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Studies in Surface Science and Catalysis 130 Part A
12th INTERNATIONAL CONGRESS ON CATALYSIS
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates
VOl.
130
12th INTERNATIONAL CONGRESS ON CATALYSIS PART A Proceedings of the 12th ICC, Granada, Spain, July 9-14,2000 Editors
Avelino Corma Francisco V. Melo lnstituto de Tecnologia Quimica, UPV-CSIC,Avda. de 10s Naranjos s/n, 46022 - Valencia, Spain
Sagrario Mendioroz Jose Luis G. Fierro lnstituto de CatAlisis y Petroleoquimica, CSlC, Campus UAM Cantoblanco, 28049- Madrid, Spain
2000 ELSEVIER
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Amsterdam Lausanne New York Oxford Shannon Singapore -Tokyo
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ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 2 1 1, I000 A E Amsterdam, The Netherlands
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2000 Elsevier Science B.V. All rights resewed.
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First edition 2000
Library of Congress Cataloging-in-PublicationData International Congress on Catalysis (12th :2000 : Granada,Spain) 12th International Congress on Catalysis : proceedings of the 12th ICC, Granada, Spain, July 9-14,2000 1editors Avelino Corma ... [et al.1.-- 1st ed. p. m. Includes index. ISBN 0444-50480-X (pt. A) 1. Catalysis--Congresses. I. Corma, Avelino, 1951- 11. Title. QDSOS .I57 2000 541.3'95--dc21
ISBN:
0 444 50480 X
@ The paper used in this publication meets the requirements of ANSIiNISO 239.48-1992 (Permanence of Paper). Printed in The Netherlands.
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PREFACE The twelfth Congress on Catalysis was held in Granada (Spain) under the auspices of the International Association of Catalysis Societies and the Spanish Society of Catalysis. Those Proceedings are the expression of the Scientific Sessions which constituted the main body of the Congress.
They include 5 plenary lectures, 1 award lecture, 8 keynote lectures, 124 oral presentations and 495 posters. The oral and poster contributions have been selected on the basis of the reports of at least two international reviewers, according to standards comparable to those used for specialised journals, among 1045 two-page abstracts received from 53 countries. The submitted camera-ready manuscripts were then evaluated by the International Scientific Board. Fortunately, most of the corrected manuscripts were received in due course and have been included as such in the Proceedings; however, in a few exceptions, no answer was obtained from the authors; in those cases, a first version of the manuscript appears in the Proceedings. In order to accommodate all these contributions, the Congress was divided in four parallel sessions and three additional sessions in which all the posters were displayed. The management of this fantastic volume of work forced us to take several decisions. As the contributions are published prior to the meeting for distribution to all delegates who attend the Congress in Granada, no discussions at the meeting have been included. Besides this, for space reasons we were restricted to expand the works to only six-page text.
Financial contribution from the Ministry of Education and Culture, the National Council of Scientific Research, other local and national institutions or corporations, chemical, refining and petrochemical companies made it possible to balance the budget of the Congress. Allowance for young students to pay a reduced registration fee was also possible from this income.
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We are grateful to the outstanding scientists, expert in different fields of catalysis, who
accepted our invitation to overview vital research areas in plenary lectures, the 1998 awardee by his illustrative conference and the keynote lecturers that introduce the various topics of the sessions covered by the Congress. The Organisers are indebted to all the scientists who accepted our invitation to come to Spain and made this meeting another outstanding success in the 44 year tradition of this event. We hope everybody will enjoy the meeting and will find these Proceedings a useful book to be added to the catalysis library. We are also grateful to Drs. A. Jongejan, managing director, Dr. P.S. Jackson, publishing director, and specially to Drs. Huub Manten of Elsevier Science Publishers for the guidance and co-operation provided in getting these four volumes printed before the Congress.
Granada, July 2000 The Editors
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LIST OF SPONSORS (by May 17, 2000)
PA TR ONS 1- Ministerio de Ciencia y Tecnologia (Spain) 2- Junta de Andalucia (Spain) 3- Ayuntamiento de Granada (Spain) 4- Consejo Superior de Investigaciones Cientificas (Spain) 5- Universidad de Granada (Spain)
DONORS
_
CEPSA (Spain)
7- REPSOL- YPF (Spain) 8- UOP (USA) 9- DEGUSSA HILLS AG (Germany) 10- PROCATALYSE (France) 11- INSTITUT FRAN(~AIS DU PETROLE (France) 12- ENGELHARD (The Netherlands) 13- NOVARTIS AG (Switzerland) 14-SHELL International Chemicals B.V.A. (The Netherlands) 15-BP AMOCO Chemicals Co. (UK) 16- CHEVRON (USA) 17- EXXON-MOBIL Research and Engineering (USA) 18- GRACE Co. (USA) 19- DSM Research (The Netherlands) 20- SI]D-CHEMIE Inc. (USA)
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EXHIBITORS 21-HALDOR TOPSOE (Denmark) 22-SE Reactor, Inc. (USA) 23- ENGELHARD (Italy) 24-ACADEMIC PRESS, Inc. (USA) 25-VINDUM ENGINEERING, Inc. (USA) 2 6 - P S R - SOTELEM (The Netherlands) 27-SPRINGER-VERLAG Ib&ica S.A. (Spain) 28-CHAMBERS HISPANIA S.L. (Spain) 29-JOHNSON MATTHEY (USA) 30-GENERAL ELECTRIC Plastics S.A. (Spain) 31- HIDEN ANALYTICAL (UK) 32-KAISER Optical Instruments Industries. (France) 33-THERMO QUEST- CE Instruments (Italy) 34-AIR LIQUIDE S.A. (Spain) 35-IBERFLUID Instruments S.A. (Spain) 36-ARGONAUT Technologies AG (Switzerland) 37- IN-SITU Research Instruments (USA) 38-VINCI Technologies (France) 39-MEL Chemicals (UK) 40- ISCOA Inc. (USA) 41- CRI Katalema (UK) 42- PARR Instrument (Germany) 43-ELSEVIER SCIENCE (The Netherlands)
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TABLE OF CONTENTS
PLENARY LECTURES
PL-1. In Situ Characterization of Catalysts H. Topsere .
1
PL-2. Pollution Abatement through Heterogeneous Catalysisfor Preserving Clean Air M.Iwamoto .
23
PL-3. Molecular Design of Heterogeneous Catalysts M.E. Davis .
49
PL-4. Millisecond Chemical Reactions and Reactors L.D. Schmidt .
61
PL-5. Catalysisfor Oil Refining and Petrochemistry, Recent Developments and Future Trends G. Martino .
83
AWARD CONFERENCE New Catalysisfiom Metal Oxide Surface Science M. A. Barteau .
105
KEYNOTE LECTURES KN- 1 From Unit Operations to Elementary Processes: A Molecular and Multidisciplinary Approach to Catalyst Preparation M. Che
115
KN-2 Acidity in Zeolite Catalysis R.A. van Santen and F.J.M.M. de Gauw.
127
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KN-3 Engineered Solid Catalysisfor Synthesis of Fine Chemicals in Multiphasic Reactant System D.E. De Vos, B.F. Sels, W.M. Van Rhijn and P.A. Jacobs .
137
KN-4 New Solid Acid Based Breakthrough Technologies S.A. Gembicki
147
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KN-5 Application of Titanium Oxide Photocatalysts and Unique Second-Generation Ti02 Photocatalysts able to Operate under Visible Light Irradiationfor the Reduction of Environmental Toxins on a Global Scale M.Anpo .
157
KN-6 Catalysis for Fine Chemicals: an Industrial Perspective P. Mktivier .
167
KN-7 Oxidative Methanol Reforming Reactionsfor the Production of Hydrogen J.L.G. Fierro .
177
KN-8 Supported Metallocenes: Monosite and Multisite Catalystsfor OIefn Polimerization . F. Ciardelli, A. Altomare and S. Bronco
187
ORAL COMMUNICATIONS Session A Preparation of Catalysts
A0 1
A02
A03 A04
A05
A New Method for Preparing Nanometer-size Perovskitic Catalystsfor CH, Flameless Combustion R.A.M. Giacomuzzi, M . Portinari, I. Rossetti and L. Forni .
197
Experimental Investigation and Modelling of Platinum Adsorption onto Ion-modified Silica and Alumina . W. Spieker, J. Regalbuto, D. Rende, M. Bricker and Q. Chen
203
Nanocrystalline Thin Films as a Model System for Sulfated Zirconia . F.C. Jentoff, A. Fischer, G. Weinberg, U. Wild and R. Schlogl
209
Nanoparticle Arrays as Model Catalysts: Microstructure, Thermal Stability and Reactivity of Pr/SiO2 Fabricated by Electron Beam Lithography . G. Rupprechter, A.S. Eppler, A. Avoyan and G.A. Somorjai
215
Epoxidation of Ole$ns on M-SiO2 (M=Ti, Fe, Catalysts with Highly Isolated Transition Metal Ions Prepared by Ion Beam Implantation Q. Yang, C. Li, S. Wang, J. Lu, P. Ying, Q. Xin, W. Shi .
22 1
Catalysis on Metals and Metal Oxides
A06
Cyclohexane Ring Opening on Metal-Oxide Catalysts L.M. Kustov, T.V. Vasina, O.V. Masloboishchikova, . E.G. Khelkovskaya-Sergeeva and P. Zeuthen
227
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A07
A08
A09
xi OleJins*om Chlorocarbons: Reactions of I , 2-Dichloroethane Catalyzed by Pt-Cu L. Vadlamannati, D. Luebke, V. Kovalchuk and J.L. d’Itri .
233
Artijkial Control of Selectivity by Dynamic Lattice Displacement of Acoustic Wave Effects: Decomposition and Oxidation of Alcohol on Ag and Pd catalysts . N. Saito, H . Nishiyama, K. Sat0 and Y. Inoue
239
The Effect of Hydrogen Concentration on Propyne Hydrogenation over a Carbon Supported Palladium Catalyst Studied under Continuous Flow Conditions D. Lennon, R. Marshall, G. Webb and S.D. Jackson
245
Refining and Petrochemistry
A10
A1 1 A12 A13
A14 A15
A16 A17
Suggestion of a New Alternative Mechanism of the Sulfuric Acid Catalyzed Isoparafln-OleJn Alkylation V.B. Kazansky and T.V. Vasina .
25 1
Hydroisomerization of n-Decane in the Presence of Sulfur and Nitrogen L.B. Galperin .
257
State of Metals in the Supported Bimetallic Pt-Pd/SOt--ZrOz System . A.V. Ivanov, A.Yu. Stakheev and L.M. Kustov
263
Dehydroisomerization of n-Butane over Pt Promoted Ga-Substituted Silico-aluminophosphates A. Vieira, M.A. Tovar, C. Pfaff, P. Betancourt, B. Mtndez, C.M. Lbpez, . F.J. Machado, J. Goldwasser and M.M. Ramirez de Agudelo
269
Pore Mouth Catalysis over Acidic Zeolites. Nature of Active Species P. Magnoux, M. Guisnet and I. Ferino .
275
Rediscovery of the Paring Reaction: The Conversion of l,Z,CTrimethylBenzene over HZSMS at Elevated Temperature H.P. Roger, M. Bohringer, K.P. Moller and C.T. O’Connor
28 1
SAPO-34 Catalystfor Dimethylether Production Gr. Pop and C. Theodorescu .
287
Catalytic Performances of Pillared Beidellites Compared to Ultrastable Y Zeolites in Hydrocracking and Hydroisornerization Reactions J.A. Martens, E. Benazzi, J. Brendlt, S. Lacombe and R. le Dred .
293
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A18
A19
A20
A2 1
A22
A23
A24
A25
A26
A27 A28
Catalyst Characterisation Effect of Ni on K-Doped Molybdenum-on-Carbon Catalysts: TemperatureProgrammed Reduction and Reactivity to Higher-Alcohol Formation E.L. Kugler, L. Feng, X. Li and D.B. Dadybujor .
299
Quantitative Determination of the Number of Active Surface Sites and the Turnover Frequenciesfor Methanol Oxidation over Metal Oxide Catalysts L.E. Briand and I.E. Wachs .
305
Metal Particles on Oxide Surfaces: Structure and Adsorption Behaviour M. Baumer, M. Frank, P. Kiihnemuth, M. Heemeier, S. Stempel and H.-J. Freund .
31 1
An Atomic XIFS Study of the Metal-Support Interaction in Pt/Si02-A1203 and Pt/MgO-A1203 Catalysts: An Increase in Ionization Potential of Platinum with Increasing Electronegativity of the Support Oxygen Ions D.C. Konigsberger, M.K. Oudenhuijzen, D.E. Ramaker and J.T. Miller .
317
Transition State and Difision Controlled Selectivity in Skeletal Isomerization of Olefns L. Domokos, M.C. Paganini, F. Meunier, K. Seshan and J.A. Lercher
323
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Development and Application of 3-Dimensional Transmission Electron Microscopy (3D-TEM)for the Characterisation of Metal-Zeolite Catalyst Systems A.J. Koster, U . Ziese, A.J. Verkleij, A.H. Janssen, J. de Graaf, J. W. Geus . and K.P. de Jong
329
Interrogative Kinetic Characterisation of Active Catalyst Sites Using TAP Pulse Experiment J.T. Gleaves, G.S. Yablonsky, S.O. Shekhtman and P. Phanawadee .
335
W Resonance Raman Spectroscopic Identijcation of Transition Metal Ions Incorporated in the Framework of Molecular Sieves G. Xiong, C. Li, Z. Feng, J. Li, P. Ying, H. Li and Q. Xin .
34 1
Surface Mobility of Oxygen Species on Mixed-Oxides Supported Metals . C. Descorme, Y. Madier, D. Duprez and T. Birchem
347
SO*-Promoted Propane Oxidation over Pt/A1203 Catalysts A.F. Lee, K. Wilson and R.M. Lambert .
353
Selective Oxidation of Toluene to Benzaldehyde: Investigation of StructureReactivity Relationships by in situ-Methods A. Brtickner, U . Bentrup, A. Martin, J. Radnik, L. Wilde and G.-U. Wolf
359
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A29
A30
A3 1
xiii
Observation of Unstable Reaction Intermediate by Picosecond Tunable Inpared Laser Pulses K. Domen, K. Kusafika, A. Bandara, M. Hara, J.N. Kondo, J. Kubota, K. Onda, A. Wada and C. Hirose .
365
Changes in Morphology of y-AI203-SupportedPt Clusters under Reaction Conditions: Evidence@om In-Situ EXAFS Spectroscopy 0.Alexeev and B.C. Gates .
371
In Situ FT-IR Investigation of Hydrocarbon Reactions over Zeolite Based Bifunctional Catalysts M. Hochtl, Ch. Kleber, A. Jentys and H. Vinek .
377
Session B Fischer-Tropsch Synthesis
B01
B02
B03
A Steady State Isotopic Transient Kinetic Analysis of the Fischer-Tropsch Synthesis Reaction over a Cobalt-Based Catalyst . H.A.J. van Dikj, J.H.B.J. Hoebink and J.C. Schouten
383
Use of Membranes in Fischer-Tropsch Reactors R.L. Espinoza, E. du Toit, J. Santamaria, M. MenCndez, J. Coronas and S. Irusta .
389
Egg-Shell Catalystfor the Synthesis of Middle Distillates C . Galarraga, E. Peluso and H. de Lasa .
395
Application of Theoretical Methods to Catalysts
B04
B05
Computer Aided Design of Novel Heterogeneous Catalysts A Combinatorial Computational Chemistry Approach K. Yajima, Y. Ueda, H. Tsuruya, T. Kanougi, Y. Oumi, S.S.C. Ammal, S. Takami, M. Kubo and A. Miyamoto .
40 1
Applications of Densily Functional Theory to IdentrfL Reaction Pathways for Processes Occuring in Zeolites and on Dispersed Metal Oxides . J.A. Ryder, M.J. Rice, F. Gilardoni, A.K. Chakraborty and A.T. Bell
407
Fuel Cells and Electrocatalysis
B06 B07
Electrocatalytic Synthesis of Ammonia at Atmospheric Pressure . G. Marnellos, G. Karagiannakis, S. Zisekas and M. Stoukides
413
Electrocatalysis at a Pt Electrode Surface in a Fuel Cell as Observed In Situpom Pt-H and Pt-0 Shape Resonances and E U F S Scattering in Pt Lz,3 XANES D.E. M a k e r and W.E. O’Grady .
419
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B08
In situ Catalytic and Electrocatalytic Studies of Internal Reforming in Solid Oxide Fuel Cells Running on Natural Gas C.M. Finnerty, R.H. Cunningham and R.M. Ormerod
B09
.
Efectof Gd203Doping and Steadcarbon Ratio on the Activity of Catalystfor Internal Steam Reforming in Molten Carbonate Fuel Cell Y.J. Shin, H.-D. Moon, T.-H. Lim and H.-I. Lee .
425
43 1
Reactor Engineering and Catalytic Processes
B10
B11 B12
B13 B14
A Microstructured Catalytic ReactodHeat Exchangerfor the Controlled Catalytic Reaction between H2 and 0 2 M. Janicke, A. Holzwarth, M. Fichtner, K. Schubert and F. Schuth
437
Catalytic Water Denitrification in Membrane Reactor O.M. Ilinitch, F.P. Cuperus, L.V. Nosova and E.N. Gribov .
443
Reactivity and Thermal ProJle of Metahne Partial Oxidation ar Very Short Residence Time F . Basile, G. Fomasari, F. TrifirC, and A. Vaccari .
449
Supercritical Synthesis of Dimethyl Carbonate@om CO2 and Methanol T. Zhao, Y. Han and Y. Sun .
455
A New Catalystfor an Old Process Driven by Environmental Issues L. Abrams, W.V. Cicha, L.E. Manzer and S. Subramoney .
46 1
Catalysis on Sulfides, Nitrides and Carbides
B15 B16
B17
B18
Mechanism of HDN over Mo and Nb-Mo Carbide Catalysts V. Schwartz, V.L. Teixeira da Silva, J.G. Chen and S.T. Oyama .
467
In Situ Characterisation of Transition Metal Surfde Catalysts by IR Probe Molecules Adsorption and Model Reactions G. Berhault, M. Lacroix, M. Breysse, F. MaugC and J.-C. Lavalley
473
Comprehension of the Promoting EfSect in the MCrzSd Mixed Surfde Catalysts . P. Afanasiev, A. Thiollier, P. Delichere and M. Vrinat
479
Potassium at Catalytic Surfaces - Stability, Electronic Promotion and Excitation A. Kotarba, G. Adamski, Z. Sojka, S. Witowski and G. Djega-Mariadassou
485
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B19
B20
B2 1
B22
B23
B24
B25
B26
B27
B28
B29
xv
Catalysis for Fine Chemical Synthesis Heterogeneous Catalysis of Aldolisations on Activated Hydrotalcites J. Lhpez, R. Jacquot and F. Figueras .
49 1
The Influence of Metal-Support Interactions During Liquid-Phase Hydrogenation of an GPUnsaturated Aldehyde over Pt U.K. Singh and M.A. Vannice .
497
Tailoring of Acido-basic Properties and Metallic Function in Catalysts Obtainedfrom LDHs for the Hydrogenation of Nitriles and of GpUnsaturated Aldehydes D. Tichit, B. Coq, S. Ribet, R. Durand and F. Medina .
503
Chemical versus Enzymatic Catalysisfor the Regioselective Synthesis of Sucrose Esters of Fatty Acids M. Ferrer, M.A. Cruces, F.J. Plou, E. Pastor, G. Fuentes, M. Bernabe, J.L. Parra and A. Ballesteros .
509
The Higly Selective Conversion of Toluene into 4-Nitrotoluene and 2,4Dinitrotoluene Using Zeolite H-Beta D. Vassena, A. Kogelbauer and R. Prins .
515
Catalytic Asymmetric Heterogeneous Aziridination and Epoxidation of Alkenes using ModiJied Microporous and Mesoporous Materials G.J. Hutchings, C. Langham, P. Piaggio, S. Taylor, P. McMorn, D.J. Willock, D. Bethell, P.C. Bulman Page, C . Sly, F. Hancock and F. King
521
Catalytic Hydrogenation of Nitriles to prim., sec. and tert. Amines over Supported Mono- and Bimetallic Catalysts Y.-Y. Huang and W.M.H. Sachtler .
527
Alkali Promoted Regio-Selective Hydrogenation of Styrene Oxide to Phenethyl Alcohol C.V. Rode. M.M. Telkar and R.V. Chaudhari
533
Production of Fatty Alcohols by Heterogeneous Catalysis at Supercritical Single-Phase Conditions S . van den Hark, M. Harrod and P. M ~ l l e r .
539
Regioselective Oxidation of Primary Hydroxyl Groups of Sugar and its Derivatives Using a New Catalytic System Mediated by TEMPO H. Kochkar, M. Morawietz and W.F. Holderich .
545
Aspects of Regioselective Control in the Hydroformylation of Methyl Methacrylate with the in situ Formed (0-Thiomethylphenyu diphenylphosphine Rhodium Complex H.K. Reinius, R.H. Laitinen, A.O.I. Krause and J.T. Pursiainen .
55 1
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mi
B30
B3 1
Highly Eficient Synthesis of N,N-Dialky~ormamides@omCarbon Dioxide and DialRylamines over Ruthenium-Silica Hybrid Gels, L. Schmid and A. Baiker .
557
Metal Complex Catalyzed Functionalization of Naturally Occurring Monoterpenes: Oxidation, Hydroformylation, Alkoxicarbonylation E.V. Gusevskaya, E.N. dos Santos, R. Augusti, A.O. Dias, P.A. Robles-Dutenheher, C.M. Foca and H.J.V. Barros .
563
Session C Environmental Catalysis. Combustion VOCs
co1
Ahocat: AhorptiodCatalytic Combustionfor VOC and Odour Control . E. Kullavanijaya, D.L. T r i m and N. W. Cant
569
c02
Structure Sensitivity of the Hydrocarbon Combustion Reaction over Alumina-Supported Platinum Catalysts T.F. Garetto and C.R. Apesteguia .
575
Ceria-Zirconia-SupportedPlatinum Catalystfor Hydrocarbons Combustion: Low-Temperature Activity, Deactivation and Regeneration . C. Bozo, E. Garbowski, N. Guilhaume and M. Primet
581
Characterisation of a y-MnO2 Catalyst Used in VOC Abatement C. Lahousse, C. Cellier, B. Delmon and P. Grange .
587
New AluminaiAluminium Monolithsfor the Catalytic Elimination of vocs N . Burgos, M. Paulis, A. Gil, L.M. Gandia and M. Montes.
593
Hydrotalcite-Derived Catalystsfor Removal of Nitrogen-Containing Volatil Organic Compounds J. Haber, K. Bahranowski, J. Janas, R. Janik, T. Machej, L. Matachowski, . A. Michalik, H. Sadowska and E.M. Senvicka
599
C03
C04 C05
C06
Environmental Catalysis. NO,
C07
C08
Surface Catalytic Reactions Assisted by Gas Phase Molecules on Supported Co-ensemble Catalysts A. Yamaguchi, T . Shido, K. Asakura and Y. Iwasawa .
605
Fresh and Used V205-W O g i 0 2 SCR EUROCAT Standard Catalyst: An European Collaborative Characterization J.C.Vedrine .
61 1
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C09 c10
c11
c12 C13
C14
C15
xvii
Lean NO, Reduction over Snl,Zr,O2 Solid Solutions J. Ma, Y. Zhu, J. Wei, X. Cai and Y. Xie .
617
Concentration Programmed Aakorption-Desorption / Surface Reaction Study of the SCR-DeNOx Reaction I . Nova, L. Lietti, E. Tronconi and P. Forzatti .
623
Bifunctional Nature of Sn02/y-A1203 Catalysts in the Selective Reduction of NO, . A. Yezerets, Y. Zheng, P.W. Park, M.C. Kung and H.H. Kung
629
SO2 Resistant Fe/ZSM-.5 Catalystfor the Conversion of Nitrogen Oxides G. Centi, G. Grasso, F. Vanzzana and F. Arena .
635
Mechanistic Studies of the NO, Reduction by Hydrocarbons in Oxidative Atmosphere S. Schneider, S. angler, P. Girard, G. Maire, F. Garin and D. Bazin .
64 1
NO Reduction in Presence of Methane and Ethanol on Pd-Mo/A1203 Catalysts L.F. De Mello, M.A.S. Baldanza, F.B. Noronha and M. Schmal .
647
Fe- Vanadyl Phosphates/TiO2 as SCR Catalysts G. Bagnasco, P. Gilli, M.A. Larmbia, M.A. Massucci, P. Patrono, G. Ramis and M. Turco .
653
Photochemistry
C16 C17
C18
Photoinduced Non-Oxidative Methane Coupling over Silica-Alumina H. Yoshida, Y . Kato and T. Hattori .
659
Photocatalytic Oxidation of Gaseous Toluene on Polycrystalline Ti02: FT-IR Investigation of Surface Reactivity of Diferent Types of Catalysts G. Martra, V. Augugliaro, S. Coluccia, E. Garcia-Lbpez, V. Loddo, . L. Marchese, L. Palmisano and M. Schiavello
665
Investigation of Environmental Photocatalysis by Solid-state NMR Spectroscopy D. Raflery, S . Pilkenton, C.V. Rice, A. Pradhan, M. Macnaughtan, S . Klosek and T. Hou .
67 1
C1 Chemistry
C19
A Study of CHd Reforming by C02 and H20 on Ceria-Supported Pd S . Sharma, S . Hilaire and R.J. Gorte .
677
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xviii
c20
c 21
c22
C23
C24
C25
C26
Catalytic Behaviour on Ni Containing Catalysts in Vaporeforming of Methane with Low HzO/CHd Ratio and Free Carbon Deposition H . Provendier, C. Petit and A. Kiennemann .
683
Carbon Deposition and Reaction Steps in CO2/CH4 Reforming over Ni-LazOd5A Catalyst J.Z. Luo, L.Z. Gao, Z.L. Yu and C.T. Au .
689
The Autothermal Partial Oxidation Kinetics of Methanol to Produce Hydrogen E. Newson, P. Mizsey, T. Truong and P. Hottinger .
695
New Reaction Mechanismfor Methane Formation in CO Hydrogenation over PdCeOr S. Naito, S . Aida and T. Miyao .
70 1
Methane Coupling over SrCo03-Based Perovskites in the Absence of Gas-Phase Oxygen Yu.1. Pyatnitsky, N.I. Ilchenko, L.Yu. Dolgikh and N.V. Pavlenko
707
Novel Conception ofthe Methanol Synthesis Mechanism on Conventional Cu-Containing Catalysts G.I. Lin, K.P. Kotyaev and A.Ya. Rozovskii
713
“Real and “Inverse’’ Model Catalystsfor Studies of Metal-Support Interactions: CO Hydrogenation on Titania and Vanadia Supported Rh W . Reichl and K. Hayek .
719
”
Environmental Catalysis. Catalysis for Clean Processes and Fuels
C27
C28
C29 C30 C3 1
Agglomeration of Pt Particles and Potential Commercial Application for Selective Hydrodechlorination of CCId Z.C. Zhang and B.C. Beard . Catalytic Diesel Soot Elimination on Co-WLa203 Catalysts: Reaction Mechanism and the Effect of NO Addition E.E. Miro, F . Ravelli, M.A. Ulla, L.M. Cornaglia and C.A. Querini
725
.
73 1
Environmentally Benign Carbonylation of Nitrobenzene and Aniline S.S.C. Chuang, M.V. Konduru, Y. Chi and P. Toochinda .
737
Nitrous Oxide - Waste to Value A.K.Uria.rte .
743
Catalytic Wet Peroxide Oxidation over Mixed (AI-Fe) Pillared Clays J. Barrault, C. Bouchoule, J.-M. Tatibouet, M. Abdellaoui, A. MajestC, I. Louloudi, N. Papayannakos and N.H. Gangas .
749
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Session D Selective Oxidation (ODH)
DO 1
DO2
DO3
DO4
DO5
DO6
A new Iron Phosphate Crystalline Phase and its Catalytic Activity in Oxidative Dehydrogenation of Lactic Acid and Glycolic Acid M . Ai and K. Ohdan .
755
Dynamic of the Oxidative Dehydrogenation of Propane over VMgO Catalysts Studied by In Situ Electrical Conductivity and Step Transients H.W. Zanthoff, J.C. Jalibert, Y. Schuurman, P. Slama, J.M. Herrmann . and C. Mirodatos
76 1
EfSect of Potassium on the Structure and Reactivity of Vanadium Species in VO/Al,O3 Catalysts P. Concepcion, S . Kuba, H. Knozinger, B. Solsona and J.M. Lopez Nieto .
767
Strategies for Combining Light Parafin Dehydrogenation (OH) with Selective Hydrogen Combustion (SHC) R.K. Grasselli, D.L. Stem and J.G. Tsikoyiannis .
773
Oxidative Dehydrogenation over Promoted Chromia Catalysts at Short Contact Times D.W. Flick and M.C. Huff .
779
Propane Oxidative Dehydrogenation by Continuous and Periodic Operating Flow Reactor with a Nickel Molybdate Catalyst C . Mazzocchia, A. Kaddouri, E. Tempesti and R. del Rosso
785
Selective Oxidation (General Papers) DO7
DO8
DO9
D10
A Novel Catalyst of Copper Hydroxyphosphate (Cu2(OH)PO,) with High Activity in Hydroxylation of Phenol by Hydrogen Peroxide F . 4 . Xiao, J. Sun, R. Yu, X. Meng, H. Yuan, D. Jiang and R. Xu .
79 1
Oxidation of Alkanes with Hydrogen Peroxide Catalyzed by di-lronSubstituted Inorganic Synzyme N. M i m o , Y. Nishiyama, I. Kiyoto and M. Misono
797
Vanadyl-Aluminum Binary Phosphate: Eflect of Thermal Treatment over its Structure and Catalytic Properties in Selective Oxidation of Aromatic Hydrocarbons F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero and M.T. Siles .
803
Direct Synthesis of Phenol ffom Benzene with 0 2 over VMo-Oxide/SiO2 Catalyst I . Yamanaka, M. Katagiri, S. Takenaka and K. Otsuda .
809
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D11
D12
D13 D14
Eu-Ti-Pt-Catalytic System for Direct Hydroxylation of Benzene by 0 2 and H2 under Mild Conditions . I . Yamanaka, T. Nabeta, S. Takenaka and K. Otsuka
815
High Yield Butane to Maleic Anhydride Direct Oxidation on New Supported Catalysts M.J. Ledoux, V. Turines, C. Crouzet, K. Kourtakis, P.L. Mills and J.J. Lerou
82 1
Propylene Epoxidation over Gold-Titania Catalysts E.E. Stangland, K.B. Stavens, R.P. Andres and W.N. Delgass
.
827
Vapor-Phase Epoxidation of Propene using HZand 0 2 over AdTiMCM-41 and Au/Ti-MCM-48 B.S. Uphade, M. Okumura, N. Yamada, S. Tsubota and M. Haruta
833
Molecular Sieves
D15
D16
D17
D18
D19 D20
D2 1 D22
Ring-Opening Reactions of Ethyl- and Vinyloxirane on HZSM-5 and Cu-ZSM-5 Catalysts A. Fasi, I . Palink6 and I. Kiricsi .
839
Controlling the Distribution of Framework Aluminum in High-Silica Zeolites . D.F. Shank, R.F. Lobo, C. Fild and H. Koller
845
Toluene Disproportionation Catalysis Using EUO Type Zeolites: Initial Optimisation and Development J.L. Casci and A. Stewart .
85 1
Influence of the Aluminium Content on the Acidity and Catalytic Activity of MTW-Type Zeolites A. Katovic, B.H. Chiche, F. di Renzo, G. Giordano and F. Fajula .
857
Synthesis of Ethylbenzenefiom 1,3-Butadiene Using Basic Zeolite Catalysts J. Ackermann, E. Klemm and G. Emig .
863
Activity/Selectivity and Ligation of the Co Ions in Zeolites in Ammoxidation of Ethane to Acetonitrile B. Wichterlova, Z. Sobalik, Y. Li and J.N. Armor .
869
Room-Temperature Oxidation of Hydrocarbons over FeZSM-5 Zeolite M.A. Rodkin, V.I. Sobolev, K.A. Dubkov, N.H. Watkins and G.I. Panov Selective Oxidation of Hydrocarbons by Active Oxygen Formedfiom NzO on ZSM-5 at Moderate Temperatures S.N. Vereshchagin, N.P. Kirik, N.N. Shishkina, S.V. Morozov, . A.I. Vyalkov and A.G. Anshits
.
875
88 1
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
D23
D24
D25
D26
D27
Redox Molecular Sieve Catalystsfor the Aerobic Selective Oxidation of Hydrocarbons . J.M. Thomas, R. Raja, G. Sankar and R.G. Bell
887
Catalytic Behaviour of H-Y Type Zeolites in the Decomposition of Chlorinated VOCs R. Ldpez Fonseca, P. Steltenpohl, J.R. Gonzilez Velasco, A. Aranzabal, and J.I. Gutitrrez Ortiz
896
Coupling Alkane Dehydrogenation with Hydrogenation Reactions on Cation-Exchanged Zeolites W. Li, S.Y. Yu and E. Iglesia
899
Water Vapor Tolerance of PdHZSM-5 in the NO-Methane-02 Reaction: Its Relation with Very Strong Solid Acidity of Zeolite Support J. Amano, 0. Shoji, K. Okumura and M. Niwa .
905
Simultaneous NO and N20 Decomposition on Cu-ZSM5 R. Pirone, P. Ciambelli, B. Palella and G . Russo .
91 1
Polymerization and Organometallics
D28
D29
D30
D3 1
Catalytic Properties of Silica-Supported Tantalum and Tungsten Hydrides in the Cleavage and Formation of C-C Bonds of Alkanes 0.Maury, J. Lefort, G . Saggio, C. Coptret, M. Taoufick, M. Chabanas, J. Thivolle-Cazat and J.M. Basset .
917
Characteristics of Ethylene Polymerization Catalyzed over Ziegler-Natta / Metallocene Hybrid Catalysts: Comparison between Silica-Based and Magnesium-Based Supports H.S. Cho, D.J. Choi, I.K. Song and W.Y. Lee .
923
Ethylene Polymerization with Zirconocene-MA0 Supported on Molecular Sieves I.S. Paulino, A.P. de Oliveira Filho, J.L. de S o u and U. Schuchardt
929
In Situ and Ex Situ Study of Propylene Polymerization with a MgC12Supported Ziegler-Natta Catalyst . V. Oleshko, P. Crozier, R. Cantrell and A. Westwood
935
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POSTER SESSIONS Session P1 Preparation of Catalysts Nanoheterogeneous Metal-Polymer Composites as the New Type of Eflective Selective Catalysts L.I. Trakhtenberg, G.N. Gerasimov, E.I. Grigoriev, S.A. Zavjalov, O.V. Zagorskaja, V.Yu. Zufman and V.V. Smimov. Design of Heteropolyacid-Polymer Composite Films as Catalytic Materials for Heterogeneous Reactions G.I. Park, S.S. Lim, Y.H. Kim, I. K. Son and W.Y. Lee . Catalyts Based on Heteropolyacids Supported on Zirconia L. Pizzio, P. Vhzquez, C. Caceres and M. Blanco . Dependence of structure andphysico-chemical properties of the catalysts containing heteropolyacids on the conjugatedpolymer matrix E. Stochmal-Pomarzanska and W. Turek . Mesoporous Silica Supported Solid Acid Catalysts S. Choi, Y. Wang, Z. Nie, D. Kambapati, J. Liu and C.H.F. Peden. Study of Acidic and Catalytic Properties of Sulfated Zirconia Prepared by Sol-Gel Process: Influence of Preparation Conditions L.B. Hamouda, A. Ghorbel and F. Figueras . Synthesis and Characterization of Ta-Pillared Ilerite Y.Ko,M.H.Kim,S.J.KimandY.S.Uh . Al-Pillared Hectorite and Montmorillonite Preparedfiom Concentrated Suspensions: Structural,Textural and Catalyic Properties R. Molina, S. Moreno and G. Poncelet . Synthesis of High Surface Area Transition Metal Carbide Catalysts A.P.E. York, J.B. Claridge, V.C. Williams, A.J. Brungs, J. Sloan, A. Hanif, H. Al-Megren and M.L.H. Green . Use of Carbon Fabrics as Supportfor Hydrogenation Catalysts Usable in Polyphasic Reactors J.P. Reymond and P. Fouilloux . Preparation, Characterization and Reactivity of Activated Carbon Supported Platinum Catalysts by Fluidized Bed Organometallic Chemical Vapor Deposition (FBMOCVD) Ph. Serp, J-C. Hierso, R.Feurer, R. CorratgC, Y. Kihn and Ph. Kalck
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
PO 12 Metal-Carbon Aerogels as Catalysts and Catalyst Supports F.J. Maldonado Hbdar, C. Moreno Castilla, J. Rivera Utrilla and M.A. Ferro Garcia .
1007
PO13 Preparation and Characterisation of Carbon Supported Pt-Ce02 Catalysts A. Sepclveda Escribano, J. Silvestre Alvero, F. Coloma and F. Rodriguez-Reinoso .
1013
PO14 Highly Dispersed Pd Based Catalystsfor Selective Hydrogenation Reactions M. Delage, B. Didillon, Y. Huiban, J. Lynch and D. Uzio .
1019
PO 15 Preparation of New Type of Supported Sn-Pt Bimetallic Catalysts Containig Lewis Acid Sites Anchored to the Platinun J.L. Margitfalvi, I. Borbith, M. Hegediis, S. GBbolos, A. Tompos, andF.Lonyi .
1025
PO16 Influence of Small Amounts of Rhodium on the Structure and the Reducibility of CdA1203 and Cu/Ce02-A1203 Catalysts X . Courtois, V . Perrichon, M. Primet and G. Bergeret .
1031
PO17 Synthesis, Characterization and Catalytic Properties of Chromium Silicate Xerogel R. Serpa da Cruz, M. de Magalhaes Dauch, U. Schuchardt and R. Kumar .
1037
PO18 Thermal Chemistry of Oxide-Supported Platinum Catalysts: A Comparative Study J.F. Lambert, E. Marceau, B. Shelimov, J. Lehman, V. Le Be1 de Penguilly, X. Carrier, S. Boujday, H. Pernot and M. Che
1043
.
PO19 Geochemistry and Catalysts Preparation: Alumina as a Chemical Reactant in the Synthesis of M0OdAl203 and WOJAl203 Systems X . Carrier, J.F. Lambert and M. Che .
1049
PO20 Chemistry of the Preparation of Silica-Supported Cobalt Catalysts@om Co(I4 and Co(II4 Complexes: Grafting versus Phyllosilicate Formation . R. Trujillano, J. Grimoult, C. Louis and J.-F. Lambert
1055
PO2 1 New Method of the Preparation of Heterogeneous Ni(0) Complexesfor Ethylene Oligomerization M.K. Munshieva .
1061
PO22 Synthesis, Characterization and Catalytic Activity of [ c u ( N H ~ ) ~ ] ~ + Supported on Hydrous Oxides D. Pate1 and A. Pate1 .
1067
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Preparation and Characterization of CuQnO and P m n O Catalysts for Partial Oxidation of Methanol. Control of Catalyst Surface Area and Particle Size Using Microemulsion Technique J. Agrell, K. Hasselbo, S. Jaras and M. Boutonnet . Mechanism of Silation of Alumina and Silica with Hexamethyldisilazane S.V. Slavov, A.R. Sanger and K.T. Chuang . Solvent Influence in Silica-Alumina Sol-Gel Synthesis A. Carati, C. Rizzo, M. Tabliabue and C. Perego . Ru, Co and RuCo/SiOz Catalyst Prepared by Sol-Gel Methods B. Botti, D. Cauzzi, P. Moggi, G. Predieri and R. Zanoni . Structure and Catalytic Properties of Co-Re Bimetallic Catalysts Prepared by SOWGEL Method and by Ion Exchange in Nay Zeolite L. Guczi, G. Stefler Z. Schay, I. Kiricsi, F. Mizukami, M. Toba and S. Niwa . On the Effect ofthe Preparation Conditions on the Localization and the Size ofPlatinum Particles on Zeolite NaX L.V. Mattos, M.C.M. Alves, F.B. Noronha, B. Moraweck and J.L.F. Monteiro Hollow Metallic Particles Obtained by OxidatiordReductionTreatment of Organometallic Precursors . F.J. Cadete Santos, R. Darji, J.F. Trillat, A. Howie and A. Renouprez Synthesis, Characterization and Catalytic Application of Inorganic Nanotubes . I. Kiricsi, A. Kukovecz, A. Fudala, Z. Konyia and J.B. Nagy Nanostructuring Surfaces by Lithography: The Production and use of Model Catalysts M. Schildenberger, Y. Bonetti and R. Prins . New Catalyst Supports and Catalytic Systems on the Basis of Metallic Gauzes Coated with 11-IV Group Element Oxides M.P. Vorob'eva, A.A. Greish, A.Yu. Stakheev, N.S. Telegina, A.A. Tyrlov, E.S. Obolonkova and L.M. Kustov .
Fischer-Tropsch Synthesis Precipitated Iron Fischer-Tropsch Catalysts: The Effect of SuFde Ions . T.C. Bromfield, T.H. Dlamini, F. Marsicano and N.J. Coville The Nature of the Active Phase in Iron Fischer-Tropsch Catalysts A.K. Datye, Y. Jin, L. Mansker, R.T. Motjope, T.H. Dlamini and N.J. Coville
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
PO35 Selective Inhibition: The Intrinsic Feature of Fisher-Tropsch Synthesis: Transient Initial Episodes in FT Synthesis on Cobalt Catalysts H. Schulz and Z. Nie .
1145
PO36 Eflects of Support on Methane Selectivity in Cobalt-Catalyzed FischerTropsch Synthesis: A Kinetic Model C.H. Bartholomew and W.-H. Lee .
1151
PO37 Does Mono-Atomic Ru Catalyse the Fischer-Trosph Synthesis? . M. Claeys. M. Hearshaw, J.R. Moss and E. van Steen
1157
PO38 Fischer-Tropsch Synthesis Using Monolithic Catalysts A.-M. Hilmen, E. Bergene, O.A. Lindvag, D. Schanke, S. Eri and A. Holmen
1163
Application of Theoretical Methods to Catalysis PO39 The Design ifCatalysts SurJaces by the Use of Stochastic Optimisation Algorithms A.S. McLeod and L.F. Gladden .
1169
PO40 Modeling of Structure and Properties of Active Centers of Catalysts on the Base of Metalorganosiloxanes A.V. Nernukhin, I.M. Kolesnikov and V.A. Vinokurov .
1175
PO41 Theoretical Study on Active sites of Molybdena-Alumina Catalystfor Ole$n Metathesis J. Handzlik and J. Ogonowski
1181
PO42 Modeling the Oxygen Activation on Dinuclear Iron MMO Mimics, a Quantum Mechanic Study P.-P. Knops-Gerrits, P.A. Jacobs and W.A. Goddard I11 .
1187
PO43 Quantum-Chemical and Experimental Study of an Interaction between C 0 2and Propylene over Rh-Co/Al~ojCatalysts L.B. Shapovalova, G.D. Zakumbaeva, A.V. Gabdrakipov, I.A. Shlygina and A.A. Zhurtbaeva .
1193
PO44 Ab-Initio Study of the Vacancy Formation in Keggin and Linqvisr Heteropolyanion J.F. Paul and M. Founier .
1199
PO45 Theoretical Studies o f the Interaction of Butane and Butene Isomers in H-Ferrierite and H-Mordenite D.E. Galindo, M.J. Goncalves and M.M. Ramirez Corredores. .
1205
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Theoretical Investigation of the Intrinsic Reaction Path (IRC) of the Methylation of mono-Substituted Aromatic Molecules over Faujasite Catalysts K.H.L. Nulens, A.M. Vos, G.O.A. Janssens and R.A. Schoonheydt Environmental Catalysis. Combustion VOCs
Catalysts or Combustion of Halogenated Volatile Organis Compounds . J. Trawczynski, J. Walendziewski and M. Kulazynski Non-Stoichiometry and Catalytic Activity in AB03 Perovskites: LaMn03 and LaFe03 A.A. Barresi, D. Mazza, S. Ronchetti, R. Spinicci and M. Vallino . Deep Catalytic Oxidation of Chlorinated VOC Mixturesfiom Groundwater Stripping Emissions A. A r d b a l , J.A. G o n d e z Marcos, R. Lopez Fonseca, M. A. GutiCrrez Ortiz and J.R. G o d l e z Velasco . Transformation of Chlorinated Compounds on Dzflerent Zeolites under Oxidative and Reductive Conditions I. Hannus, A. Tamhi, Z. Konya, S.-I. Niwa, F. Mizukami, J.B. Nagy and I. Kiricsi . Biofiltration of Gasoline VOCs with Dzflerent Support Media I. Ortiz, M. Morales, C. GobbCe, S. Revah, V.M. Guerrero, I. Shifter, R. Auria, G.A. PCrez and L.A. Garcia Environmental Catalysis. NOx
An Unified View on Nitrogen Oxide Abatement J. Paul . Measuring the Adsorption of Reactive Probes as a Toolfor Understanding the Catalytic Properties of De-NO, Catalysts A. Gervasini and A. Auroux . Reactivity of the NO, Sur$ace Species Formed Afrer Co-adsorption of NO + 0 2 on a WO&'rOz Catalyst:An FTIR Spectroscopic Study S. Kuba, K. Hadjiivanov and H. Knozinger . The Mechanism of the Selective NO, Adsorption on 12- Tungstophoshoric Acid hexa-Hydrate (HPW) S. Hodjati, C. Petit, V. Pitchon and A. Kiennemann. Activity and DRIFT Spectroscopy Studies of NO Oxidation and Reduction by C3Ht in Excess 0 2 over A1203and AdAl203 G.R. Barnwenda, A. Obuchi, S. Kushiyama and K. Mizuno
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
PO57 Transient Study of NO, N20, C3H6 Interactions over a Silica-Supported Pt Catalyst P. Denton, Y . Schuurman, A. Giroir-Fendler, H. Praliaud, M. Primet and C. Mirodatos . PO58 Electrochemical Promotion of NO Reduction by C3H6 on RWYSZ Catalyst-Electrodes and Investigation of the Origin of the Promoting Action Using TPD and WF Measurements C. Raptis, Th. Badas, D. Tsiplakides, C. Pliangos and C.G. Vayenas
1277
.
1283
PO59 The Reaction of Propane with Mixtures of NO, N2O and 0 2 Over Platinum, Palladium and Rhodium Catalysts D.C. Chambers, D.E. Angove and N. W. Cant .
1289
PO60 Kinetics of NO Reduction by CO on Rh(l1 I): A Molecular Beam Study F. Zaera and C.S. Gopinah .
1295
PO61 The Surface Migration of NO, Adspecies as a Factor Determininig the Reactivity of Supported Pt Catalystsfor the Treatment of Lean Burn Engine Emissions G.E. Arena, A. Bianchini, G. Centi and F. Vazzana .
1301
PO62 Pd/Sn02 as deNO, Catalysts: Preparation, Characterization and Activity D. Amalric-Popescu, J.-M. Herrmann and F. Bozon-Verduraz .
1307
PO63 Decomposition of NO Over Copper-Manganese Oxide Catalysts at Room Temperature I. Spassova, M . Khrishtova, N. Nyagolova and D. Mehandjiev .
1313
PO64 Catalytic Performance of SrTi03-Based Catalystsfor NO Direct Decomposition Y. Yokoi, I. Yasuda, H. Uchida, 0 . Okada, Y. Nakamura, H. Ishikawa and H. Kimura 1319 PO65 CO and NO Elimination over Pd-Cu Catalysts M. Fernhdez Garcia, A. Martinez Arias, J.A. Anderson, J.C. Conesa and J. Soria
1325
PO66 NO, Storage and Reduction over Pt/Ba/A1103 J.A. Anderson, A.J. Paterson and M. Fernhdez Garcia
1331
.
PO67 Nitrogen Emission Process in the Decomposition of Nitrogen Oxides on Pd(1 lo): An Angular and Velocity Distribution Stua'y I . Kobal, K. Kimura, Y. Ohno, H. Horino, I. Rzeznicka and T. Matsushima
1337
PO68 Reduction of NO by Propylene over ModiJied RhITi02 Catalysts in the Presence of Oxygen. T.I. Halkides, D.I. Kondarides and X.E. Verykios .
1343
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E'ect of sulfirr on the oxygen storage/release capacity of RWCe02 and RWCeO2-Zr02 model TWCs M. Boaro, C. de Leitenburg, G. Dolcetti and A. Trovarelli . Thermal Stability and Oxygen Storage Capacity of Noble Metal/CeriaZirconia Catalystsfor the Automotive Converters with the On-Board Diagnostics (OBD) P. Fornasiero, R. Di Monte, T. Montini, J. Kaspar and M. Graziani
.
N 2 0 Formation over Ce02-Zr02Mixed Oxides Supported Metal (Pt, Pd Rh), Catalystsfor Automotive Exhaust Control J.L Marc, J.R. G o d e z Velasco, M. A. Gutierrez Ortiz, J.A. Botas, M.P. G o d e z Marcos and G. Blanchard . Model Studies of Automobile Exhaust Catalysis Using Single Crystals of Rhodium and CeriaRirconia C.H.F. Peden, G.S. Herman, S.A. Chambers, Y. Gao, Y.-J. Kim and D.N. Belton . Thermally Stable, High-Surface-Area, Pro,-CeOz-Based Mixed Oxides for Use in Automotive-Exhaust Catalysts A.N. Shigapov, H.-W. Jen, G.W. Graham, W. Chun and R.W. McCabe . Improvement of Three Way Catalytic Perfomance by Optimizing Ceria and Promoters in Pd only Catalyst Prepared bu Sol-Gel Method H.-S. So, 0.-B. Yang, D.H. Kim and S.I. Woo . Flue Gas NO, Removal by SCR with NH3 on CuO/AC at Low Temperatures Z.P. Zhu, Z.Y. Liu, S.J. Liu, H.X. Niu, T.D. Hu, T. Liu and Y.N. Xian . Low temperature Monolithic SCR Catalystsfor Tail Gas Treatment in Nitric Acid Plants J. Blanco, P. Avila, L. Marzo, S. Suarez and C. Knapp . Effect of La203 Concentration in La203-A1203Supports and Pd/La203-AG03 Catalysts in Reduction of NO by H2 N.E. Bogdanchikova, A. Barrera, S. Fuentes, G. Diaz, A. Gomez-CortCs, A. Boronin, M.H. Farias and M. Viniegra R.. Molecular Basis for the Design of Transition-Metal Oxide Catalystsfor Selective Catalytic Reduction of NO by Hydrocarbons K. Shimizu, H. Maeshima, H. Kawabata, H. Yoshida, A. Satsurna and T. Hattori . Silver as Promoter for the Catalytic Decomposition of NO, under Oxygen-Excess Condition: Evidence for Oxygen Spilloverfrom Noble Metals to Silver W.X. Huang, J. W. Teng, T.X. Cai and X.H. Bao .
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
PO80 Reactiviry of Surface Species in the Reduction of NO, by Ethanol on Ag/AI203 Catalyst T . Chafik, S. Kameoka, Y. Ukisu and T. Miyadera.. PO8 1 Modelling of Uncatalysed and Barium Catalysed NO Reduction by Activated Carbon S.A. Carabineiro, F.B. Fernandes, J.S. Vital, A.M. Ramos and I.F. Silva PO82 Selective Catalytic Reduction of NO, with C3H,j under Lean-Burn Conditions on Activated Carbon-Supported Metals J.M. Garcia Cortts, M.J. Illin Gbrnez, A. Linares Solano and C. Salinas Martinez de Lecea. PO83 Role of Acidic Sites in the DENO, Process on Supported Tin Oxides A. Auroux, D. Sprinceana and A. Gervasini . PO84 Sulphated-Zr02 Prepared by Impregnation with Ammonium, Sodium or Copper Sulphate: Catalytic Activity for NO Abatement with Propene in the Presence of Oxygen V. Indovina, M.C. Campa and D. Pietrogiacomi . PO85 Co.Based exTHlcfor the Decomposition of N20: Tailoring Catalystsfor Active and Stable Operation J. Perez Ramirez, G. Mul, X. Xu, F. Kapteijn and J.A. Moulijn .
1445
PO86 Cold Start Vehicle Emission Control Using Trapping and Catalyst Technology N.R. Burke, D.L. Trirnm and R. Howe .
1451
PO87 Oscillation of the NO, Concentration in its Selective Catalytic Reduction on Platinum Containing Zeolite Catalysts Y . Traa, B. Burger and J. Weitkamp .
1457
PO88 Experimental and Theoretical Description of Transition Metal Ion Structures in Zeolites Relevant to DENO, Catalysis Z . Sobalik, J.E. Sponer and B. Wichterlova .
1463
PO89 Cooperation of Pt and Pd over H-Mordenite for the Lean SCR of NO, by Methane C. Montes de Correa, F. Cordoba C. and F. Bustamante L. .
1469
PO90 The Selective Catalytic Reduction of NzO by NH3 on a Fe-BEA Catalysts M. Mauvezin, G. Delahay, B. Coq and S. Kieger .
1475
PO91 A New Mechanismfor the Selective Catalytic Reduction of NO, by NH3 on Cu-zeolite Catalysts B. Coq, S. Kieger, G. Delahay, D. Berthomieu and A. Goursot .
1481
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PO92 In Situ FT-I. Shrdy of the Selective Catalytic Reduction of NO by Propane on Cu-ZSM-5: Evidence of a Reaction Pathway by Oxygen Pulses F. Poignat, J.L. Freysz, M. Daturi, J. Saussey and J.C. Lavalley .
1487
PO93 Catalytic Removal of NO: the Eflect of Cu Loading and Type of Binder on Catalytic Properties of Cu/ZSM-5 Catalyst V. Tomasic, A. Gerzina, Z. Gornzi and S. Zrncevic .
1493
PO94 Changes in Cation Coordination during Deactivation of Cu-ZSM-5 deNOx Catalysts . S.A. Gbmez, A. Campero, A. Martinez Hernindez and G.A. Fuentes
1499
PO95 On the Role of Oxygen in the Selective Reaction ofNO Reduction with Methane over Co/ZSM-5 Catalyst E.M. Sadovskaya, A.P. Suknev, L.G. Pinaeva, V.B. Goncharov, C. Mirodatos and B.S. Bal'zhinimaev
1505
PO96 Eflect of SiLAl Ratio of Mordenite and ZSM-5 Type Zeolite Catalysts on Hydrothermal Stabilityfor NO Reduction by Hydrocarbons . S.Y. Chung, B.S. Kim, S.B. Hong, I.-S. Nam and Y.G. Kim
1511
PO97 Precipitation of Mn02 onto the External Surface of Zeolite Microcrystals. Structure of the Manganese Oxide and its Role in the Removal of NO, by NH3 M. Richter, H. Kosslick and R. Fricke .
1517
PO98 Catalflic Properties of Mesoporous Molecular Sievesfor the Reduction of NO, 1523 A. Jentys, W . Schieber and H. Vinek PO99 Catalytic Activity of High Surface Area Mesoporous Mn-Based Mixed Oxidesfor the Deep Oxidation of Methane and Lean-NO, Reduction V.N. Stathopoulos, V.C. Belessi, C.N. Costa, S. Neophytides, P. Falaras, A.M. Efstathiou and P.J. Pomonis .
1529
PlOO Characterization of the Role of Pt on Copt and InPt Ferrierite Activity and Stability upon the SCR of NO with CH4 L. Gutierrez, L. Cornaglia, E. Mir6 and J. Petunchi .
1535
Environmental Catalysis. Catalysis for Clean Processes and Fuels P 101 Cu-Cr Oxide Catalystsfor Complete Oxidation of Aromatic Hydrocarbons V. Georgescu . PI02 Ag/Lao.&r,dMnO/y-A1203 Catalystfor Complete Oxidation of Methanol at Low Concentration H.-B. Zhang, W . Wang, Z.-T. Xiong and G.-D. Lin .
1541
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P103 Treatment of Aqueous Solutions of Organic Pollutants by Heterogeneous Catalytic Wet Air Oxidation (CWAO) . M. Besson, J.-C. Beziat, B. Blanc, S. Durecu and P. Gallezot P104 Efect of the Supportfor Pt Catalysts on Soot Oxidation J. Obuchi, J. Oi-Uchisawa, R. Enomoto, S. Liu, T. Nanba and S. Kushiyama P105 Dimethyl Carbonate Synthesis From Carbon Dioxide and Methanol over Ni-Cw'MoSiO (VSiO) Catalysts . S.H. Zhong, J.W. Wang, X.F. Xiao and H.S. Li P106 Promotion of Support in Synthesis of Propylene Carbonatefrom COr and Propylene Oxide T. Zhao, Y . Han and Y. Sun . P107 A Novel Process for the Removal of Nitrates from Drinking Water A. Pintar, J. Batista and G. Bercic . PI08 Greenhouse Gases and Emissions Control by New Catalysts Free of Precious Metals V. Gryaznov and Y. Serov . P 109 Study on the Initial Steps of the Polyethylene Cracking over DifSerent acid Catalysts D.P. Serrano, R. Van Grieken, J. Aguado, R.A. Garcia, C. Rojo and F. Temprano . P 110 Catalytic Conversion of Traces in Biomass Gasijkation Fuel Gases with Nickel Activated Ceramic Filters D.J. Draelants. H.-B. Zhao and G.V. Baron . Pl 1 1 An Environmental Friendly Catalystfor the High Temperature Shift Reaction G.C. Araujo and M.C. Range1 P112 Direct Catalytic Conversion of Chloromethane to Higher Hydrocarbons over Various Protonic and Cationic Zeolite Catalysts as Studied by In-Situ FTIR and Catalytic Testing D. Jaumain and B.-L. Su . P 113 Catalytic Conversion of 2-Chloropropane in Oxidizing Conditions: A FT-IR and Flow Reaction Study C. Pistarino, F. Brichese, E. Finocchio, G. Romezzano, R. di Felice, M. Baldi and G. Busca P114 Catalyst Degradation of Polychlorinated Biphenyls at Low Temperature D.-K. Lee, E.-S. Byun, I.-C. Cho and S.-W. Kim .
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P 1 1 5 Microwave Eflects in Exhaust Catalysis M. Turner, R.L. Laurence, K.S. Yngvesson and W.C. Conner
.
1625
Selective Oxidation (General Papers)
P 1 16 A New Way to Ti-containingZeolite Beta Catalystsfor Selective Oxidation F. Di Renzo, S. Gomez, R. Teissier and F. Fajula .
1631
P117 Synthesis of Organically ModJied Titania Silica Aerogels: Application for Epoxidation of Cyclohexenol A. Gisler, C.A. Miiller, M. Schneider, T. Mallat and A. Baiker .
1637
Pll8 Characterization and Catalytic Properties of Titanium Pillared Clays in the Epoxidation of Allylic Alcohols . L. Khalfallah-Boudalli, A. Ghorbel, F. Figueras and C. Pine1
1643
PI 19 Epoxidation of Allylic Alcohol by Vanadium-Montmorillonite Catalyst I. Kheder and A. Ghorbel .
1649
P120 Activity and Stability of Thermally Stable Polyimide Supported Mo(V.. Complexesfor Epoxidation of Olefins J.H. Ahn, C.G. Oh, S.K. Ihm and D.C. Sherrington .
1655
P121 Epoxidation of Soybean Oil Catalysed by CHjReOdH202 H. Sales, R. Cesquini, S. Sato, D. Mandelli and U. Schuchardt
1661
.
P122 Epoxidation of Limonene with Layered Double Hydroxides as Catalysts M.A. Aramendia, V . Borau, C. JimCnez, J.M. Luque, J.M. Marinas, F.J. Romero, J.R. Ruiz and F.J. Urbano .
1667
P 123 Epoxidation of Electron-Deficient Alkenes Using Heterogeneous Basic Catalysts J.M. Fraile, J.I. Garcia, D. Marco, J.A. Mayoral, E. Shnchez, A. Monzon and E. Romeo.
1673
P124 Hydroxylation of Benzene to Phenol with Nitrous Oxide on Fe-Silicalites R. Monaci, E. Rombi, M.G. Cutrufello, V. Solinas, G. Berlier and G. Spoto
1679
P125 Ammoxidation of Propane to Acrylonitrile over V-Sb-Al-Oxide Catalysts - Influence of Catalyst Preparation on Reaction Kinetics 0.Ratajczak, H.W. Zanthoff and S. Geisler .
1685
P126 Ammoxidation of Propylene to Acrylonitrile Catalyzed by Multimetal Molybdateand Iron Antimoniate-BasedActive Compounds Dispersed in Oxidic Matrixes: The Efect of the Dispersing Oxide on the Catalytic Performance R. Catani, F. Cavani, U. Cornaro, A. Del Bianco, E. Frontani, D. Ghisletti, 1691 A. Tasso and F. Trifiri,
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P127 Mastering of Chemical State of Mo at the Surface of Oxide Catalysts in the Selective Oxidation of Hydrocarbons: Towards a Fine Optimization of Catalytic Performances E.M. Gaigneaux, M-L. Naeye, E. Godard, H.M. Abdel Dayem, P. Ruiz and B. Delmon
1697
P128 Oligomerization of Higher OleJins and Oxidation in a Series of Isobutanol - Isobutyric Aldehyde - Isobutyric Acid in the Presence of Some Heteropoly Compounds S.M. Zulfugarova, M.K. Munshieva, D.B. Tagiev and P.S. Mamedova .
1703
PI29 ModiJications of Vanadium Phosporous Oxides by Aluminium Phosphate for n-Butane Oxidation to Maleic Anhydride S. Holmes, L. Sartoni, A. Burrows, V. Martin, G.J. Hutchings, C. Kiely and J.-C. Volta
1709
P130 Selective Oxidation of n-Butane over Novel V-P-O/Silica-Composites Prepared Through Intercalation and Exfoliation of Layered Precursor N. Hiyoshi, N. Yamamoto, N. Terao, T. Okuhara and T. Nakato .
1715
P 131 Molecular Structure-Reactivity Relationships in n-Butane Oxidation over Bulk VPO and Supported Vanadia Catalysts: Lessons for Molecular Engineering of New Selective Catalystsfor Alkane Oxidation . V.V. Giulants, J.B. Benziger, S. Sundaresan and I.E. Wachs
1721
P132 The Nature of the Cobalt Salt Affects the Catalytic Properties of Promoted VPO 1727 . L. Cornaglia, C. Carrara, J. Petunchi and E. Lombardo P133 Oxidation of Toluene to Benzaldehyde over VSb,,Ti,Od Kinetic Studies A. Barbaro, S. Larrondo and N. Amadeo .
Catayst.
P134 Interaction of Surface- and Bulk-Oxygen in Mo/V-Mixed Oxides during the Partial Oxidation of an UnsaturatedAldehyde A Concentration-Programmed-ReductionStudy R. Bohling, A. Drochner, M. Fehlings, K. Kraub and H. Vogel .
1733
1739
P135 Steady State and Transient Kinetic Experimentsfor Rational Catalyst Design: Dzfferences of Acrolein and Methacrolein Oxidation on Mixed Oxide Catalysts 1745 H. Bohnke, J.C. Petzoldt, B. Stein and J.W. Gaube . P136 The Effects ofParticle Size on the Performance of Er203and 30 mol% BaC12/Er203 Catalysts in the Selective Oxidation of Ethane to Ethene W. Zhong, H.X. Dai, C.F. Ng, and C.T. Au .
1751
P137 Perovskite Type Chloro Oxide S ~ C O O ~ - ~ACNovel I ; and Durable Catalystfor the Selective Oxidation of Ethane to Ethene H. X . Dai, C.F. Ng, and C.T. Au .
1757
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P138 Preparation and Activity of Copper, Nickel and Copper-Nickel-A1Mixed Oxides via Hydrotalcite-Like Precursorsfor the Oxidation of Phenol Aqueous Solutions A. Alejandre, F. Medina, X. Rodriguez, P. Salagre and J.E. Sueiras
.
1763
P139 Influence of Temperature and Catalyst Loading on the Aqueous-Phase Catalytic Oxidation of Phenol . A. Santos, P. Yustos, B. Durbhn and F. Garcia Ochoa
1769
PI40 Glyoxal Synthesis by Vapour - Phase Ethylene Glycol Oxidation on a Silver and Copper Catalysts O.V. Vodyankina, L.N. Kurina, A.I. Boronin and A.N. Salanov .
1775
P 141 Production of Alkenes through Oxidative Cracking of n-Butane over OCM Catalysts M. Nakamura, S. Takenaka, I. Yamanaka and K. Otsuka .
1781
P142 Selective Oxidation of Monosaccharides using Pt and Pd-Containing Cctalysts E. Sulman, N. Lakina, M. Sulman, T. Ankudinova, V. Matveeva, A. Sidorov and S. Sidorov 1787 PI43 Synthesis of New Neocarboxylic Acids via Rearragement and Oxidation of I , 3-Dioxanes J . Fischer and W F. Holderich
1793
P 144 Low Temperature Radical Oxidation of Butane over In-Situ Prepared Silica Species A. Satsuma, N. Sugiyama. Y. Kamiya, T. Kamatani and T. Hattori
1799
P145 Mechanochemical Preparation of V-Ti-0 Catalystsfor o-Xylene Low Temperature Oxidation V.A. Zazhigalov, J. Haber, J. Stoch, A.I. Kharlamov, A. Marino, L. Depero and I.V. Bacherikova .
1805
P146 A Common Concept Accountingfor Selectivity in Mild and Total Oxidation Reactions: The Optical Basicity of Catalysts P. Moriceau, B. Taouk, E. Bordes and P. Courtine .
1811
P 147 Copper Species in CuC12/y-Alr03Catalystfor Ethylene Oxychlorination M . Garilli, D. Carmello, B. Cremaschi, G. Leofanti, M. Padovan, A. Zecchina, G. Spoto, S. Bordiga and C. Lamberti .
1817
P 148 A Study of the Main Path and of Side-Reactions upon Ethylene Oxychlorination over CuCI2-Al203 Based Catalysts A. Marsella, D. Carmello, E. Finocchio, B. Cremaschi, G. Leofanti, M. Padovan and G. Busca .
1823
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Selective Oxidation (ODH)
P149 Oxidative Dehydrogenation of Ethane on Lithium Promoted Oxide Catalysis S. Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki . PI50 Oxidative Dehydrogenation of Ethane to Ethylene over NiO/AI203 Catalysts X. Zhang, G. Yu, Y. Gong, D. Jiang and Y. Xie . PI5 1 Oxidative Dehydrogenation of Ethane over Catalysts Prepared Via Heteropolycompounds N . Haddad, C. Rabia, M.M. Bettahar and A. Bararna
P152 The Importance of Nonstoichiometric Oxygen in NiO for the Catalytic Oxidative Dehydrogenation of Ethane T. Chen, W. Li, C. Yu, R. Jin and H. Xu
.
P153 Oxidative Dehydrogenation of Ethane on Vanadium-PhosphorusOxide Catalysts J.M. L6pez Nieto, V.A. Zazhigalov, B. Solsona and I.V. Bacherikova
.
P154 High Throughput Synthesis and Screening of Nb and Ta Containing
Mixed Metal Oxide Librariesfor Ethane Oxidative Dehydrogenation Y. Liu, P.Cong, R.D. Doolen, H.W. Turner and W.H. Weinberg .
P 1 55 Chromium-ImpregnatedMesoporous Silica as Catalystsfor the Oxidative Dehydrogenation of Propane J. MCrida Robles, M. Alciintara Rodriguez, E. Rodriguez Castellbn, J. Santaman'a Gonzdlez, P. Maireles Torres and A. Jimenez L6pez.
P156 Transient Kinetic Studies of the Propane Oxidehydrogenationon V~OdTi02( S. Pietrzyk and F. Genser
.
PI 57 Vanadium-Doped Titania-Pillared Montmorillonites as Catalystsfor
the Oxidative Dehydrogenation of Propane K. Bahranowski, R. Dula, R. Grabowski, E.M. Senvicka and K. Wcislo .
P158 Oxidative Dehydrogenation of Propane over Alkali-Mo Catalysts Supported on Sol-Gel Silica-Titania Mixed Oxides R.B. Watson and U.S. Ozkan.
P 159 Vanadium Oxide Supported on Gallium and Indium Oxides: Synthesis, Physicochemical and Catalytic Properties B. Sulikowski, A. Kubacka, E. Wloch, Z. Schay, V. CortCs Corberiin and R.X. Valenzuela .
P160 Oxidative Dehydrogenation of Cyclohexane over Heteropolymolybdates . S. Hocine, C. Rabia, M.M. Bettahar and M. Fournier
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P161 Transient Response Studies of lsobutane Oxidative Dehydrogenation over Molybdenum Catalysts N.V. Nekrasov, N.A. Gaidai, Yu.A. Agafonov, S.L. Kiperman, V. Cortes Corberhn and M.F. Portela
P 162 Olefins Formation by Oxidative Dehydrogenation of Propane over Monoliths at Short Contact Times V.A. Sadykov, S.N. Pavlova, N.F. Saputina, I.A. Zolotarski, N.A. Pakhomov, E.M. Morov, V.A. Kumin, A.V. Kalinkin, A.N. Salanov, I.G. Dalinova and E.A. Paukshtis .
1907
P 163 Experimental and Theoretical Investigation on the Roles of Heterogeneous and Homogeneous Phases in the Oxidative Dehydrogenation of Light Parafins in Novel Short Contact Time Reactors A. Beretta, E. Ranzi and P. Forzatti .
1913
Fuel Cells and Electrocatalysis
P 164 Composite Nickel-Oxide Surfaces, Modified by Palladium R.G. Baisheva, Z.K. Kanaeva, K.A. Zhubanov and Zh.K. Kairbekov
.
1919
Photochemistry P 165 Photocatalyst-CoatedAcrylic Waveguidesfor Oxidation of Organic Compounds L.W. Miller, M.I. Tejedor Tejedor, M. PCrez Moya, R. Johnson and M.A. Anderson . 1925
P166 Preparation of Eflcient Titanium Oxide Photocatalysts by an Ionizer Cluster Beam Method and Their Application for the Degradation of Propanol Diluted in Water H . Yarnashita, M. Harada, A. Tanii, M. Honda, M. Takeuchi, Y. Ichihashi and M. Anpo .
1931
P167 Thermal Treatment of Titanium Alkoxides in Organic Media: Novel Synthesis Methods for Titanium (IV) Oxide Phot~catalystof Ultra-HighActivity H. Kominami, J. Kato, S. Murakami, M. Kohno, Y. Kera, S. Nishimoto, . M. Inoue, T. Inui and B. Ohtani
1937
P 168 Photocataliytic Water Decomposition by Layered Perovskites T. Takata, A. Tanaka, M. Hara, J.N. Kondo and K. Domen.
1943
P 169 Catalyst Preparation and Reactor Designfor Gas-Phase Photocatalytic Oxidation of Trichloroethylene (TCE) Pollutant A.J. Maira, K.L. Yeung, C.K. Chan, J.F. Porter and P.L. Yue
.
P170 Formation of Ethylene Oxide by Photooxidation of Ethylene over Silica ModiJied with Copper Y. Ichihashi and Y. Matsumura .
1949
1955
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P171 Photocatalytic Oxidation of n-Butane at a Steady Rate over Rb-Mod$ed Vanadium Oxide Supported on Silica T. Tanaka, T. Ito, T. Funabiki and S. Yoshida .
1961
P 172 Photocatalytic Oxidation of Toluene on PlatinudTitania Catalysts M.C. Blount and J.L. Falconer .
1967
PI73 Photocatalytic Degradation of Toluene in Aqueous Suspensions of Polycrystalline Ti02 in the Presence of the Surfactant Tetradecyldimethylamino-Oxide V . Augugliaro, V . Loddo, G. Marci, L. Palmisano, C. Sbriziolo, M. Schiavello and M.L. Turco Liveri
1973
P174 Photooxidation of Benzene to Phenol by Ru Complex Occluded in Mesoporous FSM-16 K. Fujishima, A. Fukuoka and M. Ichikawa .
1979
Session P2 Catalysis on Metal and Metal Oxides
P 175 Hydrodechlorination of CC12F-CF3 (CFC-114a) over Silica-Supported Noble Metal Catalysts T. Mori and Y. Morikawa .
1985
P176 Hydrodechlorination of CC12F2(CFC-12) by Carbon- and MgF2Supported Palladium and Palladium-Gold Catalysts A. Malinowski, W. Juszczyk, J. Pielaszek, M. Bonarowska, M. Wojciechowska and Z. Karpinski. .
1991
P177 C-C Bond formation during Hydrodechlorinarion of CCI4 on PdContaining Catalysts E.S. Lokteva, V.V. Lunin, E.V. Golubina, V. I. Simagina, M. Egorova and I.V. Stoyanova .
1997
P178 Hydrodehalogenation of Aryl Halides by Hydrogen Gas and Hydrogen Transfer in the Presence of Palladium Catalysts M.A. Ararnendia, V . Borau, I.M. Garcia, C. JimCnez, J.M. Marinas, A. Marinas and F.J. Urbano .
2003
PI79 Carbon-Supported Palladium Catalystsfor Liquid-Phase Hydrodechlorination of Carbon Tetrachloride to Chloroform L.M. G6mez Sainero, J.M. Grau, A. Arcoya and X.L. Seoane
2009
P180 Dispersed Pd-Ag Alloys for Selective Production of 0leJinsJi.om Chlorinated Alkanes B. Heinrichs, J.-P. Schoebrechts and J.-P. Pirard .
.
2015
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P18 1 Surface Structure and Methylcyclobutane Hydrogenolysis Activity of Pt/AlzO3 and Pt/CeOz a+ Reduction at Increasing Temperature (3 73 to 973 K) G. Rupprechter, J.J. Calvino, C. Lopez-Cartes, M. Fuchs, J.M. Gatica, J.A. PCrez Omil, K. Hayek and S. Bernal .
PI 82 Hydrogenation ofXylose to Xylitol :Three Phase Catalysis by Promoted Raney Nickel, Catalyst Deactivation and In-situ Sonochemical Catalyst Rejuvenation J.-P. Mikkola, T. Salmi, R. Sjoholm, P. M&i-Arvela and H. Vainio PI 83 Immobile Silica Fibre Catalyst in Liquid Phase Hydrogenation T . Salmi, P. M&i-Arvela, E. Toukoniitty, A.Kalantar Neyestanaki, . L.E Lindfors, R. Sjoholm and E. Laine PI 84 Lowering the Trans-Isomer Content in Hydrogenation of Triglycerides of Unsaturated Fatty Acids at Ambient Temperatures I.A. Makaryan, O.V. Matveeva, G.I. Davydova and V.I. Savchenko PI 85 Size Particle Effect and Copper or Silver Addition Effect on Catalytic Properties of Rhodium Supported onto Amorphous Silica Z. Ksibi, A. Ghorbel, B. Bellamy and A. Masson . P186 Kinetic Studies on the Hydrogenation of 1,3- Butadiene, I-Butyne and their Mixtures P. Shafer, N. Wuchter and J. Gaube . P187 Selective Hydrogenation of Nitrobenzene in Phenylhydroxylamine on Silica Supported Platinum Catalysts L. Pernoud, J.P. Candy, B. Didillon, R. Jacquot and J.M. Basset . PI88 Hydrogenation of Carbonylic Compounds on Pt/Si02 Catalysts Modified with SnBu4 G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferretti, A. Moglioni and G. Moltrasio Iglesias . PI 89 Catalytic Behaviour of Several Ni/NiAl20 Catalysts in the Hydrogenation of 1,2,4-Trichlorobenzeneand Benzene to Cyclohexane Y . Ceseros, P. Salagre, F. Medina and J.E. Sueiras . P 190 Catalytic Lifetime of Amine Metal Complexes Supported on Carbons in Ciclohexene Hydrogenation J.A. Diaz Auiion, M.C. R o m b Martinez, P.C. l'hgentiere and C. Salinas Martinez de Lecea. P 191 Promoting Effect of Ni in Semi-Hydrogenation of 1,3-Butadieneover Ni-Pd Catalysts A. Sarkany .
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P192 Influence of the Surface Structure of Co on the Selective Hydrogenation of Crotonaldehyde E.L. Rodrigues, C.E.C. Rodrigues, A.J. Marchi, C.R. Apesteguia and J.M.C. Bueno . P193 Hydrogenation of Olejns in Aqueous Phase, Catalized by LigandProtected and Polymer-Protected Rhodium Colloids A. Borsla, A.M. Wilhelm, J.P. Canselier and H. Delmas . P194 Preparation and Characterisation of Ni-Mg-A1 Hydrotalcites as Hydrogenation Catalysts E. Romeo, C. Royo, A. Monzbn, R. Trujillano, F.M. Labajos and V. Rives P 195 Hydrogenation of Acrylonitrile-Butadiene Rubbers with Palladium Loaded Mesopore-Size Controlled Clay Minerals M. Shirai, K. Torii and M. Arai . P196 Single-Stage Liquid Phase Hydrogenation of Maleic Anhydride to y-Butyrolactone, 1,4-Butanediol and Tetrahydrofurane on Cu/ZnO/Al203catalysts A. Kuksal, E. Klemm and G. Emig . PI97 Methanol Decomposition to Synthesis Gas Over Supported Pd Catalysts Preparedfiom Synthetic Anionic Clays T. Shishido, H . Sameshima, T. Hayakawa, S. Hamakawa, E. Tanabe, K. Ito and K. Takehira P198 Methanol Decomposition on Unpromoted and Zn Promoted Cu/Si02 Catalysts M . Clement, Y . Zhang, D.S. Brands, E.K. Poels and A. Bliek . P199 Comparative Study of P and Mn Mod$ed CuO/Si02Catalystfor Methanol Dehydrogenation R. Zhang, Y . Sun and S. Peng P200 Decomposition of Ethanol on Rh(100) and Rh(100)c(2x2)-Mn Surfaces . R. Zhai, Z. Tian, H. Luo, D. Liang and L. Lin P201 Use of New Tin orthophosphates as Catalystsfor the Gas Phase Dehydrogenation-Dehydrationof Alcohols M.A. Aramendia, V . Borau, C. JimCnez, J.M. Marinas, M.L. Ortega, F.J. Romero, J.R. Ruiz and F.J. Urbano . P202 Steam Reforming of Ethanol Using Cu-Ni Supported Catalysts . F. Man'fio, M. Jobbagy and G. Baronetti, M. Laborde P203 Activity and Stability of Single and Perovskite-Type Manganese and Cobalt Oxides in the Catalytic Combustion of Acetone A. Gil, N . Burgos, M. Paulis, M. Montes and L.M. Gandia.
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P204 Efect of Copper Addition on the Activity and Selectivity of Oxide Catalysts in the Combustion of Carbon Particulate C. Pruvost, J.F. Lamonier, D. Courcot, E. Abi Aad and A. Aboukai's
.
2159
P205 The Efect of Mn Substitution on the Catalytic Properties of Ferrites L.C.A. Oliveira, R.M. Lago, R.V.R.A. Rios, P.P. Sousa, W.N. Mussel and J.D. Fabris
2165
P206 Oxygen Nonstoichiometry and Linear Deformation of La-Sr-Fe-Co-0 Perovskites within the Redox Cycle L.A. Rudnitsky, V.V. Aleksandrovskii and S.Y. Stefanovich .
2171
P207 Molecular Mechanism of Surface Recognition during the Adsorption/ Degradation of Organic Compounds on Iron Oxides J. Bandara, J. Mielzcarski and J. Kiwi .
2177
P208 Characterization and Catalytic Activity of Mixed Cu-Cr/Ce02 Catalyst W. Daniell, P. Grotz, H. Knozinger, N.C. Loyd, C. Bailey and P.G. Harrison .
2183
P209 Acid Strength of Support Materials as a Factor Controlling Catalytic Activity of Noble Metal Catalystsfor Catalytic Combustion Y. Yazawa, H. Yoshida, N. Takagi, N. Kagi, S. Komai, A. Satsurna, Y. Murakami and T. Hattori .
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P210 Structural Features and Activity for CO Oxidation of LaFeXNil,03+6 Catalysts 2195 H. Falcbn, J. Baranda, J.M. Campos Martin, M.A. Peiia and J.L.G. Fierro . P2 11 Amorphous Alloys as Catalystsfor Hydrogen Oxidation M. Stancheva, St. Manev, D. Lazarov and E. Stancheva
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220 1
P2 12 Precursor of Iron Catalystfor Ammonia Synthesis: Fe3O4, Fel,O, Fe203 or their Mixture? L. Huazhang and L. Xiaonian
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P213 Production ofAlkenes over Chromia Catalysts: Effect of Potassium on Reaction Sites and Mechanism S.D. Jackson, I.M. Matheson, M.-L. Naeye, P.C. Stair, V.S. Sullivan, S.R. Watson and G. Webb .
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P2 14 Acid-base Site Requerimentsfor Elimination Reactions on Alkali Promoted MgO V.K. Diez, C.R. Apesteguia and J.I. Di Cosimo .
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P2 15 Structure-Reactivity Correlations Across the Pressure-Gap Studied on Epitaxial Iron Oxide Model Catalyst Films C. Kuhrs and W. Weiss .
2225
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P216 Catalytically-Mediated Generation of HNO2 in Highly HN03 Concentrated Media S. Guenais-Langlois, C. Bouyer, J.-C. Broudic, A. Ananiev and B. Coq . P2 17 Catalytic Nitrate Reduction: Kinetic Investigations . U . Priisse, J. Daum, C. Bock and K.-D. Vorlop P218 Liquid Phase Isomerization of Propadiene to Methyl Acetylene on Modi3ed Alumina Catalysts R. Dotzel, M. Reif and E. Klernm . P2 19 The Eflects of Raney Alloy Structure on the Activation Process and the Properties of the Resulting Catalyst S. Knies, M. Berweiler, P. Panster, H.E. Exner and D.J. Ostgard . P220 Raney-Nickel-Iron Catalysts Obtained by Mechanical Alloying: Characterization and Hydrogenation Activity J. Salmones, B.H. Zeifert, J.A. Hernandez, R. Reynoso, N. Nava, J.G. Cabafias Moreno and G. Aguilar Rios . P22 1 Reduction of Benzaldehyde on Copper Supported on SiO2. Effect of Method of Preparation . A. Saadi, M.M. Bettahar and Z. Rassoul P222 Some Phenomenological and Mechanistic Aspects of the Use of Copper as Catalyst in Trichlorosilane Synthesis H. Ehrich, T . Lobreyer, K. Hesse and H. Lieske . P223 The Role of Oxygen Vacancies in Zirconia on the Dispersion, Stabilisation and Reactivity in the Presence of 0 2 of Supported Rh Particles G. Centi, B. Panzacchi, S. Perathoner and F. Pinna . P224 Interactions among Rh, Mn and Li Components in the Rh-Based Catalysts Y . Wang, Z . Song, D. Ma, H. Luo, D. Liang and X. Bao . P225 Direct Synthesis of Propionamide and Propiononitrilefrom Ethylene Carbon Monoxide, and Ammonia Using Supported Ru Catalyst S.-P. Zhao, S.-I. Sassa, H. Inoue, M. Yamakazi, H. Watanabe, T. Mori . and Y. Morikawa P226 Structure, Surface State and Catalytic Properties of a Model Platinum Catalyst J. Find, Z. Pail, H. Sauer, R. Schlogl, U. Wild and A. Wootsch . P227 Structure of Ultradisperse Pt Powders and their Performance in the Partial Oxidation of C-H Bonds by Molecular Oxygen A.P. Suknev, V.B. Goncharov, V.I. Zaikovskii, A.S. Belyi, N.I. Kuznetsova, V.A. Likholobov and B.S. Bal'zhinimaev .
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P228 Eflect of Water Vapour on the Catalytic Activity of Supported Platinum Catalysts S. Lunzer and R. Krarner .
2303
P229 Stabilization of Molecular Pd Species in a Heterogenized Wacker-Type Catalystfor Low Temperature CO Oxidation E.D. Park and J.S. Lee
2309
P230 Surface Properties of Palladium Supported on Cerium Oxide and its Catalytic Activity for Methanol Decomposition Y . Matsurnura, Y . Ichihashi, Y. Morisawa, M. Okurnura and M. Haruta .
2315
P23 1 Deactivation of Palladium Catalysts Supported on Functionalized Resins in the Reduction ofAromatic Nitrocompounds M. Krhlik, V . Kratky, M. Hronec, M. Zecca and B. Corain .
2321
P232 Metal Palladium Dispersed Inside Macroporous Ion-Exchange Resins: Textural Characterization and Accessibility to Gaseous Reactants A. Biffis, K. Jerabek, A,A, D'Archivio, L. Galantini and B. Corain
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P233 Novel Supported Heterogeneous Palladium Catalystsfor Carbon-Carbon Forming Reactions E.B. Mubofu, J.H. Clark and D.J. Macquarrie .
2333
P234 Passivation of Nickel Activity in the Nickel-Nb205-Si02System at Low Metal Contents M.M. Pereira, E.B. Pereira, Y.L. Lam, L.T. dos Santos and M. Schmal
2339
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P235 Gold Based Environmental Catalyst L.A. Petrov .
2345
P236 Evolution of a New Reaction Pathway in Propene-Deuterium Exchange Reaction by the Co-adsorption of CO over Rh, Ir, and 0 s Carbonyl Cluster Derived Catalysts Supported on Alumina S. Naito, K. Oguni, T. Naito and T. Miyao .
235 1
P237 Studies on a ChromidLanthanalZirconiaAromatization Catalyst D.L. Hoang, A. Trunschke, A. Briickner, J. Radnik and H. Lieske .
2357
P238 The Reforming Catalytic Properties of Bulk and Supported Molybdenum and Tungsten Dioxides, X02 (X = Mo, W). A. Katrib, L. Urfels and G. Maire .
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Refining and Petrochemistry
P239 Active Sites of the Isomerisation of n-Butane over Oxygen-Modijed Molybdenum Carbide and Molybdenum Oxides S . Liebig, T. Gerlach, Kh. Doukkali and W. Griinert
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AlzOj-Promoted Sulfared Zirconia Catalystsfor the Isomerization of n-Butane R. Olindo, F. Pima, G. Strukul, P. Canton, P. Riello, G. Cerrato, . G. Meligrana and C. Morterra n-Pentane Isomerization over Mesoporous Sulfated Zirconia Catalysts M. Risch and E.E. Wolf . Bzfunctional Ni, Pt Zeolite Catalystsfor the Isomerization of n-Hexane M.H. Jordao, V. Simoes, A. Montes and D. Cardoso About the Importance of Surface Hydrogen Availability during n-Hexane Isomerization over Platinum on Tungsten Oxide Promoted Zirconia M.G. Falco, S.A. Canavese, R.A. Comelli and N.S. Figoli . Highly Dispersed Framework Zirconium Phosphates-Acid Catalysts of Hexane Isomerization S.N. Pavlova, V.A. Sadykov, G.V. Zabolotnaya, D.I. Kochubey, R.I. Maximouskaya, V.I. Zaikovskii, V.V. Kriventsov, S.V. Tsybulya, E.B. Burgina, A.M. Volodin, N.M. Ostrovskii, A.M. Simakov, T.A. Nikoro, V.B. Fenelonov, M.V. Chaikina, N.N. Kmetsova, V.V. Lunin, D. Agrawal and R.Roy . Zeolites and Natural Materials as Catalystsfor n-Heptane Hydroisomerization Reaction A. Brito, M.C. Alvarez, F.J. Garcia, M.E. Borges and M. Torres . Influence of the Zeolite Structure on the Hydroisomerization of n-Heptane A. Patrigeon, E. Benazzi, Ch. Travers and J.Y. Bernhard . Hydroisomerization of Octane on Pt/AI-Pillared Vermiculite and Phlogopite and Comparison with Zeolites F.J. del Rey P.C., M.L. Shchez Henao and G. Poncelet . Slurry Isomerization of n-Hexadecane over Moo3-Carbon-Mod$ed, Pt/PZeolite, Pt/ZSM 22 and Pt/SAPO 1 I Catalysts at Medium Pressure S. Roy, C. Bouchy, C. Pham-Huu, C. Crouzet and M.J. Ledoux . Zinc Oxide Modified by Alkylsilylation as an Efficient Catalystfor Isomerization of Hydrocarbon Y . Imizu, T. Narita, Y. Fujito and H. Yamada . Eflect of Redox Treatments on the Surface State of Pd- W-Based Catalysts and on the Skeletal Rearrangement of Hydrocarbons. C. Bigey, F. Garin, L. Hilaire and G. Maire .
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P25 1 Acid or Bzfunctional Mechanism in ParafJin Isomerization Reaction and Pt/W03-Zr02 Catalysts on P~/SO;--Z~O~ J.C. Yori, C.L. Pieck and J.M. Parera
P252 Catalytic Cracking of Heavy Oil Fractions over Natural Zeolite Contained Composites R.H. Ibrasheva and K.A. Zhubanov . P253 Evidence for DifSerent Reaction Mechanisms as the Cause of the Enhanced Hydrocarbon Cracking Activity of Steamed Y Zeolites . B.A. Williams, W. Ji, J.T. Miller, R.Q. Snurr and H.H. Kung P254 TPR and C02-TPDof Composite Titania (Anatase)-Alumina Systems as Potential Vanadium Trapsfor Fluid Catalytic Cracking (FCC) F. Hernimdez Beltrim, E. Mogica Martinez, E. L6pez Salinas and M.E. Llanos Serrano . P255 Formation of SulJitr-Containing Compounds Under Fluid Catalytic Cracking Reactions P. Leflaive, J.L. Lemberton, G. Perot, C. Mirgain, J.Y. Carriat and J.M. Colin P256 The Use of Catalytic Cracking Technologyfor the Creation of Novel Processes for Oil Refinery and Petrochemistry M.I. Levinbuk and N.Y. Usachev . P257 Transformation of Methylcyclohexane over Fresh HFA U and HMFI Zeolites: Reaction Scheme and Kinetic Modeling H.S. Cerqueira, M.G.F. Rodrigues, P. Magnoux, D. Martin and M. Guisnet P258 Mechanism of n-Hexane Cracking on MFI (Si/Al= 10) L. Isernia, A. Quesada, J. Lujano and F.E. Imbert . P259 Hydrocracking of Heavy Straight Run Naphtha with Pt Supported on Zeolites Y, USY, ZSM5 and Beta H. Ortiz Soto and L.J. Hoyos Marin . P260 Hydrocracking of Phenanthrene over Pt/Si02-A1203,Pt/H-Y, Pt/H-Beta and Pt/H-ZSM-5 Catalysts: Reaction Patway and Products Distribution L. Leite, E. Benazzi, N. Marchal-George and H. Toulhoat . P261 Hydrogenation and Dehydrogenation of Hydrocarbons over Ni Supported on Alumina- and Silica-Promoted Titania G. Ptrez and T. Viveros . P262 Dehydroisomerization of N-Butane on PT-ZN/H-Mordenite M.M. Ramirez-Coreedores, T . Romero and M. Gordlez .
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P263 Bimetallic Pt-M(M=Ga,ln,T1) Silica Supported Catalystsfor lsobutane Dehydrogenation J. Llorca, P. Ramirez de la Piscina, J. Lebn, J. Sales, J.L.G. Fierro and N. Homs .
2513
P264 Aromatization of Ethane on Modijied Zeolites in the Presence of co-reactans F. Roessner, A. Hagen and J. Heemsoth .
2519
P265 Effect of Barium and Lanthanum Oxides on the Properties of Pt/KL Catalysts in the n-Heptane Dehydrocyclization J.M. Grau, L.M. Gomez Sainero, L. Daza, X.L. Seoane and A. Arcoya
2525
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P266 Effect of Lanthanum Doped y-A1203 in the Selectivity of Reforming Pt-Pb Supported Catalysts. G. del Angel, G. Torres and V. Bertin .
2531
P267 Regeneration and Oxidation-Reduction Cycles of Vapor Phase and Incipient Wetness Impregnation Pt/KL Catalysts F. Ghadiali, G. Jacobs, A. Pisanu, A. Borgna, W.E. Alvarez and D.E. Resasco
2537
P268 Mechanism of Iso-butane Alkylation by Butenes over H3PW12040, HflTiW11039and Zr Supported Catalysts E.A. Paukshtis, V.K. Duplyakin, V.P. Finevich, A.V. Lavrenov, V.L. Kirillov, L.I. Kuznetsova, V.A. Likholobov and B.S. Bal'zhinimaev.
2543
P269 Porous 12-TungstophosphoricAlkaline Salts for Isobutane/Butene Alkylation. Influence of Protonic Density and Surface Polarity of the HPA. Effects of the Supercritical Phase P.Y. Gayraud, N . Essayem and J.C. Vedrine
2549
P270 Influence of the Metal Content on the Amount and the Nature of the Coke Formed during Isobutane/2-Butene Alkylation over Ni-Y Zeolite P.A. Arroyo, C.A. Henriques, E.F. Sousa Aguiar, A. Martinez and J.L.F. Monteiro .
2556
P271 The Role of Hydride Transfer in Zeolite Catalyzed Isobutane/Butene Alkylation 2561 . G.S. Nivarthy, A. Feller, K. Seshan and J.A. Lercher P272 Mechanistic Routes of Low Temperature Alkane Activation over Zeolites J.A.Z. Pieterse, K. Seshan and J.A. Lercher .
2567
P273 Catalytic Conversion of n-Parafins in Supercritical Phase W. Wei, F. Li, J. Ren, Y. Sun and B. Zhong.
2573
P274 Sulphur Resistant Palladium-Platinum Catalysts Preparedfiom Mixed Acetylacetonates A.J. Renouprez, A. Malhomme, J. Massardier, M. Cattenot and G. Bergeret .
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P275 Production of High Cetane Diesel Fuels by Simultaneous Hydrogenation of Aromatics and Ring Opening of Naphthenes over Bzfinctional Molecular Sieves Based Catalysts M.A. Arribas and A. Martinez
P276 Formation of Diisopropyl EtherJLoin 2-Propanol Using Keggin-Type H3[W12P040]and H4[W12Si040]Heteropolyacids Supported on Zirconia E. Lopez Salinas, I.G. Hemiindez Cortez, J. Navarrete, M. Salmon and I. Shifter . P277 The Mechanism of Diisopropyl Ether Synthesispom a Feed of Propylene and Isopropanol over Ion Exchange Resin F.P. Heese, M.E. Dry and K.P. Moller . P278 The Conversion of 2,3-Butanediol to Methyl Ethyl Ketone over Zeolites J. Lee, J.B. Grutzner, W.E. Walters and W.N. Delgass . P279 Effect of Internal Dzffusion on Liquid-Phase Synthesis of MTBE R. Pla, J. Tejero, F. Cunill, J. Felipe Izquierdo, M. Iborra and C. Fit6 P280 Acid and Catalytic Properties of Supported Sulfpolyphenylketones in the Formation of DIPE, MTBE and TAME Ethers T. Jarecka, St. Miescheriakow and J. Datka . P28 1 Effect of the Addition ofAcidic and Basic Compounds on the Catalytic Behavior of Exchanged Zeolites in the Alkylation of Toluene with Methanol . A. Borgna, J. Sepulveda, S. Magni and C. Apesteguia P282 Zeolite Beta: Selective Molecular Sievefor Synthesis ofXylenesfiom Trimethylbenzenes J. Cejka and A. Krejci P283 Characterisation and Reactivity of Supported and Unsupported B/P/O Catalysts in the o-Alkylation of Diphenols with methanol F. Cavani, T . Monti and D. Paoli . P284 Oxidative Alkylation of Isobutene, Propene and Toluene with Methanol . I.P. Belomestnykh and G.V. Isaguliants P285 Side Chain Methylation of Toluene and Ethylbenzene with Dimethylcarbonate over Alkaline X-Zeolite R. Bal and S. Sivasanker . P286 The effect of the particle size on methanol conversion to ligh olefins De Chen, K. Moljord and A. Holmen
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P287 Synthesis of N-Isopropylacrylamide Synthesis Acrylonitrile and Isopropyl Alcohol over Solid Acids X . Chen, H. Matsuda and T. Okuhara
2657
P288 Pumice as Catalyst in the Claus Reaction A. Brito, R. Larraz, R. Arvelo, M.T. Garcia and M.E. Borges
2663
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P289 Supported Alkali Salt Catalysts Active for the Guerbet Reaction between Methanol and Ethanol K. Gotoh, S . Nakamura, T. Mori and Y. Morikawa .
2669
P290 Catalyst Porous Structure and Acidity Effects on Alkylphenol Transformations 2675 . J.C. Hernhdez, F.E. Imbert, P. Ayrault, M. Guisnet and N.S. Gnep Reactor Engineering and Catalytic Processes
P291 Natural Gas Utilization through CO2 Reforming in a Membrane Reactor T.M. Raybold and M.C. Huff.
268 1
P292 Activity and Selectivity of a Pd/y-A1203 Catalytic Membrane in the Partial Hydrogenation of Acetylene C. Lambert, M. Vincent, J. Hinestroza, N. Sun and R. Gonzalez .
2687
P293 Catalysis of Palladium Salt Reduction in a Gas-Liquid Membrane Reactor S. Miachon, A. Mazuy, J.-A. Dalmon .
2693
P294 Propane Aromatization in a Silicalite-1 Membrane Reactor W . Yang, P. Yang, X. Xu and L. Lin. P295 Effects of Operation Modes on the Oxidation of Propane to Acrolein in a Membrane Reactor W . Yang, P. Yang, W. Fang and L. Lin .
2705
P296 The Conversion of Methanol to Hydrocarbons over ZSMS: Fixed Bed vs. Recycle Reactor K.P. Moller, W . Bohringer, A.E. Schnitzler and C.T. O'Connor
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P297 Catalytic Dehydrogenation of n-Butane in a Fluidized Bed Reactor with Separated Coking and Regeneration Zones C. Callejas, J. Soler, J. Herguido, M. Menendez and J. Santarnaria.
2717
P298 Study in a Riser Simulator Reactor of the Role of a HZSM-5 Zeolite as FCC Catalyst Additive J.M. Arandes, I . Abajo, I. Fernhdez, J. Bilbao, H.I. de Lasa .
2723
P299 Difision Analysis of Cumene Cracking over ZSMS Using a Jetloop Reactor P. Schwan, K.P. Moller and J.P. Henry .
2729
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Monolithic Catalysts as more Eficient Reactors in Three-PhaseApplications T.A. Nijhuis, F. Kaptjein and J.A. Moulijn . Synthesis of Ethylene Oxide in a Catalytic Microreactor System H. Kestenbaum, A. Lange de Oliveira, W. Schmidt, F. Schiith, W. Ehrfeld, K. Gebauer, H. Lowe and Th. Richter Develoment of a Novel Structured CataIytic Reactorsfor Highly Exothermic Reactions . G. Groppi, C. Cristiani, M. Valentini and E. Tronconi Simulation of Alternative Catalyst Bed Configurations in Autothermal Hydrogen Production A.K. Avci, D.L. Trimm and Z.I. 0nsan . Development and Study of Metal Foam Heat-Exchanging Tubular Reactor: Catalytic Combustion of Methane Combined with Methane Steam Reforming Z.R. Ismagilov, O.Yu. Podyacheva, V.V. Pushkarev, N.A. Koryabkina, V.N. Antsiferov, Yu.V. Danchenko, O.P. Solonenko and H. Veringa . Optimization in Design, Testing and Reactor Use by Statistical Modeling of the Reaction Variables, Energy Input and Reaction Time E. Balanosky, F. Herrera and J. Kiwi Catalysis on Sulfides, Nitrides and Carbides
Synthesis of Highly Dispersed Molybdenum and Ruthenium Sulfides Using Tetraalkylammonium Surfactant P. Afanasiev, G.-F. Xia, B. Jouguet and M. Lacroix. Properties of Sonochemically Synthesized, Highly Dispersed MoS2/AI203 Catalysrsfor the Hydrodedulfurization of Dibenzothiophene and 4,6Dimethyldibenzothiophene J.J. Lee, C. Kwak, Y.J. Yoon, T. Hyeon and S.H. Moon . Structure and Catalysis of Intrazeolite Co-Mo Binary Sulfide Model Clustersfor Hydrodesuljkrization Y. Okamoto, H. Okamoto and T. Kubota . An Inelastic Neutron Scattering Study of the Interaction of Thiophene with a Hydrodesulfurisation Catalyst P.C.H. Mitchell, D.A. Green, E. Payen, J. Tomkinson and S.F. Parker Characterization of Oxide Catalysts Using Time-Resolved XRD and XANES: Properties of Pure and Sulfided CoMo04 and NiMoO4 J.A. Rodriguez, J.C. Hanson, S. Chaturvedi and J.L. Brito .
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E$ect of the Support Porosity on the Thiophene and Dibenzothiophene Hydrodesulfirization Reactions. A1203-Ti02Mixed Oxide Support T . Klimova, J. Ramirez, R. Cuevas and H. GonAlez . TPR-S and TPS Studies of CoMo and NiMo Catalysts Supported on Al203-Ti02 Mixed Oxides L. Cedefio, R. Zanella, J. Ramirez and A. L6pez Agudo . Characterization of Reduced and Sulphided Ru-V/A1203Catalysts C.E. Scott, J. Guevara, A. Scaffidi, E. Escalona, C. Bolivar C., M.J. PCrez Zurita and J. Goldwasser . Sulfir Effect on Mo2N/y-ALO3Catalyst Studied by In Situ FT-IR Spectroscopy Z. Wu, Y. Chu, S. Yang, Z. Wei, C. Li and Q. Xin . Concerted Mechanisms in the Heterogeneous Catalysis by SulJides A.N. Starsev . Selective Promoting Effect on Alkylbenzothiophenes HydrodesulJitrization Pathways F. Bataille, J. Mijoin, J.L. Lemberton, G. Perot, C. Berhault, M. Lacroix, . F. Mauge, S. Kasztelan and M. Breysse Deep Hydrodenitrogenation on Pt Supported Catalysts in the Presence of Hfi, Comparison with NiMo Sulfide Catalyst E. Peeters, C. Geantet, J.L. Zotin, M. Breysse and M. Vrinat . Hydrotreating of Heavy Gas Oil on Unsupportesd and Supported Mo-, W-, Nb-Nitrides J.A. Melo Banda, J.M. Dominguez and G. Sandoval Robles . Evidence for the Effect of Phosphorus on Molybdenum Oxynitrides and Oxycarbides, Activity and Selectivity in Propene Hydrogenation and n-Heptane Isomerization P. PCrez Romo, C. Potvin, J.-M. Manoli and G. Djeda-Mariadassou Degradation of a Spent Hydrotreating Cata1yst:interaction with Enviroment J.C. Afonso, T . Siqueira de Lima and P.C. Campos . Influence of Sulfur Containing Compounds on High Temperature Coke Formation on Hydrotreating Catalyst R. Lebreton, S. Brunet, G. Ptrot, V. Harle and S. Kasztelan Deactivation of MoS2 Cataysts during the HDS of Thiophene. Effect of Nickel Promoter F. Pedraza, S. Fuentes, M. Vrinat and M. Lacroix .
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P323 Deactivation and Testing of Molybdenum Nitride Catalysts in Hydrodesulfirization of Dibenzothiophene M. Nagai, M. Kiyoshi and S. Omi .
P324 Development of a Method for the S t u 4 of the Deactivation of HDS Catalysts J.M. Trejo and F. Albertos . P325 Deactivation by Oxygen and Subsequent Activation of Bulk Mo2C for Benzene Hydrogenation at 298 K J.-S. Choi, G. Bugli and G. Djega-Mariadassou . P326 Silicon Carbide Supported NiS2 Catalystfor the Selective Oxidation of H2S in Claus Tail Gas M.J. Ledoux, C. Pham-Huu, N. Keller, J.B. Nougayrede, S. Savin-Poncet and J. Bousquet . Molecular Sieves
P327 Novel Method of Preparing Zeolite Filmsfrom Colloidal Zeolite Nay H-Q. Lin, Z-S. Chao, T-H. Wu, G.-Z. Chen, F-X. Zhang, H-L. Wan And K.-R. Tsai P328 Evidence of the Supermicropores Creation in Zeolites. Dealumination of Narrow-Pore Zeolites P. Hudec, A. Smieskova, Z. Zidek and E. Rojasova . P329 Transition through Various Regimes of Reaction Kinetics and Selectivity Driven by Progressing Modification of the External Surface of Zeolites H.P. Roger, K.P. Moller, W. Bohringer and C.T. O'Connor P330 Polycarbonylic and Polynitrsylic Species in ~ u ' - ~ x c h a n ~ZSM-5, e d Beta, Mordenite and Y Zeolites: Comparison with Homogeneous Complexes G. Turnes Palomino, A. Zecchina, E. Giamello, P. Fisicaro, G. Berlier and C. Lamberti . P33 1 The Characterization of Zeolite Acidity by Dinamic Methods Gy. Onyesty&, J.Valyon and L.V.C.Rees . P332 Textural Characterization of Zeolites by Adsorption of Molecules of Different Size Followed by Adsorption Microcalorimetry J.M. Guil, R. Guil Lopez and J.A. Perdigon Mel6n . P333 Unusual bomerization Routes of n-Butenes on the Acidic OH(0D) Groups on Ferrierite Zeolite Studied by FT-IR J.N. Kondo, E. Yoda, M. Hara, F. Wakabayashi and K. Domen .
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P334 Use o f ' 2 9 ~N eMR Spectroscopyfor the Study of Gaseous Reactant Dzfision in a Fixed Bed of Zeolite; Application to Benzene Difision in HZSM-5 P. N'Gokoli-Kekele, M.-A. Springuel-Huet, J.-L. Bonardet and J. Fraissard
P335 Ti-0-Si and Fe-0-Si Valence Angles of Tetrahedral Ti and Fe Sites in Porous Silicas of Diflerent Structures F. BCland and B. Echchahed and L. Bonneviot . P336 FTIR Study of the Proton Transfer in Al-MCM-22 Zeolite to Insaturated Hydrocarbons B. Onida, F. Geobaldo, F. Testa, F. Crea and E. Garrone . P337 Sorption of Methanol in Alkali Exchanged Zeolites M. Rep, A.E. Palomares, J.G. van Ohrnmen, L. Lefferts and J.A. Lercher . P338 Structural Lewis Sites in Zeolite Beta - Role on Coking of the Catalyst A. Vimont, F. Thibault-Startzyk and J.C. Lavalley . P339 Mechanism of Coking and Regeneration of Catalysts Containing ZSM-5 Zeolite V.G. Stepanov, E.A. Paukshtis, V.V. Chesnokov and K.G. Ione . P340 Effect of Dealumination of Mordenite Catalystfor Amination Reaction of Ethanolamine K. Segawa and T. Shimura . P34 1 Synthesis and Characterization of Pd-Zr- and Pd-Rh-Beta Zeolite Catalystsfor Removal of EmissionsJi.om Natural Gas Driven Vehicles N. Kurnar, F. Klingstedt and L.-E. Lindfors . P342 Catalytic Behavior of the Microporous Hexagonal Zincophosphate CZP in Base-Catalyzed Reactions L.A. Garcia Serrano, J. PCrez Pariente, F. Shchez and E. Sastre . P343 Basic CsBeta - A New Support for Pt Nanoparticles Active in Aromatization of Parafins C. Jia, A.P. Antunes, J.M. Silva, M.F. Ribeiro, M. Lavergne, M. Kermarec and P. Massiani P344 Hydroformylation of Olefins Using Encapsulated HRh(CO)(PPhj) 3 in Nay Zeolite as a Catalyst K. Mukhopadhyay, V.S. Nair and R.V. Chaudhary . P345 Spectroscopic and Catalytic Investigation ofthe NO Reactivity on CoAPOs with Chabasite-Like Structure L. Marchese, E. Gianotti, B. Palella, R. Pirone, G. Martra, S. Coluccia . and P. Ciambelli
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P346 Bifinctional Zeolite Catalystsfor the Selective Synthesis in One Step of Various Ketones P. Magnoux, N. Lavaud, L. Melo, G. Gianetto, A.I. Silva, F. Alvarez and M. Guisnet
301 1
P347 Considerations about the Bzfunctionallity and the Gallium Species Role in the Propane Aromatization Reaction over [Gal- and [Ga,AI]-ZSM-5 Catalysts A. Montes and G. Gianetto .
3017
P348 Novel Shape-Selective Catalystsfor Synthesis of 4, 4'-Dimethylbiphenyl J.-P. Shen, L. Sun and C. Song .
3023
P349 Investigation of n-Pentane and Cyclohexane Total Oxidation on the Cu-Containing ZSM-5 Zeolites M.A. Botavina, M.-V. Nekrasov and S.L. Kiperman
3029
P350 Textural Properties, Stability and Catalytic Activity of Acidic Mesoporous Materials Obtained By Direct Incorporation of Aluminum Y.-H. Yue, A. GCdCon, J.-L. Bonardet, J.B. D'Espinose and J. Fraissard .
3035
P35 1 Templating Fabrication and Catalysis of Platinum Nanowires in Mesoporus Channels of FSM-I6 A. Fukuoka, N . Higashimoto, M. Sasaki, M. Harada, S. Inagaki, Y. Fukushima and M. Ichikawa .
3041
P352 Physico-Chemical and Catalytic Properties of Ni-Containing Mesoporous Molecular Sieves of MCM-41 Type M. Ziolek, I. Nowak, I. Sobczak and H. Poltorak . P353 Preparation of Highly Structured V-MCM-41 and Determination of its Acidic Properties S. Lim and G.L. Haller
3053
P354 Post-Synthetic Preparation of Ti-MCM-41 Oxidation Catalysts with Titanium Alkoxides A. Hagen, K. Schueler and M. Voskamp .
3059
P355 Fe-MCM-41 as a Novel Sulfuric Acid Catalystfor SO2 Rich Feeds A. Wingen, N . Anastasieviec, L. Hollnagel, D. Werner and F. Schiith
3065
.
Session P3 Catalyst Characterisation
P356 In situ Inpared Spectroscopic Study of the Reaction between CO and Hz over an Sm203Catalyst . Y. Sakata, T . Arimoto, H. Imamura and S. Tsuchiya
3071
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liii
P357 In Situ Inpared Spectroscopic Investigation of 12-Tungstphosphoric Acid and its Cesium Hydrogen Salts in Relation to Catalytic Activiry G. Koyano, T . Saito, M. Hashimoto and M. Misono
3077
P358 The Oxidation State of Ru Catalysts under Conditions of Partial Oxidation of Methane Studied by XPS and FTIR Spectroscopy C. Elmasides, D.I. Kondarides, S. Neophytides and X.E.Verykios .
3083
P359 Correlation between "In Situ " XPS Characterization and Catalytic Activity of Mo03/cr-A1203for Hexenes and Hexanes Isomerization. Mechanistic Approach Using Probe Molecules V. Keller, F. Barath, F. Garin and G. Maire .
3089
P360 In Situ Observation of a Surface Catalysed Chemical Reaction by Fast X-Ray Photoelectron Spectroscopy A.F. Lee, K. Wilson and R.M. Lambert .
3095
P361 The Structure of Vanadium Oxide on y-Alz03: An In Situ X-Ray Absorption Spectroscopy Study during Catalytic Oxidation M . Ruitenbeek, F.M.F.de Groot, A.J. Van Dillen and D.C. Koningsberger
3101
P362 Flux Response Analysis - A New In Situ Techniquefor Catalyst Characterization K. Hellgardt, B.A. Buffham, M.J. Heslop, G. Mason and D.J. Richardson
3107
P363 New Method of the Surface Characterisation of a Metal Catalyst under Real Reactor Conditions using Electron Microscopy W. Arabczyk, K. Kalucki, U. Narkiewicz, D. Moszynski and A.W. Morawski
31 13
P364 Direct Observation of Reduction of PdO to Pd Metal by In Situ Electron Microscopy P.A. Crozier and A.K. Datye .
31 19
P365 Identification and Roles of the Different Active Sites in Supported Vanadia Catalysts by In Situ Techniques M.A. Baiiares, M. Martinez-Huerta, X. Gao, I.E. Wachs and J.L.G. Fierro
3125
P366 Design of an SFG-Compatible Uhv-High Pressure Reaction Cell: Studies of CO and NO Adsorption on Ni and NiO(l00) by IR-Vis Sum Frequency Generation Vibrational Spectroscopy G. Rupprechter, T . Dellwig, H. Unterhalt and H-J. Freund .
3131
P367 The Combined Use of Acetonitrile and Adamantane-Carbonitrilefor FTIR Spectroscopic Characterization of Acidity in Zeolites C. Otero Areh, M. Rodriguez Delgado, M. Peiiarroya Mentruit, . F.X. Llabres i Xamena and C. Morterra
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P368 Infiared Spectroscopic Evidence for Two Ways of Adsorbed CO Coordination A.A. Tsyganenko, C. Otero Arein and E. Escalona Platero .
3143
P369 Study of Layer-by-Layer Growth of Silica on Alumina and Alumina on Silica Using IR Spectroscopy of Adsorbed CO A.A. Tsyganenko, O.V. Manoilova, K.M. Bulanin, P.Yu. Storozhev, S. Haukka, A. Palukka and M. Lindblad .
3 149
P370 Applications of Inelastic Incoherent Neutron Scattering in Technical Catalysis P. Albers, G. Prescher, K. Seibold and S.F.Parker .
3155
P371 Hydrogen Species in Cerium - Nickel Oxide Catalysts: Inelastic and Compton Neutron Scattering Studies C. Lamonier, E.Payen, P.C.H.Mitchel1, S.F. Parker, J. Mayers and J.Tornkinson .
3161
P372 Using Atomic Force Microscopy (AFW to Study the Surface Structure of Oxide and Metal-Decorated Oxide Particules E. Dokou, W.E. Farneth and M.A. Barteau .
3167
P373 Study of Catalyst Preparation Processes by Atomic Force Microscopy (AFM: Adsorption of a Pt Complex on a Zeolite Surface M.KomiyamaandN.Gu .
3173
P374 Chemistry of SO2 on Model Metal and Oxide Cata1ysts:Photoemission andXANES Studies J.A. Rodriguez, T. Jirsak, S. Chaturvedi, J. Hrbek, A. Freitag and J.Z. Larese
3 177
P375 Changes in the Electronic Structure of Pt/CeOz Based DENOXCatalysts as a Function of the Reduction Treatment. Detection of a Pt-H Anti-Bonding Shape Resonance and Pt-H EXAFS J.H. Bitter, M.A. Cauqui, J.M. Gatica, S. Bernal, D.E. Ramaker and D.C. Koninsberger .
3183
P376 Bimetallic Nano-Particle Formation in the Pt-Re Reforming Catalysts Revealed by STEWEDX XANES/EAYFS and Chemical Characterization Techniques. EfSects of Water and Chlorine T . Gjeman, M. bnning, R. Prestvik, B. Tstdal, C.E. Lyman and A. Holmen
3189
P377 Hyperfine Interactions on Pt-In Catalysts F.B. Passos, P.R.J. Silva and H. Saitovich .
3195
P378 Oxygen Atom Radical Formation on the Sol-Gel Molybdenum-Silica Catalysts Characterized by X-Ray Absorption Fine Structure Spectroscopy Y . Izumi .
3201
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P379 Structural Characterization of Pd-Ag and Pd-Cu Bimetallic Catalysts by means of EX4FS WAXS and XPS A. Longo, A. Balema, F. Deganello, L.F. Liotta, C. Meneghini, . A. Martorana and A.M. Venezia P380 Super Acidity Confirmed on a Monolayer of Sulfare Species Loaded on Zirconia N. Kadata, J-I. Endo, K-I. Notsu, N. Yasunobu, N. Naito and M. Niwa
.
P38 1 Characterization and Catalytic Properties of Aerogels Suyated Zirconia . M.K.Younes, A. Ghorbel, A. Rives, R. Hubaut P382 Structure and Surface Properties of Zr02-Supported W03Nanostructures C.D. Baertsch, R.D. Wilson, D.G. Barton, S.L. Soled and E. Iglesia . P383 Proton Afinity in Heterogeneous Acid Base Catalysis. Measurements and Usefor Analysis of Catalytic Reaction Mechanism E.A. Paukshtis P384 Basic Sites on Mixed Nitrited Galloaluminiophosphates "AIGaPON" Confrontation of Spectroscopic and Catalytic Data S.Delsarte and P. Grange . P385 Study of Relevant properties injluencing the catalytic activity of layered double hydroxides in the Meixnerite-likeform F. Prinetto, G. Ghiotti and D. Tichit . P386 Copper Redox Chemistry in CuZSM-5 Zeolites: EPR, IR and DFT Investigations B. Gil, J. Datka, S. Witkowski, Z. Sojka and E. Broclawik . P387 Dynamics of Adsorbed Benzene on Ag-Y Zeolite Studied by Solid-State NMR A. GedCon, D.E. Favre and B.F. Chmelka . P388 Interaction of CO and NH3 with Noble Metal Cations Dispersed in ZSM-5 Zeolites. Spectroscopic and Microcalorimetric Investigation V . Bolis, S. Bordiga, V. Graneris, C. Lamberti, G. Turnes Palomino and A. Zecchina . P389 The Use of NMR Imaging and Mercury Porosimetry in the Modeling and Measurement of Coke Profiles in Deactivated Catalyst Pellets S.P. Rigby . P390 Low-loaded Metal Pd-Au Supported Catalysts on Active Carbon. Recent Developments of the X-Ray Dzji-action Analysis to Detect Simultaneously Nanoclusters and Larger Particles . P.Riello, P. Canton, A. Minesso, F. Pinna and A. Bennedetti.
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P39 1 Characterisation of Aukorbates and Surface Functional Groups on Polycrystalline Oxides by UltravioletPhotoelectron Spectroscopy (UPS) M. Heber and W.Griinert .
P392 Evidence of an Alloying Eflect in Zeolite Supported Pt-Pd Systems as Seen by Hidrogen Chemisorptionand Proton NMR T . Rades, M. Polisst-Thfoin and J. Fraissard. P393 Catalytic Probe ofAg Venters on Silver Halide and Supported Silver 1.1. Mikhalenko and V.D. Yagodovskii . P394 Change in the Redox Properties of PdRWCe02-ZrO2 Catalysts afier Ageing at 1323 and 1423 K S. Salasc, V. Perrichon, M. Primet, M. Chewier and N. Mouaddib-Moral . P395 XPS Study of AuITiO2 Catalytic Systems J.W. Sobczak and D. Andreeva . P396 Procedure-Sensitive HTR for Au/TiOz M.-Y. Lee, Y.-J. Lee and S.D. Lin . P397 Characterization of Amino-ModiJied Silicate Xerogels Complexed with Copper (IQ, Cobalt (III) and Chromnium (IIQ W.K. Jozwiak, E. Szubiakiewicz, A.M. Klonkowski, T. Widemik, W. Ignaczak and T. Paryjczak . P398 Active Sites on Well-CharacterizedPd/Si02 Determined by (-)-Apopinene Deuteriumation, Cyclohexene Hydrogenation and CS2 Titration G.V. Smith and D.J. Ostgard . P399 Structural and Catalytic Properties of Zr-Ce-0 Mixed Oxides. Role of the Anionic Vacancies S. Rossignol, C. Micheaud Especel and D. Duprez . P400 An X-Ray Photoelectron Spectroscopy Investigation of a-Alumina-Supported Nickel Catalysts Preparaedfiom Nickel(I~Acetylacetonate R. Molina, M. Genet and G. Poncelet P401 The Role of Oxide Promoters in the Dissociation of CO and its Reaction with Hydrogen on Pd (111) and Rh (I 1I): A Molecular Beam Study B. Klozter and K. Hayek . P402 Characterization of the Active Sites of Ni-Si-A1 Sol-Gel Hydrogenation Catalysts C. Guimon, N. El Horr, E. Romero and A. Monzon.
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Catalysis for Fine Chemical Synthesis
P403 A New Type of Anchored Homogeneous Catalyst R.L. Augustine, S.K. Tanielyan, S. Anderson, H. Yang and Y. Gao
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P404 Regioselective Homogeneous Hydrogenation of Heteroaromatic Nitrogen as the Precatalyst Compounds by Use of [OSH(CO)(NCM~)~(PP~~~)Z]BF~ M. Rosales, F. Arrieta, J. Castillo, A. Gonzalez, J. Navarro and R. Vallejo P405 Catalyst Selection and Solvent EfSects in the Enantioselective Hydrogenation of 1-Phenyl-l,2-propanedione E. Toukoniitty, P. M&i-Arvela, A.Kalantar Neyestanaki, T. Salmi, A. Villela, R. Leino, R. Sjoholm, E. Laine, J. Vayrynen and T. Ollonqvist P406 Catalytic Asymmetric Hydrogenation and Hydroformylation Reactions with Chiral N or N,S Ligands M.L. Tomrnasino, M. Casalta, J.A.J. Breuzard, M.C. Bonnet and M. Lemaire P407 Heterogeneous Asymmetric Hydrogenation of Ethyl Pyruvate on Chirally modified Pt/A1203Catalyst with Fixed-Bed Reactor X. You, Xi. Li, S. Xiang, S. Zhang, Q. Xin, Xu. Li and C. Li . P408 EfSect of Coacid Acidity on the Cinchona-Mod$ed Pt-Catalized Enantioselective Hydrogenations B. Torok, K. Balkzsik, K. Felfdldi and M. Bartok . P409 Lactones 8. [I] Enantioselective Hydrolysis of y-Acetoxy-&lactones T. Olejniczak and C. Wawrzenczyk . P410 New Environmentally Friendly Base Catalystfor Enantioselective Reactions. Heterogenisation of Chiral Amines to USY and MCM-41 Zeolites M . Iglesias Hemhndez and F. Shnchez Alonso . P411 Selective Enzymatic Production ofAmide EmulsifiersJi.om Ethanolamine and Fatty Acids M . Femhndez Ptrez and C. Otero . P412 Rapid Solvent-Free Esterification of Conjugated Linoleic Acid and Glycerol in a Packed Bed Reactor Containing an Immobilized Lipase J.A. Arcos and C.G. Hill Jr. . P4 13 EtheriJication over a Novel Acid Catalyst R.S. Karinen, A.O.I. Krause, K. Ekman, M. Sundell and R. Peltonen P4 14 Esterification of Lauric Acid with Glycerol Using Modified Zeolite Beta as Catalyst M. da Silva Machado, D. Cardoso, J. Perez Pariente and E. Sastre .
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P415 Preparation of Alkylglucosides using Al-MCM-41 Molecular Sieves as Catalysts H.M.C. Andrade, A.R.E. Gonzaga and L.G. Aguiar .
3423
P4 16 Novel Supported Solid Acid Catalystsfor Environmentally Friendly Organic Synthesis K. Wilson, J.K. Shorrock, A. Renson, W. Hoyer, B. Gosselin, D.J. Macquarrie and J. H. Clark .
3429
P417 Catalytic Activity of Polymer-Anchored Pd(I4 SchlflBase Complexes: Hydrogenation of Styrene and Oxidation of Benzyl Alcohol M.K. Dalal and R.N. Ram .
3435
P418 Highly Promising Activity of Na$on/Silica Composite Catalyst in Acylation Reactions A. Heidekum, M.A. Harmer and W.F. Hoelderich .
344 1
P419 Alcohol Coupling to Unsymmetrical Ethers over Solid Acid Catalysts K. Klier, H.-H. Kwon, R.G. Herman, R.A. Hunsicker, Q. Ma and S.J. Bollinger 3447 P420 Gas versus Liquid Phase a-Pinene Transformation over Acid Molecular Sieves of Different Topolog~and Composition C.M. Lbpez, F.J. Machado, K. Rodriguez, D. Arias and M. Hasegawa .
3453
P42 1 Ruthenium-Catalyzed [2+2] Cross-Addition of Norbornene Derivatives and Dialkyl Acetylenedicarboxilates H. Suzuki, I. Hashiba, T.-A. Mitsudo and T. Kondo.
3459
P422 Shape-Selective N-Alkylation of Melamine Using Alcohol as a Alkylating Agent with RuIMordenite Catalyst in the Liquid Phase . S. Shinoda, K.-I. Inage, T. Ohnishi and T. Yamakawa
3465
P423 Diastereoselective Hydrogenation of a Prostaglandin Intermediate over Ru Supported on MCM-41 and MCM-48 S. Coman, R. Caraba, F. Cocu, V.I. Parvulescu, H. Bonnemann, B. Tesche, C. Danumah and S. Kaliaguine .
3471
P424 Generation of Uniformly Sized and Dispersed Copper Particles in New Cu-A1 Based Mesoporous Catalysts and their Role in Selective Hydrogenation of Conjugated Unsaturated Carbonyl Groups . S. Valange, Z. Gabelica, A. Derouault and J. Barrault P425 Selective Hydrogenation of Furfural on Ni-P-B Nanometals S.-P. Lee and Y.-W. Chen . P426 A Sulfur Based Catalytic Systemfor the Carbonylative Reduction and the Reductive Carbonylation . V . Macho and M. Kralik
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P427 Rapid Synthesis of Ketenes From Carboxylic Acids on Functionalized Silica Monoliths R. Martinez, M.C. Huff and M.A. Barteau .
3495
P428 Nitroaldol Condensation Promoted by Organic Bases Tethered to Amorphous Silica and MCM-41-Type Materials F. Bigi, S. Carloni, R. Maggi, A. Mazzacani and G. Sartori.
3501
P429 Alcoholysis of Ester and Epoxide Catalyzed by Solid Bases H. Hattori, M. Shima and H. Kabashima . P430 Beckmann Rearrangement over Anionic Clays L. Forni, G. Fornasari, R. Trabace, F. Trifiro, A. Vaccari and L. Dalloro .
3513
C1 Chemistry P43 1 Partial Oxidation of Methane over Supported Nickel Catalysts A.M. Diskin and R.M. Ormerod .
3519
P432 Partial Oxidation of CH4 into Synthesis Gas on Ni/Perovskite Catalysts Prepared by SPC Method K. Takehira, T. Shishido. M. Kondo, R. Furukawa, E. Tanabe, K. Ito, S. Harnakawa and T. Hayakawa .
3525
P433 Methane Partial Oxidation in New Iron Zeolite Topologies P.P. Knops-Gerrits and W.J. Smith .
353 1
P434 Investigations of the Selective Partial Oxidation of Methanol and the Oxidative Coupling of Methane on Copper Catalysts H.-J. Wolk, A. Scheffler, G. Mestl and R. Schlogl .
3537
P435 Partial Oxidation and Chemisorption of Methane over Ni/A1203 Catalysts Ya. Chen, C. Hu, M. Gong, Yu. Chen and A. Tian .
3543
P436 Carbon Deposition on Ni/A1203 Catalyst during Partial Oxidation of Methane to Syngas Q.-G. Yan, Z.-S. Chao, T.-H. Wu, W.-Z. Weng, M.-S. Chen and H.-L. Wan
3549
P437 Mechanistic Studies of Methane Partial Oxidation to Synthesis Gas over Si02-Supported Rhodium Catalyst Z.-S. Chao, Q.-G. Yan, T.-H. Wu, W.-Z. Weng, H.-Q. Lin, L. Yang, J.-L. Ye, M.-S. Chen, H.-L. Wan and K.-R. Tsai .
3555
P438 Reaction Kinetics and Product Selectivity in the Oxidation of Methane over Pd/Si3N4 D. Whang, F. Monnet and C. Mirodatos .
3561
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P439 Sustainable Ni Catalystfor Partial Oxidation of CH4 to Syngas at High Temperature S. Liu, G. Xiong, H. Dong, W. Yang, S. Sheng, W. Chu and Z. Yu
3567
P440 Improvements of Ceria Promoted Nickel Catalystsfor Natural Gas Oxidation to Syngas W. Chu, Q. Yan, S. Liu and G . Xiong .
3573
P441 Partial Oxidation of Methane Catalyzed by Mo-Incorporated SBA-I T. Tatsumi, N . Hamakawa and L.X. Dai .
3579
P442 Kinetic and Mechanistic Study of the Partial Oxidation of Methane to Formaldehyde on Silica Catalysts . F. Arena, F. Frusteri, A. Mezzapica and A. Parmaliana
3585
P443 Two Step Synthesis of Methyl Formatefi.om CH4and Air via Formaldehyde: Surface Reactivity of Oxide Catalysts towards HCHO G. Martra, F. Arena, S. Coluccia, F. Frusteri, A. Mezzapica and A. Parmaliana 3591 P444 Oxidation of Methanol on Sodium Mod$ed Mo-Based Catalysts . K. Ivanov, S. Krustev, P. Litcheva and I. Mitov
3597
P445 Novel Rhenium Based Catalystsfor Direct Dehydroaromatization of Methane with CO/C02 towards Ethylene and Benzene R. Ohnishi, K. Issoh, L. Wang and M. Ichikawa .
3603
P446 H-ZMS-8 Supported Mo-Based Catalystsfor Methane Conversion under Non-oxidative Condition S . Li, C.-L. Zhang, Q.-B. Kan, D.-Y. Wang, Y. Yuan and T.-H. Wu
.
3609
P447 Methane Dehydro-aromatization and NO Adsorption on Mo/HZSM-5 W. Liu and Y.-D. Xu .
3615
P448 The Location, Structure and Role of MOO,and MoC, Species in Mo/H-ZSM5 Catalystsfor Methane Aromatization . W. Li, G.D. Meitzner, Y.-H. Kim, R.W. Borry and E. Iglesia
3621
P449 Oxygen-Free Conversion of Methane in the Presence of Intermetallic Hydrogen ~ b c e ~ t o r A. Sauvage, M. Mercy, H. Amariglio, J.-C. Gachon and P. Pareja .
3627
P450 Conversions of Substituted Methanes Over ZSM-Catalysts C.E. Taylor .
3633
P45 1 A Study of Coke Formation Kinetics by a Conventional and an Oscillating Microbalance on Steam Reforming Catalysts R. Lerdeng, D. Chen, C.K. Jakobsen and A. Holmen
3639
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P452 Methanol Reforming over CuO/ZnO under Oxidizing Conditions T.L. Reitz, S. Ahrned, M. Krumplet, R. Kumar and H.H. Kung .
P453 Methane COz Reforming over a Stable Ni/Al203 Catalyst A. Castro Luna, A. Becerra, M. Dimitrijewits and C. Arciprete
.
P454 Optimization of Ni and Ru Catalysts Supported on LaMn03for the Carbon Dioxide Reforming of Methane E. Pietri, A. Barrios, M.R. Goldwasser, M.J. Perez Zurita, M.L. Cubeiro, J. Goldwasser, L. Leclercq, G. Leclercq and L. Gingembre. P455 Metal Support Interaction on Pt/ZrOz Catalystsfor the C 0 2 Reforming of CH4 S.M. Stagg-Williams, R. Soares, E. Romero, W.E. Alvarez and D.E. Resasco P456 Methane Reforming with C02 over Ni/ZrO2-CeO2 and Ni/ZrO2-MgO Catalysts Synthesized by Sol-Gel Method J.A. Montoya, E. Romero, A. Monzon and C. Guimon . P457 Support Effect over Rhodium Catalysts during the Reforming of Methane by Carbon Dioxide P. Ferreira Aparicio, M. Femindez Garcia, I. Rodriguez Ramos and A. Guerrero Ruiz . P458 C 0 2Reforming of CH4over Mo Promoted Nickel-Based Catalysts C.E. Quincoces, S. Ptrez de Vargas and M.G. Gonzalez . P459 Stable Ni/Zr02 Catalystfor Carbon Dioxide Reforming of Methane J.-M. Wei, B.-Q. Xu, Z.-X. Cheng, J.-L. Li and Q.-M. Zhu. P460 Dependence ofActivity of ZrO2 Catalystsfor Isobutene Formation in CO Hydrogenation on the Phase Structure K-I. Maruya, T. Komiya and M. Yashima . P461 Long-chain Alcohols fiom Syngas A. Frennet, C. Hubert, E. Ghenne, V. Chitry and N. Kruse . P462 Preparation and Catalytic Properties of Copper-Ytterbium Oxide System for CO Hydrogenation Y. Sakata, S. Tsuchiya, N. Kouda, F. Takahashi and H. Imamura . P463 Mechanistic Studies of CO and C02 Hydrogenation to Methanol over a 50Cd45Zd5Al Catalyst by In Situ FT-IR, Chemical Trapping and Isotope Labelling Methods L.Z. Gao, J.T. Li and C.T. Au P464 Eflect of Nb2Os Addition to Co/A1203Catalyst on CO Hydrogenation Reaction 3717 F.T. Mendes, F.B. Noronha and M. Schmal..
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P465 Investigation of Alternative Support Materials for Monometallic and Bimetallic Catalystsfor CO Hydrogenation T. Biilbiil, A.I. Isli, A.E. Alsoylu and Z.I. dnsan .
3723
P466 Isotopic Transient Inffared Study of Reaction Pathway and Intermediates for CO Insertion and Hydrogenation S.A. Hedrick and S.S.C. Chuang .
3 729
P467 Relationship between Formation of DME, CH30Hand i-C4Hs in Isosynthesis . C.-L. Su, D.-H. He, J.-R. Li, B.-Q. Xu, Z.-X. Cheng and Q.-M. Zhu
3735
P468 CO Hydrogenation on RWAI-PILC: The Effect of the Metallic Precursor S. Mendioroz, B. Asenjo, P. Terreros, P. Salerno and V. Muiioz .
3741
P469 Enhancement of the Catalytic Performance to Methanol Synthesis @om C02/H2 by Gallium Addition to Palladium/Silica Catalysts A.L. Bonivardi, D.L. Chiavassa, C.A. Querini and M.A. Baltanas .
3747
P470 A Comparison of the Catalytic Performancefor Low-Temperature Methanol Synthesis in a Liquid Medium S. Ohyama .
3753
P471 A Novel Effect of Li Additive: Dynamic Control of Rh Mobility during C 0 2Hydrogenation Reaction . K.K. Bando, H. Arakawa, N. Ichikuni and K. Asakura
3759
P472 Structure and Characteristics of Supported Metal SPR Catalystsfor C02 Hydrogenation T. Uematsu, D. Li, N. Ichikuni and S. Shimazu .
3765
P473 The Nature of the Active Species on the Fe/MgO Used in the Combustion of Methane R. Spretz, S.G. Marchetti, M.A. Ulla and E.A. Lombardo .
3771
P474 Oxidation of Methane over Pt-Cr(Mo, W)/A1203Catalysts Z . Sarbak and S.L.T. Andersson . P475 Monolith Honeycomb Mixed Oxide Catalystsfor Methane Oxidation L.A. Isupova, V.A. Sadykov, G.M. Alikina, 0.1. Snegurenko, . S.V. Tsybulya, A.N. Salanov and V.A. Rogov P476 Catalytic Properties ofActive Phase of Glass Crystal Microespheres in the Reaction of Methane Oxidation A.G. Anshits, E.V. Kondratenko, E.V. Fomenko, A.M. Kovalev, O.A. Bajukov and A.N. Salanov .
3783
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
Catalytic Methane Combustion over Alumina Supported Palladium Catalysts Prepared by Sol-Gel Method: Investigation of the Activity Evolution S . Fessi, A. Ghorbel, A. Rives and R. Hubaut . Study of PdO/Pd Transformation over Alumina Supported Catalystsfor Natural Gas Combustion G.Groppi, C. Cristiani, L. Lietti and P. Forzatti . Catalytic Activity of Tungsten Phosphate ( I v , (V), (VI) at Carbon Monoxide Oxidation V.V. Lesnyak, N.S. Slobodyanik, V.K. Yatsimirsky and N.A. Boldyreva . A Study of Pt/ZrOz Catalystsfor Water-Gas Shift Reaction in Presence of H2S E. Xue, M. O'Keeffe and J.R.H. Ross Reduction Requirementsfor Ru(Na)/Fe203 Catalytic Activity in Water Gas Shift Reaction W.K. Jozwiak, A. Basinska, J. Goralski, T.P. Maniecki, D. Kincel and F. Domka . Probing the Elementary Steps of the Water-Gas Shift Reaction over Cu/ZnO/AI203 with Transient Experiments . 0 . Hinrichen, T. Genger and M. Muhler Highly Selective and Highly Efficient Catalytic Conversion of Methane into Ethylene by the Oxidative Coupling . A. Machocki, A. Denis, J. Gryglicki and H. Mlynarska Oxidation of Methane to Methanol by Hydrogen Peroxide on a Supported Hematin Catalyst T.M. Nagiev and M.T. Abbasova . Polymerization and Organometallics
Investigation of a Cr/Silica Polyethylene Polymerisation Catalyst by In-situ Inpared Spectroscopy S.F. Parker, C.C.A. Riley and J.E. Baker . Supported (Cp)2 ZrCI2 as a Catalystfor Ethylene Polymerization S. Teixeira Brandao, M.L. dos Santns Correa, J.S. Boaventura, . and S. Carneiro Vianna On the EfSect ofAdding a Lewis Acid as a ModiJier of Metallocene Based Polymerization Catalysts P.G. Belelli, M.L. Ferreira and D.E. Darniani Surface Science Studies of Model Ziegler-Natta Polymerization Catalysts S.H. Kim, E. Magni and G.A. Somorjai .
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
P489 Ethylene Polymerization Catalyzed over Single Site Ni Complex Catalysts Supported on Silica D.S. Hong, T.S. Seo and S.I. Woo .
P490 DRIFTS, TPRS and NMR Investigations of Unsupported and Supported Pentamethylcyclopentadienyldiallyl-Complexes-Catalystsfor the Polymerization of Butadiene . H. Landmesser, H. Berndt, D. Miiller and D. Kunath P491 SchrfJBases Containing Metal ComplexesAnchored on Aerosil as Catalysts of Low-Temperature Ozone Descomposition T.L. Rakitskaya, A.A. Golub, A.A. Ennan, L.A. Raskola, V.Ya. Paina, A.Yu. Bandurko and L.L. Ped P492 Carbon Nanotubes-Supported Rh-Phosphine Complex Catalystsfor Propene Hydroformylation . H.-B. Zhang, Y . Zhang, G.-D. Lin, Y.-2. Yuan and K.R. Tsai P493 Model Rhodium Organometallic Complexes as Catalyst Precursors for CO Hydrogenation P. Terreros, R. Fandos, M. L6pez Granados, A. Otero, S. Rojas and M.A. Vivar Cerrato . P494 An OrganometallicApproachfor Tin Promotion Enhancement over Pt/y-A1203 Alkane Dehydrogenation Catalysts G.F. Santori, M.L. Casella, G.J. Siri, O.A. Ferreti and J.L.G. Fierro P495 Il9sn Mossbauer Study and Catalytic Properties of Magnesia-Supported Platinum-Tin Catalysts Prepared by Surface Organometallic Chemistry L. Stievano, F.E. Wagner, S. Calogero, S. Recchia, C. Dossi and R. Psaro. AUTHORINDEX
.
SUBJECT INDEX
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
In Situ Characterization of Catalysts Henrik Topsoe Haldor Topsoe Research Laboratories, DK-2800 Lyngby, Denmark
Abstract The state of a catalyst depends intimately on the conditions under which the catalyst operates. Such dependencies are often related to adsorbate induced relaxations or reconstructions, and small variations in conditions may result in dramatic changes in the catalyst structure. As a consequence, characterization studies should ideally be carried out in situ during catalysis and studies performed in the absence of the reactants and products may yield limited insight. This present paper will discuss some recent developments of in situ methods. Some examples will be given to illustrate how the access to in situ information may greatly facilitate the understanding of the catalysis and contribute to more rational design of catalysts. 1. INTRODUCTION In order to perform rational catalyst research, it is highly desirable to have atomic-scale information on the state of the catalyst inside the catalytic reactor. With such information, one has the possibility to establish structure-activity relationships, which can provide useful guidelines for rational catalyst research and developments. Without such insight one is very much left with treating the reactor as a black box. Heterogeneous catalysts are typically very complex solids and a multitude of structural features may coexist at the atomic-scale. In order to characterize catalysts, it is therefore a goal to find suitable techniques, which can provide detailed chemical and structural information for such systems. Besides the intrinsic complexities discussed above, one also has the added complexity that the catalyst structures may change dynamically depending on the reaction conditions. This is typically related to adsorbate induced restructuring of the catalysts, and it has been observed that even small changes in the environment may result in dramatic changes in the structures. Clearly, the state of the catalyst inside the reactor will generally be quite different from that after removal from the reactor. It is therefore important that the techniques employed in catalyst studies can both deal with the intrinsic complexities and also provide insight under in situ conditions. Over the years, great efforts have been devoted to develop methods, which allow in situ studies (see e.g. the reviews and conference proceedings (1-14)). In spite of this, many important techniques used in catalyst research are not suited for studies at the high pressures and temperatures encountered in many catalytic reactions. In view of these difficulties, simplifications have often been adopted. Thus, the investigations termed in situ in the literature often refer to studies carried out under conditions, which deviate from those encountered during catalysis. Frequently, the characterization is performed on the catalyst
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atter its transfer from the reactor to an analysis station. For example, in order to utilize many of the surface sensitive spectroscopies, the investigations are typically carried out after transfer of the catalyst to UHV conditions. Although, important insight may be obtained from such studies (see sec. 3), the state of the catalyst after transfer may, as discussed above, be quite different from that existing during catalysis. The differences may not only be limited to simple differences in the coverage of adsorbates, but may involve complete surface and bulk structural changes. The following sections will give several examples, which illustrate the dynamic nature of catalysts and the need to carry out in situ studies. Some experimental requirements and new possibilities for performing combined catalysis and in situ on-line characterization studies will also be discussed. The range of applications is illustrated using a few selected studies of catalysts for ammonia synthesis, methane conversions, hydrotreating, DeNOx and methanol synthesis. The present paper will not attempt to give a complete overview of all in situ techniques. In fact, many important techniques, like Raman spectroscopy, isotopic tracing, microscopy, NMR, SAXS/WAXS, ASAXS, gravimetry and SFG, will not be discussed here. For these topics the reader is referred to other recent papers and reviews (11-25). 2. ADSORBATE INDUCED RECONSTRUCTION Although reconstruction of surfaces by adsorbates has been known for a long time, in situ insight for real catalyst systems has been limited. One of the first techniques, which provided in situ information about such phenomena, was M6ssbauer spectroscopy (1,3). Studies using this technique revealed, for example, that the surface composition of alloy catalysts (26,27), the surface structure of ammonia synthesis catalysts (28-30) and the magnetic properties of supported catalysts (31,32) depend critically on the reaction environment. Scanning Tunneling Microscopy (STM) is one of the newer techniques, which has provided detailed atomic scale insight into adsorbate induced restructuring. The studies show that structural relaxations and reconstructions are far more common and complex than often assumed in the past. Figure 1, taken from the work of Ruan et al. (33), illustrates some of the complexities, one encounters during the reaction of H2S with preadsorbed oxygen on a Ni(110) surface. It is observed that the starting Ni surface has reconstructed upon the exposure to oxygen (Fig. 1a) and as the reaction proceeds, a significant surface roughening takes place and islands are also formed. The final surface (Fig. l f), which only contains sulfur, is reconstructed again. It is interesting that the structure of this surface is different from that observed upon direct exposure of the Ni (110) surface to H2S under the same conditions. Thus, the initial reconstruction caused by oxygen activates the Ni surface for new reaction pathways, and this leads to the production of structures which otherwise would not have formed. The above results also suggest that kinetic treatments using for example Langmuir-Hinshelwood mechanisms may provide too simplified descriptions of complex catalytic reactions. Ertl has shown that due to time and spatial dependencies of the surface reconstructions, unusual reaction behaviors (oscillations and chaos) may be observed (34). The interesting effect of oxygen reconstruction on the final structure of the S on Ni (110) may have analogies to effects observed on real catalysts. For example, it was observed (28) that an ammonia treatment of a reduced Fe/MgO catalyst prior to exposure to a H2:N2 synthesis gas gave rise to special highly active surfaces, which could not be produced by direct exposure of the reduced catalyst to synthesis gas. In a recent study, Nerlov and Chorkendorff (35) presented a nice example, which demonstrated that the presence of molecules, which do not participate in the catalytic
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
i Fig. 1. STM images of the reaction of H2S with preadsorbed oxygen on Ni(110). The images are taken after progressively higher exposures to H2S. (a) O L, (b) 3 L, (c) 8 L, (d) 20 L, (e) 25 L, (f) 35 L. (a)-(e) recorded on a 85x91A2 area. (f) shows an area of 59x63A2. According to Ref. (33). cycle, may still alter the catalytic properties dramatically through adsorbate induced reconstructions/segregations. Multicomponent catalyst systems (supported catalysts, alloys, mixed oxides, promoted catalysts, etc.) are expected to exhibit even more complex behaviors, since an even larger variety of adsorbate interactions may exist. The Cu catalyst system discussed in sec. 7 illustrates that dynamic changes may involve structural changes in the support as well as changes in the surface and bulk structure of the metal. All the above-mentioned studies clearly demonstrate the necessity to perform in situ studies on actual operating catalysts and it is suggested that many of the controversies existing in the literature regarding the structure and performance of catalysts are related to the lack of in situ insight. 0
COMBINED SURFACE SCIENCE AND CATALYSIS STUDIES. AMMONIA SYNTHESIS OVER Ru
In order to elucidate the role of different surface structures in catalysis, it may be attractive to place a high-pressure reactor inside a typical surface science UHV chamber (Fig. 2). In this way one can perform catalysis experiments using well-defined starting surfaces like single crystals. By transferring the catalyst from the reactor to the UHV system, one can directly perform post analysis. Furthermore, one may also modify the surface in the UHV system before doing new catalysis experiments. There has already been
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many successful applications of this approach. However, such studies - and others requiring sample transfer - are of course still not proper in situ studies, and it is not certain to what extent, the surfaces characterized before and after transfer resemble those present during the catalysis. Nevertheless, many useful features may be addressed and the recent studies by Dahl et al. (36,37) demonstrate nicely the special advantages of this approach. They used the above type set-up to study the N2 dissociation on Ru(0001) and examined to what extent, the presence of small amount of steps may play a role. The results shown in the top half of Fig. 3 are data for the clean Ru(0001) surface. The observed activation energy is much lower than that predicted from DFT calculations on this surface
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
(Fig. 4). On a clean Ru(0001) surface one may always have up to one percent of steps and in order to estimate the possible role of such step sites, the N2 dissociation experiment was repeated after such sites were "blocked". This could be done by depositing a few percent of a monolayer of gold, which is known to preferentially decorate the steps. This resulted (Fig. (3)) in a many order decrease in the activity. The large difference in activation energy for N2 dissociation (the rate-limiting step in NH3-synthesis) is in-line with that predicted by DFT calculations (Fig. 4). It can be estimated that the rate on the (0001) surface is about 9 orders of magnitude smaller than that on the step sites. This is probably the largest measured difference in structure sensitivity, and it is clear that for Ru, the catalysis will be dominated by the presence of a few sites resembling the step sites. The insight was used to construct a microkinetic model which can account for the reactions both over single crystals and supported Ru catalysts (Fig. 4). It is likely that other reactions reported in the literature may also have been influenced by small concentration of step, edge or defect sites, and this old topic needs to be reexplored further. It is indeed the goal of many in situ studies to obtain information about the fraction of the total amount of surface sites, which are responsible for the catalysis. Studies discussed in subsequent sections also show that for many systems the active sites may only represent a small fraction of the total concentration of surface sites.
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4. EXPERIMENTAL CONSIDERATIONS
In view of the strong dependence of the catalyst structure on the environment, catalyst characterizations should ideally be done under the exact conditions of the catalysis, and in order to relate the obtained information to the catalytic activities, it is furthermore desirable to perform simultaneous activity and characterization studies. However, many sample cells used in catalyst studies have geometry, which is not ideal for in situ studies (9). Examples of typical XRD cells are shown in Figure 5. Such cells may provide good quality diffraction data, but one will typically have large temperature and concentration gradients and poorly defined conditions at the position of the catalyst. Many designs for in situ cells for other techniques also suffer from such problems. In order to circumvent these difficulties, we have aimed at adapting the same geometry in the in situ cell, as that encountered in a plug flow reactor (38,39). An attractive solution based on miniaturization is shown in Figure 6 (39). This simple design facilitates simultaneous spectroscopy and on-line activity measurements. This approach has been used for in situ XRD (39) and combined XRD/QEXAFS experiments (40) but can also be adapted to other techniques. In order to obtain sufficient penetration of the radiation, thin capillary reactors were utilized. These also allow studies to be carried out under high pressures. The small mass of the reactor makes it ideal for transient studies and such capillary reactors have, for example, been used in TPR, TPS or transient catalysis studies. In Ref. (39), it was shown that good catalytic activity measurements can be obtained in the in situ cell. X-ray diffraction is probably the most widely used technique in catalysis R&D for obtaining structural information. Beside the normal angle-dispersion mode, the energydispersive mode can also be used and it has certain advantages for small particle and in situ studies (41). Nevertheless, it is important to note that XRD is not sensitive to structures, where the dimension of order is less than about 2 nm. This is a serious limitation since in catalysis research, it is often the goal to prepare highly dispersed catalysts with dimensions less than this value. For such systems, EXAFS has become an indispensable tool (4,5,9,10) but this technique also has several limitations since it is a local environment technique. It is clear that XRD and EXAFS in many respects provide complimentary information, and it is therefore desirable to combine these two methods in order to obtain a more complete structural description of catalysts. An attractive solution is the combined XRD/QEXAFS technique (Fig. 7)
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developed by Clausen et al. (40). By use of a position sensitive detector, the complete XRD pattern and fast EXAFS data are recorded simultaneously. Using the Quick EXAFS mode, the time resolution is about 0,01 to 2 s on a routine basis. Recently, time resolutions of about lms have been demonstrated using a so-called Piezzo EXAFS mode (42). The combined XRD/QEXAFS set-up (Fig. 7) also uses the in situ cell described above for simultaneous reaction measurements. e-
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DEACTIVATION Heterogeneous catalysts are non-equilibrium solids and their structures depend critically on the choice of preparation and activation parameters. In situ studies can also be used to obtain information about such parameters. For example, a small angle x-ray scattering (SAXS) has been used to obtain information about nucleation and crystallization processes (25). Figure 8 illustrates how the combined XRD/QEXAFS technique can be used to elucidate the processes occurring during the reduction of the precursors to a Cu-based methanol synthesis catalyst (40). Product analysis was monitored by simultaneous gas analysis. In situ studies may also be used to examine whether certain catalyst design strategies have resulted in the desired structures in the final catalyst. For example, in situ EXAFS was recently used to test whether one could succeed in preparing small Ni-Au catalysts particles, having the Au atoms present as a surface Ni-Au alloy (43). The starting point in this project was the observation by STM of the existence of a Ni-Au surface alloy in single crystal model
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systems (44). The STM picture indicates Figure 9 that the Ni atoms neighboring the Au atoms have altered electronic properties. Subsequent DFT calculations and molecular beam experiments (43) predicted, that these atoms should exhibit different properties for activating the methane molecule compared to pure Ni. In view of these results, preparation of porous, high surface area catalysts containing small Au-Ni nano-crystals with the same surface structures was initiated. In situ EXAFS studies played an essential role in examining the results of different preparation strategies (43,45). This was important, since many procedures failed due to, for example, segregation of separate Au particles. Figure 9 shows the EXAFS spectrum of a preparation yielding high amounts of the desired small Ni-Au nano-particles. In situ studies may also be used to elucidate processes occurring during catalyst deactivation. Sintering is one of the important processes leading to catalyst deactivation. Figure 10 shows the result of in situ XRD studies of the sintering behavior of two Cu-based catalysts (46). The insight gained from such studies can be used to elucidate the sintering mechanisms and aid in the development of catalysts in which sintering is minimized. It is
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6. I n situ FTIR STUDIES OF V/TiO2 DeNO,, CATALYSTS. REACTION INTER-
MEDIATES AND SPECTATORS Infrared spectroscopy is one of the most important techniques in catalysis research, and it has been used extensively to provide information about surface functional groups and adsorbed species. Recently, combined in situ FTIR and on-line reaction studies were performed to elucidate the SCR deNOx reaction over V/TiOz catalysts (48-51). It was found very useful to perform both steady state and transient studies. By comparing the mass spectrometric analysis of the reactants and products with the FTIR information (one example is shown in Figure 11, the changes in reaction rates could be related to the changes in the concentration of different surface sites and adsorbed species. Thus, combining the information from several such in situ on-line experiments, one could distinguish between the active reaction intermediates and spectator species, which were present in quite large amounts, but did not contribute significantly to the catalysis. It is a common problem in catalysis research to distinguish between reaction intermediates and spectator species, and simple adsorption experiments do not allow this discrimination. Some of the FTIR studies mentioned above also showed that Temperature Programmed Surface Reaction (TPSR) studies may also have difficulties in distinguishing between reaction intermediates and spectator species. Specifically, it was observed in some TPSR experiments that NO could react with certain adsorbed ammonia species to yield the deNOx reaction products (49,50). Nevertheless, these species should still be regarded as spectator species since the FTIR measurements showed that the species are not involved in the catalytic cycle under steady-state catalysis. The above studies demonstrate the variety of insight, which can be gained from in situ on-line studies. The studies formed the basis for the
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situ STUDIES OF THE DYNAMICAL BEHAVIOR OF Cu/ZnO BASED M E T H A N O L SYNTHESIS CATALYSTS
7. In
Recent in situ studies of Cu/ZnO based methanol synthesis catalysts have revealed that structural changes may occur as a result of small changes in reaction conditions (52-55). It has furthermore been shown that the structural changes are not restricted to the surfaces, but involve complete changes in the morphology of the Cu particles. The insight gained from the in situ studies has allowed the formulation of dynamic microkinetic models for the industrial performance. Some of the recent studies will be discussed below. Previous in situ EXAFS studies of Cu/ZnO based catalysts showed that Cu is present as metallic Cu particles (56). In recent years, there has therefore been an interest in exploring to what extent, surface science information obtained on single crystal Cu surfaces may be used as a basis for formulating a microkinetic model capable of explaining the industrial macrokinetics. Using results from Maddix's and Campbell's groups and their own results, Chorkendorff, Stoltze, Norskov and co-workers formulated a microkinetic model including both the water gas shift reaction and methanol synthesis (57,58). It was shown that such a model could account for many of the observed laboratory pilot plant results, and the model also gave an adequate quantitative description of the observed rates. Nevertheless, the model did not account for all the apparent reaction orders and the presence of special transient phenomena for the ZnO containing catalysts (52, 54). Furthermore, when analyzing the results of the microkinetic modeling of the industrial rates on Cu/ZnO/A1203 catalysts, it was noticed (see Fig. 12) that in the most reducing synthesis gas mixtures, the measured rates were larger than those predicted by the microkinetic model (53). The opposite was the case for the experiments using more oxidizing synthesis gases. The different observations suggest that the catalysis cannot be explained completely by only considering a stationary copper metal component in the catalyst. Specifically, it appeared that ZnO also plays a key role. The results in Figure 12 indicate that the state of the catalysts may depend quite sensitively on the reduction potential of the synthesis gas. Detailed insight into this feature was obtained by the
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Co I (Co + Mo) Fig. 14. Illustration of the variety of promotional behaviors encountered in Co-Mo/A1203 hydrodesulfurization catalysts. According to Ref. (63). 8. HYDRODESULSULFURIZATION CATALYSTS Hydrodesulfurization catalysts belong to some of the most intensely studied catalysts, but a detailed understanding has been lacking for many years (63). In retrospect, it is evident that this was due to the fact that the characterization tools applied could not deal with both the structural complexities, and the requirements of in situ studies under proper sulfiding conditions. The catalytic behavior of Co-Mo/A1203 hydrodesulfurization catalysts is very complex (Fig. 14). Catalysts with similar overall composition may exhibit very different promotional behaviors. In situ MGssbauer Emission Spectroscopy (MES) was pionering in providing detailed insight into the nature of the promoter atoms in the active state of the catalysts (64,65). A complication encountered is the coexistence of many different promoter phases in the same catalyst. Nevertheless, by obtaining in situ MES information for many different catalysts with different activities, it was revealed that the activity is dominated by the fraction
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Number of Co atoms present as Co-Mo-S (x1020/g catalyst) Fig. 15. Top panel" DFT calculations of the active Co-Mo-S structure ((a) top view, (b) side view). According to Ref. (67). Bottom panel: Correlation between the HDS activity and the number of Co atoms present in Co-Mo-S as determined by in situ MES. According to Refs. (63, 65). of the promoter atoms present in the so-called Co-Mo-S structure (Fig. 15)(63,65). This structure can be considered as Co located at the edges of small nano-clusters of MoS2 (63,66,67). Important insight into these structures has subsequently been obtained by combining the MES studies with in situ studies using other techniques such as XAFS, FTIR, EPR etc. (63). It is evident from all these studies that the catalytic activity is related to the edge CoMo-S structures in promoted catalysts and to the edge MoS2 structures in unpromoted catalysts. This in situ insight has provided important guidelines for many subsequent studies, and it is clear that a more detailed understanding of hydrodesulfurization requires atomic-scale
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insight into the structure of the edges and the catalytic processes occurring there. Recent DFT calculations are interesting in this respect, since these are now capable of treating a variety of realistic edge structures (66-68)(see also Fig. 15). Recently, it has also been possible for the first time to obtain direct atom-resolved STM images of the MoS2 edges. By choosing the herringbone reconstructed Au (111) surface as a template, Helveg et al. (69) succeeded in preparing small single layer nano-crystals of MoS2 (Fig. 16). The nano-crystals are single layer thick and this allowed the recording of atom-resolved STM images of the active edge structures of MoS2. It is apparent from the images that the structure of the edges is reconstructed with respect to that of the bulk. This new insight is interesting since it implies that the edge structures may be quite different from those typically assumed in most studies. In order to get further insight into the nature of the active sites, the sulfided nanocrystals shown in Figure 16 were also treated with atomic hydrogen and this resulted in the creation of vacancies at the MoS2 edges. The observation of such sites is interesting, since in the literature (63) it is commonly assumed that vacancies are the active sites for hydrodesulfurization, but direct evidence of the nature of such sites has been lacking. At present, the conditions during the STM experiments are quite different from those during catalysis, and it is therefore uncertain if the structures observed under the present conditions are typical for an active catalyst. Nevertheless, developments in STM are occurring rapidly
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and measurements under higher pressures and temperatures have become feasible. It should also be possible to carry out studies with more realistic supports. In this connection it is interesting that Hojrup Hansen et al. (70) recently have been able to record atomic resolved images for Pd clusters deposited on thin A1203 films (Fig. 17).
Fig. 17. An atom-resolved STM image of a small Pd cluster on a thin aluminum oxide film. According to Ref. (70). 9. CONCLUSION The possibilities of obtaining detailed in situ information about the state of a catalyst and the processes occurring inside a reactor have had an important impact on catalyst research and developments. Thus, today the catalytic reactor no longer has to be regarded as a black box and this opens new opportunities for moving away from the trial-and-error era. Due to dynamic reconstructions, the state of catalyst during reaction may be quite different from that present without exposure to reactants and products. Also, small changes in reaction conditions may result in quite dramatic structural changes, and the structure of a catalyst'may change with the position in the reactor. All these effects further emphasize the need to carry out in situ studies. The present paper discussed some of the novel in situ on-line techniques, which can address the complex problems encountered in catalyst research. The paper also demonstrated that it can be very beneficial to employ multidisciplinary approaches and integrate in situ studies into research efforts that also involve theoretical calculations and surface science experiments. ACKNOWLEDGEMENTS The author would like to acknowledge the many colleagues and collaborators involved in the research discussed in this paper. Particular thanks go to Nan-Yu Topsoe, Bjeme Clausen, Soren Dahl, Jan-Dierk Griinwaldt, Charlotte Ovesen, Ib Chorkendorff, Stig Helveg
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and Flemming Besenbacher for comments and assistance during the preparation of the manuscript. REFERENCES
1. J.A. Dumesic and H. Topsoe, Advances in Catalysis 26 (1977) 121. 2. K. Tamaru, Dynamic Heterogeneous Catalysis, Academic Press, 1978. 3. H. Topsoe, J.A. Dumesic, and S. M~rup: "Applications of Mrssbauer Spectroscopy", (R.L. Cohen, Ed.), Vol. II, 55 (1980). 4. F.W. Lytle, H. Via and J.H. Sinfelt, in "Synchrotron Radiation Research" (H. Winick and S. Doniach, eds.), Plenum, New York (1980) chapter 12 5. P. Gallezot in "Catalysis Science and Technology" (J.R. Anderson and M. Boudart, eds.), Springer-Verlag, Berlin, vol. 5 (1984). 6. In situ Methods in Catalysis, Catal. Today 9, No. 1-2 (1991) 1-236. 7. J.W. Niemantsverdriet, Spectroscopy in Catalysis, VCH 1993. 8. Frontiers in Catalysis: Ammonia Synthesis and Beyond (H. Topsoe, M. Boudart, and J.K. Norskov, eds), Topics in Catalysis, Vol. 1, No. 3,4, (1994). 9. B.S. Clausen, H. Topsoe, R. Frahm, Advances in Catalysis 42 (1998) 315. 9. R. Prins and D.C. Koningsberger, in X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, and XANES, (D.C. Koningsberger, and R. Prins, Eds.) Wiley, New York, (1998), 321. 10. E.G. Derouane, S.B. Derouane-abd Hamid, I.I. Ivanova, H. He, and J.C. Vedrine, Combinatorial Catalysis and High Throughput Catalyst Design and Testing (E.G. Derouane, C. Adams, F. Ramoa Ribeiro, A, Corma, Eds). (1999), 37. 11. J.C. Vedrine, and E.G. Derouane, Combinatorial Catalysis and High Throughput Catalyst Design and Testing (E.G. Derouane, C. Adams, F. Ramoa Ribeiro, A, Corma, Eds). (1999), 63. 13 Catalyst Characterization under Reaction Conditions, Topics in Catalysis, Vol. 8 (1999) No. 1,2. 14. G.A. Somorjai, CaTTech, Vol. 3, No. 1 (1999) 84. 15. R.T.K. Baker, Catal. Rev. Sci. Eng. 19 (1979) 161. 16. F. Herschkowitz and P.D. Madiara, Ind. Eng. Chem. Res. 32 (1993) 2969-2974. 17. S.C. Fung, C.A. Querini, K. Liu, D.S. Rumschitzki, T.C. Ho, Stud. Surf. Sci. and Catal. Vol. 88 (1994) 18. D. Chen, H.P. Rebo, K. Moljord, and A. Holmen, Chem. Eng. Sci. Vol. 5, No. 11 (1996) 2687. 19. S. Xie, G. Mestl, M.P. Rosynek, J.H. Lunsford, J. Am. Chem. Soc. 119, No. 42 (1997) 10186. 20. F. Eisert and A. Rosrn, Surface Science 377-379 (1997) 759. 21. W. Zhu, J.M. van de Graaf, L.J.P. van den Broecke, F. Kapteijn, and J.A. Moulijn, Indu. Chem. Res. Vol. 37 (1998) 1934. 22. X. Gao, M.A. Banares, I.E. Wachs, J. Catal. 188 (1999) 325. 23. P.L. Gai, Topics in Catal. 8 (1999) 97. 24. F. Berg Rasmussen, A.M. Molenbroek, B.S. Clausen and R. Feidenhans'l, J. Catal. (in press). 25. P.-P.E.A. de Moor, T.P.M. Beelen. B.U. Komanschek, O. Diat, R.A. van Santen, J. Phys. Chem. B 101 (1997) 11077. 26. C.H. Bartholomew and M. Boudart, J. Catal. 29 (1973) 278.
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27. P.H. Christensen, S. Morup, B.S. Clausen, and H. Topsoe, in "Proc. 8th Int. Congr. Catal.", Verlag Chemie: Weinheim, 1984, Vol. II, 545. 28. J.A. Dumesie, H. Topsoe, S. Khammotana and M. Boudart, J. Catal. 37 (1975) 503. 29. J.A. Dumesie, H. Topsoe and M. Boudart, J. Catal. 37 (1975) 513. 30. M. Boudart, H. Topsoe, and J.A. Dumesie, in "The Physical Basis for Heterogeneous Catalysis" (E. Draughlish & R.I. Jaffee, Eds.) pp. 337, Plenum Press, New York (1975). 31. B.S. Clausen, S.Morup and H. Topsoe. Surface Science 82 (1979) L589 32. S. Morup, B.S. Clausen and H. Topsoe. J. de Physique, 41 (1980) C1-331 33. L. Ruan, F. Besenbaeher, I. Stensgaard and E. L~egsgaard, Phys. Rev. Lett. 69 (1992) 3523. 34. G. Ertl, Topics in Catalysis 1 (1994) 305. 35. J. Nerlov and I. Chorkendorff, J. Catal. 181 (1999) 271 36. S. Dahl, A. Logadottir, R.C. Egeberg, J.H. Larsen, I. Chorkendorff, E. T6mqvist and J.K. Norskov, Phys. Rev. Lett. 83 (1999) 1814. 37. S. Dahl, J. Sehested, C.H.J. Jacobsen, E. T6mqvist and I. Chorkendorff, Submitted to J. Catal. 38. B.S. Clausen and H. Topsoe, Catalysis Today 9 (1991) 189 39. B.S. Clausen, G. Steffensen, B. Fabius, J. Villadsen, R. Feidenhans'l, and H. Topsoe, J. Catal. 132 (1991) 524. 40. B.S. Clausen, L. GrAb~ek, G. Steffenen, P.L. Hansen, and H. Topsoe, Catal. Let., 20 (1993) 23. 41. L. Gerward, S. Morup, and H. Topsoe, J. Appl. Phys. 47 (1976) 822. 42. H. Bomebusch, B.S. Clausen, G. Steffensen, D. Liitzenkirchen-Hecht and R. Frahm, J. Synchrotron Rad. (1999). 6, 209 43. F. Besenbacher, I. Chorkendorff, B.S. Clausen, B. Hammer, A.M. Molenbroek, J.K. Norskov and I. Steensgaard, Science 279 (1998) 1913. 44. L.P. Nielsen, F. Besenbacher, I. Stensgaard, E. La~gsgaard, C. Engdahl, P. Stoltze, K.W. Jacobsen and J. K. Norskov, Phys. Rev. Lett. 71 (1993) 754 45. A.M. Molenbroek and B.S. Clausen, unpublished results 46. L .GrAbaek, B.S. Clausen, G. Steffensen, H.Topsoe, Mat. Sci. Forum 133-136 (1993) 255 47. H. Kn6zinger and E. Taglauer, A Specialist Periodical Report, Catalysis, 10, (1993), 1 48. N.-Y. Topsoe, Science, Vol. 265 (1994) 1217 49. N.-Y. Topsoe, H. Topsoe and J.A. Dumesic, J. Catal 151 (1995) 226. 50. N.-Y. Topsoe, J.A. Dumesie and H. Topsoe, J. Catal 151 (1995) 241. 51. N.-Y.Topsoe, CaTTech, Vol. 1 (1997), No. 2, 125. 52. B.S. Clausen, J. Schie~z, L. GrAbsek, C.V. Ovesen, K.W. Jacobsen, J.K. Norskov and H. Yopsoe. Topics Catal. 1 (1994) 367 53. C.V. Ovesen, B.S. Clausen, J. Schi~z, P. Stolze, H. Topsoe and J.K. Norskov. J. Catal. 168 (1997) 133 54. H. Topsoe, C.V. Ovesen, B.S. Clausen, N.-Y. Topsoe, P.E. Hojlund Nielsen, E. T6rnqvist, J.K., Stud. Surf. Sci. Vol. 109, (1997), 121. 55. J.D. Grunwaldt, A.M. Molenbroek, B.S. Clausen, C.V. Ovesen, N.-Y. Topsoe and H. Topsoe. To be submitted. 56. B.S. Clausen, B. Lengeler, B.S. Rasmussen, W. Niemann and H. Topsoe, J. Phys. (Paris) C8 (1986) 237 57. P.B. Rasmussen, P.M. Holmblad, T.S. Askgaard, C.V. Ovesen, P. Stoltze, J.K. Norskov and I. Chorkendorff. Catal. Lett. 26 (1994) 373 58. T.S. Askgaard, J.K. Norskov, C.V. Ovesen and P. Stoltze. J. Catal. 156 (1995) 229
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59. N.-Y. Topsoe and H. Topsoe, J. Mol. Catal. A 141 (1999) 95 60. T. Fujitani and J. Nakamura. Catal. Lett. 56 (1998) 119 61. M.M. Viitanen, W.P.A. Jansen, R.G. van Welzenis, H.H. Brongersma, D.S. Brands, E.K. Poels and A.Bliek. Phys. Chem. B 103 (1999) 6025 62. E.K. Poels and D.S. Brands, Appl. Catal. A 4816 (1999) 1 63. H. Topsoe, B.S. Clausen, and F.E. Massoth, "Hydrotreating Catalysis: Science and Technology", in Catalysis, Science and Technology (eds. J.R. Anderson and M. Boudart), Springer Verlag, Vol. 11 (1996). 64. H. Topsoe, B.S. Clausen, R. Candia, C. Wivel and S. Momp, J. Cat. 68 (1981) 435. 65. C. Wivel, R. Candia, B.S. Clausen, S. Momp and H. Topsoe, J. Catal. 68 (1981) 453. 66. L.S. Byskov, B. Hammer, J.K. Norskov, B.S. Clausen, H. Topsoe, Catal. Lett.,47 (1997) 177. 67. L.S. Byskov, J.K. Norskov, B.S. Clausen, and H. Topsoe, J. Catal. 187 (1999) 109. 68. P. Raybaud, J. Hafner, G. Kresse and H. Toulhoat, Phys. Rev. Lett. 80 (1998) 1481 69. S. Helveg, J.V. Lauritsen, E. L~egsgaard, I. Steensgaard, J.K. Norskov, B.S. Clausen, H. Topsoe, and F. Besenbacher, Phys. Rev. Lett. 84(2000)951. 70. K. Hojrup Hansen, T. Worren, S. Stempel, E. L~egsgaard, M. B/iumer, H.-J. Freund, F. Besenbacher, I. Stensgaard, Phys. Rev. Lett. 83, No. 20 (1999) 4120.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Air Pollution Abatement through Heterogeneous Catalysis Masakazu Iwamoto*
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan Recem progresses in catalytic technologies for preserving clean air have been reviewed. The major part of this paper was devoted to the novel catalytic removal of nitrogen monoxide from exhausts, especially the selective catalytic reduction of NO with hydrocarbons in the presence of excess oxygen. At present there are two suggestions for the NO reduction. First is the development of a catalyst which can yield N2 selectively in the continuous flow of mixture of NO, oxygen, and hydrocarbons. Second is the separation of oxidation process of NO to NO2 with oxygen and subsequent reduction process of NO2 to N2 with hydrocarbons. In the first strategy the catalytic activities, durability, and characterization of Cu- and Fe-zeolites were summarized. In the second one the storage-reduction method and the intermediate addition of reductant have been introduced.
