehyde and alloying nickel with copper does not show any effect on the selectivities. The same is observed when alloying platinum with copper, the selectivity to the unsaturated alcohol only shows a slight enhancement if the copper-to-platinum ratio in the Cu-Pt/A200 is 1 to 4. Fig. 1 also shows that the hydrogenation of acrolein over the presulfurized Europlatinum catalyst does not increase the formation of allylalcohol significantly either,
30 h
$
Sn
I
111
IV
v
VI
VII
Vlll
x
XI
XI1
Xlll
Fig.2: Selectivity in allylalcohol formation as a function of the position of the promoting compound cation in the periodic table.
Influence of promotors upon Selectivity and Activity: The hydrogenation of acrolein was hrther carried out over a series of silica-supported platinum catalysts, promoted by various metal oxides. The saturated aldehyde (propanal) was still the dominant product over all these catalysts. We were, however, particularly interested in the influence of the different promoters on the selectivity to the unsaturated alcohol. The selectivity to allylalcohol of various promoted platinum catalysts is shown in Fig. 2. These data are the average selectivities which were obtained from a reaction versus temperature scan. Of the different groups in the Periodic Table, representative promoter compounds were chosen. The alkali metal salts (Na, K)did not show any positive effect on the selectivity. In the case of the transition metals a slight increase in the
1215
selectivity (2<Sa,,,,,,,,,,, Mn = Ni = La = Ca = Pd > Cr. Among the most active Cu-, Co-, H-, Ag-, and Zn-MFI zeolites, the order of active temperature regions was Cu (the most active temperature, 523 K) < Co (623 K) < H (673 K) < Ag (723 - 873 K) < Zn (873 K). It is noted that copper ion efficiently enhanced the activity at the temperatures below 573 K. The effect of metals on the catalytic activity of alumina has been classified into three groups; the elements promoting the activity, showing little effect, and decreasing the activity. It is clear that the activity of A1203 at the temperatures of 573 - 773 K was enhanced by adding Cu, Fe, Co, or Cr, while Ni, Mn, Zn, and V little affected the activity and the activity was decreased by adding Ag, Ca, and K. It should be noted in the first group that copper, cobalt, and iron (or chromium) lowered the active temperature region of A1203 and the first two metals increased the maximum catalytic activity of Al2O3. Clearly the most active temperatures are depending on the cations supported. The order of active temperature regions was Fe (the most active temperature, 673 K) = Cr (673 K) < Cu (723 K) < Co (773 K) I Ni (773 - 823 K) I Mn (823 K) = V (823 K) = Zn (823 K ) = K (823 K) = none (A1203 only, 823 K) c Ag (873 K) = Ca (873 K). Copper also promoted the activity of Si02-AI203. The activities of Si02-AI203 for the selective reduction of NO-at the temperatures of 573 - 873 K was increased by a factor of 3 - 5 when 3.3 wt% copper was added. For example, the conversion into N2 was increased from 9% to 31% at 773 K in a NO (1000 ppm)+C2H4 (250 ppm)+02 (2%) system: The latter value is comparable to that on Cu/A1203. It follows that copper improves the catalytic activity of Si02-Al203 at 573-773 K for the reaction. Based on the results of Table 1, one can recognize that the supporting of Cu or Co metals or ions on various supports such as zeolites, Al2O3, and Si02-AI203, resulted in the generation and/or enhancement of the catalytic activity for the selective reduction of NO. By contrast, noble metals or non-transition elements do not show such effects. This
1292 Table 1 Catalytic activity for selective reduction of NO Catalyst gas CU-MFI-1O2b)
Reductant conc./ppm 250 166 1000 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 250 500 250 250 250
500 500 250 1000
d) e)
9
g)
0 0 2
5 0 0 0 0
5
0
0
0
0 3
0 6
2 6
6 0 0
15 9 0 9
16 9
0 2
0
0
0
1 1000 0 indicates no data available. NO, 1000 ppm; C2H4, 250 ppm; 02, 2%; W/F, 0.2 gscm-3. NO, 1000 ppm; &He, 166 or 1000 ppm; 02, 1YO;W/F, 0.3 gscm-3. The exchange level of cation has not been measured. The amount of metal loaded is omitted. NO, 1000 ppm; C2H4, 500 ppm; 0 2 , 2%; W/F, 0.6 gmm-3. NO, 2000 ppm; C3H6, 1000 ppm; 0 2 , 1%; W/F, 1.5 gmm-3. -I’
b) c)
Conversion into N2 / %a) 473 K 523 K 573 K 673 K 773 K 9 41 32 16 5 28 8 5 3 82 80 32 20 7 33 39 33 0 0 7 0 0 2 5 10 1 2 0 3 17 16 23 27 6 40 25 36 21 11 38 30 11 25 6 47 21 13 5 3 7 11 2 11 2 2 7 8 6 0 5 6 22 7 0 0 0 7 0 0 0 17 8 3 5 11 14 3 10 2 22 7 2 14 21 5 19 28 4 0 13 24 4 6 0 26 21 0 6 21 4 6 0 4 0 10 2
8 8 3
1293 Table 2 Activities of cation-exchanged MFI zeolites for selective reduction of NO Max. conv. into N2 High ( > 40%) Medium(l0 - 40%) Low( < loo/,)
Temperature for maximum activity Low temp. Medium temp. High temp. ( > 823 K) ( < 573 K) (573 773 K) cu Co, H, Ag in Pd Mn, Ni, La Ca Cr
-
Total-flow rate, 150 cmamin-1 possibly suggests that the selective reduction includes a redox cycle of the active site(s) and the metal ion reducible and reoxidizable more easily is more active for the reaction. On the other hand, perovskite-type oxides such as La0.8Sro.2Co03 and Laj.sSro.sCu04 active for the direct decomposition of NO [8, 91 showed little activity for the selective reduction of NO. This concludes that the activity for selective reduction of NO is not correlated with the activity for the direct decomposition. This is further supported by the observations that the dependence of reduction activity of the CU-MFI zeolites on the exchange level of copper ion [20] is quite different from that of decomposition activity [9]. The role of the acidity in the selective reduction of NO was examined. Hamada et al.[l6] reported that the -acidity of the catalysts is one of the 100 important factors controlling the €8 catalytic activity for the selective reduction. The conversions into N2 on Al2O3, Si02-AI203, TiO2, Cs2.5H0.5PW12040, and SiO2Ti02, and proton-exchanged zeolites having various acidities [21] are less than 10% under the present condition except for A1203 and proton-exchanged zeolites. It is well known that A 1 2 0 3 has mainly the Lewis . . . ... n acid sites while the proton10 10 10 10 exchanged zeolites have the Brnrnsted acidity. These results Space Velocity I suggest that the catalytic activity Fig. 6 Correlation between the catalytic activities of oxides for this reaction is not and space velocity. a simple function of the acidic Cat. weight, 0.025-1.O g; Total flow property. Finally, the correlation rate, 80-200 cmsmin-1; PNO=1000 between the catalytic activity of ppm; Po2=2.0%; P ~ 2 ~ 4 = l 0 ppm. 00 CU-MFI-137, H-MFI-100, C ~ ( 0 . 2 0 , C~-MFI-137(573K); A , H-MFIwt%)/AI203, or A1203 and space lOO(723 K); 0, Cu/A1203(773 K); velocity was measured and A , A1203(773 K). I
1
h”
1294 summarized in Fig. 6. Each reaction temperature was set at the most active temperature; 573, 723, 773 or 773 K. The activity of proton-exchanged zeolites and Cu(0.2 wt%)/A1203 greatly decreased at high GHSV region (2 10000 h-1). A1203 was much less active in such higher GHSV region. By contrast, Cu-MFI-137 showed a characteristic dependency on the GHSV. The conversion level into N2 increased a little up to 48000 h-1 and decreased at higher GHSV region. It is noted that the conversion into N2 was 74% even at 48000 h-1 of GHSV. The results indicate that the copper ion-exchanged zeolites are the most active at high GHSV region.
4. Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan and Nissan Science Foundation.
5. References
1 A. Crucq and A. Ferennet (eds.), Catalysis and Automotive Pollution Control, Elsevier, Amsterdam,l987, p. 1. 2 J. W. Hightower and D. A. Van Leirsberg, The Catalytic Chemistry of Nitrogen Oxides, ed. by R. L. Klimish and J. G.Larson, Plenum, New York, 1975, p. 63. 3 M. Iwamoto, S. Yokoo, K. Sasaki, and S. Kagawa, J. Chem. SOC.,Faraday Trans. 1, 77 (1981) 1629. 4 B. Harison and M. Wyatt (eds.), Catalysis, Royal Society of Chemistry, London, vol. 5 1982, p. 127. 5 M. Iwamoto, H. Yahiro, K. Tanqa, N. Mizuno, Y. Mine, and S. Kagawa, J. Phys. Chem., 95 (1991) 3727. 6 H. Shimada, S. Miyama, and H. Kuroda, Chem. Lett., (1988) 1797. 7 T. Uchijima, Hyomen, 18 (1987) 132. 8 Y. Teraoka, H. Fukuda, and S. Kagawa, Chem. Lett., (1990) 1. 9 H. Yasuda, N. Mizuno, and M. Misono, J. Chem. SOC.,Chem. Commun., (1990) 1094. 10 H. Hamada, Y. Kintaichi, M. Sasaki and T. Ito, Chem. Lett., (1990) 1069. 11 M. Iwamoto, Proc. of Meeting of Catalytic Technology for Removal of Nitrogen Monoxide, Tokyo, Jan. 1990, pp. 17-22. 12 W. Held, A. Konig, T. Richter, and L. Puppe, SAE Paper 900496 (1990). 13 H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito, and M. Tabata, Appl. Catal., 64 (1990) L l . 14 Y. Kintaichi, H. Hamada, M. Tabata, M. Sasaki, and T. \to, Catal. Lett., 6 (1990) 239. 15 M. Misono and K. Kondo, Chern. Lett., (1991) 1001. 16 E. Kikuchi, K. Yogo, S. Tanaka, and M. Abe, Chem. Lett., (1991) 1063. 17 M. Iwamoto, M. Tajima, and S. Kagawa, J. Catal., 101 (1986) 195. 18 S. Tatematsu, T. Hibi, T. Okuhara, and M. Misono, Chem. Lett., (1984) 865. 19 H. Bosch and F. Janssen, Catal. Today, 4 (1989) 369. 20 S. Sato, Y. Yu-u, H. Yahiro, N. Mizuno, and M. Iwamoto, Appl. Gatal., 70 (1991) Ll. 21 K. Tanabe, M. Misono, Y. Ono, and H. Hattori (eds.), New Solid Acids and Bases, Kodansha, Tokyo, 1989, pp. 27-214.
1295
DISCUSSION
Q: K. C. Taylor (USA) I wish to compliment the authors on their very interesting resufts. My question is what are your thought regarding the role of the Cu in the zeolite lattice which leads to low temperature reaction of NO in contrast to Cu on other supports such as alumina.
A: M. Iwamoto There would be several reasons for the high catalytic activity of Cu zeolites. For example, we have clarified that the amount of NO adsorbed on the Cu zeolites are much greater than those on Cu/A1203 and Cu/SiO2-AlzO,. This would result in the easier reaction of NO. In addition, it is clarified that the redox behaviour of copper ions on the zeolite lattice is very different from that on the other support. Although we don't know at the present that the redox cycle is included in the reaction of NO, by hydrocarbon, this might be a reson for the difference. We will have more experiments to clarify the reaction mechanism and the role of catalysts.
Q: J. Blanco (Spain) Why is it necessary to feed such a large amount of hydrocarbons to obtain significant levels of NO conversion ?
A: M. Iwamoto We have usually used 250 pprn or 1000 pprn of C2H4 for the reaction of 1000 pprn NO. There is a side reaction of complete oxidation of hydrocarbon by molecular oxygen; that is, the reduction of NO or the oxidation of hydrocarbon by NO, is the competitive reaction with it. This results in the necessity of a large amounts of hydrocarbon. Q: In-Sik Nam (Korea) 1) Did you observe catalyst deactivation by the hydrocarbons you need ? 2) How long did it take for one series of tests for a catalyst ? 3) Any particular reason why you measured N2 concentration instead of NO concentration ? 4) What was the product distribution, particulary CO and C02 ?
A: M. Iwamoto 1) Under the present condition we have not observed the catalyst deactivation. However, the activity decreased gradually when the catalyst was used at higher temperature in the presence of water vapour. 2) The longest one is about 100 h. 3) We wished to measure quantitatively the formation of N2 and N S . If we used a NO, meter alone, we cannot measure the concentration of N@. 4) The major part of COXis COP CO was produced at low temperature on at very high GHSV.
Q: C. H. Bartholomew (USA) 1) What is the effect of a propenern0 ratio of greater than one ? Is breach through (slip) of hydrocarbons observed at higher ratios ? 2) We have observed > 95% conversion of NO with propane at a space velocity of 100000 h-'. Accordingly we think that propane is a more effective reducing agent than propene. Would you comment on this please ? A: M. Iwamoto 1) On CuZSM-5 most of hydrocarbon were converted into CO2 above 350 OC even at larger HC/NO ratios. On A1203 much amount of partially oxidized products were
1296
formed at all teme rature range. This would be due to the difference between the catal tic activities o CuZSM-5 and A1203 for oxidation of hydrocarbon. $) We have already reported that propane is also active for the SCR-HC reaction. The efficiency of the reducin agents is dependent on the reaction conditions (in particular, the partial pressure o oxygen), the reactivity of hydrocarbon, and the activity of the catalyst for the oxidation of hydrocarbon. Therefore, it is reasonable that propane is more efficient than propene under a specified condition.
P
P
Q: I. S. Metcalfe (United Kingdom) You show conversion increasing with increasing space velocity for Cu-MFI-137 (see Figure 6). Can thus be explained in terms of mass transfer effects ? If so, how does the presence of mass transfer effects at low space velocities inflence your comparison of catalyst activities 1 A: M. Iwamoto We think that the phenomenon can be explained by the mass transfer effects. When we compare the catalytic activities of various metal ion-exchange ZSM-5 zeolites, the mass transfer effect would not be so changed with the kinds of metal ion exchanged into the ZSM-5 lattice be cause the effect would be mainly affected by the pore structure.
Q: E.Iglesia (USA) 1) Is it possible that the role of low 0 concentrations is to remove deactivating carbonaceous deposits that form readil on H h M - 5 during hydrocarbon reactions ? 2) A roposed mechanism for SC - N H 3 is the reaction of protonated ammonia with adsorbed 0.
ii
A
NH4+ + NO* -C What is the evidence against a similar mechanism using carbenium ions in SCR-HC, e.g. C3H7+ + NO* -P ? This would account for the SCR properties of unpromoted HZSMJ (Hamada, et al.) and possibly for the role of Cu. ions. The role of Cu ions would become similar to what we have proposed for Ga in HZSM-5 [Iglesia, et al. (1992)], namely to catalyze the hydride removal ste required for the formation of carbenium ions. In our case H* is removed as Hz by $a ions; in your case, it is an oxidative removal (as H ) using Cu ions. Would you comment on the available evidence against this simple mec anism ?
f
A: M. Iwamoto 1) The ro osed reaction mechanism mi ht have possibility. The oxidation of hydrocarbon y 2, however, is not so easy on!I ZSM-5 and therefore we think the effect of 0 addition would be to oxidize NO into NO , $) The addition of 0 2 into the NO-HC?reaction is necessary to ach ieve the reduction of NO, the temperature dependence of the oxidation of HC b 0 on uZSM-5 is similar to that in the resence of NO, and the oxidation of hydrocargon%y NO is very slow. These facts woulif indicate that the first step of the SCR-HC reaction on CuZSM-5 in the oxidation of HC by 02 to yield some active intermediateswhich can reduce NO.
16
e
Q: M. Sinev (Russia) The influence of 0,on the rate of NO, reduction is a rather general phenomenon. According to our experimental data, the rate - determining step of hydrogen - containing
1297 molecules oxidation over oxide catalysts is a dehydroxylation of the surface with water formation. In other words, the equilibrium in the reaction
20Hs
-.
Os2-
+ s + H20g
(1)
is shifted to the left side. In the presence of oxygen the rate of formation of water and the regeneration of the active oxidized sites on the surface increases drastically. It means that there is another mechanism of surface reoxidation whithout dehydroxylation via reaction (1) by a process which can be summarized as
40H- t 0 2 4 40-t 2H20 (2) The acceleraion of active site regeneration is another alternative explanation of the influence of 0, on NO, reduction. Do you have any experimental evidence for participation in NO reduction of active oxidized intermediates or it is just a proposition ? A: M. Iwamoto Your suggesting reaction, the regeneration of active sites by the 02 addition, may occur on the surface. However, in such case the oxidation of hydrocarbon by 02 would be accelerated and the conversion of NO would decrease. At the present, we believe that the role of O2 in the formation of some intermediates active for the reduction of NO.
Q: D. Duprez (France) I have a preliminary remark concerning the com arison ou made between the reduction of NO by propene and the reduction by 80. 1008 ppm of propene can potentially transform a much larger quantity of 02 than 1000 p m of CO can do. Therefore, it is normal that under your conditions the reduction by 0 is more sensitive to 0 than the reduction by propene. fn connection with this, how do you interpret the reversible effect of steam? Is it a shift of CO to C02 by WGS ? (Copper is a very active catalyst in WGS.)
8
A. M. Iwamoto We have already confirmed that 166 ppm of propene gives higher conversion than 1000 ppm of CO. The reversible effect of steam would be due to the adsorption of water on the active sites for the oxidation of hydrocarbon.
Q: J. Armor (USA) For A1203 as a catalyst you suggest N o t 0 2 + NO2 is important but for CuZSM-5 an “NCO”species is important. 1) What is the role of the proton in HZSM-5 which is also a good catalyst, like CU(I1) ’? 2) Is the mechanism for a CuZSM-5 catalyst the same as for HZSM-5 ’?
-
A: M. Iwamoto I have no detailed experiments and no evidence for the reaction on HZSM-5. In my speculation, the reaction mechanism on HZSMJ would be similar to that on Alfla, since both AI20, and HZSM-5 are not ood catalyst for the oxidation of hydrocarbon by 0, and need a stronger oxidant like N 8 2 to proceed it. I will have more experiments to clarify the reaction mechanism.
Q: M. Misono (Japan) In relation to the reaction intermediates, we have carried out oxidations of several organic molecules containing oxygen and nitrogen such as nitroalkanes, alkyl nitrite, and
1298 alkylamines over a series of metal-ion exchanged zeolites under similar conditions. We found, to our surprise, product compositions (N2, N 0, NO,, (NO)) very similar to those observed for the reduction of NO in the presence ofhydrocarbons and oxygen. This fact may lead to an idea that definite organic compounds containing nitrogen and ox gen are the common intermediates for those reactions. Would you comment on this idea .
‘7
A: M. Iwamoto Thank you very much for your comments. Your suggestion indicates one possible reaction mechanism for the SCR-HC. Now we are trying to detect directly the surface active intermediates.
Guni, L.er al. (Editors), New Frontiers in Curalysis Proceedings of :he 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights resewed
THE MAKING OF CATALYSTS BY CONTROLLED OXIDATIVE DEGRADATION OF PLANAR METAL COMPLEXES ON ALUMOSLLICATE SUPPORTS: EXHAUST GAS PURIFICATION CATALYSTS FOR POWER PLANTS, AUTOMOBILES AND SMALL OUTFITS F. Steinbach, A. Brunner, H. Miiller, A. Drechsler, S. Fromming, W. Strehlau and U.Stan Institut fur Physikalische Chemie der Universitat Hamburg, Bundesstrasse 45, D-W2000 Hamburg 13, Germany
Abatmct A new principle of preparing catalysts on alumosilicate carriers is described. The carriers are loaded by impregnation out of solution or suspension with planar or nearly planar complexes of transition metals, e.g., CU, Mn. VO, Fe. The complexes are decomposed and oxidized; meanwhile, additional ligands are present in the oxidizing gas mixture. The products. though showing the overall composition of the respective metal oxide, exhibit fundamental differences with respect t o structure and catalytic activity as well. The essential feature of the formation of the active catalysts is found t o be the incorporation of part of the central metal into the alumosilicate surface immediately at the start of the oxidative decomposition of the adsorbed planar complexes. Those metal atoms merged into the surface of the alumosilicate structure are centres for the further growth of the active compound at the surface in the course of the oxidative degradation of further complex molecules. A t the start of the oxidative degradation i t is the planar shape of the precursor complexes adsorbed on the alumosilicate surface that keeps the metal atoms at long distance. Once the metal atoms are anchored and merged into the alumosilicate surface they act as centres for degradation of further precursor complexes. This fact, as well as the temporary binding of the transient ligands offered in the gas phase during degradation, prevent the final formation of an ordered oxide structure. Instead. a highly disordered material with many centres of stressed ligand fields is generated granting high activity and selectivity at low temperatures.
1. INTRODUCTION
Catalysts for the purification of exhaust and waste gas have t o be improved in two directions: The decrease of the working temperature in the purification of plant exhaust may cause drastic savings in energy; the substitution of platinum in the catalysts of automobiles may happen t o be necessary for reasons of heavy metal pollution. Both demands are met by a new class of catalysts made by controlled oxidative degradation of planar complexes on alumosilicate supports HI.
1300 2. PRINCIPLE OF CATALYST M”C
s
The new catalysts are generated on alumosilicates, e.g. c rdierite, mullite, various types of zeolites. The carriers are used as honeycombs - so in industrial scale pellets or powders. The carriers are loaded by impregnation with a solution or a suspension of planar complexes ( e.g. oxalates. acetylacetonates. phthalocyanines 1 of transition metals, in particular VO. Cu, Fe or Mn. The catalyst precursors are transformed into the active catalysts by controlled heat treatment in an oxidizing gas mixture which also contains small amounts of compounds ( e.g. CO. NO, NH, or mixtures of those 1 which are able t o serve as transient ligands t o the metal ions during the degradation of the catalyst precursors. Frequently, one of those compounds. e.g. CO. is formed during degradation of the complex: then, an addition t o the gas phase is not necessary.
3. MECHANISM OF TRANSFORMATlON OP PRECURSOR INTO CATALYST By diffuse reflectance FTlR spectroscopy, PES. and electron microscopy the transformation of the precursors into the catalysts is characterized and an understanding is derived. This is done for the most important central metals V, Mn, and Cu and the ligands acetylacetonate. oxalate. tetraaquahydrogencarbonate, and phthalocyanine. The essential feature of the formation of the active catalysts is found t o be the incorporation of part of the central metal into the alumosilicate matrix of the support immediately at the start of the oxidative decomposition of the adsorbed planar complexes. Those metal atoms merged into the surface of the alumosilicate structure are centres for the further growth of the active compound at the surface in the course of the further oxidative degradation. A t the start of the degradation. it is the planar shape of the adsorbed precursor complexes that keeps the metal atoms at long distance. Once the metal atoms are anchored and merged into the alumosilicate surface, they act as centres for degradation of further precursor complexes. This fact, as well as the temporary binding of the transient ligands offered in the gas phase, introduce a misfit and prevent the formation of an ordered oxide structure inthe final product of complex degradation. Instead, a highly disordered material with many centres of stressed ligand fields is generated granting high activity and selectivity at low temperatures.
T h VO-catnlya la comparl8on with the 0xld8~of v d u m The catalyst made out of VO-oxalate - H2CVO(C20,),1 - on cordierite is chosen t o illustrate the characteristic features which are in contrast t o regular V-oxides. V,O, is characterized by two Intensive bands at 1024 and 840 cm-’; the most significant band of V 2 0 4 Is observed a t 1004 cm-’. None of these bands is pronounced in the spectrum of the material generated after thermal oxidative degradation of the VO-oxalate loaded on the cordierite support. Instead, the most intensive band is observed at 533 cm-l, a band usually barely detectable in the spectra of V20,. This Intensive band is likewise observed after loading the cordierite with VOPc and corresponding thermal oxidative treatment. Apart from these most intensive differences in the spectm also the complete features of the spectra of the vanadium oxides are significantly different from the spectra of the substances which are generated by thermal oxidative degradation of VO-complexes. e.g., oxalate. acetylacetonate, phthalocyanine, on cordierites. 3.1,
1301 On the other hand, if NH,VO, is used as catalyst precursor instead of one of the VO-complexes mentioned, the thermal oxidative treatment results in V,O,. No other bands except those of V,O, and traces of undecomposed NH,VO, are detected in the spectra. The unambiguous assignment of bands and structures is ascertained by extensive series of mixted loadings of the cordierite under variation of the shares of the different components.
a of Cu-catdyat out of CuPc-pmcur~or The active role of the cordierite support in the pathway of the generation of irregular and abnormous structure is most effectively studied by diffuse reflectance spectra of catalysts made by degradation of CuPc on cordierite. What actually happens can be traced by model experiments with small loads of Cu(NOJ2. Cordierite plates are loaded with small amounts of either Cu(NO.J, or CuPc; after thermal oxidative treatment the load in both cases is 0.4 weight% CuO. The CuO-load is exposed repeatedly t o test gas mixture ( NO, NH,, 0,.N, 1 at temperatures of up t o 320 OC for 12 h. Prior to exposure t o test gas and after each exposure diffuse reflectance spectra are taken. CuO la characterized by a group of bands at SO0 cm-I: S99, 543, 487 cm-l; Cu,O ts characterized by a very strong band at 632 cm-' together with further bands at 1460, 1128, 881 cm-I. However, seven bands are registrated for the product of thermal degradation of Cu(NO,), on cordierite when substracted from the spectrum of plain cordierite: 1300 cm-I ( Si-OH and AI-OH surface groups 1, 1030 and 794 cm-' ( Si-0 and/or A1-0 1. 541 cm-' ( CuO 1, 519 cm-I ( structural properties of the porous alumosilicate surface 1, SlS cm-' and 446 cm-' ( CuO 1. The 1300 and S19 cm-' band decrease with increasing load, the other bands increase. That shows, that CuO is anchored on and in the surface on OH-groups. furthermore, the porous structure of the alumosilicate is blurred by the CuO-deposit. Upon heating the loaded cordierite in test gas, i.e., using the catalyst for the denox reaction. the CuO-bands slowly disappear completely: the difference spectrum of the loaded cordierite against plain cordierite exhibits negative bands in the OH-region: 1300 and also even 3600 cm-'. Minor changes are seen a t 1100 and SO0 cm-'. Whlle no traces of CuO can be detected spectroscopically after repeated exposure t o test gas, the catalytic activity remalns unchanged. I t must be concluded, that CuO, generated by decomposition of Cu(NO,), Is anchored by surface OH flrst. and under thermic and catalytic treatment gradually migrates into the subsurface of the alumosilicate. thereby affecting further OH-groups and A1-0- and/or Si-0-bonds as well as the alumosilicate structure - the latter indicated by shifts between 33 t o 71 cm-' of the respective bands - but maintains its catalytic activity. The formation of Si-0-Cu and AI-0-Cu units in the alumosilicate network is indicated further by the bands at 700 and 550 cm-*. This pattern of final formation of "copper oxide" incorporated Into the subsurface of the cordierite support is in fact the pattern of formation of the final active catalyst. When CuPc or Cu-acetylacetonate are used as catalyst precursor, the final spectroscopic results as well as the catalytic activity are the same as just described, and the process of catalyst formation follows substantially the same sequence as just outlined, however, intermediate formation of significant amounts of CuO is not detected. Apparently, Cu is incorporated into the subsurface t o an equal extent and rate as the 3.2. m
1302 CuO-intermediate is generated by oxidative degradation of the anchored complexes without being able t o form a spectroscopically detectable oxide lattice. Similar results have been observed on cordierite supports loaded with CoPc. VOPc, VO-oxalate. MnPc, Mn-tetraaquahydrogencarbonate and mixtures of Mnacetylacetonate. Cu-nltrate and CuPc as well.
3.3. PES aud eiectron mkrowopy In the PE-spectra the electron binding energies of the metals incorporated in the catalysts show the features common for CuO and V,O,; however, the peaks are strongly blurred and asymmetric indicating nonuniform and irregular ligand fields surrounding the different metal sites. By electron microscopy oxide loads on cordierite are compared with catalyst preparations on cordierite. A comparison of V,O, -loads. generated by decomposition of NH,VO, on cordierite, and of VO-catalysts. as produced by degradation of VO-oxalate or VOPc, shows the failure in order t o generate a regular microcrystalline deposit in the latter cases. On the contrary, a rather amorphous, i.e.. submicrocrystalline deposit is formed. Similar observations are made when CuOloads, generated by decomposition of Cu(N0,J2 on cordierite, are compared with Cu-catalyst preparations, generated by decomposition of CuPc on cordierite.
The performance of the catalysts has been tested in laboratory and technical scale as well. Three examples of environmental catalysts have been optimized hitherto: The selective reduction of NO by NH, is performed by a catalyst made by oxidative degradation of VO-oxalate. The oxidation of CO. hydrocarbons and other organic compounds is performed by a catalyst made by oxidative degradation of a mixture of Cu- and Mn-complexes. The purification of automobile exhaust is done b y catalysts made by oxidative degradatlon of Cu-complexes on zeolites. Activity, selectivity and long time stability of the catalysts have been tested in laboratory scale using catalyst preparations on 32 cm3 honeycomb carriers or fixed beds of pellets, extrudates. and grains of about the same volume. Space velocities of up t o SO 000 h-I have been applied. Modelled multicomponent mixtures of gases have been used or the exhaust of a gas burner, theamounts of the essential compounds - including SO, - added further equal t o power plant exhaust, waste gas or automobile exhaust, respectively. Some catalysts have performed test runs of a duration of hitherto up t o 2000 hours without losing activity. Also tests in large scale on 1000 cm3 and on 1s cm x 15 cm x 100 cm honeycomb carriers have been performed. 4.1. The dmox d y a t mads by degndation of H,CVO(C2OJ21 The essential feature of the denox catalysts made by degradation of complexes are the very broad windows of high activity at temperatures ranging from I50 t o 400 OC together with only small degrees of conversion t o N,O as well as small backlash of NH, ( figures 1 and 2 1. The K-values are as hlgh as 55 t o 60 m/h. In order t o minimize conversion of SO, t o SO, i t has been found advantageous t o add a small percentage of Na2WO; 2 H,O ( about ten percent of the final VO-load 1 t o the impregnation solution Apparently. this addition gives further support t o the generation of misfits and distortions in the catalyst structure.
1303
CXI NO-conversion
CPPmJ
r I
400
300
zoo
401
Figure 1. Conversion of NO and side reaction t o N,O on VO-catalyst at different space velocities. Load: 34.1 weight% VO-oxalate converted t o 17.1 weight X V-oxide ( including wolframate I. Test gas: NO, MI,: 1000 ppm each; 0,:1 X ; N,: rest. SVtot C h-I 1 NO-conversion C X I N20-formation C ppm 3 3590 8300 10560
*
0 8
0 X
+
In contrast t o the catalytic features of the VO-catalysts are the properties of V,O,. generated out of NH,VO, on cordierite: conversion of NO is low. the temperature window is centred around 300 OC and rather narrow. MI,-backlash and conversion t o N,O are untolerably high ( figure 2, right I. The use of a gas burner exhaust ( NO, NH,: 1000 ppm each; 0,: 8 X ; CO,: 8 X ; H,O: 10 X ; N,: rest I does not change the catalytic properties of the VO-catalyst as given in figures 1 and 2 within the limits of accuracy. Admixture of 100 ppm SO, to the burner exhaust has no influence whatsoever on catalytic activity at temperatures above 220 OC. This has been tested for over 600 h. At temperatures around 200 OC the catalytic activity declines t o zero w i t h hours due t o the well known deposit of syrupy ammoniumsulphites. Less than 1 X of the added SO, Is oxidized t o form SO,. The stability of the catalytic activity as shown in figures I and 2 has been tested in long duration runs of up t o 2000 h without traceable losses. 4.2. The oxidation a l d y a t ma& by degird.tlon of IUn[(HIO),(HC0,),3 Mangantetraaquahydrogencarbonateis generated in aqueous suspensions of MnC0, after addition of small amounts of nitric acid. The cordierite honeycomb structures are impregnated with these solutions. The optimal load with the complex is about 5 weight X . The Mn-catalyst is active in oxidizing residual organic compounds. The Mn-catalyst is also an active denox catalyst. However, in contrast t o the VO-catalyst, the Mn-catalyst is not stable with respect t o admixtures of SO,; it is t o be used In exhaust free from SO, only. Nevertheless, by means of the denox reaction a critical indicator is available in order t o discriminate catalysts made out of Mn-complexes from those MnOz-loads. made out of, e.g., Mn(NO,I,. ULI
1304 EX1 NO-conversion
Cppml NH,
PO0
100
COCI
coc1 0 NOCXI NzO C ppm 1 * NH, C ppm 1
Figure 2. Left: Conversion CXI of NO and backlash of NH, Cppml on VO-catalyst at different space velocities. Same catalyst, test gas, and space velocities as in figure 1. Corresponding symbols for ppm MI, instead of ppm N,O. Right: Conversion of NO, side reaction t o N,O , and backlash of NH, on V,O, on cordlerite. Load: 15.5 weight% V,O,. 7est gas as given in figure 1. Space velocity: 3000 h-'. Same unlts as on main figure. In test gas as given in figure 1 the activity of the Mn-catalyst is characterized by a broad window of a degree of NO-conversion of at least 90 X ranging from 10s t o 250 OC. The side reaction t o form N,O shows formation of about 100 ppm N,O at 105 OC ( 11 X of the NO-conversion 1 and increases linearly t o about 400 ppm N,O at 250 O C ( 42 X of the NO-conversion 1. In contrast, the degree of NO-conversion on an equal load of Mn02. prepared out of Mn(N0J2, resembles a narrow Gaussian, centred around 270 OC, with a maximum degree of conversion of 90 X ; however, at any temperature the reaction t o form N,O amounts t o 70 X of the respective total NO-conversion. These markedly different patterns in actlvity and selectivity show further the different surface and lattice parameters of the catalysts made by degradation of complexes in contrast t o preparations of oxides. 4.3. Tln orldrLbn a t d y a t M6 by &gradation of Cu-phthdocymb, Mn-wstylUmtmate, d CUQIJoJ* Even higher catalytic activities in the oxidation of various classes of organic compounds than am shown by the Mn-catalyst are exhibited by a Cu-Mn-catalyst. The cordlerite honeycomb is first Impregnated with a mixture of Mdacac), and Cu(NO,), , both compounds are degradated together. Subsequently, the coated honeycomb is loaded with CuPc which is degradated thereafter. Catalytic properties are tested at a space velocity of 3000 h-' with an addition of 1000 t o 1200 ppm of the respective organlc compound t o either air or a rnlxture of 2 X 0, in N,. I t is seen that the formation of unwanted side products IS either tolerable ( fig. 3 1 or can be neglected ( figs. 4 and 5 1. In particular, activity is higher with small contents of oxygen, Le., admixture of air t o industrial exhausts is not necessary as long as small amounts of oxygen are still present.
1305 [%I
Conversion of benzylamine
Cppml CO. NO
COCI Temperature Figure 3. Conversion [ X I of 1100 ppm benzylamine in air ppm CO ( 0 1, ppm NO ( ).
CXI
Cppml CO ( 0 1
Figure 4. Conversion of 1000 ppm C,H, in air ( o 1 and in 2 X 0,in N, ( x 1. Same units as in figure 3.
(
o 1 on Cu-Mn-catalyst;
co
(01
Figure 5. Conversion of 300 ppm C6H6 in air ( o ) and In 2 X 0, in N, ( x 1. Same units as in figure 3.
4.4. The automobile exhnuat catalyat made by cbgrrd.tlon of Cu( CH&OCHCOCH,I, The automobile exhaust catalyst exhlbits better activity and selectivity when prepared on ZSM-5 support rather than on cordierite. Either ZSM-5 extrudate is used or coatings of ZSM-5 on cordlerite honeycombs. The H-form only is used for impregnation with Cu(acac12 in organic solvents. The load is about 2 to 4 weight %. It is ascertalned that no ion exchange occurs. Also, spectroscopic results show again that after degradation no bulk lattice of CuO or Cu,O is formed. The outstanding feature of the Cu-catalyst is its high selectivity. Hence, it is not necessary to work with a stoichiometric exhaust with lambda = 1. but lambda values considerably higher than 1 are tolerable. Optimal working temperatures are between 450 and 500 OC; thermal stability is conserved up t o well above 580 OC. The catalyst can be considerably improved as is indicated by occasionally observed results not yet reproducible. Systematic work t o further progress is going on.
1306
* [XI
I
t
i
KO+
BOO
1400
I
1200
{
600
1000 .40-
Boa ..
400
600
i
L
400
'~
200
'
200
PC1
Figure 6. Conversion LXI of synthetic automobile exhaust on Cu-catalyst. Honeycomb carrier; space velocity = 12000 h-l; lambda = 1.1; NO: 1000 ppm; C,H,: 700 ppm; CO: 1500 ppm; H,O: 3 X .
Flgure 7. Conversion [XI of synthetic automobile exhaust on Cu-catalyst. Carrier ZSM-S extrudate; space velocity = 12000 h-l; lambda = 1.4; NO: 1000 ppm; C,H,: 700 ppm; CO: 1500 ppm; H,O: 3 X .
5. MBCHANISM OP THR CATALYTIC REACTIONS The essential features of the catalytic mechanisms, in particular the reasons
for the low temperature performance of the new catalysts, have been investigated by use of FTIR-spectroscopy. Adsorption, co-adsorption, thermodesorption, and time-resolved recording of spectra have been used. The mechenisms elucidated differ from those reported for t h e known environmental catalysts. The differences in
1307 adsorption of reactants between the pure oxides on the one hand and the catalysts on the other hand give further support t o the distinctions between both substances. 5.1. Adl3orpuon of Reactulta O11 Yo-Cdslyst Cordierite plates loaded with VOPc only do not exhibit any adsorption of NH,. The same is observed for NH,VO, as well as V,O, made by degradation of NH,VO, as long as there are traces of metavanadate still not converted. The catalysts made by degradation of either VOPc or VO-oxalate as well as the oxides V,O, and V,O, adsorb NH, strongly as NH,+-species: bands at 3195. 2977, 2027. and 1425 cm-l. There are addltional bands during adsorption on the pure oxides in the range between 500 and 1000 cm-'. NO is adsorbed only by those preparations which still contain small amounts of NH,VO,. Either nitrate ( 1x32 cm-' 1 or nitrite ( 1236 cm-' 1 are formed. A significant adsorption of NO is observed only when NH, is preadsorbed or in adsorption of gaseous mixtures of NO and Mi,. The spectra for adsorption on V20s as well as on catalysts are quite similar and differ considerably from those for adsorption on V,O,. Catalysts, V,O,: Bands of ammonium as stated, in addition a small band of weakly coordinated NH, at 1666 cm-'. Bands of NO at 1365 cm-' with satellites at 1761 and 833 cm-' indicating nkrate,to a markedly smaller amount at 1251 crn-' indicating nitrite. V20,: The ammonium band is observed at 1413 cm-l; bands of NO occur at the wavenumbers lust stated for V,O, and catalysts but in addition with very high intensity at 1820 and 2401 cm-', as a sharp needle at 717 cm-'. and with smaller intenslties at 2767, 2221. and 2070 cm-'. Hence, adsorption of NO together with NH, is a clear tool in order t o distinguish the different preparations; furthermore, it can be excluded that the particular properties of the VO-catalyst are due t o an admixture of V,O,. Most significant are the differences in thermodesorption experiments of co-adsorbed NH, and NO, monitored by continuous time-resolved FT-1R-spectra,between V,O,. prepared by degradation of NH,VO,. and catalyst preparations by degradation of VOPc or VO-oxalate. Temperatures of disappearance of bands of ammonium foreboded by band shifts are considerably lower for catalyst preparations. e.g. 137 OC for the catalyst made of VO-oxalate, than for V,Os; here the decrease of ammonium bands starts at about 1SO OC and is still not completed at 260 OC. Furthermore, the 1022 cm-' band of V,O, decreases at temperatures above 110 OC, a new band 1s generated and increased a t temperatures above 140 C owing t o generation of V,O,-shares in the substance. This is due t o a partlal reduction of the oxide owing t o catalytic work. No such changes in intensity nor shifts are observed with the 533 cm-' band of the catalyst prepared out of complexes. This indicates a stability of the catalyst preparations towards degradadion of activity due t o irreversible redox processes. S.2. Mech.nllm 011 VO-Cdslyst The identified adsorbed species lead t o the conclusion that among the different mechanisms derived hitherto for V20s-catalysts C2-61 the modified IMMmechanism (51 comes closest t o a description of what happens at the surface of the VO-catalysts made by degradation of VO-complexes. The mechanism of Takagi C41 can be ruled out since only small amounts of NO are oxidized in the gas phase t o form NO,. The regeneration of surface V-OH out of surface V=O due t o reaction with water would lead t o increasing activity with increasing addition of water to the test gas. This has not been observed. The mechanisms
1308 hccordlng t o Otto, Shelef and Kummer C61 are based on the involvement of the surface lntermedlate NHOCds). No such surface group was traced. The simultaneous presence of surface V=O ( 1022 cm-' 1 and surface V-OH ( 1244 cm" 1, as well M the Increase of V-OH during the catalytic conversion of NO and NH, In the absence of 0,. are eatabllshed by the respective bands. In the absence of oxygen In the test gas the Increme of surface V-OH develops synchronously to the decrease of catalytlc actlvity. In the presence of oxygen in the test gas no increase of surface V-OH Is observed. Furthermore, catalytic actlvity is higher with increasing contents of oxygen In the exhaust gas, it Is only at contents of about 10 X oxygen that this Increase In activlty levels out t o constancy. Hence, oxygen is involved in the mechanism and oxygen takes care that the surface, due t o a balanced content of V = O and V-OH groups. keeps Its high actlvlty as expressed in the 1MM-mechanism. The lnsensltivlty of the catalyst8 towards water, as well M the very low share of the side reactlon leadlng t o N 2 0 even at high oxygen contents. are further supports for the IMM-mechmlsm. One mlght speculate that the superior actlvlty, selectivity and stablllty of the catalysts on cordlerlte over pure V,O, are due t o the most favourable slmultaneousnesa of V=O and -OH, the latter might be even Si-OH or AI-OH as well. The mechanlsm of the denox reactlon on a Cu-catalyst made by degradation of CuPc or other Cu-complexes Is of slmllar pattern. however, additional adsorbed specbs are observed In dlfferent temperature regimes: NH,, apart from being adsorbed as ammonlum species, Is also adsorbed In a dimeric hydrazinelike structure. NO Is adsorbed as nitrate and nltrite, however, in contrast t o what is observed wlth the VO-catalyrt, without the need of preceding formation of adsorption sites due t o NH3-adsorptlon. This la the cause for the strong inhibition of the denox reaction on the Cu-catalyst by water whlch competes for the equal sites as NO. 6. CONCLUSION In laboratory teats the catalysts made accordlng t o the prlnclple outlined have proven actlvltks and selectlvltles comparable t o the well known industrial SCR catalysts wlth platlnum and additives In the automotive section and with TiO, and V,O, in the power plant sectlon. The new catalysts are superior t o the catalysts known hltherto wlth respect t o the temperature reglme of catalytlc action: the optimum worklng temperatures of the new catalysts are about 100 t o 150 OC lower. This means In warte gas purlficatlon an enormour savhg of energy and In automotive catalysis a better adaptatlon t o clty trafflc. Furthermore, the pollution with traces of platinum metals can be stopped. The extonalon of' the principle of catalyst making by degradatlon of planar complexes onto other classes of reactions has t o be studied further.
1 F. Stelnbach, R Ellmerr-KutzInskI, A. Brunner, H. MUller, DE 39 17 890, DE 39 17 900, EP 90 110 214.5, EP 90 110 213.7. foreign patents pending. 2 M. Inomata, A. Mlyamoto, Y. Murakami, J. Catalysls 64 ( 1980 1 140. 3 M. Inomata, K. Mort, A. Mlyamoto, Y. Murakaml, J. Phys. Chem. 87 ( 1983 ) 761. 4 M. Takagl, T. Kawal, M. Soma, T. Onlshi, K. Tamaru. J. Catalysis SO 1977 1 441. 5 W. C. Wong and K. Nobe. Ind. Eng. Chem. Prod. Res. Dev. 25 ( 1986 1 179. 6 K. Otto and M. Shelef, J. Phys. Chem. 77 f 1973 1 308.
1309 DISCUSSION Q: J. Blanco (Spain) Is the- alumosilicate surface affected by the different pH's of the active phase precursor solutions? A: F. Steinbach It is well known that adsorption or interaction of active phase precursors on support surfaces is influenced by pH of the precursor solution or acidity of the precursor. However, in the method used in our preparations these effects whenever present at all are overruled by the properties of the disordered oxide generated during degr?dation. A fine experimental evidence is provided by the preparation of active CO-catalysts on cprdierite support out of different precursors dissolved in solutions of stron ly different pH. VOcatalysts, having equal activity and comparable loads of active -oxide, are generated out of VO-phthalocyanine, dissolved and suspended in formic acid, out of VOacetylacetonate, dissolved in CHC13, and out of VO-oxalate, dissolved in water [l]. Similar results are found with other central metals on other supports. F. Steinbach, H. Miiller, to be published [l]
-
-
#
Q: F. Bozon-Verduraz (France)
My question concerns the preparation stage of your catalysts. Do you have experimental information about the fixation mode of the precursors ? That is, does the precursor release li ands out of the coordination sphere through anchoring on the alumosilicateor not $in this case it would be a deposition) ?
A: F. Steinbach It is not to be ex ected that ligands can be released out of the coordination sphere when the complex is [xed on the alumosilicatesurface for two reasons: the coordination sphere is a unit of one e.g. phthalocyanine or two e.g. oxalate or acetylacetonate large and very stable or8anic molecules. With these complexes it is not possible to substitute the ligand of just one coordination position; rather the complex would be destroyed in total. This does not happen at the moderate temperatures of precursor deposition. On the other hand, substitution of ligands is not necessary when addition is possible, since the complexes used are planar: at least one VOcomplexes or two other metals coordination places above and below the planar configuration are free and capable to interact by additional coordination with, e.g., the OH-groups of the surface. However, it is rather difficult to obtain experimental evidence on that interaction, e.g., by IR-spectroscopy. This is due to the fact that, prior to degradation, there is a considerable surplus of complex molecules deposited on top of the anchored ones. In addition, the spectra of the undistorted complexes are rich in bands, so detection of additional weak bands or small shifts is rather hard to achieve. Some evidence is seen in difference spectra of FTIR: (i) A light shift in the position of the OH group of the cordierite support; the decrease in intensity of the OH bands due to metal bonding is observed during and after degradation on1 . (ii) Spectra of metal phthalocyanines as precursors on cordierite show a broadening ofythe aromatic bands and slight shifts to lower wave numbers. (iii) The most significant spectroscopic indication of the fixed phthalocyanine precursors are weak bands in the region of 1580 and 1608 cm-l. These bands are due to a formic acid linking the phthalocyanine to the surface They are not observed in the absence of formic acid or phthalocyanine either [2]. F. Steinbach, S. Fromming, to be published [2]
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-
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1310
Q: M. Schmal (Brasil) Can you comment on the behavior of the CuESM-5 catalyst by comparing your results with the previous paper of Iwamoto (see p.12 8 5 ) ? Are your results comparable and what is the influence of ZSM-5?
A: F. Steinbach Though we do not have extended spectroscopic details on adsorbed species and intermediates yet, I do not see substantial disagreement with the mechanistic framework reported in the fine work of Iwamoto, since the reported influences on activity by the partial pressures of the different compounds correspond to our own findings. With respect to the role of the ZSM-5 support my opinion is somewhat different from that of zeolite enthusiasts. I feel that rather than to be an active part of the catalyst, the ZSMJ is an effective support, guaranteeing a highly disturbed and rnisfitted oxide lattice and an appropriate porosity of the oxide as well, both properties being presuppositions for an outstanding activity of the Cu-oxide. Figure 1 shows the dependence of catalytic activity from the degree of exchange.
[XI Maximum conversion of NO
w
ma
1W
1w
1Y
X M
1-
*em
as@
zu
Degree of exchange by CU
[XI
Figure 1 . Maximum conversion of NO in automotive model test gas in dependence of degree of exchange of ZSM-5 by Cu. Cu was measured by atomic emission spectroscopy Test gas: 1000 ppm NO, 1650 ppm CO, 350 ppm C,H,, 350 ppm C,H,, 4500 C31 ppm 0,. 2.5 X H,O, rest N,. Lambda = 1.2. Space velocity: 18 000 h-'.
It is seen that activity rises sharply with the so called exchange up to a value of 200 %. In fact, good activity is achieved by a substantial surplus of the Cu deposit over the capacity of real exchange. Due to the oxidative treatment during the activation and, apart fiom that, due to the oxygen surplus during catalytic action, there is no doubt that the excess copper is in the form of something oxide. Also with cordierite support, a similar catalyst can be prepared, however, less active and unable to be operated apart from h equal unity. Its main deficiency is the poor activity with respect to the oxidation of hydrocarbons. The same catalyst has high activity Why is it of so poor activity with respect to in SCR with NH3 and NO,/O,.
1311 hydrocarbons? We think, mainly because the material porosity - as it is generated by the highly irregular and microcrystallinic material owing to the highly misfitted and distorted oxide lattice - is too narrow; it is sufficiently wide for NH3, NO,, 0 but insufficient for hydrocarbons. Hence, the activity of these preparations is high in SeR but insufficiently low in automotive tasks. On the other hand, what makes an automotive catalyst selective? There is sufficient activity for the complete oxidation of unoxidated traces by 0 2 , however, the most difficult to oxidize molecules, i.e., the hydrocarbons, remain untouched by 02;they remain to be oxidized by NO. Hence, we need a large augmentation of highly active surface of the misfitted Cu-oxide rather than a new kind of surface quality. This is delivered by the coating of the ZSM-5 framework by stressed Cu-oxides, thereby creating a catalyst, porous also for large hydrocarbon molecules. Of course, these feelings and hypothetical sketches have to be substantiated - or rejected - by forthcoming studies of surface species.
Q: J. Armor (USA) As a follow up to Professor Schmal's question - I suspect that the difference between supported vanadium oxides for SCR with NH and NO /02 and CuZSM-5 for NO,/hydrocarbon/Op is the presence of NH ?n SCR, Id33 probably blocks the inhibition by H 0 which we see in CuZSM-2: Without NH3 in CuZSM-5 the H20 becomes an inhifitor - of course with a different mechanism.
A: F. Steinbach This is only part of the description; in fact, the situation is far more complex, as is disclosed by FTIR investigations. The VOcatalyst - no matter whether prepared from VO-phthalocyanine, VO-acetylacetonate or VO-oxalate - does not adsorb water. Even after wetting with liquid water at room temperature, there are no traces of bands of chemisorbed or otherwise strongly adsorbed water. Only weak intensities of the bands of gaseous water indicate some physical deposit of water; it is completely removed by a dry stream of air within seconds. No water whatsoever is held by the VOcatalyst at the elevated temperatures of catalytic action. Hence, there is no need for NH3 in order to compete with water adsorption. On the contrary, small amounts of water are weakly adsorbed on the adsorbed NH species. Thereby the very small infliction of NO conversion by very high contents of A 2 0 (above 15% H2O) in SCR can be understood [2]. Unfortunately, VOcatalysts are unable to oxidize C O or hydrocarbons, hence, their stability against inhibition by water is of no use in automotive application. On Cu-oxide catalysts - nonregarding the kind of support but one exception yet (see below) - water is adsorbed rather strongly; there is a competition between adsorption of NH3 and of H f l [41. However, with the ZSM-5 supported Cu-oxide catalyst there are two different influences of water. One is the reversible blocking of active sites by adsorption; the second one is the irreversible destruction of the active structure by slow structural breakdown of the essential ion positions of the supporting Z S M J . This is shown by comparison of two different preparations of active Cu-oxide/ZSM-S out of different precursors. Two equal Z S M J supports were loaded to the optimal load of Cu-excess of 200 %: one by "ion exchange" out of Cu-nitrate and the other by depositing Cu-acetylacetonate. Equal oxidative degradation treatment was applied on both. The nitrate descendent catalyst is less inhibited by 15 % water content in the test gas than the acetylacetonate descendent catalyst. This is due to the fact that the active phase is divided more uniform within the ZSM-5 framework in the nitrate descendent catalyst; it is concentrated to the outer surface and the pore mouths of the ZSM-5 in the acetylacetonate descendent catalyst owing to the lateral extension of the planar complex.
1312 The reversible inhibition by water adsorption is accompanied by a slow process of irreversible loss of activity accompanied by a severe decrease of BET surface. The irreversible slow loss of activity and surface is faster and more drastic for the acet lacetonate descendent catalyst, slower and smaller for the nitrate descendent catai(yst. It is due to an irreversible breakdown of the Z S M J structure, thereby enabling the misfitted active Cu-oxide to heal to normal, less active structure. The breakdown of the ZSM-5 structure is seen by X-ray diffraction. its impact on material on the outer surface is naturally more drastic and faster than to material deposited in the Z S M J channels [3]. A final remark on the important role of the nature of the supporting alumosiiicate, which, nonregarding the phenomena outlined in the previous answer, is still not fully understood by us: a Cu-oxide catalyst prepared out of Cu-acetylacetonate on Y-zeolite is an excellent SCR catalyst, even in the presence of 15 % water [5]. The Y framework seems not to be destabilized by water. More astonishing is the fact, that there exists no inhibition by water in the N O W 3 reaction, whereas that same catalyst on cordierite rt is severely blocked by water. Detailed investigations are going on. F. Steinbach, N.Probst, W. Strehlau, to be published F. Steinbach, A. Drechsler, to be published F. Steinbach, U. Stan, to be published
Q: J. Armor (USA) With SCR (usin NH3/NOx/0 ) the level of water must be important in the nature of the surface species. b h i l e excess f I 2 0 has little impact on the catalysis, does it impact the species you see via your surface science studies '? It is important to look at the surface under typical industrial levels of H20, that is in large excess of 3 % H@. A F. Steinbach Your comment has my strong support. However, what is essential in investigations of surface species and their competition and interaction is not the partial pressure in the gas Phase but the coverage of the surface. Investigations on surface species usually are carried out at temperatures far less than reaction temperatures in application. Hence, it is not necessary to carry out all experiments with high partial pressure of water. However, it is essential that at least some bridging experiments are carried out in order to ensure and determine the coverage of the surface with, e.g., water under realistic high pressure and high working temperatures at one hand and the usually significantly lower pressure necessary to ensure a comparable coverage under that low investigation temperatures usually used in spectroscopic experiments. As reported, this has been done with the VO-catalyst up to 15 % water; a similar extension with the automotive catalysts well above the 3 % mark has to be done in forthcoming experiments.
Guczi, L el of. (Editors), New Frontiers in CafalysiF Proceedings of the 10th International C o n g m on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
OXIDATION OF CO ON Pd PARTICLES ON u - A I ~ O ~REVERSE : SPILLOVER L. Kieken and M.Boudart
Department of Chemical Engineering, Stanford University, Stanford, CA 94305,USA
Abstract For a reactant ratio close to unity, the turnover rate vt of the catalytic reaction between CO and 0,on Pd increases with temperature T until a maximum is reached at Tmax,then decreases. With model catalysts consisting of Pd clusters of size d and number density n evaporated onto a single crystal support of a-A1203,it has been reported previously that, above Tmm,vt at -10-6mbar varies with T, d, and n because of reverse spillover of CO striking the A1,03 surface, diffusing on it, and reaching the Pd cluster. In this paper, we report similar phenomena below Tmu,Thus, reverse spillover of CO is important for CO oxidation on Pd at low pressure in two different kinetic regimes.
Introduction The rate of reaction between 0, and CO at -106mbar on single crystals and on a polycrystalline wire of Pd goes through a maximum at a temperature, ,T [I, 21. Below Tmm,the rate is proportional to the ratio of concentrations [02]/[CO] denoted by 6 and increases with T. Above Tmm,the rate is proportional to [CO] and decreases with increasing T. For 6-1, T, is ca. 500K. The rate was found to be independent of crystalline ,,,) on a model catalyst anisotropy [ 11. At 6=1.1, the turnover rate at 445K (below T consisting of Pd particles supported on (7012) a-A1203single crystals was found to be independent of particle size between 1 and 8 nm [3,4]and equal to that on Pd(ll1) under the same conditions [2]. The turnover rate vt is defined as the number of CO molecules consumed per second per surface Pd atom. The model catalysts that will also be the object of the present study have an average metal particle size d and a particle number density n. In this introduction, we shall review the concept of reverse spillover that happens above T,, on model Pd supported catalysts. A key observation is that at 518K, 6-1, and 2.5~10-~mbar, vt is proportional to [CO] and increases by a factor of -3 as d decreases from 5 to 1Snm to become greater than the collision rate of CO molecules from the gas phase [3]. In a related work, it was reported that the areal rate of CO oxidation at 530K,6=1.1, and 1.8~10-~mbar, on 4.5 nm Pd particles supported on mica, increased by a factor of -6 when n dropped by a factor of -10 [5]. This observation was explained as follows: molecules of CO are chemisorbed on a Pd particle either by striking it directly from the gas phase, or by first striking the support, then diffusing on the support to the surface of a Pd particle, The same
1314
argument was used to explain values of the initial effective sticking coeffihent of co greater than unity measured on mica-supported Pd particles less than 6.5 nm [6]. Thus CO molecules diffusing on the support are captured by metal particles with an efficiency that depends on both d and n. This phenomenon of reverse spillover was examined in detail by measuring vI as a function of d and n between 525 and 650K for a model supported metal catalyst consisting of Pd particles on (1012) a-Al,O, single crystals [7]. At a given T,vt not only increased with decreasing d but also with decreasing n. A simple quantitative kinetic model that takes in account the reverse spillover of CO from the support to the metal accounts for all the results. This model for reverse spillover is based on the existence of a secalled ,, collection zone surrounding each Pd particle. This circular zone extends to a distance X from the particle whose interface with the support is assumed to be circular (Fig. 1).
d
0
0
D
Figure 1.Pd particles (0)and collection zones (0).(A) Effect of Pd particle size on area of the collection zone. (B and C) Overlapping and isolated collection zones. (D) With the particles being distributed uniformly on a square array, the nearest-neighbor distance is 2L. The distance X,,,m travelled by CO physisorbed on the support before it desorbs is:
where 1 is the hop distance for surface Volmer diffusion, R is the gas constant, ED,,, and EQcOare the activation energy for CO surface diffusion and desorption on and from the alumina support, respectively [71. The model assumes that all CO molecules striking the collection zone are trapped on the support and reach the particle. In addition, the model requires the Pd particles to be sufficiently far apart on the support (low n) so that the collwtion zones do not overlap. The model fit the data for Edc,-ED,,0=23.5 Hmol-', which is the only adjustable parameter [7]. A complete theoretical treatment of the problem showed that the solution can be approximated by the one for a simplified collection zone model [8,9]. This simplified model was also used [lo, .11] to fit the data of Kumpf et al. [7] with different boundary conditions, i.e. no net flux at the boundary of the circular zone enclosing the Pd particle and zero CO concentration at the perimeter of the Pd particle. The trapping probability of CO on the support was allowed to vary as well. All other assumptions were the same as in ref. 7. Thus above Tmax,the catalytic observations on the model catalysts seem to be well understood, in terms of reverse spillover. The present work was undertaken to elucidate some anomalies reported below, ,T and
1315
'
assess the role of reverse spillover of CO and 0, in explaining them. Indeed, reverse spillover of 0, above Tm, has been suggested [12, 131. Thus, below T, Landry et al. [14,15] extracted an apparent activation energy E of 73kJmol-' from the variations of vt with T, as measured on 4.9nm Pd particles supported on (T012) a-Al,03 single crystals at 2 . 5 ~ 1 0 - ~ mbar and k 1 .1 [3]. This value is considerably smaller than the value of 126kJm01-~extracted from the rate data of Engel and Ertl on Pd( 111) measured at similar conditions, 9.3~10-~mbar and k 1 . 3 [2]. Is this discrepancy due to reverse spillover on the model supported catalysts? The present work was initiated to answer this question.
Experimental The dual UHV system for sample preparation, TPD of CO, and steady-state CO oxidation, has been described [3, 161. Two model Pd catalysts supported on (7012) a-Al,03 single crystals were prepared. The samples are denoted dN and DN and are described in ref. 16. Their average particle diameter and number density were measured by transmission electron microscopy (TJZM). A continuous, polycrystalline Pd film was prepared as well. After preparation, the samples were submitted to CO TPD cycles, in which CO was adsorbed at 310320K, 2.7xlO-'mbar. Desorption was measured with a UTI lOOC quadrupole mass spectrometer (QMS). After these preliminary experiments, the sample was transferred in the reaction chamber. This chamber was equipped with an EAI QMS, a separate ion pump, and two leak valves for the introduction of 0, (Matheson, 99.997% pure) and CO (Matheson, 99.99%). Procedures for TPD and determination of turnover rate at steady-state were similar to those detailed elsewhere [3]. Before an experiment, the EAI QMS was calibrated against an ionization gauge, which measured the pressure of N,. The sensitivity for N, was measured at 0.64PdA ( S O % ) at a QMS emission current of 1mA. The sensitivity of the QMS to CO, 0,, and CO, was calculated from the sensitivity of N, [3], and allowed to monitor respective partial pressures with the QMS. The reaction between 0, and CO was studied between 410 and respectively. The number of 700K at 0, and CO partial pressures of 1.3 and 1.2~10-~mbar, surface Pd atoms used in the calculation of the turnover rate was counted by CO adsorption at 310-320K followed by TPD [3]. The temperature was measured by a Chromel-Alumel thermocouple, which was calibrated with an infrared pyrometer.
Results Electron microscopy Electron micrographs of samples dN,DN, and the Pd film are shown in ref. 16. The preparation conditions, i.e. Pd vapor flux R, deposition time t, and support T, are summarized in Table 1 with particle average diameter d and number density n from TEM. The continuous, polycrystalline Pd film was prepared at room temperature and subsequently annealed in vacuum at 600K for 300s to minimize the effect of thermal treatment during TPD and reaction. The continuity of the film, -50nm in thickness, was confinned by the absence of any oxygen peak at -51OeV from the Al,03 support in Auger electron spectra [16]. The average particle
1316
diameter d is defined as a number-average diameter, which is the median of the particle size distribution [ 171. The latter is bell-shaped for samples dN and DN with a full width at half maximum of 4 . 9 and 1.9, respectively. The mean Pd thickness, which was deposited on each sample, is given by Rxt because the sticking coefficient of Pd on (7012) a-A1203 is unity at support temperatures up to 750K [ 181. This quantity can be calculated from values of d and n and is equal to (nxd3/6)for spherical particles. Results in Table 1 show that calculated values differ from experimental ones by only 7% for sample DN and 33% for sample dN. The agreement is remarkably good for sample DN. The deviation for sample dN is larger but can be accounted for by a decrease in d of only 0.25nm. well within the particle size range measured by E M . The value of n is usually more reliable because its measurement depends less on the microscope defocus and requires less contrast between the particles and the support in the micrograph [19]. The total area A, exposed by the Pd particles is equal to the deposition area on the alumina support -km2 times (n xd2). Values of A, are tabulated in Table 1. For the Pd film, A, is equated to the geometrical area.
Table 1 Deposition of Pd on (7012) a-A1203: Support temperature, T; deposition rate, R; deposition time, t; average particle size, d particle number density, n; measured Pd thickness, (Rxt); calculated Pd thickness, (nmI3/6);total Pd surface area; A, Sample
T/K
Wnm s-l
r/s
750 3 . 9 ~ 1 0 . ~30 dN DN 750 3.2~10" 200 Film (*) 300 1.1~10-1 480 (*) annealed at 600K for 300s. a>
d/nm d10" cm-2 (Rxt)/nm (nnd3/6)/nm Ap&m2 2.6 4.2
17.2 17.9
-----
-----
0.12 0.65 51.3
0.16 0.69
-----
0.73 1.98 2.00
I dN
LcY5
0 0310
350 400 450 500 550 3
Figure 2. CO TPD spectra of samples dN, DN, and the film: (a) before, (b) after reaction.
Temperatureprogrammed desorption The TPD spectra, corresponding to Pd surfaces saturated with CO at 310-320K. are shown in Fig. 2 for all samples. Spectra labelled (a) were obtained in the reaction chamber after a series of TPD cycles performed in the main chamber with a different QMS [16]. Spectra labelled (b) were recorded immediately after steady-statereaction experiments. The area under the TPD curve is approximately the same before and after catalytic tests. This suggests no loss
1317
in Pd surface area during steady-state CO oxidation in agreement with earlier findings [20]. Typically, the TPD peak has a broad, asymmetric shape resulting from the convolution of at least two peaks at -490K and -400K. For sample DN and the film,which correspond to large particle size (Anm), the high-temperaturepeak is dominant and the low-temperaturepeak appears only as a small shoulder with no resolved peak maximum. Also, spectra (a) and (b) have the same shape except for the film. The contribution of the low-temperaturepeak is larger before the reaction than after for the Pd film for an unknown reason. The main features of the TPD spectra in Fig. 2 are equivalent to those previously reported on similar Pd/a-A1203 systems except for higher peak temperaturesbecause of larger heating rates employed in this study, i.e. -8Ws instead of 4Ws [3,4, 16, 201. The CO adsorption sites on Pd particles responsible for the high-temperaturedesorption peak are believed to be of the same type (2-, 3.4-fold coordination)as on large Pd single crystals with low Miller indices [3]. The lowtemperature peak, which was found to increase in magnitude as panicle size decreased,has been attributed to CO desorbing from comer and edge Pd atoms on the particles [3]. The lowtemperature shoulder is present in the TPD spectrum for the Pd film as well, which is not surprising because the film was grown at room temperature and, although it was annealed, is still polycrystalline. Moreover, the presence of a low temperature shoulder has been observed in the CO TPD peak from Pd( 100) [21,22] saturated with CO, and explained by repulsive interactions between adsorbed CO molecules on the surface. The low temperature peak becomes dominant for sample dN, which corresponds to 2.6nm Pd particles. However, in this case, spectrum (a) is not representative of the original Pd surface because the series of TPD cycles performed in the main chamber with a different QMS showed a decrease by -50% in peak area paralleled with a 30K shift of the peak maximum to lower temperature (no decrease in peak area occurred in the case of the other samples) [161. Similar observations have been reported for Pd particles, less than 1-2nm in diameter, supported on (OOO1) and (7012) aA1203 single crystals [4,20]. Results have been explained elsewhere in more detail by the disproportionation and/or dissociation of CO during TPD adsorption-desorptioncycles [ 161. This suggests that the surface of the Pd particles of sample dN is partially covered with carbon, which is not reacted off during CO oxidation because the areas under the TPD curves (a) and (b) in Fig. 2 are approximately the same. The total number of CO molecules Nco which saturates the surface of the Pd particles at 310-320K and [CO]=2.7~10-~mbar is measured from a CO mass balance on the reaction chamber during TPD [3]. The surface density of adsorbed CO on Pd is given by the ratio NcdApd, in which Apd iS the totid Pd area exposed by the Sample. Values Of N&Apd are summarizedin Table 2. The correspondingcoverage by CO ec, is calculated from eq. 2.
in which ns is the average surface density for Pd(l1 l), Pd(lOO), and Pd(ll0) and is equal to -1.3~1O'~cm-~ [23]. The parameterE is an area correction factor to take in account the fact that comer and edge atoms are counted at least twice in the measurement of the area of a facetted
1318
metal particle. Values of the parameter E (Table 2) were calculated [31. For particles with a diameter larger than -4nm. ~ =because l of a small proportion of edge and comer atoms. In Table 2, the CO coverage ec0 corresponding to sample dN is corrected for the 50%decrease in the TPD area as compared to the first TPD spectrum, and the overestimationof the particle volume by 33%. and therefore A, by 21%. With varying particle size 8,,-0.3. The value of ,8 for the film is probably overestimated because it does not take in account the roughness factor of its surface. The fact that, 8 is constant as d increases above 2.6nm, is consistent with earlier results [24]: eco%co,hkl was calculated for various fcc particle shapes and found to be -constant for d greater than -3nm but increasing fast as d becomes less than -2.5nm. The quantity eCoMcorresponds to the CO coverage of the various single crystal planes exposed by the particle. However, the absolute value, i.e. 8,,-0.3, is smaller than the average CO coverage 0.7 on Pd(l1 l), (110), and (100). by a factor of almost 2.5. Such a discrepancy is difficult to explain in light of all the assumptions made to calculate €Ico Ladas et al. reported a total CO coverage of 0.6 on Pd particles (d4.9nm, n=2.2~10"cm-~)prepared on the same alumina at 823K and similar Pd flux R [3]. The authors used the same procedure in measuring Nco and A,.& However, they assumed a sticking coefficient of 0.5 for Pd on aAl,03 to calculate the average Pd thickness on the sample from (Rt), and a cylindrical particle shape of diameter d whose height was calculated from d, n, and the mean Pd thickness (0.5Rt). When the sticking coefficient of Pd on a-Al,O, is unity [18], the Pd mean thickness of their sample can be re-calculated from n and d assuming spherical shape, and is found to be within 4% of the experimental measurement of 0.1 3nm, in agreement with our findings. The new value of A,, yields a coverage eC0-0.37, which is close to 0.3 computed in this work.
Table 2 Chemisorption of CO on Pd particles supported on (7012) a-Al,O,: average particle size, d; total density of adsorbed CO. N&Apd; area correction factor, E; coverage by CO,, 8 Sample
d/nm
(~,.j~,)11014 cm-,
E
, ,e
~ 0 % )
2.5 -1.2s 0.25 * dN 2.6 3.6 -1.00 0.27 DN 4.2 0.43 Film _--5.6 -1 .00 (*) corrected for decrease in area of the CO TPD spectrum and overestimation of A, (see text).
Adsorption of 0,on a-Al,O, The isosteric heat of adsorption of 0, was measured on the same a-Al,03 as described in refs. 7 and 16. It decreases from 8.2k1.7 to 7.MO.5k.Jmol-' with increasing 0, coverage. Steady-state catalytic oxidation of CO The turnover rate vI, defined as the number of CO, molecules produced per second and per surface Pd atom, was measured and calculated as in ref. 3 except that, ,e was obtained from eq. 2.
1319
Variations of vt with sample temperature for CO oxidation on samples dN,DN, and the film are represented in the form of Arrhenius plots in Fig. 3. Below T, values of E were calculated from the least -squares fit of the variations of ln(vJ versus 1 F in the temperature intervals indicated in Table 3. Results in Table 3 show that T, increases and E decreases as d decreases from "a film" to 2.6nm. Samples dN and DN have approximately the same particle number density so that the variations of E and and T, are caused only by a change in d. Values of E for Pd( 111) and 4.9nm Pd particles supported on a-AbO, (sample Dn) were extracted from rate data collected by Engel and Ertl[2] and Ladas et al. [3], respectively. Turnover rates at 445K for all samples are equal to -O.Ols-' within a factor of two. Above Tmax,vt increases as d decreases in agreement with results of Rumpf et al. [7].
Table 3 CO oxidation on Pd: average particle size, d; particle number density, n; temperature at max vt, Tma; apparent activation energy, E temperature range AT turnover rate vLUsKat 445K; all at po2=l.l pco=1.3 x mbar, except for Pd(ll1): poZ=l.3 pco =5.3 x 10-7mbar
SampIe
d/nm
4.9 Dn, ref. 3 dN,this work 2.6 DN, this work 4.2 Pd( 11l), ref. 2 ---Film, this work ---(*) from refs. 3 and 16.
l k
I
1
T,K
n/lO** cm-*
2.20 17.2 17.9
510 530 500 480 470
---------
'
A ' ~ ~ 4 I
1
AA A A
-r
0.1 g I
ooo 0 0
0
ODOOOaD 00
0
onoo A %O
I
*-
---
A@
0.01 y
-
OO
e 0.001
DB
0
E/kJmol-'
ATK
73 8%10 112f8 126 137fll
445-490 460-495 420-455 425-470 420-455
vL445K/s-1 0.008 * 0.009 0.008 0.010 0.012
are gathered in Table 3 and Fig.3. First of all, it has been confirmed that vt at 445K,well below Tmm, is the same within 50% on Pd( 11l), a Pd film and a large number of model supported Pd catalysts with particles between 1 and 8nm, as documented in refs. 1-4 and in this study. Second,
1320
of supported catalysts above 445K by the effect of reverse spillover of the reactants from the support to the metal. A clue as to the effect of reverse spillover is provided by a study [21 of CO oxidation on Pd(ll1) at decreasing values of 6, the reactant ratio [O,]/[CO]: as 6 decreased by increasing [COI at constant [O,], the vt vs T curve reached its maximum at increasing values of Tmm. This is due to the fact that, below Tmul CO inhibits the rate while, above Tmax,it is proportional to [CO]. Now, we see in Table 3 that BS we go from the film or single crystal to the three A$03 supported model catalysts at consrunt value of& Tmmincreases distinctly. It is as if, with supported Pd, [CO] were effectively higher than for unsupported metal, although it is actually the same in both cases. Let us now define the effective concentrationof CO and 0,.[CO],, and [O,],,The number per second of CO (or0,)F ,. (M=CO or 0,),reaching a Pd particle is proportional to a sum between brackets of 2 terms,the second one being due to reverse spillover:
I
I
I
= EP 46 :- \ :
\
X
202
U.,"
0
Pd atoms per particle as measured by TPD of CO after reaction; ns as in eq. 2; w.5is the sticking probability of CO [ I O , 11 ~ or 0,on ~-A$o,; s /cm2 is n(0.5d + X,), except when the collection zones overlap ( X g L ) , in which case S=(OSd+L), [ 10, 111. In the calculation of X, (eq.t), a = 0.4nm1Ed,co=28kJmol-1
Table 4 Parameters for calculation of [O,],d[CO], (all defined in the text, X, from Fig. 4)
Dn
870
8.2
dN 70 2.5 DN 655 1.7 (*) depends weakly on temperature.
1
732
400 460
600
660
800
610
100
760
REDUCTION TEMPERATURE
Figure 4 . Reduction temperature profiles of De-SOX materials
A SOX activity o f approximately 10 was arbitrarily chosen as an acceptable performance level o f a De-SOX catalyst in the commercial FCC unit. Therefore, the sulfate reduction temperature range in which these catalysts maintain suitable SOX activity can be found from Figure 4 . The minimum temperature required to obtain a SOX activity of 10 was defined as the critical reduction temperature, T,; the temperature above which there was no further improvement in the SOX activity was designated as the maximum temperature, TH. The effect of iron content in the steamed mixed stoichiometric spinel and mixed solid solution spinel on the SOX activity Is illustrated in Figure 5 . It is not surprising to see that the mixed solid solution spinel shows higher activity than the mixed stoichiometric spinel counterpart. High-iron solid solution materials having the composition
1397 of x-1.0 -1.8 uxhibited slightly higher SOX activity than the material of x < 0.4, but these might riot be suitable for the De-SOX process simply because these materials affected cracking reaction adversely due to coke formation. The catalyst stability study was conducted in an automated continuous unattended cycle reactor. The steamed Ce/Mg0.MgAlz-xFe,04 where x-0.4, 0.1, and 0.03, and the steamed Mg0.MgA11,0Feo.104without cerium were evaluated in the continuous cycle reactor. The results are compared with Ce/Mg0.MgA120, in Figure 6. The Ce/Mg0.MgAll.aFeo,404exhibited the best steam stability among these materials including the proto-type solid solution catalyst, Ce/Mg0.MgA1204, prepared by Katalistiks, Inc. The results also brought out the distinctive advantage of a combination of cerium with iron in mixed spinels over cerium alone in the single solid solution spinel.
Reduotion a t 677.C
8L
I t a a n d N@,n@l,,,~a,.,O,
00
0.8
1
1.8
I
o
ATOMIC FRACTION OF Fe (X)
Figure 5. SOX activity vs Fe content in MgO MgA12-,Fe,04 and MgA12.xFe,04
.
10
ao
ao
UI
80
NUMBER OF CYCLES
Figure 6. Catalyst stability study of iron-mixed spinels in continuous cycle runs with steamed samples
3 . L Vanadium-Mixed Solid Solution Spinels Both virgin and steamed vanadium-mixed solid solution spinels were blended in Watson FCC equilibrium catalyst and the resulting blends were tested in an automated continuous cycle reactor for a prolonged period. The re'sults are shown in Figure 7. The vanadium-mixed solid solution sample without cerium, Mg0.MgAll.BV0,204,exhibited an excellent virgin activity and stability, but its steam stabilfty was poor. The temperature profile of the sulfate reduction half cycle of V/Ce/MgO.MgAlzOI was carried out with propane instead of H2 at 677'C. The results are included in Figure 4. The critical reduction temperature for the sulfate reduction found from Figure 4 are listed in Table 2. The results confirmed that impregnation of V onto the single solid solution spinel with cerium lowered the critical reduction temperature from 640°C to 570'C, and remarkably improved the catalyst performance. The
1398 comparative study of V-end Fe-mixed solid solution spinels will be published in the future.
-
0 0
10
20
30
80
60
NUMBER OF CYCLES
Figure 7 . Stability study of vanadium-mixed spinels by continuous cycle runs.
Finally, these iron- and vanadium-mixed solid solution spinels also showed a significant activity for the removal of NOx in the emission from the FCC regeneration unit in addition to the De-SOX activity. In short, a simultaneous removal of both SOX and NOx was achieved with these mixed spinels.
''
4. Conclusions 1. Iron-mixed solid solution spinels, Mg0.MgAlz-,Fe,04 ( x - 0 . 0 3 - 0 . 4 ) exhibited an excellent De-SOX activity, and the same spinel Impregnated with cerium showed even better activity and steam stability. 2 . The temperature profile data for the sulfate reduction clearly
indicated that iron and vanadium in mixed spinels played a key role in lowering the sulfate reduction temperature. It is remarkable to observe a drastic lowering of the critical reduction temperature to 455'C for iron-mixed solid solution spinel, Ce/Mg0.MgA11,6Fe,.40,, from 690'C for its counterpart without Iron, Ce/MgO.MgAl20,. 3 . The incorporation of iron into the solid solution spinel matrix by coprecipitation preparation tends to improve the physical property, such
1399 as mechanical strength and attrition characteristics, of the product. The iron-mixed solid solution spinel with cerium also exhibited better steam stability over the corresponding vanadium counterpart. 4. The long term catalyst stability of iron-mixed solid solution spinel materials have been demonstrated via prolonged runs in an automated continuous cycle reactor. 5. In addition to the De-SOX activity, these materials reduced a substantial portion of NOx in the effluent gas from FCC regenerator. In short, a simultaneous removal of SOX and NOx was achieved with mixed spinels. 5 . Acknowledgement
This work was carried out at Harvey Research Center, ARC0 Petroleum Product Co. Harvey, IL. Currently this technology is owned by UOP/Katalistiks Inc. We appreciate UOP, Des Plaines, IL., for granting permission to publish this work. Special thanks go to Amoco Chemical Company for providing us the opportunity to present this work. 6 . References
1 2 3
4
5 6
7 8 9 10 11 12 13
J.S. Yoo, A.A. Bhattacharyya, C.A. Radlowski, and J.A. Karch, I&EC Res., 30, 1444,(1991). J.S.Yoo, A.A. Bhattacharyya,and C.A.Radlowski, Material Research Society Fall Meeting, Boston, MA, November 26-December 1 (1990) paper S6.7. J.S. Yoo, J.A. Karch,C.A. Radlowski, and A.A. Bhattacharyya, First Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, Japan, July 1-5, (1990) Paper 0-16, p 111. J.S. Yoo, C.A. Radlowski, J.A. Karch, and A.A. Bhattacharyya, U.S. Patent, 4,963 520 (1990); 4 957 718 (1990). J.S. Yoo, and J.A. Jaecker, U.S. Patent 4 957 892 (1990); 4 642 178 (1987); 4 495 305 (1985); 4 495 304 (1985); 4 472 267 (1984). V.V. Pechkovsky, J. Appl. Chem., USSR 29, 1229 (1956). A. Katagiri, K. Watanabe, and S . Yoshizawa, Bull. Chem. SOC., Jpn., 54, 1 (1981). K.H. Lau, D. Cubicciotti, and D.L. Hildebrand, J. Chem. Phys., 66, No.10,4532 (1977). A.K. Vijih, J. Mater. Sci., 13, 2413 (1978). F. Habashi, S.A. Mikhail, and K.V. Van, Can. J. Chem., 54 3646 (1976). M. Waqif, 0. Saur, J.C. Lavalley, Y. Wang, and B.A. Morrow, Appl. Catal., 71, 319 (1991). V.B. Kazansky, A.N. Pershin, and B.N. Shelimov, New Horizons in Catalysis, Part B, Seiyam, B. T. Tanabe, K. Eds. 1210 (1981). W. Kwestroo, J. Inorg. Nucl. Chem., 9, 65 (1959). C.S. Narasimhan and C.S. Swamy, Phys. Stat. Sol. (a) 59, 817 (1980).
1400 14 15 16
17
C.S. NarPSimhAn and C.S. S w m y , Current Science, 46, 795 (1976). C.S. Narasimhan and C.S. Swamy, Appl. C a t a l . , 315 (1982). J.A. Lehcher, C. Columbier, and H. Noller, Mixed Oxides, 80, 949 (1984). J . S . Yoo, C.A. Radloweki, J.A. Karch, and A.A. Bhattacharyya, U.S. Patent;4 790 982 (1988).
DISCUSSION
Q: M.Levinbuk (Russia) You have used metal oxides for the transport of SO, from regenerator to reactor. However, in my opinion, it would be more interesting to use as an oxygen trap oxides of metals with variable valence included with in the aluminosilicate matrix or zeolite to combine acid-base and redox reactions. We propose calling these systems reagent catalysts, since they function as carriers of oxygencontaining reagents. A catalyst used in this process circulates between reactor and regenerator, thus providing the partial oxidation of feed in the first vessel with further oxidation of reduced metal oxides in the second vessel. Our experimental data have proved that reagent catalyst are useful for the decrease of carbon oxides and sulfur in regeneration flue gases and increasing of MON of gasoline by 3-4; moreover, they are resistant to heavy metals. A: J. S. Yo0 In the De-SO development work, two concepts, namely, discrete De-SO particle separated from F& catalyst and incorporation of De-SO, component into FC?!catalyst have been pursued. For the technical and practical reasons, the first concept, which uses De-SO, catalyst as a discrete particle, has currently been adopted for the commercial operation. You suggested the incorporation of the oxygen trap metal oxides with variable valence into zeolite or aluminosilicate matrix. I believe that this concept is uite similar to our second approach in principle. As I presented in my talk, the De- 0, catalyst should perform other functions includin the oxygen carrier activit to be successful in reducing SO and NO, emissions from CC regenerator. These are i r oxidation of SO;!to SO ; ii) upt& of SO3 as sulfate; iii) release of sulfate as H$ via catalytic reduction of sulite with a transition metal incorporated into the spinel structure in the cracking unit, and iv) removal of NO,. Strictly speaking, the De-SO, agent or additive may be a correct terminology than the De-SO, catalyst. Of course, your (suggested term) "reagent catalyst" can also be partially applied to this system as well. As my final slide describin the reaction paths indicates, it may be reasonable to anticipate some reduction of CO rom the same FCC regenerator emissions in the presence of the De-SOx agent in view of its various functions, in particular, oxidation activity.
8
FB
B
Q: D. E. Stobbe (The Netherlands) You showed that, in some instances, gradual sulfur build-u is occurring as a function of the number of cycles. Do you think this can be explainec r by the formation of MgS in the regeneration step:
MgSO, t 4 H2 + MgS + 4 H20
s
You did not consider this reaction in your resentation. This possibility could be checked by doing a second reduction after a reoxi ation treatment. A: J. S . Yo0 In an effort to overcome the drawback, which is inherent to the solid solution spinel, GYMg0.MgAl2O4, I have checked the possibility of MgS as a possible culprit for the
1401
problem along with MgSO, at the initial stage of my work. The results of this study led to conclude that magnesium sulfate, not MgS, is a principal contributor to the sulfur build-up process. The formation of MgS in the reduction step via the reaction of MgSO, t H2 ---. MgS + H 0 is unlikely. Contrary to other alkaline earth metal sulfates, which are reduced to suffide, magnesium sulfate is in general reduced to oxides according to: MgSO,
+ H,
4
MgO t SO2 t H 2 0
It requires a 2electron reduction process for MgSO,, whereas sulfates of Ca, Sr, and Ba require 8electron reduction process to form sulfide. 2eMgS04-
A [M~SOS] A MgO t SO2 + H20 thermally unstable
At this point, sulfur leaves from the De-SO, matrix as SO, and the liberated SO2 may be reduced in the gas phase reaction.
SO,
+H
or HC 4 H S (in the gas phase) k?eO/Fe304 +k2S -,FeS
However, sulfide becomes part of sulfur build-up problem with the high iron mixed spinels, Ce/MegO.MgAlc Fe,04, which does not assume a solid solution state when x becomes 1. An additionaf-Eleaming step was necessary to remove the remaining sulfur completely. I speculate that iron oxide, independent of the solid solution structure, reacts with H2S to form FeS, which can be retained on the catalyst, and can be released with steam in the subsequent step. Steaming is required to release sulfide as H S . However, this problem was not encountered with low high spinel samples. I expect that this problem may not exist in the commercial operation because steam (substantial portion of the gas components) is present in the FCC unit. Q: H. J. Lovink (The Netherlands) How much SO, can the commercial deSO, material take up or how much of the MgO in the material can read in the equilibrium state of the material 85 % ?
A: J. S. Yo0 About 80-85 % of MgO in the spinel structure was available for SO pickup at the saturation level. The equilibrium level of sulfur retained on the De-Sb agent was calculated by determining the sulfur level in the spent De-SO, agent. The #CC blended with De-SO agent was withdrawn from the commercial unit for the commercial trial, and the De-80, fraction was then isolated from FCC catalyst by the float-sink method in a high density medium. This results suggest that about 10-30 % of the accessible MgO in mixed solid solution spinel is participated for the SO3 pickup under the commercial conditions employed in this trial. Of course, it depends to a great extent on the original SO, level of the FCC regenerator emission and the FCC operation mode as well.
Q: F. Trifiro (Italy) It seems to me that the key reactions of the De-SO, catalyst you have been reacting with is its stability during regeneration. This property must be related to some peculiar property of the MgO or excess of the spinel composition. Do you think that excess MgO remains inside the spinel structure, forming a non-stoichiometric spinel, also present after the many regeneration steps, or forms a layer of MgO with peculiar reactivity on the surface of the spinel ?
1402
A: J. S. Yo0 This is a good question, I have been struggling to get some insight into the identity of the active component, and met with little success SO far. I will tell you the experimental facts related to this question that I know as of now. In the solid solution region where y = 0 to 1 in Ce/MgAl2O4.yMg0, all MgO remained in the expanded lattice framework of solid solution spinel structure, and the De-SO, activity increases as the MgO content increases, and reaches at a maximum level of D e S O activity when y became 1. After the SO3 was picked up to a saturation level (14-16 % on the agent), a discrete MgSO, phase was clearly detected by XRD, but it disa peared when the sulfate reduction was performed, Also, we failed to detect the Mg phase in the reduced (regenerated) sample. It may indicate that MgO formed from the sulfate reduction may still associated in the solid solution spinel structure. This is nothing but a speculation on my part. In the solid solution range, 0 < y < 1, MgO remains in the solid solution structure even after the severe steam treatment and the mgO phase does not exsolve out of the surface. We starts to observe this phenomenon when y approaches to 1. However, when y 1, the simple calcination produced two phases, the stoichiometric spinel, MgAl and MgO, and it became much more serious when steam treatment was applied. steamed catalyst showed a significant deactivation in the De-SO activity. Although it is conceivable that some MgO may eventually exsolve out on tte stoichiometric spinel surface after many prolonged De-SO, cycles, it is uncertain whether or not the De-SO agent remain in the FCC unit long enough to see this phenomenon. Probably, most of Dc-SO, agent ma leave the FCC unit via attrition before this reaction occurs to a significant extent. & rryIthat I can not provide you more reasonable response at this time, and hope that a more definitive answer will emerge in the near future.
4
8
Q: E. Hums (Germany) SO,H activity is optimum for a solid H solution of 0.5 mol %. How can that be explained mechanistic with respect to the active sites during the process '? A: J. S . Yo0 As shown in my explanation, the solid solution spinel displayed the best De-SO, activity and steam stability. Contrary to the conventional wisdom, the steamed materials exhibited higher SO, activity than those of the corresponding calcined counterparts. In the 50 mol% solid solution spinel, the same phenomenon was also observed, and the enhanced SO, activity may be attributed to finely dispersed MgO exsolved on the surface during the steaming process. As far as active sites are concerned, I believe that MgO provides sites for the SO, pickup to form sulfate, both Ce and transition metal incorporated in the mixed spinel function as the sites for the oxidation of SO2 to SO,. I briefly talked about a syner y existing between Ce and transition metal for the catalyst performance. Moreover, becomes a center for NO, removal, while transition metal plays as a catalyst for the sulfate reduction cycle.
82
Q: C. J. Cameron (France) Does De-SO,catalyst interact with V in the FCC feed ?
A: J. S. Yo0 This is a very good question. Certainly it is quite probable in terms of interaction between spinel and oxides of vanadium based on the available information in the literature, that MgO and related Mgcompounds have been claimed as a V-scavenger for the FCC operation. I also recall one US patent 4,889,615; Mobil Oil (12/26/1989), which claimed a dehydrated magnesium-aluminum hydrotalcite as a vanadium trap as well as an agent to reduce SO, in the regenerator flue gas. When this dehydrated hydrotalcite
1403 material is thrown into the FCC unit, it may quickly become the material, whose composition is similar to the MgO excess spinel. In short, it is quite possible theoretically in view of metal oxide interaction. But I am not sure whether one can achieve a significant level of V accumulation on the solid solution spinel, MgO.MgAl O,, to transform it into the in-situ mixed spinel De-SO, material under the current FZC operating conditions. This view is based on the FCC sites for Vdeposition in the cracking reaction, the concentration of V in the gas oil feed, and the duration of De-SO, materials remaining in the FCC unit. of course, the mobility of vanadium pentoxide may help to realize this goal. We also found that the metal levels on the spent De-SO, agent withdrawn from the commercial trial runs and separated in the heavy medium, remained unchanged and that observed some silica migrated on the DeSO catalyst surface. If the gas oil containing in a high level of vanadium, for example, the%enezuelan crude, is used, I imagine that the chance of realizing this concept may improve. I should, however, remind you that these metal (V, Fe, Ni) act as a poison for the cracking reaction.
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Ouczi, L. et al. (Editom), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elscvier Science Publishem B.V.All rights reserved
SINTERING, POISONING AND REGENERATION OF P W g O J. Adamieca, J. A. Szymurab and S.E. Wankc ahstitUte of Chemistry, A. Mickiewicz University, 60-780 Poznan, Poland bDepartment of Technology & Chemical Engineering, Technical and Agricultural University, 85-326 Bydgoszcz, Poland %partment of Chemical Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
Abstract Ten magnesium oxide supported platinum catalysts, with P t loadings of 0.5 to 5.9 wt%, sulfur contents of 0.00 to 1.2 wt% and chlorine contents of 0.01 to 6.0 wt%, were characterized by hydrogen chemisorption, wide-angle x-ray diffraction and transmission electron microscopy after various high temperature treatments in oxygen and hydrogen. It is concluded that the H/Pt ratios for Pt/MgO do not, in general, correspond to the Pt dispersion, even for catalysts with very low levels of impurities. Reduction at temperatures L4OO0C(HTR), of sulfur-containing Pt/MgO causes large decreases in hydrogen adsorption, but the hydrogen adsorption capacity can be restored by oxygen treatment at 500 - 550°C followed by reduction in hydrogen at 250 to 300'C (LTR). Treatment of Pt/MgO in oxygen at 550 to 800°C results in decreases in average P t crystallite size (redispersion); no significant P t particle growth (sintering) occurs during oxygen treatment at temperatures as high as 8OO0Cc.Chlorine increases the rate of P t redispersion, but chlorine is not required for redispersion of Pt on MgO. 1. INTRODUCTION
Magnesium oxide is not a commonly used commercial catalyst or catalyst support, but in recent years MgO-based catalysts have frequently been used for oxidative coupling of methane [l]. Very recently it has been reported that Pt on MgO is a selective catalyst for aromatization of light naphtha [2-31.However, relatively little hm been published on the use of MgO as a support for metal catalysts, and Davis and Derouane [2]point out that the physical properties, stability and resistance to sulfur poisoning of Pt/MgO require investigation. Efstathiou [4] and Arena et al. [5] recently showed that treatment history significantly affects the behavior and properties of MgO-supported catalysts. We have previously reported on various aspects of the Pt/MgO system [S-111,and in the current paper the influences of treatments in oxygen and hydrogen, at temperatures of up to 8OO"C, on Pt dispersion and structure Pt/MgO containing various amounts of sulfur and chlorine are discussed. Special emphasis is given to the factors affecting the reproducibility of catalyst characterization results.
1406 2. CATALYSTS AND EXPERIMENTAL METHODS 2.1 Catalysts Ten Pt/MgO Catalysts, described in Table 1, were prepared by impregnation of various magnesias. The impregnation methods, with aqueous solutions of H2PtCl, and acetone solutions of Pt(C5H702)2,have been described previously [7]. After impregnation the catalyst were dried in air for 24 h (110°C for aqueous impregnations and 75°C for acetone impregnations), reduced in flowing hydrogen at 150°C for 16 h and at 250°C for 2 h and then stored in air until use. CAT-7s was prepared by adding 2 cm3 of distilled water to a 1.6 g sample of reduced CAT-7, stirring this mixture and then adding 2 cm3 of a 0.20M solution MgSOd to the paste. The water was allowed to evaporate and the catalyst WM dried in air for 2 h at 120°C. The sulfur content of the dehydrated CAT-7S, calculated from the amount of MgS04 added to the hydrated and reduced CAT-7 ( i e . Pt/Mg(OH)a), is 1.2 wt%. CAT-8C1 was prepared in a similar fashion starting with 1.0 g of reduced CAT4 and using 1.6 cm3 of 1N HC1 instead of the MgSOI solution. CAT-8Cl was dried in air at 100°C for 24 h. The calculated C1 content of the hydrated CAT-8C1 is 4.3 wt%, including the initial C1 content of CAT-8. Neutron activation analysis yielded a C1 content of 4.1 wt% for the freshly dried, but still hydrated, CAT-8CI (Note: the fresh CAT-8 is Pt/MgO, but the treatment with aqueous solutions and subsequent drying converts the MgO to Mg(OH)*). The Pt, C1 and S contents in Table 1 are based on a dry MgO basis.
Table 1 Description of catalysts Platinum P t Loading Precursor (wt%) 0.48 CAT-1 MgO-1% HzPtCls CAT-2 MgO-I1 H,PtCI, 0.59 CAT-3 MgO-I1 Pt(C5HTOZ)z 0.56 CAT-4 MgO-I11 Pt(C5H70Z)z 0.55 0.60 CAT-5 MgO-IV HZPtC1, CAT-6 MgO-IV Pt(CsH702)a 0.53 5.9 CAT-7 MgO-I1 HzPtCb CAT-7s MgO-I1 CAT-7 5.8 CAT-8 MgO-I1 Pt(CsH7Oi)a 5.6 5.5 CAT-8Cl MgO-I1 CAT-8 See Table 2 for description of supports. Catalyst
Sumort .A
*
Initial C1 and S Contents
c1 (wt%)
s (wt%)
0.5 0.6 0.4 0.03
0.30 0.05
0.05 0.05 0.4 mol ratio by atomic absorption spectroscopy. A Cu-free precipitate was also prepared, The Cu-containing samples had Cu contents ( % mol ratio Cu/(Cu + Zn) of 0.7%, 1.2%, 2.4% and 5.8770, respectively. All samples contained less than 0.1 mol % Na. X-ray powder diffractometry showed all samples to have the reflections characteristic of hydrozincite. The preparation of Cu$nl-xO solid solutions containing divalent copper is not straightforward. A temperature high enough (T > lo00 K) to obtain an adequate rate of Cu2+-Zn2+ interdiffusion is needed, but not so high that Cu2+ becomes unstable. A compromise has to be sought. In the present work, two series of oxides were prepared by heating the hydroxycarbonates in air for 18 h at two different temperatures, namely Series A at 1053 K and Series B at 1173 K. The resulting samples will be designated as A-0.71, A-1.2% or B-0.7%, B-1.2% accordingly, and the corresponding Cu-free samples as A-ZnO and B-ZnO.
...
...
1509 The oxides were c h a r a c t e r i z e d by X-ray powder d i f f r a c t o m e t r y u s i n g N i f i l t e r e d Cu Ka r a d i a t i o n . Samples were a l s o i n v e s t i g a t e d by UV-visible-NIR d i f f u s e r e f l e c t a n c e spectroscopy, u s i n g a Perkin-Elmer 330 spectrometer equipped w i t h a r e f l e c t a n c e attachment (BaS04 a s r e f e r e n c e ) . For c a t a l y t i c t e s t i n g , the oxide samples were p e l l e t e d , crushed and sieved t o 6 0 0 - 8 5 0 ~ m p a r t i c l e s i z e , then i n s t a l l e d i n a microreactor and pre-reduced i n s i t u f o r 16 h i n H2/He (20% Ha). The s t a n d a r d r e d u c t i o n temperature was 560 K . The a c t i v i t y f o r C02/H2 conversion was determined a t 1.25 bar p r e s s u r e i n a conventional flow system u s i n g GC a n a l y s i s . A standard temperature of 538 K was chosen f o r t h e main series of experiments. Two d i f f e r e n t r e a c t i o n mixtures were used: ( i ) C02:H2:He t 1:11:8, w i t h both CH30H y i e l d (methanol s y n t h e s i s a c t i v i t y ) and CO y i e l d ( r e v e r s e s h i f t a c t i v i t y ) being measured; ( i i ) m2:H2:He = 1:1:12, w i t h CO y i e l d being measured (no CH30H observable i n t h i s c a s e ) , thus determining r e v e r s e s h i f t a c t i v i t y alone. The space v e l o c i t y was v a r i e d from 0.8 t o 1.7 s-l, and y i e l d s of CH30H and/or CO were p l o t t e d a s a f u n c t i o n of r e c i p r o c a l space v e l o c i t y t o determine rate c o n s t a n t s . Methane was observed under some c o n d i t i o n s , probably formed i n a p a r a l l e l r e a c t i o n , u n r e l a t e d t o methanol s y n t h e s i s or the r e v e r s e s h i f t r e a c t i o n (1). The c a t a l y s t s were c h a r a c t e r i z e d both i n t h e o x i d i c form p r i o r t o loading ( a s i n d i c a t e d above) and a l s o a f t e r reduction and d i s c h a r g e from t h e reactor. 3. RESULTS AND DISCUSSION 3.1. C a t a l y s t c h a r a c t e r i z a t i o n UV-visible-NIR d i f f u s e r e f l e c t a n c e s p e c t r a showed that a l l t h e c a l c i n e d copper-containing samples, both of t h e A and t h e B series, contained copper i n s o l i d s o l u t i o n , a s i d e n t i f i e d by t h e c h a r a c t e r i s t i c Cu(I1) t e t r a h e d r a l absorption ( r e f l e c t a n c e minimum) at -1500 nm ( 1 6 ) . However, only B-0.7% contained all i t s copper i n s o l i d s o l u t i o n . This deduction is made on t h e basis of t h e r e f l e c t a n c e spectrum of B-0.7% showing no o t h e r a b s o r p t i o n between t h e broad 1500 nm band and t h e absorption edge of ZnO a t -400 nm (Fig. 1 ) . O t h e r samples showed t h e presence of a r e s i d u e of CuO, as revealed by i t s absorption edge which occurs a t 850 nm (16). Fig. 1 ( r e f l e c t a n c e vs. wavelength) shows t h e c l e a r d i s t i n c t i o n between B-0.7% (complete s o l i d s o l u t i o n ) and B-2.4% ( s o l i d s o l u t i o n p l u s r e s i d u a l CuO). XRD showed t h e r e f l e c t i o n s of ZnO i n a l l samples, a s expected, and t h e (111) r e f l e c t i o n of CuO ( t h e s t r o n g e s t i n t h e CuO p a t t e r n ) was detectable i n t h e most Cu-rich samples. The a r e a of t h e CuO(ll1) peak i n t h e diffractogram of B-5.8% was about 20% smaller than t h a t of A-5.8%, r e f l e c t i n g t h e e f f e c t of t h e higher temperature i n d r i v i n g more copper i n t o s o l i d s o l u t i o n . There was no evidence f o r Cu20, n e i t h e r from DRS nor from XRD, i n any of t h e samples. Furthermore, t h e r e was no i n d i c a t i o n of a charge-transfer spectrum due t o Cu(1) i n s o l i d s o l u t i o n i n ZnO. The r e s i d u a l Cu content f o r a l l samples showing i t s presence could a l s o be decreased by h e a t i n g for longer t i m e than 18 h a t 1173 X. However, even prolonged h e a t i n g d i d n o t succeed t o e l i m i n a t e CuO from 8-2.4% o r B-5.8%. The i n f e r e n c e is that t h e s o l i d s o l u t i o n l i m i t f o r Cu(I1) i n ZnO is below 2.4 mol % Cu. The BET s u r f a c e a r e a s of the B-series oxides (18 h a t 1173 K) were a l r e a d y below 1 m2g’l, so i n view of y e t f u r t h e r s i n t e r i n g t o be expected i t was n o t p r a c t i c a b l e t o extend t h e h e a t i n g time beyond 18 h f o r
1510
:R 0.5
0.25
0
1
I
400
800
I
1200
I
1600
hlnm
F i g u r e 1. D i f f u s e r e f l e c t a n c e s p e c t r a ( r e f l e c t a n c e v e r s u s wavelength) o f 0.7% Cu/(Cu + Zn) and 2.4% Cu/(Cu + Zn) samples prepared a t 1173 K. 1 - B-0.7%, as prepared; 2 B-2.48, as prepared; 1' B-0.7%, a f t e r u s e as c a t a l y s t ; 2' B-2.4%, a f t e r u s e a s C a t a l y s t .
-
-
-
samples t o be used i n c a t a l y t i c tests. The A-series samples had BET a r e a s )1 m g but. a s a l r e a d y s t a t e d , a l l of them showed evidence of b i p h a s i c by DRS. The amount of CuO i n A - 0 . 7 1 , however, s t r u c t u r e (CuO +. CuXZnl-,O) was very s m a l l . The above c o n s i d e r a t i o n s mean t h a t i t is f e a s i b l e t o make comparisons of c a t a l y t i c a c t i v i t y and s e l e c t i v i t y between two classes of c a t a l y s t s : Cu. The ( a ) t h e 0.7% samples, and ( b ) t h e samples w i t h Cu c o n t e n t s > l % former a r e p r i n c i p a l l y Cugnl,,0 s o l i d s o l u t i o n s (wholly i n t h e case of B-0.7%), w h i l s t t h e l a t t e r c o n t a i n CsZnl-,O supplemented by i n c r e a s i n g a t 560 K g i v e s only amounts of CuO. T h e 16 h p r e t r e a t m e n t i n $/He s o l i d s o l u t i o n , thereby y i e l d i n g a very s u p e r f i c i a l r e d u c t i o n of Cu,Znl-,O s m a l l amount of copper b u t i n a h i g h l y d i s p e r s e d s t a t e : by c o n t r a s t , t h e f r e e CuO w i l l be t o t a l l y reduced. The A-series and B-series samples were used i n c a t a l y t i c tests ( r e s u l t s i n S e c t i o n 3.2 below) and were t h e n r e - c h a r a c t e r i z e d . A l l samples a f t e r d i s c h a r g e from t h e r e a c t o r contained metallic copper. T h i s was i n f e r r e d from t h e appearance of a new band n e a r 570 nm i n t h e DR spectrum ( F i g . 1 ) :
1511 a b s o r p t i o n i n t h i s r e g i o n is a p r o p e r t y of copper metal ( 1 7 ) . Some Cu20 and perhaps Cu(1) i n s o l i d s o l u t i o n i n ZnO is probably c o n t r i b u t i n g absorpt i o n a t 600-650 nm. Concomitant w i t h development of t h i s 'd-hump' n e a r 570 nm is t h e disappearance of t h e CuO edge f o r t h o s e samples, e.g., B-2.4% ( F i g . 11, which i n i t i a l l y e x h i b i t e d a s t r o n g edge. There is a l s o an apparent g e n e r a l i n c r e a s e i n a b s o r p t i o n (more c o r r e c t l y a g e n e r a l d e c r e a s e i n r e f l e c t a n c e ) superimposed on t h e whole spectrum. The e f f e c t a p p e a r s t o be wavelength-dependent, i n c r e a s i n g a s wavelength d e c r e a s e s , and may arise from a change i n t h e s c a t t e r i n g . The r e d u c t i o n w a s confirmed by XRD. Cu metal r e f l e c t i o n s , e.g. C u ( l l l ) , were c l e a r l y e v i d e n t i n t h e d i s c h a r g e d samples of A-2.4%, B-2.4%, A-5.8% and B-5.8%. D e f i n i n g t h e C u ( l l 1 ) peak a r e a f o r A-2.4% as 1.0, - t h e r e l a t i v e a r e a s of t h e C u ( l l 1 ) r e f l e c t i o n f o r e q u i v a l e n t weights of t h e f o u r samples were 1.0, 0.7, 3.8 and 2.9, r e s p e c t i v e l y . I n t h i s s e t of r a t i o s , t h e lower f i g u r e s f o r t h e 2.4% samples mainly r e f l e c t t h e lower amounts of f r e e CuO p r e s e n t i n i t i a l l y , as indeed does t h e lower f i g u r e f o r t h e B-series sample i n each e q u a l Cu-content p a i r , b u t some copper metal is d e r i v e d by extract i o n from t h e s o l i d s o l u t i o n . T h i s is made c l e a r by t h e f a c t t h a t even B-0.7% develops a 'd-hump' i n i t s DR spectrum a f t e r u s e i n t h e r e a c t o r ( F i g . 1). F u r t h e r confirmation of e x t r a c t i o n of Cu from t h e s u r f a c e l a y e r s of t h e s o l i d s o l u t i o n phase d u r i n g r e d u c t i o n / c a t a l y s i s is provided by t h e d e c r e a s e i n i n t e n s i t y of t h e Cu(I1) t e t r a h e d r a l a b s o r p t i o n band a t 1500 nm. 3.2.
C a t a l y s t a c t i v i t y and s e l e c t i v i t y
3.2.1. Methanol s y n t h e s i s The f i r s t important o b s e r v a t i o n was t h a t b o t h A-ZnO and B-ZnO showed no a c t i v i t y f o r methanol s y n t h e s i s , but a l l t h e copper-containing c a t a l y s t s produced methanol. The y i e l d s were small, a s was t o be expected f o r experiments conducted a t near-normal p r e s s u r e , and were o n l y o b s e r v a b l e f o r t h e hydrogen-rich f e e d (C02/$ = 1/11). a more f a v o u r a b l e r a t i o for CH30I-I production than t h e 1/1 f e e d . N e v e r t h e l e s s , i n s p i t e of t h e small convers i o n s , t h e r e s u l t s on t h e Cu-containing c a t a l y s t s were i n t e r n a l l y s e l f Two a s p e c t s d e s e r v e c o n s i s t e n t a s between t h e A-series and t h e B-series. to be h i g h l i g h t e d , i l l u s t r a t e d r e s p e c t i v e l y i n Figs. 2 and 3. These show t h e methanol y i e l d v e r s u s r e c i p r o c a l space v e l o c i t y ( c o n t a c t t i m e ) , p l o t s whose l i n e a r i t y w i t h t h e o r i g i n t e s t i f y r e l i a b l e c a t a l y t i c rate d a t a and whose s l o p e s d e f i n e t h e c a t a l y t i c a c t i v i t y . F i r s t , i n Fig. 2, t h e p l o t s show t h e s t r i k i n g r e s u l t that t h e c a t a l y s t s d e r i v e d from t h e s u p e r f i c i a l r e d u c t i o n of t h e s o l i d s o l u t i o n i n t h e n e a r o r t o t a l absence of any f r e e phase CuO, v i z . A-0.7% and B-0.7%, have much l a r g e r a c t i v i t i e s than t h o s e o b t a i n e d from t h e more c o p p e r - r i c h o x i d e s The A-0.7% and B-0.7% systems are A-1.2% and B-1.2% which were b i p h a s i c . t h o s e f o r which t h e r e is l i k e l y t o be maximum d i s p e r s i o n of i n t i m a t e l y s u p p o r t e d copper. There is a close analogy here w i t h t h e h i g h a c t i v i t i e s for methanol s y n t h e s i s r e p o r t e d f o r f i n e l y - d i s p e r s e d copper i n Cu/Ce02 c a t a l y s t s prepared from Cu-Ce a l l o y s ( 1 8 ) . U n f o r t u n a t e l y t h e copper s u r f a c e areas w e r e t o o low t o be q u a n t i f i e d by t h e N20 decomposition method (191, as w a s a l s o t h e case w i t h t h e Cu/CeOZ c a t a l y s t s . Secondly, i n Fig. 3, comparison is shown between t h e c a t a l y s t s d e r i v e d from b i p h a s i c oxides, namely A-1.2%. A-2.4% and A-5.8% on t h e one hand, and B-1.29, B-2.4% and 8-5.8% on t h e o t h e r . A c l e a r p a t t e r n emerges whereby t h e methanol s y n t h e s i s a c t i v i t y i n c r e a s e s as t h e copper c o n t e n t is
1512
(SV)"/s
(SV)PS
F i g u r e 2. CH30H y i e l d a t 538 K f o r 0 . 7 mol % Cu and 1.2 mol % Cu c a t a l y s t s . Feed: C02/H2 E 1/11.
F i g u r e 3. CH30H y i e l d a t 538 K f o r 1.21, 2.4% and 5.8% Cu c a t a l y s t s . Feed: C02/% = 1/11.
r a i s e d , confirming t h e e s s e n t i a l r o l e played by copper. I n t h i s sequence, however, b o t h i n t h e A-series and t h e B-series, i t is t h e r o l e of copper derived from f r e e CuO which is mainly being developed, s i n c e t h e very f i n e hyperactive p a r t i c l e s produced from t h e s u p e r f i c i a l r e d u c t i o n of t h e s o l i d s o l u t i o n g i v i n g t h e bahaviour shown i n Fig. 2 w i l l almost c e r t a i n l y be subsumed i n t o t h e l a r g e r p a r t i c l e s of Cu o b t a i n e d from t h e easy r e d u c t i o n of t h e f r e e CuO. The a c t i v i t i e s of t h e A-series c a t a l y s t s were uniformly This r e f l e c t s t h e e f f e c t higher than t h e i r c o u n t e r p a r t s i n t h e B-series. of t h e h i g h e r p r e p a r a t i o n temperature f o r t h e B-series (1173 K) having given r i s e t o g r e a t e r s i n t e r i n g of t h e f r e e CuO. The subsequent low-temperature t o t a l r e d u c t i o n of t h e s e l a r g e r CuO p a r t i c l e s a c c o r d i n g l y produces less s u r f a c e a r e a of copper f o r any given copper c o n t e n t . Thus both t h e t r e n d w i t h i n each series and t h e comparison between t h e series is c o n s i s t e n t w i t h s y n t h e s i s a c t i v i t y being dependent on copper s u r f a c e a r e a , a s indeed has been found f o r t h e high p r e s s u r e s y n t h e s i s ( 1 0 , 2 0 , 2 1 ) . 3.2.2. Reverse water-gas s h i f t a c t i v i t y The a c t i v i t y of t h e c a t a l y s t s f o r r e v e r s e s h i f t a t 538 K and C02/H2 r a t i o of 1/11 (pco2 = 48 t o r r , pH2 = 528 t o r r ) was found t o be about two o r d e r s
of magnitude g r e a t e r than t h a t f o r methanol s y n t h e s i s . I t i a c l e a r l y a f a c i l e r e a c t i o n . The r e s u l t s f o r t h e B-series c a t a l y s t s a r e shown i n Fig. 4. The c a t a l y s t w i t h t h e g r e a t e s t a c t i v i t y is t h e s o l i d - s o l u t i o n d e r i v e d c a t a l y s t B-0.71, w h i l s t B-1.2% and 8-2.42 have t h e Lowest a c t i v i t i e s . I n s h a r p c o n t r a s t t o t h e methanol s y n t h e s i s r e s u l t s , ZnO was markedly a c t i v i t y f o r r e v e r s e s h i f t . The a c t i v i t i e s of t h e A-series c a t a l y s t s showed s i m i l a r t r e n d s , namely a high v a l u e f o r A-ZnO and A-0.7%, low f o r A-1.2%, and r a i s e d a c t i v i t y a s t h e copper c o n t e n t i n c r e a s e d t o A-5.8%. The A-series c a t a l y s t s were more a c t i v e than t h e i r B-series c o u n t e r p a r t s ( e x c e p t f o r A-1.2% where t h e r e was a m a r g i n a l l y l o n e r v a l u e than f o r B-1.2%). This can a g a i n be a t t r i b u t e d t o t h e higher p r e p a r a t i o n
1513 temperature used f o r t h e B-series having l e d t o more s i n t e r i n g and hence lower s p e c i f i c areas, both f o r copper and t h e ZnO matrix.' Both series gave good l i n e a r p l o t s of CO y i e l d v e r s u s r e c i p r o c a l s p a c e v e l o c i t y , Copper has an ambivalent e f f e c t . Free-phase CuO, i f p r e s e n t i n s m a l l amounts, has t h e r a t h e r s u r p r i s i n g e f f e c t ( a f t e r r e d u c t i o n a t 560 K) of s u p p r e s s i n g t h e a c t i v i t y of t h e s u p p o r t . A t h i g h e r l e v e l s , however, as i n t h e 5.8% c a t a l y s t s , t h e copper behaves p o s i t i v e l y , and i t seems l i k e l y t h a t t h i s enhancement would be s u s t a i n e d a s copper s u r f a c e a r e a were f u r t h e r i n c r e a s e d by more copper l o a d i n g towards t h e t y p i c a l v a l u e s used i n t h e i n d u s t r i a l s h i f t c a t a l y s t ( 1 ) . Among t h e p r e s e n t c a t a l y s t s , t h e most a c t i v e of a l l f o r r e v e r s e s h i f t w a s , f o r i n s t a n c e , A-5.8%. Reverse s h i f t was i n v e s t i g a t e d f u r t h e r u s i n g t h e C02:H2:He = 1:1:12 mixture (pco2 = 67 t o r r , pH2 = 67 t o r r ) a t 538 K. The conversions t o CO
were lower than i n t h e C02:H2:He
= 1:11:8 mixture (pco2 = 48 t o r r , pH2 =
528 t o r r ) f o r a given space v e l o c i t y , but i n a l l o t h e r r e s p e c t s t h e behaviour of t h e catalysts c l o s e l y p a r a l l e l e d t h a t found w i t h t h e hydrogenr i c h f e e d . A c t i v i t y w a s a minimum i n t h e middle range of copper c o n t e n t The c a t a l y s t w i t h t h e ( a t 1.2% i n t h e A-series and 2.4% i n t h e B - s e r i e s ) . h i g h e s t r e v e r s e s h i f t a c t i v i t y i n t h e B-series was a g a i n B-0.7%. a s observed w i t h t h e 1/11 f e e d ( F i g . 4 ) . The r e l a t i v e a c t i v i t i e s i n t h e two m i x t u r e s (1/1 and 1/11) i m p l i e s a higher p o s i t i v e o r d e r of r e a c t i o n f o r Hz than f o r C02. The f u l l s e t of d a t a f o r rates of formation of CO i n r e v e r s e s h i f t are compiled i n Table 1, which f o r completeness a l s o i n c l u d e s t h e r e s u l t s f o r t h e r a t e s of methanol formation i n t h e C02:H2 = 1/11 mixture d e s c r i b e d i n S e c t i o n 3.2.1. I n a l l c a s e s , t h e g r a d i e n t of t h e l i n e a r p l o t of y i e l d versus r e c i p r o c a l s p a c e v e l o c i t y ( c f . F i g s . 2-41 has been taken a s t h e measure of c a t a l y t i c a c t i v i t y , shown a s k/mol g:its-' i n Table 1.
Table 1 Rates of formation of CH30H (methanol s y n t h e s i s ) and of CO ( r e v e r s e s h i f t ) a t 538 K Rate of formation Sample
A-ZnO A-0.7% A-1.2% A-2.4% A-5.8% B-ZnO B-O.7% B-1.2% B-2.4% B-5.8%
C02:H2 = 1:11
k/mol g;its-l CO2:H2 = 1:l
CH30H
co
co
lO'lk
109k
109k
0
13.4 11.5 2.8 12.9 19.3
6.0 3.0 0.8 6.3 9.2
7.2 8.5 3.3 1.8 5.8
2.3 6.1 1.0 0.6 2.8
5.8 0.8 8.8 15.8 0
5.1 0.5 2.8 6.3
1514
X
/
/ 8-0.7%
/,
B-ZnO
,/+
-15
CD
I
0.5
1 .o
(SV)"/S
Figure 4 . CO y i e l d i n t h e r e v e r s e water-gas s h i f t r e a c t i o n over t h e B-series c a t a l y s t s a t 538 K. Feed: CO2/H2 = 1/11.
3.2.3.
(SV)-'Is
(SV)-'/S
Figure 5. CH30H y i e l d ( l e f t ) and CO y i e l d ( r i g h t ) f o r B-0.7% Cu c a t a l y s t . Pre-reduction a t 560 K (open circles) and 580 K ( f i l l e d circles).
E f f e c t of pre-reduction temperature
The compilation i n Table 1 shows t h a t t h e c a t a l y s t derived from t h e pure
s o l i d s o l u t i o n (B-0.7%) is e x c e p t i o n a l . Bearing i n mind i t s l o w copper content, and t h e f a c t t h a t only a small f r a c t i o n of i t ( t h a t i n t h e outermost l a y e r s of t h e c r y s t a l l i t e s ) i s a c c e s s i b l e for reduction i n t h e pretreatment i n % : H e a t 560 K , t h e r e s u l t i n g copper must indeed be hypera c t i v e . The pretreatment temperature of 560 K had been chosen as the best compromise for t h e series as a whole: a higher temperature was thought l i k e l y t o a f f e c t t h e c a t a l y s i s adversely by s i n t e r i n g t o a g r e a t e r degree t h e r e s u l t i n g supported copper. T h i s , however, is more r e l e v a n t t o t h e reduced b i p h a s i c oxides where t h e f r e e CuO reduces completely. For t h e monophasic solid s o l u t i o n , w i t h i t s g r e a t e r p o t e n t i a l f o r anchoring t h e reduced copper, a higher pre-reduction temperature is on t h i s reckoning more admissible. A few experiments were t h e r e f o r e conducted on B-0.7% pre-reduced a t t h e higher temperature of 580 K. R e s u l t s a r e shown i n Fig. 5 for methanol s y n t h e s i s and r e v e r s e s h i f t , s t u d i e d u s i n g t h e C02/H2 1/11 mixture a t 538 K, t h e p l o t s a l s o showing f o r comparison t h e previously-presented r e s u l t s ( F i g s . 2 and 4) for t h e corresponding c a t a l y s t pre-reduced a t 560 K. There are two p o i n t s t o note. F i r s t , the methanol s y n t h e s i s a c t i v i t y and t h e r e v e r s e s h i f t a c t i v i t y are both enhanced by the s t r o n g e r reduction conditions. Secondly, t h e f a c t that there is a p o s i t i v e i n f l u e n c e on r e v e r s e s h i f t a t a l l confirms t h a t there is a copper-catalyzed c o n t r i b u t i o n to a c t i v i t y a t t h i s low l e v e l of copper content for catalyst derived from s o l i d s o l u t i o n , as i n d i c a t e d a l r e a d y i n F i g . 4 in t h e comparison of 8-ZnO w i t h B-0.7%.
1515 3.2.4. Temperature-dependence of reverse shift activity The question arises as to whether there is a significant mechanistic difference between the reverse shift mechanism at the low-copper level and the higher-copper level. The effect of temperature on reverse shift was therefore briefly investigated using the hydrogen-rich feed and temperatures between 495 and 560 K. The catalysts were given the standard pre-reduction treatment (16 h at 560 K). The CO yield at a space velocity (SV) of 1.1 9-1 was determined and taken as the measure of the rate constant k. This was a median SV in the range which had been investigated in detail in the main studies at 538 K and for which linearity of yield vs. (SV)-l had been established. Plots of In k vs. 1/T yielded the activation energies Ea shown in Table 2. Table 2 Activation energy of reverse shift reaction at 495-560 K ~~~~~
A-series catalysts Catalyst A-ZnO
A-0.7% A-1.2% A-2.4% A-5.8%
Ea/kJ mol-1
75 100 85 90 90
B-series catalysts Catalyst B-ZnO B-O.7% 8-1.2% B-2.4% B-5.8%
Ea/kJ mol-1 80 80 85 80 80
The values cluster in the range 80-90 kJ mol-l. In view o f the singlepoint nature of the measurements (one SV only) the values must be taken as approximate. Nevertheless, they serve to show that there are no large differences between the catalysts indicative of different mechanisms. The magnitudes endorse the view that reverse shift is an easy reaction on both ZnO and the copper-containing catalysts. In forward shift, ZnO does not rank highly among oxide catalysts (22). The high-temperature pretreatment afforded to ZnO in this work appears to have favoured a mechanism for reverse shift of particularly low activation energy.
4. CONCLUSIONS Monophasic CuGnl-xO solid solution containing 0.7 mol % Cu can be prepared by calcining Cu-Zn hydroxycarbonate for 18 h at 1173 K. Less strong calcination or use of a significantly higher concentration of Cu leads to a biphasic product consisting o f free CuO and CuXZnl-,O solid solution. The limit of solubility of Cu2+ in ZnO is of the order of 1 mol %. Copper-zinc oxides with Cu contents between 0 . 7 and 5.8 mol % Cu prepared by calcination at 1053 or 1173 K can be reduced at 560 K to give catalysts which are active for methanol synthesis. Especially high activity is shown by catalyst derived by superficial reduction of the monophasic 0.7 mol % solid solution. This is attributed to very finely-divided Cu particles anchored in the surface o f a ZnO matrix. No methanol synthesis activity is observed for similarly-calcined and reduced ZnO.
1516 The same materials are a c t i v e c a t a l y s t s for t h e r e v e r s e water-gas s h i f t r e a c t i o n , b u t i n t h i s case ZnO i t s e l f i s a l s o a c t i v e , Among t h e copperc o n t a i n i n g c a t a l y s t s , t h e monophasic 0.7 mol % Cu s o l i d s o l u t i o n a g a i n l e a d s t o material w i t h very high r e l a t i v e a c t i v i t y . An i n c r e a s e i n t h e pre-reduction temperature from 560 K t o 580 K for t h e 0 . 7 mol % Cu m a t e r i a l enhances i t s a c t i v i t y f o r both methanol s y n t h e s i s and r e v e r s e water-gas s h i f t , The v a l u e s of t h e a c t i v a t i o n energy for t h e r e v e r s e s h i f t r e a c t i o n a t 495-560 K c l u s t e r i n t h e range 80-90 k J mol-l, i n d i c a t l v e of a f a c i l e reaction. The a u t h o r s acknowledge t h e support of S.E.R.C. and I.C.I. PLC f o r work. They thank I . C . I . Billingham C a t a l y s i s Centre f o r t h e u s e of f a c i l i t i e s i n t h e p r e p a r a t i o n of samples.
this
5 . REFERENCES
W i g g ( a d . ) , C a t a l y s t Handbook, 2nd e d i t i o n , Wolfe, London, 1989. K. Klier, Adv. C a t a l . R e l a t . Subj., 31 (1982) 243. 3 G.C. Chinchen, M.S. Spencer, K.C. Waugh and D.A. Whan, J. Chem. SOC. Faraday Trans. 1, 83 (1987) 2193. 4 J . C . J . Bart and R.P.A. Sneeden, C a t a l . Today, 2 (1987) 1. 5 R.G. Herman, K. Klier, G.W. Simmons, B.P. Finn and J.B. Bulko, J. Catal.,
1 M.V.
2
56 (1978) 407. J.B. Bulko, R.G. Herman, K . Klier and G.W. Simmons, J. Phys. Chem., 83 (1979) 3118. 7 Y. Okamoto, K. Fukino, T. Imanaka and S . T e r a n i s h i , J . Phys. Chem., 87 (1983) 3747. 8 R. Buroh, R.J. Chappell and S.E. Golunski, J . Chem. SOC. Faraday Trans. 1, 8 5 (1989) 3569. 9 R. Burch, S.E. Golunski and M.S. Spencer, J. Chem. SOC. Faraday Trans., 86 (1990) 2683. 10 G.C. Chinchen and M.S. Spencer, C a t a l . Today, 10 (1991) 293. 11 G.C. Chinchen, P.J. Denny, D.G. P a r k e r , M.S. Spencer and D.A. Whan, Appl. C a t a l . , 30 (1987) 333. 1 2 A.Ya. Rozovakii, Kin. K a t a l . , 21 (1980) 97. 1 3 D.S. Newsome, Catal. Rev. Sci. Eng., 23 (1980) 275. 1 4 T. van Herwfjnen and W.A. d e Jong, J. C a t a l . , 63 (1980) 8 3 and 94. 1 5 D . Waller, D . S t i r l i n g , F.S. Stone and M.S. Spencer, Faraday Disc. Chem. SOC., 87 (1989) 107, 1 6 F.H. Chapple and F.S. Stone, Proc. B r i t . Ceram. SOC., 1 (1964) 45. 17 S. Roberts, Phys. Rev., 118 (1960) 1509. 1 8 R,M. Nix, T. Rayment, R.M. Lambert, J.R. J e n n i n g s and G. Owen, J . Catal., 106 (1987) 216; G. Owen, C.M. Hawkes, D. Lloyd, J . R . Jennings, R.M. Lambert and R.M. Nix, Appl. Catal., 33 (1987) 405. 19 G , C . Chinchen, C.M. Hay, H.D. Vandervell and K.C. Waugh, J. C a t a l . , 103 (1987) 79. 20 G.C. Chinchen, K.C. Waugh and D.A. Whan, Appl. Catal., 25 (1986) 101. 21 W.X. Pan, R. Cao, D.L. Roberts and G.L. G r i f f i n , J. C a t a l . , 114 (1988) 440. 22 D.G. Rethwisch and J . A . Dumesic, Appl. C a t a l . , 2 1 (1986) 97. 6
1517
DISCUSSION Q: K. Klier (USA) I noticed that you called the form of cop er formed by surface reduction of the (e.g.) 0.7 9% solid solution "hyperactive copper". 8 0 you think that this "hyperactive copper could be stabilized or optimized by some special preparation methods ? What do you conclude regarding the chemical and physical nature of this "hyperactive copper" ? A F.S. Stone We regard the hyperactive copper as reduced copper, whose surface atoms are capable of cycling between oxidation state 0 and 1. We consider it to be a finely-divided copper in which almost all atoms are surface atoms: these are in contact with the ZnO matrix/support, or close enough to be influenced by it and thereby stabilized a ainst mobility. The contact could be epitaxial, as you yourself have suggestecf [l]. Optimization of our catalyst would be achieved by preparing a copper-zinc oxide solid solution containing the maximum possible content of dissolved copper, and then reducin it at a temperate high enough to extract copper from the surface layers, but not so hig that mobility causes copper nuclei to grow beyond a size where their atoms lose the influence of the zinc oxide. K. Klier, Adv. in CuruL, 31,243 (1982) [l]
a
Q: V. A. Sadykov (Russia) What is your opinion: is all copper active or is there a small number of active centers ?
A: F. S. Stone We believe that all the copper which is reduced and accessible to the reactants is active, but that copper close to the zinc oxide boundary may be the most active.
0:J. Haber (Poland) Taking into account that copper has a lower work function than the zinccopper oxide solid solution, one could expect that highly dispersed copper clusters will have the tendency to give off electrons to the suppprt and therefore the oxidation state of copper atoms will be between 0 and t1 generating this superactivity. A F. S. Stone A priori this is a possible mechanism for promotion by the zinc oxide matrix, but the view has often been expressed that there is little electron transfer from Cu metal to zinc oxide [2,3].We do not see Cu(O), i.e. totally-reduced copper, as the essential species for activity in CH30H synthesis, but rather than an oxidation state between 0 and 1 is the key. The adsorption of oxygen, producing negatively charged species, is probably a more si ificant factor in increasing the oxidation state of copper. K. Klier, Appl. Surf: Sci., 19,267(1984) H.-W. Chen, J. M.White, J. G. Ekerdt,J. Curul., 99,293 (1986)
Q: L. Leclercq (France) Your results about conversion to methanol versus reciprocal space velocity clearly show that CH30H from COP t H2 is a primary product and that CO is not an intermediate of the reaction (confirmed by the reverse water gas shift reaction). Can we expect that the intermediates should be different in the CO t H z reaction than those in the CO + H2 reaction ? Can you tell us what kind of intermdiates it could be ?
1518
A: F. S. Stone We have no reason to doubt the widely-held view that there is a direct route to methanol via a surface formate intermediate when CO and H2 react over a Cu/ZnO catalyst. The same intermediate may well occur in CO iydrogenation to methanol, but the mvera e in that case may not be as great and its orientation may be different [4]. hf E. Ekley, J. R. Jennings, M. S. Spencer, 1.CafaL, 118,483 (1989) [4] Q: H. Topsoe (Denmark) Being one of the first to develop methods for Cu metal surface area determination, I would like to know to what extent you have used such methods. The larger activity of the solid solution catalyst with respect to the ones you produced from catalysts with separate Cu oxides may reflect larger Cu metal surface areas. Are your turnover frequencies larger than those reported for other CuEnO or Cu/ZnO/A1@3 catalysts ?
A F. S. Stone We have put most effort into the N 2 0 method, whereby reaction is selective for surface co per [5,6]. However, in the present case where even the total surface area was 2 , where B [Fe(2.5+) + Fe(2+)]0~tb and A [Fe(3+)tetm]. Formally this leads to negative values of ( x ) and hence to anion deficiency in the magnetite phase [8], which can be expressed as follows: 2Fe3+( Fe:+xFe::xox) OQ
o:i
Fe304
ed Fe2+Fe3+ x I - x ( Fe::xFe3+I - x
2-
O4-x
The symbathic dependence between the parameters (x, FWHM) and the catalytic activity (W) shown on figures 2, 3 allows to con clude that it is caused by one and the same process. This is the process of Fe(2+) Fe(3+) electron transfer, which is corn patible both with the "redox" and with the "associative" mecha nisms of the reaction [l]. The study of the line widths of the B sublattice of magnetite in the absence of the reaction shows that they do not depend on temperature [9]. This allows to suggest, that in reaction conditions, beside the Verwey's electron "hopping" [ 101 in magnetite , a "catalytically stimula C)
1525
B [Fe2'5+]ocT I I 1 A [Fe'+]rn~I A, [ ~ e ~I + 1 ~ I~ ~ I a Fe203I SPM I I
I
I
I 1
I I
-
I
1 I I
I
I I1 I
100
1
I 1
I
I
1
I
n
1R
U
Z
0 v) I
96
v)
3
c
I
W
2 92
3
I -6
I
-8
I -4
I
I
I
I
I
I
1
-2 0 2 VELOCITY [rnrn/s]
W K
I
I 4
I
I
1
6
I 8
Fig.1. Moessbauer s p e c t r u m r e c o r d e d at 623K, P(H20)=31 .2kPa CU~POPX CO/Ar
l w x
tCO/HJ
ODPQQW ...a,W
CO/Ar IC0,HJI n n
"E
i 1 '.
< 0.65 n
25
E E
22
U
E
,--E---
u
I
-0.02 523 573 623
0
673 723
T IK1
F19.2. Vacancy concentratlon x and catalytic activity W
*
0.55
'
4 I
0.45
5 2.
13
Y
M
0.35 10 5125 573 623 673 7 213 T rK1
Fig.3. B-slte llne wldths FWHM and catalytlc activity W
-
&
16
19
Y
$
n
Fig.4. A double-exchange process Fe(3+) without (A) and with (a) participation of CF0"b"'d H2O
F
-
*a
3
1526
ted hopping" might also exist, which has effect on (x) and (FWHM). A visual idea about the proceeding of the "catalytical ly stimulated hopping" in magnetite during the reaction is shown on fig.4. It is based on Rosencwaig's model [lo] a model of two electron transfer between Fe(2+) and Fe(3+) with the participation of oxygen 2p orbitals. Of basic interest is the question about the advantages for the catalytic process provided by the "catalytically stimulated hopping" between the two ions with their simultaneous partici pation in the oxidation and reduction steps of the reaction. If the two steps proceed simultaneously with the participation of the pair of iron ions in B position, favourable conditions are created for ideal inter-step compensation of the second order energy [ll]. This results in a decrease of the activation ener gy of the endothermal (limiting) step, due to a shift of its initial energy level. Having in mind the polaron character of the electron transfer in magnetite, one could suggest, that the energy compensation between the reduction and the oxidation steps of the reaction takes place not only within the ion pair, but between the ions of the lattice as a whole. The B sublattice of magnetite allows electron transfer between and in Fe(2+)-Fe(3+) ion pairs which can be defined by the terms "interpair hopping" and "intrapair hopping" [ 101. The existence of "cooperative interpair hopping" [lo] is also suggested. Hence, the above mentioned energy compensation can take place within the whole lattice. Such a type of cooperative lattice effects has a more general meaning in catalysis and is observed in other redox catalytic systems too.
-
&
4.
REFERENCES
1 A. Andreev, T. Halachev, D. Shopov, Bulgarian Academy of Sciences, Commun. Department of Chemistry, 21 (1988) 307. 2 H. Topsoe, M Boudart, J. Catal., 31 (1973) 346. 3 J. Phillips, Y. Chen, J.A. Dummesic, Am. Chemical Society Symp. Ser. 1985, 288 (Catalyst Characterization Sci.) 518. 4 Ding Ying-Ru, Yen Qi-Jie, Hsia Yuan-Fu and all, J. Phys. , 41 (1980) C1-341. 5 I. Mitov, T. Tabakova, D. Andreeva, T. Tomov, 2.Phys.D Atoms, Molecules and Clusters, 19 (1991) 257. 6 H. Topsoe, Ph.D.Dissertation, Stanford university, 1972. 7 J. Daniels, A . Rosencwaig,J. Phys. Chem. Solids, 30 (1969) 1561. 8 A . Matveev, Iu. Maksimov, I. Suzdalev, Izvestia AN USSR, Ser. Chem., (Russian), 7 (1976) 1439. 9 B.J. Evans, Proceed. Conf. Magnetism Magnetic Materials, San Francisco, Dec.1974, p.2. 10 A . Rosenwaig, Canad. J. Phys, 47 (1969) 2307. 11 G . K . Boreskov, Heterogeneous Catalysis, (Russian), "Nauka", Moskow, 1986, p.29.
Gwzi, L et al. (Editors), New Frontiers in Catafysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
IN SITU F"-IR STUDY OF 0 2 , CO, C02, CH4 AND C2H4 ADSORITION OR
REACTION ON THE La203/MgO CATALYST S.Shen, R. Hoy , W. Ji, Z. Yan and X. Ding State Key Laboratory for 0 x 0 Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Introduction Rare ep-th/alkaline earth metal binoxides have been found to be In these very active for oxidative coupling of methane [I1 systems the nature of the active oxygen species is still not clear and has aroused much speculation. Superoxide G was the most abundant species seen by ESR c21 and XPS csl for rare earth-alkaline earth metal composite oxide. The possibility of such a superoxide ion being involved in oxidative coupling of methane has been raised by several authors. In this paper, the adsorption and reaction of 02, CO, C02, CH, and CzH, on the La20S/Mg0 catalyst has been studied using in situ FT-IR spectroscopy. In combination with the behavior of OCM over the La20,/Mg0, the nature of surface oxygen species is discussed.
.
Experimental 10~owt-La20s/Mg0 catalyst was prepared by coprecipitation from a solution of La(NO& and Mg(NOs)2 with (NHJ2COs as the precipitating agent. The dried catalyst was calcined at 1M3K for 4 h before use. The BET surface area was 5.6m2/g. experiment, the catalyst was pressed into a self For FT-IR supporting disk of OlOmmxO.lmm, and placed in a quartz IR cell of 0 15mm x Ll2Omm with water cooling KBr windows. The sample was pretreated by evacuating at 1050K for 1 h, followed by oxidation in 4x104 Pa O2 at 1050K for 0.6 h, then cooled in vacuo to room temperature prior to the each FT-IR experiment. In this paper, O-, -sample represents the pretreated sample by the above mentioned procedure. IR spectra were recorded on a Nicolet 1ODX-FTIR spectrometer in the single beam mode at a resolution of 4 cm-I, and plotted in transmittance. Catalytic properties of OCM were evaluated in a fixed bed quartz reactor packed with 200 mg catalyst in cofeed mode. The gases were of the following purities and further purified: C&(99.99%), C2H,(99.95%), O2(99.99%), CO(99.99%), CO2(99.99%).
1528
Results and discussion Figure 1 shows the change of IR spectra for O-, -sample as function of temperature in vacuo. A strong i.r. band at 1113 cm-I appeared at room temperature. Following the original assignment of Li et al [*I, the i.r. band at 1113 cm-' is assigned to superoxide species G. This assignment is supported by the X P S spectra of 0, B'. E. at 632.2 eV on well-outgaased 100/wt-La,Oa/MgO sampleL6'. The band of 0; decrease with increasing temperature and disappeared at 1060K . Upon cooling, the bands of G could be fully retrieved. It seems likely that a n equilibrium between the 0; and 0- may exist.
Oi
+ e
-
20-
(1)
Figure 2 shows JR spectra for adsorption and desorption of CO, on %-sample. When 7 x W Pa CO, was introduced at 3M)K, strong bands at 1643 and 1346 cm-l due to bidentate carbonate, and 1470 and 1402 cm-l due to unidentate carbonate grew rapidly, accompanied by weakening of band. Formation of such bidentate carbonate suggests the existence and accessibility of considerable anion vacancies on the surf ace. Evacuation at successively increasing temperatures caused a narrowing of the carbonate bands and consequent shift of the C& bands toward the frequencies of unidentate carbonate. The bidentate carbonate transformed almost into unidentate carbonate at 673K. C G bands began to decrease at 673K and were reduced to a eufficiently low level at 1060K. When the decarbonated sample was cooled to 300K, the Oi bands returned to the original intensity. This feature is agreement with that obtained by Lunsford et alLzlfor ESR study of LaaOs. They observed a considerable ESR singnal of O-, on a rigorously decarbonated La,Os. When La,Oa was carbonated, the O-, singnal w a ~substantially reduced and eventually destroyed. "his means that a surface equilibrium involving oxygen species and carbonates may exist as proposed by Duboie et alcal.
a
co-, + 0;
G +
co, + 0-
Figur 3 shows IR spectra for adsorption and reaction of 1 . 3 ~ 1 0Pa ~ CO on %-sample. Formate ion in i.r. bands at 2840( v (CH)), 1376( v. (OCO)) and 1699cm-l( v as(OC0)) appeared at temperature range of 400
-600K. Following heating sample up to 673K, the formate ion transformed into unidentate carbonate. I.R. spectra of 1 . 3 ~ 1 0 Pa ~ C&, CJ-L, and 1.3~104Pa HI reaction with %-sample are respectively shown in figure 4, The peaks of gas phase CH, and C,K, began to decrease at 770K and 690K respectively, accompanied by appearance of C a bands. No any evidence for ~urface hydrocarbon species was observed during the reaction of CH, and CzH,. A considerable amount of CH, and CJL was consumed by reaction with %-sample at high temperature. The % band still remained itEl initial intensity after
1529 0
0
.w
0
0
T
w
u z
NO d&sdation may procetxl acuxdhg to eq. (1) clzl :
*
C4NO
+
cu+
-
ao.N +
cu2*o-
(1)
1542
T I YE, mkr
We suggest this interaction of surface species as rate-controhg step in the NO reducrion to N20 and N2. The dfwt d Oxygen OII NO
-
r e d ~ c o u l d b e t h e m x i d a t i o n o f C u o a n d C usites: ~
2cu0
+
2Cu"
+
Q
2a+o-
0, - 2 a 2
'0-
ca)
rn
Since oxygen hinders simultaneously both of reacfiolls of NO reduction, one may attribute Cu0Ssurface Species to the prenvsats for N2O and N2 formation.
1 M. Shelef, K. Otto and H. Gandhi, J. Catal, 12 (1%8) 361. 2 T. A l k h a ~ ~ G.~ --Zade, , M. (kma.110~ and M. Sultamw, Kinet. CataL, 16 (1975) 1230. 3 M. shelef and K. Otto, Atmos Environ., 3 (1%9) 107. 4 V. Sadykov, S Tikhov and V. Popovskii Kinet. CataL, 27 (1986) 147. 5 G. Gassan.de, U Osmanov and M. sultano~,Azerb. Khim Zh.,
3 (1974) 60. 6 A. Davidov and A. &IdneM, React Kinet. CataL Lett, (1984) 121. 7 D. Panayotov, M. Khristova and D. Mehamijiev, Appl CataL, 34 (1987)49 8 €L ciandhi and M. shelef, J. catat, 28 (1973) 1. 9 A. Rozovskii, V. Sticenko and V. Tretjakov, K b t . Cat& 14 (1973') 1082 10 T. Atkhazov, G. Gassan-zade and D. Khoii Kinet. Catat, 28 (1987) 1370, 11 M. Kobayashi, them. Eng. Sci, 37 (1982) 393 12 J. London and A. Bell, J. CataL,.31 (1973) %.
Guczi, L.et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
CHARACI'ERIZATIONAND ACTIVITY OF VANADIUM OXIDE CATALYSTS IN SELECTIVE CATALYTIC REDUCTION OF NITRIC OXIDE
U.S. Ozkan, Y.Cat M. W. E(umthekar and L. Zhang Department of Chemical Engineering, The Ohio State University, Columbus, OH 43210-1180, USA
Abstract Nitric oxide reduction with ammonia and direct oxidation of ammonia were studied over unsupported vanadia catalysts, having preferential exposure of different crystal planes. Surface species and morphological factors were investigated by using various characterization techniques. The activity and selectivity measurements were performed using a steady-state fixed-bed reactor system. 1. INTRODUCTION
Nitrogen oxides have been known as serious air pollutants for the last two decades and there is increasing concern about their negative impact on the environment through acid rain, forest decay, and the so called 'greenhouse effect'. Selective catalytic reduction (SCR) of nitrogen oxides has proven to be an effective technique to reduce the NOx emissions from the stationary combustion sources. Vanadia catalysts have been studied quite extensively because of their high activity and selectivity in selective catalytic reduction of nitric oxide [l-91. Several review papers have summarized most of the work done in this area [lo, 111. In previous papers we have reported the results of our studies dealing with the effect of the nature of the support material and the reaction parameters on the activity and selectivity of the supported vanadia catalysts [12-141. In addition to studies dealing with supported vanadia catalysts, there have been efforts in investigating the nature and the catalytic behavior of unsupported vanadium pentoxide [15, 16, 1 4 . In our more recent studies, we focussed on the structural specificity of vanadia catalysts in relation to their activity in selective catalytic reduction of nitric oxide and in oxidation of ammonia. In addition to characterization studies with X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, 3-D imagingkcanning electron microscopy techniques, temperature programmed reductionloxidation experiments were also used to gain insight into the differences in catalytic behavior exhibited by vanadia crystals that preferentially exposed different crystal planes.
1544 2. CHARACTERIZATION AND REACTION STUDIES
The unsupported VO ,, catalysts having preferential exposure of crystal planes were prepared in two different ways. Ammonium metavanadate (Aldrich) was calcined at 520' C for 50 hours in a flow of oxygen to give sample D. Pure VO ,, (99.6%) from Aldrich was melted at 695O C for 2 hours and then subjected to temperature programmed recrystallization to give sample M. The specific surface areas of the catalysts were measured by the BET technique with a Micromeritics 2100 E Accusorb instrument using Krypton as the adsorbate. X-ray powder diffraction patterns were obtained using Scintag PAD diff ractometer. Cu K, radiation ( h. = 1.5432 A ) was used as the incident X-ray source. The nature of the surface species was examined using a laser Raman spectrometer (SPEX 1403 Ramalog 9 - I spectrometer ), which used a 5 W Argon ion laser (Spectra Physics, model 2016 ) as the excitation source. The in-situ Raman spectroscopy experiments were performed using a specially designed high-temperature, controlled-atmosphere cell. A Hitachi S-510 scanning electron microscope using a voltage of 25 KV was used in combination of 3-D imaging technique to examine the surface morphology and crystal dimensions of the crystals. X-ray photoelectron spectra of the samples were obtained using a Physical Electronics/Perkin Elmer ( Model 550 ) ESCNAuger Spectrometer, operated at 15 KV, 20 mA. The X-ray source was Mg K, radiation (1253.6 eV ). The binding energy for C 1s at 284.6 eV was used as reference in these measurements. The reactor system used in this study was described previously [13]. Analyses of feed and product streams were performed by combining gas chromatography, chemiluminescence and titration techniques. Activity measurements for SCR of NO were performed using a feed mixture that consisted of 1465 ppm NO, 1418 ppm NH, and 0.88% oxygen with helium as balance gas. For direct ammonia oxidation, the feed mixture composition was 1418 ppm NH3, 0.88% oxygen and balance helium. Temperature programmed reduction (TPR) experiments were performed using 6.0% H, in N, as the reducing agent. The reducing gas flow rate was set at 60 cm3 (STP)/min. The temperature in the sample batch increased from 200 to 950' C at a rate of S0C/min. Samples were evacuated insku for 2 hours prior to each run. X-ray diffraction and laser Raman spectroscopy were used to determine the nature of the reduction intermediates.
-
3. RESULTS AND DISCUSSION
The surface areas of the samples D and M were 4.2 and 0.25 m2/g, respectively. The comparison of the areas exposed by various crystal planes over V205 crystals was done by scanning electron microscopy combined with 3-D imaging technique. The results clearly show that the sample prepared by decomposition and subsequent calcination of ammonium metavenadate produced crystallites with roughly equal side and basal planes, and the sample prepared by melting and temperature programmed recrystallization showed preferential exposure of the basal plane (010). X-ray diffraction patterns supplement the information of preferred orientation from the scanning electron microscopy studies. The relative intensity of
1545 (OkO) reflections was consistently higher in case of sample M. Table 1 shows the
ratios of relative intensities for various (hkl) reflections.
XI(h00)/1(010)
0.04
1.07
~1(001)/1(010)
co.01
0.26
1(101)/1(010)
co.01
0.73
............................................................................................................ The Raman spectra of both samples showed that the ratios of the intensities of 995 cm-l band to those of 702 cm-l, 528 cm-l, and 483 cm-l were consistently higher for sample M which preferentially exposed the basal plane. Since 995 cm-l is associated with V=O stretching vibration and 702 cm-ll 528 cm-I and 483 cm-l bands are associated with bridging oxygen sites [18], this observation strongly suggests a higher concentration of L O sites on sample M. This is in agreement with the SEM studies and the X-ray diffraction results, as well as the literature reports which proposed that V=O species were concentrated on the (010) plane 1161. ,, in SCR reaction, both samples To investigate the structural specificity of VO were subjected to reaction conditions over a temperature range of 150 to 400" C. At lower temperatures ( c 250" C ) sample D was more active than sample M for converting NO, whereas at higher temperatures ( > 250" C ) sample M was found to be more active than sample D. Both samples exhibited a maximum activity for NO conversion at 350" C. A careful investigation of the production rates of N20 and N, for both samples over the whole range indicated that the basal plane (010 ) promoted , whereas the side planes contributed to the formation of N2. the formation of NO Direct ammonia oxidation studies were performed over both samples under the same reaction conditions that were used for NO reduction. Both catalysts showed very low ammonia conversion below a temperature of 250" C. At temperatures of 300" C and above, a dramatic ammonia consumption rate increase was observed over sample M. Although the same trend was observed for sample D, the ammonia consumption rate over this sample was much lower than over sample M. In addition to N,O, NO and N, were also observed. Very little NO was formed on sample D even at 400" C. However, NO started to form at 300" C over sample M. The production rate of NO over sample M increased dramatically with increase in temperature. The production rates of NO and N2 reached a maximum at 350' C over sample M, and at 400" C over sample D. Temperature programmed reduction studies over samples D and M also showed a difference between the reduction behavior of basal and side planes. Three reduction steps were observed with sample D, while four were observed with
1546
sample M. The X-ray diffraction patterns of the intermediates showed that the peaks of sample D at 660,690 and 850" C are due to reduction of V205 to v6013, V02 and V2O3 respectively. For sample M, although the final reduced species was the same as that of sample D at 870" C, the presence of a low temperature reduction peak at 430 O C indicated a higher tendency for reduction through the oxygen sites located on the basal plane. The total hydrogen consumption for both samples was comparable, which is in good agreement with the theoretical value of 10.99 mmol/g assuming the final species to be V,O,. When combined with characterization data and TPR measurements, the results from NO reduction and ammonia oxidation studies prqvide important clues about the catalytic phenomena involved in selective reduction of NO versus complete oxidation of ammonia and the role of terminal and bridging oxygen sites in these competing reactions.
Acknowledgment This material is based upon work supported by the Environmental Protection Agency under Award No. R- 815861-01-0. Partial financial support form Exxon Corporation is also gratefully acknowledged. 4. REFERENCES
H. Bosch, F.J.J.G. Janssen, F.M.G. van den Kerkhof, J. Oldenziel, J.G. van Ommen and J.R.H. Ross, Appl. Catal., 25 (1986) 239. 2 G.L. Bauerle, S.C. Wu and K. Nobe, Ind. Eng. Chem. Prod. Res. Dev., 14(4) (1975) 268. G.L. Bauerle, S.C.Wu and K. Nobe, Ind. Eng. Chem. Prod. Res. Dev., 17(2) 3 (1978) 117. S. Morikawa, H. Yoshida, K. Tahahashi and S. Kurita, Chem. Lett., (1981) 251. 4 In-Sik Nam, J.W. Eldridge and J.R. Kitrell, Ind. Eng. Chem. Prod. Res. Dev., 25 5 (1986) 186. J. Haber, A. Kozlowska and R. Kozlowski, J. Catal., 102 (1986) 52. 6 7 M. Inomata, A. Miyamoto, T. Ui, K. Kobayashi and Y. Murakami, Ind. Eng. Chem. Prod. Aes. Dev., 21 (1982) 424 F.J.J.G. Janssen, Kema Scientific 81Technical Reports, 6(1) (1988) 1. 8 M. Kotter, H.G. Lintz, T. Turek and D.L. Trimm, Appl. Catal., 52 (1989) 173. 9 10 H. Bosch and F. Janssen, Catal. Today, 2 (1988) 369. 11 G.C. Bond and S.F. Tahir, Appl. Catal., 71 (1991) 1. 12 Y. Cai and U.S.Ozkan, Int. J. Energy, Environment, Economics, l(3) (1991) 229. 13 Y. Cai and U.S. Ozkan, Appl. Catal., 78 (1991) 241. 14 U.S. Ozkan, Y. Cai and M.W. Kumthekar, ACS symposium series (1992) in press. 15 M. Gasior and T.J. Machej, J. Catal83 (1983) 472. 16 M. Che and G.C. Bond (eds.), Adsorption and Catalysis on Oxide Surfaces, Amsterdam, The Netherlands, 1985. 17. M. Gasior, J. Haber, T. Machej and T. Czeppe, J. of Molecular Catal., 43 (1988) 359. 18 I.R. Beattie and T.R. Gilson, J.Chem. SOC.(A), (1969) 2322. 7
Guczi, L. et ol. editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis,19-24 July, 1992,Budapest,Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
THEORETICALSTUDY OF CO CHEMISORPTIONON Rb AND Pd CLUSTERS A. Goursotn, I. Pdpaib and D. R. Salahubc
aUA 418 CNRS, Ecole de Chimie, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 1, France bInstituteof Isotopes of the Hungarian Academy of Sciences, 1525 Budapest, P.O.Box 77, Hungary 9epartrnent de Chimie, Universite de Montreal, C.P. 6128, Succ. A, Montreal, Canada
INTRODUCTION Modelling of chemisorption, even for very simple adsorbates, is one of the most appealing areas of modern quantum chemistry. The nature and strength of the adsorption bond, its relation with different possible sites on the catalyst, the influence of the interactions between adsorbate molecules are all basic questions to answer on the way to understanding chemisorption phenomena. The chemisorption of CO on Pd and Rh surfaces and supported particles has been extensively studied experimentally, since CO is very often used as a probe molecule, studied by means of infrared spectroscopy. Major differences between Rh and Pd catalysts have been pointed out. At similar coverages, the CO adsorption sites are different, and also the ordering of the overlayer. Hydrogenation of CO yields methanol on Pd catalysts, through a non dissociative mechanism. In contrast, Rh catalysts can also promote the dissociation of CO, leading to alkane formation. Numerous papers on the chemisorption of CO on supported Rh aggregates have been devoted to the problem of disruption of Rh particles, yielding isolated Rh(C0)2 entities. This phenomenon has also been reported for Ru, but has never been mentioned for Pd particles. In order to address all these questions, we believe that first principles quantum chemical calculations have to be performed. CO bonding has been extensively studied by a In fact, metal great variety of theoretical approaches, however, only a few studies have compared different metals and different sites of adsorption (1-9). In a first attempt to understand why CO chemisorption is different on Rh and Pd catalysts, we have performed accurate quantum chemical calculations on M4CO models, where M = Rh, Pd and CO is located at three different possible sites : top, bridge and 3-fold.
-
1548 DISCUSSION OF THE RESULTS
Our c a l c u l a t i o n s have been performed w i t h t h e Linear M o d e l Core P o t e n t i a l Combination of Gaussian Type Orbitals Density Functional ( LCGTO-MCP-DF) method, using t h e deMon program package ( l O , l l ) * Accurate equilibrium geometries and v i b r a t i o n a l frequencies can be obtained by means of t h e a n a l y t i c a l . energy g r a d i e n t s and numerical second derivatives. The use of non local exchange and c o r r e l a t i o n energy f u n c t i o n a l s allows, as shown r e c e n t l y ( 1 2 - 1 4 ) , a realistic d e s c r i p t i o n of binding e n e r g i e s , even for small model clusters. We t h u s expect t h a t examination of t h e d i f f e r e n c e s between sites f o r t h e same c l u s t e r or between Rh and Pd models w i l l g i v e a good i n s i g h t i n t o t h e t r e n d s which determine t h e e n e r g e t i c s of chemisorption on real systems. Pd4 and Rh4 c l u s t e r s have first been s t u d i e d i n a tetrahedral geometry, as a model for a (111) s t r u c t u r e . The metal-metal bond d i s t a n c e s have been kept equal t o t h e bulk values, i.e. 2.75 A for Pd4 and 2.69 A f o r Rh4. The t o t a l Mulliken population a n a l y s i s leads t o t h e following atomic c o n f i g u r a t i o n s : pd 580.54 5 ~ 0 . 1 2 4d9.34 and Rh 580.59 5 ~ 0 . 1 7 4d8.24 The c a l c u l a t e d configurations a r e , v e r y close t o each other, i f one excepts t h e one e l e c t r o n d i f f e r e n c e . The c a l c u l a t e d binding e n e r g i e s p e r atom are 2.32 ev and 1.58 ev f o r Rh4 and Pd4, r e s p e c t i v e l y . The r a t i o of these values ( 1 . 4 7 ) compares very w e l l w i t h t h e r a t i o of t h e experimental cohesive e n e r g i e s of t h e bulk materials (1.54). For PdqCO and Rh4CO models, t h e M 4 p a r t s have been k e p t f i x e d , w i t h bulk bond d i s t a n c e s , w h i l e t h e CO geometry and o r i e n t a t i o n were optimized f o r t h e t h r e e possible sites : top, b r i d g e and 3fold. A f t e r optimization, t h e 3-fold and bridge models kept t h e i r i n i t i a l symmetries, r e s p e c t i v e l y C3v and C2v. I n c o n t r a s t , CO i n a t o p p o s i t i o n does not remain on t h e C3 a x i s . I t t i l t s away from it and becomes roughly perpendicular t o a t r i a n g u l a r facet, y i e l d i n g a g l o b a l CS symmetry. The c a l c u l a t e d M-C and c-0 bond l e n g t h s , t o g e t h e r w i t h t h e related s t r e t c h i n g frequencies are reported elsewhere and compared w i t h available experimental data on supported p a r t i c l e s or s i n g l e - c r y s t a l (111) s u r f a c e s ( 1 4 ) . The d i f f e r e n c e s between Pd and Rh c l u s t e r s a t t h e same s i t e have been shown t o be f u l l y c o n s i s t e n t w i t h t h e experimental data and c h a r a c t e r i s t i c of l a r g e r Rh-CO t h a n Pd-CO bond s t r e n g t h s ( 1 4 ) . The c a l c u l a t e d CO adsorption e n e r g i e s for PdqCO and Rh4Co c l u s t e r s are compared i n table I. The atomic c o n f i g u r a t i o n s for t h e metal atom(s) bonded t o CO, evaluated from t h e Mulliken population a n a l y s i s , are also reported. For PdqCO, t h e binding e n e r g i e s t o t h e 3 sites are d i f f e r e n t 8 adsorption a t t h e 3-fold s i t e is favored, while it i s t h e least probable a t t h e t o p site. On t h e c o n t r a r y , t h e binding e n e r g i e s for t h e three Rh4Co c l u s t e r s are very close t o each other. For P d ( l l 1 ) s u r f a c e s , I R and TPD s p e c t r a i n d i c a t e t h a t CO adsorbs a t 3-fold sites for l o w coverage values, a t b o t h 3-fold and bridge sites for i n t e r m e d i a t e coverage, t o p sites being only populated near s a t u r a t i o n . The c a l c u l a t e d values compare s u r p r i s i n g l y w e l l w i t h t h e experimental r e s u l t s obtained a t half-coverage. For R h ( l l 1 ) s u r f a c e s , there are no a v a i l a b l e CO adsorption energies. However, I R and TPD experiments i n d i c a t e t h a t t h e t o p s i t e i s f i r s t occupied, followed by t h e bridge site a t i n c r e a s i n g
-
1549
coverage. CO adsorption at 3-fold site has not yet been reported without the presence of a coadsorbate. Beyond the absolute values of the binding energies.and their comparison with experimental results obtained for infinite systems, we see that the site preference is pronounced for Pd4 and very weak for Rh4. Moreover, since multiple Pd-CO bonds are favored, we can say that the x bonding which involves essentially 4d electrons is more efficient than the bonding through spa electrons which concerns the Pd-CO bond at the top site. In contrast, we can conclude that these different types of bonding are of equivalent strength for Rh4CO models. Moreover, we see from table I that the configuration of the metal atom(s) of the sites is characteristic of the adsorption site and independent of the metal itself. An obvious correlation between the binding energy of the site and the related metal configuration appears if we admit that the metal atoms of the surface (having a configuration close to (sp)O o 6 dn-o*6) keep some "mmory"of the relative energies of the ground and various excited states of the isolated atom, when it responds locally to the CO chemisorption. Indeed, the difference between the bonding capabilities of Pd and Rh atoms has been shown to be related to the large energy gap (1 eV) between the 4dlO Pd ground state and its 4d95s1 configuration, which favors essentially the x bonding with CO, while, for the Rh atom, both (I and x bonding6 can occur, due to the low ener difference (0.35 ev) between the 4d9 first excited and 4d85s ground state configurations (15). Under the assumption that CO induces essentially a local rearrangement on the metal atom(s) of the site, we thus expect that Pd atoms will adopt more easily configurations with the lowest 5s and highest 4d populations, favoring the 3-fold site to the detriment of the top one. The situation for Rh models is different, due to the ability of the Rh atom to adopt either low or high 5(sp) population. This correlates with the close binding energies obtained for the 3 sites with a slightly less favorable energy for the %fold site, related to a larger contribution of the 4d9 configuration.
Y
CONCLUSION These LCGTO-MCP-DF calculations, including non local corrections, have shown that, even for very small model clusters, the experimental trends of the CO binding energies can be reproduced. Moreover, on the basis of these results, we propose that the site preference for the adsorption of a CO molecule essentially depends on the enrgetic properties of the metal atom(s) of the site. Calculations on larger-size clusters are needed to complete this rough picture,of the electronic rearrangements in the orbitals of the metal site (and those of its neighbors), demanded by the bonding process. Further studies are in progress to verify if this reasoning can be extended to other metal atoms in the periodic table.
1550 REFERENCES 1-D. P o s t , E.J. Baerends, J. ChemPhys. 78, 5663 ( 1 9 8 3 ) D.E. E l l i s , A.J. Freeman, Q.Q. Zheng, S.D. Bader, 2-P.L.La0, Phys. Rev. B 30, 4146 (1984) 3-A.B. Anderson, M.K. Awad, J. Amer. Chem. SOC. 107, 7854 ( 1 9 8 5 ) 4-5. Andzelm, D.R. S a l a h u b , P h y s i c s and C h e m i s t r y of small C l u s t e r s , P. Jena, B.K. Rao, S.N. Khanna Eds, Nato A s i P h y s i c s vol 158, 867, Plenum (1987) 5-5. Koutecky, G. P a c c h i o n i , P. F a n t u c c i , Chem. Phys. 99, 87 (1985) 6-G. P a c c h i o n i , J. Koutecky, J. Phys. Chem. 91, 2658 ( 1 9 8 7 ) 7-A. de Koster, R.A. van Santen, S u r f . Sci. 233, 366 (1990) 8-D. Drakova, G. Doyen, S u r f . Sci. 226, 263 (1990) 9-Y.T. Wong, R. Hoffmann, J. Phys. Chem. 95, 859 ( 1 9 9 1 ) 10-A. St-Amant, D.R. S a l a h u b , Chem. Phys. L e t t . 169, 387 (1990) 11-A. St-Amant, Ph. D. T h e s i s , Universitd de Montreal ( 1 9 9 1 ) 1 2 4 . F o u r n i e r , D.R. S a l a h u b , S u r f . S c i . 245, 263 (1991) 13-P.Mlynarski, D.R. S a l a h u b , J. Chem. Phys. 95, 6050 ( 1 9 9 1 ) 14-A. Goursot, I. P a p a i , D.R. S a l a h u b , t o be p u b l i s h e d 15-1. P a p a i , A. Goursot, A. St-Amant, D.R. S a l a h u b , T h e o r e t i c a Chimica A c t a , i n p r e s s
Guni, L et al. (Editors), New Fronriers in Catalysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992. Budapest, Hungary 6 1993 Elsevier Science Publishers B.V.All rights reserved
NEW DYNAMIC METHOD APPROACH TO THE ROLES OF REVERSIBLE AND IRREVERSIBLE ADSORPTION IN HETEROGENEOUS CATALYSIS
G.Ly S.Chen and S.Peng Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China
INTRODUCTION The catalytic behavior of reversibly adsorbed species, which presents on catalyst surface under reactant stream and desorbs upon evacuation or purgation, is ambiguous because of the lack of suitable characterizing method. It has been found that the reversible adsorption takes an important role in ethylene and acetylene hydrogenation, and the relative reactivity of reversibly and irreversibly adsorbed acetylene determines the selectivity[l-3]. But little is now available about the kinetic behavior of respective adsorbed species. Recently, we developed a new dynamic model to elucidate the dynamic behavior of reversible and irreversible species in catalytic reaction[4]. In this communication, we report the results of the application of this new dynamic model to the hydrogenation of acetylene on Pt/A1203 catalyst.
EXPERIMENTAL AND THEORETICAL Experimental The Pt/A120 catalyst with 1 wt.% platinum was prepared by conventional impregnation metlod. After calcination in air at 45OoC for 2h, the catalyst was reduced in flow hydrogen at 450OC for 2h. The catalyst was then purged with argon at 45OoC and cooled in argon to reaction temperature. The dynamic experiments were conducted in a flow reaction system at atmospheric pressure. After changing the Pt/A1203 catalyst (1.0 g) from Ar to reactant, the transient curves were obtained by analyzing the effluent continuously (for adsorption) or at intervals (for reaction) with GC. The reversible adsorption (Figure 1b) was measured on the catalyst preadsorbed irreversible acetylene. Hydrogenation of irreversible acetylene was conducted by changing the catalyst preadsorbed this species from Ar to H + Ar mixture (Figure 2). And the overall reaction transient curves were obtainezby using CzHz + H mixture (Figure 3). After the overall reaction reached to the steady state, the acetsene conversion was measured with different line velocity (Figure 4). The premixed gases were used, which consisted of 1% C H + Ar for adsorption, 10% Hz + Ar for irreversible acetylene hydrogenation, an% 1% C ~ H +Z 10% H2 + Ar for overall reaction respectively. Theoretical Based on the experimental conditions and results, the dynamic equation (1) for frontal transient curve of acetylene (Figure 3a) was obtained with the following assumption: (1). linear equilibrium adsorption (K) and zero order surface reaction (k3 for reversible acetylene; (2). first order adsorption (k,) and reaction (k') for irreversible species.
1552
-E- -
a2c
u-
a ax2
aQ -=i
at
aqi at
.
K-
ac - ac ax at
ac
+kr
(X
at
aqi
+-
at
*
1-a aQ - p--=o
at
.
ka ( qlt - q1 )
-
kiqi
t=O, x20,c=o
x=o,tro, c=co The analytical solution of equation (1) was obtained and the expressions for adsorption (Figure 1) or irreversible acetylene hydrogenation (Figure 2) under simplified conditions were derived from the solution. The detailed theoretical analysis of the dynamic behavior of respective adsorbed species was given elsewhere[4]. For steady-state reaction, the expression for conversion was obtained at t - w :
RESULTS AND DISCUSSION
4
The relative amount of reversible and irreversible acetylene (qr and q' 1. measured from the adsorption curves in Figure 1. The results are shown in Ta lewas Reversible acetylene is the predominant species at the temperature examined. In Figure 2, the effluent curve for hydrogenation of preadsorbed irreversible acetylene indicates that this species is hydrogenated directly to ethane (the sole product). The serious tailing of the effluent curve and the k' calculated imply that this species is strongly adsorbed and is not easy to be hydrogenated. By using C2H2 + H2 mixture, the transient curves of overall hydrogenation reaction were obtained as shown in Figure 3. After the reaction reached its steady state, the linear relationship between acetylene conversion and l/u was found, indicating that the overall reaction was zero order with respect to acetylene. The overall rate constant, kT = kr f k'q' was then calculated by equation (2). The kinetic parameter of reversible acetylene (r;q and the amount of irreversible acetylene on catalyst surface under steady state reaction condition (qlS)were estimated from Figure 3a with known k' and kT. The results are summarized in Table 1. The relative amount of q'dq', in column 3 of Table 1 refers to the percentage of irreversible acetylene remained on catalyst surface under steady state reaction condition. By using infrared spectroscopic method, Beebe and Yates[S] also found that about ten percent of irreversible acetylene remained on Pd/$140i3 catalyst surface under steady state condition. In column 4, the ratio of k /k q represents the relative reactivity of reversible and irreversible acetylene under steady state condition. For reversible acetylene, the percentage which was hydrogenated to ethylene could be estimated from the obtained kinetic parameters and the ethylene selectivity under steady state condition. The value of kr'/kr indicates that the reversible acetylene is mainly hydrogenated to ethylene.
1553 Table 1 Relative adsorption amount and kinetic parameters of reversible and irreversible acetylene on Pt/Al2O3
- V°C)
qi 1 i sqo
s'/qi0
kr/kiqis
krlpp
0.09 1.65 0 8.5 0.10 1.70 5 9.2 15 11.1 0.09 2.0 *percentageof reversible acetylene hydrogenated to ethylene 1.2
-1.0 0.98 0.93
60
0.8 0
0.4
0.0 260
360
310
0
60
100
Time (a)
Time (a)
Fig. 1 Effluent Curves of Acetylene Adsorption at O°C on Pt/Al2O3. (A) Overall Adsorption; ( 0 ) Reversible Adsorption. Fig. 2 Ethane Effluent Curve for Hydrogenation of Irreversibly Adsorbed Acetylene at O°C on Pt/Al203.
1.o
0
0.6
0 0.2
-0.2
-
300
1200
760
Time
(8)
0.0
0-1
0.2
0.a
l / u (s/cm)
Fig. 3 Effluent Curves of Overall Acetylene Hydrogenation at O°C on Pt/Al203. ( 0 ) Acetylene; (0)Ethylene; (A) Ethane. Fig. 4 Linear Relations for Acetylene Conversion and l/u under Steady State Reaction Condition. ( 0 ) O°C; (A) 5OC; ( e ) 15OC
1554
Based on the theoretical and experimental results, we conclude that in acetylene hydrogenation on Pt/A1203, the irreversible acetylene is mainly responsible for ethane formation. The contribution of reversible acetylene, which is abundant on catalyst surface and is mainly hydrogenated to ethylene, is 1.5-2.0 times the irreversible species. From the relative amount of respective species and their reactivity, together with the additional results for acetylene and ethylene competing adsorption, it seems that, in addition to taking part in the hydrogenation, the irreversible acetylene also serves to be the modifier of catalyst surface, while the reversible acetylene acts as competing adsorption agent for ethylene intermediate, which results in the high ethylene selectivity of metal catalyst.
NOMENCLATURE C E K
k
gas phase concentration axial dispersion coefficient reversible adsorption equilibrium constant reaction rate constant total adsorption amount adsorption amount reaction rate time line velocity bed length
moyml cm /s ml/g mol/g/s (zero order) l/s (first order) mol/g mol/g mol/g/s S
cds cm
Greek Symbols
a
P
bed void particle density
g/cm3
Subscripts 0
a t
initial adsorption initial total amount
Superscripts i r T
irreversible reversible overall
REFERENCES 1. S. J. Thomoson & G. Webb, J. Chem. Soc. Chem. Comm., 526(1976). 2. A. S. Al-Ammar & G. Webb, J. Chem. SOC.Faraday Trans I, 73,195(1978); 74,657( 1978); 75, 1900(1979). 3. A. Sarkany, L. Guczi & A. H. Weiss, Appl. Catal., 10, 369(1984). 4. G. Lu, S. Chen & S. Peng, Chinese J. Catal., 12,301(1991). 5. T. P. Beebe & J. T. Yates, J. Am. Chem. Soc.,108,663(1986).
Guni. L ef al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 6 1993 Elsevier Science Publishers B.V. Ail rights reserved
DISCRIMINATIONAND REGULATION OF MULTI-REACTIONPATHWAYS IN HETEROGENEOUS CATALYSIS
M.Kobayashi T.Kanno and M.Hakozaki Department of Industrial Chemistry, Kitami Institute of Technology,090 Kitami, Hokkaido, Japan
ABSTRACT Anomalous behavior of heterogeneous catalysis a t the steady state and transient state in CO oxidation on ZnO and propene oxidation on F%/Si02 has been interpreted by the multi-reaction pathway models based on the different key species(Is). CO oxidation consists of an E-R part in which Is is O(neutrally adsorbed oxygen) and a L-H part in which Is is 0-. The mode of transient response curve is sensitively shifted between a monotonous, an overshoot, a false s t a r t and a complex type according to the concentration of CO. Propene oxidation produces propanal which exhibits an oscillation and C 0 2 which reaches to a stable rate progressing on two different active sites. 1 s is weakly adsorbed propene for oscillatory propanal and strongly one for COa production. The multi-reaction pathways can clearly be distinguished and regulated from 10 to 90% by the controlling of the concentration of reactants and the catalyst metal particle sizes supported. The computer simulation used exhibits the validity of the models proposed.
1.INTRODUCTION Recent progress of the techniques for characterizing solid surfaces has visualized surface structure in detail and revealed divers degree of m r d i n a tive unsaturation. The heterogeneity of surface ions of metal oxides and supported m e t a l s may reaeonably induce multi-reaction pathways caused by the variety of adsorbed species. The specific species(design8te Is) adsorbed on the surface will selectively control the reaction pathway according to the surrounding conditions'). The complexity of the reaction routes progressed in heterogeneous surfaces has brought about a complexity of the mode of steady state rate kinetics and transient response curves obtained and thereby a difficulty of the discrimination of multi-reaction pathways. The regulation of reaction routes is, however, very available to raise reaction yield and selectivity to important products. In the present study, our interest is focused on the discrimination of multi-reaction pathways and the elevating of the selectivity to a desired reaction route, and the specific species which can control both the reaction pathways and selectivity to desired products, by applying the transient response method to CO-oxidation on ZnOas) and propene oxidation on
Pt/sio?',.
1556
2.EXPERIMENTAL The zinc oxide used in this study w a s commercially prepared by Kadox 25. The catalyst particle size was 20-42 mesh and its BET surface area was 20 m2/g, and it w a s confirmed to be an n-type semiconductor by measuring the electrical conductivity of the catalyst. All the catalysts of Pt/SiOa w e r e kindly supplied by Professor Burwell Jr. of Northwestern University, The dispersion of platinum w a s measured by the chemisorbed amount of Ha evaluating to be Dh=7,16,27,40, and 81%. The amounts of platinum loaded were 0.825-1.91wtX. A tubular flow reactor which was made of a Pyrex glaas tube was used under atmospheric pressure at the temperature range of 190225°C far CO and 290-320% for CsNs oxidation. All gas components in the effluent streams were analyzed by three gas chromatographs to follow the transient state as continuously as possible. The intraparticle and external particle mass transfer effect w e r e reconfirmed to be negligible by testing the catalytic activity as a function of catalyst size or flow rate.
3.CO-OXIDATION ON ZnO Curve I in Figure 1 illustrates a typical example of steady state rate data on ZnO at 150'C. The rate clearly exhibits a maximum and a minimum which is an anomalous tendency as a function of POO, suggesting the simultaneous progression of multi-reaction pathways as has been reported in the photooxidation of secondary alcohol on ZnO5). Our speculation comes on that a Langmuir-Hinshelwood mechanism is available at PCO lower than 0.2 atm(Region 1) whereas an Eley-Rideal mechanism becomes dominant at higher than 0.4 a t m (Region 2). The trial and error calculation based on the two reaction paths model can evaluate the two parts, E-R and L-H, as shown by curves I1 and III. The same calculation has been applied tn whole data obtained at the temperature range of 130-170'C, and the model consistently explains all of them. Using these data, the activation energy of regions 1 and 2 are separately estimated to be 105 and 121 KJ/mol, respectively. Five specified CO-concentrations are marked on curve I as A-E in Fig.1 and five transient experiments are designed between A-E. The results obtained are presented in Fig.1 by response curves 8'8. The mode of the curves obtained is classified into four different types,a complex mode (see curves B and h) a monotonous(8ee curve c), a false etart(see curve $1 and an overshoot mode(aee curve Q ) . The monotonous mode is resulted f r o m the advantage of the L-H part. The false start and overshoot mode are caused by the f a s t response of E-R part at the initial stage of the curves followed by the slow L-H part. Curves B and k exhibit an anomalous behavior, a minimum which is due to the E-R part and a maximum due to the L-H part, indicating the complex mode. The E-R part can be evaluated by the experimental response curves obtained as shown in curve p; by A r . From t h e d i f f e r e n c e , r t 0 t . i - r E it,one can e a s i l y e v a l u a t e t h e L-H p a r t ( s e e c u r v e 111). The model proposed c o n s i e t e n t l y e x p l a i n s t h e anomalous tendency o f t h e s t e a d y s t a t e r a t e d a t a w i t h maximum and minimum, and t h e s h i f t of the mode of t r a n s i e n t r e s p o n s e c u r v e s between t h e monotonous, f a l s e s t a r t , o v e r s h o o t , i n s t a n t a n e o u s and complex t y p e s depending on t h e c o n c e n t r a t i o n r a n g e o f CO. The s u r f a c e s p e c i e s ( I.) c o n t r o l l i n g t h e r e a c t i o n pathways may be c o n s i d e r e d t o be s u r f a c e oxygen s p e c i e s . I n o u r p r e v i o u s p a p e r s ) , i t
1557 h a s been demonstrated t h a t two adsorbed oxygen species 0- and O ( n e u t r a 1 ) are a v a i l a b l e f o r CO o x i d a t i o n w i t h t h e two r e a c t i o n pathways on ZnO, each of which h a s d i f f e r e n t r a t e . Although w e have no c o n c r e t e e x p e r i m e n t a l e v i d e n c e s d i s c r i m i n a t i n g t h e two species f o r t h e two p a t h s , o n e may presume 0- t o be a v a i l a b l e f o r t h e E-R p a r t because t h e e l e c t r i c a l c o n d u c t i v i t y change o f c a t a l y s t d u r i n g t h e r e a c t i o n ie f a s t c o r r e s p o n d i n g t o br in curve 8. The selectivity of the reaction pathways can be regulated by CO concentration,the proportion of 0- and 0 and reaction temperature. A t PcsO.70 and 150'C, the selectivity of E-R part is about 80% and the L-H part is about 90% at Pu~O.10, regulating the proportion of the two reaction pathways. A computer simulation technique exhibits the validity of the model proposed.
A+B+A
fq E l B-tD-rB
D+E+D
Figure 1. Schematic explanation for the mode of transient response aurves (CO oxidation over ZnO)
4 . PROPENE OXIDATION ON Pt/S102 Propene oxidation on Pt/SiOz at the temperature range of 290-320°C gives two products propanal and COz The partial oxidation exhibits a typical oscillation where- the total oxidation reaches a etable rater). The amount of weakly adsorbed pmpene shows an oscillation synchronizing with the oscillatory rate and it is thought thereby to be the species(1s) for the oscillation, whereas the strongly adsorbed progene(1s) is available for the total oxidation. The total oxidation exhibits a typical structure insensitive reaction and
1558 the oscillatory propanal formation shows a strong structure sensitive reaction, reconfirming two different path reactions on the respective active site. The selectivity of the reaction path can be controlled by the reaction conditions and the platinum dispersion(Db) as shown in Fig.2. A t 310'C, Pcm~0.20atm, PNzsO.OSatm and D r 8 l % , the selectivity of the propanal route is evaluated to be 85%.
20 0. 6
0
L
L W I 0
rn
Y
0.4 T
=31OC
0. 2
0
Figure2. Plots of Fcrmsum
0. 1
0. 2
and Sczusum vs. Pcsa ( C d h oxidation over Pt/Si03
5 . References 1) Kherbeche,A.,Hubaut,R.,Bonnelle,J,P. and Grimblet,J.,J.Catal.~,ZO4(1991). 2) KobayashLM. ,Kobayashi,H. and Kanno,T. ,Chem.Eng.Comm. ~,23(1988). 3) Kobayashi,M., Kanno,T. and Kimura,T., J.Chem.Soc., Fareday Trane.1
,a,
2099(1988). 4) Kobayashi,M., Kanno,T., Takeda,H. and Nakagawa,A., "Unsteady State Proc esses in Catalysis" Edited by Yu.Sh.Matros. p.133(1990). 5) Cunningham,J. ,Hodnett,B.K. ,J.Chem.Suc.Faraday Trans.1, 2277(1981).
n,
Guczi, L ef al. (Editors), New Frontiers in Caralysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1 M , Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
ADSORPTION STUDY BY TRANSIENT TRACING METHODS.THEORY AND MODELING P. SzedlacsePJ,A. Efstathioub, C.O. Bennettb and S. L. Suibb aCentral Research Institute for Chemistry of the Hungarian Academy of Sciences, 1525 Budapest, P.O.Box 17, Hungary bUniversityof Connecticut,Department of Chemical Engineering, Storrs, CT 06269-3060, USA
1. INTRODUCTION AND BACKGROUND
The advantages of transient tracing methods ('ITM) in studying heterogeneous catalytic systems have been described by many authors [1,2,3]. In this communication we would like to present a new application of 'ITM for studying adsorption. The basic idea of the new method is that the surface concentrations and the adsorption and desorption rates can be obtained from the measured transient response curves during the tranrient period. Generally, the temperature programmed desorption (TPD) is used for characterization of adsorption [4,5]. From experimental point of view the method is simple but the interpretation of TPD spectra are often complicated because a lot of simultaneous process are involved [6]. Usually, in transient tracing experiments gradientless reactors (CSTR) are used in order to linearize the system and reduce mathematical complexity. In model calculations the gradientless system is represented by compartmental network. While the species in the gas phase can be considered as homogeneous and can be represented by compartments under proper conditions, there is always an inherent inhomogeneity of the surface states. This inhomogeneity, originated either from the surface sites or from the adsorbate interactions, is the main object of the different types of adsorption studies. In a transient tracing experiment a preadsorbed labeled material will be replaced by chemically equivalent unlabeled one. The surface will serve information about its energy distribution through the rate of desorption process. The desorption rate of preadsorbed labeled material will be decreased in time because the species with smaller desorption energies ie. higher desorption rates will leave the surface first. This rate shift is measurable and will serve information about the energy distribution of the surface sites. 2. THE MATERIAL BALANCES Let us consider the compartmental model of an adsorbate in a CSTR. There are two compartments, represent the gas and surface phases respectively. At time = ,t a feed containing unmarked species is substituted for the previously marked one. The fractional amount of marked species is measured from time = t, until it has reached the stationary
1560
(in this case zero) level again. For simplicity, 100 % marking level and ideal step function concentration changes will be considered. This has not restricted the validity of results presented. The details of TTM has been described elsewhere [7]. The response curves resulted by this kind of experiment are plotted in Figure 1. The forcing function, which represent the tracer concentration at the system inlet 0 40 00 120 160 decreased to zero by an ideal step jump at the beginning of the experiment. The thm lsecl curves, marked M and A are the response of a nonadsorbable and an adsorbable tracer. The steady state capacities No and N, can be calculated by the help of the mixing and adsorbing curves according to Equation 1 Flgm 1.
Z
and 2
N,=
1“(Act)-M(t))d? 0
where M and A stand for the mixing and adsorbing curves. Equation 3. is the material balance for the total amount of the tracer in the gas and surface phase in integral form.
where:
N, and N, are the capacities of the gas and surface compartment in moles Z, and Z, are the fractional amount of tracer of the gas and surface phase compartments F is the feeding rate of traceable material in mol/sec. Substituting No and N, into Equation 3. &(t) can be calculated at any time during the transient period because Z,(t) is measured. It has to be noted to carry out this calculation there is no need any assumption about the surface energy distribution.
3. THE NUMERICAL MODELS
The response curves of a homogeneous and an inhomogeneouscompartmental model was calculated by numerical integration of the corresponding differential equation system. In the inhomogeneous model the surface is divided into two parts with 20 and 22 kcal/mol desorption energies respectively.
1561
The Z,(t) function can be evaluated for both cases as described above. Inserting these
values into the material balance equation of the gas phase (Equation 4.) the exchange rate r between the gas and surface phase can be calculated formally. This rate is plotted in dependence of Z, in Figure 2. As can be seen r varies in the inhomogeneous case reflecting the desorption energy shift during the depletion. The significance of readsorption can be rationalized considering the effect of flow rate in Figure 2. The readsorption is Exchage rate of adsorbed tracer, (1) increasing when the flow rate is homogeneous surface, (2) inhomogeneous decreasing. The redsorption redistributes surface, low readsorption, (3) inhomogeneous P a of the tracer originally Present in the system over and over again during the surface, high readsorption experiment, ie. the experienced rate shift will be compressed. The experimental set up is fast enough when the feeding rate and the exchange rate between the gas and surface phases have the same magnitude. In that case the steady state rate of adsorption and desorption can be estimated for inhomogeneous surfaces at Z, = 1. In a transient tracing experiment the flow rate, the partial pressure of adsorbate and the capacities of the gas and surface phases can be varied in complex way. Additionally the rare of adsorption-desorption can be adjusted also through its temperature dependence. From the point of view of experimental design the TTM has more degree of freedom as TPD. Conducting the experiment at different temperatures the calculated steady state surface capacities could serve as a TPD spectrum. This kind of reconstructed TPD spectrum of the inhomogeneous model and the variation of coverage are plotted in Figure 3. The TPD spectrum obtained in this way is not distorted by temperature and concentration gradients and there are no diffusional effects in it. Even this TPD Amount of adsorbed tracer (1) and the spectrum does not reflect the surface reconstructed TPD spectrum inhomogeneitywhat was easily assessed by the desorption rate shift.
1562 4. CONCLUSIONS
We have shown that an adsorbate-adsorber system can be characterized by the help of mM. Based on the material balances, the coverage, the adsorption and desorption rates separately can be obtained from the response curves. The 'ITM is more sensitive to the surface energy inhomogeneities as the TPD. 5. m FERENcES
1. Bennett, C. O., in "Catalysis Under Transient Conditions", ed. A. T. Bell and L. Hegedus, ASC Symp. Ser., Vol. 178, p. l., Am. Chem. Soc.,Whasington, DC (1982) 2. Happel, J, "Isotopic Assessment of Heterogeneous Catalysis, Academic Press , (1986) 3. Stockwell, D. M.,Chung, J. S., Bennett, C. O., J. Catal., 112, 135 (1988) 4. Cvetanovic, R. J., Amenomija, Y., Adv. in Catal. 17, 103 (1967) 5. Richards, R. E., Rees, L. V. C., Zeolites, 6, 17 (1986) 6. Gorte, R. J., J. Catal., 75, 164 (1982) 7. Efstathiou, A. M., Bennett, C. O., J. Catal., 120, 118 (1989)
GueLi, L ct al. (Editors), New Frontiers in Catalysis
Ptoccedings of the loth International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elscvier Science Publishers B.V. All rights reserved
IN SLTU DETERMINATION OF SURFACE CARBON SPECIES FORMED ON WA1203 DURING Corn2 REACTION BY USING VARIOUS TRANSIENT AND ISOTOPIC METHODS A. M.Efitathiou? T. Ch@, D. Bianchib and C. 0.BennetP
Wepartment of Chemical Engineering, University of Connecticut, Storm, CT 06269, USA Present address: Institute of Chemical Engineering and High Temperature Chemical Processes, 26110 Patras, Greece bLaboratoiredes Materiaux et Procedes Catalytiques, Universite Claude Bernard Lyon I, 43 Boulevard du 11Novembre 1918,69622 Villeurbanne W e x , France * Present address: Institute of Chemical Engineering and High Temperature Chemical Processes, GR-26110, Patras, Greece
Abstract The evolution of surface species formed during the CO/H, reaction at 22OOC on a lwt% WA1203 catalyst was studied by traqsient methods using both mass spectrometry and FTIR. There is a very small amount of active carbon, C, (eC ~0.05) and a large amount of surface CO (€lC0=0.95). Also present on the Rh surface are a%ydrogenated carbon and an inactive carbon, Cp,species which do not exchange with gaseous or surface 13C0. The amount of these two species grows with time on stream. The kinetics of isothermal hydrogenation of carbon species formed after C O h reaction depends on the Rh particle size. 1 . INTRODUCTION
Knowledge of the state of a catalyst surface during reaction is an ultimate goal in order to understand catalytic reactions. Various transient methods and others with isotopes have proven in the past powerful tools in achieving this goal 11-51. These methods can provide the amounts and reactivities of surface intermediates that truly participate in the reaction sequence, and also those intermediatesthat are not in the reaction pathway [ 1-31. Most of the transient work in heterogeneous catalysis has been done in the past using mass spectrometer as a detector,permitting an accurate quantitativeinsight into the dynamics of a surface reaction [l-51. On the other hand, it is sometimes difficult to know the nature of adsorbed species using only mass spectrometry. Transient FTIR spectroscopy is another powerful tool to study the dynamics of a surface reaction and identify surface adsorbed species [6,7]. The focus of the present work is to apply various transient methods using both mass spectrometry and FITR to study the in situ surface state of a 1.5 nm Rh particles supported on Al,O, for the CO/H, reaction at 1 bar. The results obtained are compared and discussed in relation to those for 9.0 nm Rh particles supported on A1203at the same reaction conditions [1,21.
1564 2 . EXPERIMENTAL
A 1 wt%RIdAl,O, catalyst was used in the transient studies with mass spectrometry and FTIR. The catalyst was prepared by incipient wetness method. Hydrogen chemisorption measurements provided a fraction exposed, FE, of 0.68 corresponding to about 1.5 nm Rh particle size. The reactor, analytical systems and the gases used were the same as reported previously [1,3,8]. In particular, the stainless steel FTIR cell was of specific design with 1 mL nominal volume and provisions to be used in the range 20-500OC [8].
3 . RESULTS
Figure 1 shows transient CH, responses after passing CO/H, over the catalyst for 30s (curve A), and from the isothermal hydrogenation of carbon species formed after various times At (30-3600s) in C O h . In curve A there is an initial transient as the reaction starts from a clean surface, followed by a period of pseudosteady-state. A switch then to pure H, produces a second transient which corresponds to the titration of the surface. This is given for 30, 60, 600 and 3600s in C o b . Much information is contained in the shapes, peak maximum positions, and areas under the curves [1,9]. Table 1 gives the amounts of total carbon species build up on the catalyst surface with reaction time, as deduced by the areas under the H, titration curves of Fig. 1. For times on stream larger than 6oos, some refractory carbon species is found. This is nmoved from the surface by programming the temperature in H, flow (TPR), and the amounts obtained are also given in Table 1. Table 1 Coverage of surface carbon-containing species isothermally hydrogenated to CH4 Time in CO/H2(At),sec
30
60 120 600 1800
3600
Total' (monolayers)c
**CHdb (monolayers)
(monolayers)
1.52 1.62 1.69 1.84(2.05)d 1.88(2.37) 1.86(2.70)
0.54
0.97
0.90( 1.05) 0.96( 1.54)
0.96 0.95
~~
I3CH,b
~
a. From the titration of Fig. 1. b. From the 3 titration as explained in text. c. One monolayer is based on 65.0 pmol RhJg. d. Number in parentheses includes the inactive carbon Cg. Figure 2 shows transient FTIR results recorded at 22OOC in the range 1150-2100cm-' for the experiments of Fig. 1. The infrared bands of adsorbed species formed during CO/H, reaction correspond to : linear CO (2046cm-'), bridged CO (1837cm-'), formate. COOH
1565
(1592, 1392, 1378 cm-') and carbonate (1460 cm-'). Figure 3 shows CH bands in the range 2700-3100 cm-l for the experiments of Fig. 1. ).H2
w
0
h b
"[ 6
Figure 1. Transient response of CH, at 22OOC and H&O=9 according to the delivery sequence: He (180s)+H2/CO(At)+H2(t).
Figure 2. Transient ITIR spectra recorded at 22BC during HdCO reaction. A=30 s; B=80 S; C=600 S; D=3600 S.
Figure 3. Transient FTIR spectra recorded at 22BC during HdCO reaction. A=30 s; B=80 S; C=600 S; D-3600 S.
The CH, transients of Fig. 1 under the isothermal titration can be deconvoluted by making use of 13C0. A switch from l2C0/H2 to 13CO/He for 30s is made, followed by H, titration. During the exposure to 13CO/He, CH, production stops, and adsorbed l 2 C 0
1566
exchanges with I3CO in the gas phase. There are also carbon species formed during reaction in l2C0/H, that do not exchange with gaseous or surface 13C0, but do react with H, to give mainly CH,. Table 1 gives these results. The sum of IZCH, and I3CH4 in Table 1 is in very good agreement with the total CH, in Table 1 as obtained from Fig. 1 (H2 titration). 4 . DISCUSSION
The present work shows that at 220°C the surface of the lwt% Rh/A1203 catalyst (1.5 nm Rh partcles) is mostly covered by undissociated CO (€lc0=0.96), independent of reaction time, and readily exchangeable with gaseous l2C0. The transient FTIR results (Fig. 2, band at 2046 cm-*)and the amount of 13CH4in Table I are, therefore, in harmony. For the 5wt% I , , decreased to about 0.6 after lh on stream Rh/A1203catalyst (9.0 nm Rh particles) the € [1,2]. These results probe for some dependence of Bco on Rh particle size during CO/H2 reaction. The amount of active carbon, C,, which is in the sequence of steps for CH, formation, is small (Oc, = oos(cy)
,
(1 1
were (p is the angle between the cnrrface normal of the sample and the given direction, I*>and I are the i n t e m i t i e s of desorption fluxes a l o w the normal and at an angle q~ t o it, cornspondbgly For other Hysterm studied the spatial distributions of the demrption flux were t o a oonsiderable extent ooncentra-
.
ted along the IsurracIe n o m l and can be w i t h high preoision described by the empirical equation:
.I/Io = coP((y)
(n :-1 )
(2 1
and by models of Van Willigen I1 1 and Corn E l . The values of activation energies were obtained for dissociative ohmhorption o r oxygen, nitrogen, carbon dioxide and carbon monoxide on F t and Ir. Equatiorm w e r e derived which allow to take account of the eiieot of surface roughness on the desorption flux angular crlistribution and thuy to correct the parametem being determined. shows W R spectra of NO decomposition on Pt and Ir. Prom p?82h€s figure one oan see that NO and nitrogen desorb from P t in oomparabla amounts, n is also desorbed. On Ir by contra& t o F t a praatically o t a l decomposition o r NO takes place. The spacial di&ributions of variow fwecies demrbing from the surfacee are fjubstentially dirremnt. Thw the angular distribution of the demrbtion flux or HO obeya the Knudsen l a w , while for nitrogen and oxygen the distributions are m o r e concentrated a l o w the surface normal and can be described by
“=T
1605
equation (2) (the values of n for a l l s y & m l i s t e d in the table).
studied are
Figure 2. TPR speotra of 190 and its deomqmsition pmcluote on Ir ( l e f t s l i d e ) and Y t (right s l i d e ) . Table Pamwtem LI (eq.(2)) of demrbing flux spatial dktributions
P i p r e 3 w h o m tlie spatial distributions of CO mleoules esaapmg fmo the platinum surfaoe obtained in t h w e different experiments: 1 ) aft-. GO ahmisorption on olean P t sux-faae, 2 ) during continuous feed of oxygen fmm a mleoular beam to the P t su-faae (T=llgOK) a o v m d by oarbn. 3 ) the same BP, (2) but without feeding oxygen from the gas plizme (the rcaotion takes plaoe at the e-ae of ahearieorbed oxyger~ or oxygen dissolved in the bulk of the sample).
1606
0 - 1 -30'
8 - 2 O0
P - 3 SO'
60°
goo
PQgm? 3. Spatial distributiom of desorbing CO i n the three types of expertnwnts on Pt (see the text)
The Knudsen distribution of the desorption flux is observed when there iY 110 activation bamier for ohmimrption (CO,HWPt,Ir). In other cams substantially narrower angular distrihutiom of deIMrhing speoies are observed. Angular distributiom a m very semitive t o the prehistory of the specien formation, t h i s being illustrated by the example of CO molecules whioh are demrhed f r o m the c h m h r b e d mleoular state and alm a8 a p r o h o t of mactiom 6 and 7. It should be noted that the measured parametem n oorrr;?tqxmd t o mu@ surfaoes of polyorystalline P t and Ir w l e s . Therefore for a Hmooth surfaces values of n can be mah higher [31. 5.
€uwlmwcl!s
1. W . V m WillQen, yhJ7s Letters No. 2 (1968) 80. 2. C.Comf%, Surfam? Soi. Reports 5 (1985) 145. 3 . Y . U . K i s l i u k , Kinetika i kataliz No. 1 (1989) 252.
GucZi, L et al. (Editors), New Frontiers in Catalysk
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 6 1993 Elsevier Science Publishers B.V.All rights resewed
SHIP-IN-BOTTLESYNTHESIS OF NaY ZEOLITE-INCLUDEDPtg AND Pt12 CARBONYL CLUSTERS: STRUCTURESAND CATALYSIS IN CO+NO REACTION G.-J. Li, T.Fujimoto, A. Fukuoka and M.Ichikawa Catalysis Research Center, Hokkaido University, Sapporo 060,Japan
4bstract
[Pt,(CO) J2-MaY(orange-brown, 2056 and 1798cm-'), and [Pt 2(C0)21]2-/NaY(darkgreen, 208d and 1824cm-') were stoichiometricallysynthesized by the reductive carbonylation at 323-373K of [Pt(NH3)J2+/NaY,and Pt2+1NaYrespectively. TPD and TPR studies reveal that Pt, and Pt, carbonyl clusters are considerably stabilized inside NaY cavities, rather than the externd complexes. Pt-L,-edge EXAFS measurement demonstrated that they are consisted with the Pt carbonyl clusters having trigonal prismatic Pt, and Pt, frameworks inferred to [NEt ]2[Pt3(C0)6]q(n=3,4). The intrazeolite Pt,and Pt,, carbonyf clusters exhibited higher catalytic activity m NO reduction by CO towards N, and N,O at 473K, compared with those on the conventional Pt/A1 0, catalysts. Removal of carbonyls with mild oxidation, and H,-reduction of zeolite-inc?uded Pt and Pt,, carbonyl clusters provide the dispersed Pt particles which chemistorb CO and in the storichiometriy of CO/F't =1.83 and WPt=1.26.
8,
1. INTRODUCTION Surface organometallic chemistry and intrazeolite synthesis of organometallics such as metal carbonyl clusters have been a recent subject of interest, because they offer many promises for molecular approaches in heterogeneous catalysis and rational preparation of tailored metal catalysts [l] having a unifonnal distribution of a discrete metaValloy clusters grafted on amorphous metal oxides [2] and entrapped inside zeolite pores. Ship-in-bottle technique for the synthesis of bulky organometallics such as metal carbonyl clusters inside zeolite cages has gained growing attention for the purpose of obtaining the catalycally active precursors surrounded with the intrazeolite constraint[3]. The zeolite frameworks may impose the molecular shape-selectivity in metal-catalyzed reactions due to the limitation of aperture sizes against in coming reactant molecules. Some intrazeolite metal carbon 1 clusters Rh,(CO) [3], Ir,(CO) [4], Pd,,(CO)r [S], [HFe (CO), 1- [6], [Fe,Rh,(CO), [7] and Rh6-xIrx(f!O)16(x=2,3,4) [8] were previously syntiesizeh and characterized. The catalysts derived from Pd, (CO) /N a y and Rh,(CO)#aY showed unique catalytic performances in COtH, and oletin hydroformylation reactions based on their molecular shape-selectivity. We report here that various Pt carbonyl clusters having general formula [Pt3(C0)3@CO) ] 2- (n=3-5) are synthesized in NaY and NaX zeolites which were characterized 6y FTId,%v-vis and EXAFS spectrocopies. The mechanism of the Pt carbonyl cluster formation and their stability was studied by in-situ FTIR and TPD. The unique redox mechanism of NO reduction with CO for [Pt (C0),I2-/NaY was proposed to be associated with the reversible formation-cleavage cycye of the Pt clusters with CO and NO inside NaY cages.
t-
,
1608
2. EXPERIMENTAL
z
[Pt(NH,) 12+/NaY(4.65 Pt wt% loading) was prepared b cationexchange of NaY (Linde LZY-52, &i/Al=S.G, Surface area 910m2/g [Pt(NH ) ] NaY was calcined in 0, flow (120mVmin) at 573K for 2h(0.5c0/min) to give Pt2*da% The reductive carbonylation of [Pt(NH ),I2+/NaY and Pt2+/NaYwere carried out in a pyrex-glass reactor under CO(40076Otorrj at 298-373K after the samples were evacuated at 353K for 2h. IR spectra wcrc rccordcd on a Shimadzu FTIR-4100 double beam spectrometer with 25100 time scans at rcsolution 2cm-'. Each sample was pressed into a self-supporting wafer. The reflectant UV-vis spectra of powder samples were recorded by using a Hitachi 330 UV-vis spectrometer. EXAFS measurements were carried out at SOR beam line 10B of the Photon Factory in the National Laboratory for High Encrgy Physics (KEK-PF) using a Si(ll1) double crystalmonochromator. The sample wafcrs were stored under Ar or CO gases in specially designed Pyrex-glass cells with Kapton film windows (500nm thick). The analysis of EXAFS data was derived using the computer program "PROGRAM 2" by FourierTransform curvefitting method, as described elsewhere. COtNO reaction was performed at reduced pressure (PcO=5Otorr,P ,=SOtorr) in a closed circulating Pyrex-glass reaction system. The products were analyzed \y on-line TCD gaschromatography (Shimadzu GC-8A) using 5A molecular sieves (60cm-long) and Porapak Q (3m-long) columns for CO, NO, N20, N, and CO, with helium as carrier gas (30ml/min). 3. RESULTS ASD DISCUSSIONS 3.1 Synthesis of [Pt (CO)l,lz-/NaY and
[Ptl,(CO)a.]L-INaY
The sample Pt2'/fiaY was heated from 298 to 373 K under CO (530-760 Torr) with a trace of H,O to gct species which show their characteristic carbonyl IR bands. It is suggested from the analogy with IR bands of previously reported platinum carbonyl complexes, e.g. Pt(C0)CI (91 and Pt3(p4-CO),(PPh3),,[10] that Pt2'/NaY reacts with CO forming PtO(CO)/$aY [2100 cm- ] and a proposed intermediate trigonal species [Pt,(CO) (p CO),]MaY [2112s, 1896m and 1841s cm-'I. They are eventually converted into the darfgreen Pt carbonyl cluster species (1) [2080vs and 1824s cm-'; 290,445 and 640 nm, UV-vis reflectance], which resemble those of the blue-green [Pt, (CO)24]2- in tetrahydrofuran (THF) solution.[ll] On the other hand, the [Pt(NH ) I2+/Na%provided stoichiometrically the orange-brown cluster species (11) [2056vs and 1 j 9 k cm-'; 300, 435 and 710 nm, UVCO (550-760 Ton) and 30 at 298-373 K,which VIS] in thc reductive . It is of interest that the terminal CO bands of the correspond to [Pt9(CO),8 to higher frequencies (26-40 cm-'), while the intrazeolitc Pt, and bridging CO bands sh%t to lower frequencies(40-50 cm-'), compared with those in THF solution indicative of the internal cluster complexes. These CO frequency shifts are interpreted in terms of the interaction between the 0 end of the cluster bridging carbonyls and the intrazeolitc acid sites such as H*and A13+,as for Rh,(CO),,/NaY.[l] 3.2 EXAFS Characterization direct evidence for In situ EXAFS data were conducted for the stoichiometric formation of trigonal (RJ, R, = 2.64 eolite cavity. For sample (I) the individual N,' = 1.7, and intertriangular Pt-Pt distances (RJ bchvcen adjacent triplatinum planes, R, = 2.99 A, N: = 1.7, were obtained by thc FT curvc-fitting of the EXAFS oscillation data, Thcse structureal values are in good agreement with those observed by EXAFS for crystalline samples of [Pt,2(CO)a][NEt,]2 mixed with boron nitride (R1 = 2.65 A, N,' = 2.0,
1,
1609
R, = 2.99 A, N," = 1.5). The X-ray structural analys's for [Pt,(C0),l2-
and [Pt,(CO) 1,crystals gave R, = 2.66-2.67 A and % = 3.01-3.03 k[13]. It 1s suggested from the E k S data that the intertriangular Pt planes of the Pt, and Pt,, carbonyl cluster anions are slightly distorted inside the intrazeolite constraint. Table 1. EXAFS evaluation for Pt, and Pt,, carbonyl clusters in NaY and parent Pt,, cluster [NEt [Pt, (CO),] Rtk) k.N Pt-Pta Pt-Ptb Pt-Ptc Pt-Cd Pt-Oe
2.65 2.99 3.88 2.05 3.32
[Pt, (CO),]2-/NaY k(A) C.N 2.64 2.99 3.87 2.05 3.28
2.0 1.5 3.0 2.0 1.0
[Pt (CO),,]2-/NaY C.N
1.7 1.7 2.9 1.3 0.5
k(A) 2.64 2.99 3.85 2.06 3.28
0
-
1.9 1.7 3.0 1.7 0.9
R Inter atomic distance: C.N.; Coordination number Estimated experimental errors are 50.03A for atomic distance and 50.2 for coordination number on the present EXAFS evaluation.
* Pt-Pta;Intra-triangular.:Pt-Ptb;Inter-triangular.:Pt-PtC;Second neighbored atoms for
inter-triangle. 3.3 Activity for NO + CO reaction The NO t CO reaction was performed at reduced pressure (Pco = 50 Tom, P, = 50 Tom) by using a closed circulating Pyrex glass reactor charged with the sample (I) an8(11) at 423523 K. As shown in Table 1, they showed high activities (10-15 times higer) for formation of N,O, N, and CO compared with those on conventional Pt/Al,O, (4 wt% Pt) catalysts. N,O was an intermeziate in the CO t NO reaction and was subsequently converted to N, in the reaction on the samples (I) and (11). The conversion of CO t NO and selectivity for formation of N, were maintained for prolonged on-stream reactions of NO t CO (40 h) on these catalyst samples. Upon exposure of [Pt,,(CO)~,l2-/NaY to NO(150 Tom), in situ FTIR studies demonstrated that NO breaks the intra-and inter-trigonal Pt-Pt bonds of the Pt, carbonyl cluster even at 298 K to give a trigonal intermediate [Pt (CO) ] (2112, 1896 and 1841 cm-') and PtO(C0) (2110 cm-') in forming N,O (2236 cm-l? and 50, (2353 cm-') in the gas phase. Furthermore, it was found that [Pt,,(CO) ],-/Nay was reversibly regenerated from the Pt carbonyl fragments such as PtO(C0) and (CO) ] inside NaY by the reaction with CO and water at 300-353 K. In contrast, in situ FTIR an8 GC-MS studies suggested that when a mixture of CO and NO was admitted onto [Pt ,(C0),I2-/NaY, N,, N,O and CO were catalytically formed at 323-473 K, with original cluster framework being retainei in zeolites.
[R
A,,
Table 2. Specific activity for NOtCO reaction at 473K on Nay-entrapped Pt carbonyl clusters and Pt/At,O, NO conversion N, formation Catalyst [Pt, (CO) r-/NaY [PtdCO),3 -/Nay Pt/Al,O,
molecules Pt-ls-' 2.00 2.83 0.58
0.56 0.68 0.05
Selectivity N,/(N,+N,O) 0.56 0.48 0.15
1610 3.4 TPD and CO/H, chemisorption studies
The TPD, coupled IR-Mass has been conducted on [Pt,(CO) :-]/NaY and [Pt, (C0),l2/Nay in flowing He(70 ml/min) and raising temperature (lOIdmin, 300-9OOK). b D patterns for zeolite-includcd Pt, and PtI2 carbonyl anion clusters mainly consisted with the CO of the Pt clusters, respcctively. A minor broad peak of CO (m/e=44) formation was obsenrcd at higher temepratures at above 720-770K possibly d2ue to WGSR between CO and intr;rzeolite OH(H,O). The TPD-FTIR studies demonstrated that the Pt, and Pt,, carbonyl clustcrs are fairly stabilizcd inside NaY upto 360K in vacuo and 400K m CO atmosphere, compared with those deposited on SiO, and Nay, where [Pt (CO) la- was decarbonylated at below 325K. After mild oxidation at 623K to remove 80, foflowed Ha-reduction at 573K, thc NaY-included Pt and Pt,, carbonyl clusters gave the highly dispersed Pt particles which chemisorb CO and in the stoichiometries of CO/Pt=1.83 and H/Pt=1.26. The atomic Pt dispersion in thc catalysts derived from the NaY-included Pt, and Pt, carbonyl clusters was fairly retained after sevcral cycles of treatments, oxidation(623K)-k2 reduction(573K) NOtCO reaction(423K), implying the intrazeolite accomodation to prevent a sintering of Pt clusters.
h,
G-J Li is on leave from Dalian Institute of Chemical Physics, Dalian, 116012, China. 4. REFERENCES 1 M. Ichikawa, Tailored Metal Catalysis, ed. I. Iwasawa, reidcl, Dordrecht, 1984, pp.183263; M. Ichikawa, L-F. Rao, T. Kimura, and A. Fukuoka, J. Mol. Card., 1990, 62, 15; L-F. Rao, A. Fukuoka, N. Kosugi and M. Ichikawa, J. Phys. Chem., 1990,94,5317. 2 M. Ichikawa, Polyhedron, 1988,1,2351. 3 P. Gelin, Y.Ben Teant and C. Naccache, J. Cufal.,1979,59,357. 4 G. Bergeret, P. Gallezot and F. Lefebure, Stud. Surf: Sci. Cutal., 1986,28, 401. 5 L.L. Shen, H. Knozinger and W.M.H. Sachtler, Cafal. Left., 1989,2, 129. 6 M. Iwamoto and S. Kagawa, J. Phys. Chem., 1986,90,5244. 7 A. Fukuoka, L-F. Rao, N. Kosugi, H.Kuroda and M. Ichikawa, Appl. Card., 1989,50, 295. 8 M. Ichikawa, L-F. Rao, T. Ito and A. Fukuoka, Faraduy Disucuss. Chem. SOC.,1989,87, 232. 9 J. Browning, P.L. Goggin, R.J. Goodfellow. M.G. Norton, A.J.M. Rattray, B.F. Taylor and J. Mink, J. Chem. SOC., Dalton Trans., 1977,2061. 10 J. Chatt and P. Chini, J. Chem. Soc. (A), 1970,1538. 11 G. Longoni and P. Chini, J. Am. Chem. SOC.,1976,98,7225. 12 A. De Mallmann and D. Barthomeuf, Curd Lett., 1990.5293. 13 J.C. Calabrese, L-F. Dahl, P. Chini, G. Longoni and S. Martinengo, J. Am. Chem. Soc., 1974,96,2614.
Ouczi, L ad.(Editors), New Frontiers in Catalysis
Procctdings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishcxs B.V.All rightp resewed
THE PREPARATIONAND CHARACTERIZATION OF HIGH-SILICA Y ZEOLITE PREPARED BY COMBINED CHEMICALAND HYDROTHERMAL DEALUMINATION X L i d , 2.Peia, L. Shea, X-W. Lia, J. Slumb, S. Linb, R. Tan? andX Maob Vatalysis Division, Chemistry Department, Peking University, 100871 Beijing, China bResearch Institute of Petroleum Processing of Lanzhou Refinery, Lanzhou, China
INTRODUCTION
To develop a FCC catalyst with high gasoline yield increased gasoline octane and good bottom conversion especially in processing heavier feedstock, a new process for preparing high-silica Y zeolite was developed. The new process consists of modification of combined chemical-hydrothermal dealumination taking chemical treatment as the first step and using new reagents. The chemical treatment removes aluminium from the framework of the Y zeolite and the defect sites are filled subsquently in the hydrothermal treatment. This procedure makes every step easy controllable.
RESULTS AND CONCLUSIONS The preparation method involves the following steps : a 1 chemical dealumination. b l calcination under self-steam condition. c> chemical treatment of the calcined zeolite. d > recalcination under self-steam condition. e l chemical treatment again to remove A1 debris from the zeolite. This process gives rise to a series of high-silica Y zeolites with controllable Si/Al radio and debris good retention of crystallinity, uniform aluminum distribution and very developped pore system. These new high-silica Y zeolites have been characterized by X-Ray diffraction, XPS, infrared spectra, adsorption study, micro-activity test and laboratory riser unit test. The X-Ray data proved that the zeolites produced by this process showed characteristic structure of faujasite with good crystalinity (>90 % , smaller unit cell size (- 24. 45 1. After calcination at 1073K ( 3 hrs in 100% steam), unit cell size reduced to 24. 15A , while the crystallinity remained unchanged. These data show that this high-silica Y zeolite has good hydrothermal stability. XPS measurement was used to study the surface composition and depth profile of 3 zeolites with similar unit cell size prepared by different methods (USY. FSEY prepared by
1612
#
1
.ooL 0
/r--
1
I
1
Iwooomlroowo Rr etohlng tlmr, S
Figure 1. Argon etching curves1 based on XPS data. NHSY 9 new high-silice Y (this work). FSEY, prepared by chemical treatment using ( NH, )&F,. USY, prepared by hydrothermal treatment.
WOO
3700
3Y00
-I
931 Figure 2. Infrared spectra of the Samples prepared at dfferent steps ammonium fluorosilicate and high-silica Y zeolite 1. Fig. 1 illustrates the Argon etching curves which show that chemical-hydrothermal dealumination process produces a more uniform composition than that prepared by other methods. It may be considered more favourable for the carbonium ion activity for catalytic cracking than the other high-silica Y zeolites. The infrared spectra of the chemically and hydrothermally dealuminated zeolits were investigated with Nicolet 5MX IR spectrometer. Some results are given in Fig. 2. The chemical dealumination (Sample Step a) led to the bands in the stretching region similar to those encountered in HY zeolite1 at about 3750, 3640 and 3540cm-'. However, the 3540cm-'band was more affected by pyridine adsorption, indicating the incompleteness of the sodalite cages after dealumination. Hydrothermal treatment of chemically dealuminated Y zeolite (Sample Step b ) generated strong absorption bands around 3700, 3670 and 36OOcm-' , while the bands at 3640 and 3540cm-lwere reduced in intersity. The appear-
1613 ance of the new absorption bands at 3700, 3670 and 3600cm-' was assumed to be the reflection of structure changes. The 3600cm and 3670cm-' bands were assigned to extraframework A1 species, while the 3700cm-' band was attributed to the OH group in the A1 defect sites in the framework. The background absorbance at 3710cm-' was used to determine the 'defect sites left in the framework after dealumination, as proposed by U.S. P. 4503023. A linear relationship between number of the defect sites per unit cell and background absorbances at 3710cm-' was found. Thus the infrared spectra of the zeolites may be used to monitor the structural changes due to chemical and hydrothermal dealumination. Stepwise study showed that the first chemical treatment step led to framework dealumination leaving A1 defect sites in the framework, the second hydrothermal treatment step caused not only filling the defect sites by silica to a large extent, but also further dealumination and structural rearrangement in the framework , which led to a zeolite with higher Si/ A1 ratio than that of precursor and with non-framework A1 species. The third step, chemical treatment, invoved primarily solubilization of non-framework A1 species generated during hydrothermal treatment although some framework A1 could also be removed. These results are summarized in Table 1. The nitrogen adsorption isotherms measured at 77. 4K showed hysteresis loops which were attributed to the formation of secondary pores with radii of 1. 5-10nm. Analysis by t-plot method gaves rise to the total surface area, surface area fo micropores, surface area of mesopores and the surface area of large pores as shown in Table 2. This adsorption study on the intermadiate and final products conduces to giving further information about the formation and location of the non-framework A1 species. Catalyst testing with laboratory riser unit and heavier feedstock showed that the catalyst (zeolite-active matrix) gives rise to a good gasoline yield, and octane advantage over hydrothermally treated catalyst. Lower coke and gas makes were also observed. Table 1 Situation of dealumination and silica resubstitution in every treatment step
A1 removed Per u. c. (Chem. Anal. ) Number of A1 defect sites per u. c.
Step b
6.1
3. 6
10. 4
6. 0
6. 3
4. 9
14. 6
8. 2
Step c
Step d
(IR)
Extent of filling A1 defect sitse % Crystallinity retention % a.
Step a
(A)
0
49
60. 2
94
110
96. 8
24.646
24.584
24.529
24.417
1614 Table 2 Influence of chemical treatment and hydrothermal treatment on surface area and pore texture Volume of Surface (Mz/g> Sample Total Micropore Secondary Pore> 1O O A micropores pores (CM3/g> 126 9.4 0. 3034 Step a 958 832 160 12.0 0. 2667 Step b 890 730 923 810 113 9. 4 0. 2964 Step c Step d 720 151 17.6 0. 2658 872 34.6 0.2831 128 Step e 887 758
Conclusion 1. A new process consists of modification of chemical-hydrothermal dealumination taking chemical treatment as the first step has been developed. This process can carry out under moderate conditions. 2. Using the procedures of chemical dealumination and hydrothermal dealurnination alternately can make the processes of the dealumination and silicon resubstitution controllable. 3. A series of high-silicon Y zeolites have been prepared. These zeolites have following characters: arbitrarily adjustable Si/Al ratio. well-distributed alumination composition, welldeveloped pore texture. 4. These zeolites are suitable for residue oil cracking.
Guai, L e~d. (Editors), New Frontiers in Curalysis
Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary CJ1993 Elsevier Science Publishers B.V. All rights reserved
ACTIVE SITES OF NOVEL IRON SUPPORTED Y-TYPE ZEOLITE
R. Iwamoto, I. Nakamura andA. Iino Central Research Laboratories of Idemitsu Kosan Co,Ltd. 1280, Kamiizumi, Sodegaura, Chiba 299-02,Japan (As a participant of Research Association for Residual Oil Prcwssing.)
A novel iron supported Y-type zeqlite prepared by treating NH4Y with ferric nitrate solution under low pH condition (FeHY) showed high activity for toluene disproportionation in the coexistence of H2S and H2. Active sites of FeHY for toluene disproportionation were investigated by elucidating function of iron cluster and zeolite separately. The activity of FeHY was significantly decreased by potassium ion exchanging (K/FeHY) Ultra stable Y-type zeolite (USY) also showed low activity. However, catalyst prepared by mixing K/FeHY and USY mechanically showed extremely higher activity than the sum of both catalysts. It was suggested that the active sites of FeHY are BrQnsted acid sites in zeolite framework and that iron clusters supply spillover hydrogen to prevent zeolite from deactivation due to coke deposition.
.
Hidaka et al. reported that a novel iron supported Y-type zeolite prepared by treating NH4Y with ferric nitrate solution under significantly low pH condition (FeHY) showed higher activity for toluene disproportionation than other metal supported zeolites in the presence of H2S [ l ] . Their results of FT-IR and Massbauer spectroscopy indicated that specific iron clusters interact with zeolite framework [ 2 - 3 1 . However, it has not been identified whether the iron clusters are catalysis active sites or not. In this work, we tried to clarify the active sites of the novel iron supported zeolite for toluene disproportionation.
1616 2. EXPERIMENTAL
FeHY was prepared by stirring NH4Y in 0.25M ferric nitrate solution at 323K for 2 hr. K/FeHY was prepared by ion-exchanging FeHY with various concentration of potassium nitrate solution at 348K for 2h. USY was prepared by treating steamed NH4Y with nitric acid solution at 348K for 0.5h. Toluene disproportionation was carried out at 623K, 6MPa, and LHSV 4h-I in a flow of H2S(0,2%)/H2.
3. RESULTS AND DI!XUSSION
Activities for toluene disproportionation over FeHY are shown in Fig.1. Though the activity decreases gradually in the flow of H2, it increases significantly in the flow of H2S/H2. It is obvious that FeHY shows high activity for toluene disproportionation in the presence of H2S. Sugioka et al. reported that new acid sites are formed by dissociation of H2S absorbed on some metal supported zeolites t41.
n
x a
. 1
'I H2S/H2
I
I
H2S/He
E
v
30 b
8
'3
b
20
6
0
lo
'
t
u--
~~
2
4
6
Time on stream
8
10
(h)
Fig.1 toluene disproportionation over FeHY under the flow of H2, H2S/H2, and H2S/He.
12
1617
The activity under the flow of H2S/He , however, decreases drastically. These results indicate that both H2S and hydrogen is necessary for the activation of FeHY. Therefore, it is suggested that H2S dose not create new acid sites in FeHY. Fig.2 shows relation between potassium content in K/FeHY and the activity for toluene disproportionation. The activity of FeHY can be almost poisoned by potassium loading of above 1.3wt% (as K2O). This result suggests that the activity is attributed to Brdnsted acidity. USY which contains no iron clusters shows considerably poor activity (Table 1 ) .
30
20
10
0 0.5
1.0
1.5
2.0
K20 ( w t % ) Fig.2 Relation between potassium content in FeHY and activity for toluene disproportionation.
However, Catalyst prepared by mixing USY and K/FeHY mechanically shows extremely higher activity than the sum of both catalysts. One possible explanation is that hydrogen molecules dissociated on iron clusters spill over on to USY and prevent USY from deactivation due to coke deposition. Many author also demonstrated that spillover hydrogen can transfer from one phase to another phase [5-71. Their results well agree with our experimental data in this work. From these results, it is considered that the active sites of FeHY are Brdnsted acid sites in zeolite framework. Fe clusters supply
1618
spillover hydrogen to prevent zeolite from deactivation due to coke deposition. H2S must be indispensable for Fe cluster to generate and/or maintain the hydrogen dissociation activity because activity changes Reversibly depending on the concentration of H2S [71.
Table 1 Activities for toluene disproportionation over prepared catalysts Catalysts FeHY K/FeHY USY USY + K/FeHY COMO/~ 1 2 0 3 CoMo/A1203 + USY
Conversion after 3h (mole%) 29.3 2.6 4.6 18.2 3.1 25.3
To confirm this investigation, the activity of catalyst prepared by mixing CoMo/Al203 and USY mechanically was evaluated since CoMo/A1203 dose not possess Brdnsted acidity but hydrogen dissociation activity as well as K/FeHY. This catalyst also showed high activity (Table 1).
Active sites of FeHY for toluene disproportionation were investigated. It was suggested that the active sites of FeHY are Brdnsted acid sites in zeolite framework and that iron clusters supply spillover hydrogen to prevent zeolite from deactivation due to coke deposition.
1 S.Hidaka et al., Proc. 7th Int. Zeolite Conf. (1986) 329 2 H,Hidaka et al., Chem. Lett. 7 (1986) 1986 3 S.Hidaka et al., Bull. Chem. SOC. Jpn., 61 (1986) 3169
4 5 6 7
M.Sugioka et al., Proc. Intern. Symp. Acid-Base Catal. (1989) 305 J.Teichner et al., Appl. Catal., 62 (1990) 1 B.Delmon et al., J. catal., 108 (1987) 294 S.Hidaka et al., J. Chem. SOC. Jap. 9 (1987) 1654
Guczi, L et al. (Editors),New Frontiers in Catalysis P m d i n g s of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary eB 1993 Elsevier Science Publishers B.V. All rights reserved
A NOVEL APPLICATION OF XRD TECHNIQUE FOR THE CHARACTERIZATION OF SECONDARY PORE STRUCTURE IN MODIFIED Y ZEOLITES
S. D. Phatak, R. P. Mehrotra, S. M. Dhir and T. S. R. Prasada Rao Indian Institute of Petroleum, Dehradun 248 005, India
In the present work (1-7) attempts have been made to correlate dealumination of Y-zeolites with the unit cell size and with the secondary pore structure and there by with increased activity and stability of catalyst (4). The present study reports investigation of the intensity changes of crystalline planes of some hydrothermally dealuminated and acid extracted zeolites and discusses the reasons for these changes. ExpwnlEwJ!AL.
Material preparation: ?he pure Sodium-Y zeolite powder used in this study as a starting material, was obtained from Linde Company (LZY-52). The details of preparation of discussed elsewhere ( 4 ) . X-ray diffraction patterns were recorded on a Rigaku (Japan) D/Max B Diffractomewter with 8-0 goiniometer using graphite single crystal reflected CuKd-radiation. The crystallinity of the samples were measured following ASTM (D-3942-80) procedure. The dealumination in samples were estimated following the procedure of Fichtner-Schmittler (4). Transmission electron photomicrographs of our samples were recorded at Philips Laboratory at Netherland on CM12/STEM Model with EDAX System attached. The pore size distribution curves were obtained from N2-adsorption, desorption isotherms by following ASTM (0-4222-83) procedure. I11
The properties estimated by x-ray diffractometry are listed in Table 1. Fig.1 shows pore size distribution curve for DYE-14 and DYE15. From Table 1 it can be seen that intensity for (Ill), (220) and (311,)are comparatively large for all samples, among them (111) is in particular very high and it increases with the degree of dealumination. This very high value apart from (111) being3$ensely packed plane may be due to a particular co-ordination of A 1 in that site, single crystal studies may reveal this more clearly. Recent NMR Studies (6) indicate that the process is very complicated and A 1 may be octahedrally co-ordinated instead the usual tetrahedral form. More work in this direction is needed. We did not observed any anamlous behaviour in the intensity of (111) reflexion in the dealumination range studied (i.e. 40% to 98%), as reported earlier (5).
It is further observed that intensity of 1st six planes [ (111) to (440)] of all samples is high in comparison to the corresponding planes .>f Nay-zeolite, irrespective of degree of dealumination. This clearly indicated removal of A13+ and subsequent replacement of Si4+ from neighbouring sites, resulting in contraction of unit cell size (4t7). Intensity of these crystalline planes is also increased suggesting that after replacement a sort of stability is attained by these planes. It is thus established that replacement resulted in a stable structure with high intensity. The fringe structure is observed for these very planes by TEM Lattice imaging Technique (Fig.2) wherein the width between the consecutive fringes gives interplaner spacing same as found by X rays thus confirming the results. The intensity of remaining five planes from [ ( 5 3 3 ) to (644)] is low compared to the corresponding planes of Nay-zeolite. It may be that in is removed a defective vacant site is produced these planes though A 1 (6) and Si4+ has not probably been able to replace it or available as most of the Si4+have already rushed to low index planes dissolving some of the sodalite units. In this process structures of these planes became unstable and this phenomenon may be responsible for the production of super micropores or meso pores (2,4). This was what we observed under TEM for DYE-15 sample or which intensity of all the planes was more or less like the other samples except that it was very low for higher index planes. Incidently this sample w a s steamed at higher temperature than to others. Further from the line broadening given only for (533) line in the table for want Suggesting further of space, shows it is higher for DYE-15. disintegration of crystallite size (5) and structural damage, this may produce pores within the zeolite crystallites ( 2 ) . The pore size distribution curve for sample DYE-15 (fig.1) clearly indicates that as the line broadening increases more and more pores of higher diameters are appearing. Our study on samples with varying crystallinity and dealumination suggests that one can produce a sample with a desired range of pore structure to improve their diffusion characteristics f o r ;-heprocessing of bulky molecules. +
ON TEM OBSERVATION The photomicrographs (Fig.2) for DYE-15 sample shows details of pore :;tructure (pore diameter ranging from 25 A to 1501) and fringe structure for low index planes which is in conformity with our results (See Fig.1). coNcLusICN
From our study it can be observed that very v g h intensity of (111) plane m y be due to different co-ordination of Al +in this site. Moreaver intensity variation of (111) with increasing dealumination (from 40% to 98%) is almost regular. From line broadening it may be apparently possible to predict the formation of secondary pores. Combined application of XRD, TEM and N2 -adsorption desorption provides the details on the secondary pore structures. More work in this direction is in progress.
-
1621
One of the authors (Dr SDP) is extremely thankful to Mr. Ashok Kaushik, Philips India and to Mr.R.P.Koster, Philips, Netherland for photomicrographs of our samples on TEM. He is also personally thankful to Prof. B.W.Wojciechowski for helpful discussions during his -visit to this Institute and to Dr.S.P.Srivastava for carefully going through the manuscript. We are further thankful to Mr. S.Suresh for X-ray diffraction records to Mr. Moo1 Chand for sample preparation and to Mr. Anand Singh for N2- isotherms. REFERw(IEs
1 2 3 4 5 6 7
Gao Zi and Tang Yi Zeolites 8 (1988) 232-237. E.G.Derouane, Ketjen, Catal Symp. (1986) G-3, 1-16. M.K.Camblor et all Appl Catal. 55 (1989) 67-74. JScherzer, Catal. Rev. Sc. EngTl(3) (1989), 215-354. -.. H.K.Beyer et a1 J.Chem.Soc.Faraday Trans 1, 81(1985) 2889-2901 J.B.Nagy and E.G.Drouaner ACS Symp. 368 (198R.L.Cottemn et al, Zeolites 11 ( 1 9 m l 27-34. TABLE 1
-
CLyStauinity 63.53
F==%?= slwF€E 7
PLAhES
W t W
91.34 98.1
- X-RAY
DIFFRACTION DATA
97.12 102.31 54.55
ME7 DYE11 MEl3 ME14 UiE14AUiE15 (cps)) 6c676 37435 19357 12748 15597 97c8 22128 28333 16783 9372 4934 8457 11978 12185 7 3 3 15228 16259 Ello 9622 9770 5284 4460 2390 3761 9262 10225 4332
,.
326
2884
401
70
91.0
91.7
1CB.23
105.7 93.33 103
W 1 4 8 DiE14cUiEl41
hBy
t35933 22335
17820 3m.5 9661
136c6 lit332 1C833 4471 lCan
2419 96.0
24.35 24.32 2 4 . 3 24.29 24.31 24.25
24.29
24.27
24.28 24.67
0.177
0.160
0.155
0.157 0.139
WarreterA W & h
(533)CINE
0.152 0.163 0.162 0.162 0.249
1622
‘*t
R
w
I OD-
-
-
10 50100 PORE RADIUS 4’
SO0
Fig. 1: Pore size d i s t r i b u t i o n c u r v e s
Fig. 2 : HREM showing secondary pores and f r i n g e s , mag: 6 , 2 5 000 x
Guczi, L ef al. (Editors),New Frontiers in Cutalysb Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
IR-SPECTROSCOPICEVIDENCE FOR ACETONITRILE INTERACTION WITH CARBENIUM IONS IN ZEOLITES D. S. Bystrop, A. A. Tsyganenkoaand H. Forsterb ahstitUte of Physics, St. Petersburg University, St. Petersburg 198904, Russia bInstitute of Physical Chemistry, University of Hamburg, D-W2000 Hamburg 13, Germany
As recently reported bv Medin et al. 111, after adsorption of acetonitrile on propene-pretreated HZTSM-5 zeolite, a new surface species appears, exhibiting an extremely high band position of the C S N stretching vibratlon, shifted by 107cni-' with respect to that of the free molecule. The authors explained this phenomenon by the appearance of strong Lewis A13' acid sites as a resrrlt of the rupture of A I - 0 bond in the bridged Si-OH-AI structure after propene adsorption. Another explanation of t h i s result was put forward by Bystrov C21, who analysed the IR spectral data for different complexes of nitriles C3,41 and came to the conclusion that such a shift cannot be caused by the coordlnation of nitrile t o any Al-containiiig Lewis acid, but should rather be assigned t o a nitrilium-like cations IR-CN-RI' produced by interaction of the nitrile with carbocatlons. The latter are known t o arise after propene adsorption on zeolites, and their interaction with nitriles can be considered as very strong coordination of nitrogen to the C t atom of the carbocation. This would mean that nitriles can be used as a good t e s t for the surface carbocation species, as their C E N frequency is also sensitive to the structure of the carbocation and to the nature of the radicals bound to the Ct atom. The aim ot this work was t o find out whether the high-frequency band of the C E N kibratioi! is really due to the species fornied as a result of acetonitrile interaction with the products of propene adsorption or i t should belong t o the nitrile molecules coordiriated to A13+ Lewis acid sites. I t is known for the complexes of acetonitrile with BHaI3 compounds that the C a N stretching fundaiiiental band reveals although small, but detectable shifts, from about 1 up t o 3 cm-' after "B by "B substitution 15,61. This means that upon using isotopically substituted propene we should expect to observe a shift of the nitrile band due t o the strong interaction of the C z N group with carbocations. For thls purpose IR spectra of CD,CN adsorbed on HZSM-5 zeolite pretreated with both nornial propene and propene-d, have been investigated at 300 K. The cell applied was similar t o that described by Basu et al. C71. Small amount of a zeolite-water slurry was spread on a tungsten mesh surving as the sample holder and dried on air. The sample was placed into the cell and outgassed in i n high vacuo a t 7 7 0 K for 5 h . Then i t was brought into contact with 2.5 torr of propene a t 300 K for S min, outgassed again and flnally exposed t o 1.2 tom of deutero- acetotiitrile. For reactivation the sample was first heated in vacuo,
1624 then in oxygen at 720 K and ultimately again evacuated a t this temperature for 1 h. The batikgrouud spectra and those obtained after propene and nitrile interaction were practlcally identical for t h e fresh and reactivated samples. The spectra were recorded on a Digilab FTS ZOE Fourier-transform spectrometer at a resolution of 4 cm-' and an acciiracy of the band maximum determination of t O . 5 cm-'. Obstruction by atmospheric carbon dioxlde was eliminated by subtratiting the reference spectrum of the beam passing the cell aside the sample. In accordance with C1 1 , two bands were observed In the region of C 1 N strethlng fuiidatneiital vibration of the spectrum of CD,CN adsorbed on propenepretreated zeolite. The band at 2301 cm-', whlch evidently corresponds t o that at 2295 cm-' reported by Medln e t al. C11, remains exactly at t h e same positioti for the sattiple pretreated either by normal or deuterated propene, whtle the 2370 cm-' band undergoes a downward shift of about 4 cm-', This result shows the frequency of the band in question t o be sensitive t o the clrarrge of masses in the species arlslng after properie adsorption. Analysis of the CSN fundamentals for benzonltrile C31 and acetonitrile C81 coinplexes wltli different Lewis acids shows that frequency shifts grow from the adducts with tertiary t o those with primary carbon atoms, and the observed frequeiice value of 2370 cni - 1 Is typical for acetonltrile bouud rather t o tertiary than t o other carbocatinons. However, the preliminary results of normal coordinate aiialysls for the CDfCN-Ct(CH3), structure predict weak, less than 1 am-', shift of the harmonic frequency of CzN stretching on the complete H-D substltutlon in carbocation. I t should be noted that theory predicts an upward shift in the harnionic approxleiatlou, the observed shift t o the lower waveiiurribers may be caused by the changes in anhamonicity on deuteration. Downward shifts up to 3-4 ciii-' should be observed for the complexes with I3C- labelled compounds if ail the four carbon atoms of carbocation are substituted, about a half of the shlft belug due t o the substitution of central atom bound directly t o nitrogen. Although we cannot yet speak about the agreement between t h e measured and calculated shift values, the observed sensitivity of the C r N frequency to isotopic substitution In propene enables u s t o conclude that the nitrlle CN group of the resulting product is really bound t o the positively charged carbon atom of propene, but not t o the AIJt ion of the zeolite framework, This is consistent with the assignment proposed in C21 and supports the idea that nitrlles may be used as a probe for surface cartwcatlons, being important iuterniediates in hydrocarbon transformation catalyzed by solid acids. Further experitiients with the labelled compounds as well as force field calculations ot model systems are in progress.
REFERENCES 1 . A. S. Medin, V. Yu. Borovkov, V. B. Kazansky, A. C. Pelmentschlkov and G.M a Zhidoinirov, Zeolites, 10 (1990) 668. D. S . Bystrov, Zeolites, 1992, t o be published. D. S. Bystrov and B. K. Nasarov, Doklady AN SSSR, 148 (1963) 1355 (in Ruse.). 4. D. S. Bystrov, Doklady AN SSSR, 154(1964) 407 (In Russ.). 5. B. Swanson and D. F. Shriver, Inorg. Cheui., 9 (1970) 1406. 6. .)l F. Shriver and B. Swanson, Inorg. Cliem., 10 (1971) 1354. 7. P. Basu, T. H. Balllnger and J. T.Yates, Jr., Rev. Sci. Instrum., 59 1988) 1321. 8. G . Olah and T.E. Klovsky, J , Amer. Chem. SOC., 90 (1968) 4666. 2,
3.
Guni, L . ef al. (Editors), New Frontiers in Catalysb
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
ACID-BASE PROPERTIES OF ZEOLITES: AN XPS APPROACH USING PYRIDINE AND PYRROLE PROBE MOLECULE R. B. Bor&, M.Huang, A. Adnot, A. Sayari and S.Kaliaguine Department of Chemical Engineering and CERPIC,University Laval, Ste-Fay,Quebec G1 K7P4, Canada
Abstract In the present investigation, the N(1s) XPS lines of pyridine chemisorbed on acidic zeolites and pyrrole chernisorbed on basic zeolites are compared. It is found that in both cases the shift in N(1s) binding energy of the component lines is large enough to provide an appropriate scale for the characterization of acid or base strength. 1. INTRODUCTION
Chemisorbed pyridine [ l ] and chemisorbed pyrrole [2, 31 have long been recognized as suitable probes for infrared investigationsof acid and Lewis base sites in zeolitic materials. Following an early publication by Defoss6 and Canesson 141 who studied the N(1s) XPS signal of pyridine adsorbed on HY, we have conducted recently a systematic study of HY [5, 61, pentasil zeolites [7, 81 and ZSM-22 [9,101 by this technique. In the present contribution, N(1s) XPS signals of pyrrole chemisorbed on alkali exchanged X and Y zeolites will be reported along with selected pyridine results. 2. EXPERIMENTAL
All XPS spectra were recorded using a V.G. Scientific ESCAlAB MARK It system operated in the constant pass energy mode (20 eV). The pyridine containing samples had their spectra recorded at room temperature, whereas in the case of chemisorbed pyrrole the XPS analysis was performed at liquid nitrogen temperature to avoid excessive desorption in the spectrometer. With all pyridine chemisorbedsamples, the Si(2p) line was used as an internal reference for the binding energy scale. The value adopted for Si(2p) binding energy is 103.3 eV which is correct for ZSM-5 and ZSM-22 samples. For HY, this may induce a small error as the accepted binding energy value for HY is 103.1 eV [Ill. The difference is however considered to be within the limits of experimental error. In the case of alkali-exchanged X and Y zeolites, the referencing of the binding energy scale was made with respect to the Au(4f7,,) line (84.0 eV). Gold was deposited onto the zeolite samples under vacuum (ca., Torr) and the thickness of the gold layer was about 20 A. In all cases the N(ls) line had to be deconvoluted into two or three component peaks. The FWHM of all component peaks was maintained at 2.4fo.1 eV.
1626 The preparation of HY I"! ( = 2.33), H-ZSM-5 (Si/AI = 39), H-FeZSM-5 (SVFe = 15), HBZSM-5, H-ZSM-22 (Si/AI = 63) and H-FeZSM--22 (Si/Fe = 43) was described in references [6. 7, 91, Prior to pyridine adsorption at room temperature all samples were thermally activated under vacuum at 300 "C (HY) or 400 "C (all other zeolites). Physisorbed pyridine was then evacuated at 100°C (BZSM-5) or 200 "C (all others). The preparation and composition of alkali exchanged X and Y zeolites were described in reference [3]. Both series were obtained by partially exchanging the sodium form of X (Si/Al = 1.25) and Y zeolite (SVAI = 2.52) , These samples were degassed under vacuum at 400 "C before pyrrole adsorption at room temperature. The desorption of physisorbed pyrrole was performed by evacuating the sample for one hour at 65 "C.
3. RESULTS AND INTERPRETATION Figure 1 shows the N(1s) XPS lines for pyridine chemisorbed on AI-ZSM-22 (figure 1A), Fe-ZSM-22 (figure lB), AI-ZSM-5 (figure lC), Fe-ZSM-5 (figure 1D) and B-ZSM-5 (figure 1E). For all these high silica zeolites, the N(1s) spectrum had to be deconvoluted in three peaks. Table 1 gives the corresponding binding energy values for the components. The low binding energy component (peak 1) is mostly associated with Lewis acid centers, whereas the two higher binding energy component llnes (peak 2 and peak 3) are associated with low and high acid strength Bronsted sites, respectively. This assignment was made after systematic comparison between Bronsted to Lewls acid site ratios calculated from IR and from XPS spectra of chemisorbed pyridine. The nature of the low strength Bronsted acid sites is not yet entirely resolved, but it is believed that this could be a Bronsted site adjacent to a silanol group and located on the external surface of the zeolite crystal. Figure 2 shows the N(1s) XPS lines of pyrrole chemisorbed on alkali exchanged X zeolites. One component (400.6iO.l eV) is formed in all spectra and as it is the dominant peak in the spectrum of Na-X zeolite, it is ascribed to pyrrole chemisorbed on a surface oxygen adjacent to a sodium cation. The other major peak (399 401 eV) is assigned to pyrrole chemisorbed on the basic sites associated with the other cation. The binding energy for these sites is in the order of increasing electroposivity: Li > Na > K > Rb > Cs. Thus the lower binding energy is associated with higher basic strength. Similar results were obtained for a series of alkali exchanged Y zeolites. A third weak component (401.W.1 eV) is present the spectra of all X zeolites. It corresponds to very weak basic sites common to all alkali exchanged X zeolites. The IR investigation of these samples had also revealed the presence of such a site with NH stretching vibration close to 3370cm-' [3].
-
Table 1 N(1s) binding energy of pyridine chemisorbed on high silica zeolites (eV)
--___________-______~----------------------------------------------------------------------------------------Catalyst AI-ZSM-22 Fe-ZSM-22 AI-ZSM-5 Fe-ZSM-5 B-ZSM-5
Figure 1A 1B 1c 1D 1E
peak 1
peak 3
peak 2 399.0 398.5 398.7 398.1 397.6
401 .O 400.2 400.0 399.7 399.0
402.9 402.1 402.0 401.8 401.3
--__-_---------------------------------------------------------------------------------------------------------
1627
3!
396
401
4M
411
Binding w r g y ,aV
Figure 2. N,, X P S lines of pyrrole chemisorbed on alkali exchanged X zeolites. Binding w r g y ,aV
Figure 1. N,, X P S lines of pyridine chemisorbed on high silica zeolites (see Table 1 for legend).
B-ZSM-5
4. DISCUSSION AND CONCLUSION
Figure 3 shows that the N(1s) binding energy of pyridine chemisorbed on the strong Bronsted acid sites in high silica zeolites is well correlated with the OH stretching vibration frequency of these sites before pyridine chemisorption. These OH frequencies are often thought to reflect the strength of the Bronsted acid sites, but they are obviously affected by other factors such as the local environment. The N(1s) binding energy might be better
AI-ZSU-5 5310 1 401.0
401 5
Al-2%-22 402.0
4OZ.6
(03 0
N l s Binding Energy (sV)
Figure 3. OH wavenumber vs. pyridine N,, binding energy for the strong Bronsted acid sites in zeolites.
1628
/
3480 correlated to acid strength because its shift Is associated with the electron density of the nltrogen atom In the '0H-Nfbrldge. Figure 4 .displays the correlatlon between N(1s) XPS blndlng energy and NH stretchlng frequency in JIb0 S3M . CI K ua pyrrole chemlsorbed on basic alkali exchanged X and Y zeolites. Here, Rb Ca the N(ls) blndlng energy shlft Is SIOO I.0 clearly related to the electron donatlng N l r Blndlng Energy (eV) ability, or Lewls baslclty of a surface oxygen forming a >O--H-N< Figure 4. Pyxrole NH wavenumber vs. N,, complex. binding energy for pyxrole chemisorbed on The O(1s) binding energy of bask (e) Y and (0)X zeolites. sold oxldes Is sometlmes used to monitor Lewis basic strength [ i f , 121. U The shifts In O(ls) and N(1s) blndlng .N K energy In basic X and Y zeolites are Ib m indeed well correlated as shown In figure 5. It may however be appreclated from thls flgure that the N(1s) shift is much larger , thus providing a more sensltlve probe of Lewis basiclty. t The chemlcal shlft of core levels in XPS Is approxlmately described as N l r Blndlng Energy (11') A(BE) IkAq - AV where Aq Is a change In electron Figure 5. 0,, vs. N,, binding energy for density and AV a change in Madelung pyrrole chemisorbed (0) Y and (A) X potential; k is a constant that depends zeolites. on the nature of the element. It seems that a large value of k for nitrogen makes both pyrldlne and pyrrole sensitive probes for the XPS monltorlng of acid and basic strength, respectively. '
5. REFERENCES 1 J. Dwyer and P.J. O'Mailey, Stud. Surf. Sci. Catal., 35 (1988) 5. 2 0.Barthomeuf, J. Phys. Chem., 88 (1984) 42. 3 M. Huang and S. Kallagulne, J. Chem. SOC.,Faraday Trans., in press. 4 C. Defoss6 and P. Canesson, J. Chem. SOC.,Faraday Trans. I , 71 (1976) 2565. 5 R. Borade, A. Adnot and S. Kaliaguine, J. Mol. Catal., 61 (1990) L7. 6 R. Borade, A. Adnot and S. Kallagulne, J. Catal., 126 (1990) 26. 8 R. Borade, A. Sayarl, A. Adnot and S. Kaliaguine, J. Phys. Chem. 94 (1990) 5989. 9 R. Borade, A. Adnot and S. Kallagulne, Zeolltes 11 (1991) 710. 10 R. Borade, A. Adnot and S. Kallagulne, Zeolites, In press. 11 Y. Okamoto, M. Maezawa and T. Imanaka, J. Catal., 112 (1988) 427. 12 T.L. Barr and M.A. Ushka, J. Am. Chem. SOC.,108 (1986) 3178.
Ouczi, L et al. (Editors),New Frontiers in Cafalysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights resewed
SULFATE AS PROMOTER OF ACIDITY OF HIGH MICROPOROUS AND THERMOSTABLE TITANIUM PILLARED MONTMORILLONITE
F.Aahaiai A. Bernier and P. Grange Catalyse et Chimie des Materiaux Divises, Universite Catholique de Louvain, Place Croix du Sud 2/17, B-1348 Louvain-la-Neuve, Belgium
Introduction Pillared titanium montmorillonite allows t o obtain large pore microporous systems (1-2). Depending on the conditions of synthesis, thermally stable catalysts (325 m2g-1 at 600°C) with a basal spacing of 228, have been prepared. In addition, knowing that solid superacids are obtained by sulfatation of bulk Zr, Fe or Ti oxides, we have synthesized new sulfated Ti PILC. In this paper, we present the role of sulfate concentration on the morphology, texture, thermal stability and acid properties of titanium pillared clays. The properties of the promoted and unpromoted bidimensional microporous systems are compared. Experimental Unpromoted Ti pillared montmorillonite are prepared in the following way: Tic14 is added to 0,125M HC1 solution (Ti4+/HCl=40)in order to prepare a 0.82M Tic14 solution, After ageing for 3h at 25"C, 10 m o l e s of the Ti solution per gram of Na montmorillonite (4gA - Wyoming-Eccagun Weston L) are slowly contacted and stirred for 16h a t 25°C. The final suspension is washed and centrifugated 5 times a t 2500 ppm for 10 11111. There are several ways to introduca the sulfate. In this work the ammonium sulfate (0.1N) was added t o the previously aged TiCldHCl solution. Three different sulfate concentrations, namely S04--/Ti4+ratios were used. The following steps of the pillaring process are identical to those used for preparing unpromoted TiPILC. After washing and drying, the samples are calcined a t different temperatures up to 600°C. Characterization Chemical analysis of the Ti and SO4 promoted Ti PILC are obtained after total acid dissolution by Atomic Absorption for All Fe, Mg, Ca, K, Na, or colorimetry for the titanium. The evaluation of the site of the pillars is obtained by X-ray Diffraction (40kV; 40mA). Few drops of the pillared clay suspension are deposited on a glass plate which his heated at different temperatures.
1630
Specific surface area is evaluated from N2 adsorption in an Automatic Micromeritics ASA 2000 equipment. The acidity of the solids is measured through TPD of NH3 and FTIR of pyrydine. In both cases, before analysis the samples are previously treated up to 400 or 600°C either under H2 flow (TPD) or high vacuum (FTIR). Results and Discussion The introduction of sulfate slightly decreased the size of the pillars from 1 to 2A whatever the calcination temperature. This is clearly evidenced on fig 1 which reports the evolution of the basal spacing (i.e. the size of the pillars + 9.6A corresponding t o the silicate layer) as a function of calcination temperature for the pure Ti pillared montmorillonite and for the sulfated Ti samples.
Fig. 1. Basal spacing of the unpromoted Fig. 2. Specific surface area of the and sulfates pillared montmorillonite unpromoted ( 0 )TiPILC and sulfated (0 without SO42-;0 SO.$- = 2.64; clay. (0): SO42- = 2.64%). W so42-= 3.60; A Sod2-= 4.90) However, there is a large difference in specific surface area according to sulfate concentration: for low sulfate concentration (S042-=2.64%), the solids still have 203 mqg-1 at 600"C, and for higher sulfate loading (3.6 and 4.4%),the promotor almost completely blocks the accessibility of the interlayer volume and the surface drastically decreases. Figure 2 reports the evolution of the specific surface area for the pure or low sulfate content promoted Ti pillared sample a t different calcination temperatures under dry air. The Ti02 and Na2O concentrations of the clays as a function of the so4 contents introduced during the preparation are reported on figure 3. The amount of ammonia desorbed for samples calcined at 400 and 600°C are reported in table 1.
1631
36
Figure 3. Ti02 and Na2O content in function of the SO42- content.
& 34 E 32
N
c-. 30 0 28
samples Na montmorilllonite
Ti PILC
SO4 % 0 0 2.64 3.6 4.9
mmo1es.g-1 400°C 600°C 150 1371 1213 1381
1254
767 1222 1040 1002
mmo1es.m-2 400°C 600°C 2
3.9 4.9 153 157
2.3 5.4 115
125
It is shown t h a t sulfates enhance the total acidity of the Ti PILC especially when the samples have been calcined a t 600°C. However, taking into account the surface area of the solids, this strong improvement is especially noted for high sulfate loadings. IR measurements indicate that Ti PILC presents mainly a Lewis acid type. However, the introduction of sulfates also promotes a weak Bronsted acidity. These results made i t clear that the introduction of sulfate in the Ti pillared montmorillonite does not drastically influence neither the thermal stability of the PILC's nor the size of the pillars. However, the highest sulfate content brings the higher basal distances a t room temperature as well a s the larger variation of the distance after calcination a t 600°C; the variations being between 1.9 and 2.6A. It is worthwile noting that after thermal treatment under air, the height of the pillar is around 10.4A. However there is not direct correlation between the sulfate loading, the size of the pillars and the total surface area or the microporosity of the samples. The solids containing 3.6 and 4.9% sulfate in the clay completely lose their surface area namely the
1632
sulfate not only completely blocks the accessibility t o the interlayer pores, but could also be partly deposited on the external surface of the clays. The Ti pillared montmorillonite with 2.64% sulfate still has around 200mzg-1 after calcination a t 600°C. At this temperature, the titanium oxide pillars obtained by dehydratation of the Ti hydroxyde precursor are amorphous, since the XRD analysis does not evidence neither the rutile or anatase structure. This would suggest that, as observed when preparing Zr-pillared montmorillonite (3-51, the titanium-clay interaction stabilizes both the clay and the oxide pillar structure. The acidic behaviour of Ti and Ti/S04 PILC has been evaluated through TPD and FTIR of pyrydine. As indicated in table 1, Ti pillars strongly enhance the total acidity of Na montmorillonite. The specific acidity (mmoles m-2) of the sulfated Solid8 is also enhanced by sulfating. It becomes obvious that these solids present a very strong acidity. The FTIR analysis shows that Lewis sites are mainly responsible for this behaviour. In addition, the 1440 cm-1 band is slightly shifted t o higher wave number. It is difficult to attribute a definite structure to the sulfated titanium pillars a8 the Ti/SO4 ratio changes with increase of the sulfate content in the pillaring solution. Additional experiments are needed in order to specify the structure and t o choose between the proposals of Tanabe (6) or Saur (7).
Conclusions In the same way as for bulk TiO2, it is possible to enhance the acidic properties of Ti PILC. In addition, the solid obtained present a high specific surface area and a high thermal stability.
Acknowledgments The "Services de la Programmation de la Politique Scientifique" (SPPS) Belgium is acknowledged for financial support. Ref(1) J. Sterte, Clays Clay Miner., 1986,34,658. (2) S. Yamanaka, T. Nishihara, M. Hattori, Mat. Chern. Phys., 1987,17,87. (3) E.M. Farfan-Torres, P. Grange, J. Chirn. Phys., 1990,87 (731,1547. (4) E.M. Farfan-Torres, 0.Dedeycker, P. Grange, in Preparation of Catalysts V (G. Poncelet, P. Grange, P.A. Jacobs, B. Delmon, Eds.), Elsevier, Amsterdam, 1991,337. (5) E.M. Farfan-Torres, P. Grange, in Catalytic Science and Technology (S. Yoshida, N. Takezawa, T. Ono, eds,), 1991,1, 103. (6) K.Tanabe, M.Itoh, K. Morishige, H. Hattori, Preparation of Catalysts I (B. Delmon, G. Poncelet, P.A. Jacobs, eds.), Elsevier, Amsterdam, 1975, 65. (7) 0.Saur, M. Ben Sitel, A.B. Mohammed Saad, J.C.Lavalley, C.P. Tripp, B.A. Marrow, J. Catal., 1986,99,104.