I. Introduction The use of catalytic processes in pollution abatemem and resource recovery is widespread and of significant economic importance for the realization of sustainable chemistry/industry [1]. As has widely been recognized, there are following five classes as environmemally benign catalyses. (1) Control of emissions of environmentally unacceptable compounds, especially in flue gases and car exhaust gases. (2) Conversion of solid or liquid waste into environmentally acceptable products. (3) Selective manufacture of alternative products that can replace environmemally harmful compounds, such as some chlorofluorocarbons (CFCs). (4) Replacement of environmentally hazardous catalysts in existing processes. (5) Developmem of catalysts that enable new technological routes to valuable chemical products without the formation of polluting by-products. The targets of environmentally benign catalyses are lying in air, water, and soil. In this paper the first topic, the heterogeneous catalysis for materials emitted into air, will mainly be taken up because the compositions and quality of fuels and emission control * Previous address: CatalysisResearch Center, HokkaidoUniversity, Sapporo 060-0811, Japan.
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during fuel utilization are strongly dependent upon the application of heterogeneous catalysis. Problems and opportunities in water, however, are also of increasing importance [2-4]. The amount of water consumption in industrialized countries is continuously increasing and in several countries the depletion of underground sources and/or their increasing level of contamination has become a central question [2]. Rational use of water resources is one of key issues for sustainable growth. Although technologies for treating recycled rinse water are available commercially, there are limitations in terms of cost of chemicals/technology, efficiency of removal of pollutants, production of side streams, severity of operation, range of conditions for operation, etc., for which innovative solutions are required. The use of solid catalysts would overcome or reduce the limitations. From these viewpoints there are two classes of studies; oxidation processes [3] and heterogeneous photocatalysis [4]. The progresses in these studies are highly expected.
2. Present Positions of Catalyses for Sulfur, Soot, and Organic Compounds Air pollution and acid rain seriously affect the terrestrial and aquatic ecosystems and therefore are very important social problems that must be solved as soon as possible. The exhaust gases from engines of vehicles and industrial boilers contain mainly carbon oxides, nitrogen oxides (NOx), hydrocarbons, sulfur dioxide, particles, and soot. In this section the studies on sulfur, soot, and organic compounds would be reviewed. 2.1. Sulfur Sulfur compounds produce SOx during combustion in engines and during catalytic regeneration in catalytic cracking units, leading to local contamination and to the poisoning of automotive exhaust catalysts [1,5]. Recent research has been conducted from two viewpoints, developments of new active catalysts for desulfurization of some organic sulfur compounds and of reduction catalysts of SO2 to elemental sulfur by CO or hydrocarbons. Very recently desulfurization of thiophene via hydrogen transfer from alkanes was reported [6]. The direct use of hydrocarbons in desulfurization reaction should be considered because of the ubiquitous presence of light alkanes in refinery and petrochemical streams and their frequent use for H2 generation. Stoichiometric hydrogen scavengers such as oxygen, CO, and CO2 can remove hydrogen formed in C-H activation steps and increase alkane dehydrogenation selectivity on H-MFI and Zn-/H-MFI. Propane coreactants (20kPa) led to desulfurization rates and H2S selectivities much higher than expected at the H2 pressures prevalent during propane-thiophene reactions (1-3kPa) and similar to those obtained at 50-300kPa HE. Catalytic desulfurization occurred without significant formation of benzothiophene or of unreactive sulfur-catalyst adducts and without requiring gas phase H2. These results show that dehydrogenation reactions can be coupled kinetically with hydrogenation reactions of thiophene or its fragments.
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Many kinds of materials have been examined as soot oxidation catalysts: single or mixed oxides [16,17], perovskite-type oxides [18], various vanadates and molybdates [19,20], and Pt-loading materials [ 15,21]. The effect of support was also tested [19,22]. The experimental conditions such as the intensity of contact between soot and catalysts may affect the activities these solids. For example, it was reported that the catalytic activity of cobalt oxide or iron oxide was dependent on the degree of contact; tight contact is better for the catalysis [23]. From this viewpoint it is interesting that Cu/K/Mo/C1 shows high soot oxidation activity under loose contact conditions [24]. Three reaction mechanisms have been suggested for the soot combustion on solid catalysts. First, the reaction is catalyzed by redox behavior of the catalytic materials [16,25]. In a recent kinetic study [26], a half-order kinetics in the partial pressure of oxygen was obtained on CuFe204 and the formation of reactive oxygenated intermediates on the soot surface is suggested to be the rate-determining step. In this mechanism the tight contact is essential for the promotion of the catalytic oxidation [16]. Secondly the formation of liquid eutectic phases is reported to be a key in determining the activity on Cu-K-V oxide [27] and Cs-V-Mo oxides [20]. The catalytic activity increased dramatically upon the improvement of the catalyst-carbon contact at or above the temperature at which liquid phases are formed. Thirdly the activity of MoO3 as a soot oxidation catalyst is claimed to be due to gas-phase contact [20]. Significant loss of catalyst through vapor phase was recognized in the experiments, which can result in catalyst emission and loss of catalytic activity in time. The catalytic activity of MoO3 is very good but it is probably not applicable as soot oxidation catalyst. In the cases of second and third reaction mechanism the tight contact is not essential. It should be noteworthy to add that NO increased the soot combustion rate on several catalysts [14,15,26] and this would be the formation of NO2 on the catalysts and the subsequent oxidation of carbon with NO2.
2.3. Organic Compounds Fully halogenated chlorofluorocarbons (CFCs) are responsible for the depletion of the ozone layer. The Program for Alternative Fluorocarbon Toxicity Testing has recommended a guide for transforming CFCs into hydrofluorocarbon compounds (HFCs). HFCs show no effect for the ozone-depletion. To recover CFCs and destroy them is a logical step forward. Many destruction techniques have been proposed [28]. Very recently, however, converting CFCs into valuable chemical compounds have been studied as a better choice. This technique involves the selective hydrodechlorination of CFCs to HFCs on supported palladium [29] or non-noble metal such as nickel [30]. As a consequence of its refractory nature and large-scale production tetrachloromethane (TCM) is environmentally ubiquitous. Since TCM is not easily decomposed under ambient conditions, and is a suspected human carcinogen, a wide variety of studies have been carried out on the decomposition of TCM over heterogeneous catalysts [31] and on selective catalytic hydrogenation of TCM to CHC13 [32]. In the
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Modem hydrocracking catalysts often consist of a combination of a sulfidic Ni (Co)-Mo or Ni-W phase [5] and an acidic zeolite. Although the reaction mechanism on the former catalysts is widely studied, the acid catalyzed reactions are not well understood. A basis to a better understanding of the reaction was provided from the theoretical study of reaction paths of sulfur comaining molecules upon contact with Bronsted acid sites [7]. It was clarified that the presence of hydrogen does not affect significantly activation barriers but dramatically changes the overall enthalpy of reaction. Currently operating desulfurization, based on the SO2 scribing with lime or limestone, is a costly process requiring a large space with complicated facilities and disposal of the used sorbents. Direct catalytic reduction of SO2 to elemental sulfur has received much attemion because it is easier to design and operate. The process can be applied directly to flue gases containing a small amount of oxygen or to the case where SO2 in the flue gas is isolated or concentrated using a proper adsorption/regeneration system. SO2 reduction with CO has been studied on several types of catalysts. In the early developmem of catalyst, a substantial amount of COS, which is much more harmful than SO2, was formed as byproduct. Recently Co304-TiO2 has been reported to show the highest catalytic activity for the reduction of SO2 by CO among catalysts reported so far [8]. There existed a strong synergistic promotional effect in the conversion of SO2 when cobalt was mixed with TiO2. It is claimed that the COS intermediate can react with SO2 to produce an additional sulfur and also behaves as a strong reductant to keep oxygen vacancies on the TiO2. On the other hand sulfided CoMo/A1203 has been found to exhibit outstanding activity for the reduction of SO2 with CO [9], though a certain amount of COS was produced. On the catalyst CO adsorbs exclusively on CoMo phase and SO2 predominantly on 7-alumina. Ceria-based catalysts have also been reported [ 10]. Methane is an attractive reductant, due to abundant and cheap natural gas. The reaction between SO2 and CH4 has been studied for a long time but the catalytic activity was insufficient. Cobalt oxide was recently reported as an active catalyst over alumina [ 11], but only at high temperatures (>973K). In contrast, La-doped and undoped ceria were found to catalyze the SO2 reduction by CH4 at 823-1023K at atmospheric pressure [12]. The addition of copper or nickel into La-doped ceria has given the improved selectivity to elemental sulfur or H2S, respectively. With proper further development, this class of catalysts offers promise for practical application to sulfur recovery from various SO2-1aden gas streams.
2.2. Diesel Soot Particulate matter as well as NOx is one of the main pollutants in diesel engine emissions. The combination of traps and oxidation catalysts appears to be the most plausible after-treatment technique to eliminate soot particles [ 13]. The possibility of promoting both oxidation and NOx reduction in a single catalyst has also been investigated [14,15]. The present position of soot combustion catalysts was summarized by Querini [ 16].
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former reaction the requirement of the input of substantial quantities of energy is the disadvantage, while in the latter catalyst deactivation is a major point to be solved. The stringent limiting value for emissions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), 0.1 ng/Nm 3 for municipal and hazardous waste incinerators, has been in effect in several European countries since the early 1990s and in Japan since January 1997. It has been shown that the TiO2-based V205-WO3 catalysts originally designed for the removal of nitrogen oxides (NOx) through selective catalytic reduction (SCR) are very effective in the decomposition of PCDD/PCDF at the same temperatures as are used for the deNOx reaction. In the last few years, the commercial SCR catalysts have been optimized for the combined dioxin~Ox destruction. This was achieved mainly by increasing the oxidation potential of the catalysts [33].
3. Catalytic Decomposition and Adsorption of Nitrogen Monoxide 3.1. Necessity of New DeNOx Technologies At present, one of the significant problems in air pollution is removal of NOx, which is produced during high temperature combustion. In particular, the removal of nitrogen monoxide (NO) is a dominant target to be achieved because NO is an inert and major component of NOx in the exhaust gases [34]. It is well known that NO is thermodynamically unstable relative to N2 and 02 at temperatures below 1200K, and its catalytic decomposition is the simplest and most desirable method for its removal. To date, however, no suitable catalyst with sustainable high activity has been found. This is due to the fact that oxygen contained in the feed or produced in the decomposition of NO, competes with NO for adsorption sites. As a result, high reaction temperature and/or gaseous reluctant is required to remove surface oxygen and regenerate catalytic activity. The catalytic reduction processes employing NH3, CO, or hydrocarbons as reductant on TiO2(-V205)-WO3 or Pt-Pd(.Rh) catalysts have been put to practical use. Although many efforts have been devoted to improve the processes, the disadvantages or problems which each of the present reduction processes suffers are summarized as follows. (a) In the selective catalytic reduction system with ammonia (NH3-SCR) there are several disadvantages such as high costs of facilities and running and leakage of unreacted dangerous ammonia. (b) The automobile catalytic converter is the only technology available for the most stringent emission standards. In this technology so called three-way catalysts are preferentially used with several limitations such as using unleaded gasoline and maintaining a specified air/fuel ratio. However, this system cannot meet the requirements of newly developed engines in which the air/fuel ratio has been made lean to an air-rich region, because the exhaust contains a considerable amount of oxygen and the present catalysts do not work under such conditions. (c) The greater use of diesel-engine vehicles is a major trend observed worldwide over the last decade. Co-generation systems using diesel engines have also been under development. Although inherently cleaner than gasoline engines from the viewpoint
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of CO and hydrocarbons, diesels produce more aldehydes, SOx, NOx, smoke, and odor. In this instance the problem similar to the above, i.e., removal of NO in the presence of oxygen and SOx remains unsolved. Much effort has recently been devoted to develop alternative methods for the removal of NO, that is, decomposition, reduction with hydrocarbons, or adsorption. Future opportunities of catalytic removal of NO can be classified as follows. This review will be described in this order. Removal of NO without Reductant Catalytic decomposition to nitrogen and oxygen molecules Adsorption-Enrichment-Aftertreatment Removal of NO with Reductant Selective catalytic reduction with hydrocarbons (HC-SCR) in the presence of excess oxygen One-stage treatment of continuous flow of NO, oxygen, and hydrocarbons Two-stage treatment separating oxidation of NO and reduction of NOx
3.2. Catalytic Decomposition
Catalysts. NO decomposition to molecular nitrogen and oxygen is the simplest, most attractive, and most challenging approach to NOx abatement. We have first reported that Cu ions exchanged in the MFI matrix exhibit unique and stable activity among Cu-zeolites [35]. In particular, the over-exchanged Cu-MFI (Cu2+/AI > 0.5) reaches very high decomposition activity [36]. These results were confirmed by Li and Hall [37]. The effects of the Si/A1 atomic ratio of the parent zeolites used to prepare the catalysts were also investigated. Since the discovery of the remarkable NO decomposition activity of Cu-MFI catalysts in 1986 [34, 35], a lot of effort has been devoted to develop active catalysts. In the case of metal oxides, CO304 is one of the most active, single component, metal oxides for NO decomposition [38]. Its activity can be enhanced by addition of Ag, presumably by modifying the extent of oxygen suppression [39]. It has also been shown that the modification of Co304 by alkali metal ions, in particular Na, is very effective for the enhancement of decomposition activity [40]. YBa2Cu3Oy[41a] and Sr2+-substituted perovskite oxides [41b] have been reported as candidates for the catalyst. Lunsford et al. have reported high activities of Ba/MgO [42]. In the case of zeolites or porous materials, it has been claimed that Co-MFI zeolite that contain Co in the framework has considerably larger maximum activity for NO decomposition than does Cu-MFI [43] though no data has been reported in a continuous flow system. Wichterlova et al. have found that Cu-MeA1PO-1 ls (Me - Mg or Zn) exhibit constant conversion in NO decomposition and the turnover frequency values at 770K are comparable to those of Cu-MFI with high silica matrix [44]. On the other hand, the decomposition activity of Pt metal has been established for a long time. Recently, the formation of Tb-nitrate intermediate was observed to be important in NO decomposition over Tb-promoted Pt catalyst [45]. The relative catalytic activities of these catalysts are roughly compared in Figure 1
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化
1.25 I
r 1.00
Cu-ZSM-5
r
r
9Lao.8Sro.2Co03
0
:.E'_ 0.75 r~
0
Na/C0304
E 0
o 0.50
9YBa2Cu3OT.x/MgO 9 ka~.sSro.sCuO4 9
"0
>
0.25 n"
0.00
Cu-MgAIPO
BaFeO3.x i C0304Pt/AI203 Ag-C0304 9 I~a/Mgu 1100
1000
900
800
700
Temperature giving maximum decomposition activity / K Figure 1.
Decomposition activity of various catalysts reported so far.
where each decomposition activity is plotted on the basis of very vague calculation since the experimental conditions are dependent on each research group. The figure indicates that the key components for direct decomposition of NO are Cu and Co and that their catalytic activities can be improved by addition of precious metal and so on. It has been reported that the increase of 1 order of magnitude in the turnover frequency could lead to a practical catalyst [46,47]. NO decomposition still offers a very attractive approach to NOx removal. However, since any combustion process is going to produce 10-20% water vapor, one must focus on a catalyst that is stable for long times in such wet environments.
Characterization o f and Decomposition Mechanism on Cu-MFI. It is clear that demonstrating the structure of the active sites, and learning the limitations of the Cu-MFI, could lead to develop new more active and stable catalysts that could find practical application. Several excellent reviews have been published and it is apparent that no general consensus of opinion exists with respect to the nature of the active site involved or indeed the reaction mechanism occurring, as pointed out by Mackinnon and coworkers [48]. The main points of dispute can be summarized as follows. (1) Considerable evidence has been provided to indicate that Cu + species participate in the reaction [49,50]. On the other hand, the reaction on Cu 2+ ion with no contribution of Cu + has also been postulated [46]. In my opinion, however, there is no doubt that the NO decomposition is a redox process. (2) The NO decomposition reaction is promoted on over-exchanged Cu-MFI catalysts and this behavior may correlate with the availability of extra-lattice oxygen
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(ELO) species [49]. The identity of the ELO is not clear. We [49] and Sachtler and coworkers [51] have proposed that it is of the form Cu2+-O2"-Cu2+, whereas Bell and coworkers have suggested that the ELO is associated with isolated Cu 2§ sites and is of the structure Cu2§ or Cu2+O2 [52]. Very Recent investigations [48,53] have supported the presence of Cu2+O" or Cu2+O2 species. (3) The mechanism for coupling of nitrogen species to form product N2 is a topic of controversy. There are two kinds of problems to be solved. First significant problem is that the number of copper ions working as active site is one or two. Second argument is the form of intermediates to produce N2; a nitrosyl species, a nitro species, a nitrate species, and dissociatively chemisorbed NO, etc. have been suggested. The first point of the third item will be discussed here in more detail. It was demonstrated that the most active catalysts are those with the low Si/AI atomic ratios and Cu exchange levels in the range of 90-150%. These results have leaded to two kinds of possibilities for copper active sites in Cu-MFI catalysts. It has been suggested the active site responsible for the high catalytic activity is a unique dimeric Cu species which is stabilized by zeolite framework. Adsorption of NO on this dimeric species to form a cuprous hyponitrite that decomposes to form N20 and then N2 is proposed to be a possible reaction mechanism [49,51,53,54]. The species Cu2+-O2-Cu 2+, Cu+-O2"-Cu2+, and Cu+...Cu2+O [53] are suggested. In contrast, the monomeric Cu site was suggested as an active site by several researchers [52,55,56]. Although Cu+(NO)2 has been proposed as precursor for N20 formation in previous studies, the lack of correlation between Cu+(NO)2 and N2 formation [57] and first principles quantum mechanical calculations [58] suggest that Cu+(NO)2 is not formed under reaction conditions. Thus, Cu+(NO)2 as precursor is ruled out. The Cu2+O or Cu2+O2 species may form on the over-exchanged Cu-MFI and act as the active sites
[56].
Detailed characterization of Cu-zeolites has now been carried out by Wichterlova and coworkers [59,60], Kuroda and coworkers [61,62] and other researchers [63,64] eagerly, being expected to solve the above controversial reaction mechanism. For example, very recently the locations of Cu § ions are proposed on the basis of experimental [60] and theoretical [63] studies and their conclusions are in good agreement with each other. In addition Kuroda et al have claimed that zeolite having an appropriate Si/AI ratio, in which it is possible for the copper ions to exist as dimer species, may provide the key to the redox cycle of copper ion as well as catalysis in NO decomposition [62]. This conclusion coincides with the results of theoretical calculation [64] in which bent Cu-Ox-Cu structures are found in Cu-MFI and these are suggested to be the part of a catalytic cycle.
3.3. Adsorptive Removal of NO It is widely accepted that selective adsorption is one of the most suitable techniques for removal and/or enrichment of low concentration pollutants. In particular, pressure swing adsorption (PSA) has widely been applied to various processes;
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Table I. NO Adsorption Properties of Various Cation-Exchanged MFI Zeolites j amount of NO adsorbed/(cm3.g -1) adsorbent
content of cation/(wt %)
reversible
irreversible
Na-MFI(23.3)-100 b Ca-MFI(23.3)-54 Sr-MFI(23.3)-105 Ba-MFI(23.3)-80 Mg-MFI(23.3)-46 Cu-MFI(23.3)-157 Ag-MFI(23.3)-90 Co-MFI(23.3)-90 Mn-MFI(23.3)-127 Ni-MFI(23.3)-68 Zn-MFI(23.3)-96 Fe-MFI(23.3)-62 Cr-MFI(23.3)-41 Ce-MFI(23.3)-8 La-MFI(23.3)-7 H-MFI(23.3)-100
2.81 1.32 5.45 6.44 0.69 5.90 10.85 3.06 4.20 2.41 3.79 2.12 0.87 0.43 0.40 0.13
0.16(0.006) c 1.81 (0.246) 2.71 (0.195) 1.50(0.143) 0.69(0.109) 4.28(0.206) 3.38(0.150) 1.52(0.131) 1.19(0.069) 1.03(0.112) 1.01 (0.078) 0.52(0.061) 0.38(0.101) 0.34(0.496) 0.25(0.388) 0.12(0.004)
0.00(0.000) c 1.56(0.212) 0.20(0.014) 1.44(0.137) 0.22(0.035) 14.90(0.716) 0.54(0.024) 19.69(1.693) 5.81 (0.339) 6.64(0.727) 0.50(0.039) 3.08(0.362) 1.16(0.308) 0.34(0.496) 0.24(0.372) 0.32(0.011)
a Adsorption time, 45 min; desorption time, 60 min; concentration of NO, 997 ppm; adsorption temperature, 273 K; adsorbent weight, 0.5 g; flow rate, 100 cm3.min -1. b Concentration of NO, 1910 ppm. c Unit, (NO molecules)-(cation) -1. therefore, the PSA is expected to be an effective method to remove or enrich NOx diluted in air. Although active carbon, carbon fiber, silica, zeolite, and chelate resin have been reported as the candidates so far, little is known of the respective amounts of reversible and irreversible adsorption of NO on metal ion-exchanged zeolites. We measured them by a fixed bed flow adsorption apparatus [65-67]. The amount of reversible and irreversible adsorption of NO per weight of adsorbent (denoted as qrev and qirr, respectively) measured at 273 K on various cation exchanged MFI zeolites are summarized in Table 1. The values in parentheses are the amounts of reversible and irreversible adsorption of NO per cation (q*rev and q*irr)- The qrev and qirr greatly changed with the metal ion. With MFI zeolites, the order of qrev was transition metal ion = alkaline earth metal ion > rare earth metal ion ~ alkali metal ion proton. Among the adsorbents listed in the table, Cu-MFI-157 and Co-MFI-90 showed the largest qrev and qirr, respectively. The dependency of the NO adsorption upon the exchange level of copper ion was studied on MFI zeolite at 273 K. qrevand qirr were linearly proportional to the exchange level of copper ion, showing that q*rev
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and q*~ are constant, approximately 0.23 and 0.64 NO molecules-Cul, respectively. It follows that the effectiveness of each copper ion in MFI zeolite for NO adsorption is independent of its loading level. The amounts of reversible and irreversible adsorption of NO were dependent not only on metal ion but also on silica/alumina ratio. Both q*rCvand q*~rrdecreased with increment of the aluminum content in zeolites. It indicates that the adsorbability of NO is mainly controlled by the aluminum content and not by the zeolite structure. The dependencies of qr~v and qirr upon the adsorption temperature are also studied. With increasing adsorption temperature q~r on Cu-MFI-157 significantly decreased. On the other hand, qr~v gradually increased with temperature, reached the maximum (4.35cm3gl ) at 243 K, and then decreased. The maximum qrev on Co-MOR-65 was 5.42cm3gl at 373 K. A high capacity for reversible adsorption of NO is required for PSA. At present Co-MOR [66] or Ag-MOR [67] are strong candidates for NO adsorbents in high or low temperature PSA. In real exhaust gases, there coexist various gases and therefore it is important from a practical point of view to clarify their influence on adsorption properties. The preadsorption of NO2 on Cu-MFI-147 resulted in the enhancement of qrev (from 4.35 9 cm 3 g-1 without preadsorbed NO2 to 7.14 after the preadsorption of NO2). At low temperature N203 is known to be in equilibrium with NO and NO2. This suggests that NO2 irreversibly adsorbed can work as new active sites for the reversible adsorption of NO. When 02, CO2, or SO2 was preadsorbed, qrCvwas little reduced (4.26, 4.25, or 3.92cm3gl, respectively). CO or H20 poisoned the adsorbability (1.39 or 0.22cm3g1, respectively). On the other hand, qirr is always decreased by the preadsorption of these gases.
4. Continuous Reduction of Mixture containing NO, Oxygen, and Hydrocarbons Cu-MFI is the most active catalyst for the decomposition of NO. However, the activity greatly decreases in the presence of excess oxygen, water vapor, and SO2, as mentioned in the previous section. Selective reduction of NO with hydrocarbons in an oxidizing atmosphere over Cu-MFI has first been reported by the present group [34]. At the same time, Held and coworkers have reported similar findings independently and Toyota Motor Co. also applied for the patents. The distinguishing characteristic of this new technology is that the presence of oxygen is indispensable for the progress of the reduction of NO. This new selective reduction of NO proceeds even in the presence of excess 02, and has the possibility to overcome the disadvantages of the present reduction systems, NH3-SCR and three-way catalytic system. Several reviews [34,68] have already summarized the progress of HC-SCR in 1996 or before. The catalysts developed before 1996 therefore will briefly be introduced here and then recent progress will be reviewed in the latter sections. Since the finding of the HC-SCR technology, a lot of catalysts have been reported to be active. Some zeolite-based catalysts including Cu, Co, Fe, Ag, or Pt show high initial activities with hydrocarbons as reducing agents as well as ammonia. So far,
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化
100
In-Fe203/H-MFI 9
80o
=
oo
60-
9 Z E 40E
o~
20 -
Au/AI203+Mn203 O
A Sn/Ga203.AI203 A Ga203-AI203
Fe2+.MFI ~ Au/AI203 Co-BEA O O
A In/Co.Al203
A Ag/Al203 Ag/Co-MFI C~3(PO4)2 ~ OCo-MFIA ln/Ga203-A1203 ~Ag/TiO2"ZrO2 ~' ln/Al203 Cu-P-MFI~ Pd-In/TiO2-ZrO2 Cu-SAPO34 9 Co-MOR in/C~lOx A AI203 Pt/MOR(~ Pt+Zn-MFI(IAR) Pd/H-MOR Ni~1203a Co/AI203 O Pt/WO3 O Cu-MFI in//~a203.A1203 A Ag/AI203 Pt-silicate O O Pt/AI203 O Ir/A1203 OCo-silicate Pt-B/YPO4 0 ~ ~ Ga/H-MFI A CoAIOx Pt-B/LaPO4 0 Fe3+-MFI ~ In/TiO2-ZrO2 Sn/Al203 A Al203 ad/Al203 O O Pt/SiO2 O Mn203+Sn-MFI ~ Pt/MFI ~ Pt/SiO2
O Pt/Co-silicate
at/gl203
A Co/A1203
A AIPO4
0 Rh/Al203 0 373
I
I
!
I
473 573 673 773 Temperature giving maximum NOx conversion / K
I
873
Figure 2. Reduction activity of various catalysts reported so far. Open and closed circles and triangles roughly correspond precious metals, microporous materials, and metal oxides. The change in catalytic activity resulting from the difference of experimental conditions has not been taken into account at all. however, the hydrothermal stability of zeolite catalysts appears to be limited. Hydrothermal deactivation can have several causes, which are structural collapse, dealumination, agglomeration of active cations to small oxide islands, and migration of the cations to inaccessible sites. As the stability is of major importance from application, improvement of zeolite catalysts should be aimed at stability too rather than initial activity only. On the other hand alumina and composite metal oxides are reported as active catalysts. Some solid acids also show catalytic activity. In the case of metal oxide catalysts the reaction rates are not sufficient, which means that a big reactor or low gas hourly space velocity is needed for the practical application. All of the catalytic activities have been measured under the experimental conditions of the respective researchers. The kinds of hydrocarbons used, the concentrations of the respective reactants, the space velocity, the shape of the reactor, and the pretreatment of the catalyst can all influence the reaction results, that is, the apparent catalytic activities. For example, we can employ ethene as reductant and probably obtain good results when we use a catalyst with high performance for hydrocarbon
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oxidation, while the use of propene could be recommended for the catalysts with low oxidation power. With the catalysts not so active for the hydrocarbon oxidation, the low space velocity can be set to get high conversion of hydrocarbons and NOx. The molar ratio of NOx and hydrocarbons also determines the apparent catalytic activity for deNOx reaction. On understanding of these situations, many results reported have been plotted in one figure to reveal general features of HC-SCR [34]. In Figure 2 the difference of experimental conditions in the respective reports has not been taken into account at all. Open and closed circles and triangles roughly correspond precious metals, microporous materials, and metal oxides, though there are many combined catalysts. The active temperature regions of catalysts are clearly depending on the type of active centers. Precious metal catalysts are active at the lowest temperature, transition metal-ion exchanged zeolites work at the middle temperature region, and the active temperatures of metal oxide catalysts are the highest. Figure 2 also indicates that the active components are Pt, Cu, Co, Fe, Ag, In, Ga, Sn and so on and that the supports frequently used are alumina and zeolites. I am guessing that the practical application might be achieved on precious metal-, Cu-, Co-, or Fe-containing catalysts. For lack of space, Cu and Fe would be reviewed here in more detail and the investigations on Pt [69], Pd [70], Rh [71], Ag [72], and Co [73] were omitted. When we consider the practical application of the present HC-SCR method, the best way is the simultaneous abatement of NOx and hydrocarbons on one catalyst bed in a continuous flow. The second best is the separation of oxidation of NO to NO2 and reduction of NO2 with hydrocarbons. In this section active catalysts for the first method will be introduced. The latter way will be described in the next section.
4.1. Copper Ion-exchanged MFI Zeolites Much effort has been devoted to the study on Cu-MFI. The major research targets are characterization of copper ions in zeolite frameworks, clarification of deactivation mechanism and development of procedures to preserve the activity, and elucidation of reduction mechanism of NO. The first target highly overlaps with that in the studies on catalytic decomposition. The latter would be discussed here. The most significant problem of Cu-MFI is deactivation during the catalytic run at high temperatures in the presence of water vapor [68]. Too much loading of copper or severe treatment of Cu-MFI has widely been reported to result in the formation of CuO particles or the destruction of lattice, and therefore one could avoid rapid deactivation if the catalyst was used under the proper conditions. The preparation of heat-resistant zeolite is a future problem. The mechanism of gradual deactivation under relatively mild conditions has not been identified. Formation of CuO particles [74] or clusters [75,76] and migration of Cu a§ ion into inert sites [77,78] have been suggested as the causes. The fresh Cu-MFI samples pretreated at 673-773 K usually show two kinds of ESR spectra with g//=2.31-2.33 and A//=140-155 G (CuA) and g//=2.27-2.29 and A//=155-175 G (CUB). The spectra have been assigned to the Cu 2§ species in square-pyramidal and
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square-planar coordinations, respectively. A few research groups [77-79] have independently reported that the treatment of Cu-MFI at 1073 K causes the elimination of the CuA and CuB species, the formation of new CuC species with gH=2.30-2.32 and Aa=155 - 160 G, and the simultaneous dealumination of the zeolite lattice. It has been suggested that the dealumination brought about the change in the location of Cu ions and the resulting migration of Cu ions to inert sites is the origin of the deactivation under the mild conditions [78,79]. On the other hand, Tabata et al. [75] have not found any dealumination under the similar conditions but observed the formation of Cu---Cu bonds by EXAFS. The formation of CuO clusters has been suggested for the deactivation. There is another report [76] in which the CuAI204 formation is associated with the deactivation. We have independently compared the ESR, IR, XRD, and 27A1 MASNMR spectra and the surface area of the hydrothermally treated Cu-MFI with those of the fresh one [80]. The results have indicated that the migration of Cu ions to inert sites without dealumination resulted in the deactivation and the change in zeolite lattice occurred under more severe reaction conditions. There are many reports for improvement of the stability of Cu-MFI. Cucontaining silicate has been reported to show better stability than Cu-MFI [81]. The coloading of La or Ce [82], Cr [83], or P [84] stabilized the catalytic activity of Cu-MFI. In particular, the addition of P is very effective. The catalyst treated at 923 K for 50 h in water vapor has still possessed the reduction activity though the active temperature region became higher. The addition of Ca onto the Cu-P zeolite is reported to be effective for the further improvement of durability. At present two kinds of reaction mechanisms have been suggested for the role of hydrocarbons. Some research groups have proposed that no direct interaction between hydrocarbons and NO is required [85]. In the mechanism, decomposition of NO proceeds first to yield N2 and surface oxygen species, and then the hydrocarbons clean up the surface oxygen adsorbates, or the hydrocarbons-O2 mixture reduce the active sites for the NO decomposition reaction which occurs by a redox mechanism. The other researchers have claimed the direct interaction between hydrocarbons and NO on the catalysts [86, 87]. In this view, carbonaceous deposits, partially oxidized hydrocarbons, hydrocarbons themselves, or ammonia are postulated as the active species, and NO, NO2, N203, and NO3" are proposed as the reactive nitrogen oxides. The latter mechanism is promising on Cu-MFI. Many types of reaction mechanisms have been suggested on Cu-zeolites, the majority of which are still controversial. It should be careful in the research on reaction mechanism that the data were obtained on over-exchanged or low-exchanged Cu-MFI [86]. For example, some types of adsorbed NO were observed on over-exchanged ones, while nitrosyl and nitrite-nitrate adsorbates were found on low-exchanged ones. The behavior of some surface N-containing intermediates such as nitrosopropane [88] was greatly dependent on the exchange level of copper and the atmosphere of the catalysts. The role of N-containing surface species in the HC-SCR has very recently been summarized by Sachtler and coworkers [89].
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4.2. Iron Ion-exchanged MFI Zeolites Numerous zeolite-based catalysts show promising activities for the reduction of nitrogen oxides with hydrocarbons, but have not yet been commercialized for this purpose, except for Co-[~ zeolite. This is due to a lack of long-term stability, especially in the presence of sulfur dioxide and water vapor [90]. Recent results indicate that iron ion-exchanged MFI zeolites exhibit remarkable stability under realistic off-gas conditions. Feng and Hall [91] reported a very high and stable catalytic activity for the reduction of NO with iso-butane at 723K in the presence of 20% H20 and 150ppm SO2. Although the very high catalytic activities could not be reproduced by other groups [92-94] and by themselves [95], Chen and Sachtler clearly demonstrated that the high activity under wet conditions continues for at least 100h at 623K [93]. The problem on reproducibility of active catalysts is attributable to the difficulty of preparation of zeolites containing unstable Fe 2+ ions as follows. Active Fe-MFI catalysts described so far have been obtained under anaerobic conditions. This is due to that Fe 2+ ions are easily oxidized in aqueous medium giving rise to the formation of iron hydroxide species [96]. In the first report, iron oxalate was used in a glass apparatus with separate supply of zeolite and iron salt under nitrogen atmosphere and the F e/AI atomic ratio reached 1.0. Chen and Sachtler [92], however, could not achieve such high degree of ion exchange in their attempt to reproduce the results. A better way for introducing iron was found to be the sublimation of a volatile iron salt, FeC13, into the hydrogen form of the parent zeolite under inert atmosphere [92,93]. Pophal et al. [97] employed iron sulphate during aqueous ion exchange at 323K under N2. On the other hand, K6gel et al. have used the solid-state ion exchange procedure [98,99] to prepare iron exchanged MFI zeolites in air [94,100]. This method using FeCI24H20 in air would be useful for the preparation of practical Fe-zeolites catalysts. The activity of Fe-MFI could be improved by the addition of La [93]. In particular the activity at higher temperature region could be much increased and the temperature window of Fe-MFI became wider. Very recently 10h exposure of Fe-MFI, prepared by sublimation of iron chloride, to wet exhaust gas at 873K was reported to cause severe deactivation of the catalyst [ 101 ]. The temperature would be too high for maintaining the zeolite structure, as has been discussed in the section 4.1. It was suggested that the second sublimation brings about an improvement in the stability of the Fe-MFI catalyst though its deNOx activity is decreased. The state of Fe dispersion in Fe-MFI was investigated by means of IR, TPD, and TPR. For samples with an Fe/AI ration NO + 3/2H20. This process has evolved to be the dominant process to produce ntric acid, now typically using -5% NH3 at - 10 atmospheres. In the 1950s Andrussow showed that by adding C H 4 to a NH3+O2 mixture in a 1/1/1 ratio, it was possible to obtain -70% selectivity to HCN in the reaction CH 4 +
NH3 + 3/202 --> HCN + 3H20
based on both CH4 and NH3, and this process is widely used to produce HCN in Nylon and MMA synthesis 8"~8. A third industrial process using millisecond reactors is the oxidation of methanol to formaldehyde 8, CH3OH + 1/202 --->HCHO + H20, which takes place on silver needles in a thin layer -5 mm thick with high gas velocities to achieve a residence time of several milliseconds. The gas composition and diluents are adjusted to that the adiabatic temperature is -600~ We and others have been exploring other millisecond reaction systems. One is the partial oxidation of CH4 to syngas 459~3 CH4 + 1/202 --->CO + H2, which competes with steam reforming (an endothermic catalytic reaction) and autothermal oxidation (a homogeneous process). The oxidative dehydrogenation of light alkanes to olefins 3 14 9 "17 C2H6 + 1/202 ---> C2H 4 + H 2 0
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is also an attractive alternative to steam cracking s C2H 6 ---)C2H 4 + H 2,
because the oxidation processes are exothermic and do not require furnaces to heat the reactants and supply the heat of endothermic reactions. Another type of millisecond reactor is the single gauze reactor where a single layer of Pt or Ptl0%Rh gauze is used as the catalyst ~922. This configuration uses the catalyst to ignite the reaction. Homogeneous reaction then occurs downstream of the gauze, and this produces both olefins and oxygen containing products with only 20% of the alkanes being converted to the total oxidation products CO and CO_,. An example we have recently investigated is partial oxidation of cyclohexane, cyclo-C6Hl2 + 1/202 ~ olefins + oxygenates + H20. In this reaction system -40% selectivity to olefins and 40% selectivity to ozygenates is observed with the dominant products being cyclohexene and 5-hexanal respectively. Figure 1 summarizes these results for C H 4 to syngas (Rh is the preferred catalyst), C2H 6 to C2H 4 (Pt
or PtSn are the preferred catalysts), and cyclohexane to olefins and
oxygenates on a single gauze. The optimal yield (selectivity multiplied by conversion) occurs near the fuel/O2 ratio predicted by the reaction stoichiometry, and the horizontal . axis of figure 1 is alkane/O2, which are optimized at 2/1 for syngas and for C2H4 and at -3 for cyclohexane/O2. The table and graph show typical results, and slightly better performance than those shown can usually be obtained using preheat or by optimizing other variables. Some reactions are clearly not suitable for millisecond reactor operation. Millisecond reaction systems must be fast and exothermic overall. High temperatures are obviously necessary for significant reaction in 103 sec. For a first order reaction we require that kz-1 or ko exp(-ER/RT)= 1/z = 103 sec ~. This can only be achieved if the rate is sufficiently high, and this requires that T be large for normal values of preexponential ko and the reaction activation energy ER, typically above 900K.
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For the systems described above, the kinetics are fast enough that reaction is
essentially complete in times less than 1 millisecond, and this requires that reaction times on surfaces are much shorter than this. The adsorption lifetimes of molecules on a surface at 1300K are extremely short. Assuming 1: = 1/k = 10~3 exp(+EJRT) with Ed the desorption activation energy, one predicts that for bonds of 25 kcal/mole (chemisorption), 10 kcal/mole (weak chemisorption), and 3 kcal/mole (van der Waals bonds) the adsorption lifetimes are 109, 10~'-, and 10~5 sec. These are extremely short adsorption lifetimes, and it is reasonable to expect that rate parameters measured at low temperature might not be accurate at high temperatures because of lack of energy equipartition at these temperatures.
Reactor Geometry The performance of millisecond reactors depends crucially on the geometry of the catalyst, the gas flow pattern, and the temperature and composition profiles created. The flow configuration and the temperatures of gases and catalysts are sensitive parameters in controlling selectivity and conversion 9~7. Foam monoliths.
Sketched in figure 2 are the temperature and concentration
profiles expected in millisecond reactors using ceramic supported metals (left) and for the single woven gauze (right). Monolith ceramics (typically low area c~-A1203 in forms such as a foam, extruded, fiber, or sphere bed with 0.1 to 1.0 mm channels) are used as supports, and metal is deposited on their surfaces from salt solutions. Metal weight loadings are 0.1 to 20%, and performance is fairly insensitive to loading because metals form micron size particles which coat the walls of the ceramic monolith. Therefore, to a good approximation the metal on a porous ceramic catalyst is a continuous film, not dissimilar to a metal gauze in size of structures and flow patterns. However, partial oxidation catalysts are quite different from conventional metals supported on high area porous oxides where surface areas determine performance and loss of surface area by sintering or poisoning is a major problem. Surface area is to a good approximation not an issue in short contact time reactors. Poisons such as sulfur are not a problem on metals
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such as Pt and Rh because temperatures are too high to form significant coverages of
most poisons. The monolith temperature is measured to be uniform to within ~ 100~ as sketched in figure 2. The gas temperature rises from -25~ (or higher with preheat) to the monolith temperature within ~ 1 mm, and to a good approximation the gas and surface temperatures are equal within 1 mm of the entrance. After the gases leave the catalyst. they cool slowly by conduction from the tube walls such that the temperature is -100~ within 20 cm after leaving the catalyst. Most reaction occurs within 1 mm of the entrance to the catalyst as predicted by detailed 2D modeling and observed by using a monolith only 1 mm thick which shows that all O~. is consumed in this distance. All reactions after the 02 is consumed must be decomposition of the remaining alkane or reaction of products with fuel molecules or products or with H:O. Full 2D modeling also confirms that the temperature reaches the surface temperature in less than 1 mm and that near the entrance a boundary layer of 90% and considerable oxygenates are formed. This must arise through homogeneous reactions which occur downstream of the gauze, and the unique temperature profile of the single gauze reactor allows homogeneous reactions which form oxygenates without subsequent reactions which would decompose them.
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The 20% of the reactants which pass near the surface are heated to ~800~
the gases that bypass the gauze wires remain at 25~
while
These gases then mix rapidly
downstream of the gauze to give a temperature profile as sketched in figure 2. The temperature measured a few millimeters downstream of the gauze is 400~
and this is
close to the calculated adiabatic temperature for the conversions and products observed, so the overall process is nearly adiabatic. The single gauze reactor is thus a rapid heat, rapid quench reactor. Rapid heating prevents reaction before the catalyst because the gases remain cold. Rapid quenching downstream of the gauze caused by mixing of cold and hot gases cools the products to a sufficiently low temperature that subsequent decomposition reactions (dehydration of alcohols, decarbonylation of aldehydes, and decarboxylation of carboxylic acids) is suppressed. In all experiments with conventional monoliths the oxygenates observed are always less than 0.1% for any conditions. We attribute this to the fact that any of these products which form would react quickly in the presence of surfaces to that none of them survive. The exact temperature axially and radially around the gauze wires is not known because it depends on the details of mixing and the reactions that occur in this region. Most of the homogeneous reactions are exothermic, and the temperature could in fact rise above the surface temperature just downstream of a wire. Detailed 2D calculations will be necessary to determine this temperature profile. While the Reynolds number is low (~ 1) so the flow is laminar and detailed flow calculations should be possible, the properties vary strongly with temperature, and the temperature profiles cannot be predicted intuitively. Further, the reactions and their rates are uncertain for these low temperatures and in excess fuels, and these must be known before detailed simulations are possible. The most important species in homogeneous reactions at low temperatures are probably the alkyl peroxy radicals and alkyl hydroperoxides which propagate chains, forming oxygenates without chain branching and further oxidation. Kinetics of reactions of these species have not been characterized in detail.
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Two-Zone Model of Millisecond Reactors Millisecond partial oxidation reactors have an unusual feature that is not seen in
most chemical reactors in that reactions always go to completion because >99% of 02 is consumed in essentially all experiments in excess 0:. It is virtually impossible to have 02 breakthrough except by having bypass because of poor sealing of the catalyst in the reactor tube or having dead zones in the catalyst. Even in the single gauze reactor, more than 90% 02 conversion is typical. Further, partial oxidation processes can be run with typically a factor of at least 10 variation in flow rate with almost no change in conversion or selectivities ~'23. Both of these limits are associated with cooling of the catalyst significantly below the adiabatic temperature. The lower limit to flow rate occurs when heat removal through the walls or by radiation is sufficient to cool the catalyst to a temperature where coke or
CO,
formation occurs. The upper limit to flow rate is where blowout begins by having the front face of the catalyst cool off sufficiently that reaction slows down so that heat generation slows. In industrial HCN synthesis no 02 is ever detected in the product stream, and in nitric acid synthesis there is no NHs in the product stream (the Ostwald process is run in excess 02) unless the gauze catalyst has holes in it. This is true for a wide range of process conditions, including large variations in flow rates, which are basically limited in commercial processes by pumps or heating or cooling limitations. It is essentially impossible to flow too fast in these commercial processes s. This is basically good in that one wants no unreacted 02 in the product because of separation problems and possible unwanted downstream reactions. It also allows the process designer great flexibility in sizing equipment. This occurs because all 02 is consumed in the entrance region of the monolith (within the first millimeter in most experiments), as sketched in figure 3. Higher flow rates simply extend the O2-containing zone farther downstream into the monolith. The limit is of course when the flow rate is so high that 02 escapes, but this is frequently associated with blowout when the front face cools.
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Millisecond reactors therefore have a short oxidation zone where the surfaces are
covered with oxygen, followed by an O2-free zone. This zone is in highly reducing conditions, and models of syngas and olefins predict that these surfaces are covered with nearly a monolayer of carbon, which may passivate the reactor against further reaction. However, additional reaction of the alkane may occur in the second zone as sketched in figure 3, and the control of these reactions is an important factor in designing partial oxidation processes. Syngas. For CH4 oxidation to syngas, CO and H2 are equilibrium products, so no further reaction of these species can occur. However, unreacted C H 4 can still react in steam reforming and CO2 reforming CH4 + H20 ~ CO +3H2, and c n 4 + C O 2 --4 2 C O +2H2,
and it has been frequently suggested 46 that syngas from methane is produced through a two-stage process where the first reaction is total combustion CH4 + 202 ---) CO2 +2H20,
followed by steam and CO2 reforming. We believe that under most situations, particularly where the temperature is >800~
most syngas is produced in a direct
reaction rather than a two-stage process, in contrast to the two-zone picture of figure 3. Addition of excess H20 and CO2 suggest that these reactions are too slow to be significant in producing syngas on Rh except at even higher temperatures. Ethylene. For ethane to ethylene C2H 6 + 11202 --~ CzH 4 + H20,
we believe that the process in fact occurs in two stages. In the absence of added Hz, the two reactions in the oxidation zone are C2H 6 + 02 ~
2 C O + 3H2,
and C2H 6 + 1/202 ----) C2H 4 +
H20
along with reaction of the H 2 produced by the first reaction to form water H 2 + 1/202 ~
H20.
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Then, when all 02 is consumed, the remaining C2H6 dehydrogenates to form more ethylene, C2H 6 ---> C2H 4 + H 2.
Since on Pt without H E added, -20% of the carbon in C2H 6 is converted to CO and CO:, this fraction of C2H 6 converted to COx in the oxidation zone. In experiments where H2 is added to the reactants, the dominant reaction in the oxidation zone is n 2 + 1/202 --> H20,
while in the second zone the major reaction is direct hydrogenation C2H 6 ---> C2H 4 + H 2.
Since with H2 added (with a Pt-Sn catalyst rather than Pt alone) the selectivity to CO falls to 5%, the direct oxidation of C2H 6 to C O must consume only this fraction of the C2H6 because most 02 reacts with H2 rather than attacking C2H 6.
In both of these situations the reactions are strongly exothermic in the first zone and strongly endothermic in the second zone, as sketched in figure 3. Thus, reaction should pull the temperature up in the first zone and down in the second, although radiation and solid conduction should make the monolith temperature fairly uniform. In the single gauze reactor there are obviously two zones because only homogeneous reaction can occur downstream of the catalyst. In this situation both sets of reactions are exothermic, and the temperature profiles are not known.
Scaleup and Scaledown One of the virtues of millisecond reactors is that they can be scaled up and scaled down very easily. In contrast to conventional packed bed reactors, millisecond reactors process large amounts of reactants, typically 1 kg/day of products from a 1 cm 3 catalyst support and HCN+H20
IO0~
Ptl0%Rh gauze 70
70
Formaldehyde CH3OH+O 2 --->HCHO+HEO
600~
Ag needles
80
90
Syngas
C H 4 + O 2 ---)
1000~
Rh on monolith
95
95
Olefins
C2H6+O2 ~ C_,H4+H2
1000~ 950~
Pt on monolith Pt-Sn +H,
65 85
80 70
800~
Pt single gauze
40 40
20
CO+2H2
Single Gauze C.H2.+2+O_, ---) olefins ---) oxygenates
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
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C a t a l y s i s for oil r e f i n i n g a n d and future trends
petrochemistry,
recent
developments
G. Martino Institut Fran~ais du Petrole l&4 avenue de Bois Preau, 92582 Rueil-Malmaison Cedex 1. I N T R O D U C T I O N Catalysis a multidisciplinary science is a very important tool for most of the industries. Industrial catalysis contributes to about 25% of the whole Gross Domestic Products (GDP) [1] and in the most developped countries the generation of a million s added GDP is linked to the spending of about 300~ of catalysts [2]. At present, worlwide catalysts demand is at about 10 Billion C. Oil refining, chemical and petrochemical industries and environmental protection are the main users of catalysts as presented in table 1. Table 1 2000 Worlwide catalysts demand (- 10 G s Refining 25% FCC HDT/HYC Others 42% Chemical processing Chemicals Polymers Emission control 33% Motor vehicules Others
10 10 5 28 14 30 3
Future projections of this demand is difficult to make but most manufacturers [3] are looking for growth in the next five years and even beyond. Heterogeneous catalysis remains the most important one, most of the developments in the fields of environnental protection are based on solids, mainly metals and oxides supported on monoliths. A lot of new polymerisation catalysts are solids and their preparation is based on several organometallic chemistry steps.
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The worldwide foreseen growing rates for the different domains are, rather low for oil refining (2 to 3% per annum), reasonable for environmental protection (4 to 5%) and rather high for chemicals (more than 6%). Regional growths may be significantly different, due to the contrasted starting points as presented in Table 2. The biggest growth rates are expected to take place in the Asia Pacific area. Table 2 2000 Estimates for specific countries (G (~) USA Refining 1.1 Chemical processing 1.1 Emission control 0.7 Total 2.9 * AP/J Asia Pacific+Japan
EUROPE 0.5 1.9 1.0 3.4
AP/J* 0.4 0.9 0.5 1.8
These trends seem reasonable for the oil refining and petrochemical industries which the present paper will be devoted to. It will be limited to the manufacturing of hydrocarbons, cuts or pure compounds like motor fuels, olefins and basic aromatics. 2. O I L R E F I N I N G 2.1. P r e s e n t s i t u a t i o n
At present, there are about 600 refineries running worldwide. Catalytic reforming and hydrotreating capacities as well as the necessary flue gas treatment are present in most of them. Thermal units like visbreaking or coking are also used. But only about one half of the refineries have a catalytic cracking unit and one on four only have implemented an hydrocracker. As a consequence, only a few families of the different catalysts are largely present in all the refineries (table 3). For each family several catalysts manufacturers offer proprietary products [4]. For most on these catalysts, important improvements have been introduced during the recent years but only a few catalysts, clearly different in concept of the existing ones, have been brought up and introduced into a refinery since 1990. These improvements have largely benefited of the tremendeous efforts made by the catalyst community in the last 20 y e a r s ; most of it has been brought together in the handbook edited by E. Ertl, H. Knozinger and J. Weitkamp [5]. Sophisticated physical technics, surface and material sciences as well a molecular modeling have allowed heterogeneous catalysis to move from , , black art to atomic understanding ~,.
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Table 3 Main basic refining catalysts Catalysts Ni, Pd, Pt/carriers Metals Pt + M/alumina Pt/chlorided alumina or mordenite Group VI + Group VIII Sulfides on carriers USY/carriers Solid H3P04/silica Acids Resins HF, H~SO 4 Modified aluminas Others Co Phtalocyanine/carbon
Processes Hydrogenations Catalytic reforming Paraffin isomerization Hydrotreating Hydrocracking FCC Polymerization MTBE Alkylation CLAUS Sweetening
Chemical engineering has made a lot of progresses in kinetic modeling and transport phenomena understanding [6] and has allowed an optimized use of industrial catalysts.
2.2. D r i v i n g f o r c e s The evolution of the refining industry is m a r k e t driven but the market rules are heavily puzzled by environmental concerns and the interference of political decisions well ahead of scientific consensus. For instance MTBE production peaked in 1998 [7a] and was hailed as the environmentaly friendly component for gasoline but it's future now looks in doubt, at least in California and other states in the USA [7b,c] and perhaps elsewhere. That means t h a t any forecast has to be done at our best knowledge on a ,, as today business ~ basis. 2.2.1. P r o d u c t d e m a n d e v o l u t i o n Worldwide oil consumption is foreseen to increase slightly beetwen now and 2010 as indicated in Table 4. For 2020, the increase may continue but it will be linked to the introduction of new kinds of transportations vehicules. Mainly the ratio of heavy ends to transportation fuels will continue to decrease ; n a t u r a l gas and renewable energies taking over as fuels for fixed power stations. Table 4 World oil consumption (GT) Transportation fuels Petrochemicals Other now energy uses Heating & ind. fuels Total
1995 1.600 0.192 0.192 1.216 3.200
2000 1.870 0.250 0.215 1.265 3.600
2010 2.320 0.300 0.250 1.430 4.300
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An other i m p o r t a n t point for the refiners is the continuous evolution of the ratio beetween gasoline and middle distillates as indicated in table 5. This raises the question of the hydrogen to carbon ratio in the refinery finished p r o d u c t s ; the hydrogen richest product is diesel oil and in the future the hydrogen content m a y even increase with the decrease of their aromatic content. The same trend is expected for gasoline with the projected aromatics and olefins content reduction.
Table 5 World distillates d e m a n d (MT) 1995 Gasoline (~) 860 Middle distillates (2) 1085 (1) including naphtha- (2) including heating oil
2000 950 1250
2010 1150 1550
Even w i t h o u t taking into account the most controversial concerns on climate change arising from Greenhouse Gases (GHG) effects, motor fuels are u n d e r p r e s s u r e in order to contribute with the engine's modification to meet the future tighter tailpipe emissions. This p r e s s u r e has led to the i m p l e m e n t a t i o n of new specifications all around the world and by j a n u a r y I st 2000 in Europe where more severe ones are expected to become m a n d a t o r y by 2005. Table 6 shows the existing specifications for gasoline. The most stringent ones concern benzene, reduced to 1% volume, sulphur, brought down to 150 ppm. The b a n of lead in most european countries gasoline has started by the i st of j a n u a r y 2000. By 2005, vapor pressure reduction will be mandatory, aromatics content will be limited to 35% volume. For s u l p h u r a m a x i m u m of 50 ppm has been decided but the possibility to move down to 30 or even to 10 ppm are u n d e r consideration. For C a n a d a and USA, 30 ppm average have been adopted. The possible lowering of the specification on olefins would severely h u r t all refiners using FCC as a work horse for conversion. Table 6 Evolution of gasoline specifications in E U Property 2000 TVR s u m m e r (Kpa) 70 Benzene (% vol) 1 Aromatics (% vol) 42 Olefins (% vol) 18 02 m a x (%wt) 2.3 S max ppm 150 Pb (max) g/1 0.005
2005 60 1 35 (i0) 50 (30) -
The situation for diesel oil (table 7) indicates the clear bend to reduce s u l p h u r in order to contribute to a reasonnable solution to NO x emissions. Europe h a s
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moved from 500 ppm to 350 by j a n u a r y this year. C e t a n e n u m b e r h a s been increased, polyaromatics are limited to 11% wt, 95% volume distillation point h a s been brought down from 370~ to 360~ Table 7 Evolution of diesel specifications in E U Property 2000 S u l p h u r m a x (ppm) 350 Cetane n u m b e r min 51 Polyaromatics m a x (%w) 11 Density (Kg/l) 0.845 T95 m a x (~ 360
2005 50 (30) (55) - (1) - (0.84) (340)
There is a definitive obligation to reach the 50 ppm level of s u l p h u r in diesel by 2005 in Europe. O t h e r countries have not yet m a d e their decision. Figures as low as 30 or 10 ppm are, even today, requested by refiners for the design of their new h y d r o t r e a t i n g units. O t h e r properties m a y give rise to new specifications as indicated in column 2 u n d e r brackets. Cetane n u m b e r of 55 and polyaromatics at 1% wt would need expensive investments. Reduction of T95 and density would reduce the a m o u n t of diesel oil available and require the implementation of several hydrocrackers. 2.2.2. O t h e r c o n s t r a i n t s As any industrial operation, oil refining generates different waste s t r e a m s which have to be h a n d l e d and induce operating costs. W a t e r -up to a few cubic m e t e r are used per ton of crude- is more and more u n d e r scrutinizing [8]. Solids like catalysts are more and more subject to r e t r e a t m e n t s . S u l p h u r which h a d a sales price in the past m a y become an issue as h y d r o t r e a t i n g is growing. Gaseous s t r e a m s are the m a i n concerns today [9]. For nitrogen oxides, sulphur dioxide, particulates and VOC's emissions new limits have been adopted in Europe for 2007 (table 8). Table 8 Limits of emissions in E U (after 2007) Pollutants Power >500 MW SO 2 mg/Nm 3 400 NO x mg/Nm 3 450 Particulates mg/Nm 3 50
Power 500~ the strained siloxanes convert to a more stable form [19]. The silicon atoms in these siloxane linkages are coordinatively saturated, and the stable siloxanes are not easily hydrolyzed, i.e., the Si-O bonds are difficult to break. The relatively inert character is easily reconciled with the need for sites of coordinative unsaturation in order to perform surface acid-base chemistry, illustrated previously by surface science experiments on the polar planes of zinc oxide [10]. An alternative to maintaining coordination vacancies at the surface is to fill these with ligands which are readily displaced by the reactant. This is standard practice in homogeneous catalysis by transition metals, where the soluble metal complex that one introduces into the reaction mixture is often not a direct participant in the catalytic cycle, but serves to introduce active centers into the cycle, often by exchanging ligands which stabilize it in the absence of the reaction mixture [20]. It is perhaps less common to see this strategy applied or identified in heterogeneous catalysis, although one could argue that common practices such as passivation of finely divided metal particles by controlled oxidation is a variation on the same theme. In any case, the maintenance of acid-base sites on silica surfaces by hydroxyl groups is essentially analogous to the example of homogeneous catalysis. Surface silanol groups can be displaced from the surface with stronger acids such as carboxylic acids:
R~
H
I
RCOOH +
O
i
>
I
C
I
O
I
i
~O + H20
~ 1 si ~
In effect, their role here is to serve as "place holders," to maintain accessible coordination sites for conjugate base ligands at surface silicons, sites which would otherwise be lost by thermally driven dehydration of the surface to form stable coordinatively saturated siloxane linkages. The role of surface OH groups in providing active sites for carboxylate formation from the vapor phase on silica is illustrated in Fig. 1. This figure depicts correlations, obtained both by infrared spectroscopy and by gravimetry, between the initial hydroxyl population on silica, and the capacity for formation of surface acetates. Gravimetric measurements, in particular permit one to determine the surface hydroxyl coverage produced by various catalyst pretreatment procedures, and thus permit the catalyst activity to be described in terms of the turnover frequency of the catalyst sites. Typical values for the turnover frequency of ketene synthesis from acetic acid with silica catalysts at 750 K are 10.3 s1. In order for the reaction to be catalytic, the active site must be regenerated in the course of the reaction sequence. The reaction of surface carboxylates to produce ketene involves the net loss of OH from the carboxylate: RR'CHOO(ad)
>
R
i C=C=O + OH (ad)
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Transmission Infrared Spectroscopy Hydroxyl Peak Area (arbitraryunits) 5
0.3
10
15
,-
0.25 r
-,m
0
0
0
0.2
9
0.1
t
9
0"
25
./
~!
....
Figure 8 - Electrostatic potential in zeolite 5A.
I
I
0.0
0.5
1.0
1.6
2.0
2.5
Exporimenta| Loading (mmolelg)
Figure 9- Comparison of predicted and experimental Nitrogen loadings for the zeolites Faujasite, Mordenite, and Linde Type A.
Combinatorial Chemistry The chemical industry must continue to improve productivity and cycle time in developing and successfully commercializing technically superior, economical products. Combinatorial chemistry has great potential to provide a step-change in the productivity of the chemical research laboratory. A combination of sophisticated statistical design, miniaturization, and parallel experimentation increases by several orders of magnitude the ability of the chemist to obtain chemical information. Particularly in the catalyst development area, where testing and evaluation are slow and costly, the successful application of combinatorial methods allows highthroughput screening to select the most-promising candidates for further development.
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The primary application of combinatorial chemistry has been in the pharmaceutical industry. The potential of this method for accelerating drug discovery is so compelling that it has been rapidly embraced by pharmaceutical companies 37. Using combinatorial methods outside of the pharmaceutical industry is in its infancy. The extension of these methods to inorganic materials has been reported 38,39. During the next few years, the application of combinatorial methods to the discovery of inorganic oxides and heterogeneous catalysts will become a reality. Successful implementation of these methodologies requires that a link from the combinatorial scale to the commercial scale be created. This means that both the preparation and testing of catalyst libraries needs to be done under pertinent reaction conditions and provide predictive performance information. A key enabler will be the ability to tie the combinatorial data together via informatics platforms. A number of labs are working diligently towards this goal. Success is in the near future. 5.
FUTURE TRENDS
Chemistry and chemical technology have been at the heart of the revolutionary developments of the 20 th century. As the scale of chemical enterprise grew and as chemistry and chemical engineering became more sophisticated disciplines, the frontiers of innovation passed to the corporations where R&D organizations risked millions of dollars on new products and processes. There is a long tradition in the chemical industry4~ of combining theory and practice, science and engineering, technology and business to improve the lot of humankind. While this central tradition remains unchanged, the methods of achieving it have become more complex and have required the utilization of new advanced methods and tools, including: new materials, improved catalyst design, rapid kinetic analysis, high throughput evaluation of ideas, miniaturization, increased emphasis on modeling, and in-situ characterization. However, innovation will still depend on creative chemists and chemical engineers that connect ideas in the "fight" sequence. REFERENCES 1. J. M. Thomas and W. J. Thomas, Principles and Practice of Heterogeneous Catalysis, 1-669, Winhein: VCH, 1997 2. B. C. Gates, Catalytic Chemistry, New York: Wiley, 1992 3. L. L. Hegedus, Catalyst Design, Progress and Perspectives, New York: Wiley, 1987 4. George A. Olah and ,Z,rp~id Moln~ir, Hydrocarbon Chemistry, New York: Wiley-Interscience , 1995. 5. J. Ertl, J. Weitkamp and H. Knozinger, Handbook of Heterogeneous Catalysis, Weinheim: VCH, 1997 6. Avelino Corma, Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon Reactions, Chem Re. (Washington, D.C.) 95, no.3 (1955):559-614 7. Kozo Tanabe et al, Study in Surface Science and Catalysis: New Solid Acid and Base, Elselvier, 1989 8. Avelino Corma, Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon Reactions, Chem. Rev. (Washington, D. C.) 95, no. 3 (1995): 559-614. 9. A. Stuve, C.P. Halsig, H. Tschorn, Handbook of Heterogeneous Catalysis, Synthesis of MTBE, J. Ertl, J. Weitkamp, H. Knozinger, ed., 1986. Weinheim: VCH, 1997.
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10. Ivano Miracea and Giorgio Fusco, Integrated Process for Producing Isobutene and Alkyl TertButylethers, Italy Snamprogetti S.p.A., Eur. EP 502265 A2 19920909. 11. Franeo Morandi, MTBE and MAS. Snamprogetti Technologies in the Oxygenated Products Field, Pet. Teeh. 332, (1987): 14-18. 12. J.A. Rabo, P.E. Pickett, D.N. Stamires, J.E. Boyle, Acidity of Zeolite X and Y Proc. 2nd Intern. Congr. Catalysis, Edition Tech. 1960. 13. Robert J. Argauer, David H. Olson, and George R. Landolt, Molecular Sieves, MOBIL OIL, USA, GB, etc. 14. Vernon C. F. Holm, Grant C. Bailey, Sulfate Treated Zirconia-Gel Catalyst, Philips Petroleum Company, USA 3,032,599. 1 May 1962. 15. C. Gosling, R. Rosin, P. Bullen T. Shimizu and T. lmai, Revamp Opportunities for Isomerization Units, Petroleum Technology Quarterly (1997-1998): 55. 16. Stephen T. Wilson, Brent M. Lok, Celeste A. Messina, Thomas R. Carman, and Edith M. Flanigen, Aluminophosphate Molecular Sieves: a New Class of Microporous Crystalline Inorganic Solids, J. Am. Chem. Soc. 104, no. 4 (1982): 1146-7. 17. Lewis, Jeffrey Michael Owen, and William Howard. Henstock, inventors, Process and Catalysts for the Manufacture of Alkenes From Alcohols and Alkyl Ethers, S. African, and USA Union Carbide Corp., assignees. ZA. 8807237. A. 1989. 74 pp. 18. Hommeltoft, Sven Ivar, and Haldor Frederik Axei. Topsoe, Preparation of Alkanes by Alkylation Process, Eur. Pat. Appl., and Den. Haldor Topsoe A/S, EP. 433954. A1. 1991. 11 pp. 19. C.D. Gosling, J. C. Sheckler P. T. Barger H. U. Hammershaimb D. J. Shields S. J. Frey, The Alkylene Process: Innovative Technology Using a Solid Catalyst, JPI Petroleum Refining Conference Tokyo 1998, 1998. 20. V.K. Duplyakin, G. A. Urzhuntsev, N. M. Ostrovskii, V. N. Parmon, and A. I. Lugovskoii, Alkylation of Isobutane by Butylenes Over Solid Superacid Catalyst, Book of Abstracts, 212th ACS National Meeting, Orlando, FL, August 25-29, Boreskov Institute Catalysis Novosibirsk 630090 Russia Federal Research Center the Russian Federation, and Washington D American Chemical Society, PETR-0361996. 21. David L. King, Michael D. Cooper and William A. Sanderson, Improved Lewis Acid-Promoted Transition Alumina Catalysts and Isoparaffin Alkylation Processes, PCT Int. Appl., and Inc. USA Catalytica, WO. 9402243. A 1.1994. 63 pp. 22. Crossland, Clifford Stuart, Elliot George Pitt, Alan Johnson, and John. Woods, Process and Apparatus for Paraffin Alkylation and Catalyst for Use Therein, Eur. Pat. Appl., and USA Chemical Research and Licensing Co., EP. 495319. A2.1992. 21 pp. 23. Sven I. Hommeltoft and Haldor F. A. Topsoe, Supported Liquid Phase Alkylation of Aliphatic Hydrocarbons Using a Fluorinated Sulfonic Acid Catalyst, U.S., and S. A. Den. Haldor Topsoe, US. 5245100, A. 1993, 5 pp. Cont.-in-part ofU.S. Ser. No. 626,956. 24. P. T. Barger and S. T Wilson, Converting Natural Gas to Ethylene and Propylene by the UOP/HYDRO MTO Process, Proc. Int. Zeolite Conf., 12th, 567-731999. 25. For review of surface science techniques and catalysis see, "Introduction to Surface Chemistry and Catalysis", G.A. Somorjai, Wiley, 1994. For STM imaging of surface reactions see e.g.X.-C. Gua, R.J. Madix, Surf. Sci., 387, 1, (1997) and W.W. Crew, R.J. Madix, Surf. Sci. 356, 1, (1996). 26. Catalyst Technology Roadmap Report in Technology Vision 2020. The US Chemical Industry, CCR/DOE/ACS Workshop, April 1997 27. Catalysis Looks to the Future, National Academy Press, 1992. 28. X-ray Absorption Fine Structure for Catalysts and Surfaces, Ed Y.Iwasawa, World Scientific, 1996 29. H. Straiger, J.O. Cross, J.J. Rehr, L.B. Sorensen, C.E. Bouidin, C.E. Woicik, Phys. Rev. Lett. 69, 3064, (1992)
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30. H.W. Deekman, J.H. Dunsmuir, K.L. D'Amico, S.R. Ferguson, B.P. Flannery, Mater. Res. Soc. Symp. Pror 217, 97,(1991). 31. B.S. Clausen, L. Grabaek, G. Steffensen, P.L. Hansen, H. Topsoe, Catal. Lett., 20, 23 (1993). 32. J.W. Couves, J.M. Thomas, D. Waller, T.H. Jones, A.J. dent, G.E. Derbyshire, G.N. Greaves, Nature, 354, 465, (1991). 33. P.-P. E.A. de moor, T.P.M. Beelen, B.U. Komanschek, O. Diat, R.A. van Santen, J. Phys. Chem. B, 101, 11077, (1997). 34. For a recent review see, J.M. Thomas, Chem. Eur, J., 3, 1557, (1997). 35. Computational results obtained using software programs from Molecular Simulations Inc., density functional calculations were done with DMOL program, and graphical displays were printed out from the Cerius2 molecular modeling system. 36. A. Gupta, R. Snuff, J. J. Low, unpublished results 37. Borman, S. "Combinatorial Chemistry," C&EN, 47, April 6, 1998 38. Danielson, E.; Devenney, M.; Giaquinta, D.M.; Golden, J.H.; Haushalter, R.C.; McFarland, E.W.; Poojary, D.M; Reaves, C.M.; Weinberg, W.H.; Wu, D.X., Science, 279 (6), 837, 1998. 39. Akporiaye, D.E.; Dahn, I.M.; Karlsson, A.; Wendelbo, R.; Angew. Chem. Int. Ed., 37 (5), 609, 1998. 40. Bowden, M.E.; Smith, J.K.; American Chemical Enterprise, Chemical Heritage Foundation, (1994).
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Applications of t i t a n i u m oxide p h o t o c a t a l y s t s and u n i q u e s e c o n d - g e n e r a t i o n TiO2 p h o t o c a t a l y s t s able to operate u n d e r visible light irradiation for the r e d u c t i o n of e n v i r o n m e n t a l toxins on a global scale M. Anpo Department of Applied Chemistry, Osaka Prefecture University Sakai, Osaka 599-8531, J a p a n The p r e s e n t investigation deals with practical applications of t i t a n i u m oxide photocatalysts and the development of unique secondgeneration t i t a n i u m oxide photocatalysts by applying advanced ionengineering techniques, enabling effective and efficient reactions not only under ultraviolet (UV) but also under visible light irradiation. Solar beams are absorbed up to 30-40% more efficiently, allowing the large scale use of these t i t a n i u m oxide photocatalysts for the reduction of environmental toxins. 1. I N T R O D U C T I O N Environmental pollution and destruction on a global scale have drawn attention to the vital need for totally new, safe and clean chemical technologies and processes, the most i m p o r t a n t challenge facing chemical scientists for the 21st century. Strong contenders as e n v i r o n m e n t a l l y - h a r m o n i o u s catalysts are photocatalysts which can operate at room t e m p e r a t u r e in a clean and safe m a n n e r while applications of such photocatalytic systems are urgently desired for the purification of polluted water, the decomposition of offensive atmospheric odors as well as toxins, the fixation of CO2 and the decomposition of NOx and chlorofluorocarbons on a global scale [1-4]. To address such enormous tasks, photocatalytic systems which are able to operate effectively and efficiently not only under UV but also under the most environmentally ideal energy source, sunlight, must be established. And to this end, we are moving in a very positive direction with the development of unique titanium oxide photocatalysts which work under solar beam or visible light irradiation [3,4]. The present paper deals with 1) various practical applications of t i t a n i u m oxide photocatalysts and 2) the design and development of unique second-generation t i t a n i u m oxide photocatalysts which operate effectively u n d e r visible and/or solar b e a m i r r a d i a t i o n for the applications in improving our environment on a large global scale.
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2. VARIOUS P R A C T I C A L A P P L I C A T I O N S OF TITANIUM OXIDE PHOTOCATALYSTS
W h e n titanum oxides are irradiated with U V light that is greater than the bandgap energy of the catalyst (about ~, < 380 nm), electrons (e" ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which together cause catalytic reactions on the surfaces, namely, photocatalytic reactions are induced. Because of its similarity with the m e c h a n i s m observed with photosynthesis in green plants, photocatalysis m a y also be referred to as artificial photosynthesis [1-4]. As will be introduced in the later part, there are no limits to the possibilitiesand applications of titanium oxide photocatalysts as "environmentally harmonious catalysts" and/or "sustainable green chemical systems". In the presence of 02 and H20, the photo-formed e" and h + easily react with these molecules on the titanium oxide surfaces to produce 02and O H radicals, respectively. These 02-and O H radicals have very high oxidation potential, inducing the complete oxidation reaction of various organic compounds such as toxic halocarbons, as shown in equation (1) [1]: CnHmOzCly
> nCO2 + yHC1 + wH20
(1)
Such high photocatalytic reactivities of photo-formed e- and h + can be expected to induce various catalytic reactions to remove toxic compounds and can actually be applied for the reduction or elimination of polluted compounds in air such as NOx, cigarette smoke as well as volatile compounds arising from various construction m a t e r i a l s , oxidizing them into CO2. In water, such toxins as chloroalkenes, specifically trichloroethene and tetrachloroethene as well as dioxins can be completely d e g r a d a t e d into CO2 and H 2 0 [1,2]. Such highly photocatalytically reactive systems are also applicable in protecting the lamp-covers and the walls in tunnels from becoming dark and sooty by emission gases. Soundproof highway walls coated with titanium oxide photocatalysts have been constructed on heavily congested roads for the elimination of NOx (Fig. 1) [5]. The reactivity of photo-formed 02- and OH radicals is high enough to decompose or kill bacteria so that new cements and tiles mixed or coated with t i t a n i u m oxides have been commercialized and are already in use in the operation rooms of hospitals to keep it sterile and bacteria-free [4]. Furthermore, titanium oxide thin films have been found to exhibit a unique and useful function, i. e., a super-hydrophilic property. Usually, the contact angle of the water droplet and surface is 50-70 degrees, therefore, metal oxide surfaces become cloudy when water is dropped on them or if there is moisture in the atmosphere. However, under UV light irradition of the titanium oxide surfaces this contact angle of the water droplets becomes smaller, even reaching zero (super-hydrophilicity), its extent depending on the UV irradiation time. Thus, under UV light irradiation, titanium oxide thin film surfaces never become cloudy, even
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Fig. 1. View of the soundproof highway walls for the elimination of NOx (the walls were constructed in Osaka, April in 1999) (20 m 3 of the polluted air was purified at the rate of I m2 photocatalyst/h)
TiO2thin film
-...,
glass[._
~0=40-60
9
water droplet
9
0=0
i I hm~v I
[
0 (degree); contact angle of water
Fig. 2a. Anti-fogging effect of TiO2 thin film coated surface. The glass mirror, whose right side was coated with TiO2 thin film, exhibits a clear image even in high water moisture like in a bath room. (b) Decrease in the contact angle under UV irradiation of the TiO2 thin film, leading an super-hydrophilic property of the mirror. in the rain. This r e m a r k a b l e function can also be applied for the production of new mirrors which can be used even in bathrooms and side mirrors for cars to protect against rain (Fig. 2) [6]. Some practical applications of t i t a n i u m oxide photocatalysts in J a p a n are as follows: 1) Air cleaners containing t i t a n i u m oxide photocatalysts White paper containing titanium oxide photocatalysts 2) Antibacterial textile fibers containing t i t a n i u m oxide 3) photocatalysts Systems for the purification of polluted air, e. g., the elimination of 4) NOx Super-hydrophilic, self-cleaning systems and coating materials for 5) cars Soundproof highway-walls covered with titanium oxide 6) photocatalysts Lamp-covers coated with titanium oxide thin film photocatalysts 7) Cements containing t i t a n i u m oxide photocatalyst powders 8) Architectural materials using t i t a n i u m oxide photocatalysts 9) Coating m a t e r i a l s using t i t a n i u m oxides for a r c h i t e c t u r a l walls I0) Self-cleaning tents II) Glass tablewares 12) 13) Outdoor a n t e n n a s coated with titanium oxide thin films
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3. D E V E L O P M E N T OF S E C O N D - G E N E R A T I O N TITANIUM OXIDE PHOTOCATALYSTS BY A P P L Y I N G A METAL IONIMPLANTATION METHOD
3. I. E X P E R I M E N T A L The main characteristics of various titanium oxide catalysts have been summarized in Table 1. Titanium oxide thin film photocatalysts were prepared using an ion cluster beam (ICB) method [7,8]. Using ICB, the titanium metal target was heated up to 2200 K in a crucible and Ti vapor was introduced into the high vacuum chamber to produce Ticlusters. These clusters then reacted with 02 in the chamber and stoichiometric titanium oxide clusters were formed. Ionized titanium oxide clusters formed by electron beam irradiation were accelerated by high electric field and bombarded onto the glass substrate to form titanium oxide thin films.
Table 1 Characteristics of titanium oxides used in this study Catalysts
Percent of BET surface anatase areas, m2/g
Particle size nm
Purity as TiO2
Bandgap eV
F-2 F-4 F-6 P-25 S-1
72.3 87.5 81.0 70.9 86.1
23.4 15.0 9.30 18.6 30.2
99.97 99.97 99.99 99.54 99.90
3.250 3.251 3.262 3.250 3.252
27.1 54.2 102 50.2 30.6
The metal ion-implantation of the catalysts was carried out by using an ion-implanter consisting of a metal ion source, mass analyzer, high voltage ion accelerator (50-200 keV), and a high vacuum pump [9]. The metal ions were expected to be injected into the deep bulk of the catalyst when high accelaration energy was applied to the metal ions. When high voltage as the accelaration energy is used, the metal ions are implanted deep inside the bulk of the catalyst. In fact, as was expected the method, SIMS analyses using a Shimadzu/Kratos SIMS1030 clearly showed t h a t the metal ions implanted into the titanium oxide catalyst exist in a highly dispersed state and are injected into the deep bulk of the catalyst, exhibiting a distribution maximum at around 1000-3000/~ from the surface and zero distribution at the surface [10-12]. Although such distribution depends on the acceleration energy and the kind of catalysts, it is one of the most significant advantages in using the metal ion implantation method to modify the bulk electronic properties of a catalyst. The metal ion-implanted catalysts were calcined in 02 at around 725-823 K for 5 h. Prior to UV-VIS diffuse reflectance, SIMS, XRD, EXAFS, ESR, and ESCA measurements as well as invesigations on the photocatalytic reactions, both metal ion-implanted and unimplanted original pure photocatalysts were heated in 02 at 750 K and then
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degassed in cells at 725 K for 2 h, heated in 02 at the same t e m p e r a t u r e for 2 h, and finally outgassed at 473 K to 10-6 Torr [10-12]. UV light i r r a d i a t i o n of the photocatalysts in the presence of r e a c t a n t molecules such as NOx was carried out using a high-pressure Hg lamp (Toshiba SHL-100UV) through water and color filters, i. e., ~, > 450 nm for visible light irradiation and )~ < 380 nm for UV irradiation, respectively, at 275-295 K. The reaction products were analyzed by GC and GC-MASS. The UV-VIS diffuse reflectance spectra were measured using a S h i m a d z u UV-2200A speetrophotometer at 295 K. The ESR spectra were recorded at 77 K with a Bruker ESP300E and a JEOL RE2X s p e c t r o m e t e r (X-band). The binding energies and the e l e m e n t composition of the catalysts were m e a s u r e d using a Shimadzu ECSA3200 electron spectrometer. The XAFS (XANES and FT-EXAFS) spectra were m e a s u r e d at the BL-7C facility of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba. 3. 2. R E S U L T S AND D I S C U S S I O N
When t i t a n u m oxide photocatalysts are irradiated with UV light t h a t is greater t h a n the bandgap energy of the catalsyst (about ~, < 380 nm), electrons and holes are produced in the conduction and valence b a n d s , respectively. These electrons and holes t o g e t h e r induce photocatalytic reactions on the surfaces. However, as can be seen in Fig. 3-a and unlike photosynthesis in green plants, the t i t a n i u m oxide photocatalyst in itself does not allow the use of visible light and can make use of only 3-4% of solar beams t h a t reach the earth. Therefore, to establish clean and safe photocatalytic reaction systems, it is vital to develop t i t a n i u m oxide photocatalysts which can absorb and operate with high efficiency under solar and/or visible light irradiation. 0.4
t" O'J
t-
Solar spectrum r r
0.3 o E i
0.2 Z
Od
(d)
0
0.1
t._
0
"0
>.. 0
300
400
500
""
9
600
Wavelength / nm
Fig. 3. UV-Vis absorption spectra (diffuse reflectance) of the un-implanted pure TiO2 (a) and the Cr ion-implanted TiO2 (b-d), the action spectrum (open circles) of the Cr ionimplanted photocatalyst (corresponding to d) for the photocatalytic decomposition reaction of NO, and the solar spectrum which reachs the earth. (amounts of Cr ionsimplanted in 10-7 mol/g, b: 2.2, c: 6.6, d: 13)
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We have applied the metal ion implantation method to modify the electronic properties of titanium oxide photocatalysts by bombarding them with high energy metal ions and discovered t h a t metal ion implantation with various transition metal ions such as Cr, V, Co, Fe and Ni accelerated by high voltage enables a large shift in the absorption band of the titanium oxide catalysts toward visible light regions with differing levels of effectiveness. However, Ar, Mg, or Ti ion-implanted titanium oxides exhibited no shift, showing that such a shift is not caused by the high energy implantation process itself, but to some interaction of the transition metal ions with the titanium oxide catalyst. As can be seen in Fig. 3-(b-d), the absorption band of the Cr ion-implanted titanium oxide shifts smoothly to visible light regions, the extent of the red shift; depending on the amount and type of metal ions implanted, with the absorption maximum and minimum values always remaining constant. Such a shift; allows the metal ion-implanted titanium oxide to use solar beams more effectively and efficiently, at up to 20-30% [4, 12]. Furthermore, as shown in Fig. 4, such red shifts in the absorption band of the metal ion-implanted titanium oxide photocatalysts can be observed for any kind of titanium oxide except amorphous types, the extent of the shift changing from sample to sample. Also, it was found t h a t such shift in the absorption band can be observed only after calcination of the metal ion-implanted titanium oxide samples in 02 at around 723-823 K. Therefore, calcination in 02 in combination with metal ion implantation was found to be instrumental in the shift of the absorption spectrum toward visible light regions. All these results clearly showed that such a shift in the absorption band of titanium oxides by metal ion implantation is a general phenomena and not a special feature of a certain kind of titanium oxide catalyst. Figure 5 shows the absoprtion bands of the titanium oxide photocatalysts impregnated or chemically doped with Cr ions in large amounts as compared with those for Cr ion-implanted samples. The Cr
hv
T hLImpuritylevel
~~,,V/S- 1
~
~ ~ ~ V/I~-4 V/P-25
I
300
I
I
'
400
I
I
500
I
600
Wavelength /nm
Fig. 4. Shifts in the absorption spectra of various types of TiO2 photocatalysts implanted with the same amounts of Cr ions. (Cr ions: 6.6 x 10-7 mol/g)
300
,
(e')
I
400
,
Wavelengh
I
500
,
I
600
/ nm
Fig. 5. Absorption spectra of TiO2 chemically doped with Cr ions. (Cr ions doped in 10-6 mol/g, a: TiO2, b': 1.6, c': 20, d" 100, e': 200)
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ion-doped titanium oxide catalysts show no shift in the absorption band, however, a new absorption shoulder appears at around 420 nm due to the formation of the impurity energy levels within the bandgap, its intensity increasing with the amount of Cr ions chemically doped. Such results indicate t h a t the method of doping causes the electronic properties of the titanium oxide catalyst to be modified in completely different ways, and thus confirming t h a t only metal ion-implanted t i t a n i u m oxide catalysts show shifts in the absorption band toward visible light regions. W i t h u n i m p l a n t e d or c h e m i c a l l y doped t i t a n i u m oxide photocatalysts the photocatalytic reaction does not proceed under visible light irradiation (~ > 450 nm). However, we have found that visible light irradiation of metal ion-implanted titanium oxide photoeatalysts leads to various significant photocatalytie reactions. As shown in Fig. 6, visible light irradiation (~, > 450 nm) of the Cr ion-implanted titanium oxide in the presence of NO at 275 K leads to the decomposition of NO into N2, 02, and N20 with a good linearity against the irradiation time. Under the same conditions of visible light irradiation, the u n i m p l a n t e d original pure t i t a n i u m oxide photocatalyst did not e x h i b i t any photoeatalytic reactivity. As can also be seen in Fig. 3 (open circles), the action spectrum for the reaction on the metal ion-implanted titanium oxide is in good a g r e e m e n t with the absorption spectrum of the photoeatalyst, indicating t h a t only metal ion-implanted t i t a n i u m oxide photoeatalysts were effective for the photoeatalytie decomposition reaction of NO. Thus, metal ion-implanted titanium oxide photoeatalysts were found to enable the absorption of visible light up to a wavelength of 400-600 nm and were also able to operate effectively as photoeatalysts, hence their name, "second-generation titanium oxide photoeatalysts" [4, 11]. It is important to emphasize t h a t the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light irradiation (k < 380 nm) retained t h e same photocatalytic efficiency as the unimplanted original pure titanium oxides. When metal ions were chemically doped into the titanium oxide photocatalyst, photocatalytic efficiency decreased 1.5 C:D m
O
E
off
on Fig. 6. Photocatalytic decomposition of NO into N2 and 02 as well as N20 on the Cr ion-implanted TiO2 photocatalyst under visible light (~ > 450 nm) irradiation at 295 K. Un-implanted original pure TiO2 photocatalyst did not show any photocatalytic reactivity under the same condition.
rD O
(Crfrio2)
O f:l. t__
*O 0.5 {D
light
>o m
(TiO2) 4 6 8 10 Reaction time / h
12
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dramatically under UV irradiation due to the effective recombination of the photo-formed electrons and holes via the impurity energy levels formed by the doped metal ions within the bandgap of the photocatalyst. These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work to modify the electronic property of the catalyst. We have conducted various field work experiments to test the p h o t o c a t a l y t i c r e a c t i v i t y of t h e newly developed t i t a n i u m oxide photocatalysts under solar beam irradiation. As can be seen in Fig. 7, under outdoor solar light irradiation at ordinary temperatures, the Cr and V ion-implanted t i t a n i u m oxide photocatalysts showed three and four times h i g h e r photocatalytic reactivity as compared with the unimplanated original pure titanium oxide photocatalyst. These results c l e a r l y show t h a t by u s i n g s e c o n d - g e n e r a t i o n t i t a n i u m oxide photocatalysts developed by applying the metal ion implantation method, we can utilize visible and solar light energy more efficiently. Figure 8 shows the relationship between the depth profiles of the V ion in the V ion-implanted t i t a n i u m oxide photocatalysts having the same numbers of V ions and their photocatalytic efficiency under visible light irradiation. As can be seen in Fig. 8, when the V ions are implanted in the same amounts into the deep bulk of the catalyst by applying high voltage as acceleration energy, the photocatalyst exhibits a high photocatalytic efficiency under visible light irradiation. On the other hand, when a low voltage is applied, this photocatalyst exhibits a low efficiency under the same conditions of visible light irradiation. It was found that increasing the numbers (or amounts) of V ioni m p l a n t e d into the deep bulk of the t i t a n i u m oxides caused the photocatalytic efficiency of these photocatalysts to increase under visible light irradiation, passing through a maximum at around 6x1016 V/cm 2 of the catalyst, and then decreased with a further increase in the number of V ions implanted. Only on samples implanted with an increased number of V ions could the presence of V ions at the near surfaces be observed by E
t,"..,... o,i
0
0 I=
6
500
1.0
400
0.8
"0
C
~0
_r
n O
Q.
=-9 4
.E| 300
E
0
> 200 .c_
0.4
"o
TiO2 Cr/TiO2
V/TiO2
Photocatalysts
Fig. 7. Effect of metal ion-implantation on the photocatalytic reactivity of TiO2 under solar beam irradiation at 295 K. (solar beams: 38.5 mW/cm2)
.o 100
0.2
~0
n
a.
E
sO "O
e
U~ r
0
.~ 0
0.6
e-
C~ z 2
O I=
0 30 keY
70 ReV
150 keY
Fig. 8. Effect of the depth profile of V ions in V ion-implanted TiO2 on the photocatalytic reactivity for the decomposion of NO at 295 K.
o W O
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ESCA measurements. Thus, these results clearly suggest that there is an optimal condition in the depth and number of metal ions implanted to achieve a high photocatalytic reactivity under visible light irradiation. The ESR spectra of the V ion-implanted titanium oxide catalysts were measured before and after calcination of the samples in 02 at around 723-823 K, respectively. Distinct and characteristic reticular V 4+ ions were detected only after calcination at around 723-823 K. It was found that when a shift; in the absorption band toward visible light regions was observed, the reticular V4§ ions could be detected by ESR. Such reticular V ions nor such a shift in the absorption band have never been observed with titanium oxides chemically doped with V ions. Figure 9 shows the XANES and FT-EXAFS spectra of the titanium oxide catalysts physically implanted with Cr ions (b and B) and chemically doped with Cr ions (a and A), respectively. Analyses of these XANES and FT-EXAFS spectra showed t h a t in the titanium oxide catalysts chemically doped with Cr ions by an impregnation or sol-gel method, the ions are present as aggregated Cr-oxides having octahedral coordination similar to C r 2 0 3 and tetrahedral coordination similar to CrO3. On the other hand, in the catalysts physically implanted with Cr ions, the ions are present in a highly dispersed and isolated state in octahedral coordination, clearly suggesting t h a t the Cr ions are incorporated in the lattice positions of the catalyst in place of the Ti ions. (a) ~ , , ~
t--
J
l :5.
(A)~ ~
Cr 6+, Cr 3+ aggregate
Cr / TiO2
t~ c~ t_ O {D c~
~I
98
3
HI3
70
> 98
4
HY
69
> 98
5
exchanged clay
14
> 98
6 7
AI clay H2PW6Mo604o
16
> 98
21
> 98
Table 1 acetylation 9 of anisole with acetic anhydride with various heterogeneous catalysts (8 h, 90~ b yield 9 on acetic anhydride (initial ratio anisol/anhydride) = 5 All tested catalysts are active in this reaction and they all show a very high para selectivity. The Rhodia company is operating an industrial process for acylation of anisole to paraacetoanisole using zeolite v". This process using a fixed bed technology is a breakthrough in this field. It allows a considerable simplification of the process as well as an increase in para selectivity, and so a reduction of its operating cost and a dramatic reduction of the effluents. Scheme 2 and 3 illustrate the simplification brought by the zeolite process just comparing the two block diagrams.
DCE
;nisole
AICI3
~ Reaction preparation I
AcCI
Reaction
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Hydrolysis
I Liquidlliquid separation I water water
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1
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9
"1
I
9effluent rAI(OH) 3 HCI I I
4,
Washing 2
I Distillation/dehydration I I
Final distillation acetoanisole
Scheme 2 simplified 9 flow chart for the conventional aluminum dichloride process using DCE (1,2 dichloroethane) as solvent
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anisole
Ac20
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"1
.,aceticacidi
Reaction(fixed bed)
I
Distillation
'j
I
I
'v
acetoanisole
Scheme 3 : simplified flow chart for the new zeolite fixed bed process using no solvent This process is a good example of how a new process can at the same time be cost efficient and environmentally friendly. This technology has now been adapted to the industrial synthesis of acetoveratrole using the same type of process.
3. Carbonylation without the use of CO The carbon monoxide chemistry has been extensively studied, leading to a wide range of methods used in small scale organic syntheses up to industrial processes v"~. Despite the versatility of carbonylation reactions, carbon monoxide suffers from major drawbacks that restricts its utilisation. From an industrial point of view, the cumbersome handling of this toxic gas necessitates very expensive facilities which prevent its use for most of fine chemical productions. An alternative process equivalent to a carbonylation reaction which avoids carbon monoxide introduction into the reactor and that can be used in standard polyvalent type units would be of great interest. Of course, catalyst cost, stability and productivity should also fulfil economical requirements. Previous studies realised in carbonylation, indicated that Iridium based catalysts are active in carbonylation under low CO partial pressure. Further more this catalyst is active for isomerisation of methyl formate with no initial CO pressure, but precipitates during reaction ~x. After more precise investigations of formates isomerisation processes we found that the hydroxycarbonylation of any carbonylable function with formic acid using an iridium based catalysts was possible without the use of CO gas. Alkenes and alcohols can be transformed to the corresponding hydroxycarbonylated products with good yields, n-hexene is carbonylated in 92% yield with a standard selectivity to linear acids neighbouring 70%. We compared this reaction with the hydroxycarbonylation of n-hexene with CO gas using the same catalyst and found similar results (Table 2). Entry
Substrate
CO source HCOOH
Pressure [bar] Autogeneous
Yield [%] 92 (68)
1
n-hexene
2
n-hexene
gas
5
86 (63)
Table 2. Hydroxycarbonylation of n-hexene. The scope of this reaction appears to be very large and examples are depicted in scheme 4.
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lineadties
O
~
C
r
O
O
~
2
o O
H
CO2H
yield> 97
O
O
H ---~H O / ~ O H
0
o HO/LV~ O 'H 0
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9
~OH_. ~CO2. Scheme 4
+ 30 %
~,CO2H
OH
H
O 70 %
CO2H
~
H
Examples 9 of reactions performed using formic acid and an Iridium based catalysis.
Two possible mechanism pathways may be involved for this reaction, Either a common pathway for all reactants passing through the alkene formation followed by a 2000~ and the XRD pattern matched perfectly the typical reflections (20 = 23, 40, 46, 54, 59 ~ of the perovskite-type structure [8]. The nature of the precursor salts on one hand may hinder the formation of a clear solution of the desired composition. On the other hand it may help in keeping the FH process to a sufficiently high temperature. Several salts with different anions were then tried, both inorganic
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and organic, for preparing the solution to be nebulised. Organic anions, acting as additional fuels, allowed a higher hydrolysis temperature. The further addition of an equimolar amount of citric acid led to a better crystallinity (Fig.la-c) and to a more uniform distribution of particle size, however accompanied by a lower surface area (Table 1, samples 1-4). Moreover, the complexing action of the citrate anion helped in obtaining a perfectly clear and stable feeding solution. As for the overall concentration of the latter, too high values (> 10 wt%) led to a higher surface area, but also to XRD amorphous powders, while too low values ( 1/2 is the absence of ethylene in the reaction products. However, as the Cu content increased the selectivity towards ethylene increased considerably, ranging from 67% on PtlCu3 to above 93% on Ptl Cu 18 (Table 1). It is worth noting that the catalysts with Pt/Cu ratio < 1/2 did not produce monochloroethane. Thus, there is a marked difference in the selectivity of the monometallic Pt catalyst, bimetallic Pt/Cu catalysts with atomic ratio _ 1/2 and catalysts with Pt/Cu < 1/2.
3.3. Infrared experiments The infrared spectra of CO adsorbed on Pt/SiO2 and PtlCu3/SiO2 catalysts at saturation coverage are shown in Fig. 3 as spectrum 1 and 2, respectively. In spectrum 1 the band at 2078 cm 1 is characteristic of linearly adsorbed 12CO on Pt ~ [13]. The bands at 2130 and 2031 cm -1 in spectruim 2 have been assigned to linearly adsorbed CO on Cu ~ and Pt ~ respectively [13]. As the v(CO) adsorbed on Cu ~ and Cu 1+ are very close, the band assignment was confirmed by the fact that the band at 2130 cm 1 disappeared when gas phase CO was evacuated. Unlike CO-Cu ~ the CO-Cu 1§ adsorption complexes are known to be stable and do not decompose upon evacuation at room temperature [ 14]. The position of absorption band of 12CO on Cu ~ for all of the catalysts was independent of the composition of gaseous ~2CO+~3CO mixture used in the adsorption experiments and was the same as for the Cu/SiO2 catalyst. The frequency of 12CO vibration on Pt for Pt/SiO2 catalyst shifted from 2078 to 2052 cm 1 when the 12CO concentration of the 12CO+~3CO mixture was decreased from 100 to 0%, but remained almost constant at around 2030 cm 1 for the Ptl Cu3/SiO2 catalyst (Fig. 4). It is noteworthy, that the v(12CO) band of CO adsorbed on Pt for the Ptl Cu2/SiO2 catalyst was close to that of Pt/SiO2 for pure lZCO and to that for the Ptl Cu3/SiO: catalyst at infinite dilution of 12CO with 13CO (Fig. 4).
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6 5
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Fig. 1. Selectivity vs. time on stream for Ptl Cul/SiO2 at 200~ 1 - ethane, 2 - ethylene, 3 - monochloroethane, 4 - conversion
Yo
TOC:~h)
30
Fig. 2. Selectivity vs. time on stream for Ptl Cu3/SiO2 at 200~ 1 -ethane, 2 - ethylene, 3 - conversion.
4. DISCUSSION 4.1. N a t u r e o f active sites in ( P t + C u ) / S i O 2 catalysts
Through the analysis of the performance of (Pt+Cu)/SiO2 catalysts the nature of active sites for the hydrodechlorination of 1,2-dichloroethane has been clarified. Even though C-CI bond scission in vicinal dichlorohydrocarbons occurs readily on a Cu surface to form an olefin [15,16], silica-supported Cu did not show dechlorination activity at 200~ (Table 1). This is most likely because dissociative H2 chemisorption is activated on Cu surfaces [17]. A slower rate of H2 dissociation on a Cu surface than the rate of C-C1 bond cleavage of a halocarbon would result in a Cu surface covered with C1 atoms. Thus, the catalytic activity is extensively suppressed due to poisoning by C1. However, the active sites for 1,2-dichloroethane hydrodechlorination cannot consist of solely Pt atoms. Monometallic Pt catalyst is unselective toward ethylene. Only the mixed PtCu sites in (Pt+Cu)/SiO2 catalysts form ethylene during 1,2-dichloroethane hydrodechlorination.
2100
2078
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v
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~) o r
21
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o
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9 .-- 1000 ppm H2S>770 ppm nitrogen > 1000 ppm H2S+770 ppm nitrogen. To achieve the same conversion in the presence of sulfur and nitrogen as for pure feed the reaction temperature was increased by about 100~ As to isomerization selectivity (total iso-Cl0), the lowest selectivity (50-55%) was observed in the presence of 1000 ppm H2S, while in the presence of 770 ppm nitrogen selectivity was aroflnd 70% and increased to 80-85% (1000 ppm H2S+ 770 ppm) nitrogen and 85-87% for pure feed. It was remarkable that at 100~ higher reaction temperature in the presence of sulfur and nitrogen, the catalyst still
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maintained very high isomerization selectivity. It is speculated that nitrogen neutralized the catalyst strong acid sites leading to the low catalyst acidity and activity. To compensate for the low activity, the reaction temperature must be significantly increased. At this temperature, the weak and intermediate strength acid sites will become active enough to achieve high paraffin conversion. On the second hand, metal activity will be significantly higher at high temperature and the perfect balance between the weak-intermediate catalyst acidity and strong metal function will be established, resulting in the high isomerization selectivity. Composition of the cracked and isomerized products reflected the proposed mechanism. If the pure n-decane used, the main isomerization products are methylnonanes, while formation of dimethyloctanes was low due to the strong steric constrains imposed by the MAPSO-31 structure and slightly increased with conversion. The addition of sulfur strongly suppressed the metal function and decreased the amount of methylnonanes relative to the pure feed run, while in the presence of nitrogen, the methylnonane selectivity was restored to the high level. Selectivity to ethyloctanes at all conditions was low and increased slightly with conversion from about 3% to 5%. At the middle conversion, the contribution of C type [3-scission is responsible for relatively low iso-normal ratio of C5-C7 (0.5). Addition of sulfur suppressed significantly metal activity (weak metal function and strong acid function) and required about 40-50~ higher reaction temperature. In spite of the fact that catalyst acidity was not affected by sulfur, the [3-scission at this high temperature is not the major cracking mechanism any more but interfered with Me hydrogenolysis, resulting in the high yield of methane. In addition, the suppressed metal function resulted in the shift to B type of I]-scission (iso/normal was close to 1). Nitrogen addition (no sulfur, weak acid function and strong metal function) deactivated the strong acid sites and decreased catalyst activity. To maintain the same conversion as for pure feed, the reaction temperature was increased about 100~ At that high temperature, the contribution of Me hydrogenolysis and type D hydrocracking was very high, methane yield was increased and iso/normal ratio dropped down to 0.2. Addition of sulfur and nitrogen to the feed affected both metal and acid functions. The metal hydrogenolysis activity was suppressed, resulting in the decreased methane formation, while isomerization selectivity was increased (iso/normal came back to 0.5). 3.2. Effect of N concentration
The effect of nitrogen concentration on performance of 0.4%Pt-MAPSO-31 catalyst in the presence of 1000 ppm HzS was studied in the range 10-770 ppm nitrogen (Fig. 3, 4). Up to nitrogen concentration of 10 ppm, no change in catalyst performance was found. The further increase of nitrogen concentration (50, 100,770 ppm) progressively suppressed catalyst activity without significant effect on total catalyst isomerization selectivity. The increase of nitrogen concentration corresponded with the increased reaction temperature and suppressed the formation of dimethyloctanes. This is in line with thermodynamics, which are favorable to methylnonanes at high temperature. Methane formation was increased with nitrogen concentration and reaction temperature.
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3.3. Effect of zeolite structure
SAPO-34 (8 member ring), SAPO-11 and SAPO-41 (10 member ring), SAPO-5 and MAPSO31 (12 member ring), MFI (10 member ring, Si/A12=38; 100; 400) as well as amorphous silicaalumina (25%A1203-75%SIO2) were studied as supports. All catalysts contained around 0.4% Pt and have been tested in the presence 1000 ppm H2S and 770 ppm nitrogen in temperature range 380-440~ (Fig. 5, 6). SAPO-34 showed poor activity which is presumably due to very small pore opening (3.8A*3.8A, diffusion limitation). It is likely that only the external surface of SAPO34 crystals was available for reaction, which affected the number of acid sites and catalyst activity. Catalyst isomerization selectivity was low, approximately 60%. SAPO-11 and SAPO-41 with the similar pore opening (3.9A*6.3A and 4.3A*7A,respectively) had excellent selectivity (90-95%) at conversions from 20-80%. 12 member ring SAPO-5 (pores 7.3A*7.3A) showed poor selectivity and stability while MAPSO-31 (pore opening 5.4A*5.4A) had performance close to SAPO-11 and SAPO-41. Product distribution corresponded with material structure. DM-C8 selectivity was highest for SAPO-5 and increased from MAPSO-31 to SAPO-41 according to pore opening. The same trend was observed for ethyloctane formation. On the other hand, SAPO-11, SAPO-41 and MAPSO-31 gave basically monomethyl isomers indicating to the steric constrains imposed by the pore structure of these materials. Silica-alumina catalyst also showed high isomerization selectivity and activity comparable to the activity of SAPO materials but with lower stability. The open structure of amorphous silica-alumina was favorable for high formation of dimethyl isomers and ethyl octanes (higher than SAPO-5). It was speculated that multibranched isomers preferably formed inside the open space of silica -alumina are not stable at high temperature and could be easily cracked to give olefins and coke, which are the cause of catalyst deactivation. Iso/normal ratios of the cracked product were around 0.5 for S APO materials, about 1 for silica-alumina and 2 for SAPO-5 and supported the above conclusions. 3.4. Correlation between catalyst performance and NH3 TPD
All SAPO family catalysts had acidity in the same range (0.25-0.33 mmol NH3/g) with similar strength distribution. Activity of SAPO-34 was discussed above and depended first on its geometric structure. As to the other SAPO materials, the range of activity (acidity) was SAPO-41 (0.315 mmol NH3/g)>SAPO-11 (0.238 mmol NH3/g)>SAPO-5 (0.327 mmol NH3/g)>(amorphous SIO2/A1203 (0.276 mmol NH3/g) > MAPSO-31 (0.255 mmol NH3/g). This range of activities was defined by the combination of many catalyst variables such as, for example, material structure, defect structure and preparation and finishing conditions, which were unique for each type of material and did not correlate with the total number of acid sites by NH3 TPD. To get a better understanding of the acidity effect, it was desirable to have a set of materials with the same structure and variable number of acid sites. To meet this requirement, the series of MFI catalysts with variable Si/A12 ratio (38; 100; 400) were tested (Fig. 7, 8). The increase of Si/A12 from 38 to 400 resulted in decreased catalyst activity and increased isomerization selectivity. MFI-38 (high acidity) behaved like a good cracking catalyst due to the high number of acid sites available for reaction even at 770 ppm nitrogen in feed. On the other hand, MFI-400 (low acidity) required much higher temperature to maintain the same conversion. At this temperature, the strong metal function and the low catalyst acidity led to the high isomerization selectivity. The decreased Si/AI2 ratio affected not only the number of acid sites but also their distribution. MFI-38 had a maximum
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on the acid site distribution curve corresponding to the strong acid sites (400-550~ while MFI100 and MFI-400 had acidity pattern similar to SAPO materials, with maxima corresponding to intermediate acid sites (300-400~ In addition, MFI-100 which had the total number of acid sites close to SAPO materials was active in the same temperature range and had significantly improved isomerization selectivity relative to MFI-38. Based on these data it was concluded that a good correlation between acidity and catalyst performance exists inside of the same class of materials (MFI in our case). It was also concluded that the strong metal function and mild acidity are required to provide the best isomerization performance in the presence of sulfur and nitrogen. Good correlation between acidity and catalyst performance was established for MFI containing catalysts. 4. Conclusions
1. Isomerization of n-decane in the presence of 1000 ppm H2S and up to 770 ppm nitrogen was studied on Pt bifunctional catalysts (MAPSO-31, SAPO-11, SAPO-34,SAPO-41, SAPO-5, MFI and amorphous silica-alumina). 2. The effect of nitrogen addition on activity and selectivity of isomerization catalysts in the presence of sulfur was established. The addition of nitrogen suppressed catalyst activity but allowed the control of isomerization selectivity. In the presence of 1000 ppm H2S and up to 770 ppm nitrogen, isomerization selectivity of SAPO-11, SAPO-41, MAPSO-31 was close to the catalyst selectivity with no sulfur and nitrogen added (pure feed). 3. Catalyst activity and selectivity correlated with catalyst acidity (NH3 TPD) within the same class of materials (MFI). 4. The mechanism of isomerization in the presence of sulfur and nitrogen over bifunctional catalysts was discussed. The carbenium ion isomerization mechanism (well-established for pure hydrocarbon feed) followed by ~-scission of the produced intermediate is consistent with the reaction products. In the presence of nitrogen (due to low activity and high reaction temperature), the contribution of metal hydrogenolysis to the cracking product distribution became significant. 5. Strong metal function and mild acidity are required for bifunctional catalysts to conduct selective isomerization of paraffins in the presence of both sulfur and nitrogen. References
1. J.Weitkamp, Appl. Catalysis, 8 (1983) 123 2. P.A.Jacobs, M.A.Martens, J.Weitkamp, H.K.Beyer, Faraday Dis. Chem Soc.,72 (1981) 353 3. J.A.Martens, P.A.Jacobs, J.Weitkamp, Appl Catalysis 20 (1986) 239 4. J.A.Martens, P.A.Jacobs, J of Catalysis 124 (1990) 357 5. J.A. Martens, R. Parton, L. Uytterhoeven, P.A.Jacobs, Appl Catalysis 76 (1991) 95 6. P. Meriaudeau, V.A.Tuan, V.T.Ngheim, C.Naccahe, G.Sapaly, Catal. Today 49 (1999) 285 7. J.M.Campelo, F.Lafont, J.M.Marinas, Appl Catalys 152 (1997) 5 8. A.K.Sinha, S.Sivasanker, P.Ratnasamy, Ind.Eng. Chem.Res. 37 (1998) 2208 9. J.M.Campelo, F.Lafont, J.M.Marinas, Appl. Catalysis 170 (1998) 139 10. S.J.Miller, 206 ACS mtg., Symp New Catal. Chem Utilizing Mol Sieves, Prepr.,(1993) 788
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Fig.1. Effect of S and N on catalyst activity
100
J
Fig. 2. Bfect of S and N on r
100
I
90
sel~
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0
320
360
400
-4- l ~ , r ~ n ~
.
.
.
.
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///
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-----:--?
80
100
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Fig.3. Effect of N concentration on catalyst activity,H 2S=1000ppm
lO0
~
20 40 60 r~Cl0 o 0 m e r s k ~ t %
440
Fig.4. Effect of N concentration on catalyst selectivity,H2S=1000ppm 100 80 o ~ . 60 ._
40
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=
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100
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350
300
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80
100
n-C10 conversion,wt% -~-- noS,noN -a--noN --e-- 50ppmN --*--100ppmN --)K--770ppmN ~ 1 0 p p m N F~6. Effect of SAPO stricture on catalyst ~,H2S--t000m~N=770mm
F~5. Elfect of SAPO stnclum on catalyst ~,H2S=1000ppmN=770m~
=eo 040
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: MAPS031 A 75% Si(~-25% A1203 "~ SaPO~
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Fig.7. Effect of Si/AI2 on MFI activity,H2S=1000ppm,N=770ppm
100
Fig.8. Effect of Si/AI2 on MFI e lectivity_,H2S_,~!_000p p m,N =770 p pm
100 80
80
o
~ 40
300
400 450 500 temDemture,C MFI-38 - 4 - MFI-IO0 ~ MFI-400
0
350
:
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MFI-38 --.I--MFI-IO0
80
:. MFI-400
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
State of metals in the supported bimetallic Pt-Pd/SO42-/Zr02 system A.V. Ivanov, A.Yu. Stakheev, and L.M. Kustov N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii prosp. 47, Moscow 117334, Russia, e-mail:
[email protected] The behavior of the metals in Pt-Pd/ZrO2 and Pt-Pd/SO42-/ZrO2 was studied by DRIFT spectroscopy and XPS. In Pt-Pd/ZrO2 partial formation of the P t ~ P d alloy takes place after reduction at 200~ For Pt-Pd/SO42-/ZrO2 the effects of alloying and interaction of the metals with the surface SO4 groups superimpose. 1. INTRODUCTION Catalytic systems based on sulfated zirconia (SO42-/ZRO2) promoted by platinum are very active in n-paraffin isomerization [1]. However, rapid deactivation of the catalyst is the main drawback that deteriorates the performance of the Pt-catalyst in the isomerization process. One of the reasons behind the deactivation of the Pt/SO42-/ZrO2 catalyst is poisoning of the platinum surface by sulfur compounds formed due to reduction of SO4 groups in a hydrogen hydrocarbon media. The enhancement of the resistance of the catalyst to sulfur should suppress deactivation. The introduction of palladium in various Pt-catalysts is known to increase sulfur tolerance as compared with monometallic systems 12]. The aim of this work is the study of the interaction of metals in the Pt-Pd/SO42-/ ZrO2 system in order to estimate the influence of palladium on the state of platinum and vice versa. The influence of the support acidity and surface sulfur species on the state of metals was also of interest. 2. EXPERIMENTAL
Pt-Pd/SO42-/MxOv and sulfiar-free systems have been synthesized by coimpregnation of S042-/MxOy(where M - Zr, Ti, A1, Si) prepared according to [3] or parent oxides (ZrO2, TiO2, A1203, and SiO2) with solutions of H2PtC16 and PdC12. The overall metal (Pt + P d ) loading was 0.5 wt. %. The atomic percentage of Pt (%Pt) in the Pt-Pd bimetallic composition was 100, 60, 50, 30, 15, or 0 at. %. Samples were reduced in a hydrogen flow at 100~300~ The electronic state of metals was investigated by DRIFT spectroscopy using CO as a probe-molecule with a Nicolet Impact 410 spectrophotometer and by XPS using a XSAM-800 Cratos spectrometer (Mg/~l,2). 3. RESULTS AND DISCUSSION
MonometaUic systems Platinum. Only Pt ~ particles characterized by the IR bands at 2070 and 1855 cm -1 corresponding to linear and twofold bridging forms of adsorbed CO are
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t
i
0.5
i
'2070
a
'
'2100
'
'
'
b
~Pd ~
0.5
1
0.4 "E
2
0.3
Pt ~* 2115
o.2
2160
0.4 2095Pt~ 0.3 0.2
0.1 0.0
0.1
1855 2200
~
,
0.0
.
' 2 oo 2100 2000 1900 Wavenumbers, cm -1
'
21oo
'
26oo
'
1 oo
'
Fig. 1. DRIFT spectra of CO adsorbed on (a) (1) Pt/ZrO2 and (2) Pt/SO42-/ZrO2; (b) (1) Pd/ZrO2 and (2) Pd/SOa2-/Zr02 reduced at 200~ observed in the Pt/ZrO2 system after reduction at 200~ (Fig. la). The position of these maxima of absorption bands are typical of the state of Pt in the systems with weak metal--support imeraction. Adsorption of CO on the Pt/SO42-/ZrO2 sample makes it possible to distinguish Pt ~+ (2115 cm -1) and Pt ~ (2095 cm -1) species as the main states of platinum. Pt + ions that can exist as near-surface ( O - - P t ) - - C O complexes were also found in a minor concemration. Two reasons can be proposed to explain the formation of the positively charged metal species in the Pt/SO42-/ZrO2 sample: (1) the modification of the Pt electronic state by adsorption of S-containing moieties and (2) the direct interaction between the acidic protons and the metal particles yielding [Pt-H] ~+ adducts [3, 4]. With the purpose to discriminate between these two effects the state of Pt supported on SO42-/TIO2, SO42-/A1203, and SO42-/SIO2 possessing different acidic properties was also studied. The addition of the SO42- anions induces the shift of the maxima of the absorption band corresponding to CO adsorbed in the linear form on Pt particles to higher frequencies for all sulfated systems and stabilizes positively charged platinum species (Pt + and Pt *+) during the reduction in the case of SO42-/ZRO2 and SO42-/TIO2 catalysts (Table). The value of the high-frequency shift of the maximum of the absorption band of CO adsorbed in the linear form on platinum particles diminishes with decreasing acid strength of the sulfated systems: SO42-/ZRO2 > SO42-/TIO2 ~ SO42-/A1203 >> SO42-/SIO2 [5]. Hence, the mechanism of the formation of positively charged forms of platinum presumably includes the interaction of the acidic protons of Bronsted acid sites (BAS) with the metal particles. Palladium. The spectrum of CO adsorbed on palladium in Pd/ZrO2 (Fig. lb) shows the presence of only Pd ~ particles with characteristic bands at 2100 cm -1
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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Wavenumbers of C O (cm -1) adsorbed on Pt supported on System
Pt +
Pt 8+
Pt/ZrO2 Pt/SO42-/ZrO2 Pt/TiO2 Pt/SO42-/TiO2 Pt/AI203
2160 2150 -
2115 2110 -
Pt/SO42-/m1203
Pt/SiO2 Pt/SO42-/SiO2
-
-
-
-
MxOyrx SO42-/MxOy
Wavenumbers, (cm-1) Pt ~ A Vco~,,+ _r~0) 2040 2095 2060 2095 2040 2080 2070 2075
75 50 -
A Vcoo%, _~0) 55 35 40 5
(CO adsorbed in the linear form) and 1970 and 1920 c m -1 with a shoulder at 1830 cm -1 (polycarbonyl bridging, monocarbonyl bridging, and three-centered forms of CO, respectively) [6]. The stability of the metal toward reduction increases u p o n modification with SO4 groups. For the Pd/SO42-/ZrO2 system, the metal particles with a partial positive charge (Pd ~+) characterized by the absorption band at 2120 cm -1 are revealed. At the same time, the formation of the polycarbonyl bridging complexes of CO is suppressed. One of the possible reasons of a decrease in the concentration of polycarbonyl bridging complexes is a decrease in electronic density on the Pd particles due to formation of Pd ~+ species. The other reason is the formation of new complexes of palladium due to the partial decoration of the Pd surface by the products of interaction with surface sulfur species [7]. These complexes are characterized by the narrow band at 1900 cm -1 in the IR spectra of CO.
Bimetallic systems
Pt-Pd/Zr02. After reduction of P t - P d / Z r O 2 (atomic ratio Pt : Pd = 1 : 1) at 100~ (Fig. 2), the main states of the metals are Pt ~ and Pd ~ with a low concentration of positively charged forms of Pt + or Pd 2+. Analysis of the spectra makes it possible to distinguish the regions of vibrations of bridging and linear carbonyl groups. Deconvolution of the first region gives the bands at 1830, 1880-1900, 1930, and 1970 cm -1 that can be assigned to Pd3(CO), PcI2(CO), Pd2(CO)2-3, and Pd2(CO)2 [6, 8]. In the second region, absorption bands characterizing adsorption on the individual Pt ~ (at 2060 cm -1) and Pd ~ (at 2095 cm -1) sites can be distinguished. All these data can indicate the existence of separate Pt ~ and Pd ~ species, i.e., give no evidence for the formation of a uniform P t - - P d alloy or for any significant mutual influence of the metals. An increase in the reduction temperatures to 200~ results in a decrease in the concentration of the Pd3(CO) complexes and an increase in the concentration of Pd2(CO)2 complexes. It may indicate the diminution of the area of Pd islands due to dilution of the Pd phase by Pt. The bands characterizing the linear forms of C O adsorption become more uniform as compared with the sample reduced at lower temperatures. In this case, partial formation of the P t - - P d alloy is possible.
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266
0.5
'
0.4
'2100
'
1
Pd 0
'
b
a
Pt ~
~9 0.3 i0.2
2095
1930
Oil/2/ 12ix60///l f~./
0.4
0.3 1
2065 75 1900
0.2
\ 1830 0.1
0.1
0.0
0.0 2200
2000
1800 2200 2000 Wavenumbers, cm- 1
1800
Fig. 2. IR spectra of CO adsorbed on Pt-Pd/ZrO2 reduced at (a) 100 and (b) 200~ (1) after adsorption and after evacuation at (2) 20 and (3) 100~ Pt-Pd/SO42-/ZrOe . The behavior of the metals in the Pt-Pd/SO4/ZrO2 systems was studied at different Pt/Pd ratios and reduction temperatures. Deconvolution of the IR spectra of CO adsorbed on the Pt-Pd/SO42-/ZrO2 sample ( P t Pd = 1 " 1) reduced at 100~ allows one to reveal two components (at 2115 and 2080 cm -1) which can be assigned to Pd ~+ and Pt ~ respectively (Fig. 3). The behavior of platinum is close to that in sulfur-free Pt/ZrO2. At the same time, the state of palladium is similar to that in Pd/SO42-/ZrO2. An increase in the reduction temperature to 200~ leads to the formation of more uniform adsorption sites, although they differ from those formed under similar conditions in the PtPd/ZrO2 system. One can assume that processes of alloy formation start in this system; however, they are overlapped by the action of surface active sites formed by SO4 groups. A further increase in reduction temperature to 300~ results in the formation of the charged forms of the metal, presumably Pd 2+, characterized by the band at 2155--2160 cm -1. The effect observed can be explained by redox processes involving metal atoms and surface SO4 groups. In the monometallic system promoted by SO42- anions, the phenomenon of metal re-oxidation was not observed and, hence, is characteristic behavior of the bimetallic system. An increase in the relative concentration of Pt in the P t ~ P d mixture results in the disappearance of the Pd ~ islands (bridging CO complexes) during the reduction even at 100~ An increase in the relative concentration of Pd leads to an increase of the intensity of the bands attributed to bridging complexes of CO adsorbed on Pd. However, reduction at 200~ causes the intensity of these bands to decrease. The behavior of the bands and during reduction resembles that in the monometallic Pd/SO42-/ZrO2 system. The main state of platinum in the systems
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0.3
267
' 21i0
2105
.~0.2
2115~
1905 2085
80
2135 IJ
( 1950~1\
~,
O.l
o.o,
b
a
21
I
2200
,
/
i
2000
,
1
1800 2200 Wavenumbers, cm -1
2000
1800
Fig. 3. IR spectra of CO adsorbed on Pt-Pd/SO42-/ZrO2 reduced at 100~ (a) the concentration of Pt is (1) 60, (2) 50, (3) 30, and (4) 15 %Pt; (b) deconvolution of spectrum 2. with different Pt/Pd ratios is Pt ~ Presumably, Pd plays the main role in surface interaction in the PtmPd mixture. The surface SO4 groups seem to interact mainly with palladium and their influence on platinum is insignificant. The additional argument in favor of the participation of SO4 groups in the metalmsupport interaction was obtained by XPS. According to XPS data, the main part of sulfur is present on the surface as sulfate groups (Fig. 4). However, the apparent shift if the XPS line is observed compared to the position of the line in bulk zirconium sulfate [9]. The XPS line of sulfur in Pt-Pd/SO42-/ZrO2 is shifted to the position close to the binding energy for organic sulfates [9]. These data are supported by S2~69.4 IR data. Hence, the metal support interaction results not only (RO)2SO 2 - 169.8 [9]/ i in modification of the properties of Zr(SO4) 2 - 1 the metal but also alters the properties of the surface sulfur groups. 9 ,,,-i r~
To describe the interactions in the Pt-Pd/SO42-/Zr02 system the following model could be proposed. The surface acid sites formed by SO4 groups are known to be responsible for the unique properties of the S042-/Zr02 system [1]. In accordance with the
160
,
I
a
I
~
165 170 Binding energy, eV
I
175
Fig. 4. XP spectra of S2p line of the Pt-Pd/SO42-/ZrO2 system.
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MonometaUic systems
0
II
S
u
//
Zr
.~
II
o- S
"-~ Zr ~",
Lr
Bimetallic systems O
0
~
o . Z
217
Zr
M0
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o S. o zr
, Zr
Zr ~"\ 0=-=-~ @ Zrf~ Zr
/ Pd~+
Scheme previous studies, these sites combine the acidic and redox properties (see Scheme). Acidic protons of BAS interact with the metal particle to form a [M--H] ~+ adduct (M = Pt or Pd). The value of the partial positive charge depends on the polarization extent of the M-.-H..-OSO3 bond and the shift of the maximum of the CO absorption band can serve as a measure of the strength of BAS. Protons can be abstracted from the acid sites to migrate over the metal surface. The OSO3groups can further react with the metal particles to form compounds of the IM + OSO3] type in which the metal atoms are positively charged. The formation of [M+--OSO3] complexes with Pt--O bond lengths which are different from those in conventional platinum oxides Pt--O--Pt was confirmed by EXAFS data [ 10]. Thus, in the bimetallic system the interaction of protons and sulfur species with metal depends on the nature of the metal. Palladium exhibiting higher reactivity interacts with the sulfate groups more easily as compared with platinum and, hence, platinum atoms are essentially not subject to the influence of surface active sites. This leads to the stabilization of the neutral state of platinum. ACKNOWLEDGMENTS
We gratefully acknowledge Dr. N.S. Telegina for XPS measurements. REFERENCES
1. 2.
X. Song and A. Sayari, Catal. Rev. - Sci. Eng., 38 (1996) 329. H. Yasuda, N. Matsubayashi, T. Sato, and Y. Yoshimura, Catal. Lett., 54 (1998) 23. 3. A.V. Ivanov, L.M. Kustov, Russ. Chem. Bull., 47 (1998) 1061. 4. C. Morterra, G. Cerrato, S. Di Ciero, M. Signoretto, F. Pinna, and G. Strukul, J. Catal., 165 (1997) 172. 5. H. Matsuhashi, H. Motoi, and K. Arata, Catal. Lett., 26 (1994) 325. 6. T. Sheppard and T.T. Nguen, Adv. Infrared Raman Spectr., 5 (1978) 67. 7. A.V. Ivanov, L.M. Kustov, Russ. Chem. Bull., 47 (1998) 57. 8. L.-L. Sheu, H. Knozinger, and W.M.H. Sachtler, Catal. Lett., 2 (1989) 129. 9. V.I. Nefedov, Rentgenoelektronnaya spektroskopiya khimicheskikh soedinenii (X-ray Spectroscopy of Chemical Compounds), Khimiya, Moscow, 1984. 10. T. Shishido, T. Tanaka, and H. Hattori, J. Catal., 172 (1997) 24.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
化
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Dehydroisomerization of n-butane silicoaluminophosphates.
over
Pt
promoted
Ga-substituted
A. Vieira, M. A. Tovar, C. Pfaff, P. Betancourt, B. M6ndez, C. M. L6pez, F. J. Machado, J. Goldwasser and M. M. Ramirez de Agudelol. Centro de Cat/disis, Petr61eo y Petroquimica, Escuela de Quimica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado Postal 47102, Los Chaguaramos, Caracas 1020A, Venezuela. INTEVEP, S.A, Apartado 76343, Caracas 1070-A, Venezuela. The catalytic transformations of n-butane were performed over a Pt-promoted Gasubstituted silicoaluminophosphate molecular sieve (Pt/GaAPSO-11 ), over a Pt-promoted Gasupported silicoaluminophosphate molecular sieve, and over a Pt-promoted SAPO-I 1 solid. The results showed similar yields for the formation of isobutane + isobutene, lower yields for the formation of C l-C3 hydrocarbons, for the Ga containing samples, particularly for the Pt/ Ga/SAPO-11 catalyst The latter also showed higher yields for the formation of dehydrogenated products. Differem techniques were used to characterize the metallic and acid functions. The possible formation of a Pt-Ga alloy and/or the formation of discrete Pt particles decorated by metallic Ga, are invoked to explain the higher dehydrogenation and lower hydrogenolysis activity shown by the Pt/Ga/SAPO-11 sample. 1. INTRODUCTION Metal-substituted aluminophosphates (MeAPO's) and metal-substituted silicoaluminophosphates (MeAPSO's) molecular sieves (Me: Cr(III), Mn(II), Zn(II), Fe(II), Co(II)), have been reported to be highly active and selective for the skeletal isomerization of nbutenes [ 1,2]. The importance of isobutene as a valuable feedstock is well documented due to it usefulness in the petrochemical industry. It can also react with methanol, producing methyl tert-butyl ether (MTBE), an important octane booster oxigenated fuel additive. Recently [3,4], we explored the possibility of producing isobutene from n-butane in a single process, thus avoiding the costs of two separate independent reactors (dehydrogenation and acid skeletal isomerization), which are presently used for the manufacture of isobutene. In addition, the direct transformation of n-butane can yield appreciable amounts of isobutane, a valuable feedstock used in the production of isooctane (by reaction with n-butenes). The reaction was performed over Pt-promoted Mn-AIPO4-11 and over Pt-promoted Mn-SAPO-11 molecular sieves. The results showed that the Pt/MnAPO-11 and Pt/MnAPSO-11 solids were more selective towards the formation of isobutene and isobutane than the Pt-promoted manganese supported counterparts. Other recent results [5] obtained over Ga silicoaluminophosphate molecular sieves, revealed that Ga(III) supported on SAPO-11 showed a much higher activity (particularly, at low times-on-stream (TOS)) towards the production of
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isobutene and isobutane, using n-butane and H2 as the reactants, than the G a - substituted counterpart. Extraframework gallium species (EFGS) species were invoked to explain the results The main purpose of the present work is to carry out the direct transformation of n-butane over a platinum promoted - gallium substituted silicoaluminophosphate molecular sieve (Pt/ GaAPSO-11) and over a platinum promoted - gallium supported silicoaluminophosphate molecular sieve (Pt/ Ga/SAPO-11). X-ray diffraction, acidity measurements (irreversibly chemisorbed pyridine, followed by infrared spectroscopy (IR)), H2 uptake at 773 K and metallic dispersion measurements (H2 chemisorption), were used as characterization techniques. 2. EXPERIMENTAL
2.1.Catalysts The synthesis procedure for the parent SAPO-11 has been reported elsewhere [5]. The specific surface area (SSA) was 190 m2/g. The GaAPSO-11 solid was synthesized according to ref 5. A final crystallization temperature of 473 K and a crystallization time of 90 h were employed. The solid was first washed with distilled water and dried at 353 K for 16 hours. The catalyst was then calcined under dry air at 773 K for 15 hours in order to remove organic residues. The molar composition formula TO2 for the calcined was (glo.42Po.42Sio.13Gao.03)O2. The SSA was 182 m2/g. A supported Ga/SAPO-11 catalyst was prepared by impregnating a sample of SAPO-11 with the same Ga promoter, using the incipient wetness technique (2.2 wt. % Ga). This sample was then dried and calcined following the same procedure as for the GaAPSO- 11 sample. The SSA for the Ga-supported catalyst was 152 m2/g. The Pt promoted catalysts were prepared by impregnating the different calcined solids with [Pt(NI-I3)4(NO3)2] (BDH, reagent grade). The experimental details have been reported previously [3,4]. The Pt loading was 0.5 wt. % Pt for all the Pt promoted solids. 2.2. Procedures Specific surface areas were determined on a commercial Micromeritics ASAP 2400 surface area analyzer at liquid nitrogen temperature. X-ray diffractograms were recorded with a Philips diffractometer PW 1730 using Co-KGt radiation (K= 1.790255A) operated at 30 KV, 20 mA and scanning speed of 2 ~ (20/rain). The H2 consumption experiments were performed in a conventional BET system identical to that used in Ref. 4 at 773 K. The experimental procedure has been outlined previously [4]. H2 chemisorption experiments were performed in the BET system mentioned above. The irreversibly held H2 was calculated using the double isotherm method. A 1:1 H:Pt stoichiometry was assumed to calculate the metallic dispersion (H/Pt). Infrared spectra were recorded at room temperature using a Perkin-Elmer 1760X FTIR spectrometer with a resolution of 2 cm1. The IR cell has been described previously [3, 4]. Before the addition of pyridine, the samples were treated with pure 02 (60 cm3/rain) at 773 K for 2 h (oxidic samples) or with pure H2 (60 cm3/rain) (reduced samples) at 773 K for 2 h. The catalysts were then evacuated (P Pt/Ga/SAPO-11 > Pt/GaAPSO-11. This result is in agreement with that obtained with the Pt-Mn analogues [4]. The catalytic results are shown in Tables 4-6. In order to estimate the role of the acid and metallic functions on the catalytic results shown, we have included data (Table 7) obtained using a 0.5 wt. % Pt supported over a non-acidic AIPO4-11 catalyst [3]. The Pt dispersion for the latter was 68 %, the SSA for the support was 162 m2/g, and the catalytic reaction was carried out under identical experimental conditions than those used in the present work.
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Table 3 H2 uptakes and Pt dispersion measurements H/Pt Catalyst H2 uptake/ ~tmoles/g SAPO-I 1 Not detected 0.83 Pt/SAPO-I 1 10.6 GaAPSO- 11 0.73 0.39 Pt/GaAPSO-11 5.11 Ga/SAPO-11 1.36 0.62 Pt/Ga/SAPO-11 8.02
H/Ga
0.0034 0.0083
Table 4. Product distribution for the transformations of n-butane over Pt/SAPO-11
TOS 30 60 90 120 180 300
X 87.4 79.0 77.0 73.3 68.7 64.4
Siso-C4 Siso-C4= Sn-c4=
S Rh > Ir, in agreement with measurements on single crystals. For Pd, it is furthermore obvious that the portion of terminally bonded CO increases as the particle size decreases ((a) ~ (c)). This change is also accompanied by a weakening of the Pd-CO bond, as deduced from photoelectron spectroscopic measurements [2,12]. Not surprisingly, the best ordered particles are obtained at 300 K. Here, the bands are clearly sharper than at 90 K. Especially, note the difference in the regime of terminally bonded CO for Ir and Rh. Only for the 90 K deposits, a shoulder signal at 2050 cm -1 appears. Since such a feature can be attributed to defect sites, i.e. kinks and steps [10], the data prove that the particles grown at 90 K exhibit a much higher defect density than the 300 K deposits. As mentioned, the Pd particles grown at 300 K are even crystalline. In fact, the corresponding bands can be assigned to specific facets on the aggregates [13]. The signals at 2108, 1956 and 1892 cm -~ are, for example, due to CO on (111) facets, whereas the peak at 2002 cm -~ is likely to be caused by CO on (100) facets and CO adsorbed on edge sites. Summing up, the results demonstrate on the one hand that IR spectroscopy with a suitable probe molecule can be a powerful tool to trace particle morphologies. On the other hand, they reveal interesting dependencies of the CO adsorption site on particle size and the position of the metal in the periodic table.
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a_)Low metal exposu.~__m_m at gO t(
b) High metal exposures at 90 K
I c) High metal exposures at 300 K I m
m
3000 _
2108
v 2002
~ 2117
soo'
2083/2052
I
,
Pd
Rh
5
2
%T
I
.
2%J
0.2 %
9
]
I
,
I
.
I
2200 2100 2000 1900 1800 Energy [cm-1]
,
I
I
"
I
,
I
,
2200 2100 2000 1900 1800 Energy [cm-1]
9
2200
l
2087
2100
.
I
.
i
,
I
2000 1900 1800 Energy [cm-1]
"" ,
1700
Fig. 3. IR spectra of Ir, Rh and Pd particles of different size and order saturated with CO at 90 K (data acquisition at 90 K). Averageparticle sizes are given next to the spectra. 6. C2H4 ADSORPTION AND REACTIVITY Molecularly adsorbed C2H4 may be present on transition metal surfaces in two forms [14]. A weakly bound species, which is thought to be the primary intermediate in ethylene hydrogenation, is usually referred to as n-bonded ethylene. It is only weakly perturbed upon adsorption. The formarion of the second type, di-o-bonded ethylene, involves a stronger rehybridisation of the carbon atoms, increasing their sp 3 character. Upon heating, it dehydrogenates to ethylidyne, C2H3. Both, di-o-bonded ethylene and ethylidyne are regarded as spectator species in the hydrogenation reaction. Ethylene rehybridisation upon adsorption results in a downshift of the strongly coupled C-C stretching and CH2 scissoring modes from their gas phase frequencies of 1623 and 1342 cm -1. As a semiquantitative measure of this perturbation, a no parameter has been proposed [ 15], defined by no = [(1623-band I)/1623) + (1342-band II)/1342] 10.366. Here, band I' refers to the higher and band II' to the lower frequency of the C-C stretch - CH2 scissors coupled pair. A higher no parameter indicates a larger extent of ethylene rehybridisation. Upon saturation of Pd, Rh, and Ir particles containing about 200 metal atoms with C2H4 at 90 K, features typical for n-bonded ethylene are observed in the infrared spectra (Fig. 4). Signatures of molecules in the di-o state are not so readily discerned. In the case of Pd, however, a weak band at ~1115 cm -~ and a C-H stretch signal at 2924 cm -~ may point to the presence of such species [16]. A comparatively high proportion of n-bonded ethylene may be due to the abundance of step and defect sites on small metal particles. Such sites have been suggested to favour the formation of the n-bonded form [17]. From the observed trend in vibrational frequencies, which decrease according to the sequence Pd > Rh > Ir, we may infer that the interaction of n-bonded ethylene with the metals increases from Pd across Rh to Ir. Making use of the n~ parameter to quantify these observations, we obtain values of 0.40 for Pd, 0.48 for Rh and approximately 0.55 for Ir. These findings agree well with results obtained on n-bonded ethylene on Me/A1203 catalysts [16,18]. This indicates that the
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Pd
Rh
Ir
I I
Fig. 4. IR spectra of Ir, Rh and Pd particles (size: --
o.o5 %
1600
i
I
1400
i
I
1200
Energy [r -1]
i
1000
200 atoms) saturated with C2H4 at 90 K (data acquisition at 90 K)
model systems are well suited for more extensive studies on hydrocarbon reactivity which are currently under way. ACKNOWLEDGEMENTS W e are grateful to a number of agencies for financial support: Deutsche Forschungsgemeinschaft, Bundesministerium ftir Bildung und Forschung, Fonds der Chemischen Industrie and N E D O International Joint Research Grant on Photon and Electron Controlled Surface Processes. This work has also been supported, in part, by Synetix, a m e m b e r of the ICI group, through their Strategic Research Fund. M.F. thanks the Studienstiftung des deutschen Volkes for a fellowship. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
M. Che, C.O. Bennett, Adv. Catal. 20(1989) 153. M. B~iumer, H.-J. Freund, Prog. Surf. Sci. 61 (1999) 127. H.-J. Freund, Angew. Chem. Int. Ed. Engl. 36 (1997) 452. D.W. Goodman, Surf. Rev. Lett. 2 (1995) 9; Surf. Sci. 299/300 (1994) 837. J. Libuda, F. Winkelmann, M. B~iumer, H.-J. Freund, Th. Bertrams, H. Neddermeyer, K. Miiller, Surf. Sci. 318 (1994) 61. Th. Schr6der, M. Adelt, M. Naschitzki, M. B~iumer, H.-J. Freund, in preparation. J. Libuda, M. Frank, A. Sandell, S. Andersson, P.A. Brtihwiler, M. B~iumer,N. M~u'tensson, H.-J. Freund, Surf. Sci. 384 (1997) 106. M. B~iumer, M. Frank, M. Heemeier, R. Ktihnemuth, S. Stempel, H.-J. Freund, submitted to Surf. Sci. K.H. Hansen, T. Worren, S. Stempel, E. l_~gsgaard, M. B~iumer, H.-J. Freund, F. Besenbacher, L Stensgaard, submitted to Phys. Rev. Lett.. M. Frank, R. Ktihnemuth, M. B~iumer, H.-J. Freund, Surf. Sci. 427-428 (1999) 288; submitted to Surf. Sci. F. Solymosi and M. P~ztor, J. Phys. Chem 89 (1985) 4789. A. Sandell, J. Libuda, P.A. Brtihwiler, S. Andersson, M. B~iumer, A.J. Maxwell, N. M~trtensson, H.-J. Freund, Phys. Rev. B. 55 (1997) 7233. K. Wolter, O. Seiferth, H. Kuhlenbeck, M. B~iumer, H.-J. Freund, Surf. Sci. 399 (1998) 190. P.S. Cremer, G.A. Somorjai, J. Chem. Soc. Faraday Trans. 91 (1995) 3671. E.M. Stuve, R.J. Madix, C.R. Brundle, Surf. Sci. 152/153 (1985) 532. S.B. Mohsin, M. Trenary, H.J. Robota, J. Phys. Chem. 95 (1991) 6657. P.A. Dilara, W.T. Petrie, J.M. Vohs, Appl. Surf. Sci. 115 (1997) 243. Y. Soma, J. Catal. 59 (1979) 239.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
An Atomic XAFS study of the metal-support interaction in Pt/SiO2AI203 and Pt/MgO-AI203 catalysts: an increase in ionisation potential of platinum with increasing electronegativity of the support oxygen ions D.C. Koningsberger a, M.K. Oudenhuijzen a, D.E. Ramaker b and J.T. Miller e a Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, PO Box 80083, 3508 TB Utrecht, The Netherlands b Chemistry Department, George Washington University, Washington, DC 20052, USA c Amoco Research Center, E-IF, 150 W. Warrenville Rd., Naperville, IL 60566, USA
The neo-pentane hydrogenolysis tum-over frequency (TOF) of platinum on macroporous acidic SiOa-A1203 is about 500 times higher than that on basic MgO-A1203 hydrotalcite clay. The TOF increases with increasing electronegativity of the support oxygen ions similar to that found for Pt dispersed in microporous supports, such as LTL and Y zeolite. In addition, for Pt on silica-alumina, the intensity of the Fourier transform of the atomic XAFS oscillations, which were isolated from the XAFS spectra is larger and shifted to lower radius. A general model for the metal-support interaction is proposed, where a Coulomb attraction causes an increase in ionisation potential of the metal d-valence orbitals with increasing electronegativity (i.e. lower electron density) of the support oxygen ions. 1.
INTRODUCTION
Numerous studies have reported enhancements in the specific reaction rates for benzene hydrogenation [1], propane hydrogenolysis [2] and neo-pentane hydrogenolysis and isomerization [3,4,5,6], on acidic supports compared to neutral supports. Several explanations for the metal-support interaction have been proposed in literature (i) Formation of a metalproton adduct [3,7], (ii) Charge transfer between the metal atoms and the nearest neighbour zeolite oxygen atoms [8,9] and (iii) Polarisation of the metal particles by nearby cations [10,11]. For each explanation, however, there is experimental evidence, which is inconsistent with the proposed model [12]. Systematic experiments carried out by our group have shown that the catalytic activity and spectroscopic properties of supported noble metal catalysts are greatly affected by H + and K + in LTL zeolite [13] and by H +, La +3, different Si/A1 ratio and non-framework A1 in Y zeolite [12]. A newly developed tool called atomic XAFS (AXAFS), which is obtained from the X-ray Absorption Fine Structure (XAFS) spectra, can provide electronic information on metalsupported catalysts [14]. The intensity of the Pt AXAFS peak increases with increasing electronegativity of the support oxygen ions. The changes in the AXAFS data could be explained by an increase in the ionisation potential of platinum with increasing electronegativity of the zeolite oxygen ions
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In this paper the metal-support interaction for platinum particles supported on high surface area, macroporous supports is further investigated. The electronegativity of the support oxygen ions was varied by using an amorphous acidic SIO2-A1203 and a basic MgO-A1203 hydrotalcite clay. The influence of the support on the neo-pentane hydrogenolysis TOF of Pt was determined. XAFS spectroscopy (EXAFS and AXAFS) was used to investigate the structural and electronic properties of the supported Pt particles.
2.
EXPERIMENTAL
2.1. Catalysts preparation The supports were commercially available: SIO2-A1203 (denoted by Si(Al)O), Davison grade 135, 510 m2/g and 0.67 cc/g) and magnesium-alumina, hydrotalcite clay MgO-A1203 (denoted by Mg(A1)O), La Roche Ind, Inc, Table 1. Dispersion and TOF values 166 m2/g and 0.18 cc/g). The supports were Catalysts Dispersion~ TOF2 calcined at 500~ and 550~ respectively, SIO2-A1203 0.85 2.7x 10 -2 for 16 hr. Platinum was added to each MgO-AI203 0.26 5.3 x 10.5 support by impregnation with [Pt(NHa)4](NO3)2 (Pt loading 2 wt%). The ~Determined by volumetric HE chemisorption catalysts were dried at 120~ calcined at assuming 1 H/Pt atom. 2Molecules/sec-surfacePt atom. 250~ and reduced in flowing hydrogen at 350~ The Pt dispersions were determined by hydrogen chemisorption after reduction at 350~ and are given in Table 1.
2.2. Neo-pentane hydrogenolysis Neopentane hydrogenolysis was conducted in a fixed bed reactor at 325~ using 0.99 vol.% neo-pentane in H2. The catalysts were re-reduced at 325~ for 1 hour, and conversion was adjusted to values less than 2.0% by varying the space velocity. The TOF was calculated based on H2 chemisorption. The analysis of the reaction products was carried out using the Delplot method [15], which gives by extrapolation to zero conversion the primary product distribution.
2.3. XAFS data collection X-ray absorption spectra have been collected at station 9.2 of the Daresbury SRS. The samples were pressed into self-supporting wafers and were then mounted in an in-situ cell equipped with Be windows. The EXAFS samples were reduced in flowing hydrogen at 400~ (heating rate 5~ for 1 hour and evacuated at 200~ for 1 hour. XAFS spectra were recorded at liquid nitrogen temperature maintaining a vacuum of better than 2xl 0 -~ Pa.
2.4. XAFS data-analysis methods By using newly defined criteria the smooth atomic background and multi electron excitations are isolated from the AXAFS and EXAFS contributions (for further details see [16]). Theoretical phase and backscattering amplitudes for the Pt-Pt and Pt-O absorberbackscattering pairs were generated utilising the FEFF7 [17] code and calibrated against reference compounds [16]. The new references can be used from a k-value of about 2.5 A -l, significantly lower than the previously used experimental references. The result is a much
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better isolation of the AXAFS peak at low R. Fitting is done in R-space, without Fourier filtering of the data. To analyse the metal-oxygen contribution to the spectra the difference file technique was used [12]. After subtracting the first metal-metal and metal-oxygen contributions, the remaining signal will be the AXAFS together with the higher order shells.
2.5. AXAFS The AXAFS, ZAx(k), is caused by the scattering of the photoelectron off the deep valence electrons in the periphery of the absorbing atom [14]. The well-known muffin-tin approximation can be used to approximate the embedded atom potential. As illustrated in Figure l, the muffin-tin approximation "clips" the exact potential at the muffin-tin radius Rmt and sets it equal to the interstitial potential Via t [ 14]. Inside Rmt the potential is assumed to be spherical, outside it is assumed to be flat and zero (i.e. no forces are exerted on the particle in the interstitial region). Vint is determined by averaging the potential at Rmt of all the atoms in the cluster, and this determines the zero of energy or the effective bottom of the conduction band. A phase corrected and k weighted Fourier transform of Zgx(k) leads to [14]" [ FT(ke2i6 ~, ZAX)[ = AV*F where AV = Vemb- VTFA with Vemb the embedded atom potential, VTFA the truncated free atom potential, and F a broadening function due to the limited Fourier transform range. The embedded potential reflects the electron distribution after embedding the free atom into its chemical environment and allowing interaction with its neighbours. This means that the FT directly reflects this chan~e in the chemical environment. More specifically, the shape and intensity of the [FT/ can be represented by the shaded area between Vfree and Vemb and below Vcut (Vcut = 2• + IEfl) as illustrated in Figure 1 (For further details see [ 12,14]). 3.
RESULTS
3.1. Neo-pentane hydrogenolysis The results for conversion of neo-pentane are given in Table 1. Analysis of reaction products by the Delplot method indicated that methane, iso-butane (hydrogenolysis), and isopentane (isomerization) were primary products. It can be seen in Table 1 that the TOF of Pt particles supported on the acidic Si(A1)O is about 500 times higher than for Pt supported on the basic Mg(A1)O.
Ev
-.-
EF
~
Vint
Vfree :
~
Vemb A
UUUUU~
Vcut
"ber
Fig. 1. Illustration of the muffin tin approximation to the interatomic potentials showing locations of Ev, EF,Vint
9
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3.2. EXAFS
In Figure 2 the Fourier transform (k2, Ak = 2.5 - 14 A "l) of the raw XAFS data (solid line) of the Pt/Si(A1)O sample is displayed. The shoulders at both the low and high R side of the first Pt-Pt peak in the Fourier transform are due to the non-linear Pt-Pt phase shift and the k dependence of the backscattering amplitude. Fitting of the experimental spectra was done in R-space over the range R - 1.6 to 3.1 A using a k2 weighted Fourier transform over the range A k - 2.5 to 14.0 Al.The results of the fit are shown in Figure 2 (dotted line). The EXAFS coordination parameters for both catalysts are given in Table 2.
0.50
0.04
0.25
0.02
~
I--: 0.00 ~ -0.25
-0.50
-0.02
..e
0
1
2 R (A)
0.00
3
4
Fig. 2. Fourier Transform (k 2, Ak=2.5-14A-~)of raw EXAFS data of Pt/Si(AI)O (solid line) and fit (dotted line).
A -0"00.0
,
i 0.5
,
i 1.0
,
R
i
1.5
2.0
Fig. 3. Fourier Transform (k1, Ak=2.5-8 Al) of AXAFS data of Pt/Si(AI)O (solid line) and Pt/Mg(AI)O (dotted line).
3.3. AXAFS It can clearly be seen in Figure 2 that at low values of R differences are present between fit and experimental data. These differences are due to the AXAFS contribution. Subtracting the calculated Pt-Pt and Pt-O contributions from the raw XAFS produces a difference file, which contains the AXAFS contribution. The k I weighted Fourier transform of this difference file is given in Figure 3 for Pt/Si(A1)O (solid line) and for Pt/Mg(A1)O (dotted line). The amplitude of the AXAFS peak is larger and the centroid is at lower values of R for the Pt/Si(A1)O sample.
4.
DISCUSSION
4.1. Neo-pentane hydrogenolysis Since neo-pentane can not form an alkene intermediate, hydrogenolysis is dependent on only the catalytic activity of the metal [ 18,19] as confirmed by the primary reaction products: methane, iso-butane (hydrogenolysis), and iso-pentane (isomerisation). Moreover, neopentane does not undergo protolytic cracking at the temperatures used for the catalytic reaction (325~ The protons present on the Si(A1)O support, therefore, do not contribute to the neo-pentane conversion. Hydrogcnolysis reactions are dependent on the metal particle
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Table 2. EXAFS co-ordination parameters Coordination
Pt-Pt
Pt-O
Parameters
N
R (A)
Aft2(A 2)
E0 (eV)
N
R (A)
Aft2 (A 2)
E0 (eV)
Pt/Si(Al)O
6.7
2.67
0.007
3.1
0.1
2.13
0.000
3.1
Pt/Mg(Al)O
7.3
2.71
0.006
2.2
0.1
2.07
-0.003
5.3
size, generally, decreasing with increasing particle size: TOF identical for catalysts with a dispersions from about 0.1 to 0.7, but a factor 2 decrease as the dispersion increased to 1.0. Thus, at an equivalent dispersion, the TOF of Si(A1)O would increase to about 1000 times higher than that of Mg(Al)O. While the particle size does influence the rate, it is not sufficient to account for the large differences in TOF in these catalysts. We conclude that the change in TOF is primarily due to the metal-support interaction consistent with previous studies [12,14].
4.2. Structure of the Pt particles. The metal particles in both Pt/Si(A1)O and Pt/Mg(A1)O catalysts have first shell Pt-Pt coordination numbers around 7; i.e. the average metal particle consists of approximately 40 atoms assuming a spherical particle morphology. A Pt-Pt coordination number of 6.7 for Pt/Si(AI)O is slightly larger than expected for a catalyst with a dispersion of 0.85 and may be due to the higher reduction temperature in the EXAFS measurements. A small interfacial oxygen contribution (only Pt atoms in the metal-support interface have oxygen neighbours) is detected within the first co-ordination sphere of Pt, but neither silicon, nor aluminum ions were found within 3 A of the platinum particles for either catalyst. Because to the small Pt-O contribution to the EXAFS, the differences in the Pt-O distances are likely not significant. Since the Pt particles in both catalysts have the same structural properties, we conclude that the change in the catalytic properties of the metal particles is due to a change in their electronic properties induced by a Coulombic interaction with the oxygen ions of the support. 4.3. Nature of the metal-support interaction Basic support Vfree"",~...
i-"
~,, Mint
Pt
R
max
Acidic support .....,
z_
Pt
R
max
Fig. 4. Schematic potential curves for a basic (a) and acidic support (b) assuming polarisation of the cluster by the support.
For Pt/Si(A1)O, the intensity of the AXAFS peak is larger and the centroid is shifted to lower R in comparison to Pt/Mg(A1)O (see Figure 3). These results are consistent with the results recently published by our group for Pt/LTL [6] and Pt/Y [12]. The support properties (acidity, basicity) determine through the Madelung potential the electronegativity of the support oxygen ions. Figure 4a and b compare the difference in the Pt-O interatomic potential as the charge (electronegativity) on
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the oxygen ion increases. The AXAFS peak in the Fourier transform of the Pt/Mg(A1)O data is schematically represented on the right side of Figure 4a with the black area and is determined by the difference between Vfree and Vemb: the black area on the left side of Figure 4a. Increasing the charge on the oxygen (8+: higher electronegativity) will change the shape of the potential of platinum since the interaction with oxygen will move platinum electrons nearer to the oxygen. This is illustrated in Figure 4b by the larger Coulomb tail on the O atom with increased charge, and hence more "roll over" of the interatomic potential and the lowering of Vcut. This causes a larger difference between Vemband the free atom potential as is shown on the left side of Figure 4b. The original difference (black area, in size and position) for the Pt/Mg(A1)O sample is given as a comparison. This larger difference causes an increase in the amplitude of the Fourier transform of the AXAFS oscillations and a shift of the centroid to lower R values as shownon the left side of Figure 4b. At the same time the platinum valence d orbitals are moved to higher binding energy; i.e. the ionisation potential of Pt is increased. Moreover, the Pt d-orbitals are radially contracted and the width of the d-band is reduced resulting in less "metallic" character. This is also reflected in the XPS 3d core level shift of Pd particles dispersed in LTL [6] and in the increase in the linear/bridge ratio of the CO FTIR spectra of Pd/LTL, Pt/LTL and Pt/SiO2. [5]. The AXAFS studies on Pt/LTL [13], Pt/Y [12] and the results presented in this paper on Pt/Si(A1)O and Pt/Mg(al)O suggest a general model for the metal-support interaction, which is based upon a Coulomb attraction between the support oxygen ions and the platinum metal particles. An increase in electronegativity of the support oxygen ions leads to an increase in the ionisation potential of the platinum metal atoms. This change in binding energy of the metal valence orbitals alters the adsorptive, catalytic and spectroscopic properties of the metal particles. This model also implies that there is no transfer of electron density between the support and the metal particles.
REFERENCES 1. 2. 3. 4. 5 6.
S.D. Lin, M.A. Vannice, J. Catal. 143 (1993) 539. J.T. Miller, F.S. Modica, B.L. Meyers, D.C. Koningsberger, Prep. ACS Div. Petr. Chem. 38 (1993) 825. Z. Karpinski, S.N. Gandhi, W.M.H. Sachtler, J. Catal. 141 (1993) 337. S.T. Homeyer, Z. Harpinski, W.M.H. Sachtler, J. Catal. 123 (1990) 60. B.L. Mojet, M.J. Kappers, J.T. Miller, D.C. Koningsberger, Proc. of the 15th Int. Cong. Catal., Baltimore, MD, (1996). B.L. Mojet, M.J. Kappers, J.C. Meyers, J.W. Niemantsverdriet, J.T. Miller, F.S. Modica, D.C. Koningsberger, Stud. Surf. Sci. Catal. 84 (1994) 909. 7. Z. Zhang, T.T. Wong, W.M.H. Sachtler, J. Catal. 128 (1991) 13. 8. G. Larsen, G.L. Hailer, Catal. Lett. 3 (1989) 103. 9. A. de Mallmann, D. Barthoumeuf, J. Chem. Phys. 87 (1990) 535. 10. A.P. Jansen, R.A. van Santen, J. Phys. Chem. 94 (1990) 6764. 11 E. Sanchez-Marcos, A.P.J. Jansen, R.A. van Santen, Chem. Phys. Lett. 16 (1990) 399. 12 D.C. Koningsberger, J. de Graaf, B.L. Mojet, D.E. Ramaker and J.T. Miller, Appl. Catal, in print. 13 B.L. Mojet, J.T. Miller, D.E. Ramaker and D.C. Koningsberger, J. Catal. 186 (1999) 373. 14 D.E. Ramaker, B.L. Mojet, W.E. O' Grady and D.C. Koningsberger, J. Phys. Condens. Matter. 10 (1998) 1. 15 N.A. Bhore, M.T. Klein, K.B. Bischoff, Ind. Eng. Chem. Res. 29 (1990) 313. 16 G.E. van Dorssen, D.E. Ranaaker, D.C. Koningsberger, submitted Phys. Rev. B 17 S. I. Zabinsky, J. J. Rehr, A. Ankudinov, R. C. Albers, M. J. Eller, Phys. Rev. B 52 (1995) 2995. 18 S.M. Davis, G.A. Somorjai, The chemical physiscs of solid surfaces and heterogeneous catalysts (D.A. King, D.P. Woodruff, eds.), Elsevier Publishers Amsterdam, 4 (1982) 271. 19 J.R. Anderson, N.R. Avery, J. Catal. 16 (1967) 315.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Transition state and diffusion controlled selectivity in skeletal isomerization of olefins L. Domokos a, M.C. Paganini a, F. Meunier a'b, K. Seshan a and J.A. Lercher b aCatalytic Processes and Materials, Faculty Chemical Technology, University of Twente, P.O. Box 217, 7500 AE, The Netherlands bTechnische UniversiRit Mtinchen, Institut ftir Technische Chemie, D-85749 Garching, Germany Microkinetic and spectroscopic analysis of skeletal isomerization of linear butenes were performed over H-FER. The effect of butene partial pressure and the rate of formation of isobutene and major byproducts are reported. Results show zero order dependence of isobutene production and first order dependence of byproduct formation with butene pressure. Biphenyl type molecules are proposed as coke species and related to byproduct formation. 1. INTRODUCTION In the last decade skeletal isomerization of light olefins has seen intense efforts in research and development, most notably for n-butene isomerization, due to new legislations on gasoline composition. Although industrially applicable processes have been developed for skeletal isomerization, a unified view on the possible active site and the mechanistic pathway is still lacking. Mechanisms proposed in the literature for butene skeletal isomerization include (i) monomolecular isomerization [1], (ii) dimerization, isomerization and selective cracking (bimolecular pathway) [2] and (iii) addition of butene to carbonaceous species, subsequent isomerization and selective cracking to isobutene (pseudo-monomolecular pathway) [3]. High selectivity to isobutene is found preferentially with medium pore zeolites [4]. The role of the acid site strength seems to be quite subtle [5]. For the most successful catalyst, H-ferrierite, selectivity to isobutene was seen to improve with time on stream, while substantial coke deposition occurred. This led to the suggestion that steric constraints caused by coke helps to enhance isobutene selectivity [3]. The present contribution aims at combining microkinetic and spectroscopic evidence to demonstrate that under usual process conditions not one of the three reaction routes prevails, but that a complex network of mono- and bimolecular reactions determines the catalytic activity and selectivity. 2. E X P E R I M E N T A L
2.1. Catalyst preparation Ferrierite (FER) was obtained from Tosoh with a nominal Si/A1 ratio of 8 and in the NaK exchanged form. Successive ion exchange with 1M NH4NO3 was carried out (three times for 4 h each) in order to obtain the ammonium form of the catalyst. Samples were successively washed with deionized water and dried at room temperature in air.
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2.2. Infrared .pectro.copy The infrared measurements were carried out in a BRUKER IFS-88 spectrometer equipped with a flow cell. The spectra were recorded in the transmission absorption mode. The zeolite was pressed as a self-supporting wafer (2-5 mg/cm 2) and placed in the cell. An activation procedure similar to the kinetic measurements was used. Activated H-FER showed a small peak attributed to terminal OH groups at 3741 cm -l and an intense band at 3584 c m "1 assigned to Bronsted acidic hydroxyl groups. 2.3. Catalytic measurements The catalytic tests were conducted in a tubular continuous flow system. All measurements were carried out at temperature ranging between 250-450~ (typically 350~ with total pressure between 1.0-1.3 bar. Catalysts were activated in situ in the reactor in large excess of dry argon (99.995%) flow at 400~ for 1 hour. Subsequently the temperature was switched to reaction temperature and then the pure argon flow was switched to a mixture of 1butene and argon. The effluent stream was analyzed in regular intervals by a CG equipped with an HPPLOT/A1203 column ("S" deactivated) and FID detector. Carbon balance was close to 98% from initial time on stream on. Conversion of 1-butene, yield and selectivity of any product was calculated according to the literature. Since linear butenes are expected to be in equilibrium at temperatures at which skeletal isomerization occurs, all linear butenes are lumped together and not counted for conversion and yields. Conversion and yield are expressed in terms of mol% on a carbon basis. 3. RESULTS AND DISCUSSION Kinetic measurements at a reaction temperature of 350~ and a partial pressure of 100 mbar butene led to 30% selectivity to isobutene (22 mol% yield, Table 1) the remaining products being mainly propene and pentene (Fig. 1). Under these conditions, the yield of propene and pentene decreased while the yield of isobutene remained constant at short times on stream (TOS). After 10h TOS a significant increase in isobutene yield occurred (up to 37%) while conversion of butene stabilized around 50 mol%. This lead to an increase in isomerization selectivity (Table 1, Fig. 1). Note that the amount of hydride transfer products (ethane, propane) was negligible after the initial period of 1Oh TOS. Table 1 Selectivities to various components at different partial pressures of 1-butene (WHSV=2h ~) Selectivity (mol%) Component C1 C2 C3i-C4 = C5-~sum) C5 + Conversion Yield of i-C4-
p(C4 =) = 1O0 mbar 3 min 50h 0.2 0.0 4.1 0.1 32.4 5.5 30.2 84.5 17.3 4.9 10.9 4.0 73.1 44.6 22.1 37.8
p(C4--) =- 5 mbar 3 min 50h 0.0 0.0 0.0 0.0 3.3 3.0 93.4 93.9 2.6 2.5 0.3 0.1 36.7 31.7 34.0 29.7
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化
mol%
100
~^^^^^^^^^^^,vuxzxzxL~.~
80
,,^zxzxzxz~
60 s
^^^^^^~
-
i-C4= selectivity
a atxaaatxtxtxtxa~'-
4O 20
ilL
:E hydride transfer
0
20
+
40
60 80 Time on stream (h)
100
120
Fig 1. Skeletal isomerization of 1-butene using H F E R as catalyst. Product yields, conversion and selectivity to isobutene vs time on stream.
When the butene pressure was lowered to 5 mbar from 100 mbar, the initial selectivity to isobutene was 93% (yield 33%) and remained stable with TOS. It is important to note that the yield of isobutene changed relatively little, when the pressure of n-butene was lowered by an order of magnitude, while the rate of formation of propene and pentene decreased by approximately a factor of 10 (Table 1). The same observation was made during a pressure transient experiment shown in Fig. 2. Monitoring the reaction with in situ infrared spectroscopy showed that most of the carbonaceous species was deposited at short TOS (Fig. 3). In contrast, selectivity to isobutene increased gradually with TOS over 100 hours (Fig. 1). In addition, at low butene pressure skeletal isomerization was very selective (Fig. 2, 0-5 hours, Table 1) from short time on stream. This indicates that high selectivity to isobutene can be reached in the absence of significant fractions of coke deposited. Additionally, during reaction a fast but only partly (approximately 40%) coverage of
60{
mol%
4
50 mbar
Iiiii 0
20
5 mbar
~
-i ..........
0
i--
-
100
20000 c ,-; 15000
~C43yielZ~__ ~
-Q
i
v
60 "~
c ~o 5000 i
20
Fig. 2. Pressure transient experiment: 5 to 50 mbar partial pressure of butene
"0 Ol t~
r-
0
d cO 0
ffl
t-
1
V
80
"5 ":. 10000
C5+ yield
5 10 15 Time on stream (h)
A
~_J
"{3:3
[]
'
'
'
0
50
100
150
40 200
Time (min) Fig. 3. Coke deposition during reaction monitored by in situ i.r. spectroscopy
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3741 cm-, 9 terminal OH groups
hydrocarbon i stretching i vibration s
3584cm., ' Bronsted ~
'
1513
1504cm-, l aromatic coke species
1
overtones ofla~ice I
8h
1504 3500
3000 2500 2000 Wavenumber (cm-,)
1500
Fig. 4. Butene isomerization reaction monitored by in situ infrared spectroscopy at 350~ and 50 mbar butene partial pressure
1700
1600
1500 1400 Wavenumber (cm-1)
1300
Fig. 5. Evolution of the infrared band of different coke species at 5 mbar butene pressure at 350~
acid sites was observed. The profiles shown in Fig. 3 indicate the diffusion plays an important role governing the molecular transport inside of the zeolite channels during butene isomerization. As n-butene tends to polymerize on acidic zeolites under these conditions, the observation that during reaction of n-butene over H-FER only 40% of the acid sites were interacting with hydrocarbon suggests that a bulky sorbate must have been formed at the pore entrance hindering further intrusion of reactant or product molecules. The nature of the coke deposited on the catalyst (main peak at 1504 cm -~) seemed to have a distinct structure typical of aromatic rings interconnected by short aliphatic chains, most likely to have a poly-4,4'-biphenyl-like skeleton (Fig. 4). This is in contrast to Guisnet et al. [3] and Andy et al. [6] who reported mostly condensed aromatic structures as coke in HFER after few hours TOS. After deactivation, low and steady amount of butadiene was found in the effluent stream. During in situ infrared measurements o overall performed in the presence of only butadiene, E 6.0 the same band at 1504 cm -1 attributed to the =,o 9.,J biphenyl structure was again observed (Fig. 4) = 4.0 "01 indicating the origin of the coke. After the initial formation of the 2.0 biphenyl species, a small shift from 1504 cm 1 to 1513 cm -1 in the corresponding band (Fig. 5) indicates a probable intramolecular 0.0 ~ rearrangement of these species driving to a 0 20 40 60 more complex structure. Furthermore, during pressure of 1-butene (mbar) purging after reaction with inert gas, it was observed that the latter species (1513 cm -1) did Fig. 6. Effect of butene pressure at 350~ not desorb from the catalyst, while the species WHSVbutene=2h1 attributed to the 1504 cm -1 band could be O .1
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removed completely. This is in a good agreement with refs. [2] and [6] suggesting a more graphitic and less reactive state of the carbonaceous deposit located probably at the outer surface of the particle. The selective isomerization reaction of butene to isobutene showed zero order dependence upon butene pressure, while the production of propene and pentenes increased in first order with butene pressure (Fig. 6). This indicates that isobutene formation occurs probably monomolecularly on acid sites that are fully covered at the lowest n-butene partial pressure. Note that the rate of conversion of n-butene depends upon the concentration of acid sites with an apparent reaction order of 0.7. Furthermore, it implies too that yield of isobutene is limited by the desorption capabilities of the molecule from the pores and not by the intrinsic rate of formation. Consequently, at higher partial pressures, the secondary reactions originating from the more reactive adsorbed isobutene will be enhanced. The Arrhenius plot of isobutene formation indicates diffusion limitation above 325~ at short times on stream (Fig. 7). The two straight sections of the graph transformed to a curve with time on stream in 3 hours. This might indicate [7] a coverage dependence of the reaction due to surface poisoning by spectator molecules. Independently, the coverage of acid sites in the catalyst was not complete, and leveled off at approximately 40% (Fig. 3) indicating the possibility of a more complex diffusion phenomenon. Using the coverage of different kind of species observed during in situ i.r. experiments, it was possible to linearize the Arrhenius plot (Fig. 8). It is interesting to note that isobutene production could be normalized by the coverage of CH3 species (mainly represents isobutene adsorbed on the surface) and not by the coverage of the aromatic deposits. This confirms again that isobutene production is limited by its concentration on the surface. The linearity of the Arrhenius plot indicates that no significant change occurred in the nature of isobutene production in the temperature region of 250-450~ This is in contrast to the literature [4-5] where a gradual shift in the mechanism of selective isobutene production from a bimolecular to a monomolecular pathway was proposed with increasing temperature. The fact that aromatic species did not provide a straight line implies that these species may not play a significant role in the selective isomerization process. Both the kinetic and i.r. 2 0
O
1.0
O
u.
0.0
1.1_ v
v t-
-1.0
-i-after 3 h
-2.0 1.3
kI
I
I
I
1.5
1.7
1.9
1/'!" (10 "3, l/K) Fig. 7. Arrhenius plot of isobutene production. Initial and 3 h TOS data represented.
-5
I --B- CH3 cov. - I - aromatic
2.0
-6 -
-7
-8
-4
-9 -10
-6
1.3
1.5
1.7
1.9
1/T (10 3, l/K)
Fig. 8. Concentration of aromatic species and aliphatic CH3 vibration over H-FER during reaction/adsorption of 1-butene
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experiments suggested that after the initial period of coke formation, only minor changes occur in the deposited species. In Fig. 1 the total amount of hydride transfer compounds decayed to zero after 10 hours TOS. Subsequently (as shown in Fig. 3) significant increase in the amount of aromatic species was not observed after 3 hours TOS. These results strongly suggest (in agreement with J. Houzvicka et al. [8]) that the isobutene production enhancement from 22% to 37% is not induced by coke formation. Indeed, this loss of approximately 15 mol% of isobutene to carbonaceous deposit or byproduct formation can be related to consecutive reaction originating from isobutene. With time on stream, as the coke is transformed into a less reactive, more bulky state (Fig. 5 after 8 h), this consecutive reaction pathway decayed and the isobutene yield increased again (Fig. 1, after 10 h TOS). This implies that the active coke species are more related to byproduct formation (first order of butene pressure) than to selective isomerization (zero order of butene pressure). We, therefore, suggest that byproduct formation occurs primarily via reaction of adsorbed isobutene and weakly adsorbed n-butene. Together with the in situ infrared spectroscopic measurements, these results suggest that the reaction occurs on Bronsted acid sites at pore entrance. The desorption of the so-formed isobutoxy species seems to be the most difficult reaction step. The selectivity and stability of the catalyst is attributed to the relative ease with which the isobutoxy species can desorb compared to the addition of n-butene to the isobutoxy group leading to byproduct formation. ACKNOWLEDGEMENT Financial support of STW/NOW under the project number of 349-3797 is gratefully acknowledged. This work was performed under the auspices of NIOK, The Netherlands Institute for Catalysis. REFERENCES 1. P. Meriaudeau, R. Bacaud, L. Ngoc Hung and Anh.T. Vu, T., J. Mol. Catal. A, 110 (1996) L 177-L 179 2. H.H. Mooiweer, K.P. de Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork, and B.C. Krutzen, in: Weitkamp, J., Karge, H. G., Pfeifer, H., and H61derich, W., (Eds.)/LZeolites and Related Microporous Materials; State of the Art 1994", Stud. Surf. Sci. Cat., 84 (1994) 2327-2334 3. M. Guisnet, P. Andy, Y. Boucheffa, N.S. Gnep, C. Travers, and E. Benazzi, Catal. Letters, 50 (1998) 159-164 4. J. Houzvicka, S. Hansildaar and V. Ponec, J. Catal., 167 (1997) 273-278 5. C-L. O'Young, R.J. Pellet, D.G. Casey, J.R. Ugolini and R.A. Sawicki, J. Catal., 151 (1995) 467-469 6. P. Andy, N.S. Gnep, M. Guisnet, E. Benazzi and C. Travers, J. Catal., 173 (1998) 322-332 7. D.W. Goodman and M. Kiskinova, Surf. Sci., 105 (1981) L265-L270 8. J. Houzvicka, S. Hansildaar, J.G. Nienhuis and V. Ponec, Appl. Catal. A: General, 176 (1999) 83-89
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Development and Application of 3-Dimensional Transmission Electron Microscopy (3D-TEM) for the Characterization of Metal-Zeolite Catalyst Systems A.J. Koster a, U. Ziese a, A.J. Verkleij a, A.H. Janssen b, J. de Graaf b, J.W. Geus b, K.P. de Jong b aMolecular Cell Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands bDepartment of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands; e-mail:
[email protected] With electron tomography (3D-TEM) a 3D-reconstruction is calculated from a series of TEM images taken at a tilt angle range (tilting range) of +70 ~ to -70 ~ The reconstruction can be visualized with contour surfaces that give information about the surface of the sample as well as with slices through the reconstruction that give detailed information on the interior of the sample. Electron tomography gives much more information than Scanning Electron Microscopy (SEM), since SEM gives only information about the surface of a sample. As a case study, the imaging of silver clusters on zeolite NaY is given. The reconstruction shows silver particles at the external surface as well as a silver particle in a mesopore of the zeolite crystallite. It is concluded that 3D-TEM comprises a breakthrough in the characterization of nano-structured solid catalysts. 1. INTRODUCTION Solid catalysts are of tremendous importance for economy and environment. The drive towards clean and efficient technology calls for precise design and characterization of catalysts. Today, many solid catalysts can be considered as sophisticated, three-dimensional nano-structured materials [1,2]. Especially zeolites and mesoporous materials are well known for their three-dimensional structures. To date, however, no method has been reported that is able to provide structural information in three dimensions with 1-30 nm resolution. In two dimensions, Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) can provide a surface image of a material at atomic resolution. Scanning Electron Microscopy (SEM) can provide a high-resolution image of a surface in three dimensions (topography), but the material below the surface is not imaged. Transmission Electron Microscopy (TEM) does give high-resolution information of a sample, but the three-dimensional information is projected into a 2D image. The information in the third dimension is lost. Figure 1 shows a TEM image of a Pt/NaY catalyst, which illustrates the absence of three-dimensional information. From Figure 1 it cannot be determined where the Pt-particles are located: inside the zeolite crystal or at the external surface. However, with the development of electron tomography (3D-TEM) it has become possible to get a 3D image of both the surface and the interior of a sample.