Guczi, L. et d.(Editors), New Fronriers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992, Budapest, Hungary D I 1993Elsevier Science Publishen B.V. All rights reserved
PLATINUM CLUSTER SUPPORTED ON ZEOLITE A BY ION EXCHANGE OF ~(NH342+
R. R y m and S.J. Cho Department of Chemistry and Center for Molecular Science, Korea Advanced Institute of Science and Technology, Taeduk Science Town, Taejon 305-701,Korea
Abstract The ion exchange of Pt(NH3)42' was found to take place easily up to 10 wt% Pt into NaA zeolite after ca. 50% of Na' was substituted with MCaZ+, due to the expansion of the a-cage aperture. The reduction of the P t ( N H 3 ) p ion exchanged in CaNaA gave a small Pt cluster supported on A zeolite (PtKaNaA), similar to the preparation of Pt cluster supported on Y zeolite (PtiY). The chemisorption of H and 0 on 1 - 2 wt% Pt/CaNaA was similar to that of PtMaY containing ca. 1-nm Pt clusters, but much lower on 10 wt% Pt/CaNaA due to partial destruction of the zeolite framework The turnover frequency for catalytic hydrogenation of cyclopropane over 1 - 2 wt% Pt/CaNaA was considerably lower than that over PtiNaY. 1. INTRODUCTION
The metal cluster encaged in the micropore of zeolite is very suitable for the study of small metal cluster in catalysis. The effect of the cluster-size variation on the chemisorptive and catalytic properties of such a small cluster is particularly interesting since its understanding may build a bridge between homogeneous catalysis dealing with a few atoms and heterogeneous catalysis taking place on bulk metal surface. However, such a study requires a very precise method to determine the cluster size to the level of the number of atoms per cluster. To this end, we recently proposed a xenon adsorption method [1,2] which determined the number of metal clusters in a given faujasite-type zeolite sample from the amount of xenon adsorbed strongly on the metal clusters. The average number of metal atoms per cluster was obtained from the xenon adsorption with the assumption that a certain maximum number of xenon atoms can be adsorbed per cluster encaged in a supercage under the experimental condition The previous result obtained by the xenon adsorption method [2] indicated that it was difficult to control the average number of Pt atoms per cluster to other than 50 60 by changing the Pt content in the range of 2 - 10 wt% Pt in NaY zeolite (PtMaY) by the ion exchange method. A similar effort to decrease the number of a t o m per cluster encaged in the supercage [ 2 ] was successful for the Ru cluster prepared by starting with the ion exchange of a Ru-red complex into N a y zeolite; the number of Ru atoms per cluster could be controlled to within about 20 - 60 by changing the preparation condition. Why was the Ru cluster containing less than ca. 20 atoms difficult to prepare in the previous study of RuMaY ? We speculate that a cluster consisting of somewhat less than 20 Ru atoms can be below the size of a supercage aperture (i.e., 0.74 nm) so that it can migrate relatively easily within the zeolite channel. Then, the cluster will grow by combining with other clusters in the channel. It thus can be thought that a zeolite with apertures significantly smaller than those of Y-zeolite supercage will be better for the preparation of smaller metal clusters. The size of a-cage in A zeolite is 1.14 nm, which is not much smaller than that (1.3 nm) of supercage in Y zeolite. The aperture of a-cage consists of an 8-membered oxygen ring with about 0.4 nm in diameter, whereas the supercage aperture is a 12-membered
-
1634 oxygcn ring A metal cluster larger than the a-cage aperture will be difficult to agglomerate, and therefore such a small size of the a-cage window will make the A zeolite suitable as a support for the preparation of very small metal clusters probably consisting of less than 10 atoms, of which the chemisorption and catalysis will be very interesting The ion exchange of Pt(NH,)P was a convenient way of initially loading Pt on Y zeolite for the preparation of P t N 131. Howevcr, this ion exchangc technique was not suitable for NaA zeolite since the a-cage aperture is too small to allow the diffusion of Pt(NH3)42’. W of Na’ in NaA zeolite with xCa2+ (i.e. CaNaA) However, the substitution of ca. 5 in the present study was found to result in the expansion of the a-cage aperture so that the ion exchange of Pt(NH,)P was performed up to ca. 10 wt% Pt. We report here the preparation of a small Pt cluster from the Pt(NH,)T in CaNaA. The chemisorption of H and 0 on this Pt cluster, as well as the catalytic hydrogenation of cyclopropane over the cluster, is also reported in this paper. 2. EXPERIMENTAL
A high-purity NaA zeolite sample was preparcd with AI(OH), (Baker Analyzed), NazSiO . 9H,O (Baker Analyzed) and NaOH (Aldrich, A.C.S. reagent) by the Breck proccdure [4J. Ca2+ was exchanged into thc NaA samplc by stimng in an aqueous solution of Ca at room tempreature (RT). The zeolite powder was then filtered and washed with doubly distillcd water. Elemental analysis by inductively coupled plasma (ICP) emission , ~ ( Sevacuation ~ ~ ~ ) ~ ~at 673 K spectroscopy gave a unit-cell formula of N ~ I ~ C ~ , ( A I O ~ ) aftcr under 1 X lo” Pa. The CaNaA sample was stirred for 14 days at 345 - 355 K (under reflux condenser) in a conc. aqueous NH, solution which initially contained the amount o f Pt(NH,),CI, requircd for a given nominal Pt loading, Intermittently, the conc aqueous NH, solution was added into the solution to refill the loss of NH,. The supernatant solution during this plriod was periodically analyzed for Pt by UVNIS spectrophotometer (Perkin-Elmer Lambda 5). Filtering and washing of this zcolitc sample with distilled water gave a precursor of the Pt cluster supported on CaNaA (Pt1CaNaA). The Pt1CaNaA precursor was pretreated (or calcined) by flowing 0, at lo00 ml g-’ rnin as the temperature was raised to 583 K over 10 h and then maintained at 583 K for 2 h, reduced by H, flowing at 200 nil g-* min-’ whilc the temperature was raised to 573 K ovcr 2 h and thcn kept there for 1 h, and evacuated under 1 X lo3 Pa at 673 K for 2 h ’fie Pt contcnt is reported hcre as a nominal wt%. Temperature programmed reduction (TPR) was carricd out for the calcined Pt sample by measuring the consumption of H, during the contact with ca. 15-kPa H, for 30 min at a given temperature and then further 15 min at RI‘, increasing the temperature in stepwise. X-ray powdcr diffraction (XRD) pattern of Pt/CaNaA sample was obtained using Rigaku instrument with Co K a sourcc. Xenon adsorption isotherm was measured volumetrically to estimate the zeolite crystallinity. The chemisorption of H and 0 was also measured volumetrically at RT. The H/Pt (or O p t ) ratio was determined from the extrapolation of the adsorption isotherm to zero pressure. The catalytic hydrogenation of cyclopropaiic over the Pt sample was carried out by using a Pyrex flow rccirculation reactor at 231 K (Pep = 0.6 kPa, PH2 = 6.0 kPa, and P,, = 94.7 kPa). ‘The product was separated in an on-line 118 in. x 6 ft Porapak Q column at 353 K and analyzcd by gas chromatography (Hcwlett-Packard 5890 Series 111) equipped with a flame ionization detector (FID), which was connected to an integrator (Hewlett-Packard 3394 A).
-’
3. RESULTS AND DISCUSSION
The NaA zeolite samplc used in this study did not contain any significant amount of Pt when the zeolite was stirred in a conc. aqueous NH, solution containing 6 X 10” M Pt
1635
(NH,)?
overnight at RT. Compared with the result, the ion exchange of Pt(NH,),’+ into N a y zeolite in aqueous slurry took place rapidly under the same experimental condition at RT; the concentration of Pt in the supernatant solution becomes negligible after stimng overnight for the ion exchange up to ca. 10 wt% Pt in Nay. The difficulty in the ion into NaA zeolite seems to be mainly due to the diffusional hindrance exchange of Pt(NH,)? through the a-cage aperture with the diameter of ca. 0.4 nm since the a-cage aperture is not large enough even for the xenon adso tion [ 6 ] , as compared with much larger supercage window of 0.74 nm Earlier works usingTz9Xe NMR [6] showed that the a-cage aperture was expanded for the xenon adsorption by substituting some of the Na+ ions with Ca2+. Similarly, we attempted to exchange the Pt(NH,),” ion into our CaNaA zeolite in which ca. 50% of the initial Na’ ions in NaA were exchanged by Ca2+. As a result, the ion exchange up to 1.9 wt% Pt was attained for the CaNaA sample by the same treatment as above at RT. This Pt content correponded to 20% of the Pt which was initially present in the ion exchange solution. The rate of Pt(NH,),2+ exchange into CaNaA was then increased by raising the temperature to ca. 350 K the Pt content in the supernatant solution became negligible after stmng in the above ion exchange solution under the same condition for 7 days. The magnetic stimng was continued for 14 days in order to ensurc uniform distribution of the Pt species within the zeolite crystal of the Pt/CaNaA precursor. Prolonged stimng over this period changed the sample color from white to grey, probably due to the decomposition of the Pt species within the zeolite crystal. The work of Dalla Betta and Ebudart on the preparation of Pt/CaY [3] found the importance of a pretreatment (or calcination) of the ion-exchanged precursor in flowing 0, under heating to 573 - 673 K prior to the reduction of Pt. The calcination treatment seemed to result in the formation of PtO [S]. Subsequent reduction treatment with H, produced a small Pt cluster encaged in the supercage [2,5]. Direct reduction by H, or evacuation under heating to e.g. 573 K without a proper calcination treatment in 0, was reported to produce a highly mobile Pt species resulting in the formation of large Pt agglomerates on the external surface of the zeolite crystal [3]. The H-chemisorption data for our 2 wt% PtMaY sample in Table 1 agreed with the earlier result; the Pt/NaY sample reduced after the calcination treatment gave a value of chemisorbed H per Pt much higher than the other 2 wt% Pt/NaY’s. In comparison with the result from PtMaY, the 1 wt% and 2 wt% Pt/ CaNaA samples gave nearly 1 W t even without the calcination treatment as Table 1 shows. Thus, the Pt species in Pt/CaNaA was considerably less mobile under the reduction conditions without the calcination than PtNaY. In Fig. 1, the TPR data of the precursors of 10 wt% PtMaY and 2 wt% Pt/CaNaA are compared after the same calcination treatment. Since the reduction ttmk place at about the same temperatures, the Pt species in the samples are considered to be identical. The decrease of the Pt mobility for PtlCaNaA is then attributed to the diffusional hindrance through the small a-cage aperture, which further suggests that the Pt cluster in Pt/CaNaA may bc significantly smaller than that in Pt/NaY. We also prepared a 10 wt% Pt/CaNaA in the same way and characterized the Pt cluster by extended X-ray absorption fine structure (EXAFS, at Pt L,,, edge, under ca. 1-atm H at RT, measured at beam line 7C in the Photon Factory, Tsukuba). ‘i’he Pt cluster size
1636 estimated from EXAFS of this sample was a. 1 nm However, the sample chemisorbed H very little as Table 1 shows. Xenon adsorption measurement, as well as XRD, indicated that the crystallinity of the 10 wt% PtlCaNaA sample was 30% less than the CaNaA zeolite. The small Wpt value for the 10 wt% PtiCaNaA seems to be due to the zeolite-structural damage which probably occurred during the heating of the zeolite containing 2 H+ produced from Pt(NH3), in the reduction and then blocked the access of H, to the 1-nm Pt cluster. The decrease of Pt loading to 1 - 2 wt% in the CaNaA zeolite avoided the zeolite-structural damage as Table 1 indicates, and the chemisorbed H/Pt for these samples was indeed very high. These samples will be further characterized by EXAFS. The turnover frequency for the catalytic hydrogenation of cyclopropane over the 1 - 2 M u Pt/CaNaA is compared with that for PtMaY in Table 1. The turnover frequency for the Pt/CaNaA samples was considerably lower. +
Table 1
Chemisorption H and 0 and cyclopropane hydrogenation
1 wt% Pt/CaNaAa) 1 wt% Pt/CaNaAb) 1 wt% Pt/CaNaAC)
2 wt% Pt/CaNaAa) 2 wt% Pt/CaNaAb) 10 wt9"t/CaNaAa) 2 wt% Pt/NaYa) 2 wt% Pt/NaYb) 2 wt% PtMaY')
1.2 1.3 1.0 1.2
0.48
100
0.31
100
0.08
70
0.001
0.8
0.15 1.3 0.23 0.63
0.73
0.01
a) reduced after the calcination. b) reduced directly. c) reduced with evacuation. d) measured from xenon adsorption and XRD, relative to CaNaA. e) turnover frequency for cyclopropane hydrogenation at 231 K. 4. CONCLUSION
This study showed that a small Pt cluster can be prepared on a CaNaA zeolite by the ion exchange method, similar to the Pt supported on Y zeolite. Hydrogen chemisorption measurement indicated that the Pt cluster was at least as small as that encaged in the Y-zeolite supercage. Further information on the cluster size (e. g., by transmission electron microscopy and EXAFS) and the catalytic activity is required for the Pt/CaNaA sample in order to correlate the chemisorption and the catalytic activity with the cluster size. 5. REFERENCES 1 R. Ryoo, in "Catalytic Science and Technology Vol. 1" (S. Yoshida, N. Takezawa, and T. Ono, Eds.), p. 405, Kodansha, Toyko, 1991. 2 R. Ryoo, S. J. Cho, C. Pak, J.-G. Kim, S.-K, Ihm, and J. Y. Lee, J. hi.Chent. Soc., 114
(1992) in press. 3 R. A. Dalla Betta and M. Boudart, in "Proceedings, 5th Int. Cong. Catalysis, Palm Beach, 1972" (J. Hightower Ed.), North Holland, Amsterdam, Vol. 2 (1973) 1329. 4 D. W. Breck, "Zeolite Molecular Sieves", Wiley, New York, 1974. 5 B. F. Chmelka, R. Ryoo. S.-B. Liu, L.-C. de Menorval, C. J. Radke, E E Petersen and A. Pines, J. Am. Chem. Soc., 110 (1988) 4465. 6 C. Tsiao, D. R. Corbin, and C. Dybowski, J. Phys. Chem., 94 (1990) 867.
O d ,L a d.(Editors), New Frontiers in Catalysis Proceedings of the 10th Intcmtional Congress on Catalysis, 19.24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishem B.V.All righB =wed
PREPARATION OF THERMALSTABLE PILLARED CLAYS
S.Mendorofl, F. Gonzalezb, C. Pesquerab, I. Benitob, C. Blancob and G. Poncelelc ahtituto de Catalisis y Petroleoquimica, Cantoblanco,28049 Madrid, Spain bDepartamentode Quimica, Universidad de Cantabria, Santander, Spain cGroupe de Physo-Chimie Minerale et de Catalyse, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium
Abstract RE-AlPILCs have been prepared by three different methods and the variations in properties with respect to the parent clay studied through various phisicochemical technics. The catalytic activity has been tested in n-heptane, h-isomerization and h-craking in the 200-400°C interval at atmospheric pressure. Some conclusions on the suitability of the proposed synthesis methods have been drawn. 1. INTRODUCTION
The main difficulty in dealing with pillared materials for the cracking of hydrocarbons arises from the need of using water vapour at high temperatures (steaming) to eliminate the coke generated during the reaction; PILCs have not shown sufficiently stable to withstand the necessary temperature (around 700°C) and they collapse with the corresponding loss of their interlamellar spaces and, therefore, of their surface area and activity (1). So, a series of attempts have been made in order to get that hydrothermal stability introducing modifications in the synthesis steps, all of which result in modifications of the physicochemical properties and catalitic activity of the resulting material. One of the possible ways to improve the hydrothermal resistance of the pillars would be to form mixed pillars with cations that would mutually stabilize, thus retarding dehydroxilation and sintering of the starting cations. When dealing with Aluminium pillars, it seems promising to introduce oxides of trivalent cations (RE: Ce, La, Ga, Nd, Sm, etc.), such as in zeolites (2), either in the structure of the pillaring cation or within the framework of the starting material, so as to prevent the breakdown of the pillars through the loss of structural water. In this work we intend to study how the method of preparation affects the catalytic performance and properties of mixed pillared clays, using aluminium as pillaring agent and Ce and La (RE) as stabilizing agents.
1638
2. EXPERMENTAL
The starting material was a Spanish bentonite from Almeria, supplied by Minas de Gador, S.A.. Its percent chemical composition was: SO2, 42.9; A1,03, 12.6; FeO,, 4.58; MgO, 6.01; CaO, 1.37; K20, 1.09; PzOlr,0.03; TiO,, 0.27; MnO, 0.02;Na20, 1.74;I.L., 29. Mineralogically it is more than 95 % montmorillonite. The Na-montmorillonite (Na-M) has a surface area of 84 m2/g, the pore volume volume, as the takeup of N2 at 0.98 relative pressure, was 0.098 ml/g (0.0196 in micropores); its CEC 61.6 meq/100 g. Atomic absorption, N, adsorption, XRD, thermal analysis, ammonia adsorption, and I R spectroscopy of adsorbed pyridine have been used for samples characterization. Hisomerization and H-cracking of n-heptane in the 200-400°Cinterval, have been used as test reaction to evaluate the catalytic activity of the materials after incorporation of 1% Pt from ammonia solution (incipient wetness method) and "in situ" reduction, 400"C, 2 h. Three different methods were used to prepare Al-RE pillared clays: I) incorporation of the A1 polycation to materials previously exchanged with RE; 11) partial exchange of A1 for marketed by RE in a solution containing the Keggin cation (Al,304(OH),4(H,0),,)7+, Hoechst as Locron; 111) "in situ" cohydrolisis of A13+ and RE3+chlorides in order to get the corresponding Al-RE mixed cation. A OH/Me ratio of 2.0 was always used except in case I) at which 1.6 was used. The polycation solutions were suitably aged in order to get the intended cation. The final pHs altogether with the main synthesis parameters are gathered in table 1. Clay concentration in solution was 2.5 %. Following Plee (3), 20 meq Me/g clay were used in all three methods. In I1 and I11 various AURE ratios, 20/0,15/5, 10/10 , 5/15 and 0/20 were used. After the exchange reaction, the materials were washed by dyalisis until Cl- elimination and freeze dryed. Table 1 Main synthesis parameters cation
AI~+/RE~+
PH
A1"IPILC mmo1/100g
4.87 4,02 5.05 7.60 5.00 7.60
294.4 322.2 235.2 333.3 221.5 329.3
M" "A1
"'A1
"A1,Ce "'N,Ce "AI,La "'A1,La
20/0 20/0 10/10 10/10 10/10 10/10
Me'+/PIL,C mmol/lOOg
14.3 145.0 30.9 225.6
%A1
q/n'
100
.21 .20 .25 .12 .24 .ll
100 94.3 69.7 87.8 59.2
'CEC/mmol Me3+incorporated 3. RESULTS AND DISCUSSION
In table 2 are gathered the basal spacings at room temperature and 500°C of all samples. Also the main textural parameters are included. As it can be observed method I is the most suitable with respect to microporosity creation, being followed by 11; method 111 has revealed as inadequate to prepare PILCs, the effect being more important in La than in Ce mixed pillars. Smaller losses in basal spacings at 500°C in samples becoming from method
1639
I1 denote a greater thermal stability of these samples with respect to those from method I and 111; again a disordered structure from the latter is patent. Since the average charge of A1 in the Keggin ion is 0.54, and the CEC of the original material was 0.616 meq/g as previously said, several types of polyoxocations may have been exchanged and/or the (A11J7+ion hydrolyzed on the clay surface (4). Table 2 Textural parameters of the pillared clays Sample
Go,, A RT
500'C
11.9 18.9 17.3 18.0 20.6 19.8 20.0 16.7 17.2
9.89 17.5 16.7 16.4 17.5 17.1 17.7 13.2 13.0
Isotherm tYPe
%E'T
sp
vP.9i
vp
m2/g
mZ/g
mllg
mllg
III
84 392 242 266 180 176 280 162 51
316 224 246 128 93 192 73
~~
Na-M 'Na-AlPM 'Ce-AlPM 'La-AlPM "Na-AlPM "Ce-AlPM "La-AlPM We-AlPM '"La-AlPM
III III
III I+III I+III I+III I
I
.0196 .236 .175 .184 .184 .I58
.214 .161 .216
.124 .ll8 .130 .05 1 .120 .076 .031
pH as high as 7 or more are not compatible with the formation of the Keggin cation, thus explaining the obtained results. An unpillared clay with microporosity due to some kind of polycations of Al, RE or boths on the external surface of the clays, may result from method I11 whereas an ideal PILC with ordereded structure results from method I; method I1 gives rise to an intermediate material as seen by the types of N2 isotherm involved (table 2). Thermal analysis detects losses at around 450°C in 'Ce-AlPM and 'La-A1PM attributable to the transformation of the aluminium oligomer into alumina. The case is not the same in the rest of the samples, in which a continuous loss is seen all along the thermogram corresponding to not well developed species. IR spectra of adsorbed pyridine show important differences among samples in acidity. Pillaring, in general, creates Bronsted and increases Lewis acidity. The latter remains up to 500"C, although it slightly decreases above 200°C. As it is known the acidic properties of the pillared materials are by no means simple summationsof those properties of the pillar and the original clay material. However, Bronsted acidity is provided mainly by structural OH groups in 2:l layers. On the other hand, the pillaring agent, forming or not pillars, is converted in more or less extension, to metal oxide by calcination contributing to the Lewis acidity of the sample (5). Then, differences among samples can be easily related with the different synthesis methods involved; samples resulting from co-hydrolysispresent more Lewis acidity, whereas the use of Locron enables the creation of Bronsted centres. A certain disorder seems to occur in the structure of the original clay when mixed pillars are introduced, less in 10/10 samples. The evolution with temperature of the total conversion on various samples is given in fig.1. Only 10/10 samples from methods I1 and I11 have been included. As it can be seen, total conversions are quite similar in all three types of samples and, in no case larger than on "a-AlPM. Only sample '%e-AlPM-10/10 presents a dramatic change in activity reaching conversions of 100% above 350°C mainly in cracking. Also mLa-AIPM-lO/10
1640
show an important decrease in activity which can be related with the loss in surface area previously detected. Product distribution at 350°C are given in table 3. Table 3. Conversions at 350°C on RE-A1 mixed PILCs Sample Conv.,96 h-ilh-c
M-Nat 'Na
'Ce
'La
"Na
"Ce
"LA
"'Na
"'Ce
'"La
19.1 ,810
39.9 .232
39.5 .219
44 1.267
38.7 2.080
49.6 1.527
43 .441
99.2 .017
27.9 .366
57.2 ,204
The pillaring process I increases cracking with respect to the original clay, more on Ce than on La samples; in contrast, method I1 increases isomerization doubling the values reached on A1-PM with Ce inclusion.The effect is slightly lower on "La-AlPM. Finally, method I11 (cohydrolysis) is on line with method I, cracking surpassing by far isomerization, more on Ce than on La samples. Fig. 1 n-heptane conversion 4. CONCLUSIONS
Mixed pillars intended for preparing thermalstable PILCs have shown fairly succesful in hitting their target. From the three methods used, only the first one gave rise to props homogeneously distributed between layers. The remainig two with a q/n ratio below 0.36 in all cases, result in incompletely pillared materials, with microporosity created through excess material on the external surface of the clays, more through I11 than through I1 method. Acidity as well as activity have shown extremely different, the method 111, and specially on samples with Al/RE 1, being the most suitable for h-cracking and method I1 for h-isomerization.
5. REFERENCES 1. M.L. Occelli, I&EC Pro.Res & Dev., 22 (1983) 553. 2. D.Tichit, F. Fajula, F. Figueras, C. Gueguen and J. Busquet, Proc., 9 O ICC, M.J.Phillips and M. Ternan (eds), I (1988) 112. 3. D.Plee, F. Borg, L. Gatineau and J.J. Pripiat, Clays and Clay Min., 35 (1987) 81. 4. R. A. Schoonheydt, SSSR, 58 (1991) 201. 5. H.Ming-Yuan, L. Zhonghui and M. Eze. Catalysis Today, 2 (1988) 321.
Guczi, L.a al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992, Budapest, Hungary 0 1993Elsevier Science Publishers B.V. All rights reserved
IN SITU X-RAYANALYSIS OF CO-AND CH30H-INDUCED GROWTH OF Pd PARTICLES ENCAGED IN ZEOLITE Y
W.Vogel Fritz-Haber-Institut der Max-Planck-Gesellschaft,Faradayweg 4-6, D-W1000 Berlin 33, Germany
Abstract
In situ X-ray diffraction of supported metal catalysts has been shown to give valuable information about the state of dispersion and intra-particle symmetry even a t extremely high metal dispersions. In a reduced P m a Y catalyst = 75 wt% of the palladium are Pd13 particles with icosahedral symmetry. Admission of CO and to a smaller amount of CH3OH leads to a growth of these particles which partly change to the fcc symmetry.
1. Introduction Recently great progress has been achieved in the understanding of the detailed mechanism of Pd particle formation in Nay, prepared by a subsequent ion exchangdcalcinatiodreduction procedure [1,21. After calcination at 500 "C and reduction a t 350 "C an average coordination number of CN=4 was found by EXAFS 131. Adsorption of CO leads to a significant increase of cluster size (CN=6). This agrees with the proposed formation of carbonyl clusters P d l ~ ( C 0 ) ~ , entrapped in the supercages [I]. We have applied in situ X-ray diffraction (XRD) to structurally characterize these extremly dispersed palladium particles. We calculated intensity curves, which describe the scattering of X-rays by an assembly of randomly oriented particles consisting of several atoms to some thousand atoms. The method is called 'Debye Function Analysis' (DFA) and is based on a distribution of 'closed shell'-clusters Me,, n having the 'magic numbers' n = 13, 55, 147,309, 561,923, 1415. It has first been applied to describe the oxidationheductionbehaviour of the standard WSiO2 catalyst EUROPT-1 141. In contrast to E M S ,XRDDFA is increasingly sensitive for n 2 100. Moreover DFA can readily determine the internal particle symmetry, i,e. fcc and icosahedral structure.
2. Experimental 7.3 and 5 wt% Pd on NaY was prepared by ion exchange of Linde NaY (LZY-52, Na57(A102)57(Si02)135.375H20 1. A detailed description of the preparation is given in 121. Samples were calcined ex situ and in situ recalcined in a flow of pure 0 2 . About 30 mg of both, the Pd/NaY catalyst and the pure NaY support were pressed to self-sustaining, thin pellets. The samples were exchanged a t every angular step of the modified HUBER Guinier goniometer. This allowed a reliable determination of the intensity difference,i.e. the intensity of the pure palladium
1642
phase. Typical measuring times for one 6-scan was 10 h. Time resolved growth processes could be followed by an 'Open Slit' ( 0 s ) technique, i.e. a fast measurement of the integral intensity around the first strong metal peak. This signal is sensitive to changes in the average nuclearity of metal particles. The 0s measurements during the intermediate treatments could be combined with an observation of reaction products via a quadrupol mass spectrometer.
3. Results Palladium metal particles could neither be detected by small angle X-ray scattering nor by wide angle scattering after the recalcination step. Reduction in flowing H2 a t 200 "C for 20 min. and subsequent cooling in He resulted in a dominant peak (= 75 wt%) in the cluster distribution Pdn at n = 13 and a second peak for n = 55 (t.20 wt%). Practically 100%of these clusters were icosahedra.
1000
800
lo
600
400
05
200 0 -0 .15 .e c L
000 800 600
ffl v)
.10
7
I
400 2 00 0
0
Figure 1. Intensity difference of Pd/NaY and DFA simulation (solid lines) before (top) and after CH30H treatment.Dashed line: Contribution of the fcc-particles. Long dashed line: Contribution of the icosahedral particles.
10 20 30 40 50 Diameter 0 , S,
Figure 2. Mass distribution versus particle diameters before (top) and after CH3OH treatment from DFA. Dashed line: Contribution of fccparticles.
1643
A slightly improved fit could be achieved if clusters with smaller nuclearity, i.e. n =4,6 were included in the simulation. Their existence, however, could not be proven
unambiguously. 20 min. exposure with CO at room temperature had the following effects: a) The fraction of Pd55 was growing to the expense of Pd13; b) Part of the Pd particles changed their symmetry from icosahedral- to the fcc arrangement. Atter prolonged expo ure to CO at ambient conditions large Pd particles with diameters D = 40200 were formed, but a fraction of the very small particles D < 10 A were still present. An increase of the Pd nuclearity was also observed during catalytical decomposition of methanol. This is demonstrated by Figure 1. Shown is the intensity difference originating from the palladium phase after H2-reduction and short exposure to CO (top drawing) and after exposure of the catalyst to ,a flow of He/CH30H for 75 min. at 212 "C. Figure 2 depicts the mass distributions versus particle diameters D corresponding to Figure 1. In this figure the discontinuous distribution originally obtained by DFA is transformed to a continuous distribution function by Gauss-smearing [41. It is evident, that the distribution of the particle sizes becomes bimodal. The corresponding X-ray dispersion dx decreased from 84% to 54%, while the fraction of particles having the fcc symmetry increased from 24% to 63%.
8:
't 5 0 0 difference intensity
3000
400
3
25004
I
partial pressure, a.u.
100
0
50
I
1
I
I
I
I
100
150
200
250
300
350
0
Time, min Figure 3 . 0 s measurement. He flow was switched to HdCH30H aRer 65 min.. Top curve: Difference intensity around the first Pdmetpeak; Intermediate curve: CO partial pressure (arbitrary units). The temperature ramp ist plotted at the bottom. If the CH30H exposure is started at room temperature, the 0s measurement indicates only a small increase of nuclearity over a period of a60 min., as shown in Figure 3. When the temperature is ramped to 220 "C ,the mass spectrometric 28
1644
amu-signal steeply increases at a180 "C. At the same time an accelerated growth of Pd particlee above 180 O C is observed.
4. Conclusions The CO-induced partial transition of icoeahedral- to close packed fcc particles could be attributed to an energetically more etable ligand formation on the surface of an cuboctaheron, which is the 'closed shell'-morphologyof an fcc particle. The latter exposes a greater variety of absorbtion sites as compared to an icosahedron. The proposed 'Ship in the Bottle'-model of palladium carbonyl clusters Pdl3(CO),, entrapped in the supercages of the Y-zeolite is supported by the observation of bimodal size distribution6 with a pertinent peak at =?A.The growth of particles much larger than the volume of the supercages is generally accompanied by the formation of an amorphoue halo, which is attributed to a disruption of parta of the zeolitic Eramework. The question whether thie dieruption is induced by a local coaleecence of mobile palladium intermedates of low nuclearity a t the interior or at the extra-zeolitic boundaries via Ostwald ripening can not be answered at the moment. The proposed CO-induced weakening of the bonds of primary Pd particles to zeolitic protons [23 as the origin of particle growth may also be adopted for the obeerved growth under methanol exposure. The deeorbtion of CO a t =180 "C induces an accelerated growth of the palladium particles. The chemistry of methanol on palladium single crystal surfaces is, however, known to be complicated, coneieting of methyl, oxygen, methoxy and cabon monoxide species [6].Very little is known about the situation on highly dispersed supported palladium. The methanol experiments presented here are preliminary, but propose a possibility to better understand the etructural interrelationship withs catalytic reactione. The preeent results do not agree in all details with previous EXAFS investigations. This mightbe partly due to diffusion limitations during treatments in the X-ray cell, which allow the reacting gas to bypass the catalyst. A combined EXAFS/XRD study with the in situ cell used in this work has been started at the CHESS synchrotron to better understand the interrelation of both techniques.
6. References 1 L.L. Sheu, H. fillzinger and W.M.H. Sachtler, Catal. Lett. 2 (1989)129 2.Zhang, H. Chen and W.M.H. Sachtler,J. Chem. Soc. Faraday Trans. 87 (1991)1413 3 2.Zhang, H.Chen, L.L. Sheu and W.M.H. Sachtler, J. Catal. 127 (1991)213 4 V. Gnutzmann and W. Vogel, J. Phys. Chem. 94 (1990)4991 6 V. Gnutzmann and W. Vogel, Z. Phys. D-Atoms, Molecules and Clusters 12 (1989)697 6 N.Kruse, M. Rabholz, V. Matolin, G.K. Chuah and J.H. Block, Surf. Sci. Lett. 238 (1990)L467 2
Guczi, L d ul. (Editors), New Froders in Cmulysis Proceedings of the 10th International Congnss on Catalysis, 19-24July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
MmAL-SUPPORT INTERACTIONSON Pd-CONTAININGZEOLITE CATALYSTS
M.F.Savchits, ,Eh. Ya Ustilovskzyq V.2,Veshtort,L. A. Agabekova and Yu. G. Egiazarov Institute of Physico-Organic Chemistry, Belorussian Academy of Sciences, 220072 Minsk, Surganov Str. 13, Belorussia
Abstract The influence of alladium on a c i d i c properties of z e o l i t e supporter and also he e f f e c t of acidio function of supporter on the s t a t e of metal have been studied. It was established that c a t a l y t i c a c t i v i t y o f Pd depends on i t s electronic state.
?
'Po date, the extensive actual data have been accumulated that r a t e the i n t e r a c t i o n between metal and support i n metalcontaining c a t a l y s t s including rteolites 1-3 However, i n the ma o r i t y of works the authors consider, as a r u l e , only one f e a w e of this phenomenon, e i t h e r a change i n electronic and c a t a l y t i c properties of metal o r aupporter
.
i!
properties including aciiiio ones. The a i m of the preeent study was systematiaed investigation of the mutual metal-su p o r t e f f e c t exemplified by palladiuma e o l i t e catalysts. I n he studies &wo s e r i e s of c a t a l y s t sank l e s were used. In the first s e r i e s 0.01-4 96 Pd wae depoei!ed on a decatinized zeolite-containing supporter (18 w t b 4% a e o l i t e Y i n the aluminosillcate matrix 208 1. In the seoond aeries, 0.5 8 Pd was introduced i n z e o l i t e Nay with a d i f f e r e n t degree of exchange o f sodium f o r ammonium cations. A l l the c a t a l y s t s were calcined and reduced a t 40O0C* The chsr a c t e r i s t i c s of synthesized c a t a l y s t s a r e given i n Table 1. In order t o study physico-chemical properties of the catalysrtla, the methods of electronic mioroscopy, thermodesorption o f ammonia and hydrogen, IR-spectroscopy of adsorbed pyridine and CO molecules, X-ray phase analysis were used. The 00 o x i dation, benzene and propylene hydrogenation, cyclohexane dehydrogenation were used as test-reactions. 'Po study the a c t i v i t y of Pd-zeolite containing c a t a l y s t s i n hydrogenation and dehydrogenation reaotions of hydrocarbons, the method o f microimpulse technique was used. In 00 oxidation the a c t i v i t y of Pd-zeolite c a t a l y s t s was determined i n f l o w reactor at_,the feed f l o w r a t e of a 1 ~01.5CO mixture with air, 20 000h It has been s t a t e d as a r e s u l t of the studiePr performed t h a t introduction of palladium i n an a c i d i c support conside-
?
-
.
1646
rably sup r e s s e s the acidic function, 0.3-0.5 wt.76 of metal being most effective. So, IVH3 quantity i r r e v e r s i b l y chemisorbed by a zeolite-containing sample at 250OC decreaaes from 0.25 mmole/g t o 0,153 mmole/g with increasPd content from 0.01 t o 0.5 $4. A f u r t h e r increase i n the Pd content praoticall y does not change t h e acidic c h a r a c t e r i s t i c s o f samples. Table 1. The c h a r a c t e r i s t i c s of studied c a t a l y s t s Sample
Cat a l y s t
NO
Na content, P a r t i c e wt. % size,
It
2 0 4 0 . .- - - 20-40 20-50 20-70
2 3 4
20-80
5
6
20-100 30-200 50-300 50-500 30-40 30-40 30-40
7
8
9
10 11 12 13
30-40
The palladium e f f e c t i s revealed mainly with decreasing o f ooncentration of proton acidic s i t e s , that i s confirmed with decreasing of adsorbed pyridine band i n t e n s i t y at 1540 cm-1 a f t e r deposition of Pd on acidic supporter (Fig*l). It is very important t h a t an equal quantity of palladium has a stronger e f f e c t on the a c i d i c function i n a z e o l i t e as compared t o a zeolite-containing support (Table 2). This sugg e s t s e i t h e r collective nature of a metaldmpport i n t e r a o t i o n o r long-range e f f e c t i n t r a n s f e r o f l o c a l disturbances in the c r y s t a l z e o l i t e l a t t i c e . An X-ray amorphous aluminosilicate matrix considerably suppresses t h a t effeot. In i t s turn, the a c i d i c s i t e s of supports have a modifying effeot on palladium. Due t o a shift of electron density from palladium t o an acidic support, the metal surface has some d e f i c i t o f electrons which manifests i t s e l f i n a shift of l i n e a r band of CO adsorption i n t o a short-wave region o f the IR-s pe c t rtam. Due t o electron deficiency on the palladium the a c t i v i t y of the c a t a l y s t s i n CO oxidation (Fig.2) and benzene hydrogenation (Fig.3 decreases, but that i n cyolohexane dehydrogenation (Fig.4 increases. The symbols on the Fig. 2-4 correspond t o numbers o f the c a t a l y s t a i n Table 1.
!
1647
$ / 12
2AN f'
u
0 Frequency, omi1
f40 160 180 200 221
Temperature,OC
Figure 1. IR-spectra o f yridine adsorbed on ZCS 7 1 3 1 ' ) and 4% Pd/ZCS(2,2') at vacuum-treatment temperature, O C : 1,2 200,
-
- 300-
l'S2'
Table 2. Influence of Pd(0.5 tions
.
_
-
_
_
_
.
_
a b
-
wt.%)
on the a c i d i c HNaY and ZCS func-
r
NH desorption t edperature, o c
----
Figure 2. Temperature dependence of the CO conversion for Pd-zeolite c a t a l y s t s o f d i f f e r e n t degree o f decationization.
__.
a
a
b
3.690 1.230 0.680 0.150
1.860 0.480 0,074 0.016
61.2 8901
a
a
0.520 0.255 0.105 0.025
0,390 0.153 0.052 0.060
b
t-
100 250 350 450
49.6 89.5
25.0 4003 49.5 73.1
-quantity i r r e v e s s i b l y cheraisorbed by a sample a t the Z J e n temperature, mmole/g i r e l a t i v e v a r i a t i o n o f supporter a c i d i t y upon 0.5 % Pd deposition, %
1648
P’
POO
500
LOO
Temperature,*C Blgure 3. T a n erature dependenoe o f r e l a ive epeoifio a c t i v i t y i n the reaction of benzene h y d r o enatlon on ZCS wlth differen oontents o f palladim.
g f
Figure 40 Tern erature depen-
denoe o f relative speolflo a o t l v i t y i n the reaotion of oyolohexaae dehydrogenation on ZCS with d i f f e r e n t Pd oont ent a.
Therefore, palladium suppresees the a o i d i o oharaoterietics of zieolite supporters..Aoidio centrea of supporters modify the Pd eurfaoe , exerting influenoe on the a o t l v i t y o f Pd i n the reaotlonls o f hydrooarbons hydrogenation and dehydrogenation, and 00 oxidation. Consequently, mutual influenoe between palladium and acid i o supporter take plaoe. REFERENCE8
Tauster, 8.C. mulg, and SOO., 100 (1978) 170.
1 9.J.
RoL. 5arten, 3. A m e n Chem.
P. Merlaudeou, OOHO Elleatad, M* Dufawr, an8 G. Naooaohe, J O Catal., 75 (1982) 243. 3 GOOI Bond, Platinum Metals RevoI 27 (1983) 16. 2
Ouczi, L Q al. (Editors), New Frontiers in Catulysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevicr Science Publishers B.V. All rights nscrved
TRANSFER OF METAL IONS BETWEEN METAL OXIDES AND ZEOLITES. PREPARATION OF HIGHLY ACTIVE CU.ZEOLITEBASED CATALYSTS FOR REDUCTION OF NO, AT LOW TEMPERATURE B. Wichterlowaa, Z. Sobalig,-M. Petrasa, I. Jirkaa and V.BosacekO
*J. Heyrovski Institute of Physical Chemistry and Electrochemistry,CzechoslovakAcademy of Sciences, 182 23 Prague 8, Dolejskova 3, Czechoslovakia bInstituteof Inorganic Chemistry,Czechoslovak Academy of Sciences, 160 00 Prague 6, Majakovskeho 24, Czechoslovakia
Abstract A solid-solid interaction between Cu oxides and NHPY or HZSMJ zeolite was investigated by means of IR, ESR, XPS, SEM and development of the catalytic activity. It is shown that Cu ions from the metal oxide are incorporated into the zeolite cationic sites. The process is controlled by temperature, Cu oxide content and, it is enhanced by water or ammonia presence. Moreover, it can be applied as a method for preparation of highly active Cu-zeolite based catalyst for selective reduction of NO with ammonia at low (470 K) temperature.
INTRODUCTION An attention has recently been paid to the high-temperature "interaction-reaction"of metal oxides with zeolites, resulting in the relocalization of metal cations from the oxide to the zeolite phase (1-8). Such processes can take place in composite metal oxide-zeolite or metal-zeolite based catalysts during catalyst activation, reaction performance and regeneration processes. Moreover, it appears that in particular cases such interaction can be applied as a simple method for preparation of metal-ion loaded zeolite catalysts (6,9). A solid-solid interaction between Cu oxides and H-forms of zeolites reported here is focused on the coordination of Cu ions introduced into the zeolite from the metal oxide, the effect of water and ammonia presence on the metal ion relocalization and, eventually, on the application of this procedure for preparation of catalyst for selective reduction of NO with ammonia. Recently, the Cu-doped zeolites have been proven as promising catalysts because of decreasing the lower temperature limit of the high activity region below 470 K, in contrast to V-Mo-W-O/TiOn catalysts which excellent performance lies usually in the temperature region above 550 K (10).
1650
EXPERIMENTAL
H-ZSM-5 (Si/Al=12.6) and NH4-Y (Si/Al=2.5) were used for preparation of Cu oxide-zeolite mechanical mixtures (mixing in an agate ball mill for 2 hours); Tables 1-3. The mixtures were heat-treated in a stream of dry oxygen or in vacuum up to the temperature (670 K for NH4-Y and 770 K for H-ZSM-5) at which zeolite dehydroxylation itself did not take place. In some cases the oxide-zeolite mixtures were treated in a water vapour (440 Torr at 358 K) or in a nitrogen stream containing 10 vol.% of ammonia at 570 or 670 K followed by the mixture oxidation in 10 vol % of oxygen in nitrogen stream at 570 K for 0.5 hours. For comparison ion exchanged CuH-ZSM-5 and CuNH4-Y zeolites were prepared by an ion exchange from CuCL solution (Table 1,3). The ESR spectra (ERS-220, ZWG, Berlin), X P S (VG 3MK 11), SEM (Jeol JEM 100B)and FT-IR spectra (Nicolet MX-1E) of skeletal and O H group vibrations were used to monitor the zeolite structural stability, changes in the number of zeolite O H groups and coordination as well as location of Cu ions. Catalytic tests were carried out in an integral isothermal reactor. The flow rate of the reaction mixture (in vol.%, 0.40 NO, 0.40 NH3, 3.00 OZ,96.20 Nz) was 30 1 (S.T.P.) h -1 per gram of a catalyst. Inlet and outlet reaction mixtures were analyzed using NO/NOx chemiluminescence analyzer (951A Beckman) and by acidimetric titration after adsorption. RESULTS AND DISCUSSION
The heat-treatment of the CuO/H-ZSM-5 mixtures of various CuO content resulted in an appearance of the ESR signal consisting of two sets of signals of axial symmetry and four hyperfine lines originated form the Cu nucleus In=3/2. This evidences formation of isolated C u 2 + ions in two coordinations (square planar and square pyramidal, cf. ref.3) in the heat- treated CuO/H-ZSM-5 mixtures similarly to the coordination of Cu ions in the ion exchange CuH-ZSM-5 (Table 1). Simultaneously, a consumption of the zeolite bridging OH groups took place and increased with the CuO content, however, approaching a limit of 30% of the number of OH groups in the parent H-ZSM-5 zeolite. These findings indicate that isolated C U ~ion + complexes located predominantly at the zeolite cationic sites are formed. A previous study ( 5 ) revealed that the level of consumption of OH groups was very similar for CuO and ( 3 2 0 . On the other hand, a substantial difference in behaviour of NH4-Y and H-ZSMJ zeolite in the metal oxide-zeolite mixtures was found. A comparable level of consumption of bridging O H groups was possible to reach for NH4-Y at a temperature 100 K lower than for H-ZSM-5. It was observed that both the water and ammonia presence in the gaseous phase enhances substantially the process of Cu oxide deaggregation already at low temperatures. It was proven by the surface Cu/Si ratio, increasing with the time of the mixture hydration and from SEM photographs, indicating deaggregation (hydrolysis) of the CuO and CUZO phases (occurring only in the oxide-zeolite mixtures). Subsequent heat-treatment of the hydrated mixture decreased in the Cu/Si ratio, reflecting migration of Cu ions into the zeolite bulk (Table 2). Very similar effects were observed when ammonia was present during CuO/NH4-Y heat treatment. Enhanced deaggregation of
1651 Table 1
ESR parameters and number of OH groups in the heat-treated CuO/H-ZSM-5 mixtures
and CuH-ZSM-5
Zeolite
Cu content (mmol/g)
treatment
H-ZSM-5
0
720 K
0.12
720 K
0.55
720 K
1.09
720 K
0.19
720 K
cuo/
AL4
-lAU 10 cm
811
bridge OH (mmol/g 1 0.94
2.062 2.077 2.056 2.072 2.054 2.070 2.048 2.063
H-ZSM-5
CUH-ZSM-5
gL
* * 2.315 2.332 2.307 2.322 2.302 2.317
* *
21.5 16.9 22.3 17.7 22.8 18.3 23.7 19.4
0.77
138.4 131.0 141.0 134.9 137.0 130.8
0.66
0.59
Table 2 XPS data of hydrated and heat-treated Cu20/NH4-Y mixture Zeolite
Cu cont. (mmol/g)
CU2O/NH4-Y
1.72
hydration 358 K, h
CuNH4-Y
treatment ( K/gas 1 b 570loxygen
CuO/NH4-Y
570/oxygenb b
-
0 0.5 11.0 20.5 20.5 20.5
Table 3 Catalytic activity of CuO/NHp-Y NO reduction with ammonia Zeolitea
treatment (K)
4.5 7.5 13.2 28.7 19.8 2.3
-
420 770
mixtures
CU/Si4XPS) 10
and CuNH4-Y
in selective
NO conversion ( % ) 470 K 570 K 99.7
100.0
33.6 36.2 22.4 36.0 40.0 98.3 68.ld 99.2 99.1 a the Cu content in both materials was 1 mmol/g heat treatment for 16 hrs in 10 wt. % 0 in N for 16 hrs in 10 vol. % NH3 in N2 then ?or 0.5 hr in 10 vol. % 0 in N 2 2 after reaction time-on-stream of 70 hours NH4-Y exhibits negligible catalytic activity
670loxygen 570/NH3/oxygenz 670/NH3/oxygen
1652
the CuO phase and migration of the Cu ions into the zeolite phase was clearly documented by the increase of the catalytic activity of the mixture in selective reduction of NO with ammonia. The CuO/NHd-Y heat-treated in ammonia exhibited the activity comparable to the ion exchanged CuNHa-Y zeolite while the activity of CuO/NH4-Y heat-treated in a dry oxygen was substantially lower but it was further developing during the catalytic test (Table 3). CONCLUSIONS
It has been confirmed that a migration of Cu ions from Cu2O or CuO phase in mechanical mixtures with H-forms of zeolites takes place resulting in their location predominantly at the zeolite cationic sites. The process is controlled by temperature, content of Cu oxide phase in the mixture, it is strongly enhanced by water or ammonia presence, and it occurs easily with Y compared to ZSM-5 zeolites. The latter fact seemingly correlate with the instability of the bridging OH groups. It can be also explained by a higher degree of deaggregation of the oxide phase owing to the higher amount of water and ammonia in NHI-Y compared to H-ZSM-5. Generally, a lattice energy of the oxides (represented by their melting points) has a controlling role over the solid-state incorporation of metal cations into the zeolite phase via oxide-zeolite interactions. The process occurs easily with the oxides of low melting point like VZOS,CrO3 (4,7) and it is very limited with the oxides of high melting point NiO, ( 3 2 0 3 , Fez01 (1,4).However, for Cu oxides the process can be strongly accelerated by the presence of ammonia (likely via Cu-ammocomplex formation ) enabling preparation of highly active Cu zeolite catalyst in selective reduction of NO with ammonia at low temperatures. REFERENCES
B. Wichterlovh, S. Beran, L. KubelkovB, J. NovBkovA, A. SmieSkov6, R. Sebik, Stud.Surf.Sci.Cata1. 46 (1989) 347. 2. S. Beran, B. Wichterlovh, H.G. Karge, J.Chem.Soc., Faraday Trans. I, 86 (1990) 3033. 3. A.V. Kucherov, A.A. Slinkin, Zeolites 7 (1987) 38. 4. A.V. Kucherov, A.A. Slinkin, Zeolites 6 (1986) 175. 5. B. WichterlovB, H.G. Karge, H.K. Beyer, J.Chem.Soc., Faraday Trans I, in press. 6. V. Kanazirev, G.I. Price, K.M. Dooley, J.Chem.Soc.Commun. 9 (1990) 712; Stud.Surf.Sci.Cata1. 69 (1991) 277. 7. M. PetrhS, B. WichterlovA, J.Phys.Chem. (1992) in press. 8. S.T. Homeyer, W.M.H. Sachtler, J.Cata1. 118 (1989) 266; 117 (1989) 9. 9. CS-patent 5413-90. 10. H. Bosch, S. Jansen, Catalysis Today 2 (1988) 369. 1.
Guczi, L et al. (Editors), New Frontiers in Catalysir
Proc#dinga of the 10th International Congm on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
MICROKINETIC ANALYSIS OF ISOBUTANE REACTIONS CATALYZED BY Y ZEOLITE
J. E. Rekoskzq R. J. Madonb, L. M. Aparicifl and J. A. Dumesica aDepartment of Chemical Engineering,University of Wisconsin, Madison, WI 53706,USA bEngelhard Corporation, 101 Wood Ave., Iselin, NJ 08830, USA
Abstract Experimental product distributions for the cracking of isobutane over USY-based fluid catalytic cracking (FCC) catalysts were analyzed by constructing microkinetic models that simulated experimental behavior. We conclude that the following reactions are kinetically significant: carbenium ion initiation, hydride ion transfers, ethylene formation, and oligomerization reactions involved in oligomerization-rearrangementcracking routes. Olefin desorptions and the accompanying proton transfers are kinetically insignificant, as are carbenium ion isomerizations and &scission reactions. The net rates of initiation and olefin desorption reactions remain fairly constant along the length of a plug flow reactor. In contrast, the rates of hydride ion transfer and oligomerization reactions are negligible at the reactor inlet but become dominant pathways at the reactor outlet. 1. INTRODUCTION
Several researchers [l-51have recently studied the initiation of carbenium ions on solid acid catalysts using small paraffin hydrocarbons as molecular probes. Isobutane, chosen initially by McVicker et aL [ 11, is an interesting molecule since its structure allows only the formation of methane and dihydrogen via any initiation route. All products larger than C4 as well as any C3 in excess of the amount of methane produced must be formed from cracking of an oligomerized intermediate. Furthermore, for our initial attempts at microkinetic modeling of acid catalyzed paraffin cracking, isobutane provides a manageable number of reaction products. The purpose of this study was to examine various acid catalyzed reactions that occur in FCC using a tractable system with the aim of identifyingthe kinetic significance of the reaction steps, and later expanding the study to larger molecules. It is not intended via microkinetic analysis to prove a mechanism but rather to see if reaction steps chosen to describe the overall chemistry are reasonable, and thus to allow us to investigate their interrelationships with changing catalyst properties and experimental parameters.
1654 2. EXPEFUMENTAL
We studied isobutane reactions on two fluid catalytic cracking catalysts made by the Engelhard in sihc crystallizationtechnique [6].One catalyst (USY-C) was calcined for 2 h at 755 K and had a unit cell size of 2.443 nm and a zeolite content of 38 wt%; the other catalyst (USY-S) was steamed for 2 h at 1060K and had a unit cell size of 2.430 nm and a zeolite content of 31 wt%. Reactions were carried out at various temperatures from 733 K to 773 K. Reaction products were collected after a reaction time of 2 min; all hydrocarbon products, greater than O.OOO5 mol%, and hydrogen were analyzed via gas chromatography. Microkinetic analysis was carried out for isobutane conversions from 10 to 40%. Details regarding experimentation and conditions are given in Ref. [7].
3. ANALYSIS AND DISCUSSION We conducted microkinetic analysis, using techniques similar to those in Refs. [8,9], with 21 reaction steps that described the formation of the main reaction products. The steps used are given in Figure 1. The model, in its present form, could not distinguish between the details of the initiation process, i.e., protolysis [2-4]versus the surface free radical route [l]. However, since either initiation reaction can be described in terms of the same overall step, we have chosen to address initiation as steps 1 and 3. Ethylene was assumed to form from n-butane, the most abundant product. Since protolysis involving Brgnsted acid sites cannot account for ethylene formation without forming primary carbenium ions, and since surface chemistry leading to ethylene formation is not known in detail, we have written this reaction as a single irreversible reaction on electron acceptor sites (step 2). All other major products formed are accounted for by @-scission(steps 7, 10, and 12), isomerization (steps 6,9,11, and 15), oligomerization (steps 5 and 8), proton transfer from a carbenium ion to the surface giving an olefin product (steps 4,14,16,17,18,and 20), and hydride ion transfer from a paraffin to a wbenium ion giving a paraffin product (13,19,and 21). The large number of steps in the reaction sequence does not allow values of preexponential factors and activation energies to be evaluated separately from the experimental data. Therefore, the preexponential factors were estimated using transition state and collision theories, and these values were not adjusted in subsequent analyses of experimental data. Thermodynamicconsistency is achieved using partition functions and transition state theory: the preexponential factor of a step is obtained by multiplying the universal frequency and the ratio of the partition function of the activated intermediate of the step to the product of the partition functions of the reactants. Furthermore, since many of the reactions in the mechanism are chemically similar, the Polanyi correlation was used to relate activation energy to the enthalpy of the reaction. The importance of using such a correlation is that the activation energies of the various steps are constrained by this correlation to be consistent with the
1655
Figure 1. Isobutane Cracking Mechanism
1.
A
+ H +
c
A
+ H a
2.
3.
4.
5.
6.
7.
8.
9.
10.
11. 12.
13.
14.
1s.
18.
17.
16.
10.
20.
21.
-A+A/
H+
- H *
+ +-
A+H',
+
1656
thermodynamics of the overall reaction. Accordingly, 7 catalyst independant adjustable parameters were used during the modeling. This procedure was particularly useful for the present study since heats of formation for the carbenium ion intermediates can be obtained from thermochemical compilations ofvalues for gaseous species [101. The gas phase heat of formation was adjusted to describe a surface carbenium ion by introducing one catalyst-dependant parameter, ie., the heat of stabilization of the surface proton relative to the surface carbenium ion. This parameter, AH + ,represents a measure of the acid strength of the catalyst: a lower value corresponds to a catalyst having stronger acid sites. Details of the reactions, rate constant estimations, and discussion of kinetic parameter adjustments for the model are given in Ref, [7]. A comparison between experimental data and values via the model is given in Table 1for the two extreme conversions studied on USY-C; we also compare experiments at low conversion for the two catalysts studied. Other comparisons are given in Ref. [7]. Table 1 shows that at approximatelythe same conversion the steamed catalyst makes less paraffinic hydrocarbons indicating that there is less hydride ion transfer. Increasing conversion increases paraffinic hydrocarbons. As can be seen, the agreement between experiment and calculated values is good; therefore the reaction scheme we have chosen appears to represent the overall chemistry reasonably well. The primary deficienciesof the microkinetic model are related to the amount of C5 species produced, especially 2-methyl-2-butene. This disagreement may be the result of additional C5 consumption pathways not taken into account. Table 1 Comparison of experimental data and model predictions at 773 K. Values compared are exit pressures (Torr) in the product stream. Catalyst
USY-s
USY-c
USY-c
Conversion, %
10
11
41
ModelEXl2tMode1mModelm Hydrogen Methane Ethylene Propane Propylene n-Butane Isobutene 1& 2-Butenes Isopentane 2-Me-2-Butene
4.52 4.17 0.68 4.48 3.81 7.27 1.92 2.66 2.89 0.22
4.99 3.92 0.75 4.97 4.21 7.57 2.32 2.77 2.80 0.10
2.94 4.56 0.99 7.07 3.35 10.39 1.62 2.25 3.86 0.23
3.16 4.28 0.88 7.04 3.39 9.73 2.05 2.40 3.46 0.09
7.33 7.54 6.56 26.5 8.89 22.9 2.44 3.39 12.5 0.56
7.16 8.04 5.89 31.2 8.22 22.4 2.40 2.88 8.73 0.28
1657
Microkinetic analyses of isobutane reactions over two USY catalysts led to the following conclusion. The kinetically significant reactions are the initiation steps, hydride ion transfer, ethylene formation, and oligomerization reactions involved in two oligomerization-rearrangement-crackingpathways: 2C4 ---> Cg ---> Cs + C3 and C4 + Cs ---> C9 ---> 3C3. The remaining reactions, &scissions, olefin desorptions, and carbenium ion isomerizations, are kinetically insignificant. We found that the net rates of the initiation and olefin desorption reactions remain essentially constant throughout the length of the reactor. In contrast, rates of hydride ion transfer and oligomerization reactions are negligible at the reactor inlet but become dominant pathways at the reactor outlet, even at isobutane conversions of 11%. For example, Figure 2 shows the relative rates of initiation and hydride transfers along the bed for 11% conversion over USY-C. Note that after a fractional distance of about 0.1, the hydride transfer reactions dominate as the reactions that consume isobutane and produce the tert-butyl carbenium ion. Increasing conversion accentuates this dominance of hydride transfer reactions leading to increased paraffinic product as shown in Table 1.
W
2 U
400 -
300
-
5
200
-
U
100
-
W
> W
I
0.00
I
I
I
0.60 0.80 1.00 Fractional Distance Along Reactor
0.20
0.40
Figure 2. Isobutane consumption rates as a function of distance along PFR for catalyst USY-C
1658
During the fitting of the 10% conversion data obtained with USY-S, we fured all parameters to be the same as for USY-C except AH+. We note from Table 1that the fit is good even with only one adjustable parameter for USY-S. Interestingly, the model differentiated between the calcined and steamed catalysts by indicating that the heat of stabilizationof the proton relative to the carbenium ion on the Bransted acid site was higher for the steamed catalyst. This means that the Bransted acid sites of USY-S are weaker than those of USY-C. Consequently, surface fractional coverage of H + increases from 0.82 for USY-C to 0.98 for USY-S at a similar conversion of lo%, whereas tert-butyl carbenium ion coverage decreases from 0.16 to 0.018. This decrease in acid strength, as suggested in Ref. [ll], and the consequential decrease in carbenium ion coverage for the USY-S catalyst is probably responsible for the decreased hydride ion transfer activity of the catalyst relative to USY-C. Thus steam treatment appears to reduce not only the number of Brflnsted acid sites in Y zeolites but also their strength. This result is in agreement with our results on similar catalysts that were studied using heat flow microcalorimetry and infrared spectroscopy [ 121, and concurs with the work of Macedo, Auroux and coworkers (13,141. Acknowledgement We gratefully acknowledge the financial assistance provided by Engelhard Corporation and the U. S. Department of Energy for work conducted at the University of Wisconsin. 4. REFERENCES
1. G.B. McVicker, G.M. Earner, and JJ. Ziemiak, J. Catal. 83, (1983) 286. 2. E.A. Lombard0 and W.K. Hall, J. Catal. 112, (1988) 565. 3. W.O.Haag, and R.M. Dessau, Proceedings of the 8th Int. Congr. on Catalysis, Vol 2, pp. 305, 1984. 4. A. Corma, J.B. Monton, and A.V. Orchilles, Ind. Eng. Chem. Prod. Res. Dev. 23, (1984) 404. 5. A. Corma, J. Planelles, J. Sanchez-Marin, and F. Tomas, J. Catal. 93, (1985) 30. 6. W.L Haden and FJ. Dzierzanowski,U. S. Patents 3,506,594 (1970) and 3,647,718 (1972). 7. J.E. Rekoske, RJ. Madon, L.M. Aparicio, and J.A. Dumesic, To be submitted to J. Catal. 8. M.D. Amiridis, J.E. Rekoske, J.A. Dumesic, D.F. Rudd, N.D. Spencer, and CJ. Pereira, AIChE Journal 37, (1991) 87. 9. S. A. Goddard, M.D. Amiridis, J. E. Rekoske, N. Cardona-Martinez,, and J.A. Dumesic, J. Catal. 117, (1989) 155. 10. M.T. Bowers, "GasPhase Ion Chemistry"Academic Press, NY, 1979. 11. RJ. Madon, J. Catal. 129, (1991) 275.
1659
12. D. Chen, S. Sharma, N. Cardona-Martinez, J.A. Dumesic, V.A. Bell, G.D. Hodge, and RJ. Madon, Submitted to J. Catal. 13. A. Macedo, A. Auroux, F. Raatz, E. Jacquinot, and R. Boulet, Perspectives in Molecular Sieve Science, W.H. Flank and T.E. Whyte Jr., (eds.) ACS Symp. Ser. 368, pp. 98, American Chemical Society, Washington, D.C. 1988. 14. A. Auroux and Y. Ben Taarit, Therm. Acta. 122, (1987) 63.
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Ouczi, L u al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All dghts reserved
PECULIARITIESOF ETHYLENE CONVERSION ON ZEOLITES AND PHOSPHORIC ACID A. G. Anshits, S. N. Vereshchagin and N.N.Shishkina Institute of Chemistry of Natural Organic Materials, K. Marx St. 42, Krasnoyarsk 660049, Russia
Temperature programmed decomposition of the oligomers of ethylene on the surface of ZSM-5, HM, HY was studied. The oligomers on HM and HY decomposed to give only the saturated hydrocarbons due to the fast reactions of hydrogen transfer while the oligomers can be desorbed without decomposition from the surface of ZSM-5. A similar behavior was observed for HY and phosphoric acid in conversion of ethylene. It was proposed that it is the multi-point adsorption centres which are responsible for the hydrogen transfer and creaking reactions.
IrnrnION Phosphoric acid and zeolites are known to convert ethylene to hydrocarbons with higher molecular weight. It has been shown that ethylene forms mainly paraffins and aromatic hydrocarbons on phosphoric acid [l], while ZSM-5 type zeolite produces olefins [2]. The conversion of ethylene over mordenite gives paraffins and coke [3]. The reaction is supposed to involve the acidic centers and the activity and the selectivity of olefins conversion depend upon the number and the nature of acid sites, Si/A1 ratio and some other factors [4-5], The aim of the paper is to study the main factors which determine the similarities and differences of ethylene conversion on different acid catalysts. A method of temperature p r o g r m e d desorption/destruction (TPDD) of surface oligomers of ethylene was used to characterize the catalytic performance of the samples.
EPEFtIMENT&U Phosphoric acid (64-85% P 0 ) and hydrogen forms of mordenite (Si/A1=5.5-30), feujasite (2.3) an2 &M-5 (19-74) were used. Before experimentsothe sample was treated in the static vacuum systemoat P=O.1 Pa and T=500 C followed by C,j14adsorption at P=8OT3 kPa and T=50 C. The amount of ethyl9p adsorbed was calculated from the pressure drop and was equal to 1 0 * l O o molec. C,j14/g. The sample was purged with hellbum flow for 90 min. at 50 C followed by linear temperature programing 3 /min to 600 C . The oligomers desorbed and the products of the oligomers decomposition were analyzed by on-line gas chromatograph on packed and capillary columng. A conversion of ethylene over phosphoric acid was carried out at 170-260 C and 0.16 MPa in a sealed glass tube with subsequent chromatographic analye. The catalysts and the products were characterized by
1662
Thermal decomposition of oligmers on zeolites. WDD curves of the zeolites s-tudiedare shown in Fig. 1. The results given clearly indicate that the zeolites differ in respect of the oligomers conversion under TPDD condition. The descption of the hydrocarbons from the surface of HZSM-5 starts at about 130 C. k l y one peak of products desorption is observed and the rage of the hydrocarbon evolution reaches the maximum value at 230-250 C (Fig. la). The principal products are C -C alkenes; small amount of alkanes, higher hydrocarbons and trace amount 3of6methane and ethane are formed (Fig. 2a). The increase of %/A1 ratio from 19 to 74 does not influence the position of the TPDD curves and decreases slightly a value of alkane/alkene ratio. The absence of the saturated hydrocarbons in the TPDD products means that no hydrogen transfer and creaking reactions occur on HZSM-5 under TF'DD condition. I
C
200
400
T, "C
ti00 0
200
400
T, "C
600 0
200
T,
400
600
O C
Fig. 1 Variation in the rate of the evolution of (1)-methane, (Z)-ethylene, (3)-propane, (4)-propene, (5)-butanes, (6)-butenes as a function of temperature for (a)-HZSM-5(Si/Al=30), (b)-HM(5.5), (c)-HY(2.3).
Thp decomposition of the oligomers formed on the surface of HM starts at 240 C (Fig. lb). Methane, ethane,ethyleneand propane are the main products and only a minor amount of $ -hydrocarbons are observed. Therefore the reaction of the redistribution of hydrogen and creaking prevail over the desorption of unchanged oligomers from the surface of mordenite. Unlike HZSM-5 changing the Si/A1 ratio from 5.5 to 308auses a shift of the maximum of the TPDD p a k from the value of 310-410 C for the native HM to the value of 250-325 C for mordenite dealuminated with the solution of HC1. This shift is accompanied by the sharp decrease of formation of methane and simultaneous increase of the yield of butanes and butenes (Fig. 2b,c). The TPDD curves for a faujasite-type zeolite displayotwo peaks of hydrocarbop evolution (Fig. lc). The first peak starts at 90 C with maximum at 150-190 C and iso-butane is the only product in this temperature region. The second peak lies at temperatures hisher than 350 C. The mixture of hYdrocarbons formed in this region contains about 75% of C 1-C2 hydrocarbons. As far as light alkanes are undesirable by-products in the processes of the conversion of the hydrocarbons it is important to know the reaction
1663 routs which are responsible for their formation. The results given in the Figs, 1,2 clearly show that the higher the temperature of the m a x i m of the TPDD curve is, the more methane and ethane are produced (except for the low temperature peak of HY) i . e . the correlation exists between the strength of the bond of the oligomers with the surface and the composition of the TmlD products. This correlation cannot be explained by the difference in the acidity of the zeolites because according to the calorimetric results and TPD of ammonia [6,7] the strength of the acid sites decreases in the order HM 2 HZSM-5 >> HY while the yield of C iC 2alkanes decreases in the order HY > HM >> HZSM-5. The structure of the framework of the zeolites itself also cannot be responsible for the differences observed as far as native and dealuminated mordenites possessing the same structure give different composition of the TPDD products (Figs. 2b,c). One can expect that the temperature of the desorption is higher in that case when the multipoint adsorption of the surface specie; occurs. This multi-pin+t3adsorption can involve the acid centres such as Hn[Si(OA1)n(OSi)4-n]n- mAl which was proposed to exist in a framework of the zeolites [8], the number of these centres depending upon the value of the Si/A1 ratio [ 91.
Fig.2 Composition (mol.%) of the C -C4 hydrocarbons desorbed from the surface of (a)-HZSM-5(Si/Al=$O); (b)-HM(5.5): (c)-HM(30): (d)-HY(2.3), high temperature peak, (el-HY. low temprature peak; (f)-ethylene conversion over phosphoric acid at 190 C, 83.2% P 0 2 5' 0 - alkanes: 1 - alkenes. Ethylene conversion over phosphoric acid. The conversion of ethylene over phosphoric acid leads to the formation of the complex mixture of 3 - C hydrocarbons. The activity of the system increases with increasing P P 8 5 concentration or temperature of the reaction. The composition of the products formed does not depend upon the concentration of the acid at given temperature. The branched alkanes are the main products and the concentration of the iso-alkanes are higher than the thermodynamic equilibrium values. For example concentration of n-butane in the Cq-fraction is equal to the value of 92-98%, while the equilibrium value is 51%. It is known that phosphoric acid alwais exists as a mixture of mono and polyphosphoric acids, relative amount of the each form being dependeg) upon P 0 concentration, temperature and presence of water waper. Using P NMR 2 5
techique it was shown that almost inactive acid (72.43 P (I ) contains most2 5
1664 ly mono specie
H3W4 (hO.Oppm),while active acids (83.2% P205) contains
predominantly polyforms (h14-75,15 06, 31 .Olppn). Moreover a linear dependence is observed betwepn the content of the polyphosphoric acid? (calculated from [lo] for T=200 C) and the conversion of ethylene at 190 C (Fig.3) I
10
6.
.
,y 83.2
/”
E 6
,*/ o/‘
5 ‘
/’
( 72.4
ethylene as a function of the contgnt of the polyphosphoric acids at 190 C. Reaction time 3h, 0.16MPa C2H4. Numbers near the points denote the total P 0 concentration, 25
79*7
0
Therefore we can conclude that it is the polyphosphoric acids which are responsible for the ethylene conversion and saturated hydrocarbons formation. The composition of the hydrocarbons obtained under TPDD condition on HY (lowotemperature peak, Fig. 2d) and one of the conversion of ethylene at 190 C over phosphoric acid (Fig. 2f) are surprisingly similar. The behaviour of the zeolites studied and concentrated phosphoric acid with the high content of polyacids shows a generality and the reason for that is a possibility to form multy-point adsorbed species. REFERENCES
1.
V.N.Ipatieffand H.Pines,Ind. and Engin. Chem., 27 (1935) 1364. 2. T.J.Gricusand R.J.Gorte.J.Cata1.,115 (1989) 233. 3 . F.Fajula and F.G.Gault. J.Catal.,68 (1981) 312. 4. V.L.Zholobenko,M.M.Kustov,B.Ju.Borovkov and V.B.Kazanskii.Kinetika i kataliz, 28 (1987) 965 (in Russ). 5 . E.A.Lombardo,T.R.Gaffneyand W.K.Hal1.Proc. 9th Int. Congr. Catal., Calgary, 1 (1988) 412. 6. E.A.Paukshtis,V.G.Stepanov,A.A.Shubin and K.G.Ione,Proc. 2 All-Union conf. on application of calorimetry in adsorption end catalysis. Novosibirsk, 1983, prepr .37 ( in Russ) . 7 . C.V.Hidalgo,H,Itoh,T.Hattory, M.Niwa and Y.Murakami, J.Cetal.,85 .(1984) 362. 8. V.G.Stepanov,A.A.Shubin K.G.Ione et.al.,Kinetika i kataliz, 25 (1984) 1225 (in Russ). 9. J.B,Nagy,Z.Gabelica,G.Debras et el., Acta.Chim.Hung. 119 (1985) 265. 10* K.N.Zag.vozdkin,M.U.bbinovich and N.A.Barilko,Dzurnal Prikladnoi Chimii, 13 (1940) 29 (in Russ).
Guczi, L d al. (Editors), New Frontiers in Catalysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights rcrervcd
BENZENE ALKYLATION IN VAPOUR-PHASE WITH ETHENE ON A ZEOLITE CATALYST G. Maria, G. Pop, G. Musca and R. Boeru
ZECASIN s.a. (the new registered name of Chemical and Biochemical Energetics Institute), Division of Catalysis, Spl. Independentei 202, Bucharest 77208, Romania
1. IN!CRODUCTION
Alkylation of benzene (B) with ethene (E) is a classical reaction for producing Ethyl-Benzene (EB), the oatalytic process in liquid- hase being applied in a oomercial-scale plant. An adequated ca alyst (AlC1 BF /alumina, phospho-dlatomyte) and specific operating condliiona ensures good selectivities in EB. The recent developments of the procesa in vapour-phase on a zeolite catalyst, in a fluid-bed reactor, present some advantages such as: better control of the process by easily dissipating the heat of reaction, a less poluant rooess than the classical one, less severe purity conditions or the rawmaterials, less expensive equipment regarding the used metallic material due to the less corrosive medium. The aim of this paper is to analyse a set of experiment8 obtained in a laboratory-scale fluid-bed reactor, in order to investigate the thermodynamics and the kinetics of the benzene alkylation in vapour-phase with ethene, and to evaluate the used zeolite oatalyst performances.
P
P
2. EXPERIMENTAL SECTION
The experiments were carried out in a glass tube, fluidbed reactor (30 mm i*d*). An electric heating-cooling syetem around the reactor maintains a constant feed mixture temperature, and forestalls an excessive reaotor ambient medium heat exchange. The commercially available p.8. benzene, continuously fed by means of a dosimetric pump, waa vaporized and mixed together with a controlled stream of ethene in a zone with inert material filled preheating. The temperature experimental profile was recorded along the axial reactor ooordinate by using four temper ture sensors. The reactor contains 30 cm3 mieroepherio catalyst (0.06 mm mean particle diameter), i,e. a ZSM-Mg zeolite. After cooling at 2'c, the reaction products were separated into two freotions: the volatile fraction (Cl-C5 hydrocarbons and H ) and the liquid one (C6+ hydrocarbons), which both are subjbied to chromatographio analysis.
-
1666
450
u
F i m 1. Selectivity of benzene in ethyl-benzene art the equilibrium state (moles EB/moles reaated benzene) vs. temperature (C degrees) and molar E/B feeding ratio.
0
Y
1.66 3 molar E/B feeding ratio
0.33
In order to study the process, the experimental program designed takes into account the follOWing independent variables: the fluid-bed (asswngd to be cvaei-isothermal) temperature in the range of 350-450 C, the WHSV in the range of 1-3.5 g / ( g catalyst.h), and the molar ethene/benzene (E/B) feeding ratio (E/BFR) In the range of 1.2-1.5 (the pressure is the normally one). The time-on-stream of 1 h, which are identical for all the runs, provides a small catalyst deactivation, The catalyst is regenerated before each run. The product analysis and the amount of fraction collected allow an overall and elementary (C H elements) mass balance for the each run. The relative input output deviation in the mass balance is lea8 than 15% for the crude data, and lese than 1% for the numerical filtered data (see details of Maria et al., 1991). The exothermicity of the overall reaction, evaluated by using the experimental dat8,varies in the range of 50-75 Kcal/ mole B reacted, being smaller for great temperature8 and mSV. 3 . T'HBRMODYHAMIC COMPUTATIONS The equilibrium computations f o r the main two reactions of the process: (1) C6Hg + CZH4T- EB, Kpl= exp(-13.46 + ll864/T) EB + C2H4DEB, K P*= exp(A5.33 f 11324/T), (2) (DEB-diethylbenzene; T=temperature,K; -the equilibrium conKPare thermodynamicapy atant,l/atm) reveal that both reactions limited, the conversion decreasing in the range of 350-450 C, from 94% to 77% for reaction (11, and from 76% to 35% for reaction (2)(1 atm and l/l molar E/BFR). By using the algorithm of overall Gibbs energy function minimization (White et al.,
-
1667
1958;Uenbigh,1961) in the chemical System Of B,E,EB, (o,m,P-) DEBocOmpO~ds,the equilibrium computations for 1 atm and 300450 C show that: i)the B conversion(xg),the B selectivity in EB(G ),and the yield of DEB( ) are small influenced by region; ii)all. these varithe EBtemperature in the ables are strong influenced by the E/BFR: for a ratio variation from 0.33 to 3 , these indices varies ap roximately 5 474 from 27% to 100%. G from 85% to 10% (Figs1 7 QDEB from to 90% respective1yFB
specs!?@
7
4. PRODUCT DISTRIBUTION
AND PROCESS KINETICS
roduct distribution. The EB selectivity is favourized by moBerate temperatures, high pressures, and small molar E/BFR.For instance, Gelbstein et a1.(1958) report for a phospho-diatomite gatalyst a 97% EB selectivity for 80% E conversion,40 atm, 325 C,O.l E/BFR, while Kozorezov et a1.(1970) re ort for a BF3 6 atm, / a l p h a catalyst a 54% B selectivity in EB for 80% 150 C, 0.2 E/BFR,1.5 WHSV. In the liquid-phase procea: Dalin et a1.(1957) recommend a 0.5 E/BFR, indicating a performance of 35%nol.EB yield, and 40% %. In our ex eriments (1 atm),in (in the range o 3040%) with a cororder to increase the responding loss in EB a l a r yield (in the range of 0.16-0.3), the E/BFR are chosen 6n the range of 1.2-1.5, at reasonable temperatures (350-450 C). The EB selectivity increases moderately with the temperature and the contact time, great values of these variables favourizing secondary product formation. Good results (0.03%H2,0.04%CH4,2.81%C2H4,3.05%C3Hg,1.44%C3Hg, 2. 08%C4H10, 1.47%CqH8, 0.13%C5H12,6%C6€I14,36 s 5%c6H6,3 .84%c7H8, 27.19%EB, 1.71%A8,5.04@9 67.44%A10, 1.2 3%A10+) with this cataly st were obtained for 400 C 0.95 WHSV and t . 4 E/BF& The Droposed kinetic modea. By analysing he reac on stoichiometry we conclude that secondary products (noted P with the mean molecular weight of 60-80,i.e. the paraffins,H ,olefins (without E),polyalkylated) proceed from E rather t h b from B. Thus, the proposed model, involving the following reduced kinetics: (3) NEB/KNI) B + E d E B , rl- kl(NBNE EB + E-DEB,r2= k2(NEBNE NDEB/KN~) (4) E0.47 P, r3= k3NE, (5) KN- PKp/ ( &Nil i = B,E,EB,DEB,P (6) (where p=pressure,atm;N=number of moles;k=kinetic constants; r=reaction rate,moles/s;K =equilibrium eonstants,l/mole;BIC6H6 Pmsecondar products ) takps into account the equilibrium reactions (17 and (2) and a secondary reaction (5) with a stoichiometry evaluated from the exp.data. The kinetic parameters k ,k ,k3 were evaluated by using an estimation stratea(Maria, 14893. .Their apparent values in a temperature (TI range of 350-450 c are the following (N are in moles/mole B fed): kll 9*64exp(=2669/T),k (mean)=0.225,k3-33.62exp(-3228/r). The experimental fluid-bgd contact time (t,s ) ,used in the estimation
B
-
-
-
1.5
Figure 2. Kinetic ode1 predictions ) and exerimental data ( 0 ) f o r temperature of' 40OoC.
a
0
2
Contact
4 6 time ( s 1
comgutatians, are defined as: t=H/u ,(H=fluid-bed height,m;u = superficial gas velocity,m/s) ,by taaing into account the mintmum fluidization and bubble characteristics of the fluid-bed (Najim 1989; Kunii and Levenspiel, 1977). The kinetic model redlchons are in good agreement with the experimental data Psee Figure 2). 5 . REPElRENCES M.A.Dalin, P.I. Marksov, R.1, Senderova and T.V. Prokofieva, Alkilirovanie benzole olefinami,Goshimizdat, Moskwa, 1957 K. Denbigh, The principles of chemical equilibrium, Cambridge Univ. Press, 1961 A.I. Gelbstein, A.A.Zansohova and G.G.Sceglova,Him. Prom., 5 (1958) 284 Y.I. Kozorezov, A.P. Ruaakova and A.N. Kulecova, Him. Prom., 11 (1970) 815 D.Kunli and 0. Levenspiel, Fluidization engineering, R.E. Krieger Publ., Huntington, 1977 G. Maria, Canad. J. Chem. Eng., 66 (1989) 825 G.Maria,G.Musca,G.Pop,G.Ignatescu,R.Boeru,D.Manoliu and G.Niculae,Proc. Intern. A W E Conf., Warsaw, July 15,1991,p.155 K.Najim,Process modeling and control in chemical engineering, M. Dekker Inc., New York, 1989 W.B. White, S.M. Johnson and C.B. Dantzig, J, Chem. P h p . , 28 (1958) 751
Guni, L. a d.(Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
ON THE NATURE OF ZEOLITE CATALYST EFFECT ON THE SELECTIVITY OF TOLUENE NITRATION BY ACYL NITRATES S. M.NagYa, R A. Yarovop, L.A. Vostrikovab,K G.Ion& and K
G.Shubina
ahstitUte of Organic Chemistry, Russian Academy of Sciences, Siberian Division, Lavrentyev ave., 9, Novosibirsk 630090, Russia bInstitute of Catalysis, Russian Academy of Sciences, Siberian Division, Lavrentyev ave., 7, Novosibirsk 630090, Russia
Abstract The substrate and positional selectivities in toluene nitration by acyl nitrates on zeolites H-Y and H-ZSM-11 have been studied. The nature of nitrating agents and the effects of zeolite framework on the reaction are discussed.
1. INTRODUCTION The use of zeolite catalysts is a prospective method to reduce the amount of acidic waste and to raise the selectivity of electrophilic aromatic substitution reactions. The most extensively studied process is the zeolite-catalyzed alkylation of aromatic compounds, the high selectivity of the process being explained using the concept of "shape selectivity", suggesting the reaction to proceed mainly in zeolite channels [ 11. Recently, emphasis has been placed on application of zeolites to perform nitration of aromatic compounds [ 11, and the possibility to achieve rather a high selectivity in this reaction has been shown [2]. For more deep insight into zeolite catalyst effects on the reaction, the selectivity of toluene nitration by different acyl nitrates (I-IV) has been studied.
2. EXPERIMENTAL Substrate and positional selectivities in toluene nitration have been studied under competitive conditions as reported earlier [2], using zeolites H-Y and H-ZSM-11 treated with tributylamine (NBu,) to block the surface acidic centers.
1670
R
R
R
=
H, Me;
R1 = Me, Et, Ph, t-Bu
3, RESULTS AND DISCUSSION As known [3,4], the relative reactivity of toluene (k,/kb) in liquid-phase nitration reactions is determined by the electronic effect of the methyl group and activity of the nitrating agent, while on zeolites diffusion limitations may proved to be substantial (Equation 1) [SI. Even in the case of close intrachannel diffusivities of
substrates (D,*Db) (evidently this is just the case in our experiments due to equal critical dimensions of toluene and benzene molecules), severe diffusion limitations may result in levelling of their reactivity. The lower (k,/kb)ObE ratio in zeolite-catalyzed nitration as compared with that observed for the liquid-phase reaction (Table 1) is apparently indicative of the very diffusion limitations. The lower relative reactivity of toluene in the case of medium-pore zeolite ZSM-11 as compared with that on the large-pore zeolite Y is in line with this assumption. As the activities of different nitrating agents involved in the reaction seem not to differ significantly, the increased para-selectivity in zeolite-catalyzed reactions (Table 1) is apparently due to specific zeolite effect, namely "shape selectivity" effect 161. In fact, the catalytically active centers of NBu,-treated zeolites are located mainly in zeolite channels, where the steric restrictions imposed by the crystalline framework must influence the isomer distribution. It is noteworthy that the highest values of para-selectivity have been observed on medium pore zeolite H-ZSM-11where the restrictions excerted by the channel walls are obviously greater than those in the large pore H-Y (Table 1). Taking into account irreversibility of the nitration reaction [31 and the absence of interconversion of isomeric nitrotoluenes under the reaction
1671 conditions, we may conclude, that it is the transition state, where the topochemical control on isomer distribution realizes.
Table 1 Selectivities in toluene nitration by acylnitrates, dCOONO,
Isomer distribution of nitrotoluenes, % Zeolite
(kt/kb) Obs
R '
para
-
-
H-Y/NBu3 H-Y/NBu3 H-Y/NBu3 H-ZSM-ll/NBu3 H-ZSM-ll/NBu3 H-ZSM-ll/NBU3
meta
Ref.
ortho
Me Et Ph
37 32 31
2 4 5
61
44.3
[41
64 64
33.3 30.7
[41 [41
Me Ph t-Bu
54 63 66
5 2 2
41
9
35 32
10 14
this work r21 this work
Me Et Ph
88 95
5
7
2 (1
3
5 5
2
7
98
this work this work [2 1
In zeolite-catalyzed nitration, the isomer ratio of nitrotoluenes depends also on the nitrating agent used (Table l ) , a tendency of increase in para-selectivity with increasing of substituent size in acyl nitrate being observed. This observation may be rationalized by the supposition that the reaction transition state involves the molecule of nitrating agent as a whole. Apparently, the electrophilic agents involved in the reaction are the protonated forms of acylnitrates (structures "A" or "B") (cf. ref. [7]) rather than the nitronium cation bound to the zeolite framework (structure "C") as it has been suggested earlier [2]. This assumption is supported by quantum chemical calculations modeling the interaction of acetyl nitrate with the acidic centers of zeolite. According to MNDO calculations performed in molecular approximation [8] with geometry optimization, the process of structure "C" formation is unfavourable (AH = 17 kcal/rnol).
1672
,g0
Me-/SiI
I'
/I
-C
I
4. CONCLUSION
The increased para-selectivity in zeolite-catalyzed nitration of toluene by acyl nitrates as compared with that in solution is accounted for by the "shape selectivity" effect in the transition state of the reaction. 5. REFERENCES 1 W. Halderich,
M. Hesse, and F. Nfumann, Angew. Chem., Int. Ed.
Engl., 27 (1988) 226. 2 S.M. Nagy, K.A. Yarovoy, M.M. Shakirov, V.G. Shubin, L.A. Vostrikova, and K.G. Tone, J. Mol. Catal, 64 (1991) L31. 3 K. Schofield, Aromatic Nitration, Cambridge Univ. Press, Cambridge,
1971. 4 G.A.Olah, H.C. Lin, J.A. Olah, and S.C. Narang, Proc. Natl. Acad. Sci.
USA, 75 (1978) 1045. 5 W.D. Haag and N.Y.Chen, in Catalyst Design. Progress and Perspectives, Ed.L.L. Hegedus, J. Wiley and Sons, New York, 1987. 6 S.M. Csicsery, Zeolites, 4 (1984) 202. 7 N. Bodor and M.J.S: Dewar, Tetrahedron, 25 (1969) 5777. 8 G.M. Zhidomirov and V.B. Kazansky, Adv. Catal., 43 (1986) 131.
OH
Guczi, L . et al. (Editors), New Fronriers in Cataljsk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights Feserved
INTERACTIONS IN MONOALKYLBENZENES DISPROPORTIONATION AMONG ZEOLITE CHARACTERISTICSAND REACTION MECHANISMS I. Wang", T.-C. Tsaib and C.-L. A F Wepartrnent of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, China bRefining and Manufacturing Research Center, Chinese Petroleum Corporation, Hsinchu, Taiwan, China
Abstract Among the monoalkylbenzenes studied, it was found that reactivity decreases in the order of cumene, n-propylbenzene, ethylbenzene and toluene. Their activation energy with bimolecular mechanism is lower than that with monomolecular mechanism. Although ZSM-5 has the greatest acidity, its activity in toluene disproportionation is lower than zeolite Beta and similar to zeolite Y and mordenite. Its ethylbenzene activity is lower than zeolite Beta and higher than zeolite Y and mordenite. It was concluded that in addition to acidity, zeolite activity is also affected by reaction mechanism and monoalkylbenzene types.
Introduction Activity is one of the major concern in zeolite catalyst development. It is widely accepted that zeolite activity is closely related to its acidity, as demonstrated in toluene disproportionation [I], ethylbenzene disproportionation [2] and cumene disproportionation [ j l ] . It was also shown that zeolite activity is linearly proportional to the number of aluminum sites no matter with the nature of zeolite active sites [q.Tsai and Wang demonstrated that disproportionation reaction mechanism depends on zeolite pore structures [A.It is unclear that how reaction mechanism affects catalytic activity. The objective of this study is to explore the understanding on the interactions in disproportionation among zeolite catalytic activity, zeolite acidity, disproportionation mechanism and monoalkylbenzenetypes in terms of reaction mechanism. Reactions procedures were reported elsewhere [5, 6, 7J.The SiqlAlzO3 molar ratio of the zeolite sample, zeolite Beta, ZSM-5, Y and mordenite were 15, 35.8, 2.1 and 7.0, respectively. The conversion of monoalkylbenzenes, XM,is defined as equation (1).
xu= 1 0 0 - ( ( B ) . , w t % ) ,
(1)
where (BM,wt%)p is the composition of monoalkylbenzene in the reactor effluents.
Results and Discussion (A) Disurowrtionation Reactivitv of Monoalkvlbenzenes Disproportionation reactivity of monoalkylbenzenes with side chains of different carbon number was examined over various zeolites. So the zeolite structural effects on monoalkylbenzene reactivity can be elucidated. There are bimolecular mechanism and monomolecular mechanism in disproportionation reactions (Figure 1). We have demonstrated [A that in disproportionation ZSM-5 catalyzes monoalkylbenzene mechanism and zeolite Beta and Y catalyze bimolecular mechanism. Disproportion over mordenite is a combination of bimolecular and monomolecular mechanism.
1674 The reactivity of monmlkylbemnes CM be gauged with the minimum reaction temperatures required for obtaining a low conversion level in the range of 1 - 3 wt% (Table 1). Over all the zeolites studied, the minimum reaction temperatures increases with increasing carbon numbers in the alkyl groups. Over ZSMJ, the minimum requirement temperatures increases in the order of cumcne < c&propylbeaxne < ethylbenzene < Qoluene. Over zeolite Beta and Y, the order incresses ps cumene < cn-propylbenzene = ethylbenzene < Qoluene. Mordenrte has mnilar catalytic properties to zeolite Y. The reactivity difference between n-propylbznzene and ethylbenzene is different for the zwlite with monomolecular and bimolecular mrLhanism. Therefore monoakylbenzene reactivity is affected by disproportionation mechanism.
Sm Mechanism
F l a r e 1 Alkylbenzene DisproportionationMechanism T 1 1 Minimum Reaction Temperatures (OC) Required for Disproportionation of & kvlbenzenes' 0110
MoDoaikvlbeazelre Toluene Ethylbenzene n-Propylbenzene Cumene
B&$
y_
230 155
320 304
167
301
112
195
1WHSV: 3.4 g hrl gat-1, Conversion: 1 - 3 ~
MordeniteZSMd 330 295 301 200
320 220 172
120
1%.
The effects can be explained in tenns of activation energy. Both monomolecular and bimolecular disproportionation proceed a monomolecular crocking reaction (Fig. 1). Although detail kinetic study was not conducted, pseudo first order kinetics can be assumed. Ethylbenzeoe disproportionation has relatively stable activity over various zeolites [Z]. Therefore the pseudo Arhenius plot of ethylbenzene disproportionation is presented in Fig. 2. The "pseudo activation energy", EO can be derived from equation (2),
1675 where X is the conversion of monmlkylbenzeaes, T is the reaction tenprature in Kelvins. The 'pseudo activation energy" of some monoalkylbenzenes over differeat zeolites were elucidated (Table 2). In the monomolecular mechanism which ZSM-5 catalyzes, with incremng the carbon number in the akyl groups, cracking of akyl groups is easier and cracking activation energy the formation -of propyl ions from n-propylknzene is lower (Table 2): A&rdingly, is much easier than the formation of ethyl ions from ethylbenzene. And the minimum reaction temperatures for n-propylbemne is much lower than that for ethylbenzene.
\
\\
\
*
\
'ieure 2 Pseudo Arhenius Plot of Ethylbenzene Disproportionation %able2 Pseudo Activation Energy in KcaVmol of the Disproportionation of Monoalkylenzenes over Various Zeolites 1 Monoalkvlbenzene Toluene Ethylbenzene n-Propylbenzene Cumene 1
11.0 7.2 11.6 10..8
Y
Mordenitp
15.8 8.6 12.6 12.2
9.8 12.5 12.3
---
zsM-5 20.3 15.2 12.3
---
WHSV: 3.4 g hrl gat-1.
In contrast, in the bimolecular mechanism which leolite Beta and Y catalyze, the pseudo activation energy of ethylbenzene is slightly lower than that of n-propylbenzene (Table 2). Therefore, the minimum disproportionation reaction temperatures of e t h y b m m e is lower than (such as zeolite Beta) or close to (such as zeolite Y) the of n-propylbenzme (Table 1). For toluene disproportionation. the cracking steps in bimoleculnr involve the breaking of beazene rings from biphenylmetbane carbeniwn ions, and the cracking steps in monomolecular dads with toluene carbenium ions. Having the highest pseudo activation energy (Table 2), therefore minimum reaction temperature (Table 1) is required for toluene disproportionation with both monomolecular and bimolecular mechanism. Intermediates of cumene disproportionation by bimoleculnr mechanism is biphenyl-propyl carbenium ions which is highly stnbiliztd by mnance structures [q and thermodynamically favored. Except ethylbename, the pseudo activation energy is the lowest
1676 one (Table 2). On the other hand, the intermediates of monomolecular mechanism is quartery carbenium ions whose cracking reactivity is greater than tertiary ions, which are formed from other monoalkylbsnzenes. Therefore, among all the monoalkylbenzenes over all the zeolites studied, cumone shows the lowest minimum reaction temperatures. Zeolite activity in disproportionation of the studied monoalkylbenzznes decreases in the order of Beta > ZSMd > >Y = mordenite (Table 1). The former w o zeolites are high siliceous in nature and have higher activity than the latter two zeolites. With the activity memmme.nte applied in this study, zeolite Beta has higher activity than ZSM-5 of 9OoC in toluene diqroportionation and of 65OC in ethylbenzene disproportionation. HoCever those lwo zeolitsr &ow Eimilar activity in disproportionation of cumene and n-propylbenzene, which both m m m t i c s with cubon number of nine. Ae being elucidated from infrared spectroscopy by Hedge et al. the acidity dscrsase~ in the order M ZSMd >Beta > >Y. This acidity ranking is reversed in the ranking of activity between zeolite Beta and ZSMJ. Therefore, acidity would not be the sole controlling factor for zeolite activity. Reaction mechanism difference could be the reason for the disagreement. It is evident that comparing with other zeolites, ZSM-5 has a higher pseudo activation energy in disproportionation. ethylbenzene disproportionation and similar value in n-propylbenzene Although hnving lower acidity, zeolite Y and mordenite has comparable activity to ZSMJ. Therefom, in addition to acidity, reaction mechanism CM affect activation energy and m l i t e activity.
[a,
Conclurion Among the monoalkylbenzenelr studied, reactivity decreases in the order of curnene, n-propylbenzene, ethylbenzene and toluene. The reactivity of n-propylbenzene are nearly but over monomolecular zeolite, the same na that of ethylbenzene over bimolecular &ite the former is more reactive than the latter. Although ZSM-5 has the greatest acidity, its activity in toluene disproportionation is nearly the same as Y and mordenite and lower than zeolite Beta. In Q monoalkylbenzene disproportionation, ZSMJ activity essentially equals to m l i t e Beta and higher than zeolite Y and mordenite. Therefore, zeolite relative activity depends on the type of monoalkylbenzenea. Activation energy for bimolscular mechanism is lower for than that for monomolecular mechanism. Accordingly the activity of monomolecular zeolites such as ZSMJ is hindered in the disproportionation of toluene and ethylbenzene due to high activation energy. It WM concluded that activity of m l i t e strongly depends on its acidity, monoalkylbenzene t y p and also reaction mechanism which is controlled mainly by zeolite pore channel structures.
Acknowledgment We would like to thank the financial
support
granted
from the National
Science
Council, Republic of China.
References 1. 2. 3. 4. 5.
6. 7. 8.
H.A. Benesi, J,. (1967) 368. H.Q. Karge, 2.Sarbak, and K. Hatnda, J., 82 (1983) 236. R.P.L. Absil, J.B. Butt, and J.S. Dranoff, J. Cat&, 85 (1984) 415. W.O. Hang and N.Y.Chen, in "Catalyst Design Progress and Perspectives", L.L. Hedgedus Ed., 1987, John Wiley & Sons, New York, p. 164. T.C. Tsai and I. Wang, J. Catal., in press (1992). I. Wang, T.C. Tsai, and S.T. Huang, Ind. Eng. Chem. Research, 29 (1990) 2005. T.C. Twi, C.L. Ay, and I, Wang, Appl. CataI., 77 (1991) 199. S.M.Hedge,R. K u m , R.N. Bhat, and P. Ratnasmy, Zeolites, 9 (1989) 231.
Guni, L u al. (Editors), New Frontiers in Catalysis Proeccdings of the 10th International Congrcss on Catalysis, 19-24 July, 1992,Budapest, Hungary 63 1993 Elsevier Science Publishers B.V.All dghts reserved
NEW SUPPORT MATERIALS FOR RHODIUM CATALYSTS: CHARACTERIZATION OF Rh/AlPO431 AND Rh/MnAP031 A. Trunschke, H.Zubowa, B. Parlitz, R. Fricke and H.Miessner
Central Institute of Physical Chemistry, Rudower Chaussee 5, D-0 1199 Berlin, Germany
1. Introduction The variety of structure types belonging to the new class of microporous materials based on aluminophosphates has been considerably increased during the last years /l/. Among the recently developed families are the MeAPO molecular sieves,which have been synthesized by incorporatingtransition metal ions into AIPO,frameworkpositions / 2/. On the other hand, early transition metal elements like vanadium, molybdenum,manganese or iron are well known to promote the hydrogenation of carbon monoxide with supported rhodium catalysts / 3/. The MeAPO molecular sieves offer a new possibility for studying support and promoter effects. The promoter (Me) can be added as usual by impregnation or may be present as structural component in the MeAPO framework. In the present work the suitability of APo4-31as a new support material for rhodium has been tested. Moreover, the influence of small amounts of manganese on the properties has been studied by incorporatingthe promoter element into the catalyst by cohpregnating AlPO,-31 with a RhC13/MnC12solution or by applying MnAPO-31as a support. NH3-TPD of dPo4-31 and MnAPO-31 was used to study the acidic properties of the support. To characterize the state of rhodium supported on the different molecular sieves infrared spectroscopy has been performed on CO after interaction with the catalyst.
2. Experimental APo4-31has been prepared according to / 4/ with the following gel composition: 1.0 Pr,NH : 1.2 A1203: 1.0 P20, : 40 H20 at 473 K under autogeneous pressure, were Pr2NH corresponds to di-n-propylamine. For the synthesis of MnAPO-31 additionally MnSO; 4 H20 was used, The catalysts were prepared by coimpregnation of APo4-31 with an ethanolicRhC13/MnC12solution or by impregnationof M M O - 31with an ethanolic RhCl,
1678
solution, respectively. For NH,-TPD ammonia was adsorbed at 373 K from a gas stream containing 3 vol-% NH3. After flushing by pure He at 393 K for 3 hours the desorption of NH? up to 773 K was started (heating rate 10 K m i d , gas flow 1 ml mid). For transmssion i.r. spectroscopicexperiments self supporting disks of the samples have been pressed and placed in an infrared cell, made from glass. The cell is equipped with CaF, windows and connected to a vacuum and gas dosing line. The spectra were recorded with a F.t.i.r. spectrometer IRF 180 (Center of ScientificInstrumentation,Berlin) whereas 1W200 scans have been accumulated. All spectra were obtained at room temperature after the followingpretreatment procedure: reduction in H, at 673 K for 2 hours, evacuation at 673 K for 30 minutes, admission of CO at 298 K (15 Torr equilibrium pressure), heating to 423 K for 30 minutes followed by evacuation at room temperature and subsequent heating in vacuum to 423 K for 10 minutes.
3. Results and Discussion AP04-31 belongs to the medium pore molecular sieves and possesses a pore diameter of about 0.6 nm. The APO4-31 crystals are spherical particles with diameters between 5 pm and 100 pm. By introduction of Mn2' the crystallinity slightly decreases as indicated by XRD and n-hexane adsorption. Because of electrostaticneutrality the aluminophosphatesare expected to have no BrBnsted acid sites. Isomorphous substitution of metals should result in the formation of acid centers. Actually, the curves of NH,-TPD of &Po,-31 as well as MnA PO-31 show a maximum at 503 K and 513 K,respectively, caused by desorption of ammonia from P-OH groups on defect lattice sites. In contrast to NP04-31a well developed shoulder is obtained with MnA PO-31 at ca.643 K.As shown by DTA/TG the presence of manganese leads to an additional endothermic peak at 578 K, which could be assigned to a new acid center. Wavenumbersand force constantsof the CO stretchingin well defined rhodium carbonyl complexes on dealuminated Y zeolites (US-&), Alpo,-5, SAPO-5, AlP04-11 and SAPO-11 have been recently used to compare the basicity of oxygen atoms coordinated to the Rh complexes and to estimate the strength of the corresponding framework acid sites in Rh loaded molecular sieves /5,6/. In Figure 1the i.r. spectra of CO adsorbed on Rh/NPO4-31, Rh-Mn/AlP04-31 and Rh/MnA PO-31 are shown. Three different types of surface complexes appear on all samples: bridge-bonded CO, Rhx(CO),with a broad feature around 1850 cm", linearly bonded CO, Rh-CO, with peaks between 2060 and 2040 cm" and different kinds of dicarbonyl species, Rh+'(CO),, characterized by typical dubletts,with the antisymmetric mode interfering in all cases with the Rh-CO band. The relative proportions of the complexes and there stability in vacuum at elevated temperature strongly depends on the support used. On Rh/AlP04-31 linearly bonded CO at 2050 cm-'and two different dicarbonylcomplexes are observed. The eak ositions of the dublett with rather narrow absorption bands at 2089 and 2027 cm- agree surprisingly well with those found on Rh/AlPO,-ll /6/. The corresponduigsurface speciescan be therefore ascribed to a well defined Rh "(CO), complex, located at weak acid sites, which could be supplied by isolated P-OH groups. Different
P P
1679 1
Rh/MnAPO-31, vacuum 573 K
Rh/MnAPO-31, vacuum 473 K
Rh/MnA PO-31, vacuum 423 K
P f i
P
1
Rh/AlF'O,-31. vacuum 423 K
0-
I
2000
I
1800
wavenumber / em.'
Figure 1. F.t.i.r. sectra of CO adsorbed on Rh/AlPO,-31, Rh-Mn/AlP04-31 and Rh/MnA PO-31 from Rh/AlP04-11 the thermal stability of the species is rather high, showing intensive absorptionbands even at 473 K invacuum. The more unspecificsymmetricstreching mode at 2102 cm-'belongs to a dublett of a Rh+'(CO), species probably located at the outher surface of the molecular sieve. In case of Rh-Mn coimpregnated Apo4-31the well defined Rh+'(CO), species are formed in only negligible small amounts,presumablydue to a blockage of the acidicsites on AlP04-31 by Mn cations. Quite interesting spectra have been obtained on Rh/MnA PO-31 after evacuation at 473 K and higher temperatures. Besides the dubletts at 2103/2040 cm-' and at 2089/2028
1680
cm-'another well defined surfacespeciesseems to appear with narrow peaks at comparatively high energy (2113/2048 cm-'). In analogy to the assignments, given in / 6 / , this dublett could arise from Rh"(CO),, located on the new acid sites, formed by incorporation of Mn into the A1Po4-31framework.The wavenumbers and the correspondingforce constants of the CO stretching are higher than those of the S A P 0 4 and SAPO-11 molecular sieves, but smaller than those of US& (Table 1). From this result it may be concluded that the Table 1 Well defined Rh dicarbonyles on different molecular sieves support
us-Ex MnAPO-31 SAPO-11 SAPO-5 AlP04-ll AlPo4-31
v(C0) (cm-')
k(CO)
2118 2113 2112 2111
1757 1749 1744 1745 1712 1712
2053 2048 2044 2046 2090 2027 2089 2027
i(C0-Cot)
Ref.
(Nm-'1 55 55 57 55 52
52
5 this work 6 6 6 this work
acidity of the framework acid sites in MnA PO-31 is in between that of US-Ex and the S A P 0 molecular sieves, in agreement with the results of NH,-TPD.
4, References 1 2 3 4 5 6
J.H. Bennett, W.J. Dytrych, J.J. Pluth, J.W. RichardsoqJr., J.V. Smith, Zeolites 6 (1986) 349. E.M. Flanigen, B.M. Lock, R.L. Patton, S.T. Wilson, Pure Appl. Chem.58 (1986) 1351. L.E. Nonnemann, V. Ponec, Catal. Lett. 7 (1990) 197. S,T. Wilson, B.M. Lock, E.M. Flanigen, US Patent 4.310.440 (1982). H. Miessner, I. Burkhardt, D. Gutschick, A. Zecchina, C. Morterra, G. Spoto, J. Chem. SOC.,Faraday Trans. 1,85 (1989) 2113. I. Burkhardt, E. Jahn, H. Miessner, J. Phys., Chem. in press.
Guczi, L.er d.(Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
TRANSFORMATION OF THIOLS AND ORGANIC SULFIDES OVER ZEOLITES
M.Ziolek and P.Decyk Faculty of Chemistry,A. Mickiewicz University, 60-780 Poznan, Poland
Abstract
The transformation of diethyl sulphide(Et2S) on sodium and hydrogen forms of X,Y and ZSM-5 type zeolites was compared with that of ethanethiol(EtSH). Acidic and basic sites were found as the active centres in both rocesses. n o reaction pathways are postulated depending on the type of the catalyst.!is M-5and NaX zeolites were concluded to be applicable for removal of EtSH and EtS, respectively, as the latter two got accordingly transformed to ethene and hydrogen sulphide.
INTRODUCTION The removal of organic sulphur compounds is usually carried out in the reaction with hydrogen (HDS) leading to the formation of H2S and hydrocarbons. Zeolites can be applied for thiols [l-41and organic sulphides [5,6] decomposition without the use of hydrogen. Hydrogen sulphide and hydrocarbons appeared to be the products of these reactions similarly as in the HDS process. The aim of the presented study was to compare the transformation of ethanethiol(EtSH) and diethyl sulphide (EtS)over different types of zeolites as well as to find the optimum catalyst and the correlation between the active sites in zeolites and the selectivity of both processes.
EXPERIMENTAL Sodium and hydrogen forms of X (Si/Al=1.13), Y (Si/Al=2.56) and ZSM-5 (Si/Al=38) zeolites were used as catalysts. The catal ic activity was determined in a pulse microreactor filled with 0.2 g of the dehydrated orm of the zeolite at 623K.Pulses of lpl of EtSH or Et2S were introduced.Products were analyzed using an on-line gas chromatographwith FID. Both reactions were also conducted after the oisoning of the active centres by the adsorption of pyridine and sulphur dioxide at 623g Moreover, the active sites in zeolites were estimated using the cumene cracking at 623K as the test reaction.
fy'
RESULTS AND DISCUSSION The decomposition of ethanethiol over zeolites can proceed via two different reaction pathways [2] : 2C2HsSH --j CzHsSC2Hs + H2S (1)
1682
gen forms ofzeolites . Catalyst NaX EISH
conv.(%) Jref.21 Product distribution(%) Ethene Butane, butenes cs + cg Thiophene Diethyl sulphide Aromatics EtzS
.mnv.(%) Product di-
HNaX
NaY
HNaY
NaZSM-5
HZSM-5
90
31
20
30
75
89
74
73 2 3
91 1 1
13 9 54
98.5 0.3 0.1 0.1 0.8 0.2 99
98.3
2
52 4 12 11 10 11 25
8
50 2
52 2
traces
0.6 0.4
traces
46 1
9 3 19 15
49 0.5 0.3 0.2 49 1
3
14 6 73
3 4 96
0.3 0.1 0.1 1'2 99.9
stribution(%)
Ethene Butane, butenes c5 + G 5 Thiophene Ethanethiol Aromatics
92
87 1 1 1
8 2
90 0.5 1 7 0.5 1
The activity of all investigated zeolites in the decomposition of diethyl sulphide is higher than that of ethanethiol excluding the NaX zeolite. The product distribution depends on the type of zeolite. In the decomposition of ethanethiol, the main reaction products, among organic compounds, are ethene and E t S (sometimes significant amounts of thiophene and aromatics appear) while in the case of diethyl sulphide transformation they include ethene and EtSH. The product distribution presented in Table I indicates that although reaction (1) occurs over NaX, HNaX and NaY reaction (2) still dominates. The simple decomposition of EtSH towards ethene and hydrogen sulphide is practically the only reaction on ZSM-5 zeolites. In the case of the E t S conversion reaction (3) is the main process which proceeds on hydrogen X and Y zeolites. NaX zeolite catalyzes only the decomposition of EtS to ethene and H B and partially the dimerization of ethene to C4 hydrocarbons. To find out the character of active sites in the applied zeolites the cumene cracking was carried out (Table 2).
1683
Table 2 Cumene cracking at 623K Catalyst Propene NaX HNaX 48 NaY HNaY 45 NaZSM-5 38 HZSM-5 34
Selectivity (%I Benzene
Styrene 100
52 100 9 27
46 35 35
31
The 100% selectivity to styrene on NaX and NaY zeolite indicates that the radical mechanism is involved [7].About 50% selectivity to propene and benzene on HNaX shows that the only sites which act in the cumene cracking on this catalyst are the Broensted acid centres. The selectivity obtained for HNaY shows the domination of the Broensted acid centres in the cumene crackin$. A comparison of these results with those obtained for the ethene and EtSH formation in the dimethyl sulphide decomposition on HNaX and HNaY suggests that Broensted acid sites are active in reaction (3). The question was whether they are the only active sites in the transformation of E m . Figure 1presents the conversion of EtSH and Et2S and the selectivityin the reactions (1-4) on pure NaX and after adsorption of pyridine (PY) and sul hur dioxide. Pyridine poisons the acid sites (in the case of NaX there are sodium cation6 and SO2 blockes the basic sites. The adsorption of both: PY and So;! causes a decrease in the conversion of EtSH.This indicates that both acidic and basic sites are involved in this process. The effect of the PY adsorption is more evident
Figure 1. Influence of pVridine (Pwand subhw d W e ahoption on the active and selectivity of NaX
DIETHYL SULPHIOE
ETHANETHIOL TRANSFORMATION
cotolyst
otter PY ods.
m E t S H conv.
ftZS select.
after ads.
92
pure
cotolyst
Et*S tmv. EtSH select.
TRANSFORflATlON
a f t e r PY ods.
otter ods.
$Hc select.
so,
1684
The results presented for the E t S decomposition show that the acid-basic sites are also involved in this process.The poisoning of some stronger basic sites with SO2 causes a significant increase in the selectivity to ethanethiol (reaction 3).Reaction (4) requires a high concentration of cationic sites and basic sites which exists in NaX zeolite. In this reaction two basic oxygens and one cation are involved in decomposition of a single molecule of EtS. The blockade of some oxygen atoms by SO2 causes an increase in the yield of reaction (3) which proceeds on one acidic and one basic site.
CONCLUSIONS Decomposition of ethanethiol and diethyl sulphide requires the presence of acidic and basic sites. The concentration of these sites affects selectivity of these reactions. Two reaction pathways are pssible in both reactions: (i) transformation of one sulphur organic compound to the other (reactions I and 3) and (ii) elimination of hydrogen sulphide leading to the formation of ethene (reactions 2 and 4). The reaction pathways depend on the type of the catalyst. ZSM-5 and NaX zeolites were concluded to be applicable for removal of EtSH and E t S , respectively, as the latter two got accordingly transformed to ethene and H2S (similarly as in the HDS process).
ACKNOWLEDGMENTS The authors acknowledge the KBN for support of this work in the project No 2 0742 91 01.
REFERENCES 1 M. Suebka, T. Kamanaka and K. Aomura, J,Catal.,52(1978) 531. 2 M. Ziol ek, P. Dec k, M. Derewiikki and J. Haber, in: "Zeolite as Catalysts, Sorbents and Detergent Buirders. Application and Innovations" (H.G. Karge and J. Weitkamp, Eds. ,Elsevier, Amsterdam, 1989, p.305. 3 M. iM ek, P. Decyk and J. Kujawa, in: "GasSe aration Technology" (E.F.Vansant and R. Dewolfs, Eds.), Elsevier, Amsterdam, 1990, p.571. 4 A.V. Mashkina, V.R. Grunvald, V.I. Nasteka, B.P. Borodin, V.N. Yakovleva and L.N. Khairulina, React. Kinet. Catal. Lett., 41 (1990 357. 5 N. Davidova, P. Kovacheva and D. Shopov, in: ew Developments in Zeolite Science and Technology" (Y.Murakami et al., Eds.), Kodansha and Elsevier, 1986, p.811. 6 C.N. Koshelev, A.V. Mashkina and N.G. Kalinina, React. Kinet. Catal. Lett.,39
k
in: "Carboniogenic Activity of Zeolites", pp.113,141.
Elsevier, Amsterdam,l973,
Guni, L et of. Pditols), New Frontiers in Catulysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elscvier Science Publishers B.V. All rights reserved
PARA-SELECTIVITYOF ZSM-5 TYPE METALLOSILICATES FOR ALKYLATION OF TOLUENE WITH METHANOL
S.Namba*, H. Ohta, J.-H. Kim and T. Yashima Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan *Present address: Department of Materials, The Nishi-Tokyo University, Uenoharacho, Kitatsum-gun,Yamanashi 409-01, Japan
Abstract
The ZSM-5 type metallosilicates exhibited higher para-selectivities for the alkylation of toluene with methanol than did HZSM-5 zeolite. The metallosilicate with weaker acid strength exhibited the higher para-selectivity, because the isomerization of primarily produced p-xylene on weaker acid sites in the narrow channel of ZSM-5 structure was suppressed more severely. 1. INTRODUCTION
It is widely known that HZSM-5 zeolites modified with oxide of magnesium phosphorous [2,4,5] or boron [2-51 exhibit a high para-selectivity for alkylation of toluene or ethylbenzene. The modification of HZSM-5 with oxide brings the reduction not only of effective pore dimension [ l - 5 1 but I n our previous papers [4-71, it has also of the acid strength [1,4,5]. been reported that the high para-selectivity of modified ZSM-5 zeolites as well as metallosilicates with ZSM-5 structure for the alkylation of ethylbenzene with ethanol i s due riot to 'product selectivity', i.e., not to the intracrystalline diffusivity of the p-isomer being much higher than that of the other two isomers, but to the suppression of the isomerization of pisomer formed primarily through 'restricted transition-state selectivity', namely the alkylation takes place even on the weak acid sites, while in the narrow pores little isomerization does without the strong acid sites. In this study, we will report para-selectivities of several ZSM-5 type metallosilicates for alkylation of toluene with methanol. The metallosilicates having weaker acid sites than HZSM-5 are expected to exhibit higher para-selectivities than does HZSM-5 as observed for the ethylation. Moreover, we will discuss that the cause of the high para-selectivity for the alkylation of toluene is distinct from that for toluene disproportionation. [1-5],
2. EXPERIMENTAL
Catalysts used were Al-(Si/A1=96), Ga-(Si/Ga=64), B-(Si/B=70), Cr(Si/Cr=260), Sb-(Si/Sb=120), and As-silicates(Si/As=92) with ZSM-5 structure. The Al-, Ga-, Cr-, and B-silicates were prepared by hydrothermal
1686 synthesis. The Sb- and A s - s i l i c a t e s were prepared by t h e atom-planting method [7,8]. A l l t h e m e t a l l o s i l i c a t e s were transformed i n t o H+-form before use. The a l k y l a t i o n was c a r r i e d o u t w i t h a continuous f l o w system under Helium atmospheric p r e s s u r e . a t 673 K and Ptoluene = Pmethanol = 0.21 atm. The apparatus and procedure f o r t h e measurewas used as a c a r r i e r gas. ments o f temperature programmed d e s o r p t i o n f o r ammonia (NH3-TPD) and t h e g r a v i m e t r i c measurement o f o-xylene a d s o r p t i o n a r e described i n t h e l i t e r a t u r e [5,61.
3. RESULTS AND DISCUSSION The p a r a - s e l e c t i v i t y , i.e., t h e f r a c t i o n o f p-isomer i n t h e xylene produced, changed w i t h c o n t a c t time o r t h e y i e l d o f xylene. I n t h e case o f t h e HZSM-5 ( A l - s i l i c a t e ) c a t a l y s t which e x h i b i t e d a extremely low paras e l e c t i v i t y under normal r e a c t i o n conditions, the para-selectivity i n creased toward 100 % w i t h decreasing c o n t a c t t i m e toward 0 [l]. This r e s u l t i n d i c a t e s t h a t t h e primary product i n t h i s a l k y l a t i o n i s o n l y pxylene and t h a t t h e h i g h p a r a - s e l e c t i v i t y can be achieved by t h e suppress i o n of the i s o m e r i z a t i o n o f p-xylene produced p r i m a r i l y . To compare t h e p a r a - s e l e c t i v i t i e s o f v a r i o u s m e t a l l o s i l i c a t e s , t h e paras e l e c t i v i t i e s a t an almost constant y i e l d o f xylene (20 %) were determined, The almost constant a l k y l a t i o n a c t i v i t y was achieved by c o n t r o l l i n g c o n t a c t time. The r e s u l t s are shown i n Table 1. The o r d e r o f t h e p a r a - s e l e c t i v i I n p a r t i c u l a r , t h e Ast i e s i s As- > Sb- > C r - > B- > Ga- z A l - s i l i c a t e . s i l i c a t e e x h i b i t e d a h i g h p a r a - s e l e c t i v i t y o f 82 %. To c l a r i f y t h e reason why t h e m e t a l l o s i l i c a t e s e x h i b i t e d t h e h i g h paras e l e c t i v i t y , t h e measurements o f t h e adsorption o f o-xylene and t h e NH3-TPD were c a r r i e d out. I n Table 1, values o f t0.3 ( t i m e t o reach 3: % o f amount o f o-xylene adsorbed a t i n f i n i t e time), which correspond t o pore t o r t u o s i t y ' , and values o f Tmax (main peak p o s i t i o n i n NH3-TPD p r o f i l e s ) , which correspond t o ' a c i d strength', are a l s o shown. The pore t o r t u o s i t i e s o f Ga-, B- and Cr-) prepared by hydrothermal t h e m e t a l l o s i l i c a t e s (Al-, synthesis were almost t h e same. On the o t h e r hand, those o f the m e t a l l o As-) prepared by t h e atom-planting method were s l i g h t l y s i l i c a t e s (Sb-, h i g h compared w i t h those prepared by hydrothermal synthesis, because o f t h e presence of nonframework Sb o r As spieces [8]. However, .a t0.3 value o f 14.7 min f o r S b - s i l i c a t e was extremely small compared w i t h t h a t f o r A l Table l. Metal l o silicate
Para-selectivity, t o r t u o s i t y (t0.3)
--------AlGaBCr-
SbAs-
a c i d s t r e n g t h (Tmax) and pore o f metallosilicates.
W/F Xylene f g h mol-1 y i e l d f X
Para-select i v i t y I%
to /m;n
----------_---------.
0.59 0.59 5.88 5.88 2.06 2.06
20.9 21.6 19.1 20.2 19.8 20.7
45.0 51.9 55.0 68.5 71.7 81.8
2.4 1.4 3.5 1.6 14.7 12.1
3
T
7!x
--------568 528 508 508 488 458
1687
5
AS-
\
>
c, .I-
>
'r
c, U
aJ
c
aJ v)
I 4 L
a
n
I
l
I
49 3
533
Al-
E I
Tmax F i g u r e 1. R e l a t i o n s h i p between p a r a - s e l e c t i v i t y and a c i d s t r e n g t h (Tmax),
s i l i c a t e m o d i f i e d w i t h a enough amount o f o x i d e t o a t t a i n a h i g h p a r a s e l e c t i v i t y ( > l o 3 min) [ 3 ] . Therefore, t h e d i f f e r e n c e i n pore t o r t u o s i t y among t h e m e t a l l o s i l i c a t e s examined h e r e a r e r e g a r d e d as n e g l i g i b l e s m a l l . I n s p i t e o f l i t t l e difference i n pore t o r t u o s i t y , t h e d i f f e r e n c e i n p a r a - s e l e c t i v i t y among t h e m e t a l l o s i l i c a t e s was much. This fact indicates t h a t t h e p a r a - s e l e c t i v i t y f o r t h e a l k y l a t i o n o f t o l u e n e w i t h methanol i s n o t due t o p r o d u c t s e l e c t i v i t y . I n F i g . 1, t h e r e l a t i o n s h i p between t h e p a r a - s e l e c t i v i t y and t h e a c i d s t r e n g t h o f t h e m e t a l l o s i l i c a t e i s shown. A c l o s e r e l a t i o n s h i p was o b s e r v ed, namely t h e h i g h e r p a r a - s e l e c t i v i t y was observed on t h e m e t a l l o s i l i c a t e w i t h weaker a c i d s t r e n g t h . T h e r e f o r e , t h e i s o m e r i z a t i o n o f p-xylene p r o d u c e d p r i m a r i l y may b e s u p p r e s s e d m o r e s e v e r e l y o n w e a k e r a c i d s i t e s , r e s u l t i n g i n the higher para-selectivity. To d e t e r m i n e t h e i s o r n e r i z a t i o n a c t i v i t i e s o f m e t a l l o s i l i c a t e s t h e i s o m e r i z a t i o n o f o-xylene was c a r r i e d o u t u n d e r d i f f e r e n t i a l r e a c t o r c o n d i t i o n s a t 673 K and t h e r e a c t i o n r a t e was determined. The r e a c t i o n r a t e measured under d i f f e r e n t i a l r e a c t o r c o n d i t i o n s c o r r e s p o n d s t o t h e x y l e n e isomerization activity. The x y l e n e i s o m e r i z a t i o n a c t i v i t y o f t h e m e t a l l o s i l i c a t e a t t h e same c o n t a c t t i m e as t h a t used i n d e t e r m i n i n g t h e p a r a s e l e c t i v i t y f o r t h e a l k y l a t i o n was e s t i m a t e d . The e s t i m a t e d v a l u e s a r e shown i n Fig. 2 as a f u n c t i o n o f t h e a c i d s t r e n g t h (Tmax). The r e l a t i o n s h i p between t h e a c i d . s t r e n g t h and t h e i s o m e r i z a t i o n a c t i v i t y i s c l o s e , i n d i c a t i n g t h a t t h e weaker a c i d s t r e n g t h p r o v i d e s t h e more s e v e r e suppression o f t h e isomerization. The a p p a r e n t a c t i v a t i o n e n e r g y f o r 0-xylene i s o m e r i z a t i o n was d e t e r m i n e d by measuring t h e r e a c t i o n r a t e u n d e r d i f f e r e n t i a l r e a c t o r c o n d i t i o n s a t 573 - 673 K. I n Fig. 2, t h e r e l a t i o n s h i p between t h e a c i d s t r e n g t h and t h e a p p a r e n t a c t i v a t i o n e n e r g y f o r t h e i s o m e r i z a t i o n a r e shown. The a p p a r e n t a c t i v a t i o n energy increased w i t h decreasing a c i d strength. These f a c t s a l s o s u g g e s t t h a t t h e weaker a c i d s t r e n g t h may p r o v i d e t h e more s e v e r e suppression o f t h e isomerization.
1688 The m e t a l l o s i l i c a t e w i t h t h e h i g h e r p a r a - s e l e c t i v i t y p o s s e s s e d t h e weaker a c i d s i t e s and e x h i b i t e d t h e lower a c t i v i t y as w e l l as t h e h i g h e r apparent a c t i v a t i o n energy f o r t h e i s o m e r i z a t i o n . From t h e s e f a c t s i t i s suggested t h a t on t h e m e t a l l o s i l i c a t e s w i t h ZSM-5 s t r u c t u r e t h e isomerizat i o n o f p-xylene produced p r i m a r i l y may be suppressed t h r o u g h ' r e s t r i c t e d t r a n s i t i o n - s t a t e s e l e c t i v i t y ' and r e q u i r e s t r o n g a c i d s i t e s t o t a k e place, compared w i t h t h e a l k y l a t i o n . Thus, t h e h i g h p a r a - s e l e c t i v i t y o f m e t a l l o s i l i c a t e s w i t h ZSM-5 s t r u c t u r e i s due t o t h e absence o f s t r o n g a c i d s i t e s which a c c e l e r a t e t h e i s o m e r i z a t i o n . I t has been r e p o r t e d t h a t t h e p a r a - s e l e c t i v i t i e s o f t h e m e t a l l o s i l i c a t e s examined h e r e f o r t o l u e n e d i s p r o p o r t i o n a t i o n a r e a l m o s t t h e same a n d e x t r e m e l y low, w h i l e t h o s e o f t h e HZSM-5 z e o l i t e s m o d i f i e d w i t h l a r g e amounts of boron o x i d e o r o f coke a r e h i g h e r t h a n t h a t o f t h e HZSM-5 as w e l l as o t h e r m e t a l l o s i l i c a t e s [3,9]. Moreover, t h e p a r a - s e l e c t i v i t y i s c l o s e l y r e l a t e d t o t h e pore t o r t u o s i t y [3,9]. These f a c t s i n d i c a t e t h a t t h e h i g h p a r a - s e l e c t i v i t y f o r t h e d i s p r o p o r t i o n a t i o n i s caused by p r o d u c t s e l e c t i v i t y [3,9]. I n conclusion, t h e m e t a l l o s i l i c a t e s w i t h weaker a c i d s i t e s p r o v i d e t h e h i g h e r p a r a - s e l e c t i v i t y f o r t h e a l k y l a t i o n o f t o l u e n e w i t h methanol as observed f o r t h e a l k y l a t i o n o f e t h y l benzene w i t h ethanol, because t h e i s o m e r i z a t i o n o f p-xylene produced p r i m a r i l y i s more suppressed on t h e weaker a c i d s i t e s i n t h e pores as narrow as those o f ZSM-5 s t r u c t u r e . The cause o f t h e h i g h p a r a - s e l e c t i v i t y f o r t h e a l k y l a t i o n i s d i s t i n c t from t h a t f o r the disproportionation.
4. REFERENCES 1. T. Yashima, Y. Sakaguchi and 5. Namba, Stud. Surf, Sci. Catal., 7 (1981) 739. 2. W.W. Kaeding, C. Chu, L.B. Young, 6. Weinstein and S.B. B u t t e r , J. Catal., 67 (1981) 159. 3. D.H. Olson and W.O. Haag, C a t a l y t i c M a t e r i a l s (ed. T.H. Whytes) ACS Symp. Ser. 248, Am. Chem. SOC., Washington, DC, 1984, p. 257. K i m , S. Namba and T. Yashima, B u l l . Chem. SOC. Jpn., 61 (1988) 4. J.-H. 1051. 5. J.-H. Kim, S. Namba and T. Yashima, Stud. Surf. Sci. Catal., 46 (1989) 71. 6. J.-H. K i m , 5. Namba and T. Yashima, Z e o l i t e s , 11 (1991) 59. 7. J.-H. Kim, K. Yamagishi, S. Namba and T. Yashima, J. Chem. SOC. Chem. Commun., (1990) 1793. 8. K. Yamagishi, S. Namba and T. Yashima, Stud. Surf. Sci. Catal., 49 (1989) 459; B u l l . Chem. SOC. Jpn., 64 (1991) 949; J. Phys. Chem., 95 (1991) 872. Kim, S. Namba and T. Yashima, Appl. Catal., submitted, 9. J.-H.
Guczi, L.ei al. (Editors), New Frontiers in CotolysiE Proceedings of the 10th International Congrcss on Qtalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
HYDROXYLATION OF TOLUENE WITH HYDROGEN PEROXIDE ON W ZEOLITES WITH VARIOUS SiAI RATIOS T. Yashima, Y.Kobayashi, T. Komatsu andS. Numbas Department of Chemistry, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152, Japan *Present address: Department of Materials, The Nishi-Tokyo University, Uenoharacho, Kitatsuru-gun, Yamanashi 409-01, Japan
Abstract HY zeolites with higher Si/A1 ratios selectively catalyzed the hydroxylation of toluene with hydrogen peroxide in liquid phase. The Brdnsted acid sites are the active sites for the hydroxylation. The electrophilic attack by (OH)+ formed from H202 and H+ onto the benzene ring results in the replacement of H+ with (OH)+ in the benzene ring to form cresols.
1.
INTRODUCTION
Selective oxidations of aromatic compounds with hydrogen peroxide are known to occur in liquid phase using peroxo complexes of W, Mo, V, etc. as homogeneous catalysts 111. Recently, titanosilicate which has MFI structure with Ti4+ in its framework has been reported as an active heterogeneous catalyst for the hydroxylation of aromatics with hydrogen peroxide [2]. Chang and Helling have reported in their patent 131 that HZSM-5 without any transition metals also catalyzed the hydroxylation of phenol to form catechol and hydroquinone. In this study, we carried out the selective hydroxylation of toluene with hydrogen peroxide in liquid phase over HY zeolites of various A1 contents. The major purpose is to clarify the role of acidic properties of zeolites as the active catalyst for the selective hydroxylation. 2. EXPERIMENTAL
NH4(93%)Y zeolite (Si/Al = 3.8, 838 m2/g) was calcined to obtain H(93%)Y, which was then treated with NaCl aqueous solution to prepare HNaY of various proton contents (76 - 23%).
1690 C a ( 6 5 % ) Y a n d L a ( 7 1 % ) Y were p r e p a r e d f r o m t h e N H 4 Y ( S i / A l = 3.8) by u s u a l cation exchange methods. P y r i d i n e p o i s o n e d HY ( H Y p ) w a s p r e p a r e d f r o m t h e Hi93%)Y. NaY ( S i / A l = 2.6, 942 m2/g) w a s treated w i t h s i l i c o n t e t r a c h l o r i d e [ 4 1 t o obtain h i g h l y s i l i c e o u s HY z e o l i t e s w i t h S i / A l r a t i o s of 5.9 - 16. The h y d r o x y l a t i o n of t o l u e n e was c a r r i e d o u t i n a f l a s k w i t h a c o n d e n s e r a t 3 8 4 K. A f t e r t h e t e m p e r a t u r e of a m i x t u r e of c a t a l y s t ( 0 . 2 5 g ) a n d t o l u e n e ( 1 5 m l , 0.14 m o l l r e a c h e d t o 3 8 4 K , 0.1 m l of 30 w t % H202 a q u e o u s s o l u t i o n (H202: 1.2 x m o l ) was a d d e d t o t h e m i x t u r e . T h r e e more doses (0.1 m l e a c h ) were added a t e v e r y 1 0 min a n d t h e n t h e m i x t u r e w a s allowed t o s t a n d for 1 0 min. P r o d u c t s were a n a l y z e d by g a s c h r o m a t o g r a phy, and H202 c o n v e r s i o n w a s o b t a i n e d by t i t r a t i o n .
3. RESULTS AND DISCUSSION As shown i n T a b l e 1 , some z e o l i t e s c a t a l y z e d t h e h y d r o x y l a HY t i o n of t o l u e n e w i t h H 2 0 2 t o f o r m 0 - , m - a n d p - c r e s o l s . z e o l i t e showed t h e h i g h e s t a c t i v i t y f o r t h e h y d r o x y l a t i o n a n d g a v e a s m a l l amount o f b e n z y l t o l u e n e s w h i c h a r e f o r m e d t h r o u g h a n o x i d a t i v e c o u p l i n g of t o l u e n e . T h e s e l e c t i v i t y t o c r e s o l s was 93% b a s e d o n t h e a m o u n t o f t o l u e n e reacted. The p e r c e n t a g e mole r a t i o of t h e t h r e e cresol isomers i s o-/m-/p- = 7 0 / 7 / 2 3 , showing t h e o r t h o - p a r a o r i e n t a t i o n . On t h e o t h e r h a n d , HZSM-5 c a t a l y z e d m a i n l y t h e o x i d a t i o n of m e t h y l g r o u p i n t o l u e n e t o form b e n z a l d e h y d e , b e n z y l a l c o h o l and p h e n o l . Therefore, t h e s e l e c t i v i t y a n d a c t i v i t y f o r t h e h y d r o x y l a t i o n were lower t h a n
Table 1 H y d r o x y l a t i o n o f t o l u e n e w i t h H202 o n v a r i o u s z e o l i t e s Catalyst ~~~
HY
HZSM-5
HMa
NH4Y
HYpb
LaY
CaY
3.8 66.5
3.8 57.8
~~
Si/Al ratio 3.8 20 5.0 H 2 0 2 conv. ( % ) 69.1 75.1 92.5 P r o d ~ c t ( x l O -m~o l ) o-cresol 3.75 0.24 m-cresol 0.35 0.04 p-cresol 1.25 0.22 0.84 0.10 CgHg CHO CsHsCH20H 0.75 C6H5OH 1.66 0.03 Benzyl t o l u e n e s 0.19 0.13 0.05 R e a c t i o n t e m p e r a t u r e = 384 K , a H-mordeni te. HY p o i s o n e d by p y r i d i n e .
3.8 63.8 0.55 0.02 0.15
-
3.8 64.0
0.09 0.01 0.05 0.10 0.10
3.61 0.27 1.24
-
-
-
0.04
c a t a l y s t w e i g h t = 0.25 g.
3.02 0.19 0.91 -
-
0.04
1691
those of HY. In the case of H-mordenite (HM), no cresol was produced and the activity for the conversion of toluene was very low though H202 conversion was the highest. NH4Y showed lower activity than HY, which suggests an important role of Brdnsted acid sites for the hydroxylation. In fact, when acid sites in HY were poisoned by pyridine treatment, the activity decreased drastically (Table 1 , HYp). In addition, LaY and CaY which have Brdnsted acid sites due to their polyvalent cations also gave the high activity and selectivity for the hydroxylation. The percentage mole ratios of o/m-/p-cresols over these cation exchanged Y zeolites are 71/5/24 and 13/5/22, respectively. These values are almost the same as the ratio for HY, which confirms the idea that Brbnsted acid sites are the active site for the hydroxylation of toluene. It is clear that H202 conversion did not change much among Y zeolites. In the case of HY, only 1 1 % of added H202 was consumed to form cresols. Therefore, the H202 conversion reflects mainly the decomposition of H202 into H20 and 02. A blank test showed that 30% of H202 was decomposed without catalyst under
S i / A 1 RATIO 20 10 5 3
n W
DEGREE OF PROTON EXCHANGE, %
Figure 1 . Effect of proton concentration of HNaY on the yields of o-cresol( 0 ) , m-cresol( A ) , pcresol(D) and benzyltoluenes(0) and on the conversion of H202(V).
A1 CONCENTRATION, mg/g
Figure 2. Effect of A1 concentration of HY on the yields of products. Symbols are the same as those in Fig. 1 .
2
1692 t h e same c o n d i t i o n s . I t c a n b e e s t i m a t e d f o r HY t h a t 2 8 % of H 2 0 2 was decomposed on t h e c a t a l y s t s u r f a c e . B e c a u s e NH4Y a n d HYp g a v e t h e s i m i l a r H202 c o n v e r s i o n t o HY, t h e d e c o m p o s i t i o n of H 2 0 2 i s n o t a c c e l e r a t e d by a c i d s i t e s . I n o r d e r t o c l a r i f y t h e role of B r d n s t e d a c i d s i t e s , w e s t u d i e d t h e e f f e c t of p r o t o n c o n c e n t r a t i o n i n H N a Y z e o l i t e s on t h e p r o d u c t y i e l d s . A s s h o w n i n F i g . 1 , t h e y i e l d s of c r e s o l s a l l i n c r e a s e d w i t h i n c r e a s i n g t h e d e g r e e of p r o t o n e x c h a n g e . T h i s a g a i n i n d i c a t e s t h a t B r d n s t e d a c i d s i t e s are t h e a c t i v e s i t e s f o r t h e h y d r o x y l a t i o n of t o l u e n e . However, t h e y i e l d of benzyltoluenes a l s o increased with proton concentration. T h e r e f o r e , t h e o x i d a t i v e c o u p l i n g of t o l u e n e i s a l s o c a t a l y z e d by B r d n s t e d a c i d s i t e s , which was a l s o s u g g e s t e d by t h e r e s u l t s o f LaY a n d C a Y ( T a b l e 1 ) . The e f f e c t o f a c i d s t r e n g t h was s t u d i e d u s i n g HY of v a r i o u s A 1 contents, A s s h o w n i n F i g . 2 , t h e y i e l d s of c r e s o l s i n c r e a s e d w i t h d e c r e a s i n g t h e A 1 c o n c e n t r a t i o n and r e a c h e d t h e i r maxima a t S i / A l r a t i o of 9 . 4 ( 3 2 m g / g ) . I t i s known t h a t t h e a c i d s t r e n g t h and t h e h y d r o p h o b i c i t y i n c r e a s e b u t t h e number of a c i d s i t e s d e c r e a s e s when H Y i s d e a l u m i n a t e d . Accordingly, s t r o n g e r a c i d s i t e s s h o u l d b e much more a c t i v e for t h e h y d r o x y l a t i o n , a n d t h e h y d r o p h o b i c p r o p e r t y of z e o l i t e s may e n h a n c e the activity. From t h e s e o b s e r v a t i o n s , w e p r o p o s e a h y d r o x y l a t i o n rnechanism as f o l l o w s ;
(OH)+
t
C6H5CHj
----+
CgHqCH3(OH)
+
Ht
where t h e B r d n s t e d a c i d s i t e s a r e a c t i v e f o r t h e f o r m a t i o n of ( O H ) + and a n e l e c t r o p h i l i c a t t a c k of ( O H ) + i n d u c e s t h e h y d r o x y lation.
4.
REFERENCES
1
G. Amato, A. A r c o r i a , F.P. B a l l i s t r e i , and G . A . T o m a s e l l , J. Mol. C a t a l . , 37 (1986) 1 6 5 ; H. Minou, L. S a u s s i n e , E. D a i r , M. P o s t e l , J. F i s c h e r , a n d R. Weiss, J . Am. Chem. SOC., 1 0 5 ( 1 9 8 3 ) 3101. B. Notari, S t u d . S u r f . S c i . C a t a l . , 37 ( 1 9 8 8 ) 413. C. Chang and S. H e l l i n g , US P a t e n t N o . 4 578 521 ( 1 9 8 6 ) . H.K. Beyer and I. B e l e n y k a j a , C a t a l y s i s by Zeolites ( B . I m e l i k , e t a l . E d s . ) , E l s e v i e r , Amsterdam, 1980. F. Haber and J . Weiss, Proc. Roy. SOC. London, Ser. A , 147 ( 1 9 3 4 ) 322.
2
3 4
5
Guczi, L e.t al. (Editors), New Frontiers in Catalysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
TOLUENE ALKYLATION OVER ALUMINOPHOSPHATE-BASEDMOLECULAR SIEVES S. H. Oh and W.Y.Lee
Department of Chemical Engineering, Seoul National University, Seoul 151-742, Korea
Abstract Of the AlP0,-based molecular sieves, SAPO-11 and MeAPO-11 were selected and prepared at 100" - 200" C by a hydrothermal crystallization method. Toluene alkylation with methanol and ethylene was performed to enhance the para-isomers of xylenes and ethyltoluenes. The crystal structure of catalysts was confirmed by means of XRD and SEM, and the acidity was examined by TPD and IR of adsorbed ammonia on the catalysts. It was found that SAPO-11 and MeAPO-11 exhibited lower catalytic activity but higher selectivity for p-xylene and p-ethyltoluene than HZSM-5. Among SAPO-11 and MeAPO-11 tested, CoAPO-11 showed the best selectivity for p-isomers of toluene. Impregnating P or B on SAPO-11, catalytic activity was decreased due to the reduction of the acidity but the selectivity for p-isomers was remarkably increased by the enhancement of the shape-selectivity. 1. INTRODUCTION Since AlP0,-based molecular sieves [1,2] developed by the Union Carbide in 1982 have manifold characteristics in their composition, structures, and acidic properties, they are expected to be applied extensively to the various catalytic processes [3,4]. Because the framework of AlPO, is composed of A1 and P instead of Si, differing from the conventional aluminsilicates, it is electrically neutral. However, by replacing a part of A1 or P in AlPO, with Si or other metals, its framework becomes electrically negative so that it has acidic properties [5,6]. It has been reported [7,8] that among the AlP0,-based molecular sieves AlP0,-11 series, of which the pore size is 0.6nm, has exhibited the high selectivity for para-xylene in the alkylation of toluene. In this work, of AlP0,-based molecular sieves, SAPO-11, MAPO- 11, and CoAPO-11 were studied for the reaction characteristics of toluene alkylation. In particular, this work was concentrated on the investigation of the effect of the impregnation of P or B on SAPO-I1 as well as thermal and steam treatments of SAPO-11 on the selectivity for para-isomers. 2. EXPERIMENTAL In this study AIP0,-based molecular sieves were synthesized at 100"- 200°C by a hydrothermal crystallization method [9,10]. For the preparation of SAPO-11, MAW-11, and CoAPO-11, aluminum isopropoxide or pseudopboehmite phase alumina, phosphoric acid, cab-o-sil M-5, magnesium oxide, and cobalt acetate were used as source reagents and di-n-propylamine as a templating agent. Also, SAPO-11 was modified by thermal treatment at 7000, 800". 9000, and 1ooo"c for 2hrs. and by steam-treatment at 6000 and 700°C for 24hrs.. and also by the impregnation of I%, 3%. and 5% of B or P,
1694
respectively. The crystallinity was confirmed by means of XRD and SEM. Acidic properties were examined by IR and TPD of the adsorbed NH,. Toluene alkylation with methanol and ethylene was conducted in a flow reactor of a 114 inch stainless tube. The reaction products were analyzed by a GC using 5% Bentone 34 and 5% DNP on 80-100 mesh chromosorb W as column materials.
3. RESULTS AND DISCUSSION TPD results of adsorbed ammonia on various catalysts are given in Fig. 1. SAPO-1 1 has two acidic sites at 184C and 2 6 W while MAPO-11 and CoAPO-11 have one weak acid site at 177°C and a small gentle peak above 4OOC. As the calcination temperature increases, both weak and strong acid peaks of SAPO-11 gradually decrease due to the dehydroxylation and the partial destruction of the framework. In particular, the reduction of strong acid peaks is significant. It has been found that the framework of SAPO-11 has been completely destroyedat 1000°C. Impregnating B or P on SAPO-11, both amount and strength of the weak and strong acidshavedecreased. Inparticular, the strong acid site on 5% B/SAPO-11 and 5% P/SAPO-11 has been mostly removed. This suggests that P and B suppress mostly the strong acid site. The impregnation effect of P on both acid sites, in particular on a strong acid site, was more significant than that of B.
184
II
Fig.] TPD of adsorbed ammonia from (a) SAPO-11, CoAPO-11, MAPO-11 and HZSM-5, (b) thermally treated SAPO-11, and (c) B/ or PISAPO-11
In IR experiment of SAPO-11, four hydroxyl groups have been observed at 3732, 3666, 3608, .and 3559 cm-'. However, by dosing NH, on it and then evacuating at lOWC, the hydroxyl group at 3559cm-' has mostly disappeared and alternately new peaks have appeared at 1410 and 1620 cm-' as represented in Fig.2. From this result it has been revealed that only 3559 cm-' hydroxyl group was related to the acid site and that both Bronsted and Lewis acid sites were present on SAPO-11. In the akylation of toluene with methanol and ethylene over the various catalysts at the temperature of 300"- 450°C. the conversion of toluene' has increased and the selectivity for xylenes and ethyltoluenes has decreased, as the reaction temperature has increased. SAPO-11, MAPO-11 and CoAPO-11 have shown lower catalytic activity than HZSM-5, but the selectivity for p-xylene and p-ethyltoluene has exceeded
1695
Fig.2 Infrared spectra of (a) SAPO-11 and (b) ammonia adsorbed on SAPO-11 at morn temperature and evacuated at 100°C
Fig.3 Isomerizationof p-xylene over various catalyst ( WHSV 1.8 hi').
-
the chemical equilibrium composition, whereas HZSM-5 has exhibited only an equilibrium composition. Of AlP0,-based molecular sieves, CoAPO-11 has exhibited the highest selectivity for p-isomers. The reason that AlP0,-based molecular sieves show
Rxn. Temp.; 400°C
Methylation: WHSV(To1.); 2.4 hr' Mole ratio; Tol./MeOH=2
Ethylation: WHSV(To1.); 8.7 hr' Mole ratio; Tol./C&,=3 *ET; Ethyltoluene
1696
the high selectivity for p-isomers is believed to be resulted partly from the high sha e-selectivity due to the elliptical pore structure (0.67 nm x 0.40 nm) which is dif erent from the pore (0.56 nm x 0.54 nm) of HZSM-5, and partly from the suppression of the isomerization reaction of p-isomers during the desorption through the pores, for the acidity of AlP0,-based molecular sieves is weaker than that of HZSM-5 as shown in TPD data Fig.3 represents the activity of isomerization reaction of p-xylene over HZSM-5 and AlP0,-based molecular sieves, Table 1 gives the conversion of toluene and the selectivities for p-xylene and p-ethyltoluene over the various catalysts at 400°C. The impregnation of B or P on SAPO-11 enhanced p-selectivity but inversely reduced toluene conversion. The maximum selectivity for p-xylene or p-ethyltoluene was above 90% over 5wt% P/SAPO-11 and 5wt% B/SAPO-ll. Thermal- and steam-treated SAPO-11 showed similar tendency with the impregnated SAPO-11 in p-selectivity and toluene conversion, because the acidity was decreased by partial dehydroxylation and the shape selectivity was enhanced due to pore plugging caused by extracted A1 from the framework.
P
4.
CONCLUSION
AP0,-based molecular sieves have lower acidity than HZSM-5 and, in particular, no distictive strong acid site is present. By calcining SAPO-11 at higher than 800C and by impregnating B or P on SAPO-11 the strong acid sites are remarkably reduced. The decrease of the strong acid sites on the catalysts and the shape-selective pore structure were found to be responsible for the enhancement of the selectivity for p-isomers.
ACKNOWLEDGEMENT Authors appreciate gratefully for the financial support from the Korean Institute of Science and Technology.
REFERENCES 1 S.T. Wilson, B.M. Lok, C.A. Mesina, T.R. Cannan, and E.M. Flanigan, 3. Amer. Chem. SOC., 104 (1982) 1146. 2 B.M. Lok, C.A. Mesina. R.L. Patton, R.T. Gajek, T.R. Cannan, and E.M. Flanigan, J. Amer. Chem. SOC.,106 (1984) 6092. 3 J.M.O. Lewis, Stud. Surf. Sci. Catal., 38 (1987) 199, 4 R.J. Pellet etal., Proc. 7th Intern. Zeol. Conf., Tokyo 843 (1986). 5 E.M. Flanigan, B.M. Lok, R.L. Patton, and S.T. Wilson, Prm. 7th Int. Zeolite Conf., Tokyo, 103 (1986). 6 E.M. Flanigan, R.L. Patton, and S.T. Wilson, Stud. Surf. Sci. Catal., 37 (1988) 13. 7 J.M.O. Lewis, Stud. Surf. Sci. Catal., 38 (1987) 199. 8 K.W. Na, M.S. Thesis, Seoul National University, 1989. 9 B.M. Lok, etal., U.S. Patent 4,440,871 (1984). 10 C.A. Messina, et al., U.S. Patent 4,544,143 (1985).
Guai, L et al. (Editors), New Frontiers in Catalysis Pmcecdings of the loth International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 0 1993 Elscvicr Science Publisher8 B.V. All rights reserved
THE CRACKING REACTION PATH IN 1-HEXENE ISOMERIZATION ON SAPO-11 AND Pd/sAP0-11
S.-X Lim and S.-J. Choung Department of Chemical Engineering, College of Engineering, Kyung Hee University, Suwon 449-701, Korea
NSlRAfl - In order to investigate into the prebinant cracking pathway on SApD-11, the activity and selectivity patterns for 1-hexene isomerization have been compared in the temperature range of 150-450'C for four different kinds of acidic molecular sieves(H-mordenite, HZSM-5, SAPO-11 and Pd/SApD-11). The predominant reaction pathway was double bond shift(DBS) at lower temperature ( m u n d 150C) without regarding the type of catalyst. Hwver, at higher temperature significant shifts in selectivity were observed from DBS to cracking(C) or skeletal rearrangement(SR) depending on the type of catalyst. The cracking reaction path was considered two ways. One is dimerization between linear hexenes and the other is dimerization between linear hexene and SR isomers. Two paths were almost equaled on %Po-11, but latter path was predominant on Pd impregnated SAPD-11, fFzsM-5 f o l l m d the former cracking path because of its characteristic small pores and complex pore structure. In the comparison between the reaction path and the acidity of catalyst, the lager the acidity of catalyst the more dominant the former path.
I-Ia Less attention has been given to skeletal rearrangement(SR) and cracking/ poly.erization(C+P) selectivities at higher reaction temperatures(150-450"C). It is not well understood and has not been reported at all on silico aluminophosphate molecular sieves(specia1 ly, sApQ-11) as a 1-hexene isomerization catalyst at higher temperatures, focusing on the interchange among DBS, SR and (C+P) selectivities. In the previous paper(l), we described the reaction of 1-hexene on those catalysts. Though, however, cracking was the main reaction in the high temperature ranges, it was remained as an unsolved problem. The main objective of present study is to establish the cracking reation path of W loaded and unloaded SAPD-11, and to coppare such results with corresponding behaviors of HZSM-5.
-
ImmumTAL A commercial SAPD-11 was employed(2); the empirical composition of this is (si0.06 Alo.49P0.46)02, with W A l about 0.12, and the structure Is equivalent to that of AlPO-11 with elliptical, one-dimensional channels of ca. 0.6X0.4 rim nominal diameter and volume of 0.1&.3/g. The HZSM-5 enployed(2) has a SUA1 lolar ratio of 38, with Na/AI of 0.045(0.08 w t L Na). H-mrdenite employed WBS Norton 9OOH. For preparation of the WSAKb11 the acid form was generated in flowing 02, 600C, 1 hr and then impregnated with aqueous [Pd(NH3)4(N03)21 following method 2 of (3).
1698 A conventional fixed bed flaw reactor system was employed for reaction measurements. In various experiments this was in operation at integral conversion, with a noma1 catalyst charge of 0.2%. 1-hexene was introduced into a feed system vaporizer via a contmlled rate syringe pump into either He flow giving a saturatued hexene-He mixture at 25°C. Typical feed flow rates here 40cm3/qin at 2OC yielding spece velosities ~a.2ooCm~/g catrin. Product was sampled directly downstream of the reactant into a Hewlett-Packard 573011 GC. with separation in a Supelco 0.19% picric acid on 80/100 nesh C a r b p a c k 4 c o l w . 2 n, at 1WC. RGULTS AND DISCESI(IW
1
(c) HMORDENITE
Reaction temperature ("C)
ZnOtH-ZSM-6 > ZnO+H-ZSM-Ei(a) ZnO/H-ZSM-5 > Zn-ZSM-5. By contrast, the number of Lewis acidic sites determined by the band at 1455 cm" increases in the same order, i.e., zinc ions form new Lewis acidic sites when
-
Normdlr*d abrwbansr of the brndr of brldgsd OH goupr, Pytl' and P p L ~prelmr
Figure 1. Comparision of different species obtained by adsorption of pyridine
Figure 2. Themperature-programmed desorption of ammonia, pd,=133 kPa, T,,,=373K, 10 Wmin
1709
incorporated into H-ZSM-5. Furthermore, after the temperature-pro- 100Ethane oonverslon 1wt.S) grammed desorption of pyridine up to 80 .................................................................................................... 870 K besides the band indicating true Lewis sites (1445 cm-'1 two additional bands at 1410 and 1480 cm" were detected in the spectra of Zn-ZSM-5 and ZnO/H-ZSM-5 zeolites which might be ascribed to pyridine associated with zinc species. According to the t.p.d. of ZnO/H-ZSM-6 ti-ZSM-6 ammonia (Fig. 2) introduction of zinc, =thermal treatmnt perhaps on cationic sites, decreases the number of Brdnsted acidic sites (high-temperature Peak around 600 K) Figure 3. Conversion of ethane at 773 K and increases the number of Lewis after2 honstream acidic centres (low-temperature peak around 400 K).However, the tailing of the curves (except those of H-ZSM-5 and ZnO) indicates an interaction of ammonia with stronger Lewis sites. The simultaneous decrease of the Bransted acidity (see Fig. 1)let us assume that solid state ion exchange takes place (11, i.e. ZnO t 2 H-OZ -------- > Zn(OZ), + H,O (1) The degree of ion exchange depends on the distance between the zinc source and the OH groups. In mechanical mixtures the contact between ZnO and the zeolite is less intimate compared with the impragnated sample. However, in the ZnO+ HZSM-5 (a), which was thermally treated at 820 K, the ion exchange obviously took place. The catalytic behavior of the ZSM-5 zeolites modified with zinc and, for comparison, of the H-ZSM-5 zeolite and ZnO is demonstrated in Fig.3 and Tab.1. The differences in ethane conversion on zeolites modified with zinc by the various methods are insignificant. In spite of the higher amount of zinc introduced by solidstate exchange during thermal treatment at 823 K, the ethane conversion is lower than without thermal treatment. However, destruction of the zeolite lattice due to thermal treatment is unlikely since the non-modified H-ZSM-5 zeolite does not show any differences in Table 1 ethane conversion or Selectivities (wt.%)for C, (methane),C,, (ethene),B selectivity. A possible (benzene), T (toluene), C, (xylenes, ethylbenzene), C,, explanation of the de(higher aromatics), N (naphthalene)after 2 h on stream crease in activity of the zinc containing catalysts c, c,. B T c.3 c.3, N upon thermal treatment Zn-ZSM-5 2.0 0.6 13.1 27.7 19.6 36.9 16,4 may be a loss of zihc. Zn-ZSM-5 (a) 2.4 0.9 12.0 22.7 19.1 42.5 27,6 Indeed, a quantitative ZnO/H-ZSM-6 1.4 0.7 14.0 24.0 22.8 37.1 18,4 determination of Zn beZnO/H-ZSM-6 (a) 2.6 1.7 13.8 23.6 16.2 43.1 26,2 fore and after thermal ZnO+H-ZSM-6 0.9 0.6 18.0 34.8 22.6 23.1 12,l treatment provided eviZnO+H-ZSM-6(a) 0.6 0.9 16.2 23.8 11.9 47.6 19,O dence for a loss of Zn in ~
~~~~~
1710
the?following sequence Zn-ZSM-5 < ZnO/H-ZSM-5 c ZnO+H-ZSM-5 (see Fig.4) which, in fact, corresponds to the observed sequence in decreasing activities (see Fig.3). Hence, the conventionally ion exchanged ZSM-6 zeolite contains Zn in the most stable form and, therefore, exhibits the highest activity after thermal treatment. As can be recognized from Table 1aromatics, especially higher ones (naphthalene) are formed as main products on ZSM-5 zeolites modified by zinc. Formation of methane, even so thermodynamically prefered, is strongly suppressed, in contrast to its formation on Pt/H-ZSM-5. The selectivities towards BTC, formation 2d 2 show the same tendencies as the ethane conversion (compare Table 1 and Fig.3). 1.I The highest selectivity for BTC, on one 1 side and the lowest one for C, on the 0.6 other is reached by the mechanical mixture of ZnO+H-ZSM-5.Whilst the selectiZn-2ZnO/H-ZSM-6 211044-ZSM-6 vities for BTC, decrease as a result of W thermal treatment thermal treatment, the selectivities for C, show no significant change, but the selectivities for C,, are higher for the thermal treated zeolites than for the Figure 4.Zn-content determined by titrauntreated ones. The selectivities for C,, tion and aromatics suggest, that Zn promotes the further reaction of ethene to aromatics more than the formation of ethene. The ethane conversion does not reflect the decrease in the number of Brmsted acid sites as shown by i.r. spectroscopy (see Fig.1). Moreover, complete elimination of the acidic OH groups in Zn-ZSM-5 by solid-state reaction with NaCl (evidenced by i.r.) did not bring about any decrease in ethane conversion. Hence, it must be concluded that the Bransted acidity does not play a significant role in ethane aromatization over Zn-containing ZSM-5 catalysts investigated. 4. REFERENCES 1 K.-H. Steinberg, U.Mroczek and F. Roessner, Appl. Catal., 66 (1990)37 2 T. Mole and J.R. Anderson, Appl. Catal., 17 (1985)141 3 M. Guisnet and N.S. Gnep, in P.A. Jacobs et al. (Editors), Zeolite Chemistry and Catalysis, Stud. Surf. Sci. Catal., Vol. 69,Elsevier, Amsterdam, 1991,p. 321 4 Y. Ono, H. Nakatani, H. Kitagawa and E. Suzuki, in T. Inui (Editor), Successful Design of Catalysts, Stud. Surf. Sci. Catal., Vo1.44,Elsevier, Amsterdam, 1989,p. 279
5 6 7 8
H.G.Karge, 2. phys. Chem. NF, 76 (1971)133 H.G. Karge and K.Doudour, J. Phys. Chem., 94 (1990)765 K. Hatada, Y. Ono and Y.Ushiki, 2. phys. Chem. [NF],86 (1982)3050 E. LoeMer, U. Lohse, C. Peuker, G. Oehlmann, L.M. Kustov, V.L.Zhobolenko and V.B. Kazansky, ZEOLITES, 10 (1990)266
Guni, L u al. (Editors), New Frontiers in Catalpis Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
IN SITU F I l R AND GC KINETIC STUDIES: COMPLEMENTARY METHODS IN THE MECHANISTIC STUDY OF BUTANOL DEHYDRATION ON ZEOLITE €I-ZSM-5
M.A. Makarovaq E. A. Paukshtis': J. M.Thomasb, C. Williams4band K I. ZamaraeVa %stitUte of Catalysis, Russian Academy of Sciences, Siberian Division, Pr. Lavrentieva 5,
Novosibirsk 630090, Russia bDavy Faraday Research Laboratory, The Royal Institution, 21 Albemarle St., London W1X 4BS, United Kingdom
1. INTRODUCTION Catalysis on zeolites has received considerable attention both in applied and fundamental studies [l-31. However, elucidation of reaction mechanisms in such systems is complicated by the fact that the course of reaction can be determined not only by the active sites of the zeolite, but also by the influence of the surrounding micropores (effect of pore confinement) [4,5]. As a result, it is often difficult to determine what actually gives rise to the kinetic parameters (activity, selectivity, activation energies) that are measured in a real catalytic experiment. In the present work we investigate this problem by using a combination of two complementary in situ methods, namely F T I R and GC kinetic studies to obtain a complete picture of molecular transformations for a model reaction in a zeolite microporous environment. From the F T I R kinetic studies it is possible to follow directly the transformation of a molecule on the active centre; in the GC studies, it is possible to observe the products that desorb from the active sites and leave the zeolite pores. The dehydration of butanol on H-ZSM-5 has been selected as a suitable model reaction. Firstly the molecular dimensions of butanol are rather similar to the dimensions of the microporous channels in ZSM-5, giving rise to a situation where we might expect to detect a considerable influence of the surrounding channels on the course of reaction. Secondly, butanol has 4 different isomers, enabling us to vary the reactant geometry and reactivity. 2. EXPERIMENTAL A series of H-ZSM-5 samples with rather similar number of active sites (Si/Al 30-42) but different crystallite sizes (4-20pm) was used in this study [6,7].In situ F T I R kinetic studies were carried out on a Bruker IFS-113V spectrometer, using a variable-temperature, static I R cell [ 6 ] . Butanol was injected into the cell in an amount corresponding to half the
-
1712 number of acid sites in the zeolite sample. Spectra were then recorded at increasing time intervals to follow the kinetics o f reaction. GC kinetic studies were carried out in a flow microreactor with on-line product analysis (alcohol concentration) typically 0.5-1 mol% in helium, overall pressure 1 atm., conversion < 5 % ) . 3. RESULTS AND DISCUSSION
At temperatures below 200'C employed in this study, the butanol dehydration reactions are relatively simple in terms of the products formed; these are butene isomers, water and in some cases dibutyl ether (more probable for primary alcohols). In the present study we compare butanol dehydration rate constants obtained by the two different methods described below. a. GC kinetic studies, using a differential flow microreactor. Reaction was studied in the temperature range 50-200°C and the rate of reaction, W , expressed as the rate of formation of organic products or of water [W(butene) + W(buty1 ether) W(water)], was determined after reaching steady state. The reaction was zero order with respect to alcohol. The reaction rate constants were determined from k-W/N, where W is the rate of reaction and N the number of active Bronsted acid site in the catalyst (see ref.[6] for a more detailed characterization of the zeolite acid sites). Experiments were carried out with samples of a wide range of crystallite sizes in order to examine the possibility of diffusion difficulties during reaction.
-
b. 3n situ FTIR kinetic studies of the transformation of adsorbed alcohol molecules on the zeolite active sites in static conditions. Dehydration was monitored by the appearance and growth of the peak at 1640cm-' (corresponding to the deformation vibration mode of water) in the spectrum for the adsorbed alcohol in the temperature range 50-100°C. The change in the intensity of this peak with time can be described by an exponential function: 1,
- I,
.[l-exp(-kt)]
where It is the peak intensity at a time t o f reaction, and I, is its maximum possibly intensity (at the end of reaction), This enables us to determine the rate constants for monomolecular transformation o f butanol on the zeolite acid OH groups. The rate constants obtained by the two methods described above for three of the butanol isomers (namely n-, iso-, and sec-butanol) are shown in Fig.l(a-c). While for n-butanol (Fig.la) they are in close agreement, the rate constants determined by the 2 methods differ significantly from one another in the case of iso- and sec-butanol dehydration (Fig.lb,c). These anomalies arise from confinement of reaction to the zeolite micropores. Thus the characteristic feature of n-butanol dehydration in the micropores of H-ZSM-5 as compared with dehydration on an open surface (for example an amorphous aluminosilicate) is the predominance of n-dibutyl ether in the reaction products [8]. Ether formation may be considered as a side reaction which is enhanced in the zeolite pores when an active .&OR surface intermediate, formed on elimination of water, reacts with a second
1713 alcohol molecule, before it succeeds in decomposing to form butane. In case of the linear n-butanol the formed ether is able to desorb from catalyst pores without difficulty and so the reaction rate constants at and the same temperatures, determined by the 2 different methods (a) (b) above under different experimental conditions are close in value: k(GC) = k (ETIR) (Fig.la)
the the one and
I
-47 -6. I
n
\-a: Y
'
c-10-
d
.
-12; -14-16'
22
2,4
2,a 103K/ T
2,6
3,0
Figure 1. Rate constants for the dehydration of butanol .I Arrhenius coordinates. The points denoted are determined from FTIR data. The remaining points correspond to rate constants determined from GC steady-state data. Points represented by different symbols are for zeolite samples of different crystallite sizes. Reactant: a) n-butanol, b) iso-butanol, c) sec-butanol
*
On dehydration of iso-butanol, then very little ether is observed in the desorbed products, In the case of this more bulky fso-butanol, the corresponding ether encounters diffusion difficulties and probably decomposes, not managing to come out of the zeolite pores. Under steady state conditions this manifests itself as a slowing down of reaction:
1714 k(GC) EU-l>ZSM-11. The xylene yield followed the order: ZSM-48>ZSM-ll>EU-l(for Al-analogs); and, ZSM-48> EU-l=ZSM-ll (for Fe-analogs). In the case of Al-analogs the above results are quite expected [5] according to the structure of these zeolites. However, higher p-xylene selectivity and xylene yield over Fe-analogs (vis-a-vis Al-) may be due to the suppression of secondary reactions like isomerization and further alkylation/transalkylation of p- and o-xylenes formed initially as primary products. Here it may be pertinent to mention that Fe-analogs did not produce any significant amount of aromatics when, under the reaction conditions used here, either pure methanol or toluene was reacted. However, one can also envisage that the higher p-xylene selectivity and xylene yield on Fe-zeolites may indicate that Feanalogs are more shape-selective than the Al-counterparts. To check this point, m-xylene was converted over same zeolites. The results are summarized in Table 1. The comparable or less p-xylene selectivity, at similar conversion levels, exhibited by Fe-analogs, particularly in the case of ZsM-ll(in accordence with the detailed studies in m-xylene isomerization over Al-, Ga- and Fe-ZSM-11 [6]) and adsorption capacities (Table 1) clearly indicate that the higher apparent para-selectivity of Fe-analogs (vis-a-vis Al-) in toluene methylation may be due t o their different chemical nature (e.g.less strong acid sites).
This work was partly funded by UNDP. A.R., K.R.R. and J.S.R. thank C.S.I.R. New Delhi for a research fellowship each. R.K. gratefully acknowledges Alexander von Humboldt Stiftung, F.R.G. for granting a research fellowship.
5.
REFERENCES
P. Ratnasamy and R.Kumar, Catalysis Today, 9 (1991) 329-416. R. Kumar and P. Ratnasamy, Stud.Surf.Sci.C Catal., 60 (1991) 41. 3 R. Szostak, Wolecular Sieves: Principles of Synthesis and Identification", pp.223, Reinhold, New York, 1989. 4 J.S. Reddy, K.R.Reddy, R. Kumar and P. Ratnasamy, Zeolites, 11 (1991) 553. 5 G.N.Rao, R.Kumar and P.Ratnasamy, Appl.Cata1. 49 (1989) 307. 6 A.Raj, J.S.Reddy and R.Kumar, To be presented at 9th Intern. Zeolite Conference, Montreal, Canada, in July 1992.
1 2
Guni, L et 1 (Editors), New FrontiCrs in Catalysis Pnwxedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
THE INFLUENCE OF TEE CATALYST PREPARATION ON THE CATALYTIC PROPERTIES OF ZEOLITESUPPORTED CATALYSTS Y. W.Chen and W.J. Wang
Department of Chemical Engineering, National Central University, Chung-Li, 32054 Taiwan, China
Abstract Two preparation techniques (incipient-wetness impregnation and ion exchange) have been used to prepare NaX and NaY zeolite-supported cobalt catalysts. These catalysts were characterized by X-ray diffraction, temperature-programmed reduction, and temperature-programmed desorption of hydrogen. The Fischer-Tropsch s nthesis was examined at temperatures in the range of 220-300 'C, a pressure of 1 atm, a CO/Hz ratio of 1, and flow rate of GHSV=lZOO. The mobility of cobalt ions inside the zeolite cages in the ion-exchanged samples caused the difficulty in reduction and less active sites for the access of CO to the metals which are entraped in the sodalite cage. The ion-exchaned catalyst thus disclosed a low catalytic activity. The absence of hydroxyl group in the impregnated catalysts results in the stabilization of the metallic form of cobalt on the external surface of zeolite which cause catalytic activity in the FT reaction. The Co/X catalysts always exhibited higher activities than the C o / Y catalysts, no matter what preparation method was followed. 1. Introduction
The development of metal/zeolite bifunctional catalysts to produce hydrocarbons with improved selectivity, from the hydrogenation of carbon monoxide, represents an important advance in Fischer-Tropsch technology [ I ] . Owing to the incorporation of the metal particles in a geometrically restricted environmeent, zeolite-supported catalysts may exhibit shape-selectivity and thus restricting chain-growth process. In addition, due to the acidic nature of the zeolites, secondary reactions such as isomerization can influence the product distribution. The dispersion of the reduced metal, which usually related to the catalytic properties, is affected by the support material, the preparation method, and the activation processes. In the present paper, we make a comparative study of F-T reactions over the cobalt catalysts supported on X- and Y-zeofites, prepared by impregnation and ion-exchnage methods.
1720 2 . EXPERIMENTAL
Two p r e p a r a t i o n t e c h n i q u e s , i n c i p i e n t w e t n e s s i m p r e g n a t i o n (IW), and i o n e x c g a n g e ( I E ) , w e r e u s e d t o i n t r o d u c e C o on or i n t o t h e z e o l i t e s u p p o r t . The i n c i p i e n t - w e t n e s s c a t a l y s t s w e r e p r e p a r e d from c o b a l t n i t r a t e d i s s o l v e d i n d i s t i l l e d w a t e r a t a c o n c e n t r a t i o n s u f f i c i e n t t o y i e l d the p r o p e r m e t a l l o a d i n g when i m p r e g n a t i n g e a c g gram o f t h e s u p p o r t w i t h 0.7 mL o f s o l u t i o n . A f t e r t h e e i m p r e g n a t i o n , the s a m p l e s w e r e d r i e d o v e r n i g h t i n a i r a t 4OoC. F o r p r e p a r i n g t h e i o n - e x c h a n g e d c a t a l y s t s , c o b a l t n i t r a t e was dissolved i n a w e a k l y a c i d i c H C l s o l u t i o n (pH 4.5). t h i s s o l u t i o n was then m i x e d w i t h z e o l i t e and s t i r r e d c o n t i n u o u s l y f o r 50 h a t a m b i e n t t e m p e r a t u r e . A f t e r t h e i o n - e x c h a n g e r e a c t i o n , t h e c a t a l y s t s w e r e f i l t e r e d and washed s e v e r a l t i m e s i n deionized w a t e r and d r i e d i n a i r o v e r n i g h t a t 4 0 OC. A f t e r preparation a l l c a t a l sts were reduced w i t h hydrogen b y h e a t i n g t o 450 OC a t 5 'CC/min and the t e m p e r a t u r e was maintained for 5 h. The m e t a l l o a d i n g s of t h e s a m p l e s w e r e determined t o be 3 w t % ( d r y b a s i s ) w i t h i n g e x p e r i m e n t a l error. A c o n v e n t i o n a l temperature-programmed r e d u c t i o n ( T P R ) a p p a r a t u s was u s e d t o s t u d y the r e d u c i b i l i t i e s o f the o x i d i c s a m p l e s . Hydrogen c h e m i s o r p t i o n d a t a were o b t a i n e d b y temperature-programmed d e s o r p t i o n m e t h o d . TPD e x p e r i m e n s w e r e erformed i n a f l o w o f a r g o n f r o m room t e m p e r a t u r e t o 5 0 0 gc a t 10 o C / m i n . F T r e a c t i o n was p e r f o r m e d a t 1 a t m , 220-300 OC, and H 2 / C O = l . 3 . RESULTS AND D I S C U S S I O N
F i g . 1 shows t h a t both i o n - e x c h a n g e d c a t a l y s t s c o u l d not be r e d u c e d even a t 7 0 0 OC, c o n f i r m i n g t h e p o o r r e d u c i b i l i t y r e p o r t e d p r e v i o u s l y [ 2 - 3 1 . However, the r e d u c t i o n p e a k s o f i m p r e g n a t e d c a t a l y s t s a r e much l o w e r . I t h a s been r e p o r t e d [ 4 ] t h a t t h e r e d u c i b i l i t i e s were m a i n l y a f f e c t e d b y the o c c u p y i n g s i t e o f c o b a l t i o n s . T h e c o b a l t i o n s on the e x t e r n a l s u r f a c e o f z e o l i t e s a r e much more e a s i l y r e d u c i b l e t h a n those c o b a l t ions i n s i d e t h e z e o l i t e c a g e . O n e c a n c o n c l u d e t h a t t h e c o b a l t i o n s a r e s p r e a d m a i n l y over t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e s i n i m p r e g n a t e d c a t a l y s t s , o n l y a s m a l l f r a c t i o n of c o b a l t i o n s g o t i n t o the i n t e r n a l s i t e s . In the i o n - e x c h a n g e d c a t a l y s t s N a ( I ) c a t i o n s are replaced by Co(II) c a t i o n s . A f e t r r e d u c t i o n w i t h h y d r o g e n , t w o B r o n s t e d a c i d s i t e s w i l l be produced f o r each c o b a l t metal atoms. F i g . 1 a l s o shows t h a t t he X - z e o l i t e s u p p o r t e d c a t a l y s t s a r e e a s i e r t o r e d u c e t h a n the Y - z e o l i t e s u p p o r t e d c a t a l y s t s . T h i s c a n be a t t r i b u t e d t o t h e h i g h e r Bronsted a c i d i t y of Y - z e o l i t e s u p p o r t . N e g l i g i b l e amount o f h y d r o g e n a d s o r p t i o n was d e t e c t e d on a l l t h e s a m p l e s . K i m and Woo [5] h a v e r e p o r t e d t h a t c o n v e n t i o n a l methods s u c h a s c h e m i s o r p t i o n , TEM, and X-ray l i n e b r o a d e n i n g f a i l e d t o d e t e c t the c o b a l t c l u s t e r s s u p p o r t e d on z e o l i t e w i t h l e s s t h a n 1 nm i n d i a m e t e r . The r e s u l t s c a n be a t t r i b u t e d t o the h i g h c o b a l t d i s p e r s i o n and the h i g h o x i d a t i o n s t a t e o f c o b a l t ions. I t i s known t h a t h i g h o x i d a t i o n s t a t e c o b a l t does
1721
not i n t e r a c t a p p r e c i a b l y w i t h hydrogen.The existence of m e t a l z e o l i t e i n t e r a c t i o n s would c a u s e t h e c o b a l t t o be i n a h i g h e r o x i d a t i o n s t a t e t h a n the presumed v a l u e o f zero. T h e electrondeficent c h a r a c t e r of C o f o r the ion-exchanged c a t a l y s t s s h o u l d be more t h a n t h a t f o r t h e i m p r e g n a t e d c a t a l y s t s .
,
1
1
1
1
1
1
300 500 700 900 T (OC) I+i s o t h e r m a l
.
1
1
1
1
1
300 500 700 900 T (OC) P isothermal
F i g u r e 1 . TPR s p e c t r a o f Co/NaX and Co/NaY C a t a l y s t s . X-ray l i n e b r o a d e n i n g m e t h o d was u s e d t o t r y t o e x a m i n e t h e c o b a l t p a r t i c l e s i n t h i s s t u d y . No XRD p e a k s i n d i c a t i v e o f c o b a l t were observed. T h i s o b s e r v a t i o n s u g g e s t s t h a t t h e c o b a l t p a r t i c l e s a r e less t h a n 4 nm f o r a l l the c a t a l y s t s . However, i t i s e x p e c t e d t h a t t h e m e t a l p a r t i c l e s i n I E c a t a l y s t s t e n d t o be more u n i f o r m i n s i z e and s h a p e d u e t o g e o m e t r i c c o n s t r a i n t s o f the z e o l i t e l a t t i c e . I n a d d i t i o n , t h e s i z e o f c o b a l t m e t a l on the IW c a t a l y s t s i s believed t o be somewhat l a r g e r t h a n t h a t o f t h e I E c a t a l y s t s . T a b l e 1 l i s t s t h e a c t i v i t i e s o f a l l the c a t a l y s t s a t 2 6 0 O C . The m o b i l i t y o f c o b a l t ions i n s i d e the z e o l i t e c a g e s i n the I E c a t a l y s t s caused the d i f f i c u l t y i n r e d u c t i o n and l e s s a c t i v e s i t e s f o r t h e a c c e s s o f CO t o t h e m e t a l s which e n t r a p e d i n t h e s o d a l i t e c a g e . The I E c a t a l y s t s t h u s d i s c l o s e d a l o w c a t a l y t i c a c t i v i t y . T h e a b s e n c e o f h y d r o x y l g r o u p i n t h e IW c a t a l y s t s r e s u l t s i n t h e s t a b i l i z a t i o n o f t h e m e t a l l i c form o f c o b a l t on t h e - e x t e r n a l s u r f a c e o f z e o l i t e which c a u s e c a t a l y t i c a c t i v i t y i n t h e FT synthesis.The Co/X c a t a l y s t s a l w a y s exhibited h i g h a c t i v i t i e s t h a n the Co/Y c a t a l y s t s , no m a t t e r what p r e p a r a t i o n method was f o l l o w e d . S e v e r a l workers [ 6 - 7 1 h a v e observed a trend o f i n c r e a s i n g a c t i v i t y f o r FT r e a c t i o n w i t h i n c r e a s i n g a c i d i t y o f the s u p p o r t . I n c o n t r a s t , o u r r e s u l t s show t h e reverse t r e n d . The d i f f e r e n c e i n a c t i v i t i e s f o r I W and I E c a t a l y s t s c a n be a t t r i b u t e d t o the extent o f t h e r e d u c t i o n o f c o b a l t . B a e s on TPR r e s u l t s , it a p p e a r s t h a t c o b a l t i n I E c a t a l y s t s i s more electron-deficient t h a n on IW c a t a l y s t s . T h i s would r e s u l t i n the s u p p r e s s i o n o f hydrogen chemisorption under r e a c t i o n
1722 c o n d i t i o n s . However, s u p p r e s s i o n o f CO c h e m i s o r p t i o n i s l e s s s i g n i f i c a n t . Therefore, the a c t i v i t i e s o f I E c a t a l y s t s w o u l d be much l o w e r d u e t o t h e l o w s u r f a c e c o n c e n t r a t i o n o f h y d r o g e n on CO.
Table 1 C a t a l y t i c a c t i v i t i e s o f C o / z e o l i t e c a t a l y s t s (umol/g c a t . sec) preparation method
Co/NaX
Co/NaY
Iw
0.416
IE
0.087
0.141 0.056
Table 2 P r o d u c t s e l e c t i v i t i e s o f Co/NaX C a t a l y s t preparation
%CH4
Iw IE
%olefin
in
method 25.23 41.22
C2-C4
63.2 78.7
%2-butene i n c4 4 52
A s shown i n T a b l e 2 , the o l e f i n f r a c t i o n of C o / N a X ( I E ) i s h i g h e r t h a n t h a t of C o / N a X ( I w ) . C o b a l t c l u s t e r s l o c a t e d i n s i d e z e o l i e c a g e s showed l o w a c t i v i t y , h i g h s e l e c t i v i t i e s t o m e t h a n e and o l e f i n content, a l l o f which a r e c h a r a c t e r i s t i c s o f h i g h l y - d i s p e r s e d n o n - r e d u c e d m e t a l c a t a l y s t s . The l o w c o n c e n t r a t i o n o f s u r f a c e hydrogen w i t h the a c i d i t y o f the s u p p o r t d e c r e a s e s t h e h y d r o g e n a t i o n c a p a b i l i t y of t h e p r i m a r i l y f o r m e d o l e f i n s , r e s u l t i n g i n the i n c r e a s e o f o l e f i n f r a c t i o n . A s i g n i f i c a n t f r a c t i o n o f C4 was i n the f o r m o f 2b u t e n e f o r Co/NaX(IE) c a t a l y s t , due t o the b i f u n c t i o n a l properties of this catalyst.
4.
REFERENCES
1 . C.P. N i c o l a i d e s and M . S . S c u r r e l l , Studies i n Surf. Sci. and C a t a l . , 35 ( 1 9 8 5 ) 3 1 9 . 2 . W . J . Wang and Y.W. Chen, A p p l . C a t a l . , 77 ( 1 9 9 1 ) 2 1 . 3 . J.B. U y t t e r h o e v e n , A c t a P h y s . C h e m . , 24 ( 1 9 7 8 ) 5 3 . 4. D . K . Lee and S . K . Ihm, J . C a t a l . , 106 ( 1 9 8 7 ) 3 8 6 . 5. J . C . K i m and S . I . Woo, A p p l . C a t a l . , 3 9 ( 1 9 8 8 ) 1 0 7 . 6 . D.K. Lee and S . K . Ihm, A p p l . C a t a l . , 3 2 ( 1 9 8 7 ) 8 5 . 7 . I.R. L e i t h , J . C a t a l . , 91(1985) 2 8 3 .
Guczi, L et d. (Editors), New Frontiers in Catalysir Proceedingsof the loth International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary
6 1993 Elsevier Science Publishers B.V. All rights reserved
REACTIONS OF ACETONE,METHANOL AND AMMONIA ON ZSM-5 ZEOLITES J. Novakova, L. Bosacek Z. Dolejsek and L. Kubelkova J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Dolejskova 3,182 23 Prague 8, Czechoslovakia
Abstract The competition of 3 bases, acetone, methanol and ammonia, for reaction with the active acid centers of IUSM-5 zeolites was investigated and compared with that at the ZnHZSM-5 zeolite. These studies were carried out under low reactant-to-active center ratios using mass spectrometric detection of the gaseous products released during the decom ition of the surface species, which were identified usingPP% MAS spectroscopy. Possible mechanigrms of these interactions are d~scussed. Introduction The reactions of alcohols with ammonia and acetone were reported to yield pyridinium bases which are important substances for the synthesis of e.g. drugs and herbicides. HZSM-5 zeolite with a Si/A1 ratio equal to 90 was found to be the most efficient catalyst for these reactions. Several mechanisms have been suggested to explain the reaction route C11. In the present paper, the reactions of acetone, methanol and ammonia were studied f r o m the viewpoint of the primary reaction steps, i.e. the formation and decomposition of surface species. Most of the reactions were performed on HZSM-5 zeolite with low Si/A1 ratio; however, the role of the number and character of the active catalyst centers was also investigated. Experimental Catalysts. HZSM-5 zeolites with Si/A1=13.5. 15 and 75, and (for details see C2.31) were employed, The treatment of zeolites is given in detail in [2]. The samples were characterized using chemical analysis, IR spectra of the skeletal vibrations, sorption capacity measurements and TPD of ammgnia. Measur_e nts. The catalysts were pretreated at 450 C1 in vacuum 10 pF’a overnight and left to react with 0.2 mmol g of ace one atlroom temgareture. Then the temperature was increased (12 C min ) to 130 C (just below the release of the first gaseous products) under evacuation and vapours of ammonia or ZnHzSn-5
8
1724
methanol (0.4 m o l g ' ) or of the mixture of methanol p d ammonia (1:l) were added. After reacting for 20 min at 130 C, the reaction volume was reconnected to vacuum, and the temperature programmed deeorption with conversion of the adsorbed specie8 (TPDC) was reaumed. The reactions of methanol and the mixture of methanol with ammonia without preadsorbed acetone were also studied. The analyeis of the gaseous products released was carried out using an on-line Finnigan 400 quadrupole mass spectrometer. For MAS NMR measurements (a Brulrer 200 NMR spectrometer), the samplee were sealed off after the isothermal pause, in some cases after further heat treatment in vacuum (for details aee C2,31). Results and Discueeion
Surface apecies - M S AlMR measurements MAS NMR spectra of eurface species formed on gZSM-5 with S i / A 1 ratio=13.5 during the iSOtht=rmal pause at 130 C revealed that ammonia react6 with the protonated carbonyl group of the acetone eurface species to imino-like complexes 121. alone as well as in the presence of methanol. Imino-like species are releaeed in dehydrogenated form as acetonitrile. No reaction of methanol with the acetone surface species was found at this temperature. If the zeolite witg acetone adeorbed a2 room temperature and methanol added at 130 C was heated to 300 C with pumping off of the gaseous products, the character of the surface species changed (Fig.2). New signals at 250-130 and 58-9 ppm can be tentatively assigned to protonated higher unsaturated ketonic aurface compounds C31 which are probably created by methylation of the protonized carbonyl compounds. The latter complexes are characterized in the spectrum by sifplals at 223 and 28 ppm. The NMR spectrum of acetone at 300 C after the simultaneous addition of methanol+ammonia exhibited mainly signals assigned to the imino-like species (200 and 23 ppm, [2]), methanol (50 and 59 ppm [3]) and mono and dimethylmines ( 2 5 . 5 and 32.0 ppm, respectively C3], not shown in the Figure). Gaseous products - maaB spectrometrf c measurements The results of th8 TPDC measurements following the isothermal pause at 130 C are shown in Fig.1 (a-d) for the acetone species alone and acetone species with the addition of: (b) ammonia, (c) - metheno1 and (d) - the mixture of ammonia and methanol, respectively. It can be seen that methanol etrongly euppreeses the effects of the addition of only ammonia (d ve. b). This can be explained by the formation and decomposition of a new, methylated surface compound according to echemes (C) and (D) which can operate simultaneously with the routee shown in ( A ) and (B), respectively. The abbreviated acheme6 below the TPDC curves in Fig. 1 (A-D) a r e also based on the previouely published infrared spectra of surface species [4], eapecially those including the formation of surface acetone dimers.
-
1725 Figure 1. TPDC for acetone species (a), and acetone species with added: (b) ammonia, (c) methanol and (d) mixture of methanol and ammonia. Abbreviated reaction schemes A - D - ieobutene,.,. acetone, - _ - _propylene, --- m2J acetonitrile, R- butadiene, aromatics C7-9, allene, methanol, dimethylether; schemes (A-D):Cl protonized surface compounde; Ac (CH3)zC0, I I ) main route, + minor route
-
C
a
d
A
r'i ! i n
B
A
C
D
[Ac1
J.1 c3H4+
i
Hzo 1
CAcH%COCH3
\1
-%O
c(cH3)2c:cHcoCH31 IL
\
i-C4H8 + CCH3co01
I
I
fc ?further conversions)
*
according to the NMR spectra probably a more condensed C=C species
1726 Figure 2.
I3c MAS spectra of surface species from acetone with the addition of methanol after the heag treatment to 300 C (Details are given in ref. [ S ] . ) 300
200
100
ppm
0
The pyridinium bases are observed only in traces under our experimental conditions: nitrogen is incorporated into surface compounds (amines and imines, detected by NMR) which are dehydrogenated to nitriles if released. HZSM-5 with a low number of acid active centers (Si/A1=75) exhibits similar product composition as the active HZSM-5 (Si/Al 13.5 or 151, but the yield of the products is much lower and those from the reaction of protonated acetone (not dimers) predominate. ZnHZSM-5 behaved quite differently, yielding mainly methane, carbon oxides and benzene from the acetone surface species; this product composition was not substantially changed by the admission of ammonia and/or methanol. These products are similar as those obtained by acetone pyrolysis. Benzene is also corresponding to the formed, with an isotope composition condensation of 2 acetone molecules. Conclusions Ammonia. if added to the surface acetone species, change8 the reaction route reacting with the carbonyl group, while the addition of methanol results in the methylation of the surface compounds. The simultaneous addition of methanol and ammonia affects the formation and decomposition of the acetone surface species-
References (and references therein)
P.J. van der Gaag, R.J.O. Adriaansens, H. van Bekkum and P.C. van Geem, Studies in Surface Science and Catalysis 52. J. Klinowski and P.J.,Barrie (eds.), Amsterdam 1989, p - 283. 2 Z. DolejPiek, J. Nov&ov&, V. Bos&ek and L. Kubelkovd. 1
Zeolites 11 (1991) 244. 3 V. Boe&ek,. Z. Dolejgek, L. Kubelkova and J. Nov&ov&, submitted for publication. 4 L. KubelkovA, J. bjka, J. Novdkov~i,V. Bosk6ek. I. Jirka and P.Jird, Zeolites, Facts. Figures, Future, P . A Jacobs and R- A. van Santens (eds.), Elsevier, Ameterdm. 1989, p - 1203.
Guczi, L er al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary tB 1993 Elsevier Science Publishers B.V. All rights reserved
REGIOSELECTIVE HYDROGENATION USING PLATINUMSUPPORT ZEOLITE MODIFIED BY CVD-METHOD H. &no, M. Shibagaki, K Tbkahashi,I. Honah and H.Matsushita Life Science Research Laboratory, Japan Tobacco Inc., 6-2 Umegaoka, Midori-ku, Yokohama, Kanagawa 227, Japan
AbSb.act Platinum-support zeolites coupled with silicon alkoxides were prepared by the Chemical Vapor Deposition (CVD) method. With this catalyst system, it was demonstrated that the terminal carbon-carbon double bond is preferentially hydrogenated in the case of several unsaturated hydrocarbons. Further, it was elucidated that highly regioselective hydrogenation of alkadiene could also be achieved in this catalyst system. 1. INTRODUCTION
It has been difficult to achieve highly regioselective hydrogenation of unsaturated compounds over heterogeneous catalysts. In previous papers,l,2) we have reported the regioselective hydrogenation of unsaturated alcohols and acids proceeded when using a platinum-silica or platinum- alumina catalyst modified by a long carbon-chain layer. However, these catalyst systems do not have a wide range of applications. In this investigation, platinum-support zeolites coupled with a variety of silicon alkoxides were prepared by CVD-treatment. These catalysts were applied t o competitive hydrogenation of terminal and internal olefins. As a result, it was found that the terminal olefin is preferentially hydrogenated. In particular, the greatest selectivity of terminal olefin could be achieved by applying diphenyldiethoxysilane as the coupling reagent on platinum-zeolite. Further, the regioselective hydrogenation of alkadiene was carried out over the CVD-modified catalyst coupled with diphenyldiethoxysilane. 2. EXPERIMENTAL
2.1. hparation ofCvDModifiedCatalyst Coupled WithTetraethoxydane A platinum-support zeolite4A was prepared by the ion-exchange method. The calcination and CVD-treatment were performed in a glass-flow reactor in flowing nitrogen gas. The CVD treatment was carried out as follows; platinum-zeolite catalyst was treated at 2OOOC for 3h with tetraethoxysilane (1 mmol h-l), and at 3OOOC for l h with deionized water (hydrolysis treatment). This series of treatments was repeated 5 times and, finally, the catalyst was treated at 200°C for l h without hydrolysis.
1728 2.2. hparation of CVDModitIed CatalystCoupledwith OrgaudconAlkoxide
The calcination and CVD-treatment were performed in a glass-flow reactor in flowing nitrogen gas. The CVD treatment was carried out as follows; organosilicon alkoxide (1mmol h-1) was deposited on platinum-zeolite catalyst for 3h at 3OOOC and, finally, this modified catalyst was calcined at 3OOOC for lh.
2.3. Hydrogenationofa Mkture~ofTerminal and JntemalOlefins The mixture of the CVD-modified catalyst (150 mg), 1-nonene (0.05 mmol), trans-4-nonene (0.05 mmol), and hexane (5 cm3) was stirred at 25OC under a hydrogen atmosphere. The reactants and products were analyzed by GLC. 2.4. HydmgenationofAlkadiene6 The mixture of the CVD-modified catalyst (150 mg), 1,ll-Octadecadiene (0.1 mmol), and hexane (5 cm-1) was stirred a t 25°C under a hydrogen atmosphere. The reactant and products were analyzed by GLC and GC-MS.
3. RESULTSAND DISCUSSION 3.1. Competitive Hydrogenation of 1-Octeneand trum-4Octeneover Several CVD-ModifiedCatalystsCoupledwith Tetraethoxydlam The hydrogenation of a mixture of 1-octene and trans-4octene was carried out over several aCVD-modified catalysts. I t was elucidated that Rl/R2 (Rl: initial rate of 1-octene, R2: initial rate of 15 trans-4-octene) is dependent on the number of times of the CVDtreatment and the results are shown in Fig. 1. In the range of 0 to 5, R1m2 increases with the number of the CVD-treatments. However, R1/R2 does not increase for values greater than 5. It is considered that 65-cycle CVD treatment is most effective to a t t a i n selective hydrogenation. ' ' ' The hydrogenation of a mix0 2 4 6 8 ture of 1-octene and other octenes CVDnumber was carried out over platinumzeolite-4A coupled with tetraethoxysilane by 6-cycle CVD-treatment (5cCVD-modifiedcatalyst) and plati- Fig. 1. Relation of Rl./R2 and CVD num-zeolite-4A (control catalyst). number. As shown in Table 1, selective R1: Initial rate of 1-octene, R2: Initial hydrogenation was observed when rate of trans-4-octene. using 5cCVD-modified catalyst in Conditions: CVD-modified catalyst, 150 all cases. Compared with the mg; control catalyst, 30 mg; 1-odene, 0.1 hydrogenation over the control mmol; trans-4-octene, 0.1 mmol; hexane, catalyst, R m 2 Was found to be. 10 cm3; reaction temperature 25°C.
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Table 1. Initial Rate of Hydrogenation for Mixture of l-Octene and Other Octenea) Other octene R1/ R2 (Initial rate of l-octene / Initial rate of other octene) Pt / Zeolite-4A (Control catalyst) 5cCVD-modified catalyst truns-4-Octene 1.61 14.90 6.31 truns-3-Octene 1.95 2.94 1.61 truns-2-Octene cis-2-Octene 1.49 3.33 a>Conditions: Pt(l.2%>/zeolite-4A,30 mg; 5cCVD-modified catalyst, 150 mg; octene, 0.1 mmol; hexane, 10 cm3; reaction temperature, 25OC. dependent on a variety of the other octenes in hydrogenation over the 5cCVDmodified catalyst. The selective hydrogenation of the double bond near the terminal site is achieved over 5cCVD-modified catalyst. 3.2. Competitive Hydrogenation of l-Nonene and trans-4.Nonene over CVDModified Catalyst Coupled with orgaaoeiliconAlkoside It is considered that the selectivity and reactivity of hydrogenation are influenced by the coupling of the organosilicon reagent on platinum-zeolite, In this investigation, the hydrogenation of l-nonene and truns-4-nonene was performed over the CVD-modified catalyst coupled with several kinds of organosilicon alkoxides. These results are listed in Table 2. As shown in Table 2, R1/R2 (Rl: initial rate of l-nonene, R2: initial rate of truns-4-nonene) is about 2 by catalysis with the control catalyst. On the other hand, R1/R2 is much greater than 2 in every CVD-modified catalyst system. In particular, the R1/R2 value was estimated as 20.3 over the CVD-modified catalyst coupled with diphenyldiethoxysilane (Catalyst-1). Thus, selective hydrogenation could be performed for a mixture of l-nonene and truns-4nonene by the CVD-modified catalyst. In addition, it was also found that the selectivity in hydrogenation is highest when using diphenyldiethoxysilane as the coupling organosilicon reagent on platinum-zeolite.
Table 2. Dependence of Hydrogenation for Mixture of l-Nonene and truns4-Nonene on Variety of Organosilicon Alkoxidea) Coupling reagent Initial rate / mol g l h - 1 R1fR2 l-Nonene(R1) truns-4-Nonene(R2) 1.49 x 10-3 7.33 10-5 20.3 Diphenyldiethoxysilane 1.92 x 10-3 3.06 x 104 6.3 Dimethyldimethoxysilane Diisopropyldimethoxysilane 2.93 x 10-3 3.73 x 104 7.9 9.33 10-5 17.0 1.59 x 10-3 Diphenyldimethoxysilane None 7.28 103 3.36 10-3 2.2 a) Conditions: catalyst, 150 mg; l-nonene, 0.05 mmol; truns-4-nonene, 0.05 mmol; hexane, 5 cm3; reaction temperature, 25°C.
1730
3.3. Hydrogenation of Alkadiene over CVD-ModifiedCatalyst Coupled with Diphenyldiethoxydam (Catalyst-1) The hydrogenation of 1,ll-octadecadiene was investigated using Catalyst-1 and platinum-zeolite-4A (control catalyst). With usual heterogeneous catalysts, regioselective hydrogenation could not be achieved, since it is difficult to discriminate between the plural double bonds in the compound. As shown in Fig. 2a, the regioselectivity in the hydrogenation of 1,ll-octadecadiene was low over the control catalyst. On the other hand, the highly regioselective hydrogenation of 1, 11-octadecadiene was achieved over Catalyst-1 and this is shown in Fig. 2b. This tendency was also found in the hydrogenation of the other alkadienes. It is considered that steric hindrance occurs around the platinum particles in CVD-treatment, and the hydrogenation would be exclusively performed in these sites. Accordingly, the double bond which is more sterically hindered in the reactant cannot easily approach the active site because of the sBric regulation caused by organosilicon layer in the catalyst-1 system.
7
0
1
2 tlme I h
3
4
0
2
4 tlme I h
6
8
Fig. 2. Hydrogenation of 1,ll-Octadecadiene: a) Control catalyst. b) Catalyst -1. 0 CHZ-CH(CH~)~CH=CH(CH~)~CH~ CH3(CH2)9CHtCH(CH2)5CH3 A:CH3(CH2)16CH3 Conditions: Catalyst, 60 mg (control catalyst 1or 250 mg (catalyst -1); 1,ll-octadecadiene, 0.1 mmol; hexane, 5 cm3; reaction temp., 25 "C.
1 H. Kuno, K Takahashi, M. Shibagaki, and H. Matsushita, Bull. Chem. SOC. Jpn., 62,3779 (1989). 2 H. Kuno,K.Takahashi, M. Shibagaki, and H. Matsushita, Bull. Chem. SOC. Jpn., 63,3320 (1990).
Guni, L ef ul. (Editors), New Fronriers in Coralys$ Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights resewed
NICKEL, COBALT AND ZINC SUBSTITUTED SYNTHETlC MICAMONTMORILLONITE: SYNTHESIS, CHARACTERIZATION AND PROPENE OLIGOMERIZATION ACTIVITY J. C. Q. Fletcher, A. P. Vogel and C. T. O'Connor
Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Private Bag, Rondebosch 7700, South Africa
Abstract
Nickel, cobalt and zinc substituted synthetic mica-montmorillonite (SMM) catalysts of differin5 metal contents have been tested for propene oligomerization. Metal incorporation occurs in the octahedral layer yielding a progressively more trioctahedral clay. CoSMM and ZnSMM yield poor activity while NiSMM exhibits an oligomerization activity greater than that of the arent material. The formation of heavier oli omer products at lower conversion leve s is surprizing and is contrary to that expected o a simple series-parallelreaction mechanism.
P
f!
1. INTRODUCTION
Synthetic mica-montmorillonite (SMM) is a dioctahedral 2: 1 layer lattice aluminosilicate. Physical and chemical characterization [1,2j of the heat treated material indicates SMM to contain both swellable (montmorillonite-like) and nonswellable (mica-like) layers, and differin amounts of Bronsted and Lewis acidity depending on the activation temperature o the ammonium form. Previous studies have shown SMM [3] and NiSMM [4,S] to be active for pro ene oligomerization to distillate range products and, after reduction, NiSMM is active or hydroisomerization [6-101 and hydrocracking [6,7] reactions. This aper reports the synthesis, characterization and propene oligomerization activity of I? i, Co and Zn substituted SMM. Results indicate the activities of these materials to be NiSMM > CoSMM > ZnSMM.
B
P
2. EXPERIMENTAL
Metal substituted SMM syntheses were carried out according to the method of Black et al. [6] with a Si/AI ratio of 1.4 and gel pH of 8.5. Metal loading was varied by addition of differing amounts of the acetate salts of Ni, Co and Zn to the synthesis gel while maintaining a constant absolute amount of aluminium in the synthesis mixture. Catalyst characterization included structural features (XRD), metal content (AA), surface area and acidity (IR of adsorbed pyridine). The number of acid sites present on the catalyst was estimated using the integrated absorption coefficients of Hughes and White [lo]. All samples emfloyed for acidity analysis and oligomerization tests were calcined overnight at 500 C under vacuum and in d flowing air, respectively. Oligomerization reactions were conducted at 130 "C and MPa in a continuous flow fixed bed reactor. A wide range of space velocities (WHSV = 2 - 90 g feed per g
7
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catalyst per hour) were employed to observe changes in oligomer product selectivi with changing conversion level. In addition, a high feed s ace velocity (WHSV = 9 4 was employed in an attempt to reduce conversion leve s in order to discriminate between the initial activities of the catalysts with differing nickel loadings.
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3. RESULTS
The results of metal content, surface area and acidity determinations are summarized in Table 1. Ni substitution results in increasing surface area up to approximately 10 wt% Ni. Higher Ni contents yield lower surface areas. This. trend is generally observed for all three metals but with Co and Zn displaying lower threshold metal contents for loss of surface area. In all cases, increasing metal content results in decreasing Bronsted acidity. As a result of the synthesis method, viz. constant aluminium content of the gel irrespective of metal addition, it may be expected that increasing metal contents would result in increased aluminium substitution in the tetrahedral layer and hence increased total acidity as is observed for the NiSMM catalysts. However, this trend is not observed for CoSMM or ZnSMM. Table 1 Catalyst Metal Content, Surface Area and Acidity Cat.
Metal
A
C D
Ni Ni Ni Ni Ni
F G H I
co co
B
E
J K
co
Zn Zn Zn
%Metal (Gel)
%Metal (Solid)
Area m2/g
Bronste: mmol/g
Lewis mmol/g*
1.0 6.8 12.0 21.6 35.7 1.0 6.8 35.8 1.1 7.5 38.2
1.0 7.1 10.1 16.4 34.4 1.0 7.2 33.1 1.2 7.7 31.4
195 216 219 196 181 176 178 126 170 139 99
0.12 0.096 0.032 0.023 0.029 0.080 0.036 0.012 0.040 0.032 0.009
0.52 0.55 0.51 0.67 0.73 0.41 0.52 0.36 0.48 0.34 0.37
* Number of acid sites as determined by adsorbed pyridine X-ray diffract0 rams of the 060 reflections are presented in Figure 1 for different is reflection, a doublet, is representative of the relative amounts of nickel contents. dioctahedral (aluminium octahedral layer) and trioctahedral (nickel oct edral la er layers in the structure as indicated by the relative intensities of the 1.5 and 1. 2 peaks, respectively. Similar results were found for CoSMM and ZnSMM. For equivalent metal loadings the propene oligomerization activity decreases as Ni > Co > Zn. For nickel the initial activity increases with increasing metal content as shown in Table 2, whereas for cobalt and zinc increasing metal content results in decreasing oligomerization activity. The oligomer product is notably influenced by overall conversion level as shown in Table 2. In particular, it has been observed that the product spectrum becomes lighter at higher conversion levels. The results for 1 wt% NiSMM catalysts at different space velocities were obtained over the first 10 hours on stream, during which period the conversion levels remained constant.
?a
2
Pd
1733
-
-TT-
1.50
A
1.52
1
-
T I
A 1.50-A 1.52 A 1.50 A 1.52 A 1.50 A 1.52 A 1.50 A 1.52 A
Figure 1. 060 X-ray reflections for NiSMM catalysts of different metal loadings. Table 2 Activity and Product Spectrum of Nickel Catalysts Cat. W H S V g/g.hr A A A A B
c
D E
2 10 50 90 90 90 90 90
Con\ersion, wt% Dimer Trimer Tetra. Init. Const." wt% wt% wt%
4.5 6.6 10 18 22
74 37 5.7 3.6
8.4 7.8 4.9 2.8 7.3 6.8 17 26
27 33 30 24 29 32 37 36
24 25 29 32 28 28 25 19
Penta. wt%
20 17 19 23 19 18 12 9.9
Hexa. Hepta+ wt% wt% 12 10 11 12 10 9.4 5.8 5.4
8.6 7.2 6.1 6.2 6.6 5.8 2.9 3.2
* Initial activity, % propene conversion # Steady state conversion during period 2 - 10 hours on stream, % propene conversion 4. DISCUSSION
A previous stud 7 reported a general increase in surface area with increasing nickel content in is M. Although the increase was not a monotonic function of metal loading, the results of this study (Table 1) differ in that they indicate increasing surface area with metal content u to approximately 10 wt% nickel followed by a loss of surface area for hieher metal loa ings. From Figure 1 it can be seen that incorporation of increasing amounts of the doubly ionized nickel ions into the octahedral layer results in a progressive increase in the trioctahedral nature of the otherwise dioctahedral clay. Furthermore, in the absence of any information to the contrary, this XRD data suggests that Ni, Co, and Zn incorporate into the crystal lattice even to the hi h metal loadinBs corres onding to fully metal substituted structures. Relative octahe ral layer reflectivities of i, Co, and Zn SMM indicate that, for similar metal loadin , the packing density of this layer is reater for zinc than for cobalt than for nickel. T ius the ease with which the octahedral fayer can accommodate a trioctahedral metal configuration is Ni > Co > Zn.
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1734
As has been found for SMM [3], oligomerization results indicate that Bronsted acidity is not playin a dominant role in catalytic activity. Both CoSMM and ZnSMM exhibit decreasin fewis acidity for increasing metal content while the high nickel loadings in NiS& yield higher amounts of Lewis acidity and thus, in general terms, the trends in Lewis acid content of the catalysts are similar to the trends in oligomerization activity. However, the overall influence of total acidity and/or surface area on catalyst activity is not yet clear as trends, for increasing Ni content of NiSMM and Ni, Co, and Zn incorporation at similar levels, are not directly reconcilable in terms of the SMM structure proposed by Wright et al. [2]. In this regard, it is worth bearing in mind that the model structure was proposed on the basis of comparisons between infrared and powder X-ray data of SMM and other 2:l layer lattice minerals and that definitive single crystal studies have not been reported. Thus, it is possible that the results of this study may be indicative of certain shortcomings in the proposed structural model. From the data of Table 2 it can be seen that for the runs at differin space velocities the oligomer product spectrum becomes lighter as the conversion leve increases. This finding is contrary to that which would be expected for a simple series or series-parallel reaction scheme for oligomerization and has been observed for other solid acid catalysts in our laboratory. Comparison of the product spectra of the catalysts A - E at WHSV = 90 (Table 2) shows that the increasing conversion level associated with increasing nickel content also results in a lighter oligomer product. This effect is more pronounced in the case of metal loading as opposed to variation in space velocity and this is possibly due to some dimerising function of the nickel incorporated into the catalyst structure.
9
5. CONCLUSION
Increased metal loading in nickel, cobalt and zinc substituted SMM results in increasing trioctahedral metal substitution for dioctahedral aluminium indicating that the metal cation is incorporated into the crystal lattice. NiSMM is more active than SMM for propene oligomerization, however CoSMM and ZnSMM show only low activity €or low metal content and are inactive at higher metal loadin . Increasing nickel content in NiSMM results in increasine oligomerization activity. lfronsted acid sites do not play a role in propene oligomerization over the metal substituted SMM catalysts and oligomerization activity is, rather, a strong function of Lewis acidity. 6. REFERENCES M. Ko'ima, J.C.Q. Fletcher and C.T. O'Connor, Applied CataZysis, 28 (1986) 169. A.C. right, W.T. Granquist and J.V. Kennedy, Journal o catalysis, 23 (1972) 65. J.C.Q. Fletcher, M. Kojima and C.T. O'Connor, Applied atalysis, 28 (1986) 181. P.G. Bercik, K.J. Metzger and H.E. Swift, Ind. Eng. Chem. Prod. Res. Dev., 17 No. 3 (1978 214. C.T. 'Connor, L.L. Jacobs and M. Kojima, Applied Catalysis, E.R. Black, A.A. Montagana, and H.E. Swift, US Patent No. 3 H.E. Swift and E.R. Black, Ind. Eng. Chem. Prod. Res. Dev., 13 No. 2 (1974) 106. J.J.L. Heinerman, I.L.C. Freriks, J. Gaaf, G.T. Pott and J.G.F. Coolegem, Journal of Catalysis, 80 (1983) 145. K.H.W. Robschlager, - C.A. Emeis and R.A. van Santen, Journal of Catalysis, 86 (1984) 1. 10 T.R. Hughes and H.M White, J. Phys. Chem., 71 No. 7 (1967) 2192.
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Guni, L ef al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
ISOBUTAN&/l-BUTENEALKYLATION ON PENTASIL-TYPEZEOLITE CATALYSTS
J. Weitkampa and P. A. Jacobsb aInstituteof Chemical Technology I. University of Stuttgart, Pfaffenwaldring 55, D-W 7000 Stuttgart 80, Germany bCentrum voor Oppervlaktechemieen Katalyse, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92,3001 Heverlee, Belgium
Abstract Alkylation of isobutane with l-butene on HZSMJ and HZSM-11 zeolite catalysts was investigated. Com ared to large pore zeolites, higher. temperatures (> 100 "C) are required and the alky ate components show a significantly lower degree of branchin . These differences are mainly attributed to shape selectivity effects, the energetical y most favourable routes of carbocation chemistry being suppressed under the spatial constraints in pentasils.
f
B
1. INTRODUCTION
Research has previously been directed towards replacing sulfuric acid and hydrogen fluoride, the catalysts used in isobutane olefin alkylation, by solid acids, especially zeolites. On large pore zeolites, such as aujasites [l-71 or beta [8], alkylate is indeed formed hitidly, typically at temperatures below 100 "C. After a relatively short time on stream, however, more and more olefins form instead of the desired isoalkanes. It has been argued that this rapid deterioration of catalyst selectivity is due to the loss of hydrogen transfer activity on account of the build-up of a low temperature coke from the olefin [7] or from coke precursors produced through conjunct polymerization [9]. Medium ore zeolites are generally considered to be less prone to coking, however, HZSM-5 {as been found to be inactive in isobutane alkylation at temperatures around 100 "C 191. In this study, the otential of HZSM-5 and HZSM-11 as catalysts in isobutane alkylation was explore at temperatures above 100 "C.
d
B
2. EXPERIMENTAL SECTION
Continuous alkylation experiments were carried out with an isobutane/l-butene feed mixture (molar ratio 1O:l) and 2,Zdimethylbutane as internal standard in a flow-type apparatus with a fixed bed reactor. S ecial procedures described elsewhere [7,10] were employed for instantaneous or di ferential product sampling in glass ampoules combined with analysis by high resolution capillary GLC. The two end members of the pentasil-type family of zeolites were both synthesized with an Si/Al ratio of 60. The zeolite powders were pressed binder-free and ground to a particle size of 0.3 to 0.5 mm.
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In all experiments, the mass of catalyst after dryin at 330 "C in a flow of nitrogen was between 1.1 and 1.5 g. The flow rate of the liquid eed mixture at ambient temperature and the pressure were 7.5 cm3/h and 3.0 MPa, respective1 With HZSMS, the reaction temperature was varied between 100 and 300 "C and witi HZSM-11 between 200 and 300 "C, hence the feed hydrocarbons were either in the liquid or in the supercritical state.
f
3, RESULTS AND DISCUSSION
With HZSM-5 at 100 "C,the breakthrough of butenes occurred after 20 to 25 min, and after 40 min their concentration in the reactor effluent had almost reached the corresponding value of 1-butene in the feed. Equilibrium between the three n-butenes was established. Besides, only some small amounts of octenes (yield below 5 %) were released from the catal st. So far, our findings are, by and large, in agreement wth the observations of Chu an Chester 191. Both HZSM-5 and HZSM-11 in the fresh state do, however, form alkanes at elevated reaction temperatures, as shown in Figure 1. At 200 "C, their catalytic performance resembles to a certain extent the one described earlier for acid faujasites at much lower temperatures, e. g. 80 "C 6,7 : With time on stream, the yield of alkanes asses through a maximum. As their yie d rops, butenes and limited amounts of ole ins with other carbon numbers, mainly octenes, a pear. Table 1 provides more detaile information on the nature of the alkanes formed on HZSM-5 and HZSM-11 in the early stage of the experiments, when olefins are still absent. Under all reaction conditions and in all carbon number fractions, the monomethyl isomers together with the n-alkane predominate in the alkylate formed on
B
\d
F
2
HZSM-11
HZSM-5 0
0
c
leO Oe8
0.6
7 P
t1 1
e
>
0.4
Alkane8
c
.
Pd3d(3/2+5/2)/Fe2,(3/2) intensity ratio versus the emer ency angle e with respect to the su ace.
B
Figure 2. b) In depth Pd concentration (%).
The in-depth diminution of the Pd concentration is rapid, but different solutions for C1 C2 C3 values generate satisfying fits. Such a result is not surprising since, even at the lowest e emergency angles the analyzed depth is quite large due to the mean free paths values for the considered photoelectrons. Nevertheless, preliminary results using the LEIS technique show a quite rapid decrease of the Pd concentration against the sputterin time. Taking into account this last observation, the solution as presented in igure 2b seems the more realistic: an important Pd concentration in the first layer with a monotonous decrease of Pd amount in the two subsequent layers and a quick decrease to reach the bulk concentration.
f
3. CATALYTIC REACTIVITY The 1.3-butadiene hydrogenation has been chosen as test reaction, since Pd is considered as the best metal catalyst for that reaction. Thi reaction was performed at room temperature in static conditions, in a 73 cm reactor in which the sample can be transferred under UHV conditions after thermal equilibration and control of the surface cleanliness and composition by AES. The reactive mixture was prepared separately in a large volume cell and introduced through a valve. The composition of the reaction mixture was analyzed by mass spectrometry with periodic sampling through a Up to the complete leak valve. The course of a run is re conversion of the 1,3-butadiene, were formed. The avera activity inc uding four reaction is 1,6xlO between the values mol.cm- .s- . Such a and Pd(ll0) o detennigd [7,4x10 rnol.cm$.s-l] at 300 K and 35 torr for the hydrogen pressure (6).
9
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f3
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When considering the constant activity along the course of the reaction (figure 3) one can conclude that the reaction proceeds with a zero order with respect to the hydrocarbon pressure and that no deactivation occurs. This agrees with the very low carbon deposit checked after the course of the run by AES .
'I
0 X
X
*
1
x
X 0
*
0
m
Butane Butenea Butadlene
0
Figure 3. Partial pressure variations of 1,3-butadiene, butenes and butane versus time (Pdiene = 3 torn, Phydrogen = 35 torn, T = 300 K). 4. SUMMARY AND CONCLUSIONS
The surface of a dilute Fe-l%at.Pd alloy equilibrated at 870 K show a large Pd surface segregation. One can estimate that the outer layer contains more than 50 % at.Pd. Such a surface is as active as pure Pd for the 1.3-butadiene hydrogenation, and its selectivity towards butenes is near unity. In conclusion, convenient treatments make possible a very large surface enrichment into the active metal, even if it is very dilute in the matrix. In a convenient morphology, such an entirely metallic material could be considered as a new catalyst. It could present better mechanical properties and have a better ability for heat evacuation than conventional catalysts in shape of small metallic particles supported on insulators.
5. REFERENCES 1 F. Williams and D. Nason, Surface Science, 45 (1974) 377. 2 P. Wynblatt and R.C. Ku, Surfacescience, 65 (1977) 511. 3 A.R. Miedema, Z. Metallkde, 69 (1978) 455. 4 J.C. Bertolini in "Les techniques physiques d'etudes des catalyseurs", Ed. Technip, 1988 p. 433. 5 F.J.Knijers, Ph. D.Thesis, Leiden University, North Holland, 1978. 6 J. Massardier, J.C. Bertolini and A.J. Renouprez in "Proceedin s 9th Intern. Congr. on Catalysis", Calgary, 1988 (M.J. Philips and M. ernan Eds), Vol. 3. p. 1222.
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Guni, L et al. (Editors), New Frontiers In Catalysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 6 1993 Elsevier Science Publishers B.V.All right3 nserved
A STUDY OF SPREADING OF VANADIA ON TITANIA POLYMORPHS USING MECHANICAL MIXTURES
M.Sanatia, A. Anderssona and L. R. Wallenbergb Wepartment of Chemical Technology, University of Lund, Chemical Center, P.O.Box 124, 221 00 Lund, Sweden bDepartmentof Inorganic Chemistry 2, University of Lund, Chemical Center, P.O.Box 124, 221 00 Lund, Sweden
Abstract Mechanical mixtures of V2O5 and each of the three titania polymorphs Ti02(B), anatase and rutile were heated in air at 450OC. Spreading of vanadia was found to occur on all three polymorphs. After 36 hours of heating, coverage with interacting vanadia (the part that is insoluble in aq. NH3) on Ti02(B), anatase and rutile, respectively, was 54,48 and 38 % of a theoretical monolayer. Raman spectra confirmed spreading and interaction. H E M images revealed no formation of multilayers of vanadia on titania. Use of the monolayer catalysts for toluene ammoxidation indicated their activity for benzonitrile formation to vary with respect to support in the order Ti02(B) > anatase > rutile. 1. INTRODUCTION Vanadium oxide catalysts are used for partial oxidation and ammoxidation of alkyl aromatic compounds [ 11. Pure vanadium oxides usually have relatively low specific surface areas. Therefore, to increase their feasibility for industrial use, they are fixed to supports. The anatase form of Ti02 is a suitable support, both because of its high surface area, and because it enhances the catalytic performance of active vanadia species [2]. This effect is due to an active phase-support interaction, which in the case of mechanical mixtures of V2O5 and Ti02(anatase) can produce a spreading of vanadia on the support surface [3,4]. Ti02(rutile) has been considered a less suitable support due to the weakness of its interaction with the active phase, which leads to formation of crystalline V2O5 [3]. Recently, use of the newly discovered (B)-phase [5] of titania as a support for vanadium oxide was reported to yield good catalytic performance in both oxidation [6] and ammoxidation [7]. The present paper reports on the spreading of vanadium oxide on TiO2(B), anatase and rutile polymorphs when mechanical mixtures are employed. The catalytic performance of the monolayers produced by this method is also compared with that obtained through impregnation followed by the dissolution of superlayer vanadia in NH3(aq) and further calcination.
2. EXPERIMENTAL Ti02(B) and rutile were prepared as described elsewhere [8]. Anatase (p.A) was obtained from Merck. The specific surface areas of Ti02(B), anatase and rutile samples were 11.0, 9.6 and 7.6 m2/g, respectively. Mechanical mixtures of V2O5 and TiOz(B), anatase and rutile, containing 7 wt.% of V2O5, were heated at 450OC in a flow of moistened air with 2 vol.% H20. The vanadia not interacting with the support was then dissolved through treatment with NH3(aq). The amount and average valence of the interacting vanadia was determined by chemical analysis after dissolution in 4 M
&so4 171.
1766 Characterization of samples was performed using FT-Raman spectroscopy. The spectra were recorded on a Bruker IFS66FRA106 instrument equipped with a low power diode pumped NdYag laser DPY 301c (1064 nm) from ADLAS, and a liquid N2 cooled Ge diode detector. The laser power was 50 mW and the resolution 4 cm-l, 1000 scans being averaged. A glass tube 5 mm in diameter was used as sample holder (1800backscattering). Complementary characterization of the samples was made with high-resolution electron microscopy (HREM) using a JEM 4000EX instrument. The performance of the catalysts in the ammoxidation of toluene to benzonitrile was studied. For comparative purposes, other samples were prepared by impregnating the support with an oxalic acid solution of NH4V03, followed by drying and calcination [8]. The loading obtained corresponded to 5 theoretical layers. The non-interacting vanadia was then dissolved by treating the samples with NHg(aq) and recalcining them at 450OC for 3 hours.
3. RESULTS AND DISCUSSION Mechanical mixtures of V2O5 and each of the titania polymorphs were heated i n air at 45OOC. Figure 1 shows the spreading of interacting vanadia as a function of heating time. Spreading is ex ressed as coverage, i.e., as the percentage of a theoretical monolayer defined as 13.4 p o l o V per m2 of support surface area [7]. As can be clearly seen, spreading on the mile and TiO2(B) phases is rapid. Saturation is already obtained after 15 hours of heating, whereas in the case of anatase coverage continues increasing up to 36 hours. The coverage obtained after 36 hours in air is 54,48 and 38 9% for Ti02(B), anatase and rutile, respectively.
P
0
10
20 TIME (h)
30
40
Figure 1. Coverage of the support surface with interacting vanadia species as a function of time of heating of mechanical V2O5 and Ti02 mixtures in air at 4500C. Curves 0 , A and 0 represent data for TiOz(B), anatase and mile mixtures, respectively. HREM images of the mechanical mixture samples after heating, but before NH (aq)treatment, did not reveal formation of multilayer vanadia on any of the titania phases. it appears that the spreading of vanadia is limited to the formation of monolayer species. Raman spectra registered for the mechanical mixture samples after the dissolution of noninteracting vanadia in NH3(aq) and recalcination, together with spectra of the pure supports and catal sts prepared by impregnation followed by dissolution of excess vanadia, are shown in Fig. As can be seen, the intensity of support bands for all supports decreases as a result of the spreading of vanadia. TiOg band intensities are also less for the catalysts prepared by the impregnationldissolution technique than for the samples resulting from mechanical mixing. This is due to the vanadia content being higher in the impregnated samples (cf. Table 1). The decrease in intensity of Raman bands when vanadia is deposited indicates the Raman technique to
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$.
1757 be surface sensitive, a matter which the subtraction spectra in Fig. 2D likewise indicate. Perturbation of the Ti02 bands can be clearly observed for all supports, which points to an interaction between the vanadia species and the surfaces. For T i 0 (B), there are two vanadium oxide bands, at 970 and at 854 c m l (cf. spectrum (a)). These %andsmay possibly be from a tetrahedrally coordinated vanadia species [7,8]. The subtraction spectrum (b) for vanadia on anatase shows the presence of a broadband at 900-1000 cm-l, a feature which has been assigned to an octahedrally coordinated vanadia species [9]. No distinct bands of vanadia on rutile appeared (cf. spectrum (c)).
I 1100
900
1100
900
I
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700 500 Wavenumber (cm )
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I
I
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Wavenumber (cm-l)
I
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900 700 500 Wavenumber (cm )
300
1100
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700 500 Wavenumber (cm -1 )
I
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300
Figure 2. FT-Raman spectra of (A) Ti02(B), (B) anatase, and (C)rutile samples. (D)shows subtraction spectra. S: pure support. IP: sample prepared by impregnation/dissolution.MM: sample prepared by mechanical mixing followed by heat treatment in air at 450OC for 36 hours, dissolution of non-interacting vanadia, and recalcination at 4500C for 3 hours. (a), (b), and (c) are subtraction spectra (MM - S ) for Ti02(B), anatase and rutile samples, respectively. Table 1 shows the results of chemical analysis of the prepared catalysts, as well as their performance in the ammoxidation of toluene. There is a striking difference between the valences of the monolayer vanadia species on the three supports. Irrespective of preparation method, the valence of vanadia on anatase is mainly +5, whereas that of vanadia on TiOz(B) is 4.For rutile, the monolayer formed with mechanical mixtures consists of both V4+ and V5+ species, whereas the impregnation/dissolution method produces only V4+ species. For Ti02(B) and anatase samples, vanadia coverage is higher for impregnatioddissolution than for mechanical mixture samples. This result, which is in general agreement with the observed Raman intensity trends, is further supported by the activity data for benzonitrile formation. A comparison of the rutile samples shows that the sample obtained by the impregnatioddissolution method is more active and selective than the sample resulting from mechanical mixing, in spite of the higher degree of vanadia coverage in the latter sample. This finding may be due to the types of vanadia species found in the two samples differing, which the differences in average vanadia valences would thus reflect. It can be concluded that monolayers on Ti02(B) and anatase are more active
1758 than crystalline V2O5 and also are similar in their selectivity for benzonitrile formation. Mechanical mixing produces a monolayer on rutile with a catalytic performance similar to that of crystalline V2O5, the impregnatioddissolution technique yielding an overlayer that is more active and also more selectivefor nitrile formation. Table 1 Chemical analysis of monolayer vanadia and its performance in the ammoxidation of toluene Catalyst
Vanadia Coverage" Valence
Benzonitrileb Rate Selectivity % clmoUn2imin % M M C , Ti02(B) 54 4.2 1.9 75 MM, anata..e 48 4.9 1.5 76 MM, mile 38 4.5 1.1 78 IPd,TiO2(B) 100 4.0 4.0 79 IP, anatase 70 4.8 3.5 83 IP,rutilee 29 4.0 1.9 88 v20.5 1.2 82 aIn fraction of a monolayer defined as 13.4 pmoI of V per mz of support surface area. b370OC, the pressures of toluene, NH3 and 02 being 0.77,2.85 and 11.4 kPa, respectively. C M M : Catalysts prepared by heating of mechanical mixtures in air at 450OC for 36 hours followed by NH3(aq)-treatment. dIP: Catalysts obtained by impregnation and dissolution of excess vanadia in NHj(aq). eIn addition, this catalyst contained V4+ dissolved in rutile (57 pmol V/g of titania), which was dissolved not in 4 M H2S04 but in conc. H2S04 + HF. It has been reported earlier that spreading of vanadia occurs when mechanical mixtures of V 2 0 and anatase or rutile are heated, this resulting in the formation of interacting V4+ species [4,16]. The present results confirm that spreading on these phases occurs. However, in con-
trast with earlier findings, our results indicate mainly Vs+ species to be formed on the anatase surfaces, and both V4+ and V5+ species to be interacting with rutile. Unlike what has been reported for o-xylene oxidation [ 3 ] ,the vanadia overlayer on mile was found here to be stable in toluene ammoxidation. We have shown for the first time a spreading of vanadia on the newly discovered titania phase Ti02(B). For toluene ammoxidation the monolayer on Ti02(B) has catalytic properties similar to that on anatase. This suggests interesting perspectives for the use of Ti02(B) as a support both for other reactions and in conjunction with other oxide phases. 4. REFERENCES 1 A. Andersson and S.L.T. Andersson, in R.K. Grasselli and J.F. Brazdil (Eds.), Solid State Chemistry in Catalysis, ACS Symposium Series, Vol. 279, American Chemical Society, Washington, D.C., 1985, pp. 121-142. 2 I.E. Wachs, R.Y. Saleh, S.S. Chan and C.C. Chersich, Appl. Catal., 15 (1985) 339. 3 M. Gasior, J. Haber and T. Machej, Appl. Catal., 33 (1987) 1. 4 G . Centi, D. Pinelli and F. Trifirb, J. Mol. Catal., 59 (1990) 221. 5 R. Marchand, L. Brohan and M. Tournoux, Mater. Res. Bull., 15 (1980) 1129. 6 J. Papachryssanthou, E. Bordes, A. Vkjux and P. Courtine, Catal. Today, 1 (1987) 219. 7 M. Sanati and A. Andersson, J. Mol. Catal., 59 (1990) 233. 8 M. Sanati, L.R. Wallenberg, A. Andersson, S . Jansen and Y. Tu, J. Catal., 132 (1991) 128. 9 I.E. Wachs, J. Catal., 124 (1990) 570. 10 G . Centi, E. Giamello, D. Pinelli and F. Trifirb, J. Catal., 130 (1991) 220.
Guni, L. et al. (Editors). New Frontiers in Catalpis
Proceedings of the loth International Conon Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All righb resewed
STRUCTUREAND ACTIVITY OF COPPER CATALYSTS PREPARED FROM AMORPHOUS Cu-Zr AND Cu-Ti ALLOY PRECURSORS: A COMPARATIVE STUD+ A. MolnW, T. Katonaa, Cs. K o p a d and Z. HegedilsC
aDepartmentof Organic Chemistry, J6zsef Attila University and Center for Catalysis, Surface and Material Science, M m 3 r 8,6720 Szeged, Hungary b e p e l Metal Works,Budapest, Hungary cAGMI Institute for Material Testing and Quality Control, Budapest, Hungary
Abstract An amorphous Cu-Ti (60:40) r i b b o n e x h i b i t e d high c a t a l y t i c a c t i v i t y i n t h e dehydrogenation of 1- and 2-propanol a f t e r a c t i v a t i o n with HF. A l l o t h er a c t i v a t i o n s a p p l i e d s u c c e s s f u l l y t o Cu-Zr proved u n s u c c e s s f u l for Cu-Ti. T h i s is i n t e r p r e t e d i n terms of ESCA d a t a i n d i c a t i n g a t i t a n i u m enrichment on both s i d e s of t h e ribbon. A f t e r HF t r e a t m e n t , both Cu(0) and Cu(I1) were found on Cu-Zr by XPS, w h i l e Cu(I1) is t h e predominant s p e c i e s on C u - T i .
1. INTRODUCTION The a p p l i c a t i o n of amorphous metal a l l o s a s new c a t a l y t i c m a t e r i a l s h a s a t t r a c t e d c o n s i d e r a b l e a t t e n t i o n r e c e n t l y 1, 2 . Most of t h e s e s t u d i e s i n d i c a t e t h a t amorphous a l l o y s undergo s u b s t a n t i a changes d u r i n g a c t i v a t i o n or r e a c t i o n , t h i s being e s p e c i a l l y true f o r b i m e t a l l i c a l l o y s . Of such mater i a l s , metal-zirconium a l l o y s have been t h e s u b j e c t of numerous i n v e s t i g a t i o n s . S u r p r i s i n g l y , t h e i n v e s t i g a t i o n of m e t a l - t i t a n i u m a l l o y s , an appare n t l y obvious c h o i c e , has been n e g l e c t e d [3]. The aims of t h i s s t u d y are t o test a Cu-Ti sample a s a c a t a l y s t and t o compare its behaviour and s u r f a c e c h a r a c t e r i s t i c s with t h o s e of an amorphous Cu-Zr a l l o y p r e c u r s o r .
t l
2.
EXPERIMENTAL
The amorphous Cu-Zr (61:39) and CuTi (60:40) ribbon samples were p r e p a r ed by r a p i d quenching i n a i r with t h e s i n g l e r o l l t e c h n i q u e [4 . The copper s u r f a c e a r e a s of t h e a l l o y Sam les a f t e r p r e t r e a t m e n t and r e a c i o n were determined by N20 t i t r a t i o n [5! (363 K, p u l s e method > . The c a t a l y t i c tests (dehydrogenation of 1- and 2-propanol) were performed a t 573 K (flow r e a c t o r , atmospheric p r e s s u r e , s a t u r a t i o n t e c h n i q u e [5 1. HF a c t i v a t i o n was performed by keeping t h e samples i n 40% HF s o l u t i o n w i h c o n s t a n t s t i r r i n g X-ray p h o t o e l e c t r o n spectroscopy (XPS) was used t o s t u d y t h e chemi[5, c a l composition of t h e s u r f a c e and t h e s u r f a c e s t a t e of t h e copper.
k]
?
11.
%
P a r t 5 o f t h e series "Amorphous Alloy C a t a l y s i s " . For p a r t 4 , see r e f . 10.
1760 3. RESULTS AND DISCUSSION
In a recent study [3], the dehydrogenation of 2-propanol on Cu-Zr was found to bring about a continuous activation of the catalyst precursor. No such activation was observed on the Cu-Ti sample, which exhibited negligible activity even after 24 h on stream at 573 K or 673 K. Other activation pretreatments (hydrogen pretreatment, heat treatment, treatment with water vapour) applied successfully to Cu-Zr were also tested on Cu-Ti. Even a long, high-temperature treatment with hydrogen prior to reaction (24 h at 673 K ) did not result in any appreciable conversion of 2-propanol. The only method which led to high and stable activity was treatment with HF solution (Figure 1). Further differences exist in the behaviour of Cu-Zr and Cu-Ti, however. On Cu-Zr, increasing duration of treatment resulted in increasing copper surface area, the activity levelling off after a longer pretreatment time (Table 1). In contrast, the surface of the HF-treated Cu-Ti samples was immeasurably low and the activity showed a maximum as a function of the duration of treatment (Table 1, Figure 1).
Figure 1. Activity of an HF-treated Cu-Ti alloy sample in the dehydrogenation of 2-propanol. * 1 min treatment I 3 min treatment + 5 min treatment 0'
1
I
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1 P
PO
a4
I
T I M ON W R I A M (h)
Table 1 Surface area and rate data in dehydrogenation of 2-propanol of HF-treated Cu-Zr and Cu-Ti Cu-Zr time of copper slrface treatment area (min) I F
Cu-Ti
rate of dehydrogenationb
I
F
0.24 0.55
7.0 7.3
2.9 6.3
0.96
8.1
9.5
1.10 1.03
7.9
9.0 9.3
~~
1 2
0.14
3
0.40 0.38 0.41
4 5
0.30
am2g-1. bmol giit s-' x
8.0
copper rate of surfage dehydrogenation area I F
--CC
-c -C C
3.8
8.0 1.7 5.6 3.5
2.6 5.0 7.5 6.3
4.2
'Unmeasurably low. I= Initial. F= after 24 h.
10
38
a
0
5
10
15
28
25
38
BINDING ENERGY (eV>
BINDING ENERGY (eV>
Figure 2 . Depth p r o f i l e s of the two s i d e s of amorphous Cu60Ti40 i n the as-received s t a t e .
u ¶
I
I
wc
1
zs
u . Z ’
c 5 4
3 2 1
SPUTTERING TIME ( m i d SPUTTERING TIME (min) Figure 3. XPS spectra of the outer s i d e of amorphous Cu60Ti40 and Cu61Zr3g a l l o y s a f t e r treatment with HF. s o l i d l i n e : imnediately a f t e r treatment, dotted l i n e : a f t e r sputtering
-. 4
a,
2
1762 An i n s p e c t i o n o f t h e depth p r o f i l e s of t h e as-received Cu-Ti (Figure 2 ) i n d i c a t e s a t i t a n i u m enrichment on b o t h sides of the r i b b o n even a t l a r g e r depths. On Cu-Zr, such an enrichment i n the second metal was observed o n l y A t t h e same on t h e o u t e r s i d e of the r i b b o n and o n l y on the surface t i m e , the i n n e r side, displayed a copper enrichment. A l l a c t i v a t i o n pretreatments used i n the case of Cu-Zr transformed t h e as-received samples i n t o a c t i v e c a t a l y s t s b y b r i n g i n g about marked changes i n t h e surface morand a s u b s t a n t i a l copper enrichment on b o t h sides o f the r i b b o n A v i s i b l e s i g n o f these changes was the appearance o f a r e d copper a f t e r a c t i v a t i o n or r e a c t i o n . No such changes took p l a c e on Cu-Ti. Depth p r o f i l e s a f t e r d i f f e r e n t pretreatments d i d n o t i n d i c a t e any s i g n i f icant changes r e l a t i v e t o the as-received s t a t e . This p o i n t s t o the i n e f f i ciency of these pretreatments as concerns t h e i n d u c t i o n o f phase separation and copper segregation i n Cu-Ti necessary f o r c a t a l y t i c a c t i v i t y . With regard t o the surface c h a r a c t e r i s t i c s of the Cu-Ti a l l o y , i t i s n o t s u r p i s i n g t h a t only the most d r a s t i c method, HF d i s s o l u t i o n , could be used f o r a c t i v a t i o n . HF treatment transforms t h e a l l o y s i n t o Raney-type copper c a t a l y s t s by d i s s o l u t i o n o f the second metal, r e s u l t i n g i n a copper e n r i c h ment on b o t h sides. I n c o n t r a s t with former observations i n d i c a t i n g t h e Cu(I1) i s the c h a r a c t e r i s t i c species on t h e presence o f Cu(1) on Cu-Zr [7], outermost surface o f Cu-Ti, w h i l e both Cu(0) and Cu(I1) appear on Cu-Zr (Figure 3). Cu(0) predominates j u s t underneath t h e surface and a t l a r g e r depths. L i t e r a t u r e data on supported copper c a t a l y s t s i n d i c a t e the i n v o l v e ment o f both i o n i c and m e t a l l i c copper species i n the dehydrogenation of a l cohols The present data are i n agreement with these observations, b u t p o i n t t o d i f e r e n t surface compositions e x h i b i t i n g s i m i l a r c a t a l y t i c a c t i v i t y and s e l e c t i v i t y .
[El.
.
[lq.
4. REFERENCES
1. A. Baiker, Faraday Discuss. Chem. SOC., 87 (1989) 239. 2. A. Molngr, G.V. S m i t h and M. Bartbk, Advances i n C a t a l y s i s , 36 (1989) 329.
3 . H. Yamashita, T. Kaminade, M. Yoshikawa, T. Funabiki and S. Yoshida, C1 Mol. Chem., 1 (1986) 491. 4 . R.W. Cahn, Glasses and Amorphous M a t e r i a l s ( M a t e r i a l s Science and Technology, Vol. 9) ( J . Zrzycki, ed.), VCH, Weinheim, 1991, pp. 493-548. 5. A. Molnar, T. Katona, M. Bartbk and K. Varga, J . Mol. C a t a l . , 64 (1991) 41. 6. J.W. Evans, M.S. Wainwright, A.J. Bridgewater and D.J. Young, Appl. Catal., 7 (1983) 75. 7. H. Yamashita, M. Yoshikawa, T. Kaminade, T. F u n a b i k i and S. Yoshida, J. Chem. SOC., Faraday Trans. 1, 82 (1986) 707. 8. A. MolnAr, T. Katona, M. Bartok, I . V . Perczel, Z. Hegedds and Cs. Kopasz, Mater. Sci. Eng., A134 (1991) 1083. 9. T. Katona, Z. Hegedds, Cs. Kopasz, A. MolnAr and M. Bartok, Catal. L e t t . , 5 (1990) 361. 10. T. Katona, A . Molnar, I . V . Perczel, Cs. Kopasz and Z. Hegedk, Surf. I n t e r f a c e Anal., i n press. 11. J. Cunningham, G.H. Al-Sayyed, J.A. Cronin, J.L.G. F i e r r o , C. Healy, W. Hirschwald, M. I l y a s and J.P. Tobin, J. Catal., 102 (1986) 160.
GUni, L u al. (Editors), Ncw Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights nselved
PREPARATION OF W/AIZOS BY CHEMISORPTION OF WOCld TO SURFACE SATURATION
M.LindbladD and L. P. Lindford' aMicrochemistry Lid., P.O.Box 45,02151 Espoo, Finland bNeste Ltd., Oil Research, Technology Centre, P.O.Box 310,06101 Porvoo, Finland
Abstract The Atomic Layer Epitaxy (ALE) technique was originally developed for thin film production.' In this paper we present an extension of ALE to the production of atomically controlled surfaces on porous materials. For producing W/AI,O, WOCl, was chemisorbed from the vapor phase onto the support. In ALE, each surface reaction is allowed to proceed to saturation. The regulation of the metal content is based on different saturation levels. Saturation levels of tungsten between 3 and 12 wt-% were obtained by varying the alvmina preheatingtemperature and the reaction temperature between 150 and 400 C. W/AI,O, samples prepared by ALE and conventional impregnation were compared. It was observed that the tungsten species were evenly distributed throughout the porous alumina particles in both of the preparation methods used. In all samples tungsten was well dispersed, XRD amorphous and present as W(W) compounds. 1. INTRODUCTION
In the preparation of heterogeneous catalysts by conventional methods, the active species are added to the support by impregnation, precipitation or ion exchange. An alternative to these wet chemical methods is the introduction of the active species from the vapor phase. A characteristic feature of the Atomic Layer Epitaxy (ALE) technique, a specific type of chemical vapor method, is surface saturation by chemisorption.' In ALE the metal loading obtained is determined by the ability of the support to bind the reactant, not by the dosing of the reactant. A further benefit of ALE is the possibility to build a surface structure of desired composition by treating the support with a sequence of different reagents in situ. The benefits are likely to become more obvious when catalysts containing several species are prepared. In this study we have prepared W/AI,O, by ALE and determined the saturation level of tungsten on alumina as a function of alumina preheating temperature and reaction temperature. W/A1,03 samples prepared by ALE and conventional impregnation were compared.
1764
2. EXPERIMENTAL 2.1. Preparation of W/AI,O, by ALE The support material used was y-Al,O, (AKZO) with a surface area of 190-200 m2 and a particle size of 0.150.35 mm. The source material was tungsten(V1)oxychloride, WOCI, (Aldrich). In the flow type ALE reactor used, the WOCI, vapor in nitrogen carrier gas was passed through a stationary bed of alumina (6g). The W/AI,O, samples were prepared by a three step ALE procedure. First, the support material was preheated in order to desorb physically adsorbed water and to attain a certain degree of dehydroxylation. In the second step, alumina was treated with WOCI, vapor. The interaction was restricted to Chemisorption by the proper choice of reaction temperature. Surface saturation was assured by using an excess of reactant. Finally, the chemical nature of the tungsten species was changed by removing chloride with a water vapor treatment. The alumina was preheated in an air oven under atmospheric pressure for 16 hours and in the ALE reactor in nitrogen at a pressure of 100~500mbar for 3 hours at 200 or 4Oo0C.The reaction with WOCI,, vaporized at 200 C, was carried out at temperatures between 150 and 400% for 1-4 hours. The reaction was ended with nitrogen flushing at the reaction temeerature used. The treatments with water vapor were performed at 200, 300 and 400 C for 2 hours. 2.2. Preparation of W/AI,O, by impregnation to incipient wetness The alumina was impregnated with an aqueous solution containing 3 wt-% tungsten as ammonium tunpstate, (NH4)2W04(Aldrich). The catalyst precursors were dried for 4 hours at 120 C in an oxidizing atmosphere. The number of cycles of the impregnation/drying procedure determined the amount of tungsten on the sarnples. Once the number of cycles was completed, the precursors were dried at 200 C for 12 hours. Three catalysts containing 5, 9 and 13 wt-% tungsten were prepared.
2.3.Analytical methods used in the characterization Tungsten was determined by wavelength dispersive X-ray fluorescence (XRF) and chloride was determined by potentiometric titration. The surface area (BET), average pore radius, pore volume and pore size distribution were determined by means of nitrogen adsorption and condensation. The size of the tungsten species was studied by X-ray diffraction (XRD). The penetration of tungsten into the centre of the particles was investigated by X-ray mapping using SEM-EDS and by electron backscattering. Cleaved surfaces were exposed by cutting catalyst particles embedded in epoxy resin with a diamond knife. The surface composition was determined by X-ray photoelectron spectroscopy (XPS). Changes in the relative amount of OH groups on the surface of the alumina were studied by infrared spectroscopy (IR), using the diffuse reflectance technique. 3. RESULTS AND DISCUSSION 3.1. Reproducibility of the ALE technique The distribution of tungsten and chloride in the samples was studied by
1765
considering the diffusion of WOCI, both in the alumina bed and 'within the alumina particle pores (Sec. 3.4). The homogeneity of tungsten and chloride in the alumina bed was confirmed by taking samples from the surface and bottom of the bed. The reproducibility of the ALE technique to obtain a desired saturation level was determined for two different metal contents. Nine 5amples were prepared with the alumina preheatingand the reaction occuring at 200,C. Four samples were prepared with the corresponding temperatures equal to 400 C. Th! mean tungsten content andothe standard deviation were 10.8k0.3 wt-% at 200 C and 3.0+0.2 wt-% at 400 C. The standard deviations are well within the analytical accuracy.
3.2.Regulation of tungsten content in the ALE samples In the ALE technique the metal content is regulated by influencing the saturation level of the surface reaction in question. The saturation level depends on the chemical nature of the reactant and the amount of favourable adsorption sites on the support. As illustrated in Figure 1, saturation levels of tungsten between 3 and 12 wt-% were obtained by varying the aluomina preheating temperature and the reaction temperature between 150 and 400 C.
h
s u z
v I
c
P)
C
c 0 0 C
-p-
13 11
preh.:SOO'C
'
0
9 . 7 .
0 C
0
.-E C
5 ' 3.
UJ
i t
Ew
+
h
Q)
C
preh.:400'C
UJ
I
E
a
!-
150
250
350
450
Reaction temperature ('C)
Figure 1. The amount of tungsten (wt-%) bound to alumina as a function of the reaction temperature.
c
E I-
3900
3700
3500
3300
Wavenumbers (cm-1)
Figure 2. The OH region of the IRspectr? of (a) pure A1,03 preheated at 400 C and (b) after the addition of WOCI, at 2 0 0 ~ ~ .
The preheatingof alumina changes both the amount and type of adsorption sites. According to the IR-spectra (Figure 2), WOCI, binds to the OH groups of alumina. The reaction temperature can be used as a regulating means provided that there are different types of bonding sites available on the surface. The steep decrease in tungsten content with the reaction temperature reflects the heterogeneity of the surface sites of y-alumina. The type of binding can change both with the support pretreatment and the reaction conditions.
1766
3.3. Removal of chloride from the ALE samples The chloride can be bound to the tungsten surface species, but also directly to alumina. The chloride bound to alumina comes from the hydrogen chloride released when the tungsten compound react? with alumina. W/AI,O, samples prepFred at 200 C contained about 5 wt-% of chloride. Water vapor treatments at 400 C reduced the chloride content below the detection limit (5 wt%). We reported previously that VO(OC2Hg)3 reacted selectively with the OH groups on the surface of S O 2 and highly dispersed V2O5 were obtained by the thermal decomposition [l]. In the present study, it was attempted to elucidate the structure of V2O5 overlayers on SiO2 and the catalytic properties in relation to the structure.
8EXPERlMENTAL In the CVD method, VO(OC2Hg)3 was adsorbed on SiO2 (Aerosil200; 203 m2.g-1) through the selective reaction with the surface OH groups and then thermally decomposed at 723 K. This adsorption-decomposition cycle was repeated several times (denoted as V205/Si02(CVD)) [ll. Another series of V2Og/SiO2 was prepared by impregnation with a solution of NH4VO3 and oxalic acid (denoted as V205/Si02(Imp)). EXAFS and XANES were taken at BL-7C of Photon Factory in the National Laboratory for High Energy Physics (KEK).Raman spectra were recorded in dry N2 by NR-1800 (JASCO). Ethanol adsorption was measured with microbalance in a high vacuum system [l]. The reactions of ethanol (493 K),2-propanol(433 K),and l-propanol(513 K)were carried out in the presence of 0 2 using a clcsed circulation system.
3. REsuLls AND DISCUSSION 3.1. CatalyticReaction
Figure 1 shows the catalytic activities for reactions (oxidative dehydrogenations + dehydration) of ethanol and 2-propanol as a function of the loading amount of V2O5. In the reaction of ethanol, acetaldehyde was formed selectively (~95%)over all catalysts. On the other hand, dehydration mainly took place in the reaction of 2-propanol (the selectivity to propene was > 85 96). As can be seen in Figure 1, the activities of V2O@iO2(Imp) were very low a t low loading levels (< 4 wt%), while above 5 wt% of V2O5, the activities increased as the loading amount increased. It is noted that V2O@iO2(CVD) exhibited high activities even at 3.4 wt% of V2O5; the activities of V2O@iO2(CVD) were 40 - 60 times as high as those of V205/Si02(Imp)at the low loadings. In the reaction of l-propanol, V205/Si02(CVD)was more selective to propanal (55 %) than V206/Si02(Imp) (42 %I, where activity changes were similar to those in Figure 1. In order to elucidate the differencein the catalytic activity, adsorption property for ethanol was measured. The irreversible amounts of adsorbed ethanol are plotted against the loading amount of V2O5 in Figure 2, where the ratio of the number of ethanol molecule to the number of V atom in the catalysts is shown on the ordinate. It was found that at low loadings, ethanol was adsorbed on V atom at nearly 1: 1stoichiometry for both series of catalysts. This indicates that V2O5 of V205/Si02(Imp)have abilities to adsorb ethanol, they are inactive for the reactions. 1.0
---.on
0.8
-
0.6
-
0.4
-
'Cb
Po
V20, loading 1wt%
-
'0. *.
0.2 -
O >
.
.*.LO
4.1-
1
VZO5loading 1 wt%
Figure 1. Catalytic activities of Figure 2. Amount of C2H6OH adsorbed V2OdSiOz for reactions (oxidative as a function of V206 loading, dehydrogenation and dehydration) 0;V20&302(CVD), of alcohols as a function of V 0, 0 ;V20&3i02(Imp). loading. o ,O :V20dSiO2(CdD) 0 , I: V20&3i02(Imp>,0, M: Ehanol, 493 K, 0, U: Z-Pr~panol,433 K.
1769
3.2 Ramanspec-pyandEXAFS Raman spectroscopy revealed the difference in the structure of V2O5 overlayers of V205/Si02(Imp). As shown in Figure 3, the peaks due to the crystallites of V2O5,997 cm-1 (V=O), 147 cm-1 (the skeletal vibration of V2O5) and 288 - 701 cm-1, were clearly observed for 5.8 wt%V205/Si02(Imp) (spectrum (a))[2]. On the other hand, 2.9 wt%V205/Si02(Imp)gave a weak peak at 1042 cm-1 (V=O)with a broad peak at 450 cm-1 due to Si02 (spectrum (b)) This demonstrates that two different V species are present on V205/SiO2(1mp); crystallites (I) for V205/Si02(Imp) (> 5 wt%) and isolated V species (11)for that (< 4 wt%), as illustrated in Figure 4. The presence of the isolated V species on SiO2 has been indicated by photoluminescence studies [31. The absence of the peaks at 997 and 147 cm-l for V205/Si02(CVD)(Figure 3(c) and (d)) shows that V2O5 are not crystallites. It has been reported that V-0-V stretching vibration of polyvanadates appears at 400 - 600 cm-l[41. The broad peak at about 500 cm-1 observed for V205/Si02(CVD) is probably due to the V-OV bond. Thus as shown in Figure 4, thin films (111) can be proposed for the structure of V2O5 of V205/Si02(CVD). Figure 5 shows the magnitude of the Fourier transform of V K-edge EXAFS. 5.8 wt%V205/Si02(Imp) gave well-resolved peaks at 1.55 (V=O), 1.89 (V-0)and 3.13 8, (V-0-V),which were also observed for the bulk V2O5, supporting that V2O5 of V205/Si02(Imp) are present mainly as crystallites. On the other hand, 5.4 wt%V205/Si02(CVD)showed a peak at 1.68, with a weak V-V
1 # ) 0 1 a m 8 0 0 6 0 0 4 0 0 m
Raman shift / cm-' Figure 3. Raman spectra of V205/Si02.
(a) 5.8 wt%V20&X02(Imp),(b) 2.9 wt%V205/Si02(Imp), (c) 7.9 wt%V20@i02(CVD),(d) 3.4 wt%V205/Si02(CVD).
1770
A.
peak at 3.13 16.9 wt%V205/Si02(CVD)gave a spectrum similar to that of 5.4 wt%V205/SiO2(C;vD)153. The weak V-Vpeak for 5.4 - 16.9 wt%V205/Si02
(CVD) indicates that the V2O5 overlayers are not in the form of crystallites, i.e., the V2O5 overlayers are either isolated V species or thin films. If one assumes a layer spread in the (010)plane of a V2O5 crystal, the layer that covers fully the surface of Si02 corresponds to 22.4 wt%. That is, most of the V2Os overlayer of 16.9 wt%V205/Si02(CVD)can not be present as the isolated species (16.9 = 22.4 wt%). Thus most probable structure is the thin film (III), which is in agreement with the previous work by XPS and XRD [ll. The weakness of the VV peak in Figure 5(b) is possibly due to the non-uniformity of the interatomic distance in the V2O5 overlayers. The results of XANES supported the model in Figure 4. In conclusion, thin films were obtained bv the wesent CVD method. and these films were very catalytically active. As the isolated species was-inactive, the sites having V-0-Vbonds are indispensable as catalysts for the oxidative dehydrogenation and the dehydration of alcohols. ~~
............................................... --_____.. ........ ::::::SiO2 :{ 96%), but saturation was about 20% lower on microporous G-03, due to geometrical restrictions [11* DRS indicated that drying in air (Tmax. = 393 K) leaves a stable diamminepalladium complex on the surface of macroporous G-59 whereas a partial decomposition follows the drying of G-03 supported preparations. Ultradispersed Pd" is the final product on both catalyst types when Nz is the decomposing atmosphere, whereas either a mixture of Pd" [SiOzjz-PdZ+ (on G-03) or pure [(Si02)]2-Pd2+ (on G-59) are the final products when air is employed in the calcining step [2]. Gpon Hz reduction a wide range of metal dispersions could be achiekd (FE= 0.200.98). Two different regions were identified, each one showing its own dependence for the FE vs. the calcining temperature (T,) pattern: (i.) at T, < 423 K the FE depends upon the Pd loading and also upon the type of Pd complex species remaining on the support right before initiating the Hz reduction, whereas (ii.) for T, > 423 K the FE is a function of the metal loading and the support structure but not of the said complex species. Hz solubility diminishes with the mean particle diameter of the Pd crystallites, reaching an asymptotic minimum value for d < 20 A [3]. The reactivity (TOF) for methanol production was similar with both support types, but TOFCH, was ten-fold higher on Pd/iG-03 (i= Low or High loading). Traces of ethane were produced on the latter catalyst series only. Although the T O F C H ~ ~decreased H smoothly by increasing the FE (0.28-0.60) regardless of the type of support, it was dependent of the Pd loading for the microporous silica. (See Fig. 1 as regards the Pd/iG-59 samples).
+
P.30 MPo
-8
60 -
0
- CHJOH _ _ CH,
-
Lo 4 0 -
503 A
1' 02
A
493
I
I
1
04
06
08
,
I
04
02
FE
A
LG-59 HG-59
06
08
FE
B
Figure 1. Specific activity (TOFCH~OH) (A) and methanol selectivity (B) vs. final exposed metal fraction, corresponding to the iG-59 catalysts (Total flow rate: 175 ml STP/min). The + symbol corresponds to a preparation for which G-59 was only purified with organic solvents.
1804 Conversely, S C H ~ O Hwaa almost independent of FE on either silica, albeit a very strong function of the support structure (G-59: 80-95%; G-03: 20-65%). Whenever acid washing of the supports was excluded the S C H ~ O Hwas higher, indicating that support impurities (totalling ca. 0.2 % w/w) give rise to significant selectivity changes (Fig. 1B). TOFs were not substantially modified, though. ECH~O= H 65.7 f 5.4 kJ/mol for samples supported on G-59 and G-03 (low Pd loading). This value, which is in good agreement with those reported for Pd black and single crystals (43.5-77.0 kJ/mol [4-6]), waa somewhat lower (49.0 f 5.4 kJ/mol) for Pd/HG-03; For methane, the other reaction product, ECH, = 85.8 kJ/mol on G-59 and 84.2 kJ/mol on (3-03. These activity (TOF) vs. FE patterns can be rationalized by recalling that the lower the crystallite size the lower the relative number of Pd atoms in planes or faces (i.e,, the mitohedrical face-edge approach, but siding with the face’s reactivity [7,8]). In addition, the higher the FE the lowef the relative number of subsurface hydrogen, as quantified by the H,,/Pd, ratio, which is directly related with the Pd surfaces’ ability to incorporate hydrogen to chemisorbed CO [3]. It is believed that these high TOFCH, and S C H , on the Pd/G-O3 catalysts are mainly caused by the lower effective molar fraction of CO at the reaction loci, due to Knudsen diffusion into the micropores. Since the effective orders of reaction with respect to CO for methanol vs. methane syntheses are substantially different on Pd (ca. 0.15 resp. -0.40 [9])a smaller inhibition of the latter is to be expected when a microporous support is used. Summarizing, being these purified supports chemically equivalent, their dissimilar catalytic behaviour rests mostly on their different structures and not on promotional effects due to anionic and/or cationic impurities. 4.
REFERENCES A.L. Bonivardi and M.A. Baltanbl J. Catal., 125 (1990) 243. A.L. Bonivardi and M.A. Baltanb, Thermochim. Acta, in press. A.L. Bonivardi, Doctoral Dissertation, Univ. Nac. del Litoral, Argentina (1991). Yu.A. Ryndin, R.F.Hicks, A.T. Bell and Yu. Yermakov, J. Catal., 70 (1981) 287. Ch. Sudhakar and M.A. Vannice, J. Catal., 95 (1985) 227. P.J. Berlowitz and W. Goodman, J. Catal., 108 (1987) 364. M. Che and C.O. Bennett, Adv. Catal., 36 (1989) 55. R.F. Hicks and A. T. Bell, J. Catal., 90 (1984) 205. R.F. Hicks and A. T. Bell, J. Catal., 91 (1985) 104.
ACKNOWLEDGEMENTS Thanks are given to Universidad Nacional del Litoral and CONICET (Grant 3075200/88) of Argentina.
Guczi, L u al. (Editon), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights mewed
THE ADVANTAGEOUS USE OF MICROWAVE RADIATION IN THE PREPARATION OF SUPPORTED NICKEL CATALYSTS
G.Bond R. B. Moyes, S. D. Pollington and D. A. W a n School of Chemistry, University of Hull, HU6 7RX Hull, United Kingdom
Abstract Comparative results are presented for the drying, by conventional means and by the application of microwave radiation, of catalyst precursors which were prepared by the impregnation of two different types of alumina with aqueous nickel nitrate solutions. Microwave drying is shown to have three significant advantages over conventional thermal drying methods. These are (1) reduced drying times, (2) more uniform metal distributions, and (3) the production of stronger catalyst pellets. EXPERIMENTAL Catalyst Preparation Alumina supported nickel catalysts have been prepared by impregnation to give catalysts with 5 and 10% nickel loadings. Two types of alumina support have been used. The first was ALC 645, an a-alumina (with a total surface area of 0.475 m2 g-1 and a total pore volume of 0.445 cm3 g-l) in the form of a tubular extrudate of diameter 8 cm and broken into lengths of approximately 9 cm. The second support was ICI type 12-2 alumina, (a y-alumina with a total surface area of 90 m2 g-1 and a total pore volume 0.19 cm3 g-1); this was pressed into 5 mm diameter pellets. The catalyst precursors were prepared by wetting the pellets in aqueous solutions of nickel nitrate of the required concentration to provide the desired metal loading. In the case of the 10% Ni 12-2 catalyst, it proved necessary to wet the catalyst pellets in nickel nitrate solution, dry the catalyst, and then re-wet with further nickel nitrate to give the appropriate metal loading. Nickel distributions were measured using a Cambridge Instruments scanning electron microscope fitted with a LINK EDXA facility. Catalyst crushing strengths were determined on equipment manufactured by Instron. Catalyst drying The following nomenclature has been used to distinguish between the different methods of preparation. The microwave dried catalysts have a suffix M and the conventionally dried catalysts a suffix CD. For example, a catalyst which has been prepared from the support ALC645 with a 5% metal loading and has been microwave dried will be referred to as 5% ALC645M. The apparatus used to monitor the weight change of the impregnated support on drying consisted of an electronic balance (Sartorius) which was mounted on top of an oven
1806
(microwave or conventional). The balance and ovens were modified so that a glass fibre could be passed through the top of an oven and on this was suspended a glass cradle containing the catalyst sample. The microwave oven was also fitted with a 500 ml water load through which a continuous flow of water was maintained. The water load is required to prevent microwaves which were not absorbed being reflected back into the magnetron where they would cause damage. With these modifications, the microwave power available to the sample was estimated to be about 240 W. For the conventional drying experiments, the air temperature surrounding the sample was 110°C
RESULTS
Our results for the catalyst precursors clearly showed that reduced drying times are achieved in the microwave system, as compared to the conventional oven, for all the samples in this study. On average, drying times were reduced by a factor of between two and three. The dried pellets were sectioned and analysed using energy dispersive X-ray analysis (EDXA) in a scanning electron microscope (SEMI. Dot maps displaying the distribution of nickel within the pellets are shown in Figure 1. The nickel dot maps show that pellets produced by conventional drying exhibit a higher concentration of nickel at the outer edges, and less in the central region, compared to the corresponding microwave dried pellets. The 10% Ni 12-2 pellets have been used in a study to determine the effect of the different drying techniques on the strengths of the pellets. The results from experiments in which the crushing strength was measured are shown in Figure 2. Analysis of the results has confirmed that the microwave dried samples are significantly stronger than those dried conventionally.
a
C
d
Figure 1. Nickel concentration as a function of position within a pellet. (a) 10% 12-2CD, (b) 10% 12-2M, (c) 5% 645CD. (d) 5% 645M.
1807
Crushing Strength I A.U.
Microwave Drying Conventional Drying Alumina Blank
Pellet Number 10
Figure 2. Crushing strengths of conventionally dried and microwave dried pellets of 12-2 alumina-supported catalyst.
- Initial -22
40
-
moisture
F
20
8
30
Y
$
U
U u0 20 Q)
L
e
v)
10
. I
5 0
I-Thickness -I
I-Thickness -1
Conventional drying profile
Microwave drying profile
Figure 3. Moisture content profile within a pellet during conventional and microwave drying [1,31.
1808 DISCUSSION The observed resr-s can be explained by considering differences in the removal of water from the pellets during microwave drying and conventional drying. During conventional drying moisture is initially removed from the external surface of the pellet which produces the required moisture gradient for outward moisture flow [l] as shown in Figure 3A. This moisture gradient causes the redistribution of the nickel ions as water is drawn from the wet interior to the dry exterior. The difference in moisture content between the surface and the interior results in the dry surface being placed under tension which causes the pellets to be weakened [2]. When a wet pellet is exposed to microwave radiation the section of the pellet which has the highest moisture content absorbs the microwaves most strongly and therefore becomes the hottest part of the pellet. Hence the rate of evaporation is the greatest from the wettest region. This effect results in a very small moisture concentration gradient within the pellet [3], as is shown in Figure 3B; this process is known as "moisture levelling". As a consequence of the minimal moisture gradient, the nickel ions are not redistributed to the same extent during the microwave drying process as they are when samples are dried conventionally. This is confirmed by the nickel distribution profiles presented in Figure 2. This lack of difference in moisture content between the surface and interior of the pellet implies that tension within the pellet is reduced and the resulting dried pellets are stronger, as shown by the crushing strengths illustrated in Figure 3. In conclusion, the drying of impregnated catalyst precursors in a microwave field offers three significant advantages over conventional drying methods. Firstly, microwave drying is more rapid. Secondly, as a result of the phenomenon known as "moisture levelling", the distribution of metal on the support is much more uniform in microwave dried catalysts. Thirdly, microwave dried materials are physically stronger than conventionally dried materials We have also confirmed that microwave radiation is effective, and offers several advantages over classical thermal methods, in the calcination of supported metal salts, such as nitrates, to yield metal oxides. These results will be published shortly. REFERENCES 1 F.H. Norton, Elements of Ceramics, Addison-Wesley Publishing Co.. (1974) 114-125. 2 A.C. Metaxas and J.G.P. Binner, Advanced Ceramic Processing and Technology, Vol 1, Ed. J.G.P. Binner, Noyes Publications (1990). 3 C.K.Wei, H.T.Davis, E.A. davis and J. Gordon, A.1.Ch.E. Journal, 31 [5], (1985) 842-848.
Guczi,L.et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest,Hungary 0 1993 Elsevier SciencePublishers B.V.All rights reserved
NEW PREPARATION METHODS FOR ACTIVE SUPERFINE CATALYSTS BY SPRAY REACTION T. Uematsua, S. Shimma, T. Kameyam& and K. Fukudab
aDepartmentof Chemistry,Faculty of Engineering,Chiba University,Yayoi-Cho,Chiba 260, Japan bNational Research Laboratory of Chemistry,Tsukuba 305,Japan
Abstract
Spray reaction method was applied for the preparation of three types of fine particle catalyst. (1) Nio/M,O,.: by reduction of Ni-M-0 binary metal oxides to develop super fine Nio on metal oxide supports; (2) direct preparation of Pdo/ZrO, catalyst by spray reaction; (3) WC fine particles by carbidization of as-sprayed WO, beads. Catalytic properties were studied f o r the hydrogenation of CO, and cis-2-butene. 1.
INTRODUCTION
Spray reaction (SPR) is an excellent method t o prepare not only singlebut also multi-component fine particles, such as ferrites, high temperature super conductors. The catalytic activity (TOF) for the methanation of CO, on Nio/ZrO, was lo2 times higher than that by those by conventional impregnation method. The activation energy greately decreased, suggesting strong promoting effects of support extended on SPR particles 111. Present paper summarizes the results for the new trials to prepare three types of super fine catalyst particles where active sites are well dispersed on the surfaces. Effects of suppors and additives to modify the charactors of active sites are disccused and the applicability of SPR was clarified. 2
EXPERIMENTALS
Mixed solutions of Ni- and other metal-salts were sprayed and calcined in air as carrier. The product particles were fine spherical beads with dimension of sub-micron and collected by a glass filter. The Ni-0 bonds were reduced by H, treatment. Pdo/ZrO, was directly formed by SPR process. WC was prepared by carbidization of W03 in a CO flow which was obtained by SPR of (NH,),,W,,O,, solution. The physical properties of these particles were studied by the observations of XRD, SEM, FT-IR, XPS, TPR and CO adsorption. Catalytic properties were studied using a closed cycle reactor.
1810 3. RESULTS AND DISCIJSSION
5K I
3.1
Nin/M.O,. As for as-sprayed binary oxides, NiOiZrO, , NiO/ZnO and NiO/Al ?O, , the XRD powder pattern indicated that the minor component of NiO is almost solved in the matrixes of major M,O,. or present in amorphous state since only traces of NiO polycrystal were detected by XRD for samples with highest Ni content (15mol-%). Uhile in the cases of NiO/TiO, and NiO/Nb,O, systems, no trace of NiO diffraction was observed and the formation of complex oxides, NiTiO, and NiNb,O!. phases was confinned respectively (Fig.1 ) . The reducibility of Ni-0 bonds in NiO/MOx were evaluated by TPR measurement. As exemplified in Fig.2, higher shifts of reduction temperature from 589K of pure NiO, upto 873K(TiO,), 903 K(Nb,O,), 658K(Zn0) and 773K(MgO) suggest that Ni-0 bonds in the binary assplayed oxide particles a re much more stable in the texture of the supports. In contrast, the reduction temperatures of Nb-0 as well as Zn-0 and Zr-0 bonds, shifted from 1096K (pure sprayed Nb?O,) down to 970K. These shifts depended on Ni content very sensitively. The stabilization effects was more prominent in the as-sprayed particles than those prepared by conventional impregnation methods. The extent of reduction was influenced by the H,-pretreatment temperature for catalytic tests, though, the complete reduction of NiTiO, and TiNb,O,, took place at 873K. The particle dimension of superfine Nin on the support was determined by SEM photograph giving rize to mean diameters 5 50,. Fig.3 shows the result for CO adsorption over Nio/M,O, after H,-treatment at 673K. The adsorption amounts
7 Nb,o, 20
60
40
Fig.1 XRD Patterns
70
28 /deg
10K/min, H, 5(hPl/tain Hi0
TiO,
473
673
873
1073
Temp. /K
Fig. 2 TPR Profiles
Fig.3 Adsorption of W on Nio/li.O,
181 1
were the order: A1,0,> ZnO> MgO> Nb,Tab. 1 Hethanation Nio/H,O, (598K) 0 5 > TiO,. This can be explained in Red. 10-Rate' Snc TON' terms of the easiness of developing Nio H,O, T/K CH, CO Rx/Rc Total sites by the reduction of Ni-0 bonds. A1,0, 773 1.53 0.0 03 The catalytic activities for CO, 1043 14.7 0.0 03 27.9 methanation over Nio,/M,O,. are listed MgO 773 0.39 0.07 5.34 31.6 1023 8.7 0.0 03 in Tab.l.In the cases of Alto, and MgO, 673 0.56 0.59 0.96 3.1 as support, the activity increased 9.6 zd) 773 0.35 0.84 0.42 and 22 times, respectively, after high a:mol*min~'~mol-Ni~ b:ain- I Osi tetempereture reduction (HTR),while for ZnO, TiO, and Nb,O,, it recovered and enhanced higher activities after suca73 598 ceeding low temperature oxidation-reThe latter two duction (LTR; Fig.4). oxides are known so-called SMSI oxides. They can form one-phase complex oxide easily by spray reaction as described above. ~~
3.2
Effects of Alkali Metals
Effects of alkali metal addition on CO, methanation is shown in Fig.5. The additives were impregnated on NiO/M,Oy and H,-treated. Apparently, the addition of K and Na promoted the catalysis in high temperature region. However, they enhanced the CO formation in consistent with the results for Ni' /MgO with basic support. Most increase in the activation energy was obtained upto 96.8kT/mol for K-NiOlZrO, suggesting small modification of electronic state of the active sites.
Ni O/TiOz Ni o/NbLO, F i g . 4 SMSI Effects on Catal. Activity
1: K-Nio/ZrOL 2: Na-Nio/ZrQ2 3: Nio/ZrO, 4: Li-Nio/ZrO,
3
3.3 Direct Preparation o f Pdo/ZrOz
The particle size distribution of Pdo/Zr02 and PdO cluster dimension can be controled by changing the concentration of spray solution or Pd content. The avarage, PdD particle sizes developed directly on the support was estimated as about 2 0 i from the dispersion. Thus, most PdD clusters may be distributed on the oxide surfaces. Pdo/ZrO, fine particles enhanced very high activity for the hydrogenation
1
0 1.6
1.7
1.8
1031~K - I
F i g . 5 Effects of Alkali Metals on Catl. Activ. of Nio/ZrO,
1812
and isomerization of butenes even at 273K, as shown in Tab.2. The amount of adsorption and the reaction rate varied with the preration condition. The catalytic rate appears higher over the particles prepared from low consentration of spray solution. The TONS, however, are within ma same level suggesting only small modification of active site induced. 3.4
Tab.2 Catal. Activity Pdo/ZrQ? cis-C,H. Hvdronenation 1273K) . , Pd SP-SO1 DSP" R o b TON mol-% mollQ CO/Pd S-' 10 0.1 0.63 2.80 0.90 10 0.5 0.35 1.60 0.93 15 0.1 0.61 7.13 1.59 15 0.5 0.60 -4.03 0.90 a:dispersn; b:rate/mmol.nin- I Sg-
wc
WO, was succesively converted to WO, and W, and finally to pure WG as shown in Fig 6 . The catalytic activities obtained for the butene hydrogenation was unexpectedly low and t h e m 1 isomerization of olefin predominated, due to the small surface area and high reaction temeratures applied (Tab.3 ) . However, produceed butene isomers were hydrogenated gradually. For the porpose of developing precious metalsubstitution catalyst by W , some quenching processes of as-sprayed fine particles are necessarly, since they were too active and sensitive in ambient atomospheres. Direct cabidization by SPR might be a possible new method for the preparation as well as plazma reacion method[2]. References [ l ] T. Uematsu and S . Shimazu TOCAT 1 , ( 1 990).
[2] T. Kamevama, K. Fukuda, T. Uematsu et al., J. Japan SOC. Powder, Metal. 38 (1991) 109.
VI
8
Y
0
U
I
1
240ain.
I
F i g . 6 Carbidization of UO, (XRD) Reactn. : 1123X CO flow
Tab.3 Catal. Activity VC(SPR) ci s-C H, Hydrogenat ion- (725K) H, -Rd 1O2XRc/m0l.dn g-cat -K n-C:,Hlotr-C,H, I-C,H, 10PSHI 50.2 a73 1.85 67.7
..
.
Ouczi, L et al. (Editors), New Frontiers in Catalysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary
Q 1993 Elsevier Science Publishers B.V.All rights nsewed
PREPARATION OF SUPPORTED Cu-NI BIMETALLIC CATALYSTS BY ALKOXIDE METHOD WITH HIGH ACTIVITIES FOR HYDROGENATION OR DEHYDROGENATION
T.Sodesawa, S.Sam and F.Nozaki Department of Applied Chemistry, Faculty of Engineering, Chiba University, Chiba 260, Japan
Abstract Various Cu-Ni-SiOe c a t a l y s t s were prepared by alkoxide method. These c a t a l y s t s showed higher a c t i v i t i e s f o r hydrogenation of benzene o r dehydrogenation of cyclohexane t h a n those by impregnation method. The surface p r o p e r t i e s of c a t a l y s t s were characterized by means of various instruments such a s XPS, SEM, EPRA, X R D , a n d TPR.
1.INTRODUCTION Hore recently, i t has been reported t h a t the dehydrogenation of methanol t o nethyl formate occurs markedly over Cu-SiOe c a t a l y s t s prepared by alkoxide method through the bimolecular reaction of methanol Furthermore, i t was found t h a t t h e dynamic parameters such a s apparent a c t i v a t i o n energy, turnover frequency, and s e l e c t i v i t y d u r i n g the reaction depend on t h e s i z e of Cu p a r t i c l e s over t h e s u p p o r t Cll. In a previous paper "21, i t was suggested t h a t Cu-SiOe prepared by alkoxide method showed higher c a t a l y t i c a c t i v i t i e s f o r the reaction In t h i s paper, t h e than those prepared by the other methods. preparation of various C u - N i bimetallic c a t a l y s t s supported on SiOe was performed i n the f i r s t place, and then u s i n g these b i m e t a l l i c c a t a l y s t s , many r e a c t i o n s such a s decomposition of methanol, hydrogenation of benzene,. and dehydrogenation of cyclohexane 'were t e s t e d . In addition, the surface properties of supported C u - N i b i m e t a l l i c c a t a l y s t s prepared by alkolride method were examined u s i n g various instruments such a s XPS, SEM, EPRA, X R D , and TPR.
.
1814
2.EXPERIMENTAL
2 . 1 Preparation of c a t a l y s t s Various Cu-Ni-SiOe c a t a l y s t s by an alkoxide method were prepared a s shown i n F i g . 1, i n the same way according t o a previous paper C3l.
2.2 C a t a l y t i c re a c ti ons The c a t a l y t i c re a c t ions were c a r r i e d o u t u s i n g a conventional flow nethod a t atmospheric pressure. The r e a c t a n t s i n l i q u i d were fed i n t o a c a t a l y s t bed w i t h a microfeeder i n a flow of helium o r hydrogen a s a c a r r i e r gas. Prior t o t he re a c ti on, t h e reduction of the c a t a l y s t s The products were was performed a t 773K f o r 2h i n a flow of hydrogen. analyzed by g a s chromatography connected w i t h r e a c t o r .
2.3 Characterization of c a t a l y s t s The s u r f ac e prope rt ie s of c a t a l y s t s were examined u s i n g both samples a f t e r and before the reduction b y h y d r o g e n . The char acterization of c a t a l y s t s was c a r r i e d o u t u s i n g various instruments such a s XPS, SEM, EPMA, XRD, and TPR.
Stirring at 8O'C
for 3 h
Mixed Solution of
M e t a l Complexes Stirring at 8O'C for 3 h
Drying at
llO'C for ZL h
Viscous S t i r r i n g X Solution
Dry G e l
Fig.
at 5oo'C for2hcu0.NioSiOz 500'C for 2h
'
1
Catalyst
Cu-Ni-Si02 ' Catalyst
Preparation o f - Cu-Ni-SiOZ c a t a l y s t s
1815 3.RESULTS AND DISCUSSION 3.1 XPS s p ec t r a Figure 2 shows XPS s p e c t r a of Cu0-Ni0-Si02 c a t a l y s t by alkoxide method without the reduction by hydrogen. As shown i n F i g . 2, i t was found t h a t t h e peaks i n XPS spe c t ra were a t t r i b u t e d t o CUO, N i O , and SiOe on t h e s u rfa c e . Furthermore, from more d e t a i l s t u d i e s about CuO and N i O on t h e s u r f a c e , i t was confirmed t h a t r e l a t i v e l y l a r g e amount After the reduction by He, new peakes of of N i O exist on t he surfa c e . C u o and N i O appeared. I t is, t he re fore, concluded t h a t N i O near CuO on the s u r f a c e is e a s i l y reduced t o N i metal.
I
Fig. 2
XPS spe c t ra of Cu0-Ni0-SiOe (Cu:Ni:SiO2=5:5:90 by a lkoride method
3.2. TPR p r o f i l e s Figure 3 e x h i b i t s TPR (Temperature Programmed Reduction)
i n wt%)
p r o f i l e s of various CuO-NiO-SiOe c a t a l y s t s prepared by alkoxide method. In t h e case of CuO-SiOe c a t a l y s t , a sha rp peak near 250 'C appeared. I t is, th e r ef o r e , understood t h a t CuO is e a s i l y reduced t o m e t a l l i c Cu. On the other hand, NiO-Sioz c a t a l y s t showed very broad peaks i n t h e temperature From these r e s u l t s , i t can be s a i d t h a t N i O range from 250 t o 750 'C. is reduced e a s i l y l e s s than CuO. The other Cu0-NiO-SiOe c a t a l y s t s
1816
showed a peak near 350 ' C which may be a t t r i b u t e d t o N i O reduced e a s i l y i n the presence of Cu. The reduction behaviors of these c a t a l y s t s by alkoxide method were d i f f e r e n t from those by impregnation method. 3.3 Catalyt'ic a c t i v i t i e s The hydrogenation of benzene was c a r r i e d out u s i n g various Cu-Ni-SiOp catalysts. As shown i n P i g . 4 , the Cu-Ni-SiOe c a t a l y s t s prepared by alkoxide method showed higher a c t i v i t i e s than those b y impregnation method i n a l l the ranges of C u / ( C u + N i ) r a t i o .
-"
0; A ' l k o x i d e Method
4:Impregnstion M e t h o d
be
Cu:Ni
20
I-
c
" CI
1'5 m + 0
7
10 0
.-
c
n
2
w
5
c 0
0
I
0
20
40
10
80
100
Cu/(Cu+Ni 1 1%)
Fig. 4
1
1
200
400
603
Temperature I oc
Fig. 3
800
Comparison of c a t a l y t i c activities c a t a i ~ m t~ . i ~ h r : o . a o ~ P o s d r a t I '2.H.t H i l l 88, 0 0 ( m a 1. r r a L 1 e ) Reeotlon tmrnparrtur.tl00F Raduotion tsrnemrrrrr~1800F Raduotlon tlmallhr ( C u + N 1 ) : 5 i 0.- 1 0 : 9 0
TPR behaviors
4. REFERENCES
m,
1 T.Sodesawa, M.Nagacho, A.Onodera, and F.Nozaki, J.Catal., 460(1886). 2 T.Sodesawa, M.Horioka, S.Sato, and P.Nozaki, J.Chem.Soc. of Japan (Nippon Kagaku Kaishi), 74(1990). 3 T.Sodesawa. Shokubai(Cata1yst) , X,311(1990).
GUni, L d al. (Editom), New Frontiers in Catalysis Proccedigs of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest,Hungary 0 1993 Elsevier Science PublishersB.V. All righfs reserved
Dy-Cu A U O Y FILMS: CATALYTICACTIVITY, COMPOSITION, STRUCTURE
K N.Zhavoronkova and 0.A. Boeva Mendelejev Institute of Chemical Technology, Moscow 125190, Miusskaja sq. 9, Russia
ABSTRACT T h e model reaction H2+D2i22HD has been used for investigation of specific catalytic activity ( K ) of alloy D y - C u films, which were found to be much more a c t i v z than pure D y , copper being absolutely inactive. T h e influence of the method of preparation of films on K was investigated, three different method being used. Results obtained were compared with the data of X - r a y s electron spectroscopy investigation of alloy films composition. Conclusions were made that the active component of all condensed films is intermetalic compound DyCu, and K s d e process pends on the depth of interaction of Dy and Cu in the of film condensation and heating. This conclusion is supported by the results of electronographical phase analysis of f i l m s . 1.INTRODUCTlON In our previous investigations C 1 1 we showed that the allying of rare earth metals (REM) with copper results in the formation of very active catalysts for the model reaction H 2 + D F 2 2 M , REW having quite low activity and Cu being absolutely inactive. In searching the reason of this effect the d e pendence of specific catalytic activity ( K ) o f alloy films o n S the method of preparation and composition was investigated in system Dy-Cu, components being able to form a t least 4 intermetalic compounds (IMC): DyCu, DyCuz, DyCus and DyCu7C21. 2 . EXPER IHENTAL
Kinetics of model reaction was investigated in high vacuum in temperature range of glass apparatus described in C1.31 7 7 - 2 0 3 K a t pressure 0 , 5 Torr. K was calculated by the equation: c -cI, -2 -1 Ks- -. In molecules cm s , S.Z
c -cz
where Cg, C , C a r e fractions of HD present a t time z e r o , Z and at equiyibrium, N - the number of molecules in reaction chemisorption volume, S - the area of the film measured by H2 a t 7 7 K. T h e following intermetalic compounds were prepared by alloying and DyCus with the exin atmosphere of pure Ar : DycU, D-2 in which the atomic ratio C u / D y - G , let us call it cess of -, D-.. All samples for investigation were prepared as filmd 10condensed o n the inner walls of glass reactor in vacuum
-
1818 Torr. We used three methods of films preparation: 1 . A r a p i d ( 1 - 7 m i n ) a n d c o m p l e t e e v a p o r a t i o n o f 5-25 m g o f t h e s a m p l e o f IMC D W , D W P or D S u a f r o m W - s p i r a l . D u r i n g f i l m deposition reactor walls were not thermostated, but after this t h e a n n e a l i n g o f t h e f i l m a t 423-473 K w a s d o n e f o r 30-60 m i n . 2. S e p a r a t e a n d s u c c e s s i v e e v a p o r a t i o n f r o m t w o d i f f e r e n t s o u r ces Dy a n d Cu, r e a c t o r w a l l s d u r i n g d e p o s i t i o n b e i n g k e p t a t 77 K . A f t e r t h i s b i l a y e r f i l m s w e r e a n n e a l e d a t d i f f e r e n t temperatures for different periods of time. After each operation of annealing surface area and catalytic activity w e r e measurod. 3. S u c c e s s i v e e v a p o r a t i o n and c o n d e n s a t i o n of 5-6 f i l m s from o n e s a m p l e ( 2 0 - 5 0 m g ) of INC o n the reactor walls thermostated a t 77 K. C h e m i s o r p t i o n o f Hr w a s m e a s u r e d a t o n c e a f t e r finis h i n g c o n d e n s a t i o n ( n o m o r e 1-2 m i n ) to a v o i d the possibility of film contamination. After all catalytic measurements were f i n i s h e d h y d r o g e n w a s p u m p e d f r o m t h e f i l m a t 473 K and the next portion was evaporated f r o m the s a m e sample and condensed above the previous film. The degree of the evaporation Q was estimated as: Zi'Ii
-
-
rli
-
'
100%
CZi.Ii
-
where r . t i m e of t h e e v a p o r a t i o n o f i - p o r t i o n (min), 1 I. c u r r e n t p o w e r in t h e s o u r c e , A . M e l h o d o f X - r a y s e l e c t r o n s p e c t r o s c o p y (XBS) w a s u s e d for the e s t i m a t i o n of t h e s u r f a c e c o m p o s i t i o n , a n d being combined w i t h i o n Ar' e t c h i n g it a l l o w e d u s t o f i n d t h e p r o f i l e o f the c o n c e n t r a t i o n in t h e d e p t h o f t h e f i l m *etching curves" C41. MQ ? - r a d i a t i o n w a s u s e d , t h e power of X-rays source was 10 k .20 m A . F i l m s w e r e p r e p a r e d b y t h e f i r s t m e t h o d in t h e Xray e l e c t r o n s p e c t r o m e t e r A D E S - 4 0 0 . R a t e o f e t c h i n g w a s @bout 40 R / m i n . E l e c t r o g r a p h i c a l p h a s e a n a l y s i s w a s m a d e with films prepared by the first method o n electrograph ER-100 (accelerat i n g t e n s i o n 75 kV).
-
-
3. R E S U L T S A N D D I S C U S S I O N .
F i g u r e 1 s h o w s d e p e n d e n c e K o n T in A r r h e n i u s c o o r d i n a t e s f o r D y - f i l m s C31, ( c u r v e l ) ,a n 3 a l s o f o r t h e f i l m s , p r e p a r e d by t h e 1 - s t m e t h o d f r o m IHC DyCum ( c u r v e 2, p o i n t s 0 ), DyCur ( c u r v e 3) a n d DyCu ( c u r v e 4, p o i n t s ) . The type of the temperature dependence of K with abrupt break, has been discussed b e f o r e [ l l . It m a y b e E e e n t h a t K s o f alloys 1s m a c h h i g h e r . then % o f Dy, but it d e p e n d s o n c o m p o s i t i o n of e v a p o r a t e d IHC. M a x i m u m i n c r e a s e o f K L a t 77 K is 1 4 0 t i m e s . B u t we h a v e no r i g h t t o s a y t h a t t h i s K S b e l o n g s t o D $ h e b e c a u s e it is the whole composition of film but not the composition o f its surfa c e . In f i g u r e 1 r e s u l t s o f i n v e s t i g a t i o n o f f i l m 1 p r e p a r e d b y t h e s e c o n d m e t h o d ( p o i n t s o ) a r e s h o w n , w h e n Dy f i l m w a s d e p o sited first and i t s Ks a t 1 4 3 K w a s m e a s u r e d (point 0-1). C u - f i l m w a s d e p o s i t e d a b o v e Dy a t 77 K , a t o m i c r a t i o C u / D y - l .
-
1819
44
2
15
14
A
13
12
F i g u r e 1 . D e p e n d e n c e of Kb of f i l m s d e p o s i t e d f r o m Dy ( l ) , DyCu. ( 2 ) . DyCuo (3) a n d D e (4) on t e m p e r a t u r e ( d e t a i l e d in the text).
94L14
13
1
F i g u r e 2. of f i l m 1 p r e p a r e d the second method in d e p e n d e n c e o n t i m e o f s i n t e r i n g a t 473 K .
!;
M s a t once increased in 1 4 times, and points 2 and also 3 and 4 situated near the c u r v e 4. Figure 2 shows the change of K of this film in dependence of the time of its sintering at 493K : K s increased maximum in 90
times and riched the same values a s K of films prepared by the first me’thod from (fig 1). In tie second experiment (film 2 ) C u - f i l m was condensated a t first and D y - f i l m - a b o v e , atomic ratio C u l D y - 5 . A f t e r measuring K film were sintered a t 373, 473 and 73:3K during 40, 30 and 90 m f n , correspondently. In fig.1 of this points X with correspondent numbers show values of K film a t 77K. K s increased totally in 400 times.After ’sintering in wide temperature interthe opportunity arisad to measure K val (alloying with Cu decreases the’dissolution of H, in D y , which made difficult K S measuring on pure REN-film a t high T 11,31). In fig 1 i t is seen that points X of this film situated straight o n the curve 2 . Thus it is evidently that interaction of D y and Cu leads t o the formation of active composition, intaking place even after the interaction a t 77K. creasing of K In sintering the degree of interaction increases a n d K reaches maximum values independently on full bulk concentratio: of c o m ponents. Profiles of concentrattons of films prepared by the first method from the samples of IHC DyCum,D y C u t , DyCu and alloys DytCu and Dy&u were found by X E S . I t appeared that character of curves depends o n the initial composition of the sample. First portion condensed on the support a r e enriched with that component which prevails in the evaporating sample. Than its concentration decreased and a t the end of evaporation the a t o mic ratio on the surface of film (on the depth o r 40A) approaches 1 lndependently,,onthe initial composition of the sample t 4 l . A ~ it’s shown in fig 3a (curve l), in evaporation from I M C DyCu t h e concentration in all depth o f films remains constant the evaporating INC, and equal t o the initial composition of except for the surface itself which is enriched by D y in the result of its interaction with the gases of vacuum when the uas support Is not cooled. Such *oxidizing segregation- of D y observed o n the surface of all films prepared from alloys of all composition. B u t if support was cooled by liquid Nt segregation of REM did not take place a s i t was found in the case of alloys of Cu wlth Tm t4l. In the same f i g 3aresults of measuthe ring K s a t 77K of films prepared from 2 sample of D W by third method (curve 2 ) a r e shown. A s i t could be supposed KS was does not depends on the degree of sample evaporation. K+ measured in the lnterval of 77-203K and middle values situated straight on the curve 4 o f fig l.(points 4 ) U e see coincidence of results for films prepared by the flrst and third methods added by points 2 , 3 and 4 of film, prepared by the second m e thod.[t must be remembered that in the first and second methods (i.e.the the ’oxisizing segregationm of Dy can’t be avoided contamlnatlon of the surface), b u t in the third method all e f forts were made to avoid it. T h e conclusion m a y , be d o n e that the oxidized surface layers I s permeable for hydrogen molecul e s , and in the case of its formation t h e under surface layer
1821 o f m e t a l t a k e s p a r t in c a t a l y s i s . E x p e r i m e n t s w i t h f i l m s p r e p a red by t h e t h i r d m e t h o d a l s o c o n f i r m t h e f a c t t h a t t h e * a c t i v e c o m p o s i t i o n * (DyCu) f o r m a t i o n t a k e s p l a c e e v e n a t 77K. In f i g u r e 3 b t h e p r o f i l e o f c o n c e n t r a t i o n s ( c u r v e s 1) of at f i l m s p r e p a r e d f r o m DyCua a n d a l s o r e s u l t s o f m e a s u r i n g K S 77K o f f i l m s p r e p a r e d by t h e t h i r d m e t h o d f r o m 3 s a m p l e s of O w . ( c u r v e 2) a r e s h o w n . I t a l l o w s t o c o m p a r e ( i f n o t q u i t e e x a c t ) K s w i t h t h e cornposition o f t h e s u r f a c e . I t . is s e e n t h a t is q u i t e l o w , b u t in t h e r e g i o n o f h i g h c o n c e n t r a t i o n o f Cu K it i n c r e a s e s a b r u p t l y w h e n C u - c o n c e n t r a t i o n S d e c r e a s e s r e a c h i n g its m a x i m u m w h e n Cu/Dy is a b o u t 4 a n d r e m a i n s c o n s t a n t a t d e c r e a s i n g u p t o C u / D y - l . V a l u e s Ks o f t h e m o s t a c t i v e f i l m ( t h e t h i r d e v a p o r a t i o n o f t h e f i r s t s a m p l e ) m e a s u r e d in i n t e r v a l o f 77-203 K s i t u a t e d o n t h e c u r v e 2 o f f i g . 1 ( p o i n t s v ) . B u t K s o f films deposited during the last evaporation of samples turned or lower. o u t to b e m u c h l o w e r a p p r o a c h i n g to t h a t o f 0 These films adsorbed much more hydrogen than the previous ones, which s h o w s t h e p r e s e n c e of f r e e Dy.
F i g u r e 3. D e p e n d e n c e o f Cu/Dy o n t h e t i m e o f ion e t c h i n g of ( a ) a n d 0-a (b) ( curves 1 ), and f i l m s e v a p o r a t e d f r o m 0d e p e n d e n c e of K o n t h e d e g r e e of e v a p o r a t i o n of t h e s a m p l e for (a) f i l m s p r e p a r e d %y 3 r d m e t h o d ( c u r v e s 2 1 f r o m 0and DycUa (b). 4. C O N C L U S I O N S
the conclusion that T h e a n a l y s i s o f a l l d a t a lead u s t o t h e a c t i v e c o m p o s i t i o n in t h e a l l o y f i l m in 0 y - m s y s t e m is t h e intermetallc compound O m , and the value of K depends o n the d e g r e e o f i n t e r a c t i o n Cu+DyrDyCu, w h i c h in i t s t u r n d e p e n d s o n
1822 C u - c o n c e n t r a t i o n a n d t e m p e r a t u r e . W h e n f i l m f o r m r a t 77K (3-rd method) the first factor plays the main role. At the big excess o f C u t h e r e a c t i o n p r o c e e d s in t h e m a x i m u m d e g r e e d u r i n g the p r o c e s s of c o n d e n s a t i o n . But t h e e x c e s s o f Cu m a k e s KS d e c r e a s e because o f the "cluster effect",i.e.too large dilution of activ e c o m p o n e n t by i n a c t i v e o n e . W h e n 4>Cu/Dyrl t h e degree of i n t e r a c t i o n s e e m s to b e q u i t e l a r g e , t h e n e g a t i v e i n f l u e n c e of 'cluster e f f e c t " b e i n g a v o i d e d and K reached the maximum values. W h e n C u / D y - 1 t h e r e a c t i o n does'not proceed completely a t l o w T, f r e e c o m p o n e n t s b e i n g n o t i n t e r a c t e d a n d ' 4 3 0 K r e s u l t s in t h e s h i f t of the e q u i l i b r i u m to t h e r i g h t to t h e f o r m a t i o n of IHC D W and g i v e s a c c e l e r a t i o n of t h i s p r o c e s s . T h i s r e s u l t s in i n c r e a s i n g of KS w h i c h w e o b s e r v e d o n f i l m s p r e p a r e d by t h e f i r s t a n d s e c o n d m e t h o d s . For f i l m s d e p o s i t e d by t h e f i r s t m e t h o d f r o m DyClurr h a ving the maximum activity the both factors were important, the e x c e s s of Cu and s i n t e r i n g f o r 1 hour at 4 7 3 K a s w e l l , which r e s u l t e d in t h e c o m p l e t e i n t e r a c t i o n o f Dy w i t h formation. of D W . T h e e x a c t c o n f i r m a t i o n of:this w a s f o u n d in researching the s t r u c t u r e of f i l m s by e l e c t r o n d i f f r a c t i o n . In f i l m s d e p o sited f r o m DyCu., DyCu and Cu p h a s e s w e r e f o u n d , and in f i l m s d e p o s i t e d f r o m DyCuz-DyCu, (=u and D y - p h a s e s .
5.ACKNOULEDGEHENTS who Ye gratefully thanks prof. V.Nefedov and dr. A.Vinogradov helped us in X E S - s t u d y i n g , a n d d r . E . L a z a r e v a n d d r . A . G o r d e e v , who p e r f o r m e d e l e c t r o n o g r a p h i c a l r e s e a r c h .
6.REFERENCES B o e v a O.A., Proc. of 9th I n t e r n a t . C o n g r . o n C a t a l y s i s , C a l g a r y , 3, ( 1 9 8 8 1 , 1330. 2. S a v i t s k y E . H . , T e r e h o v a V.F., Metalovedeniye redkozernelnyh m e t a l o v , H., N a u k a , 1 9 7 5 , p . 1 0 1 . 3. Z h a v o r o n k o v a K . N . , B o e v a O . A . at a l l , K i n e t i c a i C a t a l i z , 23, 4, ( 1 3 8 2 ) , 8 8 1 . 4. N e f e d o v V . I . , V i n o g r a d o v A . R . , Z h a v o r o n k o v a K.N., B o e v a O . A . , P o v e r k h n o s t , 1 0 , (1987). 123. 1 . Z h a v o r o n k o v a K.N., P e s h k o v A . V . ,
Gud, L ef al. (Editors),New Frontiers in Catalysis proceedings of the 10th International Conon Catalysis, 19-24 July, 1992, Budapest,Hungary Q 1993 Elsevier science PublishersB.V.All righb mewed
ADSORPTION AND CATALYTIC PROPERTIES OF HIGHLY DISPERSE SILVER CATALYSTS
N,E. Bogdanchikovcl, D. A. Bulushev, Ya. D. Pankratiev m d A . V. Khasin Institute of Catalysis, Russian Academy of Sciences, Siberian Division, Pr. Akademika Lavrentieva 5, Novosibirsk 630090,Russia
Abstract This work is devoted to studies of the adsorption and catalytic properties of highly dispersed catalysts (with silver particle sizes < 1 0 nm).
1. INTRODUCTION The available literature data on the dependence of catalytic activity in ethylene oxidation on size of silver particles are often conflicting. The catalytic activity is reported to change over a surprisingly wide range of size variations, up to 200 nm. Moreover, different dependences have been found: the activity decreases with increasing the particle size [1,2] , pa2 [ Sajkowski sses through a maximum fi,4J or minimum 123 , or increases ] and Boudart having analyzed the literature on ethylene oxidation catalyzed by silver concluded that the reaction is structure insensitive 151 . Our group have shown [6] that for particles of more than 30 nm in size, the specific catalytic activity of silver in complete and partial oxidation of ethylene is approximately constant. In present work adsorption and catalytic properties of silver particles with the sizes typical for exhibition of structural sensitivity of the reactions (i.e. 4 10 nm) have been studied. The methods of preparation, structural and electron properties of the desc-ribed catalysts (Table 1) were reported in 1 7 3
.
.
Table 1. Characteristics of catalysts
No
C a t a l y s t
Silver content (wt.%)
Mean size of silver particles (nm) ~
1 2 3
4 5
Ag/Si02 Ag/Si02 Ag/Si02 Ag/Si02 Ag powder
10.0 4.1 2.4 2 .o 100
~
59
5.8 3.5 4.0
1700
1824
2. EXPERIMENTAL TECHNIQUES Adsorption of g a s e s , i n t e r a c t i o n of adsorbed oxygen w i t h hydrogen rind e t h y l e n e , i s o t o p e exchange and c a l o r i m e t r i c measurements were performed i n t h e s t a t i c vacuum i n s t a l l a t i o n of a volume t y p e , connected w i t h mass-spectrometer ( f o r s t u d y i n g r e a c t i o n s ) and m i c r o c a l o r i m e t e r ( f o r measuring a d s o r p t i o n h e a t s ) . The c a t a l y t i c e t h y l e n e o x i d a t i o n was s t u d i e d i n a r e c i r c u l a tion installation.
3. RESULTS AND DISCUSSION 3.1. Oxygen a d s o r p t i o n I n F i g . 1. one c a n see l g W/P as a f u n c t i o n of t h e s u r f a c e coverage by oxygen ( 8 ) f o r powder and f i n e l y d i s p e r s e d s i l v e r c a t a l y s t s . The r a t e of 02 a d s o r p t i o n on f i n e l y d i s p e r s e d silver samples i s c o n s i d e r a b l y lower,
Lg bl p& 15
1 2 3 O O A
om
a b c
13
11
as a f u n c t i o n of @ a t 473 K (sample F i g u r e 1. lg(W/Po2 ) molec/m2 Ag.s.Pa 3(3) and 4 ( 4 ) ) ; a-d - r e s u l t s of some c o n s e c u t i v e e x p e r i No 5 ( l ) , 2 ( 2 ) , ments; e - e x p e r i m e n t a l r e s u l t s i n flow c o n d i t i o n s . F i g u r e 2. Adsorption h e a t of 02 a s a f u n c t i o n of 8 ( s a m p l e s No 2 ( a ) , 3 O2 ( l ) , a t H i n t e r a c t i o n (b) and 5 ( c ) ) i n t h e experiments on a d s o r p t i o n w i t h adsorbed oxygen ( 2 ) , a t t h e repeated a d s o r p t i o n of O2 ( 3 3 .
1826 t h a n . t h a t g n n i l v e r powder o v e r t h e wide c o v e r a g e r a n g e . Thus on s a m p l e s 24 t h e r a t e is l o w e r a t least by a f a c t o r of 30 t h a n on powder. The rate o f oxygen d e s o r p t i o n from t h e s i l v e r s u r f a c e of f i n e l y d i s p e r s e d s a m p l e s is a l s o l o w e r by more t h a n a n o r d e r o f magnitude a s compared t o t h e s i l v e r powder surface. However, calorimetric measurements showed t h a t t h e r e is no s h a r p d i f f c r e n c e s i n t h e c h a r a c t e r o f dependences o f oxygen a d s o r p t i o n h e a t on 8 f o r f i n e l y d i s p e r s e d s a m p l e s and s i l v e r powder ( F i g . 2 ) . 3.2. C02 a d s o r p t i o n A t 423 K , Pco2 = 1.9-29.0 Pa a n d 0.15 < 6 < 0 . 4 t h e v a l u e s o f t h e s u r f a c e c o v e r a g e by ads.orbed c a r b o n d i o x i d e ( 8 on sample 2 d o n o t e x c e e d 0.002. T h e s e c o v e r a g e s are s u b s t a n t i a l l y f&’a f a c t o r o f 20-50) lower t h a n 6 ~ 0o b t a i n e d u n d e r t h e same c o n d i t i o n s on s i l v e r powder. It is known t h a t 2 8 co is e q u a l t o 0.31 and 0,38 on s i l v e r powder a t room tempera665 and 1795 P a , r e s p e c t i v e l y [S] Under t u r e when 2 @ = 0.69 a n d Pco2 t h e same c o n d i t i o n s @ co i s ca. 0.02 on a f i n e l y d i s p e r s e d s i l v e r sampl e , i . e . i s by more t h a n Zn o r d e r of magnitude l o w e r t h a n COP a d s o r p t i o n on t h e s u r f a c e o f a s i l v e r powder.
*
3.3. Hydrogen i n t e r a c t i o n w i t h a d s o r b e d oxygen Reaction 0 + H2 = H20 on a s i l v e r f i l m i n c l u d e s 2 s t e p s ads
(1)
2 Oads
(2)
2 OHads
+ H2 + H2
.
191 :
= 2 OHads = 2 H20
At room t e m p e r a t u r e o n b o t h s i l v e r f i l m a n d f i n e l y d i s p e r s e d s i l v e r samp l e 3 t h e i n i t i a l rate o f t h e 1st s t e p i s h i g h e r by 2-3 o r d e r s t h a n t h a t o f A t t h e same time, t h e rates o f b o t h t h e 1st and t h e 2nd t h e 2nd o n e [9] s t e p s o v e r f i n e l y d i s p e r s e d s a m p l e s are l o w e r by more t h a n a n o r d e r o f mag n i t u d e as compared w i t h t h o s e on t h e f i l m s .
.
3 . 4 . I n t e r a c t i o n of a d s o r b e d oxygen with e t h y l e n e It is known t h a t a t T & 2 9 3 K e t h y l e n e is n o t a d s o r b e d on c l e a n s u r face o f t h e s i l v e r m a s s i v e s a m p l e s . The r e s u l t s o f o u r e x p e r i m e n t s show t h a t e t h y l e n e a d s o r p t i o n d o e s n o t p r o c e e d on SiO s u p p o r t a t room temper a t u r e . However, e t h y l e n e a d s o r p t i o n p r o c e e d s a t 2 room t e m p e r a t u r e on f i n e l y d i s p e r s e d s i l v e r sample 2 b e i n g s u b j e c t e d t o t h e s t a n d a r d hydrogen trea t m e n t . At.Pc2H4 =vu 190 Pa t h e s u r f a c e c o v e r a g e by a d s o r b e d e t h y l e n e is M 0.1 a n d t h e f u r t h e r i n c r e a s e o f e t h y l e n e p r e s s u r e res u l t s i n increase of 8 c 2H4 ’ E t h y l e n e a d s o r p t i o n e x p r e s s e d by t h e d i f f e r e n c e o f t h e a d s o r p t i o n v a l u es on t h e o x i d i z e d ( 8 = 0.3-0.7) a n d r e d u c e d s u r f a c e s , r e a c h e s s a t u r a t i o n a t low p r e s s u r e s a n d , t h e r e f o r e , e t h y l e n e a d s o r p t i o n a t h i g h e r p r e s s u r e s ( P c ~ H) ~ p r o c e e d s on the s i t e s o f t h e r e d u c e d s u r f a c e o f t h e f i n e l y d i s p e r sed s i l v e r samples. The i n i t i a l r a t e of e t h y l e n e i n t e r a c t i o n a t 4 2 3 a n d 473 K w i t h t h e p r e l i m i n a r y a d s o r b e d oxygen a t v a r i o u s 8 on s i l v e r powder i s 1-2 o r d e r s of magnitude h i g h e r t h a n on t h e f i n e l y d i s p e r s e d s a m p l e s . E t h y l e n e o x i d e f o r m a t i o n on f i n e l y d i s p e r s e d s a m p l e s was n o t o b s e r v e d and t h e s e l e c t 5 i i t y o f e t h y l e n e o x i d e f o r m a t i o n on silver powder r e a c h e s 15%u n d e r t h e same c o n d i t i o n s ( 6 = 0 . 7 ) .
1826
3.5. Isotope exchange of ethylene and hydrogen On reduced finely dispersed samples 2 and 3 at 423 K the isotope exchange was found to proceed in the systems: C2D4-C2H4, C2D4-H2, C D -the same 2 4 reduced surface. After both reductive and oxidative treatments with the same mixture composition at 423-473 K on silver powder and also on the support, the isotope exchange was not observed. Probably, on finely dispersed samples reduced by hydrogen, in contrast to large crystalline silver, the binding of some quantity of hydrogen occurs ( 8 , < 0.01). It was found f o r sample 2 that during the C2D4-C2H4 exchange on the oxidized surface (
8
=
0.26 and 0.62) the initial rate of the C2D3H formation is approximately 3 times as high than on the reduced surface. This may probably be explained by higher concentration of adsorbed ethylene on the oxidized surface as compared with the reduced one.
3.6. Catalytic reaction of ethylene oxidation The rates of steady-state reaction of ethylene oxidation were measured at
448 and 473 K and partial pressures of ethylene 1.5-2.0 kPa and oxygen 6.1 kPa in nitrogen or helium. The reaction rate decreases with silver dispersity increase. On finely dispersed samples 3 and 4 it is lower than on silver powder by more than 2 orders. Low activity of small silver particles on S-i02 as compared to that of silver powder (studied by us) was reported in 11. 3,101 . On finely dispersed samples the selectivity of ethylene oxide formation increases reaching the steady-state value close to 8-20%. On silver powder the selectivity in the steady-state condition is 30-40%. In steady-state regime the reaction rate is lower and the selectivity is higher than these parameters in unsteady-state (step-wise) regime. This is probably determined by accumulation on the silver surface during the catalytic process of the carbon-containing particles exerting inhibitory and modify- ' ing effect. Based on the results obtained and taking into account XPS and Auger spectroscopic data [7] it i s suggested that effects of silver dispersion on adsorption and catalytic properties result from changes in both local surface and collective electron properties of silver. The former properties mainly determine the activity rise towards ethylene chemisorption and hydrogen exchange upon changing silver states from macro- to microcrystalline, while the latter properties determine the total decrease in activity with respect to reactions with oxygen participation.
REFERENCES 1. J. Wu and P. Harriott, J. Catal. 39 (1975) 395. 2. X.E. Verikios, F.P. Stein and R.W. Coughlin, J . Catal. 66 (1980) 368. 3. M. Jarjoui, P.C. Gravelle and S . J . Teichner, J. Chim. Phys. Physiochim. Biol. 75 (1978) 1069. 4. S. Cheng and A. Clearfield, J . Catal. 94 (1985) 455. 5. D.J. Sajkowski and M. Boudart, Catal. Rev.-Sci. Engin., 29 (1978) 325. 6. S . N . Filimonova, S . N . Goncharova and A.V. Khasin, React. Kinet. Catal. Lett. 34 (1987) 303. 7. N . E . Bogdanchikova, A.I. Boronin, V.I. Bukhtiyarov et al., Kinet. Katal. 31 (1990) 145. 8. A.W. Czanderna, J. Colloid. Interface Sci. 22 (1966) 482. 9. A.V. Khasin and G.K. Boreskov, Kinet. Katal. 10 (1969) 613. 10. J.K. Lee, X . E . Verykios and R. Pitchai, Appl. Catal. 50 (1989) 171. '
Guni, L ef al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
NEW INSIGHT INTO THE CHANGING CATALYSTPOLYMER MORPHOLOGY DURING OLEFIN POLYMERIZATION: THE APPLICATION OF TOMOGRAPHY W. C. Connee, M. FerrerOa,S. Webbb,R. SommeF, M. Chiovettab, K.Jonesc and P. SpanneC aDepartment of Chemical Engineering, University of Massachusetts, Amherst, MA 01002, USA bINTEC, Universidad Nacional del Litoral-CONICET, Santa Fe, Argentina CBrookhavenNational Laboratories, Upton, NY,USA
Abstract Heterogeneous olefin polymerization increasingly dominates the production of polyethylene, polypropylene and mixed polyolefins. Our studies of the morphology of these catalysts reveal that several different mechanisms occur for the changes in the accessible active surface, the void volume and the distribution of catalytically active particles within the growing polymer. These differences in morphological dynamics are dictated by the initial catalyst pore structure and the related subsequent catalyst fragmentationprocesses. MgC12 supported catalyst tend to be uniform in their microporosity (i.e., 4 nm); although, there is evidence for multi-modal p-pore dimension due to the agglomeration of small subparticles. The initial fragmentation proceeds readily and uniformly to yield a multi-grain growth of sub-particle agglomerates.The surface of these sub-particle agglomerates is accessible through the void-space between growing catalyst/pdcle grains. Silica supported catalysts exhibit a network of porosity from meso- to micro-pores. These catalysts fragmentprogressively from the larger to the smaller meso-pores following the f i i n g of the available pore space with growing polymer. Since this fragmentation is progressive and is dictated by pore filing, the surface area and pore volume can decrease rapidly. In the extreme, these mechanisms result in both loss and retention of available surface area and pore volume. As the polymer grows these differences can result in uniform as well as nonuniform distributions of catalyst fragments within the growing polymer. However, the differences between the morphological mechanisms can depend on the conditions (pressure, temperature and gas phase V.S. slurry) of the polymerization process and, thus, no simple conclusions may be drawn that relate one catalyst system to a specific mechanism for the changes in morphology.
-
1. BACKGROUND The phenomena, and influence of, catalyst disintegration within the polymer particle during polymerization has never been experimentally addressed before. Many models have been proposed for catalytic polymerization kinetics which make varying assumptions regarding the catalyst fragment size and distribution within the polymer particle. Ray and coworkers [l] completely ignore fragmentation and tacitly assume fragments are negligibly small and uniformly distributed. Chiovetta [2] uses a single particle analysis of fragmentation with a receding front of fragmentation from the exterior toward the catalyst particle center. These models are mainly directed at the titaniudchloride supported systems which should be more friability than a silica supported catalysth the silica-supportedsystems, fragmentation dynamics cannot be disregarded in the analysis of the polymerizationkinetics. Our prior studies [3] have shown that the influence of diffusion during the early polymerization is significant and can increase with inhibited fragmentation of the silica phase and
1828
agglomeration of polymer particles if the fragments are relatively uniformly dispersed within the polymer particle in these analyses. At high polymer yields the distribution and size of fragments within the polymer particles is not known. It is customary and convenient to assume, without any experimental evidence or validation, catalytic sites and fragments are uniformly distributed within the polymer particle. Models by Schmeal and Street [4] and Singh and Merrill[5] postulate that fragments may be distributed in three ways: (1) a "hard-core" model in which polymerization occurs around a center catalyst particle which is non-friable, (2) "uniform" site model is which fragments are small and uniformly distributed within the particle, and (3) "expanding core" model is which fragments are convected to the exterior particle surface by internal expansion. Each model must assume a different mechanism for catalyst disintegration and has different implications for the influence of monomer transport on the kinetics. The presence of voids in the polymer particles is highly undesirable since it increases the cost of shipping of the polymer. Yet the internal surface created by modest void structure can facilitate monomer transport to the active catalytic insertion sites. Clearly, an experimental technique that describes fragment and void size and location within the polymer particle as a function of polymer yield is needed. Heretofore, no technique has been found to study the size and spatial distribution of catalyst fragments without substantially modifying the system in some manner. 2. EXPERIMENTAL The surface areas of the initial catalyst and the catalyst/polymer agglomerate as a function of polymer yield were characterized by nitrogen adsorption in an Omicron 360 automatic sorption system. The void structure was characterized both by nitrogen ad-desorption and by Mercury porosimetry employing both low (vacuum -> 30bar ) and high (1 -> 4000bar) Quantacrome Omnisorb porosimeters. We have characterized, for the first time, the disintegration and convection of fragments in high yield polymer particles using X-ray microscopy and computed microtomography using Synchrotron radiation at the NSLS in Brookhaven National Laboratories. A collimated (40nm in dimension [7]. The MgCl2 supported catalysts only give evidence for microporosity with pores -4nm in diameter. Most of the samples give evidence for a monomodal distribution of pores; however, some commercial samples exhibit several distinct p-pore dimensions. It is apparent that the silica
1829
based catalysts have a complex network of pores and the MgC12 based catalysts represent an agglomeration (sometimes complex) of microcrystals. Dramatic changes occur during the initial stages of olefin polymerization over heterogeneous catalysts. As polymer accumulates, the catalyst fragments and the void space within the growing particle becomes filled with polymer. The changing monomer transport rate to the active sites, dissipation of heat and stress with the particle, and eventually, dispersion of catalyst fragments within the growing particle can control the polymerization.
3.1 Changes in Accessible Surface and Pore Volume For the silica supported catalysts, the surface area increases slightly and then decreases dramatically as polymer accumulates on the surface. Simultaneously, the pore volume decreases dramatically. As an example, for IOg.polymer/g.catalyst yield, the surface area decreases from 160m2/g to less than 5m2/g and the pore volume decreases from 1.7cm3/g to less than O.Olcm3/g. This is easily understood as the polymer is both filling the pores as well as blocking the surface. Further, the shape of the pores changes to more slit-like in shape. For the MgC12 supported catalysts, the surface area does not decrease as rapidly as in the silica system. Although the initial surface area (-130m2/g) and pore volume (-0.2-0.5 cm3/g) are less than for the silica supported catalyst, after lOg.polymer/g.catalyst yield the surface area is greater than 20m2/g and the pore volume is greater than O.lcm3/g.
8 2.0
3 c 3 1.5
8
300
2-m 200
a
*zf 1.0
8
4 100 a4
2 3 0.5
a
CA
0
0.0
0.001 0.01 0.1 1.0 10 100
Yield (&)
0.1
1
10
100 1000
Yield (g/g)
Figure 1. Total Pore Volume, in cm?g (on the left) and Surface Area, in m2/g (on the right) versus polymer yield in g.polymer/g.catalyst for each system, the Cr/silica supported and the Ti/MgC12 supported catalyst.
3.2 Tomographic Analyses Different tomographic images that were found for each of the catalys systems. For some of the silica supported catalysts, the silica fragments are found near the periphery of the growing catalyst/polymer particles. There is little evidence for an extensive pore network except for a very few pores of < 1Opm in dimension [8]. This distribution of catalyst fragments is unexpected and conflicts with all prior models employed in the analyses of olefin polymerization. The reason for the segregation of polymer/ catalyst phases is not known (indeed, the phenomena had not been seen in any prior studies), but could be related to the nature of the interactions between growing polymer chains in comparison with their potential wetting of the catalyst surface. Large fragments were evident within the growing polymer for the systems where segregation occurred. However, we were unable to detect fragments for some silica supported systems, e.g., for those run as a slurry at -15bar pressure. It should be noted that the current spatial resolution for tomography is
1830
-2pm and, thus, uniform fragmentation to smaller particle dimensions will not allow detection of the relative positions of the fragments. Tomographic analyses of the MgC12 supported catalysts shows no evidence for radial segregation between the catalyst fragments and polymer. In addition, an extensive macro-pore network is found within the growing catalyWpolymer particles. The particle (agglomerate) dimensions evident in the tomograms are smaller than those seen for the silica based systems for most of the samples; however, some MgC12 supported catalysts did give evidence for larger catalyst agglomerates in the catalyst/polymer particles. Detection of larger particles was not found for specific polymerization conditions, e.g., slurry vs. gas phase or silica vs. MgC12 as examples. We do, however, conclude that the segregation between catalyst and polymer may be related to a loss of porosity which breaks up the extensive polymer network found for some of the silica supported catalysts. 4. SUMMARY AND CONCLUSIONS We find that the course of the polymerization depends on the initial stages yet is seldom considered in polymerization models. The distribution of catalyst fragments within the growing polymer can differ from uniform to segregation of fragments near the periphery of the growing particles. This seems to be related to the creation of a continuous polymer phase that would thus allow polymer-polymer interactions to balance against polymer/support interactions. At the same time, catalysts which fragment uniformly can retain internal porosity in the growing polymer/catalyst system. This will maintain accessible surface and a pore network that facilitates transport of monomer to the active polymerization sites. Some catalysts can, however, fragment progressively induced by the almost complete filling of the void network with the growing polymer.
ACKNOWLEDGEMENTS This research was supported by the National Science Foundation under grant CTS 8921381. Drs F. Karol and B. Wagner (Union Carbide), T. McKenna (ELF) and McDaniel (Phillips) are gratefully acknowledged for their support and useful discussion.
6. REFERENCES W.H. Ray, S. Floyd, Choi, K. Y., and Taylor, T. W., J. Appl. Polym. Sci., 21, 2231, (1986). M.G.Chiovetta, Ph.D. Thesis, University of Massachusetts, 1983. W.C. Conner, E.W. Webb, E. Weist, M.G.Chiovetta and R.L. Laurence, Can. J. Chem. Eng. Silver Anniversary Issue on Polymer Reaction Engineering D. Singh, and R.P. Merrill, Macromolecules, 4(5), 599, (1971) Schmea1.W.R. and Street,J.R., J. of Polym. Sci., Polym. Phys. Ed., 10, 2173, (1972), AICHE J., 17(5), 1188, (1971) W.C. Conner, E.L. Weist, A.H. Ali and B.G. N&k, Macromolecules, 22-8,3244-50 (1989) E.L. Weist, B. Naik, S.W. Webb, R.L Laurence and W.C. Conner, Proceedings on the 9th International Congress on Catalysts, Phillips & Ternan, eds., Chem. Instit. of Canada, pbs., v.4, p. 1866-73 ,11988). W.C. Conner, S.W. Webb, K. Jones and P. Spanne, Macromolecules 24 (21) (1990)
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Ouczi, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th Intcrnarional Congrcs on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
CATALYTICAL AND HIGH RESOLUTION ELECTRON MICROSCOPY STUDIES OF THE SYSTEM Pt/ZnAlzOd WITH SEVERAL PLATINUM CONTENTS
G.Aguilar-Rwsa, M.A. Valemuel&, D. R. Acostab and I. SchifteF ahtituto Mexican0 del Petroleo, A.P. 14-805,07730 Mexico, D.F., Mexico bhstituto de Fisica, UNAM,A.P. 20-364, 010oO Mexico, D.F., Mexico
Abstract Pt/ZnAlz 0 4 catalysts with contents ranging from 0.1 to 1.17 wt% were characterized by temperature programmed reduction (TPR), hydrogen chemisorption (HC), high resolution transmision electron microscopy (HRTEM) and tested in isobutane dehydrogenation, using helium or hydrogen as reaction media. For low metal contents, platinum diffuses into the lattice spinel and when concentration become important, metal-metal interactions are stronger leading to particle formation. Catalytic behaviour is explained by means of the formation of a stable (Pt-K)active complex.
1. INTRODUCTION Supported Pt have been studied by several techniques (1,2], in contrast Pt on spinel materials has received less atention (31. In this work, HRTEM observations, TPR ahalysis, HC and catalytic activity measurements in isobutane dehydrogenation were carried out on the system Pt/ZnAlz04 with different metal contents, in order to clarify the role and localization of platinum in the spinel. 2. EXPERIMENTAL Catalysts preparation
Zinc aluminate was prepared by coprecipitation of the metal nitrates then calcined at 800 OC and later impregnated with a solution of H2PtC1, .6H2O to obtain Pt wt% from 0.1 to 1.17. Characterization techniques
TPR experiments were performed in a conventional equipment, from -80 to 600
OC. HC of prereduced samples was determined at 25 OC on a volumetric installation. HRTEM were carried out in a 400 KPImicroscope. Also, TEM observations were performed in a 100 Kv microscope equipped with EDS system.
1832
Catalysts testing Isobutane dehydrogenation was studied in a continous reaction system at 550 'C, feed stream consisted of a equimolar blend of isobutane and He or H2 as reaction media. In all the cases, a contact time of 75 g - $11 was used. Before the reaction, catalysts were reduced at 550 'C for two hours. 3. RESULTS AND DISCUSSION
BET area of about 20 m2/g and pore volume of 0.12 cm3/g was obtained for all the samples. There is not any apparent correlation between texture and metal loading. The normal spinel structure of zinc aluminate was corroborated by XRD techniques. HC data are resumed in Table-I, it is clear that the total hydrogen (pmol H2/g) is almost independent of platinum up to 0.5 wt% Pt;for higher platinum concentrations, chemisorption is highly sensitive to platinum content. Calculated dispersions (%D) decrease continously as wt% P t increases. Table I Hydrogen chemisorption results of reduced PtlZnAl204 catalysts Sample 1 2 3 4 5 6 7
wt% Pt
pmol H2/g
0.00 0.10 0.16 0.47 0.59 0.81 1.17
0.00 1.94 2.01 1.93 1.99 2.20 3.60
%D -75.9 49.0 16.0 13.2 13.0 12.0
H2IPt
-1.go 1.22 0.40
0.33 0.32 0.30
T P R results are condensed in Figure 1,it is evident the presence of two P t species with reduction bands located at 300 OC and 500 OC respectively. In samples with wt% Pt lower than 0.6, the mol ratio HZ/Pt calculated is near of 2, suggesting that Pt is in an oxydation state of +4. This behaviour is evident in Figure 2, in which total amount of H2 used are compared with the theoretical value needed for the reaction PtOz t 2H2 4 Pto t 2H20. TEM mesurements in the 100 Kv microscope were carried out to obtain selected area electron diffraction patterns and EDS determinations. Zinc aluminate was fully identified in all the cases. In samples with low wt% Pt, the metal was not detected. HRTEM-400 Kv images me shown in Figure 3, sections 3a and 3b were taken from the 0.16 wt% Pt sample. Section 3c is an image from the 1.17 wt% Pt, section 3d is an elargement of the particle arrowed in section 3c. Aggregation of zinc aluminate crystallites with many boundaries were observed. Platinum particles were not detected in low wtX Pt samples. In those with 1.17 wt% Pt particles were detected like embedded in the support, suggesting some kind of matching between crystallographic planes.
1833
7
I
i
150-
-2
3 -
100-
-
4-
f i d "2
Q-cat 50-
56-
7100 300 500 700
Figure 3. HRTEM images of the Pt/ZnAh04 catalysts. 3 a and 3b 0.16 wt% Pt;3c and 3d 1.17 wt% Pt. Results from catalytic conversion are shown in Figure 4. Turnover Frecuency numbers (TOF) are aproximately three times higher when H2 is used instead of He. Notice also that TOF numbers reach a maximum value for a platinum content about 0.5 wt%. In contrast, in helium the activity decreases continously as metal increases. Concerning selectivity, in addition of the olefin, propylene and methane were detected. When hydrogen/isobutane mixture is fed, selectivity attains the highest value (97%), remaining
1834 constant for platinum loadings higher than 0.5 wt%. Moreover, when helium/isobutane is used, selectivity shows a well defined maximum at 85% and 0.5-0.6 wtX Pt. On the other hand, the deactivation constants are more important in helium than in hydrogen.
Yo s
0
05 w t % Pt
1.2
Figure 4. Turnover Frequency (TOF) numbers and selectivity (%S) for isobutane dehydrogenation obtained for Pt/ZnAl204 catalysts: ( 0 ) H2 and (V) He. 4. CONCLUSIONS
TPR experiments showed the presence of two well defined platinum species on the zinc aluminate surface. The hydrogen consumption corresponds to Pt in oxidation state +4 until 0.6 wt% Pt is reached. For a larger Pt loadings, the H*/Pt ma1 ratio is less than 2, indicating that not all of the platinum present is reduced or that platinum is in a lower valence state. Hydrogen chemisorption results suggest that there is a transition of chemisorption properties when the wt% Pt is about 0.6. &om HRTEM and EELS [4] studies it is proposed that platinum diffuses into the zinc aluminate spinel structure for low contents, showing a strong interaction with the support lattice oxygen; with increasing platinum contents particle formation is observed. Catalytic activity shows that the hydrogen as reacting medium is very important to achieve high activity and selectivity values, which means that Pt-H complexes play an important role in dehydrogenation reactions. The low activity observed when helium is used as diluent gas suggest that the hydrogen produced during reaction is not enough to form the active Pt-H species. The same idea could explain the maximum in the selectivity versus wt% Pt plot. 5. REFERENCES 1 Hiuzinga T., Van Grondelle J., Prins R., Appl. Catal., 10 (1984) 199. 2 Lieske H., Lietz G., Spindler H., Volter J., J. of Catal., 81 (1983) 8. 3 Rennard R.J., &eel J., J. of Catal., 98 (1986) 235. 4 Dom'nguez E.J.M., Aguilar-Rios G . , Valenzuela Z.M.A., Aetas del XII Simp. Bemum. de Catal. (Brasil), 111 (1990) 94.
O d ,L d al. (Editors),New Frontiers in Catalysis Procttdingr of the 10th International C o n p on Catalysis,19-24 July, 1992,Budapest, Hungary Q 1993 E W e r Science Publishers B.V.All rights reserved
THE APPLICATION OF DILATOMETRY FOR INVESTIGATION OF HETEROGENEOUS CATALYSTS L.A. Rudnitsky and A. M. Alekeev The State Institute for Nitrogen Industry, Zemljanoi Val 50, 109815 Moscow,Russia
Introduction Dilatometrio analysis of pellets of heterogeneous catalysts and adsorbents,oarried out @'tn sttu", allows the effeot of pellet @frespirationtf (variations of linear dimensions,the relative value of whioh is in the limits from hundredths of per oent up to several per oents) to be deteoted whioh aooompanies nearly all types of the physiooohemioal transformations taking plaoe at the different st es of formation, at replaoement of gas media, aging and deaot vation. In many oases suoh transformation results in the ohanges not only of the real density of pellet materia1,but also of the maorostruoture featwee, speoifioally, porosity. For this reaBon,the dilatometrio effeot proves to be s m a r y and usually does not oorrespond to relationship between the real densities of initial and final phase. If the transformation takes plaoe in one phase of a multiphase system the value ( and sometimes the sign) of the effeot are determined by and porosity of this not only looal ohanges of density phase,but also by the features of the struoture of the multiphase system, partioularly, by those on whioh the pellet strength depends. Henoe, in a number of oases, the results of dilatometrio analysis not only allow information on the oharaoter of prooeeding of a ohemioal or a physioal transformation to be obtained, but also are oorrelated with strength and destruotibility of pellets. This report oonsiders the applioation of dilatometry analysis to the problems of strength and stability of the f l o a m y i n g etruoturellof a pellet,i.e.of its framework. The results of study of porous pellets are given : a)whioh a r e exposed to alternat inner stress (ooking and gasifioation of the Ni reforming oa alyst,deoarbonation of oarbon dioxide dolomite baged absorbent), b)whioh a r e in the oourse of reoonstruotion of its struoture (reduotion of hematite based oatalyst for ammonia synthesis), o)whioh are in the oourse of its formation from a multiphase system(oa1oination of magnetite and aluminum hydroxide mixture). The results shown in this work are obtained with dilatometers with induotive displaoement transduoers. The elongation Al/l of the pellet height 1 was determinated with aoouraoy 10-2
P
Y
1836
1 ,Coking and g a e l f l c a t i o n of NI reforming catalyst While heati the reduoed Ni oatslyst for reforming of hydrooarbons in mix ure of GO2 and CH4 in a wide range of oonoentrations of .oomponents,one may observe a #looking peak",i.e.aooumulation of oarbon in the temperature range T < T(e) and its gaeifioation at T > T(e), where T(e) ie the thermodynamio equilibrium temperature of the oarbon deposition and asifioation reThe study of ooking of the oatalyef (Ni and promoaotions tors onto oorundum oarrier ) has been oarried out in parallel with dilatometer and miorobalanoe. The de endenoe of the pellet elongation on the deposited oarbon quan ity related to the Ni mass ie shown in Fig.1. The initial stage of oarbon deposition C / m g Ni) prooeeds with (up to 0.25 no obvious d latometrio effeot, i.e. doe6 not oreate oonsiderable internal BtreBB.A BUbSeqUent oarbon deposition A1/1 oauses a sharp expansion of the pellet what witnesses the development of oonsiderable inner stress. When T(e) is aohieved the prooess of gasifioation begins whioh oomes to end with a full removal of oarbon.The oarbon removal ourve is looated substantially higher than the o w e of 2.10-3 its deposition what indioates an ir0 reversible ohange of the pellet struoture.By varying the experimental oon10 20 30 40 ditions, one may establish the value of reeidual deformation and the quantity of depolsited oarbon whioh oawe deetruotion of pellets for a number 1.The dependenoe of types of the reforming oatalyst and, ation Al/l in suoh a way, o a r r y out the analysis on deposit of the stability and deformability of quantity G,% different porous struotures
.
P
T
.
.
2 Jecarbonatlon of the carbon dioxide abeorbent The transition from CaO to CaOOgis aooompanied by inorease of the volume of the reaoting phase, the reverse prooess is aooompanied by its oontraotion. Henoe,solid C02absorbents operating based on +hie reaotion in the oyolio mode are subjeoted to alternating internal streas in eaoh absorption/desorption oycle. Let's study the reaotion of the framework of pellets to prooeeding of the seoond prooess of the oyole (oarbon dioxide desorption).Compare the dependenoiee of the linear expansion ooeffioient on temperature iven in Fig.2 for heating in air of natural dolomite(1 and )and of the thermal-oyoled dolomite -"absorbentI4 (la and 2a).Dilatogramm 1 and la are obtained in desorption at the first, clilatogramm 2 at the seoond(and the last), The ourve 1 shows a and dilatogramm 2a at the fourth oyole.
B
1837
intensive oontraotion at 800 C whioh oorresponds to oaloium oarbonate deoomposition. The ourve 2 shows a oeaseless oontraotion whioh is sign of the pellet destruotion.On the oontrary,%bsorbent", as the quantity absorption/deeorption oyoles inoreases, shows a greater and greater smoothing of the deeorption dilatogramms. Desorption at 4th oyole(the ourve 2a)ie aooompanied by negligibly small oontraotion after 800 C.It is importantly that the oarbon dioxide oapaoity of I1absorbentl1does not praotioally ohange and remains about suoh as natural dolomite has. Sosthe struoture is aooeptable at whioh the pellet framework reaote in the minimum extent to the above inner 6tresses.Suoh a framework possibly forms a matrix in whioh the phase reaoting with oarbon dioxide is looated.
3,Reductlon of ammonia synthesis catalvets The study of reduotion in hydrogen flow of ammonia synthesis oatalysts of preoipitated type has been oarried out with pellets of hematite(di1ato amm 1 in Fig 3 ) , and 3% aluminum oxide shfle-promoted oatalys (dilatogramm 2 in F i g 3 ) in the linear hea ing regime. The dilato amm 3 f o r the single-promoted oatalyst has been reoeived in $1 he isothermal regime at 450 C. The is represented by the first stage of reduotion(up to m-etite) solid and the seoond (up to iron)-by the dotted lines. In the
F
100 200
'I;,min
1-
1
-1
-3
370 570 770 T,K -5
Fig.3 The dilatogramm of reduotion in hydrogen flow of ammonia 6yntheSi6 oatalysts.
-7
1573
773
973 T s K
Fig.2 The dilatogramm of deoarbonation of natural dolomite and ltabs~rbent I' (a)
.
ideal ( and simplified ) sohemesthe final struoture (metal) whioh grows in the first struo ture (oxide) shall reproduoe the initial pellet volume.However,the data the existenoe of two effeots:the initial expan-
1838
oion and subsequent oontraotion show f o r the eaoh stage.The 88oond effeot often soreens the first one. The expansion is evidently oaused by a tension of the pellet framework whioh is provoked by formation of oxygen vaoanoies in oxide. The pellet oontraotion effeot rerleots prooeeding of the th-eoonstruotion't prooess from the initial framework to final one. The oontraotion value may oharaoterize the degree of deviation of the real 'lreoonstruotion'l prooess from the ideal soheme, i.e.may be one of the oriteria of opthality of the eaoh reduotion stage reghe.The utilization of the ieothemal regime ( o w e 3)oauses a powerful oontraotion in the first stage beoause of overstating the temperature of its realization. The utilization of linear heating with a small rate ( 2 K/min ) regime allows the conditions of eaoh stage to be improved(ourve 2).The oomparison of the dilatogramms 1 and 2 shows that, besides the temperature regime,an important faotor of the pellet framework stabilization is the struoture promoting.
4,Calclnation of two-~haee system The predried pellets formed from dispersed partioles of m netite and alumhum hydroxide with water as a binder are oalo nated in air in a llnear heating regime.Fig 4 shows the dllatofor magnetite with 7 ( 2 ) and gramm fragments for magnetite (l), 14 ( 3 ) per oents of the hydroxide, for magnetite with 14 % of the hydroxide and with addition of different surfaotant ( 4 , 5 ) . The magnetite oaloination dilatogramm shows up to about 300 C only temperature expansion,but the hydroxide oalohation dilatogramm shows only contraotion oaused by removal of adsorbed water and by dehydration. Henoe,the de ee of deviation of mixture oalo ation dilatogramm from the o w e 1 depen& on the two faotors :oonoentration of hydroxide and oharaoter of its distribution around The oomparison of dila ogramms 1 , 2 and 3 shows that deviation degree from o w e 1 oorrelates t o hydroxide oonoentra tlon.0ne oan see that the addition of surfaotants(the ourves 4 , 5 ) oauses approaohing of mixture diFig.4 The m etite and latogramm to magnetite oaloinetion o w e 1 .The partial pushinghydroxide mix "$"ure oaloi-out of the hydroxide from the nation dilatogramms. oontact plaoes between the magnetite partioles may be evidently the reason of this effeot.Henoe, dllatometrio analysis data may oorrelate to the meohanioal properties of the pellets.
E
.
L a al. (Editors),New Frontiers in Catafysk loth International Congress on Catalysis, 19-24July, 1% Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rightq ~cscrvcd GI&,
procacdinp of the
ARTIFICIAL CONTROL OF CATALYTICACTIVITY OF Pd BY A SHEAR HORIZONTALSURFACE ACOUSTIC WAVE
Y.Znoue, H.,Kawaeuchi, M.Matsukawa and K Saw Department ofChemistry & Analysis Center, Nagaoka University of Technology, Nagaoka, Niigata 94-21, Japan
ABSTRACT A device type catalyst with controllable functions is desinged by employing a ferroelectric LiTaOg substrate which generates a shear-hopieontal leaky surface acoustic wave(SH-LSAW) The propagation of the SH-LSAW on a deposited Pd film caused an increase in the catalytic activity for ethanol oxidation. The effects of the SH-LSAW are accounted for in terms of a high electric field produced by the displacement of the substrate lattice.
.
1. INTRODUCTION The artificial control of catalytic activities in heterogeneous catalysis is one of the interesting issues. This would become feasible if the geometric arrangement of surface atoms and their electronic states are modfified according to external signals. We have drawn attention to surface acoustic waves(SAWs) generated on ferroelectric crystals by electric power, since their propagation on the crystal causes the displacement of lattice atoms near the surface. Recently, we have shown that the Rayleigh type SAW generated on ferroelectric LiNb03 permits the activation of Pd and Cu catalysts deposited on the propagating path. 1-3) Rayleigh SAW has the propagation direction parallel to the crystal surface and the vibration vertical to it. In order to extend the research on the effect of SAWS upon the catalytic activity, it is of interest to use the SAW which has the different vibration mode. In the present work, we have employed a shear-horizontal leaky SAW(SH-LSAW) in which both the prow'gation direction and the vibration are parallel to the crystal surface, as is schematically shown in Fig. 1. Changes in the catalytic activity of a Pd catalyst deposited on the propagation Fig. 1. SH-LSAW path of the SH-SAW were investigated for ethanol oxidation, and the results were compared with those previously obtained
1840
NWA
I; \"Mp/
Y
ID?
Pd
1 F LDT'
I
I
Fig.2. A system for the SAW generation and catalytic reaction. NWA: network analvser
I
ocs
R,"I.G..".L*S.
tric furnace
for Rayleigh wave propagation. The generation of a high electric field on the crystal during the SH-LSAW propagation was examined by a luminescence technique. 2 . EXPERIMENTAL A 36'-rotated Y cut LiTa03 single crystal with propagation along X axis4) was used as a substrate for the SH-LSAW. The input and output interdigital transducer(IDT1 A1 electrodes were photolithographically fabricated on each end of the crystal. Pd was deposited by evaporation in the middle of two sets of the IDT electrodes, which were 16mm apart. The thickness of the Pd film was 10 or 100 nm. Figure 2 shows a system for the investigation of the catalytic activity. A source of radio frequency(RF) excitation was a network analyser which was also used to monitor the extent of the propagation of the SHLSAW through the Pd thin film. The RF power was once amplified E 0 before introducing to the device catalyst. A CA thermocouple was brought into contact with the --20 backface of the substrate, and 6 the temperarture of the catalyst was controlled with an external -40 electric furnace. The products were analysed by a gaschromatograph directly connected to the 18.0 21.5 25.0 reaction system. Frequency I MHz For measurements of acoustoluFig.3. Band pass characteristics minescence, a thin layer of Of the device* electroluminescent Cu-doped ZnS 1. SH-LSAW, 2. BBSW. was deposited on the SH-LSAW
3
8
f
1841 propagation path, instead of Pd catalyst, and luminescence was measured by a photomultiplier tube.
343 z 333
x
SHLSAW
b
3. RESULTS AND DISCUSSION Figure 3 shows the band pass characteristics of the SAW device fabricated in this research. There were two peaks between 18 and 25 MHz. A main peak at 20.8 KHz was due to the SH-LSAW. A concomitant peak appearing at higher frequency by 2.1 MHz was assigned to be surface skimming 0 1 2 3 4 bulk wave(SSBW). The center tlh frequecny of the SH-LSAW, f , was in agreement with that calculated value, 20.6 MHz, according to an Fig.4. Effects of SH-LSAW and SSBW equation of f = v/d where v is upon 'the catalytic activity. the velocity of the wave and d is a wavelength of the Sh-LSAW. The a. ;lo0 nm-Pd, reaction at 333K, ;10 nn-Pd, reaction at 343K. insertion loss remained within a b. temperature of catalyst. level of -lOdBm, which was small enough to permit effective propagation of the SH-LSAW. Figure 3 shows the ethanol oxidation with and without the waves. For the SH-LSAW, the reaction rate at 333 K on a 100 nm-thick Pd film increased by a factor of 1.8 immediately after the SAW was turnd on at power of 1 W. The catalyst temperature rose by 10 K in 2 min after the SAW was turned on, but gradually lowered to the original level within 12 min by temperature control with the electric furnace, while the high reaction rate continued until the RF power was turned off. The fluctuation period of the catalyst temperature was not long enough to affect the reaction rate. The ratio of reaction rate enhancement after SAW-on to before SAW-on, Q, decreased to 1.4 at 343K. This shows that the SH-LSAW causes intrinsic changes so as to lower the activation energy of the reaction. These results excludes a possibility that the activity increase would be due to a thermal effect. The previous results on Rayleigh propagation wave showed that the value of Q was 3.5 for the same reaction at 343 K on the 10 nm-thick Pd film.1v3) Thus, it follows that the value of Q caused by the SH-LSAW was smaller than the value for the Rayleigh SAW. When the thickness of the Pd film deposited was decreased to 10 nm, however, the Q value obtained for the SH-LSAW was close to 3 for the reaction at 343 K. Thus, these results indicate that the effects upon the increase of the catalytic activity are similar between the SH-LSAW and the Rayleigh SAW, and diminishes with larger thickness of
1842 the Pd film deposited on the w30w 30 % C propagation path of the SAWS. d This is related to phenomenon 220that insertion loss occurs more d significantly for the thicker deposition of the Pd films. 10When the SSBW was turned on, E E the reaction rate increased by a 3 3 factor of 2.1(Fig. 4). The higher 0-0-0 rate decreased nearly to the original level with SSBW-off. Under the experimental conditions Fig.5. luainescence intensity vs. employed there was no significant burst frequency of the SH-LSAW. difference in the Q value between the surface and bulk waves. There are two factors to be considered for the activation of the catalyst; one is a geometric effect due to the lattice displacement, and the other is a high electric field effect caused by the distortion of the ferroelectric substrate having no center of symmetry. As for the geometric effect, it would be considered that there are considerable differences in the activation effects between the vertical(Ray1eigh SAW) and parallel(SHLSAW)movement of the crystal lattice. In order to ascertain whether the SH-LSAW is able to generate the high electric field, a thin layer of Cu-doped ZnS was deposited on the propagation path of the SH-LSAW, and the SH-LSAW was introduced as a pulse( burst frequency was changed f r o m lo5 to 10' s-'. Fig. 4 shows t hat luminescence has a threshold burst frequency at lo4 s-l and its intensity increased remarkably with decreasing burst frequency. This indicates that the SHLSAW has the ability to generate a high electric field. In the previous study,') the magnitude of the electric field caused by Rayleigh propagation Provided that the equation derived by wave was shown to be lo4 V cm-'. Lakin5) holds for the SH-LSAW, as is valid for the Rayleigh SAW the electric field produced by the SH-LSAW is calculated to be lo4 V cm- i , which is the same as that of the Rayleigh SAW. Such similarity accounts for the result that the activation of Pd catalyst was similar between the SH-LSAW and the Rayleigh SAW. Thus, the effect due to high electric field seems more responsible for the activation than the geonetric one. From the present study, it is concluded that the SH-LSAW, in addition to the Rayleigh SAW, has the effect on the increase of catalytic activity.
-
f
I
4 . REFERENCES
1 Y. Inoue, M. Matsukawa, and K. Sato, J. Amer. Chem. SOC., 111 (1989) Y. Inoue, M. Matsukawa, and K. Sato, J. Phys. Chem., in press. 2 Y. Inoue and M. Matsukawa, J . Chem. SOC,,Chem. Commun., 296 (1990). 3 Y. Inoue, M. Matsukawa, and K. Sato, Catal. and Technol, 1 (1991) 119. 4 Y. Inoue, Y. Kato, and K. Sato, J. Chen, SOC., Farad. Trans. I in press. 5 K. M. Lakin, J. Appl. Phys., 42 (1971) 899.
8965;
Guni, L u al. (Fditors),New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elscvier Science Publishers B.V. All rights reserved
CHARACTERIZATIONOF SILICA-SUPPORTEDPALLADIUM-COBALT
ALLOYS U? Juszczyka, Z. Karpinskia, Z. Padb, J. Pielaszeka %stitUte of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland bInstitute of Lsotopes of the Hungarian Academy of Sciences, 1525 Budapest, P.O.Box 77, Hungary
Abstract Probing Pd-Co alloy particles supported on SiO, by a variety of techniques (H, and CO chemisorption, temperature programmed reduction (TPR), reactions of various alkanes, and x-ray diffraction) shows that the employed preparation and pretreatment result in the formation of fully reduced Pd-Co alloys characterized by good dispersion and lateral homogeneity. The surface enrichment in palladium is evidenced. 1. INTRODUCTION
Because of their versatility in hydrogenation reactions [I], supported palladium have been recently the focus of our studies. Changes in the palladium surface induced by high temperature reduction and metal-support interactions were monitored by using neopentane conversion as a probe reaction and by chemisorptionof various gases [2,3]. The next step in the chemical probing of supported palladium catalysts is the presented work, where the effect of adding a second metal to Pd can also be monitored by the same techniques. Cobalt is an ideal candidate, not only because Pd-Co alloys may be superior to Pd and Co in hydrogenation reactions (of unsaturates and CO, [4]), but it exhibits quite different behavior than Pd. Differences in reducibility, chemisorptive properties towards H, and CO, selectivities in reactions of alkanes and CO t H, constitute a solid basis for effective probing of Pd-Co surfaces and are the focus of these investigations. 2. EXPERIMENTAL
2.1. Catalyst preparation and pretreatment Five Pd-Co/Si02 catalysts with metal loading of 1 h t % were prepared by impregnation of silica (Sewa, reinst, 80-100 mesh, acid washed and dried) with aqueous solution of PdCl, and CoCl2 using incipient wetness technique. The atom percent ratio of Pd to Co was: 100/0, 75/25,50/50, 25/75, and 0/100. After drying at 120°C for 9h the impregnated materials were precalcined in air in a fluidized bed reactor from room temperature to 450°C at 2.5"C/min, and held at 450°C for 3h. After cooling the catalysts were stored in a desiccator. Before each experiment the sample was pretreated in oxygen at 300°C for 0.5h and reduced according to a temperature program presented below. 2.2. Temperature-programmed reduction (TPR) and chemisorption TPR of Pd-Co/Si02 was conducted in flowing H,(6%)/Ar mixture (35 ml/min) from -45 to 640°C with a heating rate of 8"C/min. Because catalytic experimentswere conducted on samples
1844 reduced at 380°C for 2 or 15h (next subsection), in order to establish the degree of reduction at reaction conditions, differential TPR spectra of prereduced Pd-Co/SiO, samples were also recorded. Metal fractions exposed, as H/M or CO/M (M=Pd,Co or Pd t Co) ratios, were obtained from chemisorption studies conducted in a static system with a Setra pressure transducer. After reduction at 380°C (2 or 15h) the samples were purged with helium and degassed at 450°C. Chemisorption of H2 was measured at 100°C: the amount of strongly adsorbed H2 was determined as the difference between total and weak adsorption isoterms. After Hz chemisorption the sample was degassed at 450°C and CO chemisorption (total and weak) was studied at 20°C. All gases used for chemisorption and sample pretreatment except oxygen were purified by passing through drying traps with final purification over MnO/SiO,. Carbon monoxide was 99.995% pure (Van Eeghen, The Netherlands). 23. X-ray difFraction (XRI))
XRD experiments were performed on a standard Rigaku-Denki diffractometer using Cu& radiation and by step-by-step technique over 29 range from 30 to 72". Resulting diffraction profiles originating from the metal phase for samples reduced in-situ in H2 at 380°C (for 2 or 15h) were obtained by method of subtraction of the support background profile. For the determination of metal crystallite size and phase composition of Pd-Co aggregates the position and half-width of the (1 11) peak were considered. 2.4. Catalytic performance of Pd-Co/SiO, The investigations of the reaction of neopentane with hydrogen were conducted in a static
circulation system in the manner described previously [2]. The alkane partial pressure was 1.33 kPa and the ratio of H2-to-alkane was 10. Samples of reaction mixture were withdrawn at periodical intervals to GC analysis (Hewlett-Packard Model 5890 Series 11,6111 squalane/chromosorb P column with a flame ionization detector). Turnover frequencies (TOF) were calculated on the basis of fraction exposed measured by Hz chemisorption. The reactions of n-hexane (nH) and methylcyclopentane (MCP) with Hz were studied in a pulsemicrocatalytic system [5]. After reduction in Hz at 380" C, Pd-Co/SiO, samples were cooled in a helium flow to reaction temperature, usually 260-280°C. After subsequent switching to hydrogen flow, 1p1 pulses of nH or MCP were injected to the gas stream before a glass reactor. GC analysis of the effluent gas was carried out on a Packard Model 427 gas chromatograph (3.2m squalane/chromosorb P column with a flame ionization detector). 3. RESULTS
TPR results showed that the silica-supported Pd and Pd-Co alloys are fully reduced below 380"C, but 10wt% Co/SiO, exhibits a hydrogen consumption peak at ca. 450°C. However, difference TPR spectra of all catalysts recorded after reduction in Hz at 380°C (for 2 or 15h) showed 100% reduction. Table 1 presents data regarding fractions exposed: H/(Pd+ Co) and CO/(Pd + Co). Metal crystallite sizes calculated from hydrogen chemisorption are further compared with the XRD diffraction data. The phase composition of Pd-Co aggregates is also included into Table 1. XRD and chemisorption as well as catalytic studies show that reduction time (2 vs 15h) has practically no effect on the samples. The results of reaction (neopentane, n-hexane and methylcyclopentane) studies with use of PdCo/SiOz catalysts are presented in Fig. 1A (overall activity) and 1B (selectivities).
1845 Table 1 Properties of lOwt% Pd-Co/SiO, catalysts nominal composition at%Pd
at%Co
100
fraction exposed
DH
'
D,,
metal particle
XRD analysis
size [nm] (from DH)
phase composition
particle size [nm]
0.50
0.44
2.2
fcc, 100%Pd
2.2
25
0.44
0.44
2.3
fcc, 31%Co
< 1.5
50
50
0.27
0.30
3.7
fcc, 62%Co
< 1.0
25
75
0.11
0.20
8.0
hcpfCo)+some fcc (PdCo)
4.0
100
0.10
0.07
9.0
hcp(Coj+ f a (CO)
8.1
75
0 atom 'Yo cobalt
atom
50 100 cobalt
"/D
-
Fig. 1. Catalytic performance of 10 wt% Pd-Co/SiO,. (A) Catalytic activity in neopentane at 242°C (v), n-hexane at 280°C (0)and MCP at 280°C (+) conversions. (B) Selectivity changes neopentane isomerization (0)and in n-hexane reactions: hydrogenolysis (o), isomerization (o), MCP formation (t), and fragmentation factor C (A), defined in Ref. [5]. 4. DISCUSSION
Si02-supported Pd-Co alloys have minimum catalytic activities at intermediate compositions. Pure Pd is more active in neopentane conversion whereas Co shows higher activity in nH and MCP reaction (Fig. 1A). The selectivity differences (Fig. 1B) indicate that the higher activity of Pd-rich alloys with neopentane can be due to higher (4560%) bond shift isomerization selectivity of neopentane. Parallel isomerization and C5-yclization selectivities of nH (Fig. 1B) may indicate that the C=,-cyclic pathway of isomerization is predominant there, as reported also for Pt black [6]. These reactions exhibit rather low (20-25%) selectivitieswhich drop to nearly zero with Co-richer alloys. More pronounced changes are seen in the Pd-richer region with
neopentane reactant while for nH (and MCP) conversion, larger changes appear in the Co-rich region. In other words, the reaction of neopentane effectively probes samples rich in Pd, whereas the reaction of n-hexane and methylcyclopentaneare sensitive probes for Co-rich samples. The underlying reason must be that different reactions prevail in those two cases. These results confirm also that the active site requirement for the bond shift and C,-cyclic reactions is different, namely the former reaction requires smaller ensembles [7]. On the other hand, Pd (and three other metals) were found to possess an inherent activity in "C5-cyclic reactions" (12,cyclization and isomerization) [S]which, however, are less favoured as opposed to hydrogenolysis. Pure Pd and 25%Pd75%Co exhibit "Pd-like propertiesnand a dramatic drop of the selectivity of C.,-cyclic reactions is observed at 50% Co content. This is accompanied by an abrupt increase of the < fragmentation factor. A value of { ~ 2(single hydrogenolysis) was found to be characteristic for metals exhibiting C.,-cyclic properties while multiple hydrogenolysis (C = 2+6) prevailed with other metals including Co. Although similar in nature, these three reactants exhibit different activity and selectivity patterns as a function of alloy composition. Their diagnosticpower changes with the Pd/Co ratio, hence, their combination is a more powerful tool for catalyst characterization than the use of a single reaction. In general, all selectivities do not demonstrate any particular "additive" behavior, especially when maximafminima in the relations between selectivity and Pd-Co alloy composition are observed. Such a behavior suggests the presence of uniform alloy particles, i.e. the preparation and pretreatment of Pd-Co/Si02 used in this work result in good lateral alloy homogeneity. Table 1 shows that in the case of three Pd-Co alloys, XRD line broadening, compared to H2 chemisorption, would imply much higher metal dispersions. For pure Pd and Co catalysts both methods give a good agreement. In addition, lattice parameters of Pd-Co alloys indicate the presence of phases with palladium deficit, compared to nominal composition of Pd-Co/Si02 samples. Thus, we conclude that the enormously broadened XRD lines likely do not result from the presence of a mixture of alloy particles differing in an overall composition, but rather from the situation where a relatively thin layer of a Pd-richer phase envelops a Co-richer kernel (detected by XRD). Surface enrichment in Pd is strongly suggested by our catalytic data and predicted by segregation theories. It should be mentioned that recent results on characterization of Swt% Pt-Co/SiO, catalysts (81 are in harmony with our data. In that work some Cb is "invisible" for XRD, because for Pt-Co alloy surface segregation of Co is expected. As metal loading, type of support, preparation and pretreatment methods employed by Dees and Ponec [8] are similar to ours, we think that both works as complementary: there is no room for speculation that Co may form an unalloyed, silica-bondedlayer which is XRD-amorphous. In our case, less Pd is 'seen" by XRD because Pd segregates to the surface of Pd-Co particles. An ultimate proof should come from XPS studies which are under way in our lab. 5. REFERENCES
1. e.g. P.N. Rylander, Hydrogenation Methods, Academic Press, London, 1985. 2. W. Juszczyk and Z. Karpidski, J. Catal., 117 (1989) 519. 3. 2.Karpidski, Adv. Catal., 37 (1990) 45. 4. T. Mall&, S.Szabd, J. Petrd, S. Mendioroz and M.A. Folgado, Appl. Catal., 53 (1989) 29 and references cited therein. 5. Z. Pad and P. Tdtdnyi, Nature, 267 (1977) 234. 6. H. Zimmer, M. Dobrovolszky, P. Tdtdnyi and Z. Pad, J. Phys. Chem., 90 (1986) 4758. 7. J.R.H. van Schaik, R.P. Dessing and V.Ponec, J. Catal., 38 (1975) 273. 8. M.J.Dees and V. Ponec, J. Catal., 119 (1989) 376.
Guai, L a al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All righti reserved
TEMPERATURE-PROGRAMMED REDUCTION IN CATALYSIS: A CRITICAL EVALUATION OF THE METHOD G. Fierrd: M. Lo Jacond: M. Inversia, G.Morettia, P. Portaa and R. Lavecchiab %ntro di Studio del CNR su "Strutturae Attivita Catalitica di Sistemi di Ossidi" (SACSO) c/o Dipartimento di Chimica, Universita "La Sapienza", Piazzale Aldo Moro 5,00185 Roma,
Italy bDipartimentodi Ingegneria Chimica, Universita "La Sapienza", Via Eudossiana 18,00184 Roma, Italy
Abstract Temperature-Programmed Reduction (TPR) has been critically tested by carrying out experiments at different operating conditions on both copper-based catalysts and pure copper oxides (CuO, Cu20). In this work we give experimental evidence that, even if interference caused by mass transfer limitations or dispersion is removed, the reduction profiles are markedly affected by artefacts when an inadequate combination of the typical TPR operating variables is used. Furthermore, an attempt of tracing back to the artefacts origin is also effected.
1. INTRODUCTION Temperature-Programmed Reduction (TPR) is a relatively new technique that in the last few years has widely been employed to study, in general, the reactivity of both bulk and supported catalysts. TPR revealed to be itself a highly sensitive method for discriminating the reducibility of any species in relation to its chemical state and its chemical 'surrounding'. However, the reduction profiles can be perturbed by artefacts if TPR measurements are carried out under inadequate experimental conditions and if the occurrence of spurious effects has not been checked in advance. As pointed out in a fundamental work by D.A.M. Monti and A. Baiker [J. Cat. 83, 323 (1985)] typical TPR results, namely the temperature which corresponds to the maximum of the reduction profile and the shape of the profile itself, are affected both by factors extrinsic to the technique, as mass transfer limitations, and by the experimental operating variables. If the extrinsic effects are removed, a characteristic number 'K', relating the TPR operating variables to each other and having dimensions of a time , was defined by these authors as follows:
1848
where 'So' is the amount of the reducible species, 'V" is the total flow rate of stream and 'cot is the initial concentration of the reducing gas. In order to obtain optimum results, the number 'K has to range within 55-140 seconds for heating rates of 6 K min-' up to 18 K min'l. Taking into account the Monti and Baiker approach, in this work we give experimental evidence that artefacts markedly perturb the TPR profiles at high 'K values; however, we demonstrate that the Monti and Baiker criterion is not always sufficient for providing the reliability of TPR results at low ' K values: in fact we show that at low ' K values, even if belonging to the correct range, artefacts can also occur. At last, an initial effort of clarifying the artefacts origin both at low and at high K values is made.
2. EXPERIMENTAL and RESULTS Provided that extrinsic effects were not controlling the chemical process, we have investigated the temperature-programmed reduction of CuO, Cu20 and some CuO-ZnO catalysts employing both adequate and inadequate ' K values. In all experiments the heating rate was fixed at 5 K min-' and a diluted mixture of H2 in N2 was used as stream. In Fig. 1 the different reduction profiles, each one obtained at the labelled K value, are reported for the studied samples:
9) tr
CUO
-e,
Q 3
$
1
373 T / K 573
-
473 T/K 673
-
Figure 1. Experimental TPR profiles obtained for copper oxides and CuO-ZnO catalysts by employing different values of the characteristic number ' K (see text).
1849
The artefacts origin at low 'K values has a different nature with respect to that at vety high 'K' values in our experimental conditions. For the latter case we have developed a kinetic model that takes into account H2 adsorption-desorption processes: this kinetic model enables us to provide a possible interpretation of either the double peak appearance in the TPR profiles of pure copper oxides and the spreading of the reduction profile for the CuO-ZnO catalysts, respectively. The kinetic model consists essentially of the following equations: V*C,,- v*ce+ r v = o
(2)
that represents the H2 mass balance equation, and r = k c + k, c cs - kaC'
(3)
that is the rate equation accounting for the overall H2 consumption and consistent with the following kinetic scheme:
Me0 + H2+ Me + H20
(4)
H ~ + o $H i
(5)
In the above equations 'co' and 'Ce' are the inlet and oulet hydrogen concentration, respectively, 'V" is the volumetric flow rate, 'v' the reactor volume,'r' the rate with which H2 disappears from the gas phase, ' k i and 'k-a' are the rate constants for the adsorption and desorption equilibrium, respectively, 'cs' is the surface concentration of the adsorption sites and 'c" the H2 concentration in the adsorbed phase.The resulting TPR profiles, i.e. H2 uptake as a function of temperature, can be easily obtained by solving the eqs. (2) and (3) and expliciting the temperature dependence of either the kinetic constants k, k, k.a and the heating rate. An illustrative plot of simulated TPR profiles derived from this computer shape analysis is shown in Fig. 2 :
Normalized hydroqen uptake
E 0
1
Normallied
0 temperature
Figure 2. Simulation of TPR profiles using a kinetic model which takes into account adsorption-desorption of H2 in parallel with the reduction (see text).
1
1850
It should be noted that the shape and the position of these profiles are depending on either the heating rate and the relative contributions of each term appearing in eq. (3).
3. DISCUSSION It is worth noting from Fig. 1 the extreme change of the TPR profile shape when inadequate values of the characteristic number 'K are used. At low ' K values the double peak that appears in the CuO reduction profile can be caused by sublimation of metallic copper on the unreduced CuO particles during the TPR run, as testified by the observed formation of a metallic copper mirror around the reactor walls. On the other hand, at K values much higher than 140 s, that represents the most common case, artefacts, such as multi-peak appearance and spreading, perturb the reduction profiles of all examined samples so that the reliability of the TPR results is completely lost. A possible interpretation of these artefacts could be drawn if either the dynamic of the TPR method and adsorptiondesorption of H2 in parallel with the reduction process are taken into account. At the maximum of the TPR profile, which corresponds to the highest value of the reduction rate, the overall H2 consumption (see eq. 3)can be equal to the feed rate owing to the very high ' K value (see eq. 1). Therefore, the H2 concentration at the reactor outlet can be almost zero for a certain time: in this situation the signal of the thermal conductivity detector remains constant and fixed at the value of the maximum reduction rate (determining a profile spreading, as for CuO-ZnO catalysts) or, if H2 desorbs, the amount of desorbed H2, small as it may be, could be revealed by the detector as an enrichment of the gas stream ( as for CuO and Cu20 samples). In the latter case, owing to the detector signal polarity, the stream enrichment results in a decreasing of the detector output producing a minimum in the TPR profiles so that an apparent double peak is forrned.The shape analysis of TPR profiles based on our kinetic model supports the suggested interpretation.
4. CONCLUSIONS
-
From this work the following conclusions can be drawn:
It was experimentally ascertained that, even if interference by mass transfer
limitations or dispersion is removed, an inadequate combination of the experimental operating variables markedly perturb the TPR profiles. - Under improper conditions, we evidenced that artefacts such as double-peak appearance or spreading characterize the reduction profiles of pure CuO, Cu20 and some CuO-ZnO catalysts, respectively. - As initial effort devoted towards outstanding of the quite complex origin of these artefacts, we ascribe their appearance to sublimation of the reduced copper or to H2 adsorption-desorption processes. In the latter case a simple kinetic model was developed that allows us to simulate the perturbed TPR experimental profiles.
Guni, L u ul. (Editors), New Frontiers in Catulysk
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 6 1993 Elsevier Science Publishers B.V. All rights reserved
SCANNING TUNNELING MICROSCOPY STUDY OF PdGRAPHITE MICROSTRUCTUREAND REACTIVITY
K L. Yeung and E. E. Worf Department of Chemical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
Abstract STM study of Pdgraphite catalyst shows a distribution of crystallite shapes indicating metastable morphologies. The crystallites also exhibits high surface microroughness. Hydrogenation of 1,3- butadiene on this catalyst gives a TOF of 0.38 at 300°K. The catalyst exhibits high reactivity even at room t e m p r a w for moderate HdC& ratios. 1. INTRODUCTION
Scanning tunneling microscopy (STM) is an important surface technique capable of imaging three dimensional surface structures with atomic resolution. Application of STM to catalyst characterization can be a significant contribution to the understanding of the effect of the catalyst microstructure on its reactivity and selectivity. The STM can probe the size and morphology of supported catalyst crystallites allowing us to determine the available catalyst area and the various types of catalytic sites present. STM can also probe the local surface structures on these crystallites to determine the nature of these catalytic sites. Previous studies done by the authors were concentratedon the catalyst preparation and in refining the STM technique for application in characterization of supported catalysts [1,2,3,4]. Statistical data on the effect of various preparations and pretreatments on the sizes and morphologies of the Pt and Pd catalyst crystallites have been obtained [2,3,4]. These efforts have enable us to achieve some limited control on the crystallite size and morphology [3] of our catalysts. The objective of this STM work is to investigate the morphologies of the Pd particles prepared by impregnation of tetraamine Pd(Q nitrate precursor on flat graphite sheets and to measure the catalyst reactivity toward 1,3- butadiene hydrogenation. 2. EXPERIMENTAL
The Pd catalyst was prepared by depositing 6.4 ml solution of 0.0105 M of tetraamine Pd(II) nitrate (Aldrich Chemical Corn any, Inc.), onto the surface of the nuclear grade high purity graphite (NGHPG,196 cm ), giving a Pd loading of 1017 Pd atoms per cm2 of the support. A thin, uniform film of liquid solution was obtained by first dispersing small droplets of the solution on the graphite surface, then the liquid was pressed between
h
1852
two graphite layers. The deposition was conducted at mom temperature for 12 h. The resultant supported precursors were dried at 373'K in air for 24 h. The dried catalysts were then decomposed in flowing Ar (UKP, Linde Union Carbide) at 873°K for 2 h, then reduced in flowing H2 (UHP,Linde Union Carbide) at 723OK for 2 h, and finally outgassed at'673OK in flowing Ar for another 2 h. The catalysts were then slowly cooled back to mom temperature in Ar. Prior to reaction, the reduced Pd catalyst was characterized with STM. The STM used in this study is a commercial Nanoscope II (Di 'talInstrument Inc.) operated in ambient conditions. A medium STM scan head of 75&Inm range was operated in the height imaging mode providing reasonable scan area with good image resolution. The operational bias voltage was kept between 50 and 150 mV with the tunneling current fixed at 1 nA.The horizontal scan frequency was maintained below 2 Hz. The hydrogenation of 1,3- butadiene was conducted in a flow reactor with a constant flowrate of reactant mixture of 110 ml/min. The reactant mixture consists of 1,3butadiene and hydrogen with nitrogen as diluent. The partial pressure of the 1,3butadiene was kept constant at 4 torr with the overall reactor pressure of 1 a m . The catalyst is in the form of 0.9 x 4 cm strip of Pd/NGHPG with overall Pd loading of 0.13 mg. The products were separated with a 7 ft chromatographic column packed with 0.19% picric acid graphpac packings (Alltech Associates, Inc.) and monitored with a FID detector.
-
3. RESULTS A N D DISCUSSION
3.1 STM characterization of Pd/NGHPG The S T M images of the reduced Pd crystallites on NGHPG (fig.1) shows two types of crystallite morphologies. Crystallites of rectangular, box-like morphology with rounded corners coexist with oblong shaped crystallites as shown in fig.1a. Such distribution of crystallite shapes is an evidence to the metastable condition of the catalyst crystallites, Fig. 1b shows the three dimensional microstructure of the supported Pd crystallite. The figure shows that the crystallite have a rough and irregular surface morphology. The size of the catalyst based on hydrogen chemisorption is 140A (assuming sph rical particles) while the average size measured by STM is -190 A with a height of -40 .
A
Figure 1. (a) Large scale STM image of Pdgraphite catalyst (b) 3-dimensional morphology of the Pd crystallite showing surface roughness.
1853
The Pd crystallites on NGHF'G exhibits an anomalous height indicating a plate-like morphology with most of the crystallites having a height to length ratio of only about 1: 6, compared to 1: 2 m e a s d in most transmission electron microscopy ("EM) studies for metal particles on oxide supports [5,6]. This height anomaly was also observed on other catalyst supported on graphites [ 1,2,3,4] and can be attributed partly to the nature of the graphite substrate which is susceptible to damages caused by the supported particles as shown by the presence of burn-holes in this and previous studies. Bum-holes are shallow pits produced by partial gasification of the graphite during catalyst pretreatment which later become exposed when the crystallites are displaced from its original location. While particle embedment can explain some of the height discrepancy,it is also conceivable that the crystallite's surfaces can be partially covered by surface impurities such as carbon or oxygen. The presence of this contamination layer can affect both the chemistry and conductivity of the surface and can distort the height measured by the STM.Although small surface details (i.e. atomic corrugations) can be drastically affected by such contaminants, it is unlikely that it will have a large effect on the overall morphology observed by the STM. 3.2 1,3- Butadiene hydrogenation The butadiene hydrogenation reaction will be used to probe the catalyst reactivity and selectivity, as the STM probes the microstructure of the Pd catalyst. Fig.2 shows 1,3butadiene conversion versus temperature at various H7&4H6 ratios. It can be seen that relatively high conversions can be attained during the reaction even at room temperam.
300
350
450
400
Temperature
5 0
(OK)
Figure 2. Conversion vs Temperature The tynover frequency (TOF) at 300'K and a HdC& ratio of 10 was found to be 0.38 which compares well with the values reported by Bertolini et. al. [8]. Likewise the selectivity is also a strong function of temperature and HdCqHg ratios. As expected increases in H2/C4&j ratio favors butane formation while at low H2/c4H6 ratio butenes are the primary praducts.The conversion increases with both temperature and HdCqHg ratio, but as shown in Fig. 3, an increase beyond HdC& ratio of -125 has no effect on the C4Hg conversion.
1854
4
0
100
H2 I C4H4 Ratio
Figure 3.
Conversion versus HdC4H4 ratio.
At low HdCqHg ratio of 4.0,we observed a switch in the selectivity from trans 2butene to 1- butene as the major butene product at low reaction temperature. Studies with catalyst prepared by different methods which have been shown to exhibit different morphologies, are underway to find if there is any relation between the catalysts morphology and its activity-selectivity behavior. 4. SUMMARY
The results show that STM is an important method for probing the micmstructure of supported catalyst crystallites. Results show that a distribution of crystallite shapes exist in all of the catalysts prepared, indicating that the crystallites have a metastable morphology. Surface microroughness observed in this study is also in agreement with the previous results on Pt [1,2,3]. Although the hydrogenation study is preliminary, the effects of major reaction parameters are thoroughly explored. This raction is sensitive to both crystallite structure [7] and dimensionality [El which is important for probing the catalyst behavior.
5. ACKNOWLEDGEMENT Funds to purchase the equipment were provided by NSF CBT 88-06640 and the research was funded by NSF C T S 90-01586.
6. REFERENCES 1 King Lun Yeung and E.E. Wolf, J. Vac. Sci. Technol. B. 9 (1991) 798. 2 King Lun Yeung and E.E. Wolf, J. Catal. In press. 3 King Lun Yeung and E.E. Wolf, Cat. Let. I n press. 4 King Lun Yeung and E.E. Wolf, J. Vac. Sci. Technol. In press. 5 A.K. Datye, A.D. Logan, and N.J. Long, J. Catal.. 109 (1988) 76. 6 J.C. Heyraud, and J.J. Metois, J. Crystal Growth, 50 (1980) 571. 7 C.-M. Pradier and Y. Berthier, J. Catal., 129 (1991) 356. 8 B. Tardy, C. Noupa, C. Leclercq, J.C. Bertolini, A. Hoareau, M. Treilleux, J.P. Foure, and G.Nihoul, J. Catal., 129 (1991) 1.
GUni, L et al. (Editors), New Frontiers in Catalysis
Pmceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 6 1993 Elsevier Science Publishers B.V.All rights reserved
IR STUDY OF ADSORPTION AND DEUTERATION OF DcACETONE ON Pt/ZnO CATALYSTS EFFECTS OF THE SAMPLE PRETREATMENTS F. Boccuui, G. Ghiorti andA. Chiorino
Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, via P. Giuria 7, 10125 Torino, Italy
1. INTRODUCl'ION
Modifications of the catalytic properties of dispersed metals by the substrate were discussed by many authors (1). Very recently Sen and Vannice (2) studied the CO and the acetone hydrogenation over many Pt supported catalysts: the HTR Pt/Ti02 samples showed the highest activity, attributed to an increase of the active site concentration, presumably defects on the titania surface near the Pt. On these sites a di-sigma bonded species as important reaction intermediate was hypothesized, but no direct spectroscopic evidences were produced. Pt/ZnO catalysts reduced at high temperature show an higher CO hydrogenation activity than mildly reduced ones, however no special sites for CO adsorption at the interface between Pt and ZnO were evidenced (3); in the strongly reduced samples an intermetallic PtZn compound, enriched in Zn at the surface, is formed while on mildly reduced samples pure Pt is present. The changes in the composition of the metallic phase can be at the origin of the reactivity modifications; we studied therefore on differently pretreated samples the adsorption and the deuteration of the deuteroacetone. 2. ExPERIlzENTAL
Pt/ZnO samples were prepared and treated as previously described (4): the mildly reduced ones ( MR) were reduced in a 0.5 % D2 in N mixture at 493 K, the strongly reduced ones (SR) were treated fn pure D at 573 K. Infrared spectra were recorded with a FTIR Perkin-hner 1760 spectrometer in a static cell allowing to treat the samples in situ. 3 . RESULTS AND DISCUSSION
IR spectra reveal that differently pretreated samples show significant differences in the reactivity both with pure deuteroacetone and with deuteroacetone-deuterium mixtures. We will discuss both these interactions in the two next sections. d6-acetone adsorption on differently pretreated samples. Table I summarizes the spectroscopic features of d -acetone adsorbed on the two differently pretreated Pt/ZnO sampfes. On the mildly reduced samples the deuteroacetone is mainly adsorbed at RT in molecular end-on forms. Upon heating at 373 K min r amounts of side-on species are formed (weak band at 1545 cm-', fig. lb, curve 3). 3.1
1856
TABLE I Mode assignments ( cm’l)
for d6-acetone adsorbed on Pt/ZnO
iildly reduced
strongly reduced
Mode rultilayer Va(CD3)
2255
end-on
side-on
iultilayer end-on unsat. diketone enolic
2225
“s(cD3)
2255
2221
2115
V(W
1705
1677
i(cD3t
1050
Va(CD3CCD3) 1255
2115 1545
1705
1670
1060
1050
1060
1278
1255
1277
1580
1350
1465
(v C=C) At the same time bands in the OD stretching region grow up, a decrease of the IR transmission is observed and some chemisorbed CO appears. 10.0
-
1795
a
1255
Xt
7. 0
-
3000
1
a00
loo0
2000
0 3000
a00
P300
d
cm -1
1500
2000 -i
em
2300
?BOO cm -i
too0
1500
1000
cm - I
Pig. la -IR transrission spectra of HR sarple: curve 1, background, curve 2, 30 TOE of d6 acetone at AT, curve 3, heated at 373 K; lb absorption spectra of MR saiple: curve 1, 30 Torr of d6acetone, curve 2, 0.5 Torr, curve 3, heated at 373 K; lc - transmission spectra of SR sarple: m e 1, background, m v e 2, 30 T o n of d6-acetone, after 5 rin, curve 3, after 15 h; ld aborption spectra of SR saiple: curve 1, 30 Torr of d6-acetone, after 5 Bin , curve 2, after 1 h, curve 3, after 15 h.
-
-
1857
The decrease in the IR transmission is destroyed by oxygen interaction. The observed phenomena indicate that the adsorption on the mildly reduced sample at RT is only molecular while some abstraction of deuterium atoms and some breaking of C-C bonds occurs by heating. Reactions like: CD3COCD 28
+ +
CO + CD4 + 2D 2D+ + 2e-
+
C
can occur: the second reaction step being responsible for the decrease in the IR transmission (5). On strongly reduced samples the interaction with d6acetone at RT causes an immediate and almost complete l o s s of IR transmission and strong bands in the OD stretching region (fig. lc, curve 2 and 3 ) ; this behaviour indicates that on this sample the deuteroacetone looses ve y easily D atoms. Moreover, a quite broad band at 1580 cm-f grows up, after a long contact time (fig. Id, curve 3). Bands at 1530-1585 c m ' l were very recently observed by Vannice et al. ( 6 ) in RAIRS experiments of adsorption of acetone on Pt(ll1). The bands were attributed by the authors to acetone side-on chemisorbed species; we think that the band observed in our experiments in the same spectral region can be related to the formation of an unsaturated diketone species, that is to a deidrogenation product of the acetone; in fact, by interaction with D2 the band reduces its intensity, at the same time the bands related to molecularly adsorbed acetone increase. A condensation and deidrogenation of the acetone can produce an unsaturated diketone, w ich usually shows a strong and broad band at 1400-1600 cm' , assi ned to C=O and C=C mixed modes (7). No bands at 1535 cm' , assigned to side-on chemisorbed species were detected on this sample.
9
1.0
*I
1705
b
A
1
1255
0.5
2300
P
2000
MOO
em -i
1170
I
0.5
100
2300
2000
1800 CI -1
1000
Pig. 2 - IR absorption spectra of D2 -d6-acetone interaction on: a) HR saiple; b) SR sanple, recorded after O2 inlet.
1858 E 6 - a c e t Q n e deuteration on Bifferentlv Dretreated
S
-
m
Fig. 2 shows the IR spectra of the deuteration of d6acetone on the two samples. A s a consequence of the fact that in D2 atmosphere the samples loose completely the IR transmission, as shown in a previous paper (51, the spectra were recorded, after 15 h of contact, after inlet of few of oxygen: therefore D20 vibrational bands at X2600 and 1180 cm-l are observed. The hydrogenation reaction with D2, on the mil ly reduced assigned samples produces at RT bands at 2226 and 1170 to v,(CD)~ and to v (C-C-C) modes of the perdeuteroisopropylalcohol respectively, in agreement with the assignments of Vannice et al. ( 8 ) . However, large amounts of unreacted deuteroacetone remain in the cell, also after many hours of contact (fig. 2a). The reaction with D2 on the strongly reduced sample produces, also in this case, as unique product spectroscopically visible, the perdeuteroisopropylalcohol (fig. 2b) but, at difference than in the previously illustrated case, the deuteroacetone present in the cell or chemisorbed on the surface is almost completely depleted.
%"
ern-',
CONCLUSIONS The previously illustrated data indicate that the more strongly reduced samples show an higher dehydrogenating and hydrogenating activity than the mildly reduced ones. The different behaviour can be mainly due to the changes in the surface composition of the metallic particles pointed out in previous paper (3,4)i the isolated Pt atoms are less active in C-C breaking and more active in dehydrogenation and hydrogenation reactions, consistently with the Sen and Vannice experiments on Au-Pt alloys (2). 4.
Acknowledgerent
We gratefully acknowledge financial support from CNR Progetto Finalizzato Chimica Fine e Secondaria III'.
I'
REFERENCES 1 G. L. Haller and D. E. Resasco, Adv. in Catal., 36, 173 (1989) and references therein 2 €4. Sen and M. A. Vannice, J. Catal.,l13,52 (1988) 3 F. Boccuzzi, A. Chiorino, G. Ghiotti, F. Pinna, G. Stru-
khl and R. Tessari J. Cata1.,126, 381 (1990) 4 F. Boccuzzi, G. Ghiotti, A . Chiorino and L. Marchese, Surf. Sci., 233, 141 (1990) 5 F. Boccuzzi, A. Chiorino, G. Ghiotti and E. Guglielminotti, Langmuir, 5, 66 (1989) 6 M. A. Vannice, W. Erley and H. Ibach, Surf. Sci., 254, 1
(1991) 7 K. Nakamoto, P. J. McCarthy and A. E. Wartell, J. A m e r . Chem. SOC., 83, 1272 (1961) 8 M. A. Vannice, W. Erley and H. Ibach, Surf. Sci., 254, 12
(1991)
Guczi, L. ef al. (Editors),New Fronfiers in Cafalysh
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
SURFACE ENERGETIC CHARACTERIZATION OF SUPPORTED METAL CATALYSTS BY GAS/SOLIDTITRATION MICROCALORIMETRY
J. M.Guy A. P. Masia, A. R. Paniego and J. M.T.Menayo Instituto de Quimica Fisica "Rocasolano", CSIC, Serrano 119,28006 Madrid, Spain
Adsorption microcalorimetry is a technique which has proved its usefulness in the characterization of catalysts, mainly using probe molecules. Its application can be extended to the study of surface reactions, thereby broadening the information that can be obtained. Here we show results of hydrogen titration of preadsorbed oxygen on iridium supported catalysts. Thermochemical calculations with the calorimetric data yielded information on the surface energetics for the two adsorbates.
1. EXPERIMENTAL Hydrogen and oxygen (Sociedad Espaiiola del Oxigeno, Spain), 99.995% pure, were used as adsorbates. Supported iridium catalysts were prepared by the incipient wetness method using hexachloroiridic acid (Alfa Ventron Inorganics.) Silica gel (BASF, D-ll- 11) and y-alumina (Girdler, T-126) were used as supports. Specific surface areas determined by the BET method, after heating in air at 973 K, were 128.0 and 149.1 m2g-', respectively. The following catalysts were prepared: Ir(5 %)/Siq, Ir(0.5 %)/SO2 and Ir(2.5 %)/A120, with 5.45%, 0.53% and 2.55% metallic content. The surface iridium amount, Irsf, was determined as described elsewhere [ 11 and is given in the first row of Table 1. Calorimetric measurements were carried out in a Tian-Calvet microcalorimeter (Model BT, Setaram, France), coupled to a volumetric apparatus with a Baratron manometer (MKS, USA.) Titration calorimetric experiments of preadsorbed oxygen with hydrogen were performed at 315 K. Differential heats of adsorption for hydrogen and oxygen were previously determined since they were needed for the thermochemical calculations described later. Calorimetric measurements were also made on the supports to take into account the adsorption on them. In the following, amounts of substance are expressed as pmol (of atoms in the case of hydrogen and oxygen) per gram of catalyst dried in vacuum at 700 K.
2. RESULTS AND DISCUSSION 2.1 Titration volumetry In figure 1 hydrogen titration isotherms for the two Ir/Siq samples are given as hydrogen uptake divided by surface iridium amount. The isotherms for the two samples nearly coincide. The dispersion degree, and therefore metal particle size, is approximately the same for the two samples (5%: 2.6 nm, 0.5%: 3.5 nm), which suggests similar surface properties. The titration isotherms show an initial vertical segment at zero equilibrium pressure. Afterwards a right angle knee appears followed by a final part where hydrogen uptake in-
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creases smoothly with pressure. Figure 1 ' also depicts a hydrogen adsorption isotherm. The knee and final part are similar to those observed in titration isotherms, which can be ascribed to the same process taking place after adsorption or titration ended. In consequence, the appearance of pressure was taken as the 2 end point of the titration reaction. The ' , same conclusion holds for the adsorption and titration isotherms on the Ir/A1203 sample. The amount of titrating hydrogen, Htit, for the three samples is given in the PIkPo third row Of 1. The amount Of Figure 1 . Hydrogen titration isotherms on Ir/Si02: oxygen taken U P in the run preceding the 0 , 0 5% and X 0.5% samples, Lower line: titration experiment, O,,, is shown in the hydrogen adsorption isotherm on the 5 % sample. second row. (Surface oxidation may proceed beyond adsorption if oxygen is present in the gas phase [l]. The two runs on the 1r/Al2O3sample, Table 1, show this clearly.) The two quantities, Htit and O,,, allow us to calculate the amount of hydrogen that remains on the iridium surface after oxygen reduction and water migration to the support (fifth row in Table 1.) H/Ir stoichiometries obtained (sixth row) are different for different supports [ 13, which justifies the need to calculate them instead of assuming a unique value of 1.
t2
Table 1
Hydrogen titration volumetry results and stoichiometries H , I Irsf
2;
Surface Ir: IrSf 'ad
-f
'ox
(plkPa) Had
hit - 2.0pr
Had / Ir,,
Ir(5%)/Si02 95 111.5 113.3 319.5 319.3 (0.56) (1.33) 96.5 92.7 1.01 0.98
Ir(0.5%)/Si02 7.5
8.2 23.5 (0.18) 7.1 0.95
@no1 of atoms/g)
Ir(2.5%)/Al2O3 80 97.8 79.2 320.4 288.4 (0.22) (0.45) 130.0 124.8 1.63 1.56
2.2 Titration microcalorimetry The hydrogen titration differential heat per mol of hydrogen atoms consumed vs. amount taken up for various experiments on the Ir/SiO, samples is plotted in Figure 2-A. The titration heat includes the enthalpy change produced in each one of the processes taking place in the overall surface reaction. The dashed line corresponds to the adsorption heat measured in a hydrogen calorimetric isotherm on the 5 % sample. The titration and adsorption curves are in good agreement, which indicates that the hydrogen adsorption involved in the titration run follows the same trend as in an adsorption experiment, i.e. the iridium sites thatflrst become empty after oxygen reduction and water migration to the support, are precisely those of higher hydrogen adsorption energy. The same conclusion is reached for the titration reaction on the Ir/A1203 sample (Figure 2-B.)
2.3 Thennochemical calculations The following thermochemical cycle can be written for the elementary reactions that take place in the overall titration process:
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where Sup stand for the support and cx is the value of the H/Ir stoichiometry: 1 and 1.6 for Ir/SiO, and Ir/A1,0? catalysts, respectively, as seen in Table 1. This thermochemical cycle was used to calculate the oxygen adsorption heat on these catalysts. (Calorimetric direct measurement of this quantity gave a constant value over most of the coverage range, since mobility of adsorbed oxygen is precluded by the strong 0-Ir bond formed. Therefore oxygen remains in the site where it first adsorbs, so that heats measured are statistical mean values.) From equations (1-5) we obtain: (6)
Adsorption and titration heats depend on coverage, so that application of equation (6) yields oxygen adsorption heats on iridium as function of coverage. A value of 242 kl/mol was used as water formation heat at 315 K in the vapour state [2]. Water adsorption heat on the supports vs. coverage was taken from the literature ([3] for silica and [4] for alumina.) The hydrogen adsorption heat and the hydrogen titration heat of preadsorbed oxygen at different coverages were interpolated from experimental values. Data and results for one experiment on the 1r/A1203 catalyst are given in Table 2. The amounts of oxygen being titrated appear in column 1. The hydrogen consumed in the oxygen titration is given in column 2: each Ir atom adsorbs one 0 atom; upon H titration, two H atoms combine with the 0 atom to form water (column 4) that migrates into the support, and 1.6 hydrogen Figure 2. Hydrogen titration heat: (A) IdSi02: o , 0 5% atoms (column 3) adsorb on the freed and X 0.5% samples. (B) Ir(2.5%)/M2O3: A , V Dashed Ir atom (one atom only in the case of Ir/Si02 samples.) lines: hydrogen adsorption heat.
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Table 2 Thermochemical calculations:
The differential heat values in columns 5, 6 and 7 were obtained by interpolation to the corresponding amounts in the preceding columns, The oxygen adsorption heat, which is the calculated value for breaking the existing O-Ir bond (column 8), increases as the titration reaction progresses. This shows that this reuction proceeds from weaker to stronger 0 - l r bonds. The same kind of calculations were made for the Ir/SiO, samples and similar results were Obtained. Figure 3 depicts Figure 3. Calculated oxygen adsorption heat on: Ir/Si02: Oxygen adsorption heats vs. coverage, 5%and X 0.5% and Ir(2.5%)/A1203:V . Dashed with O n e experiment for each sample. line: measured values on the 5% sample. They are in good accord and reveal the heterogeneity not shown by direct calorimetric measurements (dashed line in Figure 3.) From the two conclusions that have been reached we can deduce that iridium sites which adsorb oxygen weakly adsorb hydrogen strongly, and viceversa. The question arises wether this opposite heterogeneity is a property of the iridium crystallites, or if it produced upon adsorption of oxygen on them, i.e. if the oxygen on the indium surface could induce in it weak adsorption sites for oxygen which would be strong sites for hydrogen. This work was partially supported by the DGICYT, Spanish Ministry of Education and Science, under Project no. PB87-0327. 3. REFERENCES
1. M. Cabrejas Manchado, J.M.Guil and A. Ruiz Paniego, J. Chem. SOC.Faraday Trans. I , 85 (1989) 1775. 2. TRC Thermodynamic Tables, Texas Engineering Experimental Station (199 1). 3. J. Fournier, B. Fubini, V. Bolis and H.PkzCrat, Can. J. Chem., 67 (1989) 289. 4. G. Della Gatta, B. Fubini and G. Venturello, I. Chim. Phys. 70 (1973) 64.
G u n i , L ei al. (Editors), New Frontiers in Colalysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights resewed
STUDY OF THE EFFECTS OF ANNEALING ON THE MORPHOLOGY OF PLATINUM CLUSTER SIZE ON HIGHLY ORIENTED PYROLYTIC GRAPHITE BY SCANNING TUNNELING MICROSCOPY S. Lee, H. Permana and K
Y. S. Ng
Department of Chemical Engineering, Wayne State University, Detroit, Michigan 48202, USA
Abstract The effects of annealing on the morphological transformation of platinum clusters supported on graphite was investigated by scanning tunneling microscopy. Without annealing, the as-deposited platinum consists of an aggregation of smaller clusters. These clusters become more uniform and spherical after annealed for 4 hours. Further annealing shows a transformation from spherical to elongated shape. Introduction The morphology of metal clusters plays an important role in determining the catalytic activity and selectivity of many structure sensitive reactions. Thus, a fundamental understanding of the particle size and the shape of supported metal clusters is essential and has been the focus of a number of studies, mainly by TEM and SEM [I-41. Recently, with the development of non-destructive scanning tunneling microscopy (STM) [ 5 ] , small clusters and surface structures can be observed in real 3-dimension space with atomic resolution. Ganz et al. [6] were among the pioneers in demonstrating STM as a very promising tool for the study of metal clusters supported on highly oriented pyrolytic graphite (HOPG). Their work was followed by a number of STM investigations on Pt cluster size, statistical analysis of bond length and bond angle of Pt clusters, effects of adsorbed Pt clusters on HOPG surface structure, and the effect of substrate functionalization on crystal size, distribution, morphology, and surface structure [7-1I]. However, the correlations among the degree of aggregation of Pt on the substrate, pretreatment conditions, and the transformation of shapes of the clusters have not been fully elucidated by STM. In this study, STM is applied to investigate the effects of annealing on the morphological transformation of vacuum vapor deposited Pt clusters on HOPG. Experimental Samples of platinum clusters supported on HOPG were prepared by a vacuum vapor deposition technique. Platinum was deposited onto newly cleaved HOPG by heating a platinum wire (Johnson Matthey, 99.99 % pure) in a tungsten boat at about 1000 OC in a vacuum of 7 X tom The HOPG substrate was rotated at different angles to control the amount of Pt deposited. The Pt-deposited samples were then annealed in a quartz tube furnace under flowing Ar at 600 OC. Four different samples were prepared and
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each sample was subjected to different annealing time of 0,4,12, or 24 hours. The samples were then imaged with a STM (Omicron) under ambient conditions without further treatment. All of the S T M images were obtained in a constant current mode, with a current of 1.5 nA and a sample bias of -15 mV. Typical scanning time was about 140 seconds for each image.
Results and Discussion Figure 1.a shows a three-dimensionalimage of an as-deposited Pt cluster adsorbed on HOPG without annealing pretreatment. The cluster appears as a bright spot in the upper left of the picture, and has a diameter of about 160 A. The cross section profile of this cluster in Figure 1.b shows that it is formed from aggregations of smaller clusters. The sizes and heights of the clusters ranges from 20 A to 50 A and 10 A to 40 A, respectively.
Figure 1.a. 3-dimensional STM image cluster of an vacuum vapor deposited Pt formed by on H O E without annealing.
Figure 1.b Cross section profile of Pt clusters shows that the large Pt cluster is aggregation of smaller clusters.
When the sample was annealed with Ar at 600 OC for 4 hours, clusters of about 150 A are observed [Figure 2.a]. The heights of the clusters are about 12 A. Interestingly, the Pt clusters are no longer an aggregation of small Pt clusters, the shape of Pt clusters has become more uniform and spherical, as shown in cross section profile [Figure 2.b] . This observation is consistent with those of Wang et al. [12] and Chojnacki and Schmidt [13] using TEM. They suggested that the catalyst morphology is influenced by the presence of gaseous species, annealing time and temperature. In their study, the particles were found to transform from cubes to spheres, and in the case of heating in H2, the spherical particles can transform back to cubes because of a significant anisotropy in surface energy produced [12].
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Distance
Figure 2.a. 3-dimensional STM image of Pt clusters (circled) on H O E annealed in Ar at 600 OC for 4 hours.
(A)
Figure 2.b. Cross section profile of the Pt cluster (left cluster on Figure 2.a).
The effects of annealing time on transformation of shapes of the Pt clusters were further illustrated when the sample was annealed for 12 and 24 hours at 600 O C . Figure 3 is a scan of a 2000 A X 1500 A area of the surface of a sample after annealing in Ar at 600 O C for 12 hours. The diameters and the heights of Pt clusters ranges from 100 A to 150 A and 10 A to 15 A, respectively. The Pt clusters are still quite spherical. However, some clusters (bottom right of the picture) now appear to have a rounding of their comers. This transformation of shape is more pronounced when the sample is annealed for a longer time (24 hours) as shown in Figure 4. In this sample the spherical shape of the Pt clusters no longer exists, and the Pt clusters have become elongated. These elongated clusters are roughly 200 A long, 75 A wide and 15 A high. The average volume of these elongated clusters is within 15%compared with spherical clusters indicates that the elongated clusters are indeed the result of transformation of spherical clusters. After 24 hours of annealing, it is reasonable to assume that the clusters have attained their equilibrium shape at 600 OC. The equilibrium shape of small clusters is not completely resolved. Based on experimental and theoretical considerations, Drechsler [ 151 concludes that the equilibrium shape of small particles should be nearly spherical, but with the formation of carbon and oxygen species on the surface, polyhedra are observed. On the other hand, Wang et al. [12] suggest that for clean surface the equilibrium shape is polyhedral; while for adsorbate-covered surface, the equilibrium shape is spherical. In this study, we observed the transformation of irregular clusters (as-deposited) to uniform sphere (4 hours), to polyhedra with rounding corners (12 hours), and to elongated polyhedra (24 hours). Though high purity argon (99.9995%) was used during the annealing, it is possible that with a long annealing time, a small amount of hydrocarbon impurities were decomposed and adsorbed on the surface. This may explain the transformation of spherical shape (relatively clean) to polyhedral shape (with adsorbates) according to Drechsler [ 151. However, it can also be argued that the purging argon gas cleans the platinum surface. If this
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Figure 3. 3-dimensional STM image of Pt clusters on HOPG annealed in Ar at 600 O C for 12 hours.
Figure 4. 2-dimensional STM image of Pt clusters on H O E annealed in Ar at 600 OC for 24 hours.
is the case, then the transformation from spherical to polyhedral shape is the result of a cleaner surface. We are i n the process of using H2 and other gases under an in-situ environment to discern these possible explanations.
References 1. 2. 3 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14.
T.P. Chojnacki, K. Krause, and L.D. Schmidt, J. Catal., 128 (1991) 1419. M. Komiyama, S . Morita, and N. Mikoshiba, J. Micros., 152 (1988) 197. H.W. Salemink, H. Meir, R. Ellialtioglu, J.W. Gemtsen, and P. Muralt, Appl. Phys. Lett., 54 (1989) 1112. S. Gao and L.D. Schmidt, J. Catal., 115 (1989) 356. G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett., 49 (1982) 57. E. Ganz, K. Sattler, and J. Clarke, J. Vac. Sci. Technol., A6 (1988) 419. M. Komiyama, J. Kobayashi, and S . Morita, J. Vac. Sci. Technol., B9 (1991) 829. U. Muller, K. Sattler, J. Xhie, N. Venkaswaran, and G . Raina, J. Vac. Sci. Technol., B9 (1991) 829. J. Xhie, K. Sattler, U. Muller, N. Venkaswaran, and G. Raina, J. Vac. Sci. Technol., B9 (1991) 833. X.C. Zhou and E. Gulari, Acta. Cryst., A47 (1991) 17. K.L. Yeung and E.E. Wolf, J. Vac. Sci. Technol., B9 (1991) 798. T. Wang, C. Lee, and L.D. Schmidt, Surf. Sci., 163 (1985) 181. T.P. Chojnacki and L.D. Schmidt, J. Catal., 115 (1989) 473. M. Drechsler, Surface Mobilities on Solid Materials, Vu Thien Binh (ed.), Plenum, New York, 1983.
Guni, L.el al. (Editors), New Fronriers In Cotalysk Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
DISTRIBUTION OF MOOXIDATION STATES IN REDUCED Mo/A1203 CATALYSTS. CORRELATION WITH CATALYTIC ACTIVITY J. Yammarub, M.YamaaW, M. HoualW and D. M. HerculeP
aDepartmentof Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA bThe Kansai Coke and Chemical Co.,Ltd., 1-1Ohama, Amagasaki 660, Japan CDepartmentof Molecular Chemistry and Engineering, Tohoku University, Sendai 980, Japan
Abstract The distribution of Mo oxidation states in reduced Mo/Al2O3catalysts (8 wt% Mo) was determined by ESCA. The results showed that Mo oxidation states ranging from +6 to 0 are produced on reduction between 200°C and 900°C. The catalytic activity of the reduced Mo/Al,03 catalysts for propene hydrogenation and propane hydrogenolysis was measured as a function of the reduction temperature. The variation of catalytic activity of the reduced catalysts was compared with the distribution of Mo oxidation states obtained from ESCA. The results indicated that Mo oxidation states I+4 are required for propene hydrogenation. Propane hydrogenolysisrequired a more severe reduction pretreatment. The catalytic activity correlated with the change in the abundance of Mo metal. This suggested that Mo metal is the most active species for propane hydrogenolysis.
INTRODUCTION It is well known (1-4) that given the appropriate reduction pretreatment Mo/Al,03 catalysts can be active for various reactions (isomerization, metathesis, hydrogenation and hydrogenolysis). This behavior suggests that the activity is associated with a specific Mo oxidation state. However, to date, direct correlation between the catalytic activity and the abundance of a specific oxidation state has been difficult to establish primarily because of inadequate means for measuring the distribution of Mo oxidation states in reduced Mo/A1,0, catalysts. A direct measure of the distribution of Mo oxidation states can, in principle, be achieved by ESCA (5,6). The objective of this study is to determine the distribution of Mo oxidation states in reduced Mo/A1,03 catalysts by ESCA and to correlate the results with catalytic activity for various probe reactions.
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EXPERIMENTAL Catalyst Preparation and Pretreatment The catalyst used for low temperature reduction studies (8% Mo/y-A1,03) was prepared by incipient wetness impregnation of y-A1203 (180 m2/g) with ammonium paramolybdate solution followed by drying at 120°C and calcination at 500°C in air. Catalyst reduction was Wried out in flowing hydrogen (50 cc/min) for 16 hrs. For high temperature reduction studies the catalyst used (8% Moly-Al,O,) and the reduction conditions were the same as those employed by Hall and Coworkers (7). ESCA Measurements X-ray photoelectron spectra (XPS or ESCA) of Mo/A1,03 catalysts were obtained with an AEI ES200 spectrometer equipped with aluminum anode (AlKol = 1486.6 eV) operated at 12KV and 20mA. For reduction or oxygen pretreatment, the catalysts were pressed as pellets and mounted on a sealable probe which permitted transfer of the treated catalyst from an external reaction chamber to the spectrometer without exposure to air. A damped nonlinear least squares fitting (NLLSF)routine was used to curve fit the Mo3d envelope. The binding energy values were referenced to the A12p line (74.5 eV). The methodology used for curve fitting the Mo3d envelope and the assumptions made for quantitative analysis of the distribution of Mo oxidation states were described elsewhere (6).
Catalytic Activity Measurements All catalytic activity measurements were carried out in a quartz fixed bed flow microreactor. The reaction products were separated and analyzed by a Perkin Elmer Sigma 2000 gas chromatograph using a column packed with chromosorb W-HP or 10% Carbowax400 on chromosorb W-HP. Propene hydrogenation activity was measured at 74°C and 1 atm. The reactants (10% propene in H,) were introduced at a flow rate of 50 cc/min. Propane hydrogenolysis activity was measured at 250°C and 1 atm. The reaction mixture (10% propane in HJ was introduced into the reactor at a flow rate of 25 cc/min. RESULTS AND DISCUSSION
Low Temperature Reduction The catalyst (8% Mo/y-Al,03) was reduced between 200°C and 400°C. The distribution of Mo oxidation states in the reduced catalysts was determined by ESCA. The Mo3d envelope for the oxidic catalyst can be curve fitted as a single doublet with B.E. values characteristic of M o + ~(Mo3d5/*: 233.1 eV, Mo3d3,, : 236.2 eV). Curve fitting of the Mo3d envelope for the catalysts reduced between 200°C and 400°C required the additional use of 3 sets of doublets with Mo3dSI2B.E. values of 231.8 eV, 229.9 eV, and 228.8 eV which can be attributed to Mots, Mot4, and Mot3, respectively. Figure 1 shows the distribution of Mo oxidation states for the Mo/A1203catalysts as a function of reduction temperature. The amount of M o + (not ~ shown in Figure 1) decreases from ca. 83 to 18% with increasing the reduction temperature from 200°C to 400°C. Also shown in Fig. 1 is the variation of propene hydrogenation activity expressed as % conversion as a function of the reduction temperature. The catalyst reduced at 200°C did not show any activity for propene hydrogenation (Fig. la). Propene hydrogenation activity appears to take
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da
01, m
‘
I
I
1
ou fi
ap
4
-s +
.- 40 3
4-
v)
a x
+ 20
8
0 500 600 700 800 900 T°C
Figure 1. (a) Propene hydrogenation activity versus reduction temperature. (b) Distribution of Mo oxidation states in Mo/A1,03 catalysts as a function of the reduction temperature.
ToC
Figure 2. (a) Propane hydrogenolysis rate versus reduction temperature. (b) Distribution of Mo oxidation states in Mo/A1,03 catalysts as a function of the reduction temperature.
off at ca. 250°C. The comparison of catalytic activity with the distribution of Mo oxidation states obtained by ESCA (Fig. lb) indicates that Mo+’ is not an active center for propene hydrogenation under the present reaction conditions and that a Mo oxidation state of +4 (or lower) is required. A sharp increase in the activity is observed for reduction temperatures > 300°C. According to Fig. lb, reduction temperatues > 300°C correspond to formation and increased abundance of M o + ~ .This may be an indication of higher intrinsic activity of M o + ~ . However, because of the small difference in the Mo3d B.E. values for MO” and Mo+, (ca. 0.6 eV), this assignment must be considered as tentative. It should also be noted that the high 96 conversion measured for catalysts reduced at T > 300°C makes the activity data for these catalysts unsuitable for kinetic study. The activity results reported in the present study are consistent with those reported by Hall and coworkers (2) on a similar catalyst and by Burwell and coworkers (1) on catalysts obtained by reaction of Mo(CO), with partially or totally dehydroxylated aluminas.
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High Temperature Reduction The catalyst (8% Mo/A1,0,) (7) was reduced between 500°C and 900°C. The distribution of Mo oxidation states in the reduced catalysts was determined by ESCA. The results reported in a previous study (6) show that Mo oxidation states ranging from +6 to 0 are formed. Figure 2 shows the comparison between propane hydrogenolysis activity and , o + ~and M o + ~are not the distribution of Mo oxidation states. The abundances of M o + ~M shown for the sake of clarity. The catalysts reduced at temperatures lower than 550°C did not show any activity. For these reduction temperatures, the Mo oxidation states present are M o + ~M0+', , M o + and ~ Mo+'. This indicates that Mo oxidation states higher than +2 are not active for propane hydrogenolysis. Propane hydrogenolysis activity begins to be detectable at a reduction temperature of 550°C (Fig. 2a) and increases steadily for catalysts reduced between 550°C and 650°C. A sharp increase in the activity is observed at a reduction temperature of 700°C which according to Fig. 2b corresponds to formation of significant amounts of Mo metal. Figure 2b shows that the change in the abundance of Mo metal is very similar to that of the catalytic activity. This strongly suggests that Mo metal is the most active species for propane hydrogenolysis. Figure 2b also shows that even catalysts which do not contain Mo metal are active. This may be taken as an indication that Mo+, is also active for this reaction. However, one cannot rule out the possibility that the activity of catalysts reduced between 550 and 650°C is due to small amounts of Mo metal present on these catalysts which, because of their relative abundance or the small chemical shift between Moo and Mo+*(ca. 0.5 eV) may escape detection by ESCA. Burwell and coworkers (8-10) have studied the hydrogenolysis of propane on carbonyl-based Mo/A1,4 catalysts. They have concluded that this reaction is characteristic of Mo metal. Similar results were obtained on conventionally prepared Mo/A1,03 catalysts (3,4). Catalysts containing Mo in an average oxidation state of +2 were much less active. These findings are clearly substantiated by the present study which also shows a direct correlation between the abundance of Mo metal determined by ESCA and propane hydrogenolysis activity.
REFERENCES 1 R. L. Burwell, Jr., in "Catalysis on the Energy Scene", S. Kaliaguine and A. Mahay (eds.), Elsevier, Amsterdam, 1984, p. 45. 2 W. K. Hall in "Chemistry and Physics of Solid Surfaces VI", R. Vanselow and R. Howe (eds.), Springer-Verlag, 1986, p. 73. 3 J. Ciiung, J. P. Zhang and R. L. Burwell, Jr., J. Catal., 116 (1989) 505. 4 J. Chung and R. L. Burwell, Jr., J. Catal., 116 (1989) 519. 5 R. B. Quincy, M. Houalla, A. Proctor and D. M. Hercules, J. Phys. Chem., 94 (1990) 1520. 6 M. Yamada, J. Yasumaru, M. Houalla and D. M. Hercules, J. Phys. Chem., 95 (1991) 7037. 7 A. Redey, J. Goldwasser and W. K. Hall, J. Catal., 113 (1988) 82. 8 R, Nakamura, R. G. Bowman and R. L. Burwell, Jr., J. Am. Chem. Soc., 103 (1981) 674. 9 R. Nakamura, D. Pioch, R. G. Bowman and R. L. Burwell, Jr., J. Catal., 93 (1985) 338. 10 R. L. Burwell, Jr. and J. S. Chuny, React. Kinet. C a d . Lett., 35 (1987) 381.
G m i , L ef al. (Editors),New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
EFFECT OF CATALYST PREPARATIONON THE PERFORMANCE OF SUPPORTED Ru-CUBIMETALLIC SYSTEMS R. MaggiorF, C. CrisafulIia,S. &irea and S. Galvagnob
aDipar&imento di Scienze Chimiche, Universita di Catania, Wale A. Doria 6, 95125 Catania, Italy bDipartimentodi Chimica Industriale, Universita di Messina, Cas. Post. 29, 98166 Sant'Agata di Messina, Italy
Abstract Ru-Cu samples prepared from different metal precursors and dispersed on supports with different surface areas have been characterized by chemisorption, TEM, TPR, BET and mercury porosimetry. Their catalytic activity has been tested in the hydrogenolysis of propane. A lower catalytic activity, caused by a larger Ru-Cu interaction, has been found on the catalysts dispersed on the higher surface area supports and on those prepared from RuC13. 1. INTRODUCTION
In order to obtain information on the nature of the interaction occurring between metal-metal and metal-support in bimetallic catalysts, Ru-Cu samples have been found of particular interest as model systems. Although copper and ruthenium are practically immiscible in the bulk, the existence of metal-metal interactions in dispersed Ru-Cu preparations is not unexpected. Indeed, formation of bimetallic clusters has been widely demonstrated[ 1-71. Notwithstanding the large number of paper published on unsupported and supported (mainly Si02) Ru-Cu systems, the results on the influence of Cu on the chemisorption properties and on the catalytic activity of Ru are often conflicting. It is likely that different sample preparation conditions lead to variations in the relative Ru and Cu interdispersion and therefore to different surface compositions. In this paper we shall discuss the influence of preparative conditions such as nature of the support and metal precursors on the chemico-physical characteristics of the Ru-Cu samples.
2.EXPERIMENTAL Bimetallic Ru-Cu samples were prepared by impregnation of SiO2 supports with surface are obtained depending on the precursor used. This is also confirmed by the
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results of the hydrogenolysis of propane reported in Fig.1. It can be observed that on increasing the Cu/Ru ratio the turnover frequency of propane hydrogenolysis decreases continuously. This decrease is higher on the samples prepared from RuC13 than on those obtained from Ru(NO)(N03)3. It has been suggested that the active sites for propane hydrogenolysis are made of ensembles of n adjacent Ru atoms present at the surface, and that the catalytic activity is related to the probability of finding such ensembles. The dilution of the active atoms with inert Cu causes a decrease in the fraction of Ru atoms exposed on the surface and more important a drastic decrease in the number of active ensembles. On these bases the lower catalytic activity has to be related to a surface interaction between Ru and Cu which decrease the number of active ensembles. On samples prepared from RuCI3 the Ru-Cu interaction is therefore larger. This can result from a higher amount of Cu on the Ru surface and/or from a more uniform distribution. The presence of a higher amount of Cu on the Ru surface is in agreement with the decrease of the chemisorption properties. According to previously published results it can be suggested that the presence of chlorine favours the spreading of Cu on the surface of Ru[3].
-
I
20 40 60 80 1 Cu (atom X)
Fig.1 - Relative rates of propane hydrogenolysis (Vrel) over Ru-Cu catalysts as a function of Cu content. (a) high surface area SiOz-supported prepared from RuC13; ( x ) high surface area SiOz-supported prepared from Ru(NO)(N03)3; (v)low surface area SiOa-supported prepared from Ru(NO)(N03)3. Tr= 200 "C . The surface area of the support has been also found to play an important role in determining the degree of interaction betwen Ru and Cu. Previous results obtained on MgO, A1203 and SiO2 have shown that a larger interaction is obtained on the support having a higher surface area[6]. In order to discriminate the effect of the chemical nature of the support from an influence of the total surface area, a detailed investigation on Ru-Cu samples supported on five different silicas having surface
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different surface area by using RuC13 or Ru(NO)(N03)3 as Ru precursor and Cu(N03)~as Cu precursor. The salt(s) concentration in the solution was adjusted to yield a total (Ru+Cu) metal content of about 2 wt.%. After impregnation the catalysts were dried at 120 "C and reduced in situ at 400 "C in flowing H2. Catalysts have been characterized by CO chemisorption, temperature programmed reduction (TPR), total surface area measurements (BET) and transmission electron microscopy (TEM). Catalytic activity was tested in propane hydrogenolysis. The reaction was carried out in a differential mode employing a tubular reactor at atmospheric pressure and using He as diluent. Results obtained at 200 "C are reported. Since preliminary runs showed a decrease in activity with time, the initial rates were used to compare the different catalysts preparations. Details on the experimental procedure used are reported elsewhere[5,6]. 3. RESULTS AND DISCUSSION
Chemisorption experiments have shown that the effect of the Cu/Ru ratio on the fraction of Ru atoms at the surface depends strongly on the ruthenium precursor used for sample preparation (Table 1). Table 1 CO chemisorption over Ru-Cu catalysts supported on Si02 Catalyst
Ru/(Ru+Cu)a
Surface area support(m2g- 1)
Ru precursor
CO/Ru
SElOO SE008 SElOON SE020N SAlOON SA020N a atomic ratios
100
490 490 490 490 25 25
RuC13 RuC13 Ru(NO)(N03)3 Ru(NO)(N03)3 Ru(NO)(N03)3 Ru(NO)(N03)3
0.36 0.07
8
100 20 100 20
0.95 0.95 0.24 0.46
Over Ru-Cu/SiOa catalysts, prepared from RuC13, Ru dispersion was lower than that measured over samples prepared from Ru(NO)(N03)3. These results have been confirmed by TEM which shows smaller particles on these latter samples. On the catalysts ex-RuC13, increasing the Cu/Ru ratio the fraction of Ru atoms at the surface was found to decrease from 36% on the monometallic Ru sample up to a value of 7% on the sample having a Cu/Ru ratio of about 11. On the samples prepared from Ru(NO)(N03)3 an increase is instead observed. Such an increase is more evident on the samples prepared on low surface area supports where the dispersion is low. On the high surface area silicas the ratio Ru&u is close to 1 and does not change with the Cu/Ru ratio. Besides the possible effect of residual chlorine on the chemisorption properties, it seems evident that particles having a different Ru-Cu composition at the
1874
areas ranging from 25 m2g-1 to 500 m2g-1 has been carried out. The precursor salts used for samples preparation did not contain chlorine. A TEM analysis of the Ru-Cu preparations has shown that, as expected, increasing the surface area of the support the average metal particle size decreases. Moreover smaller metal particle sizes are obtained on addition of Cu. This is in agreement with a dilution by the copper ions of the local concentration of the ruthenium ions. A comparison of the relative turnover frequency of propane hydrogenation on samples supported on Si02 with different surface area is reported in Fig.1. It is noted that the larger effect of the Cu/Ru ratio on the catalytic activity is obtained on the support having the highest surface area. This is in contrast with the smaller particle size measured on these samples. In fact it would be expected that on the more dispersed particles a higher Cu/Ru ratio should be required to achieve a given inhibiting effect[l]. Experiments carried out with different total metal loadings indicates that the RuCu interaction is related to the surface concentration of the metal precursors The higher interaction beeing observed on the more diluted systems. It is suggested that different arrangement of Cu within the bimetallic catalysts are obtained depending on the preparation conditions. On the samples ex-RuC13, Cu atoms are mainly present as two dimensional layers on the Ru surface. In the absence of chlorine instead a more uniform distribution of Cu within the metal particles is obtained. These latter particles are however unstable and tend to form separate aggregates as the crystallite size increase. 4. REFERENCES
1. J.H. Sinfelt, Y.L. Lam, J.A. Cusumano and A.E. Barnett; J.Cata1. 42 (1976) 227. 2. A.J. Hong, B.J. McHugh, L. Bonneviot, D.L. Resasco, R.S.Weber and G.L. Haller, in "Proc. Intern. Congr. on Catal., Calgary, 1988" (M.J. Phillips and M. Ternan, Eds), Vol. 3 p.1198, Chem.1nst. of Canada. Ottawa, 1988. 3. D.E. Damiani, E.D.P. Millan and A.J. Rouco; J.Cata1. 101 (1986) 162. 4. G.C. Bond and X. Yide; J. Molec. Catal. 25 (1984) 141. 5. C. Crisafulli, R. Maggiore, G. Schembari, S. Sciri? and S.Galvagno; J. Molec. Catal. 50 (1989) 67. 6. C. Crisafulli, S.Galvagno, R. Maggiore, S. Sciri? and A. Saeli; Catal. Lett. 6 (1991) 77. 7. M.W. Smale and T.S. King; J. Catal. 120 (1990) 335.
Guni, L. d d.(Editors), New Frontiers in Cmolysb
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
REVISITING DIFFUSE REFLECTANCE SPECTROSCOPY FOR THE CHARACTERIZATION OF METAL AND SEMICONDUCTINGOXIDE CATALYSTS A. Rakai, A. Bensalem, J . C. Muller, D. Tessier and F. Bozon-Verduraz
Laboratoire de Chimie des Materiaux Divises et Catalyse, Universite Paris 7, 2 Place Jussieu, 75251 Paris Cedex 05,France
INTRODUCTION Although diffuse reflectance spectroscopy (DRS)has been employed for many years in catalytic studies, difficulties arising from generally large bandwidths and specular reflectance have somewhat limited its development ;however the use of various model coordination compounds should reinforce the capabilities of this technique. The aim of this paper is to bring a contribution concerning two important fields : (i) the control of the preparation of a metal catalyst (alumina-supported palladium) on going from the precursor, viu the calcirrutioiz stuge, to the active phase (ii) the characterization of a supported oxide (silica supported ceria) with an attemps to link the optical spectrum to the dispersion of the oxide phase.
EXPERIMENTAL The palladium-alumina catalysts (0,4 to 1,5 % Pd) were prepared from various precursors (chloride, nitrate, acetylacetonate) different methods of precursor bonding (capillary impregnation, ion exchange, grafting), before calcination in oxygen at 400 "C (15 hr) and reduction by H2 at different temperatures Tr. Supported ceria was obtained through anchoring of cerium (IV) acetylacetonate (benzene solution) on silica aerosil and subsequent calcination at 400 "C (15 hr). The D.R. spectra were recorded on a BECKMAN 5270 spectrometer equipped with a microcomputer.
1876
RESULTS AND DISCUSSION
8)P a l l a d i u m . 1) SamDles before pretreatment D.R.S showed than an ion exchange occurs upon drying
the sample prepared from PdC12
by capillary impregnation. Evidence was also given that Pd acetylacetonate is grafted on alumina through release of an acetylacetonate moiety acac- which is retained by the carrier. On the other hand, impregnation by Pd nitrate gave a broad band ascribed to Pd hydroxide. 2) Oxidized samples The D.R. spectra recorded after calcination showed also peculiarities related to the nature of the precursor.On fig. 1 are presented the spectra of PdO, pure or diluted in alumina, and supported palladium samples calcined in 02,in the whole NIR and UV-Visible range. For pure PdO, the absorption threshold is located near 1650 nm (about 6000 cm-1 or 0.75 eV), in agreement with a value previously obtained by transmission (0.8 eV) (1). While the sample ex-nitrate exhibited a broad band near 380 nm (26000 cm-l), samples prepared from Pd(acac)2 and PdC12 showed a narrow band at 450 nm (22 000 cm-1) with a stronger absorption in the UV part of the spectrum. These result are ascribed to the semiconducting character (p type) of bulk PdO whose band gap width is about 0.8 eV as measured on fig. 1. It is known, indeed, from works performed in the field of semiconductor physics that deccreasing the particle size entails an increase of the band gap width and the appearance of discrete levels. Data presented in table 1 suggest that this so called quanti4rn confinemetit eflecr (2) may be responsible for the absorbance variations shown in fig. 1, that is the shift of the absorption threshold towards shorter wavelengths. Further information concerning the dispersion of oxide clusters was obtained by examination of the ability of NH3 ligand to exchange with oxygen linked to Pd. Such substitution was immediate only for oxidized samples ex PdC12 and Pd(acac)2. R size of various g&idiu m-containing svstems Table 1 : Estimation of D ~ icle
Samples Bulk PdO ex nitrate ex acetylacetonate ex chloride
Mean oxide particle size (A) * - 1000 - 60 not measurable 4'
* estimated from electron microscope observations
Metal dispersion after reduction (4) 0.16 0.72 1.o
1877
fie.1 DRS of PdO and oxidized SuDDorted samples 1) pure PdO 2) 1 % PdO diluted in alumina 3) sample ex Pd nitrate 4) sample ex Pd acetylacetonate 5 ) sample ex PdC12
I
300
700 1100
h(nm)
-
3) Reduced samples After reduction at 300" C, Pd2+ entities were still detected, especially when starting from PdC12. D.R.S. coupled with IR data (4) allowed to c o n f m the following scheme : strong camer-precursor small oxide high metal dispersion with interaction particles electron deficient character 'j
1)PureCe02 ffi(fie.2)
Pure CeO2 showed a strong absorption with two maxima at 275 nm (36 000 cm-I) and 340 nm (30 000 crn-1) with the absorption threshold near 400 nm (25 000 cm-1) in agreement with previous results obtained by transmission of films (Ce02 is a n-type semi conductor) (5).
Heating in vacuo at 800' C gave rise to an additional broad band at about 650 nm (15000 cm-l), enhanced by outgassing at 1000 O C, whose intensity is sharply de creased upon oxygen admission at Ta. This band is ascribed to an intervalence transition Ce 3+ - Ce4. Heating in oxygen at 600" C restored the initial spectrum. fig 2 : DRS of pure CeO7 1) initial sample
2) after heating at 800" C in vacuo (15hr) 3) after heating at 1000" C in vacuo (15h) 4) after contacting with 0 2 (1OOTorr) at ambient temperature 5 ) after heating in 0 2 (100 Torr) at 600" C 300
400
500
600
700
h(nm)
1878
Fig.3 DRS of various silica-ceria samples: 1) ceria diluted in siIica (2.4 % Ce) 2) silica supported ceria (2.4 % Ce) with small particle size (- 25 A) 3) silica supported ceria (0. I8 % Ce) of large particle size (- 1000 A) Supported ceria (0.04 % Ce) prepared from : 4) hexane 5) benzene 6) methanol
2) Silica SUP - Ported ceria
Vnm)
On silica supported ceria, D.R.S. was able to detect less than 100 ppm of cerium. The spectral features suffer drastic changes in comparison with bulk Ce02 when the particles size decreases. Fig.3 (sp.1-3) compares the spectra of bulk Ce02 diluted in silica (2.4 % weight Ce, oxide particle size of about 1000 A) with supported samples presenting very different particle sizes (near 1000 A and 25 fi respectively). The “blue shift” observed on well dispersed ceria may still be ascribed to the quarituni confinement effect (2,3), although the contribution of low coordination oxygen-cerium 4+ charge transfers may also be involved (6).Fig. 3 shows also the influence of the solvent used for grafting the cerium acetylacetonate on silica (sp.4-6).Three oxidized samples presenting the same cerium content (0.04 %) exhibit different optical spectra, suggesting that the dispersion of the oxide phase (cerium-oxygen clusters) increases when the solvents are taken in following order : hexane < benzene c methanol. No estimation of the particle size was possible from electron microscope obseivations. CONCLUSION The diffuse reflectance spectra of supported semiconducting oxides are very sensitive to the particle size and bring informations when X ray diffraction or even electron microscopy experiments are not conclusive. The spectral variations are believed to arise from the vanishing of collective properties, which leads to the enhancement of the band gap width and to the predominance of localized (surface) charge transfers. References (1) P.0 Nilsson, M.S Shivaraman , J. Phys. C., Solid State Phys, 12,(1978), 1423 (2) AT, Ekimov, AIL. Efros, A.A. Onushcenko, Solid State Comm. ,.& (1985), 921 (1989), 10935 (3) P.E Lippens, M. Lannoo., Phys. Rev. B, (4)D. Tessier, A. Rakai’, F. Bozon-Verduraz, J. Chem. SOC,Faraday Trans. (in press) ( 5 ) C.A. Hogarth, ZT A1 Dhar., Phys Rev. B, M , (1986), K157 (6)A. Bensalem, J.C. Muller, F. Bozon-Verduraz, J. Chem. Soc. Faraday Trans.(in press)
Guai, L.ef al. (Editors), New Frontiers in Caalysk Proceedings of the 10th International Congress on Catalysis,19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights resewed
THE THERMAL STABILITY OF THE ADSORBED/LATI'ICEOXYGEN SPECIES ON THE OXIDE CATALYSTS SURFACES
M.Calahraru and N.I. Ionescu Institute of Physical Chemistry of the Romanian Academy of Sciences, 202 Spl. Independentei, 77208 Bucharest, Romania
Abstract The in s i t u e l e c t r i c a l conductivity measurement i n argon and a i r flow is proposed a s a method f o r evaluating the range of thermal s t a b i l i t y of oxygen s p e c i e s on oxide s u r f a c e s presenting n-type semiconducting p r o p e r t i e s . The ranges of s t a b i l i t y a r e c l o s e l y r e l a t e d w i t h the conditions of the experiment (i.e. i n o x i d a t i v e o r d e p l e t i v e atmospheres) and w i t h t h e h i s t o r y of t h e sample. Some examples a r e presented i n the case of molybdate catal y s t s f o r propylene oxidation.
1. INTRODUCTION T h e complex behavior of oxide m a t e r i a l s i n i n t e r a c t i o n w i t h ambient atmosphere becomes p a r t i c u l a r l y important when they a r e used a s oxidation catalysts. Various mechanistic pathways have been proposed f o r oxidation of hydrocarbons, involving d i f f e r e n t oxygen specimens (adsorbed - n e u t r a l or charged - and/or l a t t i c e oxygen) a s the r e a c t i v e s p e c i e s , b u t i t is s t i l l d i f f i c u l t t o prove which one is i n f a c t responsible f o r the product formation. One o f the main causes of t h i s s i t u a t i o n i s the l a c k of informations concerning t h e thermal s t a b i l i t y of oxygen s p e c i e s on r e a l oxide surfaces.
2. NETHOD
The c a t a l y s t g r a i n may b e imagined a s a core material ( b u l k ) surrounded by t h e s u r f a c e l a y e r which must b e d i r e c t l y influenced by the changes i n ambient atmosphere. The g r e a t influence of t h e b u l k on the s u r f a c e properties cannot be, of course, neglected. However, a p a r t i c u l a r e q u i l i b r a t i o n between oxide s u r f a c e and ambient atmosphere w i l l always occur w i t h a much higher r a t e than t h e r a t e of e q u i l i b r a t i o n between s u r f a c e and b u l k , w h i c h involves s o l i d s t a t e d i f f u s i o n phenomena. I t has t o be emphasized t h a t a s long a s adsorption-desorption-reaction s t e p s involve charge t r a n s f e r processes, t h e y must be r e f l e c t e d by changes i n e l e c t r i c a l p r o p e r t i e s of the oxide surface. The enrichment of t h e s u r f a c e layer i n charge c a r r i e r s due t o t h e above mentioned phenomena w i l l provide an e a s i e r path f o r conductivity, i.e. s u r f a c e layer conduction w i l l take place p r e f e r e n t i a l l y . The measurement of e l e c t r i c a l conductivity of ground (pordered) oxide m a t e r i a l s w i l l speak l i t t l e about changes i n e l e c t r i c a l p r o p e r t i e s of b u l k , b u t much more about s u r f a c e phenomena, p a r t i c u l a r l y due t o the h i g h surface/volume r a t i o .
The o v e r a l l c o n d u c t i v i t y o f o x i d e g r a i n s can be described by t h e relationship:
general
where Go denotes an o v e r a l l term i n c l u d i n g a f u n c t i o n o f i n t e r g r a n u l a r contact area, p a r t i c l e diameter and t h e d e n s i t y o f c a r r i e r s a t the s i i r f a c e Ell, k i s Roltzmann constant and T the a b s o l u t e temperature. E denotes here the o v e r a l l a c t i v a t i o n energy o f condiiction, c o n t a i n i n g a l l the f a c t o r s determining the thermal a c t i v a t i o n o f charge c a r r i e r s , inclusively a l l p o s s i b l e l a t t i c e c o n s t r a i n t s . By u s i n g comparative experiments performed on the same sample, the pre-exponential f a c t o r can be considered as a constant. The c o r r e c t d e s c r i p t i o n o f charge t r a n s f e r phenomena on o x i d e s u r f a c e s encounters many d i f f i c u l t i e s , p a r t i c u l a r l y due t o t h e i r defected s t r u c t u r e , whose d e t a i l s a r e f r e q u e n t l y unknown. However, i m p o r t a n t f o r c a t a l y s i s i s the o v e r a l l r e s u l t , i.e. the e l e c t r i c a l p r o p e r t i e s o f t h e s u r f a c e i n r e l a t i o n w i t h c a t a l y s t performances. The e q u i l i b r a t i o n o f n-type o x i d e semiconductors w i t h surrounding atmosphere c o n t a i n i n g oxygen induces the f o r m a t i o n o f ionosorbed oxygen species (which l e a d s t o f r e e c a r r i e r s consumption). Desorption o f these species by thermal e x c i t a t i o n and/or due t o d e p l e t i v e c o n d i t i o n s p r o v i d e s excess e l e c t r o n s determining the i n c r e a s e o f e l e c t r i c a l c o n d u c t i v i t y . Obviously, the thermal s t a b i l i t y o f t h e oxygen t h e change o f the o v e r a l l species on the s u r f a c e i s d i f f e r e n t ; so, of a c t i v a t i o n energy o f conduction can be associated w i t h the removal d i f f e r e n t oxygen species from the o x i d e surface. 3. EXFEHIMEHTAL
I n +itu a.c. e l e c t r i c a l c o n d u c t i v i t y measurements have been done on a s e r i e s o f molybdenum - based o x i d e c a t a l y s t s CMo0,/Si02 (MS), Ei2M0301 (EM), Fe2M03012 c a l c i n e d a t 723 and 833 K (FM-1 and h - 2 , r e s p e c t i v e l y ) a n i Mo19.70 /SiO., catalyst (MC)I, by a multicomponent Ni4,BFe3,2Ei3, ZnO 7P u s i n g a semiautomatic p r e c i s i o n k r i d i e h % L A A M 484, a t t h e t i r e d frequency o f 1592 kHz, w i t h a s p e c i a l r e a c t i o n c e l l p e r m i t t i n g simultaneous e l e c t r i c a l c o n d u c t i v i t y and c a t a l y t i c a c t i v i t y measurements i n dynamic c o n d i t i o n s C23 The temperature dependence o f e l e c t r i c a l c o n d u c t i v i t y was f o l l o w e d on 4 cm 3 o f f r e s h c a t a l y s t g r a i n s (0.6-0.8 mm) i n a i r o r argon f l o w (1.03 cm3/5, h e a t i n g r a t e lK/min) between 523 and 673 K. The same sample was examined i n argon f l o w (which was chosen as a standard experiment) a f t e r i t s submission argon m i x t u r e and t o d i f f e r e n t treatments (e.g. r e d u c t i o n w i t h propylene r e o x i d a t i o n w i t h a i r and/or c a t a l y t i c t e s t i n g w i t h 1/10 p r o p y l e n e / a i r mixture). The c a t a l y s t s p r e p a r a t i o n and t h e i r c h a r a c t e r i s a t i o n have been presented elsewhere C3-51.
-
4. RESULTS A l l the c a t a l y s t s show n-type c o n d u c t i v i t y . As a r u l e , i n t h e temperature range s t u d i e d the p l o t s I n 0 versus 1 / T show two o r more d i f f e r e n t slopes. The corresponding a c t i v a t i o n energies, as w e l l as t h e temperature of the s l o p e break a r e presented i n Table 1, where €lor F20 denote t h e a c t i v a t i o n energies i n a i r f l o w , and Eal, EZa, Esa denote t h e a c t i v a t i o n e n e r g i e s i n argon f l o w . The data i n Table f show the change o f t h e a c t i v a t i o n energy o f conduction i n t h e temperature range studied. The v a l u e o f temperature corresponding t o the change o f a c t i v a t i o n energy (which l i m i t s t h e "low" and " h i g h " tem-
1aai Table 1 The o v e r a l l a c t i v a t i o n e n e r g y of c o n d u c t i o n c a t a l y s t s i n a i r and a r g o n f l o w Catalyst
El0
eV PIS
f20 eV
fla K
f
r
ro EN
Y f
0.37 0.44
1.63 0
603 623
0.40
563
t
FM-1
to f fo
0
t rt FN-2
fo
0.25
0.44
603
NC
r t
0.36
0.36
-
eV
%a eV
0.14 0.33 0.21
0.41 1.01 0.44
0.44 0.52 0.40
0.91 1.86 1.38 0.65 0.77 0.75 0.92 0.77 0.62
0.65
0.44 0.75 0.58 0.40 0.62 0.23
0.51
for
%a eV
3.41 0.77
-
-
molybdenum
T1 K
T2 K
-
based
oxide
c1 :.'
1 _ 1 _
588 573 573
c2 7
3.0
8.1
8.3
35.5
18.3
28.8
-
32.6
603 603
-
623 573
-
611
-
563
608 603
-
573
0
-
3.9
(1 2.8 35.0
f=fresh! rereduced3 o=reoxidizedg t=tested; t o s t e s t e d and r e o x i d i t e d ; f o n f r e s h , h e a t e d i n a r g o n flow, m e a s u r e d i n a i r flow and t h e n m e a s u r e d a g a i n i n a r g o n ! r t a r e d u c e d and t e s t e d ! $ v a l u e s m e a s u r e d d i r e c t l y i n t h e r e a c t i o n c o n v e r s i o n i n t h e "low" and " h i g h " t e m p e r a t u r e r a n g e s . m i x t u r e # C1, C2
-
p e r a t u r e r a n g e s ) v a r i e s d e p e n d i n g on t h e c a t a l y s t s t r u c t u r e , on t h e c o n d i t i o n s o f t h e e x p e r i m e n t ( i . e . i n o x i d a t i v e o r m i l d d e p l e t i v e c o n d i t i o n s ) and on t h e h i s t o r y of t h e s a m p l e . Sometimes, i n t h e h i g h t e m p e r a t i i r e r a n g e t h e a c t i v a t i o n e n e r g y of t h e f r e s h s a m p l e i n a i r is almost i d e n t i c a l w i t h t h a t f o u n d i n a r g o n i n t h e low t e m p e r a t u r e r a n g e ; f o r r e d u c e d or t e s t e d s a m p l e s t h e l i m i t i n g t e m p e r a t u r e is s h i f t e d t o l o w e r v a l u e s , sometimes n o t e v e n b e i n g n o t i c e d i n t h e r a n g e of t h e s t u d i e d t e m p e r a t u r e s ( f o r e x a m p l e t h e FM s a m p l e s ) . The v a l u e s measured on t e s t e d o r p r o p y l e n e reduced samples i n d i c a t e t h a t t h e h i g h e r t h e o x i d a t i o n l e v e l o f t h e s u r f a c e , t h e lower t h e a c t i v a t i o n e n e r g y of t h e same s t e p . I n T a b l e 1 t h e p r o p y l e n e c o n v e r s i o n d a t a i n t h e n e i g h b o u r h o o d of t h e t e m p e r a t u r e d e t e r m i n i n g t h e c h a n g e o f t h e a c t i v a t i o n energy a r e presented too.
-
5. DISCUSSION
The p o s s i b l e s o u r c e s o f free c a r r i e r s ( e l e c t r o n s i n o u r c a s e ) a r e t h e i o n i z a t i o n o f a n i o n v a c a n c i e s ( n e u t r a l or m o n o - i o n i z e d ) and d e s o r p t i o n of a d s o r b e d i o n i c oxygen a n d / o r l a t t i c e oxygen s p e c i e s . D i f f e r e n t a c t i v a t i o n e n e r g i e s of c o n d u c t i o n f o u n d o v e r d i f f e r e n t t e m p e r a t u r e r a n g e s must r e f l e c t t h e d i f f e r e n t o r i g i n of c h a r g e c a r r i e r s . E a s e d on t h e , f a c t t h a t o u r e x p e r i m e n t s h a v e been p e r f o r m e d w i t h o u t p r e v i o u s e v a c u a t i o n , t h e s u r f a c e oxygen v a c a n c i e s w i l l n o t b e c o n s i d e r e d i n t h e f u r t h e r d i s c u s s i o n , The a l m o s t i d e n t i c a l v a l u e o f t h e a c t i v a t i o n e n e r g y m e a s u r e d i n a r g o n w i t h i n lower t e m p e r a t u r e r a n g e a n d i n a i r w i t h i n h i g h e r t e m p e r a t u r e r a n g e s u g g e s t s t h e c o n n e c t i o n of t h i s p r o c e s s w i t h t h e d e s o r p t i o n of a weaklybonded oxygen s p e c i e s , which i n m i l d d e p l e t i v e c o n d i t i o n s is e a s i e r removed
1882 from the surface than i n oxygen c o n t a i n i n g atmospheres. We t e n t a t i v e l y propose here t h a t t h i s s t e p i s represented by t h e d e s o r p t i o n o f the 0species already e x i s t i n g on the surface. The h i g h e r temperature s t e p i n a1;gon f l o w must be connected w i t h the tran s f o r m a t i o n o f the n o n - o x i d i z i n g o'- species i n the h i g h l y o x i d i z i n g 0- species (which i s expected t o be governed by higher l a t t i c e c o n s t r a i n t s ) . Thi s assumption i s c o n s i s t e n t w i t h t h e increased c a t a l y t i c a c t i v i t y i n t h i s temperature range. Similarly, we could advance the o p i n i o n t h a t the t h i r d a c t i v a t e d process i n argon f l o w corresponds t o the involvement o f the l a t t i c e oxygen through a s i m i l a r t r a nsformation. The increase/decrease of the a c t i v a t i o n energy o f conduction f o r t h e corresponding steps depending on the r e d u c t i o n / o x i d a t i o n l e v e l o f t h e s u rfa ce demonstrates the c o l l e c t i v e characte r o f t h e o v e r a l l s u r f a c e conduct i o n (i.e. re duction over some l i m i t s induces l a t t i c e c o n s t r a i n t s ) . The data i n argon f l o w on reduced o r tested samples suggest t h a t t h e hydrocarbon i n t e r a c t i o n w i t h the o i d e surface a c t i v a t e s t h e s t r o n g l y bonded oxygen species), f a c i l i t a t i n g t h e i r i m p l i c a t i o n even i n species (i.e. l a t t i c e 0 argon flow. Such observation i s c o n s i s t e n t w i t h t h e data r e p o r t e d by L i b r e e t a l . C61 f o r the surface p o t e n t i a l v a r i a t i o n o f the EN sample. Following the same i n t e r p r e t a t i o n , i t can be stressed t h a t the low temperature calcined i r o n molybdate (FM-1) seems t o undergo supplementary o x id ation, since the f r e s h sample shows s i m i l a r temperature dependence as the reduced high temperature calcined sample FM-2, w h i l e supplementary o x id ized f r e s h FM-1 sample shows i d e n t i c a l t e n p e r a t u r e dependence as t h e f r e s h supplementary o x i d i z e d FM-2 sample.
h-
6. CONCLUSIONS
The t h e r n a l s t a b i l i t y o f the oxygen species on t h e o x i d e c a t a l y s t surfaces seems t o be s t r o n g l y dependent on t h e h i s t o r y o f the sample and on the c o n d i t i o n s o f the experiment (i.e. the temperature range and t h e o x i d a t i v e or d e p l e t i v e atmosphere). This suggests t h a t depending on the h i s t o r y of sample, d i f f e r e n t c a t a l y t i c behavior must be expected from . t h e same sample i n the same temperature range, and pleads f o r s t a n d a r d i z a t i o n i n c a t a l y t i c experiments intended t o mechanistic purposes. 7. REFERENCES.
1 2. 3. 4. 5.
6.
S.R.Norrison, The Chemical Physics o f Surfaces, Plenum Press, New 'fork, 1977, p.70. N.I.1onescu and N.Caldararu, React.Kinet.Catal,Lett., 8 (1978) 477. Ph.h.Patist, J.F.H.Houwens and G.C.A.Schuit, J.Catalysis, 23 (1972) 1. R.Manaila, N.I.1onescu and M.Caldararu, Z.anorg.allp.Chem., 466 (1980) 221. G . F i l o t i , R.Nanaila,, H.Caldararu, M.Caldararu, N.I.Ionescu, M.Selenina and K.-H. Schnabel, Z.phys.Chemie ( L e i p z i g ) , 271 (1990) 309. J.M.Libre, Y.Earbaux, E.Orzybowska, P.Conf1an-k and J.P.Eonnelle, Applied Ca talysis, 6 (1983) 315.
Guczi, L.et al. (Editon), New Fronriers in Cafalysb
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
CHEMICAL ANCHORING OF NOBLE METAL M I N E PRECURSORS TO SILICA AN IN SITU UV DIFFUSE REFLECTANCE STUDY
W.Zou and R.D. Gonzalez Chemical Engineering Department, Tulane University, New Orleans, LA 701 18, USA
ABSTRACT The effect of pretreatment on the dispersion of supported noble metal catalysts was studied using in-situ diffuse UV reflectance spectroscopy and transmission electron microscopy. The following amine precursors were used in this study: Pt(NH3),(N03),, Pd(NH3),(N0&, Ru(NH3),C13 and IRh(NH3)5Cl]C12. Pretreatment in oxygen prior to reduction in H2 at 400C resulted in poor dispersion for Ru and Rh, moderate dispersions for Pd and high dispersions for Pt. Direct reduction in H2 resulted in poor dispersions for Pt and Pd but high dispersions for Ru and Rh. These results are explained by considering the surface chemistry of the oxidation and reduction process as studied using in-situ diffuse UV reflectance spectroscopy. INTRODUCTION To obtain a better understanding of the factors which influence the dispersion of supported metals, we have prepared a series of catalysts using a family of noble metal amine precursors with similar chemical properties. In addition to a choice of pH which maximizes the adsorption of the precursors to silica, pretreatment and drying conditions were chosen i n a way that enabled u s to focus on the pretreatment chemistry rather other pretreatment variables. EXPERIMENTAL Catalyst preparation. The method of preparation has been published in detail elsewhere (1). Briefly, it was prepared by ion exchanging the corresponding noble metal amine precursor (Strem Chemical, Newburyport, MA) onto a Cab-0-Sil support (grade M5, 200m2/g, 14nm pores). The pH which maximizes complex adsorption without dissolving the silica support was found to be about 9. This was determined by measuring the zeta potential as a function of pH ( 2 ) . A Coulter Delsa 440 was used to perform these measurements. Following drying and washing with deionized water to remove weakly adsorbed precursors, the actual metal loading was determined by ICP (intercoupled plasma). The metal weight loading of the Pt catalyst was 2.68%, the Ru was 1%, the Rh 1.3% and the Pd 1.45%. The samples were dried i n a vacuum desiccator and either stored or pretreated for future studies. Pretreatment. Following drying and prior to a chemisorption experiment the samples were heated to the desired PretreatmenUreduction temperature at a heating rate of 10C/min. Reduction timi: in flowing H, was 3 hours at 40OC following by an hour purge in flowing He at the same reduction temperature. Except where indicated, treatment in oxygen at temperatures in excess of 1 S0C were avoided for fear of sintering or possible metal loss through the formation of volatile oxides, as was the case for Ru (1). Chemisorption measurements werc performed by the dynamic pulse method as described by Sarkany and Gonzalez (3).
1884
In-situ diffiue UV reflectance studies. The UV-VIS reactor system including an insitu cell reactor hiis been detailed i n a previous report (1). Diffuse UV reflectance spectra were obtained through the use of :in integrating sphere which was externally interfaced to the U V specbophotomcter through the use of fiber optic cables. Electron niicroscoiiy. Pnrticlcs size distributions were obtained using a Phillips EM 410 transmission electron microscope by techniques which are well established (1). RESUL1'S Dispersion results as a function of pretreatment show similarities for R u and Rh and for Pt and Pd. The Pt and Pd data are shown i n Table 1 and the R U itnd R h data are shown i n Tdbk 2. TABLE 1 Pt and Pd metal dispersion as a function of pretreatment
Pretreatmen t
Dco -
H, 400C 0, 3OOC, H, 400C Ar 300C, H2 400C H2 400C 0, IOOC, H, 400C 0, 200C. H2 400C 0 2 'JOOC, H, 400C He(Ar) 3OOC, t 1, 400C
40
70 74 11
40 42 37 67
56 84 85 26 44 37 38 71
45 70-80 70-80 20 37 32 24 70-80
The agreement between chemisorption nieitsiiremeiits and TEM is quite satisfactory. When the Pt and Pd aniine precursors were reduced directly in H2, the metal dispersions obtained were a modest 40% for Pt and only 1192 for Pd. However, pretreatment in oxygen prior to reduction i n H, resulted i n :I StIbstiintial increase i n dispersion for both metals. Decomposition of the Pd m i n e complex in either Ar or He resulted in still higher dispersions. Decomposition of Pt itmine complex i n Ar or He on the other hand resulted in dispersions which were similar to those obtained following oxygen pretreatment. To understand the rather poor dispersions obtained for both Pt and Pd amine precursors reduced direclly i n H,, the iii-siiit U V diffuse reflectance spectra were obtained. The results obtained for Pt ;ire shown i n Figure 1 . Interesting is the appearance of an absorption band centered at 21 3nm for the Pt complex. This band has been assigned to a neutral complex having the structure [ Pt(NH2)2(H)2]0.It bears no charge and has been proposed by Dalla Bettii and 13oudart (4) and By Zou and Gonzrilez (1) to be a highly mobile intermediate fornicd during rcduction. The high mobility of this intermediate results i n relatively low metal dispersions. This intermediate wits not observed during the reduction of the ;&orbed Pd coinplcx. However, its absence may, i n part, be due to the very strong silica absorption band centered ;it 208nm.
1885
A
208
W A
hke LLJ
100c
90C
210
B
+-
h
.C
150C
Fresh samp 240 270 300 330 Wavelength (nm)
c
70C r
3
Im
l L \ L
h0C ---
I
180 240 300 360 420 480 540 600 Wavelength (nm)
Figure 1. The U V spectra of different precursors treated in hydrogen: (A) [Pt (NH3)4J 2+/Si0,; (B ) [Pd(N H3)4J *+/Si0,. Pretreatment of the Pt precursor in an argon atmosphere rather than oxygen appears to have no effect. However, pretreatment of the Pd precursor in argon leads to a large increase in metal dispersion. We attribute this to the reducing atmosphere present during the decomposition of the Pd amine complex in Ar. This reducing atmosphere occurs as a result of the decon~positionof the amine complex at low temperatures. The ammonia ligands reduce the complex and inhibit the mobility of any oxide intermediates that may be formed. In the case of the Pt amine complex there is very little room for improvement as the decomposition in oxygen prior to reduction already yields high metal dispersions.
TABLE 2 Ru and Rh metal dispersion as a function of pretreatment. Adsorbed precursor
Pretreatment
Ru/Si02 Ru/Si02 Ru/Si02 [Rh(NH3)5C1]2+/Si02 [Rh(NH3)5C1]2+/Si02 [Rh(NH3)5C11*+/SiO2 [Rh(NH3),CIJ2+/SiO2 [Rh(NH3)5C1l2+/SiO2
0 2 2SC, H, 400C 0, 150C, H2 400C 0 2 250C, H2 400C
H2 400C O2 100C, H, 400C 0 2 15OC, H2 400C O2 200C, H, 400C 02 300C, H2 400C
Dco
68 30 16 73 68 50 17 16
DisDersion (%) DH, TEM (100/d(nm))
74 76 58 31 27
66 33 18 70-80 44 30 28
Reduction of the Rh and R u precursors i n H2 result in highly dispersed supported metal catalysts. Apparently these zero charged complexes do not occur during reduction and the intermediates formed are strongly bound to the surface. In the case of Ru, strongly bound surface nitrosyls have been identified using diffuse UV reflectance spectroscopy (1) and are an important aspect of the surface chemistry.
In marked contrast to Pd and Pt, the metal dispersion of Ru and Rh are strongly dependent on the pretreatment remperature in 02.Ru and Rh dispersions decrease with increasing 0, preti-catmenttemperature. The decrease i n dispersion is due to the formation of volatile oxidcs which lead to metid particle sinrering during 0, pretreatment. In the case of Ru, considerable loss in metal content, occurs as a result of the formation of R u 0 4 (2). Rh appears to behave i n a similar manner to that observed for Ru except that the metal loss as a function of increasing 0, temperature is not observed. The volatility of the oxide Rh203 or Rh207 does not appear high enough to result in metill loss.
DISCUSSION At pH=9.0, the negatively charged silica surface adsorbs metal cationic precursors according to the following general equiition : [M(NH,),]rl+
+ n SiO-H+ ----------->[M(NH3),IniISiO],- + n H+
Where I: is the charge on the cationic precursor. This surface cornplex can react with the reactants used in prctreatment in several Wi\yS ;ISfollows: Treatment in H 2 . For the case of iidsorbed Pd or Pt cationic precursors, this surface complex reacts with Hz to form mobile intermediate complexes with zero charge. For Pt, strong spectroscopic evidence supports its formation. For Pd, the CT band in the UV is not observed due to the strong silica absorption band centered at 208nm. These mobile surface complexes were not observed for Rh or Ru. Strongly bound surface nitrosyl bands were identified for the K u surface complex by UV. These surface complexes are not mobile and do not lead to metal particle growth. Trearmenr in 02. Pt and Pd metal dispersions were not affected by treatment in tlowing oxygen. ?‘he diffuse UV rcflcctance spectra of Pt and Pd differed in that a band centered :it 24511111persisted for Pd iit higher temperatures. A similar band for Pt disappcarcd at lower temperatures. Because this band wiis assigned to a charge transfer electronic transition between the support and the metal, it was sumiised that the Pt complex decornposed at lower temperatures. When the decomposition was performed in argon, reduction of the Pd complex due to the evolution of NH, occurs at a lower temperature. This lower temperature reduction results i n the inhibition of the formation of volatile surface oxides which lead to panicle growth. Poor dispersions were obtained for the Ru :md Rh complexes due to the formation of volatile metal oxide intermediate. This resulted in metal loss for Ru but not f o r Iih. ACKNOWLEDGEMENT The authors acknowledge support from the U . S. Department of Energy (Grant DOE FW2-86EK-13.51) for this research. We also gratefully acknowledge help from the staff at the Electron Microscope Facility at ‘rulme University. REFERENCES 1 W. Zou and R. D. Gonzalez, J. Catal., i n press. 2 W. Zooti and R. D. Gonzalez, Catalysis ?’odiIy, i n press. 3 J. Sarkany and R. D. Gonzalez, J. Catal., 76 (1982) 7.5. 4 R. A. Dalla Betta and M. Boudart, i n “Proceedings, 5th International Congress on Catalysis” ( f . W. Hightower, Ed.), p. 1329, North-I Iolland, Amsterdam, 1973.
Guczi, L.ef al. (Editors),New Frontiers in Catalysis Proceedingsof the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
REACTION SITES ON THE A1203 SUPPORT OF Pd/Al,O,
J. L. Falconer, B. Chen, S. A. Larson and E. C. Hsiao Department of Chemical Engineering, University of Colorado, Boulder, CO 80309-0424, USA
Abstract The hydrogenation of CO adsorbed on Pd/A1203can proceed through the forsupport by spillover. The rate of formation mation of a methoxy species on the A1203 of the methoxy species and the effect of coadsorbed water on CH30hydrogenation are presented. Methoxy hydrogenates faster than CO adsorbed on Pd, and though H20 decreases the rate of spillover, methoxy hydrogenation may contribute to the high rate of steady state methanation observed on Pd/Al,03. 1. INTRODUCTION On Ni/Al,03 catalysts we have observed that CH30 readily forms on the A1203 support in the presence of CO and H2 at elevated temperatures (1). The resulting CH30 can be hydrogenated to CH,, but the reaction rate is dower than hydrogenation of CO adsorbed on Ni. On Pd/A1203we have observed a similar formation of CH30 on the support (21, and Palazov et al(3) detected CH30 on Pd/Al,03 by IR, but in contrast to the results on Ni/A1203,the adsorbed CH30 hydrogenates faster than CO adsorbed on Pd. This paper will discuss measurement ofthe rate of CH30 formation and show that this formation is faster than the rate of CO hydrogenation on Pd. Since H20 is one of the products of methanation, the effect of coadsorbed water on the methanation rate will also be presented. Temperature-programmedreaction (TPR) was used for these measurements. A unique combination of TPR, isotope labeling, and isotope exchange allows CO adsorbed on Pd to be distinguished from CH30 adsorbed on the support. 2. EXPERIMENTALMETHODS
Temperature-programmed reaction (TPR) and desorption (TPD) with mass spectrometerdetectionwere used to study the reaction processes on a 3.7% Pd/A1203 catalyst. The apparatus and procedures, which have been describedpreviously (1,4), use a quadrupole 111~18s spectrometer with computer data acquisition. A catalyst sample of 100 mg is located in a downflow quartz reactor and carrier gases flow at approximately 100 cm3/min. Products are detected as the catalyst sample temperature is raised at a rate of 1 W Sin H2 flow. Carbon monoxide was adsorbed at 300 or 385 Kin H flow, and TPR was carried out in H,flow at ambient pressure. Isotope labeling ("to)and exchange with CO adsorbed on the Pd were used to distinguish CO on Pd from CH30on Al,03.Carbon
1888
monoxide on Pd readily exchanged with gas phase CO at 300 K but CH,O did not. The CH30was formed by CO and Ha coadsorption at 385 K or during TPR. Interrupted TPR,in which the catalyst temperature was raised to a temperature at which spillover occurred but essentially all the CO remained on the catalyet, was used to measure the transfer rate from Pd to Al,03. ARer the catalyst was cooled, exposure to 13C0at 300 Rdisplaced any "CO that remained on Pd, so that during a subsequent TPR the amounts of ',CHI and "CO provided an accurate measure of how much laCHSOwas on the A120,.These experiments were carried out at a series of interruption temperatures to measure the spillover rate as a function of temperature. Isothermal experiments were also carried out. Water was adsorbed before or after CO adsorption and TPR was then carried out while observing the HzO signal in addition to the CH,, CO, and COOsignals. The 3.7%Pd/Al,Os catalyst was prepared by impregnation to incipient wetness of aqueous H,PdCl, onto Kaiser A-201A120s.The procedure, described previously (11,is essentially the same as that used by Palazov et a1 (3). The final reduction temperature was 773 K. At the beginning of the experiments, the reduced catalyst was pretreated for 2 h at 773 K in ambient pressure H, flow. To maintain a clean surface, the catalyst was held in Ha at 773 K for 10 min at the completion of each TPR experiment.
3. RESULTS AND DISCUSSION When CO was adsorbed on Pd/A1203at 300 K, most of the CO hydrogenated to CH, during "PR;less than 5% of the CO desorbed unreacted (2). The relatively high activity of Pd/A1203(relative to Pd/SiO,) may be due to the formation of CH30
on Al,O,. In TPR experiments, as the initial CH30 coverage increased by coadsorption at 385 K, the rate of hydrogenation of the CO that was on the Pd (at the start of the experiment) decreased, so that only 14% of the CO on Pd was hydrogenated to CHI for a high CH,O coverage. The high coverage of CH30either prevents CO on Pd from transferring to active sites on the support, or prevents the CH,O that forms from hydrogenating. A series of interrupted TPR experiments were used to measure the rate of this transfer process. In these experiments, "CO was adsorbed at 300 K, an interrupted TPR was carried out, and "CO was then adsorbed. T h e resulting TPR spectra in Figure 1shows that much of the "CO originallyon the Pd was not displaced by "CO exposure after interrupted TPR. That is, the "CO was not on the Pd aRer the interrupted TPR. Without an interrupted TPR, 12CH,was not observed during the subsequent TPR because "CO on Pd really exchanges with gas phase "CO. In Figure 1,the amount of "CH, is almost the same as the amount of "CO adsorbed on clean Pd. T h e amount of "CH, from Figure 1correspondsto the amount of "CH,O that formed during the interrupted TPR since no "CO remained on the Pd after the exchange. The rate of spillover was estimated from a series of TPR experimentscarried out for different interruption temperatures. By plotting the amount that spilled over by each interruption temperature versus temperature an estimate of the spillover rate was obtained and is shown as a TPR peak for spillover in Figure 2. Details of the calculations to obtain this plot were described previously (5). Figure 2 shows clearly that the rate of spillover is fast relative to the rate of methanation. Almost all of the CO originally on the Pd had apilled over onto the AlzOs before the methanation rate became significant.
1889
Following interrupted TPR and "CO adsorption, the Al O3 surface was not saturated; many sites were available on the support so the "CO was also hydrogenated by first spilling over onto the support. Thus,the 'CHI and 13CH4spectra in Figure 1 look quite similar because they correspond to the same process. The A1203 could be saturated by CO adsorptionat 385 Kin H, flow. As shown in Figure 2, at this temperature the spillover process was fast but the methanation rate was slow. The resulting TPR spectra for two exposures are shown in Figure 3, which also includes the spectrum for CO adsorption a t 300 K Three times as much CO adsorbed on Pd/A1203at 385 Kin H, as at 300 K in He or H,.
0.9 h
d 0.60
8
v
2 Oe3-
PL 4 '
Or'
Temperature (K)
Figure 1. Methane TPR spectra on 3.7% Pd/A1203.The "CO was adsorbed a t 300 K After an interrupted TPR to 450 K, "CO was adsorbed at 300 K and the TPR was carried out.
to Temperature (K)
Figure 2. The spillover rate during TPR (dashed line) is compared to the methanation rate during TPR (solid line) on 3.7% Pd/A1203.
When a TPD was carried out following CO adsorption at 385 Kin Hzflow, then CO and H,desorbed simultaneouslyin a peak at 495 K. The W C O ratio was greater than 3 and is at least consistent with the presence of CH30 on the support (2). Infrared studies on Pd/Al,03 (3)and WAl,03(6) catalysts under similar conditions have observed CH30 on the surface. Water affectedthe formationof CH30,as reportedby Robbins and Marucchi-Soos (6)for WAlzOs. However, as shown in Figure 4, even for high coverages of H,O, the methanation rate for CO originallyadsorbed on Pd was the same. Curve a in Figure 4 is the CHI spectrum following CO adsorption in H, at 385 K to obtain a high coverage of CH30. When 1 pL HzO(550 pmollg catalyst)was adsorbed on the surface first, the amount of CHI formed during TPR (curve b) was only 25% of that seen in curve a and the amount of unreacted CO also decreased. This much H,O apparently so that the totalamount of adsorbedC0 was equal to the amount saturated the Also3, that adsorbed at 300 K on the catalyst in the absence of H,O. That is, H,O adsorbed on sites on Al,O3 and prevented CH30 formation at 385 K. As H,O desorbed, sites on the A1203became available for spillover so that above 500 K, the rate of methanation in Figure 4b was similar to that in Figure 3a.
1890
2.4
s
h
0
bD
1.6
3 Y
p) 0.8
2 0 300 -~
700
Temperature (K) Figure 3. MethaneTPRspectraon3.7% Pd/A1203.Carbon monoxide adsorption conditions: a) He flow, 30 min at 300 K, b) Ha flow, 30 min at 385 K, c) Haflow, 60 min at 385 K. The curve8 are displaced vertically for clarity.
Temperature (K)
F'igure4. MethaneTPR spectraon3.7% Pd/Al,Os: a) CO adsorbed in ambient pressure Ha at 385 K for 30 min. b) 1 pL HaO adsorbed in He at 300 K and then CO adsorbed in Ha at 385 K for 30 min.
Since some H,O desorbs at methanation temperatures, the enhancement of CH, formation by the formation of CHsO may provide an alternate pathway to methanation that explains the hi h steady-state methanation activity of Pd/AlaO,. The combination of isotope la%eling, isotope exchange, and TPR provides a new technique for characterizing reaction processes that take place during catalytic reactions. For methanation, these techniques demonstrate that the support may play a predominant role. Even in the presence of HaO,which forms during steady state reaction, the spillover to form CH,O takes place. 4. ACKNOWIZDGMENT
We gratefully acknowledge support by NSF Grants CBT-8616494 and CTS9021194. 6. REFERENCES
1 P.G.Glugla, K.M.Bailey, and J.L. Falconer, J. Phys. Chem. 92,4474 (1988). 2 E.C. Hsiao and J.L. Falconer, J. C a d . 132,145 (1991). 3 Palazov, A., Kadinov, G., Bonev, C.H., and Shopov, D., J. Catal. 74,44 (1982). 4 Schwan, J.A. and Falconer, J.L., Catal. Today 7,1(1990). 5 B. Chen and J.L. Falconer, J. Catal., i n press (1992). 6 Robbins, J.L. and Marucchi-Sous, E., J. Phys. Chem. 93,2885 (1989).
Guczi, L.ef d.(Editors), New Fronfiersin Cololysb Proceedingsof the 10th International Congress on Catalysis, 19-24 July, lLB2, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
THE EXTERNAL MAGNETIC FIELD EFFECT ON THE H2-0, REACTION ON THE Sn02 SURFACE
H. Ohnishi, H. Sasaki and M.Ippommatsu Fundamental Research Laboratories, Osaka Gas Co.,Ltd., 6-19-9Torishima, Konohana-ku, Osaka 554, Japan
Abstract The authors report on the findings regarding external magnetic field effects on the H2+ reaction on a S n Q surface by the measurements of the electric conductivity of S n a thin films. Further, we have succeeded in directly measuring external magnetic field effects on H2-Q reaction on an SnQ surface and found that the rate of increase in the reaction rate reached approximately 14% under the condition of 623 K and 5 T.
1. INTRODUCTION Extensive studies of external magnetic field effects on catalytic reactions have been made by Misono [I], Selwood [2], and other researchers. However, the reaction systems were limited to the ortho-para conversion of hydrogen molecule on magnetic catalysts. Making use of the property that the conductivity of SnQ semiconductors changes when in contact with flammable gases, SnO;! semiconductor gas sensors are widely used for domestic gas leak alarm systems. The conductivity changes of SnQ depend on the rate of the reaction between flammable gas and surface-adsorbed 02- [3 - 51. This presentation report on the external magnetic field effects on the HyOz reaction on the SnO2 surface. This is one of the first finding of the magnetic field effect on an industrially important catalytic reaction.
2. EXPERIMENTAL 2.1. Sample Preparation. SnOz thinfilm. A SnOz thin film was deposited on a sapphire substrate by the reactive RF magnetron sputtering technique. The Sn02 thin film (2.2 pm thick) had a columnar structure. Pt electrodes (1 pm thick) were deposited on the SnO2 thin film by RF magnetron sputtering technique. The SnOz thin film was used for electric conductivity measurement.
1892 SnOl powder. A 99.999% pure Sn metal was dissolved in nitric acid, and the solution was neutralized by an ammonia solution, then obtained gel was washed by water, dried, and calcined for 24 hr, at 1073 K in air atmosphere. The produced SnOz powder, primary particle size of 20 to 30 nm, was used for directly measurement of the H2-02 reaction rate.
2.2. Measuring the conductivity A diagram of the measurement system is shown in Figure 1. The magnetic field was applied to the sample vertically using a superconductive magnet made by Kobe Steel Co. The magnetic field intensity was varied in the range from -5.0 to 5.0 T. Sweeping velocity of the magnetic field was 20 T/min.
2.3. Measuring the reaction rate The measuring apparatus using flow method is shown in Figure 2. The magnetic field was applied to reaction cell by a pulse magnet capable of producing magnetic field excitation from 0 to 5 T in 15 sec. The material gas that is dry air containing 0.1% H2 was supplied into the reaction cell, and the H20 molecules in the produced gas were adsorbed by molecular sieves 3A. By measuring the increases in weight of H20 adsorbed using a microbalance CAHN-2000, the reaction rate was successfully obtained with an accuracy o f f 0.1%. The amount of Sn02 sample was controlled to obtain the reaction conversion rate of between 5 and 10%.
Figure 1. Conductivity measurement system.
Figure 2. Reaction rate measurement system for H2-02 surface reaction.
3. Results and Discussion 3.1. Backgrounds The electric conductivity of SnO2 is proportional to carrier electron density [ 5 ] . The carrier electron density is determined by the balance between the rate of the reaction producing electrons (reaction A, B in Figure 3) and the rate of the reaction consuming electrons (reaction C in Figure 3) [3 - 41. Finally, the reaction rate between flammable gases and surface-adsorbed oxygen is proportional to the conductivity of SnOZ in air containing Hz [6].
1893
Figure 3. Reaction mechanism's model on the SnO2 surface.
3.2. Observation of magnetic field effect by the conductivity measurements An increase in the conductivity of SnO2 thin film by applying magnetic field was observed only when Hz-02 reaction was proceeding on the SnO2 surface in an oxygen atmosphere at 773 K. The rate of increase in the conductivity was proportional to the square of the magnetic field intensity and was independent of H2 concentration in the range of 0.1 1 vol.%. This phenomenon was distinguished from changes in electron mobility, such as magnetic resistance or Hall effect, by measurement of the dependence of increase in the conductivity on magnetic field direction and on surrounding gases (Table 1). Because this phenomenon was a characteristic feature when H2-02 reactions on SnO2 surfaces occurred, it can be concluded that this phenomenon was caused by the external magnetic field effect characteristics of the H2-02 reaction on the SnO2 surface. The increase rate of conductivity has indicated the increase in reaction rate.
-
Table 1 Dependence on field direction. Surrounding Gases 1) Air (21%02 + N2) in Air 2) H2 (0.35 vol.%) 3) C& (0.35 vol.%) in Air 4) H2 (0.35 vol.%) in N2 5 ) H20 (1.50 vol.%) in Air 6) H2 (0.35 vol.%) + H20 (1.50 vol.%) in Air
A ~ l a o('3%) 0.00 2.32 0.00 0.00 0.00 1.43
3.3. Direct measurement of magnetic field effects Table 2 shows the results of the experiment to obtain H 4 2 reaction rates with and without the application of 5 T magnetic field in dried air containing 0.1% H2 at 623 K. The results shows that its reaction rate increased by approximately 12%(approximately 14% after the blank value was deducted) by the application of 5T.
1894
Table 2
Results from reaction rate measurement
BlankTest Sn02powder
Increase rate of molecular sieve's weight (pg 1 sec.) B=OT B=5T 1.921 x 1.921 x lo-* 1.680 x 10-1 1.886 x 10-1
Increase percentage of reaction rate (%) 0.0 12.3 (13.8*) ~~
~
* After the blank value was deducted. 3.4. Mechanism of magnetic field effects These results have strongly suggested that an activated complex of this reaction has a structure in which there is only a weak interaction between hydrogen molecules that do not dissolve nor separate and surface-adsorbed 02-,and the magnetic field affects the electron spin of quasi radical-pair H-H in activated complexes. (Figure 4) The following two models may explain the experimental results. (i) The singlet-triplet conversion in the electron spin of the activated complex in the reaction between adsorbed oxygen and hydrogen on the $1102 surface occurs due to the magnetic field. Then, the more reactive species is produced by the singlet-tripletconversion. (ii) Since any hypertine interaction effects can be neglected, the singlet-triplet conversion rate is determined only by the electric Zeeman effect. In these models, the singlet-triplet conversion rate is proportional to the square of the magnetic field intensity [7, 81. Consequently the increase in the rate of rate of the reaction between H2 and surface-adsorbed0 2 - is also proportional to the square of the magnetic field intensity, thus explaining the experimental results.
Figure 4 . Transition state of H 4 2 surface reaction on the SnO2 surface.
REFERENCES 1 2 3 4
5 6 7 8
M. Misono and P. W. .Selwood, J. Am. Chem. SOC. 90 (1968) 2977. P. W. Selwood, Adv. Coral. 21 (1978) 23. M. Ippommatsu and H. Sasaki. J. Electrochem. SOC. 136 (1989) 2123. M. Ippommatsu and H. Sasaki, J . Materials Sci. 25 (1990) 259. M. Ippommatsu, H. Ohnishi, H. Sasaki and T. Matsumoto, J. Appl. Phys. 69 (1991) 8368. H. Sasaki, H. Ohnishi and M. Ippommatsu. J . Phys. Chem. 94 (1990) 4281. H. Hayashi and S. Nagakura, Bull. Chem. SOC.Jpn. 51 (1978) 2862. Y. Sakaguchi and H. Hayashi, J. Phys. Chem. 88 (1984) 1437.
Guczi, L et al. (Editors),New Frontiers in Catalysk
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
CHARACTERIZATIONOF DIFFERENT SURFACE MOSPECIES IN Mo/A1203 CATALYSTS BY TIME DIFFERENTIAL PERTURBED ANGULAR CORRELATION S. GuiaiP, Y. Tingyuna, Y. Fushana, R. LiguoO andN. Xinbob akpartment of Petrochemical Engineering, Fushun Petroleum Institute, Fushun, Liaoning, China bShanghai Institute of Nuclear Research, Academia Sinica, Shanghai, China
Abstract Different Mo species in Mo/Al,03 catalysts were characterized by time differential perturbe angular correlation ( TDPAC) and their HDS activities were measured. The Mo/AI,03 catalysts in oxidic and reduced states contain at least two kinds of Mo species. One of them is the precussors of the active sulfided state: which is represented as Mo ( N ) p for the oxidic catalysts and as Mo ( N ) P for the reduced catalysts. A maximum amount of the M o ( N ) p appears in a catalyst with 9-l0%MoO3 Mo loading, calcination temerature and promotor Co atoms affect significantly the amount of Mo ( ) p species in the Mo/Al,O, catalysts. There is a strong interaction of Mo( N ) p with the support . 1. INTRODUCTION
Mo/AI,03 catalysts are of great technological importance for hydrodesulfrization and hydrodenitrogenation reactions. Therefore, the alumina supported molybdenum sistem has been studied expensively[ 1 3 and many techniques have been applied to elucidating the surface structure of the catalysts[l,2]. Obviously the understanding of the nature of HDS catalysts and of their function has inproved in the last two decades. However, we must also admit that many important aspects still remain to be solved. It is reported that octahedrally and tetrahedrally coordinated Mo species were observed using different technigues[2,3]. Moreover, Knozinger has concluded that a high percentage of the surface Mo species are structurally very similar to the heptamolybdate Mo70246- ( HM) and small “island “with seven Mo atoms and two layers thick exists on the catalysts[l] . Among the techniques used, TDPAC is perhaps the most suitable technique for the study of the adsorbed molybdates. In this paper we present the TDPAC determination results of the Mo/A1203 catalysts in oxidic, Hz-reduced and NH3 - extracted states, which give some interesting informations for explaining the surface structure of the catalysts. ( HDS)
1896
2. EXPERIMENTAL 2.1 TDPAC measurement A four -detector slow - fast - coincidence TDPAC spectrometer was used. The interaction between the nuclear quadrupole moment and the electric field gradient produced by all extranuclear charges was measured. The nuclear quadrupole interaction (NQI) parameters are given by a frequency w and an assymmetry parameter q. The theory, setup and data reduction of 99 M o 99TcTDPAC were described in detail elsewhere[4].
-
2.2. preparation and treatment of Mo/AI2O3catalysts The Mo/ A1203 catalysts with 4, 9, 11, 22% MOO, (Mo4, Mo9, M o l l and Mo22) were prepared by impregnation of y - A1203 in ( NH,) 6 M ~ 7 0 2( AHM) 4 solution , then dried at 120 ‘c for 16h and calcined at 450‘c for 10h. The CoMo/A1,OS catalyst with Co/Mo= 1: 2 (atom) was prepared by impregnation of Mo/Al,03 cata. lyst in Co( NO,) solution. The reduction of the oxidic catalysts was performed by contacting them with H2 al 500 ‘c for 2h. The extraction of the surface Mo species was carried out by treating the oxidic catalysts with NH,-NH4NO3 solution (8YoNH3) at 25 ‘c for 24h. 2.3. Thiophene ( C,H,S) HDS activity measurement The HDS activity of the catalysts was determined at 275 C using a continous flow MR - GC80 microreactor with a 50%( w ) thiophene and 50% benzene mixture under H2 pressure 1.5MPa. The oxidic and reduced catalysts were presulfided with a H2 and CS2mixture at 270 C . 3. RESULTS A N D DISCUSSION
3.1. Mo/A1203 catalysts in oxidic state Table 1 shows that the oxidic catalysts have two or more kinds of Mo species with different chemical environments ,which depends on Mo loading and preparation conditions . The Mo species with interaction frequencies w = 130- 170Mrad/s are octahedrally coordinated ( Mo[O]) like AHM, Moo3 and Mo ( VI ) - EDTA complex[5]. The Mo species with w = 218 - 340Mrad/s are not able to be associated with any reference compounds. We represent them as Mo( VI ) p . Table 1 NQI parameters and C4H4Sconversions for oxidic Mo/A1203catalysts CatawI I1 a1 w2 tl2 a2 a: Mo(VI CC.,H~S lyst (Mrad/s) (%) (Mrad/s) (%) (%) antent(%) (%) Mo4 Mo9 Mollb Mo22 CoMo9 AHM
230(9) 302( 11) 333(21) 453142) 218(13) 416( 10)
0.36(7) 0.44(4) 1 0.63(11) 0.24( 14) 0.28(4)
78 59 33 17 54 21
l68(9) 133(27) 160(11) 165(21) 180(3)
22(2) 1 41(4) 0 1 49 0 0.35 7 9 ( 5 ) 4(3) 1 31(6) lS(3) 0.45 76(2) 3(1)
3.1 5.3 3.6 2.8‘ 4.9
a , a0 denotes the fraction of frequency distribution around w = 0 b . ws=460Mrad/s , q3=0.77(1 5 ) , a3= 18( 8) % c . it is obtained from the data of the extracted Mo22 in Table 3 by calculation
34.6 60.1
-
11.2 58.2
1897
The Mo4 sample shows a predominent fraction( 78%) of Mo( VJ ) species and a small fraction( 22%) of Mo species with u=O, which is observed only in the Mode compounds with isolated tetrahedra Mo0?-[4]. In the cast of Mo22 catalyst, a high fraction of Molybdate species with w,=453Mrad/s and w2= 16OMrad/s is structurally similar to the HM anion , which is octahedrally coordinated and has several unequivalent Mo sites[6]. The Mo9 sample contains both Mo[O] and M O ( R ) ~ species and its Mo( VI ) amount decreases with increasing calcination temperature. The above discussion shows that for the Mo/A1203 catalysts with bw and medium loadings( < ll%MoO,) the Mo[O] species with o 450Mrad/s similar to those in AHM were not observed, and that only in higher loadings ( > 10%Mo03) they appeared, which implies that there exist polymolybdate species in multilayer form. We note that the amounts of Mo( VI ) species and the HDS conversions at first increase with increasing Mo loading , then decrease. The maximum appears in a Mo/Al,O, sample with about 9 - lO%MoO, , Therefore, we can assume that the Mo( VI ) p species are the precussors of the active sulfided state.
-
3.2. Reduced Mo/AlzOs catalysts
Table 2 NQI parameters and C,H,S conversions for reduced Mo/A1203 catalysts and AHM Catalyst ol rll Ul a 2 rlz % 06 W N ) , ccsts (Mrad/s ) ( % ) (Mrad/s) (%) (%) Content(%) ( % ) Mo4 458(27) 0.30(8) 60 llY14) 0 40(3) 0 2.4 39.1 1 62(3) l(3) 3.3 41.0 Mo9 39x17) 0.51(6) 37 147(9) 12(5) 6.6 51.4 Mo22 34y43) 0.4q14) 30 12x14) 1 58(3) CoMo9 289(19) 0.26(11) 54 11x8) 1 46(3) 0 4.9 58.3 AHM 73(8) 1 85 1x 13) Mood unreduced) 73(2) 0 100 It is indicated from Table 2 that the reduced Mo/A1203 catalysts also contain two or more kinds of Mo species which show lower NQI frequencies than the oxidic catalysts. The NQI frequencies w = 110 - 140Mrad/s with q = 1( or w = 70 - llOMrad/s with q = 0) could be contributed to octahetrally coordinated Mo4+species ( Mo4+[O] ) , which are almost identical to MoOz.whereas , we can not decide which reference compounds the NQI frequencies w = 280 - 450Mrad/s could be contributed t o , so we assum that they are associated with coordinatedly unsaturated tetrahedral M04+species or with Mos+[O] species in a double oxigen bridge structure Molo,)M0[5]. Comparing the C,H,S HDS activeties with the NQI parameters, we find that the reduced Mo species with w = 280 - 450Mrad/s could be the precussors of HDS active sites for the reduced Mo/Al,O, catalysts, which are represented as Mo(TV ) p , and that the Mo( Iv ) amounts and the HDS activeties increase with increasing Ma loading. In medium loadings the HDS activities of the reduced catalysts are lower than those of the corresponding oxidic catalysts, whereas in high loadings they are higher. The great fraction of the Mo( Iv ) species of the CoMo9 catalyst shows that the important effect of Co atoms on the catalyst is to increase the Mo( Iv ) P amount.
1898 Table 3 NQI parameters and C4H$ HDS conversions for NH3- extracted Mo/Al,O3 catalysts )I1 ec, w2 )I2 bi M4VI)P c J w 4 s Catalyst (Majs) ( % ) (Mradls) (%) content(%) (%) Mo4 252(10) 0.4y9) 65 3x2) 1.7 23.8 Moll 29q10) 0.43(4) 60 153(14) 1 37(4) 3(3) 3.3 54. I M022 288(22) 0.64(10)40 168(16) 1 34(7) 32) 3.1 56.8 CoMo9 21q 10) 0.1% 19) 95 3 2 ) 3.4 57.7 a 03=512( 35) Mrad/s ,q3=0.36( 8 ) ,u3=24( 5 ) YO
(2)
3.3. NH, -extracted Mo/AI,O, catalysts We have determined the extraction degrees of the catalysts and find that about 30 - 40% Mo in the Mo4 and about 45 - 50% Mo in the Moll and about 60 - 70% Mo in the Mo22 and CoMo9 catalysts have been removed in the NH, - extraction p r o cedure. whereas, the AHM and MOO, have been completely dissolved in the NH, solu tion, This indicates that the surface Mo species are more dificult to dissolve in NH: solution than AHM and MOO,. The results in Table 3 show that the Mo[O] species are more easily dissolved than the Mo (VI ) p and MOO.,- species, which indicates that there is a stronger in. teraction between the Mo( VI ) p and the Al,03 support. The higher HDS activities of extracted catalysts and a great fraction of w=216-296Mrad/s confirm that the Mo( VI ) p species are the precussors of the active sulfided state. The promotor CO atoms seem to weaken the interaction between the Mo[O] and the support. From the fraction 40% of w = 288 Mrad/s of the extracted Mo22 cata lyst we can obtain by calculation that the corresponding unextracted oxidic Mo22 catalyst contains about 10 - 15% Mo( VI ) species. 4. CONCLUSIONS
The TDPAC results show that the Mo/AI,O,catalysts in oxidic and reduced statescontain at least two kinds of Mo species, Mo6+[O] and Mo( VJ ) for oxidic state and Mo4+[0] and Mo (N)p for reduced state , The Mo ( VJ ) p and Mo( IV)P are the precussors of the HDS active sites . For the oxidic catalysts ,there are stronger interactions of the Mo ( VJ ) p species with the support than those of M06+[0] species and the maximum Mo ( M)P amount appears in the medium loading ( about 9 - 10 % MOO,).Whereas, for the reduced catalysts the Mo ( IV ) pamounts increase with increasing Mo loading. In the catalysts with more than 11% Moo3 ,the Mo[O] species exist partly in multilayer form. The important roles of the promotor Co atoms are to increase the amount of Mo ( VI ) pand Mo ( IV)pand to weaken obviously the interaction between the Mo[O] species and the support. Acknowledgements The authors thank prof. H. Knozinger, Dr. T. Butz and Dr. A. Lerf for their gui dance and hospitality. References 1 H. Knozinger, "Genesis and Nature of Mo-Based HDS Catalysts": Proc. 9th Intern Congr. Catal., Calgary( 1988) 2 N. Giordano et al, J. Catal., 36( 1975) 81. 3 C. V. Caceres et d,J. Catal., 9 3 1985) 501. 4 T. Butz, A. Lerf and H. Knozinger, J. Catal., 11q 1989)31 5 Ni Xinbo, Sun Guida, T. Butz and A. Lert Chemical physics, 123( 1988)455. 6 H. T. Evans et al, J. C. S. Dalton Transact., ( 1975)505
Studies in Surface Science and Catalysis 75
NEW FRONTIERS IN CATALYSIS PART C
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J. T. Yates
Vol. 75
NEW FRONTIERS IN CATALYSIS Proceedings of the 10th International Congress on Catalysis, Budapest, July 19-24,1992 PART C Editors
L. GUCZI Institute of Isotopes of the Hungarian Academy of Sciences P. 0. Box 77, H - 1525 Budapest, Hungary F. SOLYMOSI Institute of Solid State and Radiochemistry, Jdzsef Attila University P. 0. Box 168, H-6701 Szeged, Hungary P. T E T ~ N Y I Institute of Isotopes of the Hungarian Academy of Sciences P. 0. Box 77, H- 1525 Budapest, Hungary
ELSEVIER
Amsterdam-London-New York-Tokyo 1993
Joint edition published by Elsevier Science Publishers B. V., Amsterdam, The Netherlands and Akadbmiai Kiad6, Budapest, Hungary Exclusive sales rights in
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CONTENTS PART A Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V Catalysis: Past, Present and Future ...... 1 J.A.Ra bo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interfacial Coordination Chemistry: Concepts and Relevance to Catalysis Phenomena . . . . . . 31 M.Che . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . To the New Catalytic Aspects of Heteropolyacids and Related Compounds Molecular Desien " of Practical Catalvsts M.Misono . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 The Catalytic Conversion of Methane to Oxygenates and Higher Hydrocarbons J.H. Lunsford . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 New Reactions in Various Fields and Production of Specialty Chemicals W.F. Holderich . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
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Structure-Function Relationships in Heterogeneous Catalysis: The Embedded Surface Molecule Approach and its Applications P. Johnston andR. W. Joyner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotopic Tracer Studies of Chain Propagation and Termination during FischerTropsch Synthesis over RuRiO2 K R. Krishna andA. T. Bell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liquid Phase Oxidation of Glyoxal to Glyoxylic Acid by Air on Platinum Catalysts P. Gallezot, F. Fache, R. de Mesanstourne, Y. Christidis, G. Mattioda and A. Schouteeten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Effect of Oxygen Binding Energy on the Selective Oxidation of Butane over VIY-AI~O, P. J. Andersen and H. H. Kung . . . . . . . . . :. . . . . . . . . . . . . . . . . . . . . . . Secondary Oxygen Exchange Reactions during the Partial Oxidation of Methane M. M. Koranne, J. G. Goodwin,Jr. and G. Marcelin . . . . . . . . . . . . . . . . . . . . Role of Free Radicals in Heterogeneous Complete Oxidation of Organic Compounds over IV Period Transition Metal Oxides 2. R. Ismagilov, S. N. Pak, L. G. Krishtopa and V. K. Yermolaev . . . . . . . . . . . . . 1H Broad-Line NMR at 4 K for Studying the Acidity of Solids: Application to Zeolites P. Batamack, C. Doremieux-Morin andJ. Fraissard . . . . . . . . . . . . . . . . . . . . Preparation of Bifunctional Catalysts by Solid-state Ion Exchange in Zeolites and Catalytic Tests H. G. Karge, Y.Zhang and H.K. Beyer . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transition Metal/Zeolite Catalysts by Design: Nucleation and Growth of Mono- and Bimetallic Particles in Zeolite Y W. M. H.Sachtler, Z. Zhang, A. Yu. Stakheev and J. S. Feeley . . . . . . . . . . . . . . Promotion of H-ZSM-5 by Alumina J. Volter, H. D. Lanh, B. Parlitz, E. Schreier andK Ulbricht . . . . . . . . . . . . . . . The Effect of Preparation Method on Metal-Support Interaction in Pd/L-Zeolite Catalysts G. Larsen and G. L. Haller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . On the Nature of Superactive Centers in H-FeZSM-5 Zeolites. Quantum-Chemical Calculations M. . I Filatov, . A. G. Pelmenschikov and G. M. Zhidomirov . . . . . . . . . . . . . . . . .
165
181 195
205 2 19
231
243
257 271 283 297
31 1
VI CO Oxidation on Pd(ll0): A Model System for Chemical Oscillations in Heterogeneous Catalysis M. Ehsasi, M. Berdau, A. Karpowicz, K Christmann and J. H. Block . . . . . . . . . . Nature of Metal-Metal Bonding in Mixed Metal Catalysts R. A. Campbell,J. A. Rodriguez and D. W. Goodman . . . . . . . . . . . . . . . . . . . The Reduction of Nitric Oxide by Hydrogen over Pt, Rh and Pt-Rh Single Crystal Surfaces H. Hirano, T. Yamaah, K. I. Tanaka,J. Siera and B. E. Nieuwenhuys . . . . . . . . . . Spectroscopic Studies on the Reaction Pathways of Methanol Dissociation on Pd Catalyst A. Berkb, J. Raskb and F. Solymosi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyclotrimerisation of Acetylene to Benzene over Single Crystal Palladium and Gold/F’alladium Surfaces and over Supported Palladium Catalysts C. J. Baddeley, R. M.Ormerod and R. M.Lambert . . . . . . . . . . . . . . . . . . . . . Surface Chemistry for Automotive Emissions Control: Interactions of Nitric Oxide on a (111) Pt-Rh Alloy Surface G. B. Fisher, C. L. DiMaggio and D. D. Beck. . . . . . . . . . . . . . . . . . . . . . . . Shape Selective Alkylation of Benzene with Long Chain Alkenes over Zeolites S. Sivasanker, A. Thangaraj, R. A. Abdulla and P. Ratnasamy . . . . . . . . . . . . . . Comparison of SAPO-37 with Faujasites in Cracking Reactions M. Brienci, M. Derewinski, A. Lamy and D. Barthomeuf . . . . . . . . . . . . . . . . . . Catalytic Activity of Modified ZSM-5 Zeolites in the Dehydrogenation and Aromatization Reactions of Propane and n-Butane P. Fejes, J. Halasz, I. Kiricsi, Z. Kele, Gy. Tasi, I. Hannus, C. Fernandez, J. B. NaRy, A. Rockenbauer and Gy. Schobel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Coke Formation during the Transformation of Propene, Toluene and Propene-Toluene Mixture on HZSM-5 P. Magnoux, F. Machaab and M. Guisnet . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrocarbon Formation from Methanol/Dimethyl Ether over Protonated Zeolites and Molecular Sieves. New Insights from Recent Experiments S.Kolboe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H-D Exchange between Zeolites and Alkanes. Evidence for Formation and Rearrangement of Pentacoordinated Carbonium Ions C. J. A. Mota, L. Nogueira, S. C. Menezes, V. Alekstich, R. C. L. Pereira and W. B. Kover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . New Surface Nb-Dimers Chemically Interacted with SiO,: Regulation of the Catalysis by Molecular Design of Reaction Sites N. Ichikuni and Y.Iwasawa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Expert Systems Approach to Catalyst Design - Application and Experimental Verification T.Hattori, H. Niwa, A. Satsuma, S. Kit0 and Y. Murakami . . . . . . . . . . . . . . . . Metal Oxide Vapour Synthesis (MOVS): A New Preparative Method for Heterogeneous Metal Oxide Catalytic Systems E. C. Alyea, K F. Brown, K. J. FisherandK D. L. Smith. . . . . . . . . . . . . . . . . Designing of New Catalysts for Olefin Metathesis on the Base of Photoreduced Silica-Molybdena V. B. Kazansky, B. N. Shelimov and K A. Vihlov . . . . . . . . . . . . . . . . . . . . . . Si02-Grafted Dinuclcar Molybdenum Catalyst Derived from MoZ(OAc), Highly Active for Olefin Metathesis Reaction M. Ichikawa, Q. Zhuang, G.-J. Li, K Tanaka, T. Fujimoto andA. Fukuoka . . . . . . . Molecular Design of Supported Metal Oxide Catalysts I. E. Wachs, G. Deo, D. S. Kim, M. A. Vuurman and H. Hu . . . . . . . . . . . . . . . .
32 1 333 345 359 371 383 397 409
421 435 449
463 477 489 503
515 529 543
VI I
Structural Characteristics of Alumina-Supported Activated Hydrodesulfurization Catalysts. An XPS, NO Adsorption and Sulphydryl Group Study L. Portela, P. Grange and B. Delmon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559 Structure-Function Relations in Layered Transition Metal Sulfide Catalysts M. Daage, R. R. Chianelli and A. F. Ruppert . . . . . . . . . . . . . . . . . . . . . . . . . 571 Elementary Steps of Hydrogenative Sulfur-, Nitrogen- and Oxygen-Removal from Organic Compounds on Sulfided Catalysts H. Schulz and N.M. Rahman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58s Sites Characterization on Model Ruthenium Sulphide M. Lacroir, C. Mirodatos, M.Breysse, T. Decamp and S. Yuan . . . . . . . . . . . . . . 597 High Resolution Electron Microscopy Characterization of the Poorly Crystalline Structure of Molybdenum Disulfide-Based Catalysts S. Fuentes, M.Avalos-Borja, D. Acosta, F. Pedraza andJ. Cruz . . . . . . . . . . . . . . 61 1 Deuterium Solid State Nh4R Study of Molecular Mobility and Catalytic Dehydration of tert.Buty1 Alcohol on H-ZSM-5 Zeolite A. G. Stepanov, A. G. Maryasov, V. N. Romannibv and K I. Zamaraev . . . . . . . . 62 1 Stationary Liquid-Phase Homogeneous Transition Metal Catalysis I.T.Horvath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635 An in situ Radioactive Tracer Technique for Studying Adsorption-Desorption Dynamics on a Working Catalyst U.Schriider, L. Cider andN. - H. Schiitn . . . . . . . . . . . . . . . . . . . . . . . . . . 643 Application of Scanning Tunneling Microscopy/Spectroscopy (STWSTS) to Catalyst Research: Pt/Si02 M. Komiyama and M.Kirrno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 659 Characterization and Catalytic Properties of Pt-Ir Small Bimetallic Cluster in NaY 0. B. YangandS. I. Woo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671 Laser Raman Characterization of Surface Phase Precious Metal Oxides Formed on G O , Micro Domains Generated within an Alumina Host by Sol Synthesis L. L. Murrell, S. J. Tauster and D. R. Anderson . . . . . . . . . . . . . . . . . . . . . . . 681 Direct Propane Ammoxidation to Acrylonitrile: Kinetics and Nature of the Active Phase A. Andersson, S. L. T. Andersson, G. Centi, R. K Grasselli, M. Sanati and F. Trifiro. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 691 Catalytic Oxidation of Fluorene to 9-Fluorenone - Development and Characterization of Catalysts F. Majunke, H. Borchert and M. Baerns . . . . . . . . . . . . . . . . . . . . . . . . . . . 707 Ammoximation of Cyclohexanone on Titanium Silicalite: Investigation of the Reaction Mechanism A. Zecchina, G. Spoto, S. Bordiga, F. Geobaldo, G. Petrini, G. Leofanti, M. Padovan, M.Mantegazza and P. Ro@ . . . . . . . . . . . . . . . . . . . . . . . . . . 71 9 On the Catalytic Oxidation of Methanol with Vanadium (IV) in Sulphuric Acid Solution R. Larsson and B. Folkesson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 1 Role of Chromium Introduced into 12-Molybdophosphates as Catalysts for Oxidation of Hydrocarbons K Briickman andJ. Haber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741 Correlation between Catalytic and Structural Properties of Modified Molybdenum and Vanadium Oxides in the Oxidation of Ethane in Acetic Acid or Ethylene M.Merzouki, B. Taouk, L. Tessier, E. Bordes and P. Courtine . . . . . . . . . . . . . . 753 The Promoting Effect of La203 on the CO Hydrogenation over Rh/Si02 765 A. L. Borer and R. Prins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Chlorine on the Rhenium-Alumina Interaction in Low-Loaded Re/Alfi3 and PtRe/A1203 Industrial Catalysts G. Munuera, P. Malet andA. Caballero. . . . . . . . . . . . . . . . . . . . . . . . . . . 781
Vlll
A New Ap roach to Loss of Alkali Promoter from Industrial Catalysts: Importance of Excited [tates of Alkali L. Holmlid K Engvall, C. Aman and P. G. Menon . . . . . . . . . . . . . . . . . . . . . The Relation between Catalytic and Electronic Properties of Supported Platinum Catalysts: The Local Density of States as Determined by X-Ray Absorption Spectroscopy M. Vaarkamp,J. T. Miller, F. S. Modica, G. S. Lane and D. C. Koningsberger . . Direct MAS/MES Evidence for Electronic Metal-Support Interactions in Dilute side and 57Fe Carbon and Alumina-Supported Catalysts C. H. Bartholomew, L. R. Neubauer and P. A. Smith . . . . . . . . . . . . . . . . . . . . Formation and Properties of Dispersed Pd Particles over Graphite and Diamond 0. S. Aleheev, L. V. Nosova and Yu.A. Ryndin . . . . . . . . . . . . . . . . . . . . . . . Kinetics of Alkane Hydrogenolysis on Clean and Coked Platinum and PlatinumRhenium Catalysts G. C. Bond R. H. Cunningham and E. L. Short. . . . . . . . . . . . . . . . . . . . . . . Toluene Hydrogenation over Supported Platinum Catalysts S.-D. Lin and M. A. Vannice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Catalytic Activity of MoOx/Zr02 in the Hydrogenation and Metathesis of Propene V. Indovina, A. Cimino, D. Cordischi, S. Della Bella, S. De Rossi, G. Ferraris, D. Gazzoli, M. Occhiuzzi and M. Valigi . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pt-Sn-Alumina Catalysts: Relating Characterization and Alkane Dehydrocyclization Data B.H.Davis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bimetallic Pt-Sn/A120j and Pt-Au/SiO, Catalysts: A Comparison of Reactivity, Adsorption behavior and Microstructure J. Schwank, K Balakrishnan andA. Sachakv . . . . . . . . . . . . . . . . . . . . . . . . Hydroformylation of 1-Hexene by Soluble and Zeolite-Supported Iridium Species J.-Z. Zhang, 2. Li and C.-Y. Wang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
795
809
82 1 837
849 86 1
875
889
905 919
PART B Bulk Tungsten Carbide as Catalyst in Hydrocarbon Reactions: Association of Selectivity Differences with Surface Composition as Compared to the Selectivity of Pt Series Metals A. Frennet, G. Leclercq, L. Leclercq, G. Maire, R. Ducros, M. Jardinier-Offergel4 F. Bouillon, J-M. Bastin, A. L o e r g , P. Blehen, M. Dufour, M.Kamal, L. Feigenbaum, J-M. Giraudon, V. Keller, P. Wehrer, M. Cheval, F. Garin, P. Kons, 927 P. Delcambe and L. Binst. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface and Catalytic Properties of Molybdenum Nitrides L. T. Thompson, C. W. Calling, D. Choi, B. G. Demczyk and J.-G. Choi . . . . . . . . . 941 n-Hexane Isomerization on High Specific Surface Mo2C Activated by an Oxidative Treatment M. J. Ledoux, C. Pham-Huy H. Dunlop and J. Guille . . . . . . . . . . . . . . . . . . . 955 Highly Dispersed Metal Colloids: Spectroscopy and Surface Chemistry in Solution J. S. Br&y, J. M. Millar, E. W. Hill, C. Klein, B. Chaudret and A. Duteuil . . . . . . 969 Preparation of Amorphous Cu-Ti and Cu-Zr Alloys of High Surface Area by Chemical Modification S.Yoshidu, T.Kakehi, S. Matsumoto, T. Tan- H. Kanai and T. Funabiki . . . . . . 981 A Mechanistic Proposal for Alkane Dehydrocyclization Rates on PtL-Zeolite. Inhibited Deactivation of Pt Sites within Zeolite Channels 993 E. lglesia and J. E. Baumgartner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroconversion of n-Alkanes and Decalin over Bifunctional PtMazzite Catalysts F. Fajula, M. Boulet, B. Coq, V. Rajaofanova, F. Figueras and T. Des Courieres . . . 1007
IX Effect of Sulfur on the Performance of Pt/KL Hexane Aromatization Catalyst J. L. Kao, G. B. McVicker, M. M. J. Treacy, S. B. Rice, J. L. Robbins, W. E. Gates, 10 1 9 J. J. Ziemiak, V. R. Cross and T. H. Vana'erspurt . . . . . . . . . . . . . . . . . . . . . . Aromatization of n-Hexane by Aluminium-Stabilized Magnesium Oxide-Supported Noble Metal Catalysts E. G. Derouane, V. Jullien-Lardot, R. J. Davis, N. Blom and P. E. Hojhnd-Nielsen . , 103 1 Effect of the Alkali Cation on Heptane Aromatization in L Zeolite R.F. Hicks, W.-J. Han andA. B. Kooh . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,1043 Reduction and Aromatization Activity of M003/A1203 Catalysts: The Identification of the Active Mo Oxidation State on the Basis of Reinterpreted Mo 3d XPS Spectra W. Grunert, A. Yu.Stakheev, R. Feldhaus, K. Ana'ers, E. S. Shpiro and Kh. M. Minachev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1053 Reaction Routes for Methane Conversion on Transition Metals at Low Temperature T.Koerts and R. A. Van Santen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065 The Mechanism of Alkane Oxidative Dehydrogenation on Chloride and Oxychloride Catalysts R. Burch, S. Chalker and P. Loader. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1079 Transient Isotopic Studies of the Role of Lattice Oxygen during Oxidative Coupling of Methane on %/La O3 and Ca/ThO, Catalysts Z. KalenikandE. E. boy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093 The Role of the Proton in Oxidation Processes on Metal-Oxygen Cluster Compounds S. Kasztelan, G. B. McGarvey and J. B. Moffat . . . . . . . . . . . . . . . . . . . . . . . 1 105 Correlations betwcen p-Type Semiconductivity and C Selectivity for Oxidative Coupling of Methane (OCM) over Acceptor Doped SrTid3 C. Yu, W. Li, W. Feng, A. Qi and Y. Chen . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 19 Mechanistic Aspects of the Selective Oxidation of Methane to C1-Oxygenates over Mo03/Si02 Catalysts in a Single Catalytic Step M. A. Banares, I. Rodriguez-Ramos, A. Guerrero-Ruu and J. L. G. Fierro . . . . . . . 1 1 3 1 On the Mechanism of Xylene homerization and its Limitations as Reaction Test for Solid Acid Catalysts A. Corma, F. Llopis and J. B. Monton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 145 Aluminum Coordination and Lewis Acidity in Aluminas and Steamed Zeolites H. Yong, D. Coster, F. R. Chen, J. G. Davis andJ. J. Fripiat . . . . . . . . . . . . . . . 1 1 5 9 Characterization of Basic Sites on Fine Particles of Alkali and Alkaline Earth Metal Oxides in Zeolites H. Tsuji, F. Yagi, H. Halrori and H. Kita . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7 1 ZrO2-SO4*- Catalysts. Nature and Stability of Acid Sites Responsible for n-Butane Isomerization P. Nascimento, C. Akratopoulou, M. Oszagyan, G. Coudurier, C. Travers, J.F. Joly and J . C. Vedrine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8 5 Catalytic Activity for Vapor-Phase Aldol Condensation and Acid-Base Properties of Metal-Oxide Catalysts M.Ai . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1199 Reactions of Multifunctional Organic Compounds - Hydrogenation of Acroiein on Modified Pt-Catalysts 12 1 1 T.B. L. W. Marinelli J . H. Vleeming and V. Ponec . . . . . . . . . . . . . . . . . . . . . Acetonitrile Synthesis from CO, H and NH3 over Fe/C and K,Fe/C M. V. Baa'ani, L. M. Eshelman and%. N. Delgass . . . . . . . . . . . . . . . . . . . . . . 1223 Reductive Amination of Diethylene Glycol to Morpholine on Supported Nickel Catalysts - Its Activity, Selectivity, Stability and Possibility of Reactivation K. Jiratova, 0. Solcova, H. Snajahfova, L. Moravkova and H. Zahradnikova . . . . . 12 35 An Improved Asymmetric Oxidation of Sulfides to Sulfoxides by Titanium Pillared Montmorillonite - The First Example in Heterogeneous Catalysis B. M. Choudary, S. Shobha Rani and Y. V. Subba Rao . . . . . . . . . . . . . . . . . . . 1247
X Hydrogenation of CO, over Copper, Silver and Gold/Zrconia Catalysts: Comparative Study of Catalyst Properties and Reaction Pathways A. Baiker, M. Kilo, M.Maciejewski, S. Menzi and A. Wokaun . . . . . . . . . . . . . . . 1 257 Shape-Selective Reactions for Methylamine Synthesis from Methanol and Ammonia K. Segawa and H. Tachibana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1273 Selective Catalytic Reduction of NO by Hydrocarbon in Oxidizing Atmosphere M.Iwamoto, N.MizunoandH. Yahiro. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285 The Making of Catalysts by Controlled Oxidative Degradation of Planar Metal Complexes on Alumosilicate Su ports: Exhaust Gas Purification Catalysts for Power Plants, Automobiles and Small utfits F. Steinbach, A. Brunner, H. Miiller, A. Drechsler, S. Fromming, W. Strehlau and
8
U.Stan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1299
Oxidation of CO on Pd Particles on a-A1203: Reverse Spillover L. Kieken andM. Boudart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1 3 Microkinetic Analysis of the Selective Catalytic Reduction (SCR) of Nitric Oxide over Vanad ianitania -Based Catalysts J. A. Dumesic, N.-Y. Topsoe, T. Slabiak, P. Morsing, B. S. Clausen, E. Tornqvist and H. Topsoe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1325 An Infrared Study of an Active NO Decomposition Catalyst J. Valyon and W. K. Hall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1339 Acidity of W O /Ti0 Catalysts for Selective Catalytic Reduction (SCR) F. Hilbrig, H. fchmejz and H. Knozinger . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 35 1 Membrane Catalysis over Palladium and its Alloys J. N. Armor and T. S.Farris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1363 Fixed Bed Catalytic Reactors Based on Sintered Metals F. van Looij, A. Mulder, A. Q. M.Boon, J. F. Scheepens and J. W. Geus . . . . . . . . 1377 Mixed Spinels with Cerium--SO, Emission Control from Fluid Catalytic Cracking (FCC) Regenerator J. S. Yoo, A. A. Bhattacharyya, C. A. Radlowski and J. A. Karch . . . . . . . . . . . . . 139 1 Sintering, Poisoning and Regeneration of Pt/MgO J. Adamiec, J. A. Szymura andS. E. Wanke . . . . . . . . . . . . . . . . . . . . . . . . . 1405 Development of a Micro Hydroprocessing Test for Rapid Evaluation of Catalysts C. Sudhakar, L. T.Mtshali, P. 0. Fritz and M. S.Patel . . . . . . . . . . . . . . . . . . . 1 4 1 9 Alkali Promoter Synergism in Selective Oxidation M. M. Bhasin and C. D. Hendrir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1431 C-C Bond Formation via @-Additionwith Oxygen Retention Reversal in Oxygenate Synthesis K. Klier, R. G. Herman, P. B. Himelfarb, C.-W. Young, S. Hou and J. A. Marcos . . . 1441 Selective Gasoline Synthesis from CO, on a Highly Active Methanol Synthesis Catalyst and an H-Fe-Silicate of MFI Structure T. h i , T. Takeguchi, A. Kohama and K. Kitagawa . . . . . . . . . . . . . . . . . . . . . 1453 The Selective Synthesis of C2+ Oxygenates from Syngas Related Reactions over Ni- and Rh-Based Catalysts M. W. Balakm, S. S. C. Chuang, R. Krishnamurthy and G. Srinivas . . . . . . . . . . . 1467 Development of New Catalysts Formulations for Higher Alcohols Synthesis. Characterisation, Reactivity, Mechanistic Studies and Predictive Correlations A. Kiennemann, S. Boujana, C. Diagne and P. Chaumette . . . . . . . . . . . . . . . . . 1479 Characterization of MoS,-K+/Si02 Catalysts for Synthesis of Mixed Alcohols from Syngas H.-B. Zhang, Y.-Q. Yang, H. P. Huang, G. D. Lin and K. R. Tsai . . . . . . . . . . . . . 1493 Catalytic Activity of Reduced Cu,Zn(l~,10 and CuO/Cu,Zn(l-,)O in CO2/H, Reactions D. Stirling, F. S. Stone and M.S. Spencer . . . . . . . . . . . . . . . . . . . . . . . . . . 1 507
XI
Reactivities of Surface Intermediates on an Sm203 Catalyst Studied by in situ Infrared Spectroscopy Y. Sakat4 M. Yoshino, T. Fukudu, H. Yamaguchi, H. Imamura and S. Tsuchiya . . . . 15 19 In situ Investigation of the Water-Gas Shift Reaction over Magnetite by Mossbauer Spectroscopy A. Andreev, I. Mitov, V. Iahkiev, T. Tomov and S. Asenov . . . . . . . . . . . . . . . . . 152 3 In situ FT-IR Study of 02,CO, CO,, CH4 and C2H4 Adsorption or Reaction on the La 03/Mg0 Catalyst S. jhen, R. Hou,, W Ji, 2. Yan andX Ding. . . . . . . . . . . . . . . . . . . . . . . . . 1527 Study by in situ Laser Raman Spectroscopy of a VPO Catalyst in the Course of n-Butane Oxidation to Maleic Anhydride J. C. Volt4 R. Olier, M. Roullet, F. B. Abdelouahab and K Bere . . . . . . . . . . . . . 153 1 Investigation of Water-Gas Shift Reaction Under Dynamic Conditions P. Capek and K Klusacek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 535 Application of the Transient Response Method to the Study of the Catalytic System NO ~ O ~ + C O / C U O D. Panayotov and D. Mehandjiev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 539 Characterization and Activity of Vanadium Oxide Catalysts in Selective Catalytic Reduction of Nitric Oxide U.S. Ozkan, Y. Cai, M. W. Kumthekar and L. Zhang . . . . . . . . . . . . . . . . . . . . 1543 Theoretical Study of CO Chemisorption on Rh and Pd Clusters A. Goursot, I. Papai and D. R. Salahub . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1547 New Dynamic Method Approach to the Roles of Reversible and Irreversible Adsorption in Heterogeneous Catalysis G. L y S. Chen andS. Peng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1551 Discrimination and Regulation of Multi-Reaction Pathways in Heterogeneous Catalysis M. Kobayashi, T. Kanno andM. Hakozaki . . . . . . . . . . . . . . . . . . . . . . . . . . 1 555 Adsorption Study by Transient Tracing Methods. Theory and Modeling P. Szedlacsek, A. Efstathiou, C.O. Bennett and S. L. Suib . . . . . . . . . . . . . . . . . 1 5 5 9 In situ Determination of Surface Carbon Species Formed on Rh/A1203 during CO/H2 Reaction by Using Various Transient and Isotopic Methods A. M. Efstathiou, T. Chafik, D. Bianchi and C. 0.Bennett. . . . . . . . . . . . . . . . . 1563 The Calculation of Surface Orbital Energies for Specific Types of Active Sites on Dispersed Metal Catalysts R. L. Augustine, K. M.Lahanas andF. Cole. . . . . . . . . . . . . . . . . . . . . . . . . 1567 Oxidation and Removal of Chlorinated Hydrocarbons J. M. Berry, H. G.Stenger, Jr., G. E. Buzan a n d K H u . . . . . . . . . . . . . . . . . . . 1571 Structure and Reactivity of Carbidic Intermediates for the Methanation Reaction on Ni(100), Ni(ll1) and Ni(ll0) Surfaces H. Hirano and K Tanaka . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 575 C1 Adsorption on Ag(l11) and its Promoter Action D. Wang, K Wu, Y. Cao,X. WeiandX. G u o . . . . . . . . . . . . . . . . . . . . . . . . . 1579 The NOtH Reaction on Pt(100): Steady State and Oscillatory Kinetics M. Slinko, Fink, T. Loher, H. H. Madden, S. J. Lombardo, R. Imbihl and G. Ertl . . 1583 HREELSRDS Identification of Intermediates in the Low-Temperature H2+O, NO+H ,NH3t02 Reactions on Pt(ll1) Surface V. V. &orodetskii, M. Yu.Smirnov andA. R. Cholach . . . . . . . . . . . . . . . . . . . 1587 Surface Science Studies on the Mechanism for H-D Exchange of Methane over Pt( 111) Surfaces Under Vacuum F.Zaera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1591 Catalysis at Experimentally Designed Surfaces: nButane Hydrogenolysis at SnlGroup VIII Surface Alloys A. D. Logan andM. T. Pafett. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1595
8
XI I Surface Science and Kinetic Studies on Model Cu/Rh(100) Catalysts J. Szanyi and D. W. Goodman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 599 Angular Distribution of Desorbing Reaction Products and Dynamics of Some Catalytic Processes on the Surfaces of Pt and Ir M. U. Kisliuk, V. V. Savkin, T. N. Bakuleva, A. G. Vlasenko, V. V. Migulin, I. I. Tretiakov and A. V. Sklyarov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1603 Ship-in-Bottle Synthesis of NaY Zeolite-Included Pt, and Pt,, Carbonyl Clusters: Structures and Catalysis in CO+NO Reaction G.-J. Li, T. Fujimoto, A. Fukuoka andM. Ichikawa . . . . . . . . . . . . . . . . . . . . . 1607 The Preparation and Characterization of High-Silica Y Zeolite Prepared by Combined Chemical and Hydrothermal Dealumination X. Liu, Z. Pei, L. She, X-W.Li, J. Shao, S.Lin, R. Tang and X. Ma0 . . . . . . . . . . . 161 1 Active Sites of Novel Iron Supported Y-Type Zeolite R. Iwamoto, I. Nakamura and A. Iino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 61 5 A Novel Ap lication of XRD Technique for the Characterization of Secondary Pore Structure in Rodified Y-Zeolites S.D. Phatak, R. P. Mehrotra, S.M. Dhir and T. S.R. Prasada Rao . . . . . . . . . . . 161 9 IR-Spectroscopic Evidence for Acetonitrile Interaction with Carbenium Ions in Zeolites D. S.Bystrov, A. A. Tsyganenko and H. Forster . . . . . . . . . . . . . . . . . . . . . . . 1623 Acid-Base Properties of Zeolites: An XPS Approach Using Pyridine and Pyrrole Probe Molecule R. B. Borade, M. Huang, A. Adnot, A. Sayari and S.Kaliaguine . . . . . . . . . . . . . 1625 Sulfate as Promotor of Acidity of High Microporous and Thermostable Titanium Pillared Montmorillonite F. Admaiai, A. Bernier and P. Grange . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1629 Platinum Cluster Supported on Zeolite A by Ion Exchange of Pt(NH3)42+ R. Ryoo and S.J. Cho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1633 Preparation of Thermalstable Pillared Clays S. Mendioroz, F. Gonzalez, C. Pesquera, I. Benito, C. Blanco and G. Poncelet . . , . 1637 In situ X-Ray Analysis of CO- and CH30H-Induced Growth of Pd Particles Encaged in Zeolite Y W.Vogel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1641 Metal-Support Interactions on Pd-Containing Zeolite Catalysts M. F. Savchits, Eh. Ya. Ustilovskaya, V. Z. Veshtort, L. A. Agabekova and Yu. G. Egiazarov. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1645 Transfer of Metal Ions between Metal Oxides and Zeolites. Preparation of Highly Active Cu-Zeolite Based Catalysts for Reduction of NO, at Low Temperature B. Wichterlova,Z. Sobalik, M. Petras, I. Jirka and V. Bosacek . . . . . . . . . . . . . . 1649 Microkinetic Analysis of Isobutane Reactions Catalyzed by Y Zeolite J. E. Rekoske, R. J. Madon, L. M. Aparicio and J. A. Dumesic . . . . . . . . . . . . . . 1653 Peculiarities of Ethylene Conversion on Zeolites and Phosphoric Acid A. G. Anshits, S.N. Vereshchaginand N. N. Shishkina . . . . . . . . . . . . . . . . . . . 1661 Benzene AIkylation in Vapour-Phase with Ethene on a Zeolite Catalyst G. Maria, G. Pop, G. Musca and R. Boeru . . . . . . . . . . . . . . . . . . . . . . . . . . 1665 On the Nature of Zeolite Catalyst Effect on the Selectivity of Toluene Nitration by Acyl Nitrates S.M. Nagy, K A. Yarovoy, L. A. Vostrikova,X G. Ione and V. G. Shubin . . . . . . . 1 669 Interactions in Monoalkylbenzenes Disproportionation Among Zeolite Characteristics and Reaction Mechanisms I. Wang, T.-C. TsaiandC.-L.Ay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1673 New Support Materials for Rhodium Catalysts: Characterization of Rh/AIPO4-31 and Rh/MnAPO-3 1 A. Trunschke, H. Zubowa, B. Parlitz, R. Fricke and H. Miessner . . . . . . . . . . . . . 1677
Xlll
Transformation of Thiols and Organic Sulfides over Zeolites M. Ziolek and P. Decyk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Para-Selectivity of ZSM-5 Type Metallosilicates for Alkylation of Toluene with Methanol S. Namba, H. Ohta, J.-H. Kim and T. Yashima. . . . . . . . . . . . . . . . . . . . . . . . Hydroxylation of Toluene with Hydrogen Peroxide on HY Zeolites with Various Si/AI Ratios i? Yashima, Y. Kobayashi, T. Komatsu and S.Namba . . . . . . . . . . . . . . . . . . . Toluene Alkylation over Aluminophosphate-Based Molecular Sieves S. H. Oh and W. Y. Lee. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Cracking Reaction Path in 1-Hexene Isomerization on SAPO-11 and Pd/SAPO-1 1 S.-Y. Lim and S.J. Choung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Epoxidation of Alkenes Catalyzed by Decatungstate as Pillars in Layered Double Hydroxides T. Tatsumi, H. Tajima, K Yamamoto and H. Tominaga . . . . . . . . . . . . . . . . . . Conversion of Ethane into Aromatic Compounds on ZSM-5 Zeolites Modified by Zinc F. Roessner, A. Hagen, U. Mroczek, H. G. Karge and K-H. Steinberg . . . . . . . . . In Situ FTIR and GC Kinetic Studies: Complementary Methods in the Mechanistic Study of Butanol Dehydration on Zeolite H-ZSM-5 M. A. Makarova, E. A. Paubhtis, J. M. Thomas, C.Williams and K I. Zamaraev . . . Catalytic Properties of Ferrisilicate Analogs of Some Medium Pore Zeolites in C, and Cs Aromatic Hydrocarbon Reations A. Ra], K R. Reddy, J. S. ReddyandR. Kumar. . . . . . . . . . . . . . . . . . . . . . . The Influence of the Catalyst Preparation on the Catalytic Properties of ZeoliteSupported Catalysts Y. W.ChenandW.J. W a n g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactions of Acetone, Methanol and Ammonia on Z S M J Zeolites J. Novakova, L. Bosacek, Z. Dolejsek and L. Kubelkova . . . . . . . . . . . . . . . . . . Regioselective Hydrogenation Using Platinum-Support Zeolite Modified by CVD-Method H. Kuno, M. Shibagaki, K Takahashi, I. Honda and H. Matsushita . . . . . . . . . . . Nickel, Cobalt and Zinc Substituted Synthetic Mica-Montmorillonite: Synthesis, Characterization and Propene Oligomerization Activity J. C. Q. Fletcher, A. P. Vogel and C. T. O’Connor . . . . . . . . . . . . . . . . . . . . . Isobutane/l-Butene Alkylation on Pentasil-Type Zeolite Catalysts J. Weitkamp and P. A. Jacobs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Active Sites in HZSM-5 and S A P 0 Molecular Sieves for Alcohol Conversion C. Bezouhanova, Yu. Kalvachev and H. Lechert . . . . . . . . . . . . . . . . . . . . . . . The Synthesis of Cobalt Supported Catalysts by Electroless Plating Techniques N. J. Coville, S. E. Colley, J. A. Beetge and S.W. Orchard. . . . . . . . . . . . . . . . A New Precursor for the Preparation of Novel Copper Chromite Catalysts R.Prasad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pd Fe : Pd Surface Segregation and Catalytic Activity J. zrtolini, Y. Debauge, P. Delichere, J. Massardier, J.L. Rousset, P. Ruu and B. Tardy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Study of Spreading of Vanadia on Titania Polymorphs Using Mechanical Mixtures M. Sanati, A. Andersson and L. R. Wallenberg . . . . . . . . . . . . . . . . . . . . . . . . Structure and Activity of Copper Catalysts Prepared from Amorphous Cu-Zr and Cu-Ti Alloy Precursors: A Comparative Study A. Molnar, T. Katona, Cs. Kopasz and Z. Hegediis . . . . . . . . . . . . . . . . . . . . . Preparation of W/AI 0 by Chemisorption of WOCI, to Surface Saturation M. Lindblad and L. Zindfors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 681 1685 1689 1693 1 697 1703 1 707 171 1 1715 1719 1723 1727 1731 1735 1739 1743 1747
b.