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During the last five years automated electron tomography has been developed in the field of biology [3,4], although the theory has been developed much earlier. Due to the automation of the data collection and the increased performance of personal computers, electron tomography can now be applied for practical assays with reasonable investments in time and hard- and software. In this paper the first application of electron tomography in material science is presented. A concise introduction in the theoretical and practical aspects of 3D-TEM is described. As a case study, the 3D imaging of an Ag/NaY catalyst is given. This system is of fundamental interest to assess metal mobility under reducing and oxidizing environment in zeolites as is apparent from the work of Beyer et al. [5]. In this paper, however, we will restrict ourselves to the study of a freshly reduced Ag/NaY sample.
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Figure l a and b: (2D)-TEM image of Pt/NaY. Figure l a is taken at a magnification of 27.5k, Figure lb is an enlargement of the left side of Figure la. 2. THEORY OF 3D-TEM A TEM image is in good approximation a projection of the 3D structure of the sample. This causes that information about the 3D ordering of the structure is lost. This is shown in Figure 2, where the projections of several 3D structures are depicted. Although stereo images of a sample can contribute to the understanding of the 3D ordering of the sample, electron tomography is the only technique that is able to provide a 3D electron microscope image of a sample. With electron tomography a 3D image is reconstructed from a series of (2D) TEM images, taken at different tilt angles. The resolution of a 3D reconstruction is approximately given by the relation: Resolution = 7t * thickness of the sample / number of images, assuming that a tilt series is taken over the full tilt range (+ 90 ~ with a constant tilt increment [3,4]. For example, when 150 images are taken of a 100 nm thick object, a resolution of 2 nm is obtained. In practice, however, the tilting range is limited to about +75 ~, due to physical limitations of the sample holder. This causes that the resolution of the 3D reconstruction is direction
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dependent and that structures are slightly elongated in the direction of the angular gap. Therefore, the tilting range should be chosen as large as possible. However, at high tilt angles the travelling path through a non-spherical sample may increase, thus causing loss of contrast due to multiple scattering of the electrons. For example, the path through a 200 nm thick sample will be 580 nm at 70 ~ tilt. 9
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Figure 2: Loss of information when projecting 3D structures into 2D images. Changing the specimen tilt angle causes a change of focus and shifting of the sample. These changes have to be corrected in order to obtain a pre-aligned data set. Correcting these changes manually is tedious, time-consuming and prune to error. Fortunately, with automated electron tomography [3], these corrections are carried out automatically. An essential aspect of this method is that the electron microscope images are collected digitally with a slow scan CCD camera 9 The digital images are used for the automatic compensation of image shift and focus change. After the acquisition of a pre-aligned data set, the data series has to be aligned more accurately 9 This can be done with the help of fiducial markers. Gold beads sprinkled on the grid or metal particles in/on the sample can serve as markers. By least-squares fitting of the positions of these fiducial markers, the data series is aligned. After the alignment of the data series the 3D reconstruction has to be computed 9The basics of the method were already proposed in 1917. It was stated that the projection of a 3D object is equal to a central section of the Fourier transform of that object. A data series thus provides many different central sections of the Fourier transform of the sample, thus filling the 3D Fourier space. By inverse Fourier transform of the obtained 3D Fourier space a 3D image of the original object is obtained. An algorithm that is often used for the computation of the 3D reconstruction is resolution-weighted back-projection [4]. Finally, the 3D image can be visualized in different ways. One way is to build a contour model in which the outer surface of the object is visible. Another possible way of visualization is cutting the 3D image in thin, nm-thick, slices. By looking at the individual slices one can exactly locate metal particles inside zeolite crystals in three dimensions with high resolution. If the sample is dose-sensitive, methods are available to collect datasets under low-dose conditions and at cryo-temperatures.
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Figure 3" Contour surface of an Ag/NaY crystal. Silver particles are coloured pink, the zeolite is coloured green. On the blue surface a black shadow-projection of the zeolite with silver particles is shown. The white arrow indicates the shadow of a silver particle that is located inside the zeolite.
80nm Figure 4" Imersection of Ag/NaY showing a silver particle (red) at or near the surface of the zeolite (yellow).
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A drawback of 3D-TEM Oust as is the case with 'normal' TEM) is that one investigates only a very small part of the sample. Therefore, additional (macroscopic) characterization techniques, such as XPS, are needed in addition to electron microscopy. 3. APPLICATION TO METAL/ZEOLITE SYSTEMS
Ag/NaY was made by suspending 500 mg NaY (LZY 54 from UOP, Si/AI ratio is 2.5) in 100 ml 4.0E-4 M AgNO3. An exchange efficiency of 100% results in a 0.9 wt% Ag catalyst. The suspension was stirred overnight at room temperature. After centrifugation from the solution, the loaded zeolite was washed and centrifuged two times with de-mineralised water and dried at room temperature. The material was dried at 150~ in argon and subsequently reduced at 150~ in hydrogen. A tilt series of Ag/NaY was taken at a magnification of 11.5k on a Philips CM 200 FEG microscope with a 1024 x 1024 CCD camera (pixelsize 1.12 nm). From a representative Ag/NaY crystal 143 images were taken from +70 ~ to -72 ~ with 1 degree intervals. For alignment purposes 7 dark features (silver particles) that could be followed throughout the whole tilt series were chosen as fiducial markers. The 3D reconstruction contains a volume of 1150xl 150xl 150 nm and has a resolution of 11 nm. X
Z
Y
X
Z
Figure 5: Slice through the reconstruction of Ag/NaY showing a silver particle of 10 nm inside the zeolite (arrow).
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In Figure 3 a contour model of the Ag/NaY sample is given. The colours of this model were obtained by selecting several bands of grey-values (e.g. the grey-values corresponding to the silver particles and the grey-values corresponding to the zeolite crystal) and assigning different colours to the different bands. The silver particles (shown in pink) are located at or near the external surface of the zeolite. The right-hand side of the image shows a shadow projection of the crystallite (black). In this shadow projection the shadow of a silver particle that is located inside the zeolite is also visible (white arrow). In Figure 4 an intersection of the same crystallite is shown. The placement of a silver particle (red) at or near the surface of the crystallite is clearly visible. The precise placement of the silver particles is best observed when the reconstruction is presented as a stack of thin slices. In Figure 5 one of these slices (X-Y) is given, showing a silver particle inside the zeolite. The orthogonal slices (X-Z and Y-Z) further support this conclusion. 4. CONCLUSIONS AND OUTLOOK To date, no techniques were available that could characterize an individual solid catalyst structure in three dimensions at high resolution. With electron tomography (3D-TEM), however, it is possible to obtain a 3D high-resolution image of a sample. From a series of 2DTEM images at different tilt angles a 3D reconstruction is calculated. The reconstruction can be visualized with contour surfaces and with slices through the reconstruction to investigate if particles are located inside or outside the porous material. Future work will involve the further development of 3D-TEM by combination with element analysis (EDAX). The Ag/NaY material will be studied more extensively to assess metal mobility under oxidizing and reducing atmosphere. Other systems currently under study are metal-loaded carbon nanotubes and mesoporous materials. ACKNOWLEDGEMENTS This work has been carried out under the auspices of NIOK, the Netherlands Institute for Catalysis Research, Report No. UU-99-3-02. The authors would like to thank the Netherlands Organization for Scientific Research (NWO) for financial support, grant 98037. The research of one of us (AJK) has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences REFERENCES
[ 1] K.P. de Jong, CATTECH 2 (1998) 87-94. [2] K.P. de Jong, Current Opinion in Solid State & Materials Science 4 (1999) 55-62. [3] A.J. Koster, R. Grimm, D. Typke, R. Hegerl, A. Stoschek, J. Walz, W. Baumeister, J. Struct. Biol. 120 (1997) 276-308 [4] J. Frank, Electron tomography, 1992, Plenum Press, New York [5] H. Beyer, P.A. Jacobs and J.B. Uytterhoeven, J. Chem. Soc., Faraday Trans. I 72 (1976) 674.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Interrogative Kinetic Characterization of Active C a t a l y s t Sites Using TAP Pulse Response Experiment. J. T. Gleaves a, G. S. Yablonsky ~ S. O. Shekhtmana, P. Phanawadee b "I)pt. of Chemical Engineering, Washington University, Campus Box 1198, One Brookings Drive, St. Louis, MO 63130, USA bDpt. of Chemical Engineering, Kasetsart University, Bangkok 10900, Thailand The approach for interrogative characterization of catalyst active sites using a combination of TAP pulse experiment is presented. This approach includes two principal steps: a state-defining experiment is used to determine the apparent kinetic parameters related to a given catalyst state, and a state-altering multi-pulse experiment is used to determine the number of active sites related to the same catalyst state. Using momentbased analysis, analytical expressions that correspond to both steps are obtained. The thin-zone TAP reactor that minimizes the influence of concentration gradient on observed kinetic characteristics and can be used to obtain information about very fast catalytic reactions is presented. I.
INTRODUCTION
The "interrogative kinetic" (IK) approach [1] that is an alternative to the traditional kinetic approach in heterogeneous catalysis. IK combines two types of nonsteady-state kinetic experiments, called "state-defining" and "state-altering" experiments. In contrast to traditional steady-state kinetics that attempts to obtain kinetic parameters for a well-determined "steady-state" of a catalytic reaction, IK attempts to systematically probe a variety of different states of a catalyst and to understand how one state evolves into another. An important dement of this approach is the rapid feedback between experiment and analysis that earl be likened to a "dialogue" between the researcher and the catalyst sample. The set of experiments that form an IK sequence represents a "question" (e. g. how does a change in the oxidation state of a catalyst changes its selectivity, or the activation energy of hydrocarbon conversion) that when answered leads to another question, and another IK sequence. A state-defining TAP experiment is one that does not significantly perturb the chemical state of a catalyst. The transient response data obtained in a state-defining experiment is a characteristic of that state. State-defining experiments involve reactions of probe molecules with a catalyst, and provide kinetic parameters such as rate
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parameters of adsorption and desorption. One example of a "probe reaction" is the irreversible chemisorption of a hydrocarbon on a metal oxide catalyst. A state-altering experiment is one in which the catalyst compositien is significantly changed. One type of a state-altering experiment is a TAP multi-pulse experiment in which the catalyst is exposed to a long series of reactant pulses. A statealtering experiment perturbs the catalyst and changes its composition or structure in some predetermined fashion. To complete an IK sequence, another state-defining experiment is performed to characterize the new state of the catalyst. 2. TAP-REACTOR CONFIGURATIONS The theoretical analysis of TAP pulse response data is based on the reactor model used to describe the catalyst bed through which the gas pulse travels. The simplest model is the "one-zone" reactor model in which the reactor is assumed to be uniformly packed with catalyst particles, and is heated uniformly over its entire length. An extensive theory for a one-zone TAP pulse response experiment has been developed and has been discussed in detail in a number of papers [ 1, 2]. A second type of reactor is the so-called "three-zone" reactor. In a three-zone reactor the catalyst zone is sandwiched between two beds of inert particles, called inert zones. Experimentally, the three-zone reactor has several advantages, and is the most commonly used reactor in TAP experimental studies. The main advantage of a three-zone reactor is that the catalyst zone can be more easily maintained in an isothermal condition. However, it is difficult to maintain uniform surface coverage in the catalyst zone because of the gas concentration gradient, which causes diffusion. It is also difficult to theoretically analyze three-zone TAP model. Currently, curve-fitting is the main approach used to describe three-zone experimental data. Recently, a new TAP reactor model that is a limiting case of a three-zone model and is called a "thin-zone" model was introduced [4]. In a thin-zone reactor, the thickness of the catalyst zone is made very small compared to the whole length of the reactor. The advantage of this configuration is that diffusional transport can be separated from chemical reaction, and influence of concentration gradients across the catalyst bed on the observed kinetic characteristics can be neglected. 3. MOMENT-BASED ANALYSIS A general theoretical approach that has been applied to an analysis of all three TAP reactor models is a moment-based approach. The feature which distinguishes the moment-based approach is that the set of TAP model equations uses moments as functions of axial coordinate rather than concentrations. The moments are related to the nature of a TAP experiment and describe the propagation of gas mixture throughout the reactor. The mathematical basis of these models is derived from the special initially and boundary conditions that reflect the "on-off" behavior of a TAP experiment. Moments have a clear physico-chemical meaning, (e.g. the zeroth and first moments are directly
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related to conversion and residence time, respectively) and can be readily measured. An important advantage of moment-based analysis is that in the case of linear or pseudolinear TAP models, moments can be calculated analytically in a compact form. The quantities of interest, e. g. kinetic parameters, can then be expressed analytically as functions of the moments of the exit flow. The results presented below are obtained using the moment-based approach. 4. STATE-DEFINING-ONE-PULSE TAP EXPERIMENT 4.1. Primary characterization of catalyst activity. Three-zone TAP reactor. The primary characterization of catalyst activity can be considered the first important step of an IK approach [2]. This characterization of catalyst activity should satisfy the following experimental and theoretical requirements: 1) insignificant change of the chemical composition and structure of the catalyst during the experimem (that is realized in a TAP state-defining experiment); 2) assumption of a first order reaction; 3) general analytical expression that relates catalyst activity and observed characteristics (e.g. conversion) The three-zone TAP reactor is the most general TAP reactor configuration, and one-zone and thin-zone TAP reactors are particular limiting cases. For the three-zone reactor, a general expression for irreversible catalytic processes on non-porous and porous materials has been obtained, and is given by:
l-X=
cosh(+) + a + sigh(V)
+=/t~Dgtkapp, where X is the conversion; D e is the effective Knudsen diffusivity; lcat is the length of the catalyst zone; r is the reactor parameter related to the geometry and transport properties (in a typical case, a=l); kopp is the apparent kinetic parameter that can be obtained from the experimental data using this expression. One- and two-step irreversible catalytic reactions on porous and non-porous catalysts were considered. 4.2. Thin-zone TAP reactor
The thin-zone TAP reactor model simplifies the interpretation of TAP data. The key idea of the thin-zone TAP reactor is to make the thickness of a catalyst zone very small compared to the length of the reactor. The rigorous mathematical basis of the thin-
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zone approach has been discussed [4]. A unique feature of a thin-zone reactor is that the influence of concentration gradients in the catalyst zone on the observed kinetic characteristics is insignificant. As a result, a thin-zone TAP reactor can be viewed as a "diffusional CSTR" For example, the conversion for irreversible adsorption/reaction in a thin-zone reactor is governed by the same relationship a for a first-order reaction in a CSTR, that is given by: kat,p "d~ff
X
~res,cat
~ .
b
~.diff
1 + ,~app ~res,cat
and ~ff tree,cat = 6b
where r,~.c~ ~ is the residence time for diffusion throughout the catalyst zone; lcat and lj,,2 are lengths of the catalyst zone and the second inert zone, respectively. The thin-zone reactor is particularly useful for investigating fast chemical reactions since the extent of reaction can be controlled by the thickness and position of the catalyst zone. 5.
STATE-ALTERING EXPERIMENT- MULTI-PUSLSE TAP EXPERIMENT. 5.1. Problem of active site number determination.
The concept of an "active site" has been a pivotal idea for revealing relationships between structure and activity in heterogeneous catalysis. The determination of the number of active site is a primary problem in heterogeneous catalysis [5, 6], and there is an important need for new techniques that can reveal the number of active sites on complex catalytic materials. Traditional methods of active site number determination use chemisorption, and determine the total amount of the reactant that can be adsorbed on the catalyst per unit surface area. However, the conditions of a complex catalytic reaction are very different from the adsorption measurement conditions. Moreover, the number of active sites is very dependent on catalyst preparation conditions, and composition/structure changes that occur during the catalytic process.
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5.2. A new method to determine a number of active sites based on TAP multi-pulse experiment.
To determine the number of active sites using TAP pulse response experiments a combination of state-defining and state-altering experiments, are used. As previously discussed, in a state-defining experiment, the chemical state of the catalyst, particularly surface coverage (number of unoccupied active sites), changes insignificantly. In contrast, in a state-altering experiment that uses a long sequence of pulses, the surface coverage (number of unoccupied active sites) as well as the quantities observed in a onepulse experiment gradually change as a function of the pulse number. Monitoring the change of the observed quantities, the number of "working" active sites at the beginning of a series of pulses can be determined. An important advantage of this method is that number of active sites can be found in a short series of pulses that produces a detectable change in observed quantities, and provides the number of active sites for a particular catalyst state. This method can be viewed as a "differential" method. In TAP studies, it is also possible to realize an "integral" method that can be likened to traditional adsorption methods. Using perturbation theory, analytical expressions for determining the number of active sites and kinetic characteristics per one site have been developed for the three main TAP-reactors types, i.e. one-, three-, and thin-zone TAP reactors. For example, in the case of irreversible adsorption, for a one-zone reactor, the following analytical expression can be obtained:
Mo(m) = 1 - X = M0(0)[1
Scatas
m
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3Ml(O)
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a'X c~
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Mo(m ) = 1 - X = Mo(O ) 1 + Scatas m ( I - Mo(O)) 2 or
_~gX= Np c~
- - - - - Mo (0)(1- Mo (0)) 2 ,
Scatas
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where M~(m)is the n.th moment of the exit flow time dependence as a function of a pulse number, m=0, 1, 2, .... ,Np is the number of molecules in the inlet pulse, $r is the surface area of the catalyst, and a, is the number of active sites per unit area (mole/cm2). The number of active sites per unit area, a,, can be determined from the above equation using the experimentally measured characteristics, particularly moments, as functions of pulse number. 6.
CONCLUSIONS
The IK approach for characterization of catalyst active sites using a combination of TAP pulse experiment has been developed. A theoretical approach using a momentbased analysis has been developed and applied to all three TAP reactor models. The IK characterization approach includes two principal steps: 1) a state-defining experiment is used to determine apparent kinetic parameters that are related to a given catalyst state, 2) a state-altering multi-pulse experiment is used to determine the number of active sites related to the same catalyst state. A special attention has been paid to the thin-zone TAP experiment that minimizes the influence of concentration gradient on the observed kinetic characteristics and can be used to obtain information about very fast catalytic reactions. REFERENCES
1. I.T. Gleaves, I. R. Ebner, T. C. Kuechler, Catal. Rev. Sci. Engng., 30 (1988) 49. 2. I. T. Gleaves, G. S. Yablonskii, P. Phanawadee, Y. Schuurman, Appl. Catal., A: General, 160 (1997) 55. 3. G.S. Yablonsldi, S. O. Shekhtman, S. Chen, G. T. Gleaves, Ind. Eng. Chem. Res., 37 (1998) 2193. 4. S. O. Shekhtman, G. S. Yablonsky, I. T. Gleaves, S. Chen, Chem. Eng. Sci., 54 (1999) 4371. 5. M. Boudart, Chem. Rev. 95 (1995) 661. 6. F.H. Ribeiro, A. E. Schach yon Wittenau, C. H. Bartholomew, G. A. Somorjai, Catal. Rev.- Sci. Eng., 39 (1997) 55.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
UV resonance Raman spectroscopic identification of transition metal atoms incorporated in the framework of molecular sieves Guang Xiong, Can Li*, Zhaochi Feng, Jian Li, Pinliang Ying, Hongyuan Li and Qin Xin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics Chinese Academy of Sciences, P. O. Box 110, Dalain 116023, China Fax: 86-411-4694447; Email:
[email protected] The transition metal 'atoms in the framework of molecular sieves are characterized by the UV resonance Raman spectroscopy. An UV laser line (244 nm) is chosen to excite the charge-transfer transition between the framework oxygen and transition metal atoms in TS-1, Fe-ZSM-5, V-MCM-41, etc. The new Raman bands at 490, 530 and 1125 cm -~ are observed for TS-1, and the bands at 515, 1017 and 1170 cm l are detected for Fe-ZSM-5. V-MCM-41 also gives a new Raman band at 1070 cm l. The appearance of these Raman bands is due to resonance Raman effect since the laser line at 244 nm locates in the UV-visible absorption of transition metal atoms in the framework. Therefore the characteristic Raman bands solely associated with Ti, Fe and V atoms in the framework are selectively enhanced by resonance Raman effect, and the transition metal atoms in the framework are definitely identified by the UV resonance Raman spectroscopy. 1. INTRODUCTION Molecular sieves with framework atoms substituted by transition metal atoms in their framework have been considered as a new class of catalysts showing remarkable activity and selectivity for a number of oxidation reactions using dilute H202 as the oxidant [1]. The catalytic property is mainly attributed to the isolated transition metal atoms in the framework of the molecular sieves. Therefore, characterization of the transition metal atoms incorporated in the framework is the most important issue of the study. However, it remains difficult to know how and whether the transition metal atoms are incorporated into the framework of a molecular sieve, despite the extensive efforts using many techniques, such as XRD, FT-IR, UV-visible, NMR, ESR and Raman spectroscopy. In the present work, a new approach, UV Resonance Raman spectroscopy, is used to identify the transition metal atoms in the framework of molecular sieves based on resonance Raman effect since there are chargetransfer transitions between the framework oxygen and transition metal atoms. The characteristic Raman bands solely associated with the framework transition metal atoms were selectively enhanced, so that the transition metal atoms in the framework of a molecular sieve
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can be definitely identified. Among the examples, titanium atoms in TS-1, iron atoms in FeZSM-5 and vanadium atoms in V-MCM-41 were successfully identified. Therefore UV resonance Raman spectroscopy has opened up the possibility to identify the framework transition metal atoms in molecular sieves. 2. E X P E R I M E N T A L TS-1, Fe-ZSM-5 and V-MCM-41 were synthesized following the methods reported in the literature [2, 3]. UV Raman spectra were recorded on a home-made UV Raman spectrometer including four main parts: a UV cw laser, a Spex 1877 D triplemate spectrograph, a CCD detector, and an optical collection system. A 244- nm line from an Innova 300 FRED laser and a 488 nm line from Spectra Physics were used as excitation sources. The laser powers at the samples were kept below 2.0 mw and 100 mw for 244nm and 488 nm, respectively. The acquisition time was usually less than 5.0 min. The spectral resolution was estimated to be 1.0 cm 4. UV-visible Diffuse Reflectance spectra were recorded on a Shimadzu UV-365 UVVIS-NIR Recording Spectrophotometer. The molecular sieves were also characterized by XRD and FT-IR spectroscopy. 3. RESULTS AND DISCUSSION As shown in Fig. 1, the electronic transition absorption of transition metal atoms substituted molecular sieves frequently appears in the UV region. The absorption is assigned to the charge transfer transition between transition metal and oxygen atoms in the framework. This transition involves the excitation of an electron from a 7t bonding molecular orbital consisting of oxygen atomic orbital to a molecular orbital that is essentially a titanium atomic d orbital. This offers an opportunity to characterize the transition metal atoms substituted molecular sieves by resonance Raman effect. When the frequency of the laser is close to and/or within the electronic absorption band of the transition metal atoms in the framework,
/ o\ / \ o
/Si\/
G o M\ /
O Si \
\
f r a m e w o r k sites 0 ( 2p ) 220 nm Y i ( 3d ) 0 ( 2p ) 250 nm F e ( 3d ) O(2p)
280 nm ~ V(3d)
Fig. 1. Charge transfer transition between oxygen and transition metal atoms in the framework of molecular sieves.
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_
N
Pt>Pd). The OSC at 400~ of Ce0.63Zr0.3702 is multiplied by a factor of almost 4 in the presence of 0.3%Rh. Metal particles are assimilated to portholes for the subsequent migration of oxygen on the support. In the same way, the activity at 350~ of 0.3%Rh/CeO2 in the isotopic exchange of ~802 is 3 orders of magnitude larger than in the absence of rhodium. 1. INTRODUCTION As pollution control problems appeared, more and more work has been done in the field of environmental catalysis. Especially for the reduction of automotive pollution, many studies dealt with the optimization of three-way catalysts (TWC). One of the clue parameters in developing such catalysts was the control of the oxygen mobility. In fact surface mobility is involved in many catalytic processes, like oxygen reversible storage or oxygen transfer in oxidation catalysts. Thus many studies have been devoted to the synthesis and subsequent characterization of new cerium-based mixed oxides supports [ 1-12]. Cerium-zirconium solid solutions were shown to be good candidates with enlarged Oxygen Storage Capacity (OSC) [2,3,5,8,9] and improved redox properties [2,12]. However, only a few studies have been devoted to the quantification of oxygen surface diffusion on cerium-zirconium mixed oxides supported metals. In addition to OSC measurements, isotopic exchange is an adequate technique for the study of chemisorbed species mobility [13]. This paper is concerned with the characterization of CexZr(l.x)O2 solid solutions based catalysts (x = 0, 0.63 and 1). Three noble metals are under study : Rh, Pt and Pd. The influence of the metal is reviewed on the basis of OSC measurements and isotopic exchange experiments.
2. EXPERIMENTAL Oxides, calcined at 900~ were directly provided by Rhodia Terres Rares (La Rochelle, France). Rh, Pt and Pd catalysts were prepared by impregnation of the oxides with aqueous
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solutions of Rh(NO3)3, Pt(NH3)4(OH)2 and Pd(NO3)2 respectively. All samples were pretreated under flowing air (30 ml.mn"l) at 450~ for 4 hours (fresh catalysts). To study their stability, catalysts were also aged at 900~ for 4 hours under flowing dry air (aged catalysts). The oxides structure was investigated by XRD using a Siemens D500 diffraetometer. Crystalline phases were identified by comparison of experimental diffractograrns with ICDD files. Crystallite sizes determination was based on the Debye-Scherrer relation. Surface areas were measured by adsorption of N2 at -196~ with a Micromeritics Flowsorb II. This apparatus uses the single point method. The metal dispersion was calculated from HE chemisorption experiments. The procedure of these measurements had to be optimized in order to prevent hydrogen spillover onto the support [ 14]. Oxygen Storage Capacities measurements, first introduced by Yao [15], were carried out on an home-made apparatus previously described [16]. Two types of information may be obtained 9the relative kinetics of the reduction-oxidation process (OSCC) and the amount of oxygen "immediately" available in the material (OSC). Isotopic exchange experiments consist in monitoring, by mass spectrometry, the oxygen isotopomers (1602, 160180, 1802)partial pressure. Exchange proceeds between labeled oxygen atoms, initially introduced in the gas phase, and oxygen atoms of the oxide support. The rate of exchange (Re) may be determined as well as the mechanism of the reaction (simple or multiple heteroexchange) depending on the relative partial pressure in 1602 and 160180 [13, 17-21]. IR studies of 02 adsorption were performed using self supported oxides wafers. Samples were pretreated in situ in flowing 02 at 400~ over 12h. Spectra were collected at a resolution of 4crnq on a Nicolet Magna 550 FT-IR spectrometer. 3. MAIN RESULTS
3.1. Materials The main characteristics of the materials used in this study are summarized below. Table 1 Main physicochemical properties of M/CexZrt~.x)O2 samples (M = Rh, Pt, Pd - x = 0, 0.63, 1) Support
0.3 o'ARh/CeO2 0.3%Rh/Ceo.63Zro.3702 0.3 % Rh/Z rO2 l%Pt/CeO2 1%Pt/Ceo.63Zro.3702 1%Pt/ZrO2 0.5%Pd/CeO2 0.5%Pd/Ceo.63Zro.3702 0.5%Pd/ZrOz
Fresh Catalyst
Structure
Crystallite size(A)
Surface area (m2.gq)
Dispersion (%)
cubic cubic mo noc linic cubic cubic monoclinic cubic cubic monoclinic
260 110 23 0 260 110 230 260 110 230
28 41 12 28 40 12 27 41 11
56 85 44 53 70 34 56 60 49
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All oxides were found to be purely monophasic. Ceria and cerium-rich mixed oxides are cubic while zirconia is monoclinic. Zirconium has a stabilizing effect on the structure. In fact, even after calcination at 900~ mixed oxides maintain a surface area of about 40 m2.g"~ compared to approximately 10 and 30 m2.g~ in the case of zirconia and ceria respectively. Looking at the metallic phase, it also appears that the dispersion is favored on mixed oxides.
3.2. Oxygen Storage Capacities OSCC measurements confirmed that the limiting step in the redox process of these bare oxides is the reduction [22]. This may be seen from the kinetics of the process during the measurements 9the reduction by CO is slow while the re-oxidation by 02 is total and instantaneous. In the case of oxides-supported metals, the reduction is still the limiting process but the re-oxidation is not complete. This observation evidences a strong metalsupport interaction, modifying the redox properties of the oxides. In close interaction with a metal, cerium could be irreversibly over-reduced. Table 2 shows the effect on the OSC of the introduction of Zr in the ceria lattice. As seen previously an optimum exists for Ce0.63Zr0.3702 [22,23]. This solid has an OSC four times larger than the one measured for ceria. Table 2 Oxygen Storage Capacity (OSC) at 400~ of M/CexZq~.x)O2 samples (M = Rh, Pt or Pd - x = 0, 0.63 and 1) OSC at 400~ (lxmolCO2.g"l) * x=0 Ce~Zro.x~O 2 0.3%Rh/CexZro.x~O 2
l*/oPtlCexZro.~02 0.5*/.Pd/Ce~Zro_~O2
X = 0.63
x= 1
Fresh
Aged
Fresh
Aged
Fresh
Aged
101 63 57
0 32 46 42
755 716 727
202 543 615 576
132 115 105
48 147 96 113
* amount of C02 produced after the first pulse of CO during alternate CO and 02 pulses. The presence of metallic particles on the surface of these oxides also modifies their OSC. All studied metals (Rh, Pt, Pd) approximately have the same effect on the increase of the OSC. The most efficient is Rh, especially in the case of the mixed oxide where an increase by a factor of almost 4 is observed. Ageing at 900~ does not considerably affect the OSC of those catalysts. However, Pt and Pd seem to be the best promoting metals for oxygen storage on aged M/Ce0.63Zr0.3702 catalysts.
3.3. 1SO2Isotopic Exchange The whole process of exchange was described, step by step, in an earlier publication [24]. Three steps may be separately studied. Direct exchange with the support, occurring at relatively high temperature, was investigated during preliminary studies on bare oxides. The
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350
results of these studies are represented in Figure 1 by line D. Informations on the oxygen activation on the metal particles may be accessed from homoexchange measurements (line A). Finally, a thorough study of the heteroexchange gives informations on the limiting step at a given temperature. In fact, during the exchange, two regimes can be differentiated 9line B = exchange is limited by the adsorption/desorption on the metal particle, line C = exchange is controlled by the diffusion on the oxide surface. The influence of the metal on the oxygen mobility at the ceria surface was investigated as a function of temperature. Results are presented in Figure 1. 4.5 3.5
Fig. 1 9Arrhenius plot of 1802 exchange on M/CeO2 samples (M = Rh, Pt or Pd) [from isotopic exchange (full symbols) and equilibration (open symbols) results, with I = Rh, & = Pt, @ = Pd, ~ = ceria]. A, B, C and D 9see text.
A
Z5 A
E
c
1.5
9 0.5 iv,
=: -0.5
B
_1
-1.5 -2.5 -3.5
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
l O 0 0 f f (K "1) I
550
I
I
450
350
,
t
250
T (~
For all M/CeOz systems, the same behavior is observed (Figure 1). The change in the rate limiting step is observed at 300, 420 and 480~ for Rh, Pt and Pd respectively. Rhodium is the most active metal for the activation of oxygen (equilibration) and oxygen exchange. For Rh/CeO2 the oxygen surface diffusivity may be accessed between 310~ and 370~ In that temperature range, direct exchange with the support may be neglected and oxygen adsorption-desorption is fast. The surface diffusion coefficient (Ds) on ceria was calculated to be 3.10~Sm2.s l at 350~ The activation energy of the process is 48kJ.mol ~. On the opposite, palladium is practically inactive for 02 exchange : all measurements are perturbed by direct exchange with the support. The modification of the surface mobility of oxygen on various oxides was also examined by looking at 3 differem systems" Rh/CeO2, RIgCeo.63Zro.3702 and Rh/ZrO2. The activity of these systems was simply studied in the heteroexchange of oxygen. At low temperature, as previously, exchange is limited by the adsorption-desorption of oxygen on the metal particles. In fact, as we can see on Figure 2, all lines merge whatever the support is. No influence of the support is observed. At higher temperature, oxygen surface diffusion is rate limiting. In that case, the influence of the oxide is clear : oxygen mobility is higher on Ceo.63Zro.3702 than on ceria and
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
351
than on zirconia. The rate of exchange on Rh/Ceo.63Zro.3702 at 350~ is about 2 and 9 times larger than on ceria and zirconia respectively.
A
E2
Fig. 2 9Arrhenius plot of lgo 2 exchange on Rh/CexZr(~.,,)O2 samples (x = 0, 0.63, l) [from isotopic exchange results, I
C
Q 1 iv =0 ,.J
=
Ce0.63Zr0.3702, ~
=
CeO2,
A = ZrO2]
3
i
1.2
1.4
i
1.6
l
1.8
i
i
2
2.2
IO001TIK") 3.4. Surface oxygen species
In a study conducted in parallel, the mechanism of exchange was also shown to vary depending on the nature of the oxide [22]. Multiple heteroexchange was related with the presence ofbinuclear oxygen species on the surface [25]. The specific behavior of mixed oxides was then tentatively correlated with the population in superoxides at the surface. These species, identified by FT-IR, are characterized by a single band at 1126 cm -l. 12
250
A ,t--
A
d
tO w o
10~~
200
150
E
6 _~ ~
o
.~
E
to
tn 0
'E
100
50
2 ~ Q. Z =
Fig. 3 :Correlation between the OSC of CexZr(l.x)02 samples (x = O, 0.63, 1) and the amount of superoxides formed on the surface upon adsorption of oxygen (0.5mbar) at room temperature.
o zirconia
mixed-oxide
ceria
The amount of superoxides was estimated from the integrated area of the band Vo-o at l126cm "1 and normalized to lg of sample. As shown in Figure 3, a good agreement is
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observed. The high oxygen mobility at the surface of mixed oxides seems to be closely related to the presence of binuclear species at the surface. Then, oxygen could be "transported" as superoxides species at the surface of these materials.
4. CONCLUSIONS Mixed oxides impregnated metals were shown to have very large Oxygen Storage Capacities and interesting activities in the isotopic exchange of oxygen. The OSC is multiplied by a factor of 4 in the presence of a metal and the mobility of oxygen on Ce0.63Zro.3702 is 9 times greater than on zirconia. Ageing does not drastically affect the OSC of these materials. Pt and Pd catalysts are the most stable systems. Their specific behavior towards oxygen was correlated with the surface population in dioxygen species (superoxides, peroxides). In fact, oxygen could be mobile on these oxides as superoxides entities. REFERENCES
1. M. Yashima and K. Morimoto, J. An~ Ceram. Soc., 76 (1993) 2865. 2. P. Fornasiero, R. Di Monte, G. Ranga Rao, J. Ka~par and A. Trovarelli, J. Catal., 151 (1995) 168. 3. P. Fornasiero, G. Balducci, J. Kagpar and M. Graziani, Catal. Today, 29 (1996) 47. 4. P. Fornasiero, R. Di Monte and J. Ka~par, J. Catal., 162 (1996) 1. 5. A. Trovarelli and F. Zamar, J. Catal., 169 (1997) 490. 6. P. Vidmar, P. Fornasiero, J. Ka~par and G. Gubitosa, J. Catal., 171 (1997) 160. 7. A. Suda, T. Kandori and Y. Ukyo, J. Mat. Sci. Lett., 17 (1998) 89. 8. D. Terribile, A. Trovarelli and J. Llorca, Catal. Today, 43 (1998) 79. 9. S. Rossignol, Y. Madier and D. Duprez, Catal. Today, 50 (1999) 261. 10. A. Trovarelli, F. Zamar and S. Mashio, Chem. Comm., 9 (1995) 11. 11. P. Fomasiero, G. Balducci, R. di Monte, J. Ka~par, V. Sergo, G. Gubitosa, A. Ferrero and M. Graziani, J. Catal., 164 (1996) 173. 12. G. Vlaic, P. Fomasiero, S. Geremia and J. Ka~par, J. Catal., 168 (1997) 380. 13. D. Martin and D. Duprez, J. Phys. Chem., 100 (1996) 9429. 14. Y. Madier, Ph-D Thesis, University of Poitiers (1999). 15. H.C. Yao and Y.F. Yu Yao, J. Catal., 86, (1984) 254. 16. S. Kacimi, J. Barbier Jr., R. Taha and D. Duprez, Catal. Lett., 22 (1993) 343. 17. E.R.S. Winter, Adv. Catal., 10 (1958) 196. 18. K. Klier, J. Novfikovfi and P. Jiru, J. Catal., 2 (1963) 479. 19. G.K. Boreskov, Adv. Catal., 15 (1964) 285. 20. E.R.S. Winter, J. Chem. Soc., 1 (1968) 2889. 21. J. Novfikovfi, Catal. Rev., 4 (1970) 77. 22. Y. Madier, C. Descorme, A.M. Le Govic and D. Duprez, J. Phys. Chem., 103(50) (1999) 10999. 23. A. Trovarelli, F. Zamar, J. Llorca, C. de Leitenburg, G. Dolcetti and J.T. Kiss, J. Catal., 169 (1997) 490. 24. D. Duprez, Stud. Surf. Sci. Catal., 112 (1997) 13. 25. S. Rossignol, F. G6rard and D. Duprez, J. Mat. Chem., 9 (1999) 1615.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
SO2-promoted propane oxidation over Pt/AI203 catalysts Adam F. Leea, Karen Wilsonb and Richard M. Lambertb aDepartment of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom. bDepartment of Chemistry, University of Cambridge, Cambs. CB2 1EW, United Kingdom. The origin of SO2-promoted propane oxidation over Pt/alumina catalysts has been elucidated utilising spectroscopic and kinetic measurements over dispersed Pt/A1203 and model y-AI203/Pt(111) systems. HR ELS, NEXAFS, TPRS and XPS studies show that under oxidising conditions SO2 promotes the dissociative chemisorption of C3H8 at both Pt and support sites via surface sulphation. Interfacial sulphoxy species play a key role in activating propane for subsequent combustion at metal sites via a bifunctional spillover mechanism. The loading dependence of the promotional effect is rationalised in the light of X-ray and TEM measurements that reveal catalyst sulphation is accompanied by the reduction and concomitant sintering of small (3 ML, -20-30 % of surface Pt(lll) sites 300 K 0 2 remain exposed. The saturation oxygen uptake over AI/Pt(111) surfaces vastly exceeds that 8 0 0 K (3= possible over clean Pt(lll). Photoemission measurements also reveal a large chemical shift in the A1 66 eV Auger (Figure 2) and AI 2s transitions following high O2 exposures at 300 K. These shifts are further enhanced upon annealing above 800 K, and their magnitude and sign are consistent with A1203 formation. Multilayer films retain some interfacial AI in . , '_ ., I i' metallic form within a disordered alloy phase 9 " do ' 6o ' 40 of approximate composition Pt2Al3 [5]. In Kinetic Energy (eV) contrast, oxidation of a monolayer Pt3AI Fig. 2. AES spectra following 300 K growth surface alloy efficiently pulls all surface A1 into and oxidation of 5 ML of AI on Pt(111) an overlayer oxide. High temperature oxidation also induces the appearance of well-defined vibrational losses at 450, 670 and 870 cm~. These modes are characteristic of crystalline yalumina, and indeed coincide with the observation of a sharp (4~/3x4~/3)R30~ LEED pattern consistent with an expanded A1203 overlayer [6].
化
|,
C l e a n
(xl/s)
Pt
3.2 R e a c t i v i t y o f 7-A1203/Pt(I 11) s u r f a c e s
The subsequent exposure of Pt-supported alumina films towards SO2 was explored under oxidising conditions. Over clean Pt(111), XPS, HREELS and NEXAFS show such treatments result in weakly-bound surface SO4, prone to electron-stimulated decomposition. In the ].2 I Inteffacial / ~ presence of Al203 films ESD is greatly reduced, and both the thermal stability and -~ ] concentration of SO4 are increased. This = stabilisation of surface sulphoxy species, "~ 0.8 particularly located at the Pt-A1203 interface, has a profound impact on the .~ 0.6 surface reactivity towards alkane activation (Figure 3). The sticking probability of both =o0.4 bare Pt [7] and A1203/Pt(111) [6] surfaces / for dissociative C3Hs chemisorption is ~0.2 essentially zero under UHV. Although adsorbed SO4 facilitates propane oxidation 0 0 1 2 3 over Pt(lll) alone, interfacial sulphate Alumina Film Thickness / ML species promote a further five-fold rise in Fig. 3. Temperature programmed reaction of C3H8 over SO4/AI203/Pt(111) surfaces r~
~
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
356
the yield of oxidation products. This unique enhancement is most likely mediated by a reverse spillover process, in which propane, dissoeiatively chemisorbed at sulphated interfacial Pt-alumina sites, migrates onto neighbouring metallic Pt centres where it undergoes reaction with coadsorbed atomic oxygen (Schemel).
. 'k...-"
,y~.. i~.,,,
so:
:'~ "~ It Q~O/d
I
Platinum
O H %
IH20 /
.,H
i1~~~1 o ? I -~ I_ P ! ~ ~ - I ..........................
~o:
/fl
)
~-
AT
Scheme 1. Mechanism for SO4-promoted C3I-Is chemisorption and oxidation at the AI203/Pt(111) interface
3.3 Structure-reactivity of dispersed Pt/Ai203 catalysts
Though the preceding single-crystal studies provide an explanation for the role of SO2 in enhancing propane chemisorption over Pt/AI203 catalysts, they do not explain the structuresensitive nature of this promotion. Light-off measurements reveal low loading (100 A between the 0.05 and 9 wt% samples. These trends coincide with changes in the near-edge (XANES) region consistent with a transformation from oxidic to metallic Pt. Sulphation lifts this loading dependence, reducing the 0.05 and 3 wt% samples' white-lines and inducing XANES features representative of their higher loading and bulk Pt counterparts. Sulphated 0.05 wt%
9 wt% Pt foil
b !
-40
60 160 Energy above edge(eV)
260
-40
!
60 160 Energy above edge(eV)
|
260
Fig. 5. Normalised Pt Lin-edge XAFS spectra of a) flesh and b) sulphated Pt/Al203 catalysts as a function of Pt loading. The corresponding fitted Pt local coordination spheres, shown in Figure 6a-b, confirm the presence of a PtO2 phase within the fresh 0.05 wt% Pt/A1203 catalyst, which is reduced to Pt metal upon sulphation. These observations may be rationalised in terms of the simultaneous sulphate-induced reduction and sintering of oxidic Pt present within fresh, low loading Pt/AI203 catalysts. Oxidic Pt surfaces exhibit lower oxidation activity than metallic Pt [9,10].
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
358
化
/
~ , ,
~
Fresh
~
0
2
4
6
Interatomic Distance (A)
8
0
/
~
2
,
Sulphated
\
~
4
3wt%
6
8
Interatomic Distance (A)
Fig. 6. Pt Lni-edge pseudo-radial distribution functions of fresh and sulphated Pt/A1203 catalysts as a function of Pt loading Our investigations suggest that SO2 promotes propane oxidation over Pt/ml203 catalysts in three ways. First, sulphation modifies the alumina support, generating crystalline aluminium sulphate and surface sulphoxy groups. Sulphated alumina surfaces greatly enhance the dissociative chemisorption and subsequent oxidation of propane on neighbouring partiallyoxidised Pt sites. Second, surface sulphate enhances direct dissociative propane chemisorption onto metallic Pt clusters in the absence of alumina mediation. Both these factors contribute to the small enhancement in oxidation rate (ATs0 = -50 K) observed for high loading (9 wt%) Pt/AI203 catalysts. Third, we have demonstrated the sulphate-induced reduction and sintering of highly-dispersed platinum oxide particles, predominant for low loading (0.05 and 3 wt%) Pt/AI203 catalysts. The resultant large metallic platinum particles exhibit higher activities towards propane activation. The principal promotional effect of SO2 upon low loading Pt/AI203 catalysts is thus the reduction of PtO2 to Pt. This eliminates the structural differences with their higher loading counterparts, thereby lifting the dependence of propane oxidation activity on Pt concentration observed for unsulphated samples. REFERENCES 1. R.W. McCabe and J.M. Kiesenyi, Chem. Ind., (1995) 605. 2. G. Sadowski, and D. Treibmann, Z. Chem., 19 (1979) 189. 3. H.C. Yao, H.K. Stepien, and H.S. Gandhi, J. Catal., 67 (1981) 23 7. 4. K.M. Adams, J.V. Cavataio, and R.H. Hammerle, Appl. Catal. B, 10 (1996) 157. 5. K. Wilson, A.F. Lee, J. Brake and R.M. Lambert, Surf.Sci., 387 (1997) 257. 6. K. Wilson, A.F. Lee, C. Hardacre and R.M. Lambert, J. Phys. Chem. B, 102 (1998) 1736. 7. K.Wilson, C. Hardacre and R.M. Lambert, J. Phys. Chem., 99 (1995) 13755. 8. H. Moselhy, G. Pokol, F. Paulik, M. Arnold, J. Kristof, K. Tomor, S. Gal and E. Pungor, J. Therm. Anal., 39 (1993) 595. 9. K. Otto, J.M. Andino, and C.L. Parks, J. Catal., 131 (1991) 243. 10. R. Burch and M.J. Hayes, J. Molec. Catal. A: Chem, 100 (1995) 13.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
Studies in Surface Science and Catalysis 130 A. C o r n , F.V. Melo, S. Mendioroz and J.L.G.Fierro (Editors) 0 2000 Elsevier Science B.V. All rights resewed.
Selective Oxidation of Toluene to Benzaldehyde: Investigation of StructureReactivity Relationships by in situ-Methods A. Briickner, U. Bentrup, A. Martin, J. Radnik, L. Wilde and G.-U. Wolf
Institut fir Angewandte Chemie Berlin-Adlershof e.V., Richard-Willstatter-Str. 12, D- 12489 Berlin, Germany (VO)2P207and potassium-doped V205catalysts have been studied in the selective oxidation of toluene to benzaldehyde by in situ-EPR, -FTIR, -XRD, -UVNIS and -XPS. In (V0)2P207, Brensted surface sites formed under reaction conditions favour strong product adsorption and, thus, total oxidation. They can be blocked by adding pyridine, which improves catalytic performance. K-V205 catalysts are markedly reduced during reaction. Both V" and v4'species are likely to be active in the catalytic redox cycle. Crystalline Ko.5V205formed on the catalyst surface under feed probably lowers the catalytic performance due to structural reasons. 1. INTRODUCTION
From the industrial point of view, benzaldehyde is the most important aromatic aldehyde. It can be obtained by the selective gas-phase oxidation of toluene using catalysts based on vanadia. In general, low conversion rates are preferred to avoid deeper oxidation. However, the benzaldehyde selectivities obtained so far do not exceed 40 - 60 % [I]. Thus, improvement of the catalytic performance by rationalized catalyst design is of considerable industrial interest. This approach requires detailed knowledge on structure-reactivity relationships which can best be obtained by investigating the catalysts under working conditions. In this work, catalysts based on vanadium oxide were studied during reaction using in situ-EPR, -FTIR, -XRD, -UVMS and -XPS. The spectroscopic results are compared to the catalytic performance elucidated in catalytic tests. 2. EXPERIMENTAL
Catalysts: Crystalline (V0)2P207 was obtained by calcination of the precursor VOHP04. 0.5 H20 (prepared in aqueous medium [2]) in N2 atmosphere at 873 K (sample VPP = 4.7 m2 g-'). Potassium doped V ~ O S 1, SBET= 6.4 m2 g-l) or 1023 K (sample VPP 2, SBET catalysts were obtained i) by the incipient wetness method using 20 g V2O5and 25 g of a 10 % aqueous solution followed by drying for 8 h at 403 K (VOK 1) and ii) by impregnation of 18.19 g V205with 50 rnl of a 1.7 % aqueous K2S04 solution, followed by evaporation at 343 K and drying at 403 K (VOK 2). These catalysts are characterized by the following properties: W S N = 0.06910.03411 (VOK 1) and 0.0510.02511 (VOK 2); SBET= 4.0 m2 g-l (VOK 1) and 4.5 m2 g-l (VOK 2); mean vanadium valence state determined by potentiometric titration [3] 4.947 (VOK 1) and 4.858 (VOK 2). Methods: In situ-EPR measurements were performed in X-band (ELEXSYS 500- 10112, Bruker) using a home made fixed-bed reactor [4]. The product stream was trapped in ethanol and analyzed by off line-GC (Shimadzu GC 17AAF). Transmission in sztu-FTIR spectra (Bruker
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
IFS 66) were recorded from self-supporting disks mounted in a heatable flow cell. In situ-XRD analyses were performed in a XRK reactor chamber (A. Paar). Quasi-in situ-XPS spectra were recorded at 293 K using a MgKa-source (ESCALAB 220 iXL, VG Instruments) after treating the catalysts under working conditions in a reaction cell installed in the lock to the analysis chamber. Peak positions were corrected with respect to the C Is signal at 284.5 eV. In situW M S measurements were performed using a Cary 400 W M S spectrometer (Varian) equipped with an in situ-difise reflectance accessory (praying mantis, Harrick). Catalysts were diluted with a-A1203 (calcined at 1473 K for 4h). The reaction cells used with the different methods were connected to a gadliquid dosing apparatus. Feed composition and contact time were chosen so as to be similar to the conditions of the catalytic tests performed separately using a fixed-bed U-tube quartz reactor. If not stated otherwise, a reactant mixture of molar ratio aidtoluene = 100 (total flow: 34 ml min-') was used. 3. RESULTS AND DISCUSSION 0 2 P 2 0 7 : The crystal structure of (VO)2P207 contains intinite double chains of VOa octahedra which are coupled by effective spin-spin exchange interactions giving rise to a narrow EPR singlet (Fjg. I ) . The stronger the exchange interaction the smaller the line width and, thus, the higher the signal amplitude. Under reaction conditions, a temporal decrease of the signal intensity due to line broadening is observed which arises from a perturbation of the spin-spin exchange between neighbouring VO" ions. A similar behaviour of the EPR signal has been observed, too, for a number of different v4?0 catalysts during the ammoxidation of toluene to benzonitrile [5, 61. The perturbation results from changes in the electron density at surface vo2' centres which are caused on the one hand by alternating reduction and re-oxidation steps according to a Mars-van Krevelen mechanism and on the other hand by the adsorption of the basic aromatic ring system [5]. The intensity loss is most pronounced for high conversion ~ ' are involved in the reaction cycle and, indicating that in this case more surface ~ 0centres thus, contribute to the spin-spin exchange perturbation (Fig. 2).
N,
toluene
N,
50
- 100
30
-90 ,
co I 0
I
I
fig. 1 EPR spectra of ~ p 2pat 658 K in N~ and under feed (time interval between spectra: 10 min)
00
70 20
80 40 60 conversion I %
a
100
. -
-60
Fig. 2 Relative EPR intensity of VPP 2 under feed (0, Inl(N2) = 100 %) and benzaldehyde selectivity ( 0 )versus toluene conversion
On switching again to N2 atmosphere, the EPR signal returns only slowly to its initial state (Fig. 1) indicating that aromatic products are strongly adsorbed on the surface. This is in line with in situ-FTIR results in which, besides benzaldehyde, cyclic anhydride intermediates were detected on the catalyst surface [7]. These intermediates are precursors for total oxidation products and, thus, responsible for the low aldehyde selectivities (Fjg. 2). FTIR data also
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
revealed that strong product adsorption is due to the interaction of benzaldehyde with Bransted surface sites generated under reaction conditions. Therefore, pyridine which is not oxidizable under these reaction conditions has been continuously dosed to the feed (molar ratio: air/toluene/water vapour containing 4 wt.-% pyridine = 100/1/1) to block acidic sites and prevent strong product adsorption. In situ-EPR spectra reveal no change of the signal when only water vapour is added to the feed while its intensity starts to increase again as soon as pyridine is present (Fig. 3). In the latter case a roughly threefold increase of the benzaldehyde selectivity was observed at the same conversion rate (Table I). In situ-FTIR spectra indicated the preferred adsorption of pyridine on Brernsted surface sites, thus, displacing benzaldehyde from the latter and preventing deeper oxidation [8]. Table 1Maximum area specific rate of benzaldehyde formation, RBA,and reaction temperature, TR,derived from catalytic tests.
Catalyst
RBA1 pm01 h-' mV2 TR/ K i
VPP 1
VPP 1 + pyridine
VOK 1
VOK 2
29 648
76 665
120 660
110 66 1
toluene j toluene air i air
:
N,
N,
time on stream I min
Fig. 3 Relative double integrated EPR intensity of VPP 1 at 658 K as a function of feed composition and reaction time
toluene
N2
time on stream I min
Fig. 4 Relative double integrated EPR intensity of fresh (0) and conditioned sample VOK 1 ).( at 623 K as a function of feed composition and reaction time
XPS data of sample VPP 1 before and after treatment in the reaction chamber are listed in Table 2. Since the binding energies of the V 2p312peak in vanadium phosphates with equal vanadium oxidation state differ slightly [9, 10, 111, reliable identification of the vanadium oxidation state is not straightforward. Therefore, the difference between the binding energies of the 0 1s peak assumed to be constant and the V 2 p 3 ~peak has been used to account for small deviations of the vanadium valence state [9, 121. Values of 15.8 eV, 14.3-14.4 eV and 12.8-12.9 eV have been found for pure v3', v4+and containing phases, respectively [9, 131. Thus, the value of 14.2 obtained for the fresh sample VPP 1 points to a slight oxidation of the catalyst surface. After reaction, the V 2p3n peak is slightly shifted to lower binding energy suggesting that partial reduction of the catalyst surface occurred. The 0 1s peak in fresh VPP 1 appears at 532.5 eV. In contrast to some literature data in which two lines are fitted to the experimental peak (531.2 eV for lattice oxygen and 533.2 eV for OH-species [9, 14]), the peak in fresh VPP 1 can well be fitted with only one component assigned to 0'- ions of the lattice. This is in good agreement with the FTIR measurements in
v5'
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which no Brransted OH-sites could be detected for fresh VPP 1. After reaction, two 0 1s peaks appear at 531.7 and 528.5 eV. The former one arises from lattice oxygen and is virtually not influenced by the catalytic reaction. The new peak at 528.5 eV is assigned to OH groups generated by hydrolysis of V-0-P andlor P-0-P bonds. This is supported by the FTIR results indicating the creation of Brransted sites during reaction. In comparison to literature data [9, 141, this OH-peak is shifted to lower binding energies. The shift might be caused by the strong adsorption of reaction products (benzaldehyde and cyclic anhydrides) on the catalyst surface still persisting after evacuation at room temperature. Recently we have shown that the surface OH-groups in (V0)2P20, generated during reaction interact with the C=O group of benzaldehyde via hydrogen bonding [8]. Thus, the effective negative charge of the OH-oxygen atom is assumed to be enhanced giving rise to the shift of the binding energy. Table 2 XPS data of samples VPP 1 and VOK 1 before and after reaction
sample
signal
binding energy /eV assignment
VPP 1 (before)
0 1s V2p3 P 2p 0 1s
532.5 518.3 135.1 528.5 53 1.7 517.5 130.9 134.2
VPP 1 (after)
V2p3 P 2p VOK 1 (before)
VOK 1 (after)
a
surface percentage "
02v4', v5+ (trace) ~ 2 0 7 "
OH02'
v4+
P-OH P~O?
0 1s V 2p3 s 2~ K 2p3 0 1s V 2p3
Difference to 100 %due to carbon
The position of the P 2p peak in fresh VPP 1 (135.1 eV) is in good agreement with values found in a number of VPO compounds containing mono-, di-, poly- and hydrogenphosphate anions, respectively (133.2 - 134.5 eV [9,10, 141). After reaction, an additional P 2p peak occurs which is well below these values. Accordingly, it is assigned to P-OH groups interacting with adsorbed reaction products. K-V20s: In the EPR spectrum of the as-synthesized sample VOK 1 a small signal with partially resolved hyperfine structure is evident which indicates the presence of some isolated and weakly interacting tetravalent vanadium ions. This is in line with the mean vanadium valence state of 4.947. The intensity of this signal at 293 K increases by a factor of = 2.5 after heating in nitrogen for 1 h at 623 K and, additionally, by a factor of = 3 after treatment under feed for 1 h at 623 K indicating partial reduction of v5' to V4'. This is confirmed, too, by XPS measurements before and after reaction (Table 2).
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
300
400
500
660
Nanometers
760
800
fig- 5 DRS Spectra of VOK 1 at 293 K: a) fresh sample; b) after heating in N2 to 653 K;C) after lh reaction at 653 K.
&,O,
2 Theta
Fig. 6 In situ-X-ray diffmctograms of VOK 2 in N2 (,) and underfeed (b),
Under feed, the conditioning process seems to be finished after = 30 min when the EPR intensity keeps constant (Fig. 4, open circles). This equilibrated catalyst has been used for another experiment (Fig. 4, filled circles). In this case, no temporal decrease of the EPR double integral is observed under feed as for (V0)2P207. Instead, slightly more intense EPR signals appear under reaction conditions which decrease again on switching to nitrogen. This can be understood assuming that both V4' and v'' surface species are involved in the catalytic reaction. In the case of EPR silent v", changes of the electron density should temporarily enhance the EPR intensity since EPR active v4'is created by the reduction step of the Mars-van Krevelen redox cycle. On the other hand, temporal intensity loss is expected in case of v4'due to spinspin exchange distortion observed similarly for (V0)2P207. It is likely that both effects contribute to the overall EPR intensity. However, the fact that V" is in excess (Table 2) could explain the observed net intensity increase (Fig. 4). The UVMS-DRS spectra of the fresh samples VOK 1 and VOK 2 are rather similar being dominated by the well known charge-transfer bands of crystalline V205 between 300 and 500 nm (Fig. 5 a) [IS]. However, in contrast to pure VzOs, these bands decrease markedly in the potassium doped samples VOK 1 and VOK 2 upon thermal treatment in N2 atmosphere. Simultaneously, a very broad appears above 600 nm which arises from d-d transitions of v4' (Fig. 5 b). In agreement with the EPR results, this indicates easy reducibility of the VOK samples. Switching from nitrogen to feed at 653 K creates a rather intense band below 300 nm which persists also after evacuation at room temperature (Fig. 5 c). Comparison with reference spectra of a number of different tetravalent vanadium phosphates all showing a similar absorption suggests, that this band arises from charge-transfer transitions of v4'.It is, however, not visible in the spectra of samples VOK 1 and VOK 2 after catalysis and storage in air for several weeks suggesting rapid reoxidation. In contrast to (VO)2Pz07,no adsorbed product molecules could be detected by FTIR. The reason may be a lower surface acidity caused by enrichment of potassium in the surface as evidenced by XPS (Table 2). The most obvious difference between (VO)zP207 and the K-V205 samples arises from the catalytic performance which is markedly higher for VOK 1 and VOK 2 (Table I). In situ-XRD measurements revealed the presence of crystalline KV3Os in fresh VOK 2 which transforms immediately into &,5Vz05 in contact with the reactant gas mixture suggesting that the crystalline vanadates are located on the surface of the catalyst (Fig. 6). For VOK 1 no reflections besides those of V205 were visible during the in situ-XRD experiment. However, the surface atomic ratios of K N = 0.34 and v5'fV4' = 3.37 detected by
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XPS in VOK 1 after reaction (Table 2) are rather similar to the values resulting from the
composition of Ko.5V20s. Presumably, the surface of VOK 1 is covered by a similar potassium vanadate phase which is, however, microcrystalline or amorphous and, thus, not detected by XRD.The beneficial catalytic effect of K-V205 in comparison to (V0)2P207 could be due to structure of the &.5V205 surface phase [16] in which the K' ions are located between the [V205In layers. Thus, they could partially block acidic sites and prevent too strong product adsorption. 4. CONCLUSIONS
In (V0)2P207 catalysts, in situ-formation of Brnrnsted sites under reaction conditions hinders product desorption and causes oxidative damage of the aromatic ring leading via cyclic anhydride intermediates to COX.This undesired process can partially be suppressed by blocking the acidic sites by pyridine which improves the benzaldehyde selectivity at constant conversion rates. K-V205 catalysts show even higher catalytic performance for which at least two reasons can be discussed: i) Marked reduction to ? occurs during conditioning which lowers the oxidation potential and improves the selectivity in comparison to a pure v5' surface. ii) Modification with potassium leads to the formation of a potassium vanadate surface phase. The enrichment of potassium in the surface in turn reduces the surface acidity, prevents strong product adsorption and, thus, diminishes total oxidation. ACKNOWLEDGEMENT
This work has been supported by the Federal Ministry of Education and Research (Germany), project no. 03C0280. REFERENCES [l] F. Briihne, E. Wright: in Ullmam, 6th Edition 1998 Electronic Release (benzaldehyde entry). [2] K. Schlesinger, M. Meisel, G. L a h g , B. Kubias, R. Weinberger and H. Seeboth, German Patent No. DD WP 256659 Al(1984). [3] M. Niwa and Y. Murakami. J. Catal., 76 (1982) 9. [4] a) A. Briickner, B. Kubias, B. Liicke and R. StOkr, Colloids and Surfaces, 115 (19%) 179; b) H. G. Karge, J. P. Lange, A. Gutze and M. Laniecla, J. Catal., 114 (1988) 144. [5] A. Briickner, A. Martin, B. Liicke and F. K. H~MOUT, Stud. Surf. Sci. Catal., 110 (1997) 919. [6] A. Briickner, A. Martin, B. Kubias and B. Liicke, J. Chem. Soc., Faraday Trans.. 94 (1998) 2221. [7j A. Martin, U. Bentrup, A. Briickner and B. Liicke, Catal. Lett., 59 (1999) 61. [8] A. Martin, U. Bentrup, B. Liicke and A. Briickner, Chem. Commun., 1999, 1169. [9] F. Richter, PhD Thesis, University Leipzig (Germany), 1998. [lo] L. M. Cornaglia, E. A. Lombardo in H. Hattori, M. Misono, Y. Ono (eds.), Acid Base Catalysis 11, Elsevier Science Publ., Amsterdam, 1994. p.429. [Ill M. Abon, K. E. Bere, A. Tuel and P. Delichere, J. Catal., 156 (1995) 28. [12] F. Garbassi, J. C. J. Bart, F. Montino and G. Petrini, Appl. Catal., 16 (1985) 612. [13] G. W. Coulston, E. A. Thompson and N. Herron, J. Catal., 163 (1996) 122. [14] S. Albonetti, F. Cavani, F. Tdir6, P. Venturoli, G. Calestan, M. Mpez Granados and J. L. G. Fierro, J. Catal., 160 (19%) 52. [15] G. Centi, S. Perathoner andF. Trifir6, J. Phys. Chem., 96 (1992) 2617. [16] J.-M. Savariault and J. Galy, J. Solid State Chem., 101 (1992) 119.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
化
365
Observation of unstable reaction intermediate infrared laser pulses
by picosecond
tunable
K. Domen*, K. Kusafuka, A. Bandara, M. Hara, J. N. Kondo, J. Kubota, K. Onda, A. Wada, and C. Hirose Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. * CREST, JST (Japan Science and Technology) The picosecond tunable infrared laser pulses enabled us to carry out the vibrational spectroscopic measurements of the species adsorbed on surfaces with picosecond timeresolution. The irradiation of near-infrared pump pulses causes thermal excitation of the surface, and the change of adsorbates was probed by sum-frequency generation (SFG) spectroscopy. We applied this method to the direct observation of unstable species of formate on NiO(111) surface. The stable bidentate formate on NiO(111) was transformed to monodentate by the irradiation of pump pulses, and the dynamic behavior of the newly observed monodentate formate was followed as a precursor for the decomposition of formate. The application of the present method has extended to the study of methoxy species on Ni(111).
1. Introduction Time-resolved spectroscopy using ultrashort laser pulses is one of the powerful methods to study the reaction dynamics of molecules on surfaces [1,2]. Irradiation of near-infrared pump pulses causes thermal excitation at surfaces and the temperature jump of the surface at 100---1000 K in a few tens picosecond is possible to populate the adsorbed molecules imo an unstable state. Although the laser heating of surface has been utilized for the analysis of desorbing molecules in the laser induced thermal desorption (LITD) method [1-5], little attemion has been given to direct observation of the surface during the reaction of molecules due to the temperature jump. Sum-frequency generation (SFG) is a nonlinear vibrational spectroscopy using ultrashort laser pulses [ 1,6,7], which enable the time resolved observation of the surface in the heating period. Formation and decomposition of formate on various catalyst surfaces are key steps in many catalytic reactions such as water-gas shift reaction, methanol symhesis, etc. [8]. We have studied decomposition of formic acid on a NiO(11 l) surface which is prepared by the epitaxial oxidation of a N i ( l l l ) crystal [9-11]. The formic acid dissociates to bidemate formate on NiO(111) at-160 K and the formate species decomposes to form H2 and CO2 at 340-390 K and H20 and CO at 410 K [10]. Two types of formates, bidentate and monodemate, have been idemified by infrared reflection absorption spectroscopy (IIL&S) under the catalytic decomposition of formic acid in a flow of formic acid gas at > 102 Pa [11 ]. However, the dynamic behavior of bidemate and monodentate formates has still not been understood.
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In this study, we demonstrate by ultrashort laser method the stable bidentate formate on N i O ( l l l ) is transformed to monodentate before the decomposition [12-14]. The monodentate formate was regarded as an intermediate species for the decomposition of formate. Recently, our application of this method has been carried out for the study on the methoxy species on Ni(111), and the tentative results is shown in the last section of this article.
2. Method
The optical setup of generation of frequency-tunable infrared pulses, generation of SFG signals, and irradiation of pump pulses is shown in Fig. 1 [12-14]. A mode-locked Nd:YAG laser (1064 nm, 35 ps pulse width, and 10 Hz repetition rate) was used as the source of light pulses. The frequency-tunable near-infrared (NIR) pulses were generated by an optical parametric generator/amplifier (OPG/OPA) using 13-BaBO4 (BBO) crystals pumped by the second harmonic generation (532 nm) output of a KDP crystal. Tunable picosecond infrared (IR) pulses (60 ~tJ/pulse and 3 cm t FWHM at 2200 cm -~) 1064 1064 for SFG were obtained from the Nd:YAGlaser NIR and 1064 nm pulses by a5 s 532 1064 nm nm difference frequency generation NIR,,~ n ~ , , 532 (DFG) using an AgGaS2 (AGS) j,'."....~~ .. crystal. A portion of the grating IIq'" "'%: 1064 nm pulses was used to p: generate the visible (532 nm, 200 [ : ~tJ/pulse) light to be used for SFG. The remaining 1064 nm ODL pulse (-~10 mJ/pulse) was used as a pump pulse after passing PMr S ~ f ' " - ' ~ , ~ . ~ l ~ ~ through a variable optical delay "L.__~ -t'"--ll__ "~ sample ] UHV line (ODL). The IR and ~ chamber visible beams for SFG and the 1064 nm beam for pumping were Fig. 1 Optical setup for time-resolved SFG crossed at the surface at an angle measurement with 1064 nm pump beam. of incidence of about 75 ~. The diameters of the IR, visible and 1064 nm beams were ca. 2, 3 and 5 mm, respectively, at the sample surface. The generated SFG photons were detected with a photomultiplier tube after passing through optical filters and a monochromator. For the experiments of methoxy species on Ni(111), we used another OPG/OPA system which generates IR pulses between 2500-4000 cm ~ [15]. The experiments were performed in an ultrahigh vacuum (UHV) chamber equipped with an Ar-ion bombardment gun, low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) devices. A Ni(111) piece could be cooled to 130 K by liquid nitrogen and heated to 1300 K by resistive heating. Clean Ni(111) surfaces were obtained by repeated cycles of Ar ion bombardment followed by annealing at 1100 K. In the preparation of a NiO(111) surface [9], the cleaned Ni(111) surface was oxidized by the repeated cycles of exposure to oxygen at 1000 L (1 L = l x l 0 6 Torr s, 1 Torr = 133 Pa) and 570 K and annealing at 650 K in vacuum.
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The schematic drawing for the sequence of the time-resolved experiment is shown in Fig. 2. The surface is heated t o - 3 0 0 K above the initial temperature by the irradiation of pump pulses. The magnitude of the temperature jump has 9 0.2 0.3 been experimentally estimated from the time / a _ pressure" 10-5 ~ 10 -4 Pa change of vibrational temperature at } OD/NiO(11 l) surface by the irradiation [ 13]. The surface temperature is cooled ooo down within several hundreds picosecond ~r I==AT following the classical heat diffusion -300 K equation [4]. The surface molecules are pump ulse transformed to another species or SFG ~ ~-~.~ii.l-.- pu,~ decomposed to desorb from the surface by the temperature jump. When a small -100 0 100 200 part of surface molecules leaves from the delay time / pa surface by the irradiation, the initial Fig. 2 Sketch of the time flow of timecoverage is reproduced before the next resolved experiment. The initial surface is irradiation of pump pulse by the flowing of recovered before next irradiation of pump the formic acid gas at 10.5 - 10.4 Pa. In pulses, so that the signals for a number of the heating and cooling period, the gas pulses are able to be averaged over. phase molecule does not come into collision with the surface at the gas pressure of 10.5 Pa, and change of the surface in this period is free from the interaction with gaseous molecules. The advantage of this method is that the surface covered with the molecules at the extremely high temperature (AT - 300 K) is able to be observed. If the surface is kept at such high temperature for a long period (>ps), all of adsorbates should be completely decomposed and desorbed from the surface. The amount of decomposition is kinetically limited by the short period in the high temperature, and the behavior of adsorbates at the high temperature is able to be monitored by SFG. v
3. Results and discussions
(1) Formate/NiO(111) system The SFG spectra obtained at various delay times from the irradiation are shown in Fig. 3. Two polarization combinations were examined. For C-D stretching mode of formate, the signal obtained by the s-polarized visible and p-polarized infrared pulses is enhanced when the C-D axis is tilted from the surface normal. In the spectra at -100 ps, the peak assignable to bidentate formate was observed at 2160 c m "1 and this peak was assigned to the C-D stretching mode of formate in a bidentate configuration. When the surface was irradiated by the pump pulses, the 2160 cm ! peak weakened and new peak appeared at 2190 cm 1. At -20 ps delay from the peak top of the pump pulse, the surface temperature takes maximum and the temperature has been estimated as 300 K higher than the initial temperature. The 2190 cm ~ peak disappeared after 100 ps. The newly observed peak was assigned to monodentate formate [11]. The intensity of the 2190 cm ~ peak for the polarization combination of s-polarized visible and p-polarized infrared was consistent with the structure of monodentate in which the C-D axis tilts away from the surface normal. We have
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368
poralization poralization actually found that vis. 9 s vis. 9 p monodentate formate is ir 9 p ir : p present on NiO(111) under 2160 the catalytic decomposition ;~, 2190 of formic acid at 102 Pa. 2160. delay S ' ,;~j time The present study using .~ 2190 er ultrashort pulse laser clearly - 1 0 0 ps shows that monodentate :2, ] ,'.1,', formate was originated from ~20 ps bidentate formate directly but not from gaseous formic ; I acid. The monodentate 100 ps formate was equilibrated i 9 i ; , 9 I 2000 2100 2200 2300 2000 2100 2200 2300 with bidentate formate on wavenumber / c m "1 w a v e n u m b e r / c m -1 the surface at the high temperature induced by the Fig. 3 SFG spectra of formate on NiO(111) at 400 K with irradiation. pump pulses at various delay times from the pump pulse. The time profiles of the The surface was in the flow of DCOOD at 10.5 Pa. SFG signals at 2160 and 2190 cm -~ are shown in Fig. 4. The changes of bidentate and monodentate signals were compensated for each other indicating that monodentate formate comes from bidentate formate and that the two species are in equilibrium. The change of bidentate formate was not completely recovered at 300 ps delay in spite of the fully disappearance of monodentate signal. This indicates that a small part of bidentate was decomposed by the 5 irradiation. Intensity of SFG signal is known to ~i 1.0 be proportional to the square of the number of ~ . ~ 0 molecules. Thus, the 7% decrease of the SFG ~t-g*** [ 2160 cm-1 signal of bidentate suggests that only 3---4% 9 C bidentate formate was transformed to monodentate -~ 0.95~,~, bidentate C formate. The SFG susceptibility of monodentate ._~ . formate was thus quite higher than that of bidentate 5 formate because such small amount of 1t l d ~ ~ 2190cm-1 monodentate gave clear SFG peak in the spectra. monodentate ~ .D We assume the elemental reaction step as i
=Jj I
"
I
"
I
"
I
I
K
DCOQ
_
OCOOm
32 + CO _,D O + CO,
C
.
~C m
"
c
O"
I
I
. . . . . . . . . . . . . . . .
.m
where K is the equilibrium constant between bidentate and monodentate species which is expressed as K = 0,n / Oh = exp(- AE / RT) and k is the rate constant defined as dOm/dt =-kO m and
k=Aexp(-Ea/RT).
0b and
0m
are
the
-100
I
0
I
100
I
200 300
delay time / ps Fig. 4 Time profiles of SFG signal at 2160 and 2190 cm t for formate on NiO(111) at 400 K.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
369
coverages of bidentate and monodentate formatcs, and AE, Ea, R, A, and T arc the potential energy difference between bidentate and monodentate, activation energy for decomposition of monodentate formate, gas constant, pre-exponential factor, and surface temperature, respectively. The values of AE = 19 kJ mol "~, Ea - 30 kJ mol ~, and A = 5x10 ~ s"~ were estimated from fitting with all of the experimental results for various initial temperatures [ 1214]. We reasonably reconfirmed that the monodentate species was a precursor for the decomposition, and the obtained kinetic values approximately agree with those derived from kinetic analysis under the catalytic conditions [ 11 ]. (2) Methoxy/Ni(111 ) system Mcthoxy on catalyst surfaces is also one of the important species in various catalytic reactions and numerous studies have been reported on this species. The SFG spectra of methoxy species on Ni(100) was reported [16], however no dynamical aspect has been investigated. Our application of the time-resolved SFG experiments has extended to the study of methoxy species on Ni(111). The primary results on the methoxy/Ni(111) are shown produced on Ni(111) by introducing methanol at 200 K. before irradiation of pump pulses is shown in the top trace in Fig. 5a. Two peaks were observed at 2825 and 2923 cm ~, which were assigned to the symmetric C-H stretching mode and combination mode of methoxy group. The assignment of 2923 cm -~ peak has controversy at present; one is due to asymmetric C-H mode [17] and the other is due to combination mode of deformation mode [18]. On the basis of recent studies on photoelectron diffraction [19], it seems more reasonable that the methoxy species stands perpendicular on the surface and that the peak at 2923 cm -~ corresponds to the combination band. When the surface was irradiated by the 1064 nm pump pulses, the two peaks weakened as shown in the bottom trace. The transient change of peak intensity at 2923 cm -~ is shown in Fig. 5b, which corresponded to the change of surface temperature. We considered the reason of the weakening of SFG peaks caused by temperature jump. Our tentative interpretation is that the methoxy species changed its structure with the transient heating. The detailed experiments suggest that the change is not explained by the thermal broadening of SFG peaks. The temperature jump in the methoxy/Ni(11 l) system was estimated as 150-200 K. The surface was thus heated to 350-400 K by the irradiation which was-~100 K higher than decomposition temperature of methoxy species. Thus, the methoxy species on Ni(111) was found to change the structure above decomposition
in Fig. 5. Methoxy species was SFG spectrum of methoxy species
~
(a)
2923
~
'
A ,
delay time
~"~-150
~,
ps
-35 ps
27:00 " 28t)0" 2<JO0 " 3000 " 31()0 wavenumber / cm -1 5 (b) ,/ ~ 1.0-
c o.9-
_~ ~0.ae"~ 0.7 -200 "-1()0"
() " 1 ~ ) " 2()0'300
delay time / ps
Fig. 5 Time-resolved SFG spectra of methoxy species on Ni(111) at 130 K (a), and transient signal change of methoxy at 2923 cm l by the irradiation of pump pulses at 110 K (b).
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temperature. The methoxy species may tilt from the surface normal or migrate to another adsorption site at such high temperatures. The study of methoxy/Ni(111) system will be published with more detailed results. In conclusion, we demonstrate the direct observation of reaction intermediate in the formate decomposition. The laser induced temperature jump technique was found to be one of the powerful tools for identification of precursor molecules prior to the activation barrier in the reaction. Monodentate formate was an intermediate in the decomposition of bidentate formate on NiO(111). This technique has been also applied to the study of methoxy species on Ni(111). References
1. Laser Spectroscopy and Photochemistry on Metal Surfaces Part I, eds. H. L. Dai and W. Ho, (World Sci. Pub., Singapore, 1995). 2. Laser Spectroscopy and Photochemistry on Metal Surfaces Part II, eds. H. L. Dai and W. Ho, (World Sci. Pub., Singapore, 1995). 3. R. B. Hall and A. M. DeSantro, Surf. Sci. 137 (1984) 421. 4. D. S. King and R. R. Cavanagh, Molecule Surface Interactions, Advances in Chemical Physics, Ii"ol. 76, ed. K. P. Lawley, (John Wiley & Sons., New York, 1989) p. 45. 5. Z. Rosenzweig and M. Asscher, Surface Science of Catalysis: In Situ Probes and Reaction Kinetics, eds. D. J. Dwyer and F. M. Hoffmann, (American Chem. Soc., Washington, 1992), Chap. 22. 6. Y. R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984). 7. T. F. Heinz, Nonlinear Surface Electromagnetic Phenomena, eds. H. -E. Ponath and G. I. Stegeman, (Elsevier, Amsterdam, 1992), Chap. 5. 8. G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, (John Wiley & Sons., New York, 1994). 9. H. -J. Freund, Angew. Chem. Int. Ed. Engel. 36 (1997) 452. 10. A. Bandara, J. Kubota, A. Wada, K. Domen and C. Hirose, J. Phys. Chem. 100 (1996) 14962. 11. A. Bandara, J. Kubota, A. Wada, K. Domen and C. Hirose, J. Phys. Chem. B 101 (1997) 361. 12. A. Bandara, J. Kubota, A. Wada, K. Domen and C. Hirose, J. Phys. Chem. B 102 (1998) 5951 13. C. Hirose, A. Bandara, S. S. Katano, J. Kubota, A. Wada and K. Domen, Appl. Phys. B, 68 (1999) 559. 14. K. Domen, A. Bandara, J. Kubota, K. Onda, A. Wada, S. S. Kano and C. Hirose, Surf. Sci. 427-428 (1999) 349. 15. T. Yuzawa, T. Shioda, J. Kubota, K. Onda, A. Wada, K. Domen and C. Hirose, Surf. Sci. 416 (1998) L1090. 16. J. Miragliotta, R. S. Polizzotti, P. Rabinowitz, S. D. Cameron and R. B. Hall, Chem. Phys. 143 (1990) 123. 17. R. Zenobi, J. Xu, J. T. Yates Jr., B. N. J. Persson and A. I. Volokitin, Chem. Phys. Letters 208(1993)414. 18. J. P. Camplin and E. M. McCash, Surf. Sci. 360 (1996) 229. 19. O. Schaff, G. Hess, V. Fritzsche, V. Fernandez, K.-M. Schindler, A. Theobald, Ph. Hoffmann, A. M. Bradshaw, R. Davis and D. P. Woodruff, Surf. Sci. 331-333 (1995) 201.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Changes in Morphology of 7-Al203-Supported Pt Clusters under Reaction Conditions: Evidence from In-Situ EXAFS Spectroscopy Oleg Alexeev and Bruce C. Gates Department of Chemical Engineering & Materials Science, University of California, Davis, CA 95616 USA Pt clusters on 7-A1203, MgO, and SiO2, with average diameters of 11, 20, and 45 A, were characterized by EXAFS spectroscopy in the presence of H2, ethene, propene, 02, and ethane as well as ethene + H2 and propene + H2 (undergoing catalytic reaction). Adsorption of alkenes (but not alkanes) or oxygen led to flattening of the smallest clusters (on 7-A1203) but to essentially no changes in the larger clusters (on MgO or SiO2); the changes were reversible. The morphology of the smallest Pt clusters depends on the gas composition in alkene hydrogenation catalysis. 1. INTRODUCTION UHV experiments have demonstrated the influence of adsorbates on single-crystal metal surface structures, leading us to expect that morphologies of supported metal clusters could change as a result of adsorption. Our goals were to test this expectation by measuring EXAFS spectra of supported Pt clusters of various average sizes in various gas atmospheres, including those undergoing catalysis of alkene hydrogenation.
2. EXPERIMENTAL METHODS The powder supports 7-A1203 (Degussa), MgO (EM Science), and SiO2 (Degussa), with BET surface areas of 100, 47, and 250 m2/g, respectively, were calcined and evacuated at 400~ prior to use. n-Pentane solvent was purified by refluxing over Na/benzophenone ketyl and deoxygenated. Pt/y-A1203, Pt/MgO, and Pt/SiO2 were prepared by slurrying [PtC12(PhCN)2] (Strem) with the corresponding support in n-pentane, with amounts giving samples containing 1 wt% Pt after removal of the pentane by evacuation. Schlenk lines and a N2-filled drybox were used to exclude air. Hydrogen chemisorption measurements were made, as before [ 1 ] . In-situ EXAFS experiments were performed at X-ray beamline X-11A of the National Synchrotron Light Source, Brookhaven National Laboratory. Each sample was reduced with H2 at 400~ for 2 h to remove organic ligands and form supported Pt clusters. Each powder sample was then placed into a sample holder and secured by Kapton tape. The sample mass was chosen to give an absorbance of about 2.5 at the Pt LIII absorption edge (11563.7 eV). The sample holder was placed in an in-situ cell connected to a gas distribution system that allowed flow of gases through the sample. H2, N2, He, ethene, ethane, and propene (Matheson, UHP grade) were purified by passage through traps containing reduced Cu/A1203 and activated zeolite to remove traces of 02
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and H20, respectively. elsewhere [ 1].
Data were analyzed with XDAP software [2]. Details are as stated
3. RESULTS 3.1. Dispersion of Pt The values of average Pt cluster size and dispersion (Table 1) were determined from hydrogen chemisorption data and the model of Kip et al. [3] relating the average cluster size to the Pt-Pt first-shell EXAFS coordination number (Npt-Pt). When Pt/~'-Al203 was exposed to H2 at 400~ Pt clusters with an average diameter of about 11 A (incorporating about 25 atoms each) formed. Similar treatment of Pt/MgO and Pt/SiO2 led to larger clusters, with average diameters of about 20 and 45 A, respectively (Table 1). Table 1 Dispersions of supported Pt catalysts after treatment in H2 at 400~ Sample Dispersion, Pts/Ptt Average Pt cluster diameter (A) from chemisorption from EXAFS Pt/~,-AI203 1.00 11 Pt/MgO 0.54 21 18 Pt/SiO2 0.25 45 -
3.2. EXAFS data characterizing interaction of H2 with supported Pt To determine the influence of H2 on the Pt morphology, samples were investigated by EXAFS spectroscopy under vacuum and in flowing H2 (Table 2). The A1203-supported clusters under vacuum are characterized by Npt-Pt of 6.4 and a Pt-Pt distance (Rpt-Pt) of 2.70 A. After exposure to flowing H2 at 25~ Npt-Pt remained unchanged, but Rpt-Pt increased to 2.76 A. After subsequent evacuation at 25~ the EXAFS data matched those observed initially for Pt/7A1203 under vacuum, showing the reversibility of the changes induced by H2. Table 2 EXAFS results at the Pt LIII edge characterizing the interaction of Pt clusters with H2 a Sample Conditions First-shell Pt-Pt contributions during scan N R (A) 103 . A~2 (A2) AE0 (eV) Pt/7-AI203 vacuum 6.4 2.70 6.77 3.5 Pt/7-A1203 H2 flow 6.5 2.76 4.87 0.1 Pt/MgO vacuum 8.3 2.75 2.00 0.0 Pt/MgO H2 flow 8.2 2.75 1.50 0.1 Pt/SiO2 vacuum 11.2 2.76 0.80 -0.9 Pt/SiO2 H2 flow 11.0 2.76 0.50 -0.7 aNotation: N, coordination number; R, distance between absorber and backscatterer atoms; A• 2, Debye-Waller factor; AE0, inner potential correction. Estimated accuracies: N, + 20%; R, + 1%; Ao2, + 30%; AE0, + 10%. Samples were scanned at 25~
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In contrast, the morphologies of the larger clusters in Pt/MgO and Pt/SiO2 were not influenced significantly by H2.
3.3. Influence of 0 2 on structure of Al203-supported Pt clusters When completely reduced Pt/qt-Al203 was exposed to doses of 02 in N2 at 25~ partial and then complete oxidation of Pt occurred (Table 3). The values of Npt-pt (4.2) and Rpt-Pt (2.68 A) represent partially oxidized Pt, indicating a decrease of the Pt cluster size as a result of 02 addition. The fully oxidized sample gave no indication of Pt-Pt contributions, indicating complete conversion of the Pt to platinum oxide. 3.4. Interaction of completely oxidized Pth/-Al203 with H2 The interaction of these supported platinum oxide clusters with H2 was tracked in a series of EXAFS experiments (Table 3). When the first portion of H2 was dosed into the N2 flowing through the completely oxidized sample, Npt-Pt increased to 2.7 and Rpt-Pt became 2.75 A, indicating conversion of parts of platinum oxide clusters to Pt clusters. After an additional dose of H2, the respective values were 3.0 and 2.77 A. Further dosing of H2 led to a gradual increase of Npt-Pt to 6.5 with the value of Rpt-Pt remaining essentially unchanged. These data correspond to complete conversion of platinum oxide clusters into Pt clusters in their original statewthe process was reversible. 3.5. Interaction of supported Pt clusters with alkenes and alkanes When propene or ethene flowed through the completely reduced Pt/)'-A1203, the value of RetPt (2.71 A) was the same as that characterizing the sample under vacuum (Table 4). The values of Npt-Pt representing the ~/-Al203-supported clusters with adsorbed ethene or propene were 3.0 or 4.1, respectively, indicating that the alkenes caused changes in the Pt cluster morphology. The decreases in Npt-Pt were accompanied by increases in the Debye-Waller factors. The adsorption of ethane on this sample led to much smaller changes in Npt-Pt, which
Table 3 EXAFS results demonstrating the influence of O2/H2 treatments on structure of ~/-A1203supported Pt clusters a Sample conditions during First-shell Pt-Pt contributions scan N R (A) 103 . Ao2 (A2) ALE0 (eV) 02 dosed in N2 flow 4.2 2.68 9.90 -2.2 02 dosed in N2 flow 0.0 0.00 0.00 0.0 H2 dosed in N2 flow 2.7 2.75 1.80 1.0 H2 dosed in N2 flow 3.0 2.77 1.95 -1.5 H2 dosed in N2 flow 3.7 2.77 2.00 -2.4 H2 dosed in N2 flow 4.4 2.77 2.29 -2.1 H2 dosed in N2 flow 4.8 2.77 2.45 -2.0 H2 dosed in N2 flow 5.9 2.77 3.00 -1.8 H2 dosed in N2 flow 6.5 2.76 4.87 -0.1 aNotation as in Table 2. See text for explanation of sequence of experiments.
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were directionally the same as those caused by the alkenes. These changes were reversed by exposure of the sample to H2. When Pt/MgO was exposed to flowing ethene, Rpt-Pt remained the same, and Npt-Pt became only slightly less (7.5) than that observed after treatment with H2 (8.3) (Table 4). Exposure of Pt/MgO to ethane led to no changes in the EXAFS parameters. The structure of Pt/SiO2 underwent no observed change as a result of interaction with ethene or ethane (Table 4).
3.6. EXAFS of supported Pt clusters during propene hydrogenation catalysis The samples were characterized by EXAFS spectroscopy during ethene hydrogenation and propene hydrogenation catalysis at 25~ with the EXAFS cell serving as a flow reactor. The data (Table 4) show that structural parameters characterizing the relatively large Pt clusters on MgO (or SiO2) remained essentially unchanged under catalytic reaction conditions. In contrast, the smaller Pt clusters on ~/-A1203 underwent structural changes depending on the composition of the reactant mixture. When H2 in the feed was not enough for complete hydrogenation of ethene (or of propene) and alkene was the predominant gas-phase component, Npt-Pt and Rpt-Pt were the same as when ethene (or propene) alone flowed through the sample. In contrast, when the H2 in the feed was enough for complete conversion of ethene to ethane, Npt-Pt and Rpt-Pt matched the values for the sample in ethane alone. Thus, the data demonstrate dynamic changes in the morphology of small A1203-supported Pt Table 4 EXAFS results at the Pt LIII edge characterizing the interaction of Pt clusters with alkenes and alkanes a Sample Conditions Detected First-shell Pt-Pt contributions during scan product N R (A) 103 . Act2 (A2) AE0 (eV) H2 flow 6.5 2.76 4.87 -0.1 C3H6 flow C3H6 4.1 2.71 5.30 4.8 2.71 5.30 4.8 C3H6/H2 (2:1) C3H6/C3H8 (1:1) 4.0 H2 flow 6.6 2.76 4.90 -0.1 C2H4 flow C2H4 3.0 2.71 5.50 3.6 H2 flow 6.5 2.76 4.70 0.1 Pt/7-A1203 C2H4/H2 (5:1) C2H4/C2H6 3.0 2.70 5.30 3.8 C2H4/H2 (1:1) C2H6 5.7 2.75 5.50 0.0 H2 flow 6.5 2.76 4.87 0.3 C2H6 flow C2H6 5.6 2.75 4.20 -0.1 H2 flow 8.3 2.75 2.00 0.0 Pt/MgO C2H4 flow C2H4 7.5 2.74 2.00 0.4 C2H6 flow C2H6 8.2 2.75 1.88 0.5 H2 flow 11.0 2.76 0.50 -0.7 Pt/SiO2 C2H4 flow C2H4 11.3 2.76 0.80 -0.8 C2H6 flow C2H6 11.0 2.76 0.50 -0.6 aNotation as in Table 2; flow rate ratios are molar.
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clusters, depending on the composition of the reaction mixture. The changes were reversible m subsequent treatment with H2 at 25~ led to values of Npt-Pt and Rpt-Pt close to those characterizing the initially reduced sample. ....
3.7. Analysis of XANES region The XANES data provide evidence of the influence of hydrogen, oxygen, alkenes, and alkanes on the electronic structure of the Pt (Figures 1 and 2). When relatively large Pt clusters were present on the support (MgO or SiO2), interaction with H2 or hydrocarbons led to no substantial changes in the XANES region. When A1203-supported Pt clusters were exposed to H2, neither the edge position nor the area under the white line changed relative to the values characterizing the sample scanned under vacuum, but interaction with H2 changed the position of the first inflection point (Figure 1). Exposure of the sample to 02 led to a change in the position of the first inflection point and a substantial increase in the area under the white line, which indicates that the sample had a lower electron density than the reduced sample. In contrast, the position of the first inflection point did not change when Pt/y-A1203 was exposed to ethene or to ethane (Figure 2). After adsorption of ethene, the white line area increased relative to that of the evacuated sample (Figure 2). Addition of H2 to the ethene changed the white line area, depending on the amount of H2 in the feed. When the H2 was sufficient to convert all the ethene to ethane, the white line area was the same as that after adsorption of ethane.
1.5
8 r
1.5
1.0
o r ..Q
._
i ~ \\
C2H4~
Complete
oxidation- - - - - - - - ~
Partial oxidation
8
1.0
o
Vacuum
Inflection point
..Q
Hydrogen flow 0.5 0.5
o Z
0.0 11540
11550
1156(
0.0 11570
11580
11590
11600
Energy, eV
Fig. 1. Comparison of XANES regions characterizing Pt/y-A1203 scanned in vacuum, H2, and after exposure to 02.
11610
11540
11550
11560
11570
11580
11590
11600
11610
Energy, eV
Fig. 2. XANES data characterizing Pt/y-A1203 sample scanned in vacuum and after interaction with hydrocarbons.
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4. DISCUSSION
The data demonstrate that although the effects of adsorbates on the structures of the larger Pt clusters (on MgO or SiO2) were negligible, the effects of adsorbates on the smaller Pt clusters (on A1203) were substantial. The adsorbates formed from H2, alkenes, and 02, which we infer were strongly bonded to Pt, caused a reversible flattening of the clusters, as indicated by the increasdd values of Npt-Pt and Npt-o. In contrast, the effect of adsorbed alkanes was very small, consistent with weak adsorption of these compounds. Changes depending on the composition of the reacting mixture of alkene and H2 imply that there were clusters of different morphologies at different positions in the flow reactor operating at steady state; catalysts near the inlet are expected to have had flattened clusters and those near the outlet (where the conversion was high and most of the gas was alkane) not being flattened. Clearly, it is an oversimplification to model the catalyst as a single Pt species. The results are contrasted with those representing Ir4/7-A1203 and Ir6/7-A1203 in propene (sometimes with H2) under the same conditions, which indicate the lack of changes of the clusters relative to those present in H2 [4]. We infer that It4 tetrahedra and It6 octahedra on 7A1203 are more resistant to morphological change in propene than the approximately 11 ,A, Pt clusters on 7-A1203. It is not clear whether the difference is associated with just the metal or whether the nearly uniform It4 and It6 structures may be more resistant to morphological change than (nonuniform) clusters of Pt containing about 25 atoms each. The results are consistent with the earlier inference [5] that the supported Ir4 and It6 clusters are different from larger clusters or particles that can be classified as (more nearly) metallic, as indicated by their catalytic activities for toluene hydrogenation. The data suggest a refinement of the classification of supported noble metals according to the average cluster or particle size (large particles which show almost no size effect on catalytic activity (turnover frequency); intermediate-sized clusters that show cluster size effects; and the smallest (nearly molecular) clusters that are chemically distinct and not metallic in character [6]. The morphological changes indicate changes of the cluster energy (with the interacting adsorbate and support ligands), and the evidence of changes with only the smallest Pt clusters suggests that the difference may represent kinetics rather than thermodynamics. ACKNOWLEDGMENT This work was supported by the National Science Foundation (grant CTS-9615257). REFERENCES
1. O. Alexeev, D.-W. Kim, G. W. Graham, M. Shelef, and B. C. Gates, J. Catal., 185 170. 2. M. Vaarkamp, J. C. Linders, and D. C. Koningsberger, Physica B 209 (1995) 159. 3. B.J. Kip, F. B. M. Duivenvoorden, D. C. Koningsberger, and R. Prins, J. Catal., 105 26. 4. G. Panjabi, A. M. Argo, and B. C. Gates, Chem. Eur. J., 5 (1999) 2417. 5. F.-S. Xiao, W. A. Weber, O. Alexeev, and B. C. Gates, Stud. Surf. Sci. Catal., 101 1135. 6. Z. Xu, F.-S. Xiao, S. K. Purnell, O. Alexeev, S. Kawi, S. E. Deutsch, and B. C. Nature, 372 (1994) 346.
(1999)
(1987)
(1996) Gates,
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
In situ F T - I R i n v e s t i g a t i o n o f h y d r o c a r b o n reactions o v e r z e o l i t e b a s e d bifunctional catalysts M. H6chtl a, Ch. Kleber a, A. Jentys b, H. Vinek a aVienna University of Technology, Institute for Physical and Theoretical Chemistry Getreidemarkt 9/156, A-1060 Wien, Austria bTechnical University of MUnich, Institute of Technical Chemistry II, Lichtenbergstr. 4, D-85748 Garching, Germany SUMMARY
The transport properties and the hydroconversion of n-heptane and 2-methyl-hexane over Pt loaded HZSM-5 and HBETA were investigated by in situ FTIR-spectroscopy combined with the evaluation of the kinetic data. Propane and i-butane were the main products observed on the catalyst surface as the temperature increased, while the number of interacting BrCnsted acid sites was decreasing. The diffusivities of n-heptane and 2-methyl-hexane were in the same range for HBETA, but a significant diffusion restriction for the monobranched isomer was observed on HZSM-5, which was reflected in a lower reaction rate. 1. I N T R O D U C T I O N Isomerization of linear C5-C8 alkanes, the main constituents of gasoline obtained from crude oil refining, is of great economic importance as branched alkanes have a considerably higher octane number than their linear counterparts. Therefore, alkane isomerization is an attractive process route to produce both, high-octane gasoline and environmentally acceptable gasoline components. Typically, alkane isomerization is performed over bifunctional catalysts, where the hydrogenating/dehydrogenating function is provided by noble metals like Pt, and the acid function by zeolites. According to the generally accepted bifunctional reaction mechanism, alkanes are dehydrogenated on the metal sites and the alkenes formed diffuse to the acid sites, where they are protonated yielding carbenium ions. After isomerization and B-cracking the products are hydrogenated on the metal sites. The rate-determining step depends on the kind and number of metal and acid sites. For metals with a low hydrogenation activity or catalysts with a low number of metal sites, the rate-limiting step is the hydrogenation/dehydrogenation. If sufficient metal sites are present or for high hydrogenation activity, isomerization of the carbenium ion on the acid sites is rate limiting. The overall reaction rate can be determined by the transport to and from the active sites, by adsorption - desorption or by the reaction itself. Therefore, the investigation of the nature, number and strength of the sites responsible for the transformation of hydrocarbons would appear to be critical for the understanding of the catalytic properties and for designing new catalysts. Furthermore, detailed experimental studies of the mechanism of catalytic reactions on a molecular level are of crucial importance.
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In situ infrared spectroscopy is certainly one of the most powerful techniques for obtaining information on the concentration of reactants, intermediates and products at the surface of the catalysts. The aim of this paper is to examine the hydrocarbon conversion over bifunctional catalysts by the determination of kinetic parameters combined with in situ FT-IR investigations.
2. EXPERIMENTAL The Pt containing HZSM-5 (2 %wt Pt, Module 28) and HBETA (2 %wt Pt, Module 27) were prepared by incipient wetness impregnation with [Pt(NH3)4]C12. The catalysts were dried for 4 h at 70~ and subsequently at 100~ for 12 h. Prior to reaction the samples were treated in flowing oxygen and reduced in hydrogen at 475~ Temperature programmed desorption (TPD) of ammonia was used to determine the concentration of strong acid sites. The platinum dispersion was obtained from a hydrogen adsorption isotherm measured in a volumetric adsorption apparatus. The morphology and the crystallite size of the catalysts were revealed by scanning electron microscopy (SEM). Infrared (IR) spectra were recorded on a Bruker IFS 28 FTIR - spectrometer, which was equipped with (i) a transmission vacuum cell and (ii) a reaction cell for in situ investigations under reaction conditions. The catalysts were pressed into self-supporting wafers that were placed in a ring shaped furnace inside the cell. To observe the molecular transport in the catalyst pores, n-heptane (n-C7) and 2-methylhexane (2-MC6) were adsorbed at 40~ and 10-i mbar in the vacuum ir-cell. Spectra were recorded every 10 seconds until the equilibrium was reached. The integrated C-H stretching vibrations were taken as measure of the amount of hydrocarbon adsorbed and the diffusion coefficients were estimated as presented in [ 1]. The in situ FTIR hydroconversion experiments were carried out in a reaction cell similar described by Mirth et al. [2] approximating a continuously stirred tank reactor with a volume of 1.5 cm 3. The effluent gas stream was sampled and subsequently analyzed by means of gas chromatography (HP 5980 II) equipped with a flame ionization detector. The hydrocarbon reactions were investigated at temperatures between 150~ and 350~ at 1.5 bar total pressure. The alkane partial pressure in the H2 stream was 25 mbar. To obtain the difference spectra of the in situ measurements, the spectra of the activated catalysts recorded at corresponding temperatures were subtracted from the spectra recorded during the reaction. 3. RESULTS In Table 1 the crystallite size, the platinum dispersion, the concentration of strong BrCnsted acid sites and the acid strength are given. Both molecular sieves exhibited similar acid strength, characterized by the frequency shift of the SiAl-OH groups after adsorption of benzene. The HZSM-5 crystallites consisted of inhomogeneous agglomerates of prism shaped particles with an average size of 0.9 ~tm, while HBETA showed homogeneous spheres with a diameter of 0.6 ].tm. The IR spectrum from HZSM-5 exhibited OH bands at 3610 cm -I and 3740 cm -I assigned to bridged SiAl-OH groups and terminal Si-OH groups, respectively. In the hydroxyl region of the IR spectrum of the activated HBETA five absorption bands were observed. The bands with the highest intensity at 3741 cm -I and 3610 cm -1 were
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assigned to terminal silanol and SiAl-OH groups, respectively. Two bands with a lower intensity at 3785 cm -l (terminal A1-OH groups), at 3668 cm l (extra framework A1) and a broad band around 3530 cm -~, attributed to internal bridging SiAl-OH groups were observed [3]. Table 1. Crystallite size, Pt dispersion, concentration of strong BrCnsted acid sites and SiAl-OH frequency shift after adsorption of benzene
HZSM-5 (M 28) HBETA (M 27)
Pt-dispersion crystallite size [%] [nm] 15 0.9 14 0.6
SiAl-OH Avon (benzene) [mmole g-l] [cm_l] 0.37 354 0.61 346
During the n-heptane conversion over HZSM-5 (Figure 1) in the IR-reaction cell the band at 3610 cm -~ decreased and the perturbed OH-groups gave rise to a band at 3447 cm -1. New bands appeared due to adsorbed n-C7 between 2700 and 3000 cm -l, which were assigned to the stretching modes of Vasy (C-H) in CH3, (2958 cm -l) Vasy(C-H) in CH2 (2939 cm -1) and fused bands of the stretching modes of Vsym(C-H) in -CH3 and -CH2- (2866 cm-l). Interaction of the alkane with the silanol groups was only found at low temperatures and high gas pressures. The formation of coke precursors could be excluded, as the spectra after reaction did not show bands at 1600, 1554, 1500 and 1343 cm -~, due to carbonaceous species [4]. Furthermore no CH-stretching vibrations above 3000 cm -~, resulting from unsaturated hydrocarbons, could be observed. At 210~ n-C7 was the main species adsorbed on the catalyst. With increasing reaction temperature the surface coverage, represented by the decrease of the SiAl-OH band and the intensity of the C-H stretching area, decreased. 1~~210~ Simultaneously, the dominating ~." surface species changed from n-C7 to reaction products as indicated by the change of the IR spectra in the f / / ~ ~ 285~ C-H stretching domain. The CH2 stretching vibration at 2939 cm -~ significantly decreased relative to a sharp band at 2965 cm -1, that can be attributed to the CH3 vibrations in i-Ca and C3, paralleled by the 3800 3400 3000 2600 decrease of the 1375 cm -~ band in wavenumber [cm-l] the CH deformation region. Hence, an increase in concentration of CH3 groups at the expense of CH2 Figure 1. Difference spectra of the in situ investigation during the n-heptane conversion over Pt/H-ZSM5. groups during reaction was shown. At 350~ only low intensities of the bands attributed to adsorbed molecules were observed. At this temperature the strong acid sites seemed to be uncovered, suggesting that only a low number of sites was actually contributing to the n-C7 conversion. Note that the reactant or product molecules occupied only 34% and 7% of the BrCnsted acid sites at 210~ and 285~ respectively.
/
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To identify the nature of the surface species, the IR spectra during the conversion of n-C7 over Pt/HZSM-5 were compared to the mathematically added spectra of the substances in the gas stream after reaction. The adsorption of the pure hydrocarbons (C3, i-Ca, n-C4, n-C7 and 2MC6) was recorded at 250~ on HZSM-5 (Module 1000), which possesses only a minor ,--, 80
,--, 40
O
O
o
60
30
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40
20
~
20
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10
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~"
0
0 0
20 40 60 n-C~ conversion [%]
80
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20 40 n-C 7 conversion [%]
60
Figure 2. Conversion/Yield dependencies of the n-C7 conversion over (a) Pt/HZSM-5 and (b) Pt/HI3 9 C3, A i-C4, V n-C4, O methyl-C6, ~di-methyl-C5 catalytic activity, in order to obtain spectra on a surface and conditions comparable to the reaction over HZSM-5 (M 28). The spectra were added according to the selectivities obtained from the analysis of the reaction products, which revealed that the main products of the n-C7 conversion reaction were C3 and i-Ca over both, Pt/I-IZSM-5 and Pt/HBETA (Figure 2). On the latter, a higher concentration of monomethyl-hexanes and dimethyl pentanes was found, which appeared as primary products. As shown in Figure 3 the C-H stretching vibrations of the "7. added spectra of propane and iso-butane (Fig. 3(a)) perfectly matched with the spectra of the n-C7 reaction over Pt/HZSM-5 at J (a) 350~ and 80% total conversion (b_. . ) ) j (Fig. 3(b)). The spectra of the (c) conversion of n-C7 over J Pt/I-IBETA (Fig. 3(c)) showed a higher concentration of heptane 3100 3000 2900 2800 2700 2600 isomers on the surface of the wavenumber [cm-1] Figure 3. catalyst, indicated by the CH2 (a) Addition of the of propane and iso-butane spectra adsorbed stretching vibrations at 2935 cm -~, on HZSM-5 (M 1000) confirming the results of the (b) IR-spectra during the n-heptane conversion over Pt/HZSM-5 product analysis. (M 28), T = 350~ total conversion = 80% In Figure 4 the time resolved (c) IR-spectra during the n-heptane conversion over Pt/H[~ (M IR spectra of the in situ 27), T = 350~ conversion = 52% measurements at 250~ are shown. When the hydrocarbon flow was stopped, the concentration of propane and iso-butane on the surface increased rapidly and the number of interacting BrCnsted acid sites decreased with the time of reaction.
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Table 2. Relative diffusivities and rates of n-C7 and 2-MC6 D(n-C7/2-MC6) Pt/HZSM-5 Pt/HBETA
100 1
rates [mol.s-~.g -l] n-C7 2-MC6 2.9 10-5 1.5 10-5 1.5 104 1.6 10-5
n-C7 22 11
iso/n-C4 2-MC6 24 12
In Table 2 the rates at conversions below 10% and the relative diffusion coeff'lcients of n-C7 and 2-MC6 are given. The diffusivity of n-C7 was the same over both catalysts, while for 2-MC6 great differences were observed. On HZSM-5 the ratio of the diffusion coefficients of n-C7 and 2-MC6 was 100, whereas on HBETA it was 1, which was reflected in the activity for the respective reactants. 4. DISCUSSION The hydroconversion of n-alkanes over bifunctional catalysts is supposed to proceed via a mechanism that was suggested by Weisz et al. [5]. In this reaction scheme the isomerization of alkenes on the acid sites is assumed to be the rate determining step. Isomerized and cracked products are hydrogenated and desorbe as saturated hydrocarbons. For the reaction step on the acid sites, S. Tiong Sie introduced the protonated cyclopropane structure as intermediate for o isomerization and cracking [6]. According 3800 3400 3000 2600 ~' to this mechanism, cracking reactions wavenumber [cm -1] should occur parallel with isomerization. Figure4. Time resolved IR spectra of the n-C7 As shown by adsorption measureconversion over Pt/HZSM-5, T = 250~ ments, all acid sites were accessible for nheptane and 2-methyl-hexane in P t ~ Z S M - 5 and P t ~ B E T A . The adsorption kinetics under non-reactive conditions (40~ 0.1 mbar) indicated that HZSM-5 imposed sterical restrictions on the transport of 2-methylhexane whereas no restrictions were observed for the large pore HBETA zeolite. This difference was reflected in the diffusion coefficients, as the same values for the diffusion coefficient were determined for n-heptane in both zeolites, while the value for 2-methylhexane was much lower on HZSM-5 than over HBETA. At 200~ the IR spectra showed n-heptane adsorbed on the Br6nsted acid sites (Fig. 1) The sorbed molecule is able to interact with the zeolite lattice (pore walls), the acid and metal sites and with other sorbed molecules. If enough metal sites are present on the catalyst the heptane-heptene equilibrium will be established. However the heptene concentration is extremely small. Therefore IR bands corresponding to alkenes could not be observed in the spectrum. At 250~ the number of Br6nsted acid occupied by the reactant decreased and the intensities of the stretching vibration bands of CH3 and CH2 changed indicating an increase of CH3 groups. Reactant or product molecules were interacting with only a small part of the acid
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sites at 300~ In agreement with the results of Mirth et al. [7] for xylene isomerization and Jolly et. al. [8], who investigated n-hexane cracking over HY zeolites, the intensity of the hydroxyl bands of the catalysts showed no decrease at 350~ compared to the spectra of the activated surface and thus a great part of the acid sites was uncovered. When the continously stirred tank reactor was converted to a batch reactor by turning off the hydrogen flow, the number of free acid sites increased as the reaction proceeded, due to the lower enthalpy of adsorption of the reaction products [9]. In the gas phase mainly cracking products, propane and i-butane, besides monomethylhexanes over Pt/HZSM-5 and monomethyl-hexanes and dimethyl-pentanes over Pt/HBETA were found. In the conversion-yield plots both, the cracked and isomerized products seem to be formed in a primary reaction. This is in agreement with Guisnet et al. [10], who found cracking as the main primary reaction for the conversion of n-heptane over Pt/HZSM-5. However, cracking products cannot be the result of the direct scission of n-heptane since ibutane and not n-butane is produced. Therefore i-butane can be only the product from cracking via a cyclopropyl-alkyl intermediate suggested by Sie. Otherwise, i-butane formed by l-cracking of dimethyl-pentanes would appear as secondary products. Molecular diffusion plays an important role in the reaction over microporous catalysts. Experimentally determined diffusion coefficients may contribute to further theoretical understanding of the molecular processes taking place on the catalyst surface. At lower temperatures cracking rates were not diffusion limited and the reaction rates showed a stronger temperature dependence than the diffusivities. At higher temperatures the activity is mainly governed by the diffusion of the products, as indicated by their increasing surface concentration, suggesting that their initial rate of formation must be higher than the transport out of the pore system. This clearly indicates that the diffusion of the products out of the zeolite pores strongly affects the observed product distribution in the gas phase.
ACKNOWLEDGEMENTS This work was gratefully supported by the ,,Fond zur F6rderung der wissenschaftlichen Forschung", Proj. Nr. 11749 and the "Oesterreichische Nationalbank" Proj. Nr. 5410. REFERENCES
1. 2. 3.
J. Crank, Mathematics of Diffusion, p. 62 ff, Oxford University Press, London 1956. G. Mirth, F. Eder and J.A. Lercher, Appl. Spectrosc., 48, 2 (1994) 194. I. Kiricsi, C. Flego, G. Pazzuconi, W.O. Parker jr., R. Millini, C. Perego and G. Bellussi, J. Phys. Chem., 98 (1994) 4627. 4. J.F. Joly, N. Zanier-Szydlowski, S. Colin, F. Raatz, J. Saussey and J.C. Lavalley, Catal. Today, 9 (1991) 31. 5. P.B. Weisz and E.W. Swegler, Science, 126 (1957) 31. 6. S. Tiong Sie, Ind. Eng. Chem. Res., 32 (1993) 403. 7. G. Mirth, J. Cejka and J.A. Lercher, J. Catal., 139 (1994) 24. 8. S. Jolly, J. Saussey and J.C. Lavalley, J. Mol. Catal., 86 (1994) 401. 9. F. Eder, Thesis, University of Twente, Enschede, The Netherlands (1996). 10. M. Guisnet, F. Alvarez, G. Giannetto and G. Perot, Catal. Today, 1 (1987) 415.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
A STEADY STATE ISOTOPIC TRANSIENT KINETIC ANALYSIS OF THE F I S C H E R - T R O P S C H S Y N T H E S I S R E A C T I O N O V E R A C O B A L T BASED CATALYST H.A.J. van Dijk, J.H.B.J. Hoebink, J.C. Schouten Laboratory for Chemical Reactor Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands A transient kinetic analysis is presented of the Fischer-Tropsch synthesis reaction using the SSITKA technique in combination with a GCMS analysis of the 13C-labeled hydrocarbon reaction products. By using the complete MS fragmentation pattern, both the total fraction labeled components as well as the transient responses for each C1 to C5 isotopomer are obtained. The experimental data give rise to a mechanism with a parallel route towards both methane and C2§ hydrocarbons or a buffer step for the Cl,ads species. 1. INTRODUCTION The Fischer-Tropsch synthesis, i.e. the conversion of synthesis gas into hydrocarbons, is an alternative route for the production of transportation fuels and for the liquefaction of natural gas. Although the synthesis is applied on industrial scale [ 1,2], the reaction mechanism is not yet fully understood. Early steady state kinetic modeling led to overall mechanistic models with only one kinetic parameter: the chain growth probability ot [3]. Recent more careful examinations of the product spectrum under different reaction conditions have led to more sophisticated models accounting for the readsorption of reactive olefins [4,5,6]. However, transient experiments are more promising for enraveling the reaction mechanism. With the Steady State Isotopic Transient Kinetic Analysis (SSITKA) technique, the catalyst is kept under steady state conditions and an isotopic transient is introduced by abruptly replacing one reactant by its isotope. Compared to other transient techniques, the surface composition of the catalyst does not change during SSITKA because of the overall steady state [7]. This technique has been widely used to study methane formation on Fischer-Tropsch catalysts [8,9]. This paper expands the technique towards the C1 to C5 hydrocarbons [10], both paraffins and olefins, while accounting for the degree of labeling of the reaction products. 2. EXPERIMENTAL SET-UP The SSITKA experiments are performed on an experimental set-up consisting of feed, reactor, and analysis sections. Mass flow controllers for every component in the feed section provide two feed streams with identical composition and total flow, which is necessary for the
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generation of an isotopic step-change without pressure effects. One feed stream contains a maximum of 5 vol% Ne as inert tracer to monitor both the quality of the isotopic step and the gas hold-up in the reactor. The other feed stream contains ~3CO instead of ~2CO. The synthesis gas is diluted with 40 vol% He or He plus Ne. A stainless steel tubular fixed bed reactor with 7 mm internal diameter and 40 mm length is used which is free of dead volumes, eliminating axial gas mixing. All product containing lines and valves are heated up to 388 K. The steady state performance of the catalyst is monitored by on-line gas-chromatography (GC), quantifying the C~ to Ct0 product spectnma. On-line quadrupole mass-spectrometry (MS) with reactor sampling via heated capillaries is used to monitor the transient responses of Ne, ~3CO, and ~3CI-h. Finally, a gas-chromatograph-mass-spectrometer (GCMS) equipped with a 12-loop sample valve is used for the determination of the transients of the C2 to C5 carbon labeled hydrocarbons. This particular analysis will be discussed in detail below. Because of the heavy overlap of the fragmentation patterns of hydrocarbons in massspectrometry, on-line MS cannot provide the transient responses for these products. For this reason a 12-loop sampling valve collects samples during the isotopic transient, which are sequentially analyzed by GCMS. The SSITKA experiment produces a hydrocarbon product mixture, containing not only different hydrocarbon types, but also the non-, partially and fully labeled variants for every hydrocarbon type. First, each sample is separated into the different hydrocarbon types. Second, the fragmentation pattern of each hydrocarbon type is recorded for the C1 to C5 range. Third, this fragmentation pattern is used to calculate the isotopic composition of the mixture of different isotopic variants for the particular hydrocarbon type. The observed fragmentation pattern can be considered as a linear combination of the fragmentation patterns for the individual isotopic components. By minimizing the difference between the observed and the calculated value for every m/z position in the fragmentation pattern, the contribution of each isotopic variant can be quantified. This requires the fragmentation pattern of each isotopic hydrocarbon to be known. These can be determined from the pattern of the non-labeled component when assuming that (i) peak intensities do not depend on the presence of 13C, and (ii) secondary fragmentations can be neglected. This only holds for symmetrical hydrocarbons (n-paraffins, ethene and 2-butenes). For asymmetrical hydrocarbons (e.g. propene), the fragmentation patterns for the different isotopomers cannot be determined from the non-labeled compound. Then, only the overall fraction labeling can be obtained by using the m/z region corresponding to the non-fragmented molecule. All experiments are performed on a Co/Ru/TiO2 catalyst with an average pellet size of 0.23 mm of which the composition is based on the work of Iglesia et al. [11]. Experiments are performed at low conversions and at intrinsic kinetic conditions. A combined set of steady state and transient data is obtained at 498 K and 1.2 bar at different bed residence times and H2:CO feed ratios. At these conditions considerable amounts of 2-olefins, 1-methyl-l-olefins, and 1-methyl-paraffins are produced besides the desired 1-olefins and n-paraffins. Low amounts of alcohols but no aromatics are detected. Typical c~4-t0 values ranging from 0.3 to 0.6 are obtained. 3. EXPERIMENTAL RESULTS Typical transient responses following a Ne/He/12CO/H2 ---> He/13CO/H2 step change are presented in figures 1 to 4. The chromatographic effect for CO, i.e. the fast exchange between adsorbed CO and CO in the gas phase, is responsible for the fast but delayed ~3CO response in figure 1. The delay is a measure for the surface concentration of adsorbed CO. Furthermore, it
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~1,o 9 "
"~,~...' 1.o
Ne
m I~ 0.6
13CH4
Q. I~ 0.6 "O QN0.4
~' NQ0.4 0.2 9
.............................................................................................................................................................. ~~ ~ 1 2 C 2 H 6 "
,s"
-~'-" 12C2H4
~ 0"2 iI~~" ~" " " ~"l b ~ , ~ ~
o.o
0
,
,
50
100
o
o.o-.
150
~
0
o.-l~
L ....................................................................................................................................................................................
~
1-'-
.
--
200
300
Time / s
Figure 1" Normalized responses for Ne, 13CO, and t3CH4 at H2/CO= 1 and W/F = 44.8 mol kgcat"l s"l.
o 0.8 m "on" 0.6
"~"~ 13C2H412C3'CH4
100
Time / s
9
""4)- 12C13CH6 -41- '3CzH6
-
Figure 2" Normalized responses for ethane ( ~ )
and ethene ( ...... ), conditions as in figure 1. l
~ 0.8 ~Q"
12C3Hs nCz13CHs
z 0.0
......................................................................................................................................................................................
I1
It
12 ,o -!1- lzCz13CzHlo
z 0.0 0
100
200 Time I s
Figure 3" Normalized responses for propane.
300
0
100
200
300
Time I s
Figure 4" Normalized responses for n-butane.
is observed that 13CH4 already evolves before 13CO is observed, indicating the absence of a strong interaction of CH4 with the catalyst. The similarity of the shapes of the transient responses for the C2 to C5 hydrocarbons (e.g. figures 2 to 4) illustrates the common surface chemistry of these products, characteristic for a polymerization-like process. From the transients for each isotopic variant in figures 2 to 4, the transient for the overall fraction labeled can be calculated for each hydrocarbon. The shapes of these responses can be compared to the methane response. They also appear to be similar in shape. The consequences of this observation are highlighted in the modeling section. The 1-olefin responses show a time lag compared to the paraffinic counterparts, as shown in figure 2 for ethane and ethene, and this lag increases with bed residence time. Similar to the reasoning of the chromatographic effect for CO, this delay without deformation of the shape is the result of a reversible process, i.e. the olefin readsorption. However, this lag is absent for the 2-olefins and the 1-methyl-l-olefins of the Ca hydrocarbons (not shown). Together with the observation that the changes in the selectivity of these latter products with bed residence time are similar to the behavior of the n-paraffins, we conclude that internal and iso-olefins may undergo readsorption, but much less than the 1-olefin. Although readsorption occurs, the Anderson-Schulz-Flory-plots for the C3 to C~0 products are essentially straight lines, indicating the absence of a strong chain length dependence of the olefin readsorption process under the applied conditions. This is also illustrated by the relative
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insensitivity of the chain growth probability with the bed residence time under the current experimental conditions, although it is sensitive to the H2:CO ratio in the feed gas.
4. M O D E L I N G The experimental results are modeled under the application of a plug-flow reactor model. It allows the incorporation of fast reversible reaction steps in the mechanism [7,12], like the CO chromatographic effect and the readsorption of olefins. When a CSTR reactor model is applied, concentration gradients over the catalyst bed due to these reversible reactions are neglected, which may lead to erroneous results. Tested kinetic models are based on a carbene mechanism, assuming a CH2,aas species as monomeric building block. A first consideration of methanation alone shows that models with a parallel route towards methane rather than a single route describe the responses significantly better. The best fit for models 1 and 2, both having a single route towards methane, is represented in figure 5. A better fit, as illustrated in figure 6, is obtained for models 3 through 6, having a parallel route towards methane or a buffer step. It is not possible to discriminate between models 3 through 6, since they result in identical fits. Model 1
Model 2
CH4,g COads-'--~ Cads
i~
COads"-'-~ Cot,ads
%
Model 4
Model 3
i~ g / CH41'g i~ g COads--'-~ Cot,ads COads-'-~
CH4 T'~ C~,ads Model 6
Model 5
CH41'g i~ g Cot,ads
> C~,ads
Cox,ads
COads
\
> C~,ads
COads Cox,ads
%
> C~,ads
> C~,ads
, 1.0 ~ 0.8
~ o.8
n,
~0.6
0.6
.~ 0.4 Z
0.2 0.0
j/
9.'7
0
~" 0.4
0.0 50
100 Time
150
0
i
50
Is
Figure 5: Fit for ~3CO (4,) and 13CH4(A) according to models 1 and 2. Symbols: data; lines: models,
100
150
Time/s
13CO(,)
Figure 6: Fit for and 13CH4(~k) according to models 3 through 6. Symbols: data; lines: models.
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On the basis of these 13CO SSITKA experiments alone, the chemical background of the Ca,ads and C~,ads species cannot be assessed. Moreover, the surface concentrations of adsorbed atomic hydrogen and of vacant sites need to be lumped into the reaction rate coefficients. The methanation models 3 through 6 are used as a basis for the formulation of models for the Fischer-Tropsch reaction. They can again be divided into models possessing a single or a parallel route towards the C2+ hydrocarbons, with examples of both types presented below. Model 3.1 contains a parallel route towards methane, but only one C-species contributes to chaingrowth, whereas in model 4.1 both C-species contribute to methane formation and to chaingrowth. The best fits for both models are presented in figure 7 and 8, where for clarity only the methane and ethane transients are displayed. Only models of type 4.1 can describe the similarity in shape of the ethane transients with the methane transient mentioned in the previous section. This implies that both types of C-species must contribute to methanation and to C2+ hydrocarbon formation. The hypothesis that one C-species leads preferably to methane and the other to C2+ hydrocarbons is therefore not supported by the data. Using model 4.1, typical values for surface coverages are calculated: the surface coverage for COads amounts to 70%, for Ca,ads to 20%, for CI3,ads to 7%, and for C2+,ads, to 1%. The surface is therefore mainly covered with CO and C1 species. Consequently, the surface concentrations of growing hydrocarbon chains are very small. Furthermore, the formation of C~,ads and C~,ads from CO are the slowest steps in the mechanism. From C~,ads onwards all processes are relatively fast, in agreement with the findings of Mims and McCandlish [ 13].
i~g
/
CH4 l ,g
C Oads--'-~ Cox,ads
co 0.8 Q. n,,
/ ~
~ oO.-""
u) 9 0.6
--
T
Model 4.1: Two paths towards methane and C2+.
:
1.0
o
I
l/,/
>
Model 3.1" Two paths towards methane, one path towards C2+.
o
c2n6,g c2n4,g
COads-''-~ Ca,ad s
I
,
CH4,g
C2H l 6,g C///2H4,g
. 1.0 q) w c 0.8 I~ 0.6
O.4
o O.2 Z 0.0 0
.....
- ..........
5O Time
/s
.
i
100
150
Figure 7: Transient responses for 13CH4 ( " and +), nCI3CH6 (& and ~ ) , and 13C2H6 (ll and El) according to model 3.1. Data: ... ; calculations: m
0.0
0
50 Time
/s
100
150
Figure 8" Transient responses 13CH4 (~l~ and +), 12C13CH6 (A and ~ ) , and 13C2H6 (Ira and I-'1) according to model 4.1. Data: .... calculations:
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Besides model 4.1, the experimental results can be equally well described by a similar scheme based on model 6. This scheme possesses a buffer step for the C l,ads species, affecting both methane and the C2+ hydrocarbons. Incorporation of a depolymerization reaction, as frequently observed by olefin cofeed studies [ 14], provides a buffer with only a small capacity since the surface concentration of growing hydrocarbon chains is small. The introduction of a surface depolymerization step in Fischer-Tropsch schemes based on models 3 and 5 does therefore not alter the responses for the C2+ hydrocarbons in the way the buffer step in model 6 does, and results in transient responses similar to those presented in figure 7. 5. CONCLUSIONS The combination of the transient SSITKA technique with a GCMS analysis of the C-labeled hydrocarbon reaction products is a powerful tool for a mechanistic investigation of the Fischer-Tropsch synthesis reaction. Only the 1-olefins display considerable readsorption onto the Fischer-Tropsch chain growth sites, whereas for iso- and 2-olefins this readsorption occurs in far less extent. The presence of a second route towards both methane and C2+ hydrocarbons or the presence of a buffer step for the Cl,ads species is required to model the experimental results. CO and C l,ads predominantly cover the catalyst surface and after the initialization of chaingrowth the surface reactions become relatively fast. ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support provided by the Commission of the European Union in the framework of the DG XII-JOULE program, subprogram Energy from Fossil Sources, Hydrocarbons (JOF3-CT95-00016). REFERENCES
10 11 12 13 14
M.E. Dry, Catal. Today, 6 (1990) 183 M.M.G. Senden, A.D. Punt, A. Hoek, Stud. Surf. Sci., 119 (1998) 961 B.W. Wojchiechowski, Catal. Rev.-Sci. Eng., 30 (1988) 629 E. Iglesia, S.C. Reyes, R.J. Madon, S.L. Soled, Adv. Catal., 39 (1993) 221 E.W. Kuipers, I.H. Vinkenburg, H. Oosterbeek, J. Catal., 152 (1995) 137 G.P. van der Laan, A.A.C. Beenackers, Ind. Eng. Chem. Res., 38 (1999) 1277 J. Happel, Isotopic assessment of heterogeneous catalysis, London, Academic Press, 1986 A.R. Belambe, R. Oukaci, J.G. Goodwin, J. Catal., 166 (1997) 8 S. Vada, E. Blekkan, A. Hilmen, A. Hoff, D. Schanke, A. Holmen, J. Catal., 156 (1995) 85 T. Komaya, A.T. Bell, J. Catal., 146 (1994) 237 E. Iglesia, S.L. Soled, R.A. Fiato, G.H. Via, J. Catal., 143 (1993) 345 R.H. Nibbelke, J. Scheerova, M.H.J.M. de Croon, G.B. Marin, J. Catal., 156 (1995) 106 C.A. Mims, L.E. McCandlish, J. Phys. Chem., 91 (1987) 929 D.S. Jordan, A.T. Bell, J. Phys. Chem., 90 (1986) 4797
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Use of membranes in Fischer-Tropsch reactors R.L. Espinoza a , E. du Toit a, J. Santamaria b, M. Menendez b, J. Coronas b and S. Irusta b a Sasol Technology, P.O.Box 1, Sasolburg 9570, South Africa b Dpt of Chemical and Env. Eng., University of Zaragoza, 50009 Zaragoza, Spain
Water is one of the primary products in the Fischer-Tropsch (FT) process for the conversion of coal or natural gas derived synthesis gas to hydrocarbons. This reaction water oxidizes the FT active sites, thereby shortening the catalyst life. For iron based catalysts it has the additional negative effect of inhibiting the reaction rate. A family of membranes has been developed for the highly selective in-situ removal of the FT reaction water. These membranes have proven to be effective at operating conditions typical of commercial fluidized and slurry bed FT reactors. The use of these membranes will not only result in longer FT catalyst life but also in a better reactor utilization. 1. INTRODUCTION In spite of its long life, the Fischer-Tropsch (FT) process for the conversion of coal or natural gas derived synthesis gas to hydrocarbons is still a very dynamic technology. Continuous improvements are being made in areas such as natural gas reforming, reactor technology and catalyst development. In the reactor technology front, some of these developments include the Slurry Phase Fischer-Tropsch reactor [e.g. 1,2] and the Sasol Advanced Synthol (SAS) process (fluidized bed reactor) [e.g. 3], while in the catalyst development front there has been numerous patents dealing with cobalt based catalysts for use in low temperature FT [e.g. 4], although the efforts placed towards the improvements of iron based low temperature FT catalysts [e.g. 5] did not receive the same degree of attention. A common feature of all FT catalysts, whether cobalt or iron based, or for low or high temperature FT application, is that they are reduced and carbided. Under these conditions, it is obvious that oxidation of the working catalyst has to be avoided or minimized. Under typical FT conditions, this oxidation can be caused by water. This water is produced directly by the hydrocarbon synthesis, according to equation 1. CO + (1 + x ) H 2 --~ CH2x + H 2 0
(1)
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2. S O M E N E G A T I V E E F F E C T S OF THE PARTIAL PRESSURE OF W A T E R ON THE FT PROCESS AND E F F E C T OF ITS IN-SITU REMOVAL
Although the effect of water on the deactivation of iron and cobalt based catalysts at low temperature FT [e.g. 2,6,7] and high temperature FT [e.g. 8] have been previously described, some comments are still necessary in order to have a clear picture. It is generally accepted that the rate of oxidation for iron and cobalt based catalysts increases with the water partial pressure. It is also accepted that the rate of oxidation is higher for iron based catalysts as compared to cobalt based ones. This means that an in-situ removal of the FT reaction water will result in a decrease of the oxidation rate, the decrease being more pronounced the higher the amount of water removed. In addition, water has an inhibiting effect on the rate of reaction for iron based FT catalysts. Therefore, for iron based catalysts, the removal of the reaction water will result in higher per pass conversions, due to a more favorable kinetic environment. In conclusion, and due to kinetic and\or oxidation considerations, the in-situ partial removal of the reaction water will result in an increase in the maximum practical per pass H2+CO conversions, therefore lowering the cost of the FT commercial plants based on both iron or cobalt FT catalysts. 3. USE OF MEMBRANES F O R IN-SITU REMOVAL OF FT REACTION WATER Several zeolite membranes and supports were tested (9) under a wide range of operating simulating conditions typical of SAS reactors (gas phase, 300 to 350 ~ 17 to 23 bar) and slurry bed reactor conditions (gas-liquid, 200 to 250 ~ about 20 bar) Mordenite, ZSM-5 and silicalite membranes were deposited on a stainless steel support. In addition, a mordenite membrane was deposited on an alumina support. They had a permeation area of about 8 to 9 cm 2. The mordenite membranes were prepared by in-situ hydrothermal synthesis onto a porous stainless steel support or onto a commercial c~-alumina support obtained from Societe des Ceramiques Techniques of France, following a procedure described by Salomon et al [10]. Significant amounts of ZSM-5 and chabazite were also found to be present in the zeolite material. The ZSM-5 membranes were deposited on a porous stainless steel tubular support following a procedure described by Coronas et al [11] while a silicalite membrane was deposited on a stainless steel support following a procedure described by Jia et al [ 12]. 4. EXPERIMENTAL RESULTS The conditions inside both the fluidized (SAS) and slurry bed FT reactors were simulated by feeding the main components inside each reactor at its typical operating
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temperature and pressure. hydrodynamic constrains.
No solids were used to simulate the catalyst due to
4.1 Gas Phase experiments The aim of the study was to evaluate the system capability to separate water (W) from other species (e.g. H2, CO2, CH4 and nCs as an example of higher hydrocarbons) at conditions similar to those found in SAS reactors. The zeolite membrane was placed in a stainless steel module and sealed by means of graphite gaskets. The gaseous components were fed as a multicomponent mixture (CO was added in some experiments) to the membrane tube side. The water and nC8 were fed as liquids by means of two mass flow controllers and passed through two evaporators operating at 400 ~ The mixed feedstream entered the tube side of the membrane module, while the shell side was swept with a flux of N2. Gas chromatography (TCD and FID) was used to analyze the composition of the permeate and retentate streams. The permeance for each component ( P e r i ) was calculated in the standard manner, using the molar flow rates at the permeate side, the partial pressure differences between the permeate and retentate side and the available permeation area. The water/speciesi selectivity (Swi) was calculated as the ratio of the permeances (equation 2), while the separation factors (Otw/i), were calculated as the quotient between the ratio of the molar fractions of water and any species "i" in the permeate and retentate side (equation 3).
Sw/i
=
Perw , i ~ W Peri
Yw / Yi Otw/i = ~ ,
i~ W
(2)
(3)
XW / Xi
Some of the data obtained for the gas phase reactor are shown in Table 1
4.2 Slurry phase experiments To simulate the conditions inside a typical commercial slurry FT reactor, the experimental set-up had to be modified in order to have a liquid phase inside the membrane tube. This was achieved by means of adding a stainless steel vessel containing nC8 to simulate the liquid hydrocarbons inside a FT slurry bed reactor. The level of liquid nC8 inside the tubular zeolitic membrane was controlled by means of a line between the nC8 vessel and the retentate side, therefore equalizing their pressure. The feed lines had to be modified accordingly. The N2 sweep gas was fed on the shell side of the tubular membrane while the water and other species (H2, CO, CO2 and CH4) were fed (bubbled) in the inside section of the tubular membrane. Some of the results obtained for the slurry bed FT reactor are shown in Table 2.
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Table 1 Results for the gas phase experiments Feed
Pt
T
P.
Memb [*C] [bar] [bar] A A A A A B B B B B B B B* B*
150 196 355 349 350 353 367 298 345 343 350 346 244 359
20.9 19.5 19.7 22.3 22.4 18.7 20.0 22.3 20.1 18.8 18.0 18.5 17.2 20.8
3.8 3.3 7.9 6.9 9.4 7.3 7.5 8.3 1.9 2.1 2.8 3.0 7.9 4.4
W
C8
H2
6.9 7.4 16.7 12.5 25.2 21.0 18.4 22.5 13.9 22.9 15.3 16.6 24.0 17.0
0.6 0.7 3.4 3.3 2.3 1.8 1.5 2.8 .1.2 1.2 0.9 1.3 0.5 0.4
13.0 14.0 12.7 10.8 14.6 13.4 12.4 13.7 47.4 71.5 24.7 26.8 14.0 32.0
Water Flux
6.4 5.9 12.8 9.1 22.4 8.3 5.4 8.4 3.1 3.3 4.2 4.1 9.5 11.7
Pt
CH 4 CO 2 CO
[bar]
N2
11.0 13.0 13.2 12.1 15.2 14.5 14.3 13.7 37.0 55.0 15.7 17.3 9.3 18.0
20.0 18.7 18.6 21.2 20.7 18.5 18.7 19.8 18.2 18.3 16.5 16.8 16.6 19.7
47.0 45.0 14.8 15.4 9.2 10.0 13.8 12.1 26.7 27.2 24.9 23.6 45.0 45.0
6.5 7.6 2.6 2.0 2.5 3.2 2.5 2.4 3.0 16.1 20.7 5.8 8.8 4.6 11.8 4.9 12.0 -
Separation Selectivity
Memb [kg .m-2.h-1] W/C8 A A A A A B B B B B B B B* B*
Permeate
Component flow [mmol/min]
80.0 63.0 9.7 8.6 1.4 8.7 4.1 5.5 12.7 20.0 11.7 9.8 25.4 23.5
72.1 44.5 5.4 5.1 1.0 7.0 4.3 4.1 14.4 16.5 6.3 5.3 41.5 25.5
58.3 10.7 10.0 5.9 1.3 3.8 3.7 4.5 4.8 5.1 10.6 5.3 13.8 36.6
W
Cs
4.7 4.3 9.7 6.9 16.9 7.8 5.1 6.0 2.3 2.4 3.1 3.0 14.0 8.6
0.0 0.0 0.6 0.7 1.4 0.1 0.1 0.1 0.0 0.0 0.2 0.3 0.0 0.0
H2
CH. CO 2 CO
0.4 0.3 0.3 0.5 3.7 3.6 3.2 3.6 10.1 10.1 3.2 1.4 2.2 1.2 1.2 0.6 1.6 0.6 1.4 0.5 1.8 0.7 1.8 0.7 0.4 0.3 2.9 1.2
0.6 0.1 0.6 0.5 1.6 0.4 0.2 0.2 0.7 0.5 0.1 0.2 0.5 0.6
Separation Factor
W/H2 W/CH W / C O W/CO 76.0 66.0 4.5 5.2 0.8 2.2 1.9 2.1 7.0 7.7 3.8 3.4 48.4 18.3
II
Component flow [mmol/min]
3.7 -
W/C 8 W/H 2 W/CH W/CO W/CO 69.5 51.7 3.7 4.7 1.3 6.8 3.7 4.6 9.8 14.9 9.8 8.5 22.4 13.0
72.9 54.1 2.0 2.9 0.9 1.8 1.8 1.8 5.5 5.9 3.3 3.0 42.9 10.2
66.0 36.6 2.3 3.8 1.0 5.4 3.9 3.5 11.1 12.4 5.4 4.6 36.7 14.1
21.0 8.7 3.8 3.3 1.2 3.0 3.4 3.8 3.8 4.0 8.9 4.6 12.1 20.3
A: Mordenite on alumina support B" ZSM-5 on stainless steel support B*" ZSM-5 on stainless steel support, sweep gas inside of membrane tube
3.0 -
2.2 0.4 0.6 -
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Table 2 Results for the slurry phase (simulation) experiments Permeate
Feed
T
Pt
Memb [~ B B C
C C C D D D
250 248 248 254 257 253 237 258 240
Pw
W
18.2 21.5 20.5 21.8 22.3 22.1 19.0 20.1 20.9
7.5 7.0 0.4 0.6 2.0 1.8 1.2 0.9 1.9
3.6 4.0 0.3 0.3 1.1 1.0 0.8 0.6 1.4
Water Flux
C s Flow
Memb [kg. m'2.h'l] [kg -m'2-h'l] B B C C C C D D D
4.9 6.1 0.2 0.7 2.0 2.2 1.1 0.6 0.8
Pt
Component flow [mmol/min]
[bar] [bar]
2.6 2.4 5.8 1.8 2.1 2.5 10.1 7.2 5.1
Ca -
-
H2
CH 4 CO 2 CO
12.0 13.0 11.9 12.8 18.3 4.5 22.5 5.4 24.6 4.1 22.5 5.0 10.4 13.0 16.2 7.8 18.3 7.4
5.4 5.7 0.4 0.4 0.3 0.4 5.3 1.7 0.4
-
0.6 -
-
[bar] 17.5 20.8 18.8 19.2 20.2 19.7 18.1 18.9 19.2
Component flow [mmol/min] N2
W
50.4 3.6 45.7 4.5 45.7 0.1 37.3 0.5 41.9 1.5 50.0 1.6 46.7 0.8 49.9 0.4 46.0 0.5
C8
H2
CH4 CO2 CO
0.3 0.3 0.6 0.2 0.2 0.3 1.1 0.8 0.6
2.3 0.7 0.4 6.8 4.0 8.1 0.0 0.4 0.0
1.8 2.0 0.0 1.7 0.5 0.7 0.0 0.1 0.0
0.5 0.2 0.0 0.1 0.0 0.1 0.0 0.4 0.0
Separation Factor W/H 2 W/CH W / C O W/CO 36.0 25.0 23.0 28.0 13.6 22.0 353.0 37.0 226
B- ZSM-5 on stainless steel support D: Mordenite on stainless steel
43.0 8.9 127.0 25.0 67.0 72.0 308.0 171.0 273.0
35.0 16.0 9.5 24.0 42.0 44.0 116.0 82.0 15.5
4.8 -
C" Silicate on stainless steel
5. DISCUSSION The following observations can be made with respect to the experimental data" The membranes are able to selectively separate the water from other species, operating at a wide range of temperature (i.e. 150 to about 360 ~ The membranes were effective with both a gas phase and two phases (liquid and gas) in the retentate side, therefore simulating a fluidized bed and a slurry bed FT reactor. The water flux tends to increase with the partial pressure of water in the feed, while the separation selectivities and separation factors tend to increase with the flow of sweep gas, in this case nitrogen.
2.4 -
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At low values (e.g. 150 and 196 ~ runs, table 1) temperature tends to increase the separation factors and the selectivity. This effect tends to diminish at temperatures of 240 ~ and higher.
For an industrial application, it would be preferable for the zeolitic membrane not to be in contact with the catalyst particles. This is to avoid possible attrition of the zeolitic layer by contact with the catalyst particles. The experiments performed by feeding the water and other species to be separated on the shell side, and the sweep gas on the zeolitic membrane side (internal part of the tubular membrane), have shown that there is no loss of water flux or separation selectivity when operating in this manner. 6. CONCLUSIONS The results obtained show that these membranes can be used for the highly selective, in-situ removal of the FT reaction water at operating conditions typical of those encountered in commercial fluidized bed and slurry bed reactors. The zeolitic membranes can be successfully deposited on a porous stainless steel support, which will facilitate their industrial application. Further developments may include the increase of the water flux rate, since this will have a direct impact on the number of tubes (membranes) to be fitted inside the FT reactor. 7. R E F E R E N C E S
1. B. Jager, R.C. Keltkens and A.P. Steynberg, Natural Gas Conversion II, H.E. Curt'y-Hyde and R.F. Howe (eds), Elsevier Science B.V. (1994) 419. 2. B. Jager and R.L. Espinoza, Catalysis Today, 23 (1995) 17. 3. B. Jager, M.E. Dry, T. Shingles and A.P. Steynberg, Catal. Lea., 7 (1990) 293. 4. R.L. Espinoza, J.L. Visagie, P.J. van Berge and F.H. Bolder, RSA Patent 952903 (1995). 5. R.L. Espinoza, P. Gibson and J.H. Scholtz, RSA Patent 982737 (1998). 6. R.L. Espinoza, A.P. Steynberg, B. Jager and A.C. Vosloo. Applied Catal., A: General 186 (1999) 13-26. 7. D.J. Duvenhage, R.L. Espinoza and N.J. Coville, Catalyst Deactivation 1994, B. Delmon and G.F. Froment (eds), Studies in Surface Science and Catalysis, Vol 88, Elsevier Science B.V. 8. A.P. Steynberg, R.L. Espinoza, B. Jager and A.C. Vosloo, in print, Applied Catal., A: General 186 (1999). 9. R.L. Espinoza, J. Santamaria, M Menendez, J. Coronas and S. Irusta, PCT/IB 99/01043 10. M.A. Salomon, J, Coronas, M. Menendez and J. Santamaria, Chem. Com., (1998) 125. 11. J. Coronas, J.L. Falconer and R.D. Noble, AIChE J., 43 (&) (1997) 1797. 12. M.D. Jia, B. Chen, R.D. Noble and J.L. Falconer, J. Membrane Sci., 90 (1994) 1.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Egg-Shell Catalyst for the S y n t h e s i s of Middle Distillates C.Galarraga b , E.Peluso b and H.de Lasa a aCREC, University of Western Ontario, N6A 5B9, London, Ontario, Canada bpDVSA-INTEVEP, Apdo 76343, Caracas 1070, Venezuela This study demonstrates that various preparation parameters of the eggshell catalyst affect the evolution of the eggshell thickness and consequently the metal distribution, the metal morphology and the metal crystallite size. In addition, it is shown that preparation conditions influence in a Co-Zr catalyst supported on silica, the production of middle distillates and particularly the C10-C20 hydrocarbon fraction. On this basis, it is established that an optimum eggshell catalyst should have 10wt% Co deposited in about the half radius of a 1.81mm diameter particle. This eggshell catalyst displays, under reaction conditions, encouraging selectivity, yielding 65wt% hydrocarbons in the diesel range. 1. I N T R O D U C T I O N The synthesis of middle distillate hydrocarbons via Fischer-Tropsch synthesis (FTS) is a process strongly influenced by intra-catalyst mass transport limitations. These mass transport limitations are due to the relatively slow diffusion of high-molecular weight paraffins inside the catalyst pores. To solve these problems eggshell catalysts have been proposed [1]. These eggshell catalysts are engineered with an active phase deposited in the outer region of the catalyst pellet and they can provide a suitable solution to overcome the diffusional problems commonly encountered in the Fischer-Tropsch synthesis. Even more, better understanding of the various preparation parameters can allow to take full advantage of the eggshell catalysts reducing intra-particle mass transport and achieving high yields of desired middle distillate paraffinic hydrocarbons.
2. EXPERIMENTAL PROCEDURES To prepare the eggshell catalysts a silica support (DAR-240 from UOP) with a 372m2/g specific surface area and a 1.81mm average particle diameter was selected [2]. The following steps were adopted: a) the desired amount of support was placed into a fritted glass funnel mounted on a flask connected to a vacuum system; b) the impregnating solution was poured onto the support in a volume ratio solution/support of about 5, c) after the desired impregnation time was reached (between 5 to 60 seconds) the excess
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solution was rapidly evacuated from to the flask by connecting the vacuum to the system. While for dry impregnation, the support was directly used as received from the silica manufacturer, for the case of wet impregnation the support was prewetted with water. No excess of water was allowed on top of the particles (or in between particles). In addition and to change the viscosity of the solution from 2 cP to 40 cP (0.1 g Co/ml solution), hydroxyethylcellulose (in a concentration of 1 wt %) was added in the water solution as viscosifying agent. The impregnating solution was constituted by a cobalt nitrate solution with close to 5 wt% zirconia promoter content (based on the combined Zr+Co metal content). Once the support impregnated it was transferred to a fluidized bed made out of sand particles (60 microns average size), and kept at 90~ The goal of this operation was to "freeze", at this temperature, the movement of the solution inside the pores of the support. This was achieved given the excellent heat transfer conditions in the fluidized bed enhancing fast drying. As well, the drying under fluidized conditions prevented the collapse of the catalyst porous structure given the close control of temperature in this unit. Completed this step the impregnated catalyst samples were calcined at 400~ for about 5 hours with the temperature being increased at the rate of 5~ This methodology was adopted following preliminary studies showing that at these conditions there is complete decomposition of the cobalt precursor [2]. Also under these conditions little interactions between the cobalt and silica are expected and as a result it is believed that most of the impregnated cobalt should be available as active species for the hydrocarbon synthesis reaction [4]. The adequacy of this method was also confirmed, in this study, using TPR and showing a high degree of reducibility of the cobalt species (72-98%). In the present study, changing various preparation conditions (refer to Table 1) five series of catalysts composed of 25 samples were prepared. In this respect, various impregnation times, viscosities of the impregnating solution, solution concentrations and state of the support before impregnation (dry or wet) were changed systematically in a quite wide range. Table 1. Preparation Conditions. Eggshell catalysts. Solution State of the Series Impregnation Viscosity of Concentration Support The solution Time (s) (gr cobalt/ml) (centipoises) Dry 0.10 4~5~6~7~8~12~16 2 A Wet 0.10 4-5~8,12,16,25 2 B Dry 0.20 4~5~12~20~40 5 C 0.20 Wet 4~10~20~40 5 D Dry 0.10 40 4,20,60 E Optical microscopy, SEM, nitrogen adsorption, hydrogen chemisorption and temperature programmed reduction were employed for catalyst characterization. The viscosity of the impregnating solution was measured using a falling ball viscometer Type 2[2].
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In addition, the prepared catalyst samples were evaluated under reaction conditions using a Berty laboratory scale reactor [3]. The experimental set-up included gas-pressurized cylinders, molecular sieves to trap gas impurities and low and high temperature separators to achieve adequate separation of the gas and liquid hydrocarbon fractions. Before the reaction experiments, the catalyst was activated under hydrogen flow with the temperature being progressively increasing as described in [2]. Following this the reactor was cooled down to 180~ The temperature was then, increased to 210~ under H/CO flow. Fortyeight hours of operation were allowed under these conditions to reach steady state. Additional details about the experimental procedures, used in this study, are reported by Galarraga [2] and Peluso[3]. 3.1. R E S U L T S AND DISCUSSION 3.1. Eggshell Formation Experiments Regarding the results obtained the following observations were established: a) Fig.1 reports the evolution of the eggshell thickness with impregnation time for selected cases of the catalysts listed in Table 1. It is shown that that the impregnating solution penetrates faster in the dry support than in the prewet particles. Thus, the state of the support (dry or wet) is of significance on the characteristics of the external particle film formed. In addition, as reported in Fig. 1, it is demonstrated that the viscosity and the cobalt concentration of the solution are two key parameters affecting the evolution of the cobalt film. Low concentration High concentration .0 AIA 1.07-0
~0.8r
=
0.6-
r
~0.4ho
0.2-
'•//
/
V /
~0.8-
I,
0
b/~ /
0
/
~0
.m HI/ / Hi
~0.6 ~0.4-
f
/vi
r
.2cP-01-D o2cP-01-W Y40cP-01-D
"~0.2-
/ /
m5cP-02-D [::]5cP-02-W
O.Q~. , , 0 20 40 60 Impregnation time (s) Fig.1. Evolution of the eggshell catalyst thickness with impregnation time. Codes: a) first digits" solution viscosity in centipoises, b) second digits" solution concentration in g Co/ml, c) D or W symbol: dry or wet support. b) Hydrogen chemisorption demonstrated that metal dispersion was in all cases smaller than 5% with wet impregnation giving the higher metal dispersions" 2-4% for wet support versus 0.3-1.1% for dry support. Crystallite sizes were in the 25-100nm range and this suggested the formation of metal 0.0,"
0 20 40 60 Impregnation time (s)
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agglomerates. Agglomeration was also confirmed w i t h SEM w i t h cobalt a g g l o m e r a t i o n being more i m p o r t a n t for c a t a l y s t s p r e p a r e d by dry impregnation versus the ones prepared via wet impregnation. c) EDX analysis was employed to confirm the distribution of Co and silicon across the spherical pellet. For catalysts prepared using wet impregnation a quite uniform metal profile with a diffuse ring of metal, gradually decreasing with the pellet radial position was observed. On the other hand, for the dry impregnated samples, the metal ring displayed a more irregular Co distribution. d) EDX was also used to confirm that the zirconium promoter was placed in the outer particle region and it was closely associated with cobalt.
3.2. Catalytic Activity and Selectivity in Eggshell Catalysts Experimental runs were developed in a Berty reactor to evaluate the catalytic activity of the prepared eggshell catalysts as follows: a) CO turnover frequency (TOF) for various impregnation times, b) product distribution for various cobalt contents. Typical operating conditions considered during these experiments were as follows: a) temperature:210 ~ Total Pressure:l.5MPa, GHSV=350h ~, H2/CO=2. These operating conditions were selected as being representative of suitable conditions to establish the effects of catalyst preparation on the CO conversion, the chain growth probability and the product distribution [3]. Table 2. Catalytic activity and selectivity for selected catalysts. Catalyst TOF Selectivity Codes
~mol CO converted]s/g Co
HC
H20
CO s
Reference
13.5
39.7
57.7
2.6
4s-2cP-01-D 12s-2cP-01-D 4s-2cP-01-W
16.7 18.7 13.3
43.7 43.6 43.3
55.3 54.2 56.8
1.0 2.2 0.0
20s-2cP-01-W
18.9
42.2
57.8
0.0
5s-5cP-02-D
3.8
42.2
57.8
0.0
25s-5cP-02-D
13.7
43.9
53.4
2.8
4s-5cP-02-W
6.1
41.4
53.8
4.8
20s-5cP-02-W
8.7
42.7
57.3
0.0
40s-5cP-02-W 6.8 42.8 57.2 0.0 Note: For catalyst codes refer to the caption of Fig.1. Regarding the CO turnover frequency, based on the unit weight of cobalt loaded, it was observed (Table 2) that the "reference" catalyst (12wt%Co) converted 13.5 ~Lrnoles of carbon monoxide per second per gram of cobalt. The observed selectivity for this catalyst was as follows: 39.9 % of hydrocarbons, 57.7 % of water and 2.6 % carbon monoxide.
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It was also noticed that the eggshell catalysts exhibited different activity trends depending on the preparation methodology and the degree of coverage (eggshell thickness). In fact those catalysts prepared from low concentration solution of cobalt (series 2cP-01) were the most actives. These catalysts had an activity comparable to the one of the "reference" catalyst (uniformly impregnated) and even more in some cases superseded the turnover frequency (TOF in ~moles]gCo.s) for the reference catalyst. In fact, the most active eggshell catalysts were those prepared with a low concentration of cobalt in the impregnating solution: series 2cP-01, wet (W) and dry (D) impregnation (refer to Table 3). In fact, for short impregnation times (4s) both catalysts (series 2cP-01) prepared from dry and wet impregnation converted as much carbon monoxide as the reference catalyst did. However, when the impregnation time was increased from 4s to 12s and 20s for both dry and wet impregnation the TOF activity was respectively improved up to 30%. Regarding the eggshell catalysts prepared from high concentrated solutions (0.2 g Co/ml) they did not compare well in terms of TOF with the reference catalyst or with the eggshell catalysts impregnated with low concentrated solutions. Generally, for series 5cP-02-D and series 5cP-02-W the TOF activity was about half of that observed, at the same impregnation time, for catalysts prepared with low concentrated solutions (0.1 g Co/ml). In addition, it appears that there is an optimum time for impregnating the eggshell catalysts since it appears there is a maximum TOF and this for both catalysts impregnated with both low and high viscosity solutions (or low and high concentration of cobalt in solution). It was also observed that the eggshell catalysts with the higher Co content exhibited a reduced surface area (about 15%) with respect to those with lower Co content. Consistent with this lower TOF were observed for the catalyst with higher Co levels. Regarding the hydrocarbon selectivity for the eggshell catalysts, it was found that the productivity of hydrocarbons ranged from 41 to 44 % while compared to about 40 % for the reference catalysts. This hydrocarbon selectivity improvement can be traced to an increased accessibility of active sites, smaller diffusivity constraints with more opportunity given to the reactants to reach active sites and to the reaction to be completed. Fig. 2 reports the performance of selected eggshell catalyst in terms of product distribution versus the reference catalyst. The following can be concluded: a) Eggshell catalysts prepared impregnating a dry support (Fig.2a), gave 5564% product yield in the Clo-C~o range versus 42% for the reference catalyst. b) Eggshell catalysts, prepared impregnating a wet support (Fig.2b), gave 4557% product yield in the C10-C~0range versus 42% for the reference catalyst. Furthermore, the C10-C20selectivity trends of Fig. 3 demonstrate that there is an optimum catalyst formulation to produce hydrocarbons in the C~o-C~0 range. This optimum meets a compromise between cobalt content (close to 10 wt%) and eggshell thickness (close to 0.5). On the other hand, when comparing the
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400 different crystallite sizes it is also noticeable that to maximize the productivity of the Cto-C2o hydrocarbons, there is an optimum value of crystallite size (in the range of 67 - 89 nm), and consequently an optimum also for metal dispersion.
Product percentage, wt (%) a
80 60
t
'
40 20 0
CI
C 2 - C 4 C5-C9 C10-C20 C21+
CI
C2-C4 C5-C9 C10-C20 C21+
Fig.2. Product distribution for eggshell catalysts: a) dry impregnation, (b) wet impregnation. For codes refer to Fig.1. l~14s-2cP-01-D, r-112s-2cP-01-D,mReference
Productivity Clo-C2o range
100 80 60 40
-
(0.41) (0.47) ~, 67nm~-89nm (0.38) (0.~7) ~ ; ~ " " ~98nm 36nm~/ /P~ ~-, N[-~. ",~(0.75) (0.27) 27nm
20 I
(0.10) J ~ 50nm (1"00) 94nm
I
I
0 5 10 15 20 Cobalt content in the catalyst (wt%) Fig.3. C~o-C2oas a function of Co content. Crystallite sizes in nm. Eggshell thickness in between brackets. (.) 2cP-01-W.r ~n)5cP-02-D
REFERENCES
1. E. Iglesia, S. Soled, J. Baumgartner, and S. Reyes, J. Catal., 153, (1995) 108. 2. C. Galarraga, Master ofEngng. Sci., Univ. of Western Ontario, Canada (1998). 3. E. Peluso, Master of Engng.Sci., Univ. of Western Ontario, Canada (1998). 4. S. W. Ho, M. Houalla, and D. Hercules, J. Phys. Chem., 94 (1990) 6396.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
化
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Computer-Aided Design of Novel Heterogeneous Combinatorial Computational Chemistry Approach
Catalysts
-
A
Kenji Yajima, Yusuke Ueda, Hirotaka Tsuruya, Tomonori Kanougi, Yasunod Oumi, S.Salai Cheettu Ammal, Seiichi Takami, Momoji Kubo and Akira Miyamoto Department of Materials Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan Combinatorial chemistry is an efficient technique for the synthesis and screening of large number of compounds. Recently, we have proposed a combinatorial computational chemistry approach and have applied it to design of deNOx catalysts. Various ion-exchanged ZSM-5 are good candidates for the removal of nitrogen oxides (NOx) from exhaust gases in the presence of excess oxygen. In the present study we investigated the adsorption energies of the NO and SOx on various ion-exchanged ZSM-5 by using a combinatorial computational chemistry. It was found that the Cu§ Fe2., Co2§ Irz* and TI3§ ion-exchanged ZSM-5 catalysts have a high resistance to SOx poisoning during the deNOx reaction. 1. INTRODUCTION Combinatorial chemistry has been developed as an experimental method which make possible to synthesize hundreds of samples at once and to examine their properties in detail. Originally the combinatorial chemistry was proposed and applied mainly to the synthesis of organic compounds. Recently it was introduced also into the inorganic chemistry fields such as thin films [1], luminous bodies [2], and magnetoresistances [3]. Combinatorial chemistry is expected to be as a highly efficient screening method even in the inorganic material synthesis. Computational chemistry is mainly used to elucidate the mechanism of catalytic reactions including also catalytic activity, deactivation, and so on. In addition to such investigations at atomic and electronic levels, computational chemistry is expected to have an important role to predict new catalysts with high activity, high selectivity, and high resistance to poisons.
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Recently we introduced the concept of the combinatorial approach to the computational chemistry for a catalyst design and proposed a new method denoted "a combinatorial computational chemistry" [4]. In this approach, the effects of a large number of metals, supports, and additives on the catalytic activity can be calculated systematically using computer simulation techniques, which can predict the best element for each catalytic reaction. Removal of nitrogen oxides (NOx) from exhaust gases in the presence of excess oxygen is a global problem. Much effort has been done on this process in the past decade. It has been reported that the selective catalytic reduction of NOx species by hydrocarbons can be catalyzed by various ion-exchanged ZSM-5 [5-13]. However, almost all ion-exchanged ZSM-5 were found to be poisoned by the water or SOx [13]. Moreover, various exchange cations presented in ZSM-5 exhibit a different activity, selectivity and durability, and one cannot compare those results directly, since the experimental conditions are often not the same. We have applied computational chemistry to the deNOx reaction on some ion-exchanged ZSM-5 [14-17]. Recently we applied combinatorial computational chemistry to search for novel effective exchange cations in ZSM-5 with high resistance to water poisoning for deNOx reaction and proposed new candidates [4]. In the present study we tried to propose novel effective exchange cations in ZSM-5 with high resistance to SOx using a combinatorial computational chemistry. The activity and durability of numerous ion-exchanged ZSM-5 were investigated and discussed. 2. METHOD OF CALCULATION AND MODELS Molecular Dynamics (MD) calculations were carded out with the MXDORTO program developed by Kawamura [18] to determine the structure of the zeolite framework. The Verlet algorithm was used to calculate the atomic motions, while the Ewald method was applied to calculate the electrostatic interactions. All MD calculations were performed under the following conditions: a temperature of 300 K, a pressure of 0.1 MPa, a time-step of 2.5 x 10-~s s, and a simulation time of 1000020000 steps. Quantum chemical calculations were based on Density Functional Theory (DFT) and were performed using the Amsterdam Density Functional (ADF) program [19] which employs a quasi-relativistic spin-unrestricted frozen-core mode. Basis sets were represented by the atomic Slater-type orbital and corresponded to a double zeta plus polarization basis. Geometry optimizations were carried out at the local density approximation (LDA) level with the VVVN exchange correlation functional.
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The corrections to the overestimated adsorption energies have been accounted for by a gradient correlation in terms of the Becke88-Perdew functional. The framework structure of the MFI-type silicalite without AI incorporation was obtained from the X-ray diffraction (XRD) data. In order to determine the framework structure of ZSM-5 with AI incorporation, one should to calculate the unit cell of ZSM5 containing all 288 atoms under the three-dimensional periodic boundary conditions. MD calculations were carried out in order to determine the framework structure of ZSM-5. The T12 sites were considered for the aluminum substitution. Eadier quantum chemical studies [20] have also reported that T12 site was energetically favorable for the incorporation of the aluminum. Various cations were selected as exchanged cations. We selected K*, Cu § Ag*, and Au § as monovalent cations, Fe2., Co2., Ni2., Cu 2., Zn 2., Pd2., and Pt2. as divalent cations, and AI3., Sc3., Cr3., Fe3., Co 3., Ga 3., In3., Ir3. and TI3. as trivalent cations. From experimental results [21-23] we assumed that monovalent cations existed as M§ (M = metal), divalent cations as M2*OH and trivalent cations as M3§ 2. During the MD calculations, the exchange cations were attached to the AI tetrahedral site. MD calculations have been performed on the framework structures of M§ M2*-ZSM-5 and M3*-ZSM-5, where M§ = Cu*; M 2. = Cu2*; M3* = Ga 3*. From the optimized structures, an AIO4-X (X -- exchange cation) cluster was extracted. Four hydrogen atoms were added to saturate the dangling bonds, so the cluster became as AI(OH)4-X. This system was used as a model for the active site in our DFT calculations. The positions of each distinct exchange cation were optimized. During the DFT calculations, AI(OH)4 fragment was fixed at the geometry of the framework structure of ZSM-5. 3. RESULTS AND DISCUSSION Many high active catalysts could not be industrialized because of their short lifetime. One of the important masons for the deactivation of catalysts is their poisoning by co-existent gases such as water and SOx (SO2, SO3). Hence, the investigation on adsorption of poisoning gases will provide an important information for the design of catalysts with high resistance to poisons. Experimentally it is well known that an ion-exchanged ZSM-5 can be easily deactivated in the presence of SOx. Hence, we calculated the adsorption energies of the NO and SOx molecules on various ion-exchanged ZSM-5. Here the adsorption energy (E,=) was defined according to the following equation:
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E,~ = E ( ~ , m , m ) -[E(zsM-s)+ E(mo~uk,)] Therefore a large negative H O value of E,~ indicates that the Ga S molecule strongly adsorbs on ZSM5. Fig.l shows the optimized configuration for the SO2 molecule on Ga3*-ZSM-5. In order to investigate the effect of exchanged cations on the Fig.l Optimized configuration for the adsorption of SO2, we analyzed the SO2 molecule on Ga3+-ZSM-5 charge on exchanged metal cations before adsorption. Fig.2 shows the correlation between the charge on an exchanged metal cation and the adsorption energy of SO2 molecule on various ion-exchanged ZSM-5. The exchange metal cations with a large positive charge attract more strongly the SO2 molecule. In order to evaluate the ability of different exchanged cations exhibiting resistance to SOx poisoning, the difference in the adsorption energies (A E) of the NO and SOx molecules on various ion-exchanged ZSM-5 were calculated and plotted in Fig.3. The positive value of A E indicates that the SOx molecule strongly adsorbs on the exchanged cation as compared to the NO molecule. Hence, the exchanged cation with the positive value is readily deactivated by SOx molecules. On the contrary, the
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Fig. 4. Pore size distributions of xG25N-M catalysts. 4. CONCLUSIONS Gd element reacts with MgO below the saturation limit of 10 wt% referenced to Ni to increase Ni dispersion, bond strength between NiO and MgO support, and OH concentration on the catalyst surface as if steam feed were increased, which resulted in suppression of coke deposition. So, Gd was a promising cocatalyst for the internal steam reforming catalyst, Ni/MgO, in MCFC. ACKNOWLEDGEMENT This work was financially supported by R&D Management Center for Energy and Resources (RACER), The Korean Energy Management Corporation. REFERENCES
1. A. L. Dick, J. Pow. Sour., 71 (1998) 111. 2. M. C. Demicheli, D. Duprez, J. Barbier, O. A. Ferreti, and E. N. Ponzi, J. Catal., 145 (1994) 437. 3. J. R. Rostrup-Nielsen, in J. R. anderson and M. Boudart (Editors), CatalysisScience & Technology, Vol. 5, Springer-Verlag, Heidelberg, 1984, p. 1. 4. K. S. Jung, B.-Y. Coh, and H.-I. Lee, Bull. Korean Chem. Soc., 20 (1999) 89. 5. Q. Zhuang, Y. Qin, and L. Chang, Appl. Catal., 70 (1991) 1. 6. A. Slagtern, U. Olsbye, R. Blom, I. M. Dahl, and H. Fjellvag, Appl. Catal. A, 165 (1997) 379. 7. M. P. Rosynek, Catal. Rev. Sci. Eng., 16 (1977) 111. 8. I. Alstrup, B. S. Clausen, C. Olsen, R. H. H. Smits, and J. R. Rostrup-Nielson, Stud. Surf. Sci. Catal., 119 (1998) 5. 9. R. T. Yang and J. P. Chen, J. Catal., 115 (1989) 52. 10. O. Yamazaki, K. Tomishige, and K. Fujimoto, Appl. Catal. A, 136 (1996) 49.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
A Microstructured Catalytic Reactor/Heat Exchanger for the Controlled Catalytic Reaction between H2 and 02 M. Janicke a, A. Holzwarth a, M.Fichtner b, K. Schubert b, F. Schtith a Max Planck Institut ftir Kohlenforschung, P.O. Box 10 13 53, 45466 MUlheim, Germany b Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany
a
A microstructured catalytic reactor/heat exchanger has been developed, based on the microstructured heat exchanger of the Forschungszentrum Karlsruhe. One channel set of a crossflow device is coated with a platinum catalyst supported on A1203, the other channel set is used for heat removal, either by a gaseous or a liquid fluid. Stoichiometric mixtures of hydrogen and oxygen can be safely combusted in this device, while heat can be transferred either to a heat exchanger oil or used to evaporate water or methanol. Such a system can be useful as part of the fuel processor for fuel cell driven cars. 1. I N T R O D U C T I O N Microstructured reactors are gaining increasing interest in different fields of chemistry [ 1]. Such microstructured reactors are characterized by a high surface-to-volume ratio, small internal dimensions and well defined flow conditions. This suggests applications in several fields, i.e. reactions which need to be quenched rapidly to prevent subsequent steps [2,3], highly exothermic or endothermic reactions to prevent hot or cold spots [3], on demand production of chemicals needed only in small quantities or on the spot synthesis of highly toxic chemicals in order to avoid transportation of such substances [4], or reactions in the explosive regime [5,6]. Such a reaction is the combustion of H2 with 02, since hydrogen and oxygen are explosive in mixtures of almost any concentration. If dimensions of the structure, in which the reaction proceeds, are sufficiently small, the homogeneous chain reaction cannot propagate, since the quench distance is longer than the reactor dimensions. A microstructure as used in this work has been tested as an explosion barrier for this reaction. It was found to prevent flame propagation even at elevated pressure, if two vessels filled with H2/O2 are connected by such a structure and the gas in one of the vessels is electrically ignited [7]. In addition, since heat can be removed extremely efficiently in microstructures, overheating of the reactor can be avoided and the heat generated can be used for other purposes. Such applications of microstructured reactors can be envisaged in a fuel processor system for a fuel cell driven car [6]. The approach chosen by most car manufacturers for hydrogen fuel cell technology is the production of hydrogen on board from methanol or gasoline by steam reforming. Such a system requires many components, which have to be small and lightweight, i.e. this is the ideal application field for microstructures. The catalytic hydrogen combustion/heat exchanger unit would be used for the rapid and efficient evaporation of the liquid fuel before passing into the reformer stage. Such a system must efficiently evaporate methanol or gasoline with a very short warm up period. One possibility is electric heating, but
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such a system would be relatively heavy. An alternative is using a hydrogen sidestream from the main hydrogen stream and using the heat generated by the combustion for methanol evaporation. Another possible use of this reactor is the coupling of the exothermal combustion reaction with the endothermal steam reforming reaction itself.
2
Experimental
2.1
Reactors/Heat Exchanger
The catalytic reactor/heat exchanger is based on the microstructured crossflow heat exchanger designed and built by the Forschungszentrum Karlsruhe [8]. It consists of stainless steel plates with micromachined channels which are stacked in an alternating manner, whereby subsequent plates are perpendicular to each other. This creates a crossflow channel system. The plates are diffusion bonded to each other, providing a pressure tight fit between the channel sets. A Fig. 1: SEM of part of the microstructured stainless steel housing provides reactor, looking on one edge. Entrance to connections to the periphery via reaction channels upper left, entrance to cooling standard tube fittings. The channels channels lower right, section. used for cooling have a cross section of 70 tam (width) x 100 tam (height), the reaction channels are 140 tam x 100 tam. The channels in both sets are l0 mm long. A SEM of the comer of the reactor showing part of both sets of channels is shown in fig. 1.
2.2
Catalyst Preparation
In order to increase the surface area in the catalytic channels, a coating of A1203 has been deposited in these channels. There are alternative pathways to produce such coatings, i.e. deposition from a sol, or chemical vapor deposition (CVD). The best method in our case proved to be CVD, since it allows the formation of relatively homogeneously coated channels. For the CVD of A1203 we used aluminum isopropoxide as the precursor. It was kept molten in a loading vessel at 160~ and carried into the 140 tam x 100 tam channel set via a stream of ll/min of nitrogen bubbling through the melt. This alkoxide loaded stream was mixed with 7 1/min of oxygen before entering the reactor. The oxygen facilitates the decomposition of the precursors and prevents build-up of carbon. The reactor was kept at 300~ by passing hot nitrogen gas through the channel set later used for cooling of the reactor. The deposition process was carried out for one hour, then the reactor was cooled down and the process repeated for another hour with the alkoxide flow entering from the opposite direction in order to obtain a homogeneous coating. To produce deposits of reproducible quality, the deposition system had to be operated for several days to reach a stable state. Following this procedure, typically coatings of 5 - 10 tam thickness could be prepared. The alumina is present in the gamma phase, krypton adsorption isotherms revealed, that the
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surface area is increased typically by a factor of 100. In a test system with removable individual foils which were not diffusion bonded, the coating was shown to be fairly homogeneous over the length and width of one foil. Platinum loading was achieved by passing hexachloroplatinic acid solution through the reaction channel system repeatedly, typically five times. After the last step, the channels were left filled with the platinum solution. Excess solution at the inlet and outlet was removed with filter paper, and the whole reactor calcined at 570~ The final reduction of the catalyst was carried out at 350~ under flowing nitrogen/hydrogen (10:1). If higher platinum loadings were desired, the loading procedure was repeated up to three times. Elemental mapping of the removable foils in the test reactor showed a homogeneous distribution of the platinum in the alumina-coated channels.
2.3
Catalytic experiments
Flows of HE, O2 and optionally N2 in different ratios - stoichiometric, net oxidizing or net reducing - and at different flow rates were mixed in a T-element, using mass flow controllers, about three centimeters before entering the reactor. The temperature of the gas leaving the reactor was measured with a thermocouple placed immediately at the outlet. Cooling was either achieved by a strong flow of nitrogen through the cooling channels or by pumping a heat exchanger oil through the cooling channels. The temperature of the exiting heat transfer medium was also measured immediately at the outlet. Conversions were determined by collecting the water formed in a cooled molecular sieve trap and weighing after a predetermined time. Since strong thermal gradients were observed over the reactor, especially when using the heat exchanger oil, the temperature profile on the external surface of the reactor was visualized using an IR thermographic imaging system. Due to the reflectivity of the reactor and problems compensating for this, the readout of the imaging system could not be fully calibrated and thus only qualitatively shows the temperature distribution on the external surface of the reactor. For high loadings of platinum (repeated impregnation) the catalytic reaction ignited already at room temperature and no external heating was necessary. At lower platinum loadings, the reactor had to be preheated to about 70~ Once the reaction had started, though, the reactor could be operated autothermally. 3
RESULTS
Before discussing the results obtained under different conditions, first some remarks about safety of the reactor operation are necessary. In numerous experiments, only one explosion occurred, however, not in the reactor, but in the bubbler through which the offgas was led to the vent. This explosion was due to an ill prepared catalyst coating, which did not well adhere to the wall. During the experiments, the catalyst coating was carried out of the reactor and accumulated in the bubbler. When the amount of catalyst in the reactor was not sufficient any longer to reach full conversion, the explosive gas mixture could ignite on the catalyst in the bubbler. Due to the low volumes handled, however, no damage except to the bubbler itself occurred. This single explosion, on the other hand, clearly shows the potential of the microstructured reactor itself to efficiently prevent the homogeneous ignition of the explosive mixture.
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3.1
Operation with gas cooling
The first experiments were carried out with a reactor loaded only once with platinum (low loading) and gas cooling of the whole setup. Since the reaction did not start at room temperature at low platinum loadings, it had to be heated to 80~ by two heating element on the top and bottom of the reactor. Once the reactor reached this temperature, the reactor could be operated autothermally. Fig. 2 shows the temperature response at the reactor outlet during such an experiment. The first data points do not appear at the preheating ~ ' 226 0.2 I/min 02 ~,~,,,~ temperature of 80~ because 0.4 I/min H2 j ~ .o 20o data were only recorded from the time when the final H2 flow had & 181J been reached. This flow was E 160 increased gradually due to safety 0.1 I/min 02 reasons, so that the temperature 9 14() 0.2 I/min H2 == already started to rise before the o 12t) final settings were adjusted. As one can see, the gas composition g 100, (u entering the reactor can be used "80 0 2 4 6 8 ~0 to control the temperature level. time [min] The cooling gas exited the reactor at the same temperature as the Fig. 2: Reactor response at different inlet gas reaction gas. For the low reactant compositions. Cooling gas: 3 1/min N2, reaction gas flows, the conversion had not diluted with 1 1/min N2. reached 100 % after 10 min. At 0.2 1/min of oxygen and 0.4 1/min 400" hydrogen, however, full _ . . . . ,, ~ ~-~..~.------ of conversion had been achieved. After 5 runs n m 0 The still rising temperature at 300" Q that point is due to the slow heat up of the reactor housing. m i 0 In a reactor impregnated three 200" 9 Q E " e First run times with platinum solution, the Ireaction started already at room 100" " I temperature at the same inlet gas J m flows. In this case, the temperature of the whole reactor was controlled by changing the 0 20 40 60 80 coolant gas flow. With coolant Time [rain] gas flows of 4 1/min, 5 1/min and Fig. 3" Reactor response for higher catalyst loading. 7 1/min steady state temperatures Inlet 0.2 1/min 02, 0.4 1/min H2, 1 1/min N2, cooling 3 of 315~ 280~ and 230~ were 1/min N2. reached. Under these conditions, a power of about 70 W is produced, of which about one third is carried out of the system via the reacting and cooling gases, the rest is lost via the reactor housing and the fittings. One should note, that the gas o L----,
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mixture corresponds roughly to the composition of air, so that air would be sufficient to start the reaction at room temperature if the microstructured reactor would be used in a fuel processor system. With a freshly prepared catalyst in the reactor, the reactor needed about 30 min before the reaction ignited. However, a very substantial shortening of this induction period to about one minute was observed after several runs. Fig. 3 shows the response of the fresh catalyst and after five runs under identical conditions. Similar observations, i.e. that exposing a catalyst to hydrogen/oxygen mixtures leads to more active materials compared to reducing in pure hydrogen, have been reported before [9,10,11]. Different explanations have been put forward in these publications, but due to the fact that the catalyst can not be analyzed under reaction conditions or after the reaction due to the inaccessibility of the reactor interior, we do not have evidence for or against either of the explanations. Experiments were also carried out without diluting the reaction gases and using different mixtures of hydrogen and oxygen. The highest flows were adjusted in an experiment with 0.8 1/min of both hydrogen and oxygen, i.e. in the middle of the explosive regime. Under such conditions, even with 14 1/min of nitrogen passing through the cooling channels, the maximum which could be used with the equipment installed, the reactor temperature rose to 300~ in five minutes and kept rising with a steep gradient. Since the homogeneous reaction could have ignited in the hydrogen/oxygen mixture before entering the reactor, this experiment was terminated before a steady state could be reached. Instead, we investigated more efficient means of heat removal.
3.2 Operation with liquid cooling Using a heat transfer oil is such a more efficient method. The oil (Haake Synth 260) was pumped through the reactor using a peristaltic pump. In a typical experiment a feed composition of 1.2 1/min H2, 0.6 1/min 02 and 2 1/min N2 as carrier gas was used. Typical oil flows were about 30 ml/min. The reaction ignited at room temperature in a reactor with a high catalyst loading, which generates a total power of approximately 250 W. Under these conditions, the oil exits the reactor at a temperature of 207~ while the reaction gas exits at a temperature of only 70~ This on first inspection surprising behavior can be attributed to the cross flow design of the reactor/heat exchanger. The reaction is so fast, that most of the hydrogen is consumed immediately after entering the reactor. At this point, most heat is transferred to the oil. When passing further through the reactor, the reaction gas is efficiently cooled by the cold oil, exiting at low temperature. The exit temperature of the oil is the average of very hot oil . . . . . . . . . . generated near the reactor entrance and colder oil Fig.4" Temperature profile of the which passed through the cooling channels near the reactor. H2/O2 entering from exit. The temperature profile of the reactor recorded bottom, cold oil entering from the with an IR imaging system is shown in fig. 4. The right. Hottest temperature at the dark colors correspond to the hottest regions. It can reactant gas entrance.
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clearly be seen, that the hottest section of the reactor is immediately at the reactor entrance, where the surface temperature measured with a thermocouple is around 260~ The heat is very efficiently removed by the coolant oil, so that the temperature drops to about 65~ near the reactor exit. Due to the high viscosity of the oil, the flow could not be increased further with the equipment available. Although it seems possible to generate much more power in the reactor by passing higher H2/O2-flows through the system, this was not possible, since the temperature limit of the oil is 270~ which was already exceeded under the conditions used near the reactor entrance, so that some decomposition of the heat transfer oil occurred which was indicated by gas bubbles present in the oil after leaving the reactor. Initial experiments were also conducted with water entering the cooling channels which is evaporated in the heat exchanger. About 10 g/min of hot steam could be generated in this fashion. However, the conditions in the reactor are fairly ill defined in this mode of operation and proper energy balancing becomes difficult. One possibility to solve these problems is the construction of a counterflow device. Work to this effect is in progress. With such a device the generation and transfer of 1 kW in the cubic centimeter sized device seems to be readily achievable, which would be sufficient to evaporate close to 4 1 of methanol per hour if fed in at room temperature which is close to the demand of a small size fuel cell in a car.
4
CONCLUSION We have demonstrated that the catalytic combustion of hydrogen can be safely run in a microstructured reactor/heat exchanger. Such a device could find use for the evaporation of methanol in the fuel processor of fuel cell powered cars. With a counterflow design, a thermal power generation by catalytic combustion and transfer of more than 1 kW in a cubic centimeter sized device seems possible. REFERENCES See the Proceedings of the International Conferences on Microreaction Technology 1-3 R. Srinivasan, I.M. Hsing, P.E. Berger, K.F. Jensen, S.L. Firebaugh, M.A. Schmidt, M.P. Harold, J.J. Lerou, J.F. Ryley, AIChE J. 43 (1997) 3059. 3. D. HOnicke, G. WieBmeier, in: Microsystem Technology for Chemical and Biological Microreactors, DECHEMA Monograph Vol. 132, VCH, New York (1995), p.93 4. J.J. Lerou, M.P. Harold, J. Ryley, J. Ashmead, T.C. O'Brien, M. Johnson, J. Perrotto, C.T. Blaisdell, T.A. Rensi, N. Nyquist, ibid., p.51. 5. U. Hagendorf, M. Janicke, F. Schtith, K. Schubert, M. Fichtner, in: Topical Conference Preprints: 2nd International Conference on Microreaction Technology, AIChE 1998, p. 81. 6. A.L.Y. Tonkovich, D.M. Jimenez, J.L. Zilka, M.J. LaMont, Y. Wang, R.S. Wegeng, ibid., p. 186 7. Fraunhofer Institut fur Chemische Technologie, unpublished 8. W. Bier, G. Keller, G. Linder, D. Seidel, K. Schubert, in DSC-19, Microstructures, Sensors and Actuators. Cho, D et al. (Eds.), The American Society of Mechanical Engineers, Book No. G00527 (1990). 9. S.J. Gentry, J.G. Firth, A. Jones, A., J. Chem. Soc. Faraday Trans. 70 (1974) 600. 10. F.V. Hanson, M. Boudart, J. Catalysis 53 (1978) 56. 11. G. Pecchi, P. Reyes, I. Concha, J.L.G. Fierro, J. Catalysis 179 (1998) 309. 1. 2.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Catalytic Water Denitrification in Membrane Reactor O.M.Ilinitch (a), F.P.Cuperus (b), L.V.Nosova (a) and E.N.Gribov (a) (a)Boreskov Institute of Catalysis, Novosibirsk 630090, Russia* (b)Agrotechnological Research Institute, NL-6700 AA Wageningen, The Netherlands* Mono- and bimetallic catalysts with Pd and/or Cu supported over y-A1203 were investigated in respect to reduction of aqueous nitrate and nitrite ions by hydrogen. Composition of the supported catalysts was analyzed using XRD, SIMS and H2-O2 chemisorption techniques. Pronounced limitations of catalytic performance due to intraporous diffusion of the reactants were observed in the reaction. Catalytic membrane containing Pd-Cu active component supported over macroporous membrane-support was prepared and investigated. Forced flow of the reaction solution through the membrane was revealed to increase the effective catalytic activity.
1. INTRODUCTION In three-phase catalytic processes, the reactants diffusion in the catalyst pores is often a ratelimiting factor. To minimize the internal diffusion limitations, such processes are typically performed in slurry reactors with powdered catalysts that must be separated from the mixture when the reaction is completed. Our approach, reducing the negative influence of the internal diffusion and avoiding the need to separate the catalysts, presumes the use of porous membranes as a catalyst support of a specific type. The catalytic membranes with an active component deposited on the pore walls can assure more intensive intraporous transport of reactants to the catalytic centers compared to the conventional solid catalysts. Intensification of the intraporous mass transfer for the reactions limited by the internal diffusion can result in the improved catalytic activity and selectivity [ 1, 2]. These improvements were suggested to be due to a different character of mass transfer in the pores of a macroporous membrane and catalyst particles OCorcedflow in a membrane vs. diffusion-drivenflow in a catalyst). In this study, interrelations between the catalytic behavior and intraporous mass transfer were explored for the macroporous catalytic membrane in the process of nitrate ions reduction by hydrogen in water. The reaction occurring at ambient temperatures in the presence of palladium-containing catalysts [3] is a potential method of purifying drinking water from toxic nitrates that are increasingly produced by the industrial and agricultural activities worldwide. An incentive to applying the macroporous catalytic membranes for water denitrification stemmed from observations of their superior catalytic behavior in three-phase reactions and techno-economic assessments of the process [4]. In the present study the catalytic and Support of this work by the grant No. 047-010-100-96 from the Netherlands Organization for Scientific Research (NWO) is gratefully acknowledged.
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structural characteristics of Pd-Cu catalysts were investigated. The macroporous catalytic membrane was employed to facilitate the intraporous mass transfer and to increase on this basis the effective activity of Pd-Cu active component.
2. EXPERIMENTAL 2.1. Preparation of catalysts and catalytic membranes The series of mono- (Pd or Cu) and bimetallic Pd-Cu catalysts was prepared by (co)impregnation of 7-A1203 granules (SaEv=197 m2/g, mean pore diameter 15 nm) with hydrochloric solution of the catalyst precursors [PdC12 and/or Cu(NO3)2]followed by oxidation in air at 300~ and reduction in aqueous solution of NaBH4 at room temperature. The samples of catalytic membrane were prepared via the same procedure. The ceramic membrane (SaET=0.9 mE/g, pore diameter lktm) developed at BIC was employed at this stage as the catalyst support. The metal content was ca. 5 wt.% for Pd-containing catalysts and 1.7 wt.% for Cu/A1203; the catalytic membrane contained 1.6 wt.% of Pd and 1.2 wt.% of Cu. 2.2. Catalytic runs The catalysts were tested at 298K in a glass apparatus with the stirred reactor described in detail elsewhere [5]. In experiments with the catalytic membrane, in-house built membrane reactor of 100 cm 3 volume equipped with a magnetic stirrer was used. Concentrations of NOaand NO2- anions in the catalytic experiments were monitored using a liquid ion chromatograph "Tsvet-3006" (Russia) with electroconductivity detector. NH4+ cations were analyzed with an ion-selective electrode ELIT-51 (Russia). Reaction rates were determined by differentiating the "concentration vs. reaction time" dependencies at low (_ 0.00 ~....
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Fig. 2. Activities of y-AI203 supported catalysts in reduction of aqueous nitrite ions at 298K, P.2=l bar, CNO2-=50mg/l, pH=6.0
Table 1. Selectivities of Pd/Al203 and Pd-Cu/A1203 catalysts in reduction of NO3Reaction conditions: temperature 298K, PH2 = 1 bar, CNO3~--200mg/1, pH=6.0 Atomic ratio of XNo3-= 50% XNO3 --- 75% XNO3= 25% XNO3-= 95% metals in catalyst SNO2-,% SNH4",% SNO2-,% SNH4-,% SNO2-,% SNH4-,% SNO2-, o~ SNH4-,o~ Pd Pd:Cu = 1:0.33 Pd:Cu = 1:0.8 Pd:Cu = 1:2.25
0 23.2 22.3 38.0
29.0 11.6 8.6 10.3
0 12.5 18.2 43.2
Analysis of the experimental dependencies "concentration vs. time" indicates that the reaction proceeds according to the consecutiveparallel scheme (1). Both target product N2 and by-product NH4+ are likely to be formed via the same intermediate species.
66.0 21.0 10.3 13.4
0 5.2 14.0 31.0
68.0 25.0 19.5 17.0
(1) NO 3
+H2
~
N O
_
0 0.9 5.0 17.8
87.0 32.3 24.8 24.3
/ --------~ N 2
2
wNH
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3~. Characterization of the catalysts and catalytic membrane
3.2.1. XRD investigation No peaks of palladium, of copper or of the combinations of the two known to form in Pd-Cu system were detected in the XRD spectra of Pd/A1203, Cu/AI203 and Pd-Cu/A1203 catalysts. This is most likely due to the pronounced line broadening typical of the nanosized metal particles, along with the overlapping of the resulting halo and the peaks of the ~-A1203 support. XRD spectrum of the catalytic membrane contains a superposition of sharp peaks belonging to the membrane-support fabricated of the natural silica-alumina mineral and a broad peak with the maximum at 20~,~,41o characteristic of nanosize metal clusters. The XRD patterns of the metallic particles in the catalytic membrane are similar to those reported for the silica-supported Pd-Cu catalysts in [8], where formation of palladium-copper alloys has been suggested. Mean size of the metal particles in the membrane calculated according to the Scherrer's approach is ca. 3 nm. 3.2.2. SIMS investigation ~u
//
0.7 0.6
" 2~ "~
0.5 0.4 0.3 0.2
.c
Pd:C u=1:0.33_ _ 9
M e m b ran e
. l l ' l ' l ' l ' l
'l.l,, "B"
Pd:Cu=1:1.25
f /m ~m/ P d :C u =I :O.8 ~~~~176176176176176176
0.1
4--I
o.o
Pd:Cu=l:2.25 .
.
.
.
I
//1'
o.oo o.;i 0.;2 o.d3 0.04
r~
-
9
''1
....
o.s
i ....
S p u t t e r i n g d e p t h , l~m
Fig. 3. Relative intensities of ion currents l~
+ vs.
depth of sputtering for
Pd-Cu/Al203 catalysts and catalytic membrane
According to the SIMS results (Fig. 3), the topmost layer of the metal particles in Pd-Cu/A1203 catalysts with the ratios Pd:Cu=1:0.33 and Pd:Cu=1:0.8 is strongly enriched with Cu compared to the integral composition. Higher copper content characteristic of the third catalyst and the membrane favors more homogeneous distribution of the metals. In the porous grains of Pd:Cu=l:0.33 and Pd:Cu=l:0.8 catalysts the ratio Pd:Cu is the lowest on the outside, gradually increasing to reach the steady level at the granule depth of ca. 0.1~tm and 0.03~tm respectively. Essentially homogeneous spatial distribution of the metals was registered for the catalyst Pd:Cu = 1:2.25 and the membrane.
3.2.3. H2-O2 titration The results of hydrogen titration were used to determine the palladium dispersion in Pd/A1203 and Pd-Cu/A1203 catalysts. The uptake of hydrogen and oxygen by the catalysts under the experimental conditions employed was assumed to result from chemisorption by the surface atoms of the metal particles without formation of bulk phases of both hydride and oxide [9]. It was also assumed that hydrogen titration involves only oxygen species preadsorbed over palladium atoms in the course of oxygen titration [6]. The values of hydrogen titration HT/Pdt, expressed as the number of gas atoms adsorbed relative to the total number of palladium atoms in a given catalyst, are listed in Table 2. Divided by the stoichiometric coefficient of hydrogen titration 3 [10], these numbers give dispersion D of palladium in the supported metal particles, i.e. fraction of the surface atoms Pds relative to the total amount of palladium atoms Pdt : D - Pds/Pdt = (HT/Pdt)/3.
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As one can see, relatively high palladium dispersion was achieved for Pd/AI203 catalyst. The average size of palladium particles calculated by the ratio dpd = 0.9/D [ 10] is 1.7 nm. Addition of copper decreases the amount of H atoms chemisorbed per one Pd atom. The resulting values of palladium dispersion are ca. 4-6 times lower for PdCu/A1203 catalysts than those for Pd/AI203. This suggests a marked decrease in the number of the surface Pd sites accessible to hydrogen adsorption in the bimetallic catalysts. The latter can be caused by screening of palladium atoms by copper on the surTable 2. Hydrogen titration of Pd/A1203 and face of the bimetallic particles Pd-Cu/A1203 catalysts at 298K and/or by increase in the size of the particles. Taking into account H2 Pd Catalyst composition titration dispersion, the above results of XRD and SIMS studies and evidence existMetal content, atomic ratio HT/Pdt D=Pds/Pdt ing in literature [11 ], it can be conwt. % Pd:Cu cluded that the most probable rea1.56 0.52 4.1 Pd son for the observed sorption be0.40 0.13 5.2 Pd + 1.1 Cu 1:0.33 havior of Pd-Cu catalysts is the 0.38 0.13 5.2 Pd + 2.5 Cu 1:0.8 screening of palladium by copper. 0.28 0.09 5.0 Pd + 6.8 Cu 1:2.25
3.3. Catalytic membrane and internal diffusion hindrance In our catalytic experiments the activity of Pd-Cu/A1203 catalysts was found to increase with decrease in the grain size down to as low as 10-20 pin, revealing a pronounced influence of the intraporous diffusion which dictates the usage of fine catalyst powders in this process [12]. As an alternative means to minimize the internal diffusion hindrance, the catalytic membrane was employed in this study. The disk-shaped membrane (diameter 45 mm, thickness 4 mm) was installed in the reactor shown schematically in Fig. 4, and comparative catalytic runs were performed at the identical experimental conditions (temperature, NO3-concentration, stirring speed). In the first run the valve at the reactor exit was closed thus preventing the reaction solution from flowing through the membrane (flow rate "0" in Fig. 5) and making the membrane
H2
.m_ 0.40
E
reaction solution
._0.35
stirrer " ~ ~
,. ,,' '
membrane ---.2 .
valve ~
i ii
lg
.
.
.
.
.
.
.
t
pump
Fig. 4. Schematic of the catalytic membrane setup
~~ o E 0.25 ,.-.. .~ 0.20 O z 0.15 o i:: 0.10
E
0.05
.>_ 0.00 U
MCr2S4+4H20
(1)
MCr207 2Py +7H2S => MCr2S4+3S + 2Py + 7H20
(2)
3Na2CO 3 + 3S + CrC13 => 3NaC1 + NaCrS2 + Na2SO4 + 2CO2 + CO
(3)
Hydrous oxide precipitates were obtained by adding aqueous ammonia to the solutions of mixtures of 0.2 M. Cr(NO3) 3. 9H20 and 0.1 M of the hydrated chloride or nitrate precursor of the second metal. Dichromate pyridinium complexes were precipitated after addition of 50 ml of pyridine to the solutions of 0.01 mol (NH4)2Cr207 and 0.01 mol of the second metal nitrate in 100 ml of distilled water. The precipitates were aged for 2 days in the solution, then filtered, washed with distilled water and dried at room temperature. Sulfidation was done in a Pyrex reactor at 673 K for 4 h under a flow of 15%H2S in N2 mixture. Preparation of NaCrS2 compound was done in the molten flux of elementary sulfur (0.2 mol) Na2CO 3 (0.04 mol) and CrC13 (0.01 mol). After the reaction at 623 K for 2 h, the product was consequently extracted with toluene and water. The solids were characterized using chemical analysis, X-ray diffraction, BET surface measurements, X-ray photoelectron spectroscopy (XPS), and low energy ion scattering spectroscopy (LEIS). Thermoprogrammed reduction (TPR) was carried out in a quartz reactor under a flow of hydrogen. Samples of mixed sulfides were linearly heated from 293 to 1073 K (5 K /min). Hydrogen sulfide evolved upon reduction was detected by means of a HNU photoionisation detector. Thiophene hydrodesulfurization (HDS) and tetraline hydrogenation (HYD) reactions were chosen as model reactions for the comparison of the catalytic properties of the solids. Thiophene HDS was carried out in the vapor phase in a fixed bed microreactor operating in the dynamic mode at the atmospheric pressure of hydrogen without addition of H2S (thiophene pressure: 2.4 KPa, total flow : 6 l/h). A catalyst charge of about 0.1 g was employed. For the tetraline gas phase HYD, the experimental conditions were so chosen to avoid thermodynamic equilibrium that would favors dehydrogenation to form naphthalene. The range of temperatures studied was 523 - 573 K, the hydrogen pressure 4.5 MPa, the tetraline vapor partial pressure 8.9 KPa and the H2S pressure 84 KPa. Extended Htickel calculations were done using BICON - EDIT program package [ 7 ]. Density of states (DOS), crystal orbitals overlap population (COOP), Fermi level and electronic stabilization energy were calculated for several mixed sulfides. 3. RESULTS AND DISCUSSION
The properties of the sulfides obtained are listed in Table 1. As follows from the characterizations, the target mixed sulfides were prepared with specific surface areas varying from 13 to 110 m2/g, depending on the preparation technique. Most of the solids had the cubic spinel structure. Na and Ni compounds had different lattice symmetry. In the case of Co and
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Ni, compound prepared from pyridinium complexes, metastable solid solutions with the pyrite structure were probably formed at 673 K but they were transformed into the stable spinel and monoclinic lattices at 873 K. Table 1. Preparation conditions, Precursor Used Mn-Cr hydrous oxide MnCr207 -Py Fe-Cr hydrous oxide
Cofr20 7 -Py CoCr207- Py CoCr204 Ni-Cr hydrous oxide NiCr207 -Py NiCr207 -Py CuCr204 ZnCrzO 4 -Py ZnCr204 CdCr204 NaCrS2
composition and Sulfidation Temperature, K 873 673 673 673 873 873 873 673 873 673 673 873 873 Molten salt
surface areas (Ssp) of mixed MCr2S4 sulfides. Structure Obtained Chemical S so m2/g (XRD) Composition Spinel MnCr2.03Sn.01 13 Spinel MnCr2.Sn.01 63 Spinel FeCrl.8783.74 50 Pyrite CoCri.99S4.01 96 Spinel CoCrl.99S4.21 22 Spinel CoCrl.99S4.01 16 Monoclinic NiCr2.07S4.15 13 Pyrite NiCr2.00S4.25 110 Monoclinic NiCr2.00S4.15 31 Spinel CuCr2.00S4.0~ 48 Spinel Znfrl.9784.02 78 Spinel ZnCrl.99S4.oo 55 Spinel CdCr2.o2S4.o2 14 Trigonal NaCrS2 51
The variations of specific catalytic activity of MCr2S 4 solids as a function of M in both HDS and HYD reactions were similar, having the maxima for the NiCr2S4 and CoCr2S4 compounds (Fig.l). These latter systems showed enhanced hydrogenating properties, specific activities being comparable to those of Mo and NiMo sulfides (1.03xl0-6.mol.min-'.m -2 in HYD of tetraline ). The HYD/HDS activity ratio was higher for the MCr2S 4 solids than for the molybdenum sulfide reference, in agreement with our previous finding on the enhanced hydrogenating activity of the chromium - based sulfide catalysts [8].The variations of both HYD and HDS catalytic activities as a function of M in MCr2S4 were in good correlation with the reducibility of MCr2S4, determined as the amount of sulfur removed from the catalysts by hydrogen at the temperature of the catalytic tests (573 K) (Fig.l). Therefore we suggested that sulfur vacancies created by reduction with hydrogen are the catalytic centers in both reactions. To clarify further the structure of active surfaces of MCr2S4 solids we performed XPS and LEIS studies. XPS measurements demonstrated that surface layer of mixed sulfide crystals was enriched with Cr, relative to the bulk composition. The only exception was Fe compound where the surface composition was close to that of the bulk. LEIS measurements carried out as a function as sputtering time suggest that the crystalline planes which are preferentially exposed to the surface of MCr2S4 dispersions, contain mostly Cr species (Fig. 2).
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10
(a) A
-r.
0
0.75
60 -50
A
a
0.50
-40
-30,.
-r
0.25
~
> 0
E
a) I_
-20"~ 10
0.00
0
Fig. 1. (a) - catalytic activity (10 .7 mol/min.m 2) of the MCr2S 4 solids at 573 K in the reaction of HDS of thiophene. Circles - samples prepared from the mixed oxides, triangles- samples prepared from the pyridinium complexes. (b) - catalytic activity (10 -6 mol/min.m 2 ) of the MCr2S4 solids at 573 K in the reaction of HYD of tetraline (circles); and the amount of sulfur removed by hydrogen at 573 K , (~tmol/m2 ) (triangles). Both reduction by hydrogen and LEIS sputtering lead to the removal of the top layer of sulfur and exposure of coordination unsaturated Cr atoms which are supposed to be active centers.
ta]
(b)
Fig. 2 L o w - index planes of thiospinels 9(111) (a), and (110) (b). Large hollow circlessulfur; black circles- chromium; gray circles - the second metal.
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The reducibility of MChS4 solids is related to their electronic structure, as illustrated by the results of EH calculations, used to determine cohesion energy of the MCr2S4 sulfides. The electronic stabilization energies obtained from EH calculations are listed in Table 2. Pronounced minimum of the cohesion energy is obtained for Co and Ni compounds. Table 2. EH electronic stabilisation energies and interatomic distances (crystallographic data) in some MCr2S4 sulfides M AE stab d(M-S) .A d(Cr-S), ,~ 2Na -558 2.798 2.454 Mn -485 2.241 2.497 Fe -451 2.209 2.461 Co -449 2.191 2.448 Ni -431 2.388 2.424 Cu -481 2.17 2.418 Zn -489 2.211 2.463
The inverse general correlation between the sulfur binding energy and the M-S or Cr-S bond lengths was observed, i.e. more ionic bonds are stronger than short (more covalent) ones. However, Cu compound having the shortest bonds was not at the minimum of stabilization energy so the correlation is not complete. The results of EH calculations can be qualitatively explained in terms of relative positions of the elements valent orbitals.. In the NaCrS2 compound, the presence of the alkali metal leads to the increase of bonds ionicity, i.e. lowering of the S 3p states. Therefore, interaction between S3p and Cr 3d t2g levels decreases i.e. the bond covalence decreases. For the transition metal sulfides, when in MCr2S4 M goes from the left to the fight of the periodic table, the electronic effects observed can be realized as the competition of two opposite trends - that of filling of the d-band of M (so that the energy the 3d electrons provided by M increased), and that of increase of M nuclei charge, shifting the band of the same 3d electrons down.
Cr3d eg P
Cr3d t 2g
Co, Ni Fe, Mn Cu, Zn
Fig 3. Schematic energy diagram, showing the relative bands positions in the MCr2S 4 compounds.
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For the chromium neighbors in the periodic table, Mn and Fe, the d - levels of the second metal are close to those of Cr (Fig 12) and the sign of the effect is not clear, at least in the frames of the EH method. Then, going to Co, we see that band filling prevails over the band shifting down, and the HOMO states of the mixed sulfide CoCr2S4 are probably those of Co 3d. At the same time the S-Cr and S-Co antibonding orbitals become populated due to the increased d - electrons count. The same situation was observed for Ni, the effect being the decrease of sulfur binding energy. However in the case of Cu, the d- band, though almost filled is already placed too low, and the result of Cu introduction into the mixed sulfide is that of the increased sulfur bonding. Indeed, CuCr2S4 was less reducible than Co and Ni compounds. 4. CONCLUSIONS The MCr2S 4 systems, being chosen rather as a model solids for basic research purposes showed enhanced hydrogenating properties, specific activities being comparable to those of Mo and NiMo sulfides. By contrast to the industrial sulfide catalysts, which are extremely difficult to characterize, MCr2S 4 mixed sulfide dispersions are stable, and have well-defined surface and bulk properties. In this case the trends of catalytic activity could be easily explained from comparison with TPR and surface characterizations data. The idea about dynamically created active centers [5] get an additional support in this work. Though different elementary steps of HYD and HDS catalytic reactions were not considered here, sulfur lability which depends on the nature of metal M, appears to be a key parameter determining the catalysts performance in both processes. REFERENCES 1. Pecoraro, T.A., Chianelli, R.R., J. Catal., 67, (1981) 430. 2. Lacroix, M., Boutarfa, N., Guillard, C., Vrinat, M., Breysse, M., J. Catal., 120, (1989) 473. 3. Raybaud, P., Kresse, G., Hafner, J., Toulhoat, H., J. Phys, Condens. Matter, 9 (1997), 11085. 4. Harris, S., Chianelli, R.R., J.Catal., 86, (1984), 400. 5. Byskov, L.S., Hammer, B, Norskov, J, Clausen,B.S., Topsoe,H., Catal. Lett., 47 (1997) 177. 6. Bacon, G.E., Zeit. Crystallogr 82 (1932) 325. 7. Brandle, M., Rytz, R., Calzaferri, G., BICON - CEDiT Extended Hiackel Band Structure and Crystal Electronic Dipole induced transitions calculations. Bern, 1997. 8. Thiollier, A., Afanasiev, P., Cattenot, M., Vrinat. M, Catal. Letters 55(1) (1998) 39.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
In situ characterization of transition metal sulfide catalysts by IR probe molecules adsorption and model reactions. G. Berhault 1 M. Lacroix 1 M. Breysse 2, F. Maug63 and J-C. Lavalley 3
1Institut de Recherches sur la Catalyse, CNRS UPR 5401, 2, Avenue Albert Einstein, 69626 Villeurbanne Cedex, France. 2Laboratoire de R6activit6 de Surface, CNRS UMR 7609, Universit6 Pierre et Marie Curie, 4, place Jussieu, Casier 178, 75252 Paris Cedex 05, France. 3Laboratoire Catalyse et Spectrochimie, CNRS UMR 6506, ISMRA-Universit6, 6, Boulevard du Mar6chal Juin, 14050 Caen Cedex, France. This work reports a detailed characterization of reduced states of RuS2/SiO2 catalyst by combining catalytic activity measurements and IR probe molecule adsorption. Depending on the solid composition monitored by a progressive reduction these surfaces gradually moves from an acid-base character to a metallic one. Both Lewis and BrCnsted acidic sites are created in mild reduction conditions and the Lewis acidic sites play an important role in the activation of sulfur containing molecules and subsequently on their transformations. The hydrogenation properties are related to Ru sites with a low sulfur coordination. 1. I N T R O D U C T I O N Transition Metal Sulfides are efficient materials for catalyzing several reactions such as the C-X (X=S, N, O, Metal) bond hydrogenolysis, hydrogenation, the selective transformation of organic disulfides into the corresponding thiols as well as the aromatization of cyclic thioethers and the selective ketones amination. Among these solids, RuS2 is one of the most active TMS [1-2]. This indicate that its surface is flexible enough to adapt the proper configuration site required to catalyze this large variety of reactions which demand different intrinsic properties. Theoretical and experimental studies have ascribed the high activities of RuS2, as for RhzS3 and PtSx to their weak metal-sulfur bond energy [3]. This property is propitious to the formation of a large number of coordinatively unsaturated sites (CUS) whose properties may be regarded as a Lewis-type center interacting with electron-donating organic substrates [4]. Beside this CUS, the surfaces of TMS also contains some sulfur anions and SH groups which simultaneously co-exist depending on the nature and on the composition of the surrounding atmosphere. However, there is a lack of characterization of the acid-base properties of these surfaces which are supposed to play an important role in the successive elementary steps involved in the above mentioned reactions. Besides this acidic-base character, Moraweck et al have demonstrated that for small RuS2 clusters encaged into a Y zeolite, some metallic Ru microdomains may co-exist at the surface of the sulfided particles leading to a metal-sulfide type interface [5]. Accordingly, the surface of a RuS2 particle may
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behave as a metal or as an acid-base material depending on reaction conditions in agreement with the highly reducible character of such a sulfide. The aim of this work was to develop the required tools for characterizing the modification of the surface properties of a silica supported RuS2 induced by a progressive reduction. For this purpose, we used a silica supported RuS2 catalyst as model system because silica is relatively neutral and does not interact too much with the supported sulfide phase. Solid characterizations were performed by combining catalytic measurements with in situ probe molecule adsorption (CH3SH, CO, pyridine and lutidine). Pyridine was used to detect the Lewis acidity while lutidine was preferred for dosing the BrCnsted acidity because of its higher basicity and CO was selected because its wavenumber is sensitive to the CUS environment. The catalytic properties were determined in two model reactions suspected to reflect different surface properties i.e. the lbutene hydrogenation and the condensation of CH3SH into CH3SCH3.
2. EXPERIMENTAL 2.1. Catalyst preparation The silica-supported RuS2/SiO2 was prepared by the pore filling method using RuC13 aqueous solutions. The impregnated and dried solid was sulfided at 673 K with a 15%H2S85%N2 mixture. After this activation procedure, the solids were cooled down to room temperature in the presence of the sulfur-containing atmosphere, flushed with an oxygen free nitrogen flow and stored in sealed bottles. The Ru loading was 7.5 weight %, the S content corresponds to RuS2.7 and the residual chlorine content was lower than 0.1%.
2.2. Catalyst reduction and catalytic properties These experiments were performed in situ in the same flow microreactor equipped with two parallel detectors, a Flame Photometric Detector (FPD) and a Flame Ionization Detector (FID) in order to detect respectively H2S and the hydrocarbons. The H2S released upon hydrogen reduction was quantified by calibrating the detector with a known concentration of H2S (573 ppm) diluted in hydrogen. The degree of reduction ot was defined by the ratio of the amount of H2S eliminated from the solid to the total sulfur content. The reduced catalysts were then contacted at 273 K with a mixture of H2(93.4%)-l-butene(6.6%) or at 473 K N2(94.4%)-CH3SH(5.6%). For both reactions, conversions were kept lower than 10% in order to avoid mass transfer limitations.
2.3. FTIR spectroscopy FTIR characterization was performed using self-supporting discs of pressed samples. The catalysts were resulfided in situ in the infrared transmission cell according to the procedure already described[4]. Solid reduction was performed with 200 Torr of hydrogen at various temperatures. Several reduction-evacuation cycles were done in order to remove the H2S formed upon reduction. Then, the samples were evacuated at 393 K for 30 min prior to molecule adsorption. Probe molecule adsorptions were performed at 100 K for CO or at room temperature for the others probes. The reduced catalysts were contacted with 1 Torr of CO, 2 Torr of pyridine (Py) and 2-6 dimethylpyridine (DMP) or 4 Torr of CH3SH and then evacuated. The IR spectra were recorded using a Nicolet 60SX FTIR spectrometer. Band intensities were corrected from slight differences in catalyst weight and adjusted to 10 mg.
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3. RESULTS 3.1 Solid reduction and catalytic properties
The starting point of this work was to examine how the RuS2/SiO2 catalyst behaves towards a hydrogen treatment. Preliminary TPR experiments have evidenced that the silica support sulfided in the same experimental conditions does not retained any detectable amount of H2S. It was also observed that over the RuS2/SiO2, H2S is mostly removed upon heating and then the solid rapidly equilibrates when treated in isothermal conditions. Accordingly all solids were reduced at the desired temperature and left in isothermal conditions for only 2h 12 ~o
100
Deg.of Red.
[-~
2 ~
N
80
"~
8
N'~~
60
o
6
~~~ 0
40
",,..a
CH3SH (10 -8 mol.s- I .g- 1)
10
I
4 1-Butene (lamol.s- 1.g- 1) w
|
,
i
,
,
,
298 423 473 523 573 623 673 Reduction Temperature (K) Fig. 1. Evolution of the degree of reduction and of the mean particle size as a function of the temperature of reduction.
0
20
40
60
80
100
Degree of Reduction (%) Fig. 2. Evolution of the catalytic properties as a function of the degree of reduction
Figure 1 shows the evolution of the degree of reduction versus the reduction temperature. At 673 K the amount of H2S released from the solid corresponds to that determined by chemical analysis indicating that the solid is entirely reduced. This figure also reports the mean particle size determined by HREM. The non-reduced solid could be considered as an assembling of spherical particle with a mean diameter of circa 35/k with a narrow distribution since the standard deviation was about 8/k. Neither the particle size nor the distribution width were affected up to a reduction temperature of 573 K. At 623 K both parameters increase and the XRD patterns reveal the concomitant presence of the RuS2 and the metal Ru phases while only the latter is detected when the solid is reduced at 673K. These data indicate that the pyrite phase preserves its morphology up to a reduction temperature of 573 K. Figure 2 shows the variation of the catalytic activities as a function of the degree of reduction. The non-reduced solid is already active for the condensation reaction. As far as sulfur is removed from the catalyst the activity increases, reaches a maximum for ot = 20% and then continuously decreases for further sulfur removal. By contrast, the butane formation follows a distinct trend i.e. the non-reduced solid is inactive and the activity increases up to = 40% and then stabilizes. The different comportment of the catalyst towards both reactions strongly suggest that they require different type of sites.
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3.2 Pyridine (Py) and 2-6 dimethyipyridine (DMP) adsorption.
Py interaction with the silica support gives rise to several bands characterizing Lewis and BrCnsted sites. However, this interaction is weak because a low signal is detected after desorption at 423 K (Fig. 3). By contrast, these bands remain on the Ru catalyst. The spectra recorded on the non-reduced sample (NR) exhibits intense bands at 1602 and 1444 cm -I involving Lewis (L) acidity as well as some weaker ones in the range 1500-1580 and 16101660 cm l characterizing the BrCnsted (B) sites while the band at 1485 cm -I arises from both L and B sites. Solid reduction up to 473 K brings about an increase of band intensities without any change in their positions suggesting an increase in the number of acidic sites without a large modification of their strength. The diminution of band intensities for a reduction temperature of 573 K is rather surprising since the amount of H2S removed has drastically increased. This unexpected behavior suggests a strong modification of the surface properties because particule size remains unaffected. ---,
1602 L 1636 B. 9
~
9
1485 1444 L L+B " 9 1540 B 9 9 ~
9
,
9
~
773K
,.Q O ,.Q
~
:'~i~"' :~i~"
:~i!~ilili~+: e; I | *ll
Fig. 1 Schematic representation of Schottky cycle. El, Ea represent measured energies for K § and K, ~ is the calculated work function, I tabulated potassium ionization potential, e~ and ev refer to electron at Fermi level and vacuum respectively. 3.3. Excitation
The most spectacular are undoubtedly the results obtained for carbides. In this case not only emission of mere K and K § species was observed but additionally a presence of highly excited potassium K* (Rydberg atoms) was detected. From the applied field gradient Fc in the detector the threshold value of their principal quantum number n > 25 was estimated using the formula [7]: FJ(Vcm -l) = 3.2 • 108/n4. Since the intensity of Rydberg atom emission strongly depends on the surface voltage [8] thermal desorption studies were conducted as a function of the sample potential Vs. The results are shown in Figs. 2a and 2b. The desorption bands of K* exhibit clear fine structure, which varies with the temperature and voltage. Following the literature we assigned this structure to K*n clusters with various numbers of potassium atoms. In general, low voltages favors larger clusters while at high field due to fragmentation smaller clusters are produced. In the Fig. 2b the same data are presented in the Arrhenius coordinates. For each peak a straight line was obtained, which means that they correspond to distinct processes. The activation energies for the K* desorption at V~ >25 V vary from 1.04 to 1.81 eV. Since the values are significantly smaller than the K desorption energies for carbides, they rather reflect the process, in which K* merge into clusters of the size governed by the magic numbers. Such clusters of Rydberg matter K, (n = 7, 14, 19, 61) were previously observed by Holmlid et al. [9] using time-offlight spectrometry. 4. DISCUSSION Surface stability of potassium and its electronic promotion appear to be inimical phenomena. Although the post doping has the greatest impact on the electronic properties of the catalyst it results in lower potassium stability compared with the nascentdoping. The work function can also be modified by varying the K loading. Thus, there are at least two variables, which can be exploited during catalyst preparation for adjustment of the Fermi edge in order to optimize particular catalyst performance.
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6
-26
1 /aI/
~
W
~ ~ / I/ ~------~/'~ ~ 4U 3 ~ 6 4
(j$/]/,] 10 (a)
/
0
7"- 720 7--700 -7" 680 660 0fO X,,\
0 620
/
-28 / -29
1.
/'
1 .12
~/
-30" 4 0 " ~ 1 . 0 4 (b)
~ ,~/k
Us /V
Fig. 2. Rydberg state signal as a function of sample voltage and temperature (a) and the corresponding Arrhenius plot (b). The mechanism, along which K atoms can be brought into Rydberg states at the catalytic surfaces still remains unclear. From our experiments it seems that the Rydberg state can be considered as an intermediate in surface ionization of potassium: K ---> K* ---> K § + e-. The formation of K* is observed when the bonding of K is stronger than K § (Table 1). This suggest that the potential energy diagram given in [3], where the minimum of potential for the surface K should be deeper and closer to the surface, while that for K § is flatter and more distant is applicable also for this case. The potential curve for K* is located higher in energy approximately by the ionization potential of potassium lowered by the cluster formation energy. If the crossing locus of K and K § curves occurs above the minimum of K* then the excited Rydberg atom may appear as an intermediate in the potassium ionization pathway (stepwise mechanism). However, if the crossing occurs below the minimum then the surface K atoms are directly ionized into K § (concerted mechanism). What is actually observed depends on the binding energies of K and K § work function of the catalyst and the potential applied to the surface, which shifts the relative position of the curves. The existence of Rydberg atoms at the surface leads to a large decrease in the work function value, due to their very low ionization potential. In this case the Schottky cycle is invalid. The relevance of Rydberg atoms to catalysis is a challenging question. An original concept of the alkali action at catalytic surface was proposed by Pettersson et al. [10]. The model includes the reaction between a Rydberg state of an alkali atom and a reactant molecule at the surface, which was proposed to explain the increase in sticking probability of the reacting molecules upon alkali doping. Along this line, the transfer of excitation energy from K Rydberg
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species to reacting molecules has recently been proved experimentally [ 11]. Emission of highly excited potassium species (K*) from industrial iron catalyst for ammonia and styrene production were experimentally observed [12, 2]. Moreover, the flux of K* was shown to correlate with the catalyst activity [5]. The emission of Rydberg atoms was high for active catalysts and low for the spent one, although the total flux of potassium in the latter case increased due to surface segregation. 4. CONCLUDING REMARKS Different aspects of the alkali promotion can be successfully studied by a simple thermal desorption method. As shown, the chemical status of alkali metals on catalytic surfaces involves several effects and the complex approach is necessary to evaluate their role in heterogeneous catalysis. In particular, the investigation on excited states of alkali metals may provide a sensitive starting point for unraveling the mechanism of promoting action. A working hypothesis has been advanced to account for the formation of Rydberg atoms at the catalytic surfaces. ACKNOWLEDGEMENTS A.K. is grateful for a "Poste Rouge" (PICS 508 - C.N.R.S.) at the Laboratoire de R6activit6 de Surface, Universit6 P. et M. Curie, Paris. REFERENCES
1. W.D. Mross, Catal. Rev.- Sci. Eng., 25 (1983) 637. 2. A. Kotarba, K. Engvall, J.B.C. Pettersson, M. Svanberg, L. Holmlid, Surf. Sci., 342 (1995) 327. 3. K. Engvall, A. Kotarba, L. Holmlid, J. Catal., 181 (1999) 256. 4. A. Kotarba, M. HagstrOm, K. Engvall, J.B.C. Pettersson, React. Kinet. Catal. Lett., 63 (1998) 219. 5. K. Engvall, L. Holmlid, A. Kotarba, J.B.C. Pettersson, P.G. Menon, P. Skaugset, Appl. Catal. A, 134 (1996) 239. 6. C. Sayag, PhD Thesis, P. M. Curie University, Paris VI, 1993. 7. R.F Stebbings, F.B. Dunning, Rydberg States of Atoms and Molecules, Cambridge University Press, Cambridge 1983. 8. J. Wang, K. Engvall, L. Holmlid, J. Chem. Phys., 110 (1999) 1212. 9. J.Wang, L.Holmlid, Chem. Phys. Lett., 295 (1998) 500. 10. J.B.S. Pettersson, K. MOiler, L. Holmlid, Appl. Surf. Sci,. 40 (1989) 151. 11. J.Wang, L.Holmlid, submitted to Chem. Phys. Lett. 12. K. Engvall, A. Kotarba, L. Holmlid, Cat. Lett., 26 (1994) 101.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Heterogeneous catalysis of aldolisations on activated hydrotalcites Joseph Lopez, Roland Jacquot* and Frangois Figueras Institut de Recherches sur la Catalyse. 2, Avenue Albert Einstein. 69626 Villeurbanne Crdex, France and *Rhodia, Centre de Recherche des Carrirres, 85 Avenue des frrres Perret, 69192 StFons Cedex, FRANCE. The aldolic condensations of benzaldehyde on acetone and acetophenone have been investigated on hydrotalcites, KF and KNO3 supported on alumina, La203, X zeolites containing Cs and Mg clusters. A Langmuir Hinshelwood kinetic mechanism with competitive adsorption of the reactants is observed. Changing the polarity of the solvent induces large differences of rate. Hydrated hydrotalcite is the best catalyst at 273 K, reaching 70% conversion with 90% selectivity to aldol. Supported KF catalyses these reactions at 273 with lower selectivity. The different solid bases have been compared at a reaction temperature of 423 K: KF treated at 723 K is the most active, reaching 90% selectivity in chalcone at 80% conversion. INTRODUCTION Aldolisation reactions are of interest in fine chemistry because they allow the formation of C-C bonds. These reversible reactions usually show an unfavorable thermodynamic equilibrium, and are exothermic which means that the equilibrium conversion decreases when the reaction temperature increases [ 1]. In most cases the aldol can be easily dehydrated so that the reaction at high temperature yields the unsaturated ketone. They can carried out by acid or basic catalysis, and the investigation of the effect of substituents showed that activated hydrotalcites (HDT) catalysed the reaction by a basic mechanism [2]. Several attempts have been performed either in the gas [3] or the liquid phase [4]. Recent work on the activation of HDT demonstrated that the aldolisation of acetone [5,6] or the aldolic condensation of benzaldehyde on acetone [7] can be performed selectively on BrOnsted bases obtained by rehydrating thermally decarbonated HDT. In addition of the basic properties of the solid the nature of the solvent and the kinetics can condition the yield of the reaction, and we report here the results of this investigation. Moreover, several basic catalysts have been described such as over exchanged zeolites [8,9], supported potassium fluoride [ 10] or nitrate [ 11 ] which relative basicity is not known. They have been compared in order to establish a pattern of basicity from their catalytic properties. EXPERIMENTAL METHODS The preparation of HDT was made by coprecipitation as reported by Miyata [12]. Aqueous solutions containing the first one 0.75 mol/L of MgNO 3 and 0.25 mol/L of A1NO3, the second one 2 mol/L of KOH and 0.5 mol/L of KzCO3, were introduced by two electric pumps to a 4 L flask and mixed under vigorous stirring at constant pH = 10, controlled by a pHstat. The mixture was aged under stirring at 338 K for 18 hours. The precipitate w a s washed several times until the solution was free of chloride ions (AgNO 3 test) then dried at 383 K. For activation, the catalyst (about 0.15 g) was first heated in a flow of nitrogen at a rate of 10 K per minute up to 723 K maintained for 8 h. The solid was then cooled in dry ni-
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trogen and contacted with a flow of nitrogen (61/h) saturated with the vapor pressure of water for 6 h at this temperature. KFla was supported on SPH 512 or-alumina from Rh6ne Poulenc (surface area 10.5 m2/g). 15 g of alumina were poured into 150 mL of water containing the desired amount of KF (1 mmol KF/g of support). Water was evaporated at 323 K then the solid was dried at 383 K, and calcined at 723 K just before use. A commercial sample from Aldrich denominated KFA (40 wt% KF, 14.9 m2/g) was used for comparison. A sample of KNO3h/AI203 (41% KNO3) was prepared using the procedure reported by Zhu et al. [ 11 ] on SCP 350 of Rh6nePoulenc (surface area 400 m2/g) as support. 3.5 g of nitrate and 5 g of alumina were dried at 373 K, then mixed by grinding in a mortar for 15 min, then 0.5 ml of water were added. Atter mixing the resulting solid was dried at 383 K overnight. Lanthanum oxide was precipitated at pH 9 from a solution of La(NO3)3 using NH4OH. The solid was washed twice, dried at 383 K and activated by slow calcination (1K/min) at 923 K just before use.The surface area of the resulting La203 oxide is 12.4 m2/g. The basic zeolites were prepared from NaX (CECA) by exchange with Cs acetate (CsAc) at room temperature: 20 g of NaX were contacted with 1 L of a 0.1 M solution of CsAc and strirred for 48 h. A second exchange was performed, then the solid was washed 3 times and dried at 353 K: the degree of exchange of Na + by Cs + was 50.4%. This NaCsX zeolite was further impregnated by AcCs: 5g of zeolite were contacted with 12.8 mL of water containing 0,736 g of AcCs, then water was evaporated at 308 K. The resulting solid contains 14.9 Cs per unit cell. A NaMgX zeolite was prepared from NaX contacting 10 g of zeolite with 150mL of ethanol containing 5.216 g of Ac2Mg, 4H20, then ethanol was evaporated at 323 K. The resulting solid contains 32 Mg per unit cell. These basic zeolites were activated by slow calcination (1K/min) at 823 K. The characterisation consisted in X Ray diffraction, XPS, DTA-DTG and nitrogen adsorption to measure the surface area and porosity. The reactions at low temperature were investigated in batch conditions using a three neck glass reactor equipped with a condensor. The solvent and the substrates were mixed at 273 K. When the temperature was stabilised the freshly activated catalyst was rapidly introduced and the measurement started. For the reactions of benzaldehyde-acetophenone performed at 423 K the solid was introduced in the solvent (12 ml of DMF), the reactor was closed, purged with nitrogen and heated to the reaction temperature, then the reactants (0.89 ml of acetophenone and 0.76 ml ofbenzaldehyde in 6.4 ml of DMF)were introduced from a vessel connected to the reactor. The standard amount of catalyst was 0.15 g. Reactants and products were analysed by gas chromatography using a polar capillary column. RESULTS
1) Characterisation of the samples. The original sample is a pure HDT. No change of the surface composition could be observed upon activation by XPS, therefore the solid is assumed to be homogeneous. Hydrotalcite treated at 723 K in nitrogen or air is converted to a mixed oxide of high surface area, which can be reversibly rehydrated to a layered structure by contact at room temperature with a stream of nitrogen saturated with water (Fig. 1). This rehydration also corresponds to large changes in surface areas as illustrated in Table 1. From XRD analysis, KFA is a mixture of K3AIF6 and KF, while the pattern of KFla contains only very weak reflexions of K3AIF6 and or-alumina. KFla looses water below 373 K, but is stable in the range 373-723 K, suggesting that fluorine is retained. The two samples
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493
show similar surface areas (8.5 m2/g for KFla and 14.9 m2/g for KFA), and the main difference is then a higher dispersion of KF in KF 1a. KNO3/TAI203 appears as an XRD amorphous solid of surface area 15.8 m2/g after aetivatinn at I~9.~1~ 8 800 , .~, 600
5
.~- 400 "~ r"
HT
5h
~ 200
o
.7"
7
,~
6
,
5
~
4
~
3
2 % -.~ 1 10 20 30 40 50 60 70 80 Angle 2Theta(~
Fig. 1. XRD patterns of hydrotalcites after synthesis, calcination and rehydration for different periods of time.
N o
0 1 2 3 4 5 15 7 Molar ratio acetophenone/benzaldehvde Fig.2. Kinetics of condensation ofbenzaldehyde with benzophenone at 298 K on rehydrated HDT
Table 1. Surface areas and porosities of a synthetic hydrotalcite after calcination at 723 K, and further rehydration at room temperature for different periods of time. HT1A Treated 723 K Rehydration 5h 15h 48h SBET [rn2/g) 95.3 265.2 50.2 19.1 11.7 Pore vol (ml/g) 0.415 0.857 0.261 0.0991 0.0636 The basic properties of HDT have been described elsewhere [13]: judged from the changes of the enthalpy of adsorption of CO2, the basic strength does not change much upon rehydration. Two types of solid can then be obtained acording to the pretreatment: a Lewis base by decarbonation and a Bronsted base by further rehydration. 2) Reaction of benzaldehyde on acetone. At 273 K in acetone as solvent, the reaction gives the aldol but also some aldolisation of acetone to diacetone alcohol. It was attempted to suppress this bimolecular side reaction by diluting the system, but this raises the problem of the choice of the solvent. Solvent effects were investigated on this reaction using activated HDT as catalyst. The results reported in Table 2 show that the initial reaction rate is related to the polarity of the solvent. The effect of acido-basic character of the solvent can be estimated using the parameters Aj and Bj related to the ability of solvation of anions and cations [14]. Ethanol and THF show the same polarity but ethanol is more acidic and less suitable as solvent. THF appears as a good solvent, less toxic than DMF and has been used for standard measurements. The kinetics of this aldolisation determined in THF obeys a competitive mechanism with a rate going through a sharp maximum for a ratio acetone/benzaldehyde =12.6. This maximum was not noticed in absence of solvent [7] which gives evidence of a competition between the reactants and the solvent for adsorption at the surface.
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494
Hydrating the sample increases activity and selectivity for the aldol of HDT but increases activity and decreases selectivity of KFA (Table 3) suggesting that the sites for aldolisation are different in the two cases.
Table 2. Effect of the solvent on the initial rate of the aldolic condensation of benzaldehyde on acetone in the liquid phase at 273 K. Solvent Polarity Aj Bj Aj+Bj Initial Rate * 106 heptane toluene anisole THF EtOH DMF water
(D) 0 0.3 1.3 1.7 1.7 3.2 1,8
0 0.13 0.21 0.17 0.66 0.30 1
0 0.54 0.74 0.67 0.45 0.93 1
0 0.67 0.96 0.84 1.11 1.23 2
Table 3. Effect of hydration for HDT and KFA catalysts. Solid KFA calc. KFA + 501aL H20 r0 (mol.g-l.s -1) % conv. benzaldehyde (lh) % selectivity aldol (lh)
1.3 10-6 40.9 76.8
1.2 10-5 28.8 31.5
(mol.8_l.sec_l) 0.31 0.34 1.3 5.2 2.85 16.4 0
HDT calc
HDT calc + r6hydr.
7.5 10-7 2.2 56.6
3.5 10-5 72.7 83.8
For hydrotalcites prepared in identical conditions, the rate of reaction goes through a maximum at a ratio Mg/A1 close to 3, thus a maximum of basicity is observed for isolated sites.
3) Condensation of benzaldehyde on acetophenone: At 298 K the equilibrium conversion from thermodynamic values in aqueous phase is 89.1% for an equimolar mixture of the reactants [ 1]. The main product of the reaction on HDT is the unsaturated ketone (chalcone) resulting from the dehydration of the aldol. HDT is the only solid base selective in these reaction conditions. KF/alumina produces large amounts of 1,3,5-triphenylpentan-l-5-dione, by Michael addition of acetophenone on chalcone. Since this reaction is more difficult, the kinetic study of aldolisation on HDT was performed in DMF in order to have shorter reaction times at 298 K. Here also a sharp maximum is observed for the initial rate as a function of the concentration of both reactants (Fig. 2). The optimum ratio is here close to 1, which illustrates the differences in acidity of the two ketones. Using the optimum ratio of reactants the yield reaches 82% after 20 h at 310 K. At higher reaction temperatures, the selectivity to chalcone decreases at the expense of the Michael addition. 4) Comparison of different solid bases The different solid bases were compared at 273 K for benzaldehyde-acetone. The only solids showing good activitiy at this temperature are HDT and supported KF, which reach complete conversion in less than 6 h. The pattern selectivities of these catalysts is: HDT calcined < KF/alumina rehydrated KF1 a (94)> KNO3/AI203(92)> MgX (89) > CsNaX (82). The lower selectivity of microporous zeolites, connected to a lower activity, is attributed to diffusional limitations which favour consecutive reactions. The low activity of HDT at 423 K is due to the dehydroxylation of the solid: for a reaction at 298 K, the conversion after 3 h decreases from 70% for the hydrated form to 4.6 % after dehydration at 423 K, the selectivity being unchanged. 0.4
0.4 i~. ~ ~
0.3
g
0.3
=
"~k - ~
0.2
= 0.2 o
0.1
g 0.1 . / / "
~ ~
benzaldehyde acetophenone olefinic ketone
/
0.0
HT21
0.0 0
60
120
180
240
300
0
60
120
Time (min)
180
240
300
Time (min)
Fig. 3. Aldol condensation of acetophenone and benzaldehyde at 273 K on KF1 (left) and rehydrated HDT (fight)
~ 100
100
9O 8O
g
7O e
~m
o
l"
c
e
c: ~ 70
* =
o u
20
"O3 0
10 0
8o
(D 09 (~
,-
,
|
,
,
,
0
60
120
180
240
300
360
r 60
-
0
60
120
KFla KNO3/TAI203 NaCsX-AcCs La20 3 NaX-(Ac)2Mg II
180
240
300
360
Time (min)
Time (rrin)
Fig.4. Conversion ofbenzaldehyde at 423 K as a function of time (let~) and selectivities into ehalcone (fight) for a series of solid bases: a) HDT calcined, b)NaMgX, c) La203, d) HDT calcined then rehydrated, e) NaCsX, f) KNO3/A1203 and g) KF1 a.
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496
DISCUSSION In spite of a rather mild basic strength hydrated HDT appear as a good aldolisation catalyst. The degree of hydroxylation of the surface is a crucial parameter. Aldolisation can be performed at 273 K either by hydrated HDT or supported KF.. The most active sample KFla is supposed to be supported KF with good dispersion. In that case the active sites are
proposed to be F- anions of low coordination. Both OH- or F- sites can catalyse aldolisation with comparable rates, but the selectivity is higher on hydrated HDT. The importance of the consecutive Michael reaction when using supported KF is attributed to a higher basicity of the solid. The comparison of the different solids in the aldolic condensation of acetophenone with benzaldehyde shows that comparable conversions can be obtained at 423 K with many solid bases. Microporous systems are not suitable as expected for this type of reactions due to the possibility of consecutive steps. However selectivities for chalcone higher than 90% can be reached with mesoporous systems. The bimolecular Langmuir-Hinshelwood mechanism has been observed in many other cases [15, 16] and requires that the reactants compete for the same sites. The hypothesis that acetone or acetopheneone forms the carbanion which reacts at the surface on benzaldehyde adsorbed by an hydrogen bond could account for the experimental observations. In this case, the position of maximum rate is related to the ratio of the adsorption coefficients of the reactants which in turn is related to the basicity of the surface. It is expected that these curves are shitted when the basic strength increases, as reported earlier [ 16]. With hydrated hydrotalcite the higher activity is observed for an equimolar mixture of reactants, and HDT is therefore an interesting catalyst from the practical point of view. It can also be recycled without loss of activity. In conclusion the success in the application of solid bases is controlled by several factors such as activation, nature of solvent and composition of the reaction medium. When these are optimised very good yields can be reached with high selectivities, close to 100%. References. 1. J. P. Guthrie, Can. J. Chem. 56 (1978) 962. 2. Tichit, M.H. Lhouty, A. Guida, B. Chiche, F. Figueras, A. Auroux, E. Garrone, J. Catal. 151 (1995) 50-59. 3. W. T. Reichle, US Patent 4.458.026 (Jul. 3, 1984) to Union Carbide. 4. M.J. Climent, A. Corma, S. Iborra and J. Primo, J. Catal. 151 (1995) 60 5. F. Figueras, D. Tichit, M. Bennani Naciri, R. Ruiz, in "Catalysis of Organic Reactions" (F. E. Herkes ed) Marcel Dekker Inc, New York 1998, p37-49. 6. R. Tessier, D. Tichit, F. Figueras, and J. Kervenal French Patent 95 00094, 1995. 7. K. Koteswara Rao, M. Gravelle, J. Sanchez Valente, F. Figueras, J. Catal. 173 (1998) 115. 8. H. Tsuji, H. Hattori, H. Kita, Proc. 10th Inter. Congr. Catalysis (1992) 1171. 9. M. Lasperas, H. Cambon, D. Brunel, I. Rodriguez, P. Geneste, Micropor. Mat. 1 (1993) 343. 10.J.H. Clark, Chem Rev. 80 (1980) 429. 11 .J.H. Zhu, Y. Wang, Y. Chun, X. S. Wang, J. Chem. Sot., Faraday Trans., 94 (1998), 1163. 12. S. Miyata, Clays & Clay Miner. 23 (1975) 369. 13. J. Sanchez Valente, F. Figueras, M. Gravelle, P. Kumbhar, J. Lopez, J-P Besse, in press. 14. C. Reichardt, "Solvents and solvent effects m Organic Chemistry" 2nd edition, VCH (1990) 402. 15. A. Aguilera, A R. Alcantara, J. M Marinas, J. V. Sinisterra Can. J. Chem. 65 (1987) 1165. 16. A. Guida, M. H. Lhouty, D. Tichit, F. Figueras, P Geneste Appl. Catal 164 (1997) 251.
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497
化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
The Influence of Metal-Support Interactions During Liquid-Phase Hydrogenation of an ~, [3-Unsaturated Aldehyde over Pt Utpal K. Singh and M. Albert Vannice Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802 Liquid-phase hydrogenation of citral was investigated over SiO2- and TiO2- supported Pt catalysts in the range of 298 - 423 K, 7 - 41 atm H2 pressure, and 0.5 - 6.0 M citral in hexane. The initial rate of citral hydrogenation over Pt/SiO2 catalysts exhibits an activity minimum with respect to temperature accompanied by an increase in selectivity for hydrogenation of the C=O bond with increasing reaction temperature. Furthermore, the initial rate of citral disappearance is strongly influenced by metal particle size since the rate of citral disappearance at 373 K decreased 20-fold from the 5 nm SiO2-supported Pt crystallites to the 1 nm SiO2-supported Pt crystallites. This difference is suppressed at 298 K and only a five-fold decrease in the rate is observed during this change in Pt crystallite size. Pt/TiO2-LTR (low temperature reduced - 473 K) and Pt/SiO2 catalysts exhibited zero- and first-order dependencies on citral concentration and hydrogen pressure, respectively, at 373 K. In contrast, the Pt/TiO2-HTR (high temperature reduced - 773 K) catalyst exhibited negative first- and zero-order dependencies on citral concentration and hydrogen pressure, respectively. The TOF on the Pt/TiOz-HTR catalyst was more than an order of magnitude greater than that on Pt/SiO2 and Pt/TiO2-LTR. In addition, the Pt/TiO2-HTR catalyst exhibited a marked enhancement in selectivity towards hydrogenation of the C=O bond. 1. INTRODUCTION It has been stated that approximately 50-100 kg of by-product are produced per kg of product in the fine chemicals and pharmaceutical sectors of the chemical industry [1]. Therefore, in light of the increased environmental awareness, it is of interest to develop heterogeneous catalysts for synthesis of pharmaceuticals and fine chemicals In the present work we report the influence of metal-support interactions (MSI) during the selective liquidphase hydrogenation of citral (3,7-dimethyl-2,6-octadienal), which contains three unsaturated bonds including a conjugated system of C=C and C=O bonds and an isolated C=C bond. From a thermodynamic perspective, the isolated C=C bond is the most favorable to hydrogenate followed by the conjugated C=C bond and lastly the C=O bond [2]. However, kinetic control of the reaction can be induced to yield high selectivity for hydrogenation of the C=O bond alone. In the present paper we examine the influence of reaction parameters and support effects on selective hydrogenation of citral.
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498
2. EXPERIMENTAL
The details of catalyst synthesis, characterization, and hydrogenation reaction procedures are described in detail elsewhere [3]. Briefly, the catalysts were prepared via incipient wetness or ion exchange using H2PtC16 or Pt(NH3)4CI2, respectively, as the precursor. SiO2 (Davison Grade 57 silica gel - 220 m2/g) and TiO2 (Degussa P25 - 47 m2/g) were dried and calcined at 773 K for four hours prior to catalyst synthesis followed by drying at 393 K overnight. The catalysts were characterized using H2 and CO chemisorption at 300 K to evaluate dispersion and average particle size. Pt/SiO2, Pt/TiO2-LTR (low temperature reduced), and Pt/TiO2-HTR (high temperature reduced) were reduced in situ at 673 K, 473 K, and 773 K, respectively. Nitrogen was bubbled through both hexane and citral prior to their addition into the reactor to remove trace quantities of oxygen from the liquid phase. The reaction progress was monitored by GC analysis of liquid samples periodically withdrawn into a N2 purged vessel as well as by the instantaneous rate of H2 uptake [3]. 3. Results and Discussion 3.1 Citral Hydrogenation over Pt/SiO2
The kinetic data obtained with Pt/SiO2 catalysts was previously shown to be free of transport limitations and poisoning effects as verified by the Madon-Boudart test [3, 4]. The influence of reaction temperature on rate and product distribution is displayed in Figures 1 and 2, respectively, for reaction over a 1.44% Pt/SiO2 catalyst at 20 atm hydrogen pressure and 1 M citral in hexane in the range of 298 - 423 K [3].
0.6 t[
-~
i 0.4 + i
~"
~ ,-
.......
L
-""
/
-- - 2 9 8 K 1
', '~ ~-
o
9
.
i i
,
0
80
| ,
'
.... 373K ~ 423 K i
i
"~ ~
"",.,.,ih,... i."
~ - ~ ~
20
:~" i /-~ 40 ~-~",~
~
7 40
60
Citral Conversion (%)
Figure 1:Temporal H2 uptake profiles for reaction at 298 K, 373 K, 423 K with 1 M citral in hexane at 20 atm H2 pressure.
20 t
.
.
.
.
.
I
....
-0 ....
,i
I o Conjugated C=C Bond (298 K);; o Conjugated C=C Bond (373 K)':'
~ 1 ~ - - - - '
O~ L
t
0
20
~ ---
40
~
---
60
I "~
80
100
Citral Conversion (%)
Figure 2" Selectivity for hydrogenation of conjugated C=O and C=C bonds at 298 K, and 373 K with 1 M citral in hexane at 20 atm H2 pressure.
It is apparent from Figure 1 that there is an activity minimum in the rate with respect to temperature. Furthermore, significant deactivation is observed at 298 K in contrast to the negligible deactivation exhibited at 373 K and 423 K. The product distribution is dramatically altered by reaction temperature as seen in Figure 2. The selectivity towards
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499
hydrogenation of the conjugated C=C bond is greater than that for the C=O bond at 298 K. This trend is reversed at 373 K where hydrogenation of C=O bond is the primary reaction pathway. This unusual kinetic behavior was rationalized by utilizing a conventional Langmuir-Hinshelwood-type model for each of the hydrogenation steps along with a concurrent inhibiting decarbonylation reaction. The activity minimum has been explained based on the lower activation barrier for the decomposition reaction yielding chemisorbed CO as compared to that for CO desorption [3, 5, 6]. Such a trend was shown to have three consequences including: strong deactivation during reaction at low temperatures i.e., 298 K, an activity minimum at approximately 373 K, and negligible deactivation at 373 K and 423 K [3]. The complex kinetics observed during liquid-phase hydrogenation is also manifested in the apparent structure sensitivity of the reaction at 373 K with 20 atm H2 pressure and 1 M citral in hexane as shown in Table 1. The differences in the rate of citral hydrogenation among catalysts with different dispersion was suppressed at 298 K in contrast to the behavior at 373 K. Due to the strong deactivation behavior at 298 K, there is significant uncertainty present in reporting a single value for the rate at this temperature. Therefore, Figure 3 displays the temporal H2 uptake profile for reaction over Pt/SiO2 catalysts with varying metal dispersion at 298 K, 20 atm H2 pressure, and 1 M citral in hexane. It should be noted that in spite of the dramatic differences in the initial rate of citral hydrogenation for catalysts with different particle sizes, the selectivity vs. conversion behavior was similar for all the catalysts, within experimental uncertainty, as shown in Figure 4. Table 1 Effect of average Pt particle size, determined from H2 chemisorption at 300 K, on the initial rate of citral hydrogenation at 373 K, 20 atm H2 pressure, with 1 M citral in hexane. Dispersion H/Pt
Metal Particle Size
Initial TOF
Pt/SiO 2
(nm)
(s")
3.59% Pt
0.04
28
0.120
6.65% Pt
0.09
2.65% Pt
0.22
5.1
0.092
3.80% Pt (sintered)
0.37
3.1
0.040
1.44% Pt
0.41
2.8
0.017
0.49% Pt
0.45
2.5
0.012
3.80% Pt
0.66
1.7
0.010
1.1
0.005
Catalyst
0.77% Pt
0.065
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0.8
r~
..............................................................................................................
0.6
o H / p t = 0.09
,., e~
9 H / Pt =
0.4
o H / Pt = 0.4
\',,,
~.
\o
~ H / Pt =0.7
0.2
~o
" H/Pt
i o~....'-~___~---~__ [-
0
0.2
-----I_
~---~_---~___^
0
= 1.0
- ....... o ~ - - - - o _ o ~ _ o
60
"-=---u--~--~~---o 120
..............
!
240
300
180
Time (min)
Figure 3" Influence of metal dispersion on the rate of H2 uptake during reaction at 298 K and 20 atm H2 with 1 M citral in hexane. 100 -;. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -=
80
-
60:
L
.
I'-OH > secondary-OH. However, under the conditions described above, the secondary hydroxyl 2-OH was the most reactive. An explanation for this regioselectivity is probably to be the activation, by general-base catalysis, of the most acidic hydroxyl of sucrose (the 2-OH) to a more nucleophilic alcoxide [5]. On the contrary, the enzymatic acylation of sucrose has been reported to occur at the I'-OH primary hydroxyl
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514
group (on the fructose ring) using subtilisin-like proteases [7] or at the primary 6-OH with different lipases [8-9].
HO H
4
-OH
6 ~
1
%-
Vinyl ,aurate /
HO'~I"~O " ~-,"~OH
i"
Lipase from Humicola I
lanuginosalCelite
I
2-methyl-2-butanol:DMSO | 40~ 1
Na2HPO4 Celite, Eupergit C DMSO 40~
Le(-OH O
o/
"OoU l •
~
yl laurate
/o
%-
9H4 9
.o-r
6-O-lauroylsucrose
o.
3o. 2"! ~-C" o
\2"
OH43._~~60 H
2-O-lauroylsucrose
Scheme 1. Enzymatic (left) and chemical (right) acylation of sucrose with vinyl esters. In conclusion, different regioisomers may be obtained with an appropriate election of the catalyst. This is remarkable since the properties of different monoesters have been reported to be significantly different [ 1]. Both methods we propose here are rather simple and suitable for the synthesis of sucrose monoesters with high regioselectivity.
REFERENCES 1. F.A. Husband, D.B. Sarney, M.J. Barnard and P.J. Wilde, Food Hydrocolloid, 12 (1998) 237-244. 2. Ver H. Baal, G. Vianen, Cosmetics News, 20 (1997) 33-37. 3. Y. Nishikawa, M. Okabe, K. Yoshimoto, G. Kurono and F. Fukuoka, Chem. Pharm. Bull., 24 (1976) 387-393. 4. O.T. Chortyk, J.G. Pomonis and A.W. Johnson, J. Agric. Food Chem., 44 (1996)1551-1557. 5. C. Chauvin, K. Baczko, D. Plusquellec, J. Org. Chem., 58 (1993) 2291-2295. 6. M. Ferrer, M.A. Cruces, M. Bernab6, A. Ballesteros and F.J, Plou, Biotechnol. Bioeng., 65 (1999) 000-000. 7. S. Riva, M. Nonini, G. Ottolina, B. Danieli, Carbohydr. Res., 314 (1998) 259-266. 8. M. Woudenberg, F. Van Rantwijk, R.A. Sheldon. Biotechnol. Bioeng., 49 (1996) 328-333. 9. J.E. Kim, J.J. Han, J.H. Yoon and J.S. Rhee, Biotechnol. Bioeng., 57 (1998)121-125. 10. F.J. Plou, M.A. Cruces, E. Pastor, M. Ferrer, M. Bernabe and A. Ballesteros. Biotechnol. Lett., 21 (1999) 635-639. 11. F.J. Plou, E. Pastor, M.A. Cruces, M. Ferrer and A. Ballesteros. Spanish Patent No. 9802086 (1998).
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
The H i g h l y Selective Conversion of Toluene into 4-Nitrotoluene and 2,4-Dinitrotoluene Using Zeolite H-Beta D. Vassena, A. Kogelbauer, and R. Prins Laboratory for Technical Chemistry, ETH-Ztirich, CH-8092 Ziirich, Switzerland The nitration of toluene was studied in the vapour and in the liquid phase to assess the potential of various zeolites and solid acids for replacing sulfuric acid. Zeolite beta in its proton form provided a higher para-to-ortho ratio in the nitration of toluene to nitrotoluene (NT) compared to other catalysts. Enhanced para-selectivity was also observed for the formation of dinitrotoluene (DNT) using H-beta and acetyl nitrate in the liquid phase. Characterisation and adsorption studies suggest that the high para-selectivity originated from steric hindrance of the ortho position induced by adsorption rather than from classical transition state selectivity. 1. INTRODUCTION Nitrotoluenes are important intermediates in the chemical industry. The typical product composition that is obtained in the industrially applied mixed-acid nitration favours the formation of the less desired ortho product [1]. A substantial number of studies has been reported trying to overcome these limitations and aiming at a higher fraction of parasubstituted products [2-7]. Mainly zeolites such as mordenite [2,3], ZSM-5 [2-4], ZSM-11 [5], beta [2,6] and Y [3,5] have been tested as catalysts, but also clay-supported metal nitrates [8] and SiO2 or A1203 impregnated with H2SO4 or H3PO4 [7] have been investigated. In the majority of studies organic nitrating agents were applied such as various acyl nitrates [5,6] or alkyl nitrates [2,4]. Few studies on nitrogen dioxide (dinitrogen tetroxide) [2,3] or nitric acid [5,7] have been done. The use of zeolites seems to be the most promising route so far and the concept of shape selectivity has been commonly invoked to explain the enhanced paraselectivity [4,5]. In the current paper we present results obtained for the nitration of toluene and 2-nitrotoluene (2-NT) using H-beta zeolite under various reaction conditions suggesting reasons other than classical shape selectivity being responsible for this unique behaviour. Our earlier work has shown that zeolites were catalytically inactive in the nitration of aromatics in the liquid phase with nitric acid as the industrially preferred nitrating agent because of the poisoning of the acid sites by water [9]. One crucial task was therefore the removal of water from the acid sites in order to keep the catalyst active. This was achieved by raising the reaction temperature and thereby evaporating the water [10-13] or by chemically trapping it with acetic anhydride [6].
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2. EXPERIMENTAL
The acid form of the different zeolites was obtained by repeated ion exchange of the parent Na zeolites (Chemie Uetikon) with 1 M aqueous NH4NO3 solution and subsequent calcination in static air. For comparison, the non-microporous solid acid Deloxan, a polysiloxane bearing alkylsulfonic acid groups (Degussa AG) was used as received. The Si and AI concentration of the zeolites was determined by AAS analysis; nitrogen adsorption at 77 K was carded out on a Micromeritics ASAP 2000M volumetric analyser. The catalysts were degassed prior to analysis under vacuum at 400~ The specific surface area was evaluated using the BET method, the external surface area is given as the difference between the BET surface area (N2 adsorption) and the micro pore surface area which was determined according to the t-plot method [14]. The average pore diameter of pores bigger than 17 .A was estimated from the desorption branch of the isotherm using the BJH method [15]. The resulting zeolite characteristics are presented in Table 1. The vapour phase nitration was achieved in a flow reactor system at 158~ and atmospheric pressure over a period of 26 h reaction time. The catalysts were pretreated in flowing N2 at 158~ during 1 h. Equimolar amounts of toluene and 65% nitric acid were fed using N2 as carder. The HNO3 conversion was determined by back-titration of the unreacted nitric acid. The organic products were collected in dichloromethane and analysed off-line by gas chromatography using a HP 5890 gas chromatograph equipped with a HP-1 fused silica capillary column and 1,3-dinitrobenzene as integration standard. An alternative method for sustaining the activity, the trapping of water by chemical reaction, was attempted using acetic anhydride [6]. Acetyl nitrate, the nitrating agent, was generated in situ from 90 wt % nitric acid and acetic anhydride. For reaction experiments 35 mmol 90 wt % nitric acid and 1.0 g of dried catalyst (130~ overnight) were mixed and stirred at 0~ 53 mmol of acetic anhydride were added, corresponding to the stoichiometric amount needed to convert nitric acid into acetyl nitrate and the water present in nitric acid into acetic acid (AcOH). 35 mmol of toluene or 3.5 mmol of 2-NT were then added dropwise in order to keep the temperature below 20~ After addition of the substrate, the mixture was stirred for 30 minutes. The organic products were separated by extraction with methylene chloride and analysed in the same way as for the vapour phase nitration. Infrared spectra of self-supporting wafers of H-beta and H-ZSM-5 were recorded at ambient temperature on a Mattson Galaxy 6020 IR spectrometer equipped with a MCT detector at a resolution of 4 c m "1. Prior to analysis, samples were degassed at 500~ for 10 h Table 1 Characteristics of the catalysts Si/AI BET surface area (m2/g) Deloxan H-beta H-ZSM-5 H-mor
11.5 16.9 4.6
external surface area (m2/g)
pore volume (cm3/g)
aver. mesopore diam. (A)
micro pore volume (cm3/g)
642 199 64 6
1.11 0.90 0.51 0.27
51 102 98 64
0.21 0.16 0.22
642 690 433 544 HH
,H...
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at a pressure below 10"4 Pa. Adsorption experiments using 2-NT and 4-NT were carded out at ambient temperature at a pressure of 10 Pa. The stepwise desorption of NT was followed by IR in a temperature range between ambient temperature and 500~
3. RESULTS AND DISCUSSION In both reaction regimes enhanced formation of 4-NT was observed using H-beta while non-microporous solid acids and also other zeolites gave product compositions similar to mixed acid. In the vapour phase nitration (Figure 1) zeolite H-beta exhibited a para-to-ortho ratio of nitrotoluenes of more than 1.1 during the first hours on stream. Selectivity to 4-NT decreased over a period of about 10 hours on stream due to pore filling/blockage by strongly adsorbed products/byproducts [10]. Another zeolite with a 12-membered ring as pore opening, Hmordenite, did not exhibit enhanced para-selectivity compared to the reaction without zeolite. Using H-ZSM-5 and Deloxan the p/o ratio of product NT was slightly higher than that observed in the absence of a catalyst (p/o = 0.7). It remained at the somewhat higher value for the whole duration of the experiment with Deloxan whereas a rapid decrease was observed
1.2 1.1 Z
r
!
Iz
,r I
9
1 09
H-mor
0.9
0.8
v
H-ZSM-5
x
Deloxan 9
0.7 0.6
H-beta
~
o
'
~
g
~ -
'
'0
'1
'5
Blank
-
2'o2'5ao
Time-on-stream / h
Fig. 1. Time-on-stream behaviour of various solid acids during vapour phase nitration (158~
65 % wt HNO3, HNOa/Tol = 1, pTol=13.2 kPa, W/F = 5 g h moll).
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Table 2 Nitration of toluene with acetyl nitrate (toluene/HNO3 = 1) NT Yield (mol %) DNT Yield (mol %)
none H-beta H-ZSM-5 H-mor
76 76 73 83
0.0 1.4 0.0 0.0
4-NT/(2+3+4)-NT 0.38 0.72 0.40 0.39
when H-ZSM-5 was used. Judging from the kinetic diameters of the nitrotoluenes (4NT = 5.2 A, 2-NT = 6.7 A [4]) in combination with the irreversible nature of the nitration reaction [16] one may conclude that 2-NT can be formed in the channel intersections of HZSM-5 but remains virtually trapped. This would lead to rapid deactivation of H-ZSM-5 and the non-selective formation of products by the homogenous vapour phase reaction. The nitration of toluene in the liquid phase with acetyl nitrate demonstrated clearly that only with H-beta a higher selectivity to 4-NT was observed (Table 2). Beta was also the only catalyst that gave a small yield of DNT. We have shown previously that the selective formation of 2,4-DNT using H-beta was not only linked to the enhanced formation of 4-NT but also to the highly selective conversion of 2-NT into 2,4-DNT [17]. The results in Table 3 show that this is also a unique property of beta zeolite. H-beta, having pore openings of 7.6 x 6.4 A, should not impose steric restrictions upon the penetration by nitrotoluenes. This was confirmed by in situ infrared measurements of the adsorption of 2-NT and 4-NT which showed that all acid sites were rapidly covered after the exposure of H-beta to 10 Pa NT (Figure 2). On the contrary, incomplete coverage of the Bronsted acid sites was observed for H-ZSM-5 during 2-NT adsorption while 4-NT was able to adsorb on all acid sites. These results are in line with the behaviour of H-ZSM-5 during nitration reactions where initially some enhanced formation of 4-NT was observed followed by a rapid decline of activity and selectivity. Since re-equilibration of products does not occur, selectivity regarding the transition state may be invoked for explaining the high para-selectivity. The size of the critical transition state, the Wheland intermediate, is very similar to that of the product molecule NT. A severe restriction regarding the formation of the Wheland intermediate that yields 2-NT should therefore also be manifested in a significant blockage of the adsorption of 2-NT. This, however, was not observed. Even more, high selectivity toward the para product (2,4-DNT) Table 3 Nitration of 2-NT with acetyl nitrate (2-NT/HNO3 = 0.1) DNT Yield (mol %) 2,4/(2,4+2,6)-DNT none H-beta H-ZSM-5 H-mor
0.2 89.6 0.8 1.3
0.94 0.60 0.60
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519
~
b ,
a
3800 3600 3400 32'o0 3o'00 2800 Wavenumber
/ c m -~
38'oo36'oo'34'oo'32'oo 30'00 2800 Wavenumber
/ c m -~
Fig. 2. IR spectra of H-beta (left) and of H-ZSM-5 (right) at ambient temperature after degassing at 500~ (a), after 30 min equilibration with 10 Pa 4-NT (b), after subsequent evacuation at ambient temperature for 24 h at a pressure below 10-4 Pa (c); after 30 min equilibration of a fresh zeolite with 10 Pa 2-NT (d), after subsequent evacuation at ambient temperature for 24 h at a pressure below 10-4 Pa (e).
was observed when 2-NT was nitrated. The steric requirements for these two reactions, nitration of toluene and nitration of 2-NT are significantly different. It is our conclusion therefore that classical transition state selectivity can not explain the observed results. The possibility that the bulky acetyl nitrate induces selectivity by not being able to approach the ortho position can be ruled out for the following reasons. Acetyl nitrate formed in situ from acetic anhydride and nitric acid gave high yields of NT even without any zeolite, however, in the typical product ratio with about 60% 2-NT. Furthermore, carrying out the nitration with simultaneous removal of the water by distillation, 2-NT washighly selectively nitrated to 2,4-DNT using H-beta and nitric acid. H-beta is unique in its behaviour because the use of other large pore zeolites such as mordenite did not give enhanced formation ofpara-substituted products. While H-beta has a high external surface due to the small crystallite size, H-mor is characterised by a small external surface and big crystallite size. Given the fact that the non-selective homogenously occurring nitration reaction is always in competition with the heterogenously catalysed reaction on the zeolites, the crystallite size is expected to play an important role regarding the overall selectivity. The smaller crystallite size of beta might therefore be beneficial by providing shorter diffusion paths and thereby enabling a larger contribution of the heterogenously catalysed selective nitration. Further experiments with a mesoporous mordenite and a macrocrystalline beta are in progress to test this hypothesis. Another peculiarity of beta is the high concentration of silanol groups. During the adsorption studies we observed strong interaction of the non-acidic silanol groups up to the temperature of desorption of NT. Similar observations have been reported by others using NMR
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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spectroscopy [18]. It is evident that these silanol groups participate in the bonding and therefore conceivably influence the steric arrangement in the adsorbed state. At present, we tentatively ascribe the observed selectivity effects to an adsorption-induced steric blockage of the ortho position of the substituted aromatics. 4. CONCLUSIONS
H-beta has been shown to be unique among solid acids with respect to the high para selectivity obtained during nitration of substituted aromatics. The enhanced para-selectivity seemed to originate from sites located inside the micropore system of H-beta and is most probably linked to steric hindrance induced by adsorption on a rigid solid surface. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Ullmann's Encyclopedia of Industrial Chemistry, A17, VCH, Weinheim, 1991. J.M. Smith, H. Liu and D.E. Resasco, Stud. Surf. Sci. Catal., 111 (1997) 199. D.B. Akolekar, G. Lemay, A. Sayari and S. Kaliaguine, Res. Chem. Intermed., 21 (1995) 7. T.J. Kwok and K. Jayasuriya, J. Org. Chem., 59 (1994) 4939. S.M. Nagy, K.A. Yarovoy, V.G. Shubin and L.A. Vostrikova, J. Phys. Org. Chem., 7 (1994) 385. K. Smith, A. Musson and G.A. DeBoos, J. Org. Chem, 63 (1998) 8448. H. Schubert and F. Wunder, US Patent No. 4 112 006 (1978). L. Delaude, P. Laszlo and K. Smith, Acc. Chem. Res., 26 (1993) 607. A. Kogelbauer, D. Vassena, R. Prins and J.N. Armor, Catal. Today, submitted. D. Vassena, D. Malossa, A. Kogelbauer and R. Prins, in: M.M.J. Treacy, et al. (Eds.), Proceedings of the 12th International Zeolite Conference, Vol II, Materials Research Society, 1999, p. 1471. L. Bertea, H.W. Kouwenhoven and R. Prins, Stud. Surf. Sci. Catal., 78 (1993) 607. L. Bertea, H.W. Kouwenhoven and R. Prins, Stud. Surf. Sci. Catal., 84 (1994) 1973. L. Bertea, H.W. Kouwenhoven and R. Prins, Appl. Catal.: A, 129 (1995) 229. B.C. Lippens and J.H. de Boer, J. Catal., 4 (1965) 319. E.P. Barret, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. A. Germain, T. Akouz and F. Figueras, J. Catal., 147 (1994) 163. D. Vassena, A. Kogelbauer and R. Prins, Proceedings of the First International FEZA Conference, accepted for publication. M. Hunger and T. Horvath, Catal. Lett., 49 (1997) 95.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Catalytic Asymmetric Heterogeneous Aziridination and Epoxidation of Alkenes using Modified Microporous and Mesoporous Materials Graham J. Hutchings, a Christopher Langham, ~ Paola Piaggio," Sophia Taylor," Paul McMorn," David J, Willock, a Donald Bethell, b Philip C. Bulman Page, c Chris Slyf Fred Hancock e and Frank King e aDepartment of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF1 3TB, UK. bLeverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool L69 3BX, UK. CDepartment of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK. dRobinson Brothers Ltd., Phoenix Street, West Bromwich, West Midlands B70 0AH, UK. eSynetix, R&T Division, P.O. Box 1, Billingham, Teeside TS23 1LB, UK. Copper-exchanged zeolite Y is a highly active catalyst for the aziridination of alkenes. Modification using bis(oxazolines) leads to the formation of an enantioselective aziridination catalyst. Using a similar approach, manganeseexchanged MCM-41 modified with a chiral salen ligand is found to be an effective enantioselective heterogeneous epoxidation catalyst for cis-stilbene. 1. INTRODUCTION The design of asymmetric catalysts is of intense current interest and procedures describing the use of chiral transition metal complexes as homogeneous catalysts have been described for epoxidation, cyclopropanation, aziridination and hydrogenation of alkenes. There is an increasing awareness that heterogeneous catalysts can offer significant advantages over homogeneous catalysts and this has prompted research activity in this field. To date, three experimental approaches have been used in the design of enantioselective heterogeneous catalysts: (i) the use of a chiral support for an achiral metal catalyst; (ii) the immobilization of an asymmetric homogeneous catalyst onto an achiral support; and (iii) modification of an achiral heterogeneous catalyst using a chiral cofactor. The first approach was pioneered by Schwab in 1932 [1]. Using Cu, Ni, Pd and Pt supported on enantiomers of quartz he demonstrated low enantioselection in the dehydration of butan-2-ol. A number of other chiral supports have been examined, e.g. natural fibres and chiral polymers. Most recently, attention has focused on using zeolite [3, since it is possible that a chiral form of this zeolite could be synthesised [2]. The second approach has been particularly effective for enantioselective hydrogenation reactions using zeolites as supports for asymmetric Ru and Rh catalysts [3]. The third approach,
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involving the creation of a chiral catalyst surface by the adsorption of a chiral modifier onto an achiral catalyst, has been successful in a number of studies, again particularly for enantioselective hydrogenation. For example, the modification of platinum catalysts with cinchona alkaloids for the hydrogenation of prochiral 0~-ketoesters [4] have been extensively studied. We have also used the third approach, and we have concentrated our design efforts on zeolite Y since we consider that, for the optimal catalyst design, the achiral catalyst should have a well defined structure. In our initial proof of concept studies we studied the modification of zeolite H-Y with chiral dithiane 1oxides and we have shown that this approach can give catalysts that are capable of enantioselection for the dehydration of butan-2-ol in the temperature range 110-150 ~ [5]. We have now extended this generic approach and have designed catalysts for the enantioselective aziridination and epoxidation of alkenes. 2. E X P E R I M E N T A L
The zeolite HY used in this study was supplied by Union Carbide (LZY 82). A1MCM-41 was synthesized according to literature methods [6], and prior to use was calcined at 550 ~ for 4hrs in nitrogen, followed by 16hrs in air. Cu-exchanged zeolite (CuHY) was prepared using a conventional ion-exchange method in which zeolite H-Y was treated with aqueous Cu(OAc)2 solution, the concentration of which was chosen so as to obtain the required exchange level (ca. 50-60%). The solids were then washed with distilled water until all u n b o u n d Cu 2. was removed and dried at 110 ~ in air. An identical method was used for the preparation of CuA1MCM-41. The aziridination of alkenes was carried out in a batch reactor using (N-(p -tolylsulfonyl)imino)phenyliodinane (PhI=NTs) and (N-(p-nitrobenzylsulfonyl)imino)phenyliodinane (PhI=NNs) as the nitrene donors. The alkene, nitrene donor and solvent were stirred together in a flask under controlled temperature in the presence of the Cu catalyst. The nitrene donor reagents are relatively insoluble under the chosen conditions and the reaction was monitored by observing the dissolution of this reagent; when the dissolution was complete the reaction was considered to be complete. In a typical experiment, styrene (500 ~tl) was reacted with PhI=NTs (0.3g) in acetonitrile together with CuHY catalyst (0.3g) at 25 ~ for 2 h. The products were isolated by column chromatography and product identification was confirmed by NMR, elemental analysis, GCMS and infra red spectroscopy. When enantioselective aziridinations were carried out, the CuHY catalyst was pretreated with a chiral modifier prior to reaction, and the products were analysed using chiral HPLC. The method described above was used to prepare MnHY and MnA1MCM41 via ion exchange with aqueous manganese acetate (0.2M, 25 ~ 24 h), followed by filtration, washed with water and vacuum dried. This procedure was repeated twice and the material was calcined (550 ~ 24 h). The calcined Mn-exchanged materials were refluxed with the chiral salen ligand, (R,R)-(-)-N,N'- bis(3,5-ditert-butylsalicylidene)-l,2-cyclohexanediamine, in CH2CI 2 (24 h, metal:salen = 1:1), cooled to 0 ~ and washed with CH2C12. In the case of MnA1MCM-41 this procedure resulted in 10% of the salen ligand being incorporated (determined by TGA and solution analysis). The Mn-exchanged material:salen catalyst was then
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investigated for the epoxidation of cis -stilbene using iodosylbenzene as oxidant in a batch reactor. 3. RESULTS A N D DISCUSSION
3.1. Heterogeneous asymmetric aziridination of alkenes with CuHY CuHY is found to be an effective catalyst for the aziridination of a range of alkenes when using PhI=NTs and PhI=NNs as the nitrene donors (Table 1). To confirm the process was wholly heterogeneous, following the reaction the zeolite catalyst was recovered by filtration and another aliquot of reactants was added to the recovered filtrate and no reaction was observed. Further, the r e m o v e d catalyst was reused with fresh reagents and solvent and the catalyst gave the same reactivity as that observed initially. It is observed that the catalyst gives best results with phenyl-substituted alkenes, and although lower yields are observed with cyclohexene and trans-hex-2-ene the reaction is still observed. Interestingly, for the aziridination of trans-stilbene no product could be observed. We consider this to be due to the relatively bulky aziridine product being too large to be accommodated within the small pores of CuHY and is further evidence that the reaction proceeds inside the pores of the zeolite. This interesting result illustrates the potential for a heterogeneous catalyst to possess size-specificity to a precise degree. Such a property could be exploited by making use of zeolites with a range of pore sizes, and could also be developed to achieve regioselectivity in a reagent containing two or more double bonds. Modification of the CuHY with chiral bis oxazoline ligands (e.g. 2,2-bis-[2-((4R)-(1-phenyl)-l,3-oxazolenyl)]propane) leads to the aziridine being obtained with up to 75% enantiomeric excess in these initial experiments (Table 1). We have found that a temperatures in the range of -10 to 20 ~ provide the highest combination of enantioselectivity, yield and reaction time when using acetonitrile solvent. In these experiments, the chiral modifier is simply added to the reaction mixture, and no special pretreatment of the catalyst system is required. To demonstrate that alternative types of silicate framework can be used for this reaction, experiments were carried out with copper-exchanged MCM-41. Yields of up to 87% of the aziridine with e.e. of 37% were obtained. Using this type of mesoporous catalyst system greatly enhances the versatility of the heterogeneous aziridination reaction. The major advantage of the use of CuHY as a catalyst for this reaction is the ease with which it can be recovered from the reaction mixture by simple filtration if used in a batch reactor (alternatively it can be used in a continuous flow fixed bed reactor). We have carried out the heterogeneous asymmetric aziridination of styrene until completion, filtered and washed the zeolite then added fresh styrene, PhI=NTs and solvent, without further addition of chiral bis(oxazoline), for several consecutive experiments. The yield and the enantioselectivity decline slightly on reuse; we have found that adsorbed water can build up within the pores of the zeolite on continued use and we believe that this is the cause of loss of activity and enantioselection. However, full enantioselectivity and yield can be recovered if the catalyst is simply dried in air prior to reuse, or alternatively the catalyst can be recalcined and fresh oxazoline
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ligand added. We are therefore confident that this catalyst system can form the basis of a commercial heterogeneous catalyst for the aziridination of alkenes. Table 1 CuHY-Catalysed Aziridination of Representative Alkenes. ....................................................................................... Bis-oxazoline Alkene a Sytrene cz-Methylstyrene Cyclohexene Methyl cinnamate trans-Stilbene trans-Hex-2-ene Sytrene Styrene Trans-~Methylstyrene trans-f~Methylcinnamate
None
Me Me ~O r ~ /~_ p, (1) Me
~ n l ~ Me~r
~M%
Styrene Styrened
Op~~/1~ h~nhO,,~Pt,,,,,,,Nr (2)
PhINTs T e m p .....~'ieid 6e.e. c ~ % % 25 90 (92) 25 33 25 50 (60) 25 84 (73) 25 25 25 -10
0 (52) 44 87 82
29 44
-10
74
36
-10 25 -20 -20 0
8 (21)
61 (70)
64 15 (89)
0 18 (63)
Styrene
25
78 (75)
10 (10)
Styrene
25
73 (74)
0 (15)
Styrene
-10
4
61
PhINNs Yield 6 e.e. c % % 93 (97)
69 100e
52 75e
30 ~ 87
59 f 64
100
34
(3)
Ph
,mPh
(4)
(5) a
b
Solvent CH3CN, styrene: PhI=NTs = 5:1 molar ratio; Isolated yield of aziridine based on PhI=NTs. Values in parentheses indicate yields obtained from homogeneous reactions; CEnantioselectivity determined by chiral HPLC; dstyrene was used as solvent, e0 ~ f25~ Absolute configurations of major products, determined by optical rotation, are (S) for trans~- and trans-~-methylcinnamate, (R) for styrene.
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3.2. Asymmetric epoxidation using modified Mn-exchanged materials.
To demonstrate the flexibility of the approach to catalyst design that we set out in this paper, the epoxidation of alkenes using iodosylbenzene has also been studied. Initial studies focused on MnHY:salen catalysts for the epoxidation of styrene, however, the reaction was slow, and low yields of styrene oxide were observed. Analysis of the reaction mixture revealed the breakdown of the salen ligand within a few turnovers. Subsequently Mn-A1MCM-41 was used with iodosylbenzene as the oxygen donor and cis -stilbene was used as substrate, and the results, together with those of control experiments, are shown in Table 2. Mn(OAc)2 in the absence of A1MCM-41 or salen ligand is not particularly reactive, and only 1.5% yield of the epoxide was formed after reaction for 24 h at 25 ~ Modification of Mn 2. in solution by the salen ligand, as expected, leads to a significant rate enhancement, and both the cis-epoxide and the trans -epoxide were formed, the latter with 78% e.e. Interestingly, immobilization of the Mn 2. within A1-MCM-41 leads to an increase in reactivity, and the epoxide is formed with an enhanced cis/trans ratio to the homogeneously catalysed Mn:salen catalyst. This effect suggests that the A1-MCM-41 is occupying part of the Mn 2. coordination sphere, restricting the cis ---~trans transformation. Further modification of the Mn-exchanged A1-MCM-41 with salen leads to a further enhancement in reactivity, and the trans epoxide is formed with an 70% e.e., very similar to that observed for the equivalent homogeneous reactions, trans Stilbene is found to be a significantly less reactive substrate, and the e.e. of the resultant trans-epoxide is significantly decreased with the salen modified Mnexchanged A1-MCM-41. The use of Mn-exchanged A1-MCM-41:salen catalyst for this epoxidation does not result in the formation of significant levels of byproducts as has been observed when manganese bypiridyls have been used as catalysts, and typically only deoxybenzoin is observed at low levels (ca. 5-10%). A further set of experiments was carried out to examine the reusability of the Mnexchanged A1-MCM-41:salen catalyst. Following the reaction, the Mn-exchanged A1-MCM-41:salen catalyst was recovered by filtration and the solid was reused in a new catalytic reaction; although the reactivity and enantioselectivity had declined, epoxide was still formed and the cis/trans ratio was unchanged. Recalcination of the recovered material and addition of new salen ligand essentially restored both the reactivity and the enantioselection. Use of the solution following the filtration did not give any activity, and furthermore this solution contained no Mn 2§ These experiments demonstrate that the reaction occurring with Mn-exchanged A1-MCM-41:salen is wholly heterogeneously catalysed. At this stage we have made no attempt to optimise the catalytic performance, but we anticipate that appropriate modification of the chiral salen ligand and the reaction conditions will lead to enhanced reactivity and e.e.. CONCLUSIONS In this paper we have described a design approach for heterogeneous enantioselective catalysts. The approach is based upon modification of the counter-cation of zeolites or mesoporous alumino-silicates with a suitable chiral
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Table 2 Epoxidation of stilbene at 25~ usin~Mn-exchan~ed MCM-41 Entry Catalyst Time Conv. Epoxide Selectivity (%) Ha Yield (%)f cis trans 1 2
None Mn(OAc)2
3
Mn(salen) complex
4
c,d
A1MCM-41 MnMCM-41
5 6 7
solution
9
25 24
0 100
0 1.5
0 0
0 100
0 0
1
100
86
29
71
78
0
0
0
0
0
2
45
3
0
100
0
ce
2 26
100 100
69 35
58 0
42 100
70 25
2
0
0
0
0
0
c
2
37
18
51
39
30
c
MnMCM-41+salen
(%)g
24
c
MnMCM-41+salen
8
c
e.e. trans
c
~
MnMCM-41 reused 10 recalcined +salen c,h 2 100 52 63 37 54 a reaction time, ~ as determined by decomposition of iodosylbenzene to iodobenzene, US~lr~ HPLC, Csolvent CH2CH 2 with molar ratio of cis-stilbene:catalyst:iodosylbenzene=7:1:0.13, reaction conducted m CH3OH, trans-stilbene used as substrate, Conversions, yields a d selectivity determined by HPLC, g enantiomeric excess determined by chiral HPLC., "MnA1MCM-41 from entry 6, recalcined and refluxed with salen ligand. 9
9
e
9
f
"
"
n
ligand. We have demonstrated the approach with two examples: (a) enantioselective aziridination of alkenes using Cu2*-exchanged zeolite Y modified with chiral oxazolines and ( b ) t h e modification of manganeseexchanged A1-MCM-41 by a chiral salen ligand for the enantioselective epoxidation of alkenes. Since there is a broad range of zeolites and m e s o p o r o u s materials available as catalytic materials, it is anticipated that the approach described in this paper can form the basis of a generic design of new enantioselective catalysts. Acknowledgements
We thank Synetix, Robinson Brothers and EPSRC for financial support. REFERENCES
1. G.M. Schwab and L. Rudolph, Naturwiss., 20 (1932) 362. 2. M.E. Davis and R.L. Lobo, Chem. Mater., 4 (1992) 756. 3. A. Corma, M. Iglesis, C. del Pino and F. Sanchez, Stud. Surf. Sci. Catal., 75 (1993) 2293. 4. G. Webb and P.B. Wells, Catal. Today, 12 (1992) 319. 5. S. Feast, D. Bethell, P.C.B. Page, M.R.H. Siddiqui, D.J. Willock, F. King, C.H. Rochester and G.J. Hutchings, J.Chem.Soc., Chem. Comm. (1995) 2409. 6. J.S. Beck, J.C. Vartuli, w.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Shepherd, S.B. McCullen, J.B. higgins and J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Catalytic Hydrogenation of Nitriles to prim., sec. and tert. Amines over Supported Mono- and Bimetallic Catalysts Yin-Yan Huang* and Wolfgang M.H. Sachtler N.V. Ipatieff Laboratory, Center for Catalysis and Surface Science Department of Chemistry, Northwestern University, Evanston, IL 60208, USA ABSTRACT The selectivity of nitrile hydrogenation to prim., sec. and tert. amines is dominantly controlled by the transition metal, similar selectivities are observed in gas phase flow and liquid phase batch runs. All amines are formed during one residence at the catalyst surface. Isotopic labeling in acetonitrile hydrogenation and co-hydrogenation of acetonitrile and butyronitrile show that the hydrogen atoms in the amine groups of the product are not provided by Ha~ at the metal surface, but by the methyl group of other acetonitrile molecules. It is concluded that N-bonded surface complexes are likely intermediates for the formation of prim., sec. and tert. amines 1. INTRODUCTION Hydrogenation of unsaturated compounds over transition metal catalysts dominated the list of intensely studied catalytic reactions for much of the 20th century. Work by Sabatier and Senderens on the hydrogenation of unsaturated acids opened the list in 1902.[ 1] In 1934 Polanyi and Horiuti proposed the mechanism for ethylene hydrogenation[2] which is still considered an adequate description of the shortest route from reactant to products. Bond and Wells showed that alkynes are hydrogenated first to alkenes, while the subsequent step of alkane formation has to wait until alkynes no longer dominate the metal surface.[3,4] In the second half of the century evidence was obtained that the hydrogenations of CO or N2 were no hydrogen additions to a double bond, but dissociative chemisorption of N2 [5] and CO [6,7] precedes the reaction of the fragments with adsorbed H atoms to form ammonia or Fischer-Tropsch products. The world-wide research effort also showed that in hydrogenation catalysis other reactions take place. Polymerization and" coke" formation are often inevitable side reactions; surface science studies also showed that adsorbed alkyl groups split offH atoms forming alkylidene and alkylidyne groups[8], while work with labeled molecules proved that allylic adsorbates occur.[9]. In view of the impressive mechanistic knowledge accumulated during a century of hydrogenation catalysis, it is surprising that the hydrogenation of a nitrile, such as acetonitrile:
*Present address: Prototech Co, 32 Fremont Street, Needham, MA 02194, USA
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CH3CN + 2H2 = > CH3CH2NH 2
(1)
presents a number of perplexing features. No catalyst is known which forms exclusively primary amines, but secondary and tertiary amines are major co-products, often they are the dominant products. Minor side products are Schiff bases, enamines and alkanes. If the analogy of adding H atoms to C - N or C - C groups were realistic one would expect formation of an aldimine such as CH3-CH=NH from CH3CN, but in reality no aldimine has been observed in nitrile hydrogenation. In the research described in this paper, the following problems will be addressed: ,
2.
Selectivity: Are sec. and tert. amines prinmry or secondary products? Alloying: Does alloying of an active transition metal with a less active metal decrease or increase the specific activity? Can selectivity changes be predicted on the basis of an ensemble effect?
.
4.
Mechanism: Are chemisorbed H atoms added in steps to the C---N group? Overlayer effects Are H atoms from the metal surface directly transferred to an adsorbed nitrile molecule, or does indirect H transfer take place via an overlayer?
Zeolite NaY supported transition metal catalysts were prepared by ion exchange; for amorphous supports impregnation was used. Catalysts were tested either in a microflow reactor at atmospheric pressure with a H 2 or D 2 flow first passing through a saturator, or in a stirred autoclave with an initial H2 pressure 0f24 bar. The hydrogenation of acetonitrile, AN, was studied in the gas phase, butyronitrile, BN, was hydrogenated both in the gas and the liquid phases. Products were analyzed by GC. For experiments with D2, GC-MS was used; moreover certain product fractions were condensed and subjected to a secondary exchange with liquid D20. As only the amine hydrons are exchanged in this process, the mass difference before and after the second exchange directly shows the number of D atoms bonded to N atoms. Deutero-AN was used in cohydrogenation with BN + H 2 in order to identify the source of hydrons in the resulting amines. Experimental details are given in references [ 10-13] 2. SELECTIVITY If aldimines and primary amines were formed in successive steps and both molecules were desorbed from the catalyst surface into a surrounding liquid, one might expect that they react with each other, forming a larger molecule that can subsequently be hydrodeamminated to a sec. anfme. Its reaction with another aldimine could lead to a tert. arrflne. Such reaction steps have been proposed for hydrogenation of nitriles in a liquid phase.[ 14] Our work shows, however, that also in the absence of a liquid phase higher amines are formed with high selectivity; even at very low conversion sec. and tert. amines are formed during a single residence of the reactant at the catalysis surface. The selectivity for a particular amine depends mainly on the nature of the metal. Comparison of different metals at the same temperature in liquid and gas phase operation leads to very similar selectivity patterns; no liquid is required for the formation of higher amines. Over Ru the prim. anfme prevails, over Pd, the sec. and over Pt the tert. anfme. The selectivities can be correlated with another catalytic parameter characterizing transition metals, viz. the multiplicity in the H/D exchange of alkanes. Among the Pt group elements the selectivity towardprim, amines is highest over Ru, which has the highest propensity for catalyzing C-C fissions.
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Adding pentylamine to liquid B N in the presence of H 2 at high pressure and a Pd/NaY catalyst strongly retards the reaction rate of BN hydrogenation, indicating that the amine is more strongly ehemisorbe~ than the nitrile. The amine participates in the reaction, dibutylamine and butyl-pentyl amine are fonmd in equal quantities. The selectivity for the sec. amines is roughly 75% of that in the absence of added pentylamine.[ 12] 3. ALLOYING Previously we showed that bimetallic clusters are formed when two transition metal precursors are co-reduced with hydrogen inside the same zeolite. The temperature at which the less noble element can be reduced is lowered by the presence of the more easily reducible rnetal.[15] Alloying Ni with Ru, Rh, Pd or Pt increases the activity for BN hydrogenation beyond the amount attributable to the enhanced Ni reduction. Mixed surface ensembles are thought to be responsible. Addition of Sn to Pt lowers the activity for AN hydrogenation but improves the selectivity for the sec. anfme at high Sn contents. Addition ofRu, Rh, Pd or Pt to Cu enhances the reduction of Cu 2+ ions and results in active catalysts with high selectivity towards the sec. anfme. For instance, at 125~ the selectivity of PdCu/NaY for this amine is 84%, while it is 64% over Pd/NaY and Cu/NaY is inactive. 4. MECHANISM
Kinetic analysis of BN hydrogenation in the gas phase reveals a positive order in H 2 of 1.2 and in BN of 1.35 and an apparent activation energy of 68.3 kJ/mole.. Upon directing a flow of AN + D 2 over NaY supported Pt, Pd or Ru at 75~ part of the AN exchanges H atoms against D and leaves the surface as partially deuterated AN, while another fraction is converted to (partially deuterated) amines. Conclusions of relevance to the mechanism are drawn by analyzing both the H/D exchange and the "hydrogenation" more carefully. The H/D exchange is of the stepwise type over Pt and Pd, the molecule with one D atom, d~, prevails after short and moderate exchange times. In contrast, multiple exchange is significant over Ru. When a mixture of ethylamine + D2 is led over R u ~ a Y the d2 product prevails. Remarkably, this H/D exchange is highly regiospecific: the H atoms in the methylene group are easily exchanged, exchange of methyl hydrons is slower but, amazingly, almost no H/D exchange occurs with the hydrons in the amino group. NMR and MS analysis after secondary exchange with D20 unambiguously show that after the primary exchange the amino group is mainly -NH 2. This apparent lack of exchangeability of the hydrons in the amino group is also confirnaed with (CH3CH2)2NH: over Ru/NaY: all 10 hydrons bonded to C atoms are readily exchanged, but the H atom bonded to the N atom is not. The primary exchange product contains all isotopomers from d~ to d~o, but virtually no dn.. Accordingly, secondary exchange with D20 causes a shift of the MS peaks by one m/e unit. These results could still be interpreted in two ways: either the N-H bonds are never broken when amines are chemisorbed at the metal surface, or they are broken and strong N-M bonds are formed with the M atoms of the surface, but desorption requires H transfer from another amine molecule. Which of these models is correct, can be decided on the basis of the data obtained by
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"hydrogenating" AN with D2. Again, dramatic deviations from the simple D addition model are observed. Initially, a large fraction of the amines contain no D atoms at all This is largely due to the interference of the overlayer, see the next Section. But even after extended reaction time, when the overlayer is in isotopic equilibrium with the adsorbates, the isotopic composition of the reaction product remains markedly different from that predicted by a simple D addition model Detailed analysis of the isotopomers shows that almost no D atoms are present in the amine group of the reaction product. As with H/D exchange of diethylamine, there is a precipitous decrease in abundance from d~0 to d~ with all catalysts.[ 11] Any interference by OH groups of the support has been excluded as a possible source of the H atoms which become attached to the N atom; only methyl groups of the AN reactant appear to act as H donors.
To check this conclusion, BN was hydrogenated with H2 over Ru/SiO2 and Pt/SiO2 in the presence of CD3CN at an AN/BN ratio ~1. It was found that all prim. and sec. amines had predominantly D atoms in their amine groups.[ 13] The results thus indicate that a precursor of the amine is bonded to the metal surface via the N atom; these M-N bonds are predominantly broken in a concerted mechanism when AN molecules transfer some of their H atoms to this precursor. The adsorbed AN will replace the lost H atoms by D atoms from the catalyst surface and leave it as a partially deuterated AN molecule. 5. OVERLAYER EFFECTS When the nitrile containing gas flow is directed over the catalyst, neither nitrile nor amines are observed in the first few minutes, indicating that an overlayer is built up. Over Pt/NaY, for instance, the first amine is detected only after 5 min TOS. Overlayer formation is confirmed by thermal gravknetry. Previously, Thomson and Webb had presented arguments that hydrogenation of ethylene occurs by H transfer between an C2Hx overlayer and the adsorbed olefin, rather than by direct addition of chemisorbed H atoms.[ 16] Indeed, the initial ethylamine product of AN + D 2 runs contains much do, the predominant isotopomer is d2. After 4 h on stream, a steady product composition is observed with 30% d 3 as the dominant compound. These findings confirm the presence of a large concentration of strongly adsorbed species and their participation in the catalysis by H donation to or exchange with molecules which are desorbed and detectable in the gas phase. For further details we refer to reference [ 17].
6. DISCUSSION AND CONCLUSION The resuks with labeled molecules clearly prove that an intermolecular H transfer takes place to the N atom of the chemisorption complex, both in the H/D exchange of amines and the "deuteration" of acetonitrile to the prim. and sec. amines over Group VIII catalysts. Apparently, this "indirect" process is more efficient than direct addition of adsorbed D atoms to the N atom.. Although the release of a strongly held intermediate from the surface will be rate limiting, this desorption is assisted by the interaction with another impinging or weakly adsorbed nitrile molecule in accordance with the positive reaction order in nitrile. This kinetics is typical for the "adsorption assisted desorption phenomenon" as studied by Yamada and Tamaru, for CO on single crystal faces of several Pt group metals. In generalizing their findings, these authors
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conclude: "'Reactant molecules can enhance the desorption of tightly bonded product molecules in certain kinds of catalytic processes".[ 18] This expectation appears to be confirmed by the present results on nitrile hydrogenation and H/D exchange of amines. In both cases a reactant molecule donates H atoms to a chemisorbed entity, assisting its desorption. A simplified scheme for this unconventional chemistry is shown below. Ruthenium is used as an example, its propensity to form strong M-N bonds is at the base of its ability to catalyze ammonia synthesis from N2 + H2. Scheme 1:
/CH3 CN CD2 I I CH 3 + N ,,~ II Ru
/CH3 CN CD2 I I I-I2C tIN \
/ Ru
~
CN I HC + II Ru
/ CH3 CD2
I HNH
During nitrile hydrogenation the metal surface will become densely populated with N bonded adsorbates Under the conditions of AN + D 2 the CH3-CD2-N=Ru surface complex will interact with a CH3CN molecule; H transfer leads to CH3-CD2-NH2 and Ru=CH-CN. The =CH-CN group will pick up two adsorbed D atoms and leave the surface as CHD2CN, so that the overall reaction, in this example, becomes: 2CH3CN + 2D 2 = CHD2CN , + CH3CD2NH 2
(2)
The CH3-CH2-N=Ru surface complex can be considered as the N analogue of a propylidene complex. Alkylidene complexes have been identified on metal surfaces (8). It is easy to see, that sec. or tert. amines can be formed on transition metals by addition of alkyl groups to the N bonded intermediates. This mechanism for the formation of prim., sec. or tert. amines appears to be in accordance with the experimental data known at present, whereas an earlier proposal, postulating condensation reactions of a hypothetical aldimine intermediate with an amine in solution or on acid sites, fails to explain the present findings. 7. A C K N O W L E D G M E N T We thank the management of Air Products and Chemicals for sponsoring this research and their kind permission to publish the results.
REFERENCES 1 P. Sabatier, and J.B. Senderens, Comptes Rend. 135 (1902) 87. 2 M. Polanyi and J. Horiuti, Trans. Far. Soc. 30 (1934) 1164. 3 G.C. Bond, P.B. Wells, Adv. Catalysis 15 (1964) 92.
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4 V. Ponec, G. C. Bond, "Catalysis by Metals and Alloys" Vo195 of"Studies in Surface Science and Catalysis" Elsevier, Amsterdam, 1995. 5. P. Mars, J. J. F. Scholten and P. Zwietering in The Mechanism of Heterogeneous Catalysis J. H. de Boer et al. (eds.) Elsevier, Amsterdam, 1959. 6. M. Araki and V. Ponec, J. Catal. 44 (1976) 439. 7. P. Bilocn and W. M. H. Sachtler; Adv. Catal. 30 (1981) 165. 8. G. A. Somorjai, "Chemistry in Two Dimensions: Surfaces". Cornell University Press Ithaca, London, 1981, p. 278. 9. S. Naito and M. Tanimoto, J.Chem Soc. Chem Commun. 1987, 363. 10. Y. Y. Huang and W. M. H. Sachtler, J. Phys. Chem. B. 102 (1998) 6558. 11. Y. Y. Huang and W. M. H. Sachtler, J. Catal. 184 247 (1999) 12. Y. Y. Huang and W. M. H. Sachtlcr, Appl Catal A 182 365 (1999). 13. Y. Y. Huang and W. M. H. Sachtlcr, J. Catal. (subm.) 14. J. Volf, and J. Pa~ek in "Catalytic Hydrogenation"; L. (~erven~, (ed.), Elsevier, Amsterdam, 1986, p 105. 15. J. Feeley, and W.M.H. Sachtler, J. Catal. 131 (1991) 573. 16. S. J. Thomson and G. Webb, J.C.S.,Chem.Comm., (1976) 526. 17. Y.Y. Huang, and W. M. H. Sachtler, Appl. Catal. (in press). 18. T. Yamada, K. Tamaru; Surf. Sci. 146 (1984) 341.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
ALKALI P R O M O T E D REGIO-SELECTIVE HYDROGENATION OF STYRENE OXIDE TO [3-PHENETHYL A L C O H O L C.V. Rode*, M. M. Telkar and R.V.Chaudhari Homogeneous Catalysis Division, National Chemical Laboratory, Pune 411 008, India. Fax: +91 20 5893260, e-mail:
[email protected] The selective hydrogenation of styrene oxide to 2-phenyl ethanol (~-Phenethyl alcohol) has been investigated using different catalysts and supports. The effect of reaction conditions such as H 2 pressure, agitation speed, concentration of substrate and temperature on the initial rate of reaction was investigated. The complete conversion of styrene oxide was obtained using 1% Pd/C, as a catalyst, under milder temperature (313K) and pressure (2.048 MPa) conditions. 2- phenyl ethanol was selectively formed when alkali was used as a promoter. A plausible mechanistic pathway has also been proposed for the hydrogenation of the styrene oxide to 2- phenyl ethanol. 1. INTRODUCTION 2-Phenyl ethanol (13-Phenethyl alcohol, PEA) is extensively used in perfumery and deodorant formulations as it possesses faint but lasting odour of rose petals[ 1]. The conventional synthetic methods for PEA involve Grignard synthesis starting from ethylene oxide and Friedel craft alkylation of benzene in presence of A1C1312]. Both these processes are multistep and suffer from the following drawbacks: Formation of side products (biphenyl) leading to poor selectivity of PEA Handling of hazardous chemicals like diethyl ether, ethylene oxide Tedious work up and recovery of pure PEA which is critical for perfumery applications. Generation of appreciable quantities of wastes due to use of A1C13. PEA can also be prepared by reduction of styrene oxide using different reducing agents in stoichiometric quantities and major side product formed in such reduction processes is secondary alcohol (phenyl carbinol)[3-6]. Recently, single step catalytic hydrogenation of styrene oxide has been reported using Raney Ni and other metal catalysts in a temperature range of 120150 oC with selectivity of PEA in the range of 60-87%. Thus, all of these routes give one or the other side products along with PEA, posing serious problems in the recovery of pure PEA which is crucial for the perfumery applications. In this paper we report a single step catalytic hydrogenation of styrene oxide using alkali promoted supported metal catalyst which gives complete conversion of styrene oxide with PEA selectivity as high as 99.9 %. The aim of our work was to screen various transition metal catalysts on different supports and study the effect of temperature, hydrogen pressure and concentration of substrate on conversion of styrene oxide and selectivity of PEA. 2. E X P E R I M E N T A L 2.1 Materials All the chemicals were procured from Aldrich, Co Ltd, USA, and the various catalysts were prepared by the procedure given elsewhere[7]. Hydrogen gas of > 99.9% purity was supplied by Indian Oxygen Ltd., Bombay.
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2.2 Experimental set-up and procedure All the hydrogenation experiments were carried out in a 300 ml capacity SS-316 autoclave (Parr, USA) the details of which are described else where [8]. In a typical experiment, known quantities of styrene oxide, solvent, catalyst along with the promoter were charged into the autoclave and the contents were flushed twice with nitrogen and then the system was pressurised with H2 to the required pressure. The reaction was then continued at a constant pressure by supply of hydrogen from the reservoir vessel. The consumption of H2 was recorded as a function of time. The liquid samples were analysed by GC for reactant and products. 3. RESULTS AND DISCUSSION
Some initial experiments on hydrogenation of styrene oxide using 1% Pd/C catalyst showed that the selectivity of PEA was only 51% due to the formation of side products such as 1-methoxy ethyl benzene, dimethoxy ethane and 2-methoxy benzene ethanol. These side products were identified by GCMS and GCIR. In order to enhance the selectivity of PEA, a systematic study on catalyst screening, role of support, and promoter was undertaken. Table 1: Screening of catalysts Catalyst Conversion Used (%) I%Pt/C 70 1%Pd/C 100 10%Ni/C 60 10%Ni/HY 10 2%Ru/C 82
Selectivity (%) 88 99 85 -87
3.1 Screening of Catalyst
Several transition metal catalysts such as Pd, Pt, Ni, Ru were tested for their activity and selectivity for hydrogenation of styrene oxide at 313 K and 2.048 MPa pressure in presence of NaOH as a promoter and the results are presented in Table 1. It can be seen from this Table that I%Pd/C catalyst selectively gives Temp :313 K, Pressure : 2.048 MPa, Solvent: PEA as the product, with 100% conversion of MeOH, Conc of Catalyst:0.375 Kg/m~, Conc of styrene oxide. In case of other catalysts, the Styrene oxide: 0.4166 Kmol/m3, Conc of NaOH : conversion of styrene oxide was much less than 0.013 kg/m~ that for 1%Pd/C with low selectivity to 2- phenyl ethanol. The other side product formed was found to be 1- methoxy ethyl benzene which, was identified by GCMS. Pt/C and Ni/C catalysts showed almost comparable activity (70% and 60% conversion respectively), while Ni/HY catalyst showed lowest activity (10% conversion). In case of Ni/HY catalyst, from the consideration of pore size of support and the particle size of supported metal, almost all metal is expected to be on the outer surface of HY zeolite, leading to a very small surface area for the supported metal causing the lowest catalyst activity [9]. It is interesting to note that the formation of neither deoxygenated (e.g. ethyl benzene) nor any isomerisation products (e.g. 1phenethyl alcohol) was observed in the present work. The formation of such products has been reported in earlier work for other epoxy compounds [10] in which mostly the gas phase hydrogenation experiments were conducted in the temperature range of 120-180~ The absence of any deoxygenated product, in this work suggests that the metal-styrene oxide interaction is weaker particularly, for the Pd catalyst. Isomerization products were also not observed because of addition of alkali, which neutralises the acid sites responsible for the isomerisation which is said to be a parallel reaction with hydrogenation of epoxy compounds [ 11 ]. 3.2 Effect of supports In this work the role of support such as carbon, alumina, silica and zeolite-ZSM-5, was investigated for 1% Pd catalyst and the results are shown in Figure 1. For all the supports
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studied, 100% selectivity to 2- phenyl Cony ethanol was obtained in presence of NaOH looSel. while the catalytic activity varied in the order C >A1203 >SiO2 >ZSM-5. It is known that in 0~80a basic medium, (pH range of this work was 11-12) the activated carbon support is stable ~> but alumina and silica are likely to dissolve ~ 60undergoing structural changes[ 12]. This may be the reason for decrease in activity of the fi 40 catalysts when supported on alumina and silica [13]. When zeolite was used as a support, the channel dimension of ZSM-5 20 (5.4 x 5.6 and 5.8 x 5.2 A ~ does not allow the penetration of styrene oxide due to its 0 Carbon Alumina silica ZSM-5 larger diameter (7.32A ~ to get adsorbed on Fig 1 Effect of supports on conversion and selectivity the entire surface (external + pore surface) of Temp: 313 K, Pressure: 2.048 MPa, Solvent: MeOH, the catalyst[13]. This is a probable explanation Conc. of Styrene oxide: 0.4166 kmol/m3, Conc. of catalyst: 0.375 Kg/m3,Conc.ofNaOH:0.013Kg/m3 and more work is required for a clear understanding of the observed trends. Since 1%Pd/C was the most active catalyst, detailed investigation on the effect of solvent, promoter, its concentration, temperature, hydrogen pressure, etc. was carried out using this catalyst, and the results are discussed in the following sections. .,I
O
3.3 Effect of solvents Solvents such as methanol, hexane and 1-4 dioxane were screened for hydrogenation of styrene oxide. In a protic solvent, methanol the conversion obtained was 100% while, in aprotic solvents such as hexane and 1-4 dioxane the conversion obtained was 75% and 33% respectively. This can be explained in two ways, i) Solubility of hydrogen is higher in methanol hence, highest conversion was obtained in methanol ii) and the protonated diol gets attacked by hydride to give 2-phenyl ethanol. As the protonation increases the hydride attack is easier therefore, leading to highest conversion of styrene oxide in methanol. 3.4 Effect of Promoters Nucleophilic promoters are believed to play a key role in the hydrogenation of epoxides. The role of various organic and inorganic promoters was investigated and the results are given in Table 2. It was found that in absence of a promoter and methanol as a solvent though, the conversion of styrene oxide obtained was 99%, the selectivity of PEA was only 51%. Besides PEA, other side products obtained were 1-methoxy ethyl benzene and 1,2- dimethoxy ethyl benzene. For all the promoters studied in this work the selectivity to 2-phenyl ethanol achieved was above 95% and in some cases even >99% however, the level of conversion of styrene oxide varied, giving complete conversion with only sodium hydroxide as a promoter.
3.5Effect of H2 Pressure Figure 2 shows the effect of pressure of hydrogen on the initial rate of reaction, for different temperatures, I%Pd/C catalyst and NaOH as the promoter. It was observed that initially as the pressure increases the rate of reaction also increases to a maximum (3.44 MPa) and then
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drops down with further increase in H 2 pressure indicating the possibility of hydrogen inhibited kinetics at higher pressure. At low pressure (< 3.44 MPa) both styrene oxide and H2 would be chemisorbed on the catalyst surface with some free active sites also available. As the H2presure increases the rate would increase until all surface sites are occupied by hydrogen. Further increase in the H 2 pressure, would cause the adsorbed styrene oxide to be swept away which 4 would result in decrease in hydrogenation rate. "7 9 40~ O t~
Table 2: Screening of promoters Promoter used
Conv.
Selec.
(%)
(%)
NaOH
99.9 Na2CO 3 47.0 Quinoline 36.8 Pyridine 64.3 Triethylamine 70.2 Diethylamine 47.5 Dimethylamine 55.0 Without 99.0 promoter . . . . . . . . . . . . . . .
50~
x
9
-'63
......
99.9 97.6 96.9 94.3 99.6 99.9 99.8 51.2
Temp. 313 K, Pressure : 2.048MPa, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst: 0.375 Kg/m 3, Conc. of promoter :0.013 Kg/m 3.
X
0
i lib
0
,
i
1
9
i
9
i
9
i
2 3 4 Pressure, MPa
9
i
,
5
6
Fig 2 Effect of pressure on initial rate of reaction. Temp. 313 K, Solvent : MeOH, Conc. of styrene oxide : 0.4166 Kmol/m 3 Conc. of catalyst : 0.375 Kg/m 3, Conc. of NaOH :0.013 Kg/m 3.
3.6 Effect of substrate concentration The effect of concentration of substrate on the initial rate of reaction was studied and the results are shown in Figure 3. Initially, the rate of reaction increases as the concentration of styrene oxide increases upto 1.2 x 1 0 - 4 K m o l / m 3 beyond which the rate decreases, with further increase in substrate concentration. This effect is more pronounced at higher temperature (333K). Similar observation made for H2 effect on rate of hydrogenation indicate that the adsorption of both styrene oxide as well as Hz is important and need to be considered. 3.7 Effect of Temperature The effect of temperature on both selectivity of PEA and the rate of hydrogenation was studied in a temperature range of 313-333 K. The selectivity of PEA was found to be unaffected at all the temperatures
.7~ I .~E3
o
0
~~~,,~
B 40~ X 50~ 60~
1 2 Concentration of substrate, kmoCm3
3
Fig 3 Effect of concentration of substrate on initial rate of reaction. Temp.: 313 K, Pressure: 2.0148 MPa, Solvent: MeOH, Conc. of catalyst: 0.375 kg/m 3, con. of NaOH: 0.013 kg/m 3
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while, the initial rate of hydrogenation increased with increase in the temperature and the activation energy evaluated from the Arrhenius plot was found to be 55.4 KJ/mol. 4. P R O P O S E D M E C H A N I S M
The reactions of epoxy compounds with H 2 in the presence of supported metal catalysts are known to give deoxygenated, isomerised and hydrogenated products. In our work, formation of ethyl benzene or styrene (deoxygenated products) was not observed hence, the strong adsorption of oxygen to the catalyst surface is not expected. Notheisz, et al. also reported that the metal-epoxy oxygen interaction was found to be weaker in case of Pd catalysts [14]. Moreover, the presence of NaOH on the catalyst surface decreases the adsorptivity of the epoxy oxygen resulting in higher selectivity of the desired alcohol (PEA). The absence of isomersied products is due to the neutralisation of acid centres (if any) by added alkali. Many authors have described mechanism of ring opening of oxiranes. Among them Bartok described the opening ofoxacycloalkanes in acidic medium to give different products with secondary alcohol as a major product [ 10]. Mitsui, et al. have explained deoxygenation of styrene oxide involving the radical cleavage reaction [15]. They have suggested two different mechanisms for explaining the formation of PEA, one on the basis of radical cleavage and the other involving SN 2 mechanism. In their work none of the intermediates could be separated or characterised and also the role of NaOH as a promoter in the reaction mechanism was not clearly understood. In our work the formation of only 2- phenyl ethanol indicates the regio selective opening of the C-O bond which is less hindered i.e. distant from the subsituents which is normally observed in the case of Pd and Pt metal catalysts [ 16]. The addition of NaOH is also responsible for the formation of PEA, because it neutralises the acidic sites reponsible for isomersization products (ketone in this case) which after hydrogenation give secondary alcohol. The regioselective formation of 2- phenyl ethanol can be explained based on two different reaction pathways as shown in schemes I and II. Scheme I, involves formation of n benzyl complex from the adsorbed styrene oxide. The rc benzyl complex (2) yields an alkoxide ion (3), which is stabilised by NaOH. The alkoxide ion on protonation with a solvent like methanol gives selectively 2-phenyl ethanol. According to this mechanism, the cleavage of C-O bond is postulated to be from the more substituted side, which is normally not the case for Pd catalyst. However this has been proposed by Mitsui, et al [ 10]. Scheme I
(1)
(2)
(3)
In scheme II S N 2 attack of OH- is proposed, leading to the cleavage of C-O bond from the less hindered side. The secondary alkoxide ion (4) formed in this ease then yields an intermediate 2-phenyl ethane diol (5) which on hydride attack gives selectively 2- phenyl Scheme II ,.
~
,
H
(4) (5) ethanol. Both the mechanistic pathways may contribute simultaneously to regioselective formation of PEA. However, the probability of Scheme II operating seems to be more because
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i) the cleavage of C-O bond is from the less hindered side. ii) C2 carbon of 2- phenyl ethane diol is more electropositive than C, carbon atom due to the electronegativity of phenyl ring hence, the hydride attack on C2 atom is favored to give selectively PEA. iii) In a separate experiment, styrene oxide was refluxed in aqueous sodium hydroxide for 3 hours to give 2- phenyl ethane diol (5) which was separated and well characterized. This diol was isolated and then further hydrogenated using 1%Pd/C catalyst, in methanol as a solvent to give 2-phenyl ethanol. 5. CONCLUSIONS The hydrogenation of styrene oxide in presence of Pd/C catalyst and NaOH as a promoter under very mild conditions was found to be regioselective to give only 2- phenyl ethanol as the product. A systematic study on screening of catalysts, promoters, solvents and the effect of major reaction parameters such as H z pressure, substrate concentration and temperature on the catalyst activity and selectivity was carried out. Speculative reaction pathways have been proposed for the regioselective formation of 2- phenyl ethanol. REFERENCES
1. B. D. Mookherjee and R. A Wilson, Kirk othmer (eds.) Encyclopedia of chemical technology, 4 ed, John Wiley, New York, Vol. 4, 1996. 2. E.T. Theimer, Fragrance chemistry, Academic Press, New York, 1982. 3. E. L. Eliel and D.W. Delmonte, J. Am. Chem. Soc.,78 (1956) 3226. 4. M. L Mihailovic, V. Andrejevic and J. Milovanoic, Helv. chim. Acta., 69 (1976) 2305. 5. A. Okawa and H. K. Soai, Bull. Chem. Soc. Jpn., 60 (1987) 1813. 6. S. Krishnamurthy, R. M. Schubert and H. C. Brown, J. Am. Chem. Soc., 95 (1973) 8486. 7. R. Mozingo and E. C. Homing, (eds) Organic Synthesis collective volume, 3, John Willey, London, 1956. 8. C. V. Rode, S. P. Gupte, R. V. Chaudhari, C. D. Pirozhkov and A.L Lapidus, J. Mol. Cat., 91 (1994) 195. 9. M. V. Rajshekharam, C.V. Rode, M. Arai, S.G. Hegde and R.V. Chaudhari, Appl. Cat. A: General 195 (2000) 1. 10. M. Bartok, F. Notheisz, A. G. Zsigmond and G.V. Smith, J. Cat. 100 (1986) 39. 11. H. Davidova and M. Kraus, J. Cat. 61 (1980) 1. 12. M. Bartok, Catalyst supports and supported catalyst: Theoretical aspects and applied concepts, (eds). Alvin Stiles Butterworth, New York, (1987). 13. R. Augustine, Heterogenous catalyst for synthetic chemist, Marcel Dekker, New York, 1996. 14. R.E. Malz and H. Heinemann (eds), Marcel Dekkar, New York, 1996. 15. S. Mitsui, S. M Imaizumi and Y. Sugi, Tetrahedron, 29 (1973) 4093. 16. F. Notheisz, A. Molnar, A.G. Zsigrnond and M. Bartok, J. Catal, 131 (1986) 98.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Production of Fatty Alcohols by Heterogeneous Catalysis at Supercriticai Single-Phase Conditions Sander van den HarkS, Magnus H~trrOd~ and Poul M~ller2 ~Chalmers University of Technology, Department of Food Science c/o SIK, Box 5401, SE-402 29 G6teborg, Sweden Fax: +46-31-83 37 82 Email:
[email protected] /Augustenborggade 21B l a, DK-8000 Aarhus C, Denmark Fatty alcohols can be produced by catalytic hydrogenation of fatty acid methyl esters. This heterogeneous catalytic reaction, traditionally performed in a multi-phase system, is limited by the mass transport of hydrogen to the catalyst. To overcome this limitation vve have used the unique properties of supercritical fluids, properties which are in between those of liquids and gases, making them a very suitable medium for reactions. By adding propane to the reaction mixture of hydrogen and fatty acid methyl esters we have created supercritical single-phase conditions. These single-phase conditions eliminate the transport resistance for hydrogen and create the possibility to control the concentration of all the reactants at the catalyst surface independently of the other process settings. In this way, extremely rapid hydrogenation can be combined with a high product selectivity. In our lab-scale experiments the catalyst activity was studied as a function of hydrogen pressure, substrate concentration and temperature. The catalyst activity was extremely high compared to the multi-phase hydrogenation. Complete conversion of the liquid substrate was reached in a few seconds. The high catalyst activity results in reaction rates which are comparable with similar gas-phase hydrogenation reactions of much smaller molecules (e.g. methylacetate). As long as single-phase conditions remain-in our experiments we have tested up to 15 wt.% substrate- the gas-phase-like activity can be maintained. Our results prove that performing hydrogenation at supercritical single-phase conditions is beneficial for this and other heterogeneous catalytic processes which are limited by mass transfer. 1. INTRODUCTION Fatty alcohols (FOH) and their derivatives are one of the major oleochemicals and widely used as surfactants. They can be produced from natural fats and oils by catalytic hydrogenation of fatty acids or fatty acid methyl esters (FAME). When dealing with such low volatile substrates, one is confronted with a multi-phase system consisting of a liquid (substrate and product), a gas (hydrogen) and a solid (catalyst). The gas liquid binary "subsystem" is a result of the low solubility of hydrogen in these liquids, especially in relation to the stoichiometric hydrogen requirement (often above 50 mol%). The hydrogenation of FAME requires severe process conditions, typically: hydrogen pressures between 200 and 300 bar, temperatures ranging from 200 to 300 ~ (1, 2). Even at these conditions the solubility of hydrogen in the liquid phase (i.e. in the FAME) is low, as can be
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化
Hydrogen
Supercritical
Catalyst
FAME
Catalyst
|
Substrate
o -~,
r.~
Hydrogen
o t~
o
o
t~ o
ll'
E ~. Hydrogen
:
O
ixM.
Substrate m
A)
Distance
B)
Distance
Fig. 1. General concentration profile over the reactor for the substrate and hydrogen in: (A) a multi-phase system (C *= solubility of H2 in FAME. See also on the base-line, representing this binary gas-liquid system, in Fig. 2). (B) Homogeneous supercritical phase.
Propane
FAME
C
,
Hydrogen
Fig. 2. Phase diagram for the system FAME, propane and hydrogen, ~ the estimated singlephase region at 100 bar and 200 ~ [4]. (---) indicates the stoichiometric amount of hydrogen needed. (--) the composition of the reaction mixtures used in the experiments. seen in the concentration profile over the reactor (Fig. 1A). As a consequence there is a lack of hydrogen at the catalyst surface. Hence, the reaction rate is low and limited by the mass transfer of hydrogen. Also, as can be seen in Fig. 1A, there is an unfavorable ratio of FAME and hydrogen in the liquid phase surrounding the catalyst. To overcome these restrictions we have added propane to the reaction mixture. Supercriticalpropane (Pc=42.5 bar and Tc=96.6~ is completely miscible with hydrogen and FAME. By choosing the fight conditions, at the given reaction temperatures, a substantially homogeneous supercritical phase can be formed, with almost unlimited a c c e s s to the catalyst surface for all the reactants (3, 4). The single-phase area, based on literature data (5, 6, 7) and our own experiments, was estimated as shown in the ternary phase diagram (Fig.2). The concentration
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profiles over the reactor can now be illustrated by Fig 1B; note that the ratio of FAME to hydrogen is inverted compared to the traditional process. Very high reactions rates are possible under these conditions, e.g. the partial hydrogenation of methylated rapeseed oil, a 400 fold increase in LHSV (substrate volume processed per hour by a unit volume reactor, m3substratc/m3reaetorh.) was obtained (8). A high selectivity is possible because the concentrations and other process conditions can be set independent of each other. Selectivity is of major interest for the product quality. In our case, besides the main reaction (i.e. the formation of FOH), "overhydrogenation" can lead to the undesired formation of alkanes. Other side products are e.g. aldehydes and wax esters (wax) which are reaction intermediates. This study demonstrates that the reaction rate is enhanced in a single-phase system. Furthermore, experiments were performed to investigate the influence of the hydrogen concentration, temperature and substrate concentration on the catalyst activity and selectivity at supercritical single-phase conditions (for details see 9, 10). 2. MATERIALS AND METHODS
A CuCr catalyst (Cu-1985 P, Engelhard, the Netherlands) with a particle size of 32-71 gm was used. Further, propane (Instrument Quality, AGA, Sweden), hydrogen (Hydrogen Plus 99.995%, AGA, Sweden) and methylated sunflower oil (C18:0-2) were used in the reaction mixtures. An experimental space was created by varying temperature, residence time, and hydrogen level (as molar ratio of H2 to FAME), according to a central composite design, see Table 1 (11). All other factors were kept constant; the total pressure was 150 bar, the FAME concentration was 0.3 mol% (~2.3 wt%), and the total flow rate was 60 retool/rain (1.40 l/rain NTP). The experiments were performed on the same equipment and analyzed as reported in earlier work (8,10). Table 1. Experimental space for the investisation, variables and levels. Variables Experimental space Low High Temperature (~ 260 300 H2:FAME 1 4 (1.8) 64 (30) Residence time (s) , 0.1 0.9 1) The correspondinghydrogenpressure is given in brackets. In each experiment the reactor productivity was measured in terms of conversionrA~ and yieldFoH. Models (i.e. goal functions) for these two dependent variables were constructed to describe the correlations with the independent variables in the experimental space. ConversionFAME was defined as the decrease in FAME concentration in the reaction mixture, in mol%. (FAMEi, = start concentration, FAMEout= concentration after the reactor)
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conversion
-- FAME~, - FAME ,u, , 100% FAME
FAMEin
(1)
The products from this conversion can be aldehydes, FOH, alkanes or wax esters. The actual yield of FOH, including conversion and selectivity, was therefore defined as the ratio between the amount of produced stearyl alcohol (FOH) from FAME: FOH , 100% yield FOH = FAME--------~
(2)
The catalyst activity, in terms of reaction rate, is expressed as the consumption of FAME at a given time (i.e. at a given point in the catalyst bed). It can therefore be calculated as the derivative of the conversionF~ with respect to time. 3. RESULTS AND DISCUSSION
The used catalyst both hydrogenates the carbon-carbon double bonds and the carboxyl groups in the fatty acid chains. Hence only saturated FOH are produced (1). Hydrogenation of carboncarbon double bonds was very fast at the applied reaction conditions. In the following, "hydrogenation" refers only to the slower reaction involving the carboxyl group. Totally 3.4 mol H2/mol FAME are needed to complete both reaction steps. The catalyst activity was very high, high conversion levels of FAME were reached with residence times below 1 s. Temperatures up to 250-260 ~ accelerate the catalyst activity, while higher temperatures only lead to a slight additional increase in the activity (9). Thus, the reaction temperature is not changed by the supercritical conditions (10). By calculating the reaction rate at a constant FAME concentration (i.e. a fixed level of conversionFAME) the effect of the ratio H2:F ME on the catalyst activity can be studied (see Fig. 3). The linear dependency of the reaction rate on the H2:FAME ratio, indicates first order kinetics with respect to hydrogen. This is in agreement with the first order in hydrogen concentration found in gas-phase kinetics (12, 13). As stated earlier, the catalyst activity should be combined with a high selectivity (i.e. mainly the suppression of "overhydrogenation"). Both these objectives are included in yieldFoH (eq. 2). Fig. 4 shows a contour plot of this yield as function of the ratio of H2:FAME and residence time. A contour plot can be regarded as a two dimensional projection of a continuous response surface. In the experimental space the selectivity was around 95%. The yield decreases from 100% to 90% in the upper-fight comer of Fig. 4. This is a result of a lower conversion not due to overhydrogenation of the product. The conversion decreases as a consequence of a large pressure drop over the catalyst bed, leading to a multi-phase system when long residence times (i.e. long catalyst beds) were used (9). All our successful experiments were performed in the single-phase region with an excess of hydrogen (see Fig. 2). As a consequence the reactor productivity was large and ranges from LHSV of 10 to 100 ( i.e. 300 to 2600 ktmol/g cat min). This can be compared with a LHSV of
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900
A
C
I
I
I
I
I
I
=m
E 2000-
A
/
"~ 1500
v
E
im
I-.
1000 0 ,,=m
,,i,,a
o 0
IZ:
E O
E
E =
U~
= 300 tJ
t.19
500
"10
0 0
I
i
i
i
10
20
30
40
H2:FAME
(mollmol)
Fig. 3. Reaction rate (pmol/g=t, ly,tmin) versus H2:FAME ratio (280~ and FAME=0.09 mol%).
(R 19
IZ
N
100 4
8 16 32 H 2" FAM E (mollmoi)
Fig. 4. YieldroH at 280 ~ FAMEi, =0.3 mol%
64
(1.5*SEE=9.1).
0,2-0,4 for the traditional fatty alcohol process (1). The catalyst activity reached at supercritical conditions with these large molecules (MW=300 g/mol) is comparable to that found for gas-phase reactions of methyl- and ethyl acetate (MW=100) under similar temperatures (12, 14). In such gas-phase reactions there is no external diffusion limitation. Hence, at supercritical single-phase conditions we have achieved the state where the catalyst activity is controlled by the "surface" kinetic also for large molecules. Only the single-phase region to the fight of the stoichiometric hydrogen demand (see Fig.2) can give these high activities. Hydrogen is an antisolvent and reduces the maximum concentration of FAME in the singlephase at a given set of conditions. Furthermore, in a future process where propane is recycled, excess hydrogen should also be recycled. Hence, a low excess of hydrogen and high substrate concentrations are favorable and were therefore tested in additional experiments. Experiments no. 1 to 3 in Table 2 verify the trends from Fig. 4; a lower hydrogen excess reduces the catalyst activity due to a lower hydrogen concentration at the catalyst surface and should be compensated by longer residence times to maintain the conversion. With the fight balance between residence time and hydrogen a high selectivity can be achieved, resulting in high yields. Table 2. Additional hydro~;enation experiments with sunflower FAME over CuCr cata!yst 1) Run H2/FAME Time ConversionF~tE YieldFoH Side-products 2) FAMEm (%) (mol%) (wt.%) (moVmol) (s) (%) Wax 66 1 0.5 ~3 10 2 94 Alkane 67 2 0.5 ~3 10 8 100 90 3 0.5 ~3 4 8 100 48 Wax 4 2 13 4 8 72 91 5 2 15 12 8 100 1,Conditions: 150 bar, 280 ~ 2~Alkaneindicates "overhydrogenation",Wax is a reaction intermediate to FOH
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When we increased the FAME concentrations in our process, we could maintain the high catalyst activity, as long as single-phase conditions remained (see Table 2). Under the applied conditions (280 ~ and 150 bar) we have measured a homogeneous phase with up to 15 wt% FAME and 24 mol% hydrogen in the reaction mixture. Unforttmately, exact phase behavior data for the reaction mixture in the region of interest are missing. Current research is focussed on this topic.
The exothermic reaction in combination with the high reaction rates would cause a temperature rtmaway in a traditional reactor. In the supercritical process the propane present as a solvent also acts as a "cooling medium" and the temperature rise can be limited. 4. CONCLUSIONS For the hydrogenation of a liquid substrate to fatty alcohols in a homogenous supercritical phase, created through the addition of propane, extremely high catalyst activities were reached. This activity is strongly influenced by the ratio of hydrogen to substrate in the reaction mixture. The selectivity could be maintained at a high level. With more solubility data and more knowledge about the influence of the substrate concentration on the catalyst activity the process can be further optimized. A pilot plant will be constructed together with an industrial partner.
Acknowledgement This work was financially supported by Daka, a.m.b.a., Losning, Denmark REFERENCES 1. F. Ullmann, Ullmann's Enzyclopaedie der technischen Chemie, 4th edn., Verlag Chemie, Weinheim, 1976. 2. M. Hoffman, K. Ruthardt, Oils and Fats International, 9 (1993) 25. 3. M. Hiirr6d, P. Moiler, PCT patent application, WO 96/01304, (1996). 4. M. Hiirr6d, M. -B. Macher, S. van den Hark, P. Moiler, In: Proc. 6 th Meeting on Supercritical Fluids, ISASF, Nancy (1999) 253. 5. H. Schiemann, PhD Thesis, Universit~it Erlangen-Numberg, Erlangen, 1993. 6. H.G.A. Coorens, C.J. Peters, and J. De Swaan Arons, Fluid Phase Equilibria, 40 (1988) 135. 7. E.Weidner, D. Richter, In: Proc. 6th meeting on supercritical Fluids, ISASF, Nancy (1999) 657. 8. M. -B. Macher, M. H~irr6d, P. Moiler and J. H6gberg, Fett/Lipid,101 (1999) 301. 9. G.A. Camorali, MSc Thesis, Chalmers Univeristy of Tecnology, Sweden, 1999. 10. S. van den Hark, M. H~irr6d, M., P. Moller, J. Am. Oil Chem. Soc., 76 (1999) 1363. 11. G. Box, W.G. Hunter, J.S. Hunter, Statistics for Experimenters. An Introduction to Design, Data Analysis and Model Building, John Wiley & Sons, New York, NY, 1978. 12. J. Evans, M. S. Wainwright, N. W. Cant, D. L. Trimm, J Catal, 88 (1994) 203. 13. N. Chikamatsu, T. Tagawa, S. Goto, J. of Chemical Eng. of Japan, 21 (1991) 604. 14. P. Claus, M. Lucas, B. Lticke, Applied catalysis A, General, 79 (1991) 1.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Regioselective oxidation of primary hydroxyl groups of sugar and its derivatives using a new catalytic system mediated by TEMPO. H. Kochkar a, M. Morawietz b and W. F. Hrlderich ~* aDepartement of Chemical Technology and Heterogeneous Catalysis, University of Technology, Rgrl'H-Aachen, Worringerwegl, 52074 Aachen-GERMANY. Phone 949 241 80 65 60/61, Fax 949 241 88 88 291, e-mail
[email protected] bDegussa-Hfds AG, Hanau, GERMANY.
Primary hydroxyl groups were oxidized regioselectively using organic oxoammonium salts generated on supported silver catalysts, which promote disproportionation of 2,2,6,6tetramethylpiperidinyl-1-oxy (TEMPO) in aqueous solution. The oxidation reactions were performed at pH 9.5 in a batch reactor at RT using heterogeneous silver catalysts and peroxides as primary co-oxidants. 99 mol.% selectivity to methyla-D-glucopyrasiduronic acid was obtained at 90 % conversion of the pyranoside over a silver-celite catalyst. The activity was increased by adding carbonates to the silver catalysts. This resuk can be explained by the increase of the electron charge deficiency on silver in presence of carbonate, which accelerates the nucle0philic attack of TEMPO and/or hydroxyl groups. This result was proved using TPD of ammonia in the case of Ag-AI203 catalyst. The observed regioselectivity is due to the sterical hindrance caused by the four methyl groups in TEMPO. 1. INTRODUCTION Metal-catalyzed oxidation of alcohols to carboxylic compounds, in conjunction with cooxidants, is an important step for synthesis of fine chemicals 1. Particularly, the oxidation of sugar and its derivatives such as starch and cellulose is important. The oxidized carbohydrates can be used as thickening, gelling agents, in paints, resins, detergents co-builders and super absorbers have an important economic impact. There are many methods for the selective oxidation of secondary hydroxyls groups in the presence of primary ones 2, but few suitable reports describe procedures for the oxidation of primary hydroxyl groups that leave the secondary hydroxyl groups still intact. Semmelhack et al3, reported that electrooxidation as well as autoxidation of alcohols, mediated by 2,2,6,6tetramethylpiperidinyl- 1-oxy (TEMPO), shows this matter of regioselectivity. The corresponding nitrosonium ion of the nitroxyl radical, which is a powerful oxidizer of alcohols 4, can be obtained with a hypochlorite/bromide system5, sodium bromite and calcium hypochlorite6, copper(i)chloride.oxygen 7, p-toluenesulfonic acid s, electrochemically 9. Depending on the reaction conditions and the substrate used, aldehyde 1~ o r carbox-ylate 11 were obtained. The drawback of this method is a large accumulation of waste salts and contamination with chlorinated compounds. Furthermore, the new European laws will not accept these "conventional" environmental unfriendly process technologies anymore. Therefore, the above cited process should be replaced by heterogeneous catalysis.
* To whomthe correspondence shouldbe addressed.
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We report here about a new method for the regioselective oxidation of primary hydroxyl groups over a silver catalyst/peroxodisulfate system mediated by TEMPO. The oxidation was performed first with methyl-a-D-glucopyranoside since it has been used as a model molecule. 2. E X P E R I M E N T A L
2.1. Catalyst preparation Ag-Na-Y, Ag-AIPO4 and Ag-AI203 were prepared by incipient wetness impregnation with silver nitrate by stirring at 25 ~ during 15 h. After filtration, washing and drying at 373 K. Then, the catalyst was calcined under air at 773 K for 6 h. The materials were characterized by ICP-AES and nitrogen sorptlon at 77 K (see Table 1). Silver carbonate celite catalyst was prepared according to M. F&izon et a112, the catalyst contains 0.17 mmol of Ag per gram of celite and a BET surface are of 6 m2.g-1. Table 1" Characterizations of the catalysts. Catalyst/support
Ag ~ (Wt %)
Surface area (m2.g -1)
Pore diameter b A
A1203 A B C D
1.1 2.0 2.8 5.0
253 247 249 241
65 65 64 64
NaY E
5.8
573
< 12
AIPO4
4.5 41 F a 9determined by ICP-AES. b: determined by using BJH method.
20
/
/
.
,.-I
/
i
A
80
!
/ / /
/
'
/
m
.
/
! g 20
/
100
y-
o o
a)
i
6040-
b)
20-
/
I
200 Temp ~
300
400
0
r-
i
200
400
600
Temp ~
Figure 2 The dependence of conversion of temperatures for a) cyclohexene oxidation, no water, 9 9 Pt, 9 Rh and b) thiophene oxidation over Pd, 9 in the absence of water, o in the presence of water. Table 3 Comparison of precious metal catalysts for the oxidation of dry and wet (2% moisture)VOCs. VOC Catalyst Temperature ~ Dry Temperature ~ Wet 20% 100% 20% 100% conversion conversion conversion conversion Methyl Pt 183 226 131 187 methacrylate Rh 227 290 214 280 Pd 130 175 130 175 168 260 157 225 Cyclohexene Pt Rh 262 360 232 335 Pd 80 158 125 220 190 315 205 340 Diethylamine Pt Rh 264 354 204 312 Pd 140 266 125 280 Pt 309 360 296 360 Thiophene Rh 337 368 326 356 Pd 340 380 300 340
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Thus it is clear that the application of Adsocat to the control of odorous impurities in the gas phase is practicable. Activated carbon is the best adsorbent in moist streams, giving an adsorption capacity of ca. 1 to 3x10 -3 mol g-1. Desorption can be achieved by heating the carbon to above 270~ and catalytic combustion, preferably over a Pd based catalyst, occurs readily at temperatures above 360~ to completely destroy the pollutant. Continuous tests based on hydrogen sulphide and sewage gas have shown no loss of activity over 2 years operation [5]. Similar long term tests for odour control are in progress.
Acknowledgment Financial support by Mahanakorn University of Technology, Bangkok, Thailand is gratefully acknowledged. The authors also acknowledge Johnson Matthey Catalytic Systems Division for providing catalyst samples.
REFERENCES 1.
R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control, Van Nostrand Reinhold (1995). 2. J.J. Spivey, Ind. Eng. Chem. Res., 26 (1987) 216. 3. C. Lahousse, A. Bernier, P. Grange, B. Delmon, P. Papaefthimoiu, T. Ioannides, and X.E. Verykios, J. Catal., 178 (1998) 214. 4. V. Meeyoo, J.H. Lee, D.L. Trimm, and N.W. Cant, Catalysis Today, 44 (1998) 67. 5. V. Meeyoo, D.L. Trimm, and N.W. Cant, J. Chem. Tech. Biotechnol., 68 (1997) 411. 6. J.H. Lee and D.L. Trimm, Fuel Processing Technol., 42 (1995) 339. 7. Y-F Yu Yao, J. Catal., 87 (1984) 157. 8. N.W. Cant and W.K. Hall, J. Catal., 16 (1970) 220. 9. J.Rostrup Neilson, Steam Reforming Catalysts, Danish Technical Press, Denmark (1975). 10. R. Burch, F.J. Urbano and P.K. Loader, Appl. Catal. A., 123 (1995) 173. 11. J.H. Lee, D.L. Trimm, and N.W. Cant, Catalysis Today, 47 (1999) 353.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Structure sensitivity of the hydrocarbon combustion reaction over aluminasupported platinum catalysts T.F. Garetto and C.R. Apesteguia* INCAPE (UNL-CONICET), Santiago del Estero 2654, (3000) Santa Fe, Argentina The reaction kinetics, structure sensitivity, and in-situ activation of cyclopentane and methane combustion on Pt/A1203 catalysts of different metallic dispersion were studied. The reaction orders in oxygen were 1 (cyclopentane) and zero (CH4). Methane oxidation turnover rates did not change significantly by changing the metallic dispersion but the cyclopentane combustion activity increased dramatically with increasing Pt crystallite size. On both reactions, the activation energies did not change by changing the Pt dispersion. Results are interpreted in basis of two different reaction mechanisms over the metallic Pt active sites. I. INTRODUCTION Platinum-based catalysts are highly active for oxidative removal of small amounts of hydrocarbon from gaseous or liquid streams. The effect of varying the platinum particle size on the catalytic combustion of different hydrocarbons has been extensively studied [1-4], but the results obtained are conflicting, probably because correlation between catalytic activity and metallic dispersion depends on the type of hydrocarbon to be abated. Several papers on light alkanes combustion, namely methane [ 1], propane [5], and butane [4] have reported that alkane oxidation turnover rates increase with increasing platinum particle size. In contrast, in a recent study on the C2H4 combustion over platinum-supported catalysts Pliangos et al. [6] proposed that turnover frequency changes, which cannot be explained by structure sensitivity considerations, are caused by interactions between the metal crystallites and the carrier. Similarly, Papaefihimiou et al. [7] reported that the benzene oxidation turnover rate on Pt/A1203 strongly increases with increasing Pt particle size but does not change by changing the Pt dispersion on Pt/SiO2 and Pt/TiO2 catalysts. Palladium and platinum catalysts are often activated on stream, ab-initio of the hydrocarbon combustion reaction [5,8,9]. This phenomenon has been widely studied on palladium-based catalysts; in the case of methane oxidation, several authors have proposed that the initial activation period is caused by reoxidation from Pd metal or oxygen-deficient PdOl.x to more active steady state PdO species [8]. In contrast, very few papers have been published using platinum-based catalysts [5] and the causes of induction periods on platinum remain unclear. In a recent paper [9], we studied the structure and reactivity of Pt/A1203 catalysts for benzene oxidation at low temperatures. In this work we report results on the oxidation of cyclopentane and methane over a set of Pt/AI203 catalysts of different metallic dispersion and chlorine concentration. Our goal was to obtain further information on the catalyst activation phenomenon and on the sensitivity of hydrocarbon oxidation turnover rates to Pt crystallite size. "Corresponding author. Email:
[email protected],fax: 54-342-4555279
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2. EXPERIMENTAL
A Pt(0.3%)/AI203 catalyst (catalyst A) was prepared by impregnation at 303 K of a highpurity 7-A1203 powder (Cyanamid Ketjen CK303) with an aqueous solution of H2PtCI6.6H20 and HCI. After impregnation, samples were dried 12 h at 393 K and heated in air stream to 773 K. Then the chlorine content was regulated using a gaseous mixture of HC1, water and air. Finally, the sample was purged with N2 and reduced in flowing H2 for 4 h at 773 K. A set of three catalysts with different Do (Pt dispersion) was prepared by treating catalyst A in a 2% O2~2 atmosphere at 848, 873 and 903 K for 2 h (catalysts B, C, and D, respectively). The Pt dispersion was measured by 1-12 chemisorption by using the double isotherm method and a stoichiometric atomic ratio H/Pt~=I, where Pts implies a Pt atom on surface. The characteristics of catalysts A, B, C, and D are shown in Table 1. Tabla 1 Characteristics of the catalysts used in this work Catalyst Pt loading CI concentration (wt.-%) (wt.-%) A 0.30 0.95 B 0.30 0.61 C 0.30 0.58 D 0.30 0.60
Pt dispersion Do (%) 65 38 24 15
Hydrocarbon oxidation reactions were carried out at 1 atm in a fixed-bed tubular reactor. Cyclopentane (0.65%) or methane (2%) were fed in a 10% O2/N2 mixture. On-line chromatographic analysis was performed using a gas chromatograph equipped with a flame ionization detector and Bentone 34 or Porapak Q packed columns. Before gas chromatographic analysis, the reaction products were separated and carbon dioxide converted to methane by means of a methanation catalyst (Ni/Kieselghur) operating at 673 K. Two experimental procedures were used for catalyst testing. The complete oxidation of hydrocarbons was studied by obtaining curves of hydrocarbon conversion (X) as a function of temperature (light-off curves). The temperature was raised by steps of about 23 K, from 25 to 673 K (cyclopentane) or 913 K (methane). More fundamental differential reactor experiments (less than 10% conversion) were performed at constant temperature. 3. RESULTS AND DISCUSSION 3.1. Catalytic tests: Light-off curves Fig. 1 shows the X vs T curves obtained on catalyst A in two consecutive catalytic tests. The cyclopentane combustion started at about 473 K in the first run and the conversion increased then dramatically at ca. 553 K reaching a value of X ~ 100% between 663 and 673 K. The reaction was maintained at 673 K for 2 h and then the catalyst was purged and cooled down in nitrogen to 373 K. Subsequently, a second catalytic test was carried out. As shown in Fig. 1, the X vs T curve corresponding to the second run was clearly shifted to lower temperatures as compared to that obtained in the first run. Such a displacement of the light-off curves typically illustrates the catalyst activation phenomenon in hydrocarbon combustion reactions. To compare catalyst activities, we measured from light-off curves the value of the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
577 temperature at X = 50 %, T i,j, 5~ where i identifies the catalyst and j indicates first (1) or second (2) runs. The difference ATSO = TSO 50 is a measure of the i,l - Ti,2
100 80
-~
.
Methane
60
activation phenomenon on catalyst i. Table 2 shows that the AT5~ value for cyclopentane combustion was about 80 K. The CH4 combustion on Pt occurs at temperatures (T5~ = 823 K) significantly
4o
r
20 9 n
o =
!
1st. run 9 2nd. run
I
higher as compared with cyclopentane combustion (TS~ = 593 K). The consecutive
=
light-off curves for CH4 combustion were similar (Fig. 1), thereby suggesting that catalyst A is not activated ab-initio of this Fig. 1" Light-off curves on catalyst A reaction. On the other hand, we measured the Pt dispersion on catalyst A atter the second runs (D2, Table 2). By comparing the Do and D2 values in Table 2 it is inferred that the metal was severely sintered in both reactions after two consecutive catalytic tests. Similar experiments were carried out on catalyst C. For cyclopentane combustion, the light-off temperature in the first run (T c5~ = 518 K) was clearly lower than that obtained on 400
600
Temperature
8O0
1000
(K)
catalyst A; the initial catalyst activation was negligible (ATc5~ 5 K) as well as the metal sintering at~er two consecutive runs (Table 2). In contrast, for methane combustion the T~5~ and AT5~ values obtained on catalyst C were similar to those found on catalyst A. Tabla 2 Catalytic activity and Pt dispersion in two consecutive catalytic runs Catalyst Reactant Temperatures at X = 50% (K) T~~ T25~ AT 5~ A A C C
Cyclopentane Methane Cyclopentane Methane
593 823 518 823
513 818 513 823
80 5 5 0
Pt dispersion (%) Do D2 65 65 24 24
15 18 20 17
3.2. Low-conversion catalytic tests In order to establish the effect of the Pt crystallite size on catalyst activity, additional kinetically-controlled catalytic tests were performed In all the cases, the initial conversion was lower than 10%. The oxidation reactions were performed over catalysts A, B, C, and D at 443 K (cyclopentane) and 713 K (methane). For cyclopentane combustion, the activity of welldispersed catalyst A slowly increased with time on stream, but the turnover frequency (TOF, sl) on sintered catalyst D was constant along the 20 hour run. Over all the catalysts, methane oxidation rates did not change with time on stream. In Fig. 2 we have plotted the initial turnover frequencies as a function of the metallic dispersion. Cyclopentane combustion turnover rates
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increased drastically with the Pt particle size; the TOF value on catalyst D (Do = 15%) was about 40 times higher than that measured on catalyst A (Do = 65%). This result shows that cyclopentane combustion on Pt/Al203 catalysts is a structure-sensitive reaction preferentially promoted on larger Pt crystallites. On the contrary, for methane combustion the effect of Pt dispersion on the catalytic activity was rather weak and the turnover rate does not change significantly with Do (Fig. 2). All these results suggest that the existence of initial activation periods is related to the sensitivity of
~
n
e
.L_._
0.1 v
I.i_
o
0.01
I
6O
o0 (%) Fig. 2: Turnover frequency (TOF) vs Do the combustion turnover rate to the Pt crystallite size. 3.3. Kinetic studies
Kinetic data were interpreted by considering a power-law rate equation: #
r o = k(P~ )~ (P~2~, where r0 (mot HC/hg Pt)is the initial reaction rate. In Figs. 3 and 4 the ro values obtained on catalyst A for the oxidation of cyclopentane and methane were represented in logarithmic plots as a function of P~c and p00 2 respectively. Reaction orders cx and 13 were determined graphically from Figs. 3 and 4. The reaction orders for cyclopentane Pg= 0.126 atm Methane
~ o
o
,_o r -1 o
Cyclopentane
-2
443 K
-3 -4
o
_~
'
713 K
Methane
e-
-2
PHc = 0.02 atm,
Cyclopentane~._~
-3
_15
'
_'4
'
_~3
In Pnc Fig. 3" Reaction orders in the hydrocarbon
I
I
-2.4
PHC= 6.5 10 .3 atm, 443 K i
In P0 2
I
-2.0
Fig. 4: Reaction orders in oxygen
combustion were ot _-- 0 and J3 -= 1 while values of cz _=_1 and J3 --- 0 were determined for CH4 combustion. Similar values for ct and 13 were measured on catalyst C. On the other hand, we plotted the In TOF values as a function of 1/T for calculating the apparent activation energy (Ea) and the preexponential factor A of both reactions on catalysts A, C and D via an Arrhenius-type function. The apparent activation energies were 11 + 1 kcal/mol (cyclopentane) and 17 + 1 kcal/mol (CH4), irrespective of the mean Pt crystallite size of the sample. For
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cyclopentane combustion, we measured a A D/A A ratio of about 60. These results suggested that increasing the Pt particle size increases the density of active sites available for the ratedetermining step but does not modify the cyclopentane oxidation mechanism. The kinetic results show that CH4 and cyclopentane are oxidized by different mechanisms. The reaction orders obtained for cyclopentane combustion are well interpreted by considering that the reaction occurs via a Mars-Van Krevelen type mechanism [ 10], being the dissociative adsorption of oxygen on Pt the rate determining step. For cyclopentane oxidation on Pt, this mechanism may be represented by the following elementary steps: 02(g) + 2 L CP(g) + L
CP.L + O.L (CP...O).L
k~ > k2 >
20.L
k3 >
(CP...O).L
O.L
>
CP.L
CO2(g)+ H20(g)
where L represents the vacant active sites. The expression of initial rate r0 results: pO o klk3 o~PcP r0 = k,po ' + vik3PO p w h e r e v i is the stoichiometric coefficient of oxygen in the overall reaction.