3.
1751 1755
1 759 1763
XIV
Highly Active V 0 Thin Films Prepared by Chemical Vapor Deposition on Silica for Oxidative Deiydrogenation of Alcohols T. Okuhara, K Inumaru, M. Misono and N. Matsubayashi . . . . . . . . . . . . . . . . 1767 Influence of the Chemical Composition on the Preparation of Cu-Co-Zn-Al Mixed Oxide Catalysts with a High Metal Dispersion A. J. Marchi, J. I. Di Cosimo and C. R. Apesteguia . . . . . . . . . . . . . . . . . . . . . 1771 Sol-Gel Derived Heterogeneous Catalysts T. Walton and P. A. Sermon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,1775 Forming of Pyrogenic SiO, and TiO, and their Applications as New Types of High Surface Area Catalyst Supports M. Bankmann, B. Despreyroux, H. fiause, J. Ohmer and R. Brand. . . . . . . . . . . 1 78 1 New Preparation Method of Small Particles in Ni/SiO, Catalysts Involving Chelate Ligands Z . X . Cheng, C. LouisandM. Che. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178s Stepwise Monitoring of Mixed Oxide Catalyst Preparations by XAS Spectroscopy 0. Clause and M. Che . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 789 Improving SO, Resistence of Base Metal Perovskite Type Oxidation Catalyst W. Li, H. DaiandY. Lieu. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1793 Carbide Catalysts: Laser Pyrolysis Synthesis and Catalytic Activity J . M. Stencel, P. C. Ekluncl, X.-X. Bi, B. H. Davis, G.T. Hager and FJ. Derbyshire . . 1797 Structural Support Effects in the Systematic Preparation of Pd/Si02 Catalyst for Methanol Synthesis by Ion Exchange Techniques A. L. Bonivardi, M. A. Baltanas and D. L. Chiavassa . . . . . . . . . . . . . . . . . . . . 180 1 The Advantageous Use of Microwave Radiation in the Preparation of Supported Nickel Catalysts G. Boncl, R. B. Moyes, S. D. Pollington andD. A. M a n . . . . . . . . . . . . . . . . . . 1805 New Preparation Methods for Active Superfine Catalysts by Spray Reaction T. Uematsu, S. Shimazu, T. Kameyama and K Fukuda . . . . . . . . . . . . . . . . . . . 1809 Preparation of Supported Cu-Ni Bimetallic Catalysts by Alkoxide Method with High Activities for Hydrogenation or Dehydrogenation T. Sodesawa, S. Sat0 and F. Nozaki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i 8 13 Dy-Cu Alloy Films: Catalytic Activity, Composition, Structure K. N. Zhavoronkova and 0.A. Boeva . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i 817 Adsorption and Catalytic Properties of Highly Disperse Silver Catalysts N.E. Bogdanchikova, D. A. Bulushev, Yu. D. Pankratiev and A. V. Khasin . . . . . . . 1823 New Insight into the Changing CatalystPolymer Morphology during Olefin Polymerization: The Application of Tomography W. C. Conner, M. Ferrero, S. Webb, R. Sommer, M. Chiovetta, K Jones and 1827 P.Spanne.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytical and High Resolution Electron Microscopy Studies of the System Pt/ZnAl 0, with Several Platinum Contents G. Aguikr-Rio, M. A. Valenzuela,D. R. Acosta and I. Schifier . . . . . . . . . . . . . 1831 The Application of Dilatometry for Investigation of Heterogeneous Catalysts L. A. Rudnitsky andA. M. Aleheev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 a35 Artificial Control of Catalytic Activity of Pd by a Shear Horizontal Surface Acoustic Wave Y. Inoue, H. Kawaguchi, M. Matsukawa and K Sat0 . . . . . . . . . . . . . . . . . . . . 1839 Characterization of Silica-Supported Palladium-Cobalt Alloys W. Juszczyk, Z. Karpinski, Z. Pa64 J. Pielaszek . . . . . . . . . . . . . . . . . . . . . . . 1843 Temperature-Programmed Reduction in Catalysis: A Critical Evaluation of the Method G. Fierro, M. Lo Jacono, M. Inversi, G. Moretti, P. Porta and R. Lavecchia . . . . . . 1847
xv Scanning Tunneling Microscopy Study of Pd/Graphite: Microstructure and Reactivity K L . YeungandE. E. Wolf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1851 IR Study of Adsorption and Deuteration of d6-Acetone on Pt/ZnO Catalysts: Effects of the Sample Pretreatments F. Boccuzzi, G. Ghiotti and A. Chiorino . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 855 Surface Energetic Characterization of Supported Metal Catalysts by GadSolid Titration Microcalorimetry J. M. Guil, A. P. Masia, A. R. Paniego andJ. M. T. Menayo . . . . . . . . . . . . . . . . 1859 Study of the Effects of Annealing on the Morphology of Platinum Cluster Size on Highly Oriented Pyrolytic Graphite by Scanning Tunneling Microscopy S.Lee,H.PermanaandKY.S.Ng.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1863 Distribution of Mo Oxidation States in Reduced Mo/A12O3 Catalysts. Correlation with Catalytic Activity J. Yasumaru, M. Yam- M. Houalla and D. M. Hercules . . . . . . . . . . . . . . . . I 867 Effect of Catalyst Preparation on the Performance of Supported Ru-Cu Bimetallic Systems R. Maggiore, C. Crisafulli, S. Scire and S.Galvagno . . . . . . . . . . . . . . . . . . . . 1871 Revisiting Diffuse Reflectance Spectroscopy for the Characterization of Metal and Semiconducting Oxide Catalysts A. Rakai, A. Bensalem, J. C. Muller, D. Tessier and F. Bozon-Verduraz . . . . . . . . . 1 875 The Thermal Stability of the Adsorbed/Lattice Oxygen Species on the Oxide Catalysts Surfaces M. Caldararu and N. I. Ionescu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1879 Chemical Anchoring of Noble Metal Amine Precursors to Silica: An in situ UV Diffuse Reflectance Study W. Zou and R. D. Gonzalez . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1883 Reaction Sites on the A1203 Support of Pd/Al 0 J. L. Falconer, B. Chen, S. A. Larson and E. C! dsiao . . . . . . . . . . . . . . . . . . . 1887 The External Magnetic Field Effect on the H2-02 Reaction on the S n 0 2 Surface H. Ohnishi, H. Sasaki and M. Ippommatsu . . . . . . . . . . . . . . . . . . . . . . . . . . 1 89 1 Characterization of Different Surface Mo Species in Mo/AI2O3 Catalysts by Time Differential Perturbed Angular Correlation S. Guida, Y. Tingyun, Y. Fushan, R. Liguo and N. Xinbo . . . . . . . . . . . . . . . . . . 1895
PART C New Catalytic Phases for the HDN and MHC Reactions in CoMoP-Alumina Catalysts A. Morales, R. PradaSilvy and V. Leon . . . . . . . . . . . . . . . . . . . . . . . . . . . Multi-Purpose Ni-Mo/Al O3 Catalysts for Gas Oil Hydrotreating A. F. Somogyv&ri,M. C. hballa and P. S.Herrera . . . . . . . . . . . . . . . . . . . . . Characterization of Phosphorus Containing Ni-W/A1203 Catalysts P. Atanasova and T. Halachev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concerted Mechanism of Thiophene Hydrogenolysis by Sulfide HDS Catalysts A. N. Startsev . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Highly Active Ni-W/A1203 Catalyst for Upgrading Unconventional Feedstocks H. Shim& T. Kameokq H. Yanase, M. Watanabe, A. Kinoshira, T. Sam, Y . Yoshimura, N.Masrubayashi and A. Nishijima . . . . . . . . . . . . . . . . . . . . . . Adsorption and Activation of Thiophene on MoS2, Co$& and RuS2 C. RongandX Qin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correlation of HDS Activity with Heat of Adsorption over Carbon Supported CoMo Catalysts S.-K Ihm, X-H.MoonandC.-D. Ihm. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 899
1 903 1 907
19 1 1 1 91 5
1919 1923
XVI Effect of Fluoride on the Surface Structure of W03/A$O3 Hydrotreating Catalysts R. L. Cordero, J. R. Solis, J. V. G.Ramos, A. B. Patricro and A. L. Aguab . . . . . . . 1 927 Hydrodesulfurization Activity of Zeolite Supported Nickel- and Cobalt Sulfide Catalysts W. J. J. Welters, T. I. Koranyi, V. H. J. de Beer and R. A. van Santen . . . . . . . . . . . 1 93 1 Enhanced HDS Activity via Multiple Impregnation of Sulfided Mo/A1203 Catalysts C.-S. Kim, F. E. Massoth, C. Geantel and M. Breysse . . . . . . . . . . . . . . . . . . . 193s Surface Structure of Molybdenum Nitride and its Activity for Hydrodesulfurization and Hydrodenitrogenation M. Nagai, T. Miyao, T. Tsuboi and T.Kusagaya . . . . . . . . . . . . . . . . . . . . . . . 1939 Hydrocracking Gas Oils from Synthetic Crude with Mixed Pillared Clay-Alumina Supported Catalysts J. Monnier, J.-P. Charland J. R. Brown and M. F. Wilson . . . . . . . . . . . . . . . . 1943 Transformations of Thiophene, Tetrahydrothiophene and Butanethiol over &-Black, &/A1 O,, Mo/AI 0, and Co-Mo/Al 0, during Temperature-Programmed Reaction V. V. iozanov, Y. %zao and 0. V. Kry?ov . . . . . . . . . . . . . . . . . . . . . . . . . . . 1947 Partial Oxidation Reaction of Methane with Oxygen or Carbon Dioxide by Transition Metal Catalysts Supported on Ultrafine Single-Crystal Magnesium Oxide 0. Takayasu, I. Matsuura, K. Nitta and Y. Yoshida . . . . . . . . . . . . . . . . . . . . . 1951 Surface Oxygen Specics and their Reactivities in the Oxidation of CH4, C2H6 and C2H4 over Cerium Oxide at Mild Temperatures C. Li, Q.Xin,X. GuoandT. Onishi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1955 Oxidative Catalytic Conversions of Tetrahydrofuran Derivatives R. Skolmeistere, L. Leitis, M. Fleisher and M. Shymanska . . . . . . . . . . . . . . . . . 1 959 Role of Mo and Sb in Oxide Catalysts for Selective Oxidation of Propylene 8. Zhou, X. Guo and K. T. Chuang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1963 Partial Oxidation of Methane at Low Pressure over Silica Gel and Silica-Supported Sn, Zr and Ge Oxides T.Ono, K I h t a and Y. Shigemura . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1967 The Influence of the Support on the Performance of Heterogeneous Catalysts for the Wacker Oxidation of Alkenes A. W. Stobbe-Kreemers, J. J. F. Scholten, M. Soede and J. W. Veenman . . . . . . . . . 1971 Optimization of NiO/MoOPeO, Catalytic System for Direct Oxidation of Propene to Acrylic Acid C. Mazzocchia, R. Anouchinsky, A. Kaddouri and E. Tempesti . . . . . . . . . . . . . . 197s Role of Amorphous Phasc and its Modification in V-P-0 Catalysts for Maleic Anhydride Synthesis from Butane N. Yamazoe, H. Morishige, J. Tamaki and N. Miura . . . . . . . . . . . . . . . . . . . . 1979 Selective Oxidation and Ammoxidation of Propane to form Acrolein and Acrylonitrile Y. Moro-oka, N. Miura, N. Fujikawa, Y.-C. Kim and W. Veda . . . . . . . . . . . . . . 1983 Oxidation of Propenc on Alkaline Metal-Doped MoOfliO, Catalysts: A FT-IR Study C. Martin, I. Martin, C. Mendizabal and V. Rives . . . . . . . . . . . . . . . . . . . . . . 1987 Oxidation of Methanol to Formaldehyde over Antimony-Molybdenum Oxide Catalyst 1991 R. S. Mann andR. A. Diaz-Real . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Comparison between Epoxidation and Degradation of Ethylene and Propylene over Silver C. Henriques, M. F. Portela, C. Mazzocchia and E. Guglielminotti . . . . . . . . . . . 1995 Liquid-Phase Oxidation of Benzene with Molecular Oxygen Catalyzed by Cu-Zeol i tes T. Ohtani, S. Nishiyama, S. Tsuruya and M. Masai . . . . . . . . . . . . . . . . . . . . . 1999
XVI I Formation of Formaldehyde from Methanol over Supported Titanium Oxide H. Imai, Y. Murakami and H. Irikawa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2003 A Study on P-Mo-As Heteropoly Compounds as Catalyst for Selective Oxidation of Methacrolein B. Zhong, W. Zheng, R. He, G. Huang and X.Li . . . . . . . . . . . . . . . . . . . . . . . 2007 Silica as an Ammoximation Catalyst for the Production of Cyclohexanone Oxime D. P. Dreoni, D. Pinelli) F. TriJro, Z. Tvaruzkova, K. Habersberger and P. Jiru . . . 201 1 Kinetics of the Redox Reactions of the 02:Propylene:y-Bismuth Molybdate System: A TAP Reactor Study D. R. Coulson, P. L. Mills, K. Kourtakis, P. W. J. G. Wijnen, J. J. Lerou and L. E. Manzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 015 Partial Oxidation of Propene in the Presence of Steam Y. A. Saleh-Alhamerl, R. R. Hudgins and P. L. Silveston . . . . . . . . . . . . . . . . . . 2 o 1 g The Kinetics of Activation of Industrial and Model Iron Catalysts for Ammonia Synthesis in Dried and Wet Atmosphere A. Baranski, A. Kotarba, J . M. Lagan, A. Pattek-Janczyk, E. Pyrczak andA. Reizer . . 2 0 2 3 The Modifying Role of Ru, Mo and Rare-brth Elements (REE) in the Creation of New Generation of Catalysts Based on Iron Hydroxides R. V. Chesnokova, L. M. Dmytrienko) I. G. Broakkuja, A. M. Alekseev, A. A. Vasilevitch, N. A. Dubyaga andl. 1. Bondartsova . . . . . . . . . . . . . . . . . . .2 0 2 7 The Role of Various Modes of Adsorbed CO in Synthesis Gas Conversion on Lanthanide Ions Promoted by Supported Pd Catalysts Yu.N. Nogin, N. V. Chesnokov and V. I. Kovalchuk . . . . . . . . . . . . . . . . . . . . . 2 o 3 1 Influence of the Interaction of Support with Active Species on Sintering and Stability of Alumina Supported Oxide Catalysts 0. A. Kirichenko, M. P. Vorob’eva and V. A. Ushakov . . . . . . . . . . . . . . . . . . .2035 Structural Transformation and Catalytic Behaviors of Rhodium Ternary Oxides during Calcination and Reduction Treatments K Kunimori, H. Oyanagi, H. Shindo, T.Ishigaki and T. Uchijima . . . . . . . . . . . . 2 0 3 9 Marked Support Effect of Dispersed ZrO, Catalysts in Propene-Deuterium Addition and Exchange Reaction S. Naito, M. Tanimoto, M. Soma and Y. Udagawa . . . . . . . . . . . . . . . . . . . . . . 2043 Acidity Generation of Binary Metal Oxide Catalysts A. Gervasini, G. Bellussi, J. Fenyvesi and A. Auroux . . . . . . . . . . . . . . . . . . . . 2047 Effects of Potassium Addition on the Performance of a Nickelmagnesia Catalyst for Steam Reforming of Methane S. Kitabayashi, Y. Ogino, T. Yamazaki and S. Ozawa . . . . . . . . . . . . . . . . . . . . 205 1 Weakening of Hydrogen Poisoning by Sm203 Promoter in Activation of Dinitrogen on Ru/AI2O3 Catalyst Y. Kaabwaki, S. Murata and K. -I. Aika . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2o5 5 XANES, EXAFS and Reaction Studies of Some Well-Dispersed Ferric Oxide Catalysts W. J i Y. Kuo,S.Shen, S. LiandH. Wang. . . . . . . . . . . . . . . . . . . . . . . . . . 2059 C02 Derivatives Adsorbed on Promoted Surfaces J. A. K. Paul, Y. Shao and 0.Axelsson . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2063 The Effect of Metallic Promoters on Supported Cobalt Catalysts A. Hofj E. A. Blekkan, A. Holmen and D. Schanke . . . . . . . . . . . . . . . . . . . . . 2067 The Structure-Activity Relationship of Re207 Metathesis Catalysts Supported on Phosphated Alumina and Silica-Alumina R. SpronkandJ. C. M o l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2071 Modification of the Site Structure of MoS,/y-Al,O, Catalysts by Addition of P, F and Alkaline Elements 0. Poulet, R. Hubaut, S.Kasztelan andJ. Grimhlot . . . . . . . . . . . . . . . . . . . . . 207s
XVI II Cerium Oxides Supported on Alumina-Crystallite Structures and Catalytic Activity M. Haneda, T. Miki, T. Mizushima, N. Kakuta and A. Ueno . . . . . . . . . . . . . . . . 2 o 7 9 The Role of Zirconium Dioxide in the Activation of Water and as the Catalytic Site for Low-Temperature Steam Reforming over Rh/Zr02 A. Igarashi, T. Ohtaka, T.Honnma and C.Fukuhara . . . . . . . . . . . . . . . . . . . . 2083 Surface Structure and Reactivity of Magnesia-Supported Nickel Catalysts: A Model System A. Parmaliana, F. Arena, F.Frusteri, N. Mondello and N. Giordano . . . . . . . . . . 2 0 a7 Effect of Tin and Iron Deposition on the Catalytic Properties of Platinum Supported on Graphite E. Lamy-Pitara, L. El Ouauani-Benhima and J. Barbier . . . . . . . . . . . . . . . . . 2 o 9 1 Properties of a Laterite Iron Mineral: Characterization, Catalytic Behavior and Promoter Effect M. R. Goldwasser, M. L. Cubeiro, M. J. Perez Zurita and C. Franco . . . . . . . . . . 2095 Platinum Catalysts Supported on High Surface Area Molybdenum or Tungsten Trioxides for Hydrogenation Reactions C. Hoang-Van, 0. Zegaoui and Y. Arnaud . . . . . . . . . . . . . . . . . . . . . . . . . . 2099 Pt-C Interaction in Catalyst Supported on a Carbon Black Subjected to Different Heat Treatments F. Coloma, C.Prado-Burguete and F. Rodriguez-Reinoso . . . . . . . . . . . . . . . . . 2 1 0 3 Mechanism of the Effect of Additives on Catalytic Properties of Palladium L. N. Edygenova, N. V. Anisimova, A. V. Korolev and D. M. Doroshkevich . . . . . . . 2 1 0 7 Pt-Ge/Al 0, Catalysts: Influence of the Thermal Treatments and the Redox Cycles J. A. M. Zorrea, S. R. ak Miguel, G. T. Baronetti, A. A. Castro and 0.A. Scelza . . . . 2 1 1 1 Effect of the Support on the Copper State in Copper-Titanium Oxidation Catalysts T. S. Perkevich, L. Ya. Mostovaya, Yu. G. Egiazarov and N. A. Kovalenko . . . . . . . 2 1 15 Effect of Addition of Pd, Co and Pd-Co on (20,. Syngas Conversion and Acetaldehyde Reaction H. Idriss, C.Diagne, J. P. Hindermann, A. Kinnemann and M. A. Barteau . . . . . . . 2 1 1 9 The Role of Electric Field Acting on the Characteristic of Adsorption of Solid Surface, the Oxygen Adsorption on Tin Oxide Film in an External Electric Field R.Zhou . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 2 3 Catalytic Behaviour of LiFeO Anode for Solid Oxide Fuel Cells R. T.Baker, I. S. Metcave, P. Middleton, P. Petrolekas and B. C. H. Steele . . . . . 2 12 7 Electrochemical Modification of the Activity and Selectivity of Metal Catalysts M. Stoukides, D. Eng, P.-H. Chiang and H. Alqahtany . . . . . . . . . . . . . . . . . . . 2 1 3 1 The Selective Hydrogenation of Acetylene by the Electrochemically Pumped Hydrogen over Cu in the Presence of Abundant Ethylene K Otsuka, T. Yagi and M. Hatano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 35 Solid Electrolytes for in situ Promotion of Catalyst Surfaces: The NEMCA Effect C. G. Vayenas, S. Bebelis, I. V. Yentekakis, P. Tsiakaras, H. Karasali and Ch. Karavasilis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 39 Theory and Experiment of Photo-Activation of Catalytic Sites and Active SiteSupport Interactions 0. Novaro and J. Garcia-Prieto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 43 Photocatalytic and Physicochemical Studies on Metallised Titania Systems B. Viswanathan, U.D. Mary and R. P. Viswanath . . . . . . . . . . . . . . . . . . . . . . 2 1 4 7 Heterogeneous Photocatalysis: Mechanistic Considerations of Photocatalytic Reductions and Photocatalytic Oxidations on Semiconductor Oxide Surfaces R.I. Bickley, L. Palmisano, M. Schiavello and A. Sclafani . . . . . . . . . . . . . . . . . 2 1 5 1 De-NO,-ing Photocatalysis - Excited States of Cop er Ions Anchored onto Zeolite and their Role in Photocatalytic Decomposition of N at 275 K M. Anpo, T. Nomura, Y. Shioya, M. Che, D. Murphy and E. Giamello . . . . . . . . . . 2 1 5 5
b.
8
XIX A Novel Series of Photocatalysts with an Ion-Exchangeable Layered Structure of Niobate K Domen, J. Yoshimura, T. Sekine, J. Konab, A. Tanaka, R Maruya and T. Onishi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalysis by Coordinatively Unsaturated Rhodium Complex Stabilized on Porous Glass for Alkane Dehydrogenation Y. Waah, C. Nakano, Y. Yamauchi and A. Morikawa . . . . . . . . . . . . . . . . . . . . Heterogeneous Photocatalysis as a Method of Water Decontamination: Degradation of 2-, 3- and 4-Chlorobenzoic Acids over Illuminated Ti02 at Room Temperature J.-C. D'Oliveira, W. D. W. Jayatilake, K. Tennakone, J.-M. Herrmann and P. Pichar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Investigation of the Electron Transfer Mechanism between Methyl Viologen Radicals and Protons Via a Noble Metal Catalyst R. Bauer and H. A. F. Werner. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal Fibre - A Novel Catalyst Material for Selective Dehydrogenation of Alcohols B. Zhang, L. Lu, Y. Xiao, D. Jin, J. Ai and Z. Zhou . . . . . . . . . . . . . . . . . . . . . Use of Electron Spectroscopy Methods in the Study of the Structure of Surface Layers of Hydride Catalysts R. Kh. Ibrasheva, R. G. Baisheva, Z. Kaizi, T. A. Solomina, G. I. Leonova and K. A. Zhubanov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shape-Selectivity of Alkali Metals Graphite Intercalation Compounds for Catalytic Isomerization, Hydrogenation and Alkylation of Aliphatic and Aromatic Hydrocarbons S.Tsuchiya, S.Sakai, M. Kikugawq T. Mitsuno and Y. Sakata . . . . . . . . . . . . . . Activation and Reactivity of Titanium Oxynitrides in Ammonia Decomposition C. H. Shin, G. Bugli and G. Djega-Mariadassou . . . . . . . . . . . . . . . . . . . . . . A Study on the Preparation and Characterization of a NiP Catalysts J.Shen,Z. Li, Q.Zhang, Y. Chen, Q. BaoandZ. L i . . . . . . . . . . . . . . . . . . . . . Investigations of Hydrodenitrogenation of Quinoline over Molybdenum Nitride K S.Lee, J. A. Reimer andA. T. Bell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . CO Oxidation, NO Decomposition and NO Reduction by CO on Superconducting and Related Cuprates I. Halasz, A. Brenner, M. Shelef a n d R Y. S.Ng . . . . . . . . . . . . . . . . . . . . . . . The Active Oxygen on the Li/La203 Catalyst Surface and its Catalytic Behavior in the Oxidative Coupling of Methane L. Wang, J. Wang, S.Yuan and Y. Wu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidative Coupling of Methane over SrO Promoted La20JCaO: A Comparative Study of the Kinetics and Mechanism Y.-D.Xy L. Yu, J.-S. HuangandZ.-Y. Lin. . . . . . . . . . . . . . . . . . . . . . . . . . Oxidative Coupling of Methane over Sol-Gel Magnesium Oxide Catalysts: Effect on Selectivity to Olefin Formation R. Gomez, T. Lopez, L. Herrera, A. A. Castro, 0. Scelza, G. Baronetti, E. Lazzari, A. Cuan, M. Campos, E. Poulain, A. Ramirez-Solis and 0.Novaro . . . . . . . . . . . Influence of the Ion Charge and Coordination State on Catalytic Properties of Barium Ferroniobate for Methane Oxidation D. Filkova, I. Miiov, L. Petrov, V. Bychkov, M. Sinev, Yu.Tulenin and P. Shiryaev . . Reaction Performances of Methane Oxidative Coupling Along Catalyst Bed with Sm-CaO Catalyst C. Tang, L. Lin, Z. Xu andJ. Zang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Pressure on the Methane Oxidative Dimerization Yu. P. Tulenin, A. A. Kadushin, V. A. Seleznev and A. F. Shestakov . . . . . . . . . . .
2159 2 163
2167 2173 2 177
2 1 a1
2 1 a5 2 i a9 2193 2197 2201 2205 2209
22 1 3 22 17 2221 2225
xx Methane Oxidative Coupling over Complex Metal Oxides Possessing K2NiF4 and Related Structure Q. Yan, Y. Jin, Y. Wang, Y. Chen and X. Fu . . . . . . . . . . . . . . . . . . . . . . . . . 2229 Oxidative Coupling of Methane over PbO/PbAI2O4 Catalysts S.-E. Park and J. -S.Chang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 33 Characterization of TiLaNa Catalysts in the Oxidative Coupling of Methane S. Rossini, S.-T. Branduo, 0.Forlani, L. Lietti, A. Santucci, D. Sanfilippo and P. Villa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2237 Catalytic Properties of Ca-Doped La203Catalysts for Coupling of Methane X.Yang, Y. Bi, K Zhen andY. Wu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2241 Synergy between High Temperature Stable Carbonates and Irreducible Oxides: Destruction of Non-Selective Surface Oxygen on Oxidative Coupling of Methane Catalysts J.-L. Dubois and C. J. Cameron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2245 Laser Stimulated oxidative Coupling of Methane to Ethene on LiC104/pb3(P04)2 2249 S-H. Zhong and H-Q. Ma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Temperature-Programmed Studies of Surface Oxygen Species in the Oxidative Coupling of Methane G. W. Keulks, N. Liao, W. An andD. Li . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2253 Oxidative Dimerization of Methane on Alkali Chloride Promoted Co304 M. Gratzel, D. Klissurski, J. Kiwi and K. R. Thampi . . . . . . . . . . . . . . . . . . . . 2257 Oxidative Coupling of CH4tCD4 Mixture over Manganese Oxide Catalysts Y. G. Borodko and L. M. lofle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2261 A Comparison of the behavior of Catalysts for Methane Coupling by Transient Analysis R. Spinicci andA. Tofanari . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2265 Oxidative Coupling of Methane to C Hydrocarbons over Doped Titania Catalysts D. Papageorgiou, D. Vamvouka and% E. Verykios . . . . . . . . . . . . . . . . . . . . 2269 The Importance of Carbon Dioxide in Oxidative Coupling of Methane A. Machocki. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2273 Kinetic Effects in Conversion of Propane, Isobutane and Propane-Isobutane Mixtures on K-Pt/y-A1203 Catalysts, Modified by Sn and In L. C. LOC,H. S.Thoung, N. A. Gaidui and S. L. Kiperman . . . . . . . . . . . . . . . . . 2 2 7 7 The Reaction of Reduction Catalyzed by Homogeneous and Immobilized Binuclear Rh(I1) Complexes with Rh-Rh Bond V. 2. Sharf and V. I. lsaeva . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2281 A Highly Active, Heterogeneous Hydroformy lation Catalyst: Rh(CO)(acac)L,L= =poly-TRIM Bound Phosphine J . Hjortkjaer, B. Heinrich, C. Anakrsson andA. Nikitidis . . . . . . . . . . . . . . . . . 2 2 8 5 Selective Hydrogenation of C, - Acetlylenes over an Ion-Exchanged Copper on Silica Catalyst J . T. Wehrli, D. J. Thomas, M. S. Wainwright, D. L. Trimm andN. W. Cant . . . . . . 2 2 8 9 Chiral Metal Transition Complexes in Zeolites: Enantioselective Hydrogenation of Dehydrophenylalanine Derivatives A. Corma, M. Iglesius, C. del Pino and F. Sanchez . . . . . . . . . . . . . . . . . . . . . 2293 Selective Vapor Phase Hydroformylation of Olefins over Cluster-Derived Cobalt Catalysts Promoted by Alkaline Earth Oxides K Takeuchi, T. Hanaoka, T. Matsuzaki, Y. Sugi, M. Reinikainen and M. Huuska . . . 2 2 9 7 Design of Platinum Based Metallic Catalysts for Selective Hydrogenation of Crotonaldehyde A. Jentys, C. G. Raab andJ. A. Lercher . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2301 1,3-Butadiene Hydrogenation in 1-Butene over Alumina Supported Pd-Ag Catalysts J. W. Hightower, B. Furlong, A. SarMny andL. Guczi . . . . . . . . . . . . . . . . . . . 2 3 0 5
XXI Oxidative Dehydrogenation of Propane in Presence of Rare Earth Vanadates J. Castiglioni, P. Po& andR. Kieffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2309 Reactions of Cyclopropane and 1-Butene over Reduced Molybdena-Alumina Catalysts I. Oliveros, C. Bolivar, P. Marcano, C. Scott, M J . Perez Zurita and J. Goldwasser . . 2 3 1 3 Effects of Mixtures of Modifiers on Optical Yield in Enantioselective Hydrogenation: a Test of the Template Model KE.Simons,P.A.Meheux,A.IbbotsonandP.B. Wells.. . . . . . . . . . . . . . . . 2317 Nib4 Type Bimetallic Reducing Systems Zs. Boahar, T. Mallrit and J. Petrb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 1 New Aspects on the Mechanism of Olefin Polymerization with Reduced Philipps Catalysts H. L. Krauss, H. A. Schmidt, B. Siebenhaar, P. Wolffand Q. Xing. . . . . . . . . . . . 232s The Role of Porosity in Ethylene Polymerization on Cr/SiOz Catalysts I. G. Dalla Lana, J. A. Szymura andP. A. Zielinski. . . . . . . . . . . . . . . . . . . . . 2329 Partial Hydrogenation of Alkynes and Dienes on Highly Selective Fe-Cu/Si02 Catalysts Y. Nitta, Y. Hiramatsu, Y. Okamoto and T. Imanaka . . . . . . . . . . . . . . . . . . . . 2333 Supported Dehydrogenation Catalysts Based on Iron Oxide D. E. Stobbe, F. R. van Buren, A. J. van Dillen and J. W. Geuss . . . . . . . . . . . . . 2 337 Sulfur Resistance of Nickel Catalysts Supported on K-Clinoptilolite Containing Iron in Ethylbenzene Hydrogenation A. Arcoya, X.L. Seoane and J. Soria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 1 Hetero eneous Ethylene Hydroformylation Catalyzed by Oxide-Supported [Rh 2(E0)30]2- Anion: Influence of the Nature of the Support C. dossi, A. Fusi L. Garlaschelli, R. Psaro and R. Ugo . . . . . . . . . . . . . . . . . . 2345 Isolated and Competitive Hydrogenation to Characterize Ni-B Catalysts G. Jannes, P. Kerckx, B. Lenoble, P. Vandenuegen, C. Verlinden and J.P. Puttemans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2349 The Role of Surface Fugacities and of Hydrogen Desorption Sites in Catalytic Reactions of Alkanes E. Iglesia, J. E. Baumgartner and G. D. Meitzner . . . . . . . . . . . . . . . . . . . . . . 2353 Heterogeneous Catalysis of a-Octene Hydroformylation. the Catalytic Behaviors of Non-Homodisperse EGG-Shell Catalysts W. Huang, L.-H. Yin andC.-Y. Wang. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2359 Hydroformylation of Ethylene over Silica-Supported Pt/Sn Catalysts P. Ramirez de la Piscina, J. L. G. Fierro, G. Muller, J. Sales and N. Homs . . . . . . . 2363 Kinetics of2,2,3,3-TetramethylbutaneHydrogenolysis over Rh/AIzO, Catalysts B. Coq, T. Tazi, R. Dutartre and F. Figueras . . . . . . . . . . . . . . . . . . . . . . . . . 2367 Surface Organometallic Chemistry on Metals: Role of a Surface Organometallic Fragment Sn(n-C4Hg)x on the Selective Hydrogenation of Citral with a Rh/SiO2 Catalyst B. Didillon, A. El Mansour, J. P. Candy, J. M. Basset, F. Le Peletier and J. P. Boitiaux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2371 Hydrogenolysis of Methylcyclobutane on Supported Pt Model Catalysts C. Zimmermann andK Hayek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237s Preparation of Egg-Shell Type Pd-Ag and Pd-Au Catalysts by Selective Deposition and Hydrogenation of 1,3-Butadiene H. Miura, M. Terasaka, K. Oki and T. Matsuda . . . . . . . . . . . . . . . . . . . . . . . 2379 Hydrogenation and Deuteration of Butadiene and Cyclohexadiene over Reduced and Sulfided Molibdena-Alumina A. R&a!ey,D. Smrz and W. K Hall. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2383 Hydrocracking Boscan Heavy Oil with Unimodal and Bimodal Catalysts 2 387 M. Ternan and J. Menashi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
XXI I A Kinetic Model for Aromatization Processes over ZSM-5 Catalysts. Aromatization of Short Chain Hydrocarbons over HZSM-5 D. B. Luk'yanov, V.I. Shtral, V. I. Timoshenko and S. N. Khadzhiev . . . . . . . . . . . 2391 On the Role of Reversible and Irreversible Adsorption Hydrogen in the Dehydrogenation and Reforming Reactions Y. Sun, S. Chen andS. Peng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,2395 Catalyst Design for the Upgrading of Australian Coal-Derived Liquids A. T. Townsend and F. P. Larkins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2399 The Use of Intermetallic Hydrides on Basis Lanthan with Nickel and Cobalt for Hydrogenation of Asphaltenes Concentrate N. M. Parfenova, I. M. Halperin, S. R. Sergienko, V. A. Pherapontov and E. V. Staroahbtzeva . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2403 n-Hexane Isomerization and Aromatization on the Catalysts Derived from AluminaSupported Pt-Sn Clusters X. Li, Y. Wei, J. ChengandR. L i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2407 The Effect of Rhodium Particle Size on n-Butane Hydrogenolysis Activity and Selectivity D. Kalakkali, S. L. Anderson andA. R Datye . . . . . . . . . . . . . . . . . . . . . . . . 241 1 Pretreatment Effects on Active State and Aromatization Activity of GaESM-5 Catalysts K. M. Dooley, C. Chang, V. Kanazirev and G. L. Price . . . . . . . . . . . . . . . . . . . 241 5 New Modification Method of Pt/L Zeolite Catalyst for Hexanes Aromatization H. Katsuno, T. Fukunaga und M. Sugimoto . . . . . . . . . . . . . . . . . . . . . . . . . 24 1 9 High Temperature Sensitivity of Paraffin Hydrocracking R. T. Hunlon, C.R. Kennedy, R. A. Ware andS. S.Wong . . . . . . . . . . . . . . . . . 2423 Effect of Modification of the Alumo-Platinum Reforming Catalyst with Dy, Cr, Ba and Nytrogen on its Catalytic and Physico-Chemical Properties G. M. Sen'kov, E. A. Skrigan, E. A. Paukshtis, M. F. Gorbatsevich, A. M. Nikitina and E. N. Ermolenko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2427 Transformation of Propane over Ga/HZSM-5 Catalyst: On the Nature of the Active Sites for the Dehydrogenation Reaction P. Meriaudeau and C.Naccache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 43 1 Study of the Sclcctive Semi-Hydrogenation of Various Carbon-Carbon Triple Bonds over a Pd/Sepiolite Catalyst M. A. Aramendia, V. Borau, C.Jimenez, J. M. Marinas, M. E. Sempere, F. J. Urbano and L. Villar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 435 n-Arylhydroxylamines Transformation in the Presense of Heterogeneous Catalysts I. A. Makaryan and V. I. Suvchenko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2439 Highly Efficient Enantio-Differentiating Hydrogenation Catalyst Prepared from Ultrasonicated Raney Nickel by Asymmetric Modification A. Tai, T. Kikukawa, T. Sugimura, Y. Inoue, S. Abe, T. Osawu and T. Harada . . . . . 2443 New Vapor Phase Process for Synthesis of Ethylenimine by Catalytic Intramolecular Dehydration of Monoethanolamine M. Ueshima, Y. Shimasaki, K Ariyoshi, H. Yano and H. Tsuneki . . . . . . . . . . . . . 2447 Catalyst for Vinyl Chloride Synthesis N. A. Prokudina, V. V. Chesnokov, B. P. Zolotolvskii, L. N. Yelesina, V. G. Yenakueva and V. F. Tarasov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 45 1 A New Process for Production of 1,3-Dimethyladamantane R Takagi and Y.Naruse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2455 Palladium Based Catalysts for Hydrogenation of Nitrobenzene E. Brazi, G. Cordier and C. N. Sauvion. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2459 Modification of Pd and Pt by Thiophene and Carbon Tetrachloride during Hydrogenation and Isomerization of (+)-Apopinene G. V. Smith, F. Notheisz, A. G. ZsigmondandM. B a r d k . . . . . . . . . . . . . . . . . . 2463
XXI I I Selective Hydrogenation of Carbonyl Groups by Means of Hectorite-Intercalated Rhodium Complexes S. Shimazu, T. Chiaki and T. Uematsu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enantioselective Hydrogenation of a-Keto Esters over Pt/AI Og Catalyst: Kinetic Aspects of the Rate Acceleration Effect Induced by Addition ofcinchonidine J. L. Margirfalvi, B. Minder, E. Talas, L. Botz and A. Baiker . . . . . . . . . . . . . . . Crotonaldehyde Hydrogenation over Pt/TiO, Catalysts. Influence of the Catalysts Pretreatments R. Makouangou, A. Dauscher and R. Touroude . . . . . . . . . . . . . . . . . . . . . . . Selective Hydrogenation of a$-Unsaturated Aldehydes over Supported Ru A. Waghray, R. Oukaci and D. G. Blackmond . . . . . . . . . . . . . . . . . . . . . . . . Hydrogenolysis of Tetrahydrofurane on Platinum EL Krewer and R. Kramer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of Polymer Support Morphology on Ion-Exchanger Catalysts Activity in tert.-Alkyl-Methyl Ethers Synthesis K. Jerabek, T. Hochmann and Z. Prokop . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Catalytic Organic Solid-Gas Reactions R. Lamartine, F. Sabra and A. Selatnia . . . . . . . . . . . . . . . . . . . . . . . . . . . . Infrared Study of Coke Deposition on Alumina J . Datka and R. P. Eischens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen Surface Concentration Effect on the Temperature-Programmed Hydrogenation of Adsorbed Carbonaceous Species on an Iron/Alumina Catalyst H. Halafi, E. Borgstedt, A. M. Ejitathiou, S. L. Suib and D. Bianchi . . . . . . . . . . . FTIR and Catalytic Studies of the Effects of Sulphur Poisons on Cu/A1203 Catalyst Selectivity M. B. Padley, C. H. Rochester, G. J . Hutchings, P. I. Okoye and F. King . . . . . . . . Enhancement of the Stability of PtSn Catalysts in Regeneration Cycles by A1203 Doping with Rare-Earth Oxides Y. Fan, L. Lin, J . ZangandZ. Xu. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Development of a New Catalyst for the Hydroconversion of Heavy Oils M. M. Ramirez de Agudelo and C. Galarraga . . . . . . . . . . . . . . . . . . . . . . . . Molecular Poisoning of Ni/SiO, Catalyst. A Magnetic and Catalytic Study of the Effect of Thiophene, Carbonyl Sulfide and Carbon Disulfide Adsorption J. B. Baumgartner, R. Frety and M. Guenin . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Sintering in 0, Atmosphere on the Surface Properties of PtRh/A1203 Catalysts S. Kacimi, C. Kappenstein and D. Duprez . . . . . . . . . . . . . . . . . . . . . . . . . . Coke Deposits on Pt/A1203 Catalysts: FTIR and HRTEM Studies L. Marchese, E. Borello, S. Coluccia, G. Martra and A. Zecchina . . . . . . . . . . . . Molecular Orbital Study of the Chemisorption of Small Molecules on MgO Surfaces H. Kobayashi, A. St. Amant, D. R. Salahub and T. It0 . . . . . . . . . . . . . . . . . . . Applications of a New Isothermal Single Pellet Diffusion Reactor S. S. Au, J . B. Butt andJ. S. Dranofl. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coke Elimination from Pt-Re/Al O3 by Ozone Containing Mixtures C. L. Pieck, E. L. Jublonski and M. Parera . . . . . . . . . . . . . . . . . . . . . . . . New Model of Deactivation of Iron Catalysts for Ammonia Synthesis W. Arabcqk and K. Kalucki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pore Restriction in Resid Hydrotreating Catalysts P.-S. E. Dai and B. H. Bartley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of the Internal Mass Transfer Resistance on the Ni/AI,O, Deactivation by Thiophene S. Zrncevic and V. Tomasic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Study of Coke Formation in Resid Catalytic Cracking T.C.Ho . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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XXlV Selective Hydrogenation of Adiponitrile (ADN)to Hexamethylenediamine (HMD) in the Liquid Phase on Raney Catalysts: Synergetic Effect between the Base in the Solution and the Iron Dope on the Catalyst J. F. Spindler, G. Cordier, J. Jenck and P. Fouilloux . . . . . . . . . . . . . . . . . . . . Acid Resistant Copper Chromium Oxide Catalysts Used in the Hydrogenolysis of Fatty Acids K. Kochloejl, G. Maletz, G. Hausinger and M. Schneider . . . . . . . . . . . . . . . . . Synthesis of Anthraquinones by Cyclization of Ortho-Benzoylbenzoic Acids in the Presence of Solid Acid Catalysts S. A. Amitina and V. C. Shubin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acid-Base Properties of Sulfided Ni-Mo-Y Zeolite Catalysts for Water-Gas Shift Reaction M. Laniecki and W. Zmierczak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Investigation on Adsorption and Acid-Base Properties of Solid Catalysts by Infrared Thermography S. Marengo, G. Raimondini and P. Comotti . . . . . . . . . . . . . . . . . . . . . . . . . Nature of the Extra-Framework Aluminum in MFI Type Zeolites Synthesized in Fluoride Medium. Influence on the Catalytic Properties of Zeolites Q. Chen, J. L. Guth andJ. Frabsard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron - an Acidity and Texture Modifier for Alumina Supported Catalysts S. Engels, E. Hero14 H. Lausch, H. Mayr, H. - W. Meiners and M. Wilde . . . . . . . . Sulfate-Modified Superacid Zr02 Catalysts: Study of the Surface Acidity and of thc Activity towards Alcohols C. Morterra, G. Cerrato, C. Emanuel and V. Bolis . . . . . . . . . . . . . . . . . . . . . Highly-Dispersed Heteropoly Anions on Metal Oxide Carriers Modified with Silane Coupling Agents and their Catalytic Properties Y. Kera, H. Nishizima and M. Kamaah . . . . . . . . . . . . . . . . . . . . . . . . . . . . New Catalytic Systems: Heteropolyanions Doped Conjugated Polymers as the Catalysts for Ethyl Alcohol Convcrsion J. Pozniczek, I. Kulszewicz-Bajer, M. Zagorska, M. Hasik, A. Bielanski and A.Pron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Thermal Treatment on Surface Properties and Catalytic Activities of a Prepared Ti0 ZrO? Catalyst J. A. Navio, d.-Macias, F. J. Marchena, J. M. Campelo and J. M. Marinas . . . . . . Gas Phase Synthesis of MTBE over Acid Zeolites A. Nikolopoulos, T. P. Palucka, P. V. Shertukak, R. Oukaci, J. G. Goodwin, Jr. and G.Marcelin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acidity-Tunable Pillared Mica Catalyst Derived from Talc K Urabe, I. Kenmoku, K. Kawabe and Y. Izumi . . . . . . . . . . . . . . . . . . . . . . . Beckmann Rearrangement over Solid Acid Catalysts T. Curtin, J. B. McMonagle and B. K. Hodnett . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Solid Superacids of Tungsten Oxide Supported on Tin Oxide, Titanium Oxide and Iron Oxide and their Catalytic Action K Arata and M. Hino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of the Method of Platinum Inclusion into Spherical Promoter of Catalytic Cracking Termofor on its Activity in CO Combustion M. I. Levinbuk, V. B. Melnikov, H. K. Shapieva, V. I. Vershinin, V. A. Kuzmin and V. Je. Varshaver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . &oxidation CO and H, on Palladium Catalyst N. A. Boldyreva and V. K. Yatsimirsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Structure of Supported Pd, Kinetics and Mechanism o f the Low-Temperature Oxidation of Carbon Monoxidc S. N. Pavlova, V. A. Sadykov, D. I. Kochubei, B. N. Novgorodov, G. N. Kryukova and V. A. Razdobarov. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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xxv Transition Metal Compound Oxide Catalysts for Lowering the Light Off-Temperature of Particles from Diesel Exhaust 2629 C.Setzer, W. SchutzandF. Schuth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FTIR Study of Reduction Mechanism of NO by C,H, and C,H, on Vanadium Oxides Layered on ZrO T. Ohno, F. Hatayama, $. Maruoka and H. Miyata . . . . . . . . . . . . . . . . . . . . . 2633 The Selective Catalytic Reduction of Nitrogen Oxides with Ammonia in a Catalytically Active Ljungstroem Heat Exchanger 2637 H.-G. Lintz and T. Turek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evaluation of Nitrous Oxide Formation over Rhone-Poulenc's DN 110 SCR Catalyst 2641 E. Garcin and F. Luck. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytically Accelerated Solid-Gas Reaction between NO and Ba-Cu-0 for Efficient NO Removal M. Machiah, S. Ogata, K Yasuoka, K Eguchi and H. Arai . . . . . . . . . . . . . . . . 2645 Catalytic Property of Perovskite-Type Oxides for the Direct Decomposition of Nitric Oxide 2649 Y. Teraoku, T. Harada, H. Furukawa and S. Kagawa . . . . . . . . . . . . . . . . . . . . TPD-Analysis of Metallic Three-Way Catalysts. Effect of Zr, Si, La and Ba on Thermal Aging M. Huuska and T. Maunula . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2653 Mechanistic Studies of CO Oxidation on Highly Dispersed Gold Catalysts for Use in Room-Temperature Air Purification M.Haruta, S. Tsubota, T. Kobayashi, A. Ueah, H. Sakurai and M. Ando . . . . . . . . 2657 Low-Temperature Oxidation of Light Paraffins and Olefins at Solid Surfaces: FT-IR Studies G. Busca, V. Lorenzelli, G. Ramis and V. S. Escribano . . . . . . . . . . . . . . . . . . . 2661 Catalytic Diesel Engine Emission Control. - Studies on Model Reactions over a EUROFT-1 (Pt/SiO ) Catalyst E.Xue,KSeshan,?G.vanOmmenandJ.R. H.Ross. . . . . . . . . . . . . . . . . . . 2665 Carbon-Oxygen Reaction on Cu/V/K Catalyst for Soot Oxidation P. Ciambelli, V. Palma and S. Vaccaro . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2669 Kinetics of the Selective Catalytic Reduction of NO by NH, over a Commercial Catalyst W. R. A. M. Robinson, J. G. van Ommen, A. Woldhuis and J. R. H. Ross . . . . . . . . 2673 Selective Catalytic Reduction of NO on CopperaAIumina in the Cleanup of High Sulfur Content Flue Gas:Catalyst Development and Design G. Centi, N.Passarini, S. Perathoner, A. Riva . . . . . . . . . . . . . . . . . . . . . . . . 2677 The Role of NO2 in the Selective Catalytic Reduction of NO, J. Blanco, P. Avila and J. L. G. Fierro . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2681 Catalysts for the Elimination of Sulphur Dioxide from Streams by the Claus Reaction at Low Temperature E. Alvarez, A. Mendioroz and J. M. Palacios . . . . . . . . . . . . . . . . . . . . . . . . . 2685 Promoter Effects on Platinum Catalysts for Automotive Exhaust Control J. R. Gonzalez-Velasco, J. Enirena, J. A. Gonzalez-Marcos, J. I. Gutierrez-Ortiz and M. A. Gutierrez-Ortiz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2689 Correlation of Bulk and Surface Thermodynamics of some Transition Metal Oxides; Application to Exhaust Gas Catalysts A. D. van Langeveli, A. C. T. van Duin, J. W. Bijsterbosch, F. Kapteijn and J.A.Moulijn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2693 &adsorption and Reaction of NO and CO on Cu/AI,O, Catalysts R. Hierl, H.-P. Urbach and H. Knozinger . . . . . . . . . . . . . . . . . . . . . . . . . . . 2697 Activity and Electronic Properties of Automotive Emission Control Catalysts B. H. Engler, D. Lindner, E. S. Lox and P. Albers . . . . . . . . . . . . . . . . . . . . . . 2701
XXVl
Alumina Supported Manganese Catalysts for Low Temperature Selective Catalytic Reduction of NO with NH3 L. Singoredjo and F. Kapteijn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduction of Nitric Oxide by Carbon Monoxide on Palladium Based Bimetallic Catalysts J. Massardier, A. El Hamdaoui, G. Bergeret and A. Renouprez . . . . . . . . . . . . . . Activity and Characterization of Palladium Catalysts for Nitric Oxide Decomposition A. Ogata, A. Obuchi, K Mizuno, A. Ohi and H.Ohuchi . . . . . . . . . . . . . . . . . . Effect of Calcination on V-0-Ti-P Catalysts J. Soria, J. C. Conesa, M. Lopez-Granados,J. L. G. Fierro, J. F. Garcia de la Banda and H. Heinemann. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methanol Synthesis over Copper Based Catalysts: Comparison of Co-Precipitated, Raney-Type and Catalysts Derived from Amorphous Alloy Precursors A. C. Sofianos, J. Heveling, M. S. Scurrell and A. Armbruster . . . . . . . . . . . . . . . Activity and Selectivity of Ruthenium-Cobalt Bimetallic Catalysts in Carbon Monoxide Hydrogenation S. A. Korili and G. P. Sakellaropoulos . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Kinetics and Periodic Operation of Higher Alcohol Synthesis J.-L. Li, Q.-M. Zhu, J.-L. Hu and N . J . Yuan . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Selective CO Hydrogenation to Isobutene over Oxide Catalyst K Maruya, A. Takasawa, T. Haraoku, M. Aikawa, T. Arai, K Domen and T.Onishi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conversion of Cl-C4 Alcohols into Aromatics on the Modified ZSM-5 Zeolites. Active Centres and Reaction Pathways D. -Z. Wang,J. -Y. Wang and X. -D. Lu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Activities and Selectivities of Supported Co-Ru, Co-Pd and Co-Pt Bimetallic Catalysts in Fischer-Tropsch Synthesis M. P. Kapoor, A. L. Lapidus andA. Yu. Krylova . . . . . . . . . . . . . . . . . . . . . . New Ternary Cu-V-Zn Catalyts for Conversion of C 0 2 by H into Methanol N. Kanoun, M. P. Astier, F. Lecomte, B. Pommier and G. M.5ajonk. . . . . . . . . . Alkali-Promoted MoSz Catalyst.. for Alcohol Synthesis: The Effect of Alkali Promotion and Preparation Condition on Activity and Selectivity H. C. Woo,T. Y. Park, Y. G. Kim, In-S. Nam, J. S. Lee and J. S. Chung . . . . . . . . . New Alkene Oligomerization Catalyst NiSOdy-A1203 and its Characteristics T. Cai, D. Cao and L. Li . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Properties of Partially Reduced Fe/SiO, in CO Hydrogenation S. H. Moon, C. W. Park and H. K Shin . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogenation of Carbon Monoxide over Rh/Zr02 Catalysts Promoted by Molybdenum Oxide E. Guglielminotti, E. Giamello, F. Pinna, G. Strukul, S. Martinengo and L. Zanderighi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Identifying the Reaction Network of the Higher Alcohol Synthesis over AlkaliPromoted ZnCrO Catalysts L. Lierti, E. Tronconi and P. Forzatti . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Formic Acid and CO, in CO Hydrogenation to Methanol over CopperBased Catalysts and Nature of Active Sites J. Cai, Y. Liao, H. Chen andK. R. Tsai. . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of ZnO-Supported Cu, Cu-Mn, Cu-Fe, Cu-Co and Cu-Ni Catalysts in CO Hydrogenation P. A. Sermon, M.A. M.Luengo (Yates) and Y. Wang . . . . . . . . . . . . . . . . . . . . Methanol Synthesis from C 0 2 and H2 over Supported Copper-Zinc Oxide Catalyst. Significant Influence of Support on Methanol Formation H. Arukawa and K. Sayama . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2705 2709 271 3 2717 272 1 2725 2729 2733 2737 2741 2745 2749 2753 2757
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XXVll C 0 2 Hydrogenation over Platinum Group Metals Supported on ( 3 0 2 : Evidence for a Transient Metal-Support Interaction A. Trovarelli, G. Dolcetti, C. a'e Leitenburg andJ. Kaspar . . . . . . . . . . . . . . . . . Potassium Promotion of Cu-ZnO-A1203 Catalysts for Higher Alcohol Synthesis I. Boz, D. Chadwick, I. S. Metcalfe and K Zheng . . . . . . . . . . . . . . . . . . . . . . Conversion of Syngas to Aromatic Hydrocarbons on Cobalt-Manganese-Zeolite Catalysts G. Baurle, K. Guse, M. Lohrengel and H. Papp . . . . . . . . . . . . . . . . . . . . . . . Light Olefins Formation from Syngas over Zr02-Zn0 Catalysts W. Zhang, R. Gaol G. Su and Y. Yin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Selectivity of Diesel Fraction in Fischer-Tropsch Synthesis with Co/Nb,O, A. Fryahan, R. R. Soares and M. Schmal . . . . . . . . . . . . . . . . . . . . . . . . . . . The Catalytic Behavior of Some Zr-Ni-Co-Cu-Ru Intermetallic Compounds in Fischer Tropsch Reactions I. R. Harris, I. T.Caga, A. Y. Tala andJ. M. Winterbottom . . . . . . . . . . . . . . . . X-Ray Powder Diffraction In Situ Characterisation of the (Cu, Zn, Al)-Hydrotalcite Phase in Cu-Zn0-A1203-Catalysts Highly Active in Methanol Synthesis K. Richter, W. Kraus, G. Nolze and B. Peplinski . . . . . . . . . . . . . . . . . . . . . . . Catalyst Component Interactions in Ag-Cu/ZnO/A1203 Catalyst for Carbon Oxygenate Synthesis E. Kis, G. Lomic, G. Boskovic, R. Neahcin and P. Putanov . . . . . . . . . . . . . . . . Cobalt Reducibility and Magnesium Promotion in Silica-Supported Fischer-Tropsch Catalysts I. Puskds, T. H. Fleisch, J . B. Hall, B. L. Meyers and R. T. Roginski . . . . . . . . . . . Transient Response Study of the Oxidation and Hydrogenation of Carbon Monoxide Adsorbed on Pd/A1203 G. Kadinov, S. Todorova and A. Palazov . . . . . . . . . . . . . . . . . . . . . . . . . . . Higher Alcohol Synthesis over Functionalized Solid-Base Catalysts from Methanol as a Building Block W. Ueah, T. Ohshida, T. Kuwabara and Y. Morikawa . . . . . . . . . . . . . . . . . . . In Situ 1H N M R Study of the Adsorption of Hydrogen and Formic Acid on Copper Based Methanol Synthesis Catalysts A. Bendada, J. B. C. Cobb, B. T. Heaton and J . A. Iggo . . . . . . . . . . . . . . . . . . Isomerisation of Long-Chain n-Alkanes on Pt/H-ZSM-22 and Pt/H-Y Zeolite Catalysts and on their Intimate Mixtures J. A. Martens, L. Uytterhoeven, P. A. Jacobs and G. F. Froment . . . . . . . . . . . . . Kinetics of Bimodal Grain Size Distribution of a Ni Catalyst During Hydrogenation of co M. Kolb, M. Agnelli and C. Mirodatos . . . . . . . . . . . . . . . . . . . . . . . . . . . . Critical Point Properties of the Surface Structure During CO Oxidation M.Kolb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selectivity Effects Related with Reaction Mechanism and Diffusion Limitations over Deactivating Catalysts K Kumbilieva, S. L. Kiperman andL. Petrov . . . . . . . . . . . . . . . . . . . . . . . . Author index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Studies in Surface Science and Catalysis (other volumes in the series) . . . . . . .
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Guczi, L. et al. (Editors), New Frontiers in Caialysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
NEW CATALYTIC PHASES FOR THE HDN AND MHC REACTIONS IN COMOPALUMINA CATALYSTS A. Morales, R. Prada-Silvy and V. Leon
INTEVEP S.A., Division General de Refinacion y Petroquimica, Apartado 76343, Caracas 1030A. Venezuela
Abstract CoMoP-Alumina catalysts were prepared and characterized by XPS, IR, Auger, DRS and surface acidity measurements. The results indicate that phosphorus improves both hydrodenitrogenation (HDN) and mild hydrocracking (MHC) activities. The hydrodesulphurization (HDS) and aromatic hydrogenation (HID) activities remain unchanged. The IR and Auger studies show there is a formation of a Mo-P heteropoly compound and a Co-Phosphate-like compound which may be actives phases in HDN and MHC reactions. The promoter role of phosphorus in CoMo-Al catalysts is complex because it affects the acidity, the surface structure and the metals dispersion. 1. INTRODUCTION A catalytic promoter is used for improving activity, selectivity and/or stability. Phosphorus has been recommended and studied for several years. Presently, it is used in a significant number of commercial catalysts [1,21. According to Hilfman 121 and Morales et al. [31, the effect of phosphorus in NiMoP type catalysts is: to inhibit the formation of NiAl,O, to enhance the nickel dispersion in sul hided catalysts, thereby increasing the formation of octahedrally coordinated N i A [31, to decrease coke deposition [41, and change the acidity of the alumina support [51. Chadwick et al. [61 observed that the HDS activity is improved by increasing P content up to about 1wt%. Recently, Topsoe et al. [71 suggested that the variations in the catalytic results with P addition may be well caused by modification of the properties and the number of CoMoS (NiMoS) sites. Eijsbouts et al. [81 have shown that phosphate is an efficient HND promoter over NiMo-Al but does not significantly influence the sulphur removal from thiophene. Consequently, the dispersion of the active phase, the surface structure and crystal morphology of the catalyst may change with P addition. This work is part of a series aimed a t systematically understanding the role phosphorus plays on the physicochemical and catalytic properties of hydrotreating catalysts.
1900
2. EXPERIMENTAL
A series of CoMoP-Alumina catalysts with various phosphorus loading and constant molybdenun and cobalt content were prepared by coimpregnation using the pore filling method [31. One commercial catalyst was also used as control. The physical properties and chemical composition of the catalysts, were determined by standard methods. Acidity was measured in a Sortorious electrobalance using ammonia as the probe molecule. Surface characterization was evaluated by XPS, IR and Auger. W-Vis. Diffuse Reflectance spectra were also recorded. The XPS measurement were performed in a Leybold-Heraeus LHS11 system using Mg and Al K a (1,284.6 eV and 1,486.8 eV). The Al(2p) peak (74.8 eV) was taken as a reference for binding energy evaluation. Atomic concentrations were determined using automatically integrated peak area and sensitivity factors determined by the equipment used. The Auger spectra were recorded in the same equipment for XPS using MgKa ( h y = 1,253.6 eV, 14 KV, 20 mA) after elimination of oxygen Auger measured using AlK a radiation [ 9 1. IR studies were obtained following the procedure of ref. 10, using a HP Fourier transform IR equipment. HDS, HDN and MHC reactions were evaluated using a typical Venezuelan heavy vacuum gas oil (HVGO) containing 1200 ppm of nitrogen, 2.18 wt% of sulphur and 62 wt% of aromatic compounds. The experimental condition were a follows: T= 633"K, P= 5.4 . 106 Nm-2, LHSV= 1 h-1 and H%/Feed= 300 Nm3/m Before the reaction, all catalysts were sulphurized using atmospheric gasoleum containing CS2 (2 wt%) and hydrogen under the following conditions: T= 623°K P= 2.7.106 Nm-2 and LHSV= 2 h-' during a 10 hour run.
I
3. RESULTS AND DISCUSSION
Table 1 Physical, chemical and catalytic activities of catalysts Catalysts CoMoPAl-1 CoMoPAl-2 CoMoPAl-3 CoMoAl COAl COPAl MoAl MoPAl
Moo3 COO P2O5 SA HDS (wt%) (wt%) (wt%)(m2gl) (wt%) 12.9 12.8 13.0 13.6 ---
4.6 4.7 4.6
4.9 4.5
-
--
4.3
7.1
13.2
-
12.8
1.1 3.5 7.0
7.4
230 200 190 190 220 190 199 188
88.9 90.3
89.6 88.7 2.0 3.3
HDN (wtL;t! 48.3 49.9 50.2 28.5
6.3 30.2
32.1
25.2
33.6
28.1
MHC
HID
(wtol~) (wt%)
25.3 25.8 26..9 10.2 5.4
18.3 9.5 18.1
17.8 17.3 17.7
17.6 1.0
2.5 10.3 10.8
1901
Physical properties, chemical composition and catalytic activities of the catalysts are reported in table 1. The molybdenum contentin all catalysts was of 13 wt% (as Moog). The Co content for the Co-containingcatalysts was of 4.6 wt% (as COO),while P vaned from 0.0 to 7.4 wt% as P205. The effect of P on the physical and chemical properties has been showed and discussed before [5,111. The activity results indicate that phosphorus improves both the HDN and the MHC reactions by 48 and 100% respectively. HDS and HID activities remains unchanged. CoPAl and MoPAl catalysts have important HDN and MHC actitities. The values of irreversible acidity determined by ammonia adsorption for P containing catalysts increasing with P addition. Because, the acidity values obtained for the commercial catalyst ( containing no P) was two times the acidity of the catalysts containing phosphorus, the phosphorus effect on HDN and MHC reaction can not be associated only with the catalysts acidity. The promoter effects of phosphorus is complex and should be related to changing metal dispersion [ l l l , morphology and structure of the active phase [7,81. DRS and W S spectra were not indicatives in the characterization studies of catalysts samples. The binding energies of Mo, P, Co, Al and 0 were the same for all catalysts. The Mo3d (229.4 eV) peak resembled that usually obtained for sulphided Mo-containing catalysts. The binding energy of the P2p peak was 134.2 eV. This means that phosphorus addition does not change the binding energy of molybdenum. These results agree with those of Chadwick et.al. 161. Regarding the C02p binding energy value, we observed a decrease of only 0.3 eV in Pcontaining catalysts. This result does not exclude the possibility of the formation of Mo-P and Co-P compounds since the difference between the BE of metal oxide and metal phosphate are small. Phosphorus addition increases Mo and Co dispersion, in oxyde and sulphyded catalysts. The improvement obtained on Co
Figure 1. Auger Spectras.
Figure 2. IR Spectras
1902
was more relevant. These results are in perfect agreement with the previously reported results using NiMoPAl catalysts [lll. To avoid interferences in the XPS analysis due to high concentration of Mo and Co we prepared two new catalysts of composition l.O%CoO + 7.5%P205-4 and l.O%CoO-Al. The XPS analysis showed a small shift for the C02p binding energy, however, the Auger spectra of this catalysts showed 2.8 eV deplacement to higher frequencies in the samples containing phosphorus (769.5eV for the Co catalyst and 772.3 eV for the COPcatalyst). On the other hand, it can be seen there are important similarities between Auger spectra of P-containing catalyst and the cobalt-phosphate-like compound adsorbed on the alumina (see fig. 1). Fig 2, shows the IR spectra of MoAl and MoPAl catalysts in the range 1300-500 cm-1. It can be seen that P enhances the formation of Mo-P heteropoly compounds on catalyst surface [lo]and decreasing Mo-Al interactions [ll]. The HDS, HID, HDN and MHC experiments demonstrate that P addition to CoMoAl catalysts does not affect the HDS activity but increases the HDN and MHC reactions. If the HDS active phase (Co-Mo-S) remains constant in our catalysts, it can be concluded that HDN and MHC reactions require more than a single type of site. Each of these two sites can play a particular role in the polyfuntional catalyst: one as a hydrogenating site and the other as a hydrogenolysis (or craking) site. In the case of the MHC reaction, a hydrogenolysis site is required. Hence, the effect of P on these types of reactions may be regarded as being due to the surface phosphate existing in phases such as Mo-P and Co-P which after sulphidization leads t o a new sulphided species with enhanced hydrogenolyzing power. 4. REFERENCES
1 A. Morales , A. Guillen, M.M. Ramirez, N. Martinez and R. Carrasquel, US Patent Nr 4 600 703 (1986). 2 L.Hilfman et al. US Patent Nr 3 617 528 (1971). 3 A. Morales and M. M. Ramirez, Applied Catal., 23 (1986)23. 4 C.W. F'itz and H.F.Rose, Ind. Eng. Chem. Proc. Res. Div., 22 (1983)40. 5 A. Morales, M.M. Ramirez and F. Hernandez, Applied Catal. 41 (1988)261. 6 D. Chadwick, D.W. Altchinson, R. Badilla and A. Josefsson ( G. Poncelet, P. Grange and A. Jacobs, Eds.), Preparation of Catalysts 111, Elsevier, 16 (1983) 323. 7 H. Topsoe, B.S. Clausen, N. Topsoe and P. Zeuthen, (D.L. Trimm et al. Eds.) Catalysis in Petroleum Refining, 1989,Vo1.53 (1990)77. 8 S.Eijsbouts, J.N.M. ven Gestel, J. A. R. van Veen, V.H.J. de Beer and R. Prins, J. Catal. 131 (1991)412. 9 V. Leon and J. Carraza. Rev. Tec. INTEVEP, No 9(1) (1989)81. 10 P. Atanasova and T. Halachev, Applied Catal., 48 (1989)295. 11 M. M. Ramirez and A. Morales , (M.J. Phillips and M. Ternan Eds.), Proc. 9th . Int. Cong. on Catalysis, 1 (1988)42.
Guczi, L. el al. (Editors), New Frontiers in Catulysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 63 1993 Elsevier Science Publishers B.V. All rights resewed
MULTI-PURPOSENi-Mo/Al,O, CATALYSTS FOR GAS OIL HYDROTREATING A. F. Somogyvciri, M. C. Oballa and P. S.Herrera
NOVA HUSKY Research Corporation, 2928-16th Street N.E. Calgary, Alberta T2E 7K7, Canada
The Bi-Provincial Upgrader (BPU) complex is a grassroots facility jointly owned by Husky Oil, the Government of Canada, and the Provincial Governments of Alberta and Saskatchewan. We were pleased to be selected to perform the initial screening of the catalysts for the BPU and briefly report herein our endeavours concerning the selection of catalysts for the gas oil hydrotreater.
For us, the most important parameters for the gas oil hydrotreating catalysts were the maximization of hydrodenitrogenation (HDN), polynuclear The aromatics (PNA) reduction and hydrodesulphurization (HDS). relationship between product quality and hydrogen consumption was a n economic consideration required only for final catalyst selection. It may be seen from Table 1 that the best combination of HDN, PNA removal and HDS have been achieved for the catalysts F and A. Table 1
Feedstock and Hydrotreated Product Properties
Density, (kg/dm3) Viscosity, (cP) Carbon, (wt%) Ha, (wt%) S, (wppm) N, (wppm) Aromatics,(wt%)
FEEDSTOCK 823.3 38 87.09 11.61 14123 1799 46.9
PROD. PROD. PROD. PROD. PROD. PROD. A B C D E F 872.8 890.2 890.2 886.3 889.2 880.4 13 22 21 19 22 16 86.60 87.04 87.02 86.91 86.78 86.50 12.74 12.15 12.26 12.47 12.38 12.65 795 894 517 393 391 247 102 276 204 62 129 30 18.4 29.1 33.4 26.9 30.5 22.7
Mo coverage appears to correlate with HDN, PNA reduction and HDS (Figure I). There appears to be a direct relationship between the phosphorus content of the catalysts (Table 2) and some component of the hydrogen uptake. Those catalysts which are highly active in HDN also have high
1904
Table 2 Composition and Properties of Catalysts CATALYST A B C Nickel, (wt%) 2.8 4.7 2.8 Molybdenum, (wt%) 11.0 12.8 11.6 Phosphorus, (wt%) 3.3 2.3 2.9 Surface Area, (m2/g) 149 159 180 Pore Volume, (cm3/g) 0.41 0.38 0.37
D 3.0 13.2 3.5 159 0.30
E 3.3 11.9 176 0.47
F 2.4 12.3 2.8 145 0.39
hydrogen consumption and consequently have been found to be high in phosphorus content. These results appear to be consistent with those found in the literature where the most beneficial effects of phosphorus have been documented for Ni-Mo/Al,O, catalysts. These positive effects include bettg: dispersion of the active components, an increase in the octahedral Ni concentration in the active phase and pytibly a reduction in the acidity of the catalyst (1-3). The increase in the Ni concentration in the active phase is believed to result in increased hydrogenation activity while the reduction in acidity has been postulated to decrease coke formation (4). Our data (Figure 11)appear to be consistent with these observations.
E
3.5"
zu 8 612
;.o
i.5 i.0 8.5 Molybdenum Coverage x l o 3 (glm')
2.560
i.5
FIGURE I: Conversion as a Function of Molybdenum Coverage
14
16
18
20
Phosphorus Coverage x 10' (glm')
FIGURE 11: Coke on Catalyst as a Function of Phosphorus Coverage
Hydrodesulphurization studies using single components resulted in first order reactions with respect to sulphur (5,s). However, for petroleum fractions, reaction orders were found to increase from 1.2 to 3 with the boiling point of the fraction (6,7). Hydrodenitrogenation of both model compounds and oil fractions have been described using first order kinetics (6,s).The kinetics of these reactions have been expressed using a power law. The integral form of this equation has been plotted in Figure I11 for catalyst A. In agreement with other authors HDN was found to follow first order kinetics while HDS was second order.
1905
The existence of a "Ni-Mo-S"phase, similar to the well established "Co-Mo-S' phase in Co-Mo catalysts, has been postulated as the active phase for HDS in Ni-Mo/Al,O, catalysts (9,10). Sulphur vacancies on molybdenum atoms most likely assoaated with the "Ni-Mo-S" phase have been implicated in the hydrogenation activity as well. The formation of this phase on the surface of the alumina support has been found to be dependent on the sulphiding conditions as well as on the nature and concentrations of the various promoter ions. The Broensted acidity of the support has been implicated in C-N hydrogenolysis as well as in cracking and isomerisation reactions. Thus, HDN should occur most efficiently at dual sites where the nitrogen containing substrate has facile and proximate access to both acidic sites on the support and to the sulphur vacancies in the "Ni-Mo-S" phase. Although we have generated no direct evidence in support of these suppositions, it is noteworthy that the catalysts which produce the highest HDN activities also have the highest PNA conversions. Moreover, in every instance where activity has been plotted as a function of a catalyst component, the curve for HDN was found parallel to that for PNA reduction.
3.5
4
-. w HDN 1st Order HDS 2nd Order
3.5
70
0
1
0.5 0
0
0.4
1.2 1.6 llLHSV (hour) 0.8
2
FIGURE ill: Kinetic Plots for HDN and HDS
%
s 65 Fz 60
s 55
50
0
400 860 800 1000 Time on Stream (hours)
200
FIGURE IV:
Effect of Catalyst
on Aromatics Conversion as a Function of Time on Stream
Catalysts A and D were chosen as the most likely candidates for use in the Bi-Provincial Upgrader. While the initial activities of catalyst A have been found to be superior to that of catalyst D, the possibility that catalyst A deactivates faster than catalyst D was of concern. It has been reported that the HDS activity of catalysts for heavy oils depends strongly on the pore structure (11). The best catalysts have sufficient large mesopores for metals deposition while retaining the accessibility of reactants to smaller mesopores. As pore size decreases with time on stream, smaller pores will be generated from the larger ones to maintain constant catalyst activity. If there is an overabundance of small pores, these will become blocked quickly resulting in rapid deactivation with time on stream. The pore size distributions of the fresh (oxidic) and "spent" catalysts have been recorded in Table 3. Aromatics conversion and PNA conversion have been plotted as a function of time on stream in Figure N for the two catalysts. The Figure shows that there is
1906
virtually no difference between catalyst A and catalyst D in terms of aromatics conversion as a function of time on stream. Similar results have been obtained for HDN and HDS. Thus, based on conversions after 1000 hours time on stream the two catalysts appear identical. However, changes in the pore size distribution of the spent catalysts relative to the fresh catalyst (Table 3) suggested that deactivation of catalyst D was imminent. Catalyst A was judged technologically superior. Table 3 Pore Volume Distribution (%) in Fresh and Spent Catalysts Avg. Catalyst Pore 1OOA Volume AV% 50A lOOA (cm3) Radius 38 1.81 97.04 0.89 0.26 0.3039 D: Fresh Spent 22 98.44 1.56 0.00 0.00 0.1344 55.7 A: Fresh 55 0.56 26.96 70.63 1.85 0.4100 Spent 43 8.76 60.36 30.22 0.70 0.2558 37.6 ACKNOWLEDGMENTS We thank Mr. L. Neumann and Ms. N. Hamza for technical assistance and Mr. W. Vandenhengel and Mr. G. Dennis for enlightening discussions. Part of this work was supported by a 50/50 cost shared program between Husky Oil Ltd. and CANMET Energy, Mines and Resources, Ottawa, under contract number 23440-8-9261101-SQ. REFERENCES 1 2
3 4 5 6 7 8 9 10 11
W. Pepperger and W. Shawn, J. Less Common Metals 54,353 (1979). R.E. Tischer, N.R. Noroen, G.J. Steigel and D.L. Cillo, Znd. Eng. Chem. Prod. Res. Dev., 26,422 (1987). A. Spojakena, S. Ramyaneva, L. Petrev and 2. Vit, Appl. Catal. 56, 163 (1989). C.W. Fitz Jr. and H.F. Rase, Znd. Eng. Chem. Prod. Res. Dev., 22, 41 (1983). B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill Inc., (1979). C.I. Chu and I. Wang, Znd. Eng. Chem. Prod. Res. Dev., 21,338 (1982). R.P. Kirchen and E.C. Sanford, AOSTRA J. Res. 5,287 (1989). H.A. Rangwala et al, Energy and Fuels 4 , 5 9 9 (1990). B.S. Clausen, W. Niemann, P. Zeuthen and H. Topsoe, Prepr. Am. Chem. SOC.Div. Pet. Chem. 35, 208 (1990). S.P.A. Louwens and R. Pnns.. Prepr. - Am. Chem. SOC.Div. Pet. Chem. 35,211 (1990). E. Myszka, J.R. Grzechoweai and G.V. Smith, Energy and Fuels 3 , 541 (1989).
Guni, L. et al. (Editors), New Frontiers in Coralysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights resewed
CHARACTERIZATION OF PHOSPHORUS CONTAINING Ni-W/A1203 CATALYSTS P. Atanasova and T.Halachev
Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Abstract C h a r a c t e r i z a t i o n o f t h e oxide form o f phosphorus c o n t a i n i n g co-impregnated Ni-W/AlzOa c a t a l y s t s by X-ray Photoelectron Spectroscopy (XPS) and D i f f u s e Reflectance Spectroscopy (DRS) i s made. The a n a l y s i s o f t h e XPS r e l a t i v e i n t e n s i t i e s i n d i c a t e s that-Fhosphorus forms a monolayer up t o concentration P/A1 = 3.10 Phosphorus i n f l u e n c e s mainly t h e d i s t r i b u t i o n and the s t a t e o f n i c k e l i n t h e c a t a l y s t s . From DRS spectra a conclusion i s made t h a t co-impregnation increases t h e amount o f Ni(0h) in p o l y t u n g s t a t e s t r u c t u r e s . The presence o f phosphorus prevents t h e i n c o r p o r a t i o n o f n i c k e l i n t e t r a h e d r a l sites.The maximal amount o f Ni(0h) i s observed a t 2 . 5 w t . X PZOS.
.
1.INTRODUCTION The e f f e c t o f phosphorus on t h e s t r u c t u r e and a c t i v i t y o f Mo based h y d r o t r e a t i n g c a t a l y s t s has attracted considerable a t t e n t i o n r e c e n t l y [l-41. The aim o f t h e present paper is to c h a r a c t e r i z e t h e oxide form o f a s e r i e s of phosphorus c o n t a i n i n g Ni-W/A1203 catalysts by X-ray Photoelectron Spectroscopy (XPS) and D i f f u s e Reflectance Spectroscopy (DRS). A comparison t o P-Ni-Mo/A1203 c a t a l y s t s C3.51 i s made.
2. EXPERIMENTAL The sampl$s-rere obtained by co-impregnation of 7-A1203 266m g ) w i t h s o l u t i o n s o f (NH4) ioHz(W207) 6 . xHzO, (COOH) 2 , N i (NO?) 2.6Hz0 and H3P04 u s i n g pore f illi n g method. Only sample PO was obtained by step impregnation. tungsten being introduced f i r s t . The samples wore d r i e d a t 393'K and c a l c i n e d a t 823 K f o r t w o hours. The content o f N i O 3.7wt.X and W03 - 13.4wt.X and t h e r a t i o N i / W = 0.85 a r e k e p t constant
(SBET
-
1908 i n a l l samples. The numbering o f t h e samples r e f l e c t s t h e phosphorus c o n t e n t as w t . X PZo5 ( T a b l e 1 ) . The s p e c i f i c s u r f a c e area S(BET) was measured on a D i g i - S o r b 2600 apparatus. The XPS s p e c t r a were r e c o r d e d on an ESCALAB MK-11 spectrometer u s i n g A 1 anode (A1 Ko: = 1486.6eV. 250W). The peak p o s i t i o n s were determined r e l a t i v e t o t h e b i n d i n g energy o f A l z p - 74.5eV. The DRS s p e c t r a were r e c o r d e d on a Beckman 5270 s p e c t r o m e t e r i n t h e 900-220 nm r a n g e u s i n g alumina o x i d e as a r e f e r e n c e . L i n e a r decrease o f S(BET) w i t h i n c r e a s i n g o f t h e phosphorus c o n t e n t i s obserye_di(Table 1). The s l o p e o f t h e S(BET) c u r v e i s a p p r o x i m a t e l y 6m g per w t . % PZOS. similar t o the results o b t a i n e d f o r P-Ni-Mo/Alz03 c a t a l y s t s 131. Table 1 D a t a on t h e c o m p o s i t i o n , s u r f a c e area s p e c t r a l parameters o f t h e samples
P
S (BET)
[at/nm2]
[m2/g]
0 0 0.2 0.4 1 .o 2-4 3.3
144 173 170 154 152 132 131
and
XPS d a t a
on
XPS
and
DRS
DRS d a t a * *
Samples
PO* PO P0.6 P1 .o P2.5 P5.9 P7.8
**
N i Zp/A12s
W4r/A12m
0.94 6.25 4.82 4.83 3.56 7.76 8.96
0.84 0 -76 0.90 1.03 0.88 1.09 1.19
R a t i o between t h e DRS Kubelka
-
F (R720)/F
(R640)
0.89 1.16 1.22 1.45 1.32 1.28 1.28
Munk v a l u e s
The a n a l y s i s o f t h e XPS peak p o s i t i o n s o f P 2 p = 134.4eY. W4r7/z = 36.0eV. N i z p 3 / z = 855.8eV (only f o r sample PO N i z p 3 / 2 = 856.5eV) i n d i c a t e s t h a t t h e r e i s no change i n the b i n d i n g e n e r g i e s w i t h t h e c o n t e n t o f phosphorus- The peak p o s i t i o n s a r e i n good agreement w i t h l i t e r a t u r e d a t a f o r NiOW03/A1203 c a t a l y s t s C6.71. Fig.1 shows t h e c u r v e o f t h e r e l a t i v e i n t e n s i t i e s of I(Pzp)/I(Alzn)-(a) and I(Ni~p)/I(W4f)-(b) vs. the phosphorus c o n t e n t . The I ( P 2 p ) / I ( A l z s ) c u r v e i s l i n e a r up to sample P2.5. Then i t s slope-ghanges. Most p r o b a b l y up t o t h i s c o n c e n t r a t i o n (P/Al = 3.10 ) phosphorus forms a monolayer and then m u l t i l a y e r s t r u c t u r e s a r e formed. This c r i t i c a l value is a p p r o x i m a t e l y two t i m e s l o w e r t g a n t h e one observed f o r P-NiMo/A1203 samples - P / A l = 6.10r3.51. This indicates the s t r o n g e r i n t e r a c t i o n between t u n g s t e n and t h e s u p p o r t compared t o t h e i n t e r a c t i o n between molybdenum and t h e s u p p o r t . Also. t h e a d s o r p t i o n s l t e s a r e p r e f e r a b l y occupied by t u n g s t e n in c o m p e t i t i o n between t h e l a t t e r and phosphorus.
-
1909
E
z I
F
O/P 0:
P 0:
PO I
i
1
1
I
I
I
700 600 500 400 30C
% [nml Figure 1. XPS r e l a t i v e i n t e n s i t i e s (a) I( P z p ) / I (A1 2 0 ) (b) - I( N i Z p ) / I ( W 4 f ) vs. t h e phosphorus content
-
F i g u r e 2. DRS spectra 2f o f samples PO and PO and t h e d i f f e r e n t i a l spectra PO/PO
The I(Nizp)/I(W4f) r e l a t i v e i n t e n s i t i e s decrease almost t w i c e w i t h t h e increase o f t h e phosphorus content (sample P 2 . 5 ) i n comparison t o t h e phosphorus f r e e sample and then f o r h i g h e r i t reaches t h e i n i t i a l values. phosphorus concentrations S i m i l a r behaviour i s observed f o r t h e I( N 4 2 p ) / I ( A 1 2 9 ) curve, w h i l e t h e values f o r t h e I(W4r)/I(A12s) i n t e n s i t t e s do not 1). This f a c t indicates undergo such d r a s t i c changes (Table t h a t i n Ni-W/Al203 samples phosphorus i n f l u e n c e s mainly the s t a t e o f n i c k e l . c o n t r a r y t o t h e case o f P-Ni-Mo/A1203 samples, where phosphorus s t r o n g l y i n f l u e n c e s t h e s t a t e both o f n i c k e l and molybdenum C3.51. Considerable d i f f e r e n c e s in the I( N i z p ) / I (A1204 and I(Nizp)/I(W4r)*values a r e observed f o r samples PO and PO For t h e sample PO (obtained by step impregnation) these values a r e 7 times l e s s than those o f sample PO. As f a r as t h e XPS r e l a t i v e i n t e n s i t i e s r e f l e c t the dispersity o f the active components one c o u l d conclude. t h a t t h e p r e p a r a t i o n method i s o f considerable importance f o r t h e f o r m a t i o n o f t h e precursor and eppecialy f o r t h e d i s p e r s i t y o f n i c k e l . A d d i t i o n a l data on t h e s t a t e o f n i c k e l was obtained by DR: spectroscopy. On F i g . 2 a r e presented DRS spectra o f samples PO and PO. The spectra o f phosphorus c o n t a i n i n g samples. obtained by co-impregnation, a r e s i m i l a r t o t h e spectrum o f sample PO. The d i f f e r e n c e s a r e o n l y i n t h e r e l a t i v e i n t e n s i t i e s o f the
.
1910 bands. The bands a t 630,580nm (PO*) and t h e s h o u l g y r at 640nm f o r a l l o t h e r samples i n d i c a t e t h e presence o f N i (Td). 2 , w h i l e t h e bands a t 72 0 , 460(sh). $?Onm a r e c h a r a c t e r i s t i c o f N i in Ni-W-0-4: s p e c i p s . where N i i s octahedrally coordinated and b o t h A1 a r e p r e s e n t i n t h e second c o o r d i n a t i o n sphere and W
'
C8.91. The r a t i o between t h e Kubelka- Munk f u n c t i o n v a l u e s a t 720nm and 640nm shows q u a l i t a t i v e l y t h e change o f NS(Oh)/Ni(Td) r a z i o vs. phosphorus c o n t e n t ( T a b l e 1 ) . T h i s v a l u e f o r sample PO i s much lower t h a n t h o s e o f samples PO - P7.8. Obviously. t h e co-impregnation increases t h e amount o f N i ( 0 h ) in p o l y t u n g s t a t e s t r u c t u r e s . which i s c l e a r l y m a n i f e s t e d i n t h e d i c f e r e n t i a l spectrum o f sample PO r e l a t i v e t o t h a t o f sample PO ( F i g . 2 ) . The a d d i t f o n o f phosphorus i n c r e a s e s t h i s tendency and p r e v e n t s t h e i n c o r p o r a t i o n o f n i c k e l i n t e t r a h e d r a l sites. The maximal amount o f N i ( 0 h ) i s observed f o r sample P2.5. F(R760)/ However. a t h i g h e r phosphorus c o n c e n t r a t i o n s the F(R64o) r a t i o remains c o n s t a n t . which s u p p o r t s t h e assumption. t h a t t h e amount o f N i i n t h e Ni-W-0-A1 phase i s l i m i t t e d C8.91.
Acknoledgements The a u t h o r s express t h e i r g r a t i t u d e t o P r o f . J . U c h y t i 1 f o r t h e p r e p a r a t i o n o f t h e samples.
3.
M.Kraus
and D r .
REFERENCES
1. R. Lopez Cordero, N . E s q u i v e l , Y . Lazaro. I.L.G. F i e r r o and A. Lopez Agudo. Appl. C a t a l . . 48 (1989) 341 2 . P.J. Mangnus. J . A . R . Van Veen. S . E i j s b o u t s . V.H.J. De Beer and J.A.Moulijn. Appl. C a t a l . . 61 (1990) 99 3. P . Atanasova, Y . U c h y t i l . M. Kraus and T. Halachev. Appl. C a t a l . . 65 (1990) 53 4. Y . Walendziewski. React. K i n e t . C a t a l . L e t t . . 43 (1991) 107 Petrov. A. Andreev. G. 5 . P. Atanasova. T. Halachev. i n L . Kadinov (eds.). Proceed. Seventh I n t . Symp.. Bourgas. 1991. P a r t I. p. 247 6 . L. S a l v a t i . L.E. Makovsky. Y.M. S t e n c e l . F.R. Brown and D.M. H e r c u l e s . J. Phys. Chem.. 85 (1981) 3700 7 . D. O u a f i . F . Mange. J . C . L a v a l l e y . E. Payen. S. Kasztelan. M. H o u a r i . J. G r i m b l o t and J.P. B o n n e l l e . Catal. Today, 4 (1988) 23 and J.A. Moulijn. J. Phys. 8 . B. S c h e f f e r . J.J. H e i j e i n g a Chem.. 91 (1987) 4752 Catal.. 46 9 . B . S c h e f f e r . P. Molhoek and J.A. MouliJn. Appl. (1989) 11
G m i , L et al. (Editors), New Frontiers in Catalysb Proceedingsof the 10th International Congress on Catalysis, 19-24July, 1992, Budapest, Hungaty Q 1993 Elsevier Science Publishers B.V. All rights reserved
CONCERTED MECHANISM OF THIOPHENE HYDROGENOLYSIS BY SULFIDE EIDS CATALYSTS A. N. Startsev
Institute of Catalysis, Russian Academy of Sciences, Siberian Division, Novosibirsk 630090, Russia
Abstract Hydrogenolysis of C-S-bond on sulfide HDS catalysts is supposed to occur in coordination sphere of sulfide bimetallic species (SBMS)- active component of HDS catalysts- without participation in catalytic cycle of structure-forming sulphur atoms. Ni (or Co) in composition of SBMS is the site of adsorption of thiophene molecule and H activation occurs on Mo (or W). High rate of catalytic reactign is provided by synchronous interaction of reacting molecules according to concerted mechanism. 1. INTRODUCTION A large number of researches is devoted to study of mechanism of the reaction of C-S-bond hydrogenolysis because of practical importance of hydrodesulphurization (HDS) catalysts. There are
several hypothesis on the nature of these catalysts action, but all the mechanisms are based on two basic theses: 1. hydrogenolysis of S-containing molecule is considered as a multistep redox process which occurs on anion vacancies of MoS or "Co-MoS-phase"; 2. active component is supposed to be Mos, (WS ) and Co(Ni) plays the role of promotors. But study of thk mec?mnism of thiophene hydrogenolysis on well-characterized highly active catalysts prepared via metalcomplex precursors [ I 1 allows us to put forward the new mechanism of HDS catalysts action [2,31.
2. STRUCTURE OF S W AND PRINCIPLE OF ELECTRONEUTRALITY In recent years it was shown that the active component of HDS catalysts has a structure of MoS2 single slab with Co(Ni) localized on its periphery [2-61. The structure of MoS2 single slab was analized on the basis
1912
of principle of electroneutrality which might be formulated as follows: in (MoS ) macromolecule of any size @d fowstoichiometry S:Mo=2 as2dll as oxidation state of Mo and S' have to be conserved. In other words, the sum of ( t ) charges on Mo atoms must be equal to the sum of ( - ) charges on S atoms and valence bonds broken on the periphery of a single slab must be compensated by correspondent number of opposite charges.
It can be shown, that in electroneutral macromolecule the single slab of MoS has to be restricted by planes (0110) and (3030) which conta3n coordinative unsaturated atoms of S and Mo respectively (fig.1) and so they carry the excess of ( - ) and (t) charges [71. However, the macromolecule as a whole has small surplus of electrons which can be compensated by protons.
0-s
Fig. 1. Top view of a MoS, single slab. L
O- Mo~w Similarly, instead
@ - N ~ , c ~of terminal Mo atoms one may take Ni or Co. But it is wellknown, that all complexes of Ni and most of Co with chelate S-containing ligands have a square-planar surrounding (see e.g. [81). Therefore, the most likely position for its location is the center of sauare formed by-S'atoms in plane (1010) (fig.1) However, to obtain electroneutral macromolecule one has to suppose formal positive charge on these atoms to be equal 4t. Indeed, according to XPS data [3,91 the binding energy of electrons on Ni2p and Co2p / levels in bimetallic sulphide catalysts is sh?f?ed by 1 e3 in comparison with NiS and Co S That means Ni(Co) atoms in composition of SBMS carry an 3d&tional ( t ) charge and their formal oxidation state can be 4t. Proceed from the structure of MoS it can be shown-by simple calculation that when Ni(Co) is locited in plane (1010) (fig.1) omic distances arise: Ni(Co)-S = 2.2 2 and Ni(Co)-Mo Namely these values were obtained by EXA.FS on studying sulphide (Ni,W)/S'O catalysts [51. But in [61 the distance of Ni(Co)-Mo=2.8 i s well as square-pyramidal surroundirq of Ni(Co) were found . This phenomenon is lmown in coordination chemistry: after adsorption of donor molecule squareplanar complexes transform into square-pyramidal and central atom goes out of square plane [lo].
.
4
'
1913
3. PARTICIPATION OF ANION VACANCIES IN CATALYTIC CYCLE The rate of H35S heteroexchange with the surface of sulphide Mo and W catalygts does not diminish [ I 1 3 and the amount of S removed at thermo-programmed reduction does not change significantly [ 3 1 after addition of Ni to these catalysts. Since the rate of thiophene hydrogenolysis is increased 15-60 times one may conclude that anion vacancies seem not to participate in catalytic cycle. By calculation in terms of interacting bond method (IBM) of enthalpy of reaction of surface S atoms with H2 and of thiophene adsorption on a vacancy formed [I21 it was found that electroneutral molecule of both MoS2(WS2) and SBMS are thermodynamically stable and anion vacancies are not formed in catalytic cycle. 4.
“DISTRIBUTION OF ROLES” OF METALS OF S W
Desulphurization of thiophene occurs on Ni Raney under very mild conditions [I31 if there is H dissolved in it. Similarly, hydrogenolysis of C-S-bond is medigted by some Ni complexes if there is a source of activated H [141. NiS is not active in reactions that require H activatign [151. Vice versa, MoS is active in hydrogenatiog of unsaturated molecules [ I 5 1 an8 H activation occurs very easy on some Mo complexes [I61. Thergfore, one may conclude that Ni(or Co) in composition of SBMS is the site of thiophene, but Mo(or W)- of H2 activation. 5. CONCERTED MECHANISM OF THIOPHWE HYDROGWOLYSIS Distinguished features of thiophene hydrogenolysis on highly active catalysts of metal complex origin are: 1 ) absence of possible intermediates am0 the reaction products and 2 ) low value of activation energyY”l4 kCal/mol ) [ 3 1 Summarizing everything said above one may suggest the following mechanism of thiophene hydrogenolysis on SBMS (fig.2). Adsorption of thiophene, which has an unshared electron pair on S atom, is carried out on the electron deficient Ni(Co) atom and it is coming out of the square of sulphur atoms. Simultaneously H activation occurs on two neighbouring Mo(W) atoms. The drivpng force o_f H2 adsorption is a slightly ( - ) charge on S atoms in plane (0110)restricting SBMS. H2 activation probably occurs independently of but simultaneously with the stage of thiophene activation and proceeds continuously as activated H2 is consumed for the hydrogenation of the thiophene ring. The calculations in terms of IBM [I71 have shown that the heat of adsorption of H S, thiophene and tetrahydrothioyhene
.
3
1914
Fig. 2. Concerted mechanism of thiophene hydrogenolysis on SBMS. (THT) on SBMS is varied within 12, 22 and 3b kCal/mol respectively. The small values of AH confirm essential reversibility of H,S adsorption in the reaction conditions and the possibility of adsorbed H,S being supplanted by thYophene. The high AH for THT reflects the practical irreversibility of adsorption of the latter. This also means @ Ni,Co Mo,W that the only way to complete the catalytic cycle i.s the rupture of C-S-bond with the desorption of reaction products (butenes, butane, H2S) in gas phase.
@
6. REFERWCES 1. Yu.I.Yermakov, Usp. Chim., 40 (1986) 499 (in Russian). 2. A.N.Startsev, V.A.Burmistrov and Yu.I.Yermakov, Appl. Catal., 45 (1988)191. 3. A.N.Startsev, Kinet. Katal., 31 (1990)869 (in Russian). 4. H.Topsoe, B.S.Clausen, R.Candia, C.Wiwe1 and Morup, Bull.
Soc.Chim.Belg., 90 (1981) 1189.
5. D.I.Kochubey, M.A.Kozlov, K.I.Zamaraev, V.A.Bmistrov A.N.Startsev and Yu.I.Yermakov, Appl.Catal., 14 (1985) 1. 6. S.M.A.M.Bouwens, D.S.Koningsberger, V.H.J.de Beer, S.P.A.
Louwers and R.Prins, Catal. Lett.,5 (1990)273. 7.P.Ratnasamy and S.Sivasanker,Catal.Rev.-Sci.Etg.,22(1980)401. 8.R.Eisenberg, Progr. Inorg. Chem., 12 (1970) 295. g.A.P.Shepelin, P.A.Zhdan, V.A.Burmistrov, A.N.Startsev and Yu.I.Yermakov, Appl.Catal., 1 1 (1984)29. lO.R.R.Holms, Progr. Inorg. Chem., 32 (1984) 119 ll.A.N.Startsev, E.V.Artamonov and Yu.I.Yermakov, Appl.Catal., 45 (1988) 183. 12.Yu.I.Yermakov, A.N.Startsev, V.A.Burmistrov, 0.N.Shumilo and N.N.Bulgakov, Appl.Catal., I8 (1985) 3 3 . 13.A.I.Meyers, Heterocycles in Organic Synthesis, Wiley Interscience, New York, 1974. 14.J.J.Eisch. L.E.Hallenbeck and K.I.Han, J.Am.Chem.Soc., 108 (1986)7763. 15.K.Tanaka and T.Olnrchara, Catal.Rev.-Sci.Eng. ,I5 (1977)249. 16.C.J.Casewit and M.R. DuBois, J.Am.Shem.Soc.,l08 (1986)5482. 17.N.N.Bulgakov and A.N.Startsev, Mendeleev Comm., 1 (1991) 97. 4
Guni, L ei d.(Editors), New Fronriers in Catalysh Proceedings of the 10th Inlemational Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights =served
HIGHLY ACTIVE Ni-W/A1203CATALYST FOR UPGRADING UNCONVENTIONAL FEEDSTOCKS
H. Shimadd, T. Kameokaq H. Yanaseb, M. Watanabeb,A. Kinoshitab, T. Sat&, Y. Yoshimuraa,N. MastubayashiO and A. Nishijirn& aNational Chemical Laboratory for Industry, Tsukuba, Ibaraki 305,Japan bCatalysts & Chemicals Industries Co.,Ltd., Kitaminato, Wakamatsu, Kitakyushu 808, Japan
Abstract A highly active Ni-W/AI,O, catalyst was developed to upgrade coal-derived liquids. Compared to conventional Ni-Mo catalysts, the Ni-W catalyst was found to have higher hydrogenation and lower hydrocracking activities. Because of these properties, the Ni-W catalyst showed high hydrodenitrogenation activity and slow deactivation for feedstocks with a large amount of catalyst poisons. 1. INTRODUCTION Establishment of an efficient and economical upgrading process is essential for the utilization of unconventional feedstocks as transportation or furnace fuels. Catalytic technology is the most important technology for improving the upgrading processes. During the course of a study on the upgrading of coal-derived liquids, the authors proposed a two-stage catalytic process [l]. Hydrogenation (HYD) and hydrodenitrogenation (HDN) are promoted over NiMo/A1,0, catalysts in the first stage and hydrocracking (HCK) is catalyzed over zeolite-based Ni-Mo catalysts in the second stage. The primary objective of the present study has been to develop a better catalyst for the first stage.
2. EXPERIMENTAL Feedstocks used in the present study were Wandoan coal-derived kerosene (W-KR, 155228"C, C; 86.5%, H; 10.4%, N ; 0.46%, S; 0.05%, 0; 2.6%), Battelriver coal-derived kerosene (BR-KR 177-240°C, C; 84.8%, H; 10.0%, N; 0.57%, S ; 0.016%, 0; 4.6%) and gas oil (BR-GO, 250-342"C, C; 89.2%, H; 9.8%, N; 0.57%, S ; 0.018%, 0; 0.4%) fractions. The support used was y-Al,03 extrudate with a diameter of 1/32 inch (surface area, 209 m'g.'; pore volume, 0.81 cm3g-'). Ni-Mo, Co-Mo, and Ni-W catalysts were prepared by impregnation. A commercial Ni-Mo/Al,O, catalyst that showed the best performance in upgrading among several commercial catalysts tested was chosen as the reference catalyst. Catalyst activity tests were performed using a continuous flow reactor with a catalyst volume of 15 cm3. Model test reactions were carried out to evaluate fundamental activities of catalysts. Detailed procedures of the reactions were described in a previous report [2].
1916 3. RESULTS AND DISCUSSION 3.1 Catalyst screening Figure 1 shows some of the results from preliminary catalyst screening tests of Ni-Mo and Ni-W catalysts. This shows that some of the prepared catalysts were more active than the reference Ni-Mo catalyst. A Ni-W catalyst with 4.2% of M O and 29% of WO, was found to be the most active both in HYD and HDN of the kerosene fraction. All of Co-Mo prepared catalysts showed comparatively low activities. Many other oxide supports, including double oxides, were investigated. The activity, however, was relatively poor. Comparison of the aging behavior of the Ni-W catalyst with that of the reference catalyst is shown in Fig. 2(A). The Ni-W catalyst showed a better initial HDN activity and less deactivation than the reference catalyst in hydrotreating the kerosene fraction. A life test of the Ni-W catalyst was also performed using the gas oil fraction as a feedstock (Fig. 2(B)). Nitrogen removal was stable for 950 h, although it slightly decreased at the beginning of the test, In contrast, the HYD activity, which was indexed by the H/C atomic ratio of the product, decreased gradually through the whole life test. HYD activity was a more sensitive index of the catalyst deactivation than the HDN activity in the present system. Several Ni-Mo catalysts, including the reference catalyst, were examined for upgrading the gas oil fraction. The initial decrease in the HDN activity, however, was much more rapid than the Ni-W catalyst. A detailed report on the screening will be published somewhere else.
100 90 n
€R 80
+** + Ref.
W
z n
I70
r
0 100 200 300 400 500 600 1.8 100 I
*
60
o 50
5
10
a
Ni-W 0 Ni-Mo
15 20 H/C ( X )
0
1.5
200 400 600 800 1000 Time on Stream (h)
Figure 2. Short term catalyst aging tests Figure 1. Catalyst screening tests for upgrading BR-KR. (A) Comparison of Ni-W and Ni-Mo catalysts AH/C( %) = ~OOX[(H/C),-(H/C)J/(H/C),Feed: W-KR, T=643 K , P=13.2 MPa, SV=2.0 h', H,/Oil=lOOO Nlll (HIC),: atomic H/C ratio of product (H/C); atomic H/C ratio of feedstocks (B) Change in HDN and HYD activities of Ni-W Catalyst Feed: BR-GO, T=653 K, Feed: BR.-KR, T=633 K, P=6.9 MPa, P=11.7 MPa, SV=2.0 h ' , H,/Oil=lOOO NI/I SV=2.0 h', H,/Oil =lo00 NU1
1917
3.2 Catalytic properties of the Ni-W catalyst Catalyst characterization was done to elucidate the difference of the properties between NiW and Ni-Mo catalysts. As a general trend obtained by model test reactions, Ni-W catalysts have higher HYD and lower HCK activities than Ni-Mo catalysts. In the HDN reaction of quinoline (Table l), the Ni-W was superior to the Ni-Mo catalyst in the opening of the heterocyclic ring to yield amino compounds. In contrast, the Ni-Mo catalyst yielded more denitrogenated compounds which were obtained only after cleavage of the C-NH,. With increasing initial H, pressure, the yield of amino compounds increased, with only small changes in the amount of denitrogenated compounds, from both of the catalysts, particularly from the Ni-W catalyst. This indicates that H, is involved in the first step of the C-N cleavage, that is, the opening the heterocyclic ring. On the other hand, the yield of denitrogenated compounds was affected primarily by the reaction temperature. HYD active sites are involved in the first step of the denitrogenation, while the final denitrogenation requires HCK active sites with acidity. The result in Table 1 indicates that the Ni-W catalyst has mild function for the cleavage of C-N bonding as compared to the Ni-Mo catalyst. It has been known that the HYD of the aromatic rings should precede the cleavage of C-N bonding [3]. Thus, HDN reactions require both HYD and HCK active sites. The optimal balance of HYD and HCK depends on the properties of feedstocks. In the case of pure quinoline, HYD of the aromatic rings readily proceeds on the HYD sites when the ratedetermining step for HDN is C-NH, bond cleavage. For coal liquids with high aromaticity, on the other hand, the total HDN reaction rate primarily depends on the HYD steps, because a large amount of aromatic compounds in the feedstock results in competitive adsorption on the HYD active sites. It should also be noted that the feedstocks used in the present study contained a large quantity of nitrogen-containing materials but much less sulfur-containing materials than petroleum fractions. It is very likely that the activities of Ni-W catalysts are enhanced by low H,S concentration in the reaction environment. Gachet et al. [4] reported that a Ni-W catalyst with high loadings of Ni and W was highly active in HDN of a coker gas oil fraction. The properties of the present feedstocks intensified the superiority of Ni-W catalysts with Table 1 Product distribution in hydrodenitrogenation of quinoline over Ni-Mo and Ni-W catalysts
613
Temp.(K)'
653
~
4.9
P. (MPa)' Product3
HY
NH, DN
Ni-Mo/Al,O, 74.4 18.4 7.2 Ni-W/AI,03 67.4 28.9 3.7
8.8 HY
NH, DN
66.3 26.1 7.6 55.4 40.9 3.7
9.8
4.9 HY
NH, DN
39.0 15.5 45.5 20.0 31.3 42.7
HY
NH2 DN
24.7 23.1 52.2 5.7 52.3 42.0
': Reaction temperature, ,: Initial H, pressure, ': Product distribution; HY =hydrogenated quinolines, NH, =with -NH, group, DN =denitrogenated hydrocarbons, Reaction was carried out using a batch-type reactor [2]. Conversion of quinoline was 100% for all reactions.
1918 conventional loadings. In conclusion, all the above discussions show that the active sites of Ni-W catalysts, which have more HYD and less HCK function as comparing to Ni-Mo catalysts, are suitable for HDN of coal-derived liquids.
3.3 Deactivation aspects Carbonaceous deposition on the used catalysts for which the upgrading results are shown in Fig. 2(A) was determined by a CHN analyzer. The reference catalyst had 5.6 wt% of carbon with a N/C atomic ratio of 0.073, while the Ni-W catalyst had 5.65 wt% of carbon with a N/C ratio of 0.046. There was no difference in the total amount of carbonaceous deposit, however, less nitrogen-containing materials were deposited on the Ni-W catalyst. This indicates that fewer active sites on the Ni-W catalyst were poisoned by basic compounds, resulting in the slower deactivation profile shown in Fig. 2(A). The reason for less nitrogencontaining materials on the Ni-W catalyst is related to the catalytic properties described in the previous session. Thus, it can be concluded that the combination of high HYD and low HCK activities, which gave a mild HDN activity, resulted in the high initial activity and slow deactivation of the Ni-W catalyst. Although the HDN activity was very stable, a gradual decline was observed in the HYD activity as shown in Fig. 2(B). An X-ray photoelectron spectroscopic study of the used catalyst suggested that the surface concentration of Ni was significantly decreased during the reaction. This indicates the removal of Ni species from Ni-W sulfide structures. Also, transmission electron microscopic observation indicated growth of WS, slabs during the life test. These changes were more pronounced than those observed for Ni-Mo catalysts. Further improvement of Ni-W catalysts can be achieved by suppressing these structural changes during the reaction. 4. ACKNOWLEDGEMENT
This work was partly supported by NED0 (New Energy and Industrial Technology Development Organization, Japan).
5 . REFERENCES
1 H. Shimada, T. Sato, Y. Yoshimura, A. Hinata, S. Yoshitomi, A. C. Mares and A. Nishijinia, Fuel Processing Technology, 25 (1990) 153. 2 H. Shimada, T. Sato, Y. Yoshimura, J. Hiraishi and A. Nishijima, J. Catal., 110, (1988) 175. 3 C. N . Satterfield, M. Modell, R. A. Hites and C. J. Declerck, Ind. Eng. Chem. Process Des. Dev., 17 141 (1978). 4 C. Gachet, M. Breysse, M . Cattenot, T. Decamp, R. Frety, M . Lacroix, L. de Mourgues, J . L. Portefaix, M. Vrinat, J. C. Duchet, S. Housni, M. Lakhdar, M. J. Tilliette, J. Bachelier, D. Cornet, P. Engelhard, C. Gueguen and H. Toulhoat, Catalysis Today, 4 (1988) 7.
Guczi, L e% al. (Editors), New Frontiers in Cofalysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishen B.V. All rights m e w e d
ADSORITION AND ACTIVATION OF THIOPHENE ON MoS,, CO&& AND Ru% C. Rong and X. Qin Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Abstract Quantum chemical calculations for model clusters of the adsorption systems have been done using the DV-Xa method. The properties of the adsorption center have been elucidated on the basis of discussing the activation pattern of thiophene.
Introduction As hydrotreating processes become more and more important in petroleum refining, more effective catalysts need to be developed. Thus the origin of the catalytic activity and the promotion effect of hydrotreating catalysts attract much attention of a great many authors. In the quantum chemical calculations of Chianelli et. a1 [11, the electronic factors are related to catalytical activity in the case of hydrodesulfurization(HDS). In the work of Zonnevylle et. a1 [2], the adsorption center on MoSz is thoroughly studied. In order to elucidate the detailed electronic and bonding properties of the adsorption systems for thiophene on MoSz, Co9Ss and RuSz, we have done some quantum chemical calculations for model clusters which represent the sulfide catalysts and their adsorption systems [3,4]. In this paper some of the properties of the adsorption center will be elucidated on the basis of discussing the activation pattern of thiophene on these three sulfides.
Methods and models The method used is the DV-Xa method in the form of SCC procedureC51. The model clusters of the substrates are depicted in Fig. 1. Fig. 1A is a model cluster of Mo3SI4,Removing the S at the origin, it becomes M03S13 with one vacancy. If thiophene is adsorbed perpendicularly with its sulfur S(T) at the origin and its ring in the yz plane, then it is the Mo3SI3-thiophene model ( M l ) . If there are only two S in the y axis left in the xy plane and thiophene is adsorbed like that in M1, then it is the M03S9 - thiophene model (M2). If the upper three S in the prism containing Mo'(the superscript represents the sequential number of metal atoms in the figure hereafter) is removed from Fig. l A , and thiophene is 12" inclined to the xy plane with S(T) at the origin and its ring above Mo' , then it is the M03S11 -thiophene model (M3).
1920 Fig. 1B is a model cluster of Cogsl4. Removing the topmost S it become C0nSl3 with one vacancy. If thiophene is adsorbed with its S ( T ) at the vacancy and the system possesses a symmetry of CzV,then it is the Cogs13 -thiophene model ( M 4 ) . If the five S above xy plane are removed, it becomes Co8Sg. When thiophene is adsorbed on Cons9 like that in M4, it is the Cogs9 - thiophene ( A ) model (M5). If thiophene is adsorbed parallel to the xy plane with appropriate distance it is the Co8S9-thiophene(B) model (M6). 2
--X
LJ
0
s
o
Mo
0 0
co
o
s
C
Ru
Fig. 1 Pictorial view of the model clusters for the substrates Fig. 1C is a model cluster of Ru& which contains 8 vacancies of S. Removing the left atom of the S pair positioned at the center of this figure, it becomes Ru4Sllwith an additional vacancy. If thiophene is 45"inclined to the xy plane with its S ( T ) at this additional vacancy, it is the Ru,$ll-thiophene model as a model in which Ru2 ,Ru3 and 1C (M9). If we take half of the atoms in Fig. the central S pair is contained, then it is a model of RuzS?. If thiophene is adsorbed parallelly to xy plane with S(T) at a vacancy above and near Ru' and its ring is symmetry with respect to the line Ru2Ru3, then it is the Ru& -thiophene model (M8). If three of the four vacancies in Ru2S7are supplemented with S and the thiophene is adsorbed perpendicularly to the xy plane with S ( T ) positioned at the remained one then it is the RuzSll-thiophene model (M7).
Results and discussion Various bond orders for all the models are listed in Table 1. In this table 6A and 6B represent respectively the change of the C (a) - S (T)and C ( u ) - C (0) bond orders after adsorption, in which positive value means increase and the bonding is strengthened while negative value means decrease and the bonding is activated. MA-S(T) or MB-S(T) is the bond order between metal atoms and in M6, in M4 and M5, S ( T ) , in which MA stands for Mo' in MoS2, Ru2 in M7 and M8 or Ru" in M9, MB stands for MoZv3 in MoS2, CO'-' in M6 or Ru2 in M9. MC-C(u) and MD-C(P) are the bond orders between metal atoms
1921
and C(a> , C(p) respectively, in which MC stands for Mo' in.M3, Co5-* in M6 in M9, MD stands for Mo' in M3, in M5 or Co5*'in M6. The and calculated results in Table 1 indicate that : Table 1
M1 M2 M3 M4 M5 M6 M7 M8 M9
Various bond orders for several atom Dairs of all the models
o 0.024 0.015 8-0.183 0.150 0-0.041 0.043 n-0.185 0.098 0-0.252 0.015 n-0. 149 -0.141 o 0.018 0.194 n 0.045 -0.014
-0.041 0.046 0.061 -0.013 -0.007 0.153 -0.020 0.010 -0.073 -0.082 -0.018 0.030 -0.034 0.000 0.020 0.000
0-0.131 0.129 n-0.168 0.042 o 0.062 0.309 n-0.189 -0.335 o 0.118 0.047 n-0. 046 0.050 o 0.099 0.004 n-0.314 -0.355 0-0.015 0.306 n-0.116 -0.306
-0.006 -0.040 0.151 0.051 -0.359 0.004 0.167 0.077
-0.029 0.198 -0.031 0.038 -0.028 0. 082 0.030 -0.015 -0.043 0.048 -0.099 -0.003 -0.392 0.017 0.045 0.025 0.096 -0.048
0.025 0.021
-0.060 0. 124
0.031 0.006 -0.081 -0.003 0.000 -1.344 -0.043 -1.482 -0.053 0.000 0.174 0.135 0.103 0. 163
0.007 0.019 -0.025 -0.003 -0.001 0.016 0.064 0.084 -0.029 -0.190 0.042 -0.027 0.016 0.027 -0.030 -0.008
-0.089 -0.867
0. 017 0.105
1. If thiophene is adsorbed on a single vacancy it must stand perpendicularly to the surface, since there is no free space to accommodate the thiophene lying parallelly or inclined to the substrate surface, such as for the M1, M4 and M7 models. For the cases of M4 and M7, no adsorption occurs as indicated by the bond order values of M-S(T). At the same time, the bonding of C(a)-S(T) and C(a) -C(p) are strengthened due to the overlap of the orbitals of the adsorbate and the substrate. As for the case of M1, the u orbitals of thiophene and the d orbitals of Mo are mainly involved in the adsorptive bonding, and this bonding is of considerable strength. The n bonding for C(a) --S(T) is activated while the u bonding is slightly strengthened, as a whole C ( a ) -S(T) is activated. At the same time it is striking that the C ( a ) - S (T) is activated at the expense of strengthening the C(a) -C(p) bonding. 2. If thiophene is adsorbed on multiple vacancies, it may stand perpendicularly, lie inclined or parallelly to the surface. For the case of M2, the thiophene is adsorbed perpendicularly to the surface. Comparing the bond orders with that of M1, we find that the u orbitals of thiophene and the d orbitals of Mo are still mainly involved in the adsorptive bonding, and the bonding strength is comparable to that of M1. At the same time C(a>-S(T) is activated at the expense of
1922 strengthening the C(a>-C(P) bonding as well. But both the (J and n bonding of C ( u )-S(T) are activated in the case of M 2 , therefore C ( a > -S(T) is activated to a large extent than that for the case of M1. Similarly thiophene is adsorbed perpendicularly for the case of M5. In this case S ( T ) is directly interacted with four Co atoms, the n orbitals of thiophene and all the s , p and d orbitals of Co are mainly involved in the adsorptive bonding. The C ( a > - S ( T ) bond is much more activated than that of M1. The activation of u orbitals is more effective than that for the case of M2. At the same time C ( a >S ( T ) is still activated at the expense of strengthening the C ( u ) -C(P) bonding. 3. For the thiophene lying inclined or parallelly to the surface on the multiple vacancies, such as for the cases of M3, M6, M8 and M 9 , some characteristics are exhibited: 1) The n orbitals of thiophene and s , p and d orbitals of metal are mainly involved in the adsorptive bonding. 2 ) All the S ( T ) , C ( u ) and C(P) may be involved in the adsorptive bonding. 3) Both the C ( a > - S ( T ) and C ( u ) C(p> are activated simultanously, at least C(u) - C(P) is not strengthened in the case for M9 in which thiophene is 45" inclined to the RuSz surface. 4) There may be another interaction of substrate and thiophene such as for the case of M 8 in which C ( u ) is strongly interacted with a surface S , and this interaction is mainly of n character. Thus the n bonding of C(u> - S ( T ) is further weakened considerably. It seems that multiple vacancies of S formed by removing the surface SH groups may afford sufficient free space to accommodate the lying flat thiophene and then the metal atoms may interact with the thiophene molecule by using their s , p and d orbitals. Thus the metal atoms with multiple vacancies may act as a favourable adsorption center. As proposed by I). Delmon and J. L. Dallons [6] that the surface SH groups which may act with C ( u ) at appropriate distance may promote the activation and further desulfurization of thiophene [7].
References 1. R. Chianelli, Catal. Rev. 26(1984)361; S. Harris and R. Chianelli, Chem. Phys. Lett. 101(1983)603; J. Catal. 86(1984)400. 2. M. C. Zonnevylle, R. Hoffman and S. Harris, Surf. Science, 1 9 9 ( 1 9 8 8 ) 320 3. Chen Rong and Xin Qin, J. Mol. Catal. 64(1991)321 4. Chen Rong And Xin Qin, Submitted to J. Mol. Catal. 5. A. Rosen, D. E. Ellis, H. Adachi and W. Averill, J. Chem. Phys. 65 (1976)3629 6. B. Delmon and J. L. Dallons, Bull. SOC.Chim. Belg. 97(1988)473 7. Chen Rong and Xin Qin, unpublished results.
Guczi, L el al. (Editors), New Frontiers in CatdysiF Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science. Publishers B.V. All rights resewed
CORRELATION OF €IDS ACTIVITY WITH HEAT OF ADSORPTION OVER CARBON SUPPORTED CoMo CATALYSTS
S.-K Ihm, Y.-H. Moon and C-D. Ihm Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, Korea
Abstract Temperature-programmed desorption (TPD) was carried out over the typical carbon-supported CoMo catalysts, and heat of adsorption for each of thiophene and hydrogensulfide was obtained from the T P D spectra with different heating rate. The activity for hydrodesulfurization HDS) of thiophene was measured in a microflow reactor at atmospheric pressure an 325 oC.It was demonstrated that the HDS activity was well correlated with the difference in heat of adsorption between thiophene and hydrogensulfide.
6
1. INTRODUCTION
Typical CoMo catalysts for HDS show synergy in reaction rate with varying the have been made in order to explain the synergistic metal composition. Man that the HDS activity can be maximum at the phenomenon. Chianelli optimum value of heat o Temperature-programmed desorption TPD) can be used to obtain the heat of adsorption for thiophene and hydro ensul de as demonstrated by Cvetanovic and Amenomiya [2] and Konvalinka et al. f3]. The present work is to report some experimental data for possible correlation between the HDS activity and the difference in heat of adsorption between thiophene and hydrogensulfide.
I,
2. EXPERIMENTAL The CoMo/C catalysts were prepared by incipient wetness technique. Ammoniumheptamolybdate and cobalt nitrate were co-impregnated on activated carbon support. The surface area and pore volume of activated carbon were 580 mz/g and 0.82 cm3/g respectively. The catalysts were presulfided with mixture of hydrogen
1924
and hydrogensulfide at 350 oC. The sulfided catalysts were used for T P D measurements metals was fixed a t 4.28 as well as thiophene HDS reaction. The total loadin was controlled to be x 1020 atom/gcai, and the mole fraction of Co, {I= 0, 0.14, 0.27, 0.5, 0.72, 0.85, and 1.0 respectively. was measured in a microflowreactor under atmospheric pressure at 325 oC. T P D of thiophene and hydrogensulfide over CoMo/C catalysts were carried out with constant helium flow and the desorbed amount was measured with thermal conductivity detector.
3. RESULTS AND DISCUSSION The intensities of TPD spectra from metallic phase were obtained by subtracting the contribution of carbon support from the spectra of CoMo/C catalysts. The heat of adsorption (AH was evaluated by the usual equation first proposed by Cvetanovic and Amenomiya [2 .
1
In ('I'm'/@) = (AH)/RTm
+C
where Tm is the temperature at maximum in TPD spectrum and p i s the linear heating rate (T=To+Pt), and C is a constant depending on the experimental condition (e.g., flow rate of carrier gas).
0
200 300 Temperature (0C)
100
400
Fi . 1-a. Thiophene TPD spectra with dikerent heating rate (r=0.5, p ; a=lOoC/min b=150C/min1 C=2OoC/min, d=250C/min).
0
200 300 Temperature (0C)
100
400
Fig. 1-b. Hydrogensulfide TPD spectra with different heating rate (r=0.5, /3 ; a=lOoC/min, b=15oC/min1 c=200C/min, d=250C/min).
Typical T P D spectra of thiophene and hydrogensulfide for r=0.5 were shown in Fig. 1-a. and 1-b. respectively. The spectra for hydrogensulfide are bimodal, and the main peak at a lower temperature around 100 OC which seems to be due to physical adsorption was not considered for obtaining the heat of adsorption.
1925
It can be seen that the temperature of maximum (Tm) is shifted t o a higher value with increasin the heating rate (p). The shift of Tm was measured as a function of for different meta compositions. The heats of adsorption were obtained from the slopes of the lots, 1n(Tm2/p) vs. 1/Tm as shown in Fig. 2. Similar analysis was made for hydrogensul de.
P
I:
10.2 r 10.0 9.8 @a 2 9.6 h
-G CJ
-
8.4
-
9.2
-
M
rb: r = Oc: r = od: r = Ae: r = Of: r =
-
0.14 0.27 0.50 0.72 0.85
Y
1.85
1.93 2.01 2.09 2.17 l/Tm x l o 3
Fig. 2. ln(Tmp/p)vs. l/Tm of thiophene.
The catalytic activity of thiophene HDS is proportional to the partial pressure of thiophene and inhibited by hydrogensulfide as shown in Fig. 3 and Fig.4 respectively. h
-
P H ~ 0.75 atm Pn.- 0.075 atm
PM.= balance
\
P T ~ I =1.0 atm 0
fi3
3 ‘S
2
0
0.04
0.08
0.12
0.16
Thiophene partial pressure (atm) Fig. 3. Effect of partial pressure of thiophene on HDS reaction rate (Temp.=3250C, W/F=234465 gcat .min/mol, r=0.5).
2
0
0.02 0.04 0.06 0.08 0.1
Hydrogensulfide partial pressure (atm) Fi 4. Effect of partial pressure of %ydrogensulfide on HDS reaction rate (Temp.=3250C, W/F=234465 gcat min/mole, r=0.5).
-
1926
This suggests that the difference in heat of adsorption between thiophene and hydrogensulfide be possibly an important parameter for deciding the HDS activity. This can be demonstrated by overlapping the activity and the difference in heat of adsorption for different compositions of metal as in Fig. 5.
30 . .
2 0
U
-2 -4
10
-6 -8 -10
0
0
0.2
0.4
0.6
0.8
1
5 -X X
a I
h
H X
v
Q
v
r = Co/(Co+Mo) Fi . 5. Correlation of HDS activity with the d i lerence in heat of adsorption between thiophene and hydrogensulfide.
It can be seen that the synergy is obvious and that the correlation between the activity and the difference in heat of adsorption is excellent. This result seems to provide a clue to the explanation of the synergistic phenomena in HDS activity over CoMo C catalysts. Accordin ly it is recommended to pursue further investigation on the a sorption strength of t iophene and hydrogensulfide on the surface of CoMoS phase [4].
d
a
4. REFERENCES 1 R. R. Chianelli, Cat. Rev. Sci. Eng., 26 (1984) 361. 2 R. J . Cvetanovic and Y. Amenomiya, Cat. Rev. Sci. Eng., 6 (1972) 21. 3 J . A. Konvalinka, J. J. F. Scholten and J. C. Rasser, J. Catal. 48, (1977) 365. 4 H. Topsoe and B. S. Clausen, Appl. Catal., 25 (1986) 273.
Guni, L o ul. (Editors), New Fronriers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights nserved
EFFECT OF FLUORIDE ON THE SURFACE STRUCTUREOF WOS/AI~OJ HYDROTREATING CATALYSTS R. L. Corder@, J. R. Sohb, J. V. G. Ramosc, A. B. Patricwd and A. L. Aguabd
aCentro de Investigaciones Quimicas, Washington 169,La Habana, Cuba bDept. Ingenieria Quimica, Facultad de Quimica, UNAM, Mexico, D.F. 04510,Mexico %st0 de Optica "Daza de Valdes", CSIC, Serrano 121,28006 Madrid, Spain dInsto de Catalisis y Petroleoquimica, CSIC, Serrano 119,28006Madrid, Spain
Abstract Tungsta catalysts supported on fluorinated-aluminahave been characterized by XRD, diffuse reflectance spectroscopy (DRS), Raman spectroscopy,TPR and acidity measurements. Fluoride incorporation favoured the formation of polystungstates at the expense of the monotungstate species. Total acidity of the catalysts was lower than that of the supports and did not significantly vary with F content, except for the catalyst with 2,5 wt% F-.
INTRODUCTION Fluoride is one of the additives most currently used to enhance the acidity of many alumina-based catalysts, especially of molybdenum hydrotreating catalysts when hydrocracking functionality is also required. It has been shown in several studies that the incorporation of fluoride to promoted and non-promoted molybdenum catalysts can have a beneficial effect on HDS, HDN and hydrogenation activity, depending on fluoride loading and catalysts preparation. These effects seem to be mainly due to changes in the dispersion and distribution of the active metal phases, as well as to increases in acidity, but there is still no consensus on the F action (1-3, and references therein). Similar studies on tungsten catalysts have not been reported, despite their importance for hydrocracking. Only the effect of the isoelectric point (IEP) of F-modified alumina on adsorption of tungsten has been reported (4). In the present study, the effect of F content on the surface structure of tungsten oxide supported on alumina is examined. 1. EXPERIMENTAL 1.1. Catalyst preparation A series of WO3/A1,O3-F(x)catalysts containing a constant loading of 20.9wt% W03 and various amounts of fluoride (x=O, 0.2,0.8,1.5 and 2.5 wt% F) were prepared by sequential incipient wetness impregnation of y-Al,O, (Girdler T-126,surface area 190 mz g-') with NI&F and (NH4)&,W1& solutions. After each impregnation step the samples were dried in air at
1928
393 K overnight and then calcined at 773 K for 4.5 h. These catalysts are denoted W/Al-F(x). Additional F-free W03/Al,0, samples (loading 20 wt% WO,), denoted as W/Al-pH(x), were also prepared at three different impregnation pH values (1.5, 7 and 9) using a solution volume higher than the pore volume of the y-Al,O,. The pH was adjusted to the desired value with either HN03 or W O H . These samples were calcined at 823 K for 4.5 h.
1.2. Characterization techniques Details concerning the equipment and conditions used for specific surface area, XRD, TPR and acidity measurements have been reported elsewhere (2,5). Laser Raman spectra were recorded with a Jobin Yvon U-lo00 spectrometer using the 514,5 nm line of an Ar' laser (Spectra Physics, Mod 165) as exciting source. DRS were recorded in the range 220-400 nm with a Lambda 9 Perkin-Elmer spectrometer. The relative reflectance was calculated using the spectrum of the A1203 support in the cell as reference. 2. RESULTS AND DISCUSION
The surface areas of the W/A1-F(x) catalysts decreased slightly with increasing fluoride content, as also observed for similar fluorinated-alumina supported molybdena catalysts (13,7). The decrease was only about 7% for the 2.5 wt% F-containing catalyst as compared with the F-free countepart (Table 1). The XRD patterns of all the catalysts showed only the broad lines of the y-Alz03support, indicating that F incorporation did not appreciably modify the amorphous dispersion of tungsten oxide on the support. The Raman spectra of the calcined W/AI-F(x) samples maintained in air (Fig. 1) showed, in general, very similar spectral features, viz., a single broad band at 972-996 cm-', which species distorted by interaction with the alumina corresponds mainly to tetrahedral WO: support (8-10). This band shifted from 972 cm", for the fluoride-free catalyst to 996 cm-' for the F-richest catalyst. According to literature (7-9) this shift indicates that fluoride incorporation reduces the hydration of the surface tungstates and favours the formation of polytungstate species. Segregation of bulk W Q or A12(W04)3phases was, however, not evident since their major lines at 811 and 1046 cm-', respectively, were not observed. However, the strong fluorescence background of the spectra could swamp weak signals of these phases. 3.0 I F -Wt'/o 2.5 1.5 0.8 0.2
A
720 800 880 960 1040
I0
0 X Inm
A 9 cm-l
Fig. 1. Raman spectra of the calcined W/Al-F(x) catalysts.
Figure 2. DRS spectra of the W/Al-F(x) catalysts: (-.-), F(0); (---),F(0.2); (....), F(0.8); (-), F(1.5); and (.-), F(2.5).
1929
All catalysts showed also similar DRS spectra (Fig. 2), exhibiting a single band centered at about 257 cm'' which was attributed to wb+ in tetrahedral coordination (10). The position of the maximum of this band did not significantly change with the F content, but the band was shifted slightly toward the octahedrally coordinated Wb' region in some of the Fcontaining samples. This observation is in line with the LRS finding that F enhances the formation of polytungstates. Further information on the changes caused by fluoride incorporation on the surface distribution of tungsten was obtained by TPR. Fig. 3 shows the TPR profile of the W/Al-F(x) catalyst series. It is evident that the single peak at 1248 K of the F-free catalyst gradually decreased with increasing F content. Simultaneously,a new peak appeared at lower reduction temperature (1153-1233 K), overlapping the peak at 1248 K aid the reduction started at lower temperatures, suggesting that tungsten species easier to reduce were formed upon the incorporation of F.Table 1 shows that th total extent of reduction erived from total H2consumption of w+ to WO, clearly increased with fluoride content. trmprroturr K
4
d
trmprroturr K
Figure 3. TPR profiles of W/Al-F(x) ca- Figure 4. TPR profiles of W/Al-F(0) catalysts and bulk WO, (**'..). prepared at pH: (a) 1.5, (b) 7 and (c) 9. talysts (-) Table 1
This increase in reduction, which occurs at lower temperature at the expense of that at higher temperature, indicates that a fraction of the surface monomeric tungstates present in the F-free catalyst may become polytungstates when the F content is increased, as Raman spectra also indicated. The surface polytungstates are easier to reduce than the WO? species, since on average the wb+ ions of the former are surrounded by a lower number of A13+ions,
1930
and consequently they are less polarized. Consistently, bulk WO, was reduced at lower temperature than the W/Al-F(x) catalysts, as shown in Fig. 3. Note also that the TPR profiles of the catalysts prepared at different impregnation pH showed (Fig. 4) a significant shift (- 50 K) towards lower reduction temperatures when the pH decreased from 9 to 1.5. The easier reduction of the low-pH catalyst, which reflects the presence of polytungstates, is consistent with the known fact that in acidic solutions the octahedral polymeric species of tungstate predominate. In contrast, at basic pH the predominant species are tetrahedral WO;+ anions, and consequently in the W/A1 catalyst prepared at pH 9 the tungsten species are more difficult to reduce. The small reduction shoulder at ca. 1023 K exhibited by this catalyst may correspond to the tungstate fraction deposited during the drying which after calcination may become poorly dispersed as WQ-like species. Scheffer et al. (1 1) also found, for W0,/A1203catalysts, a higher reducibility with higher tungsten loadings or calcination temperatures, due to loss of dispersion. These changes in the distribution and nature of the tungsten surface species, induced by fluoride incorporation, are considered to result from several factors. One is the lower adsorption of tungstate species produced by the decrease in the IEP of the alumina by fluoridation, and another is the decrease in the number of alumina sites for tungstate adsorption, caused by the substitution of the surface OH groups by F, and by the concurrent loss of surface area. Total acidity of the catalysts expressed in mmol n-butylamine per m2 was almost constant with the increase in F, except for the 2.5 wt% F--containingcatalyst which increased slightly. Since the fluoridated-alumina showed relatively larger acidity values, it Seems that tungstate incorporation eliminates the acidic OH groups previously created by fluoridation of the A1,0,.
ACKNOWLEDGMENTS We acknowledge support from the DGICYT Project PB87-0261 and the Programa CooperaciQ con IberoamCrica, MEC (Spain), DGAPA-UNAM and CONACYT (MCxico).
REFERENCES 1 J. Ramirez, R. Cuevas, A. Mpez Agudo, S. Mendioroz and J.L.G. Fierro, Appl. Catal. , 57 (1990) 223. 2 J.L.G. Fierro, R. Cuevas, J. Ramirez and A. Mpez Agudo, Bull. SOC. Chem. k l g . , 100 (1991) 945. 3 Ch. Papadopoulou, A. Lycourghiotis, P. Grange and B. Delmon, Appl. Catal., 38 (1988) 255. 4 F.M. Mulcahy, M. Houalla and M. Hercules, J. Catal. 106 (1987) 210. 5 R. Mpez Cordero, S. Mpez Guerra, J.L.G. Fierro and A. Mpez Agudo, J. Catal., 126 (1990) 8. 6 P.M. BOOrman, K. Chong, R.A. Kydd and J.M. Lewis, J. Catal., 128 (1991) 537. 7 R. Thomas. F.P.M. Kerholf. J.A. Mouliin. - . J. Medema and V.H.J. de Beer, J. Catal., 61 (1980) 559. 8 J.M. Stencel, L.E. Makosky, J.R. Diehl and T.A. Sarkus, J. Raman Spectrosc. 15 (1984) 282. 9 D. Onagi, F. Mauge, J.C. Lavalley, E. Payen, S. Kasztelan, M. Houari, J. Grimblot and J.P. Bonnelle, Catal. Today 4 (1988) 23. 10 B. Scheffer, J.J. Heijeinga and J.A. Moulijn, J. Phys. Chem., 91 (1987) 4572. 11 B. Scheffer, P. Molhoek and J.A. Moulijn, Appl. Catal., 46 (1989) 11.
Guczi, L ef al. (Editors), New Frontiers in CafalysiS Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights resewed
HYDRODESULFURIZATION ACTIVITY OF ZEOLITE SUPPORTED NICKELAND COBALT SULFIDE CATALYSTS W. J. J. Welters, T. I.Kordny( V. H.J. ak Beer and R. A. van Santen
Schuit Institute of Catalysis, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands
Abstract Various zeolite (Nay or Cay) supported metal sulfide (Ni or Co) catalysts were prepared (impregnation or ion exchange) and characterized by means of thiophene HDS, '29XeNMR and TPS. Especially the acidic zeolites showed a very high initial activity. The observed activity differences are discussed in terms of sulfidation, dispersion, position of the metalsulfide relative to the zeolite pore system and acidity, the latter two being the most important. It is concluded that small Ni and Co sulfide clusters are very efficient thiophene desulfurization catalysts. 1. INTRODUCTION
Zeolites activated by transition metal sulfides are currently widely applied in hydrocracking. The aim of our study is to provide more insight in the significance of parameters such as the position of the metal sulfide relative to the pore system (in- or outside) and the metal sulfide - zeolite interaction strength on the initial properties of these catalysts. Here we report thiophene hydrodesulfurization activities determined as a catalytic characterization of the metal sulfide function in the zeolite. 2. EXPERIMENTAL
NaY (Ketjen LZY-52), NaY derived Cay, y-Al,O, (Ketjen 001-1.5E) and carbon (Norit RX3-extra) supported oxidic Ni or Co catalysts were prepared via pore volume impregnation (with Ni (Co) chloride solutions), drying (393 K, 16 h, in air) and (except for the carbon supported catalysts) calcination (648 K, 24 h, in air). Ni or Co containing catalyst precursors were also prepared via ionexchange of NaY with aqueous solutions of NiCI, or CoCl,, washing to remove C1- and drying (393 K, 16 h, in air). The initial activity and deactivation were measured using thiophene hydrodesulfurization (microflow reactor, 673 K, 1 bar, 2 h runtime). 'On leave from Institute of Isotopes of the Hungarian Academy of Sciences, Budapest, Hungary.
1932
Prior to the activity test the catalysts were sulfided (1 bar, 10 % H,S in H,, in 1 h from 293 K to 673 K and 2 h at 673 K). The sulfidation of an ion exchanged NiNaY and impregnated NiNaY and NiCaY was studied by means of temperature programmed sulfidation (TPS)[l].In addition the ore system of the sulfided ion exchanged and impregnated NiNaY was characterized by 'Xe NMR according to a method described by Fraissard and Ito [2].
P
3. RESULTS AND DISCUSSION
The supports showed only very low thiophene HDS activity. However, when combined with Co or Ni sulfide, catalysts with a very high initial activity could be prepared. As shown in Table 1 both metal sulfides had similar activities, considerably higher than those of their A1203supported counterparts and comparable to that of carbon supported catalysts. As in the case of carbon supported catalysts the presence of Mo or W is not required to obtain high activities. This finding supports models for HDS that pro ose a high intrinsic activity also for the so called 'promoter ions', viz. Co2+ and NiP* [3]. Whereas for Co this was illustrated on carbon [4] the zeolite catalysts also show the same for Ni. A striking difference between the zeolite and the carbon supported catalysts is the low stability of the former. It is found that the presence of acidic zeolite sites causes high initial activity as well as strong deactivation (Nay (ion ex.) and CaY (impreg.)) due to coke formation, In the absence of these sites the catalysts are more stable, however their initial activity is considerably lower. Table 1 Quasi Turn Over Frequency (mol thiophene converted per mol metal per second) for A1203,carbon and zeolite supported metal sulfide catalysts. The metal loading is 4 wt% metal for each catalysts. Ni
co
runtime (min.)
5
120
5
120
NaY (impreg.) CaY (impreg.) NaY (ion ex.) C
3.0 5.5 9.1 3.1 0.9
2.2 2.5 3.3 2.2 0.8
3.0 4.4 6.6 4.0 1.4
2.4 2.0 3.0 3.3 1.4
The difference in initial activity may be due to differences in the metal sulfide dispersion. The ion exchanged catalysts showed linear activity increase with increasing metal loading (Fig. 1). The fact that this does not apply to the impregnated NaY catalysts indicates a decrease in dispersion with increasing Ni loading.
1933 25 20
b
6
15 10
5 0 0
2
4
6
0
1
0
metal loading [wt%]
Figure 1. Quasi Turn Over Frequency (mol thiophene per mol metal per second) as function of the metal loading for ion exchanged( + ) and impregnated (0)zeolites.
Figure 2. TPS patterns of zeolite supported Ni catalyst (metal loading is 4 wt%).
In order to verify this we examined the dispersion and the location of the nickel sulfide phase relative to the pore system by means of EXAFS, TPS and lZ9Xe NMR measurements. From preliminary EXAFS data obtained for 4 wt% Ni containing impregnated NiNaY and NiCaY as well as ion exchanged NiNaY catalysts we can conclude that the nickel is fully sulfided, with by far the main sulfide species being Ni,S,. In all three catalysts the nickel sulfide dispersion appears to be comparable. The TPS results of the same catalysts showed that although for the impregnated samples the H2S uptake at room temperature was relatively high, the total H,S consumption at 673 K was the same for all three catalysts (Fig. 2). In accordance with the EXAFS results the S/Ni ratio was almost equal to that for Ni3S2 which is the most stable compound under HDS reaction conditions in the presence of an inert support [5]. In the case of the ion exchanged sample the maximum H,S uptake occurs at a somewhat higher temperature indicating a more difficult sulfidation. Various nickel containing ion exchanged NiNaY zeolites and a 4 wt% impregnated NiNaY sample were examined by Xe NMR in their sulfided state. The results show a linearly increasing NMR shift with increasing density of adsorbed Xe which is to be expected if the Ni is fully sulfided. The diameters of the supercage spheres were calculated by the formula given by Derouane and Nagy [6] from the NMR shifts extrapolated to zero Xe pressure. These diameters decrease with increasing Ni loading as shown in Figure 3. This strongly suggests that nickel sulfide is at least partially present in the supercages and that its amount increases with increasing metal loading. The impregnated sample exhibited two kinds of supercage diameters, both of which were
1934
0NaY €3 11 10
’
.
1.0
m 1.7 m 5.5
0 .
7.6
8 .
0
7 -
1 3 4
Figure 3: Calculated supercage sphere diameters (A) of NaY and sulfided ion exchanged (x wt% Ni) and impregnated (IM) NiNaY zeolites.
4 &i
larger than the comparable (3.7 wt%) ion exchanged zeolite. So, in this case less nickel sulfide is present inside the zeolite pore system compared to the ion exchanged sample, and the catalyst is less homogeneous. From the above findings one can conclude that the difference in thiophene HDS activity is not primarily caused by variations in degree of sulfidation or nickel sulfide dispersion. It is more likely that this phenomenon is caused by differences in the position of the nickel sulfide relative to the pore system in combination with differences in acidity. The latter might influence the thiophene HDS directly or indirectly by enhancing the adsorption of thiophene in the zeolite.
Acknowledgement: These investigations are supported by the Netherlands’ Foundation for Chemical Research (SON) with financial aid from the Netherlands’ Technology Foundation (STW). 1
2 3 4
5 6
P. Amoldy, J.A.M. van den Heijkant, G.D. de Bok and J.A. Moulijn, J. Catal., 96 (1985) 35. J. Fraissard and T. Ito, Zeolites, 8 (1988) 350. J.C. Duchet, E.M. van Oers, V.H.J. de Beer and R. Prins, J. Catal., 80 (1983) 386. J.P.R. Vissers, V.H.J. de Beer and R. Prins, J. Chem. SOC.,Faraday Trans. 1, 83 (1987) 2145. R. Burch and A. Collins, J. Catal., 97 (1986) 385. E.G. Derouane and J.B. Nagy, Chem. Phys. Lett., 137 (1987) 341.
Guni, L et al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1W, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights rcserved
ENHANCED HDS ACTIVITY VIA MULTIPLE IMPREGNATION OF SULFIDED Mo/A1203 CATALYSTS
C.-S.Kima, F. E. Massotho, C. Geantefi andM. Breysseb aDepartment of Fuels Engineering, University of Utah, Salt Lake City, Utah 841 12, USA bInstitut de Recherches sur la Catalyse, CNRS, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France
Abstract Catalysts, prepared by mu1 tiple impregnations of previously sulfided catalysts (SIP), together with conventionally prepared catalysts (CON), were characterized by high resolution electron microscopy, NO chemisorption and HDS activity. The SIP catalysts displayed similar morphology as the CON catalysts, but exhibited lower NO adsorptions and higher HDS activites. It is proposed that the higher activities are due to the creation of more catalytically active sites from the repeated exposure to air during their preparation. 1. INTRODUCTION
One of the 1 imitations of hydrotreating catalysts for hydrodesulfurization (HDS) of sulfur-containing feeds is that the HDS activity reaches a maximum
at about 10-12% Mo, with no further increase in activity with additional amounts of Mo. The object of the current research was to improve the HDS activity by sequential additions of Mo to presulfided catalysts. The rationale for this approach came from prior studies, where it was found that the MoS, coverage of the alumina support remained low, even at high levels of Mo [ l ] . Thus, it was thought that addition of Mo to a presulfided catalyst would allow additional coverage the uncovered alumina with MoS,, giving a superior dispersion and catalytic activity. 2. EXPERIMENTAL
Sulfide-impregnated catalysts (SIP) were prepared starting with a 7.7% Mo/Al,O,, which was presulfided at 400' C. Additional Mo was added by an incipient wetness impregnation, followed by drying in He at 100' C and stream up to 400' C, holding at this temperature sulfiding with a 10% H,S/H, for 2 h. Additional incremental amounts of amounts of Mo were again added and the catalysts sulfided as before. Another series o f catalysts (CON) were prepared by a conventional method, consisting of multiple impregnations with
oven drying between additions, followed by air calcination at 500' C for overnight. Catalysts (CON) were sulfided at 400' C prior to testing. The sulfided catalysts were examined by high-resolution electron microscopy, as described previously [ 2 ] , and were characterized by NO chemisorption at '0 C, using a pulse technique [3]. Catalytic activities for thiophene HDS were measured in a fixed-bed reactor under vapor-phase conditions at atmospheric pressure and 350' C [4]. Pseudo first-order rate constants were calculated from conversions obtained after 18 h on stream. 3. RESULTS AND DISCUSSION
Electron microscope pictures of a sulfide CON and SIP catalyst are given in Figure 1. No morphological differences were observed between the two sets
Figure 1. TEM micrographs of 14.8% Mo CON (a) and 15.5% SIP (b) catalysts. o f catalysts. With increasing loading, the stacking of CON catalysts remained
low, but a growing number of bulk crystallites was observed. These particles of bulk MoS, probably did not represent a large fraction of the Mo, as XRD failed to detect MoS,. For SIP catalysts, stacking was slightly promoted, but few bulk crystallites were visible in this series. Measurements o f the average size and number of layers of MoS, slabs, given in Table 1, show an increase in both with increasing Mo content. The SIP catalysts exhibited slightly larger slab sizes and number of layers than the CON catalysts at a given Mo content. Evidently, the SIP procedure did not give a better dispersion with respect to monolayer slabs as hoped. The NO adsorption for the SIP catalysts was significantly less than for the CON catalysts (Table 1); the former were independent o f Mo content, while the latter showed a slight maximum. Previous literature has suggested that edge sites of the MoS, slabs are responsible for adsorption and catalytic reaction, and these are believed to be associated with sulfur anion vacancies
1937
Table 1 Characterization results of sulfided catalysts %lo
Prep'
Lb
nc
NOd
kC
7.7 11.3 14.8 18.2 12.9 15.5 19.5
CON CON CON CON SIP
3.0 3.5 3.0 3.5 3.2 3.7 4.0
1.4 1.5 1.8 2.0 2.1 2.2 2.6
0.252 0.305 0.276 0.263 0.182 0.180 0.186
14.7 17.3 16.5 16.1 20.7 24.7 31.4
SIP SIP
'CON-conventi onal preparation; SIP-sul f ide impregnated preparation bSlab length, nm 'No. layers in cluster dNO chemisorption, mmol/g ePseudo first-order rate constant, cc/min-g cat
1 2
I
%MOIL
I
I
4
% Mo/L
Figure 2. NO adsorption ( a ) and rate constant (b) vs. %Mo/L. [5-71. The total edge area per gram of catalyst is proportional to %Mo/L. A plot of NO/g vs. Y/dlo/L is given in Figure 2a. The NO adsorption for the first two CON catalysts is proportional to their relative edge area (%lo/L is not the true area but is proportional to it), but then drops off with increase in relative edge area for the higher Mo-loaded CON catalysts; whereas it is constant, for the SIP catalysts. This shows that NO adsorption is not proportional to edge area for either set of catalysts, and suggests that the number of adsorption centers do not increase proportionally with increase in edge area. As seen in Table 1, catalytic activities (as first-order rate constants) increased and then decreased with Mo content for the CON catalyts. These
1938 r e s u l t s a r e s i m i l a r t o t h o s e found i n t h e l i t e r a t u r e . On t h e o t h e r hand, q u i t e s u r p r i s i n g l y , t h e c a t a l y t i c a c t i v i t e s f o r t h e SIP c a t a l y s t s c o n t i n u e d t o i n c r e a s e w i t h Mo l e v e l . The r e l a t i o n s h i p between c a t a l y t i c a c t i v i t y and r e l a t i v e edge area i s shown i n F i g u r e 2b. While k m i r r o r e d t h a t o f NO ( F i g u r e 2a) f o r t h e CON c a t a l y s t s , k showed a r a p i d r i s e w i t h edge area f o r t h e S I P c a t a l y s t s , and showed no r e l a t i o n t o NO. The r e l a t i o n s h i p between a c t i v i t y and NO c h e m i s o r p t i o n i s shown i n F i g u r e 3, i n terms o f i n t r i n s i c a c t i v i t y , k/NO ( s i m i l a r t o a t u r n o v e r number) vs. independent o f edge area, i n d i c a t e s a d i r e c t c o r r e l a t i o n between NO a d s o r p t i o n s i t e s and c a t a l y t i c sites, t h a t i s , regardless o f the change i n s i t e c o n c e n t r a t i o n w i t h edge area. T h i s r e l a t i o n s h i p i s n o t observed f o r t h e SIP c a t a l y s t s , where the intrinsic activity i n c r e a s e d w i t h edge area ( F i g . 3). The unusual increase in i n t r i n s i c a c t i v i t y n o t e d above f o r
200
150
-
0
5 100 50 -
J
1
I
I
I
F i g u r e 3. I n t r i n s i c a c t i v i t y vs. s u l f i d e d Mo/A1,0, c a t a l y s t s exposed %Mo/L . t o a i r and t h e n r e s u l f i d e d , develop more a c t i v e r e a c t i o n s i t e s due t o i n c o r p o r a t i o n o f oxygen, f o r m i n g more a c t i v e O-vacancy s i t e s as compared w i t h S-vacancy s i t e s . It i s suggested t h a t t h i s may be t h e reason f o r t h e h i g h a c t i v i t y o f t h e SIP c a t a l y s t s , s i n c e a f t e r each a d d i t i o n o f Mo, t h e p r e v i o u s l y s u l f i d e d c a t a l y s t was exposed t o air.
4. REFERENCES
1 W. Zmierczak, Q. Qader and F.E. Massoth, J. C a t a l . 106 (1987) 65. 2 C. Mauchausse, H. Mozzanega, P. T u r l i e r and J-A. Dalmon, 9 t h I n t . Cong. C a t a l . 2 (1988) 775. 3 J. M i c i u k i e w i c z , W. Zmierczak and F.E. Massoth, B u l l . SOC. Chim. B e l g . 96 (1987) 915. 4 F.E. Massoth, C.-S. K i m and J-W. Cui, Appl. C a t a l . 58 (1990) 199. 5 H. Topsoe and B.S. Clausen, Appl . C a t a l . 25 (1986) 273. 6 S.J. T a u s t e r , T.A. Pecararo and R.R. C h i a n e l l i , J. C a t a l . 63 (1983) 265. 7 C.B. Roxlo, M. Daage, A.F. Ruppert and R.R. C h a n e l l i , J. C a t a l . 100 (1986) 176.
Guczi, L
al. (Editors), New Frontiers in Cololys~ Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992. Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights rcsclved
SURFACE STRUCTURE OF MOLYBDENUM NITRIDE AND ITS ACTIVITY FOR HYDRODESULFURIZATIONAND HYDRODENITROGENATION M. Nagai, T. Miyao, T. Tsuboi and T. Kusagaya Department of Chemical Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
Abstract An alumina-supported molybdenum nitride catalyst was prepared and tested t o determine its activity and selectivity in t h e hydrdenitrogenation (HDN) of carbazole and the hydrdesulfurization (HDS) of dibenzothiophene. 1.
INTRODUCTION
An increasing interest has developed in exploring the catalytic properties of transition metal nitrides, such as MoZN and W2N. Mo2N is among the most active catalysts for NH3 synthesis [I] and ethylene hydrogenation [21. More recently, @am et al. [3] and kirkel and Zee [4] reported that unsupported gMo2N powders were active for the HDN of quinoline and the HDS of thiophene. In this study, an alumina-support& molybdenum nitride catalyst was prepared and tested to determine the activity and selectivity in the HDN of carbazole and the HDS of dibenzothiophene. The effects of catalyst pretreatment on the canposition and structure of the catalyst have been studied.
2.
EXPERIMENTAL
The alumina-supported molyWena catalyst was prepared by a mixture of hexamnium molybdate and Ealumina and calcined in air at 45OoC for 24 h. The catalyst pretreatment unit was used to prepare the samples for nitriding at 7OO0C and for characterizing the pmder samples by XPS and XRD. Nitriding of the samples was done according to the procedures of Vople and Boudart [5]. The freshylybdena alumina catalyst was nitrided in pure NH3 flowing at 4 litersh- at 35OoC, (500 or 70OoC) for 3 h; LTN catalyst, ( m or H?N catalyst). The surface structure and canposition of the molybdenum nitrides were characterized by means of XPS, TPD, XRD, an?. elmental analysis. The activity measurements for carbazole HDN and dibenzothiophene HDS were carried out in a high-pressure stainless-steel flow system, fixed-bed microreactor. The molybdena alumina-supported catalyst (2.09, 10-20 mesh) was nitrided in-situ under the MTN and H reducing conditions after preheating at 45OoC in air. The reaction consisting of 0.25 w t % carbazole in xylene, was introduced at 20 mlmh-1 All experiments were performed at 280-360°C and 10.1Mpa total pressure, The HDN activity of the
.
1940
catalyst represents the rate of the formation of bicyclohexyl per unit of surface area. 3. RESULTS AND DISCUSSION
.
3.1 Characterization The data of the elemental analysis and the N2 BET surface area of the 11.7%Mo/Al 0 catalyst evacuated at 2OO0C are sham in Table 1. The H" and MlN cagajysts contained 1.5 and 1.6 w t % nitrogen, respectively. These values are larger than the anticipated stoichianetric amount of 0.6wt%. The nitrogen contents for the 97.1% Mo/A1203 samples for the H" and MlN conditions were also 1.3 and 1 .I times the stoichianetric value, respectively. This result showed that excess nitrogen was present on the surface of the prepared Mo/A120 catalyst, especially on the alumina surface. The Mo 3d XP spectra $or the nitrided and reduced Mo/A1203catalysts are shown in Fig. 1. F'ran the XPS data of the Mo 3d spectra for the 11.7% Mo/A1203 catalyst, the broad and ill-resolved spectra were attributed to the presence of a considerable proportion of Mo(V) species. However, the M o 3d spectra for the 97.1% M o / A l O3 catalyst showed a relatively sharper double-peak structure. Althougi the molybdenum state for the 11.7% Mo/Al O3 catalyst was decreased to a much lower state, the molyWenum state for t$e 97.1% catalyst is decreased to the I11 or IV states by the H?N and MTN treatment. The A1 Is XPS spectra for the 11.7% Mo/AI O3 catalyst before and after the reaction show that all nitrided samples L v e the same spectra of 76.3 eV of Al Is emission as the Mo/Al,O3 catalyst even before nitriding. This result shows that alumina in the Mo/AlZO3 catalyst is not nitrided under these conditions. Furthermore, the measurement of the MI3 TPD for the catalysts suggested that the 11.7%Mo/Al O3 nitrided catalyst held much less acidity than that reduced at 4OO0C, afthough it held more acidity than alumina alone. This result showed that the nitrided catalyst has a lower acidity than the reduced catalyst.
Wle 1
The cunposition and HDN activity of the 1 1 .7%Mo/A1203 catalysts ~
Nitridation Temp. (OC)
*
Nitrogen*
BET s face area*
wt%
(mY/g)
500
1.5
700 Reduced
1.6
267.2 195.4
-
-
HCN activity (molg-cat/hr/m' ) 3OO0C 32OoC 340°C
1.58
1.54 0.68
3.6 4.6 1.5
10.6 12.3
5.0
The catalysts were evacuated at 2OO0C before the measurement.
The X-ray diffraction data for the 97.1%Mo/Al 0 catalyst shmed that produced in the H" catalyst. Since the XPS data proved the presence of Mo2N on the 11.7%Mo/A1203 catalyst even under the MTW conditions, Md) on the A1203 was reduced by NH to form amorphous or highly dispersed moly&enum nitrides as well as trans2omtion to Mc02 on the catalyst under the MTN conditions.
Moo2 was mainly formed in the MTW catalyst, Mo2N &?ng
1941 3.2. Activity for HDN and HDS The activity of the IFIN, M" and reduced M0/A120 catalysts for carbazole HDN is also shown in Table 1. The HDN activity the datalysts decreased in the following sequence: the HTN catalyst > the MTN catalyst > the 40OoCreducing catalyst. The HDN activity of the H" catalyst is 3.1 and 2.4 times higher than that of the 3 0 0 and 320°C reducing catalysts, respectively. Nitridation enhanced the activity of the Mo/Al O3 catalyst for the HDN reaction canpared with the reducing method. In $he reactior. prducts, the major product, bicyclohexyl, was prduced from the C-N bond. scission of perhydrmrbazole which was produced through the successive hydrogenation of carbazole (Scheme 1 ) [61. A nonlinear least-squares regression technique was used to fit the data with a suitable rate equation of the Lanqmuir-Hinshelwd type. The mechanism of carbazole HDN is best correlated by a dual-site model. The kinetic results demonstrate the ccmpetitive adsorption of tetrahydrccarbazole on one kind of catalytic site and of hydrogen on another. This result suggests that the creation of two independent active sites gives rise to higher activity of the molybdenum nitrided catalyst than that of the reduced catalyst [61.
02
I
W==qo(=~
,2794
H
Carbarole
H
1,2,3,4-Tetrahydrocarbazole
1,2,3,4,4a,9aHexahydrocarbaz
Bicyclohexyl perhydrocarbazole
1. NH3
Scheme 1. The reaction network of the HcN of carbazole. 1,2,3, 4,40,9o-Hexohydrod 1benzoth 1ophene
A,2,3,4-TetrOhYdrodlbenzothlophene 248
BINDING ENERGY [ e V l
\
228
Cyclohexvl benzene
Figure 1. &I M XPS spectra of the nitrided Mo/A120 catalysts. Loading ( w t % ) : a-c, f1.7. d-f, BlPhenyl Nitriding temp. ( 6C): a 97.1. and d, non-treatmentib 4e, Scheme 2. The reaction network of the HDS of dibenzothiophene. 500; c and f, 700. In the study of the HDS of dibenzothiophene, the activity and selectivity of the nitrided 11.7%MoAl 0 catalyst canpard with those of the sulfided catalyst is sham in Fig.2z The bTIN catalyst is 1.3 and 1.2 times more active than the 400°C-sulfiding catalyst at 300 and 34OoC, respectively.
1942 The selectivity of the MTN catalyst for the formation of biphenyl to cyclohexylbenzene are 32.5 and 16.3 times more active than those for the sulfided catalyst at 300 and 340°C , respectively. Moreover, since the presence of biphenyl does not increase the formation of cyclohexylbenzene in the HDS of dibenzothiophene as shown in Fig. 3 , biphenyl is not hydrogenated to form cyclohexylbenzene from the cleavage of the S-C bond of hexahydrdibenzothiophene in the hydrogenation of dibenzothiophene (Scheme 2 ) [71. Therefore, the nitrided catalyst is an extremely active catalyst for direct sulfur removal frcm the heterocyclic sulfur canpounds with less consumption of hydrogen. 4.
( A , A ):selectivity (0,.):activity (A,Ol:nitrided
loor
100 / O
80
-ap Y
60
.u)
L
40
2 c
0
20 h
-A-A 260
280
0 300
320
340
360 380
Reaction Temperature [ " C ]
Figure 2. The activity and selectivity of the nitrided Mo/M2O3 ( 0 , A ) and the sulfided l%/fi203 ( A ) for the hydrdesulfurization of dibenzothiophene.
References
[ I 1 L. Volpe and M. Boudart, J. Phys. Chem. , 9 0 ( 1 9 8 6 ) 4874. [ 2 1 G . S . Ranhotra, G.W.
Haddix, A.T. Bell, and J.A. Reimer, J. Catal., 108 (1987) 24. [ 3 1 J. C.
Schlatter, S. T. Oyama, J. E. Metcalfe, and J.M. Lambert, Jr., Ind. Ihg. Chem. Res., 27 (1988) 1648.
[41 E. J. Markel and J.W. Van Zee, J. Catal., 126 ( 1 9 9 0 ) 643. 151 L. Volpe and M. Boudart,
J. Solid State Chem, 59 (1985) 332. [ 6 1 M. Nagai, T. Masunaga,
and N. Hanaoka, Energy & Fuels, 2 (1988) 645. [71 M. Nagai, T. &to, and A. Aiba, J. Catal., 97, (1986) 52.
Reaction t i m e [ h r l
Figure 3. m e effect of added biphenyl on the formation of cyclohexylbenzene in the hydrosulfurization of dibenzothiophene.
Guczi, L ef d.(Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1!S3 Elscvicr Science Publishers B.V. All rights reserved
HYDROCRACKING GAS OILS FROM SYNTHETIC CRUDE WITH MIXED PILLARED CLAY-ALUMINA SUPPORTED CATALYSTS
J. Monnier, J.-P. Charland J. R. Brown and M. F. Wilson Energy Research Laboratories,CANMET, Energy, Mines and Resources Canada, 555 Booth Street, Ottawa, K1A OG1 Ontario, Canada
Abstract Nickel-molybdenumcatalysts supported on mixed pillared clay-alumina extrudateswere prepared and tested for hydrocracking denitrogenated Canadian synthetic crude gas oil using an automated microreactor system. Compared with Ni-Mo on Y-zeolite-alumina, the catalysts were found to be superior as they utilized H,more efficiently and produced more liquids in the combined middle distillate and naphtha ranges. Very little hydrocarbon gases were formed over pillared clay-alumina supported catalysts. X-ray photoelectron spectroscopy provided information on the nickel and molybdenum active sites in the catalysts. Basal spacings of pillared clays were determined using X-ray diffraction. 1. INTRODUCTION
Synthetic crude gas oils derived from Canadian oil sands are difficult to hydrocrack due to the presence of nitrogen compounds which poison acidic sites and cause rapid deactivation of the catalyst [ 11. To overcome this problem, a two-stage process has been developed whereby synthetic crude gas oil is first hydrotreated to eliminate most of the nitrogen compounds and then hydrocracked to middle distillates and naphtha [2-51.The performance of nickel-molybdenum catalysts supported on mixed pillared clay-alumina (PILC-Al,O,) as second-stage hydrocracking catalysts is discussed in terms of product distribution, quality, and catalyst characteristics. 2. EXPERIMENTAL
Two montmorillonite clays were obtained from American Colloid Company. The starting materials were either Polargel NF (PGL) or Microfine Panther Creek (MPC) and were pillared by ion-exchanging aluminum polynuclear cations [A11304(OH),(H,0),,]7t between the clay lamellas, followed by drying and calcining. This led to increases in the basal spacings as monitored by X-ray diffraction: from 9.92 to 19.33 8,for PGL and from 12.05 to 18.85 8, for MPC. Nickel-molybdenum catalysts (16 wt % MOO, and 4 wt % NiO) were then prepared by the incipient wetness method from extruded supports
1944
containing 50 or 70% Al,O, mixed with PILC or Y-zeolite (Linde LZY-72). The catalysts were then characterized by proton-induced X-ray emission spectrometry (PIXE) and Xray photoelectron spectroscopy (XPS). Synthetic crude gas oil (SGO) from northern Alberta was hydrotreated with a commercial Ni-Mo/y-Al,O, catalyst at 10.3 MPa and 375°C to reduce the nitrogen content from 1100 to 92 ppm. The performance of Ni-Mo on PILC-AI,O, catalysts was then compared with Ni-Mo on Y-zeolite-Al,O, for hydrocracking denitrogenated synthetic gas oil (DSGO)using an automated 10-mL microreactor system at 36OoC, 380"C, 400°C and 420°C, 15.2 MPa pressure, 0.5 and 1.0 h-' LHSV and lo00 L H2/L DSGO feed ratio. Catalyst presulphiding was performed in situ with a gas mixture of 10% H2Sin H, flowed through the microreactor at 4WC, 0.3 MPa and 1000 h-' GHSV for 2.5 h. The liquid products obtained after a minimum of 50 h at steady state conditions were characterized to give information on gas oil conversion, product yields, N and S contents and 13C-NMR aromaticity.
3. RESULTS AND DISCUSSION The calcined Ni-Mo catalysts on PILC-A120, and Y-zeolite-Al,O, supports were examined by XPS. The Mo 3d spectra indicated that molybdenum was in the Mo6+oxidation state (Mo 3d,,, = 232.6 eV) for both catalysts, indicating the presence of MOO, 50 A 40
lot - 0 0
10
20
30
40
50
10 - 0
60
c 0
10
CONVERSION (%)
20 30 40 CONVERSION (%)
60
50
35
C
D
-
25
z
9 w *
10
20
30 40 CONVERSION (%)
50
15 5 -5
10
20 30 40 CONVERSION (%)
50
,
0
Figure 1. Yields of heavy middle distillates (A), middle distillates (B), naphtha (C)and gases (D)as functions of DSGO conversion using catalysts supported on mixed PILCAl,O, (A) or zeolite-Al,O, [experimental (0) or commercial (o)].
1945
and/or AI,(MoO,), phases. However, the catalysts differ with regard to the Ni 2p3/2 photoline peak energy. The curve-fitted XPS Ni 2p3 envelopes suggest that the state of nickel was more electronegative on PILC-Al,O, (Ni 2p3/2 = 857.2 eV) than on Yzeolite-Al,03 (Ni 2p3,, = 856.8 ev). In both cases, nickel was likely present as Ni-silicate and Ni-aluminate phases; a small amount of Ni-hydroxide was detected in PILC-Al,O,. Figure 1 presents the yields of (A) heavy middle distillate (290-360"C), (B) middle distillate (195-360"C), (C) naphtha (IBP-195°C) and (D) gases as functions of DSGO conversion. Table 1 compares characteristics of products obtained at similar conversions over the two different catalyst supports. It was found that below 30% DSGO conversion, slightly more naphtha was produced over PILC-Al,O, supports whereas at higher conversions, the use of Y-zeolite-Al,O, support favoured high production of naphtha. For middle distillates, the yields obtained over Y-zeolite-Al,O, support remained constant at about 28-30 wt % up to 50% conversion and then dropped. For mixed PILC-N,03 supports, the middle distillate yields steadily increased from 30 to 50 wt % with DSGO conversion. As expected, the Y-zeolite's high cracking activity caused excessive gas production (Fig. 1, D) which did not fall below 5 wt % and actually increased at higher conversions. In contrast, PILC-Al,O, supported catalysts were more selective towards liquid products and their production of gases remained close to zero. No major difference in catalyst selectivity was observed in using PGL or MPC PILC or in varying concentration from 30 to 50%. In summary, Ni-Mo catalysts on mixed PILC-Al,O, supports showed superior selectivity compared with Y-zeolite-Al,O, supported catalysts as indicated by substantially lower gas yield, higher liquid production and better utilization of H, feedstock.
,
Table 1 Performance of Ni-Mo on experimental mixed PILC- or Y-zeolite-Al,O, support as second-stage hydrocracking catalyst used at 420"C, 0.5 h ' LHSV and 15.2 MPa
Gas oil feedstock (DSGO)
Gas oil conversion (%) Liquid yield (wt %) IBP-195°C 195-290°C 290-360°C 2 360°C Gas yield (wt %), by diff. Liquid products H/C (atomic) N (PPm) s (PPm) "C-NMR fa
0.0 0.0 28.7 71.3
1.72 92 80 0.114
Supported Ni-Mo catalysts containing 30% Polargel NF
30% Y-zeolite
52.9 96.6 12.1 19.4 31.5 33.6 3.4
56.6 84.7 22.2 14.2 17.4 30.9 15.3
1.83
s0.5 1.7 0.078
1.83 0.6 2.1 0.093
1946
PILC-AI,O, supported catalysts produced liquid fuels containing less aromatic hydrocarbons as indicated by the "C-NMR aromaticity fa which varied between 0.04 and 0.08. The fa values followed an ascending trend with temperature which is consistent with a shift of thermodynamic equilibrium which favoured the dehydrogenation of naphthenes to form aromatics above 380°C [6]. Conversely, the fa values for the hydrocracked products from Y-zeolite-AI,O3 supported catalysts remained high between 0.09 and 0.12. The superior selectivity of mixed PILC-AI,O, supported catalysts in terms of product distribution and quality may be attributed to a reduction in the molecular diffusion limitations which are usually encountered in Y-zeolites. Compared with the smaller diameter size of Y-zeolite pore openings (7.4 A) which limits diffusion of small hydrocarbon molecules [7], mass transfer appears to be less impeded inside the porous structure of the PIE-AI,O, support. It is also possible that the improved selectivity is due to the formation of macropores within the support. The macropores may be formed from edge-to-face aggregation of clay platelets creating a "house-of-cards''type structure [8]. Thus polynuclear aromatics which are present in gas oil and middle distillate fractions might penetrate the porous structure of the PILC-AI,O, more easily. Increased diffusion rates implied shorter residence times for hydrocracked products which had a direct impact on gas formation and hydrogen utilization. 4. CONCLUSIONS
Nickel-molybdenum catalysts supported on mixed PILC-AI,O, were found to be very effective for hydrocracking synthetic crude gas oil having reduced nitrogen content. Their liquid yields and product quality were significantly better than those obtained using Yzeolite-aluminasupports. The results obtained using pillared clays were attributed to the larger pore diameter of the molecular sieve structure and the possible formation of macropores from aggregation of clay lamellas, which are both expected to enhance the diffusion of multi-ring compounds. 0 Minister of Supply and Services Canada, 1991. 5. REFERENCES
M.F. Wilson, R.A. Simmons and H. Notzl, Am. Chem. SOC.Div. Petr. Chem., Prepr., 32 (1987) 383. J. Monnier, Y. Yoshimura and J.F. Kriz, 11th North American Meeting of the Catalysis Society, Dearborn, Michigan (1989), Paper PD14. J. Monnier, J. Christensen, J.-P. Charland and M.F. Wilson, 12th North American Meeting of the Catalysis Society, Lexington, Kentucky (1991), Paper D36. J. Monnier, G.T. Smiley and M.F. Wilson, 12th North American Meeting of the Catalysis Society, Lexington, Kentucky (1999 Paper (22. A. Mahay, J. Chmielowiec, I.P. Fisher and J. Monnier, Fuel Proc. Technol., (1992) in press. M.F. Wilson, I.P. Fisher and J.F. Kriz, J. Catal. 95 (1985) 155. R.M. Moore and J.R. Katzer, AIChE J. 18 (1972) 816. M.L. Occelli, S.D. Landau, and T.J. Pinnavaia, J. Catal. 104 (1987) 331.
Guczi, L ef al. (Editors), New Frontiers in Caralysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Sciencc Publishers B.V. All rights resewed
TRANSFORMATIONS OF THIOPHENE, 'ITI'RAHYDROTHIOPHENE AND BUTANETHIOL OVER CO-BLACK,C0/A1203, M0/A1203 AND CO-MO/AI~O~ DURING TEMPERATURE-PROGRAMMED REACTION V. V . Rozanov, Y. Tzao and 0. V . Krylov
Institute of Chemical Physics, Russian Academy of Sciences, Moscow,Russia
Abstract TPR spectra of thiophene ( T I , tetrahydrothiophene (THT) and butanethiol (BT) initially adsorbed on sulphidized Co-blaok, Co/A1203, Mo/A1203 and Co-Mo/A1203 were registered and analyzed. The oonversion of T was found to prooeed only over Co-Mo/A1203 resulting in the formation of BT. THT undergoes transformation over Co-Mo/A1203 as well as over Mo/A1203. The main produots of its oonversion were BT and T. Initially adsorbed BT does not undergo any appreoiable transformatiom over any one of the oatalysts studied. The ooinoidenoe of kinetio parameters of desorption for initially adsorbed BT and the one whioh is formed during TPR from preliminarily adsorbed T and THT testify that intermediate oompounds in T transformation in our experiments may be THT and BT while the limiting step is desorption of surfaoe butylthiolate. 1 IN!l!R~Uc!cIoN
Despite the wide praotioal w e of supported oobalt-molybdenum oatalysts for hydrotreating the stage meohanism of thiophene (T ) hydrodesulfurization (HDS ) remains up to now open to disouesion. Several different routes of thiophene HDS are being postulated [1,21. For studing the stage meohanism of thiophene HDS reaotion we used the temperature-programmed reaotion [TPRI method
1948
whioh oonsiets in adsorption of the reagent6 on the oatalyst surfaoe and registering the deeorbing reaotion produote in the gas phase whioh are formed during the programmed heating of the oatalysts. TPR speotra of preliminarily adsorded T, tetrahydrothiophene [THTI and butanethiol [BTI are obtained and analyzed.
2.ExPERmAL The experiments were oarried out in a high vaouum flow apparatue. The desorbed produote were analyzed by a monopolar mase-epeotrometer ROW-2. The TPR epeotra obtained represented the dependenoe of the desorption rate of the oompounde on the oatalyet temperature. The rate of the inoreaee of the oatalyet temperature was equal to 35deg/min. The oatalyste were Co-blaok, 20%Co/A1203, I5%Mo/Al2O3, and 15%Co-1 5SbMo/A1203. The oatalyste were oaloinated immediately before the TPR experiments, then reduoed Cn 8 t h in hydrogen at 5OO0C (in 0888 of Co-blaok at 35O0C) and eulphided in a mixture of thiophene and hydrogen during two hours. The eample weight was 0,lg. The reagents: T, THT and BT were ohromatographio pure. 3,REsuLTs resulted in The adsorption of T on Co-blaok and Co/A%O 3 the appearanoe TPR speotra of the initial eubstanoe peake with maximum temperatures T, equal to 120 and 125OC oorrespondingly (see Figure6 l a and 1 b) In 0888 of Mo/A1203 only the desorption of initial T wae also observed but a6 two badly resolved peaks with Tm roughly determined a6 equal to 160 and 25OoC (see Figure lo). The bimetallio oatalyet Co-Mo/A1203 produoed a more oomplex TPR speotrum. Besides the desorption of initiial T with T peake at 160 and 25OoC the epeotrum oontahed a peak at 30013C belonging to the reaotion produot BT (eee Figure Id), Similarly to T the THT did not undergo any transformations
.
1949
in the experiments performed on Co-blaok and Co/A1203(see Figures 2a and 2b). In the speotrum of ThT adsorbed on Mo/A1203 besides the desorption of the initial oompound whioh produoed two partially resolved peaks with Tm equal t o 200 and 29OoC the peaks of the produots - T and BT w e m observed (see Figure 2 0 ) . Their T, values were oorrespondingly equal to about 400 and 375OC.
n ZT
200 300 400 100 200 300 400 100 200 300 400 T,OC T,OC T,OC Fig.1. TPR speotra Fig.2. TPR speotra Fig.3. TPR speotra of T on a - d. of THT on a - d. of BT on a - d. a - Co-blaok, b - Co/Al2O9, o - Mo/A1203, d - Co-Mo/Al2O3 100
The TPR speotrum of THT for Co-Mo/A1203 was very similar to the one observed in oase of Mo/A1203. The only di6tinotion was that T and BT were evolved i n the first oase at lower
1950
temperatures: the Tm value f o r BT was equal to 300°C, while in oase of T it was observed at about 35OoC (see Figure 2d). In oase of BT on all the four oatalysts examined no deteotable transformations were observed. The desorption speotrum of BT from Co-Mo/A1203 was in essenoe a oombination of separate oharaoteristios f o r Co/A1203 and Mo/Al 0 the 8, main amount of BT being desorbed as a peak with Tm 300 C (see Figures 3a, 3b, 30, 3d). 4, coNcLusIoN
When analyzing the above results one may note several regularities. In oase of T its transformations during a TPR experiment took plaoe only on a Co-Mo/A1203 oatalyst while THT oould be transformed into T and BT both on Co-Mo/A1203 as well as on Mo/A1203. It should be noted that in suoh experimental conditions when there were no substantial amount of hydrogen in the gas phase only hydrogenolysis of T and THT took plaoe with the oleavage of one S-C bond and formation as the end produot of BT. The initial BT did not undergo any transformation. For the oleavage of the seoond S-C bond and the formation of C -hydrooarbons it is probably neoessary to have substantial 4 amounts of hydrogen to be present on the oatalyst surfaoe. The oohoidenoe of kinetio parameters of desorption for initially adsorbed BT and the one whioh is formed during TPR from preliminarily adsorbed T and THT testify that intermediate oompounds in T transformation in our experiments may be THT and BT while the limiting step is desorption of surfaoe butylthiolate. 5,FmERFNcBs 1 M.W. Vrinat, Appl. Catal., 6 (1983) 137. 2 Zdrakil, Appl. Catal., 4 (1982) 107.
Guni, L ef al. (Editors),New Fronriers in Caalysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
PARTIAL OXIDATION REACTION OF MmHANE WITH OXYGEN OR CARBON DIOXIDE BY TRANSITION METAL CATALYSTS SUPPORTED ON ULTRAFINE SINGLE-CRYSTALMAGNESIUM OXIDE
0.Takayasua, I. Matsuuraa, K Nittab and Y. Yoshidab aFaculty of Science, Toyama University, Toyama 930, Japan bCernent Research Center, Ube Industries Ltd., Ube 755, Japan
Abstract It was demonstrated that transition metal (especially Ru, Rh and Ni) catalysts supported on ultrafine single-crystal magnesium oxide (uscMgO) were better performing catalysts for the partial oxidation reactions of methane with oxygen or carbon dioxide to give nearly pure syngas stoichiometrically under the condition of lOs/h in space velocity and over 750 OC. The uscMgO not only has extremely great surface area but also heat stability in comparison with the MgO made from Mg(N03)2.
1. INTRODUCTION It is well known that C 0 2 and CH4 are the most serious greenhouse gases involved in environmental global warming. If C 0 2 and CH4 are converted into a pure syngas by the carbon dioxide reforming reaction of CH4 (CH4 + C 0 2 42CO + 2H2). it could be converted into many useful chemicals, for example, alcohols, hydrocarbons, aldehydes, etc. by using already operating industrial plants. In this regard, the partial oxidation reaction of CH4 with C 0 2 to give nearly pure syngas is supposed to be one of the useful chemical reactions for diminishing the amount of carbon dioxide emitted from industrial plants, especially power plants, iron works, and cement factories. Even though many catalysts for the reaction have been proposed by a few groups [ 1-31, we developed better performing transition metal catalysts. We used ultrafine single-crystal magnesium oxide (uscMgO) as a supporting material for the metals, not only because of the well-known support effect [4] but also because of its extremely great surface area and heat stability due to its hexahedral crystals in comparison with MgO made from Mg(N03)2 [5-71.
2. EXPERIMENTAL The uscMgO, 99.98% pure, was commercially available from Ube Industries Ltd. The metal supported on the uscMgO, i.e., Ru, Rh, Ni, Pt, and Pd, was introduced through the corresponding metal-acetylacetonate (M-AA)(Dojindo Lab. or Aldrich Chem. Co., Inc.). The catalysts were prepared in a special way in order to avoid MgO dissolving in water. That is, at first, M-AA and the MgO were mixed with each other, made into pressed pellets, and then heated at 300 OC for 4 h in a He stream to decompose M-AA. The pellets were crushed into small grains (25-30 mesh). The grains of 0.1 g was charged into a standard flow reactor. The ratios of feed gases were CH4/C02/He=15/15/70 in the case of the carbon dioxide reforming reaction of CH4 and CHq/Ofle=30/15/55 in the case of the oxygen reforming
1952
reaction of CH4 (CH4 + 1/202 -+CO + 2H2). Their overall flow rates were controlled to be 100 mL/min by mass-flow control systems (Stec Inc.). The reaction temperature was raised from room temperature to 800 OC at a 40-50 K interval. At each temperature after 10 min., 2 mL of gas was sampled and submitted to G.C. Since it was difficult to analyze the amount of H20 by G.C., it was calculated by material balance equations. 3. RESULTS AND DISCUSSION Figure 1 shows the result of 5-mol%Ni/uscMgO in the oxygen reforming reaction of CH4. The vertical axis shows the relative amounts of gases under the condition that the amount of inlet CH4 is normalized to be 100. It is obvious in the figure that the complete combustion of CH4 occurred to give C 0 2 and H 2 0 above 300 OC. However, the C 0 2 and H 2 0 disappeared above 750 oC to give CO and H2. In detail, the temperature that gave the maximum amount of H 2 0 was higher than that for C02. Hence it is considered that the oxygen reforming reaction of CH4 consists of complete combustion of CH4 followed by the carbon dioxide reforming reaction of CH4, the steam reforming reaction of CH4 ( CH4 + H 2 0 + CO + 3H2), and the water gas shifi reaction (C02 + H2 CO + H20). Figure 2 shows the results of 5-mol%Ni/uscMgO in the carbon dioxide reforming reaction of CH4. The products were CO, H2, and a small amount of water. CO and H2 were produced over 300 OC, and around 800 OC almost all C 0 2 and CH4 were stoichiometrically converted into CO and H2. That is, almost pure syngas was obtained around 800 OC. Two broken lines show the values of calculated thermodynamic equilibrium. Therefore, it is recognized that these reactants and products were approximately in equilibrium. Water was formed around 600 OC. The water is considered to be produced by the water gas shift reaction. However, over 700 OC the water disappeared to give CO and H2. That is, the steam reforming reaction of CH4 is considered to occur. Therefore, it is also recognized that the carbon dioxide reforming reaction of CH4 involves both the water gas shift reaction and the steam reforming reaction of CH4.
300
400
500
600
700
Reaction temperature /OC
800
300
500 600 700 800 Reaction temperature /oC
400
Fig. I . Result with 5-mol%Ni/uscMgO in Fig. 2. Result with 5-mol%Ni/ uscMgO the oxygen reforming reaction of CH4. in the C 0 2 reforming reaction of CH4. Inlet methane is normalized to be 100. Inlet CH4 is normalized to be 100. Catalyst=O. lg, GHSV=50,000 /h. Catalyst=O.l g , GHSV=50,000 /h.
1953 Table 1. Catalytic activities of the catalysts in the carbon dioxide reformine reaction At 750 OC Activation C07 CI-€4 H7 CO Energyof WeiehtGHSV Catalyst g 1000/h ;let “0; and CH4 are kcal/mol 700 - 800OC ormali ed to be 100 11.1 97 98 197 198 3-mol%Ni/uscMgO 0.1 50 50 92 91 183 181 9.6 1-mol%Ru/uscMgO 0.1 9.9 90 88 179 174 1-mol%Rh/uscMgO 0.1 50 50 87 88 174 171 18.6 1-mol%Pt/uscMgO 0.1 50 86 84 171 167 14.5 1-mol%Pd/uscMeO 0.1
.
m
(
)
150
hl €
8
100
c
0
c
l
200 1. 1-rnol%Ru/uscMgO 2. 1-rnol%Rh/uscMgO 3. drnol%NVuscMgO 4. 1-rnol%PVuscMgO 5. 1-rnol%Pd/uscMgO 6. 5-rnol%NVSiO2 7. 5-mol%NVA1203
hl €
22 0
;loo u) L
C
i!
50
0 300
0 400
500
600
700
800
Reaction temperature P C Fig. 3. Comparison of catalytic activities of the catalysts in the C02 reforming reaction of CH4. Catalyst=O.O2g, GHSV=250,000/h.
0
20
40
60
80
100
Time /h Fig. 4. Results of the endurance tests of 5-mol%Ni/uscMgO(-) and of 5-mol%Ni/hmMgO(----) for the CO2-reforming of CH4 ( 0 , ~and ) the 02-reforming of CH4 ( 0 ,B). Catalyst=O.l g, GHSV=50,000/h.
Figure 3 shows the activities of different metal catalysts by the amounts of CO formed in the carbon dioxide reforming of CH4. For distinctive comparison, the GHSV was increased to 250,000 /h, which is 5 times greater than that of Fig. 2. It is recognized that the order of the activities of these catalysts is in the sequence of Ru > Rh > Ni > Pt > Pd. This sequence is the same as that of the steam reforming reaction [8]. This agrees with the former conclusion that the carbon dioxide reforming reaction of CH4 involves the steam reforming reaction. Even though the catalytic activities were compared with those indicated by unit weight, the sequence was the same as that obtained from their activation energies as described below. Here, 5mol%Ni/uscMgO was used instead of 1-mol%Ni/uscMgO, because 1-mol%Ni/uscMgO showed poor reproducibility. In the figure, 5-mol%Ni/Si02 and 5-mol%Ni/A1203 are also shown. The Si02 was prepared from Snowtex-N colloidal silica (Nissan Chemical Industries, Ltd.). The A1203 was JRC-ALO-2. The catalysts were prepared in the same way as that for MgO. It is obvious that the activities of Ni/Si02 and Ni/A1203 are quite small compared with that of Ni/uscMgO. The differences in the activities of the Ni-catalysts
1954 supported on different materials were considered as folows. When C 0 2 reacts with CH4, at first, CH4 transforms into an adsorbed species and H2. The adsorbed species reacts with C 0 2 to give CO and H2. After the formation of H2, the water gas shift reaction takes place to give CO and H20. Then the water reacts with CH4, i.e., the steam reforming reaction takes place. Regarding the adsorbed species, Matsumoto [9] reported that, when hydrocarbon adsorbed on nickel, the adsorbed species are CH0.08(a) on nickel foil, CHo.s(a) on Ni/Si02. and CH2(a) on Ni/MgO. Since the adsorbed species on Ni/Si02 involves a much smaller amount of hydrogen in comparison with that on Ni/MgO, the adsorbed species on Ni/Si02 is readily transformed into kinds of coke. That is, Ni/Si02 has suffered from a deactivation problem by the formation of cokes. In other words, Ni/MgO has scarcely suffered from the deactivation. This would be one of the reasons that the catalysts supported on MgO are better performing catalysts in comparison with the catalysts supported on S O 2 or A1203 On the other hand, the water gas shift reaction takes place on MgO even without metal, but scarcely takes place on S i 0 2 or Al2O3. This effect could not be neglected, when we consider the difference in the catalytic activities of catalysts supported on different materials. Table 1 lists the catalytic activities at 750 OC. It is recognized that conversions of C 0 2 and CH4 are around 90%, and selectivities to H2 and to CO are nearly 100% on those catalysts. The 100% was obtained from the stoichiometric result that the amounts of CO and H2 formed were almost twice as much as those of C 0 2 and CH4 consumed. The activation energies for the formation of CO agree with the sequence of activities as described above. Figure 4 shows the results of endurance tests with 3-mol%Ni/uscMgO and 3mol%Ni/hmMgO in the reactions of oxygen reforming of CH4 and carbon dioxide reforming of CH4 at 800 OC. The hmMgO indicates the home made MgO made from Mg(N03)2. It is obvious in the figure that the life of Ni/uscMgO is much longer in the oxygen reforming reaction than in the carbon dioxide reforming reaction. Since the temperatures were the same in the two reactions, the extent of sintering of both Ni and uscMgO in the Ni/uscMgO can be considered to be the same in the reactions. Therefore, it cannot be considered that the extent of sintering of Ni and uscMgO in the carbon dioxide reforming reaction exceeded the deactivation of Ni/uscMgO in the oxygen reforming reaction. Hence, the difference in the activities of the Ni/uscMgO in the two reactions is attributable to deactivation by the formation of coke. On the other hand, deactivation proceeded faster with the Ni/hmMgO than with the Ni/uscMgO in the C 0 2 reforming reaction. This difference shows that Ni/hmMgO has suffered much from deactivation by the formation of coke than Ni/uscMgO. It needs further investigation. It is concluded that the transition metal catalysts, especially Ru, Rh and Ni supported on uscMgO are better performing catalysts than the metal catalysts supported on hmMgO. However, even Ni/uscMgO has suffered from a deactivation problem by the formation of coke.
4. REFERENCES 1 A. Saito, T. Sodesawa, and F. Nozaki, 52th CATSJ Meeting Abstracts A (1983) 46. T. Sodesawa, A. Dobashi and F. Nozaki, React. Kinet. Catal. Lett., 12 (1979) 107. Y. Sakai, H. Saito, T. Sodesawa and F. Nozaki, ibid., 24 (1984) 253. 2 J. T. Richardson and S. A. Paripatyadar, Appl. Catal., 61 (1990) 293. 3 J. Nakamura, S. Umeda, K. Kubushiro, T. Ohashi, K. Kunimori and T. Uchijima, Shokubai.. 33 (1991) 99. . A. D. Logan and A: K. Datye, J. Catal., 1 12 (1988) 595. 0. Takayasu, T. Hata and I. Matsuura, Chem. Express, 5 (1990) 829. Y. Hashimoto, 0. Takayasu and I. Matsuura, 6 (1991) 81. I. Matsuura, Y. Hashimoto, 0.Takayasu, K. Nitta and Y. Yoshida, Appl. Cat., 74 (1991) 273. E. Kikuchi, S. Tanaka, Y. Yamazaki and Y. Morita, Bull. Jpn. Petrol. Inst., 16 (1 974) 95. H. Matsumoto, Shokubai, 16 (1974) 122, 18 (1976) 71, Hyomen 15 (1977) 226
Guczi, L efal. (Editors), New Fronriers in Curalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
SURFACE OXYGEN SPECIES AND THEIR REACTIVITIES IN THE OXIDATION OF CH4, C2Hg AND C2H4 OVER CERIUM OXIDE AT MILD TEMPERATURES
C.LP, Q.Xinu, X. Cud and T.Onishib aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China bResearch Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan
Abstract The o x i d a t i o n o f CH, C2He and C2H4 on CeO, surface has been followed by i n s i t u F T - I R spectroscopy f r o m r o o m temperature t o 6 7 3 K . These hydrocarbons are oxidized on well-outgassed CeO, a t 473 K o r even lower temperatures. The surface common i n t e r mediate i s formate species which i s converted t o carbonate species a t temperatures exceeding 573 K . The i n i t i a t i o n o f t h e hydrocarbon o x i d a t i o n i s independent o f t h e presence o f t h e gaseous O 2 but depends l a r g e l y on the pretreatment o f CeO,. It i s proposed t h a t t h e surface l a t t i c e oxygen, most p o s s i b l y i n the c o o r d i n a t i v e l y unsaturated s t a t e created by t h e outgassing a t high temperatures, i s responsible f o r t h e m i l d o x i d a t i o n o f the hydrocarbons. Isotope experiments confirmed that the a t mild adsorbed 022- could react w i t h the hydrocarbons temperatures but t h e adsorbed 0,- i s almost i n e r t toward t h e hydrocarbons a t temperatures below 573 K.
1 . INTRODUCTION The studies on oxygen species and t h e i r r e a c t i v i t i e s have long been the important subjects i n the c a t a l y s i s o f o x i d a t i o n . I n p a r t i c u l a r , f o r t h e c a t a l y t i c o x i d a t i o n o f l i g h t alkanes, t h e a c t i v a t i o n o f t h e C-H bond i s f a c i l i t a t e d by t h e surface oxygen a t y p i c a l nonstoichiometric oxide, has been species. CeO,. f r e q u e n t l y used as an a c t i v e component o f t h e c a t a l y s t s f o r reducing automobile exhaust emissions owing i t s h i g h a c t i v i t y i n t h e o x i d a t i o n o f hydrocarbons. I n our previous work 1 1 1 adsorbed dioxygen species, superoxide(0,-) and ~ e r o x i d e ( O , ~ - ) , have been w e l l i d e n t i f i e d f o r CeO,. I n t h i s study, t h e r e a c t i v i t i e s o f surface oxygen species i n c l u d i n g both t h e dioxygen and surface C2Hs and C2H4 on CeO, have been l a t t i c e oxygen toward CH., investigated by using isotope l e 0 2 and i n s i t u FT-IR spectroscopy w i t h a view t o c l a r i f y t h e r o l e o f t h e d i f f e r e n t oxygen species played i n t h e hydrocarbon o x i d a t i o n .
1956 2 . EXPERIMENTAL Ce02 was prepared as described i n a previous paper [II . An oxygen-18 exchanged cerium oxide denoted as Ce'eO'eO was obtained by r e o x i d i z i n g a p a r t i a l l y reduced CeO, w i t h l e O z . The p a r t i a l l y reduced cerium oxide was obtained by t r e a t i n g CeO, i n HZ a t 673 K f o r 4 h. To remove t h e surface contaminants t h e sample was outgassed a t 1 0 0 0 K p r i o r t o t h e adsorption and r e a c t i o n studies. Surface oxygen species were detected by F T - I R and ESR spectroscopies. The o x i d a t i o n o f CH,. C2He and CzH4 on Ce02 was c a r r i e d out a t elevated temperatures and t h e surface species formed from the r e a c t i o n were followed by i n s i t u FT-IR spectroscopy.
3 . RESULTS AND DISCUSSION The o x i d a t i o n products o f CH, C2He and CH ,, over CeOn are and HZO which are g e n e r a l l y evolved a t temperatures mainly CO, above 573 K . However t h e surface r e a c t i o n takes place a t temFig.l(a) peratures f a r below 573 K on a w e l l outgassed CeO,. spectra o f surface species formed i n t h e shows the i . r . The i . r . bands o x i d a t i o n of CH, a t temperature range 373-673 K .
I
1800
I
I
lboo
1200
I
1600
cm-1
1800
1600
1400
c m-1
1200
F i g . 1 . I R spectra o f surface species derived from t h e o x i d a t i o n o f C H 4 ( a ) and o f C2H4(b) on CeOe a t elevated temperatures i n t h e presence o f 02. F , formate; C, carbonate.
1957 a t 1544, 13 71 and 1355 cm-’(two other bands a t 2933 and 2844 cm-’ are not shown) due t o formate species c a n be c l e a r l y observed a t 473 K . A t elevated temperatures carbonate species ( 1 4 5 4 and 1355 cm-’) were produced w h i l e t h e formate species disappeared. The s i m i l a r spectra have been recorded f o r t h e C2H4 o x i d a t i o n 121 as displayed i n F i g . l ( b ) . The surface formate o x i d a t i o n on species can be produced even a t 3 7 3 K f o r t h e C,H4 CeO, and t h e amount o f the formate species increased markedly a t 473 K . I n the absence o f gaseous 0, almost the same t h e r e s u l t s Fig. 3 c l e a r l y shows could be observed as w e l l as i n F i g . 1 . There i s the e f f e c t o f gaseous O2 on t h e C2H6 o x i d a t i o n on CeO,. no n a t u r a l d i f f e r e n c e f o r the observed surface species between the absence o f and the presence o f gaseous 0,. The gaseous O2 only enhances the amount o f t h e surface species. The surface formate species, i s commonly detected on CeO, f o r t h e o x i d a t i o n o f CH, C2H6 and C2H4 as reviewed i n [31. A t temperatures above 473 K , t h e surface formate species g r a d u a l l y decomposes i n t o surface carbonate species and i s f i n a l l y desorbed as CO, and H20. The f a c t t h a t t h e m i l d o x i d a t i o n of CH,. C2H4 and C2He on CeO, i s independent o f the presence o f 0, i n d i c a t e s t h a t t h e hydrocarbons are f a c i l e l y oxidized by the surface l a t t i c e oxygen which i s possibly i n the c o o r d i n a t i v e l y unsaturated s t a t e created by t h e outgassing a t h i g h temperatures. The adsorbed 02having an i . r . band a t 1126 cm-’ was d e f i n i t l y detected when introducing O2 onto t h e Ce02 surface a t temperatures below 373 K .
QI U
673 K
0, 0
C
C
0
O
n
20 573 K ul
n
U
173 K
L
0
4
373 K I
I
1I 10
1600
cm-1
1100
-I 12bo
ul
n
a 473 K 373 K 11 10
1
1600
1
1400
1: I0
c m-1
F i g . 2 . I R spectra o f surface species derived from t h e o x i d a t i o n o f C2H6 on CeO, a t elevated temperatures i n t h e absence o f 0 2 ( a ) F, formate; C, carbonate. and i n the presence o f O,(b).
1958 Table 1 Formate species derived f r o m the oxidation o f C2H4 on Ce’s02 and Ce’sO’eO a t 3 7 3 K i n the absence o f 02.
________________________________________---------------
Catalyst
.
Observed i r . bands(cm-’ )
Assignment
________________________________________--------------2933 2844 1544 1371 1355 HC’ eO’ soCe’ Ce’ -0’ eO
2 9 3 3 ( ~ )- 2929 2838 2919 2838
1 5 4 4 ( ~ )-1534 1369 1522 1361
1355(w) 1334 1319
HC’ HC’ HC’
eO’ eO-
________________________________________----------------------w,
weak, - - not observed.
The 0,was also confi rmed by ESR as reported i n [41. However the i . r . band o f adsorbed 02- w a s hardly affected by the presence o f CH, C2He and C2H4. The formate species derived f r o m the oxidation o f CH, C2H6 and C2H, i n the Presence o f ls02 and l e 0 2 e x h i b i t s the same i . r . spectra i n d i c a t i n g t h a t the produced HCOO- contains no oxygen-18. This enables us t o conclude t h a t the adsorbed 02- i s not the a c t i v e species f o r the o x i d a t i o n o f C2H, a t temperatures below 473 K. Table 1 gives the i . r . bands o f formate species produced f r o m the C2H4 o x i d a t i o n on the Ce’sO’eO surface a t 373 K. The i . r bands i n the Table 1 are majorly a t t r i b u t e d t o the formate species w i t h oxygen-18. namely, HC’sO’eO and HC’eO1eO, which o r i g i n a t e d f r o m the surface l a t t i c e oxygen anions of Ce’eO’eO instead o f the adsorbed 02- species. When CH4, C2He and C2H4 were introduced onto a p a r t i a l l y reduced cerium oxide, no any i . r . band due t o the oxidized species was observed a t elevated temperatures. But the formate species could be formed when the p a r t i a l l y reduced cerium oxide was exposed t o 02+CH4, 02+C2H4 or 02+C2H6 mixtures a t 473 K. The i . r . bands o f surface formate species were also s h i f t e d t o lower frequencies by using le02 i n the reactant mixtures. The 02,species can be c l e a r l y detected when the p a r t i a l l y reduced cerium oxide i s exposed t o O2 [ I ] . I t i s speculated t h a t the 02,- species has the r e a c t i v i t y i n the hydrocarbon o x i d a t i o n a t m i l d temperatures. This work was supported by the Natural Science Foundation o f China(NSFC).
4 . REFERENCES
1 2 3 4
C . L i , K. Domen. K. Maruya and T. Onishi, J . Am. Chem. Soc., 111(1989)7683. C. L i . Q. Xin and X . - X . Guo, ( a ) Chinese J. Mol. C a t a l . . 5 ( 1 9 9 1 ) 1 9 3 ; ( b ) Catalysis L e t t e r s . i n press. V. D. SokolovskII, Catal. Rev.-Sci. Eng., 3 2 ( 1 9 9 0 ) 1 . M. Gideoni and M. Steinberg. J. S o l i d State Chem. 4 ( 1 9 7 2 ) 3 7 0 .
Guni, L d al. (Editors), New Fronriers in Curolysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights resewed
OXIDATIVE CATALYTIC CONVERSIONS OF TGTRAHYDROFURAN DERNATIVVES R. Skolmeistere, L. Leitis, M. Fleisher and M. Shymanska
Institute of Organic Synthesis, Latvian Academy of Sciences, Riga, Latvia
Abstract During the oxidation of tetrahydrofuran derivatives on vanadium -molybdenum oxide catalyst (V:Mo = 1:9) at 360-430' the oxidative dehydrogenation of nucleus and also the oxidation or reduction of substituents are observed. The oxidation rearrangements of these substances are accompanied by reduction of vanadium ions. The oxygen of catalyst lattice is suggested to take part in the mentioned processes. 1. INTRODUCTION
The oxidation of furan and tetrahydrofuran (THF) derivatives with air oxygen on V-0 and V-Mo-0 catalysts (vanadia always being predominant) at 250-560" and 7 = 1-3 s results in the formation of maleic anhydride, the yield amounting up to 78% [ 11. In the present work we have compared the reactivity of THF, 2-methyl (THFMe), 2-hydroxymethyl (THFHy) and 2-formyl tetrahydrofurans (THFCHO) in the oxidation on V-Mo-0 catalyst containing the excess of MooJ (V:Mo = 1:9). This catalyst shows high selectivity in methylpyridine oxidation to pyridine aldehyde [2]. The binary vanadium magnesium system containing vanadium oxide from 9 to 12% was found to be active in the oxidative dehydrogenation of alkylaromatic and alkylheterocyclic compounds to the corresponding vinyl derivatives [3]. 2. EXPERIMENTAL
The V-Mo-0 catalyst (V:Mo = 1:9) consisting of vanadium oxides in molibdenum trioxide matrix was prepared according to [2]. The catalytic vapour phase oxidation by air was carried out in a microreactor (1 ml). Pulse volume - 0.4 pl. The reaction products were analyzed by GLC (Porapack Q, molecular sieves 13 X and column loaded with 10% of SE-301 and 2.5% of Replex 400 on Chromosorb W AW). Chromatographic massspectrometric analysis was carried out on a Kratos MS-25 spectrometer. The method described in [4] was used for vanadium valency determination. Quantumchemical calculations were made by the GEOMO method in CND0/2 approximation.
1960
3. RESULTS AND DISCUSSION
The investigation of the oxidation of THF derivatives on V-Mo-0 catalyst (V:MO = = 1:9) at 360 and 400" shows that the composition of the reaction products depends on
contact time. If the contact time is small (0.2 s) the oxidation of the substituents in THF derivatives, as well as the nucleus dehydrogenaiion and the complete oxidation occur (Table 1). Table 1 Catalytic rearrangements of the tetrahydrofuran derivatives in the presence of gaseous oxygen Contact time, s Conversion, %
Feed
T, "C
THF THFMe
360 360 400 360 360 360
0.2 0.2 0.2 0.6 0.2 0.6
8 9 27 70 49 78
360
0.2
15
THFHy THFCHO
Products (%)* DHF (6), co2 (1) THFCHO(4),COl(3) DHFMe(20),COz(2) FMe (S), MA (17), C 0 2 (40) THFCHO(22),COz(20) DHFMc(2O),DHF(16), THFCHO(3),CO,(28) co, (12)
*FMe - 2-methylfuran, DHF - 2,3-dihydrofuran, DHFMe - 2-methyI-2,3dihydrofuran, MA - maleic anhydride The oxidation of the substituents occurs at the temperature lower than that for the nucleus dehydrogenation. At the oxidation of THFHy the increase of the contact time up to 0.6 s results in the formation of DHFMe. Thus, under the experimental conditions the reduction of hydroxymethyl substituent to the methyl group takes place. The water does not influence the reduction process. The methyl derivative of DHF formed in the presence of heavy water does not contain deuterium. This fact means that the only source of hydrogen may be the nucleus hydrogen. DHFMe is found to be the main product at THFHy oxidation in He atmosphere (Table 2). The data given in Table 2 show that the vanadium oxide lattice oxygen is able to take part in tetrahydrofuran nucleus dehydrogenation and in the complete oxidation. In the absence of air oxygen up to 20% of THFMe were converted into DHFMe with 80% selectivity. The selectivity increases due to the decrease of the complete oxidation in comparison with the oxidation in the presence of oxygen. The maximum degree of the vanadium reduction is reached in the case of THFMe and equals 78%. In the steady state in the presence of air oxygen the reduction degree of vanadium is equal to 18-22%. The above reactions do not occur on the individual Moo3. The formation of positively charged surface complex of furan derivatives coordinated on the reduced vanadium ions (Lewis acid centres) has been suggested to promote their oxidation to maleic anhydride [ 5 ] . V-Mo-0 (V:Mo = 1:9) catalyst contains the highest
1961
Table 2 The catalytic rearrangements of the tetrahydrofuran derivatives on'V - 0 - Mo catalyst (V:Mo = 1:9) in the absence of the gaseous oxygen (T = 0.6 s) v5 + :v4 + 5 7 3 + Feed T, "C Conversion, % Products (%)
THF THFMe THFHy
400 360 400 430 360 420
22 10 25 75 47 80
DHF (lo), COz (8) DHFMe(9) DHFMe (20), COz DHFMe(42),C02(28) DHFMe (18), COz (23) DHFMe(36),FMe(24), THFCHO (l), COz (10)
50:50:0
22:21:57 32:33:45
amount of vanadyl ions (basic centre) in comparison with other V-Mo-0 catalysts [ 6 ] .If tetrahydrofurans coordinated on this centre form negatively charged surface complex C4-H3 bond in THFMe is very weakened and the 2,3-dihydrofuran formation may be expected (Table 3). In the case of THFHy the dehydrogenation of hydroxymethyl group is possible. In the THFCHO molecule C-0 and C = O bonds are very weakened, thus causing the destruction of the whole molecule. Table 3 The change of THF derivatives surface complex bond strength, %
2H,
1
lHNC\
R
-C6-
-C6-
-C6=
/ H8 H9 HI0
' /
'
O+H10 H9
/ H8 02
I
,H7
/c2b
01
Bond
The change of bond strength, %
c 3 -c 4 c4 -c 5 C4 -H3 C4 -H4 C5 - H2
+8 +7 4 9 -8 -3
C6 - 0 2 02-H10
+9 -79
0 1 -a C2-H7 C6 = 0 2
-2
4 -20
1962
The results given above permit to conclude that in the oxidation of THF derivatives on vanadium-molybdenum oxide catalyst (V:Mo = 1:9) the oxygen of the basic centre takes part in nucleus dehydrogenation. The formed hydrogen ions reduce the hydroxymethyl group. The gaseous oxygen takes part mainly in the oxidation of nucleus and the substituents.
e.REFERENCES 1 2 3 4
5 6
M.Shimanska, Zh.Yuskovets, VStonkus, Wlavinska, D.Kreile, A.Avots. Contact Reactions of Furan Compounds, Riga, 1985. R.A.Skolmeistere, O.V.Orbidane, L.Ya.Leitis, M.V.Shymanska, V.S.Aizbalts. Invent. Certif. 925951 (USSR). Bull. Izobr. 17 (1982) 118. A.V.Simakov, N.N.Sazonova, S.A.Veniaminov, D.F!Belomestnykh, N.N.Rozhdestvenskaya, G.V.Isaguliants, Kinetika i Kataliz, 30 (1989) 684. M.Nakamura, K.Kawai, Y.Fujiwara, J. Catal., 34 (1974) 345. L.O.Golender, M.V.Shimanskaya, React. Kinet. Catal. Lett., 13 (1980) 85. Yu.Sh.Goldberg, M.V.Shymanska, React. Kinet. Catal. Lett., 7 (1979) 193.
Guczi, L d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1 9 2 , Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
ROLE OF Mo AND Sb IN OXIDE CATALYSTS FOR SELECTIVE OXIDATION OF PROPYLENE B. Zhofl, X. Cud and R T.Chuangb aNational Laboratory of Fundamental Research in Catalysts, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China bDepartment of Chemical Engineering, University of Alberta, Edmonton, T6G 2G6 Alberta, Canada
Abstract
The activity of Moo3 - Sb2O4 catalyst for propylene oxidation was found to relate with the acidic property of Moo3 and redox property of Sb2O4. Results also indicate that the adsorption of propylene occurs on the Lewis sites, and the subsequent abstraction of a-hydrogen, which is the rate-determining step, takes place on the Bronsted sites. Both sites are located on the surface of Moog. The presence of SbzO4 in the catalyst facilitates the dissociation of molecular oxygen into active species which can subsequently migrate onto the surface of Moog, resulting in the reoxidation of the reduced active sites. INTRODUCTlON
.Mo and Sb have been postulated as the most important catalytic elements for the amm)oxidation of propylene [l]. Many studies have focused on the relations ip between surface property and activity for catalysts containing Moo3 or SbzO4, but their role in these catalysts is still not clear. The objective of our study is to elucidate the effect of these two elements in the selective oxidation of propylene; more specifically, the influence of acidic property of Moo3 and redox property of Sb2O4 on different elemental step of propylene oxidation.
h
EXPERIMENTAL
The catalysts were prepared by dispersing MOO and Sb2O4 in n-pentane. The detailed procedure has been described elsewhere ?2] The catal sts were characterized by BET, XRD, SEM, AEM, XPS and ISS techniques whic were also reported in ref [2]. Pyridine adsorption was studied by infrared spectroscopy. The spectra were obtained on a Pekin E er 580B infrared spectrophotometer operating in the range of 800 to 4000 cm-? The results were used to identify the Lewis and Bronsted acid sites on the catalysts. TPD of ammonia was carried out in order to determine the amount of Lewis and Bronsted sites resonance. The reoxidation of reduced catalyst was studied by electron spin
K
1964 resonance (ESR). A previously reduced Moog. gfs mixed with Sb2O4 in this mixture gave a accordin to a procedure described in ref [2]. the d o ! signal. The mixture was then reoxidized by oxygen (1% 0 2 in 99% strong ER He t 403K. The ESR spectra were recorded over a period %f 8 hours. The Mc$$ concentration was expressed as the relative intensity ([Mo '1 rel.) [Mo5+] rel. = (I/I0)hH2. (l/m)(l/MoO3wt%) where I is intensity of the Mo5+ ignal,; lo is the intensity of the internal standard; AHis width of the si nal for Mo5'; m is mass of the sample ar&MoOg wt% is the weight percent of 003 in the sample. The decrease of [Mo ] rel. represents the reoxidation degree of the mixture. Catalytic activity was measured by on-line gas chromatography. Reaction products were acrolein, Cop and water.
d
RESULTS and DISCUSSION
-
The structure of the Moo3 Sb2O4 catalyst was extensively characterized by the physico-chemical techniques, and the results were used to elucidate the reaction mechanism. Figure 1 presents the microanalysis data by AEM. A reat number of SbpO4 and MOO particles was analyzed and the results show t at each particle is pure, i.e. mutua contamination is not detected. The characterization results obtained from XRD, SEM, XPS and ISS 21 lead to a conclusion that (1) the catalyst is composed of pure Moo3 and Sb2 4 articles, (2) these particles are well dispersed and exist with an intimate contact etween each other. The IR spectra of a sample containing 50 wt% Moo3 before and after adsorption of pyridine are iven in Figure 2,, After the adsorption, the bands at 1640, 1617, 1580, 1542, 14 2 and 1451 cm' appear (compared Fig.2a with 2b). The bands at 1617, 1580, 1492 and 1451 cm; disappear after outgassing at 383K (Fi , 2c) and those at 1640 and 1542 cm' disappear only after evacuation at 503K Fig. qd). According to the literature [3,4], the bands at 1617, 1580, 1492 to pyridine chemisorbed on the Lewis acid sites; those at and 145 cm' are 1640 and 1542 cm' are due to pyridine chemisorbed on Bronsted acid sites. The bond between pyridine and Bronsted site is stronger than that with Lewis site because the desorption of pyridine from Bronsted site occurred at a temperature (503K) higher than that for the desorption from Lewis site (383K). Similar results were obtained for the temperature programmed desorption Two desorption peaks were of NH3 from the catalysts containing MOO observed, one at 383K which corresponds to d l 3 desorbed from Lewis sites and another at 498K for the desorption from Bronsted sites. The amount of NH3 desorbed from both sites was determined by comparing the peak area with the integral thermal conductivity signals obtained from known amounts of NH . Figure 3 presents the catalytic activity as a function of t e NH3 concentration on different sites. It shows linear relationship between the propylene conversion and the amount of NH3 desorbed from Lewis sites. Similarly, the acrolein yield is found to be directly proportional to the amount of NH3 desorbed from the Bronsted sites. It should be mentioned that pure Sb2O4 does not possess any acidic properties, 1.e. pyridine and ammonia are not adsorbed on the Sb20 surface. As a result, pure SbpO4 is not active for the propylene oxidation. 8 u t when
p1
9
b
E
8
I
ye
ii
1965
Sb2O4 is mixed with Moog, a dramatic increase of catalytic activity can be observed. To understand the role of Sb2O4 in the catalyst, a series of experiments was carried out. A previous reduced molybdenum oxide, MOO^-^, was mixed with Sb 04 and then reoxidized by gaseous oxygen. The reoxidation was foll w e t b y ESR measurements. Figure 4 presents the relative intensity of the Mo9+ signal (g = 1.93, width: ca. 100 Gauss) as function of reoxidation ti?? Before reoxidation (time < o), all samples have practe$ly the same Mo concentration. As soon as the reoxidation begins, the Mo concentration in the mixture is alwaygdower p f n that in pure Moog. This indicates that the reoxidation of Mo to Mo in the mixture occurs more rapidly than that in the pure Moog. An increase of the Sb2O4 content in the mixtures progressively accelerates the reoxidation. It is generally accepted [I] that the selective oxidation of propylene consists of four elementary steps: (1) chemisorption of gropylene, (2) abstraction of a-hydrogen to generate allylic species, a rate- etermlning step, (3) insertion of lattice oxygen into the allylic intermediates and (4) the reoxidation of the active site back to its initial and active state once it has undergone a reduction cycle during the oxidation of propylene to acrolein. Step (4) is vital in maintaining the catalyst activity. Our results suggest that propylene, bein a weak base, can be chemisorbed on the Lewis acid sites and converte to either acrolein or C02. The abstract of a-hydrogen which is the rate-determinin step leading to the acrolein formation is certainly related to Bronsted sites. Bot sites are located on the surface of MOO The role of Sb2O4 is to dissociate the gaseous oxygen into active species whict can migrate, at the contact region between the two oxides, onto the surface of Moog, where they complete redox cycle by reoxidiring the reduced molybdenum oxide. This results in the regeneration of deactivated sites on the surface of Moo3 during the reaction, and thus maintains the catalyst in an active and selective state. The catalysts for (amm)oxidation of propylene used in the industrial applications usually contain Mo and/or Sb oxides. Our study gives a clue for understandingthe behaviors of these kind of catalysts.
8
R
REFERENCES
1. 2. 3. 4.
R.K. Grasselli, G. Centi and R. Trifiro, Appl. Catal. 57 (1990) 147. B. Zhou, S. Ceckiewicz and B. Delman, J. Phys. Chem. 91 (1987) 5061. E.P. Parry, J. Catal. 2 (1963) 13. K. Tanabe, "Solid Acids and Bases", Academic Press, New York, N.Y. 1970.
1966 NH3 desorbed from Lewis sites (M mol/m2 cata)
fl0
2
I
I
I
Energy I kev
I 01 5
Figure 1 AEM spectraof (1) Sb204 partlcles (2) MoQ particles
I
1.5
10
NH3 desorbed from Bronsted sites mol/rn* cata)
Figure 3 Relationship between (a) propylene conversion and N& desorbed from Lewis sites @) acrolein yield and N* desorbed from Bronsted sites.
150
L Y
*-
100
-a
-r so
I
2000
1500
1000
I
I
Wavenumber
I C1-l 1
Figure 2 Infrared spectra of the sample containing -96 Mom: (a) before pyridine adsorption; @) after pyridine adsorption and outgassing at 3531(; (c) @) t outgassing at 383% (d) (c) t outgassing at 503K
0
2
L
6
Time ( h l Figure 4 Concentrationof ~ o 5 in Mom Sb204 as a function of reoxidation time.
-
8
Guni, L u al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Calalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights nserved
PARTIAL OXIDATION OF METHANE AT LOW PRESSURE OVER SILICA GEL AND SILICASUPPORTED Sn, Zr AND Ge OXIDES
T.Ono, K Ikuta and X Shigemura Department of Applied Chemistry, University of Osaka Prefecture, 804 Mom-Umemachi 4ch0, Sakai, Osaka 591, Japan
Abstract The methanol and formaldehyde as well as CO and COa were produced by the oxidation of methane uaing Oa over some silica gels and silica-supported Sn, Z r , and Ge oxides a t around 10 t o r r and a t 600°C. The role of these oxides as catalysts in the partial oxidation of methane was discussed.
The conversion of methane t o methanol o r formaldehyde has been studied by many worker&-2). Kastanas e t a1.,(3) have reported t h a t formaldehyde is selectively formed on various silica. The direct oxidation of methane into methanol a t high pressures has been reported by Burch e t a1.(4) and Hunter e t al.6). The role of glass o r silica surface seems t o be still unclear. In this work, we have studied the methane oxidation a t low pressure(l0 Torr) using silica gel and some silica-supported group IV oxides such as SnOa, ZrOa, and GeOa i n order t o understand the role of these oxides as a catalyst in methane oxidation. The homogeneous and heterogeneous processes in methane oxidation are also discussed.
The oxidation of methane was carried o z t using a quartz reactor and a closed circulation system(ca. 300 cm 1. The reaction was done a t around 600°C. The quartz reactor showed a l i t t l e activity t o methane oxidation, but its contribution w a s below 10% a t around 600°C. The t r a p a t liquid nitrogen temperature was placed next t o the reactor in the circulation path t o collect the products such as methanol, formaldehyde, COa, and HaO. In this case, the maximum pressure of methane is Limited t o a vapor pressure a t -196”C,i.e.,
1968 ca. 10 Torr. The amounts of methanol and formaldehyde were determined by gas chromatography using TSR-1 column. Some commercial silica gels were used as catalysts. The Ge, Zr, and Sn oxides were supported on Aerosil 300 by an impregnation method, using the chlorides dissolved in ethanol.
3.RESULTSand DISCUSSION 3.1 Partial oxidation of methane over some silica gels Table 1 shows the results of methane oxidation over various silica gels. The CO formation is negligibly small in these cases. Table 1 Methane oxidation over various silica gels Silica gel
Surfaace Rate of Area/m /g conversion /%/g h Fr a ct o sil^’ 40 2.0 vy c o r”’ 150 1.3 ADM”’ 12 1.9 ~i02*’ 240 4.2 0x50’’ 50 2.4 A130 130 2.4 MoXl70 170 1.6 A300 300 1.4 380 1.3 A380
Product selectivity/% COa HCHO CHSOH
60 77 40-76 62 100 96 84 48 57
8 0 0 12 0 t t 2 21
32 23 60-25 26 0 4 16 50 22
Experimental conditions: 600”C, reaction time 1 h, 7% of Oa, and catalyst 0.3g. a) Merk, b) Corning, c) Shin-Etsu Chemicals, dlprepared by sol-gel method in this work, and e) Japan Aerosil(0X t o A): Numerals denote surface area. On some silica gels, methanol is formed as shown in Table 1. On SnOa and ZrOa, no methanol formation was observed although they have 100-1000 times higher activities than SiOa. GeOa had a l i t t l e activity t o methanol and formaldehyde. It resembled t o SiOa. AerosilOX50, however, has no selectivity t o methanol but A300 has high selectivity t o it. Such differences of the selectivity t o methanol among various silica gels seem t o come from t h e i r surface characters. The r a t e of methane conversion on A300 depends on oxygen concentration as shown in Fig. 1. The methanol selectivity has small dependency on the oxygen concentration. According t o J.
1969
Mathias e t al.(B), the f r e e silanol group density is higher f o r 0x60, while the bridged silanol group increases f o r MOO and 380 which have high surface area. These characters seem t o be related t o the reaction selectivities a f t e r the evacuation of silanol groups as Ha0 a t around reaction temperatures. According t o Kastanas e t al.(3), formaldehyde is the main product over A800 silica. This difference might be caused by the oxygen concentration and the reaction method.
3.2 Partial Oxidation of Methane over Silica-supported oxides. Methane oxidation over SnOa/SiOa was carried out a t 600-600°C. The rates over these catalysts were several times higher than those on silica gel(A300). A t low Sn content between 0.01-0.3 wt% of SnOa, fonnaldehyde and methanol were formed in ca.6096, while at Sn content between 1-6%, methane wa8 mainly oxidized t o COa and CO. As shown in Fig. 2, over SnOa(O.lwt%)/SiOa,methanol selectivity is ca. 60% below 1 % of Oa concentration. Formaldehyde formation increases above 1%of O a and passes through a maximum a t several% of 00. This situation is very different from t h a t over ASOO, i.e.,
02 I OIO Fig. 1. Methane conversion and selectivity as a function of Oa concentration over A300 a t 600°C.
Fig. 2. Methane conversion and selectivity as a function of Oa concentration over SnOa(O.l wt%)/SiOa(A800) a t 600'C.
1970
the methanol selectivity is kept high even a t 10% of oxygen on it. With the SnOa/SiOa oxide, methanol is oxidized rapidly t o formaldehyde. The activity and selectivity of methane oxidation was compared with other silica-supported oxides. The order of methane oxidation activity was obtained as SnOa> ZrOa> GeOa= SiOa. Among these catalysts, the SnOa/SiOa has the highest activity and the product selectivities varied with oxygen concentration. The ZrOa/SiOa showed a l i t t l e lower activity than the SnOa/SiOa. Their selectivities t o methanol and formaldehyde were ca.10 and 30%, respectively, over O.lwt%catalyst. But they had no selectivity above the 2wt% catalysts even a t 1.6% of Oa concentration. With GeOa/SiOa, its activity was similar t o t h a t of A300, the selectivity to formaldehyde being a l i t t l e higher. AccordingtothemechanismofgasphaseoKidationofmethane(l), methanol is formed a t lower oxygen concentration rather than a t higher concentration. In this work, the product selectivities depend on the catalysts used as well as on oxygen concentrations. The heterogeneous process seems t o be important over silica and silica supported oxide catalysts. The initiation (methyl radical formation) seems t o occur by the surface oxygen. "he methoxy radical may be formed in both heterogeneous and homogeneous processes. The former possibility is present in this case. Methanol should be produced by the reaction between methoxy radical and methane homogeneously. With SnOa/SiOa, the rate increase seemsto be originatedfromthe enhancementinthemethyl radical formation by the presence of t i n oxide. However, methanol is also dehydrogenated consecutively by its surface oxygen.
1 R. Pitchai and K. Klier, Catal. Rev.-Sci. Eng. 28 (1986) 13. 2 John C. Mackie, Catal. Rev.-Sci. Eng. 33 (1991) 169. 3 G. N. Kastanas, G. A. Tsigdinos and J. Schwank, Appl. Catal., 44 (1988) 33. 4 Robert Burch, Gravin D. Squire and Shik C h i Tsang, J. Chem. SOC., Faraday Trans. 1, 85 (1989) 3561. 6 N. R. Hunter, H. D. Gesser, L. A. Morton, and P. S. Yarlagada, Appl. Catal., 57 (1990) 45. 6 J. Mathias and G. Wannemacher, Technical Bulletin Pigments No.28, Degussa AG, Frankfurt/M. 1986.
Guni, L u af. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International C o n p on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights nsewed
THE INFLUENCEOF THE SUPPORT ON THE PERFORMANCE OF HETEROGENEOUS CATALYSTS FOR THE WACKER OXIDATION OF ALKENES A. W. Stobbe-Kreemers,J. J. F. Scholten, M.Soe& and J. W.Veenman Department of Chemical Process Technology, Delft University of Technology, Jvlianalaan 136,2628 BL Delft, The Netherlands
Abstract The reactivity of palladium sulfate impregnated vanadium oxide monolayers on yAl,03, TiO, (anatase), ZrO, and SiO, has been investigated with temperature programmed reduction. A relation has been found between the activity of the catalysts in the Wacker oxidation of ethene and the reducibility of the vanadium oxide monolayers. 1. INTRODUCTION
Recently we found that the activity of heterogeneous catalysts for the Wacker oxidation depends on the type of support material. Catalysts supported on TiO,(anatase) showed a much higher activity in the oxidation of ethene than y-Al,O,-supported ones [ 11. These catalysts, developed by van der Heide [ 11, consist of a porous support material, covered with a monolayer of vanadium oxide. On this monolayer a palladium(II)salt, such as PdCl, or PdSO,, has been deposited. Mechanistic and kinetic studies revealed that the reaction sequence over these catalysts is independent of the support [ 11: * Pd + acetaldehyde 4 HCl (1) H,PdCl, + ethene + H,O(g) Pd + V,O, + 4 HCl * H,PdCl, H,V,O, (2) H,V,O, + Yi 0, 4 V,O, H,O (3) Since the supports themselves are catalytically inactive, their effect on the activity of the catalysts is probably due to their interaction with the vanadium oxide. This interaction might result in a better reducibility of the vanadium oxide, thus enhancing step (2). In this paper the reducibility of vanadium oxide layers on different support materials has been investigated with temperature programmed reduction (TPR).
+
+
+
2. EXPERIMENTAL
Catalysts were prepared on y-Al,03 (AKZO, OOl-lSE), TiO, (anatase) (RhGnePoulenc, CRS-3l), ZrO, (Engelhard) en SiO, (Degussa, Aerosil 200V). Vanadium oxide
1972
monolayers on y-Al,03, TiO, and ZrO, were prepared by adsorption from an aqueous NH,VO, solution (10 g/l, pH=4) at 340 K. On silica the vanadium oxide was deposited by precipitation of an electrochemically prepared V(II1) solution [2]. The catalysts were washed with distilled water, dried 20 h at 350 K and calcined in air for 4 h at 673 K. After calcination palladium was applied by impregnation to incipient wetness with a PdSO, solution. The catalysts were dried and stored in air at 350 K. Pd loadings were chosen in such a way that the V/Pd atomic ratio was approximately constant for the four catalysts as shown in Table 1. The activity of the catalysts in ethene oxidation has been determined at 373 K. The feed was composed of 1% ethene, 7% water vapor and 92% air, total flow 100 ml/min. TPR was performed in a 67% hydrogedargon mixture at a heating rate of 10 Wmin. Table 1 Composition of the catalysts
~~
TiO, (anatase) ZrO, Y-A1203
SiO, V,O, (bulk)
112 55 250 186 4
7 2 9 13 99
0.6 0.1 0.6 1.o 1.0
3. RESULTS AND DISCUSSION
The activity of the catalysts, shown in Table 2, clearly depends on the type of support material. The TiO, supported catalyst shows the highest activity followed by the ones on ZrO, and Al,O,. The catalyst on SiO, showed almost no activity and after one hour the production of acetaldehyde stopped completely. The total amount of acetaldehyde produced by this catalyst corresponds roughly with the starting amount of Pd2+ in the catalyst, indicating that no reoxidation of palladium took place. To find out whether these differences in activity are related tot differences in reducibility of the vanadium oxide, the catalysts were investigated with TPR. Figure 1 shows the reduction profiles of the catalysts. The profiles show two reduction peaks, one sharp peak at ca. 373 K and one at higher temperatures, ca. 600 K. The low temperature peak represents the vanadium oxide reduction, while the high temperature peak can be attributed to either further reduction of the vanadium oxide, reduction of the support or of the impregnated sulfate. The reduction of palladium proceeds very rapidly at room temperature and is not visible in these profiles. Figure 1 and Table 2 clearly show that the temperature of vanadium oxide reduction varies with the support. The lowest reduction temperature, 360 K, is found for the TiO, a slightly higher temperature of 385 K is found. The profile catalyst. For ZrO, and A120,
1973
Table 2 Activity at 373 K and reducibility of supported vanadium oxide catalysts support
relative activity
TiO, (anatase) ZrO, Y- 4 0 3
SiO, V,O,( bulk)
T,,(K)
355 370 380 >400 475
10 2 1 0
Vanadium reduction vS+ vS+
vs+
+
~
+
v5+ +
v2.4+ v2.4+ v4.3+
?
v5+ + v3+ 4 v2+
of the SiO, catalyst consists of several broad low peaks and the temperature of vanadium oxide reduction is at least 400 K. The reduction of bulk vanadium pentoxide impregnated with PdSO, proceeds in 2 steps, the first at 475 K and the second at 675 K. Apart from the difference in reduction temperature also a difference in the degree of vanadium oxide reduction is found for the four supports. The TiO, and ZrO, catalysts both show a reduction of V5+ to V2.,+. The Al,O, supported catalyst on the contrary, shows only reduction to V4.3+.For the silica supported catalyst no final oxidation state can be calculated since no distinct peaks are present. Vanadium pentoxide is reduced via v3+to v2+. The reduction of vanadium oxide is strongly catalysed by palladium. Without palladium, vanadium oxide reduction occurs between 600 and 950 K [3]. However, in the presence of palladium vanadium oxide reduction already proceeds at 373 K [4]. 1000 r
200
400
600
800
1000
Temperature (K)
Figure 1.TPR profiles of supported PdSOY,O, catalysts. Heating rate 10 Wmin.
1974
Van der Heide[4] showed that the reduction proceeds via a hydrogen spill-over process from palladium to the surrounding vanadium oxide. Several studies on supported vanadium oxide catalysts revealed that the structure of vanadium oxide is almost the same on different supports, but is largely dependent on the vanadium oxide coverage [3,5]. The differences in reactivity of the vanadium-oxide monolayers found in this paper and by others [3,5], however, point to essential differences of the layers on different types of supports. Deo and Wachs [5] recently suggested for supported vanadium oxide catalysts that oxygen atoms positioned between vanadium and the support are the active ones during methanol oxidation. This implies that the differences in reactivity of vanadium oxide can be explained by the different bonding strengths between vanadium and the cations of the supports. This view is also supported by the fact that the TiO, and ZrO, supported catalysts show the lowest reduction temperatures. Since it is known that the surfaces of titania and zirconia can bereduced relatively easily [6,7] it is likely that the oxygen bonds between these oxides and the vanadium oxide are more reactive than V-0-AI or V-0-Si bonds. A comparision of the activities in ethene oxidation (Table 2) with the temperatures of reduction reveals that the catalysts with the most reactive vanadium oxide layer show the highest activity in Wacker oxidation. The bad performance of the silica suppported catalyst can easily be explained. The TPR profile shows that the vanadium oxide in this catalyst can only be reduced at temperatures above 400 K, whereas the ethene oxidation is carried out at 373 K. At this temperature the redox system is not active, which results in a rapid deactivation of the catalyst. 4. CONCLUSIONS
The reducibility of palladium-sulfate impregnated vanadium oxide layers varies with the support, and is highest on TiO, (anatase) and ZrO,. This higher reducibility is possibly related to the fact that both these supports can be reduced relatively easily. Furthermore, the vanadium oxide layers on TiO, and ZrO, are reduced to an average oxidation state "f v2.5+ ,whereas on y-Al,O, reduction to only V4.3t is found. The results reported in this paper prove that a better reducibility of the vanadium oxide layer leads to higher activities in ethene oxidation.
5. REFERENCES 1
2 3 4 5 6 7
E. van der Heide, M. de Wind, A.W. Gerritsen arid J.J.F. Scholten, Proc. 9th I.C.C. Calgary, 4 (1988) 1648 E. Vogt, Ph.D thesis Utrecht 1989 F. Roozeboom, A.J. van Dillen,J.W. Geus and P.J. Gellings, Ind. Eng. Chem. Prod. Res. Dev. 20 (1981) 304 E. van der Heide, Ph.D. thesis Delft 1990, chapter !> G. Deo, I.E. Wachs, J. Catal. 129 (1991) 307 S.J. Tauster, S.C. Fung, R.T. Baker and J.A. Horsley, Science 221 (1981) 1121 S.J. Tauster, Acc. Chem. Res. 20 (1987) 389
Ouczi, L et al. (Editors), New Frontiers in Caalysb
Proceedings of the 10th International Congrcso on Catalysis,19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights rcservcd
OPTIMIZATION OF NiO/MoOfle02 CATALYTIC SYSTEM FOR DIRECT OXIDATION OF PROPENE TO ACRYLIC ACID C. Mazzocchiaa, R. Anouchinsky4 A. Kaddouri" and E. Tempestib
aDipartimentodi Chimica Industrial ed Ingegneria Chimica del Politecnico, P.za Leonard0 da Vinci 32, 20100 Milano, Italy bDipartimentodi Ingegneria Meccanica, Universita di Brescia, Via Valotti 9,25060 Brescia, Italy
Dehycfogmation of alkanes to alkenes is an impartant route to convert them into more useful chemicals. Starting from propane, the propene obtained can be used to conventionally produce acrylic acid in two stqm (via aadein). We me obviously interested in single-step procedwes which globally convert propane to popene (first step) and propene to acrylic acid (second step). This has not yet been realized on an industrial scale.Accordngly we decided to investigate on the t m m y catalytic system NiO/MoOfle@ by using the same methodological approach used in the oxidative dehycfogenation of propane with binary system Ni01Mo03 [l], e.g. by emphasizing the rde of any single component 121.
The catalysts have been prepared in three ways: a) d i d state reaction - b) coprecipitation - c) mechanical mixtures. The rationale which presides over the propwative procedures must be rigorously controlled in ardor to obtain pue phases [3,4].Several analytical techniqes have been employed to characterize the samples; ptxt~cularly X ray diffractometry, IR and L a w Raman spectroscopy, electronic miaoscopy, thermal analysis (TG-DTA) and X ray photoelectron spectroscopy (XPS). Speufically the a or fl phases of NiMQ have been defined by using high temperature XRD [2], while the presence of MoOg has been evidenced by using Laser Raman Specdr.oscopy. TepMoO7 has been studied by DTA (melting point), XRD (to a s m a its puity with respect to the presence of the starting o x i h in the final product) and I.R. Catalytic tests have been run in a quartz flow reactor (1 20 cm - 4 10 mm) with the catalyst finely dispersed into a ganular cerbarundum matrix. The feed composition was 10% propone, 10% oxygen and 80% nitrogen. For some kinetic teat8 a feed mixture composed of 5% aadeine, 15% oxygen and 80% nitrogen.w&s also employed.
-
-
and Diw&gn After the assessment of the catalytic performances of the pub phases (table 1, run 1,2 and 3), the optimal ratio between MoO3 and N i M a has been defined (runs 4 - 7)
1976
following the improvement of the C3H402lC3H40ratio. Indeed a NiMo04 (run 2) is not selective while with Te2MoO7 (run 3) only a very small C3H402/C3H40 ratio is obtained. Table 1 Conversions and selectivities found in propene oxidation -___
RunSb Surface T(’C) C Area [ m2/g]
C3H4O C3H402 C0zC COc C2H40 C2H402
-
-___
400
0.3
60.6
40.0 420
10.5 1.7 14.2 17.6 9.4 2.6 78.3 97.0 96.0
5.1 76.6 6.1 13.0 30.0 47.3 53.0 28.4 11.8
1)
2.3 1.o 32.0 21.4 8.1 6.0 21 .o
420 420 420
440 450 420
33.3
-
91.1 10.2
8.7 1.5 30.0 22.2 8.7 27.0 45.4 58.0
0.9 0.8 1.0 1.6 2.7 0.2 0.2 0.3
ei5.o
47.0 41.1 39.1 12.5 6.1 16.1 8.4 13.8 10.0
-
I catalysts used :1) M a g ; 2) NiMo04; 3) Te2MoO7;
1.0 3.5 4.0 7.3 4.5 3.2 0.3 0.6 4.0
--
4) NiMo04 0.1 M a g ; 5) NiMoO4 2MoO3; 6) NiMo04 5Mo03; 7) NiMoOq 12MoO3;8. 9, laid) NiMoO,* 2MoO3 Te2MoO7 (7.25% wlw) b Contact time ( g h/l) is 0.2 in run n.1, 0.3 for runs n. 9 and 10, 0.1 in the other cases. c - CO and C02 are iointly evaluated in runs n. 1, 2, 4, 5, 6 and 7. d - 10% water in the feed composition in run n. 10.
-
Effect of To on the rate of formation of QH4O This effect has been evaluated by using catalyst 5 and 10 of table 1 in the presence of 7.25% wlw % of Te2MOOi (Catalyst 5 : VC3H4O 2.8 nimdeslh g; Catalyst 10 vC~H~O 11.0 mmdedh g at W C ) . A preliminery kinetic study has thus been performed with, catalyst 10 in order to discriminate between the rate of formation of acrolein and acrylic ac:id as reported in table 2.
-
-
Table 2 Ratee of formation of aadein and acrylic acid Catalyst n.10
-
T 350”: T WC:
-
-
vC3H40 1.8 mmdedh g
”
-___
-
VC3H402 4.2 mmdes/h g
5.9
10.0
”
-~
It may be observed that the rate of formation of aadein is dower. Accordingly we have extended the kinetic study by using a power-law rate expression of the form :
1977
r=k0*e%~*pOp*pC3H6n The experimental data we repcfted in table 3
Table 3 Kinetic parameters of propene oxidation over the tancry catalytic system
co co2
17.752
21.4
0.392
0.427
0.029
15.362
17.4
0.233
0.496
0.038
Acrdein
25.160
30.1
0.477
0.324
0.05
a: Standard deviation As for the partial pressure dependence with respect to CO,CO2 and C3H40 famation, it may k aeon that : 1) the yidd of acrdein can k improved by lowering propene and by increasing oxygen paid posewe8 and 2) the observed values do not justify a conventional ox.redox mechanism
Optirnizrtion of the CaH~OICSH~O2 ovdution. The C3H40/&H402. ratio is influenced by different factas, paiarlwly the reaction conbtions, the feed and phaes composition. Table 1 shows the rde of contact time (run 8 and 9) and of water vtrpou (run 9 and lo), while table 4 illustrates the importance of the phase composition. Table 4 Catalytic activity as a function of phase composition T (‘C)
c
aNiMoO4
400
65
NiMoO4.7.5XTe2Md37
380
54
C3H402
cW02 65
64
14
Feed : 5% acrdein, 15% oxygen, 80% nitrogen. Thew data d d y demonstrate that mdybdic anhydide damatidly increases the selectivity of the biphasic system, and that the preeenco of tdlurium, while deaeasing the conversion, improves the selectiwty of tho reaction.
1978
Synergy effects of t m q catalytic systems In order to rationalize the observed catalytic resutts synergy effects between the aNiMo04 and MoO3 phases have to be postulated, see XPS data (table 5), these effects may be due to a geater amount of Ni on the d a c e of the catalyst or, more specifically, to the presence of NiMoO) decorating the surface of MoO3 ayaallites. Similrv condusions have been independently obtained by other researchers using Raman miaoprobe spectroscopy [5]. As for the r d e of Te as a promoter, based on kinetic results, we assume that Te contributes in creating andlor in mantaining the Ni and Mo centers at the high oxydation state (i.e. MoQ + Te4+ --> M o b + Tek).requked for easy desorption of aadein , whose formation probably represents the rate determining step. Table 5 XPS Data and theoretical value in atomic ratios. NilMo
NiMoO4 NiMo04 NiMoO4 NiMoO4 NiMoO4
2MoO3: 2Md3 + 5% Te2MoO7 5MoO3 5Mo03 + 5% Te2MoO7
TelO
T.V.
XPS
1 .o
1.08
0.3
0.67 0.67
0.3 0.2
0.32
0.2
0.28
T.V.
XPS
0.0127
0.004
0.0124
0.004
Conclusions
These optimisation steps, if compared with the results obtained by other authors [6] have led us to: 1) an increase of 20%in conversion and 2) a much better production of acrylic acid with respect to aadein (2 : 1 against 1 : 1).
Acknowledgement The authors wish to thank C.N.R. and the Minister0 della Pubblica Istuzione (60%) for the financial support. References 1 C.Mauocchia, E.Tempesti and C.Aboumrad, Fr.Pat. 8900522 , 18/1/1989 to NorsdW 2 C.Mazzocchia, C.Aboumad, C.Diagne, E.Temp@, J.M.Herrmann and G.Thomas, Catalysis Letters ,lo (1 991 ) 181 3 C.Mauocchia. F.Di Renzo, P.Centda, R.Del Rosso, Chemistry and Uses of Molybdenum, Ed. by H.F.Barry and P.C.H.Mitchell, Climax Molybdenum Co, Gdden (USA).( 1 982) 4. L.Corona, G.Raffaglio. Thesis Pditecnico of Milan (1W) 5 U.Ozkan and G.L.Schrader, Jounal of Cat., 95 (1985) 120 6 J . M , A.Bossi, G.Peirini, G.Battiston, ACastellan , Appl.Cat. 4 (1982) 153
Guczi, L a al. (Editors), New Frontiers in CaIalysk Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
ROLE OF AMORPHOUS PHASE AND ITS MODIFICATION IN V-P-0 CATALYSTS FOR MALEIC ANHYDRIDE SYNTHESIS FROM BUTANE
N. Yamazoe, H. Morishige, J. Tamaki and N.Miura Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan
Abstract V-P-0 catalysts were investigated for the maleic anhydride (MA) synthesis from butane. The V-P-0 catalysts with P/V ratios larger than unity were a mixture of crystalline (vO)2P207 and an amorphous phase having P/V=2. The amorphous phase, when separated, showed MA formation activity per unit surface area comparable to that of the unseparated catalyst. The addition of some metal cations to the catalysts markedly increased the rate of butane oxidation and/or MA selectivity. Particularly, Bi-added catalyst prepared from the organic solution showed the highest MA yield of 62 mol% at 395 OC. 1. Introduction The mixed oxide V-P-0 is a sole catalyst industrially employed for the synthesis of maleic anhydride (MA) from butane. Although many researchers have claimed that the MA synthesis is catalyzed by the major crystalline phase of (VO)2P2O7 in the catalyst [l-41, this claim does not seem to be well consistent with why the industrial catalysts always contain phosphonis (P/V=l.05-1.20) in excess of (VO)2P207 composition. We haye shown from X P S study that the catalyst surface tends to be covered with a phosphorus rich layer having the P/V ratio of 1.6-1.9, even when the bulk P/V ratio is set close to unity, and proposed that the phosphorus rich layer is responsible for the catalysis [ 5 ] . Later we have shown that an amorphous phase having P/V=2 can be separated from the V-P-0 catalyst by washing the catalyst precursor with water [6]. This study aims at elucidating the role of the amorphous phase as well as at modifying it with foreign metal cations for the improvement of the catalytic performance. 2. Experimental
V-P-0 catalysts were prepared by aqueous as well as non-aqueous methods. The aqueous method started from an aqueous solution dissolving V2O5, H3P04 and NH20H.HCI at desired compositions. The solution was evaporated to dryness to obtain solid powder
1980 (Precursor I). Precursor I was again mixed with water, boiled for 15 min and the insoluble part (Precursor U) was separated by filtration. The filtrate was evaporated to dryness (Precursor 111). These precursors were calcined at 500 OC for 2 h under N2 flow to obtain three kinds of V-P-0 catalysts denoted Cat. I, 11, and 111, respectively. To prepare modified catalysts, metal cations (M) were added to the starting solution as metal chlorides at the composition of P/V=l. 1 and M/V=O. 1, followed by the standard procedures for Precursor I and Cat. I. The catalyst preparation from a non-aqueous solution was carried out according to the method of Trifiro et al [7]. To prepare modified catalyst, each acetylacetonate of Mg, Mn, and La, or Bi chloride was added to the organic vanadium solution ( M N a . 1 for Mg and Mn, 0.05 for La and Bi) immediately after the addition of H3P04. The resulting precipitate was collected, dried, and calcined at 400 "C for 1 h in air. The contents of Mg, Mn and La in the prepared catalysts were confirmed to be close to the nominal compositions by means of X-ray fluorescence analysis. However, the content of Bi was found to be as low as Bi/V=O.02. The oxidation of butane was performed in a conventional fixed bed reactor at temperature from 330 to 450 OC at an atmospheric pressure. The effluent was analyzed by NaOH titration and gas chromatography.
3. Results and Discussion
3.1. Role of the amorphous phase The crystalline state of the precursors and catalysts prepared from the aqueous solution was evaluated by XRD. Fig. 1 shows the XRD patterns for the starting P/V ratio of 1.7. Precursor I consisted of two crystalline phases, VOHP04.0.5H20 and VO(H2P04)2, and their amounts varied with the starting P/V ratios. Since these phases are insoluble and soluble in water, respectively, they could be separated from each other to Precursor I1 (VOHP04.0.5H20) and Precursor 111 (VO(H2P04)2). On calcination, these phases were converted to a crystalline phase of (VO)2P2O7 (Cat. 11) and an amorphous phase with P/V=2 (Cat, 111), respectively. Obviously Cat. I should be a mixture of these phases, although the latter phase is invisible in the XRD pattern. Table 1 lists the catalytic performances of Cat. I, 11, and 111 at 450 OC. It is remarkable that Cat. I11 containing the amorphous phase only showed the butane oxidation activity per unit surface area as well as MA selectivity comparable to Cat. I although the butane conversion was very low (1 1 mol%) because of its small specific surface area (2 m2-g-l). Although Cat. I1 consisted of the (VO)2P2O7 phase as revealed by XRD, its surface was still rich in phosphorus (P/V=1.7) as revealed by XPS. These results suggest that the amorphous phase is an active phase for the MA formation, and that (VO)2P207 provides an effective support for increasing the surface area of the amorphous phase in the actual V-P-0 catalyst.
Precursor I 0
0
Cat. I Calcination in N2 flow (500 "C)
0
30
20 I deg.
40
20
Precursor I1
Cat. I1
zP 0
d
10
20
30 20 I deg.
40
30
50
20 I deg.
L
I
I
I
I
I
I
I
I
I
10
20
30
40
50
I0
20
30
40
50
.:
20 I deg.
28 I deg.
Fig. 1. XRD patterns of various precursors and catalysts prepared from the aqueous solution. VOHPO4 0.5H20, o:VO(H2P04)2, A: (V0)2P207
Catalyst
Table 1 Butane oxidation over Cat. I, 11, and I11 Conversion Sel ectivity/ mol% rba) I mol% co co2 MA
S.S.Ab) 1m2.g-1
8.7 8.4 63 26 12 62 Cat. I 13 62 5.0 16.6 Cat. I1 53 25 14 64 8.6 2.0 Cat. 111 11 21 Reaction temperature :450 OC, SV=2400ml.h-l.g-l-cat. a) the rate of butane oxidation per unit surface area (10-5 molm-2*h-l),b) specific surface area.
3.2. Modification by additives It was found that the V-P-0 catalysts (P/V=l. 1) loaded with some metal cations such as Mg, Mn, Fe, and Co by the aqueous preparation method gave far larger MA yields than the unloaded catalyst. Particularly Mg effectively improved the initial selectivity to MA, while Mn appeared to prevent the consecutive oxidation of MA to C02. In each case, no significant changes in the XRD patterns of (VO)2P207 were detected, and therefore these foreign cations were likely to be incorporated in the amorphousphase covering (V0)2p207.
1982
The promoting effects of additives on the catalytic performance were also clear for the catalyst prepared from the organic solution. The V-P-0 catalyst prepared in this method had large specific surface area (39 m2.g-l) and showed better catalytic performances in butane conversion and MA yield than the catalyst prepared from the aqueous solution. Despite its bulk composition close to P/V=l, its surface P/V ratio as revealed by XPS was 1.6, suggesting again the existence of the phosphorus rich surface layer. Figure 2 shows the MA selectivitybutane conversion profiles observed at various temperatures over the V-P-0 catalyst and metal cation-added catalysts (V-P-M-0). MA yields are also scaled. In accordance with the case of the catalyst prepared from the aqueous soluMA yield / mol% tion, Mg increased MA selectivity at low con20 30 40 50 60 70 80 90 version level, while Mn kept the MA selectivity of 68 mol% up to 70 mol% conversion, both giving the maximum MA yields of 55 mol% at 430 OC. The most marked effect was exerted by Bi however. Over VPi.oBio.02 the highest MA selectivity of about 75 mol% was preserved up to about 90 mol% conversion, attaining the highest MA yield of 62 mol% at 395 "C. The addition of Bi not only increased the initial selectivity but also suppressed the consecutive oxidation of MA. These favorable effects seem to benefit from 20 30 40 50 60 70 80 90 100 Conversion / mo1% the high activity of Bi-added catalyst which made it possible to lower the reaction temperature as indicated. It is considered that Fig. 2. Relationships between MA selectivity the high activity is contributed in parts from and butane conversion for modified and unthe large surface area (43 m2/g) and the pres- modified catalysts. (Parentheses mean the reaction temperatures). ence of Bi ions.
References 1 T. Shimoda, T. Okuhara, and M. Misono, Bull. Chem. SOC.Jpn., 58 (1985) 2163. 2 F. Cavani, G. Centi, and F. Trifiro, Appl. Cutal., 15 (1985) 151. 3 B. K. Hodnett and B. Delmon, J . Cutal., 88 (1984) 43. 4 H. Morishige, Y. Teraoka, N. Miura, and N. Yamazoe, J. Chem. SOC.Jpn., 1989 (1989) 1074. 5 H. Morishige, J. Tamaki, Y. Teraoka, N. Miura. and N. Yamazoe, J. Chem. SOC. Jpn., 1989 (1989) 1983. 6 H. Morishige, J. Tamaki, N. Miura, and N. Yamazoe, Chem. Lett., 1990 (1990) 1513. 7 G. Busca, F. Cavani, G. Centi, and F. Trifiro, J. Catal., 99 (1986) 400.
Guni, L a al. (Editors), New Frontiers in Caralysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993Elsevier Science Publishers B.V.All rights reserved
SELECI'IVE OXIDATION AND AMMOXIDATION OF PROPANE TO FORM ACROLEIN AND ACRYLONITRILE
!I Moro-oha, N.Miur&, N.Fujikawaa, Y;-C. Kima and W. Uehb aResearch Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 227, Japan bDepartment of Environmental Chemistry and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 227, Japan
Abslmct Partial oxidation of propane to acrolein and ammoxidation to acrylonitde with molecular oxygen were carried out over complex metal oxide catalysts made up of bismuth oxide, molybdenum oxide or vanadium oxide and further modified with another metal oxide. The selective formations of acrolein and acrylonitrilerequired high reaction temperature around 5OO'C and reactant feeds of high partial pressures of propane. Under the conditions, the reactions involve homogeneousreactions in the gas phase where thermally activated propane converts into propene. In the case of low propane partial pressure condition where the contribution of the homogeneousreaction is negligible, vanadium oxide-basedcatalysts were mostly active for the formation of acrylonitrile in the ammoxidation. Bismuth molybate catalyst showed fairly high selectivity to acrylonibile,however the activity being quite low. 1. INTRODUCI'ION Many attentionshave been paid to chemical reactions of alkane, one of which is partial oxidation of propane. The oxidations of propane are, however, usually accompanied by many diffcultiesdue to its low reactivity. The low reactivity requirs severe m t i o n conditions, which brings about subsequent oxidations of the partially oxidized products which are desired. Therefore, depelopmentsof novel selectiveoxidation catalysts are necessary. Many industrial and scientific works have already been done and mported on the partial oxidation to oxygenates[l-21and the catalyticammoxidationof pmpane to acrylonitrile[3-6]. We recently found that propane can be oxidized selectively to acrolein with molecular oxygen by the use of silver-doped bismuth vanadomdybdate catalystsp-91. The catalysts are also active and selective for the ammoxidation of propane to acrylonitrile[lO]. In this paper we report the catalytic performance of various complex metal oxides in the (amm)oxidation of propane in high or low partial pressure.
2 EXPERIMENTAL 21. Catalyst preparation One of the catalysts used in this study is scheelite-type metal mdybdates of a variety of constituents and was prepared by a slurry method. More details of the preparation were
1984
reported previously[lO]. The prepared catalyst samples were confirmed to be monophasic as scheelite-type structure by powder X-ray diffraction patterns. The other group is ternary complex metal oxides and was prepared by a conventional copmipitation method. All catalysts were used in the form of powder of 100-200 mesh after five-fold dilution by quartz chips. 2 2 Reaction sy&m and procedure The reaction was carried out at atmospheric pressure in a conventional flow system equipped with a quartz reactor (210 mm x 18 mm I.D.tube). A 6 mm O.D.quartz tube runs longitudinally through the center of the reactor to serve as a thermowell. Catalyst powder, mixed with quartz chips, occupies the lower 20 mm of the reactor tube. The remaining sections are empty. Reactants were fed in fmm the top of the reactor. Conditions of the reaction under high partial pressure of propane are as follows: The feed compositions for the oxidation were 32 mol% of propane, 59 mol% of oxygen, the remainder being nitrogen and for the ammoxidation 44 mol% of propane, 41 mol% of oxygen, the remainder being ammonia. The reaction temperature was in the range of 480-520 "C and the space velocity is 1200 or 3000 ml/g-cat.h. Conditions of the ammoxidation under low partial pressure of propane are as follows: The feed compositions were 5 mol% of propane, 12 mol% of oxygen, 8mol% of anmonia and the remainder being nitmgen. The space velocity was 2400 ml/gcat.h.