(1) If k I < < k 3
Eq. (1) reduces to: klP~ 2 ro = - Vi
(2)
and the orders with respect to cyclopentane and oxygen predicted by Eq. (2) are 0 and 1, respectively, which are the approximate orders determined from our experiments. According to Eq. (2), any increase in rate constant k 1 accelerates the cyclopentane oxidation rate. The observed turnover rate increase with increasing Pt particle size would reflect therefore an increase in the density of reactive Pt-O species resulting from higher Pt oxidation rates. This assumption is consistent with previous work which showed that the number of Pt-O bonds of lower binding energy, i.e. the site density of more reactive surface oxygen, increases on larger Pt particles [2]. The initial activation of well-dispersed Pt catalysts in cyclopentane combustion would be caused by sintering of the metallic phase, which occurs in reaction conditions even if the cyclopentane the combustion reaction is performed at low-temperature and low-conversion regimes. The reaction is highly exhotermic and the Pt crystallite temperature is significantly increased in reaction conditions. Hot-spots on the metallic particles together with the presence of gaseous water cause the metal phase sintering at mild reaction conditions and the formation of larger, more active, Pt particles. The methane combustion has been interpreted by considering a Langmuir-Hinshelwood mechanism, where the rate-determining step is the abstraction of the first hydrogen on the
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580
adsorbed methane molecule and oxygen chemisorption steps are not kinetically significants [8]. The proposed reaction pathways"
202+4.L
". K, > 4 0 . L
CI-I4+L
< K2 )
k
CH4.L + O.L CH3.L + 3 0 . L
>
CH4.L CH3.L+ OH.L
". K3 > C.L+3OH.L K4 < > C O2.L+2L K5 < > 2 H20(g) + 20.L + 2 L K6 < )' C02(g)+ L
C.L+20.L 40H.L
C02.L
leads to a complex kinetic rate expression: k K34K,K2 Pc., [Po~1,2
r= ,
+4K,K
,
+I
+2 K6
qKKK6 [p02]1/2Pc0212
When hydroxyl groups are the most abundant species, the initial rate expression becomes:
~ r0 -
PH2o
(3)
which is consistent with the observed experimental rate equation.
REFERENCES 1. K. Otto, Langmuir, 5 (1989) 1364. 2. P. Briot, A. Auroux, D. Jones and M. Primet, Appl. Catal., 59 (1990) 141. 3. M. Kobayashi, T. Kanno, A. Konishi and H. Takeda, React. Kinet. Catal. Lett., 37 (1988) 89. 4. R.F. Hicks, H. Qi, M.L. Young and R.G. Lee, J. Catal., 122 (1990) 280. 5. P. Mar6cot, A. Fakche, B. Kellali, G. Mabilon, M. Prigent and J. Barbier, Appl. Catal. B: Environmental, 3 (1994) 283. 6. C. Pliangos, I.V. Yentekakis, V.G. Papadakis, C.G. Vayenas and X.E. Verykios, Appl. Catal. B: Environmental, 14 (1997) 161. 7. P. Papaefihimiou, T. Ioannides and X.E. Verykios, Appl. Catal B: Environmental, 15 (1998) 75. 8. K. Fujimoto, F. Ribeiro, H.M. Avalos Borja and E. Iglesia, J. Catal., 179 (1998) 431. 9. T.F. Garetto and C.R. Apesteguia, J. Catal., in press. 10. P. Mars and D.W. van Kravelen, Chem. Eng. Sci., 3 (1954) 41.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Ceria-zirconia-supported platinum catalyst for hydrocarbons combustion : low-temperature activity, deactivation and regeneration Christine Bozo, Edouard Garbowski, Nolven Guilhaume* and Michel Primet Laboratoire d'Application de la Chimie/L l~Environnement (UMR 5634), Universit~ Lyon 1 (B~.t. 303), 43 Boulevard du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France ABSTRACT A Ce0.67Zr0.ssO2 solid solution has been prepared by coprecipitation, and used as support for platinum, to investigate the effect of this support on the total oxidation of hydrocarbons. The total oxidation of C3I-I6 (in the presence of an excess 02) occurs at low temperature (120180~ but the catalyst deactivates rapidly when kept isothermally at 155~ This deactivation is reversible, and the activity is fully restored after decomposition of large amounts of surface carbonates. Methane conversion occurs at higher temperatures, but strong deactivations under isothermal conditions (especially at 350~ are also observed. Ex-situ characterizations of the deactivated catalyst as well as in-situ study of the effect of oxidizing and reducing treatments suggest that the deactivation in methane combustion is related to the presence of oxidized species linked to the support and/or to the metal. 1. INTRODUCTION Ceria-zirconia solid solutions tend nowadays to replace ceria as noble metals supports in three-way catalysis, because they show improved thermal stability and oxygen storage capacity compared to pure ceria [ 1]. The introduction of zirconia in the ceria lattice leads to a higher mobility of surface and bulk lattice oxygen [2], and to the enhancement of the catalytic activity under reducing conditions and after ageing at 1000~ [3]. Since the ceria-zirconia support seems to play an important role in oxidation reactions, we investigated its effect on the activity of a Pt/Ceo.67Zro.3302 catalyst for the combustion of two hydrocarbons: propene and methane. 2. RESULTS AND DISCUSSION
2.1. Experimental part The Ce0.67Zr0.ssO2 support was prepared by coprecipitation of an aqueous solution of cerium nitrate and zirconyl nitrate in ammonia. The precipitate was washed with water, dried and calcined at 700~ for 3 hours in an air flow. Pt(NH3)4(NO3)2 was impregnated from an aqueous solution, followed by calcination at 400~ for 12h under 02. The catalyst was then reduced at 300~ under hydrogen. The Ce0.67Zr0.s302 formula was deduced from chemical analysis. Temperature-programmed reduction (TPR) of the catalysts was performed under 1 vol.% H2 in argon, with a temperature ramp of 20~ 1, from ambient to 1000~ The solids were pretreated at 400~ in flowing air for 1 hour, then flushed with argon at the same * Corresponding author. Fax: (33) 4 72 44 81 14, E-mail:
[email protected] 家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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temperature (1 h.), and cooled to room temperature under argon. Hydrogen consumption was measured with a TCD detector. 0.50 g catalysts were used in the activity measurements. Methane combustion was measured under isothermal conditions, between 200 and 800~ the temperature being increased by steps of 50~ and the catalysts kept at each temperature for 3 hours. The feed consisted in 1 vol.% CI-I4, 4 vol.% 02 and balance nitrogen (total flow: 6.4 l.h-1). The analysis of the products and unreacted methane was performed as described in ref. [4]. In the propene combustion experiments (0.3 vol.% C3I-I6, 2 vol.% 02, balance N2, total flow 12 l.h-1), infrared analyzers were used for the detection of C3H6, CO and CO:, while O: was analyzed with a paramagnetism analyzer. 2.2. Preparation and characterisation of the solids
The X-ray diffraction of the support alone after calcination at 700~ shows the diffraction lines corresponding to cubic ceria, with a slight displacement of the lines due to the incorporation of the smaller Zr4§ cation (0.84 A in cubic coordination) in place of Ce4§ (0.97 A in cubic coordination) [5]. The support pattern is not modified after impregnation. The amount of platinum, determined by chemical analysis, is 1.6 wt.%. The dispersion calculated from 1-12chemisorption isotherms is 40 %, which should correspond to an average particle size of 2.8 nm.. The surface area of the ceria-zirconia support (70 m2.gl aider calcination at 700~ is not modified aider Pt impregnation and subsequent thermal treatments (69 m2.gl). Temperature programmed reduction profiles of the ceria1200 zirconia support alone and Pt140 loaded are shown in Fig. 1. - 1000 Like pure ceria [6], Ceo.67Zr0.3302 is reduced in 800 90 t~ two peaks under hydrogen : 600 the first peak (maximum at 680~ is attributed to the 400 reduction of "surface" cerium, N 40 d while the second one 200 (~1000~ corresponds to the -j reduction of bulk Ce4+. -10 Actually, the hydrogen 0 5000 10000 consumption of the first peak Time (s) corresponds to the reduction of ~ 4 surface layers of Fig. 1: H2-TPR profiles of the Ceo.67Zro.3302 support (thin cerium, and to ~38 % of the total Ce4+ amount in the line) and Pt/Ce0.67Zr0.3302 catalyst (thick line). sample. The total H2 consumption in the TPR experiment represents the reduction of 62 % of the total cerium. Balducci et al. [7] obtained a similar reduction of cerium after H2-TPR of a Ce0.sZr0.502 sample of comparable surface area. The TPR profile of the Pt/Ceo.67Zr0.3302 catalyst is different : a very important hydrogen consumption occurs at ambient temperature, when the H2 / Ar mixture is sent on the catalyst. At the beginning of the temperature ramp, a small negative peak is due to the desorption of hydrogen chemisorbed on platinum. Two reduction
A
i
i
i
i
]
i
i
I
i
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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peaks are seen at 150 ~ and 565~ (weak), while the reduction of bulk cerium at 1000~ is not modified compared to the support alone. The important feature is that the 1-12consumption at room temperature corresponds not only to the reduction of surface platinum, but also to that o f surface Ce 4+ (1.6 to 1.9 layers according to the hypotheses made for taking into account the hydrogen consumption at room temperature, i) surface reduction of Pt particles, ii) reduction of bulk PtO aider the oxidizing pretreatment). At the end of the TPR, more than 70 % of the total cerium is reduced into Ce ~+. Recently, Fornasiero et al. [8] studied the redox behavior ofRh-, Pt- and Pd-loaded Ce0.5Zr0.502 catalysts of high surface areas (53, 42 and 35 m2.g~ respectively) by TPR. No hydrogen consumption at room temperature was mentioned in the work of Fornasiero et al. and the amount of Ce 3§ was estimated to 6 1 % after the TPR. This difference may be ascribed to several points : the surface area of our Pt/Ce0.67Zr03302 catalyst is larger (69 m2.gl), and the Pt loading ~3 times higher. Furthermore, the Pt/Ce0.sZr0.502 catalyst (0.5 wt.% Pt) in ref. [8] was prepared from a H2PtCI6 precursor, and it has been shown for Rh/CeO2 catalysts that the use of RhC13 as precursor leaves a significant amount of chloride species on the support [9], which affects the chemisorption and redox properties of rhodium [9, 10], but also strongly disturbs the redox behavior of ceria [ 11 ]. The presence of platinum, nevertheless, promotes the support reduction in two ways : the surface cerium is reduced at a much lower temperature, and the overall reduction of cerium occurs in a larger extent. 2.3. Activity in propene combustion The low-temperature activity of the P t / C e 0 . 6 7 Z r 0 . 3 3 0 2 catalyst was tested in the oxidation of propene. Fig. 2 shows the evolution of the C3H6 conversion during a light-off experiment (temperature ramp 5~ Followed by stabilization of the temperature at 155~
The propene conversion increases rapidly between 130 and 155~ 100 180 (typical "S" conversion curve), where it reaches 77% (the catalyst temperature is shown on 120 o the figure, and it slightly r~ 50 temperature because of the o t~ 0 60 .-.o exothermicity of the reaction). :ff After the temperature ramp, the temperature was stabilized at 155~ The conversion decreases then very quickly, and is only 0 30 60 90 10% after 30 minutes dwell. It Time (min) further decreases and is only 7% at the end of the experiment. Since ceria is a basic oxide and Fig 2: Propene conversion in a light-off experiment carbonates rapidly in the (5~ followed by a temperature dwell at 155~ presence of CO2, the catalyst was heated under nitrogen between 20 and 300~ 9a CO2 desorption peak appears, which is maximum at 270~ This peak corresponds to ~ 250 ~tmol. CO2 per gram catalyst (or 3.6 l,tmol. CO2 per m2). Table 1 shows
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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the results of a series of light-off measurements, followed by an isothermal deactivation at 155~ and separated by intermediate desorptions under nitrogen. The amount of CO2 desorbed is reproducible, and the light-off activity is fully restored after each desorption. In order to correlate these results with a poisoning of the catalyst by carbonates, the adsorption and desorption behavior of CO2 was studied by infrared spectroscopy. CO2 adsorbed at ambient temperature gives bands at 1587 and 1298 cm1, which can be attributed to bidentate carbonates [12, 13], and two broad bands in the 1550-1480 and 1415-1330 em~ domain, which probably correspond to bulk carbonates. Table 1 : Light-off activity and CO2 desorption from Pt/Ce0.67Zr0.3302 catalyst. Conversion temperature Desorption Experiment T2o (~ Ts0(~ CO2 desorbed Temperature of peak n~ (l.tmol/g. catalyst) maximum (~ 1 153 167 2 244 270 3 160 171 4 246 271 5 130 157 6 252 267 7 136 159 Upon desorption under vacuum at increasing temperatures, the bands due to surface bidentate carbonates decrease progressively between 100 and 300~ At 400~ only bulk carbonates (1460, 1400 cm 1) are present, and at 500~ nearly all the carbonate species are eliminated. This strongly suggests that the low-temperature poisoning of the propene oxidation reaction could be related to the formation of surface carbonates which remain adsorbed at 155~ Aider desorption of these species, the activity is fully recovered. No deactivation was usually observed for propene oxidation over Pt supported onto alumina. The loss of activity here observed is due to a poisoning of active sites present of the ceria-zirconia support. This conclusion stresses the participation of the CeO2 - ZrO2 support in the combustion process. 2 . 4 . A c t i v i t y in m e t h a n e c o m b u s t i o n
The activity of the Ce0.67Zr0.3302 support and of the Pt/Ce0.67Zr0.3302 catalyst for methane total oxidation is shown on Fig. 3. The solids were kept at each temperature for 3 hours. The most important feature to notice is that the ceria-zirconia solid solution is active for methane combustion : the conversion starts at 400~ and is total at ca. 800~ This means that the Ce0.67Zr0.3302 support presents active oxygen species which can oxidize methane. Addition of platinum leads to a very active catalyst (the conversion starts at 200~ which shows important deactivations under isothermal conditions, particularly between 350 and 450~ Since the deactivation is the most pronounced at 350~ we chose this temperature for in situ study of the parameters influencing it, and characterizations of the deactivated catalyst were performed at~er isothermal tests at the same temperature for 12 hours.
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100 Fig. 3 9Methane conversion on (ZX) Ceo.67Zro.3302 and (0)
~0 r~ l,,,
,~ 50
Pt/Ce0.67Zr0.3302.
O
0 200
400 600 Temperature (~
800
Because of the carbonatation of the support observed at low temperature, we tested the effect of a nitrogen purge at 500~ which should remove surface carbonates, as well as the effect of CO2 addition in the feed-steam (Fig. 4). A nitrogen treatment of the deactivated catalysts at 500~ has no effect on catalytic activity in methane oxidation. Similarly the introduction of 1 vol. % CO2 for three hours in the feed of reactants at 350~ does not modify the conversion 1 0 0 -~
1
5000 4000
COz
.o
3000
50 r~
O
2000
|
1000
m r~
0 0
5
10 T i m e (It)
15
2t3
'
'
'
I
I
2000
4000
6000
8000
10000
T i m e (s)
Fig. 5: Effect of intermediate catalyst Fig. 4: Effect of CO2 addition (1 vol.%) in oxidation (thick line) and reduction (thin the feed (e), and of intermediate N2 purge line) on the deactivation at 350~ at 500~ (/x ) on the deactivation at 350~ of methane. Thus the deactivation observed at 350~ is not connected with a poisoning by surface carbonates. The catalyst was characterized in its deactivated state. The surface area of the deactivated solid is not different than that in the flesh state. The platinum accessible area measured by 1-12 chemisorption is also unmodified (40 %) for a sample aged at 600~ under reactants and subsequently hydrogen reduced. This suggests that the loss in the activity is not related to the catalyst sintering, or to metal encapsulation, as proposed for Pd/CeO2 upon reduction [14], and for Pd/Ce0.5Zr0.502 [15,16] and Pd or Rh/Ce0.vZr0.302 [17] upon high temperature redox aging. The effect of in-situ oxidation or reduction of the catalyst on the activity at 350~ is shown on Fig. 5. Starting from a deactivated state, in which the methane conversion is ~ 20%, the catalyst was first treated under 4 vol. % 02 for 2 hours, then the CH4 + 02 mixture was sent again. The conversion remains the same as before the treatment under oxygen. When an
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intermediate reduction under H2 for 1 hour is performed at 350~ the catalyst is strongly activated : the conversion recovers the value observed at the beginning of the deactivation, before decreasing again slowly. Finally the activity in CH4 combustion of the Pt/Ce0.67Zr0.3302 catalyst initially preoxidized under oxygen at 350~ is very similar to that of the reduced sample and the same deactivation on stream occurs. 3. CONCLUSION The ceria-zimonia support modifies the properties of platinum in oxidation reactions " it is active for methane oxidation, and probably participates to the reaction in the presence of Pt. At low temperature (ca. 200~ the support is quickly carbonated, which leads to a rapid but reversible deactivation. At temperatures higher than 300~ the carbonates species are not longer adsorbed. Nevertheless a loss in activity for CI-I4 total oxidation is observed. The activity is only recovered by a reduction step under hydrogen at the same temperature. The deactivation in isothermal conditions is due to the formation of poisoning species linked to the support and/or to the platinum particles, such species are reducible in the conditions of the combustion reaction. ACKNOWLEDGMENTS
The financial supports of GAZ DE FRANCE and ADEME are gratefully acknowledged. REFERENCES 1. J.P. Cuif, G. Blanchard, O. Touret, A Seigneurin, M. Marczi and E. Qu6m6r6, SAE paper 970463. 2. P. Fomasiero, R. Di Monte, G. Ranga Rao, J. Kaspar, S. Meriani and A Trovarelli, J. Catal., 151 (1995) 168. 3. J.G. Nunan, W.B. Williamson and H.J. Robota, SAE paper 960798. 4. N. Guilhaume and M. Primet, J. Chem. Soc. Faraday Trans., 90 (1994) 1541. 5. R.D. Shannon, Acta Cryst., A32 (1976) 751. 6. V. Perrichon, A. Laachir, G. Bergeret, R. Frdy, L. Toumayan and O. Touret, J. Chem. Soc. Faraday Trans., 90 (1994) 773. 7. G. Balducci, P. Fomasiero, R. Di Monte, J. Kagpar, S. Meriani and M. Graziani, Catal. Lett., 33 (1995) 193. 8. P. Fomasiero, J. Kagpar, V. Sergo and M. Graziani, J. Catal., 182 (1999) 56. 9. D.I. Kondarides and X. Verykios, J. Catal., 174 (1998) 52. 10. D.I. Kondarides, Z. Zhang and X. Verykios, J. Catal., 176 (1998) 536. 11. S. Bemal, J.J. Calvino, G.A. Cifredo and J.M. Rodriguez-Izquierdo, J. Phys. Chem., 99 (1995) 11794. 12. A. Trovarelli, Catal. Rev. Sei. Eng., 38 (1996) 439. 13. C. Binet, A. Jadi and J.C. Lavalley, J. Chim. Phys., 89 (1992) 1779. 14. A. Badri, C. Binet and J.C. Lavalley, J. Chem. Sor Faraday Trans., 92 (1996) 1603. 15. G.W. Graham, H.W. Jen, W. Chun and R.W. MeCabe, Catal. Lett., 44 (1997) 185. 16. J.C. Jiang, X.Q. Pan, G.W. Graham, R.W. MeCabe and J. Sehwank, Catal. Lett., 53 (1998) 37. 17. G.W. Graham, H.W. Jen, W. Chun and R.W. MeCabe, J. Catal., 182 (1999) 228.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Characterisation of a ~(-MnO2 catalyst used in VOC abatement C. Lahousse, C. Cellier, B. Delmon, P. Grange* Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Universit6 catholique de Louvain, 2/17 P1. Croix du Sud, B 1348 Louvain-la-Neuve, Belgium, Fax: 32.10.47.36.49 This study presents the results of a comprehensive characterisation of a very efficient VOC abatement catalyst. The changes suffered by a ~,-MnO2 catalyst during VOC oxidation are determined and their impact on the catalytic activity is discussed. Sintering and partial reduction are detected. But they respectively have only a limited and no effect on the stability of the catalytic activity. Conversely, water vapour adsorption appears to cause very long (12h) stabilisation delays. 1. I N T R O D U C T I O N In a recent paper [ 1], we have shown that the nsutite (T) form of MnO 2 is a very promising VOC removal catalyst which is more active and in many respects superior to conventional catalysts based on noble metals. A 150-hour test showed that this catalyst was able to maintain its activity over a long period of time. However, it sometimes presented an important decrease of activity during the first hours of operation. In some conditions T-MnO_~ activity was decreasing during several hours before stabilising while in other conditions, the activity was stable after a few minutes. The aim of this paper is to characterise the modifications suffered by this catalyst as a function of the reaction conditions in order to determine the origin of this decrease of activity. Surface area, XPS and IR measurements are performed to detect and evaluate possible sintering and reduction phenomena. Specific tests are performed to determine the effects of reduction and reactant adsorption on the catalytic activity.
2. EXPERIMENTAL 2.1. XRD Changes in the T-MnO 2 lattice parameters were assessed using a Siemens D5000 powder diffractometer operating at 20 kV with copper Ka wavelength. The shift of the XRD pattern is quantified in table 1 by pointing the position of the most intense peak.
* Financial support by the Commission of the European Union (Contract No. EV5V-CT93025) is gratefully acknowledged. Special thanks are also due to Mrs Piton of SEDEMA-SADACEM Belgium for providing the catalyst. The authors also thank the R6gion Wallonne is for funding on-going research in this area (Convention 971/3667).
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2.2. N 2 physisorption The textural characteristics of fresh and used catalysts were measured on a Micromeritics ASAP 2000 sorptiometer. The measurement was performed on samples outgassed at 150~ As manganese oxides are known to be sensitive to outgassing [2], the applicability of this treatment was checked. A measurement on a sample outgassed at 80~ which gave consistent results confirmed that the treatment at 150~ was still appropriate. 2.3 XPS XPS was used to detect possible changes in the catalyst surface composition (e.g. coke deposition or alkaline segregation) and to evaluate the modifications of the oxidation state of the surface Mn ions. As indicated by the literature [3,4], the most sensitive measurement of Mn oxidation state is given by the distance in eV between the Mn3s main peak and its shakeup satellite. This is the value reported in table 1. The apparatus used is a Surface Science Instrument spectrometer (SSI 100) working with monochromatised AI ka radiation(1486,6 eV). 2.4 FT-IR The lattice vibration of fresh and used catalysts were studied using IR spectroscopy. The results were interpreted using the work of Potter and Rossman [5]. For this study, 1 mg of catalyst was diluted in 120 mg of dried KBr. The diluted powders were pressed into cardboard supported pellets and placed in a small MIDAC FT-IR spectrometer.
2.5 Catalytic activity measurement In this work, 2 types of tests are presented. The variation of conversion as a function of time is measured either using a low concentration mixture of ethylacetate (ea) and n-hexane (hex) (250 ppm each) or with a very high concentration of n-hexane (20 000 ppm). Low concentration tests are performed at 150~ (unless otherwise specified), high concentration ones at 220~ For the evaluation of water effect, 20 000 ppm of water vapour were added to the low concentration stream. The VVH is, in all cases, 72 000 h -l. Catalyst activation and reactivation were performed using a flow of O 2 (or when specified of N2) for 30 minutes. For some of the tests, conversion was first measured as a function of time, then as a function of temperature from 0 to 100 % conversion. These tests are marked as "final conversion 100%" in table 1 as this has an influence on the characterisation results. More details concerning test procedure can be found elsewhere (1,6). 3. R E S U L T S The characteristics of the used catalysts are presented as a function of the reaction conditions in Table 1. This table shows that : - Surface area decreases (from 100 m2/g to 80 m2/g) in the presence of added water vapour or when the VOC concentration is very high (20 000 ppm). - The XRD pattern of catalysts shifts when the samples are coming from high - concentration experiments finished below 100% conversion. - A reduction of the average oxidation state of Mn ions at the surface is detectable by XPS. This reduction is observed in all conditions as long as the test is stopped below 100% conversion. XPS and IR characterisations also showed that no coke formation occurred whatever the conditions. XPS carbon content does not increase and no new band is observed when
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performing the IR analysis of used samples. IR spectra nevertheless change. The IR lattice vibration bands are significantly broadened in the spectra of the sample with a high VOC concentration. The initial spectrum is recovered after catalyst reactivation with 0: but not when reactivation is performed with N 2. As underlined in the introduction, once stabilised, ?-MnO~ is able to maintain its activity over a long period of time but an important decrease of activity was observed at the beginning of the test. The length of this stabilisation period varies enormously (between l0 minutes and more than 12 hours) and depends upon the reaction conditions. Our experiments showed that : the lower the VOC concentration, the longer the stabilisation the higher the conversion, the quicker the stabilisation the addition of water in the inlet stream dramatically shortens the stabilisation time. -
-
-
4. DISCUSSION The surface area was measured at each step of a typical reaction procedure with a high concentration. The results are presented in figure 1. The variation of surface area as a function of time shows that sintering, when it happens, mostly occurs during the activation and at the beginning of the experiment. The surface area rapidly reaches 80 m:/g and seems very stable afterwards. The catalytic activity diminishes accordingly. The stabilisation period can be observed in the absence of sintering (low VOC concentration, no water addition) or with presintered (reactivated) catalyst.
Hexane conversion on a fresh sample and after 1,2 and 3 reactivations 70% m2/g
60% tO (/)
..,..,
8
50% 89 m2/g
I.,,,
(D
> t-
40%
80 m2/g
O
o
30%
20% 10% 0%
I
0:00
I
0:30
time on stream
Fig 1. Effect of sintering on the catalytic activity
1 "00
I
1:30
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The shift of the XRD pattern of ?-MnO: has been observed previously in the literature [7]. It happens when this oxide gets partially reduced. This partial reduction is also detectable by IR and results in the broadening of the IR structural vibration bands. When working with nonrealistic concentrations of VOC, partial reduction of the catalyst occurs. However, the nsutite structure is always conserved. ?-MnO 2 is not reduced to lower oxide (Mn203) and keeps its activity. Partial reduction of the bulk does not impair long term stability. As shown by IR and XRD, bulk reduction is reversible. Treating a used catalyst for 30 minutes under oxygen (or letting it work 1 hour at 100% conversion) is enough to recover the original XRD pattern and IR spectrum. XPS shows that bulk reduction begins by a surface reduction. Unlike bulk reduction, surface reduction takes place whatever the concentration. It remains a reversible phenomenon since it can not be observed any more on catalysts used in experiments finished at 100% conversion. In order to determine the effect of reduction on the stabilisation period, the stabilised catalyst was reactivated by a flushing with either 02 or N 2. Figure 2 compares the results of the two reactivation methods. The reactivation proves to be equally efficient with either gas. Although XRD, XPS, and IR indicate that N 2 reactivated samples are as reduced as the original used ones, they regain as much activity as the reoxidised ones. This clearly indicates that the long stabilisation period is not due to the reduction of the catalyst surface or the consumption of MnO 2bulk oxygen. By elimination, it is thus possible to attribute the appearance of the very long stabilisation period to the adsorption of one of the reaction products, namely water vapour. This conclusion is in agreement with all our observations and is particularly confirmed by the fact that, as shown in figure 3, water vapour addition leads to a very rapid stabilisation.
Catalyst reactivation with oxygen I initial activity
Catalyst reactivation with nitrogen
I 7oi 60
I initial activity ~c; n a 1 ~ t ; x r ; t x r
50 = O
= O 40 30
>
> 20 = o o10
=
o
o
fresh 1 2 3 number of reactivations Fig 2. Comparison of reactivation by 02 and
fresh 1 2 3 number of reactivations N 2
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conversion of 250 ppm n-hexane as a function of time
100% ~~llli~ m hex alone 75% = 50% O
~lm -A
A hex+water l~
44m Into ~
mtm ~
mi ~
nm mtm mtm n ~ ~
25% -- A ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ A A A ~ A ~ ~ A 0%-
0:00
8:00
time on stream (hours)
16:00
Fig. 3. Effect of water vapour on the duration of the stabilisation period. 5. CONCLUSIONS The modifications of our catalyst in different conditions have been determined. Sintering takes place at the beginning of the experiments and does not impair the long term stability of the catalyst. A reversible partial reduction of ~,-MnO2 occurs but seems to have no effect on the catalytic activity. The decrease of activity recorded during the first minutes or day is certainly due to the hydration of the catalyst surface. This communication provides an example of the unusual phenomena encountered when dealing with low temperature processes in environmental catalysis. In these types of application, air moisture is almost always present and begins to significantly affect the catalyst behaviour when the reaction temperature approaches 100~ REFERENCES
1. 2. 3. 4. 5. 6.
C. Lahousse, A. Bernier, P. Grange, B. Delmon, J. Catal., 178 (1998) 214. F. Kapteijn, L. Singoredjo, A. Andreini, J.A. Moulijn, Appl. Catal. B., 3 (1994) 173P. D.A. Shirley, Phys. Scripta, 11 (1975) 117. B.W. Veal, P. Paulikas, 51, 21 (1983) 1995. A.M. Potter, G.R. Rossman, Am. Mineralogist, 64 (1979) 1199. C. Lahousse, A. Bernier, E. Gaigneaux, P. Ruiz, P. Grange, B. Delmon, in " 3 rd World Congress on Oxidation Catalysis" (R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons, Eds.), Stud. Surf. Sci. and Catal., 110 (1997) 777. 7. R. Ruetschi, J. Giovanoli, J. Electrochem. Soc., 135-11 (1988) 2663.
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Table 1 Conversion and characterisation results as a function of reaction conditions test conditions: catalyst m a s s d , l g ; VOC=n-hexane, VOC concentration= 20000 ppm (reactivation experiments) or 250 ppm (TvaFiation experiments) ; canier flow=l00 cmYmin ; carrier nature=air +(when specified) 20000 ppm H 2 0 .
procedure 220°C, activation & reactivation 0 2 22OoC, activation & reactivation 0 2 220°C. activation 0 2 & reactivation N2 220°C, activation 0 2 & reactivation N2 220°C. activation & reactivation N2 220°C. activation & reactivation N2 220°C, activation & reactivation 0 2 220°C, activation 0 2 & reactivation N2 220°C. activation & reactivation N2 stabitisation 16h 150°C + T variation stabilisation 16h 150°C + T variation + H 2 0
conversion % after 1 minute I h 30 12h 5 minutes I hour after reactivation
63 59 57 100 94
45 38
38 79 22
52 46 46 37 20
41
37 36
sample catalyst changes Remarks characterised S BETl(m2tg) XRD dhkl1A XPS DMn3sIeV after before after before after before after 95 2.42 2.41 4,9 4,8 activation 100 reactivation 100 84-80 2.42 2,41 4,9 4,7 activation 100 95 2.42 2,41 4.9 4.8 reactivation 100 80 2.42 2.44 4.9 5.2 activation 100 2.42 2.42 4.9 4.9 reactivation 100 80 2.42 2,44 4,9 5,2 full test 100 80 2.42 2.44 4,9 5,2 full test 100 80 2.42 2.44 4.9 5,2 full test 100 80 2.42 2,44 4,9 5,2 full test 100 100 2.42 2,42 4.9 4,9 final conversion 100% full test 100 80 2.42 2.42 4,9 4,8 final conversion 10096
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
New alumina/aluminium monoliths for the catalytic elimination of VOCs N. Burgosa, M. Paulisa, A. Gilb, L.M. Gandia b and M. Montes a aGrupo de Ingenieria Quimica, Dto. de Quimica Aplicada, Fac. de C. Quimicas, UPV/EHU, Apdo 1072, E-20080 San Sebasti~in, Spain bDto. de Quimica Aplicada, Universidad POblica de Navarra, Campus de Arrosadia, E-31006 Pamplona, Spain Metallic monolithic catalysts have been prepared and tested for the abatement of VOCs. The monoliths are based on a A1203/A1 cermet produced by the controlled anodization of aluminium foils, and have been impregnated either by a noble metal (Pt or Pd) or by a transition metal oxide (Mn203). Both kinds of monoliths presented high activity for the complete oxidation of toluene, above that of the powder catalysts. I. INTRODUCTION Catalysts for VOCs oxidation are usually prepared over monoliths. The advantages of monolithic catalysts are the very low pressure drop, the high external surface area, the uniformity of the distribution of the flow within the honeycomb matrix to improve the pollutant-active site contact, etc. Ceramic monoliths are most commonly obtained by extrusion, and the cost of largescale production is very low [1 ]. The manufacture of metallic monoliths is easy and cheap for small series. However, the fixation of a porous catalyst on the surface of the metal is not an easy matter. With regard to this adhesion problem, the surface oxidation properties of aluminium offer a very interesting choice to grow up a porous and adherent layer of alumina, by a controlled anodization process. This work deals with the different variables of the anodization process, to produce a catalytically suitable support. Monoliths prepared with these alumina/aluminium cermets were used to prepare catalytic devices by impregnation with noble metals or manganese. The samples were characterised by different physico-chemical techniques and the catalytic activity was measured in the complete oxidation of toluene.
Financial support by MEC (CICYT-QUI97-1040-CO3), UPV/EHU, Gobierno Vasco, Departamento de Educaci6n y Cultura del Gobierno de Navarra (Ordenes Forales 557/1996 y 143/1998) and Universidad Pfiblica de Navarra are gratefully acknowledged. Aluminium sheets supply by INASA is acknowledged.
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2. EXPERIMENTAL PROCEDURE 2.1 Preparation of the monolithic substrate: anodization Four variables were chosen to study the anodization process: electrolyte concentration, current density, anodization time and temperature. The choice of the electrolyte can vary the A1203 final properties: surface area, porosity, thickness of the layer, etc. Based on literature data [2] H2SO4 has been chosen as the electrolyte in order to obtain satisfactory A1203 layers. An air bubble assisted cooling system was used to eliminate the local temperature rises produced by the anodization process [3]. The study of the influence of the variables on the properties of the alumina produced was carried out by N2 adsorption, gravimetry and SEM. The amount of formed A1203 was calculated by gravimetry, dissolving the A1203 layer with a phosphoric-chromic mixture. The process yield was calculated as the ratio between the experimentally measured layer of alumina and the theoretical one calculated using Faraday's rule, and the dissolved amount of A1203 was calculated as the difference between those values. Figures l, 2 and 3 show the amount of A1203 formed per m E of aluminium foil, the amount of A1203 dissolved per mE of aluminium and the specific surface area of the alumina formed during the anodization process as a function of: time, current density, and electrolyte concentration. The effect of the temperature follows the same trend as the electrolyte concentration does.
40
200~ m
~, 120
30 -]
i
I
o,,.~
"~..~ 80
~
o
....
9
-
/il
20
f
//
o o
o
Fig.2. Amount ofA1203 generated (O), A1203 dissolved (ll) and m ~ A1203/g A1203 formed ( , ) for varying current densities.
50
4o i, 30 ~ >
0
~1o~>
60
20 ~ lO
o
I- --"----"~
current density (A/dm2)
L
.-----'.120
-I !s
20
~ .... ' .... 1 . . . . . . .
60
Fig.1. Amount of A1203 generated (O), A1203 dissolved (11) and m" A1203/g A1203 formed (0) for varying anodization times.
"~i so 2_
..... o--
io2 o
-
40 time (minutes)
i I o ilOO
1 2 3 electrolyte concentration (mol/1)
Fig.3. Amount of A1203 generated (0), A1203 dissolved ( I ) and m~ A1203/g A1203 formed (0) for varying electrolyte concentrations.
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The properties of the alumina/aluminium cermets obtained are the result of two opposite processes: the anodization of the aluminium that increases the alumina layer from the top of the aluminium surface to the inside, and the dissolution of the alumina formed inside the pores of this layer that reduces and modifies the texture of the oxide layer. Then, as anodization time increases, the amount of A1203 generated increases but the amount of A1203 dissolved increases too (Figure 1). In this case the generated alumina has higher specific surface area at higher anodization times. However, the higher the current density, the lower the specific surface of the generated alumina (Figure 2), due the generation of a less porous alumina layer. On the other hand the A1203 production decreased on increasing electrolyte concentration (Figure 3) due to an enhanced dissolution process. A global view of the result of these analysis, together with the decrease of the process yield with the increment of all the variables led as to choose the following conditions as a balance between the best alumina properties and the lowest process cost" anodization time, 50 minutes; current density, 2.06 A/din; electrolyte concentration 1.64 mol H2SO4 per litre; temperature, 303K. Once the aluminium sheets were anodised, they were rolled together with alternate corrugated sheets to prepare the monoliths. The properties of the monoliths prepared in this way, similar to that of the commercial ones, are presented in Table 1. Table 1 Structural properties of the AI/O3/A1 monoliths prepared by anodization Geometric volume 6 cm 3 Cell area Total exposed surface 40 m 2 Surface to volume area ratio Number of cells 355 cell/in2 Empty fraction Specific surface area/cell 0.36 m2/cell Wall thickness
1.9 10-4 mZ/cell 1900 m -l 81% O.lmm
2.2 Impregnation of the active phase The impregnation of the monoliths with platinum or palladium was done from a (NH4)2PtCI6 (Fluka, puriss) or Pd(NO3)2 (Jonhson Matthey, Alfa) solution over 70 minutes. The noble metal impregnated monoliths were dried at 393K for 2h and calcined at 723K for 2h. In the case of manganese, the monoliths were dipped in a solution containing Mn(NO3)2 (Merck, PA) and citric acid (Panreac, PA) for 30 minutes. The dipping process was carried out one to three times to follow the catalytic properties of the monolith after several impregnations. Afterwards they were dried in air for 30 minutes, dried in vacuum at 343 K for 4 hours and calcined at 723 K for 2 hours. 2.3 Characterisations and catalytic tests Textural characterisation by N2 adsorption at 77K (Micromeritics ASAP2000) and metallic dispersion by chemisorption of H2 pulses (Micromeritics PulseChemisorb 2700) were carried out using the complete monoliths and not small specimens. The catalytic activity was measured in the complete oxidation of toluene in air obtaining both, the ignition curves at increasing temperature (2.5 K/min) and isothermal tests at high conversion. Catalytic tests were carried out in a plug flow reactor, using mass flow controllers to control the feed mixture. A He stream was bubbled through two thermostated
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and pressurised saturators containing toluene. This stream, fimher diluted with air, was passed through the monolith placed inside a furnace with temperature controller. The temperature of the reactant mixture was continuously monitored by a thermocouple placed at the inlet of the monolith. Conversion was calculated by three ways: by the disappearance of toluene and the appearance of water followed by GC-TCD containing a semicapilar column (TR-WAX, 30m), and by the appearance of carbon dioxide followed using a specific IR detector (SENSOTRANS, IR). The adsorption-desorption of reactants and products was studied by TPD and Temperature Programmed Surface Reaction (TPSR) using a MS detector (Omnistar, Balzers). 3. RESULTS AND DISCUSION N2 adsorption analysis of the AI203/A1 monoliths prepared with the chosen anodization conditions, showed the formation of a porous layer of A1203 with specific surface areas from 35 to 50m2/g and very homogeneous mean pore diameters of about 18nm. The alumina thickness is 15 to 20 lam allowing to obtain more than 3500m2 of surface area per m2 of aluminium sheet. The toluene ignition curves of the Pt and Pd monoliths are presented in Figure 4, together with the toluene ignition curves of powder Pt/A1203 and Pd/A1203, all of them pretreated in air at 573K before the reaction. Both noble metals, Pt and Pd, monoliths present excellent activity showing ignition temperatures (Ts0, temperature at which conversion is 50%) between 455 and 485K, and complete conversion of toluene below 530K. These temperatures are similar or lower than those corresponding to the conventional powder catalysts presenting comparable surface area and metal content.
.,o 0.8 r~
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Fig. 4. Toluene ignition curves for Pt and Pd supported on powder A1203 (~ Pt and Q Pd) and A1EO3/A1 monoliths (O Pt and O Pd) (225ppm of toluene).
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Fig. 3 DMF conversion over calcined HTs.
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and b) 550 ~ C;
9 - Cu~Cr,
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results in a gradual retardation of NOx evolution. Similar observation was reported recently for the non-hydrotalcite derived Cu-Cr-V catalyst [9]. Interestingly, the spectrum of various nitrogen oxides is affected in a different way depending on the V content in the sample. Thus, initially the later onset of NOx formation is due to the reduced evolution of N20 while higher amount of V reduces the evolution of NO and NO2. Further modification by replacing half of chromium with aluminium lowers evolution of N20 and so does subsequent replacement of part of copper with zinc. Varying shapes of the conversion and selectivity curves cause that in each of the series different ~nples show best characteristics. Table 3 compares the temperature ranges and the temperature windows in which the catalysts show the activity higher than 80% and the yield of NOx lower than 20%. Table 3 Temperature ranges and temperature windows of>80% conversion and 840 --> 0 ppm) in flowing 02 (2%) + balance He in the temperature range 550-680K. A typical result of a NH3-CPAD experiment is shown in Figure 1, where the dotted line and the symbols represent the inlet and the outlet NH3 concentration, respectively. Fig. 1 shows that upon linearly increasing the inlet N[-I3 concentration, the outlet concentration exhibits a dead time of about 400 s and then a rapid increase with a knee near 500 s. During the whole rise phase (0-1500 s) the outlet NH3 concentration is lower than the inlet one, due to the adsorption of NH3 on the catalyst surface. Only during the phase at constant inlet NH3 concentration (15003400 s) the outlet NH3 concentration equals the inlet NH3 concentration because the system has reached a steady state. On the other hand, upon decreasing the NH3 inlet concentration (3400-3800 s), the outlet NH3 concentration is higher than the inlet one since adsorbed NH3 is being released from the catalyst surface. Notably, ' ' ' ' ' ' ' ' " ' i the area between the inlet 800 and the outlet NH3 concentration curves is -~ smaller than in the rise o. 600 phase, thus indicating = that some NH3 remains ._o adsorbed on the catalytic 400 surface at the end of the ,-~ exper~ent (4500 s) ..
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upon subsequent heating of the catalyst in He + 2 0 ] % 02 (TPD exper~ent 0 1000 2000 3000 4000 5000 6000 up to 823 K at 15 K/min), Time (s) indicated by an arrow in Figure 1- NH3-CPAD experiments performed at 573 K. Dotted the figure. line: inlet NH3 concentration; circles: outlet NH3 concentration; NH3-CPAD solid line: model fit (k%= 33.87 m3/mol s, k%=2.2 10 6 l/s, E~ = experiments were 22.0 kcal/mol, a=0.256, ~ = 2 7 0 mol/m3). The arrow indicates performed at different the start of the TPD run. temperatures (in the range 573-673 K) and results similar to those shown in figure 1 were always obtained. _o
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However the initial dead time progressively decreases upon increasing the temperature, in line with an exothermic adsorption phenomenon. Furthermore, at temperatures above 630 K, the formation of N2 is also observed, due to catalytic NH3 oxidation by oxygen. No formation of nitrogen oxides is observed at any investigated temperatures. The effect of H20 on the NH3 adsorption-desorption characteristics has also been investigated. For this purpose, experiments were performed in the presence of 0.8 % v/v H20 in the feed. The results, not reported for the sake of brevity, indicated that no significant changes occur in the NH3 adsorption/desorption characteristics, in spite of the fact that H20 content is nearly 1 order of magnitude higher than that of NH3. This indicates that for the investigated H20 level water does not appreciably compete with NH3 for the adsorption on the catalyst surface. In order to gain quantitative information on the NH3 adsorption-desorption characteristics, the experimental data obtained at different temperatures have been analyzed according to a dynamic one-dimensional isothermal heterogeneous PFR model of the test reactor. On the basis of diagnostic criteria, the influence of both intraparticle catalyst gradients and external mass transfer limitations were found negligible. Under these hypotheses, the unsteady mass balance of NH3 on the catalyst surface and of NH3 and NO in the gas-phase were written [1]. The following kinetic expressions for the NH3 adsorption/desorption processes (ra and rd) and for NH3 consumption by oxidation to nitrogen (ro~) were used: ra=k~ CNH3 (1-0NH3),
rd=k~ exp (-Ed(0NH3)/RT) 0NH3,
rox= kox 0NH3
where k~ k~ and kox are the kinetic rate constant for NH3 adsorption, desorption and oxidation, respectively, Ed is the activation energy for NH3 desorption and 0NH3 is the NH3 surface coverage. A non-activated NH3 adsorption process has been considered, on the basis of preliminary results. Different dependencies of EO on 0NH3 have been used, including a Langmuir (Ed=constant) and a Temkin-type (Eo=Ed~ dependency, this latter taking into account the catalyst surface heterogeneity. When the dynamic kinetic model was fitted to the data, the Langmuir-type kinetics failed in describing the results, whereas both the NHaCPAD data and the final TPD experiment could be satisfactorily represented (solid line of Figure 1) by a Temkin-type desorption process with a value of the activation energy for desorption at zero-coverage (E~ of 22 kcal/mol. Estimates of the other kinetic parameters are given in the caption. Notably, the NH3 oxidation occurring at high temperatures (and the corresponding N2 formation) could be nicely fitted by the dynamic model as well. b) N O - C P A D - No adsorption-desorption of NO was observed in this case, thus suggesting that NO, as opposite to NH3, does not appreciably adsorb on the catalyst surface. 3.2. NO-NI-Ia dynamic surface reaction experiments (CPSR) The study of the SCR reaction under unsteady-state conditions was carded out in flowing 02 (2%) + balance He by performing" i) linear variations of the NH3 inlet concentration (0 -~ 840 ppm -~ 0) in constant NO (750 ppm) (NH3-CPSR); and ii) NO variations (0 -~ 750 ppm -~ 0) in constant NH3 (840 ppm) (NO-CPSR). a) N H j - C P S R - A typical result of a NH3-CPSR experiment is shown in Figure 2 where the NH3 (circles), NO (up triangles) and N2 (squares) outlet concentrations are shown as a function of time. The NH3 inlet concentration (dotted line) is also reported. During the rise phase, the NH3 outlet concentration shows a long dead time (about 750 s),
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and then an increase till the end of the transient phase, reached at t=1500 s. On the other hand, the outlet NO concentration shows completely different dynamics: in fact it immediately decreases upon admission of NH3, it shows a weak minimum near 750 s and then it slightly increases up to the end of the NH3 rise phase (t=1500 s). The concentration of molecular nitrogen (squares in fig. 2), formed in the reaction along with water (not reported the figure), is specular to that of NO, thus suggesting 800 that neither NO nor N2 adsorption is 700 involved in the SCR reaction. The different NH3 and NO dynamics observed during the NH3 rise phase is in v500 line with a mechanism involving the e~9 400 reaction between adsorbed NH3 and gaseous NO. As a matter of facts, NO is ca00 ID o consumed as soon as NH3 is fed to the o 200 f catalyst, thus showing that the adsorption of NH3 is a very fast process. Notably, the outlet NO concentration (and hence the 0 1000 2000 3000 4000 5000 6000 NO conversion as well) shows a complex Time (s) dependence on the NH3 inlet Figure 2 - NH3-CPSR experiment at 573 K. Dashed lines: inlet NH3 concentration; symbols: concentration, being nearly linear with the experimental data (a: ammonia, b" NO inlet NH3 concentration for low NH3 levels and showing a weak inhibiting effect at concentration); solid lines" model fit (k~ high NH3 concentrations. This l0 s l/s, E~ = 19.2 kcal/mol, 0~3=0.06, other phenomenon, which has been reported by oarameters as in fie. 1). other authors [4], has also been confirmed by steady-state experiments, and accordingly it is not related to a transient feature. It is also noted that this inhibiting effect, weak at the lowest investigated temperatures, vanishes upon increasing the reaction temperature. The data obtained during the NH3 decrease phase (t>3300 s) confirm the results obtained upon the rise phase. Indeed also in this case the NO consumption shows a weak inhibiting effect by NH3 at high N H 3 concentrations and a nearly linear dependence with the inlet NH3 concentration for low NH3 levels. To quantitatively describe the transient reactivity data, the dynamic reactor model used to fit the CPAD experiments was modified by including a term accounting for NH3 consumption by the SCR surface reaction (rNO): I
0
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: ......
| ....
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I
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0~3 rso = k so C so 0 ~ , (1 - e ) In line with the results shown in Figure 2, this kinetic expression accounts for the complex dependence of the rate of NO consumption on the NH3 surface coverage, but not for the observed weak inhibiting effect of NH3. The solid lines shown in Figure 2 represent the model fit, based on the parameter estimates reported in the caption. A good agreement between experimental data and model fit has been obtained: in particular, the initial dead time in the concentration of NH3 and the levelling off of the NO conversion during the rise phase are well represented. Notably, in the fit of the data shown in figure 2 the parameters for the NH3 adsorption-desorption dynamics obtained during the NH3-CPAD experiments (figure 1) have been used. This confirms the adequacy of the adopted model for the description of the
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transient adsorption-desorption and reaction kinetics, as well as the virtual superposition of 90o the two processes. ' ' ' ' ' ' ' ' ' ;' ~ !_. b) N O - C P S R - A t y p i c a l r e s u l t o f a 8oo NO-CPSR experiment is shown in ~" Figure 3 The NH3 and NO outlet ~. 700 600 concentrations (circles and up triangles, "o 500 respectively) are reported as a function "~ 400 ~' ' " of time, along with that of the NO inlet 300 " concentration (dotted line). The NI-'I3 200 " and NO outlet concentration curves o 100 exhibit very different features from those observed during the NH3-CPSR
~
~
0
..........
! .............
1...
1000 2000 3000 4000 5000 6000-7000
experiment shown in figure 2. Indeed in
Time (s) Figure 3 - NO-CPSR experi~nent performed at 573 K. Symbols: experimental data. a): NO, b): ammonia concentration. Dotted line: inlet NO concentration. Solid lines: model predictions, Kinetic parameters as in figure 2.
this case the NH3 consumption and the N2 production (not reported in the figure) start as soon as NO is fed to the
0
reactor and they continue to increase during all the rise phase. The curve of the NO outlet concentration does not show any dead time, thus indicating that NO does not appreciably adsorb on the catalyst surface. The curves of the different species are symmetrical, pointing out a direct dependence of the reaction rate on the gaseous concentration of NO. The kinetic model used to fit the NH3-CPAD and NH3-CPSR experiments was also used to analyze the NO-CPSR experiments on a purely predictive basis. It appears that these experiments are nicely described by using the kinetic parameter estimates obtained in previous fits (see Figure 3, T=573K). c) Effect of H20 and S02 in the feed stream - To ~100 ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' investigate the reactivity of the commercial catalyst ~v 80 .:"" -.. NO under more represemative operating conditions, NH3- and NO-CPSR experiments were performed 8 60 b ......... ::~..in the presence of H20 (0.8 and 5 % v/v) and of E 4o SO2 (500 ppm) in the feed stream. As an example, o 2o figure 4 compares the results of NH3-CPSR tO experiments performed in the absence of 1-I20 and 0SO2 (curves a), in the presence of H20 (5% v/v, ' ' ' ' ' . . . . . . . ' ' ' " .. curves b), and in the presence of H20 (5% v/v) and ~= 80 SO2 (curves c). In all cases similar dynamics are t::: observed, i.e. the presence of water and SO2 in the .o 60 feed does not modify the dynamic features of the 40 reaction. On the other hand, figure 4 points out that to water and SO2 addition to the feed stream strongly 20 tO affects the reactivity of the catalyst, since very different NO conversion levels are attained at 0 50 100 150 200 250 300 350 400 steady-state in the presence of water and water + Time (a.u.) SO2. In particular, water inhibits the SCR reaction, Figure 4 - NH3-CPSR experiment at 593 K. Dashed lines: inlet NH3 concentration; whereas the presence of SO2 enhances the catalyst curves a: NH3 in NO; curves b: NH3 in activity. Notably, the presence of SOz in the feed NO+ H20 (5%); curves c: NH3 in NO+ H20 overcomes the inhibiting effect of water, since the (5%) + SO2 (500 ppm) .,- ...................
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reactivity in the presence of H20 + 502 is higher than that measured in the absence of both species (compare curves a and c). It is also noteworthy that a strong inhibiting effect of water can be observed already for low water contents, e.g. 0.8 % v/v (data not reported). The well known inhibiting effect of water on the SCR reaction has been interpreted in different ways, e.g.: i) competition of H20 with NH3 on the adsorption on the active sites; ii) modification of the structure of the active sites (e.g. conversion of Lewis into Bronsted acid sites [5]); and iii) retention of high catalyst oxidation state [6]. As a matter of facts, the NH3-CPAD data reported in fig. 1 indicate that the presence of H20 in the feed (0.8 % v/v) does not appreciably modify the adsorption-desorption characteristics of N H3: this apparently rules out any competition effect of H20 with NH3 on the adsorption on the catalyst surface. The beneficial effect of SO2 on the catalyst activity is also well documented in the literature, and it has been associated with the strengthening of the Lewis acidity of the vanadyl sites [7], or to the formation of new acid sulfate sites (either Lewis or Bronsted) [8] close-by to the vanadyl sites, that has been suggested to favor the SCR reaction [9]. Work is currently in progress to arrive at a quantitative analysis of the effect of H20 and SO2 on the SCR reaction aiming at a better understanding of the related mechanistic implications. 4. CONCLUSIONS Our work demonstrates the potential of the CPAD/CPSR technique in evaluating kinetic and mechanistic aspects of the SCR process. In particular, it has provided a way to study the adsorption-desorption of reactants separately from their surface reaction, thus allowing separate investigation of the sequence of steps of the reaction. The data confirmed that over the investigated V205-WOa/TiO2 commercial catalyst NI-I3 is stored on the catalyst surface, and that the reaction occurs between adsorbed NH3 and gaseous or weakly adsorbed NO. The dynamic study clearly showed that H20 does not compete with NH3 in the adsorption on the surface acid sites at any surface coverage, but significantly inhibits the SCR reaction; an inhibiting effect of adsorbed NH3 on the reaction has also been pointed out. SO2, on the other hand, enhances the reactivity of the catalyst: in all cases similar dynamics are observed, i.e. the presence of H20 and SO2 in the feed does not modify the dynamic features of the reaction but affects the reactivity of the catalyst. It is worth of note that such aspects could not have been established so conclusively neither based on steady-state techniques nor on the usual transient step response methods, since by imposing a finite rate of change of the operating variables the system d ~ ~ c s can be analysed over the full range of intermediate conditions. The overall set of data could be nicely described according to a dynamic kinetic model of the SCR reaction which superimposes the reaction to the NH3 adsorption-desorption processes. A complex dependence of the rate of NO consumption on the ~"I3 surface coverage has been established, and the related mechanistic implications will be addressed. REFERENCES 1. L. Lietti, I. Nova, S. Camurri, E. Tronconi, P. Forzatti, AIChE Journal, 43 (1997) 2559. 2. E. Tronconi, A. Cavanna and P. Forzatti, Ind. Eng. Chem. Res., 37 (1998) 2341. 3. E. Tronconi, C. Orsenigo, A. Cavanna, P. Forzatti, Ind. Eng. Chem. Res, in press. 4. M. Koebel, M. Elsener, Ind. Eng. CherrL Res., 37 (1998) 327 5. G.Ramis, C.Cristiani, P.Forzatti and G. Busca, J. Catal. 124 (1990) 574 6. S.A. Selim, Ch.A.Philip and R.Mikhail, Sh. Thermochirn. Acta, 36 (1980) 287 7. G. Ramis, G. Busca and F. Bregani, Catal. Lett., 18 (1993) 299 8. J.P. Chen and R.T. Yang, J. Catal 139 (1993) 277 9. C. Orsenigo, L. Lietti, E. Tronconi, P.Forzatti and F. Bregani, Ind. Eng. Chem. Res., 37 (1998) 2350
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Bifunctional Nature of Reduction of NO~
SnO2/Y-Al203Catalysts in the Selective
A. Yezerets*, Y. Zheng*, P.W. Park #, M.C. Kung* and H.H. Kung*. * Center for Catalysis and Surface Science, Northwestern Univ., Evanston, I1 60208, USA. # Current address: Caterpillar Inc., Peoria, IL, USA. ABSTRACT In lean NOx reduction by C3H6 over SnO2/A1203, both SnO2 and A1203 participate in the reaction. C3H6 is activated on SnO2 active sites to form oxygenated intermediates such as acrolein and acetaldehyde. The oxygenates subsequently react with NOx on A1203 to yield N2. 1. INTRODUCTION The push for more fuel-efficient vehicles and the increasingly stringent environmental regulations have spurred intensive research in lean NOx catalysis. Lean NOx reduction is challenging because the selective reduction process (Eq. 1) has to compete with the combustion reaction (Eq.2). 2C3H6 + ((12+y)/x)NOx ~((12+y)/2x) N2 +yCO2+(6-y)CO+6H20 C3H6 + 3(1+x)/202 ~ 3COx+ 3H20
(1) (2)
The selective reduction ofNOx (Eq.1) is a multi-step process involving the activation of the hydrocarbon and its subsequent reaction with NOx to yield N2. It has been proposed that the activated hydrocarbon may be an oxygenated one [ 1,2]. Hamada et al., have demonstrated that oxygenates, like alcohol, are very effective reductants of NOx over inert oxides such as A1203 [2]. SnO2/A1203 is one of the most active and stable lean NOx catalyst reported in the literature [3]. Extensive characterization shows that both amorphous and crystalline SnO 2 co-exist on the A1203 support [4]. The size distribution of the amorphous oxo-tin clusters is very broad and the ratio of the amorphous and crystalline SnO2 changes with Sn loading. Surprisingly, the maximum NOx conversions over a large range ofSn loadings (1-10 wt.%) are very similar [4]. Since for most lean NOx catalysts, the maximum NOx conversion occurs when hydrocarbon consumption is near completion, its value obtained under similar reaction conditions reflects how effective an active site is in promoting the selective NOx reduction. Thus, it appears that the ability to promote selective NOx conversions over the combustion reaction is not sensitive to the dispersion of SnO2. This is in contrast to many other A1203- supported lean NOx catalysts, where the best performance occurs at relatively low metal loadings of 2 wt.% or less [5,6]. The objective of this study is to understand this unusual feature of SnO2/A1203 catalyst via careful delineation of the roles of SnO2 and A1203 in the lean NOx reduction process.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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2. EXPERIMENTAL
2.1. Catalyst Preparation The supported SnO2 catalysts were prepared by incipient wetness impregnation with an ethanolic solution of SnC12 on y-A120313] or SIO217], and calcined in air at 800~ for 2h. XRD showed that on the both supports SnO2 exists in rutile form. The catalysts are labeled according to the nominal Sn loading and support. Thus, Sn5/A1203 is a catalyst with 5wt% Sn loading, supported on A1203. 2.2. Catalytic Reaction The catalysts were tested in a flow of 200 cc/min gas feed composed of 15% 02, 10 % H20, 0.11% C3H6,0.1% NO and balance He. Two designs of the reactor assembly were used: 1. single-bed configuration with the option to reverse flow direction through the reactor. 2. double-bed: two identical reactors were placed in series so that the feed can pass through one or both of the reactors. The temperature of each reactor was controlled separately. The void space of the fused silica microreactor was packed with quartz chips to minimize gas phase reactions. Gas phase reaction is greatly promoted by the presence of 0.1% NO and the extent of gas phase reaction in an empty reactor as a function of temperatures, detected as C3H6 conversions, are as follows: 1% at 525~ 6% at 550~ and 40% at 575~ No N2 production was observed accompanying gas phase reaction of C3H6. The reaction products were analyzed using a HP 6890 gas chromatograph equipped with two parallel columns packed with Haysep Q and molecular sieve 5A. The exit gas from the former column was analyzed with a HP5973 massselective detector. NO, concentration was measured using a Beckman 951 NOx analyzer.
3. RESULTS AND DISCUSSION 3.1 Evidence of Bifunctional nature of SnO2/Al203 Figure 1 compares the NOx conversions over 0.04 g of Sn5/AI203 (curve a) and 0.2 g of Snl/A1203 (curve b). Although the same weight of SnO2 was used for both experiments, the latter catalyst was much more effective in NOx reduction. This is unexpected, as the ability to promote
60
%
Figure 1. Conversion of NOx b
50-
C
4030-
d
20-
a
100 400
,
450
,
500
~.
#
#
e
550 600 Temperature,~
% 80 70 60 5O 40 30 20 10 0 400
Figure 2. Conversion of C3H6
450
500
550 600 Temperature,~
Figures 1 and 2: Catalytic Performance of a) 0.04 g Sn5/A1203, b) 0.2 g Snl/A1203, c)physical mixture of 0.04 g Sn5/A1203 + 0.16g A1203, d) 0.16 g A1203, and e) no catalyst.
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selective NOx conversions over the combustion reaction of supported SnO2 appears to be independent of its dispersion [4]. However, the performance of 0.04 g Sn5/A1203 can be elevated almost to the level of0.2g Snl/A1203 by simply mixing it with 0.16g A1203 (curve c). The NOx conversions of the physical mixture of 0.04 g Sn5/A1203 and 0.16 g A1203 were substantially higher than the sum of the conversions of the individual components (curves a and d). Figure 2 shows the corresponding C3H 6 conversions over these catalysts. Supported SnO2 readily promotes the activation of C3H6as significant C3H 6 conversions were observed using only 0.04 g Sn5/A1203 (space velocity of 160,000 h-l). It is interesting to note that the C3H6conversion over A1203 at 575~ was lower than the gas phase reaction. This is because gas phase reactions are free radical reactions and the presence of catalysts effectively arrests the chain propagation. The importance of A1203 for NOx reduction is further demonstrated when the NOx and C3H 6 conversions were compared over Sn5/A1203 and Sn5/SiO2 (Table I). NOx conversions were observed only for the Sn5/A1203 catalysts, although C3H 6 was effectively converted on the Sn5/SiO2 catalyst as well. Table I. Comparison of NOx and c 3 n 6 conversions (%) over 0.2g Sn5/A1203 and Sn5/SiO2 Catalyst
NOx Conversion
NOx Conversion
C3H 6 Conversion
C3H 6 Conversion
Sn5/A1203
54 (450~
52 (450~
48 (525~
88 (525oc)
Sn5/SiO2
0.8 (450~
16 (450~
3 (525oc)
92 (525~
The above results indicate that the activation of C3H 6 occurs on the SnO 2 site and the production of N2 occurs on A1203 sites. To ensure that the synergistic effect observed for the physical mixture (Fig.l) was not due to migration of SnO2 from Sn5/A1203 to A1203, an experiment was performed in which 0.1 g Sn5/SiO/was physically separated from 0.1 g A1203by a layer of quartz wool (Fig 3). The flow direction of the reaction feed was controlled such that it could pass from the layer of Sn5/SiO2 to A1203 or vice versa In the absence of any synergistic Fig. 3a:NO x Conversions
30 25
Figure 3b: C3H6 Conversions
60 o 50
40
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9 Sn5/SiOa first
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420
440 460 480 Terroerature (~
500
420
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440 460 480 500 Temperature (~
520
Figure 3" NOx and C3H 6 conversions over physically separated Sn5/SiO2 and A1203:" a) Flow direction from Sn5/SiO2 to A1203; b) feed from A1203 to Sn5/SiO2.
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effect between the two beds, the overall conversions should be similar regardless of the direction of the flow. This was seen for the C3H6 conversion (Fig.3b). However, significantly higher N2 yield was observed with the feed flowing from Sn5/SiO2 to A1203 catalyst than when the flow was reversed (Fig.3a). That a synergistic effect was observed even when the beds were physically separated suggests that some stable intermediates are generated on the SnO2 site and these intermediates can react over A1203 to form N2.
3.2 Cause of Synergistic Effect To uncouple the catalytic characteristics of SnO2 and A1203, a two bed configuration was employed and the compositions of the exit gas from different catalysts were tabulated in Table II. Significant C3H6conversions was observed over 0.1 g Sn5/SiO2 or 0.04 g Sn5/A1203, but not over 0.1 g A1203. C3H6conversion was stable with-time-on stream over 0.04g Sn5/A1203. The catalyst was white after reaction and within experimental uncertainties the carbon appeared balanced. However, C3H6 conversion over Sn5/SiO2 decreased slowly with time. The decrease was faster for C3H6 conversion and CO2 production than for the generation of acetaldehyde and acrolein. Deactivation of Sn5/SiO2 may in part be due to coking as the catalyst was grey after reaction and there was a carbon imbalance which decreased with time-on-stream. Trace amounts of products are not listed in Table II and these included acetonitrile, propenenitrile, nitromethane and acetone. Table II. GC-MS analysis of exit gas from reactors containing 0.1 g Sn5/SiO2 or 0.04 g Sn5/A1203 at 500~ and 0.1 g A1203 at 475~ or combination of these two. Product Concentrations in ppm C2H4 C3H6 Ac a
% Conv.
Acl b
HCN
N2
C3H6
NO
42
120
0
0
43
0
894
27
89
0
0
23
0
8
809
29
2
22
85
30
17
0
1
1122
2
0
0
19
3
4
497
434
35
699
0
0
0
106
40
21
Sn5/SiO2 (50 h ) +A1203 c
264
531
12
843
4
0
0
85
27
17
Sn5/A1203 + A1203c
463
690
10
731
2
0
0
180
37
36
Catalysts
CO 2
CO
Sn5/Si02 (flesh)
421
0
26
662
Sn5/Si02 (after 50 h)
134
0
14
Sn5/A1203
402
391
A1203
20
Sn5/SiO2 (flesh) +A1203 c
a. acetaldehyde; b. acrolein: c. supported SnO2 was upstream of A1203 and in separate reactors The distributions of carbon containing products differed significantly for Sn5/SiO2 and Sn5/A1203. CO2 was the only combustion product for Sn5/SiO2, whereas comparable amounts of CO and CO2 were produced over Sn5/A1203. Besides CO2, acrolein was the major product detected over Sn5/SiO2. However, it was not detected over Sn5/A1203. On the other hand, HCN was detected from Sn5/AI203 but not from Sn5/SiO2.
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The absence of acrolein in the product stream from Sn5/A1203 catalysts might be due to its facile reaction over A1203. Indeed, when the product stream from the Sn5/SiO2 catalysts was passed through a second reactor containing only A1203, the exit gas no longer contained acrolein, This ease of reaction of acrolein is in agreement with reports that oxygenated compounds usually react at significantly lower temperatures than alkene [2] and proposals that oxygenated hydrocarbons are important intermediates in the pathway towards N2 production [1]. The observation of N2 yield on Sn5/A1203 and not on Sn5/SiO2 is consistent with the proposal that acrolein is able to react with NOx to produce N2 on A1203. Furthermore, with time-on -stream over Sn5/SiO2, although C3H 6 conversion declined by as much as 47%, acrolein production only dropped by 26% and concomitantly N2 yield over Sn5/SiO2 + A1203 combination only decreased by 20% (Table II). Acrolein reaction with NOx on A1203 appeared to result in HCN formation, as the latter was detected in the product stream of 0.04 g Sn5/AI/O3 but not Sn5/SiO2. In the 2-bed configuration of Sn5/SiO2 and A1203, HCN was detected when a reduced amount of AlzO3 (0.01 g) was used, but was absent with 0.1 g A1203 present in the second bed. Since the TCD sensitivity factor of HCN is not known at present, its concentration was estimated by assuming a sensitivity factor identical to water. It is possible that HCN reacts with NOx on A1203 to form N2, as higher NOx conversion was observed with 0.1 g than 0.01 g of A1203. However, this reaction can only be confirmed with further experiments using isotope labeling. Baiker, et al. [9] have proposed formation ofallyl oxime from propene, and that trans-elimination reaction of the oxime can result in the formation of acetaldehyde and HCN. HCN has also been observed in the exit gas of some lean NOx catalysts such as Cu-ZSM-5 and Cu/A1203 [9,10] and thus may play an important role in the selective reduction of NOx by hydrocarbon. The other major product from the reaction ofC3H 6 on supported SnO2 is acetaldehyde. As discussed, acetaldehyde can also be produced from allyl oxime on A1203, and at the same time, being an oxygenate, it can readily react with NOx or 02. That acetaldehyde was being produced and consumed at the same time when the exit gas from supported SnO2 was passed over a bed of A1203 was demonstrated by changing the temperature of A1203. When A1203 was at 475 ~ the acetaldehyde produced on the SnOz site was consumed over A1203, but at 400~ the acetaldehyde concentration at the exit of the A1203 bed was higher than at the inlet. Thus, a number of potential reaction intermediates were detected in these experiments. They include various oxygenates such as acrolein and acetaldehyde, as well as HCN. All of these have been shown to react on A1203 and with their consumption, concomitant increases in the production of N2 was observed. Additional experiments will be performed to further identify their detailed role in the NOx reduction process. For many bifunctional catalytic systems, enhanced N 2 yield is achieved by using one component to generate NO2 and a second component to facilitate the reaction of N O / w i t h hydrocarbon to produce N2 [8]. In this study, the contribution of NO2 to the observed synergistic effect was minor since its concentration measured after Sn5/SiO2 was the same as the background level and that measure aider Sn5/A1203 was only between 20 and 30 ppm above that.
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4.
CONCLUSIONS
Figure 4 shows a possible reaction scheme for the selective reduction of NOx over SnO2/A1203 catalyst. The detected molecules are depicted in bold. This is a scheme that applies to the observed synergistic effect in experiments where the supported SnO2 and A1203 are separated. However, it may not be a complete picture as it is not understood why in the doublebed configuration, NOx conversions are higher for the SnO2/A1203 + A1203 configuration than the SnO2/SiO2 + A1203 one. However, the presence of synergistic effect is very promising. It suggests that tremendous flexibility in the design of lean NOx catalysts can be introduced by separating the hydrocarbon activating function from the N2 forming function. The optimization of the former function may be achieved by designing a good partial oxidation catalyst.
CH3_CHO NOx 02, NO~ ~ CH2=CH-CHO ...................> CH2=CH-CH=N-OH---->
C3H~
'~
A1203
SnO2 L
HCN
>N~
AI203
NOx CH3_CHO
'~
A1203
Figure 4. Proposed reaction scheme for the selective reduction of NOx over SnO2/AI203. ACKNOWLEDGMENT This work was supported by the Department of Energy, Basic Energy Sciences. Y. Zheng acknowledged partial support from the EMSI program of the National Science Foundation and the Department of Energy [CHE-9810378] at the Northwestern University Institute for Environmental Catalysis. REFERENCES AND .
2. .
4. 5. 6. 7. .
9. 10.
NOTES
T. Truex, Autocatalyst News, No 8 (1991) ( Johnson-Matthey Plc, Royston, U.K.) H. Hamada, Y. Kintaichi, T. Yoshinari, M. Tabata, M. Sasaki, and T. Ito, Catal. Today, 17,111 (1993). M.C. Kung, P.W. Park, D.-W. Kim, and H. H. Kung, J. Catal., 181, 1 (1999). P. W. Park, H. H. Kung, D.-W. Kim, and M. C. Kung, J. Catal., 184, 440 (1999). J.Yang, M.C. Kung, W.M.H. Sachtler, and H. H. Kung, J. Catal., 172, 178 (1997). K. A. Bethke, and H. H. Kung, J. Catal., 172, 93 (1997). Davidson-62 silica (250 m2/g) was washed with 4MHNO3. dried at 120~ and then calcined in air at 450~ for 12h. C. Yokoyama, and M. Misono, Catal. Lett., 29,1 (1994). F. Radtke, R.A. Koeppel, and A. Baiker, J. Chem. Soc. Chem. Commun., 427 (1995). F. Radtke, R.A. Koeppel, E. G. Minardi, and A. Baiker, J. Catal., 167, 127 (1997).
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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
SO2 resistant Fe/ZSM-5 catalysts for the conversion of nitrogen oxides G. Centi, G. Grasso, F. Vazzana and F. Arena Dip. Chim. Ind. ed Ing. Materiali, Univ. Messina, Italy. Salita Sperone 31, 98166 Messina, Italy. Phone: +39-090-393134, fax: +39-090-391518, e-mail:
[email protected] Fe/ZSM-5 catalysts prepared by CVD (chemical vapor deposition) show a stable activity in the selective catalytic reduction of N20 and NO with propane in the off-gas from chemical processes. These catalysts show a better propane economy and a considerable higher resistance to deactivation by SO2 especially at lower reaction temperatures than using Fe/ZSM-5 catalysts prepared by ion-exchange or impregnation, especially when the parent zeolite is pretreated to create structural defects. The peculiar activity and stability characteristics of these catalysts are suggested to be related to the presence of iron-oxide nanocluster, whereas with other methods highly clustered Fe 3+ species and Fe203 particles also form.
Introduction The requested reduction of greenhouse gas emissions (Osaka agreement) will lead soon to new regulation limits on the emission of N20, a powerful greenhouse. Therefore, efficient technologies for its removal, in particular in the off-gas of the production and use of nitric acid and the industrial combustion of waste, must be developed. [ 1]. Current catalysts are not stable and active enough, and the process cannot be applied economically, when N20 is present in a diluted concentration (typically below 0.1%) and in the presence of H20, 02 and poisoning agents such as SO2 and NOx. Recent data, however, have shown that Fe/MFI catalysts may be successfully applied for the reduction of nitrogen oxides (N20 and NO) in such a type of conditions, when an hydrocarbon is cofeed [1-6].
Experimental Fe/ZSM-5 catalysts were prepared by impregnation (FeIMP/ZSM-5), ion-exchange (FelE/ZSM-5) and CVD (Chemical Vapour Deposition) using a parent ZSM-5 zeolite unpretreated or pretreated to create structural defects in the zeolite (FecvD.UNP/ZSM-5 and FecvDp/ZSM-5, respectively) are studied. The parent zeolite for the preparations was the Na-form of a commercial ZSM-5 sample synthesized with a template-free method (SN27 from ALSIPenta, SIO2/A1203 = 27). The zeolite pretreatment prior to Fe addition (FecvD_p/ZSM-5) was a hydrothermal treatment (6h at 650~ in a flow of N2 containing 3% steam) followed by washing with ammonium acetate aqueous solution to remove extra-lattice aluminium. 3+ The impregnation was made using incipient wet impregnation method and Fe -nitrate as the salt. The ion-exchange method was made using an 0.02 N aqueous solution of ironammonium-sulphate heated to 80~ The CVD method was made in a N2 flow after anhydrification of the zeolite and using FeCl3 as the reactant. Further details on the preparation were reported previously [6]. The catalytic behavior was studied in a flow reactor apparatus with on-line mass quadru-
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Table I Characteristics of the Fe/ZSM-5 samples used for the catalytic tests. (wt.) ~
Si/Al molar ratio I
Surface area, me/g
Crystallinity, % (from IR2; ._.+10)
phases 3
FelMv/ZSM-5
2.27
27
286
100
MFI
FeIE/ZSM-5
3.75
27
237
100
MFI
Fecvo.tmv/ZSM-5
0.81
27
362
100
MFI
35
306
90
Sample
%Fe
XRD
MFI 0.63 FecvD.v/ZSM-5 1 determined from atomic adsorption (AA) spectroscopy 2 crystallinity of the zeolite estimated from the intensity ratio of the bands at 450 and 550 cm~ in the infrared (IR) spectrum with respect to the parent Na-ZSM-5 zeolite. 3 No phases associated to iron were detected by X-ray diffraction (XRD). pole analysis. Tests were made using 0.2g of zeolite in the form of pellets with 40-60 mesh of dimension and a gas hourly space-velocity (GHSV) of 23.000 h 1. The typical gas composition for the tests was 0.05% N20, 2% 02, 3% H20, 0.1% C3H8 and the remaining helium. The same composition, but in the presence of also 0.05% SO2, was used for the tests of durability. The analysis of the composition of the inlet and outlet gas streams of the reactor was made by an on-line mass quadrupole apparatus, after correction of the mass intensities to consider multiple fragmentations, when necessary. The line from the reactor to the mass quadrupole was heated at 150~ to prevent the condensation of the products. The conversion of the reactants (N20 and C3H8) was estimated on a molar basis. Samples were characterized by AA, IR, XRD, UV-Visible-diffuse reflectance spectroscopy, temperature programmed reduction and electron microscopy analysis (SEM-EDX). Results and Discussion
The catalysts characteristics are summarized in Table 1. Three methods of loading the iron were used, by (1) incipient wet impregnation (FelMp/ZSM-5) using iron-nitrate, (2) ionexchange (FelE/ZSM-5) using a very soluble salt (iron-ammonium-sulphate), and (3) chemical vapour deposition with FeC13 on the anhydrified zeolite. In the latter case, either the zeolite as such in the sodium form (FecvD-LrNp/ZSM-5) or the zeolite partially dealuminated by hydrothermal treatment (FecvD.p/ZSM-5) was used. No residual C1 in the samples prepared by CVD was detected after calcination. In all cases, XRD show a good crystallinity and the absence of crystalline phases other than the zeolite itself (MFI structure). IR analysis in the region around 500 cm l confirms that, within the experimental error, a loss of crystallinity does not occurs, apart a slight decrease for FecvD_p/ZSM-5. All the samples show a larger surface area than the parent zeolite in the sodium form (196 m2/g), probably due to the removal of some clustered sodium. The analysis of the pore distribution in the samples do not shows significant changes in the micro-meso porosity distribution, in particular in the samples prepared by CVD. Electron microscopy with electron dispersion X-ray (EDX) analyses confirm the absence of modification in the morphology of zeolite crystallites (mean dimension 1-2 lam) and of segregation of iron out of the zeolite crystallites, the latter apart for the sample prepared by impregnation. Activity in N20 reduction. The activity curve for N20 reduction to Nx of the various Fe/ZSM5 catalysts is reported in Figure 1. All samples show a good activity in the selective reduction of N20 to N2 with conversion above 80% at low reaction temperatures (below 400~ not-
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80
Z
60
~
4o
g
8
-41-- FezE/ZSM-5 - O - Fecvo.,JZSM-5 - - ~ - FeMJZSM-5
2O
Fecvo.uNP/ZSM-5
0
200
~
,
300
i
,
i
,
|
500
400
Reaction temperature, *C
100
80 td
"1" e,)
o c ._o r
> t-
60
40
O
o
o 200
300
~
~
~
Fecvo.uNP/ZSM,
400
5
500
Reaction temperature, ~
Fig. 1 Catalytic behavior of Fe/ZSM-5 catalysts in the reduction of N20 with propane/O2 in the presence of steam in the feed. Fig. 2 Turnover frequency at 350~ of of Fe/ZSM-5 catalysts in N20 and C3H8 depletion.
I--.~o II r~C3H8 < o o
z~ 14.
o
I-
II CVD-P Fex/ZSM-5
IMP
CVD-UNP
9
withstanding the low concentration of propane in the feed (1000 ppm). All the samples show also a comparable activity, apart from Fecvo.tmv/ZSM-5 slightly less active. The activity in N20 conversion does not parallel that in propane conversion, where two group of samples can be identified: (i) samples prepared by ion-exchange or impregnation and (ii) samples prepared by CVD. The latter show a lower activity in propane conversion. Based on a first order rate equation that was found to satisfactory model the kinetic of the reaction and a plugflow reactor model, it is possible to estimate the turnover frequency (TOF) assuming that all iron ions are available for the catalytic activity. The resuits for a reaction temperature of 350~ are shown in Figure 2. The sample prepared by CVD on the pretreated zeolite (FEcvD_p/ZSM5) shows the highest specific activity in N20 reduction. Furthermore, differently from the other samples, the rate of N20 depletion is higher than the rate of propane depletion. This indicates that on this catalyst it is possible to use very low amounts of propane as selective reductant, without affecting the rate of nitrogen oxides selective reduction, an important effect in terms of process economics. Durability in the presence of SO_2. The Fe/ZSM-5 catalysts prepared in the different modes showed a different durability in the presence of SO2 especially at temperatures below about 450~ which are the more interesting for practical applications. Reported in Figure 3 is the comparison of FelMP/ ZSM-5 and FecvD.p/ZSM-5 durability in accelerate tests using a high SO2 concentration in the feed (500 ppm). FeIMP/ZSM-5 catalyst shows a very
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638
=~
F%
80
G) C
|~
u~
60
._
U
400"C
o
,",
60 ~
u"~
o
~
c
o
40
433~ (+ 430 ppm NO)
"~
>e
,
~8
>
9 FeuJZSM5 (NzO) O Fecvo.p/ZSM-5(N20) 9 Fec~JZSM5 (NO)
20
0
i
0
200
,
,
.
,
,
400
800
600
Time on stream, h
Fig. 3 Comparion of the conversion of N20 of FeIMp/ZSM and Fe CVD-P/ZSM-5 during durability tests in the presence of a high SO2 conc. (500 ppm). Other conditions as indicated in the experimental section. In the case of Fe CVD-P/ZSM-5 aider 600h of time on stream 430 ppm NO were also added to the feed. 120
t 100~___
9 Conv.N20 O Conv.C3H8
|
fast deactivation at 380~ Increasing the reaction temperature to 410~ the rate of deactivation decreases, but still a large deactivation could be observed in about 100 h of time on stream. Only at much higher reaction temperatures (above 480~ a nearly constant activity could be observed for over 500 h of time on stream. A different behavior was detected for FecvD.p/ZSM5. In this case, even at low temperature (400~ a constant catalytic behavior is observed after an initial minor decay of the activity occurring approximately during the first 100 h of time on stream. Then a constant activity was observed up to 600 h of time on stream. Increasing the reaction temperature to about 430~ a constant conversion of N20 of over 80% is observed.
A temperature dependence of the durability behavior in the presence of SO2 was observed also for .o_ 60 500"C FecvD.tmp/ZSM-5 and FeIE/ZSM-5. T= 480 *C c The behavior of the latter which is o 0 40 T= 450 *C similar to that of FecvD_UNP/ZSM-5 is reported in Figure 4. At 500~ a 20 stable catalytic activity is observed, both in N20 and propane conver0 20 40 60 80 100 sion. Decreasing the reaction temTime, h perature to 450~ a progressive decrease of the activity is detected. Fig. 4 Durability tests in the presence of 802 (500 ppm) at A stable behavior requires a reacdifferent reaction temperature of Fe~E/ZSM-5. Other tion temperature higher than about conditions as in Fig. 3. 470~ Therefore, although these two samples show a lower deactivation rate by 8 0 2 than FelMP/ZSM-5, they have a stable activity at temperature only above about 470~ Only FecvD.p/ZSM-5 show a stable activity in the presence of SO2 at reaction temperatures lower than 450~ ~
8o
0
,
,
,
,
,
,
i
,
,
i
,
,
,
,
,
,
,
,
,
l
l
,
,
,
Conversion of both N20 and NO. The simultaneous conversion of NO and N20 was tested in the aged FecvD.p/ZSM-5 sample by adding 430 ppm NO to the feed (Figure 3). Although activity of the catalyst in converting NO is worsen with respect to N20 (at 430~ about 46% with respect to over 80% conversion of N20), a stable activity in converting NO in the presence of 500 ppm SO2, H20 and 02 is also shown, indicating the applicability of these catalysts for the simultaneous conversion of N20 and NO in industrial emissions.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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8
,
,
Nature of the iron species. Figure 5 show UV-Visible 2 - - - - FemlZSM-5 after cat tests diffuse reflectance (UV-Viscalc 6 / "~'": 3 ....... Fecvo-,JZSM-5 DR) spectra (normalized to L/ .:'~. 4-Fecvo..*/ZSM'5 after cat tests ~" " ~, 5 .... Fe,Mp/ZSM-5 calc the same Fe content) of ~" 6 ~ Fecvo-u. #ZSM'5 calc Fe/ZSM-5 samples before "10 ' "5 '~ | . ,~ .N_ 4 ' . and after the catalytic tests. The spectra are characterized by two intense bands at about 210 and 260 nm due to ligand to metal Fe 3§ charge L . . . . ~.~.~.~..;.~ ~ ~ I transfer in isolated species in o [ . . . . . . . , ~ ' " = ....... ~ " ~ ........... distorted octahedral coordi200 400 800 800 nation [7]. These bands are not specific of a type of iron Fig. 5 UV-Visible diffuse reflectance spectra (normalized to the species and are detected in a same iron content) of Fe/ZSM-5 samples before or after catanumber of compounds, such 3+ lytic tests in N20 reduction with propane/O2 (absence of SO2). as Fe 2§ or Fe-sulphate, Fe 3 + -hydroxide or oxide, etc. 1000 The latter two compounds show in addition a band at 2 ,ecv. S.-s !i about 380 nm which shifts 3 ~ Fecvo.u~p/ZSM-5 750 4 Fe,E/ZSM-5 ! i. to higher frequencies (330 nm) in nanosized crystallites [7]. A broad diffuse absorption between 400-600 nm with a maximum at about 250 550 nm could be also detected upon partial reduction, due to intervalence 0 ' ' charge transfer. 200 300 400 500 600 700 800 The spectra after calcinaTemperature, *C tion (Figure 5) show all a Fig. 6 H2-TPR (temperature programmed reduction) curves (norbroad absorption in the 300malized to the same iron content) for calcined Fe/ZSM-5 sam600 nm region, apart from ples. 80 mg sample, 6% HE in He, 20~ FecvP_p/ZSM-5 for which only a shoulder centred at near 340 nm could be detected. In the other samples, the broad tail can be roughly deconvoluted in two peaks centred at about 380 and 550 nm. This suggests that while in FecvP.p/ZSM-5 small clustered Fe 3§ in (hydr)oxide-type species is present, larger, partially reduced iron-oxide species are present in the other samples. Note, however, that the intervalence band indicates the presence of Fe 2+ ions in an Fe3+-oxide matrix and not the presence of reduced iron-oxide species. In samples prepared by impregnation and ionexchange the 260 band is more intense than the 210 nm band, differently from CVD sample, indicating a different coordination of iron ions, in agreement with the presence of iron-oxide nanoparticles in the latter sample. After catalytic tests, the spectra remain nearly unchanged (for clarity, only spectra of FeIMP/ZSM-5 and FecvP.p/ZSM-5 after catalytic tests have been reported in Figure 5), although in some cases spectra may result apparently more intense (compare, for example, spectra 3 and 4) due to a darkening of the sample related to trapped 1 ~
9
. . . . .
nm
0 r-
|
FemlZSM-5 calc
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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electrons. The same effect prevented the analysis of the samples after durability tests. Figure 6 show hydrogen temperature programmed reduction (H2-TPR) experiments. The curves have been normalized to the same iron content. The sample prepared by impregnation shows the typical H2-TPR behavior of supported iron-oxide [3]. The two peaks correspond to the reduction of Fe 3+ to Fe2+ (max. at 373~ and Fe 3§ to Fe ~ (max. at 534 ~ In the sample prepared by ion exchange the start of the reduction shifts to higher temperatures, with two reduction peaks, the first of which is the more intense and the second corresponds to the higher temperature peak of Fe~Mp/ZSM-5. The total area of the two peaks roughly corresponding to that of FeIMP/ZSM-5. The change in the H2-TPR curve indicates the presence of smaller ironoxide crystallites (in agreement with UV-Vis-DR spectra; Fig. 5) reasonably inside the zeolite crystallites, as confirmed by SEM-EDX analysis, and differently from FeIMP/ZSM-5 where part of iron is also outside the zeolite crystals. In the samples prepared by CVD on the unpretreated zeolite the lower temperature peak is also absent, but the reduction occurs only at high temperature, in coincidence with the higher temperature peak of FelMP/ZSM-5. The area of the curve is around 40% of that of the previous two samples, and being the H2-TPR curves in Figure 6 normalized to the same iron content, this indicates that part of the iron could be not reduced. The sample prepared on the pretreated (partially dealuminated) zeolite shows a lowering of about 100~ in the maximum of the reduction curve. The area of the reduction curve also decreases, indicating a further lowering of the fraction of reducible iron species. Relationship between nature of iron species and catalytic behavior. Figure 2 shown that all Fe/ZSM-5 samples, apart from FecvD.p/ZSM-5 show a higher rate of propane depletion than that of N20. This indicates that two parallel pathways of propane conversion are present, one which leads to the selective reduction of N20 forming N2 + CO2 as the final products and a second of direct oxidation of propane to CO2. The comparison of these data with the UV-VisDR spectra (Figure 5) suggests that the species responsible of the direct propane oxidation and of the lowering of TOF (Figure 2) is the iron-oxide species having larger crystal dimensions (UV-Vis-DR band above 350 nm) and higher temperature of reduction (580~ The more selective iron species shows a similar TPR curve (a single TPR peak centred at about 480~ of the dinuclear oxo-hydroxo iron species suggested by Sachtler et al. [3,4] to be the active species in Fe/ZSM-5. UV-Vis-DR spectrum and the role of zeolite pretreatment in the formation of this iron species, however, suggests that a small nanosized iron-oxide is a preferable interpretation [8]. Iron ions during CVD treatment probably initially react with the zeolite defects created during pretreatment (or by the HC1 generated during CVD method itself) forming pseudo-framework ions which probably are not active (a pure iron-silicalite was found inactive in the reaction), but acts as the nucleation center for the iron-oxide nanocrystals. This species also shows a higher resistance to deactivation by SO2 during durability tests, especially at reaction temperatures below 450~ (Figures 3 and 4), and is active in both N20 and NO conversion (Figure 3). [1] [2] [3] [4] [5] [6] [7] [8]
G. Centi, S. Perathoner, F. Vazzana, CHEMTECH, 29(12) (1999) 48. M. Krgel, V.H. Sandoval, W. Schwieger, A. Tissler, T. Turek, Catal. Lett., 51 (1998) 23. H.Y.Chen, W.M.H. Sachtler, Catal. Today, 42 (1998) 73. T.V.Voskoboinikov, H.Y. Chen, W.M.H. Sachtler, Appl. Catal. B, 19 (1998) 279. W.K.Hall, X. Feng, J. Dumesic, R. Watwe, Catal. Lett., 52 (1998) 13. F. Vazzana, G. Centi, Catal. Today, 53 (1999) 683. S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola, G. Vlaic, J. Catal., 158 (1996) 486. R.W. Joyner, M. Stockenhuber, Catal. Lett., 45 (1997) 15.
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Studies in Surface Science and Catalysis 130
A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
化
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M E C H A N I S T I C STUDIICS O F T H E N O x
R E D U C T I O N BY H Y D R O C A R B O N IN O X I D A T I V E
ATMOSPm~RE
St6phanie Schneider, Sandrine Ringler, Paule Girard, Gilbert Maire, Frangois Garin, Dominique Bazin * LERCSI, UMR 7515, ECPM-ULP, 25, rue Becquerel, 67087 Strasbourg Cedex 2, France Tel : 33 (0)3 88 13 69 44, Fax : 33 (0)3 88 13 69 68, e-mail:
[email protected] *LURE, Centre Universitaire Paris-Sud, Bat 209 D, 91405 Orsay France This paper gathers an ensemble of experiments realized on platinum-based catalysts for the reaction of reduction of NOx by hydrocarbons in the presence of an excess of oxygen. We have analyzed the influence of two parameters on the reactivity : the metal loading of the catalyst and the acidity of the support. Through the combined used of labeled compounds and characterization techniques, we were able to explain differences observed in the various mechanisms.
1. INTRODUCTION The removal of NOx from exhaust gases has been widely studied since several years. The catalytic post-combustion treatment represents the only available solution to diminish the NOx emissions [1,2]. Despite the numerous catalysts tested, none satisfies fully the required conditions in the selective catalytic reduction (SCR) of NOx by hydrocarbons. In order to improve their compositions, it is necessary to understand the reactional mechanisms which govern the SCR by hydrocarbons. Several propositions have been made [3-6] but it appears that the mechanism(s) are not fully understood. Indeed, the various pathways implicated in the reaction make it very difficult to understand. Platinum-based catalysts represent one of the most promising materials to achieve the reduction of NOx by propene and propane in the presence of an excess of oxygen [6]. To bring some new responses on the mechanism(s), we have engaged a comparative study on a set of platinum-based catalysts deposited either on alumina or on zeolites and used labeled compounds such as 15NO and 1802 in order to follow potential intermediates. Among the work performed with the platinum catalysts on the various SCR mechanisms, still remain several aspects concerning the influence of i) metal loading, i.e. particle size and shape and ii) support acidity; for such reactions.
2. EXPERIMENTAL PROCEDURE 2.1. Catalysts used: preparation and principal characteristics The studied catalysts are composed of platinum deposited either on alumina or on zeolites. They are listed in Table 1.
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2.1.1. Alumina- based catalysts: The preparation method was described earlier in a previous paper [7]. What is important to note is the difference in the heating mode during the reductive treatment between the two 0.2wt% Pt/A1203 : one was heated following a conventional manner with a Joule effect heating (0.2%AL), the other one was heated by the use of microwaves (0.2%ALMO).
2.1.2. Zeolites based catalysts: The catalysts were prepared by ionic exchange of the zeolite with a solution of hexachloroplatinic acid complexed with NH3. The catalyst is calcined under air at 450~ for 2 hours before the experiments. Table 1 : Catalyst description: platinum percentage, support used, specifications in function of their coded name. Specifications Coded name Support Ptwt% I%AL H2 reductive treatment-Joule effect heating 1 )' A1203 H2 reductive treatment-Joule effect heating 0.2%AL 0.2 ]I A1203 0.2%ALMO H2 reductive treatment-MICROWAVE heating 0.2 7 A1203 EMT 02 oxidative treatment NaEMT 0.5 0.5 H-ZSM5 02 oxidative treatment ZSM
2.2. Characterization techniques used 9 The mean particle sizes of platinum were determined either with TEM measurements on pre-oxidized samples or with CO or H2 chemisorptions on reduced samples. 9 The 1%AL catalyst was analyzed by in situ EXAFS measurements at the platinum Lm edge under a reactional atmosphere composed of NO (500 ppm), C3H6 (500 ppm) and 02 (14%). The experiments were done at the LURE synchrotron (XAS 4 station). The catalyst was first reduced under pure hydrogen at 450~ for 2 hours before being cooled down at room temperature. The gaseous mixture was then introduced and, at three temperatures: 100, 200 and 300~ the XAS spectra were collected. 9 The 0.2%AL and 0.2%ALMO were analyzed by in situ Infrared experiments realized under CO. Infrared spectra were recorded on a Nicolet 5 DCX Fourier transform apparatus with a 4 cm ~ resolution and 35 scans. The catalyst was a pellet of 13 mm diameter. 2.3. Catalytic tests: experimental set-up The experiments were performed in a catalytic reactor working under recirculation conditions directly coupled to a magnetic mass spectrometer, as analyzer. With this set up, we were able to follow the catalytic activity versus the time at various constant temperatures between 150 and 250~ under 550 torr. The gaseous mixture was composed of 3.6% 1802, 727 ppm 15NO, 727 ppm C3H8 and 363 ppm C3H6 with He as balance. The quantity of catalyst used for the experiments was 100 and 300 mg. Before each test, the catalysts were pre-treated in situ under a flow of ~602 at 350~ for 3 hours. 3.
RESULTS
3.1. Initial rates and NO conversion: general remarks The analysis of the initial rates, at each temperature, reveals how complex the mechanism of NO reduction is. Indeed, if we consider that total oxidation reactions such as the oxidation
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of propene by 1802 or 15NO only occurs, we should obtain an equality between the disappearance rates of the reactants and the formation rates of the products concerned, which is never the case. This implies that several reactions occur simultaneously at the same temperature [7]. In Table 2 are listed the initial rates in mol.(gpt.S)"1 for 15N2formation and for 15NO, C3H6 and 1802 disappearance at 200~ for each catalyst. Table 2 : Comparison of initial rates ('104) of 15NO, C3H6 and 1802 disappearance (minus sign) and 15N2 formation with NOx conversion and 15N2selectivity at 200~ Catalyst 15NO 1802 C3H6 15N2 Conversion NOx % Selectivity 15N2 % 1%AL -5.6 -9.3 -4.1 +0.2 75 30 0.2%AL -2.8 -14.4 -4.6 +0.5 68 99 0.2%ALMO -24.4 -2.6 +0.6 67 1011 EMT - 7. 3 -3.3 -4.5 +0.2 97 24 ZSM -5.6 -3.3 -10.0 +0.4 78 53
We may notice important differences in reactivity on one hand, between alumina supported catalysts and zeolite supported ones and, on the other hand, between the two 0.2%Pt catalysts which differ from their heating mode during the H2 reductive treatment. The most striking points which arise from these experiments are : -The initial rate of 15NO disappearance is a function of platinum content, i.e. of metallic dispersion. -The selectivity in 15N2 is high for the microwave catalyst. At the opposite, the EMT catalyst exhibits a poor selectivity in 15N2. -A high consumption of 1802 for the microwawe treated catalyst is linked with a high 15N2 formation. -The presence of zeolite enhances the initial reactivity of propene. These above points developed in the following parts. 3.2. Influence of platinum loading Three important effects related to platinum loading seem to influence the reactivity of the catalysts: i) the dispersion, ii) the growth of the particles during the catalytic test, and iii) the influence of the particle shape. 3.2.1. Influence of the metallic dispersion on the initial rates The catalysts used for the DeNOx reaction exhibit different platinum particle sizes as shown in Table 3. Table 3 9Particle size values and dispersion for each catalyst. Catalyst 1%AL* 0.2%AL 0.2%ALMO EMT* ZSM* Particles size (A) 70 13 18 110 80 Dispersion (%)** 16 90 60 10 14 9The TEM measurements were done on pre-treated samples under 10% O2 (balance He) for 2 hours at 200~ (0.2%AL and 0.2%ALMO were reduced before the TEM). 9* Dispersion was estimated from the mean particle diameter with the spheric model. Even if the particle sizes are high, we do not exclude the presence of small particles, with a diameter lower than 10 A which could not be detected by TEM. In Table 4 are reported the evolutions of the initial rates of disappearance of 15NO and formation of 15N2 and C1802 at different temperatures in function of the platinum dispersion.
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Deliberately we did not express the rates in TOF because we do not know about the state of the active platinum sites: are they metallic, oxidized or both of these two cases? However, it appeared that the rates of 15NO reaction is greatly enhanced when the particle sizes are important as was already mentioned in the literature [7,8]. The EMT catalyst exhibits the highest initial rates for lsNO disappearance whereas the initially well dispersed 0.2% Pt/Alumina catalyst leads to the lowest ones. A similar tendency is observed for the formation rates of C 1 8 0 2 . Furthermore, we have also to take into account the influence of the nature of the support. Table 4 : Initial rates of disappearance of 15NO and formation of 15N2 and CISo2 between 180 and 250~ in mol.(gPt.S) 1.104in function of the dispersion in perce atage. T~ Catalysts EMT ZSM 1%AL 0.2%ALMO 0.2%AL
Dispersion 180
200
250
'~No
15N2 C1802 15NO 15N2 C1802 15NO
15N2 C1802
10% -6.7 +0.7 +0.5 -7.3 +0.2 +1.1 -10 +0.6 +16.0
14%
16%
60%
90%
-0.8 +0.7 +0.8
-5.6 +0.4 +2.1 -5.4 +0.2 +10.0
-1.3 +0.3 +1.4 -5.6 +0.2 +4.6 -6.5 +1.3 +11.9
-1.6 +0.2 +0.9 -2.8 +0.5 +2.1 -3.2 +0.9 +3.2
+0.6 +1.4
-8.2 +2.3 +7.2
A question may arise : do the mean particle sizes change during the catalytic reaction? We noticed that particles sintered after a DeNOx reaction. For example, platinum particles dispersion has decreased from 60 to 18% for the 0.2%ALMO and from 90 to 21% for the 0.2%AL after an experiment realized at 250~ This sintering was pointed out through XAS experiments. 3.2.2. EXAFS results: The experiments were done on the I%AL catalyst following the procedure described previously. In Figure 1 are given the evolution of the Fourier transform moduli associated to the first coordination shell of the platinum. By comparison with Pt-Pt and Pt-O reference moduli, oxygen neighbors appear in the first co-ordination shell of platinum. The results of the numerical simulations (not given here) confirm this qualitative observation. Moreover, even if oxygen neighbors are present, still platinum-platinum bonds remain when the temperature increases. In fact, the particles sinter. This led us to suggest that platinum particles are constituted of a metallic core surrounded by one or more platinum oxide layer as represented in Scheme 1. At this stage, we cannot deduce if the oxygen atoms are chemisorbed or take part entirely to a platinum oxide phase [ 14]. We have already observed by in situ XAS the sintering of platinum particles for the same catalyst when it was submitted to NO only [10]. However, in this latter case we have not noticed the formation of an oxidic phase. Moreover, even in presence of an excess of oxygen, when the gaseous flow contains either NO or propene in low quantity, the particles are not oxidized in the bulk.
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Pt
0
I
2
3
4
5
6
O
(c) co) (a)
7R( ~
Figure 1: Fourier transform moduli for the reduced catalyst at 25~ (a), under NO+ C3H6+O2 at 100~ (b), 200~ (c) and 300~ (d)
Scheme 1 : Evolution of the reduced platinum when the catalyst is submitted to a flow of NO+C3H6+O2
3.2.3. Influence of the particle shape Another point to put emphasis on is the particular behavior of the microwave catalyst: indeed, it appears that this catalyst leads to a poor formation of 15N20 between 150 and 250~ which means that the selectivity in 15N2 is high. In fact, compared to the "classical" catalyst containing 0.2 wt %Pt, the metallic cristallites of the catalyst present a more important proportion of comer and edge atoms. This result was pointed out through measurements by infrared spectroscopy of CO adsorption [11]. Ertl and coll. [12] have identified the active sites responsible for NO dissociation on a (0001) ruthenium surface. They concluded that NO dissociates on the step atoms of the metal. This result could explain the better reactivity of NO and the better selectivity in N2 for the microwave catalyst. 3.3. Influence of the support Not only the general morphology of the platinum aggregates plays a key role in the DeNOx reactions. Indeed, we have noticed that the presence of Br6nsted acid sites has a favorable effect on the catalyst efficiency. Experiments were done on the 0.2%AL, ZSM and EMT catalysts in a continuous flow reactor in the same conditions as for the recirculation reactor. In Table 5 are gathered the evolution of the temperature range of "NOx mildconversion", which represents the width at half of the height of the conversion peak of NO versus temperature, and the quantity of Br6nsted and Lewis acid sites. These latest results were obtained by pyridine adsorption measured by IR at 150~ [ 13]. Table 5: Temperature range of "NOx mild-conversion", and quantity of Br6nsted and Lewis acid sites. Temp. range of"NOx Catalyst Br6nsted acid sites Lewis acid sites mild-conversion" ~tmoles/g 9moles/g ZSM 160-335~ 1231 80 EMT 205-240~ 250 10 0.2%AL 210-310~ 0 116 It appears that the ZSM catalyst shows a high quantity of Br6nsted acid sites when the alumina catalyst exhibits none acidic sites in the conditions of the measurement. EMT is in between these two catalysts. The ZSM catalyst gives the largest temperature range of "NOx mild-conversion",. This could be linked to the propene reactivity. In fact, the propene remains adsorbed in the pores of the ZSM zeolites, which allowed to enlarge the NOx conversion
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temperature ranges. Moreover, less the catalyst has Br6nsted sites, more rapidly the formation of C1SO2 occurs, which characterizes the oxidation of propene with 1802, as shown in Table 4.
3.4. Apparent activation energy determination The analysis of the apparent activation energy (Ea) allowed us to notice differences between the catalysts. Indeed, the Ea values vary in function of temperature :there is a break in the slope of the Arrhenius plots. One can distinguish the low temperature range and the high temperature range. These domains are quite similar for all the catalysts. From Ea values, one may deduce that several mechanisms occur on these catalysts in these two domains. For I%PTAL, 0.2%PTAL, EMT and ZSM, at low temperature, an additive process between propene and 15NO takes place giving nitroso and oxime intermediates. By oxidative degradation, the formation of 15N20 is favored. At high temperature, for alumina SU~sPOrted platinum, a partial oxidation of propene occurs, giving a ketone, before reacting with 5NO to form preferentially 15N2. For EMT, this reaction does not seem to take place. This explains why we observed a poor formation of 15N2 for this catalyst. For the microwave catalyst, these two mechanisms occur following parallel routes for the whole temperature range. The formation of 15N2 is however favored on the microwave catalyst. This could be due to the special shapes of the platinum aggregates which exhibit different crystallographic orientations [ 16, 20]. 4. CONCLUSION From this study, we pointed out that large particles with the presence of defects favor the reaction NOx to N2 in presence of light hydrocarbons. Moreover, such reaction leads to a sintering of the Pt particles. Two mechanisms were proposed an additive one and a "partial oxidation" one which proceed consecutively on quite all catalysts except on the microwave treated one where these mechanisms occur in parallel. REFERENCES: A. Fritz, V. Pitchon, Appl. Catal. B, 13 (1997) p. 1 V.I. Parvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) p. 233 A.P. Walkers, Catal. Today, 26 (1995) p. 107 M. Shelef, Chem. Rev., 95 (1995) p. 209 F. Acke, B. Westerberg, L. Eriksson, S. Johansson, M. Skoglundh, E. Fridell, G. Smedler, CAPOC IV, April 1997, Stud. Surf. Sci. Cat., 116, Eds N. Kruse, A. Frennet, J-M. Bastin, Ed. Elsevier (1998) p. 285 [6] R. Burch, T.C. Watling, Catal. Lett., 37 (1996) p. 51 [7] S. Ringler, P. Girard, G. Maire, S. Hilaire, G. Roussy, F. Garin, Appl. Catal. B, 20 (1999) p. 219 [8] R. Burch, P.J. Millington, A.P. Walker, Appl. Catal. B, 4 (1994) p. 65 [9] G.C. Bond, "heterogeneous Catalysis : Principles and Applications, Clarenton Press, Oxford (1987) [10] S. Schneider, D. Bazin, F. Garin, G. Maire, G. Meunier, M. Capelle, R. Noirot, Appl. Catal. A, 189 (1999) p 272 [ 11] S. Ringler, Ph. D Thesis, University Louis Pasteur, Strasbourg, 1998 [12] T. Zambelli, J. Winterllin, J. Trost, G. Ertl, Science, 273 (1996) p. 1688 [ 13] Professor Michel Guisnet, personnal communication
[1] [2] [3] [4] [51
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
NO Reduction in Presence of Methane and Ethanol on Pd-Mo/AI203 Catalysts L.F. de Mello% M.A.S. Baldanza a, F.B. Noronha b and M. Schmal a* aNUCAT/COPPE-PEQ - Universidade Federal do Rio de Janeiro Caixa Postal 68502 - CEP 21945-970 - Rio de Janeiro - RJ, Brazil The reduction of nitric oxide by methane and ethanol on Pd/ml203 and Pd-8%Mo/AI203 was studied. TPSR analyses of NO + CH4 revealed that above 560 K methane reacted with NO on palladium sites. On Pd-8Mo/alumina, NO decomposition was also observed. The NO+CH4 reaction at 723 K on both catalysts showed high NO conversion and selectivity for N2. The presence of water did not affect the NO conversion, however, selectivity towards N2 was lower on the Pd-8Mo/alumina catalyst. The NO + ethanol results suggested that ethanol and NO competed for the same active sites. However, the presence of MoO3 improved the selectivity for N2 formation during reaction at 593 K. I. INTRODUCTION The catalytic reduction of NOx produced by both stationary and automotive combustion processes is of great importance, due to severe restrictions for NOx emissions. The automotive three-way catalysts have in their basic formulations noble metals (rhodium, platinum or palladium) dispersed over washcoated 7-A1203 and are used for controling NOx, CO and hydrocarbon emissions. Earlier studies [ 1,2] have shown that molybdenum oxide when associated to Pd and Pt presented good NOx reduction activity and high selectivity for N2 formation. This behavior evidenced that molybdenum oxide might substitute Rh, in view of its high cost and scarce resources. Recent papers confirmed the high selectivity for N2 in the CO+NO reaction on PdMo/AI203 catalysts [3]. Based on TPD analysis of NO and CO adsorption, a redox mechanism for NO reduction to N2, involving partially reduced molybdenum oxide was proposed [3]. However, it is interesting to study the reduction of NO using different hydrocarbons as reductants. Methane is considered a convenient reductant because of its low costs and it is also the main alkane in lean bum exhaust gases and emissions from natural gas powered vehicles. It is noteworthy studying oxigenated organic compounds as reductants, since alcohols and ethers are widely used as fuel additives. Therefore, this work focuses attention on the surface reactivity and adsorption capacity of NO in presence of CH4 and ethanol. The NO + CH4 and NO + ethanol reaction, in presence and absence of 02 and water, on a Pd/Al203 and Pd8Mo/AI203 catalysts were studied. TPD and TPSR analyses were the main techniques used.
Corresponding author. Tel. 55-21)590-2241, fax. (55-21)290-6626, e-mail:
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2. EXPERIMENTAL
The 8%Mo/AI203 catalyst was prepared by A1203impregnation with aqueous solution of (NH4)6Mo7024.4H20. The sample was dried at 383 K and calcined under air flow at 773 K for 213. Pd/Al203 and Pd-Mo/AI203 samples were obtained by impregnation of A1203 and Mo/A1203, respectively, with a solution of Pal(NO3)2 (Aldrich). Catalytic tests, TPD and TPSR analyses were performed in a multi-purpose apparatus coupled to a quadrupole mass spectrometer and on-line gas chromatography. Prior to all experiments, the catalysts were purged under helium flow (50 cnaa/min.) at a heating rate of 10 K/rain from room temperature (RT) up to 823K The catalysts were cooled and then reduced at 773K for 2h, with pure H2 (3 0cna3/min.). Following reduction, the system was outgased with helium flow at the reduction temperature for l h and cooled. All samples were characterized using TPD analysis atter NO or ethanol adsorption. The adsorption of NO or ethanol was performed in the same way as discussed elsewhere [4]. The TPSR experiments for NO+CH4 were performed similarly, however, alter adsorption of NO, the samples were heated under a flow of 3.94% CH4/He mixture. In the TPSR of NO+ethanol, the adsorbed gas was ethanol and the samples were heated under a flow of 1% NO/He mixture. The catalytic reduction of nitric oxide by methane was carried out at 723 K using a feed mixture consisting of 0.5% CH4 and 0.3% NO. After reaction, oxygen at 0.8% was added to the feed, then removed and followed by adding 10% water. A catalyst weight of 100 mg and a total flow rate of 150 cc/min were used. For the reduction of nitric oxide by ethanol, the feed mixture consisted of 0.2% ethanol and 0.3% NO. Oxygen (0.6%)was then added. The reaction was carded out at 593 K and the total flow rate and catalyst weight were 250 cc/min and 140 nag, respectively. 3. RESULTS AND DISCUSSION
3.1. Temperature Programmed Desorption (TPD) After adsorption of ethanol on alumina, TPD analyses showed ethanol desorption at 395 and 500 K and a great formation of ethylene around 550 K. No dehydration to acetaldehyde was detected. The Pd/AI203 and Pd-8Mo/AI203 catalysts exhibited similar desorption profiles. There was a large decrease of ethylene production together with the formation of CO, CH4 and H2 around 495 K, which is attributed to the decomposition of ethoxy species adsorbed on alumina, diffusing to palladium particles [4, 5]. On the Pd/Al203 catJyst, ethanol underwent dehydrogenation to acetaldehyde at 530 K, while on Pd8Mo/AI203 (figure 1), two desorption peaks of acetaldehyde at 466 and 516 K were observed. The peak at 466 K is due to the oxidative dehydrogenation of ethanol on partially reduced molybdenum oxide. Further desorption of CO, HE and CO2 was observed on both catalysts above 723 K, which was associated to the decomposition and/or reaction with surface hydroxyls of an intermediate carbonaceous species [4]. The 8Mo/AI203 catalyst showed desorption of ethanol at 386 and 476 K, ethylene formation at 540 K and also two peaks corresponding to acetaldehyde at 455 and 530 K. Unlike the other catalysts, no CO, CH4 and HE were observed.
3.2. Temperature Programmed Surface Reaction (TPSR)
The TPSR profiles of NO + CH4 on the Pd/Al203 catalyst are displayed in figure 2. Nitric oxide desorption was observed up to 570 K with no decomposition products. This initial
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behavior was very similar to the TPD profile of adsorbed NO [3]. At 570 K, however, NO desorption (m/e = 30) decreased drastically with simultaneous formation of N~ (m/e = 28 ) and CO2 (m/e = 44) besides methane (m/e = 16) consumption. It indicates that at 570 K, methane is activated on palladium, starting to react with NO. Above 720 K, CO2 formation decreased, exhibiting mainly CO and H2. Thus, methane probably reacted with adsorbed NO up to 720 K, when NO was totally consumed. After that, methane probably decomposed and/or reacted with surface hydroxyls yielding CO and H2.
' m./o=28
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m/e = 16 (x 0 . 5 ) ~ ~ ' - " ~ f r o / ( = 2 (x 0.25)
.__.....~dehyde(m/e=29 t ~ ~. . . . C O (m/e=44)
--7-'7-i
3 00
5 00
7 00
|
iisothermic
Temperature (K) 823 Figure 1: TPD of adsorbed ethanol on Pd-8Mo/A1203 catalyst.
. ~ j
~~~__ m/e,=44 rn/e = 30 (x 4) I'
473
,
I
673 [ isothermie
Temperature / K
Figure 2: (a) NO + CH4 TPSR on Pd/Al203 (b) amplified detail from (a).
The TPSR lineshapes for Pd-8Mo/AI203 are displayed in figure 3. Initially, nitric oxide was desorbed around 395 and 555 K (m/e=30), with simultaneous formation of N2 (m/e=28) and N20 (m/e=44) at 555 K (figure 3 b). Once again, the initial behavior was similar to the TPD profile aider adsorption of NO [3]. NO desorption and decomposition decreased sharply above 560 K, together with methane (m/e=l 6) consumption. As for the Pd/A1203 catalyst, at 560 K, methane is activated on palladium, beginning to react with NO. However, at this temperature NO was already being decomposed on the MoOx surface. Therefore, it seems that NO decomposition decreased, prevailing the reaction of NO with CI-h on Pd. Above 560 K, TPSR profile for both catalysts were very similar, indicating that methane probably reacted with adsorbed NO until 730 K, when NO was totally consumed and CO2 formation decreased. Then, methane was decomposed and/or reacted with surface hydroxyls forming CO and H2. The 8Mo/A1203 catalyst did not evidence any reaction between the adsorbed nitric oxide and flowing methane. The TPSR and TPD profiles were very similar [3], exhibiting NO desorption at 375 and 580 K with small formation of N20 and N2 between 340 and 700 K. This is attributed to the decomposition of NO on the partially reduced molybdenum oxide.
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-28
.,~
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o~,,~
m ' ~ 2'x I
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m/o--.
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I ....... 473
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Temperature / K Figure 3" (a) NO + CH4 TPSR on Pd-8Mo/Al203. (b) amplified detail from (a).
~~ ~
_
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Temperature / K Figure 4' NO + ethanol TPSR on Pd8Mo/A1203.
The TPSR of NO + ethanol on Pd/AI203 catalyst showed that at low temperature the profile was very similar to the TPD profile of ethanol. However, during TPSR two ethanol desorption peaks were observed around 400 and 495 K. NO consumption was observed only at 590 and 695 K, with simultaneous increase of signals m/e= 28, 44 and 12, that may be assigned to CO (m/e= 28 and 12), CO2 (m/e=44, 28, and 12), N2 (m/e=28) and/or Y20 (m/e = 44). This suggested that NO probably reacted on Pd particles with carbonaceous species deriving from ethanol decomposition at lower temperatures, as discussed previously [4]. The TPSR results of Pd-8Mo/AI203 are shown in figure 4. Ethanol desorbed around 400 and 495 K. Ethylene was observed at 550 K and acetaldehyde at 480 and 540 K, quite similar to the TPD profile after adsorption of ethanol (figure 1). The NO consumption was observed only above 565 K, in a wide temperature range, showing one peak at 695 K and shoulders at 645 and 600 K. However, the TPD profile of NO on Pd-8Mo/AI203 [3], showed that NO decomposed below 565 K. This suggests that ethanol and NO are competing for the same active sites and that the NO decomposition occurred only aider the desorption and/or decomposition of ethanol. The NO consumption around 600 and 695 K coincide with the consumption peaks on the Pd/AI203 catalyst and are probably deriving from the reaction of NO with the carbonaceous species on Pd particles. In fact, the signal for the m/e=12 fragment showed two peaks at 600 and 695 K (figure 3), indicating CO and/or CO2 formation. The NO consumption around 645 K was only observed on the Pd-8Mo/A1203 sample, probably due to the NO decomposition on reduced molybdenum oxide, since N2 formation (m/e=28) was also observed at 645 K. This is reinforced by the fact that at this temperature the signal (m/e=12) did not increase. Results on 8Mo/AI203 showed that below 575 K the profile was very similar to the TPD of adsorbed ethanol. Above that NO was decomposed and N2 and N20 were observed.
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3.3. Catalytic Activity
The catalytic results of the NO reduction by methane in the absence and presence of water are presented in table 1. The 8Mo/AI203 did not show any activity for this reaction at 723 K. For the Pd-containing catalysts, NO was completely reduced to N2. Methane conversion was higher on Pd/AI203, although the Pd-8Mo/AI203 catalyst showed better selectivity for CO2 production. In a previous work [3], the NO + CO reaction was studied and a redox mechanism was proposed. It was shown that NO decomposed on reduced molybdenum oxide and the oxygen was available at the palladium sites to oxidize CO to CO2. Therefore, for the Pd-8Mo/AI203 catalyst, a similar mechanism may occur and a greater amount of oxygen is available for palladium to oxidize methane. That would explain the higher selectivity of this catalyst for CO2 formation. In fact, the main difference observed on the TPSR analyses of CI-I4 + NO on Pd/AI203 and Pd-8Mo/AI203 was that, besides the reaction with methane on Pd particles, NO was also decomposed on the Pd-8Mo/AI203 catalyst, in contrast with the Pd/AI203 catalyst, where no decomposition was observed. The reaction feed containing water was passed through the catalyst bed for 1 hour before data acquisition. The presence of water did not change the activity and selectivity for NO conversion to N2 on the Pd/ml203 catalyst. However, methane conversion increased and the catalyst was more selective for CO2 when compared to the reaction feed without water. This is probably due to the reforming of methane on Pd particles. On the Pd-8Mo/AI203 catalyst, N2 selectivity dropped while N20 was formed. Methane conversion did not increase as much as for the Pd/AI203, although CO2 selectivity was further enhanced. This indicates that water may be competing with NO for adsorption sites on the reduced molybdenum surface, decreasing NO decomposition and, hence, the selectivity for N2. However, the oxygen from water and also from nitric oxide was still available for palladium to oxidize methane, showing a higher selectivity for CO2 production (table 1). Table 1 - NO reduction by CH4 in absence and presence Absence of water Conv. (%) Selectivity (%) NO CH4 Nspecies C species Catalyst N2 N20 CO CO2 Pd/A1203 100 33 100 0 47.5 52.5 Pd-8Mo/Al203 100 21 100 0 36.2 63.8
of water at 723 K Presence of water Conv. (%) Selectivity (%) NO CH4 N species C species N2 N20 CO CO2 100 52.5 100 0 21 79 100 25 84 16 9 91
In the presence of oxygen, NO reduction ceased, prevailing methane reaction with oxygen, forming CO2 as the only product. For the Pd/A1203 catalyst, methane conversion was higher in the presence of oxygen (61% conversion) than in the absence of oxygen. However, the Pd-8Mo/AI203 catalyst was less active for oxidation of methane by oxygen (11% conversion) than for the oxidation of methane by nitric oxide (21% conversion). The results for the reduction of nitric oxide by ethanol in the absence of oxygen are presented in table 2. Nitric oxide and ethanol conversion were approximately the same for both Pd/AI203 and Pd-8Mo/AI203 catalysts. However, the Pd-8Mo/AI203 catalyst showed a much higher selectivity for N2 formation than the Pd/AI/O3 catalyst which, on the other hand, was more selective for CO2 production. Ukizo et. al. [6] studied the reaction of NO+ethanol+O2 on alumina-supported silver catalysts and through IR analyses they observed the appearance of
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characteristic bands of isocyanate species adsorbed on m1203 and Ag, above 573 K These species were assigned to be intermediates of the NOx reduction. In this work, it was observed through IR analyses of adsorbed ethanol and NO (not shown), that the presence of Mo covered great part of the alumina surface area. Although this would hamper the formation of isocyanate species on alumina, the NO conversion was the same on both catalysts. As seen from the TPSR analyses, NO not only reacted on the Pd particles but also decomposed (forming mainly N2) on the MoOx surface. Hence, this would explain the similar activity on both catalysts and also the higher selectivity for N2 formation on the Pd-8Mo/AI203 sample. The 8Mo/AI203 catalyst showed lower activity and 100 % selectivity for N20 and acetaldehyde formation, which is most likely due to the decomposition of ethanol and NO on the surface of the catalyst, and not the reaction between ethanol and NO. Table 2 - NO reduction by ethanol in absence of 02 at 593 K Conversion (%) Selectivity (%) NO Ethanol N species ,,, C species Catalyst N2 N20 CO CO2 Acetaldehyde Pd/AI203 54.1 89.9 47.9 52.1 23.8 76.2 0 Pd-8Mo/AI203 55.8 85.7 83.2 16.8 6 8 . 3 31.6 0 8Mo/AI203 a 19 83 0 100 0 0 100 a activity measuremems for this catalyst were taken at 623K In presence of oxygen, the Pd-containing catalysts were not active for the reduction of NO. Instead, all ethanol reacted with oxygen to form carbon dioxide. For the 8Mo/AI203 catalyst there was still some formation of N20, while part of the ethanol was now oxidized to CO2 (12% CO2 and 88% acetaldehyde). 4. SUMMARY The effect of MoO3 on the Pd/Al203 catalyst for the reduction of NO by CH4 and ethanol demonstrated that the selectivity towards N2 was enhanced due to the promoting effect of NO decomposition on reduced MoOx surface. The presence of oxygen decreased drastically the conversion of NO, affecting the selectivity. On the other hand, water did not affect the reduction of NO by methane. Indeed, it increased the conversion of methane and the selectivity towards CO2 as well. The redox mechanism proposed for the CO+NO reaction [3] also plays an important role for the NO reduction by methane, however the reduction by ethanol was accomplished by an important promotion of the NO decomposition. REFERENCES 1. H.S. Ghandi, H.C. Yao and H.K. Stepien, "Catalysis Under Transient Conditions" (Bell, A.T. and Hegedus, L.L., Eds.), ACS Symposium Series No. 178 (1982) 143. 2. I. Halasz, A. Brenner, M. Shelef and Ng. Simon, Applied Catal. A : General 82 (1992) 51. 3. M. Schmal, M.A.S. Baldanza and A. Vannice, Journal of Catalysis 185 (1999) 138. 4. M.A.S. Baldanza, L.F. Mello, F.B. Noronha, A. Vannice and M. Schmal, submitted to Applied Catal. B : Environmental (1999). 5. E.M. Cordi and J.L. Falconer, Journal of Catalysis 162 (1996) 104. 6. Y. Ukisu, T. Miyadera, A. Abe and K. Yoshida, Catal. Letters 37 (1996) 265.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Fe-vanadyl phosphates/TiO2 as SCR catalysts G. Bagnasco l, p. Galli 2, M. A. Larrubia3, M. A. Massucci 2, P. Patron04, G. Ramis 3, M. Turco ~ ~Dipartimento di Ingegneria Chimica, Universith "Federico II", P.le Tecchio 80, 80125 Napoli, Italy. 2Dipartimento di Chimica, Universith "La Sapienza", P.le Aldo Moro 5, Roxlm, Italy. 3Dipartimento di Ingegneria Chimica e di Processo "G.B. Bonino", Universifft di Genova, P.le J.F. Kennedy 1, Genova, Italy. 4IMA[ CNR, Via Salaria Km 29.600, 00016 Monterotondo Stazione, Roma, Italy. Fe-vanadyl phosphate (FeVOP) was precipitated in the presence of different amounts of TiO2. The samples also contained Ti(HPO4)2"H20 and probably amorphous FePO4. In the catalysts heat treated at 450~ besides TiO2 and anhydrous FeVOP, amorphous layered TiP207 was likely present. NH3 adsorption sites with medium and high strength were detected, that were related to Bronsted acidity of a pyrophosphate phase and Lewis acidity of FeVOP. The catalysts were noticeably more active and selective than pure FeVOP. NO conversion was increasing with FeVOP content, reaching 90% value at 400~ with unit selectivity to N2 and NH3/NO reaction ratio close to 1. 1. INTRODUCTION VOPO4 phases have been widely investigated, due to their role in VPO oxidation catalysts (1). Recently new materials have been obtained by isomorphous substitution of VO groups of VOPO4-2HzO with a trivalent metal such as A1, Cr, Fe, Ga, Mn (2). Such substitution modifies the adsorption properties of VOPO4 phase (3, 4). Moreover Fe-vanadyl phosphate gave high activity for NO reduction by NH3 (SCR process) if compared with conventional SCR catalysts (5). The activity of this compound can be related to the dehydrogenation properties of Fe, promoting the formation of species like amide, that reacts with gaseous NO (2,5). However the catalytic activity of Fe-vanadyl phosphate is limited by its low surface area (5 m2/g).Therefore these systems could be improved by dispersing the active phase on a suitable material. In this work, we have studied catalysts obtained by precipitating Fe-vanadyl phosphate in the presence of titanium dioxide. Such catalysts that were never reported before, are not simple supported systems, due to the presence of other phases formed during the preparation. These systems were studied for SCR catalytic properties and characterized for physical and chemical properties by means ofEDS, XRD, NH3 TPD and FT-IR techniques.
2. EXPERIMENTAL [Fe(H20)]o.2(VO)o.sPO4"2.25H20 (FeVOP) was prepared by refluxing for 16 h a suspension of V205, Fe(NO3)3"9H20 in H3PO4 3.3 M, with a yield of 60% (2). Materials A, B and C were
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prepared by refluxing for 16 h the above suspensions in the presence of 12, 6 or 3 g of TiO2 (s. a.=125mEg ~) respectively. The catalysts A-450, B-450 and C-450 were obtained by treating the materials at 450~ for 12 h in He flow. A reference material TiP-TiO2 (s.a.-35mEg 1) was prepared by refluxing TiO2 with HaPO4 3.3 M in the same conditions. Elemental analysis was effected by EDS on a Philips XL30 apparatus. BET surface areas were measured on a Quantachrom Chembet 300. XRD measurements at room temperature (r. t.) and at 450~ were performed by Philips diffractometers PW 1100 and 1710 (HT-A.Paar diffraction camera) respectively. NI'-I3 temperature programmed desorption (TPD) was carried out in a flow apparatus at a rate of 10~ min -~. FT-IR spectra were recorded with a Nicolet Proteg6 460 instrument, using conventional IR cells with evacuation-gas manipulation apparatus. Catalytic activity tests were carried out in a flow apparatus with a fixed bed reactor at T=200-450 ~ contact time=8 9103 s. The feed mixture contained 700 ppm of NO and NH3, 27000 ppm of O2~He as balance. NO and NH3 were measured by continuous analyzers, N2 and N20 by gaschromatography. The nitrogen balance was verified within 5% error.
3. RESULTS AND DISCUSSION Table 1 Composition and surface areas of the materials Sample
FeVOP
P Ti mol% b) mol% b) A 10 34.0 61.7 B 20 30.7 64.4 C 40 27.0 56.2 a) nominal, assuming 60% yield in FeVOP; wtO~ a)
V mol% b) 2.12 2.10 11.5 b) EDS analysis,
Fe Surface area mol% b) m2g-~ 2.09 56 2.80 59 5.21 61 on oxygen free basis
Composition and surface areas of the samples are reported in Table 1. Surface areas are markedly higher than pure FeVOP and are unchanged after treatment at 450~ The vanadium content of the samples A and B would correspond to about 7wt% FeVOP, that of C to about 30wt% FeVOP. These percentages, quite lower than the nominal ones, suggest that the presence of TiO2 hinders in some way the formation of FeVOP, as more as higher is the TiOz amount in the preparation mixture. The phosphorous content is always largely exceeding the amount corresponding to FeVOP and the Fe/V ratio is higher than 1/4, that is the value usually obtained (2,5). These data indicate that the samples cannot be described by a simple FeVOP/TiO2 composition, and that other phases are produced when FeVOP is precipitated in the presence of TiO2. The XRD patterns are reported in Fig. 1. TiO2 shows the characteristic signals of the anatase and brookite phases. TiP-TiO2 shows the reflexions of layered cz-Ti(HPO4)z-H20 (TIP) (6), besides weak signals of TiO2. This indicates that refluxing TiO2 with H3PO4 leads to partial conversion of TiO2 into TIP. A similar behaviour was found in the treatment of TiO2 supported catalysts with H3PO4, leading to formation of titanium hydrogenphosphate (7). FeVOP shows its characteristic pattern (3). Different phases are detected in the XRD of A, B and C. The signals of TiO2 and TIP are always present, the intensity of TiP signals decreasing from A to C. The signal with d-7.11 A of FeVOP is not observable in the pattern of A, while it appears as a shoulder in B and is clearly evident in C that shows also other reflexions of FeVOP. Thus on going from A to C the amount of the FeVOP phase increases while that of
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TiP decreases. This confirms that decreasing the amount of TiO2 in the preparation mixture
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.
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Fig. 2. XRD patterns of A-450, B-450, C-450 and reference materials.
favours the precipitation of FeVOP in respect to the formation of TiP. The presence of TiP in all samples explains why the phosphorous amounts exceed those corresponding to FeVOP (Table 1). Moreover the high Fe/V ratio suggests the presence of other Fe containing phases, such as FePO4, not detected by XRD because in too low amount or in an amorphous state. Fig. 2 reports the XRD patterns of the materials heated at 450 ~ After this treatment TiO2 shows an unchanged pattern, while FeVOP transforms into a well crystalline anhydrous phase (3). The pattern of TiP-TiO2 treated at 450~ (TiP-TiO2-450), shows the signals of TiOz and of layered titanium pyrophosphate (L-TIP207), formed by condensation of the TiP phase. In the XRD of the catalysts, besides the signals of TiO2, the strongest signal of anhydrous FeVOP with d=4.21 ,~ is well evident, while the reflexion with d=3.08 A becomes evident only in C-450. This suggests an increase of the amount of anhydrous-FeVOP from A-450 to C-450, that agrees with the increase of hydrated FeVOP in the corresponding precursors. It is worthnoting that the reflexions of L-TiP207 are not observable in the XRD of the catalysts, although they are present in the pattern of TiP-TiO2-450, suggesting that the condensation process of TiP in the catalysts is influenced by the presence of FeVOP (or other unidentified phases). It can be supposed that some reaction between TiP and FeVOP (or other phases) can occur during heat treatment, leading to formation of a disordered phase, not detected by XRD. NH3 TPD spectra of the catalysts and reference materials treated at 450~ are reported in Fig. 3. TPD spectrum ofTiO2-450 shows two bands due to Lewis acid sites of different strength,
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due to coordinatively unsaturated Ti4+ ions (8). FeVOP-450 exhibits a broad band due to superficial Fe 3§ and VO 3+ ions acting as .~ ._ Lewis acid sites, with a large variety of acid strength (3). The TiP-TiO2-450 sample 't1 'r'= 1 ~ gives a spectrum very different from that of -~ TiO2. However the spectrum is very similar ~ to that reported for TiP phase treated at ~, • 450~ that is partially transformed into ~ L-TiP207 with signals due to adsorption g on Bronsted acid sites (9). This suggests ~, ~. that TiP-TiO2-450 has adsorption properties similar to L-TiP207. It can be g supposed that the TiO2 particles are ~0 o=~ z . , . z surrounded by TiP phase, formed by the reaction of T i O 2 with H3PO4, and 0 200 400 600 transformed into L-TiP207 during heat Temperature, ~ treatment. TPD spectra of the catalysts show broad desorption peaks due to NH3 Fig. 3. NH3 TPD of A-450, B-450, C-450 and adsorbing sites with strength from medium reference materials (right axis for FeVOPto very high. By taking into account surface 450) area values, the amoums ofdesorbed NH3 correspond to similar concemrations of surface sites (abt. 2-1014 cm2). The shape of the curve of A-450 is very similar to that TiPTIO2-450, suggesting the presence of a titanium pyrophosphate phase in the catalysts. XRD failed to detect this phase probably because it was amorphous. The shape of the curves gradually changes from A to C, as the 400~ component increases, while that at 550~ decreases. XRD shows that the content of FeVOP increases from A-450 to C-450. Therefore the increased intensity of the signal at 400~ can be related to an increase of the amount of FeVOP, since this phase shows a noticeable concentration of sites desorbing NH3 at 350400~ The decrease of the 550~ signal can be related to a decrease of the amount of the amorphous pyrophosphate phase. The nature of surface acid sites of the catalyst has been investigated by FT-IR technique (Fig. 4). According to (10,11) the band observed at 1605 cm1 is assigned to asymmetric deformation mode (Sas NH3) of ammonia coordinated to Lewis acid sites; the corresponding symmetric deformation (Ssym NH3) is not detectable because obscured by the cut-off of the transmittance of the sample due to the absorption of the bulk. Moreover, bands of NH4+, due to the adsorption of ammonia over Bronsted acid sites, can be observed at near 1680 and 1440 cml; these bands are respectively due to symmetric (Ssym NH4) and asymmetric (Sas NH4) deformation mode and the associated stretching. The relative intensities of the 8as NH4 with respect to the 8as NH3 seems higher in the spectrum of TiP-TiO2-450 and lower in the spectrum of pure FeVOP-450; in the case of C-450 and A-450 an intermediate trend is observed. These data indicate that ammonia adsorbs over all the catalysts in the form of molecularly coordinated species and of ammonium ions. The former are due to Fe 3§ and VO 3§ groups, the latter to HPO4 groups present on the surface of L-TiP207 (6). Moreover protonation of NH3 by water molecules coordinated to Fe 3+ ions cannot be excluded. The results of catalytic tests for the SCR reaction are reported in Fig. 5. The catalysts are very active, giving NO conversions up to 90%. TiO2 is inactive, FePO4 (5) and TiP-TiO2-450 have
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very low activity, appreciable only at temperatures higher ), 13" than 300~ Thus the (/) 0 catalytic activity must o" I1} be related to the -.1 t") anhydrous FeVOP I11 phase. The catalysts r appear more selective than pure FeVOP. In fact the NH3/NO reaction ratio is close to 1 and no formation of N20 is observed in all conditions, except Wavenumbers (cm -1) for C-450 that gives conversion to N20 of Fig 4. FT-IR spectra of adsorbed species arising from contact of about 15% at 450~ NH3 over pure FeVOP-450 (a), C-.450 (b), A-450 (c) and TiPOn the other hand TIO1-450 (d). with pure FeVOP conversion to N 2 0 was observed starting fi'om 300~ (5). The catalytic activity towards NH3 oxidation has also been investigated, in the same conditions as SCR tests, but in the absence of NO. NH3 oxidation activity is negligible up to 300~ and markedly increases at higher temperatures (Fig. 6), giving N2 as the only product (traces of NO are produced at 450~ with 100
100
80.m L_
60-
E:
o r
40-
806040-
Z
20-
o'-9. 200
300
400
20-
50(:
200
Temperature, ~
Fig. 5. SCR reaction: NO conversion on A-450 ( O ) , B - 4 5 0 (E3), C-450 (A) and FeVOP-450 (V)..
300 400 Temperature, ~
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Fig. 6. N H 3 oxidation: N H 3 conversion on A-450 (O) ,B-450 ([2) and C-450
(a).
C-450). However under SCR conditions, such NH3 oxidation activity is ineffective suggesting that NH3 reacts preferentially with NO rather than with 02. The activities of the three catalysts can be directly compared, since their surface areas are almost the same. The catalytic activity increases from A-450 to C-450, suggesting that it is related to the content of FeVOP phase.
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However, since the catalysts are more selective than pure FeVOP, it can be supposed that either the FeVOP phase is someway modified by interaction with titanium phosphate, or other phases are involved in the catalysis. It can be supposed that a modified pyrophosphate phase, in which some V4+ ions replace T1.4+.ions, catalyzes NO reduction with higher selectivity, taking into account the catalytic properties observed for V4+ species in V2OJTiO2 catalysts (12). REFERENCES
1. G. Centi, Catal. Today, 16 (1993) 5. 2. K. Melanov~, J. Votinsky, L. Bene~ and V. Zima, Mat. Res. Bull., 30 (1995) 1115. 3. G. Bagnasco, L. Bene~, P. Galli, M. A. Massucci, P. Patrono, M. Turco and V. Zima, J. Therm. Anal., 52 (1998) 615. 4. G. Bagnasco, G. Busca, P. Galli, M. A. Larrubia, M. A. Massucci, P. Patrono, G. Ramis, M. Turco, J. Therm. Anal., in press. 5. G. Bagnasco, G. Busca, P. Galli, M. A. Massucci, K. Melanova, P. Patrono, G. Ramis, M. Turco, submitted to Appl. Catal. B: Envir. 6. G. Alberti, P. Cardini-Galli, U. Costantino and E. Torracca, J. Inorg. Nucl. Chem., 29 (1967) 571. 7. J. Blanco, P. Avila, C. Barthelemy, A. Bahamonde, J. A. Odriozola, J. F. Garcia de la Banda, H. Heinemann, Appl. Catal. 55 (1989), 151. 8. N. Y. Topsoe, J. Catal., 128 (1991) 499. 9. G. Bagnasco, P. Ciambelli, A. La Ginestra, M. Turco, Thermochim. Acta, 162 (1990) 91. 10. A. A. Tsyganenko, D. V. Pozdnyakov, V. N. J. Filimonov, Mol. Struct., 29 (1975) 299. 11. K. Nakamoto, in "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 4 th ed., Wiley, New York (1986). 12. H. Bosch, F. Janssen, Catal. Today, 2 (1988) 369.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Photoindueed non-oxidative methane coupling over silica-alumina Hisao Yoshida, a Yuko Kato, a Tadashi Hattori b a Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
b Research Center for Advanced Waste and Emission Management, Nagoya University, Nagoya 464-8603, Japan* Photoinduced non-oxidative methane coupling proceeds around room temperature any oxidant molecules on silica-alumina. No oxygenated products are formed. dispersed aluminum species, which are generated on the surface of silica-alumina dehydration at high temperature and provide the characteristic phosphorescence function as the active sites for the photoinduced non-oxidative methane coupling.
without Highly through spectra,
I. INTRODUCTION In order to convert natural gas into useful chemicals, the oxidative methane coupling is an expedient reaction. However, it is quite difficult to obtain the coupling products in high selectivity, because the oxidation of coupling products to COx proceeds more selectively than the coupling reaction. If the oxidant molecules are removed to avoid complete oxidation, the reaction requires very high temperature [1]. Photoinduced reaction are one of the most available reactions taking place at low temperature where complete oxidation could be minimized. Actually, it was reported that photoinduced methane coupling proceeded at 373-473 K in the presence of oxygen over TiO2 [2]. However, the selectivity of COx was very high and the yield of coupling products was only ca. 0.5%. The possibility of the photoinduced non-oxidative coupling, where no oxidant molecules are employed, was examined by using transition metal oxides. However, the highest yield was on Mo/SiO2 only ca. 0.007 % in gaseous phase and less than 0.4% even after forced desorption by heating or by admission of water vapor [3]. It seems that employment of transition metal oxides is not the only solution. Recent years, some kinds of photore act ions were successively discovered on silica and silica-based materials, e.g., photometathesis of alkene over amorphous silica or mesoporous silica [4], photooxidation of propene over mesoporous silica, SiO2 and Mg/SiO2 [5]. In addition, silicaalumina was found to be photoactive; it exhibited a characteristic phosphorescence emission spectra [6]. In this study, photoinduced non-oxidative methane coupling was examined on silica, * Present address is the same as that of the other authors.
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silica-alumina and alumina and it was found that it occurs on silica-alumina in yields as high as 5 % without formation of CO and CO2 [7]. The active sites on silica-alumina were assigned to be the phosphorescence sites.
2. EXPERIMENTAL
The silica sample was prepared from Si(OEt)4 by sol-gel method followed by calcination in dry air at 773 K [8], and its specific surface area of was 679 m2g"1. The alumina sample was a reference catalyst of the Catalysis Society of Japan, JRC-ALO-4 (174 m2g"1) [9]. The silicaalumina samples employed, SiO2-A1EO3(L) and SiO2-AIEO3(H), were also JRC samples, JRCSAL-2 (560 m2g"1) and JRC-SAH-1 (511 m2g'l), respectively. The alumina contents were 13.8 wt% and 28.6 wt%, respectively [9]. Phosphorescence spectra were recorded at 77 K with a Hitachi F-4500 fluorescence spectrophotometer using a UV filter (transmittance X > 330 nm) to remove scattered light from UV source, where the excitation light was cut off by chopper and emission light was recorded after a time lag to collect phosphorescence emission which was free from fluorescence. Before measurement of spectra and the reaction test, the sample was heated in the presence of oxygen and evacuated at desired temperature. The reaction test was carried out under irradiation by using a 250 W Xe lamp for 3, 18 or 30 h. No oxidant molecules were introduced into the reaction system. Under photoirradiation, the temperature of sample bed was measured to be around 310 K. Products in the gaseous phase were collected with a liquidN2 trap and analyzed by GC. Then adsorbed products were thermally desorbed by heating (573 K 15 min), collected, and analyzed. 3. RESULTS AND DISCUSSION 3.1. Activities in photoinduced non-oxidative methane coupling Table 1 shows the product yields in photoinduced non-oxidative methane coupling in the absence of gaseous oxidants over silica, silica-alumina and alumina [7]. Note that no oxygenates such as MeOR HCHO, CO and CO2 were detected in the absence of oxygen molecules (runs 1-7). For the empty reactor (run 1), only a trace amount of C2H6 was formed upon photoirradiation. On the silica sample (run 2), a small amount of CEH6 and C3H8 were obtained in the gaseous phase, and a trace amount of C2H4 and C3H6 were observed as the thermally desorbed products. On the alumina sample (run 3), the conversion was obviously much higher than that over silica; total yield was 5.33%. In the dark, alumina exhibited no activity (run 6), which indicates that photoirradiation is necessary for the reaction over alumina. However, the products upon irradiation were dominantly obtained through thermal desorption. It would be important point to release products without heating for lowtemperature methane coupling. Over the silica-alumina samples (runs 4, 5), the coupling products were also obtained as high yields as on alumina. In contrast to the case of alumina, however, a large amount of gas phase products of C2-C 4 alkanes were obtained without heating while smaller amount of C2-
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C 6 alkanes
and alkenes were desorbed upon heating. Among the thermally desorbed products, alkenes were the major products [7]. In the dark at 473 K (in an electric furnace, run 7), no products were detected, clearly indicating that photoirradiation is necessary for the above reaction. On SiO2-A12Oa(L), total yield went up to 5.90 % and even the yield of gaseous phase products reached 4.53 %. They are extremely higher than those in any other reports on photoinduced coupling of methane [2,3]. It should be noted that no oxidant molecules were introduced in the reaction system and that the temperature of the sample bed, measured by a the rmocoupl e, was 310 K. In the presence of oxygen (run 8), though photoinduced coupling reaction of methane also occurred, a large amount of CO2 (11.5 C%) was mainly produced. Table 1 Results of photoinduced non-oxidative methane coupling over the samples pretreated at 1073 K a Run Sample Surface Gaseous phase products Desorption Total area b (C%) c products (C%) c
(m2g"1) C 2 H 6 C 3 H 8 C4Hlo Total (C%) c 1 (blank) trace 0.00 0.00 trace trace 2 SiO2 526 0.08 0.01 0.00 0.09 trace 0.09 3 A1203 134 0.48 0.02 0.00 0.50 4.83 5.33 4 SiO2-A1203(L) 223 3.54 0.85 0.14 4.53 1.37 5.90 5 SiO2-A1203(H) 273 1.82 0.27 0.03 2.12 0.90 3.02 6 d A1203 134 0.00 0.00 0.00 0.00 0.00 0.00 7 d SiO2-A1203(L) 223 0.00 0.00 0.00 0.00 0.00 0.00 8 e SiO2-A1203(L) 223 0.24 0.01 0.00 11.75 f 1.30 13.05 f a Reaction temperature was ca. 310 K. Reaction time was 18 h. Initial amount of CH4 was 100 lamol. Production of H2 was not monitored in this case. b The specific surface area was measured after reaction test. c Based on the initial amount of CH4. d Reaction at 473 K without UV-irradiation. e Photoinduced oxidative coupling (CH4 = 100 ~tmol, 02 = 30 ~tmol). y CO2 of 11.5 C% was included. Time course of the reaction was recorded for 30 h (initial amount of methane was 300 lamol) on SiO2-A1203(L) evacuated at 1073 K (Fig. 1). Amount of produced H2 increased linearly with time. Amount of alkane in the gaseous phase, such as C2H6, C3H8 and C4Hl0, also increased, but not linearly. Fraction of higher hydrocarbons also increased with time; after 30 h, even 0.5 mol% of C5-C7 fractions were detected as thermal desorption products. Figure 2 shows Schultz-Flory plot of the products in this coupling reaction. The results after irradiation for 18 h (Fig. 2a) gave a good straight line as expected from the equation (1) with the assumption that the reaction proceeds successively with the same probability for coupling reaction though the number of data is not enough for strict discussion: log (Nx/No) = log (1-p)2/p + x log p
(1)
where x is number of carbon in the hydrocarbon, Nx is amount of hydrocarbon which has x of
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8
0.0-
(a)
r..) t_ o
-0.5 -
"-66 E
~~(b)
-1.0 Z (b) ~2 0
(c) (d)
< o
I
0
I
10
I
20
Time / h
I
30
Fig. 1 Time course of produced amount of
H2 (a), C2H6 (b), C3Hs (c) and C4Hlo (d) in photoinduced non-oxidative methane coupling on SiO2-A1203(L) evacuated at 1073 K. The unit is ,umol for (a) and C% for (b)-(d).
o
-1.5-
-2.0 -2.5 -
I
1
I
2
I
3
I
4
Numberof carbons
I
5
Fig. 2 Schultz-Flory plot derived from the results in Fig. 1. The reaction time was 18 h (a) and 30 h (b).
carbons, No is amount of the converted methane, p is the probability for coupling of methane with another hydrocarbon. This linearity of the plot suggests that the photoinduced nonoxidative coupling proceeds successively with the same probability. However, the results after 30 h gave the non-linear line (Fig. 2b), while C5-C7 fractions were produced . Coupling reaction of higher hydrocarbons might predominantly occur when the reaction proceeds. 3.2. P h o t o a c t i v e sites on s i l i c a - a l u m i n a
Fig. 3 shows the dependence of the total products yield in this reaction on the pretreatment temperature of SiO2-A1203(L). It is clear that the products yield was affected by the pretreatment temperature; when the pretreatment temperature was high such as 1073 K, high products yield was obtained. Probably dehydration occurring on the surface of the silicaalumina at high temperature would produce the active sites. Phosphorescence spectra of SiO2-A1203(L) evacuated at various temperature are shown in Fig. 4. The characteristic spectrum which has the free structures [6] appeared when evacuated at higher temperature such as 873 or 1073 K, its intensity increased with an increase of pretreatment temperature. This dependence of phosphorescence intensity seems to be related to the dependence of activity for the photoinduced reaction on pretreatment temperature (Fig. 3). Thus, the luminescence sites, which are generated through dehydration at high temperature and provide the characteristic phosphorescence spectra, are proposed as the active sites for the photoinduced non-oxidative methane coupling. The luminescence sites of silica-alumina which exhibit the characteristic phosphorescence spectra have been proposed to be highly dispersed aluminum species on the surface of dehydrated silica-alumina [6]. The results on the reaction listed in table 1, i.e., the
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0.60.5-
0
E 0.4=I. 7_=o . 3 r
[.-o
(d)
0.2-
0.10.0--
!
400
I
600
I
800
I
1000
Pretreatmenttemperature/ K
Fig. 3 Total yield in photoinduced nonoxidative methane coupling on SiO2A1EOa(L) evacuated at various temperature. Irradiation time was 3h. Initial amount of methane was 200 ~tmol.
400
500 600 Wavelength /nm
7t
Fig. 4 Phosphorescence emission spectra of SiO2-A1203(L) evacuated at 473 K (a), 673 K (b), 873 K(c) and 1073 K(d). Excitation wavelength was at 300 nm.
total yield on SiO2-A1203(L) was higher than that on SiO2-A1203(H), also suggested that the highly dispersed aluminum species are responsible for the activity in the photoinduced reaction. Therefore, the highly dispersed aluminum species on the silica-alumina would be the active sites for both the phosphorescence and the photoinduced reaction. In order to confirm that the luminescence sites correspond to the active sites for this reaction, interaction between the photoexcited sites and methane molecules was studied. In the presence of gaseous molecules the luminescence would be quenched if the r~ ID molecules interact with the photoexcited sites. Fig. 5 shows quenching effect by methane on phosphorescence spectra of SiO2-A12Oa(L) evacuated at 1073 K. The intensity was obviously reduced with 400 500 600 700 Wavelength /nm increase of methane pressure, indicating that methane interacted with the photoexcited sites on the surface of Fig. 5 Phosphorescence spectra of S iO2SiO2-A1EOa(L). Introduced methane of A1203(L) evacuated at 1073 K in vacuo (a) only 2 Torr was enough to quench a hal f and in the presence of methane: 1.0 Torr (b), of the phosphorescence intensity, while 3.1 Torr (c) and 4.0 Tort (d).
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nitrogen (inert gas) of 15 Torr was required. This indicates that methane molecules interact with the photoexcited sites and receive the excitation energy from the photoexcited sites. Probably this step is the key step for the activation of methane molecules. From the above results, the conclusion of this section is that the highly dispersed aluminum species on the surface of silica-alumina evacuated at high temperature act as not only the luminescence sites for the characteristic phosphorescence spectrum but also the active sites for the photoinduced non-oxidative methane coupling.
4. CONCLUSION It was found that the coupling of methane proceeds successively upon photoirradiation over the silica-alumina evacuated at high temperature without using any oxidant molecules. No oxygenated products such as MeOH, HCHO, CO and CO2 are observed in this system. The silica-alumina exhibits the high products yield in the gaseous phase in contrast to the alumina. The active sites for this photoinduced reaction are identical to the phosphorescent luminescence sites, which are highly dispersed aluminum species on the surface of the silicaalumina evacuated at high temperature. The energy transfer to methane molecules from photoexcited sites on the silica-alumina, which would be the key step for methane activation, is observed by phosphorescence study. REFERENCES
1 L. Guczi, R. A. Van Santen and K. V. Sarma, Catal. Rev. Sci.-Eng., 38 (1996) 249. 2 K. Okabe, K. Sayama, H. Kusama and H. Arakawa, Chem. Lea., 1997, 457. 3 W. Hill, B. N. Shelimov and V. B. Kazansky, J. Chem. Soc., Faraday Trans. 1, 83 (1987) 2381. 4 (a) H. Yoshida, T. Tanaka, S. Matsuo, T. Funabiki and S. Yoshida, J. Chem. Soc., Chem. Commun., 1995, 761. (b) T. Tanaka, S. Matsuo, T. Maeda, H. Yoshida, T. Funabiki and S. Yoshida, Appl. Surf. Sci., 121/122 (1997) 296. (c) H. Yoshida, K. Kimura, Y. Inaki and T. Hattori, Chem. Commun., 1997, 129. 5 (a) H. Yoshida, C. Murata, Y. Inaki and T. Hattori, Chem. Lea., 1998, 1121. (b) H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki and S. Yoshida, Chem. Commun., 1996, 2125. (c) H. Yoshida, T. Tanaka, M. Yamamoto, T. Yoshida, T. Funabiki and S. Yoshida, J. Catal., 171 (1997) 351. 6 H. Yoshida, T. Tanaka, A. Satsuma, T. Hattori, T. Funabiki and S. Yoshida, Chem. Commun., 1996, 1153. 7 Y. Kato, H. Yoshida and T. Hattori, Chem. Commun., 1998, 2389. 8 S. Yoshida, T. Matsuzaki, T. Kashiwazaki, K. Mori and K. Tarama, Bull. Chem. Soc., Jpn., 47 (1974) 1564. 9 (a) Y. Murakami, Stud. Surf. Sci. Catal., 16 (1983) 775. (b) T. Uchijima, Catalytic Science and Technology, (eds.) S. Yoshida, N. Takazawa and T. Ono, Kodansha, VCH, Tokyo, vol. 1, (1991) 393.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Photocatalytic oxidation of gaseous toluene on polycrystalline TiO~: FT-IR investigation of surface reactivity of different types of catalysts G. Martraa*, V. Augugliarob, S. Colucciaa, E. Garcia-L6pezb, V. Loddo b, L. Marchese c, L. Palmisa._~oband M. Schiavellob di Chimica IFM, Universit~ di Torino, Via P. Giuria 7, I-10125 Torino, Italy ~ ipartimento ipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit~ di Palermo, Viale
_ delle Scienze, 1-90128 Palermo, Italy ~Dipartimento,, di Scienze. e Tecnologie Avanzate, Universit~ del Plemonte" Orientale" "A. Avogadro, C.so Borsalino 54, I-15100 Alessandria, Italy
Commercial TiO2 Merck and TiO2 Degussa P25 powders were employed as the catalysts for the photo-oxidation of toluene. By using TiO2 Merck benzaldehyde was found in gas phase as the main product of the toluene partial oxidation. After an initial transient period, this catalyst exhibited a high stability in the presence of water vapour in the gaseous mixture, whereas the photoproduction of benzaldehyde sharply decreases after removal of H20 fi'om the feed. IR spectra of the used catalyst revealed that in the absence of water vapour benzaldehyde is molecularly held on the catalyst surface. This feature was confirmed by co-adsorbing benzaldehyde and water on the fi'esh catalyst. By contrast, when toluene photo-oxidation was carried out on TiO2 Degussa P25 no products in the gas phase were detected. In this case benzaldehyde adsorption, monitored by IR spectroscopy, mainly resulted in the formation of benzoate-like species, strongly adsorbed on the catalyst surface. 1. INTRODUCTION Heterogeneous photocatalysis by semiconductors is a fast growing field of basic and applied research, especially for the case of the oxidation processes of organic pollutants in waste waters [1-3], or in air [4-6]. Among the photocatalytic processes in gas-solid regime, the photo-oxidation of gaseous toluene (the most abundant aromatic volatile organic pollutant in indoor and industrial emissions) to benzaldehyde on TiO2 powders has been recently reported as an effective method to transform noxious C6HsCH3 molecules in a valuable chemical [7]. However, the product distribution and the catalyst stability strongly depend on the nature of the catalyst and the experimental conditions. Ibusuki and Takeuchi [8] obtained the complete photo-oxidation of toluene on TiO2 in the presence of water vapour carried out at room temperature. They used four photoreactors in series and they found that the presence of water was beneficial in order to achieve the almost complete mineralisation of toluene, benzaldehyde having been detected only in very small amounts. Furthermore, Obee and Brown [9] studied the photooxidation of toluene and other organic pollutants in gas-solid regime by using polycrystalline TiO2 as catalyst, evaluating the influence of the competitive adsorption of water and toluene vapours on the photo-oxidation rate.
*Author to whom correspondence shouldbe addressed; tel.: +39-011 670 7538; FAX: +39-011 670 7855; e-mail:
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In a previous paper we reported a combined catalytic and FT-IR study of the partial photooxidation of toluene in gas-solid regime by using a commercial anatase TiO2 Merck powder as catalyst [7]. In that case benzaldehyde in gas phase was obtained as the main product, and a strong dependence of the photocatalytic behaviour on the presence of water vapour in the gaseous reaction inlet was observed. In the present study we investigate more deeply the phenomena occurring during the process on the surface of the semiconductor particles by FT-IR spectroscopy, focusing on the role of molecular water in the interaction between photo-oxidation products and the catalyst surface. Furthermore, the fate of the photo-oxidation products in dependence of the strface features of the catalyst was also studied, carrying out the toluene photo-oxidation on a different type of TiO2 powder (i.e. TiO2 Degussa P25). 2. EXPERIMENTAL Photo-oxidation runs were carried out by using polycrystalline TiO2 Merck (anatase phase, BET specific surface area 10 m2.gq) as catalyst. Some selected rims were carded out by using TiO2 Degussa P25 (80% anatase, 20% rutile, BET specific surface area 50 mE'g-l). The reactant mixture was obtained by bubbling air through saturators containing bidistilled water and toluene (Carlo Erba, RS) at room temperature. The gas flow rate was 0.42 cma.s-1 and the toluene and water molar fractions were 1.3x10-2 and 2.5xl 02 respectively. Water vapour was kept in the feed until steadystate condition of toluene fractional conversion to benzaldehyde was attained. After that, water was removed fi'om the inlet gaseous mixture and then readmitted in the final part of the run. A 400 W medium pressure Hg lamp (Polymer GN ZS, Helios Italquartz) was used to irradiate a continuous fixed-bed photoreactor consisting of a Pyrex cylinder described in ref. 2, thennostatted at 140 ~ For the IR measurements (Bruker IFS 48, 4 cm -1 resolution), the TiO2 powders were pressed in form of self supporting pellets, and then placed in a IR quartz cell equipped with KBr windows, permanently connected to a conventional vacuum line (residual pressure: 1.0xl0 ~ Torr: 1 Tow= 133.33 Pa) allowing all thermal and adsorption-desorption experiments to be carded out in situ. 3. R E S U L T S and D I S C U S S I O N 3.1. Photoreactivity results
The UV irradiation of the toluene\air~I20 mixture flowing through a catalytic bed of TiO2 Merck mainly resulted in the production of benzaldehyde, but also benzene, benzyl alcohol and traces of benzoic acid were detected. In Figure 1 the toluene fractional conversion to benzaldehyde versus the irradiation time is reported for consecutive runs carded out in the presence ofH20 vapour (Fig. 1A) and after its removal (Fig. 1B) and re-admission (Fig. 1C).
,.o O .,..,
0.10
OeO00000eO00000
oo .o
Fig. 1. Toluene fi,actional conversion to benzaldehyde vs. UV irradiation time for consecutive rtms carried out in the presence of H20 vapour (section A), after removal of H20 from the feed (section B) and after H20 readimission (section C).
0.15.!
-~o
+ H20 ...............................
0.05-
0.0
I0 9 ! O0 000 i
0
]60
260
+ H20
p.
360
Time (h)
460
le i
| '1
9
500
9
600
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By using a wet feed, steady state conversion of toluene to benzaldehyde was achieved after a transient period of 70 h of irradiation (Fig. 1A). After 340 h no decrease of the benzaldehyde photoproduction was observed, showing that, in the presence ofH20 vapour, TiO2 Merck behaves as a highly stable photocatalyst. By contrast, a sharp decrease of toluene conversion to benzaldehyde occunvxt after removal of water from the feed, followed by a further slight loss in activity for longer time of irradiation (Fig. 1B). When water vapour was readmitted in the reaction mixture, a limited increase of the amount of benzaldehyde in gas phase was observed, but it quickly turned down to the level achieved in the absence of H:O (Fig. 1C). By contrast, for the runs carded out by using TiO2 Degussa P25 as photocatalyst, no significant amount of products of toluene conversion was detected in the gas phase during the irradiation time. Moreover, the catalyst became brown coloured after the runs, indicating that toluene photo-oxidation products remained adsorbed on the catalyst surface. In order to determine the nature ofthese species, the catalyst was stirred in acetonitrile for 24 hours at the end of the run and, after its separation, the liquid phase was analysed by HPLC. Benzaldehyde, benzyl alcohol and benzoic acid were found. In a similar experiment on TiO2 Merck much less benzoic acid was found. 3.2. FTIR studies 3.2.1. Nature of the reaction products adsorbed on the surface of the TiO2 Merck catalyst In order to mtionalise the catalytic behaviour commented on above, a study of the nature of species present on the catalyst surface at different stages of the run was performed, comparing the IR spectra of a sample atter a photocatalytic nm in the presence of H20 vapour (Fig. 1, stage A) with the spectra of a sample after irradiation in the absence of H20 (Fig. 1, stage B). The results are reported in Figure 2, where the spectrum of fresh TiO2 Merck powder simply outgassed at r.t. (dotted lines) is also reported to better evidence the spectral features related to the species adsorbed on the used catalysts. This spectrum simply exhibits a broad band at ca. 1630 cm-~, due to the bending vibration of adsorbed molecular water, and weak components in the 1500-1400 cm -~ range, due to carbonate-like groups [7].
10,
// ./
o ,.Q
95 %) and very selective (CO selectivity > 94 %) at 800~ H2/CO ratio is close to 3 which means that water gas shift reaction is little active and confirms that the reduced iron located outside the perovskite is linked to nickel (cf. TPR) or with lanthanum as LaFeO3 (XRD).Yet, those systems can become differentiated for each others when studying their ageing (figure 3). LaNi0.3Fe0.7 is very stable and after a slight initial decrease, the catalyst keeps an yield over 90% after 200 working hours. At the opposite, CO yield decreases with time (from 95% initially to 40% after 200 hours) on catalyst that contains more nickel (LaNi0.TFe0.303). - LaNiO3 that rapidly forms a carbon deposit in our procedure test. H20/CH4 ratio value is a crucial point. With a H20/CH4 = 3, even if water excess allows the limitation of carbon formation [8], the selectivity turns from CO to CO2 (71% conversion, 60% CO and 40% CO2 selectivity) at 800~ for LaNi0.3Fe0.703. 3.4 Characterisation of LaNixFel.xO3 after catalytic test Table 1 gathers the crystalline phase detected by XRD, and the percentage of free nickel (XRD) and the ratio NirejNifree calculated from magnetism measurement. On studying XRD data, it can be concluded that poor nickel content systems (0.1 _< x _ 0.3) keep their LaNiFe trimetallic structure even if the nickel ratio strongly decreases. The absence of free nickel characterisation (Ni ~ or NiO) shows its high dispersion. On the nickel rich systems, the LaFeO3 perovskite is the only one to be conserved with an increasing formation of La203. Whereas nickel is on a NiO form for x = 0.5 and 0.6, nickel is on a Ni ~ form for 0.7 _< x _ 1. For x = 0.9 and 1, we can observe carbon deposition. The regular increase of specific area with x from 3.2 m2.g -1 to 37.6 m2.g -1 (from 4.2 to 5.5 m2.g -~ for the catalysts before reactivity) is mainly due to carbon formation. This specific area increase becomes important from x _> 0.6 (9.4 m2/g for 0.5; 15.9 for x = 0.6). An elementary analysis of carbon on all the catalysts after reaction gives a carbon percentage below 0.3% for x _< 0.5 and 13.4% for x = 0.9.
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~
100 . .. -A ....
80
e~
I
._o 60
/ ~- , -
I,..
>
40
cO
0
0 9C H 4
ta
_ _,_
~.--~
t
d
t
2On
/
1
\
0 "-0
X a H20/CH4
" - ~--
.ItI
- - e - - CO2 selectivity
i
. "~--.----o--0.2 0.3 0.4
" Cselectivity CO selectivity
~i
~
~-:--_...-~.,,----*--~ _ 0.5 0.6 0.7 0.8 0.9 1
Fig. 2 : Selectivity into CO, CO2 and carbon at 800~
ratio = l.
'~!
i\"
" 0.1
800oc
01
k "\
700oc
~ ' ~ ,
/~"\
'\
_
-
',,
\,,/
40 / 9
-i\
650oc
e--
---A--- 750oc
conversion at various temperature versus x with
60 i_ ~
--,,.--*..
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
801~ \,
.~
_. ~_. 600~ -.
"'-.,I ~ '.. ;
./.--7 .." . ~...... . ~ ,........ . ~ ., . ~ . ,. ~ . , ,~: ;'._~.-.~,
-r
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.~,
-A . . . .
/."':"" ,,.- ., - . _ , ~ - , ~ - , .". ',, /3./,\ ~:'/
~ 20
~
."
A...
X
versus X (H20/CH4 = 1).
100 80 60
---n-LaNio.3Feo.703
"~,
40
~LaNio.7Feo.303
O O
20
"o (1}
0 0
50
100
150
200
T i m e (h)
Fig. 3 : CO yield versus time for LaNio.3Feo.703 and LaNio.7Feo.303.
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Table 1 Crystalline phases detected by XRD after test : Crystalline phases detected by XRD after test
X 0 0.1 0.2 0.3 0.4 0.5 0.6
Nired/Nifree
50 65 73 100 100 100
15 19 28 43 47 57
100 100
64 67
100
74
100
85
(%)
LaFeO3
LaFeO3 LaFeO3 LaFeO3 0.7 LaFeO3 0.8 LaFeO3
LaNio.05Feo.9503 LaNio.ovFeo.9303 LaNio.08Feo.9203
La203 La203 La203 La203 La203
NiO NiO NiO NiO*
Ni* Ni
La203
Ni
La203 1.0 La203 * Corresponds to phases present as traces.
Ni Ni
0.9
% of free Ni
C C
On a micrograph after reactivity and EDS analyses of LaNi0.3Fe0.703, we confirm the presence of LaNiFeO3 perovskite (figure 4a). Some NiO particles (several nm) have also been evidenced (figure 4b). For the rich nickel systems (x = 0.7), some 10 to 20 nm particles of NiO can be seen on the edge of perovskite structure. LaFeO3, La203 and turbostatic carbon around big nickel particles can also be observed. The study of the elementary distribution (EDS) of these three elements for x = 0.3 shows that La/Fe ratio is constant (perovskite structure is preserved), and presence of nickel enriched zones (free nickel). For x _> 0.7, catalysts are much more heterogeneous with a variable La/Fe ratio and some very rich nickel zones. These observations can explain the catalysts ageing (figure 3). As CH4/H20 is more oxidant than CH4/CO2 it must be noticed that unlike TPR and dry reforming [4] results, in the reaction conditions no Ni-Fe alloy formation can be found and evoked for restricted the carbon formation. For the nickel poor systems (0.2 _< x ___0.4), the presence of nickel in the bulk and on the surface of the perovskite structure leads to a strong interaction between the small metal particles and the crystal lattice. This strong interaction results in a slow progressive formation of active species decreasing the sintering and hence the carbon deposition. The carbon deposit is activated in presence of the LaFeO3-La203 oxidizing system in vicinity of the metal particles and its accumulation will be limited. Further, the oxidizing character of LaFeO3 near the metal particles lowers the sticking coefficient of CH4. The strong interaction of Ni with the crystal lattice permits the regeneration of the system by a mere calcination. For the nickel rich systems, such an interaction is not observed, and the systems are not sufficiently reducible to form a Ni-Fe alloy as with CH4/CO2. The higher the nickel content, the more nickel particles without strong interaction will be formed and the more carbon will be deposited with time on stream leading to deactivation. These latter catalysts are not regenerated by reconstruction of the initial perovskite.
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Fig. 4 : HRTEM micrographs of LaNio.3Feo.703 after vaporeforming (a) : mixed La, Ni, Fe perovskite (b) : NiO particles on La,Ni,Fe perovskite 4. CONCLUSION Systems active for vaporeforming of CH4 with low H20/CH4 ratio have been obtained starting from a trimetallic LaNixFel_xO3 perovskite and then characterised before and after reactivity. The system are particularly efficient for x=0.3 which the trimetallic structure is preserved together with LaFeO3 perovskite and small Ni particles. By this way, carbon deposition is limited. ACKNOWLEDGEMENTS
The authors thank the European Community for financial support of the JOULE contract JOR3-CT95-0037. REFERENCES
1. J.R. Rostrup-Nielsen, , Catalytic Steam Reforming >>in J.R. Anderson and M. Boudart (Editors), Catalysis, Science and Technology Vol. 5, Springer Verlag, Berlin (1983). 2. Alstrup, B.S. Clausen, C. Olsen, R.H.H. Smits and J.R. Rostrup-Nielsen, Stud. Surf. Sci. Catal., 119, (1998) 5. 3. Z. Zhang and X.E. Verykios, Catal. Lett. 38 (1996) 175. 4. H. Provendier, C. Petit, C. Estourn6s and A. Kiennemann, Stud. Surf. Sci. Catal., 119 (1998), 141. 5. H. Provendier, C. Petit, A.C. Roger and A. Kiennemann, Stud. Surf. Sci. Catal., 118 (1998) 285. 6. J. Garcia, J. Blasco, M.G. Proietti and M. Benfatto, Phys. Rev. B. Condensed Mat., 52 (1995) 15823. 7. M . Houalla, F. Delannay, I. Matsuurai and B. Delmon, J.C.S. Faraday 176 (1980) 2128. 8. H. Provendier, C. Petit, C. Estourn~s, S. Libs and A. Kiennemann, Appl. Catal. A, 180 (1999) 163. 9. D.L. Trimm, Catal. Today, 37 (1997) 233.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Carbon deposition and reaction steps in CO2/CH 4 reforming over NiLa203/SA catalyst J.Z. Luo", L.Z. Gao "'b, Z.L. Yu a'b, and C.T. Au a'* "Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong, China. Emaii:
[email protected] bChengdu Institute of Organic Chemistry, Chinese Academe of Sciences, Chengdu, Sichuan, China The cause of carbon deposition and the reaction pathways for COJCH4 reforming over NiLa203/5A have been investigated. XRD results revealed that due to the formation of perovskite-like La2NiO4 in Ni-La203/5A, the small-size (ca 9 nm) Ni ~ crystallites formed in H2 reduction remained unsintered during 48 h of on stream reaction. The accumulation of carbon on the active sites is the main reason for catalyst deactivation. The detection of ~3CO2 and CO2 in 02 pulsing onto a sample pretreated with 13CH4/CO2 confirmed that the deposited carbon was from both CH4 and CO2. In CO and COJCH4 atmospheres, we observed similar TGA patterns and identical TEM images (carbon nanotubes) of deposited carbon; we propose that carbon deposition is mainly via CO disproportionation. The observation of CD3COOH and CD3CHO in CD3I chemical trapping experiments and the detection of formate/formyl bands in DRIFT suggested that HCOO and HCO were intermediates. The amount of CO2 converted was roughly proportional to the amount of H present on the catalyst. These results indicated that CO2 activation could be H-assisted. Pulsing CH4 onto a H2-reduced sample and a similar sample pre-treated with CO2, we found that CH4 conversion was higher in the latter case. Hence, the idea of oxygen-assisted CH4 dissociation is plausible. As for the rate of methane conversion, a kH/kD ratio of 1.2 and 1.1 was observed at 600 and 700~ respectively, implying that C-H cleavages are slow kinetic steps. Based on these experimental results, we have derived reaction pathways for CO2/CH 4 reforming. In the proposed mechanistic model, CHxO (x-1 or 2) decomposition is considered to be a rate-determining step. 1. INTRODUCTION CO2 reforming of methane for the production of syngas with HJCO ratios suitable for F-T synthesis has aroused renew interest in recent years. Many transition metal-based catalysts have been investigated [1]. It has been reported that catalyst deactivation due to carbon deposition is a serious problem [2,3]. Noble metals such as Rh and Ru can reduce carbon deposition [3]. However, considering the high cost and limited availability of these precious metals, it would be more attractive to develop a nickel-based catalyst. In general, carbon deposition via CH4 decomposition or CO disproportionation could be reduced if fine nickel particles are supported on a metal oxide of strong basicity [4-6]. In the present work, COJCH4
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reforming over Ni-La203/5A was studied by in situ TGA, pulse experiments, chemical trapping, TEM, and in situ DRIFT.
2. EXPERIMENTAL The Ni-La203/5A catalyst was prepared by adopting the citric acid complexing method. The loading of Ni was 8 wt%. The catalytic activity was measured in a fixed-bed quartz-reactor under reaction conditions of temperature = 800 ~ GHSV = 48,000 mL h ! g~, and CO2/CH 4 molar ratio = 1 / 1. The phase structures were determined by X-Ray Diffractrometry (XRD, Rigaku D-MAX). Ni particle size was estimated according to the half height width of the Ni (111) peak obtained in XRD investigation. The deposition and reactivity of carbon on the catalyst were examined using a Thermal Gravimetric Analyzer (TGA, Shimadzu DT-40). TEM images of deposited carbon were taken by means of a JEM-1000CX (JEOL) equipment operated at 100 kV. Pulse and CD3I chemical trapping experiments were performed in a micro-reactor equipped with a mass spectrometer (HP G1800A). In order to identify the products correctly, we have taken into account the interference due to isotopes and fragments. High purity He (99.99%) was used as carried gas. 13CH4 (>99.9%) and CD4(>99%) were employed for the investigation of isotope effect. For the pulsing experiments, the volume of each pulse was 67.5 pL. As for the in situ DRIFT experiments, a Nicolet Magna 550 FT-IR Spectrometer was used. 3. RESULTS AND DISCUSSION
3.1. Phase structure and catalytic performance According to the XRD patterns, strong signals of La2NiO4 and 5A phases as well as weak signals of A1203 and SiO2 phases were observed. After reduction in H 2 at 500~ nickel existed mainly as Ni ~ particles, with diameter estimated to be ca 9 nm. The formation of La2NiO4 in the fresh catalyst indicated that with the aid of complexing ability of citric acid, Ni 2§ and La 3§ ions could be evenly distributed. The size (ca 9 nm) of Ni ~ particles generated in H2-reduction is much smaller than that (> 100 nm) observed over Ni/La203 [5]. It indicates that La2NiO4 is a good means for producing fine Ni ~ particles. Supports which are basic have been reported to be capable of activating CO2 and are favourable for the elimination of deposited carbons [ 1,6]. 5A molecular sieve can adsorb and activate CO2 due to its great affinity towards the gas. At 800~ the Ni-La2OB/5A catalyst showed 90.0% CO2 conversion and 91.7% CH 4 conversion. With the advance of time from 10 rain to 48 h, CO2 and CH 4 conversions decreased from 90.0% and 91.7% to 82.2% and 82.0%, respectively. These catalytic performances are comparable to those observed over the 1 wt% Ir/A1203 catalyst [3]. Generally speaking, there are two reasons for the deactivation of nickel-based catalysts in CO2/CH4 reforming: (i) the blocking of active sites by carbonaceous deposits; (ii) the sintering of nickel particles. Since the size of nickel particles remained unchanged in 48 h of on-stream reaction, we suggest that the main cause of catalyst degradation is carbon deposition. 3.2. Carbon deposition Figure 1 shows the TGA profiles of carbon deposition as related to reaction time at various temperatures in a c a 4 , CO or CO2/Ca 4 (molar ratio - 1:1) atmosphere. In ca4, the amount of
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carbon deposited increased when the temperature was changed from 700 to 800~ However, in a CO/N 2 or CO2/CH4 atmosphere, the extent of carbon deposition decreased with the same change in temperature. The morphology of carbon deposited on the catalysts at 800~ in different atmospheres was investigated by TEM. The carbon coming from CH4 was mainly the encapsulated type and EDX demonstrated that it was amorphous. The deposited carbon formed in CO or CO2/CH 4 existed mainly as carbon nanotubes. These carbon nanotubes were more or less twisted with an outer diameter of 10-~20 nm and a length of up to 10 ~tm; EDX revealed that they were mainly graphitic carbon. We purpose that CO disproportionation is the main cause for carbon deposition in CO2/CH4 reforming. 150 100 50
~o 150
E 100 ~
~
50
.,~
15o 100
a)CH~
.,,__,
// / m _ _ m - ~
m--'m
/A~x/X-~--'x~X--X
~m~
14 =.12
10
(b) CO
_•AX.I.I--i, ~ x
.
'1--1
x- - x
.&--&
(c) COdCH4
~
8
0 ~
6
y-
(a) / -
(b)
o o
<E
50
0
20 40 60 80 100 On-stream time (min)
Fig. 1. TGA profiles of Ni-La203/5A versus time on stream at 600~ (=), 700~ (• and 800~ (A) in various atmospheres.
(c)
2 A~A~A_---------&
~
&
Ordinal number of 13CH4/CO2 pulse Fig. 2. The amount of 13CO2 generated versus 13CH4/CO2 pulse number at (a) 600~ (b) 700~ and (c) 800~ over Ni-La203/5A.
In order to investigate the origin of deposited carbon and the reaction mechanism of CO2/CH 4 reforming, we pulsed 13CH4/CO2 (molar ratio = 1:1) at a desired temperature onto a Ni-La203/5A sample which had been Hz-reduced at 500~ for 1 h. After 5 pulses of 13CH4/CO2, the deposited carbon was treated with 4 pulses of 02 at the same temperature. Throughout the experiment, the 13COJCO2 ratio and the amount of '3CO2 generated were monitored. With the increase of 13CH4/CO2 pulse number, the amount of 13CO2 generation increased; such an increase was the most obvious at 600~ (Fig. 2). The generation of 13CO2 affirms the accumulation and oxidation of surface 13C species. When 02 was then pulsed onto the sample, both ~3COz and CO2 were detected. We conclude that both CH4 and CO2 contribute to carbon deposition. In the 4 pulses of 02, the total amount of 13CO2 produced and the '3COJCO2 molar ratio decreased with the increase in temperature. It is clear that carbon deposition above 700~ is mainly due to CO2 dissociation and CO disproportionation.
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3.3. Reaction mechanism for syngas formation
3.3.1. CO2 and C H 4 activation The amount of C H 4 converted in 5 pulses of C H 4 at 600~ over a H2-reduced Ni-La203/5A sample was 1.3 pL (Table 1) and there was no generation of C2H6, C2H4, or C O x. It indicated that the carbon generated in CH4 dissociation remained completely on the catalyst. When CH4 was pulsed onto a H2-reduced sample pretreated with 5 pulses of CO2 at 600~ (Table 2), CO was detected, indicating that there was interaction between C H 4 and the oxygen released in CO2 dissociation. Taking into consideration that the amount of converted C H 4 (8.8 pL) over the CO2-treated catalyst was much more than that (1.3 ~tL) over the reduced catalyst, we propose that surface oxygen species such as O and OH promote the decomposition of CH4. Similar trends were observed at 700 and 800~ Hence, we advocate the idea of oxygenassisted CH4 dissociation.
Table 1 The amounts of converted CH 4 and CO2 and that of CO generated in first 5 pulses of CH4 followed by 5 pulses of CO2 over a H2-reduced Ni-La203/5A sample. Temp. (~
CH 4 pulse C H 4 (~L)
600 700 800
1.3 67.5 113.4
CO (~tL) 0.0 0.0 4.2
CO 2 pulse C02 (~tL) 13.0 101.2 303.8
CO (~tL) 14.0 114.8 336.8
Table 2 The amounts of converted CO2 and CH 4 and that of CO generated in first 5 pulses of CO2 followed by 5 pulses of CH4 over a H2-reduced Ni-La203/5A sample. Temp. (~ 600 700 800 * produced from
CO2 pulse C H 4 pulse CO2 (~tL) CO (~tL) CH 4 (ktL) CO (~L) 8.1 6.0 8.8 1.4 58.7 55.3 114.3 36.5 293.0 283.5 308.5 219.4 5 pulses of C O 2 / C H 4 (molar ratio = 1/1) at 600~
C H 4 / C O 2 pulse*
CO (~tL) 140 240 307
When CO was pulsed over a H2-reduced Ni-La203/5A sample at or above 600~ CO2 and detected in the effluent. The generation of CH4 was due to CO methanation, a result of CO interaction with the hydrogen adsorbed during the reduction of the catalyst. We envisioned that the amount of C H 4 generated should be proportional to the amount of H species present. In order to vary the concentration of H adspecies, we purged the H2-reduced sample with He for 10 min at 600, 700, and 800~ respectively. We then cooled the sample to 600~ in He and pulsed CO onto the catalyst until there was no observable change in CO peak intensity. Table 3 shows the amount of CH4 produced during CO pulsing. One can observe that with the rise in purging temperature, the amount of CH4 generated decreased. Similarly, we observed that the amount of CO2 consumption during CO2 pulsing at 600~ over the HeCH 4 were
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purged samples decreased with the rise in purging temperature. It was a clear indication that surface hydrogen promoted the dissociation of CO2. Following the introduction of 5 pL of CD3I onto a sample pretreated with CO2/CH4, w e detected CD3COOH. Formate signals at 2915 and 2885 cm -~ were observed in DRIFT studies. These results suggested that HCOO was an intermediate in C O 2 / C H 4 reforming. The existence of HCOO on the sample indicated that CO~ reacted with the adsorbed H on nickel. From Table 2, it can be seen that the total amount of CO (7.4 ktL) formed in the first 5 pulses of CO2 and in the following 5 pulses of C H 4 w a s much smaller than that (140 pL) generated in 5 pulses of C O 2 / C H 4 at 600~ Similar trends were obtained at 700 and 800~ Therefore, we suggest that C H 4 and CO2 can mutually activate each other. _
Table 3 The amounts of CH 4 formed in CO-pulsing and that of CO2 consumed in CO2-pulsing over a H2-reduced Ni-La203/5A sample. He purging Temperature (~ Amount of C H 4 formed (~tL)* Amount of CO2 consumed (pL)* * detected at 600~
600 4.7 9.0
700 2.7 5.2
800 0.5 2.0
3.3.2. Reaction intermediates Deuterium isotope effect on methane conversion was observed at 600 and 700~ but not at 800~ There was no obvious isotope effect on the conversion of CO2. The ratios of C H 4 and CD4 conversions were 1.2 and 1.1 at 600 and 700~ respectively. The results are in accord with the expected CH4/CD 4 isotope effect [7]. At 800~ C H 4 and CO2 conversions over NiLa2OJ5A were, respectively, 91.7% and 90.0%, suggesting that the reaction was close to thermodynamic equilibrium. In other words, the reaction was controlled by thermodynamics rather than by kinetics and n o C H 4 / C D 4 isotope effect was observed. The isotope effect observed at 600 and 700~ indicated that C-H cleavages are slow steps. There are three possible ways for C-H cleavage in C O 2 / C H 4 reforming: C H 4 ---) C x +
4H
(i)
CHx + O --) CO + xH (ii)
CH•
--) CO + xH (iii)
Among them, step (i) was believed to be reversible [1,8]. As described above, direct C H 4 dissociation could take place at 600~ over a reduced Ni-La203/5A sample. After CO2/CH4 exposures, no IR bands at around 3000 cm ~ were observed, indicating the absence of CHx adspecies; this could be a result of fast CHx interaction with surface oxygen species. The 13CH4/C02and C H 4 - C O 2 pulsing experiments also demonstrated that the deposited carbon deriving from C H 4 reacts with CO2 quite readily. Thus we deduced that step (ii) was a combination of two elemental steps: CHx + O --) CHxO and CHxO --) CO + xH. Since the former elemental step is rather facile, we propose the latter to be rate-determining. The observation of CD3CHO in CD3I chemical trapping indicated that there was HCO on the sample. The broad and weak IR bands in the range of 2700-3000 cm ~ could be due to CHxO (x = 1-3) species [7,9]. Since no CD3OCH3 was detected in CD3I trapping and there was no ethane detected in C O 2 / C H 4 reforming, we speculated that there was no methoxy or CH3 on the surface during the reforming reaction. It was reported that formate dissociated quite
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readily to formyl and formaldehyde in H 2 at high temperatures [9]. Also, IR studies demonstrated that H2CO adsorption would give rise to formyl [9]. Based on these understandings, we suggest that there were HxCO (x = 1 or 2) species on the sample and the decomposition of HxCO is rate-determining.
3.3.3 Mechanistic model Based on the above results and discussion, we propose the following mechanistic model for C O J C H 4 reforming (s, surface; g, gas phase):
(1) (2) (3) (4) (5) (6)
CH4, g + CH4, s CO2, g "-) CO2, s CH4,., "-) CH,,, s + (4-x)H, s c o . , , , --> CO, s + O,s CO2, s + H,~ + HCOO, s HCOO, s ---) HCO, s + O,s
(7) (8) (9) (10) (11) (12)
HCOO, s --> CO, s + OH, s CHx, s + O,~ --) CH,,O, ~ CH4, s + O, s '--) CHxO + (4-x)H, s CHx, s + OH, s "--) CHxO, s + H,s CHxO, s "-> CO, s + xH, s H,s + OH, s "-> H20, s
In this model, CH4 decomposition on Ni ~ is assisted by the oxygen generated in CO2 dissociation via CHxO (x = 1 or 2) formation. The CO2 adsorbed on basic sites dissociates with or without the aid of H species originated in CH4 decomposition to give CO and O or CO and OH. The rate-determining step is the decomposition of CHxO to CO and H. ACKNOWLEDGMENT
The work described above was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region, China ( Project No. HKBU 2053/98 P). REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
M.C.J. Bradford and M.A. Vannice, Catal. Rev. -Sci. Eng., 41 (1) (1999) 1. T. Nishiyama and K.I. Aika, J. Catal., 122 (1990) 346. A.T. Ashcroft, A.K. Cheetham, M.L.H. Green, and P.D.F. Vernon, Nature (london), 35 (1991)225. O. Yamazaki, Y. Nozaki, K. Omata, and K. Fujimoto, Chem. Lett., (1992) 1953. Z.L. Zhang, X.E. Verykios, S.M. Macdonald, and S. Affrossman, J. Phys. Chem., 1O0 (1996) 744. G.J. Kim, D.S. Cho, H.H. Kim, and H.J. Kim, Catal. Lett., 28 (1994) 41. H. Burghgraef, A.P.J. Jansen, and. R.A. van Santen, J. Chem. Phys., 101 (1994) 11012. C.T. Au, M.S. Liao, and C.F. Ng, J. Phys. Chem., A 102 (1998) 3959. J.F. Edwards and G.L. Schrader, J. Phys. Chem., 89 (1985) 782.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
The Autothermal Partial Oxidation Kinetics of Methanol to Produce Hydrogen E. Newson, P. Mizsey, T. Truong and P. Hottinger General Energy Deptartment, Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland, Fax: +41 56 310 2199, Email:
[email protected] The kinetics of autothermal methanol partial oxidation are investigated to produce hydrogen for fuel cell systems. Two reactor systems are used to determine the kinetic parameters under isothermal conditions. The originally supposed six-reaction system (dimethyl ether formation, methanol decomposition, water gas shift, steam reforming, methanol partial oxidation (POX), hydrogen total oxidation) could be simplified, because the water gas shift reaction is slow in comparison to the others and the total oxidation of hydrogen is mass transfer limited with the commercial copper/alumina catalyst used. Previously determined kinetic data for methanol decomposition [7] were also used to facilitate the evaluation of the kinetic data. Respective activation energies in kJ/mol are 117, 76, -, 81 and 65 (POX), with the standard deviations of 6-24%. Turnover frequencies at 250~ for the POX reaction were calculated from copper surface area measurements. They were the same order of magnitude (460 min -1) as literature values [ 1,11 ]. Under non-isothermal "hotspot" operation, hydrogen production rates were 10000-13000 litresH2/hour/ litre reactor volume(lrv), which is equivalent to 30-39 kWth/lrv, providing significant power densities from the fuel processor. Hydrogen yields of 72% or 2.2 moles of hydrogen per mole methanol feed, with 1-2% CO in the exit gas, were measured. 1.
INTRODUCTION
The autothermal partial oxidation of methanol or hydrocarbons is being intensively studied on the catalytic [ 1,2] and reaction engineering scale [3,4] to produce hydrogen for stationary and mobile fuel cell systems. The mobility aspect requires reaction engineering of the exothermic partial oxidation for fast startup and endothermic steam reforming for higher hydrogen concentrations and system efficiencies [3]. Kinetics for the latter reaction have been published [5] and apparent activation energies for the POX reaction (482-71 kJ/mol) were dependent on Cu-Zn catalyst composition [ 1]. The valence state of Cu-Zn catalysts is critical to maximise hydrogen yields [1] whilst high methanol conversions and hydrogen selectivities were obtained with zinc oxide supported palladium catalysts using sub-stoichiometric oxygen/methanol feed ratios [2]. Targets for the catalyst performance are suggested by analysis of full fuel cycle efficiencies (ffc) from well to wheel [6]. The fuel processor-fuel cell subsystem must reach an energy efficiency of 40% so that the ffc is 23%. This exceeds comparable values of 18% for mobile systems using internal combustion engines. With a 50% efficiency for the polymer electrolyte fuel cell, an 80% efficiency of the fuel
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processor is required which is determined primarily by catalyst kinetics and subsequent reaction engineering. This paper develops an applied kinetic model derived initially on the basis of six kinetic constants based on isothermal measurements in two laboratory reactor systems. Non-isothermal measurements were also made to illustrate the power densities of the commercial catalyst. 2.
REACTION STOICHIOMETRY
The reaction system based on product analyses from preliminary work, is limited initially to six simultaneous reactions with equilibrium limitations as shown, together with the corresponding heats of reaction (AHR) at standard conditions. Table 1 Reactions considered for autothermal methanol partial oxidation Nr.
3.
Name
Reaction
1.
DME formation
2.
Me decomposition
2 CH3OH r
3.
Water gas shift (WGS)
4. 5.
Steam reforming (SR) Partial oxidation(POX)
6.
Hydrogen burning
AHR (kJ/mol)
CH3OCH3 + H20
CH3OH r
CO + 2H2
CO + H20 r
-21 99
CO2 + H2
-39
CH3OH + H20 r CO2 + 3 H2 CH3OH + 0.5 02 ~ CO2 + 2H2
60 -184
H2 + 0.5 02 ~ H20
-245
EXPERIMENTAL
The laboratory microreactor system is shown in Figure 1 as three major parts: feed preparation, reactor, analytics and PC control. The system is computer controlled for unattended, continuous operation with safety features for temperature and pressure. The control system monitors and saves the data on process parameters every minute for
gt7
.... ~
~ :
Balance
n
.!.
o
n
r
o
cco~
Fig. 1. Laboratory microreactor system for autothermal methanol partial oxidation.
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subsequent analysis. Methanol feed quality was analysis grade (99.8%) and gas purities were 99.995%. The feed mixture is vaporised and contacted in a static mixer with nitrogen, hydrogen or oxygen as required. The reactor has an internal diameter of 4 mm with a catalyst bed length of 14-120 mm depending on catalyst dilution and an internal thermowell for temperature measurements. 100 mg of commercial copper-alumina catalyst was used in the 250-500~tm particle size range. Catalyst activation began with outgasing in nitrogen at 300~ followed by air oxidation at 450~ a nitrogen flush, and reduction in hydrogen for one hour at 450~ GC analysis utilised three different columns and two detectors (FID, TCD). Condenser water content was analysed by a 737KF Coulometer. Reactor temperatures were between 220300~ weight hourly space velocities (WHSV) based on methanol were between 5-50 h -l, pressures between 102-103 kPa and data were taken continuously for up to 300 hours. Only data with elemental mass balances for C,H,O _+10% were considered. 4.
RESULTS AND DISCUSSION
For the kinetic model, each reaction is first studied to determine if equilibrium limitation is significant. Based on Gibbs free energies the equilibrium constants are determined. Reaction 1" Dimethyl ether formation (DME) Keq,DME =0.106243 exp(21858.37/R*T)
(1)
Reaction 2: Methanol decomposition (Me) Keq,Me =1.71791e14 exp(-95417.89/R*T)
(2)
Reaction 3" Water gas shift (WGS) Keq,WGS =9.54335e-3 exp(39876.31/R* T)
(3)
Reaction 4: Steam reforming (SR) Keq,SR =1.8493e10 exp(-56087.19/R* T)
(4)
The kinetics are significantly influenced by the WGS equilibrium, less so for DME and Me, and insignificant for SR. Reactions 5 and 6, partial oxidation of methanol and hydrogen burning, are not equilibrium limited. The determination of the kinetics of the 6-reaction-system were facilitated by investigating the reactions individually. Considering the complexity of the system only reactions 4 and 6 can be studied alone. To simplify the simultaneous solution of kinetic parameters for the six parallel reactions, auxiliary isothermal experiments were made for these reactions (WGS and hydrogen burning) in a microreactor system. 4.1 Kinetics of WGS A reformate gas mixture (2% CO, 10% CO2, 20% H2, 68% N2) is used to determine the kinetics of the water gas shift (WGS) reaction. The measurements at different temperatures, water compositions (water contents: 4.2, 20, and 30% of the total mixture), and conversion rates show that the rate of reaction of WGS depends both on the water ( PH20 ) and the carbon
monoxide composition ( "~ ) and can be described by the following equation:
rWGS=2"25682e-3exp ( 50,000 / R * T PCO * PH20 * Eqco
[
mol ] gcat sec
(5)
where the partial pressures are in kPa, and Eqco is:
Eqc 0 = 1- PCO~ * PH2/Keq,WGS * PCO * PH20
(6)
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During the evaluation of the experimental data for the whole 6-reaction-system, it was found that the WGS reaction rate is 1-3 orders of magnitude less than those of the other reactions and therefore this reaction can be neglected at the conditions studied.
4.2 Kinetics of hydrogen burning The hydrogen burning reaction was also individually studied. Isothermal kinetic measurements were made in the microreactor system, the results being evaluated in the temperature range 25-65~ At higher temperatures (200-300~ required for autothermal pox, this reaction is so fast that mass transfer is the rate limiting step. The rate on the pellet surface can be written: rH2 : k m a m ( C b - C s )
(7)
where km is the mass transfer coefficient, am is the external unit surface area per unit mass of pellet, Cb and cs are the concentrations in the bulk gas and at the pellet surface, respectively. Because the reaction rate at 200-300~ derived from the kinetics is much greater than the mass transfer rate, the concentration at the surface (cs) approaches zero. Ruthven [8] recommends a method to estimate the mass transfer coefficient which is dependent on the flows and the external surface area per unit mass of pellet, but the temperature dependence (< 5%) is neglected in the temperature range investigated.
4.3 Utilisation of the results of methanol decomposition kinetics Previous investigations of the kinetics of methanol decomposition [7] showed that the rate equation for dimethyl ether formation could be expressed as second order with equilibrium limitations, while the remaining methanol decomposition and steam reforming could be assigned their stoichiometric order with equilibrium terms being necessary for reactions 2 and 4 [5,6]. The relative rates of the four reactions are rDME > rSR > rMe >> rWGS . The rate of the WGS reaction is significantly lower than those of the others and can be neglected. 4.4 Kinetics of autothermal methanol partial oxidation (WPOX) The measured isothermal data points were evaluated with the SIMUSOLV software package. Arrhenius temperature dependence is assumed and the activation energies and preexponential factors are determined by the multidimensional optimisation method of the gradient type. As starting values, the parameters obtained for the kinetics of methanol decomposition are used. The multidimensional optimisation showed, that the activation energies of the previously studied DME formation, methanol decomposition, and steam reforming reactions are practically the same as those for the autothermal methanol partial oxidation (WPOX), Table 2. However, the pre-exponential factors for WPOX are different from those obtained for methanol decomposition kinetics. In the second step of the optimisation, the activation energies obtained earlier are accepted and only the pre-exponential factors and the activation energy of the methanol partial oxidation are determined. The results with standard deviations are shown in Table 3. Figure 2 shows the Arrhenius plot of the reactions. Figure 3 shows a typical rate of reaction profile at 250~ in the isothermal microreactor, WGS is neglected.
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Table 2 Comparison of activation energies Reaction Activation energies [J/mol] MeOH decomposition system WPOX data DME formation 117,000 116,157 MeOH decomposition 76,000 75,800 Steam reformin~ 81,000 81,100 Table 3 Pre-exponential factors and activation energies for WPOX Reaction Activation energy [kJ/mol] Pre-exp. factor [mol/gcat sec kPa x] DME 117 130.0 (17%) MeOH decomp. 76 1.138 (24%) SR 81 34.5 (12%) POX 65 (6%) 0.466 (11%)
The commercial catalyst used did not contain zinc oxide, which could explain the DME found in the reaction products. The estimated activation energy of 117 kJ/mol is similar to values obtained (95kJ/mol) with gamma alumina [10]. The values estimated for methanol decomposition (76kJ/mol) and steam reforming (81kJ/mol) are close to values found for copper-zinc catalysts, 77 and 78 kJ/mol, respectively [ 11 ]. Turnover frequencies at 250~ for the POX reaction were calculated from surface copper area measurements by the N20 pulse technique and rates of reaction, Figure 2. Values of 460 min -~ were estimated compared to 250 min -~ for experimental Cu-Zn catalysts [ 1,11 ].
....
DME
MeOH . . . . . . WGS . . . . SR POX H2 burning
-10
-15
CI-I3OH DME
In(k)
+ 20
CI-I3OH CO + 2 Id CO + I-IO CO + I-Iz CI-I3OH + H20 CO2+ 32
---..~ ~....: : . . . . . . . . . . . .
-20
CI-I3OH + 0.5 Q => CO2 + 2 Ial -25 0.0017
0.0018
300~
0.0019
0.002
1 IT
0.0021
0.0022
200~
Fig. 2. Arrhenius plot of the 6-reaction-system, k [mol/gcat sec kPa x]
H2 + 0.5 Q => I~O at G=0.08 kg/m 2 sec
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8.00E-04
5.00E-05 r~
l.d
=1 6.00E-04 ,.=
~
@
4.00E-04
2.00E-04
3.75E-05
1"'"'""
~
~ 0
2.50E-05
~
POX
......
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7"=''''=" 50
75
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....
25
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Length of reactor [ %] Fig. 3. Rates of reactions of the 6-reaction-system [mol/g sec] at 250 ~ 5. NON-ISOTHERMAL OPERATION TO PRODUCE HYDROGEN Under nonisothermal, "hot spot" operation, hydrogen production rates were 10000 - 13000 litres H2/hour/litre reactor volume (lrv), which is equivalent to 30-39 kWth/lrv, providing significant power densities. In a single reactor tube, methanol conversions were about 90%, yields to hydrogen 72% or 2.2 moles hydrogen per mole methanol feed. Scale-up on a catalyst weight basis of a factor of 25 has been achieved with a small loss in hydrogen yields due to the presence of both a hotspot and a coldspot in the same reactor. ACKNOWLEDGEMENTS
The project was financially supported by the Swiss Federal Office of Energy (BFE), commercial catalysts were supplied by Johnson-Matthey plc (UK) under a confidentiality agreement, P. Binkert was responsible for constructional work. REFERENCES
1. L. Alejo et al., 3 rd World Congress on Oxidation Catalysis, Ed. R.K.Grasselli et al., Elsevier (1997) 623. 2. M.L. Cubeiro and J.L.G. Fierro, J.Catal. 179 (1998) 150. 3. E. Edwards et al., J. of Power Sources, 71 (1998) 123. 4. W.L. Mitchell et al., SAE 1999-01-0535. 5. B. A. Peppley, Ph.D. Thesis, RMC Kingston, Ontario, Canada, May (1997). 6. B.L. H6hlein, lEA Adv. Fuel Cell Workshop, Wislikofen, Switzerland (1997) 43. 7. E. Newson, P. Mizsey, T. Truong, P. Hottinger, EUROPACAT-IV, Rimini (1999),P/II/114. 8. D. M. Ruthven, Chem. Engng Sci., 23 (1968) 759. 9. SIMUSOLV, Version 3.0-150, Dow Chemical Company (1993). 10. M. Wittman, Diss. Techn. Univ. Mtinchen, Germany (1991). 11. J. L. G. Fierro, Pers. Commun (1999)
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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
New Reaction Mechanism for Methane Formation in CO Hydrogenation over Pd/CeO 2 Shuichi Naito*, Shigeru Aida and Toshihiro Miyao Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan.
[email protected] Drastic changes in the activity and selectivity of methane and methanol formation were observed in CO-H2 reaction over Pd/CeO2 catalysts by high temperature reduction (SMSI effect). A new reaction mechanism for methane formation is proposed, which involves (CO)2(a) and /or (HCO)2(a) species as intermediates for isotopic exchange and methane formation processes, respectively. I. INTRODUCTION It has been well known that the selectivity for the formation of methane or methanol in CO hydrogenation over palladium catalysts is highly sensitive to the composition of supports[I,2], as well as the particle sizes of Pd[3,4] and the presence of promoters[5,6]. The structural difference of active sites for both products is debatable, and the mechanism of this reaction is still controversial, especially concerning the question of whether non-dissociative CO(a) is associated with the formation of methane or not. Cerium oxide is an important support for group 8-10 metal catalysts, because of their application in automotive pollution control and in syngas conversion. The promotive SMSI effect has been reported for hydrogenation of CO2 to methanol over Pd/CeO2 under high pressures[7]. We have also reported that the selectivity of CO-H 2 reaction over Pd/CeO2 changes drastically by higher temperature reduction, which is restored to its original state by lower temperature reduction atter oxidation (SMSI phenomena) [8]. In this paper we have studied the influence of SMSI effect to the sequential elemental steps involving CO-H2 reaction over Pd/CeO2 by isotopic exchange reactions as well as infrared spectroscopic investigation during the reaction. We propose a new reaction mechanism for methane formation, which may involve (CO)2(a) and/or (HCO)2(a) type intermediates at the interface of Pd and reduced ceria. We also propose the transformation of active sites during CO-H2 reaction is the key step for the acceleration of methanol over these catalysts. 2. EXPERIMENTAL Ceria (20 m2/g) supported catalysts (0.5 and 4 wt %) were prepared by a conventional impregnation method, employing (NH4)2PdC14 as a precursor (Pd/CeO2). In order to investigate the effect of residual chloride ions on the reaction, some of the catalysts employed in this study were washed thoroughly by distilled water to remove a trace amount of chlorine (Pd(w)/CeO2). The reaction was carried out in a closed gas circulation system (CO:H2 = 1:3, total = 120 Torr). Before each run the catalyst was reduced by hydrogen at 573K (low temperature reduction: LTR) or 773K (high temperature reduction: HTR). To gather primary
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products, a liquid nitrogen cold trap was employed in the circulation system, and the products were analyzed by gas chromatography as well as mass spectroscopy. 3. RESULTS AND DISCUSSION 3.1 C h a r a c t e r i z a t i o n of the catalysts
Table 1 summarizes the results of H 2 and CO adsorption at room temperature over 4 wt % Pd/CeO 2 after various sequential pretreatments, as well as the particle sizes and dispersions estimated by XRD line width and TEM photograph. It was confirmed from XRD and TEM as well as the amount of sorbed hydrogen H(b) into Pd bulk, that these treatments did not affect Pd particle sizes. But the amounts of adsorbed H(a) or CO(a) on the Pd surface decreased drastically by Table 1. Amount of Sorptionand Particle Size of4wt%Pd/CeOEat 300K HTR treatment, and Redt~on Amount of Sorpfion Particle Size were restored to their original values by 02 T ~ (xlO-Smol.g.cat-') (A) treatment at 723K (K) H(a) (H/M) H ( b ) CO(a) XRD TEM followed by LHR. 790 30(0400 573 8.1 (025) 12 8.0 These results clearly 790 5(g)-6~ 773 3.0(0.08) 13 1.7 indicate the modification atter oxidation at 723K of reduced ceria(Ce203) 573 8.2 (0.25) 12 onto the top layer of 773 2.9(0.08) 11 surface Pd atoms (SMSI effect) by HTR, and its removal by re-oxidation. The occurrence of SMSI was confhxned by the infrared spectra of adsorbed CO, whose intensity decreased drastically by HTR(see Fig. 5). High resolution TEM photographs as well as EDX analysis represent that particle sizes of Pd increased slightly (from 300 to 500 A) by HTR, and their surface seems to be covered by thin layers of reduced ceria. It is worth noticing that the dispersion determined from hydrogen adsorption (H/M) was much larger than that estimated from XRD or TEM particle sizes, which suggests a spillover of H(a) or CO(a) onto ceria support. 3.2 Kinetic study of C O - H 2 reaction
When a mixed reaction gas (CO:H2=1:3, total pressure = 120 Torr) was introduced at 413K onto 4 wt % Pd/CeO2 which was freshly reduced at 573k (LTR), both methane and methanol were formed from the beginning of the reaction as shown in Fig. 1. However, the rate of methanol formation was slow and exhibited an induction period of one hour. After the induction period, its formation rate increased drastically and was accompanied by a decrease in the rate of methane formation. After 12h, the gas phase was evacuated briefly and the same mixed reaction gas was reintroduced. No induction period was observed this time and --
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2,000 hr) durability of the Pt/A1203 catalyst for selective HDC of CC14 to CHC13. Platinum clusters in commercial 0.3wt% Pt/A1203 catalyst are predominantly smaller than 1.5 nm and are stable against thermal sintering at temperatures as high as 500~ in H2. These clusters exhibited a high degree of electron deficiency. After treating it with NH4C1, however, the agglomeration of Pt primary particles took place in H2 at temperatures as low as 320~ as a result of de-anchoring of Pt clusters from Lewis acid sites. The small Pt clusters are highly susceptible to HC1 poisoning. On the other hand, the agglomerated large Pt particles exhibited high resistance toward HC1, therefore affording a stable catalyst for the HDC of CC14. The selectivity for CHC13 over the supported large Pt particles was also improved. A commercial viable process for the selective HDC of CC14to CHC13 was developed.
Introduction The production and use of carbon tetrachloride (CC14) has been phased out according to the Montreal Protocol effective as of January 1, 1996, due to its high potency in depleting the stratospheric ozone layer [ 1]. However, in the production of CH2C12 and CHC13, CC14is still formed as a major by-product. The disposal of this CC14waste by-product, typically by incineration, has become an environmental challenge and a major economic burden to manufacturers of CH2C12/CHC13. As CHC13 is much less destructive than CC14 for stratosphere ozone layer and is used as a precursor for the manufacture of replacement HCFC's and fluorinated polymers, an attractive alternative to CC14 disposal is to convert it to CHC13 by a catalytic HDC process. However, the selectivity to CHC13 can be limited due to a high exotherm toward CH4 in the process. In addition, a catalyst has to be resistant to HC1, a byproduct of the HDC process. *Author to whomcorrespondence on this paper shouldbe addressed.
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Previous researches on the subject dating back to the early seventies as well as recently indicated prolonged catalyst life only at a rather low conversion level and at a high H2/CC14 ratio [2,3,4]. Such conditions would preclude commercial viability of this process. Supported Pt catalysts were reported with reasonable selectivity to CHC13 in liquid phase (typically under pressure) [5,6] and vapor phase [3,4,7] CC14 HDC. However, heavier byproducts (C2C14 and C2C16) are usually produced. In addition, the catalysts in vapor phase reaction typically deactivated over a short period. Alumina supported Pt catalysts have been extensively used in many important industrial processes including the abatement of automotive and industrial plant emissions, naphtha reforming [8,9], etc. Techniques for preparing supported noble metal catalysts are typically chosen to achieve high metal dispersions in order to obtain high activity for a given metal loading. For various applications, the sintering of metal particles has been a major cause for catalyst deactivation [ 10,11 ]. Therefore, re-dispersion of sintered metals on deactivated catalysts such as Pt/A1203 has been the theme of many studies [ 12,13,14]. In this paper, we report the performance of several commercial noble metal catalysts for CC14 HDC. In particular, a commercial 0.3wt% Pt/A1203 egg-shell catalyst was studied that led to intriguing discoveries on the reductive agglomeration of Pt in the presence of NHaC1 and on the durability and selectivity of the catalyst as a result of the Pt particle agglomeration.
2. Experimental 2.1.
Catalysts and Pretreatment A 0.5% Pd/C and a 1.0% Pd/C were from Engelhard Inc. No additional pretreatment was performed on these catalysts except standard activation in H2 prior to catalyst evaluation as described below. A Pt/A120 3 pellet (1/8") catalyst with 0.3 wt% Pt was obtained from Johnson Matthey (JM). Platinum is predominantly distributed at the exterior of the pellets as a thin dark layer, and therefore they are often referred to as an "egg-shell" type catalyst. The fresh catalyst is also referred to as "untreated catalyst" to distinguish it from the NHaC1 treated catalyst as described below. NHaC1 treatment was performed on the JM Pt/A1203 catalyst by soaking it in a saturated solution of NHaC1 at ambient temperature for 89hr. The excess solution was drained, followed by drying at 100~ over night in air. The catalyst pretreated with NHaC1 is hereat~er referred to as "treated catalyst" or "NHaC1 treated catalyst". 2-2. Activation and Reaction Catalyst were evaluated in a fixed bed glass reactor with a frit to support 1.0 g of Pt/A1203 catalyst (for Pd/C catalyst, 0.5g) and a concentric thermocouple to measure the catalyst bed temperature. Prior to catalytic evaluation, each catalyst was treated with H2 at a flow rate of 20 ml/min at 350~ for 2 hr and then cooled to 90~ in H2. The HDC reaction was started at 90~ and 1 atm by passing a mixture of H2 with vaporized CC14 (25 ml/min in total) at a H2/CC14 molar ratio of 7.0 through the reactor. An on-line gas chromatograph (Varian 3400) with a DB-1 capillary column and flame ionization detector was used to analyze the downstream gases.
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2-3. X-ray Photoelectron Spectroscopy (XPS) Surface compositional analysis of the catalyst pellets was performed with a Physical Electronics 5600ci x-ray photoelectron spectrometer. Due to overlap of the strong Pt(4f) peaks with those of Al(2p) from the support, the Pt(4d,5/2) peak was used to evaluate the concentration and chemical state of the active Pt phase. Sample charging correction was performed based on the C(1 s) peak from adventitious hydrocarbon at 284.4 eV. Charge correction consistently yielded an Al(2p) binding energy of 74.6 + 0.1 eV, as expected for alumina. In-situ activation of the pellets (2 hrs, 350~ was performed in a side chamber attached to the XPS instrument, equipped with a sample heater, manifold, thermocouple, and vacuum pumps. Details in sample handling can be found eslewhere [ 15].
3. Results 3.1. Pd/C catalysts The results obtained with Pd/C catalysts for CC14 HDC are shown in Figures 1 and 2. The conversion of a 1% Pd/C at 90~ decreases rapidly on stream due to catalyst deactivation. The activity of a 0.5% Pd/C is less than 5% under similar conditions and CH4 was the only product detected. The selectivity to CHC13 on the 1.0% Pd/C was 20% with 80% CH4. Conv (%) 100
Selectivity
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20 40 60 80 100 120 140 160 Time on Stream (min)
Figure 1. Conversion of Pd/C catalysts for CC14 HDC
(%)
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CH4
Figure 2. Selectivity of Pd/C catalysts for CC14 HDC
3.2. Pt/A1203 catalysts 3.2.1. Untreated Pt/A1203 catalyst The fresh commercial 0.3% Pt/A1203 egg-shell catalyst that was activated at 350~ in H2 showed an initial conversion of CC14 over 90%. But the catalyst was rapidly deactivated; the conversion dropped to 315.3 ~'~"~'~'-
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Figure 6. Pt(4d, 5/2) binding energies of Pt/A1203 catalysts
3.2.3. Pt particle size and Pt (4d, 5/2) binding energy The TEM results as reported earlier [ 15] showed that Pt particles in the untreated Pt/A1203 catalyst are predominantly smaller than 1.5 nm, even after activation in Hz at temperatures as high as 500~ On the NHaC1 treated catalyst after activation at 320~ the Pt particles are narrowly distributed at 5-8 nm range. It was concluded that Pt particle agglomeration in the presence of NHaC1 was due to the de-anchoring of primary Pt particles from the strong Lewis acid sites and the formation of mobile chlorinated Pt precursors on the catalyst surface. XPS data of the untreated and NH4C1 treated Pt/A1203 catalysts are shown in Fig. 6. For the untreated Pt/A1203, the binding energy of Pt after activation in H2 is 314.4 eV. In comparison, the binding energy of Pt on the NHaC1 treated Pt/A1203 catalyst is 313.9 eV, which is identical to that of bulk Pt metal.
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4. Discussion 4.1. Catalyst performance As shown in Fig.2, the Pd/C catalysts produce mainly CH4 for CC14 HDC. In addition, the more active 1.0% Pd/C catalyst was found to rapidly deactivate during reaction. The activity of the 0.5% Pd/C was considerably lower than that of the 1.0% Pd/C. A selectivity of 60% to CHC13 on the untreated commercial 0.3% Pt/A1203 catalyst indicated that Pt/A1203 is a better catalyst for the desired product. However, it became rapidly deactivated (Fig. 3). In addition, C2C16was produced as a byproduct, as well as CH4. Most strikingly, the NHaC1 treated Pt/A1203 catalyst not only became stable, it was also more selective for CHC13. The selective for CHC13 reached 80% on this catalyst. In addition, no heavy byproduct, such as C2C16, was produced. While Pt on different support materials showed different rates of deactivation, [2] the NHaC1 treated Pt/A1203 catalyst represents the most selective and stable catalyst ever reported for the HDC of CC14 to CHC13. The dramatic difference in catalytic performance for CC14 HDC between the untreated and NHaC1 treated Pt/A1203 catalysts is mainly due to Pt particle size effect. While small Pt particles smaller than 1.5 nm are readily poisoned by HC1 that was produced from the reaction, the metal-like large Pt particles of 5-8 nm are highly resistant to HC1. 4.2. Agglomeration of primary Pt particle in the presence of NH4C1 Sintering of Pt particles of 4 nm size on A1203 support in pure oxygen was reported at a higher temperature (600~ after a longer heating period [ 16]. In this work, it is found that thermal sintering in pure H2 did not take place even after treatment at 500~ The stability of the small Pt clusters at this high treatment temperature is attributed to their strong interaction with the predominantly Lewis surface acid sites. Such interaction is proposed to be the cause for the strong anchoring of the Pt clusters onto the alumina surface. Compared to bulk Pt metal, the small Pt clusters are highly electron deficient, as indicated by its higher electron binding energy from XPS measurement. The electron deficiency may be ascribed to two sources. (1) Strong interaction with Lewis acid sites leading to a profound electronic polarization of the Pt clusters, forming a stable Ptn+AI(O)3 local structure, analogous to Ptn-H + adduct as proposed for electron deficient Pt particles in zeolites [17]. And (2) the intrinsic electronic character of small metal particles, which is correlated to electron binding energy [18,19,20]. It is particularly significant that Pt in the NHaC1 treated Pt/A1203, prior to activation, has the highest BE ( 315.3 eV). This increase of Pt BE as a result of NHaC1 indicates that an interaction of small Pt clusters with NHnC1 has already taken place. Such an interaction may have set the stage for Pt agglomeration during subsequent thermal treatment in H2. It is possible that some PtnClx, e.g. in the form of Cl'Ptn + Al(O)3 was formed. The negatively charged chloride ions may induce the deanchoring of Ptn clusters, which is responsible for the agglomeration of primary Pt clusters [21 ]. Upon activation at 320~ the Pt particles agglomerated to a narrow distribution of 58 nm in the presence of NH4C1. The agglomerated 5-8 nm Pt particles exhibit purely metallic Pt characteristic, as indicated by XPS results. At the initial low temperature, during the agglomeration process, ammonia could play a role in neutralizing the acid sites. However, no significant Pt agglomeration was observed
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by treating the Pt/A1203 catalyst with NH4OH and NH4NO3. The experimental results indicate that ammonium ions alone are insufficient to cause Pt cluster agglomeration. The fact that LiC1 can also result in Pt cluster agglomeration (results not shown in this paper) is a further confirmation on this point.
5. C o n c l u s i o n s HDC of CC14 on Pd/C catalysts produced mainly CH4. In addition, an active 1.0% Pd/C catalyst deactivates rapid on stream. A batch of typical commercial 0.3wt% Pt/A1203 egg-shell catalyst was found to deactivate rapidly due to HC1 poisoning. C2C16 was produced as a byproduct. The Pt particles on the catalyst are predominantly smaller than 1.5 nm. The high stability of the small Pt particles on this catalyst against thermal sintering at 500~ suggests the formation of stable Ptn+AI(O)3 local structure as the cause for the strong anchoring of small Pt clusters at the A1203 surface. Treatment with NHaC1 resulted in highly stable catalyst for CC14HDC with 80% selectivity for CHC13, balancing CH4. The increased durability is ascribed to the increased Pt particles in 5-8 nm due to NHnC1 induced agglomeration of Pt at 320-350~ under a reducing environment (H2). The agglomeration is ascribed to the deanchoring of Ptn clusters from surface Lewis acid sites by chloride ions.
6. REFERENCES 1 United States Environmental Protection Agency Air and Radiation Stratosphere Division, Section 606 of the Clean Air Act Amendments of 1990 (1993) 2 H.C. Choi, S. H. Choi, J. S. Lee, K. H. Lee, Y. G. Kim, J Catal., 166 (1977) 284 3 A.H. Weiss, B. S. Gambhir, R. B. Leon, J. Catal., 22 (1971) 245 4 A.H. Weiss, S. Valinski, J. Catal., 74 (1982) 136 5 S. Morikawa, M. Yoshitake, S. Tatematsu, US Patent 5,334,782 (1994) 6 E.T. Miquel, J. M. S. Gimenez, A. C. Arroyo, X. L. S. Gomez, A. A. Martin, US Patent 5,208,393 7 S.Y. Kim, H. C., Choi, O. B. Yanga, K. H. Lee, J. S. Lee, Y. G. Kim, 3. Chem. Soc., Chem. Comrnun., 2169 (1995) 8 J.P. Brunelle, A. Sugier, and J. F. le Page, J. Catal., 43 (1976) 273 9 C. Corolleur, S. Corolleur, D. Tomanova, and G. Gault, J. catal., 24 (1972) 385 10 G.A. Mills, S. Weller, E.B. Comelius, Proc. 2nd Int. Congr. Catal., p.2221 (1960) 11 S.F. Adler, J. J. Keavney, J. Phys. Chem., 64 (1960) 208 12 I. Guo, T. T. Yu, and S. E. Wanke, in Catalysis, J. W. Ward ed., p21 (1987) 13 E. Ruckenstein, Y. F. Chu, J. Catal., 59 (1979) 109 14 E. Ruckenstein, M. L. Malhotra, 41 (1976) 303 15 Z.C. Zhang, B. Beard, Applied Catalysis, 174 (1998) 33 16 A. Bellare, D. B. Dadyburjor, and M. J. Kelley, J. Catal., 117 (1989) 78 17 W.M.H. Sachtler, A. Y. Stakheev, Catalysis Today, 12 (1992) 283 18 R.C. Baetzold, M. G. Mason, and J. F. Hamilton, J. Chem. Phys 72(1) (1980) 366 19 M.G. Mason, Phys. Rev., B, 27(2) (1983) 27 20 J. Goetz, M. A. Volpe, A. M. Sica, C. E. Gigola, and R. Touroude, J. Cat., 153 (1995) 96 21 Z.C. Zhang, B. C. Beard, Appl. Catal., 188 (1999) 229
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Catalytic Diesel Soot Elimination on Co-K/La203 Catalysts: Reaction Mechanism and the Effect of NO Addition E.E. Mir6, F.Ravelli, M.A.UIIa, L.M.Cornaglia, C.A. Querini Instituto de Investigaciones en Cat/disis y Petroquimica- INCAPE- Fac.Ing. Quimica (FIQUNL, CONICET) - Santiago del Estero 2654 - (3000) Santa Fe - ARGENTINA Catalysts containing Co and/or K supported on La203 have been studied for diesel soot catalytic combustion. While supported Co provides redox sites for the reaction, potassium and the support itself contribute to create additional sites for soot consumption by forming carbonates intermediates. The formation of a perovskite structure after high temperature treatment leads to the lost of activity. The presence of NO in the gas phase improves the catalytic activity for soot elimination. NO is oxidized to NO2 on the catalyst surface, and NO2 is a stronger oxidizing agent than 02, therefore decreasing the temperature needed to burn the soot. 1. INTRODUCTION Particulate matter and NOx are the main pollutants in diesel engine emissions. The combination of traps and oxidation catalysts appears to be the most plausible after-treatment technique to eliminate soot particles (1). Since the temperature of typical exhausts is 400~ or below in light duty applications, and diesel stack temperatures can exceed 600~ at full load even for a turbocharged/afiercooled engine, a potentially useful catalyst has to operate efficiently at low temperatures and be thermally stable. Studies with a large number of formulations have been reported during the last few years. However, the reasons for the better or poorer performance of a given material have been scarcely studied. We have previously reported studies with Co-K/MgO catalysts, establishing a relationship between the catalytic behavior for soot combustion and the properties as determined by a variety of characterization techniques (2,3). It is the objective of this work to establish such relationship for Co-K/La203 catalysts, in order to determine the mechanisms of catalytic soot combustion. 2. EXPERIMENTAL Catalysts were prepared from a La203 suspension in water, to which C o ( N O 3 ) 2 and/or KOH were added, in order to obtain 12wt.%Co and 4.5 and 7.5wt.%K. After evaporation and drying, the samples were calcined at 400 and at 700~ The soot was obtained by burning commercial diesel fuel (YPF, Argentina) in a glass vessel, as described in (2). TPR experiments were carried out in an Okhura TS-2002 instrument. The XPS spectra were obtained at room temperature with a computer-controlled Shimadzu ESCA 750 instrument, using MgK~ radiation. X-ray diffractograms were obtained with a Shimadzu XD-D1 instrument with monochromator using CuK~ radiation. In order to study the interaction of the
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catalyst surface with CO2, pulse experiments were carried out. At selected temperatures, consecutive pulses (0.25 cc) of 1.2% of CO2 were sent to the reactor. The catalytic activity for the combustion of soot was determined from TemperatureProgrammed Oxidation (TPO), of carefully prepared mixtures of catalyst and soot (20:1). To study the effect of nitric oxide, some experiments were carried out at constant temperature. These were performed by heating the sample under helium flow until the desired temperature was reached. Then, the feed was switched to the oxygen-nitric oxide-helium reacting mixture. Samples were taken and stored in a sixteen-loop Valco valve for chromatographic TCD analysis. 3. RESULTS AND DISCUSSION
Table 1 shows activity results measured at 350~ hydrogen consumption during TPR experiments (carded out up to 600~ and XRD main phases obtained with catalysts calcined at 400 and 700~ (number between brackets). All the samples calcined at 400~ have poor crystallinity, showing only La202CO3 and La(OH)3 as the main XRD crystalline phases. This is not the case for samples calcined at 700~ which show better crystallinity. La203 and La(OH)3 peaks are seen in XRD spectra of K/La203 samples; and La203 , C0304 (trace level) and LaCoO3 (perovskite) in the case of Table 1 Soot burning rate, H2 consumption during TPR, and XRD main phases. Catalysts
K(400)
Reaction Rate b lamol/s, gsoot 1.33
K(700)
1.56
Co (400) Co (700)
Co-K(4.5) (400)
0.78 0.00 1.31 0.63
Co-K(4.5) (700) Co-K(7.5) (400) 1.29 Co-K(7.5) (700) 068 Co-K(4.5) (400) a 4 00 awith 0.5 % of NO in the feed. bmeasuredat 350~
H2 Consumption mmol/g 0.0 0.0 4.9 19 4.9 2.9 5.4 2.0 4.9
XRD main phases La202CO3, La(OH)3 La202CO3, La(OH)3 La202CO3, La(OH)3 La203, La(OH)3, LaCoO3 La202CO3, La(OH)3 La203, La(OH)3, LaCoO3 La202CO3, La(OH)3 La203, La(OH)3, LaCoO3 La202CO3, La(OH)3
Co,K/La203 samples. In the latter case, the crystallinity of the perovskite phase is favored by the increase of potassium content. Pure La203 burns the soot starting at 450~ The catalysts containing K are more active than Co/La203 catalysts. When the catalysts are treated at high temperature (700~ a decrease both in activity and in the amount of Co that can be reduced is observed, except in the case of K/La203. This catalyst displays a very good thermal stability. A shift in the TPR profiles (not shown) to higher temperatures is also observed upon the high temperature treatment. As said above, the XRD diffraction patterns of the catalysts shown in Table 1
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indicate that after the treatment at 700~ a perovskite-type structure is developed in those catalysts containing Co. Therefore, the lower amount of Co304 observed by TPR, is due to the formation of this mixed oxide compound, which does not reduce below 600~ This is the reason why Co,K/La203 deactivates while K/La203 does not, upon high temperature treatment. When Co is present, it reacts with the support forming perovskite with a decrease in the amount of surface La203 which as will be shown below plays a role during the soot burning reaction. We have previously suggested that one of the roles of K in Co-K/MgO catalysts is to consume C from the soot, to form an intermediate carbonate which then decomposes regenerating the K active site (3). La203 at 400~ interacts with CO2 similarly to CoK(4.5)/MgO and Co-K(4.5)/CeO2. Figure 1 shows results obtained in the CO2 pulse experiments. The highly distorted pulses of CO2 coming out from the cell containing Co/La203 or Co,K/La203 catalyst at 400~ indicate a strong interaction between CO2 and the catalyst surface. More likely, a carbonate type compound is formed, and as CO2 is leaving the reactor, the carbonate decomposes generating a broad peak. This behavior is not observed at low temperature. Since this behavior is also observed in absence of K (Fig. 1C), an important role of the support (La203) is to provide more active sites for soot consumption through the formation of a carbonate, which is a different role from that we have previously observed for the CeO2 supported catalyst. The role of the latter, is to provide oxygen for soot combustion, through a redox mechanism (4).
A
B
5
C
5
~ 3
25~
,ooocI 2- Cl
jl\
,ooocl
40
80
TIME,S EC
120
oooc/
C
3oocI
- ~I
~
54
0
i 40
TIME,S EC
" ~~176 80
120
3
25oc
~IJl \~----
2000c
/;
~I/-------~
I-------J
0L
40
....
i
80
....-
4-0-~c i~
12D
TIME,S EC
Figure 1:CO2 pulses at different temperatures on La203 supported catalysts. A: Co/ La203 ;B: Co,K/La203 ; C: La203.
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Table 2 shows the surface composition of each catalyst, as measured by XPS. The highest K/La ratio is found on K/La203, which has an activity similar to the Co-K catalysts (see Table 1). On the other hand, these catalysts have C0304 Which reduces with a main peak below 400~ The high temperature treatment leads to the deactivation of the La203 supported Co-K catalysts. The XPS results indicate that an enrichment in Co and K occurs alter such treatment. According to the XRD results, this is due to the formation of a Table 2 XPS signal intensity ratios for La203 supported catalysts Catalysts a
Calcination Temperature
Co/La
K/La
O/La
400 400 400 400 700 700 700
0.49 -0.16 0.10 0.42 -0.24 0.17
-0.75 0.09 0.12 -0.90 0.50 0.90
6.3 5.5 5.1 5.7 5.9 6.3 5.5 6.1
(~
Co/La203 K(4.5%)/La203 Co, K(4.5%)/La203 Co, K(7.5%)/La203 Co/La203 K(4.5%)/La203 Co, K(4.5%)/La203 Co, K(7.5%)/La203 "Co/La)bulk = 0.40
700
K/La)bulk = 0.23
perovskite oxide, which is not as active for this reaction as cobalt oxideand lanthanum oxide. The high temperature treatment increases the amount of potassium on the surface but due to the formation of the mixed oxide, it decreases the amount of active Co and La203. The final result is a loss in activity. It has to be emphasized that the support (La203) in this case plays an active role during the 0,02 reaction. Therefore, the formation of the perovskite NO 0.5% decreases the amount of free La203 and also the amount of C0304, decreasing both the 23 redox property and the ability O 0,01 to form carbonate-type O intermediates. Since nitrogen oxides and particulate matter emissions are the main pollutants present in diesel engine emissions, it is interesting to explore the 0,00 25O 300 350 400 450 500 550 60 possibility of the simultaneous Temperature (C~) NO and soot removal reaction. Figure 2: TPO analyses of mechanical mixtures of Shangguan et al (5) reported Co,K(4.5%)/La203 catalyst and soot. II soot +02 (6%)that perovskite-related oxides and spinel-type oxides are +NO(0.5%) reaction. D soot+O2 (6%) reaction. active for the above mentioned
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reaction. On the other hand, Mul et al. (6) reported that the addition of NOx enhances the catalytic carbon black oxidation rate, which is attributed to bifunctional catalysis: NO2 reacts with carbon black yielding NO and NO is reoxidized to NO2 over a transition metal oxide. In this work, we have found that the La203 - supported Co-K catalyst is effective for the simultaneous elimination of soot and NO. In Figure 2 it can be seen that there is a decrease of c.a. 25~ in the combustion temperature when NO is introduced in the feed, if compared with the soot combustion with oxygen. Additionally, the TPO profile becomes sharper. In the last row of Table 1 it can be seen that when 0.5% of NO are added to the feed, the soot combustion reaction rate increases three times, the NO being partially converted into N= and N=O. Figure 3 shows the evolution of the soot conversion under isothermal conditions when temperature or reactant concentrations are changed. As it can be seen, an induction period is present in most of the cases, suggesting the m situ formation of new active sites, probably from the interaction of NOx and the soot particles. In the same figure it is observed that soot conversion increased when either NO or O= were increased. It can also be seen ( Fig. 3B) that when oxygen is not present in the feed, the soot conversion is very low, indicating that NO is not a good oxidant and the formation of NO= through the NO + 89O= ~ NO= reaction is necessary. A fundamental role of the catalyst surface would be to increase the oxidation rate of NO to NO=, as suggested by Teraoka et al. (7). These authors also found that the doping of potassium to CuFe204 was effective in promoting the catalytic performance for the reaction under study. The catalytic performance of Cul.xKxFe204 significantly depends on the doping amount of potassium, and the highest activity and selectivity were simultaneously attained at 0,8
400 ~
/
Lt'r (D > r 0,4 o
/
,'
i
~,'i5~o(~
/
~;::: .... m
50
HI /"
"/.="
/m
=,,,
': ,,/,I~ o sS jJ |
100
m
150
m
,
-
:
0
50
:
: :
0%0
," /
' /
..,.i ......,m~" 100
.o
:
,"i
,,/
50 ~
;'
/m
:
;
m'
I
,'f
"/ -
NO..:...N"../..IN"
I%
3% 0 2
m':/~
,, /' / , .. :
0
C 6% 0 2 i ../:'.. .
i .,"; ; 9 I: ,:m /
0
0,0
.
/';
t- 0,6
.9
0 0 cnO,2
B
./.1
A
150
0
50
...HH"'
100
~I
/
150
200
time (sec.)
Figure 3 : Isothermal combustion of soot. Mechanical mixture of Co,K/La203 and soot. (A) Effect of temperature. 6% 0 2 , 0.5% NO. (B) Effect of oxygen concentration. T=400~ 0.5% NO. (C) Effect of NO concentration. T=400~ 6% 02.
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x=0.05. From Fig. 2 it can be seen that our catalyst shows similar soot ignition temperature (c.a. 250~ if compared with the best potassium promoted CuFe204 studied by Shangguan et al. (7). According to this study, La203 supported catalysts have good activity for both soot and soot+NO elimination. Therefore, additional studies are now being carried out in our laboratory in order to obtain more information about the thermal stability of these catalysts and the influence of the potassium content on the catalytic properties. 4. CONCLUSIONS
La203 is a good support for Co and K to be used both for soot oxidation and for the simultaneous NO and soot removal. This support provides additional sites for soot consumption, by forming carbonates intermediates. Potassium, due to its high mobility, is much more efficient than La203 for improving the rate through this path. Cobalt provides additional oxygen by a redox mechanism. However, since all the support surface plays a role by forming carbonate-type intemediates, the influence of Co on the activity for soot combustion with oxygen is very low, K/La203 having as good activity as Co,K/La203 . Moreover, the formation of a perovskite structure LaCoO3 after high temperature treatment leads to activity loss. Then cobalt accelerates the deactivation of the La203 supported catalysts, and should not be present in an optimal formulation.
Acknowledgments The authors wish to thank Universidad Nacional del Litoral, CAID program and to ANPCyT (PICT N ~ 14-00000-00720 ) for its support to this project. Thanks are also given to Prof. E. Grimaldi for her help in the edition of the English manuscript, and to Mr. Claudio Maitre for his technical assistance. The experimental work and technical assistance of Ms Maria L. Pisarello and Clara Saux are also acknowledged. REFERENCES 1. J.Widdershoven, F.Pischinger, G.Lepperhof, SAE paper 860013, 1986.
2. C. Querini,M.Ulla, F.Requejo,J. Soria,U. Sedran,E.Mir6,Appl. Catal.B :Environmental 3. 4. 5. 6. 7.
11(1997)237 C.Querini, L.Cornaglia, M.Ulla, E.Mir6, Appl. Catal B :Environmental 20(1999)165. F.Ravelli, C.Querini, M.Ulla, L.Cornaglia, E.Mir6, Catalysis Today, in press. W.Shangguan, Y.Teraoka, S.Kagawa, Appl.Catal.B:Environmental 12 (1997) 237. G.Mul, W.Zhu, F.Kapteijn, J.Moujlin, Appl.Catal.B:Environmental 17(1998)205. Y.Teraoka, K.Nakano, S.Kagawa, W.F.Shangguan, Appl. Catal. B:Environmental 5(1995)L181.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Environmentally Benign Carbonylation of Nitrobenzene and Aniline Steven S. C. Chuang', Mahesh V. Konduru, Yawu Chi, and Pisanu Toochinda Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906
Abstract Carbonylation of nitrobenzene and aniline on supported metal catalysts provides an environmentally benign route to isocyanate synthesis. Infrared studies suggest that both oxidative and reductive carbonylation proceed via the insertion of adsorbed CO into adsorbed nitrene. Adsorbed nitrene is formed from the deoxygenation of nitrobenzene in the reductive carbonylation pathway and from the dehydrogenation of aniline in the oxidative carbonylation pathway. With appropriate catalysts, oxidative carbonylation can take place at significantly milder conditions than reductive carbonylation. I.
INTRODUCTION
Carbonylation is the process in which a CO molecule is attached onto a parent molecule as a carbonyl functional group. The interest in carbonylation stems from the fact that the reaction provides a pathway for the conversion of organic substrates to high-added value products. Advances in catalyst development have allowed the use of carbonylation to bypass a number of traditional synthesis pathways for increased yields and reduced environmental risks. One notable example, illustrated in Figure 1, is the use of the reductive carbonylation of nitrobenzene and oxidative carbonylation of aniline to replace phosgene as a feedstock for isocyanate synthesis (1). The reaction of phosgene with a primary amine is currently the most important process for the synthesis of isocyanates (1). The environmental impact of the process arises from not only the highly toxic and corrosive nature of phosgene but also the use of chlorinated hydrocarbons as a solvent for the reaction. The catalytic carbonylation of nitrocompounds and amines provides a direct route to isocyanate without the use of phosgene. This reaction process has been considered as the most promising environmentally benign process to replace the conventional method of isocyanate synthesis (2). Extensive efforts have been directed toward development of effective catalysts for oxidative and reductive carbonylation (1,2). Depending on catalysts and the type of reactants used, the process can lead to either isocyanate or carbamate in a single step. Carbamate synthesis from the carbonylation of nitrocompounds and amines has been the most attractive for its high selectivity, high "To whom all the correspondence should be sent
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stability, and low toxicity of the carbamate products as well as selective conversion of carbamate to isocyanates. In contrast, the direct isocyanate synthesis from carbonylation of nitrocompounds suffers from the polymerization side reaction (1). Extensive carbamate synthesis studies have been carried out in liquid media with the metal-complex catalyzing the reductive carbonylation of nitrobenzene and the oxidative carbonylation of aniline under high-pressure conditions. Although some of these studies have shown high product yields, these reaction processes not only require the use of high pressure but also involve significant catalyst recovery and product separation problems.
a. Synthesis of Isocyanate from Aniline and Phosgene RNH2 + COC12 ~ RNCO + 2 HC1
R = alkyl or phenyl group
b. Reductive Carbonylation of Nitrobenzene to Carbamate RNO2 + R'OH + 3 CO ~ R-NH-CO-O-R' + 2 CO2 c. Oxidative Carbonylation of Aniline to Carbamate RNH 2 +
R'OH + CO + 890
2 ---'> R-NH-CO-O-R' + H 2 0
Fig. 1. Isocyanate and Carbamate Synthesis The ability to catalyze carbonylation on the surface of heterogeneous catalysts under low pressure conditions provides a novel route for eliminating the catalyst recovery and solventuse problems, simplifying the synthesis process, and reducing environmental risk. The present study was aimed at determining the reactivity of adsorbed CO in reductive carbonylation ofnitrobenzene with Ce-Pd/A1203 at 373 K and 0.101 MPa and oxidative carbonylation of aniline with CuC12-PdC12/ZSM-5 at 0.101 MPa and 438 K (mild reaction condition compared with those reported in literature).
2.
EXPERIMENTAL
2.1
Catalyst preparation
The Ce-Pd/A1203 was prepared by sequential impregnation of PdC12 on to the Ce/A1203 catalyst that was prepared by the incipient wetness impregnation method. The ratio of Ce to Pd was 20 to 1. The CuC12-PdC12/ZSM-5 was also prepared by the sequential impregnation of PdC12 onto the CuC12/ZSM-5 that was prepared by the incipient wetness impregnation of
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CuC12into H-ZSM-5. The ratio of Cu to Pd was 1.4. Both catalysts were dried in air at 298 K for 24 h and the Ce-Pd/Al203 was further calcined at 673 K for 4 h. 2.2
In situ IR studies
A self-supporting disk of the catalyst (Ce-Pd/A1203 or CuC12-PdC12/ZSM-5) weighing 25 mg, was pressed by a hydraulic press at a pressure of 4000-4500 psi. The disk was placed in the IR beam path of the reactor cell (3). Prior to the injection of the organic reactants into the reactor cell, the Ce-Pd/AI203 was pretreated in flowing H2 in the in situ IR cell. The catalysts were exposed to 5% CO at the reaction temperature and allowed to stand for 30 min. The surface was flushed with He and the volatile organic reactants such as nitrobenzene, aniline, ethanol, and methanol were injected into the reactor with the aid of a liquid syringe through a septum present at the reactor inlet. IR spectra were collected at regular time intervals.
3.
RESULTS AND DISCUSSION
3.1
Reductive Carbonylation
Figure 2 shows the IR spectra taken during the addition of CO (g), nitrobenzene, ethanol, and aniline on Ce-Pd/A1203 at 373 K and 0.101 MPa. CO adsorbed as linear CO (Pd-CO) at Pd 2071 cm ~ and bridged CO ( Pd > CO ) at 1905 cm -~ (4) during CO flow and these species were retained on the surface even after flushing with He. The strongly adsorbed CO species disappeared on nitrobenzene addition (325 ~tl) while bands due to vibrations of the NO2 species of nitrobenzene appeared at 1344 and 1526 cm ~ (5). Subsequent addition of ethanol (50 lal) led to the appearance of ethanol bands at 1071 cm -~ and did not cause any significant change in the 1344 and 1526 cm ~ bands. Further addition of 500 lal of aniline caused an increase in the nitrobenzene species (1344 and 1526 cm-'), appearance of aniline species at 1605 cm ~, and formation of methyl phenyl carbamate (C6Hs-NH-COOC2Hs) as observed by the presence of a weak band for the carbonyl species (-C=O) at 1740 cm -~ (5).
3.2
Oxidative Carbonylation
IR spectra of CuC12-PdC12/ZSM-5 taken during the addition of CO (g) and 200 ~tl of aniline/methanol/NaI (17.1/79.3/3.6) at 438 K and 0.101 MPa is shown in Figure 3. Addition of the reactants led to the appearence of linear CO at 2082 cm ~ and 2054 cm l, NH stretching of aniline at 1618 cm ~, C=C aromatic stretch of the aromatic species at 1502 cm ~, C-H bending at 1454 cm ~, C-N aromatic stretch at 1345 cm l, and C-N stretches at 1279 and 1268 cm ~. Increase in reaction time from 10 to 27 min led to a decrease of all the adsorbed reactant species and formation of CO2 (g) as well as increase in the C=O stretching
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
740
~'~ ~ ~
~" ~
, ~
~,~, ; ~
NH i: fillil COCHj lil II 2 ~, hi
O
"
1
"-~
[il it
CH / 2 OH ,
I!1 :I
Ng u --~"O ~ p/d~d o,ii ,~j[I~x~fI~COI~NO2 ~ ~ + t~
::
r
'.'.
'
:
'.
'.
~: " ~ i____~:~,(~"1 -He flus'~(x3)~_ N ~
{~
"-"~
C2 H OH i~,-,,-, .
CO flow (x3) I
2267
11i33 Wavenumber (cni l) IR spectra of Ce-Pd/A1203 during reductive carbonylation at 373 K and 0.101 MPa ~
Fig. 2
1r
~
r
~
oO
_'2 ~
ee~
ee~
'
:
,
:
',
"Iime (min)
V i
?
t'q
~ "
::
~
anol ,r~
Fig. 3
2400
2200
2000
:
I
I
I
I
1800 1600 1400 1200 Wavenumber (cm l) IR spectra during oxidative carbonylation on CUC1E-PdCIJZSM-5 at 438 K and 0.101 MPa
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( 1765 cm~) of the methyl phenyl carbamate species suggesting that the carbamate species can be produced via the oxidative carbonylation of aniline at 0.101 MPa.
3.3
Reaction Pathways
Decrease in IR intensity of adsorbed CO accompanied by the increase in IR intensity of the carbamate species observed in Figures 2 and 3 suggests the involvement of adsorbed CO in both oxidative and reductive carbonylation. Figure 4 provides a schematic of the proposed mechanism which delineates the steps required for carbamate synthesis. In reductive carbonylation, the reaction begins with N-O bond dissociation for deoxygenation of nitrobenzene, CO insertion into adsorbed nitrene, dehydrogenation of alcohol to form the alkoxide, and O-C bond formation between the carbonyl species and alkoxide to form carbamate. In oxidative carbonylation, the reaction involves dehydrogenation of aniline and alcohols to produce the same type of adsorbates postulated for reductive carbonylation. a. Reductive carbonylation pathway
N02~
CH
CO----~*CO + N
~
H
~
CO + H OCH3
-
NHCOOCH3
///////
b. Oxidative carbonylation pathway
CH
CO----~*CO + N
Fig. 4.
~
I
CO +
H
OCH3
~
Reaction pathways during the carbonylation reactions
NHCOOCH3
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The rapid growth of the carbamate band in Fig. 3 demonstrates the high activity of C u C I 2PdCl2-NaI catalyst in oxidative carbonylation. The excellent activity of CUCIE-PdCI2NaI/ZSM-5 catalysts for oxidative carbonylation can be attributed to the high dehydrogenation activity of C u C I 2 and PdC12 and promoter effect of NaI. The redox cycle of C u C I 2 and PdCl2 observed in Wacker catalysis (6) may play a role and remains to be investigated.
4.
CONCLUSIONS
This study demonstrates that the adsorbed CO is involved in both reductive carbonylation on Ce-Pd catalyst and oxidative carbonylation on CuC12-PdC12-NaI catalysts to produce carbamate. The latter provides a potential route for an environmentally benign synthesis of isocyanate under mild reaction conditions. The absence of harmful organic solvents in this reaction process not only reduces the environmental risk but also eliminates the catalyst recovery step. The catalyst remained to be tested for a large number of analogus oxidative carbonylation reactions.
ACKNOWLDEGMENT
This work has been supported by the United States National Science Foundation under Grant CTS 9816954. The authors also wish to thank Dr. Khalid A1-Musaiteer for providing the catalyst.
REFERENCES
.
.
~
5. ~
Cenini, S., Pizzotti, M., and Crotti, C., "Metal Catalyzed Deoxygenation Reactions by Carbon Monoxide of Nitroso and Nitro Compounds" in Aspect of Homogeneous Catalysis: A Series of Advances, vol. 6, Ugo, R., Ed.; D. Reidel Publishing, Holland, 1988; pp 97-198. Parshall, G. W., and Lyons, J. E., "Catalysis for Industrial Chemicals" in Advanced Heterogeneous Catalysts For Energy Applications; U. S. Department of Energy, 1994; Chapter 6. Chuang, S. S. C., Brundage, M. A., Balakos, M., and Srinivas, G., Appl. Spectrosc. 49 (8), 1151 (1995). Grill, C. M., and Gonzalez, R. D., J. Phys. Chem. 84, 878 (1980). Silverstein, R. M., and Webster, F. X., in "Spectrometric Identification of Organic Compounds", John Wiley and Sons, Inc., New York, 1998. Park, E. D., and Lee, J. S., J. Catal. 180, 123 (1998).
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
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N i t r o u s O x i d e ( N 2 0 ) - W a s t e to V a l u e Anthony K. Uriarte Solutia Inc., P.O. Box 97, Gonzalez, FL 32560-0097, USA
Waste N20 from adipic acid process is used a feedstock for novel single-step technology of manufacturing phenol from benzene (AlphOx process). The AlphOx process is discussed both as an integral part and key step of the new adipic acid manufacturing concept and as a stand-alone technology. Major developments in N20 recovery and purification from the adipic process waste stream, as well as in N20 on purpose manufacturing are presented. 1. INTRODUCTION
The worldwide adipic acid industry capacity is approximately 2000k mt/yr with almost all produced by the nitric acid oxidation of cyclohexanone (A) and/or cyclohexanol (K), KA-oil, (eq. (1)). OH
0 HNO3
HO2C(CH2)4CO2H + NOx + N20 (1)
The HNO3, NOx and N20 stoichiometry is complex and dependent upon the reaction conditions and feedstock composition. In general, 0.8 to 1 mole of N20 is produced per mole of adipic acid. This equates to a potential emission total of 600k mt/yr. A few adipic producers employ thermal reduction furnaces to decrease NOx emissions. The furnaces coincidentally destroy 99+% of the N20 contained in the waste offgas from these adipic plants. However, this represented only about 35% of the total adipic process generated N20. In 1991, Thiemens and Trogler recognized the magnitude of adipic-related N20 emissions and reported that adipic acid manufacture might contribute up to 10% of the growth of atmospheric N20, a potent greenhouse gas and suspect ozone depleter [ 1 ] . Although not regulated at the time, the heightened awareness of the environmental impact, prompted the major adipic acid and catalyst manufacturers to form a consortium to develop N20 abatement
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technologies [2]. By 1998, Asahi, Bayer, BASF, Rhone-Poulenc and DuPont were implementing various forms of abatement from catalytic decompostion to N2 and 02 to thermal oxidation for nitric acid recovery value [3]. R&D and capital investments for implementation of this effort have been over $40M.
Solutia, at their Pensacola adipic plant, was already practicing complete N20 abatement in a thermal reduction unit. In the early 90's, the decision was made to pursue a more desirable path of value-added utilization of N20. Over 100k mt/yr was available. The direct hydroxylation of benzene to phenol by N20 over ZSM-5 catalysts was receiving much attention at this time [4]. This appeared to be good candidate process. Particularly, when the phenol process was incorporated into overall new adipic acid process (eq. (2)).
3H2~ C
~O2) ~../COOH N20
............................................................................................ J
(2)
Uniqueness of nitrous oxide is determined by several factors the most important being the fact that it is a carrier of a single oxygen atom and its metastability. When contacted with the catalyst, N20 decomposes giving N2 and highly active form of oxygen bound to the catalyst, called a-oxygen, a-Oxygen, upon contact with organic molecules, inserts into C-H bonds giving products of selective partial oxidation. In collaboration with the Boreskov Institute of Catalysis (BIC), a program was initiated in 1994 to commercialize the direct benzene to phenol process. The two major steps: (1) the hydroxylation reaction and (2) the N20 recovery and purification were developed in parallel. All aspects of the hydroxylation process, including all gas and liquid recycle effects, were successfully demonstrated in Solutia's pilot facility at Pensacola [5]. 2. ADIPIC ACID PROCESS DESCRIPTION AND N20 SPECIFICATIONS Figure 1 shows a simplified diagram of a typical adipic acid process. In the reactor, KA-Oil and excess HNO3 are intimately mixed in the presence of a soluble Cu and V catalyst system. A large recycle of the HNO3 liquid stream improves product selectivity and temperature control. A side stream is
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w i t h d r a w n from the recycle stream and sent to a crystallization train for purification. The off-gas from the reactor [A] is sent to a water and air scrubber to convert the NOx to recoverable HNO3. The waste gas from the scrubber [B] is sent to a NOx a b a t e m e n t system. Table 1 shows the approximate compositions of the two gas streams from which N20 could be recovered. NOx Abaterr~nt
H20
,
I?
Air
)
hllgtllP
~
HNO3 recovery r
KA-Oil
:l
NO3
..-
~
~ Adipic acid purification
Fig. 1 Table 1. Approximate compositions of the gas streams in process streams [A] and [B] (see Fig.l), their temperatures, and pressures Parameter
Stream A
Stream B
NeO, vol.%
75
35
Ne, vol.%
4
61
02, vol.%
0
3
CO2, vol.%
6
3
CO, vol.%
0.2
0.1
NOx, vol.%
5
0.3
HeO, vol.%
5
3
VOC, ppm
300
100
Temperature, oC
75
40
Pressure, psia
45
40
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Despite its high reactivity, the N 2 0 - Fe-ZSM-5 system is very delicate and sensitive to impurities that may be present in the feed stream. Thorough studies of the influence of these impurities on the hydroxylation of benzene have revealed that such contaminants as NOx, ammonia, and N-containing compounds are catalyst poisons. Others, like CO, although not catalyst poisons, can compete for a-oxygen, thus reducing the selectivity of the process. O2 was found to lead to complete benzene combustion. Therefore, the nitrous oxide purification requirements are stringent with respect to these species. However, there are many compounds that are inert under the reaction conditions (N2, CO2, etc.). Solutia's new phenol process, in fact, requires that inert components be present in the feed stream to avoid the flammable region. This leads to a unique nitrous oxide purification process - while some of the components have to be removed completely, it is desirable to leave the others. In summary, the poisons should be reduced by 99% whereas the CO and O2 by 95%. 3. NaO RECOVERY AND PURIFICATION OPTIONS
A large-scale adipic acid plant (> 200 mt/yr) generally has multiple reactor and purification trains. Depending upon the adipic acid plant configuration and purification/recovery scheme chosen, the use of one stream will have overall cost advantages over the other. The more concentrated stream [A] is more amenable to absorption, membrane, and/or cryogenic separation technologies. Catalytic purification is more suited for the dilute stream [B]. Fig. 2 shows the schematic of the catalytic process. Nitrous oxide stream similar in composition to stream B in Table 1 is processed in the first reactor using low temperature selective catalytic reduction (SCR) with low ammonia slip to remove NO and NOz. Because the feed stream to this reactor is coming from multiple adipic acid trains operating under varying conditions, the system is designed for rapid response to fluctuations in NOx levels as high as +20%. It is also highly resistant to catalyst poisons.
Gas From Adipi~~ Acid Adsorber
1
SCR
DeOx
T NH3
l H2
Fig. 2
N20 to AlphOx
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The gas s t r e a m leaving the SCR unit is fed to the DeOx unit to remove oxygen. This is a special case of oxygen removal using a noble m e t a l catalyst differing from traditional DeOx applications in t h a t the s t a r t i n g levels of oxygen are higher (2-4% vs. usual 100 ppm) and oxygen is selectively removed in the presence of a n o t h e r oxidant - N20. There is an additional benefit in using the DeOx system in this a p p l i c a t i o n - it destroys VOCs in the feed stream.
Both SCR and DeOx systems have to operate below 350~ to prevent N20 loss. The Pilot P l a n t using the actual adipic acid off-gas has been in operation for more t h a n 1.5 years and d e m o n s t r a t e d t h a t fast-response control logic systems are critical for stable and efficient purification of N20. The Pilot P l a n t P r o g r a m established a n u m b e r of i m p o r t a n t process features: 95 to 97% recovery of the N20, 90% destruction of the organic carbon, and 98% destruction of the NOx. It also showed how well the technology could ride out u p s t r e a m upsets and led to the evaluation of non-traditional regeneration, startup, and shutdown procedures.
4. ON-PURPOSE N20 One should not be mislead to assume t h a t the novel hydroxylation technology should only be of interest to the parties having access to large a m o u n t s of waste N20. On the contrary, our studies have shown t h a t competitive processes can be designed even if one has to make N20 on purpose. Traditional route for m a n u f a c t u r i n g N20 - decomposition of a m m o n i u m nitrate - would require 100 mt of molten a m m o n i u m nitrate (!) to be handled at any time to supply 90K mt/year N20 sufficient to make 150K mt/year phenol. Direct oxidation of a m m o n i a by oxygen is practiced by Mitsui Toatsu Chemicals to m a k e 450 mt of medical grade nitrous oxide per year [6]. The direct oxidation reaction shows the most promise for economically viable route, especially t a k i n g into account t h a t the hydroxylation process does not require neat N20. Solutia and BIC t e a m e d up on the N20 on-purpose project with the goal of developing a stand-alone hydroxylation process independent of adipic acid off-gas. Oxidation of a m m o n i a to N20 is a strongly exothermic reaction producing ca. 80~ of adiabatic t e m p e r a t u r e rise per 1 mole % of converted NH3 in feed (eq. (3)). The process t e m p e r a t u r e needs to be kept below 400~ to minimize N20 decomposition. These constraints make fluidized bed reactor design the preferred technology. 2NH3 + 202 -~ N20 + 3H20
(-180 kcal/mole)
(3)
The program, t h a t included both catalyst and process development, has had some significant advances and it now entering a Pilot P l a n t stage. Ammonia oxidation to nitrous oxide is accomplished using a Mn/Bi/A1 oxide catalyst [7] and selectivity to N20 as high as 92% have been achieved at almost complete conversion of ammonia.
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5. CONCLUSION
What started as a search for an innovative way to deal with a waste steam evolved into a collection of novel technologies with great potential. Single-step hydroxylation of benzene to phenol provides a commercially viable and environmentally friendly alternative to the traditional cumene process. It also serves as the basis for a step change in adipic acid manufacture, which allows to convert waste N20 to valuable product. The need to provide N20 of certain specifications has led to the development of new efficient purification technique. Other hydroxylation processes using N20 are being developed and will emerge in the near future. The general attractiveness of partial oxidation using nitrous oxide dictated the necessity to produce it in the most economical way possible and this need was met by the development of direct ammonia oxidation process. Success in one field opens opportunities in other fields, and we believe that the industrial chemistry of nitrous oxide as reagent is just beginning. The fact that the cost of active oxygen in N20 is about 88of its cost in hydrogen peroxide should provide significant driving force for search for new applications in all fields of chemical manufacturing from commodity to fine chemicals.
REFERENCES 1. M.H. Thiemens and W.C. Trogler, Science, No. 251 (1991) 932 2. R.A. Reimer, C.S. Slaten, M. Seapan, M.W. Lower and P.E. Tomlinson, Environmental Progress, No. 13 (1994) 134. 3. Chemical Week, No. 160 (1998) 37. 4. (a) E. Suzuki, K. Nakashiro and Y. Ono, Chem. Lett., (1988) 953; (b) M. Gubelmann and P.J. Tirel, US Patent No. 5 001 280 (1991); M. Gubelmann, J.M. Popa and P.J. Tirel, US Patent No. 5 055 623 (1991); (c) R. Burch and C. Howitt, Appl. Catal., A, No. 86 (1992) 139; (d) A. S. Kharitonov, T.N. Aleksandrova, L.A. .Vostrikova, K.G. Ione and G.I. Panov, USSR Patent No. 4 445 646 (1989); A. S. Kharitonov, G. I. Panov, K. G. Ione, V. N. Romannikov, G. A. Sheveleva, L. A. Vostrikova and V. I. Sobolev, US Patent No. 5 110 995 (1992). 5. A.K. Uriarte, M.A. Rodkin, M.J. Gross, A.S. Kharitonov and G.I. Panov, In Proc. 3rd Intern. Congress on Oxidation Catalysis, R.K. Grasselly, S.T. Oyama, A.M. Gaffney and J.E. Lyons, Stud. Surf. Sci. Catal., Elsevier Science B.V., No. 110 (1997) 857. 6. Jpn.Chem. Week, No. 35(1758) (1994) 9. 7. V.V. Mokrinskii, E.M. Slavinskaya, A.S. Noskov and I.A. Zolotarskii, PCT Int. Appl. WO 9825698 (1998), priority: RU 96-96123343.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Catalytic W e t Peroxide Oxidation over mixed (AI-Fe) Pillared Clays J. Barraulta, C. Bouchoule a, J.-M. Tatibouet a, M. AbdellaouP, A. Majest6 a, I. Louloudi b, N. Papayannakos b and N. H. Gangas b a LACCO, ESIP, 40, Av. Recteur Pineau, 86022 Poitiers cedex, France, e-mail
[email protected], fr b Dept. of Chem. Eng., NTUA, 9 Heroon Politechniou, GR 15780 Zografou, Athens, Greece
Mixed (AI-Fe) pillared clays, now prepared on pilot scale are very efficient solid catalysts for the oxidation of organic compounds with hydrogen peroxide in water. It is demonstrated in this study that in rather mild experimental conditions (atmospheric pressure, T _< 70~ and with a low excess (20 %) of hydrogen peroxide, phenol was highly converted to CO2 without catalyst leaching. Indeed the (AI-Fe) pillared clay (called FAZA) was recycled and used several times without any change of their catalytic properties. Iron strongly bonded to aluminium in the pillars characterized by M6ssbauer spectroscopy were suggested as the active sites in the phenol oxidation. I. INTRODUCTION Numerous wastewater streams containing organic pollutants are generated by many industrial processes and agricultural activities. As environmental regulations and health quality standards are becoming more restrictive there is a need [1 ] for defining different strategies in order: (i) to develop new "clean" technologies; (ii) to improve existing processes; and (iii) to build closed industrial water purifying and recycling systems. With respect to toxic and refractory pollutants~ catalytic wet oxidation process (CWO) employing either oxygen, ozone, hydrogen peroxide (CWPO) or a combination thereof could prove useful, since the CWO process appears to be more suitable for achieving high conversion or/and high rates at lower temperature and pressure [2]. Homogeneous catalysts are in general very effective but their recovery from the treated ettluents requires additional separation costs. This drawback can be overcome by using heterogeneous catalysts if they are stable. Among all the possible processes for catalytic wet oxidation, depending on the oxidising agent, the CWPO process using hydrogen peroxide as oxidant was chosen and investigated in our laboratory. This choice is based on the idea of a porous catalyst having active centres in order either for inducing the homolytic activation of hydrogen peroxide, followed by a reaction initiated by radical species, or for forming hypervalent species useful perhaps in a follow up non-radical mechanism. Up to now very different solids were proposed as catalysts for the oxidation of various organic compounds in water [3-7] but one often observes a declining catalytic activity and a
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concomitant leaching of the catalytically active elements. Previous studies with some clay based catalysts showed that these materials could represent an alternative if their activity could be improved [3, 4]. In the present study, the characteristics and the catalytic properties of natural clays intercalated or pillared with aluminium or mixed (aluminium-iron) polyoxocations, now prepared on pilot scale [8], are reported and phenol oxidation is chosen as a model reaction.
2. EXPERIMENTAL 2.1. Phenol oxidation Phenol oxidation was carried out in a thermostated pyrex [3] or in a stainless steel reactor at a constant pH. A 100 ml phenol solution containing 5 10-5 M of phenol was placed over the solid catalyst in the presence of hydrogen peroxide generally at atmospheric pressure and at 293 K. Hydrogen peroxide was added either at the beginning of the reaction or continuously. At the end of the experiment a molar ratio of 1.2 was reached for H20~ added versus carbon of phenol; [H~O2] / 6 [phenol]. Samples of the reaction medium were analysed in order to obtain the phenol and the hydrogen peroxide conversion. Total organic carbon (TOC) was measured with a DC 190 Dohrmann equipment. Phenol and products of phenol conversion were analysed with a Waters HPLC equipped with a Cls column. The main products detected were 9catechol, hydroquinone, quinones, muconic acid, maleic acid, oxalic acid and acetic acid. The H202 concentration was obtained by a colorimetric method with a UV/Visible DMS 90 Varian spectrophotometer. The content of metal ions in the solution was determined from atomic absorption analysis (PerkinElmer 2380). 2.2. Catalysts preparation and characterisation The catalyst FAZA was a mixed AI-Fe pillared clay. The starting clay was a Greek bentonite with the commercial name Zenith-N. The preparation route for FAZA was developed at laboratory scale and was further scaled-up to 1 Kg/batch quantity [8, 9]. It comprised the following steps : A 2 % wt Zenith-N clay suspension in water was prepared. An intercalant solution was prepared by titration of an A13+/Fe 3+. cationic solution with 0.2 M NaOH. The cationic solution consisted of 0.18 M AICI3 and 0.02 M FeC13. The addition of the NaOH solution to the cationic solution was attempted at a controlled rate and at 70~ while the amount of the NaOH solution added was such that the final OH/cation ratio was equal to 1.9. The intercalant solution was added to the clay suspension under stilxing. The final (Al+Fe)/clay ratio was equal to 3.8 mol/Kg dry clay. The pillared clay precursor was then washed till chloride free, dried at 60-70~ and calcined at 500~ for 2 h to receive the FAZA catalyst pillared clay. The do0~-spacing of FAZA was determined as 1.84 + 0.02 nm by XRD analysis. Nitrogen adsorption data were used to estimate a BET specific surface area equal to 240 + 10 m2/g. The pore volume of FAZA was estimated as 0.14 cm3/g. The composition of FAZA on dry basis is : SiO2 : 52.50 wt %, A1203 : 27.56 wt %, Fe203 : 7.02 wt ~ CaO : 0.35 wt %, MgO : 2.50 wt %, K20 : 1.38 wt %, Na20 : 0.27 wt %, TiO2 : 0.17 wt % and LOI : 7.50 wt %.
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3. RESULTS
3.1. Phenol oxidation in presence of a homogeneous [Fen, Fe m, 02, H202] system Before studying clays based solid catalysts, the homogeneous phenol transformation in the presence of soluble iron species was investigated and the results are presented in figure 1. In agreement with previous works [10, 11] a maximum rate of phenol elimination is obtained for a pH value between 2.5 and 3.5 which corresponds to the pH range where the rate of H202 decomposition is minimum [ 10]. Indeed for such pH values, iron is in a Fe(OH) ++ complex form [ 12] while (i) for a more acidic pH, the Fem content is more important increasing the consumption of HO" and HO2" radicals; (ii) for a more basic pH the ionic decomposition of H202 is preponderant and produces molecular oxygen without radicals formation. H202 + O H ~ HO2 + H20 (1) HO2- + H202 --~ 02 + OH" + H 2 0 (2)
10
100
-*- Phenol
8 .o 6
.or/3 60
4
=~ 40
r,r l-q O
/~
80
-*- Phenol
o
o
20 0
2
3
4
pH
5
Fig. 1. Phenol (100 mE, 5 10.5 M) oxidation by H202 (2 mE.h "l, 0.01 M) with iron (5 mg.L -l) and a nitrogen flow (2L.h l) at 25~ at 60 min
,
2
3
4
pH
5
6
Fig. 2. Phenol (100 mL, 5 10.5 M) oxidation by H202 (2 mL.h l, 0.01 M) with iron (5 mg.L l) and an air flow (2 L.h~) at 25~ at 60 min
In the presence of oxygen the reaction rate is much more important either for phenol elimination or TOC conversion (figure 2). In fact in agreement with work of Uri [13] and Debellefontaine [ 14], the (Fe 2+, H202) system initiates the reaction via the formation of radicals and oxygen could increase the propagation steps via the following and simplified reaction scheme, Fe 2+ + H202 --~ H O ' + Fe 3+ + O H (3) Fe 3+ + H 2 0 2 ~ Initiation steps HO2"+ Fe 2+ + H + (4) RH + HO" ~ R" + H20 (5) RH + HO2" ~ R" + H202 (6) Propagation steps
~ ~
R" + 02 ROO" + RH -~ ROOH + Fe 2§ ~ ROOH + Fe 3+ ~
ROO" ROOH + R" RO" + Fe 3+ + O H ROO'+ Fe 2§ + I-I+
(7) (8) (9) (10)
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In such a way, at room temperature, the phenol is completely eliminated from the solution and mainly transformed into catechol, hydroquinone and maleic acid, the TOC conversion being of 15 % and 35 % after respectively 60 and 120 min of reaction. 3.2. Phenol oxidation over clay based catalysts Preliminary experiments carried out with (AI-Cu) or (AI-Fe) pillared clays [3, 4, 15] showed that such solids catalysed the phenol oxidation in similar conditions with a TOC conversion greater than that obtained with homogeneous catalysts. It was also demonstrated that the final result is greatly dependent on the preparation process of the catalyst and on the metal content. Moreover all these variables also influence the metal leaching during the reaction which is one of the main problems to be solved. From these researches we discovered that mixed (A1-Fe) pillared clays prepared according to the procedure described in the experimental section are among the more stable catalyst in aqueous phase. 3.3. Phenol oxidation over (AI, Fe) pillared clays Table 1 and results presented in figures 1 and 2 show that phenol and specially TOC conversion in the homogeneous catalysis experiments (Fenton - type reaction) are lower than the values obtained in heterogeneous catalysis with FAZA solids. Table 1 Phenol oxidation with hydrogen peroxide over mixed (A1-Fe) pillared clays catalysts Experiment Catalyst T (~ P (MPa) Phenol TOC Soluble (g) conversion (%) conversion iron (ppm) (time) (%) 1 0 25 0.1 0 0 0 2 Homog. 25 0.1 100 (2h) 10 (2h) a 2 3 0.5 25 0.1 100 (90mn) 50 (2hf 0.4 (4h) 4 0.5 40 0.1 100 (90mn) 70 (2h) a 0.8 (4h) 5 1 70 0.1 100 (15mn) 80 (90mnf 0.2 (2h) The experiment was done in a batch reactor (100 ml of phenol, 5 10.5 M) with a continuous introduction o f H202 (2 mL.h -1 , 0.01 M) for 4h, an.air flow of 2 L.h1 and a pH = 3.5 - 4 . a The main products obtained at the end of the experiment were oxygenated (oxalic, acetic and formic acids). Moreover, in the presence of iron homogeneous catalysts, the amount of iron in the phenol solution was 2 or 5 ppm, i.e. about 4 to 30 times higher than the amount detected in the experiments with the FAZA samples. Indeed from experiment 3 carried out at 25~ one can observe that besides the total conversion of phenol in less than 60 min, the TOC conversion reaches 50 % over the pillared clay. In similar conditions and with an iron homogeneous catalyst the TOC conversion was only of 10 %. If the experimental conditions are modified i.e. by increasing the temperature (experiment 4) and the catalyst weight (experiment 5), the TOC conversion is higher than 80 % after 90 min. This result means that in such conditions, the more resistant light acids are at least partially transformed into CO2. The comparison of theoretical and experimental TOC conversions seems to indicate that these results are not the consequence of a lack of hydrogen peroxide
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even though some direct transformation of hydrogen peroxide to oxygen occurs during the reaction.
3.4. Recycling of FAZA sample used in the phenol oxidation with H20, A 1 g of FAZA sample was used in 3 consecutive experiments and the results are reported in Table 2. It is rather evident that there is neither catalyst deactivation during the reaction nor catalyst leaching. Indeed the iron analyzed in the solution after the phenol oxidation represents less than 0.05 % of the total iron content of the FAZA catalyst. Free iron oxides on the clay outer surfaces are most probably the main source of the iron leached in the above experiments. Table 2 Recycling of FAZA sample in the phenol oxidation Experiment Time Phenol TOC convers~n Solub~ kon (min) convers~n (%) (%) (ppm) 15 3 3 1 60 55 20 0.2 120 100 60 15 30 5 2a 60 100 25 0.25 120 100 60 15 50 7 3a 60 100 35 0.35 120 100 60 The experiment was done in conditions presented in table 1 at a temperature of 25~ with a catalyst weight of 1 g. Before reuse, the sample was dried in an oven at 80~ for 12 h The low iron leaching shown by FAZA sample is in line with the conclusion of a study by Komadel et al [16] that iron in this material is strongly fixed with aluminium in the pillars. This conclusion was based on measurements of the amount of iron extracted upon treating two different AI-Fe pillared clays with acids, and complexing and/or reducing agents. More specifically, it was found that less iron was extracted from FAZA than from the other AI-Fe pillared clay that contained about 25 wt % of iron. The existence of strong bonding of iron to aluminium in the pillars was also concluded by Bakas et al [ 17] in a M~Jssbauer study of an A1Fe pillared clay containing-20 wt % of iron. In the case of the FAZA samples used in this study M~ssbauer and chemical data have revealed that - 60 % of the iron is bonded in the pillars together with aluminium [9]. 4. CONCLUSION In this paper it is shown that [Iron-Aluminl"um] oxides pillared clays are very efficient catalysts for the total conversion of phenol to carbon dioxide in rather mild conditions (atmospheric pressure, temperature _< 70~ Of course, the use of hydrogen peroxide as oxidant favours the oxidation reactions in such experimental conditions, but one must notice that it is the first time that so signifieative results are obtained with a so low excess of hydrogen peroxide (1.2) and without catalyst leaching.
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ACKNOWLEDGEMENTS
The authors gratefully acknowledged the European Community, (the HYDROCONV network) for financial support. REFERENCES
1. Y.I. Matatov-Meytal and M. Sheintuch; Ind. Eng. Chem. Res., 1998, 37, 309. 2. F. Luck, Chem. Biochem. Q., 1996, 27, 195. 3. J. Barrault, C. Bouchoule, K. Echachoui, N. Frini-Srasra, M. Trabelsi and F. Bergaya; Appl. Catal. B - Envi, 1998, 15, 269. 4. M. Abdellaoui, J. Barrault, C. Bouchoule, N. Frini-Srasra and F. Bergaya, J. Chim. Phys., 1999, 96-3, 419. 5. M. Falcon, K. Fayerwerg, J.N. Foussard, E. Puech-Costes, M.T. Maurette and H. Debellefontaine; Environmental Technology, 1995, 16, 501. 6. K. Fayerwerg and H. Debellefontaine; Appl. Cat. B. Env., 1996, 10, L229. 7. C. Hemmert, M. Renz and B. Meunier; J. Mol. Cat. A. Chem., 1999, 137, 205. 8. V. Kaloidas, C.A. Koufopanos, N.H. Gangas, N.G. Papayanakos, Micr. Mat., 1995, 5, 97. 9. N.H. Gangas and N.G. Papayannakos; Sections 111.4 to 111.6 in the Final Report (1993) of Feasibility Award: FA- 1010-91; Contract Number: BRE2-063. 10. D.F. Bishop, G. Stern, M. Fleischmann and S. Marshall, Ind. Eng. Chem. Process Res. Dev., 1968, 7-1, 110. 11. H.R. Eisenhauer, J. Wat. Poll. Control. Fed., 1964, 36-9, 1116. 12. M. Pourbaix, "Atlas d'~quilibres 61ectrochimiques", Gauthier- Villars Eds, Paris, 1963. 13. M. Uri in "Autooxidation and antioxidants", W.M. Lundberg Ed, Interscience, New York, 1961. 14. H. Debellefontaine, M. Chakchouk, J.N. Foussard, D. Tissot and P. Striollo, Env. Poll., 1996, 92-2, 155. 15. M. Abdellaoui, J. Barrault, C. Bouchoule, M. Touchard and F. Bergaya, submitted. 16. Komadel. D. H. Doffand J.W. Stucki; J. Chem. Soc. Chem. Commun., 1994, 1243. 17. T. Bakas, A. Moukarika, V. Papaefthymiou, A. Ladavo, N.H. Gangas, Clays and Clay Minerals, 1994, 42-5, 634 and 1996, 44-6, 851.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
A n e w iron p h o s p h a t e c r y s t a l l i n e p h a s e a n d its c a t a l y t i c a c t i v i t y in 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 l a c t i c a c i d a n d glycolic acid M. Ai ~ and K. Ohdan b ~Department of Applied Chemistry and Biotechnology, Niigata Institute of Technology, 1719 Fujihashi, Kashiwazaki 945-1195, Japan bUbe Laboratory, UBE Industries Ltd., 1978-5 Kogushi, Ube 755-8633, Japan Iron phosphates are effective as catalysts for oxidative dehydrogenation of lactic acid and glycolic acid. The structure of iron phosphate is transformed into a new crystalline phase during the use as catalyst in these reactions. This induces a marked increase in the catalytic performances. The formation, characteristics, and catalytic properties of the new iron phosphate phase were studied.
1. INTRODUCTION Phosphates of vanadium and molybdenum, such as (VO)2P207 and H3PM012O40, are widely used as oxidation catalysts for producing carboxylic acids and anhydrides. Indeed, a great number of studies concerning them have already been reported. On the other hand, little attention has been paid to iron phosphate which possesses both acidic and redox properties like (VO)2P207 and H3PMo12040, though iron phosphate catalysts have been known to be effective for the oxidative dehydrogenation of isobutyric acid to form methacrylic acid. Recently, it was found that iron phosphates show a unique catalytic performance for several oxidative dehydrogenation reactions, such as formation of pyruvic acid from lactic acid [1], glyoxylic acid from glycolic acid [2], and pyruvaldehyde (2-0xopropanal) from hydroxyacetone (acetol)[3]. More recently, it was also found that a part of iron (III) orthophosphate (FePO4) is transformed into a new crystalline phase during the use as catalyst in the oxidative dehydrogenation of both lactic acid [4] and glycolic acid, and that the performances of catalyst are markedly enhanced by the change in structure. XRD patterns of the new phase are very similar to those of clay minerals such as kaolinite, halloysite, dickite, and nacrite. From the studies on DTA/TG, FT-IR, oxidation states of iron in the bulk and on the surface, XRD patterns, and elementary analysis, the chemical composition of the new crystalline phase was estimated as FePO4-0.5H20 or Fe2P2OT(OH)2 [5]. The aim of this paper is to get more insight into the new crystalline phase concerning the formation, the characterization, and the catalytic properties.
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2. EXPERIMETAL
2.1 C a t a l y s t The original iron phosphate sample was prepared according to the procedures described previously [3,6,7]. The P/Fe atomic ratio was in the range of 1.05 to 1.2. It was calcined in a s t r e a m of air at 400~ for 8 h. It consisted of tridymite-type FePO4. The presence of quartz-type FePO4 was not detected in the XRD patterns. 2.2 Reaction procedures The reaction was carried out with a continuous-flow system at atmospheric pressure. The reactor was made of a stainless steel tube, 50 cm long and 1.8 cm I.D., m o u n t e d vertically and immersed in a lead bath. The iron phosphate sample was placed n e a r the bottom of the reactor and porcelain cylinder, 3 m m long and 1.5 m m I.D./3.0 mm O.D, were placed both under and above the sample. Air, oxygen, nitrogen, a mixture of oxygen and nitrogen, hydrogen, or helium was fed in from the top of the reactor; an aqueous solution containing a fixed a m o u n t of an organic compound was introduced into the p r e h e a t i n g section of the reactor by a syringe pump. The effluent gas from the reactor was led successively into four chilled scrubbers to recover the w a t e r soluble compounds. The products were analyzed by GC and LC. 2.3 Characterization XRD p a t t e r n s were studied using a Shimadzu 6000 diffractometer with Cu I ~ radiation. FT-IR spectra were recorded from 4000 to 400 cm 1 with a PerkinE l m e r 1700, using KBr disk technique. E l e m e n t a r y analyses were performed with a Yanagimoto CHN corder MT-5. Surface areas were m e a s u r e d by the BET method using nitrogen as adsorbate a t - 1 9 6 ~ The amounts of Fe 2§ and Fe 3§ ions in the bulk were determined by the redox titration method [8]. 3. RESULTS AND DISCUSSION 3.1 F o r m a t i o n o f n e w iron p h o s p h a t e phase The effects of reaction variables on the formation of the new iron phosphate crystalline phase were studied. The typical procedures were as follows. A 5 g portion of the tridymite-type FePO4 sample was placed in the stainless steel reactor. A mixture of nitrogen and oxygen was fed in the reactor with the feed rates of 200 and 5.0 ml/min, respectively. An aqueous solution containing about 10 wt% of an organic compound was introduced into the reactor with a feed rate of 35 ml/h. The t e m p e r a t u r e was 220~ and the reaction time was 24 h. 3 . 1 . 1 E f f e c t s o f organic c o m p o u n d Vapors of various kinds of organic compounds were passed over the tridymitetype FePO4 sample in the presence of a large a m o u n t of steam and a limited a m o u n t of oxygen. The results are s u m m a r i z e d Table 1
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Table 1 Effects of organic compound Organic compound n-propanol Propylene glycol Formic acid Isobutyric acid Pyruvic acid Oxalic acid Lactic acid Glycolic acid Hydroxyacetone (vs) = very small
24. 5 ~
Phase observed in XRD 12.2 ~ (new p - new phase) Quartz + new p(vs) 32.4 ~ Quartz + new p(vs) "Quartz + new p(vs) Quartz + new p(s) Tridymite + new p(vs) New p 10 15 20 25 30 35 New p + Fe2P2OT(s) 2 theta / o New p + Fe2P207 Fig. 1 XRD p a t t e r n s of new phase. Fe2P207 amount; (s) = small a m o u n t c-
t
The best results were obtained from oxalic acid; the new phase was the sole crystalline compound observed. The next best results were obtained from lactic acid; a small a m o u n t of Fe2P207 was always formed together with the new phase. A mixture of the new phase and Fe2P207 was obtained with glycolic acid. In the case of hydroxyacetone (acetol), the iron phosphate sample was reduced totally to Fe2P207 and the new phase was not obtained. On the other hand, the new phase was scarcely obtained with n-propanol, propylene glycol, formic acid, isobutyric acid, and pyruvic acid. The transformation from tridymite-type to quartz-type is ascribable to the steam [9], but not to the organic compounds. Typical XRD p a t t e r n s of the new phase obtained using oxalic acid are shown in Fig. 1. The relative intensities of the three main peaks at 20 of 12.2 ~, 24.5 ~, and 32.4 ~ were 60, 100, and 65, respectively. No clear peaks assigned to any known iron phosphate crystalline phases were detected in the spectra.
3.1.2 Effects of temperature The effects of reaction t e m p e r a t u r e were studied using oxalic acid as the organic compound. The optimum t e m p e r a t u r e was found to be in the range of 210 to 240~ At a t e m p e r a t u r e below 200~ the vaporization of oxalic acid became difficult. The FePO4 sample was not transformed into the new phase at a t e m p e r a t u r e above 300~ This finding may be consistent with the fact t h a t the new phase contains crystalline water [5]. 3.1.3 Effects of oxygen The effects of oxygen in the feed were studied by changing the concentration from zero to 20 mol% using oxalic acid as the organic compound. In t h e absence of oxygen, a small amount of crystalline Fe2P207 was obtained beside the new phase at 220~ While at 200~ the new phase was the sole phase observed in XRD patterns. Moreover, when the oxygen concentration was more t h a n 1 mol%, the new phase was the sole phase observed even at 220~
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3 . 1 . 4 Effects o f reaction period When the amount of sample used was 5 g, the whole sample was transformed within 5 h on stream. With further time on stream, no change in the structure was observed. However, when the amount was 40 g, a reaction period of more t h a n 24 h was required to transform the whole sample into the new phase.
3 . 1 . 5 Effects o f starting iron p h o s p h a t e The oxalic acid solution was passed over different iron phosphate samples prepared from the original tridymite-type FePO4, such as amorphous FePO4, tridymite-type FePO4, quartz-type FePO4, Fe3(P2OT)2, Fe2P2OT, and Y-phase. The Y-phase was obtained by reoxidation of Fe2P207 in air at 400~ for 8 h [10,11]. It was found that, independent of the change in the structure, all of the FePO4 samples can be transformed into the new phase, while that the Fe3(P2OT)2, Fe2P2OT, and Y-phase samples cannot be transformed into the new phase. 3.2 C h a r a c t e r i s t i c s o f new phase 3 . 2 . 1 T h e r m a l stability The new phase was found to be stable enough up to 300~ in both air and nitrogen. However, it decomposed gradually at 400~ in both air and nitrogen; within 4 h the majority was decomposed to form amorphous phase with a small amount of either Y-phase (in air) or Fe2P207 (in nitrogen). 3 . 2 . 2 R e g e n e r a t i o n of new phase The sample consisting of the new phase was calcined in nitrogen at 440~ for 46 h. The peaks assigned to the new phase were disappeared. Then, a mixture of nitrogen and steam was passed over it at 250~ for 23 h. The new phase was regenerated in a small part and another unknown phase, which is characterized with two clear peaks in the XRD patterns at 20 of 16.3 ~ and 23.4 ~ was formed. 3.3. Catalytic a c t i v i t y Activity of the new phase was studied in the following two reactions; CH3-CH(OH)-COOH + 0.502 (HO)CH2-COOH + 0.502
~
-~
CH3-CO-COOH + H20
O=CH-COOH + H20
(1) (2)
3 . 3 . 1 O x i d a t i o n of lactic acid to pyruvic acid The catalytic performances of the new phase were compared with those of various other iron phosphates prepared from the original tridymite-type iron phosphate. The results and the reaction conditions used are shown in Table 2. The catalyst consisting of the new phase was clearly higher than that consisting of the original tridymite-type FePO4 in both the activity and selectivity. The Fe3(P207)2 and Y-phase iron phosphate show a higher activity than the new phase does, but the new phase is higher in both the yield and selectivity t h a n the Fes(P207)2 and Y-phase iron phosphate.
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Table 2 Comparison of the catalytic performances of various iron phosphates in the oxidation of lactic acid to form pyruvic acid * Catalyst Temperature = 225~ Temperature = 230~ Iron phosphate Cony/% Yield/% Selct/% Cony/% Yield/% Select/% Tridymite-type FePO4 29.5 19.6 66. 41.1 25.5 62 New phase 53.2 40.0 75. 62.3 45.0 72 Y-phase 58.0 34.8 60. 67.0 35.5 53 Fe3(P207)2 61.0 37.8 62. 78.0 42.1 54 Fe2P207 32.4 25.0 77. 42.2 31.5 75 *Feed: lactic acid/air/steam = 19.2/350/962 mmol/h; contact time = 1.6 s.
3 . 3 . 2 O x i d a t i o n o f glycolic acid to glyoxylic acid The catalytic performances of the new phase in the oxidation of glycolic acid was studied under the following reaction conditions; feed rates of glycolic acid/oxygen/water/nitrogen = 12.3/25/480/500 mmoYh, temperature = 240~ amount of catalyst used = 5.0 g (contact time = 1.25 s). It was found th at the catalyst consisting of the new phase is higher t h a n that consisting of freshly prepared tridymite-type FePO4 in both the catalytic activity and selectivity as in the case of the oxidation of lactic acid. On the other hand, it has been reported t ha t the partially and fully reduced iron phosphates are clearly lower in both the catalytic activity and selectivity t h a n the iron phosphate consisting of tridymite-type FePO4 [2]. It is important to note that the structure and the oxidation states of iron phosphate change during the use as catalyst in the oxidation of glycolic acid much as in the case of the oxidation of lactic acid. That is, after 20 h on stream the original tridymite-type FePO4 is transformed into quartz-type FePO4 and after 100 h on stream all of the quartz-type FePO4 is transformed into the new phase. The variation in XRD patterns are shown in Fig. 2. Figure 3 shows the variations in the conversion, the yield of glyoxylic acid, and the oxidation states of iron in the bulk as a function of the time on stream. As the time on stream increases, the catalytic activity increases, shows a maximum at around 20 h on stream, and then falls with a further increase in the reaction time. After about 110 h on stream, the catalyst was calcined in a stream of air at 400~ for 1 h. It was found that the calcined catalyst consisting of the new crystalline phase, shows a markedly higher catalytic activity. The presence of a large amount of steam is essential to every oxidative dehydrogenation reaction performed with iron phosphate catalysts [7,12,13,14]. This may be related closely to the finding that the new phase containing crystalline water shows a performance better t han that of the iron phosphates without crystalline water. Similar view has been proposed in the case of oxidation of isobutyric acid by Millet et al. [12]. It is considered t ha t as the time on stream increases the catalyst is deactivated gradually due to a poisoning by deposit of some nonvolatile products on the
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surface. Possibly, iron phosphate catalysts cannot burn out the nonvolatile products at low temperatures used. This may be the reason why the catalytic activity was totally regenerated by the calcination in air at 400~ 9
-=O
~
|
1
S t r u c t u r e change t r idymi r e - > q u a r t z ~ new phase
I
90
'/!
GA conv GXA y i e l d
"O
-~ ,4~
0
Ca I c i nat ion at 4()0~
,
_=
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15
20
25
30
35
0
0
2 theta / o 9T r i d y m i t e FePO. o New phase
20
I
,
I
40
60
80
100 120
Reaction time / h zl Ouartz FeP04 0 Fe2P207 Fig. 3
Fig. 2 Variation of structure during
Variation of catalyst performances
and oxidation states during the oxidation
the use in the oxidation glycolic acid. of glycolic acid. REFERENCE
1. M. Ai and K. Ohdan, Appl. Catal. A, 150 (1997) 13. 2. M. Ai and K. Ohdan, Stud. Surf. Sci. Catal., 110 (1997) 527. 3. M. Ai and K. Ohdan, Bul. Chem. Soc. Jpn., 72 (1999) in press. 4. M. Ai and K. Ohdan, Appl. Catal. A, 165 (1997) 461. 5. N. Ishizawa, A. Saeki, K. Ohdan, and M. Ai, Powder Diffract., 13 (1998) 246. 6. M. Ai, E. Muneyama, A. Kunishige, and K. Ohdan, Bul. Chem. Soc. Jpn., 67 (1994) 551. 7. E.Muneyama, A.Kunishige, K.Ohdan, and M. Ai, J. Mol. Catal., 89 (1994) 371 8. R.A.J. Day and A.L. Underwood, "Quantitative Analysis," 4th ed. PrenticeHall, Englewood-Cliffs, N.J. (1980). 9. M. Ai and K. Ohdan, Appl. Catal. A, 180 (1999) 47. 10. J.M.M.Millet, J.C.V~drine, and G.Hecquet, Stud.Surf.Sci.Catal.,55(1990) 833. 11. E. Muneyama, A. Kunishige, K. Ohdan, and M. Ai, J. Catal., 158 (1996) 378. 12. J.M.M. Millet, D. Rouzies, and J.C. V~drine, Appl. Catal., A, 124 (1995) 205. 13. M. Dekiouk, N. Boisdron, S. Pietrzyk, Y. Barbaux, J. Jrimblot, Appl. Catal. A, 90 (1992) 61. 14. E.Muneyama, A.Kunishige, K.Ohdan, and M. Ai, Catal. Lett., 31 (1995) 209.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Dynamics of the oxidative dehydrogenation of propane over VMgO catalysts studied by in situ electrical conductivity and step transients H.W. Zantho~, J.C. Jalibert a, Y. Schuurman a, P. Slamaa, J.M. Herrmann b and C. Mirodatos a a Institut de Recherches sur la Catalyse, 2, ave.nue Albert Einstein, F-69626 Villeurbanne C6dex, France, Fax : 33 (0)4 72 44 53 99 / E-mail :
[email protected] b U.R.A., C.N.R.S., Photocatalyse, Catalyse et Environnement, Ecole Centrale de Lyon, BP 163, F-69131, Eeully, France The oxidative dehydrogenation of propane (ODHP) over redox VMgO catalysts was studied under non-steady state conditions using in-situ electrical conductivity and steptransient kinetic experiments. It was found that higher yields to propene can be obtained compared to steady state operation. The reaction of propane over these catalysts occurs by a fast reduction of the surface VO34 polymers and slow reduction of the bulk Mg3V2Os. Important kinetic parameters for the bulk diffusion of oxygen necessary for a possible scale up of transient operation can be obtained from the modelling of the conductivity and kinetic transient experiments. 1. INTRODUCTION The oxidative dehydrogenation of alkanes is an attractive alternative way for obtaining olefins, as compared to the applied non oxidative processes : cheap and abundant feedstock, relatively low temperature and almost no deactivation. However, no process has been developed industrially so far, mostly due to yield limitation. The lack of product selectivity at reasonable conversion levels is due to parallel and consecutive formation of undesired products, i.e., carbon oxides. A typical example is the oxidative dehydrogenation of propane over VMgO catalysts which are active and selective for properle production [ 1,2,3]. The reaction occurs via a Mars-van Krevelen mechanism that implies surface and bulk reactions in which catalytic redox cycles participate. The selective surface involves V 5§ cations, lattice oxygen and ionic vacancies [2,3]. However, in the steady-state mode where hydrocarbon and oxygen are co-fed the undesired total oxidation can hardly be suppressed due to the presence of loosely bonded oxygen species [4,5]. Therefore, it has been recently proposed [5] and demonstrated [6] that higher yields and selectivities can be obtained if the reaction is carried out in anaerobic transient operation mode, e.g. in a riser-regenerator or moving bed reactor. Carrying out a reaction in transient operation, however, requires a more detailed knowledge on the surface and bulk kinetic processes compared to steady state operation. In the present study we demonstrate how a combination of transient in-situ electrical conductivity experiments and conventional transient kinetic studies lead to improved understanding and important kinetic parameters for possible scale-up of ODHP in the non-steady state regime.
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2. EXPERIMENTAL
2.1. Catalyst preparation and characterisation VMgO catalysts with vanadium contents from 0 to 25 wt.% (referred to as 0, 2, 5, 10, 14 and 25V/VMgO in the text) were prepared from [Mg(OR)2]4 and [VO(OR)312 alkoxides with R = CH(CH3)CH2OCH3 in 1-methoxy 2-propanol and calcined up to 1073 K, as described in [7]. The catalysts were crushed and sieved to 200 to 300 ~tm for the experiments. At steady state conditions (C3Hs/O2/He = 64:90) these catalysts give optimised propene yields from about 10 % to 13 % at 823 K in a micro-catalytic fixed bed reactor. BET surface areas range from 88 to 37 m2.g~, decreasing with increasing vanadium content. Detailed physico-chemical bulk and surface characterisation using HREM, XPS, UV-vis and XRD techniques (as discussed in [3,9]) reveals that the activated catalysts are essentially biphasic with i) platelets of MgO covered with an amorphous scattered overlayer of VO34 units as isolated and polymeric species and ii) crystalline Mg3V208 large particles in lower amounts. Increasing vanadium loading favours the fraction of the crystalline Mg3V2Os phase [8]. Under reducing atmosphere the amorphous layer can easily and reversibly polymerise and form precursors of spinel-like surface structures with MgO. In the mean time, the Mg3V208 phase also transforms slowly to spinel but less reversibly [9]. 2.2. In-situ electrical conductivity measurements A static conductivity cell described elsewhere in detail [10] was used. Step changes in electrical conductivity were followed m situ by contacting the fully oxidised samples (treated under oxygen atmosphere to reaction temperature for at least 30 min) with a reducing propane atmosphere (Pc3a8 = 45.6 Torr). Prior to propane admission, the solids were kept under vacuum for 1 min. Temperatures between 723 and 773 K were applied. 2.3. Transient kinetic experiments Non steady-state step-response experiments simulating a circulating bed were carried out in a micro catalytic fixed bed reactor fed by automated valves allowing to switch abruptly from an oxidising mixture (O2/He) to a reducing one (C3Hs/He) and vice-versa with a continuous analysis of the effluent by on-line mass spectrometry. Intensity of the masses 4, 28, 29, 32, 41, 43, and 44 were used for quantification. Conversion was calculated from the sum of products formed. The reaction was carried out at TR = 823 K. The exposure time for each gas amounted to three min before switching to the other gas mixture. 2.4. Modeling of the transient conductivity and catalytic experiments The electrical conductivity step changes with time (reduction and oxidation) were modelled by accounting for i) the primary reducing process of propane oxidation to propene (rrcd) ii) the surface reoxidation by bulk oxygen diffusion (Do) and gaseous oxygen activation (rox) to replenish the created anionic vacancies (Fig. 1). The kinetic step transient experiments were modelled assuming two parallel propane-topropene reactions corresponding to i) the fast consumption of the initially present surface oxygen (from amorphous VOx units) and ii) the diffusion limited consumption of bulk oxygen provided by the orthovanadate phase. The ratio between these two pools of oxygen was varied according to the vanadium content.
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Call8 + Os ~ CsFI6 + H20 + Ds 02 + 2Ds -4 2 0 s Oi + [--]i-I ~
O = kr
1
L
Oi-I + [--]i
i
~ [ 1-0i]/n
i+l
?/
dOs d20s - DO dL2 - k~ed0sPCSH8 + 2. kox[lr _0~]2.1 PO2 dt dOi andfori~s:-~-
_
Ir
n
d20 i D O dL2
Subscript s represents the surface layer. where Oi = concentration of lattice oxygen and [ l-0i] = concentration of vacancies, in layer i.
Fig. 1. Mechanistic scheme and continuity equations used for modeling step changes in electrical conductivity. 3. RESULTS AND DISCUSSION In oxygen atmosphere the initial conductivity values range between In o = -9.8 ~'2l-cmq and -8.5 f~q-cm1. Under reducing propane atmosphere, o instantaneously increases by several orders of magnitude (proportionally to the V content) as typical for n-type semi-conductors [2,10]. Figure 2a reports the relative changes in electrical conductivity obtained in propane atmosphere at 773 K for the applied V/VMgO catalysts. A very rapid increase occurs during the first minutes of propane introduction and then the conductivity increases only slowly, the slope being a function of the catalyst V content. On pure MgO no significant change in the conductivity is observed on introducing propane. A similar fast primary increase is observed if performing the step responses at different temperatures (Fig. 2b), but the second slow process becomes more important with increasing temperature. According to the theory of n-type conducting materials the strong increase in conductivity can be assigned from the formation of anionic vacancies V0 in place of surface lattice oxygen anion 0 2- due to the reduction of the metal ions. Paraffin + 0 2- -40lefin + H 2 0 + V 0 The anionic vacancies can easily release free electrons into the conduction band, via:
(1)
v0 v~ + e(2) which are responsible for the increase in conductivity. In the presence of an oxidising agent, e.g. oxygen, the number of vacancies is reduced, due to: 1 / 2 0 2 + V0 r 0 2(3) and the conductivity decreases again. For numerous oxide catalysts it was observed that more than one monolayer of oxygen is participating in the redox process, i.e. bulk lattice oxygen is able to diffuse to the surface to participate in the reaction [ 11]. In turn, this means that surface oxygen defects (anionic vacancies) "diffuse" into the bulk of the catalyst. In consequence, the observed change in conductivity during a redox transient is due to both surface and bulk phenomena. If the kinetics of the limiting steps of the bulk and surface processes are different they can be observed separately. From the occurrence of two regimes with different time constants, we assume that the primary fast increase in conductivity is due to surface
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phenomena, i.e. the formation of surface anionic vacancies, whereas the second slower increase is due to bulk effects, i.e. the diffusion of oxygen vacancies into the volume of the catalyst (that means reoxidation of surface active sites by bulk lattice oxygen ions). These two phenomena may also be related to changes in phase structure. 9
!
9
!
~EB~
'
'6-1
V content 1%
/ - - o - 475"C
j -zx- ~zooc
.
--13--
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,,,~'~
- - A - - 10 - - e - - 14 - - 0 - - 25 r
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9~
"1~0"1~0
at
20
time I Bin
_Lx~
'
;0
'
~0
'
~0
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time I min
Fig. 2. Relative changes in electrical conductivity during a step transient of propane (a) for different V-content (TR = 773 K) and (b) for a 10V/VMgO catalyst at different temperatures For low V contents (up to 5 wt.%), which corresponds to samples with mainly surface polymer VO34 and only minor bulk Mg3V2Os, only the primary increase in conductivity is revealed (Fig. 2a). According to our characterisation study [3,9] this fast change in conductivity can be ascribed to the reduction of the amorphous VS+Oxsurface polymers into a V3§ phase which is completely reversible. For higher V contents (>5 wt.%), which corresponds to samples still containing surface polymer VO34 but with much larger amounts of bulk Mg3V2Os, the reduction of both surface and bulk V ions occurs together, but at distinct rates. The fast increase of conductivity (surface reduction) is now followed by a slower one corresponding to the reduction of the Mg3V208 phase to a bulk V 3+ spinel phase. Since oxygen/vacancies diffusion increases with increasing temperature, the second slow part becomes faster at elevated temperatures (Fig. 2b). Log (~
-6 Fig. 3. Experimental and calculated electrical conductivity transient response when switching from oxygen to propane over a 14V/VMgO catalyst (TR = 773 K; Pc3m = 45 Torr)
9
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-OI
0
10
I
I
I
20 30 40 time (min)
I
50
60
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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Using a model that accounts for reactive surface oxygen and bulk oxygen diffusion (see experimental part) the modelled change in electrical conductivity during a propane step transient gives a diffusion coefficient of the lattice oxygen close to 1021 m2.s"~(Fig. 3). This is actually an average value of surface and bulk diffusion ions. It is comparable with literature data in oxide materials [12]. In the transient kinetic step responses carried out in anaerobic conditions, C3I-I6, CO and CO, are observed as products, similar to steady state conditions (i.e. cofeeding C3I-h and O2) but with distinct yields. In a 3 min pulse (TR = 823 K; C3Hdinert = 0.05; ptot= 0.1 MPa) over the 14V/VMgO catalyst the propene, CO2, and CO yields decrease from 18.5 % to 2.4 %, from 7.0 % to 0.5 % and from 7.9 to 2.2 %, respectively. The conversion therefore drops during the experiment from 33.4 % to 5.1%. In a steady state operation experiment over the same catalyst a conversion of 30.5 % is obtained. Yields towards C3I-E, CO2 and CO amount to 15.1%, 8.9 % and 6.5 %, respectively. These experiments clearly show that in an anaerobic step transient a significantly higher selectivity and yield towards propene can be obtained initially, compared to steady state operation. The advantage of the transient operation, however, depends on the exposure time of the transient which should be lower than approx. 2 min. Similar data were obtained by Creaser et al. [6]. 9
1.0
I
9
I
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I
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|
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'
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s'0
' 160
'
time I s
' 2 6 0 ...... 2 S 0
0
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' 16o
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' 260
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time I s
Fig. 4. Response signals for C3H6in a step transient of propane over a (i) 5V/VMgO and a (ii) 14V/VMgO catalyst. (solid line: experimental; dashed line: simplified model) Like for the electrical conductivity experiments the shape of the anaerobic step transient responses depends on the catalyst V content (Fig. 4). With a low V content (2 to 5 wt.%), a sharp initial peak of propene formation occurs after propane admission, exhibiting a low time constant, followed by a period with a slow decrease with an approximately 10 times larger constant. On a catalyst with a higher V loading (14V/VMgO) the sharp decrease of the initial propene peak is overlayed by a broad slow decrease with a larger time constant. Again, the sharp peak in the beginning of the step response is attributed to the fast reduction of the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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surface V 5§ polymers and the formation of the V 3§ spinel phase whereas the broad tailing is related to the reduction of the bulk Mg3VEOs phase and the respective bulk spinel formation. As shown in Figure 4a and b the simplified model based on two distinct pools of oxygen (see experimental part) is able to describe satisfactorily the propene formation in the propane transient. For low (5 wt.%, Figure 44) and high (14 wt.%, Figure 4b) V content, ratios of bulkto-surface oxygen of 60 and 150 have been found, respectively. These values are quite consistent with the advanced characterisation of these biphasic materials [3,9]. A complete set of kinetic parameters (rate constants, diffusion constants, amount of reactive oxygen available) was derived, but still has to be confirmed in a full model accounting for all products. 4. CONCLUSIONS In-situ electrical conductivity and step transient experiments of the ODHP over VMgO catalysts reveal that the reaction occurs in a redox cycle in which different surface and bulk oxygen species participate. For low V content (up to 5 wt.%), surface Vs+Ox species forming an amorphous adlayer can be rapidly and reversibly reduced into a spinel-like structure. At higher V contents also a slower reduction process takes place which can be attributed to the conversion of the MgaVEOs phase into a bulk V 3+ spinel phase. Very close experimental and modelled data were obtained for the non steady-state kinetics experiments simulating the circulating bed. They demonstrated that such a process would both lead to improved propene yields and decrease the costly step of propene/COx separation which is required for any conventional co-feed process. Optimized ODHP catalysts for non-steady state operation would consist in a well dispersed ovedayer of amorphous VO4 3" units. Bulky mixed phases should be avoided.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8 9. 10. 11. 12.
M. Chaar, D. Patel and H.H. Kung, J. Catal., 109 (1988) 463. A. Pantazidis, A. Auroux, J.M. Herrmann and C. Mirodatos, Catal. Today, 32 (1996) 81. A. Pantazidis, A. Burrows, C.J. Kielly and C. Mirodatos, J. Catal., 177 (1998) 325. A. Pantazidis, S.A. Buchholz, H.W. Zanthoff, Y. Schuurman, and C. Mirodatos, Catal. Today, 40 (1998) 207. H.W. Zanthoff, S.A. Buchholz, A. Pantazidis, and C. Mirodatos, Chem. Eng. Sci., 54 (1999) 4397. D. Creaser, B. Andersson, R.R. Hudgins and P.L.Silveston, Chem. Eng. Sci., 54 (1999) 4437. L. Albaric, N. Hovnanian, A. Julbe, C. Guizard, A. Alvarez-Larena, and J. Piniella, Polyedron, 16 (1997) 587. A. Pantazidis, and C. Mirodatos, ACS Symp. Series "Heterogeneous Hydrocarbon Oxidation" ed. B. Warren and T. Oyama, 638 (1996) 207. J.C. Jalibert, PhD thesis, Lyon 1 University, 1999. J.M. Herrmann, in: Catalyst Characterization- Physical Techniques for Solid Materials; Eds. J.C. Vedrine and B. Imelik, Plenum Press, London, (1994) 559. A. Bielanski and J. Haber, Oxygen in Catalysis, Dekker, New York, USA, 1991. D. Martin and D. Duprez, J. Phys. Chem., 100 (1996) 9429.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Effect of Potassium on the Structure and Reactivity of Vanadium species in VOx/Al203 Catalysts P. Concepci6n l, S. Kuba ~, H. Kn6zinger l, B. Solsona2, J.M. L6pez Nieto 2 Institut f'tir Physikalische Chemie, LMU. Miinchen, 80333 Mtinchen, (Germany). 2 Instituto de Tecnologia Quimica, UPV-CSIC, 46022 Valencia, (Spain). Undoped and potassium doped (K/V atomic ratio of 0.7) alumina-supported vanadia catalysts (3.5 wt% of V-atoms) have been characterized by laser Raman spectroscopy, FTIR-spectroscopy of adsorbed CO at low temperature, temperature-programmed reduction (TPR) and electron paramagnetic resonance (EPR). All results suggest a higher dispersion of the vanadium species on the A1203 support by addition of potassium. This, together with the possibility of the presence of small amounts of K20, leads to a decrease in the amount of A13+ Lewis acid sites and a lower reducibility of the vanadium species. Moreover, the reoxidation ability of the vanadium species seems to be reduced by the presence of potassium. All these results can explain the higher selectivity to oxydehydrogenation products observed over K-doped catalysts in the oxidation of C4 hydrocarbons. 1. INTRODUCTION Alkali metals have been used as promoters of supported vanadia catalysts [ 1, 2]. This is the case of Na-doped [3-5] or K-doped [6-8] supported vanadia catalysts. Recently, it has been observed that the incorporation of potassium strongly modify the catalytic behaviour of VOx/A1203 catalysts [ 1,8]. There is little structural information available on the vanadium species formed in the presence of potassium additives [6, 7]. The aim of this paper is to determine the influence of potassium on the nature of the vanadium species in VOx/AI203 catalyst. The catalyst were characterized by laser Raman spectroscopy, temperature-programmed reduction (TPR) and electron paramagnetic resonance (EPR). In addition, the acid properties of the catalysts were studied by FTIR-spectroscopy of adsorbed CO at low temperature (77K). Carbon monoxide is one of the best probe molecules for sensitive determination of the nature and strength of Lewis acid sites [9]. 2. EXPERIMENTAL 2.1. Catalyst preparation. VOx/A1203 (3.5 wt% V) catalyst was prepared by impregnation of 7-A1203 support with an aqueous ammonium metavanadate solution. The VOx-K/A1203 catalyst (KJV atomic ratio of 0.7) was prepared by impregnation of the alumina-supported vanadia catalyst with an aqueous solution of potassium nitrate. The catalysts were calcined at 600~ for 6h.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
768
2.2. Catalyst Characterization. The laser Raman spectra were measured with an OMARS 89 triple monochromator spectrometer (Dilor) using the 488nm line of a spectra physics 2020 Ar + laser for excitation. A laser power of 50mW and a spectral resolution of 10 cm -I was used. IR spectroscopy studies were carried out with a Bruker IFS-66 spectrometer at a spectral resolution of 1 cm l and a number of 128 scans. Before the measurements the samples were activated for lh in a flow of oxygen at 773 K, followed by lh evacuation at the same temperature. TPR experiments were carried out with a Micromeritics apparatus loaded with 20 mg of catalyst and an H2/Ar mixture (H2/Ar molar ratio of 0.15 and a total flow of 50 ml min -l) and heated at a rate of 10 ~ min -! to a final temperature of 1000 ~ EPR spectra were recorded on an Varian E9 spectrometer using the X band. The spectra were obtained at 80K and with a power of 10mW. 2.3. Catalytic test. The catalytic test were carried out in a fixed bed tubular reactor, and the analysis of reactant and products were achieved by gas-chromatograhy [8, 10]. An alkane/oygen/ helium molar ratio of 4/8/88 and different contact times were used in order to obtain similar alkane conversions.
3. Results and Discussion 3.1 Catalytic tests Figure 1 shows the selectivity to oxidehydrogenation products obtained during the OXDH of C2-C4 alkanes on undoped and K-doped catalysts. On undoped catalyst, the longer alkane chains gave the lower selectivity to OXDH products. In contrast, the opposite trend is observed on K-doped catalysts. Thus, the higher selectivity to olefins on ] 1 C-2 I-1 C-3 ~ C-4 ] the undoped catalyst was obtained from ! J ethane, while the better selectivity to olefins on K-doped catalyst was t obtained during the OXDH of n-butane. Si
(%
1
u
undoped
K-doped
Figure I. Selectivity to oxidehydrogenation products during the OXDH of ethane, propane and n-butane on undoped and K-doped VOx/AI203 catalysts at 550~
3.2. Raman laser results Raman spectra of VOx/A1203 and VOx-K/A1203 catalysts are shown in Figure 2. Some differences in the molecular structures of the surface vanadium oxide species on unpromoted and K-promoted catalysts are observed in the hydrated and dehydrated states, respectively. Bands at 823, 890, 997 and 1023 cm -I can be observed on the dehydrated VOx/Al203 sample (Fig. 2a). The sharp band at 823 cm ~ is attributed to isolated tetrahedral vanadium species such as
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orthovanadate (VO4) 3", and the band at 1023 cm l is assigned to isolated vanadyl species (OaV=O). Raman bands in the region of 870-1000 cm ! characterize polyvanadate species, where a shift to higher frequencies is related to an increase in the average chain length and coordination number of the vanadium species [ 12, 13 ]. Thus, the band at 890 cm ~ can be attributed to dimeric pyrovanadate-type species and the band at 997 c m -1 t o octahedral polyvanadate species. Previously reported 51V-NMR results [11] show the presence of mainly polymeric tetrahedral species. On the other hand, from the FT-IR spectra no bands associated with octahedral polyvanadate ~o o are observed. Therefore, we may infer that the band at 997 cm -! could also be due to a perturbation of the V=O bond of the isolated vanadyl species by the coordination of small amounts of readsorbed water. Adsorption of water (Fig. 2b) leads to a shift of the band at 1023 cm -~ to lower frequencies and to the appearance of a broad band near 950 cm -~ due to coordination of water to the vanadium species.The Raman spectra of the dehydrated K-doped VOx/A1203 sample shows two strong bands at 823 and 997 cm -I (Fig. 400 600 800 1 0 0 0 1200 2c). As indicated above, the band at 823 cm ~ c,m-1 characterizes isolated tetrahedral vanadium species. The band at 997 cm -~ is assigned to octahedral polyvanadate species but can also be attributed to the V - O stretching mode of the vanadyl species (1023 cm -I) shifted to lower frequencies due to an interaction with the potassium ions. Otherwise no band at 890 cm -I characterizing dimeric vanadate species cannot be observed. These results suggest a higher dispersion of the vanadium species in the presence of potassium, which has also been observed on samples with a higher amount of vanadium [14]. Addition of water lead to the appearance of a 400 600 800 1 0 0 0 1200 strong band at 945 cm ~ and a decrease of the cr.n-1 intensity of the band at 823 cm l. This may be due to a solvation of the vanadium species with an Figure 2. Raman spectra of dehydrated (a, c) and rehyincrease in the coordination number and/or chain drated (b, d) samples of length. It must be noted that the hydratationundoped (a, b) and K-doped rehydratation process is completely reversible. VOx/AI203 (c, d) catalysts.
3.3. Acid properties of cata!ysts IR spectroscopy of CO adsorbed at low temperature has been employed in order to determine the influence of potassium on the number and nature of acid sites in relative to undoped VOx/A1203 catalysts. The IR spectra in the CO stretching region of the A1203, VOx/A1203 and K-doped VOx/A1203
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samples, obtained after adsorption of CO at low temperature (equilibrium pressure of 2-0.01 hPa CO and evacuation of the sample for increasing times) are shown in Fig. 3. 0.0
57
A
0.05
I
0.05
2157
C
v-
_J 2250
2200
2150
2100
Wavenumber (cm-1)
2050
2250
22'00 ' 21's0 ' 21'00 ' 2 0 5 0
Wavenumber (cm-1)
2250
2200
2150
2100
2050
Wavenumber (cm-1)
Figure 3. IR spectra of CO adsorbed on A!203 (a), undoped (b) and K-doped (c) VOx/Al203 catalysts.
Three bands, at 2204, 2195 and 2157 cm -~ are observed on pure A1203 support after adsorption of CO (Fig. 3A). According to the literature [10] and in agreement with a simultaneous shift of the OH groups, the band at 2157 cm l is due to CO interacting with OH groups. The bands at 2204 and 2195 cm ~, shifted to lower frequencies (2189 cm -l) by increasing amount of adsorbed CO, are characterizing to the adsorption of CO on A13+ Lewis acid sites. Chemical inductive effects are responsible for this kind of shift. Adsorption of CO at low temperature on undoped VO• sample (Fig. 3B) shows two bands, at 2204 cm 1 and 2195 cm ~. Both bands can also be assigned to V 4+ species [15]. Therefore an assignement of both bands to A13+ or V 4+ is controversial, taking into account a similar stability of both bands towards evacuation. A band at 2157 cm -~, associated with CO interacting with hydroxyl groups, is also observed. The addition of potassium leads to a strong decrease of the Lewis acid sites (Fig. 3C). Only a very small broad band associated with V 4+ and A13+ species is observed. The decrease in the amount of Lewis acid sites could be due to a higher dispersion of the vanadium species as observed from the Raman results and/or to the presence of small amounts of K20 species blocking the Lewis acid sites. Changes are also observed in the v(OH) region of the A1203 sample after addition of vanadium and potassium (spectra not shown) [ 14]. According to the literature five different OH groups are observed on the A1203 sample [10]. Addition of vanadium (VOx/A1203 sample) leads to the dessapearance of the more basic OH bands (3792 and 3770 cm-l). Moreover, addition of potassium leads to a decrease of the intensity of the more acidic OH bands (3729, 3683 and 3670 cm l ) probably due to ion exchange of the proton by potassium cations. 3.4. TPR and EPR results TPR results of VOx-K/AI203 catalyst show the presence V-species with a different reducibility as on VOx/A1203 catalysts (Figure 4)" (i) vanadium species with
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lower reducibility than in VOx/AI203 samples, probably due to an interaction of K § with the V-O-A1 anchoring bond, and (ii) vanadium species with higher reducibility due to a more dispersed vanadium species, as observed by Raman results. It has been observed that the addition of Na § on VO• catalysts leads to a dramatic change in the shape of the EPR spectrum of V 4§ spectrum [4]. In order to obtain more information about the interaction of the K § cations .1-, with the vanadium species a serie of Q. :3 EPR experiments was carried out on the t" unpromoted and K-promoted catalysts. 0 i."0 The behaviour of catalysts after >., -1addition of 1-butene at 200~ have also been studied. The EPR spectra on unpromoted and K-promoted catalysts are shown in Figure 5. 2C)0 ' 4C)0 ' 6C)0 ' 8C)0 Only a slight difference in the Temperature, ~ nature of the V 4§ species due to the presence of potassium was observed on Figure 4. TPR patterns of undoped (a) oxidized samples (spectra _a in Fig.5B). and K-doped (b) alumina-supported After addition of 1-butene at 200 ~ an vanadia catalysts. increase in the amount of V 4§ species is observed (Spectra b, in Fig. 5B). The nature of these species is similar in the undoped and doped samples. Addition of oxygen at r.t. on the reduced samples and subsequent evacuation leads to a slight broadening and a decrease of the V 4§ signal on
.w~-~-~
~
b
(x30)
200000
300000
400000
(xlO)
200000
i
a
300000
400000
B B Figure 5. EPR experiments of un-doped (A) and K-doped (B) catalysts: a) oxidized; b) after adsorption of 1-butene at 200~ c) reduced in H2 at 480~ d) reoxidized at 25~
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the VOx/AI203 sample (Fig. 5A, spectrum c), whereas no changes in the shape and intensity of the signal in the promoted catalyst has been observed (Fig. 5 B, spectrum c). 0 2. species were not observed on neither the unpromoted nor the promoted catalysts. This confirms that lattice oxygen ions participate in the reaction, whereas adsorbed oxygen is not involved. An increase of the V 4+ concentration in the sample is induced by the presence of potassium while the reoxidation ability (at 25~ of the vanadium species seems to be suppresed by the presence of K+. 4. CONCLUSIONS
Thus the results presented here seem to indicate a higher dispersion of the vanadium species on the A1203 support by addition of potassium. This, together with the possibility of the presence of small amounts of K20, leads to a decrease in the amount of A13+Lewis acid sites. An interaction of K+ with the V=O bond of the vanadyl species and with the V-O-AI anchoring bond is proposed leading to a lower reducibility of the vanadium species. The reoxidation ability of the vanadium species seems to be reduced by the presence of potassium. All these results are in accordance with the higher selectivity to oxydehydrogenation products observed over K-doped catalysts in the oxidative dehydrogenation of both n-butane and 1-butene [1, 8, 14]. Thus, both the absence of acid sites and the presence of V-species with a low oxidation state appear to be important aspects in the selective oxidehydrogenation of Ca-hydrocarbons. REFERENCES
1. 2. 3. 4.
T. Blasco, J.M. L6pez Nieto, Appl. Catal. A, 157 (1997) 117. G. Deo, I.E. Wachs, J. Haber, Crit. Rev. Surf. Chem. 4 (1994) 141. C. Martin, V. Rives, A. R. Gonzalez-Elipe, J. Catal. 114 (1988) 473. R. Ficke, W. Hanke, H.-G. Jerschkewitz, 13. Parlitz and G. Ohlmann, Appl. Catal. 9 (1984) 235. 5. a) G.T. Went, S.T. Oyama and A.T. Bell, J. Phys. Chem., 94 (1990) 4240; b) S. Irusta, A.J. Marchi, E.A: Lombardo, E.E. Mir6, Catal. Lett. 40 (1996) 9. 6. a) J. Zhu, S.L.T. Andersson, J. Chem. Soc., Faraday Trans.1, 85 (1989) 3629; b) G. Deo and I.E. Wachs, J. Catal. 146 (1994) 335; 7. G. Ramis, G. Busca, F. Bregani, Catal. Lett. 18 (1993) 299; 8. A. Galli, J.M. L6pez Nieto, A. Dejoz and M.I. V~izquez, Catal. Lett., 34, 1995, 51. 9. H. Kn6zinger, "Handbook of Heterogeneous Catalysis" (G. Ertl, H. Kn6zinger and J.Weitkamp, Edts.), Wiley-VCH, Weinheim, p.707 (1997). 10. T. Blasco, A. Galli, J.M. L6pez Nieto, F. Trifir6, J. Catal. 169 (1997) 203. 11. J.M. L6pez Nieto, R. Coenraads, A. Dejoz, M.I. Vfizquez, Stud. Surf. Sci. Catal. 110 (1997) 443. 12. a) G.T. Went, S.T. Oyama and A.T. Bell, J. Phys. Chem., 94 (1990) 4240; b) J.M. Jehng, G. Deo, B.M. Weckhuysen and I.E. Wachs, J. Mol. Catal. A., 110 (1996) 41. 13. a) G.T. Went, L.J. Leu and A.Bell, J. Catal, 134, 1992, 479; b) J. Haber, A. Kozlowska and R. Kozlowski, J. Catal., 102 (1986) 52. 14. J.M. L6pez Nieto, P. Concepci6n, A. Dejoz, F. Melo, H. Kn6zinger and M.I. Vfizquez, Catal. Today (2000) in press. 15. P. Concepci6n, K. Hadjiivanov and H. Kn6zinger, J.Catal., 184 (1999) 17.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
STRATEGIES FOR COMBINING LIGHT PARAFFIN DEHYDROGENATION (DH) WITH SELECTIVE HYDROGEN COMBUSTION (SHC)
Robert K. Grasselli *a, David L. Stem b and John G. Tsikoyiannis b a Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA; and Institute of Physical Chemistry, University of Munich, Butenandtstr. 5-13 (Haus E), D-81337, Munich, Germany b Mobil Technology Company, Strategic Research, 600 Billingsport Road, Paulsboro, NJ 08066-0480, USA
By combining catalytic dehydrogenation (DH) with selective hydrogen combustion (SHC), higher than equilibrium olefin yields can be obtained from the corresponding light paraffins. Towards this goal, two different process approaches were explored: A cofed DH->SHC->DH method with three reactors interconnected in series, neat paraffin fed to the first reactor and oxygen or air cofed to the second reactor; and a redox DH+SHC method using a single reactor containing a mixture of both catalysts, with no oxygen or air cofed, where the lattice oxygen of the SHC catalyst is the sole oxidizing agent for the conversion of the in situ produced hydrogen. Periodic regeneration with air is required in the latter method. In microreactor cofed operation, (0.7 wt% Pt-Sn-ZSM-5 DH catalyst and 10 wt% In203/ZrO 2 SHC catalyst) with propane as feed, propylene yields between 29.7% (97% sel.) and 33% (89% sel.) were obtained at 550~ compared to an equilibrium yield of 25% at 99% selectivity; i.e., an olefin yield improvement of 19 to 32% over equilibrium. Similarly, an 18% yield improvement over equilibrium was realized for isobutylene from isobutane. In redox mode operation (0.7 wt% Pt-Sn-ZSM-5 DH catalyst mixed with 42 wt% Bi203/SiO z SHC catalyst) initial propylene yields of 48.2% at 90% selectivity were obtained from propane at 500~ compared to an equilibrium yield of 20% at 95% selectivity. The resulting olefin yield enhancement is 140%. Although the olefin yield declines with on stream time, it is still in excess of 65% over equilibrium after 10 redox cycles. More stable SHC catalyst systems are suggested for future consideration. 1. INTRODUCTION Conventional catalytic dehydrogenation (DH) of paraffins is equilibrium limited, while oxidative dehydrogenation (ODH) is not. Catalytic dehydrogenation is well established and practiced commercially on large scale. Best known are the Oleflex (UOP) , STAR (Phillips Petroleum Co.) , Catofin (Houdry United Catalysts Inc. - ABB Lummus Crest), Linde-BASF and FBD (Snamprogetti Yarsintez) processes, which have been recently reviewed in depth by Buonomo, et. al., [1 ]. * Corresponding Author: Fax: + 1-302-831-2085 (USA)/+49-89-2180-7568 (Germany) e-mail: rkgrasse @ olymp.cup.uni-muenchen.de
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Oxidative dehydrogenation of light paraffms has recently received considerable attention. However, olefm yields obtained by ODH are still halting, because the selectivity to olefins declines precipitously as conversion is increased. Propylene yields as high as 35% at about 80% selectivity have been claimed by some investigators [2,3], however, a more realistic level is a 20% yield at 70% conversion [4-6]. An alternative approach to DH and ODH is a combination process of dehydrogenation with selective hydrogen combustion (DH/SHC), a concept pioneered by Imai, et.al., [7,8]. In this latter process, a paraffin is converted in a first reactor by conventional dehydrogenation to the corresponding olefin in an equilibrium process, the effluent therefrom led to a second reactor containing a selective hydrogen combustion catalyst, to which oxygen or air is cofed. The effluent from the second reactor is then fed to a third reactor which contains again a conventional dehydrogenation catalyst. In this manner higher than equilibrium yields of olefins are attained. We have substantially expanded on this latter concept and summarize our findings here. 2. E X P E R I M E N T A L
2.1 Catalyst Preparation The SHC catalysts were prepared by well known methods described earlier [9], entailing in general the refluxing of given metal-nitrates in the presence or absence of molybdenum heptamolybdate and/or silica sol (Ludox AS-40). The resulting slurries were refluxed for 16 hours, dried at 120~ pulverized, air dried at 290~ for 4 hour, and calcined in air at 600~ for 4 hours. The calcined solids were crushed, pelletized and sized to 20-40 mesh, before use. The 0.7 wt% Pt-Sn-ZSM-5 DH catalyst was prepared according to known procedures [10]. 2.2 Gravimetric Experiments A Cahn 1000 Vacuum Electrobalance with 1/100 mg sensitivity was employed for the gravimetric experiments. Typically, the oxide catalysts were reduced in hydrogen, propane, propylene or H2/C3 mixtures at 1 atm and 500~ and were then reoxidized in air at the same temperature. Further details of operation have been reported elsewhere [9].
2.3. Experimental Setup for Catalyst Evaluation The cofed DH->SHC->DH and redox DH+SHC experiments were carried out in automated microreactor units described earlier [ 11]. For the DH->SHC->DH experiments a three reactor setup was used (lg DH->0.1g SHC, diluted with quartz->lg DH; Fig. 1), for the DH+SHC experiments a single reactor (lg DH + l g SHC catalysts mixed; Fig. 2). 3. RESULTS and DISCUSSION
3.1. DH->SHC->DH (Cofed) and DH+SHC (Redox) Process Modes In the DH->SHC->DH cored process mode (Fig. l), conventional dehydrogenation catalysts are placed into the first and third reactor, where equilibrium dehydrogenation takes place. Into the second reactor a selective hydrogen combustion catalyst is placed to which oxygen or air is cored to selectively combust the hydrogen produced in the first reactor. Herewith, the hydrogen content of the feed entering the third reactor is reduced, allowing the dehydrogenation catalyst of the third reactor to produce overall olef'm yields in excess of equilibrium. The DH catalysts are generally Pt/AI203 based systems, the SHC catalysts Cs/Pt/(Sn)/AI203 [7,8] or Cu, Fe, Ni-ZSM-5 systems.[ 12]. We studied a 0.7 wt% Pt-Sn-ZSM5 composition as the DH catalyst [9] and selected metal oxides as the SHC catalysts [9, 11].
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775 DH
r-.:l
/::1
;
/ "'1
C3 = + H=O
DH
SHC
l l liB
: o Oo oe
I"I
"
i
C30 + C3 = + H2 + H20
Oo~
0 2 (air)
9 DH Cat.
9 SHC Cat.
I 'l
!'.[
9 DH Cat. 9 SHC Cat.
C3~
Fig. 1. Schematic of DH->SHC->DH cofed process mode [9 ].
Fig. 2. Schematic of DH+SHC redox process mode [9].
In the DH+SHC redox process mode, the DH and SHC catalysts are commingled in a single reactor (Fig. 2). No oxygen or air is cofed to the reactor. The lattice oxygen of the metal oxide SHC catalyst is the sole oxidizing agent to combust the hydrogen produced in situ by the commingled DH catalyst. The catalyst mixture must be periodically reoxidized to replenish the lattice oxygen of the reduced SHC catalyst. This process concept is operable with SHC catalysts of the redox type containing removable lattice oxygen which preferably oxidizes hydrogen over the paraffin feed and olefin product. Such redox SHC catalysts have recently been identified by us [9, 11]. Because of the combined redox and SHC 101 ._= requirements, Pt-based SHC catalysts used '-E 0 2 by Imai, et. a1.,[7,8] and the Cu,Fe,Ni-ZSMpane "= o ~o 10" 3 5 catalysts used by Lin et.al.,[12] in the DH0-3 VzOs >SHC->DH process, cannot be used in the "67' redox DH+SHC process. O O1
vEl 0 -s ~ 0
!
!
I |
3.2. S e l e c t i o n of S H C C a t a l y s t s
Our gravimetric studies using a Calm balance show large differences in the SHC ,.-,.. behavior of the different metal oxides. For .__. 10 example the preference of metal oxides such cE O as V205 to combust propane over hydrogen ~x,~ydrogen = 1 contrasts the performance of Bi2Mo3Ot2 BizNIo3Ot2 which combusts hydrogen with great ~ ' O lO-1 opane preference to propane (Fig. 3). tr,~, The reducibility of several metal oxides m 1 0.a E 0 0.'05 0.1 0.15 is intercompared in Table 1. In this table the ' ' Fractional Weight Loss percent of the lattice oxygen removed by hydrogen is compared to that removed by propane after exposure of a given metal Fig. 3. Reduction behavior of V205 and Bi2Mo3Ot2 oxide to neat hydrogen or propane for five at 500~ (Calm Balance). minutes at 500~ The selectivity for hydrogen combustion (SH) in preference to propane combustion is also recorded. It is seen that compositions such as Bi203, In203, Bi2M%O,2 and In2M%O,2 are both, very selective towards preferred hydrogen combustion (SHC selectivities ranging from 95.3 to 99.0 %) and 0.05 0.1 0.15 Fractional Weight Loss
~
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are very active as well (47.7 to 85% of the lattice oxygen being removed in 5 minutes). Compositions such as A12Mo3O12, Fe2MO3Ol2 and Cr2Mo3012 are of intermediate SHC selectivity (88.4 to 95.5%) and are substantially less active (10.2 to 21.2% lattice oxygen removal) than the first group. La2Mo3Ol2, Ce2Mo3Ol2 and MoO 3 are less selective (Sn ranging from 78.3 to 85.7 %) and much less active (3.0 to 4.1% oxygen removal). In contrast to the above stands the poor hydrogen selectivity of V205 which is only 6.1%, and its rather high propane activity (in a five minute experiment 30.9% of the lattice oxygen is removed by propane, while only 2% is removed by hydrogen). Table 1 SHC behavior of some metal oxides over a five minute reduction span at 500~ [9] Catalyst BizMo~Ou In2Mo~Oi2 InzO~ Bi20~ AlzMo~Oiz Fe2MoaOtz CrzMo~O~2 LazMo~Otz CezMo~Olz MoO~ V20~
Percent Lattice Oxygen Removed By Propane [O]p By Hydrogen [O]H (in 5 min) (in 5 min) 0.47 47.7 47.7 0.78 82.6 3.70 85.9 4.27 14.9 0.70 10.2 0.65 21.2 2.77 4.0 0.67 4.1 1.07 3.0 0.85 2.0 30.9
SH 99.0 98.4 95.7 95.3 95.5 94.0 88.4 85.7 79.3 78.3 6.1
These gravimetric experiments suggest that Bi203, In203 , Bi2Mo3012 and In2Mo3O12 are good SHC catalysts and should be applicable as such in DH-SHC processes, while the remaining systems are less well suited, with V205 being totally unsuitable as an SHC catalyst. As an aside, all good redox SHC catalysts identified thus far [9,11] contain elements in their prevailing oxidation state during reaction having a lone pair of electrons.
3.3 DH->SHC->DH Cofed Process Approach As shown in Fig. 1 a three reactor arrangement, placing DH catalysts into the first and third reactor and an SHC catalyst into the second reactor constitutes the simplest cored arrangement of combining conventional catalytic dehydrogenation with selective hydrogen combustion. In our study we employed a 0.7 wt% Pt-Sn-ZSM-5 as the DH catalyst and a 10 wt% In203/ZrO2 as the SHC catalyst. The performance of this assembly, using neat propane as feed to the first reactor and cofeeding various amounts of air to the second reactor are shown in Fig. 4. At 550~ atmospheric pressure and WHSV of 2h l, propylene yields of 29.7% at 97% selectivity (0.3 air/propane in SHC reactor) and 33% at 89% selectivity (0.6 air/propane ratio in SHC reactor) were obtained. These results compare favorably to the equilibrium yields of 25% at 99% selectivity when only the DH catalyst was used under identical operating conditions. The yield improvements over equilibrium are 19 and 32%, respectively. Similarly, under the same operating conditions, but with isobutane as feed (Fig. 5), higher than equilibrium yields are also obtained using the DH->SHC->DH process arrangement.
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0.3 A
A
_e
0 =Z
0.2
O
"I:3
>"
>-
o.1
o 0
0.3
0.6
Air/Propane
Fig. 4. Propylene yield from propane.
0,4
0.3
0.2
0.2
0.4
0.6
Air/Isobutane
Fig. 5. Isobutylene yield from isobutane.
DH->SHC->DH cofed process mode. DH catalyst = 0.7wt%Pt-Sn-ZSM-5, SHC catalyst = 10wt%In203/ZrO2 550~ atm pressure,WHSV=2h", feed = neat paraffin to reactor 1, cofed artificial air (21%O,./79%He) to reactor 2 [11]. With a 0.2 air/isobutane ratio in the SHC stage, the isobutylene yield from isobutane was 47.5% at 99+% selectivity, compared to a 40% yield at 99+% selectivity when the DH catalyst was used by itself. The yield improvement over equilibrium is 18%. 3.4. D H + S H C Redox Process Approach As already mentioned, it is also possible to operate a version of the DH/SHC processing in a redox mode (Fig. 2) where the DH and SHC catalysts are commingled in a single reactor, and where the SHC catalyst must be a reducible and readily reoxidizable (i.e., redox) catalyst. The catalysts reported earlier [9,11] and mentioned above in section 3.2 fulfill these requirements. The lattice oxygen of the metal oxide is the sole oxidizing agent for the in situ generated hydrogen. A demonstrative example of this redox DH+SHC DH + SHC process configuration was carried out in a 5 cc microreactor containing a physically well DH + SHC .................................. commingled mixture of a 0.7 wt% Pt-Sn-ZSM-5 as 50the DH catalyst and a 42 wt% Bi203/SiO2 as the SHC 40" catalyst. The test conditions were 540~ WHSV of 2 hr ~, neat propane as feed and a cycle time of 2.8 minutes between regenerations. The results of the 20first cycle are shown in Fig. 6. It is seen, that an 10initial propane conversion of 59.9% is obtained using the DH+SHC redox process method, which is I . . . . . . . i. . . . . ! ! ....... substantially higher than the 27.5% conversion Conversion Yield obtained with the DH catalyst alone. Similarly, the Fig. 6. Comparison of DH and DH+SHC propylene yield is 48.2% for the DH+SHC method redox operations. Initial propylene yields. while it is 20.0% for the DH alone. The propylene Neat propane as feed, 540~ atm pressure, yield increase is a respectable 140% over 2h~ WHSV, 2.8 min run time [11]. equilibrium. ...........
i ...................
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While the observed initial results are extremely encouraging, it must be mentioned, that the high yields are not sustainable with time on stream using the catalyst mixture studied. Over a period of ten consecutive redox cycles, the propylene yields stay still well (i.e., 65+%) above the equilibrium yield obtained using DH alone; i.e., the propylene yield declines from the initial 48.2% to 33.0% after 10th cycles, or about 2.2%/cycle. The decline is attributed to a loss of Bi203 dispersion on the SiO2 support, rather than Bi poisoning the Pt of the DH catalyst [ 11 ]. In order to overcome this observed decline in performance, it will be necessary to identify more rugged, redox SHC catalysts. 4. CONCLUSIONS
Catalytic dehydrogenation (DH) of light paraffins can be combined with selective hydrogen combustion (SHC) in various process modes to obtain higher than equilibrium yields of olefins. In a DH->SHC->DH cofed process mode arrangement, using a 0.7 wt% Pt-Sn-ZSM-5 DH catalyst and a 10 wt% In203/ZrO2 SHC catalyst, yield improvements of 19 to 33% over equilibrium were realized for propylene from propane, and 18 % for isobutylene from isobutane at 550~ In a DH+SHC redox process mode arrangement, using the same 0.7 wt% Pt-Sn-ZSM-5 DH catalyst and a 42 wt% Bi203/SiO2 SHC catalyst, propylene yield improvements over equilibrium as high as 140% were realized at 540~ These initial high yield improvements cannot be sustained over time and decline to about 65% (i.e., still well above equilibrium) after 10 redox cycles. In order to overcome this observed decline in performance with cycle time, more rugged redox SHC catalysts need to be identified. Possible candidates are complex mixed metal Bi-molybdates, -tungstates or-phosphates. Such catalysts are extremely rugged in various catalytic processes, e.g., (amm)oxidation of olefins and can be reoxidized rather readily, even if severely reduced [13, 14]. Once stability and compatibility problems of catalysts are worked out, it is believed that the redox process mode will win out over the cofed mode. REFERENCES
[1] [2] [3]
[4] [5] [6] [7] [8] [9] [10] [ 11] [12] [13] [14]
F. Buonomo, D. Sanfilippo and F. Trifiro, "Dehydrogenation of Alkanes" Handbook of Heterogeneous Catalysis (G. Ertl, H. Knoezinger, J. Weitkamp, Eds.), 4 (1997) 2140. V.D. Sokolovskii, Catal. Rev.-Sci. Eng., 32 (1990) 1. B. Delmon, P. Ruiz, S.R.G. Carrazan, S. Korili, M.A. Rodriguez, and S. Sobalik, Catalysis in Petroleum Refining and Petrochemical Industries, M. Absi-Halabi et. al., (Eds), Elsevier, Amsterdam (1996) S. Albonetti, F. Cavani and F. Trifiro, Catal.Rev.-Sci. Eng. 38 (1996) 413. J.C. Vedrine, J.M. Millelet and J.C. Volta, Catal. Today, 32 (1996) 115. D.L. Stern and R.K. Grasselli, Stud. Surf. Sci. Catal., 110 (1997) 357; ibid. J. Catal. 167, (1997) 550; ibid., J. Catal. 167 (1997) 560. T. Imai, U.S. Pat. 4,435,607 (1984). T. Imai and D.-Y. Jan, US. Pat. 4,788,371 (1988). J.G. Tsikoyiannis, D.L. Stern and R.K. Grasselli, J. Catal. 184 (1999) 77. R.M. Dessau and E.W. Valyocsik, U.S. Pat. 4,868,145 (1989). R.K. Grasselli, D.L Stern and J.G. Tsikoyiannis, Appl. Catal. 189 (1999) 1 and 9. C.-H. Lin, K.-C. Lee and B.-Z. Wan, Appl. Catal. 164 (1997) 59. R.K. Grasselli and H.F. Hardman, U.S. Patent 4,503,001 (1985). D.D. Suresh, M.S. Friedrich and M.J. Seely, U.S. Pat. 5,212,137 (1993).
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Oxidative Dehydrogenation over Promoted Chromia Catalysts at Short Contact Times D. W. Flick and M. C. Huff Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA The oxidative dehydrogenation of ethane and propane at millisecond contact times was studied using Cr203 and Pt coated ceramic foam monoliths. The supported Cr203 catalyst was able to achieve a higher selectivity to C2H4 at higher conversion of the hydrocarbon feed than the Pt coated monolith. I. I N T R O D U C T I O N The selective dehydrogenation of alkanes still remains a formidable obstacle in the wider use of lower alkanes as feedstocks for large industrial processes. Currently, olefins are used as important chemical intermediates for a large number of industrial processes. Thermal dehydrogenation of light alkanes to olefins is thermodynamically favorable at high gas temperatures (800-900~ but often leads to high yields of smaller hydrocarbons and coke [ 1]. In the industrial cracking process, steam is added to retard the deposition of coke on the walls of the reactor tube that eventually require the periodic shut-down of the process for decoking [ 1]. As a result, there has been much interest recently in trying to find a catalytic alternative to the current industrial steam cracking process. The catalytic oxidative dehydrogenation of hydrocarbons offers a promising alternative to thermal pyrolysis and catalytic dehydrogenation. In this study, we investigate C2H4 and C3H6 production by catalytic partial oxidation of C2H6 and C3H8 over chromium oxides supported on cx-A1203 and Mg-stabilized ZrO2 foam monoliths at millisecond contact times. In addition to identifying optimum feed conditions, the study examines the effect of the support on the activity of the metal oxide catalysts. The performance of these metal oxide catalysts is compared to the results from oxidative dehydrogenation over Pt coated monoliths which have been examined for the oxidative dehydrogenation of C2-C6 hydrocarbons [2-11 ].
2. EXPERIMENTAL 2.1 Reactor Configuration The reactor is essentially identical to that previously described for the production of syngas [ 12] and oxidative dehydrogenation of light alkanes [2]. The reactor consists of a quartz tube with an inner diameter of 20 mm. The catalyst is sealed in the tube with high temperature silica-alumina felt which prevents the bypass of gases around the catalyst. To reduce the radiation heat loss in the axial and radial directions and to better approximate adiabatic operation, inert foam monoliths are placed in front and behind the catalyst as heat shields, and the reaction zone is externally insulated. The reaction temperature was measured
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by type K (chromel/alumel) thermocouples placed at the center of the reactor tube between the catalyst and the heat shield. The gas mixture for the reactor consists of C2H6 and 02 with N2 as the diluent, and are controlled by electronic mass flow controllers. The level of dilution ranged from 20 to 50%. The total feed flow rate to the reactor ranged from 1 to 3 SLPM which corresponds to an approximate contact time of < 10 milliseconds for the monolith catalyst. For all the experiments, the reactor pressure is maintained at 1.2 atm (18 psi). The autothermal reaction takes place over the catalyst around 900~ and a sample of the product gases is fed to a HP 6890 Gas Chromatograph (GC) through heated stainless steel lines. While the reaction operates autothermally at steady state, an external heat source is necessary to initially ignite the reaction. Over the Cr203 catalyst, the reaction ignites at approximately 350~ which is significantly higher than the 220~ preheat needed to ignite that reaction over a Pt coated monolith catalyst under the same conditions. After ignition, the external heat source is removed, the composition is adjusted to the desired value and steady state is established ( 8 kPa. The conversion rates of benzene and 0 2 also showed maxima at P(C6H6) ~ 7kPa and decreased sharply at higher P(C6H6). In order to get information about the reaction paths for the formation of products, the effect of contact time (W/F) on the oxidation of benzene was studied for the two catalysts. I
I
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I
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.