3. REsuLTsANDDIscussION 3.1 Oxidation and ammolvidation of propane under high propane partial pressure A number of the complex metal oxides containing of both bismuth and molybdenum were found selective for the formation of acrolein in the propane oxidation under the high propane pressure condition adopted in this work. The notable catalysts are shown in Figure 1. Among the bismuth molybdate catalyst family, the complex metal oxides having scheelite
350
20 10
ll1III C
-
A
B
A : Bi2M%O12 B : Bi3(FeO&h400J2 C : Bi,(GaOJ(MoO& : Bi0.8fib0.55M00.4504 : Ag0.01Bi0.85V0.54 MQ.4504
[ ] : Conversion of propane
Sei. to m l e i n \&I.to acwionitriie
D
E
Fig. 1. Comparison of acrolein and acrylonitrile selectivities in the oxidation(5oo'C) and ammoxidation(480"C) under high propane partial pressure condition.
1985
[I : Convemionofpmpne Space velocity (mVg-cat.h)
structure showed the highest catalytic performance in respect to the comparative yields of d e i n and acrylonitrile. By the use of the best selected catalysts, the acFolein selectivityof 64moI% and the acrylonitrile selechtity of 63 mol% were achieved at the 13 % propane conversion in the oxidation and 17% ppane conversion in the ammoxidation, respectively. It is notable in Figure 1 that all these scheelite-typecatalysts show good catalytic performance in both reactions and the achieved propane conversions are roughly the Same irrespectiveof the reactions and of the constitutingmetal element of the sheelite-type catalysts. On the other hand, the attained selectivitiesare mostly higher for the formation of acrylonitrilethan of acrolein. This might be because that acrolein is more reactive than acrylonitrile in the oxidation conditions 90 that the subsequentreactions of acrolein could not be restricted. We have speculated from the catalytic perfomances and the reaction behaviors that the propane oxidation involves the homogeneous reaction of propane to propene in the gas phase before propane contacts with catalyst suIface[lo]. In order to clear the speculationwe compare the propane oxidationsin the presence and absence of the catalyst. The results are shown in Figure 2. The main product at high temperature(around 5OO'C) was propene under non-catalytic condition(absence of the catalyst) while acrolein was a main product when the catalyst was present. Striking is that the selectivity of both products at the same reaction temperature wellcoinaded with each other. A similar wincidence of the selectivity was also observed when the reactionsin the presence or absence of the catalyst were canied out under the different flow rate condition. As a consequence, the main route for the formation of acmlein may consist of the first step of pmpene formation fmm propane via the homogeneousreaction and of the subsequent step where the formed ppene was selectively oxidized to acrolein over the metal oxide catalyst. On the other hand, the extent of the involvement of homogeneous reaction in the ammoxidation of propane to acrylonitrile is slightly different from that in the oxidafion, because the selectivity to pmpene formed in the non-catalytic condition was a p p n t l y low compared with the selectivityto acrylonitrileformed in the presence of the catalysb(Rg. 2). 3.2 Ammoxidation of propane under low propane partial pressure Various ternary complex metal oxide catalysts have been tested here for their ability to participate in the ammoxidation of propane to acrylonitrile under the condition of low partial pressure of propane. The obtained catalytic performance data listed in Table 1. It is known that complex oxide catalysts based on vanadium pentoxide generally show very high oxidation ability. In fact, the conversion of propane pceeds over most of the vanadium-based
1986
Table 1. Ammoxidation of propane over various complex metal oxide catalyst under low partial pressure of propane" ~
catalyst
Reaction temperatunf'C)
4
Conversion %
Selectivity(% Aaylonitrile Acetonide ~
~~
cox
~
V-Mq-Ox v-P-0, v-Sb-0, V-W24,, V-BiQ
510 510 514 509 510
27.0 184 7.5 21.5 6.1
76.3 76.8 39.3 89.7 61.1
17.9 26.9 13.0 7.7 0
11.6 4.3 0 7.4 0
49.2 58.3 65.8 42.0 64.6
Mo,,-P-O, MeBi-Ox Mq-Bi-0,
510 509 509
4.7 5.2 4.3
621 51.8 581
11.9 56.2 30.7
14.7 6.2 0
188 15.7 9.0
catalysts under the present reaction condition as can be seen in Table 1. However, the attained selectivity to acrylonitrile was quite low in all cases, compared with the reactions under the high propane partial p ~ s s u r econdition. Carbondioxides were the major products, although one can seen clear dependency of the acrylonitrile selectivity on the constituting element of the vanadium-based catalysts. For example, vanadium and phosphorous mixed pentoxides which are well-known catalysts for the n-butane oxidation to maleic anhydride displayed the activity for the formation of acrylonitnle with 27% selectivity. The catalyst containing molybdenum oxide showed high activity for propane conversion. The Sb-V-0 catalyst which has been well-studied so far for propane ammoxidation under various conditions[3,q showed poor catalytic performance under our reaction conditions. Interestingly, the complex metal oxide containing of bismuth and molybdenum, which is selective catalyst for pmpene oxidations, was found selective for the formation of acrylonitrile in the propane ammoxidation too. Since it was comfirmed by the separate experiments that no homogeneous reactions of propane can take place under the present reaction conditions, it is obvious that acrylonitrile is formed through surface reaction pathes directly. 4 References
1 N.Giordano, J.C.J.Bart, P.Vitarelli and S.Cavallar0, Oxid. Comrmoz.,7 (1984)99. 2 T. Komatsu, Y.Utagami and Kotsuka, Chem. Len., 1903 (1988). 3 T.G. Attig, KJ. Kurut and RKGmselli, The Standard Oil Company, US Patent 4 736 054 (1988). 4 M.A.Toft, J.F.Brazdil and LC.Glaeser, The Standard Oil C o m p y , US Patent, 4 879 264 (1989). 5 G.Minow, K.-H. Schnabel and G.Ohlmann, 2. Phys. Chem. (LeipZig), 265 (1984) 145. 6 G. Centi, D.Pesheva and F. Trifro, Appl. Catal.,33 (1987) 343. 7 Y.-C.KIm,W.UedaandY.Mom-oka,J.Chem.Soc.Chem.Commun,652(1989). 8 Y.-C. Kim,W. UedaandY. MOCDQJ(ZL,Chem.Lett.,531(l!B9). 9 Y.-C. Kim, W. UedaandY. Mom-oki, Chem.Lett.,2173(1989). 10 Y.-C. Kim, W. UedaandY. MorDoka,Appl. Cata1,,70(1991)175,189.
Guczi, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier science Publishers B.V. All rights resewed
OXIDATION OF PROPENE ON ALKALINE METAL-DOPED MoOfliO, CATALYSTS: A m-IRSTUDY
C. Martin, I. Martin, C. Menduabal and V. Rives Dpto. de Quimica Inorganica, Universidad de Salamanca, Facultad de Farmacia, Salamanca, Spain
Abstract The reactivity of MoOfli02 systems (pure or Na- or K-doped) with propene has been studied by FT-IR spectroscopy. Reactivity decreases in the presence of the alkaline cations, and thus reactive adsorption (leading to acrolein formation) is only observed in the doped catalysts above room temperature. This behaviour has been related to the decrease in surface acidity. 1. INTRODUCTION
Molybdena-based catalysts is a set of materials widely used nowadays for partial oxidation of olefins [ 1,2]. The most of the papen existing in the literature on this subject demonstratethe relationship existing between the nature of the active phase (stnrcture, dispersion, etc.) and the activity and/or selectivity of the catalyst. In the present paper, a FT-IR study has been carried out on the adsorption and oxidation of propene on titania-supported molybdena, doped with alkaline cations (Na and K), in order to correlate the reactivity of these systems and their surface acidity 2. EXPERIMENTAL Catalysts were obtained by impregnation of the support (titania P-25 from Degussa, Germany, a.50 mVg) with aqueous solutions of ammonium heptamolybdate (Merck, p.a.), then calcined in oxygen at 770K for 3 h. In some cases, calcinationwas carried out at 1 100K, in order to analyze the effect of molybdena melting on the properties of the solids. The alkaline dopants were incorporated (1 or 3% in weight) to the support, before incorporation of molybdenum, by impregnation with aqueous solutions of KNO3 or NaOH in a rotiry vacuum evaporator. The amounts of molybdena correspond to one or two monolayers of MoO3. Samples are named m M(number of monolayers of Mo03)Alkaline(percentage).The sample calcined at 1 lOOK is named M lT, and does not contain any alkaline dopant. Adsorption of propene (from Sociedad Castellana del Oxigeno, 99.95%), acroleine and acetone (Carlo Erba, p.a.) were followed by FT-IRspectroscopy in special cells with CaF2 windows, after outgassing the sample in situ at 670K for 2 h.
1988 Physicochemicalcharacterization of the catalysts had been canied out by X-ray diffraction, Exafs and Visible-Ultravioletiffuse reflectance spectroscopies and specific surface area determination [3], as well as by FT-IRmonitoring of pyridine adsorption [41. According to the results obtained, molybdenum species exist mainly as [Moos]. Alkalinefree samples contain MoO3, its dispersion increasing as the Mo content decreases and in sample M1T. Polymolybdate species are detected in the presence of alkaline cations, its dispersion degree being higher for the low alkaline-loaded samples; for sample M2K1 both molybdena and polymolybdatesexist. Pyridine adsorption indicate the presence of both Bronsted and Lewis surface acid sites for the alkaline-free samples. As the percentage of alkaline doping cation increases, the number of Bronsted acid sites decreases, being completely cancelled in those samples containing 3% of alkaline cations; in these systems, the Lewis acid sites are weaker than in the absence of alkaline dopants. However, for sample M2K1 the amount of Brdnsted sites is similar to that existing in the alkaline-free samples, due to the presence of MoO3 species (as detected by Xray diffraction). 3. RESULTS A N D DISCUSSION
Adsorption of propene was carried out at room temperature (r.t.),373 or 573 K, and the samples were then outgassed at r.t.. The spectrum recorded after adsorption and outgassing at r.t. (Fig. 1) on the bare SUpDort, indicates the presence of molecularly adsorbed propene, with bands at 1625, 1615 (uC-C), 1452, 1429, 1414 and 1373 cm-1 (bCH3), due to olefinx-bonded to exposed surface cations (Ti4+),and no perturbation of the bands due to uOH modes is observed. At 373K the bands are recorded at 2963,2927 (UGH),1625, 1609, 1463, 1448, 1386, 1164 and 1097 cm-1, due to the mentioned x-bonded species and to isopropoxy species (uC-0, 1097 cm-I), formed upon reaction of propene with surface hydropxyl groups at this temperature. At 573K the intensities of these bands decreases, and new bands develop with maxima at 1567 and 1445 cml; these bands can be ascribed to ua(CO0) and us(C00) modes of carboxylate species, but, contrary to the results reported by Graham et al. [S], formation of acetone is not observed. For samples Ml and (i.e., with one or two monolayen of Mo03, but with no doping), the spectra recorded upon propene adsorption show, in addition to the bands above described for x bonded catiodoleh complex (The most characteristicbeing that at 1620 cm-1,u(C-C), a medium intensity bandat 1678 cm-1 (Fig. I), similar to that recorded upon adsorption of acetone on these samples, and has been ascribed to acetone coordinated to surface Lewis acid sites through the oxygen atom (uC-0 for gaseous acetone is recorded at 1734 cm-1 [a]). The shape and position of the other bands (1465, 1390, 1378, 1326 and 1270 cm-1, due to uC-C and BCH3) indicate that they are originated by materials formed upon propene polymerisation [7,8], a reaction easily taking place on these systems due to their high acidity (both Br6nsted and Lewis sites). Upon increasing of the adsorption temperature, the intensity of the acetonerelated band increases, while those due to x bonded olefin species decreases; acrolein nor carboxylate species are detected.
-
-
1989
For the sample calcined at 1100 K 0 ,
no reactive adsorption is observed at room temperature, and only the n-bonded olefin bands at 1623 and 1615 cm-1 (uC=C) are recorded. As the temperature is increased, surface coordinated acetone is detected (uC-0 at 1678 cm-1). The lack of formation of polymerised species can be due to the lower surface acidity of this sample, as shown by pyridine adsorption [41. The & and 5-doDed samples (MlKl, MlNal) show similar spectra upon propene adsorption. At room temperature several bands are recorded in the 1640-1620cm-1 range, due to the uC=C mode of n - bonded complexes, where the olefin molecule is coordinated to different surface cations: the main absorption at 1639 cm-1 has been ascribed [7] to a n-bonded K+/olefin complex. Other bands at 1457, 1442, 1383 and 1314 cm-1 are due to NCH3). At 373K, Fig. 1, the intensities of these bands decrease, while a new one arises at 1690 cml; this band, together with those at 1618, 1426, 1365, 1279 and 1165 cm-1, have been also recorded upon adsorption of acrolein on these samples, and are due to the u(C=O), u(C=C), 6(C-H) and u(C-C) modes of acrolein weakly adsorbed on cationic sites (uC=O for gaseous acrolein is recorded at 1724 cm-l[91).
Ti02
J
q2K1
Samples with a larger alkaline content, M1K3 and MlNa3, are not reactive at r.t., but at 373 and 573K a very complex spectrum is recorded, with several bands between 1682 and 1659 cm- 1, probably due to different carbonylcontaining species adsorbed on the surface. 1
(where X-ray difFinally, for sample fraction indicated the simultaneouspresence of polymolybdatesand molybdena),again both acetone bonded to Lewis sites (uC=O at 1678 cm-1) and Ir-bonded catiodolefm species (uC=C at 1635-1610cml) are detected at r.t., but nopolymeric species, Fig. 1. At 373K several bands
1800
I
I
1500
1200
cm-l Figure 1.-Spectra recorded upon adsorption of propene on the samples indicated at the given temperatures.
1990
that can be originated by carbonyl-containingspecies (acrolein, acetone, etc.) coordinated to Lewis sites, are recorded at 1695, 1675, 1659 and 1651 cm-1.
4. CONCLUSIONS
The different behaviour shown by these samples upon propene adsorption should be related to their different surface acidity. The results obtained agree with the mechanism proposed by several authors [10,11] for olefins oxidation on these systems. According to these authors, Bronsted sites (Mo6'-OH) are responsible for propene hydrationdehydrogenation to acetone, while coordinatively unsaturated molybdenyl species are responsible for the allylic oxidation (acrolein formation). So, on our samples, acetone is only detected in Bronsted sites rich-samples (alkaline-free samples and M2KI), and acrolein in samples where these sites do not exist or in a small amount. This larger selectivity to acrolein upon alkaline doping is also in agreement with previous results with V205niO2 systems doped with sodium [ 121. So, it should be concluded that addition of alkaline species (K+ or Na+)decreases the activity of MoOfliO2 systems (no reactive adsorption is observed at r.t.), although increases the selectivity to acrolein.
5. ACKNOWLEDGMENTS Authors thank finantial support from CAICYT (MAT88-556)and Consejeria de Culturn de la Junta de Castilla y Leon. I.M. acknowledgesa grant from DGICyT.
6. REFERENCES R. Grabowski, S. Sloczynski, K.Dyrek and M. Labanowska, Appl. Catal., 32 (1987) 103. 2 T. Ono, Y. Nakagawa, H. Miyata and Y. Kubokawa, Bull. Chem. SOC.Japan, 57 (1984) 1205. 3 A. Muiioz-Paez, P. Malet, C. Martin and V. Rives, J. Catal., in press. 4 C. Martin, I. Martin, C. Mendizabal and V. Rives, Proc. 7th. Int. Symp. Heterogeneous Catal., Bourgas, Bulgaria (Eds. L. Petrov, A. Andreev and G . Kadinov), Vol. 1, p. 163 (1991). 5 J. Graham, R. Rudham and C. H. Rochester, J. Chem. Soc. Faraday Trans 1,80 (1984) 895. 6 G.Delle Piane and J. Overend, Spectrochim. Acta, 22 (1966) 593. 7 G.Busca, G.Porcile and V. Lorenzelli, J.Mol. Struct., 141 (1986) 395. 8 G.Ramis, G.Busca and V. Lorenzelli, Appl. Catal.,32 (1987) 305. 9 R. K.H a m s , Spectrochim. Acta, 20 (1964) 1129. 10 V. Shchez-Escribano, G.Busca, V. Lorenzelli, J. Phys. Chem., 94 (1990) 8939. 11 J. fiber, Proc. 8th Intern. Congr. Catalysis, West Berlin (1984) Vol. 1, 88. 12 C. Martin and V. Rives, J. Molecular Catal., 48 (1988) 381. 1
Guni, L.et al. (Editors), New Frontiers in Coralysir Proceedings of the 10th International Congnss on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
OXIDATION OF METHANOL TO FORMALDEHYDE OVER Ah'TIMONYMOLYBDENUM OXIDE CATALYST R. S. Mann and R. A. Dim-Real Department of Chemical Engineering, University of Ottawa, Ottawa, Canada
Abstract The air oxidation of methanol to formaldehyde was investigated over Moos, Sb2O4 and their mixtures in an integral flow reactor at atmospheric pressure, between 623 and 698 K. The effects of several operating variables SbzOr/MoO3 ratio, temperature, space time, and methanol/air ratio on the conversion of methanol and the selectivity of the catalyst for formaldehyde formation were determined. Selectivity as high as 100% was obtained. Best operating conditions were found to be W/F of 27.5,methanol/air ratio of 0.06 a t temperature of 698 for a SbzO4/MoO3 ratio of 2. The rate expression, based on a two stage irreversible redox expression fitted the data best.
1
Introduction
Formaldehyde is commercially manufactured by the partial oxidation of methanol, using a metal (generally silver) or a mixture of oxides as catalyst. Moo3 has been found to be invariably a constituent of mixed oxides, which give high conversions and high selectivity for formddehyde formation. The use of Moo3 with Fe203 (l),MnOz (2), V205 (3), wo3 (4), BiO and Te02 has been widely investigated. However, information regarding the use of Sb204-Mo03 mixture in the partial oxidation of methanol is very scarce. This paper reports the results of a detailed kinetc study and rate equation for methanol oxidation over a catalyst containing Sb204/Mo03 in a 2:l ratio. This catalyst was unique and was made in our laboratory. It converted methanol to formaldehyde completely without the formation of any CO or C 0 2 in the product.
2
Experimental
The air oxidation of methanol over Sbz04-Mo03 catalyst was investigated in an isothermal flow reactor made of 316 SS and was similar to the one used earlier (4). Heat and mass transfer effects were negligible. Sb204-Mo03 catalyst waa prepared by dissolving known amount of ammonium rnolybdate completely in water to which required amount of antimony pentoxide was added. The required catalyst (Sb204/Mo03= 2) was of 60-80mesh (0.246 to 'PraKntly at CANMET-ERL, Ottawa, Canada
0.175 mm clear opening size) size and had a surface area of 6.1 m2/g and density of 1.678 g/cm3. The gases were analyzed for C02, CO, 02 and N2 by periodic injection of the sample gas into HP Model 5700 gas chromatograph using a 305 cm x 0.3175 cm 0.d. SS tube containing Hayesep T. The column separated Nz, CO, COz, 02 and CHI. Liquid products were analyzed using a 458 cm x 0.3175 cm 0.d. SS tube containing 15% Sucrose-octa-acetate coated on Chromosorb T. This column could separate, methanol, formaldehyde, water, dimethyl-ether, methylformate, dimethoxy methane and formic acid.
3 3.1
Results and Discussion Antimony Oxide/Molybdenum Oxide Ratio:
Figure 1 shows the effect of percent Moos in the Sb204-Mo03 catalyst on the conversion
of methanol and selectivity for the formation of formaldehyde at 673 K , W / F of 27.5 and 6 percent methanol in air. Conversion and selectivity both increased with percent Mo03, reaching a maximum of 82% conversion and 100 % selectivity, for a SbzO4/MoO3 ratio of 2. Beyond this though the selectivity remained nearly loo%, conversion decreased. Therefore Sbz04/Mo03 ratio of 2 was considered as optimum and chosen for the detailed kinetic study.
1.0d
a
*-
0.8-
0- Conversion 0 = Selectivity
0.70.8
0.0 0.0
20.0
40.0
60.0
X MOO, Figure 1. Effect of Catalyst Compodtion on Conversion
3.2
60.0
100.0
-
0.8-
VI
0.6
-
I[
0- Conversion A - selectivity O=Yield
0
Temperature K
Figure 2. Effect of Temperature on Conversion, Selectivity and Yield at W / F of 27.5 and R of
Temperature:
The effect of temperature on conversion, yield and selectivity was studied a t 623-698 K at several values of space time (W/F) and percentage of methanol in air. Figure 2 shows that the conversion of methanol increased to nearly 100% at about 673 K . Selectivity remained nearly 100% for the range studied.
3.3
Space Time (W/F):
The effect of W / F on conversion, selectivity and yield was studied at temperatures 623-698 K and methanol concentrations of 4-10 %. Figure 3 shows the effect of W / F a t 673 K with
1993 6% methanol in air. At small W/F ratios conversion increased sharply with a slight increase in W / F and then became asymptotic (~100%)at higher values of W/F. Similar trend was noted at other temperatures.
1.0-
B w a
0.0-
-
a
0.0-
m 0'
0.4 0.2 0.0
-
--
/
0 Conrerrion A = Seleotirity 0 Y ield
- 0
0.0 10.0
20.0
m
ao.0 40.0 00.0
Space time W/n
Figure 3. Effect of Space Time on Conversion, Selectivity and Yield at T of 673 K and fi of 6 3.4
0.80
3
-
0- Conrerrion A = Seleotitity
0.86
i.0
4.0
0.0
Y iold 0.0
10.0
1 ,O
Yethrnol to rir flow ratio Figure 4. Effect of Methanol Air Flow Rate at T of 648 K and W / F of 38.75
Methanol Concentration in Air:
The effect of methanol to air flow ratio waa studied on conversion, selectivity and yield between 623 and 698 K and for space time between 5 and 50. The conversion increased with an increase in methanol concentration. Figure 4 shows the effect of methanol to air flow rate at 648 K and W / F of 38.75. 3.5
Catalyst Particle Size:
The effect of the catalyst particle size (0.17-0.67 mm) was studied on the conversion of 6% methanol in air at 673 K and W / F of 27.5. There was no noticeable change in the conversion with particle size. The catalyst that waa used in the study had a particle size between 0.17 and 0.24 mm, for which internal diffusion can be considered negligible and the effectiveness factor to be nearly one.
3.6
Methanol Flow Rate:
There waa no appreciable change in conversion of methanol with flow rates in the range of 0.0154 and 0.154 mol/hr a t 673 K and W/F of 27.5.
3.7
Kinetic Analysis:
A kinetic analysis was made of the experimental data using the approach suggested by Hougen and Watson ( 6 ) . The effect of external diffusion waa kept to a minimum by using high velocity of the gaa and the diffusion in pores was eliminated by using small particle size.
1994 The temperaure and partial pressure gradients between the flowing fluid and the external surface of the catalyst was evaluated by method of Yoshida et al. (6). A maximum temperature difference of 0.1 K and maximum partial pressure of 10 P a suggested that heat and mass transfer were negligible. Method of Yang and Hougen (7) was used to eliminate some of the rate controlling steps. A number of rate equations for the oxidation of methanol to formaldehyde were derived based on two and three stage Redox mechanism for orders of 0 to 2 for met.hano1 and 0 to 1 for oxygen. The data fitted best to a two stage Mars and van Krevelen's (8) Redox mechanism. According to this mechanism a steady state is assumed between the following two steps:
-
mCH30H(,) t So, 5 mHCHO(,) t mHzO t SEd (1) kz mOz(,)t Sred so, (2) Assuming the rate of adsorption to oxygen to be of the same magnitude as that of the rate of chemical reaction, the following rate equation was obtained rate = klPM/(l t 0 . 5 k l P ~ / k z P o , ) where PM and Poz are partial pressures of methanol and oxygen in feed, and kl and kz are rate constants for step 1 and 2.
4
Conclusions
The catalytic air oxidation of methanol over oxides of Mo and Sb was investigated between 623-698 K at atmospheric pressure, W / F of 5 to 50 &at/molcH30Hh-' and methanol concentration of 4 to 10%. A maximum yield of nearly 100% of formaldehyde with a selectivity of 100% and conversion of 100% was obtained at 698 K with a W / F ratio of 27.5 and 6% methanol in air. A two stage Redox mechanism satisfied the data.
5
References 1. P. Jiru, B. Wichterlova and J. Tichy, In Proceedings of the 3rd International Congress of Catalysis, North-Holland Pub. Co., Amsterdam (1966) 199.
2. R. S. Mann and K. Hahn, J . Catalysis 15 (1969) 329. 3. R. S . Mann and M. Dosi, J. Catalysis 28 (1973) 222. 4. R. S. Mann, S. Jain and M. Dosi, J . Appl. Chern. Biotech. 27 (1977) 198.
5. 0. Hougen and K. M. Watson, Chemical Process Principles Part 111, p. 251, John Wiley, New York, 1947. 6. F. Yoshida, D. Ramaswami and 0. Hougen, A.Z.Ch.E. J. 8 (1962) 5. 7. K . Yang and 0. Hougen, Chern. Eng. Pmg. 46 (1950) 146.
8. P. Mars and D. van Krevelen, Chern. Eng. Sci. (Special Supplement on Proceedings of Conference on Oxidation Processes) (1954) 41.
I
Guni, L el al. (Editors), New Fronrkrs in Catalysis
Proceedings of the 10th International Congrcsa on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1!393 Elsevier Science Publishers B.V. All rights rcserved
A COMPARISON BETWEEN EPOXIDATION AND DEGRADATION OF El'HYLENE AND PROPYLENE OVER SILVER C. Henrquesa, M.F. Porteld, C. Mauocchiab and E. GuglielminoniC aGrupo de Estudos de Catalise Heterogenea, Centro de Processes Quimicos (INIC), Dep. Enga Quimica, Instituto Superior Tecnico, Av. Rovisco Pais, 1096 Lisboa Codex; Portugal bDipartimentodi Chimica Industriale ed Ingegneria Chimica Giulio Natta, Politecnico di Milano, P.za L da Vinci 32, 20133 Milano, Italy CDipartimentodi Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, via P. Guria 7, 10125 Torino, Italy
Abstract Oxidation of ethylene and propylene by dioxygen has been studied over two different silver catalysts, wich exhibited markedly different selectivities for propylene epoxidation. A kinetic study shows that orders in olefin and oxygen do not change with the catalyst, for ethylene reactions, but they clearly change with the catalyst for propylene degradation. TPD of oxygen evidence that the amounts of oxygen desorbed from the two catalysts, although of the same types, are very different and infrared study shows that C02 formation proceeds via acetate and formate intermediates, with both olefins.
1. INTRODUCTION
Silver catalysts have very significant industrial application for the epoxidation of ethylene by dioxygen. However, epoxidation of propylene over the same catalysts, leads to very low selectivities. Experimental evidence indicates that epoxidation mechanims for ethylene and higher olefins should be similar [l-21. To shed light on such matter, a comparative study of two different unsupported silver catalysts with markedly differenr propylene epoxidation selectivities, was carried out with ethylene and propylene.
2. EXPERIMENTAL Two unsupported silver catalysts were prepared: catalyst A, by precipitation from a silver nitrate/ammonium hydroxyde aqueous solution, with formaldehyde 131 and caralysr B by gas phase reduction of Ag2O by a 10%olefin in nitrogen flow at 408K. TPD of oxygen was carried out in a GC-MS system. FTlR experiments were undertaken in a Pyrex IR cell and performed with a supported silver catalyst (18%wt. silver on y-alumina). This step proved to be necessary to obtain acceptable oprical rransparency ; a blank test with y-alumina was carried out.
1996
3.RESULTS AND DISCUSSION The reaction products observed in the catalytic tests were only ethylene oxide (EO), propylene oxide (PO), COP and H20. With both catalysts and olefins, the rates of C02 and epoxides pass through a maximum with increasing olefin pressure, at constant oxygen pressure; the maximum is observed for 2.5-3% of feed composition, for both oleflns. On the other hand, catalysts activity always increases with oxygen pressure. Tables 1 and 2 shows the apparent orders for the reactions with ethylene and propylene. The olefin orders change to negative, in the higher pressure range, evidencing a competition between reactants on silver surface. Table 1 Apparent reaction orders for the oxidation of ethylene Reaction QtalYwl C2H4 02 0.50 r(CO2) 0.21 *-0.27 0.33 r(E0) 0.48 * -0.1 5
CatalvrtR C2H4 0.21 0 - 0 . 2 7 0.48 m -0.22
Table 2 Apparent reaction orders for the oxidation of propylene Reactlon CWYUA Catalvrta C3"6 02 C3H6 r(CO2) 0.21 --0.19 0.50 0.50 0 -0.13 0.44 0.36 m -0.1 5 rPO) 0.36 0 -0.1 7
02 0.50 0.33
02 1.1 0.44
Table 1 shows that the reaction orders are similar for ethylene with both catalysts. As shown in Table 2, C02 formation, from propylene, is the reaction that exhibits the major differences, with respect to the orders in 0 2 and olefin, with both catalysts. Conversely, for PO formation, the orders are similar for both catalysts. For ethylene, the global activity (measured as olefin reaction rate) of catalyst A is about 2-3 times higher than that of catalyst B. For propylene, it is considerably higher, depending markdly on the oxygen partial pressure (from about 25 times at low pressures to less than 10 times for the higher pressures). The acriviry for degradation , for catalyst A is quire similar for ethylene and propylene and for epoxidation is about 30 times higher for ethylene. For catalyst B the degradation activity is about 1 0 times higher with ethylene and, concerning epoxidation, about 30 to 75 times higher depending on oxygen partial pressure. With catalyst A, at 473K the mean values for PO selectivity, are 1-1.5%, and 6-15% with catalyst B. At the same temperature , the EO selectivity is 1030% with catalyst A and 20.50% with catalyst B. With this catalyst, the EO and PO selectivities decrease, when 02 pressure increases, towards the lower levels exhibited by caralysr A. The increase of EO and PO selectivities, observed with catalyst B, is due to a decrease of COP formation fare, as epoxidation fares are similar for both ceraiysrs and with borh olefins.
1997 Oxygen TPD tests were carried out with both catalysts. Figure 1 shows 0 2 TPD spectra obtained withy a mass spectrometer (mass 3 2 fragments). The TPD oxygen spectra evidences that, for both catalysts, the desorption processes occurs in the same temperature range 450-773 K: t w o main desorption processes were recorded, the first with a maximum at 538K and the second at 583K, but for catalyst B the global amount of desorbed oxygen is much smaller than with catalyst A. 2.0)
a Tomprratura (K)
Figure 1. TPD of oxygen spectra for catalyst A and catalyst B Figure 2 shows FTlR spectra obtained with oxygen/olefin mixtures over AglyA1203 at different temperatures. and exposed to 1.2 KPa of oxygen and 0.96 KPa of olefin. The spectra were obtained by subtraction of a room temperature spectrum.
A
1
2000
iaoo
I
I
1
1600
1400
1200
c rn-’
0
I
1
I
1
1800
1000
1400
1200
’
c m-’
Figure 2. FTlR spectra of Agly-A1203 catalyst: (a) after reaction with C3H6/02 mixture,heated at 373K for 3 0 min (spectrum 11, at 478K for 45 min (spectrum 2) and at 543K for 45 min (spectrum 3); (b) after reaction with C2H4/02 mixture, heated at 423K for 90 min (spectrum 1 ) and at 473K for 60 min (spectrum 2).
They show that basically three groups of IR bands are present: (1) bands at 1650 cm-l, 1435 c m - l and 1230 c m -l , assigned to carbonates andlor bicarbonates, formed by C 0 2 14-51; (2) bands at 1 5 8 0 c m - l and 1378 crn-l; (3) bands at 1570 cm-l and 1455 c m - l. These t w o last groups could be assigned, respectively, to formate and acetate structures. In fact these types of structures has been detected as surface complexes in the propylene oxidation over different oxides 151. Bands assignable to the CO at = 1700 crn-l, found by other authors and assigned to acrolein 161, were not detected. With ethylene similar spectra was obtained: (11 bands at 1646 crn-l, 1425 cm-l and 1225 crn-l assignable to carbonates and bicarbonates; (2) bands at 1594 crn-l and 1376 crn-l and (3) bands at 1579 c m - l and 1461cm-l, assignable to acetates and formates. This evidence that acetate and forrnate species are involved in C 0 2 formation 12, 7-91; no bands assignable to EO were detected.
4. CONCLUSIONS The increase of the epoxides selectivity in, with catalyst 6, is due, not to an increase of the epoxide formation rate, but to a decrease of the C 0 2 formation rates, specially with propylene 1101. This selectivity is probably related to the different amounts of available oxygen, evidenced by TPD experiment; in fact, when the amount of surface oxygen is increased (by increasing 0 2 pressure in the feed mixture), selectivity drops markedly. FTIR experiments indicate that both olefins proceeds to degradation via formate and acetate intermediates. The presented results evidence that epoxidation depends not only on the olefin but simultaneously on the olefin and catalyst.
5. REFERENCES C. Mukoid, S. Hawker, J.P.S. Badyal, R.M. Lambert, Catal. Letters, 4 (1990) 5 7 R.A. van Santen and H.P.C.E. Kuipers, Adv. Catal., 3 5 (1987) 265 K.P. de Jong and J.W. Geus, Appl. Catalysis, 4 (1982) 41. N.D. Parkyns, J. Chem. SOC. (A), (1969) 410. A.A.Davidov, V.G.Mikhaltchenko, V.D.Sokolovskii and G.K.Boreskov, J.Catalysis, 55 (1978) 299. 6 I.L.C. Freriks, Robert Bowman and Peter V. Geenen, J. Catalysis, 65 (1980) 31 1. 7 A.G. Sault and R.J. Madix, J. Catal., 67 (19811 118 8 T. Kanno and M. Kobayashi, Proc. Int. Catal. Conf., 8th, Berlin 111 (1984) 277. 9 E.L. Force and A.T. Bell, J. Catal., 3 8 (1975) 440 E.L. Force and A.T. Bell, J. Catal., 40 (1975) 356 E.L. Force and A.T. Bell, J. Catal., 44 (1976) 175 1 0 M.F. Portela, C. Henriques, M.J. Pires, L. Ferreira and M. Baerns, Catalysis Today, 1 (1987) 101
1 2 3 4 5
L el al. (Editors), New Frontiers in Catalysh Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rightp reserved
G&,
LIQUID-PHASE OXIDATION OF BENZENE WITH MOLECULAR OXYGEN CATALYZED BY CU-ZEOLITES T. Ohtani S. Nishiyama, S. Tsuruya and M. Masai Department of Chemical Engineering, Faculty of Engineering, Kobe University, Nada, Kobe. 651, Japan
Abstract The liquid-phase direct oxidation of benzene to phenol with molecular oxygen was studied using heterogeneous Cu(II) ion-exchanged zeolite catalysts in the presence of ascorbic acid under mild reaction conditions, from which phenol was selectively ielded. Cu(1) ions were found to be the active species for the formation of henol. he turnover numbers of Cu(II) ion-exchanged zeolites, defined as the mo e ratio of produced phenol to Cu ions in the catalyst, were more than unity, in contrast to those of non-anchored Cu(II) catalytic systems of which the turnover numbers were extremely less than unity.
P
I
1. INTRODUCTION The direct synthesis of phenol from benzene has been recent1 focused on, not only from the point of view of an organic synthesis, but also rom a chemical industrial oint of view. Recently, the oxidation of benzene to form phenol using cuprous ch oride as a catalyst has been reported by Sasaki and his co-workers (11, in which reaction system oxygen molecules have been used as the oxidant in place of hydro en eroxide. Takehira and his co-workers have studied benzene oxidation with mo ecu ar oxygen catalyzed by Cu(II) salts in the presence of ascorbic acid (2). Cu(II) ion-exchanged zeolites have been reported to be useful as catalysts for the as-phase oxidation of some organic materials (3-51, and copper ions in zeolites aave been indicated to have redox properties according to physico-chemical measurements (6). Armed with this knowledge, we have tried the liquid-phase oxidation of benzene with molecular oxygen under atmospheric pressure a t room temperature using Cu(I1) ion-exchanged zeolite catalysts in the presence of ascorbic acid as a reducing reagent for Cu ions. The catalytic behavior of the Cu(II)-zeolitesfor the benzene oxidation was compared with those of non-anchored Cu(II) catalysts.
P
P
B P
2. EXPERIMENTAL
The Cu(II)ion-exchanged zeolites were re ared b ion-exchanging each parent zeolite with an aqueous solution of Cu(CI$sC80)2 foylowed by filtration, washing with deionized water, dryin a t 393 K overnight, and calcination at 773 K for 5 h under an air atomosphere. T e liquid-phase oxidations of benzene were carried out in a 50 cm3 flask immersed in a thermostatted water bath, which was usually maintained at 303 K, under an oxygen atmosphere (1 atm). The reaction products were analyzed by a gas chromatograph with a flame ionization detector using 3 m
‘6
2000 stainless column (3 mm diameter) filled with Silicon OV-17IChromosorb GAW DMCS and using 2-propanol as an internal standard. 3. RESULTS AND DISCUSSION
The main product of benzene oxidation with molecular ox gen was henol, irres ective of the zeolite supports when using the Cu(II) ion-exc anged zeoyites as cata ysts. The zeolite supports themselves were found to have no catalytic activities for the benzene oxidation. Also i t was found that no oxidation of benzene occurred using the Cu(II) ion-excahnged zeolites under a nitrogen atmosphere in place of an oxygen atmosphere.
K
P
3.1. Oxidation Activity of Benzene Catalyzed by Noa-Anchored Cu(I1) Salts To compare the oxidation activity for the li uid-phase benzene oxidation to form phenol with the corresponding supported Cu( ) catalysts, some copper salts, which were not anchored on the supports, were tested as catalysts in both water and 1 moll1 acetic acid aqueous solvents, with the results shown in Table 1. No benzene was produced without ascorbic acid, irrespective of the copper(lI) salts used here. However, benzene was obtained using ascorbic acid as a reducing reagent for Cu(II) ions, though the amounts of benzene roduced were considerably low. The mole ratios of the phenol produced to the u(II) ion are considerably less than unity for all the Cu(II) salts used here as catalysts. The increase in the ascorbic acid amount was also found to cause no increase in the phenol yield in the catalytic systems with the non-anchored Cu(II) salts. The Cu(II) salts in the aqueous solvent including acetic acid had larFer catalytic activities for benzene oxidation than in a pure water solvent, irrespective of the Cu(II) salts used, as shown in Table 1.
3
8
Table 1.Benzene Oxidation with 0 2 Catalyzed by Cu(II) Saltsa). Catalyst
Solvent
cuc12
H2O
Cu(CH3C00)2
Ascorbic acid Phenolb) PhenoYCuc) (mmol) (mmol)
0 4
1N CH3COOH aq. sol.
0
0
HzO
4 0 4 0
0.12 0 0.12 0 0.15
1N CH3COOH aq. sol.
4 ~~
0 0.10
0 0.02 0 0.03 0 0.03 0 0.04
~
a) Catalyst, 4 mmol; benzene, 2 cm3 (22.5 mmol); solvent, 20 cm3; temp, 303 K; time, 24 h. b) The amount of benzene roduced. c) (The phenol produced / t e amount of Cu in the catalyst) mole ratio.
R
3.2. Oxidation Activity of Benzene Catalyzed by Cu(I1) Ion-Exchanged Zeolites The results of the oxidation of benzene with oxygen molecules usin the Cu(II) ion-exchanged zeolites in the presence of ascorbic acid are indicated in able 2.As indicated from the phenol/ Cu mole ratios in Table 2, the Cu species ion-exchanged on the zeolites was found to have greater activity for benzene formation than the non-anchored Cu(II) salts shown in Table 1. Fayjasite type zeolites such as Y- and
fi
2001
X-t pes among the zeolites studied here were particularly effective for the
oxiCYation activity. The comparisons of the oxidation activit between the proton type and sodium type zeolites were erformed using the u!II) ion-exchan ed fa 'asite and ZSM-6 zeolites. Thaorffer of the phenoVCu mole ratios were N& > Cu-NaX > Cu-Mordenite > Cu-NaZ, which order is 'ust opposite from the SiIA1 atomic ratios of these zeolites. One of the factors whic govern the benzene oxidation is thus considered to be the hydrophilicity of the zeolite used as the support. An ascorbic acid molecule as the reducing reagent for Cu ions will be able to easily enter into the pores of zeolites with high hydro hilicity such as faujasite t e zeolites and reduce the Cu(II) ions in the zeolites. t is evident from Table 2 t& the Cu-Na type zeolites have higher oxidation activities than the Cu-H type zeolites, irres ective of the zeolite structures. Also the Cu-zeolites in the a ueous solvent inclu 'ng acetic acid were found to have higher catalytic activity or the benzene oxidation than those in water solvent, irrespective of the zeolites studied.
E i
Eu-
P
1
1
Table 2. Oxidation of Benzene with 0 2 Catalyzed by Cu(II) Ion-Exchanged Zeolitesa). Catalyst
Cu ion-exchanged
(%I
Solvent
Phenol PhenoUCu (mmol)
~~
67
CU-HZ
67
Cu-NaY
9.6
CU-HY
9.6
Cu-NaX
14
CU-HX
14
Cu-Mordenite
23
~
H2O 1N CH3COOH aq. sol. H2O 1N CH3COOH aq. sol. H2O 1N CH3COOH aq. sol. H2O 1N CH3COOH aq. sol. H2O 1N CH3COOH aq. sol. H2O 1N CH3COOH aq. sol. H2O 1N CH3COOH aq. sol.
0.07 0.13 0.08 0.17 0.26 0.38 0.21 0.27 0.21 0.32 0.13 0.21 0.26 0.32
1.21 2.12 1.34 2.70 6.10 7.47 3.68 4.74 4.71 7.24 2.91 4.71 2.50 3.20
~~
~~
a) Catalyst, 0.4g; benzene, 3 cm3 (22.6 mmol); ascorbic acid, 4 mmol; solvent, 20 cm3; temp, 303 K; time, 24 h. 3.3. Effect of Amount of Acetic Acid on Benzene oxidation The de endence of the phenol formation on the amount of acetic acid in the aqueous so vent was inveshgated using the Cu-NaZ, Cu-Nay, and non-anchored CuCl2 as illustrated in Fig. 1. The phenol formation increased with increased concentration of acetic acid assed throu h a maximum and inversely decreased with a further increase in 8 e acetic aci in the CuCl2 catalytic system. Phenol formation catalyzed by the Cu-NaZ zeolite tended to be slightly promoted with an increase of the ascobic acid concentration. The oxidation activity of the Cu-NaY for phenol formation was found to appreciably increase with increased acetic acid concentration.
P
d
3.4. Effect of Reducing Reagent on Benzene Oxidation
The Cu(1) ions, which are formed through the reduction of the Cu(II) ions by ascorbic acid, are considered to be the active species for the oxidation of benzene to
2002 phenol since no oxidation occurred without ascorbic acid. The influence of the ascorbic acid amount on phenol formation is illustrated in Fig. 2 using Cu-NaZ, Cu-Nay, and non-anchored CuClz as the catalyst. The amount of henol increased only slightly with an increase in ascorbic acid when the non-anc ored CuC12 was used. An increase in ascorbic acid with the Cu-NaZ catalyst caused an increase in the amount of phenol formed but a further increase in ascorbic acid tended to inversely decrease the amount of phenol formed as illustrated in Fig. 2. The phenol formation was found to be profoundly promoted with an increase in the amount of the reducing reagent using Cu-NaY as the catalyst.
r:
0,
Ti 1 0 E
0
c
;
5
0
0
2 4 6 8 1 0 Concentration of acetlc acid (M
0
2 L 6 Amount of ascorbic acid(m m d 1
Fig. 1. Effect of Concentration of Acetic Acid on Benzene Oxidation. Benzene, 2 mI(22.5 mmoI); solvent, 20 ml of acetic acid (1 moU1) aqueous solution; reaction temperature, 303 K; reaction time, 24 h; A ,0.53 g (4 mmol) of CuClz,0,0.20 g of Cu-NaY(9.5) (Cu, 0.8 wt%); ,0.40 gof Cu-NaZ(67) (Cu,O.9 wt%). Fig. 2. Effect of Amount of Ascorbic Acid on Benzene Oxidation. Benzene, 2 ml(22.5 mmol); solvent, 20 ml of acetic acid (1moU1) aqueous solution; reaction temperature, 303 K; reaction time, 24 h; A ,0.53 g (4 mmol) of CuC12, 0,0.20 g of Cu-NaY(9.5) (Cu, 0.8 w t l ) ; H ,0.40 g of Cu-NaZ(67) (Cu,O.9 wt%). 4. REFERENCES
Perkin 1 S. Ito, T. Yamasaki, H. Okada, S. Okino, and K. Sasaki, J. Chem. SOC. Trans. II 1988,285. 2 H. Orita. Y. Hayakawa, M. Shimizu, and K. Takehira, J. Mol. Catal. 42 (1987) 99. 3 C. Naccache, and Y.Ben Taarit, in "Zeolite: Science and Technology'' (eds. F.R. Ribeiro, A.E. Roodrigues, L.D. Rollman, and C. Naccache), Martinus Nijhoff Publishers, Ha e, 1984, pp 373-396. 4 S. Tsuruya, Y. kamoto, andT. Kuwada, J. Catal. 60, (1979) 52. 5 S. Tsuruya, M. Tsukamoto, M. Watanabe, and M. Masai, J. Catal. 93, (1985) 303. 6 J.H. Lunsford, Catal. Rev. 12, (1975-1976) 137.
r
G d ,L a ol. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congrcac on Catalysis,19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publisher8 B.V.All n@i rcsclved
FORMATION OF FORMALDEHYDE FROM METHANOL OVER SUPPORTED
TITANIUM OXIDE
H.Imai, Y.Murakami and H.Irikawa Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama 227, Japan
Abstract Decomposition o f methanol was s t u d i e d o v e r t i t a n i u m o x i d e / a c t i v e c h a r c o a l c a t a l y s t s i n t h e temperature range between 600 and 800K. The formaldehyde/ Moreover, d i m e t h y l e t h e r r a t i o approached z e r o as W/F approached zero. formaldehyde and methane were formed from d i m e t h y l e t h e r on t h e c a t a l y s t s . These f a c t s i n d i c a t e t h a t formaldehyde i s formed v i a d i m e t h y l e t h e r i n t e r m e d i a t e . The a c t i v e s i t e s o f t h i s r e a c t i o n a r e discussed on t h e b a s i s o f t h e r e s u l t s o b t a i n e d by XPS measurements. 1. INTRODUCTION Both anatase and r u t i l e s e l e c t i v e l y dehydrate a l c o h o l s [l-31, although Wheeler e t a l . [ 4 ] r e p o r t e d t h a t t h e o x i d e s evacuated a t 673K showed dehydrogenation s e l e c t i v i t y . Our r e c e n t s t u d y on t h e d e c o m p o s i t i o n o f methanol o v e r a t i t a n i u m o x i d e [ 5 ] demonstrated t h a t b o t h t h e a c t i v i t y and t h e s e l e c t i v i t y depended g r e a t l y on t h e t e m p e r a t u r e o f r e d u c t i o n . The s t u d y showed t h a t t h e s u r f a c e area o f t h e t i t a n i u m o x i d e decreased g r e a t l y d u r i n g hydrogen t r e a t m e n t a t h i g h temperatures. The r e a c t i o n o v e r a supported t i t a n i u m o x i d e was undertaken i n o r d e r t o p r e v e n t s i n t e r i n g o f t i t a n i u m o x i d e a t h i g h temperature. An a c t i v e c h a r c o a l was s e l e c t e d because o f i t s c a t a l y t i c i n a c t i v i t y a t t h e reaction conditions. 2. EXPERIMENTAL The c a t a l y s t s were prepared b y an i m p r e g n a t i o n method w i t h t i t a n i u m t e t r a i s o p r o p o x i d e and a coconut a c t i v e c h a r c o a l (Tsurumi Coal HC-30S. 30-60 mesh). The c h a r c o a l was t r e a t e d b y hydrogen a t 1273K f o r 16h b e f o r e t h e impregnation. Decomposition o f methanol was c a r r i e d o u t b y an atmospheric f l o w method [ 5 ] i n t h e temperature range between 600 and 800K i n a stream o f h e l i u m (20 ml/min) c o n t a i n i n g 30.1 T o r r o f methanol vapor. Reaction o f dimethyl e t h e r was a l s o c a r r i e d o u t b y an atmospheric f l o w method i n t h e temperature range between 600 and 900K. The c a t a l y s t was t r e a t e d w i t h hydrogen a t a The p r o d u c t s were g i v e n temperature f o r 90 m i n b e f o r e t h e r e a c t i o n . Molecular analyzed b y gas chromatography w i t h 4-m APSEOl/Flusin T and 0.5-m Sieve 5A columns. The s p e c i f i c s u r f a c e a r e a was measured b y t h e BET method w i t h . a d s o r p t i o n
2004 of n i t r o g e n a t l i q u i d n i t r o g e n temperature. The XRD spectrum was r e c o r d e d on a Rigaku Denki powder X-ray d i f f r a c t o m e t e r w i t h n i c k e l f i l t e r e d Cu KK r a d i a t i o n . The XPS spectrum was r e c o r d e d on a Shimadzu ESCA-850 s p e c t r o meter. T i t a n i u m t e t r a i s o p r o p o x i d e (>95%), methanol ( s p e c t r o s c o p i c grade) were o b t a i n e d from Wako Pure Chemical, w h i l e d i m e t h y l e t h e r (>99%) was o b t a i n e d from Tokyo Kasei Kogyo. Methanol was d r i e d w i t h M o l e c u l a r S i e v e 4A b e f o r e use i n t h e r e a c t i o n . Helium, o b t a i n e d from Japan Helium Center, was p u r i f i e d t h r o u g h a M o l e c u l a r Sieve 5A column and a r a r e gas p u r i f i e r (Model RT-3, Japan Pure Hydrogen Co. ), s u c c e s s i v e l y . 3. RESULTS AND DISCUSSION 3.1.
Decomposition o f methanol methane, formaldehyde (FA), carbon Products a r e d i m e t h y l e t h e r (DME), monoxide and hydrogen i n t h e temperature range s t u d i e d , as shown i n Fig. 1. The c a t a l y s t was reduced a t 1173K b e f o r e t h e r e a c t i o n . Decompositions o f d i m e t h y l e t h e r and formaldehyde a r e observed a t h i g h e r temperatures. F i g u r e 2 shows t h e e f f e c t o f t i t a n i u m o x i d e c o n t e n t on t h e c a t a l y t i c a c t i v i t y a t 723K. The a c t i v e c h a r c o a l i s i n a c t i v e a t t h e r e a c t i o n c o n d i t i o n s . The f i g u r e i n d i c a t e s t h a t t h e a c t i v e s i t e s a r e formed on t h e supported t i t a n i u m oxide. E f f e c t o f r e d u c t i o n temperature was s t u d i e d a t 723K w i t h 5 w t % Ti02/C c a t a l y s t . The t o t a l decomposition a c t i v i t y decreased w i t h simultaneous s l i g h t i n c r e a s e o f formaldehyde s e l e c t i v i t y (SF) w i t h i n c r e a s i n g r e d u c t i o n temperature, as shown i n Fig. 3. The r e a c t i o n s were F i g u r e 4 shows t h e e f f e c t o f W/F on FA/DME r a t i o . c a r r i e d o u t a t 723K w i t h 5% Ti02/C c a t a l y s t reduced a t 1173K. The r a t i o approaches z e r o as W/F approaches zero. T h i s i n d i c a t e s t h a t formaldehyde i s a secondary p r o d u c t and t h a t i t i s formed v i a d i m e t h y l e t h e r i n t e r m e d i a t e .
n
FD 2oo C E
.ti
r
1501
n
0
c
-?-I
E
\
\
r(
.-i
0
E a.
100-
0
E
a.
" Il
J
" 50 a
700 Temperature
800
(K)
Fig. 1 Reaction o f methanol over 5 w t % Ti02/C. Reduction t e m p e r a t u r e = l l 7 3 K 0 : CO, 0 : CH4. (>: FA, 0 : DMA
5
0 TiO,
10
15
content (wt%)
Fig. 2 E f f e c t o f t i t a n i u m o x i d e content. Reduct i o n temperature=ll73K 0: CO, 0 : CH4, @: FA, DMA
a:
2005 The above o b t a i n e d r e s u l t s i n d i c a t e f o l l o w i n g mechanism. ECH3OH
->
CH30CH3
CH30CH3
->
H2CO
+
CH4
H2CO
->
CO
t
H2
t
that
the
reaction
takes
place
by
H20
A
?
t
0
-rl
-4
E
A
\
N
"
0
E
-. "
4
a L
LL
cn
u
(u
LL
4
a
LT
0
600
800
1000
1200 W/F
Reduction temp. ( K >
Fig. 4
Fig. 3 E f f e c t o f r e d u c t i o n temperature.
(g.min/l)
FAIDME r a t i o vs.
3.2.
W/F
Reaction o f dimethyl e t h e r Reaction o f d i m e t h y l e t h e r was s t u d i e d i n o r d e r t o c o n f i r m t h i s mechanism. Table 1 shows an example o f r e s u l t s o b t a i n e d b y a r e a c t i o n a t 723K on 5 w t % Ti02/C c a t a l y s t reduced a t 1173K. Formaldehyde and methane were formed from d i m e t h y l e t h e r on t h i s c a t a l y s t . Carbon monoxide, hydrogen and s m a l l amounts o f ethane, e t h y l e n e and methanol were a l s o formed i n t h i s reaction condition. Table 1 Reaction o f d i m e t h y l e t h e r o v e r 5 w t % Ti02/C* Product methane forma 1dehyde carbon monoxide hydrogen ethane ethylene methanol
*
-
Rate(pno1/ m i n g ) 240 227 18.4
20.0 3.9 1.6 1.0
Reaction temperature=723K,
DME f l o w r a t e = 1 5 ml/min
2006 60
3 n
A
h
w
5
;
"
A 3
2
40
-r(
>
d "
; C
x
0
-.-I
"
3
';
20
al
Pi
il 0)
0
0
-* u)
m C
-
4 0 k-
4
I
2
W/F
4
0
462
(g.min/l>
Fig. 5 E f f e c t o f W/F. Reaction temperature=723K 0: CO, 0: FA, CH4,
a:
0: Conv.
460 Binding energy
458 (ev)
Fig. 6 XPS o f reduced 5 w t % Ti02/C c a t a l y s t . a: o r i g i n a l , b, c: r e s o l v e d
E f f e c t o f r e a c t i o n temperature was s t u d i e d w i t h 5 w t % Ti02/C c a t a l y s t a t c o n s t a n t f l o w r a t e s o f d i m e t h y l e t h e r (5 ml/min) and h e l i u m (25 ml/min). Decomposition o f formaldehyde was observed a t h i g h e r temperatures. Effect o f W/F was a l s o s t u d i e d w i t h t h e same c a t a l y s t a t 723K. h e l i u m / d i m e t h y l e t h e r r a t i o b e i n g k e p t c o n s t a n t a t 5. The r e s u l t s , shown i n Fig. 5, i n d i c a t e t h a t carbon monoxide and hydrogen a r e secondary products. These f a c t s s u p p o r t t h e above mentioned mechanism. XRD measurements showed t h a t a l l samples were amorphous even a f t e r hydrogen t r e a t m e n t a t h i g h temperatures. F i g u r e 6 shows a r e s u l t o f wave r e s o l u t i o n a n a l y s i s o f a Ti2p3/2 XPS spectrum o f reduced (67310 5 w t % Ti02/C c a t a l y s t . The b i n d i n g energy o f t h e e l e c t r o n s i s observed a t 459.8 ev w i t h A standard T i 0 2 sample ( a a shoulder a t a lower b i n d i n g energy (459.2 ev). r e f e r e n c e c a t a l y s t o f C a t a l y s i s S o c i e t y o f Japan, JRC-TIO-5 prepared by a gas phase method) showed t h e Ti2p3/2 spectrum a t 458.9 ev w i t h o u t shoulder. So t h e e x i s t e n c e o f t h e shoulder peak i n d i c a t e s t h a t t h e t i t a n i u m o x i d e was reduced a t 673K o v e r t h e a c t i v e carbon surface. The reduced t i t a n i u m i o n s may be r e s p o n s i b l e f o r t h e new c a t a l y s i s f o r f o r m a t i o n o f formaldehyde from methanol v i a d i m e t h y l e t h e r i n t e r m e d i a t e . 4. REFERENCES 1 2 3 4 5
P. Jackson and G.D. P a r f i t t , J. Chem. SOC. Faraday Trans. 1, 68 (1972) 1443. I . C a r r i z o s a and G. Munuera, 3. Catal., 49 (1977) 174. D.J. C o l l i n s , J.C. Water and B.H. Davis, , Ind. Eng. Chem.. Prod. Res. Dev., 18 (1979) 202. D.J. Wheeler, P.W. Darby and C. Kemball, J. Chem. Sot.. (1960) 332. H. Imai and K. Nakamura, J. Catal., 125 (1990) 571.
.
Guai, L a al. (Editors), New Frontiers in Catabsk
Proceedings of thc 10th International Congress on Catalysis, 19-24 July, 1992,Budapcst, Hungary Q 1993 Elsevier Science Publishers B.V.All righe mewed
A STUDY ON P-Mo-AS HETEROPOLY COMPOUNDS AS CATALYST FOR SELECTIVE OXIDATION OF METHACROLEIN* B. Zhongb, W.Zhengb, R. Heb,G.HuangC andX L% aSupported by NSFC bepartment of Chemistry,Jinan University, Guangzhou 510632,China Wuangzhou Institute of Chemistry,Chinese Academy of Sciences,Guangzhou 510650,
China
Abstract
P-k-As heteropoly compounds, as catalyst for oxidation of methacrolein to methacrylic acid, have been prepared and characteried.The structure of these catalysts were studied by IR, Ranaa spectra and "P NWR. The characteristic cheaical shift values were observed for As-containing highly efficient catalyst. Apparently, in it a new active chemical species appeared, which is different from P H o I ~ O ~ O (Pk14 -~ anion with Keggen structure. The application of heterowly compounds (HE) as catalyst in the oxidation of methacrolein (HAL) to methacrylic acid (MA)was known as one of the rost important advance in heterogeneous catalysis [ll. The essential role of the Kessin structure of heteropolyanion (HPA) is generally assumed [2l, however it proves difficult to interpret the experimental result reported in this paper.The present study attempts to correlate chemical constituent,structure of HPC and their catalytic functions. The catalysts were prepared by conventional procedures. H3Pk1204028H20 dissolved in water and the required quantities of arsenic acid and metal oxide were added to the solution with stirring,subsequently the solution was refluxed at 343 K for 24 h. Final product was obtained by evaporation to dryness. The catalytic reaction,by which activity and selectivity data were obtained,was carried out in a f ixed-bed aicroreactor-gas chromatograph combination around 573 K and at atmospheric pressure.The reaction products were detected by FID in GC. The data obtained were processed with a GC Data Processor. was
2008 The catalysts were charcterized by IR,Raman spectra and3'P NMR. IR spectra were recorded.with a PE - 683 infared spectrophotometer. Raman spectra were obtained on a SPEX-1403 laser Ramam spectrophotometer. A JNM-FX 9OQ spectrometer was used to acquired =lP NMR spectra. Ni2', Co2' and Cu2' with nearly equal ionic Four countercations Zn", radii were chosen for a given HPA, the rate for MAA at optinua condition were almost the same for catalysts containing the above cations except Cu". The redox properties of Cu" nade it more active.Similar oxidizing abilities for different countercations provided evidence that the selective oxidation occurred on HPA.
For catalyst sample X-9, which was prepared with phosphoric acid instead of arsenic acid. The oxidation activity and selectivity for MAA were substantially lower than X-4, which contained As atom. The presence of As atom in hetropolyanionic constituent increased the rate for MAA. The IR spectra of HPA looked alike despite of their difference in constituent due probably to the similarity in bond character of the X-0 , M-0-M and M.O bonds of HPA. In contrast,drast ic deviations occurred among Raman spectra of these catalysts. Fig. 2 shows the different spectra in the 500 -1200 ern-' region. The extra bands 977 cm" and 906 cm-' observed for As -containing sample X-4 suggest difference of chemical species in As-containing catalyst.
Fie.1. IR Spectra A:X-~,B:X-~,C:PMOI~,D:X-~
Fig.2. Ramam Spectra
2009
Chemical species have been investigated in aqueous solution before vawrization by NUR. Fig. 3 shows the corresponding spectra for P7, PZ and PO sample. P7 is solution of HSUolnOlo. The composition of Pa is the same as X-4 catalyst and PO as X-9. The appearance of resonance at u = -2.4 ppm peak indicated the presence of new chemical species only on Pa sample.Al1 spectra were referenced externally to 85 X orthophosphoric acid peak as 0 ppm.
Fig.3. "P NUR spectra of salples
A
Fig.4. Variation of slP NUR spectra in the hydrolysis of HsPUolAro
The resonance at u = -2.4 PPI was also observed for aqueous solutions of X-4 catalyst. The hydrolysis process of HsPWoilOa takes place in the presence of H a 4 or HM4. 3HaPUoizOro + HaPOlo + 12Hd) 4HaPWod)ai(OHo)s 2HsPwOdlai (OHz) a HsPoUoinOsn + 6Hn0 According to the refenerce [31 and in connection with applied solvent the rwnances of "P are assignable to chemical species in aqueous solution as seen in table 3. Thus the resonance at u = -2.4 ppm must be attributed to PAsUOin-" (PAdbin)
.
2010
Table 4. Relative persantase of different species Species
a
-PWQ
38.2 24.9 5.1
PO P2 P7
PzWote
B
37.2
52.7 29.9 6.1
-PH012
9.2 8.0 85.3
............................................................................
Ihe 31P NMR spectra were recored in the hydrolysis process of H3PHo1~040 after addition of HsAsO4.Following the proceeding of reaction, the intensity of resonance assigned to PUo12 was weakened. In contrast, the resonance at a =-2.4 ppm, attributed to PAsWola appeared and its intensity was gradually strengthened with tire. Quantitative study by "P NMR clarified the behavior of aqueous solutions of H3PUo1~040 in the presence of H3ASO4 or HsPO4. For samples PZ and PO PI4012 was decreased owing to hydrolysis, the decrease being greater for PZ sample due to partial replaceaent of P by As. 'he content of PA~UOISproduced in hydrolysis of H s P H o ~ d r oreached 37 % for PZ sawle. Earlier studies have shown that for HPC molecular structure and the type of behavious in the crystalline state were similar in solution [41, so that chemical species in solid state liked in solutidon. Apparently, for the PZ catalyst, as a result of high relative concentration of PAsHole, oxidation activity and selectivity toward UAA were remarkable. With these facts as above rentioned taken into account,the authors conclude that these catalysts are not single structure materials, but it contains several chemical species in equilibrium. For selective oxidatidon reaction of unsaturated aldehydes the catalystic agent is probably a HPA-Pkk~$lsn-~ (PAsHold with Dawson structure rather than PUo1z with Kegeen structure.
REFERENCES 1
B. K. Zhong, R. J. He et al., J. Sci. Ued. Jinan Univ. (Nat. Sc . Ed.),
(1986) 35. 2 H. Hisono, Catal. Rev.-Sci. Ens., 29 (1987) 269. 3 A. Aoshim and T. Yamaguchi, NiPwn Kagaku Kaish , (1985) 2237. 4 I. V. Kozhevnikov, Russ. Chem. Rev. 56 (1987) 81
Ouczi, L d d. (Editors), New Frotuiers in Catalysis Proceedings of thc 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elscvier Science Publishers B.V. All righa lesctved
SILICA AS AN AMMOXIMATION CATALYST FOR THE PRODUCTION OF CYCLOHEXANONE OXIME D. P. Dreonia, D. Pinellia, F. Trifro", Z. Tvaruzkovab,K. Habersbergerb and P. Jirub 9epartment of Industrial Chemistry and Materials, V.le del Risorgimento 4,40136 Bologna, Italy bHeyrovsky Institute of Physical Chemistry and Electrochemistry, Dolejskova 3, 18223 Praha, Czechoslovakia
Abstract Amorphous silica samples exhibiting different Bronsted acidities were tested in the gas-phase ammoximation of cyclohexanone with molecular oxygen. Silicalite and titanium silicalite were also tested in the same reaction. The results of the catalytic tests and of the acidity characterizationsconfiied the importanceof Bronsted sites in order to obtain good activity and showed that the introduction of transition metals into the crystalline framework of silicalite may influence the selectivityconsiderably and in particular suggested that Ti-silicalitemay be a good catalyst for the gas-phase ammoximation of cyclohexanone with 02.
1. INTRODUCTION Cyclohexanone oxime is an important intermediate for the production of caprolactam and nylon-6. Recently a new reaction has been studied for the synthesis of cyclohexanone oxime. In the new reaction (called ammoximation) the cyclohexanone is transformed into the oxime by reaction with ammonia and an oxidizing agent. A first process, patented by Montedipe (l),is very near to industrial application. It is a liquid-phase process using titanium silicalite as the catalyst and hydrogen peroxide as the oxidizing agent. A possible alternative to this process has been patented by Allied Chemical Corporation and studied by Armor et al. (2,3) and more recently by our research group (4- 6). We studied the gas-phase ammoximation with molecular oxygen as the oxidizing agent and a commercial amorphous silica as the catalyst. In our previous studies the reaction network and the mechanism of the steps involved were investigated by flowreactor experiments and FT-IR measurements. They showed that the ketone and the ammonia react on the Bronsted acidic silanols of the catalyst surface to produce the corresponding imine which is transformed by three main parallel pathways: i) a parasitic formation of aldol condensation products catalyzed by the same Bronsted sites of the silica, ii) selective oxidation
201 2
4 6 +NH3
aotiv.
0
2
-H20
Flg.1: Simplified reaction network for cyclohexanone ammoximation with 02.
to the oxime by some activated oxygen species generated by the silica, and iii) the formation of heavy products which deposit on the catalyst as tars by reactions in which the same active oxygen species are involved. A simplified reaction network is shown in Fig.1. Results from a previous study (6) suggested that, in order to have good performance in the gas-phase ammoximation of cyclohexanone with 0 2 , the catalyst must possess a bifunctional nature and, in particular, must exhibit a calibrated Bronsted acidity and as well as good capability to activate molecular oxygen to radical species responsible for the oxidation processes. In regard to the latter aspect, the presence of Ti was shown to produce an increase in oxime selectivity without, however, decreasing the rate of production of the tars. In this paper we present some data relative to the comparison of the catalytic behaviours of some commercial amorphous silicas, with different acidities. Since titanium gave good results in the case of amorphous silicas (6) and since Ti- silicalite is the best catalyst for the process with hydrogen peroxide, the catalytic behaviours and the acidities of pure silicalite and Ti-silicalite were also investigated. 2. EXPERIMENTAL
The flow-experiments were carried out in a glass tubular fixed-bed plug-flow micro-reactor. The products were collected in a solvent (n-hexane) and analyzed by gas-chromatography. A complete description of the reactor and of the analytical procedures is reported elsewhere (5). The typical reaction conditions were :NH3=34% mol, @=lo% mol, cyclohexanone CH=2.8% mol, the remainder nitrogen, T=170-250C, catalyst weight W=0.5-1.3 g loaded as powder (0.100-0.150 mm) or grains (0.3- 0.6 mm), contact time=3.0-5.0s (GHSV=720- 1200 h-'), The following sampleswere tested in the ammoximationof cyclohexanone:i) commercial amorphous silica samples: AKZO F- 7, Grace Nr.2, Cabosil, Titania Grace Nr.3, ii) a pure silicalite and a Ti-silicalite prepared according to patent (8). An I.R. characterization of the surface acidity of the catalysts was carried out by recording the FT-IR spectra of self-supporting pellets of the samples (thickness 10-12 mg/cm2) activated in vacuum. The IR spectra were recorded in the conventional IR cell, in the region where the vibrations of OH and CN groups of adsorbed CD3CN are found (3000-4000 and 2100- 2400 ern-', respectively), by a Nicolet MX-1E spectrometer. Further details on the technique can be found in the experimental section of ref.7.
201 3
2
(9)
I
GraceNr.2")
175
0.5
Cabosil
1
W/F Cow. Y-CHO S-CHO Y-tars Bronsted (g/h.mol) (%mol.) (%mol.) (%mol.) (%mol.) acidity
W
Catalyst
Nr.
I
0.5
I
175
42.0
I
55.4
1.3
I
13.2
3.1
I
23.8
13.6
I
25.9
m
I
s
3
AKZOF-7(')
0.5
175
72.1
32.2
44.7
18.9
vs
4
AKZOF-7(*)
0.5
175
46.3
12.4
26.8
6.8
vs
5
Ti-GraceNr.3")
0.5
175
75.8
25.4
33.5
26.9
S
6 7
I I
Silicalite Ti-Silicalite
I I
1.3 1.3
I I
456 456
I I
35.7 40.9
I I
2.4 13.3
I I
6.7 32.5
I 1
6.6 5.3
I I
w W
I
I I
Notes: reaction temperature = 220'C except in Expt. Nr.4 T=19O'C. ketone conc. in the gas phase=2.8%mol., W=weight of fresh catalyst, F=molar flow rate of cyclohexanone in the test. Acidity characterization: v'svery strong, sstrong, m=medium, w=weak. ; (*) complete catalytic data in ref.5, (#) complete catalytic data in ref.6.
3. RESULTS AND DISCUSSION The behaviour of each of the catalysts tested was followed as function of time-on-saam up to deactivationcaused by the deposition of the tars. The best performances obtained during the tests are reported in Table.1. A rough comparison between the acidities of the samples was attempted by measuring the absorption in the region between 3000 and 4000cm-' corresponding to the OH stretching of free and H-bonded silanols normally present on the silica surface which arc responsible for Bronsted acidity. Silica samples usually exhibit a sharp band at 3745 cm" due to the OH stretching of free surface silanols. Other broad absorption bands are also observed at lower wave numbers which are assigned, in the literature, to H-bonded and/or internal hydroxy groups. The IR characterization of the AKZO F-7 sample, which proved to be the best catalyst, revealed an unusual intensity of the absorption in the OH region, in particular with regard to the component at lower frequencies, evidencing an anomalous concentration of surface Si-OH groups, expeciallyof H-bonded silanols. A complete and exhaustiveacidity IR characterizationof AKZO silica is reported elsewhere (7). The same characterizationprocedure was also applied to the other commercial amorphous silica samples and to the silicalites. A fist classification of the surface Bronsted acidity of the samples (from very strong to weak) was attempted on the basis of the comparison of the intensities of the absorptions in the 3000-4000cm-' region. The results are reported in Table.1. The Cabosil sample, which exhibited an acidity very similar to that usually reported in the literaturefor amorphous silicas, proved to be the least active and by far the least selectivecatalyst
2014
among the amorphous silicas tested (Expt. Nr.1). A medium activity was obtained in the case of Grace Nr.2 (Expt. Nr.2), which, on the other hand, exhibited a medium acidity. On the basis of these results, the simple correlation between the acidity and the catalytic performance suggested in ref.6 may be drawn. The comparison between the catalytic behaviours of pure silica lite and Ti-silicalite (Expts. 6 and 7) confirmed the positive effect on oxime selectivity of the presence of Ti found in the case of amorphous silicas (comparing Expts. 2 and 5 , ref.6), and suggested that Ti-silicalite may be a good catalyst not only in the liquid-phase ammoximation with H202 but also in the gas-phase reaction with 0 2 , the oxime selectivity obtained being near to that found with the AKZO sample. On the other hand, the conversion obtained with this catalyst is too low to permit high yields of the oxime although they are obtained with higher selectivities than that found for AKZO silica at nearby the same conversion but obtained at lower temperature (190 C, Expt. Nr.4). The I.R. spectroscopy measurements showed that the Ti-silicalite has a lower Bronsted acidity than the AKZO sample and this evidence accounts for the lower activity, since the first step of the process is the production of the Si- bonded imine on the catalyst silanol groups (5,7). It should be noted that the activities of the silicalite samples are not affected by changes in the Bronsted acidity since both the silicalites exhibited about the same acidity, irrespective of the presence or the absence of Ti inserted in the framework. The comparison between the increases in selectivity obtained by introducing Ti into amorphous silica (Grace Nr.2 and Ti-Grace Nr.3 samples, Expts. Nr.2 and 5 ) and into silicalite (Expts. Nr. 6 and 7) showed that the presence of Ti in the crystalline lattice is fundamental to obtain a high oxime selectivity. Furthermore, it was shown that in the case of silicalite, as well as in that of Grace amorphous silicas (6), the yield of tars is not affected by the presence of Ti, confiiing that the role of Ti regards only the selective pathway (6).
Acknowledgments. The financial support from C.N.R. - “Progetto Finalizzato - CHIMICA FINE 2“ (Rome) is gratefully acknowledged.
4. REFERENCES 1. P. Roffia et al., US Patent 4,745,221 (1988). 2. J. N. Armor, J.Catal.,70 (1981) 72. 3. J. N. Armor, E. J. Carlson, S. Soled, W. D. Conner, A. Laverick, B. De Rites and W. Gates, J.Caral., 70 (1981) 84. 4. D. P. Dreoni, D. Pinelli, F. Trifub, in ”12 Simposio Ibero American0 de Catalise”, Rio de Janeiro 1990, v01.2, p. 305. 5. D. P. Dreoni, D. Pinelli, F. Trifirb, J . Mol. Catal., 69 (1992) 171. 6 . D. P. Dreoni, D. Pinelli, F. Trifirb, in Studies in Surface Science and Catalysis - New Development in Selective Oxidation by Heterogeneous Catalysis 111” procedings of the “Third European Workshop Meeting”, Louvain-la-Neuve 1991, Elsevier Science pub. 7. D. P. Dreoni, D. Pinelli, F. Trifirb, G . Busca, V. Lorenzelli,J. Mol. Catal., accepted. 8 . G. Bellussi, A. Giusti, A. Esposito, F. Buonomo, European Patent 226,257 (1986)
Guni, L et d.(Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24July, 1W2, Budapest, Hungary 8 1993 Elsevier Science Publishen B.V.All rights reserved
KINETICS OF THE REDOX REACTIONS OF THE 0,: PR0PYLENE:vBISMUTH MOLYBDATE SYSTEM: A TAP REACTOR STUDY D. R. Coulson, P.L. Mills, K Kourrakis, P. W.J. G.Wijnen,J. J. Lerou and L. E. Manzer DuPont Central Research and Development, DuPont Company, Experimental Station, Wilmington, Delaware 19880-0262,USA
lntroduct ion In an attempt to better understand the interaction of oxygen and propylene with ybismuth molybdate, y-Bi2MoO6, we have taken the approach of using a TAP (Temporal Analysis of products) reactor to study these processes. Previous appmaches to the study of this system have involved processes which resulted in significant disruptions of the catalyst surfaces. In all cases, the redox processes involved amounts of oxidant or reductant equivalent to cu. a monolayer or more of coverage. In addition, the time scales of the processes examined were of the order of minutes. In this study, we demonstrate the use of the TAP reactor to examine the redox reactions associated with yBi2MoO6 occurring over time scales one to two orders of magnitude smaller than previous studies and involving less than 1/1000th of the catalyst surface. In this manner we are able to monitor the activity of isolated sites of the catalyst at the earliest stages of reaction. Several gr0ups~9~ have reported that the reductions of yBi2MoO6 by olefins proceed by different pathways. Recently, Glaeser and coworkers2 reported a model, based on a Raman IR study of reduced yBi2MoO6, for these reductions where different oxygens of y-Bi2Md6 are involved in the oxidations of propylene and 1-butene. In this model, the oxygen in acrolein, arising from reduction by propylene, comes from an oxygen bridging two Mo atoms whereas only an oxygen from a Bi-0-Mo bridge is involved in the oxidation of 1-butene to butadiene. Other workers4, from examinations of reactions of 180-labelled yBi2Mo06, have given contrary evidence that the oxygen contained in the acrolein formed from propylene oxidation over yBi2MoO6 comes from an oxygen attached only to Bi and not Mo. Finally, Ueda and coworkm3 claimed that the reaction of propylene with 18Oenriched yBi2MoQj gives different extents of l80 enrichment in the acrolein produced, depending upon whether the y-BizMoO6 was previously enriched (to the same extents) by propene/1802 or l-butendl802 treatments. We report here our attempts to use the TAP reactor to examine these claims.
Experimental
Materials Pure-phase yBi2MoOg (B.E.T. surface area: 1.4 m2/g; average pore diameter 13.2 nm; particle size 354-425 pm.), prepared according to the method reported by Batid, was used in
2016
this study. All reactant gases were obtained from commercial sources and used as received without further treatment. The gases used here include propylene (99.2%),1-butene (99%), '602(99.98%),1802 (97%)and krypton (99.92%). TAP Reactor Svstem All experiments were performed using a TAP (Temporal Analysis of Products) reactor system. A detailed description of this system is available elsewherelb. All reductions of yBi2MoO6 were accomplished by reacting a fixed number of pulses of the desired olefin (propylene or 1-butene) with 0.5 g samples of the catalyst at 723 K. The extents of reduction were determined by measuring the total oxygen consumed from pulsing oxygen over the reduced catalyst. The total amounts of oxygen required for complete reoxidation were found to be 4.0 nmoles/g for 1-butene and 2.24 nmoles/g for propylene. Assuming that y-Bi2MoO6 contains 6 x 1018 0 atoms/m2. this implies that only .053%and .030%,respectively, of the surface oxygens had been removed by the olefin reductions. Reoxidations of the reduced yBi2MoO6 samples were followed by measuring initial oxygen consumptions. The average amount of oxygen consumed during the initial five pulses (out of a total of at least 60 pulses) was taken to be a measure of the initial reoxidation rates. Following a reoxidation, the catalyst was again reduced at 723 K. This redox cycle was repeated several times using a different oxygen pulse intensity or temperature for each measurement. '80-enriched samples of yBi2Md6 were prepared by flowing the desired olefins over 0.5 g samples of y-Bi2MoO6 held at 723 K while pulsing 1802 to maintain a fully oxidized system. Approximately 2.0 x 102 pmoles of 18% were consumed by these processes for each olefin.
Results and Discussion If the reductions of y-Bi2MoOg by 1-butene or propylene involve different sites, then the reoxidations of separate propylene- and 1-butene-reduced y B i 2 M d 6 samples by oxygen might be expected to show differences in kinetic behavior. To test this hypothesis, we measured the kinetic order in oxygen of the reoxidation of a sample of 1-butene-reducedyBi2MoOg and compared it to a previously-determinedlvalue of the oxygen order for reoxidation of propylene-reduced yBi2MoO6. A plot of the logorithms of initial oxygen consumption vs. initial oxygen pulse intensity gave a slope of 0.95 indicative of a first order reoxidation in oxygen. Previously1, we found that the reoxidation of propylene-reduced yBi2MoO6 was also first order in oxygen (see Table 1). The activation energies of these reoxidations were also determined. Samples of propylene- and 1-butene-reducedyBi2Md6 were reoxidized by pulses of oxygen at a series of temperatures in the TAP reactor yielding initial oxygen consumption rates. Table 1 summarizes the typical values of the activation energies found. As seen from Table 1, the activation energies found for these two processes are identical within experimental error. In preliminary studies, we found that 1802, when pulsed over yBizMoO6 at 723 K, did not undergo exchange. This finding agreed with that found previously by other workers7 who also reported that acrolein did not measurably exchange with 1802 over yBi~MoO6. When propylene was pulsed alone or simultaneously with 1802 over yBizMoO6 at 723 K, negligible
201 7
Table 1. Kinetic orders in @ and Activation Energies of Reoxidation of Reduced y-Bi2MoO6 y - B i f l G Reductant
Kinetic Oxygen Order
Activatioh Energy, kcaVmol
1Butene
0.95 (0.05)a
4.1 (0.6)a(493 to 723 K)
Propylene
1.02 (0.06)a
4.8 (0.5)a (493 to 723 K)
a) E m s arc 2 x std. errors. incolporation of 180 into the reaction products was also observed, as found previously by other workers? As a test of possible differences in the reductions of yBizMoO6 by propylene or 1butene, we examined the ductions of 180-enriched samples of y-Bi2M006 The 180-enriched sample of y-Bi~MoO6 produced by the l-butendQ treatment was examined first. Ueda and coworkd, w d n g with a similarly-preparedenriched sample of y-Bi~MoO6, had found that this material gave a 180/160 ratio for acrolein (from reaction with propylene) that was significantlylower than the corresponding ratio found for water (from reaction with l-butene). This was reported as direct evidence that the oxygens of yBi2MoO6 react differently when treated with propylene or l-butene. However, when we pulsed l-butene or propylene over our sample of 18O-enriched y-Bi2Mo06, we found these corresponding l80/l6O ratios to be identical, within expenmentalemr. When, in a separate experiment,the y-Bi~Md6was fully reduced prior to reoxidation with 1802, reaction with l-butene and propylene again produced identical 1@/1% ratios in the respective products. Similarly,when the 180-enrichedsample of yBizMoO6 produced by the propylene/%2 treatment was examined, the same results were obtained (see Table 2). The amounts of acrolein and water (from l-butene) produced during each ratio determinationwere ca. 0.2 and 4.0 pmoledg, respectively. Attempts to measure the l@/l% ratios for the carbon dioxide, carbon monoxide and water (frompropylene) produced during these oxidations were unsuccessful due to contaminationof the d e values obtained by other compounds. Ratios in Mutts obtained from Reactions of 1Butene and Propylene Table 2. with 180-labeled.y-Bi2MoO6. 180 Enrichment Method
kductant
l-Butend8@
1-Butene
Water
Acrolein
0.23, 0.26, 0.138
Ropylene ~ o p ~ l ~ c / l t ?l-Butene ~
'80/'% Ratios
0.21, 0.25, 0.138 0.24
Propylene a) Ratio obtained from srepwise l-butenereduction
0.21 mxidation.
201 a
The reaction parameter values presented in Table 1 do not allow us to distinguish between the reoxidations of 1-butene- and propylene-reduced y-Bi2MoOg. Furthermore, our 180-labelling findings do not distinguish between the different oxygens of y-Bi2Mdg ending up in acrolein and water (from 1-butene). These latter results are in apparent conflict with claims24 that the oxygens of y-Bi2MoOg are distinguishable in these reduction processes. We conclude that either the reactive oxygens of y-Bi2MoOg react identically in these reactions or that they do behave differently, but that oxygen exchange between the various structural types of oxygen proceeds more rapidly, erasing any differences that might be seen by '80-labelling. As noted previously1, the finding of unit oxygen order for the oxidation of reduced yBi2MoOg is in disagreement with Brazdil et a1.8 who reported one-half order oxygen dependence. As noted by Wragg et a1.7 and confirmed by our findings, the absence of 1802 exchange with y-Bi2MoOg under reaction conditions is evidence that a rapid, reversible chemisorption of oxygen to form 0,Oor Om2is unlikely. This fact, coupled with our kinetic findings', suggests the following mechanism for the reoxidation of reduced y - B i ~ M d :6 M+@
-
+V M-0 + V M*@
M*&
(K1=j$ kl
M-0 + V-0
( Slow )
M+V-0
( Rapid )
(1)
M and V represent different types of oxygen vacancies. V sites are created by the reductant and cannot be directly reoxidized with oxygen while M represents a site that can be reoxidized with oxygen and can also reoxidize a V site. & is the initial concentration of M sites. This mechanism predicts a reoxidation that is first order in both oxygen and oxygen vacancies at low partial pressures of oxygen, as found in the TAP reactor. The fractional oxygen orders reported by Brazdil er al. for the reoxidations could have resulted from their examination of the kinetics at oxygen partial pressures high enough to exhibit pseudo-half order kinetics.
References 1) A preliminary report of portions of this work may be found in the Proceedings of the Third European Workshop Meeting on Selective Oxidation by Heterogeneous Catalysis held in huvain-la-Neuve on April 8-12, 1991. 2) L. C. Glaeser, J. F. Brazdil, M. A. Hazle, M. Mehicic and R. K. Grasselli, J. Chem. Soc., Faraday Trans. I, 81 (1985) 2903. 3) W. Ueda, Y. Moro-oka and T. Ikawa, J. Chem. Soc.,Faraday Trans. I, 78 (1982) 495. 4) (a) H. Miura, T. Otsubo, T. Shirasaki and Y. Morikawa, J. of Catal., 56 (1979) 84. (b) ibid, 36 (1975) 240. 5) Ph. A. Batist, J. Z. H. Bouwens, and G. C. A. Schuit, J. Catal., 25 (1972) 1. 6) J. T. Cleaves, J. R. Ebner and T. C. Kuechler, Catal. Rev.-Sci. Eng., 30 (1) (1989) 43. 7) R. D. Wragg, P. G. Ashmore and J. A. Hockey, J. Catal., 22 (1971) 49 , 8) J. F. Brazdil, D. D. Suresh and R. K. Grasselli, J. Catal., 66 (1980) 347.
Guczi, L d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights mewed
PARTIAL OXIDATION OF PROPENE IN THE PRESENCE OF STEAM
Y.A. Sakh-AlhamedO, R. R. Hudginsb and P. L. Silvestonb T h e m . Eng. Dept., King Abdulaziz University, Jeddah, Saudi Arabia bChem. Eng., University of Waterloo, Waterloo, Ontario, Canada Abstract Addition of steam in the partial oxidation of propene improves selectivity and is frequently mentioned in the patent literature. Despite its use, little has been published on the effect of steam on partial oxidation kinetics or on the mechanism through which steam alters selectivity. This contribution addresses these questions employing results from steady state and step-change measurements of rates of product information, isotopic transient measurements and temperature-programmed desorption studies. Two catalysts were investigated: a Sb/Sn/V oxide, described in the literature as capable of forming acrylic acid, and a B i m o oxide that does not form the acid. For the B i m o oxide, steam addition reduces the rates of product formation, but does not affect selectivity markedly and leaves the reaction order with respect to reactants unchanged. The role of water is to block reaction sites by competitive adsorption on these sites. The effect of steam addition on the Sb/Sn/V oxide is complicated. Low levels of steam (< 2 %) sharply alter selectivity by suppressing total oxidation and increasing acrolein and acrylic acid formation rates. Total oxidation products arise solely from a C-C bond scission side-reaction which produces acetaldehyde and acetic acid. Steam levels greater than 5% decrease product formation. Step-change data indicate total oxidation of propene or partial oxidation products must occur and that acrylic acid must form through acrolein. Use of an "0,isotope indicated 0,exchange between steam and the catalyst and suggested rapid formation of weakly adsorbed acrolein on the surface. The Sb/Sn/V oxide as a source of O2for acrolein formation was indicated. TPD measurements suggested both weak and strong adsorption sites participate in the partial oxidation reactions. Acrolein was associated with the former, while propene and oxygenates where found on the larter sites. Acrylic and acetic acid are strongly adsorbed. These results are consistent with partial oxidation occurring on both weakly and strongly adsorbing surface sites. Acrolein appears to be produced mainly on the former; C-C bond scission and total oxidation occurs primarily on the latter. Acrylic acid is formed on these sites as well but via acrolein. The role of steam seems to be to selectively block the strongly adsorbing sites and enhance re-oxidation of the catalyst surface. 1. INTRODUCTION
Presence of steam in the reaction mixture during selective oxidation of propene (qis known to improve selectivity and depress formation of carbon oxides. Thus, steam is added to the feed for industrial-scale production of acrolein and acrylic acid by catalytic oxidation. Relatively little has been published on the effect of steam and on its mechanistic role for different catalysts. This investigation addresses these questions. 2. EXPERIMENTAL
Bismuth molybdate (Bi,M%O,& and Sb/Sn/V oxide catalyst were used. The former was a 99.5% pure, 200-mesh material. Sb/Sn/V had a nominal composition 2/1/1 as Sb/Sn/V and was prepared as outlined in British Patent 1,034,914 (1966). With the Bi/Mo catalyst used in this study, acrolein, acetaldehyde, CO, and acetone were formed. With Sb/Sn/V, acrylic acid and acetic acid were also produced. A conventional flow reactor with GC analysis was used for all experiments (Saleh, 1990).
2020 Most experiments were performed with Sb/Sn/V at 340°C and atmospheric pressure. Data for Bi/Mo were obtained at 360°C. Propene concentrations up to 20 (vol) % at three levels of O2(5, 10 and 20%) were employed. Steady-state results for the Sb/Sn/V catalyst were corrected for catalyst deactivation. Total conversion did not exceed 10% and was usually much lower. 3. RESULTS
Under steady state with 10% 4 and variable C; in the feed, the presence of steam made a substantial difference in the partial oxidation kinetics. Rate of acrylic acid formation in the presence of steam is approximately twice as great as in its absence and the rate continues to grow with increasing C,. concentration. Formation of CO, is strongly suppressed by a factor of about 2. Acetone is formed only in the presence of water. When expressed in yields, steam has only a small effect on the acrolein and q acrylic acid, but suppresses yields of acetaldehyde and COP Yields of acetaldehyde, acetic acid and C02 in the presence of water fall at the same rate with respect to the q / O , ratio, suggesting these products form from a common intermediate. The CO, yield equals the sum of the yields of acetic acid and acetaldehyde. Thus, these products arise from C-C bond scission. In the absence of steam, this sum is about half the C0,yield. The remaining half arises from the furlher oxidation of acrolein or acrylic acid. An 0,-rich surface seems necessary for acrylic acid formation. Unless water is present, this condition also leads to product degradation. Different trends for acrolein and acetaldehyde with the C,'/O, ratio suggest different sites are involved for H, abstraction 0, addition and C-C bond scission. Examination of the dependence of the rate of product formation on water concentration at 20% 0, shows that these rates become independent of or slightly inhibited by water for and 5% or 10% concentrations greater than 2 to 5% for all reaction products except acetone. A maximum for acrylic acid indicates water must adsorb on sites where acrylic acid is formed. The strong influence of steam on rates of oxygenate formation at low steam levels suggest two roles for water on the Sb/Sn/V surface: creation of new oxidation sites and site blockage. We speculate from the immediate reduction in CO, formation at low water levels, that blockage of sites for total oxidation by one or more water molecules creates a site capable of 0,insertion. concentration. BiMo catalyst shows different dependencies of product rates of formation on Except for acetone, steam suppresses rates of formation, but does not change reaction order with respect to C;. In the absence of steam, COz formation indicates some total oxidation of C, or one of the products. When steam is added, total oxidation is suppressed; also the acetaldehyde rate exceeds CO, production by about 30%. Thus, the side-reaction to acetaldehyde must yield an oxygenate other than cop Sb/sn/V is more active than BiMo by 8 to 15 times in the presence of steam, whereas in its absence, the activity advantage disappears when C,- exceeds 10% vol. The transient response when 8% steam is added shows a sharp fall in CO, with an immediate jump in oxygenates. This indicates some of the CO, must arise from total combustion of a strongly bonded acrylic acid, an intermediate for this acid, and possibly acetaldehyde and acetic acid. These strongly bonded species are displaced when water is introduced. Organic acids seem to compete with water for the same adsorption sites. The opposite response of acetaldehyde and C0,may mean that water adsorption interferes with oxidation of the C, species formed by the C-C bond scission. The rapid rise in product concentration within 10 to 20 seconds signals an adsorption/desorption phenomenon. Water displacement of oxygenates (except acrolein) is likely. Local minima observed following product peaks suggest inhibition of product formation by water adsorption. The slow increase in concentration that follows these minima can be explained either by surface reorganization or changes in the oxygen co-ordination of the metal, extending below the catalyst surface. These changes generate new active sites on the catalyst surface. Perhaps these are nascent sites, unable to function in the absence of water because they are blocked by strongly bonded acids and acetaldehyde
2021 species. These species must desorb before the sites become active. On removal of steam, aldehydes, acids, and COzbehave differently. A sharp overshoot of acrolein may be explained by rapid desorption of water from the surface which frees sites for C; adsorption. Most of this C,’ is rapidly transformed into acrolein and accounts for the initial rise. The following decline has two possible sources: slow regeneration of acrolein sites through re-oxidation by gas phase 0,or diffusion of acrylic acid (or strongly adsorbed intermediates) to these sites, choking off acrolein formation. The local maximum in the acrylic acid concentration following a sharp undershoot arises from the availability of acrolein. The acrylic acid maximum follows the acrolein maximum by less than a minute. Except for CQ, the new steady-state concentrations are below their initial values. A loss of active sites has occurred, possibly because of reorganization of the surface, or slow poisoning by a build-up of the product acids. All temperature-programmed desorption (TPD) measurements were made on catalyst samples pretreated in air for at least 1 h at 460°C.After sample adsorption at a chosen temperature, the catalyst was flushed with helium. Desorption was obselved for a heating rate of 10°C/min using a flame ionization detector. A large desorption peak obtained at about 200°C with both and acrolein lowtemperature adsorption was found to be acrolein. A second large peak at about 320°C was a mixture of propene and oxygenates. Acids desorb at temperatures higher than 30O0C,indicating they are strongly adsorbed on the catalyst. TPD measurements suggest that the Sb/Sn/V catalyst ought to be covered with adsorbates at the 340°C temperature used in the rate experiments. A TPD experiment for adsorbed on Bi/Mo catalyst at 150°C showed no desorption peaks above 250°C. Thus, B W o is not heavily populated by adsorbed species at the 360°C temperature used with that catalyst. Results for Sb/Sn/V indicate two sets of active sites. On the lower temperature (LT) sites, C,’ abstracts oxygen from the surface to form acrolein. Most of the acrolein originates with the LT sites at the reaction temperature, whereas acrylic acid forms on higher temperature (HT) sites. Total oxidation probably occurs only on the latter. The degree of metal co-ordination of the Sb/Sn/V surface controls the adsorbate composition: at a high degree of oxidation, acids or aldehydes predominate, whereas on a reduced surface, C; predominates. Some acetaldehyde appears to originate with the LT sites but, since acetaldehyde desorbs at higher temperature, most C-Cbond scission occurs on the strong adsorption sites. Acrolein adsorbs rapidly on the HT sites; adsorption on the oxidized HT sites may be limited. TPD results suggest acrylic acid formation may be desorption limited. The formation of acrolein is not controlled by desorption, since the LT peak for acrolein adsorption has a lower temperature than the peak for C,’ adsorption. Because reactions associated with the HT peak sites seem involved in acrylic acid formation, C-Cscission and probably total oxidation, the role of water was examined for this peak only by adsorbing at 200OC. The HT peak is substantially reduced by pre-adsorption of water. Apparently, water competes with propene for strong adsorption sites. Isotopic transient experiments were performed with I8Ozfor the Sb/Sn/V catalyst. When steam was added, 0.4% was used. Response for acrolein, acetaldehyde, and COz isotopes was obselved following an isotopic switch at 20% Oz and 5% propene. The Cl6O2signal indicated either strongly adsorbed CO, or its formation from a hydrocarbon-lattice oxygen reaction. The C’60180 response passed through a maximum and then decreased slowly. Initially the signal for C”0, increased at about half the rate for CL60”O.Since the gas-surface exchange was slow, CO, appears to form from further oxidation of partially oxygenated species. The C’60’80 concentration was twice as high and did not decay, suggesting the existence of a constant source of l 6 0 , presumably from water present in the feed of the reactor. Evidently, oxygen from the water exchanges with the surface or can be incorporated into a surface intermediate which can be further oxidized to COP The acrolein I6O species dropped nearly linearly and reached zero at 4 x the time needed by a He tracer. Acrolein formation must thus draw on an I6O reservoir: lattice Oz or 0, adsorbed on the Sb/Sn/V surface. Whatever the source of 0,, the rapid replacement of I6O by “0is consistent with acrolein formation on weakly adsorbing LT sites in the TPD measurements. Acetaldehyde behaves like acrolein. This seems to indicate that the
2022 carbon bond scission may occur to a larger extent on LT sites than W D measurements implied. A possible intermediate for COz formation is total combustion of adsorbed acrylic acid. Novakova et al. (1976) have found that water could participate in the formation of acrylic acid: therefore, it could be the source of the higher content of C1601*0in the reaction products. Also, no I6O acrolein was present in the product after 48 s; thus, acrolein cannot be the source of l 6 0 in C160180, since this species continues to be observed at high levels 180 s after the “Ozto ‘*O, switch. 4. DISCUSSION
For B@Io the reaction order was unity with respect to C,’ and zero with respect to oxygen. Thus. the reaction appears to be reduction-controlled under our experimental conditions. Presence of steam did not enhance the activity of this catalyst. Re-oxidation controls with the Sb/Sn/V catalyst in the absence of water when the C,-/O, ratio is greater than unity. The reaction order changes from half- to zero-order or less with respect to C,- for this condition. The largest increase in catalyst activity due to water occurs. Evidently, water increases catalyst activity by enhancing the catalyst re-oxidation rate. Further evidence is that reaction order with respect to Oz decreases by a factor of 3. Experimental results are consistent with the formation of most of the acrolein from oxidation of adsorbed on weakly adsorbing LT sites. Propene adsorbed on more strongly adsorbing HT sites undergoes C-C bond scission under oxygen attack to yield acetaldehyde and COz or oxygen addition to form acrolein. This acrolein probably does not desorb, but is further oxidized to acrylic acid. If there is sufficient oxygen on the surface or if acrylic acid is so strongly bonded that desorption cannot occur, further oxidation to combustion products takes place. Water acts via adsorption on the strongly bonding sites, blocking these sites and preventing funher oxidation. Water can exchange oxygen with the surface and may promote re-oxidation of the redox catalyst in this way, or by decreasing the amount of strongly adsorbed oxygenates on the surface that act as reductants. 5. ACKNOWLEDGEMENTS Support for this project came from NSERC operating grants to two of us (RRH, PLS). Y.A. S-A was funded by the Government Saudi Arabia to Y.A. S-A. Prof. Goodwin generously permitted use of his isotopic transient equipment. 6. REFERENCES
Novakova, J., Dolejsek, Z., and Habersberger, K., React. Kinet. Catal. Lett. 4 (3), 389-395 (1976). Saleh. Y.A.A., PhD thesis, Univ. of Waterloo. Waterloo, Ont., Canada (1990).
Guni, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elscvier Science Publishers B.V. All rights reserved
THE KINETICS OF ACTIVATION OF INDUSTRIALAND MODEL IRON CATALYSTS FOR AMMONIA SYNTHESIS IN DRIED AND WET ATMOSPHERE A. Baranski, A. Kotarba, J. M.Lagan, A. Pattek-Janc& E. Pyrczak and A. Reuer
Jagiellonian University, Karasia 3, 30-060 Cracow, Poland
The oxide precursor of the catalyst is activated by a hydrogen reduction in situ in industrial reactors or in especially built installations [l]. The kinetic data are of primary importance for the technological design of the reduction process Kinetic equations for the reduction of industrial catalyst in dried gas phase were previously proposed by us [2,3]. Their significance was emphasized in the review articles [4,5]. Water evolved during the reduction significantly retards the reduction rate. The retarding effect is due to the alumina [6]. It implies a necessity of a study of industrial and model catalysts in dried and wet atmosphere.
.
1. ABBREVIATIONS
Mlcatl-42 is a typical label of a model catalyst. It reads: doubly promoted (by K and Al) catalyst containing 42 wt % of wustite. Letter M will end the label if wustite is lacking, i.e. the catalyst is based on magnetite only. Letter u at the beginning will denote an unpromoted catalyst. 2. EXPERIMENTAL
Model catalysts were prepared in industrial conditions. If necessary, they were preoxidized or prereduced in CO/CO, mixture at 1370K in order to remove wustite or magnetite. The alumina and K,O content amounts to 3-4 and to 1.5-2 w t % respectively. Reductions were carried out in a flow Mc Bain thermobalance [2]. The water content (ppm) in the 3H,/N, mixture amounts to ca. 100 (dry atmosphere), -2600, -6000 and -10000 (wet atmosphere). 3. THE KINETICS OF REDUCTION OF THE INDUBTRIAG CATALYSTS IN
THE DRY ATMOSPHERE
Two mixed-control equations: Seth-Ross (SR) (see [2]) and were previously used for the kinetic Spitzer (S) (see [3]) description of the reduction of the industrial iron catalysts
-
2024 in dry atmosphere. A complex linearized form of (SR) equation was replaced by the relation R=R(t) (where R was the reduction degree and t was time ) . The simple linear regression method was replaced by the direct numerical solution of the nonlinear equation. The parameters of the equation were calculated by a minimization procedure. This treatment proved that SR was valid also for the initial part of the kinetic curve. The discrimination procedure reveals that SR is a more adequate empirical equation than S. The detailed description of the outlined results will be presented in a separate paper.
Dry atmosphere (-100
ppm HzO)
Wet atmosphere
A1catl-M
Time/mCn
(-lo Oo0
ppm H2O)
Time/min
KM I
'oar 80
Time/min
Figure 1. The kinetic curves of the reduction of the iron catalysts.
2025 4. THE EBBECT OF WATER OU THE REDUCTIOU RATE
Two industrial catalysts (Danish KM I and Polish PS3INS) and Alaatl-Y were chosen for the study of the effect. The catalysts were reduced at varying temperatures and atmospheres. The kinetic curves are shown in Fig. 1. Some of them are averaged basing on several (2-10) kinetic runs. A spectacular decrease of the reduction rate can be observed in wet atmosphere for industrial catalysts (for T< 500OC) and for Alaatl-W (for T400 A). ?his structure sensitive character for CH, steam reforming is explainable by considering a reaction pathway, the rate determining step of which is the recombination of the chemisorbed carbide (C') intermediate species with oxygen adatoms (0') originated from the catalytic decomposition of a water molecule adsorbed on the MgO surface. Therefore, it can be ar ued that the stronger interaction between defective Ni sites of low coordination (d8C 50 ) and 0' (structure sensitive) [7] leads to the lowest TOF values, while the more regular structure of large Ni particles (d>50 A) favours a faster recombination between C' and 0' species well accounting for higher TOF values.
i
x
CONCLUSIONS
1. The practical suitability of Ni/MgO catalysts is strongly dependent on the activation (i.e., calcination and reduction) treatments. 2. The unique structural features of the Ni/MgO system, leading us to span a wide range of metal dispersion (0.7 - 90%), has allowed to evidentiate a negative particle size eflect for methane steam reforming (structure sensitive character). REFERENCES 1. J.R.Rostrup Nielsen, in S t e m Reforming Catalysts, Danish Technical Press, Copenaghen, 1975. 2. P.A.Hagan, M.G.Lofthouse, F.S.Stone and M.A.Trevethan, in Preparation of Catalysts ZZ, B.Delmon, P.Grange, P.A.Jacobs and G.Poncelet (Editors); Elsevier, Amsterdam (1979) p. 417. 3. T.Borowiecki, Appl. Catal., 10 (1984) 273. 4. A.Parmaliana, F.Arena, F.Frusteri and N.Giordano, J. Chem. SOC.,Faruday Trans, 86 (1990) 2663. 5. F.Arena, A.Licciardello and A.Parmaliana, Card. Lett., 6 (1990) 139. 6.F. Arena, B.A.Horrel1, D.L.Cocke, A.Parmaliana and N. Giordano, J . Catal. 132 (1991) 58. 7. R.F.Hicks, H.Qi, M.L.Young and R.G.Lee, J.Catul., 122 (1990) 280.
Guni, L d d. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1 W 3 Elsevier Science Publishers B.V.All rights rcserved
EFFECT OF TIN AND IRON DEPOSITION ON THE CATALYTIC PROPERTIES OF PLATINUM SUPPORTED ON GRAPHITE E. Lamy-Pitara, L. El Ouazzani-Benhimaand J. Barbier Labratoire de Catalyse en Chimie Organique, URA CNRS 350,40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
IN'I'RODUCTION
Bimetallic catalysts Pt-Sn and Pt-Fe are known for their good selectivities in the catalytic hydrogenation of insatured aldehyds, leading to insaturated alcohols (1-9). However, many questions are still open in relation to the characterization and the effects of tin and iron on the catalytic properties of platinum. In this work platinum catalysts supported on graphite were modified by tin and by iron deposits and characterized "in situ" by different methods : cyclic voltammetry, adsorption and hydrogenation of maleic acids, used as probemolecules. EXPERIMENTAL
The base catalyst was a 5 ?i platinum supported on granular graphite (E.G. Carbone Lorraine) (11). The deposition of tin and iron was a chieved at controlled potentials, higher than the Nernst potentials of bulk deposition, in the so-called range of under-potential (Prolabo) and Fe,(S0,),.5H20 deposition (10). SnC1,.5H,O (Aldrich-Chemie) were used for this deposition. The real surface of platinum, before and after deposition of tin and iron, and also the coverages by these adatomes, were evaluated by cyclic voltammetry (11). All the experiments were carried out at 25"C, in an aqueous acid solution (O.5MH2S0,) and with pH,=latm. RESULTS AND DISCUSSION I
-
Effect of tin
Deposited tin was first chdracterized by cyclic voltammetry. Tin induces a decrease of weakly adsorbed hydrogen (peaks between OV and 0,25 V in fig.11, when strongly adsorbed hydrogen is slightly enhanced (shoulder at 0.25-0.40 V in fig.1). At potentials higher than 0.5 V/RHE begins oxydation and dissolution of the deposited tin, but it
2092 is very difficult to regenerate the platinum surface. The free A G O for energy of adsorption of maleic acid on platinum ( e,=O) decreases for 8, < 0.1, pointing out a lowering of the chemical affinity between this olefin and platinum. Such evolution can be explained by an electronic donor effect of tin. The catalyti c activity of platinum for the reaction of hydrogenation of maleic acid decreases also sharply at low tin < 0.051, giving a high "initial toxicity" at coverages (&, 8,, = 0) equal to about 30-40 atoms of platinum deactivated by one atom of surface tin (fig.2). This surprising high toxicity can be explained by a fast diffusion of tin on the surface of platinum or by a ligand effect.
0,O
0.1
0.4
0.6
0.8
Fig. 1 : Effect of tin deposition on the adsorption of hydrogen on platinum/graphite (in 0.5 M H2S04, 25"C, v = 5 mV/sec) - voltammogram of pure platinum/graphite voltamnogram of platinum/graphite modified by tin
---
Fig. 2
I1
-
Dependence of the intrinsic rate of maleir acid hydrogenation on the degree of coverage by tin ( C , = M, pH2= 1 atm, 25°C)
Effect of iron
Iron, introduced as Fe,(SO,),, can be deposited on platinum/graphite at a controlled potential equal to OV/RHE. This potential being higher than the Nernst potentials of Fe3+/Fe (Eat - 0.037 V ) and of Fe2+/Fe (E,= - 0.44 V), the iron deposition could occur only in underpotential conditions (10) Iron induces a reconstruction of the initial platinum surface, namely the shoulder associated to the strongly adsorbed hydrogen increases, while the two main peaks
2093
associated to more weakly adsorbed hydrogen species decrease (fig.3). The deposited iron species are stable up to 0.5 V ; then they are oxidized between 0.6 and 0.9 V / W E and desorb from the surface (fig.3). The reconstruction of platinum surface is irreversible and persists after the regeneration of platinum surface and also after application of potentials as high as 1.5 V/RHE. The activity of platinum/graphite, for the hydrogenation of maleic acid, is enhanced in presence of deposited iron and a maximum is obtained between 0 < 8,- < 0.30 (fig. 4 ) . The observed restructuration is not sufficient to explain this result. Two opposite effects of deposited iron should also occur : an inhibiting geometrical effect and another favorable effect which could be ascribed to a surface segregation of Pt, as it is found in the case of Pteo Fe,, (111) and Pdl Fess alloys (8,121, or to an electronic modification of platinum induced by iron, as it is proved by EXAFS and by the measure of edge shapes in the case of a Pt,,Fe-20/charcoal catalyst (9). A X ~~Qmol/mincm*
I
" . . 0.0 0.1
, 0.1
. 0.3
, 0,4
4
.
i.
0.5
0.6
R
081
Fig. 3 : Effect of iron deposition on the adsorption of hydrogen on platinum/graphite (in 0.5 M H2S04, 25"C, v=5 mV/sec) a) . . . voltammogram of pure Pt/graphite b) - - - voltammogram of platinum/graphite modified by iron c) - voltammogram obtained after the oxidation of iron Fig. 4 : Dependence of the intrinsic rate of maleic acid hydrogenation on the degree of coverage by iron (CM=10-3M, pH*= 1 atm,2S°C)
The main conclusions of this study relative to the modification of platinum/graphite catalysts by tin and iron are : - tin causes a strong deactivation of platinum at 8,, < 0 . 0 5 (for the C=C hydrogenation reactions). - iron induces a restructuration of Pt/graphite catalysts and an enhancement of their catalytic activity for the hydrogenation of C=C bonds.
2094
Tn spite of those different effects, iron and tin act in the same way for competitive hydrogenation of C=C and C=O bonds. Such modification could be in relationship with a similar modification, induced by iron and tin, of hydrogen adsorption. REFERENCES 2. POLTARZEWSKI, S. GALVAGNO, R. PIETROPAOLO and P. STAITI, J. Of Catalysis, 102 (1986) 190.
W.F. TULEY and R. ADAMS, J.A.C.S., 47 (1925) 3061. P.N. RYLANDER, H. HIMELTEIN and M. KILROY, Engel. Ind. Techn. Bull. 4 (1963) 40.
D. GOUPIL, P. FOUILLOUX and R. MAUREL React. Kinet. Catal. Lett., 3 5 (1-2), (1987) 185 D. RICHARD, J. OCKELFERD, A. GIROIR-FENDLER and P. GALLEZOT, Catalysis Letters, 3, (1989) 53.
D.V. SOKOLSKII, N.V. ANISIMOVA, A.K. ZHARMAGAMBETOVA, S.G. MUKHAMEDZHANOVA and L.N. EDYGENOVA React. Kinet. Catal. Lett., 33, (1987) 399 P. BECCAT, J.C. BERTOLINI, Y. GAUTHIER, J. MASSARDIER and P. RUIZ, J. of Catalysis, 126, (1990) 451 P. BECCAT, Y. GAUTHIER, R. BAUDOING-SAVOIR and J.C. BERTOLINI, Surface Science, 238, (19901 105 B. MORAWECK, P. BONDOT, D. GOUPIL, P. FOUILLOUX and A.J. RENOUPREZ, Journal de Physique, Colloque C 8 , supplkment au no 12, 47, (28-297 (1986) 10) D.M. KOLB, Advances in Electrochemistry and Electrochemical Engineering, ed H. Gerischer and C.W. Tobias, vol. 11, p. 125, Wiley, 1978 11) L. El OUAZZANI-BENHIMA, thGse, Universit6 de Poitiers 6 novembre 1991. 12) Y. DEBAUGE, P. DELICHERE, J. MASSARDIER and J.C. BERTOLINI, GECAT 1991 (15-18 avril 1991, Blainville sur Mer), resum6 p. 21.
OUai, L et al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
PROPERTIES OF A LATERITE IRON MINERAL:CHARACIXRIZATION, CATALYTIC BEHAVIOR AND PROMOTER EFFEm M. R. Goldwasser, M. L. Cubeiro, M. J. Perez Zurita and C. Franc0 Universidad Central de Venezuela, Facultad de Ciencias, Bcuela de Quimica, Apartado 47102, Los Chaguaramos, Caracas, Venezuela
Abstract A laterite iron mineral has been modified to obtain a K and t4n promoted laterite based catalyst for CO hydrogenation reaction. Catalytic tests were conducted at atmospheric and higher pressure at 280-300 'C in fixed bed reactor system. Characterization of the solids indicate a high influence of! prorotors on the Fe phases present on the catalysts. Hn inhibits formation of carbidic phases while the presence of k: accelerates this effect. The double proroted laterite producea an active and stable solid with high selectivity to the C2-C,, olefin fraction. I
~
I
O
W
There has been renewed interest in recent years in the Fischer-Tropsch (F.T.) synthesis specially for the selectivct production of petrochemical feedstocks such as C2-C olefins directly from synthesis gae( 1-5). The aost active cataiysts for CO hydrogenation are group VIII elements such as Co, Nil Ru and Fe. After reduction, with the exception of iron, those elements remain in zero valent state under a variety of procesii conditions ( 6 , 7 ) . However, during F.T. synthesis, conventional. reduced bulk iron catalysts evolve into different phases, including metal carbides and metal Oxide8 which are present at steady state catalytic conditions(8,9). Production of low molecular weight alkenes calls for new catalytic systems to bet developed. It is well known that addition of promoters tcb crupported metal catalysts has an effect on the light olefin selectivity(4,10,11). In the present work a laterite mineral. has been modified to obtain a K and Mn proroted laterite based1 catalyst to achieve high activity, thermal stability, long life and selectivity to the production of light olefins.
The original laterite was washed to eliminate soluble materials. It was use as such and promoted. The solids were, calcined at 450 'C. The Mn(5%) and K(4%) promoters were added in two steps by the method of incipient wetness using
2096
respectively. The catalysts w e r o charadterlzed by means. techniques such as atomic absorption, XR fluorescence analysis, XRD, BET surface area measurement, mercury intrusion poroslmetry, SEn analysis, EPR and Mossbauer Effect Spectroscopy (MES) at room terperature. n o C!O hydroqcnation reantion. was studied in two flow systerrn with fixed bed reactors at atmospheric.pressure (280 * C I 40 q catalyst) and 1.1 EtPa (290 300 C, 2 g catalyst). The composition of the reaction mixture and effluents wacl determined by on-line CONTHO9 and BINOS meters and by gar, chromatography.
M I I ( N C ~ ~ ) ~ . ~and K ~ GK 3 C 0
02
-
RESULTS AND DISCUSSION
The chemical composition and the physical properties of tha initial mineral show Fe, All Si and Ti as major constituents in the form of hematite, Bagnetite, ilme ite, quartz, gibbsite and kaolin, with a surface area of 140 a /g with a b oad pore sizu distrib tion and a pore volumen V = 0.12 cm /g and V 0.19 cmY /g. The composition of t%v 4K5Mn laterite cataY3st: determined by XFt fluorescense analysis was 30.02% Fe 0 34.45% A1 03, 14.17% 9102, 6.46% MnO, 5.32% KzO, 2.62% 0.191: P , 0.16% CaO. 'Azter promotion with K and Mn no important changes on the textural propertlee of the solids were observed. The most: relevant fact from the elemental radial dlstribution, as; determined by SEn, is the parallelism observed for the signal. of Fe and Mn not observed for K. This fact seems to Indicate that Fe and Mn are associated, probably as solid solutions of! Mn in a iron oxide structure. MES results seem to correlate this observation. Table 2 showa the spectral composition for calcined unpromoted laterite, llr~ and KMn promoted laterite, after exposure to synthesis gas for 72 h.
E
5
=t
&a;,
8
Table 1 Spectral composition of MES for the laterite based solids %
x Fe5C2
Later1te Mn/Laterite m-terite
36
---37
E
Fe2.2C
----
Fed+
-304
FeJ+
17
20
27
----
48
21
31
19
16
8
20
Reaction conditions : 1.1 m a , 300 * C , 5.0 nL/gcat./h In the presence of Mn no carbidlc suppression of carbide fornation by results observed by SEH and tends interaction. The MES spectra for the
phases were observed. The
Mn is consistent with the to indicate a high F W t IWn promoted catalyst
2097 LndiCatea a rapid oarburatluh to both X and E ' oarbidem g i v i n q r h o to at leamt licit of thome phase.. Table 2 6howrr th. catalytic results for M e Knn promoted latorite at atmmphoric and higher promsuro.
Table 2 Hydrocarbon Distribution H2:CO:Na
6:3:1
p (ma)
0.1
T ('C)
280
~(nl/gcat/n)
0.28
tima on atroar
16 day6
xco ( 8 )
4.75 : 4.75 : 0 . 5
3:3:4
290
0.14
0.07
22 day6
42
**
1.1
27 day8
38
300
1.70
1.75
1.32
92 h
140 h
187 h
29
40
52
1.51
1.47
60
A (1OO*molcoOonv.
/g cat / h) rat I cH4
'2"4
Cz'c4
0.16
0.07
0.06
1.07
17.1
15.3
14.3
12.2
13.3
16.3:
54.1
59.1
58.2
63.9
63.4
61.9
OhfiIU(8) 82.6
86.9
76.5
92.4
92.1
90.4
Activation procedure : * H at 350 'C, 16 h during tho test ** at 450 'C fol1ow.d by CO at 150-300 'C before the test
1,
In all the catalytic tea- performed, deactivation of thu latorito ba6.4 molib. warn n w o r okorved, on the contrary, an increase in activity w i t h tima on stroar wa6 alway6 attained. Thim in-oitu activation could k rolatod to tho proaonce of' unreduced Fe apeci.6. Mgs of the calcined Km laterite showe a broad hyperfin. field dimtribution, a6mign to iron oxide specie. with different onvirorentm, which cau6os different: rducibility of tho original mpaciem. W o n when tho sarplerr were mubjected to more drastic treatments much am H2 reduction (450 'c, 48 h) followvd by co activation (159-300 'c, 7 1 h) only 76 8 of xFe5C2 w a m okerved. For tho unproroted latorite the initial catalytic activity u a m very low with high CHI formation. The single prorotion by either Hn or If increases that olefin/paraf fin ratio, whilo tho doublo promoted laterito gavel very high 8olectivitiom for light olefins. This soleativity of
2098
the iiXn promoted catalyst was higher than that observed for :i catalyst indicating that Mn promotion of the K promoted laterite further enhances selectivities. The role of Mn as Fe promoter is still under discussion( 5 ,6 ,10 ,11). It remains an open question whether an alect.ton.i.c tntersctf.oa occurs,. as for the case of alkalins metals, or whether a specific iron structure is stabilized. Our SEM, MES and catalytic results indicate that it acts as both: It inhibits formation of large a-Fe particles which could bar carbided, favoring Fe3O4 phases and enhances light olef Ins selectivities, probably by increasing the basicity of the solid or demetallizing iron particles by electron withdrawall. effect(4). For the catalytic tests, better results (less CH4 and higher C -C4 oleflns) were obtained when the solids were pretreated wfth H2 followed by CO activation, than when they were used an such and activation occurs in-situ by synthesis gas exposure. One of the outstanding features of these laterite iron mineral based solids is its high production of C2-C olefinn (up to 922) and low CHI formation at high conversions 152%), irr comparison to commercial fu8ed iron catalysts. In conclusion, an active catalyst that shows very stable behavlour with high olefln production has been developed.
The Mossbauer effect spectroscopy study was conducted by Dr. Fernando Gonzdlez and Lic. Edgar Jaime8 at the Magnetisar Laboratory. The atmospheric pressure tests were conducted at Engler Bunte Institut of Karlsruhe University. REFERBNCBS 1. M.L. Cubeiro, M.R. Goldwasser, V. Baez, F. Navas, M.J. M r e z Zurita, Rev. Soc. Vzlana. Catal., 3 (1) (1989) 38, 81. 2. K.B. Arcuri, L.H. Schwartz, R.D. Plotrowski and J.B. Butt, J. Catal., 85 (1984) 349. 3 . M.E. Dry, The Fischer-Tropsch Synthesis in : Catalysis, Science and Technology, eds., J.R. Anderson and M. Boudart (N.Y.1981) vol 1, p. 159. 4 . R. Snel, Catal. Rev. Sci. Eng., 29 ( 4 ) (1987) 361. 5. G.C. Maiti, R Malessa, U. Lochner, H. Papp and M. Baerns, Appl. Catal., 16 (1985) 215. 6. J.B. Butt, Catal. Lett., 7 (1990) 83. 7. D.B. B u k w , X. Lang, J. A. Rossin, W.H. Zimmenuan, M.P. Rosynek, E.B. Yeh and Ch. Ll, Ind. Eng. Chem. Res. 20
.
(1989) 1130. 8. S. Soled, E.Iglesia and R.A. Fiato, Catal. Lett., (1990) 271. 9. L. Guczi and K. Lazar, Catal. Lett., 7 (1990) 53. 10. J.J. Venter and M.A. Vannice, Catal. Lett., 7 (1990) 219. 11. J. Barrault and C. Renard, Appl. Catal. 14 (1985) 133.
c1
Guczi, L et al. (editors),New Frontiers in Catalysis Proctcdings of the 10th Internationst Congress on Catalysis,19-24July, 1992, Budapest, Hungary 0 199J Elscvicr Science Publishers B.V. All rights mewed
PLATINUM CATALYSTS SUPPORTED ON HIGH SURFACE AREA MOLYBDENUM OR TUNGSTEN TRIOXlDES FOR HYDROGENATION REACTIONS C. Hoang-Vana, 0.Zegaouib and Y.Arnaud au CNRS "Photocatalyse,Catalyse et Environnement",Ecole Centrale de Lyon, B.P. 163,69131 Ecully Cedex, France bFaculte des Sciences, Universite Mohamed ler, Oujdar, Marocco WRA au CNRS "ChimieAppliquee et Genie Chimique", Universite Claude Bernard, Lyon I, 6%22 Villeurbanne Cedex, France
Abstract
Finely divided MoO3 and WO3 supported Pt catalysts prereduced at temperatures up to 773 K were tested in the hydrogenations of allylic alcohol, acrolein and carbon monoxide. For reduction temperatures < ca. 473 K, the catalytic properties of these hydrogen bronze-based catalysts were not significantly different from those of conventional catalysts (e.g. Pt/Al2O3 or Pt/Si%). In contrast, an enhancement of catalytic activities and certain selectivities were observed when the pretreatment temperature increased above 473 K, which was tentitatively correlated with the occurrence of a strong metal support interaction (SMSI)state between Pt and hydmgen bronzes.
INTRODUCTION
Hydrogen bronzes of Moo3 or WO3 can be formed in the presence of molecular hydrogen, when small metal particles of Pt or Pd are deposited on the surface of these oxides. The Pt or Pd metal particles are expected to interact with the modifled oxides, since a fraction of the occluded hydrogen electron is transferred into a conduction band as in an alkali-metal bronze (1). Furthermore, partial reduction of Moo3 or W03 by the metal particles via hydrogen spillover can lead to new SMSI catalysts.
This study was undertaken to determine possible effects of finely divided Moo3 or WO3 hydrogen bronzes on the catalytic properties of deposited platinum.
21 00
EXPERIMENTAL
The catalysts, prepared by H2PtCl6 impregnation of high surface area Moo3 or WO3 (40and 45 m2g-1 respectively), obtained in a flame reactor (2). were characterized by XRD. TEM. volumetric measurements of H2 consumption and CO chemisorption in a pulse flow system. The Pt content was 0.2 wt% for Pt/MoO3 and 0.1 w t o ! for Pt/W03. For comparison purposes, 0.4wtohPt/AlO3 and 1 wt% Pt/Si02 catalysts were used. These catalysts are designated throughout this text as Pt-Mo. R-W, Pt-Al and. Pt-Si, respectively. Kinetic measurements were performed in the dynamic regime, under atmospheric pressure for the hydrogenations of allylic alcohol and aerolein and under lMPa (10 bars) for that of carbon monoxide. RESULT8 AND DISCUSSION
Comporition and rtructure of hydrogen bronze6 The compositions and structures of hydrogen bronzes deduced from volumetric measurements of H2 consumption and XRD are summarized in the Table 1. The volumetric compositions were not confirmed by XRD analyses because : i) H1.6 Moo3 is transformed into H0.g Moo3 and HyWO3 into WO3 (2).by exposure to air during the transfer to the X-ray diffractometer and ii) HxMo03 prepared at temperatures above ca. 433 K are amorphous. Table 1. Hydrogen contents and structures of Moo3 and WO3 hydrogen bronzes Catalyst
Reduction temperature
H content*
Structure
(K) Pt-Mo
298 433 473 573 673 773
1.44 H0.9MoO3 1.67 H0.9M003 + Hl.6MOO3
1.86 2.10 2.00
---
amorphous amorphous amorphous Mo+unidentifled peaks
Pt-W
* x or y values (in bronzes HxMo03 or HyWO3) determined volumetrically at equilibrium compositions. Initial H2 pressure : 101.3 kPa
21 01
The surface areas were ca. 30 m2 g-1 for H,MoO3 and 40 m2 g-1 for JIyWO3. Chemisorption of CO at 298 K on bronzes Pt/H,Mo03 or Pt/HyWO3 was markedly smaller than expected from platinum particle diameters observed on 'IEM micrographs (1.5 - 3 nm).In addition, the presence of metallic Mo following the reduction of Pt-Moat 773 K and those of W h . 8 and W30 obtained by reduction of Pt-W at 673 K and 773 K respectively (see Table 1) strongly support the occurence of a SMSI state between Pt and hydrogen bronzes. ACTMTIES AND SELECTIVITIES
The activities and selectivities of Pt/HxMo03 and Pt/HyWOs, prepared
at different temperatures, were compared with those obtained for conventional Pt-Al and R-Si catalysts prereduced at 773 K and Pt black prereduced at 433 K. For the hydrogenations of aUylfc alcohol or acrokin a t 323 K, the activities of hydrogen bronze-based catalysts increased with the pretreatment temperature from 323 to 673 K and then sharphy decreased for 773 K particularly in the hydrogenation of acrolein ('hble 2, columns 3,4.5 and 6).When pretreated between 433 and 673 K, these catalysts were more active per mass unit of Pt than the conventional catalysts, especially in the hydrogenation of acrolein (Table 2) Table 2. Hydrogenations of allylic alcohol and acrolein at 323 K Catalyst Reductiona Allylic alcohol Acrolein
Pt-W
433 573 673 773
2150 699 1750 801 2150 2463 4056 1319 2250 100 822 2527
569 699 732 33
323 433 573 673 773
1363 1909
483 677
2273 700
806 248
99 194 226 226 48
- _ - ---
280 546 637 637 136
Pt-Al 773 1545 509 335 109 Pt-Si 773 2535 1978 473 369 Pt black 433 8 26 3 9 a For 3 h. b Units are millimoles allylic alcohol or acrolein per gram Pt per hour. C Turnover number, based on CO adsorption. Units are molecules per hour per surface Pt atom. I'
21 02
For increasing pretreatment temperature up to ca. 773K. the selectivities of Pt-Mo and Pt-W catalysts towards propanal increased at the expense of propanol. in the reaction of allylic alcohol. Also, in the hydrogenation of acrolein. the presence of allylic alcohol and propanol was observed in the products for prereduciton temperatures in the range 673-773K. It is noteworthy that conventional Pt catalysts and PtMo or Pt-W prereduced at temperatures c ca. 573 K hydrogenated acrolein into propanal almost exclusively. In the hydrogenation of CO at 523 K, the activities of Pt-Mo and Pt-W catalysts and their selectivity towards C2+ hydrocarbons and the ratio of the selectivities S C 2 + / S C H 4 increased with the temperature of pretreatment in the range 573-773K. These activities and selectivities are generally much higher than those observed for Pt-Al and Pt-Si catalysts. The kinetic results obtained in this work tend to show that catalytic properties of Pt are not signicantly modified by the hydrogen bronze support when Pt-Mo and Pt-W are prereduced at temperatures c ca. 473 K. In contrast, a large increase of their catalytic activity is observed when the pretreatment temperature increases above 473 K. Under these conditions, it is likely that complexes of the type Pt" - M4, (with M : Mo or W : x c 3) are formed and the value of x decreases wlth increasing pretreatment temperature (metallic Mo. W02.8. W30 have been detected, see Table 1). These complexes may constitute new activite sites at the interface Pt-support and they are probably at the origin of the high activity observed, because of the electron-attracting character of reduced Mo or W ions in these sites which would promote the activation of the C = 0 bond (3).Also, the occurrence of a SMSI state in these catalysts is likely responsible for the enhancement of the selectivities towards propanal (from allylic alcohol), allylic alcohol (hydrogenation of acrolein) and C2+ hydrocarbons (hydrogenation of CO). In particular, an increase in the ratio SC2+/SCH4 with the reduction temperature of hydrogen bronzebased catalysts as observed in this work is indicative of an increasing metal-support interaction (4). REFERENCES
1. D. Tinet. P. Canesson, H. Estrade and J.J. Fripiat, J. Phys. Chem. Solids, 41 (1979)583. 2. C. Hoang-Van. 0. Zegaoui, B. Pommier and P. Pichat. Studies in Surf. Sci. and Catal. No 63. "Preparation of Catalyst V", G. Poncelet. P.A. Jacobs, P. Grange and B. Delmon (eds.). Elsevier, p. 679, 1991. 3.S.Galvagno and 2. Poltarzewski. J. Molecular Catal., 35 (1986)365. 4. C.H. Bartholomew, R.B. Pannel. J.L. Butler and D.G. Mustard, I a EC Product Res. a Devel., ACS. 20 (1981)296.
O h , L d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1% Q 1993 Elsevier Science Publishers B.V. All rights rcserved
Budapest, Hungary
Pt-C INTERACTION IN CATALYST SUPPORTED ON A CARBON BLACK SUBJECTED TO DIFFERENT HEAT TREATMENTS
F. Coloma, C.Praab-Burgueteand F.Rodriguez-Reinoso Departamento de Quimica Inorganica e Ingenieria Quimica, Universidad de Alicante, Alicante, Spain
Abstract Increasing heat treatment in He (up to 2200°C) of a carbon black results in supports which after impregnation with H,PtClqproduce catalysts with very high Pt dispersion (up to 0.99). The evolution in Pt dispersion and sintering resistence (in an H, atmosphere up to 500°C and for up to 84h) has been explained in terms of the evolution of crystal size and sulfur loss upon pregraphitization and their effect on the Pt/C interaction.
1. INTRODUCTION The selection of a carbon for the preparation of carbon-supported catalysts must be based not only on the surface area and pore size distribution but also on the chemical nature of the surface. Thus, it has been recently shown that inorganic impurities and the nature and amount of oxygen surface groups in carbon play an important role on the dispersion and catalytic activity of the active phase [l-31. In the particular case of platinum supported on carbon we have reported the effect of oxygen surface groups introduced in a furnace carbon black in the large dispersion and sintering resistance of Pt/C catalysts [3,4]. In such report a catalyst supported on a carbon black heat treated in helium at 2000°C exhibited a very large dispersion (0.92) and it was then decided to analyse this Pt/C system in more detail. This paper presents the results found when comparing the Pt catalysts supported on the same carbon black subjected to a range of heat treatments.
2. EXPERIMENTAL A furnace carbon black, CC-40-220 from Columbia Chemicals Co., was heat treated in helium for 30 min. at 1600°C and for 60 min. at 1800, 2000 and 2200°C in order to modify both the structure and the porosity. The supports were characterized by physical adsorption of N2 (77K) and CO, (273K), X-ray diffraction and temperature programmed desorption (TPD).
21 04
The carbon supports were loaded with l w t % Pt by impregnation with an aqueous solution of HZPtCl6.6H20as described in [3]. The exact Pt content was determined by burning away the carbon in air at 973K and analysing by UV spectrophotometry the residue dissolved in aqua regia. Platinum dispersion was determined by chemisorption of H, and CO at 298K in a volumetric system, assuming that each P t surface atom chemisorbs one atom of hydrogen or a molecule of CO. The catalysts were previously reduced in hydrogen and outgassed following the procedure described in [3]. The chemisorption isotherms were determined in the conventional way: by extrapolation to zero pressure the linear hydrogen isotherm and by the dual technique in the case of CO [4]. A further check of dispersion was carried out by transmission electron microscopy (TEM) with a Zeiss EM10. In order to compare the interaction between the metal and the support all catalysts were subjected to a common sintering procedure under H, using different steps of increasing temperature (35O0-5OO0C) and soaking time (1236h).
3. RESULTS AND DISCUSSION Increasing temperature of the heat treatment of the carbon black results in important modifications of the parameters defining the structure and the porosity. Thus, the results of Table 1 show that there is a continuous increase in the values of L, and L, dimensions of the cystallites and the interlayer spacing (d), indicating the increasing pregraphitization produced by the heat treatment. This evolution is paralled by a decrease in surface area mainly caused by the closure of small micropores as detected by comparing the adsorption of N2 and COz [ 5 ] . Table 1 Some data for carbon supports. Carbon C C1600 c 1800 c2000 c2200
S,,,(m2/g) 956 43 1 300 212 175
Smic(mZ/g)
La(nm)
630 269 155 80 63
7.63 7.84 8.62 8.90
--
Lhm) -2.23 2.71 3.21 3.61
d(nm) -0.353 0.348 0.349 0.347
In order to check the possibility of introduction of oxygen surface groups by simple oxidation of the carbons with air all supports were analyzed by T P D but no desorption of CO or CO, was detected, this meaning that the amount of surface groups had to be very small. The Pt dispersion is very high in all catalysts, with a maximum value (0.99) for the catalyst supported on the carbon treated at 1800°C; these dispersions are
21 05
among the highest reported up to now in the literature for the Pt/C system. It is important to mention that the dispersion for the catalyst suported on the original carbon black (0.30) or in the carbon black reduced in H 2 at 1000°C (0.21) are much lower. Although a previous report [3] showed that oxidation of the same carbon black with H202considerably increased the Pt dispersion, the values were lower than those reported here. Table 2 ChemisorDtion results of Pt catalvsts. ~~
Heat treatm.('C)
H/Pt
d(nm)
1600 1800 2000 2200
0.67 0.99 0.92 0.83
1.6 1.1 1.2 1.3
CO/Pt
d(nm)
H/CO
0.58
1.9 1.4 1.7 1.9
1.16 1.29 1.48 1.46
0.77 0.62 0.57
The high dispersion is not caused by spillover as checked during the chemisorption measurements; on the other hand, TEM analysis on catalysts supported on carbons heat treated at 1800 and 2000°C showed Pt particles very homogeneously dispersed with an average particle size of 1.2 and 1.5 nm, respectively, in good agreement with the chemisorption values (1.1 and 1.2 nm). The chemisorption of CO results in lower values of dispersion (Table 2), as described for other Pt/C systems for which H/Pt was larger than CO/Pt for dispersions above 0.3 [4]. However, there is again a maximum value of dispersion for the catalyst supported on the carbon heat treated at 1800°C. The information provided by the adsorption of H, and CO can be used to explain the evolution of dispersion for these catalysts. There are two factors controlling the adsorption of H,PtCl, and the corresponding dispersion: the surface area and the chemical nature of the support. There is a direct relationship between the dispersion and the non-microporous surface area (taken as the difference between the BET and the CO, surface area, since the latter measures the narrow micropores only [4]) but small changes in this surface area produce too large changes in dispersion. This means that the chemical nature of the support must be important and since the role of oxygen surface groups is almost neglegible in this case, one has to consider the possible effect of sulfur. The sulfur in the carbon support partially inhibits the total reduction of the Pt with an effect in CO chemisorption [4]; the heat treatment of the original carbon reduces the sulfur content from 1.OSwtfR;to less than 0.01wt96 in the carbon treated at 2000°C and as a consequence the H/CO ratio for the chemisorption on the four catalysts varies from less than one (0.46) in the original carbon black to 1-16 in the carbon treated at. 1600°C and larger values for higher temperatures with a slight maximum at 2000°C. The loss of sulfur results in a more effective pre-graphitization, with a maximum increase in L, at 1800-2000"C, probably coincident with the maximum loss of sulfur. This will mean an increase in the number of a electrons in the carbon crystallites able to form a-complexes with Pt and a subsequent increase in Pt dispersion [a].
21 06
On the other hand, the dispersion is proportional to the percentage of surface area outside the narrow micropores and this means that the best support is one loosing as many as possible heteroatoms (oxygen and sulfur in this case) without much loss of the proportion of non-microporous surface area. This is the case for the carbon heat treated at 18OO'"C, the one providing larger Pt dispersion. The sintering behaviour of these catalysts shown in Figure 1 is a further confirmation of the explanation given above. The sintering decreases with increasing temperature of the support, that of 2000 and 2200°C being very similar after 84h; consequently, incresing pre-graphitization results in a larger Pt/C interaction and a more effective anchoring of Pt particles. In fact, Figure 1 also shows the behaviour for a catalyst supported on carbon heat treated at 1800°C but later oxidised in H202to introduce oxygen surface groups; the sintering resistence is lower (although still much larger than in the original untreated carbon black, due to the anchoring sites introduced by oxidation [31).
I-
0.8
-
as -
6
Figure 1. Effect of the sintering treatment on the dispersion of the catalysts.
Acknowledgements: This work was supported by DGICYT (Project PB86/279). 4. REFERENCES 1 D. Richard and P. Gallezot, Preparation of Catalyst IV, Elsevier (1987) 71. 2 F. Rodriguez-Reinoso et al., Appl. Cat., 15 (1985) 293. 3 C. Prado-Burguete et al. J. Catal., 115 (1989) 293. 4 C. Prado-Burguete el al. J . Catal., 128 (1991) 397. 5 F. Rodriguez-Reinoso, Pure & Appl. Chem., 61 (1989) 1859. 6 H. E. van Damm and H. van Bekkum. J. Catal., 99 (1991) 335.
GUni, L et al. (Editors),New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishen B.V.All rights rcserved
MECHANISM OF THE EFFECT OF ADDlTIVES ON CATALYTIC PROPERTIES OF PALLADIUM L. N. Edygenova, N. V. Anisimova, A. V. Korolev and D. M.Doroshkevich Institute of Organic Catalysis, Academy of Science of Republic Kazakhstan, 142 K. Marx Str., 480100 Alma-Ata, Kazakhstan
The reduction of either olefinic bond or carbonyl group, conjugated with the latter, in a molecule of a,@-unsaturated crotonic aldehyde, relates to the problem of intramolecular selective hydrogenation.Introduction of oxide additives into the catalytic system is one of the methods of controlling the activity and selectivity of catalysts. A s was shown by us earlier [l], the addition of iron oxide as a mechanical mixture to Ptblack alters drastically the direction of hydrogen addition within a molecule of crotonic aldehyde (CA) providing high selectivity of its C=O bond hydrogenation (80% yield of crotyl alcohol). On the contrary, use of supported palladium catalysts is effective for selective hydrogenation of olefinic bond of CA which is of great applied and scientific interest. We have found that promotion of Pd-containing catalysts by basic oxide additives in the process carried out in liquid phase, causes an increase in its activity by 2-3 fold involving a considerable growth of stability during hydrogenation of CA portions successively introduced. The tracer method, microcalorimetry, and quantum-chemical calculation by CNDO/BW method were used for studying the mechanism of the effect of oxide additives. The tracer study in the process of CA hydrogenation was carry out in the presence of Pd-black, and Pd + ZnO, and Pd/ZnO + ZnO mechanical mixture. A deuterium label was introduced into a solvent (H2 in D20). By special experiments on isotope exchange of CA molecules on zinc oxide it was found that -30% of atoms of aldehyde group hydrogen was substituted by deuterium atoms during CA adsorption on deuterooxylated oxide surface (in D20). This
21 08 rather important fact points to adsorption of CA molecule on surface of zinc oxide by an aldehyde group. It is known [2] that surface hydroxyl groups determine the reversible adsorption of organic molecules forming hydrogen bonds with their polar groups.In our case,a CA molecule,by adsorption on oxide, forms a hydrogen bound surface compound.Under these conditions an aldehyde proton obtains a possibility t o exchange with deuterium atoms from surface deuteroxyl groups. An increase in deuteroexchange of an aldehyde proton of butyric aldehyde occurs on addition of ZnO as a mechanical mixture to Pd black or Pd/ZnO catalyst (Table 1). This confirms the assumption of additional adsorption of CA on an diluant oxide. Table 1 Distribution of deuterium in butyric aldehyde forming during CA hydrogenation by hydrogen in heavy water (H2 in D20) at 298K. Content of deuterium in crrows Catalysts of butyric aldehyde atoms, %CH3 CH2 CH2 CHO Pd-black 0 0 8 50 0 25 30 65 Pd + ZnO 0 15 15 50 Pd/ ZnO 0 20 20 58 Pd/ZnO + ZnO
-
-
-
The appearance of isotope atoms in reaction products is indicative of presence of the process of atomic deuterium migration from deuterooxylated surface of oxide t o metal particles (a reverse spillover) The involvement of spillover deuterium in hydrogenation leads to formation of isotope-substituted isomers of butyric aldehyde, -CHD-CH2-. Thus the results of the tracer study prove the existence deuterium spillover when the process is carried out in H20. On the basis of the tracer study, the direct evidences of involvement of an aldehyde group of molecule in the interaction with the catalyst surface have been obtained: during adsorption on zinc oxide a CA molecule is coordinated with the hydroxylated catalyst surface by an aldehyde group to form hydrogen-bond complexes providing H-D exchange with surface hydroxogroups.
.
21 09 The shape of isotherm curve of CA adsorption on ZnO from H20 suggests reconstruction of the'adsorption layer- a bend in the one monolayer coverage region at planar adsorption of molecules:
50
30 10
20
60
I00
cint r.,pwte .9-'
Figure 1. Isotherm of CA adsorption from water at 303K on ZnO.
The amount of CA absorbed is 48 m o l e g-l which makes -3 monolayer of the value theoretically calculated from 3 , l nm2 molecular plateau at planar adsorption of the molecules or the one monolayer at aldehyde group adsorption. It can be assumed that in the first moment the molecule is adsorbed in a planar fashion and then ( a bend on isotherm curve) is coordinated vertically by an aldehyde group to the surface (Ilpaling"). The initial differential heat of CA adsorption is 85 kJ/mole which is sufficient for formation of hydrogen-bond compound with the hydroxylated surface of zinc oxide.The result of the calorimetric study supports the conclusion of the tracer study about the character of adsorption of CA molecules on the surface of ZnO. Quantum-chemical calculation of the electronic structure of CA molecule hydrogen-bond by a carbonyl group with hydroxyl or water as well as of a molecule coordinated to the basic or acid centres, has shown that rr-acceptor ability of the whole n-system of CA molecule increases on coordination of the latter to the basic centre (C4H60:H-) or with hydroxyl (C4H60:OH) (Table 2).
2110
Table 2 Population of n-orbitals on CA molecule atoms in different systems calculated by CNDO/BW method C ~ H ~ OC ~ H :~H+O C ~ H :~H-O C ~ H :~H O~ O C ~ H :~OHO System 0.688 0.674 0.716 0.870 0.840 0.812 0.829 0.792 0.851 c3 0.847 0.851 C4 0.950 1.052 0.855 0.947 0.870 1.847 1.826 1.875 O5 1.877 1.812 ............................................................. 41 23 5 Numbering of CA molecule atoms : C C =C C =O
ClC2
0.718
0.950
0.869
0.860
.
,-
I-
The C4H60: OH-system is characterized by the highest increase in n-acceptor ability of C1 atom (C1=C2,Table 2), i.e. its activation for nucleophile attack or for interaction with a n-donor site. Coordination of a CH=O group of CA molecule with hydroxyl leads to loosening and growth of n-acceptor ability of C=C bond conjugated with it. This facilitates considerably activation of the latter by a metallic activator. Calculation of bond energies and the charging state of CA molecule atoms in the basic and exited states has shown the anionic state of CA to be the most reactive which accounts for the promotion effect of oxides donor with respect to Pd. Combination to the obtained data gives grounds to assume that the oxide surface together with boundary metal atoms constitute a single adsorption site participating in the activation of CA molecule. The forming bifunctional complex can be presented: Scheme 1 The scheme of possible adsorption of CA molecule, proposed by us, includes the surface of oxide support and a metal centre as a single adsorption assembly. This agrees to a certain extent with "the model of special active sites available at the interface of metal and support1I,proposed by Burch and Flambard [3]. 1 D.Sokolskii, ets. React. Catal.Lett., 33 (1987) 399. 2 T.Morimoto, Y.Suda, Langmuir, 1 (1985) 239. 3 R.Burch, A.R.Plambard, J.Cat., 78 (1982) 389
Guczi, L d d.(Editors), New Frontiers in Catalyb Proceedings of the 10th international Congrcss on Catalysis, 19-24 July, 1992, Budapest, Hungary
0 1993 Elscvier Science Publishers B.V. All rights reserved
Pt-Ce/A120~CATALYSTS: INFLUENCE OF THE THERMAL TREATMENTS AND
THE REDOX CYCLES J. A. M.Correa, S. R. de Miguel, G. T. Baronetti A. A. Castro and 0.A. S c e h INCAPE, Institute de Investigacionesen Catalisis y Petroquimica, HQ,UNGCONICET, Santiago del Ester0 2654,3000 Santa Fe,Argentina
Abstract When Pt-Ge/Al203 catalysts are reduced at 300"C, Ge(I1,IV) species would produce only a geometric modification of Pt, but after reduction at 500"C, a great proportion of metallic Ge species could interact with Pt to produce alloy particles with very low dehydrogenation and hydrogenolysis activities. The successive oxidation-reduction cycles at high temperature decrease the dehydrogenation and hydrogenolysis activities and the H2 chemisorption capacity due to a germanium segregation from the Pt-Ge alloy with a surface enrichment of Ge.
1. INTRODUCTION Naphtha reforming bimetallic catalysts are composed of P t and a second metal. The effect of Re, Ir or Sn addition to Pt has been extensively studied, but few papers on the effect of Ge addition have been published [1,2]. In this paper we study the influence of both thermal treatments (calcination and reduction) and oxidation-reduction cycles on the characteristics of the metallic phase of Pt(0.3wt%)-Ge/AlzO3 catalysts having different Ge contents.
2. EXPERIMENTAL Pt(O.3%)-Ge(O-O.3%)catalysts were prepared by successive impregnation of a commercial y-alumina (SBET =180 m2/gr). Alumina was first impregnated with a solution containing HzPtCls and HCl, dried at 12OOC and then impregnated with a hydrochloric solution of GeC14 . After impregnations, samples were dried at 120"C, calcined in an air stream at different temperatures and reduced in Hz at 300 or 500°C. Oxidationreduction cycles consisted of successive treatments of calcination in air at 500°C for 4 hours and reduction in H2 at 600°C during 17 hours. After cycles, a portion of each catalyst was submitted to an oxychlorinntion treatment with a HC1-HzO-air mixture (H20/HC1 molar ratio = 60) at 500°C in order to adjust the chlorine content. Temperature Programmed Reduction (TPR) experiments were carried out by us-
2112 ing a 5% Hz-Nz mixture and a heating rate of 5°C min-' from 0 to 850°C. Hydrogen chemisorption was volumetrically determined at room temperature by using the double isotherm method (total and reversible isotherms). X-Ray Photoelectron Spectroscopy (XPS) spectra were taken in an spectrometer equipped with a MgK X-Ray source. The catalytic activity for cyclohexane dehydrogenation (CHD) and cyclopentane hydrogenolysis (CPH) WM measured at atmospheric pressure in a differential flow reactor. CHD was carried out at 300"C, by using a Hz/CH molar rat,io of 26. Reaction conditions for CPH were: temperature = 300 or 500"C, and a Hz/CP molar ratio of 22.5.
3. RESULTS AND DISCUSSION Pt-Ge/AlzOa catalysts submitted to different calcination temperatures (300, 400 and 500°C) showed similar TPR profiles and initial activities in CH dehydrogenation, which indicates that there is no influence of the calcination temperature on the characteristics of the metallic phase. On the other hand, there were important differences in the behavior of the metallic phase when the reduction temperature was modified between 300 and 500°C . In fact, the hydrogen consumption values (Table 1) obtained in the TPR-isothermal experiments at 300 and 500 "C (the temperature was increased at 5°C min-' up to 300 or 500"C, and then kept constant until the Hz consumption was null) showed a catalytic effect of Pt in the Ge reduction, which could be attributed to a strong interaction between both metallic components. Besides, the reduction degree of Ge in these bimetallic catalysts increased with both the reduction temperature and the Ge content (Table 1). Additionally, germanium addition to platinum produced a more pronounced effect on the CHD and CPH activities as the reduction temperature increased (Figure 1). In both cases, Ge addition to Pt decreased both the dehydrogenation and the hydrogenolysis activities, although this effect was higher for the C-C break reaction. Table 1. Reduction degrees of Ge in TPR isothermal experiments at different temperatures (Tr) of the isothermal step % Ge in different oxidation states after reduction (*) CATALYST Tr, "C GeO Ge" G e" 300 100 0 0 Ge(0.3wt%)/Alz 0 3 0 500 76 24 300 73 27 0 Pt-Ge(0.15wt%)/A120 3 94 6 500 0 70 0 Pt-Ge(0.29wt%)/A1~03 300 30 58 500 0 42 ~~~
~
~~
~
~
(*) The percentage of Ge in different oxidation states was calculated assuming that all Pt" was reduced to Pto and all GelV was first reduced to Ge" , and then to Ge'.
2113
CP HY DROGENOLYSIS
CH DEHYDROGENATION
, REDUCED AT W O ° C REDUCED AT 300%
REDUCED AT 500%
0.0
0.1
0.2
0.3
01 CONTENT ( W % )
0.0
0.1
0.2
0.3
00 CONTENT (wto/a)
Figure 1. Initial CH dehydrogenation and CP hydrogenolysis rates for PtGe/Al203 with different Ge contents and reduced at 300 and 500°C.
1
Figure 2. R = Initial rate or H2 uptake for fresh, after cycles or after oxychlorination samples relative to those corresponding to fresh samples. Pt(0.3 wt %)Ge(0.15 wt %)/A1203
1
Figure 3. TPR profiles of Pt(0.3 wt %) Ge(0.15 wt %)/A1203 after different treatments.
2114
Besides, when these bimetallic catalysts were reduced at 500"C, the turnover numbers for CHD (a structure-insensitive reaction) dropped as the Ge content increased. Pt/A1203 catalyst reduced either at 300 or 500°C showed similar activities in both reactions (Figure 1). These results could be explained considering that after reduction at 300"C, Ge(I1) or (IV) species are present in bimetallic catalysts, and these species would produce only a geometric modification of Pt. But, after reduction at 500"C, a portion of Ge, mostly in the zerovalent state, could interact with P t to produce alloy particles with very low dehydrogenation and hydrogenolysis activities. The concentration of the alloy particles would increase as the the Ge content in the catalyst increases. In fact, XPS spectra showed that the binding energies of the Pt4da12 and Ge2p312lines, after reduction at 500"C, were shifted to values which were very close to those of PtGe, intermetallic compounds. Nevertheless, in these bimetallic catalysts reduced at high temperature, other effects, such as dilution and probable formation of Ge aggregates on the particles of free platinum, must be considered. Figure 2 shows that Pt-Ge/Al203 catalysts submitted to severe oxidation-reduction cycles at high temperatures showed lower dehydrogenation and hydrogenolysis activities and chemisorption capacities than the fresh ones. The effect of redox cycles on the hydrogen chemisorption is lower than on the CHD, which means that dehydrogenation is more demanding than the H2 chemisorption. Besides, it was observed that these dehydrogenation and hydrogenolysis activities and the chemisorption capacity were restored after the oxychlorination treatment with a HC1-H2O-air mixture at 500°C. The effect produced by redox cycles was more pronounced in catalysts with high Ge contents. Figure 3 shows TPR profiles of fresh Pt-Ge/Al203 and those after redox cycles and oxychlorination. The fresh catalyst showed a peak at about 250"C, corresponding to the platinum reduction, another peak at higher temperatures (about 620"C), corresponding to the Ge reduction, and an intermediate reduction zone between 400 and 600°C. It must be noted that the reduction peak of Ge in the Ge/A1203 catalyst appears at 720°C [3]. When bimetallic Pt-Ge/AlzOs catalysts were submitted to the redox cycles, the Ge reduction peak was shifted to a higher temperature (approximately at 695"C).On the other hand, the TPR profile after oxychlorination treatment was similar to that corresponding to the fresh sample (Figure 3). The oxidation-reduction cycles at high temperatures would produce a segregation of germanium particles from the alloy, as it is shown by the TPR experiments, and these segregated Ge species would cover free Pt particles. This effect would be responsible for the drop both in dehydrogenation and hydrogenolysis activities and in the H2 chemisorption capacity. On the other hand, the oxychlorination treatments of these catalysts (after the redox cycles) would produce a metallic surface similar to that of the fresh sample.
4. REFERENCES 1 R.Bouwman and P.Biloen, J.Catal., 48 (1977) 209. 2 S.de Miguel, 0.Scelza and A.Castro, Appl.Catal., 44 (1988) 23. 3 S.de Miguel, J.Martinez Correa, G.Baronetti, A.Castro and O.Scelza, Appl. Catal., 60 (1990) 47.
Guczi, L d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1!B2, Budapest, Hungary
Q 1993 Elsevier Science Publishers B.V. All rights reserved
EFFECT OF THE SUPPORT ON THE COPPER STATE IN COPPER-TITANIUM
OXIDATION CATALYSTS T. S. Petkevich,L. Ya Mostovayq Yu. G. Egiazarov and N. A. Kovalenko Institute of Physico-Organic Chemistry, B e l o m i a n Academy of Sciences, Surganov Str. 13, Minsk, Belorussia
Abstract The formation of active sites in copper-titanium and copper-titanium-calcium catalysts has been studied. The presence of two copper-containing eltee differing in the nature of association with the support surface has been found. It has been shown that interaction between copper and a titaniumcalcium support manifests itself to a lesser degree than with titanium dioxide.
The genesis of a catalytically active phase of the TI02 and Ti02* CaSO supported copper catalysts has been studied. 4 The samples were prepared by impregnation of supports with aqueous solutions of copper nitrate. A copper quantity m o u n ted to 0.5-10.0 wt.%, a temperature range was 380-970 K. The copper phase state was studied by the DRS, EPR and IR-spectroscopy of adsorbed CO. Based on the DB9 data, a conclusion has been made about the presence of two types of copper-containing sites in copper-titanium catalysts differing in the nature of copper-Ti02 association. The first type of sites is characteriaed by the 380 nm band which can be considered as interaction between copper iona and Ti02 by charge transfer between them. This ie supported by a decrease I n the edge intensity of the inherent
2116
Ti02 absorptuon after copper desorption. The second type is characterized by the 760-780 nm band specified by the d-d tra nsitions in Cu2+ ions with distorted octahedrical coordination. Depending on a copper content in samplea the formation of those sites during thermal treatment has some featuree. In the 0.5-2.0 wt.% Ou samples 8 temperature dependent curve of 380 nm intensity band (I-type sites) crossee a 520 K maximum while for 5.0-10.0 wt.% au samples the band intensity drops with increasing calcination temperature (Fig. la). The comparison of two conoentration ranges shows that a strong interac tion with a support occurs to a greater degree at small quantities of copper. Por the II-type sites, the 760-780 nm inten sity band slightly depends on the calcination temperature (copper oontent is 0.5-2.0 wt.%), while for samplee of a high er copper content the intensity drops with temperature (Big. 1B). Besides, in the spectra of samplee oontaining over 2.0 wt.% Cu after thermal treatment at 720 K, the adsorption is observed cat 240-280 nm of charge transfer to copper associates which increases upon further heating to 870 K. The spectnun of a sample, containing 10.0 wt.%Cu, represents a wide un structural absorption which is characteristic of unstoichiometric oxides of transition metals. After deposition of copper to a titanium-calcium support and calcination, it is spectrally reflected at the same wave lengths as in spectra of the Ti02-supported samples. However, the 760-780 nm bands of Cu2+ ions are slightly expressed and the 380 nm band intensity decreases as compared to the Ti02supported eamplee. Thus, the comparison of spectral characteristice of two sample serkee obtained on titanium oxide and titanium-calolum supports evidences that in the latter case a strong interaction between copper and a eupport manifeste itself to a lesser degree. In the EPR epectra of copper-titanium oxide catalyete dehydrated at 380 K, an anisotropic signal is observed with $*= P 2.05, 2.21-2.23 of Cu2+ions in coordination of a Plat square. Upon calcination of the eamples at 720 K the signal
9,,=
2117
is broaaened, the value oft,, decreases to 2,16. Such a variation i n the EPR signal form and parameters evidences the appearanoe of clusters or associates of oopper with a weak exchange interaction. In tne epeotra of samples containing 5.0 wt.48 Cu, an additional narrow signal appears with %= 1.991 of Ti3+ions. The same signal is present in all the samples of copper-titanium catalysts calcined at 870 K. The appearance of a trivalent titanium eignal leads to a conclusion that already at stages before reduction, a strong interaction between titanium dioxide and copper ions occurs. The calcination temperature increase to 970 K sharply decreases the EPR absorption intensity of Ti3+, apparently, due to implantation of copper ions into the Ti02 lattice to give CuxTiO compouY nds evidenced by the X-ray phase analysis. If copper is deposited to a titanium-calcium support, a low-intensity signal of !Ti3+ ions appears only after calcination at 870 K and disappears completely at 970 K. Thus, the EPR data are indicative of a weaker interaction between copper and Ti02*Ca804 as compared to Pi02.
Big. 1. Dependence of the 380 nm (a) and 760-780 nm (b) band intensity on the calcination temperature of copper-titanium catalysts. The Ou content (wt.%)t 0.5 ( 1 ) ; 2.0 ( 2 ) ; 5.0 ( 3 ) .
2118 For information about copper state in the catalysts, the IR-spectroscopy of low-temperature CO adsorption was applied. The catalysts were studied in the oxidized form after oxygen treatment at 720 K and in the reduced form after carbon-monoxide treatment at 620 K followed by CO adsorption at 173 K.In the IR-spectra of CO adsorbed on oxidized samples, the 21102130 cm-'and 2140-2150 cm-'absorption bands appear, specified by formation of surface complexes with two states of Culfons. The 2110-2130 om-'absorption is to be referred to complex CO with Cu"3.n associates, possibly, Cu2+-O-Cu2+. The appearance of that complex in spectra of oxidized samples shows the reduction of a part of cu2+ions even upon low-temperature interaction with CO. The absorption in the 2140-2150 cm-'range isspthe reduced-Cu0 cified by formation of complex CO with Cul'in state. It is to be noted that copper ions on a titanium-calcium support are capable for reduction to a lesser degree at CO adsorption than in the case of Ti02(at an equal percent Cu content). T h i s especially manifests itself after reduction trea tment of samples with carbon monoxide. For instance, the depth of the reduction of the 5.0 wt*% Cu/Ti02 sample is so great that it does not permit us to register the complexes of adsorbed CO due to a high level of CuO absorption. All the spectra show the 2185-2190 cil bands of complex CO with Cu2+. Attention is drawn by a decrease (or disappearance) o f the 2115 cm-'band intensity at CO adsorption on reduced samples with simultaneous band intensity incsease of complex CO with 0 ern-'), This seems to be explained by Cul+ in CuO ( 4 ~ =2140 aggregation of copper associates to CuO affected by CO, Thus, based on the above data it can be concluded that interaction between the deposited copper and Ti02*CaSOqoccurs to a lesser degree than with Ti02, as a result of which the probability of finding copper in the lowest oxidation degrees decreases. The data obtained can be used to explain the catalytic properties of copper catalysts baaed on the titaniumcontainlng support8 .
G d ,L d d. (Editors), New Frontiers k Caralpis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishem B.V. All rights reserved
EFFECT OF ADDITION OF Pd, CO AND Pd-CO ON CeO,. SYNGAS CONVERSION AND ACmALDEHYDE REACTION
H.Idrissb, C. DiagneO, J. P. Hindermanna, A. KinnemannO and M.A. Barteaub ‘Laboratoire de Chimie Organique Appliquee, Universite Louis Pasteur, 1 rue Blaise Pascal, 67000 Strasbourg, France bDepartment of Chemical Engineering, University of Delaware, Newark, DE 19716, USA
Abstract: Ethanol was observed from the reaction of CO/H2 + CH2C12on Pd/Ce02 as well as from the reaction of CO/H2 on Pd-Co/Ce02. This result clearly indicates that Co sites provide CH, species responsible for ethanol formation. Intimate contact between Pd and Co was most likely formed since TPR indicated that both metals were reduced at the same temperature. Moreover, acetaldehyde TPD showed that ethanol produced by acetaldehyde hydrogenation desorbed at only one peak temperature which might be indicative of a single type of active site. 1. INTRODUCTION:
The investigation of bimetallic catalysts is of importance in understanding numerous catalytic reactions, including syngas conversion, selective oxidation and have shown the performance of autocatalysts. Supported Pt and Pd catalysts [l] significant activity for conversion of CO + H2 to methanol mainly because they do not dissociate CO at moderate temperatures (200-230 OC, 1 atm.). Addition of Li to Pd/Ce02 [2] and of Co to Pt/AI2O, [4] results in ethanol formation from CO + H2. Higher alcohol synthesis is believed to be due to the formation of CH, species on Co or Pd-Li sites; these CH, species react with non-dissociated, adsorbed CO on Pd or Pt to yield higher oxygenates. In an effort to understand the site requirement for these reactions the formation of ethanol, on three different catalysts, 3%Pd/Ce02, 3”/oCo/Ce02and 3%Pd-3%Co/Ce02was studied.
2. EXPERIMENTAL:
:Ce02 was precipitated from an aqueous solution of cerium nitrate by NH,OH at pH 8. Cobalt nitrate and palladium chloride were deposited by conventional co-impregnation methods. All catalyst were dried at 120 OC for ca. 12 hours, and calcined at 500 OC for 3 hours. All catalysts were reduced with H2 at 300 * was studied at OC for ca. 12 hours before further investigation. 1 atm. pressure using a standard fixed bed reactor previously described [2]; products were analyzed by GC (FID) using a chromosorb 102 column. CO uptake was determined by the pulse method at room temperature as described previously ! T P ~ was crried out as follows: after (31. ~emoeratureP ~ V reduction under H2, the catalyst was cooled to 100 OC under He, oxidized with 5% O2 in N2 for 1 hour, and cooled to room temperature. The catalysts was, then, flashed under H2 at 15 OC/min. up to 850 K. w e p r o m e dd e s m
21 20 [TPDl: after acetaldehyde adsorption at RT, catalysts were flashed under He at 30 Wmin up to 850 K. As many as 50 masses were followed during TPD, using a UTI mass spectrometer interfaced with a PC computer. The flow reactor apparatus was described previously [5].
3. RESULTS: 3.1 COIH2 + CH2C12 While 3%Pd/Ce02 converted CO/H2 to methanol with >90% selectivity at 215 oC, 1 atm. with no ethanol formation [3], addition of CH2CI2, figure 1, to the CO/H, flow resulted in ethanol formation. Clearly this result indicated that the addition of CH2(a) ((a)adsorbed) species, provided by the dissociation of CH2C12, was responsible for ethanol formation on the Pd/Ce02 catalyst. This is in agreement with similar experiments on PdN205 [6] where the authors also observed ethanol formation from the reaction of CO/H2 and CH2C12.
Figure 1: Ethanol formation from CO/H2 + CH2CI2 on Pd/CeO, catalyst. Conditions: 250OC, 1 atm, CO/H&H2C12(1/2/0.04), 2 I/h. g Cat-l.].
0
40
Time
80
120
(minutes)
3.2 CO adsorptlon and Syngas conversion: Table 1 presents CO/H2 conversion as well as CO adsorption data for Co/CeO,, Pd/Ce02 and Pd-Co/Ce02 catalysts. CO was only converted to hydrocarbons on Co/Ce02. On Pd/Ce02 negligible amounts of ethanol were formed, the major
21 21
product was methanol. However, on Pd-Co/Ce02ethanol selectivity was ca. 43 % of the total alcohol production. Thus, addition of Co to Pd/Ce02 resulted In similar behavior as co-feeding CO/H2 with CH2C12. In other words, Co provided CH2(a) species in close contact with CO(a) on Pd. Similar results were also obtained on Pt-Co/A1203where the authors [4] indicated that addition of Pt to Co/Ai203 resulted in the formation of ethanol from syngas. For CO uptake, the Pd-Co/Ce02 catalyst adsorbed ca. 6 times more CO than did Pd/Ce02 and Co/Ce02 together (table 1) clearly indicating an intimate contact between both metals. It was previously observed by XPS [7] that addition of Pd to Co304 resulted in the formation of Coo during hydrogen reduction while no Coo was observed in the absence of Pd. The increase in the adsorption capacity of Pd-Co/Ce02 mlght thus be explained by Increased reduction of Co cations to Coo which would increase the amount of adsorbed CO. However, intimate contact between Pd and Co might also result in preventing Pd sintering which would also increase the amount of adsorbed CO. In order to further study this behavior, TPR and TPD of acetaldehyde were studied. Table 1 Catalyst
CO-IHC CO (x 108) uptake in mo1ec.g cat1
3%Co/Ce02 2.97 3%Pd/Ce02 1.44 3%Pd-
I-a:&&&&) CO % HC conversion
MethanalEthanol
n.d. 0.21
0 55(97.2)
1.97
100' 43.4b
alcohols ( ): % eelectivitywlth respect to alcohol8 only
0 1.6(2.6)
-s67
3.3 TPR and TPD of acetaldehyde: TPR (figure 2A) results indicated that Pd is reduced In a single peak at 385 K, while in the case of Co two peaks were observed at 445 K and 533 K. However PdCo catalysts showed the presence of one broad peak at 415 K. Clearly Pd enhanced the reduction of Co cations on the surfaces of ceria. The reduction of acetaldehyde to ethanol was similarly affected, since two peaks for ethanol from acetaldehyde TPD (figure 28) were observed in the case of Co/Ce02 while only one, at lower temperature was observed on Pd/Ce02 and Pd-Co/Ce02. 4. SUMMARY
Addition of CH2CI2to the CO/H2 flow over Pd/Ce02 resulted in ethanol formation (figure 1). Ethanol (43% of oxygenate selectivity) was also observed from direct syngas conversion on Pd-Co/Ce02. Two peaks were observed by TPR (figure 2A) during the reduction of Co/Ce02, at 445K and 533K, probably indicating the reduction of cations In two different oxidation states. For Pd/Ce02, only one reduction temperature was observed at 385K. Moreover, the Pd-Co/Ce02catalyst was reduced at only one reduction temperature close to that of Pd and lower than those of Co. This resuit indicated that further Co reduction most likely occurred and was assisted by Pd. TPD of acetaldehyde showed that ethanol desorption also occurred at one temperature on Pd-Co/Ce02 while two peaks for ethanol were observed in the case of acetaldehyde TPD on Co/Ce02. The similarity between the
2122 temperatures for reduction of Pd-Co/Ce02 and of acetaldehyde hydrogenation to ethanol might indicate that both metals are within close contact on the surface, forming a single type of site. Figure 2: A- TPR of 3%-Pd/Ce02, 3% Co/CeO2 and 3%Pd-3%Co/Ce02. Bethanol desorption during acetaldehyde TPD on the same catalysts of A. B (acetaldehyde TPD)
A (TPR) 1
3%' PdlCeO
400K
2fi
c 0) )
e C 0
.-
c)
3% ColCe02
P L
zw
I I
U
410K
L Y 0)
B 2
0
200
400
600
800 200 Temperature
400
600
800
(K)
5. REFERENCES 1 G. van der Lee, V. Ponec, Catal. Rev. Sci., 29 (1987) 183 2 C. Diagne, H. Idriss, J.P. Hindermann, A. Kiennemann, Appl. Catal., 51 (1989) 165. 3 C. Diagne, H. Idriss, 1. Pepin, J.P. Hindermann, A. Kiennemann, Appl. Catal.., 50 (1989) 43 4 L. Guczi, T. Hoffer, Z. Zsoldos, S. Zyad, G. Maire, F. Garin., J. Phys. Chem. 95 (1991) 802 5 K.S. Kim, M.A. Barteau, W.E. Farneth, Langmuir, 4 (1988) 533 6 T.L.F. Favre, G. van der Lee, V. Ponec, J. Chem. SOC.Chem. Comm. (1985) 230 7 V.M. Belouzov, J. Stoch, L.V. Batcherikova, E.V. Rozhkova, L.V. Lyashenko, Appl. Surf. Sci. 35 (1988-89) 481
Ouai, L et al. (Editors), New Frontiers in Catalyis
Proceedings of the 10th International Congnss on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993Elscvicr Science Publishers B.V.All rights resctved
THE ROLE OF ELECTRIC FIELD ACI'ING ON THE CHARACI'ERISTIC OF ADSORPTION OF SOLID SURFACE, THE OXYGEN ADSORPTION ON TIN OXIDE FILM IN AN EXTERNAL ELECI'RIC FIELD R. Zhou East China University of Chemical Technology, 130 Meilong Road, P.O.Box 384,200237 Shanghai, China
Abstract It is assumed that the electric field can affect the adsorption by changing the surface barrier caused by electron transfer. An adsorption model is founded to elucidate the mechanism of the adsorption and the model is substantiated by the adsorption of 0,on SnO, film. 1. INTRODUCTION
It has been reported the external electric field (EEF) has some appealing effects on both adsorption and catalysis [l-31, but there was no detailed study on the mechanism of the adsorption process. In this paper the adsorptive property of surface is correlated with EEF from surface barrier point of view, and then an adsorption model is established to elucidate the mechanism of the adsorption affected by EEF. The model is substantiated experimentally by the adsorption of 0, on SnO, film. Formation of surface band bending caused by the internal electric field (IEF) .in the space charge region of the surface is a common occurrence hen adsorption takes place with charge transfer on a semiconductor or an isolator. Th band bending forms a surface barrier that prevents electrons from traveling between surface and adsorbates, so that further adsorption is hindered [4]. On the basis of this view point, it is reasonable to deduce that EEF can affect the adsorption in the following way: if IEF and EEF are same in direction, IEF is enhanced resulting in an increase in the barrier and a decrease in adsorption; conversely if IEF and EEF are opposite, IEF is reduced resulting in a decrease in the barrier and increase in adsorption. According to the aforementioned mechanism, an adsorption model concerning EEF can be established. For convenience, only the case that the electronegative species adsorbed on an n-type semiconductor is taken in to consideration. The rate of the change of electron density a t the surface state due to adsorption can be assumed as dn, / d t = Kn(Nt - n,)n, - K n n , n t- KPp,n,
F
(K, - rate constant for electron capture a t a surface state, K, - rate constant for hole capture at a surface state, N, - density of surface state, n, - density of electrons in a surface state, n, - density of electrons in the conduction band edge at surface, nI emission constant for electrons, pc - density of holes in the valence band edge a t surface) where the three terms represent the rate of electron transfer to the surface state, the rate of electron injection into the conduction band and the rate of hole transfer from valence band to the surface state respectively. Substituting the expression of n, , p, , and n, [4]
21 24 into Eqn. 1, and assuming the surface barrier eV, is given by eV, = e(rV + V’,)
(2)
- voltage of EEF, r - a factor representing the contribution of EEF to surface potential, V’, -surface potential introduced only by adsorption) an expression for the rate of adsorption associated with EEF is obtained
(V
dn, / dt = Kn(N, - n,)n,exp[ - e(rV + Vl)/ KT] - K,,N,n,exp[ - (Ec - El) / KT]exp[ - e(rV + Vi)/ KT] - Kpn,P,exp[e(rV + V:) / KT]
(3)
(nb - bulk electron density in the conduction band, N, - effective density of states in the conduction band, E, - energy of conduction band edge, E, - energy level of surface states, pb - bulk hole density in the valence band, K -Boltzmann’s constant, T -temperature) When dn, / dt = 0, an equilibrium equation of the adsorption can be derived from Eqn. 3 n Aexp[ - e(rV + V’,) / KT] I * (4) N, - n l Bexp[e(rV + V’,) / KT] + Cexp[ - e(rV + V’,) / KT] (A = K,nb, B K&, C = K,N,cx~[-(E,-E,) / KT]) In Eqn.4, N, is a constant associated with the adsorption. If the surface is clean, n, / (N,-nJ = 0; If the amount adsorbed arrives at 50% of its maximum, n, / (N,-nJ = 1. 2. METHODS
The adsorption was carried out under 1.00 atm a t 434K in a fluid gas adsorption system. The amount of 0;adsorbed on Sn02 film was determined by measuring the conductivity betweerr two gold plated electrodes prepared on the surface of the film. The film was coated on a glass flat substrate by CVD method [S]. The EEF perpendicular to the surface was provided by two metal foils at a distance of 1.1 mm. The direction of EEF from the surface of the film to the substrate is defined as positive, and conversely, as negative. The SnO, film placed in between the two foils was packed in a test cell which was evacuated at 434K prior to the adsorption. Then a t 434K the film was exposed to a continuous flow (100ml/min) of N2 of 99.99% purity and O2 of 99.8% purity mixture which was dried prior to entering the system, A typical adsorption test requires 12 hours to reach at its equilibrium. 3. RESULTS AND DISCUSSION The field effect on adsorption is depicted in Fig. 1. As we known the increase of O2 adsorbed results in the rising of n, and hence causes the decrease of conductivity (a)
-
in the solid. The conductivity is determined by u A n o - n,)
01 - electron mobility, no -initial electron density in bulk)
(5)
21 25
-f
Po, (atrn) 0.8
0.
5
l3
a
m
\
;0.20
a
a
0
-
4
0
Poa=0.05 atm RUN-2 EEF=O V
a 0. a
a
0.15
W
b
b
0.0
0.5
1 .o
-40
Po, (atm) Fig.1. Effect of EEF and Po, on the atm. adsorption of 0, (P,,,,=l.OO T = 434 K)
0
40
POTENTIAL (V) Fig.2. Comparison of effect of EEF (RUN-1) with Po, (RUN-2) on adsorption. (Pmd = 1.00 atm. T = 434 K)
Fig.1 shows the amount of O2 adsorbed increases with the rising of Po, and the field effect on adsorption is significant. When V>O, EEF and IEF being opposite in direction, the amount of 0, adsorbed increases; conversely, when V e 0, the 0, adsorbed decreases, which quite accord with the mechanism assumed. The change caused by varying EEF from -35 V to +35 V is greater than that by increasing Po2 from 0 to 1 atm as shown in Fig. 2. The experiment shows the contribution of the non-adsorption effect to the conductivity of the film is so small (O, b>l) (10) where a, b, and c are constants to be simulated. V’,(V) varies in between the value of 0 and -2a. The result of the simulation (S-1) is shown in Fig. 3. and Table 1. The model fits the voltage isotherm very well. The other simulation approach (S-0) simplifying V’,(V) as a constant (b = 1, V’,(V)= -a) is shown in Fig. 3 and Table 1 to compare the effect of V’,(V) on the simulation. In conclusion, the effect of EEF on adsorption is based on two major facts: the formation of double layer and the existence of space charge region. EEF can affect surface adsorption process by altering surface barrier to change the apparent amount adsorbed while the thermodynamic equilibrium of the adsorption can not be moved. So EEF is to a surface adsorption what a catalyst is to a chemical reaction. It is predictable that the ”field effect” on catalysis will most probably appear in redox reaction on the surface of semiconductor or isolator. Fig.3. Voltage isotherm (RUN-3) and simulation results. (PtoId= 1.OO atm. T = 434 K)
Table 1 Results of simulation Simulation Parameter fiNt A B C r a b x lo3 x 10’ xi0 x103 xi02 ~ 1 0 ) xi02 1 S-0 2.060 2.168 1.0002 3.756 1.033 0.890 1.011 S-1 1.253 1.274 1.0000 4.079 1.019 1.480 1.568 1.080 uo
Correlation Coeficient \ 0.9984 9.00 0.9996
4. REFERENCES 1 S.A. Hoenig and J.R. Lane, Surf. Sci. 11 (1968) 163. 2 W.W. Lincoln and J.L. Olinger, AIChE Symp. Ser. 71 (1975) 77. 3 S. Kristyan and R.B. Timmons, J. Catal. 101 (1986) 331. 4 S.R.Morrison, ”The Chemical Physics of Surfaces” Plenum Press, New York and London, 1979. 5 G. Yao, J. Han and W. Yuan, J. East China Inst. Chem. Technol. 16 (1990) 245.
Guczi, L d al. (Editors),New Frotuiers in Coralysh
Proceedings of the 10th International Congrtss on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elscvier Science Publishers B.V.All rightp reserved
CATALYTIC BEHAVIOR OF LiFeO, ANODE FOR SOLID OXIDE FUEL CELLS R. T.Bakera, I. S. Metcalfea, P. H. Middletonb, P. Petroleha and B. C. H. Steeleb
aDepartment of Chemical Engineering, Imperial College, SW7 2BY London, United Kingdom bDepartment of Materials, Imperial College, SW7 2BP London, United Kingdom
Abstract
A novel combination of electrochemical and temperature programmed techniques have been used to investi ate the electrode material LiFeO2. The behaviour of this catalyst in the re 'on of 500-650k a pears to be influenced by the presence of surface hydroxyl groups. %is combination o electrochemical and temperature programmed techniques appears to be particularly a propriate for the investigation of new electrode materials for use in solid oxide fuel cel s.
F P
1.
INTRODUCTION
related to that of charge transport. 2.
EXPERIMENTAL
The oxide electrode materials were made b the solid state diffusion process, using high purity oxide precursor powders obtained i o m Aldrich Chemicals Ltd.
2128 After mixing the appropriate proportions and calcining the resultant catal st powders were either added to a tape casting slur in order to make the film electrodres, or pelletized and then crushed for use in the Tymicroreactor. The electrodes were made by spreading the slurry over a pre-sintered zirconia electrolyte sheet and sintering at the appro riate temperature. #he electrochemical cell housing was made of a machinable ceramic, and bonded
3.
RESULTS AND DISCUSSION
Figure 1shows a cyclic voltammogram for LiFe02 in helium at 560°C. At high positive otentials the current rises due to o gen evolution, whereas at high negative otentiaE t h e cathodic currents are due to t e reduction of the oxide catalyst. The steresis reflects the non-equilibrium changes in the composition of the catal st as the erectrode potential is varied over a comparatively short time scale (ty ically minutes per scan). The most significant observation is the cathodic peak at +fO mV. This was observed after scanning first in the anodic direction, then reversing a t +400 mV and scannin back in the cathodic direction. The magnitude of the currents appear to be too ,mat for this phenomena to be due to bulk oxygen. Thus it a pears that a surface species is undergoing irreversible reduction at this temperature. T e reduction appears to be irreversible as there is no anodic peak observed on the cyclic voltammogram corresponding to reoxidation of the surface species. Moreover, the peak was found to disappear completely when the temperature was raised to 620°C see Figure 2). When the temperature was then decreased to 560"C, the cathodic re uction peak was not recovered indicating that the surface species had been completely destroyed.
%
3
g
6
21 29
5
0
-600
-400
-200
200
0
400
600
Potential (mv)
Figure 1. Cyclic voltammogram for LiFe02 in helium at 560°C.
10 0 6
E
4
4
il
4
I
w
E=
2
5
0 -2
L
-4
-6 1 -400
,
I
0
-200
I
200
400
Potential (mv)
Figure 2. Cyclic voltammogram for LiFeO2 at 620°C. Tem erature programmed reduction of the same material was carried out in flowing car on monoxide (see Figure 3). A low tem erature carbon dioxide peak was observed at 370"C, however, by repeating the run in felium this peak was shown to be due to desorption of residual carbon dioxlde and not reduction by the carbon monoxide. Reduction of the electrode material began to take place at a temperature of around 450°C. Simultaneous evolution of hydrogen and carbon dioxide was observed. At hi her temperatures a lar e reduction peak is observed, possibly due to the onset of bu k reduction. The simu taneous production of carbon dioxide and hydrogen in the
\
s
d
21 30
TP experiment may be indicative of the decomposition of surface hydroxyl groups. This explanation would fit in with the electrochemical results. The electrochemical reduction of surface hydro 1 groups would be irreversible, either hydrogen or water being evolved. The hydroxy groups would be readily destroyed by the of a cathodic current in the temperature ran e of the ex eriments explaining w reduction peak is observed at 6 b C (or in t e later experiment at
r
1
n
c
3
2.5
P
-
0
-0
W
2-
’ C
0,
1.5
L Q) c
:
-
1-
0 L c U
g
0.5
-
v) u)
:
z
oo
200
600
400
800
1000
1: 00
Temperature( ‘C)
Figure 3. Temperature programmed reduction of LiFe02 in carbon monoxide. 4.
CONCLUSIONS
The electrode material LiFe02 has been investigated usin a novel combination of electrochemical and TP techniques. The behaviour of this cata yst in the re ion of 500650°C appears to be influenced by the presence of surface hydro 1 groups. f t is expected that, in the future, these techniques will be employed to yiel more information on the catalytic behaviour of new electrode materials for use in solid oxide fuel cells. Furthermore, it is intended to develop a system in which a working electrode can be subjected to programmed operation.
4 7
5.
ACKNOWLEDGEMENTS
We acknowledge the suppoJt of the SERC, the Ceramic Electrochemical Reactors Club, and the State Scholarships Foundation of Greece (PP). 6.
REFERENCES
1
D.C. Fee and J.P. Ackerman pp 11-14, Abstracts of 1983 Fuel Cell Seminar, Orlando FL, November 1983. B.C.H. Steele, I. Kelly, H. Middleton and R. Rudkin, Solid State Ionics 28-30 1988 1547. S. Idetcalfe, P.H. Middleton, P. Petrolekas, B.C.H. Steele, 8th International Conference on Solid State Ionics, Lake Louise Canada, October 1991. R.J. Chater, S. Carter J.A.Kilner, B.C.H. Steefe, 8th International Conference on Solid State Ionics h k e Louise, Canada, October 1991. D. Chadwick & P.J.k O’Malley, J.Chem.Soc., Faraday Trans. 1,a(1987) 2227.
2
3 4
5
i.
Guczi, L a al. (Editors), New Frontiers in Caalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights teserved
ELECTROCHEMICALMODIFICATION OF THE ACTIVITY AND SELECTIVITY OF METAL CATALYSTS
M.Smkidks, D. Eng, P.-H.Chiang and H. Alqahtany Chemical Engineering Department, Tufts University, Medford, MA 02155, USA
1. INTRODUCTION The partial oxidation of methane has become a topic of ma'or interest in heterogeneous catalysis. Over the past decade, the num er of studies on oxidative coupling of methane has increased Interest in other methane partial oxidation tremendously [ 11. reactions (methanol formation, synthesis gas formation) has also As an alternative to conventional catalytic studies, increased [2]. several investigators have recently studied the possibilit of oxidizing methane with the aid of 0- transported through so id electrol te membranes rather than using gaseous 0 2 [3]. In this case, an - conducting solid electrolyte, such as yttria-stabilized zirconia (YSZ), is used. Finally, the possibility of electrochemical dehydrogenation of methane has been recently investigated by using H+ solid state conductors [3,4]. In the resent brief communication, two solid electrolyte aided studies of 8H4 are presented, namely non-oxidative dimerization of CH4 on Ag with the use of H+ conductors and 0- aided study of CH4 conversion to synthesis gas on Fe electrodes.
Y
i;
2. EXPERIMENTAL Three electrolyte-reactor configurations were used for the CH4 stud : a single-chamber AglSCYlA cell, a single chamber Agl S Ag ce 1, and a double-chamber AgISC'fIAg cell. Figures l a and 1b show the single-chamber and double-chamber configurations respectively. The strontia-ceria-ytterbia (SCY) electrolyte pre aration has been described in detail in previous communications [4,5f The Ag electrode was prepared from a silver suspension in but 1 acetate in a method presented elsewhere 6 . The su erficial area o each Ag film electrode was about 3 c m i land the fi ni thickness was in the range of 25-40pm. A two-chamber YSZ cell with Fe electrodes was used for the synthesis gas foriiintion studies. The cell configuration was similar to that shown in figure lb. The Fe catalyst-electrode had an avera e particle size of 1 5 p m with a corresponding surface area of about 2 0 cm2/g. Details on the Fe catalyst prcparation have been Reactants and products were provided in previous works [7]. analyzed using on-line as chromatograph with a molecular sieve 5A column to separate H %2, N , CH4 and 60 and a porapak Q (or N) to separate C02, C2H4, C2%6 and &O.
r
c0u2;li%
P
s
Y
zu 19uz3 .. -
- '
I t
tPHZ3 'PH3
ZE 1z
21 33
be seen that upon increasing I, the reaction rate can become as hi h as 8 times the open-circuit rate while the C2 selectivity is about 100 o for currents lower than 50mA. At higher currents some 0 2 reacted to form C 0 2 . Figure 2b shows the rate of methane consum tion vs the cell voltage V. It can be seen that after reaching a thresh old voltage value (~1.6volts in Figure 2b), there is a linear relationship between reaction rate and cell voltage. It was of interest to investigate the importance of the ionicall conductive species in enhancin the reaction rates. T,o this end the SC electrolyte was re laced by SZ disc. Silver electrodes were used again. Despite t e ionic species being 0--, the rate of methane dehydro enation was increased again. At I=40 mA, the rate was enhance by a factor of 2.5. A YSZ cell o erating at 9OO'C and atmospheric total pressure was used in the stu y of synthesis gas formation. The feed consisted of 7.5% CH4, 8% steam and the balance was helium. Figure 3 compares the effect of ionic 0- to that of gaseous 0 added to the methane feed. It can be seen that for either 0 or 0- the $0 formation rate (Fi 3a) is essentially the same. The sefectivity to CO was ver high (EOlCO2 exceedin 1OO:l). Similarly, the H /H2O ratio was 280:l. Again, 0-and 0 2 tehaved the same towards iydrogen formation.
8
K
i
f
B
5
B
4.bb .
3
002
%
v
a
8
'ts
I
1.bq
0.W
0.0
0.2
0.4
0.6
0.b
b
supplied oxygen (% of total flow)
Figure 3ab. Effect of oxygen feed on (a) CO and (b) H2 formation.
21 34
The presented results seem to confirm two interesting characteristics of solid electrolytes in heterogeneous catalysis. First, the electrolyte needs not to have a s ecific conducting species in order for rate enhancement to be observe as it has been recently pointed out by other research groups as well [S]. Second, the one-chamber cell could be easily incorporated into existin industrial reactor designs since reactants do not have to be se arated The solid electrolyte can simply replace the conventional cata yst support.
s P
ACKNOWLEDGEMENT We gratefully acknowled e the National Science Foundation and the De artment of Energ for suggort of this work under Grants Partial support by C€3%-8815927 and DJ-89-CE90 8 respectively the Amoco Oil Co. is also gratefully acknowledged:
REFERENCES l.Y. Amenomi a, V.I. Birss, M. Goledzinowski, J. Galuszka and A. San er, Cat. iev.-Sci. Eng., 32(3) (1990) 163. 2.R. fitchai and K. Klier, Cat. Rev.-Sci. En 28 (1986) 13. 33 (3/4), in press. 3.D. Eng and M. Stoukides, Cat. Rev.-Sci. 4.P.-H. Chiang, D. Ens and M. Stoukides, Electrochem. SOC., 138(6) (1991) L11. 5.11. Iwahara, T. Esaka, M. Uchida and N. Maeda, Sol. St. Ionics, 3/4 (1981) 359. G.S. Seimanides and M. Stoukides, J. Catal., 88 (1984) 490. 7.D. En and M. Stoukides, Proc. 9th Int. Con r. Catal., Calgary 1988, The C eniical Institute of Canada, Vol. 2, p. 9 4. 8.C. Vayenas, S. Bebelis and S. Ladns, Nature, 343 (1990) 625.
kf.,
E
9
O d ,L a al. (Editors), New Frontiers in Caalysis Proceedings of thc 10th Inkmationel Congress on Catalysis, 19-24 July, 1% Budapest, Hungary Q 1993 Elstvier Science Publishers B.V. All rights ~~scrvcd
THE SELECTIVE HYDROGENATION OF ACETYLENE BY THE ELECTROCHEMICALLY PUMPED HYDROGEN OVER Cu IN THE PRESENCE OF ABUNDANT ITHYLENE
K
Otsuka, T. Yagi and M.Hatano
Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan
Abstract The a c t i v e hydrogen, which is generated on t h e cathode of Cu by t h e electrochemical pumping through a H3P04-membrane. can hydrogenate acetylene t o e t h y l e n e a t 353 K i n t h e p r e s e n c e o f a b u n d a n t e t h y l e n e where t h e production of ethane is minimized. The electrochemically pumped hydrogen on t h e c a t h o d e s o f d-metals (Pd, P t , Rh, and I r ) h y d r o g e n a t e s e t h y l e n e preferentially. The r e s u l t s of k i n e t i c s t u d i e s f o r t h e cathode of Cu have s u g g e s t e d t h a t t h e h y d r o g e n a t i o n s o f e t h y l e n e and a c e t y l e n e o c c u r It is suggested t h a t t h e s t r o n g e r i n t e r a c t i o n of independently each o t h e r . Cu with acetylene enables the p r e f e r e n t i a l hydrogenation of acetylene.
1. INTRODUCTION The sp-metals such as Cu, Ag, and Au are notably less e f f i c i e n t f o r hydrogen e q u i l i b r a t i o n r e a c t i o n s and h y d r o g e n a t i o n o f u n s a t u r a t e d hydrocarbons than d-metals due t o t h e i r i n a b i l i t y t o i n t e r a c t with hydrogen molecules. Hydrogenation of o l e f i n s and benzene on d-metals with t h e hydrogen supplied by electrochemical pumping through proton-conducting One can expect membranes has been demonstrated by Langer e t a l . [1,2]. t h a t t h e electrochemical pumping of hydrogen t o t h e sp-metals a l s o generates Therefore, t h e purpose of t h i s an a c t i v e hydrogen on such i n a c t i v e metals. work are t o d e m o n s t r a t e t h e a c t i v a t i o n o f hydrogen o n Cu by t h e e l e c t r o c h e m i c a l pumping and t o a p p l y t h i s method f o r t h e s e l e c t i v e The hydrogenation of acetylene i n the presence of abundant ethylene. removal o f a c e t y l e n i c compounds i n o l e f i n s is o n e o f t h e i n d i s p e n s a b l e processes f o r t h e s y n t h e s i s of o l e f i n s as polymerization feedstock.
The membrane reactor and t h e p r i n c i p l e of experimental procedure are shown schematically i n F i g . 1. The method of preparations o f t h e cathode, the anode and of t h e proton conducting membrane w a s described i n d e t a i l To be b r i e f , t h e anode w a s prepared f r o m P t black (20 mg) elsewhere [3]. mixed with g r a p h i t e ( P t : Carbon = 2 : 5 ) and t e f l o n powder (5 mg) by t h e hot-press method. The cathode prepared from the g r a p h i t e impregnated with a
21 36 metal c h l o r i d e was reduced i n The hydrogen a t 473 K. c o n t e n t o f t h e metal was 20 wts. The p r o t o n c o n d u c t i n g membrane was a s i l i c a - w o o l disk (diameter 20 mm, thickness 1 mm) holding 0.59 m l H3P04(aqe 14.7M). A g a s mixture of C2H4 and C2H2 (C2H4 : C2H2 = 51 : 1 kPa) was c a r r i e d w i t h helium t o t h e cathode compartment. Hydrogen (81 P a ) 'with water vapor (20 electrode Pt -electrode kPa, added t o k e e p t h e H3PO4in (anode) (cathode) e l e c t r o l y t e always wetted) was wool silica flowed in the anode compartment. The pumping rate Figure 1. The membrane r e a c t o r f o r hydroof hydrogen was controlled by genation of C2H4 i n abundant C2H4. means of t h e externally applied p o t e n t i a l between both electrodes. The r e a c t i o n was c a r r i e d o u t a t t h e temperatures 293-373 K . The e s s e n t i a l f e a t u r e of the reaction d i d not change with temperatures. Therefore, only t h e r e s u l t s observed a t 353 K w i l l be described below.
I
3. ResULTS
AND DISCUSSION
Table 1 shows the r e s u l t s observed f o r the cathodes added with various metals a t a pumping rate of hydrogen of 21.8 p o l / m i n (corresponding t o 70 mA). Here, t h e s e l e c t i v i t y t o C2H6 and t h e c u r r e n t e f f i c i e n c y o f hydrogenation were defined as follows, C2H6 S e l e c t i v i t y
(The r a t e of C2H6 formation) 8
Current e f f i c i e n c y =
(The r a t e of C2H2 conversion)
x
(The r a t e of C2H2 conversion plus t h a t of C2H6 formation) (The pumping r a t e of H2)
100 %
x
100
%.
The favorable cathodes a r e the ones giving higher r a t e of C2H2 conversion with lower C2H6 s e l e c t i v i t y . The g r a p h i t e without metals showed only small c a t a l y t i c a c t i v i t y f o r the hydrogenation of acetylene. Addition of metals t o t h e g r a p h i t e enhanced appreciably the conversion r a t e of C2H2 a s shown i n Table 1. The h i g h c u r r e n t e f f i c i e n c y and t h e l a r g e C2H6 s e l e c t i v i t y observed f o r the P t / G r , Pd/Gr, R h / G r , and I r / G r i n d i c a t e t h a t these dmetale a r e very a c t i v e f o r hydrogenation of C2H4 a s w e l l known. Among the cathodes examined i n Table 1, the highest conversion of C2H2 and the lowest C2H6 s e l e c t i v i t y obtained f o r the Cu/Gr show t h a t Cu is the b e s t cathode f o r s e l e c t i v e hydrogenation of C2H2 i n t o C2H4 with t h e lowest conversion of C2H4 i n t o C2H6.
21 37 Table 1.
Hydrogenation of the gas mixture of C2H4 and C2H2 by t h e pumped hydrogen a t 353 K.
Cathode
C2H2 conversion
C2H6 selectivity
1% Qr(graphite) Pt/Qr Pd/Qr Rh/Qr
Ir/Qr Ru/Qr Ni/Qr Cu/Qr Ag/Qr
Au/Qr
1% ,a
0.9
Current efficiency
1% < 1
8.4 17.9 4.4
1140 610
98 117
3.4 1.3
1420 ,a
3
1.0
180 1 ,a
11 1
1.6
70
2
530
1.8 11.8
26 51
5
a not measurable due t o the low conversions of C2H2 and C2H4. The r a t e of C2H2 conversion and t h a t o f C2Hg f o r m a t i o n H2 pumping rate / pmol.mid observed f o r t h e Cu/Qr a r e 0 15 30 p l o t t e d a s f u n c t i o n s of t h e current or of t h e pumping rate of H2 i n Fig. 2. The r e s u l t e i n .E IS .-IC Fig. 2 show t h a t C2H2 i s p r e f e r e n t i a l l y h y d r o g e n a t e d by 2 2 % t h e hydrogen pumped t o t h e Cu\ cathode. The C2H6 s e l e c t i v i t y u) U was remarkably low (C2H6 s e l e c t i v i t y 70 mA i n ,, F i g . 2. Thus, t h e c u r r e n t U ’% e f f i c i e n c y of hydrogenation decreases from 35% ( a t 5 mA) t o 8 % ( a t 100 mA). 0 0 When t h e hydrogen was n o t 0 50 100 pumped t o t h e Cu-cathode, Current / mA addition of hydrogen to the gas Figure 2. E f f e c t of the pumping r a t e mixture of C2H4 and C2H2 i n the cathode compartment d i d cause of H2 on the r a t e s of C2H2 convers i o n and C2H6 formation. n e i t h e r the hydrogenation of C2H2 nor t h a t o f C2H4. T h i s was confirmed by mixing hydrogen with a flow r a t e of 187 Fmol/min which was s i x times greater than the highest pumping r a t e of hydrogen i n Fig. 1 (100 mA or 31 H2 pmol/min). T h i s o b s e r v a t i o n i s n o t s u r p r i s i n g b e c a u s e t h e low
%* ~
4
SJ
d
!
21 38
!L Lt
0A 0 Figure
0.5 1.0 Partial pressure of C 2 4 / kPa
3. E f f e c t of t h e p a r t i a l pressure of C2H2.
0
0
20 40 Partial pressure of C2H4 / kPa
Figure
4. E f f e c t of t h e p a r t i a l p r e s s u r e of C2H4.
c a t a l y t i c a c t i v i t y of Cu has already been pointed o u t by many r e s e a r c h e r s [4]. However, i t should be emphasized t h a t t h e a c t i v e hydrogen generated on t h e Cu-cathode by t h e electrochemical pumping can hydrogenate C2H2 t o C2H4. The r e s u l t s i n Fig. 3 show t h a t the rate of conversion of C2H2 on t h e Cu-cathode i n c r e a s e s proportionally with t h e p a r t i a l p r e s s u r e of C2H2 (0-1.0 kPa) a t a constant pumping rate of hydrogen (70 mA) and a t a p r e s s u r e of However, t h e rate of C2H6 formation was n o t a f f e c t e d by C2H4 of 51 kPa. i n c r e a s i n g t h e pressure of C2H2, i n d i c a t i n g t h a t t h e hydrogenation occurs stepwise as C2H2 4 C2H4 4 C2H6. The results i n Fig. 4 shows t h a t t h e rate of C2H6 formation depends on the pressure of C2H4 (10-50 kPa) by f i r s t order. The presence of C2H2 (1.0 kPa) d i d not change t h e rate of C2H6 formation a t a l l t h e p r e s s u r e s of C2H4 The observations i n Figs. 3 and 4 s t r o n g l y show t h a t examined (Fig. 4 ) . t h e hydrogenations of C2H2 t o C2H4 and of C2H4 t o C2H6 occur independently each o t h e r . The s e l e c t i v e h y d r o g e n a t i o n o f C2H2 w i t h a minimized hydrogenation of C2H4 over Cu can be ascribed t o t h e s t r o n g e r i n t e r a c t i o n of C2H2 with Cu than t h a t between C2H4 and Cu. I n c o n c l u s i o n , t h e e l e c t r o c h e m i c a l pumping o f h y d r o g e n c a n s u p p l y a c t i v e hydrogen onto Cu, c a t a l y z i n g t h e s e l e c t i v e hydrogenation of acetylene W e b e l i e v e t h a t t h e membrane r e a c t o r i n Fig. 1 can i n abundant ethylene. be applied f o r many s p e c i f i c hydrogenations.
4.
REFERENCES
1 2
S.H. Langer and H.P. Landi, J . Am. Chem. SOC., 86 (1964) 4694. S.H. Langer and S. Yurchak. J . Electrochem. SOC., 116 (1969) 1228. K. Otsuka, T. Shimizu, and I. Yamanaka, J. Electrochem. SOC., 137 (1990) 2076. G.C. Bond, C a t a l y s i s by Metals, Academic P r e s s , New York, 1962.
3 4
GUai, L a 111. (Editors),New Frontiers in Cafalysis
Proceedings of the 10th International Congnss on Catalysis, 19-24 July, 1992,Budapest,Hungary 0 1993 Elsevier Science Publishers B.V.All rights nserved
SOLJD ELECTROLYTES FOR IN SITU PROMOTION OF CATALYST SURFACES: THE NEMCA EFFECT C. G. Vayenas, S.Bebelis, I. V. Yentekakis, P. Tsiakaras, H. Karasali and Ch. Karavasilis Institute of Chemical Engineering and High Temperature Chemical Processes, University of Patras, 26110 Patras, Greece
Abstract Solid electrolytes can be used as active catalyst supports to dramatically and reversibly alter the catalytic properties of porous metal films by electrically polarizing the metal-solid electrolyte interface. The common features of this effect, termed Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA), are summarized in the present work, together with the origin of the effect, which is due to the controlled change in catalyst work function and to the concomitantchanges in the strength of chemisoxptive bonds 1 . INTRODUCTION It was recently found (1-10) that the application of current or voltage in solid electrolyte cells of the type “gaseous reactants, metal catalystlsolid electrolytelmetal, 0,” and the concomitant pumping of ions through the gas-impervious solid electrolyte to or from the catalyst surface can induce dramatic changes in catalytic reaction rates, which can be orders of magnitude larger than the correspondingrate of ion transfer through the solid electrolyte. This phenomenon, called Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA), corresponds to a dramatic change in catalyst properties and can be viewed as an electrochemical catalytic promotion (1 1) induced by a slow electrocatalyticreaction at the gascatalyst-solid electrolyte three phase boundaries. This electrochemical type of promotion permits continuous controlled in situ variation and monitoring of the amount of the promoter. The NEMCA effect has been studied so far for more than fifteen catalytic reactions on Pt, Rh, Pd and Ag surfaces (1-10). Both stabilized Zro, (an @-conductor) and P’-Al2O3(a Na+ conductor) have been used as the ion donor. It has been found that the NEMCA effect can induce changes in the catalytic reaction rate up to 3x105 times larger than the rate of ion transfer through the solid electrolyte and up to a factor of 70 larger than the regular (open circuit) catalytic rate r,.Significant changes in product selectivity have been also observed (9). It was recently confirmed (1-10) that NEMCA is due to a controlled change in catalyst film work function caused by an electrochemicallydriven spillover of ions onto the catalyst surface. In fact it was shown both theoretically (4,5,9) and experimentally by means of a Kelvin probe (1,lO) that for solid electrolytecells where the catalyst and reference electrode are made of the same metal it is Ae@ = eAVW where e@ is the work function of the gas
21 40
exposed, i.e. catalytically active, electrode surface and,V is the ohmic-drop-free catalyst (W) potential with respect to a reference (R) electrode. The change in catalyst work function caused by polarizing the metal-solid electrolyte interface, affects the strength of chemisorptive bonds on the gas exposed catalyst surface and that causes the observed dramatic changes in catalytic activity. 2 . EXPERIMENTAL The continuous flow atmospheric pressure CSTR-type reactor used in NEMCA studies has been described previously (2-9)along with details of the analysis unit and the three electrode configuration used. Porous catalyst films used in NEMCA studies are typically 5 to lOpm thick, have superficial surface areas of 1 to 2 cm2 and true surface areas of 50 to 500 cm2. Details of catalyst preparation and characterizationcan be found elsewhere (2-9).
3 . GENERAL FEATURES OF NEMCA There are three common features of NEMCA which have bten observed in all the systems studied so far. (ICatalytic ) rates depend exponentially on the ohmic drop free catalyst potential VWRor, equivalently, on the work function ecP of the gas-exposed catalyst surface: h(r/rO) = OE(vWR-v~)/kbT= CXC(@-a*)/kbT (1) where kb is Bolzmann's constant and a, V&R and @* are catalyst- and reaction specificic constants. Activation energies of catalytic reactions arc also generally found to change linearly with e0. An example is given on Fig. 1 for the cases of 5H4and CH, oxidation on Pt.
A ie 9 I , eV
Fi ure.1. Effect of catal st work function on the ackvatton ener and c a h tic rate enhancement ratio r/r, for (a) an CHq (b).
&
2
ZfG / I o
Figure 2. Comparison of predicted and measured enhancement factor A values for the catalytic reactions already found to exhibit the NEMCA effect.
21 41
I
(n)The order of magnitude of the absolute value A1 of the enhanceinent factor A defined as the ratio of the catalytic rate change to the corresponding rate of ion transfer through the solid electrolyte can be estimated for any catalytic reaction from:
IA1 = 2FrJo
(2)
where I, is the exchange current of the catalyst-solid electrolyte interface. As shown on Figure 2 there is an excellent agreement between Equation (2) and experiment over at least five orders of magnitude. Equation (2) underlines the fact that in order to obtain a strong nonfaradaic rate enhancement, i.e., [A[>>1 one has to use a highly polarizable, i.e. low I,, catalyst-solid electrolyte interface. (111) The catalytic rate relaxation time constant z during galvanostatic transients is comparable to the time required for a certain ion coverage to be established on the catalyst surface. This implies that the NEMCA effect is not an electrocatalyticeffect localized at the threephase boundaries (TPB)but a catalytic effect related with the enthe catalyst surface. 4. THE ORIGIN OF NEMCA
The experimental observations of all NEMCA studies have been adequately explained by a semiquantitative model based on the macroscopically uniform change of the catalyst work function due to an elecmhemically induced spillover of ions onto the catalyst surface and on the concomitant change in the strength of chemisorptive bonds of covalently bonded reactants and reaction intermediates (4-9). If the rls of a catalytic reaction involves cleavage of a metal-adsorbate bond it is expected that the rate of this reaction will change exponentially with changing heat of adsorption of the
@@.
CATALYSIS-ELECTROCHEMISTRY Figure 3. Effect of catalyst work function on the selectivity to H2CO during m 0 H oxidation on pt, Inlet conditions: PcH~oH:O.~W~, Po,=19 Wa.
Figure 4. Origin of NEMCA effect: U n polaxization of the metal-solid electrolyte in& the catalyst potential chanrs by m d the catalyst surface work func on by V due toion Spillover, both the.catalytic rate r a J 7 q elecmaplyhc rate V2F (I is the current and F is Faraday s constant) are exponential1 dependent on catalyst work function and potentidchange.
21 42 adsorbate (-AH,) and consequently with AeO, if one takes into account the semitheoretical linear relation between A(-AH,) and m,proposed by Boudard (12). This explains Eq. (1) and the generally observed quasilinear change in activation energy with eV, or e 4 (Fig. 1). Different catalytic reactions are influenced to a different degree by changing catalyst potential (Eq. 1) and this implies that the NEMCA effect can also influence the selectivity to a specific product. This is shown on Fig. 3 for the case of methanol oxidation on Pt (7). Equation (2) can be explained on the basis of Fig. 4 by noting that both rho and I/Io are exponentiallydependent on Ae@hT=eAVw&T. Also the observation that NEMCA is due to ion spillover in conjunction with the fact that the formation of the spillover dipoles at the three-phase-boundaries at high activation overpotential (and not their surface diffusion on the catalyst surface) is the rate limiting step for the ion-compensatingcharge spillover, provides a direct explanation for the third of the general features of NEMCA.
5. CONCLUSIONS Solid electrolytes can be used as ion donors or acceptors to control the work function of metal catalysts and to promote in this way in situ catalyst surfaces, inducing dramatic and reversible changes in catalytic activity and selectivity. The use of NEMCA, i.e., of in situ electrochemical promotion of catalyst surfaces, allows for new areas of surface catalytic chemistry to be explored and could lead to technological applications by influencing catalyst performance in desirable directions.
Acknowledgements We wish to thank the EEC Non-nuclear Energy, JOULE and SCIENCE Programmes, the VW Foundation of Germany and the Hellenic Secretariat of Research and Technology for financial support of NEMCA studies during the last few years.
6. REFERENCES C.G. Vayenas, S. Bebelis and S. Ladas, Nature (London) 343 (1990) 625 I.V. Yentekakis and C.G. Vayenas, J. Catal. ill(1988) 170 C.G. Vayenas, S. Bebelis and S. Neophytides, J. Phys. Chem. p2 (1988) 5083 S. Bebelis and C.G. Vayenas, J. Catal. JU(1989) 125 S. Neophytides and C.G. Vayenas. J. Catal. lljl (1989) 147 C.G. Vayenas, S. Bebelis, I.V. Yentekakis, P. Tsiakaras and H. Karasali, Platinum Metals Rev. 3 (1990) 122 [7] C.G. Vayenas and S. Neophytides, J. Catal. 122 (1991) 645 (1991) 415 [8] C.G. Vayenas, S. Bebelis and M.Despotopoulou, J. Catal. [9] C.G. Vayenas, S. Bebelis. I.V. Yentekakis and H.-G. Lintz, Catalysis Today, (1992) 303-442 [lo] S. Ladas, S. Bebelis and C.G. Vayenas, Surf. Sci. 251/252 (1991) 1062 (1990) 592 [ 113J. Pritchard, Nature (London) [ 121M. Boudart. J. Am. Chem. Soc. 24 (1952) 3556
[ 11 [2] [3] [4] [5] [6]
Guni, L et al. (Editors), New Frontiers in Caralysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
THEORY AND EXPERIhlENT OF PHOTOACTIVATION OF CATALYTIC SITES
AND ACTIVE SITE-SUPPORTINTERACTIONS 0.Novaroa and J. Garcia-Prietob %stituto de Fisica, UNAM, A.P. 20-364, Mexico OlOOO, D.F., Mexico bhtituto Mexican0 del Petroleo, A.P. 14-805, Mexico 07730, D.F., Mexico
Abstract A close collaboration between theory and experiment, the first in the form of precise quantum chemical calculations the second consisting on matrix isolation lowtemperature experiments supported by multi-spectroscopic techniques, allows to understand the site-support interactions and the power of photecatalytic techniques. 1. INTRODUCTION
It has been some time since the first evidence correlating the studies of the reaction between molecules and isolated metal atoms and the chemisorption data on heterogeneous catalyst surfaces, was published [l]. Our proposal [2] that the active sites on metallic surfaces are related to metal atom excited states added a new dimension to the problem. The clue in this type of studies is the close interaction between theory and experiment. The use of low-temperature techniques make the experimental results to be accessible to direct comparison with quantum chemical calculations of the highest precision. In both cases the fundamental interactions between metal and molecule are isolated from entropy, temperature or transport effects. This allows to study the essential features of the chemisorptive bond. Furthermore the effects introduced by different supports on the active sites can be clearly distinguished. Finally the metal atom states involved in the catalytic phenomenon can be predicted theoretically and confirmed by photo-excitation experiments. All this will be exemplified here. 2. METHOD
The methods are two-fold; experimental and theoretical. The former consist of matrix isolation techniques supported by infrared, UV-visible and ESR spectroscopies. Wavelength selective photochemistry, thermal annealing, isotope substitution of reactants and alternative use of different supports, are all used [3]. The theoretical studies include full-CI calculations on hundreds of geometrical conformations for the reactant-catalyst and catalyst-product system. The method uses linear combinations of atomic orbitals followed by a configuration-interaction scheme including of the order of one million determinants [2]. All of the quantum electronic states of the catalyst
21 44
that might conceivably be reached by the photoexcitation in the matrix-isolation experiments, are included in the calculations. The potential energy surfaces and reaction pathways are thusly obtained and interpreted to compare with the experimental results.
3. RESULTS Let us start with the photolysis of copper atoms in methane matrices, a system of considerable practical interest (the activation of natural gas having a great potential as an important energy source) and a relatively long history [3]. The reaction of Cu with methane occurs only after the system is subjected to a light source of precisely the energy (320 nm) to send Cu from its 2S ground state to its second excited state ( 2 P ) . This is confirmed by UV-visible absorption spectra of the photolysis. Infrared spectra on the other hand (Fig. l a ) show the formation of a metastable intermediate HCuCH3 which by a second-photon process leads to the final products: CuH+CHs and a smaller fraction of H+CuCH3. The time delay in the formation of the products and the need for a second-photon process for its formation can be understood from Fig. lb. The reaction pathway depicted there explains the following: the ground state of Cu 2S is energetically incapable of activating CHI but the 2P state, reached by the first-photon process readily does so. The deep well of the HCuCH3 intermediate evidently explains
CUH
1
19w
1700
'
1203 IW IWO
"
6W
FREQUENCY [ cm-'I
llo'o
. 80.0
CuI%I+CH, 3ZOnm
HtCuCH3
40.0
00
Cu@I+CH4 Cd SIKH
PRIMARY PHOTOLYSIS
HCutCH, SECONDARY PHOTOLYSIS
PRODUCTS
(b) Figure 1. Two-photon process of activation of methane by cooper. a) Infrared spectrum, b) reaction pathway.
(b) Figure 2. One-photon process for the Cu+H2 reaction. a) ESR spectra before and after photoactivation, b) reaction pathway.
2145 its great stability, and the need of a second photon to reach the products. The most abundant products are reached without any activation barrier, but the others are not only more endothermic but show a high activation barrier, explaining the much larger (80:l) abundance of CuH and CHJ. Reasons of space preclude us to give here more details which will appear elsewhere [4]. Let us now talk about experiments [3] and calculations [2] concerning the cocondensation of Cu atoms and hydrogen molecules at 12 K over diverse matrices. When the matrix used is methane, a competition is established between the Cu+H2 and Cu+CH*. This is not the case when noble gas matrices are used. Again the reaction of the ground-state state 2S of Cu is not observed (see the left-hand side of Fig. 2a where ESR spectra of different Cu isotopes are depicted) and the presence of hydrogen atoms (right-hand side of Fig. 2a, again with different isotopes, H, D) occurs only after irradiating with 320 nm=85 kcal/mol photons, thus suggesti that the second excited 2P state or the first excited 2D+2P and considering that 2S-i"gzD is a forbidden transition. The precise quantum mechanical calculations [2] however, lead to Fig. 2b where it is shown that it is in fact the state 2P which, as in the case of methane, activates the hydrogen (or deuterium) molecule. An interesting fact of Fig. 2b is that even though the 2D and 2S potential energy curves originally are repulsive to H2, after a series of avoided crossing with the descendinpi 2P+H2 curve, they become attractive. Therefore, even if the 2P state should decay to D or 2S, the energy gained by the photoexcitation would be converted to kinetic energy for the Cu+H2 reaction and the activation barriers of the two lower reaction paths in Fig. 2b could easily be overcome. Notice also that the HCuH intermediate is llpt stable in this energy conditions, in contrast with the HCuCH3 intermediate in Fig. lb. This had already been remarked in the matrix isolation experiments [3]. This seems to be the explanation of why the Cu*+H2 reaction readily occurs in noble gas matrices but is greatly impeded in methane matrices. This being a one-photon process, the Cu is trapped in the methane matrix and cannot attack Ha.
34.0 42.0
t .".I
(b)
Figure 3. 9l and 92 configurations of PQH2. a) Infrared spectrum, b) reaction pathway.
21 46 As a final example of the interaction between theory and experiment let us consider the Pd+H2 reaction, for which quantum mechanical calculations predicted [4] that two configurations 9' and q2 existed. Even if the interaction of two closed-shell systems as Pd(dlo) and H2 (lo,,) at the low temperature of lOoK might seem unexpected, the matrix isolation experiments using H and D isotopes and different supports (Kr or Xe led to immediate confirmation, as shown in Fig. 3. In effect the two structures q1 and q for which the attractive curves of Fig. 3b were calculated, are observed in the infrared spectra of Fig. 3a. This in fact is the first case of ligand free endcm and side-on molecular complexes in the literature. The end-on (Pd-8' HD) case where hydrogen molecules with one deuterium substituting an H atom shows two peaks, one for the Pd-H-D and one for the Pd-D-H linear complexes. The results of Fig. 3a correspond to,Kr, if a Xe support is used only the q2 is observed imp1 ing that for the latter the steric hindrance may be smaller and that the shift from q to q2 may be downhill in energy, a fact recently confirmed [4]theoretically. Also this is an indication that the energy difference between the q1 and q2 complexes must be of the order of magnitude of matrix interactions, a fact quite evident from Fig. 3b. This explains that a small matrix change (Xe for Kr) disappears the second peak in Fig. 3a.
a
r
4. CONCLUSIONS We have shown a few selected examples of the combined use of quantum mechanical and matrix isolation techniques for the interaction of small molecules with metals. The very rich interaction between theory and experiment ranges from the theoretical explanation or reinterpretation of matrix isolation data, to the confirmation of a quantum chemical prediction by the experiment. The present results on the photoactivation of metal atoms are of course deeply associated to the new field of photocatalysis. We have shown low-temperature matrix-isolation experiments free of masking effects present in industrial catalysis (bulk and solvent hindrance, temperature and transport effects, etc.). The choice of the light source is here guided by extremely precise theoretical calculations on the metal atom excited states. The interaction of the catalyst with the support is of great actuality. A few examples of support effects in the matrix isolation experiments were mentioned in the previous section. We hope that this few selected examples serve to draw the attention of people involved in the practical aspects of catalysis towards the very fertile ground of isolated metal atom-adsorbate interactions and to the wealth of experimental and theoretical data that combined low-temperature experiments and quantum-theoretical calculations can provide. 5. REFERENCES
1. 2.
3. 4.
G. A. Ozin, Acc. Chem. Res. 10 (1977) 21. J. Garcia-Prieto, M. E. Ruiz and 0. Novaro, J. Am. Chem. SOC. 107 (1985) 5635 and references therein. J. M. Parnis, S. A. Mitchell, J. Garcia-Prieto and G. A. Ozin, J. Am. Chem. SOC. 107 (1985) 8169 and references therein. 0. Novaro and co-workem, to be published.
Guczi, L et al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1% Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights rcserved
PHOTOCATALYTIC AND PHYSICOCHEMICAL STUDIES ON MmALLISED TlTANIA SYSTEMS B. Viswanathan, U.D. Mary and R. P. Viswanath Department of Chemistry, Indian Institute of Technology, Madras 600 036, India
Abstract The pho toca ta.I yt i c a.ot i v i ty for dehydrogenation of met hano 1 on metallised titania systems was found to be dependent on the pretrea.tment conditions employed. 1 t is shown that the surface metal dispersion or agglomeration which effectively reflects the controlled by basic thermodynamic catalytic a.ctivity is parameters I ike the 6UPfaue f e e etiepay, the heat q u a t i t i t i e 5 1 i k a hea.t o f sublima.tion of the metal or the hrot of formation of the oxides.
1.
I NTRODUCT I ON
Various optimisation and preparation procedures have been for use in employed to obtain semiconductor catalysts photoassfsted processes C l - 4 1 . The physics of the interface especial ly that between the semiconductor and metal often controls the oxidation reduction processes that take place on mod-ified semiconductor photocatalysts 15-63. The present study Pocurses o n these aspects with respect to metal lised titania sys tems.
2. EXPER I MENTAL The catalysts na.mely metallised titania systems were prepared by impregnation method. The photocatalytic dehydrogeimtion of methanol was followed in a closed system b y theestimation of formed formaldehyde spectrophotometrlcally 171. The diffuse ref 1ecta.ance spectra of the samples were recorded using a varian spectrophotometer and the photoelectron spectra were recorded using a n ESCALAB Mark I I unit with M g Ka radia.tion.
3.RESULTS AND DISCUSSION The essential aspects of the results generated are: The photocatalytic autiuity uf the metallized tltanla systems (where metal is Pt,Pd,Rh,and R u ) was found to depend on the nature o f the pretreatment employed, namely optimum aotivity
21 48 could be achieved i n the case of Pt or Pd loaded titania systems after pretrea.tment of the catalyst system in oxygen at 673 K followed by treatment in hydrogen at 673 K (OH) while R h and Ru loaded systems exhibited higher activity after prstrea,tments in nitrogen followed by hydrogen treatment at 673 K for 1 2 hrs.(NH or H). Typical data on the photoca.talytic activity for dehydrogenation o f methanol o f Pt/Ti02 a.nd Ru/Ti02 systems are The order of given in Table. 1 f o r different metal loadings.
Table 1 Photocatalytic activity of P t / l I
OJ. and R u / T i O 2
I
wt z of Pt in Pt/TiO .-.-..--2-,
H,IHCHOI'
in mic
W t X of
I
ayatercr'
HdHCHOI' in micromol/h
0.04 0.08 0.22 0.38 0.8Q 1.48
0.12 0.28 0.60 0.68 0.87 1.00 1.65 2.00 1
I
I
1
1
ml of methanol is i r r a z ed in the presence catalyst for 1 h at 303 K ( a ) HCHO formed are given 02/H2 treatement in O2 and then in H2 at 673 K for 1 2 treatment in N2 and then Hg at 673 K for 12 h and ( d ) in H2 a t 673 K for 1 2 h.
1
of 0.1 g o in brackets h, ( c ) N2/H2 H 2 treatment
activity of metallised titania systems after OH treatment is P t > P d > R h > R uwhile in the case o f NH and H treatments the activity follows the order Rh>Ru>Pt>Pd. The reactivity order for the OH treated system i s in accordance with the Fermi level positions of the metals and the redox potential of HCHO/CH$H couple in The fact that the deposited metal particles aqueous phase 1 8 1 . a f t e r J certain metal loading level function as recombination centres for the photogenerated charge carriers a s well a s act as active sites f o r the back reaction C Q l . The metallisatfon of titania is found to Increase the rate of hydrogen evolution thus The Fermi indicating that metals form ohmic contacts C l O I . l e v e l of the metal 1 ised aemiuonduutor is determined b y the nature I J ~the metal. As the conduction band 8nd Fermi level are close to each other, the semiconductor bands w i l l be lowered in the is created in the contact zone and a band asymmetry semiconducotr. This leads to a n electrical field which separate the photogenerated electron-hole pairs. Then the electrons flow to the metal thus favouring the separation of the oxidation end ruduction sites and thus accounting for the enhancement of photocatalytic activity. Typlcal diffuse reflectance sprectra obtained for Pd/titania system are given i n F i g . 1. K i w i and Cratzel t 1 1 1 as we1 I a s Kiwi s t a l El83 h a v e alfio k-eported similar diffuse reflectance spectra for Pt loaded titania systems prepared by impregnation and
21 49
Fi ure 1. Diffuse reflectance spectra of pure Ti0 and Fd/TiOZ syfems prepared b y various treatments ( 1 ) TiOZ, $2) (0.36wtX Pd/TiOZ)OH, ( 3 ) (0.36wtX Pd/Ti02)NH, ( 4 ) (0.36wtX Pd/TiOg)H, ( 5 ) (1.5wtW Pd/TiOZ)OH, ( 8 ) (1.5wtX Pd/TiOZ)NH, ( 7 )(l.5wtW Pd/TiOZ)H. exchange -impregnation methods. The absorption at higher wavelengths have b e e n attributed to the uniform dispersion of the metal particles o f optlmum size C71. In order to account for the reactivity changes w i t h pretreatment, the surface metal concentrations were determined by XPS. These data a r e given in Table. 2. It is seen that Pt and Pd loaded systems showed a n
Metal loaded
(nM/nTi of treated catalyst) X i 0 0 (nM/nTi of untreated - catalyst)
-. Pt Pd Rh Ru
OR
OW
113
82 97 66 16
141 72
14
-OHB 34
-
74 6
(nM/nTi)* X 100 (nH/nTi I e
HH
OHM
HH
18
68
38 85 60 40
6 12 56
-
30 120
I
enrichemnt whereas Rh and Ru loaded systems exhibited a n impoverishment on the surface after oxidation. One of the reasons for this behaviour is the valuer oP the surface free energy of the oxides of the noble metals in relation to that of The validity of this argument is titania (725 ergslcm 2 ) . supported by the fact that after treatment In hydrogen the surfa.ca metal concentratlons in the systems are reduced considerably as the surface free energy of these metals are in the range of 2100-3050 erg/cmz. The migration of the metal particles Into the bulk of the semiconductor in accordance with the surface free energy values is also seen from the metal concentration values obtained after sputtering the systems after hydrogen treatment. However, i t should be remarked that these
arguments are not unique and similar observations can also be accounted for in terms of the values of heat of formation of the oxides, heat of sublimation and heat of vapourisation of metals. The 5lJrfa.ce metal concentrations determined b y XPS and the activity data are given i n Table.3. I t is seen that the activity Table 3 T h e s u r f a c e metal concentration determined by photocatalytic activity data for various noble systems
XPS metal
and the loaded
I
Pretreatments
Metal ( M ) loaded on Ti02 (wt % ) nM/nTi ~
Pt(1.65%) Pd ( 1.50%) Ru( 1.46%) Rh(O.82%)
----.
0.3500 0.1827 0.0228 0.1350
i
Rate of H2 evo lut inn (-N m o l h-')
nM/nTi
132 79 19 45
0.0186 0.0103 0.1967 0.0215
Rate of H2 evo I u t ion ( M m o l h-l)
"-___--__ 19 9 51 39
has direct correlation with metal oonoentrations for Pt,Pd,and I t has been proposed that metal clusters of appropriate size may control the activity. I t is probable that i n the optimum metal 1oa.ded systems, the number of clusters of appropriate size may be maximum thus accounting for the observed activity order. The size of the metal clusters though could be guessed from the distinct emissions in the valence band region in XPS, it is proposed that a distribution of metal clusters of varying sizes are always formed on the surface thus preventing the identification of the enact size responsible for optimlJm activity.
Ru.
4. REFERENCES
1 2 3 4
5 6
7 B
9
10 11
12
N.Gratra1, red), Energy Resources through Photochemistry and Catalysis, Academic Press, New York, 1983. I. A i t - Ichou, H. Formenti, B.Pommier and S. J. Teichner, J.Catal., 91 (1985) 293. J.Kiwi and M.Gratzel, J.Mol.Catal., 39 (1987) 63. H.Bensi, M.Curtis and P.Studer, J.Catal., 10 (1968) 328. A.J.Nozik,Appl.Phys.Lett., 30 (1977) 567. G.A.Hope and A. J.Bard, J.Phys.Chem., 87 (1983) 1979. B.Viswanathan, U.D.Mary and R.P.Viswanath, Indian J.Chem., 29A, (1990) 1138. G . @ l i l a z t o and S.Caroli, Tables of Electrode Potentials, John Uiley and Sons, N e w York, 1978. H.Courbon, J.M.Herrmann and P.Pichat, J.Phys.Chem., 88 (1984) 6210. D.E.Apnes and A. He1 lrr, J. Phys. Chem., 87 (1983) 4919. J.Kiwi and H . C r a t ~ r l , J.Phye.Chrm., B e (1986) 1302. P.Albers, K.Seibold, A.J.McEvoy and J.Kiwi, J.Phys.Chem., 93 (1989) 1510.
Guni, L ei al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevicr Science Publishers B.V.All righb resewed
HETEROGENEOUS PHOTOCATALYSIS: MECHANISTIC CONSIDERATIONS OF PHOTOCATALYTIC REDUClYONS AND PHOTOCATALYTIC OXIDATIONS ON SEMICONDUCTOR OXIDE SURFACES R.I. BickIeya, L. Palmismob, M.Schiavellob and A. Sclafanib Wepartment of Chemical Engineering, Chemistry and Chemical Technology, University of Bradford, BD7 1DP Bradford, United Kingdom bDipartimentodi Ingegneria Chimica dei Processi e dei Materiali, Univesita di Palerrno, 901% Palerrno, Italy
Abstract
In the present paper the roles of the interfaces formed in gas-solid and in gas-liquid-solid regimes, during two photocatalytic processes are reported. The photoreduction of dinitrogen in presence of H20 vapour and the photooxidation of phenol in aqueous dispersions have been examined at interfaces formed with powdered polycrystalline Ti02 (anatase) and with irondoped titanium dioxide respectively.
1.INTRODUCTION
Photocatalysis is a catalytic process in which the electromagnetic radiation plays a role at least in one of the steps of the reaction. In the present paper we refer to heterogeneous processes involving oxide semiconductors in which we report the photocatalytic behaviour of two polycrystalline systems, one of pure TiO2, the other the FexO -Ti02 system both used for performing two photocatalytic reahions : namely the photoreduction of N in gas-solid regime and the photodegradation of phenol in gas-liquid-solid regime. Several aspects are of interest, but, principally, the roles of the interfaces, for the two photoreactions, on the photoreactivity will be discussed.
21 52 2.EXPERIMENTAL 2.1.Photocatalyst
preparation
Coprecipitation and wet impregnation methods were used for preparing two series of iron-doped titanium dioxide powders. The nominal iron concentration in the specimens designated as TF/CP and TF/IM, respectively, ranged from 0.2 to 10 atom 'b. For preparing the TF/CP powders, iron and titanium hydroxides were coprecipitated from an aqueous soiytion of with an Tic13 containing the required nominal quantity of Fe aqueous ammonia solution. The solids were filtered and washed until disappearence of C1' in the effluent liquid, dried at 393 K for 24 h and then fired in air at 823 K for 24 h. A "home prepared" Ti02 (hereafter named "TD hp" 1 was obtained exactly in the same way as the TF/CP samples cited, but without Fe. For preparing the TF/IM powders, the "TD hp" sample was added to an aqueous solution of ferric nitrate containing the required nominal amounts of iron; after standing at room temperature for 48 h, the liquid phase was evaporated at 393 K for 24 h, and the dried solids fired in air at 823 K for 2 4 h. 2.2.Dinitrogen
photoreduction
The reactivity runs were performed in a continuous flow, fixed-be photoreactor made of a Pyrex tube having a volume of ca. 5 cm4 .filled with 1 g of the powder which was vertically positioned inside a water bath and thermostatted at 313 K. The reactor was irradiated by a 160 W high pressure Hg-lamp (OSRAM H W L ) . Dinitrogen (99.95% purity) was bubbled through distilled water at room temperature and fed to the reactor. The gas leaving the reactor was bubbled through an aqueous solution 0.01 N H C 1 in order to determine the quantity of ammonia evolved, which was analyzed Using the standard indophenol colorimetric method [ l l . The runs usually lasted between 1 and 3 hours, but some runs were prolonged for > 2 0 hours for determining any possible decline in activity. At the end of the photoreactivity runs the reactor was purged with N at 573 K in order to desorb and to analyze the ammonia which iad remained adsorbed on the photocatalyst at room temperature. 2.3.Phenol
photo-oxidation
A pyrex batch-reactor with an immersed lamp (500 W high pressure mercury lamp, Helios Italquartz B-type) was used for all of these pho ocatalytic experiments. The volume of the Cooling water was circulated through a reactor was 2 dm pyrex glass jacket surrounding the lamp to avoid it becoming overheated, to warm the reactor to the temperature of ca. 309 f l K and to remove the IR component of the incident beam as well as any radiation below 300 nm. The annular reac-
f.
21 53
tor had five ports at its top; for sampling, gas inlet and outlet, pH and temperature measurement. The pH of the solution was adjusted by adding NaOH or H2SO4 as necessary. 'For all the runs 1 g ~ d m ' ~of catalyst powder was used. The photodegradation experiments lasted 1.5 hours and the phenol concentration in the supernatant liquid was measured by means of a standard colorimetric method [21. In order to determine quantitatively the extent of phenol mineralization, the outlet gas was bubbled through a saturated barium hydroxide solution to trap C02, during some selected reactivity runs.
3 -RESULTS
3-1-Photocatalyst structural charace-r i z a t ion
Solid state chemistry fjpdies have shown that the specimens to 1% Fe by weight are solid solutions in containing u which the Fey+ is dispersed in the lattice of TiO2. Depending upon ( i ) the prepara5kon method (impregnation or coprecipitation) and (ii) the Fe ion content, these sp imens differ in several respects: the penetration depth of Fey' into the lattices, the ratiy of anatase/rutile, the mechanisms of accommodating ions, etc. The specimens having higher than ca. 1% FefQe weight accomodate any excess of iron, in addition to the formation of solid solution, as iron oxides (Fe2O3 was revealed by x-ray in some specimens) and/or Fe2Ti.05 as minute particles or small aggregates, which appear, at the surfaces of the particles of solid solution.
E!
3 - 2 - D i n i t r o g a n photoreduction
Minimal activities for pure Ti0 (anatase and rutile), u-Fe203 and Fe2Ti05 were detected. Tze results have clearly shown that only the solid solution specimens are photo-active for the photo-transformation of N2 to ammonia. Differences between the photoactivities of these specimens, are obse ved ranging the production t f NH31 from 1 . 5 1 0-8 g s f g-i to about 0.7*10' 0g.s *g-l, depending on the specimen preparation. 0
-
3,3-Phenol photo-oxidation
The results showed that all the TF specimens, the pure a Fez03 and Fe Ti05 display similar activities to Ti02 (anatase) or lower. It was also shown that the photoactivity is pH dependent.
4,DISCUSSION
A close examination of the results reveals several points that are worthy of discussion. It is3$learly evident that there are contrasting behaviours of the Fe ions, dispersed in Ti0 : the resulting photocatalysts indeed, are photocatalyticalfy active for the dinitrogen reduction whereas for the phenol photodegradation they display a similar activity (or lower) to the host matrix, It must be observ:Ao&at Ti02, from the thermodynamic point of view, is able to perform the photoreduction of N2 [ 3 1 . The fact that the reaction does not occur on Ti02 is a clear indication of kinetics hindrance in, for example, the fficulty of the charge separation step. The addition of Fe4' ions appears to enhance the formation of a space charge region which improves the charge separation step. Moreover Fe3+ ions play an essential role in some of the NH3 photoproduction steps, as recently reported in the literature [ 4 1 . The addition of Fe3+ is without influence in the liquid-solid phenol photodegradation reaction possibly due to the presence of an electrolytic solution which is sufficient to create the space charge region [ 5 1 .
5,ACKNOWLEDGMENT
L.P., M.S. and A.S. wish to thank the Minister0 della Universita e della Ricerca Scientifica e Tecnologica for financial support.
6,REFERENCES
1 J.M. Henry, M.M. Cannon and T.A. Winkelmann, Clinical Chemistry: Principles and Techniques, Harper and ROW, New York, 1978. 2 H.J. Taras, A.E. Greenberg, R.D. Hoak and M.C. Rand, Eds., APHA-AWWA-WPCF, Standard Methods for the Examination of Water and Wastewater, 13th Edn., Washington D.C., 1971. 3 A. Sclafani, L. Palmisano and M. Schiavello, J. Phys. Chem. 94 (1990) 829. 4 J. Soria, J.C. Conesa, V. Auqugliaro, L. Palmisano, M. Schiavello and A. Sclafani, J. Phys. Chem. 9 5 (1991) 274. 5 L. Palmisano, V. Augugl'iaro, A . Sclafani and M. Schiavello, J. Phys. Chem. 92 (1988) 6710.
Guczi, . L a ul. @ditors), New Frontiers in Ckalysb Proceedings of the 10th International Congress on Catalysis,19-24 July, 1992, Budapest,Hungary Q 1993 Elsevier Science Publishers B.V.All right?nserved
-
De-NOX-INGPHOTOCATALYSIS EXCITED STATES OF COPPER IONS ANCHORED ONTO ZEOLITE AND THEIR ROLE IN PHOTOCATALYTIC DECOMPOSITION OF NO AT 275 K M. Anpd: T. Nomura", Y.ShwyaO, M. Cheb, D. Murphyf and E. GiamelloC Qepartment of Applied Chemistry, University of Osaka Prefecture, Mozu-umemachi, Sakai, Osaka 591, Japan bLaboratoire de Reactivite de Surface et Structure, Universite P. et M. Curie, URA 1106, CNRS,4 Place Jussieu, Tour 54,75252 Paris Cedex 05, France CDipartimentodi Chimica Inorganica, Chimica Fisica e Chimica dei Materiati, Universita' di Torino, Via Pietro Giuria 7, 10125 Torino, Italy
Abstrgt Cu ions anchored onto ZSM-5 zeolite prepared by an ion-exchange method were reduced to Cu' by progressive evacuation at high temperatures. The Cu+/ZSM-5 catalyst decomposed NO molecules photocatalytically and stoichiometrically into N Dynamic photoluminescence, ESR, and IR studies of the excite8 and O2 at 275 K. states of Cu ions and the adsorbed species of NO on the catalyst have indicated that the Cu+ ions located as isolated Cu' monomer species in ZSM-5 and NO molecules are adsorbed on them to form Cu+--NO adduct species. From these results, it has been clarified that a local chargeteparation, i. e., an electron transfer from the excited state of the Cu' (3d 4s1) to an anti-bonding fl orbital of NO plays a significant role to initiate the decomposition of NO.
Reducing global air pollution caused by NOx, as well as C02 and SOX, is currently an urgent and demanding challenge. Ion-exchanged Cu+/zeolite catalysts have attracted a great deal of a ention as potential catalysts for direct decomposition of NOx into N and O 2 j 5 However, they operate only at temperatures above 673 K. On the otier hand, utilization of photocatalytic proces s in gassolid systems also seems promising and potentially of vital importance!' We have reported that Cu2' ions supp rted on Si02 prepared by an ion-exchange method are $+ reduced to Cu' ions when a Cu /Si02 sample is evacuated at temperatures above 573 K, and thus prepared Cu+/Si02 catalysts d compose NO photocatalytically and stoichiometrically into NZ and O2 at 275 K?,e, In the present work we report the characteristics of the excited states of the Cu' ions anchored onto ZSM-5 and their roles in the photocatalytic decomposition of NO into N2 and O2 on the Cu'/ZSM-5 catalyst at 275 K. Particular emphasis is given to understanding the reaction mechanism of the photocatalytic decomposition of NO at a molecular scale.
ation of catalysts Two Cu /ZSM-5 samples ( W A 1 ratio = 23.3) having different copper loadings were prepared by ion-exchange with an aqueous (CU(NH~)~)~' solution. After 2.1. R e
T+
21 56 washing and drying i n air, the copper loading of the samples were determined by means of an inductively coupled plasma emission spectrometer. The copper contents were 1.9 and 0.13 wt%. 2. 2. Apparatue and procedurae P r i o r t o t h e s p e c t r a l measurements and p h o t o - r e a c t i o n s , t h e s a m p l e s were pretreated a s follows: they were degassed a t 673 K f o r 1 h, heated at t h e same temperature under 20 Torr O2 f o r 1 h, and then f i n a l l y evacuated a t the desired temperature. F'hotoluminescence of t h e c a t a l y s t and t h e l i f e t i m e s were recorded a t 77 and 293 K with a Shimadzu RF-501 spectrofluorophotometer and an apparatus f o r t h e l i f e t i m e measurement, respectively. ESR s p e c t r a were recorded on a JES-RE2X and a Varian E-109 s p e c t r o m e t e r (X-band) a t 77 and 293 K. I R s p e c t r a were recorded on a Bruker IFS 11% spectrometer a t 298 K. Photo-reactions were c a r r i e d out a t 275 K (a1280 nm). Reaction products were analyzed by gas chromatography and maes-spectrometry.
3. RESULTS AND DISCUSSION 3.1. p C a 1 reduction of cu2+ t o cu+ Cu /ZSM-5 samples exhi i t e d a x i a l , broad and scarcely resolved ESR s p e c t r a due t o the typical hydrated ions i n z e o l i t i c frameworks. Thermovacuum treatment a t increasing temperature,f+ l e d t o a change i n t h e s p e c t r a l p r o f i l e du t o the appearance of d i s t i n c t Cu species. The observed spectrum f o r t h e Cu /ZSM-5 sample (three species with s l i g h t l y d i f f e r e n t g and A values) which was dehydrated a t 873 K was t h e same a t se previously reported f o r t h e Cu2+/ZSM-5 evacuated a t A s shown i n F i g u r e 1 (curve: a ) , i n c r e a s i n g t h e t h e same t e m p e r a t u r e . evacuation temperature of t h e amples leads t o a d r a s t i c decrease i n t h e i n t e n s i t y of the ESR s i g n a l s due t o C>+ ions with c h a n g y + i n t h e i r g values and shapes, the t o Cu+ p r o c e e d s d u r i i n d i c a t i n g t h a t t h e c h e m i c a l r e d u c t i o n of Cu progressive evacuation of the samples above 373 K. The reduction of t h e Cu" ions by e v a c u a t i o n t r e a t m e n t a t h i g h e r t2e+peratures h a s a l r e a d y been ~ ~ p ~ Although ~ the ~ r ~e d u c ~i b i l i tey of3 t h e Cu ion was d i f f e r e n t from t h a t of ZSM-5, a s i m i l a r p r o f i l e of t h e ESR !pectra of t h e chem c a l re u i o n of the Cu2+ ions was a l s o observed f o r t h e Cu /Y-zeolite and Cu /Si02. I n these stages, a colour of t h e samples c l e a r l y changed from blue t o white.
CJ+
9+
s5-pp
'+
0 273
473
673
Dcgosrlng
873 D?3 tamperalure, K
1273
Figure 1. Effects of tv+degaeeing temperature of t h e Cu2+/Zs+M-5 on t h e i n t e n s i t y of ESR signal due t o Cu (a), of photoluminescence due t o Cu and t h e r a t e of the photocatalytic decomposition of NO a t 275 K (c) on t h e Cu /ZSM-5 c a t a l y s t .
(b),
21 57 3. 2. b i t & states of the cu' ioas in ~ ~ 4 - 5 As show in Figure 1, only very weak ESR signals of the Cu2+ can be observed for the Cu R /ZSM-5 samples which were evacuated at temperatures higher than 673 K. With only these Cu+/ZSM-5 catalysts, photoluminescence becomes observable when the catalysts are excited at around 300 nm beams. Figure 2 shows a typical photoluminescence spectrum of the Cu+/ZSM-5 (spectrum: a) at 77 K, which was prepared by evacuation at 1173 K. Figure 2 also shows the typical photoluminescence spectra of the Cu+/Y-zeolite (spectrum: b), Cu+/Si02 (spectrum: c) and Cu+/Vycor glass (spectrum:d) for reference. The Cu+/ZSM-5 shows a major photoluminescence at around 440 nm with a weak shoulder at around 510 nm. As reported elsewhere, the photoluminescence at around 510 and 440 nm can be attributed to the radiative species and of d electronic transition from the excited state of the Cu+--C the isolated Cu+ monomer species in zeolite, respectively. As shown in Figure 1 (curve: b), the photoluminescence yields increase with evacuation temperature of the Cu2+/ZSM-5 sample, passing through a maximum at around 1173 K, and then decrease in the regions of much higher degassing temperatures due to the deep reduction of Cu+ to Cuo (a colour changed to red).
1-4,Ityl) +
I
IM
4w
1
I
I
I
h50
Wl
SSO
Mx)
I
6SO
Wavelength, nm Figure 2. Typical photoluminescence spectrum p,,-1.90 &I.ILV~ of the Cu+/ZSM-5 (a), Cu+/Y-zeolite (b), Cu+ Figure 3. ESR signal of the / S i 0 2 (c), and Cu+/Vycor glass (d) at 77 K, cu --No adduct species on wh ch were prepared b evacuation of the the Cu+/ZSM-5 at 77 K. Cu /ZSM-5 at 1173 K, Cu" Y-zeolite at 973 K, Cu2+/Si02 at 973 K, and Cu /Vycor glass at 973 K, respectively.
k
4+
3. 3. Interactloa of ND with the cu+/zsn-5 catalyst The addition of NO onto the Cu+/ZSM-5 catalyst led to the efficient quenching of the photoluminescence in its intensity and lifetime, its extent depending on the NO pressure. These results clearly indicate that the Cu' ions in ZSM-5 strongly interact with NO molecules not only in the ground state but al;o in the excited state. These results suggest that NO molecules react with the Cu /zeolite catalysts under UV-irradiation, even at low temperatures. After the quenching of the photoluminescence by added NO, the evacuation of the system at around 300 K led to a recovery of most of the photoluminescence intensity, suggesting that NO molecules are adsorbed on the Cu+ emitting sites by a weak interaction. The addition of NO molecul s on the Cu+/ZSM-5 leads to the appearance of the ESR signal shown in Figure 3.A The spectrum is basically understood i n terms of axial partially overlapped quartets because of the hyperfine interaction between the unpaired electron and the Cu nucleus (I = 3 1 2 ) . A further splitting to produce some triplet hype ine lines is also observable due to the interaction Spectra similar to that in Figure 3 were previously with the N nucleus (I = l).sf
21 58 o b s e r v e d on t h e Cu+/Y-zeoli+te12) and C U + / S ~ O ~ , b~e' i~n g) a t t r i b u t e d t o t h e n i t r o s y l i c a d d u c t , I. e., Cu --NO s p e c i e s . Being i n a g r e e m e n t w i t h t h e ESR observation, a dominant band i n t h e i n f r a r e d spec rum due t o t h e c o n t a c t of NO molecules w i t h t h e Cu+/ZSM-5 wa found a t 1811 cm- , w h i l e a t higher NO pressure o t h e r bands became ~ b s e r v a b l e . ~These ~ r e s u l t s c l e a r l y suggest t h a t t h e major adsorption s p e c i e s formed i n t h e e a r l y + s t a g e s of the i n t e r a c t i o n of NO w i t h t h e w i t h a very weak i n t e r a c t i o n between Cu+/zeolites is a n i t r o s y l i c adduct, Cu -NO, NO and t h e Cu+ ion, becaue they a r e e a s i l y removed by evacuation a t 280 K.
I
P h o t o c a t a l y t i c decompos+ition of No on the oU+/zsK-5 c a t a l y s t UV-irradiation of t h e Cu /ZSM-5 c a t a l y s t a t 275 K i n t h e presence of NO was found t o lead t o the p h o t o c a t a l y t i c decomposition of NO i n t o N2 and O2 w i t h a good stoichiometry (N2:02 = 1: 1). P h o t o c a t a l y t i c a c t i v i t y of t h e Cu+/ZSM-5 a t a l y s t was found t o s t r o n g l y depend on t h e evacuation temperature of t h e Cuf+/ZSM-5 s a m p l e s . A s shown i n F i g u r e 1 ( c u r v e c ) , t h e y i e l d o f t h e p h o t o c a t a l y t i c decomposition of NO i n c r e a s e s w i t h t h e evacuation temperature of t h e sample, These changes e x h i b i t a passing through a maximum a t 1173 K, and then decreases. good p a r a l l e l w i t h those of photoluminescence due t o t h e Cu' species, i n d i c a t i n g t h a t t h e presence of t h e Cu+ s p e c i e s on t h e s u r f a c e plays a s i g n i f i c a n t r o l e i n t h e p h o t o c a t a l y t i c decomposition of NO on t h e Cu+/ZSM-5. W - i r r a d i a t i o n of t h e Cu+/ZSM-5 on which NO was adsorbed l e d t o a decrease of t h e ESR s i g n a l due t o Cu+--NO adduct s p e c i e s without appearance of any new ESR s i g n a l s . After ceasing W - i r r a d i a t i o n , the ESR s i g n a l of t h e Cu+-NO recovered t o its o r i g i n a l l e v e l i n i n t e n s i t y . These r e s u l t s n o t o n l y s u g g e s t t h a t t h e Cu+--NO s p e c i e s a c t a s r e a c t i o n s p e c i e s but a l s o support t h e f a c t t h a t t h e photo-decomposition of NO molecules proceeds c a t a l y t i c a l l y . From these r e s u l t s , t h e following mechanism is p5oposed f o r t h e p h o t o c a t a l y t i c d i r e c t decomposition of NO i n t o N2 and O2 on t h e Cu ZSM 5 a t 275 K: an e l e c t r o n t r a n s f e r from t h e e x c i t e d s t a t e of t h e Cu+ (3d 9 4 s I s t a-t e ) t o a n a n t i - b o n d i n g (Ttorbital of 0 nd simultaneous e l e c t r o n t r a n s f e r from t h e bonding z o r b i t a l of NO t o the 3d 4s s t a t e of t h e Cu+ i o n occur and t h e s e l o c a l charge s e p a r a t i o n r e s u l t i n t h e weakening of t h e N-0 bond and i n i t i a t e t h e decomposition of NO. 3.4.
B a
1 2
3
4 5 6
7 8 9 10 11 12 13
M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. M i k u r i y a , and S. Kagawa, J. Chem. SOC., Chem. Commun., (1986) 1272. M. Anpo, i n 'Photochemical Conversion and Storage of Solar Energy," Us., E. P e l i z z a t t i and M. Schiavello, Kluwer Academic Publ., Dordrecht, (1991) 307. M. Anpo, T. Nomura, T. Kitao, E. Giamello, M. Che, and M. A. Fox, Chem. Lett., (1991) 889. M. Anpo, T. Nomura, T. K i t a o , E. G l a m e l l o , D. Murphy, M. Che, and M. A. Fox, Res. Chem. Intermedi., 15, (1991) 225. E. Giamello, D. Murphy, G. Magnacca, C. Morterra, Y. Shioya, T. Nomura, and M. Anpo, submitted t o J. Catal. M. W. Anderson and L. Kevan, J. Phys. Chem., 91, (1987) 4174. Y. Sendoda and Y. Ono, Z e o l i t e s , 6, (1986) 209. P. A. Jacobs, W. DeWilde, FL.Schoonheydt, J. B. Uytterhoeven, and H. Beyer, J. Chem. SOC., Faraday Trans. I , 72, (1976) 1221. P. A. Jacobs and H. Beyer, J. Phys. Chem., 83, (1979) 1174. D. H. Strome and K. K l i e r , J. Phys. Chem., 84, (1980) 981. J. D. B a r r i e , B. Dunn, G. H o l l i n g s w o r t h , and J.I. Zink, J. Phys. Chem., 93, (1989) 3958. C. Naccache, M. Che, and Y. Ben T a a r i t , Chem. Phys. Lett., 13, (1972) 109. C. C. Chao and J. H. Lunsford, J. Phys. Chem., 76, (1972) 1546.
Guai, L ef al. (Editors), New Frontiers in Cutalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1!W3 Elsevier Science Publishers B.V. All rights rcscwed
A NOVEL SERIES OF PHOTOCATALYSTS WITH AN ION-EXCHANGEABLE LAYERED STRUmURE OF NIOBATE
K Domenu, J. Yoshimur#, T.Sekin@, J. KonaW, A. Tanakaq K MaruyaD and T. Onishia aResearchLaboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan bNikon CO.,1773 Asarnizodai, Sagarnihara 228, Japan
Abstract Some ion-exchangeable niobates with a layered perovskite structure were found to be a new class of heterogeneous photocatalysts due to the unique structure of those materials, and were suggested to work as "twodimensional" photocatalysts in which catalytic reactions mainly proceed at interlayer spaces. 1. INTRow(;TIoN
Recently the authors found that GNbs017 and R b N b s O 1 7 , which ionexchangeable layered structure, showed remarkable photocatalytic activity T
0
C
0
0
0
t @Nb06,
OCa''. @K+,
0
H'
.
Figure 1. Schematic structure of KCaz N b 3 01o (a) and HCa2N b s 01o (b)
2160 f o r overall water splitting into Hz and
0 2 . In this work, another type of ion-exchangeable layered perovskite type niobates formulated as A(Mn1Nbn03nt1:A Na, K, Rb, Cs; M = La, Ca, etc. The schematic structures of the original form as well as H+-exchanged form are shown in Figure 1. Alkaline metal ions at the interlayers can be replaced by other cations. Especially in acidic solution, almost 100% of alkaline metal ions are exchanged by H+ ions. The H+-exchanged forms of those compounds are easily hydrated while the original ones are not. More interestingly, some of them showed some photoresponse under visible light irradiation.
2.
EXPERIMENTAL
Catalysts were prepared according to the previous publications [6-101. The structure of those products were confirmed by X-ray diffraction which coincided with those in the literature. The catalyst powders were 1-10pm in diameter, The photocatalytic HZ evolution from aqueous methanol solution and 02 evolution from 6000 aqueous silver nitrate solution 0 0 over these compounds was examined under irradiation of a high pressure Hg lamp ( 4 5 0 W or 5000 a Xe lamp ( 5 0 0 W) with a W cut filter ( 4 2 0 nm < ) . H+-exchange was carried out in 5 N HNOJ solution f o r 3 days at room 4000 temperature.
-
. C
.-0 3 . RESULTS AND DISCUSSION Although all layered compounds showed moderate activity for HZ evolution in those original forms, marked enhancement of the HZ evolution rate by two o r three orders of magnitude were observed by the replacement of alkaline metal ions with H i ions. Time courses of HZ evolution over KCaz N b 3 01o , K S r z N b 3 01o and their H+-exchanged forms are shown in Figure 2. Further increase of the activity by several times occurred with the loading of Pt in each case. The activities were stable and the total amounts of evolved Hz exceeded the equivalent amounts of used catalysts which confirmed the catalytic cycle
.w
3
5> 3000 a,
I" ir 2000 +4
c
3
a
1000
0 0
1 .o
0.5 Time /
h
Figure 2. Time course of Hz evolution from methanol aqueous solution. High pressure Hg lamp ( 4 5 0 W). 0 : KCazNbsOlo, 0 : H+/KCazNbsOlo, H: ~ ~ r z N b s O 1 0 0 ,: H+/KSr~Nbs010
21 61 of the reaction for every H*-exchanged form. As is mentioned above H*exchanged forms of these compounds are hydrated and the c-axis lengths corresponding to the interlayer spacing increase with hydration. The marked increase of the HZ evolution rates, therefore, is considered to be due to the migration of the reactants, i.e. HZO and W O H into the interlayer spaces. To examine this speculation, the dependence of the HZ evolution rate on the degree of H*-exchange for KCa~NbaOloin aqueous methanol solution was pressure Hg lamp (450 W) studied. The rate of HZ evolution increased drastically with H*-exchange degree of ca. 60%. From XRD measurements, the 'interlayerspace length (c-axis) was found to be expanded by ca. 0.8 A at the same degree of H*-exchange as shown in Figure 3 . In Figure 4 , UV-vis diffuse reflectance spectra of these compounds are shown. This structural change is caused by an increase of hydrated water molecules at the interlayer spaces and is responsible for the increase of the HZ evolution rate. Among these layered perovskites, some compounds such as CsRbNb3Oio and K N W showed marked photoresponse in visible light irradiation. Actually CspbzNb~Olowith intercalated R metal particles at the interlayer spaces evolved & efficiently under visible light ( > 420 nm) irradiation in methanol aqueous solution. Furthermore the same catalyst evolved 01 in silver aqueous solution, which means this material has a potential to decompose water into HZ and 01 by visible light.
.
. .. ...
15.5
ma \
L Y
m C al
.--X v)
15.0
0 V
0
20
40
60
80
100
Degree of exchange / '10
Figure 3 . Dependence of interlayer space length of KCa~Nb3010on the degree of H* exchange.