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Figure 6 shows the effects of W/F on 60 [ I the selectivities of products for VsMo4-.. (a) VaMo4-Oxide/SiO2 oxide(4wt%)/SiO2 (a) and V4Mos40 1 ~~,,,Phenols oxide(4wt%)/SiO2 (b). In the case of VsMo4-oxide/SiO2, the selectivities of phenols increased with decreasing contact time. The selectivities to PhOH, HQ, and BQ were extrapolated to 30, ~ 0 100 , , , 15, and 15%at zero contact time. The (b) V4Moa-Oxide/Si02 selectivities to COx and M A were '-o 8 0 " ' extrapolated to 35 and 5 %. These results suggest that each product is 60 ' ,,'~ PhenoIs produced via parallel reaction paths at the initial stage of the oxidation of benzene. In the case of V4Mos-oxide/SiO2 catalyst (Fig. 6(b)), the selectivity to 0 BQ I I " i --~ PhOH remarkably increased with 0 0.2 0.4 0.6 0.8 W/F / sec g cm-3 decreasing W/F. The selectivities to Fig.6. Effect of W/F on the selectivities of PhOH, HQ, and BQ were extrapolated products. to 85, 10, and 5% at zero contact time. In contrast to these Phenols, the selectivities to COx and M A were extrapolated to zero. These results suggest that phenols were selectively produced on the V4Mos-oxide/SiO2 catalyst at the initial stage of the oxidation of benzene. The COx and M A are the secondary oxidation products from the Phenols. The results described above strongly suggest that the reaction paths for the oxidation of benzene are very different between the two catalysts. The apparent activation energies for the oxidation of benzene were evaluated from the effect of temperature on the oxidation rates of benzene for the two catalysts. Good straight lines were obtained for the plots of the formation rates of products versus 1/T (T=723-823K) for both catalysts. The apparent activation energies for the conversion of benzene and the formation of phenols were evaluated from the slopes and they are shown in Table 1. The apparent activation energies are quite different between the two catalysts. The quite different kinetic results obtained for the two catalysts suggest different reaction mechanisms or different active oxygen species responsible for the hydroxylation of benzene on the two catalysts. I
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3.4 Mechanistic information for benzene oxidation In order to get more information about the reaction mechanism, the oxidation of a mixture of C6D6 and C6H6 were carried out at 823K to estimate kinetic isotope effect (kH/kD). The isotope effect of 1.3 was observed for the formation of PhOH over the VsMo4-Oxide/SiO2, suggesting that the cleavage of C-H bond of benzene might be the rate-determining step. In
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contrast, the kinetic isotope effect was 1.0 for V4Mos-oxide/SiO2 catalyst, which suggests that the cleavage of C-H bond is not the rate-determining step. These results also support the idea that nature of the active oxygen species or the reaction mechanisms are very different between the two catalysts. The nature of the active oxygen species can be discussed on the
Table 1. Nature of catalysisofVMo-Oxide/SiO2.
basis of the regioselectivity in the oxidation of toluene. The oxidation of toluene was studied at 723K. The
VMo-Oxide /SiO2
Ea / kJ mol 1 C6H6 Phenols
benzene toluene kH/kD Cr/Bz
ratio of the amount of the phenyl V4Mos145 111 1.0 1.6 ring oxygenated products (o, m, pcresols) and that of the Me-group VsMo477 33 1.3 0.5 oxygenated products (benzaldehyde and benzyl alcohol) was defined as Cr/Bz for the evaluation of the regioselectivity. As shown in Table 1, Cr/Bz value of 1.6 was observed for the V4Mos-oxide/SiO2, suggesting that the active oxygen species on this catalyst prefers the oxidation at the stronger C-H bond (111 kcal mol l ) in phenyl ring. Probably addition of oxygen species to the aromatic C=C bond proceeds at the initial stage of the oxidation. On the other hand, Cr/Bz ratio for the oxidation over VsMo4-oxide/SiOz indicates a preferential attack of the active oxygen at the weaker C-H bond (88 kcal mol "l) of Me-group, which results in the abstraction of hydrogen atom from the Me-group. The discussion on the basis of the regioselectivities in the toluene oxidation suggested the reactivities of the active oxygen species, i.e., the oxygen on V4Mo8-oxide/SiO2 is susceptible to add on the aromatic ring, but that on VsMoa-oxide/SiO2 has a strong ability to abstract hydrogen. If the rate determining step is an electrophilic addition of oxygen to the aromatic ring, we do not observed the isotope effect (kH/kD~l.0) as this was the case for the hydroxylation over V4Mos-oxide/SiO2 catalyst. On the other hand, if the rate determining step is the abstraction of hydrogen atom from the phenyl ring by the active oxygen having a radical character on V8Mo4-oxide/SiO2, the experimentally observed isotope effect (kH/kD=1.3) for the hydroxylation of benzene may be well explained. REFERENCES 1. M. Iwamoto, et al., dr. Phys. Chem., 87 (1983) 903; E. Suzuki, K. Nakashiro, Y. Ono, Chem. Lett., (1988) 953; G.I. Panov, et al.,Appl. Catal. A, 98 (1993) 1; Chemical Week, 159, (1997) 11. 2. N. Herron and C.A. Tolman, J. Am. Chem. Soc., 32 (1987) 200; K. Sasaki et al., Bull Chem. Soc. Jpn, 62 (1989) 2613; T. Miyake, et al.,Appl. Catal. A, 131 (1995) 33. 3. A.K. Uriarte et al., Stud. Surf. Sci. Catal., 110 (1997) 857. 4. W.Ueda, et al., Ind. Eng. Chem. Res., 28 (1989) 1587.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
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Eu-Ti-Pt-Catalytic System for Direct Hydroxylation of Benzene by 02 and Hz under Mild Conditions I. Yamanaka*, T. Nabeta, S. Takenaka and K. Otsuka Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan One step synthesis of PhOH from benzene was performed by Eu-Ti-Pt-catalytic system with 02 and H2 at atmospheric pressure and 40~ The best combination of the elements for the hydroxylation of benzene was Eu(OTf)3-TiO(acac)2-Pt-oxide/SiO2. The Eu3§ 2+ redox catalyses the reductive activation of 02, which is enhanced by TiO(acac)2. Pt-oxide/SiO2 works as a catalyst for the reduction of Eu 3+ to Eu 2+ by H2. The reductively activated oxygen by the concerted actions of the catalytic components is responsible for the hydroxylation of benzene to phenol. 1. INTRODUCTION In the current chemical industry, a large amount of PhOH is manufactured by the Cumene Process. The Cumene Process requires multi-step operations as well as a process for the utilization of co-product of acetone. Therefore, a new catalytic system for direct hydroxylation of benzene to PhOH has been desired. Under these circumstances, the direct hydroxylation of benzene to phenol (PhOH) by 02 and H2 with catalysts such as Pd-Fezeolite, Pd-CuSO4/SiO2, Pt/V205/SiO2, etc. [ 1-3] attracted much attention. We have recently reported a new catalytic system based on Eu salts, Zn powder and acetic acid for the partial oxidation of hydrocarbons by 02 [4]. This Eu-catalytic system can oxygenate benzene to PhOH, propene to propene oxide, and cyclohexane to RH ROH cyclohexanol and cyclohexanone at room Zn0 eu 3 temperature. If we use CF3CO2H for the Eucatalytic system instead of MeCO2H, methane can be hydroxylated to MeOH at C02 room temperature. As shown in Scheme 1, we Zn2+ consider that Eu 2§ species formed by the reduction of Eu 3+ with Zn powder (step 1) o2 MeCCH activate 02, forming active oxygen species Scheme 1. Reaction scheme for the oxidation (step 2) which oxidize hydrocarbons to of hydrocarbon with a system of EuC13oxygenates (step 3). One of the problems of MeCO2H-Zn powder. this Eu-system is to use an expensive
step21~E-u2+~'~
Eu2+f ; (
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reducing agent of Zn powder. If 1-12,a cheaper reducing agent, can be used for the Eu-catalytic system instead of Zn powder, a new process of the direct PhOH synthesis might be brought into an industrial application. The purpose of this work is to demonstrate hydroxylation of benzene by 02 and 1-12with Eu compounds-based catalytic system, and to get informations about the catalytic functions of each catalyst component for the hydroxylation. 2. EXPERIMENTAL The standard experimental procedures were as follows; Eu(OTf)3, (120 ~tmol) and bis(2,4pentanedionate)TiO (TiO(acac)2, 20 ~tmol) was dissolved in a mixture of benzene (4 ml) and MeCO2H (16 ml) in a round bottom flask with a reflux condenser. After Pt/SiO2 or Ptoxide/SiO2 (lwt%Pt, 0.1 g) was added, a gas mixture of 02, H2 and Ar (1:1:1, 30 ml min-l) were introduced to the reaction mixture. The reaction was carried out for 2h by stirring the reaction mixture with a magnetic spin-bar at 40 *C. Products were analyzed by HPLC with a UV detector and GC with TCD and FID detectors. Eu(OTf)3 was prepared from Eu203 and CF3SO3H. Pt-oxide/SiO2 was prepared by an impregnation method from H2PtCI6 aq. and SiO2 (SIO-8, supplied from Catalysis Society of Japan). A precursor of the catalyst was calcined in air at 573K for 2h (Pt-oxide/SiO2). Ptoxide/SiO/was reduced by H2 at 673K for 2h. Other reagents (special-grade) were used without purification. The H2-efficiency used for the hydroxylation of benzene was defined as Eq. 1. H2-Efficiency = [sum of hydroxylated products (mol) / H20 formed (mol)] x 100% (1) Eu and Ti species in the reaction mixture were quantitatively analyzed by an UV-visible spectrometer after the mixture had been reduced by H2 with PtJSiO2 catalyst and after subsequent oxidation of the reaction mixture by 02. 3. RESULTS AND DISCUSSION
3.1. Hydroxylation of benzene by 02 with Eu-catalytic system As described above, EuC13 catalyst can hydroxylate benzene to phenol (PhOH) with 02 in the presence of Zn powder (e- donor) and MeCO2H (H§ donor and solvent) under mild conditions [4]. When a gas mixture of 02 and H2 was introduced to the Eu-catalytic system in the absence of Zn powder, the oxidation of benzene did not take place at all. We considered that the absence of oxidation was due to the inability of the system for the activation of H2. Therefore, several catalysts (Pt, Pd, Rh, Ir and Ni supported on SiO2, A1203, active carbon, etc.) for the activation of H2 were added to the reaction mixture, and tested for the hydroxylation of benzene by 02 and H2. The hydroxylation of benzene took place with EuC13-catalytic system by addition of Pt/SiO2 at atmospheric pressure and 40~ Hydroxylated products were PhOH > hydroquinone (HQ) > catechol (CT). Other byproducts were cyclohexane due to the hydrogenation of benzene. CO2 was also produced from oxidation of MeCO2H.
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Although the EuCI3-Pt/SiO2-MeCO2H-O2EuCl3, Pt/SiO2 1-12 system could hydroxylate benzene 9 R1OH directly, the yield of the sum of PhOH, HQ Eu(OT03, Pt/SiO~ m and CT was very low, turnover number less Eu(ClO4)3, Pt/SiO 2 than 1 in 2h. In order to enhance the productivity of phenols, the catalytic system Eu(acae)3, Pt/SiO2 has been developed further as follows. First, the effects of various Eu compounds Eu(TFA)3, Pt/SiO2 on the hydroxylation were studied. Figure 1 Eu(OTf)3, Pt-oxide/SiOz shows the product yield for several Eu-salts Eu(OTf)3, TiO(acac)2 and complexes with Pt/SiO2 for the oxidation Pt-oxide/SiO2 of benzene by 02 and H2. Eu(OTf)3, TiO(acach, Eu(CIO4)3, and EuC13 were active for the Pt-oxide/SiO 0 i i hydroxylation. However, when Eu(acac)3, 5 100 150 2()0 Eu(TFA)3 and Eu(NO3) 3 were used, the Product yield //tmol oxidation of benzene did not take place. Fig. 1. Hydroxylation of benzene by 02 Second, the preparation conditions of and H2 with various catalytic systems at 1 atm and 40 ~ Pt/SiO2 (calcination and reduction temperatures, amount of Pt loading, etc.) 200 were important. We found that Pt-oxide/SiO2 -" Jl~l~ II phenols o II ,., ~. o PhOH showed better performance for the ~-150 zx HQ hydroxylation of benzene than Pt/SiO2. The results for different Pt loading (1 to 5wt%) on ~1oo SiO2 indicated that the yield of phenols did not change appreciably. The combination of o Eu(OTf)3 and Pt-oxide/SiO2 showed a better 50 results, as shown in Fig. 1. Third, enhancing effect of the addition of o TiO(acac)2 to Eu(OTf)3-Pt-oxide/SiO2 was 0 60 120 180 240 Amount of TiO(acac)2 added //tmol examined because TiO(acac)2 worked as a cocatalyst for the previous Eu-catalytic system Fig. 2. Effect of TiO(acac)2 on the benzene using Zn powder [4]. The yield of phenols hydroxylation by O2 and H2. remarkably increased with the addition of TiO(acac)2, as shown in Fig. 1. The TiO(acac)2-Pt-oxide/SiO2 in the absence of Eu(OTf)3 was inactive for the hydroxylation of benzene. These results suggest that TiO(acac)2 works as cocatalyst of Eu(OTf)3 for the hydroxylation of benzene by 02 and H2. Figure 2 shows the effect of the amount of TiO(acac)2 added to the Eu(OTf)3-TiO(acac)2Pt-oxide/SiO2 catalytic system on the oxidation of benzene. When a small amount of TiO(acac)2 (10~tmol) was added to the catalytic system, the hydroxylation activity was enhanced considerably. The maximum yield of phenols was obtained at 20~mol addition of TiO(acac)2. This quantity was just 1/6 of that of Eu(OTf)3. An excess addition of TiO(acac)2 decreased the catalytic activity of the Eu(OTf)3-TiO(acac)2-Pt-oxide/SiO2 system.
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Figure 3 shows the yields of phenols as 10 ...--m--..../--'>" O functions of the partial pressure of H2 6~ (P(H2)) for the oxidation of benzene with 6 ID the Eu(OTf)3-TiO(acac)E-Pt-oxide/SiO2 4"~ "7, 4 catalytic system. These experiments were 9 =72 Eft performed without an inert gas of Ar 0, o (P(H2) + P(O2) = 101kPa). The yields of 400 cyclohexane, CO2 and H20, and the HE--,9 9 phenols Q E~ [] CsH12 ~ efficiency estimated from Eq.1 were also :r 3 0 0 a 002 / -- " ~ plotted in Fig. 3. The results in this figure u indicate that the oxidation of benzene did not take place at all at P(H2)-0 by Eu(OTf)3-TiO(acac)E-Pt-oxide/SiO2 N100 ~ ~ catalytic system at 40~ Yield of phenols (PhOH + HQ + CT) increased with 0~ ' ~-"~ ' ' ' ' ' 0 ' 20 40 EF- 60 80 100 increasing P(H2) and reached to the P(H2) / kPa maximum at around 67kPa. Main product Fig. 3. Effect of P(H2) on the benzene was PhOH at all P(H2), and the hydroxylation by 02 and H2. selectivities to HQ and CT were lower than 5%. A considerable amount of cyclohexane, that is a hydrogenation product of benzene was produced at higher P(H2). The yield of cyclohexane increased with increasing P(H2), decreasing the PhOH selectivity. Thus, higher P(H2) was not favorable on the view point of selective hydroxylation. The formation of H20 was accelerated with increasing P(H2) and reached the maximum at P(H2)=50 kPa. The maximum efficiency of H2 was obtained at 75 kPa. Under the experimental conditions in this work, a P(H2)~(O2) ratio of 3 was the most suitable gas composition from the view point of the effective and selective hydroxylation of benzene. Kinetic curves for the oxidation of benzene by 02 (75kPa) and H2 (25kPa) with the Eu(OTf)a-TiO(acac)2-Pt-oxide/SiO2 catalytic system were studied. Hydroxylation of benzene took place for more than 8h. The formation rate of phenols was decelerated after 2 h due to the accumulation of H20 in the reaction mixture. Effects of total amount of catalysts with different ratios of each catalytic component, concentration of benzene, and reaction temperature on the formation of phenols were studied for the Eu(OTf)a-TiO(acac)2-Pt-oxide/SiO2-system. The maximum formation rate of phenols was 1.5 (mmol g-cat. "1 h "l) with a H2-efficiency of 10% at atmospheric pressure and 40 ~ This is fairly good productivity of phenols comparable to that obtained with Pt/V2Os/SiO2catalyst [3]. i
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3.2. Character of active species of Eu-Ti-Pt-catalytic system As mentioned above, Eu(OTf)3-TiO(acac)2-Pt-oxide/SiO2 catalytic system was the most active one for the hydroxylation of benzene by 02 and H2. Under the standard reaction conditions, the yield of phenols for each Eu-catalysts was Eu(OTf)3 (310~tmol in 2h) > Eu(CIO4)3 (260)> EuC13 (150)>> Eu(acac)3, Eu(TFA)3, Eu(NO3), Eu(OAc)3 (-10).
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Therefore, the three Eu-compounds, Eu(OTf)3, Eu(CIO4)3, and EuCI3, were chosen as the Eu(OTf)3, catalysts for further studies on the nature of TiO(acac)2 active species. First, the kinetic isotope effect on the Eu(CIO4)3, TiO(acac)2 formation of PhOH from a mixture of C6D6 m-Cr and C6H6 (1:1 mol) was measured for the three EuC13, catalysts. The isotope effect on the formation TiO(acac)2~ of PhOH (kH/kD) were 1.0 for Eu(OTf)3, 1.1 for Eu(C104)3, and 1.2 for EuCI3. These small I , I , I , I , 0 200 400 600 kH/kD-values suggest that a cleavage of C-H Products Yield //tmol bond of benzene may not be involved in the Fig. 4. Oxidation of toluene by 02 and H2 rate determining step of the hydroxylation of with (Eu-compounds + TiO(acac)2 benzene for the three catalytic system. + Pt-oxide/SiO2). In order to get information for the reactivity of active oxygen species, oxidation of toluene was carried out with the three Eu-components. Figure 4 shows the product yields of toluene oxidation by 02 and H2 with the Eu-compounds-TiO(acac)2-Pt-oxide/SiO2. Products were cresols (o-, m-, p-Cr), benzyl alcohol (BzOH) and benzaldehyde (BzO). Product yields and distributions were very different among the three Eu-compounds. In the case of Eu(OTf)3, the major product was cresols (o:m:p = 3:1:5). The ratio of the benzene ring oxidized products (o-, m-, p-Cr) and the Me-group oxidized products (BzOH, BzO) was 7.0 (Cr/Bz). The active oxygen species preferes to attack at benzene-ring rather than Me-group. This result suggests that active oxygen species has electrophilicity. In contrast to Eu(OTf)3, the Me-group oxygenated products (BzOH, BzO) were the major ones for Eu(C104)3 and EuC13 catalysts. The ratios of Cr/Bz were 0.6 and 0.2 for Eu(C104)3 and EuC13, respectively. The small Cr/Bz values for these two Eu-catalysts suggest that active oxygen species is favor for the abstraction of H. from the Me-group rather than it's addition to benzene ring. Thus, this oxygen species may have a radical property.
/L~
3.3 Oxidation state of Eu and Ti species
Figure 5 shows the UV-visible spectra of Eu species after reduction by H2 with Pt/SiO2 and after oxidation by 02. Spectrum (i) is a UV spectrum of solution of EuC13 in MeCO2H. Spectrmn (ii) has been measured after reduction of the solution of EuC13 (6 mmol ll) and MeCO2H by H2 with Pt/SiO2. Spectrum (ii) shows the broad absorption bands having two peaks at 270 and 320 nm. These absorption bands were identified as UVabsorption of Eu2+ (f-d) [5]. Very similar spectrum to that of (ii) was observed for the
(i) before reduction (ii) afterreductionby H2 (iii) after oxidation by 0 2
(ii)
O) o
(ii
300 400 ' 5()0 ' Wavelength / nm Fig. 5. UV spectra of Eu species.
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solution of EuCI3 and MeCO2H after reduction by Zn powder. Spectrum (iii) was measured after the oxidation of the solution (ii) by 02. The spectnma (iii) is almost same as the spectrum (i). The changes as spectra (i) to (iii) were observed repeatedly by the reduction and the oxidation. These results strongly suggest that Eu 3+ is reduce to Eu 2§ by H2 in the presence of Pt/SiO2 and the Eu 2§ species is oxidize back to Eu 3+ by 02. Similar UV measurements were performed for the co-catalyst TiO(acac)2. The UV spectra were not changed before and after the reduction treatment by 1-12 with Pt/SiO2 and the oxidation treatment by 02. These results suggest that Ti 3§ (or Ti 2§ species is not produced during the hydroxylation. Thus, the redox of Ti4§ 3§ (or Ti 2§ may not be involved in the hydroxylation.
3.4 Model of reaction scheme As described above, Eu 3+ is reduced to Eu 2+ by 1-12with Pt/SiO2 . This Eu 2+ species is oxidized by 02 to Eu 3+, which suggests that 02 is reduced by Eu2+, generating active oxygen species. While, Ti 4§ (TiO(acac)2) is not reduced by 1-12with Pt/SiO2. Ti 4§ must enhance the oxygenation by keeping its oxidation state (IV). On the bases of the above experimental facts in this work, we propose a tentative model of reaction scheme in Scheme 2. The Eu 2§ generated by reduction with H2 on Ptoxide/SiO2 (or Pt/SiO2) reductively activates 02, which is assisted by TiO(acac)2. The active oxygen species may \ [Eu,tT! I be generated by a concerted action of Eu 3+ Eu3+ and Ti 4§ The reactivity of the active ~,~ Eu 2§ \ H+ H2 oxygen species is strongly affected by the ~Ti4+ ligands such as TfO-, C104-and C1-. Further studies are needed to clarify the state of the active oxygen species and the SiO 2 role of the ligands. For the application of the catalytic systems in this work, we have Scheme 2. Model of the hydroxylation of benzene to improve remarkably the catalytic by 0 2 and H2 with Eu-Ti-Pt system. activity and, especially, the H2-efficiency for the production of phenol.
Q
( ou
o2 [..-~
REFERENCES 1. N. Herron and C.A. Tolman, J. Am. Chem. Soc., 32 (1987) 200. 2. K. Sasaki et al., Bull Chem. Soc. Jpn, 62 (1989) 2613. 3. T. Miyake, et al., Appl. Catal. A, 131 (1995) 33. 4. I. Yamanaka, K. Otsuka, et al., J.C.S., Perkin 2, (1996) 2551; Chem Lett., (1996) 565. 5. G. Adachi, J. Shiokawa, et al., Inorg. Chim. Acta, 113 (1986) 87.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
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High yield butane to maleic supported catalysts.
anhydride
direct oxidation
on new
M. J. LEDOUX a V. TURINES a, C. CROUZET a, K. KOURTAKIS b, P. L. MILLS b and J. J. LEROU b a Laboratoire de Chimie des Mat4riaux Catalytiques GMI-IPCMS, UMR7504 du CNRS, ECPM, Universit6 L. Pasteur 25 Rue Becquerel, 67085 STRASBOURG Cedex 2 (France) bE.I. du Pont de Nemours and Co, Experimental Station, Route 141 WILMINGTON, Del. 19880-0262, USA The preparation and the characterization of VPO supported on a new SiC support are described. This catalyst provides a very significant gain in maleic anhydride yield by reacting air and butane, because of the control of the surface temperature. I. I N T R O D U C T I O N The direct oxidation of butane by oxygen (air) into maleic anhydride (MA) is a wellestablished industrial process (1,2) using vanadium pyrophosphate (VO)2P2OT, (VPO), in its bulk form as a catalyst. Hundreds of papers and patents have addressed the method of preparation and the characterization of the active site(s) (3-6), but very few deal with the process itself. Two different configurations of reactor are used: - fixed-bed, cheap in capital investment and easy to run, however, strongly limited by the release of a large amount of heat (a very exothermic reaction), and consequently only able to work at a low butane concentration ; - transported-bed with a very short contact time between the catalyst and the feed mixture in a riser tube, costly in capital investment and tricky to run. Because of the very short contact time, the reoxidation step of the catalyst taking place outside the reactor, and the efficient heat transport by the outlet gases, it is possible to work at high MA yield provided by a high concentration of butane. The main inconvenience of these systems is the poor ability of bulk VPO, because of its low thermal conductivity, to conduct heat outside the reactor, and also to absorb the heat generated at the surface to avoid hot spots. Many attemps to disperse VPO on different conventional supports (alumina, silica, TiO2) were not very successful because of the strong interaction (7) between the oxygen atoms of the support and the oxygen atoms of the active phase, especially during the activation step transforming the hemihydrate precursor into VPO. In addition, the thermal conductivity of these oxidic supports is quite low and large improvements in heat transfer were not expected (see Table 1). Table 1 Thermal conductivity of different materials used as catalyst supports Material Thermal Conductivity in W.moll.K "1 SiO2 A1203 SiC
0.015 - 1 1- 8 146 - 270
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A new generation of supports based on ceramics with high thermal conductivity and without oxygen atoms has been prepared and tested in our laboratory. The best example is a new relatively large specific surface area SiC (> 20 m2/g) prepared via the "shape memory synthesis"(8-10). Conventional SiC (carborandum) of low specific surface area (< lmE/g) prepared via the Acheson process was patented a long time ago as support for VPO (11,12) but it does not appear to disperse the active phase because of too little interaction between the two solids. In addition, because of this lack of interaction the supported catalyst very quickly demixes under heating, and VPO and SiC are irreversibly separated after a short time. The objective of this article is firstly to describe the preparation and the characterization of VPO supported on the new SiC and secondly to show the very significant gain in MA yield obtained on this catalytic material because of the control of the surface temperature. 2. E X P E R I M E N T A L
SiC was prepared in different shapes; fine powder, grains (diameter 0.5-1 mm) and extrudates (1 cm long, 2 to 4 mm diameter), according to the two methods already described and referred to by the genetic term "shape memory synthesis". The first method (8-10) is based on preformed activated charcoal reacted with SiO vapor generated in a different chamber of the reactor, the second (13) being based on a mixture of carbon, oxygen containing polymer and Si powder, preshaped and heated under a neutral atmosphere. Because of the relatively low temperature of the reaction, between 1200 and 1300~ compared to the 1800~ which is necessary for the synthesis of carborandum or a-SiC, the product obtained is mainly I$-SiC with a surface area varying between 20 and 180 m2/g according to the reaction conditions and the doping agents, with no microporosity. The profile of the pore distribution is centered around 80-100 A (10) and the surface is made either of pure SiC outside the pores or a thin layer (2000 hours) of reaction under different conditions of temperature and butane/air flows and ratios. There are no significant changes between this diagram and the preceeding one, in agreement with the fact that the catalyst was stabilized after 100 hours of activation, but a slight increase in the background noise should be noted. 700
1
600
1
0
Figure la
2
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Fig. 2. HRTEM of VPO/SiC
In order to obtain more information on the nature of the interaction between VPO and SiC, HRTEM was performed using a Topcon Model EM200B operating at 200kV with a point-topoint resolution of 1.7 A. The sample of catalyst was deposited on a carbon grid and introduced into the analysis chamber without specific precautions as all these samples seem to be stable for a short time in air, otherwise they were kept under dry nitrogen for long periods. VPO is very sensitive to the energy of the microscope beam and cannot be observed more than 10s without starting to decrystallize. For this reason the pictures were blindly taken by switching the beam on and off for the time of the photo and moving the grid in different directions after the beam had been focused on one spot of the sample followed by a partial destruction of the crystal. Luckily 2 to 10% of the resulting pictures were usable and could provide excellent information. For EDS analysis, when an area was selected, the subsequent decrystallization was not detrimental as only the elemental composition was sought. Fig 2 shows a microcrystal of VPO
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in contact with the SiC support. At the junction, the crystal order of VPO progressively disappears into an amorphous phase (between arrows) which contains V, P, O, Si and C elements as found by EDS. This phase does not have the same appearance as the amorphous layer found on the top edge of the crystal which only contains V, P and O. It is suggested that the interphase is a solid solution of VPO in a silicon oxycarbide acting as glue. On a different support this glue phase can be observed by XRD because it is partly crystallized (16). The same sort of interface cannot be observed on carborandum or conventional ct-SiC and explains the very poor mechanical strength of the VPO supported on this ct-SiC. An improvement of the attrition resistance when compared to bulk VPO reinforced by a coating of silica (17) was researched with the SiC impregnation. This was not obtained, VPO/B-SiC presented a poorer attrition resistance than the V P O + S i O 2 c o a t . Because of the relatively small amount of oxygen contained in the upper layer of the SiC pores and probably because of the peculiar chemical property of this oxycarbide, it seems that there is the fight balance of interaction, strong enough to form the glue phase, but weak enough to avoid oxygen interference during the formation of the VPO phase from the hemihydrate precursor.
3. R E S U L T S AND D I S C U S S I O N The catalysts were tested in a micropilot already described (18). Each gas of the feed was independently monitored before mixing, i.e. O2, butane, He ( N 2 w a s replaced by He in the air mixture to ease the analysis) and a known amount of CH 4 was injected after the reactor to be used as standard to connect the two chromatographic analyses made on-line. The mass balance in oxygen and carbon was always better than 99%. The reactor was of a fixed-bed type with a thermocouple measuring the temperature inside the catalyst bed and another the temperature outside the reactor wall at the heart of the oven. The temperature measured in the bed was used as reaction temperature. At steady state and with the bulk VPO reference catalyst the difference in temperature between the two thermocouples could be as high as +30~ while with the SiC supported sample, the difference was found to be generally much lower, between 0 and +5~ the temperature inside always being higher than outside. The reference bulk catalyst was a state of the art industrial sample. The standard conditions of reaction were: O2/butane volume ratio (or mole)=l.4-1.5/1, partial pressure: 12 to 14 % of butane, total flow: = 40 cc/min for a catalyst weight o f - 0.5 g giving WHSV = 1.7 h -~ (all these figures were very accurately measured for each experiment). In Fig 3 the specific rate of MA formation on each catalyst (bulk VPO and VPO/SiC) as a function of the reaction temperature per gram of VPO and not of total catalyst is reported. At low temperature, below 350~ where the selectivity in MA is very high (see Fig 4), but the conversion very low, the rate of reaction seems to be equivalent on the two catalysts. However at 370~ for the same selectivity of 79%, the rate on VPO/SiC (3.10- 6 m/g.s) is almost 2 times faster than on bulk VPO (1.7.10 .6 m/g.s). This could be attributed to a dispersion effect due to the support. At higher temperature the comparison is impossible as a sharp drop in selectivity is observed on the bulk VPO. Conversions and yields in MA are reported in Figs 3 and 4 as a function of the temperature. The main by-products are CO 2 and CO and to a minor extent some acrylic acid and methacrolein. If the yield of MA at 370~ reaches 24% on the bulk and only 78% on the supported catalyst catalyst, this is only due to the fact that VPO/SiC contains 27% of VPO instead of 100%. At 420~ the two catalysts produce the same yield (16%), above this temperature most of the products found on bulk VPO are CO and CO 2 while the selectivity in MA remains relatively stable on VPO/SiC, above 70%, up to 470~ which explains the yield of 38% observed at this temperature. The simplest hypothesis to explain these results is to consider that the temperature of the surface of the bulk catalyst does not correspond to the temperature measured in the bed, and that the MA formed after a certain limit of temperature is burnt into CO and CO 2, while on the supported catalyst the surface temperature is quickly equilibrated by the heat sink effect of the conductive SiC support. If one assumes that the surface temperature equals the temperature of
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the full bed with the supported catalyst, Fig 4 explicitly shows that MA is not extensively burnt even at a temperature as high as 470~ which means that on bulk VPO the surface temperature should be much higher considering the very poor selectivity in MA observed above 420~ It was interesting to see if higher yields could be reached on the supported catalyst. In the standard conditions all oxygen was consumed at 4700C and the limitation in yield could only be due to this shortage in oxygen. To check this, a series of measurements were made on a slightly more active catalyst than the preceeding series by increasing the O2/He f l o w , keeping constant the butane flow, thus working at constant butane WHSV but at higher total flow. The results are presented in Fig.5. It is clear that the reaction is only limited by the amount of oxygen available for the reaction; the selectivity remains more less constant, the conversion increases faster at the beginning but quite linearly with the 02 avaibility. The maximum yield obtained for a ratio O2/butarie of 10 reaches 55% for a butane conversion of 70% and it is probably possible to go higher, but the micropilot was not equipped to do so. Many other parameters have been studied and will be published in a longer article later. 50
1 10.5
40
~ ;;
~VPO/SiC - - I - - - ' ~ - V P O bulk
/ I1r
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8
100
10. 6
90
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"
0 300
350
400 T (~
-o--~vPo/sic
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300
350
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Fig. 3. Compared reaction rates on VPO bulk Fig. 4. Compared MA selectivity on VPO bulk and VPO/SiC and VPO/SiC 90
Figure 5 Selectivity
80 70 6050" 40 30
Air/butane flow ratio 15
I
20
I
25
I
30
I
35
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55
Fig. 5. Maleic anhydride yield and selectivity 4. C O N C L U S I O N The new [3-SIC with a large mesoporosity is an excellent support for VPO. The interaction between the active phase and the support is well balanced, not too strong which would perturb
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the formation and the selectivity of the active phase, not too weak in order to keep a good mechanical strength. The high thermal conductivity of SiC allows an homogeneisation of the temperature in the catalyst bed, particularly by strongly decreasing the surface temperature at steady-state. Consequently the total oxidation can be better controlled and very high yields can be reached in a fixed-bed configuration twice as high as for the conventional bulk catalyst- i.e. 55% instead of 24% with a partial pressure of butane of about 14%. REFERENCES
1. J.T. Wrobleski, J.W. Edwards, C.R. Graham, R.A. Keppel and M. Raffelson, US Patent 4562 268 (Monsanto) (1985). 2. R.M. Contractor, H.E. Bergna, H.S. Horowitz, C.M. Blackstone, B. Malone, C.C. Torardi, B. Griffiths, U. Chowdhry and A.W. Sleight, Catal. Today, 1 (1987)49. 3. E. Bordes, Catal. Today, 3 (1988) 163. 4. G.J. Hutchings, Applied Catal., 72 (1991) 1. 5. F. Cavani and F. Trifiro, in Catalysis, The Royal Society, Vol. 11,246 (1994). 6. G. Centi (Eds), Catal. Today, 16 (1993) 5. 7. M. Ruitenbeek, A.J. Van Dillen, D.C. Koningsberger and J.W. Geus, in Prep. of Catalysts VII, Elsevier Sciences B.V., Studies in Surface Science and Catalysis, Vol 118, 549 (1998). 8. M.J. Ledoux, S. Hantzer, C. Pham-Huu, J. Guille and M.P. Desanaux, J. Catal., 114 (1988) 176. 9. M.J. Ledoux, S. Hantzer, J. Guille and D. Dubots, Eur. Patent 0 313 480 (1992) and US Patent 4 914 070 (P6chiney) (1990). 10. N. Keller, C. Estoumes, C. Pham-Huu, S. Roy, J. Guille and M.J. Ledoux, J. of Mat. Sc., 34, 3189, (1999). 11. R.O. Kerr, US Patent 3 156 705 (Petro-Tex Chem. Corp.) (1964). 12. R.A. Schneider, US Patent 3 864 280 (Chevron) (1975). 13. B. Grindatto and M. Prin, Eur. Patent 0 543 752 A1 (Pechiney) (1992). 14. N. Keller, C. Pham-Huu, M.J. Ledoux, J.B. Nougayrede, S. Savin-Poncet and J. Bousquet, Catal Today (in press). 15. K. Katsumoto, D.M. Marquis, US Patent 4 132 670 (Chevron) (1979). 16. M.J. Ledoux, C. Crouzet, C. Bouchy, K. Kourtakis, P.L. Mills, J.J. Lerou, a patent application is in progress. 17. H.E. Bergna, US Patent 4 677 084 (Dupont) (1987). 18. F. Meunier, P. Delporte, B. Heinrich, C. Bouchy, C. Crouzet, C. Pham-Huu, P. Panissod, J. J. Lerou, P.L. Mills and M. J. Ledoux, J. Catal., 136 (1997) 33.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Propylene Epoxidation over Gold-Titania Catalysts Eric E. Stangland, Kevin B. Stavens, Ronald P. Andres, and W. Nicholas Delgass School of Chemical Engineering, Purdue University, West Lafayette, IN 47907 We present here kinetic isotope effect support for a hydroperoxy- intermediate, data that suggest consideration of oxidized Au in the active site, and XPS evidence for extended TiO2 as sites for propylene oxide oligomerization in the direct epoxidation of propylene to propylene oxide over Au/TiO2 catalysts in H2/O2 mixtures. 1. INTRODUCTION Haruta et al. have shown that deposition-precipitation (DP) gold-titania catalysts produce greater than 90% selectivity to propylene oxide (PO) from propylene, 02, and H2 at temperatures of less than 400 K [ 1,2]. Their suggestion that a peroxy- intermediate is a likely source of this unexpectedly high selectivity in the presence of allylic hydrogens [2] is supported by Sellers et al. who compute that gold is the best metal for producing peroxy radicals from H2/O2 mixtures [3]. In our work, gas phase propylene epoxidation using O2/H2 has been performed over a wide variety of gold-titania catalysts resulting in TOFs that vary over 4 orders in magnitude. A maximum in PO TOF is observed for most catalysts at 413 K as higher temperatures promote further oxidation products. XPS suggests that alkali promoters may control some of these further reactions, particularly PO oligomerization, at the catalyst surface. A decrease in the steady state PO TOF when D2 is substituted for feed H2 suggests that hydrogen from H2 is present in the complex involved in the rate limiting step and supports the assumption that peroxy intermediates are involved. Kinetic studies involving Au(OH)3 show that oxidized gold species stabilized by titania may play a role in creating the active site for epoxide formation. 2. EXPERIMENTAL METHODS Gold-titania catalysts were prepared and characterized by a variety of spectroscopic techniques [4]. Titanium tetra-isopropoxide (TTIP)-modified-Au(OH)3 (Alfa Aesar) was prepared by impregnating Au(OH)3 with TTIP in pentane, which then evaporated. Propylene oxidation was carried out differentially in a stainless steel 1/2" packed bed reactor over the temperature range of 323-473 K using an atmospheric 10/10/10/70% mixture of H2 (or O2), O2, hydrocarbon, and He at a flow rate of 35 cc (STP) min -l. Ethanal, PO, acetone, propanal, acrolein, propane, and CO2 were analyzed in the product stream by gas chromatography. XPS analysis was performed using a Mg anode powered with 15 keV at 300 W. A reaction chamber connected to the XPS UHV chamber allowed samples to be exposed to the normal propylene oxidation reaction conditions mixture and then analyzed without being exposed to air. A saturator was also used to expose catalysts to a similar number of PO molecules to that which they produce in 6 hours of propylene oxidation. Additionally a
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heating stage could warm samples in v a c u o during analysis if desired. XPS of Au(OH)3 and Au powder was performed by burnishing samples into Ag foil, charge compensating with a low energy electron gun, and referencing all binding energies to Ag 3d5/2 at 368.0 eV. Silica supported catalysts were referenced to the Si 2p in at 103.4 eV. Further details concerning the experimental methods can be found in ref. [4].
3. PREVIOUS RESULTS A detailed analysis was performed on catalysts prepared by a variety of techniques, seen in Table 1 [4]. This work has shown that PO activity can be generated from almost every Au-TiO2 preparation method and Au particle size above 2 nm in diameter. The PO selectivity for the representative Au/TiO2 catalysts, Figure 1, clearly shows there are optimal preparation methods that maintain better selectivity over the temperature range studied.
Table 1: Characterization results for gold-titania catalysts [4]. Abbreviations: Titanium tetra-isopropoxide (TTIP), Distributed Arc Cluster Source (DACS), Degussa P25 TiO2 (P25), deposition-precipitation (DP). The ratio in parenthesis, for the DACS Au-Ti cluster, e.g. (Au:Ti, 4:1), corresponds to the Au:Ti atomic ratio as determined by XPS. Catalyst Preparation Method Gold Loading Dp (wt. %) (nm) T/Au TTIP on Au Powder 99.5 -1000 (Au)/P25 DACS Au clusters on P25 1.7 7.6+3.1 Au(DP)/P25 DP on P25 1.1 7.4+2.3 Au(DP)/A DP on anatase 0.2 4.5+1.0 Au(DP)/T-S DP on TiO2-modified-SiO2 0.5 4.2+1.0 (Au:Ti, 4:1)/S DACS Au-Ti clusters on SiO2 0.3 11.1+10.0 (Au:Ti, 200:1)/S DACS Au-Ti clusters on SiO2 0.2 3.0+2.0 (Au:Ti, >300:1)/S DACS Au-Ti clusters on SiO2 0.6 5.0+2.8 Pure gold or pure titania have no intrinsic activity, but Au powder modification with TTIP creates a few PO active sites. The catalyst created by supporting gold DACS particles onto P25, (Au)/P25 had similar selectivity and TOF. Better gold-titania contacting, and therefore better PO activity and selectivity, was created by using the DP method of Haruta [1]. It is interesting to note, as did Haruta [2], that DP of Au onto three different titania supports, P25, anatase, and titania-modified-silica, did not produce materials of equivalent activity and selectivity at temperatures above 413 K. It appears the type of titania Support has dramatic effects on the formation of active Au-Ti interfaces. The most active catalysts in this study included Au(DP)/T-S and (Au:Ti, 200:1)/S with TOFs of 0.016 s 1 and 0.005 s 1 respectively at 413 K. Nijhuis et al. have suggested that PO yields are capped over Au/TiO2 catalysts at about 2% as a result ofPO oligomerization and rate inhibition [5]. While PO oligomerization may be responsible for some loss of PO yield, the sequential oxidative cracking of PO to ethanal and CO2 represents a critical loss of PO selectivity at temperatures higher than 373 K. A similar effect is observed with decreasing weight-hourly-space velocity [4]. Both PO formation and its oxidative cracking apparently occur at the Au-Ti interface, suggesting that maximizing PO formation sites may not be the true solution to a high yield catalyst. Further
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PO oxidization must also be controlled. Higher sustained PO selectivities with increasing temperature are achieved b y depositing Au onto a welldispersed, non-crystalline TiO2 phases, either as DP Au on TS-1 [5], T-S supports, or by using certain DACS bi-metallic Au-TiO 2 clusters on SiO2 [4]. Figure 1 shows even for the DACS bimetallic catalysts a variation in selectivity is observed with Au:Ti cluster composition. The lack of extended TiO2 phase on both DACS and TS-1 catalysts help control further reactions of PO, demonstrating the importance of not only the type, but amount, of titania in contact with Au.
829
1 ~ ~
Temperature (K)
500
455
2.0
2.2
417
385
357
2.4 2.6 IO00/T (K1)
2.8
>
Q~
0
Figure 1. PO selectivity over various Au/TiO2 catalysts. Reaction conditions were C3H6/O2]H2/ He= 10/10/10/70% at 35 STP cc min ~ ( O ) Au(DP)/T-S, ( A ) Au(DP)/A, ( + ) Au(DP)/P25, ( V)(Au)/P25, ( 0 ) (Au:Ti, 4:1)/S, ( N ) ((Au:Yi, 200:1)/S, ( X ) (Au:Yi, >300:1)/S, ( O ) T/Au. Data taken from ref. [4]
4. RESULTS AND DISCUSSION 4.1 Control of PO oligomerization Oligomerization may be responsible for loss of PO yield and catalyst deactivation over many DP Au/TiO2 with bulk titania phases [5]. Figure 2 shows the C:Ti or Si atomic ratios from XPS spectra of carbon deposition over Au(DP)/T-S during steady state propylene oxidation and PO exposure. Column a) shows the normal amount of carbon deposited during six hours of propylene oxidation in O2/H2. Column b) shows that a fresh catalyst exposed at 473 K to a similar number of PO molecules as in a) results in a larger amount of carbon deposition. Bulk gold foil gave no carbon retention, but the T-S support used to create Au(DP)/T-S not exposed to the DP method, and therefore having no
1 0.9 0.8
~o.7
~cY ~cY
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~ O oO
~cY
0.4 0.3 0.2 0.1 0
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b)
c)
~cq
I
d)
Figure 2. XPS of C ls region after PO exposure over Au(DP)/T-S, the untreated T-S support, an Au foil, and (Au:Ti, 200:1)/S. Layer models were used for calculation of the atomic ratio, Nc/Nyi/si. Black bars are C ls < 289 eV; white bar C ls > 289 eV.
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surface alkali, collects about 1 carbon atom per Ti atom as shown in Column c). XPS detects that the Au(DP)/T-S catalyst in b) has about 0.1 Na atoms per surface Ti atom. It would appear that alkali promoters may block TiO2 surface sites responsible for oligomerization. The reactivity of Au(DP)/T-S, however, suggests that sites for the sequential PO oxidative cracking reactions are much less affected. Studies on the effects of surface promoters, which are known to alter the reactivity of Ag for gas phase propylene epoxidation [6,7], are just beginning for AuffiO2 materials [8]. The C:Si atomic ratio on (Au:Ti, 200:1)/S in column d), calculated assuming a similar number of Si atom as Ti atoms in the T-S monolayer, shows without an extended TiO2 phase, carbon deposition from PO is reduced.
4.2 D2 Kinetic Isotope Effect (KIE) Temperature (K) Upon the substitution of D2 for H2 in the feed, the rates to all 500 455 417 385 357 -1 products except acrolein were 10 suppressed. This is clearly shown for PO in Figure 3 for the Au(DP)/T-S "7 and (Au:Ti, 200:1)/S catalysts. The u_l 0.2 o PO rate maximum with temperature is preserved with H2 or D2, but the O 13.. differences in TOFs diminish as temperatures approach 473 K. The 10-3 observation that D2 affects the 210 212 2.'4 216 2.'8 addition rate of one oxygen atom to 1000/T (K"~) propylene forming PO is strong evidence that hydrogen from H 2 is Figure 3. Effect of D2 substitution on PO involved in the formation of the formation rate for Au(DP)/T-S and (Au:Ti, critical reactive intermediate, perhaps 200:1)/S. Reaction conditions were C3H6/02/I--I2 or through a hydroperoxy- species. In a Dz/He=10/10/10/70% at 35 STP cc min -1. related system, Laufer et al. have Au(DP)/T-S: ( O ) H2, ( 9 ) D2 (Au:Ti, 200:1)/S: concluded that the in situ generation ( D ) H2,( m ) D2. of H202 from gas phase H2 and 02 at the Pd site in liquid phase epoxidation of propylene with Pd/TS-1 is the rate determining step [9]. The lack of a similar isotope effect on acrolein production during propylene oxidation suggests that it occurs by a different route; perhaps one that starts with loss of allylic hydrogen from the propylene and subsequent oxidation by adsorbed O atoms rather than the selective peroxy- intermediate. Acrolein, however, makes up a just a small portion of the overall product distribution at these reaction conditions. Further evidence for a possible PO selective hydroperoxy-- intermediate is offered by results in our laboratory that show the Au(DP)/T-S catalyst epoxidizes ethylene in a gas phase reaction with H2/O2 at a TOF (413 K) of 0.003 s ~ with a selectivity of 84%. This parallels TS-1 which utilizes H202 in the selective epoxidation of both ethylene and propylene [10]. The different temperature sensitivity of the KIE between Au(DP)/T-S and (Au:Ti, 200:1)/S suggests that these catalysts differ by more than just the number of PO active sites. While there are small changes in apparent activation energies (Ea) with D2 substitution for reactants and most products, a large change in Ea is observed for ethanal, from 18.9+0.7 to 24.8+1.8 kcal/mol with H2 and D2 respectively, over (Au:Ti, 200:1)/S. A smaller 1.5 kcal/mole difference for ethanal is observed over Au(DP)/T-S. Since ethanal formation I
v
I
i
I
I
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appears to occur at the Au-Ti interface [4], this suggests that there are distinct differences in the energetics at the active site for different catalyst preparations. 4.3 Studies with Au(OH)3
100 DP Au/TiO2 catalysts, prepared by the deposition and 80 ~--I subsequent reduction of Au(OH)x C o 3 species at the catalyst surface, 6 0 "o {1:) generally produce very selective catalysts for PO formation [ 1,2,4,5 ]. Dehydration and reduction of Au(OH)3 occurs 0 .... ,'" 20 0 through a gold oxyhydroxide, Au(O)OH, phase [11] which has 0 0 been linked to enhanced low 0 2 4 6 8 10 temperature CO oxidation activity Time (hours) [12]. Although a direct link between the sites active for low Figure 4. Au(OH)3 reaction profiles at 35 total STP temperature CO and propylene (balance He) cc min l. Symbol Key: ( O ) PO, ( ~ ) epoxidation over Au catalysts has CO2, (0) propane, ( ..... ) temperature profile. not yet been established, we have Reaction conditions: a) 0.3 lg Au(OH)3 with 10% probed the Au(OH)3 surface for C3H6, b) 0.34g Au(OH)3 with 0% C3H6/10%O2/ POactivity. Figure 4b) shows 10%H2, c) 0.30g 1.0 wt.%-TiO2/Au(OH)3 with that the propylene/Oz/H2 mixture 10%C3H6/10%O2/10%H2. reacts with Au(OH)3 to form PO and other oxygenates (not shown) at temperatures as low as 298 K. The initial PO selectivity was about 60%. The PO formation rate decreases as the reaction mixture eventually reduces the hydroxide to metallic gold, resulting in subsequent hydrogenation activity to propane. The amount of PO produced integrated over the entire duration of the experiment results in a total turnover number (TON) of about 0.08 PO molecules produced per atom of Au in the reactor, or about 10 turnovers per Au surface atom as measured by BET. With propylene only, Figure 4a), no propylene oxidation is observed at 298 K. Slightly higher temperatures are necessary to form reaction products which are sustained for a shorter time than with 02/H 2. A slightly lower PO TON of 0.06 is observed. Figure 4c) shows that modification of the Au(OH)3 with TTIP, despite an initial PO selectivity of only 30%, maintains PO activity over the entire duration of the experiment in contrast to the non-modified Au(OH)3. The PO TON in this case is 0.1. The formation of large amounts of propanal over the modified-Au(OH)3 accounts for the primary difference in initial selectivity between Figure 4b) and 4c. Since isomerized PO is the likely source of the propanal in 4c) [4], it is reasonable to believe that a combined PO and propanal total TON of 0.2 is a more appropriate value. While the estimations of these TONs still suggest thatthe reaction with the Au(OH)3 is largely stoichiometric and not catalytic, the presence of TiO2 appears slightly enhance the reactivity before ultimate reduction of the gold. Most interesting, however, is the ability of pure Au(OH)3 to produce PO in the absence of Ti, suggesting a role for oxidized Au in the active site. XPS of model Au(OH)3 compounds and the Au(DP)/T-S catalyst shows that TiO2 may help stabilize oxidic gold states. In Figure 5a) we see the typical 4f lines expected for
~
t
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Heating the Au(OH)3 to 373 K in vacuo results in partial gold reduction and a complex mixture of states that eventually fully reduce Au ~ When a sample of TTIP-Au(OH)3 is studied in >, a similar fashion, the time required for w reduction of the Au species is extended c beyond the expected reduction time c based on pure Au(OH)3. Figure 5d) shows a sample of modified-Au(OH)3 at a heating time one hour longer than 96 94 92 90 88 86 84 82 80 the Au4f spectrum for 5c), a time at Binding Energy (eV) which complete reduction was expected. Figure 5e) shows that fresh Figure 5: Au 4f XPS of a) Au Powder, b) Au(DP)/T-S vacuum dried overnight at Au(OH)3 at 298 K, c) Au(OH)3 at 373 K, d) 393 K retains some unreduced gold TTIP-modified-Au(OH)3 wafer at 373 K, e) A species unlike catalysts were gold has 1.0 wt.%. Au(DP)/T-S. The Sicts.6 intensity been deposited by AuC13 [13]. While has been subtracted from the spectrum in e). the presence of oxidic gold species have not been detected after calcination over Au/TiO2 prepared by DP [2,4] or as precipitated Ti(OH)4 supported Au(PPhB)(NO3) [14], the selective catalysts formed by deposition of Au(OH)x species may retain as yet undetected amounts of non-metallic gold species stabilized on the catalyst surface as those observed for Au on rI-A1203 [15]. Alternatively, the oxidized gold could form the Au-O-Ti linkages that are the precursors to the active site. A
====
l
REFERENCES
M. Haruta, CataL Today, 36 (1997) 153. T. Hayashi, K. Tanaka, and M. Haruta, J. Catal. 178 (1998) 2. P. Paredes Olivera, E. M. Patrito, and H. Sellers, Surf Sci. 313 (1994) 25. E. E. Stangland, K. B. Stavens, R. P. Andres, and W. N. Delgass, J. Catal., submitted. T. A. Nijhuis, B. J. Huizinga, M. Makkee, and J. A. Moulijn, Ind. Eng. Chem. Res. 38 (1999) 884. 6. A. Gaffney, A. Jones, R. Pitchai, and A. Kahn, US Patent 5,698,719. 7. G. Lu and X. Zuo, CataL Lett. 58 (1999) 67. 8. B.S Uphade, M. Okumura, S. Tsubota, and M. Haruta, App. Catal., A 190 (2000) 43. 9. W. Laufer, R. Meiers, and W. H61derich, J. Mol. Catal., A 141 (1999) 215. 10. B. Notari, Adv. Catal. 41 (1996) 253. 11. R. J. Puddephatt, "The Chemistry of Gold", Elsevier, Amsterdam, 1978, 38. 12. R. M. Finch, N.A. Hodge, G. J. Hutchings, A. Meagher, Q. A. Pankhurst, M. R. H. Siddiqui, F. E. Wagner, and R. Whyman, Phys. Chem. Chem. Phys. 1 (1999) 485. 13. Y. -S. Su, M. -Y. Lee, and S. D. Lin, Catal. Lett. 57 (1999) 49. 14. H. Liu, A.I. Kozlov, A.P. Kozlova, T. Shido, K. Asakura, and Y. Iwasawa, J. Catal. 185 (1999) 252. 15. W.N. Delgass, M. Boudart, and G. Parravano, J. Phys. Chem. 72 (1968) 3563.
1. 2. 3. 4. 5.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Vapor-Phase Epoxidation of Propene using H2 and 02 over Au/Ti-MCM-41 and Au/Ti-MCM-48 B.S. Uphade, M. Okumura, N. Yamada, S. Tsubota and M. Haruta Osaka National Research Institute, AIST, Midorigaoka 1-8-31, Ikeda 563-8577, Japan Tel.: +81-727-51-9550, Fax: +81-727-51-9629, E-mail:
[email protected] Vapor-phase epoxidation of propene using H 2 and 0 2 o v e r homogeneously dispersed gold particles deposited by deposition-precipitation (DP) on the supports Ti-MCM-41 (hexagonal) and Ti-MCM-48 (cubic) is studied at a space velocity of 4000 h-l.cm3/g(cat.) and 100~ or 150~ Better performance of Ti-MCM-48 over Ti-MCM-41 in the propene epoxidation reaction was observed. Influence of the various parameters were investigated: in the case of Ti-MCM-48 support, Si/Ti ratio, precipitating agent for Au deposition, Au loading, calcination temperature and in the case of Au/Ti-MCM-41 the influence of CsC1 addition as a promoter. GC-MS investigation of the desorbed species from the used catalyst revealed the presence of acidic as well as oligomeric species accumulated on the catalyst surface responsible for the catalyst deactivation. Based on the above experimental results, a probable reaction mechanism is explained. 1
INTRODUCTION
2
EXPERIMENTAL
2.1
Synthesis of Ti-MCM-41 and Ti-MCM-48 Supports and Au deposition
2.2
Catalytic Activity Measurements
The existing two commercial processes for the production of PO, using either chlorohydrin or hydroperoxide, have many shortcomings [1]. Our research work on the catalysis of gold [2] has opened a new stage for the direct epoxidation of propene using hydrogen and oxygen. We have reported recently in a series of papers the vapor-phase epoxidation of propene over highly dispersed nanosize Au particles supported on TiO 2 [3, 4], TiO2/SiO 2 [3, 4], and titanosilicates such as TS-1, TS-2, Ti-~ and Ti-MCM-41 [5-7]. This finding is being followed by few other researchers [8]. We also reported very recently ~e formation of propanal from propene, H 2 and 02 at high temperatures (_> 200~ [9]. Here, we attempt to compare two different titanosilicate supports in the epoxidation of propene over Au supported on Ti-MCM-41 and Ti-MCM-48.
The mesoporous supports were prepared under hydrothermal conditions at 100~ in a static Teflon bottle for 10 days according to the literature procedures [ 10, 11] and were finally calcined at 540~ for 6h. They were characterized by XRD, UV-Vis, FT-IR, and specific surface area measurements. Finely dispersed gold was deposited on the supports by deposition-precipitation (DP) method [12] and by calcination at 300~ tbr 4h. TEM observations were made to know the Au particles size and its distribution. Actual Au and Ti contents in the catalysts were analyzed by ICP. Catalytic tests were carried out in a vertical fixed-bed quartz reactor (i.d. 10 mm) using a feed containing 10 vol% each C3H 6, H2 and 02 diluted with Ar at a space velocity of 4000 h
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~.cm3/g.cat. passed over the catalyst (0.5 g) bed. As a promoter, CsC1 was impregnated or physically or mechanically mixed. Prior to testing, the catalysts were first pretreated at 250~ for 30 min in a stream of 10 vol% H 2 in Ar, followed by 10 vol% 02 in Ar. The feed and products were analyzed by using three on-line GCs equipped with TCD (AC and porapak Q columns) and FID (HR-20M column) detectors.
3
R E S U L T S AND D I S C U S S I O N
3.1
Characterization of Supports and Supported Au Catalysts
XRD, UV-Vis, FT-IR and very high surface area (970 m2.g -1 and 1170 m2.g ~ for TiMCM-41 and Ti-MCM-48, respectively) confirmed the structural integrity and mesoporous nature of the supports [13]. UV-Vis confirmed the absence of segregated TiO 2 phase at ca. 330 nm. However, the presence of anatase TiO 2 at >__4 wt.% Ti in mesoporous supports was observed. FT-IR spectra showed the presence of sharp peak at 963 cm ~ indicative of Ti in its tetrahedral coordination. This peak increased in intensity with an increase in Ti content and disappeared for MCM-41 and MCM-48 when calcined at 1000~ ICP analysis gave an actual Au loading of about 1 wt.% for most of the samples. TEM observations showed a homogeneous dispersion of Au with an average Au particle size of 2.0 nm and 2.1 nm for Au/Ti-MCM-41 and Au/Ti-MCM-48, respectively.
3.2 Epoxidation of propene
The results of propene epoxidation at 150~ over Auffi-MCM-41 and Auffi-MCM-48 are compared in Fig. 1. For both the catalysts initial propene conversion is about 5 mol% and it decreases with time due to catalyst deactivation. Superior performance of Auffi-MCM-48 in terms of activity, PO selectivity and also the H 2 efficiency than Auffi-MCM-41 is attributed to its three-dimensional pore system which can be more resistant to blockage by extraneous materials like oligomers of PO than the one-dimensional pore system of Ti-MCM-41. Koyano and Tatsumi [ 11] also have shown in the liquid phase epoxidation of cyclododecene with H202 the higher activity of Ti-MCM-48 as compared to Ti-MCM-41. The results of the influence of Si/Ti ratio in Ti-MCM-48 on the epoxidation of propene over 8%Auffi-MCM-48 at 100~ are given in Fig. 2. Conversions of H 2 and 02 increases whereas PO selectivity decreases with an increase in Si/Ti ratio. This can be ascribed to the increase in Au loading due to higher Ti content. The propene conversion is maximum at Si/Ti ratio of 50. Further increase in Si/Ti ratio causes a decrease in propene conversion, which is most probably due to the presence of polymeric anatase TiO 2 phase as observed in UV-Vis and TEM observations. We have also found that the best results in the propene epoxidation are obtained when NaOH is used as a precipitating agent for depositing gold on the supports. The results are presented in Fig. 3. One probable reason for this could be uniform and high dispersion of Au nanoparticles coupled with traces of alkali content. More basic nature of Na than Cs may also be the other reason for higher PO selectivity. The results of the influence of Au loading in the catalyst, Au/Ti-MCM-48 (Si/Ti - 50), on the epoxidation of propene at 150~ are given in Fig. 4. With an increase in Au loading propene, H 2 and 02 conversions increase whereas PO selectivity decreases. Propene conversion almost levels off at the 16% Au loading in the solution. At the Au loading below 2% in solution formation of propane was observed. We have reported earlier [4] that at the lower gold loading of < 0.2 wt.% propane formation takes place over AuffiO 2 due to the formation of gold particles smaller than 2 nm in diameter. The results of the catalyst calcination temperature on the epoxidation activity are presented in Table 1. Propene conversion passes through maximum for the catalyst calcined at 300~ The increase in PO selectivity with an increase in catalyst calcination temperature is expected due to the stronger contact between the support, Ti-MCM-48, and the gold particles.
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835 -
.
-
.
-
.
-
_
.
-
_
.
.
.
.
.
.
~'92
~ 86 ~, 60,~so
H2 02
s.o -9
4.5
E 4.0
8 3.5 3.0 2.5
20
40 60 80 Time (min)
100 120
Fig. 1 : Influence of support (Ti-MCM -41 vs Ti-MCM-48) in the epoxidation of propene [Au in solution: 12wt.%, Temp. 150~
30
40 50 60 70 80 sifri ratio in support
90
100
Fig.2 : Influenece of Si/Ti ration in Ti-MCM-48 on the epoxidation of propene over 8wt.%Au/Ti-MCM-48 at I(X)~
The surprising fact is that even the catalyst calcined at 150~ also produce PO with a propene conversion of about 1.7%. At such a low calcination temperature the reduction of Au(OH~3 into its metallic state is highly unlikely. Our attempt to detect the presence of Au + and Au " species for the catalyst calcined at 150~ were unsuccessful due to highly dispersed Au particles mostly inside the channels eventhough the Au loading was very high. The results in the epoxidation of propene, however, may indicate that the reduction of Au(OH) 3 can take place easily during pretreatment and/or reaction. Table 1: Influence of catalyst calcination temperature on the epoxidation of propene [catalyst: 16 wt.%Auffi-MCM-48 (Si/Ti = 50), Reaction Temp.: 150~ Temp. (~ for Conversion (%) off Selectivity (%) for calcination a pretreatment b C3H 6 Ha 02 PO CO2 150/4h 150/4h 1.73 27.3 200/4h 200/4h 2.86 26.9 300/4h 250/4h 3.03 21.8 400/4h 250/4h 1.87 17.3 500/4h 250/4h 1.50 15.5 ain air, Din 10vol% 02 in Ar, Cafter 20 min of reaction
19.5 15.2 13.5 11.6 10.0
89 92 92 93 94
8.0 6.0 5.6 5.0 4.8
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95 90
9
""
50
~
45 g~
~"
SO "~
40
90 60
~. 40
~
20
~" 6.0
4.5
4.0
3.$;
~
3.11
~
2.o
LiOH NaOH KOH RbOH CsOH Precipitating agent
5
10
15
20
25
Au (wt. %) in solution
Fig.3 :Influence of precipitationg agent during Au deposition on TiMCM-48(16wt. % Au in solution, Si/Ti=50) on the epoxidation of propene at 150~
Fig.4 :Influence of Au loading in the catalyst Au/Ti-MCM-48 (Si/Ti=50) on the epoxidation of propene at 150~
3.3
Drastic Depression in H z Consumption An important parameter for the feasibility of application o f these catalysts in the epoxidation of propene is the efficiency for H 2 and 02. At present though the propene conversion is around 3-4%, the conversions of H~ and O~ are very high, usually above 50%. The addition of CsC1 successfully lowers the consumption of H~_ and 02 by about 90%, while maintaining the propene conversion above 1.7%. Among the methods for adding the promoter, the best way is to just mix physically (simple mixing) the supported gold catalyst with a finely crushed CsC1 powder. The optimum loading of CsC1 as a promoter is found to be about 1.0 wt.% (Fig. 5). An appreciable increase in the size of Au particles was noticed by TEM analysis when CsC1 content was more than 1.0 wt.%. Therefore, even in the case of physical mixing, the Au particles tend to agglomerate in the presence of C1- anions. It was observed in H 2 oxidation experiments that the Au/Ti-MCM-41 with CsC1 require 350~ for quantitative H 2 oxidation as compared to 150~ Auffi-MCM-41 alone (Fig. 6). The results, therefore, indicate that the presence of CsC1 markedly depressed the reaction of H 2 with 0 2, while giving little influence on the reaction of propene with selective oxygen species. _
3.4 Catalysts deactivation and their regeneration
Another major problem associated with this system is the fast catalyst deactivation. The experimental results of the preadsorbed PO or propene on the catalysts before the start of the reaction suggests that the catalyst deactivation occurs mainly due to the adsorption of PO but not of propene. GC-MS study indicated the presence of large number of organic compounds adsorbed on the catalyst surface. These are: acetic acid, propanoic acid, 1acetoxy-2-propanol, 2-acetoxy-l-propanol, 1-hydroxy-2-propanone, propylene glycol, propylene carbonate, 2,5-dimethyl-l,4-dioxane, etc. We believe that these species are formed due to initial PO adsorption on the catalyst surface and its further oligomerization,
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rearrangement, cracking, coupling etc. on the free titanium sites. Acidic species such as acetic acid, propanoic acid may also contribute to the cause of catalyst deactivation. Our attempts to regenerate the deactivated catalysts by thermal treatment at > 250~ in the oxygen stream and also by dissolving organic moieties from the catalyst surface in organic solvent such as ethanol and acetone failed to achieve original activity of the catalysts.
100 -96
94
|
O ="
80
92
60 e~
o
1.o
40
20
g
e~ 10 0
o~
.i.
0
2
4 6 CsCI (wt.%)
8
10
0
50 1 0 0 1 5 0 200 250 300 350 Temperature (~
Fig.6 : Oxidation of l vol% H 2 in air over a) Au/Ti-MCM-41 and b) lwt.% CsC1 + Au/Ti-MCM-41 (a physical mixture) catalysts
Fig.5 :Influence of promoter CsC1 content physically mixed with Au/TiMCM-41 in the propene epoxidation at I(X)~ 3.5
Probable Reaction M e c h a n i s m Based on the theoretical prediction by Olivera et.al. [14] that the surface of gold is capable of generating H202 in-situ, we have proposed the following reaction mechanism which is different from that we proposed for Au/TiO 2 [4].
C3H6 + [S]
~
~-- [S]- - .C3JH6 S: the surface of Au and [Ti4+-SiO2] H2 + 02
H202 + [Ti4+-SiO2] [Ti4+-SiO2]- - -C3H6
[Aul
~ H202
~- HOO- - -[-Ti4+-SiO2] + HOO- - -[-Ti4+-SiO2] H3C---~"--CH2 ",O /
+ H20 + 2[Ti4+-SiO2]
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CONCLUSIONS 1 Superior performance of Au/Ti-MCM-48 than Au/Ti-MCM-41 in the vapor-phase epoxidation of propene using H 2 and 02 is attributed to its three-dimensional (cubic) pore system which can be more resistant to blockage by extrarieous materials like oligomers of PO and bulkier organic compounds as evidenced in GC-MS study than the unidimensional pore system of hexagonal Ti-MCM-41. 2 Physical mixture of CsC1 and Auffi-MCM-41 brings about 90% decrease in H~ consumption; however, the presence of c r anions also causes agglomeration of gold particles thereby also reducing propene conversion to certain extent. 3 Catalyst deactivation takes place not only due to the large number of organic compounds on the catalyst surface but also due to acidic compounds such as acetic acid.
REFERENCES 1. S.J. Ainsworth, Chem. Eng. News. (1992) 9 2. M. Haruta, Catalysis Surveys of Japan, 1 (1997) 61; Catal. Today, 36 (1997) 153 3. T. Hayashi, K. Tanaka and M. Haruta, Shokubai, 37 (1995) 72 4. T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 178 (1998) 566 5. Y.A. Kalvachev, et.al., Stud. Surf. Sci. Catal., 110 (1997) 965 6. M. Haruta, B.S. Uphade, S. Tsubota and A. Miyamoto, Res. Chem. Internaed., 24 (1998) 329 7. B.S. Uphade, M. Okumura, S. Tsubota and M. Haruta, Appl. Catal. A: Gen. (in print) 8. T.A. Nijhuis, et.al., Ind. Eng. Chem. Res., 38 (1999) 884 9. B.S. Uphade, S. Tsubota, T. Hayashi and M. Haruta, Chem. Lett., (1998) 1273 10. K. Watanabe, Y. Onoda, H. Tsuneki and T. Tatsumi, Jpn. Patent App. H07-300312 (1997), to Nippon Shokubai Co. Ltd. 11. K.A. Koyano and T. Tatsumi, Stud. Surf. Sci. Catal., 105 (1997) 93 12. M. Haruta, et.al., J. Catal., 144 (1993) 175 13. A. Corma, N.T. Navarro and J. Perez-Pariente, J. Chem. Commun., (1994) 147 14. P.P. Olivera, E.M. Patrito and H. Sellers, Surf. Sci., 313 (1994) 25
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Ring-opening reactions of ethyl- and vinyloxirane on HZSM-5 and CuZSM-5 catalysts A. FfisP, I. Pfilink6 b and I. Kiricsi c* aChemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, Pusztaszeri t~t 59-67, H-1025 Hungary bDepartment of Organic Chemistry, J6zsef Attila University, D6m t6r 8, Szeged, H-6720 Hungary CDepartment of Applied and Environmental Chemistry, J6zsef Attila University, Rerrich B. t6r 1, Szeged, H-6720 Hungary
Transformations of ethyloxirane, vinyloxirane and their mixture were studied over HZSM-5 and CuZSM-5 zeolites in the presence of hydrogen at 363 K. Ethyloxirane underwent single ring-opening and deoxygenation, while ring expansion was the main transformation pathway for vinyloxirane. Reactions of the mixture revealed that the active sites for adsorbed oxygen mediated ring transformations can be separated on HZSM-5 from those, where the adsorption of the vinyl group and the ring oxygen is a requirement. This cannot be done on CuZSM-5.
1. INTRODUCTION Epoxides, due to their strained three-membered ring, can undergo easy ring opening, making these compounds versatile intermediates e n r o u t e of preparing more complicated organic molecules. The ring may be opened both stoichiometrically and catalytically. Catalytic ring opening may be performed by a nucleophile in the presence of a Lewis acid activator [3], on solid acids [4] and on supported metals in the presence of added hydrogen as well [5]. The electrophilic catalytic way of ring opening may involve solid acids of various kinds. These acids may contain Bronsted or Lewis acid centres or their combination. Both types of sites may be involved in the reactions, however, evaluating their specific roles may not be an easy exercise. Zeolites can be used as effective solid acid catalysts for promoting ring opening and the same parent material can be transformed to a form containing appreciable amount of Bronsted sites or overwhelmingly Lewis acid centres, therefore their role may be studied more or less separately. In this contribution the ring-opening reactions of ethyloxirane and vinyloxirane are described on HZSM-5 prepared by wet ion-exchange or CuZSM-5 zeolites prepared by ion exchange in the solid state.
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2. EXPERIMENTAL
Ethyloxirane and vinyloxirane were commercial products (Fluka) and were used as received, except for some freeze-evacuation-thaw cycles before reactions. The parent zeolite was NaZSM-5. The H-form was prepared via NH4ZSM-5 by de-ammonization at 873 K for 6 hours in vacuum. CuZSM-5 was made from HZSM-5 using the solid-state ion exchange method [6]. Certain amount (5 mol %) CuC12 was intimately mixed with well-powdered HZSM-5 in an agate mortar. The mechanical mixture was heat-treated at 873 K for 8 hours in air. The product was cooled to ambient temperature and washed free of chloride, then dried at 373 K. The zeolites were characterized by X-ray diffractometry and BET measurements and the metal content was determined by X-ray fluorescence spectroscopy. X-ray diffractograms were registered on well-powdered samples with a DRON 3 diffractometer in order to check crystallinity. BET measurements were performed in a conventional volumetric adsorption apparatus at the temperature of liquid N2 (77.4 K). Prior to measurements the samples were pretreated in vacuum at 573 K for 1 hour. The ratio of Bronsted to Lewis acid sites was determined by pyridine adsorption followed by FT-IR spectroscopy. Self-supported wafers were pressed and degassed in situ in the optical cell at 573 K for 1 hour. Then, they were cooled to 473 K and pyridine was loaded. The wafers were kept in pyridine for 1 hour followed by evacuation at the same temperature. For determining crystallinity from IR measurements the ratio of bands in the range of 440-480 crn-1 and 550-650 cm-~ was used [7]. For samples of good crystallinity the value should be around 0.72. FT-IR measurements were performed with a Matson Genesis spectrometer and 128 scans for collected for one spectrum. Characteristic data on the catalysts collected by the methods described above are displayed in Table 1. The reactions were run a static closed circulation reactor in the presence of hydrogen to slow down deactivation. A mixture of 1.33 kPa of the respective oxiranes and 20 kPa of H2 was prepared and allowed to react on 10 mg of the dehydrated zeolite (1-hour evacuation at 573 K). The reaction temperature was 363 K. In a series of runs the mixture of ethyl and vinyloxiranes (0.67 kPa of each) was allowed to react in the presence of 20 kPa H2 over the usual 10 mg zeolite at 363 K. Analysis was performed by the GC-MS method (Hewlett Packard (HP) 5890 gas chromatograph equipped with a HP 5970 quadrupole mass selective reactor; 50-m long HP-1 capillary column, 523 K and 423 K as the temperature of the injector and the oven, respectively) on samples withdrawn at certain time intervals. Table 1 Characteristic data on the catalysts HZSM-5 a
CuZSM-5 a
-
2.7
BET surface area/m2g -1
336
318
Crystallinityb/%
89
83/79
Bronsted/Lewis c
0.88
0.09
100
46
Characteristics C u 2+
content/weight %
Degree of exchange/%
a - Si/AI= 13.8 b - by XRD/IR c - by pyridine adsorption
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3. RESULTS 3.1. Reactions of ethyloxirane Ethyloxirane underwent ring opening on both catalysts either by single or double C-O scission (Scheme 1).
~/
~~/CHO 2
0
0 1
1 Scheme 1. Transformations of ethyloxirane on HZSM-5 and CuZSM-5 On the whole, CuZSM-5 was more active than HZSM-5. Double C-O cleavage, which means deoxygenation, was an important transformation pathway on both catalysts, but more significant on CuZSM-5 than on HZSM-5. Out of the two ways of single C-O rupture, the one leading to butanal formation was predominant on both catalysts, however, 2-butanone was also found. Selectivity towards butanal formation was higher on CuZSM-5 than over HZSM-5, nevertheless, on the absolute scale more 2-butanone was formed on CuZSM-5 than on HZSM5. For details, see Table 2. Table 2 Product distribution (mol %) in the reactions of ethyloxirane on HZSM-5 and CuZSM-5 zeolites (10 ms zeolite, 363 K, 20 kPa Hz) Comp.
HZSM-5
CuZSM-5
0 min
5 min
15 min
25 min
5 min
15 min
25 min
1
100
96.5
93.8
92.1
95.2
89.1
85.3
2
0
1.7
3.7
5.0
2.2
5.7
7.1
3
0
0.5
0.8
1.0
0.7
1.2
1.5
4
0
1.3
1.7
1.9
1.9
3.0
6.1
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3.2. Reactions
of vinyloxirane
More complicated, but in a way more selective reactions took place with vinyloxirane (Scheme 2 ). CHO
\__/
~ C H O
/
6
7
0 5
O Scheme 2. Reactions of vinyloxirane on HZSM-5 and CuZSM-5 zeolites As far as the overall ring opening is concerned, the HZSM-5 catalyst was more active than CuZSM-5. Single C-O cleavage was the only transformation route on HZSM-5, while double C-O bond rupture was also observed on CuZSM-5. It led to 1,3-butadiene. Single C-O scission provided with 2,5-dihydrofurane, trans-2-butenal and cis-2-butenal in this order of decreasing significance on both catalysts. Furane formation was faster on HZSM-5 than on CuZSM-5, while the other two products formed with similar rates and relative selectivities. For details see Table 3. Table 3 Product distribution (mol %) in the reactions of vinyloxirane on HZSM-5 and CuZSM-5 zeolites (10 mg zeolite, 363 K, 20 kPa Hz) Comp.
HZSM-5
CuZSM-5
0 min
5 min
15 min
25 min
5 min
15 min
25 min
5
100
49.2
20.3
13.2
54.2
35.1
25.7
6
0
2.1
10.8
11.9
10.9
14.4
14.7
7
0
2.0
2.9
3.5
2.1
2.9
3.4
8
0
46.7
66.0
71.4
32.7
47.5
56.0
9
0
-
-
-
0.1
0.1
0.2
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3.3. Reactions of ethyloxirane - vinyloxirane mixture
When a 1-1 mixture of the oxiranes was allowed to react, new product was not formed compared with the reactions of the neat compounds on any of the catalysts. The relative rate of transformations on HZSM-5 was not altered either. On CuZSM-5, however, the activities and selectivities of product formation were modified (Table 4). Table 4 Relative concentrations at various sampling times in a 1"1 mixture of ethyloxirane and vinyloxirane to the neat ethyloxirane or vinyloxirane on CuZSM-5 in the presence of hydrogen (20 kPa H2 at 363 K; data and the notations for the oxiranes are in italics) t/min
c(mixture)/c(neat) 1
2
3
4
5
6
7
8
9
5
0.9
0.9
0.8
0.7
1.1
0.4
0.8
1.3
1.2
15
0.6
0.7
0.7
0.6
1.1
0.5
1.1
1.3
1.2
25
0.6
0.3
0.7
0.4
1.1
0.6
1.1
1.3
1.2
Ethyloxirane transformed slower in the mixture than alone, while vinyloxirane reacted somewhat faster. Each product from ethyloxirane in the mixture was formed slower than in the neat form, while for vinyloxirane in the mixture the picture is mixed. Ring expansion accelerated in the greatest extent, while the proportion of the single ring opening route became smaller. Within this reaction pathway the formation of trans 2-butenal decelerated the most.
4. DISCUSSION Products from the ring opening of vinyloxirane are formed by the scission of the sterically more hindered C-O bond (or with double C-O rupture when 1,3-butadiene, a reaction pathway only existing on CuZSM-5, but even there of minor importance). This means more selective transformations than those of ethyloxirane. However, ethyloxirane underwent simpler single C-O scission reactions, since there were no secondary reactions after isomerization. In vinyloxirane two functionalities, the olefinic double bond and the C-O bond in the highly strained ring are combined. Both can be attacked by acid centres. Therefore, parallel with ring opening double bond migration also takes place. Beside the formation of internal olefinic bond either cis-trans isomerization or, as the more important transformation pathway, ring enlargement occurs. Each reaction occurring via single C-O scission with vinyloxirane and the one producing butanal from ethyloxirane can be considered typical acid-catalyzed transformations. The intermediates are the most stable carbenium ions the system allows, secondary carbenium ions, those are. Interpreting 2-butanone formation is more problematic. Since the reactions were run under hydrogen atmosphere to slow down deactivation, certain copper ions may have reached metallic state. In HZSM-5 similar things might have happened with metal ion impurities present in minute amounts. These centres transiently being in zero oxidation state may be responsible
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for 2-butanone formation. Comparison of products and their distribution on HZSM-5 and CuZSM-5 catalysts reveal, that both types of acid centres catalyze single as well as double C-O scission. Lewis acid sites alone, however, seem to be less active in the ring expansion reaction leading to 2,5-dihydrofurane than the mixture of Bronsted and Lewis sites (Scheme 3). @
@
(~
(~0"" s'o
///
ili
O-/~1--=-
,
o
/// "-
e
~
~
, ,
,
,~ eili
, eH
.
e
,
~
e
~
Ill
Scheme 3. The formation mechanism of the ring expansion product on a combination of Bronsted and Lewis sites Experimental results indicate that the active sites necessary for the reactions of the two oxiranes can be separated when HZSM-5 is applied. The mixture of the oxiranes reacts as if in their neat forms. Since this catalyst contains about equal amounts of Bronsted and Lewis sites, because of the availability of two nearly equally good choices, the molecules do not disturb each other. When practically only Lewis sites are available (either in the form of Cu § ions due to partial reduction by, e.g., hydrogen, or Cu 2§ ions due to the oxidizing ability of either oxirane when they are adsorbed through their ring oxygen, or extraframework AIO+), the adsorption of vinyloxirane suppresses somewhat the adsorption of vinyloxirane, accelerating especially the route leading to the ring expansion product. Conclusions
Although HZSM-5 and CuZSM-5 zeolites showed similar performance in the reactions of ethyloxirane and vinyloxirane in their neat forms, the reactiviy and selectivity of transformations of a 1:1 ethyloxirane-vinyloxirane mixture revealed that there was no competition between the two oxiranes when Bronsted and Lewis sites are equally available, however, when Lewis sites predominate, the active sites overlap, thus competition between the two oxiranes occurs. REFERENCES 1. W.F. H61derich, in "Catalytic Science and Tehnology", (S. Yoshida, N. Takezawa, and T. Ono, eds.), Kodansha/VCH, Weinheim/New York/Cambridge/Basel, 1991, Vol. 1, p. 31. 2. B. T6rSk, Gy. Sz6116si, M. R6zsa-Tarj~ni, M. Bart6k, Mol. Crys. Liq. Crys., 311 (1998) 289. 3. S.E. Denmark, P.A. Barsanti, K.-T. Wong, R.A. Stavenger, J. Catal., 63 (1998) 2428. 4. M. Bart6k, in "The Chemistry of Functional Groups." Supplement E2" The Chemistry of Hydroxyl, Ether and Peroxide Groups: (S. Patai, Ed.), Chap. 15, p.843. Wiley, Chichester/ Brisbane/New York/Toronto/Singapore, 1993. 5. A. F~isi, I. P~link6, J. Catal., 181 (1999) 28. 6. H.G. Karge, H.K. Beyer, Stud. Surf. Sci. Catal., 69 (1991) 43. 7. G. Coudurier, J.C. Naccache, J.C. Vedrine, J. Chem. Soc., Chem. Commun., (1982) 1413.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Controlling the Distribution of Framework Aluminum in High-Silica Zeolites D. F. Shantz, a R.F. Lobo, a C. Fild, b H. Koller b a Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, U.S.A. b Institut for Physikalische Chemie, Westf~lische Wilhelms Universit~it MOnster, Schlosspaltz 7, D-48149 MOnster, Germany
Using CP/MAS, REDOR, and heteronuclear correlation NMR spectroscopy we have investigated the geometrical relationships between the positive charge of organic structuredirecting agents and the negative charge in the framework of zeolite ZSM-12. In samples of all-silica and aluminum containing ZSM-12 prepared using benzyltrimethyammonium cations as structure-directing agents (SDA), we have found that the positively charged segment of the SDA is preferentially ordered near the negative framework charge. This result implies that the distribution of A1 sites in the zeolite is distinct from the random distribution often assumed for high-silica zeolites. Our results suggest that the distribution of acid sites can be controlled using SDAs with different charge distributions. 1. INTRODUCTION Control of the distribution of acid sites in microporous materials is an important objective in the synthesis of more selective catalysts as well as in the preparation of model catalysts. Controlling the average concentration of acid sites in zeolites (i.e., the Si/A1 ratio) by synthetic or postsynthetic methods (or both) can be done in a straightforward manner. However, obtaining materials with identical concentrations of catalytic sites, while at the same time containing a different spatial distribution of sites is still a synthetic objective that has not been demonstrated. The catalytic properties of zeolite samples with, e.g., individual, isolated aluminum sites could be contrasted to other samples with 'pairs' of adjacent acid sites. In this way, materials with similar Si/A1 ratios could be used to determine if postulated mechanisms of catalytic reactions (like relative rate of cracking vs. hydride transfer) depend not only on acid strength but also on acid site distribution. How could we systematically control the distribution of acid sites? One path towards achieving this goal was recently suggested by an investigation of the clathrate nonasil[ 1]. We have shown that small trimethylalkylammonium molecules, molecules that are not hindered by the cage to rotate, are permanently 'attached' to the negative charge of the cage walls up to at least 100 ~ In cases where the conformation of the organic structure-directing agent is
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fixed because of steric constraints, the location of the negative charge should be severely limited by the geometry of the organic molecule. The objective of this work is to determine if the location of negative charges in the framework is controlled by the location of the positive charge of the structure-directing agent. To answer this question we have used samples of the one-dimensional large-pore zeolite ZSM-12 (MTW, see Fig. 1) in its all-silica form and in its Al-containing form (with an A1/R+-I where R += benzyltrimethylammonium). This latter organic molecule has the advantage that it can be selectively deuterated in different locations (either on the methyl, phenyl, or methylene groups or combinations thereof). Subsequently, the dipolar coupling between different hydrogen nuclei (1H) and different nuclei (298i, 27A1) in the zeolite framework can be studied to estimate relative distances. From these data we can infer the local geometry of the organic molecule. We will use cross-polarization (CP) and rotational echo double resonance (REDOR) to investigate our samples. The strong spatial dependence on the rate of CP (~l/r 6) makes CP/MAS spectroscopy a useful qualitative tool for studying solids. REDOR is in contrast a difference experiment that gives rise to a curve for each signal of the 1D NMR spectrum. The stronger the dipolar interaction between the coupled nuclei (i.e., the closer they are to each other), the steeper the initial slope of the REDOR curve. The interpretation of CP/MAS and REDOR experiments depends on the mobility of the nuclei of interest, so we have also performed 2H NMR experiments on the samples to quantify the relative mobility of the different segments of the SDA. 2. EXPERIMENTAL The samples of ZSM-12 are prepared as reported elsewhere[2,3]. Benzyltrimethylammonium chloride is prepared by the reaction of trimethylamine with benzyl chloride. The selectively deuterated molecules are prepared from suitably labeled precursors (d9trimethylamine and/or d2, dT, ds-benzyl chloride). For A1-ZSM-12, the chemical analysis gives a Si/Al=28, consistent with one framework aluminum per structure-directing agent.
1-13
// Fig. 1. Projection of the structure of the zeolite ZSM-12 (left) and the benzyltrimethylammonium SDA.
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1H MAS spectra were acquired using a Bruker MSL 300 spectrometer, a spinning rate of 10 kHz, 5 s recycle time and a 5/2s 90 ~ pulse. 2H static and MAS spectra were acquired using the quadrupole echo pulse sequence. 29Si-{1H} CP/MAS spectra are recorded using a 6 ,as 1H 90 ~ pulse, a spinning rate of 3 kHz, 5 s recycle delay and high-power decoupling. 29Si-{1H} CP-REDOR experiments are performed using a 5 s recycle delay, a 6 ,as 1H 90 ~ pulse length and a contact time of 10 ms. The spinning rate was 3 kHz with a 32 step phase cycling routine. More experimental details can be found in the references[2,3]. 3. RESULTS AND DISCUSSION
3.1. Mobility of the Structure-Directing Agent 2H NMR of d9-ZSM-12 shows the methyl groups have a quadrupole coupling constant of 15.9 kHz, consistent with methyl groups that undergo two rapid rotations; one about the methyl C3 axis and one about the nitrogen C3 axis. An additional smaller narrow signal is observed indicating the presence of molecules with isotropic motion. The cause of this small component is not known at this time and continues to be investigated; but it is not due to decomposition of the SDA or chemical exchange during the synthesis process. 2H NMR shows the d2- and ds- samples have quadrupole coupling constants of 110 and 100 kHz, which are consistent with essentially immobile deuterons. The small reduction of the QCCs from the static values are due to small angle 'wobbling' motions because the organic groups do not fit tightly in the pores of ZSM-12. 3.2. Siliceous ZSM-12 Figure 2 shows the ~H MAS spectra of all-silica ZSM-12 and A1-ZSM-12. The major difference is the presence of a line at 10.2 ppm that has been assigned to silanol groups that are strongly hydrogen bonded to siloxy groups (SiO--..HO-Si). Also, there is a line at 7 ppm
~
.
.
.
.
.
.
.
.
.
,
20
.
.
.
.
.
.
.
.
.
,
10
.
.
.
.
.
.
.
.
.
,
.
.
0 d/1H (ppm)
.
.
.
.
.
.
.
.
.
-10
.
.
.
.
.
.
.
.
,
-20
Fig. 2.1H MAS NMR of as-made A1-ZSM-12 (top) and all-silica ZSM-12 (bottom).
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848
due to the aromatic protons, and the resonances between 4.5 and 3 ppm are due to the aliphatic protons. 29Si MAS NMR of several as-made all-silica zeolites has shown the positive charge of the structure-director is balanced by a siloxy group coordinated to two or three silanol groups[4]. Heteronuclear correlation spectra of selectively deuterated samples of ZSM-12 made with a different SDA as well as samples of nonasil[2] show that the methyl groups of the structure-directing agent are in close proximity to the defect site composed of one siloxy group and three silanol groups. We have recently confirmed several aspects of this defect model using multiple quantum NMR. The results obtained with a sample prepared with a perdeuterated SDA show that indeed the defects consist of one siloxy group and three silanol groups. Double and triple quantum NMR experiments performed are consistent with a model where three silanol protons are engaged in strong hyrdogen bonding to the charge compensating siloxy group. These three hydrogen bonded silanol protons are responsible for the resonance observed at 10 ppm. Also, the double quantum NMR experiments performed show that the methyl and methylene protons of the SDA are in close proximity to the defect site, consistent with heteronuclear correlation spectroscopy results for several as-made all-silica zeolites[2]. 3.2. AlUminum-containing ZSM-12 As observed in Fig. 2, our samples of A1-ZSM-12 contain no defects within the detection limits of NMR since there is no signal intensity at 10 ppm. Single pulse 27A1 MAS NMR reveals that all the A1 atoms are in tetrahedral conformation: only one sharp and slightly asymmetric signal is observed at 56 ppm. We have obtained 27A1 - {IH} REDOR curves for samples of A1-ZSM-12 prepared with do, d7, d9, and d14-SDAs. Not surprisingly, the initial slope of the REDOR curve scales with the number of protons present in the SDA. That is, no structural information can be obtained from these experiments. Figure 3 shows the 29Si-{1H} CP/MAS spectra obtained with a 400/ts contact time for
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-60
-70
-80
-90
-100 -110 6' 29Si / (ppm)
-120
-130
-140
Fig. 3 29Si- {1H} CPMAS NMR of (a) d7-, (b) dg-, and (c) d14- AI-ZSM-12
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samples prepared with dT, d9 and dI4-SDAs. For the dI4-ZSM-12, we see a clear difference between the relative intensity of the Si(1A1) signal and the Si(0A1) signal as compared to the other samples. The Si(1A1) signal is substantially enhanced when only the methylene protons are not labeled (Si(1A1)/Si(0A1)=0.98 as compared to the one-pulse 29Si MAS spectrum value of 0.17). We conclude that the methylene protons are preferentially ordered near silicon atoms adjacent to the framework aluminum. We have obtained 29Si- {1H} CP/REDOR curves (see Figure 4) for the same set of materials and the results are fully consistent with the interpretation obtained from the 29Si-{1H} CP/MAS experiments[3]. The CP/MAS and CP-REDOR results both show that there are preferential dipolar interactions between the methylene protons of the SDA and the Si(1A1) silicons which are adjacent to the framework aluminum. Because the dipolar interaction is related to the internuclear distance, the results show that the methylene protons of the SDA are preferentially ordered with respect to the Si(1A1) silicons. The preferential ordering observed is consistent with coulombic forces between the cationic SDA and the framework aluminum. These results, therefore, indicate that there is a direct spatial association between the charge center of the SDA and the framework aluminum atoms. While the SDAs are not ordered in the crystallographic sense, for one-dimensional materials they are packed in close contact down the length of the pore. We believe these results demonstrate that is possible to exhibit some control over the distribution of catalytic sites in high-silica zeolites based on the synthesis conditions. Further, we believe that a combination of altering the charge distribution of the SDA and a basic knowledge of the SDA's organization in the zeolite micropores should be a useful starting point for a more rational approach to altering the distribution of framework heteroatoms via synthesis. In the context of zeolite synthesis, this is another positive step towards the goal of tailoring the physical properties of microporous
0.8
0.6
9
A
r./3
( 0 ) a + N2
(1)
The n u m b e r of loaded oxygen atoms was determined from the amount of N2 evolved, as well as from the isotopic exchange with 1802, and found to be 2.0 2.2• m oxygen atoms per gram of catalyst in all cases. After a-oxygen loading the microreactor was isolated from the rest of the reaction volume and cooled to room temperature. Gas in the reaction volume was replaced with organic vapor which contacted with the catalyst sample for 10-15 min. The excess organic starting material was removed by evacuation, the sample was then taken out of the reactor and extracted with 2 ml of aqueous solution of acetonitrile (CH3CN : H20 = 1 : 1). The composition of the extract was determined using GC and GC/MS methods. The overall reaction is selective hydroxylation described by eq. (2). R-H + N20
-~ R-OH +N2
(2)
Additional information on the experimental details can be found in references [7, 8].
3. RESULTS AND DISCUSSION In our previous w o r k [7, 8] a similar technique has been used for detailed studies of the reaction of benzene and methane with a-oxygen at room temperature. This interaction has been shown to yield selectively phenol and methanol, respectively. The amounts of detected products within experimental error matched the amounts of reacted starting materials. Accurate quantitative t r e a t m e n t of the results of such studies is quite challenging. Zeolites are known to strongly adsorb both the reaction products and the starting materials. The main goal of this work was to expand the scope of compounds that can react with a-oxygen, which inevitably led us to focus on the qualitative rather than quantitative information. Thus, we did not optimize the extraction procedure for every case. The extracted products accounted for 515 ~mole/g of catalyst, which enabled us to reliably identify them and determine
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their relative ratios. However, taking into account the possibility of incomplete extraction for different products, the ratios of the products reported below should be treated as preliminary results, that will be refined in our more detailed studies. 3.1 Oxidation of alkanes and alkenes
In view of our earlier results with methane [7], it was no surprise that oxidation of ethane yielded ethanol as the only product. When more complex alkanes are subjected to interaction with a-oxygen, secondary alcohol formation is predominant over the hydroxylation of the primary C-H bond. Thus, when propane is reacted with a-oxygen, 1-propanol and 2-propanol are formed m 1:2 ratio. When the reaction was performed with n-hexane only secondary positions of the molecule have been affected, giving an approximately equimolar mixture of 2-hexanol and 3-hexanol (eq. (3)). In most cases, the secondary alcohols can be oxidized further and the corresponding ketones were also observed, usually in small quantifies. OH
a-oxygen
:
1
OH
(3)
In the case of branched 2-methylhexane, the hydroxylation exhibits high sensitivity to steric hindrances: hydroxylation affects neither the tertiary C-H bond, nor the secondary C-H bond next to the branching. Hydroxylation attack on position 7 to branching is much easier than that on the ~-position (eq. (4)). OH or-oxygen
1
:
4
(4)
Similar observations were made for cydoalkanes. Cyclohexane gives cydohexahol exclusively. In the case of methylcydohexane, having a methyl substituent on the cyclohexane ring led to almost complete blocking of positions 2 and 3 of the ring leaving position 4 as the only available for attack on the ring (2and 3-methylcydohexanols are formed in trace quantities). Cydohexanemethanol is another major product, the formation of which is probably also determined by specific steric environment of methylcyclohexane in the zeolite micropore space (eq. (5)).
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878
?CH3
ct-oxygcn HOX
?CH3
?CH2OH
3 : 1 (5) The reaction of a-oxygen with olefins was studied using cyclohexene as the starting material. Allylic oxidation was found to be the predominant process with 2-cyclohexene-l-ol as the major product (along with minor amounts of the corresponding a,~-unsaturated ketone- 2-cyclohexene-l-one, the formation of which can be explained by enhanced reactivity of allylic alcohol towards oxidation). 3-Cyclohexene-l-ol was also detected; the ratio of the products of allylic and homoallylic attack being ca 4:1 (eq. (6)). OH
O
0
OH
(~-oxygen
4
:
1
(6)
3.2 Oxidation of aromatics
As has been shown previously [8], the interaction of benzene with ~oxygen resulted in selective hydroxy]ation of the aromatic nucleus. The competition between the hydroxy]ation of aIiphatic and aromatic carbons as we]]
as the balance of electronic and steric factors influencing these transformations was studied using alkylaromatic compounds as starting materials. In all cases, hydroxylation of the aromatic nucleus was observed to be much more facile than hydroxylation of the side chain. Interaction of toluene with a-oxygen leads to both the products of benzylic and aromatic hydroxylation: benzyl alcohol and cresols (o:m:p = 1:1:2.5) (eq. (7)). CH2OH a-oxygen + 1
:
H 2.5
(7)
The oxidation of ethylbenzene (eq. (8)) and isopropylbenzene (eq. (9)) shows that an increase in the bulk of the substituent strongly suppresses hydroxylation of both ortho- and meta-positions. For ethylbenzene, ortho- and
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meta-ethylphenols are found in trace quantities, whereas for isopropylbenzene, para-isopropylphenol is the only ring-hydroxylated product observed.
Hydroxylation of the side chain of alkylaromatic compounds provides another good example of steric influences in a reaction happening in a zeolite micropore space. Hydroxylation of the side chain of ethylbenzene leads predominantly to ~phenetol. With the increase of the steric bulk of the alkyl substituent in isopropylbenzene, a-hydroxylation is strongly suppressed and 2-phenyl-1propanol is the predominant product of side chain hydroxylation, even though the tertiary C-H bond in the alkyl substituent of cumene is generally the most reactive. Even with severe steric restrictions, its reactivity is sufficient enough to make formation of 2-phenyl-2-propanol the only example in this study of the hydroxylation of the tertiary sp3-carbon. HO a-oxygen
O~
k
1
OH
9
3
(8)
OH
a-oxygen +
+
~" 1
OH 9
4
(9)
Hydroxylation of halobenzenes was also studied in the reaction with ~oxygen at room temperature (eq. (10)). In the case of fluorobenzene (X= F), fluorophenols were the major products (o:m:p = 1:2.6:5.1). When chlorobenzene (X=C1) is hydroxylated by a-oxygen, ortho- and para-chlorophenols in a 1:5.1 ratio are the major products. X
X a-oxygen v
(io)
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In a,a,a-trifluorotoluene (X=CF3), the CF3 group, being a substituent with a strong -I effect and large steric bulk, directs hydroxylation predominantly in the meta-position (m:p = 1.5:1).
4. CONCLUSIONS A screening study of alkanes, alkenes and aromatic compounds in room temperature reactions with a-oxygen formed by N20 decomposition on the surface of Fe-containing zeolites is reported. Though the results are of qualitative nature, they significantly expand our knowledge of the chemistry of (z-oxygen and provide leads to other catalytic oxidations by N20. N20/FeZSM-5 zeolite was shown to be a versatile system for selective hydroxylation of a variety of organic compounds, effecting O-insertion into both aromatic and aliphatic C-H bonds. The reactivity of aromatic nucleus was found to be higher than that of the aliphatic substituents in all the studied alkylaromatic compounds. The regioselectivity of hydroxylation is determined by steric and electronic factors and is strongly influenced by the constraints imposed by the micropore space of the zeolite matrix. The reported results demonstrate the possibilities of novel routes to alcohols and phenols, most of which currently are manufactured via multi-step processes. To make these transformations commercially viable catalytic versions of these reactions need to be developed, a formidable challenge. We hope that the reported results will inspire researches to take a closer look at this novel and exciting field of oxidative catalysis.
REFERENCES 1. G.I. Panov, A.K. Uriarte, M.A. Rodldn and V.I. Sobolev, Catalysis Today, No. 41 (1-2) (1998) 365. 2. G.I. Panov, V.I. Sobolev, and A.S. Kharitonov, J. Mol. Catal., No. 61, (1990) 85. 3. G.I. Panov, V.I. Sobolev, K.A. Dubkov and A.S. Kharitonov in Proc. 1 lth Intern. Congr. Catal. J.Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (eds.), Stud. Surf. Sci. Catal., Baltimore, 1996, Elsevier Science B.V. No.101 (1996) 493. 4. G.I. Panov, A.S. Kharitonov and V.I. Sobolev, Appl. Catal. No. 98 (1993) 1. 5. A.K. Uriarte, M.A. Rodkin, M.J. Gross, A.S. Kharitonov and G.I. Panov m Proc. 3rd Intern. Congress on Oxidation Catalysis, R.K. Grasselly, S.T. Oyama, A.M. Gaffney and J.E. Lyons (eds.) Stud. Surf. Sci. Catal., Elsevier Science B.V., No. 110 (1997) 857. 6. V.I. Sobolev, K.A. Dubkov, Ye.A. Paukshtis, L.V. Pirutko, M.A. Rodkin, A.S. Kharitonov and G.I. Panov, Appl. Catal. No. 141 (1996) 185. 7. K.A. Dubkov, V.I. Sobolev and G.I. Panov, Kinet. Katal., No. 39 (1998) 79. 8. V.I. Sobolev, A.S. Kharitonov, Ye.A. Paukshtis and G.I. Panov, J. Mol. Catal., No. 84 (1993) 117.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
S e l e c t i v e o x i d a t i o n of h y d r o c a r b o n s by a c t i v e o x y g e n f o r m e d f r o m N 2 0 on ZSM-5 at m o d e r a t e t e m p e r a t u r e s S.N.Vereshchagin a, N.P.Kirik a, N.N.Shishkina a, S.V.Morozov b, A.I. Vyalkov b and A.G.Anshits a a Institute of Chemistry and Chemical Engineering SB RAS, K.Marx st. 42, Krasnoyarsk, 660049, Russia; e-mail:
[email protected] b Institute of Organic Chemistry SB RAS, Novosibirsk, 630090, Russia Conversion of methane, ethane, propane and benzene on HNaZSM-5 in the presence of N20 at 350-450~ was studied. The reaction rate was shown to be determined by the strength of C-H bond attacked and zeolite acidity had no influence on methane and benzene reactivity. Oxidative methylation of aromatic ring with methane occured under reaction condition giving toluene from benzene. Hydrogen form of zeolite favored the reaction of oxidative methylation whereas reaction of deep oxidation prevailed on non-acidic samples. 1. INTRODUCTION It is known that the interaction of N20 with ZSM-5 (or their iron analogs) produces surface oxygen [1] called a-oxygen [2]. This active oxygen species shows an extraordinary ability for selective oxidation: at moderate temperatures it converts ethane to ethylene or benzene to phenol with 96-98% selectivity. Though the process of benzene to phenol hydroxylation with N20 was demonstrated by Solutia on a pilot plant scale, the mechanism of these reactions is still unknown and there is a lack of data on the chemistry of a-oxygen. Despite of the high cost of N20 for selective oxidation of the most hydrocarbons the understanding of regularities of a-oxygen formation and its interaction with different hydrocarbons could be very useful for general concept of selective conversion of low alkanes and design of active catalysts. The objective of the present paper is to consider the peculiarities of light alkane transformation over ZSM-5 in the presence of nitrous oxide to reveal the chemistry of the oxygen species produced from N20 under condition of catalytic reaction.
2. EXPERIMENTAL Zeolites H-Na-ZSM-5 (SIOJA1203=33, the contents of iron 0.06 wt.%, sodium 0.01-2.5 wt.%) were prepared from NaZSM-5 by triple ion exchange with 1M NH4C1 followed by impregnation with NaOH solution to reach desired content of sodium. Catalytic runs were carried out at atmospheric pressure and 350-450~
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in a conventional set-up with stationary catalyst bed as described elsewhere [3]. GC-MS analyses of the liquid products of 13CH4+C6H6+N20 conversion were performed on capillary column HP-5MS (5% phenyl methyl siloxane). 13C isotope distribution and location of labeled atoms were calculated from mass spectra (GC-MS data) and from 1H and 13C NMR spectra [4].
3. RESULTS AND DISCUSSION 3.1 Reactivity of hydrocarbons in respect of oxygen species Three distinct types of transformation were found for catalytic conversion of methane, ethane, propane and benzene in the presence of N20 on ZSM-5 (Table 1). Methane undergoes basically deep oxidation to CO2 and CO with a small amount of aromatics; ethane and propane are selectively dehydrogenated to olefins, whereas phenol is formed from C6H6, the selectivity of dehydrogenation and hydroxylation being very high (90-98%). The conversion of the hydrocarbons drops rapidly with time on stream but the selectivity remains almost constant. It is interesting that the highest rate of activity decrease is observed for methane (about 4 times for the first 20 min of run), the lowest- for ethane. Table 1 Main routs of transformation of RH-N20 feeds on HZSM-5 at 380-420~ 15%vol, CN2o=3-10%vol, He balance, RH conversion less than 10%). RH
C2H6 C3Hs C6H6
main product (selectivity, %) > > >
C2H4 (>90) C2H4 (>80) C6HsOH (95-98)
(CRH--3-
minor products (selectivity, %)
+ + +
C3-C6aliphatics+aromatics(5-10), COx (1-5) C4-Csaliphatics+aromatics (5-15), COx(l-5) CO2(1-5)
To understand the differences in catalytic behavior we have studied a reactivity of hydrocarbons towards surface oxygen formed from N20. The values of relative reactivities expressed as krd=kRH/kCH4 (where kcH4, kRH- kinetic constants of reactions (1) and (2)) are calculated from equation (3), where P~ PORH, PCH4, PRH, - partial pressures of methane and R H before and after reactor, provided that feed contains simultaneously methane and R H (e.g. C2H6-CH4N20) [3]. The values of krel calculated in such a way decrease in the order C3Hs>C2H6>CH4~H2~C6H6. High selectivity to principal products (Table 1) indicates that a-oxygen attacks only C-H bond of hydrocarbon molecule (H-H for hydrogen), therefore, the properties of C-H bond can influence the course of the reaction. From the experimental data one can estimate the reactivity of the single C-H bond as k'rel- k,.el/N, where N-number of equivalent H atoms. CH4 + [O] ~kcH4--~ products (1) R H + [O] mkRH---->products, where R H - C3H8, C2H6, H2 or C6H6 (2)
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PCH4 kcH4 ln( PRH ) In( POH4)- kRH POH
(3)
An excellent linear correlation between energy of C-H bond and corrected relative reactivity k'rel (Fig.l) 1,6 indicates that it is the strength of C-H bond (H-H for hydrogen) that determines 1 2 1,0 the rate of alkanes conversion. The reactivity of benzene is found to be close 0,4 v to that of methane and fits in with the correlation line for alkanes though the -0,2 3 value of k'c6H6 is slightly less than predicted. The data presented do not -0,8 allow to distinguish between C-H and -1,4 C=C double bond attack for hydroxy92 96 100 104 lation of C6H6. In our case the low reactivity of benzene can be explained E, kcal/mole not only by the distinction of reaction Fig. 1. Variation of k'rel a s a function of mechanisms but also by transport energy of C-H bond E for: 1-propane, 2- limitation that decreases the actual ethane, 3-methane, 4-benzene and 5- concentration of C6H6 in the vicinity of hydrogen. active centres inside the channels. 2,2
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Fig 2. (a)- NH3 TPD profile (Tads=200~ ~=10~ and (b)- relative reactivity 0 of methane and benzene for: $ - HZSM-5; D- 1.5)ANaHZSM-5; A- 2.5%NaZSM-5 Influence of acidity on the relative reactivity of methane and benzene was studied over three samples of ZSM-5 possessing different amount of sodium and, therefore, distribution of acid sites (as can be seen from TPD profiles of ammonia, Fig.2a). It is evident that the experimental points for all the samples satisfy the same correlation line in coordinates ln(PRH/pORH)-ln(Pcm/pOcHr(Fig.2b). Therefore, the relative reactivity of methane and benzene does not depend on acidity and no
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additional activation on acidic site is necessary for the first step of the interaction of surface oxygen with the molecule of a hydrocarbon. It is reasonably to suggest t h a t the different product composition (Tabl.1) is caused not by peculiarities of aoxygen interaction with hydrocarbon, but by subsequent transformation of activated surface hydrocarbon species.
3.2 R o u t e s o f h y d r o c a r b o n t r a n s f o r m a t i o n in the p r e s e n c e of N20 Though preferred formation of COx occurs over HZSM-5 for neat m e t h a n e (Tabl.1) only traces of deep oxidation products are found for CH4-C6H6-NeO feed. Instead the enhanced amount of toluene (Tol) and diphenylmethane (DPhM) are observed (Table 2), xylenes (Xy), cresols (Cre), indene (Ind) and methylated naphtalene (Naph) are also formed in minor amount. Composition of the products obtained over 2.5% NaZSM-5 differs Table 2. Amount of products formed on HZSM-5 from t h a t on HZSM-5. For acidic at 418~ with different reaction feeds. sample the formation rates of phenol Time of s t r e a m 2000s, GHSV=26500 h -1. and toluene are about equal (Table 2), but for non acidic zeolite the rate Reaction feed (vol.%, He balance) of toluene formation is more t h a n an CH4 14 14 order of magnitude lower (Figs.3a) C6H6 3.5 3.5 and only traces of aromatic NeO 5.7 5.8 5.8 compounds are found. The A m o u n t of products (mmole/g) conversion of m e t h a n e remains the COx 200 kPa H2 on H-ZSM5 and to 50-100 kPa H2 on Co/H-ZSM5 and much higher than at the H2 pressures prevalent during alkane reactions (1-2 kPa). Alkane reactions on M/H-ZSM5 are limited by hydrogen desorption bottlenecks that lead to surface hydrogen chemical potentials higher than those in the contacting H2 gas phase. The addition of these hydrogen species to thiophene leads to the observed desulfurization, which provides an alternate path for hydrogen removal and leads to higher alkane dehydrogenation rates. This synergistic coupling occurs on H-ZSM5, but cations appear to provide a promotional effect. Cations provide recombinative desorption sites that decrease the availability and chemical potential of adsorbed hydrogen, but they also appear to be involved in providing alternate binding sites for hydrodesulfurization reactions. 1. INTRODUCTION Sulfur compounds in crude oil often remain in fuels, leading to their release and to the poisoning of exhaust catalysts. Environmental concems and legislation will lead to a significant decrease in the sulfur content of diesel and gasoline fuels (400 to 98.0%) at 353 K for 16 h four times using fresh NH4NO3 solutions to ensure complete exchange. The zeolite sample was filtered and washed with deionized water. Samples were dried in air at 398 K for 20 h and calcined at 773 K for 20 h in flowing dry air to convert to the proton form of the zeolite (H-ZSM5). Co/H-ZSM5 was prepared by ion exchange of H-ZSM5 at 353K for 16 h in a 0.05 M Co(NO3)2 solution (Aldrich, 99%). The sample was filtered, washed with deionized water, dried in air at 398 K for 20 h, and then treated in flowing dry air at 773 K for 20 h. Atomic absorption spectroscopy (Galbraith Laboratories, Inc.) measured a Co content of 0.91 wt% (Co/A1 - 0.15). Co§ isolated cations are located at the exchange site and replace two Bronsted acid sites during exchange. These characterization data will be reported elsewhere [7]. Zn/H-ZSM5 was prepared by aqueous exchange methods that lead to the predominant formation of Zn+2 cations bridging two A1 sites [8]. Mo/H-ZSM5 was prepared by solid-state exchange methods using MoO3/H-ZSM5 physical mixtures, which lead to the formation of (Mo205) +2 dimers interacting with two A1 sites [9].
2.2. Catalytic Thiophene Desulfurization Thiophene desulfurization studies were performed in a tubular reactor with plug-flow hydrodynamics. Rates, selectivities, and deactivation behavior were measured at 773 K using C3H8 (20 kPa)/C4H4S (1 kPa), H2 (0-200 kPa)/C4H4S (1 kPa), or pure C4H48 (1 kPa) reactants. C3H8(Praxair, >99.5%) was purified using O2/I-I20 traps (Matheson) and C4H4S (Aldrich, >99%) was used without purification. HE (Praxair, UHP) and He (Praxair, UHP) were purified using 02 and 13X sieve traps (Matheson). Reactant and product concentrations were measured by gas chromatography (Hewlett-Packard 6890) using capillary (HP-1 Crosslinked Methyl Siloxane, 50m x 0.32mm, 1.05 ~t film) and packed columns (Hayesep-Q, 80/100 mesh, 1 0 ' x 0.125") and flame ionization and thermal conductivity detection. Thiophene reaction rates are reported as molar thiophene conversion rates per A1 or per residual OH group after exchange. Isotopic exchange (DE-OH) experiments were used to determine the number of residual OH groups [8, 9]. Sulfur selectivities are reported on a sulfur basis as the percentage of sulfur from the converted thiophene that appears in each sulfur-containing product. Propane conversions were 200 kPa), because Co cations catalyze hydrogen adsorption-desorption steps. As a result, the availability of surface hydrogen is lower than on H-ZSM5 and H2 can be more effectively used as a source of hydrogen than on H-ZSM5. Thiophene desulfurization rates for H2/CaHaS mixtures with H2 pressures (1-2 kPa) typically present during propane reactions, however, were lower than for C3H8/C4HaS mixtures; therefore, desulfurization does not require gas phase H2 on Co/H-ZSM5. Zn cations in H-ZSM5 show similar effects on the rate of hydrogen removal and on the equivalent H2 pressure obtained during propane-thiophene reactions [ 12].
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3.4. Effects of C3Hs and H2 on Catalyst Deactivation Rates 0.5 Catalyst deactivation rates also depend on
A
the availability of surface hydrogen species that decrease the level of unsaturation and increase ~ , -~, , '~ ~ H 2 pressure during the reactivity of strongly adsorbed carbonaceous "'' C3H8 reaction deposits. Deactivation rates are proportional to ~ 0.3 the concentration of undeactivated catalytic sites. r176~ Treatment in 20% 0 2 at 773 K restores initial ~o 0.2 desulfurization rates, suggesting that deactivation w can be reversed by combustion of carbonaceous C3H8 / 0.1 C4H4S A H 2/C4H4S deposits. Deactivation rate constants (kd) with propane-thiophene reactants (0.014 h 1) are much l ~ 0.0 lower than with pure thiophene (0.49 hl). H2 coo 2'5 50 75 125 reactants also decrease deactivation rate H E Pressure (kPa) constants, approaching the values obtained with Figure 3: Comparison of first order deactivation rate propane at 50-100 kPa of H2 (0.060-0.021 h -1) constant (kd) in the presence of propane or H 2 on (Figure 3). Thus, reactions of propane-thiophene Co/H-ZSM5 [773 K, 1 kPa C4H48, 20 kPa C3Hs mixtures on Co/H-ZSM5 show deactivation or 0-100 kPa HE, balance He, Co/Al=0.15] behavior that reflects a surface with hydrogen chemical potential similar to that present with 50-100 kPa HE in the contacting gas phase. 0.4
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REFERENCES
.
4. 5. 6. 7. .
9. 10. 11. 12. 13. 14. 15.
EPA Staff Paper on Gasoline Sulfur Issues, 1998, U.S. Environmental Protection Agency Office of Mobile Sources. Wormsbecher, R.F., Weatherbee, G.D., Kim, G., and Dougan, T.J., National Petroleum Refiners Association Annual Meeting 1993, San Antonio, TX. Biscardi, J.A. and Iglesia, E., J. Phys. Chem. B, 102 (1998) 9284. Biscardi, J.A. and Iglesia, E., J. Catal., 182 (1999) 117. Yu, S.Y. and Iglesia, E., J. Phys. Chem., manuscript in preparation. Iglesia, E. and Baumgartner, J.E., Catal. Lea., 21 (1993) 55. Li, W., Yu, S.Y., Meitzner, G.D., Yu, G. and Iglesia, E., J. Phys. Chem., manuscript in preparation. Biscardi, J.A., Meitzner, G.D., and Iglesia, E., J. Catal., 179 (1998) 192. Borry, R.W., Kim, Y.-H., Huffsmith, A., Reimer, J.A., and Iglesia, E., J. Phys. Chem. B, 103, (1999) 5787. Saintigny, X., van Santen, R.A., Clemendot, S., and Hutschka, F., J. Catal., 183 (1999) 107. Benders, P.H., Reinhoudt, D.N., and Trompenaars, W.P., "Cycloaddition Reactions of Thiophenes, Thiophene 1-Oxides, and 1, 1-Dioxides." Thiophene and Its Derivatives, ed. S. Gronowitz, Wiley, New York, 1985. Yu, S.Y., Li, W., and Iglesia, E., J. Catal., 187 (1999) 257. Garcia, C.L. and Lercher, J.A., J. Phys. Chem., 96 (1992) 2669. Weitkamp, J., Schwark, M., and Ernst, S., J. Chem. Soc., Chem. Comm., (1991) 1133. Meitzner, G.D., Iglesia, E., Baumgartner, J.E., and Huang, E.S., J. Catal., 140 (1993) 209.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Water vapor tolerance of the Pd/HZSM-5 in the NO-methane-O2 reaction: its relation with very strong solid acidity of zeolite support Jiro Amano, Osamu Shoji, Kazu Okumura, and Miki Niwa 1 Department of Materials Science, Faculty of Engineering, Tottori University Koyama, Tottori 680-8552 Japan Pd/HZSM-5 with very strong acid site due to the extra-framework AI has the high water vapor tolerance in the NO-methane-O2 reaction. The very strong solid acid site is required to prevent the active species of highly dispersed Pd-O from agglomeration into the large particle. I. INTRODUCTION Methane is the most preferable reductant, because it is contained in natural gas. However, catalyst with a high activity is required for the reduction of NO with methane due to the low activity of methane. From this viewpoint, much attention has been paid to the catalysis of Pd loaded on the zeolite ZSM-5 and mordenite, because these are active and selective for the reduction of NO with methane in the presence of 02, as first reported by Nishizaka and Misono [1,2]. Acid site of support zeolite is inevitable for the selective reduction of NO on the Pd catalyst, and without the acidity, only the combustion of methane occurs. The catalysis of the acid site in the reaction is however not clarified, and a possible role of the acid site as for oxidation of NO was reported [2]. Another problem to the catalytic system is the loss of activity by water vapor, although the Pd/zeolite has the relatively high stability against the water vapor. In general, enhancement of the water vapor tolerance is important for development of the environmental catalyst, because it will be used at the atmospheric conditions. We have already proposed that the chemical vapor deposition of silica on the external surface on mordenite [3] and ZSM-5 [4] is effective in obtaining the durable activity in the presence of water vapor. Silica deposited on the external surface of zeolite suppressed the sintering of Pd: i.e., the deposited silica not only affords the hydrophobicity, but also retards the stabilization of Pd large particles on the external surface. Rh- [5] or Co [6] -modified Pd/zeolite were reported for the same strategy, but these modifiers were added in order to enhance the oxidation of NO into NO2. In this investigation, various ZSM-5 zeolites with 25 of Si/AI2 molar ratio were synthesized under the different conditions for synthesis, and the proton-type ZSM-5 were used as a support of Pd. The activity of NO-CH4-O2 was measured on these Pd/HZSM-5 with and without added water vapor, and the dependence of the activity and stability upon the characterized property, particularly the solid acidity will be studied.
l Corresponding author: e-mail,
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Table 1. Preparation conditions and fundamental data of the selected ZSM-5 species No 1-120/ Na § Time Rotate Size Si/Al2 Pd SiO 2 mol h rpm ~tm wt % 0 Provided by Tosoh 1.4, 0.7 23.8 0.20 1 50 0.095 120 500 0.3 23.2 0.34 3 60 0.07 168 100 0.5 26.3 0.29 4 60 0.05 168 100 0.6 23.9 0.02 6 80 0.07 168 100 0.36 25.1 0.20 8 120 0.046 168 100 2.3, 1.1 23.4 0.28 12 60 0.09 168 100 0.6 23.2 0.31
2. EXPERIMENTAL Zeolites ZSM-5 were prepared under various hydrothermal conditions. Colloidal silica Ludox HS-40, A12(SO4)3, and template molecule tetrapropylammonium bromide were used, and the molar ratio of Si/AI2 was adjusted to 25; on the synthesis, such conditions as temperature, H20/SiO2, and rotate rate were varied. Six species of them were identified as the highly crystallized ZSM-5, and these and one more sample supplied from Tosoh Co (named, No. 0) were used. After it was calcined to remove the template molecule, the sample was ion-exchanged with NH4NO3, and calcined to form the proton-type ZSM-5. [Pd(NH3)4] 2+ cation was then exchanged. Finally, the sample was calcined in a N2 flow at 773 K to remove ammonia. Preparation conditions and fundamental data of the selected fine species were shown in Tab. 1. The catalytic reaction of NO-CH4-O2 (NO and CH4, 1000 ppm, 02, 1%, and He balance) with 0 or 10 % H20 was performed at 473 - 823 K, and the outlet gas was analyzed by GC and NOx meter. Temperature programmed desorption (TPD)of ammonia was measured using all-glass apparatus with a mass spectrometer detector. After ammonia adsorption, water vapor was admitted at 373 K, and water was adsorbed in place of removable ammonia. After the water vapor treatment, the sample bed temperature was raised from 373 K in a ramp rate of 10 Kmin ~. Desorbed ammonia was detected using 16 of m/e. Detailed method was described in our review [7]. Pd K-edge EXAFS was measured at BL01B1 station of Japan Synchrotron Radiation Research Institute (SPring-8). The storage ring was operated at 8 GeV with a ring current of 44-65 mA. The sample was transferred to glass cells with two Kapton windows connected to a flow reaction system without contacting air. For the curve fitting analysis, the empirical phase shift and amplitude functions for Pd-O and Pd-Pd were extracted from the data for PdO and Pd foil, respectively. 3. RESULTS AND DISCUSSION 3.1. Reduction of NO The activity of the catalysts in the reaction of N O - C H 4 - O 2 w a s measured, as shown in Fig. 1. The reduction activity showed the maximum against the reaction temperature at 423 - 473 K. N2 and CO2 were products all over the catalysts, and neither N20 nor CO was formed. The degree of conversion of NO were different from species to species, and the sequence of the catalyst with respect to the reduction activity was observed, i.e., No. 3 > Nos. 0, 12 > Nos. 6, 8 > No. 1.
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No. 4 showed the least activity due to the low Pd loading, and this catalyst was skipped in following experiments. On the other hand, the addition of 10 % water vapor to the reaction system resulted in the decrease of activity, and the different sequence of the activity was obtained for the reaction of NO-CH4-OE-H20, as shown in Fig. 2. No. 3 > No. 0 > No. 8 > No. 6 > No. 1 > No. 12. One can identify, from the comparison between them, that the loss of activity due to the water vapor was outstanding on the samples No. 12 and 6. To the contrary, the loss of activity of Nos. 3 and 0 was small; and, these catalysts showed the highest reduction activity in the presence of water. In our previous paper [4], two kinds of deactivation behavior due to the added water were found, i.e., reversible and irreversible decrease in the catalyst activity. From the study of transient experiment, the irreversible change of activity was outstanding on the sample No. 12. Therefore, the loss of activity due to the added water vapor was caused by the irreversible change of activity, and this was the serious problem encountered on this catalytic system. 3.2. C h a r a c t e r i z a t i o n a n d correlation with the activity
These catalysts were characterized by SEM, infrared spectroscopy (OH profile), and N 2 adsorption. The size of zeolite crystal was 0.3 - 2.3 ktm, as shown in Table 1. Infrared spectra showed the usual acidic hydroxide and isolated silanol at 3600 and 3745 cm ~, respectively. Nitrogen adsorption showed the typical type I isotherm with adsorbed amount of ca. 85 to 110 cmS/g at p/p0, 0.5. These characterization data were not related with the reduction activity shown above. Acidity of these Pd loaded catalysts was then characterized by TPD of ammonia. In the present study, the water vapor treatment was performed after the adsorption of ammonia, and the low temperature peak (/-peak) which was weakly adsorbed ammonia was completely removed. Because only ammonia molecule adsorbed on acid site is desorbed after the water vapor treatment, the number of acid sites is measured quantitatively. Even with this technique, however, two desorption peaks were observed in the TPD of ammonia, as shown in Fig. 3. One was a typical desorption peak of HZSM-5 at about 600 K, and ascribable to the
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acid site due to framework AI (h-peak). One more desorption peak at about 800 K was identified as the very strong acid site due to the non-framework AI in combined with the framework AI (h+-peak) [8]. A weak shoulder at 480 K was observed on a few catalysts, however, the assignment of the weak peak was ambiguous. Distribution of the two desorption peaks depended on the Table 2 sample, as shown in Fig. 3. Such a difference in the Total amount of acidity distribution was found to be closely related with the desorbed NH~ activity and the decay due to the added water vapor. The No Acid site samples No. 12 and 6 on which the activity was most mol/kg severely suppressed by the water vapor were characterized to 0a) 0.44 have the high concentration of acid site of h-peak. On the 1~) 0.39 other hand, the samples Nos. 3 and 0 with a small loss of 3~) 0.47 activity showed the high intensity of h+-peak. Thus, a 6~) 0.75 strong relation was observed between the loss of activity due 8~) 0.57 to the added water and the distribution of acid sites. The 12a) 0.63 HZSM-5 with the usual acid site of the framework AI is 0b) 0.48 therefore not enough for the support of Pd. The very strong a)Pd/HZSM-5 acid site of h+-peak is required to obtain the active and b) HZSM-5 durable catalyst. The strengths of acid sites of h- and h+-peaks were calculated to be 130-135 and 180-190 kJ/mol, respectively, based on the theoretical equation derived by us [7]. Total desorbed amount of ammonia was small on the samples with the high intensity of h+-peak, as shown in Tab. 2. This can be explained by the stoichiometry that two A1 cations are required to form the very strong acid site. 3.3. Change of Pd and acid site due to the water added Furthermore, we characterized the change of activity of Pd/HZSM-5 using EXAFS and TPD. Fourier transforms of the k3~(k) EXAFS of Pd on HZSM-5 (No. 0) were shown in Fig.
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4. Atter calcination in N 2 at 773 K, only Pd metal was observed at 0.26 nm / ' ~ Pd-Pd(PdO)z.a ~ (Fig. 4-a). Pd amine complex was, therefore, so easily reduced by the high 40~,u) temperature calcination in N2. _ (c) However, the Pd metal was completely 30 oxidized upon oxidation at 773 K. After the oxidation (Fig. 4-b), Pd-O was ~ ' (b) 20 observed at the distance of 0.16 nm with few of neighbor Pd atoms. Therefore, the PdO species was dispersed so well in 10 al) the zeolite framework. Because the PdO could be regarded as a basic metal oxide [9], the PdO could be interacted 0 1 2 3 4 5 6 with acid sites. The difference in Distance/0.1 i n chemical properties is a driving force to keep the highly dispersed PdO. One Fig. 4. EXAFS of Pd on HZSM-5 (No.0) taken can indicate the interaction with the PdO atter: (a), calc in N2, (b), oxidized (c); used for species as the important role of the solid NO-CH4-O2; (d), used for NO-CH4-O2-H20. acid site in the present catalyst. The observed EXAFS of PdO could not be interpreted with the exchanged Pd cation in the zeolite as reported previously [ 10,11 ], because the distance of Pd-O was in good agreement with that of a standard PdO, and coordination number of oxygen around the Pd was indeed 4. These parameters obtained from EXAFS measurement are elucidated with the structure of dispersed Pd-O species in the zeolite framework [ 12]. Atter used for the reaction of NO-CH4-O2 for 3 h, it seemed that the coordination number of Pd decreased somewhat (Fig. 4-c). However, the Pd-Pd bond appeared, atter it was used for the reaction of NO-CH4-O2-H20 (Fig. 4-d). Therefore, the PdO was agglomerated into 50
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the large particle after the reaction with 1-120. In other words, the sintering of PdO occurred during the reaction with added water, thus resulting in the irreversible deactivation of the catalyst activity. Recent study using Raman spectroscopy [13] also showed the agglomeration of PdO during the reaction with water, which was in consistent with the present observation. On the other hand, TPD of ammonia was measured atter the reaction, and compared with that before the reaction. As shown in Fig. 5, the TPD of sample No. 12 changed drastically atter the reaction. On the other hand, the change of TPD was not significant on the sample No. 0. The decrease of h-peak intensity on the No. 12 was obvious, whereas that of h+-peak was not outstanding. Change of solid acidity suggests the restructuring of aluminum cation in the zeolite. The decrease in acid site during the reaction seems to indicate the dealumination of aluminum in the framework. It could be therefore indicated that the stronger acid site of h+-peak is stabilized more than that of usual h-peak. Usual acid site of H-ZSM-5 is not sufficient to stabilize the PdO highly dispersed and active for the reduction of NO. In conclusion, very strong acid sites due to the non-framework AI are required to prevent the active PdO from sintering under the humidified conditions. REFERENCES
1. Y. Nishizaka and M. Misono, Chem. Lett. (1993) 1295. 2. M. Misono, Catteeh, 2 (1998) 183. 3. M. Suzuki and M. Niwa, Chem. Lett. (1996) 275. 4. M. Suzuki, J. Amano, and M. Niwa, Mieroporous Mesoporous Mater., 21 (1999) 541. 5. M. Misono, Y. Nishizaka, M. Kawamoto, and H. Kato, Stud. Surf. Sei. Catal., 105 (1997) 1501. 6. M. Ogura, Y. Sugiura, M. Hayashi, and E. Kikuehi, Catal. Lett., 42 (1996) 185. 7. M. Niwa, and N. Katada, Catal. Surveys Jpn., 1 (1997) 215. 8. T. Kunieda, N. Katada, and M. Niwa, "Proceedings of the 12th International Zeolite Conference", Vol. 4, M. M. J. Treaey, B. K. Marcus, M. E. Bisher and J. B. Higgins, Eds., Materials Research Society, Warrendale, p 2549 (1999). 9. R. T. Sanderson, "InorganicChemistry"; Reinhold Publishing Corporation: New York, 1967. 10. A. Ali, W. Alvarez, C. J. Loughran, and D. E. Resaseo, Appl. Catal., B: Environmental, 14 (1997) 13. 11. A. W. Aylor, L. J. Lobree, J. A. Reimer, and A. T. Bell, J. Catal., 172 (1997) 453. 12. K. Okumura, J. Amano, and M. Niwa, manuscript in preparation. 13. H. Ohtsuka and T. Tabata, Appl. Catal. B: Environmental, 21 (1999) 133.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Simultaneous NO and N20 Decomposition on Cu-ZSM5 R. Pirone a, P. Ciambelli b, B. Palellac and G. Russo a aIstituto di ricerche sulla combustione, CNR, P.le Tecchio 80, 80125 Napoli (Italy) bDipartimento di ingegneria chimica e alimentare, Universit~ di Salerno, via Ponte Don Melillo, 84084 Fisciano, SA (Italy) CDipartimento di ingegneria chimica, Universi~ di Napoli "Federico II", P.le Tecchio 80, 80125 Napoli (Italy) The mutual interaction between NO and N20 on Cu-ZSM5 has been studied in experimental conditions in which both decomposition reactions occur. The oscillating behaviour observed in N20 decomposition is strongly influenced by the presence of NO, as the latter quenches the oscillations and stabilises the highest active state of the catalyst. A reaction mechanism is proposed to explain the experimental observations, based on the assumption that NO reduces the catalyst sites from Cu +2to Cu +, via the formation of adsorbed species and gaseous NO2. 1. INTRODUCTION Nitrogen oxides, mainly produced in combustion processes, are very dangerous pollutant gases for the environment. One of the most attractive ways to solve the problem of NOx atmospheric pollution is their catalytic decomposition to molecular nitrogen and oxygen. CuZSM5 is a very active catalyst in both NO and N20 decomposition [ 1,2]. Many efforts have been devoted to the study of each decomposition reaction on this catalyst [1,2], but at the moment few investigations exist on their simultaneous proceeding on Cu-ZSM5 [2-7] and report contrasting results. Kapteijn et al. [2] showed that the presence of NO does not affect N20 decomposition on Cu-ZSMS, although it is able to increase N20 consumption rate on Co- and Fe-ZSM5 catalysts via the formation of NO2 and N2 according to the following stoichiometry: NO + N20 ~
NO2 + N2
(1)
On the other hand, very different results have been reported by Lintz and Turek [4] and Turek [5]. They found an oscillating behaviour of N20 decomposition rate on Cu-ZSMS, quenched by the presence of 20 ppm of NO. The activity of the catalyst towards the decomposition of nitrous oxide was also found to be increased by the presence of NO, due to the stabilisation of the most active state of catalyst sites. Moreover, in a further paper, Turek [6] hypothesised the occurrence of the reaction (1) also on Cu-ZSMS, as proposed by Kapteijn for Fe- and Co-ZSM5 [2], in order to explain the high consumption observed for N20.
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We found a different effect of NO, that is a decrease of N20 decomposition rate on CuZSM5 with high Si/A1 ratio (80) as a consequence of NO addition to the feed [3]. Similar results were reported by Mascarenhas and Andrade [7] in the investigation of the effects of several additional gases on the catalytic decomposition of N20 over Cu-ZSM5. We also observed the presence of self-sustained and isothermal oscillations of N20 and 02 outlet concentrations during the decomposition of N20 on over-exchanged Cu-ZSM5, disappearing in the presence of NO [3]. The genesis of the dynamic behaviour was assumed kinetic, i.e. related to a particular reaction mechanism [8]. More specifically, we have proposed that periodic changes of copper oxidation state occur on the catalyst surface, due to the interaction of Cu sites with N20, according to the following reaction steps [8]:
N20 + Cu + --~ [N20"-Cu +2] [N20"-Cu +2] + Cu + ~ [Cu+2-O'2-Cu +2] + N2 N20 + [Cu+2-O'2-Cu +2] "-~ N2 + 02 + 2 Cu + [Cu+2-O'2-Cu +2] ~ Oads + 2 Cu + 20,ds ~ 02
(2) (3) (4) (5) (6)
The reaction mechanism hypothesised by us is based on the existence of two different decomposition steps, i.e. N20 reaction with either Cu § (2) and Cu § sites (4). Moreover, a kinetic model was formulated, involving a heterogeneous description of the step (3) kinetics, in which the activation energy is a linear function of the Cu § molar fraction on the catalyst surface [9]. The kinetic model is able to describe the behaviour of the system in different experimental conditions and, in particular, to reproduce the effect of varying N20 inlet concentration and contact time or adding 02 to the feed [10], but the behaviour of the system in the presence of NO remains unknown. In this paper, we have studied the mutual interaction between NO and N20 when simultaneously fed to a Cu-ZSM5 catalytic bed, in conditions in which both decomposition reactions occur. The aim of the work is to give a further insight into the reaction mechanism of these reactions. 2. EXPERIMENTAL The Cu-ZSM5 catalyst (Si/AI = 25) was prepared by ion-exchanging H-ZSM5 powder (Zeolyst) with copper acetate aqueous solution to 114% exchange (Cu/A1 atomic ratio = 0.57). Catalytic activity tests were carried out with a fixed-bed flow microreactor heated by an electrical oven. The reactor temperature was monitored by a 'K' thermocouple, placed in a quartz tube, coaxial and internal to the reactor. The gas analysis was carried out with continuous analysers (Hartmann & Braunn) to measure the concentrations of NO, NO2, N20 and 02. Brooks mass flow controllers were used to maintain the flow rates of high purity gases: NO (1% vol.) + He, N20 (1% vol.) + He, He (99.995%) and 02 (99.7%), allowing to feed a gas mixture with controlled composition. Before each test, the catalyst has been treated in He (35 N1/h) for 2 h at 550~ to obtain a reduced form of Cu-ZSM5.
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3. RESULTS AND DISCUSSION
The measurements of catalytic activity were carried out at two different temperatures (343 and 394~ by feeding a gas mixture containing NO (0-5000 ppm) and/or N20 (0-2000 ppm) diluted by He. When simultaneously fed, both nitrogen oxides are converted to N2 and 02 at 394~ although a very different catalytic activity for the two decomposition reactions is exhibited by Cu-ZSM5. Fig. 1 shows the influence of the presence of N20 on the conversion of NO to N2 at different values of NO/N20 ratio in the feed.
o,,0. 4( & Z o .~ 3
o
9-
l,
0
500
,
~
,,I,,
IF
1000
1500
2000
N20 inlet concentration, ppm Fig. 1. NO conversion to N2 as a function of N20 inlet concentration at 394~ (a, 5000; b, 2000; c, 1000 ppm), N20 and balance He. W/F = 0.02 g-s-Ncm"3.
Feed: NO
In the absence of N20, NO is decomposed to N2 and 02 at 394~ with a very low conversion, due to the low activity exhibited by Cu-ZSM5 for this reaction in the experimental conditions investigated [ 11]. Furthermore, the co-presence of N20 reduces NO conversion, probably due to the competitive adsorption of the two species. Therefore, it is possible to exclude the occurrence of reaction (1) in these experimental conditions, since the consumption/production ratios between the four species involved are very different from those expected by the stoichiometry of the reaction. In Fig. 2 the conversion of N20 to N2 is plotted as a function of the co-fed NO concentration for different values of N20 inlet concentration, reaction temperature and contact time. In the absence of NO, the decomposition of N20 is a much faster reaction than that of NO and shows an oscillating behaviour, in agreement with previous observations [3-6,8]. The copresence of NO in the feed, even at very low amount, results in two main effects: it (i) quenches the oscillations of both reactant and product concentrations, and (ii) dramatically increases the conversion of N20 to N2. With respect to the first effect, similar results were previously reported by us [3] and Lintz and Turek [4]. They showed that at 450~ the
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oscillations are still observed in the presence of 10 ppm of NO, but not yet at higher NO concentrations, and proposed that NO, even fed in low amount, stabilises the catalyst in its most active state for N20 decomposition. 100
"
// A
80 60 40 0 r~
t
~ w...!
20
I
i
I
I
//
80
0 0
0 Z
C,,1
60 40
~
~--._____
6
20
I
I
I
J
//
0
500
1000
1500
2000
/
/
i
5000
NO inlet concentration, p p m Fig. 2. N20 conversion to N2 as a function of NO inlet concentration at 394~ (A) and 343~ (B). Feed: N20 (O, 1000; El, 2000 ppm), NO and balance He. W/F - 0.02 (A) and 0.073 (B) g.s.Ncm"3. In the absence of NO, the time-averaged conversion has been reported. Closed symbols represent the maximum and minimum values attained in the presence of oscillations. Fig. 2A proves that at 394~ N20 conversion is higher at lower N20 inlet concentration, in agreement with our previous results [8]. Moreover, with both values of N20 concentration in the feed (1000 and 2000 ppm), in the presence of NO, N20 conversion is enhanced above the maximum value attained in the oscillating regime which establishes in the absence of NO. By increasing NO concentration, N20 conversion tends to decrease, but at 5000 NO ppm, it is still higher than in the absence of NO for both values of N20 inlet concentration. A slightly different behaviour is observed at lower temperature. Fig. 2B shows that at 343~ the addition of NO still results in increasing N20 decomposition rate, but less strongly than at 394~ In the presence of 5000 ppm of NO, the conversion of N20 reached a quite similar value to that measured in the absence of NO. An inhibiting effect of NO on N20 decomposition rate, likely due to the different experimental conditions and catalyst involved in the measurements, has been reported by
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Mascarenhas and Andrade [7] and by us [3] at 370~ In particular, in our previous experimental investigation, N20 inlet concentration was 300 ppm, while the range of NO concentration investigated was 500-5000 ppm, i.e. the feed ratio NO/N:O was much higher than that involved in the present investigation and in the results of Turek [4-6]. Owing to these considerations, it is possible that at 370~ the positive effect of the presence of NO is not observable at high NO/N:O ratio, but only the second decreasing part of the conversion plot shown in Fig. 2 is detected. However, the presence of different copper species in the highly overexchanged Cu-ZSM5 catalyst at Si/AI = 80 could also explain the different experimental results obtained. Since contrasting results have been reported on the effect of the co-presence of NO in the feed on N20 decomposition rate, no clear explanations still exist about the phenomena involved when both nitrogen oxides are fed to Cu-ZSM5. Ochs and Turek [12] hypothesise a relation between the genesis of oscillations and the byproduction of NO measured during N20 decomposition above 400~ They propose a reaction mechanism in which N20 can react with either Cu + and Cu+2-O2-Cu +2 sites, as in our mechanism (step 2 and 4) [8], and the product of N20 reaction with the oxidised sites is NO. Consequently, the role hypothesised for NO is to restore catalyst active species Cu+ by reducing the catalyst through the formation of surface nitrate species. Moreover, Turek [6] also proposes that the increase in N20 conversion in the presence of NO is due to the occurrence of the redox reaction (1) between NO and N20, following the experimental results of Kapteijn et al. in similar conditions [2]. We do find the formation of NO2 in our experimental conditions, which increases by increasing NO concentration in the feed, but it is lower than that expected by the stoichiometry of reaction (1). In particular, the excess of N20 converted is much higher than the amount of NO reacted or NO2 produced in all conditions. Actually, we explain the appearance of NO2 in the reaction products as result of NO oxidation with 02 produced by NO and N20 decomposition reactions: 2 NO + 02 ~--- 2 NO2
(7)
Our previous results, which evidenced the high activity of Cu-ZSM5 catalyst in this reaction [11] allow us to conclude that the rate of NO oxidation is so high that the equilibrium condition between NO, 02 and NO2 is reached in these experimental conditions. Due to the proven occurrence of reaction (7) on Cu-ZSM5, we believe that the formation of NO2 could be related to the catalyst reduction, as proposed by Ochs and Turek [12], but through the following simple reaction step: NO + [Cu+2-O'2-Cu+2] ~ NO2 + 2 Cu+
(8)
Our previous results [ 11], proving that Cu-ZSM5 is very active in reaction (7), support the hypothesis that the catalyst is reduced by NO, reaction (8) being a step of NO oxidation to NO2. Furthermore, the above assumption is congruent with the "complex" role played by NO when interacting with the catalyst. Indeed, it is known that in the absence of O2, NO could be strongly adsorbed on both oxidised and reduced sites, and at low temperature oxidises Cu + to Cu +2 [ 13-15]. Finally, by assuming the reducing ability of NO towards Cu +2 sites, the experimental results here presented could be explained as follows:
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i. the stabilising effect of NO on N20 decomposition rate is justified, having attributed the occurrence of oscillations to the capability of N20 to react with either Cu + and Cu +2 sites, but with different rates. In fact, in the presence of NO, copper sites mostly exist as Cu + if reaction (8) rate is very fast. This phenomenon is observed even with very low NO concentrations, when the rate of reaction (8) is much higher than those catalyst of the oxidation steps in the reaction mechanism (2-6), in agreement with our previous results [9-11 ]; ii. the enhancing effect of NO on N20 decomposition rate is likely due to the increased concentration of Cu + sites. This effect is highly effective already at low NO concentration, having assumed a very fast kinetics for reaction (8); iii. at higher concentrations of NO, the inhibiting effect of nitric oxide on N20 decomposition can be expected. In fact, an increase of NO concentration increases the amount of NO adsorbed on the catalyst surface, resulting in the reduction of the fraction of the active sites available for N20 decomposition.
4. CONCLUSIONS Decomposition into elements of N20 is much faster than that of NO over Cu-ZSM5. This difference is dramatically increased when both nitrogen oxides are fed to the catalytic reactor, since the conversion of NO is decreased by the presence of nitrous oxide, while N20 decomposition rate is strongly enhanced by the presence of nitric oxide. The oscillating behaviour shown by N20 decomposition on Cu-ZSM5 is not observable in the presence of NO. Even 50 ppm are enough to quench the oscillations and stabilise the highest active state of the catalyst. These phenomena have been explained by assuming that NO quickly converts the less active Cu § species into reduced copper sites (Cu+), which are the most active for N20 decomposition, through the formation of gaseous NO2. REFERENCES
1. V.I. Pftrvulescu, P. Grange and B.Delmon, Catal. Today, 46 (1998) 233. 2. F. Kapteijn, G. Marb/m, J. Rodriguez-Mirasol and J. A. Moulijn, J. Catal., 167 (1997) 256. 3. P. Ciambelli, E. Garufi, R. Pirone, G. Russo and F. Santagata, Appl. Catal. B, 8 (1996) 333. 4. H.-G. Lintz and T. Turek, Catal. Lea., 30 (1995) 313. 5. T. Turek, Appl. Catal. B, 9 (1996) 201. 6. T. Turek, J. Catal., 174 (1998) 9808. 7. A.J.S. Mascarenhas and H.M.C. Andrade, React. Kinet. Catal. Lett., 64 (1998) 215. 8. P. Ciambelli, A. Di Benedetto, E. Garufi, R. Pirone and G. Russo, J. Catal., 175 (1998) 161. 9. P. Ciambelli, A. Di Benedetto, R. Pirone and G. Russo, Chem. Eng. Sci., 54 (1999) 2555. 10. P. Ciambelli, A. Di Benedetto, R. Pirone and G. Russo, Chem. Eng. Sci., 54 (1999) 4521. 11. R. Pirone, P. Ciambelli, G. Moretti and G. Russo, Appl. Catal. B, 8 (1996) 197. 12. T. Ochs and T. Turek, Chem. Eng. Sci., 54 (1999) 4513. 13. R. Pirone, E. Garufi, P. Ciambelli, G. Moretti and G. Russo, Catal. Lett., 43 (1997) 255. 14. E. Giamello, D. Murphy, G. Magnacca, C. Morterra, Y. Shioya, T. Nomura and M. Anpo, J. Catal., 136 (1992) 510. 15. Y. Li and J. N. Armor, Appl. Catal., 76 (1991) L 1.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Catalytic Properties of Silica-Supported Tantalum and Tungsten Hydrides in the Cleavage and Formation of C-C Bonds of Alkanes. O. Maury, L. Lefort, G. Saggio, C. Cop6ret, M. Taoufik, M. Chabanas, J. Thivolle-Cazat* and
J.M. Basset*
Laboratoire de Chimie Organom6tallique de Surface, UMR CNRS-CPE 9986 43 Bd du 11 Novembre 1918, 69616 VILLEURBANNE, C6dex, France
Heterogeneous catalysts are widely used in industrial applications; they are often easily prepared at a low cost and can be conveniently separated from the reaction medium. However it is always difficult to define and control the active site and then to determine the various elementary steps of a catalytic reaction. On the contrary, homogeneous catalysis has been based on the rules of molecular organometallic chemistry, which affords a better understanding of what are the active species and the elementary steps of a process. Surface Organometallic Chemistry (SOMC) is aimed at the preparation of a new type of heterogeneous catalysts for which the concepts and the rules of molecular organometallic chemistry could be transposed. The reaction of molecular complexes with functional groups of a surface affords the formation of highly reactive well-defined supported organometallic species, usually unprecedented in solution. 1. INTRODUCTION Various silica-supported transition metal hydrides of Ti,[ 1] Zr,[2] Hf,[3] Ta,[4] Cr and W[5] have been prepared in the laboratory. In particular, the Ta(-CH2CMe3)3(=CHCMe3) complex reacts with the OH groups of a silica dehydroxylated at 500~ to form a mixture of two surface species: (-Si-O-)xTa(-CH2CMe3)3_x(=CHCMe3) (x=l: =65%; x=2: =35%). Upon treatment under hydrogen at 150~ overnight, these two complexes lead to a surface tantalum(Ill) monohydride (=Si-O-)2TaH, which has been fully characterised by IR spectroscopy, EXAFS, elemental analysis and quantitative chemical reactions.[4] Silicasupported tungsten hydride can be obtained similarly from the reaction of W(-CH2CMe3)3(-CCMe3) with the surface OH groups, followed by a treatment under hydrogen at 150~ overnight.[5] These surface metal hydrides are strongly anchored to the silica surface, coordinatively unsaturated, and highly electron deficient; thus showing unprecedented properties in the electrophilic activation of alkanes either in a stoichiometric or a catalytic way.
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2. EXPERIMENTAL
2.1. Preparation of the silica-supported tantalum hydride (-=Si-O-)2TaH Tris(neopentyl)neopentylidene
tantalum Ta(-CI-I2CMe3)3(=CHCMe3) is prepared
according to the literature description.[6] Silica (Aerosil Degussa 200m2/g) can be used as a compacted powder or pressed into a wafer for IR studies; it is treated under vacuum at 500~ overnight; Ta(-CI-LzCMe3)3(=CHCMe3) is reacted with silica either via sublimation or impregnation from a solution in pentane previously degassed and dried over Na/K; the excess of complex is desorbed via back sublimation or washing in pentane. The grafted complexes (---Si-O-)xTa(-CI-I2CMe3)3_x(=CHCMe3) are then treated under hydrogen (600 torr, 150~ 15h) to form the tantalum hydride (-Si-O-)2TaH.
2.2. Stoichiometric and catalytic experiments (-Si-O-)2TaH can be prepared in situ in IR glass cell on self-supported silica disk, or used as powder in glass reactor. After evacuation, gas reagents are introduced at the desired pressure through a zeolithe-deoxo purifying trapp. Liquid hydrocarbons are degassed and kept on zeolithe in small glass containers; they are transferred as a vapour into reactor. The reactor is then heated at the desired temperature in an oven; during the reaction, aliquots are expended in a small volume, brought to atmospheric pressure and analysed by GC or GC-MS (A1203/KC1 on fused silica column-50m x 0.32mm). 3. RESULTS AND DISCUSSION
3.1. Stoichiometric C-H bond activation of cycloalkanes. The (-Si-O-)2TaH complex can activate a C-H bond of cyclic alkanes (cyclopentane to cyclooctane) at room temperature to stoichiometrically form the corresponding tantalum(m)cycloalkyl species along with the evolution of one equivalent of hydrogen: (-Si-O-)2Ta-H + (cyclo-RH)
> (-Si-O-)2Ta-(cyclo-R) + H 2
(1)
3.2. Metathesis reaction of acyclic alkanes. The surface tantalum and tungsten hydrides catalyze at moderate temperature (25200~ a novel reaction: the metathesis of acyclic alkanes, which transforms an alkane into its higher and lower homologues.[5] In the case of ethane, there is formation of propane and methane in comparable amounts and traces of butane and isobutane (figure 1); this process implies the transfer of a methyl fragment from one molecule of ethane to a second one. A distribution of higher and lower homologues is also obtained from propane, butane, pentane as well as branched alkanes such as isobutane or isopentane according to the general equation: 2 CnH2n+2 > Cn+iH2(n+i)+2 + Cn-iH2(n-i)+2 where i = 1, 2 .... n-l, but with i = 1 generally favoured.
(2)
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Ia-
919
40
Methane
100--
.,.30
0 G
o 30 E Propane
o
80
m,,~
(b)
20 *
(a)
10 ~
E: 20 60 o 10
Butane + Isobutane
o
~
0
24
48
72
96
d 4O
0
24
Time (h)
48
72
96
0 120
Time (h)
Fig. 1. Metathesis of ethane catalyzed by (=Si-O-)2Ta-H (550 Torr, 150~ CEH6]Ta = 500)
Fig.
2.
Degenerated Metathesis of (120 Torr, 150~ laCH3-CHa/Ta- 120).
lacHa-CH3
Mechanistic investigations with 13C monolabelled ethane showed the occurrence of a degenerated process along with the formation of metathesis products;[7] indeed, during the metathesis reaction, production of non- and dilabelled ethane was observed (figure 2) (degenerated metathesis) together with that of methane, propane and butanes (productive metathesis). This detectable degenerated metathesis is necessarily accompanied by an undetectable process (fully degenerated metathesis) where two 13C monolabelled ethane molecules exchange their methyl groups with each other to form two different lac monolabelled ethane molecules (scheme 1):
Scheme1 *CHa-CHa *CHa_CHa
*CHa ~ H3
*CHa-CHa
*CHa CH a
CHa-*CHa
Detectable degenerated metathesis
*~H3 ~ H3 CH a *CHa
Fully degenerated metathesis
The total scrambling process of 13C methyl groups can be written as follows (eq. 3) and proves to be at least 5 times faster than the productive metathesis:
4 13CHa-CHa
> CHa-CH3 + 2 13CHa-CHa + 13CHa-13CH3
(3)
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[Ta]s-H CH3-CH 3
Scheme 2
H2
[Ta] s-CH2-CH3 CH 4
CH3-CH 3
A Productive Metathesis
CH3-CH3
CH3-CH2-CH3
[Ta] s- CH 3 CH3- CH3
B
CH3-CH3
Degenerate Metathesis CH3-CH3
CH3-CH 3 [Ta] s- CH 3
Productive and degenerated metathesis both require the cleavage and the formation of a C-C bond. The productive reaction is assumed to involve first a C-H bond activation of ethane to form a surface (-Si-O-)2Ta-Et species similarly to the activation of cycloalkanes. In a second step a new molecule of ethane may react with this surface species to transfer a methyl group affording the liberation of propane and the formation of a (-Si-O-)2Ta-Me species. This last species can be displaced by an incoming molecule of ethane to liberate methane and regenerate the (-Si-O-)2Ta-Et species (scheme 2 A); however, the (-Si-O-)2Ta-Me species which is very similar to the (-Si-O-)2Ta-Et species can also be involved in a C-C bond cleavage process, thus giving rise to the degenerate metathesis (scheme 2 B).
3.3. H/D exchange reaction on (-=Si-O-)2TaH. Since a scrambling reaction involving C-C bonds has been evidenced during the metathesis reaction, it was interesting to verify if a similar process could occur in the case of C-H bonds. The study was performed with methane in order to avoid a metathesis reaction of C-C bonds. Different mixtures of C I ~ C D 4 were heated with (-Si-O-)2TaH at 150~ and the formation of the different isotopomers of methane (dl, d2, da) was monitored by IR spectroscopy (figure 3); GC/MS was also used at the end of the reaction to confirm the composition of the gas mixture. Formation of CHaD and CHD3 was first observed before that of CH2D2 suggesting that the H/D exchange proceeds stepwise with only one H or D atom exchanging at a time. After about 10 h, a new stable composition was obtained corresponding
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to the statistical distribution given by the coefficients of the polynom (do + d4)4. This exchange process proves to be approximately 10 times faster than the productive metathesis. , cx 4
6O
= CH3D
A
w I_ G
50
E
40 -=
o
30
0
" 0
9 CH2D 2 x CHD 3 x CD 4
O o
z
o
(4
|
.
c
=:9 q)
0
=
10 0
II -
0
x
0
|
.
9
9 I
200
I
400
I
600
I
800
I
I
1000
1200
I
1400
T i m e (min)
Fig. 3. Evolution of the different isotopomers of methane during the heating of a 53/47 mixture of CH4/CD4 with (-Si-O-)2TaH at 150~ An H/D exchange reaction can also be performed with a CD4/I-I2 mixture to produce a similar distribution. During the reaction with CH4/CD4 at 150~ the IR spectra of the catalyst showed the decrease of the v(Ta-H) band at 1830 cm -~ for the benefit of a new band at 1323 cm -1 consistent with a v(Ta-D) vibration mode. No band for the methyl groups is observed, but the IR intensity of such a band is usually very weak. Three different mechanisms can be proposed for the H/D exchange between CH4 and CD4 depending on whether (-Si-O-)2TaH or (-Si-O-)2TaMe or both species are the active intermediates. 3.4. Hydrogenolysis of alkanes. Surface tantalum and tungsten hydrides also catalyze the hydrogenolysis of light alkanes. Our laboratory has previously reported that silica-supported group 4 metal hydrides catalyze the hydrogenolysis of neopentane, butane, isobutane, propane into a final mixture of ethane and methane; ethane was not cleaved by these metal hydrides. The mechanistic pathway which fits nicely with these observations is based on the key step of 13-alkyl tranfer, which corresponds to the microreverse process of a double C=C bond insertion into a metalcarbon bond: R R \
\1M
\ R'
-
--
- - R'
This key step may logically occur on a metal-alkyl species after a C-H bond activation of the alkane by any of these metal hydrides; since a metal-ethyl species arising from the activation
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of ethane does not contain any alkyl group in 13-position, no C-C bond cleavage can occur. In the opposite, tantalum and tungsten hydrides catalyze the hydrogenolysis of the abovementioned alkanes and also ethane; a [3-alkyl tranfer process can still be invoked to explain the cleavage of all the alkanes except ethane for which another mechanism has to be proposed.
4. CONCLUSION Surface tantalum and tungsten hydrides present unsual properties in the activation of alkanes. The metathesis reaction of alkanes is an unprecedented process affording the formation of higher and lower homologues; it is accompanied by degenerated processes leading to the scrambling of carbon and hydrogen atoms in the molecules; these hydrides also catalyse the hydrogenolysis of ethane. None of these two reactions is catalysed by the group 4 metal hydrides; this tremendous difference can be related to the electronic structure of these surface hydrides which is do for group 4 metals and d2 for tantalum or tungsten.
REFERENCES 1. C. Rosier, G.P. Niccolai and J.M. Basset, J. Am. Chem. Soc. 119 (1997) 12408. 2. J. Corker, F. Lef~bvre, C. IAcuyer, V. Dufaud, F. Quignard, A. Choplin, J. Evans and J.M. Basset, Science 271 (1996) 966. 3. L. D'Ornelas, S. Reyes, F. Quignard, A. Choplin and J.M. Basset, Chem. Lett. (1993) 1931. 4. V. Vidal, A. Th6olier, J. Thivolle-Cazat, J.M. Basset and J. Corker, J. Am. Chem. Soc. 118 (1996) 4595. 5. V. Vidal, A. Th6olier, J. Thivolle-Cazat and J.M. Basset, Science 276 (1997) 99. 6. R.R. Schrock and J.D. Fellmann, J. Am. Chem. Soc. 100 (1978) 3359. 7. O. Maury, L. Lefort, V. Vidal, J. Thivolle-Cazat and J.M. Basset, Angew. Chem. Int. Ed. 38 (1999) 1952.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Characteristics of ethylene polymerization catalyzed over Ziegler-Natta/ Metallocene hybrid catalysts: Comparison between silica-based and magnesium-based supports Han Seock Cho a, Dae Jung Choi a, In Kyu Song b, and Wha Young Lee a* a Division of Chemical Engineering, College of Engineering, Seoul National University, Shinlim-Dong, Kwanak-Ku, Seoul 151-742, Korea* b Department of Industrial Chemistry, Kangnung National University, Kangnung, Kangwondo 210-702, Korea Two types of inorganic supports, silica-based and magnesium-based, were prepared by the sol-gel and the recrystallization method, respectively. The polyethylene produced by the Ziegler-Natta/Metallocene hybrid catalysts showed two melting temperatures and a bimodal MWD, corresponding to products arising from each of the catalysts. This suggests that the hybrid catalysts acted as individual active species and produced a blend of polymers. 1. INTRODUCTION Metallocene catalyst systems hold great promise as the next generation of catalysts for olefin polymerization[l]. Such systems show a high activity and are capable of producing polymers which possess special properties. Although metallocene catalysts have the advantages of high activity and the capability to produce polymers with special properties, the polymers produced via these catalysts have a very narrow molecular weight distribution (MWD) which affects the processability of polymers. In polymer processing, molecular weight (Mw) and MWD are important factors, since they play a major role in mechanical and rheological properties, respectively. On the one hand, polymers with a narrow MWD lead to products with high impact resistance and a higher resistance to environmental stress-cracking, while, on the other hand, polymers with a broad MWD show greater melt flowability at high shear rates. These properties are important for blowing and extrusion techniques[2]. The narrow MWD of polymers produced via metallocene catalysts can be broadened by preparing Ziegler-Natta/Metallocene hybrid catalysts because of the relatively higher Mw of polymers
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derived from Ziegler-Natta component. Such hybrid catalysts can take advantage of the
properties of both metallocene and Ziegler-Natta catalysts. In addition, hybrid catalysts would be expected to enhance polymer processability and to be utilized in prevailing processes without significant process modifications. In this study, two types of supports, silica-based and magnesium-based, were prepared for the impregnation of hybrid catalysts, and ethylene polymerization was carried out to investigate the characteristics of the hybrid catalysts. 2. EXPERIMENTAL
2,1. Materials Toluene and ethylene were further purified by usual procedures. CpEZrCI2 (Strem Chem), TiCl4 (Aldrich Chem), MAO (methylaluminoxane, type 4, Akzo Chem), TiBAL (triisobutylaluminum, Aldrich Chem), MgCI2 (Aldrich Chem), colloidal SiO2 (LUDOX HS-40, Dupont), CH3OH (Farmitalia Carlo Erba) were used without further purification. 2.2. Preparation of magnesium-based support and catalysts 0.10 mol of MgCI 2 was introduced into a reactor and 100 mL of methanol was then added. 100 mL of n-decane was then added to this solution and the mixture was stirred at 2000 r.p.m. under vacuum at 80 ~ This recrystallized support (MgC12 94CH3OH) was pretreated with MAO according to the methanol content (18mmol MAO/g-Support). 2g of the MAO-treated supports were suspended in 100 mL toluene, and reacted with 0.10 g of Cp2ZrCI2 at 50 ~ for 2 h (Cp2ZrCIE/MAO/MgC12, Zr:0.14 wt%). 3mL of TIC14 was introduced into this supported catalyst and the mixture was stirred at 70 ~ for 2 h, and then washed (TiCI4/Cp2ZrCIE/MAO/ MgC12, Zr:0.12, Ti:2.30 wt%). The support was reacted with 3 mL of TiCl4 at 70 ~ for 2 h and then washed (TiC14/MAO/MgC12, Ti:2.78 wt%). 2.3. Preparation of silica-based bisupports and catalysts 0.035 mol of MgC12 was dissolved in 20 mL of distilled water (pH 6.70), and was then introduced into 1.4 L of corn oil media. It was uniformly dispersed and colloidal
SiO 2 was
then added to initiate gelation. The particles were separated, washed, and dried at 80 ~ for 24 h. The bisupport (MgCI2/SiO2) was suspended in toluene and MAO was added according to the hydroxyl content (10mmol/g-Support). 2g of the MAO-treated bisupport was suspended in 100mL of toluene and reacted with 0.10g of CpEZrC12 at 50"C for 2 h and then washed (CPEZrCIE/MAO/MgCI2/SiO2, Zr:0.10wt%). This catalyst was reacted with 3mL of TiCl4 at 70~
for 2 h
(TiC14/CpEZrCIE/MAO/MgCI2/SiO2,Zr:0.06,
Ti:0.71wt%). The bisupport was
reacted with 3 mL of TiC14 at 70 ~ for 2 h and then washed (TiCI4/MgCI2/SiO2, Ti:0.70 wt%).
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2.4. Characterization and ethylene polymerization The morphology of the support was observed by SEM (JEOL JSM-840A). The elemental content ratio on the surface of support was measured by SEM-EDS (JEOL JSM-840 A). The surface area and pore volume were determined by N2-BET (Micromeritics ASAP-2000). The Ti and Zr content of the catalysts were determined by ICP (VG PQ2-Turbo, VG elemental). Polymer analysis by DSC (Dupont V 4.0B) was carried out at a heating rate of 10 ~ The Mw and MWD were determined by GPC (PL-210, Polymer Lab. Ltd.). 300 mL of toluene and a cocatalyst were introduced into a 1 L reactor and the reactor was then evacuated to remove N2. Ethylene was fed into the reactor at a constant pressure of 1.3 atm, which contains hydrogen partial pressure of 0.2 atm. The polymerization reaction was initiated by introducing the catalyst suspension into the reactor at 700(]. The polymerization was terminated after 50 min by adding an excess of dilute hydrochloric acid solution, and the resulting polymer was isolated and dried. The polymerization rate was determined from the amount of consumed ethylene, as measured with a mass flowmeter. 3. RESULTS AND DISCUSSION
3.1. Characteristics of the bisupport (MgCI2/SiO2) Fig.1 shows a proposed mechanism for the formation of the bisupport. Particles of the colloidal sol are approximately 12 nm in diameter. These particles contain negative charges on their surfaces, which serve to suppress the gelation of particles. When a MgCl2 solution is added to this stable sol, the dissolved magnesium salt neutralizes the negative charge on the surface of the stable sol and the silica undergoes gelation via dimerization, trimerization and further polymerization. Since the agglomerated bisupport is formed in a dispersed media, the particle growth is limited, resulting in the formation of solid particles within a few minutes. As shown in Fig.2, the bisupport has a spherical morphology and the particle size varies with agitation speed. This morphology and the size of the support are important in heterogeneous catalysis because the produced polymers replicate that of the supported catalyst. Table 1 shows the characteristics of the bisupport. The difference in the relative weight ratio of Mg/Si Cl
Si(
__OH O'N~ I~ 0 jOH HO ..
Stable Sol
~.-"
--~ ( SiO2 MgC,2 ~ O H
Solution
~'~
H IVkJ H 0 ~. 0' 0 " - ~
SiO2 '~
etc.
H O ~
Mg
Fig. 1. Proposed mechanism of the reaction between colloidal Si02 and MgCI2 solution.
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.
.
Table 1. Characteristics of the bisupport with respect to agitation speed Agitation Average Particle Surface Pore Si,Mg Contents Speed Size Area Volume in Bisupport (r.p.m.) (~tm) (m2/g) (cc/~) (wt%) 1000 61.7 138 0.229 Si:43.1 Mg:l.34 2000 39.6 135 0.217 Si:43.9 Mg:l.47 3000 13.5 134 0.238 Si:44.5 Mg:l.39 .
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SurfaceRatio Si: Mg: CI (wt%) 89.1" 1.3:9.6 89.4: 1.7:8.9 87.2: 1.7:11.1 .
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between the bulk and on the surface of the bisupport was about 50 (wt%/wt%), suggesting that magnesium is reasonably well distributed throughout the interior and exterior portions of the bisupports. In addition, the relative mole ratio of Si: Mg: C1 on the surface of the bisupport indicates that hydroxyl groups on the surface of colloidal silica interact with Mg +2 during the formation of the bisupport, thus generating magnesium oxide (-Si-O-Mg-CI).
3.2. Characteristics of the recrystallized MgCI2 In the case of recrystallized MgC12, considerable amount of methanol (ca. 55wt%), which was employed as a solvent, exists. The methanol content of the support is important because the chemical complexes between methanol and alkyl aluminum create the impregnation sites for the metallocene catalyst[2]. The methanol in the support also serves as a deactivation material, that is Lewis base, for the metallocene catalyst, and, therefore, should be eliminated. An alternative method is to heat the support, in order to reduce the methanol content. In this case, however, the methanol serves as a source of methoxy groups, which do not serve as impregnation sites, as shown in equation (1)[3]. Thus, stoichiometric amounts of MAO were reacted with the recrystallized MgC12 to create the impregnation sites. MgC12 * 4CH3OH
~ MgClx(OCH3) v + mHC1 + nCH3OH
(n+m=4, X+Y=2)
A
(a) (b) (c) Fig.2. SEM photographs of the bisupport with respect to the agitation speed: (a) 1000 r.p.m. ; (b) 2000 r.p.m. ; (c) 3000 r.p.m.
(1)
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3.3. Ethylene polymerization over hybrid catalysts Tables 2 and 3 show the results of ethylene polymerization over the metallocene supported catalyst, the hybrid catalysts, and the Ziegler-Natta supported catalyst. The activity decreases, compared between the metallocene supported catalyst and the Ziegler-Natta supported catalyst, due to the relatively low activity characteristics of Ziegler-Natta catalysts. The polymers produced via the hybrid catalysts with MAO cocatalyst showed different patterns for each support. The polymers produced via the hybrid catalysts supported on MgCI2 showed one melting temperature (Tm), which was mainly produced via Ziegler-Natta catalyst. In contrast to this pattern, the polymers via the silica-supported hybrid catalysts showed two Tm. In this case, the metallocene portion is more dominant than that of Ziegler-Natta catalyst, and the bimodality could be adjusted by varying the cocatalyst ratio, TiBAL to MAO. With corresponding to TiCl4,
increasing amounts of TiBAL, the peak intensity around 140~ increased, and the peak around 130~
corresponding to Cp2ZrCI2, decreased, as shown in
Fig.3 (A). This is due to the fact that aluminum alkyls form complexes with the active zirconium, thus reducing the number of active sites[4]. Two characteristic peaks can be clearly observed in Fig.3 (A), suggesting that the polymers are composed of two lamellar structures, each of which are polymerized by one of the catalysts. The variation of GPC profiles is similar to that ofDSC thermograms as shown in Fig.3 (B). The positions of the two peaks via the hybrid catalysts are consistent with that of polymers produced by metallocene and Ziegler-Natta supported catalysts, respectively. It is noteworthy that this variation in modality affects the molecular weight as well as the molecular weight distribution, and can be cotrolled by varying the cocatalyst ratio. 4. CONCLUSIONS Hybrid catalysts appear to be compatible with the supports examined herein, and produce a blend of polymers. The produced polymers showed bimodal patterns in DSC and GPC analyses, suggesting that the hybrid catalysts acted as individual active species on the support. Table 2. Analytical data on polyethylene via various catalysts supported on magnesium-based support Mole ratio Tm Xc b Mw Catalysts Cocatalyst (Al/[Metal]) Activitya (~ (%) (x 105) MWD Cp2ZrCIJMAO/MgC12 .
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MAO
TiCI4/Cp2ZrCI2/ MAO/MgCI2 .
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0.48
3.8
MAO A1/Zr=3000 6.21 137.0 57.2 6.40 "-- M.~t3-- - - -/(1/ZIL--~3()()(~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 137.1 60.2 6.09 TiBAL AI/Ti=25 TiBAL A1/Ti=100 9.1 137.3 59.8 5.88 TiBAL A1/Ti=100 12.5 137.3 61.4 5.45
8.6
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TiCI4/MAO/MgCI2
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A1/Zr=3000
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a Activity: kg-HDPE/g-[Metal].atm.hr,
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b
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38.8 .
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129.5 .
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70.8 .
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7.4
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Crystallinity: Xc(%)= 100(AHm/AHm*); AHm*=282.84J/g.