21 62 I.V
,-*,
--'
\ \
C
.-0 !0? 0.5I
VI
n
0
400
600
800
wavelength I nm
UV-DRS of Various Niobate Compounds Figure 4. W-vis d i f f u s e r e f l e c t a n c e spectra of RbPbzNbs010 and K N m O l o . REFERENCES
1. K. Domen, A. Kudo, A. Shinozaki, A . Tanaka and T . Onishi, J . Chem. SOC., Chem. Cominun., ( 1 9 8 6 ) 356. 2. A. Kudo, A . Tanaka, K . Domen, K . Maruya, K. .4ika and T. Onishi, J. Catal., 111 ( 1 9 8 8 ) 67. 3. A. Kudo, A. Tanaka, K. Domen, K . Maruya and T. Onishi, J. Catal. 120 ( 1 9 8 9 ) 337. 4. K. Sayama, A. Tanaka, K. Domen, K. Maruya and T . G n i s h i , Catal. L e t t . , 4 ( 1 9 9 0 ) 217. 5. K . Sayama, A. Tanaka, K. Domen, K. Maruya and T . Gnishi, J . Phys. Chem., 95 ( 1 9 9 1 ) 1345. 6. M. Dion, M. Ganne and M . Tournoux, Mat. Res. B u l l . , 16 ( 1 9 8 1 ) 1429. 7. M. Dion, M. Ganne and M. Tournoux, Rev. Chim. Miner., 2 1 ( 1 9 8 4 ) 92. 8. A . J . Jacobson, J . W . Johnson and J . T . Lewandowski, Inorg. Chem., 24 ( 1 9 8 5 ) 3729. 9. M. Dion, M. G a m e and M. Tournoux, Rev. Chim. Miner., 23 ( 1 9 8 6 ) 61. 10. J . Gopalakrishnan and V . Bhat, Mat. R e s . B u l l . , 2 2 ( 1 9 8 7 ) 413.
Ouczi, L et al. (Editors),New Frontiers in Colafyst
Proctcdings of the 10th Inkmatiom1 Congrcss on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights resclved
PHOTOCATALYSIS BY COORDINATIVELY UNSATURATED RHODIUM COMPLEX STABILIZEDON POROUS GLASS FOR ALKANE DEHYDROGENATION
Y.
C.Nakano, X Yamauchi andA. Morikawa
Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1Omkayama, Meguro-ku,Tokyo 152,Japan
Abstract The Vaska type complex of rhodium, RhCl (CO)(PMe,) *, supported on porous glass showed the catalytic activity for the alkane dehydrogenation under light irradiation. It was revealed that the complex was converted to the coordinatively unsaturated species by losing CO ligand on the glass surface under the irradiation and was stabilized to be long-lived species under dark. A mechanism of the photocatalytic dehydrogenation of alkane by the coordinatively unsaturated species on the surface is proposed.
1. INTRODUCTION Acitvation of alkane C-H bond by homogeneous catalysts is an important target from the point of view of synthetic chemistry and utilization of natural resources[l]. One of the effective catalysts to such purpose is a rh0di.m complex, RhCl (CO)(PMe,) *, which photocatalyses several reactions including the dehydrogenation or the carbonylation of alkanes [2-51. For these C-H activation reactions, a tricoordinate species, RhCl(PMe,) *, formed by photoinduced dissociation of CO from the complex, is believed to be an active species[2-51, detected with the flash photolysis technique in a benzene solution by Wink et al. [6]. The authors have found that a coordinatively unsaturated species was formed by photoirradiation of RhCl(CO)(PMe,), on a porous silica glass surface with a long life and showed photocatalytic activity for the dehydrogenation of alkane. 2. EXPERIMENTAL
A piece of porous Vycor Glass(Corning 87930) was calcined in air and degassed at 973 K. The Vaska type rhodium complex, RhCl(C0) (PMe,) *, was supported on the glass by the impregnation method using a n-pentane solution of the complex in Ar atmosphere. The reaction experiments were carried out with a glass-made closed gas-circulating system at 323 K. A medium pressure mercury lamp was employed for photoirradiation.
21 64
3. RESULTS AND DISCUSSION While the rhodium comlex supported on the porous glass (denoted by [RhlCO/PVG, where [Rh] expresses RhCl (PMe,) p , hereafter) was inactive for dehydrogenation of 2-methylpropane under dark, it became catalytically active for the- reaction under the photoirradiation as displayed by (A) in Fig.1. The yield of 2-methylpropene, an only product, was increased monotonously with the increase of the irradiation time. The reaction was stopped immediately, when the irradiation was ceased. The re-irradiation made the reaction started again at the almost same rate as observed before the cease of the irradiation. The yield of the product far exceeded the values estimated from chemical equilibria of the dehydrogenation of the alkane to produce 2-methylpropene at 323 K. A 370 nm-cut low-pass-filter (50 X cut at 390 nm) for the irradiation hardly conducted the photoreaction. Thus, the wavelength of the light effective to the reaction was less than - 3 8 0 ~ 1 ,well corresponding to the W-VIS absorption spectrum of [Rh]CO/PVG, (a) shown in Fig.2. It is concluded that the complex absorbs photons and carries on the reaction. The MLCT band at 5340 nm was clearly observed on the UV-VIS spectrum of the [Rh]CO/PVG, (a) in Fig.2, similar to that observed for a cyclooctane solution of the complex[7], indicating that the complex is weakly adsorbed on the glass surface and restores the structure in the solution. When Rh[CO]/PVG was irradiated under degassing for 60 min, the spectrum (a) in Fig.2 was changed into (b). The MLCT band almost disappeared and a new broad band grew at -400 11111. Desorption of CO gas was detected by mass spectrometry during the period, less than 15-20 min, corresponding to the MLCT band decrease under the irradiation, but no desorption of trimethylphosphine was observed. Therefore, it is stressed that the complex is converted to a coordinatively unsaturated species like RhCl (PMe,) I , or [Rh], by losing CO ligand, which is a fast process and is finished in the early stage of the photoirradiation (see Fig. 1).
For the introduction of 2-methylpropane, neither the substantial change in the absorption spectrum (b) nor the reaction were observed as shown by (c) in Fig.2, suggesting no interaction of the alkane to [Rh] under dark. After the catalyst was irradiated in the presence of the alkane, almost the same spectrum as (b) or (c) in Fig.2 was observed and the dehydrogenation was resulted. Therefore, it is stressed that [Rh] is present on the glass surface during the photo-dehydrogenation and the electronically excited state of [Rh] plays a key role on the photocatalysis. As shown in Fig.1, the original catalyst was photoactive but its activity decreased during the photoirradiation, as shown by the curve (B), but the preliminary irradiation longer than 75 min gave steady photoreaction as indicated by (C) in the figure. This behavior suggests that the active
21 65
species formed in the course of the photoirradiation, [Rh], is eventually converted to a stable active species giving a steady photocatalytic activity, temporarily denoted by [Rh],, as follows. [Rh] + h v [RhIs. Little recovery of the MLCl' band by CO introduction on the catalyst of spectrum (c) in Fig.2 was observed. It seems that the recoordination of CO to [Rh]/PVG or [Rh],/PVG is rather difficult. The following scheme can be proposed, in which the expression of the sruface of the glass, /PVG, was omitted.
where I, is the light absorption intensity by [Rh]; 4 the quantum yield of [Rh]*, and k's are the rate constants of the corresponding equations. When CO gas was mixed in the reacting gas, the photoreaction was retarded as shown in Table 1. Thus, the equation (5) competes with the equations (3) and (4). When the reaction mixture containing CO gas was replaced by the pure reactant in the course of the photoreaction, the activity was recovered nearly to that of the fresh catalyst. Thus, the retardation of the reaction by CO is not irreversible. Table 1 Activity retardation by CO, and Activity dependence on reactant pressure. Mole ratio Activity" I Reactant Pressure Activity U
2 5 12
4.9 1.9 1.3 0.9
6.7 10.0 13.3 26.7
4.8 6.1 5.9 4.6
The steady state approximation on [Rh]* for the equations (2)-(5) gives the following equation to the alkene formation rate, R , which may be a qualitative explanation of the experimental results shown in Table 1. R =
1. 4 k, [alkane] k, + k,[alkane] + k5[CO]
*
It has been reported that [Rh] shows an absorption band over the range of wavelength, 390-550 nm and produces its dimer through the investigation of [RhICO benzene solution by flash photolysis technique[6]. It is likely
21 66
that the W-VIS absorption spectrum of (b) observed in the present work may correspond to the coordinatively unsaturated species formed by photoliberation of CO ligand, but has not been clearly identified to tricoordinate species, RhCl (PMe,) 2 . However, it is stressed that such coordinatively unsaturated species is present on the porous glass surface with a life long enough to photocatalyse the dehydrogenation of alkanes. Fig. 1 Photo-dehydrogenation of 2methylpropane (6.7 kPa) on [Rh]CO/PVG at 323 K.
5. 1.0
;
“I
E
x
0 ’ 0
2t 01
1 ,/I:.,
‘ 0- d’ ’
0
2
0
300
; 0
E
400 Wavelength / nm
(8):
A 1.25 w-[ah]oO/E-Pvc.
(C):
1.25 w-[Rh]W/g-PVC, preliminarily irradiated under degassing, A 75 nin,
90.5
. N
W 90 .in. 3I
I
4
5I
6I
I7 0
1
500
Fig. 2 W-VIS absorption spectra of [Rh]CO/PVG for various treatments, (a): 1.25 mg- [Rh]CO/g-PVG, (b): (a) was irradiated 60 min under degassing, (c): 6.7 kPa CH,-CH(CH,), was added on (b).
REFERENCES 1 Activation and Functionalization of Alkanes, C.Hil1 Ed., John Wiley and Sons, New York, 1989. 2 T.Sakakura and M.Tanaka, Chem.Lett.,(1987)249. 3 K.Nomura and Y.Saito, J.Chern. Soc. ,Chem.Commun. , (1 988)161. 4 T.Sakakura, T.Sodeyarna and M.Tanaka, New J.Chem.,13(1989)737. 5 J.A.Maguire, Y.T.Boese and A.S.Goldman, J.Am.Chem.Soc.,111(1989)7088. 6 D.Wink and P.C.Ford, J.Am.Chem.Soc. ,107(1985)1794; ibid, 109(1987)436 7 R.Brady, B.R.Flynn, G.L.Geoffroy, H.B.Gray, J.Peone,Jr. and L.Vaska, Inorg.Chem.,15(1976) 1485.
Guai, L a al. (Editors),New Frontiers in Cruolysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elscvier Science Publishers B.V. All rights reserved
HETEROGENEOUS PHOTOCATALYSISAS A METHOD OF WATER DECONTAMINATION: DEGRADATION OF 23 3- AND 4-CELOROBENZOIC ACIDS OVER ILLUMINATED TiOz AT ROOM TEMPERATURE J.-C. D'OliveiraO, W D. W. Jayatilakeb, K Tennakoneb,J.-M. Hemmanna and P. Pichato WRA CNRS "Photocatalyse, Catalyse et Environnement", Ecole Centrale de Lyon, BP 163, 69131 Ecully Cedex, France bInstituteof Fundamental Studies, Hantana road, Kandy, Sri Lanka
Abstract
Chlorobenzoic acids(CBA) are completely mineralized into C02 and C1over W-illuminated Ti02 in water according to the following reactivity order : 4-CBA>2-CBA>3-CBA.The identification of various intermediates of 2-CBA shows the decarboxylation and the progressive hydroxylation of the aromatic ring.?his study confirms the potentialities of photocatalysis for water detoxification.
-
1 INTRODUCTION
Heterogeneous photocatalysis appears as an efficient method to destroy and totally mineralize organic pollutants in water (1,2), For example, chlorophenols are totally converted into C02 and C1- over illuminated titania (3.4) whereas benzamide and nitrobenzene are converted into CO2 and NO3- (5). The degradation of various carboxylic acids has been reported, either aliphatic (6,7), or aromatic, such as benzoic acid (8). salicylic acid (9, 10). and chlorinated phenoxyacetic acids( 13.14) which are herbicides. This paper describes the total mineralization by heterogeneous photocatalysis of the three chlorobenzoic acid isomers chosen as model compounds, in particular to study the influence of the relative positions of the substituents on the aromatic ring.
2168
2 - EXPERIMENTAL
The organics were obtained from Aldrich . The photocatalyst was Degussa P - 25 Ti02 (50 m2 g-1, non porous).The degradations were carried out in a static batch photoreactor containing 50 mg of "YO2 in 2Ocm3 of solution illuminated at 340 nm with a radiant flux of 50 m W cm-2. Ti02 was stirred in the dark for 40 minutes to reach the adsorption equilibrium .Organics were analyzed by HPUS using a UV and a photodiode array detector. The eluent was 40 % methanol/60 % water buffered by acetic acid. Some of the intermediates were also identified by GC-MS. C1- and C02 were analyzed by HPLC (conductivity detector) or GC,respectively.
3 - RESULTS AND DISCUSSION 3.1 Photocatalytic degradation of 2-CBA
The concentration C of 2-CBA decreases linearly with illumination time from Co = 0.5 mM.i.e.80ppm (mg 1-l) with an apparent zero order and disappears within about 2 h (Fig.1). The kinetic order can be explained by a strong adsorption with saturation of the adsorption sites or
v = - d(a-CBA)/dt= k KC/1+KC = k C=Co -kt
where k is the true rate constant of 2-CBA disappearance and K is the adsorption constant of 2-CBA. This is in line with the important fraction of adsorbed molecules in the dark prior to illumination : decrease in C from 80 ppm (nominal concentration) to 60 ppm . This behavior is different from that of other compounds whose extent of adsorption is lower (1-5) C1- are progressively released in the solution (Fig.l) The final amount is reached within 2h and is equal to 90 % of the expected value if no chlorinated organic compound remained .which can be explained by C1- adsorption on TI02 . The final level of carbon dioxide formation (Eq.3)is reached within 5h
(Fig. 1). Cl-CgH4-COOH+7 0 2
-.
7COz+HC1+2H20
(3)
The period of time necessary to reach the total mineralization is thus ca. 2.5 times longer than that necessary to eliminate 2-CBA and to reach the maximum of chloride formation . For a potential application, one will have to consider the stage where the treatment has to be stopped as a function of the cost and the regulations.
21 69
0
2
t k f h
4
0
0
tw / n
2
Fig. 1. Kinetics of 2-CBA disappearance and of (2% and C1- formation (C) = normalized concentrations
;
Fig. 2. Photocatalytic disappearance of the three chlorobenzoic acids (A,B,C for 2-, 3- and 4-CBA, respectively).
Chlorobenzene and 2-chlorophenol were identified as intermediates , the former being present only as traces because of its very poor solubility in water and its high reactivity with OH.radicals to yield 2- and 4- chlorophenol by attack at ortho- and para- sites (15). Chlorohydroquinone was also detected. All these intermediates disappear within about 2h. No hydroxybenmic acids such as salicylic acid were found contrary to the case of benzoic acid (81, probably because of the presence of a chlorine substituent, which contributes to deactivate the aromatic ring. This is in agreement with the initial easy decarboxylation (cf.the case of aliphatic acids (6,ll)).Taking into account the nature of these identified intermediates and the degradation pathways of chlorophenols (3,4), the following degradation scheme is proposed :
co2+
CI'
21 70
3.2 Comparison of the degradation of the three isomers.
2-, 3- and 4- CBA were tested separately under identical contitions (50 ppm because of the very poor solubility of 4-CBA). The disappearance of the three pollutants (Fig.2) exhibits an apparent zero-order kinetics with rate constants equal to 4.1 ,2.4 and 5.3 pmol h-1 for 0. ,m. and p. CBA,respectively.The same order of reactivity is also observed for the total mineralization .The para-position induces a higher reactivity than the ortho- and the meta-one. This is in agreement with what has been found for monochlorophenols (3.4).
-
4 CONCLUSION
This study confirms the potentialities of heterogeneous photocatalysis to decontaminate waste waters. In particular, the high efficiency for dechlorination enables one to suggest this method as a complementary process for biological treatment of water, since chlorinated products are often toxic for bacteria.Additionally, photocatalysis appears, up to now, as the only example of heterogeneous catalytic treatment of used waters working at room temperature.
5 - REFERENCES D.F. Ollis, E. Pelizzetti, N. Serpone in "Photocatalysis", N. Serpone, E. Pelizzetti (eds). J. Wiley (1989)603. 2 R.W. Matthews in "Photochemical Conversion and Storage of Solar Energy", E. Pelizzetti, M. Schiavello (eds). Kluwer(1991) 427. 3 J.C. D'Oliveira, G. Al-Sayyed and P. Pichat. Env. Sci. Technol. No 24 (1990) 990. 4 G. Al-Sayyed, J.C. D'Oliveira and P. Pichat, J. Photochem. Photobiol. A : Chem. No 58 (1991) 99. 5 J.C. D'Oliveira. C. Maillard. C. Guillard, G. Al-Sayyed and P. Pichat Proc. Intern. Cong. "Innovation, Industrial Progress and Environment", MCI, Paris, (1991) 421. 6 M. Bideau. B. Claudel and M. Otterbein, J. Photochem. No 14 (1980) 29 : M. Bideau, B. Claudel, L. Faure and H. Kazouan, J. Photochem. Photobiol. A : Chem. No 6 (1991) 269. 7 J.M. H e m a n n , M.N. Mozzanega and P. Pichat, J. Photochem. No 22 (1983) 333 8 R.W. Matthews, J. Chem. Soc.,Faraday "Yans I. No 80 (1984) 457 9 M. Abdullah, G.K.C. Low and R.W. Matthews, J. Phys. Chem. No 94 I19901 6820. 10 J. Cunningham and G. Al-Sayyed, J. Chem. Soc.. F a r a d a y m s . No 86 (1990) 3935. 11 D. F. Ollis. C. Y. Hsiao, L. Budiman and C.L. Lee, J. Catal. No 88(1984) 89. 1.
21 71
12. A. Chemseddine and H.P. Boehm, J. Mol. Catal. No 60 (1990) 295. 13 J.C. D'Oliveira, These, Lyon (1992) 14 M. Barbeni, M. Morello, E. Pramauro, E. P e k t t i , M. Vicenti, E. Borgarello and N. Serpone, Chemosphere No 16 (1987) 1165 15 T. Sehili, G. Bonhomme, P. Boule and J. Lemaire, Proc. IUPAC Symposium on Photochemistry (Bologna),(1988). p 692.
.
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Guni, L d al. (Editors), New Frontiers in Cafolysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
INVESTIGATION OF THE ELECTRON TRANSFER MECHANISM BETWEEN MEITIYL VIOLOGEN RADICALS AND PROTONS VIA A NOBLE METAL CATALYST
R. Bauer and H.A. F. Werner Institute of Physical Chemistry, TU Vienna, Getreidemarkt 9, 1060 Vienna, Austria
Abstract Methyl viologen radicals underlies an acid-base equillibrium with pK, = 4.4 f 0.2 and can reduce protons without catalyst in acidic media. 1. INTRODUCTION
The photochemical reduction of water is imagined to occur via a noble metal catalyst (eg. Pt) working as an electron-pool [l]. There are, however, reasons to suggest that the electron transfer process occurs intramolecularly (equation (2)) within an electron relay radical-proton complex. Such a complex has been presented before [2], but was short thereafter rejected [3,4,5]. In this paper we present new evidences, supporting this hypothesis. 2. EXPERIMENTAL
The investigationof the uncharging reaction rate of reduced methyl viologen radicals at different pH-values (between H=4,2 and pH=8,0) and catalyst concentrations (between 1,7 and 13,4 mg Pt*dm$ [6,7] was performed utilizing a 450 W Oriel Xenon lamp with quartz optic fitted with a water jacket and a cut-off filter (for 1 Rb > Cs. s u g g e s t i n g t h e charge t r a n s f e r e f f e c t .
A!
tf;
k4;
Table 1 Rate constants(k) a t 473 K and apparent a c t i v a t i o n e n e r g i e s (E). Hydrogenation Catalyst
KKE8
Rbc; RbC24 csc8 csc74
Rate Consts. k/(mol/g h Torre) 4. 83x108 3.64 3.80 2.67 3.37 2.74
Decomposition
A. E. E/(kJ/mol) 32.6 31.6 13.8 50.8 85.2 125.7
Rate Consts. k/(mol/g h T o r r 2 ) 6 . 8 8 ~1O8 3.71 3.34 1.83 1.09 1.79
A. E. E/(kJ/mol) 63.9 53.0 58.7 42.0 32.4 120.3
The a l k y l a t i o n o f t o l u e n e and ethylbenzene b y e t h y l e n e t o o k p l a c e o v e r t h e MC8 and MC24. The r e a c t i o n proceeded f i r s t o r d e r w i t h r e s p e c t t o t h e pressure o f e t h y l e n e , and minus f i r s t o r d e r w i t h r e s p e c t t o t h e p r e s s u r e o f t o 1 uene. n - P r o p y l b e n z e n e and 3 - p h e n y l p e n t a n e w e r e f o r m e d b y t h e e t h y l a t i o n o f t o l u e n e . s e c - B u t y l b e n z e n e was s e l e c t i v e l y f o r m e d b y t h e e t h y l a t i o n o f ethylbenzene. The t e n d e n c y t h a t t h e benzene r i n g s o f aromatic hydrocarbons were n o t a1 k y l ated. b u t t h a t t h e i r side-chains were a l k y l a t e d i s t y p i c a l i n t h e base c a t a l y s i s .
of
3. M o d i f i c a t i o n graphite layer lattice. Because t h e p o r e s i z e o f a z e o l i t e a r e n a r r o w e d b y a C V D method, t h e s t r u c t u r e - s e l e c t i v i t y due t o t h e m o l e c u l a r - s i e v i n g e f f e c t can be v a r i e d [7].
By m e a n s o f an a n a l o g o u s m a n n e r , we t r i e d t o m o d i f y a g r a p h i t e i n t e r c a l a t i o n compound i n o r d e r t o narrow t h e entrance and/or t h e d i s t a n c e o f l a y e r l a t t i c e f o r r e a c t i n molecules. We employed a monomers t o form i t s polymer a t t h e edge andfor between t h e l a t t i c e l a y e r s . As a t e s t r e a c t i o n , t h e c o m p e t i t i v e i s o m e r i z a t i o n o f l - b u t e n e and 3m e t h y l - l - b u t e n e o v e r a KC and a m o d i f i e d KC24 a t 400 K has been i n v e s t i g a t e d , w h e r e t h e 2?10w t y p e s y s t e m was used. The r e a c t a n t s e l e c t i v i t y , l-butene/3-methyl-l-butene, was v a r i e d depending on t h e amounts o f and k i n d s o f t h e monomer t a k e n up. The monomer may b e e i t h e r c h e m i s o r b e d o r p o l y m e r i z e d , o r b o t h . T h i s t e c h n i q u e may be c a l l e d t h e polymer adding m o d i f i c a t i o n (PAM) method. The r e a c t a n t s e l e c t i v i t y depending on t h e k i n d s o f monomer employed was sumarized i n Table 2, where t h e optimum amounts o f monomers were t a k e n up. Table 2 The e f f e c t o f monomer on t h e s e l e c t i v i t y * a ) monomer selectivity
4.24
PMS*')
AMS*3)
MMA*4)
4.20
3.53
4.40
none 4.02
2.94
~
* l ) : s t y r e n e . "2): para-methyl styrene. methyl methacrylate. "5): 2 - v i n y l p y r i d i n e .
*a):l-butene/3-methyl-l-butene. "3):
a-methylstyrene.
"4):
Acknowledgment T h i s work was p a r t l a l l y s u p p o r t e d b y "a G r a n t - i n - A i d f o r S c i e n t i f i c Research from t h e M i n i s t r y o f Education, Science and Culture", f o r which t h e authors' thanks are due.
4. REFERENCES T. Kondow, H. I n o k u c h i , and N. Wakayama, J. Chem. Phys., 43 (1965) 3766. M. A. M. Boersma, i n J. J. B u r t o n and R. L. G a e t e n (eds.) Advanced M a t e r i a l s i n C a t a l y s i s , Academic Press, New York, 1977, p.67. 3 M. A. M. Boersrna, C a t a l . Rev. --- S c i . Eng., 10 (1974) 243. 4 K. Watanabe, T. Kondow, M. Soma, T. O n i s h i and K. Tamaru, Proc. Roy. SOC., Ser., A333 (1973) 51. 5 S . T s u c h i y a , S. Yamamoto, H. Imamura, J. Catal., 88, 225 (1984). 6 S. T s u c h i y a , T. M i s u m i , N. Ohuye and H. Imamura, B u l l . Chem. SOC., Jpn., 55 (1982) 3089; S. Tsuchiya and H. Imamura, Shokubai ( C a t a l y s t ) Tokyo, 25 (1983) 293. 7 H,. H a t t o r i , N. Y o s h i i and K, Tanabe, Proc. 5 t h I n t . Congr. C a t a l . (Maiami), 10 (1973) 233. 8 S. Tsuchiya and H. Imamura, Shokubai ( C a t a l y s t ) Tokyo, 25 (1983) 133; S. T s u c h i y a , i n K. Tanabe, H. H a t t o r i , T. Yamaguchi and T. Tanaka (eds.) Acid-Base C a t a l y s i s , V. C. H. Weinheim, 1989, p.169. 9 M. Niwa and Y. Murakami, Hyomen, 22 (1984) 319. 1 2
Guczi, L d al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
ACTIVATION AND REACTIVITY OF TITANIUM OXYNITRIDES IN AMMONIA DECOMPOSITION C. H.Shin, G. Bugli and G. Djega-Mariadassou Universite P & M Curie, Laboratoire Reactivite de Surface et Structure, CNRS URA 1106, 4 Place Jussieu T 54,75252 Parix Cedex 05, France
Abstract
Titanium oxynitrides TiN,O, were prepared by reaction of titania with gaseous ammonia between 890 and 1120 K. Optimized operating conditions yielded specific surface areas ranging from 58 to 87 m2.g-1. A kinetics study of catalytic ammonia decomposition at 900 K showed that these oxynitrides presented a tungsten-type (or iron) behaviour at low hydrogen pressure and a Temkin-Pyzhev behaviour characteristic of platinum, at higher hydrogen pressures.
1. INTRODUCTION Transition metal oxynitrides or oxycarbides are non-stiochiometric materials [ 1-33 and have been claimed to present metallic properties [4,5]. Recent researches [6-91 have shown temperature programmed synthesis, in a stream of ammonia using metal oxides as precursors, to be efficient in providing transition metal oxynitrides with high specific surface areas. Tamaru [ 101 have recently shown that already published data about catalytic synthesis and decomposition of ammonia could be classified into two classes. Low hydrogen pressure and high temperature working catalysts. These materials obey a rate law such as:
~ K P N--aH , ~
V = l+KP,,
3
mathematically equivalent to: where k, K and k are constants. Let us note that this kinetic law is not dependent on hydrogen pressure. It has been more particularly verified over tungsten taken as a reference. Tamaru has shown that eq.( 1) is not resulting from a Langmuir-Hinshelwood sequence, but rather from a dynamic balance between production and desorption of chemisorbed nitrogen, the most abundant reaction intermediate (mi)[ 10, 111. High hydrogen pressure and low temperature active catalysts. In this case, a TemkinPyzhev rate law is observed :
where k and a are constants. The rate determining step (rds) is the desorption of nitrogen, a quasi-equilibrium existing between gaseous ammonia, gaseous dihydrogen and chemisorbed nitrogen. In these conditions, dihydrogen acts as an inhibitor of the reaction.The present work describes the activation of titanium oxynitrides and their catalytic activity in the decomposition of ammonia at atmospheric total pressure to test their metallic behaviour.
21 90
2. EXPERIMENTAL Temperature Programmed Desorption (TPD) experiments on oxynitrides were carried out in vacuum using a quadripole mass spectrometer as detector (fiber Q S 200). After nitridation the solid sample was either passivated or stored in the reactor without any exposure to air. The TPD heating rate was 18 K min-1 to about 970 K. Catalytic decomposition of ammonia was carried out in a conventional dynamic differential microreactor, at atmospheric pressure, between 880 and 940 K. Initial partial pressures varied between 3.8 and 22.3 Torr (1 Torr = 133 Pascal) for ammonia and 6.2 and 27.6 Torr for hydrogen. A constant total flow rate was maintained at 10 1. h-1. Partial pressures were established by dilution with helium. The sample weights were in the range between 30 to 185 10-3g. Each activity step was maintained for 90 minutes in order to get a steady state of the system. No deactivation of the catalysts was observed. 3. RESULTS AND DISCUSSION Activation of tiranium oxynifrides. Temperature Programmed Desorption (TF'D) of samples showed that ammonia and water were totally evolved at 960 K. Hydrogen did not desorbed easily, even after several TPD up to 1300 K. No desorption of dioxygen was observed. Nevertheless nitrogen desorbed at high temperature (1 110 K), in a higher ratio with respect to hydrogen than that corresponding to ammonia stoichiometry, showing that the oxynimdes could decompose at high temperature. As a consequence, a standard pretreatment of the oxynimde was processed at 950 K, first in the nitriding mixture, then in helium at the same temperature before catalytic test. Catalytic decomposition of ammonia. First case: no excess of hydrogen. Figures 1 of reaction with respect to ammonia is positive and the rate law shows that the order (a) obeys eq. 2. The apparent activation energy was determined to be 139 kJ.mo1-1. The preexponential factor was found to be 6.96. In the second case, hydrogen was added to the reaction mixture. In these conditions, Fig. 2 shows that p is negative and the rate law obeys eq. 3.
lnV(899K)
>
'1
c
lnV(908K) lnV(921K)
.. m
-1
1
2
3
4
= O-1i 1l . -
In 2 P(H2) 3
4
In P(NH3) Figure 1. Positive order with respect to ammonia (a= 0.57 at 880 K to 0.84 at 941 K) [V = k pMI3 a ] (eq.2) (V is x 10-17molecules NH3 g1.s-1)
Figure 2. Negative order with respect to hydrogen ( p = - 0.76 at 908 K to -0.64 at 941K)[V = k' h 3 ( q . 3 ) for h H 3 = 9.6 TOIT.
21 91 As a conclusion, it appears that the order p vaned from zero without added hydrogen, to a negative value in excess of H2 and the titanium oxynitride behaves like transition metals in ammonia decomposition. Remaining with the Tamaru's model, the two corresponding sequences can be treated, according to Boudart and Dj6gaMariadassou [ 111, as two step reactions. At low hydrogen pressure, concentration and contact time of hydrogen are not sufficient to permit NH3 to form by rehydrogenation of nitrided surface species. As hydrogen does not play a kinetic role, the following two step sequence can be considered:
.................unspecified
steps ..................... kinetically nonsignificant
k2
2 *N
> N2+2*
where *, *NH2 and *N are respectively a free active site and surface adsorbed species, *N being the mari.. Step ( 5 ) represents the desorption of nitrogen as dinitrogen. Application of the quasi-steady state approximation leads to : 1/2
(ki/2k2 PNHJ
V = k2 [Ll(
(6)
1 + Ck1nk,PNJ)
which is formally identical to eq. 1 [l 11 : v = k [L] K h ~ / (13 + K P N H ~ )
w i t h k ~ k zand K =k1/2k2.
It also behaves mathematically like eq. 2 [ 111 : V = k' h ~ a 3 To complete the description of the metallic behaviour of the titanium oxynitride, it is interesting to compare the present results with those of Uffler and Schmidt (over Fe) [ 121. The linearized expression of eq. 1 gives the values presented Table 1. Table 1. Comparison between constant values k'l= ki[Ll, k'2 = k~[Lland K TiN,O, (880 IT I 9 4 0 K) P w 3 = 4 to 23 TOK
K = 1.58 x 10 -loexp(145464/RT) TOK-1 k'1= 1.89 x l@Oexp(-162184/RT) k'2 = 0.60x lO30exp(-307648/RT) (*) In Ref (12), K
Fe (600IT I1250 K) (12) PNH3 = 0.05 tO 1 TOR
(*)
K = 6.70x 10-9exp(165528RT) Tor1 k'i = 1.05 x lO2Oexp(-41800/RT) k'2 = 1.57 x 1@8exp(-207328/RT)
kl / k2. Energy unit : Joules.
At a higher hydrogen pressure, rehydrogenation of nitrided adsorbed species in ammonia can occur. According to Tamaru model, the following two step sequence can then be assumed to take into account the inhibitor effect of hydrogen :
21 92
2NH,
+ 2*
2 *N
Kl
h
2*N
+ 3%
(7)
> N2+2*
Step (7) is now quasi-equilibrated; it is not an elementary step, but represents a sum of quasi-equilibratedsteps. Step (8) far from equilibrium is the rds and the rate equation is :
Eq.(9) is formally equivalent to
v = kexp PNH3" PH2P From experimental data it was found that : exp(-214852/RT) PNH3' v = 2.15 X
(10) PH2P molecules g1S - l
(11)
This data can be compare to the equation given by Liiffler and Schmidt over platinum [ 121. Platinum presents a Temkin-Pyzhev type behaviour : V = 1.0 x 1024 exp (-133760mT) P N HP~~ 2 3 ' 2 (energy in Joules) formally equivalent to eq. (3) : v = kexp ( P N H ~ *PHz3 )v. As a conclusion, it can be seen that titanium oxynimde clearly shows, in excess of hydrogen, a behaviour near from that of platinum whereas, at low hydrogen pressure, it behaves like tungsten or iron.
4. REFERENCES R. Collongues, La Non-Stoechiometrie, Masson et Cie (dd.),Paris, 1971. W.S. Williams, Sciences, 152 (1966) 34. P. Pascal, Nouveau Trait6 de Chimie Minerale, Tome IX,Masson et Cie (a.), Paris, 1962. 4 R.B. Levy, Advanced Materials in Catalysis, J.J. Burton and R.L. Garten (eds.), Academic Press, New York (1977) 101. R.B. Levy and M. Boudart, Sciences, 181 (1976) 547. 5 6 H.C. Jaggers, N.F. Michaels and M.A. Stacy, Chem. Mater., 2 (1990) 150. 7 L. Volpe and M. Boudart, J. Solid State Chem., 59 (1985) 332. S.T. Oyama, J.C. Schlatter, J.E. Metcalfe and J.M. Lamben, Ind. Eng. Chem. 8 Res., 27 (9) (1988) 1639. 9 C.H. Shin, G. Bugli and G. Djtga-Mariadassou, J. Solid State Chem., 95 (1991) 145. 10 K. Tamaru, Acc. Chem. Res., 21 (1988) 88. 11 M. Boudart and G. DjCga-Mariadassou,CinCtique des RQctions en Catalyse HCt&ogtne, Masson (&I.), Paris (1982). Kinetics of Heterogeneous Catalytic Reactions, Princeton University Press, Princeton, NJ (1984). 12 D.G. Liiffler and L.D.Schmidt, J. Catal., 41 (1976) 440 ;44 (1976) 244. 1 2 3
Guczi, L ef af. (Editors), New Frontiers In Cafalysb P m d i n g s of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
A STUDY ON THE PREPARATION AND CHARACTERIZATION OF A NIP CATALYSTS J. Shena, 2.Lia, Q.Zhang, Y. Chena, Q. B d and 2. Lib aChemistry Department, Nanjing University, Nanjing 210008, China bBeijing Research Institute of Chemical Industry, Beijing 100013, China
Abstract a(amorphous)-Ni,,P,, ultrafine alloy particles (Al) with even particle size distribution (ca. 130 nm) have been prepared by the reaction of aqueous solutions of NiC1, and NaH,PO, at room temperature initiated by an inducing reagent. The reaction described by an autocatalytic mechanism is completed in 45 min with an initial pH value of 12, and the A1 particles are found to be more active and stable than the quenched rib bon catalyst with similar compositon in the hydrogenation of styrene to ethyl benzene. 1. INTRODUCTION
Amorphous alloys have attracted much attention because of their unique properties resulted from the disordered long-range structure, and the catalytic behavior of quenched alloys have been reviewed[ 11; however, the small surface area and rather se vere pretreatments needed for activation are their disadvantages. Chemical methods have been used to prepare ultrafine amorphous alloys[2], and in particular, a-NiP particles were obtained by the reaction of NiCI, and NaH,PO, in aqueous solution at 373 K[3]. Recently, we have developed a new method to carry out the above reaction at room temperature, and the a-NiP (Al) particles thus prepared show excellent catalytic properties for styrene hydrogenation. The aim of this work is to investigate the preparation kinetics and to characterize the a-NiP samples in comparison with those prepared by conventional methods.
2. EXPERIMENTAL a-NiP (Al) samples were prepared by the reaction of aqueous solutions of NiCI, and NaH2P02 at room temperature, initiated by adding a small amount of inducing reagent into the mixture, the basicity of which had been adjusted with NaOH solution.
21 94
The black precipitates produced were filtered, washed with NH,OH and water sequencially, and then dried. For comparison, another a-NiP (A2) sample was pre pared by the method reported in literature[3]. The preparation kinetics was studied by measuring the volume of H, evolved versus time. The composition of the samples was analyzed by ICP. The amorphous structure of the samples was identified by X R D using CuKa radiation. The morphology and particle size of the samples were determined by TEM.XPS was used to analyze the chemical states on the surface, using CIS (284.6 eV) as a reference to calculate the binding energies. The catalytic activity and selectivity for styrene hydrogenation of the A1 catalyst were tested in a fixed-bed microreactor and the products were analyzed by G. C.
3. RESULTS AND DISCUSSIONS The preparation reaction can be qualitatively expressed by: N?++ H,PO;-
a-NiP
+ H,
f
+ other by-products
The reaction rate is found to be second order and can be described by a self-catalyzed mechanism: -dVt / dt oc -dq / dt
=
k’ F,POJ
q
(1)
where k‘ is the rate constant of the autocatalytic reaction, Vt the volume of H, evolved during the reaction at time t, and q the quantity of the a-NiP product formed. The integral equation is
where, [H,PO;I, is the initial concentration of NaH,PO,, qo can be taken as the quantity of a-NiP produced by inducing reagent and Ve is the total H, volume evolved during the reactin. The equation can be rewritten into a linear form, from which the constants, k’and qo can be obtained from the slope and intercept. Fig. 1 shows the dynamic process (line) expressed by equation (2), which fits the experimental results (dots) perfectly. It is seen that the reaction can be completed within 45 min. at 293 K with an initial pH value of 12. It is therefore not necessary to boil the reaction mixture for a long time as reported in the literature[3]. In addition, we have revealed that the reaction actually stops due to exhausting of NaH,PO, , and part of the nickel ions remain unreduced. In such case, the resulted solid product is inevitably a mixture of a-NiP and Ni(OH), . Thus, the unreduced residue Ni(OH), must be washed out with ammonia solution before further treatment. The sample (Al) as-prepared has the composition of Ni,,P,, found by ICP. The XRD pattern in Fig. 2(a) has only a broad peak around 45 ,indicating the amorphous O
21 95
10
30 50 t(min)
Figure 1. Dynamic process of the reaction, line: according to equation(2), dots: experimental results.
35
45 28/O
55
Figure 2. XRD patterns of the a-Ni,,P,, (Al), (a) as-prepared, (b) after 40h reaction.
Figure 3. TEM graphs of (a) A1 and (b) A2 samples of the a-Nip.
855 850 131 129 127 Binding Energy(eV)
Figure 4. XPS spectra of (a) Ni2p,, and (b) P2p in the A1 sample.
Figure 5. The hydrogenation activity (a) the A1 sample, (b) a ribbon catalyst[5].
21 96 structure of the sample. The TEM results of the A1 sample as well as an A2 sample are graphically shown in Fig. 3, and the predominant difference in comparing the two samples is that the A1 sample has an even size distribution (ca. 130 nm), whereas the particles of the A2 sample are larger (ca. 150 nm) and not even. Fig. 4 shows the XPS spectra of the A1 sample, in which only one N i 2 ~ , , ~peak and one P2p peak are observed with their binding energies of 852.4 eV and 129.4 eV, respectively. These results suggest that only metallic nickel and elementary phosphorus exist on the surface[4]. No oxidized nickel or phosphorus has been found in this sample, which is different from that r e ported by Okamoto et al. in reference[4], where samples were prepared by the conventional method without washing with ammonia solution and their XPS results evidenced the coexistence of a-NiP and Ni(OH),. It is interesting to note that, as shown in Fig. 5(a), the a-Ni,,P,, (Al) gives 100% conversion and selectivity for the hydrogenation of styrene to ethyl benzene at 433 K and a liquid space velocity of 46h-', and its catalytic properties is stable for more than 40h. In comparison, a ribbon sample of similar composition, Ni,,P,,, needs higher reaG tion temperature (553 K) and lower liquid space velocity (6h-') to reach the similar conversion, and, as can be seen in Fig. 5(b), its activity decreases dramatically after 3h on stream. In addition, the ribbon catalyst needs complex pretreatments for activation[5]. In contrast, for the A1 catalyst, rather mild pretreatment, i.e., 2h in H, at 523 K, is enough. It has been revealed that the amorphous NiP ribbon is more active than corresponding crystallized NiP or Ni foil in the hydrogenation of styrene to ethyl benzene[5]. Thus, the loss of amorphous structure on the surface of the ribbon catalyst, which may related to the rather severe pretreatment conditions, may be one of the reasons for the rapid decrease of its activity. In fact, the Ni,,P,, ribbon catalyst was partially crystallized after 10h reaction[S], while the A1 catalyst still maintained its amorphous structure, as shown in Fig. 2 (b), and its XPS results have shown that Ni and P on the surface are mainly in their elementary states after 40h reaction. Obviously, as a hydrogenation catalyst, the relative higher surface area and simple pretreatment conditions are the advantages of the a-NiP ultrafine particles produced by chemical reduo tion over the corresponding ribbons prepared by the quench method. We thank SINOPEC for financial support. 4. REFERENCES 1 A. Molnar and G. V. Smith, Adv. Catal., 36 (1989) 329. 2 J. van Wonterghem, S. Morup, C. J. W. Koch, S. W. Charles, S. Wells, Nature, 322 (1986) 622. 3 S. Sada, Kogyo Kagaku Zasshi, 71 (1968) 957. 4 Y.Okamoto, Y.Nitta, T. Imanaka and S. Teranishi, J. Chem. SOC. Faraday Trans. 1,75 (1979) 2027. 5 B. N. Zhong, E.Z. Min, S. Z. Dong and J. F. Deng, Acta Chimica Sinica, 47 (1989) 1052.
Guni, L. et al. (Editors),New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1W2, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
INVESTIGATIONS OF HYDRODENITROCENATION OF QUINOLINE OVER MOLYBDENUM NITRIDE
R S. Lee, J. A. Reimer and A. T. Bell Center for Advanced Materials, Materials Science Division, Lawrence Berkeley Laboratory and Department of Chemical Engineering, University of California, Berkeley, CA 94720, USA
Abstract An investigation has been carried out of the reaction pathway for the hydrodenitrogenation of quinoline over Mo N. Quinoline is found to undergo a rapid hydrogenation to form 1,2,3,4,-tetrahydroquinoline.This product then reacts more slowly to form 2-propylanaline, which in turn undergoes hydrogenolysis of the C-N bond in the saturated ring to form propylbenzene. No evidence is found for propylcyclohexane.
1. INTRODUCTION The removal of nitrogen from organonitrogen compounds present in petroleum is carried out by hydrodenitrogenation (HDN). The catalysts most frequently used for this purpose are sulfided CoMo/Al,O, or NiMo/Al,O,. While effective, these catalysts require complete saturation of all aromatic rings prior to removal of nitrogen, resulting in a consumption of hydrogen that is significantly higher than the minimum required for the removal of nitrogen atoms. Recent work by Schlatter et al. (1) has shown that molybdenum carbides and nitrides might offer an interesting alternative to existing HDN catalysts. Both Mo C and Mo2N are reported to have HDN activities comparable to that of commercial NiMo/Af,O, catalysts, but ehxibit much higher selectivities for the formation of aromatic products. The aim of the present study is to gain some insight into the reaction pathway for nitrogen removal from quinoline over Mo2N. 2. EXPERIMENTAL The fcc phase of Mo N was prepared following the procedure of Volpe and Boudart (2). X-ray diffraction of a freshly prepared sample showed a characteristic pattern for MozN and no evidence of residual Moo3. The BET area of a freshly prepared sample was typically in excess of 200 m2/g All reaction were carried out in a quartz microreactor heated by a tube furnace. Hydrogen was purified by passage through a catalytic oxygen remover and the rate of delivery was set using a mass flow controller. Liquid feeds were delivered to the flow manifold using a syringe pump. To assure complete vaporization of liquid feeds, the portion of the flow manifold located downstream from the point of liquid introduction was maintained at a temperature above 500 K. The flow rate of H, was maintained at 110 cm3/min and the liquid flow rate was 0.1 cm3/h. The effluent from the reactor was analyzed by on-line gas chromatography. Products were separated using a 60 m long, 0.25 mm i.d. capillary column coated with a 1 pm thick film of polydimehylsiloxane. Product identification was carried out by off-line gas chromatography/mass spectrometry.
3. RESULTS AND DISCUSSION The time dependence of the activity of Mo,N for quinoline HDN was determined for freshly prepared Mo2N, Mo2N passivated by exposure to a 1% 02/Hemixture, and passivated
Mo2N that had been air exposed. At 723 K, the conversion of quinoline over the freshly prepared sample decreased from 30 % to 10 % i n 30 h and then remained constant for an additional 20 h. Passivation and air exposure reduced the initial converiion to about 25 % but did not significantly affect the long-term activity of the catalyst. Measurements of the BET surface area before and after 50 h of reaction indicated a decrease from 230 m2/g to 145 m2/g, from which it is concluded that most of the loss in activity with time is due to a loss in the BET surface area of the catalyst. Consistent with this interpretation it is observed that the product selectivity is virtually the same, independent of catalyst pretreatment and duration under reaction conditions. The dependence of quinoline conversion on temperature over freshly prepared Mo,N is given in Figure 1. Between 523 and 623 K, the conversion decreases with increasing temperature, whereas above 623 K, the conversion increases with increasing temperature. This pattern suggests that at temperatures below 623 K, the reaction is equilibrium limited by an exothermic process, whereas at temperatures above 623 K the kinetics for converting the products of the equilibrium reaction to other products become sufficiently rapid to give rise to an increase in the conversion of quinoline with increasing temperature.
1
Irn 80
523
S73
623
673
Temperature
723
773
(K)
Figure 1. Quinoline conversion versus temperature The distribution of the principal products is given in Figure 2. In Figure 2, the group of products referred to as analine total is comprised of 2-methylananline (2-MA) and 2propylanaline (2-PA), whereas the group referred to as benzene total is comprised of benzene, methylbenzene (toluene), ethylbenzene, propylbenzene (PB), and 2.3-dihydroindene. The distributions of individual products in the total analine and total benzene groups are shown in Figures 3 and 4, respectively. Figure 2 shows that at 523 K, the principal product is 1,2,3,4-tetrahydroquinoline(1THQ). Experiments conducted with 1-THQ as the feed indicate that for temperatures greater than or equal to 523 K thermodynamic equilibium is attained between quinoline and 1-THQ. Since the hydrogenation of quinoline is an exothermic process, the observed decreases in quinoline conversion as the temperature is increased from 523 to 623 K is attributable to a decrease in the equilibrium conversion of quinoline. Above 623 K, the kinetics of 1-THQ conversion to products other than quinoline is suffcient to to achieve an increase in quinoline conversion with increasing temperature. It is evident from Figures 2-4, that with increasing temperature 1-THQ reacts first to 2PA and 2-MA. Studies conducted with 2-PA as the feed indicate that 2-MA is formed by hydrogenolysis of the propyl group. At temperatures above 623 K, 2-PA can undergo ring closure to form 2-methylindole. The formation of benzene and alkylbenzenes from quinoline occurs above 553 K (see Figures 3 and 5). Here again, experiments conducted with 2-PA as the feed indicate that benzene and alkylbezenes are formed by the release of the Mi,group of 2PA to form PB and ammonia, and the subsequent hydrogenolysis and dealkylation of PB to form methylbenzene and benzene, respectively.
21 99
Figure 2. Product selectivity versus temperature
Figure 3. Analine compound selectivity versus temperature
2200
-L
40
2
523
S13
623 613 l'crnperamrc (K)
113
113
Figure 4. Benzene compound selectivity versus temperature It is significant to observe that the reaction products contain no evidence of cyclohexane or alkylcyclohexanes. The absence of these products is assumed to be due to a weak interaction of the benzene ring with the catalyst surface. Experiments conducted using benzene or propylbenzene as the feed indicate that at temperatures where these products are formed from quinoline, hyrogenation of the benzene ring occurs to only a very limited degree, and when ammonia is present in the feed stream the hydrogenation activity of Mo,N is totally suppressed. CONCLUSIONS The hydrodenitrogenation of quinoline HDN over Mo,N is initiated by the hydrogenation of quinoline to 1-THQ. This reaction is very rapid and achieves thermodynamic equilibrium already at 523 K. No evidence is found for 5,6,7,-tetrahydroquinoline, even though the Gibbs free energy for forming this compound is significantly greater than that for the formation of 1THQ. The removal of nitrogen from 1-THQ is initiated by cleavage of the C-N bond in the saturated ring, to form 2-PA. 2-PA then undergoes a loss of the amine group to form PB. Hydrogenolysis of the alkyl group in PB leads to the formation of toulene and ethylbenzene, whereas dealkylation produces benzene. In strong contrast to sulfided NiMo/Al,O, [3,4] the standard catalyst for HDN, Mo,N does not produce decahydroquinoline or propylcyclohexane. This difference is attributable to the weak chemisorption of aromatic rings on Mo,N. 4.
5. ACKNOWLEDGMENT This work was supported by the Materials Sciences Division, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC03076SF00098.
6. REFERENCES 1 J.C. Schlatter, S.T. Oyama, J.E. Metcalf, 111 and J.M. Lambert, Jr, Ind. Eng. Chem. Res. 27 (1988'1 1648. 2 L. Volpe'and M. Boudart, J. Solid State Chem., 59 (1985) 332. C.N. Satterfield and J.F. Cocchetto, Ind. Eng. Chem. Process Des. Dev., 20 (1981) 53. 3 4. C.N. Satterfield and S.H. Yang, Ind. Eng. Chem. Process Des. Dev., 23 (1984) 11.
Ouczi, L u d.(Editors), New Frontiers in CaalysiS Proceedings of the 10th International C o n g m on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elscvier Science Publishers B.V. All rights reserved
CO OXIDATION, NO DECOMPOSITION AND NO REDUCTION BY CO ON SUPERCONDUCTING AND RELATED CUPRATES
I. Halrisf', A. Brennefl, M. Shelef and K. Y.S.N f aDepartment of Chemistry, Wayne State University, Detroit, MI 48202; Inquiries should be addressed to A. Brenner, USA bScientificResearch Staff, Ford Motor Co.,Dearborn, MI 48121, USA CDepartment of Chemical Engineering, Wayne State University, Detroit, MI 48202, USA *Permanent address: Central Research Institute for Chemistry of the Hungarian Academy of Sciences, 1525 Budapest, P.O.Box 17, Hungary
Abstract Catalytic activities of five non-conducting and three superconducting cuprates were compared with each other and to CuO, a hlghly active catalyst, for the oxidation of CO, decomposition of NO, and reduction of NO by CO. The concentration of the reactants and the space velocities approximate the conditions used for automotive exhaust catalysts. The insulating barium cuprates, Ba2Cu305 and BaCuOp, were about 10 times more active for oxidation of CO than CuO and about 103 times more active than the least active cuprates, T12CaBa2Cu20g-x and Ba2Cu03.5-X. Only the perovskite-like superconductor, Y B a 2 C ~ 3 0 7 - xand ~ the non-perovskite insulator, Y2Cu205, gave high rates (similar to CuO) for the reduction of NO by CO. None of the catalysts showed significant activity for the decomposition of NO. In contrast to earlier results, it appears that a perovskite-like structure, electrical conductivity, and the presumed presence of Cu3+ ions do not enhance the activity of cuprates for the reactions studied here. 1. INTRODUCTION
New superconducting and related complex cuprates containing two to six metallic elements have recently attracted wide attention. Most high temperature superconductors can be considered to belong to one of four families based on complex La, Y, Bi, or TI cuprates. A small number of superconducting and some related non-conducting cuprates of the first three families have been reported to be active catalysts for several reactions including the oxidation of CO [l-41, toluene [5],
2202 and C1 to C3 alcohols [6], decomposition of NO and N 2 0 [7],hydrogenation of CO [a], and the dehydrogenation of alcohols [9]. The catalytic properties of all other complex cuprates have not been previously investigated. A comparison is reported here of the catalytic activity for CO oxidation, NO decomposition, and NO reduction by CO of eight superconducting and nonconducting cuprates and the activities are compared to the highly active catalyst, CuO. Many of these compounds had not been previously investigated as catalysts. The concentrations of reactants and the space velocities resemble the conditions of catalysts used for automotive exhaust pollution control [lo].
2. EXPERIMENTAL Catalysts were prepared from well ground solid oxides, nitrates, and carbonates. Stoichiometric amounts of the ingredients were calcined in air at temperatures from 88OOC to 940°C in a porcelain crucible for different lengths of time. Specific surface areas and bulk densities ranged from 0.1 to 1 m2/g and from 1.2 to 3.4 cm3/g, respectively. Details of preparation procedures as well as the physical parameters, crystal structures, and oxygen contents of the catalysts have been reported previously [2, 111. Catalytic experiments were done with equal contact times for the catalysts using 0.43 f 0.01 cm3 of samples. The samples were contained in a fused quartz reactor (10 mm i. d.) of the flow-through type and were preheated in a flow of He (- 35 Uh) at 5OOOC for 2 h. For the reactions of oxidation of CO, decomposition of NO, or reduction of NO, about 1Yo 02 and 1% CO, 0.1 % NO, or 0.1 Yo NO and 1.5% CO, respectively, were mixed with He carrier gas to achieve a total pressure of 1 atm (1 x 105 Pa). Each gas flow was controlled by a Brooks mass flow controller. Space velocities varied from 5 x 104 to 10 x 104 h-1. For the oxidation of CO, the reactant gases, CO and 02, and the product, C02, were analyzed using a Gow Mac Series 550P gas chromatograph. For the direct decomposition of NO, N2 (and possibly N20) and 0 2 are the expected products, while in the reaction of CO and NO, the formation of C 0 2 and N2 (and possibly N20) is expected. For the latter two reactions, a Beckman Model 951 chemiluminescent NO-NOx gas analyzer was used to compare the concentrations of NO at the inlet and outlet of the reactor. Details of measuring procedures, equipment, and product analysis have been reported elsewhere [2]. 3. RESULTS AND DISCUSSION
Perovskite type oxides, such as LaCoOg and LaMnOg, and some copper based oxides, such as CuO, CuCr204, and CU-ZSM5, are among the most active base metal catalysts for several important reactions for automotive exhaust abatement [12, 131. Many reports suggest that the catalytic activity of cuprates is associated with the presence of Cu3+ ions in a perovskite-like crystal lattice [ l , 3, 4, 7, 91. Oxygen vacancies and the associated electrical conductivity of cuprates have also
2203 been suggested to be important for the catalytic activity [l,41. In order to elucidate the importance of these factors, the activities of the perovskite-like superconductors, YBa2Cu307-~,Bi2CaSr2Cu20i 0-X,and TI2CaBa2Cu20g-x, and of the perovskite-like insulator, B a 2 C ~ 0 3 . 5 - xwere ~ studied for the three reactions. These catalysts may contain some Cu3+ ions and non-stoichiometric 0ions which can be removed from or replaced into the crystal lattice without significant changes in the perovskite-like structure. Activities of the non-conducting, perovskite-like Ba2Cu305, the non-conducting, non-perovskite catalysts, Y2BaCuO5, Y2Cu205, BaCu02, and, for comparison, CuO, were also investigated. These oxides do not contain Cu3+ or 0-ions. 3.1. Oxidation of Carbon Monoxide
The activities of catalysts for oxidation of CO to COP at 3OOOC are compared in Fig, 1. All of the superconductors, YBa2Cu307-~,Bi2CaSr2Cu201 0-X, and T12CaBa2Cu20g-x, and the perovskite-like insulator Ba2Cu03.5-x were found to be less active than CuO. All other cuprate catalysts are more active than CuO. The maximum reaction rate on the most active catalyst, Ba2Cu305, is similar to that of platinum wire [ l o ] and is about three orders of magnitude higher than the least active catalyst. Although the reaction rates vary with varying reaction temperatures, the relative activities remain unchanged. These results suggest that the perovskitelike structure and electrical conductivity of cuprates are not essential for high catalytic activity in CO oxidation. Cuprates containing presumably some Cu3+ and 0-are in general less active for oxidation of CO than cuprates containing only 02-.
1
2
3
4
5 6 Catalyst
7
8
9
Figure 1. Comparison of activities for the oxidation of CO and the reduction of NO at 300°C 1 = Ba2Cu305 4 = Y2Cu205 7 = Bi2CaSr2Cu201 0-x 2 = BaCu02 5 = CUO 8 = T12CaBa2Cu20g-x 6 = YBa2Cu307-x 9 = Ba2Cu03.5-x 3 = Y2BaCuOg
2204
3.2. Decomposition of Nitric Oxide
None of eight cuprates tested, including five perovskite.-like cuprates, showed significant activity for the decomposition of NO to N2 (N20) and 0 2 at reaction temperatures up to 700OC. This result indicates that earlier reports of reasonable activity of some perovskite-like lanthanum and yttrium based cuprates was due to their activity being measured at 20 to 30 times higher partial pressures of NO and at 20 to 100 times lower space velocities than used here [13]. The absence of significant activity at more realistic conditions suggests that they probably are not practical catalysts for automotive exhaust applications. 3.3. Reduction of Nitric Oxide by Carbon Monoxide
Only Y2Cu205, YBa2Cu307-x and B a 2 C ~ 0 3 , 5showed -~ activities for reduction of NO by CO which resembled the high activity of CuO (Fig. 1). The reaction rates on these catalysts are about two orders of magnitude higher than on the least active catalysts, Bi2CaSr2Cu20i 0-x and T12CaBa2Cu20g-x. All cuprates lost activity in the presence of oxygen. Since some compounds of the lanthanum based perovskite-like superconductor family were reported to be less active catalysts for NO reduction (141 than BI2CaSrpCu2010-x, it appears that the perovskite-like structure, electrical conductivity, and Cu3+ content do not enhance the activity of cuprates for this reaction. The different trends in relative activities of cuprates for oxidation of CO and reduction of NO suggest that the oxidation of CO is probably not rate determining in the NO+CO reaction. Since the rate of the NOtCO reaction is much higher than the rate of NO decomposition, the direct decomposition of NO to N2 is not a step in the former reaction. 4. REFERENCES
1. A. R. Jiang, Y. Peng, 0. W. Zhon, P. Y. Gao, H. Q. Yuan, J. F. Deng, Catal. Lett. 3, 235 (1989) 2. I. Halasz, A. Brenner, M. Sheief, K. Y. S. Ng, Catai. Lett. 6, 349 (1990) 3. J. C. Otamiri, S. L. T. Anderson, Appl. Catal. 73, 267 (1991) 4. S. Rajadurai, J. J. Carberry, B. Li, C. B. Alcock, J. Catal. 131, 582 (1991) 5. J. C. Otamiri, A. Anderson, S.L. T. Anderson, J. Chem. SOC.Faraday Trans. 87, 1265 (1991) 6. I. Halasz, React. Kinet. Catal. Lett. 41, 115 (1990) 7. H. Yasuda, T. Nitadori, N. Mizuno, M. Misono, J. Chem. SOC.Jpn. 604 (1991) 8. J. A. Brown Bourzutschky, N. Homs, A. T. Bell, J. Catal. 124, 52 (1990) 9. 1. Rodriguez-Ramos, A. Guerrero-Ruiz, M. L. Rojas, J. L. G. Fierro, Appl. Caral. 68, 217 (1991) 10. J. T. Kummer, Prog. Energy Combust Sci. 6, 177 (1980) 11. 1. Halasz, H. W. Jen, A. Brenner, M.Shelef, S. Kao, K. Y. S. Ng, J. Solid State Chem. 92, 327 (1991) 12. M. Iwamoto, H. Yahiro, Y. Mine, S.Kagawa, Chem. Left. 213 (1989) 13. H. Shimada, S.Miyama, H. Kuroda, Chem. Lett. 1797 (1988) 14. N. Mizuno, H. Toyama, M. Tanaka, M. Yamato, M. Misono, Bull. Chem. SOC. Jpn. 64, 1383 (1991)
Guczi, L. ef ul. (Editors), New Frontiers in Cufalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary (6 1993 Elsevier Science Publishers B.V. All rights reserved
THE ACTIVE OXYGEN ON THE Li/La,O, CATALYST SURFACE AND ITS CATALYTIC BEHAVIOR IN THE OXIDATIVE COUPLING OF METHANE L. Wang, J. Wang, S. Yuan and Y Wu
Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022, China
Abstract by adding Li to The 02species was found on the La203, compared with the single La203. Li/Laz03 but not on the single La203. In low-temperature desorption, ethane desorbed from the Li/La203 but was not detected with the single La203. It is considered that the addition of Li gave rise to some basic sites which are favorable for the coupling reaction.
The coupling selectivity was greatly enhanced
1. INTRODUCTION Since the early work of Keller and Bhasinl’ a considerable progress has been made in the oxidative coupling of methane to ethane and ethylene. Several selective catalysts are composed of basic oxides promoted by alkali metal ions such as lithium or sodium ions. Lunsford et a12’ reported the reaction mechanism and the nature of the active site which is responsible for the activation of methane over the Li/MgO and Na/CaO catalysts, and it is suggested that centers of the type [Li’O-I or [Na’O-1 are effective for the formation of methyl radical. In the present paper, comparison between the Li/LazO3 and single La203 is made on the basis of an activity test, by the ESR study, and the low-temperature desorption, followed by discussions about the active oxygen and the activation of methane on the catalyst surface. 2 . EXPERIMENTAL
The single La203 catalyst was prepared by immersing commercial La203 powder 0 9 9 . 9 9 % ) in distilled water, and heating while stirring to dry. The Li/La203 catalyst was prepared by immei.sing La203 with a solu-
2206
I--" A
C
Fig. 1 Apparatus for quenching and ESR measurement A-ESR tube B-liquid N;! trap C-sample holder D-rotatable valve E- thermocouple F-stopping valve G - spher i ca 1 er i nd H - manometer J - water I - oxygen K - heater L- to vacuum
Fig.2 The ESR signal of 02formed on the Li/La~03 (0.6wtX) surface
ation of LiOH 1120, and heating while stirring Lo dry, then calcinatins. An apparatus shown in Fie.1 was made for the detection of Lhe active oxygen by the quenching technique and ESR measurement. In the low-temperature desorption experiments, He was used as the carrier gas, and the desorbed products were also trapped at different temperature ranges for g a s chromatography analysis. The activity test results at 780'C over the Li/La203 catalysts with different Li-contents are listed in Table 1 , where CH4:02:N2=3:1:6 (in mole) and SV=8800 h-'.
3. RESULTS A N D DISCUSSION In comparison with the single La203, all the Li-doping catalysts gave rise to the C2-selectivity and the Cz-yield. The conversion of methane decreased with the Li-content, but the Cz-selectivi ty remained c0ns ta I1 t . Nolicing the decrease of catalyst surface area due to Li-doping, we
2207
consider that the reaction mechariism is in agrecinent with tlmt methyl radicals couple in g a ~ phtlsc:”. l’hc loword surfaca a r ~ i1 imi I s Lhc contact of the radicals onto the surface so tl~atthe i‘urlher oxidation is inhibited. Table 1 Activity of the Li-Laz03 catalysts with different Li-contents Li-Content wtX
Surface Area m2/g
Conv. of CH4
X
C2-Selec.
x
&-Yield ?:
In the Li/La203 catalyst calcined at 350‘C, six phases were found Li~C03, L i d , and LiLaOa. Although some of them disappeared in the sample calcined at 850.C. these transit ion phases wou Id probab 1y emerge again under the reaction con -dition and play as some basic sit,es favorable for the coupling. BY using the apparatus given in F i e . 1 , the sample of Li/La203 (O.GwtX) was treated in vacuum (L) at 800% for 8 h. The surface-cleat1 -ed sample was recalcined in situ at G50’C in an atmosphere of oxygen I ( I ) , then poured into 1 iquid oxygen below for quenching at 77 K. After recxhausting, the sample was transferred into the ESR tute(A) and examined by ESR. The signal thus ob -tained is shown in Fig.2, which is the typical 02- signal with g,= F i g . 3 Desorption profile of the 2.031, gyy=2.000, g,,=1.995. The La203 (02 pretreatment temperature: same procedure was repeated using 1--400’C ; Z--GOO*C ; 3--800’C, nitrogen instead of oxygen (I), but CH4 adsorption: 1.33 kpa, R.T.) no similar signal like 02- was by
XRD, i.e. La2O3, La(011)3, La(C03)011,
I
\
2208
x
observed, indicating that the 02species came from the o x m e n adsorb -ed on the catalyst surface. On the CHd(I6X) CH, (18:) other hand, no 0 2 - species was co ( 1 5 x 1 CO ( 7 6 % ) found in the same condition in the CzHs (9x1 CzHs(6X) case of single L a d s instead of I L i /La 203. 298K The activation of methane on the surface of La203 and Li/LazOs Fig.4 Desorption profile of the catalysts was also investigated by Li/La203 ( 02 pretreatment: means of a low-temperature desorp 13.3 kpa, 500%; CHn adsorption: -tion from 77 K to room tempera1.33 kpa, R.T. 1 ture. After the pretreatment of La203 catalyst in vacuum for 8 h and the preadsorption of oxygen for 2 h and exhausting gaseous oxygen, methane (1.33 kPa) was adsorbed on the catalyst which was cooled with 1 iquid nitrogen. The low-temperature desorption started when the 1 iquid nitrogen was moved away. There are 2-4 peaks in the profile on LazOs, as shown in Fig.3, suggesting that different active sites for methane activation exist on the catalyst surface. From the analysis of products collected at different desorption tem -perature ranges, as given in Fig. 3, it was noted that methane was activated on the La203 surface and converted into complete oxidation products only even at low-temperature. In the case of Li/LazO3 (O.Gwt%) catalyst., three desorption peaks occurred in the desorption profile, as shown in Fig.$. The desorption products responding to the former two peaks contained 9% of ethane in addition t o CO and methane, and that responding to the last big peak contained ethane (6%) too. It was evident that the Li-doping in the Li/La~03catalyst did promote oxidative coupling of methane and repress the complete oxidation.
4. REFERENCES 1) G. E.Ke 1 ler , M. M. Bhas in, J. Cata 1. ,73,9 (1 982) 2 ) J-X. Vang and J.H. Lunsford, J. Phys. Chem., 90(17), 3890(1986): ibid, 90(22) ,5883(1986) 3) T.Ito, J-X.IJang, C-H.Lin, J.H.Lunsford, J. Am. Chem. SOC., 107(18) ,5062(1985)
Gwzi, L ef al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congrese on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
OXIDATIVE COUPLING OF METHANE OVER SRO PROMOTED La20$aO: COMPARATIVE STUDY OF THE KINETICS AND MECHANISM
A
X-D. Xu, L. Yu, J.S.Huang and Z.-X Lin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O.Box 110, Dalian, China
Abstract A comparative kinetic analysis of the oxidative coupling of methane over Laz03/Ca0 and SrO-La203/Ca0 catalysts shows that the SrO additive can cause changes in the oxygen adspecies from a molecular form to an atomic form. The reaction mechanism changes from the Langmuir -Hinshelwood type to the Rideal -Redox type.
1. INTRODUCTION In a previous study [l] , we found that SrO addition affects the oxidative coupling of methane over 20% Lazo~/CaOin many ways : a formation of a phase of mixed but highly dispersed SrO and La203 oxides, a decrease in the amount of weak basicity sites and a modification of the distribution of strength of basicity, and an increase in a species of oxygen released at higher temperatures. A comparative kinetic analysis over La203/Ca0 (LC) and SrO-LazO3/CaO (SLC) catalysts is reported here to show the promotional effects in terms of reaction kinetics and the nature of the reaction mechanism.
2. EXPERIMENTAL CaO was impregnated with a solution of Sr and La nitrates with the desired ratio, left overnight and dried and calcined at 1173K for 2h. The sample was ground, tableted, crushed and sieved into 40-60 mesh for further use. Kinetic analyses were carried out in a fixed bed reactor of quartz of 4 mm i. d. The experiments were generally performed with methane conversion of less than 10% and oxygen conversion less than 30%. The effluents were analysed by GC.
.
3. RESULTS AND DISCUSSION 3. 1. The influence of the residence times on the reaction parameters.
2210 Over LC, an increase in the residence times increases selectivities to CzHd and COz but decrease the selectivities to CZH6 and CO. Selectivities to Cz (C2H4 CzHs) , SC,, and COx(CO COz), SCO., did not change much (see Fig. 1. ). The feature over SLC is that the selectivity to C O , so,increases with an increase in residence times. In addition, &, clearly decreases, while Sax increases with an increase in the residence time as shown in Fig. 2. The difference over LC and SLC implys a difference in the reaction mechanisms and the reaction paths for the formation of COX. The dependence of the conversion of methane and the yields of C2H6, C2Hd, CO and COZon residence time all gave straight lines through the origin in the range of temperatures and residence times studied. This is evidence that the rate determining step is a surface reaction and the intermediates for the formation of Cz and COXare formed on the catalyst surface [2]. The rate constants for methane conversion, Cz and COXproduction are listed in Table 1. These suggest that the SrO additive suppresses the total oxidation of CHI.
+
+
65.00-
52.00
-
k-\ [ I
39001
75.00
60.00
45.00
3dOO
Figure 1. The residence times ver sus the selectivities over LC.
I
4
Figure 2. The residence times versus the selectivities over SLC.
221 1 Table 1 The values of kc,/kcH, and kcox/koc, at various temperatures. Catalyst
LC SLC
*
Ratio
Temperatures, K
998
1023
kc,/kcH,
0. 56
kcox/kcH, kc,/kcH,
0. 5 5
1048 0. 61
1073 --
0. 42
0. 44
0. 44
--
0. 51
0. 6 3
0. 69
0. 74
kcoX/ka, 0. 35 0. 31 0. 32 0. 35 kc,, kCH,, and kco, are rate constants for methane conversion and the production of Cz and COX.
3. 2. The reaction mechanism over the LC catalyst. Since the ratios of Cz/CO, increase with the reaction temperature and Cz and COXas products are formed simultaneously over both LC and SLC, it is reasonable to suppose that COXmainly results from the total oxidation of the CH3 radicals. If the reaction over LC follows the Langmuir-Hinshelwood mechanism and the adsorption of CH4and 0 2 on the surface can be expressed by the Freudlich formula, with the assumption of the stationary state we can obtain the following equations: 1% (2Rc2+ Rco,) = p log PO,+ q log PCH,+const.
c11
= X log Po,+ const.
PI
log (Rcz/R&,,)
By plotting log (2Rc2+ R c ~ , )against log Po, and against log P c ~,, we can obtain the values of p and q from the gradients. Also, by plotting log (Rc2/R&,x) against log Po,, the X value of about 2 can be obtained. The data are listed in Table 3. For comparison, the experimental results of the reaction orders in Table 2 The apparent reaction orders and the X values over the LC catalyst. Reaction order with respect to Temp. K
O2 Exper.
X value
CH4 Derived,
Exper.
Derived,
998
1. 0
0. 8
1. 0
1. 0
1. 9
1023
1. 1
1. 0
1. 0
0. 7
2. 0
1048
1. 0
0. 9
0. 8
0. 8
1. 8
221 2
oxygen and CHI are also listed. The two sets of data are consistent with each other. This suggests that the oxidative coupling of methane over the LC catalyst follows the Langmuir -Hinshelwood mechanism and oxygen adspecies in a molecular form are responsible for the reaction.
3. 3. The reaction mechanism over the SLC catalyst. It was found that over the SLC catalyst the reaction follows the Rideal-Redox mechanism, and can be expressed as follows:
Again, based on the assumption of the stationary state, we obtain the following equation :
By plotting (Rcyo.5/Rco,)against (PCH,/POy0.5) , a straight line can be obtained for the SLC catalyst. The values of (kzk4".'/klk3) and k4°.5/k3 are listed in Table 3. This is evidence that the reaction over the SLC catalyst follows the RidealRedox mechanism and an oxygen adspecies in its atomic form is involved. Table 3 The values of (k2k40.5/klk3)and k40.5/k3over the SLC catalyst. Temp. , K 998 1012 1048 k~k4'.~/kik3 k4".5/k3
0. 38
0. 30
0. 49 0. 32
0. 50 0. 49
4. REFERENCES
1 Y. -D. 2
Xu, L. Yu, J. - S . Huang, W. Li and Z. - Y . Lin, Proc. the Intern. Conf. on Petrol. Refining and Petrochem. Processing, Vol. 3, p1373, September 11-15, 1991, Fkijing, China. J.B. Deboy and R.F. Hicks, J. Catal. , 113 (1988) 517.
GUni, L a al. (Editors), New Frontiers in Catalysis Proceedingsof the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights aserved
OXIDATIVECOUPLING OF METHANE OVER SOL-GEL MAGNESIUM OXIDE CATALYSTS: EFFECT ON SELECTIVITY TO OLEFIN FORMATION R. GomeP, T. Lopez': L. Herreraa, A. A. Castid, 0.Scelzab, G.Baronetfib,E. Lazzarib, A. Cumc, M. CamposC, E. Poulainc,A. RamirezSolisC and 0.Novarod aUniversidad Autonoma Metropolitana-I, CBI, A.P. 55-534, Mexico 09340, D.F., Mexico bINCAPE, Santiago del Ester0 2654,3000 Santa Fe, Argentina CInstitutoMexican0 del Petroleo, S.G.I.A., A.P. 14-805, Mexico 07730, D.F., Mexico dInstitutode Fisica, UNAM,A.P. 20-364, Mexico 01000, D.F., Mexico
Abstract A comparative study on the partial oxidation of methane was made in Li-doped magnesium oxide catalysts prepared by the sol-gel method, (i.e. gelation of magnesium ethoxide) or by wet impregnation of commercial MgO support. Activities were comparable in both preparations. However, selectivities to ethylene are up to five times higher in sol-gel than in impregnated catalysts. Theoretical calculations on selected model clusters were made through semi-empirical and ab-initio methods. Semi-empirical results show a trend towards the formation of [Lit 0-1 centers when the Li concentration increases. HF-SCF calculations using large gaussian basis sets on linear clusters indicate a loss of more than 1 electron on oxygen when a magnesium atom is replaced by lithium. 1. INTRODUCTION
The partial oxidation of methane is a useful reaction in the utilization of natural gas. Its industrial application strongly depends on the yield to olefins or high hydrocarbons. From a large number of catalysts, the Li-doped magnesium oxide is the more promising one [1,2]. Modifications to this catalyst have already been made using metal oxides as additives, i.e., CaO, NiO, SnO and T i 0 [3]. However, only slight selectivity improvement WEB found. The special feature of Li/MgO catalysts is related to the (Li+ 0-1 species which are supposed to be the active sites [2]. These active sites are created by substitution of a Mg ion of the MgO structure by a Li. The methyl radical formation mechanism is done by abstraction of a hydrogen atom from the methane on a 0 ion site. The coupling, in gas phase, of two methyl radicals forms ethane. Successive oxidative dehydrogenation of ethane on the 0 ion site drive the reaction to ethylene formation. The activity and selectivity of Li/MgO catalysts seems to depend on the number of [Li+ 0-1 species formed during their preparation. Li-doped magnesia catalysts were usually prepared by wet impregnation of the magnesia support with lithium salt. The
2214 substitution of a Mg ion by a Li ion depends then on the hydrolysis of MgO during the impregnation step. An improvement in selectivity has been observed when the lithium ion is added to a Mg(OH)2 precursor [3]. In the present work, a promising alternative is reported: the Li/MgO catalysts were prepared by the homogeneous hydrolysis of magnesium alkoxide in a solution containing ethanol, water and LiC1, i.e., the sol-gel method [4]. Additionally, theoretical semi-empirical and ab-initio methods were made to calculate the oxygen ion charge in a Li-doped MgO cluster and the formation of [Li+ 0-1 centers. 2. EXPERIMENTAL
Li/MgO sol-gel catalysts: A solution containing Mg(0Et)z (0.05 mole), 0.85 mole of ethanol and 0.02 mole of ammonium hydroxide was refluxed under constant stirring. After 10 min of reflux a LiCl aqueous solution was added by drops in the appropriate amounts to obtain 1 and 2 wt% of Li. The solution was kept at reflux and under
constant stirring until the gel was formed. Impregnated catalysts: Sol-gel (labeled Li/MgO-I-sol-gel) and as a reference, the commercial (Degussa) MgO supports (labeled Li/Mg-I-com) were impregnated with aqueous solutions of LiCl. The products were dried at 70' C for 12 h and then treated at 750' C for 12 h. Catalytic activity: The activity of the oxidative coupling of methane was measured at atmospheric pressure in a flow quartz reactor at 750' C. Gas analysis was carried out with gas chromatography. The feed gas ratio composition was CH4 / 0 2 and the contact time was 3.35 g s/ml CH4PTN. 3. RESULTS AND DISCUSSION
3.1 Catalytic Activity
Activity and selectivity to C2 products are reported in Table 1. It can be seen that the activity and selectivity of sol-gel preparations is quite different from the commercial catalysts. The most important result was obtained in the high selectivity to ethylene shown by the Li/MgO sol-gel preparations. Catalysts prepared by impregnation of an MgO sol-gel support or by the hydrolysis of magnesium ethoxide have the same behavior. For these catalysts the activity is generally lower than that shown by the commercial one. However, selectivity to ethylene is up to six times higher in sol-gel
Table 1. Activity and selectivity for the oxidative coupling of methane in Li/MgO catalysts. Catalyst Conversion S% C2H6 S% C2H4 CzH4/CzHti Li/MgO Li/MgO Li/MgO Li/MgO Li/MgO Li/MgO
sol-gel 1% sol-gel 2% I-sol-gel 1% I-sol gel 2% I-com 1% I-com 2%
11.6 20.2 24.0 21.0 21.4 32.9
24.0 6.6 21.2 6.4 36.2 17.1
19.0 35.4 25.4 40.2 28.8 12.8
0.8 5.4 1.2 6.3 0.8 0.7
221 5 catalysts when the Li content is 2 wt%. A t low metal content (1 wt%) all catalysts show the same behavior. The high selectivity of sol-gel catalysts should be, originated in the preparation method. The incorporation or deposition of the promoter Li in a sol-gel magnesia support seems to have an important effect in the nature of the active centers rather than in their number. The exchange of a Mg ion by a Li ion in the gel results in the formation of centers that produce the successive oxidative dehydrogenation, and hence, high yields to ethylene. 3.2 Theoretical Calculations In order to test the hypothesis of a large charge variation on oxygen when a magnesium atom is replaced by lithium we performed semi-empirical and ab-initio calculations. Semi-empirical Extended Huckel (EH) calculations were made on the 41-atom cluster proposed by Mehandru et al [5], which consists of three planes of 9, 16 and 16 atoms. The first plane has a magnesium atom in the center. For different Li concentrations (from 2.5 to 15 wt%) all structures were optimized with the FCC constraint. The oxygen Mulliken charge population was found to decrease from -1.57 to -1.1 as lithium concentration increased. Larger doping can diminish the oxygen charge below -1.0. At high concentrations, the most stable structures have the lithium atoms placed on the edges but these structures are not the ones having the lowest oxygen charges. [Li+ 0-1 centers can be observed from 5 wt% onwards. This concentration effect is in agreement with the experimental results of methane activation by Li/MgO catalysts of Driscoll et al. (61. The ab-initio Hartree-Fock Self Consistent Field (HF-SCF) calculations using large gaussian basis sets were made on linear clusters of MgO containing 7 and 11 atoms (one central oxygen). Prior tests of charge stability were made on one, two and three-dimensional clusters with the same number of atoms on each direction using Extended Huckel calculations. It was found that one-dimensional clusters are good models as far charge density concerns. The SCF calculations have been made with the PSHF code [7] which includes the pseudopotential method of Durand et al. [8]. The gaussian basis sets are of (5s5p/4s4p) type for 0, (4slp/3slp) for Mg and Li [9]. A pseudoatom(X) at both ends of the linear cluster was used obtaining almost identical charge density for the central oxygen when magnesium atoms were placed a t the ends. Once the MgO linear clusters were optimized, one of the Mg atoms bound to the central oxygen was substituted by a Li atom allowing geometry optimization (in Cmv symmetry) to compare the resulting charge distribution with that previously obtained for pure MgO. Comparative analysis of the 7 and ll-atom charge distributions reveals almost identical results for the central oxygen atom on pure MgO clusters. The calculated charge distribution for 0 was -1.53 and t1.52 for the nearest Mg atoms. This Mulliken population analysis agrees very well with the experimental values of f1.6 [lo]. Therefore, subsequent calculations were made using the 7-atom cluster with 24 electrons for MgO (23 for Li/MgO). When lithium was substituted a great charge distribution modification was obtained leading to a loss of more than 1 electron on the central oxygen. Mulliken populations are shown in Table 2. Table 2. Mulliken population for the linear Li/MgO cluster. X 0 Mg 0 Li
to38
-1.12
t1.58
-0.32
-0.61
0
X
-0.52
t0.61
221 6 4. CONCLUSIONS
The Li/MgO catalysts prepared by the sol-gel method produce more selective solids to ethylene formation. Semi-empirical and ab-initio calculations point towards a charge loss of oxygen atoms when the lithium concentration grows. For a 15 wt% Li doping [ L i t 0-1 centers are observed using an EH semi-empirical method. 5. REFERENCES
T. Ito and J.H. Lunsford, Nature 314 (1985) 721. T. Ito, J.-X.Wang, C.-H. Lin and J.H. Lunsford, J A C S 107 (1985) 5062. S.J. Korf, J.A. Roos, L.J. Veltman, J.G. Van Ommen and J.R.H. Ross, Appl. Catal. 66 (1989) 119. 4 T.Lopez, I. Garcia-Cruz and R. Gomez, J . Catal. 127 (1991) 75. S.P. Mehandru, A.B. Anderson and J.F. Brazdil, JACS 110 (1988) 1715. 5 D.J. Driscoll, W. Martir, J.-X.Wang and J.H. Lunsford, JACS 117 (1985) 58. 6 J.P. Daudey, based on the HONDO version QCPE program 338, M Dupuis, J. 7 Rys and H.F. King, J . Chem. Phys. 65 (1976) 111. 8 Ph. Durand and J.C. Barthelat, Theoret. Chim. Acta 38 (1975) 283; J.C. Barthelat, Ph. Durand and A. Serafini, Mol. Phys. 33 (1977) 179. 9 G.F. Pacchioni, P. Fantucci, G. Giunchi and J.C. Barthelat, Gazz. Chim. Ital. 110 (1980) 183. 10 B. Deprick and A. Julg. Nouveau Journal de Chimie 11 (1987) 299. 1
2 3
Ouczi, L a al. (Editom), New Frontiers in Catalysis
Proceedings of the 10th International C o n p on Catalysis,19-24July, 1992, Budapest, Hungary Q 1993 Elscvier Science Publishers B.V. All reaewed
INFLUENCE OF THE ION CHARGE AND COORDINATION STATE ON CATALYTIC PROPERTIES OF BARIUM FERRONIOBATE FOR METHANE OXIDATION D. Filkovaa, I. MitovO, L. PetrovO, V. Bychkovb, M.Sinevb, Yu. Tuleninb and P. Shiryaevb
*Institute of Kinetics and Catalysis, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria bInstitute of Chemical Physics, Russian Academy of Sciences, Moscow 117049, Russia
Abstract Three sampler of r a t i o (B%FeXNb2-X0b,
perovskite structure, X ~ 0 . 9 ,1.0,
1.11,
differing
i n FdNb
have been synthesized.
They were characterized by XRD and Moessbauer spectroscopy.
The
c a t a l y t i c p r o p e r t i e s of t h e three sampler i n methane o x i d a t i o n 0
range 600-850 C.
r e a c t i o n wore studied i n t h e tempW8tW. explanation
of
the
higher
selectivity
of
the
samples
An
with
r e l a t i v e i r o n or niobium excess was proposed.
INTRODUCTION
Mixed oxides of perovskite s t r u c t u r e are o f t e n used f o r t h e i n v e s t i g a t i o n of s t r u c t u r a l and chemical m o d i f i c a t i o n i n f l u e n c e on t h e i r
c a t a l y t i c p r o p e r t i e s by p a r t i a l s u b s t i t u t i o n of
cations i n & s u b l a t t i c e
the
f o r t h e c a t i o n s w i t h close i o n i c radium
and d i f f w e n t charge C 1,23. In Used
-
our case where X
barium 0.9,
ferroniobate
1.0,
and 1.1.
BafleXNb2,X06
There
is a
(BFN)
wu
porsibility t o
obtain t h e c a t i o n s i n non-convmtional charge and/or
coordina4+ t i o n s t a t e f o r such system by varying t h e Fe/Nb r a t i o . The Fo
cations may be s t a b i l i z e d
i n the
mixed oxides o f
perovskite
s t r u c t u r e as a r e s u l t of charge c o m p m u t i o n t31. Thus, r i a t i o n of t h e o x i d a t i o n degree
of the
t h e va-
c a t i o n s may be reached
without an a d d i t i o n o f t h e f o r e i g n cations.
EXPERIMENTAL To prepare the BFN, iron niobate FexNb2-x04 was synthesized first by Fe2OO3-Nb2O5 mixture calcination successively at BOO, 900, and 1100 C for 8 hours with intermediate grinding. The re-
action of this compound with BaC03 leads to BFN formation. Phase composition of the mixed oxides was investigated
with
Cukd-radiation on DRON-2 X-ray diffractometer. The state of Feions was studied by Moessbauer spectroscopy on CNCA-1000 s p u trometer (Wissenschaftliche Electronic GMBH) at room
tempera-
ture. Catalytic experiments were performed on a flow type quartz reactor with on line gas chromatographic analysis. Methane to air ratio was 111. More detailed description is given by Sinev et al. L41. The temperature range investigated was 600
- 850°C.
RESULTS AND DISCUSSION The three investigated samples contain a single phase o$ cubic perovskite structure with a lattice parameter a = 8.094 A The most of the Fe-ions in the single-phase BFN occupy the octahedral B-positions
and their
oxidation degree
depends on
Fe/Nb ratio to make the mean value of the charge in
B position
equal to (4+). The increase of X from 0 . 9 to 1.1 tortion of octahedral
leads to dis-
3+
Fe
-cation environment (five coordinato Fe4+ cated 5C-Fe3+ with an oxygen vacancy) and further
-
tion formation. High spin Fe
3+
ions are detected also (Tabla 1)
The catalytic activities are slightly dependent on X value but the C2 selectivity varies from one sample to another (Figs 1, 2 ) . The sample with X = 1.0 is the most active in Carbon oxides formation. The samples with the deficiency or the excess of iron are more selective in C2 hydrocarbon% formation. This fact
indicates that
the
non-stoichiometric
samples contain
additional sites able to activate the methane molecules with formation of C%-radicals hydrocarbons formation 153.
which are the intermediates f o r Ca
2219 rable 1 floesobauer
Spectra
Characteristics
of
the
Studibd
Samples,
Percent Content o f t h e D i f f e r e n t I r o n Species
- (0.05-0.07)
Isomer Shift (0.43-0.45) (mm/s)
3+
X
Fa
Fe
doubl.
4+ doubl.
(0.33-0.36)
HS-Fe
3+ sext.
(0.17-0.20)
SC-Fe
3+
rewt.
40C2SEL.o/o
I
T9:
TOC
.
F i g . 2 C2 s e l e c t iv i t y dependan-
F i g 1 Methane conversion depen-
ce on t h e temperature
dance on t h e temperature
x
->
x=o.9,
+ ->
I n previous papers t S , 6 3 02-
coupled
CH3-radicalr
with
the
formation.
x=1.0,
0
->
X=l.l
-
i t was proposed t h a t 0
reducible
cations
The presence o f
s i t e s and
might be a c t i v e i n 4+ Fe i o n s makes b o t h
these types of a c t i v e oxygen species p o s s i b l e t o appear i n t h e sample w i t h X
= 1.1 by an e l e c t r o n t r a n s f e r l e a d i n g to tFe3'O-l
p a i r format ion.
2220 It is more complex to explain the high efficiency of the iron-deficient sample. T h u o are at least two ways to reach tho charge compensation in this case: to decreaso tho charge of iron ion8 (Fe3+-> Fo2+ ) or of niobium tNb5+-> Nb 4+ The
>.
energetic proforability botween tho two ions reduction may be evaluated approximately from the enthalpios of oxygon 8b.tr.Ction reactions of the typo: MmOn--> MmOn,l+ COI, for Fe2% and Nb2%. Basing on these calculations on0 can say that the reduction of Fe3+ cation is preferable. However, in both cases high ionic radii of the decreased charge cations must lead to lattice strain. It should be more pronounced in the case of iron reduction ( A r = 0.13 A) than in the came of niobium reduction ( A r = 0.01 The absence of F.~+ cations in the M O e 8 8 b ~ U ~ spectra indicates that the socond factor is more important.
a,.
The lattice strain compensation should be simplified in the surface layer. of the solid. It means that the excoss of Nb ions when X < 1.0 may be concentrated on the surface. The high negative mthalpy of Nb4+ oxidation may cause an oxygen chemisorption M (n-l)+ 02--> Mn+02
-. The
participation
of O2
- in
me-
thano activation was proposmd in C73. Moreover, tho transformation of O2 at elevated temperatures may lead to the form&-
-
tion of additional 0- sites# 2#+02
--->
2Ff+O-+
02.
REFERENCE8 1. R.J.H. Voorhoeve, Advanced Materials in Catalysis, Burton and D.L. Garten (ed.), A?, New-Ywk, 1977, p. 129
J. J.
2. K. Otsuka, T. Komatsu, K. Jinno, Y. Uragami, and A. Morikawa Proc.IX Intern. Congr. on Catalysis, V.2, Calgary 1988, p.913 5.C.T. Luiskutty, P.J.Ouseph.So1id State Commun.,l3 (1973) 405 4. M.Yu. Sinev, G.A. Vorob'eva, and V.N. Korchak, Kinet. Catal. (Russ), 27 (1986) 1164 5 . D.J. Driscoll, W. Martir, J.-X. Wang, and J.H. Lunsford, J. Am. Chom. SOC., 107 (1985) 58 6.D.J. Driscoll and J.H. Lunsford,J. Phys, Chrm.,89 (1983) 4415 7. J.X. Wang and J.H. Lusford, J. Phyd. Chom., 90 (1986)3890
Guczi, L et al. (Editors), New Frontiers in Cafalysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights resewed
REACTION PERFORMANCES OF METHANE OXIDATIVE COUPLING ALONG CATALYST BED WITH Sm-CaO CATALYST
C.Tang, L. Lin,Z. Xu and J. Zang Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O.Box 110. Dalian 116023, China
Abstract Mass spectrometer has been used to measure in situ the reactant and products (CH4, 02, CO, COZ, C2H4, C2H6) distributions of methane oxidative coupling over 2%Sm/CaO. The results indicate that the rate of CH4 conversion does not vary with the continual consumption at 1023K. At 873K, the poisoning effect of COz in the catalyst bed must be considered. 1. INTRODUCTION
Methane oxidative coupling(M0C) is a complex reaction comprising of homogeneousheterogeneous processes[ 11. With the conventional reaction system, it is difficult to estimate the relative contribution of heterogeneous vs homogeneous reactions during CH4 oxidative coupling to C2 hydrocarbons. In this work, the product distribution along 2%Sm/CaO catalyst bed has been measured in situ and the characterization of MOC reaction is illustrated. 2. EXPERIMENTAL
The 2%Sm/CaO catalyst was prepared by impregnating calcium oxide with an aqueous solution of samarium nitrate followed by drying and calcination. The product distribution was measured by locating a molecular leak which was at the end of a lmm-I.D. quartz tube at different height of the catalyst bed during the reaction. Part of the effluence of the reacting gas was directly led through the leak into a quadrupole MS analyzer. The reaction was carried out at ambient pressure, while the pressure at the molecular leak was below 1 ~ 1 0Torr. . ~ Moreover, the length of the leak was less than lmm to avoid further reaction of inlet material. The amount of catalyst was about 250mg(30mm bed height) and the flow of reaction gas (46%CH4+ 16%02+38%He)was 46ml/min(STP). 3. RESULTS AND DISCUSSION
The product distribution of the temperature programmed reaction of MOC over a 2%Sm/CaO catalyst shows that the conversion of CH4 to COX is initiated at about 823K,
2222
the coupling reaction starts at 923K and the yield of Cz reaches a maximum at about 1073K. When the temperature exceeds 1073K, the yield of Cz drops. Fig.lA shows a set of data of product distribution along 2%Sm/CaO catalyst bed at 1073K. From the slopes of the CH4 curve of Fig.lA, it can be calculated that the rate of CH4 conversion in the catalyst region is 40-times larger than that in the preheating zone of packed A1203. This result clearly reveals that the MOC reaction mainly occurs in the catalyst bed and the contribution of the homogeneous reaction of CH4 and 02 can be neglected in our experiment.
'+.+l+ Bp
M
4
10-
i -
z" 0
F:
2b
5
0
4-
2
-
w
D
u
z 0 u
=='t
--
1.0 -
O
0
0.5 -
0.0 1-
' 0
A
0.05
1-
1,
d,A
t" 0
A
A
-
-
0
-
I
10
15
20
25
BED H E I G H T , m m
(A) 1073K
5
I
1
I0
15
I
20
BED H E I G H T , m m
(B) 873K
A1203 Fig. 1 Reactants and products distribution of MOC reaction along catalyst bed. @%@2%Sm/CaOmempty,CH4(+),a( O),CO( n),CO2( N),CzH6( A),CzH4( A).
Fig.lA also shows that the concentrations of reactants and products along the catalyst bed vary significantly only within a certain height of the catalyst bed where 02 is not
2223 completely eliminated. Behind this region, all the curves for concentration vs bed height reach an inflexion point, then only a slightly change can be. observed. In other experiment(see Fig. lB), MOC reaction and CzHa dehydrogenation can not proceed behind the catalyst bed where remaining 0 2 is still present. These results indicate that the transformation of CH4 to C2 and COX and dehydrogenation of CzHa to C2H4 proceed remarkably only when both the catalyst and 02 are co-existing. All the results suggest that C2 formation and other main reactions occur mainly over the catalyst surface. In Fig.lA the concentration of 02 decreases linearly along the catalyst bed until it reaches zero. Apparently, the rate of CH4 conversion does not vary with the continual consumption of 02, even near zero concentration of oxygen. From this phenomenon we can conclude that MOC reaction is a zero order reaction with oxygen and the forming of oxygen active sites is not the rate determining step of the MOC reaction at 1073K for the 2%Sm/CaO. The same relationship can also be observed on a Sm203 catalyst even at a lower temperature of 873K. Fig.lB shows the product distribution of MOC reaction over the 2%Sm/CaO catalyst at 873K. In Fig. 1B oxygen is not consumed linearly at the catalyst bed, and at the end of the catalyst bed the rates of CH4 and 02 conversion approach zero, though the catalyst and large amount of CH4 and 02 are still existing. For studying the reason of these fact, after the reaction at 873K, the temperature of the catalyst bed was increased to 1273K. A large amount of C02 was desorbed from the catalyst bed during temperature raising and C02 was wholly desorbed at 1073K. Therefore the inhibition scheme of active sites by C02 can be proposed as follows: ki 02 ---+2[0-] k2
CH4
+ [O-]
CH3.
+ [OH-]
lo c02
+ [o-]
__+
[CO3-]
According to the work of Lunsford et al[2], the methane activation is due to the [0-] active sites detected from the Li/MgO catalyst in the MOC reaction. In the Sm/CaO catalyst, the active sites may be [O-] too. In our experiments we find that the rate of CH4 conversion is independent from the gas phase concentration of 02 at 1073K so that it may be true also at 873K. If we assume that this kind of active sites adsorbs C02 at lower temperature, which causes the deactivation of the catalyst, and the amount of the [O-] sites covered by C02 is proportional to the partial pressure of COz, elsewhere, the rate of CH4 conversion is first order with methane in nature, the relation between reaction rate (dx/dh)/Pc~4along the bed and C02 partial pressure Pc02 can be expressed as: (dx/dh)/Pc~4= k2(h- lOPc02) where P C His~ the partial pressures of CH4 and C02 respectively. kz denotes the rate
2224
constant of CH4 conversion and b is a constant correlated to COz adsorption. 80 is a constant correlated to zero order of 02 consumption.
0.018 0
0.016
0.014
-
n
‘E
0.012
J
rd
0.010
7
E E 0.008
v
‘d
X
0.006 ?c
x
0.004
-Q
0.002
0.000
0.0
0.2
0.4 0.6 pco2 ( K
0.8
1.0
1.2
W
Fig.2 The relation between (dx/dh)/Pcw and the partial pressure Pc02. The plot of (dx/dh)/Pc~4vs Pc02 gives a linear relation as shown in Fig.2 which indicates that the above speculation is right. This result implies that when the catalysts with basic supports or promoters for MOC reaction are studied at lower temperature, the poisoning effect of CO2 in the catalyst bed must be considered. 4. REFERENCES 1 T. Ito, J.Wang,C. Lin,and J.H. Lunsford, J. Am. Chem. Soc.,107(1985)5062. 2 C-H. Lin, J-X. Wang, and J.H. Lunsford, J. Am. Chem. Soc.,109(1987)4808.
Guni, L et al. (Editors), New Frontiers in Colalysir
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
EFFECT OF PRESSURE ON THE METHANE OXIDATIVE DIMERIZATION
Yu.P. Tulenin, A. A. Kadushin, V.A. Seleznev and A. F.Shestakov Institute of Chemical Physics of the Russian Academy of Sciences, Kosygin Str. 4, 117334 Moscow,Russia
Abet rac t It is Shown that i n methane oxidative dimerizstion over 1% Ia$3/?dgO c a t a l y s t a t 1123 K i n conditions of gas phase process suppression the increase of pressure from 0.1 t o 0.8 Mra
gives r i s e t o an increase of methane conversion.which is accompanied by a more than t r i p l e increase of ethylene/ethane ratio. The s e l e c t i v i t y of the r e a c t i o n products formation does not d i r e c t l y depend on presaure being influenced only by the degree of me thsne conversion. INTRODUCTION
I n the previous work (11 the p o s s i b i l i t y was shown of volu-
me process heterogeneous i n h i b i t i o n by magnesium oxide i n oxidative methane transformation under gas pressure. A scheme was suggested of the gas phase r e a c t i o n i n h i b i t i o n mechanism
based on the double r o l e of MgO surface which i s simultaneously a source and an adsorbent of a c t i v e species. I n paper [2] i t was &own that on increase of pressure i n methane oxidative dimerieation an increase of the r o l e of the gas phase (non-catalytic 1 reaction takes place. A t low l i n e a r velocit i e s of the gas flow i n the region of pressures 0.4-0.6 W a the r o l e of the c a t a l y s t c o n s i a t s only i n the change of the r a t i o CO/C02 i n the o u t l e t stream. The e f f e c t of t h e gas phas e r e a c t i o n was decreased by increasing the l i n e a r v e l o c i t y of the gas flow. It should be noted that i n the work [ 2 1 there was no f i l l e r behind the c a t a l y s t layer. This could lead t o f u r t h e r transformations of the r e a c t i o n products i n the hot zone. I n the present work the e f f e c t of pressure on the catalysed r e a c t i o n was studied i n conditions of heterogeneous i n h i b i t i o n of the gas phase r e a c t i o n and of decrease of the r e a c t i o n products residence time i n the hot zone.
EXPERIMENTAL
The experiments were c a r r i e d out i n a laboratory set-up which allowed t o work at pressures up t o 2.5 MPa and temperat u r e s up t o 1200 K. The r e a c t o r was a quartz tube of 4 mm ln-
n e r diameter. The temperature i n s i d e t h e r e a c t o r w a s c o n t r o l l e d by a chromel-alumel thermocouple i n s e r t e d i n t o a q u a r t z c a p i l l a r y of 1.2 mm o u t e r diameter. The zone of s t a b l e h e a t i n g was 6 mm long and beyond i t s l i m i t s t h e temperature s h a r p l y decreased. The l i q u i d p r o d u c t s were condensed immediately a f t e r t h e r e a c t o r . The o t h e r components of t h e mixture were analyzed by g a s chromatography. The g a s mixtures used had t h e following composition (vol.%): CH -67.5; 0 -29.5; N -3.0 and CH4-55:7; - C2H6-1 1.0; 02-33. 3. 1.08wt L a 2 ~ 3 ? ~ (gs ~u r l a c e a r e a f r a c t i o n of 0.25-0.50 mm) was used as c a t a l y s t i n 16.0 mdg-'t h e s t u d y t3]. 5 m g of t h e c a t a l y s t was loaded i n t o t h e react o r and i t occupied 20% of t h e h e a t e d zone volume. A s f i l l e r of t h e r e a c t o r f r e e volume magnesium oxide ( s u r f a c e a r e a 0.4 d g - l f was used. The experiments were c a r r i e d o u t i n t h e range of p r e s s u r e s 0.1-0.8 MFa and temperatures 1073-1123 K. Small amounts of hydrocarbons C3 o r h i g h e r were n o t taken i n t o account i n t h e c a l c u l a t i o n s . RESULTS AND DISCUSSION
In t a b l e 1 t h e dependences a r e shown of t h e c a t a l y z e d r e a c t i o n parameters on p r e s s u r e at T=1123 K. Table 1 The e f f e c t of p r e s s u r e on t h e parameters of o x i d a t i v e methane dirnerization over 1% k2O3/MgO (CH4:02:N2=67.5:29.5:3.0; T=1123 X) T r e s s u r e , Contact Conversion, ol 70 time, s MPa CH4 O2
Selectivity, % C2H6
C2H4
CO
c 2'421. GO2
C2H6
H2
Prom t h e t a b l e i t f o l l o w s t h a t a n i n c r e a s e of p r e s s u r e at a f i x e d c o n t a c t time c a u s e s a n i n c r e a s e of methane and oxygen conversion. Under t h e s e c o n d i t i o n s t h e y i e l d of e t h y l e n e i n t h e i n d i c a t e d p r e s s u r e range i n c r e a s e s and r e a c h e s a l i m i t i n g value t h i s being accompanied by a n i n c r e a s e of t h e e t h y l e n e / ethane r a t i o ( s e e f i g . l a ) . I n f i g . Ib t h e dependences a r e shown of t h e C2-sel.ectivity and t h e r a t i o C H /C2H6 on methahese curves is ne conversion. The c h a r a c t e r i s t i c f e a t u r e o
$2
2227
the presence of a kink a t methane conversion of 30-3s. It i s important t o note that the C - s e l e c t i v i t y values and C2H4/C2% r a t i o s obtained i n experiments c a r r i e d out at atmospheric pressure and d i f f e r e n t contact times 2 and i n experiments
0
€R
+
cu
6
u
i 4
H
M2
E.l
40
-
-4 3
Bp
w
PRESSURE, MPa
Q xN
?& xcu
zi
2"
5w
1 2
0
CONVZXSION, %
Figure 1. Dependence of the y i e l d and the r a t i o C2H4/C2H6 on pressure (a), the C - s e l e c t i v i t y and the r a t i o C2H4/C2H6 on methane converaion ?$) a t Tall23 K. performed at fixed 2 and pressures from 0.1 t o 0.8 MFa f a l l on one and the same curve. Thia evidences t h a t at fixed val u e s of the temperature and conversion the r e a c t i o n products composition does not depend on the way by which the conversion is achieved (by changing the pressure o r the contact time). T h i s i s probably due t o s a t u r a t i o n of the c a t a l y s t surface by i n i t i a l substances. Under these condition8 varist i o n s of the contact time and pressure e x e r t a s i m i l a r e f f e c t on the f i l l i n g up t o the surface. It may a l s o be noted that there is no c l e a r c o r r e l u t i o n between the dependences of hydrogen, ethylene and ethane y i e l y i e l d s on conversion. I n order t o tarst the possible assumptions concerning the contribution of processes developing i n the r e a c t o r behind the c a t a l y s t layer r e a c t i o n s were studied f o r ethane-containi n g mixture and the same other parameters on s u b s t i t u t i n g the c a t a l y s t by magnesium oxide (see t a b l e 2). The r e s u l t s obtained show only a negligible contribution of methane and Cg-hydrocarbons t o t a l oxidation although a high conversion of ethane i n t o ethylene takes place reaching 30%. However i n conditions of c a t a l y s i s the conversion of ethane i n t o ethylene and the t o t a l oxidation of C -products behind the c a t a l y t i c l a y e r w i l l be much l e s s due $0 the amaller cont a c t time of the C2-products with MgO.
2228
Table 2 The e f f e c t of p r e s s u r e on t h e parameters of o x i d a t i v e methane and ethane t r a n s f o r m a t i o n s over MgO (CH 4 :C 2H6 :Q2 =55.7:11.0:33.3; T=1073K,Z=0.228, V=2.8 cm.8”) ~~
?re ssure , MI’a
O u t l e t g a s e s composition, 4’0 vol.
CH4
O2
2H6
2H4
co
co2
H2
2+
~
0.1 0.4 0.6 0.8
52.5
54.3 53.4 55.0
31.0 30.2 30.4 29.3
9.3 8.9 7.9 7.8
2.1 2.2 3.0 2.9
0.6 0.5 0.5 0.5
1.7 1.5
2.8 2.4
1.8 1.6
2.9
3.0
11.4 11.1 10.9 10.7
CONCLUSION The following conclusions can be drawn from t h e s e r e s u l t s when t h e p r e s s u r e i s i n c r e a s e d from 0.1 t o 0.8 MPa: 1. The methane conversion i n c r e a s e s . 2. The r a t i o C2H4/C2H6 i n c r e a s e s by more than 3 times. 3. The s e l e c t i v i t i e s of r e a c t i o n products formation do n o t d i r e c t l y depend on p r e s s u r e and depend o n l y on conversion. REFERENCE Yu.P. Tulenin, V.A. Seleznev, V. N.Korchak, A.A.Kaduehin. Third European Vorkshop on C a t a l y t i c Methane Conversion, A b s t r a c t s , P o s t e r S e s s i o n B, 27-29 May, 1991, V i l l e u r banne-France. 2 A.!?xtrom, R.Regtop, S.Bhargava, Appl.Catal., 62 (1990) 253. 3 A.S. USSR N 14.82905 (1987). USSR Pat. Bul. N 20 (1989). 1
Guczi, L.er al. (Editors), New Frontiers in Catalysis Prcceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
METHANE OXIDATIVE COUPLING OVER COMPLEX METAL OXIDES POSSESSING KtNiFs AND RELATED STRUCTURE”
Q. Yana, Y. Jinb, Y. Wanp, Y. Chena andX. Fua aDepartmentof Chemistry, Nanjing University, Nanjing 210008, China bCenter of Material Analysis, Nanjing University, Nanjing 210008, China
Abstract The catalytic performance of complex layered oxides with K,NiF, structure composing of alternating perovskite (ABO, ) and rock salt (AO) layers was studied for the oxidative coupling of methane to C, hydrocarbons. The partial substitution of transition metal ions in B position of the oxide by low valence metal ions such as Li, Zn and Mg ions was found to greatly improve the C, selectivity. A C, yield of 22% was obtained under 1073 K over Ca(Sr,Ba)LaFeLiO catalyst. Study of the deactivition showed that the CaLaFeLiO sample can maintain a C, yield of 19.2% after 420 hr on stream. The results are discussed on the basis of catalyst structure,valence state of transition metal ions in B site, and surface oxygen species.
1. INTRODUCTION
Oxidative coupling of methane to give C2 hydrocarbons has attracted much attention recently because of the increasing interest of the direct utilization of natural gas. A large number of metal oxides have been reported to be active for the reaction [I]. Among those, the complex oxide with perovskite or rock salt structure has shown good catalytic properties [2-31. This paper is to report methane oxidative coupling (MOC) over complex layered oxides with K,NiF, structure composing of alternating perovskite (ABO, ) and rock salt (AO) layers. Effect of partial substitution of transition metal ions in B position on the activity and selectivity for MOC is discussed.
2. EXPERIMENTAL
Three series of catalysts with K,NiF, structure containing Fe, Ni, Cu ions in B site were prepared by solid state reaction. the catalytic reaction was carried out in a quartz * Project Supported by National Natural Science Foundation of China
2230 fix bed microreactor at 1073 K, 1 atm. and CH,/ 0, ratio of 2. The eMuent was analyzed by two on line GC. XRD measurements were performed on a D / MAX-RA X-ray diffractometer for phase analysis and lattice parameters determination. Transmission Moessbauer spectra were obtained in a constant acceleration mode, and zero velocity was defined with respect to the centroid of the metallic iron spectrum. XPS and SIMS were recorded using a VG ESCALAB / SIMSLAB MK I1 surface spectrometer. A Al Ka X-ray ( hv= 1486.6 eV ) was used. Binding energies were referenced to CIS peak at 285.0 eV. The primary ion beam for SIMS experiment is generated by FAB 61 (0-5 kV) ion / atom gun, the resolution of A MM 12-12 quadrupole mass spectrometer is 1 in 800.
3. RESULTS AND DISCUSSION
The catalytic properties of ten complex oxides are given in Table 1. It can be seen that SrLaFeO, is not very active for MOC, C, yield of only 6.3% is obtained. When the Fe atoms are partially substituted by Zn or Mg, the C, yield over SrLaFe,,, Zn(Mg),, 0, (designated as SrLaFeZn(Mg)O) increases to 15%. Similar results have been obtained on La,Ni(Cu)O,. On La,NiO,, methane conversion is very high, but no C, selectivity is observed. However, when half of the Ni atoms are substituted by Li, the C, yield over La,Ni,,,Li,,,O, (designated as LaNiLiO) increases to 15%. While for Ca(Sr,Ba)o,,La,~,Feo,5Lio,504 (designated as Ca(Sr,Ba)LaFeLiO) catalysts, the variation of A site cation does not show much effect on the selectivity, as shown by the fact that C, selectivities of 54-58% and C, yield ca. 22% are observed
Table 1 Catalytic activity and selevtivity of complex oxides with K,NiF, structure catalyst conversion(%) selectivity(%) 0, CH4 C, C2H4 CO CO, CaLaFeLiO 81.9 39.2 54.8 41.3 6.6 38.6 SrLaFeLiO 94.1 41.8 53.6 37.1 0 46.4 BaLaFeLiO 89.7 38.7 57.5 41.3 1.9 40.6 SrLaFeO, a 95.2 28.7 22.1 8.2 0 77.9 SrLaFeZnO * 95.3 36.2 41.5 19.5 1.2 57.3 SrLaFeMgO a 96.2 35.8 39.0 0 18.7 61.0 La, NiO, 95.3 83.1 0 0 99.3 0.7 LaNiLiO 94.2 35.3 42.3 0 21.9 57.7 La, CuO, 95.2 25.2 0 2.3 0 97.7 LaCuLiO 95.0 35.0 0 60.0 40.0 23.2 CaLaFeLiOb 88.0 41.2 46.5 32.6 Reaction condition: T = 1073 K, S.V.= 6600 hr-’, CH,/ 0, = 2. a: T = 1023 K; b: after 420 hr reaction.
yield(%) C, 21.5 22.4 22.3 6.2 15.0 14.0 0
14.9 0.6 14.0 19.2
2231 on the above three catalysts. Table 1 also reveals that CaLaFeLiO is quite stable, after 420 hr reaction a C, yield of 19.2% is maintained.
In order to correlate the catalytic activity and selectivity with the catalyst structure, surface state and valence state of the cation, the catalysts were characterized by XRD, Moessbauer spectroscopy, XPS and SIMS. XRD measurements demonstrate that all the samples as prepared possess the K,NiF, structure. The lattice parameters a, and c, of the samples containing iron are in the range of 0.3750-0.3876nm and 1.2758-1.31 66nm respectively, confirming the tetragonal symmetry in the samples studied is formed [4].
VELOCITY(mm / s) Figure 1. Room-temperature Moessbauer spectra (a)CaLaFeLiO, (b)SrLaFeO,
BINDING ENERGY (eV) Figure 2.01s XPS spectrum of CaLaFeLiO
The typical Moessbauer spectra of CaLaFeLiO and SrLaFeO, samples are shown in Fig.1. The spectrum of CaLaFeLiO consists of a doublet with isomer shift (IS)= -0.19 mm / s and quadrupole splitting (QS) = 1.04 mm / s; and for Sr(Ba)LaFeLiO samples similar Moessbauer parameters have been detected. The result reveals that iron exists as Fe4+ in all those three catalysts [5]. However, for the case of SrLaFeO, , Moessbauer parameters show that iron exists as Fe3+ (IS = 0.30 mm / s, QS = 0.98 mm / s); noteworthy, when half of the iron atoms are substituted by Zn or Mg ions, near 50% of the Fe3+ ions in the samples are converted to Fe4+ with
2232 IS=-0.17 mm/s, QS=0.30 m m / s for the substitution by Zn ions, and with IS =-0.12 mm / s, QS = 0.28mm / s by Mg ions. XPS results show a binding energy of Fe2p3/, at 712.3 eV and 711.2 eV for Ca(Sr,Ba)LaFeLiO and SrLaFeO, respectively. The above results further demonstrate that both in the bulk and on the surface of the two catalysts, iron exists as Fe" and Fe3+,respectively. The X R D results of LazNi(Cu)04 catalysts also confirm the formation of K,NiF4 structure. When Ni(Cu) is partially substituted by Li, the XPS spectrum of Ni2p,,,(Cu2p3,, ) gives a broad peak, which can be fitted respectively by Ni2p,/, peaks at 856.1 eV and 854.6 eV( Cu2p3,, peaks at 935.8 eV and 933.7 eV). The intensity ratio of higher binding energy peak to lower one is about 1.5:1. The peaks at 856.1 eV and 935.8eV have been assigned to Ni3+ and Cu3+ respectively [6-71. These facts implie that about 60% of Ni3+ (Cu" ) and 40% Ni2+ (Cu2+ ) exist in La,Ni(Cu)LiO catalysts. The Ni,03 - peak (m / z- = 164) in SIMS negative ion spectra of LaNiLiO once again indicates the existence of Ni3+.It seems reasonable to agree that for the case of existing metal ions such as Ni3+,Cu3+ and Fe4+ in oxides, a single-elec tron might transfer from the 0'- to the metal ion with the production of 0-. As shown in Fig.2 the 01s XPS spectrum of CaLaFeLiO can be fitted by two peaks at 531.9 eV and 529.4 eV, and which could be assigned to 0-and 0'- respectively [8]. Similar 0 1 s XPS spectra are obtained for LaNiLiO and LaCuLiO catalysts. The above results indicate that although the ten catalysts listed in Table, all have K,NiF4 structure, very different MOC activities are obtained. The substitution of transition metal ions in B position by low valence cations results in the formation of Fe4+ , Ni3+ , Cu3+ and ' 0 species on the surface and a great improvement of the C, selectivities. Apparently, the existence of surface 0-species and Fe4+, Ni3+, or Cu3+ plays an important role for the increase of the C, selectivity,
4. REFERENCES
G,J.Hutchings, MSScurrell and J.R.Woodhouse, Chem.Soc.Rev.,l8 (1989) 251. K.Asami, SHashimoto, T.Shikada and K.Kiyimoto, Chem.Lett.,(l986) 1233. A.Kaddouri, R.Kieffer, A.Krennemenn, P.Poix and J.L.Rehspringer, Appl.Catal., 51 (1989) Ll-L6. P.Ganguly and C.N.Rao, J.Solid State Chem., 53 (1984) 193. G.Demazeau, L.M.Zhu, L.Fournes, M.Pouchard and P.Hagenmuller, J.Solid State Chem., 72 (1988) 31. M.Oku, H-Tokuda and K.Hirokawa, J.Electron Spectrosc. Relat. Phenom., 50 (1990) 61. E.Sacher, J.E.Klembery-Sapieha, ACambron, A.Okoniewski and A.Yelon, ibid., 48 (1989) C7. P.Ganguly, M.S.Hegde, Phys. Rev., 37 (1988) 5107.
Guni, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congrcss on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
OXIDATIVE COUPLING OF METHANE OVER PbO/PbA1204 CATALYSTS
S. -E. Park and J.4.Chang Catalytic Research Divison, Korea Research Institute of Chemical Technology, 305-606 Taejon, Korea
Abstract The loading of PbO on y -alumina support could make the various phases of spinel between PbZtand Al)' with Pb,O, phase. These spinel phases between Pb2*and A13+were confirmed as magnetoplumbite-likestructure (PbA1,,OI9) and lead aluminate (PbASO,). At the 60wt.% loading of PbO on y -alumina, when it was treated at 750 "C for 4 hrs, it seem like to be mainly lead aluminate-supportedPbO catalyst. And this showed the highest activity and selectivityon C,, in the oxidative coupling of methane. Thus, in the case of PbO-y -alumina, the species responsible for the selective oxidative coupling of methane may be rather a PbO/ PbAl,O, catalyst, perturbed due to such a high loading and thermal treatment. 1. INTRODUCTION
Various kinds of low melting oxides have been reported as active catalysts in the oxidative coupling of methane since the pioneering work of Keller and Bhasin [11. Keller and Bhasin [11 as well as Hinsen and Baerns [2] reported that supported PbO catalysts are also active for the oxidative coupling of methane as oxygen supplier. Baerns' group [3] reported that for supported PbO catalysts, surface acidity influenced the C,-selectivity in oxidative coupling of methane. Also, Fujimoto's group [4] observed that when they used basic oxides as supports they got p a t improvements in methane conversion and C,-selectivity. Besides the effect of surface acidity, they postulated that the selectivity may be dependent on the ability of the catalyst to transfer lattice oxygen, which occurred by interaction with the lattice oxygen of PbO. Wendt et al. [5] ascertained that there would be a key role of reducibility of lead oxides and amorphousPbAl 120,9 phase, which has magnetoplumbite-likesmcture, as an active site. However, Marcelin's group proposed an "isolated site"-type mechanism which means that the isolation of strong oxidizing sites (PbO) on MgO could enable high selectivity [6]. In previous study the following phases on y-alumina-supported PbO catalysts were detected: lead aluminate (PbAl,O,), magnetoplumbite-likephase (PbA1,,OI9),red lead (Pb,O,), and solid solutions of PbO in y -alumina, lead aluminate or/and PbAl,,O,, [7]. At the 60wt.% loading of PbO on y -alumina, when it was treated at 750 "C for 4hrs, PbO orthorhombic and PbAl,O, spinel phases were dominant. The purpose of this study was to investigate the behavior of this PbO/PbAl,O, catalyst in the reaction of oxidative coupling of methane.
2234
PbO/PbAlz04catalyst was prepared by the incipient wetness method using an aqueous solution of lead nitrate. The mixture was dried at 120 "Cand calcined at 750 T for 4hrs.. For the preparation of solid-mixed catalyst, 6Owt.% of PbO and 40wt.% of ')' -alumina were mechanically mixed in an agar m o m and calcined at 750°C for 4hrs. The catalytic experiments were carried out in a conventional fixed-bed reactor (quartz, i.d. 10mm)at atmosphericpressure via cofeed method. The reactants and products were analyzed by an on-lined gas chromatograph (FID, TCD) with Porapak N and MS 5A columns. The Raman spectrawere obtained with a Jobin Yvon UlOOO spectrometerusing a Coherent's INNOVA 70 Argon laser, where the exciting line was typically 514.5 nm. For the ESCA study, binding energies of Pb 4f photoelectron lines, were obtained by VG ESCALAB Mark II photoelectron spectrometerusing A1 K, radiation. Spectra were obtained at 50 eV analyser pass energy and calibrated with the reference of adventitous C 1s at 284.6 eV.
3. RESULTS AND DISCUSSION The PbO loaded y -alumina catalyst contained magnetoplumbite-like(PbAl 12019) and lead aluminatephases mixed with Pb,04 and PbO othorhombicand PbO tetragonalphases depending on the contents of PbO. These phases seemed to be formed by the diffusion of Pb2' and AP* as well as lattice oxygen at the high temperature. Among them, the 60wt.96 loaded catalyst had mainly PbO and lead aluminate, which meant that it didn't have other spinel phases such as y -alumina and megnetoplumbite [7]. The solidmixed catalyst which same composition as the &t.% loaded catalyst showed much lower conversion of methane and selectivity on the C,, hydrocarbons. (Table 1) We already found the volcano-type patterns not only in (3H4conversion but also in C; selectivity were observed at this 60wt.% loaded catalyst among the various loadings [7]. Comparison of the catalytic activities over PbO/PbAl,04 (60% PbO on -alumina) and the solid-mixedcatalystsare shown in Table 1. WO/PbAI,04catalyst gave much higher activity and c,' selectivity. Laser Raman spectrum of this catalyst, coincided with XRD data, showed the large characteristic bands of PbAl,04 at 79,99,209,291, and 372 cm-' with 87,143,288, and 385 cm-I bands of PbO orthorhombicphase.(Figure l(a)) Table 1 Comparison of the activities of catalysts in the oxidative coupling of methane Catalyst
Conv. % PbO/PbAl,04 Solid-mixed
Selectivity(%)
(334
12.9 6.0
co* 32.3 47.7
CZH6
Z ' H4
47.2 38.9
16.4 13.4
',' 4.1
2235
104 x 3 (d) after reaction
Solid-mixed
Ub
a f t e r reaction
RAMAN S H l F T ( c 6 ' )
Figure 1. The laser Raman spectra of supported PbO catalysts.
I
I
I
I
I
I
150
146
142
138
134
130
Binding Energy (ev)
Figure 2. XP spectra of Pb 4f core levels in (a) PbO/PbAl,O, and (b) the solid-mixed catalysts.
Bindingenergiesof A1 2p and 0 1son the low PbO loading catalysts were very close to those of 7-alumina [7]. However, they changed to 73.2 and 529.7 eV at the 60% PbO loading, respectively, which values are quite similar to those of CoAl,04 spinel [8]. Also the binding energies of Pb 4f 7R at low loading and high loading catalysts were close to the value of PbO orthorhombic. X-ray photoelectron spectra of Pb 4f in PbO/PbAl,O, and the solid-mixed catalysts are shown in Figure 2. Pb 4f 7R peak in PbO/PbAl,O, catalyst observed at 137.4 eV. It means that supported PbO is not merely PbO itself but strongly interacted with the interacted phase, which is lead aluminate. So, we could call this Owt.96 loaded catalyst as lead aluminatesupported PbO (PbO/PbAl,O,) catalyst. And also, we would say that PbO orthorhombicphase on Pb%04 which was formed by the interactionof PbO with -alumina would be active species in the oxidative couplingof methane contrary to the results of Wendt and co-workers [5],who proposed the active site as an amorphous magnetoplumbite phase. On the other hand, in the case of the solid mixed catalyst PbO phase was major before and after reaction with much smaller content of PbAl,O,. The Raman spectrum of this solid-mixed catalyst showed mainly the bands of PbO due to the poorly scatteringproperty of 7 -alumina.(Figure 1(c)) After the reaction this spectrum had changed abruptly into (d) spectrum,but the Raman spectrum of PbOPbAl,O, catalyst after the reaction seemed to be relatively similar to that of before the reaction. The (d) spectrum showed
2236
the bands of Pb,04 which is known to behave chemically as a mixture of PbO and PbO, [9],that showed the existence of Pb4+cations. The solid-mixed catalyst could be assumed that the interactions of the components occurred only in the boundaries of the component places, which were mainly between ')'-alumina and PbO instead of PbA1,04 and PbO. That's why his mechanical mixture gave poor activity and selectivity in the oxidative coupling of methane. X-ray photoelectron spectra in the solid-mixed catalyst showed the Pb 4f,, bands at 138.5 eV with the shoulderat 136.5eV.(Figure 2(b)) This indicatesthat two phases existedseparately. It means that there was little possibility of interaction between PbO and PbAl,04. So, these experiments on the solid-mixed catalyst indicate that there should exist an interaction between PbO and PbA1,04 for obtaining high activity and C,-selectivity in methane coupling reaction. The formation of spinel phases such as lead aluminate and magnetoplumbite could be the evidence of the diffusion of both cations and lattice oxygen during not only calcination but also methane coupling reaction at such a high reaction temperature. Thus,lead specieson the surface could be interactingwith supportedmaterials during the oxidativecoupling of methane. It means that the movement of lead cataions and lattice oxygens of the support are possible on the surface and PbO on lead aluminate would be the preferable phase for the oxidativecoupling of methane. And lead aluminate would be adequate for the diffusion of oxide ion to PbO, which would be the site of hydrogen abstraction. This also indicated that ')' -alumina is characterized by fast bulk diffusionof oxygen [ 101.Lead aluminate is known as a spinel which is preferably formed at such high PbO composition and high temperature [ 111. Therefore, lead aluminate would be appropriate in the oxygen mobility for getting good catalytic activity and C,, hydmcarbon selectivity with the low CO, selectivity. And lead aluminate-supported PbO catalyst is a more proper expression for the 60wt.95 PbO loaded catalyst than ')' -alumina-supported PbO catalyst. 4. REFERENCES
1. G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. 2. W. Hinsen, W. Bytyn and M. Baerns, Proc. 8th Int. Congr. Catal., 3 (1984)581. 3. W. Bytyn and M. Baems, Appl. Catal., 28 (1986)199. 4. K. Asami, S.Hashimoto, T. Shikada, K. Fujimoto and H. Tominaga, Chem. Lett., 1233 (1986). 5. G. Wendt, C.-D. Meinecke and W. Schmitz, Appl. Catal., 45 (1988)209. 6. S.S.Aganval, R.A. Migone and G. Marcelin, J. Catal., 121 (1990)110. 7. S.-E. Park and J.-S. Chang, Catal. Sci. Tech., 1 (1991)435. 8. C. D.Wagner, W. M. Riggs, L. E. Davis and J. F. Moulder, "Handbook of X-ray Photoelectron spectroscopy." Perkin-Elmer Cop., USA, 1979. 9. A.F.Wells, "Structural Inorganic Chemistry", Clarendon Press., Oxford, 1984,p.558. 10. G.I. Golodets, "HeterogeneousCatalyticReactionshvolving Molecular Oxygen." (translated by J.R.H. Ross), Chap. 3;Elsevier, Amsterdam, 1983. 11. K. Torkar, H.Krischner and H. Moser, Ber. Deut. Keram. Ges., 43 (1966)259.
GwZi, L et al. (Editors), New Frontiers in Caralysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights resewed
CHARACI'ERIZATION OF TiLaNa CATALYSTS IN THE OXIDATIVE COUPLING OF METHANE S. Rossinia, S.-X Bran&&,
0.ForIania, L. Liettib, A. SantuccP, D.Sanfilippoa and P. Villab
aSnarnprogetti, Via Maritano 26, 20097 S. Donato Milanese, Italy bDipartimenlo di Chimica Industriale "G.Natta" del Politecnico di Milano, Piazza L. da Vinci 32,20133 Milano, Italy
1. INTRODUCTION Recently many efforts have been addressed to methane direct conversion to chemicals. One of the most promising ways seems to be the Oxidative Coupling of Methane ( O C M ) , making methane react with oxygen in presence of oxide catalysts to give mainly Cz+ hydrocarbons. A Methane Oxidative Coupling tricomponent model catalyst (Titanium-Lanthanum-Sodium mixed oxides) has been defined as member of a more general family [l] having the composition Q4'/T3'/B, where Q4* is a tetravalent element (IV A and IV B groups), T 3 + is a trivalent element (I11 B group) and B is a basic component (I and I1 A group). The aim of the present work is to characterize the catalyst surface in order to obtain information about gas-solid interaction in situation as close as possible to the OCM working conditions. Preliminary results coming from surface characterization techniques (XPS and on/off switching of reactants) performed on TiLaNa catalysts will be reported. 2 . EXPERIMENTAL
TiLaNa catalysts used in this study were prepared by mixing solid TiOp with a water solution of lanthanum and sodium nitrates (Fluka reagents). The so obtained slurry was concentrated by evaporation under stirring and then dried at llO'C. The dried samples were calcined in air at 8OO'C f o r 4h and sieved at 20-40 mesh. On/off switchings of the reactants were performed in a quartz fixed bed microreactor (I.D. = 7 mm). The catalyst was kept at the reaction temperature under a flow of He and one of the two reactants while the second was then stepwise added for 200 9. The concentration profiles of reactants and reaction products with time was monitored by a UTI lOOC quadrupole mass detector (QMD) coupled with an IBM AT PC for data processing. The XPS measurements were performed on a VG ESCALAB 200 Spectrometer equipped with high pressure gas cell. The XPS
2238 reference compounds were: Na2C03 (Carlo Erba), TiO, (Fluka), La,O, (BDH) and La203 (calc) obtained from calcination of lanthanum nitrate (Fluka) with the same temperature profile used for the catalyst. 3 . RESULTS AND DISCUSSION
3.1 - On/off switching of reactants. a) methane switch in He+O, flow - A blank run was performed by on/off switching of methane in flowing He+O, in the reactor filled with quartz chips at 750'C. No formation of products was observed when methane was admitted to the reactor. A completely different picture emerges in the presence of the TiLaNa catalyst: in correspondence of the methane switch on(Fig. 1 a-e) a sharp decrease in the concentration of oxygen was observed, with the simultaneous formation of CO, and ethane. However, while ethane formation is immediate, a slow rise in the C02 outlet concentration is observed. When the CH4 addition was stopped, a very fast termination of ethane occurred, whereas C02 outlet concentration showed a pronounced tail as shown by the 4 4 amu monitored peak. Fig. 1 e shows that the shape of the outlet concentration of ethylene (represented by amu 25, which was corrected for ethane overlapping in the cracking pattern) is the same of ethane (Fig. 1 c). This fact indicates that the formation of
a
b
C
d
e
0
-
100
200
t
i)
On/off switching of CHI in He + 0 , . Catalyst: Ti/La/Na a) amu 16 (CH,); b) amu 32 (02);c) amu 30 (ethane); d) amu 44 (CO,); e) amu 25 (ethylene).
Fig. 1 1/1/2;
T=750'C.
2239 ethylene with time parallels that of ethane, and thus no hypothesis could be advanced (with our time-resolution) if either ethylene or ethane are the primary product in the OCM reaction. The experimental data show that the formation of products occurs only when the catalyst is present and immediately upon admission of the reactant (methane). The outlet concentrations of products change with time, possibly due to rapid variations in the superficial characteristic of the catalyst. In particular, the slow appearance of CO, in the gas phase is likely attributed to the formation of carbonate-like species, which were found on the used catalysts by XPS analysis and in some cases also by XRD (presence of carbonate-phases). Also, decomposition of carbonate species is possibly related to the delay in C O P concentration in the outlet stream at the end of methane switch. As shown in Fig. 1 c, the ethane profile vs time in the gas phase is characterized by a transient response which is opposite to COz: this may be interpreted either as an inhibiting effect of carbonates on the oxidative coupling of methane, or as a minor ethane gas-phase combustion at the beginning of CHA switch due to the lower oxygen concentration. b) On/off switching of 0, in Hetmethane flow - The addition of 0, in the gas phase immediately caused ethane and CO, formation, with the same characteristic previously seen for CHA addition. Thus ethane appeared almost immediately in the outlet stream, whereas a delay was observed for C02. At the 0, switch off, while CO, decreased gradually, ethane formation stopped immediately, thus indicating that no lattice oxygen partecipates in the reaction. The instantaneous termination of ethane formation at the end of either the oxygen or methane switch off indicates that there is not a pool of oxygen or CH, species on the catalyst surface which undergoes the OCM reaction without the simultaneous presence of the reactants in the gas-phase. Qualitatively similar results were obtained using catalysts with different TiLaNa atomic ratioes. However a more pronounced delay in the COz appearance was observed for the Lanthanum rich catalysts: this was attributed to the formation of lanthanum carbonate phases which were confirmed by XPS. 3.2 - XPS measurements. The XPS measurements have been performed on untreated, C02 and 0,-treated pure oxides as reference (TiO, and LazO3) and on a TiLaNa catalyst having an atomic ratio 1/1/2. Results are reported in Table 1. The Ti(2p) peak both in the pure oxide and in the catalyst is assigned to Ti in a 4+ oxidation state. Small differences in the binding energy (BE) are observed upon treatment of the pure oxide with gases (0, and CO,), which have been attributed to a different oxygen vacancies at surface. XPS analysis on La,03 samples (BDH and calc) showed a good agreement with literature data [2] f o r lanthanum oxide. However, La(3d5,,) peak shifted to lower BE upon 0, treatment of La,03 (calc), which eventually indicates the conversion of
2240
Table 1 - Results of XPS measurements. A: Ti0,; B: La20, (calc); C: 0,-treated La,O, (calc); D: fresh Ti/La/Na 1/1/2; E: discharged Ti/La/Na 1/1/2.
____________________----------------------------------Sample A B.E. Na( 1s) La ( 3d5 1 2 1 O(ls c a r b O ( ~ S ) O X
Ti ( 2P3 / 2 1 C(IS)carb
B
C
834.7 531.4
833.4 531.4 528.6
531.4
289.3
289.1
289.0
D 1071.3
529.7 458.4
E 1071.4 834.5 531.2 529.3 457.6 289.6
F 1071.4 834.5 531.5 529.6 458.1 289.2
___________________--------------------------------------
B.E.: Binding Energy (ev). All B.E. were calibrated to C ( l s ) a d v (284.6 eV). some hydroxy or oxycarbonate species to oxide. Indeed, the C(1s) signal of carbonate was present in the untreated sample while the O 2 treated sample shows two O(1s) peaks, typical of ) oxide-like ( O o x ) forms. carbonate-like ( O c a r band The presence of the carbonate C(1s) signal as well as two types of surface oxygen ( O c a r band O o x ) were evident also in the XPS spectrum of the TiLaNa fresh catalyst, although the latter at BE values slightly different. The same features are observed in the discharged catalyst even if different relative surface atomic ratioes are computed. In particular we observed an r increase of the surface content for Na, C c a r band O c a r b while Ti, La and O o x decreased. 4. CONCLUSIONS
Tha catalyst surface may be regarded as a C02 acceptor forming carbonate species, mainly of lanthanum but also of sodium due to its surface enrichment with respect to the bulk. These partially reversible carbonate species seem to depress the selectivity to C 2 hydrocarbons. Some XPS peak shifts have been indeed observed in the catalysts with respect to the single oxides: studies are still in progress to confirm them comparing with some binary and ternary TiLaNa single-phase oxides and to check if the surface is a mixture of one component oxides or a single ternary compound. 5. REFERENCES 1
0. Forlani, S. Rossini, D. Sanfilippo; US Patent Application 9/650968
2
Handbook of X-Ray Photoelettron Spectroscopy, Perkin Elmer Corp.
OUai, L rt al. (Editors), New Frontiers in Catalysir Proceedings of the 10th International C o n g m on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elscvier Sdence Publishers B.V. All rights reserved
CATALYTIC PROPERTIES OF Ca-DOPEDLatOJCATALYSTS FOR COUPLING OF METHANE X Yang,X Bia, K Zhena and X Wub
aJ3epartment of Chemistry, Jilin University, 130023 Changchun, China bThe Changchun Institute of Applied Chemistry, 130021 Changchun, China
Abstract In this paper, the catalytic properties of a sericn of Ca-doped La808 catalysts are reported: The experimental results indicate that the Ca doped lanthana prepared by co-precipita-
tion method p o e ~ e better ~ e ~ selectivity for C, hydrocarbon formation, and the activity is apparently higher. These catalysts can be used at a large space velocity, for example, 100000 h-l. It is undoubted that the series of catalysts are promising ones. A so-called hot spot effect appears over this type of catalyst for the given reaction. Additionally, XRD and AES measurements show that the substitution of La ion by Ca ion is satisfactory.
INTRODUCTION It has been shown that deactivation of Li-containing catalysts is unavoidable due to loss of lithium ion at high temperatures"'. It is necessary to develop and search for new types of catalysts without lithium for methane coupling in commercial scale. It is necessary to develop and search for new types of catalysts without lithium for methane coupling in commerical scale. In this paper the present authors synthesized a series of Ca-doped La203oxides. The catalytic properties and the bulk structure of these catalysts were measured, particularly, 1% Ca containing lanthana was investigated in more detail. EXPERIMANTAL All the catalysts used were prepared by co-precipitation method. The catalysts were calcined at 1273 K for 10 h. Catalytic properties were mea-
2242
sured in a quartz continuous flow reactor. The reaction was carried out under atmospheric pressure and at temperatures ranged 773- 1073 K. The reactant mixture of desired molar ratio(without diluent typically CH, : 0 2 = 80 was used. The total flow rate was 100 ml/min. Analysis of inlet and exit gases was performed on a gas-chromatograph. XRD determination of the samples was carried out on a SHIMATDU diffractometer. The AES measurements were performed on a set of ESCALAB MK I electronics. : 20)
RESULTS AND DISCUSSION Catalytic properties of Ca-doped lanthana : methane conversion and CB hydrocarbon selectivity over these catalysts are given in Fig. 1 and Fig. 2 showing that 1% Ca-Laz03 possesses the highest activity for methane conversion and Czselectivity at the temperatures ranged 873-1073 K. This fact
Iu v,
20
ry
V
I
8+3 913 953 993 1033 10’73 T, K Fig.1 CH4 Conversion over
Ca-La20
Fig. 3
H g . 2 C 2 S e l e c t i v i t y o v e r Ca-La20, I
Methane Conversion over 1% Ca-La203
2243
73
773
873 T,IC
Fig.4 Methane Conversion over 1$ Ca-La203
might be correlated with the formation of La3+-O--Ca2+
component in high
concentration , which gives 0- species for activation of methane molecule as suggested by several researchers based on their ESR experimentsC2'although in our work no ESR data were provided. The determination of the catalytic properties of 1% Ca-lanthana reveals an interesting phenomenon as drawn in Fig. 3: an S-shape curve is responsible for the dependence of methane conversion on the reaction temperature. One can find that the larger the amount of catalyst, the sharper the S-shape curve, for example, in case where 160 mg of 1% Ca-lanthana was embedded the mathane conversion quickly increases from zero at 723 K to more than 20% at 748 K. In addition, the smaller the amount of catalyst , the slower the increase in the methane conversion. Besides, quite different profiles of the activity curves obtained for 1% Ca-lanthana were observed at raising and lowering reaction temperatures ranged from 523 to 773 K and higher as shown in Fig. 4. One can conclude that the reaction occurred over the catalyst would be conducted by the exothermal effect of methane coupling under the given conditions. This fact means that as far as a high conversion of methane achieves, a large amount of heat may be produced, which is enough to retain the reaction continued at much lower temperatures. Based on the above experimental results one can also consider that there exists a so-called "hot spot effect". The obtained data also show a high stability of the catalyst and a high GHSV of 100000 h-lFor instance, under the reaction conditions 58% of C2 selectivity and 0. 28 mol/g h of STY reached, and even after stream on time of 500 h no deactivation took place.
2244 Table 1 Lattice Parameters of Ca-La,O, ~~
rnol(%) ofCa in Ca-LalO,
a(A>
c(A>
V(A')
0
3.9319
6.1382
82.181
0.1
3.9356
6.1324
82.258
0.5
3.9369
6.1306
82.290
1
3.9373
6.1335
82.343
Characterization of Ca-doped lanthana : in order to correlate the catalytic properties with the bulk structure of these oxieds the authors calculated the lattice parameters of the catalyst crystals as listed in Table 1. The Table shows that with the increase of calcium ranged from 0-1 % the parameter , a , and cell volumn, V , of the crystal increases, however, a further increase in calcium content may lead to irregular change of the two parameters, It is very clear that the crystal of 1% Ca-lanthana has the largest a and V values. This may probably be one of the factors for this catalyst to have the highest methane conversion and selectivity for Cz hydrocarbon formation. Based on the comparison of the catalytic properties and the lattice parameters of Cadoped lanthana it is clear that there exists an apparent correlation between the data. It should be pointed out that although the ion radii of CaZ+is smaller than that of Las+ , the cell volumn of the Ca doped lanthana crystal still increases due to that the charge of these two ions does not match each other. It is also indicated that the composition of the solid solution containing CaO and La203is limited in a narrow region. AES determination of the 1% Ca doped lanthana shows that the surface atomic ratio of Ca/La is 0. 9% exhibiting a coincidence between the surface and bulk composition , in other words , the substitution is very satisfactory.
ACKNOWLEDGEMENT This work is supported by the National Natural Science Foundation of China. REFERENCES 1 C.E. Keller and M.M. Bhasin, J. Catal. , 73, 9(1982) 2 G. S.Lone and E. E. Wolf, J. Catal. , 161, 119(1989)
Guczi, L a al. (Editors), New Fronriers in CaolysiF Proceedings of the 10th International Congrcss on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights resewed
SYNERGY BETWEEN HIGH TEMPERATURESTABLE CARBONATES AND IRREDUCIBLE OXIDES: DESTRUCTION OF NON-SELECTIVE SURFACE OXYGEN ON OXIDATIVE COUPLING OF METHANE CATALYSTS J.-L. Dubois and C. J. Cameron Division de Recherche Cinetique et Catalyse, Institut Francais du Petrole, 1 & 4 avenue de &is-Preau, BP 31 1,92506 Rueil-Malmaison, France
Abstract The addition of high temperature stable group IIA carbonates (Sr or Ba) to irreducible oxides not known to exist as carbonates (Th and Y) significantly reduces CO synthesis activity during the oxidative coupling of methane reaction. Differential gravimetric analyses of mixed oxide/carbonates and mixed carbonates indicate that the temperatures corresponding to the maximum rates of decarbonatation are shifted with respect t o the pure carbonates. Surface carbonate may play an important role in destroying superoxide, thus lowering the production of CO. 1. INTRODUCTION The most active and selective oxidative coupling of methane (OCM) catalysts have a number of common features, they are: high basicity, stability of surface carbonate species, oxygen ion conduction, absence of reducible surface cations and high temperature, p-type semiconductivity [l]. The precise reason for the apparent requirement of an irreducible cation capable of forming high temperature stable carbonates (i.e. Li, Na, K, Sr or Ba) on OCM catalysts is not clear. We had previously suggested that the peroxycarbonate anion ( C O T ) may be formed on the surface of La203 containing catalysts based on XPS O(1s) binding energies [2]. It was suggested that surface carbonate might assist the decomposition of superoxide via COT as an intermediate. This work presents both OCM catalytic and thermal gravimetric results on catalysts composed of high temperature stable group IIA carbonates (Sr and Ba) and/or oxides which do not exist in carbonate form (Y and Th) and/or oxides which possess intermediate carbonate stability (Ca and La). These results agree with the hypothesis that surface carbonate significantly lowers the selectivity toward CO in the OCM reaction by decomposing superoxide. Heterogeneously generated CO may result from the reaction of the primary selective product, methyl radical, with surface bound superoxide.
2246 2. E X P E R I M E N T A L
2.1. Catalyst p r e p a r a t i o n The catalysts CaC03 (Merck), SrC03 (Prolabo), BaC03 (Prolabo) and Tho2 (Johnson Matthey) were used without pretreatment. La202C03and Yz03 were prepared from La2C03.9Hz0 and Y2(C2O4)3.9H20 by thermal decomposition in air at 570°C for 2 h. All of the catalytic mixtures were prepared by mechanical mixture of 1 : l mole ratios of the starting compounds, with the exception of Sr/Y203 which was prepared by impregnation of the oxide with aqueous Sr(N03)2followed by the same thermal treatment. 2.2. Catalyst testing The procedure and apparatus used for these experiments have been previously described [3]. Both the reactor and the thermocouple well were made from sintered alumina. The quartz grains serving in the preheat zone of the reactor (tests 1-14) and those in the post-catalytic zone (tests 2, 9, 10, 14) were calcined for 4 h at 1100 OC in air before use to avoid the interference of unwanted reactions with the silica surface leading to CO formation [3]. The reactions were not performed under isothermal conditions. The feed gas was composed of methane (1 l/min) and oxygen (0.1 l/min) without an inert gas diluent. The weight of catalyst used in the experiments was varied to approach either 75 % (tests 1-3, 5, 6, 10) or 94 % (tests 11-13) 0 2 conversion. The temperatures indicated in Table 1 are those required to attain a catalyst bed hot spot temperature of 880 OC. The bed was composed of powdered catalyst diluted in 3 ml of tabular alumina. 2.3. T h e r m a l gravimetric analysis TGA was carried out using 10-11 mg of the catalyst samples described above. The analyses were performed by placing the sample in an alumina crucible in a Tag 24 Setaram thermobalance and heating at a rate of 10 OC/min under a helium flow of 4 l/h. DGA provided the temperatures of maximum rate of C 0 2 loss, see Table 2. The experimentally obtained weight losses were slightly lower than the theoretical values probably due to oxide impurity in the samples. The mixture SrC03-La202C03, test 14, exhibits two carbonate losses. Samples 10 and 11 were not examined.
3. DISCUSSION The catalytic results shown in Table 1 are classified in order of decreasing CO selectivity. Two features are particularly noteworthy. First, the oxides which do not carbonate (Th and Y) are found at the top of the table along with the group IIA carbonate most easily converted to its oxide (Ca). Second, in general, the selectivity t o C2+ products increases as the selectivity to CO decreases. Thus, the decrease in CO selectivity is not compensated by a corresponding increase in COz. The ratio of CO to COZ decrease from an average of 1.7 (tests 1-5) to 0.6 (tests 6, 8-10) to 0.35 (tests 11-14) with only BaC03 deviating significantly from this trend. The high temperature stable carbonates (Sr and Ba) are not particularly active and
2247 Table 1 Oxidative coupling of methane over various catalytic compositions. Test Catalyst Weight Temp. Conv. (%)
1
y203
2 3 4 5 6 7 8 9 10 11 12 13 14
Tho2 Tho2 y2 0 3
CaC03-Y203 CaC03 BaC03 SrC03 LL-O~CO~ SrC03-Th02 Sr/Y203 SrC03-Y203 BaC03-Y203 SrC03-La20&0,
Selectivity (%)
(g)
(‘C)
CH4
0 2
c2+
co
c02
0.2 1.0 1.0 0.4 0.3 0.5 0.3 0.3 0.3 1.0 0.3 0.3 0.3 0.3
828 802 821 807 826 818 856 868 759 807 786 778 772 759
11.3 10.5 10.1 12.6 11.1 10.3 11.1 13.3 14.1 12.0 14.6 14.7 14.7 15.4
76.4 74.4 73.7 86.9 74.5 79.7 68.3 88.0 99.6 74.1 94.5 92.0 94.3 99.6
66.3 62.0 64.1 67.4 69.0 62.6 73.7 70.2 69.7 75.0 74.9 77.4 76.7 77.8
24.0 23.5 21.9 20.0 18.0 15.2 12.6 11.9 10.4 9.2 7.4 6.8 5.6 4.4
9.7 14.4 14.1 12.5 13.0 22.2 13.8 17.9 20.0 15.8 17.7 15.8 17.7 17.8
Tablc 2 Differential gravimetric analysis of some OCM catalysts used in this study Catalyst Weight loss (%) Decarbonatation Temp. Test No.
1 and 4 2 and 3 5 6 7 8 9 12 13 14 14
y203
ThOz CaC03-Y203 CaC03 BaC03 SrC03 La2 0 2 C 0 3 SrC03-Y203 BaC03-Y2O3 SrC03-La202C03
Exper. 0 0
Theor. 0 0
13.4 42.5 21.3 28.7 11.5 11.3 9.5 7.5 8.5
13.5 44.0 22.3 29.8 11.9 11.8 10.4 8.5 8.5
Tmaz,
(“1
725 759 1130 988 762 933 1060 768 922
2248 must be heated to substantially higher temperatures to attain similar activities as those exhibited by the other catalysts, which are either totally or partially in the oxide form under reaction conditions. It has been previously suggested that carbonate decomposition serves as the site for oxygen chemisorption [4]. This would explain the low activity of SrC03 and BaC03. The combination of one of these carbonates with a more active (but less selective) oxide permits both a gain in activity (lower heating temperature) and a decrease in CO selectivity, tests 10-13. This synergy can be attributed to two factors: 1) increased p-type conductivity enabling increased oxygen adsorption and therefore higher activity and 2) increased selectivity due to the presence of surface carbonate. The latter is known to play a decisive role in decreasing surface 0;. Both thoria and yttria exhibit strong 0; EPR signals by simply heating in an oxygen containing atmosphere. This is in sharp contrast to lanthana which only exhibits a signal when prepared under rigorous conditions excluding carbonate. The 0; signal on lanthana is rapidly destroyed upon exposure to a COZ containing atmosphere [S]. It would be expected that if surface carbonate is to play an important role it should be highly mobile. The high temperatures required for elevated OCM activity are in the range where carbonate (C02 chemisorbed onto a strong Lewis basic site, 0,) will easily exchange with gas phase C02. Surface bound COz is likely to hop from one 0: to another under reaction conditions. DGA results, Table 2, indicate that individual carbonate entities, such as SrC03, are significantly perturbed or no longer exist in mixed oxide/carbonates. The maximum rate of C 0 2 loss for SrC03 is significantly lowered when it is mixed with YzO3 and La203 (LazOzC03 is almost completely converted to lanthana before SrC03 begins to decompose). The decomposition temperature of La202C03 is only slightly increased when mixed with SrC03. If mobile surface carbonate is important, it may be possible t o increase the selectivity of poorly cabonated catalysts, such as that used in test 5, by the addition of COZ to the feed gas. The lowered decomposition temperatures of the group IIA carbonates in the presence of an oxide suggest an oxide assisted decomposition of the carbonate. Among the catalysts used in this study, the difference between T,,, and the hot spot temperature is the lowest for SrC03-LazOzC03, (42 'C). This catalyst also exhibits the lowest CO selectivity. Group I carbonates were excluded from this study due to the high volatility of their corresponding oxides; however, a similar trend would be expected. 4.
REFERENCES
1 2
J.-L. Dubois and C.J. Cameron, Appl. Catal., 67 (1990) 49. J.-L. Dubois, M. Bisiaw, H. Mimoun and C.J. Cameron, Chem. Lett. 6 (1990) 967. B. Kooh, H. Mimoun and C.J. Cameron, Catal. Today, 4 (1989) 333. S.J. Korf, J.A. Roos, N.A. de Bruijn, J.G. van Ommen and J.H.R. h s s , J. Chem. SOC.Chem. Commun., (1987) 1433. T.L. Chang, M. Che, M. Kermarec, T. Le Van, J.-M. Tatibouet and C. Louis, Catal. Today, in press.
3 4
5
Ounl, L d al. (Editon), New Fronriers in CatalysiF Proceedings of the 10th International Congrese on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishen B.V.All dghts nserved
LASER STIMULATED OXIDATIVE COUPLING OF METBANE TO ETHENE ON LiC104/Pb3(PO& S-H. Zhong and H-Q. Ma Department of Chemical Engineering, Tianjin University, Tianjin, China
Abstract IR, TPD and pulse CO, laser techniques have been used to investigate the behaviors of the title reaction system. The methane conversion of 35% and the ethene selectivity of 93% are obtained with the 9.27 pm laser excited the solid surface bonds for 1000 times at 473 K and atmospheric pressure. A mechanism of this reaction and a model of energy transfer and relaxation in such a process are proposed based on the experimental results. INTRODUCTION Considerable interest in the oxidative coupling of methane through the use of heterogeneous catalysts has been recently generated by many works [l-41. However, the low selectivity of ethene and the high temperature of reaction are not favorable to an industrial development of the catalysts. Laser stimulated surface reaction (LSSR) have been studied in many reaction systems [5-81 with IR photon. The high selectivity and mild temperature are the essential characteristics of this reaction technique. This prompted 0s to investigate the oxidative coupling of methane through the use of LSSR. Our results are briefly summarized and discussed here. EXPERIMENTAL The solid materials LiCIO, and Pb3(P04), were prepared as a common chemical method and 5% LiCI0,/Pb3(P0,), was prepared from the Pb,(PO,), powder immersed in a propyl ketone solution of LiCIO,. After vaporization, the sample LiClO, / Pb3(P0,), was ground and pressed in a form of wafers with 20 mm in diameter and 0.4 rnm in thickness, Infrared spectra were obtained using a stainless-steel cell equipped with KBr windows held in place by threaded caps. The cell is connected to a gas handling and vacuum system. A Hitachi 270-30 spectrometer was employed in the experiments. TPD experiments were performed in the same apparatus as mentioned above. The gaseous products of desorption was measured by a time-of-flight mass spectrometer. The procedures for the IR and TPD experiments are described in more detail elsewhere [9].The experiments of LSSR were carried out using a laser-surface reactor equipped with Ge windows and connected to a gas handling and vacuum system. A grating turned TEA CO, laser ( A 9-11 pm) was employed in the experiments. The mixture of CH, and 0,(CH,:O,= 2:l) diluted with N, was fed to the laser-surface reactor and the reaction products were analyzed using a 103 gas chromatograph.
2250 RESULTS AND DISCUSSION 1. Infrared Spectra and TPD Results Fig. l a shows-the spectrum of methane taken a t room temperature, 50 Torr does of methane. Five strong bonds (at 1344, 1299, 1263, 1245 and 1221 cm-') appear in the range of 1400 to 1170 cm-' of the spectrum. These bonds were assigned to the bending vabration modes of C-H bonds in the methane molecules. Fig. I b shows the spectrum of LiClO, / Pb,(PO,), taken after its drying, reduction at 473 K, evacuation and cooling to 298 K. Six bands (at 1145, 1100, 1030, 1000, 942 and 900 cm-') appear in the range of 1250 to 850 cm-' of the spectrum. The bands at 1145 and 1 100 cm-' were assigned to the stretching vibration modes of P = 0 and C1= 0 bonds respectively, the bands at 1030 and 1000 cm-' were assigned to the stretching vibration modes of P-0 bonds and the bands at 942 and 900 cm-' were assigned to the stretching vibration modes of C1= 0 bonds. Fig. Ic shows the spectrum of the adsorbate-solid system taken at 298 K, 10 Torr does of methane exposed for 1 min then evacuated. Two new bands at 1070 and 1040 cm-' occured when methane adsorbed on LiCIO,/Pb,(PO,),. These bands are the bending vibration modes of C-H bonds in the adsorbed methane molecules on the surface sites of P = 0 and C1= 0 bonds respectively.
c-
0 wavcniirnber
(CIII')
Figure 1. IR spectra
Figure 2. TPD result
TPD result obtained for CH, chemisorption on LiClO, / Pb,(PO,), is shown in Fig. 2. Four desorption bands at 470, 425,255 and 235 C appear in the thermal desorption trace. The bands at 470 and 425 C were assigned to the CH, adsorbed on the surface sites of P = O bonds and the bands at 255 and 235 C were assigned to the CH, adsorbed on the surface sites of C1= 0 bonds. 2. Reaction Behaviors of Methane Oxidation The results obtained for the LSSR (at 473 K and 1 atm, laser excited 1000 times) of methane oxidation on LiClO, / Pb,(PO,), are shown in the Fig.3a and 3d (laser 19.27 pm, pulse interval time 3 S ) , 3b (CH, 2%, pulse interval time 2.5 S ) and 3c (CH, 2%, laser 19.27 pm) . The conversion of methane, X, and the selectivity of ethene, S , in the reaction products depend linearly of the concentration of reactant methane (see 3a and 3d). It is clear that the concentrations of reactant methane and reaction products have little influence on the efficiency of the laser photon energy. The conversion of methane,
2251
X,depends upon the frequency (3b) and the pulse interval time (3c) of laser. It decreases with the increase of the laser pulse interval time as a consequence of decreasing the adding rate of the laser photon energy. And it has the optimum laser frequency in association with the competition between the utilization rate and the damping rate of the laser photon energy in the course of LSSR. Under the optimum conditions the reaction products are ethene, propane and propylene, the conversion of methane is 35% and the selectivity of ethene in the reaction products is 93%.
f
x
so
100
40
80
30
60
20
40
10
20
CH,%
Figure 3. LSSR results
Figure 4. Model of energy transfer
3. Model of Energy Transfer and Relaxation An energy transfer and relaxation model has been elaborated as shown in Fig. 4. It is suggested that in an utilization path the laser photon energy is resonantly adsorbed by the solid surface bonds C1= 0 with an adding rate I , then transfers immediately from the solid surface bonds to the C-H bonds in the adsorbed methane molecules by way of V-V energy transfer with the energy transfer rate II , and that the loss paths of the laser photon energy are mainly caused by the energy transfer from the solid surface bonds CI = 0 to the solid bulk bonds Pb-0 with the energy relaxation rate Ill and the energy transfer from the adsorbed methane molecules to the gaseous methane molecules with the energy damping rate N . In order to utilize efficiently the laser photon energy to promote the methane oxidation reaction, the adding rate I and the transfer rate 11 should be high in order to vibrationally excite enough adsorbed molecules, the relaxtaion rate III and the damping rate N must be small enough to prevent the energy loss from the excited solid surface bonds and the excited C-H bonds in the adsorbed methane molecules. The derection and rate of the energy transfer and relaxation comply with the rule of high frequency to low frequency and with the approaching principle of the vibrational frequency. 4. Reaction Mechanism of Oxidative Coupling of Methane to Ethene.
In general terms, three steps are involved in the course of LSSR. At first, the reactant methanc molecules are adsorbed on the solid surface sites of CI = 0 bonds. Secondary, the solid surface bonds CI = 0 are excited by the laser photon and the energy is transfered immediately from the excited C l = O bonds to the bonds C-H in the adsorbed
2252 methane molecules. With a result, the adsorbed methane molecules react with the crystal-frame oxygen and rupture to form some spieces OH and CH, . Finally, the adsorbed spieces occur the surface reaction each other to form the reaction products H,O and CH,= CH, on the solid surface sites of P = 0 and 0-Pb-0, then the reactant oxygen adsorbs on the solid surface sites and comes into the crystal-frame to supple ment the expended crystal-frame oxygen. H
0
H
2 CH,
-0-
Figure 5. A Reaction mechanism of oxidative coupling of methane to ethene CONCLUSIONS This investigation has shown that the LSSR is an excellent reaction technique for the oxidative coupling of methane to ethene on LiC104/ Pb,(PO,), , Under the conditions of 473 K and 1 atm., the reaction products are ethene, propane and propylene, the conversion of methane is 35% and the selectivity of ethene in the reaction products is 93%. The solid surface plays an important part and the vibrational excitation of the solid surface bonds is an effective mode for the LSSR. In the LSSR, the efficiency of the laser photon energy depends strongly on the frequency and the pulse interval time of laser, on the infrared characteristics of the solid materials and on the adsorbing capability of the reactant molecules. Three reaction steps (reactant chemisorption, laser excitation and surface reaction) are involved in the course of LSSR. The crystal-frame oxygen of the excited C1=0 bonds takes part in the oxidative coupling reaction of methane to ethene. The reactivity of methane oxidation depends essentially on the rate of energy transfer from the excited solid surface bonds to the bonds to be excited in the adsorbed methane molecules. REFERENCES 1. C. H.Lin, J. X.Wang and J. H. Lunsford, J. Catal, 111,302 (1988) 2. J. Solymosi, I. Tombacz and G. Kutsan, J. Chem. SOC.,Chem. Commun., 1455 (1985) 3. J. A. S. P. Carreiro and M. Baerns, J. catal., 117,258 (1989) 4. H, Imai, T. Tagawa and N. Kamide, J. Catal., 106, 394 (1987) 5. T. J. Chuang, Surf. Sci., 178, 763 (1988) 6. B. Fain and S.H. Lin, Chem. Phys. Lett., 114,497 (1985) 7. B. N. J. Persson, Phy. Rev. Lett., 48 (8), 549 (1980) 8. D. C. Wang and S.H.Zhong, J. Atom. and Mol. Phys., 5(4), 897 (1988) 9. S. H.Zhong, J. Catal., 100,270 (1986)
Guczi, L et al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congnss on Catalysis, 19-24July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
TEMPERATURE-PROGRAMMEDSTUDIES OF SURFACE OXYGEN SPECIES IN THE OXIDATIVE COUPLING OF METHANE
G.W.Keulks, N.Liao, W.An andD. Li Department of Chemistry, Laboratory for Surface Studies, University of WisconsinMilwaukee, Milwaukee, Wisconsin 53201, USA
Abstract The reactivity of surface oxygen species for the oxidative coupling of methane over various catalysts, [Sm203,Li(5%)-Sm203,Li-MgO, Bi-P-K/MgO, Bi-P-Na/MgO, and a series of catalysts of the general composition MBi,,LixO /MgO, where M=Sr, Ba; x= 0, 0.05, 0.15, 0.20, 0.40, 0.501, has been investigated by means of temperatureprogrammed desorption (TPD), temperature-programmed reaction (TPRX), and temperature-programmed reduction (TPR) between 100-800"C. The experimental results suggest that simple, adsorbed oxygen species are not the active species for oxidative coupling of methane. Instead, they are responsible for deep oxidation. Simple-oxide catalysts are active only in the presence of gaseous oxygen. For multicomponent-oxide catalysts, surface lattice oxygen directly participates in the reaction, giving a high selectivity toward C2 products. 1. INTRODUCTION
The oxidative coupling of methane has been widely investigated in the last decade, after the reported findings of Keller and Bashin [l]. Several general features of the reaction have emerged. The products of the reaction are primarily ethane and ethylene with minor amounts of propylene and other higher hydrocarbons. The C2 yield is almost always reported below 30%, and modeling calculations suggest that there is a "30% limit" to the reaction [2]. Both simple oxides and multicomponent oxides catalyze the reaction. Active and selective catalysts are active both in the presence and in the absence of gaseous oxygen. The importance of reactive, surface oxygen species also is well accepted. Various forms, such as O;,Oi', 0-,and 02-, have been proposed [3] to be involved in the oxidative reaction, but the exact nature of the reactive, surface oxygen is still open to speculation. Surface oxygen species have been proposed to be responsible for the activation of CH,. On the other hand, unwanted, highly reactive, surface oxygen species lead to low C2yields. Thus, the identification and control of reactive, surface oxygen species are key factors in the development of improved catalysts for the oxidative coupling of CH,.
2254
The objective of this work is to obtain further information on the reactivity of oxygen species which are involved in selective oxidation and complete oxidation. 2. EXPERIMENTAL For temperature-programmed desorption (TPD) experiments, the catalysts were preoxidizedinitially with a stream of 02/He (10/10 cm3/min) at a temperature of 700°C for 2 hr. The preoxidized catalysts then were quenched at liquid nitrogen temperatures in an atmosphere of O,/He. The inlet 0, flow was stopped and the system flushed with He (35 cm3/min). The He flow was adjusted to 6 cm3/min, and the temperature program begun. A portion of the gas-phase effluent was leaked into Similar a UTI quadrupole mass spectrometer to monitor desorbed species. procedures were followed for the temperature-programmed reaction (TPRX) and temperature-programmedreduction (TPR) experiments. 3. RESULTS AND DISCUSSION 3.1. Surface Oxygen Species In the TPD experiments, 0, desorption was detected in three distinct temperature regions: a (250-450"C), p(450-600" C) and y(600-800"C). The O2 desorption characteristics for simple-oxide catalysts (Sm2O3, Li(5%)-Sm203,and Li/MgO) vary considerably from those for the multicomponent-oxidecatalysts (Bi-P-Na/MgO, Bi-PK/MgO, and MBi,,LixOy/MgO). The TPD spectrum obtained for 0, desorption from Sm203is characterized by a single a-desorption peak. When Li is added to Sm203, a y-desorption peak is detected and the a-desorption peak decreases. For Li-MgO, the y-desorption peak is the main species detected. These results suggest that the a-state is produced by active centers associated with Sm sites and the y-state is produced by active centers associated with Li sites. For multicomponent-oxide catalysts 0, desorbs mainly from the p and the y temperature regions. However, without additional information regarding the nature of the active sites present on these multicomponent-oxidecatalysts, it is not possible to assign the desorption peaks to specific sites. 3.2. Reactivity of Surface Oxygen Species 3.2.1. Simple-Oxide Catalysts The reactivity of the surface oxygen species was examined by TPRX, both in the presence and absence of gas-phase 0,. From the TPRX without gas-phase 02, we found that the a-oxygen on Sm,03reacts with CH, and only produces CO. This result indicates that at low temperatures, the weakly adsorbed a-oxygen cannot selectively oxidize CH, to C2 hydrocarbons. According to the results obtained from the TPRX without gas-phase O,, the reaction of y-oxygen with CH, also mainly produces CO and CO,. Only a small amount of C2 hydrocarbons is detected and the amount of C, hydrocarbons is not directly related to the amount of y-oxygen over both Sm203andLi/MgO. This result implies that the
2255
surface adsorbed oxygen (a- or y-oxygen) species are inactive for oxidative coupling of CH, and are responsible for complete oxidation. The TPRX in the presence of gas-phase 0, is quite different in that a large amount of C, hydrocarbons is produced in the y region. This result suggests that gas-phase 0, must be present in order to generate active oxygen species for the oxidative coupling of CH,. Furthermore, if y-oxygen species are responsible for abstracting hydrogen from CH, to form methyl radicals, then the y-oxygen species can only be regenerated by gas-phase oxygen, not by the lattice oxygen of Sm203 or Li/MgO. The active oxygen species might be intermediate oxygen species which are produced from the interaction of gas-phase 0, with the catalyst surface.
3.2.2. Multicomponent-Oxide Catalysts From the TPRX without gas-phase O,, we found that starting at 500" C the reaction of surface oxygen with CH, over Bi-P-Na/MgO, Bi-P-K/MgO, and MBil-,Li,Oy/MgO produces not only CO, but also C, hydrocarbons. In contrast with simple-oxide catalysts, when the temperature is increased, a significant amount of C, hydrocarbons is produced. According to these results, we can speculate that if surface active oxygen species are important for the production of C, hydrocarbons, then the oxygen species are continually regenerated. That is, the lattice oxygen in these bulk oxides is able to diffuse to the surface to replenish the surface active oxygen species. The results obtained below 600"C, where only a small amount of C2hydrocarbons is produced, can be explained by assuming that the diffusion rate of lattice oxygen at low temperatures is too slow to regenerate the surface lattice oxygen efficiently for oxidative coupling of CH,. From the experimental results of TPRX with gas-phase 02, we found that in the pregion, the reaction of CH, with surface oxygen species mainly produces total oxidation products. When the temperature is increased to 600"C, a large amount of C2 hydrewbons is produced. We can explain these results by suggesting that surface lattice oxygen is responsible for selective oxidation and that gas-phase 02,when present, forms a large amount of weakly bound adsorbed oxygen which inhibits the formation of active surface lattice oxygen. These weakly bound adsorbed oxygens are responsible for the total oxidation and retard the diffusion of lattice oxygen to the surface for selective oxidation at low temperatures. When the temperature is increasedto over 600"C, the mobility of lattice oxygen increases and the weakly bound adsorbed oxygen species are removed so that selective oxidation becomes the primary reaction.
3.3.Temperature-Programmed Reduction of Multicomponent Catalysts Because the weakly bound surface oxygen species that we associate with the poxygen are responsiblefor complete oxidation, the C, selectivity should improve if we first desorb these oxygen species using TPD. After desorbing surface oxygen from Bi-P-K/MgO, a TPR experiment, using CH4 as the reducing agent, was conducted. We found that even though a significant amount of C, hydrocarbons still appears over 600"C, a small amount of C2hydrocarbons now begins to appear at 400" C. We also found that a much smaller amount of COPis produced. This result strongly suggests that the surface oxygen species derived from lattice oxygen are responsible for the
2256 selective oxidation. A large amount of C, hydrocarbons is still produced if a second TPR experiment is conducted. This result indicates that Bi-P-K/MgO has a large pool of lattice oxygen that can provide the active oxygen species for the oxidative coupling reaction. 4. CONCLUSIONS
1). For the simple-oxide catalyst, Sm20g, the principle oxygen species is the weakly adsorbed a-oxygen. For the simple-oxide catalyst, Li/MgO, the primary oxygen species is the strongly bound y-oxygen. For the multicomponent-oxide catalysts, Bi-P-Na/MgO, Bi-P-K/MgO and MBil~xLixOy/Mg.O,the p-oxygen is adsorbed oxygen and y-oxygen is associated with the surface lattice oxygen. 2). For SmO ,, the weakly bound a-oxygen species produces complete oxidation products. For Li-MgO, if gas-phase 0,is absent, the y-oxygen reacts with CH, to produce mainly CO and CO,. For multicomponent-oxide catalysts, p-oxygen species produce CO and C02. 3). For simple-oxide catalysts, gas-phase 0,is necessary to generate the active oxygen species for oxidative coupling of CH,. For multicomponent-oxide catalysts, lattice oxygen in the bulk oxide regenerates the active oxygen species for the oxidative coupling of CH, and gas-phase 0, reoxidizes the reduced oxide. 4). For simple-oxide catalysts, the interaction of gas-phase 0, with the catalyst For surface produces the active intermediates for the oxidative coupling of CH,. multicomponent-oxide catalysts, the mobility of lattice oxygen produces the active surface lattice oxygen for the oxidative coupling of CH,. The high selectivity toward C, hydrocarbons depends not only on the amount of surface lattice oxygen, but also on the mobility of lattice oxygen from the bulk oxide to the surface. REFERENCES 1 G.E. Keller and M.M. Bhasin, J. Catal. 73, (1982) 9. 2 J.G. McCarty, A.B. McEwen, and M.A. Quinlan, New Developments in Selective Oxidation. G. Centi and F. Trifiro’(Eds)-1990, p. 405. 3 G.J. Hutchings, M.S. Scrurrell and J.R. Woodhouse, Chem. SOC.Rev., 18, (1989) 251-283.
GUni, L d al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
OXIDATIVE DIMERIZATION OF METHANE ON ALKALI CHLORIDE PROMOTED C0304 M.Gratzelo,D. Klissurskib, J. Kiwi0 and K. R. Thampia BInstitut de Chimie Physique, EPFL, 1015 Lausanne, Switzerland bhtitute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1040, Bulgaria
Abstract Oxidative dimerization of CH4 has been studied using Cog04 catalysts promoted with chlorides of Li, Na, K and Cs. Chemical modification of cog04 LiCl catalyst at the reaction conditions was evidenced by ESR spectroscopy. A decrease. in the optimum temperature, at which C2- product yields were maximum, was noticed when the promoter was changed from Li t o Cs through group IA alkali metal chlorides.
INTRODUCTION It has been shown by Komatsu et.al., (1)that for oxidative coupling of CH4, LiCl promoted Cog04 exhibits high activity, C2 selectivity and ethylene/ethane ratio. This has motivated us to undertake a systematic study of the catalytic behaviour and long term stability of cog04 catalysts promoted with different alkali chlorides.
The experiments were carried out using a conventional flow reactor (a quartz U-tube of i.d.=8mm) a t atmospheric pressure (2). ESR spectra of the catalysts were registered at room temperature in the X-band, using a 220-Q unit, X-ray diffractograms (XRD) were recorded on a Siemens X-ray unit. The alkali metal (M = Li, Na, K or Cs) promoted Cog04 catalysts were prepared by wet impregnation techniques (11, followed by calcination at 973 K for 2 hours in air. As shown by Komatsu et.al. (11, in the case of CogO4/LiC1catalyst with -17 at% Li content, C2-yields reached their maximum values. In view of this, a M:Co atomic ratio 1:5 was chosen for comparative studies of the different alkali metal chloride promoters. The flow rate of the reaction mixture and CH4 : 0 2 ratio have been vaned between 15 to 50 ml/min., and 5 to 0.8, respectively. Most of the experiments were performed with l g catalyst using a gas flow rate of 50 d m i n and a CH4 to 0 2 ratio 1:l.
2258
Fig.1. CH4 conversion, C2 selectivity ( S ) and yield (Y)as a function of temperature in an empty reactor (Flow = 50 mVmin and CH, : O2 =1: 1) 0
680
m
780
Temperature (OC)
RESULTS AND DISCUSSION The percentage conversion, C2-yield and selectivity due t o the gas phase homogeneous reaction are shown in Fig. 1, as a function of temperature. The maximum C2 selectivity was 18%,with C2 yields approaching -5%. During the initial period of testing at 600 "C, the catalysts showed three types of behaviour depending on the nature of the promotors (Fig.2) :(a) The CogO&iCl catalyst showed a drastic decrease in C2 selectivity (S) (from 25 t o 6%) and yields (Y) (from 16 to 3%)with respect to time on stream. The decrease in selectivity and yield were almost exclusively caused by a decrease in the yield of ethylene, which dropped by a factor of -10. The yields of all other reaction products remained stable. (b) The CogO4/NaC1 and c0304/Kcl showed small, but steady decrease in S and Y values. (c) With the CogOq/CsCl catalyst, after a small initial drop the S and Y values remained steady for a long time. The behaviour of the CogO4/LiC1 catalysts could be related not only t o structural, but also t o significant surface and bulk chemical modifications. This was revealed by XRD and ESR data. The presence of C1- and HC1 in the eMuent gases during the initial period of catalytic evaluation has been noticed. Fig. 3 presents the ESR spectra of a fresh C030q/LiCl catalyst calcined for 2 hours a t 700 "C in air (spectrum 11, the same catalyst after being used for 30 hours (spectrum 2) and a fresh catalyst prepared using LiOH instead of LiCl as the promotor and calcined for 2 hours at 700 "C (spectrum 3). The ESR spectrum of the Li-CogO4 system shows (i) a single Lorentzian line with g = 2.22, typical of Co2+ in tetrahedral positions of the spinel lattice; (ii) a singlet Lorentzian line with g = 2.142 and AH1 = 5 mT, which corresponds to Ni3+ impurity ions in a low-spin state (an ESR signal of Ni3+ ions with the same spectral parameters was also registered in the LiCoO2 matrix); (iii) an axially anisotropic line with 4,O 2 g 2 3,5 and g = 2,O assigned to Co2+ ions (3).
2259
y; X1 pa 2.142
0 s
E
% I
3
0
BD
1#) Time (mid
180
Fig.2. CH4 conversion, S ((32) and Y (C,) us reaction time, for Co304 catalysts promoted with (a) LiC1, (b) NaCl or KC1 and (c) CsC1. Fig.3. ESR spectra of (1) fresh C0304/LiC1(2)used C0304LiC1 and (3) fresh Co304/LiOHcatalysts. The sample corresponding to ESR spectrum 1 in fig. 3, exhibits signal A only, while the samples corresponding to spectra 2 and 3, showed signals B and C in addition to signal A. This indicates that among fresh samples, unlike in LiOH-CogO4 system, no interaction takes place between both components in the Cog04LiCl mixture (spectra 3). However, this interaction proceeds at a later stage during the catalytic reaction (spectra 2). Such an interaction could be favoured by partial hydrolysis of LiC1, during the initial stages of the catalyst application. The relationshia between the degrees of conversion, selectivities and yields and the temperature of the reaction zone are shown in Fig. 4. The data shown were reproducible and have been obtained after 15 h on stream at 700 "C. When the promotor was varied from Li to Cs chlorides, the reaction temperature for maximum C2 selectivity and yields, moved favourably downwards (fig.4). In general, this tendency could be related to the difference in the ionic radii and electronegativity of the differentalkali metals. CsCl promoted catalyst showed long life and relatively high C2 yields and selectivities during the entire period of testing (120 h). The drop in C2 selectivity and yield were respectively 7 and 15% of the initial values. It is important to note that the decrease of C2- yield was mainly due to the drop in selectivity, because the total conversion of CH4 remained practically constant. When shifted to higher reaction temperatures, the maximum C2 yield and selectivity exhibited by LiCl-Cog04 catalyst also increased to noticeably higher values (S = 23% and Y = 14%).
2260 BD
c-w----
ad 8
rp.
d '6
12
Q b
6
k
d
o
0
i
" 3 30
12
0
0
sb
.4 sr
Mx)
8u)
100
m600
Temperature("0
800
m
8x)
Fig.4. Conversion, yield and selectivity data as a function of temperature for C03O4 catalysts promoted with (a) LiC1, (b) NaCl, (c) KCl and (d)CsCl. A variation of the CH4:O2 ratio from 1 : l to 4:l resulted in a n increase in Cz selectivity from 25 to 35-40%. However, the maximum yield remained at 15%. With a CH4:O2 ratio below 1:1,the total oxidation of CH4 to C02 is increased.
CONCLUSION The observed results are in general agreement with the concepts related t o redox type catalytic reactions on oxide surfaces, where the binding energy of chemisorbed surface oxygen plays a key role (4). Pure Co304 is able to oxidize CH4 totally, because of the highly reactive excess oxygen on its surface layer. The binding energy of these oxygen species depend on surface coverage and vary between 14 and 80 kcal/mole (5).The alkali metal chloride promotors seem to block partly the most reactive surface sites either physically (by CsC1) or chemically by surface or bulk interactions with cOg04, resulting in a change in the surface oxygen reactivity.
1 T. Komatsu, T. Amaya and K. Otsuka, Catal. Letts., 3 (1989) 317. Chem. Comm.,(1990) 2 J. Kiwi, K.R. Thampi, and M. Gratzel, J. Chem. SOC. 1690. 3 S. Angelov, C. Friebel, E. Zheeheva and R. Stoyanova, J. Phys. Chem. Solids, 51 (1990) 1157. 4 D.G. Klissurski, 'Proc. 8th Int. Cong. Catalysis', Berlin 1984, Verlag Chemie, Weinheim, vo1.3, p-165. 5 V.A. Sazonov, V.V. Popovski and G.K.Boreskov, Kinet. Catal., (Engl.), 9 (1968)251.
Guni, L ff al. (Editors), New Frontiers in Catalysysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights Reserved
OXIDATIVECOUPLING OF CHd+CDd MIXTURE OVER MANGANESE OXIDE CATALYSTS
Y. G.Borodko and L.M.Ioffe Institute of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow region, Russia
INIRoW(;TIoN "he oxidative coupling of methane, the major constituent of natural gas, to C2 hydrocarbons [l] or to transportable liquid hydrocarbon fuel [ 2 ] is the subject of catalytic research. Enhancement of selectivity formation of C2-products mainly ethylene as more desirable product of methane oxidative coupling over metal oxide catalysts is rather difficult problem since the reaction is radical in nature and non-selectivity is inherent of it. Mechanism of methane oxidative coupling to C2's is currently under investigation, especially the nature of primary C2-hydrocarbons and intermediate species. At present a few reaction routes from CH4 to C2-hydrocarbons are under consideration which involved CH3 and CH2 radicals as primary species [ 13. We studied the composition and structure of isotopic ethanes and ethylenes-the products of oxidative coupling of CH4/CD4 equimolecular mixture over a complex manganese oxide catalysts to provide mechanistic information about radicals are generated in the primary C2-hydrocarbons[3] If only CH3 and OCM reaction the products will have to contain three isotopic (HD)ethanes: H3CCH3, H3CCD3, hm, the result of CH3 and species dimerization. I f both CH3 and CH2 species are involved in %-hydrocarbons formation the reaction products will contain full set deuterated molecules. Previously such approach have been used by P.Nelson and N . b t to study OCM over several catalysts [ 4 ] .
a
MpERIKENrAL AND RESULTS Natural manganese minerals (NMM) was used both as unpromoted catalyst and promoted one. Catalysts included besides manganese oxides (10-35% wt) inherent a number of accompenied elements: Na, Mg, Be, Ca, Al, Si, which are chemical and structural promoters [ 51. The catalytic experiments were performed in redox mode by cycling methane and air independently in fixed-bed flow reactor under atmospheric pressure. The ethane, ethylene and carbon oxides were analyzed and summed to give methane conversion. Chemical states of surface and bulk of fresh as well as used but active catalysts were studied by XPS, Auger and IR-spectroscopic techniques. It was k 2 0 3 oxides to Mn304, MnO is mainly responsifound that reduction of Mr&, ble for catalysts deactivation, whereas the formation of carbonaceous material on a catalyst surface is negligible. The cumulative product of oxidative coupling of CH4tCD4 equimolecular mixture was seperated with chromatograph and (HD)ethanes and (HD)ethylenes were gathered using cooling traps as collecting box. Then mixture (HDIethanes only and mixture (HDlethylenes only were collected in cryocell and spectra of %-products were registered both in gas and solid states at -190OC. IR spectroscopic analysis of frozen (HD)Q-hydrocarbons has an advantage over meas-
urements their IR and mass-spectra in gaseous state since all deuterated ethylenes and ethanes, including isotopic stereoisomers, in solid state are characterized by well resolved absorption bands [6]. Illustration of one run experiments in redox mode at 85OoC is shown in Table 1. As seen NMM-catalysts activity are comparable with synthetic manganese oxide catalysts. Especially amazing their high selectivity formation of C2-hydrocarbons in spite of complex composition of catalysts. Table 1
REDOX RUNS OVER NATURAL MANCANESE OXIDE MINERALS feed gas
V,h-'
convers.,%
selectivity,%
a 4
c2
cox 14 3
a a a
7300 11400
11
86
9
6200
27
atCHc13
11400
24
97 80 82
yield,% c-2
9
20 18
8.7 21.5
20
IR spectroscopic analysis of isotopic C2-products of the oxidative coupling of CH4tCD4 mixture have been made for two kinds of product: at low (3%) and high (25%) methane conversion (Table 2) Table 2
REDOX RUNS OVER NATURAL MANGANESE OXIDE MINERALS feed gas
V,h-'
convers.,% CH4tCD4
CH4 +m4 CH4 +CD4
6200 16000
25 3
selectivity,% ethane ethene Cox 8 83
67 16
25 1
CH4/04=l, t=850°C, p=latrn, Mcat=4g
It was established that the value of HD-exchange between CH4 and CD4 is insignificant at low methane conversion under reaction condition. In order to obtain experimentally more reliable data about composition of (HD)Cz's mixture (HDlethanes only and (HDlethylenes only were registered in gas and solid state. It enables to eliminate rotational structure of vibrational bands and induce appearance of narrow Q-branches and thus make better precision of spectral analysis. The experimental position of IR bands g m a x of solid and gaseous (HD)ethanes and (HDIethylenes are summarized in Table 3 As illustrated in Figure 1 spectrum of (HD)ethanes in solid state is more informative that one in gaseous state. In the case of high methane conversion IR-spectrum contains full set of absorption bands which are assigned to isotopic ethanes: do, di , d2, d3, d 4 , dS, d6, including rotational isomers. But only three isotopes: H3CCH3. H3CCD3. kCCD3 (do,d-d3,ds) were observed at low methane conversion (Fig.1,spectrum D). Very close result obtained from analysis of IR spectra of (HDIethylenesproduct of C&/CD4 mixture oxidative coupling. All set of deuterated ethylenes have been identified in spectra at 25% methane conversion, but marked amount of isotopes:H2CCH2, HzCCq, &only were exhibited at low methane conversion. The concentration ratio was estimated using values of bands integral intensity. Ratio [D303]:[H3CCD3]:[H3CCH3] as determined by IR-spectroscopy is: 1 : 2.7 : 1 . 7 , respectively, and roughly reflects kinetic isotope effect in methyl radical formation (Table 4 ) .
2263 INFRARED SPECIRA OF GASWS AND SOLID LABELLED (HDIEMYLENESAND (HD)ETHANES.cm-1
Table 3
3 2 ,C-H(D) 0,
,C-H(D) bending out-of-plane gas solid gas solid
*
JTHANES
*
\)CCH(D) bending gas solid
~
C2H4
do
1444 1438
QH3D
dl
1403 1396
H2CCq 0(-d2
1384 1378
~is-(cHD)~ d2
1342 1336
tr-(CHD)2 C2HD3
1300 1295 1292 d3 1290 1283
C2D4
d4 1078 1073
949 947
821
715 806 791 715 805 685
d2
635 720 618 651 599 631 594
819 823 714 805 716 803 680 676 631 720 620 599 631 594 592
* ) number of deuterium atoms
A
8
-4
C
ln
do
gas
*rl
solid
D
&
H3ccD3 I
900
I
I
800
1
I
700
1
I
I
600 wavenumber, cm-'
Figure 1. IR-spectra of (HDlethanes: A ) gaseous, 25% methane conversion. B) sample A in solid state at -190OC. C) gaseous, 3% methane conversion. D) sample C in solid state at -190OC.
2264 Table 4 Concentration ratio [D3CCD31: [H3CCD3]: [ H 3 m 3] calculated versus value of kinetic isotope effect (KH/KD)in methyl radical formation (in frame of collision theory)
1.2 1.3 1.4 1.5
1 1
1 1
2.53 2.7 2.94 3.16
1.57 1.86 2.15 2.47
The comparison experimental finding with calculated data shows that distribution of (HD)ethanes at low methane conversion approximately corresponds with value of KH/KD = 1.3 Evidently, that marked amount of deuterated ethanes other than do,O(-d3. d6 at high methane conversion is the consequence of secondary Wexchange at high residence time.
CxlNcLUsION Natural mnnganese minerals may be used as starting material for preparation of promising catalytic system for methane oxidative coupling. Investigation of oxidative coupling of CH4/CD4 mixture over catalysts on the base of natural manganese mineral indicates that in the case of low methane conversion formation of C2's may be described by reaction network,where ethane is the primary product.
Infrared spectroscopic analysis of frozen (HD)Cz-products of oxidative coupling of C&/CD4 mixture over heterogeneous catalyst is convenient technique to get more definite experimental finding about composition of all (HD)-hydrocarbons.
1 G,J.Hutchings, M.S.Scurrel1. J.R.Woodhouse,Chem.Soc.Review,18 (1989) 251 2 J.M.Fox,T.P.Chen, B.D.Degen,Chem.Fhg.Progress,April 1990, pp 42-50 3 L.Melander,W.H.Saunders,Jr. Reaction Rates of Isotopic Molecules, J.Wiley and Sons, N.Y. 1980. 4 P.F.Nelson,C.A.Lukey,N.W.&t, J.Phys.Chem. 92 (1988) 6176 Journ. Catalysis, 120 (1989) 216 5 L.M.Ioffe,Y.G.Borodko, Catalysis Todey. 13 (1992) 597 6 Y.C.Borodko, L.M.Ioffe,A.Y.Borodko,Jr. Catalysis Today,l3 (1992) 549
O h , L u al. (Editors),New Frontiers in Catalysis Proceedings of the 10th IntemationaI Congrcas on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elscvicr Science Publishers B.V.All rights reserved
A COMPARISON OF THE BEHAVIOR OF CATALYSTS FOR METHANE COUPLING BY TRANSIENTANALYSIS
R. Spinicci and A. Tofanari Dipartimento di Energetica, Universita di Firenze, Via S. Marta 3,50139 Firenze, Italy
Mmtract
Two catalysts f o r methane coupling, namely 2% Li,O/Y,Q and 3.5% N&O/CaO have been studied by mans o f transient techniques: transient response nethod and steady state isotopic transient k i n e t i c analysis. Both techniques and mainly the second one allow us t o obtain important informations about the r e l a t i v e importance o f the gas phase reaction i n respect o f the surface heterogeneous reaction and about the surface species which contribute t o the progress of the selective and t o t a l oxidation.
Transient techniques can be usefully employed i n studying heterogeneous catalytic reactions, i n order t o gain an insight I n the adsorptive properties of the catalysts and therefore i n the surface processes, i n reaction condftions. Translent response nethod i s a non steady state technique which can give indications about the preferential pathway followed by reactants i n presence o f the catalyst; steady state isotopic transient k i n e t i c analysis i s a steady state technique, which can give Indications about the adsorption and desorption processes accolpanying the reaction: they can provide therefore meaningful and valuable k i n e t i c i n f o m t i o n . ( 1 ) Table 1 Yelds o f the different products obtained i n methane coupling reaction on the two catalysts investigated
x co
Temperature
710 *C 740 :C 770 c
%
co,
Na-Ca Li-Y
Na-Ca L1-Y
0.5 0.6 0.7
7.0 9.2 9.0
0.7 0.6 0.5
0.7 9.0 11.4
%
ClHI
%
cx
Na-Ca Li-Y
Na-Ca Li-Y
7.0 0.0 0.6
5.6 0.2 9.6
7.9 7.6 6.5
4.6 7.1 0.0
2266 T h ~ ntwo techniqites appears very useful in studying methane coup1 ing reaction, in order to obtain important informations about the controversial problem of the importance of the true catalytic reaction in respect o f the gas phase reaction. This is even more important for two catalysts, 2x1 Li,O/Y,O, arid 3.5% b,G/'Curj, which exhibit a remarkable activity, and a good seiectivity towards the format ion o f C,-hydrocarbons. As it is shown in Table 1, the results obtained in previous inyestigations with these two catalysts in the temperature range 700 - 800 C are quite different from other catalysts, as for example ZnO based catalysts, which are not so selective and reach comparable levels of activity at temperatures over 800 'C.(2) EXPERIWENTM
The two catalysts have been prepared by impregnating comercia1 Y,O, and respectively CaO (obtained forn marble decolaposition and containing 0.5% of strontium oxide) with a solution of lithium nitrate, and respectively, of sodium nitrate: in both cases the excess solution has been evaporated and the precursor has been dried and subsequent1y calcined. The experiments have been performed in a tubular flow reactor connected to a UTI mass analyzer. Catalyst samples of about 0.08 g have been employed, after a pretreatment in a m i x p e of helium plus oxygen at temperatures ranging between 600 C and 800 C. The reaction has keen then carried out at a temperature, chosen in the interval 700 - 800 by employing a ratio methane/oxygen of 5/2. Transient response experiments have been performed by the sudden adriss ion of mthane in a flow of helium plus oxygen, or by taking off suddendly methane or oxygen fron the reactant mixture. Steady state isotopic transient analysis experimnts have been carried out by making abrupt switches between "CH, and "CH, in the feed under steady state reaction conditions and by monitoring the decay or respectively the development of the isotopically labeled species (reactant and products), resulting from the switching. RESULTS AllD
OISCUSSION
Prel iminary temperature programed desorption experiments with adsorbed methane show that on 2% Li,O/Yp, a great part of methane remains ancho!ed on the surface until 750 - 770 'C, after a peak which starts at about 550 C and which is characterized by desorption with total oxidation to carbon dioxide. On 3.5% Na#/CaO desorption of methane is chara$terized by two distinct processes, the first in the interval 300 - 550 C with a small production pf ethane and carbon dioxideL and the second one starting from about 700 C and completed over 1000 C, which gives completely carbon dioxide. In Fig. 1 it is possible to see that transient method experiments with 2% Li&/Yp, show that the response of C,-hydrocarbons reaches quickly a maximum and is characterized by a marked overshoot (the rate of consumption of oxygen is in accordance with this increase). The response of methane too, increases rapidly enough to reach its maximum in a short time and this seems
2267 t o stretighton t h o hypothesis t h a t it i s not adsvbed i n a gpeat amount on the catalyst surface: this.feature i s g r e a t l y en inced i n the experiments a t temperatures over 730 C. On the contrary thd response o f carbon dioxide increases slowly and therefore the r a t e o f ii . desorption coud a f f e c t the r a t e o f i t s fonnation; moreover when i t s l e v e l i s a i i t s tiiyiiest values, the response o f C, s t a r t s decreasing, probably because o f a slow.regeneration o f active sites. (Experiments a t higher temperatu-es up t o 750 C show t h a t t h e r a t e of increase o f the carbon dioxide respon e i s progressively greater). When methane i s eliminated from the feed, the response o f hydrocarbons i s s u f f i c i e n t l y rapid, even i f t h a t o f methane i s a b i t slower; on the contrary the response o f carbon dioxide i s very slow. I J Fig. 2 i t i s possible t o see that the transient response r e s u l t s f o r 3.5% N&O/CaO do not show many differences i n comparison w i t h 2% Li,O/Y,O,: the methane response shows some transient behaviour and therefore i t s adsorption occurs t o a greater extent, conf inning the temperature progranined desorption results. C, response increases r a p i d l y and i t s overshoot i s less marked, but i t i s coincident again w i t h the maximum o f the l e v e l o f the carbon dioxide response, whose r a t e o f increase i s again low. a)
M.A. signal
,--2
-- -.-..-..6
b)
M.A. s i g n a l
1
CHq
....*'-'---*--.. .................. .....-.. ,//.*/-*
.. . 2
6
l o t (min)
Fig. 1 Reactants and products responses during the very beginning of the reaction a f t e r the admission o f methane on 2% Li,O/Y,O, (a), and 3.5% N&O/CaO I n Fig. 2 it i s possible t o see the main r e s u l t s o f steady s t a t e isotopic transient k i n e t i c experiments w i t h 2% Li,O/Y,O,. I n steady conditions the adsorption o f methane, followed by mans o f the response o f "CH,, switched i n the feed, Is characterized by a small induction period and i s sonewhat slower than i m transient experiments a t the very beginning o f the reaction. It seems important t o underline t h a t also the response o f carbon dioxide shows a small induction period; i n p a r t i c u l a r the response o f "CQ increases slowly and, above a l l , t h a t one o f "CO, decreases very slowly: t h l s obviously seems t o strenghten the hypothesis t h a t the production o f carbon dioxide i s followed
2268 by R slow desorptinn. Ths r e s u l t s obtained w i t h 3.5% M&O/CaO demonstrate a somewhat greater c a p a b i l i t y o f t h i s c a t a l y s t I n adsorbing methane and i n r e t a l n i n g carbon dioxide on i t s surface. I1.A.
signal
a)
N.A. signal
b)
T 13CH4
200
so0
1000
t (sec)
200
600
1000
t (sec)
Fig. 2 Steady state isotopic transient k i n e t i c responses a f t e r the switching o f "CH, I n t o the feed on 2% Li,O/Y,O, (a), and 3.5% N&O/CaO These r e s u l t s seem t o show t h a t the formation o f C,-hydrocarbons during steady s t a t e reaction i s l a r g e l y due t o gas pahase reaction expecially f o r 2% Li,O/Y#, above temperatures o f about 715 C, since the amount o f carbon dioxide produced by a surface reaction i n temperature programed desorption experiments i s too much smaller than i n steady s t a t e reaction. Methane is not e a s l l y adsorbed a t the beglnning o f the reaction on the fresh surface, b u t when the presence o f carbon dioxide on the surface i s s l g n l f i c a n t , methane interactions w i t h t h i s type o f surface are somewhat stronger: carbon dioxide i s desorbed very slowly and remains adsorbed, w i t h the consequence o f forming carbonate surface species, which a f f e c t the progress o f the reaction. This i s expecially t r u e f o r 3.5% Na&/CaO, where carbon dioxide i s stable up t o very high temperatures. Methane interacttons w i t h t h i s type o f surface do not favour however the formation o f C,-hydrocarbons. On 2% Li#/Y#, th? formation o f C,-hydrocarbons i s due mainly t o gas phase reaction beyond 715 C, because o f the decreased p o s s i b i l i t y o f methane o f onteracting with the surface. The forination o f C,- hydrocarbons indeed is i n every case s t r i c t l y r e l a t e d t o the presence o f gaseous oxygen, as t e s t i f i e d by the r a t e o f appearance of C,hydrocarbons and by the s i m i l a r r a t e o f disappearance o f oxygen i n transient experimnts a t the reaction beginning, and by the absence i n TPD spectra REFEREKES 1 K.P. Peil, J.G. Goodwin, 6. Marcel in, Natural Gas Conversion (1991),73 2 R. Spinicci, Proceedings S.C.I. Mat. Congr., (1991), 257
Guczi, L. d al. (Editors), New Frontiers in Catalysis Promdings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
OXIDATIVE COUPLING OF METHANE TO Cz HYDROCARBONS OVER DOPED TITANIA CATALYSTS
D. Papageorgiou, D. Vamvouka and X. E. Verykios Institute of Chemical Engineering and High Temperature Chemical Processes, University of Patras, 26110 Patras, Greece
Abstract The performance of altervalent doped TiO, catalysts under oxidative cou ling of methane Ta5+)of TiO, conditions is investigated. It is shown that higher-valence doping (W6+, enhances significantly methane conversion activity and reduces selectivity to nearly zero levels. On the other hand, doping TiO, with lower valence cations (LP, Zn+,) has a small influence on activity but results in a significant enhancement of selectivity. These results are attributed to alterations of the bulk electronic structure of the semiconductivity catalysts as well as to alterations of surface parameters such as aciditybasicity characteristics,oxygen ion mobility and lattice oxygen reactivity.
d+,
1. INTRODUCTION The catalytic conversion of natural gas, which is mostly composed of methane, to liquid fuels and chemical feedstocks has attracted considerable attention in recent years, primarily due to significant economic and environmental incentives. Most efforts have concentrated on the oxidative coupling of methane ( E M ) for production of C, hydrocarbons. Selective catalysts for this reaction route include rare earth oxides [1,2], alkaline earth and alkalialkaline earth oxides [3,4] as well as pure and doped transition and main group metal oxides [ 5 ] .The mechanisms of the various reaction routes and surface uansfomations as well as gas phase contributions are still hampered with uncertainly and speculation. However, certain important structural and chemical parameters of effective OCM catalysts have been identified [ 6 ] , and these are related to type of conductivity of the solid, surface aciditybasicity characteristics,oxygen ion mobility and exchangability of gas phase and lattice oxygen. 2. EXPERIMENTAL METHODS
TiO, was doped with foreign cations following the procedure of high-temperature diffusion. The oxides or nitrates of the doping cations were thoroughly mixed with TiO, in a slurry, dried and heated to 9 W C at a rate of 4OC/min. They were maintained at 900°C for 5h and were subsequently cooled very slowly. The resulting catalysts were characterized in terms of total surface area, employing the BET method. Their electrical conductivity and acti-
2270 vation energy of electron conduction were determined in a specially designed cell [7] under N, flow, in the temperature range of 30-45OOC, using the two probe direct current technique. Kinetic parameters of OCM were determined in a fixed bed quartz tubular reactor in the temperature range of 700-9OOOC.Temperature was monitored by a thermocouple inserted in a thermowell which run along the catalyst bed. Feed flowrates were monitored by thermal mass flow meters. Conversions and selectivities were determined as a function of residence time with variable feed composition. Intrinsic rates were determined at a methane conversion of 1%.
Surface acidity and acid strength dismbution were measured employing the method of amine titration using Hammett indicators. The method involves titration of benzene suspensions of powdered catalysts with n-butylamine, using Hammett indicators of various strengths to determine end-points.
3. RESULTS AND DISCUSSION Doping of a semiconductive metal oxide with cations of valence different than that of the parent cation results in alterations of the electronic structure of the solid (Fermi level, work function, n or p-type semiconductivity) while parameters such as oxidationheduction potential, surface aciditybsicity characteristics and oxygen ion mobility may also be affected by doping. The performance of altervalent doped TiO, in terms of activity and selectivity under OCM conditions is illustrated in Figure 1, in which methane conversion and C, selectivity are shown as a function or reactor space-time. It is apparent that upon doping TiO, with a higher valence cation (W6+) activity for methane conversion increases significantly. However: selectivity decreases to nearly zero 60 levels. Similar results were also obtained with -0other higher-valence dopants tested (Nb+5, 40 Ta+5). Doping TiO, with h a cation of lower valence ae W (Li+) results in a small In reduction of methane X conversion activity and a 20significant enhancement of selectivity towards hydrocarbons formation. Similar results were also 0obtained upon doping I TiO, with Zn+, cations. t 0 00 005 0 10 015 Alterations in activity WIF and selectivity of doped TiO, catalysts were Figure 1. Influence of higher and lower-valencecation doping found to be a strong of TiO, on methane conversion activity and C, hydrocarbons function of dopant selectivity.
2271 concentration in the TiO, matrix. This is illustrated in Figure 2a in which initial rates, at 1% CH, conversion, of methane consumption, C, hydrocarbons formation and COXformation, are shown as a function of Li,O content of the catalyst. Selectivity towards hydrocarbons formation is shown in Figure 2b as a function of LizO content of TiO,. It is apparent that initial rates of methane conversion and hydrocarbons formation go through a maximum at approximately lwt%Li,O content of the catalyst while initial rates of COXformation decrease significantly at Li,O dopant levels between 0 and 2wt96. As a result, selectivity increases significantly with increasing Li+-dopantand goes through a weak maximum at approximately 2 wt% Li,O content. These experiments were conducted at 750% under a CH, partial pressure of 0.15 bar and an 0, partial pressure of 0.05 bar. The influence of temperature on selectivity is also illustrated in Figure 2b. Selectivity is shown to increase with increasing temperature, a result which was also observed at high methane conversion levels. This is due to the fact that apparent activation energy of hydrocarbons formation is higher than that of COXformation and probably due to the fact that longer contact times are required at lower temperaturesfor the same conversion, which might lead to combustion of the hydrocarbons formed.
-
P,,-O.lbbmr
n
f
0:'
a
1-750 C P,-a05
t=: 01 0
1
2
3
y o .(*I%)
bar
4
I
5
Figure 2. Effect of Li+-dopant content in the TiO, mamx on (a) initial rates of methane consumption,C,hydrocarbons and COXformation and (b) selectivity towards hydrocarbons formation at various temperatures. Measurements of electrical conductivity and activation energy of conduction [8] have shown that higher-valence doping of TiO, results in enhancement of conductivity and reduction of the activation energy of conduction. It has further been shown that the activation energy of conduction increases monotonically with increasing Li+-dopantcontent of TiO? These results demonstratethat the electronic structure of the semiconductivecatalyst is altered upon doping with altervalent cations. However, this parameter alone is not sufficient to explain the observed alterations in kinetic parameters. For this reason, surface acidity of doped TiO, catalysts was determined as a function of acid strength and is shown in Figure 3. The acid strength of W6'-doped TiO, is significantly higher than that of undoped TiO, while surface acidity of Li+doped TiO, is significantlysmaller and decreases with increasing Li+ concentration in the TiO, matrix. These observations, compared with the kineticresults
2272
L
9 -
u.ut
EE
v
Acid Strength, H,
Figure 3. Surface acidity of altervalent doped TiO, at different acid strength. presented earlier might imply that surface acidity is detrimental to selectivity towards hydrocarbons formation. Surface acidic sites might be responsible for the formation of methoxy intermediate complexes which lead to combustion products. The same sites might participate in cracking reactions of the hydrocarbons formed, a process which would also lead to combustion products. It has been observed in this laboratory that altervalent doping of TiO, also influences oxygen ion mobility, exchangability of gas phase and lattice oxygen, surface basicity charateristics, and reactivity of lattice oxygen (in the absence of gas phase oxygen) towards CH,, C,H, and qH,. Thus, the observed alterations in kinetic parameters of the OCM reaction with altervalent doping of TiO, catalysts must be attributed to both, changes in the bulk electronic structure of the semiconducting solid (Fermi energy level) as well as changes in surface parameters. The latter are a result of the incorporation of foreign cations in the crystal mamx of TiO, and of the presence of these cations on the metal oxide surface.
REFERENCES 1. K. Otsuka, K. Jinno. and A. Morikawa. J. Catal.. 100 (1987) 353. 2. K. D. Campbell, H. Zhang, and J.H. Lunsford, 92 (1988) 750. 3. J.A.S.P. Carreiro, and M.Baerns, J. Catal., 117 (1989) 258 and 396. 4. C.H. Lin, J.X. Wang, and J.H. Lunsford, J. Catal., 111 (1988) 302. 5. S. Becker and M. Baems, J. Catal., 128 (1991) 512. 6. J.L. Dubois, and C.J. Cameron, Appl. Catal., 67 (1990) 49. 7. E.C. Akubuiro and X.E. Verykios, J. Phys. Chem. Solids, 50 (1989) 17. 8. D. Papageorgiou, D. Vamvouka, D.Boudouvas, and X.E.Verykios, Catal. Today,
in press.
Ouczi, L u al. (Editors), New Frontiers in Catalysis Procccdinga of the 10th International Congma on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elncvicr Science Publishers B.V. All righk'rescrved
THE IMPORTANCE OF CARBON DIOXIDE IN OXIDATIVE COUPLING OF METHANE A. Machocki
Department of Chemical Technology, Faculty of Chemistry, Maria Curie-Sklodowska University, 20-031 Lublin, Poland
1. INTRODUCTION
In the process of the oxidative coupling of methane a larger part of the catalyst is in contact with a gas containing carbon dioxide, whose amount depends on the degree and selectivity of methane transformation. It may cause a decrease 11-81 or increase [4,5,71 in the rate of the process and of the yield of C2+ hydrocarbons. The aim of the present paper is to determine the effect of carbon dioxide not only on the activity but also on the selectivity of one of the best catalytic systems of the process, i.e. sodium promoted calcium oxide catalyst [91. 2. EXPERIMENTAL
The precursor of the catalyst containing 1 . 7 wt.% of Na+ was prepared by impregnating calcium carbonate ( 0 . 6 - 1 . 2 mm) with aqueous solution of sodium carbonate. After drying at 383 K, the precursor was calcined in pure air (10 ml/min) for 1 hour at 1023 K just before the reaction. Only calcium oxide bulk phase was found after such calcination. The examination of the catalyst activity and selectivity was carried out in a quartz flow reactor at the temperature of 1023 K using the reaction mixture containing 50 % of methane, 10 % of oxygen, and nitrogen and/ /or carbon dioxide at varied amounts with the contact time of 0.317 s*g/ml. All presented results were obtained at the stable pro ertles of catalyst. Sorption and desorption of carbon dioxide under the reaction conditions and composition of the catalyst, as a func ion of carbon dioxide concentration in the gas phase, were measured by using the mass spectrometry and X-ray diffraction methods. 3. RESULTS
The presence of carbon dioxide radically decreases the catalyst activity (Figure 1 ) and the greatest changes are observed with carbon dioxide contents up to about 20 %. Along with the decrease of the amount of the reacting methane, the selectivity of the process also undergoes significant changes (Figure 2 ) .
2274
\ &
0
10 20 30 LO CO2 CONTENT, d.%
C 0 2 , CONTENT, molo/o
Figure 1. The catalyst activity as a function of carbon dioxide concentration in the reaction feed.
Figure 2. Carbon dioxide effect on the selectivity of oxidative coupling of methane.
There rises the selectivity of the methane conversion to ethane and carbon oxide, while that of the conversion of methane t o ethylene, C3+ hydrocarbons and carbon dioxide - decreases. Although changes in the selectivity t o particular products are considerable and various, the selectivity to the two main products, namely, hydrocarbons and carbon oxides, does not undergo any change under the influence of CO (Figure 3). However, there occurs a significant change in 2
the
mutual
ratio of
products within
unsaturated hydrocarbons (C,/Ci)
these
groups
-
saturated
and
and of the products of incomplete and
complete methane oxidation ( C O / C 0 2 ) .
The increase in the ratio CO/COz,
in
comparison with the value i t had in the case of the mixture without carbon dioxide, is greater than the rise of the ethanel'ethylene ratio (Figure 4).
J
40-
0
L x3 20 30 do
0
C02
CONTENT
C02 CONTENT, mol.
rnol.%
Figure 3. Selectivities to major products as a function of carbon dioxide amount.
O/O
Figure 4. The changes in CO/C02 and C-/C= 2
2
ratios caused by CO
2'
2275
-
10 YO without c02 co2
ahout 40% without c02 CQ co2 I-1
I
I
\
1
\
I
\ \
I
\
I
'4
0"
3 ET
e
I
I
\
The inhibition of the process and selectivity changes are reversible after the removal of carbon dioxide from the reaction mixture, the catalyst regains its initial properties. The phase composition of the catalyst also depends on the presence of carbon dioxide in the gas phase (Figure 5). When the concentration of C02 is
-100s (without catalyst I
+
0
Figure 5. Dependence of the catalyst properties (activity) on its phase composition influenced by carbon dioxide.
small (the reaction product), only calcium oxide was found, but when carbon dioxide contents is high (component of the input reaction mixture) the catalyst consists of two phases, calcium carbonate (calcite) and oxide.
4. DISCUSSION
The results clearly indicate that carbon dioxide is a poison of the catalyst Na+/CaO and that this poisoning is reversible and associated with the equilibrium of the reaction CaO+CO =CaCO 2
3'
The influence of carbon dioxide on the amount of the reacted methane and reaction selectivity may become understandable after accepting the scheme of the process in Figure 6 . It assumes that the decisive factor, determining the course of the process, is oxygen concentration on the catalyst w=ti@ surface, which is decreased when 9 - U j calcium carbonate is formed. The amounts of the reacted methane and formed products depend on the efficiency of the first stage of the dehydrogenation of methane molecule. Figure 6 . Scheme of the oxidative The stage requires the existence of one coupling of methane site on the catalyst surface containing oxygen reactive in . the process. As a consequence of blocking active sites by carbon dioxide, surface oxygen concentration diminishes, which decreases the amount of dehydrogenated methane and of products formed in subsequent stages. For the further course of the process another site on the catalyst surface with oxygen reactive in the process is necessary, regardless of the direction of the reaction of the CH species. In the case of oxidation to
~
~
~
3
carbon oxides such a necessity is obvious, but ethane formation also
2276 requires oxygen to provide another CH3 species needed for dimerization. The change in surface oxygen concentration, which is not accompanied by a change of the mechanism of the process, does not alter the ratio of the probabilities of contact of the CH3 species with another CH3 species or with surface oxygen. Hence, the selectivity of methane transformation to hydrocarbons or carbon oxides undergoes no change. A decrease in oxygen concentration on the catalyst surface diminishes a possibility of the dehydrogenation of ethane to ethylene and of the oxidation of carbon monoxide to carbon dioxide. The consequence is the observed increase in the ratios ethane/ethylene and CO/C02. The smaller increase of the ethane/ethylene ratio than that of CO/COz may indicate that ethane dehydrogenation also occurs in the way different from that which takes place through the reaction with surface oxygen. Iwamatsu [lo] has shown that this reaction can take place without a catalyst. The poisonous effect of carbon dioxide introduced along with the reaction mixture may be of great importance for the technical performance of the process. For the full utilization of methane it will be necessary to repeat several times its circulation through the catalyst layer. Unless carbon dioxide is removed, the process will be slowed down considerably. At the same time, the products will include a much smaller amount of ethylene, the most desirable product. 5. CONCLUSIONS
The presence of carbon dioxide in the reaction mixture of the process of oxidative coupling of methane exerts an important influence on the decrease of the efficiency and poor composition of the products - they include a smaller amount of ethylene. The cause of the poisonous effect is the calcium carbonate formation, which is associated with the blocking of the catalyst surface sites participating in supplying oxygen capable of activating and transforming methane molecules. The poisoning is reversible and depends on the carbon dioxide concentration in the gas phase. 6. REFERENCES
1 K. Aika and T.NishiJama. J. Chem.SOC. Chem.Commun., (1988) 70. 2 K.Aika and T.NishiJama, in: M.J.Phillips and M.Ternan (eds.1, Proc. 9th Intern.Congr.Catal., Calgary 1988, Chem. Inst.Canada, 1988, vol.2, p. 907. 3 K.Aika and T.NishiJama, Catal. Today, 4 (1989)271. 4 S. J.Korf, J.A.Roos, N.A. de BruiJn, J.G.van Ommen and J.R. H.Roos, J.Chem. SOC.Chem. Commun. , (19871 1433. 5 S. J.Korf, J. A.Roos, N. A. de Brui Jn, J.G.van Ommen and J.R. H.Roos, Catal. Today, 2 (1988)535. 6 J.A.Roos, S. J.Korf, N.A.Vechof J.G.van Ommen and J.R.H.Roos, Appl. Catal., 52 (1989)131. 7 T.Suzuki, K.Wada and Y.Watanabe, Appl.Catal., 59 (1990) 213. 8 X.D.Peng and P.C.Stair, J.Catal. , 128 (1991) 264. 9 J.A.S.P.Carreiro and M.Baerns, J.Catal., 117 (1989) 258. 10 E. Iwamatsu and K.Aika, J.Catal., 117 (1989) 416.
Guai, L d d. (Editors),New Frontiers in Cutaiysis Procccdinp of the 10th International C o n p on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights =served
KINETIC EFFECTS IN CONVERSION OF PROPANE, ISOBUTANE AND CATALYSTS, MODIFIED PROPANE-ISOBUTANEMIXTURES ON K-pt/y-Al~O~ BYSnANDIn
L. C. La+, H.S.Zbang, N.A. Gaidaib and S.L. Kipermanb ahstitUteof Flavor Chemistry and Catalysis, 1, Mac Dinh Chi Str., Ho Chi Minh City, Vietnam bInstituteof Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, Moscow 117939, Russia
INTRODUCTION Earlier we studied the kinetics and mechanism of iso- and n-butane dehydrogenation over platinum - alumina catalysis, containing 0.35% Pt without and with different additives [l-41.This paper deals with an investigation of dehydrogenation of propane, isobutane and their mixtures and side reactions (cracking and isomerization) on catalysts, containing 0.6% Pt and 1%K (K-I), with 2% of Sn (K-11) and In (K-111) as additives. EXPERIMENTAL The experiments were carried out in a gradientless flow circulating system at atmospheric pressure in the presence of hydrogen. The reaction mixtures were analyzed using GLC with 0.7 m column of silicagel and with 8 m column of 20% ester of tributeneglicol and butyric acid. The kinetic isotopic effects were determined as the ratio of reaction rates in Hz and D2 under the same conditions. RESULTS AND DISCUSSION Kinetics of dehydrogenation of propane, isobutane and their mixtures was studied at different values of the initial partial pressures of the paraffins and hydrogen, as well as of specially added propene and isobutene. Reactions temperatures ranges from 773 to 893 K. The conversion in side reactions did not exceed 24%. The back reaction was taken into account by introducing a correction coefficient y [5]. All the data for y calculation was taken from [6].
2278 Dehydrogenation of individual hydrocarbons Analysis of experimental data leads to the following kinetic equations for all the catalysts:
where ki - constants. In eq. (2) Pn-QH8 represents the partial pressure of n-butene formed in the result of isomerization.
Dehydrogenation of propane-isobutanemixtures On all the catalysts kinetic equations for propane dehydrogenation in the presence of isobutane and for the contrary case have been found as follows: k1 PC3H8 (r/y)c3 = (3) Po*'H2
+ klPC3H6 +k2Pi-QH8 + k3Pn-QH8
It has been shown that the experimental data are in acceptable coincidence with the curves calculated on the basis of eqs (3) and (4). Consequently, the total rate of mixture dehydrogenation is described by the sum of eqs (3) and (4). Also the kinetic isotopic effects in propene (isobutene) hydrogenation (B = 1.3 - 1.7) and in propane (isobutane) dehydrogenation @' = 1) confirm that in all the processes we have a common mechanism for C.1-C4 dehydrogenation [ 1-41.
Side reactions: Cracking and Isomerization As it follows from experiments, the rates of light hydrocarbon formation can be described as follows:
+ k5PC3H6 + k6PC3H6
k4 PC3H8
(rcr)c3 = P0"H2
where k'i - constants. The isomerization rate should be expressed as follows:
(5)
2279
(7)
-
where k"i constants. Promotion effects of metallic additives As it follows from kinetic data, additives did not change the form of kinetic equations of the processes, but an influence on kinetic constants has been observed (an increase in the slow step rate, a decrease of inhibition by olefins), but Sn and In in this case have weaker effect than in the case of the catalysts, containing 0.35% Pt without K [l-31.For the side reactions Sn and In have the same effect as in [1-31,which is expressed in increasing the selectivity in dehydrogenation. For catalysts K-I and K-I11 K increases dehydrogenation rate two times and an increase of 64% in selectivity is expected in result of a cracking depressing from 30 to 8%and from 10 to 4% respectively, but over the unmodified catalyst the isomerization extent rised from 0 to 14%. On K-I1 analogous effects are not expected. Thus, among Pt-K modified catalysts, Pt-K-In is the best for C3-C4 dehydrogenation.
CONCLUSION The role of additives consists in increasing the determining step rate, decreasing the adsorption of reaction producs and reducing the rate of side reactions. K decreases cracking, increases dehydrogenating capacity of K-I and K-111, but not of K-11, at the meantime isomerization is raised on K-I and K-11. REFERENCES L.C.Loc, N.A.Gaidai, S.L.Kiperman, Proc. sthIntern. Congress on Catalysis, Calgary, v.4, 1261, 1988. L.C.Loc, N.A.Gaidai, B.S.Gudkov, S.L.Kiperman, S.B.Kogan, Kinet. i Katal. 27
(1986) 1365. L.C.Loc, N.A. Gaidai, S.L.Kiperman, S.B.Kogan, Kinet. i Katal. 31 (1990)483. L.C.Loc, B.S.Gudkov, N.A.Gaidai, S.L.Kiperman, H.S.Thoang, S.B.Kogan, Unsteady State Processes in Catalysis, Proc. Intern. Conference, Novosibirsk, 391,1990. S.L.Kiperman, V.S.Gadzhi-Kasimov, Izv. AN SSSR, ser. khim. 6 (1965)1110. A.A.Ravde1, A.M.Ponomareva (eds.), Kratkii Spravochnik Phiziko-Khim. Velichin, Leningrad, "Khimiya'l, 1983.
This Page Intentionally Left Blank
Guczi, L a al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congm on Catalysb, 19-24July, 1992,Budapest, Hungary Q 1993 Elscvicr Science Publishers B.V. All rights mewed
THE REACTION OF REDUCTION CATALYZED BY HOMOGENEOUS AND IMMOBILIZED BINUCLEAR Rh(Il) COMPLEXES WITH Rh-Rh BOND
V.Z. Sharfand V. I. Isaeva N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, 117913 Moscow, Russia
The wide applioation of Rh oompound as oatalysts bases on
the ability of this metal to ohsngs the oxidation degree from 0, t1 to t3. In this oonneofion the qwstion of the oatalytio properties of Rh(I1) oomplexes the potential internediates in this prooesses is of great bporfanoe. 8VSTEM8. It hss been established thsf the binuolearfiexafluoroaoetylaoetonate Rh(I1) oomplexes with different axial lQand8 (Py, 5 0 ) in the mild oonditiono(0.1 MPa, 50’0) oatalyze the hydrogenation and isornoriastion of allylbeneene and styrene. The oatalytio aotivity of his oompound8 kzoreases after the pph3 htrodwtion in the system. Probably,the original oomplex undergoes the tramsfoxmation in the highly aotivcr Rh(I1) mnomer ugon the pph3 aotion. Aotually, it was oonfinned by mean6 of %3Rspeo trosoopy/I/ It has been found that the oatalytio aotivity of Rh(I1) tetraaoetate solution inoreases twenty-fold times after the treatment of this solution by NSBH4. It has been established that in these oonditiono the forn of Rh(I1) oomlexes with two-[%(0200H3)212t and three [ 9 ( 0 2 0 0 € 5 ) 3 1 t aoetate bridges operates. It was o o n f h e d by mean6 of eleotronio speotrosoopy the oharaoteristio band8 of these oomplexes are 1550 and 1610 om’ respeotively /2/. Obviosly, the eleminstion of 1-2 aoetate br-s takes plaoe. This leads to
-
.
-
2282
the appearenoe of vaoant sites in ooordination sphe re of the oomplex. HE 0SWTEMS The new oatalyerts on the baee of bhuolear oomplexes fixed on the aminOProPJr1 &rows oontahing silioa gel ( F - d Y p S ) and polhere modified by heterooyolio amhgroupa. The oxidation degree Rh(t2) not ohanges after the fixation of [+(02CCH3)41: RE-speotrum of the oatalysts has one maximum with the energie Er Rh 365,2=309.5eV oharaoteristio for Rh(t2). Thia aJmtem is dimmgnetio (the ab13enOe of BSR-signal). It is lndioated the presenoe of Rh-Rh bond. The absorbtion band6 of aoetate $ 8 (000)1410 om-' &g~~ent group6 $86 (000)1550 and in IR-speotrum of the oatalyata. The wide band in the region 1600 om-' in eleotronio epeotrum of the oatalpts indioates the presenoe of aoetate oomplex with blnuolesr etruoture Rh-Rh bond and 2-4 aoetate bridges. The Rh(t2) oxidation degree does not ohange after the hbilit3atiOn of tetraaoetate dirhodium(I1) on N-oontaining polhere. The one maximum oharaoteristio f o r Rh(t2) (XI-Rh 3dg,2=309.5eV for one oatalyst and 308.9eV for seoond oatalyst) present in RE-speotra. !!!he both oataly6ts are diam8gnetio. Thw, the binwlear atrvoture Rh-Rh bond aoetate 1-d~ (2-4) retain after the imnobilisation of oomplex [wl,(02CCH3I,) on y-AMPS and N-oontaining polhers ( s t m t u r e A-C).
I
1
I
-m-r% P I
(k)(74)3 3 I Si si I I 0
0
A
-Rh-
I
-r-
I
I
C--F
P 4
( p V 3 (74)3
3I;(
sbi B
si
'8
I
0
0
I C
I
-rF
(p13
si 0
I D
2283 The ligends of original oomplex mbstituted by a~minogroups of the support after the fixation of the aoetonitrile, sulphate and hexafluoroaoetylaoetonate oomplexes on the support. lPIl0 maxima with Er Rh 3%,2=310.7 eV(80%) and 308.3 eV (20%) present in the IR-apeotra of aoetodtrile oomplex ~ b i l i a e d on r-AMPS. Thw, h this oa6e the oxidation-redwtion prooesa: 2Rh(II) Rh(1) + RH(II1) takd plaoe. The values of Er Rh 3ag,2 of hexafluoroaoetylsoetomte and milphate oomplcxes -310.8 eV and 310.1 eV respeotively arc ohamotcristio for Rh (+3). Therefore, the Rh(+2) oxidation a g r e e ohange.8 and the rupture of Rh-Rh bond takes plaoe after the fixation of Rh(I1) oomplexes with labile bridges (eulphate) and bridgle136 p u p s (aoetonitrile and hexafluoroaoetylaoetonate (8tluoture D). Heterogenieed Rh(I1) oomplexes oatalgee the hydrogenation and iaomeriaation of o l e f h , redwtion of ketone6 h o l u d h g hydrogen tran6fer f r o m iso-propanol in KOH media and the reduotive hydrodchalogenation of p-bromtoluene and gem-dihsloderivatives of phenyloyolopropanea with hydrogen tran6fer from aloohole (I ) and NaBH4 (2 )
-
The elimination of two halogen atoms f r o m oyolopropanea prooeed oomeoutive with different rates. The
2284
hydrodehalogsnation of 1 ,l-dibromo-2-phsngloyolopmpane in the ~ F O B O ~ Oof O oolqplex t (02am3 I f %xed ‘on the modified polgmer hsv3(5)-!fldhJrlp~olJrleFouesleads t o p ~ f O ~ d fozmation (65%)of ois-isomer monobromide. The yield of this isomer ashieves 98% in the OMO of 6 i l i O a gel fired oompleres. BtheFeoseleotivitJr of the prooess ohanges after the introdwtion of mthylgrougs in oyolopmpane r-. In the 0-0 of pwtidl dehalogsnst ion Of l,l-dibFOw, -2-fIl~thylO~lOprOp8Il098% O f fzlans-iBOInOI- fOxmS. The destruotion of oyolopropane rooouro i n omall d o p e or does not take plaoe in the pmsenoe of the oatal-ts at the reduotive dohalogenation of &em-bihalO&FiVatiVO~ O f ph~lO~OlOprop8tleB. Noteworthy, in hveatigated reaotionn hmbilised binwlesr Rh(I1) sostate oolqplex suFpasse& a o t i v i t y the momnwlear Rh oomplexes obtained by the fixation of aoetonitrile, sulphste and he~fluomaoetylaoetonste oon?plexes. -8
:
1. A.N.Priljaev, A.V.Rotov, I.V.KuSmsnk0, I.B.Bsranovskii, Dool. AN USBR, N.3(1988), 61. 2. O.R.Wilson, H.TCaube, Inorg.OheUI. , N.9(1975), 2276.
Guczi, L ei al. (Editors),New Frontiers in Catalysb
Proceedings of the 10th International Congnss on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
A HIGHLY ACTIVE, HETEROGENEOUS HYDROFORMYLATION CATALYST: Rh(CO)(acac)L,L=poly-TRIMBOUND PHOSPHINE J. Hjortkjaera, B. Heinrich4 C.An&rssonb and A. Nikitidisb
9epartment of Chemical Engineering, The Technical University of Denmark, Building 227, 2800 Lyngby, Denmark bOorganiskKemi 1, Kemicentrum, Box 740,220 07 Lund, Sweden
INTRODUCTION The immobilization of catalytically active complexes may lead to catalysts, which combine the advantages of homogeneous and heterogeneous catalysts, namely high activity and selectivity together with separability, so that these catalysts may readily be used in liquid or gas-flow reactor systems. This "heterogenization" may be established in several ways: By chemical anchoring of the catalyticad active complex to an inorganic support (I), to an organic (polymer) support (27, or by supporting a liquid phase - in which the catalytically active complex is dmolved on a solid surface (SLP-catalyst) (3). This report describes our experimental examination of a new pol er supported catalyst, which with respect to activity and stability compares favora ly to previously described, heterogeneous hydroformylation catalysts.
r
EXPERIMENTAL
6
Catalyst preparation: TRIM-particles poly-(trimethylolpropane) trimethacrylate) were prepared as previously describe (4). These particles, which contain unreacted double bonds in the pore system, were functionalid with pol -acrylic acid chloride followed by reaction with N-methyl-4-4(diphenylphos hinof-benzylamine, leading to TRIM-particles with a phosphorous content of 2.9 0 . Finally, Rh was introduced by reaction with Rh(CO)z(acac), leading to Rh(cO)(a~ac)L,L= polyTRIM bound phosphine (5).
P
Kinetic ezamination: The catalyst was tested for activity and stability relative to propene hydroformylation in a microcatal ic flow system operated as a differential plug-flow reactor. A known quantity o the catalyst was placed in the reactor, where it formed a short bed, and a mixture of the gaseous reactants (C3H6, CO and Hz) was allowed to flow through the reactor. Flow rate control was by means of a precision metering valvc, and after reduction to atmospheric pressure, the exit gas was periodically analyzed by injection of samples into a gas chromatograph (Shimadzu GC-SA).
?
2286
RESULTS AND DISCUSSION
As outlined earlier (6, 7) for differential conversion of propene in the reactor the rate of hydroformylation is defined as:
where x is the fractional conversion of propene in the reactor for a flow rate with respect to propene of F, and W, represents the weight of rhodium present in the reactor. The effect of time on catalytic behavior assessed at various temperatures is depicted in Fig. 1. It is clear that the activity of this catalyst is very high when compared to the activity of other polymer-bound hydroformylation catalysts (8, 9), and even relative to SLP-catalysts (7) the activity is very high: At the conditions given in Fi . 1 at 333K the rate is 1.1-10-4 mo1.s-1.g Rh-1 which compares favorably to a rate o 6.7.10-6 mol-s-1.g Rh-1 for the SLP-catalysts (7). It is also noteworthy that after 140 hours on stream there is no observable loss in activity.
d
t
'0°1
333 K
1 323 K
I-
Fig. 1: Hydroformylation of propene heterogeneous1 catalyzed by Rh CO)(acac)L, osphine, in L=poly-TRIMa microcatalytic plug-flow
6
against time on stream at various temperatures. Total pressure: 5 atm. - C~HG:CO:H~:N~ = 5.0:1.0:3.4:4.7.
2.7
3.0 5.9 6.1 4.0 4.2 6.8 7.0 7.2 7.4
Time/ min
lo3
The influence of total pressure on reaction rate was examined at 333K with a reactant composition as given in Fig. 2: When the total pressure of reactants was raised from 2,3 to 11 atm. the rate went up from 8,4-10-6 moles-1.gRh-1 to 53.10-6 mo1.s-1.gR.h-1. Within this pressure ran e there was a linear relationship between the logarithms of the rate and of the tot pressure, indicating a constant total order of reaction with respect to the reactants. To identify further the rate dependence on pressure, catalytic behavior was also examined as a function of the partial pressure of the reactants - here exemplified (in
9
2287
Fig. 2: Logarithm of hydroformylation rate (r/mol-s-l.gRh-l) at 333K against logarithm of total pressure (Ptot./atm.). Reactant stream com 37,6%propene 22,2 o hydrogen, 4,Od carbon monoxide and 36,2% nitrogen
Fion:
Fig. 3: Logarithm of reaction rate (r/mol.s- g 333K and 6 atm. l. tot pressure at against logarithm of carbon monoxide artial pressure (p,,/atm!) Reaction rate is corrected for the influence of the partial pressures of propene (p /atm.) and of
Rh2
c3
hydrogen (pH /atm.). 2
Upper curve: Total hydroformylation rate, slope: 4 7 5 . Middle curve: Rate of n-butanal formation, slope: 4,79. log r
- alog
Pc
3
- b log
PH
2
Lower curve: Rate of iso-butanal formation, slope: -0,68.
2288 From Fig. 3 it is clear that the rate is reduced on rising carbon monoxide pressure (pco), and similar experimental examination of the influence on reaction rate of the partial pressures of propene (p, ) and hydrogen (p ) reveals the experi3
mental rate equation at 333 K: r =
1,7-10-5.pc 0,63. pH2-pco 1 -0,79 3
where rn is the rate of n-butanal formation (mol -s-1-gRh-1)
where rise is the rate of iso-butanal formation (mol-s-1.gR.h-1) rtot =
2,6-10-5.pc0,59. pH2*pco 1 -0,75 3
where rtot is the rate of butanal (n- and iso-) formation (mol-s-1.gR.h-1) pc; PH2 and pco are the partial pressures of propene, hydrogen and carbon monoxide, respectively. These kinetic parameters are applicable when the reactant partial pressurea are varied in the range 0,5-2 atm. Hydroformylation rate was found to be very sensitive to temperature, leading to an apparent activation energy of app. 90 kJ-mol-1. In the temperature range 295-333K) the activation ener 'es for n- and iso-butanal formation app. 90 kJ-mol-1 , so that tFIere was no effect from temperature on regioselectivity (rate of n-butanal ormation/rate of iso-butanal formation).
1
REFERENCES 1
2 3 4
B. Delmon, G. Jannea (editors), Catalysis: Heterogeneous and Homogeneous, Elsevier, Amsterdam 1975. C.U. Pittman, Jr. in G. Wilkinson, F.G.A. Stone and E.W. Abel (editors), ComprehensiveOrganometallic Chemistry, Vol. 8, Pergamon, Oxford, 1982. P.R. Rony, J.F. Roth, J. Mol. Cat. l(1973) 13. P. Reinholdsson, T. Hargitai, R Isaksson, B. T h e l l , Angew. Chemie
t%z!idsson, T. Hargitai, R Isaksson, A. Nikitidis, C. Andersson, React. Polymers (submitted). 6 J. Hjortkjaer, M.S. Scurrell,P. Simonsen, J. Mol. Cat. 6 (1979) 405. 7 J. Hjortkjzer, M.S. Scurrell, P. Simonsen J. Mol. Cat. 12 (1981) 179. 8. K.S. Ro, S.I. Woo,J. Mol. Cat. 61 (1990) 27 9. J. Hjortkjaer, Y. Chen, B. Heinrich, 3rd Nordic Symposium on Catalysis, Helsinki, Finland 1990. 5
Ouczi, L u al. (Editors),New Frontiers in Cataljsis Proceedings of the 10th International C o n p on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishem B.V. All rights nserved
-
SELECTIVE HYDROGENATION OF C4 ACETLYLENES OVER AN IONEXCHANGED COPPER ON SILICA CATALYST
J. T. Wehrlia, D. J. Thomasa, M.S.Wainwrighto, D. L. Trimma and N. W C a d aSchool of Chemical Engineering and Industrial Chemistry, University of New South Wales, P.O.Box 1, Kensigton, NSW 2033, Australia bSchool of Chemistry, Macquarie University, NSW 2109, Australia
Abstract An ion-exchanged CdSiO, with high activity for the gas phase hydrogenation of C4acetylenes has been investigated. It has a very low tendency for overhydrogenation to nbutane, or for butene isomerisation, but there is significant acetylene oligomerisation. This tendency is lower at elevated temperature and if high concentrations of butenes and hydrogen are present. The catalyst can be used for the removal of acetylenes from an indusmal C4 stream, but butadiene will be hydrogenated as well if not removed separately. 1. Introduction
Olefin streams used industrially are normally produced by the steam cracking of ethane or higher hydrocarbons (1). Acetylenes are undesired byproducts which are usually removed by catalytic hydrogenation to avoid interference with downstream processes (2). The situation is particularly complicated for C4 streams. In addition to undesired 1- and 2butynes and vinylacetylene (l-butene-3-yne) they contain 1.3-butadiene which is valuable and easily lost by hydrogenation. Removal of butadiene by extractive distillation followed by liquid phase hydrogenation of the remainder over palladium is commonly used but activity tends to decline with use due to deposition of oligomeric material and loss of palladium by complexation (3). A gas phase process using a copper based catalyst has been reported to be advantageous (4). Aspects of the hydrogenation of C4 acetylenes and dienes over palladium have been investigated in considerable detail (3,5,6). Apart from early work using a static system (7) there is very little corresponding work with copper catalysts. In this work we have investigated the characteristics of an ion-exchanged WSiO, catalyst for the hydrogenation of pure and mixed C,-acetylenes and dienes streams. Previous work (8) had established that the same catalyst was very selective for the corresponding reaction of C, compounds. 2. Experimental The WSiO, catalyst was prepared as described previously (8) with Degussa Aerosil 200 as the support. The catalyst contained 6.94% copper, had a copper area of 4.2m2/g
2290
by a pulse N,O method (9) and a total area of 243mZ/g. Catalyst testing was carried out using a computerised flow system involved 0.5 g samples of a 250-400 pm particle size fraction contained in a vertically mounted 9mm ID copper tube. Oligomeric material draining from the tube was trapped in an ice bath and the C, stream leaving the trap was analysed using a Shimadzu GC-8A chromatograph fitted with a 4m VZN-1 column operated at 30°C. The standard test mixture for the individual C, compounds (from standard commercial sources) was 1% C,, 4% hydrogen, balance helium with a total pressure of one atmosphere and flow rate of 130 cm3(STP)/min. The industrial C, stream was from Altona Petrochemicals, Victoria to which hydrogen alone was added to the same total flow. 3. Results and Discussion Figs. 1,2 and 3 show the temperature dependence of reaction using standard mixtures in which the amount of hydrogen is more than sufficient to allow complete saturation to butane. With 1-butyne (Fig.1) conversion reaches 100% at 140°C. Oligomers then form 75% of the product but this percentage falls to 20% when the temperature is increased to 280°C. The major product is then 1-butene. Both 2-butenes and n-butane were undetectable (4.3%) at all temperatures. The reaction of 1-butene-3-yne is somewhat slower with complete conversion reached at 170°C (Fig. 2). Oligomerisation is then 60% falling to 45% when the temperature is increased to 280°C. Butadiene is the largest C, product at conversions less than 100% but is overtaken by 1-butene at higher temperatures. 1-Butyne is just detectable below complete conversion while 2-butenes with a cis-rruns ratio of -1 are formed at higher temperature. No n-butane is produced. The situation with 13-butadiene (Fig. 3) is quite different. Oligomerisation is barely detectable below complete conversion at 140°C and undetectable above that. The dominant product is 1-butene, reaching 72% at 150" but declining to 65% at 280°C. The yield of 2butenes, again with a cislrruns ratio near to one, increases slightly with temperature. These results indicate that the 2-butenes observed during the reaction of 1-butene-3-yne arise from reaction of butadiene, the intermediate product. Table 1 summarises the selectivities observed at the temperature at which complete conversion was first reached together with apparent activation energies and rates calculated from data obtained under differential conditions at lower temperatures. The apparent activation energy for 1-butyne is very similar to that found for propyne (8) while that for 'OO1\ + 2-bulene A ethylacetylene
-90
130
iio
2io
2%
TEMPERATURE ("C)
Fig. 1 Hydrogenation of 1-butyne (ethylacetylene)
" I
80
-
li0
160
200
Temperature
2io
280
(OC)
Fig. 2 Hydrogenation of 1-butene-3-yne (vinylacetylene)
2291
Table 1 Characteristics of hydrogenation of pure C4 compounds over WSiO,
E,- Rate at 13OOC Selectivity at temperatures for 100% conversion kJ/mol
mmovkds
1-butyne =65 1-butene-3-yne -78 1,3-butadiene =50
1-butene 2-butenes butadiene butane oligomers
1.21 0.25 1.08
25 10 72
4.3 1.2 20.0
4. 1
4.1 4.1 4.1
13.0
75 56
:.
:.
0.5
--
2
&
K. Ha@ New Hydrogenation Catalysts. John Wiley, New York, 1971. R L.Augustine, Catalytic Hydrogenation, M. Dekkcr, New York. 1965. h4. Fnifelder, Ractical Catalytic Hydrogenation, Wiley-Intcrsci., New York, 1971. K. Sakai. M. Ishige, H. Kono, I. Motoyama, K. Watanabe and K. Ham, Bull. Chem. SOC. Japan, 41 (1968) 1902. 5 T.Mallat, Zs. Bodnar and J. P e w , Tetrahedron, 47 (1991) 441. 6 2s. Bodnar, T. Mallat and J. Petro, J. Mol. Catal., 70 (1991) 53.
1 2 3 4
Guczi, L.et al. (Editors),New Frontiers in Caralysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
NEW ASPECTS ON THE MECHANISM OF OLEFIN POLYMERIZATION WITH REDUCED PHILLIPS CATALYSTS
H.L. Krauss, H.A. Schmidt, B. Siebenhaar, P. Wolfland Q. Xing Laboratorium fur Anorganische Chemie der Universitat Bayreuth, Postfach 10125 1, D-W 8580 Bayreuth, Germany
Abstract The polymerization of olefins with reduced Phillips systems is hindered (induction period) or fostered (fast start) by residues of the reducing agents. The catalytic activity is restricted to a small part of the surface chromium centres which are coordinatively unsaturated and carry Cr-C o-bonds both prerequisites of chain propagation. These active organometallic centres contain probably a saturated metallacycle loop which is still reflected in the Cr(lV) end product of the high temperature redox reaction between the surface metal and the olefin.
-
Introduction The polymerization of 1-olefins with surface chromium is usually formulated with the coordinatively unsaturated Cr(ll) species as the active centre or its precursor. Recent discussions of this reaction were based on models either with carbene complex intermediates [2] or with a reactive Cr-alkyl species [3] both emerging from a reaction of the metal site with the monomer. But none of the typical carbene complex reactions was verified experimentally [4] and on the other hand no IR evidence for a methyl end group in the primary complex was found [5], not to mention the lack of one hydrogen atom. Consequently both models were combined assuming a fast equilibrium of a carbene complex and a metallacycle with two chromium-to-carbon ubonds [2b]. - The deactivation of the catalyst at high temperatures proceeds by an irreversible redox reaction to a Cr(lV)-complex and hydrogen and/or alkanes [6]. In the present work these statements are discussed in the light of new experimental evidence: (i) determination of Cr-C o-bond and C content of the catalya:s, (ii) correlation to their polymerization activity, (iii) analysis of the Cr(lV)-complex.
-
2326 Experimental
The catalysts were prepared as described elsewere [7]. As a standard, a contact with 1 % Cr on silica Merck 7733 was used, activated in oxygen at 8WC, reduced with CO at 350°C. Variations included support, Cr concentration, temperatures of activation and reduction, reducing agent and further thermal treatment. The reaction with the olefins was carried out either in a gas solid system (ethene) or in a n-heptane slurry (1-0ctene). Temperature, olefin concentration and reaction time were varied. The process was followed by GC analysis of unreacted monomer. For the preparation of the Cr(lV) complex the catalyst was reacted with ethene at 300°C for ca. 20 min at normal pressure, followed by purging with argon, cooling to room temperature, purging with HCI and again argon; the complex was extracted with methanol, separated on sephadex and fractionated by HPLC on a diol column. The determination of the Cr-C a-bond was carried out by a modified "aldehyde method" [8] as described in detail in [9]: By reaction of the "working catalyst" (slurry) with oxygen the polymerizationwas stopped and the catalyst isolated. After treatment with HCI the formed aldehyde was extracted and analyzed quantitatively by IR. For the C determinationthe catalysts were heated in a stream of oxygen from room temperature to 600°C; the C02 was collected as BaCOg and analyzed by titration. The catalytic activity of the samples was measured as l / t after 10, 50 and 90 % turnover of the monomer (l-octene) as described in [lo]. The MS analysis of the Cr(lV) compounds was carried out by MS/MS (Finnegan MAT TSQ 70 and Vacuum Generators TS-250E) and HPLC/MS (Hewlett-PackardMS Engine).
Results
Surprisinglythe "standard catalysts" showed to be contaminated with carbon (from the reducing agent) to about 4 3 % mol/mol referred to surface chromium. Almost the same number is found for the content of a-bonds (33 %) immediately after starting the reaction. Since the effective concentration of the carbon compound is too low to be detected by IR, this correlation was overlooked hitherto.- If ethane or ethene are used for reduction, the C and the a values are higher; in contrast a short application of an argon diluted oxygen pulse on the CO reduced samples diminishes both values. These modificationsare paralleled by an increase of the mean oxidation number, from ca. 2.15k0.02 (standard) to ca. 3.0i0.5. During the first minutes of the polymerization reaction, ketones and aldehydes, e.g. acetone, propanal and butanal are evolved from the CO reduced systems. The u values increase during the polymerizationtill the maximum rate is reached. Catalysts which were reduced by ethane and ethene (short contact at 4WC) showed not only
2327 higher values of C content and (I, but a correspondingly higher activity. As well the CO reduced samples gained activity by the short contact with the argon/oxygen pulse mentioned. The optimum oxidation number for a high polymerization activity ist near 3.0 in all these samples [lo]. This behaviour is mainly due to shorter induction periods. If the catalyst is reacting with the olefin (ethene) at high temperatures (T>12O"C, preferably 30OoC),Cr(lV) organometallic surface compounds are formed together with hydrogen and/or alkanes, as reported earlier[6]. By treatment with HCI/MeOH the chromium species can be removed from the surface and isolated as a blue oil, [ C~(C~H~)CI~ ]P.SOIV with different values for n,m according to the polymerization. By means of MS/MS and HPLC/MS it was shown that n is 8, 10, 12, ..., while m is 2n - 4 and p is 1 or preferentially 2. HPLC was used to isolate an individual compound Cr2(C8H11)2c14*
Discussion The basic reaction in the preparation of the catalyst is obviously the formation of a set of coordinatively unsaturated chromium sites [l 11. The most active of them hold vigorously parts of the reducing agent, e.g.CO. With the first olefin added, an oxygen containing organometallic surface species is formed, which is not active in polymerization (compare [12]). This species causes the induction period: It is slowly replaced by an oxygen free organometallic surface compound which is now the active catalyst, most probably with a metallacycle (no H needed, no methyl end group found) [12]. Oxygen pulses "clean" the centres partially from CO - resulting in a shorter induction period. - Ethane and ethene on the other hand may leave organic groups at the catalytic centre which have a positive effect similar to the "co-catalyst" AIRnXm. The metallacycle model for the active centre, postulated in [12] on the basis of spectroscopic evidence, gets further support from the MS results on the Cr(lV) high temperature products: They contain obviously an organic loop x-bound to the metal centre (C/H ratiol) and therefore inactive vs. insertion of olefin monomers. It should be emphasized that genuine Cr2 units appear, indicating that pairs of chromium are present already from the very beginning of the metal fixation at the surface - according to a dichromate model. This should be considered when the structure of the active centres is discussed.
-
2328 We thank the Deutsche Forschungsgemeinschaft, SFB 213, and the "Fonds der Chemischen Industrie" for financial support, the Hoechst AG, .the Kali-Chemie AG, Fisons-Vacuum Generators and Hewlett-Packard GmbH for technical help.
References 1 Part XXXVII: H.L. Krauss and G. Langstein, J. Mol. Catal. 65, 101 (1991) 2 a) H.L. Krauss and E. Hums, Z. Naturforsch. 34b, 1628 (1979), b) G. Ghiotti, E. Garrone, S. Coluccia, C. Morterra and A. Zecchina, J.C.S. Chem. Comm. 1979,1032 3 M.P. McDaniel and M.M. Johnson, J. Catal. 101,446 (1986); P. Cossee, J. Catal. 3,80 (1964) 4 K. Weiss and H.L. Krauss, J. Catal. 88,424 (1984) 5 E. Garrone, G. Ghiotti and A. Zecchina in "Olefin Metathesis and Polymerization Catalysts", Nato AS1 Series C, 326, 393, Kluver Acad. Publ. Doordrecht, 1990 6 H.L. Krauss, K. Hagen and E. Hums, J. Mol. Catal. 28, 233 (1985) 7 H.L. Krauss and U. Westphal, Z. Anorg. Allg. Chem. 430,218 (1977) 8 a) H.L. Krauss and H. Schmidt, Z. Anorg. Allg. Chem. 392, 258 (1972), b) G. Ghiotti, E. Garrone and A. Zecchina, J. Mol. Catal. 46,61 (1988) 9 Dissertation P. Wolff, UniversitEit Bayreuth 1991 10 Dissertation B. Siebenhaar, Universitat Bayreuth 1991 11 H.L. Krauss and H. Stach, lnorg Nucl. Chem. Letters 4,393 (1968) 12 G. Ghiotti, E. Garrone and A. Zecchina, J. Mol. Catal. 65, 73 (1991)
Ouczi, L u a& (Editors), New Fronriers in Catalysis
Proceedin@ of the 10th International Congress on Catalysis,19-24 July, 1992,Budapest, Hungary 6 1993 Elsevier Science Publishers B.V. All rights resewed
THE ROLE OF POROSITY IN ETHYLENE POLYMERIZATION ON Cr/Si02 CATALYSTS I. G. Dalla Lanaa, J. A. Szymurab and P.A. ZielinskF aDepartment of Chemical Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada bDepartment of Technology & Chemical Engineering, Technical and Agricultural University, 85-326 Bydgoszcz, Poland %stitUte of General & Ecological Chemistry, Technical University of Lodz, 90-924 Lodz, Poland
Abstract Examination of three silica-supported catalysts during the ethylene polymerization indicated a very strong influence of the the silica support upon the transient yield of polymer in a s Polymerization within pores is a dominant factor. Acceleration continual up to high polymer yields per gram of catalyst if pore sizes facilitate particle fragmentation. The growing polymer encapsulates the catalyst fragments even though more rupturing occurs, suggesting insertion of ethylene into the polymer chain occurs at the catalyst surface. 1. INTRODUCTION
Published research describing the Cr/SiOp catalyzed polymerization of ethylene in slur reactors does not resolve the relation between the kinetics and processes invo ved [l]. Several gas-phase studies [2-51 suggest that the morpholo y of the catalyst/polymer particles plays an important role. The porosity o the support may inflence the transient kinetics [2] and polymer yields [3,4] by facilitating fracturing of the particles thereby exposing additional active sites. This work examines the role of porosity in slurry reactor polymerization by using both porous and non-porous forms of silica supports.
B
r
2. EXPERIMENTAL DETAILS
Nonporous Cab-O-ql (BET area=163 m2/g) gnd two porous siliqas: Davison ( B q area=277 m /g, pore volume=1.57 cm g) and PQ (330 m g and 1.5 cm /g) were used in preparin 0.5 and 5. wt% Cr catalysts y impregnation with aqueous solutions of Cr 3. Both catalyst activation (usin 0.1 0.29 and sequential treatment: N2 pt 5OO0C, 0 2 at 5OO0C, and CO at 35OoC! ang ethylene polymerization (in 50 cm of n-hexane) were done in 100 or 1 0 cm Vycor reactors. In these studies, polymerization was carried out at 25 C and near-atm pressure to slow the sequential changes in morphology of the
8
d
6-
8
2330 catalystlpolymer particles. The higher Cr loading of 5 wt% enabled stable catalytic activities and high polymer yields within roughly 2h reaction times. Under such reaction conditions, the course of the polymerization was tracked by gravimetric determination of polymer yields (g polymer/g catalyst) at various times and examination of the resulting polymer/catalyst particles using transmission (TEM with Philips EM300, 80 kV) and scanning electron microscopy (SEM with ISI-60, 15 kv) . Polymers were also examined by XRD (Philips diffractometer), qel permeation chromatography (GPC with Waters 150-C) and differential scanning calorimetry (DSC with Perkin-Elmer DSC-4). 3. RESULTS AND DISCUSSION
Polymer yields for four catalysts have been plotted versus time in figure 1. Regarding curve b as being representative, one sees an initial rapid growth in the polymerization rate (step 1) declining after 5 min before gradually increasing between 5 to 30 min (step 2). After 30 min, the rate accelerates increasingly (step 3) up to ver)! high yields . Curves a and b include results from,tests using both the 100 cm reactor (yields up to 26.5g/g) and the 150 cm reactor (up to 126g/g). The continuity of the plots shows that the contacting between the ethylene gas and the catalyst particles in the n-hexane liquid medium was not altered by a 50% scaleup in reactor size. Curves b and b', 5 and 0.5 wt% Cr on Davison silica, respectively, exhibit the same characteristic steps but clearly the additional active Cr sites on the 5 wt% catal st provide much hi her ields at the same reaction time in step 3. Use of 5 wt% r also enabled muc hig er polymer yields to be obtained within reaction times of 2 to 3h (maximum of 126g/g obtained). Curve a (Cab-O-Sil) shows an almost constant rate (slope of curve) over the entire range of yields shown, somewhat higher than that of the Davison support. All catalysts show essentially identical initial accelerations (step1) but only PQ support (curve c) fails to exhibit the rapid increase of step 3! Curve c suggests that the catalyst is deactivated by the processes involved in steps 1 and 2. The suppression of polymerization following step 1, common to curves b, b and c, all porous supports, in contrast to continued polymerization on the nonporous support (curve a ), suggests that deactivation is being caused by filling of the pores by the growing polymer chains. After 9 min, the BET pore volume of the polymer/ 5 wt% Cr on Davison catalyst particles measured 1% of the initial pore volume of the support, confirming that pore blockage was nearly complete. DSC did not detect branching in the polymer; XRD patterns indicated considerable crystallinity; and, GPC indicated a weight average of molecular weight of ca. 500,000. This suggests that polymer formed up to at past 9 min is linear high density polyethylene (PE), whose bulk density is 0.96 cm /g [ 6 ] . Use of this density gives a ratio of 1.42 g PE/g cat which is very close to the yield of 1.32g/g obtained after 9 min of polymerization in the slurry reactor. The slight difference may be attributed to looser packing within the pores relative to that in high yield industrial polymer. The overlap in yields for curves b, b' and c during step 2 probably results from the similar pore volumes of Davison and PQ supports and, presumably, comparable Cr active sites initially exposed on their surfaces. The increasing rate of polymerization of curve b is attributed to fracturing of the particles from the pressures generated within the pores by the growing polymer. The failure of curve c to exhibit step 3 is attributed to the smaller pores of PQ support becoming filled, unable to fracture and thus deactivated. The Cr active sites on the external
8
a i
2331
3
TIME, min
Figure 1. Polymer yield v time of Dolvmerization in 100 cm (full symbols) and 150cm3 (open symbols) reactors for: a - 5 wt% CrjCab-0-Sil; b, b' - 5 and 0.5 wt%, resp., Cr/Davison; c - 5 wt% Cr/PQ.
d
Figure 2. SEM micrographof PE/cataiyst (5wt% Cr/Davlson) taken after 75 mln of polymerization showing effects of the support fracturing. Magnification 200x.
Figure 3. TEM micrograph of 5 wt% Cr/Davison catalyst before (left) and after 75 min of polymerization (right), showing decrease in sizes of silica gel globules due to fragmentation. Magnification 110,000~.
2332 surface of PQ are probably negligible to those exposed on internal pore area. However, the Cab-O-Sil support, even though non-porous, has particles much smaller and of sufficient external area to create an active catalyst. The 5% Cr/Davison silica catalyst particles became covered with PE, in the form of cauliflower-like structures growing during step 3 (see SEM micrograph, fi ure 2). The fracturing and resulting fragmentation of the particles is clearly s own at the higher magnification in the TEM photo raph in figure 3. The particle fragments are encapsulated within the cauliflower4 e structures which also tend pol mer chain interlocking at their surfaces. Ethylene transport t o t e active sites within the polymer mass is clearly not limited. and TEM showed little indication of fracturing in the Cab-O-Sil support [a]. On curve a in figure 1, steps 2 and 3 are nearly absent, the slight curvature resulting from separation of agglomerated very small particles. Curve a initially showed the highest catalytic activity of the three silica-supported catalysts. At 170 min (not shown), curves a and b cross and curve b shows continuing acceleration of the polymerization rate. The extensive fragmentation of the Davison silica yielded a catalytic activity of 200 (g PE)/(g cat)(h)(atm C2H4), comparable to industrial rates [7].
fi
a
i
4. CONCLUSIONS
The rate of polymerization of ethylene on Cr/Silica is enhanced by reducing the size of the individual catalyst particles and, more importantly, by increasing porosity. The pore size distribution is critical; micropores become blocked and deactivated by the PE mass, whereas larger pores weaken the particle structure, facilitating fracturing and fragmentation. 5. REFERENCES
1. 2.
M.P. McDaniel, Adv. Catal., 33 (1985) 47 A. Follestsad, S. Helleborg and V. Almquist, Stud. Surf. Sci. Catal., 56
3.
!.Eoh%b,
4.
5.
6. 7. 8.
E.L. Weist, M.G. Chiovetta, R.L. Laurence and W.C. Conner, Can. J. Chem., 69 (1991 665 E.L. Waist, A.H. All, B. . Naik and W.C. Conner, Macromol., 22 (1989) 3244 A. Munoz-Escalona, J.G. Hernandez and J.A. Gallardo, J. Appl. Polym. Sci., 29 (1984) 1187 H.L. Hsieh, Catal. Rev. Sci. Eng., 26 1984) 631 R. Merryfield, M.P. McDaniel and Par s,J. Catal., 77 (1982) 348 J.A. Szymura, P.A. Zielinski and I.G. Dalla Lana, Electron Microscopic Characterization of Cr/Silica System Active in Ethylene Polymerization, Fall Meeting of MRS, Dec. 2-6, 1991, Boston, Massachusetts.
d
b
6. ACKNOWLEDGMENTS
Financial support was provided by the Natural Sciences and Engineering Research Council of Canada and by NOVACOR Chemicals.
Guczi, L. ef al. (Editors), New Fronriers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary @ 1993 Elsevier Science Publishers B.V. All rights reserved
PARTIAL HYDROGENATION OF ALKYNES AND DIENES ON HIGHLY SELECTIVE Fe-Cu/SiOz CATALYSTS
Y. Nitta, Y. Hiramatsy Y. Okamoto and T. Imanaka Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
Abstract A Fe-Cu/SiO, c a t a l y s t , p r e p a r e d by c o p r e c i p i t a t i o n of i r o n and c o p p e r s u l f a t e s o v e r u l t r a f i n e s i l i c a g e l w i t h a n e x c e s s amount o f sodium o r potassium c a r b o n a t e a t a low t e m p e r a t u r e , was found t o b e h i g h l y s e l e c t i v e i n t h e p a r t i a l h y d r o g e n a t i o n of v a r i o u s a l k y n e s and c o n j u g a t e d d i e n e s . The u s e o f p o t a s s i u m c a r b o n a t e a s t h e p r e c i p i t a n t r e s u l t e d i n a catalyst w i t h an improved s e l e c t i v i t y i n a t a i l - e n d h y d r o g e n a t i o n of p h e n y l a c e t y l e n e , p r o b a b l y because of a n e l e c t r o n i c e f f e c t of t h e K i o n s r e m a i n i n g on t h e catalyst surface. INTRODUCTION I r o n c a t a l y s t s a r e known t o have h i g h s e l e c t i v i t y , b u t e x t r e m e l y low a c t i v i t y , f o r p a r t i a l h y d r o g e n a t i o n of a l k y n e s t o a l k e n e s . Development of i r o n c a t a l y s t s w i t h h i g h a c t i v i t y f o r a l k y n e h y d r o g e n a t i o n is r e q u i r e d f o r industrial applications. R e c e n t l y w e have r e p o r t e d t h a t , i n t h e p a r t i a l h y d r o g e n a t i o n of p h e n y l a c e t y l e n e (PA) t o s t y r e n e (ST), t h e a c t i v i t y and s e l e c t i v i t y o f F e / S i O z c a t a l y s t a r e e x t r a o r d i n a r i l y i m p r o v e d by The e f f e c t s o f t h e s i m u l t a n e o u s a d d i t i o n o f c o p p e r and s u l f a t e i o n s [ l ] . p r e p a r a t i o n v a r i a b l e s , e s p e c i a l l y t h e p r e c i p i t a t i o n t e m p e r a t u r e , and t h e doping of c o p p e r and s u l f a t e i o n s h a v e been s t u d i e d by u s i n g XPS, TPR, XRD, a n d TGA [ 2 ] . I t was f o u n d t h a t t h e h i g h a c t i v i t y a n d s e l e c t i v i t y a r e a t t r i b u t a b l e t o t h e i n c r e a s e d d i s p e r s i o n o f F e metal, I n dehydrogenation of e t h y l b e n z e n e t o s t y r e n e , p o t a s s i u m d o p i n g o f i r o n c a t a l y s t s are known t o i n c r e a s e a c t i v i t y and s e l e c t i v i t y [ 3 , 4 1 . It i s i n t e r e s t i n g t o know i f t h e potassium-promoted i r o n c a t a l y s t s are a l s o e f f e c t i v e i n t h e p a r t i a l h y d r o g e n a t i o n o f a l k y n e s and o t h e r r e l a t e d r e a c t i o n s . I n t h i s work, we r e p o r t t h a t t h e u s e of p o t a s s i u m c a r b o n a t e i n s t e a d o f sodium c a r b o n a t e as t h e p r e c i p i t a n t much improves t h e s e l e c t i v i t y of t h e r e s u l t i n g c a t a l y s t i n a tail-end h y d r o g e n a t i o n of p h e n y l a c e t y l e n e . P a r t i a l h y d r o g e n a t i o n s of d i f f e r e n t a l k y n e s and d i e n e s were a l s o examined on these c a t a l y s t s . EXPERIMENTAL
Catalyst p r e c u r s o r s were p r e p a r e d by a p r e c i p i t a t i o n method as f o l l o w s : a s o l u t i o n of a n e x c e s s amount of sodium or p o t a s s i u m c a r b o n a t e (1.5-1.7 m o l a r e q u i v a l e n t ) i n 25 m l o f d i s t i l l e d water was a d d e d a t 2 O o C t o a n a q u e o u s s u s p e n s i o n (150 ml) c o n t a i n i n g 2 g o f non-porous u l t r a f i n e s i l i c a
2334 g e l (Nippon A e r o s i l , A-300, 320 m'g-' ), i r o n ( I I 1 ) s u l f a t e , and c o p p e r ( I 1 ) s u l f a t e ( t o t a l amount o f metal = 36 m m o l , Fe:Cu = 7 : 3 i n atomic r a t i o ) i n 1-2 min under v i g o r o u s s t i r r i n g . The p r e c i p i t a t e was a g e d f o r 15 min a t 75-80°C and f i l t e r e d , f o l l o w e d by w a s h i n g t h r e e times w i t h h o t water and d r y i n g a t 110 " C f o r 20 h. One gram o f a d r i e d p r e c u r s o r was r e d u c e d by h e a t i n g i n a stream o f hydrogen t o 500 " C and h o l d i n g a t t h i s t e m p e r a t u r e f o r 1 h. The h y d r o g e n a t i o n o f a l k y n e s o r d i e n e s (0.6 m l ) w a s c a r r i e d o u t u s u a l l y i n e t h a n o l ( 2 7 m l ) a t 60 " C u n d e r 1.0 MPa o f hydrogen by u s i n g a g l a s s a u t o c l a v e e q u i p o e d w i t h a m a g n e t i c s t i r r i n g system. RESULTS AND DISCUSSION A s shown i n T a b l e 1, t h e u s e o f p o t a s s i u m c a r b o n a t e i n s t e a d of sodium c a r b o n a t e as t h e p r e c i p i t a n t r e s u l t e d i n a c a t a l y s t w i t h h i g h e r s e l e c t i v i t y t o ST i n t h e h y d r o g e n a t i o n of PA b u t l o w e r a c t i v i t y f o r b o t h PA and ST h y d r o g e n a t i o n s . The s e l e c t i v i t y was improved e s p e c i a l l y a t t h e end o f t h e Therefore, a tail-end hydrogenation r e a c t i o n : 99.6% a t 99.9% c o n v e r s i o n . w i t h a m i x t u r e of PA and ST (5:95) i n n-propanol was examined on t h e s e two c a t a l y s t s a t 60 "C under a n a t m o s p h e r i c p r e s s u r e o f h y d r o g e n . A s shown i n F i g u r e 1, catalyst-B e x h i b i t e d h i g h e r s e l e c t i v i t y (99.6%) t h a n catalyst-A (98.7%) a t 100%c o n v e r s i o n o f PA a l t h o u g h t h e a c t i v i t y o f catalyst-B was a l i t t l e lower t h a n t h a t of c a t a l y s t - A . TGA and XRD measurements showed t h a t b o t h c a t a l y s t s are similar t o e a c h o t h e r i n t h e r e d u c t i o n b e h a v i o u r o f t h e p r e c u r s o r s and i n t h e d i s p e r s i o n o f Fe metal i n t h e r e d u c e d c a t a l y s t s . T h e r e f o r e , t h e h i g h e r s e l e c t i v i t y o f c a t a l y s t - B may b e a t t r i b u t e d t o t h e e l e c t r o n i c e f f e c t o f p o t a s s i u m i o n s r e m a i n i n g on t h e c a t a l y s t s u r f a c e (ca. 0.2% of F e ) r a t h e r t h a n t h e d i f f e r e n c e i n c a t a l y s t morphology. T h e r e l a t i v e a d s o r p t i o n s t r e n g t h o f a l k y n e a n d a l k e n e o n t h e s e two c a t a l y s t s a r e a s s e s s e d f r o m i n d i v i d u a l a n d c o m p e t i t i v e r e a c t i o n r a t e s o f PA and ST. Assuming t h a t t h e h y d r o g e n a t i o n r a t e of r e a c t a n t PA ( o r ST) is p r o p o r t i o n a l t o t h e amount o f a d s o r b e d PA ( o r ST) and t h a t t h e a d s o r p t i o n of PA (or ST) f o l l o w s t h e Langmuir i s o t h e r m , t h e f o l l o w i n g e q u a t i o n i s o b t a i n e d when t h e r e a c t i o n r a t e i s i n d e p e n d e n t of t h e c o n c e n t r a t i o n o f PA ( o r ST) [ 5 , 61 as was c o n f i r m e d i n s e p a r a t e e x p e r i m e n t s .
K(r) = K(PA)/K(ST) = [A(PA)/A(ST)]/[r(PA)/r(ST)] Table 1 H y d r o g e n a t i o n o f p h e n y l a c e t y l e n e and s t y r e n e on Fe-Cu/SiOn p r e p a r e d w i t h Na,C03 (A) a n d K,CO, (B) r o a / m o l . rnin-lg-'
Catalyst A
B
a b c d
Precipitant
S,,,/Zb r(PA)
r(ST)
Na,C03
0.36
K2CO3
0.23
1.o 0.82
99.4 99.8
r(PA)
catalysts
r(ST)
A(PA)' A(ST)
K(r)d
0.36 0.28
2 . 7 ~ 1 0 ' ~7.6~1023 5.0~10 1.8~10
I n i t i a l h y d r o g e n a t i o n rates of p h e n y l a c e t y l e n e (PA) and s t y r e n e (ST). S t y r e n e s e l e c t i v i t y a t 50% c o n v e r s i o n o f p h e n y l a c e t y l e n e . C o n v e r s i o n r a t i o of PA t o ST i n c o m p e t i t i v e h y d r o g e n a t i o n . Relative adsorption c o e f f i c i e n t of phenylacetylene t o styrene.
2335 5
a)
3 -
- 100 \
2 -
-+%
A
u
.rl
>
98.7%
1-
.d
c,
u
aJ rl
b)
4
L
i
r
LA
L 1
3 -
F i g u r e 1. T a i l - e n d - h y d r o g e n a t i o n of p h e n y l a c e t y l e n e o n Fe-Cu/SiOn c a t a l y s t s p r e p a r e d by u s i n g a ) NazC03 a n d b ) K2C03 a s t h e p r e c i p i t a n t . Ethylbenzene, .:Selectivity to styrene. O : P h e n y l a c e t y l e n e , A:S t y r e n e , 0: w h e r e K(r) is t h e a d s o r p t i o n c o e f f i c i e n t o f PA r e l a t i v e t o t h a t o f ST, r(PA)/r(ST) t h e r a t i o o f t h e i n d i v i d u a l r e a c t i o n r a t e s , a n d A(PA)/A(ST) t h e r a t i o o f t h e c o n v e r s i o n o f PA t o t h a t of ST i n a n i n i t i a l s t a g e of t h e c o m p e t i t i v e r e a c t i o n . T h e v a l u e o f K(r) i s u s e d a s a m e a s u r e o f t h e r e l a t i v e a d s o r p t i o n s t r e n g t h of PA t o ST. A s l i s t e d i n T a b l e 1, t h e K(r) v a l u e s f o r b o t h c a t a l y s t s a r e much l a r g e r t h a n u n i t y , m e a n i n g t h a t t h e M o r e o v e r , K(r) f o r a d s o r p t i o n o f PA is much s t r o n g e r t h a n t h a t of ST. c a t a l y s t - B is l a r g e r t h a n t h a t f o r catalyst-A. This indicates that the r e l a t i v e a d s o r p t i o n s t r e n g t h of PA t o ST is l a r g e r o n c a t a l y s t - B t h a n on catalyst-A. I n o t h e r w o r d s , t h e i n t e r m e d i a t e p r o d u c t ST on c a t a l y s t - B is more e a s i l y r e p l a c e d by t h e r e a c t a n t m o l e c u l e PA b e f o r e b e i n g h y d r o g e n a t e d t o e t h y l b e n z e n e , which w i l l l e a d t o t h e i m p r o v e d s e l e c t i v i t y of c a t a l y s t - B .
P o t a s s i u m i o n s r e m a i n i n g on t h e s u r f a c e of c a t a l y s t - B seem t o i n c r e a s e e l e c t r o n i c a l l y t h e a d s o r p t i o n s t r e n g t h o f a l k y n e s rathe'r t h a n a l k e n e s . The s e l e c t i v i t i e s i n p a r t i a l h y d r o g e n a t i o n o f d i f f e r e n t a l k y n e s and d i e n e s were examined on t h e s e two c a t a l y s t s and found t o b e e x t r e m e l y h i g h f o r b o t h c a t a l y s t s , a l t h o u g h t h e a c t i v i t i e s o f c a t a l y s t - B were a l i t t l e l o w e r t h a n those o f catalyst-A. T a b l e 2 lists t h e a c t i v i t i e s and I t is n o t e w o r t h y t h a t s e l e c t i v i t i e s a t v a r i o u s c o n v e r s i o n s on catalyst-A. 1 , 3 - c y c l o o c t a d i e n e i s e a s i l y h y d r o g e n a t e d t o c y c l o o c t e n e w i t h 100% s e l e c t i v i t y , w h e r e a s t h e catalyst is almost i n a c t i v e f o r t h e h y d r o g e n a t i o n of 1 , 5 - c y c l o o c t a d i e n e . I n t h i s case, c y c l o o c t e n e i s p r o b a b l y h y d r o g e n a t e d from 1 , 3 - c y c l o o c t a d i e n e a f t e r t h e i s o m e r i z a t i o n r e a c t i o n of 1,5cyclooctadiene. T h u s , t h e Fe-Cu/SiOz c a t a l y s t i s h i g h l y u s e f u l n o t o n l y f o r p a r t i a l hydrogenation of alkynes i n t h e presence of a l k e n e s b u t a l s o f o r s e l e c t i v e h y d r o g e n a t i o n of c o n j u g a t e d d i e n e s t o m o n o e n e s i n t h e presence of nonconjugated d i e n e s . Table 2 P a r t i a l h y d r o g e n a t i o n o f v a r i o u s a l k y n e s and d i e n e s on Fe-Cu/SiOzcatalyst-A S e l e c t i v i t y b /mol% roa/mmol- min-lg-'
Reactant
s2 0
s50
99.4
99.4 99.7 100 100 100
SlOO
~
Phenylacetylene 1-Phenyl-1-propyne 1-Heptyne 2-Butyne-1,4-diol 1,3-Cyclooctadiene 1,5-Cyclooctadiene -~
a b c
3 . 6 x lo-' 2.9 x lo-' 1.8 x lo-'
4.8 2.2 x 7.4
lo-"
99.2 100 100 100 2 100( 3)'
-
9 9 . 4 ( 90)' 99.7(92) 2100 (100) 1100 (100) 2100 (100)
-
~~
~
I n i t i a l hydrogenation rate. S e l e c t i v i t i e s o f monoene a t c o n v e r s i o n s o f 2 0 % ( S 2 0 ) , 50%(Sso ) , a n d a r o u n d ) lOO%(S Numbers i n p a r e n t h e s e s a r e e x a c t c o n v e r s i o n s of t h e r e a c t a n t .
.
REFERENCES
1 Y . N i t t a , S. M a t s u g i a n d T. Imanaka, Catal. L e t t . , 5 (1990) 6 7 . 2 Y . N i t t a , Y. H i r a m a t s u , Y. Okamoto a n d T. Imanaka, S t u d . S u r f . S c i . C a t a l . , 63 ( 1 9 9 1 ) 103. 3 W. D. Mross, Catal. Rev.-Sci. Eng., 2 5 ( 1 9 8 3 ) 591. 4 T. H i r a n o , Appl. Catal., 26 ( 1 9 8 6 ) 65 and 81. 5 T. C h i h a r a and K . T a n a k a , Chem. L e t t . , (1977) 843. 6 M. K a j i t a n i , N . S u z u k i , T. Abe, Y. Kaneko, K. Kasuya, K. T a k a h a s h i and A. S u g i m o r i , B u l l . Chem. SOC. J p n . , 5 2 ( 1 9 7 9 ) 2343.
Guczi, L el al. (Editors), New Frontiers in CaialysQ
Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992, Budapest, Hungary 6 1993Elsevier Science Publishers B.V. All rights reserved
SUPPORTED DEHYDROGENATION CATALYSTS BASED ON IRON OXIDE D. E. StobM, F. R. van Burena, A. J. van Dillenb and J. W.Geussb aHOC R & D, Dow Benelux N.V.,P.O.Box 48,4530 AA Terneuzen, The Netherlands bDepartmentof Inorganic Chemistry, State University of Utrecht, P.O.Box 80083,3508 TB Utrecht, The Netherlands
Abstract A magnesia-supported iron oxide catalyst promoted with potassium was prepared for the dehydrogenation of hydrocarbons, such as, e.g. ethylbenzene or l-butene. With high-field magnetic measurements it was established that potassium ferrite, KFe02,is the active phase of the catalyst under dehydrogenation conditions. 1. INTRODUCTION
Generally the catalytic dehydrogenation of hydrocarbons, such as, e.g. ethylbenzene or l-butene, can be carried out in the presence of steam usingunsupported iron oxide catalysts promoted with potassium [11. An undesirable phenomenon with these unsupported catalysts is the degradation of mechanical strength due to the reduction of the iron oxide phase and migration of the potassium promotor under process conditions [2]. This degradation of the mechanical strength is expected to be obviated by the use of a preshaped support material, onto which iron oxide is applied homogeneously and in finely divided form [3]. The paper deals with the development of a magnesia-supported iron oxide catalyst. Magnesium oxide has been chosen as a support material because of its basic properties and the fact that magnesium oxide does not react with the potassium promotor. The activity of this supported catalyst in the dehydrogenation of 1-butene and the role of the potassium promotor have been determined. The phase composition of the catalyst under dehydrogenation conditions has been studied using high-field magnetic measurements. 2. EXPERIMENTAL
Iron and potassium were applied homogeneously onto a preshaped magnesium oxide support (Engelhard Mg-0601, tablets 1/8") by incipient wetness impregnation [4,5]. As iron precursors ammonium iron(II1) citrate or ammonium iron(II1) EDTA were used. Potassium was applied as potassium carbonate. After each impregnafion the catalyst was dried, first at room temperature followed by drying at 393 and calcination at 973 K. The activity and selectivity of the catalyst for the dehydrogenation of l-butene to 1,3-
2338 butadiene were studied using an automated flow apparatus, which has been described in detail elsewhere [6]. The reactor feed consisted of 5 vol.% l-butene and 30 vol.% water in nitrogen (molar steam/hydrocarbon ratio of 6). The catalysts were tested in the temperature range between 725 and 900 K at a F/W of 50 ml m i d g-'. The nature of the iron oxide compounds in the catalysts, both in the fresh state and under dehydrogenation conditions, has been studied with high-field magnetic measurements using the Weiss-extraction method [7,8]. A special reactor has been constructed which allowed for the treatment of a catalyst under 1-butene dehydrogenation conditions prior to the magnetization experiments without intermediate exposure to air. 3. RESULTS
In the fresh catalyst without potassium the iron oxide is present as magnesium ferrite, as observed both with thermomagnetic analysis (TMA) and X-ray diffraction. The TMA diagram of the fresh catalyst is shown in figure lA.A magnetic compound is observed with a Curie temperature (Tc) of 595 K, as indicated in the figure by the dashed line. This temperature is characteristicof femmagnetic MgFe,O,. With selective oxygen chemisorption and X-ray line broadening it has been determined that the average size of these MgFe,O, particles is 20 nm [9]. As shown in figure 2, this unpromoted catalyst shows a 1-butene conversion of 29% at 873 K. After an initial increase, the selectivity to 1.3-butadiene is ca. 62%. The catalyst is rapidly deactivated due to the deposition of carbon. The activation energy for this unpromoted catalyst is 47 kcal mol-'. After dehydrogenation, the TMA figure 1' shows a T, of 790 K. This points to the presence of Fe,O,. However, on the basis of the relative magnetization at room temperature it is concluded that only 17 % of the iron oxide is present as Fe,O,. The remaining 83 wt.% of the iron oxide i s present as anti-ferromagnetic FeO, which must be stabilized by the formation of a solid solution with MgO [8].Hence, it can be concluded that under dehydrogenation conditions the MgFe,O, phase in the unpromoted catalyst is reduced mainly to FeO stabilized in the magnesium oxide support. However, at the surface the stabilization of FeO is less strong and consequently the MgFe,O, is only reduced to Fe,O,. Evidently, the Fe,O, provides the dehydrogenation activity of the unpromoted catalyst. The reduction of the catalyst can also account for the initial increase in selectivity. Promotion with potassium results in the almost complete disappearance of the ferrimagnetic MgFe,O, phase, as can be seen in figure 1'. With X-ray diffraction and thermogravimetical analysis it has been established that the MgFe,O, reacts with K2C03 to form KFeO, and CO,above ca. 923 K [ 101. KFeO, evidently is antiferromagnetic. In figure 2 the conversion and selectivity of the promoted catalyst are shown also. The potassium promoted catalyst shows both a higher conversion and a higher selectivity when compared with the unpromoted catalyst. The activation energy found for the potassium promoted catalyst is 37 kcal mol-'. In contrast to what was observed for the unpromoted catalyst, TMA diagram of the promoted catalysts shown in figure lDhas not changed after dehydrogenation. Evidently, under dehydrogenation conditions no reduction of the potassium ferrite phase occurs. This leads to the conclusion that KFeO, must be the active phase, which is supported by the different activation energy of 37 kcal mol-'.
2339
200
400 600 Temperature (K)
800
-+
Figure 1. TMA in helium of magnesia-supported iron oxide catalysts before and after 4 hours in 1-butene dehydrogenation at 873 K. The solid curve in the figures indicates the signal of the empty reactor. The vertical dashed line indicates the Curie temperature. A) 5.6 wt.% Fe/MgO catalyst, fresh; B) 5.6 wt.% Fe/MgO catalyst, after dehydrogenation C) 3.1 wt.% Fe, 3.0 wt.% K/MgO catalyst, fresh; D) 3.1 wt.% Fe, 3.0wt.% K/MgO catalyst, after dehydrogenation Although KFeO, appears active and selective in the dehydrogenation the catalyst is still deactivated by coking. Evidently, KFeO, is not sufficiently active in catalyzing carbon gasification to entirely suppress coking. As is described elsewhere [lo], suppression of coking requires the additional presence of highly dispersed K2C0,. 4. CONCLUSIONS
A magnesia-supported iron oxide catalyst promoted with potassium has been prepared for the dehydrogenation of hydrocarbons, such as, e.g. ethylbenzene and 1-butene. In the
2340
O ! 0
1
I
I
5
10
15
Time (hours)
20
+
Figure 2. The 1-butenedehydrogenation activity of a 3.1 wt.% Fe/MgO catalyst measured isothermally at 873 K, before (A) and after (B) promotion with 6 wt.% K. I) 1-butene conversion ; 11) 1,3-butadiene selectivity unpromoted catalyst the magnesiumferrite phase reduced to Fe,O., under dehydrogenation conditions. On the other hand, in the potassium promoted catalyst no reduction occurred. This means that KFeO, is the active phase in the potassium promoted catalyst. 5. REFERENCES
E.H.Lee, CataLRev. 8(2) (1973) 285 B.D.Herzog and H.F.Rase, 1nd.Eng.Chem.Prod.Res.Dev. 23 (1984) 187 D.E.Stobbe, Ph.D. thesis, Utrecht 1990 D.E.Stobbe, F.R. van Buren, P.E.Groenendijk, A.J. van Dillen and J.W.Geus, J.Mater.Chem. l(4) (1991) 539 5 D.E.Stobbe, F.R. van Buren, A.W.Stobbe-Kreemers,J.J.Schokker, A.J. van Dillen and J.W.Geus, J.Chem.Soc., Faraday Trans. 87(10) (1991) 1623 6 D.E.Stobbe, F.R. van Buren, M.S.Hoogenraad, A.J. van Dillen and J.W.Geus, J.Chem.Soc., Faraday Trans. 87(10) (1991) 1639 7 P.W.Selwood, 'Chemisorption and magnetization', Acad-Press,New York 1975 8 D.E.Stobbe, F.R. van Buren, A.W.Stobbe-Kreemers, A.J. van Dillen and J.W.Geus, J.Chem.Soc., Faraday Trans. 87(10) (1991) 1631 9 D.E.Stobbe, F.R. van Buren, A.J.Orbons, A.J. van Dillen and J.W.Geus, J.Mater.Sci., in press 10 D.E.Stobbe, F.R. van Buren, A.J. van Dillen and J.W.Geus, J.Catal. in press
1 2 3 4
Guczi, L.et al. (Editors),New Frontiers in Catalysb Pmceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
SULFUR RESISTANCE OF NICKEL CATALYSTS SUPPORTED ON K-CLINOPTILOLITECONTAINING IRON IN ETHYLBENZENE HYDROGENATION A. Arcoya, X L. Seoane and J. Soria Instituto de Catalisis (CSIC), Universita Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spain
Abstract The d e a c t i v a t i o n o f N i c a t a l y s t s supported on a K - C l i n o p t i l o l i t e c o n t a i n i n g i r o n species was studied i n t h e hydrogenation o f e\hylbencene i n presence o f thiophene. The c o o r d i n a t i o n and l o c a t i o n o f t h e Fe + species was m o d i f i e d by thermal treatments. A c o r r e l a t i o n between t h e n i c k e l - i r o n i n t e r a c t i o n , evidenced by ESR, and t h e s u l f u r - r e s i s t a n c e was found. So, t h e sample c a l c i n e d before t h e N i impregnation has shown a stronger i n t e r a c t i o n and h i g h e r s u l f u r r e s i s t a n c e . The c o n t r a r y was observed f o r a sample from which i r o n had been p r e v i o u s l y extracted. An i n t e r p r e t a t i o n of t h e c a t a l y s t r e s i s t a n c e towards thiophene poisoning based on e i t h e r t h e formation o f sugface e l e c t r o n d e f i c i e n t N i species o r t h e decoration o f N i p a r t i c l e s by Fe segregated by c a l c i n a t i o n i s discussed. 1. INTRODUCTION
I t has been reported i n t h e l i t e r a t u r e t h a t c a t a l y s t s c o n t a i n i n g e l e c t r o n d e f i c i e n t metal atoms (Me”) on t h e surface are more r e s i s t a n t t o s u l f u r poisoning than those w i t h metal atoms w i t h a h i g h e r e l e c t r o n d e n s i t y [1,2]. E l e c t r o n - d e f i c i e n t species can be formed by i n t e r a c t i o n o f t h e metal w i t h electron-acceptor o r a c i d s i t e s present on t h e support [3]. I n t h e present communication we r e p o r t t h e r e s u l t s concerning t h e e f f e c t o f i r o n i o n s on t h e r e s i s t a n c e o f n i c k e l c a t a l y s t s t o thiophene poisoning i n t h e t i t l e reactioy; As support we have used a modified n a t u r a l c l i n o p t i l o l i t e c o n t a i n i n g Fe species, which chemical nature, coordination, and l o c a t i o n , and t h e r e f o r e t h e i n t e r a c t i o n w i t h N i , can be changed by d i f f e r e n t thermal treatments. C a t a l y s t s were c h a r a c t e r i z e d by gas chemisorption and i n f o r m a t i o n on t h e n a t u r e o f t h e n i c k e l - i r o n i n t e r a c t i o n s obtained by E l e c t r o n Spin Resonance (ESR). 2. EXPERIMENTAL
A K - C l i n o p t i l o l i t e (K-CL) was obtained from a Cuban n a t u r a l c l i n o p t i l o l i t e (wt% composition: Si02=77.5; Al2O3=12.4; Fe O,=Z.O; Na,O=2.9; K C l . The f i n a l K20=1.3; Mg0=1.3; Ca012.6) by extensive i o n exchange exchange was about 95%, w h i l e o n l y 1.6% o f t h e i n i t i a l Fe was removed. Four c a t a l y s t s w i t h 8 w t % N i were prepared by soaking t h e support w i t h Ni(N0,) 6HZ0 s o l u t i o n s . C a t a l y s t s B and C were c a l c i n e d a t 773 K before and a f t e r t i e impregnation, r e s p e c t i v e l y . Sample A was n o t c a l c i n e d and sample
with
2342 D was prepared from K-CL o f which Fe3* was p r e v i o u s l y e l i m i n a t e d by c a l c i n a t i o n a t 773 K followed by e x t r a c t i o n w i t h o x a l i c acid. The f o u r c a t a l y s t s were reduced a t 623 K and 3 MPa i n H stream d u r i n g 16 and t h e t i t r a t i o n . The Fe c a t i o n s number o f accesible N i atoms determined by O,-l!, and n i c k e l p a r t i c l e s i n t h e c a t a l y s t s were s t u d i e d by ESR. Poisoning experiments were conduced i n a fixed-bed t u b u l a r r e a c t o r c o n t a i n i n g 10 g o f 10/1 c a t a l y s t (1.41-2.0 mm p a r t i c l e s i z e ) , - t t 3 MPa, 523 K , H /Ethylbenzene mole/mole, l i q u i d space v e l o c i t y 3 h and 100 ppm o f t i i o p k e n e (Th) i n t h e feed. I n these conditions, we have v e r i f i e d t h a t e x t e r n a l and i n t e r n a l d i f f u s i o n a l l i m i t a t i o n s were absent and t h a t an uniform d i s t r i b u t i o n o f t h e l i q u i d i n t h e r e a c t o r was a t t a i n e d . Product samples were analyzed by GLC. S e l e c t i v i t y t o ethylciclohexane was always 100 X , p r a c t i c a l l y .
9,
-
3. RESULTS I n t h e absence o f poison, we have v e r i f i e d experimentally t h a t a l l t h e c a t a l y s t s have a s i m i l a r and very s t a b l e a c t i v i t y . I n t h e presence o f 100 ppm o f thiophene, however, t h e conversion decays as a f u n t i o n o f t h e t i m e on stream by t h e t o x i c e f f e c t o f s u l f u r , b u t w i t h s i g n i f i c a n t d i f f e r e n c e s . Thus, w h i l e B and A samples e x h i b i t a r e l a t i v e l y h i g h i n i t i a l r e s i s t a n c e , f o r C and D samples t h e a c t i v i t y f a l l s a b r u p t l y r i g h t from t h e begining. I n Figure 1 we have represented t h e normalized a c t i v i t y versus t h e number o f molecules o f thiophene f e d per i n i t i a l exposed n i c k e l atom (e), which i s a parameter p r o p o r t i o n a l t o time on stream.
0
0.2 0.4 0.6 0.8 1 0 ( l h moloc /Ni atom1I
Figure 1. D e a c t i v a t i o n curves o f the catalysts.
Figure 2 . FMR spectra o f t h e c a t a l y s t s recorded a t 77 K .
Since s u l f u r - r e s i s t a n c e o f metal c a t a l y s t s i s a p r o p e r t y t h a t depends on t h e physicochemical c h a r a c t e r i s t i c s o f t h e metal atoms i n t h e c a t a l y s t but a l s o on t h e number o f exposed metal atoms, t h e q u a n t i t a t i v e comparison of t h e t h i o r e s i s t a n c e was done on t h e basis o f both i n t e n s i v e and extensive d e a c t i v a t i o n parameters. Thus, from t h e curves, t h e poison i n i t i a l t o x i c i t y (To number o f n i c k e l atoms d e a c t i v a t e d per molecule o f thiophene f e d a t
-
2343 t - - > O , c a l c u l a t e d from t h e slope o f t h e curve a t t h e origen) and t h e semid e a c t i v a t i o n time (tl ) were obtained (Table 1). T i s a very v a l u a b l e parameter because i t a%ows t o evaluate t h e e f f e c t on !he n i c k e l atom o f t h e f i r s t molecule o f poison t h a t a r r i v e s t o t h e c a t a l y s t , f r e e o f t h e p o s s i b l e i n f l u e n c e o f o t h e r neighboring poisoned metal atoms. From these values, t h e f o l l o w i n g o r d e r o f s u l f u r - r e s i s t a n c e r e s u l t s : C ( D (< A ( B.
Table 1. Characterization Catal v s t lO-I9x N (Ni at/g c a t ) To ( N i at/Th molec) t9112 ( f o r a/a, = 0.5) t1,2 (h)
and
deactivation
parameters
of
A
B
c
D
4.4 0.56 0.32 7.5
7.5 0.23 0.45 18.0
3.4 8.26 0.055 1.1
4.5 5.68 0.083 2.0
catalysts
Furthermore, data o f Table 1 show t h a t , i n t h i s case, t h e r e s i s t a n c e t o thiophene i s an i n t r i n s i c p r o p e r t y o f t h e n i c k e l atom r a t h e r than an extensive p r o p e r t y o f t h e c a t a l y s t , thus i n d i c a t i n g t h a t an e l e c t r o n i c e f f e c t i s operating. For example, i n s p i t e o f having both c a t a l y s t s A and D p r a c t i c a l l y the same number o f exposed N i atoms, those o f sample A are i n i t i a l l y about t e n f o l d more r e s i s t a n t than those o f sample D. S i m i l a r conclusions can be drawn i f one compares t h e o t h e r c a t a l y s t s . It i s i n t e r e s t i n g t o p o i n t o u t t h a t c a l c i n a t i o n o f c l i n o p t i l o l i t e p r e v i o u s l y t o i t s impregnation provides a c a t a l y s t (sample B ) t h a t have, f o r example, 1.7 times more surface N i atoms than iampie A: I t s s u l f u r - r e s i s t a n c e (To'or-tl12) i s , however, more than t w i c e t h a t o f A. 4. DISCUSSION
On the b a s i s o f t h e accepted mechanism f o r metal poisoning by thiophene a t about T > 423 K, t h a t i n v o l v e s t h e s t r o n g chemisorption o f t h e poison molecule on t h e metal, e v e n t u a l l y followed by hydrogenolysis, l e a d i n g t o an i n a c t i v e m e t a l - s u l f u r species [ 4 ] , an i n t e r p r e t a t i o n o f our r e s u l t s can be suggested. The lower t h e e l e c t r o n d e n s i t y o f t h e n i c k e l atoms t h e weaker t h e i r i n t e r a c t i o n w i t h thiophene and t h e r e f o r e t h e h i g h e r t h e t h i o r e s i s t a n c e . E l e c t r o n - d e f i c i e n t N i atoms can be formed indeed through t h e i n t e r a c t i o n o f N i w i t h t h e electron-acceptor Fe3+/Fe2' species present i n t h e c l i n o p t i l o l i t e , since t h e Lewis a c i d character o f t h e i r o n c a t i o n s c o o r d i n a t i v e l y unsaturated present on t h e supports surface has been proved [ 5 ] . Such an i n t e r a c t i o n was i n d i c a t e d by the ESR r e s u l t s . ESR spectra and Mossbauer spectroscopy r e s u l t s [6] i n d i c a t e t h a t t h e i r o n i n t h e s t a r t i n g c l i n o p t i l o l i t e i s e s s e n t i a l l y present as two types o f octahedral Fe3+ cations, probably occuping two d i f f e r e n t s i t e s i n t h e channels o f t h e z e o l i t e . These s i t e s have two o f t h e i r s i x l i g a n d s d i f f e r e n t i n n a t u r e an$+ consequently t h e i r p r o p e r t i e s are d i f f e r e n t . Thus, one o f these types o f Fe c a t i o n s i s more e a s i l y removed t o t h e o u t e r surface o f t h e z e o l i t e framework by thermal treatments [ 7 ] . The surface i r o n can then be e x t r a c t e d by an o x a l i c a c i d treatment. Sample D d i d n o t show Fe3* s i g n a l s . The ESR spectra o f t h e reduced samples showed a t 77 K t h e FMR s i g n a l s o f
2344 N i p a r t i c l e s , depicted i n Figure 2. The spectra o f samples A and B are mainly formed by a very asymmetric signal w i t h a maximum a t very low f i e l d . A s i m i l a r signal has been observed on reduced N i / T i O samples and was assigned t o planar N i p a r t i c l e s w i t h shape ( o r m a g n e t o s t r i c t i o n ) induced magnetic anisotrapy, produced by a strong i n t e r a c t i o n w i t h t h e support [a]. This p a r t i c u l a r s i g n a l i s n o t observed f o r sample D, which d i f f e r s from sample B o n l y because i t was t r e a t e d w i t h o x a l i c a c i d t o e x t r a c t t h e segregated i r o n content. This r e s u l t i n d i c a t e s t h a t t h e i n t e r a c t i o n o f t h e n i c k e l p a r t i c l e s i s n o t t a k i n g place w i t h t h e c l l n o p t i l o l i t e b u t with t h e i r o n c a t i o n s . Such an i n t e r a c t i o n can arJ+Ses by a p a r t i a l charge t r a n s f e r o r e l e c t r o s t a t i c p o l a r i z a t i o n from N i O t o Fe /Fez+ Ions, o r by t h e d e c o r a t i o n o f t h e n i c k e l p a r t i c l e s w i t h a porous overlayer o f i r o n especies [9]. I n any case, t h e i n t e r a c t i o n Ni-thiophene becomes s t r o n g l y hindered by decreasing t h e electron-donor character o f N i and/or by d i m i n i s h i n g t h e s i z e o f t h e ensembles o f Nio r e q u i r e d f o r t h e hydrogenolysis o f thiophene. Consequently, t h e N i atoms are p r o t e c t e d against t h e poison a t t a c k but m a i n t a i n t h e i r hydrogenation a c t i v i t y . The s i g n a l o f t h e p l a n a r N i p a r t i c l e s was stronger f o r sample B than f o r sample A and was n o t observed f o r t h e o t h e r two samples, thus e x p l a i n i n g t h e order o f s u l f u r - r e s i s t a n c e observed. I n c a t a l y s t B, c a l c i n a t i o n o f K-CL segregates t h e i r o n t o t h e e x t e r n a l surface o f t h e c l i n o p t i l o l i t e i n form o f h i g h l y disperse aggregates, a l l o w i n g a stronger i n t e r a c t i o n with N i d u r i n g t h e r e d u c t i o n step than i n c a t a l y s t A. I n sample C, c a l c i n a t i o n a f t e r impregnation probably produces a segregation o f l a r g e r aggregates o f n i c k e l oxide t o t h e i r o n - f r e e p a r t o f t h e c l i n o p t i l o l i t e surface, i n such a way t h a t i n t e r a c t i o n a f t e r r e d u c t i o n does n o t occur. T h i s I n t e r a c t i o n i s n o t p o s s i b l e i n c a t a l y s t D. I n these two l a t t e r cases, t h e surface N i O atoms with a h i g h e r e l e c t r o n d e n s i t y have a lower t h i o r e s i stance. 5. ACKNOWLEDGMENTS F i n a n c i a l support from C I C Y T , Spain ( P r o j e t s MAT90-0808 and MAT88-0223) i s g r a t e f u l l y acknowledged. 6 . REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
M. Guenin, M. Breysse, R. Frety, K. T i f o u t i , P. Marecot and J. Barbier, J. Catal , 105 (1987) 144. A. Arcoya, X.L. Seoane, N.S. F i g o l i and P.C. L'Argentiere, Appl. C a t a l . 62 (1990) 35. G.N. Sauvion, 1. Akelay, M.F. Guilleux, J.F. Tempere and D. Delafosse, J. Chim. Phys. 80 (1983) 769. K. Koyama and Y. Takduchi, Z e i t . K r i s t a l l o g r . , Bd. 145 (1977) 216. G. Connell and J.A. Dumesic, J. Catal., 101 (1986) 103. Results t o be published. K. Sugiyama and Y. Takduchi, i n "New Developments i n Z e o l i t e s Science and Technology", Y. Murakami, A. I i j i m a and J.W. Ward (eds.), E l s e v i e r , Amsterdam, 1986, p. 449. J. Soria, M.T. Blasco and J.C. Conesa, Surf. Sci., 251/252 (1991) 1018. A.R. Gonzalez-Elipe, P. Malet, J.P. Espinos, A. Caballero and G. Munuera, i n " S t r u c t u r e and R e a c t i v i t y o f Surfaces", C. Morterra, A. Zecchina and G. Costa (eds.), E l s e v i e r , Amsterdam, 1989, p. 427.
.
Ouczi, L et al. (Editors),Nm,Frontiers in CatalysQ
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All dghts reserved
HETEROGENEOUS ETHYLENE WDROFORMYLATION CATALYZED BY OXIDE-SUPPORTED ~ h l ~ ( C O ) &ANION: INFLUENCE OF THE NATURE OF THE SUPPORT
C.Dossia,A. Fusia, L. Garlaschellia, R. Psarob and R. Ugoa BDipartirnento di Chirnica Inorganica e Metallorganica, Via Venezian, 21,20133 Milano, Italy bCentro CNR, Via Venezian, 21,20133 Milano, Italy
Introduation Heterogeneous olefin hydroformylation has widely been demonstrated to be an ideal model reaction for studying the promotion effects of both electropositive [1,2] and electronegative [3] metal ions on supported Rh catalysts. In particular, high activity and selectivity towards oxygenates could be rationalized on the basis of an intimate contact between the metal phase and the promoting ion [l]. Conventional Rh catalysts show no or very low selectivity in the heterogeneous hydroformylation of olefins. In the case of electropositive promoters, e.g. Na+, K+, Zn2+, their utilization as countercations of metal carbonyl clusters anions as metal precursors of supported catalysts has been proposed as an elegant way to solve the problem [4]. The close contact between the metal framework and the promoter is inherently obtained in the starting precursor. In this contribution, the effect of the nature of the support on the catalytic properties of oxide-supported K2 [ M 1 2(CO)30] will be investigated using ethylene hydroformylation as model reaction. ~xperimental The K2[Rh12(C0)30] [ 5 ] cluster has been prepared by literature's method. Commercial oxides .(SiO, Davison 6 2 , yA 1 2 0 3 Akzo, MgO Carlo Erba W E ) have been used as supporte.
2346
The supports have been overnight evacuated in high vacuo and impregnated under argon by incipient wetness technique with a H20/acetone solution of the cluster. The metal loading was 1% by weight in all samples. Ethylene hydroformylation was investigated at 18OoC and atmospheric pressure (‘C2H4:CO:H2 = 1:l:l)h a continuous-flow glass microreactor interfaced to a HP5890 gas-chromatograph. Infrared spectra of the samples, as pressed wafers or nujol mulls, have been recorded on a Fourier Transform IR spectrometer (Digilab FTS 40) operating at a constant resolution of 2 cm-l. Results and Disoussion The silica support shows negligible reactivity at room temperature towards the K2[Rh12(CO)30] cluster. The initial red-violet color of the cluster is mantained, and the infrared spectrum in the vC0 region shows relatively broad bands at 2086(w), 2040(vs) and 1834(s) cm-l, which resemble those of the pure cluster in THF solution. The broadness of the bands is likely to be related to a partial interaction with the surface, via exchanging Na’ ions with the surface protons. On the contrary, alumina and magnesia show a completely different behavior. The initial red-violet color of the solution immediately disappeared upon impregnation, indicating the occurrence of a fast reaction with the support. After removal of the solvent, a grayish powder was left in both cases, showing only negligible vC0 bands in the IR spectrum. The extensive degradation of the anionic cluster precursor is thus suggested. Segregation of the K+ cation, as the consequence of cluster-support interactions, can be reasonably proposed as the first reaction step, followed by the decomposition of anionic Rh12 framework on the surface. The supported catalysts have been tested in ethylene The hydroformylation at atmospheric pressure and 1 8 O o C . silica-supported cluster is an active and stable catalyst. A selectivity to C2H5CH0 of 35% is in fact observed at the beginning of the catalytic test. In addition, the selectivity to C2H5CH0 is constant after long time on stream (Table).
2347
On the contrary, both the alumina- and magnesia-supported catalysts show no initial selectivity to C2H5CH0, the only reaction being ethylene hydrogenation to ethane. Significant changes then occur with time on stream; hydrogenation rapidly decreases, whereas hydroformylation slightly increases. After 50 h on stream, a selectivity to C2H5CH0 of about 15% is observed, together with a non-negligible formation of npropanol. TABLE.
Catalytic activity in ethylene hydroformylation of K2[Rh12(C0)30] cluster supported on different oxides.
OXIDE
Catalytic activity' lh 50h
Selectivity to C2H5CH0 lh 50h
SiO,
0.75
0.51
35
37
A1203
0.43
0.31
4.8
15
MgO
0.33
0.25
3.9
16
Reaction conditions : 1 atm (C2H4:CO:H2 = I:I:I), 18OoC Activity as Mol(CO conv.)/M~l(Rh) min
The catalytic data can be fully explained on the basis of the chemical interaction taking place between the [Rh12(C0)30]K2 cluster and the oxide surfaces. In the case of silica, the high initial selectivity to propionaldehyde indicates that a stable and active catalyst is formed and stabilized on the surface just upon impregnation. Although a direct chemical evidence is still lacking, a preliminary infrared investigation on pressed wafers seems t o suggest that the Rh12 cluster itself, or a closely related molecular species, is the active catalyst for ethylene hydroformylation [4]. On the A1203- and MgO-supported samples, the absence of any selectivity to aldehyde at the beginning of the catalytic test
confirms that the Rh12 precursor is completely decomposed during impregnation, and no metal-promoter ihteractions are left. Accordingly, unpromoted Rh catalysts show negligible activity in olefin hydroformylation [l]. The slow increase in the hydroformylation activity with time on stream would therefore be related to a partial reconstruction of the cluster framework under CO atmosphere. This kind of reactivity has been recently demonstrated to be a general feature of many oxide-supported platinum-group metals [6,7]. Our results seem to be in marked contrast with those reported by Ichikawa [ a ] , where basic supports were required to enhance the hydroformylation activity. However, this lack of agreement is more apparent than real; in both cases, anionic Rh species are thought to be the catalytically active species in olefin hydroformylation. Starting from neutral clusters, as in Ichikawa's work, anionic species can be only formed and stabilized on basic surfaces.
conaluriona The potassium salt of the [Rh12(CO)30]2- anion, has been demonstrated to be an efficient precursor of heterogeneous hydroformylation catalysts only when supported on unreactive oxides, such as silica. In the presence of strong metal-support interactions the benificial effect of the intimate contact between cationic alkaline metal and RhI2 framework is inhibited.
Roforonaor 1 W.M.H.Sachtler and M.Ichikawa, J.Phys.Chem., 90 (1986) 4752. 2 S.Natio and M.Tanimoto, J.Catal., 130 (1991) 106. 3 Y.Izumi, K.Asakura and Y.Iwasawa, J.Catal., 127 (1991) 631. 4 C.Dossi, A.Fusi, L.Garlaschelli, D.Roberto, R.Ugo, R.Psar0, Catal.Lett., in the press. 5 P.Chini and S.Martineng0, Inorg.Chim.Acta, 3 (1969) 299. 6 C.Doesi, A.Fusi, R.Psaro, D.Roberto and R.Ugo, Mater. Chem. Phys., 29 (1991) 191. 7 H. H. Lamb and B. C. Gates, J. Am. Chem. SOC., 108 (1986) 81. 8 M. Ichikawa, J. Catal., 59 (1979) 67.
Gwzi, L. er al. (Editors),New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights resewed
ISOLATED AND COMPETITIVE HYDROGENATION TO CHAl$ACTERIZENI-B CATALYSTS G. Jannes, P. Kerckr, B. Lenoble, P. Vanderwegen, C. Verlinden andJ.P. Puttemans CERMnstitut Meurice, av. E. Gryzon, 1, 1070 Brussels, Belgium
Abstract. The hydrogenation of cyclohexene and cyclooctene in the liquid phase is proposed as test reactions system to probe electron transfer on the metallic catalytic center. Noble metal catalysts (Pd, Pt, Rh) are studied to delineate the limitations of this methodology. From its application to Ni-B catalysts, it is tempting to reverse generally accepted ideas and to propose an electron transfer from the nickel atom to its neighbors. 1. IIVTRODUCTION
Several papers have recently shown that kinetic analysis of well-chosen catalytic reactions, as well of isolated reactants as in competitive systems, may help evidence the r6le and the nature (geometric, electrostatic, electronic) of modifications induced in metallic catalysts by promoters, poisons, alloying or support [l]. When compared to physical methods, this simple approach presents the advantage of making it possible to study the catalyst in the liquid phase, i.e. in most cases, the real working conditions. We have applied this methodology to the study of nickel b r i d e catalysts, prepared by reduction of nickel acetate with sodium borohydride. Hydrogenation reactions, insensitive to the StrUCtUre, are well-suited to characterize the nature of the metal site. We propose, as a teat reactions system, the isolated and competitive hydrogenations of cyclohexene and cyClOOCtene. If it is assumed that the two hydrocarbons are competing for the same sites on the metal surface, different from these of hydrogen adsorption, the hydrogenation rate of cyclohexene, at constant hydrogen pressure, may be written, according to the Langmuir-Hinshelwood mechanism : V ’
kc n bc rn cc n
where k, is the rate constant including the constant hydrogen
2350 term, b the adsorption coefficient and C the concentration, and the subscripts Q and OD stand for cyclohexene and cyclooctene respectively. The terms concerning the products and the solvent were neglected. A similar rate equation may be written for cyclooctene hydrogenation. Taking the inverse of the rate equations, linear transforms are obtained as :
-- VCa
+
bcmCcn
b C
cn c m
cn
+-
bco cn c n
.-C
cCO
cn
In experimental conditions where the surface is not completely covered, we may determine the two rate constants k a and , k and the two adsorption coefficients ba and ,b from the slope and the intercept of the plot l/vg vs C , / C , at constant cyclohexene concentration and of the plot l/V, vs Ca/C, at constant cyclooctene concentration. The values of these four parameters may be validated by comparison with the experimental value of the Wauquier-Jungers selectivity, S = kODb,/k,b, [ 4 ] , determined from the logarithmic plot of the relative concentrations. 2.ExpERIHENTAL
The P-2 nickel boride catalysts, according to Brown's terminology [ 3 ] , are prepared Inin situU1: the nickel salt solution, a 95% ethanol solution of nickel acetate and the borohydride ethanolic solution are outgassed before mixing by successive cycles of pumping and nitrogen introduction. During reaction time, the hydrogen pressure is kept constant at 120 kPa. Hydrocarbon concentrations are in the range 0.1-0.5 M. The evolution of the reaction mixture composition is followed by G.L.C. 95% ethanol is also used as solvent for the hydrogenation on supported noble metal catalysts. Reproducible preparation of NIB catalysts is demonstrated, provided that all preparation parameters, and namely, the mixing rate during the reduction of the nickel salt which determines the relative importance of growth and germination, is kept constant. 3. RESULTS and DISCUSSION
Hydrogenation on alumina supported noble metal catalysts Isolated and competitive hydrogenation of cyclohexene and cyclooctene are performed first on a series of alumina supported noble metals (Pd,Pt,Rh) from Engelhard, in order to probe this test reactions system. Some results on cyclohexene are given in table I. 3.1.
2351
Table I Kinetic rate constants and adsorption coefficients values of cyclohexene at 25'C on alumina supported noble metals
metals* Pt Pd Rh
bc m (M-') 3.3
4.2 34.1
ka(M.min-tg-' 1 . t . I
)
1.2 lo-' 9.2 lo-' 10.9 lo-'
Neglecting the entropic term, the adsorption forces scale as deduced from the b c m values, is Pt>>trans-2- b u t e n e > c i s - 2 - b u t e n e . M e t a t h e s i s p r o d u c t s were n o t o b s e r v e d u n t i l most of t h e d i e n e had been removed. n-butane w a s formed c o n t i n u o u s l y , presumably as t h e r e s u l t of s e c o n d a r y hydrogen a t i o n of t h e b u t e n e s . The d e u t e r a t i o n of b u t a d i e n e was a l s o s t u d i e d . A slow s t e p w i s e exchange w a s o b s e r v e d w i t h t h e
i n i t i a l d o > d l > d 2 > d e t c . S u b s t a n t i a l amounts of b u t a d i e n e -d w e r e p r o d u c e d i n 20 min. S i m i l a r r e s u l t s were o b s e r v e d wihh t h e s u l f i d e d c a t a l y s t . The h y d r o g e n a t i o n of c y c l o h e x a d i e n e g o e s i n t w o s t e p s : a t f i r s t c y c l o h e x e n e i s formed a n d t h e n t h i s i s f u r t h e r hydrogen a t e d t o c y c l o h e x a n e ( F i g u r e 1).
0 10
50
10 0
Time(min)
F i g u r e 1. D e u t e r a t i o n o f c y c l o h e x a d i e n e o v e r r e d u c e d molybdena-alumina a t 25 OC. I n i t i a l p r e s s u r e s o f d e u t e r i u m were 36 kPa (open symbols) a n d 10 kPa ( s o l i d s y m b o l s ) r e s p e c t i v e l y . I n i t i a l p r e s s u r e o f c y c l o h e x a d i e n e was 3 . 6 kPa. I n t h e h y d r o g e n a t i o n o f c y c l o h e x a d i e n e a t 25 OC o v e r reduced and s u l f i d e d c a t a l y s t s , t h e d a t a showed o n l y a m i n i m a l i n c r e a s e i n r a t e on i n c r e a s i n g t h e h y d r o g e n p r e s s u r e . T h i s i s i n sharp c o n t r a s t with t h e approximately f i r s t o r d e r e f f e c t w i t h b u t a d i e n e . The s u l f i d e d c a t a l y s t h a d a much l o w e r a c t i v i t y t h a n t h e r e d u c e d o n e . The e x p o s e d a l u m i n a s u r f a c e of t h e reduced c a t a l y s t can c o n t r i b u t e t o t h e exchange r e a c t i o n o f D 2 w i t h u n s a t u r a t e d h y d r o c a r b o n s , t h u s d i s g u i s i n g t h e mecha n i s t i c d a t a . However, t h i s c o u l d be e l i m i n a t e d by chemis o r p t i o n o f C 0 2 on t h e a l u m i n a s i t e s . When t h i s w a s d o n e , exchange o c c u r r e d w i t h c y c l o h e x a d i e n e p r o d u c i n g almost e x c l u s i v e l y t h e d 4 s p e c i e s . Only t r a c e amounts o f c y c l o h e x a d i e n e -d2 were o b s e r v e d a f t e r 6 0 min. The c y c l o h e x e n e s formed ref l e c t e d n i c e l y t h e cyclohexadiene data a s d i d t h e cyclohexa n e s . I n e a c h case t h e s e p r o d u c t s are t h o s e e x p e c t e d i f D2 were added t o c y c l o h e x a d i e n e - d o and -d4 or t o c y c l o h e x e n e - d 2 and -d6 ( F i g u r e 2 ) .
2385
s
- 50o,
E
1
50
1
0 10
50
1
010
I
50
Time (min)
-Figure 2 . Deuterium d i s t r i b u t i o n v s . r e a c t i o n t i m e f o r t h e r e a c t i o n p r o d u c t s of t h e d e u t e r a t i o n o f cyclohexadiene o v e r reduced molybdena-alumina a t 25 OC. The c a t a l y s t was poisoned w i t h C 0 2 b e f o r e the experiment.
w i t h c y c l o h e x a d i e n e , t h e i n i t i a l hydrogenation rates were compared w i t h t h o s e f o r exchange. I n a l l cases t h e i n i t i a l hydrogenation r a t e was from t h r e e t o f i v e times f a s t e r t h a n t h a t of t h e exchange which i s c o n t r a r y t o t h e s i t u a t i o n observed w i t h b u t a d i e n e ,
4 . DISCUSSION
The e x p e r i m e n t a l d a t a o b t a i n e d f o r t h e hydrogenation and d e u t e r a t i o n of b u t a d i e n e o v e r reduced and s u l f i d e d c a t a l y s t s i n d i c a t e t h a t t h e same mechanism is followed w i t h both catal y s t s , S i n c e t h e deuterium exchange i n t o the t e r m i n a l v i n y l p o s i t i o n of b u t a d i e n e i s o v e r t e n times f a s t e r t h a n i n t o t h e i n t e r n a l p o s i t i o n s , it may be assumed t h a t t h e s e c - b u t e n y l l i g a n d i s formed p r e f e r e n t i a l l y t o t h e primary one. The p r o d u c t d i s t r i b u t i o n s s u g g e s t e d t h a t 2-butenes c o n t r i b u t e d l i t t l e t o t h e formation of n-butane u n t i l almost a l l o f t h e 1-butene was used up'. The p r o d u c t d i s t r i b u t i o n of t h e cyclohexadiene hydrogen a t i o n as a f u n c t i o n of t i m e i s v e r y s i m i l a r t o t h a t o f b u t a d i e n e . The formation of cyclohexadiene-d can be e x p l a i n e d by t h e e q u a l exchange i n t h e f o u r e q u i v a l e n t v i n y l p o s i t i o n s of cyclohexadiene o v e r t h e CUS of t h e molybdena. The s u l f i d e d molybdena-alumina e x h i b i t e d a much lower hydrogenation r a t e f o r t h e cyclohexadiene as was t h e case of b u t a d i e n e .
2386 5. REFERENCES
1 W.K.Hal1, in "Chemistry and Physics of Solid Surfaces VI," p. 73 ff. Springer-Verlag, New YorkIBerlin, 1 9 8 5 2 J.Goldwasser, J.Engelhardt and W.K.Hal1, J.Cata1. 70, 2 7 5 (1981)
6
A.Redey and W.K.Hal1, J.Cata1. 1 1 9 , 5 3 4 ( 1 9 8 9 ) ~.Redeyand W.K.Hal1, J.Catal. 1 0 8 , 1 8 5 ( 1 9 8 7 ) w.S.Millman and W.K.Hal1, J.Cata1. 5 9 , 3 1 1 ( 1 9 7 9 ) W.S.Millman, K.I.Segawa, D.Smrz and W.K.Hal1, Polyhedron
7
J.Valyon and W.K.Hal1, J.Cata1. 92, 1 5 5 ( 1 9 8 5 )
3 4
5
5,
169 (1986)
ACKNOWLEDGEMENT This work was supported by the National Science Foundation at the University of Pittsburgh under Grant DMR-9 0452 2 8.
Guczi, L d d.(Editors), New Fronriers in Catalysis Pmeedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
HYDROCRACKING BOSCAN HEAVY OIL WITH UNIMODAL AND BIMODAL CATALYSTS M. Ternana and J. Menashib aEnergy Research Laboratories/CANMET, Energy, Mines and Resources Canada, 555 Booth Street, Ottawa, K1A OG1 Ontario, Canada bBillerica Technical Center, Cabot Corporation, 157 Concord Road, Billerica, Massachusetts 01821, USA
1.
INTRODUCTION
Experimental data have shown that diffusion in hydrocracking catalyst pores affects the conversion of heavy oils and bitumens. For example, the influence of diffusion was demonstrated (1) during the hydrocracking of Athabasca bitumen using unimodal catalysts of different pore diameters, d,. The following equation describing the diffusion of liquids in pores has been derived (2) using theoretical concepts. D,
/
D, = (1 - A ) *
/
(1
+
PA)
where A = d,/d, and d, is the diameter of the diffusing molecule. Unlike other equations that have been proposed in the literature, in addition to correlating the data, the above equation satisfies two essential criteria. At very large values of d, the effective diffusivity in the pore, D, becomes equal to the diffusivity in the bulk, D, and when d, = d ,, then D, = 0. Other experimental data (3) have shown that conversion can be improved by altering the chemical compostion of catalysts that have comparable surface areas and pore dimensions. This indicates that both surface reaction and pore diffusion affect conversion during residuum hydrocracking, although neither "controls" or dominates. Bimodal commercial catalysts for hydrocracking residua contain both mesopores (MEP), that provide surface area for reaction, and macropores (MAP), that provide large pore cross-sections for enhanced diffusion rates. In our laboratory, well known concepts have been combined to develop an equation for the reaction rate in terms of surface reaction, diffusion in both MEP and MAP, chemisorption, and film diffusion. Three of those terms, surface reaction plus diffusion in both MEP and MAP are shown in Equation 2, where D, L, and A representthe effective diffusivity, the pore length, and the pore cross-sectionalarea in the MAP and MEP respectively, whereas ,k, and A, represent the rate constant for the surface reaction and the catalyst's internal surface area.
2388 r
=
PI L
In this work, the above concepts have been used to explain vanadium hydrodemetallization using catalysts of various pore geometries. 2. EXPERIMENTAL Steady state hydrocracking experiments were performed in a continuous flow hi h pressure reactor (internal length to diameter ratio of 24) at 13.9 MPa, 455OC, 4 LSV, and 0.8 L H,/mL heavy oil. Similar equipment has been described previously (1). The feedstock and fresh catalyst properties are shown in Tables 1 and 2 respectively. The catalysts contained 16-19% MOO,, 3.1-3.5wt % Co,O, with the balance being primarily AI,O, plus a few trace elements. Catalysts 1 to 9 were laboratory prepared, and had diameters of 1-2mm. Catalyst 10 was an extrudate obtained from American Cyanmid, having a diameter of 1 mm.
8.'
Table 1
Table 2
Boscan heavy oil properties
Catalyst properties
t525OC vacuum resldue (wt %) 54.6 Speclflc gravity 15OC 1.002 Pentane lnsolubles (wt %) 20.1 Toluene lnsolubles (wt %) co.1 Carbon (wt %) 81.8 Hydrogen (wt %) 10.4 Sulphur (wt %) 5.38 Nitrogen (wt %) 0.64 Ash (wt %) 0.22 Vanadium (ppm) 1385 Nickel (ppm) 107 Iron (PPm) 13 Aromatic carbon (%) 24.2
BET N, HgPoro MAP MEP Catalyst Surface Surface d, d, Number Area Area m2/g m2/g nm nm 1 2 3 4 5 6 7 8 9 10
175 121 232 165 195 140 217 226 214 295
229 163 161 81 289 189 292 290 273 256
370 380
-
110 62 140 250
7 10 9 18 12 21 12 8 9 6
CATALYST Bulk Density g/mL 0.733 0.739 0.427 0.427 0.489 0.486 0.381 0.497 0.456 0.561
3. RESULTS AND DISCUSSION Diffusion phenomena in macropores and mesopores can be compared by calculating the effective diffusivities,, ,D and DMEp after substitutingvalues of d, and d, into Equation 1. The pore diameters, d, in Table 2 were obtained from mercury porosimetry distributions. Although the molecular diameter is a function of molecular weight (4), d, = 4 nm was selected after considering several measurements (5,6). The calculations showed that for almost all of the macropores, ,D , = D, whereas DMEp cD,. For this reason the macropore term in Equation 2 was ignored with respect to the mesopore term, on the basis that both macropore and mesopore domains had similar L/A (porosities).
2389 The vanadium conversions in Figure 1 can be divided into two groups. The bimodal catalysts (open circles) had conversions that were greater than the = 0. Therefore, the unimodal catalysts (solid circles). For unimodal catalysts LAP mesopore length must be at least equal to the radius of the catalyst particle. Bimodal catalysts are probably composed of domains of macropores adjacent to domains of mesopores. Molecules can diffuse through the macropores with the same effective diffusivity as in the bulk liquid. Thus the role of the macropores is to the length of the mesopores. As Equation 2 indicates smaller values decrease hEp, of hEp increase the reaction rate which would explain why the greater conversions were obtained with the bimodal catalysts in Figure 1.
0
4 10 0
2
0
.
6
0
5
1
0
2000
3000 TOTAL
5000 REACTOR
4000
SURFACE
AREA IN
6000 (m2)
Figure 1: Vanadium Conversion (wt %) Versus Total BET Surface Area in Reactor (m2) The role of the mesopores can be seen by rearranging Equation 2 after having deleted the macropore term. (ATOF)''
=; r
=
]
[k A,, cB
&EP
DMEP B ,
1
t ,
I31
k
Equation 3 can be considered in terms of the structure of the alumina catalyst support. By raising the calcining temperature during catalyst preparation, sintering occurs causing d, to increase and A,, to decrease. In the limit, the combination of very small A,, and very large d, would make the diffusion term in Equation 3 negligible so that the reaction rate would only be governed by the surface reaction rate. Equation 3 also shows that if A,, increased and d, decreased the diffusion term would become more important and the overall reaction rate would be affected by both surface reaction and diffusion. The vanadium hydrodemetallization (HDM) data have been plotted in Figure 2 as the inverse of the area turnover frequency, (ATOF)-', versus AlJ(DMEp/DJ. The thermal conversion in the absence of a catalyst was subtracted from the total
2390 vanadium conversion to obtain the catalytic conversion that was used to calculate ATOF. 150
125
100
75
50
0
10 SURFACE AREA
20
/
40
30
50
( D ~ ~ ~ / (m2 D ~x )
Figure 2: Inverse Area Turnover Frequency (ATOF).’ (Atoms Vanadium Removed per nm2of surface area per second) Versus Surface Area Divided by the Ratio of the Effective Diffusivity in the Mesopores to the Diffusivity in the Bulk Liquid, AIS/(DMEP/DB) (m2X l o 3 ) The points for all of the laboratory prepared catalysts fall on a straight line, as indicated by Equation 3. The commercial catalyst, No. 10, does not fall on that straight line, perhaps because its L/A could be different. The results can be summarized as follows. The role of the macropores is to The combination decrease the length of the diffusion path in the mesopores, hEP. of surface reaction and diffusion in mesopores is described by Equation 3 and Figure 2. The conflict betweensimultaneouslymaximizingboth catalyst surface area and pore diameter will produce a different optimum for each size of heavy oil molecule. 4
REFERENCES
1. M. Ternan, Can. J. Chem. Eng. 61,689-696 (1983).
2. M. Ternan, Can. J. Chem. Eng. 65, 244-249 (1987). 3. P.M. Boorman, J.F. Kriz, J.R. Brown, and M. Ternan, Proc. 8th Intern. Congr. Catal. 2,281-291 (1984). 4. R.L. Nortz, R.E. Baltus, and P. Rahirni,. Ind. Eng. Chem. Res. 29, 1968-1976
(1990).
5. J.G. Speight, The Chemistry and Technology of Petroleum, Marcel Dekker, New York, 1980, p.202.
6. J.C. Ravey, G. Ducouret, and D. Espinat, Fuel=,
1560-1567 (1988).
Guczi, L.er al. (Editors), New Frontiers in Caralysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
A IUNETIC MODEL FOR AROMATIZATION PROCESSES OVER ZSM-5 CATALYSTS. AROMATIZATION OF SHORT CHAIN HYDROCARBONS OVER HZSM-5 D. B. Luk'yanofl, V. I. Shtrala, V.I. Timoshenkoaand S. N. Khadzhievb aKarpov Institute of Physical Chemistry, Ulitsa Obukha 10, 103064, Moscow K-64, Russia bGroznyi Petroleum Institute, 364902 Groznyi, Russia
INTRODUCTION Aromatlzatlon of VarIOUS feedstocks over ZSM-5 catalysts can be schematlcally represented I1,21 as a sequence of the 3 main etages: ( 1 ) conversion of feed to the mixture of oleflns, (2) lnterconverslon of oleflns, ( 3 ) olefln aromatizatlon. Each stage Is a complex catalytic reaction, Alongslde with the numerous chemlcal transformations of reagents In the reaction system the processes of coklng and catalyst aglng take place a6 well. In the present paper we propose an approach to the development of a general kinetic model for aromatlzatlon processes over ZSM-5 catalysts. A kinetic model for aromatlzatlon of short chain hydrocarbons over HZSM-5 zeolite (a part of the general model) is dlscussed In detail. EXPERIMENTAL The converslons of C i and C; olefins and C,-C, paraffins were In a flow carried out over HZSM-5 zeolite (SiO,/A1,0,=240) micro-reac tor at 500°C and atmospheric pressure. The reaction products were analyzed by GC. RESULTS AND DISCUSSION Our approach to the development of the general kinetic model for aromatlzation reactions over ZSM-5 catalysts Is based on
2392
splitting of the whole model into 5 modules (Fig. 1 ) . Each module is a kinetic model f o r one of the main stages of aromatization process. It should be noted that module 1 (Fig. 1 ) in its turn consists of a few modules (paraffin cracking, methanol conversion to olefins, etc.). conversion
interconversion
aromatizatIon
p Deac z tivat Gionq Figwe 1. The module structure of the general kinetic model. Kinetic model for aromatizatlon of short chain hydrocarbons Includes 3 modules ( 3 kinetic models): ( 1 ) paraffin Cracking, (2) olefin Interconversion, ( 3 ) olefin aromatization. The model describes the rates of formation and conversion of 40 components (hydrogen, 9 olefins C i - C y o , 10 paraffIns C, -C, 5 alkylcyclohexanes Y,,-Y, o, 5 alkylcyclohexenes Yi-Yio, 5 alkylcyclohexadlenes YE-Y;o, and 5 alkylbenzenes +,-A, ) in the following steps of the reaction: 1. Protolytic cracking of C-H bond in paraffin molecule op
2Sn610 2 . Protolytlc cracking of C-C bond in paraffin molecule k(npm ) cn + z C I Z + c,,-m 3QnS10, 2 G m 6 9
-
3. Hydrogen transfer between paraffins and olefins
4.
Olefin ollgomerization and cracking 2 6 n, m 6 8, 4
< n+m
6 10;
2393 5.
Olefin cyclization 6c n
c
10;
6. Olefin aromatization (hydrogen transfer) k(m) 6cnc10, 2cmS10; Yn t cz: Y,;Z t cm
-
6Qnc10, 2cme10; 6 ~ n ~ 1 0 2 d,m c l O ; 7 . Hydrocarbon adsorption and deeorption
2
Q
n c 10;
6 c n d 10; 6c n
c
10;
where Z is zeolite catalytic site. The model was developed with the following assumptione: (a) Adeorption equilibrium is established; (b) Adeorption constants K, (1 )-Ka ( 4 ) are independent of hydrocarbon molecular weight. The kinetic model for light hydrocarbon aromatization reaction consists of a set of 40 equations describing the rates of traneformations of 40 group components in 237 reaction steps: 237
R, = c visrs h -I
(1 < 1 < 40),
where R, is the rate of transformation of the i-th component, vis is the stoichiornetric coefficient of the I-th component in the e-th reaction step, r, is the rate of the s-th reaction step. The change of the reactivity of hydrocarbons with 811 increase of their molecular weight is reflected by the dependence of the rate constants on the number of carbon atoms In the reagent molecules. The values of the kinetic constante were determined by means
2394 of comparison of the mathematical modeling results with the experimental dependence of the product distribution on contact time f o r the conversions of C; and C; olefins and C,-Ce paraf-
fins over HZSM-5 zeolite. Fig. 2 demOnstrate6 that the proposed kinetic model fits the experimental data quite well (agreement between 6imUlated and experimental data was practically the same f o r all feedstocks Used in this study).
c, wt%
c, wt%
Figure 2, Product distribution vs. contact time: experimental data (points) and simulated curves f o r ethene (A), n-hexane (B) conversions over HZSM-5 zeolite. CONCLUSIONS The approach to the development of the general kinetic model f o r aromatization processes over ZSM-5 catalysts is proposed. The main advantage of this approach is the open character of the model. Such character allows to extend the model by an addition of new modules f o r conversion of various feedstocks. Specially developed algorithms and programs make it pO66ible to perfect the kinetic model by means of introducing of new reaction steps in already finished modules (when new data on the reaction mechanism are obtained). On the basis of the proposed approach the kinetic model f o r light olefin and paraffin aromatization reaction over HZSM-5 zeolite is developed. REFERENCES 1. W.O.Haag, in D.H.Olson and A.Bisio (eds.), Proc. 6th Intern. Zeolite Conf. , Butterworths, Guildford, UK, 1984, p.466. 2 . P.B.Weisz, Ind. Erg. Chem. Fund., 25 (1986)53.
Guni, L a ul. (Editors), New Fronriers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 19!B Elsevier Science Publishers B.V.All rights reserved
ON TEE ROLE OF REVERSIBLE AND IRREVERSIBLE ADSORPTION HYDROGEN IN THE DEHYDROGENATION AND REFORMING REACTIONS Y. Sun, S. Chen andS. Peng
Instituteof Coal Chemistry,Chinese Academy of Sciences,P.O.Box 165, Taiyuan,Shanxi 030001, China
The conversion of hydrocarbon over Pt catalysts is very important in petroleum processing. Their catalytic performance, particularly product selectivity, depends strongly on the pressure of H2 and its sorption behavior. Thus, much attention has been paid to investigate the effect of H2 adsorption on hydrocarbon conversion over supported Pt catalysts[l] . However, only irreversible adsorption hydrogen has been considered in almost all conclusions. Under a real reaction condition, in our opinion, irreversible and reversible adsorption species co-exist on catalyst surface. The phenomenon has been observed in our experiments[l-3]. Furthermore, it is found that the irreversible and reversible species could play different roles in a catalytic reaction. The reversible adsorption species refers to a type of adsorbed species existing on the catalyst surface at reaction temperature when there is its gas phase pressure in the system and once the pressure is removed by evacuation or purging. it would be completely desorbed, while the irreversible adsorption species can not be removed from the catalyst surface. Therefore, it is necessary t o investigate deeply the roles of the two adsorbed species. The present paper is fucosed on the roles of irreversible and reversible adsorbed hydrogen in dehydrogenation and reforming reaction over supported Pt catalysts.
Catalysts containing 0.5%(wt) platinum were prepared by impregnation of
11 -A1203 of surface area of 239m2/g in HzPtCls solution. The composition and pretreatment of catalysts were given in Table 1. All catalysts were reduced at 450.c in flowing H2. Both static chemisorption by Chemisorb 2800 (Micromeritics Co., USA) and dynamic adsorption by pulse and FTPS methodl41 was used to measure the hydrogen isobars at different H2 pressure and temperature from 35.c to 550.c. Here the reversible and irreversible uptake of H2 was represented
by Hr/Pt and Hi/Pt respectively, and the fraction of reversibly and irreversibly adsorbed hydrogen in total uptake was symbolized by Fr and Fi respectively. Table 1 Composition and pretreatment conditions catalysts Ga/Pt atom ratio Cl/Pt atom ratio PH value of soiution Calcination temp. (%I
AH1
AH4 ---- AH21 ---- AH2 --- ----
G11 1.04 1.41 2.28 1.32 1.24 2.17 2.54 1.94 1.94 1.15 2.54 450 120 450 450 450
GP2 CP3 2.08 2.89 1.72 ---2.54 2.54 450 450
GP4 3.78 1.46 2.54 450
The data of the coversion of n-hexane at 360.c and 450'c and the dehydrogenation of cyclohexane at 300% in the pressure of H2 were determined with 150 mg of catalyst in a continuous flow microreactor. The feed rate of H2 and n-hexane with volume ratio 1OOO:l was 60 ml/min, and for dehydrogenation, the mole ratio of Hz and cyclohexane is 1O:l. The reliable data were taken after reacting 10 hours. The selectivity of isomerization, dehydrocycl ization and hydrocracking was represented by Si, Sb and Sh respectively. The deactivation amount was given by the difference AX of conversions at initial time Xo and after running 10 hours Xi00
3.RESULT AND DISCUSSION 3.1.The Effect of the Irreversible Hydrogen on the Dehydrogenation of Cyc 1ohexane At 300 'c and atmospheric pressure, benzene was the unique product of cyclohexane dehydrogenation over supported Pt catalysts, and after catalyst running for ten hours, no deactivation was 0bserved.h this case, it was very interesting to note that there was linear dependence between the conversion of cyclohexane and irreversible uptake of hydrogen Fi, which could be expressed empirically as follows: X=1.89-2.44Fi (1) This equation revealed that the irreversibly adsorbed hydrogen on the catalyst surface at the reaction temperature suppressed the dehydrogenation of cyclohexane, i.e., with the increase of the fraction of irreversible hydrogen, catalyst activity showed a 1 inear decrease. Furthermore, the relationship was independent of catalyst preparation. It could, therefore, be reasoned that although the dehydrogenation could also take place on the modified catalyst surface by the irreversible hydrogen, the rate was slower than that on the surfase of fresh catalyst. 3.2. The Effect of Reversible and Irreversible Hydrogen on Conversion of N-hexane For the reaction of n-hexane over Pt catalyst in the presence of hydrogen. the product .was quite sensitive to the reaction temperature. And
2397 therefore, 360'c and 450'c of temperature were selected to study the isomerization and the aromatization of n-hexane. Surprisingly, it was also found that there existed the linear relationships between the reversible hydrogen and the product selectivities and catalyst deactivation amount. They could be empirically expressed by the following equations: at 360.c Si=2.68Fi-. 8354=1.845-2.68Fr r=O.996 (2) r=o.991 (3) 1202-8.3Hr/Pt at 450.c Sb=l.67Fi-0.0334=1.647-1.67Fr r=O.993 (4) r=O.999 (5) SitSh=2.17Fr-1.0472 (6) r=O.996 32-15.OHr/Pt It could be seen from these equations that adsorption type had a crucial effect on the product distribution of n-hexane conversion over supported Pt catalyst, i.e., the increase in the relative amount of irreversible hydrogen promoted the isomerization at 360.c and the aromatization at 450*c,while that in the relative amount of reversible hydrogen did the hydrocracking and isomerization at 450.c ; meanwhile, the reversible hydrogen could protect the deactivation of catalysts. These results demonstrate that not only irreversible but also reversible adsorption species did play important roles in a heterogeneous catalytic reaction, and further indicate that the reaction could take place on the catalyst surface modified by the irreversibly adsorbed hydrogen. This implied that the irreversible species were not only an active reactant but also a modifier of catalyst surface and even could induce reaction or active catalylic sites, while the reversible species mainly act as an active reactant or/and precursor of irreversible species. Furthermore, the negative intercept in equations (2), (4) and (5) implied that the reaction could occur only when there was certcain relative amount of irreversible hydrogen on the surface of the catalyst.and the negative slope in equation (3) and (6) suggested that the supported Pt catalyst would be not deactivated if there was sufficient amount of reversibly adsorbed hydogen on the surface of catalysts.
nX=O.
nX=O.
4. CONCLUSIONS
Not only irreversibly but also reversibly adsorbed hydrogen at the reaction temperature plays important roles in catalytic conversian of hydrocarbon over supported Pt catalyst. The irreversible hydrogen could also induce the reaction o r even modify activate catalytic sites, while the reversible hydrogen acts an active reactant or/and precursor of irreversible species.
The financial support of the National Natural Science Foundation of China is gratefully acknowledged.
2398
REFERENCES: Z. Paol. in Hydrogen Effect in Catalysis - Fund. Prac. Appl(Z. Paol and P.C.Menon, Eds.1 , Marcel Dekker Inc. New York, 1988. 2 . Y.H. Sun, Study on the Influence of Preparation Parameters on the Sorption of H2 and Catalytic Comersion of Cs Hydrocarbons over Supported Pt catalyst, Ph. Dr. Thesis, Institute of Coal Chemistry, 1989. 3 . G.M. Lou, Study on the Effect of Reversibly and Hydrogen on Selective Hydrogenation of Unsaturated Hydrocarbon, Ph. Dr. Thesis, Institute of Coal Chemistry, 1990. 4 . Y.H. Sun, L.X. Zhou, and S.Y. Chen et a l . , Acta Petrolei Sinica (Petro Processing section), 6 ( 3 ) ,(1990) 30. 1.
Guczi, L d al. (Editors), New Fronriers in CurolysiF Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary (a 1993 Elsevier Science Publishers B.V. All rights resewed
CATALYST DESIGN FOR THE UPGRADING OF AUSTRALIAN COAL-DERIVED LIQUIDS A.
T. Townsenda and F. P, Larkinsb
aDepartmentof Chemistry, University of Tasmania, Hobart, 7001 Tasmania, Australia bDepartment of Chemistry, University of Melbourne, Parkville, 3052 Victoria, Australia
Abstract A range of Ni-Mo/A1203catalysts with varying surface areas and active species loadings were tested in a batch autoclave reactor for their activity to upgrade Australian coal-derived liquids (CDLs). Their ability to facilitate aromatic hydrogenation and hydrodeoxygenationwas of particular interest. The catalysts prepared were compared with the Shell 424 hydrotreating catalyst, currently in commercial use. Results indicated that those catalysts of near 2-3 wt% Nil 8.2-12.3wt% Mo supported on alumina of surface area -200 m2/g possessed the highest initial activities. This was attributed to their optimum active site dispersion on the support surface. The catalysts were characterised by a number of techniques including XRD and TPR, along with surface area and acidity determinations.
1. Introduction Traditional catalysts for the upgrading of CDL's have usually consisted of Mo or W promoted by Ni or Co supported on alumina. In recent times there have been numerous studies into finding an optimum catalyst design by varying catalyst parameters including metal types [ll. metal ratios [2], support types [3], catalyst acidity [4] and pore size [5]. However to our knowledge catalyst surface area and catalyst active species loading have received only limited attention in the literature. This paper will detail a preliminary study into the effect these parameters have on the initial activity of catalysts employed for the upgrading of Australian CDL's. A batch autoclave reactor was used for the screening of the catalysts, as although continuous reactor studies are necessary to obtain lifetime and steady state effects, they are both time consuming and expensive for survey work.
2. Experimental A number of CDL's were used in this study, derived from Morwell and Wandoan coals. All were of similar elemental composition and were supplied from the continuous coal liquefaction plant at BHP, Melbourne. All possessed high aromatic and oxygen contents, which was present predominantly as phenols. This is a characteristic of Australian Brown coals and their subsequent CDL's [6]. The catalysts were prepared by the sequential impregnation of Ni and Mo using Ni(acetate), and Moo3 salts, on alumina supports supplied by the manufacturers Norton and Strem. Prior to use the catalysts were calcined in air at 45OoC, and then sulphided under a flowing Stream of H2S (10% by vol.) and hydrogen (90%by vol.) at 35OOC for four hours.
2400
Upgrading reactions were undertaken using a 70 ml rocking autoclave charged with CDL and a 10% loading of the sulphided catalyst. The initial hydrogen pressure was 10 MPa and the reaction temperature was maintained at W C for a period of 30 minutes. The original and upgraded oils were analysed by a number of techniques. Phenol removal was monitored by measuring the intensity of the 3200 cm-' band found in the FI'IR spectra of both original and upgraded oils. This measurement has been found to correlate well with the phenol content of such materials [7]. Aromatic hydrogenation was followed by both proton and I3C NMR. Catalyst characterisation included the measurement of catalyst surface area by the standard BET technique with nitrogen adsorption. X-ray diffraction (XRD) spectra of the calcined catalysts were obtained, giving an indication to the degree of catalyst crystallinity. Temperature programmed reduction (TPR) profiles of the calcined catalysts were also collected, at a heating rate of 12°C/min to 900OC in a mixed argon (95%)/hydrogen (5%) atmosphere. Acidity measurements were undertaken via the temperature programmed desorption of tbutylamine (TBA). Acidity values were calculated as relative acid site densities (RADs).
3. Results The results obtained from the experiments with the catalysts of varying surface area and active species loading will be detailed separately in the following discussion. 3.1. The effect of surface area Six alumina based catalysts of varying surface areas (up to -220 m2/g) were prepared by using support materials of different initial surface areas. Each had a constant metal loading of 3.2 wt% Ni and 13.2 wt% Mo. This loading is the same as that employed by Shell in the production of their Shell 424 catalyst, used as a reference for this work. The effect of surface area on phenol removal and the hydrogenation of a CDL are shown in Table 1. The aromatic content of the oils is represented as the fraction as aromatic carbons (fa).
Table 1 The initial activities of catalysts of differing surface area with constant loading* ~~~
SUPpofl type
~
Catalyst surface area (m2/g)# %OH removal
Blank reactor (ie no catalyst) Norton SA5151 Norton SA3232 Strem 132550 Strem 132500 Norton SA6275 Norton SA6173 Shell 424 reference
c1 39 83 154 198 222 166
33 41
60 80 80 89 87 85
fa
71 67 65 58 60 53 56 54
~
* Constant loading of 3.2 wt% NiA3.2 wt% Mo
# Surface areas of the calcined catalysts
It is evident from Table 1 that as the surface area of the catalyst increases, the higher the initial activity for phenol removal. The optimum phenol removal and aromatic hydrogenation was displayed by those catalysts with surface areas near 200 m2/g. Both catalysts of surface areas 198 and 222 m2/g respectively had initial activities comparable with the commercial Shell 424 reference.
2401 Changes in the catalyst surface area were found to cause only minor differences in the physical character of the catalysts. The catalysts were found to possess similar acidities, were all amorphous in character (except the catalyst of lowest surface area that was slightly crystalline) and showed similar metal oxide-supportinteractions (from P R studies).
3.2. The effect of active species loading A number of Ni-MoIAl 0 catalysts of varying active species loadings were prepared on an alumina support of 198 m2 2Ig3 (Norton SA6275). This support was chosen as it had been found in our previous work to possess an optimum surface area for the upgrading of CDL's. The loading range considered was from 1 wt% Ni/ 4.1 wt% Mo through to 15 wt% Ni/61.9 wt 5% Mo. The metal ratio (ie of Ni:Mo) was held constant (at 1:4.1)for all catalysts preparcd and is the same as that used in the production of the Shell 424 catalyst. For ease of subsequent discussion these catalysts will be r e f d to by their wt% Ni content alone. The effect of active species loading on phenol removal and aromatic hydrogenation is shown in Table 2. It should be noted here that a different CDL was used as a feedstock for the testing of these catalysts, so the magnitude of results cannot be directly compared with those shown previously in Table 1. Table 2 The initial activities of catalysts of differing active species loading Catalyst loading*
Catalyst surface area (m2/g># %OH removal
Blank alumina 114.1 218.2 3112.3 6124.6 10141 15162 *Values npnsent wt% Ni/ wt% Mo
198 205 181 173 118 55 25
14 48 79 79 78 66 47
fa
53 44 41 39 38 46 53
# Surfacemas of the calcined catalysts
From Table 2 it is apparent that as the active species loading became greater there was an increase in the initial activity of the catalysts, reaching a maximum, then a decrease at subsequent higher loadings. Similar activities were displayed by those catalysts with loadings of between 2 and 6 wt% Ni, indicating that an activity plateau may have in fact been reached. The effect of metal addition is also noticeable when the activity of the catalysts are compared with the activity of the blank alumina alone. It was the particular aim of this section of work to investigate the effect of active species loading on the activity of catalysts for CDL upgrading, while keeping all other catal sts parameters constant. Table 2 would indicate that this was not possible as the surface area o the catalysts was observed to decrease with increased loading. This provided additional complicationthat will be discussed further in a subsequent section. The characterisation of these catalysts was not as straight forward as for the previous suite. As expected catalyst crystallinity was found to increase with greater loading. Similarly as the loading increased the interaction between the metal oxides and the alumina became weaker (TPR). Eventually the loading became so high that the reduction profiles of the catalysts mimicked the profiles of the individual metal oxides alone. Catalyst acidity was found to increase with loading, until at 6 wt% Ni the RAD was observed to decrease. This was attributed to the drop in catalyst surface m a , and is a limitation of the TPD technique.
r
2402
4. Discussion Both suites of catalysts (0-varying surface area and active species loading) represent changes to the same basic catalyst variable- the active site dispersion. High dispersions can be obtained by increasing the catalyst surface area, or by lowering the active species loading; and vice-versa for low dispersions. Two variables were alteredwhen the loading of the second suite of catalysts was increased. Hence, a new value of activity per unit surface area was calculated to enable the determination of highest initial activity. This value is related to the active species dispersion of the catalysts. Catalyst dispersions could not be obtained directly by the usual CO chemisorption technique as complete reduction of the catalysts could not be obtained even at temperatures in excess of 900°C under a hydrogen atmosphere. Measures of catalytic activity were taken from the values of % phenol removal (FTIR), fa (13C NMR) and H f l ('H NMR). When this was carried out it was found that the catalysts of 2-3 wt% Ni possessd the highest activity per unit surface area. It is apparent from the results obtained that those catalysts with loadings of 2-3 wt% Nil 8.2-12.3 wt% Mo supported on alumina of surface area -200 m2/g possessed the highest activity of all the prepared catalysts considered. It is postulated that these catalysts possessed an optimum active species dispersion on the alumina support material, allowing a synergistic effect between both metals oxides. As the active site dispersion is either increased or decreased (by altering the surface area or loading), the activity of the catalysts was observed to fall away.
5. Conclusion Catalysts of 2-3 wt% Nil 8.2-12.3 wt% Mo supported on alumina of -200 m2/g have been found to offer increased initial activity for the upgrading of Australian CDL's. This was attributed to their optimum active site dispersion on the support surface.
6. References 1. 2.
3. 4. 5.
6. 7.
M. F. Wilson and J. F. Kriz, Fuel, 63 (1984) 190. R. E. Tischer, N. K. Narnain, G. J. Steigel and D. L. Cillo, ACS Div. of Pet. Chem. Preprints, 30 (1985) 459. R. L. McCormick, J. A. King, T. R. King and H. W. Haynes Jr, Ind. Eng. Chem. Res., 28 (1989) 940. J. R. Baker, R. L. McCormick and H. W. Haynes Jr, Ind. Eng. Chem. Res., 26 (1987) 1895. C. Song, T. Nihhonmatsu, K. Hanaoka and M Nomura, ACS Div. Fuel Chem. Preprints, 36 (1991) 542. P. J. Redlich, W. R. Jackson, F. P. Larkins. A. L. Chaffee and 1. Liepa, Fuel, 68 (1989) 1538. S. Supaluknari. F. P. Larkins, P.J. Redlich and W. R. Jackson, Fuel Process. Tech., 19 (1988) 123.
Guai, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights resewed
THE USE OF INTERMETALLICHYDRIDES ON BASIS L4NTHAN WITH NICKEL AND COBALT FOR HYDROGENATION OF ASPHALTENES CONCENTRATE
N. M. Parfenovaa, I. M. Halperina, S. R. Sergienkoa, V. A. Pheraponiovb and E. V. Starodubizevab ahstitUte of Chemistry, Turkmenian Academy of Sciences, Ashkhabad, Turkmenia
bN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia
ABSTRACT
The present paper reports t h e results of investigation of c a t a l i t i c effect of intermetallic hydrides with composit i o n LaNi CO (where ILxrLc5) f o r hydrogenation of asphaltene 8 o n k d k a t e . The asphaltene concentrate ( a s p h a l t i t e ) obtained a f t e r deasphaltization of West Siberian o i l residium consisted of 70 $ of asphaltenes and 30 $ of aromatics and resins. The high c a t a l i t i c a c t i v i t y of hydrides i n hydrogenation reaction of a p h a l t e n e concentrate was observed. A t temperatures 25O-LcOO C , pressure of hydrogen I 4 MPa and reaction time 5-10 hr 75 % yield of l i q u i d products maltenes soluble i n n-hexane) w a s achieved. The sharp drop i n molecular weight from I434 t o 632 and 700 accordingly f o r maltenes and s o l i d s (unsoluble i n n-hexane) and considerable desulfurization from 2.82 t o 0.45 % i n maltenes and 0.81 56 i n s o l i d s was registered i n products of hydrogenat i o n of asphaltenes concentrate. The s p e c t r a l data have showed t h a t maltenes consist of compounds with low aromaof compounds w i t h high aromaticit i c i t y , and s o l i d p a r t t y (not l e s s then 5 rings) and s h o r t alkyl chains. The r e s u l t s first obtained of this exsperiment are supported the idea about exceptional hydrogenative properties of IMC hydrides stimulated by t h e presence of activated hydrogen i n t h e i r matrix.
-
-
-
I. INTRODUCTION Catalysis i s one of t h e important branches using t h e intermetallic compounds (IMC) of r a r e e a r t h metals (REEII) with 3-d t r a n s i t i o n metals /1-8!. This compounds are capable of absorbing large quantities of hydrogen at room temperature and moderate pressures. H i g h c a t a l i t i c a c t i v i t y of i n t e r metallic compounds, t h e i r hydrides and complex systems, formed upon high temperature oxidation-reduction treatment,
2404
i n t h e hydrogenation o f - C d and C=O bonds has been established l a s t years /1-81. On t h e b a s i s o f data obtained /I/ t h e authors concluded t h a t IMC and t h e i r hydrides are t h e s p e c i f i c c l a s s of hydrogenation c a t a l y s t s . The hydredcs of IMC R E M with t h e i r unique sorptlve and hydrogenative properties have a t t r a c t e d our a t t e n t i o n a s the catalgnts f o r hydrogenation o f the-most i n t e r e s t natur a l substances petroleum asphaltenes. The p e c u l i a r i t i e s of asphaltenes l i e s i n t h e i r high molecular weight and large condensed polyaromatic r i n g systems and t r a d i t i o n a l catal y t i c poisons (S,N, V and Ni) i n i t s molecules. The struct u r e and composition of petroleum asphaltenes have been the subdect of many investigations using mostly methods of thermo- and hydrocrycking. The aim of t h i s study was t o investigate t h e c a t a l y t i c e f f e c t of IMC hydrides f o r t h e asphaltenes concentrate hydrogenation reaction i n the reaction conditions excluding the crycking of asphaltenes macromolecules.
-
2. EXPERIMENTAL
Hydrides with composition LaNi C O have been prepared bx s a t u r a t i o n k t 8 i n i t i a l a l l o y a t 20 C and 1 - 4 ma. The asphaltene concent r a t e obtained a f t e r deasphaltization of West Siberian o i l residiwn consisted of 70 % asphaltenes and 30 % of aromat i c s and resins. It has a molecular weight of 1434. The hydrogenation of asphaltenes Boncentrate was c a r r y out a t temperatures from 250 t o 400 C t pressure o f H 1-4 MPa, and reaction time 5-10 h. The products of hygrogenation were separated on l i q u i d (mtiltenes, soluble i n n-hexane) and s o l i d (insoluble i n n-hexane but soluble Maltenes were separate4 i on compounds i n benzene-)once by the method of l i q u i d chromatography. For a l l products the next values have been estimated8 molecular wei@ (by cryoscopy), H,C ,S content and IR-spectra.
.
3. RESWPS AND DISCUSSION I n the reaction conditions: temperature 380 OC pressur e of H 3 ma,reaction time .5 h 75 % yield of l i q u i d products (maltenes) was achieved. The sharp drop i n mol e c u l a r weight from 1434 t o 632 and 700 accordingly f o r maltenes and s o l i d s insoluble i n nLhexane, and considerable desulfurization from 2 82 t o 0.45 % in maltenes and 0.81 % j.n s o l i d s was regisiered i n products of hydrogenat i o n of asphaltenes concentrateo The s p e c t r a l data have showed t h a t maltenes consist of compounds with l o w aromaof compound vdtn h i aromaticity t i c i t y , and s o l i d p a r t ( not l e s s then 5 rings) and short alkyl cha ns.
-
-
-
P
2405
The r e s u l t s of investigations of t h i s t w o products by complex methods indicated t h a t they are very dJffered from l i q u i d and s o l i d products obtained by hydrogenation of asphaltenes concentrate over supported N i and Co catalysts. Some parameters of products asphaltenes concentrate hydrogenatton over hydrides and supported c a t a l y s t a r e given i n Table
.
Table Comparison of data obtained a f t e r asphaltenes concentrate hydrogenation over IMC hydrides and supported Co c a t a l y s t Products I n i t i a l asphaltenes concentrate Maltenes a f t e r hydrogenation: over
Supported Co/Si02 cat alys8 (T=380 C,P= I 0 MPa)
Molecular weight
S content,
H c, at.
I434
2,82
1 9 3
632
0,45
195
712
I,24
193
56
Even from t h i s data are indicated t h a t t h e i n t e r m e t a l l i c hydrides a r e more active i n hydrogenation of asphaltenes concentrate than supported C o c a t a l y s t : maltenes a r e more saturated by hydrogen (H/C= I , 5 ) , desulfurization degree ia above, molecular weigh f a lower. The r e s u l t s first obtained of this experiment a r e support e d the idea about exceptional bydrogenative properties of IMC hydrides stimulated by the presence of activated hydrogen i n t h e i r matrix. T h i s conclusion i s confirmed by absen; ce the light hydrocarbons, coke, wd other products of d i s t r i b u t i o n cracking of asphaltenes. The presence of i n t e r metallic hydrides contribute t h e reactions of hydrogenation, desulf urization and depolimeriaation and suppress the reaction of condensation. This results are opened the new p o s i b i l i t i e s t o get the new data about structure and reactions such complex compounds of o i l s as asphaltenes using I M hydrides as c a t a l y s t s f o r t h e i r hydrogenation. 4. CONCLUSIONS
Intermetallic hydrides w i t h composition LaNi Co were found effective f o r hydrogenation of as&a?!fdks
concentrate. Their liquid products yields, molecular wei& of liquid products, and S content in liquid proflucts compared favourably with the supported Co catalyst. They hainterve shown advantages which can be attributed using metallic hydrides for hydrogenation of as haltenes concentrate: high yield of liquid products Pmaltenes), sharp drop in uolecular weight and considerable desulfurization of products.
5. REZERENCES 1.R.Konenko; E.A.Starodubtseva, Yu.P.Stepanov, E.A.Fedorovskaya, A.A.Slinkin, E,I.Klabunovski and V,P.Mordovin, Kinetika and Kataliz, 27 (1986) 456, Starodubtseva E.V., I,R.Konenko, E.A.Fedorovskaya, Yu.P. Stepanov, A.A.Slinkin. E,I.Klabunovski and V.P.Mordovin, Kinetika ail Kataliz, 26 (1985) 340. E.V,Starodubtseva, I.R.Konenko, E.A.Fedorovskaya, E.I. Klabunovski, and V.P.Mordovin, Izv~ANSSSR. Ser.Klnim., (19861, 2960. J.Barrault, A.GtuTleminot , A,Percheron-Guegan, V.PaulBoncour and J.C.Achard, J.Less-Common Metals, I31 (1987) 425 K,N.Semenenko, L.A.Petrova and V.V.Burnasheva, J.Neorg. Khim., 28 (19833, 195. T.Shirotsuka, K,Onoe and A.Yo,.oyana, J.CheK.EnC.Jq., i9 (1986) 376. H.ILaamura Y.Kato and S.Tsuchiya, Z.Phys.Chem. (BRD), I41 (1984-)129. A.Shams1, W.E.Wallace, Ind.Ehg.Chem., Product Research and Development, 22 (1983) 583.
Gwzi, L. et al. (Editors), New Frontiers in Cafalysb
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
n-HEXANEISOMERIZATION AND AROMATIZATION ON THE CATALYSTS DERIVED FROM ALUMINA-SUPPORTEDPtSn CLUSTERS.
X Li, Y. Wei, J. Cheng and R. Li Department of Chemistry, Sichuan University, Chengdu 610064, China
INTRODUCTION : The homogeneous distribution of two metal components on the support surface is very important for high catalytic activities or selectivities of bimetallic catalysts. However, the conventional preparation method using metal salts as the catalyst precursors is difficult to obtain the homogeneity. A method has been tried to prepare the catalysts with high dispersion and homogeneous distribution of Pt and Sn by pyrolysis of well -defined Pt - Sn clusters. In this paper the comparative studies of the catalytic properties in n - hexane conversion and cumene hydrocracking using the catalysts derived from different Pt - Sn clusters and the catalysts prepared from H2PtCI6and SnClz are reported , and the effect of Sn/Pt ratio on the activity and selectivity is discussed.
EXPERIMENTAL Three Pt - Sn ClustersPtCl ( SnCL ( PPh, lZl ( Me4N l 2 CPtC1, (SnC13)z3and (Me4N>31Pt(SnC13)53-were synthesised as reportedI1 - 31. Seven catalysts (with 0. 5% of platinum) were prepared by impregnating r AIz03with the solutions of HZPtCl6and SnClz as well as Pt - Sn clusters, respectively. T h e precursors of the catalysts were I -HzPtCls, 1 -H2PtC16+ SnC1, lU -H2PtC1, -k 2SnClZ, lV -HzPtCls 5SnC12, V -PtCl ( SnC1,) (PPhj12, M-(Me,NIz CPtC1, (SnC1,)23 and W-(Me,N)3 CPt (SnC13)53 (hereafter referred to as the catalysts I -W 1. The catalysts were reduced at 500'C for 4h then their activities in n - hexane conversions and cumene hydrocracking were measured in a fixed bed microreactor in hydrogen stream at atmo-
+
.
This Project Supported by NSFC.
2408
spheric pressure. T h e compositions of the products were determined by GLC in a 3m X 3mm 0.d. column packed with 25% squalane on 6201 carrier. A T C detector was used, and peak areas were measured by a data processor for chromatography. The catalysts were characterized by Hz - O2 titration and XPS with XSAM 800 Electron Spectrometer.
RESULTS AND DISCUSSION The product distributions of n - hexane conversions on the catalysts were listed in Table 1. The data showed that the isomerization and aromatization activities of the catalyst V - W derived from P t - Sn clusters were much higher than that of the catalyst I -IV prepared from HzPtCl6and SnClz, moreover the change of the catalytic activities of II -lV with Sn/Pt ratios was opposite to that of V - W . Although tin was an effective promoter for enhancing platinum activities when the Sn/Pt ratio was lower than one, as shown in comparison I with II , tin would become an inhibitor of the activities for the catalysts prepared from H2PtC16and SnClzif the Sn/Pt ratio increased further, as shown by the catalyst 1 and N , whose activities were obviously lower than that of the In contrast with above - mentioned , the activities of V - W incatalyst I creased with Sn/Pt ratio increase. It was interest to compare the catalytic behaviours of lV with that of W in n - hexane conversions. The Sn/Pt ratio and platinum content in both catalysts were the same, but the conversion of n hexane over N was 3. 7 % and that 49. 3% overW at 510°C. T h e yields of the isomerization (2MP-t 3MP) and the aromatization (C6H6)over W were about 34 and 7 times greater than that over lV ,respectively. The similar results could be observed when compared 1 with VI .
.
Table 1
c6 product
distributions of n - hexane conversions (mol% )
Catalyst
~
Sample
530% 2MP n -C, +3MP MCP Ben
'
89. 4
34.0
2.9
4.5
58.6134.4
4.0
11.0
n -C,
3.2
2.8
3.390.7
87. 7
6. 9
3. 1
5. 1 84. 9
94. 7
0.7
1.1
0.697.6
96. 3
0.8
0.9
1.297.1
78. 0
5.2
2.0
5.087.8
52. 8 23. 3 50.7128.3
4. 7 4.7
14. 0 23.8
58.0 43.0
2MP-2 - methylpentane. 3MP-3 - methylpentane, MCP-methylcyclopentane, Ben-benzene ,n - C6-n - hexane.
2409
The catalytic activities in cumene hydrocracking reaction which was usually used to characterize the acidity of the catalysts, decreased in order of I > I[ > V > Iv W as listed in Table 2. The results showed that the introduction of tin into the catalysts suppressed the hydrocracking activities. The determination of XPS of the catalyst examples revealved that tin oxide formed on alumina surface without metal state of tin after the reduction of the catalyst 1-W at 500°C in hydrogen stream. The bonding energies of Sn 3d512and 3d312were 486. 7 - 487.0 ev and 495. 2 - 495. 5 ev , respectively. The formation of tin oxide would decrease the acidity of the catalyst thereby inhibited the hydrocracking reactions. The higher Sn/Pt ratio in the catalysts, the lower the acidity. Therefore, the high isomerization activities of V -W derived from Pt - Sn clusters were not due to acidity increase of the catalysts.
>
Table 2 Conversions of cumene in hydrocracking reaction (mol% 1. Reaction temperature, 'c
Catalyst sample
490
510
530
550
7. 9 5. 1
9. 8
11. 9
5. 8 5. 7
7. 7 7. 6 5. 2 1. 5
I
5. 6
I
4. 9
v
3. a 4.0 0. 9
VI
w
4. 6
3.9 1. 0
4. 0
1. 1
The data of H2 -02 titration at 25 - 500% for five catalysts were showed in Table 3. The results indicated that the H/Pi ratios of V -1were higher than that of I - II and the H/Pt ratios increased with increasing titration temperature. A maxium value of H/Pt ratios was observed at about 300 - 400'C , except the catalyst VII whose H/Pt ratio increased always with increasing titration temperature up to 500%. It was clear that hydrogen spillover on the surface of VI and VII occurred at high temperature, for example, H/Pt ratios for VI and VII were 1. 42 and 1. 6 1 at 400%, respectively. Therefore, the high dispersions of platinum on the catalyst surface were obtained. The excellent properties for activation and adsorption of hydrogen over VI and Vn were related with the high dispersions of platinum and were consistent with their high activities in n - hexane conversions. The large differences of the activities between two types of above - mentioned catalysts were connected with the structure of their original platinum
241 0
Table 3 H/Pt ratios measured at different Hz - 0, titration temperature Catalyst sample
titration temperature, 'C
25
100
200
300
400
500
I
0. 22
0. 24
0. 35
0. 43
0. 45
0. 4 3
I
0. 2 3
0. 25
0. 53
0. 59
0. 38
0. 29
V
0. 24
0. 36
0. 58
0. 87
0. 34
0. 33
VI
0. 27
0. 37
0. 67
0. 87
1 . 42
0. 89
w
0. 31
0. 53
0. 86
1. 46
1. 61
2. 64
compounds. The homogeneous distribution of Pt and Sn on the surface of the catalysts prepared from H2PtC16and SnClz was very difficult to be obtained. Platinum crystallite could be covered with the large aggregate of tin oxide if Sn/Pt ratio was high , thus the catalytic activities of platinum would decrease with Sn/Pt ratio increase as showed by the catalyst and N in Table 1. When Pt - Sn clusters were deposited on alumina the coordination bond between Pt and Sn could be maintained. Although the cluster structure was broken up after reduction at 500'C in hydrogen stream, the original structure of Pt - Sn clusters was favourable for the homogeneous distribution of Pt and Sn on alumina surface. We infer that tin oxide in the catalyst VI and W with high Sn/Pt ratios could form an array which seperated the small P t crystallite and prevented platinum atom from migrating on alumina surface. This would create a suitable microcircumstance in which tin oxide would play full role in modification for platinum catalytic properties and would promote P t crystallite activation at high temperature. The metal centers instead of acid centers on the surface of the catalyst VI and W could play the principal role in the isomerization of n - hexane. Therefore the both catalysts derived from Pt - Sn clusters exhibited excellent activities in the isomerization and aromatization of n - hexane althouhg their acidities were weaker.
REFERENCES 1. M. C. Baird, Inorg. Nucl. Chem. ,29 (1967) 367. 2. Rosario Pietropaolo, et al. , Inorg. Chem. , 8 , (1969) 1560. 3 . P. G. Antonov, et al. , J. Inorg. Chem. , (RUSS), 27, (1982) 3130.
Guczi, L ei al. (Editors), New Fronriers in Corolysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights nserved
THE EFFECI' OF RHODIUM PARTICLE SIZE ON n-BUTANE HYDROGENOLYSIS ACTIVITY AND SELECI'IVITY D. Kalakkad S.L.Anderson and A. EL Datye Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131, USA
Abstract Rh metal was supported on alumina and silica supports and the particle size was varied (from c 1 to >5 nm) by changing the metal loading from 0.15 to 7.2 pnoles of RWmZ of support. Metal particle sizes were determined by transmission electron microscopy. Hydrogenolysis of n-butane was used as a probe reaction. The ethane selectivity of the highly dispersed Rh was very different from that of the low index single crystal surfaces of Rh. As particle size was increased, the selectivity of the catalysts approached that of Rh (1 1l), and annealing the catalysts at 773 K led to better agreement between the supported Rh and single crystal Rh( 111). None of the catalysts after initial H2 reduction resembled the (100) or (1 10) surfaces in their hydrogenolysis selectivity. However, preoxidation of the Rh catalysts before reaction did cause the selectivity to approach that of the more open Rh single crystal surfaces. The results show that changes in surface structure caused by preoxidation of Rh do not resemble those caused by changing particle size. 1. INTRODUCTION The effect of particle size on catalytic activity has been a subject of much study in the literature. Much of the interest stems from the extensive compilation by Van Hardeveld and Hartog' who calculated the coordination of surface atoms in small metal particles as particle size was varied. For a cuboctahedron, they found that particles having less than 6 atoms along the edge (4.0 nm in the case of Rh) would have the majority of the surface atoms as corner and edge atoms. Only with particles larger than 4.0 nm in diameter would the atoms on the low index faces constitute the majority. One would expect therefore that as particle size in a supported metal catalyst is increased, the catalytic behavior should start to resemble that of the low index facets of the metal. Experimental verification of such behavior was provided by Engstrom et al.2 in their study of n-butane hydrogenolysis over single crystals of Ir. The hydrogenolysis selectivity of Ir (1 11) agreed with that reported previously by Foger & Anderson3 on large particles of Ir. On the other hand, the behavior of small Ir particles was modeled very well by the Ir (1 10) surface which is known to undergo a (110)-1x2 surface reconstruction. When a similar comparison between small particles and single crystals was attempted in the case of Rh, the agreement was not as good4. While the Rh( 111) surface was indeed a good model for the larger particles, the Rh( 110) surface did not model highly dispersed Rh catalysts. It was argued that the absence of a (1 10)-1x2 surface reconstruction may explain the inability of the (1 10) surface of Rh to model highly dispersed Rh particles4. However, since the environment of surface atoms varies with particle diameter*, there ought to be some intermediate particle diameter where the coordination of surface metal atoms would resemble that of the more open Rh( 110) or Rh( 100) surfaces. The objective of this study was to explore the effect of
241 2
particle size and determine the particle size range where the behavior of small particles agrees with that of specific low index facets of Rh . Hydrogenolysis of n-butane was used as a test reaction since it is known to be sensitive to particle size of Rh. The mechanism of the alkane hydrogenolysis reaction was studied by Kempling and Anderson5 who found that the product selectivity did not depend on conversion as long as conversions did not exceed 50 %. The effect of temperature on the hydrocarbon cleavage patterns is also well understood6, and while the cleavage of C-C bonds at low temperatures is selective and only a single C-C bond is broken during a turnover, at higher temperatures multiple hydrogenolysis occurs and non-selective formation of methane is observed. Therefore, hydrocarbon cleavage patterns at low temperatures and conversions can be used as a probe for the study of surface structure in small Rh metal particles. For instance, highly dispersed Rh preferentially cleaves the central bond in n-butane yielding ethane as the major product’. On the other hand, the product distribution on Rh (1 11) is more statistical yielding 50%ethane and equal amounts of methane and propane. Rh(ll0) and Rh (100) yield about 30 % ethane from n-butane in the single hydrogenolysis regime. Particle size has also been reported to affect alkane hydrogenolysis activity of Rh. Y a m and Sinfelt* observed that specific activity for ethane hydrogenolysis went through a maximum as the particle size was increased, the maximum occurring at ~ 1 . nm. 2 Particle sizes were deduced from chemisorption measurements and variations in particle size were introduced by increasing the metal loading from 0.1 wt% to 10 wt%. Such a maximum in specific activity was not seen by Yao et a19 when they studied n-pentane hydrogenolysis on alumina supported Rh. As the metal loading of the catalyst was increased, they found that the activity per mole of Rh remained constant up to a loading of 3 p o l e RWm2 of alumina surface area. The Rh was assumed to be highly dispersed when loading was kept below 3 pmoldm2 of support surface area, and was termed the ‘6’ phase. Metal loadings greater than this threshold value were believed to cause formation of Rh particles, and the specific activity declined markedly at higher metal loadings. In this study, we have used TEM to determine the metal loading at which particles begin to form on the support. 2. EXPERIMENTAL The catalysts were prepared by aqueous impregnation using RhC13.xH20 and Rh(N03)2 precursors with Cabosil HS-5 silica (300 m2/gm) and ALON C alumina (100 m2/gm) as the supports respectively. The preparation followed the procedures used previously by Yates and Sinfelt8 for W S i 0 2 and by Yao et al.9 for WA1203. . The catalyst weight loadings were similar to those used by these workers and ranged from 0.15 to 10 wt%. Table 1 provides a summary of the catalysts investigated in this study. The metal loading per m2 is based on the BET surface area of the support. The average particle sizes were determined using a JEOL 2000 FX transmission electron microscope (TEM). Hydrogenolysis reactivity of n-butane was measured in a quartz U-tube flow reactor with a flow of 20 sccm of H2 and 1 sccm n-butane. The total pressure in the reactor was 630 Torr. The catalysts were prereduced at 473 K in flowing H2 before use. For some experiments, the catalyst was preoxidized at 773 K in 10% 0 2 and used directly for reaction, this is referred as the ox-red state of the catalyst. The effect of high temperature reduction (HTR) at 773 K in H2 was also studied. Other experimental details have been reported previously.10
3. RESULTS and DISCUSSION As seen in Table 1, the average Rh metal particle size is greater on the silica support at comparable metal loadings. Metal particles could be clearly Seen on the 1.0 wt% RWsilica catalyst, though faint scattering centers = 1 rim in diameter were visible even on the 0.5 wt% RWsilica catalyst. In order to facilitate the detection of small metal particles on alumina, we have used a crystalline support (Degussa ALON C). On the 0.63 wt % Walumina catalyst, metal particles could be seen in a few areas, however they were unambiguously identified only on the
241 3
2.2 wt% catalyst. The loading of this catalyst, 2.2 p o l e Rh/m*, is below the saturation concentration reported previously for the dispersed '6' phase of Rh? We conclude therefore that the '6' phase did not form under our conditions. Fig. la shows the activity of these catalysts as a function of metal loading. The activity is expressed in terms of pmoles of n-butane converted per gm of catalyst per s. Activity increases linearly with loading on both supports, the differences between the silica and alumina catalysts are caused by the the different surface areas of these two supports. The levelling off in the activity versus loading curve is associated with the formation of larger metallic particles of Rh, whose lower surface to volume ratio causes a fall off in activity. &oxidation causes an increase in activity while HTR causes a decrease in activity as shown by the triangles on fig. la. These activity cycles are noticeable only on the higher loaded metal catalysts. We have shown previously that the activity increase is caused by a 'surface roughening'. The increase in volume if the metal particle upon oxidation, and the inability to restructure into a single crystal when Educed at low temperatures leaves behind a 'roughened' surface." Similar activity changes were recently seen also on single crystals of Rh.12 Table 1
~~
10.0 I 3.3 I 5.5 I 7.2 d. no particles detected :) faint scattering centers = lnm in diameter were seen.
.1
1
10
Loading pmole/sq.m.
.1
I
7.2
1
I
3.0
10
Loading pmole/sq.m.
:. 1 Activity and Selectivity of the supported Rh catalysts at 448 K (H2:n-butane 201).
1
241 4
The selectivity for ethane formation from n-butane is shown in fig. lb. All data are reported at 448 K where cleavage of a single C-C bond occurs during hydrogenolysis. Our results show that highly dispersed Rh preferentially cleaves the central bond in n-butane leading to selective formation of ethane. This is true on both the silica and alumina supports. As metal loading is increased, there is a drop in ethane selectivity. The drop in ethane selectivity on silicasupported Rh occurs when particle size exceeds 4 nm, but on alumina, the ethane selectivity drops even though the particle size is below 4 nm. At the highest metal loadings, the selectivity approaches but does not quite match that of the single crystal Rh( 111) surface. Annealing at high temperatures in H2 does cause the selectivity to match that of Rh( 111). Such treatments are commonly used in studies of particle size effects when larger particles are desired. Since the (1 1 1) close packed surface would be favored during particle growth, it is understandable that these high temperature treatments lead to better agreement between the behavior of small particles and single crystals. Closer examination of previous work on Ir2 also shows that the large particles of Ir whose behavior matched that of Ir( 1 1 l ) 3 were obtained by treatment of the catalyst at elevated temperatures. We suspect that such thermal treatments may cause the formation of faceted particles having annealed close packed surfaces. Oxidation-reduction cycling has a pronounced effect on the product distribution, but only on the catalysts having the larger metal particles. The selectivity after preoxidation is closer to that of Rh (1 10) or Rh (100). Such changes in selectivity due to preoxidation are not seen on the catalysts having smaller metal particles where particle size may dictate surface structure. CONCLUSIONS The TEM evidence shows that Rh forms small particles on both supports. Our data does not provide any evidence for the formation of a disperse Rh phase. As particle size is increased, the hydrogenolysis selectivity of Rh approaches but does not quite match that of Rh (1 11). Annealing in H2 at high temperatures improves the agreement between the small particles and the single crystal. None of the catalysts after standard pretreatment (H2 reduction at 473 K) show a selectivity similar to the more open Rh( 100) or Rh(ll0) surfaces. Oxidative restructuring of the Rh particles causes the selectivity to approach that of the open surfaces of Rh such as (100) and (1 10). Selectivity changes induced by oxidation-reduction cycling are, however, quite different from those caused by a decrease in Rh metal particle. Acknowledgements Financial support for this work was provided by the American Chemical Society Petroleum Research Fund grant 23995-AC5-C. Electron microscopy was performed at the microbeam analysis facility within the Department of Geology at the University of New Mexico. REFERENCES l R . V. Hardeveld and F. Hartog, Surf. Sci. 15 (1969) 189.
*J. R. Engstrom, D. W. Goodman,and W. H. Weinberg, J. Am. Chem. Soc. 108 (1986) 4653. 3K. Foger and J. R. Anderson, J. C a d . 59 (1979) 325. 4A. K. Datye, B. F. Hegarty & D. W. Goodman, Faraday DiscussChem. Soc.87 (1989) 337. 5J. C. Kempling and R. B. Anderson, Ind. Eng. Chem. Prod. Res. Dev. 9 (1970) 116. 6J. R. Engstrom. D. W. Goodman and W. H. Weinberg, J. Am. Chem. Soc. 110 (1988) 8305. 7T. C. Wong. L. C. Chang, G. L. Hailer, J. A. Oliver, N. R. Scaife and C. Kemball, J. C a d . 87 (1984) 389. *D. J. C. Yates and J. H. Sinfelk I. C a d . 8 (1967) 348.
". C. Yao, Y.-F. Yu Yao and K. Otto, J. Catal. 56 (1979) 21. 1°E. J. Braunschweig, A. D. Logan, S. Chakraborti and A. K. Datye, Proc. 9th Inll. Congr. Catal. (M.J. Phillips and M. Ternan. eds.) The Chemical Institute of Canada, Ottawa. 1988, vol3, p 1122. l l S . Chakraborti. A. K. Datye and N. J. Long, J. C a d . 108 (1987) 444. I2A. D. Logan, K. Sharoudi and A. K. Datye, J. Phys. Chem. 95 (1991) 5568.
Guai, L d al. (Editors),New Frontiers in Catalysis Proceedings of the 10th International Congrcss on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elscvicr Science Publishem B.V.All rights reserved
PRETREATMENT EFFECTS ON ACTIVE STATE AND AROMATIZATION ACTIVITY OF GdZSM-5 CATALYSTS
K M. DooleYq C. Chanf, V. Kanatirevb and G. L. PricC aDepartment o f Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803, USA bInstituteof Organic Chemistry, Bulgarian Academy of Sciences, 11 13 Sofia, Bulgaria
1.
INTRODUCTION
Recent work on Ga-containing ZSM-5 has shown that H,-reduced Ga,O, /ZSM-5 mixtures a r e active and selective for propane aromatization [ 1-31. Extraframework Ga in ZSM-5 enhances rates of olefin and diene dehydrogenation [4,5] ; Ga,O, or steamed gallosilicates aromatize olefins and dienes a t 5006OOOC [ 5 , 6 ] . The dispersion and oxidation state of Ga affect relative rates of dehydrogenation and hydrocracking. The aims of this work were to show that Ga-exchanged materials fall into our theory on G a reduction and t h a t , although reduction disperses Ga, the form of the catalyst best for aromatics production does not consist of Ga+' cations. 2.
RESULTS AND DISCUSSION
The HZSM-5 (UOP MFI, SiO,/Al,O, = 40) was ball-milled with f3-GaZO, to produce physically mixed Ga/HMFI (5.0 wt%Ga) The second material (GaMFI) was.prepared by two ion exchanges of 5 equiv. Ga(NO,), with HMFI, with heating to 5OO0C after exchanges. An exchanged/physically mixed material (Ga/GaMFI) was prepared b y ball-milling GaMFI with 3.1 wt% Ga as B-Ga,O,. A TGA was used to determine the H, reduction potentials. After drying in N, at 65OoC, the sample was cooled to the desired T in N, and the flow switched to A DSC was used to follow the redox behavior of dried Ga/HMFI 20-50%H,/N,. b y heating in 30%H,/N, (575'C), then in N,-( 55OoC), then in air ( 200-560'C). For catalytic activity measurements, 0.8 g of 20-40 mesh particles were loaded in a tubular reactor and reduced in 30%H z / H e a s follows: l0O'C for 2 h , 100'C to 575'C (l'C/min), 575'C for 2 h . For reactions of propane (WHSV= 1.3) or ethane (WHSV = 1.01, the feed was 123, paraffin in He a t 1 b a r . Calcined GaMFI contained 2.22 wt%Ga by XRF, and Ga/GaMFI 5.17%. The Si/Al ratios of these and the starting material w e r e identical. The X R D patterns in the range 2 0 24-36' can be used to determine roughly ( + 0.2 wt% Ga) the amounts of Ga a s R-Ga,O, [ 2 ] . According to this technique, GaMFI contained 0.5 wt% Ga as B-Ga,O, and Ga/GaMFI 3.4% (3.6% expected assuming 3.1%added B-Ga,O, plus 0.5%from calcination of GaMFI) . The maximum loading of exchanged Ga in GaMFI was therefore 1 . 7 wt%. Assuming 1 . 7 wt% exchanged Ga+3 in GaMFI (with remaining Ga as Ga,O,) , complete reduction of this material with loss of water should result in a 0.10% weight decrease assuming the stoichiometry (where Z - is the anionic zeolite framework) :
.
241 6 GaCn(Z-)nt H, Ga,O, + 2 H, + 2 H'Z-
---> --->
Ga'Z- + 2 H'Z2 Ga'Z- -t 3 H,O
Reaction ( 2 ) is the previously determined reduction for R-Ga,O,/ZSh1-5 mixtures [ 2 ] ; it is applicable to Ga,O, a s long a s Z - a r e available. If the exchanging ion is [ Ga(OH),]" , the following reactions should occur instead of ( 1 ) : [Ga(OH),]'Z(GaO)'Z- + H,
--->
(GaO)'Z- f H,O Ga'Z- t H,O
--->
(3) (4)
Reactions ( 2 ) - ( 4 ) should give a 1.01% weight loss for GaMFI upon reduction; however, if the drying process completed ( 3 ) , then the loss would be only 0.58%. The final loss upon reduction a t 62OoC was 0.96?0.12% (Fig. l ) , suggesting that both [ Ga(OH),]'Z- and (Ga0)'Z- were present after drying. For Ga/GaMFI, according to ( 2 ) - ( 4 ) , Z- limits the reaction, and w e compute an expected weight the loss of l.61%, in fair agreement with loss observed (1.43+0.06%,Fig. 1 ) . Reduction curves for Ga/HMFI a t less than 6OO0C could only be successfully correlated by the stoichiometry of reaction ( 2 ) [ 2 ] . The product of the reductions of Ga/HMFI and Ga/GaMFI according to o u r theory was the same, and the Ga contents were also similar, 5.0% (Ga/HMFI) and 5.2% (Ga/GaMFI). Therefore we expect the materials to have similar catalytic properties. Direct comparison of activity is difficult because the materials continue to undergo activation even in H e , but the propane aromatization results in Table 1 demonstrate that activity and selectivity for Ga/GaMFI (Run 1) fell roughly between Runs 3 and 4 for Ga/HMFI. Therefore reduction, X R D and reaction results suggest that ion-exchanged materials underwent reduction of Ga, as did physically mixed materials. However, the reaction studies suggest that the as reduced form was not optimal. The differences between r u n s 4 and 5 in Table 1 merited f u r t h e r s t u d y , The because it appeared a s if He treatment activated the catalysts. performance of Ga/ HMFI for ethane aromatization was therefore examined following various pretreatments A-D carried out after the normal reduction. For sample A there was a 3 O O O C purge in He; sample B w a s contacted with air ( 5 h , 550'C); sample C with H e ( 5 h , 55OOC); and for D the same treatment was used as for B , but followed by a second ttnormal" reduction. The activity and selectivity data are in Table 2. The activity and aromatics selectivity for this series followed the order B>C>A>D. This order can be explained a s follows : the first reduction (sample A ) dispersed some B-Ga,O, in the zeolite b y converting it to Ga,O and then to Ga' by reaction ( 2 ) . Reoxidation (sample B ) then converted the reduced framework Ga' to dispersed, amorphous Ga,O, , GaO' cations, o r a mixture of the two. The oxidation also took place with trace 0, and H,O in the He (sample C ) , although not as effectively a s with air (sample B ) . Then Ga"' was re-reduced (sample D) to give a material of low activity and selectivity; more Ga' must have been present after the second reduction. The dispersion of Ga in the zeolite has been confirmed b y observation upon reduction of an almost ten-fold increase in the Ga(3d) line intensity in XPS. TEM and EDAX studies have shown that B-Ga,O, crystals disappear upon reduction with simultaneous transfer of Ga to within zeolite particles, accompanied by diminution of the framework -OH band intensity [ 71. The low amounts of methane obtained upon treatment D also suggest Ga' zeolitic cations a r e present. For Ga/ZSM-5 materials, methane formation is associated with framework Bronsted acid sites [ 5 , 6 ] . However, aromatization of the lower paraffins also requires the presence of a t least some Bronsted sites [ 4-61. By
241 7
Run'
5
HOP,
HOH"
12
36
c,
Selectivity (carbon %'>'
7.9
% Conv.
Arom.
C,
17.7
67.5
20.8
First run Ga/GaMFI, others GalHMFI Total hours of propane feed at reaction conditions Hours of He treatment a t 5 3 O O C after initial activation
% Conv.
Sample
Figure 1
I
Selectivity (carbon%) Cl Arom
Figure 2
.
241 8 X R D the wt%Ga a s R-Ga,O, was determined; fresh Ga/HMFI-A contained 2.5%, fresh Ga/HMFI-B 2.2%, and used B a n d C 2.1-2.4%. Therefore initial reduction was incomplete, reoxidation did not regenerate crystalline R-Ga,O,, and the dispersed Ga created by initial reduction was not augmented b y on-line reduction. Pretreatment C was duplicated in the TGA (Fig. 2 ) . Reoxidation of GalHMFI was possible using trace amounts of 0, (here -100 ppm); 75%of the weight loss due to reduction was regained upon reoxidation for 5 h . Prolonged treatment resulted in 9424% weight regain. Ga/GaMFI could also be almost completely (9724%) reoxidized a t 55OoC, but only 70% of t h e Ga was rapidly reoxidized (Fig. 1). Re-reduction of reoxidized material (simulating sample D , Table 2 ) gave the same weight loss intitially, 1.44f0.09% (compare 1.78%for complete reductionaccording to reaction ( 2 ) ) ; complete stoichiometricreduction was found to be possible only a t higher T (620OC). The catalysts used in the ethane experiments (Table 2 ) should therefore have been only partly reduced b y the "normal" pretreatment, in agreement with the XRD results. The redox cycle of sample B was also reproduced in the DSC; the heat of the endothermic transition was 58 m J / m g . For complete reduction of R-Ga,O, to Ga,O, the heat would be 73 m J / m g for this sample. The actual heat should be < 73 m J / m g , because the second step of the overall reduction ( 2 ) is: Ga,O t 2 H'Z-
--->
2 Ga'Z- t 2 H,O
(5)
This step is exothermic, using either gas phase or aqueous heats of formation for H' and Ga'. When aqueous values a r e used, the computed heat of reaction ( 2 ) is 49 mJ/riig. It is unlikely that (Ga0)'Zis the ultimate product of reoxidation, because the weight regains are too high; a 64% regain would correspond to formation of (Ga0)'Z- in GalHMFI, and 57-73% in Ga/GaMFI, while 94+4 and 9724% regains were observed, respectively. It is possible that the rapid weight regains in the f i r s t few hours were associated with GaO'Zformation. So the highly active and selective Ga/MFI-B probably included both (GaO) framework cations and extra-framework Ga,O,. +
REFERENCES
1. 2.
3. 4.
5. 6.
7.
V . Kanazirev, G . L . Price, and K . M . Dooley, J . Chem. SOC. Chem. Conim. (1990) 712. G.L. Price and V . Kanazirev, J . Catal., 126 (1990) 267. G.L. Price and V . Kanazirev, J. Molec. Catal., 66, (1991) 115. T . Changyu, C . Chengri, 2. Lixin and P. Shaoyi, in M.J. Philips and ill. Ternan ( e d s . ) , Proc. 9th Int. Cong. Catal., Chemical Institute of Canada, 1988, p.445. P. Meriadeau, G . Sapaly and C . Naccache, in T . Inui et al. ( e d s . ) , Chemistry of Microporous Materials, Elsevier, Amsterdam, 1991. C . R . Bayense, A.J.H.P. van der Pol, and J.H.C. van Hooff, Appl. Catal., 72 (1991) 81. V. Kanazirev, G .L. .Price and K . M . Dooley, in P.A. Jacobs et al. ( e d s , ), Zeolite Chemistry and Catalysis, Elsevier, Amsterdam, 1991, p.277.
Guni, L et al. (Editors), New Fronriers k Caialpis P m d i n g s of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 6 1993 Elsevier Science Publishers B.V. All rights mewed
NEW MODIFICATION METHOD OF Pt/L ZEOLITE CATALYST FOR HEXANES AROMATIZATION
H.Katsuno, T. Fukunaga and M.Sughoto Central Research Laboratories, Idemitsu Kosan Co.,Ltd., 1280 Kamiizumi, Sodegaura, Chiba 299-02, Japan (As a member of Research Association for Utilization of Light Oil [RAULO], Japan.)
Abstract
The treatment of KL zeolite with halocarbons prior to its impregnation with Pt(NH3) C12 improves the activity and selectivity The treatment is also effecfor the aromatization of hexanes. tive for the stability of the catalyst. The halocarbon treated catalyst shows a unique advance in the industrial production of aromatics from hexanes.
.
1 INTRODUCTION
Conventional reforming catalysts have poor activities and selectivities for the aromatization of hexanes and heptanes. Bernard reported that platinum supported KL zeolite is much more active and selective than conventional Pt/A1203 reforming catalyst for the aromatization of n-hexane(1). It has been also demonstrated that acidic sites decrease the selectivity and stability of the Pt/L zeolite for the hexane aromatization (1)(2). In this paper we report that the treatment of KL zeolite with halocarbons prior to its impregnation with Pt(NH3)4C12 improves the activity and selectivity for the hexanes aromatization. The treatment is also effective for the stability of the catalyst. The chemical , physical and catalytic properties of the treated catalysts (Pt/FKL) are evaluated by the application of a variety of techniques. 2.EXPERIMENTAL
Catalyst Preparation KL zeolites used in this study were obtained from Tosoh corporation. Halocarbon treatment was carried out in a fixed bed quartz reactor at 500°C for 2 hours. CF C1, (CF2C1I2, CFC13 were used as a halocarbon,respectively. PZatinum was supported on the halocarbon treated KL zeolite (FKL) by incipient wetness impregnation with an aqueous solution of Pt(NH3I4Cl2. The
2420
impregnated zeolites were dried at 80°C for 3 hours , and then calcined at 300 "C for 1 hour. Surface Property The behavior of surface OH groups was examined by FT-IR spectroscopy after the evacuation of KL zeolites at 500 'C, for 1 hour. Electronic State of Platinum The electronic state of platinum has been studied by means of CO adsorption and infrared spectroscopy. The catalysts prereduced under flowing H were reduced in the infrared cell at 540"C, for 1 hour and &en evacuated at same temperature for 1 hour. CO gas was introduced with 3 torr at room temperature for 30 minutes. The spectra were recorded after evacuation for 1 hour. Catalytic Measurement The aromatization reactions of hexanes were carried out in a continuous flow micro reactor. The catalyst was activated under H2 flow before introducing hexanes. 3.RESULTS AND DISCUSSION
Chemical and Physical Properties of the Zeolites Table 1 lists the BET surface area and the composition of the FKL and KL zeolites. The BET surface area of FKL zeolites are lower than that of KL zeolite. Fig.1 shows X-ray diffraction patterns of KL zeolite and FKL zeolite( CF3C1 and CFC13). CF3C1 treated zeolite retained its crystalinity The decrease of intensity and the presence of AlF3 (29=25.3')in the X-ray spectram of CF3C1 treated zeolite show that some aluminum is removed from the zeolite framework. It is suggested that AlF3 formed by halocarbon treatment plugged a part of channel of the zeolite. Fig. 2 shows IR spectra of KL zeolite and FKL zeolite treate with CF3C1. After the CF3C1 treatment , the band at 3745 cm
.
-9
Table 1 . Chemical Composition and Surface Area of KL and FKL zeolite. Sample Composition Halogen (Molar Ratio) Cont.(wt%) BET Area F c1 (m2/g) K20 A1203 S i O 2 KL FKL(CF Cl) FKL((C$ C1)2) FKL(CFC~~)
1.09 1.06 1.06 1.00
1.00 1.00 1.00 1.00
5.99 5.91 5.96 5.97
0.0 0.5 0.4 6.7
0.0 0.5
195 110
0.3
133
6.4
19
20 25 30 35 26 Figure 1 . X-ray Diffraction Spectra of FKL and KL Zeolites 5
10
15
assigned to terminal OH was considerably decreased,suggesting that the terminal OH groups were replaced with €7 or C1 (3)(4). Transmission electron micrographs of Pt/KL and Pt/FKL(oCF3C1) catalysts showed that platinum particles on $F3Cl (70), the diesel fuel produced via this route is low in both aromatics and sulfur. An important consideration in the design of a commercial reactor for such operation is the apparent activation energy, ,,E of the dominant heat-release reaction, which is cracking. Our own in-house studies have shown that the value of E, for n-paraffin hydrocracking in the presence of basic-nitrogen poisons can be unusually high, being on the order of 200 kcal/gmole. Such a high,,E has significant implications for the control of commercial reactors. What follows is a simplified model development for n-paraffin cracking that serves to explain how such poisoned reaction pathways can lead to such high E, values.
2. MODEL DEVELOPMENT The primary reaction network in n-paraffin hydrocracking is the conversion of n-paraffins to the corresponding i-paraffins, which then crack. Extensive studies of n-paraffin (C+-C17)
hydroisomerization over PtNS-Y zeolite [2,3] have lead to the following proposed mechanism: n-paraffin
monebranched isoparaffin
+multi-branched + cracked isoparmin
products
The normal paraffin must convert to at least a dimethyl isomer before cracking can occur due to the nature of the carbenium-ion mechanism involved in catalytic acid cracking. While these studies did not address the issue of acid-site poisoning, studies in our own laboratory have shown that such poisoning (with organically-bound nifrogen) increases the temperature requirements and activation energies of the individual reaction steps for nhexadecane (n-Cl& hydroconversion over a WUS-Y catalyst.
2424
Using the reaction pathway proposed in [2,3] (reverse reactions were assumed insignificant) together with results from our own studies, a simple model was written for the hydroiosmerization of an n-paraffin. The reaction steps were assumed to be first-order in the reactant and to have individual activation energies of 35 kcal/gmol. Organically-bound nitrogen was assumed to both strongly adsorb (heat of adsorption equal to 25 kcal/gmol) and react, with the conversion product, NH,, having low surface coverage due to both a relatively low adsorption constant and a low liquid-phase concentration. A fourth-order Runga-Kutta program was written to determine for a set space-time and temperature the relative concentrations of the n- and i-paraffins and cracked products. The temperature was then varied at constant space-time to determine the temperature sensitivity of, in particular, cracked product formation. The apparent activation energy of n-paraffin cracking was then determined by assuming that cracked products were formed from the fiistorder reaction of total paraffins (normal- plus iso-).
3. RESULTS AND DISCUSSION Shown in Table 1 are the apparent activation energies for cracked product (D) formation, denoted as E,,,,,,, for a range of conditions. The conditions were chosen to show how varying degrees of basic-nitrogen poisoning, varying isomerization selectivities, and a combination of both all affect the calculated Erpp,Dfor a feed of 100% n-paraffin (A). Table 1 Predicted Apparent Activation Energies for Cracked Product Formation (Eapp,D, kc al/gmole).
kl k2 > BAn-paraffin mono-branched paraffin
Feed = 100% A A->D A->B>D
A -> B->
C-> D
kz/k2
k&,
c-
>
multi-branched paraffin
k3
>
D cracked products
D e m e of Poisoning Poisoning NoPoisoning NoRxn
Poisoning W/Rxn
m
m
35
60
78
.1 1 10
m
97
146
OD
41 59 45
.1 1 1
1 1 .1
54 69 54
124
200
m
2425 Starting in the upper left-hand comer of the table, by setting both and k3 very high, the main reaction simplifies to A-->D; thus, E,pP,Dis equal to EaPPfor non-poisoned A conversion, 35 kcaVgmole. When this system is poisoned by basic nitrogen, E,,D increases significantly. For the case where the nitrogen is non-reactive (second column), EaPP,D increases from 35 to 60 kcaUgmole, while for the case where the nitrogen reacts, E, ,D increases further to 78 kcaVgmo1. The physical interpretation of this phenomenon is tiat since both the adsorption constant and concentration of the basic-nitrogen compound decrease with increasing temperature, due to the exothermic heat of adsorption in the first case and to the HDN reaction in the second, and since the reactant conversion rate is inversely proportional to the product of the two, then the sensitivity of the conversion rate to temperature, as reflected by E,,,, can be quite high. As the intermediate species become more significant in the non-poisoned reaction scheme, i.e., as k2 and k3 move towards k,, E increases by close to a factor of two. This is shown in the first column. For the case ofr->B-->D (kd, = 1.0, kfil = ap), E,g is 59 kcaVgmole, while for the case of A-->B-->C-->Dwith all three rate constants equal, E,,D is 69 kcdgmol. The predictions of such high values of En, ,D for non-poisoned feeds have been confirmed by analysis of numerous n-paraffin hydkracking studies published in the open literature, such as those by Weitkamp [4]. Typically in such studies, only the activation energies for the individual reaction steps are calculated and reported. However, by combining the published activity and selectivity data, it is possible to calculate the apparent activation energy for cracked product formation. In this way, E,mD'~ ranging from 60 to 90 kcaVgmol w m calculated from Weitkamp's data. The physical interpretation of the phenomenon that leads to the high temperature sensitivity of the cracked product formation for non-poisoned feeds is as follows. For a normal first-order A-->D reaction at a set space time, the concentration of the reactant, A, decreases with increasing temperature, thus countering the increase in the reaction rate constant and effectively "quenching" the rate of D formation. However, for an A-->B-->C->D reaction, the concentration of the reactant, in this case C, increases with increasing temperature. The increase in rate constant and reactant concentration make the rate of D formation especially temperature sensitive. Thus, as indicated above, both poisoning and the inherent reaction chemistry of n-paraffin hydroisomerizationcan independently lead to high values of By combining the two effects, even higher E,,D values are predicted. As shown in the table, for the case where kl=k2=k3and where N-poison both strongly adsorbs and reacts, E,,D values on the order to 200 kcaVgmole are predicted. Such an extremely high prediction was confirmed in our studies of n-hexadecane conversion over Pt/US-Y in the presence of catalyst poisons. As predicted, the poisoning caused an observed increase in EaPqfor the conversion of the nparaffin from 35 to 92 kcaVgmole (similar case to that in top nght comer of table), and also caused an increase in EaPPfor the rate of cracked product formation from 60 to -240 kcdgmole.
2426
4. CONCLUSIONS AND IMPLICATIONS The predominant chemistry in n-paraffin hydrocracking is unique in that at least two intermediates of significant concentration must form before cracking occurs. Accordingly, apparent activation energies for the formation of cracked products are high, being on the order of 60-70 kcaugmol for non-poisoned feeds. Poisoning of the system by, for example, organically-bound nitrogen, in which the poison both strongly adsorbs and reacts, increases this apparent activation energy even further. The combination of these two phenomena can lead to apparent activation energies of -200 kcal/gmol for cracking of poisoned nhexadecane feeds. A high apparent activation energy has significant commercial implications, since this variable is a measure of the temperature sensitivity of the main heat-release reaction, i.e., cracking, and high values can cause control problems with reactor temperature and efficiency. Naturally, reductions in the apparent activation energy, such as by removing feed poisons, are desirable, but as discussed above, such reductions are limited since the fundamental process chemistry cannot be altered.
5. ACKNOWLEDGEMENTS We would like to acknowledge B. J. Gallagher for her contribution to the experimental studies. We would also like to thank the following people for their helpful discussions: G.G.Karsner, R.B. Lapierre, and S.J. McGovern. Finally, thanks to M.R. Apelian for providing help with the computer programming. 6. REFERENCES 1) J. Eilers, S. A. Posthuma, and S. T. Sie, "Shell Middle Distillate Synthesis (SMDS) Process," presented at the 1990 Spring National AIChE Meeting in Orlando. 2) G. F. Froment, Cat. Today, 1.455 (1987). 3) J. A. Martens, P. A. Jacobs, and J. Weitkamp, Aml. Cat., 20,239 (1986). 4) Weitkamp, J., Ind. Enn. Chem. Prod. Res. Dev., 2l. 550 (1982).
Guni, L. cf al. (Editors), New Frontiers in C~rafysis
Proceedings of the 10th International Congrcss on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
EFFECT OF MODIFICATION OF THE ALUMO-PLATINUM REFORMING CATALYST WITH Dy, Cr, Ba AND NYTROGEN ON ITS CATALYTIC AND PHYSICO-CHEMICALPROPERTIES G. M.Sen'kov, E. A. Skrigan, E. A. Paukshtis, M. F. Gorbatsevich, A. M. Nikitina and E. N. Ermolenko Institute of Physico-Organic Chemistry, Belorussian Academy of Sciences, Surganov Str. 13, Minsk 220073,Russia
Abet ract The main means in increasing the efficiency of the alumoplatinum reforming catalyst is its modification with different elements. The theoretical principles of APC promotion are developing [?I. The literature of the last years, including our works, stresses a dete minative role of the electron Pt state (electron-ex essive Pt5-, in particular along with electron deficient Pti+ or ultndispersed platinumf in cyclization activity of modified APC 11-31. It was proved quantitatively that active sites of hydrocarbon dehydrogenation contain the null-valent platinum (Pto)[2]. The influence of Dy, Cr, Ba additions into APC on its stabilsty to the poisoning effect of nitro-organic compounds in the process of reforming practically has not been studied. There are only single works dealing with the effect of nitrogen on Pt in APC without allo wance for simultaneous influence on acidic surface sites [4]. The work was aimed at investigation of catalytic APC properties after modification with Dy, Cr, Ba and poisoning with n& trogen in dehydrogenation of cyclohexane, dehydrocyclization of n-octane, benzine fraction reforming at 65-102OC, i n v o l a physico-chemical properties, such as electron state of Pt and promoter, strength and concentration of Lewis acidic surface sites using the methods of CO-IRS, ESDR, TPD of reagents and reaction products with registration by a mass-spectrometer. 1. EXPERINENTAL
The catalysts were prepared by impregnation El). Upon investigation of catalytic activity and TPD of cyclohexane with poison, the poisoning of catalysts with nitrogen was carried out by introduction of piridine into a reactant. To take the CO IR-spectra, the catalyst was poisoned with NH3 by the method described in [ 6 ] . To take the ESDR spectra, the sample^ were treated with a mixture of hydrogen and ammonia in a flow reactor at 100-5OO0C.
2428 2. RESULTS
AND DISCUSSION
In cyclohexane dehydrogenation the APC modification with Dy, Cr, Ba has 8 little effect on its stability to nitrogen: at low nitrogen concentrations in the feedstock (0.001-0.1 wt.% N) the additions provide the APC stability to poisoning, while at 1 wt.% N all the catalysts decrease their activity tending to higher APC stability, especially with increasing quantity of a promoter from 0.2 to 0.6-1.0 wt.% In conversion of n-octane at atmospheric H pressure upon 0.01 wt.96-nitrogen poieoning, the APC modific&ion with Dy, Cr, Ba does not charge, however, at 0.1 wt.$ somewhat decreases the n-octane conversion degree, aromatic hydrocarbon yield, aromatization selectivity, though the promoting effect of an addition and dependence on its concentration manifest themselve. In this aase the hydrocracking and isomerization seleotivity, C to C -dehydrocyclisation(DHC)ratlo decrease 8s com pared to %he A d control (with a 0.2-1.0 wt.96 Cr addition and 0.01 wt.% N poisoning the isomerieation degree is almost unaffected). The beet charaoteristlcs were obtained with APC+Cr. An Increase of the piridine concentration in n-ootane ( l w t , % B) markedly deareases all the data of the process. However, the modified samples have a higher aromatization activity and n-octane conversion degree in comparison with APC. For allthe catalysts under Investigation upon poisoning with nitrogen, the o-ylol/ethylbenzene ratio decreases in the n-octane conversion catalysate evidencing the amplification of electrondonor properties of Pt-sites. The tests on n-octane conversion under 0.6 MPa of H and at minimum piridine aoncentration in the feedstock (0.801 wt. f% N) showed (Table 1) a marked effect of the addition nature in APC on its nitrogen stability. A poison addition into n-octane decreases the APC and APC+Ba activity on which low feedstook conversion, high catalysate and low gas yield were observed. The aromatieation activity with APC+Cr does not increase but selectivity on the latter is highest among the catalysts tested while isomerization selectivity is not high. The best results were obtained on APC+Dy, involving also refomhg of the straight-line benzine fraction. The sample has a high aromatizing activity and selectivity that considerably increase after introduction of 0.001 wt.% N into n-octane with simultaneous increase in the n-octane conversion. The isomerization seleativity decreases with the catalysate and gas yields being unaffected (Table 1). A study has been made of the effect of the nature of N-organic compounds on the activity of modified APC. According to the APC deactivation degree, they represent the following seriee: piridine 7benzilamine >diethylamine. By use of the adsorbed CO IRS method it was shown that at modification of APC with Dy, Cr, Ba and BH poisoning the electron density shifts from a addition ta Pt to form the electron-excessive platinum Pt%- (Fig. 1) The treatment of APC+Cr with NH3 leads to a decrease in
2429
Table 1 Conversion of n-octane on modified APC under hy$ogen pressure. P=0.6MPa; T=5OO0C; H2/n-octane=3tl; Vp1.5 h ; 0.001wt.N Catalysts
Characteristics
~
APC+O. 6% Dy UC+l .OR Cr
0.4% Pt
Catalysate comp.,wt.%: isoparaffines n-paraffines aromatic Conversion C -DHC/C -DHC ratio* Agomatidtion select. Isomerization select.
*
24.4
31.9 18.7 80.6
15.8 40.2 13.1 60.7
1.0
23.2 30.3
21.6 26.0
20.0
20.3 19.6
30.6 25.7 80.8 0.8
36.5 89.6 0.8
24.3
40.2 22.6
31.8
20.9
18.2
35.6
46.4
26.1
19.2 62.0 0.6 31.0 29.3
7.17 0.6
36.4 9.1
m-xylol+p-xylol/o-xylol+ethylbenzene ratio
the quantity of Pt6+ sites evidenced earlier by the CO IRS of APC+Cr 111. -Thisis associated probably with a deeper reduction of chromium to CrO in A P C , a donor of electronsp I?& and, correspondingly, deerease in the quantity of Cr an acceptor of electrons by P t Cl] confirmed by the ESDH method. The donation degree o f electrons on Pt from the promoters under investigation is lower than from NH and decreases in the series: NH3 > Ba > Dy > Cr. The quantitdof chemosorbed NH has been found to decrease with introduction o f additions add gives a series: APC + Dy(APC + Cr<APC + Ba> para > me&-isomer. On the other hand, the rate for RhCI(PPh3, varied within about 13%. We have also studied the regioselectivity of these hectorite-supported rhodium complexes for the hydrogenation of unsaturated aldehydes such as crotonaldehyde, 2-hexenal, cinnamaldehyde, citronella1 and cyclohexenecarboxaldehyde. For each catalytic system, the main products from the hydrogenation were saturated aldehydes. The unsaturated alcohols was detected mainly for RhPPWNHT. Especially, for the hydrogenation of trans-cinnamaldehyde, citronella1 and 3-cyclohexene-I-carboxaldehyde, RhPPhlNHT is superior to RhCI(PPhJ, in the formation of unsaturated alcohols.
.
4. DISCUSSION 4.1 Characterization of catalysts XRD studies indicated that the large expansion of basal spacings took place after the intercalation of rhodium complexes. Since it was considered that solvent molecules remain in the interlayer space, we have measured the temperature programmed desorption(TF’D) of these clay-supported complexes. TPD studies show that the water molecules hydrated to interlayer cations diminished in the supported complexes. The presence of acetonitrile can also be neglected from IR spectra. Thus it is concluded that the intercalated rhodium complexes are present in the interlayer and the clearance spacings of the supported complexes are nearly consistent to the molecular size of RhCl(PPhJ, (n=2 or 3).
4.2. Catalytic property The results of the hydrogenation for saturated aldehydes implies the clear shape selectivity to the chain length and the shape of tolualdehyde isomers. This catalytic behavior indicates that the rhodium complexes are located in the interlayer. Although R h P P W ” is superior to RhCI(PPhJ, in the formation of unsaturated alcohol, discrete difference between the clay-supported catalysts and the homogeneous catalyst was not observed for the hydrogenation of unsaturated aldehydes. Probably it is thought that the main form of the rhodium
2470
complex in the interlayer can be RhCl(PPh3pand cationic species such as [Rh(PPha,I+ is partially produced in the RhpPhMHT.
Aliphatic aldehyde
Aromatic aldehyde
Figrure. Relative rates and selectivities of hydrogenation for aliphatic RhCl(PPh3, aldehydes and aromatic aldehydes: QRhPPhnHT (VJ; (0) (VJ. V1(100)=2.34x104mol (mol-Rh h)-I for propionaldehyde and 1.17 xl@ mol (mol-Rh h)-l for benzaldehyde. V,(100)=1.15x1O3 mol (mol -Rh h)-l for propionaldehyde and 4.61xlO%nol (mol-Rh h)-I for ben zaldehyde.
5. REFERENCES 1 T. J. Pinnavia, in "Chemical Reactions in Organic and Inorganic Constrained Systems", R. Setton, Ed., Reidel, Dordrecht, 1986, pp. 151-164. 2 S. Shimazu, T. Hirano and T.Uematsu, Appl. Catal., 34 (1987) 225. 3 S. Shim-, T,khida and T.Uematsu, Proc. 9th ICC, 4 (1988) 1913. 4 S. Shimazu,W. Teramoto and T.Uematsu,Cad.Today, 6 (1989) 141. 5 S. Shimazu,T.bbkia andT. Uema~su,J. Mol. Catal., 55 (1989) 353. 6 A. Weiss, Angew. Chem., 75 (1963) 113. 7 G. MestrOni, A. Camus and G. Zassinovich ,in "Aspectsof Homogeneous Cad.vol4" R. Ugo, Ed., Reidel, Dordrecht, 1981, p ~71-98. . 8 M. Gargano, P. Giannoccaro and M.Rossi, J. Organometal. Chem., 129 (1977) 239.
GI&, L el d (Editors),New Frontiers in CalalpQ Proceedings of thc 10th Inkmtional CongnSs on Catalysis, 19-24July, 1992, Budapest, Hungary 0 1993 Elstvier Science Publishers B.V. All rights rcscrvcd
ENANTIOSELECI'IVEHYDROGENATION OF a-KETO ESTERS OVER WA1203 CATALYST: KINEI'IC ASPECI'S OF THE RATE ACCELERATION EFFECT INDUCED BY ADDITION OF CINCHONIDINE
J. L. Margicfalvib, B. MindeF, E. T d d , L. B o l p andA. BaikeF 'Swiss Federal Institute of Technology, Department of Chemical Engineering and Industrial Chemistry, 8092 Zurich, Switzerland bOn leave from Central Research Institute for Chemistry of the Hungarian Academy of Sciences, 1525 Budapest, P.O.Box 17, Hungary
Abstract The enantioselective hydrogenation of ethylpyruvate catalysed by Pt/Al2O3 modified with cinchonidine has been investigated with the aim to gain information about the relevant interactions occurring in this complex catalytic system. Transient kinetic experiments with separate injection of the modifier (cinchonidine) and substrate (ethylpyruvate) into the reactor revealed that the performance of this catalytic system depends significantly on the contacting of the different interacting species. Highest initial hydrogenation rates were measured when cinchonidine was injected into the reactor. In contrast, lowest initial rates were measured with simultaneous introduction of all reaction components prior to the addition of hydrogen. This behaviour is attributed to undesired side reactions of the substrate and the cinchonidine. The experimental results indicate that the interactions occumng in the liquid phase have to be taken into account t o explain the global catalytic behaviour of this system. 1. INTRODUCTION
A characteristic feature of the enantioselective hydrogenation of a-keto esters
[ll is the strong rate acceleration induced by addition of the enantio-differentiating chiral modifier [2,31.Unfortunately, the reasons for this phenomenon are still not understood, since very little is known about the interactions occurring between substrate, modifier, catalyst and solvent. The aim of this work is t o promote knowledge about the role of these interactions. Here we report some kinetic observations which were made when the contacting of the interacting species was changed deliberately. The results indicate that the interactions occurring in the liquid phase influence the catalytic behaviour of this system. 2. EXPERIMENTAL
The hydrogenation of ethylpyruvate (ETPY) in ethanol was carried out in a SS stirred autoclave equipped with an injection chamber for separate introduction of either the substrate (ETPY)or the modifier (cinchonidine(CD))into the reactor.
2472
The reaction was carried out at room temperature at 70 bar using a commercial Pt/Al203 catalyst (Engelhard E 4759 with 5 wt% Pt). The reaction conditions were: stirring rate, 1600 rpm; solvent, ethanol (Fluka), 88.5 ml; ethyl pyruvate (ETPY) (Fluka), 1.0 mol (fresh distilled); amount of catalyst, 0.25 g; cinchonidine (CD) , 0.05 g. Changes fiom these standard conditions are indicated in the tables. The optical yield was expressed as enantiomeric excess ee [%I = (R-S)/(R+S)xlOO and determined by gas chromatography as described in detail elsewhere [31. HPTLC and HPLC were applied to follow the transformation of the modifier. NMR spectroscopy was used t o investigate interactions involving the substrate.
3. RESULTS AND DISCUSSION In a series of experiments, the influence of deliberate changes in the contacting of the interacting reaction components (substrate, modifier, solvent) on the catalytic behaviour of the Ptfcinchonidine system has been studied. Experimental conditions and results of these investigations are summarized in Table 1. Figure l(a) shows the kinetic curves measured after injection of CD (runs 4-8, Table 1). The ETPY conversion versus time curves show that the hydrogenation rate changes drastically upon injection of CD to the reaction mixture, confirming the rate acceleration effect observed in previous investigations [2-41.Rate acceleration in this system is also observed in presence of other nitrogen bases such as quinuclidine and quinoline (Table 1, run 10 and 111, as previously reported [23. The kinetic curves of enantioselective hydrogenation (Fig. l a ) show two distinct regions, which appear t o obey first order kinetics with respect to the substrate (Fig. lb). Upon injection of CD a period with larger rate constant (kl) starts which then after several minutes is followed by a period with significantly lower rate constant (k2). A possible explanation for this behaviour emerged from separate studies of the interactions occurring between CD and ETPY using NMR and HPTLC. It was found that when ETPY is mixed with CD both in the presence or absence of hydrogen, an adduct is formed. During hydrogenation of ETPY both CD an the adduct undergo undesired chemical transformations. Upon mixing ETPY and alcohols (methanol or ethanol) ketal formation takes place. In the presence of CD the ketal formation is very fast and in addition transesterification of the methyl or ethyl pyruvate and deuterium exchange both in the MEPY and its corresponding ketal was observed. These side reactions are also likely t o occur during the premixing of the reactants, i.e. under reaction conditions as they were used in earlier investigations [l-41. Under these conditions (represented by run 2, Table 1)the hydrogenation rate is relatively low, probably due t o partial poisoning of the catalyst by high molecular weight compounds formed. In contrast, the hydrogenation rate is high, when the modifier is separately introduced into the reactor containing only ETPY and ethanol (Table 1, runs 4-8). The high hydrogenation rates observed in these cases indicate that undesired side reactions are suppressed under these conditions. Figure l(c) shows the time dependence of the development of the enantiodifferentiation. It is interesting t o note that the development of the enantiodifferentiation (Fig. l(c)) does not coincide with the rate acceleration (Fig. l(a)). This behaviour indicates that the rate acceleration and the chiral induction may
2473 1001
a
0 -0.5 n
"0
3
-1
-1.5 -2 -2.5
0
50
100
150
0
15
30
45
60
time [min]
time [min]
Fig. 1 Influence of cinchonidine injection on kinetic behaviour and optical yield. (a) Time dependence of ETPY conversion. Symbols refer t o experiments of Table 1 (D run 1;mrun 4;. run 6;x run 6; o run 7; run81 (b)Periods with different apparent rate constants kiC-1 and k2(- 1. Same s y m bols as in Fig. l(a). Correlation coefficients of linear regressions 2 0.99). (c) Time dependence of optical yield. Same symbols as in Fig. Ua).
-
0
50
100
time[min]
150
not be of the same origin, although they appear to be correlated [3,4].Another interesting result emerging from Fig. l(c) is the lowering of the maximum achievable optical yield with increasing injection time. This indicates that products formed during racemic hydrogenation influence the enantioselective reaction. Table 2 shows the influence of the initial concentration of CD and ETPY on the initial rate, optical yield and the calculated rate constants, k l and k2, respectively. Note that a rate acceleration can already be observed at extremley low concentration of CD. Different concentration dependence was observed for k l and k2, depending on CD concentration. Upon increasing CD concentration, the ratio kl/k2 strongly decreases. The high kl/k2 value is attributed to the fast transformation of CD at low concentration. Another interesting observation is the low optical yield obtained at low concentration of ETPY. The observations made in these investigations strongly indicate that the interactions of the reaction components occurring in the liquid phase can not be ignored for explaining the global catalytic behaviour of the Pt/CD system. Undesired side reactions of the substrate (ETPY)and the chiral modifier (CD) are likely to influence the course of the enantioselective hydrogenation and may disguise the intrinsic catalytic behaviour of this system. The undesired side reactions can be partly suppressed by appropriate contacting of substrate, modifier and solvent.
2474
Table 1 Influence of the mode of introduction of the reaction components on the kinetic parameters and optical yield ~
~
run No 1 2 3
4 5 6
7 8 9 10 11
~~
Initial rate ro [mo~gCatsecl
Mode of introduction no modifier added premixing injection of CD+ETPY+EtOH injection of CD (O)(a) injection of CD (15) injection of CD (22) injection of CD (60) injection of CD (90) injection of ETPY injection of QN (15) injection of QL (15)
Rate constants k l [min-l] k2
Optical yield ee[%l 3 65
0.03 0.20
0.43 0.85(b) 0.73(b) 0.75(b) 0.54(b) 0.30(b) 0.37 0.23 0.14
C)
0.04
C)
0.05 0.06 0.05 0.06 0.03 0.05 0.02 0.03 0.03
0.11 0.10 0.10 0.10 C)
0.06 0.04 0.02
60 64(d) 63(d) 65(d) 53(d) 57(d) 48
a) Injection times during racemic hydrogenation are given in parenthesis; b) rate measured after injection of CD; c) to short to be measered; d) formation of racemate before injection of CD is taken into account; QN=quinuclidine; QL=quinoline.
Table 2 Influence of the concentration of cinchonidine (CD) and ethylpyruvate (ETPY) on the kinetic parameters and optical yield No 12 13
14 15 16
substrate concentration [moMI
modifier conc. x 10-3[moMI
1.0 1.0 1.0 0.22 0.50
0.034 1.70 5.10 1.70 1.70
Rate constants k l [min-11 k2 0.13 0.10 0.08 0.10 0.11
0.01 0.05 0.06 b) 0.05
ratio klA2
Optical yield ee[%l(a)
13 2 1.3
59 63 63 23 55
2.2
a) formation of racemate before injection of CD is taken into account; b) to short to be measered; introduction of CD after 15 minutes of racemic hydrogenation.
$: REFERENCES
1 2
3 4
Y. Onto, S. Imai, and S. Niwa, J. Chem. SOC. Jpn. (1979)1118. H.U. Blaser. H.P. Jalett. D.M. Monti, and J.T. Wehrli, Stud. Surf. Sci. Catal. 41 (1988)153. . J.T. Wehrli. A. Baiker. D.M. Monti and H.U. Blaser, J. Mol. Catal. 61, (1990)207;J. T. Wehrli, Ph.D.-Thesis No. 8833,ETH - Zurich, (1989). M. Sutherland, A. Ibbotson, R.B. Moyes and P.B. Wells, J. Catal. 125 (1990)77.
Guczi, L u al. (Editors), New Fronfiersin Catalysis P d i n g s of the lOth International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishen B.V.All rights resented
CROTONALDEHYDE HYDROGENATION OVER pt/Ti02 CATALYSTS. INFLUENCE OF THE CATALYSTS PRETREATMENTS R. Makouangou, A. Dauscher and R. Touroude Labratoire de Catalyse et Chimie des Surfaces, URA 423 du CNRS, Universite Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasburg W e x , France
Abstract The hydrogenation reactions of crotonaldehyde (CROALD), butyraldehyde (BUTNAL) and crotyl alcohol (CROALC) have been performed in gas phase over a 2 wt % Pt/TiO2 catalyst after reduction at 200, 400 or 5OOOC. Changes in activities and selectivities were observed during one pulse, that are different in the first pulse and in the following pulses, showing the presence of strongly and weakly poisoned sites. Strong metal-support interaction sites, induced by high reduction temperature, have been found to be very sensitive to poisoning.
INTRODUCTION PtPTiO2 catalysts in the strong metal-support interaction (SMSI) state are well known to enhance the methanation reactions, by activation of the C O adsorbed at the metal-support o r metal-suboxide interface [l]. This property can therefore be useful to selectively hydrogenate the C=O rather than the C=C bonds of unsaturated a$ aldehydes, which is a difficult task t o achieve. Vannice and Sen [2], by studying the hydrogenation reactions of crotonaldehyde (CROALD) over Pt"iO2 catalysts in gas phase, have obtained an increase in the formation of the unsaturated alcohol after reduction of the catalysts a t high temperature. In order to investigate further about SMSI effects in such reactions, the hydrogenation reactions of CROALD were performed on PtR"i02 catalysts with other experimental conditions. A fine study of such catalysts put forward the crucial point of the stability concerning catalytic activity and selectivity. Additional studies with crotyl alcohol (CROALC) and butyraldehyde (BUTNAL) were performed in order to investigate about the reaction scheme.
EXPERIMENTAL The PtJI'iO2 catalyst (2 wt % Pt) was prepared from T i 0 2 (Tioxide) and H2PtCl6,6H20 as already described [31. Different samples were reduced either at 2OOOC (R200),4OOOC (R400) or 500°C (R500) under purified hydrogen, at atmospheric pressure, "in situ" in the catalytic apparatus during 16 h. Pulses
2476
of 25 p1 CROALD (Fluka, puriss), CROALC (Sigma, purum) and BUTNAL (Fluka, puriss) at a constant partial pressure of 8 f l Torr were passed over the catalyst at temperatures comprised between 20 and 100OC. The changes of activity and selectivity were followed by gas chromatography (column: carbowax 1640, 4.5 m length, 1/8 inch diameter) during each pulse (duration: 18 m i d . Several pulses of reactant were performed one after another to look at the stability of the catalyst. When the reactant was changed, the catalyst was re-reduced for 16 h at the initial reduction temperature.
RESULTSAND DISCUSSION
As the study of the hydrogenation of CROALD was progressing, we noticed that whatever the reduction or reaction temperature, (i) the results obtained during the first pulse are different from the following ones (ii) from the second pulse, the catalysts present very reproducible behaviours and (iii) the activities and selectivities both vary during every pulse. We are therefore going to distinguish the first experiment from the following ones. Most of the experiments have been performed at 80"C, temperature at which the selectivity in the reaction CROALD + CROALC is the greatest, especially for overall conversion higher than 20%. The global activities sharply decrease during the 8 first min in the first pulse of CROALD (figure la). Roughly 75% (only 60% on R500) of the initial catalytic sites are poisoned. These sites remain inaccessible for further experiments but can be partly regenerated by a re-reduction treatment. These sites can be considered as strongly poisoned sites. In the reproducible state, the decrease in activity is less important (figure lb): 30 to 40% (20% on R500) are still poisoned, but in a totally reversible mode, with poison formation a t the beginning of the pulse and poison desorption at the end of the same pulse. These sites are called weakly poisoned sites. During the hydrogenation reaction of BUTNAL, the same poisoning behaviour occurs while during the hydrogenation reaction of CROALC, no strong poisoning appears in the first experiment. We can think that either CROALD or the BUTNAL formed block the sites in CROALD hydrogenation reaction. After 15 min under stream, the activities are quite stabilized, whatever the reduction temperature. It was checked by performing longer pulses. The activity linearly decreases with an enhancement of the reduction temperature. Actually, there is a site blocking due to the SMSI state induced by higher reduction temperature. The sites generated by SMSI are either strongly or weakly poisoned sites, as defined previously. The activity of each of the 3 reactants CROALD, BUTNAL or CROALC depends on the reduction temperature. CROALC follows the same trend as CROALD but is more reactive. BUTNAL is the less active and becomes completely inactive on B O O . The changes of selectivity in products during the first and the following experiments (HC=butane+butene, BUTNAL, butanol (BUTNOL) and CROALC) are reported in figure 2. Some other products, such as propanal or dibuthylether, have also been detected mainly in the first experiment or at the beginning of the other pulses, to an extent of roughly 3f2 %.
2477
0 ' . 0
'
5
.
" ' 10
. ' 15
20
O;
'
time on stream (min)
5' . 10 ' . 1'5 time on stream (min)
'
'
20
Figure 1. Changes in total activity (pls-lgpt-1) during hydrogenation of CROALD as a function of time on stream on Pt/Ti02 catalysts reduced at 200, 400 or 500°C.a: first experiment, b: following experiments (reproducible state).
-1 5 60
40
0' 0
5
'
10
.
'
15
80 I
"
20
I 60
40
-
0
-
20:
0
5
10
15
I
80 I
"0
20
5
10
15
time on stream (min)
20
80 I
0 0
1
5 10 15 time on stream (min)
20
Figure 2. Changes in selectivity (0HC, A BUTNAL, 0 BUTNOL, + CROALC) as a function of time on stream during CROALD hydrogenation on Pt/Ti02. a,c,e: first experiment on R200,R400 and R500,respectively. b,d,f: following exps. (reproducible state) on R200,R400 and R500, respectively.
2478
In the first experiment (figures 2a, 2c and 2e), the selectivities in BUTNAL increase while the selectivities in BUTNOL decrease, whatever the reduction temperature, meaning that the BUTNAL + BUTNOL transformation mainly occurs on a clean surface. On R200, the amounts of HC formed are more important than on R400 and R500, mainly at the beginning of the reaction where the overall conversion is very high (between 20 and 50%). On the other hand, the selectivity in CROALC sharply increases during the 10 first min of reaction on R200 , while not on R400 and R500. In the first moments of the reaction, CROALC would be quite totally transformed into HC, as it is the case at high conversion when starting with CROALC. This reaction would better occur on the non SMSI sites. BUTNAL, used as reactant, is quite selectively transformed into BUTNOL and doesn't lead to the formation of HC. A remarkable fact is the very high selectivity in CROALC obtained on R500 (S=44%). In the following experiments (figures 2b, 2d and 20, the changes in selectivity during one pulse are less important, and even less as the reduction temperature is high. The selectivities in HC and BUTNOL are always lower. On R400 and R500,the CROALC selectivities increase while the BUTNAL selectivities decrease all along the time on stream. Taking into account the decrease of total activity, it is obvious that the rate of CROALC formation remains quite constant while the BUTNAL formation is hindered on the weakly poisoned sites. On R200, the situation is more complicated, due to the further transformation of BUTNAL into BUTNOL. A great decrease in the selectivity in CROALC can be noticed between the first experiment and the others on R500 (44+ 29 %), whereas it is not the case on R200 and R400 (S=25f4 %), so that there is no more influence of the reduction temperature on the CROALC selectivity. The SMSI sites would be readily poisoned if we consider, as postulated by Vannice and Sen [21 that the CROALD + CROALC reactions occur on these sites.
CONCLUSION The study of the hydrogenation reaction of CROALD put forward that the stability in catalytic activities and selectivities are strongly dependent on the experimental procedure: repeated short aldehyde pulses on hydrogen stream permited to show the presence of strongly and weakly poisoned sites that were not observed in a continuous mode of injection [2]. Moreover, the creation of SMSI sites leads to higher selectivities in the CROALD + CROALC reaction during the first pulse. We have to postulate that these sites disappear under hydrogen stream so that the further observed selectivities become similar to that obtained on catalysts not in the SMSI state.
1 M.A. Vannice and C.C. Twu, J. Catal., 82 (1983) 213. 2 M.A. Vannice and B. Sen, J. Catal., 115 (1989) 65. 3 A. Dauscher and G. Maire, J. Mol. Catal., 69 (1991) 259.
Guczi, L d d.(Editors), New Frontiers in Clrrolysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 6 1993Elsevier Science Publishers B.V. All rights reserved
SELECTIVE HYDROGENATION OF a,P-UNSATURATED ALDEHYDES OVER SUPPORTED Ru A. Waghray,R. Oukaci and D. G. Blackmond
Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261. USA
Abstract The hydrogenation of 3-methyl 2-butenal over Ru was investigated to study the influence of alkah promoters and of the type of support on the activity and selectivity. Alkali species increased selectivity of Ru to the unsaturated alcohol, an effect which was observed both for alkali-promoted Ru/Si& and for Ru supported on Y zeolites with Merent alkali compensating cations. The gas-phase reactions were strongly diffusion limited over the zeolite-supported Ru catalysts.
1. INTRODUCTION The role of heterogeneous catalysis in the production of fine chemicals is becoming increasingly more important [ 1,2]. Selectivity in the hydrogenation of molecules having more than one unsaturated function is a key issue in these reactions. For example, unsaturated alcohols are the desired products in the hydrogenation of a,P-unsaturated aldehydes. The intrinsic behavior of a transition metal in these reactions may be altered through the addition of promoters or by appropriate choice of a support. In the present studies, Ru supported on modified Y zeolites and Si02 was used in the gas-phase hydrogenation of 3-methyl 2-butenal to investigate both support and promoter effects on selectivity to the unsaturated alcohol 3-methyl 2-butenol.
2. EXPERIMENTAL Zeolite-supported Ru was prepared by ion-exchange of Ru3(NH3)&13 to a nominal 3% by weight. The effect of the type of compensating cation in the zeolite was studied by exchanging the parent Na cation for K and Cs. 3 weight% Ru/SiO;! was prepared by incipient wetness impregnation of RuNO(NO~)~. Ru particles measured by hydrogen chemisorption were about 2.5 nm for Ru/Si02 and 1.4-1.9 nm for zeolite-supported Ru . Gas-phase hydrogenation reactions were carried out under differential reaction conditions under a constant total flow rate of 200 ml/min. Reactant aldehyde concentrations were varied between 7 and 35 pmol/min by bubbling He through a saturator at 298 K. Hydrogen flow rates were varied from 20 to 180 d m i n to keep a constant total
2480 flow rate of 200 ml/min. Reactions were carried out at temperatures ranging from 313 to 393 K. Products were quantified by on-line G.C. analysis and were measured under steady state conditions. Conversion of the organic substrate at 343 K were less than 5%.
3. RESULTS AND DISCUSSION Product selectivity in the reaction of 3-methyl 2-butenal over Ru catalysts is shown in Figures 1 and 2. Figure 1 shows that Ru supported on SiO, showed the same high selectivity to the saturated alcohol (SAL) as the unsupported Ru catalyst. Addition of a potassium promoter to the Si02-supponed Ru caused a dramatic shift in product selectivity towards the unsaturated alcohol (UOL). Figure 2 shows that all of the zeolitesupported Ru catalysts showed higher selectivity to UOL than the unsupported Ru and that the UOL selectivity increased in the order Ru/NaY c Ru/KY c Ru/CsY. The presence of alkali species, whether in the role of the charge compensating cation in a zeolite structure or added to the catalyst as a potassium salt, caused a shift in reactivity at the C=C function of the organic substrate to hydrogenation at the C=O function.
Catalyst
Catalyst
Figure 1. Selectivity in 3-methyl 2butenal hydrogenation over unpromoted and K-promoted Ru/SiO, compared to unsupported Ru.
Figure 2. Selectivity in 3-methyl 2butenal hydrogenation over Ru/NaY, Ru/KY, and Ru/CsY compared to unsupported Ru.
UOL = unsaturated alcohol SAL = saturated aldehyde SOL = saturated alcohol Cation effects in zeolites have been seen before in other reactions, including CO hydrogenation where exchanging Na for the more electronegative K and Cs leads to
2481
the suppression of olefin hydrogenation [3-51. These effects have been attributed to an electronic interaction between the alkali cation and the small metal particles within the zeolite pores which increases the electron density on the metal. The electron-rich metal may alter the manner in which the organic substrate is adsorbed, leading to a more polarized and hence more reactive C=O bond. Similar electronic effects have been invoked to describe the influence of alkali promoters on the adsorption and reaction of molecules on alkali-promoted supported metals similar to the Ru-K/SiOz catalyst in the present study [6]. In addition, both geometric effects and the direct interaction of adsorbates with alkali species have been suggested [6,7]. The polarization of the organic substrate leading to increased reactivity of the C=O function may result from interaction of the adsorbed molecule with alkali species in its proximity [8]. 4.5 I 0
-
1
Ru-K/SiO,
4.0 v Ru/KY
v
Ru/KY
0
Ru-K/SiO,
3.5 3.0
4
v
G 2.6
d
2.0
0
20
40
P,,./PyX
Figure 3. Arrhenius plots for reaction of 3-methyl 2-butenal over Ru-K/Si02 and Ru/KY.
60
,
80
100
10
Figure 4. Rate of hydrogenation of 3methyl 2-butenal over Ru-K/SiO, and Ru/KY as a function of the partial pressure of the substrate and hydrogen.
Apparent activation energies measured for the reaction of 3-methyl 2-butenal over Ru-K/Si02 and Ru/KY are shown in Figure 3. The value measured for the silica-supported catalyst is in the range reported for reactions of this type [8], while the zeolitesupported catalyst exhibited a much lower activation energy. Low values for Ea are often reported for diffusion-limited reactions where the controlling rate process is less temperature-sensitive than is the intrinsic reaction rate. Figure 4 shows another trend that suggests that diffusion limitations were more important in the zeolite-supported catalysts. The rate of formation of the totally hydrogenated product, the saturated alcohol (SOL), was much more sensitive to the relative partial pressures of the organic substrate (UAL) and hydrogen for the zeolite-supported catalyst than for the silicasupported Ru. There was a strong shift in product selectivity towards the total hydrogenated product under conditions of low substratehgh hydrogen concentration for the
2482
zeolite-supported Ru, while no such change in selectivity with reactant concentrations was observed in the case of the Ru supported on Si02. For Ru-K/Si02, in fact, the production of the total hydrogenation product SOL was very low under all conditions of reactant concentrations and reaction temperatures [9]. It appears that the availability of hydrogen in the pores was important in controlling activity for the zeolite-supported Ru and that the opportunity for prolonged adsorbed substrate-hydrogen contact due to significant residence time w i h n the zeolite pores played a role in determining selectivity [ 101. Readsorption and secondary reactions of the half-hydrogenated products within the pores were more significant in the zeolite-supported systems than for silicasupported Ru where diffusion limitations were absent. 4. CONCLUSIONS
The presence of alkali species in supported Ru catalysts led to changes in the selectivity of hydrogenation of 3-methyl 2-butenal. Dramatically increased production of the unsaturated alcohol was suggested to be due to increased polarization of the adsorbed substrate either through electronic interactions between the alkali species and the Ru particles or by direct interactions between the alkali species and the adsorbate. The zeolite catalysts exhibited much lower apparent activation energies for the overall reaction and hgher production of the totally saturated product in comparison with the silica-supported Ru, leading to the suggestion that the reactions were diffusion-hted over the Ru/zeolite catalysts. 5. REFERENCES 1 a) M. Guisnet, et al. (eds.), Heterogeneous Catalysis and Fine Chemicals, Proceedmgs from the 1'' International Symposium, 1987, Elsevier, Amsterdam, 1988; b) M. Guisnet, et al. (eds.), Heterogeneous Catalysis and Fine Chemicals 11, Proceedings from the 2"d International Symposium, 1990, Elsevier, Amsterdam, 1991.
2 See, for example, the following patents: Cordier, G., Fouilleux, P., and Grosselin, J.M., French Patent App. No. 882919 (1988); Homer, M., and Irgang, M., German Patent No. 81-313805 (1981); Ichikawa, Y., Suzuki, M., and Sawaki, T., Japanese Patent No. 52084193 (1977). 3 Cavalcanti, F.A.P., Blackmond, D.G., Oukaci, R., Sayari, A., Erdem-Senatalar, A., and Wender, I., J. Catal., 113 (1988) 1. 4 Oukaci, R., Sayari, A., and Goodwin, J.G., J. Catal., 102 (1985) 126. 5 Blackmond, D.G., Oukaci, R., Blanc, B., and Gallezot, P.,J.Catal., 131 (1991) 401. 6 Kesraoui, S., Oukaci, R., and Blackmond, D.G.,J. Card., 105 (1987) 432. 7 Hoost, T.E., and Goodwin, J.G., Jr.,J. Catal., 130 (1991) 283. 8 Vannice, M.A., and Sen, B., J. Catal., 115 (1989) 65. 9 Waghray, A., Wang, J., Oukaci, R., and Blackmond, D.G., submitted for DU blication. 10 kaghray, A., Oukaci, R., and Blackmond, D.G., submitted for publication.
Guczi, L et al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights resewed
HYDROGENOLYSIS OF TEXRAHYDROFURANE ON PLATINUM
K. Kreuzer and R. Kramer Institut fur Physikalische Chemie, Universitat Innsbmck, 6020 Innsbruck, Austria
Abstract The hydrogenolysis of tetrahydrofurane on platinum proceeds by scission of an ether bond to form a butanol-species that is decomposed to either propane and CO or to butane and water. The rupture of the CO bond is promated by the support, whereby titania exhibits a stronger effect than silica. The reaction is inhibited by selfpoisoning with product CO. Removal of CO can occur by methane formation, by desorption, or by CO spillover to the support.
1. INTRODUCTION In the hydrogenolysis of methyltetrahydrofuraneon platinum it was found that the support affects the selectivity of the reaction. In particular, on nonsupported platinum the ether bond to the secondary C atom was selectively broken, while on silica supported platinum the ring opening occurred also at the ether bond to the tertiary carbon atom [I]. In order to study specifically the scission of the ether bond to a secondary carbon atom, we investigated the ring opening of tetrahydrofurane (THF)on platinum. Additionally, the reaction of l-butanol- the primary product of the hydrogenolysis of THF - was studied with respect to the selectivity to possible secondary reactions. These are either CO-abstraction yielding propane, or C-0 bond rupture, leading to butane as final product.
2. EXPERIMENTAL A platinum foil, the well characterized Pt/Si02-catalyst EUROPT-1, and two platinum catalysts prepared by impregnation of SiOz (Aerosil 130 - Degussa) and of Ti02 (P-25 Degussa) were used in the experiments. The catalysts were characterized by measuring the uptake of hydrogen and CO in a volumetric adsorption apparatus and by TEM-inspection (Table 1). Prior to every reaction the catalysts were pretreated by heating in oxygen at 400°C followed by reduction in hydrogen at 400°C. With Pfli02 reduction at 250°C instead of 400°C did not lesd to a distinct change in the catalytic performance. The measurement of catalytic conversion was performed in a tubular flow reactor and in a
recirculating reactor. The pressure of the reactants was 50 mbar in case of THF and 5 mbar in case of butanol. Hydrogen was added to attain atmospheric pressure. For investigation of the reaction mechanism, intermediate products were monitored by a dispersive infrared spectrometer (Perkin Elmer 180).
EUROPT-1 F'tJSiO2 Wi02
I
6.3 5
2.5
1.02 0.56 0.84
0.6 0.46
1.21
I
18 21 16
3. RESULTS. With both silica supported catalysts and with the Pt-foil the onset of the reaction was at about 150°C while on Pt/Ti02 the reaction starts at a far lower temperature (80°C). The primary products (butyraldehyde or n-butanol) are found in the product stream in their equilibrium composition which is largely on the side of n-butanol under the experimental conditions. These primary products reacted via two main routes producing either butane and water or propane and carbon monoxide. When n-butanol or n-butanal were used as reactant instead of THF, nearly the same product pattern was obtained indicating that n-butanol is an intermediate product in the reaction. On platinum foil propane was formed nearly exclusively, while on silica supported platinum both propane and butane were formed in almost equal proportions (Fig. 1). However, on Pfli02 the selectivity was strongly shifted to more butane formation and only traces of propane were obtained (Fig. 2). On this catalyst the intermediate product butanol was not observed temperatures lower than 180°C, presumably due to its adsorption at the support. With all catalysts the proportion of propane increased with increasing temperature. This temperature effect is seen most clearly on the EUROPT-1 catalyst (Fig. 1). The catalytic activity and selectivity of the Pt/Si02 catalysts changed distinctly with timeon-stream. After admission of reactants to a freshly regenerated catalyst the high initial activity decreased rapidly approaching a constant "steady-state'' activity. This fast deactivation process is most likely due to selfpoisoning by product CO. Because of the faster selfpoisoning at higher conversions the activity increases only slightly with increasing temperatures and calculation of activation energies becomes meaningless. In contrast to Pt/Si02, the conversion on the titania supported catalyst increased nearly linearly with increasing contact time. The higher time-on-stream stability and also the higher activity of the Pfli02 catalysts may be due to the lower formation of propane and CO, i. e. to a lower degree of selfpoisoning. The effect of CO-poisoning on the activity and selectivity of the reaction was demonstrated by prepoisoning the EUROPT-1 catalyst with various amounts of CO prior to
2485
$
propane
% 50 butane "
9)
CI
al
m
0. 150
200
temperature/C
250
/ butanol
-=
150
200
250
temperature/C
Figurel. Variation of selectivity with temperature on a) EUROET-1and b) PtiTi02 the reaction. When the amount of CO admitted corresponded to a monolayer coverage of the platinum surface, the initial high activity was suppressed. Butane was formed with the rate of the "steady-state'' activity, while the onset of propane formation was delayed by about 10 minutes. However, when the prepoisoning was occumng due to CO doses corresponding to five monolayers of CO, the onset of butane formation was about 30 minutes after admission of the reactants, and formation of propane started again 10 minutes later. The effect of prepoisoning was, however, smaller at higher reaction temperatures. During the "induction" time no methane formation was detected. On the pure supports Si02 and TiOz no conversion was observed at temperatures lower than 280°C. Above this temperature formation of propane, butane and of butenes was found when either THF or butanol was fed to the reactor. By infrared spectroscopic investigation of the EUROPT-1 and of the Pt/Aerosil-catalyst we could show that the ringopening of THF can start even at room temperature at these catalysts. After room temperature admission of THF and hydrogen to a regenerated Pt/SiOz catalyst an adsorbed aldehyde species absorbing at 2685 cm-1 was monitored. Increasing the temperature to about 80°C caused the decomposition of the aldehyde species accompanied by the appearance of a CO absorption band at 2060 cm-1. The absorbance of this CO-band approached a saturation value after a short time. If the temperature was then stepwise increased by 20°C the CO-absorbance also increased each time, but again a constant COabsorbance was soon attained. 4. DISCUSSION
The hydrogenolysis of tetrahydrofurane on Pt proceeds in two consecutive reactions. Butyraldehyde or n-butanol are formed in a fist step. These intermediate products decompose in two parallel secondary pathways, leading either to butane and water or to propane and CO. Among the secondary reactions, the product distribution appears strongly dependent on the
2486
kind of support. While on Pt-foil propane and CO are formed nearly exclusively, the supported catalysts exhibit a high selectivity for butane formation together with a distinctly higher overall activity. The higher overall activity of the supported catalysts together with higher selectivity towards butane is most likely due to activation of CO bond scission, which enhances the rate of the primary rupture of the ether bond and shifts the product distribution of the secondary reaction. However, the pure supports are able to catalyse both pathways, but at far higher temperatures. Thus, a classical bifunctional mechanism proceeding via dehydrogenation of butanol adsorbing from the gas phase is not likely to occur. An activation of carbon monoxide and carbonyl bonds by Pt/TiOz has been reported repeatedly [2-41 and was explained by the presence of catalytic ensembles composed of platinum and support sites situated at the metal-support phase boundary. We propose to extend this support effect of Ti02 also to the scission of the CO bond present in ethers or alcohols. The enhanced activity of the phase boundary sites may be caused by the electric potential gradient operating at the interface in order to adjust the Fermi-level of the two phases [ 5 ] . On the other hand, the support could provide the adsorption sites where THF or butanol are hydrogenated by hydrogen spilled over from the platinum. The overall reaction is retarded due to selfpoisoning by product CO. This is deduced from the fast deactivation with time and from the fact that the deactivation is stronger when the selectivity towards CO is higher. The IR-spectroscopic data show that THF-ringopening and CO formation starts already at a far lower temperature than does the overall reaction. Thus, the rate-limiting step is most likely the desorption of product CO. This assumption is supported by the results of CO poisoning experiments. The overall reaction is severely inhibited by CO prepoisoning. The reaction starts after an induction period, but surprisingly no sink for the poison CO is detected: CH, is not formed and in the recirculating reactor CO desorption does not occur under the experimental conditions. We assume that CO adsorbed on the Pt spills over to the support thus regenerating free sites on the platinum surface. Spillover of CO to the titania could also explain the high CO uptake of the PVTiO, catalyst. Similar spillover states for CO on alumina or titania have been reported recently [6,7]. The clean up of the Pt surface from CO (either by desorption or by CO spillover) is facilitated by higher temperatures and therefore the pathway occumng on the Pt surface (leading to propane and CO) becomes more important at higher temperatures.
5. REFERENCES 1 U. Gennari, R. Kramer, and H.L. Gruber, Appl. Catal. 44 (1988) 239. 2 M.A. Vannice, J.Cata1. 74 (1982) 199. 3 M.A. Vannice and B. Sen, J.Catal. 115 (1989) 65. 4 M.A. Vannice, J.Mol.Cata1. 59 (1990) 165. 5 R. Kramer and M. Fischbacher, J.Mol.Catal. 51 (1989) 259. 6 B. Sen, J.L. Falconer, T.F. Mao, M. Yu, and R.L. Flesner, J.Catal. 126 (1990) 465. 7 T.F. Mao and J.L. Falconer, J.Cata1. 123 (1990) 443.
Guczi, L d al. (Editors),New Fronriers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights rcserved
INFLUENCE OF POLYMER SUPPORT MORPHOLOGY ON ION-EXCHANGER CATALYSTS ACTIVITY IN terL-ALKYL-MKTHYL ETHERS SYNTHESIS
K Jerabek, T.Hochmann and Z. Prokop Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 16502 Prague 6, Czechoslovakia
Abstract The influence of ion exchanger-catalysts morphology on their activity was investigated in the synthesis of methyl tert.buty1 ether (MTBE) and tert. amyl methyl ether (TAME). In both cases the catalytic reaction proceeds in swollen polymer mass. However, differences in the polymer morphology affect the MTBE and TAME syntheses relatively weakly. Steric effects playing important role e.g. in ion exchanger catalyzed reactions in aqueous medium are here suppressed by forces accompanying adsorption of reactants on the polymer chains. 1. INTRODUCTION
One of the most important industrial application of ion exchanger catalysts is the production of valuable octane boosters methyl tert.buty1 ether (MTBE) and tert.amy1 methyl ether (TAME). The influence of polymer backbone morphology on the catalyst activity in these reactions is not yet fully understood [ l ] . The starting components for the MTBE and TAME synthesis (methanol and isoolefin) have very different affinity and swelling ability toward the ion exchanger catalyst. Macroreticular ion exchangers are usually recommended as catalysts; it is supposed that their porosity partially independent of swelling, can improve the accessibility of active centers. On the other hand, in some tests the gel-type resin had similar catalytic activity as the macroreticular one [2]. Therefore, we studied in detail the relations between morphology of ion exchangers and their catalytic properties for MTBE and TAME syntheses. 2.
EXPERIMENTAL
A series of commercial ion-exchanger catalysts (Lewatit - Bayer AG, Germany; Amberlyst - Rohm and Haas, USA) spanning in morphology from lowcrosslinked gel-type to highly crosslinked macroreticular resins were used. Their properties are summarized in Table 1. Prior use all ion exchangers were washed with 0.1 M HC1, distilled water, dried at 110 OC overnight and kept over phosphorus pentoxide. Before the test the weighed catalyst was placed in the reactor and preswelled either in methanol (MTBE synthesis) or in the reaction mixture (TAME synthesis) for at least 12 h. Catalytic activity was measured as reaction rates of MTBE and TAME syntheses at defined conditions in the CSTR microreactor directly connected
with a gas chromatograph. Catalyst grain size was 0.16 - 0.315 mm and in separate series of experiment it was found that the results were free of both external and internal diffusion effects. Methanol (p.a., Lachema) was dried with magnesium and distilled. Reaction mixture for MTBE synthesis was prepared from pure isobutylene (Slovnaft) and methanol in the molar ratio 1 : 1 and tests were performed at 70 O C . For TAME synthesis the C5 petrochemical fraction (Chemical Works Litvinovl contained 25 % of active isopentenes and was mixed with methanol in molar ratio 1 : 1 in resgect to the content of reactive olefines; the reaction temperature was 80 C. For both reactions, the pressure in the reactor was maintained at 1.5 MPa. Table 1 Properties and catalytic activities of the examined ion exchangers. Trade name
Typea Divinylbenzene BET surface content, %
m2/g
Lewatit SC 102 G 2 Lewatit SC 108 G 8 Lewatit SPC 108 M 8 15 Lewatit SPC 118 M 18 45 Amberlyst XNlOlO M unspecified,but high 580 aG - gel, M - macroreticular; bin (mol/g.h), measured in a conversions 7 % (MTBE) or 25 % (TAME), respectively.
b Reaction rates MTBE
TAME
0.57 0.73 0.81 0.80 0.31
0.13 0.13 0.21 0.17 0.07
CSTR reactor at
3. RESULTS AND DISCUSSION
The results of catalytic tests (Table 1 ) show relatively weak influence of ion-exchanger morphology on the rate of both reactions. Neither the occurrence of pore network independent of swelling in macroreticular types nor the substantial differences in the degree of crosslinking, are able to induce any dramatic changes in activities. The lowest activity had the ion exchanger with the highest BET surface area and hence, the highest portion of active groups located on the polymer-mass surface. Therefore, the direct accessibility of the acidic centers from the fluid phase is is not a significant factor and these reactions proceed predominantly in the swollen polymer mass. Small influence of polymer morphology on catalytic activity of the ionexchanger catalysts is an indication of small differences in the catalytic activities of acidic centers located in various parts of the polymer mass. It is in agreement with Rehfinger and Hoffmann [ l l , who, in their interpretation of the kinetics of MTBE synthesis used such supposition but without experimental evidence. However, for ion-exchanger catalysts opposite observations are usually made, e.g. in the reesterifications of ethyl acetate by methanol and propanol or bisphenol A synthesis [31 and especially, the ethyl acetate and sucrose hydrolyzes performed in aqueous medium [4,51. In these reactions were found (and even on the basis of swollen polymer morphology characterization quantitatively described) more than an order of magnitude differences in the activities of low-crosslinked gel and highcrosslinked macroreticular ion exchangers, in spite of the fact that water is the best swelling solvent and hence, the accessibility of the acidic centers inside of swollen polymer mass should be the best. The explanation
2489 for the low influence of catalyst morphology on the rates of MTBE and TAME syntheses must be therefore sought in the points which are special for these reactions. In a non-aqueous environment the reactions catalyzed by strong acidic ion exchangers proceed with the participation of assembles of sulfonic groups which are much more active then isolated acidic centers [ a ] . An influence of the polymer morphology on the formation of these assembles should be therefore also considered [31. We have tested the influence of changes in acid centers concentration on the reaction rate of MTBE synthesis. Results of these experiments, which due to insufficient space cannot be here fully reported, showed that the MTBE and TAME syntheses are in this respect similar to other ion exchanger catalyzed reactions. In the study of the catalytic activity of a series of catalysts prepared from a macroreticular polymer sulfonated to different degree 171 the acidic centers were selectively introduced to various depths of the polymer mass and their catalytic activity was then separately tested. Now we have used the same series of partially sulfonated ion exchangers (prepared from a macroreticular styrene-divinylbenzene copolymer containing 10 % DVB) also for the investigation of MTBE and TAME syntheses. The results are in Fig. 1 compared with the previous data on bisphenol A synthesis 171. They are expressed as relative numbers computed from the values of exchange capacity and of specific catalytic activity (per mole of acid) as the fully sulfonated member of the series as reference. While with increasing degree of sulfonation the activity of sulfonic groups for bisphenol A synthesis steeply diminished, in both MTBE and TAME syntheses, the activity of acidic groups located in the low-sulfonated catalyst near the surface only, was very similar to average activity of all groups in the fully sulfonated sample. This is a direct evidence that for these reactions is the accessibility of active centers within the ion exchanger polymer skeletons exceptionally little sensitive to the polymer morphology.
1_
4
\
0
-
0
O%.!O
0.5 Sulfonation degree
1 .b
Figure 1. Dependence of the relative catalytic activity on the sulfonation degree. MTBE, TAME, bisphenol A. Synthesis of:
Polymer morphology can influence the accessibility of the interior of polymer mass in the two ways. The first is a molecular sieve effect preventing the molecules of reactants t o enter the very dense, poorly swollen polymer mass if they are larger than the spaces between the polymer chains. This phenomena in the MTBE and TAME synthesis is significant maybe for the very dense Arnberlyst XNlOlO but it is probably of a low importance for the other ion exchanger catalysts. The second mechanism is the steric influence of the polymer chains diminishing the concentration of solute molecules inside of pores bigger than the size of solute molecules, in comparison with their concentration in the free fluid phase. This effect is well known from the steric exclusion (or gel) chromatography and it is the chief factor influencing the activity of ion exchanger catalysts in aqueous environment 14.51. The reactions are there catalyzed by hydrated protons moving in the fluid phase in the vicinity of polymer chains 161. The steric hindrance is then exercised by the polymer chains acting as inert objects. Evidently, in the MTBE and TAME synthesis this effect is virtually absent. In the ion exchanger catalyzed reactions in non-aqueous medium as the protonating agent, the undissociated acid groups act catalytically and the reaction proceeds in the direct contact with the polymer chains. Resulting adsorptive forces between the polymer chains and the reacting molecules effectively eliminate the steric exclusion effect. In spite of poorer swelling of ion exchangers in organics in comparison with water, the accessibility of active centers is then very good. 4. REFERENCES A . Rehfinger and U. Hoffmann: Chem. Eng. Sci. 45, (1990) 1605. M. Imaizumi and Y. Fujiwara: J. Japan Petrol. Inst. 27, (1984) 356. K. Jeiabek and K. Setinek: J. Mol. Catal. 39, (1987) 161. K. Jefabek: Proc. 8th Int. Congr. Catalysis, Berlin 1984, Verlag Chemie Weinheim 1984, Vol. 4. p.781. 5. K. Jeidbek and K. Setinek, J. Polym. Sci. Part A Polym. Chem. 28, (1990)
1. 2. 3. 4.
1387. 6. 9. C. Gates and W. Rodriguez, J. Catal. 31 (1973) 27. 7. K. Jeiibek: Coll. Czech. Chem. Commun. 46, (1981) 1577.
Guni, L et al. (Editom), New Frontiers in Catalysu Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
MECHANISM OF CATALYTIC ORGANIC SOLID-GAS REACTIONS
R. Lamartine, F. Sabra and A. Selatnia Universite Claude Bernard, Lyon I, Laboraoire de Chimie Industrielle, 43 Boulevard du 11 Novembre 1918,69622 Villeurbanne Cedex, France
Abstract The solid state hydrogenation, deuteration and alkylation of 4-tertbutylphenol have been studied under mild conditions : at room temperature and one bar gas pressure. These catalytic organic solid-gas reactions are completely heterogeneous. Experimental kinetic studies show that the reaction rates depend on the interface nature. Theoretical results are in good agreement with a diffusional process and allow to depicte catalytic solid-gas reactions by a three steps mechanism.
1. INTRODUCTION
Numerous reactions involve an organic solid and a gas. These reactions are observed just as well in natural processes : oxidation, hydrolysis, carbonation as in industrial processes : hydrogenation, halogenation, alkylation [l -41. Most of the time to achieve such reactions a catalyst must be used. The catalyst can be solid, liquid and possibly gaseous. Therefore the number of phase increases and the composition of the reaction mixture becomes complicated. Consequently the knowledge and the study of the catalytic gasmolecular solid reactions will require to take into account and to solve problems of : organic solid, catalyst, heterogeneous catalysis, kinetics, organic chemistry. Despite these difficulties gas-solid reactions offer unique possibilities. First of all, the trouble and costs of addition and removal of a solvent is avoided, moreover the lack of a solvent may give rise to new reactions with new selectivities and lastly the abscence of interaction between a solvent and the catalyst leads to a more simple catalytic system and this might lead to an easier understanding of the reaction mechanism. In this study we compare hydrogenation, deuteration and alkylation of solid 4-tert-butylphenol. Hydrogen, deuterium and isobutene gas react with the solid substrates in the presence of various catalysts under mild conditions. Mobil species are formed by dissociation of molecular gas on the catalysts and then migrate towards the organic solids and react.
2492 2. EXPERIMENTAL
All the reactions are carried out in mild conditions : room temperature and one bar hydrogen, deuterium or isobutene pression. The mixture, solid catalysffsolid substrate is placed at the bottom of the batch reactor to form a thin layer and then evacuated to approximately 10-3 Torr for 10 mn. After introducing gas, the reaction occures and the products formed are analysed and characterised by gas chromatography, mass spectrography, 1HNMR and 15CNMR Hydrogenation-Catalysts : PVA1203, Pt/C, Rh/A1203, Rh/C ; products : cis 4-tertbutylcyclohexanol, trans 4-tert-butylcyclohexanol and 4-tert-butylcyclohexanone. - Deuteration-Catalyst : Rh/A1203 ; products cis 4-tert-deuterocyclohexanol and trans 4-tert-deuterocyclohexanol[4]. - Alkylation-Catalyst : Si3 (PO& - Si (HP04)2 ; products : 2,4-ditert-butylphenol and 2,4,6-tritert-butylphenol.
-
3. RESULTS AND DISCUSSIONS To establish the played part by the various phases on the course of the reaction all the parameters which can act upon the reactivity and the selectivity have been studied. So the gas volume absorbed varies inversely with the substrate grain size. When the grain size decreases the number of contacts between organic solid/catalyst increases. With 4-tert-butylphenol grains of small size, the number of contacts with the catalyst is high and the reactivity increases. Hydrogen, deuterium and isobutene volumes absorbed vary also with the ratios of catalysts. When the ratios of catalysts increase the transformation rates increase. By increasing the ratios of catalysts the numbers of contact between catalysts and organic solids increase. These results show that organic solid-gas reactions are contact reactions between solid catalysts and organic solids. The reactions take place at the vicinity of the catalyst in a three phases system gas/solid CatalysVorganic solid. To localize the reaction several kinds of contacts between catalyst and organic solid have been considered : a- refer mixture : the catalyst and the 4-tertbutylphenol in the weight proportion of 0.1 are mechanically mixed. This mixture is placed at the bottom of the reactor to form a thin layer (conversion 80%) ; b- refer mixture pressed (conversion 10%) ; c- only organic solid is pressed, the catalyst is placed loosely on the organic surface (conversion 5%) ; d- powder of organic solid covered by a layer of catalyst (conversion 7%). In spite of the decreasing of the intergrain distances and the increasing of the average number of contacts (compare a and b) the reactivity decreases. One can think that others parameters than distance play a part in the reaction. For example the access of gas to the catalyst and the possibility of reaction in the interface. Whatever the mixture we observe a localisation of the products. For example the needles of cis and trans 4-tert-butylphenol appear at the edge of the catalyst layer or on the external line of the pressed powders where the catalyst grains are on contact with hydrogen. So the reaction occures in the interface organic solidkatalyst. The intergrain distances and the average number of contacts are
2493 significant parameters but it would appear that the nature of the interface playsalso a part in the organic solid-gas reaction. The reactions occure between and adsorbed gaseous reactive and an organic solid which cannot be adsorbed on the catalyst. It appears that such reactions do not follow a classical mechanism. To explain the course of the solid-gas reactions various hypotheses have been suggested [5.7]. The catalytic process can be attributed to moving reactive species. A diffusion occures between a donor (the solid catalyst) and an acceptor (the organic solid). So, to describe the solid-gas reactions one can envisage that the diffusion of mobile reactive species controls the rates of the reactions. In the case of solid state hydrogenation and deuteration a theoretical model is established. The mathematical representation of the model is based on solution of two differential equations. - Equation of material balance or mass which gives the quantity of mobil species passing through the solid by diffusion
-
Equation of rate formation of products which depends on the quantity of mobil species reacting with the solid and on the concentration of the solid and/or its surface.
[Cp]=[So] [ 1 - exp (-kp Cst]
(2)
From (1) and for the whole solid we obtain the final expression of [Cp]
Where [C,] represents either the transformation of the solid or the formation of products ; [S] refers to the concentration of the organic solid and/or its surface. D is the diffusion coefficient and 0 is the flux density or the quantity of gas entering into is the rate constant of transformation of the solid substrate or of the the formation solid. o solid products ; k is the rate constant of the gas disappearrence. The dependence of [Cp] on these different parameters kdk, D and 0 have been simulated by computational techniques. The theoretical cutves obtained are similar to the experimental ones. So the proposed model gives a good description of the kinetical and physical parameters of the gas-solid systems and confirms that catalytic organic solid state hydrogenation occurs according to a diffusional process. In the case of solid state alkylation two methods of investigation are considered : The polynomial approach without any pre-established model. The treatment of the conversion ratio versus time according to pre-established models. The mathematical fit of the experimental points of each kinetic curve is carried on with a polynomial function
Y
2494 All the curves are fitted with a three order polynomial. It is established that the curves giving the conversion ratio versus reduced time are affine. Hence, the alkylation of solid 4-tert-butylphenol occurs according to the same process whatever the conditions. All the kinetics can be depicted by the same model. In order to find the kinetic model the best adapted to describe the physical and chemical phenomena which occure during the alkylation reaction, we have tested several pre-established models. The analysis of results shows that the best model for alkylation of solid 4-tert-butylphenol is the diffusion one (1 - a ) I n ( l - a ) + a = k t
(5) It would appear that gas-solid alkylation is controlled by the diffusion in a two dimensional process.
CONCLUSIONS
4.
The experimental and theoretical studies of the catalytic solid state hydrogenation, deuteration and alkylation of 4-tert-butylphenol allow us to propose a three steps mechanism taking into account the part played by the catalyst, by the diffusional process of gaseous species and by the reaction between organic solid and the gaseous reactive species.
-
1 Dissociation : Gaseous molecules are adsorbed on the catalyst to form dissociated gaseous specis 2 - Diffusion : Dissociated gaseous species migrate from the catalyst (donor) until the surface of the organic solid compound is reached (acceptor) 3 - Reaction : Dissociated gaseous species react with the organic compound to form products. 5. REFERENCES
1 2 3 4 5 6
7
M.D. Cohen and B.S. Green, Chem. Brit. 9 (1973) 490. R. Perrin, R. Lamartine, M. Perrin and A. Thozet Studies in Organic Chemistry, Vol. 32, G.R. Desiraju Ed., Elsevier, Amsterdam 1987, p. 271. R. Perrin and R. Lamartine, Studies in Physical and theoretical Chemistry, Vol. 69, M. Pierrot Ed., Elsevier, Amsterdam 1990, p. 107. R. Lamartine, F. Lamsouber and R. Perrin, Mol. Cryst. Liq. Cryst. 161 (1988) 163. R. Lamartine and R. Perrin, Studies in Surface Science and Catalysis, Vol. 17, G.M. Pajonk, S.Teichner, Eds., Elsevier, Amsterdam 1983, p.251. B.K. Hodnett and B. Delmon, Studies in Surface Science and Catalysis, Vol. 27, Cerveny Ed., Elsevier Amsterdam 1986. A.V. Filikov and N.F. Myasoedov, J. Phys. Chem., 93 (1989) 2502.
Guni, L ef al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
INFRARED STUDY OF COKE DEPOSITION ON ALUMINA J . Datkaa and R. P. Eischensb aFaculty of Chemistry, Jagiellonian University, Crakow, Poland bZettlemoyer Center for Surface Studies, Department of Chemistry, Lehigh University, Bethlehem, PA 18015, USA
In previous work bands near 1580 and 1460 cm-' have been observed in the spectrum of coke deposited on alumina by exposure to acetylene or ethylene at elevated temperatures [1,2]. Isotopic substitution experiments, involving an oxygen18 exchanged alumina, have shown that these bands are due to an oxidized species [ 2 ] . This species appears to be a carboxylate, rather than a formate or a carbonate, because identical bands are observed after chemisorption of acetic acid on alumina. Observation of an oxidized species in coke was unexpected because coke deposition involves reducing conditions. The study of coke on alumina was part of a study of coke deposition on alumina-supported metals such as platinum, rhenium, and tin. The metal containins samples were studied after reduction in hydrogen at 350OC. - Because of this factor, the alumina was also reduced in hydrogen prior to the coke deposi0.5 tions. Prior reduction of alumina did not affect carboxylate formation. The absence of any effect due to w prior exposure of the alumi0 z na to hydrogen suggests that a residual active oxygen is m az not the source of the oxygen 0 which produces the carboxylv) m ates. For this reason, the a current study focused on the surface hydroxyls of the alumina to determine whether these hydroxyls are involved in carboxylate formation. Figure 1 shows the carboxylate bands observed after exposure of DeGussa Aluminum Oxide-C, surface area 100 m2/gl to 10 Torr of acetylene
u I600cm-1
I400
Fig. 1. Carboxylate Bands on Alumina.
2496
at 250OC. The spectra were observed by transmission through alumina discs weighing 75-100 mg and having a face area of 2.6 cm2. Similar bands are observed after exposure to ethylene, or mixtures of 10 Torr of ethylene and 760 Torr of hydrogen [2]. The entire spectra also show small bands due to carbon-hydrogen stretching and bending vibrations. Acetylene and ethylene were chosen for these coke deposition studies in order t o minimize hydrocarbon bands because these bands complicate the 1600-1400 cm-'region. In prior work it was estimated that fifteen percent of the carbon atoms in acetylene coke were present as carboxylate at total coke loadings of one percent [l]. Figure 2 shows the effect of carboxylate formation on 0.3surface hydroxyls. Spectrum A was observed before exposure to acetylene. It shows bands at w 0 3765, 3715, 3670, 2 and 3527. The 3765 band should be re- e solvable into two 0 m bands [3] but res- m olution of the FTIR Q. spectrometer is poor in this region. It is evident that the high-frequency bands decrease during car1 I I I boxylate formation while the low-fre3700cm-1 3500 quency bands are not affected. The high Fig. 2. Effect Of Carboxylate Formation On frequency hydroxyl Surface Hydroxyl Bands of Alumina. bands of alumina are due to the most basic hydroxyl groups [4]. Boehm has found that basic surface hydroxyls of titanium oxide react with olefins to produce alcoholates [5]. A corresponding reaction between ethylene and alumina hydroxyls would produce A1-0-CH,H,. Alcoholates have been visualized as intermediates in the formation of carboxylates on alumina after exposure t o alcohols [6,7]. Thus, it appears reasonable to conclude that the carboxylates of Figure 1 are produced by the reaction of surface hydroxyls with the hydrocarbon.
2
There is a question as to whether results of coke deposition studies involving acetylene and ethylene can be extrapolated to higher hydrocarbons. Studies of coke from a variety of sources do not indicate infrared bands which would be assigned to oxidized species. For example, Blackmond and co-workers [8], Karge [ 9 ] , and Eisenbach and Gallei [lo] have not found carboxy-
2497 lates in infrared studies of coke deposition from hexene on zeolites. Instead, they report a conventional coke which resembles carbon from many sources such as coal and hexane soot [ll]. This conventional coke has a strong band near 1600 which is commonly, but not conclusively, assigned to the carbon-carbon stretchings in aromatic rings, a moderate band in the mid 1300 region and a very small band near 1460. Because of the differences between these literature reports and the results shown in Figure 1, an HY zeolite was exposed to acetylene under the conditions which had produced the carboxylate bands of Figure 1. In this case carboxylate bands were not observed. The result was a conventional coke with a strong band at 1590, a moderate band at 1382, and a weak, barely detectable band at 1460. These results show that, on zeolites, hexene and acetylene produce the same results. It is the differences between alumina and zeolite which produce the contrasting results. The most simple explanation is that the zeolite lacks the basic hydroxyls which are found on alumina. At this time this explanation is favored. However, it is not possible to completely exclude the possibility that the differences are not due to the A1-0-A1 structure of alumina and the Si-0-A1 structure of the zeolite. This research was supported by the Division of Chemical Sciences of DOE'S Office of Basic Energy Science. We are grateful to Mrs. Marge Sawyers for assistance in preparing this manuscript. lK.H. Ludlum and R.P. Eischens, Preprints of the Petroleum Division, American Chemical Society Meeting, April 1976. 2J. Najbar and R.P. Eischens, Proceedings of the 9th International Congress on Catalysis, Calgary 1988, paper 184A6. 3J.B. Peri, J. Phys. Chem. a,220 (1965). 4H.P. Boehm and H. Knozinger, "Catalysis Science and Technology," M. Boudert, ed., vol. 4 (1983). 5H.P. Boehm, Kolloid Z., 2. Polymers W , 17 (1968). 6R.0. Kagel, J. Phys. Chem. 844 (1967). 716 (1969). 7A.V. Deo and I.C. Della Lana, J. Phys. Chem. 8D.G. Blackmond, J.G. Goodwin, Jr., and J.E. Lester, J. Catal. D ,34 (1982). 9 H.G. Karge, Studies in Surface Science and Catalysis, vol. 58, Introduction to Zeolite Science and Practice, eds. H. Van Bekkum, E.M. Flanigan, and J.C. Jansen, Elsevier: Amsterdam, 1991. 10D.Eisenbach and E. Gallei, J. Catal. 86, 377 (1979). llM.S. Akhter, A.R. Chughtai, and D.M. Smith, Applied SpeCtrOScopy 39, 143 (1985).
n,
n,
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Guni, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
HYDROGEN SURFACE CONCENTRATION EFFECT ON THE TEMPERATUREPROGRAMMED HYDROGENATION OF ADSORBED CARBONACEOUS SPECIES ON AN IRON/ALUMINA CATALYST H. Halafa, E. Borgsredrb, A. M. Efsrarhioub, S. L. Suibb and D. Bianchia %boratoire des Materiaux et Procedes Catalytiques, Universite Claude Bernard Lyon I, 43 Boulevard du 1 1 Novembre 1918,69622 Villeurbanne W e x , France bInstituteof Material Science, Department of Chemical Engineering and Chemistry, T 06269, USA University of Connecticut, Storm C
Abstract Temperature-programmed hydrogenation (TPH) of adsorbed carbonaceous species on metallic catalysts is a useful method to measure their numbers and reactivities. In the present paper it is shown that changes in the T.P.H spectrum must also be interpreted in terms of the change in the hydrogen surface concentration.
1.INTRODUCTION Characterization of adsorbed carbonaceous species by isothermal or temperature-programmed hydrogenation (TPH) conditions is a method widely used particularly with metal supported catalysts (1-2). The adsorbed species formed during catalytic reactions such as CO/H2 or cracking mainly give CH4 and small quantities of other alkanes by hydrogenation. In the case of Fe-supported catalysts the reactions which involve CO lead to the carburization of the iron which is also hydrogenated to CH4. For isothermal and temperature-programmed hydrogenation the response of the rate of CH4 production may produce several peaks. Kinetic models ( 2 - 4 ) have been used to interpret these responses and to reveal the effect of the main kinetic parameters such as the number of elementary steps with the same rate constant k (k= A exp(-Er/RT), the activation
2500 energy of hydrogenation Err and the atomic hydrogen surface concentration H. These three parameters do not have the same importance either for isothermal or TPH. It has been shown ( 3 ) that in isothermal conditions the mechanism of the hydrogenation process strongly affects the results. The activation energy and the hydrogen surface concentration act to a lesser extent. In temperature-programmed conditions the mechanism of the process is a second order factor. The temperature T, at the maximum of the peak is related to kinetic parameters by Er=RTmLn(AH/am) [I] (with am the heating rate ) and is independent of the number of elementary steps involved (4). TPH results are mainly affected by the activation energy of hydrogenation but to a lesser extent by the hydrogen surface concentration H. On the other hand, the effects of this parameter H on the TPH is rarely considered and the shift of a TPH peak toward higher temperatures is generally interpreted as due to the formation of a less active species (similarly a shift of T, to lower temperatures as due to the formation of a more active species). In the present paper results obtained from a 10% CO/H2 reaction with an Fe/A1203 catalyst show that, the modification of the TPH spectrum by a specific treatment must first be related to the change in the hydrogen surface concentration and second to the nature of the adsorbed species. 2.EXPERI13ENTAL WETHODS A N D RESULTS
The catalyst used is a 10 wt%Fe/Al203 (Alon-C Degussa) obtained by a precipitation method. The main analytical system, using a quadrupole mass spectrometer (Leybold-Heraeus) as detector, permits to make in flow conditions, various types of transient experiments such as: TPH and H2 chemisorption on the same catalyst sample (3-4). The changes in the bulk iron are followed at room temperature by MUssbauer spectroscopy (35 plus multichannel analyzer from Canderra Industries with the M6ssbauer drive and MUssbauer transducer from Austin Scientific Associates ,Inc). After reduction of the catalyst at 773 K for 14 h, MUssbauer spectroscopy shows that the iron is a mixture of metallic iron and irreducible Fe+2 as observed in a previous study (5). The introduction of 10% CO/H2 mixture at 558 K for 20 min leads to the carburization of the bulk iron and the disappearance of the metallic iron. After the CO/H2 reaction at 558 K and a switch to He ( ~ O S ) ,the reactor is cooled to room temperature, followed by a switch to hydrogen. The TPH response (Fig.1, curve a) is obtained with a linear heating rate of 5 K / s . In a second experiment, after the switch to He at 558 K, the temperature is raised to 673 K for 10 min prior to the TPH experiment (Fig.1, curve b) This treatment gives
.
2501
only small differences in the Mossbauer spectrum. In a third experiment after the switch to He at 558 K the temperature is raised to 873 K for 10 min prior to the TPH experiment (Fig.1, curve c). This treatment leads to a Mossbauer spectrum similar to the one obtained after the reduction of the catalyst ( the carbide dissapears by the treatment at 873 K). During heating in He, CO formation is detected. In relation to these experiments the hydrogen surface concentration is measured before each TPH according to the procedure described earlier (6). The amounts of atomic hydrogen chemisorbed are : H=29 pmol/g of cat., after 20 min of CO/H2, Hf=45 pmol/g of cat., after He treatment at 673 K and Hff=47pmol/g of cat., after He treatment at 873 K. 900
c
0 Kl
700
E a (0
1
nl
rt
C
500
1
m
X Y
a)c, m
p:
300 0
60
120 Time ( s )
Figure 1. catalyst : a) same as a) but same as a) but
TPH of adsorbed species formed on a Fe/A1203 After 20 min in 10%CO/H2 mixture at 558 K, b) followed by 10 min treatment in He at 673 K, c) followed by 10 min treatment in He at 873 K.
3.DISCUSSION
In curve a) of Fig.1 the peak at Tm1=840 K is due to the carbide, the peak at Tm2=805 K corresponds to an adsorbed carbonaceous species and the shoulder at Tm3=660 K to a more active adsorbed carbonaceous species. After the treatment at 673 K, the three peaks are clearly detected but at lower temperatures (Tm1t=762 K, Tm2t=700 K, Tmjt=576 K). Note that the first peak increases whereas the second peak decreases. The observed shifts are not attributed to the formation of more active species with a lower activation energy. Assuming that the activation energy Er is constant, and using: A=10-2 crn2/(site*s), metallic surface area=15 m2/g of cat., and am=5
2502 K/s equation [I] provides a ratio of (Ln(AH/a,))/(Ln(AHI/am))= 0.98 which must be equal to the ratio Ra=Tm/Tmr. The experiments give for the three peaks the ratios: Ra1=0.91, Ra2=0.87 et Ra3=0.87. These values are in good agreement with the value of 0.98. Therefore, the shifts observed are due to the change in the hydrogen chemisorption and not to the change in the nature of species. The quantity of CH4 produced in each peak shows that the treatment in He at 673 K converts parts of the less active adsorbed species to the more active species which increase from 50 pmol.C/g cat. to 160 pmol.C/g cat. Curve c) differs from curve b) but the hydrogen surface concentration H is almost constant after the two experiments. This agrees well with the fact that the first peak appears at the same temperature as in curve b). Mossbauer spectroscopy shows that carbide decomposed and that it is the initial is actually recorded. spectrum after reduction which Therefore, the second peak reveals the presence of a new adsorbed species with a high activation energy of hydrogenation. The total quantity of CH4 produced decreases according to the treatment in He. This comes from the formation of water by the hydroxyl groups of the alumina support. The water decomposes on the iron surface (6) and oxygen reacts with surface carbon to give CO which desorbs. It has been observed that the TPH show a shift of the CH4 peaks (Fig.1 curve a) toward higher temperatures when the time on stream in CO/H2 increases (4). Atomic hydrogen surface concentrations measured after each time of reaction show a decrease from 45 pmol/g of cat. on a fresh reduced catalyst to 38 pmol/g of cat. after 8 min in CO/H2 and to 32 pmol./g Of cat after 15 min. The observed shift comes from the decrease in the hydrogen surface concentration with time on stream and not from the formation of less reactive species. 4
CONCLUSIONS
The present study shows that measurement of the hydrogen concentration is necessary in order to understand changes in the temperature-programmed hydrogenation spectra. 5 REFERENCES
1 Falconer,J.L and Schwartz,J.A., Catal. Rev 25,141,(1983) 2 Bhatia,S., Beltramini,J., and Do,D.D., Catal Today 7, 309, (1990) 3 Bianchi,D. and Gass,J.L., J Catal. 123, 298, (1990) 4 Bianchi,D. and Gass,J.L., J.Cata1. 123 I 310, (1990) 5 Tau,L.M., Borcar,S., Bianchi,D. and Bennett,C.O. J of Catal. 87, 36 , (1984) 6 Ahlafi,H., Bianchi,D. and Bennett,C.O., Appl-Catal. 66, 99, (1990)
Guczi, L.d al. (Editors), New Frontiers in Caralysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
FTIR AND CATALYTIC STUDIES OF THE EFFECTS OF SULPHUR POISONS ON Cu/AI,O, CATALYST SELECTIVITY
M. B. Padleya, C. H. Rochestera, G. J. Hutchingsb, P. I. Okoyeband F. Kingc aDepartment of Chemistry, University of Dundee, Dundee DD14HN, United Kingdom bLeverhulmeCentre for Innovative Catalysis, University of Liverpool, P.O.Box 147, Liverpool L69 3BX, United Kingdom Katalco R & T Group, P.O.Box 1, Billignham, Cleveland TS23 lLB, United Kingdom
INTRODUCTION The adsorption on catalyst,s of materials normally regarded as poisons may lead to significant changes in catalyst selectivity. The extent of poisoning may differ for different possible reaction pathways in a particular system, and therefore influence product distribution. Changes in selectivity may be affect,ed by t,he compounds undergoing reaction, the kind and concentrat.ion of added poison, the conditions of poisoning and of reaction, and the catalyst properties. The main objective of our present research is to study the modification of catalytic selectivity by the cont,rolled addition of poisons at levels below those required for total loss of catalytic activity. It was decided t,o study copper catalysts supported on alumina, because these are relevant to methanol synthesis and water-gas shift react,ions. Copper catalysts are also active for a wide variety of other reactions, and therefore provide good opportunity for flexible choice of systems for testing the effect of poisons. The present, paper will report selectivity data for the hydrogenation of crotonaldehyde over Cu/A1 0 in the presence and 2 3 absence of controlled amount,s of SO as a sulphur poison. The data are correlated with infrared results, cgaracterising the effects of the poison on the copper surface by monit,oring the changing mode of CO adsorption.
EXPERIMENTAL Catalyst precursor was2pregared by impregnation of Condea SCF yalumina (surface area 136 m g- ) with aqueous copper(I1) nitrate followed by drying at. 383 K. The dried precursor was mounted either as a pressed disc in an infrared cell or in a micro-reactor before calcination (573 K, 16 h) in flowing dry air and subsequent reduction (483 K, 16 h) in flowing hydrogen to give a reduced 5 wt.% C u / A 1 0 catalyst. Infrared spectra were recorded with a Perkin Elmer 1720 $ ?TIR spectrometer. Catalyst selectivities were measured using a fixed bed microreactor, coupled to an online gas chromatography fitted with both TCD and FPD.
2504
al
Pk
P
2300
2150 2OOO wavenumber/cm-l
Figure 1. Reduced Cu/A1203 catalyst2exposed to ( a ) 530 N m o_f2C0,followed by (b) 270 N m of SO2. Sample then allowed to stand for (c) 5, (d) 10 and (e) 20 min. (f) Difference spectrum (e)-(a).
2300 2150 2000 wavenumber/cm-1
Figure 2. Reduyed Cu/A1 0 exposed 2 to ( a ) 530 N m- of CO fol?owed by (bh - 13.5, (c) 26.5, and (d) 40 N m of SO (spectra were recorded 2 . after equilibration).
RESULTS
Consecutive and competitive experiments have been carried out involving addition of CO and a sulphur poison to Cu/A1 0 . Effects of 2 3 oxidation state of the Cu, surface coverage, gas pressure, time and temperature have been monitored. Figure 1 shows some time-dependent behaviour of adsorbed CO on Cu after subsequent addit'on of sulphur di-i oxide. Spectrum (a) shows bands at 2094 and 2140 cm , due to CO on Cuo and Cu', respectively. Spectra (b)-(e) are for increasing time ( < 2 0 rnin, 295 K ) after the addition of sulphur dioxide. The difference spect,rum (f) showf the slow removal of Cuo-CO and the growth of bands at, 2148 and 2121 cm , due to CO on oxidised Cu sites. Further experiments show that the oxidative effect of SO on Cu does not chemically involve CO, although 2 the latter does impede the reaction via blocking of surface sites. In the absence of CO the oxidative reaction of SO with reduced catalyst was com2 paratively rapid ( X* and enrichment in Pt for X < X* (X* = 25 at
2521 700'C and 40 at 900'C). In oxidizing atmosphere, Pt-Rh bimetallics must normally be enriched in Rh which forms the most stable oxide. This is verified at X > X*. The apparent enrichment in Pt at low Rh content would be due to the subsurface diffusion of Rh-ions in alumina 11, 4-51.This phenomenon which tends to decrease the surface concentration of Rh becomes preponderant at X < X*.
Transient CO oxidation (OSC) Table 1. OSC measurements : amounts of CO, formed at 450'C preoxidized samples (1st pulse of CO). Catalyst
PtRhlOO (pure Rh) PtRh45
137
33 26
x,
0
75 41
27
x*
c
64.0 31.0 12.4 6.4 5.6 5.4
115 102
84 63 52 40
PtRh28 PtRhl9 PtRhl2 PtRhO (pure Pt)
sintered at 900'
sintered at 7 0 0 ' ~
fresh
(pmo1.g-l) on
Rh surface %
50
100
x
0
50
100
Rh at. %
Figure 1. Surface composition of PtRh bimetallics sintered at 7OO'C (a) and 9OO'C (b); * * transient CO oxidation (OSC); -* XPS .
-1602 + l*02 ;
- --
-
-
A part of the active "oxygen" is stored in the metal oxides and another part is stored by spillover on the support. This is clear for pure Rh (PtRhlOO)which requires 75 pmol CO g'l for the reduction of Rh2O3. Rhodium is more apt than platinum to store active "0"(in metal oxide and on A1203), so that OSC values can give an accurate picture of the metal surface in the reaction. Assuming that there exists a linear relationship between the OSC values and the surface composition, we have :
2522
xs = 100 [OSC(PtRh) - osc(Pt)]/[osc(Rh) - OSC(Pt)]
(3)
For the fresh samples, Xs is close to X thus conforming that there is no surface enrichment in these samples. The values of Xs deduced from Eqn 3 for catalysts sintered at 700'C and 900'C are shown in Fig. 1. The curves Xs vs X are close to those which are obtained from isotopic measurements. In particular the same zones of surface enrichment and the same transition point at X = X* are observed.
XPS results. The surface weight-% is higher than the bulk one for Rh and lower for Pt; both contents decrease by heating as a consequence of the sintering (Table 2). Nevertheless there remains a surface enrichment of rhodium for all the samples and there is no change between 700 and 900'C (see figure 1). The presence of platinum helps to keep a reduced state of rhodium for the samples calcined at 700'C (shift of the Rh3d5/2 peak from 309.5 to 306.8 eV), whereas the calcination at 900°C leads to an oxided state for the most part of rhodium (peak at r. 309.8 eV). Table 2. XPS data : bulk and surface compositions after sintering at 700 and 900'C; evolution of the Rh3d5/2 peak (binding energies in eV). bulk composition surface composition
700 OC sample wt%-Rh wt%-Pt Rh3d5/2 wt%-Rh wt%-Pt PtRhl00 PtRh45 PtRh28 PtRhl9 PtRhl2
0.51 0.17 0.09 0.05 0.04
0.55 0.55 0.50 0.52
309.5 307.6 306.9 306.8sh 307.6
1.49 0.75 0.39 0.62 0.21
0.23 0.23 0.59 0.32
Rh3d5/2 309.8 309.7* 310.0 309.8 309.6
900 OC wt%-Rh wt%-Pt
1.oo 0.48 0.18 0.21 0.16
0.14 0.11 0.24 0.24
* shoulder of the main peak at 306.9 eV. REFERENCES 1 S. Kacimi and D. Duprez, in "Catalysis and Automotive Pollution Control, CaPoc2" (A.Crucq, Ed.) Stud. Surf. Sci. Catal., Vol71 p 581, Elsevier, Amsterdam (1991) 2 D. Duprez, H. Abderrahlm, S. Kacimi and J. Riviere, in "Proc. 2nd Int. Conf. Spillover" (K.H. Steinberg, Ed.). Karl-Marx Universitit Publ., Leipzig, p 127 (1989). 3 J.R. Anderson, in "Structure of metallic catalysts", Acad. Press, New York (1975). 4 H. C. Yao, S. Japar and M. Shelef, J. Catal., 50 (1977) 407. 5 D. Duprez, G. Delahay, H.Abderrahim and J. Grimblot, J. Chim. Phys., 83 (1986) 465.
Guczi, L.et ul. (Editors), New Frontiers in Caalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
COKE DEPOSITS ON Pt/AI,O, CATALYSTS: FTIR AND HRTEM STUDIES L. Marchese, E. Borello, S. Coluccia, G. Mama and A. Zecchina Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, Via P. Giuria 7, 10125 Torino, Italy
Abstract Graphitic and amorphous coke deposits on Pt/A1203 catalysts are evidenced by HRTEM. Their interaction with the metal phase and the support is studied by the spectra of CO adsorbed at 300 and 77K. A band due to li uid-like CO stabilized on coke at 77K is observed at 2136 cm-P
.
1. INTRODUCTION
Coking and sulfidation are two main deactivation processes for reforming catalysts [l]. It is relevant to determine the structure of the deposits, their location, the treatments to remove them and the structure of the metal phase after reactivation treatments. These points are examined in this report by Infrared Spectroscopy and High Resolution Transmission Electron Microscopy. CO adsorption at 300K and 77K has been adopted as a test for measuring the overall surface activity both of the metal phase and the support. It is found that CO also interacts with the coke phase. 2. EXPERIMENTAL
The results refer to a Pt/A1203 sample (CK 306, Pt 0.6 wt % by Cyanamid Ketjen) which, after catalytic runs, showed a 6.7 wt % of coke content. The reforming feed was a H /n-eptane mixture (molar ratio =2) ; the n-eptane contained g.4 ppm of sulfur. Infrared spectra were obtained by Bruker IFS 48 and the HRTEM micrographs by a Jeol 2000EX instruments. 3. REBULTS AND DISCUSSION
Electron Microscopy Carbonaceous compounds on a particle of the coked catalyst are visible in the high resolution micrograph in Fig. 1. Curved fringes 3.6 apart in zone A are associated with graphitized carbon black (interplanar spacing d 002 - 3.4 A ) . Coke deposits with amorphous structure (as aigueh-by their 3.1.
2524
Figure 1. High resolution electron micrograph showing graphitized ( A ) and amorphous (B) coke deposits on Pt/A1203 catalyst.
disordered arrangement [2]) are present in the zones B. HRTEM analysis of the Pt phase is not discussed in this short report. However, Pt particles size was determined to be 10-15 A. IR spectra of t h e a a t a l y a t Figure 2 A shows the IR spectra of the coked Pt/A1203 catalyst reactivated by gradual oxidation (150 Torr 0 ) at increasing temperature. Disc ete, though broad, absorpsions are observed at 1700-1100 cm-' in the spectrum of the original coked sample (curve a). The overall transparency in t h e IR region increases gradually as the oxidizing temperature increases (curves b-d) and the sample turns rom black to whitish. Moreover, the bands at 1700-1100 cm-' are strongly reduced in intensit and in spectrum d only weak components in the 1600-1450 cm-' ra ge are left. By contrast, the intensity of a band at 1370 cm-' increases as the oxidizing temperature increases up to 773K. This band disappears upon heating in H2 at 973K. The bands which disappear by oxidation are certainly associated with oke. In particular, two correlated bands at 1580 and 1470 cm-' are due to the asym. and sym. stretching vibrations of carboxylate groups [3], while the other components are due to deformation modes of residual CH groups as found in carbonaceous deposits with llmolecularll form and with high H/C ratio [4]. Noticeably, bands in this region were also attributed t o carbon-carbon stretching vibrations in distorted graphitic structure in dust carbon particles and in soccerball shaped st uctures (c60, fullerene-like molecules) [5]. The 1370 cm-' band is due to sulfate groups [6] which originate from oxidation of the sulfur containing compounds which are fed into the reactor during the catalytic runs. 3.2.
2525
CO adsorption at 300K on Pt. Figure 2B shows the IR spectra of CO adsorbed (80 Torr) at successive stages of removal of coke from the surface. Before adsorption experiments, samples were treated in H2 t o reduce the metal phase to%--' and fhen outgassed at 773K. The bands i the 2120-1950 cm- range monitor CO linearly adsorbed on Pt8 sites [7]. These absorptions are very weak in curve a, showing that on the original sample the Pt sites are almost completely poisoned by coke. They are much more intense in curve b, indicating that, by oxidation at 473K, a large fraction of Pt sites are freed from carbon deposits though the majority of these contaminants are still present on the sample (cfr. Fig. 2A,b). Some increase of the intensity of the is observed in the last stages (Fig. 2B,curves c-d). Apparently, the bonds between Pt sites and carbonaceous contaminants are destroyed in the first steps of oxidation, whereas the bulk of coke deposits burn off at higher temperature. The spectrum of CO adsorbed on the sample regenerated at 773K (Fig. 2B,d) is similar to that of the fresh uncoked catalyst [7] and this suggests that dimension and morphology of the metal particles did not change significantly upon the catalytic runs and regeneration procedures. 3.3.
CO adsorption on the AlZ03 support t 300K. Only a small fraction of cationic All' sites stabilize CO at 300K €;I. The two bands at the highest frequencies (21802250 cmin the spectra at 300K are due to CO adsorbed on the alumina support [8]. They are already present in Fig. 2B,a, showing that surface A13+ sites are available for CO adsorption even pn a heavily coked sample. Only the very we k band at 2235 cm- , due to CO adsorbed on the most acidic A1 9+ sites, appears after the last stage of regeneration (oxidation at 773K, Fig. 2B,d), indicating that such sites were previously interacting with carbonaceous deposits. 3.4.
CO adsorption at 77K on A1 O3 and on Coke. Pratically all A13+ sites a8sorb CO at 77K [ 8 ] . The spectrum in the inset in Fig. 2B refers to CO adsorption on the original coked sample. The bands at 2185 and 2160 c m ' l are due to CO on tetrahedral and octahedral A13+ sites respectively [8]. Their intensity slightly increases upon regeneration, confirming that only a small fraction of the surface sites of the support are involve in the stabilization of coke. The band at 2136 cm-' is assigned to liquid-like CO stabilized on the coke phase. In fact, its intensity progressively declines as the carbonaceous contaminants are gradually burnt off and, consequently, CO appears to be a useful probe molecule also for characterizing carbonaceous contaminants. 3.5.
Acknowledgement. The authors are grateful to CEE and ASPPiemonte for financial support and t o prof. G. Froment for providing the coked sample.
2526
I
I
I
I
2200 1800
'
I
L
1400' 100 -1
Wavenumbers cm
Wavenumbers cm-
'
Figure 2. Infrared spectra of coked Pt/A1203 catalyst gradually regenerated by oxidation at increasing temperature. Sect. A : background spectra (transmittance) of the original coked sample (curve a) and after oxidation at 473,573 and 773K respectively (curves b-d). Sect. B: spectra (absorbance) of CO adsorbed at 300K on the sample at the successive stages of regeneration quoted in Sect. A. Inset: CO adsorbed at 77K on the original coked sample. 4.
1
2 3 4
5 6
REFERENCES J. Oudart and H. Wise (eds.), Deactivation and Poisoning of Catalysts, Marcel Dekker, Inc., New York, 1985. P. Gallezot, C. Leclercq, J. Barbier and P. Marecot, J. Catal. 116 (1989) 164. J. Najbar and R.P. Eischens, in M.J. Phillips and M. Ternan (eds.), Proceed. 9th ICC, Calgary 1988, vol. 3, p. 1434. M.J.D. Low and C. Morterra, Structure and Reactivity of Surfaces, C. Morterra, A. Zecchina and G. Costa (eds.), Elsevier, Amsterdam, 1989. W. Krkitschmer, K. Fostiropoulos and D. R. Huffman, Chem. Phys. Lett., 170 (1990) 167. C.R. Apesteguia, T.F. Garetto and A. Borgna, J. Catal., 106 (1987) 73.
7
8
L. Marchese, M.R. Boccuti, S. Coluccia, S. Lavagnino, A. Zecchina, L. Bonneviot and M. Chef Structure and Reactivity of Surfaces, C. Morterra, A. Zecchina and G. Costa (eds.), Elsevier, Amsterdam, 1989. A. Zecchina, E. Escalona Plater0 and C. Otero Arean, J. Catal., 107 (1987) 244,
Guni, L et al. (Editors), New Frontiers in Catalysis Proceedingsof the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 6 1993 Elsevier Science Publishers B.V.All rights reserved
MOLECULAR ORBITAL STUDY OF THE CHEMISORPTION OF SMALL MOLECULES ON MgO SURFACES H. Kobayashia,A. St. Amantb, D. R. Salahubb and T. Itoc BDepartment of Applied Chemistry, Kyoto Prefectural University, Shimogamo, Kyoto 606, Japan bUniversite de Montreal, Departement de Chimie, C.P. 6128, Succursale A, Montreal, Quebec H3 6 5 7 , Canada cDepartment of Chemistry, Faculty of Science, Tokyo Metropolitan University, Minamiohsawa, Hachiohji, Tokyo 192-03, Japan
Amract
Electronic structures of Mg0 and chemisorption of methane molecules on it were investigated by theoretical calculations based on the local density functional
method. Both the MgrlO4 duster and two layer slab models were employed. For the latter model, the importance of long range electrostatic interactions was discussed,and the point charge comction method was found to be pmmising to improve the slow convergence of total energy. 1. INTRODUCI'ION
Hyd~ugenand methane dissociate easily on MgO surfaces. Many researchers have paid attention to this interesting pmperty. The outline of the reaction mechanism which is consistent so far with many experimental results, is an ionic dissociative o n e the molecule is adsorbed onto the surface Mg2+ and 02-ions with a low (most probably three) cootdination number. In our recent paperxIl2L the dissociative adsorption of hydrogen and methane molecules were examined using a M u 0 4 cluster model and the Hartree-Fock and Wer-Plesset (2nd order) perturbation methods. The next step to be elucidated may be the methanation step which is an important reaction in relation t o efficient use of natural gas. However, it is well known that the methanation does not pmceed as smoothly as simple dissociative adsorption. Many experimental workers have been trying to impmve catalysts, and theoretical approaches are also expected to contribute t o this field The MgqO4 cluster model consists of only the three coordination site. To
2528
extend the research to include the other coordination sites, larger cluster models are necessary. This point is a bottle neck in the general theoretical approaches. The local density functional(LDF)method has developed so much in the 1980s. Using the LDF method, the computing time is reduced to N3 order from N4 order of the Hartree-Fock method. (Nis the number of basis functions.) nlerefore calculations with larger clusters become much easier. Furthermore the slab model calculations with realistic atomic arrangements become'possible. In this short communication, we will present (1) the theoretical approach on methanation with a comparison to our previous H a m - F o c k calculations, and (2) its application to periodic systems, espedany the treatment of the long range electrostatic interactions (Madelung energy) and the differences in the electronic structures calculated by the cluster and slab models. 0.96i 2 MODELS AND METHOD OF CALCULATION We used an MgqO4 cubic cluster (shown in Fig11 as a model of three coordination sites on the surface. For the slab model calculations, we employed the unit cells containing two Mg and Fig.1. Configuration of Mg 404 two 0 atoms, which cover the upper two layers cluster with two 0-H bonds on the surface. As to the program,we used a program package 'deMon' [3,41 and its extended version to the periodic systems.
p..
*
3.RBSULlS AND DISCUSSION 3.1. AppUcatim to methanation Fig2 shows the relative total energies for the isolated and chemisorbed systems. The sum of total energy of M&O4 and two methane molecules is taken as the Mg 0 -CH4 2(Mg-CH3) 0-CH.3 reference. On adsorption of one 4 4 Mg 0 -H +CH4 4 4 2 methane molecule with the 2 ( 0 - H+) -+C2H6 formation of Mg-CH3 and 0 - H Mg404- (CH4) bonds, the energy lowers by 25 Pig.2. Relative total energies for kcal/mol. This value is reactant (Mg 0 +2CH4), product comparable to our previous (Mg 0 -H + and intermediates 4 4 2 Hartree-Fock result (21.6 kcal/mol) [2]. The adsorption leading to the formation of 0-CH3 and Mg-H bonds is however found to be unstable. In the case of two molecule adsorption, two configurations are considered. The adsorption energy is more than twice the single molecule case for the product having two Mg-CH3 and two 0 - H bonds. For
$,A,)
2529
the other product where Mg-CHb 0-H, Mg-H and 0-CH3 bonds are formed, the adsorption energy is very small but not zero. The Anal product with an Mg-H and an 0-H bonds which is formed by desarptlon of ethane, is more stable by 19 kcal/mol than the reactant. In our previous study 121 we showed that there is a high energy banier (more than 60 kcal/mol) in the came of adsorption forming two 0 - H bonds directly (Figl). Here we assume the same for the coupling of two methyl: The direct formation of ethane from the intermedlate with two Mg-CH3 and two O-H bonds ls difficult. The formation from the intermediate with Mg-CHg 0-H,Mg-H and O-CH3 bonds is expected more favorable. An Important consequence is how the MgO catalysts should be implwed to stabilize the latter tntemrediate compared to the f m e r . Calculations with alkaline atom impurlties will be a future work
3.2 Effectsdlmgrangeekctrmtatictnteractlm For the perladic systems, we checked convergency in total energy with interaction ranges up to 43 Angstrom. Prom the result of our energy decomposition analysis, we knew that the reason of slow convergence is a turbulence by the electrwtatlc interactions. We replaced the Mg and 0 ions at distant sites by point charges. This appmxhnation is applied for the ions beyond the interaction range and the ions up to 43 Angstmm. Tables 1 and 2 show the energy differences in calculations wtth and without point charge correction. For the M g 0 chain, te., one dtmensional case, the point charge correction remarkably improve the convergence. For the slab, the calculations requtre much more computing time and Table 2 is not yet completed. However we can read fran Table 2 that the improvement ls less remarkable
aDtstance is Angstrom unit, and energy is atomic unit.
2530
compared to the linear chain case but it is still promising. The calculations with the interaction range of 13 Ansupplemented by the point charge correction up to 43 Angstrom are expected to give reliable results.
33.-in lonkity betweenperbcttcand dustermodels The charge on atoms is estimated using Muniken population for the cluster and slab model calculations The posttive charge on Mg atoms are +0.19, + O X , and +I21 for the duster, (100) slab, and (110) slab, respectively. The charge on 0 atoms is the negatlve and same magnitude. We easily understand that the cluster model @vesa very covalent character. This result is at least qualitatively reasonable since it is well known that the covalency ts enhanced at the low coordinationsites such as steps or kinks on the surfaces The results for the slab calculations indicate that the high ionic states of component atoms am stabilized by the higher coordtnation number of surrounding ions in the two layer slab (5 or 4) compared to 3 in the MgqO4 cluster. CRBFBRENCBS 1 H. Kobayashi, M Yamaguchi and T. Ito, J. Phya Chem. 94(1990)7206. 2 T. Ito, T. Tashlm, M.Kawasaki, T. Watanabe, K Toi and H. Kobayashi, J. Phys. Gem. 931991M76. 3 A St. Amant and D. R Salahub, RDgram package 'deMon'. 4 D.R Salahub, R Foumier, P. Mlynarski, I. Papal, A St. Amant and J. UsMo, in Density Functional Methods in Chemtstry, J. K. Labanowski and J. W. Andzehn (edk Spmgir-Verla&1991, -77.
Guczi, L.et d.(Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Cahlysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights resewed APPLICATIONS OF A NEW ISOTHERMAL SINGLE PELLET DIFFUSION
REACTOR S. S. A y J. B. Butt and J. S. Dranoff
Department of Chemical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3 120, USA
Abstract A new isothermal single pellet diffusion reactor is developed for reaction engineering studies under severe intraparticle mass transfer limitations. It is equipped with two operational modes for analyzing the centerplane concentration: Attenuated Total Reflectance-FI'IR, and GC analysis. With benzene hydrogenation on nickellkieselguhr as a model reaction, several studies are underway: reactor transient and steady state behavior, kinetic and diffusivity modeling, and induced activity gradients. Nomenclature Ac: Cross-sectional area of slab, cm2 0: Concentdon of benzene, moles/cm3 IM: Effective diffusivity, cm% k Rate constant, moledg-s KB: Adsorption constant of benzene, llatm L: Thickness of pellet, cm PB: Partial pressure of benzene, atm 1.
Rate of reaction of benzene, moledg-s rBobs: Observed rate of reaction, moles/g-s Wc: Weight of catalyst, g xH2: Mole fraction of hydrogen z: Distance in catalyst pellet, cm pc: Catalyst pellet density, g / c d superscripts s.*: surface, centerplane rg:
INTRODUCTION
The single pellet diffusion reactor is a system designed for direct study of catalytic reaction kinetics under severe transport limitations. Operated under conditions where significant concentration and/or temperature gradients exist, it represents the opposite extreme of "gradientless" reactors. An experimental configuration which allows direct measurement of the intraparticle gradient enables more accurate conclusions to be drawn about mass transport influences, especially in systems where complex interactions exist between simultaneous rate processes, such as reaction, diffusion, and catalyst deactivation. Previous isothermal reactor designs by Petersen [ I ] and non-isothermal configurations by Butt [2] have been applied in studies ranging from kinetic and diffusivity measurements, studying transient and steady-state boundary-layer and intraparticle gradients, and identifying catalyst deactivation models. The current isothermal single pellet reactor is a first attempt at constructing a system which may operate in a more severe operating regime than previously investigated, with intended applications such as hydrotreating or hydrocracking reactions. The current system is designed for temperatures up to 350°C and pressures up to 15 atm. It makes use of two alternative techniques for analyzing the catalyst centerplane concentration: Attenuated Total Reflectance-
2532 Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Gas Chromatography (GC).In the ATR-FTIR mode, a reflectance element is mounted flush against the catalyst pellet which allows in situ analysis of the centerplane concentration with minimal necessary sample volume. This technique serves to eliminate a substantial stagnant gas space which may contribute to thermal gas-phase reactions, disguising the real catalyst activity. Although thus far we have had only partial success with the ATR-FTIR analysis, we are able to demonstrate operation of the system using a traditional GC technique. Using benzene hydrogenation on nickel/kieselguhr as a model reaction, we have been able to study reactor transients, evaluate kinetic and diffusivity parameters, as well as begin a new study on nonuniformly active catalysts.
2. THEORETICAL MODEL The current reaction under study is benzene hydrogenation on nickel/kieselguhr at 60-200°C and 0-200 psig (1-15 atm). The reaction is essentially irreversible in this temperature regime. Butt [ 2 ] ( 1 atm), and later Marangozis 131 (up to 4 atm) found the kinetics well described by a single-site adsorption Eley-Rideal model. This model will be tested for validity at higher operating pressures:
The three equations governing the system are presented below. Experimentally, kinetic and transport parameters k, KB, and Deff, as well as the effectiveness factor and Thiele Modulus, are evaluated by substituting in the experimentally measured quantities C B ~CB', , mobs, and system conditions. (1) Single Particle Mass Transport Equation, Slab geometry, Fick's Law Diffusion: De.ff-=rB d2C
pc
dz2
with boundary conditions: z = 0 (center),
=o
dz z = L (surface), CB = Ck
( 2 ) Surface flux: (3) Centerplane concentration:
z = 0 (center), CB = CB'
3. EXPERIMENTAL 3.1. Process Conditions Gas phase benzene hydrogenation is carried out with a vapor feed of hydrogen (Great Lakes Airgas Co., ultra high purity) and benzene (J. T. Baker Chemical Co., "Baker Analyzed Reagent") mixture, typically in a molar ratio of 1911, at a total flow rate of 400 SCCM. The catalyst is impregnated nickelkieselguhr powder (Engelhard Corp. Ni-0104P) diluted with an inert kieselguhr powder (Engelhard Corp.), pelletized at 1O,OOO-5O,OOO psi (680-3400 atm). 1 5 1 cm) thickness. Typical pellet dimensions are 0.356"(0.9cm) diameter, and 0.06611-0.4''(0. Catalyst pre-treatment includes 1 hour of N? purge at room temperature, 4 hours of N. purge at l W C , and 4 hours of reduction under H? flow at 180°C. at gas flows of 100 SCCM for a pellet typically composing of 0.02 g nickeVkieselguhr catalyst and 0.13 g inert kieselguhr powder. Reaction conditions are 60-200"C. and 0-200 psig (1- 15 atm).
2533 3.2. Reactor Design The single pellet reactor is shown in Fig. 1. All essential parts are constructed from 3 16 stainless steel. High pressure copper gaskets in a Conflat closure design seal the reactor flanges from the external atmosphere. Viton O-rings are currently used at various crystal-metal interfaces to minimize mechanical stress. The catalyst pellet is formed by pressing catalyst powder into a titanium ring. The titanium cell is then used in situ, with one face of the slab catalyst exposed to a sweeping bulk reactant mixture and the other face sealed against a closed centerplane chamber. The reactor assembly is installed in an FTIR spectrometer for in situ analysis. In the IR analysis mode, a germanium reflectance element mounted flush against the catalyst centerplane enables an infrared beam passing through it internally to generate a reflectance spectrum which gives valuable centerplane information. In the GC analysis mode, all optical crystals are eliminated and a septum port is added to the centerplane chamber. A small sample is drawn out by syringe from this closed chamber and injected onto a GC column for analysis. The reactor is allowed to return to steady state between centerplane samples. The reactor is heated with a temperature-controlled heating tape.Three thermocouples monitor respectively the bulk vapor, catalyst wall, and centerplane temperatures. Due to the large thermal mass of the reactor assembly, temperature control is generally very steady, and the three reactor temperatures are within 1-2°Cof each other. 3.3. Flow System A continuous flow system (Fig. 2) feeds H? and liquid benzene to a vaporizer packed with glass beads. A Brooks mass flow controller and an 1 x 0 syringe pump keep good respective control of the H?and benzene feed rates. The vaporized benzene442 mixture is then sent to the reactor where it sweeps across the bulk reactor chamber to which one face of the catalyst slab is exposed. The product mixture is sampled by GC at the outlet and represents the bulk composition at the catalyst surface, in the absence of fluid phase diffusion effects. The catalyst centerplane composition is analyzed alternatively by ATR-FTIR or GC sampling depending on which operation mode is in use. 1
2
3 4
5 6
Viton O-ring Catalyst pellet Germanium reflecmce element Copper Conflat gasket KRS-5 (TBrI) window Thermocouple
Reactat/
1 Benzene 2 Vaporim 3 H2 4 Single Pellet Reactor 5 C c n t c r p l ~mrly~is ~ GC 01 ATR-Fl’R 6 Roduct~d~U-Gc
-
Roduct out 6 t
m
, , i
Fig. 1 Single Pellet Reactor
Fig.2 Flow System
4.
DISCUSSION
In the process of developing the single pellet reactor as a rejiable tool for reaction engineering studies, we are faced with a number of practical problems. Catalyst preparation is a major concern. One time-consuming task that we had was to find a workable combination of a dilution recipe and pelletizing conditions to prepare pellets which maintain good physical integrity under reaction conditions. Dilution with inert kieselguhr is a must since pellets formed from the impregnated nickel catalyst alone was found to fail under thermal treatment. The fraction of active catalyst must also be adjusted to allow us to work in a reasonable diffusion regime where centerplane concentration/surface concentration is neither close to 0 nor 1. Petersen [ I ] suggested a desirable Thiele Modulus range of 0.5-5. Other fundamental issues that we are concerned with are data reproducibility, effect of pre-treatment conditions, boundary-layer diffusion influences, and catalyst deactivation effects. F i g 3 and Fig.4 show typical transient and steady state behavior of this catalyst. F i g 3 is a plot of the intrinsic activity data collected from a differential powder reactor, whereas Fig.4 shows a typical pellet reactor run. The differential reactor data are collected as an independent confirmation of the kinetic behavior observed in the pellet reactor. Transient data show an initial deactivation of the catalyst activity which may last for up to 30 hours dependent on the space velocity. This activity decline is more pronounced when the initial conversion is high. When the activity reaches a steady state, it remains stable for at least 5 days. Surface and centerplane conversion data are collected at steady state. The kinetic data that we obtained experimentally agree very well with previous investigators' results. Our catalyst activity is found to be close to Kehoe's [2] who reported a rate constant pre-exponential factor of 134.79 mole/g-s, an activation energy of 12290 cal/mole, an adsorption constant pre-exponential factor of 6.7676 x 10-3 atm-1, and a heat of adsorption of 8263 cal/mole. Our reaction rates are slightly higher because our pre-treatment conditions call for a H? reduction temperature of 180"C, whereas Kehoe used 120°C. We operated in a Thiele Modulus range of 1-2. For pellets formed under 10,OOO psi (680 atm) pelletizing pressure, we obtained effective diffusivities of 0.035-0.042 cm%, at 60-80"C and 1.2 atm. For pellets formed under 20,000 psi (1360 atm) pelletizing pressure, the effective diffusivities ranged from 0.016-0.023 cm% at 60-180°C and 1.2 atm.
References 1. Hegedus and Petersen, Cat. Rev. - Sci. Eng., 9(2), 245 ( 1974). 2. Kehoe and Butt, AIChE J., 18(2), 347 (1972). 3. Marangozis, Mantzouranis, and Sophos, Ind. Eng. Chem. Prod. Res. Dev., 18( I ) , 61, (1979).
1.00
p ':. .
= 19/1
-
0
.I ' 0.80 * 0.60
B
OAO
0.10 0.00
:. ' a
z
*
Total Flow I 400 SCCM H%Bcnzens = 19/1 W V I 195 T=180'C P=1.2atm ?-.
... .,. '4 A.
m
.
Guni, L I al. (Editors), New Frontiers in Cluolysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 199'2, Budapest, Hungary 0 1993 Elsevier Science Publishem B.V. All rights reserved
COKE ELIMINATION FROM Pt-Re/A1203BY OZONE CONTAINING MIXTURES C. L. Pieck, E. L. Jablonski and J. M.Parera INCAPE, FIQ, UNL - CONICET,Santiago del Ester0 2654,3000 Santa Fe, Argentina
Abstract The elimination of coke deposited on Pt-Re/AlzOj by burning with ozoneair mixtures was studied at different temperatures, flow rates, and times. Coke elimination presents a maximum at about 400 K and, regarding the metal or acid functions, is not selective. 1 . INTRODUCTION
Coke deposited on naphtha reforming catalysts is commercially eliminated by burning with very low pressures of oxygen diluted in an inert gas and starting at a low temperature. In this way, the increase in temperature due to the great exothermicity of the combustion reactions is controlled and the irreversible catalyst sintering is low, but the process consumes a very long time. One possible alternative is the use of a endothermic reaciton [ l ] , a slightly exothermic reaction 121, or ozone as oxidant at low temperatures [3]. The objective of this paper is t o study the influence of temperature, gas flow rate and time on coke elimination from a commercial Pt-Re/AlZOj catalyst using a stream of ozone in air as oxidant. Benzene hydrogenation (metal function) and n-pentane isomerization (acid function) test reactions are used to deduce how catalytic functions are recovered during coke elimination by ozone. 2 . EXPERIMENTAL
catalyst coked during a A commercial Pt(O.3%)-Re(O.3%)/A1203-C1(0.84%) commercial cycle up to 13.5% carbon was used. The catalyst has a specific surface area of 180 m2/g and was ground to 35-80 mesh. The coke combustion was performed in a 1 2 mm i.d. quartz reactor loaded with 0.5 g catalyst, at atmospheric pressure, temperatures between 298 and 473 K, gas (03-air mixtures) flow rate between 35 and 100 STP ml/min and times of operation between 4 and 15 h. The oxidant mixture was obtained by passing air through an ozone generator [ 4 ] , and the residual carbon on the catalyst was analized by combustion-volumetry.
.
2536 3 . RESULTS AND DISCUSSION
F i g u r e 1.a shows t h e ozone c o n c e n t r a t i o n a t t h e r e a c t o r o u t l e t w i t h t h e empty r e a c t o r o r c h a r g e d w i t h 0.5 g of c a t a l y s t f r e e of coke. I t c a n be seen t h a t ozone c o n c e n t r a t i o n a t t h e r e a c t o r o u t l e t d e c r e a s e s w i t h t h e i n c r e a s e i n t h e r e a c t o r t e m p e r a t u r e due t o t h e t h e r m a l i n s t a b i l i t y of ozone. T h e r e is n o t d e c o m p o s i t i o n o f ozone a t 298 K , and t h e c o n c e n t r a t i o n ' i s h i g h e r f o r t h e 35 ml/min g a s f l o w r a t e t h a n f o r 54 ml/min b e c a u s e o f t h e h i g h e r c o n t a c t t i m e i n t h e ozone g e n e r a t o r . A t e a c h flow r a t e , t h e d i f f e r e n c e between t h e p o i n t s w i t h o r w i t h o u t c a t a l y s t i s small and due t o t h e c a t a l y s t c a p a c i t y t o decompose ozone. I n t h e c a s e of t h e h i g h e r c o n t a c t t i m e i n t h e r e a c t o r ( 3 5 ml/min), t h e ozone d e c o m p o s i t i o n i s h i g h e r . The a c t i v a t i o n e n e r g v of ozone d e c o m p o s i t i o n , c o n s i d e r i n g a f i r s t o r d e r K i n e t i c s , f o r t h e empty
1.2
# 0.8
B
c
0
8 0.4 .
P2 0
4
i
E 01.o2
32
4
k!
0'
v)
I
b)
250
I
I
I
I
300
350
400
450
500
TEMPERATURE, K F i g u r e 1. a, ozone c o n c e n t r a t i o n a t t h e r e a c t o r o u t l e t as a f u n c t i o n of t e m p e r a t u r e . Gas f l o w r a t e 3 5 mllmin: empty r e a c t o r ; A , r e a c t o r charged w i t h c a t a l y s t w i t h o u t coke. Gas flow r a t e 54 m l / m i n : O , empty reactor;., r e a c t o r charged w i t h c a t a l y s t w i t h o u t coke. b , r e s i d u a l c a r b o n on t h e c a t a l y s t a f t e r b u r n i n g w i t h t h e m i x t u r e o z o n e - a i r as a f u n c t i o n of b u r n i n g t e m p e r a t u r e . Gas f l o w r a t e 54 m l / m i n : O , b u r n i n g time 4 h;., b u r n i n g t i m e 10 h . , g a s f l o w r a t e 35 ml/min and 10 h b u r n i n g t i m e
x,
2537 reactor is 10,200 cal/mol, and for the reactor with the catalyst is 9,400 cal/mol (average values considering both flow rates). Figure 1.b shows the residual carbon after burning the catalyst coke as a function of the burning temperature. The burning of coke by ozone starts at about 300 K, which is 300 K lower than the temperature of burning by oxygen [S]. The residual carbon has a minimum at about 400 K for the operating conditions used. This minimum is a compensation between the influences of temperature on reaction and decomposition of ozone. At 298 K, carbon is not eliminated by ozone; increasing the temperature up to 348 and 3 7 3 K, the carbon elimination increases with the increase in temperature due to the increment in the rate of burning, and ozone concentration is little affected by decomposition. At 398 K, a large part of ozone is decomposed but its concentration is still enough to produce a great elimination of carbon. At higher temperatures, ozone is decomposed rapidly and a smaller amount of coke can be eliminated. At 448 K and higher temperatures, at the top of the catalyst bed there is a layer of catalyst completely free of coke and the decomposition of ozone is completed in that layer. In the lowest part of the bed, coke remains unaltered. At the same time of reaction (10 h), coke elimination is greater in experiments with a higher flow rate because of the larger amount of ozone passing through the reactor. At the same flow rate (54 ml/min), coke elimination is greater for a larger time of reaction, and the larger difference in residual carbon is at the minimum, where the burning rate is at the maximum. Figure 2 shows the relative catalytic activity of burnt samples as a function of the carbon eliminated. It can be observed that the activities for the test reactions of both catalytic functions increase similarly with the decrease in coke on the catalyst. This means that there is no selective
I .v
$
0.8
8 0 .
0.6 0
0.4
. 0
0.01 0
Figure 2 .
I
%'
0
I
5 10 13.5 ELIMINATED CARBON , X Relative activity in test reactions of catalyst samples partially burnt with ozone-air mixtures as a function of the eliminated carbon. Values relative to the value of the sample with total elimination of carbon ( 1 3 . 5 % ) . 0 , benzene hydrogenation. , n-pentane isomerization
2538 coke e l i m i n a t i o n from t h e c a t a l y t i c f u n c t i o n s . 'This b e h a v i o r is t h e o p p o s i t e t o t h e coke b u r n i n g w i t h oxygen where coke e l i m i n a t i o n i s s e l e c t i v e , and coke on t h e metal f u n c t i o n i s e l i m i n a t e d b e f o r e t h a t on t h e a c i d f u n c t i o n [ 5 ] . The l a c k of s e l e c t i v i t y c o u l d b e due t o t h e r a p i d ozone d i s s o c i a t i o n forming v e r y r e a c t i v e r a d i c a l s t h a t r a p i d l y and n o n s e l e c t i v e l y react w i t h coke. I n c o n c l u s i o n , t h e b u r n i n g of coke on Pt-Re/Al2O-j naphtha r e f o r m i n g c a t a l y s t by d i l u t e d ozone c a n b e performed a t v e r y low t e m p e r a t u r e s , a b o u t 300 K lower t h a n t h e b u r n i n g w i t h d i l u t e d oxygen. There i s a n optimum t e m p e r a t u r e t h a t i s a compensation between t h e i n c r e a s e i n b u r n i n g r a t e and i n ozone decomposition w i t h t e m p e r a t u r e . Coke e l i m i n a t i o n i s n o t s e l e c t i v e , b o t h c a t a l y t i c f u n c t i o n s are s i m u l t a n e o u s l y and i n a s i m i l a r e x t e n t r e c o v e r e d by coke e l i m i n a t i o n .
4 . REFERENCES 1 Yu. ti. Zhorov and R. K e p s e l , K i n e t . Catal. 25 (1984) 1271. 2 P. Marecot, S . P e y r o v i , D . B a h l o u l and J. B a r b i e r , Appl. C a t a l . 66 (1990) 181. 3 G . J . Hutchings, R.G. C o p p e r t h w a i t e , T . T h e m i s t o c l e o u s , G . A . F o u l d s , A . S . B i e l o v i t c h , B . J . L o o t s , G . Nowitz and P. Van Eck, Appl. C a t a l . 34 (1987) 153. 4 N.L. My and P.N. Sahghal. The Chem. Eng. J o u r n a l 40 (1989) 1 7 5 . 5 C.L. P i e c k , E.L. J a b l o n s k i and J.M. Parera, Appl. C a t a l . 70 (1991) 1 9 .
Guczi, L ef al. (Editors), New Frontiers in Carolysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
NEW MODEL OF DEACTIVATION OF IRON CATALYSTS FOR AMMONIA SYNTHESIS
W.Arabczyk and K Kalucki Technical University of Szczecin, ul. Pulaskiego 10, 70-322 Szczecin, Poland
Abstract The process of oxygen and nitrogen adsorption on the iron surface covered with potassium has been studied. A new model of iron catalyst deactivation in the synthesis of ammonia has been suggested. 1. INTRODUCTION
In spite of many studies 11-61 on surface phenomena of iron catalyst for ammonia synthesis reaction, any elucidation of deactivation mechanism of this catalyst does not exist. In this work we try to explain the process of oxygen and nitrogen adsorption on the iron surface covered with potassium and to present the model of potassium promoted iron catalyst active surface. Basing on the presented model the mechanism of stable and reversible deactivation of catalyst in the synthesis of ammonia makes clear. 2. EXPERIMENTAL
All experiments were performed in an UHV chamber equipped with CMA analyzer on iron (111) surface. The cleaning method of the iron samples has been described before [71. Potassium was produced by heating a zeolithe K+ ion emitter. Adsorption was carried out under pressure of 2 , 6 6 x Pa for oxygen, and 10-2Pa for nitrogen, using pure gases of the 1’Air Liquide. Sulphur was introduced on the iron surface by segregation at 900 K. 3. RESULTS AND DISCUSSION.
Fig.1 shows the variation of the intensity of the Auger peak height versus the exposure time of potassium ions on a clean iron surface as well as on a covered with sulphur or oxygen one. Adsorption of potassium ions on the iron surface at room temperature proceeds with constant rate till reaching saturation by coverage eK = 0 . 5 5 [81, yielding a surface concentration of 3.8 * loi4 atoms/cm
2
. Adsorption of potassium on iron surface covered with oxygen or sulphur with B = 1 s,o
proceeds with identical rate as on the clean iron surface till reaching 2 saturation at coverage 43 = 0.8 (5.6 1014 atoms/cm 1.
The change of L23VV sulphur Auger peak shape as a result of potassium adsorption on the iron surface covered with sulphur is observed [91. The change of Auger peak shape is connected with a change of local free electron density in the vicinity of sulphur and indicates on electron transport from potassium atom to the valency band of sulphur atom [91. Any changes of KVV oxygen Auger peak shape is observed as aresult of potassium adsorption on a surface covered with monolayerof oxygen atoms [91.
Fig.2 shows variation of intensity of Auger peak of iron and potassium as result of oxygen adsorption on the iron surface covered with potassium. With increasing coverage with oxygen (increasing Auger peak intensity of oxygen) the iron peaks decreases and at the same time potassium peak height undergoes no changes. Similar effecthas been observed by oxygen and nitrogen adsorption on a surface covered earlier with oxygen (eo=l) and potassium (8=0.8) [ l o ] . Taking into account theeffect of attenuation of electrons leavingthe sample it can be stated that oxygen and nitrogen atoms are adsorbed on the iron atoms under the potassium atoms. The heating of the iron sample covered with the potassium atoms leads to fast potassium desorption just at 470 K and obtaining of a clean surface (fig.3). Complete potassium removal from the surface covered with sulphur was observed at about 700 K. The amount of sulphur remaining on a surface is equivalent to this one before potassium adsorption and heating of the sample. Potassium is fastly binded in the presence of oxygen . Fig.4 shows variation of Auger peaks height of sulphur, oxygen and potassium as a result of heating of the sample covered with sulphur (es=O.l), oxygen (00=0.9)and potassium (eK=0.8).
Change of sulphur peak shape points out that the potassium bounded on surface through sulphur atoms (T < 520 K) is removed first of all. Above this temperature oxygen and potassium are removed, and the concentration ratio of potassium to oxygen remains constant for defined temperature (for APPH(K)/APPH(O)=2.4) and decreases with the example for T = 620 K increase of temperature. Thermal decomposition of that adsorption layer depends on oxygen concentration in bulk. The temperature of decomposition increases with an increase of bulk oxygen concentration and the ratio of the potassium atoms amount to the oxygen atoms amount decreases simultaneously [111. This fact proves that thermal decomposition of an oxygen-potassium layer proceeds through oxygen diffusion into the bulk. Basing on the studies on the interactions between potassium, oxygen, nitrogen, and sulphur on the iron surface, one can propose the simplest model of an active surface of iron catalyst (fig.51, in which potassium atoms were adsorbed on an iron atoms layer and active centers able to adsorb atoms of non-metal are situated between potassium layer and iron crystal. A part of those centers is occupied by oxygen atoms which stabilize the presence of potassium on the catalyst surface at temperature of ammonia synthesis. In the process of ammonia synthesis the free adsorption centers are occupied by nitrogen atoms and they are released after reaction with hydrogen and ammonia desorption. The occupation of those sites by others
2541
,m
owerape
APPMX) lub. unltal
(I,
Fig.2. Changes of Fe and K Auger peek height as rewit of oxygen edrorptlon
Fig.l.Potassium coverage versus exposure
APPIUK) [arb. unite1
'?
-
-
lo ::K APPWY) Irrb. unltri
370
470
670
610
I
400
500
F.1rnl.M F.iml*i1x1le.c
100
40
2060
270
1. I'C1600
100,
770
Tarnprature 1K1
mo
100
100
-
APPH(Fa M,,VV) [arb. unltrl
h t l o . Ol."In#
Fig.3. Changes of potassium Auger peak height versus sample temperature
so
Fig.4. Potassium, oxygen end sulphur AES peak height vs sample cleaning degree
atoms leads to its poisoning. An oxygen adsorption which results in occupation of all sites under the potassium monolayer could be an example of that poisoning. It creates stable structure Fe-0-K. Hydrogen reduction causes reactivation of that surface by partly removal of oxygen atoms and creation of adsorption centers. If the atoms of poisons (e.g. sulphur) adsorbed on an active surface displace oxygen atoms from the surface then the binding energy of potassium with the surface increases and it follows up with complete or partial potassium desorption from the surface. That surface will not possesses any original catalytic properties even after removing from it poisonous atoms (stable poisoning of catalyst). In thatmodel stable poisoning of catalyst is not related to blockage of adsorption centers with poisonous atoms but to removing from itpotassium atoms. 4. REFERENCES
1. R.Bril1, J.Kurzidim, Colloq. Int. C.N.R.S., 187 (1969) 99. 2. R.Bril1, E.L.Richter, E.Ruch, Angew.Chem.-Int.Ed., Engl., 6 (1967) 882. 3 . F.Bozso, G.Ert1, M.Grunze, M.Weiss, J.Catal., 49 (1977) 18. 4. C.Ert1, S.B.Lee, M.Weiss, Surface Sci, 114 (1982) 527. 5. D.R.Strongin, J.Carrazza, S.R.Bare, G.A.Somorjai, J.Catal., 103 (1987) 213. 6. E.V.Albano, Surface Sci., 215 (1989) 333. 7. W.Arabczyk, B.Liebscher, F.Storbeck, H.-J.MTssig,Pol.J.Chern., 65 (1391) 95. 8. S.B.Lee, M.Weiss, G.Ert1, Surface Sci., 108 (1981) 357. 9. W. Arabczyk, U.Narkiewicz, E.Slaboszewska, to be published. 10. W.Arabczyk, U.Narkiewicz, to be published. 11. W.Arabczyk, U.Narkiewicz, to be published.
Guczi, L.ef al. (Editors), New Frontiers in Coralysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
PORE RESTRICTION IN RESID HYDROTREATING CATALYSTS P.S.E. Dai and B. H. Bartley Texaco R & D Department, Chemical & Catalysis Research Section, P.O.Box 1608, Part Arthur, Texas 77641, USA
Abstract The minimum effective pore diameters (MEPD) for the HDS, HDNi anf HDV reactions in resid hydroprocessing were determined using data on catalyst pellet sizes, metal contents, and intrinsic HDS activities of CoMo and Mo alumina catalysts. For Arabian Light/Medium vacuum resid feedstock, MEPD is ca. 4 nm for the HDS reaction and ca. 5 nm for HDNi and HDV reactions. A model was developed to estimate the sizes of sulfur and metal bearing molecules and to determine the optimum pore diameters. The results were successfully applied to the development of an improved bottoms desulfurization catalyst, HDS-2443, for the H-Oil process.
1. INTRODUCTION Recent advances in catalyst design focused on the modification of the pore structure and shape of the catalyst supports. Our previous work (Dai et al., 1990) showed that the incorporation of macropores with pore diameters in the range of 25 to 100 nm is beneficial to the resid hydrodesulfurization (HDS) activity. An important factor in the design of resid hydrotreating catalysts is the minimum effective pore sizes and the sizes of molecules. In this paper, the minimum effective pore diameters (MEPD) were experimentally investigated to aid the prediction of optimum pore diameters for respective HDS, nickel removal (HDNi), and vanadium removal (HDV) reactions. 2 . EXPERIMENTAL SECTION
2.1. Catalyst Preparation Gamma-alumina supports in both bead and cylindrical extrudate forms, provided by Akzo Chemie America and Davison Chemical Co., were thermally treated at temperatures higher than 927 C to modify the pore size and surface area. Ten CoMo catalysts were prepared using both the as-received and the
2544 heat-treated gamma alumina. Five Mo catalysts were prepared with the alumina supports calcined at 538 C for one hour. Co and Mo were introduced into the supports with cobalt nitrate and ammonium heptamolybdate by the absorption method. The catalysts were dried at 120 C for 24 hours and calcined at 482 C for 4 hours. 2.2. Physical characterization and catalyst activity The surface area and pore volume distribution were determined using Micromeritics Digisorb 2500. The details of HDS-MAT Test and Berty Resid Hydrotreating Test were reported in the previous paper (Dai et al., 1990).
2.3. Minimum effective pore diameter The MEPD of the CoMo alumina and Mo alumina catalysts were determined by noting the minimum pore size which brought the adjusted rate constants for the HDS, HDNi and HDV reactions into agreement. That is, a minimum pore diameter, D i , was found for each set of catalysts which minimized the variation of the G term in the equation:
3. RESULTS AND DISCUSSIONS
3.1. Minimum effective pore diameter The MEPD are given in Table 1. With standard adjustment, it was assumed that all of the catalysts in a set (CoMo alumina or Mo alumina) had the same activity per square meter. However, this was not be the case. Therefore, two attempts at adjusting for differences in the intrinsic rate constants per square meter among the catalysts in a set were made. For an HDS-MAT rate constant per square meter adjustment, it was assumed that the HDS rate constants measured under conditions of no diffusion control in the HDS-MAT test corresponded to the relative intrinsic HDS activities per square meter of the catalysts under Berty reactor conditions. The other, way of adjusting the intrinsic rate constants was to assume the rate constants were proportional to the Mo oxide content per square meter of the catalysts. The lowest Std G/Avg G shown in Table 1 indicates which range of pore diameter best fits equation 1. The smallest pore diameter of this pore range is the minimum effective pore diameter. The MEPD obtained from three types of adjustments to the rate constants are similar for the both CoMo alumina and Mo alumina catalysts. For instance, all three of the adjustment methods gave 40 A as the MEPD for the HDS reaction over the CoMo alumina catalysts. With each reaction over the two different kinds of catalysts, the MEPD was the same or within 5 A for the three methods of activity adjustment. For the CoMo alumina catalysts, the best value for the MEPD for respective HDS, HDNi, and HDV reactions of AL/AM vacuum resid are 4 0 , 4 0 , and 50 A. Likewise for the Mo alumina catalysts, the MEPD for HDS, HDNi, and HDV are 2 5 , 55, 60 A, respectively. 3.2. Theoretical analysis on optimum pore diameter The mathematical analysis reported by Do (1984) was modified to give a model consisting of a single, first-order irreversible reaction occurring in an isothermal cylindrical extrudate. This model was used to determine the effects of diffusivity and bulk resid concentration on the optimal reactant
Table 1 M i n i m u m effective pore d i a m e t e r
I
I
Catalyst R e a c t ion
A
Pore D i a m e t e r .
I
A v g G x lo7 Std G
/ Avg
G
Pore D i a m e t e r ,
A
AVR G x lo6 Std G
1
/ Avg G
Pore D i a m e t e r .
A
I
I
COHO A l u m i n a HDS
I
HDNi
40
I
45
I
HDV
HDS
1
HDNi
50
70 50 100
500
1000
30
T/% 10
0.1
0.3
0.5 0.7
P/Po
C.5
Figure 1. N2-absorption isotherms on the binary Ti02-Zr02 oxide powders. Figure 2. Variation of the surface area of the TiO2-ZrO2 catalysts with the pre-calcination temperature.
of N2 and SEM-EDX techniques. Fig.la, shcws the adsorption isotherm on the original material (HXPZT) outgassed at llOT,lh. It can be seen that the ad sorption isotherm is of the type 11; the practically absence of hysteresis indicates that possibly the fresh amorphous material have not mesoporous % ture. However, when sample (HXPZT) is heated up to 700T or 8000C there is signs of hysteresis (Fig.lb) which may be regarded as type H3 (formerly teg med type B). The type of isotherm is observed with aggregates of plate-like particles [ 7 ] giving rise to slit-shape pores. The specific surface area of thermal treated samples were measured by applying the BET method. Changes in surface &ea were observed (Fig.2) depending upon the calcination -:emperat! re. Fran SEM technique applied to sample (HXPZT) after being heated at sevs ral temperatures, a very large distribution of shape and dimnsions of m i cles were observed. In the temperature range of 100-3000-Cparticles showed cracks by SEM technique which could be associated with the loss of v a t m f m n both the surface and the structure,during the thermal evolution observed by DTA and TPD-MS [6] In fact, specific surface area increase up to a rmxinm value at 250pC. Particularly interesting is the observation that at 700-8000-Cthe particles are constituted by well defined aggregates (Fig.7 in ref.6) of spherical particles (2p,&am.). The specific surface area does not changes practically in the temperature range of 600-75OT in which the crystallization process was observed, possibly by a capensation betwe6-1W opposite effects, the developping of small spherical particles and the sintering process. In fact the morphology observed at 700-8OOT dissapears af ter heating at above 800T possibly by the sintering between small spherical particles, leading thus a decrease in the surface area which is observed above 8002C (Fig.2). One important observation is that belw the crys? llization temperature at 6500-C,SEM-EDX study s h m d particles in which the zrO2 seem to be in a higher proportion with respect to TiO2[6];this c d d h e toae&kt.After heating at 700% the proportion of both oxides was
.
2599 50% of each, with a surface area of 39.5 rn2g-l and spherical particles in shape giving rise to slit-shape pores. acidity and catalytic activity The acidic and basic properties of thermal treated samples was measured by a spectrophotanetric titration method [8]using pyridine (pKa=5.3) or bnaaic acid (pKa=9.2) as adsorbates. The surface acidity and basicity of the Ti02-2r-02 binary oxide powders pre-calcined at several temperatures, are listed in Table 1. It is worthy to note that the procedure for the preparation of this catalyst [61 enable to modify the acidbase ratio of the sample by calcination treatment. Surf-
Table 1 Surface properties of the binary oxide Ti02-Zr02 pre-calcined at the indic2 ted temperatures Basicity Surface Area Acidity Calcination Temperature ( pols m-2) (m2g-l) ( p l s rn-2) (eC) 700 300 1000
39.5 29.0 9.5
0.06 0.04
0.13 0.02
0.06
0.00
(3-1the other hand, the catalytic activity at 300-400eC tested for the dehy dration of isopropanol over the pre-calcined catalysts, s h m d a practically total selectivity to propene, following conversion a i3assett Habgood first order kinetic equation [ 91 Kinetic and thennodynamic parameters have been calculated and are reported on Table 2.
.
Table 2 Kinetic and thermcdynamic parameters for the conversion of isopropanol to propene over Ti02-Zr-02 binary oxide catalyst pre-calcined at the indicated temperatures -C mrp-ame (TI
EMiING Equation
ARRHENIUS Etptmn arstant kKx lo6 g.s.1
Ea
1nA
( Kcal/ml) ~~~
700
78.4
800 1000
27.9 15.7
H+
Sf
Gf
(Kcal/nol) (cal/fmlK)(Ycidhml)
~~~~
6.8 13.2 21.1
-9.9 1.2 7.5
6.8 12.0 19.8
-66.8 -59.4 -46.9
45.1 46.1 46.7
A plausible model for explain the selective conversion of isopropanol to propene must be found in a concerted two-center mechanism based on the generation of active sites (Zr+ - 0 - Ti-) in which the reaction is believed to proceed by adsorption of the C, at the acidic (Zr+) site and the Cg at the
2600 basic (Ti-) site. The facilitation of proxy-structure formation at Z r 4 + h [ l o ] , which are known to be m r e stable in higher than six oxygen-ligand coordinatj.on is in favour with this mechanism. 3 .c(3NwsI(rJs
In conclusion, our prepared Ti02-Zr-02 binary system have lower acidity ;nd basicity properties than those reported by other authors [ 2-51 , h m v e r our results prove that this catalyst, particularly pre-calcined at 7000-C, could be used for the selective conversion of isopropanol to pro pene, in spite of i.tslow acidity. A point to be emphasized is the absence of ether as descanposition product; traces of ketone (practically negligable) was observed. This fact suggest that on our ZrTio4 catalyst, mnlecular dehydration of the alcohol is produced more easily than is the b i F lecular process. On the other hand, none other products which could be gens rated by a Ziegler-Natta oligmrization process were observed. From the variation of the'catalytic activity with the pre-calcination t q perature we conclude that for a totally ciystalline zrTiO4 material the amounts of acid and base sites are =he major factor affecting the catalytic activity for the dehydration of isopropanol to pro pene. Work is now in progress by us in order to increases the acidity by adding selective species to the surface of our prep: red ZrTiO4 catalyst.
-
The Ministerio de Educaci6n y Ciencia,Madrid (Spain) is gratefully acknowledged for support of this research into the scope of "Acci6n Integrada" (Grant HF'-048,1992). One of us (J.A.N.) also thanks the Ministerio de EducG ci6n y Ciencia for supporting his visit to the URA au CNRS n21383,Ecole Cen_ trale de Lyon (France), in 1990. The financial support for research groups-1991 of the "Junta de Andalucia" is gratefully acknowledged. 1 2 3 4
M.A.Butler and D.S.Ginley, J.Electrochem.Soc.,125 (1978) 228. K.Araka,S.Akutagawa and K.Tanabe, Bull.Chem.Soc.Jpn.,49(2) (1976) 390. K.Araka and K.Tanabe, Bull. Chem. Soc.Jpn., 53 (1980) 299. Ikai Wang, Wun-F'u Chung, Ru-Jen Shiau,Jung-Chung Wu and Chung-Sun Chung, J.Catal.,83 (1983) 428; Jung-Chung Wu, Chung-Sun Chung,Ching-Lan Ay and Ikai Wang, J.Catal., 87 (1984) 98. 5 Hirashima Yutaka, Nishiwaki Katsuhiko, Miyakoshi Akihiko, Tsuiki Hideyasu Ueno Akifumi and Nakabayashi Hirotoshi, Bull. Chem. Soc.Jpn.,61 (1988) 1945. 6 J.A.Navio, F.J.Marchena, M.Macias, P.J.Sanchez-Soto and P.Pichat,J.Mater. Sci., 26 (1991) 7 K.S.W.Sing in: "Cannision on Colloid and Surface Chemistry including Cat2 lysis", Pure and Applied Chem., 57,No 4 (1985) 603. 8 J.M.Campelo, A.Garcia, J.M.Gutierrez,D.Luna and J.M.Marinas, J.Colloid :1 terface Sci., 95 (1983) 5 4 4 , and references cited therein. 9 D.Basset and H.W.Habgood, J.Phys. Chem., 64 (1960) 769. 10 R.J.H.Clark, D.C.Bradley and P.Thornton, in:"The Chemistry of Titanium, Zirconium and Hafnium", Pergamon Press, New York, 1975.
Ouai, L a al. (Editors),New Frontkrs h Catafysir Proceedings of the 10th Intcmtional Congrew on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elscvicr science Publishers B.V.All rights reserved
GAS PHASE SYNTHESIS OF MTBE OVER ACID ZEOLITES A. Nikolopoulo.9~ T. P. Paluckab) P. V.Shertukdea, R. Oukacia) J. G. Goodwin,Jr.0 and
G. Marcelinab *Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA bAltamira Instruments, Inc., 2090 William Pitt Way, Pittsburgh, PA 15238, USA
1. INTRODUCTION
Methyl tert-butyl ether (MTBE) is made commercially by the acid-catalyzed equilibrium reaction of methanol and iso-butylene using an ion-exchange resin catalyst, viz.:
CH,OH
+ (CH,)$=CH,
-
(CH,),COCH,
This reaction is typically carried out in the liquid phase at relatively low temperatures (T < 100°C). This low temperature limit is imposed partly by the increasingly unfavorable thermodynamics and partly by the instability of the resin at elevated temperatures. Although the reaction proceeds via an acid catalyzed pathway, little is known about the requirements of the catalyst to form the MTBE product selectively. .This work examines the effect of acid properties of various catalysts, namely zeolites, on the selective gas phase MTBE synthesis. Zeolites are ideal materials for this study since they have high acidity and can be readily prepared with a range of acid characteristics. Previous studies have addressed the use of zeolites for MTBE synthesis [l-31. However, a systematic investigation of the cffect of acidity on the reaction has not been reported. In this work, a number of HY zeolites with different acid properties were studied in an attempt to investigate their catalytic performance and correlate it with their acid properties. 2. EXPERIMENTAL
Six acid catalysts were studied. Four of these were HY zeolites with different acid properties. Zeolites LZ210-12, Y62, and Y82 were commercially obtained, while zeolite S(LZ12)8 was obtained by mild steam dealumination of LZ210-12. A silica-alumina catalyst (Si-Al-0) with lower acidity and Amberlyst-15 resin were also evaluated for comparison. The zeolites were initially in the ammonium form and were dearninated prior to reaction by heating to 400°C at a rate of 2"C/min under He flow and holding for 12 hours.
2602 The Si/Al ratios of the zeolites were determined by X R D and 2'Si and 27AI MASNMR. Temperature programmed desorption (TPD) of pyridine was carried out on all catalysts except Amberlyst in order to estimate acid strength. Adsorption was conducted at 100'C and desorption was conducted from 1 0 0 ° C to 7 0 0 ° C at a rate of 1O0C/niin. Acid strengths were also determined by n-pentane cracking reaction at 400'C [4]. The MTBE synthesis reaction was carried out using a small fixed bed reactor with on-line GC-FID analysis. Experiments were performed at atmospheric pressure, 100 C, i-butylene/methanol ratio of 1.8, and a space velocity of about 16 h-'. 3. RESULTS AND DISCUSSION
The structural and acid properties of the catalysts studied are summarized in Table 1. For the zeolites, acid site densities were calculated based on each structural Al resulting in a Bronsted site. In the case of the Si-AI-0, i t was not possible to accurately determine the acid site density, but a working range could be determined by taking the lower limit as the number of pyridine molecules adsorbed per gram of catalyst and the upper limit as the aluminum site density. I t should be noted that these catalysts have ;I different number of Brijnsted sites and acid strength of these sites, thus allowing comparison of the catalytic behavior t o both acid strength m c l site density. Table 1 Summary of structural and acid properties of catalysts Catalyst
Y 62 Y82 LZ2 10-12 S(LZ12)8 Si-AI-0 Am her ly s t - 15
Source Comniercia I (Linde) Com nie rcial (Lintle) Coin me r ci ;i I (Linde) Steani Dealum. LZ2 10-12 Co mme rc i a1 (Am .Cyannmicl ) Co m nie rci al (Rohm 24 Hans)
Si/AI Ratio 2.5"
Brijnstetl Site Density ( 102"/g) 27;1
Relative Acid StrengthC 1.0;'
Pyridi ne TI'D (pmol/g) 2200
5.1"
16"
-5.8"
1850
0.Oi'
l5;l
4.0"
I700
8.3"
10;'
8.8;'
1200
1.5
20-30''
< < 1'1
1650
> > 1'1
-
-
3oc
a. From reference [4]; b. From reference [ 5 ] ; c. See text; d. Estimate. e. Defined as ratio of TOF's over TOF for Y62 for n-pentane cracking at 400°C.
2603
Steady-state reaction data for all the catalysts studied are presented in Table 2. In general the zeolites were found to be superior to the Amberlyst-'15 resin in terms of activity and MTBE selectivity. The poorer resin selectivity is primarily due to formation of the C8 i-butylene dimer and dimethyl ether (DME). This may be due to the very strong acid character of the resin which favors many secondary reactions. The lower steady-state conversion value may simply be due to fast poisoning of the strong acid sites. An important characteristic of the zeolite which may also be important is shape selectivity. The resin structure is quite open having average an pore radius of 160A [6] allowing all reactions to occur within the pores. In contrast, the small zeolite pores can inhibit i-butylene diffusion and thus dimer formation, thus enhancing MTBE selectivity. Table 2 MTBE Synthesis over Acid Catalysts at 100°C. Comparison of Ion Exchange Resin and Zeolites at Steady-State Y82 LZ210-12 S(LZ12)8 Si-AI-0 Amberlyst YG2 Cat.Weight (g) 0.10 0.05 0.05 0.05 0.10 0.05 i-C,HIo/CH3OH 1.0 1.8 1.8 1.8 1.7 1.8 WHSV (hi') 15 17 16 16 14 18 CH30H conv.(%) 8.1 12.4 7.9 12.6 10.1 5.6 MTBE select.(%) 54.5 99.0 99.0 98.6 99.9 99.5 ANALYSIS MOL% Hydrocarbons: 0.0 0.0 0.0 0.6 0.0 C1-C7 1.5 2.5 2.1 0.9 39.4 3.7 1.4 C8 Oxygenates: 0.0 DME 0.0 0.0 17.1 0.0 0.0 1.4 0.1 0.4 t-butanol 0.3 0.9 0.9 MTBE 41.3 95.4 97.7 96.1 97.2 98.7
The effect of acid strength on the initial and steady-state turnover frequency (rate of MTBE production per acid site) for all zeolites is shown in Figure 1. A linear correlation is observed between initial TOF and acid strength in the range of values studied. In contrast, no clear correlation is observed when the steady-state results are used. Interestingly, the change in activity between initial and steady-state, which is an indication of catalyst deactivation, is also an increasing function of acid strength. These results suggest that acid strength exerts a major influence on the catalytic behavior of zeolites for MTBE synthesis with increased acid strength is favoring the formation of MTBE. However, high acid strength also facilitates deactivation. Consequently, there seems to be an optimum set of values for acid strength and acid density. High acid strength results in a higher MTBE production rate but can lead to significant deactivation. Low site density tends to reduce overall production. Among the catalysts tested, zeolite LZ210-12 appeared to possess the optimum acidity characteristics.
2604 30 h
v
x 25 X
aJ
5 20
3
LLm
P
5 l5 0 Q1
5 10
E
/
A
Figure 1. Relationship between TOF for MTBE synthesis and acid strength. 4. CONCLUSIONS A kinetic study of gas phase MTBE synthesis reaction over acid zeolites showed that HY zeolites are in general superior to Amberlyst-15 resin in terms of activity and MTBE selectivity. This is due both to the acid and shape selective characteristics of the zeolites.
5, ACKNOWLEDGMENTS
We thank Ann McGowan for assistance in the TPD experiments. Funding for this research from the U.S. Department of Energy, under contract DE-AC22-90PC90047, is gratefully acknowledged. 6. REFERENCES 1. P. Chu and G.H. Kiihl, Ind. Eng. Chem. Res., 26 (1987) 365. 2. S.I. Pien and W.J. Hatcher, Chem. Eng. Comm., 93 (1990) 257. 3. R. Le Van Mao, R. Carli, H. Ahlafi, and V. Ragaini, Catal. Lett., 6 (1990) 321. 4. P. Shertukde, Ph.D. Dissertation, University of Pittsburgh (1991). 5 . F. Ancillotti, M. Massi Mauri, and E. Pescarollo, J. Catal., 46 (1977) 49. 6. G. Marcelin, D.C. Cronauer, R.F. Vogel, M.E. Prudich, and J. Solash, Ind. Eng. Chem. Process Des. Dev., 25 (1986) 747.
Guczi, L ef al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 19!B Elsevier Science Publishers B.V. All rights reserved
ACIDITY-TUNABLE PILLARED MICA CATALYST DERIVED FROM TALC
K Urabe, I. Kenmokq K
Kawabe and Y.Izumi
Department of Applied Chemistry, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan
Abstract A novel pillared clay can be prepared from talc via a swellable fluor-mica, and its catalytic activity for cumene cracking is not only greatly enhanced through the modification of cation-exchange sites by various cations but also tunable by choosing the type of cation. 1. INTRODUCTION
Recently, a new family of microporous crystals referred to as 'pillared clays' have attracted considerable interest as advanced catalysts [ l ] because they have uniformly distributed active sites in the bulk and molecular-sieve microporosity like zeolites. The number o f materials has been increased further with the use of specific clays, such as synthetic fluor-mica [2]. However, pillaring has hitherto been restricted to swellable clays as starting materials. In the previous communication [ 3 ] , we reported that a novel pillared clay could be .synthesized from non-swellable talc via a swellable fluormica and its c talytic activity for cumene cracking was greatly enhanced by La4+ cation modification. Here we demonstrate that its catalytic activity is tunable by choosing the type of cat ion. 2. EXPERIMENTAL
The original talc was collected from Kanshi deposits in China. Its basal spacing, dOo1, of 9 . 3 A is not expanded after attempts to cause pillaring using the ever-popular ' Keggin ion. The swellable nature was conferred on the tal?'& reaction with NaZSiFg at 1 1 2 3 K f o r 2h [ 4 ] . The reaction product, o r fluo -mica (FM), has a double-layer hydrated basal spacing o f 1 5 . 4 and the cation-exchange capacity (CEC) o f 8 0 90 meq. per 100g-clay. This materi 1 gives a strong (001) reflection with a dO0 value of 1 8 . 3 after pillaring [ 3 ] . The modification ok cation-exchange sites by various cations was followed by the same method for the pillared tetrasilicic !ica (TSM) [5] as follows. First, cation-exchanged FM (Mn+-FM) \s prepared from the original Na+ form by the common cation
i
w
exchange method, followed by calcination at 573 K for 3 h (procedure of calc. #l). Then, the calcined Mn+-FM was dispersed in an acetone-water equivolume mixture and an 'Al13' Keggin ion solution was added and stirred at room temperature for 12 h. The resultant cation-modified pillared FM (Mn+-PFM) was finally calcined at 573 K for 3 h (procedure of calc. #2) and used for the cracking reaction of cumene as a catalyst and for various characterizations [5]. The catalytic reactions were carried out in a conventional flow system with a fixed bed of catalyst under atmospheric pressure. 3. RESULTS AND DISCUSSION
cations The talc-derived pillared clay, o r pillared fluor-mica (PFM), is unusually inert as a solid acid catalyst des ite its microporous structure and high surface area of 320 m 9 / g . In the cracking of cumene , i t manages only a 0 . 2 % conversion of cumene at 573 K. In catalytic efficiency, PFM strongly resembles the pillared tetrasilicic mica (PTSM) which gives the same low conversion, but contrasts with the conventional pillared montmorillonite (PM) which exhibits a high conversion of 15.4% [3]. In fact, both PFM and PTSM have low acid contents of 50-60 pnol/g estimated using the temperature programmed desorption (TPD) method with adsorbed ammonia a a basic molecule. These results, together with their MAS "Si NMR data [3], prove that FM derived from talc is structurally very similar to TSM. We found previously [5] that the catalytic activity of PTSM was greatly enhanceg+ through the modification of cation exchange sites ty La . Following the same modification [5] for PFM, the La3 modified catalyst (La3+-PFM) displays a high 3 . 1 . Modifying effect of various
Table 1 . Catalytic activitiesa for cumene cracking and physical properties of various cation-modified catalysts (Mn+-PFM). Catalyst
Hydration energy Basal spacing of cation/kJ g-ion-' dool/A
PFM
L$:-PFM Ba Sr
-PFM
+
- PFM
Pb - PFM C~~+-PFM Mg2+-PFM Ni2+-PFM Be 2+-PFM La3+-PFM +
-
18.3 28 . O ,14.1 18.7 18.4 18.8 19.2 9.8 9.9 17.0 18.8
552 1376 1519 1554 1666 1996 2168 2550 3390
Surfac area/m'g-l 320b 163b 343b 360b 370b 364b 33c 2OC 26Eb 215b
XCumene conv. 0.2 14.1 1.8
5.2 8.2 5.6 0.4 0.2
11 . o
13.9
~
aReaction temp.=573 K. W/F=33 g-cat .h mol-l(W=catalyst weight, feed rate o f reactant), averaged cumene conversion after lh. 'Langmuir-type N2 adsorption isotherm. 'BET-type isotherm.
2607
ac ivity, with a cumene conversion of 13.9%, twice as much as La' -PTSM and comparable to that of an active PM. Table 1 shows the catalytic activities of various cation-modified catalysts (Mn;;PFM) a l o g with the physical properties. Except for Mg and Ni , every catalyst has a distinct pillared structure but a different 2$tivity epending on the o r Ni , the catalyst type of cation. In the case of Mg shows a very low activity because it does not have a pillared i t is noticeable that Li+-PFM has a structure. For Li', different pillared structure from others as seen from X-ray rational reflections of dQ01=28.0, but has a very similar structure to pillared rectorite [6] which belongs to regularly pillared interstratified clay. The activity sequence o f catalysts seems to relate with the hydration energy of the cation, o r the acidity of the aquo-cation. as shown in the first column of Table 1 . In this respect, the catalytic activity of Li+-PFM is extraordinarily high despite its small hydration energy.
8+
3.2. Activity enhancement with cation fixation
In order to know what happen in the Mn+-PFM catalysts with di ferent activities in Table 1 , both amounts of fixed Mn+ and A1' incorporated as pillars were determined analytically. Figure 1 shows these amounts as a function of the ionic radius (r) of fixed cation. The values of these amounts are expressed as a unit of meq. per 100g-clay. For Mn+-PFM with pillared structures, a large amount of A1 pillars were fixed. resulting in their high surface areas as shown in Table 1 . Obviously, A1 content can be a measure of the micropores formed by pillaring. With a decrease in r value of modifying cation, the amount of
0s- 0
1.0
15
Figure 1 . Amounts of fixed Mn+ and A 1 incorporated as pillars.
Figure 2. TPD spectra of adsorbed ammonia on Mn+-PFM.
2608 fixed Mn+ in Mn+-PFM tends to incr ase. 19,the cases of Mg2+ and N i 2 + , were fixed a lot of Mg' and Ni up to about 90 meq/100g, equivalent to the CEC value of FM, and i t resulted in an extensive reduction of layer charge, o r a failure in up $ke in A1 pillars. The Mn+-PFM catalysts (Mn+= Li+,Be , La' ) with high activities are characterized by a definite fixation of Mn+ as well as a considerable amount of A1 uptake. The high efficiencies for cumene cracking are confirmed by the measurements of solid acidity. Figure 2 shows the TPD profiles of adsorbed ammonia over several Mn+-PFM catalysts. The amount of acid sites increases from 60 pmol/g of nonmodified PFM to 110-150 by every cation modification. IR studies of adsorbed pyridine proved also the genesis of strong Lewis-type acid sites by the modification, in accord with the previous result on Mn+-PTSM catalyst [5]. However, there is no apparent correlation between the acid content and the catalytic activity, suggesting the importance of the difference in acid strength for each Mn+-PFM. In fact, it is obvious from TPD peak shape :hat stronger acid sit 5 are cr ated on, especially, Li+- o r La3 -PFM compared with Ba' - o r Sr' -PFM. In conclusion, the Mn+-PFM catalyst gives a tunable activity for acid-catalyzed reactions by the choice of cation (Mn+).
Ee
4.
ACKNOWLEDGMENT
We thank Dr H.Tateyama (Government Industrial Research Institute of Kyushu) for supplying the fluor-mica and the Nippon Sheet Glass Foundation for Materials Science , the Nitto Foundation and the Ministry of Education, Science and Culture (Grant-in-Aid for Science Research, No.03805068) f o r financial support of this work. 5. REFERENCES
J.M.Thomas, Angew.Chem.Int.Ed.Eng1. Adv.Mater.,28(1989)1079. (a) K.Urabe,H.Sakurai and Y.Izumi, J.Chem.Soc.,Chem.Commun., (1986)1074. (b)J.W.Johnson and J.F.Brody.in 'Microstructures and Properties of Catalysts'(MRS Proc.lll),ed.,M.M.J.Treacy, J.M.Thomas and J.M.White, MRS, Pittsburgh, (1988) 257. K.Urabe,I.Kenmoku,K.Kawabe and Y.Izumi. J.Chem.Soc., Chem. Commun.,(1991) 867. H.Tateyama,K.Tsunematsu,H.Hirosue,K.Kimura,T.Furusawa and Y. Ishida, Proc.9th 1nter.Clay Conf.,Strasbourg,ed.,V.C.Farmer and Y.Tardy, Sci.G6ol.,M&m.,86 (1990) 43. H.Sakurai,K.Urabe and Y.Izumi, Bull.Chem.Soc.Jpn., 62 (1989) 3221; 63 (1990) 1389. (a) G.J.Jie,M.E.Ze and Y.Zhiquing, Eur.Pat.Appl., #197012 (1986).(b)K.Urabe,N.Kouno,H.Sakurai and Y.Izumi, Adv.Mater., 3 (1991) 558.
Guni, L. et al. (Editors), New Frontiers in CatalysiE
Proceedings of the 10th International Congress on Catalysis,19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Sciencc Publishers B.V. All rights nserved
BECKMA" REARRANGEMENT OVER SOLID ACID CATALYSTS
T.Curtin, J. B. McMonagle and B. K Hodnett Department of Chemical and Life Sciences, University of Limerick, Plassey Technological Park, Limerick, Ireland
Abstract
Beckmann rearrangement of cyclohexanone oxime to caprolactam has been investigated by passing the oxime vapour in helium over a series of modified alumina or zeolites held between 250 and 380'C in a continuous flow fixed bed micro-reactor. The range of solid acids studied included alumina modified with boron, sodium, hoe horus, sulphate or chloride, and the zeolites H-mordenite, Each catalyst studied exhibited an initiation period, a H-A, H-Y and period of steady state activity and a deactivation period. The duration of each phase of activity depended on experimental conditions but for the boron modified alumina operating at 300°C the initiation period was ca. 1 hour. This phase was characterised by a high conversion of the oxime but very poor selectivit to caprolactam and was accompanied by a build up of coke on the surface o the catalyst. The stead state eriod was ca. 8 hours and featured high conversion and selectivity. d e finaf phase featured a slow irreversible lowering of conversion and selectivity and was accompanied b the formation of a boronalumina glassy phase. The boron modified alumina eatures a hiqh concentration of acidic sites of intermediate acid stren h, which are assoclated with high selectivity to caprolactam. Deactivation an loss of selectivity are associated with the loss of boron through its incorporation into the alumina solid support.
8ZS8-5.
ry
f
P
INTRODUCTION Caprolactam is an industrially im ortant intermediate in the manufacture of Nylon-6. Conventional methods for t e production of the compound involve the li uid phase Beckmann rearrangement of cyclohexanone oxlme usin fuming s phuric acid as the catalyst. An alternative method involves passing t!ke oxime over a solid acid catalyst [1,2].This method avoids one of the main shortcomings of the conventional method, i.e. the generation of stoichiometric amounts of ammonium sulphate. However, the solid acid method must be hi hly selective if it is to be competitive with the conventional method because o the extremely high selectivity associated with the latter. The performance of solid acids ap lied to this problem have, to date, been less than satisfactory from an industriJpoint of view. In particular, there have been problems of deactivation after relatively short times on stream and poor selectwit to the lactam, with excessive amounts of coke formation 131. Here we present e results of a study of a series of solid acid catalysts which examines the relationship between deactivation, selectivity to caprolactam and surface acidity.
R
3
f
x
2610
EXPERIMENTAL A series of alumina catalysts were pre ared by impregnation of the oxide with boric acid, sul hate, phosphate or ch oride f as the ammonium salts, or sodium carbonate. Ap1 zeolites used were obtained from commercial sources and were exchanged with ammonia and heated to 550'C prior to use. Testing was performed by passing cyclohexanone oxime as a vapour (partial pressure = 2 torr) in helium or nitrogen (total flow = 30ml min-1) over lOOmg of catalyst held in a Pyrex microreactor in the temperature range 250-380°C. Analysis was by condensing the reactor emuent on a cold finger or collecting in methanol followed by gas chromatographic analysis as outlined full elsewherer31. Characterization was by X-ray di action and by temperature programmed desorption (TPD) of ammonia or carbon dioxide. These involved passing pulses of ammonia or carbon dioxide (0.2 ml) over the fresh or worked catalysts at room temperature until saturation; the system was then flushed with helium (25 ml min-1) and the temperature was increased to 550°C at the rate of 10°C min-1. Desorbed NH, or C02 was detected by a thermal conductivity detector.
d
RESULTS AND DISCUSSION The conversion of cyclohexanone oxime and selectivity to caprolactam, cyclohexanone and 5-cyanopent-1-eneover a B20dA1203 catalyst at 300'C is
Conversion or Selectivity
40
-
20
0 0
2
4
6
8
Time /hours
Figure 1.Conversion of cyclohexanone oxime (4 and selectivity to caprolactam (+), cyclohexanone (*) and 5-cyanopent-1-ene(0)over B20dA1203at 300°C. presented in figure 1. Other minor products observed included analine and cyclohexanol. During the initial period of contact with the catalyst a good deal of
261 1
coke built up on the catalyst surface. Thereafter a period of stable activity and good selectivity to caprolactam was observed, but after about 8 hours on stream a slow irreversible deactivation occured which was characterized by a lessening in selectivity to caprolactam. This behaviour was typical of that observed for a wide range of solid acids studied. Conversion, selectivity and yield of caprolactam over this range of catalysts during periods of steady state activity, i.e. after about 1-3 hours on stream is presented in table 1. These data show that B20dA1203was by far the best Table 1 Solid Acids for Cyclohexanone Oxime Conversion to Caprolactam Solid Acid
A12°3
2%Na-A1203
TempPC
%Conv*
%&l*
%Yield*
300 300 350 350 350 300 380 380 380 380
98 17 88 88 75
20
20 2 41 21 38 72 43
10 44 24 54 75 43 62 3 48
96
100 100 16 84
H-Y 62 H-A 1 H-Mordenite 40 * Molar conversions of cyclohexanoneoxime; selectivity and yield to caprolactam. catalyst tested for this reaction in terms of yield of caprolactam and it was also superior to the others in terms of duration of steady state activity. The zeolites tended to deactivate more readily than the aluminas. Results of acidity assessment by TPD of NH3 are summarised for some of these solid acids in figure 2. These data show a very strong uptake and subsequent desorption of NH3 from B30 Al 0 b comparison with the other solid acids tested in this way. In partacufarftie 6203/A1p3catalyst showed a very strong NH3 desorption peak in the temperature range 250-35o'C. NH, desorption was absent from the 2Na-A1203 catal st in this particular temperature range and it was one of the poorest catafysts tested. By contrast some NH, was desorbed from P04-A1203in this mid-tem erature range and its performance as a catalyst, although not as good as B 2 0 b 0, was superior to that of A1203. Hence an em herical observation can be maJe that selectivity to caprolactam during the stea y state period of o eration is associated with surface acidic sites of intermediate strength from whic NH3desorbs in the temperature range 250-350°C. This mid-tem erature range peak diminished in intensity when the B20 Al O3 catalyst ecame deactivated. This finding is consistent with previous res ts or this system which showed a reduction in the fraction of water soluble boria on the alumina support from a value of close to 1 for the fresh catalyst to a value of 0.7 for the deactivated catalyst [3] indicatin that a part of the boria had entered into solid solution with the alumina. The eactivation phenomenon observed was then associated with the non-availablity of the boria
B
dI
R
1
5
261 2
TCD Response
P04lA1203 A1203 I
1
0
200
400
600
Temperature / 'C Figure 2. TPD of NH3 from the solid acids indicated. on the surface of the alumina to modify the surface acidity. The TPD data confirm that when this process occurs the NH3 can no longer contact the boria component of the catalyst. No adsorption occured during exposure of B204Al2O9 to C02, nor were peaks detected during subsequent TPD. By contrast, large amounts of CO adsorbed onto 2Na-A1203,the poorest catalyst tested, which was readily detected during TPD analysis. In conclusion selectivity over B204Al2O3 is associated with the presence of brou ht about by boria on the surface acidic sites of intermediate stren alumina support; there is also a marked a sence o surface basic sites on this material. Irreversible deactiviation involves the diffusion of boron into the support so becoming unavailable for surface acidity modification.
P9 f
REFERENCES 1 2
3
A. Aucejo, M.C. Burguet, A. Corma, and V. Fornes, Appl. Catal., 22 (1986)187 S.Sato, S.Hasebe, H. Sakurai, K. Urabe, and Y. Izumi, AppLCatal., 29 (1987)107 T.Curtin, J.B. McMonagle and B.K.Hodnett, Studies in Surface Science and Catalysis, 59 (1991)531
GUni, L a al. (Editors), New Fronriers in Caaljsb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992. Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights rcserved
SYNTHESIS OF SOLID SUPERACIDS OF TUNGSTEN OXIDE SUPPORTED ON
TIN OXIDE, TITANIUM OXIDE AND IRON OXIDE AND THEIR CATALYTIC ACTION
K. Arataa and M. Hind 9epartment of Science, Hokkaido University of Education, Hachiman-cho 1-2, Hakodate 040,Japan bHakodate Technical College, Tokura-cho 226, Hakodate 042, Japan
Abstract Solid superacid catalysts, which were active for reaction of isopentane, were obtained by impregnating hydroxides or amorphous oxides of Sn, Ti, and Fe with ammonium rnetatungstate and then calcining in air; the maximum activity was observed with calcination at 1000°C for the material of Sn and 7OO0C for those of Ti and Fe (11-13 wt.% W).
1. INTRODUCTION
Acids stronger than Ho=-12, which corresponds to the acid strength of 100% H2S04, are known as superacids.
We have synthesized solid superacid
catalysts with an acid strength of up to Hoz-16.04 by exposing hydroxides or oxides of Zr, Sn, Ti and Fe to sulfate ion followed by calcination in air over 50OoC; the surface was shown to be SO4 co-ordinated to the metals [2,31. The superacid catalysts were satisfactorily active in a heterogeneous system for reactions which are generally catalyzed by strong acid, especially superacid: the skeletal isornerization of butanes and pentanes, and others. We have then synthesized another type of superacid, not containing any sulfate but consisting of metal oxides, which was W03 supported on Zr02 with an acid strength of Hoz -14.52 [4-51. This preparation method of catalyst I
Table 1. Activities of the catalysts for reaction of isopentane at 250°Ca)
Catalyst
Calcination temp. ("C)
W03/Sn02
900
Products CO+Co2
C3
i-C4
0.3
(%)
Quantity of W
b) c4
c5
Cl
0.1
1.0
Trace
1.7
Trace
1000
0.2
5.3
0.1
3.0
0.8
1100
Trace
1.6
Trace
2.0
0.1
1200 W03/Ti02
100
WO,/Fe,O,
I00
0.2 0.1
3.8
2.0 Trace
(Wt.%)
1.0 Trace
3.2
13.2
12.1 0.6
11.3 13.4
0.6
a) Flow rate of He 10 rnl/min; catalyst 0.5 g; pulse size 0.05 r n l (gas). b) The 3rd pulse values.
was applied to other supports, and W03 supported on Sn02, Ti02, and Fe20H, as observed with the sulfate superacids, were found to show superacidity on their surface; the catalysts were active for reactions of pentanes.
2. EXPERIMENTAL
The catalysts were prepared as follows.
Sn(OH)q, H4Ti04, and Fe(OH)3 were
obtained by hydrolyzing SnC14, TiC14, and Fe(N03)3, respectively, with aqueous ammonia, washing, drying at 3OO0C, and powdering the precipitates (32-60 mesh).
The hydroxides were impregnated with aqueous ammonium
metatungstate [(NH4)6(H2W12040)1 calcining in air for 3 h.
followed by evaporating water, drying,
The concentration was 15 wt% W based on the
hydroxides [ l l - 1 3 wt.% after calcination (Table 1 ) l .
Reactions were carried
out in a microcatalytic pulse reactor with a fixed-bed catalyst.
3. RESULTS AND DISCUSSION
The catalysts were examined in the dehydration reaction of ethanol to ethylene and ethyl ether and showed quite high activities, much higher than that of Si02-A1203 which is well known as one of the catalysts with the highest surface acidity.
The maximum activity was observed with calcination
261 5 at the exceedingly high temperature, 1000°C, for W03/Sn02: that was 7OO0C for W03/Ti02 and W03/Fe203.
The relative activity of W03/Sn02 calcined at
1000°C [W03/Sn02 (lOOO°C)l, W03/Ti02 (7OO0C), W03/Fe203 (7OO0C), and Si02A1203 (500°C) for the dehydration was 4 4:2:1. Reaction was carried out for the less reactive isopentane (i-C5) and the results are shown in Table 1.
Pentane
C5) and heptane (C,) were observed
as products in addition to propane (C3) isobutane (i-C4), and butane (C4). The maximum activity was again observed with calcination at 1000°C for W03/Sn02.
The Si02-A1203 catalyst was completely inactive for the reaction.
Acid strength of Si02-A1203 used was in the range of -12.70< HoL-11.35, determined by the visual color change method of the Hammett indicators [ 2 51.
The present catalysts were themselves colored; the acid strength was
not estimated by the visual color change method.
The highest strength of
the surface acidity was estimated to be Ho=-14, -13, and -13%-12 for W03/Sn02 (lOOO"C), W03/Ti02 (7OO0C), and W03/Fe203 (7OO0C), respectively, judging from the reaction results on the basis of catalytic activities of other superacids 12-51.
Since the acid stronger than Ho=-12 is known as
superacid, the present catalysts would be solid superacids. Superacid sites were not created by impregnation of the tungstate on the crystallized oxides. 100O'C
Namely, the catalysts prepared by calcining Sn(OH)4 at
and H4Ti04 and Fe(OH)3 at 700°C to the crystallization, impregnating
each with the tungstate and finally calcining at the same temperature were not active at all for the reaction of isopentane (under 1/10 in activity for the dehydration of ethanol).
The XRD pattern of the material prepared from
the crystallized oxide was completely different from that prepared from the hydoxide; the former showed the degree cf crystallization to be much high in addition to spectra of the crystallized W03, while the latter was not, W 0 3 being highly dispersed into the supports in the case of the latter. Specific surface areas of the catalysts prepared from the hydroxides were quite large considering calcination at elevated temperatures and also much larger compared with those of the materials prepared from the crystallized oxides and of the oxides without tungsten oxide as shown in Table 2; results agree with the retardation of crystallization for the former.
the It is
concluded that tungsten oxide combines with tin, titanium, and iron oxides to create superacid sites at the time when the supports are crystallized.
Table 2. Surface areas and binding energies of XPS spectra of the catalysts
Catalyst
Surface area
Binding energy (eV) W 4f7/2
W 4f5/2
0 Is
qb)
34.7
36.7
530.1
48b.lC)
494.6d)
lb)
34.8
36.7
530.0
486.OC)
494.5d)
13a)
34.8
36.7
529.6
458.2e)
464.2f)
2a)
34.9
36.8
529.8
710.99)
724.Sh)
(m2/g) WO3/SnOZ (900'C)
42
(1000°C) 33 (1100°C) 18 (1200°C) W03/Ti02 (70O0C)
6
38
W03/Fe203 (700°C) 34
-a )
Prepared by impregnation of the tungstate on the crystallized oxide.
b) Sn02 without the tungstate treatment.
c)-h) Binding energies for c)
Sn 3d5/2, d) Sn 3d3/2, e) Ti 2p3/2, f ) Ti 2p1/2, g) Fe 2p3/5, and h ) F e 2 ~ 1 1 2 .
Experiments using XPS were carried out in order to elucidate the surface property, and binding energies obtained for several electron levels of the catalysts are shown in Table 2.
The spectra of W 4f7/2 and W 4f5/2 for all
the materials were consistent with those for W03 [41.
The binding energies
of Sn 3d, Ti Zp, and Fe 2p were in agreement with the literature values of Sn02, Ti02, and Fe203, respectively.
Thus, the surface is composed of W03
and Sn02, Ti02 or Fe203. To the best of our knowledge this is the first synthesis of solid superacid which is prepared by calcination at temperatures above 100OoC, whose temperature is in the range of calcination to prepare stable materials, ceramics.
It is also concluded that solid superacids can be
synthesized by supporting W03 as well as SO4 on oxides of Zr, S n , T i , and Fe.
4 . REFERENCES 1
Superacids by metal oxides, 4.
For previous publication in the series,
M. Hino and K. Arata, Chem. Lett., (1989) 971. 2
K. Arata, Adv. Catal., 37 (1990) 165
3
K. Arata and M. Hino, Mater. Chem. Phys., 26 (1990) 213.
4
K. Arata and M. Hino, Proc. Int. Congr. Catal., 9th, Calgary (1988) 1727.
5
M. Hino and K. Arata, J. Chem. SOC., Chem. Commun., (1988) 1259.
Ouczi, L.ct al. (Editors), New Frontiers in Catalysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevicr Science Publishers B.V.All righfs reserved
EFFECT OF THE METHOD OF PLATINUM INCLUSION INTO SPHERICAL PROMOTER OF CATALYTIC CRACKING TERMOFOR ON I'IS ACTIVITY IN CO COMBUSTION M. I. Levinbukj V.B. Melnikov, H. R Shapieva, V. I. Vershinin, V. A. Kuzmin and V.Je. Varshaver Grozny Oil Scientific Research Institute, Br. Dubininih 23, Grozny, Russia
ABSTRACT Oxidative activity in CO cornbustion of spherical cracking catalyst samples containing Pt introduced separately into zeolite and matrix has baen studied. On the basis of laboratory scale and commercial tests t h e greater effect of Pt inclusion into zeolite component of spherical cracking catalyst is shown. Microspherical promoters of CO combustion in catalytic cracking units added in small amounts into catalyst are used all over the world. In the USSR CO emission in the cracking unit with spherical catalyst is 10 times less than in the unit with microspherical catalyst. However Soviet Thermofor units process great amount of cracking feedstock and CO combustion in them is a real problem. Lower catalyst circulation ratio in Thermofor units requires the use of CO combustion promoter in quantities comparable with spherical catalyst loading into reactor-regenerator block (Table 1). Therefore CO combustion promoter itself is a n active component of petroleum fraction cracking as compared t o microspherical promoter the support of which is inactive in cracking alumina. To simplify the technology of spherical catalyst promoter production Pt compounPs are intruded into the catalyst at the stage of shaping of wet spheres with gelling solutions. Upon separate inclusion of constant Pt compounds quantities into zeolite and catalyst matrix with Corresponding gelling solutions the authors revealed different catalytic activity of the contacts in CO combustion (Table 2 ) in laboratory-scale plant which is a possible result of specific distribution of Pt in the samples.
261 8 Table 1 Comparison of some parameters of catalytic cracking units in the USSR with spherical and microspherical catalyst.
............................................................ catalytic cracking units
.................................... with spherical catalyst
with microspherical catalyst
............................................................ Unit capacities, re1 .% Average emission of carbon oxide in regeneration gases, t/day
69.5
30.5
8.0-14 0 80-120 ............................................................
Content of platinum in CO combustion PROMOTER, wt.% Quantity of CO combustion promoter in mixture with catalyst in reactorregenerator block in cracking units, wt.%
0.0005-0.0015
0.05-0.10
10-25
0.1-0.2
Table 2 Activity of spherical catalysts carbon oxide combustion.
promoted
by
platinum
in
To evaluate Pt distribution over a catalyst granule there were synthesized samples of zeolite zirconium silicate catalysts with separate Pt inclusion into zeolite and wt%. For zirconium silicate matrix in amount of 0.12 investigation there were used well polished mechanical
spalls of two samples which were put into the chamber of raster electron microscope JSM-820 (Japan) with microprobe analyzer ("Link" company Great Britain)/Q/. The X-ray Patterns were preliminary set t o characteristic spectrum lines of aluminium, zirconium and platinum. The areas of high element concentration in scanning range in the photograph will be white and black in those points where the given element is absent. Distribution of zeolitb particles (2-4 p m ) in matrix was determined from X-rays characteristic of zirconium and aluminium because in given samples these elements are concentrated in matrix and zeolite respectively. If silica alumina matrix were used it should be hardly possible to distinguish matrix from zeolite because both of them contain aluminium and silicium. Conglomeration of zeolite particle t h u s determined was studied in X-ray characteristic of platinum. As one can see in figurea 1 . 2 platinum included into the sample with zeolite suspension is mainly concentrated in zeolite particle.
Figure 1. X-ray patterns of zirconium, aluminium and platinum distribution in spall of catalyst sample with Pt introduced into zirconium silicate matrix.
Figure 2. X-ray patterns of zirconium, Platinum distribution in spall of catalyst introduced into zeolite suspension.
aluminium and sample with Pt
The authors relate higher oxidative activity of samples with platinum introduced into zeolite t o formation of AlaPt phase inactive in CO combustion. AlaPt phase was formed upon thermal steam treatment of the catalyst in regenerator according t o the hypothesis /3/. Obviously, its formation is more preferable in amorphous silica alumina matrix than in regular crystalline structure of zeolite. On the basis of the results acquired commercial lots of spherical promoters of CO combustion were produced and tested in Thermofor unit at Salavat Refinery. As can be seen from Table 3 spherical promoters ( a s an additive t o Zeokar-3 catalyst) containing platinum introduced into zeolite are more effective under commercial conditions used in operating cracking units.
Table 3. Changes in parameters of Thermofor cracking unit of Salavat Refinery using Zeokar-3F
............................................................ Mixture, catalyst/ promoter
Residual coke an catalysts, wt5
Average CO emission into atmosphere,t/day
Increase of gaso1 ine yield, wt.5
............................................................
---
Zeokar-3 + Zeokar-3tZeokar-3F2 Zeokar-3+Zeokar-3F4
base -0.25 -0.45
8 -0-14.0 4.0-6.0 2.0-3.5
base 1.1 1.8
1. F.D.Hartzwel1 and A.W. Chaster. Oil and Gas J., v 77, N 16 ( 1 9 7 9 ) 83 2. H.K.Magomadova, M.1. Levinbuk. Zeolite Usage in Catalysis, I V All-Union Conference, Moscow (1989), 204 3. N.A.Zakarina, G.D.Zakumbaeva, Petroleum Chemistry, N 1 ( 1 9 9 0 ) 40 4 . Raster Electron microscopy. Edited by Goldstein and H.Jakovit6. M i r Moscow ( 1 9 7 8 ) 29-466.
Guni, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 8 1993 Elsevier Science Publishers B.V.All rights reserved
COOXIDATION CO AND HZ ON PALLADIUM CATALYST N. A. Boldyreva and V. K Yarsimirsky Chemistry Department, Kiev State University, Volodymyrska 64,252017 Kiev, Ukraine
Unstationary phenomena such as existence of critical temperature and oxidized agent concentration, "the surface memor y o f earlier reaction", multiplicity of system stationary states are observed in CO and €I2 separate oxidation in oxygen excess. In particular, according to our data two stationary states occur on palladium catalyst; the low active state (-10% CO or H2 conversion) corresponds to the process proceeding by heterogeneous mechanism and high active state (-100% CO or H2 conversion) - by heterogeneous-homogeneous one. Transition from one state to another sharply takes place in the narrow (3-5 K) temperature range [l] In the present work the study of H2 and CO mutual influence during their cooxidation in gradientless reactor has been carried out. Palladium foil (Spd= 0.002 m 2 ) and the system 0.5% mass 2 Pd on silica gel (Spa= 0.6 m ) have been used as catalysts. Palladium surface has been determined by impulse method using CO chemisorption. CO and H2 conversion degree temperature dependences are shown in Fig. l(a) on palladium foil and Fig. l(b) on Pd/Si02 system. CO and H2 mixture is oxidized at the temperatures inside the temperature range for H2 and CO separate oxidation. Both CO and H2 conversion degree dependences are described by the same curve during their cooxidation curve 3 - 3 ' ) . Hysteresis is (Fig. l(a) - curve 3; Fig. l ( b ) not observed on curves corresponding to Pd foil, while f o r Pd/Si02 catalyst H2 addition into reaction mixture ( C O t 0,) leads to hysteresis loop expansion and this effect grows with hydrogen concentration increase.
.
-
-
2622
320
400
480
7
F i g u r e 1 . H2, CO and m i x t u r e (CO+H2) c o n v e r s i o n d e g r e e temper a t u r e s dependences on I’d f o i l ( a > a n d on Pd/Si02 c a t a l y s t ( b ) . a : 1 -H2, 2-C0, 3-CO+H2; b: 1-1’ -E2, 2-2’ -CO, 3-3’ -CO+H2 Gas f l o w r a t e 0.1 l / m i n . C = 0.89*10-4mol/l; C = H2 0 . 4 4 ~ 1 0 - 4 m o l / l ; C = 8.9.10 -5O mol/l.
-
O2
o x i d a t i o n r a t e dependences on o x i d i z e d compo2 n e n t s c o n c e n t r a t i o n i n g r a d i e n t l e s s r e a c t o r c y c l e a r e shown i n F i g . 2 , d u r i n g s e p a r a t e o:,idation and c o o : i d a t i o n on Pd f o i l ( a ) and Pd/SiO;, c a t a l y s t ( b ) . R e a c t i o n r a t e dependence o n 112 and C O c o n c e n t r a t i o n on Pd f o i l h a s maximum which i s s h i f t e d t o t h e h i g h e r i n i t i a l c o n c e n t r a t i o n a r e a w i t h temper a t u r e r i s e . There i s no h y s t e r e s i s on Pd f o i l . H2 a d d i t i o n t o t h e C O and O 2 m i x t u r e l e a d s t o r e a c t i o n r a t e i n c r e a s e . T h i s e f r e c t i s most n o t i c c a b l e i n t h e m a x i m u m v i c i n i t y . 112 o x i d a t i o n r a t e dependence on H2 c o n c e n t r a t i o n on Pd/Si02 cat a l y s t ( F i g . 2 ( b ) , c u r v e s 1 - 1 ’ ) i s c h a r a c t e r i z e d by h y s t e r e s i s . If H2 i n i t i a l c o n c e n t r a t i o n i n c r e a s e s ( c u r v e 1 ’ ) t h e r e a c t i o n p r o c e e d s w i t h s t e a d y r a t e a n d c a n be t r a n s P e r r e d i n t o low-active s t a t e o n l y w i t h s u b s t o n t i a l t e m p e r a t u r e decrease. H2 a d d i t i o n e f f e c t s C O o x i d a t i o n ( F i g . ? ( b ) , c u r v e s 3-3’) n o t o n l y i n the l o w co c o n c e n t r a t i o n s r e g i o n Lriiere r e n c i-ion o r d e r e q u a l s t o 1 , b u t also i n c a r b o n monooxide high c o n c e n t r a t i o n a r e a , w i t h r e a c t i o n o r d e r by CO b e i n g c l o s e t o 0. CO and H
2623
8tt
a
L
F i g u r e 2. Reagents c o n c e n t r a t i o n dependence o f t h e C O a n d H2 o x i d a t i o n r a t e on Pd f o i l ( a ) and Pd/Si02 c a t a l y s t ( b ) . a: 1 -H2 ( 3 7 3 K ) ; 2-CO (508 K ) ; 3-CO+H2 (509 K , CH = 0.44*10'4 m o l / l > 2 b: 1-1'-H2 (323 K); 2-2'-CO (443 K); = 0.44*10'4 m o l / l ) . 3-3'-COtH2 (443 K, C H2 T h e r e f o r e , i n t h e c a s e of CO and H2 c o o x i d a t i o n on Pd CO o x i d a t i o n t o t a l rate i n c r e a s e i s observed b o t h f o r t h e c a t a l y s t w i t h low p a l l a d i u m s u r f a c e and f o r t h e sample w i t h developed m e t a l surface. The d a t a o b t a i n e d cannot be d e s c r i b e d u s i n g t w o - i t i n e r a r y h e t e r o g e n e o u s mechanism, t h a t w a s proposed i n 121 and that supposes mutual i n h i b i t i o n i n t h e CO a n d H2 o x i d a t i o n ( o n P t w i t h P = 10 -5 Torr). Homogeneous component makes s u b s t a n t i a l c o n t r i b u t i o n t o t h e t o t a l C O and H2 o x i d a t i o n r a t e and c a u s e s d e v i a t i o n from r e a c t i o n t o t a l rate d e c r e a s e e f f e c t t y p i c a l f o r heterogeneous c a t a l y t i c process.
2624
Our i n v e s t i g a t i o n p o i n t s t o t h e i n i t i a t i o n r o l e of H2 add i t i o n i n C O o x i d a t i o n on Pd, t h a t i s t y p i c a l f o r homogeneous CO o x i d a t i o n . A c t i v e p a r t i c l e s r e s p o n s i b l e f o r CO o x i d a t i o n by heterogeneous-homogeneous mechanism a r e g e n e r a t e d p r o v i d e d H and O 2 a r e i n v o l v e d i n t h e r e a c t i o n . I t i s known t h a t H 2 and 0 atoms, OH'and HO; r a d i c a l s a r e a c t i v e p a r t i c l e s i n CO and H2 homogeneous o x i d a t i o n . I n p a r t i c u l a r , i t i s ehown i n 131 t h a t s o l i d s u r f a c e s ( r e a c t o r w a l l s ) s a t u r a t i o n by s o r b e d atoms and r a d i c a l s t a k e s p l a c e d u r i n g r a d i c a l - c h a i n r e a c t i o n s and t h i s l e a d s t o c h a i n s growth and brttnching. A p p e r e n t l y , s u c h phenomenon ( s u r f a c e g a r t i c i p a t i o n i n h e t e r o g e n e o u s c h a i n r e a c t i o n s due t o t h e s o r b e d atoms a n d r a d i c a l s ) must t a k e p l a c e on a c t i v e c a t a l y s t s t o o , i n p a r t i c u l a r , on Pd where t r a n s i t i o n o f h e t e r o g e n e o u s r e a c t i o n i n t o heterogeneous-homogeneous regime i s observed. Taking i n t o c o n s i d e r a t i o n t h e p o s s i b l e r e a c t i o n s o f s o r b e d a c t i v e c e n t r e s on c a t a l y s t s u r f a c e [ 3 ] , we c a n suppose t h a t CO o x i d a t i o n on Pd i n t h e H2 p r e s e n c e p r o c e e d s a c c o r d i n g t o t h e f o l l o w i n g scheme: Ils + O2 H02 HS + H02 --c 20H OH + CO C02 + H H + O2 + M H02 + M H + HOP 20H Hs s o r b e d hydrogen atom.
-
-
REFERENCES 1 N.A. Boldyreva, Khirn.Phys., 9 ( 1 9 9 0 ) 1538. 2 V.D. Kuchaev, L.M. N i k i t u s h k i n a , M.I. Tiomkin, 11. Conf. Chem.Heact.I(inetilts, Moscow, 2 ( 1 9 7 5 ) 1 2 . 3 V.A. A z a t i a n , Usp.l;himii, 54 ( 1 9 8 1 ) 33.
Guczi, L. er d.(Editors), New Frontiers in Caralysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
THE STRUCTURE OF SUPPORTED Pd, KINETICS AND MECHANISM OF THE LOW-TEMPERATURE OXIDATION OF CARBON MONOXIDE
S.N. Pavlova, V.A. Sadybv, D. I. Kochubei, B. N. Novgorodov, G. N. Kryukova and V. A. Razdobarov Institute of Catalysis, Russian Academy of Sciences, Siberian Division, pr. Lavrentyeva 5, 630090Novosibirsk, Russia
ABSTRACT By wing trarwniwion electron m i c m ~ EXAFS, , thermodcmrption and unsteadyrtate kinetic Sxpsrimentr the influence of the rtructura of mpported Pd on its admp tion propertier and activity in the reaction of low-temperature oxidation of carbon monoxide h u been inveetigated. The problem of d r u c t m sedtivity of t b reaction has ban d i e c d .
1 Introduction It h a been shown (1-21 that at temperaturea > 673 K the reaction of CO catalytic oxidation on Pd L structure hnoitive. However, h low-temperatureregion ( < 300 K)mpported Pd b wry active in thh reaction, while bulk metal ia practically inactive I3-41.In order to elucidate the nature of the diffmce between the activity of bulk and supported Pd in low-temperature region, we have carried out a complex invertigation of structure, reactivity and h r p t i o n propertier of Pd mpported on Ti&,SiO2 and 7 - A1208 wing such methods aa TEM, EXAFS, thermodeeorption combined with unoteady-date kinetic experiments.
2
Experimental
The detaih of the eamplea preparation can be found in [a], Pd content in all samples wan ca 2.6 %. TEM inveetigations were carried out uaing JEM-1OOC electron microm p e in bright-field mode. EXAFS-spectra were collected in t d i o n mode on the opedrometer of Siberian Center of SB 161. Sampler after reduction or rubaequent p r a treatment in CO/naction mixtmwere trannferred into the vacuum-tight cello without air contact. Khetic measmmentr and thermdemrption acperimento were made using pulae/tlow microcatalytic inatallation and reactor with vibrofiuidircd bed of catalysts.
2626
After standard treatment at 673 K in hydrogen the catalyst was usually cooled down to the temperature of experiments in the He flow.
3 Results and discussion 3.1 The structure of supported Pd particles According to TEM, the particles of Pd on various supports have narrow sine distribution,
the average s b e being close to 20 A. For Pd/silica the uniformly distributed Pd particles have nearly spherical form, the clusters being elevated over the surface of mpport. The particles of Pd on anatase were flattened and 6xed on the moat developed (110) faces of support. For 7-alumina the particlea of Pd have not definite forms and are distributed nonunifomly. The study by EXAFS of the catalysts reduced in hydrogen at 673 K has demonstrated that for all sup rte Pd is in a metal state. The first Pd-Pd distance in all casee b in a range 2.66 (Pdlanatase) 2.79A(Pd/silica), that b close to one in Pd foil (2.74A). Decreased Pd-Pd distance for Pd/anataee and flattening of clusters were explained by epitacy with thie support. Supported Pd was characterbed by an increased value of Debye factor as compared to Pd foil. Analysis of the Debye factors by a method p r o p d in [S] enabled to establish that main peak in EXAFS-spectra could be approximated by two Pd-Pd distances differing by AR= 0.040.07b depending on the support nature. One of them could be assigned to the d a c e layer of Pd particle, the other - to the core. A compression of the eurface layer was explained by reconstruction of the faces of (100 ) and (110) types into more deneely packed corrugated layera with local hexagonal arrangementsof Pd atoms. A part of Pd atoms is 'squeered" from the rmrface layer, while some atoms of the second layer get exposed resembling "missing row" model After CO adsorption for all catalysta the distances approached each other, the Debye factor increaaing. For Pdlalumina peak corresponding to Pd-C distance appeam. The treatment by reaction mixture for Pd on all eupporb leads to the strong disturbance of the surface that waa reflected in the increase of AR to 0.08-0.11d, accompanied by the increase of Debye factor. After thia treatment for Pd/silica and Pd/alumina Pd0 distance (1.94A) and increased average Pd-Pd distance have been observed. These facts indicate the incorporation of oxygen into the subsurface layer of Pd clusters. The approaching of the Pd-Pd distance for the shell to that of the core caused by CO adsorption could probably indicate the rearrangement of the structure throughout the particle due to the increase of the degree of Pd coordination on the d a c e .
K"
-
.
3.2 Themodesorption According to TPD data (FigJ), an amount of weakly bound CO deeorbing up to 400 K is suflBciently high approaching 10-60 96 of monolayer, while in the c w of bulk Pd it is not higher than 0.196 of monolayer 141. For Pd/anatase total coverage at 298 K wa8 close to 60-70 % of monolayer, while for other mpporte this d u e wa8 dose to monolayer. Note the appearance of unique low-temperatun, peak at ca 268 K for Pd/oilica As poseible model for the centere with low bonding drength, ' g q u d " Pd a t o m on
2
2627
reconstructed faces could be considered. The decrease of the heats of admrption could be explained by rimdtaneous rupture of a part of bonds with underlying Pd atoms ("breathing raft model"). Note also (Fig.l) high-temperature peala a k n t for bulk Pd and Pd/ a-alumina Ill. They could originate from CO disproportionation on defect cent= with high bonding drength with subsequent C+O recombination at elevated temperatune. 25
15
Figure 1: Thermal demrption of CO from supported Pd catdysts, after CO adsorption at 263 K from the stream of .66% CO in He: 1 -Pd/TiOl, 2 - Pd/Al108,3 - Pd/Si02.
S.S Reaction of CO and oxygen titration The titration of adsorbed oxygen by gaeeoua CO wao characterirred by the monotonous decrease of the rate of carbon dioxide formation a a function of a pulse number indicating impact type interaction. In the cam of the titration by oxygen of CO adoorbed on free surface, the rate of carbon dioxide production has typical a-form for all catalysta. The titration of CO adoorbed on the surface pmcmered by oxygen or treated in reaction mixture WM characterbed by a rate of carbon dioxide production running through the maximum at ca 0.5. In these casee also the dependence of initial rates of titration on oxygen p m u r e WM found, indicating thm the dagea of impact type, in contrary to eingle crystal iacea where only mdaniom of Langmuir-Hinshelwimd type operates. 'lbmperatu.re-programmed reaction in adlayer has a h revealed that oxygen incorporated into wburface layer has low reactivity and can be removed by reaction with CO only at temperatures higher than 400 K.
3.4
Mechanism of law-temperature carbon monoxide d d a tion
An found from the comparison of the rates of carbon dicucide evolution from the surface of catalysts under the action of p u b of carbon monaotide, q g e n and reaction mixture, 3
2628
at ambient temperatures catalytic activity in eteady-etate conditions is determined by interaction of weakly bound CO with weakly bound or gaa-pham axygen. A cornlation of activity with an amount of weakly bound CO b found.The operation of thir type of mechanism in low-temperature region wae a h manifested by alteration of kinetic parameten aa compared with those typical for the caae of h g l e cxyotd faca. A dynamic features of low-temperature reaction (Fu.2) an determined by attainment of the deadyetate coveragea of the d a c e by CO, being dependent upon the nature of the eupport. For Pd /alumina a rapid fall of the reaction rate could be explained by attainment of nearly monolayer average of the d a c e by otmngly bound CO dominating for thir catalyet (cf. FQ. 1). It mema quite obviow that otructure rensitivity of the reaction of low-tempmture CO oxidation b determined by the fact that a density of defect centera s t a b w i g reactive forma of CO and oxygen could vary in broad limitr. At temperaturea higher than 400 K demrption of weakly bound forma provideo for the transfer to wual type of L H mechanism without dructural oenaitivity. a
w 40-17 20
A1
10 -
1
-
2 "
Y
n
Y
Figure 2: COa evolution in p h of l%CO 1 -Pd/TiOa, 2 Pd/Al&, 3 Pd/SiOa Standard pretreatment. [W]=mol(CO,)/ma(Pd) oec
-
4
-
0 3 ,
m
+ 19%01 ih He at 283 K
References
l.S.LadaS, H.Poppa, M.Boudart, Surf. Sci., 102 ( 1981 ) 161 2.MmBoudart,F.&umpf,R.eact. Kinet. Catal. Lett.,36 ( 1987) 96 3.S.N.Pavlova, V.A.Swnov and V.V.Popovok& B,eact.Kinet.Catal.Lett., 37 (1988) 326 4.T.Engel and G.Ert1, in The Chemical Physics of &lid Surfand Heterogeneous Catalysis, (DAKingand and D.P.W&ff, Eds.),4 (1982)76. 6. D.I.Kochubei, YuABabanov and K.I.Zamaraev. The Study of the Structure of Amorphow Solids by X-Ray Spectral Method. Nwoaibimk, 1988. b.C.Y.Yang, M.A.Paeater and D.E.Sagem ,Phyr. Rev. B 39 (1989) 10342 .
4
Guni, L. et ol. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July. 1992,Budapest, Hungary 8 1993 Elsevier Science Publishers B.V. All rights reserved
TRANSITION METAL COMPOUND OXIDE CATALYSTS FOR LOWERING THE LIGHT OFF-TEMPERATUREOF PARTICLES FROM D m E L EXHAUST
C. Setzer4 W.Schiktzb and F. Schiltha aInstitut f i r Anorganische und Analytische Chemie, Johanna-Gutenberg-UniversitatMainz, Johanna-Joachirn-Becher-Weg 24, D-W 6500 Mainz, Germany bDidier Werke AG, Wiabaden, now: Fachhochschule Bremerhaven, D-W 2850 Brernerhaven, Germany
1. Introduction Over the last two decades it was realized that particle emissions from Diesel engines are a severe environmental problem which is difficult to solve by engine modifications alone. Even if the combustion occurs in a high excess of air, appreciable amounts of soot particles are found. Cracking processes, dehydrogenation and polymerisation play an important role in the formation of soot [l-31 . To reduce the soot fraction several methods have been developed in the past [3], but most of them have fundamental disadvantages. Thus in the last few years a new procedure has become more important. It is based on the catalytic burning of the soot in the exhaust gas. In this study we investigated the effect of several compound oxides for this purpose.
2. Experimental The catalysts useful in this process are transition metal compound oxides. They were all synthesized by solid state reactions. A mixture of the oxides or carbonates was heated to temperatures between 800 K and 1150 K [4-91, for times between 16 h and 96 h. The reaction products were characterized by XRD (X-Ray Powder Diffraction) measurements.The light-off temperatures of mixtures containing soot and these different catalysts were determined by a DTA/TG method (Differential Thermal Analysis/Thermo Gravimetry). For this purpose a "standard" of soot, obtained from a Diesel motor under varying engine loads, was mixed with the concerning catalyst in ratio one to one. These mixtures were heated to 1070 K using a heating rate of 20 K/min. The light-off point was determined from the peak in the derivative of the DTA Signal which proved to be most reproducible (+/- 5 K). Corundum was used as a reference.Using DRIFT spectroscopic measurements (Diffuse Reflexion Infra Red Fourier Transform) with some of the catalysts, kinetic studies were also carried out. For that purpose a mixture of the catalyst and soot was heated to 970 K in atmospheres with different oxygen/nitrogen ratios. A flow of 120 ml/min was passed over the samples, the oxygen concentrations were adjusted between 2.5 and 10%. Varying the flow rate ensured, that at 120 ml/min the system was operating in a differential mode. The C02 signal of the IR spectrometer was used to monitor the reaction
2630 rate.In order to obtain some information about the mechanism of the catalytic soot combustion DRIFT measurements were applied again. For that purpose the catalysts were mixed with soot in ratio 1 to 1 and afterwards the samples were mixed with KBr in ratio 1 to 4 (necessary to reduce absorption losses). The samples were then heated to 870 K with a rate of 20 K/min and spectra of the catalysts were recorded during the burn-off. The room temperature spectrum of each run was used as background.
3. Results The light-off points that were obtained by DTA measurements are collected in table one. Table 1: Catalyst
Light-off point [K]
I "1
I
Catalyst
Light-off point [K]
I
Temperature [K]
Figure la: Integral absorbance over C02 band versus temperature at different 0 contents (1 : 2.5%, 2 : 5 % , 3 :21.5%, 4 : 10%).
I Figure lb: Integral absorbance of CO2 band at a temperature of 659 K versus 0 2 concentration
2631
As can be seen from table one the light-off point of the soot is lowered in every case where a catalyst is used. It is decreased for more than 100 K when the catalyst is a copper vanadate. The results of the kinetic studies for copper orthovanadate can be seen in figure la. The intensity of the C02 band is a measure for the reaction rate at a given temperature, since the C02 is continuously purged out of the system by the N*/O*-flow. Figure la shows that the reaction rate at the ignition point increases with increasing oxygen content, while the ignition temperature remains unchanged. In order to obtain information about the reaction order the integral absorbance over the C02 band at 659 K was plotted versus oxygen content. Figure lb shows a linear correlation between oxygen content and CO2 peak intensity. Although the measurements were not performed under steady state conditions (which is not feasible in a reaction like this) a reaction order close to unity can be deduced from this plot. DRIFT spectra of the catalysts at different points of the catalytic soot combustion are shown in figure 2. In this e am le the catalyst was copper metavanadate. During heating a peak appears at 980 he! intensity reaches a maximum at 724 K and decreases at temperatures higher than that. While the peak at 980 cm-1 disappears, another peak grows at 900 cm-l. The maximum intensity of this peak is reached at 792 K. At higher temperatures it diminishes again. To show this effect more clearly, the peak intensities were plotted over the temperature in figure 3. A similar behavior was observed for all copper containing catalysts, while catalysts containing other cations also showed new peaks emerging during the bum-off which are, however, appreciably shifted in wavenumber. -
'1 0
6
8
om
om
03
03
Wavenumbers [M-I]
Figure 2: IR spectra of the copper metavanadate catalyst during soot combustion. (1 : 298 K, 2 : 724 K, 3 : 792 K)
Temperature [K]
Figure 3: Intensity of the peaks at 900 cm1 (1) and 980 cm -1 (2) versus temperature.
2632
4. Discussion When examining table one it can be gathered that the light-off point is correlated with the ability of the catalyst components to change their oxidation state. Copper- cobalt- and iron cations can change their oxidation state easily between +I1 + +I or between +I11 + +I1 respectively. Thus these vanadates show lower light-off points than the corresponding nickel or zinc compounds. The same tendency can be seen for the copper compounds with different complex anions. Vanadium can change its oxidation state easily between + V -+ +IV or between +V + +I11 res ctively, while niobium prefers an oxidation state of +V, and a similar tendency holds or molybdenum and tungsten. These interpretations are supported by the DRIFT investigations. The IR-measurements in figure 2 show that around the ignition point new peaks appear and increase. IR investigations with mixtures of soot and KBr and catalyst and KBr, respectively, proved that these peaks appearing during the ignition process can neither be attributed to the catalyst nor to the soot alone. Most probably, at the ignition points the catalysts undergo a structural change and/or a change of the oxidation states of the metal ions when in contact with reducing materials. This is confirmed in experiments where the copper vanadates mixed with KBr, but without soot were heated in N2 or CO. In the inert N atmosphere we also observed a peak at 980 cm-', the intensity of which, however, was onfy ibout 10% of the intensity obtained during soot burn-off. The full intensity of the 980 cm- peak, though, could be observed while heating in the reducing CO atmosphere. Since the behavior was the same for all copper vanadates, but different for the Co-compounds, we conclude that these bands are coniiected with a reduction process of the Cu-ions. Further investigations concerning the importance of the process for the catalytic soot removal using isotope labelling and further DRIFT studies are being carried out.
p"
Acknowledgement We wish to express our thanks for the permanent and generous support we received from Didier-Werke, Wiesbaden.
5. References A. Thomas, Combustion and Flame, 6 (1962) 46 B.B. Chakraborty and R. Long, Combustion and Flame, 12(5) (1968) 469 W. Huhn, MTZ, 31(3) (1970) 109 A.G. Nord and G. Aberg, Chemica Scripta, 25 (1985) 212 J.C. Pedregosa, E.J. Baran and P.J. Aymonino, Z. Anorg. Allg. Chem., 408 (1974) I!?
6-A.G. Nord, P. Kierkegaard and T. Stefanidis, Chemica Scripta, 28 (1988) 133 7 G.V. Bazuev and E. I. Krylov, Russ. J. Inorg. Chem., 14(12) (1969) 1686 8 E. Gebert and L. Kihlborg, Acta Chem. Scand., 21(9) (1967) 2575 9 J. Haber and K. Jamroz, Revue Chimie MinCrale, 20 (1983) 712
Ouczi, L ct al. (Editors), New Fronricrs in Catalysic Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
FTIR STUDY OF REDUCTION MECHANISM OF NO BY C2H4 AND C3HB ON VANADIUM OXIDES LAYERED ON ZrO2 T. O h n 4 , F. Hatayamaa, T.Maruokab and H. Miyatab aSchool of Allied Medical Sciences, Kobe University, Suma, Kobe 654-01, Japan bDepartment of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka 591, Japan
Abstract The reduction mechanism of NO by C a L o r CSHS on V-Zr oxide prepared by a gas-phase method and t h e s t r u c t u r e s of the catalysts under reaction conditions have been studied by FTlR spectroscopy. It has been proposed t h a t hydrocarbon is oxidized by the c a t a l y s t t o form COa via carbonyl and carboxylate species and NO reoxidizes t h e catalyst t o form Na. 1. Introduction Supported vanadium oxide catalysts a r e widely used as selective catalysts f o r t h e oxidation of various hydrocarbons and NO reduction. Catalyst s t r u c t u r e s may change under reacton conditions through such redox o r hydration/rehydration processes. We have recently reported t h e characterization of vanadium oxides layered on zirconium oxides prepared by a gas-phase method and reaction of olefin and 2-propanol on it [1,2]. In t h e present work, in o r d e r t o clarify the mechanism of NO reduction by CaH4 or c&8 on V-Zr oxide and t h e s t r u c t u r e of t h e catalyst under reaction condition have been studied by FTlR spectroscopy as well a s t h e analysis of the reaction products. 2. Experimental The zirconium hydroxide was prepared from ZrOCla. The hydroxide was calcined at 383 K, followed by decomposition a t 673 K. V-Zr oxide catalysts were prepared by a gas-phase method using
2634 VOCla (6.3 wt% vanadium as VaOs, 63 m"/g). The catalyst was heated t o 723 K under evacuation and kept a t 723 K under a flow of oxygen (ca. 4 kPa) for 2 h. This pretreatment was repeated several times before each experiment. The apparatus was a conventional closed-circulation system equipped with an IR c e l l in the circulation loop. Details of the apparatus and the data acquisition and analysis system were described previously [l-31.
3. Result and Discusaion
IR spectra of V-Zr catalyst in the oxidation state showed no bands due t o adsorbed NO species. Therefore, the catalyst was reduced by a flow of H a (3 Wa) a t 623 K f o r 30 min. This treatment caused 24 % reduction of the integrated intensity of the V=O band of the catalyst. The spectra of reduced catalyst showed a number of bands a f t e r introduction of NO (650 Pa) a t room temperature [Fig. l(a)]. These bands a r e attributable t o NO' (>1900 cm-') o r NOspecies ( ? i = l would j u s t i f y t h e value o f t h i s r a t i o obtained by XPS f o r sample 6P; they might a l s o be responsible f o r t h e peaks o f t h e A-phase detected w i t h XRD. C a l c i n a t i o n a t 973 K produces a b e t t e r c r y s t a l l i z a t i o n o f t h e d i f f e r e n t components, and probably t h e decomposition o f some o f t h e T i phosphates which w i l l r e s u l t i n a growth o f t h e anatase p a r t i c l e s and t h e segregation t o t h e surface o f some phosphorus ( i n t h e form of amorphous PO,), carrying with i t V ions. For Tc11073 K these e f f e c t s are more marked w i t h t h e f o r m a t i o n o f r u t i l e , Ti,O(PO,), and a vanadium oxide, w h i l e TiP,O, i s decomposed. These phases being however covered by a P - r i c h l a y e r (mainly amorphous P O , , probably) according t o t h e XPS data. The vanadium agglomeration i n t o an oxide phase and t h e phosphorus enrichment o f t h e surface l e a d t o t h e low V/(PtTi) r a t i o f o r t h i s sample. I n conclusion, t h e present r e s u l t s i n d i c a t e t h a t t h e c a l c i n a t i o n temperature a f f e c t s t h e c a t a l y s t by i n f l u e n c i n g t h e c r y s t a l 1 i z a t i o n o f t h e d i f f e r e n t components, t h e decomposition o f t h e l e s s s t a b l e ones and t h e types and d i s t r i b u t i o n o f phosphate phases i n the c a t a l y s t . The surface, formed mainly by T i phosphate and (amorphous) PO ,, f o r Tc=773 K, i s depleted o r enriched i n P upon i n c r e a s i n g Tc depending on t h e f o r m a t i o n and decomposition o f T i phosphates. F o r sample 7P, which has maximum c a t a l y t i c a c t i v i t y f o r SCR o f NO,, decomposition o f t h e main T i phosphate leaves on t h e surface a V-P-0 phase, where each vanadium i o n has i n t h e average, according t o EXAFS data [9], two s h o r t V=O bonds. I t may w e l l be t h a t such V species w i t h more than one s h o r t V=O bond are t h e ones p a r t i c u l a r l y a c t i v e f o r t h e SCR r e a c t i o n .
k
5. ACKNOWLEDGMENTS We thank D r s . J. Blanco and P. A v i l a f o r p r o v i d i n g t h e samples. 6. REFERENCES
H. Bosch and F . Janssen, Catal. Today 2 (1988) 1. P. A v i l a , A. Bahamonde, C. Barthelemy and J. Blanco, Anales G.E.C. (Madrid, 1989) 167. J. Blanco, P. A v i l a , C. Barthelemy, A. Bahamonde, J.A. Odriozola, J.F. Garcia de l a Banda and H. Heinemann, Appl. C a t a l . 55 (1989) 151. J. Soria, J.C. Conesa, M. Lopez-Granados, R. Mariscal, J.L.G. F i e r r o , J.F. Garcia de l a Banda and H. Heinemann, J. Catal. 120 (1989) 457. N.G. Chernorukov, I . A . Korshunov, and M . I . , Zhuk, Russ. J. I n o r g . Chem. 27 (1982) 1728. A. C l e a r f i e l d , J. S o l i d S t . Inorg. Chem. 28 (1991) 37. J. Soria, J.E. I g l e s i a s and J. Sanz, i n p r e p a r a t i o n . J. Soria, J.C. Conesa, C . Gomez, V.M. V i l l a l b a and M. Castro, Actas XI1 Simp. I b e r . C a t a l . 2 (1990) 183. M. Lopez-Granados, J.C. Conesa, P. Esteban, H. Dexpert and D. Bazin, Proc. 2nd. Eur. Conf. Progr. X-ray Synchr. Rad. Res., A. Balerna e t a l . Eds., S.I.F., Bologna 1990, p. 551.
Guczi, L a al. (Editors), New Frontiers in Caralysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All righb reserved
METHANOL SYNTHESIS OVER COPPER BASED CATALYSTS: COMPARISON OF CO-PRECIPITATED, RANEY-TYPEAND CATALYSTSDERIVED FROM AMORPHOUS ALLOY PRECURSORS A. C.SofianoS(l,J. Hevelingb, M.S.Scurrella and A. Armbrusterb
aCatalysis Programme, Energy Technology, CSIR, P.O.Box 395, Pretoria OOO1, sbuth Africa bLonza AG,Walliser Werke, 3930 Visp, Switzerland
Abstract This paper presents comparative results obtained for the methanol synthesis reaction from CO/CO2/H2 over copper-based catalysts prepared according to three different methods: co-precipitation, leaching of Cu-Zn-Al alloys (Raney-type systems), and systems derived from amorphous Cu-Zr alloy precursors. The activity and selectivity for methanol synthesis obtained over these novel catalysts (Raney-type and activated amorphous Cu-Zr alloys), despite being relatively high in some cases, are not yet able to challenge the catalytic performance of the CuO-ZnO-Al203 co-precipitated systems which was further enhanced by zirconia addition.
1. INTRODUCTION Hitherto, co-precipitated catalytic systems consisting of mixed Cu,Zn and Al or Cu, Zn and Cr oxides have been exclusively used as catalysts for the production of methanol according to the low-pressure process. In recent years, however, Raney-type copper catalysts, prepared by leaching ternary Cu/Zn/Al alloys with strong sodium hydroxide solutions have been claimed [ 11to demonstrate specific catalytic activities comparable with those of commercial methanol catalysts. More recently, amorphous alloys based upon the combination of copper or palladium with transition metals of mainly the Group IV (Ti, Zr or Hf) have been proposed as catalyst precursors for the methanol synthesis, mainly from C o n 2 [2]. This paper presents comparative results obtained for the methanol synthesis over catalysts prepared according to the three methods described above using industrially relevant experimental conditions.
2. EXPERIMENTAL The basic CuO-ZnO-Al203 co-precipitated catalysts, containing CuO and ZnO invarious ratios (Cu0:ZnO ratios of 2:1,2:3 and 3:7 were tested), were prepared by co-precipitation of hydroxycarbonate precursors from the nitrate solutions of the components. Depending on the Cu0:ZnO ratio, the pH, the temperature and the mode of precipitation (simultaneous or reverse), the precursor consisted either of pure hydrotalcite (HTlc), (Cu,Zn)6A12(0H)16c03.4H2Or or a mixture of hydrotalcite, aurichalcite, (Cu0.3Zn0.7)5. (OH)6(CO&, and/or rosasite (R), a malachite-type phase, Cu2-,ZnX(OH)2C03. The catalysts were further modified by the incorporation of zirconia which was added to a level of 1-10% as ZrO2 (by mass), as a nitrate solution, during the precipitation stage.
2722
Raney-type catalysts were prepared by caustic leaching of Cu/Zn/Al precursor alloys of different compositions (Cu~Zn15Al49and Cu43Zn18Al3g). The leaching process was carried out under a hydrogen atmosphere, at a temperature of 50 "C, using a 1,5 times excess of a 20% NaOH solution for time-periods of 1-4 h. The catalysts were extensively washed with demineralized water to a pH of 7, then dried in situ at 100 "C with flowing nitrogen, and finally passivated with an N20 stream. As in the case of the co-precipitated systems, the Na content of the catalysts was constantly monitored [3]. The last class of catalyst investigated was obtained from amorphous copper-zirconia (cU70Zr30) precursor alloys via suitable activation. The amorphous precursor alloys were prepared from the pre-mixed melt of the pure metals using the melt-spinning method (copper wheel). The ribbons obtained from melt-spinning were cut, ground to size under liquid nitrogen, and then activated under carbon dioxidehydrogen at different temperatures (Table 1). Table 1 Physical properties of various methanol synthesis catalysts BET cu CUO Catalyst surface area surface areaP XRD (A) (m2/d (m2/d Co-precipitatedb 24 A1 56 55 AlZr3 69 25 51 45 AlZr5 78 26 128 27.5 42 A2 168 29 40 A2Zr5
Raney catalystsd Cu~ZnlsAh Cu43Zn1~Al39
55 52
9.5 8.6
Catalysts from amorphous precursors CumZrm (C33) 59.3 0.8/3.75e 70 1.2/4.23e Cu7oZrw (C25) a
Pulse chemisorption method with N2O; back-titration with CO
Hydroxycarbonate precursor phases
HTlc + R + (G)' HTlc + R + (G) . . HTlc + R HTlc+R HTlc + R + amorph.
80 92 240 240
+
Al:CuO-ZnO-A1203;metal atom ratio Cu:Zn:Al=60.75:30:8.85;AlZr3: A1 3%Zr as ZrO2; A1 series: fixed pH batch co-precipitation; A 2 Reverse co-precipitation, alkaline pH; same composition
HTlc:Hydrotalcite, R:Rosasite, G:Gerhardite Cu2(OH)3N03 Composition of initial alloy; lcach time: 1h Before and alter the reaction (induction period ca. 24 h)
Catalyst activity tests were performed using a a high-pressure, fixed-bed microreactor described elsewhere [4] and a feed gas mixture containing H2,CO and C 0 2 in a ratio of 63:32:5. Analysis of the reaction products was performed by on-line gas chromatography. For activation of all catalytic systems tested, the usual stepwise procedure was adopted (thermal treatment of the catalysts first under nitrogen and subsequently with a 2 % H f l 2 mixture at 200 "C for 16 h). As measures of the catalytic performance, and for purposes of comparison, the conversion of carbon oxides (in C-mol-%), the selectivity towards methanol (C-mol-%), the space-time-yield (STY) of methanol (expressed as kgkg/h or as mol/kg/h) and the turnover frequency (s-1) are used. The experimental conditions of the tests were: Temp.: 175-350 "C, Press.:4-10 MPa, GHSV:3000-20000 (dSTp/g,th).
2723
3. RESULTS AND DISCUSSION Comparative results for the catalytic activity of representative samples of the three catalyst series, for the methanol synthesis reaction, are presented in Table 2 and, as a function of the reaction temperature, in Fig. 1. Table 2 Results of the methanol synthesis over various catalysts Turnover kequency Space-TimeRate&, Rate/Sc, (l0-2.molec/ Catalyst Yield (lO2.g/m2/h) (102.g/m2c,/h) s.Cu-atom) &g/kg,/h) Co-precipitatedb 0.60 1.07 2.50 0.782 Ala A1Zr3a 0.83 1.20 3.32 1.039 AlZr5' 0.86 1.10 3.31 1.036 Mb 0.90 0.70 3.27 1.024 MZrSb 1.15 0.69 3.97 1.243
Raney catalysts' CUjaZnuA49 CU4Zn18A139
0.29 0.25
Catalysts from amorphous precursors CU'IOZ~M (C33) 0.35 CUmZr3o ( ( 2 5 ) 0.41
0.53 0.48
3.05 2.92
0.955 0.914
0.59 0.58
9.33 9.69
2.921 3.034
Reaction conditions: Temp.:225 "C, Press.: 4 MPa, GHSV 8ooo h-'. GHSV 12ooo h l ; Temp.:250 'C, Press.: 5 MPa, GHSV 8ooo h i
Temp.:250 "C,Press.: 5 MPa,
The results in Table 2 and Fig. 1 clearly show that the co-precipitated systems are far more active in regard to the test reaction than the catalysts from the two other series (Raney-type and those derived from amorphous precursors). For comparison purposes, the performance of a co-precipitated 10% Cu-ZrO2 system is shown as well. In all cases, Zr-containing co-precipitated catalysts were superior to their CuO-ZnOA1203 counterparts, no matter what method of preparation was used. With these systems a high activity and selectivity (at temp. 97%). In contrast, the Cu-Zr02 sample produced some methane, even at lower temperatures, and generally showed an overall poor activity for methanol synthesis. The Raney-type catalysts showed a high initial catalytic activity, which can be further enhanced by the addition of zinc during the leaching process as sodium zincate, or via impregnation after the leaching process has been completed [3]. However, after prolonged exposure to synthesis gas, these systems tend to deactivate mainly because of the deficiency in aluminium (and the low zinc level) in the finished catalyst.
2724 1 .o
0
G:
7f WIJ'
0.7
Y"
0.6
2 z nvr
0.5
5
0.4
2
0.3
g k! a
A1 Z r 5 (coprec.)
/------o,
0.9 0.8
Fig. 1: Comparison of the activity of various methanol catalysts vs reaction temperature (P = 5 MPa, GHSV = 8000 h-1)
0.2 0.1
nn
Y
175
ZOO
225
250
275
300
325
350
REACTION TEMPERATURE IN OC
The Cu70Zr30 catalysts exhibit surprisingly high turnover frequencies which implies that systems obtained from amorphous alloy precursors have a high potential (higher than Raney-type systems) to yield highly active methanol synthesis catalysts if prepared under strictly controlled conditions. In addition to the proven possibility of using the amorphous state as a catalyst precursor, various solid-state reactions, occuring during the activation procedure and the induction period of the syngas hydrogenation, lead to the formation of a supported microcrystalline phase of the active metal. Hence, in contrast to the Raney-type catalysts, these systems do not seem to have stability problems, since, in their active form, they have a large surface area which contains finely dispersed crystallites of the active component. The activity for methanol synthesis obtained over this catalyst class, as well as the selectivity towards methanol were relatively high and stable. Methane formation was observed but it was limited (at temp. < 275 "C). In the case of the catalysts derived from amorphous precursors, it is clear that there is no relation between the exposed elemental copper surface area and the activity of these systems in methanol synthesis, Despite relatively high activities in the test reaction, rather low copper surface areas were measured.
4. CONCLUSIONS These results show that at this stage of development, Cu-based catalysts of the Raney type and systems derived from amorphous precursors are not yet able to challenge the catalytic performance of the CuO-ZnO-Al203 co-precipitated system in methanol synthesis. The systems obtained from amorphous alloy precursors have a higher potential than Raney-type systems to yield highly active methanol synthesis catalysts. The use of ZrO2 as an additional support enhances mainly the textural properties of the system thereby improving the overall catalytic performance and the mechanical and thermal stability of the catalyst.
5. REFERENCES 1 G.C. Chinchen, P.J. Denny, J.R. Jennings, M.S.Spencer and K.C. Waugh Applied Catalysis, 36 (1988) 1 and references therein. 2 D. Gasser and A. Baiker, Applied Catalysis, 48 (1989) 279. 3 A.C. Sofianos, J. Mellor, M.S. Scurrell and N. Coville, to be published. 4 A.C. Sofianos and M.S. Scurrell, Ind. Eng. Chem. Res. 30 (1991) 2372.
Guczi. L.d al. (Editors), New Frontiers in Catalysis
Proccedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 83 1993 Elscvier Science Publishers B.V.All rights reserved
ACTIVITY AND SELECTIVITY OF RUTHENIUM-COBALT BIMETALLIC CATALYSTS IN CARBON MONOXIDE HYDROGENATION S. A. Korili and G.P. Sakllaropoulos
Department of Chemical Engineering, Aristotle University of Thessaloniki and Chemical Process Engineering Research Institute, Thessaloniki 54006, Greece
A series of bimetallic ruthenium and cobalt catalysts on a zeolite NaY support have been prepared and tested in carbon monoxide hydrogenation reaction. In all cases, c 1 - C ~hydrocarbons were formed. Selectivity shifted towards lower molecular weights ( 4 4 ) with increasing cobalt content. The presence of cobalt caused a decrease in overall catalytic activity but enhanced stability with time-on-stream. Increased methane rates at intermediate metal compositions may be the result of modified adsorbate coverages due to ruthenium-cobalt interactions. INTRODUCTION Ruthenium is an active catalyst for CO hydrogenation, yielding a broad hydrocarbon distribution with high average molecular weight. Cobalt is less active than Ru but it can form light olefins, especially at low loads [l-31. Activity and selectivity differences of monometallic Ru or Co have been attributed to a number of factors, including catalyst dispersion, metal-support interactions, degree of metal reduction and shape selectivity of the support. The latter distinctive feature of zeolites has proven advantageous in controlling chain length growth [1,21. A combination of cobalt and ruthenium on a zeolite substrate could exploit geometric and electronic interactions to enhance cobalt reduction and to control reactant adsorption, as demosntrated for other systems with Group VIII bimetallics [4-61. In the present work, ruthenium-cobalt bimetallic catalysts on zeolite NaY are prepared and tested for their stability, activity and selectivity of CO hydrogenation towards light hydrocarbons.
The catalysts were prepared by loading the active metals from aqueous solutions of their salts Co(N03)2.6H20 and Ru(NH3)&13 to a type NaY zeolitic support (Valfor CP308-66). All catalysts had a total metal loading of 2% wt, as
2726
verified by atomic absorption spectrometry measurements performed on the solids after dissolution. Activity and selectivity tests were conducted in a continuous flow differential reactor, operating at 1 atm. About 0.3 g of catalyst were placed in the reactor, reduced in situ in flowing hydrogen a t 723 K and tested in the temperature range 448-673 K. The existing gas chromatographic analysis system permitted determination of reactant concentrations a t the reactor inlet and outlet and full range. separation of all hydrocarbons and isomers in the c1-C~ Catalyst characterization included determination of their total and metal surface area by nitrogen and hydrogen adsorption, respectively. Total surface areas were measured a t 77 K for fresh and used catalyst samples. Hydrogen chemisorption measurements were performed in a static volumetric apparatus a t 308 K after catalyst reduction in hydrogen at 723 K and subsequent evacuation a t 10-5 Torr. Chemisorption isotherms were of typical Langmuir type, exhibiting a linear region at 50-300 Torr. Extrapolation of this linear part to zero pressure gave the amount of gas adsorbed. A second isotherm was obtained for each catalyst, after completion of the first adsorption cycle and sample evacuation for about thirty minutes, in order to determine the amount of gas reversibly adsorbed by the solid.
RESULTS AND DISCUSSION Catalyst composition and adsorption properties are shown in Table 1. All catalysts exhibited good crystallinity and structure integrity, as judged from their high total surface areas. The latter remained practically the same after catalyst testing, very close to that of the support (700 m2/g). Hydrogen adsorption increases with increasing ruthenium content for all catalysts. This is in accordance with the fact that ruthenium is usually highly dispersed in similar catalytic systems [7,81, while cobalt reduction is usually low and difficult [91, unless catalysts are prepared by special methods. For dispersion calculations, total hydrogen adsorption values would be more appropriate here, since the assumption of H(tot)/M(s) = 1 has been found to be suitable for cobalt which chemisorbs hydrogen reversibly [91, and for ruthenium at particle sizes above 1 nm. Hydrogen adsorption on our catalyst support was negligible, especially at low pressures. Table 1. Composition and Adsorption Characteristics of Ruthenium-Cobalt Catalysts Catalyst 2 Ru 1.5Ru 0.5Co 1Ru 1Co 0.5Ru 1.5co
Ru:Co
Ru atoms
(wt. ratio) (% in metal) only Ru 100 3:l 64 37 1:l 16 1:3
H, uptake (pmol/g cat.) Total 45.2 36.5 29.9 14.2
Irrev.
25.7 19.3 17.1 8.6
BET area (m2/g cat.) fresh cat. 665
687 690 677
used cat. 656 658 665 659
2727 Table 2. Catalyst activity and selectivity (H2:C0= 3:1, 1atm, GHSV=12500 h-1) Temp.
Conv.
( K)
(96)
C,
C2
C,
C,
>C,
fractiona
2Ru
473 523 573
0.2 1.7 4.1
45.1 59.1 82.9
11.3 15.2 11.0
20.3 14.7 4.5
14.5 6.0 1.1
8.8 5.0 0.5
0.86 0.48 0.20
1.5Ru 0.5Co
473 573
0.1 1.2 3.0
51.2 59.3 78.8
11.3 15.0 13.2
17.9 14.7 5.8
12.2 6.3 1.5
7.4 4.7 0.7
0.82 0.41 0.15
473 523 573
0.05 1.0 2.7
67.5 60.8 78.5
10.1 14.3 13.4
13.1 14.4 5.9
9.3 6.1 1.4
4.4 0.8
0.31 0.11
10.0 16.1 15.5 15.4 15.1 8.7 propane).
13.2 7.0 2.6
2.6 0.7
0.49 0.16
Catalyst
523
lRu 1co
0.5Ru 1.5co
473
Product distribution (wt 96)
0.05 60.7 0.3 59.5 1.1 72.9 573 a : defined as mol propene / (mol propene + mol 523
C, olefin
0.80
0.80
Results concerning catalyst activity and selectivity at various temperatures are shown in Table 2. Conversions were kept deliberately below 5%, to ensure differential reactor operation. Catalyst activity increases with increasing ruthenium content. Hydrocarbons in the c 1 - c 6 range are produced in the temperature range of Table 2. At temperatures below 473 K, light hydrocarbons (C1-C3) are produced, while at temperatures above 573 K the product spectrum shifts again t o low molecular weights, and carbon dioxide appears at 623 K. In all cases, the presence of cobalt limits hydrocarbon formation to less than five carbon atoms. Olefin selectivity is expressed as the propene fraction in C 3 hydrocarbons. This term is representative of olefin fraction in the c 3 - C ~range, since C2's at most temperatures consist usually of ethane. Olefin production is favoured at low temperatures, Table 2, in agreement with results in similar, monometallic systems [7,81.It is interesting to note that a minimum in olefin formation is observed at intermediate metal compositions, namely for catalysts lRulCo and 1.5Ru0.5C0, with a concomitant increase in paraffins. For these two bimetallics, the production rates of methane are significantly higher than those expected if the two metals acted proportionately, especially at 523-573K. These results suggest a synergistic effect of Ru upon Co and/or its oxidized forms on the substrate. Cobalt reduction may be facilitated by Ru, as it occurs with other Possible bimetallic cluster formation would modify two-metal catalysts [5,6,101. the strength of CO and H 2 adsorption on the catalyst and, hence, the relative surface coverage during reaction. Since a number of metal ensembles is considered necessary for CO dissociation, in our case a 37-64% Ru atom composition may enhance surface hydrogen coverage. This would facilitate further hydrogenation of olefins and surface carbon, resulting in lower olefin selectivity and higher paraffin (especially methane) formation rates.
2728
Figure 1. Rate of CO hydrogenation with time-on-stream Carbon monoxide reaction rates at 523 K are shown in Figure 1 for all catalysts. An activity decrease is evident at the beginning of the reaction, probably due t o formation of carbonaceous species on the catalyst surface. Although initial activity can easily be restored by hydrogen bracketing techniques, it is interesting that in the presence of cobalt this initial decay is diminished. This could also be attributed to the above described kind of metalmetal interactions.
1. R.B. Anderson, The Fischer-Tropsch Synthesis, Academic Press, New York, 1984. 2. G.A. Mills, Catalysts for Fuels from Syngas, IEA Coal Research, London 1988. 3. B.G. Johnson, C.H. Bartholomew, and D.W. Goodman, J . Catal. 128 (1991) 231. 4. S.Y. Lai and J.C. Vickerman, J.Catal., 90 (1984) 337. 5. D.J. Elliott and J.H. Lunsford, J. Catal., 67 (1979) 11. 6. W.C. Conner Jr., in Z. Paal and P.G. Menon (eds.), Hydrogen Effects in Catalysis, M.Dekker, New York, 1988. 7. Y.W. Chen, H.T. Wang and J.G. Goodwin, Jr., J. Catal. 83 (1983) 415. 8. I.R. Leith, J. Catal. 91 (1985) 283. 9. W.H. Lee and C.H. Bartholomew, J. Catal. 120 (1989) 256. 10. L.Guczi, Z. Schay and I. Bogyay, Stud. Surf. Sci. Catal. 16 (1983) 451.
Ouczi, L et al. (Editon), New Frontiers in Catalysk Proceedings of the 10th International Congma on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishen B.V. All rights nserved
SURFACE KINETICS AND PERIODIC OPERATION OF HIGHER ALCOHOL SYNTHESIS
J.-L.Lia,Q.-M. Zhua, J.-L.Hua and N.J. Yuanb 9epartment of Chemistry, Tsinghua University, Beijing, China bDepartment of Chemical Engineering, Tsinghua University, Beijing, China
Abstract Concertrations of the surface species were measured at intervals by in situ IR after introducing a concentration jump of CO / H, into a in situ IR cell containing a Cu / Zn / Mg / K catalyst under working conditions, Based on the mechanism we proposed elsewhere and the data on concentrations of the surface species, the rate constant of each elementary step was calculated. The results indicate that the rate determining step is probably the insertion of adsorbed CO into the metal-carbon bond of bidentate oxymethylenic species. The effect of periodic parameters on the fromation rate of alcohols over a Cu / Zn / Mg / K catalyst in forced CO, concentration cycling was studied under 2.0MPa and 320C.It was shown that periodic dosing with CO, raised the time-average formation rate of CgOH by up to 22.7% and that of total alcohols by 10% compared to those obtained under the steady state. Mechanistic explanation of the periodic operation has been presented.
1. INTRODUCTION Considerable attention has been paid to the chemistry of higher alcohol synthesis and many mechanistic models have been proposed[l-41. However, these models were mainly based on the analysis of the products and very little about the surface species and their behaviors was known. As for the rate determining step, the knowledge is even more lacking. By means of a novel technique-pulse in situ IR, concentrations of surface species were measured and therewith the rate constant of each elementary step calculated. Based on the behaviors of surface species and our earlier work [5], 'it was expected that periodic dosing of the catalyst with CO, might enhance the formation of alcohols. Our experiments have proved the prediction.
2. EXPERIMENTAL The catalyst used in the experiments was a Cu / Zn / Mg / K type prepared by co -precipitating. In the experiments on surface kinetics, the catalyst powders were tableted into a thin wafer, which was then put into the in situ IR cell. Before experiments, the wafer was reduced with a mixture of N, and H, and then purged with N,. After introducing a concentration jump of syngas under
2730 conditions of 300C and 2.0MPa, IR spectra were recorded every 5 seconds. Correlating the band intensity with the surface concentration of the surface species and conducting data fitting according to a simplyfied mechanistic model based on our earlier postulation[6], the rate constants were calculated. The experiments on periodic operations were performed under 320C and 2.0MPa by periodically injecting CO, into a fllow fixed-bed reactor filled with the catalyst and being operated in an CO / H, atmosphere. The formation rate of ClOH and total alcohol was measured at intervals when different parameters such as periodp), cycle split (S), amplitude of CO, concentration (A) were adopted. To get the information about the mechanism of periodic operations, concentrations of some major surface species were measured using in situ IR cell operating in a periodic mode as described above.
3. RESULTS AND DISCUSSION 3.1 Surface kinetics
The simplified mechanistic model is as follows:
Accordingly, 7 rate equations and one matter-balance equation could be written. In terms of the IR spectra recorded, the coverage of all species appearing in the model was calculated at different moments affter injecting CO / H, pulse. Fig. 1 shows an example. Using Marquart nonlinear regression, the rate constants were calculated to be as follows:
k,= 1 . 2 4 4 ~IO-’mol/g k,= 3.682 x 1O”mol/ g k,= 3.683 x 10%0l/ g k,= 2.666 x 10-7m~l/ g k,= 3.609 x 10dmol / g k,= 3.710 x 10-5mol/ g k,= 7.691 x 10-5m~l/ g
atm s atm s
k’, = 2.173 x lO-’mol/ g s k’,= 1.531 x 10dmol / g s
s
V3=3.367 x 10dmol / g v,= 3.674 x ~ O - ~ ~ I I O/I g k’, = 5.583 x lO-’mol/ g k’, = 8.003 x 10-7m~l/ g k’, = 5.740 x 10-7m~l/ g
s s s s
s s
s s s
Comparing the values of the rate constants, it is reasonable to conclude that forward step of reaction 4 is probably the rate determining step because its rate constant is much smaller than the others.
2731
@.@
4 0.0 2010 4010 6010 SO!O 104.0 12d.0 14d.0 164.0 TIME, s
Fib. 1 R e s p o n s e of M-OC 2 H5 t o e c o n c e n t r a t i o n jump o f C O / H 2 .
3.2 Periodic operation The results of the experiments on the periodic operation are summarized in Table 1. Normalized formation rate (NFR),i.e. the time-average formation rate of products in the periodic operation divided by the formation rate obttained under the steady state with a syngas containing CO, equel to the time-average content (TAC)of CO, in the periodic operation and other conditions unchanged. It can be seen from the table that periodic dosing with CO, can considerably enhance the formation of alcohols, especially that of higher alcohols provided that parameters are proper. Fig.2 shows concentration variations of surface hydroxyl, formyl and formate within one period. In the duration of COz pulse, the concentration of surface formate
1 =tl
a
4
9
4
m
2732 greatly increases due to the insertion of CO, into M-H bond, thus promoting the formation of alcohols and M-OH via further reactions. M-OH in turn reacts with CO to form surface farmate. Therefore, during the latter half of the period, surface formate increases again. After CO, pulse seases, the surface is no longer dominately occupied by CO,, allowing CO and H, to adsorb. This fact is indicated by the increase of surface formyl. So M-H and M-CO needed for related reactions are still available. That's why periodic dosing with CO, is more favorable to the alcohol formation than the steady operation. Table 1 The effect of periodic parameters on the NFR NFR TAC P A S NFR of total of co, min YO % Of alcohols 0.1 0.5 0.920 1.010 0.2 1.o 1.150 1.035 5.0 1.5 1.276 1.076 0.3 0.4 2.0 0.852 0.884 5 0.1 1.o 0.812 0.885 0.15 1.5 0.932 0.953 10.0 0.2 2.0 0.790 0.889 0.3 3.0 0.949 0.975 0.1 0.5 1.227 1.100 0.2 1.o 1.088 1.063 5.0 0.3 1.5 0.932 0.904 0.4 2.0 0.901 0.933 10 0.1 1.o 1.088 1.030 10.0 0.1 5 1.5 1.178 1.061 0.3 3.0 0.782 0.889 ~~
5.0
0.1 0.2 0.3
0.5 1.o 1.5
1.187 1.012 1.067
1.116 1.010 0.998
10.0
0.1 0.2
1.o 2.0
0.938 0.790
0.950 0.879
20
REFERENCES 1.
2. 3. 4. 5. 6.
A. T. Bell, "Heterogeneous Catalysis" ,Edited by B.A. Shapito, Texas A and M University Press, 1984. G. D. Graves, Ind. Eng. Chem., 23 (1931) 1381. M. A. Vannice, J. Catal., 37 (1975) 462. T. J. mazanec, J. Catal., 98 (1 986) 1 15. D. Q. Liu, Q.M. Zhu and J. L. Li, Proce 9th Intl. Cong. Catal., 1988. J. L. Hu, Q.M. Zhu and J. L. Li, J. of Catal. (China) 12 (1991) 173.
Guczi, L.d al. (Editors), New Fronriers in Caralysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest,Hungary Q 1993 Elsevier Science Publishers B.V.All rights resewed
MECHANISM OF SELECTIVE CO HYDROGENATION TO ISOBUTENE OVER OXIDE CATALYST R Maruyaa, A. Takasawaa, T. HaraoW, M. Aikaw&, T. Araia, K. Domena and T. Onishib BResearch Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan "Tokyo Polytechnic College, 2-32-1 Ogawanishi-machi, Kodaira City 187, Japan
Abstract The CO hydrogenation mechanism of s e l e c t i v e i s o b u t e n e formation over Zr02 Ce02 catalysts was investigated. The chemical trapping '":na 3C tracer studies showed that branched chain is formed by the carbonylation of C1 species produced from C-0 bond cleavage follwed by the Aldol condensation type reaction of the C2 species formed. 1. INTRODUCTIOIN
There have been a number of studies on the CO hydrogenation over t r a n s i t i o n metal c a t a l y s t s t o form mainly l i n e a r hydrocarbons. On the other hand, only few studies on the reaction over oxide catalysts have been reported. Oxide catalysts selectively form branched carbon chain products[l-61. On the formation of branched carbon chain products there have b e e n o n l y f e w t e n t a t i v e mechanisrns[71, i n w h i c h t h e carbonylation at carbon atom of aldehyde and ketone groups is a key step. H o w e v e r , it i s a l s o w e l l k n o w n t h a t t h e carbonylation at carbon position of the groups is very hard, although the carbonylation of coordinated acetone is reported in the homegeneous zirconium systems[81. Since the addition of acetone to the CO-H2 reaction under mild conditions leads to the decrease in the yield of isobutenet51, there should be another mechanism for the formation of isobutene. Here we describe the adsorbed species during the CO hydrogenation reaction over Zi02 and Ce02 and discuss the mechanism. 2. EXPERIMENTAL
Catalysts were prepared by the hydrolysis of zirconium oxynitrate with about 5% aqueous ammonia solution and the washing with deionized water, the dryness at 393 K overnight, and the caLcination of the precipitates at 773 K for 3 h.
2734 Reactions were carried out in a glass vacuum system with a gas-circulating pump. The catalyst was evacuated at 973 X for 3 h before the reaction of CO and H at 67 kPa. Metal carbonyl in CO gas was removed by active carson trap at liquid nitrogen temperature. NMR spectra were recorded on a JEOL GX 500 spectrometer for the products and JEOL GX 270 for solid samples, which were prepared by the rapid cooling of the catalysts during the CO hydrogenation reaction and the filling into a holder under an Ar gas. The isolation from air was confirmed by the repeated measurement after a week. Chemical trapping was carried out by the modified method of literaturesl91. Products except for methane were collected at liquid nitrogen temperature. Quantitative analyses were conducted by GC equipped with a Porapak Q column for most of the products, VZ-7 and -10 for isomers of C4 and C5 hydrocarbons, and a molecu sieve 5A for CO and N 2 internal standard. The distribution i isobuteneatfaq 3C0 hydrogenation products was determined byC ' NMR.
3.
RSULTS AND DISCUSSION
T h e CO-H2 r e a c t i o n o v e r Z r O a t 5 2 3 e x c l u s i v e l y f o r m s methanol, while the reaction agove 6 2 3 K changes into the selective formation of isobutene[41. IR spectra during the reaction at these tem rature showed only formate and methoxide species[lOl. CP MAS "C NMR spectra showed only the resonances due to formate species at 169.5 ppm and methoxide species at 55.1 ppm. These results suggest that the formation of C 1 species is fast and the formation process of C2 species is slow with regard to the CO hydrogenatioan over Zr02 catalyst. The treatment of OH-covered ZrO surface with C o forms formate species on the Zr02 surface[l8]. The CO-H reaction on formate ion-preadsorbed surface formed a little amount of hydrocarbons with 46% of C4 selectivity in a initial 20 min and gradually reached the steady state. O n the other hand, the reaction o n dimethyl ether-treated Zr02 surface initially formed about 20 times higher yield of C4 hydrocarbons with a high selectivity than in the steady state. These results indicate that formate species is not an intermediate and that methxide species is an intermediate or a surface species which deeply relates to an intermediate. Thus, it would be important t o know the way for the surface species t o incorpolate into produc , especially isobutene. The "C0-H2 reaction over Zr02 evacuated at 973 K was carried out at 623 K for 3 h. After evacuation of gas phase except for a small catalyst part, the 12CO-H reaction started again at the same temperature. The producgs were c lected at liquid nitrogen temperature. The concentration of '3C at each carbon in isobutene wh by the analyses of splitting pattern due to coupling are shown in Figure
2735 1. These results suggest that the carbon atom from methoxide is concentrated at the center position and the methyl and methylene c a r b o n s c o m e from g a s phase C O r a t h e r t h a n preadsorbed species.
y 3
y 3 a)
CH =C(584)
CH (87%)
CH
(542)
Figure 1. 3C concentration at carbon positions ' isobutene from the 12CO-H reaction over ZrOZ treated with "CO and H2. a) 0-20 min. bf? 60-120 min. The CO-H2 reaction over Ce02 at 673 K forms isobutene as a main product. Although the C4 hydrocarbon selectivity ( 2 2 % ) is not so high as that of Zr02 (68%), isobutene selectivity in C4 hydrocarbons is as high as that of Zr02 (88%). The most important difference between CeO and Zr02 catalysts is that the CO-HZ reaction over CeO at 5223 K forms C4 aldehyde and C ketone as main products. h e surface species were determine2 by the chemical trapping method. Table 1.
Products obtained by chemical trapping.a)
Zr02
52 3
Ce02
643 523 673
140 30 0.7 0.2
-
-
-
+
0.08b)
0.9c) 0.2d)
-
a) The catalysts, which were quickly cooled by liquid nitrogen and evacuated, were warmed to room temperature and treated with a vapor of diluted aqeous HC1 solution. b) Small amounts of CH3CH0 and CH CH2CH0 were detected but C7 aldehyde or ketone was not. c) small amount of C2H4 was detected. d ) Small amounts of C2-C5 were detected. The formation of 2-methylpropanal and the detection of C2 and C aldehydes from the Ce02 surface may show that there are adsorbed C2, C3 and C4 acyl species, because the acid treatment of Zr-acyl complex forms aldehyde[ll]. These results suggest that C3 and C4 aldehydes are formed by the aldol condensation type reaction. The formation of methane by the chemical trapping may indicate the presence of methylene and/or methyl species as a surface species. Thus, branched C4 species are formed through two step aldol condensation type reaction of C2 aldehyde, which is fromed from the carbonylation of surface methylene or methyl species. Since the thermal decomposition -formaldehyde zirconium complex produces ethyleneIl21, ylene species could be an C1 intermediate. The high selectivity of ethylene in the CO hydrogenation over Ce02
2736 catalysts[ 13 I may indicate the methylene ies formation of the Ce02 catalysts, while the results ofsP"C tracer studies shown in Fig. 1 may indicate the methyl species formation on the Zr02 catalyst. Although the formation of the C species is the key step for the CO hydrogenation over oxide cakalysts, the mechanism is not clear yet. The formation process of isobutene is tentatively written as follows. CH20(a) CO
+ H2> -
&
>-
CH30(a) CH2O
CH2(a) or> -,
co
CH2O CH3CO(a 1
+CH3CH2CO(a)
CH3(a)
CH3 H2 >CH3CHCO(a)/CH3CHCH20H
-H20
> CH3-C=CH2
Figure 2. Tentative formation process of isobutene from CO and H2 over oxide catalysts. REFERENCES
1 2
3 4 5
6 7 8
9
10
11
12 13
.
H. Pichler and K. H. Ziesecke, Brennst. Chem.,30 (1949) 13. R. B. Anderson, J. Feldman, and H. H. Storch, Ind. Eng. Chem., 44 (1952) 2418. R. Kieffer, J. Varela, and A. Deluzarche, J. Chem. SOC., Chem. Commun., (1983). K. Maruya, A. Inaba, T. Maehashi, K. Domen, and T. Onishi, J. Chem. SOC., Chem. Commun., (1985) 487. K. Maruya, T. Fujisawa, A. Takasawa, K. Domen, and T. Onishi, Bull. Chem. SOC. Jpn., 62 (1989) 11. K. Maruya, T. Arai, M. Aikawa, K. Domen, and T. Onishi, "Catalytic Science and Technology," ed. by S. Yoshida, N. Takezawa, and T. Ono, Kodansha, Vol.1 (1991) p.457. T. Mazanec, J. Catal., 98 (1986) 115; S. C. Tseng, N. B. Jackson, and J. G. Ekerdt, J. Catal., 109 (1988) 284. S. A. Matchett, J. R. Norton, and 0. P. Anderson, Organometallics, 7 (1988) 228; D. M. Roddick and J. E. Bercaw, Chem. Ber., 122 (1989) 1579; 11 P. Hofmann, P. Stauffer, M. Frede, and K. Tatsumi, Chem. Ber., 1 2 2 (1989) 1559. A. Deluzarche, R. Kieffer, and A. Muth, Tetrahedron Lett., (1977) 3357; A. Deluzarche, J. P. Hindermann, and R. Kieffer, Tetrahedron Lett., (1978) 2787; J. SaUSSey, J. c. Lavalley, T. Rais, A. Chakor-Alami, J. P. Hindermann, and A. Kiennemann, J. Mol. Catal., 26 (1984) 159. H. Abe, K. Maruya, K. Domen, and T. Onishi, Chem. Lett., (1984) 1875. D. W. Hart and J. Schwartz, J. Am. Chem. SoC., 96 (1974) 81 15. G. Erker, C. Kruger, and R. Schlund, Naturforsch., B42 (1987) 1009. T. Arai, K. Maruya, K. Domen, and T. Onishi, J. Chem. Soc., Chem. Commun., (1987) 1757.
Ouni, L er al. (Editors), New Frontiers in Caralysir Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 6 1993 Elscvicr Science Publishers B.V. All rights rcscrved
CONVERSION OF Cl-C, ALCOHOLS INTO AROMATICS ON TEE MODIFIED ZSM-5 ZEOLITES. ACTIVE CENTRES AND REACTION PATHWAYS
D.-2.Wane, J.-Y.W a n 8 and X.-D. Lua %stitUte of Coal Chemistry, Academia Sinica, P.O.Box 165, Taiyuan, Shanxi, China bPresent address: Henan Institute of Chemistry, Zhengzhou, Henan, China
Abstract Activity tests, IR spectroscopy of adsorbed pyridine (Py-IR) and TPD / mass spectroscopy have been used for comparative studies of the Cu-, Zn-, Cd- and Ga-exchanged ZSM-5 zeolites in the CI-C4 alcohols to aromatics conversion. The results obtained have led to a better understanding of the nature of the active centres on the modified zeolites and their role in the alcohols to aromatics conversion.
1. INTRODUCTION Conversion of lower alcohols to aromatics is of great interest with regard to the use of non-petroleum raw materials for the prodution of aromatics. H-ZSM-5 zeolite is known to convert methanol and C alcohols selectively to high octane gasoline [1,2]. It has been reported that ZdM-5 modified with ZnZ+,CdZ+and Ga3+ ions are effective in the aromatization of methanol [3,4]. However, few reports have concerned with the aromatization of Cz+ alcohols. Here, we describe the acidic properties of theCu-, Zn-, Cd- and Ga-ZSM-5, their catalytic behavior in the conversion of C1-C4 alcohols to aromatics and the TPD behavior of alcohols. Differences in the catalytic performance and the role of metal cations are explained on the basis of various evidences and a discussion of the nature of the active centres on the zeolites.
+
2. EXPERIMENTAL The Cu-, Zn-, Cd- and Ga-ZSM-5 samples were prepared by ion-exchanging the acidic form (HZ) with corresponding salt solution. The metal content (wt%) is: Cu, 1.4; Zn(l), 1.0; Zn(2), 2.0; Cd, 0.9 and Ga, 0.7. Their acidic properties were measured with Py-IR. Catalytic tests were conducted with a pulse reactor at 450C using methanol, ethanol, n-propanol, n-butanol and cyclohexane as reactants. TPD products of adsorbed alcohols (ROH) were directly analyzed by a mass spectrometer.
2738
3. RESULTS AND DISCUSSION 3.1. Acidic properties of the zeolites The results of Py-IR show that the introduction of Cuz+ , Zn2+ , Cd2+ and Ga3+ to HZ, except Ga3+ ,all decrease the B acidity, increasing the L acidity, and produce a band between 1609 and 1616 cm-' due to pyridine-cation interactions. It follows that Cu2+ ,Zn2+ and CdZ+occupy the ion exchange position; Ga3' does not so, probably as Ga203at the outer surface of ZSM-5. 3.2. Catalytic performanceof the zeolites Conversion data for ROH shown in Table 1 indicate that for the conversion of MeOH, the introduction of ZnZ+,CdZ+and Ga3+ ,especially Zn2' and Cd2+ ,to HZ greatly improves the selectivity to aromatics (Sa), and si nificantly suppresses the formation of C,-C, gaseous products, while that of Cu" decreases both aG tivity and Sa, and shows a long induction period. The Sa in the C2-C4 ROH conversion is higher for the Zn-ZSM-5 than for the corresponding HZ too, and is lower than that observed for MeOH, with decreasing order: MeOH > PrOH, BuOH > EtOH. The results of cyclohexane conversion to Ar give evidence that the increased Sa is related to the increasing dehydrogenation activity of the modified ZSM-5.
Table 1 Conversion data for Cl-C4 alcohols on different zeolites at 450'c and 100% conversion (< 100% on Cu-ZSM-5) Selectivity wt % c1-c4
c5+
Ar
MeOH
EtOH
HZ
Cu
Cd
Ga
Zn(2) Zn(2)
56.3 43.7 31.0
34.8 65.2 25.0
41.7 58.3 51.2
47.1 52.9 44.7
37.8 62.2 52.3
40.8 59.2 44.2
PrOH
BuOH
35.8 64.2 48.6
35.6 64.4 49.0
3.3 TPD of C,-C, alcohols From the profiles of TPD products for MeOH (Fig. 1,a-f) we see that (i) there exist two types of active centres on all the modified ZSM-5, as evidenced by an overlapping desorption peak as that for Ar on Ga-ZSM-5, for MeOH on Znand Cu-ZSM-5, and by two single desorption peaks for Ar on Zn-ZSM-5. The first peak (1) corresponds to the B acid site, the second peak (2) to the active metal species; (ii) two Ar peaks on Zn-ZSM-5 are peak (1) < peak (2). This is reverse to those on Ga-ZSM-5. In connection with the Py-IR results, it follows that the contribution of active metal species is greater than B acid site on Zn-ZSM-5, to be contrary to that on Ga-ZSM-5, in the aromatization of alkenesl and that the zinc species detectable by pyridine adsorption at 1616cm- band is a dehydrogenation active site; and (iii) no hydrocarbon products but DME, CO and CO, released from Cu-ZSM-5. Moreover, CO and CO, is detected on Zn-ZSM-5 too.
2739
1 P a
MeOH/HZ
MeOH/Zn(l)
MCH
A
Ad
Ar
l7e.f E t O H , DEE H20 I
60 210 MeOH/Ga
360
I
bl
MCH
A
210 360 MeOH/Zn(2)
f I
510
I
510
7
60 210 PrOH/Zn(2)
1I
360
2 t
Ar
Ar
A
MCH
P
I
PrOH
210 MeOH/Cu
360
1
51[ t
co
H20
210 360 MeOH/Ar(xlO) 1
510
3 210 BuOH/Zn(2)
I
I
360
510 i
-------E!!
% DBE
Me01
A
BuOH H20
'
I
I
I
I
210
360
510
660
I
60
210
360
510 60
210
360
510
T (OC)
Figure 1. Mass chromatograms of TPD products for C -C, alcohols on different zeolites. (MCH) Me-cyclohexenes, (P) paraffins, (0)ofefins.
2740
+
The TPD profiles of C,-C, ROH on Zn(2) (Fig. 1, e and g-i) show that (i) C ROH to alkenes are much easier than MeOH, especially PrOH and BuOH; Ji) dehydration of C,+ ROH proceeds both mono- and bi-molecularly. Higher ROH prefers to the former way; (iii) a lot of released ethene for EtOH evidences that ethene is not the main precursor of Ar. This explained the least Sa shown in the EtOH conversion; and (iv) the relative importance of Zn"(Ar peak(2)) to H+(Ar peak(1)) seems to be reduced as the chain length of ROH increases. This may be the reason of the Sa being C3-C4 ROH < MeOH. 3.4. Nature of active centres and their role in methanol to aromatics The above results indicate that the activity and selectivity of ZSM-5 in the MeOH conversion closely depend on the cations chosen for modification. For MTA, the Zn- and Cd-ZSM-5 are the most active, Ga-ZSM-5 the less, and Cu-ZSM-5 is the least. Znz+and Cdz+as zeolite cation8 and Gaz03at the outer surface of ZSM-5 are the active species for the dehydrogenation of alkene intermediates. CuZtas a zeolite cation catalyses the decomposition of MeOH; so does Zn2+.The increased yield of Ar is mainly due to the active species leading to the fornation of Ar through a direct dehydrogenation of alkene intermediates formed on H+, thus limiting the formation of alkanes, which is inevitable when Ar are formed via H-transfer between alkene precursors and carbenium ions. Zn2+and Cdz+replace H+, inhibitig both the H-trasnfer and cracking activity while Ga3+behavesdifferently. Therefore, as shown in the TPD profiles, the formation of Ar is preferentially via the direct dehydrogenation pathway on the Zn- and Cd-ZSM-5 (in analogy with Zn), in opposite to that on the Ga-ZSM-5, thus leading to a higher Sa for the two formers. In view of the fact that the Cu-ZSM-5 gives the TPD products only DME, CO, CO and H, for MeOH and shows an induction period during MTA, it is postulated &at in that period active copper species is formed initially by reduction by the first reaction products, e.g. H,and CO, accompanying the release of H+, increasing the B acidity on ZSM-5. Then, the decomposition of MeOH, which is catalyzed by Cuz+, can be suppressed, and the reaction sequence of MTA can proceed. In analogy with filled 3d- and 4d-orbital of Zn"and CdZt, it is postulated that the active copper species is Cu+. Finally, the reaction scheme for CI-C4 alcohols conversion on the modified ZSM-5 is summarized.
4, ACKNOWLEDGEMENTS This work was supported by the NNS Foundation and NSF of Shanxi of China. The authors thank Associate Professor Y.-X. Xiao for TPD/MS measurements, and also thank Professor S.-Y. Peng for helpful discussion.
5. REFERENCES 1 C.D. Chang, and A.J. Silvestri, J. Catal., 47 (1977) 249. 2 N.Y. Chen, CHEMTECH, 13 (1983) 488. 3 C.D. Chang, W.H. Lang and A.J. Silvestri, Aromatization of Hetero-atom Substituted Hydrocarbons, US Patent No. 3 894 104 (1975). 4 Y.Ono, H.Adachi and Y.Senoda, J.Chem. SOC.Faraday Trans. I, 84 (1988) 1091.
Guni, L a aL. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
ACTIVITIES AND SELECTIVITIESOF SUPPORTED CO-Ru,Co-Pd AND Co-Pt BIMETALLIC CATALYSTS IN FISCHER-TROPSCH SYNTHESIS
M. P. Kapoor, A. L. Lapidus and A. Yu. Krylova N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Russia Prospect 47, MOSCOW,
Abstract T h e 1 0 % C o + ( O - 0 . 5 % M ) / A 1 2 ~( M = R u , P d & P t ) c a t a l y s t s w e r e s t u d i e d to d e t e r m i n e t h e i r
a c t i v i t y and
selectivity.
T h e p r o m o t e r s effect i n t o
t h e Co-alumina c a t a l y s t w a s e x p l a i n e d w i t h using T e m p r a t u r e Programmed Reduction(TPR)
and
methods.
maximum
The
Diffuse
Reflactance
selectivity
h y d r o c a r b o n s f r o m CO+H2
Infra-Red
obtained
for
spectroscopic(1RDR) the
formation
of
w e r e f o u n d m o r e t h a n 80% f o r Co-Ru s y s t e m .
T h e o x i d e p h a s e of t h e c a t a l y s t is s u p p o s e d to b e r e s p o n s i b l e for t h e variation i n activity and selectivity. INTRODUCTION
A
limited
of
number
reports
have
appeared
describing
the
catalytic
properties
process.
It is w e l l k n o w n t h a t s u p p o r t is n o t i n e r t c o m p o n e n t of t h e
Fischer-Tropsch pretreatment cobalt
cobalt
and
containing
Fischer-Tropsch
of
bimetallic catalysts( 1).
reduction
of
phases
could
reaction(2).
catalysts
For
in
instance,
IO%Co/Al, O3 t h e be
the
formed
new
which
Fischer-Tropsch
in
the
hardly can
course
influence
T h e a d d i t i o n of n o b l e m e t a l s ( R u ,
of
reducible on
Pd & Pt)
i n t o t h e C o - a l u m i n a c a t a l y t i c s y s t e m c o u l d lead to c h a n g e i n c o b a l t state and enhance t h e i r activity and
selectivity.
Therefore,
t h e r e a s o n for
t h i s is t h e s u b j e c t of on g o i n g r e s e a r c h . EXPERIMENTAL Catalysts
were
prepared
on
the
commercial
Y-/A12O3
by
c o n v e n t i o n a l w e t n e s s i m p r e g n a t i o n m e t h o d b y u s i n g a n a q u e o u s s o l u t i o n of
2742 C C ( N O ~.6H20 )~ a n d a n a c i d i c s o l u t i o n of R u , P d a n d Pt c h l o r i d e s i n t w o s e q u e n t i a l s t e p s . Details of c a t a l y s i s , t h e a n a l y s i s of g a s e o u s a n d l i q u i d reaction
products
and
method
of
TPR
and
analysis
IR
are
given
e l s e w h e r e ( 1,3). RESULTS AND DISCUSSION
The catalyst
catalytic y i e l d s total
test
data(Tab1e)
shows
that
unpromoted
cobalt
153 g m / m * h y d r o c a r b o n s w h i c h c o n s i s t s a b o u t 34%
l i q u i d h y d r o c a r b o n s ( C g ) , w i t h m o r e t h a n 90% n o r m a l p a r a f f i n s .
Table E f f e c t of p r o m o t e r s i n t h e s y n t h e s i s of h y d r o c a r b o n s f r o m CO a n d
HZ o v e r flA120j 0.1
supported cobalt catalyst.
MPa, C O : H 2 = 1 : 2 ( v o l . ) ,
463-5301>C4H,. lyst can be deactivated by the addition of NaOH, and the fatal amounts of NaOH for oligomerization of ethene , propene and 1- butene (nearly) correspond to the same value of about 0. 85 mmol per gram catalyst(Fig. 1). On the other hand, the acidity of HOG- 3.0 decreases linearly with increasing amount of NaOH and finally approaches zero at about 0. 85 mmol NaOH/g-cat. (Fig. 2). So it can be concluded that acid sites related to alkene oligomerization are those with strength of H 6 - 3. 0.
1.0 n
50.8
$0.6
5 0.4 25
c
a 'G 42 4:
0 0
0.2 0.4 0.G 0.8 1.0 NdOH ( rnmovg-cat I
Figure 1. Effect of NaOH poisonng to NiSO,/Y - A120, on alkene oligomerization. CzH,. at 308 K i C,H,. a t 283 K , lh-'(I,HSV); 1-C,H6, at 353 K, Ih-' (LHSV).
0 0.2 0.4 06 0.8 1.( NaOH ( mmol/g-cat
1
Figure 2. Effect of NaOH addition to NiSO,/Y -- A1,0, on the distribution of acid strength of the catalyst.
An important information about active site can be obtained by comparing
NiSO,/Y - AI2O3with the catalysts derived from FeSO, Al, (SO, l 3 and (NH,),SO, which are generally considered as acid catalysts(see Table 1 ) . It was found that the latter three catalysts are almost inactive for ethene, and less active for propene with product distribution quite different from that of the former (Table 1). However, their catalytic behavior in the oligomerization of 1butene, especially FeS04/Y- A120, is fairly similar to that of NiSO,/Y A1203. The above result revealed that the catalytic behavior of NiSO,/Y A1203is different from that of the catalysts derived from the other sulfates.
2755
Table 1 Propene oligomerization over sulfate derived catalysts" Catalyst
Conversion ( %
Product distribution(wt.
%)
c,,
c 1 5
c,,
SO,'- /7-Al,0,2'
C, 54. 5 9. 52 4. 17 5. 50
cc
98. 0 56. 0 26. 7 30. 7
19. 7 57. 0 42. 3 47. 2
23. 3 25. 7 38.7 35. 1
1. 86 7. 76 14.1 12. 2
0. 75 0 0.72 0
NiS04/7- A1203 (CO poisoned )
53. 1
14. 9
49. 7
25. 6
9.84
0
NiSOJ7- AI,O, FeSOJ7- AI,O, Alz (SO4)3/7-AIzO,
Note :1)Reaction conditions: T= 303 K , P= 2.5 MPa, LHSV= 1 h-' 2)it was prepared by calcination of (NH,)zS0,/Y-A1203
CO poisoning method was applied to identify active site of the reaction and Ni oxidation state by using CO as a probe molecule since it forms a stable complex with the lower valence nickel(Ni+, Ni"). The CO poisoning effects on oligomerization of various alkenes appear to be remarkably different. The most serious poisoning effect was observed for ethene dimerization where the activity was lost entirely. The propene conversion was considerably decreased, the selectivity was greatly changed and became close to that of FeS04/Y - A1203 (Table 1). In contrast , 1- butene conversion decreased only a little and the distribution of the product remains almost unchanged. Thus it is inferred that the site attracting CO, probably Ni', acts as an active site in ethene and propene oligomerization. The generation of Ni', arising from the contact between catalyst and ethene or propene , was clearly detected by ESR. A s shown in Fig. 3 , only one signal of g=2. 0027 exists when the catalyst was treated at 773 K in oxygen atmosphere. But after introducing ethene or propene new signals of ~ = 2 .18 and gr= 2. 082 appeared. The signal of g= 2.0027 and the new signals may be ascribed to NO2+readical and Ni+ , respectively143. This result is in good conformity with the existence of the induction period in ethene dimerization(Fig. 41, during which Ni2+ is to be reduced to Ni+. It is evident that Ni+ is generated in ethene and propene oligomerization and acts as an active site, as demonstrated by CO poisoning. It is necessary to point out that Ni+ can be developed only by evacuation at temperature as high as 773 K(Fig. 3, d). In such case, the induction period was disappeared (Fig. 4). In the 1- butene oligomerization, no signal of Ni+ can be detected. This implies that the mechanism of 1butene oligomerization is different from that of ethene and propene. Since the decrease in 1-butene conversion versus NaOH amount added is parallel to the change in acidity of H O G- 3. 0 , it is suggested that 1-butene oligomerization proceeds via acid catalysis.
2756
20
7
0
20
40
GO
t (min) Figure 3. ESR mearument. ( a ) , the catalyst calcined at 773 K in 0, followed by evacuation at room temperature. ( b ) , After introduction of ethene(80kPa) on(a>. ( c ) , After introduction of propene on(a). ( d ) The catalyst evacuated at 773 K for 4h.
Figure 4. Ethene dimeization over NiSO,/ Y - A1203catalyst. ( a ) , T h e catalyst calcined at 773 K for 4h followed by outgasing at room temperature. ( b ) , The catalyst evacuated at 773 K for 4h.
In view of the foregoing results, it can be concluded that ethene dimerizaton might be attributed to coordination catalysis, the active site of which consists of Ni+ and acid site. Propene oligomerization can proceed via both coordination and acid catalysis. In the oligomerization of 1- butene, the reaction proceed via acid catalysis. Moreover, it was sutggested that the mechanism of alkene oligomerization over NiS04/Y - A1,03 catalyst changes from coordination catalysis to acid catalysis while alkene changes from ethene to 1-butene. 4. REFERENCES
1 D. Wang, Ch. Jin and T. Cai, Shiyou Huagong(China1 , 17(1988)594. Y. Chauvin, D, Commereuc, F. Hogues and J. Thivolle-Cazat, Appl. Catal. , 42(1988)205. 3 T. Cai, L. Zhang, A. Qi, D. Wang, D. Cao and L. Li, Appl. Catal. , 69 (199111. 4 L. Bonneviot, D. Olivier and M. Che, J. Mol. Catal. , 21(1983)415. 2
Guni, L. et al. (Editors),New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
CATALYTIC PROPERTIES OF PARTIALLY REDUCED Fe/Si02 IN CO HYDROGENATION S. H. Moon, C. W. Park and H. K Shin Department of Chemical Engineering, Seoul National University, Shinlimdong San 56-1, Kwanak-ku, Seoul 151-742, Korea
Abstract Effect of partial metal reduction on the properties of supported iron catalysts has been studied using 9.lwt% Fe/SiO, as a model catalyst and CO hydrogenation as a test reaction. The extent of iron reduction does not alter the distribution of the hydrocarbon products of specific carbon numbers, but changes the olefidparaffin ratio among the products. This kinetic behavior agrees with the result of gas chemisorption that the HJCO adsorption ratio changes with the extent of metal reduction. 1. INTRODUCTION Iron is difficult to reduce completely particularly when it is dispersed as fine particles on the catalyst support. Accordingly, most of the iron catalysts are used as partially reduced ones containing the unreduced iron oxides even after reduction at elevated temperature for long periods. Turnover rates are commonly used to specify the intrinsic activity of the surface-exposed metal, but this is based on the assumption that the activity is not influenced by the coexisting unreduced metal oxide. However, validity of this assumption should be examined more carefully because a few previous studiesrl-31 suggest that the unreduced metal oxide or the surface oxygen species alter the activity of the surface metal, sometimes by an order of magnitude. With the above background considerations, we have studied the changes in the kinetic behavior of supported iron catalysts in CO hydrogenation due to different extents of metal reduction. We have changed only the extent of iron reduction by preparing the iron catalyst in one batch and then by reducing it under different conditions. The kinetic behavior of the partially-reduced catalysts has been correlated with the results of gas chemisorption.
2. EXPERIMENTAL 2.1. Catalyst Preparation and Reduction The sample catalyst used in this study is Fe/SiO, with the nomial metal loading of 9.1 wt%. Silica support is a standard material(JRC-SiO-6) obtained from Japanese Catalysis Society[4] with the reported surface area of 109 m’/g. Iron salt, Fe(NO3);9H,O, with the purity of 99.997% has been purchased from Aldrich and used without further purification. The catalyst has been prepared by the incipient wetness method as described by Vannice[S], and reduced by heating in dihydrogen stream. The extent of reduction has been estimated by oxygen titration at 400°C[6], assuming that iron forms Fe,O, after oxidation[7].
2758
The extent of reduction is determined mostly by the final reduction temperature but not by the reduction period as far as the reduction period is longer than 10 minutes. The catalysts have been reduced to 3, 21, 34, and 63% by reduction at 320, 350, 420 and 450°C for 10 minutes. For convenience, the catalysts will be coded in this paper according to their percentage reduction, e.g., Fe(3%).
2.2. CO Hydrogenation The rates and the product distribution in CO hydrogenation over the iron catalysts have been measured in a conventional microreactor unit with the reactant stream containing H, and CO at the 5:l ratio. The reactant gases have been purified by passing them through Molecular Sieve 5A and MnO traps in series. CO has been purified further after flowing through an activated carbon column. The products of the reaction have been analyzed by Hewlett Packard 5890A GC equipped with a flame ionization detector. A relatively low reaction temperature, 270°C. has been used to prevent further reduction of the catalysts during reaction experiments. 2.3. H, and CO Chernisorption Since adsorption of dihydrogen on iron is an activated process[8], the amount of dihydrogen adsorption has been obtained after cooling the catalyst in dihydrogen from the temperature 30°C lower than the final reduction temperature to room temperature. The amount of CO adsorption has been measured at room temperature. 3. RESULTS AND DISCUSSION
3.1. Kinetic Behavior in CO Hydrogenation Two aspects areobserved with the CO hydrogenation rates on the partially reduced iron catalysts. One is that the rates are higher on the relatively well-reduced catalysts, and the other is that the initial rates decrease with the reaction time. High reaction rates on the well-reduced catalysts are due to their large metal surface area, and the rate decrease with time is due to the surface carbon deposition as reported by others[7]. Although the catalysts are deactivated during the reaction, the distribution of the hydrocarbon products of specific carbon numbers remains almost constant as exemplified for Fe(3%) in Table 1. The product distribution is also unaffected by the extent of iron reduction. This result indicates that the chain propagation step in CO hydrogenation is not Table 1. Product distribution in CO hydrogenation on 9.lwt% Fe/SiO, catalysts Catalyst (% reduction)
Hydrocarbon distribution (mol%)
c,
c,
Fe(3%)
49.9 50.6 49.0
16.1 18.7 19.4
18.8 19.2 18.9
9.5 7.3 7.2
5.7 4.3 5.5
0.58 3.55 6.87
Fe(2 1%) Fe(34%) Fe(63%)
52.1 51.1 52.8
17.2 16.6 19.0
17.5 17.7 19.6
8.3 7.5 2.7
4.9 7.3 5.7
0.77 0.63 0.50
c, c,
c,
Time on stream (hr)
2759
Figure 1. Changes in the ethylene/ethane ratios among products with the conversion and the extent of 9.1wt% Fe/SiO, catalysts. Solid symbols are for Fe(63%) and open symbols are for Fe(3%). influenced by the surface carbon deposition and by the extent of metal reduction. Contrary to the distribution of the products of specific carbon numbers, that of the olefinic and paraffinic hydrocarbons among the products changes with the reaction period and with the extent of metal reduction. In Figure 1 are plotted the ethylene/ethane ratios among the products obtained with the two catalysts of different metal reduction, Fe(3%) and Fe(63%). Since the olefidparaffin ratio in CO hydrogenation changes with the conversion[9], the data have been plotted against the conversion. In the figure, the data obtained from each reaction test have been represented by an identical symbol. A close observation of the figure reveals that the product-ratio data fall on a single curve as far as they are for the same catalyst. In other words, the characteristic dependence of the product ratio on the conversion is not affected when theconversions become smaller due to the catalyst deactivation. Figure 1 clearly shows that the extent of iron reduction is a distinct parameter to modify the olefin/paraffin ratio among the products. The olefin/paraffin ratio decreases when the iron catalyst is reduced poorly. For example, the ethylene/ethane ratios on Fe(3%) are about half of those on Fe(63%) in the conversion range shown in Figure 1. Difference in the product ratios has also been observed with the cobalt catalysts[3], but in that case the ratios have been higher on the poorly reduced catalysts.
3.2. H, d C 0 Chemisorption To study further the effect of the partial metal reduction, we have measured the amounts of H, and CO chemisorption on the catalysts. Table 2 shows that the amounts of CO and H, chemisorption decrease when the catalysts are reduced poorly. This is
I
2760 Table 2. Amounts of gas chemisorption on 9.lwt% Fe/SiO, catalysts Catalyst (% reduction)
Fe(3%) Fe(63%)
H (pmodg-cat) 20.7 30.1
co
HJCO ratio
4.28 11.87
4.83 2.54
OlmoVg-cat)
normal because the number of iron atoms exposed to the surface decreases as the catalysts are reduced to lower extents. A result to be noted in Table 2 is that the HJCo ratio is higher on Fe(3%) than on Fe(63%). This result agrees with the kinetic result of Figure 1 that the ethylene/ethane ratio is lower on Fe(3%) than on Fe(63%). As the iron surface of Fe(3%) is relatively rich with the adsorbed hydrogen than that of Fe(63%), the reaction intermediates on the catalyst surface are easily hydrogenated into paraffinic products instead of being desorbed simply as olefins. The observed HJCO ratios in Table 2 are high considering that the stoichiomeuic number of H, chemisorption is 0.5 and that of CO chemisorption is between 0.5 and 1.0 on most transition metals. But, a similar result has been obtained by Rankin and Bartholomew[lO] who explained it to be due to small values, less than 0.5, of the stoichiometric number of CO chemisorption on iron.
4. CONCLUSION We have studied the properties of the partially reduced iron catalysts in CO hydrogenation and have obtained the following conclusions. The extent of iron reduction does not alter. the intrinsic reaction rate nor the distribution of the hydrocarbon products of specific carbon numbers in CO hydrogenation but changes the olefin/paraffin ratio among the products. The change in the olefidparaffin ratio agrees with the chemisorption result that the HJCOadsorption ratio increases as the catalyst is reduced poorly. Inconclusion, the extent of metal reduction should be considered as an important variable in characterization of supported metal catalysts.
5. REFERENCES 1 R.L. Palmer and D.A. Vroom, J. Catal., 50 (1977) 244. 2 D.J. Dwyer and G.A. Somorjai, J. Catal., 52 (1978) 291. 3 S.H. Moon and K.E. Yoon, Applied Catal., 16 (1985) 289. 4 S. Yoshida, N. Takezawa and T. Ono (eds.), Catalytic Science and Technology, vol l., Kodansha, Tokyo (1991) 393. 5 M.A. Vannice, J.Catal., 37 (1975) 449. 6 C.H. Bartholomew and R.J. Farrauto, 45 (1976) 41. 7 G.B. Raupp and W.N. Delgass, J. Catal., 58 (1979) 337. 8 Z.Paal and P.G.Menon(eds.), Hydrogen Effects in Catalysis, Marcel Dekker, New York (1988) 139. 9 J.A. Amelse, L.H. Schwartz, and J.B. Butt, J. Catal., 72 (1981) 95. 10 J.L. Rankin and C.H. Bartholomew, J. Catal., 100 (1986) 533.
Ounl, L u al. @litom), New Frontiers tr Catalysk Proceedings of the 10th Intcrnatioml Congress on Catalysis, 19-24July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishen B.V. All dghb reserved
HYDROGENATION OF CARBON MONOXIDE OVER Rb/ZrOz CATALYSTS PROMOTED BY MOLYBDENUM OXIDE E. Guglielminottia, E. Giantell&, F.Pinnab, G. Strukrtlb, S.Martinengoc and L.Zanderighid aDipartimentodi Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, Via P. Giuria 7, 10125 Torino, Italy bDipartimentodi Chimica, Universita di Venezia, Donoduro 2137, 30123 Venezia, Italy CDipartimento di Chimica Inorganica e Metallorganica, Univenita di Milano, Via G. Venezian 21,20133 Milano, Italy dDipartimentodi Chimica Fisica ed Elettrochimica, Universita di Milano, Via G. Venezian 21,20133 Milano, Italy
Abstract
Rh-Mo/Zre catalysts, with different Mo loading, were caracterized by TEM, ESR, TPR FTIR, H2 and 0 2 chemisorption and their catalytic activity was studied in the CO + H2 reaction. The Mo effect strongly depends on the activation temperature of the catalysts. 1. INTRODUCTION
Supported rhodium catalysts are active for the production of C2 oxygenates in the carbon monoxide hydrogenation reaction, although the activities and selectivities reported to date are far from any practical interest. Among the different promoters added to the basic Rh catalyst, Mo oxides have shown a significant improvement on the activity and selectivity of the reaction. In spite of the large amount of experimental data, the role of the promoter on the reaction path to oxygenates is still controversial. In this study we report a thorough characterization of the system Rh/Mo/zr02 based on spectroscopic (ESR, FTIR), TPR, chemisorption and reactivity data, with the aim of shedding more light on the fundamental steps of the reaction. 2. EXPERIMENTAL Catalysts were prepared by adsorption on 2102 (70 m2/g) of Mo(C0)6 from a hydrocarbon solution under inert atmosphere, followed by adsorption of Rh4(C0)12 under the same conditions [l]. The Rh content for all samples was 1.0 wt 9h while the Mo/Rh atomic ratio changed from 0.0 to 2.0. Supported carbonyls were decomposed in H2 at 523 K (LTR) and 773 K (HTR), passivated in 5% 02 in Ar at 298 K and the resulting catalysts stored in air. The TEM experiments were performed with a JEOL 2000 EX working at high resolution (6OO.OOO magnification at 200 KV) and equipped with a top entry stage. The ESR spectra were taken by a v a r i ~E-109spectrometer operating in the X-band mode and equipped with a dual cavity. Varie pitch was used for g-values calibration. The FTIR spectra were recorded at RT by a Perkin Elmer 1760 spectrometer. The samples were nduced "in situ" in static conditions at 523 or 773 K for 2 h, changing the H2 and with a final outgassing at the same temperanue. H2 and 02 chemisorption were performed at 298 K using a pulse flow system [2]. Rior to measunments, catalysts were reduced for 2 h at 523 K (LTR) and 773 K (HTR).
2762
In the TPR experiments [2] all samples were calcined at 773 K for 1 h, cooled to 298 K and then heated (10 K/min) to 1023 K in a 10% H2 in Ar mixture. Catalytic activity was studied in a flow reactor at 493 K, 101 KPa, HgCO = 3/1. Before testing, the catalysts were activated in situ at 523 and 773 K in H2 flow for 2 h.
3. RESULTS The TEM results show the presence of Rh2O3 and probably of Rh(0)OH phases with a = 3 nm size on the Rh/zro2 and Rh-Mo(2.O)EQ reduced (773 K) samples exposed to air.On the Mo containing sample a slightly reduced Mo,,O3,,-, Magneli phase is present. The ESR analysis of the reduced samples reveals the presence of Mo5+ ions, the concentration of which increases with the Mo content and with the reduction temperature. The TPR profile of the unpromoted catalyst shows a H2 consumption peak centered at about 373 K that was assigned [2] to the reduction of Rh2O3 and a broad band centered at about 773 K, which is also present in the TPR profile of ZrO2 alone, probably due to a rearrangement of the Zr@ surface with formation of coordinatively unsaturated Z#+ [2]. The TPR profile of the Moo3 promoted catalysts show a peak at 373 K and a broad band with two maxima centered at about 773 and 1025 K. The latter, which increases with the Moo3 content, has been assigned to the reduction of MoO3, by comparison with the TPR profile of a Mo03Er02 sample. It has to be pointed out that the peak at 373 K increases in intensity with the Mo content without any temperature shift. Since the Rh content of the catalysts is the same (I%), a redox reaction at the borderline between reduced Rh and Moo3 could take place with formation of MOO, (x c 3) species and a further reduction of the oxidized rhodium. The H2 and 0 2 chemisorption measurements (Tab. 1) carried out on the catalysts (atomic ratio Mo/Rh = 0.0. 0.5, 1.0 and 2.0) reduced at two different temperatures (523 and 773 K), show i) a high dispersion of Rh after LTR, with a mean particle size of 1.4 nm; ii) that the H2 chemisorption decreases progressively with the increase of Mo content, while the 0 2 chemisorption increases, the effect being more evident on the HTR samples.
-
TABLE 1
Chemisorption data after H2 activation at 523 K (LTR ) and 773 K (LTR ). Catalysts LTR HTR H/Rh O/Rh 0.7 1 1.1 0.75 Rh-MO(OS)/ZI@ 0.65 1.4 0.19 1.3 Rh-Mo(1.0)/ZrO2 0.32 1.6 0.15 1.8 Rh-M0(2.O)/Zr02 0.22 1.9 0.04 2.1 All catalysts have 1% Rh (w/w) on the number in brackets is the atomic ratio Mo/Rh. H/Rh and O/Rh are the number of hydrogen and oxygen chemisorbed per Rh atom in the sample.
m;
On the basis of the TEM, ESR, TPR and chemisorption results we suggest that Rh may favor the Moo3 reduction with the formation of a Magneli phase that, due to a large surphace mobility, may partially cover the Rh particles, thus decreasing the Rh sites available to the H2 chemisorption and increasing the total sites available to the @ chemisorption. The FTIR results of chemisorbed CO on HTR samples show a strong decrease in intensity of the adsorbed CO with the increase of Mo content (Fig.1) in agreement with previous results [3] reported for Rh/Mo03/Si02 catalysts. However the spectrum of adsorbed CO is not qualitatively changed by increasing the Mo content, so excluding an electronic or an alloying effect of the Mo on Rh.
2763
The spectrum of adsorbed CO is similar to that previously found on other supported Rh catalysts [4,5] except for the presence of a couple of bands at 2082 and 2070 cm-1, related to a reversible CO species as demonstrated by desorption experiments. The same bands have been found on Rh-Mo0dSiO.L [5] and were associated to Rh states perturbed by the Moo3 coating and partially oxidized. We agree with this assignement and consider these CO species to be peculiar of the Rh-MOO, supported systems. The linear CO species at 2060 cm-1, remaining after RT outgassing, and the bridged CO species at 1890, are typical of CO adsorbed on Rh0[4,5]. The couple of bands at 2100-2090 and 2045-2035 cm-l are assigned to Rhl+(C0)2 moieties [4]. A broad band at 1650 cm-1 (curve a), shifting at 1600 cm-1. and strongly declining with the increase of M o a , has been already found on the RhE1-02 system [4] and assigned to Rh2-C0-@+ bordeline complexes. Its behaviour is in agreement with the M a x Rh coating model here advanced and is also c o n f i e d , in the sample with a high Moo3 content, by the strong intensity of the band at = 2190 cm-l assigned to @+-CO species [4]. However no bands of CO adsorbed on reduced MoX+(x=4,5) sites 161 can be observed, probably because of the overlappingwith the intense @+-CO band at 2190 cm-l.
AI0.15
A
- 2
Fig Iz. FTIR spectra. normalized to the Rh (1%) content, of CO (pco= 5 KPa) adsorbed at RT on RhIMolZrO2 reduced for 2 h and outgassed for 15 min. at 773 K: curves a, b and c refer to MoRh ratios = 0.5, 1 and 2 respectively.
On heating in CO atmosphere at 423-473K. the reduction of Rhl+ to Rho occurs and the bordeline species at 1650-1600 cm-l disappears, paralleled by the formation of carbonate groups. Therefore the CO dissociation with C@ formation can be suggested on these sites. An experiment of C02 reduction with H2 c o n f m the data reported 171 on supported Rh: C@ is reduced to CO by H2 already at RT and strongly at 523 K with the formation of linear (2055 cm-1) and bridged (1890 cm-1) RhO-CO species. CO is formed at the expenses of C@ linear species (2350 cm-1) and also of carbonate and formate species adsorbed on ZrOz. A C@ C02 equilibrium, as in W.G.S. reaction, is therefore operating during both the CO.L-H2 and the C&H2 reaction. In the latter, new surface species are formed by H2 reduction at 523 K with the decrease of adsorbed CO: hydroxyl groups and water (3770-3630 cm-l), formates (2987,2882,1568, 1380 and 1368 cm-1). bands at = 2930 and 1450-1422 cm-1 which can be assigned to the stretching and bending modes respectively of CH, (x = 2,3) groups of alcoholates formed by formates reduction.
2764
The results of the CO hydrogenation on LTR and HTR catalysts are reported in Table 2. The data clearly show that i) the activity of LTR catalysts increases with the Mo content while that of HTR ones does not change; ii) the CO2 formation depends only on the amount of Mo on the catalysts; iii) the general trend in carbon efficiency of total hydrocarbons and oxygenates does not change significantly. Nevertheless, within the two fractions the selectivities of the C2 hydrocarbons and MeOH show a marked increase with the Mo content.
TABLE 2
Activity and selectivity of catalysts for carbon monoxide hydrogenation as a function of the activation temperaturea Catalysts co C Q Selectivitv as carbon efficiency b conv. evdiated without C% -Em (%) (%) c1 c2 c3 Cq-8 Me R M
LTR Rh/zroz Rh-Mo(O.S)/Zr02 Rh-Mo(1.0)/Zr02 Rh-M0(2.O)/Zr02 HTR
4.7 7.6 11 14
2.8 5.7 14 32
60 57 49 36
5.0 8.0 16 26
11 10 11 12
12 11 11 16
1.5 1.5 2.0 4.0
10 11 10 5.8
0.5 1.5 1.0 0.2
3.1 2.8 57 4.0 9.0 14 1.8 12 1.2 RhJzro2 14 52 8.2 8.0 7.0 4.0 19 1.8 Rh-MO(O.S)/Zr02 3.2 Rh-Mo(l.O)/2102 2.5 29 40 21 10 7.6 11 10 0.4 Rh-M0(2.0)/ZrO.r 3.4 36 26 33 13 8.0 16 4.0 -aReaction conditions: T = 493 K; H2:Co = 3:l; P = 101 Wa. bCarbon efficiency = 100(niAi/hiAi), where ni = carbon number of product ; Ai = moles of product i. Co;! not considered in &Ai. 4. CONCLUSIONS The promotion effect of Moo3 on Rh/ZrO2 catalyst strongly depends on the activation temperature: at high temperature (773 K) the formation of a mobile Magneli phase gives rise to a masking effect of the Rh particles, with consequent reduction of Rh activity The MOO, species favor the chain growth to the C2 hydrocarbons and promotes the C02 formation by both CO dissociation and WGS reaction. In our experimental conditions the MOO, species do not promote the alcohol formation and shift the selectivity from ethanol to methanol.
5. REFERENCES 1 A. Benedetti, A. Carimati, S. Marengo, S. Martinengo, F. Pinna, R. Tessari, G. Strukul T. Zerlia and L. Zanderighi, J. Catal., 122 (1990) 330. 2 C. Dall’Agnol, A. Gervasini, F. Morazzoni, F. Pinna, G. Stmkul and L. Zanderighi; J. Catal., 96, (1985) 106. 3 B.J. Kip, E.G.F. Hermans, J.H.M.C. Van Wolput, N.M.A. Hermans, J. Van Grondelle and R. Prins; Appl. Catal., 35, (1987) 109. 4 E. Guglielminotti, J. Catal. 120 (1989) 287 and references therein. 5 M.D. Wardinsky and W.C. Hecker; J. Phys. Chem. 92, (1988) 2602. 6 J.B. Pen, J. Phys. Chem., 86 (1982) 1615. 7 F. Solymosi and H. Kntlzinger, J. Catal. 122 (1991) 166.
Guczi, L et d.(Editors), New Frontiers in Cutulysis
Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
IDENTIFYING THE REACTION NETWORK OF THE HIGHER ALCOHOL SYNTHESIS OVER ALKALI-PROMOTED ZnCrO CATALYSTS L. Lietti, E. Tronconi and P. Forzatti Dipartimento di Chimica Industriale "G.Natta" del Politecnico, P.zza L. da Vinci 32, 20133 Milano, Italy
1. INTRODUCTION
Mechanistic aspects of the Higher Alcohol Synthesis (HAS) over modified methanol catalysts have been extensively investigated in the last few years [1,2,3]. It has been demostrated that the formation of higher oxygenates is controlled by a slow first carbon-carbon bond formation (C,-->C, step), followed by fast chain growth processes involving aldol condensations and aadditions reactions, both occurring in the "normal" and "reverse" modes. An "a posteriori" thermodynamic analysis has been attempted [ 4 ] to identify the classes of reactions which are limited by chemical equilibria. It has been proved that thermodynamics play a role in determining the relative abundance of many products of the HAS, including methanol, C02, aldehydes, ketones and esters. A kinetically controlled reaction network, providing routes for chain growth to higher oxygenates, is superimposed to a thermodynamic background. Aspects still requiring clarification are the mechanism of the C,-->C, step, i.e the slow step of the HAS, the reactivities of the oxygenates involved in the chain growth according to their nature and molecular structure, and the origin of hydrocarbon by-products. Such aspects are preliminary to a detailed reaction network and to a mechanistic kinetic analysis of the HAS. They are addressed in the present work by a number of appropriate techniques. 2. EXPERIMENTAL
Unpromoted and K- and Cs-promoted ZnCrO catalysts were used in this study. Reactivity and mechanistic information was collected by catalytic activity tests carried out under actual synthesis conditions (P up to 100 bar), chemical enrichment experiments where compounds were added to the C O / H p feed, TPSR and flow-microreactor experiments of C 1 - C5 oxygenates at atmospheric pressure. 3. RESULTS AND DISCUSSION
a) C1-->Cz step - The reactivity of various oxygenates which may be involved in the C,-->C, step (e.g. CO, methanol,
2766
formaldehyde, formic acid, methyl formate) has been addressed by means of flow microreactor experiments performed at 300'C over a K-promoted ZnCrO sample. The results reported in Table 1 indicate that all the investigated molecules are mainly decomposed to COX and HI, but significant amounts of higher oxygenates (particularly isobutanal) are also formed in the case of methanol, formaldehyde, methyl formate. This indicates that carbon-carbon bond formation is effective already at atmospheric pressure over the K-doped ZnCrO catalyst.
Inspection of Table 1 indicates that: i) methanol and formaldehyde originate the largest amounts of isobutanal, thus indicating the high reactivity of these molecules; ii) methyl formate originates significant amounts of isobutanal but its decomposition is accompanied by formation of methanol, from which isobutanal may be formed consecutively; iii) CO/H2 mixtures are not converted at atmospheric pressure over K-doped ZnCrO; iv) formic acid is mainly decomposed to C02 and only traces of isobutanal have been detected in this case. It can be concluded that species directly related to methanol or formaldehyde are possibly involved in the formation of the first carbon-carbon bond. It is worth mentioning that formation of small amounts of isobutanal have been also observed at temperatures as low as 150'C when feeding formaldehyde, thus indicating that either formaldehyde or species strictly related to formaldehyde are very reactive in the formation of carboncarbon bonds. b) Chain growth to C 2 + oxygenate6 A series of TPSR and flow microreactor experiments were performed with C3, linear and branched C4 oxygenates (primary alcohol, aldehyde and acid) , 2butanone and 3-pentanone. TPSR experiments performed with n-propanal and n-butanal indicated that a number of different reaction products are
-
2767 obtained over the K-doped ZnCrO. Table 2 shows the main identified desorbed species and the associated catalyst functions which are responsible for their formation.
CN alcohol
,
RICH0 t H2 -->
RICHZOH
C2 unsaturated aldehyde
2 RICHZCHO --> R1CH,C=CH(Rl)CHO t H2O
C2 ketone
2 R,CH2CH0 --> R,CHZ(CO)CH(R,)CH3 t H20 2 R,CH,CHO + O ( l a t t i c--> e) RlCH2(CO)CH2R, t Cop t H2)
C N t I 2-ketone
R,CH2CH0 t CO -->
CN olefin
RlCHzCHpOH
C N - lolefin
-->
hydrogenation "Normal" aldolic condensation "Reversal" aldolic condensation decarboxylative condensation
RlCH2(CO)CH, R,CH=CHp
RICHZCHO+ O ( 1 a t t i c e ) --> R,=CH2 + C02
a-addition dehydration
decarboxylation
TPSR runs of the alcohol and acid molecules confirm the catalyst functions previously discussed and provide further evidence for the reactivity of acid molecules towards decarboxylation and ketonization reactions. Flow microreactor experiments performed with 1-propanol, 2butanone and 3-pentanone compare well with TPSR data of C3 and C4 oxygenated molecules as far as the nature of the reaction products are considered, indicating that the same catalyst functions account for both transient (TPSR) and steady-state (flow microreactor runs) conditions. However, under steadystate conditions the different reactivity of the species participating in the numerous chemical reactions can be appreciated, as detailed in the following: 1) "Normal" and "Reversal" aldolic-type condensations of aldehydes and ketones - Only aldehydes (linear and branched) participate in aldolic-type condensations as electrophilic reactants. Both linear C 2 + aldehydes and linear ketones can be involved as nucleophilic species, aldehydes showing higher reactivity than ketones. Branching in the 2-position reduces the reactivity of electrophilic reagents and prevents the participation of both aldehydes and ketones as nucleophilic species in aldolic-type condensations; 2) ketonization reactions - C 2 + aldehydic or carboxylate surface species are involved in ketonization reactions. Also in
2768 this case the presence of substituents in the a-position to the carbonyl group hinders the reactivity of the reagents; 3 ) "reversal" a-addition reactions - C 2 + aldehydes and a C1 nucleophilic species are involved in this reaction. The C1 intermediate may be associated with nucleophilic oxygenated C1 species originated during decarboxylation reactions; 4) hydrogenation/dehydrogenation reactions - All primary alcohols/aldehydes and secondary alcohols/ketones pairs approach hydrogenation/dehydrogenation equilibria. The signs of the deviations from chemical equilibrium observed at low temperatures indicate that carbonyl compounds are intermediates in the chain growth process whereas alcohols are produced by subsequent hydrogenations; 5) dehydration reactions - all the alcohol molecules have been found to undergo dehydration to the corresponding olefins. The following order of reactivity is observed: 2-alkanols > secondary alcohols > primary alcohols. Olefins may also be formed to a certain extent by decarboxylation of surface carboxylate species. c) Reaction network of the HAS Upon comparing the reaction pattern identified by TPSR and microreactor flow experiments with information obtained by catalytic activity tests and chemical enrichment experiments, it is concluded that the catalyst functions identified in the present study are adequate to describe the reactions occurring under pressure as well. However, some of the characteristic reactions are limited by chemical equilibria. This is for example the case of ketonization reactions, which occur to a large extent during TPSR and flow experiments but are strongly depressed under pressure, and of hydrogenation reactions of aldehydes and ketones to the corresponding alcohols which are on the contrary Thus, a peculiar and complex greatly fovoured in HAS. interaction is established between kinetically controlled chemical routes for chain growth and thermodynamics, which must be taken into account to adequately describe the product distribution under HAS conditions. This approach has been successfully used to develop a mechanistic kinetic analysis of the HAS over a Cs-promoted ZnCrO catalyst which accounts quantitatively for over 50 oxygenates in the product mixture as function of temperature, contact time, pressure and feed composition [5].
-
4 . REFERENCES
1
P. Forzatti, E. Tronconi, I. Pasquon, Catal. Rev.-Sci. Eng., 33 (1&2), 109 (1991), and references therein contained.
2
4
J.G. Nunan, C.E. Bogdan, K. Klier, J. Smith, C-W. Young, 410 (1988) and U, 195 (1989). R.G. Herman, J. Catal., U, D.J. Elliott, F. Pennella, J. Catal., U,90 (1988) and 119, 359 (1989). E. Tronconi, P. Forzatti, and I. Pasquon, J. Catal., U,
5
376 (1990). E. Tronconi, L. Lietti, G. Groppi, P. Forzatti and I.
3
Pasquon, J. Catal., in press.
Guczi, L er d.(Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
EFFECTS OF FORMIC ACID AND C 0 2 IN CO HYDROGENATION TO METHANOL OVER COPPER-BASEDCATALYSTS AND NATURE OF ACTIVE SITES
J. Cai) Y.Liao) H.Chen and K R. Tsai Department of Chemistry & Institute of Physical Chemistry, Xiamen University, Xiamen 361005, China
ABSTRACT Effect6 of small amounts of formic acid and/or COz on methanol yield in CO hydrogenation to methanol over Cu/ZnO, Cu/ZrOz, and Cu/MgO catalyete and the i n - e i t u FT-IB spectra of the chemisorbed epeciee have been investigated. Formate adepeciee appears to be a chemieorbed intermediate in CO hydrogenation to methanol over the three copper-baaed catalyste; CO hydrogenation over Cu/MgO ia inhibited by even very small proportion of COz; over Cu/ZnO, the inhibit on by COz becomes apparent at slightly higher COz proportion, while over Cu/ZrOz it ie unaffected by small amounts of CO. The inhibition is probably due to carbonation of the baeic MgO site and tbe slightly basic ZnO site. The nature of the active site and mechanism of synergistic catalysis in CO hydrogenation to methanol over copper-based catalyste ie discuaeed. INTRODUCTION The industrial euccees of Cu/ZnO-baaed catalyete for low preesure conversion of CO-COz-l$ into methanol has prompted a great deal of fundmental reeearch work, in many laboratoriee throughout the world, on this type of catalyets and the reaction syeteme, aa shown by recent reviews [l]. It has now been generally accepted that COz hydrogenation to methanol plus HtO over Cu/ZnObaaed catalyete takee place on the copper eurface without the aid of any metal-oxide promoter, but no unified view about the nature of the active site for CO hydrogenation to methanol over such catalysts has been reached so far. It haa been propoeed by Klier and hie coworkere [ 2 ] that the active site may compriee Cut dispersed in ZnO lattice, and they have shown that remarkable eynergy between the two components appears to exiet. However, more recently Chinchen and Waugh 131 pointed out that, with COz and CO preeent in comparable proportions in the eyngaa feed (aa in the conventional low-preeaure methanol-
This mrk IU supported by tho National Natural k i r m Fwnhtim of China, and by a grant for spactrwropic chrracttrizationfrw tho Sktt b y laboratory tor ?hysicrl Chmistq of tho Solid Surfccr, Xiurn Uliwnity
2770 synt,hesis practice), there is an approximately linear relationship between methanol-synthesis activities and the initial copper surface areas of copperbasedcatalysts prepared on various supports, and that it is the hydrogenation of the Cot component of the feed which is responsible for practically all the methanol produced; in other words, with such a feed composition, CO hydrogenation becomes insignificant in comparison with C02 hydrogenation over Cu/ZnObasedcatalysts. Recently, IR spectral evidence [ 4 , 5 1 has shown that the major chemisorbed species in methanol synthesis with such syngas feeds appear to be So it seems to us that, with such carbonate and formate adspecies, besides a feed composition, the slightly basic ZnO site, a requisite component of the active site for CO hydrogenation based on a mechanism proposed previously [ 6 ] , might be inhibited due to formation of carbonate or formate. In the present work we focus our attention on the effects of C02 and small amount of formic acid in the feed on the yield of methanol and on the in-situ FT-IR spectra of the chemisorbed species, in order to obtain further insight about the nature of the active site and the mechanism of CO hydrogenation to methanol over copper-based catalysts.
a.
EXPERIMENTAL Preparation of catalysts: Low-copper loading catalyst, 15molXCu/ZnO, was prepared by constant-pH coprecipitation method as described before [ 2 , 7 ] , followed by drying, ignition, and activation by mild reduction with hydrogen diluted with Nl, as reported previously. Cu/Zr02 and Cu/MgO catalysts were similarly prepared. Effect of formic acid (2~01%) in feed upon catalytic activities: A conventional flow reactor-G.C.(TCD) outfit w a s used. Formic acid was introduced by means of gas-bubbling saturator. Catalyst evaluation was carried out under atmospheric pressure and 190°C, using a very sensitive solidelectronic temperature-controller,XCC-1000. In-situ Fl'-IR detection of chemisorbed species: An IR cell has been designed for the spectroscopic detection of chenisorbed species under syngas reaction conditions at elevatedpressure and temperature. In the present work, however, the in-situ FT-IR spectra were taken only at atmospheric pressure t o correlate with the catalyst evaluation data. Catalyst samples (30mg) for the in-situ IR spectroscopy were compressed into thin wafers and carefully reduced at temperature up to 4 2 3 K in the IR cell with hydrogen (HI : N2 = 1 : 10 v/v) stream. The IR spectra were recorded using a Nicolet FT-IR 740 spectrometer. RESULTS AND DISCUSSION The variation of methanol-synthesis activity with feed composition at 190°C and atmospheric pressure is shown in Table 1. It can be seen that the addition of 2.01% formic into the feed (with CU/$ around 3/7 mole ratio and 0-5.01% Cot) always resulted in an increase in methanol yield for each of the three
2771 catalysts, while Cot at 2.01% enhanced methanol yield in the case of Cu/ZnO and Cu/ZrOz catalysts, but depressed it in the case of the C u m catalyst. In the case of the Cu/ZnO catalyst, with 5.01% C02 in the feed, the methanol yield began to drop slightly below the level with 2.01% COz in the feed. Table 1 Variation of catalyst activity with feed composition at 190°C & 1 bar CHSV
I. 5x10'
wso
0.1
Syngas composition COz: CO: 5 (.Ol%) 0 0 2 2 5 5 2 2 0 0 2 2 0 0
HCOOH added
: 33 : 6 1
: : : : : :
Methanol yield l0'xg/gut. h 0.007 0.012 0.009 0.017 0.M 0.016 0.007 0.014 0.W 0.009 0.002 0.007 0.015 0.021
33 31 31 20 20 31 31 33 33 31 31
:61 : 67 : 61 :67 :61 : 61 : : 67 : :61 : :61 : : 61 : : 61 : 33 : 6 1 : 33 : 6 7
I R spectra for cherisorbed species on Cu/ZnO catalyst:
The adspecies arising from (COz t 5 ) at 150°C ave rather strong IR band due to bidentate formate (1590, 1395, 1370 cm ) and also bands due to carbonate ( 1520, 1425cm-'). For the CO2-containing syngas at 25OoC, the IR bands showed that formate and carbonate adspecies appeared together when CO/CO .01 ratio 2 5/28, while the IR bands due to bidentate carbonate were hardly observable when COz/CO < 3/30. As soon as syngas without COz was introduced over the catalyst wafer at l5O0C, a transient appearance of formyl (HCO) IR bands at 1675, 168Ocm-I occurred, but persisted for only about 30 seconds; after that the IR bands of bidentate formate and methoxy adspecies (2945, 2836, 1096cm-l) became dominant. For 2.01% HCOOH-containing and COz-free syngas over the catalyst wafer at 15OoC, the IR bands due to formyl adspecies could still be detected during the first minutes. IR spectra of cherisorbed species on Cu&O catalyst: With COztHz at l5O0C, bidentate carbonate adspecies was detected by its IR bands, but no formate adspecies; while with COtH , not only formate adspecies, but also H B transient adspecies were detected by the IR bands. Some inferences can now be made from the above results: (1) Since introduction of small amount of formic acid into the eyngas feeds was found to increase the methanol yield for all the three catalysts, and formate adspecies was detected by in-situ IR spectroscopy, bidentate formate adspecies is most probably an intermediate species of CO hydrogenation to methanol over all the three copper-based catalysts. (2) With Cu/ZnO catalysts and slightly higher proportion of COz in the syngas feeds, COz changes from being a promoter to an inhibitor of CO hydrogenation over Cu/ZnO, probably due to carbonation of the slightly basic site on the ZnO component, while COz hydrogenation remain uninhibited; this accounts for the fact that C02 hydrogenation becomes the predominate source of methanol produced with such feed compositions. (3)
-f
2772 Under steady state reaction condition, the first step of CO hydrogenation may conceivably lead to the formation of a transient for yl adspecies aided by dipole-charge interaction with the promoter cation Zn' (or Mezt, or Zrlt), as previously proposed by us [6] ; but this species appears to rapidly incorporate a interfacial bridging oxide ion to form a formate ion, probably still bridging the interfacial Cu' and the Zn", further hydrogenation may lead to the formation of dioxymethylene and then methoxy ligand (attached to Zntt) which is finally liberated as methanol by accepting a protonic hydrogen from a neighboring hydroxy ligand. (4) It may be speculated that, with freshly activated Cu/ZnO catalysts, the interfacial bridging ligand between Cut and Zntt might be a hydroxyl, rather than an oxide ligand, therefore the formyl ligand formed in initial step of CO hydrogenation can not easily incorporate an oxide ligand to become a formate ligand, thus in the first 30 seconds or so of the methanol eynthesis reaction, a transient formyl adspecies can be detected. These mechanism of bidentate-formate intermediate formation through the incorporation of a transient formyl intermediate with a surface-lattice p 0 may be illustrated as follows:
-CO
B(Cu) H a L
- -
Y - (pa
HCm-
H(Cu) H~CQQZCH,Q-
ZnQHbt CH30H t ZnQ
where the underlined atoms in the chemical symbols signify those atoms (of the ads ecies) directly bound to Cult (for @), or to Cu (as indicated), or to a probably as Cu and Zntt in the case of pQ, or to Znzt (for the second ZnH(OH)), of surface lattice of the (Ca)Cur( @)Zn(H)(OH) active-center previously proposed by us [6]. The above mechanism is similar to that in [6], except with the slight modification that here the H a (with the terminal ''Q interacting through dipole-charge interaction with a neighboring Znft to to form a account for the promoter action [6]) readily incorporates a bidentate-formate intermediate, ticoo_', to account the present experimental observation; the HCQQ- intermediate may then further hydrogenated (probably first to HzCQQ- 141) to methanol. If the proton transfer step is rate controlling, then the deuterium isotope effect is likely to be very small and positive; on the other hand, if one the hydrogenation steps (e.g., the formation of H a ) is rate controlling, then a significant deuterium inverse isotope effect may be observable.
rP
REFERENCES [l] J.C.J.Bart and R.P.A.Sneeden, Catal. Today, 2, l(1987) R.G.Herman, Studies in Surf. Sci. & Catal., Vol. 64, 265(1991) [ 2 I P. G Herman , K K1 ier , G W Simmons , B. F Finn, J B Bul ko and T. P Roby 1inski, J.Catal., 56, 407(1979) [3] G.C.Chinchen, R.C.Waugh and D.A.Whan, Appl. Catal., 25,101(1986) [4] C.Chauvin, J.Saussey, J.C.Lavalley, H.Idriss, J.P.Hindermann, A.Riennemann, P.Chaumette and P.Courty, J. Catal., 121, 56(1990) [51 J. Saussey and J.C.Lavalley, J. Mol. Catal., 50, 343(1989) [6] H.P.Chen, S.J.Wang, Y.Y.Lia0, J.X.Cai, H.B.Zhang and K.R.Tsai, Proc. 9th ICC., Vol. 2, 537(1988) [7] J.X.Cai, H.B,Chen, D.H.Chen and H.B.Zhang, J. Mol. Catal.,(China) 4, 139 (1990)
.
.
..
.
..
.
Guczi, L ef d.(Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
COMPARISON OF ZnO-SUPPORTEDCu, Cu-Mn, Cu-Fe, Cu-Co AND Cu-Ni CATALYSTS IN CO HYDROGENATION
P. A. Sermon, M. A. M.Luengo (Yates) and Y. Wang Surfaces and Solids Research Group, Department of Chemistry, Brunel University, Uxbirdge, Middx. UB8 3PH, United Kingdom
Abstract C a n 0 catalysts prepared by precipitation have been modified by Mn, Fe, Co and Ni. After characterisation these have been assessed for their activity and selectivity in CO hydrogenation. The dispersion of the ternary catalysts remains similar to C a n 0 alone and temperature-programmed reduction suggests that the IB and VII-VIII metals are interacting in these catalysts. The conditions of reductive pretreatment of the samples critically affects their activity and selectivity in CO hydrogenation. Mn increases the rate of methanol synthesis of W n O after reduction at suitable temperatures. However, Fe and Co decrease activity, increase alkane production and increase ethanol production, and decrease methanol production compared to cu/znO. The causes of this are considered in some detail. Finally, Ni suppesses both the rate of CO hydrogenation and the selectivity to methanol of C a n 0 and this might be because of the formation of a partial Cu-Ni alloy. The realtive importance of Cu-M”’ and Cu-M pairs in CO hydrogenation upon these catalysts is still the subject of discussion. 1. INTRODUCTION
While C a n 0 and Cu/ZnO/Al,O, produce methanol with high selectivity, when Co is also added there is an increased selectivity to alkanes and ethanol [l]. The reasons for this are explored using CuEnO catalysts to which Mn, Fe, Co or Ni are added.
2. EXPERIMENTAL The catalysts shown in Tables 1 and 2 have been prepared by coprecipitation. In this the aqueous solution containing zinc acetate and the nitrates of copper, manganese, iron, cobalt and/or nickel were treated with NaOH and NaHCO,. The washed precipitates were dried (403K, 16h), calcined (3h, 523K) and reduced in flowing hydrogen (6kPa, 50cm3.min”) as described in the footnote to Table 2. Total surface areas (SBBT)were determined on samples (0.5g) which were outgassed (2h, 473K) by adsorbing N, at 77K and applying BET theory. Average crystallite sizes d for CuO and ZnO in the pre-calcined precursors to the catalysts were determined by applying the Scherrer equation to the line broadening seen in X-ray diffraction. X-ray reflections for Mn, Fe and Co phases were not seen. Temperatureprogrammed reduction (TPR) profiles were measured in a traditional flow system. A microreactor described previously was used to measure the isothermal activities and selectivities in CO hydrogenation at 20atmospheres pressure under conditions given in a footnote to Table 2, together with the reductive conditions used. In addition under temperature-programmed conditions the same reactive atmosphere was saturated with HCOOH at 293K flowing at
2774
Table 1 Physical Properties of ZnO-supported Catalysts (a) %Cu %Mn %Fe
8.0
-
8.0
-
2.0 8.0
8.0
-
8.0
-
8.0
-
SBET (m2.g.')
G E T
d(nm) CuO ZnO
T,, (c) (K)
-
66
248
5.3
9.8
470
-
75 51 72 43 91 73 62
465 807 293 847 24 1
b
8.5
-
12.5
4.8
8.7 15.4 7.8 8.7 18.8
495 542,673 483,673 667,873 489 401,492,821 488,586
%Co %Ni
2.0 8.0
-
-
2.0 8.0
-
-
2.0
-
4.5
-
a: where X-ray diffraction line broadening is used to determine the average crystallite size in nm except for peaks which are too broad or small (b), adsorption of N2 at 77K is used to measure total surface areas ,S and temperature programmed reduction (TPR) is used to measure temperatures of rates of reduction. c: MnO, reduces to MnO and FeO, to FeO. Unsupported oxide CuO had maximum rates of reduction at 590 and 648K, MnO, at 572 and 654K and COO at 641 and 675K. Table 2 Activity and Selectivity of ZnO-supported Catalysts in CO Hydrogenation (x) % selectivity
r %Cu %Mn %Fe
8.0 8.0
-
8.0 8.0 8.0 8.0 8.0 8.0
2.0 2.0
-
-
2.0 2.0
-
-
-
-
alkanes
c,
c,
c,
95.0 90.2
0.8 1.9
0.4 0.2 (*)
0.4 96.6 0.4 96.7 16.9 32.0 19.3 20.6 8.2 59.8 0.5 84.4
1.2 1.0 9.0 4.0 8.4 1.4
0.2 0.4 (#) 4.5 (*) 0.8 (*) 1.2 (*) 0.0
c,
54.7 45.7
1.3 3.1
1.3 2.3
1.2 2.3
-
114.2 64.7 67.4 66.2
2.0
-
5.5
-
2.0
8.9
0.8 0.8 25.4 37.1 11.5 9.2
0.8 0.7 12.2 18.2 10.9 4.6
%Co %Ni
-
c,
alcohols
c,
(+)
x after 4h at 20atmos, 50cm3.min-',0.lg cat, 67%C0+33%H2,523K after pre-reduction for 30min at 573K + pmoVgcat/min, while selectivity is on a C mol basis * measured at 20cm3.min-'; for Cu/Fe/ZnO (*) the catalytic activity was measured at 503K after pre-reduction at 498K # measured after reduction at 543K
2775 latmosphere pressure. All product and reactant compositions were determined chromatographically.
3. RESULTS The total surface areas of the ZnO-supported catalysts are shown in Table 1 and these clearly do not decrease when Mn, Fe, Co or Ni is added to W n O and at the same time the average crystallite size of the CuO and ZnO is decreased slightly. Table 2 shows that Mn, Fe and Co additives raise the temperature of reduction of CuO on ZnO compared to that for CuOEnO alone; in other words there is likely to be an interaction between these group VIIVIII cations and ZnO-supported Cu”. At the temperature used for in-situ reduction prior to catalysis (i.e. 573K)the copper is largely reduced, but the precise extent of Mn. Fe and Co reduction is less certain. Comparison of catalytic results in Table 2 show that the precise flow rate and temperature of reduction affect the activities and selectivities seen. When Mn is added to Cu/ZnO, activity is increased in CO hydrogenation if the temperature of reduction is suitably chosen, and the selectivity to methanol is not decreased, but rather may benefit a little. However, the same Table shows that when Fe or Co are added alkane selectivity is increased and methanol selectivity is decreased, and in addition ethanol selectivity is increased. While Co suppresses activity, iron appears not to do so. Finally, the addition of Ni suppresses both the total activity and the methanol synthesis rate upon Cu/ZnO. 4. DISCUSSION AND CONCLUSIONS Mn, Fe, Co and Ni additives all appear to interact with ZnO-supported Cu; their different effects upon the selectivities and activities in CO hydrogenation can be explained by the importance of Cu-Mx+ or Cu-M pairs. Several points need to be made:
(a) It is noteworthy that Mn can induce even higher selectivity and activity in Cu/ZnO to methanol synthesis. (b) Both Co and Fe induce increased alkane and higher alcohol production in CUnnO. It was long ago suggested [2] that in the catalysed hydrogenation of CO by metals their selectivity to methane or oxygenated products was defined by their ability to dissociate CO. Thus formate species on copper and CH,- species on the group VIII metal might interact to produce a higher alcohol than methanol, in a heteronuclear approach to the coupling of surface intermediates [3].However, the effect of the present group VII-VIII metal additives is more complex since it is certain that these metals are not entirely reduced under reaction conditions. Thus for example Coz+can catalyse the insertion of CO into copper-derived adsorbed formaldehyde [4].In addition the promoter m a y affect the manner of adsorption of CO even on the Cu. Figure 1 shows that this is not likely at least at 1 atmosphere in that all catalysts produce methanol, rather than ethanol, when treated with HCOOH+CO+H, Interestingly, the enhancement in methanol production is greatest for Cu-Mn/ZnO at about 483K and is greatest for the catalyst reduced at the higher temperature. (c) Finally, Ni suppesses both the rate of CO hydrogenation and the selectivity to methanol of Cu/ZnO and this may be because of the formation of a partial Cu-Ni alloy.
2776
5. REFERENCES [ 11 DOE report DOE/ER/13392-1; G.R.Sheffer, R.A.Jacobson and T.S.King J.Catal.
116,(1989),95; J.G.Nunan, C.E.Bogdan, K.Klier, K.J.Smith, C.W.Young and R.G.Herman
J.Cata1. 116,(1989),195 1977),176 [2] R.W.Joyner J.Cata1. [3] J.G.Nunan, C.E.Bogdan, K.Klier, K.J.Smith, C.W.Young and R.G.Herman J.Cata1. 113 (1988),410 [4] D.A.Katahira, K.G.Moloy and T.J.Marks Organometallics L(1982),1273
a,(
-9
pmol CH,OH/g caVmin 15
10
-...5
-..-.-
8%Cu-2%Co/ZnO 8%Cu-Z%Mn/ZnO 8%Cu-2%Mn/ZnO
(t)
8%Cu/ZnO
Figure 1 Rate of methanol production from a 67%CO-33%H2 stream saturated with HCOOH and passed over samples of the catalyst at 1 atmosphere. All catalysts were pre -reduced at 573K except that highlighted * which was pre-reduced at 543K.
Ouczi, L et al. (Editors), New Frontiers in Catalysis Proccedings of the 10th International Congnss on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
METHANOL SYNTHESIS FROM C02 AND H2 OVER SUPPORTED COPPERZINC OXIDE CATALYST. SIGNIFICANT INFLUENCE OF SUPPORT ON METHANOL FORMATION
H.Arakawa and K Sayama National Chemical Laboratory for Industry, Tsukuba Research Center, Tsukuba, Ibaraki 305, Japan
Abstract The support effect of Cu-ZnO catalyst on CH30H formation from H2/C@ was studied. It has proved that support affects CH30H formation activity and selectivity significantly. In genaral, basic oxide support such as rare earth metal oxide was favorable to selective synthesis of CH30H. From the viewpoint of effective CH30H production, however, Ti@ was the most promising support. The role of support and possible reaction mechanism was discussed using an in situ FT-IR spectroscopy for surface species over catalyst. 1. INTRODUCTION Catalytic hydrogenation of Co;! to CH30H has been recently recognized as one of promising recycling technologies of emitted Co;! in Japan. Cu-ZnO catalyst, which is a typical catalyst for CH30H synthesis from syngas, is known as one of useful catalysts for C@ conversion to CH30H[1]. However, more advanced catalysts are uested to develop. Only a few studies with the support effect of Cu-ZnOcatalyst reported so ar[2-3]. Therefore, here we would like to report a extensive study on support effect of Cu-ZnO catalyst on CH30H formation from H2/C@.
?
2. EXPERIMENTAL Catalysts used in this study were prepared by an impregnation of mixed aqueous solution of Cu(N03)23H20 and Zn(N@)2.6H20 onto various kinds of oxides. Impregnated catalysts were dried and calcined under an air stream at 450 'C. Prepared catalysts were characterized by XRD, CO chemisorption, N20 consumption and so on. A high pressure C 0 2 hydrogenation reaction was conducted with a flow type fixed bed micro-reactor. A reaction mixture of Ar/H2/C@=10/60/30 was used as feed gas. Before reaction, 1 g of packed catalysts was pretreated with H2 stream at 350 'C for 30 min. The effluent gas was analyzed by on line gas chromatograph. To clarify the reaction mechanism for CH30H formation from H2/C@, dynamic behavior of surface species over catalysts during reaction was observed using an in situ FT-IRspectroscopy.
3. RESULTS AND DISCUSSION More than 20 oxides were tested as catalyst support of Cu-ZnO for Co;! conversion to CH3OH. From Table 1, it is clear that catalyst support influences catalytic behavior significantly,that is, such as total yields of products, CH30H yields, CH30H selectivitiesand Turnover frequency of CH30H formation(TOF(CH30H)). Total yield of Cu-ZnO catalyst was improved very much by the use of support. The increase of total yield is roughly proportional to the surface area of sup rt andlor the amount of CO chemisorbed by catalyst. This result shows that Cu metal su ace area, which is the active sites for this reaction, of
K"
2778 Table 1 Support effect of Cu-ZnO catalyst on CH30H formation from H2/C@ mixture Catalyst4 Support ZnOf)
Total Yieldd) CH30H S.A.b) C0ad.C) Yield Selec. TO@) (m2/g) (pmollg) (pmo1lg.h) (pmo1lg.h) (%) (llh) 2 242 275 54 26 3 14 2
0.5
300
280
3390 2070 1640 1610 1250 1050 790 790
1330 1360 1280 910
92
622
co Yield Selec. (pmo1lg.h) ( %) 20
8
39 25 2060 61 43 66 34 710 205 78 360 22 202 57 700 43 960 153 77 290 22 830 79 267 220 21 650 82 209 140 18 650 82 361 140 18 -0 530 79 660 130 21 5 9 590 650 63 91 60 9 630 41 290 7 43 340 52 500 90 3 2 560 185 60 10 470 1 500 94 6 2 213 30 3 450 480 30 7 93 166 -0 69 320 460 140 31 2 420 30 450 94 6 190 1 400 380 94 20 6 0 2 2 0 3 reductionprocess [4-61. The presence of metal does not effect the high temperature peak but it lowers and splits the first peak; it is well known that noble metals can activate and accelerate the reduction process by hydrogen spillover to the support. This appears to be the case with Me/Ce02; the high H2 uptakes (Table 1) can be explained in the same way. Observation of CO TPD desorption peak for HTR catalysts but not after LTR, strongly suggests that CO is produced from C02 adsorbed on Ce3+ sites (i.e. support oxygen vacancies) which are formed during the high temperature reduction. A similar behaviour was found by Jin et al. [7]. The role of metal is to facilitate the reduction process (i.e. formation of Ce3+) and to activate H2 for the hydrogenation reaction. The high transient activities ob&ved after high temperature reduction for the Rh, Ru, Ir and Pt catalysts could be attributed to hydrogenation of C02 moieties activated by the Ce3+ ions. TPD results show that C O 2 dissociation on the HTR catalysts provides O s d species which reoxidize Ce3+ to Ce4+ and thus re-estabilishing the steady state catalytic properties. Partial reoxidation of surface Ce3+ sites in catalytic conditions was detected by X.P.S. [2]. The active site for the reaction is suggested to be located at the adlineation perimeter between the metal and Ce02, where lattice oxygen vacancies may play an important role. Such mechanistic picture is similar to that proposed for noble metals supported on other reducible oxides [8]. Acknowledgments. The authors thank CNR and MURST 40% and 60% for financial support.
REFERENCES 1.A.Trovarelli. C.de Leitenburg and G.Dolcetti,, 1991. 472. 2.A.Trovarelli, G.Dolcetti, C.de Leitenburg, J.Kaspar, P.Finetti and A.Santoni, J.Chem.Soc. ,submitted for publication. 3.F.Solymosi and A.Erdohely, J.Mol.Catal. 1980.8.471. 4.M.D.Yao and Yu.Y.F.Yao,J.Catal., 1984,95,227. S.A.Laachir, V.Pemchon, A.Badri, J.Lamotte, E.Catherine, JCLavalley, J.E1 Fallah, L.Hilaire, F.le Normand, E.Quemere, G.N.Sauvion and O.Turnet, 1991,87,1601 1988,92,4964 6.J.Z.Shyn. W.H.Weber and H.S.Gandhi,J.Phvs.Chem., 7.T.Jin, T.Okuhara, G.J.Mains and J.M.White,J.PhYs.Chem., 1987,91,3310 8.J.D.Bracey and R.Burch,J.Catal.,1984,86,384.
u,
Ouczi, L ef al. (Editors), New Fronriers in Caralysb Proceedings of the 10th International Congress on Catalysis, 19-24July, 1992,Budapest, Hungary 0 1993 Elscvicr Science Publishers B.V. All rights mserved
POTASSIUM PROMOTION OF Cu=Zn0-AI2O3CATALYSTS FOR HIGHER
ALCOHOL SYNTHESIS I. Boz, D. Chadwick, I. S.Metcalfe and K Zheng Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, London SW7 2BY, United Kingdom
Abstract K promotion of Cu-ZnO-Al,O, catalysts is found to increase selectivity to higher alcohols at low loadings. Total activity and selectivity to hydrocarbons decreased with loading. XPS has been used to study the state of potassium under reaction conditions. 1. INTRODUCTION Alkali metal promotion of methanol synthesis catalysts has long been known to increase the selectivity to higher alcohols [1,2]. However, the role of the alkali metal remains uncertain. The present paper addresses the distribution of promoter within multicomponent K-Cu-ZnO-Al,O catalysts, the state of potassium under reaction conditions and the effect on the selectivity to higher alcohols.
2. EXPERIMENTAL Potassium carbonate was added to CuO-ZnO-Al,O, catalysts (60/30/10 mo1%) by incipient wetness impregnation to produce nominal loadings of 0.83, 2.5 and 4.2 wt% K. Catalysts were dried at 60°C and calcined in air at 300°C. K-ZnO and K-Al,O,, each 4.2 wt% K,were prepared in a similar manner. Catalysts were tested for alcohol synthesis activity in a 0.3 1 internal recycle reactor using an impeller speed of 1500 rpm with a 6 gm charge diluted with glass beads. Catalysts were prereduced in 5% H in N with a 10°C m i d temperature ramp up to 220°C for 12 hours. Premixed 57% H,, 3 b % 60. 10.5% N, was fed to the reactor via a heakd alumina bead bed which served to remove any carbonyls and preheated the reactant stream. Activities were measured at 40 barg, with space velocities of 2000 to 30000 hrl. Product analysis was by online GC. XPS analysis was performed in a VG ESCALAB MkII with Al Ka radiation. Catalysts were pretreated in situ using a gas reaction cell. Binding energies are quoted relative to Zn 2p = 1022 eV.
3. XPS STUDIES Catalysts were analysed following calcination in air at 300°C and after in situ reduction in H, at 220°C and exgosure to a flow of 10% CO,, 20% CO, 60% H,, 10% N at the reaction
temperature of 275 C for up to 24 hours. The CO,/CO ratio was slightly higher than the
2786 outlet composition from the internal recycle reactor for the lowest space velocities and highest temperature used in the kinetic work which gave the highest selectivity for higher alcohols. 3.1 Intensities One of the aims of the XPS analysis was to study the distribution of the promoter in the multicomponent catalysts under reaction conditions. Samples of K promoted alumina and zinc oxide (4.2 wt% K) were studied for reference. The measured intensity ratios for the dried samples, the calcined precursors, and after in situ reduction and exposure to the synthesis gas mixture are given in Table 1. The K2pIA12p ratio was found to be almost constant for all the treatment conditions and was close to the value expected for a monolayer catalyst indicating good dispersion of the promoter in K-AI,O,. In contrast, the K2p/Zn3p ratio from dried KZnO was lower than expected for a monolayer catalyst. However, the ratio increased significantly on calcination and showed negligible change following reduction and exposure to the synthesis gas mixture at reaction temperature. Previous studies of the K-ZnO system have noted an increase in relative intensity on heat treatment consistent with an increase in K dispersion and wetting of the oxide surface by the promoter 131. "
Table 1 XPS intensity ratios for 4 . 2 ~ 1 %K-ZnO and 4.2wt% K-Al,O, Catalyst Ratio Precursor K-ZnO K2pIZn3p 0.23 K-Al,O, K2pIA12p 0.58
Calcined 0.36 0.53
CO,/CO/H, 0.38 0.52
XPS data for the oxide components of the catalysts is given in Table 2. The A12p/Zn3p ratios for the K containing catalysts in the calcined state were found to be higher than that for the unpromoted catalyst. The most likely explanation for this observation is a reduction in the surface area of the zinc oxide component of the catalysts in the presence of potassium since it is well-known that zinc oxide is sintered by alkali. The Al2plZn3p ratios for the promoted catalysts decreased significantly following in situ reduction and exposure to the synthesis gas mixture. This is an indication of strong segregation effects. The data given in Table 2 is for reduction for 1 hour followed by 1 hour exposure to the gas mixture. However, exposure of the 2.5 wt% K catalyst to the synthesis gas mixture was extended to 24 hours without further changes in relative intensities. Table 2 XPS intensity ratios for K promoted catalysts K (wt%) Ratio 0.0 Al2pIZn3p A12p/Zn3p 2.5 A12pIZn3p 4.2 2.5 K2pIZn3p 4.2 K2pIZn3p 2.5 K2pIA12p 4.2 K2pIA12p
Calcined 0.82 1.12 1.19 0.28 0.43 0.25 0.36
CO,/CO/H, 0.97 0.7 1 0.67 0.42 0.82 0.59 1.22
2787
The K2p/Al2p ratio for the 4.2 wt% K catalyst in the calcined state was lower than for the K-Al,O, reference, whereas a higher ratio would be expected for a yell-dispersed system given the composition of the catalyst. This suggests that the dispersion of potassium in the calcined state of the catalysts was rather poor compared with the reference. Nevertheless, when the promoted catalysts were reduced and exposed to the synthesis gas mixture, the K2p/Al2p and K2p/Zn3p ratios increased significantly. Again, this is a clear indication of strong segregation of potassium in the catalysts. As noted above, exposure to the synthesis gas mixture for 24 hours did not result in any further intensity changes. Studies of the reduced state indicated that the segregation of potassium commences during prereduction and continues under the initial exposure to the reaction conditions. The increased K2p/Al2p and reduced Al2p/Zn3p ratios under reaction conditions are consistent with segregation of potassium towards the alumina surface. The catalyst K2p/Al2p and K2p/Zn3p ratios under reaction conditions are higher than for the K-Al,O, and K-ZnO references, which indicates that the potassium segregates to both the alumina and ZnO surfaces with a preference for the alumina surface. Given the catalyst composition and reasonable assumptions concerning the oxide surface areas, at 4.2 wt% K loading there would be insufficient alumina surface area to accommodate all the potassium. 3.2 State of potassium under reaction conditions The potassium 2p spectra from K-Al,O, and the K promoted catalysts were rather broad and poorly resolved under all treatment conditions, in contrast to the spectra from K-ZnO which was clearly defined. The most significant aspect of the spectra, obtained from catalysts exposed to the synthesis gas mixture was the observation of a relative strong C l s signal at around 290 eV which is consistent with the presence of potassium carbonate, Figure 1. No carbonate C l s peak was detected for the unpromoted catalyst under reaction conditions. The carbonate CldK2p intensity ratios obtained from catalysts exposed to reaction conditions are given in Table 3. The CldK2p intensity ratio obtained from potassium carbonate was 0.16. Comparison of this value with those of Table 3 suggest that under the reaction conditions used here, the potassium promoter in the catalysts was almost entirely in the form of potassium carbonate.
Table 3 Carbonate CldK2p intensity ratios under reaction conditions. Catalyst K-ZnO K-A1 K-CdZdAl 2.5 wt% 4.2 wt% K Loading 4.2 wt% 0.15 0.10 0.17 C 1dK2p
K-CdZdAl 4.2 wt% 0.17
4. HIGHER ALCOHOL SYNTHESIS
Catalysts containing 0, 0.83, 2.5 and 4.2 wt% K were tested for higher alcohol synthesis activity from 270-300OC. As expected, the overall selectivity to higher alcohols increased with temperature and lower space velocity. The product distribution contained a high fraction of branched alcohols with 2-methyl-l-propanol being the most prominent. Overall the product distribution was similar to that reported previously [2,4]. Carbon selectivities for a space velocity of 3000 hrl and 275OC are given in Figure 2. The addition of the promoter reduced the overall activity from 152 to 78 pmole/gm min for 0% and 4.2% K catalysts respectively. At 275"C, the effect of increasing K from 0 to 0.83 wt% was to slightly increase the selectivity to higher alcohols from 5% to 6%. Higher K loadings decreased the selectivity to higher alcohols. At 290OC. the same pattern was obtained with selectivity to higher alcohols increasing from 15% to 20% for 0 to 0.83 wt% K loading. A major effect of K at all conditions was a large reduction in the selectivity to hydrocarbons, Figure 2. This finding is
2788 consistent with the XPS results which indicate that the K is mainly located at the oxide surfaces. R 20
9 3 P)
8-l
4 10 P
ua!
275
285
295 305 Binding Energy (eV)
Figure 1. The XPS C l s and K2p region of catalysts (4.2 wt% K) under reaction conditions: (a) K-Cu/Zn/Al and (b) KZnO.
" 0 C1
)C2
C l O H ) Q O H ALD
Figure 2. The carbon selectivities of various K-promoted catalysts. Note that the methanol (C1OH) selectivities are scaled down to 114.
5. CONCLUSIONS
Potassium promotion of CuO-ZnO-Al,O, catalysts results in an increase in selectivity to higher alcohols for low loadings, but a decrease in overall activity. A major effect appears to be strong redution of the selectivity to hydrocarbons. Under reaction conditions, it would appear that the potassium is preferentially located on the oxide surfaces, with a bias towards the alumina surface, and, for the particular synthesis gas composition employed, is present as potassium carbonate. Of course, given the semi-quantitative nature of XPS, the possibility that small amounts of potassium may be located on the copper surface, and may not be in the from of carbonate, cannot be excluded. REFERENCES
1 2 3 4
G.Natta, U.Colombo and I.Pasquon, "Catalysis", Reinhold, N.Y,5, (1957). 131. K.J.Smith and R.B.Anderson, J.Catal., 85 (1989), 428. D.Chadwick and K.Zheng, Proc. 11th Can. Symp. Catal., 121,1990, Halifax, Canada. E.M. Calverley and K.J. Smith, Proc. 11th Can. Symp. Catal., 68, 1990, Halifax, Canada.
Guczi, L e.f al. (Editors), New Frontiers in Cufalysir Proceedings of the 10th International Congrcss on Catalysis, 19-24 July, 1992,Budapest, Hungary 6 1993 Elscvicr Science Publishers B.V. All rights resewed
CONVERSION OF SYNGAS TO AROMATIC HYDROCARBONS ON COBALTMANGANESE-ZEOLITE CATALYSTS
G.BcrUrle, K
Guse, M. Lohrengel and H.Papp
Lehrstuhl fir Technische Chemie, Ruhr Universitat Bochum, D-W4630 Bochurn, Germany
Abstract A Co/Mn oxide catalyst system was investigated, tested in the Fischer Tropsch reaction and characterized by X P S and CO chemisorption. This catalytic system was mixed with pentasil zeolites to form aromatic hydrocarbons. Two modes of operation were tested: a single bed reactor with a mechanical mixture of the components and a dual bed operation with FT component and the zeolite respectively in seperate reactors. The active component in the &/Mn oxide system is cobalt, manganese acts as promoter which supresses the formation of methane and enhances the production of olefins. The combination of these catalysts with ZSMS zeolites with low Si/Al ratio led to the formation of aromatic hydrocarbons. due to the transformation of olefinic and oxygen containing products. Additionally high molecular weight hydrocarbons were cracked to lower alkanes. The dual bed arrangement with seperate FT and zeolitic component is to be preferred since it is possible to adjust the optimum temperature for the FT component and the zeolite respectively and to regenerate the zeolitic component independently. 1. INTRODUCTION Bifunctional catalysts composed of a FT component and a shape selective zeolite can be used to produce aromatic hydrocarbons from synthesis gas. The basic idea was the development of a FT catalyst that generates products which can easily be converted into aromatic hydrocarbons. As known from literature such precursor products are olefins and alcohols. The combination of Co as FT-component and HZSM-5 seems to be a very promising approach to produce highly aromatic gasoline from syngas /1-3/,Therefore a Co/Mn oxide catalyst was optimized for the production of a high percentage of olefins from syngas. This improvement of the Co/Mn oxide catalyst was achieved by variation of the reaction conditions and catalyst composition (Co/Mn ratio). The FT component and zeolite (HZSM-5, GaZSM-5) were combined either in mixed or in dual bed arrangements to investigate the catalytic behaviour. 2. EXPERIMENTAL The Co/Mn oxide catalysts were prepared by continuous coprecipitation from the
comsponding metal nitrate solutions with aqueous ammonia solution at a constant pH value of 9,2 and constant temperature of 70 'C /4/. The composition of the catalysts was determined by ICP and is given together with the notation in Table 1. Commercially available H-ZSMS and Ga-ZSM5 zeolites with different Si/Al ratios (14 to 50) were used. In the two step
2790 process the synthesis apparatus consisted of two sequentially arranged down flow fixed bed microreactors; the fmt contained the FT catalyst, and the second the zeolite. For the one step process zeolite and FT catalyst were each sieved to -100 mesh, thoroughly mixed and pelletized. Product distribution was determined by gaschromatography. The catalysts were characterized in their surface by XPS in a LHS 10 apparatus after different treatments. The specific surface area was determined by the one point BET method. The active surface area was measured gravimeaic by carbon monoxide chemisorption.
3. RESULTS AND DISCUSSION The active component in the Co/Mn oxide system is cobalt, manganese acts as promotor which suppresses the formation of methane and enhances the production of olefins. These results are illustrated in the Figures 1 and 2. A strong increase in the reaction temperatures to obtain comparable CO-conversions was observed with increasing Mn content (cf. Figure 1 and Table 2). Figure 2 shows that the olefidparaffin ratios decrease strongly with increasing cobalt content. The chemisorption data presented in Table 3 show an increase in the amount of adsorbed CO with an increase in cobalt content. X P S experiments showed that only the catalyst ColOO contained metallic cobalt in the surface before and after synthesis, whereas the Mn containing catalysts consisted of varying amounts of Co*Iand Co"'. These results indicate that a-cobalt is responsible for the high activity, the low olefidparaffin ratios and the high methane selectivity of the catalyst Col00 (see Table 2). The catalysts CoMn4060 and Cohh1981 were further investigated. High reaction temperatures favour the formation of methane and carbondioxide. Using syngas of low CO& ratio led to a decrease in methane formation and a strong increase in olefidparaffin ratios. Furthermore a decline in olefidparaffin ratios was observed with an increase in CO conversion. High total pressures are connected with lower olefidparaffin ratios and higher CO conversions
:I\
O/P ratio
Temperature ('C) 250
210 230
i\
190 1 0
I
50
Co-content (at %)
100
0
50 Co-content (at%)
100
Figure 1: Necessary reaction temperatures to Figure 2: Dependence of the olefidparaffin obtain comparable CO-conversions with ratios on the Co-content of the catalysts at identical Eaction conditions (identical SV catalysts of different Co-content with respect to mass of cobalt).
2791 C3&3 ratio
S ( C 5 hydrocarbons)
70
4l 2
0' 0
4
10
20
30
Synthesis time (h)
40
0
30 Synthesis time (h)
10
20
40
Figure 3: Change in product distribution switching between FT catalyst alone and dual bed operation (0-20 FT alone, 20-32 dual bed, 32-40 FT alone) The influence of zeolite addition to FT catalysts on product distribution is shown in Table 4 and Figure 3. Figure 3 shows the dramatic change in product composition upon switching from FT operation alone to a combination with the zeolite (dual bed operation). In this experiment the temperature of the zeolite containing reactor was held at 470 'C. In all cases no changes in the avtivities (CO conversion) of the FT catalysts due to zeolites were observed, but the C3 and C4 olefin/paraffin ratio decreased strongly. This is due to the ability of ZSM-5 to oligomerize propene and butene under these conditions. A large increase in the selectivity to C1 to C4 paraffins and much higher i/n-butane ratios were observed, employing either mixed or dual bed operation. This is typical for the cracking of higher molecular weight hydrocarbons and isomerization reactions on the acidic centers of ZSM5. A sharp decrease in the fraction of Cg+-hydrocarbonsis seen in Figure 3. Ga exchange of the zeolites, very promising in the case of aromatization of lower alkanes /5/, did not lead to better results. since these catalysts deactivated strongly and high amounts of methane were formed. The main aromatic products were toluene, xylenes and Cg+ aromatic hydrocarbons. Benzene was in all cases only a byproduct. The highest selectivity towards aromatic hydrocarbons was obtained with the dual bed operation (about 18 96.) Table 1: Composition of the C o M oxide Table 3: Amount of chemisorbed CO as catalysts prepared by continuous precipitation function of catalyst composition and reduction temperature 200'C 300'C 400'C T(Reduc.) catalyst Co (at 96) Mn (at 96) catalyst CO adsorbed (mgk) MnlOO 0.0 100,o MnlOO 0,015 0,026 0,074 Coho595 433 957 CoMn0595 0,082 0,154 0,038 CoMn2080 19.0 81.0 CoMn2080 0,365 0,437 0,386 CoMn3664 36.4 63.6 coMn3664 0,081 0,543 0,697 CoMn4060 40.2 59,8 CoMn6634 0,133 1,894 1,316 CoMn6634 65.5 343 15.0 CoMn8515 85.0 CoMn8515 1.291 2.590 1,340 1oo;o 0;o ColOO 1,877 1,950 1,117 ColOO
2792
Table2: Product distributions of Co/Mn oxide catalysts (H2/C0=2, p=10 bar, SV=2800 /h) Catalyst Coloo coMn6040 CoMn4060 CoMnl981 Temp. .$'CY 200 210 220 230 19,9 14,9 11,l 12,6 Conv. CO % S(i) w%-C CO2 -0.8 1.4 194 cH4 13,8 13,2 83 13,2 c2 3,1 2.8 2.3 193 C2= 092 0.4 19 0,7 c3 3-2 3.1 13 03 c3= 2,4 2,7 58 3,4 n-C4 1,9 12 1,o 0.8 a= 192 192 292 28 cs+ 74,2 74,6 74,7 75,4 c2=/c2 0,05 0-1 0,8 0,6 c3=/c3 0,74 03 32 4,1 c4=Ic4 0,61 1,o 2,l 3,4 Table 4: Product distribution of a Co/Mn oxide catalyst in combination with ZSM-5 zeolites Catalyst CoMn1981(a) CoMn1981/ CoMn1981/ CoMn4060(b) CoMn4060/ GaZSM-da) GaZSM-da) HZSM-S(b) Mode single bed single bed dual bed single bed dual bed S(i) w%-C cH4 8 18 22 19,O 19,6 S(OX0) 1 0 0 1 0 S (Aromatics) 0 9 11 0 18 i/n-C4 0 1,s 1,7 0 13 o/Pc 3 28 091 0.1 10,4 3,O o/Pc 4 1,9 0,1 8
A
OO
1
2
: Sel. to propanal
3
4
5
6
W/F (g-cat.min/mi)
Fig. 2. Condensation of pmpan- 1-01 with methand over MgO-(NO) catalyst at 653 K under different W/F conditions.
3 3 Condensation of pmpan-1-01 with methanol over MgO-(NO) catalyst Figure 1 and 2 show catalyst deactivation as a function of reaction time and effect of contact time, respectively. MgO(NO), the most selective catalyst, was used here. The catalyst deactivated gradually over the initial reaction period and then maintained steady state activities. Since water is co-product in the mction(eq. I), the deactivation of the MgO catalyst may be caused by the change of its surface state with water formed during the reaction. The contact time slightly affect the product selectivities; at the short contact time(low conversion) the selectivity to 2-methylpmpan-1-01 is down and that to pmpanal is up. The present synthetic reaction of higher alcohol with methanol seems to consist of consecutive reactions of dehydrogenation, condensation, dehydration, hydrogenation over catalyst surface. It is interesting that the MgO catalyst found in the work can promote such a multi-step reaction smoothly. 4 References 1 RA. Sheldon, Chemicalsj?om Synlhsis Gas,Reidel, Dordrecht, 1983. 2 W. Ueda,T. Yokoyama, H. Kurokawa, Y. M m k a , and T. Ikawa, J. Jpn. Petrol. Inst., 29 (1!%36) ?2. 3 H. Kumkawa, T. Kato, W. Ueda,Y. Morikawa, Y. M m k a , and T. Ikawa, J. Cufd.,126 (1990) 199. 4 W. Ueda,T. Kuwabara, T. ohshida, and Y. Morikawa, J. Chem. SGC.,Chem. Commun., (1990) 1558. 5 W. Ueda,T. Ohshida, T. Kuwabara, and Y. Morikawa, Catal. Lett., in press
Ouczi, L et al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elscvicr Science Publishers B.V.All rights mwvcd
IN SITU lH NMR STUDY OF THE ADSORPTION OF HYDROGEN AND FORMIC ACID ON COPPER BASED METHANOL SYNTHESIS CATALYSTS A. Ben&&
J. B. C.Cobb, B. T.Heaton and J. A. Iggo
Department of Chemistry, University of Liverpool, Grove Street, P.O.Box 147, Liverpool M9 3BX, United Kingdom
Abstract The adsorption of hydrogen on Cu/ZnO/A1203 and CdZnO is shown by lH NMR spectroscopy to give a single copper bound species, however adsorption of hydrogen on Cu/A1203 under the same conditions, gives two distinct species. The decomposition in uucuo of these species is characterised by an initial rapid loss of some of 'hydrogen on copper' followed by more leisurely loss of the remaining copper bound hydrogen. This effect is more pronounced on aged catalysts. The adsorption, and subsequent reactions of formic acid, on the copper containing catalysts were studied. The adsorption reactions were followed in situ and reaction intermediates detected. 1. INTRODUCTION
We have previously described a cell for reactive atmosphere, in situ NMR studies of heterogeneous catalysts that allows variation of the pressure of a reactive gas inside the reactor, evacuation, admission of a second species, etc [l].We now report preliminary results on the adsorption of hydrogen and formic acid on copper based methanol synthesis catalysts. !&RESULTSANDDISCUSSION 21. Adsorption of hydrogen Figure 1 show the lH NMR spectra on admission of H2 t o the reactor containing Cu/ZnO/A1203, Cu/ZnO and Cu/Al, O3 severally. In each case resonances in the region 85 to 100 ppm relative to TMS = 0 ppm are observed.
2826
In agreement with Dennison et al. we assign these Knight shifted resonances to hydrogen adsorbed on copper [21. Interestingly, on the adsorption of hydrogen on to CdA1203 we observe two resonances in the Knight shifted region indicating two adsorption sites on copper are available to hydrogen in this catalyst- these may be Cuo and partially oxidised copper, different sized particles or adsorption on different planes. Evacuation of the reactor results in a reduction in intensity of the Knight shifted resonances indicating decomposition of the copper-hydrogen species. A variable fraction of the intensity is lost in an initial rapid decomposition followed by a more leisurely loss of all or part of the remainder, Figure 1. This 'sticky hydrogen' is most clearly seen on samples that have undergone several activation procedures indicating that sintering of the catalyst increases the number of this type of adsorption site. Partial oxidation of the copper surface may also be important
1
I
1 100
ppm
60
100
I
I
PPm
6o
Figure 1.'H NMR spectrum of adsorption of % on (a) Cu/ZnO/Al203;(b) Cu/ZnO; and (c) CdAI2O3: and (d) tenacious hydrogen on CdA1203 after evacuation at 300 K.
2827
in generating sticky hydrogen. Thus, cleaning the surface i n flowing H, (99.9995 %) at 500 K substantially reduces the amount of this adsorbed species whereas deliberate oxidation with air can increase the amount of this tenaciously adsorbed hydrogen. This species may be sub-surface hydrogen 'locked in' by the surface reconstruction resulting on oxidation as proposed by Waugh and others [31.
22Adsorption of formic acid The reactions on adsorption of formic acid on CdAl203 have been followed with time, Figure 2. Initially two resonances of equal intensity are observed at 13 and 7 ppm. As the reaction proceeds these resonances a r e replacedlobscured by broader resonances at 11 and 5 ppm and a very weak Knight shifted peak at 88 ppm grows in. Ultimately an intense resonance at 11 ppm with a shoulder at 5 ppm and a weak resonance at 88 ppm are observed.
x 16
I
80
PPm
40
I
0
Figure 2. 'H NMR spectrum of adsorption of formic acid on Cu/Al,O,.
(b)6 min; (c) 30 min; and (d) 60 min.
(a) 2min;
2828
The adsorption of formic acid on CdSiOz has been investigated by in situ IR by Rochester [4].They reported that formic acid adsorbs on the copper to give bidentate formate species. In a parallel study of the adsorption of methyl formate Rochester reported the intervention of a unidentate species, bonded through the carbonyl oxygen to copper, on route to a bidentate formate [61.A similar sequence can explain our observations- the initial resonances at 13 and 7 ppm being due to the unidentate formic acid which converts with time to a bidentate formate whose proton resonates at 6 ppm. Eventually the resonance of physisorbed formic acid (11ppm) dominates the epectrum.
Catalyst samples, provided by ICI Katalco, were activated at 600 K in flowing H2/N2. Experimental procedures and instrumentation were as described previously [13.
4. ACKNOWLEDGEMENTS
The authors thank Drs. G. Chinchen and N.J. Clayden for helpful discussions and the SERC for financial support.
1. A. Bendada, G. Chinchen, N. Clayden, B.T. Heaton, J.A. Iggo and C.S.
Smith, Catalysis Today, 1991,9,129-136. 2. P.R. Dennison, K.J. Packer and M.S. Spencer, J. Chem. SOC.,Faraduy Trans. I , 1989,86,3537-3560. 3. M. Bowker, R.A. Hadden, H. Houghton, J.N.K. Hyland and K.C. Waugh, J. Catal. 1988,109,263-273and ref0 therein. 4. G.J. Millar, C.H. Rochester and K.C. Waugh, J. Chem. Soc., Faraday Trans.I , 1991,87,1491. 5. G.J. Millar, C.H. Rochester and K.C. Waugh, J. Chem. SOC.,Faraduy Trans. I , 1991,87,2785-2793.
Guczi, L.et al. (Editors), New Frontiers in Caralysb Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.AII rights nselved
ISOMERISATION OF LONG-CHAIN n-ALKANES ON PVH-ZSM-22 AND Pt/H-Y ZEOLITE CATALYSTS AND ON THEIR INTIMATE MIXTURES J. A. Martens': L. Uytterhoeven4 P. A. Jacobsa and G. F. Fromentb
aCentrum voor Oppervlaktechemieen Katalyse, K.U. Leuven, Kard. Mercierlaan 92, 3001 Heverlee, Belgium bLaboratorium vmr Petrochemische Techniek, R.U.G., Krijgslaan 281,9000 Gent, Belgium
Abstract Octane, decane and undecane are converted over bifunctional Pt/H-Y and Pt/HZSM-22 zeolites and on intimate 50-50 and 80-20 wt/wt mixtures of these zeolites. Compared to the individual zeolites, the zeolite mixtures exhibit an enhanced catalytic activity, and produce higher yields of feed isomers. The dependence of the synergetic effects on the chain length of the feed and on the relative catalytic activities of the two zeolites is discussed. 1. INTRODUCTION
The selective conversion of long-chain n-alkanes (octane and larger) into their branched isomers is a desirable conversion in petroleum refining. A drawback of the use of bifunctional zeolite catalysts for the isomerisation of long-chain n-alkanes is their tendency to catalyse hydrocracking at high levels of feed conversion [11. Zeolite ZSM-22 has a particular micropore structure. The micropores are so narrow that intracrystalline branching of n-alkanes is not possible, but occurs at the micropore mouths [2]. In previous work on decane isomerisation [3], a synergetic effect was obtained upon mixing Pt/H-ZSM-22 and Pt/H-Y zeolite catalysts. Compared to the individual zeolites, an intimate mixture of equal weights of bifunctional ZSM-22 and zeolite Y crystals produces enhanced yields of isomers from decane [3]. To achieve this synergism, ZSM-22 has to be more active than zeolite Y, but it is irrelevant whether platinum is present on both zeolite phases, or on only one [3]. On this mixed zeolite catalytic system, decane is dehydrogenated on the platinurn metal. The resulting ndecenes diffuse towards acid sites on both types of zeolite crystals. H-ZSM-22 transforms n-decenes selectively into 2-methylbranched isodecenes. Due to the limited access of branched C10molecules to the micropores of ZSM-22, methylshift, further branching as well as cracking occur mainly on the zeolite Y crystals. The high concentration of 2-methylnonene that is generated on H-ZSM-22 favors the desorption of other isodecenes from the zeolite H-Y catalyst component and prevents their cracking. Finally, all alkenes are hydrogenated on platinum to saturation. In this work we have studied the influence of the n-alkane chain length, and of the relative catalytic activity of the two zeolites on the synergetic effects.
2830 2. EXPERIMENTAL
NH4-ZSM-22 with a Si/Al ratio of 30 [2] was impregnated with Pt(NH3)&12 solution to obtain a Pt loading of 0.5 wt.-%. NH4-Y with Si/Al ratio of 2.8, obtained by steam-dealurnination of Na,NH4-Y [3], was exchanged with Pt(NH3)4CI2 solution to obtain a 0.5 wt.-% Pt loading. Intimate 80/20 and 50/50 wt/wt mixtures of the zeolite powders were obtained by ball-milling. The catalysts were activated by oxidation in 0 followed by reduction in H2 at 673 K.The conversion of n-alkane vapors was performe in a continuous flow reactor over a catalyst bed of 200 mg of dry 0.3-0.5 mm catalyst pellets. The pressure was 0.35 MPa, the HZ/n-alkane ratio was 388 and the space time, W/Fo, 0.5 h kg mol”. The separation of ono- and multibranched isomers and cracked products was carried out on-line by GC on a 50 m capillary CPSil-5 column from Chrompack.
3
9
3. RESULTS AND DISCUSSION
The conversion curves for the n-alkanes are shown in Fig.1. They are steeper on pure Pt/H-Y compared to Pt/H-ZSM-22. At low feed conversion, Pt/H-ZSM-22 is more active than Pt/H-Y, while from medium conversion levels on, the relative catalytic activities are reversed. With increasing chain-length of the feed, the differences in catalytic activity between Pt/H-ZSM-22 and Pt/H-Y vanish. On H-ZSM-22, the reactivity of the n-alkanes increases less with increasing carbon number than on Pt/H-Y. This can be explained by the reduced number of additional reaction pathways on Pt/ZSM-22 with increasing n-alkane chain length, due to the severe sterical hindering in the narrow pore mouths of this zeolite. At high feed conversion levels, mixing of the zeolites in 50/50 as well in 80/20 wt/wt proportions results in a synergetic effect on the catalytic activity. Such effect was not observed in the isomerisation of n-Cl0 over a 50/50 wt/wt mixture containing a significantly more active Pt/H-ZSM-22 phase [3]. The higher activity in the present zeolite mixture can be explained by the comparable activity of both zeolites for n-alkane conversion, and the higher reactivity of isoalkanes compared to n-alkanes on Pt/H-Y, The yield of isomers is plotted against the feed conversion in Fig.2. With all catalysts and up to 30% conversion, the feed is converted quantitatively into isomers. At higher conversions, due to the occurrence of hydrocracking, the isomerisation yield curves do no longer coincide with the diagonal of the figures. The maximum yield of isomers that can be reached with the zeolite mixtures is significantly higher than on the individual zeolites. To obtain the synergism, mixing of the two zeolites in 50/50 and 80/20 wt/wt proportions is equally effective. The yield curves of multibranched isomers from the 3 different n-alkanes are shown in Fig.3. In each instance, mixing of zeolites leads to an important improvement of the maximum yield of multibranched feed isomers. When the zeolites are mixed in a 80/20 wt/wt ratio, the maximum yield of multibranched feed isomers is slightly lower compared to the 50/50 wt/wt mixture, and it occurs at slightly higher feed conversion. An advantage of the zeolite mixtures is that high yields of multibranched feed isomers are obtained at high (>go%) feed conversion. The amount of unconverted n-alkane remaining in the products at optimum catalyst operation conditions with respect to feed isomerisation is low.
2831
460 476 600 626 660 676 800
Tuyrrotm
aC)
Figure 1. Conversion curves of nC n-C and n-C on Pt/H-Y, P9H-Zs#-22, Wd-hM-22 H-Y (50/50) and WH-ZSM-22 H-Y (80/20). (The hatched areaindicatesthesynergetic effect of mixing the zeolites).
+
425 460 476 600 626 650 676 800
Tlcluwrtuo
aC)
100
I
C8
+
c10
I
80
€
60 40 20 100 0 80
-+ WDIY t
60 \
40
Figure 2. Isomerisation yield against feed conversion for the experiments of Fig. 1.
20 0
0
20
40
80
Convrrkn (lb)
80
100
2832
I 1
I
c10
26
L
f
6
80 20
0
40
60
80
100
26
Conroroh (%I
'
16
t ...
Figure 3. Yield of multibranched feed isomers against feed conversion.
L
0
0
20
3
40
60
80
100
ConrrJon 1%)
4. CONCLUSIONS
The mixing of H-ZSM-22 and H-Y zeolites with comparable catalytic activities results in (1) an increased activity at high levels of feed conversion, (2) enhanced yields of total feed isomers and (3) enhanced yields of multibranched feed isomers. The first of these synergetic effects was not observed in previous work on decane isomerisation in which a ZSM-22 catalyst component was used which was substantially more active than the zeolite Y component [3]. Further investigations on these synergetic effects for a broader range of feed carbon numbers and mixed feedstocks are in progress. 5. ACKNOWLEDGMENTS
J.A.M. acknowledges the Belgian N.F.S.R. for a position as Research Associate. This research has been sponsored by the Belgian government within the frame of an IUAP Center of Excellence programme.
6. REFERENCES 1. J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 550. 2. J.A. Martens, R. Parton, L. Uytterhoeven, P.A. Jacobs and G. Froment, Appl. Catal. 76 (1991) 95. 3. R. Parton, L. Uytterhoeven, J.A. Martens and P.A. Jacobs, Appl. Catal. 76 (1991) 131.
Guczi, L Q al. (Editors), New Frontiers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary 0 1993 Elsevier Science Publishers B.V.All rights reserved
KINETICS OF BIMODAL GRAIN SIZE DISTRIBUTION OF A Ni CATALYST DURING HYDROGENATION OF CO M. K o l B M. Agnelli" and C. Mirodatosa aInstitut de Recherches sur la Catalyse, 2 ave A. Einstein, 69626 Villeurbanne Cedex, France bLaboratoire Chimie Theorique, Ecole Normale Superieure, 46 Allee d'Italie, 69364 Lyon Cedex 07, France
Abstract Several basic growth mechanisms are analyzed in order to model the bimodal size distributions observed in catalytically induced sintering processes. It is shown how 1 ) kinetic equations may generate such distributions as a transient phenomenon and 2) how Monte Carlo simulations may lead to broad monomodal distributions.
1.Introduction Sintering is a widely observed phenomenon in catalysis. I t is usually associated either with thermal effects or with chemical reactions. The latter mechanism is at the origin of the experimentally observed changes in particle size distributions for several supported, heterogeneous catalysts. It often leads to an important decrease in There have been many empirical and fundamental studies on sintering and redispersion and on ways to counteract the aging of catalysts. The basic mechanisms, kinetic or thermodynamic, used to model the coarsening are well known and theoretically well understood4. In specific experimental situations more than one mechanisms may be needed to explain the observed coarsening phenomenon. In the present study we investigate the appearance of bimodal size distributions of a Ni catalyst supported on SiO, which develops during methanation of H2 at low temperature (230°C). Similar results were found for other catalysts, but the explanation given here differs from the one proposed previouslyss6. The quantitative analysis of our experiments is based on magnetic measurements and on electron microscopy'. Our samples were prepared by impregnating silica with [Ni(NH&I2+ as described in Ref. 3. Measurements were performed at different times up to % hours. Starting from completely reduced nickel with an average particle size of 4 n m there is only a slight restructuring for the first hour. Beyond three hours, rapidly growing faceted particles appear and grow beyond 50nm during the experiments. Figure 1 shows the corresponding size distribution, parametrized by the time on stream.
2834
10'
1 o-2
1Q - 4
t1 10
Fig.1 Number of Ni particles as a function of size at different stages of the sintering process. The small 'spherical' particles and the large 'faceted' particles are plotted separately, for times up to % hours. The distribution of the initial spherical particles does not change appreciably during this process. EELS measurements suggest that the coarsening mechanism is via atomic transport (probably via the formation of nickel carbonyl) rather than via diffusion and coalescence7. Furthermore they indicate that the atomic diffusion is restricted to the support.
2. Mean field theory
We have investigated theoretically the origins of a bimodal size distribution. Two types of models were considered. In a purely kinetic approach, a Smoluchowski-type equation was considered including a fragmentation term8 . ~ - a&k, c,, + f,,(n+ l ) k n +I - fOnflcn d,c, = qJn- ~ ) ( I C , ,cg
(1)
where c, is the concentration of particles with n atoms, cg the vapor concentration and a$ and ao.fo are parameters. Mass conservation yields an equation for cg. The kernel i n the equation was chosen such that only particle-atom processes but no particle-particle processes are allowed. The classical model of 'Ostwald ripening' as described by Lifshitz-Slyozov and by Wagnerg-I1,is a special case of this model. Both analytical considerations and numerical computations suggest that for a single particle species and an appropriate choice of the initial distribution and the parameters, the system does not destabilize spontaneously and evolve into a bimodal distribution. To model bimodal distributions we are therefore led to consider a) a more complicated dynamics invoking a change in kinetics at some given size or b) coupled equations for two or more types of particles.
2835 Numerical solution for the kinetic model for two coupled species each obeying equation ( l ) , and coupled through a common vapor phase, does lead to a bimodal distribution as a transitory phenomenon between an initial stage dominated by small (spherical) particles and a final stage dominated by the more stable, large (faceted) particles. Figure 2 shows a calculation which models the evolution of the ‘spherical’ towards the ‘faceted’ distribution.
1 o-2
10-6
1 oo
10’
1o2
k
1
o3
1o4
Fig.2: Size distributions C as a function of the number of atoms k for two particle types coupled by the vapor phase. The parameters for both phases are a=2/3,8=-1/3, %,fo=l (‘spherical’ phase), %,fo=10(‘faceted’ phase).
3. Monte Carlo results Monte Carlo simulations of atomic diffusion with modified (enhanced) surface diffusion at the surface of the large particles have been performed for a two dimensional model system. The following features were included to favor the formation of large particles:
1) facilitated evaporation of atoms from irregular particle surfaces 2) enhanced surface diffusion of freshly condensed atoms to favor smoothing of the surface of the larger particles. This process leads to the coexistence of many small and a few large particles - the size distibution becomes broad and develops a long tail. However, it is not a bimodal distribution. The broadening is caused by the anomalously large growth rates of the largest particles. Experimentally it is very difficult to distinguish a bimodal distribution from a monomodal distribution with a long tail, as the number of large grains is orders of magnitudes lower than the number of small grains .
2836 A typical configuration of the later stages during this enhanced diffusion process is shown in Figure 3.
P
l
i
p
Fig.3: Typical late stage configuration for two dimensional diffusion with strong attractive interactions between the atoms and enhanced surface diffusion. The latter also leads to a smoothing (faceting) of the grain surface for large grains. Conclusions Simple coarsening mechanisms based on atomic transport do not reproduce the experimentally observed bimodal size distributions. Introducing two species with different surface properties does explain the appearance of a bimodal size distribution. It is a transitory phenomenon that eventually leads to the complete disappearance of the small particles. Monte Carlo results show that the apparently bimodal distribution may be a broad monomodal distribution caused by an anomalously high growth rate of the large particles. References 1 B.Pulvermacher and E.Ruckenstein, J.Cata1. 35, 115 (1974) 2 P.C.Plynn and S.E.Wanke, J.Catal. 37,432 (1975) 3 C.Mirodatos, H.Praliaud and M.Primet, J.Catal. 107,275 (1987) 4 D.B.Dadyburjor, in ‘Catalyst Deactivation 1987’, B.Delmon and G.F.Froment (eds.), Elsevier, Amsterdam 1987 5 N.L.Wu and J.Phillips, J.Catal. 113, 129 (1988) 6 A.Bellare, D.B.Dadybujor and M.J.Kelley, J.Catal. 1 17, 78 (1989) 7 M.Agnelli, C.Nicot and C.Mirodatos, to be published P.G.J.van Dongen and M.H.Ernst, J.Stat.Phys. 37,301 ( 1984) 8 9 W.Ostwald, Z.Phys.Chem. 37,385 (1901) 10 I.M.Lifshitz and V.V.Slyozov, J.Phys.Chem.Solids 19, 35 ( 1%1) 11 C.Wagner, Z.Elektrochemie 65,581 (1961)
Guczi, L et d.(Editors),New Frontiers in Cufalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 0 1993 Elsevier Science Publishers B.V. All rights reserved
CRITICAL POINT PROPERTIES OF THE SURFACE STRUCTURE DURING CO OXIDATION
M.Kolb Institut de Recherches sur la Catalyse, 2 ave A. Eisntein, 69626 Villeurbanne Cedex, France and Laboratoire Chimie Theorique, Ecole Normale Superieure, 46 Allee d'Italie, 69364 Lyon Cedex 07, France
Abstract A geometrical model which captures the basic features of CO oxidation on a planar surface has been investigated in the bistable regime between low and high CO surface coverage. For a specific value of pressure and temperature one observes properties that are similar to those found at second order phase transitions in equilibrium systems: long range spatial correlations, divergence of thermodynamic quantities and slow relaxation times.
1.Introduction Experimental observations and model calculations have demonstrated that for many catalytic reactions - and notably for the oxidation of CO on Pt - the reaction rates and the surface coverages there is a rich variety of phenomena such as bistability and oscillations, bifurcation schemes and chaos, solitary and spiraling wavesl-11. Modeling based on the elementary reaction steps has been successful, at least for some of the experiments. Here we investigate a modified version of an irreversible geometrical model which describes the surface structure of a catalyst for a Langmuir-Hinschelwood monomer-dimer process.
2. Model The irreversible model is defined as followss: 1) Each site of a flat surface is either empty, occupied by a CO or occupied by an 0, 2) Gaseous CO ( 0 2 )adsorbs (adsorbddissociates) randomly on a single (two nearest neighbor) surface site(s), 3) Any neighboring CO and 0 reactldesorb instantaneously. The sole parameter of the model is the partial pressure of the gaseous CO (pco) resp. 0, (po=I-pc,). For large pco, a transition towards a CO poisoned state is predicted. A low to high CO surface coverage transition is also seen experimentally.
2838 In order to remove the artifact of total CO poisoning, and to introduce a parameter that allows to search for a critical point, another step is included in the reaction: 4) CO desorbs randomly, with desorption rate dco.
3. Monte Carlo results Monte Carlo simulations were performed for pco close to the poisoning transition of the irreversible model and for small desorption rates dco. Figure 1 shows the CO, production, the oxygen coverage and the CO pressure pco vs. the CO coverage. Note that the CO2 production is largest for dco>O, for pco in the range where the surface would poison without desorption.
-
DCO
-
c OXY coz
-0
N
0 0
0
0.2
0.4
co
0.8
v.6
1
Fig. 1: Desorption rate (dco) (x), oxygen coverage (+) and C 0 2 production (A) at steady state for a CO partial pressure ~ ~ ~ ' 0 . 6 .
0 0
0.52
0.53
0.54
0.55
0.56
0.57
0.58
p,
Fig. 2: Phase diagram: CO coverage vs. CO partial pressure pco, with dco (.Ol