6.4 5.9
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Table 3. Analytical data on polyethylene via various catalysts supported on silica-based support Catalysts Cp2ZrCIJMAO/
Mg_Cb_/Si9~_
TiCI4/Cp2ZrCI2/ MAO/MgCI2/SiO2
TiCI4/MgCI2/SiO2
Cocatalyst
Mole ratio (Al/[Metal])
Activity~
Tm (~
Xc b (%)
MW (x 10-5)
MWD
MAO
AI/Zr=3000
20.2
131.0
60.1
0.51
2.3
MAO
AI/Zr=3000
4.43
59.6
2.10
3.6
MAO TiBAL MAO TiBAL TiBAL TiBAL
Al/Zr=3000 Al/Ti= 100 Al/Zr=3000 Al/Ti=300 AI/Ti=100 AI/Ti=100
44.3
5.84
8.1
40.4
8.33
42.0
39.8 42.5
7.34 7.60
5.9 6.3
a Activity: kg-HDPE/g-[Metal].atm.hr,
b
3.58 2.92 2.50 1.97
Crystallinity: Xc(%) = 100(AHm/AHm*); AHm*=282.84 J/g.
f A
131.0 137.8 128.3 139.5 127.9 139.5 139.7 139.0
0.8 I1
e
=1
0.6 o .~ 0.4
~ r~
0.2 0.0 100
120
140
160
2.5
3.5
4.5
Temperature (~
5.5
6.5
7.5
logM
(A) (B) Fig. 3. (A) DSC thermograms: (B) GPC profiles; Cp2ZrCIJMAO/MgCI#SiO2: (a) MAO; TiCL/ Cp2ZrCI2/MAO/MgCIJSi02: (b) MAO; (c) MAO & TiBAL (AI/Ti=100); (d) MAO & TiBAL (AI/Ti= 300): (e) TiBAL (AI/Ti=100); TiCL/MAO/MgClJSiO:: (f) TiBAL (AI/Ti=100). ACKNOWLEDGMENT
The authors wish to acknowledge financial support from the Korea Science and Engineering Foundation (KOSEF) and the Korea Institute of Industrial Technology (KITECH). REFERENCES
1. W. Kaminsky and H. Sinn, Adv. Organomet. Chem., 18 (1980) 99. 2. H. S. Cho, J. S. Chung, J. H. Han, and W. Y. Lee, J. Appl. Polym. Sci., 70 (1998) 1707. 3. D. N. T. Magalhaes, O. D. C. Filho, and F. M. B. Coutinho, Eur. Polym. J., 27 (1991) 1093. 4. D. Fischer and R. Mtilhaupt, Makromol. Chem. Makromol. Syrup., 66 (1993) 191.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Ethylene polymerization with zirconocene-MAO supported on molecular sieves Icaro S. Paulino § Antonio Pedro de Oliveira Filho, Jos6 Luis de Souza and Ulf Schuchardt* Instituto de Qufmica, Departamento de Quimica Inorg~nica, UNICAMP P.O. Box. 6154, 13083-970, Campinas, SP- Brazil. The catalytic activity of Cp2ZrC12 supported on three types of molecular sieves (MCM41, VPI-5 and Y Zeolites) was evaluated in the polymerization of ethylene. The supports were dehydrated, pre-treated with methylaluminoxane (MAO), and then reacted with Cp2ZrCI2. After heterogenization of Cp2ZrC12 on the molecular sieves, the MAO concentration could be reduced without significant effect on the catalytic activity. The polymers obtained with the heterogeneous catalysts showed higher melting points and molecular weights, as well as narrower polydispersion than those obtained with the homogeneous catalyst precursor. 1. INTRODUCTION In the presence of methylaluminoxane (MAO), zirconium metallocene dichloride catalyses the polymerization of olefins with high activity [ 1]. However, the industrial use of this catalyst is hampered by the large quantities of MAO needed. Furthermore, it is not possible to promote gas phase polymerization reactions [2]. In order to reduce the amount of MAO used, the immobilization of metallocene on the surface of oxides such as alumina and silica gel has been attempted by Soga et al. [3] and by Sacchi et al [4]. A different approach involves the incorporation of the catalyst and co-catalyst in the channels of molecular sieves. Molecular sieves are promising supports for catalysts because of their large surface area and well defined pore diameter. Furthermore, deactivation of zirconocene by dimerisation is avoided because the narrow channels of the molecular sieves do not allow the formation of dinuclear species. Molecular sieves are available with one, two and three-dimensional structures and different pore sizes. In Zeolite Y the pore diameter is 7.4 .A and the supercage is 13.7/k wide [5]. A1PO-VPI-5 is an aluminophosphate containing onedimensional channels with a diameter of 12.4 ik [6]. MCM-41 is a silicate with hexagonal array of one-dimensional mesopores, whose diameters can be tuned from 20 to 100 A [7]. In this work, the polymerization of ethylene catalyzed by bis(cyclopentadienyl)zirconium(IV) dichloride supported on three molecular sieves (Zeolite Y, A1PO-VPI-5 and MCM-41) was studied. 2. EXPERIMENTAL
2.1. Synthesis of the Catalytic Precursors The zirconocene (Cp2ZrC12) was prepared by reaction of cyclopentadienylsodium with +Correspondingauthor:
[email protected] 家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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zirconium(IV) chloride, in freshly dried tetrahydrofuran [8]. AIPO-VPI-5 was synthesized according to the method described in the literature [9]. MCM-41 was synthesized according to a method developed in our laboratory using sodium metasilicate Na2SiO3 and Aerosil 200 as silica sources. Tetramethylammonium hydroxide (TMAOH) was used as a mineralizer and cetyltrimethyammonium bromide (CTMABr) as a template. The reaction gel was prepared by suspending the silica sources in an aqueous solution of TMAOH (25%) and combining this suspension with CTMABr dissolved in water, obtaining a final gels with the following composition: 1 SiO2 : 0.15 Na20 : 0.08 TMAOH : 0.21 CTMABr : 100 H20. The gel was refluxed in a glass flask for 16 h.. The obtained solid was filtered off, dried at 100~ and finally calcined at 540~ for 1 h under a flow of nitrogen and subsequently for 6 h under a flow of synthetic air. Zeolite Y samples were provided by Degussa and used as received.
2.2. Preparation of the Supported Catalysts The catalysts were prepared by stirring the molecular sieves and MAO (10% in toluene, Akzo Nobel) in toluene for 4 to 20 h, filtering and washing with toluene. The slurry obtained was dried in v a c u u m (10 "2 mmHg) and the resulting solid impregnated with bis(cyclopentadienyl)zirconium(IV) dichloride, under vigorous stirring in toluene. The catalyst was then washed and dried in vacuum. Zr and Al contents of the supported catalyst were determined by inductively coupled plasma (ICP) spectroscopy. 2.3. Polymerization The polymerization experiments were carried out in a 1 L Biachi reactor at 70~ and 2 bar of ethylene using 50-100 mg of catalyst and 0.5-2.0 mL of MAO in 200 mL of toluene. Polymerization was also accomplished in homogeneous phase using 4.8 mmol of Cp2ZrC12, 3.0 mL of MAO and 200 mL of toluene. The polymerization temperatures were varied at 70~ After 30 min, the polymerization reactions were interrupted with the addition of ethanol. The polymers were filtered, washed with ethanol and dried in a stove at 60~ for 4 h. 2.4. Determination of the Melting Point The thermal characteristics of the polymers were examined using a DSC 4 (Perkin Elmer) with a heating rate of 10~ in the range from 50 to 200~ Melting points (m.p.) are related to the 2 nd heating process. 2.5. Determination of the Molecular Weights The molecular weight averages, Mw and Mn, of the polyethylene samples were determined by gel permeation chromatography (GPC), using 1,2,4-trichlorobenzene as a solvent at 140~ in a Waters 150C chromatograph. Viscosity-average molecular weights (Mv) of the polymers were measured in decalin at 135~ using the equation of MarkHouwink [ 10]. 2.6. Analysis of the Polymer Morphology The morphology of the polymers was analyzed by scanning electron microscopy using a JEOL model JSM-T300 microscope, with a tension of 25 kV and amplification of 1500 to 7500 times. Qualitative measures of distribution of the metals in the samples were made with a Noram dispersive energy X-ray spectrometer (EDS), coupled to the electron microscope. Larger particles of the polymer were analyzed with a Micronal-CBA-K optical microscope.
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3. RESULTS AND DISCUSSION The amounts of Zr and AI supported on the molecular sieves were measured by ICPOES. The results are shown in Table 1. Table 1. Zr and A1 contents in the catalysts as determined by ICP-OES. Catalyst A1PO-VPI-5/MAO/Cp2ZrC12 NaY/MAO/CpzZrCI2 MCM-41/MAO/CpzZrCI2
btmol Zr/g cat. 41 65 140
mmol Al/g cat. 2.92 a) 2.95 a) 4.02
AI/Zr 75 45 29
a) Aluminum belonging to MAO only. The amount of Zr and Al supported on the different molecular sieves varied from 41 to 140 ~tmol Zr/g cat. and 2.92 to 4.02 mmol A1/g cat. To determine the amount of aluminum belonging to MAO in zeolite Y and in AIPO-VPI-5, an analysis of the pure molecular sieves was done previously. 0.26 mmol A1/g was found in zeolite Y, and 6.80 mmol Al/g in A1POVPI-5. The largest amount of MAO immobilized in MCM-41 can be attributed to the larger number of hydroxyl groups present in this molecular sieve. This phenomenon was also observed by Tait et al. [ 11 ] in the reaction of MAO with alumina and with silica. They found that alumina adsorbs twice as much MAO as silica does, because of the higher concentration of superficial hydroxyls in alumina. With an increased amount of MAO on the support, it is possible to form a large number of the complex zirconocene/MAO inside the pore system. Furthermore, larger pore diameters would allow zirconocene to diffuse more freely into the molecular sieve.
3.1. Polymerization Reactions Typical results obtained in the polymerization reactions are shown in Table 2. With the heterogenization of CpEZrC12 on the molecular sieves, the amount of MAO used to promote the polymerization could be reduced without loss of activity, since the immobilization of zirconocene on the molecular sieves hampers its deactivation by bimolecular processes [ 12]. Table 2. Typical results obtained in the ethylene polymerization using the catalyst Cp2ZrC12 in homogeneous phase and supported on A1PO-VPI-5, MCM-41 and zeolite y.a) A1/Zrb) Activityc) Prod. a) Mv e) Catalyst NaY/MAO/Cp2ZrC12 423 2300 298 182 VPI-5/MAO/CpzZrC12 1000 2470 201 216 MCM-41/MAO/CpzZrC12 425 2560 704 350 CpzZrClz/MAO 2000 2980 ..... 86 a) The polymerization was carried out at 70~ b) Rate Al/Zr during the polymerization. c) kg PE/molZr.bar.h. d) Productivity- g PE/g catalyst. e) kg/mol. f) polydispersity Index Mw/~n.
Mn e) 269 363 539 126
Mw e) PDi t) m.p.(OC) 553 2.1 139 753 2.1 141 1070 2.0 145 280 2.2 135
2 bar of ethylene, for 1 h.
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The polymers synthesized with the heterogeneous catalysts present a polydispersity similar to that obtained with the homogeneous catalyst. This indicates that zirconocene molecules are homogeneously distributed on the support, forming active sites with similar steric and electronic characteristics. The homogeneous distribution is probably due to the regularity of the pore system in the molecular sieves, which promotes the formation of roughly uniform catalytic sites inside the channels. The opposite is observed when supports with wide distribution of pore size are used, such as silica or alumina. The polydispersity of polyethylene produced with these catalytic systems range between 3 and 5 [ 13]. The heterogeneous catalysts produce polymers having viscosimetric molecular weights between 180 and 350 kg/mol, which are 2-4 times higher than those obtained with the homogeneous system. In a similar fashion, the melting points of the polyethylene obtained with the heterogeneous catalysts were found to be 139-145~ while the homogeneous system produces polyethylene with melting points around 135~ This increase in the melting point is related to the increase in the molar mass of the polymers. MCM-41 proved to be the best support for Cp2ZrC12, showing the highest values for activity and productivity. Additionally, polyethylene obtained with this catalytic system presented better properties such as larger molecular weight and higher melting point.
3.2. Polymer Morphology The morphology of the polyethylene obtained with the catalysts was analyzed by scanning electron microscopy and optical microscopy. Figure 1 shows micrographs of the polymers and of the respective supports.
. . . . . . . . . . . . . .iI!. . . . .
9
. .i!.::,,,,. iL
!
................ ..L..s ................................................................ (a~
(b)
(c)
(d) (r (f) Figure 1. Scanning electronic micrographs of the supports: MCM-41 (a), Zeolite Y (b) e A1PO-VPI-5 (c). Optical micrograph of the polyethylene obtained with the catalysts MCM41//MAO/Cp2ZrC12 (d), NaY/MAO/Cp2ZrC12 (e) e VPI-5/MAO/Cp2ZrC12 (f).
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One of the most important characteristics of olefin polymerization using heterogeneous catalysts is the replication of the polymer particles following the original morphology of the particles of the support. The replication rate (rate between the medium diameter of polymer particles and the medium particle diameter of the catalyst) for third generation Ziegler-Natta catalysts varies between 40 and 50 [ 14]. With the catalysts supported on the molecular sieves studied in this work, it was possible to obtain polymers with up to 4 mm diameter and replication rates equal to 600, which are much higher than those obtained with the conventional Ziegler-Natta catalysts. The polymers obtained with the heterogeneous catalysts present a granulated form, unlike those obtained with the homogeneous catalyst, which produces powdered polyethylene. The granulated morphology is of great industrial interest because it facilitates the processing of the material. Assuming that the polymerization reactions occur inside the pores of the supports, the particles of the polymer formed will fill these spaces gradually. The tension inside the support increases due to the accumulation of polymeric chains in the pores, eventually causing the structure of the catalyst to collapse. Niegisch et al. [ 15] suggested that the fragmentation happens inside due to the build-up of hydraulic force of the pores of the support in the beginning of the polymerization, but it is difficult to examine the remains of the support, since the small fragments of the support remain in the large particles of the polymer. To overcome this problem, the polymerization was interrupted after five minutes, with the objective of obtaining small particles of the polymer that could be analyzed in the electron microscope. When the particles of the polymer were examined by backscattering, fragments of the support A1PO-VPI-5 were found dispersed on the polymer, confirming that the catalyst is broken into fragments during the polymerization. Figure 2a shows the micrograph of a polyethylene particle, where the points in white are fragments of the support. Figure 2b is a amplification of one fragment of the support (detail A). The presence of a fragment of the support was confirmed by the analysis of EDS.
(a)
(b)
Figure 2. Micrography of the Polyethylene (a) and fragments of the support A1PO-VPI-5, detail A (b). 4. CONCLUSION The molecular sieves used in this work proved to be good supports for the catalyst Cp2ZrC12, since they led to catalytic activities similar to those obtained with its homogeneous precursor, using a lower ratio AI/Zr. Additionally, the polyethylene obtained with the
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supported catalysts presented higher melting points and molecular weights, still maintaining a narrow distribution of the molecular weight. With the heterogenization of the zirconocene it was possible to obtain polyethylene granulated, which is not possible with the homogeneous catalysts. A small decrease in the catalytic activity was observed probably because of the difficult access to the active sites, which at least in case of zirconocene seems to be confined in the inner pores of the molecular sieves. Particularly, MCM-41 was shown to be the best support for Cp2ZrCI2, leading to larger activity and higher productivity. Additionally, the polyethylene obtained with this catalytic system presented better properties such as larger molecular weight and higher melting point. ACKNOWLEDGEMENTS The authors would like to thank FAPESP and CNPq for financial support and Petroquimica Uni~o, Degussa AG, Akzo and Condea for supplying the starting materials. REFERENCES
1. H. Sinn and W. Kaminsky, Adv. Organomet. Chem., 99 (1980) 18. 2. S. S. Reddy and S. Sivaram, Prog. Polym. Sci., 20 (1995) 309. 3. K. Soga and M. Kaminaka, Makromol. Chem., 194 (1993) 1745. 4. M. C. Sacchi, D. Zucchi, I. Trito and P. Locatelli, Macromol. Rapid Commun., 16 (1995) 581. 5. M. M. J. Treacy, J.B. Higgins e R. von Ballmoos, Zeolites, 16 (1996) 323. 6. M. E. Davis, C. Saldarriaga e C. Montes, Zeolites, 8 (1988) 362. 7. S. Bis e M. L. Occelli, Catal. Rev., 40 (1998) 329. 8. G. Wilkinson and J.M. Birminghan, J. Am. Chem. Soc., 76 (1954) 4281. 9. M. E. Davis, C. Montes, P. E. Hathaway and J.M. Garces, Stud. Surf. Sci. Catal., 42 (1989) 199. 10. R. B. Seymor and C. E. Carraher Jr., "Polymer Chemistry- An Introduction", Marcel Dekker, New York, 1981. 11. P. J. T. Tait, A. I. Abozeid and A. S. Paghaleh, Proceedings of the Worldwide Metallocene Conference, 1995. 12. W. Kaminsky and F. Renner, Macromol.Chem, Rapid Commun., 14 (1993) 239. 13. M. R. Ribeiro, A. Deffiex and M. F. Portela, Ind. Eng. Chem. Res., 361 (1997) 1224 14. H. F. Mark, N. M. Bikales, C. G. Overberger and G. Menges, Encyclopedia of Polymer Science and Engineering, vol. 13, John Wiley, New York, 1988. 15. W. D. Niegisch, S. T. Crisafulli, T. S Nagel and B. E. Wagner, Macromolecules, 25 (1992) 3910.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
In situ and ex situ study of propylene polymerization with a MgCI2-
supported Ziegler-Natta catalyst V. Oleshkoa, P. Crozier a, R. Cantrell b and A. Westwood ~ aCenter for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA. bpolypropylene Catalyst R&D, Unipol | Systems Department, Union Carbide Corporation, Westhollow Tech. Ctr., PO Box 4685, Houston, TX 77210, USA. CAnalytical Technology Section, R&D, Union Carbide Corporation, PO Box 670, Bound Brook, NJ 08805, USA. In situ light and TEM microscopy observations of polypropylene formation over a TiC14MgC12 supported Ziegler-Natta catalyst have been performed by video recording in real time. Polymer formation was confirmed by TG-DTA. The morphologies of the catalyst and polymer particles were characterized by the combination of ex situ light microscopy, SEM/EDX, conventional and high-resolution TEM, and STEM/PEELS/EDX.
1. INTRODUCTION The large-scale production of polyolefins (polyethylene, polypropylene) continually stimulates the development of high activity heterogeneous Ziegler-Natta catalysts with the third and fourth generation systems in production, such as, high surface area defective MgC12 with TiCI4 chemisorbed on to the surface. Currently, Ziegler-Natta polymerization is the only major process for producing highly stereospecific polypropylene. However, in spite of intensive research on the kinetics and mechanisms of the process, the present level of understanding of the catalyst is still incomplete because of its complex composition, comprising multiple components (i.e., TiCI4, AIR3, electron donors) leading to a multitude of local active site environments [ 1]. To our knowledge, this paper presents the first in situ video microscopy study of propylene polymerization over a MgC12-supported Ziegler-Natta catalyst combined with ex situ characterization by light microscopy (LM) and advanced electron microscopy techniques.
2. Experimental 2.1. Catalyst preparation and characteristics The catalyst was prepared in accordance with the procedures outlined in an earlier patent [2]. As prepared, the procatalyst contains 19% magnesium, 2.9% titanium and 12.4% diiso-
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butylphthalate. Catalyst performance was evaluated in a standard one gallon LIPP (liquid propylene polymerization) reactor. Reaction conditions were as follows: Ti - 0.01 mmol, H2 45 mmol, triethylaluminum (AIEt3) - 1.0 mmol, a silane donor- 0.25 mmol, temperature 65~ and duration - one hour. Polymer yield was 28.8 kg polypropylene/g of the catalyst.
I
Color TV-camera VK-C370 i I Video recorder I I IA/I HS-U760 ~ 1_.___[Infini--var
Color monitor
PVM-1351Q a
Lig~V ~V5
~3
~nI-I: V4~
IRP!
Fig. 1. (a) Setup for in situ video recording of catalytic polymerization of propylene: V 1-5 valves, P- manometer, RP - rotary pump; (b, c) instantaneous growth of polymer; arrows indicate growing polymer beads, the two images were recorded within 1/34 s interval; (d) the same area after 29 s; (e) the same area after 129 s. The catalyst specimens were stored in a glove box under a He atmosphere (< lppm H20/O2). All sample preparation for microscopic analysis was conducted in the dry box under dry conditions. Samples were transferred to specimen holders in the dry box and then transferred into the microscopes under high purge N2 conditions to prevent poisoning of the catalysts by air and moisture.
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2.2. In situ video recording of polymerization
A schematic of the experimental setup used for the in situ study of propylene polymerization is shown in Fig. 1. The procatalyst was combined with the co-catalyst, A1Et3, and gas phase catalytic polymerizations were run at 22-25~ in a glass cell with an input of dry propylene at a pressure of~20 psi. Monomer was periodically added to the cell to keep the same reaction conditions. The process was monitored by an InfiniVar-ZOOM video microscope at a magnification of x80-160. Recently direct real-time observations of the polymerization were also performed using a Philips 430 transmission electron microscope (TEM) at 300 kV accelerating voltage fitted with an environmental cell at gas pressures of 0.5-1 torr.
2.3. Ex situ characterization Thermogravimetric and differential thermal analysis (TG-DTA) was carried out on a SETARAM-TG-DTA 92-16 derivatograph with an alumina tube furnace and a graphite heater at a heating rate of 2~ min -1 over the temperature range 20-200~ in an Ar atmosphere (Fig.
2). 40 56t ~ - ~
60
80
100
~
120
140
^ [I
4-1
~
160
200
Mass Loss, mg (1,2) -6.5 Heat Flow, ~tV (3,4)_ f
- -
II
180
~
~
/
F-4
..3~
E 3 o
2
2 o
I
133"C
I
o
t0
-1-1
~
I--1
~_._,~.-.~l,
9
-L 2
-3
-. 2
-3 40
60
80
100
120
140
160
180
200
Temperature, ~
Fig. 2. TG-DTA of the catalyst and polymer in argon. TG, left axis" 1 - heating cycle, 2 cooling cycle; DTA, right axis: 3 - heating cycle, 4 - cooling cycle. LM was performed on a Mitutyo Ultraplan FS-110 video microscope equipped with a long focus lenses in bright- and dark-field transmission, reflection and differential interference contrast modes at working magnifications from x100 to x1000. Images were recorded by a color video TV-camera and digitized on-line. Scanning electron microscopy (SEM) was carried out at 15 kV accelerating voltage on a JEOL JSM-840 scanning electron microscope equipped with an EmiSPEC Vision digital data acquisition system and a Noran energy-dispersive x-ray (EDX) analyzer. For SEM, catalyst powders were deposited onto conductive supports and carbon coated. For TEM, the material was dry crushed in an oxygen-free He atmosphere and deposited onto lacey carbon films. Samples were analyzed using a JEOL-4000EX microscope operating at 400 kV accelerating
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voltage with a point-to-point resolution of 0.17 nm. Scanning transmission electron microscopy (STEM) and compositional nanospectroscopy by parallel electron energy-loss spectroscopy (PEELS) and windowless EDX analysis were performed on a VG HB-501 STEM operating at 100 kV accelerating voltage with resolution of 0.4 nm.
3. RESULTS AND DISCUSSIONS
Figs. l b and 1c present successive images of the reacting catalyst recorded with an interval of 1/34 s, which revealed growing polymer beads 9-10 ~tm in size. Primary beads could then aggregate forming much larger conglomerates of 20-40 ~tm in size. The amount of polymer increased drastically within the localized area during the next 129 s as shown in Figs. 1d and le.
z0~m
...~i . ii :z
Fig.3. Morphology of initial (al, bl, el) and "working" catalyst (a2, b2, c2): al, a 2 - LM, bright-field, transmission mode; b 1, b 2 - SEM, SE; c 1, c2 - SEM, BSE, "compo" mode.
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As a result of the polymerization, the volume of material increased by a factor of three during the reaction. The formation of polypropylene was confirmed by TG-DTA on the appearance of a narrow exothermic peak corresponding to polypropylene melting at 133~ [3] (Fig. 2). Fig. 3 compares the morphology of the initial and "working" catalyst as revealed by LM (al and a2) and by SEM in secondary electrons (SE, b l and b2) and in backscattered electrons (BSE, c l and c2), respectively. The average particle size in the initial catalyst is about 9 lam. Polypropylene growth increases the particle size in the "working" catalyst to 3050 lam. Fig. 3c2 shows 2-6 lam-sized bright features (higher backscattering coefficient) on the surface of large catalyst/polymer particle of 40 ~tm in size suggesting a partial fracturing of the catalyst in the course of polymer growth. The microscopic observations indicate a complicated fragmentation of the catalyst and formation of aggregated hierarchical structures exhibiting a variety of morphologies [4].
Fig.4. Low-magnification TEM (a, b) and HRTEM (c, d) of catalyst (a, c) and polymer/catalyst (b, d) particles. The TEM image and diffraction pattem of the catalyst particles in Fig. 4a shows crystalline domains of 2-5 nm in size and amorphous areas. The combination of ring reflections in the
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diffraction pattern (insert) was assigned to a hexagonal structure of MgCI2. This is in agreement with high-resolution TEM (HRTEM) observations (Fig. 4c) which exhibit partially disordered lattice fringes with spacings of between 0.26-0.31 nm and 0.56-0.66 nm. On the contrary, the diffraction pattern of a supposed polymer particle (Fig. 4b, insert) revealed only an amorphous halo and the HRTEM image in Fig. 4d showed no evidence for crystallinity. As one can see from the images b and d, this large particle consists of many randomly aggregated polymer globules of 20-40 nm in diameter. 150[
CK
"~ [/ sot
I I
250
300
10001[
-~ 350
Tin23 400
Energy Loss, eV
450
CIKctI
|it~,[MgKl3, II 500
Ib
,,.o
Fig. 5. (a) PEEL and (b) EDX spectrum of catalyst/polymer particles. Lines of interest are marked. Combined PEELS/windowless EDX analyses in the STEM mode (Fig. 5) confirmed the presence of carbon rich polymer particles. PEELS/EDX data point to that polymer and catalyst are often present in all parts of the analyzed particles. CONCLUSIONS We have observed for the first time in situ gas phase polymerization of propylene over a TiCIa-MgCla supported Ziegler-Natta catalyst by video LM and TEM microscopy. Real time observations show that 9-10 ~tm-sized polymer beads appear to form rapidly ( PMo-PES-MC > PMo-PPO-MC > PMo. The enhanced conversions over the composite catalysts were due to the enhanced PMo dispersion. PMo-PPO-MC showed the smallest conversion among the film catalysts. This may be partly resulted from partial agglomeration of PMo throughout PPO matrix. PMo-PSF-MC and PMo-PES-MC showed both enhanced oxidation and acidic catalytic activities compared to the bulk PMo. The selectivity to oxidation over PMo-PPO-MC was three times or more compared to the other film catalysts, but that to dehydration was 50% or less. The structural flexibility of PMo in PMo-PSF-MC and PMo-PES-MC may be also responsible for the enhanced catalysis of non-porous composite film catalysts. The macroporous PMo-PSF-MC-M composite catalyst showed the higher ethanol conversion than the non-porous PMo-PSF-MC. This might be due to the welldeveloped macropores of PMo-PSF-MC-M providing a reduced mass transfer resistance. Table 1. Catalytic activity of composite film catalyst for the ethanol conversion at 170~ Catalyst
EtOH conversion
Bulk PMo a) PMo-PSF-MC b) PMo-PES-MC b) PMo-PPO-MC b) PMo-PSF-MC-M c)
(%)
6.9 39.5 33.7 13.4 46.0
Amount of EtOH converted to product (xl04 moles/g-PMo-hr)(carbon selectivity)
CHaCHO
C2H4
0.52(12.8) 4.67(20.0) 1.79(9.0) 4.71 (59.4) 2.95(10.8)
0.34(8.4) 3.76(16.1) 6.46(32.4) 0.78(9.8) 9.67(35.5)
C2HsOC2H5 3.22(78.8) 14.93(63.9) 11.68(58.6) 2.44(30.8) 14.60 (53.7)
W/F=169.1 g-PMo-hr/EtOH-mole ; air=5cc/min ; film thickness=17 ktm ; a)bulk heteropolyacid treated at 300~ ; b) casted and dried in ambient condition (56% relative humidity) ; r casted and dried at 85% relative saturation of methanol vapor
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The composite film catalysts were also applied to the liquid-phase TBA synthesis from isobutene and water in a semi-batch reactor. As shown in Fig. 5, all the composite film catalysts showed the enhanced TBA yields compared to the homogeneous PMo. The catalytic activities were in the following order; PMo-PPO-MC > PMo-PES-MC > PMo-PSF-MC > homogeneous PMo. In order to confirm such a catalytic behavior of the composite film catalysts, the absorption amounts of isobutene in/on the composite film catalysts were measured at room temperature, as shown in Fig. 6. The PMo-PPO-MC showed the highest absorption amounts of isobutene. It was also observed that PMo-free PPO-MC film showed the highest absorption amounts of isobutene among the PMo-free polymer films. This indicated that polymer matrix was not a simple support for PMo but an active carrier for the reaction. It is believed that the high absorption capability of PMo-PPO-MC for isobutene played an important role in enhancing the isobutene concentration in/on PMo-PPO-MC composite catalyst in the reaction medium. The absorption capability of PMo-PPO-MC was high enough to overcome the low solubility of isobutene in water, which was encountered in a normal liquid-phase TBA synthesis 3
8
0
EL ,
O3
2
0
I-0
o3
o~ .0~ ' ~ 4 ,.-g
0.6 ), the isotherm can be associated with a type 4 isotherm indicating the presence of mesopores within the Ta-pillared ilerite. The hystersis is observed in the relative pressure region of P/Po>0.6 due to the capillary condensation in mesopores, which originates pillaring layered silicates.
3.6. Chemical analysis The metal contents of the samples are ranged from 5 ~ to 3.8 % by weight as determined by an Inductively Coupled Plasma (ICP) spectrometer and an Atomic absorption spectrometer. The Si/Ta ratios in several points of the sample are checked by EPMA (electron probe micro analysis) method and the Si/Ta ratios are constant for the entire samples from same batch. This data indicates a homogeneous dispersion of Ta in the Ta-pillared ilerite samples. Acknowledgments This work is supported by the Korea Institute of Science and Technology under the contract 2E15210 and Brain Pool Program of Korean Government. REFERENCES 1. H. Annehed, L. Faith and F.J. Lincoln, Zeitschriftff~r Kristallographie, 159 (1982) 203G. 2. S. Inagaki, Y. Fukushaima and K. Kuroda, J. Chem. Soc. Chem. Commun., (1993) 680 3. S. Inagaki, A. Koiwai, N. Suzuki,Y. Fukushaima and K. Kuroda, Bull. Chem. Soc. Jpn., 69 (1996) 1449 4. Z. Johan, G.F. Maglione, Bull. Soc. Fr. Mineral Cristallogr., 95 (1972) 372 5. H. Gies, B. Marler, S. Vortmann, U. Oberhagemann, P. Bayat, K. Krink, J. Rius, I. Wolf and C. FyFe, Micropor. andMesopor. Mater., 21 (1998) 183 6. S. Vortmann J. Rius, S. Siegmann and H. Gies, J. Phys. Chem., B 101 (1997) 1292 7. F. Agnes, K. Imre, S.I. Niwa, M. Toba, Y. Kiyozumi and F. Mizukami, Applied Catalysis A: General, 176 (1999) L153 8. K. Kosuge and A. Tsunashima, J. Chem. Soc. Chem. Commun., (1995) 2427 9. J.S. daily and T.J. Pinnavania, Chem. Mater., 4 (1992) 855 10. R. Sprung, M.E. Davis, J.S. Kauffmann and C. Dybowski, lnd. Eng. Chem. Res., 29 (1990) 213 11. S.T. Wong and S. Cheng, Chem. Mater., 5 (1993) 770 12. A.M. Parkash and L. Kevan, Jr. Am. Chem. Soc., 120 (1998), 13148 13. Y.S. Ko and W. S. Ahn, Micropor. andMesopor. Mater., 30 (1999) 283 14. G.N.Vayssilov, Catal. Rev.-Sci. Eng., 39 (1997) 209 15. G. Guiu and P. Grange, J. of Catal., 156 (1995) 132 16. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd Edition, Academic Press, 1982
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Al-pillared hectorite and montmorillonite prepared from concentrated clay suspensions: structural, textural and catalytic properties. R. Molina a S. Moreno b and G. Poncelet I
l
Unit6 de Catalyse et Chimie des Mat6riaux Divis6s, Universit6 Catholique de Louvain, Place Croix du Sud 2/17. 1348 Louvain-la-Neuve, Belgium. Concentrated suspensions of hectorite and montmorillonite have been pillared with base-hydrolyzed aluminum solutions with different OH/A1 molar ratios. The pillared clays have been characterized, and their catalytic performances have been evaluated over Pt-impregnated samples using the hydroisomerization of heptane. 1. INTRODUCTION Pillared clays with different types of pillars have been abundantly investigated over the last 20 years. Al-pillared interlayered days (PILCs) prepared in dilute clay suspensions are the most documented ones. However, in industrial conditions, preparations in diluted systems are uneconomical [1]. In recent years, a few studies on similar materials obtained in concentrated medium have been reported [2-6]. Schoonheydt et al. [2] described the preparation of Al-pillared saponite using 6-10 % clay suspensions. Previous works have shown that Al-pillared clays obtained from concentrated suspensions exhibited structural, textural, and catalytic properties similar to those prepared in dilute conditions [6]. In this study, suspensions of commercial hectorite (6 %) and montmorillonite (2 and 40 %) have been pillared with partially hydrolyzed aluminum solutions. The main structural and textural characteristics of the pillared clays have been determined. The influence of the clay concentration and the OH/A1 molar ratio of the pillaring solution on the catalytic performances has been examined in the isomerization of heptane. Present address: aDepartamento de Qufmica, Facultad de Ciencias, Universidad de Antioquia, Medellfn, Colombia. bLaboratorio de Cat~ilisis, Facultad de Ciencias, Universidad Nacional de Colombia. Ciudad Universitaria, Santa Fe de Bogot~i, Colombia.
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2. EXPERIMENTAL
2.1. Pillaring procedure
Pillaring experiments were carried out with commercial hectorite and Westone-L montmorillonite. They were used as received without previous fractionation. Partially hydrolyzed solutions were prepared by slow addition of 0.5 M sodium hydroxide to 0.2 M aluminum nitrate to achieve OH/A1 molar ratios of 1.2, 1.6, and 2.0 for the hectorite, and 1.6 for the montmorillonite. The concentration of A13+ in the final solution was adjusted to 0.1 M. Vigorous stirring was maintained throughout the hydrolysis. The pillaring solution was aged for 24 h at room temperature. Dialysis bags containing weighed amounts of 2 and 40 % clay slurries of WLmontmorillonite, and 6 % clay slurry of hectorite (the highest possible concentration for this clay) were dipped in glass vessels containing the required volumes of the pillaring solution in order to supply 50 mequiv. A13+per g clay. Each system was aged under mild stirring for 48 h at room temperature. Washing was performed by dipping the dialysis bags containing the clay slurry in distilled water, renewing the water until the conductivity was reduced to its initial value. 2.2. Characterization methods
The basal spacings (001 reflection) of the pillared materials were obtained from the X-ray patterns recorded with a Philips equipment fitted with copper anticathode (Cu-Ko~ radiation, Ni-filtered). The specific surface areas and micropore volumes were determined from the nitrogen adsorption isotherms established at the boiling point of nitrogen, using an ASAP 2000 Sorptometer from Micromeritics. The samples were previously calcined at 400 ~ and degassed in situ at 200 ~ for 6-8 h prior to the measurements. The micropore volumes were determined with a method recently proposed [7]. The catalytic performances of the pillared clays were evaluated in the hydroisomerization reaction of heptane over samples previously impregnated with 5 10-3 M tetrammineplatinum(II) chloride solution in order to load the solid with I wt % Pt. The pretreatment and reaction conditions were similar to those detailed elsewhere [8,9]. The catalytic tests were performed in a continuous flow micro-reactor operated at atmospheric pressure. The catalyst (200 mg) was activated in situ in flowing air for 2 h at 400 ~ followed by helium purge, and reduction of the metal in flowing H 2 for 2 h at 400 ~ The reactor was then cooled to 150 ~ and stabilized at this temperature. A stream of hydrogen saturated with heptane vapor was generated by passing hydrogen through a glass saturator containing heptane thermostated at 27 ~ The total flow was 10
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ml min 1, and the WHSV was 0.9 h -1. The reaction was carried out in temperature-programmed mode, with a ramp of 1.7 ~ min "1. On-line gas phase analysis was done in a HP-5890 gas chromatograph provided with TC detector and 50 m long CPSil-5 capillary column (Chrompack). The treatment of the results (conversion, selectivity) was done as detailed elsewhere [9]. 3. RESULTS AND DISCUSSION. 3.1. Characterization of the pillared clays
The interlayer distances (001 reflections), the specific surface areas, and the micropore volumes of the A1-PILCs previously calcined at 400 ~ are given in Table 1. The two pillared montmorillonites exhibited stable spacings of 17.7 ,/k, and between 16.1 and 17.7 A for the hectorites. These spacings are in the range of values usually found for adequately pillared materials. The hectorite pillared with a solution with OH/A1 ratio of 1.2 had a smaller interlayer distance (16.1 A.). The XRD patterns of the pillared hectorites showed a reflection at ca. 10 ,/k, indicating that a fraction was not pillared. Table 1. Spacings (d), surface areas (SBET), and micropore volumes (V~) Sample (%) Mont.-2% Mont.-40% Hect. (1.2) Hect. (1.6) Hect. (2.0) * shoulder
d (001) (400 ~ (A)
SBET
V~
(m 2 g-l)
(cm 3 g-l)
17.8
263
0.074
17.7 16.1 - (10.0)* 17.7- (10.1)* 17.4 - (9.8)*
277 97 222 164
0.079 0.040 0.076 0.070
The surface area and micropore volumes of the pillared clays determined from the nitrogen sorption isotherms are shown in Table 1. The BET surface areas and the micropore volumes of the montmorillonites prepared with concentrated suspensions were similar to those of the sample pillared in dilute suspension. The textural characteristics of the pillared hectorites differed according to the OH/A1 ratio of the pillaring solution. As expected from the d spacings, a smaller micropore volume was obtained for the hectorite treated with the solution with OH/A1 ratio = 1.2. The sample prepared using a pillaring solution with OH/A1 of 1.6 exhibited similar surface area and micropore volume to those found for the montmorillonites pillared both in diluted or concentrated suspension.
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3.2. Catalytic properties
Hydroisomerisation-hydrocracking of n-paraffins has been largely used for the evaluation of activity of acid solids. Three types of reaction products are obtained: isomerization, cracking and cyclization produts. In these bifunctional catalysts, the role of the metal is to dehydrogenate the paraffin and rehydrogenate the iso-olefins. Isomerization to mono- and di-branched C7 occurs on the protonic sites via protonated cyclo-propane intermediates. 100
5O
9 A 9
80
[]
0
~ 0
U
9
60
4ot
Hect (2.0) Hect (1.6) Hect(1.2) W L - 40~ WL-2%
30
40
20
20
10
0
9 Hect (2.0) 9 Hect (1.6) 9 Hect (1.2) o W L - 40% 9 WL-2%
0 150
200
250
300
350
Temperature
Fig. 1. Variation of total conversion vs reaction temperature
400 (~
150
200
250
300
350
Temperature
400 (~
Fig. 2. Variation of isomerization vs reaction temperature
Figure 1 shows the variation of the conversion of heptane vs reaction temperature obtained over the Al-pillared montmorillonites and hectorites. The results obtained over the two samples of Al-pfllared montmorillonite were consistent with previous studies [8-11]. None of these catalysts reached total conversion. A higher conversion was achieved over the sample prepared with the diluted clay suspension. For the Al-pillared hectorites, differences were observed according to the OH/A1 ratio of the pillaring solution. The clay prepared with the solution with OH/A1 = 2 ratio was the most active while the lowest conversion was found for the day obtained with a solution with OH/A1 = 1.6. The decreasing conversion noticed above 350 ~ reflected the loss of stability of the acid sites. This effect was not found for the Al-pillared montmorillonites. The two montmorillonites and the hectorite prepared with the pillaring solution with OH/A1 = 1.6 were less active than the pillared hectorites (1.2) and
(2.0).
The isomerization vs reaction temperature curves obtained over these samples is shown in Fig. 2. This figure is another illustration of the influence of the OH/A1 molar ratio of the pillaring solution. The two montmorillonites and
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the hectorite (1.6) exhibited similar results, while hectorites (1.2) and (2.0) produced more C7 isomers, particularly the later one. Note that slightly more isomers were produced over the montmorillonite pillared in concentrated suspension. Characteristic values taken from the experimental results have been compiled in Table 2, where T10iso is the temperature at which the yield of heptane isomers reached 10 %. It provided an estimation of the relative activities. At this conversion, cracking and cyclization products were not formed. The values in the following columns were taken at maximum isomerization conversion, namely, the temperature (TMax.), the conversion of heptane (Conv. %), the yield of isomers (Yt~o,% ), and cracking products (Ycr, %), essentially C3 and iso-C4, and the selectivities to C7 isomers (S, %). Table 2. Catalytic results at maximum isomerization Clay Mont.-2 % Mont.-40 % Hect. (1.2) Hect. (1.6) Hect. (2.0)
T10Iso TMax. Conv.
(oc)
(oc)
(%)
265 268 262 267 262
335 327 325 341 312
54 44 66 56 63
YIso
YCr
(%)
(%)
34 36 39 35 47
14 5 22 18 2
63 82 59 63 75
(%)
S
With respect to the yields of isomers, the Al-montmorillonite prepared with 40% clay slurry exhibited a significantly higher selectivity (82 %) than the sample prepared with a 2 % suspension (63 %). At the opposite, higher yields of cracking products (14 %) were formed over the sample Mont.-2 %, than for the sample Mont.-40 % (5 % cracking). In this Table, the difference between the conversion percentage and the sum of the percentages of the isomerization and cracking products represents the percentages of cyclization products (mainly benzene and toluene). For the Al-pillared hectorites, Table 2 clearly shows the influence of the OH/A1 ratio used in the preparation of the catalysts. For the higher OH/A1 ratio (2.0), both higher yields and selecfivities of C7 isomers were obtained compared with the other two samples. Conversely, the lower yield of cracking products (only 2 %) was observed for hectorite prepared with the higher OH/A1 while the others two samples exhibited significantly higher yields of these products (18 and 22 %). This last result should be interpreted as a direct influence of the OH/A1 ratio used in the preparation of catalyst, as it has been reported [12]. This effect is attributed to the higher proportion of Al137§ species with Keggin structure in solutions with OH/A1 = 2.0 than in those with lower molar ratios (1.2 and 1.6) assuming, of course, that this polymeric species is the one constituting the pil!ar precursors [12].
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4. CONCLUSIONS
Al-pillared clays prepared from concentrated suspensions exhibited heat resistant basal spacings, high surface area and micropore volume that did not differ from similar materials obtained in dilute conditions. Hectorite and montmorillonite pillared with hydroxy-aluminum solutions with OH/A1 molar ratio of 1.6 showed similar catalytic performances in the isomerization of heptane. The OH/A1 molar ratio of the pillaring solution plays a role on the catalytic properties. The yields of C7 isomers were significantly improved when using a pillaring solution with OH/A1 = 2.0. The Bronsted acid sites were less stable in M-pillared hectorite than in A1pillared montmorillonite. REFERENCES
1. D.W.E. Vaughan, Catal. Today, 2 (1988) 187. 2. R.A. Schoonheydt and H. Leeman, Clay Miner., 27 (1992) 249. 3. R. Molina, A. Vieira-Coelho and G. Poncelet, Clays Clay Miner., 40 (1992) 480. 4. W. Diano, R. Rubino and M. Sergio, Microporous Mater., 2 (1994) 179. 5. R.A. Schoonheydt and H. Leeman, Clays Clay Miner., 41 (1993) 598. 6. S. Moreno, E. Gutierrez, A. Alvarez, N.G. Papayannakos and G. Poncelet, Appl. Catal., 165 (1997) 103. 7. M. Remy, A. Vieira-Coelho and G. Poncelet, Microporous Mater., 7 (1996) 287. 8. S. Moreno, R. Sun Kou, R. Molina and G. Poncelet, J. Catal., 182 (1999) 174. 9. S. Moreno, R. Sun and G. Poncelet, J. Catal., 162 (1996) 198. 10. A. Schutz, W.E.E. Stone, G. Poncelet and J.J. Fripiat, Clays Clay Miner., 35 (1987) 251. 11. R. Molina, A. Schutz and G. Poncelet, J. Catal. 142 (1994) 79. 12. S. Moreno, R. Sun, and G. Poncelet., J. Phys. Chem. 101 (1997) 1569.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Synthesis of high surface area transition metal carbide catalysts A.P.E. York, J.B. Claridge, V.C. Williams, A.J. Brungs, J. Sloan, A. Hanif, H. AI-Megren and M.L.H. Green
Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, U.K. The synthesis of molybdenum and tungsten carbide from their oxides using the temperature programmed reaction method with methane and ethane is presented. Lower reaction temperatures are required for metal carbide formation with ethane, and the highest surface areas materials are also prepared by this method. In addition, the use of the different hydrocarbons enables synthesis of various phases of molybdenum or tungsten carbide. Thermogravimetric analysis studies demonstrate that the carbide formation mechanisms for the two transition metal carbides in ethane differ. Finally, a similar technique has been employed for the synthesis of uranium monocarbide with moderately high surface area. I. INTRODUCTION High surface area transition metal carbides are active catalysts for a wide range of reactions, including hydrodesulphurisation (HDS), hydrogenation, Fischer-Tropsch synthesis, hydrocarbon isomerisation and methane oxyforming [1-5]. Synthesis of these potentially important materials has attracted considerable attention over the years, and a number of procedures have now been described for the production of carbides with surface areas suitable for catalyst materials (i.e. > 30 m E g-l) [6]. The most commonly employed method is temperature programmed reaction (TPRe) of metal oxide with a hydrocarbon (usually methane), which was developed by Boudart and co-workers [7-9]. In spite of all the interest in these materials, comparatively few detailed studies of the material synthesis have been reported. Indeed, it may be possible to prepare carbides with improved properties by modification of already known methods. For example, replacement of methane by ethane for TPRe should lead to higher surface areas due to the more reactive nature of the latter hydrocarbon; the carbon activities can be expressed by the following equations, respectively (1 and 2) [ 10]. dC --
ar
(1)
)2
=I I___L__.
)3
(2)
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The reaction with an oxide is more complicated, but relationships between methane activity and gas composition for higher hydrocarbons can still be obtained (3) [ 11 ].
C2H6tg) + H2(g) ---> 2CH4(g)
acH ' = Ke(cH,).
Ke(c~H,)
(3)
In this paper, we will demonstrate that molybdenum and tungsten carbides with higher surface areas can be synthesised using ethane as the reactant in place of methane, and also that the choice of hydrocarbon determines which carbide phase is produced. This may be important for the application of the carbide materials as catalysts. In addition, the synthesis of uranium carbide using methane will be presented; these materials have been almost neglected outside of the nuclear industry, while uranium shows some similarities to the group VI metals. 2. EXPERIMENTAL 2.1. Synthesis of molybdenum and tungsten carbides The apparatus used in this work was an up-graded version of the commercial Labcon microreactor which has been described in detail previously [5]. The TPRe metal carbide syntheses were carried out as follows: 100 mg of oxide was heated at 1 K min -~ from room temperature to a final temperature, chosen according to the hydrocarbon and metal oxide being used (see Table 1). The reactant gases, 20 vol.% CH4/H2 or 10 vol.% C2H6/H2, were passed over the oxide at 250 cm 3 min ~. After reaction the samples were quenched to room temperature under argon. Before exposure to the atmosphere the carbide samples were passivated in flowing 1 vol.% O2/Ar. Material surface areas were then determined using nitrogen BET. Methane (Union Carbide, >99.95%), ethane (BOC, C.P. grade), hydrogen (BOC, C.P. grade) and argon (BOC, C.P. grade) were used as received without further purification. MoO3 (Johnson Matthey, Puratronic| 99.998 %) and WO3 (Johnson Matthey, Puratronic| 99.998 %) were used as supplied, and had surface areas less than 1 m 2 g~. High-resolution analytical electron microscopy (HRTEM) was carried out using a JEOL-4000FX microscope, XRD on a Phillips PWl710, and thermogravimetric analysis (TGA) using a Rheometric Scientific PL-STA.
2.2. Synthesis of uranium carbide A small piece of uranium metal was cleaned and placed into the furnace tube. This was hydrided under a low flow of hydrogen and then heated under vacuum at 623 K, affording a fine pyrophoric uranium metal powder. The surface area of this powder was not determined because crushing caused it to ignite under an atmosphere of purified nitrogen gas and so handling it in a nitrogen filled glove-box was problematic (02 < 10ppm). The uranium powder was carburised at 903 K under methane (50 cm 3 min l) for 6 h and then allowed to cool to room temperature.
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3. RESULTS AND DISCUSSION 3.1. Molybdenum and tungsten carbides Table 1 summarises the surface areas and final synthesis temperatures employed in this study. Ethane TPRe gave materials with surface areas much higher than from the methane method for both the molybdenum and tungsten carbides. In addition, the reaction temperatures when ethane was employed were much lower than those with methane, as expected from the thermodynamics Table 1 discussed in the introduction. The Surface areas and final reaction temperatures for the surface areas obtained here for TPRe synthesised metal carbides. ethane TPRe synthesised CH4 TPRe C2H6 TPRe molybdenum carbide are as high as S~ (m2 g~) T t (K) S~ (m2 g~) Tt (K) any reported previously [8,12]. 91 1020 174 900 MoO 3 TGA studies of the 39 1150 71 900 W03 synthesis of W2C and M02C from t Final synthesis temperature. the reaction of their respective oxides with ethane are shown in Figure 1. The traces for the two systems are clearly very different, with three peaks in the tungsten trace, compared to the molybdenum system which displayed only one weight loss. This indicates a different carburisation reaction mechanism for the two metal oxide-carbide transformations. 22~ 21
,m0;I
23 (mg) 22
181
17t b)
a) 20
500
600
700 800 Ol)0 Temperature (K)
1000
11oo
16
,
soo
6;0
700 8;o 9;o Temperature (K)
lo'oo
11oo
Fig. 1. TGA of the ethane TPRe of a) WO3 (10 K min -~) and b) MoO 3 (13 K min~). Oyama has proposed that the route to niobium carbide is via an oxycarbide [ 13]. This may also be the ease for the synthesis of molybdenum carbide presented here (reaction (4)), and indeed Ledoux and co-workers have shown that for higher hydrocarbons (C4§ an oxycarbide can be formed under certain conditions [ 14,15]. For example, using hexane as the carburising gas, Ledoux showed that molybdenum oxycarbide was formed at 623 K, while the carbide was favoured at slightly higher temperature, 673 K. For the tungsten system, we propose that the most likely route is as follows; the first step is the reduction of tungsten (VI) oxide to a shear phase oxide, followed by further reduction to WO2, and finally carburisation to W2C (reaction (5)). MoO 3 ~ [MOOxCy] --9, Mo2C WO3 ~ WO3.x ~ WO2 ~ W2C
(4) (5)
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a)
MoO3
C2HdI-I29 fl- Mo2C(P631mmc) /
~
b)
~/'O3
21020K fl- Mo2C(P631mmc)
2II50K
C2H6~I2
-
/,.,~c)
~-
wc(:6~2)
/ / NH3980K W2N(Fm3m)
I
_~.-~-.~
fl- WC,_x(Fm'3m) CH4/H2 1050K
Fig. 2. Routes to phases of a) molybdenum carbide and b) tungsten carbide via TPRe. In addition, it should be noted that, where several different phases can be formed, the choice of route can allow the synthesis of comparable surface area systems, e.g. 13-Mo2C and [3-MoCk.x, and the various tungsten phases. Figure 2 shows the phases that are formed by TPRe with methane or ethane, and also via a two step route involving conversion of the oxide to the nitride, by ammonia TPRe, and then carburisation to the carbide under methane. This may be important since the phase formed may effect the catalytic properties of these materials. From the above results, it can be seen that higher hydrocarbons should be promising reagents for the conversion of transition metal oxides to the high surface area carbides under comparatively mild conditions for application as catalysts. 3.2. Uranium carbide
Figure 3a shows the XRD pattem of the material formed after the reaction of uranium hydride with methane. Uranium dioxide was rapidly formed on contact with air, water or even carbon dioxide, severely hampering data collection. Despite this, however, comparison of the pattern with standard XRD data indicates that uranium monocarbide has been synthesised; this is the favoured phase under the conditions used (i.e. 903K) [16]. Uranium carbide only exists over a narrow range of compositions; the C/U ratio is 1.00 + 0.03, and any deviation outside this stoichiometry is due to either residual metal or free carbon. The UC material synthesised here is f.c.c., with a lattice parameter of 4.962 A, in close agreement with the published value of 4.9605(2) A for a pure, highly crystalline material. However, the relatively broad line widths in the XRD pattern indicate that the crystallite size is rather small (r~), 530 A). The surface area of the uranium carbide measured using N2 BET was 5 m 2 g~, and the particle size calculated assuming spherical particles was 880 A. When the high
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density of UC is taken into account (13.6 g cm3), this surface area, although not exceptional, is certainly moderately high. A small sample of the synthesised uranium carbide was carefully passivated so that it could be briefly handled in air, allowing study by HRTEM. In Figure 3b a typical micrograph of the UC material is shown. The UC synthesised here is made up of micrometre sized particles, composed of aggregates of smaller particles, consistent with the XRD and BET measurements.
a)
I00
l
l
' ~./~
:''
,
. 'l
i:ii
b)
l
~. ,, ~. ,.. ,:.;:,,: ..l.O-nm I,!,!)
(2,0,0)
6O
(2,2,0)
(3,1,I)
(2,22)
'L
30
9
3'5
9 4~
'
45
,
,
!
,
!
5 !0
Fig. 3. Characterisation of the uranium monocarbide by a) XRD and b) HRTEM. 4. CONCLUSIONS TPRe is a versatile method for producing high surface area early transition metal, or uranium, carbides, and reactions with ethane tend to proceed at lower temperatures than is the case with methane. The potential of molybdenum and tungsten carbides as future industrial catalysts has already been demonstrated. However, using the ethane TPRe method shown here it may now be possible to extend the catalytic applications to a wider variety of molybdenum and tungsten phases. The suitability of these materials for the important industrial reactions mentioned earlier is currently being determined. REFERENCES
1. J.S. Lee and M. Boudart, Appl. Catal., 19 (1983) 207. 2. G.B. Raupp and W.N. Delgass, J. Catal., 53 (1979) 361. 3. M.J. Ledoux, C. Pham-Huu, A.P.E. York, E.A. Blekkan, P. Delporte and P. Del Gallo, in "The Chemistry of Transition Metal Carbides and Nitrides", S.T. Oyama (ed.), p.373, Blackie Academic and Professional, Glasgow, 1996. 4. A.P.E. York, J.B. Claridge, A.J. Brungs, S.C. Tsang and M.L.H. Green, Chem. Commun., (1997) 39. 5. J.B. Claridge, A.P.E. York, C. M~irquez-Alvarez, A.J. Brungs, J. Sloan, S.C. Tsang and M.L.H. Green, J. Catal., 180 (1998) 85.
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6.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
S.T. Oyama in "The Chemistry of Transition Metal Carbides and Nitrides", S.T. Oyama (ed.), p.1, Blackie Academic and Professional, Glasgow, 1996. L. Volpe and M. Boudart, J. Solid State Chem., 59 (1985) 332. L. Volpe and M. Boudart, J. Solid State Chem., 59 (1985) 348. J.T. Wrobleski and M. Boudart, Catal. Today, 15 (1992) 349. M. Katsura, J. Alloys Comp., 182 (1992) 91. J.B. Claridge, A.P.E. York, A.J. Brungs and M.L.H. Green, Chem. Mater., submitted. M.J. Ledoux and C. Pham-Huu, Catal. Today, 15 (1992) 263. V.L.S. Teixeira da Silva, M. Schmal and S.T. Oyama, J. Solid State Chem., 123 (1996) 168. E.A. Blekkan, C. Pham-Huu, M.J. Ledoux and J. Guille, Ind. Eng. Chem. Res., 33 (1994) 1657. S.T. Oyama, P. Delporte, C. Pham-Huu and M.J. Ledoux, Chem. Lea., (1997) 949. E.K. Storms, The Refractory Carbides, Academic Press, New York, 1967, Vol. 2.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Use of carbon fabrics as support for hydrogenation catalysts usable in polyphasic reactors. J.P. Reymond and P. Fouilloux L.G.P.C.; E.S.C.P.E.; 43 Bd du 11 Novembre ; F-69616 Villeurbanne cedex; France. In order to improve the performances of fixed bed reactors or slurry reactors in polyphasic catalytic hydrogenation of organic molecules, routes allowing to deposit noble metals on bidimensionnal supports, which could transformed further in structured catalytic systems (monolithic reactors), have been studied in this paper. Carbon fabrics have been selected as bidimensionnal support and the obtained catalytic systems have been tested in two triphasic reactions 9hydrogenation of acetophenone and hydrogenation of 4-chlorophenol. 1. INTRODUCTION A great number of catalytic conversions of organic molecules take place in triphasic reactors (gas-liquid-solid). Compared to conventionnal fixed bed or slurry reactors, the monolithic reactor is an interesting alternative [1 ]. Due to its particular structure, constituted of a great number of small parallel channels (up to 100 per cm2), a monolith presents some advantages compared to the usual reactors : mass and heat transferts are more effective; pressure drops are smaller; as the active phase is anchored on the channel walls the liquidcatalyst particles separation is eliminated. Typically, the channel walls of a monolith are covered by a porous oxide layer (washcoat), over which the catalytic phase is deposited. It is also possible to deposit the metal rightly on the channel walls. We have studied two routes to deposit metal clusters on a bidimensionnal support, support which could be further shaped into monolith, making up the channel walls. Use of stainless steel grids, or sheets, has been described elsewhere [2], use of carbon clothes is described in this paper. 2. E X P E R I M E N T A L
2.1. Preparation of catalysts Two routes of deposition of active metal have been investigated (the amount of metal precursor is calculated to lead to catalysts containing 5 wt% of metal). - the well known impregnation method has been used to deposite ruthenium. Four fabrics pieces (each of 4x4 cm, total weight ~ 0.45g) are immersed in an aqueous solution of RuC12 for 15 hours, drained, dried (3 hours at 443 K under nitrogen flow). An activation treament, reduction under H2 flow, is necessary to reduce the deposited metal complex and generate the zerovalent active metal. - the autocatalytic deposition method [2], deriving from the electroless plating method [3] has been used to deposit palladium. This process comprises four steps 9 a- cleaning of carbon fabrics pieces (4 pieces; each of 4x4 cm; 0.45g), which is operated in a Kumagawa apparatus with acetone
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b- preparation of an aqueous PdCI2 solution : adding of hydrochloric acid (0.5 to 1 g HCI 36 wt% in 25 cm 3 of water) is necessary to dissolve PdC12 c- deposition of the metal : the carbon clothes are immersed in the PdC12 solution. A given volume of a reductant solution (sodium hypophosphite) is then added in well controlled conditions (temperature, reactant concentrations and hypophosphite adding rate). d- drying : 2 hours in air at 298 K. This process is similar to a galvanic process (anodic and cathodic reactions take place in the mechanism) in which the electrons are provided by the chemical reductant [3]. It leads to the zerovalent metal which is catalytically active without activation treatment. 2.2. M a t e r i a l s
9
Carbon clothes (CECA and Actitex) are obtained from carbonization (hydrogen consumption) and activation (porosity formation) treatments of viscose fabrics (rayon). The diameter of carbon fibers is 20 ~tm and cloth thickness is ~ 0.5 mm. Some textural characteristics of three carbon clothes and of an active carbon in grains are given in table 1. Table 1 Characteristics of carbon supports. Support (type and manufacturer)
SBET
(m2/g)
Smicr~176 T~ Vp~ (m2/g) (cm3/g)
Vmicr~176 d micropore (cm3/g) (nm) 0.215 0.7 0.475 0.5
TE80 : Active carbon in grains (Degussa)
1098
495
0.550
IS : woven carbon cloth (CECA)
1250
948
0.505
RS 1301 : woven carbon cloth (Actitex)
1348
1004
0.731
0.462
0.5
FC 1201 : non woven carbon cloth (Actitex)
1000
889
0.444
0.410
0.6
1.3. C a t a l y t i c r e a c t i o n s :
The best way to evaluate the catalytic efficiency of a solid is to study its activity in a test reaction. Two reactions have been used : hydrogenation ofacetophenone (palladium and ruthenium catalysts) and hydrogenation of 4-chlorophenol (ruthenium). Catalytic tests take place in a 125 c m 3 stainless steel stirred reactor (semi-batch). In the case of acetophenone hydrogenation, catalyst samples are immersed in cyclohexane, hydrogen pressure is 2.5 MPa and reaction temperature is 393 K. Extent of reaction is evaluated from hydrogen and acetophenone consumption and product formation. Catalyst activity is characterized by initial activity and selectivity values. Acetophenone (AC), has an aromatic ring with a ketone function and reaction products depend on the nature of the active metal [4, 5]. On ruthenium catalysts the aromatic cycle is hydrogenated, leading to the formation of methylcyclohexylketone (MCC) and cyclohexylethanol (CE). On palladium the ketone function is hydrogenated in phenylethanol (PE) and ethylbenzene(EB). Hydrogenation of 4-chlorophenol is part of a two steps process of depolluting waste water which has been developped in our laboratory [6]. The first step is the adsorption of chlorophenol on the carbon support of the catalyst (Ru/carbon). The second step is the catalytic dechlorination by hydrogenation of the adsorbed chlorophenol, in an aqueous basic media (soda) and under mild operating conditions (313 to 353 K, and hydrogen pressure 0.3 to 0.5 MPa) [6]. The reaction products are cyclohexanol and hydrochloric acid (neutralized by soda).
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3.1. Catalysts based on ruthenium deposited on carbon clothes: 3.1.1. Hydrogenation of acetophenone :
The IS carbon cloth (see table 1) has been used as support to prepare ruthenium catalytic systems, following the impregnation method. Figure 1 depicts the conversion of reactants and products (Ru/carbon cloth; 393 K; 2.5 0,3MPa H2; A i is 7.29 molH2/g.h), as a function of time. Acetophenone is first hydrogenated in 0,2 methylcyclohexylketone (MCC) which, in g turn, is hydrogenated in cyclohexylethanol e(CE) which is the main (or only) product at 9 0,1 0 tthe end of the reaction (no more hydrogen O 0 consumption). Light formation of phenyl 0~ ethanol (PE) and ethyl benzene (EB) is 0 20 40 60 noted, indicating hydrogenation of the Time (mn) ketone function of acetophenone. Fig. 1. Product and reactant conversion during The influence of several operating acetophenone hydrogenation over Ru/C cloth. parameters of preparation on the activity of Ru/Carbon cloth have been evaluated. Table 2 depicts some results. For example, there is an optimal reduction temperature (773 K). Above 773 K the support reacts with hydrogen and sintering of ruthenium particles probably occurs, leading to a poor catalytic activity. ~nH2
---.Zk--AC
t'-
Table 2 Effect of preparation operating parameters of Ru/carbon cloth catalysts on initial activity. Reduction temperature (K) Reduction time (h) mass (g) Activity (moIH2.glRuh-l) 573 573 573
3 20 6
693 773 873
6
0.46 0.45 0.9 0.45 0.22 0.1 0.5 0.235 0.09 0.47 0.44
5-9 7-9 5.5 6.5 6.1 5.9 12.15 13.14 12.2 21-25 7
As it is observed in table 2 the initial activity does not depend on the catalyst mass. Moreover, the activity of Ru/carbon cloth is egal to that of the same catalyst finely grinded. It can be concluded from the experiments that there is no internal diffusion problem with the catalytic system, that the reactor is perfectly stirred and that the reaction occurs in chemical regime. Carbon cloth appears as a convenient support to prepare catalytic systems.
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However, it has to note that in same conditions of activation and reaction, a commercial catalyst (Ru over carbon powder, from Degussa), is twice more active (up to 53 molm.gtRu hl) than our catalysts but presents similar selectivities. 3.1.2. Adsorption and hydrogenation of 4-chlorophenol on Ru/Carbone: The depolluting process, mentioned above, was first operated over a conventional catalyst : ruthenium deposited on active carbon in grains (Pica, TE 80). To improve this process, the properties of two carbon fabrics, RS1301 (woven cloth) and FC1201 (non woven), towards adsorption of 4-chlorophenol have been evaluated and compared to that of the carbon in grains. Adsorption isotherms of 4-chlorophenol on the supports, presented on figure 2, are similar (in fig.2, Q is the amount adsorbed at a given time and Qmaxis the maximal amount adsorbed). The adsorption rates, determinated in a stirred batch reactor, are also similar while adsorption rate of 4-chlorophenol is slightly greater on FC 1201.
1
1 0,8
0'8 t E0,4--O--FC1201 ---i~--TE80 ---X-- RS1301
0,2 C
f
o,6
.~ o,6 E0,4
0,2
I
0~ o
750 1500 2250 Equilibrium concentration (ppm)
Fig.2. Adsorption isotherms of chlorophenol on adsorbents (293 K).
- -
--~---TE 80 ~FC 1201 ---X'-- RS 1301
I
2000 4000 Time (s)
6000
Fig.3. Breakthrough curves of adsorbents (3g; 1l/h; initial concentration" 4000 ppm; 293 K)
Two important parameters, Q~ and t~, are determined from the so-called breakthrough curves which are presented on figure 3. These curves, obtained from in-line adsorption experiments (column), give the pollutant concentration in the outlet flow versus time. t~ is the critical time, time for which the pollutant concentration becomes higher than the maximun value allowed for pollutant rejection (0.1 ppm for 4-chlorophenol) and Qr is the corresponding critical amount of adsorbed pollutant. Table 3 Comparison of adsorptive properties of carbon supports. Qmax tc (s) Qc (g/g) FC1201 RS1301 TE80
0.489 0.577 0.455
0.5 1/h 2070 1710 840
1.5 1/h 614 175 37
0.5 1/h 1.43 1.04 0.48
1.5 1/h 0.43 0.26 0.06
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The values of Qmax, tc and Qc are given for two flow rates in table 3. For FC1201 the outlet flow remains below the limit value for a longer time and then rises more stiffly than for the other adsorbents. With FC 1201 the in-line adsorption of 4-chlorophenol could be carried out with a higher flow rate. From these results it can be concluded that adsorption properties of carbon fabrics are slightly better than that of carbon grains. Compare to active carbon grains, carbon fabrics present improved characteristics : greater adsorption capacity and stiffness of the breakthrough curves. Moreover, the packing of an adsorption column with carbon cloth disks leads to an ordered structure presenting a lower pressure drop than it is in the case of carbon grains (the same result will be obtained in an inline reactor). Then, ruthenium has been deposited by the impregnation method on the RS 1301 cloth which presents the higher adsorption capacity. The resulting catalyst has been tested in the hydrodechlorination of 4-chlorophenol and its activity has been compared to that of the conventional catalyst (Ru/TE80). Catalytic activities (transformation rates of 4-chlorophenol in cyclohexanol) are very similar over the two tested catalysts : 0.4molH2gRu-lhI in each case. As evidenced by batch and in-line adsorption experiments carbon clothes show interesting performances for chlorophenol adsorption. They are also useful as support to prepare effective catalysts in the further hydrogenation of the adsorbed 4-chlorophenol.
3.2. Catalysts based on palladium: hydrogenation of acetophenone. The autocatalytic deposition method, usable on all kinds of support, has been used to deposit palladium on the IS carbon fabrics (see table 1) instead of the impregnation method. The influence of several preparation operating parameters on the activity of obtained catalytic systems has been studied in the hydrogenation of acetophenone. As evidenced from chemical analysis and X-ray emission analysis [2], the autocatalytic deposition process leads to the formation of a layer of palladium-phosphorus alloy coating the whole surface of the support and to the formation of discrete particles of the same alloy. If sodium borohydride is used as the reducer, instead of sodium hypophosphite, the deposit is an alloy of palladium and boron. Mechanism of phosphorus and palladium codeposition in electroless plating baths is not really elucidated, it could occur via interaction of hypophosphite ion with atomic hydrogen or with hydride ions or with palladium metallic surface [7]. Catalytic properties of Pd/carbon fabrics have been compared to that of industrial hydrogenation catalysts (Degussa and Doduco) in which palladium (5 wt%) is supported on powdered active carbon (SBET'~1000 m2g~). Results are presented in table 4. Table 4 Comparison of initial activity and selectivities of industrial and laboratory Pd/carbon catalysts. Catalyst Reduction Ai Phenylethanol Ethylbenzene treatment mo1H2g-lpd hl % % Pd/C (Degussa) N 52 99 1 Pd/C (Doduco) N 123 18 82 Pd/C cloth N 68 0 100 Pd/C (Degussa) Pd/C cloth
Y Y
219 45
76 15
24 85
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All catalysts have the behaviour of palladium : they catalyze the hydrogenation of the ketone function. However, our catalytic system appears quite different to industrial catalysts. When catalysts are not prereduced by hydrogen, Pd/C-fabrics and Pd/C-Degussa have quite similar initial activity (hydrogen consumption rate: ~ 60 molH2gpdl.h-~) while Pd/C-Doduco is twice more active (~120 molH2gpdl.h~). On the other hand, selectivities at the end of the reaction are more different. Pd/carbon cloth leads only to ethylbenzene while industrial catalysts have a more lighter hydrogenolysis activity, producing only phenylethanol (Degussa) or phenylethanol and ethylbenzenze (Doduco). A reducing pretreatment (H2, 2h, 493 K) decreases slightly the initial activity of Pd/C fabrics (45 molH2gpd~.h~), while initial activity of industrial catalyst (Pd/C Degussa) is strongly enhanced (220 molH2gpdl.hl). This is not surprising because the autocatalytic preparation method leads to the deposition of zerovalent palladium which is catalytically active (the influence of phosphorus is not elucidated). On the contrary, impregnation of a support from a metal salt solution leads to the formation of a surface metal complex which must be decomposed in order to generate the zerovalent active metal. Nevertheless, reduced Pd/C-Degussa has a great hydrogenation activity (formation of phenylethanol) and a weak hydrogenolysis action (conversion of phenylethanol in ethylbenzene). Selectivities observed with our catalyst are different, it leads mainly to the formation of ethylbenzenze, its hydrogenolysis action is greater, but phenylethanol is not fully conversed. Combined to the autocatalytic deposition process carbon clothes allow to prepare interesting bidimensionnal catalytic systems which could shaped as structured catalyst (monoliths and column packings are intended). 4. CONCLUSION
Compare to carbon powders or granulates the implementation of carbon fabrics in polyphasic reactors present several improved caracteristics : good textural characteristics and adsorptive properties, possibility to prepare structured catalytic systems. Carbon clothes have been used as support to prepare efficient hydrogenation catalysts. Nevertheless, the activity of actual catalytic systems on carbon fabrics has to be improved to reach the activity level of industrial catalysts. REFERENCES 1. A. Cybulski and J.A. Moulijn, in "Structured Catalysts and Reactors" (A. Cybulski J.A. Moulijn Eds., M. Dekker, Amsterdam, 1995), 1. 2. J.P. Reymond, D. Dubois and P. Fouilloux, Studies in Surface Science and Catalysis, (1998), 63-72. 3. D. Dukes in "Electroless Plating : fundamentals and Applications" (G.O. Mallory, Hadju eds., A.E.S.F., Orlando, 1992), 511. 4. N.S. Barinov, D.V. Mushenko and E.G. Lebeva, Zh. Prikl. Khim., 39 (1966), 2599. 5. J. Mason, P. Cividino, J.M. Bonnier and P. Fouilloux in "Heterogeneous Catalysis Fine Chemicals II" (1991), 245. 6. V. Felis, C.de Bellefon, P.Fouilloux and D. Schweich, Appl. Catal.,20 (1999), 91. 7. G. Salvagno and P.L. Cavalotti, Plating, 59 (1972), 665.
and 118 J.B.
and
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Characterisation and Reactivity of Activated Carbon Supported Platinum Catalysts Prepared by Fluidised Bed Organometallic Chemical Vapour Deposition ( F B O M C V D ) Ph. Serp", J-C. Hierso a, R. Feurerb, R. Corratg6 ~, Y. Kihn~ and Ph. Kalcka a) Laboratoire de Catalyse, Chimie Fine et Polym~res b) Laboratoire de Cristallographie, R~activitO et Protection des Mat~riaux Ecole Nationale Sup~rieure de Chimie de Toulouse, 118 route de Narbonne, 31077 TO UL0 USE Cedex, FRANCE. c) C.E.M.E.S.-C.N.R.S. no. 8011, 2, Rue Jeanne Marvig, 31055 Toulouse, FRANCE
A.E. Aksoylu, J.L. Faria, A.M.T. Pacheco and J.L. Figueiredo Laborat6rio de Catdlise e Materials, Faculdade de Engenharia, Universidade do Porto, Rua dos Bragas, 4050-123 Porto, PORTUGAL
I. INTRODUCTION In the course of developing new methods for the preparation of heterogeneous catalysts, we have recently shown that the combination of organometallic chemical vapor deposition (OMCVD) and the fluidizati0n of a bed of porous particles is a powerful method to prepare supported catalysts [1]. Highly dispersed deposits of rhodium, palladium and platinum on metal oxide supports were obtained in a single step using a fluidized bed reactor [2]. The method of chemical vapor deposition (CVD) allows direct deposition of the active phase onto the catalyst support by means of the reaction between surface sites containing oxygenated groups and the vapor of a suitable organometallic compound. Using silica or alumina with large specific areas- around 200 m2g1- the presence of hydroxyl groups on the surface allows the facile grafting of small and dispersed particles of the noble metal (2-5 nm). On such supports, the high concentration of anchoring groups presumably permits the maintenance of a high nucleation rate which favors the growth of the particles. For carbon supports the situation is somewhat different, since less active sites are available and these have a larger diversity in their chemical nature. Through careful oxidation processes, it is possible to control the nature and concentration of the oxygenated functions on the surface. As carbon supported catalysts play an important role in many catalytic reactions, we have undertaken a study in which we have examined the suitability of the FBOMCVD method for the direct elaboration of such catalysts. The present report concerns the deposition of platinum on various carbon types. Atter preliminary OMCVD experiments on planar model supports, various activated carbon powders were examined for the elaboration of platinum supported catalysts. The activity of these catalysts was compared with that of conventional wet impregnated catalysts in benzene hydrogenation.
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2. EXPERIMENTAL
2.1- Preparation of carbon supports - Planar supports: highly oriented pyrolytic graphite (HOPG) was used as received or oxidised in refluxing 5N HNO3. Rough planar graphitic carbon disks were obtained by cutting high purity carbon rods for spectrography and were used directly or alter the same oxidative treatment as previously. - Powder supports a commercial activated carbon, Hydraffin, was ground and sieved to 200300 gm particle size prior to its use. Then the following pre-treatment procedures were applied" (i) Hydraffin was washed with 2N HCI solution for 12 h, and then washed with distilled water under reflux for 6 h. Then they were dried overnight at 383 K (AC1). HC1 washed activated carbons were used as such and also taken as the base for the subsequent oxidation treatments as follows: (ii) AC1 was oxidised in 5 % 02-95 % N2 mixture at 723 K for 10 h (AC2) and (iii) AC1 was directly oxidised in 5 N HNO3 solution for 3 h and washed with boiling distilled water till the pH of the rinsed solution reached 5.5. These treatments were followed by overnight drying at 383 K (AC3). 2.2- Preparation of platinum catalysts - CVD catalysts: the deposition of platinum on planar supports has been performed in a classical horizontal hot-wall CVD reactor. Vapours of PtMe2(COD) were carried on by a 3% molar H2/Helium mixture, under reduced pressure, onto the support heated at 110~ For the powder supports, the previous conditions were adapted to a vertical fluidised bed CVD reactor. Impregnated catalysts: three 1 wt% Pt/AC catalysts were prepared on AC1, AC2 and AC3 supports by incipient wetness impregnation of aqueous solution of hexachloroplatinic acid. The impregnation was conducted under vacuum with ultrasonic mixing. The precursor solutions (2 ml.g'lsupport) with calculated concentration were added via a peristaltic pump. After impregnation step, the samples were dried overnight at 373 K. Transmission electron microscopy (TEM) analyses (Philips CM12, 120 kV) and scanning electron microscopy (SEM) observations (Jeol JSM-35 C)were performed on both CVD and impregnated catalysts. Atomic force microscopy (AFM) was performed on a Nanoscope II Scanning Tunneling Microscope (Digital Instrument Inc). Temperature-programmed desorption (TPD) profiles were obtained in a home maid set-up consisting of a U-shaped tubular reactor mounted inside a furnace. The amounts of CO and CO2 desorbed from the samples were monitored by mass spectometry (Spectramass Dataquad quadrupole). 2.3- Catalytic experiments All six catalysts were tested in benzene hydrogenation at 393K with a H2:C6I-I6 molar ratio of 11, and 17 gl/min Cd-I6 flow in the feed. All gas flows were controlled by using mass flow controllers (Bronkhorst) and the flow of benzene was controlled by an HPLC pump (Gilson 305) connected to an Autoclave Engineers BTRS reactor. Prior to reaction, impregnated samples were reduced at 623 K for 14 h under pure hydrogen. In reaction tests, 250 mg fresh sample was placed in a tubular microreactor whose temperature was controlled within a range of + 0.1 C by a temperature controller. 3. RESULTS AND DISCUSSION
3.1- Platinum deposits on model supports
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Figure 1: AFM image of Pt deposited by CVD on oxidised HOPG On the untreated HOPG support, the surface of which does not contain any oxygenated functions nor physical defects (e.g. steps or edges), no deposition was observed by SEM (xl 0 000) and AFM, whatever the duration of the runs (3 to 20 minutes). Deposition was noted on the reactor walls. However, oxidation of the HOPG support by HNO3 solution provides anchoring sites, which allows the deposition of platinum particles. SEM examination reveals the presence of particles, some of them isolated on planar surfaces, others laid out presumably on crystal defects. Analysis by AFM of the deposits obtained aRer 3 min confirms this observation and reveals the presence of particles (roughly 50-60 nm in diameter after 3 min of deposition) localised on surface defects created by the oxidation (Figure 1). On the unoxidised graphitic disks, deposits are observed mainly on the defects induced by sawing. After nitric acid oxidation a greater number of particles are deposited on the whole surface and the crystallite size distribution is more homogeneous (centred around 50 nm for both samples). 3.2- Platinum deposits on activated carbon powders
The results displayed in Table 1 show that air oxidation treatment led to a slight increase in the BET surface area, whereas HNO3 oxidation led to a significant decrease. The decrease in the BET surface area aRer HNO3 oxidation can be explained by the collapse of the pore walls due to the attack of highly concentrated nitric acid during oxidation. As a consequence, the microporous volume (Wo) of the nitric acid oxidized sample, AC3, dropped by35 %. Table 1: Characteristics of the supports Support AC1 AC2 AC3
BET (m2/g) 1415 1589 845
Adsorption Wo (cm3/g) 0.597 0.689 0.399
Smeso (m2/g) 40.3 89.0 32.0
CO 1325 4315 4616
TPD (~tmol/8) CO2 CO/CO2 393 577 2732
3.37 7.48 1.69
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The results indicated that air oxidation doubled the mesoporous surface area (Smeso) of the hydraffin support. On the other hand, AC3 (HNO3 oxidised) showed a mesoporous surface area 20% smaller than that of AC 1. The concentration of oxygenated groups, which release CO during the TPD run, is almost four times higher for AC2 than for AC 1. In the TPD of AC2, the CO-type groups which decompose above 800 K are mainly carbonyl, phenol, ether and quinone. The CO~ release curves indicate that, although AC 1 has carboxylic groups, AC2 does not present the carboxylic acid, function, but instead it has carboxylic anhydride groups which are indicated by the high temperature CO2 release. The concentration of oxygen containing groups is greater for the oxidized samples, as shown by larger amounts of CO and CO~ released compared to AC1. In particular, the carboxylic acid groups (CO~-type groups that decompose at around 523 K) are predominantly formed by HNO3 oxidation. For AC3, a decrease of the CO/CO~ ratio was observed when compared to AC1. Since CO~ releasing groups have more acidic nature than CO releasing surface groups, CO/CO~ ratios point out that air oxidation led to a more basic support, whereas HNO3 oxidation led to a relatively more acidic surface, compared to AC 1. HCl-washed activated carbon powders (AC1) submitted to CVD in a fluidised bed, give rise to a loading of 1% of platinum aggregates whose sizes range mainly from 5 to 20 nm. A similar distribution was observed for the catalysts prepared by impregnation (Figure 2). Thus, whatever the technique used, the distribution appears to be equally poor in the absence of any oxidative treatment of the support. For the two techniques, and aider an oxidising treatment in air of the support (AC2), the particle size distribution appears narrower (1-9 nm, Figure 3). It is worth to mention that only a few large particles were detected by TEM analyses. Thus the use of the combined technique FBOMCVD for the preparation of highly dispersed small particles appears very suitable. Finally, nitric oxidation of AC1, providing more oxygen-containing surface groups, particularly carboxylic ones (AC3), results in a satisfactory platinum particle size distribution (Figure 4) since they have a homogeneous morphology, their size is centred on 3-5 nm (CVD) or 2-4 nm for impregnation and no large particle was detected. Examination of the catalysts on AC2 and AC3 shows mainly that the CVD method affords a roughly Gaussian distribution of the particle size whereas for the impregnation method, the number of particles of size higher than 5 nm increases significantly and regularly.
i ii iiii liii;iiiiiliii(ii!ii!ii!!ii!i!iiiiiiiiiiiii;i 4o
iiiiiiiiiiiii~iii;liiiii::iiiiiiiiii!iiii:iiiiiiiiiiiiiiii'.iil;;:;ii!ii ~
0 .
50
Particle size (rim)
Figure 2: Particle size of Pt/AC1 prepared by CVD (black) and impregnation (white).
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........................................... " ........... ' ~ * ~ - " - ~ ' - ~ " ~ ' ~ ............... ~ ' - §
..... ~-~{"-
40 =
30
10
,
e..~
0 ummm 9
---
Particle size (nm)
Figure 3" Particle size of Pt/AC2 prepared by CVD (black) and impregnation (white).
:.:........... ...:.~. :-.....-~::::: ::: :~:.
~..:..:,...:.:.~ ........ ~-.:.,..:.~..,.., ~..:.::~ :: ::::: :::::....:..:..:.:.:..:. ~ ::~:~:..z,.::::......:..~ :=9 .7,
::~.:::..:.::.~::~:::-.-:.::::..:. 9
:.....&-:.?':.'..
I~%-~,'.._'..-:~r ~
0 ~~~-.:~ I['-}'.:iB~U' "~::'=:~":~*::
...... -"~...............
10 ' ~ ~ H i :~~"'i": ~ " 0
; Si-O-SiMe 3 + "Me3SiNH2"
(R3Si)2NH + NH3
~ Si-OH
(5) (6)
---) ~ Si-O-SiR 3 + NH 3
(7)
formed essentially from the initiation of silation. At 400~ HMDSO is not formed until 4 h after initiation of silation. Further, the amount of HMDSO formed decreases with increasing the temperature of silation. Thus the reaction by which HMDSO is formed is not an initial reaction, and is not thermally promoted in a direct manner. It is also unlikely that HMDSO is formed by reaction of HMDS with pendant - O - S i M e 3 groups, as both are present from initiation of silation but HMDSO is not formed immediately for reactions at temperatures of at least 300~ It is noteworthy that the delay in formation of HMDSO coincides with the formation of small amounts of CH 4 , and the length of the delay increases with the amount of CH 4 formed. These data suggest that a competitive reaction is occurring. Acidic hydroxyls at the surface of oxides react with pendant - O - S i M e 3 groups to liberate CH 4 and form bridging - O - S i M e 2 - O - groups (eq. 8) [9-11]. /SiMe 3 O
si
Si
i
/
/H
O I
i
\
O
/
I
/ SiMe2\ ---)
O
O
Si
Si
i
/
I
t
x
O
/
+ CH 4
(8)
I x
The rate and extent of reaction 8 increases with increasing temperature. Any silyl group that so reacts will no longer be available to form HMDSO. These data suggest that, at temperatures of 300~ or higher, acidic hydroxyls on the surface of silated SiO 2 initially react rapidly with neighboring pendant - O - S i M e 3 groups to liberate CH 4 . Thereafter, the concentration of intact - O - S i M e 3 groups begins to increase, and neighboring - O - S i M e 3 groups are then able to react to form HMDSO (eq. 9). 2 ", ,S i - O - S i M e 3
--->
(Me3Si) 2 O +
,-
Si-O-Si
(9)
No CH 4 is produced during silation of SiO 2 by HMDS at 200~ and so HMDSO is produced by reaction 6 immediately after initiation of silation. At 300~ after formation of HMDSO has
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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commenced, HMDSO and CH 4 are produced concurrently for a short time. HMDSO and CH 4 are produced concurrently for an extended period at 350~ and 400~ The rate of formation of CH 4 and HMDSO at 350~ and 400~ each decline coincidentally with the decline in rate of consumption of HMDS. These data suggest that reactions 8 and 9 each continue, in a competitive manner, at these temperatures until silation of SiO 2 by HMDS is completed. The net effect is that the amount of HMDSO formed, as a proportion of HMDS consumed, decreases with increasing temperature, and that the number of surface-bound silyl groups increases with increasing temperature. The surface silyl groups include both pendant - O - S i M e 3 groups, and bridging groups from which methyl groups have been removed by elimination of methane. The predominant nitrogen-containing product at all temperatures is NH 3 . The majority of the remaining N from HMDS forms N 2 , and a small amount of N remains bound to the surface after silation at temperatures 300~ or higher. The oxidative reaction to form N 2 involves surface oxide species, possible formed during calcination or dehydroxylation at 500~ which are not readily removed by thermal desorption or decomposition.
REFERENCES
Paul, D.K.; Ballinger, T.H.; Yates, J.T., Jr. J. Phys. Chem. 1990, 94, 4617. Kurth, D.G.; Bein, T. J. Phys. Chem. 1992, 96, 6707. Kang, H.-J.; Blum, F.D.J. Phys. Chem. 1991, 95, 939. Paul, D.K.; Yates, J.T., Jr. J. Phys. Chem. 1991, 95, 1699. Yates, J.T., Jr.; Paul, D.K.; Ballinger, T.H.U.S. Patent 5,028,575 (1991). 5. Armistead, C.G.; Hockey, J.A. Trans. Faraday Soc. 1967, 63, 2549. 6. Angst, D.L.; Simmons, G.W. Langmuir 1991, 7, 2236. 7. Gonzalez, R.D.; Miura, H. Catal. Rev. - Sci. Eng. 1994, 36, 145. 8. Zecchina, A.; Otero Arean, C. Catal. Rev. -Sci. Eng. 1993, 35, 261. 9. Slavov, S.V.; Chuang, K.T.; Sanger, A.R. Langmuir 1995, 11, 3607. 10. Slavov, S.V.; Chuang, K.T.; Sanger, A.R., J. Phys. Chem. 1995, 99, 17019. 11. Slavov, S.V.; Chuang, K.T.; Sanger, A.R.J. Phys. Chem. 1996, 100, 16285. 12. Barthel, H. Chemically Modified Surfaces,. Mottola, H.A.; Stteinmetz, J.R., Eds.; Elsevier, Amsterdam (1992), p. 243. 13. Proctor, K.G.; Blitz, J.P. Chemically Modified Surfaces, Mottola, H.A.; Stteinmetz, J.R., Eds.; Elsevier, Amsterdam (1992), p. 209. 14. Hancock, R.D.; Howell, I.V.; Pitkethly, R.C.; Robinson, P.J. Catalysis: Heterogeneous and Homogeneous, Delmon, B., Jannes, G., Eds.; Elsevier: Amsterdam, 1975; p. 361. 15. Morrow, B.A.; Cody, I.A.J. Phys. Chem. 1976, 80, 1998. 16. Blomfield, G.A.; Little, L.H. Can. J. Chem. 1973, 51, 1771. 17. Slavov, S.V.; Sanger, A.R.; Chuang, K.T.J. Phys. Chem. B 1998, 102, 5475. 18. Liu, C.L.; Chuang, K.T.; Dalla Lana, I.G.J. Catal. 1972, 26, 474. 19. Lappert, M.F.; Power, P.P.; Sanger, A.R.; Srivastava, R.C. Metal and Metalloid Amides; Wiley: Chichester, 1980.
1. 2. 3. 4.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
S O L V E N T I N F L U E N C E IN S I L I C A - A L U M I N A S O L - G E L S Y N T H E S I S A. Carati ~, C. Rizzo, M. Tagliabue, C. Perego EniTecnologie, Via Maritano 26, 1-20097 San Donato Milanese, Italy Fax 0039-02-520-56364:
[email protected] Textural properties are often a target of heterogeneous catalyst design: indeed surface area and pore size determine the accessibility to active sites and this is o~en related to catalytic activity and selectivity in catalysed reactions. In this work amorphous silica-aluminas with different textural properties have been synthesised modifying the type and the amount of solvent. Information about the pore structure of these materials has been obtained by physisorption isotherms, that are able to discriminate between micro and mesoporosity and associated border line situations. Formation mechanism for micro- and mesoporous silica-aluminas is proposed. I. INTRODUCTION Amorphous silica-alumina materials represent an important class of porous inorganic solids which have not long-range order and usually have a wide distribution of the pore size, in the micro and mesopore region. Since early 90's a strong synthetic effort has been devoted to developing amorphous silica-aluminas with high pore volume, high specific surface area and pore size larger than that of zeolites: MCM-41 [1], MSA [2], ERS-8 [3], AI-HMS [4] and FSM-16 [5]. They show outstanding catalytic behaviours in several acid catalysed reactions [2, 61. Syntheses performed via sol-gel are particularly interesting, thanks to their versatility. In a typical sol-gel preparation the reagents are mixed to form a homogeneous solution that then gels to give a highly porous oxides and with good homogeneity. The type of precursor, the solvent, the gelation temperature and gelation catalyst can give rise to materials with different properties. Our group has recently claimed amorphous silica-aluminas, having acidic properties as catalysts [7], prepared from alkoxide precursors by sol-gel synthesis route [2, 3]. The use of gelling agents, as tetraalkylammonium hydroxide (NR4-OH), permits to control the porosity distribution. Depending on the N~-OH/alkoxides molar ratio and on the alkyl chain length of R groups two families of silica-aluminas have been obtained: 9 MSA is mainly mesoporous with a small contribution of micropores responsible of the adsorption observed at very low relative pressure, p/p~ < 0.1, in agreement with a Type IV + (I) isotherm [8]. 9 ERS-8 is a microporous solid, in agreement with a reversible Type I isotherm. A broad peak in the low angle region of XRD pattern is present, due to a very low structure order.
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In this work the influence of the alcoholic aliphatic chain and the H20/alcohol molar ratio has been investigated. The formation mechanism of different silica-aluminas is discussed on the base of their textural properties. 2. EXPERIMENTAL Samples were prepared via sol-gel in alkali-free medium using Si(OC2H5)4 (Dynasil-A, Nobel), Al(sec-OC4H9)3 (Fluka), tetrapropylammonium hydroxide (TPAOH, Sachem), alcohol (ROH) selected among C2HsOH, n-C3H7OH, n-C4H9OH, n-CsHllOH, n-C6H13OH (Fluka). All preparations were performed at the same molar ratio: SIO2/A1203=300, TPAOH/SiO2=0.09. The molar ratio H20/alcohol was changed between 0.5 and 5. When alcohols with a long aliphatic chain are used, only samples with low water amount can be synthesised, in order to obtain a homogeneous solution. In all syntheses the volume fraction of TPA § in the dried precursors is 40% (calculated by assuming a mass density of 0.9 g/cm 3 for TPA § and 2.0 g/cm3 for SiO2) [3]. A typical synthesis preparation is following described. ml(sec-OC4n9)3 was dissolved in Si(OC2H5)4 at 60 ~ The obtained homogeneous solution was cooled at room temperature, then the required alcohol and TPA-OH in aqueous solution were added in sequence. Monophasic clear solutions were obtained, then transformed in homogeneous compact gel without separation of phases. The molar compositions of reagent mixtures are reported in table 1. The sample acronyms indicate the number of carbonium atoms in the alcohol used and the molar ratio H20/ROH. Table 1 Molar compositions of reagent mixtures. SAMPLES
ET/C2-5 ET/C2-1 ET/C2-0.5 !ET/C3-5 ET/C3-1 ET/C3-0.5 ET/C4-0.5 ET/Cs-0.5 ET/C6-0.5
ROH
C2HsOH
Molar ratio HzO/ROH
Molar ratio H20/SiO2
Molar ratio ROH/Si02
5
10 8 2 10 8 2 2 2 2
2 8 10 2 8 10 10 10 10
1
0.5 5 n-C3H7OH
1
n-C4H9OH n-CsH~ 1OH n-C6HlaOH
0.5 0.5 0.5 0.5
After 15 hour ageing at room temperature, the gels were dried at 100 ~ and calcined 8 hour in air at 550 ~ The textural properties of all calcined samples were determined by nitrogen isotherms at liquid N2 temperature, using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples (-~ 0.2 g) were outgassed for 16 h at 350 ~ under vacuum. The specific surface area (SBET)was evaluated by 2-parameters linear BET plot in the range p/pO 0.01-0.2. The total pore volume (VT) was evaluated by Gurvitsch rule. Medium pore size (dDvr) and pore size distributions were calculated using DFT method for all materials.
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Indeed, DFT, based on molecular statistical approach, is applied over the complete range of the isotherm and is not restricted to a confined range of relative pressure or pore sizes. Pore size distribution is calculated by fitting the theoretical set of adsorption isotherms, evaluated for different pore sizes, to the experimental results. 3. RESULTS All samples synthesised by using ethanol (ET/C2) and propanol (ET/C3) are characterised by irreversible Type IV isotherms, with H2 Type hysteresis (Figure 1 and 2 respectively). Higher pore volume and surface area are always obtained for ET-C3 compared to ET-C2 samples, furthermore, a shift towards upper relative pressure of the hysteresis loop is generally observed. For each alcohol, samples prepared at H20/ROH = 1 show the highest oore volume and the highest surface area. 750
A
E v
"o o .Q L O I/) "o
2000 ppm S) at low pH also serves to precipitate iron hydroxide/ferrihydrite. The iron sulfide species presumably aggregate the precipitated iron species, giving rise to the highly crystalline material indicated by XRD.
a)
b) 9O
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,
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i
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pH of s u l f i d a t i o n
Figure 3: Effect of solution pH on the iron species found in solutions containing different amounts of S2 (a) 500 ppm, (b) 20000 ppm.
In these figures sulfide-containing species are expressed as a percentage of total sulfide added, and ferric hydroxide species expressed as a percentage of total Fe 3+ added. Therefore, X refers to sulfide, ferrous or ferric. a
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4. CONCLUSION Chemical speciation studies permit for an understanding of the influence of S2" on the physical properties of precipitated iron oxide. Thus, both the concentration and the pH of addition of S2 to the iron/carbonate solutions have been shown to influence the crystallinity and pore properties of iron oxides. This study shows that it is possible to control the physical properties of the precipitated iron complexes by variation of the above parameters. The precipitation process also influences the position of the sulfide ions within the crystallites and this impacts on the catalytic behaviour of the iron catalysts.
A clmowledgements We wish to thank the University, the FRD/NRF and Sasol for financial assistance. REFERENCES
1. 2. 3. 4.
R.A. Diffenbach and D.J. Fauth, J. Catal., 100 (1986) 466. R.M. Cornell and R. Giovanoli, Clay and Clay Minerals, 33 (1985) 424. U. Schwertmann and E. Murad, Clay and Clay Minerals, 31 (1983) 277. D.R. Milburn, R.J. O'Brien, K. Chary and B.H.Davis, Studies in Surface Science and Catalysis, 87 (1994) 753. 5. M.E. Dry, Catalysis: Science and technology, eds. J.R. Anderson and M Boudart, Springer-Verlag, New York, 1 (1981) 159. 6. T.C. Bromfield and N.J. Coville, Appl. Surf. Sci., 119 (1997) 19. 7. T.C. Bromfield and N.J. Coville, Appl Catal, A: General, in press (1999). 8. T.C. Bromfield and N.J. Coville, to be published. 9. K. Lazar, W.M. Reiff, and L. Guczi, Hyperfine Interactions, 28 (1996) 87. 10. JCPDS (40832 FeS). 11. S. Yunes, Explanation and Application of the Physisorption and the Chemisorption techniques in the Characterisation of solids in general and catalysts in particular, Micromeritics (1996). 12. J.R. Duffield, F. Marsicano and D.R. Williams, Polyhedron, 10 (1991) 1105.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
The nature of the active phase in iron Fischer-Tropsch catalysts Abhaya K. Datye a, Yarning Jin a, Linda Mansker a, R. Thato Motjope b, T. Humphrey Dlamini b and Neil J.Coville b aCenter for Microengineered Materials and Dept. of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131, USA. bDepartment of Chemistry, University of the Witwaterstrand, Johannesburg, South Africa. Working Fe Fischer-Tropsch (F-T) catalysts were studied using Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and M0ssbauer spectroscopy. The catalysts were removed from a slurry phase Fischer-Tropsch reactor. Instead of using Soxhlet extraction, which may expose the highly reactive catalyst to atmospheric oxidation, we have used an alternative pretreatment involving room temperature extraction that allows us to concentrate the catalyst, but at the same time protect it against atmospheric oxidation. Catalysts from two slurry reactor F-T runs were analyzed, one in its active state and the other after deactivation. Major changes in phase composition as well as in particle size can be seen in these two catalysts. In its active state, the Fe F-T catalyst contains highly dispersed carbide particles of z-FesC2 and Fe7C3. The deactivated catalyst, on the other hand, shows a different carbide structure that is indexed by MSssbauer as the E' carbide. The combination of XTEM, XRD and Mtissbauer spectroscopy helps resolve some of the discrepancies in the literature about the identity of the phases seen in a working F-T catalyst. 1. INTRODUCTION F-T Synthesis is recognized as a viable route for conversion of syngas to liquid fuels [1]. Due to their high water gas shift activity, Fe catalysts are the preferred choice for coal derived syngas having a H2/CO ratio 4000 min is noticed (with the rhenium promoted catalyst, the respective decline of desorption probability is ca. 0.35 to ca. 0.14). Thus we conclude: The polymerization nature of the FT synthesis
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1148
z
1
z
Q.
13.
~0.8 .._1 rn
10 h and are representative of stabilised catalysts, i.e. after about 90 hours on stream. 3.
RESULTS AND DISCUSSION
Table 1 illustrates the effect of increasing intraparticle mass transfer restrictions for conventional (powder) catalysts. The table shows the selectivity for two powder catalysts with different particle size. Increasing the catalyst particle size from (53-751.tm) to (425-8501am) results in low C5+ selectivity and high CH4 and CO2 selectivities as a result of significant mass transfer restrictions. CO depletion leads to increased termination by hydrogenation and increased intraparticle residence time for produced water results in increased secondary watergas-shift reaction producing CO2. Due to the kinetic negative order dependency of CO, the effect of CO transport limitations on the overall reaction rate is delayed and therefore the presence of transport limitations will first be seen on the selectivity, due to the well known relationship between H2/CO ratio and selectivity [ 1]. The increase in particle size in Table 1 corresponds to a 10-fold increase in diffusion length. Even larger particles (>1000 lam) are necessary in commercial fixed-bed reactors to avoid unacceptable pressure drops. Table 1 Comparison between powder CoRe/A1203 catalysts of different pellet size. K, pressure 13 bar, H2/CO=2.1 CH4 C2-4 Catalyst Relative rate* sel. sel. Powder catalyst (53-751am) 1.00 9.0 7.4 Powder catalyst (425-8501.tm) 0.80 21.5 12.7 * Rate (gHc/gcat.'h) relative to the rate for powder catalyst (53-75 lam).
Temperature: 483 C5+ sel. 82.9 64.4
CO2 sel. 0.7 1.5
The thermal stability and the ability to withstand rapid temperature variations are of great importance in many monolith applications. Monolith structures are therefore usually made from a low-surface area ceramic material. The surface area can be increased by depositing a high surface area material such as y-alumina on the monolith surface and subsequently add the catalytically active material or the supported catalyst can be deposited directly on the monolith surface by washcoating.
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Table 2 Comparison between a conventional powder CoRe/A1203 catalyst and the corresponding monolithic catalysts. Temperature: 483 K, pressure 20 bar, H2/CO=2.1 Catalyst Relative CH4 C5+ C3JC3- CO2 Rate* sel. sel. ratio sel. 1.00 8.3 82.3 2.4 0.2 Powder catalyst (38-53 I,tm) Wascoated cordierite 0.92 8.9 82.5 1.9 0.3 (washcoat thickness 0.04mm) * Rate (gHc/gcat.'h) relative to the rate for powder catalyst (38-53 lttm). The performance of washcoated cordierite monoliths is shown in Table 2. Calculation of the relative rate to hydrocarbons shows that the washcoated monolithic substrates are as active as the conventional (powder) catalyst. They also have similar CH4 and C5+ selectivities, but a slightly lower olefin to paraffin ratio for the cordierite-based catalyst indicates higher diffusion resistance than for the powder sample. Figure 1 shows the effect of increasing the washcoat layer thickness on the cordierite substrate. The washcoat layer thickness was calculated assuming an even distribution of the washcoat on the monolith surface. The rate is relative to the rate of powder catalyst (38-53 ~tm), Table 2.
1,5
83 I1)
(~ 82 o
~
o.o--'" ~:'::,".... .o........
t~
~ 8~
.u> t~
>
~
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',,
...o... Relative rate 79
0,00
i
I
i
0,05
O,10
O,15
0
0,20
appr. washc0at layer thickness [mm] Figure 1. Effect of washcoat layer thickness on the rate and C5§ selectivity of monolith catalysts (483 K, 20 bar, H2/CO=2.1).
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3 r
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9
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(a)
(b)
Figure 1. (a) Evolution of the optimal solution to the A + B--+ 0 problem with an A:B ratio in the bulk ph~e of 1:1 using the GA. A adsorption sites are shown in white. (b) The evolution of the most active surface and population average for the A + B ~ 0 reaction. VFSR algorithm an exponential cooling schedule is used where Tk -- To exp(--ckUD). The parameter c specifies the rate of cooling. Starting with a randomly generated surface, the activity of the surface is first calculated by MC simulation. A new surface is then generated at random. The probability of accepting this new surface as the next step in the search is given by the generating function N
1
gk - iII= 1 2(lyil + Tk) ln(1 + 1/Tk)"
(4)
The parameter lYil is equal to the number of differences between the sites of the two surfaces. Small steps in search space are therefore more likely to be successful than large leaps. Note that gk and Tk must satisfy the condition Egk = c~. This condition requires all sites must at least be able to be visited, assuming that the search is ergodic. If the new surface is accepted as the next trial location, then the activity of the new surface is calculated. The probability, P, of accepting the new solution in place of the previous solution is given by the Metropolis condition [
p _ ~exp(AR/Tk)
(
1
ifAR < 0;
(5)
otherwise,
where A R is the difference in catalyst activity between the current catalyst surface and the new surface. Initially, at high system temperatures, most changes are accepted allowing for large areas of the parameter space to be searched. As the optimisation progresses, and the system cools, the acceptance criterion becomes more difficult to satisfy and only successive improvements are accepted. 3.
RESULTS AND DISCUSSION
The evolution of the solution to the A + B--+ 0 problem for an A:B ratio of 1"1 using the GA is shown in Figure 1. The A adsorption sites are shown in white in the figures or as the
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0.22 0.2
-'-"
0.21
.-
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0.15 I--
E = Z
E
z
P
0.19
=._
> O
> O r
= I--
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0.18 0.17
o Simulated annealing r~ Genetic algorithm 0
[]
0.1
5 10 15 F u n c t i o n E v a l u a t i o n s [x10 -3]
20
I-
0.05
o Simulated annealing
Genetic algorithm
0 F u n c t i o n E v a l u a t i o n s [x10 -3]
(b)
Figure 2. (a) A comparison of the GA and SA solutions to the A+B --+ 0 problem. (b) The corresponding data for the A + B2--+0 problem '0' sites in the matrix notation. The optimal solution with the most active catalyst surface, a checkerboard site distribution, is located after 50 generations. The spreading of the highly reactive [~ 0] group, which maximises the mixing of the reactants on the surface, illustrates the ability of the GA to identify and propagate the desirable characteristics of the solution through the population. A comparison with solution obtained by the SA algorithm for the A + B - + 0 problem is given in Figure 2(a). Although it may have been possible to predict the checkerboard solution, the solution for other bulk phase compositions is not as intuitive. As a further test of the methodology, the design of a catalyst surface for the A + B - + 0 reaction was repeated using a range of bulk phase compositions. Altering the bulk phase composition such that the A species becomes twice as likely to make an adsorption attempt leads to very different solution. The most active surface in this case is given by a tiling of the [ 0 ~ ] group. The site density changes as to increase the probability of adsorbing the 110 component that is deficient in the bulk phase. This diagonal pattern was maintained for increased A:B ratios of 3:1 and 4:1. Results for the A+B2--+ 0 reaction are shown in Figure 2 (b). In this case the optimal solution, obtained by both the GA and SA, is found to be similar to that obtained for the A+B--+ 0 reaction. The most active surface found was a checkerboard tiling of the [0 0 ~ 00] group. A comparison of the two algorithms demonstrates that, for both reactions, SA is the more efficient optimisation method. For the A+B--+ 0 reaction the first occurrence of the optimal solution required approximately 5 x 10a function evaluations, as compared to 1 x 104 evaluations for the GA. Similarly, for the A + B 2 ~ 0 reaction the SA algorithm required only 2.7 x 10 a evaluations as compared to the 1.1 x 104 required by the GA. Despite the efficiency of the SA algorithm, however, there is a compelling reason for using the GA for practical design problems, where the catalyst surfaces will be more complex that those discussed here. The advantage of using the GA is that the GA does not seek out a single solution, but instead aims to propagate specific site groups throughout the solution population. These groups correspond to the desirable traits of the catalyst surface. Thus, even if a single optimal solution is not obtained for a complex problem,
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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the aspects of the catalyst surface that most influence the catalytic activity or selectivity may be identified. In the A + B - + 0 reaction for example, 50 generations are required to find the optimal site distribution. However, examination of the population after only a few generations revealed that the majority of solutions in the population had an $1 : $2 ratio of 1:1. Thus, the principle influence on the catalyst activity is the proportion of $1 :$2 sites rather that their geometric distribution on the surface. 4.
CONCLUSIONS
A rational methodology for the optimal design of simple catalyst surfaces at the molecular level has been introduced. The method has been applied to design of two-component catalysts for the A+B--+ 0 and A+B2--+ 0 reactions. For the A+B--+ 0 reaction, where the bulk phase is a mixture of equal amounts of each component, the surface yielding the highest catalytic activity is found to be a checkerboard distribution of the two active sites. Deviations from this configuration are observed for cases in which the bulk phase is not composed of an equal mixture of the reactants. As the relative proportion of one of the species increases, the number of adsorption sites for that species tends to a ratio of 2:1 in favour of the species that is deficient in the bulk phase. Extension to the more complex A+B2 ~ 0 reaction with equal amounts of each component in the bulk phase produces a similar checkerboard pattern consisting of a tiling of pairs of adjacent adsorption sites. While the SA approach has been found to be more efficient than the GA, the GA is expected to be more suited to practical catalyst design problems. The advantage of the GA is that the GA identifies the desirable characteristics common to a large population of solutions. For complex optimisation problems, where qualitative trends are more likely to be identified that unique optimal solutions, the GA is will be the preferred optimisation algorithm. 5.
ACKNOWLEDGMENTS
ASM thanks Peterhouse, Cambridge for the award of the Rolls Royce Frank Whittle research fellowship. LFG thanks the Innovative Manufacturing Initiative and EPSRC.
REFERENCES
[1] J.H. Holland, Adaptation in Natural and Artificial Systems, MIT Press, Cambridge, 1994. [2] D.E. Goldberg, Generic Algorithms in Search, Optimisation, and Machine Learning, AddisonWesley, New York, 1989. [3] L. Davis, Generic Algorithms and Simulated Annealing, Pitman, London, 1987. [4] S. Kirkpatrick, C.D. Gelatt and M.P. Vecchi, Science, 220 (1983) 671. [5] R.M. Ziff, E. Gulari and Y. Barshard, Phys. Rev. Lett., 56 (1986) 2553. [6] K. Fichthorn, E. Culari and Y. Barshard, Phys. aev. Lett., 63 (1989) 1527. [7] A.S. McLeod, M.E. Johnston and L.F. Gladden, J. Catal., 167 (1998) 279. [8] A.L. Ingber, Mathl. Comput. Modelling, 12 (1989) 967.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Modeling of structure and properties of active centers of catalysts on the base of metalorganosiloxanes
A.V.Nemukhin *~ I.M.Kolesnikov b and V.A.Vinokurov b a- Department of Chemistry, Moscow State University, Moscow, 119899, GSP, Russia b - Russian State University of Oil and Gas, Leninsky Prospect, 65, Moscow 117296, Russia Quantum chemical calculations provide support to the reaction mechanisms suggestinq chemisorption of the ethylene molecule on the A104 site of alumophenylsiloxane accompanying a n-bond rupture and a creation of cation-radicals. Studies of elementary chemical processes occurring at the alumosilicate compounds are of importance due to a great significance of these species as catalysts in the conversion of oil fractions~. Along with wellknown solid alumosilicates related soluble substances, in particular alumophenylsiloxane (APS) and metallphenylsiloxanex possess similar structural properties and are viewed as perspective catalysts as well 2. Recent experimental studies 3 show that in the presence of APS in a liquid reaction volume an propilene-benzene interaction occurs yielding propyl-benzene. It is suggested that one of the stages of the process may be related to the chemisorption of unsaturated hydrocarbons on APS ahd MPS accompanying a creation of cation- and anion-radicais. Considerable efforts are being performed to clarify mechanisms of catalytic activity of alumosilicates starting from different viewpoints 4. According to the theory catalysis by polihedra developed by one of the authors 5 active sites of alumosilicate compounds are the tetrahedrons {A104} and {SiO4}. This work presents a quantum-chemical contribution in some support to this idea taking the ethylene-APS system as an example.
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All calculations described here have been carried out with the GAMESS package 6. We have considered the interaction of the ethylene molecule with APS by means of a group of atoms representing the local chemisorption site. Namely, the {AIO4} fragment have been selected as an ac,tive species in environments simulating the original APS complex. A special attention has been paid to such parameters as partial charges on atoms, effective electronic configurations and compositions of bonding orbitals. At the preliminary stages the structure of APS, (C6Hs(HO)2SiO)2AI(OSi(OH)2C6Hs), has been studied by using molecular mechanics and semiempirical AM1 quantum chemistry techniques 7 followed by ab initio MO LCAO calculations with the 3-21G basis set. According to these approaches the central part of APS may be considered as a distorted AIO4 tetrahedron with two A1-O bonds directing to the Si(OH)2C6H5 fragments and two AI-O bonds forming a well-known aluminum-oxygen-silicon cycle with a bridged OH group 4. A partial electronic charge on aluminum is estimated as +2.0, on oxygen as-1.2 (on average), and on silicon as +2.4. A composition of the bonding A1-O orbital may be described in terms of natural bond orbitals 8 as 0.35 sp a3 (A1) + 0.95 sp 13 (O) indicating that the orbital is strongly polarized towards the oxygen hybrid. A strongly ionic character of the compound is consistent with the data common to aluminum oxide molecules 9'~~ To simulate an active site of APS, namely, the {A104} fragment, several reduced systems of APS have been considered: A104Li3, AI(OH)4, AI(OSiN)202Li, AI(OH)2(OSiH3). In all cases the tetrahedral arrangement of oxygen atoms placed at a distance 1.72 A from the aluminum center has been assumed. Detailed results will be presented below for the interaction C2H4 + AIO4Li3 although most impirtant qualitative conclusions are similar for all species. The lithium atoms play here the role of "pseudoatoms" often used in attempts to select a finite cluster from an extended system when studying complicated processes like chemisorption 4. In the A104Li3 moiety partial charges on atoms (+ 1.4 on A1 and -0.9 on O) resemble those calculated for APS (+2.0 and-1.2) as well as effective electronic configurations (3s ~ 3p ~~ in A104Li3, versus 3s ~ 3p ~ in APS for aluminum, and 2s19 2p5.0 in A104Li3 versus 2s 1"8 2p 5"4 in APS for oxygen). A composition of the bonding AI-O orbitals in A104Li3 (0.34sp z3 (AI) + 0.94sp 1"6(O)) also reproduces nicely that of APS. We conclude that the local electronic properties of the A104 site are reproduced successfully by the A1 O4Li3 molecule.
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Chart I shows geometry arrangements of the entire reactive complex C2H4 + AIO4Li3. The C2v symmetry has been assumed for the system. Other spatial configurations have been rejected since their energies are considerably higher. We have varied only one geometry parameter, a distance R between A1 and a center of the C-C bond in C2H4. Other parameters have been optimized with the RHF/3-21G (a restricted Hartree-Fock model within the 3-21G basis set) wavefunctions for C2H4, and A104Li3 in separate calculations.
H~
J
c
CL..
t
H j
H H
R
O Li /
O ~ ~
A1 ~ ~
/\ \/
O
Li
O
Li Chart 1
Within the natural borM orbital formalism 8 the electronic structure of C2H4 is described as a configuration with the doubly occupied bonding orbitals: [Core]cy2(C-C)[cya(C-H)]4n2(C-C). When interacting with the A104, site of APS or related reduced systems specific changes in the electronic structure of C2H4 are expected. For the goals of the present study the most important is a process of a n-bond rupture in C2H4 which should be reflected by a substantial reduction of population of the n (C-C) orbital. The data presented in Table I show the main result of this work, namely, the changes in total energy and in the electronic structure of the ethylene molecule during its interaction with the A104Li3 species. The numbers have been computed at the RttF/3-21G level of the theory.
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Table 1. Interaction energies C2H4 + A104Li3 (AE), total charge on the C2H4 fragment (Q), and population of the n (C-C) orbital (n~) versus intermolecular distance (R) calculated at the RHF/3-21G approximation.
R,A
AE, kJ/mol
Q
nn
50.0 5.0 4.0 3.5 3.0 2.75 2.50 2.25
0.0 +3.3 +26.8 - 114. -413. -527. -4601 +54.8
0.0 0.0 +0.194 +0.496 +0.638 +0.656 +0.668 +0.676
2.00 1.98 1.77 < 1.5 --'--
First of all we note that the interaction C2H4 + AIO4Li3 leads to a strongly bound system at the distance R about 2.75 A with a binding energy above 527 kJ/mol. A small barrier in the entrance channel (about 27 kJ/mol) may disappear and the well depth may increase if a geometry relaxation is taken 71-110 account in calculations. A bound C^H + A10 Li complex evidences that a chemisorption may take place when ethylene is contacting APS. A total natural charge on the ethylene fragment is increasing from zero to a value (+0.68) close to +1 indicating that reaction mechanisms which assume creation of cation-radicals gain some support by these quantum chemical simulations. We also see. that along the reaction pathway a population of the n-orbitaj of C2H4 is reducing from 2.00 to a boundary value of 1.50 generally accepted as a critical one for regularly occupied MO's 8 immediately after passing the potential barrier. We can interpret the result as a rr-bond rupture in this region. An analysis of electron density in terms of natural orbitals shows that .instead of the n(C-C) orbital the ~(C-O) orbitals are getting populated which results in a cycle extending over the entire complex.
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In conclusion the present study confirms that the {A104} tetrahedron may serve as an active site of the alumophenylsiloxane complex. According to the quantum chemical model calculations the ethylene molecule reacts efficiently with this site yielding a singly bonded cation-radical as an intermediate species. Acknowledgments The authors thank Prof. J.Almlof for the making these computations possible, Professors F.Weinhold for using their computer programs.
financial support M.Schmidt and
References 1. Kreking neftyanykh frakzyi na tseolitsoderzhschikh katalizatorakh (Cracking of oil fractions on ceolit containing catalysts), ed. S.N.Khadjiev, Khimia, Moscow, 1982, p.280 (in Russian). 2. I.M.Kolesnikov, G.M.Panchenkov and V.A.Tulupov, Zh.Phys.Khim., 1965, 39, 1869 (in Russian) 3. I.M.Kolesnikov and N.N.Belov, Zh.PrikIadnoy Khim., 1990, 63, 162 (in Russian) 4. G.M.Zhidomirov, A.A.Bagaturyanz and I.A.Abronin, Prikiadnaya kvantovaya khimia (Applied Quantum Chemistry), Khimia, Moscow, 1979, p.296 (in Russian) 5. I.M.Kolesnikov, Kinetika i kataliz v gomogennykh i geterogonnykh uglevodorodsoderzhschikh sistemakh (Kinetics and catalysis in homogeneous and heterogeneous hydrocarbon containing systems), Textbook, Moscow Institute of Oil and Gas, Moscow, 1990, p. 198 (in Russian) 6. M.W.Schmidt, K.K.Baldridge, J.A.Boatz, J.H.Jensen, S.Koseki,M.S.Gordon, K.A.Nguyen, T.L.Windus and S.T.Elbert, Quantum Chemistry Program Echange Bulletin, 1990, 52 7. M.J.S.Dewar, E.G.Zoebisch, E.F.Healy and J.J.P.Stewart, J.Amer.Chem.Soc. ,1985, 107, 3902 8. A.E.Reed, R.B.Weinstock and F.Weinhold, J.Chem.Phys., 1985, 83,737 9. A.V.Nemukhin and F.Weinhold, J.Chem.Phys., 1992, 97, 3420 10.A.V.Nemukhin and L.V.Serebrennikov, Uspekhi Khimii, 1993, 62, 566 (Russian Chemical Reviews, 1993, 62, 527)
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A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Theoretical study on active sites of molybdena-alumina catalyst for olefin metathesis J. Handzlik and J. Ogonowski Institute of Organic Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Krak6w, Poland Theoretical investigations of ethylene and propene metathesis reactions proceeding on molybdenaalkylidene and molybdacyclobutane centres of the molybdena-alumina catalyst were done. The density functional theory in Gaussian 94 package was applied. The energetic effects of the reactions of the alkene molecule with Mo TM and Mo vI carbene centres were calculated. Reaction pathway of ethylene addition to Mo vl alkylidene complex was also investigated 1. INTRODUCTION It has been generally accepted that transition-metal-catalysed olefin metathesis proceeds according to carbene mechanism [1]. Metal carbene reacts with olefin, thus forming the metallacyclobutane complexes. Subsequent decomposition of the metallacyclobutane leads to formation of a new olefin and a carbene structure. Several theoretical studies on the olefin metathesis reaction and the structures of molybdenum homogeneous catalysts have been performed [2-4]. Those works concem Mo vI alkylidene and molybdacyclobutane complexes. However, we have not found examples of theoretical modelling of olefin metathesis active sites on heterogeneous molybdena catalysts. The active catalyst centres contain probably M o vI [ 5 - 6 ] , but other Mo valences are also possible (e.g., Mo TM [7]). In this work theoretical studies on possible ethylene or propene metathesis reactions proceeding on molybdenaalkylidene centres of molybdena-alumina catalyst are reported. Two variants of theoretical models of the active sites have been developed. In the first case, simple structures of the carbene and molybdacyclobutane complexes are proposed, where the bonds between molybdenum and the carrier are replaced by hydroxyl groups. In the second case, molybdenum is attached to a small cluster of formula A12(OH)6, which represents alumina. 2. COMPUTATIONAL Calculations were carried out with the GAUSSIAN 94 program [8], installed on SPP1600/XA computer in ACK CYFRONET AGH (grant No. KBN/SPP/PK/099/1998). Density functional theory was applied. Geometry of the structures was optimized with the Slater local exchange functional and the VWN5 local correlation functional [9], or, with the B3LYP nonlocal functional [10]. Harmonic vibration frequencies were calculated for each structure to confirm the potential energy minimum or the transition state involved, and, to obtain the thermal energy, enthalpy, entropy and Gibbs free energy. The electronic energy of
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each structure was calculated using B3LYP functional. Three basis sets were employed: basis A - LANL2DZ basis set (the Hay-Wadt effective core potential [11 ] plus double-zeta basis for molybdenum and aluminium atoms, Dunning-Huzinaga valence double-zeta basis set D95V - for other non-transition elements), basis BI and B2 - LANL2DZ basis set for molybdenum atom and D95V(d) or 6-31G(d) basis set for non-transition elements, respectively. Calculations of the alumina clusters alone were carried out with MOPAC 7 program using semiempirical AM1 Hamiltonian. 3. RESULTS AND DISCUSSION
3.1. Simple models of the carbene and molybdacyclobutane complexes The following Mo vl complexes: Mo(O)(CH2)(OH)2 (1), Mo(O)(CH2CH2CH2)(OH)2 (2a and 2b), syn-Mo(O)(cncn3)(on)2 (3), Mo(O)[CH(CHa)CH2CH2)(OH)2 (4), Mo(O)[CH(CHa)CH(CHa)CHE](OH)2 (Sa and 5b), and Mo Iv complexes: Mo(CHE)(OH)2 (6a and 6b), Mo(CH2CHECHE)(OH)2 (7a and 7b) were studied. Calculated geometry of 1 is shown in Fig. 1. In Table 1, structures of 1 obtained using different functionals and basis sets are compared. The results indicate that optimized geometry almost does not depend on used functional or basis set. Calculated structures of other Mo vx complexes and Mo TM structures, singlet (6a, 7a) and triplet states (6b, 7b), are shown in Fig. 2. 01 Hl Mo
~
C
-M 1
Table 1 Optimized structures of 1. Bond distances are in angstroms, angles are in degrees. SVWN5/A B3LYP/B1 B3LYP/B2 Mo-C 1.886 1.896 1.891 Mo-O l 1.716 1.691 1.696 Mo-O 2 1.880 1.916 1.917 C-H l 1.102 1.092 1.089 C-H 2 1.109 1.099 1.097 OI-Mo-C 103.0 103.8 103.9 Mo-C-H 1 124.9 126.6 127.0 Mo-C-H 2 119.3 118.0 118.0 O2-Mo-O 3 110.5 110.3 109.9
Fig. 1. Optimized geometry of 1. Four-coordinate molybdenum alkylidenes I and 3 have pseudotetrahedral structure with Cs symmetry. Mo vl molybdacyclobutanes have a square pyramidal (SP" 2a, 4, 5) or a trigonal bipyramidal (TBP: 2b) structure. Calculated structures of Mo vI complexes are in a very good agreement with other published results for Mo alkylidenes and molybdacyclobutanes [2-4]. SP molybdacyclobutane (2a) is predicted to be more stable than TBP structure (2b) by 66 kJ/mol with the B3LYP/B1 method and by 69 kJ/mol with the B3LYP/B2 method. For Mo TM methylidene complexes, B3LYP/B1 and B3LYP/B2 calculations give a 33 kJ/mol and 35 kJ/mol preference to the triplet state (6b), respectively. Triplet state of molybdacyclobutane 7b is more stable than the singlet state 7a by 57 kJ/mol and 60 kJ/mol, according to B3LYP/B 1//SVWN5/A and B3LYP/B2//SVWN5/A calculations, respectively.
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@ =Mo
O
=C
9 =H
~!~3 = 0
1.889
4+
I
C)
F+
6a
5b +..
.+__ 1.890
6b
i
7a
t ~ ' k ' - 1.868
~.~
/g+
~
,--.
7b
g (
Fig. 2. Optimized structures of Mo vl and Mo TM complexes. Mo=C bond lengths (in angstroms) were obtained from SVWN5/A calculations. Thermodynamic effects of the reactions involving Mo vl and Mo TM centres are given in Table 2. Reactions of alkene and Mo vI alkylidene complex leading to formation of SP molybdacyclobutane (I, III, IV, V) are exothermic with relatively small absolute value of the
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change of Gibbs free energy. On the other hand, the reaction VIII of Mo TM methylidene complex with ethene is very exothermic and exoergic. The enthalpy of formation of trigonal bipyramidal molybdacyclobutane from methylidene complex and ethene (reaction II) is close to zero, but the reaction is endoergic. Molybdacyclobutane leading to trans-2-butene has lower energy than the corresponding complex giving cis-2-butene (reactions IV and V). However, the decomposition of the latter structure is energetically preferred, in comparison to the decomposition of the former one (reactions VI and VII). Table 2 Calculated changes of thermal energies (AE~ kJ/mol), enthalpies (AH~ kJ/mol) and Gibbs free energies (AG~ kJ/mol) for the metathesis reaction of MoVl and Mo Iv complexes. Entry Reaction Method AE~ AH~ AG~ I 1 + C2H4 "-) 2a B3LYP/B1//SVWN5/A -69.1 -71.6 -16.8 B3LYP/B1 -65.6 -68.1 -14.3 II 1 + C2H4 --) 2b B3LYP/B1//SVWN5/A -4.9 -7.3 44.9 B3LYP/B1 0.7 -1.8 49.8 B3LYP/A 2.5 0.0 52.4 III 1 + C3H6 ") 4 B3LYP/B1//SVWN5/A -57.3 -59.8 -0.1 B3LYP/B1//SVWN5/A -52.2 -49.7 8.2 IV 3 + C3H6 ") 5a B3LYP/B1//SVWN5/A -40.2 -37.8 22.0 V 3 + C3H6 ") 5b VI 5a --) 1 + trans-2-butene B3LYP/B1//SVWN5/A 55.8 58.2 -0.1 VII 5b --) 1 + cis-2-butene B3LYP/B 1//SVWN5/A 49.0 51.5 -9.7 VIII 6b + C2H4 "-) 7b B3LYP/B1//SVWN5/A -130.1 -132.6 -79.7 Reaction pathway of ethylene addition to Mo vl alkylidene complex 1 was investigated. n-Complex 8 of ethene with the alkylidene complex and transition state 9, leading to TBP molybdacyclobutane, were localised (Fig. 3). The efforts to find a square-pyramidal transition state have failed. The thermodynamic parameters calculated for the reactions studied are given in Table 3.
9
=Mo
~
=C
. =0
Q =H (--~
8
9
"
C
~,..5
Mo
- C
1 --
2.744
Mo - C l = 2.306
Fig. 3. Optimized structures of zc-complex 8 and transition state 9. Mo - C distances (in angstroms) were obtained from B3LYP/A calculations.
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Table 3 Calculated changes of thermal energies (AE~ kJ/mol), enthalpies (AH~ kJ/mol), kJ/mol) for the reaction entropies (AS0298, J/(molK)) and Gibbs free energies (AG~ pathway of ethene addition to Mo vl methylidene complex. Entry Reaction Method AE0298 AH~ AS~ AG~ IX 1 + C2H4 --) 8 B3LYP/A 22.3 19.8 -148.0 63.9 X 8 --) 2b B3LYP/A -19.8 -19.8 -27.9 -11.5 XI 8 --) 9 B3LYP/A 12.7 12.7 -30.1 21.6 3.2. Model structures including alumina
The following Mo vl and Mo TM (triplet state) complexes: Mo(O)(CH2)(H4A1206) (10), Mo(O)(CH2CH2CH2)(H4A1206) ( l l a and lib), Mo(CH2)(H4A1206) (12) and Mo(CH2CH2CH2)(H4AI206) (13) were studied (Fig. 4). Their geometries are very similar to the corresponding geometries described in chapter 3.1. To verify correctness of the small alumina cluster of formula A12(OH)6, deprotonation energies and charges on the hydrogen atoms in this cluster were compared with the respective deprotonation energies and charges in a larger cluster of A1203 consisting of over one hundred atoms. 9 =Mo
9 =C
10
=O
lla
9 =H
Q
=A1
lib
Fig. 4. Optimized structures of Mo w and Mo TMcomplexes including alumina. Mo=C bond lengths (in angstroms) were obtained from SVWN5/A calculations.
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Thermodynamics of the metathesis reactions is given in Table 4. As in the case of the previous results, the SP molybdacyclobutane (lla) is predicted to be more stable than the TBP structure (lib). Formation of Mo Iv molybdacyclobutane 13 is exothermic and exoergic. Table 4 Calculated changes of thermal energies (AE~ kJ/mol), enthalpies (AH~ kJ/mol) and Gibbs free energies (AG~ kJ/mol) for the metathesis reaction of MoVl and Mo TM complexes. AE~ AH~ AG~ Entry Reaction Method XII 10 + C2H4 ") l l a B3LYP/B1//SVWN5/A -49.3 -51.8 1.5 XIII 10 + C2H4 "-) l i b B3LYP/B1//SVWN5/A 7.6 5.1 59.9 XIV 12 + C2H4 "-) 13 B3LYP/B1//SVWN5/A -132.4 -134.9 -82.2 4. CONCLUSIONS 1. SP structure of the molybdacyclobutane has lower energy than TBP structure. The formation of the former structure from molybdenaalkylidene complex and alkene is exothermic with relatively small absolute value of the change of Gibbs free energy. The formation of the latter structure is clearly endoergic. 2. During propene metathesis, the formation of the molybdacyclobutane leading to trans2-butene is energetically favoured, in comparison to the formation of the corresponding complex leading to cis-2-butene. On the other hand, decomposition of the molybdacyclobutane leading to cis-2-butene is more exoergic than the analogous process leading to trans-2-butene isomer. 3. Formation of the Mo TM molybdacyclobutane is very exothermic and irreversible. REFERENCES
1. J. L. Herisson, Y. Chauvin, Makromol. Chem., 141 (1970) 161. 2. T. R. Cundari, M. S. Gordon, Organometallics, 11 (1992) 55. 3. E. Folga, T. Ziegler, Organometallics, 12 (1993) 325. 4. Y.-D. Wu, Z.-H. Peng, J. Am. Chem. Soc., 119 (1997) 8043. 5. K. A. Vikulov, I. V. Elev, B. N. Shelimov, V. B. Kazansky, J. Mol. Catal., 55 (1989) 126. 6. W. Griinert, A. Yu. Stakheev, R.Feldhaus, K. Anders, E. S. Shpiro, K. M. Minachev, J. Catal., 135 (1992) 287. 7. K. Tanaka, in: Y. Imamoglu (ed.), Olefin Metathesis and Polymerization Catalysts, NATO ASI Series C326, Kluwer Academic Publishers, Dordrecht 1990, p. 303. 8. Gaussian 94, Revision E. 1, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. A1-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez, and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1995 9. S. H. Vosco, L. Wilk, M. Nusair, Canadian J. Phys., 58 (1980) 1200. 10. A. D. Becke, J. Chem. Phys., 98 (1993) 5648. 11. P. J. Hay, W. R. Wadt, J. Chem. Phys., 82 (1985) 299.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Modeling the Oxygen activation on Dinuclear Iron MMO Mimics, a Quantum Mechanic Study Peter-Paul H. J. M. Knops-Gerrits a-b, Peter A. Jacobs b • William A. Goddard III a aMaterial & Process Simulation Center, Beckman Institute (139-74), California Inst. of Technology, Pasadena CA 91125, USA Tel 626-395-2731, Fax 626-585-0918, email
[email protected] bCenter for Surface Science and Catalysis, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium, Tel: 32-16-32 1597; Fax : 32-16-32 1998, email
[email protected] Methane Mono-Oxygenase (MMO) is a di-iron active site containing enzyme that catalyzes the dissociative binding of molecular oxygen. To mimic the MMO active site we chose to study the heptapodate coordinated binuclear iron (II or III)-complexes of N,N,N'N'tetrakis(2-benzimidazolylmethyl)-2-hydroxy- 1,3-diamino-prop ane (HPTB), N,N,N',N',Tetrakis(2-pyridylmethyl)-2-hydroxy-l,3-diamino-propane (HPTP) in experiments and their finite cluster model N,N,N',N',-Tetrakis(2-iminomethyl)-2-hydroxy-l,3-diamino-propane (HPTM) in theoretical calculations. These have active sites of the form [Fez(HPTL)(p~-OH)]4+ or 2+. Quantum Mechanic structures are compared with experimental EXAFS data. For the O2 binding on the reduced active site the g-q~:ql-O 2 mode seems to proceed formation of the O=Fe-O-Fe=O bis-ferryl active site that reacts exothermally with methane. The nature of the ferryl groups are these of a reactive two center three electron bond. 1. INTRODUCTION Methane Mono-Oxygenase (MMO) [1] and Deoxyhemerythrin [2] are examples of diiron enzymes that catalyze the dissociative and non-dissociative binding of molecular oxygen. Dissociative binding of oxygen via a peroxo intermediate to a diamond core structure [3] leads to a reactive species active in the oxidation of alkanes [4-5]. Non-dissociative binding of oxygen via a side-on peroxo intermediate such as in the active site of deoxy-hemerythrin does not allow the splitting and allows binding/release of oxygen as a function of the physiological conditions [2]. These active sites are among the growing list related to O- and OH-bridged di- or poly-iron cores in biological systems [8-9]. MMO has a binuclear iron active site with two histidines and four glutarates. Both iron ions are coordinated by a histidine, an oxygen from a bridging carboxylate and a ~-oxo bridge [5]. Theoretical modeling of such enzyme active sites has been recently reported [10-16]. Yoshizawa et al. studied the dioxygen cleavage and methane activation on diiron enzyme models with the extended Hfickel method, an approximate molecular orbital method, the ~t']]1:']]1-02 o r ~.~-']]2:']']2-0 2 binding modes are distorted to the corresponding dioxygen complex. The p-rl~:q~-O2 mode is more effective for electron transfer to the d-block orbitals. Regarding methane activation Crabtree [11] reviewed the recent data. According to Siegbahn et al. [12].
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the most significant structure is the FeH~-O-FeV=O oxo structure. The ground state of this structure is ~A and the iron spins are 4.00 and 2.94, the spin on the bridging oxygen is 0.76 and on the oxo ligand is as high as 1.13. In reactions with ethane there is 35% of inversion of configuration, the 65% that remains unchanged is difficult to realize if free radicals have more than a transient existence. The mimicking of MMO by immobilization of the model complexes in the voids of clays or mesoporous silica and silica-alumina has been our ongoing interest [21-23]. The characterization and theoretical quantum structure of [Fe2(HPTP)(~tOH)(NO3)2](C104)2 (1) and [Fe2(HPTB)(g-OH)(NO3)2] (C104) 2 (2) and their oxygen and methane activation, is investigated here. 2. E X P E R I M E N T A L 2.1.SYNTHESIS 2.1.1.[Fez(HPTB)(OH)(NO3)2](N03) z N,N,N',N'-tetrakis(2-benzimidazolylmethyl))-2-hydroxy-
1,3-diaminopropane (HPTB) is prepared [8-9]. To an ethanol solution of Fe(NO3)3.6H20 (0.31 g) the HL (0.30 g) is added and the precipitated complex is collected. 2.1.2.[Fez(HPTP)(OH)(NO3)z](C104) 2 N,N,N',N',-tetrakis(2-pyridylmethyl)-2-hydroxy -1,3diamino-propane H(HPTP)* as perchlorate is prepared from p-chloropicoline and 2-hydroxy1,3-diamino-propane after [10]. As in the previous synthesis, Fe(NO3)3.6H20 (0.31 g) and H(HPTP)(C104)2 (0.28 g) are solved in ethanol. The complex is washed with acetonitrile/diethylether and recrystallised in diethylether.
Figure 1. Structure of [Fe2(HPTM)(O2)] 3+optimized by Quantum Mechanics. 2.2.COMPUTATIONAL ANALYSIS The ab initio calculations used involve full geometry optimization of the clusters with density functional theory (dft) as implemented in Jaguar [15] (Jaguar 3.0, Schrodinger, Inc., Portland, Oregon, 1997) at the B3LYP method level (Becke3 hybridization functionals, Slater/Becke88 non-local exchange and Li, Yang, Parr local and nonlocal correlation corrections to the local potential energy functionals of Vosko, Wilk and Nusair ) using the Los Alamos effective core potential and valence double Zeta for iron ( LACVP** basis sets ). The molecular mechanics calculations involve a new molecular mechanics force field, the Universal force field (UFF) of Rapp6 et al. [13-14]. The force field parameters are estimated using general rules based only on the element, its hybridization and its connectivity. The force field functional forms, parameters, and generating formulas for the full periodic table have
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been published [13]. For charge equilibration used in molecular dynamics simulations the charges in the complexes were determined [16] to readjust charges based on geometry and experimental atomic properties. The initial structures were energy minimized using suitable sets of parameters appended to UFF, to better describe coordination around iron. The quantum mechanical study was performed at the ab initio level starting from a UFF optimized structure. UFF minimization is carried out on an Origin2000 machine. (16 MIPS R10000 (IP27) CPU's, 195 MHz w/4 MB secondary cache each, with an IRIX 6.4_$2MP + OCTANE operating system) using a Newton-Raphson minimization scheme with a norm of the gradient convergence criteria of 1 x 10-1~ k c a l / m o l / A . In order to accommodate iron in five coordinate form, according to EXAFS data, after UFF optimization a QM calculation is performed. Atom types for iron and other transition metals in the UFF is given with its symbol, hybridization geometry and valence state. UFF contains 126 atom types, the force constants are generated using Badgers rules as described, the van der Waals parameters are computed based on a Lennard-Jones type potential [ 13-14]. 3.RESULTS AND DISCUSSION 3.1. EXAFS S P E C T R O S C O P Y The Fe K-edge EXAFS-XANES analysis gives direct information of the coordination environment of the complexes as salts. The Fe K-edge XAS on [F%(HPTP)(pOH)(NO3)2](NO3)2 and [Fe2(HPTB)(p-OH)(NO3)](NO3)2 show pre-edge features indicative of a distorted five coordinated iron with a sixth Fe ligand, the edge-shifts of 13.1 eV and 15.5 eV are characteristic for its high spin ferric form. The Fe K-edge EXAFS indicates that the [Fe2(L)(p-OH)] 4+ gives a coordination number (CN) of 1 for the Fe-Fe bonding and an Fe-Fe distance of 3.020 A for L=HPTP and of 3.223 A for L=HPTB in accordance with the crystallographic data [18-19]. For complexes with less bulky ligands e.g. HPTP the two species in the Mossbauer spectra are observed, i.e. a p-OH and a p-O species. Shorter Fe-OH inter-atomic distances are seen of 1.92-1.99 A with EXAFS compared with computations and crystallographic structures [5]. The HPTP pyridine and amine groups give Fe-N bonds of 2.11 A~and 2.327 A~. The HPTB benzimidazole and amine groups give Fe-N bonds of 2.185 A and 2.384 A~. The large Debye-Waller factor arises from many different species in the samples.
3.2. C O M P U T A T I O N A L ANALYSIS 1. Modeling the Structures. The [F%(HPTP)(p-OH)] 4+(1), and [F%(HPTB)(p-OH)] '+ (2) complexes are used in catalysis. These are models that have a structure that combines all the features of the ligand and iron active site. The [Fe2(HPTM)(p-OH)] '+ complex was used in the calculations and an imine group is positioned where a pyridine or a benzimidazole occur in the actual ligands as seen in Figure 1. Both the ferric [Fe2(HPTM)(p-OH)] 4+and the ferrous [Fe2(HPTM)(p-OH)] 2+ cores are then optimized with Quantum Mechanics (QM). The QM optimized structure of [Fe2(HPTM)(p-OH)] 4+ is reacted with oxygen in an acidic medium to give intermediates P and Q . [21]. The charge of the [F%(HPTM)(p-OH)] •247cluster is +4 or +2, depending on the simulation of a Fe(III) or an Fe(II) active site and the energy levels of different multiplicity are studied. The relative energies of some important catalytic reactants were analyzed, as are the effects of their solvation again obtained by QM calculations. In UFF due to the change of a ferrous to a ferric type the decrease in the bond-length and the increase in the force constant for the Fe-X distances are seen and some changes in the angle bending parameters are observed. Only a slight increase is seen for the angles of the N_2 from 111.2 to
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111.3 o, these for the O R increase from the standard 110 to 128.0 and 126.4 o in the diferric core. Also, a decrease of the O_R Fe O_R angle can be seen from 90 to 74,5 ~ The N_2 Fe3+3 N_2 angles remain constant at 109.47 ~ The complex has C2h symmetry and both ferrous ions occur either in a high-spin quintet state, and intermediate-spin triplet state or a low-spin singlet state. When these states couple we obtain a nonuplet state if the two iron ions are high-spin, the quintet state Q if the two iron ions are intermediate-spin, or singlet S when the two iron ions are in low-spin form. N (dxz) 2 (dxy)~(dyz)l(dz2)l(dx2-y2)l Q (dxz) 2 (dxy)2(dyz)l(dz2)l S (dxz) 2 (dxy)E(dyz)2 The ferric ions occur either in a high-spin sextet state, and intermediate-spin quartet state or a low-spin doublet state. When these states couple we obtain the undecuplet state if the two iron ions are high-spin, the septet state SP if the two iron ions are intermediate-spin, or triplet T when the two iron ions are in low-spin form. U (dxz) ~ (dxy)l(dyz)~(dz 2) l(dx2-y2)~ SP (dxz) 2 (dxy)~(dyz)l(dz2)l T (dxz) 2 (dxy)2(dyz) ~ In the ferric case (Fe I") the septet (SP) and the undecuplet (U) are its ground state and low lying exited states. The equatorial (trans) and axial (cis) effect of the N atoms with respect to the bridging ~t-oxo groups dictate their bond lengths that are 2.03-2.04 ,~, 2.31 A and 1.951.99/~ respectively. The distances of 1.95 and 1.99/~ obtained from QM seen in the direction of the equatorial unprotonated and protonated O groups compared to the calculations, are accompanied by Fe-O-Fe angles of 90-93 ~ and an O-Fe-O bite angle of 74.5 ~ 2. Modeling the Oxygen Activation. Geometrical en electronic properties affect the relative catalytic properties such as the hydrogen bond abstraction energies of the binuclear cores of these iron complexes. In the QM optimized structure the iron core has two ~t-oxo and six terminating nitrogen atoms. In acid aqueous reactions, the bridging by a deprotonated ligand alcohol group remains strong, the bridging (kt-OH) can be protonated and removed as water. This helps the binding of molecular oxygen and the consequent transformation into a peroxo (022-) group (P) with formal change of charge of iron from +2 to +3. This leads to a diamagnetic singlet state S (dxz) 2 (dxy) 2 (dyz)~(O2rr) ~ (dz 2) ~(O2~) 1 here the dyz orbital is coupled to the two orthogonal three electron pi-system of the 02 ligands. An alternative is a paramagnetic quintet state Q (dxa-y2)~(dxz)a(dxy)l(dyz)l(OEr01 (dz 2) ~(O2~) ~ The two ferryl bonds become stronger by transfer of ~ bonding electrons between the two Oxygen atoms to their anti-bonding orbitals and the pairing of these with an extra electron from the iron ion. In a consequent step the peroxo (022-) group is transformed into two ferryl (02-) bound groups (intermediate Q), with the formal change of the charges of iron from +3 to +4. In an alternative step the peroxo (O22-) group of the complex can also transform to yield two bridging (~t-O) oxo groups. The superexchange coupling for these complexes is fairly small, this agrees with the experimental observation of a J value of 12 cm -~ for the diferrous OH bridged model compound [18-19]. The geometrical implications on these reactions are probed with QM analysis and the results are shown in Table 1 and 2. The Fe-O bond length of 1.21 ,~ increases to 1.31 ,~ on the model compound and is smaller than the 1.49 .~ distance in H202, consequently this bond is broken in the model compound. The Fe..Fe distances of 3.14/~ increases to 3.43 A in intermediate P and 3.63 ,~, in intermediate Q. Upon transformation into the bis (~t-O2-) oxo bridged dimer, it decreases to 2.65/~. The peroxo form P is slightly more stable than the ferryl form Q by about 18.9 kcal/mol. The formation of the bis (kt-O2-) oxo is unfavorable since it is 44.4 kcal/mol higher in energy.
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Table 1. Bond len[~ths (A) obtained b~, QM calculations ~li~and L= HPTM) bond L O(=O)2 L O(O2) L O3 L o(on)2 L o(on) (OCH~) 3.60 Fe Fe 3.628 3.433 2.647 3.639 Fe O 1.607-1.611 1.843-2.037 1.824, 1.909 1.724, 1.756 1.729, 1.747 Fe O (C) 1.971-1.982 1.876-2.039 2.071 1.958-2.008 1.993-1.993 O O 1.309 Fe a N(sp 2) 2.020-2.034 2.000-2.085 1.956-2.078 2.093-2.109 2.010-2.072 Fe b N(sp 2) 2.093-2.095 2.031-2.071 2.094 Fe a N(sp 3) 2.101-2.166 2.077 2.056 3.120 Fe b N(sp 3) 2.270 2.107 O1-H1 0.974-0.984 0.974-0.984 1.432 C1-O2 1.437 1.435 1.420 1.433 i
However, solvation stabilization is higher in complexes with increased charge transfer i.e. the Fe(IV) complexes are better stabilized by solvation than the Fe(III) complexes. Shaik et al. [17] showed that iron oxide cations have a high spin ground state and..,adjacent low spin excited state. The adjacency of the two spin states together with the poor bonding of the high spin state and the good bonding of the low spin state, leads to a spin cross-over along the reaction coordinate and opens a low-energy TSR (two state reactivity) path fo.r hydroxylation. Table 2. Bond an~;les (o) obtained b~, QM calculations bond angles HPTM HPTM HPTM O(:O)2 0(02) 03 Fe O Fe 133.2 122.4 79.4- 93.0 O Fe O 97.8-98.7 80.3-89.9 72.2- 78.2 Fe O O 119.9-123.7 H O Fe C O Fe 109.9-116.8 117.9-119.6 116.9 Ne-Fe-Ne 115.6-115.9 108.9-110.1 118.0 Na-Fe-Ne 81.0-81.3 81.7-83.4 80.6-83.9 Na" axial nitrogen, Ne 9equatorial nitrogen.
HPTM O(OH)2 133.1 93.2-101.9 113.0-117.1 112.2-114.5 116.9-119.2 80.0-80.7
HPTM O (OH)(OCH~) 133.7 95.9-99.4 112.2 112.7-113.3 117.6-117.8 80.2-81.3
3. Modeling the Alkane Activation. Upon interaction with C H 4 the geometry of the two ferryl (02-) bound groups (intermediate Q) to the ferric (FenI-OH, FenI-OCH3) groups does not change substantially, the Fe..O distances change, as the system becomes more asymmetric since one OH group that is formed will show hydrogen bonding with the neighboring methoxy group. For the ground state (multiplicity 7) the bridging Fe-O distances change from 1.97 and 1.96 ,~ to 1.993 and 1.993 A and the terminal Fe-O distances increase from 1.61 and 1.61 A to 1.747 (FeHIOH) and 1.729 (FemOCH3) /~, the CH and OH distances are 1.097, 1.096, 1.095 and 0.982 ,~, respectively. Overal the reaction with methane is exothermic by 50.56 kcal/mol, the consecutive substitution of the methoxy group by a hydroxo group is endothermic by 7.50 kcal/mol and the regeneration of the active site with H202 is again endothermic by 3.24 kcal/mol. The solvation calculations are very important to obtain good quantitative data. The solvation energy is about 150, 300 and 500 kcal/mol for the 2+, 3+ and
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4+ complexes, respectively. The active bis-ferryl ~-oxo-bridged site (intermediate Q) shows eight unpaired electrons. The localization of these free d-electrons occurs partially on the iron (Fe TM like with a spin density of 2.6 to 2.8) and on the oxygen (spin density of 0.80 to 0.85).
4. CONCLUSION. The reaction of the MMO binuclear heptapodate coordinated iron (III)-complexes of N,N,N',N'-tetrakis(iminomethyl)-2-hydroxy-l,3-diamino-propane model with methane is exothermic by 50.56 kcal/mol. The [Fe2(HPTP)(~t-OH)]4+ and [Fe2(HPTB)(~t-OH)] 4+ model complexes give a coordination number of 1 for the Fe-Fe and a distance of 3.02 A and of 3.22 A respectively, in accordance with the QM value of 3.135 A obtained on the [Fe2(HPTM)(~tOH)] 4+ model complexes. The o- and n-bonds of the ferryl Fe=O in the plane of the Fe-O-Fe bridge, have the properties of a two atom three electron bond. ACKNOWLEDGEMENTS
PPKG thanks the FWO-Flanders for a post-doctoral fellowship, A.Fukuoka & M.Ichikawa from the CRC, at Hokkaido University, Sapporo, Japan for a collaboration on EXAFS. PPKG and WAG wish to thank BP Amoco for financial support. REFERENCES
1. L., Que, Y., Dong, Acc. Chem. Res., 29 (1996) 190. 2. Loehr, T. M.Ed., Iron carriers and Proteins; VCH, Weinheim, 1989, 373. 3. L. Shu, J.C.Nesheim, K.Kauffrnann, E. Munck, J.D. Lipscomb, L. Que, Jr., Science, 275 (1997) 515.4. Y. Dong, S. Yah, V.G.Young Jr., L.Que Jr., Angew. Chem., 108 (1996) 674. 5. A.C., Rosenzweig, C.A., Frederick, S.J., Lippard, P., Nordlund, Nature, 366 (1993) 537. 6. K.E. Liu, D. Wang, B.Huynh, D. Edmonson, A. Salifoglou, S.J. Lippard, J.Am.Chem.Soc., 116 (1994) 7465.7. K.E. Liu, A.M. Valentine, D. Qiu, D.Edmonson, E.Appelman, T.Spiro, S.J. Lippard, J.Am.Chem.Soc., 117 (1995) 4997. 8. S.J. Lippard, Angew.Chem. Int.Ed. Engl., 27 (1988) 344. 9. D.M. Kurtz, Chem. Rev., 90 (1990) 585. 10. K. Yoshizawa, K. Ohta, T. Yamabe, R. Hoffmann, J.Am.Chem.Soc., 119(1997) 12311. 11. R.H., Crabtree, Chem.Rev., 95 (1995) 987. 12. P.E., Siegbahn, R.H., Crabtree, J. Am. Chem. Soc., 119 (1997) 3103. 13. A.K. Rappe, C.J. Casewit, K.S. Colwell, W.A.Goddard, W.M.Skiff, J.Am.Chem.Soc., 114 (1992) 10024. 14. A.K., Rappe, W.A., Goddard, J. Chem. Phys., 95 (1991) 3358. 15. References in the Jaguar User Guide. 16. P.P. Knops-Gerrits, F. Faglioni, W. Goddard III, J. Am. Chem. Soc., submitted. 17. S. Shaik, M. Filatov, D., Schrrder, H., Schwarz, Chem. Eur. J., 1998, 4, 193-199. 18. S. Menage, B.A.Brennan, C.Juarez-Garcia, E.Munck, L.Que, Jr, J.Am.Chem.Soc., 112 (1990) 6423.19. R.G.Wilkins, Chem. Soc. Rev., (1992) 171-178; P.C.Wilkins, R.G.Wilkins, Coord. Chem. Rev., 79 (1987) 195-214. 20. R.E.Norman, R.C.Holz, S.Menage, L.Que, jr., J.O'Connor, Inorg. Chem., 29 (1990) 4629. 21. P.P.Knops-Gerrits, A.Weiss, S.Dick, P.Jacobs, Stud.Surf.Sci.Catal., 1997, 110, 1061. 22. P.P.Knops-Gerrits, A.M.Van Bavel, G.Langouche, P.Jacobs, NATO ASI 3.44 (Derouane et al. Eds.), 1998, 215. 23. P.P.Knops-Gerrits, A.Verberckmoes, R.A.Schoonheydt, M. Ichikawa, P.A. Jacobs, Micro- and Mesoporous Mat., 1998, 21, (4-6) 475.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Quantum-chemical and experimental study of an interaction
between C02 and propylene over Rh-Co/AI2-Oa catalysts
L.B. Shapovalova, G.D. Zakumbaeva, A.V. Gabdrakipov, I.A. Shiygina and A.A. Zhurtbaeva The Institute of Organic Catalysis & Electrochemistry of the Ministry of Science and Higher Education of the Republic of Kazakstan, 142, Kunaev str., Almaty, 480100, Kazakstan
1. INTRODUCTION The studies of catalytic activation of CO2 and its interaction with hydrocarbons intensively increase at present time. Because it needs to decrease of carbon dioxide in atmosphere. Utilization of carbon dioxide can solve the problem of green-house effect and as well as the involving of carbon into circulation for production of artificial oil, different organic compounds and synthesis-gas (CO+H2). Recently the results of study of interaction between hexene-1 or propylene and carbon dioxide on Ru-Co/AI203 have been published [1,2]. In this paper the process of interaction between CO2 and propylene over Rh-Co/AI203 cluster type catalysts has been studied.
2. EXPERIMENTAL The process was carried out in flow type reactor at variable temperatures from 423 to 627K and pressure from 0.1 to 1.5 MPa. The space velocity was 100-120 hr -~. The mixture C3HJCO2=1/4 was used. Catalysts were prepared by impregnation of AI203 support with mixture of RhCI3 and Co(NO3)26H20 solutions. Then they were reduced by hydrogen at 773K during 3 hours, washed from CIx and NO3x ions and dried up in the air at 303-323K. Catalyst was additionally reduced directly in the reactor at temperatures from 473 to 673K during 1 hour before the reaction between CO2 and propylene. The reaction rate was controlled on propylene decrease by using a chromatographic analysis. IR-spectra of reactants adsorbed on catalyst surface were recorded in a UR-75 spectrometer in the 1200-3500 cm -~ range.
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Quantum-chemical calculations have been made on the basis of extended Huckel method complemented with Anderson core-core repulsion as item of total electron energy. Cluster approximation was used for calculations. Minimal quantity of metal atoms (2-7-13) was taken into account. This approach seams a reasonable, because, as is known, chemisorption is high Iocalizated phenomenon.
3. RESULTS AND DISCUSSION It has been established that the conversion of propylene and carbon dioxide depends on catalyst composition and pressure of mixture CO2+propylene. Propylene conversion on Rh-Co(I:I)/AI203 at Tex~=523K and P=I.8MPa is 25.0%. The reaction products are methanol (8.4%), formaldehyde (0.8%), acetone (10.3%), ethanol (8.7%), i-propanol (13.9%), buthanol (1.7%), butyric acid (37.3%), traces of butylaldehyde and other compounds. A decrease of pressure up to 1.5 MPa (Tex,=523K) implies the propylene conversion decreases to 8.3%. Also the product composition changes. The yield of methanol, formaldehyde and i-propanol increases up to 22.7, 3.4 and 31.0% consequently. The yield of acetone and butyric acid decreases to 5.4 and 20.3% at the same time. It was shown that the degree of propylene conversion is 7.9-9.3% when 523 to 673K (P=1.5 MPa). But with temperatures increase the ratio of formed products significantly changes. It is observed the tendency to decrease of yield of methanol from 22.7 to 18.4, formaldehyde from 3.4 to 1.7 and ethanol from 7.9 to 0.7%. The yield of butyric acid changes extremely. The maximum yield reaches 33% at Tex~=573K. Then the yield of butyric acid sharply decreases to 7.3.% at Texp=673K. In these conditions the yield of acetone and i-propanol increases from 5.4 and 31.0 (Tex~=523K) and to 17.5 and 38.3% (Texp=673K) consequently. Propylaldehyde are formed at 623-673K (2.8-4.0%).
Texp increase from
In IR-spectra absorption bands at 3500, 3300 (OH-groups), 3060, 3000 (=CH2- groups), 2980, 2920, 2850 (-CH, -CH2, -CH3 - groups), 2380, 2350 (CO2~g~), 2030 and 1900 cm ~ corresponding to linear and bridge forms of GOads on Rh n+ and Rh~ are presented at chemisorption of CO2 + C3H6mixture (T=473K) on Rh-Co(9:I)/AI203 catalyst. In addition it was observed the intensive absorption bands in the range 1700-1300, 1580 and 1440 cm -~. The absorption bands at 1580 and 1440 cm ~ can be correspond to chemisorbed CO2 molecule. With an increase of Co concentration in catalyst composition to 30-50 mas.% the intensity of absorption bands in the range of 2400-2000 cm ~ increases. The position of absorption bands varies slightly with the temperatures increase from 473 to 573K. However intensity of absorption bands at 1950, 2380-2350 and especially 1550 and 1430 cm -~ significantly increases at the
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adsorption of mixture on Rh-Co(I:I)/AI203 catalyst. These structures are sufficiently strongly bonded with catalyst surface and don't disappear under vacuum. The structure and state of surface of Rh-Co/AI203 catalyst were studied by physico-chemical methods of analysis. The size of metal particles in RhCo(9:1)/AI203 catalyst is 9-10 nm. With increase of cobalt concentration in catalyst composition the particle size decreases. The particle size of RhCo(1:9)/AI203 catalyst is 1.5-2.0 nm. It has been established that Rh and Co form the cluster structure. Their state and stability are determined by ratio Rh:Co.
By quantum-chemical method it has been shown that replacement of rhodium on cobalt in 13-atoms cluster results to increase of cluster stability. It grows with increase of quantity of cobalt atoms. The formation of bimetallic structures in catalyst is more energetically favourably than monometallic ones. The stability of 13-atoms Rh-Co-clusters decreases in the series: 12Co-Rh(~) < 3Co(~,4,7)10Rh
meta for toluene, as was found experimentally. This work presents a theoretical investigation to obtain fundamental insight in the mechanism of EAS over acid zeolites, more specifically whether a one-step (concerted)
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or two-step mechanism is operative. To find this, the complete Intrinsic Reaction Co-ordinate (IRC) is calculated.
2. AMl-method The systems under investigation consist of a cluster and an aromatic molecule. The cluster is composed of 4 Si tetrahedra and one A1 tetrahedron. These tetrahedra are terminated with hydrogens. The bridging hydroxyl is replaced by a methoxide, which acts as the electrophile. The aromatic molecules under study are benzene, toluene and nitrobenzene. The adsorption of the aromatic molecules on the methoxide, the Transition State (TS) of the EAS reaction and the adsorption of the methylated aromatic molecules are studied at AMi level. These three states are linked via the intrinsic Reaction Coordinate (iRC). it is the coordinate of steepest descent in energy which links the TS with the reagents and products and is calculated with the TS as starting point.
2.1. Dehydration of Methanol The Transition State (TS) is CH3OH2+ with a longer C-O bond, 2.13 A, than in methanol. However there is no interaction between the oxygen of the cluster and the protons of the activated complex, CH3OH~+ (fig l a). This is not in agreement with the literature where a proton still has some interaction with an oxygen of the cluster [7]. After the TS the C-O bond breaks, water is expelled and a methoxide is formed. Fig.l b, which gives the energy change along the reaction path during the reaction, shows that this reaction occurs according to a one-step mechanism with one activation energy of 262.53 kJ/mol. There is only one maximum. Another interesting feature is the position of the bond breaking and formation along the reaction path (fig. 2a). Before the TS is reached, the Or bond is lengthened and the Hmah-Om~ bond is shortened, indicative of the jump of the acid proton of the bridging hydroxyl towards the methanol. After the TS the Cm~h-O~,~te~ distance is shortened and the Cmeth-Omethbond is lengthened, indicative of methoxide formation. As there is no simultaneous breaking and forming of bonds, it is - strictly speaking - not a concerted reaction. Fig. 2b presents the change of the sum of the three HCH angles of the methyl fragment: in the adsorption stage of the reagents and products, the sum equals +/- 327 ~ la
lb
-226850
0 Si
OAI -~-~
O
O
.~_~o .._...
IL--
E^=262"53 kJ/m?1 i /
E -227060
-227100 -40
-20
0
path lehgth [Bbhi~.ah~u 112]
fig. 1: a; Transition State, b; energy for the dehydration of methanol
20
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Cm~-Oa~ \ ~
2a ....... - . . . . . . .
DIC>DCM>TCE) coincides with those reported in the literature for single component streams, indicating that the oxidation mechanism of each chlorohydrocarbon is not altered by the presence of the others.
Agarwal et al. [4], investigated the same chlorinated stream but half concentrated, over a Cr203-A1203 catalyst and found that the oxidability order was: DIC>TCE>DCE>DCM. This indicates the active sites of each catalyst shows different affinity to each compound, so that the destructibility of the hydrocarbons in the mixture is strongly influence by the catalyst type. The addition of non-chlorinated hydrocarbons, i.e. hexane and toluene to the multicomponent chlorinated feedstream, (feed 2 and 3), caused the change in the oxidability order of the chlororganics as shown in conversion vs. temperature curves in Fig. 3 to 6. As a result, DIC and TCE oxidation were promoted, while DCE oxidation was inhibited, resulting in the following destruction order: DIC > TCE > DCE > DCM. Ts0 (temperature for the 50% conversion) for DIC oxidation, in the presence of hexane over Pt/A1203, was reduced from 350~ to 250~ However, Ts0 for DCE increased from 300~ to 350~ while DCM decomposition curve was not affected by the presence of hydrocarbons. TCE experienced the major oxidability change, specially in the temperature range 200-400~ where Ts0 was reduced from 450~ to 290~ being the second compound with higher reactivity; however, above 350-400~ reaction got slower, becoming the compound with the lowest oxidability. This behaviour can be better analysed in the Fig. 4, reaction in the presence of hexane and catalysed by Pt/A1203. In general terms, DIC and TCE oxidation were the most promoted reactions (Fig 3
lOO
I ~ f
iOOb
l ,/li I1' II
8 20f
lOO
y
II
~
I I
60
:t.
9
8>:~ 7TCE
0 ~ ............. 200 250 300 350 400 450 500 550 Temperature, ~ Fig. 3. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 2 over 0,42 wt%Pd/A12Os.
20[-~.
,,~/f
~'/
:-" TOE OCM
0 ~ 200 250 300 350 400 450 500 550 Temperature, ~ Fig. 4. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 2 over 0,44 wt%Pt/A1203.
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100 f
100
80 f
80 c" 60 o
" f 0 .... 200
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, -
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), ~ /~
9 DCM
...................
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400
Temperature, ~
4~o
~oo
/ /
20
_//'7/ . . . . . . . . . . . .
~5o
Fig. 5. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 3 over 0,42 wt%Pd/A1203.
= TCE ,, DcM
;.,.D'.C,
O= 200 250 300 350 400 450 500 550 "
"
"
" m
,m
~,
Temperature, ~
Fig. 6. Ignition curves for the oxidation of DCE, TCE, DCM y DIC in the feed 3 over 0,44 wt%Pt/A1203.
and 5), whereas in the presence of toluene DCE oxidation is most inhibited (Fig 4 and 6). In both cases, using palladium catalyst higher conversion of TCE was achieved, specially in the temperature range between 300 and 550~ Apart of the inhibition or promotion, the CVOCs experienced faster ignition in the presence of hexane and toluene, specially DCE and DCM. Their conversion increased from 10 to 90% in a range of 10~ as a consequence, the DCM oxidation was completed at 400~ (Fig. 3 and 4) when hexane was present in the feed, requiring 50~ less than in the absence of the hydrocarbons (feed 1). The addition of toluene inhibited the oxidation of DCM at low temperatures, but due to the quick ignition, temperature for complete conversion was not altered. The results obtained with the feeds 2 and 3 indicate the complexity of the reaction mechanism: the presence of non-chlorinated hydrocarbons (hexane and toluene) modifies the work function of the catalyst, leading the different oxidability of the chlorohydrocarbons with respect to their oxidation in the absence of them. At first sight, the reduced DCE conversion suggests that competitive adsorption effects play the major role. Both hexane and toluene are more competitive for catalytic sites than DCE. This conclusion would also led to the inhibition of TCE oxidation, since in a previous research about the catalytic oxidation of two component ~ D C E and T C E - - feedstreams, inhibition for TCE was observed, concluding that TCE was competing for the same active centres than DCE. However, TCE and DIC oxidation are highly promoted when hexane and toluene are present in the feed. Therefore, it seems that the oxidation of nonchlorinated hydrocarbons changes the properties of the catalytic sites, which
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enables the chlorinated ethylenes (TCE and DIC) to be adsorbed faster in such sites than DCE and DCM. Nevertheless, when ignition of DCE and DCM started (350~ TCE oxidation was inhibited, requiring higher temperatures for its total oxidation. In such case, palladium catalyst was able to reach higher TCE conversion in the range 300-550~ than platinum catalyst. All these with promotion and inhibition effects, indicate that the catalyst surface is energetically non-uniform, leading to active sites with different affinity to each reactant depending on the reaction conditions, i.e. temperature and environment. Somorjai [5] reported the existence of different crystal faces, edges and corners demonstrating the heterogeneity of the metallic surface. He observed carbonaceous overlayer was formed over the faces of the platinum crystals reducing their activity, while the edges and the corners, much more reactive, remained clear. The heterogeneity of the catalytic surface was also demonstrated by variation of the chemisorption heat of oxygen [6,7], concluding that the rate of oxygen chemisorption was different on energetically different sites. Gangwal et al. [8] found different oxygen chemisorption constant (ko) values for the oxidation of benzene and n-hexane over Pt,NYT-A1203. Since ko is the kinetic constant for the oxygen chemisorption on the catalyst, it should be independent of the nature of the organic compound oxidised. Gangwal et al. explained this fact by assuming that the active catalyst species were in different oxidation sates. Likewise, Minot and Gallezot [9] reported that the selectivity to adsorption of a certain compound depended also on the electrodonating or electron-accepting species interacting with the metal surface, since they change the work function, leading to the "mixture effects" observed in this work. ACKNOWLEDGEMENTS The authors wish to thank Universidad del Pais V a s c o / E H U (UPV069.310G40/98) Gobierno Vasco (GV069.310-0111/94) and Ministerio de Educaci6n y Cultura (QUI96-0471) for the financial support. One of the authors (A.A.) acknowledges Gobierno Vasco for the grant BF194.010. REFERENCES 1. J.R. Gonz~lez-Velasco, A. Aranzabal, J.I. Guti6rrez-Ortiz, R. L6pez-Fonseca and M.A. Guti6rrez-Ortiz, Appl. Catal. B, 19 (1998) 189. 2. H. Windawi and M. Wyatt, Platinum Metals Rev., 37 (1993) 186. 3. G.C. Bond and N. Sadeghi, J. Appl. Chem. Biotechnol., 25 (1975) 241. 4. S.K. Agarwal and J.J. Spivey, Appl. Catal. A, 82 (1992) 259. 5. G.A. Somorjai, Adv. Catal., 26 (1997) 1. 6. G.L. Golodets, Heterogeneous Catalytic Reactions Involving Molecular Oxygen, Elsevier, Amsterdam, 1983. 7. P. Briot and A. Auroux, Appl. Catal., 59 (1990) 141. 8. S.K. Gangwal, M.E. Mullins, J.J. Spivey, P.R. Caffrey and B.A. Tichenor, Appl. Catal. A, 36 (1988) 231. 9. C. Minot and P. Gallezot, J. Catal., 123 (1990) 341.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Transformation of chlorinated compounds on different zeolites under oxidative and reductive conditions I. Hannus a, A. Tamfisi a, Z. K6nya a, S.-I. Niwa b, F. Mizukami b, J. B.Nagy c and I. Kiricsi a aDepartment of Applied and Environmental Chemistry, J6zsef Attila University, Rerrich t6r 1, H-6720 Szeged, Hungary* bNational Institute of Materials and Chemical Research, Department of Surface Chemistry, 1-1, Hagashi, Tsukuba, Ibaraki 305, Japan CLaboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium
Decomposition of carbon tetrachloride was investigated over mono- and bimetallic forms of ZSM-5 zeolite catalysts. Independently on the reduction methods used for generating platinum-containing samples two main types of reactions were found to occur. Under inert or oxidative conditions a simple reaction takes place between the zeolite and the reactant leading to the destruction of zeolitic framework. In hydrogen flow, in reductive atmosphere hydrodechlorination took place and no detectable dealumination of zeolite skeleton occured.
1. INTRODUCTION Due to their high stabilities chlorofluorocarbon (CC12F2) and carbon tetrachloride (CC14) are suitable materials in great many applications. This character is advantageous in daily life, however, harmful from environmental point of view since they play significant role in ozone depletion. This fact gave rise to environmental concerns and eventually resulted in their negotiated phase-out under the terms of the Montreal Protocol [1] and its subsequent revisions. Nowadays, there is another problem, the storage, treatment and safe destruction or transformation of CC12F2 produced until 1996. Furthermore, the decomposition or transformation of chlorinated by-products, such as CC14, in manufacturing of methylene chloride (CH2C12) and chloroform (CHC13) are also urgent problems [2]. Two main potential ways are in attention: their transformation to potentially valuable chemical compounds by hydrodechlorination (HDC) [3] or their (oxidative) destruction to form environmentally safe products. This work was performed with the help of grants OTKA T 025248 and F25245, Hungary.
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In this paper we report on the investigation of the oxidative decomposition and reductive hydrodechlorination reactions of CC14 over modified ZSM-5 zeolites. 2. EXPERIMENTAL
Mono-ionic samples were prepared by ion-exchange of Na,HZSM-5 zeolite (Si/A1=32.95 and unit cell composition Na0.87Ha.13A13.0Si93.00192). The zeolite samples were exchanged in solutions of Co(NO3)2 or [Pt(NH3)4](NO3)2 with stirring for 24 h at room temperature to prepare the corresponding CoZSM-5 and PtZSM-5. For preparing two-ionic Pt,CoZSM-5 sample an equivalent amount of the Pt(NH3)4]2+ was dissolved in distilled water. While stirring, a solution containing the Pt(NH3)4]2+ ions was added dropwise to the filtered and dried CoZSM-5 solid suspended in water. After 24 h stirring the solution was evaporated. The metal loading of the samples are summarized in Table 1. Table 1 Meta!=!oad!ng and ac!d!~,of the s ~ p ! e s ........ ZSM-5 samples Metal loading (mmol/gzeolite) Reduction with Br6nsted/Lewis Co:~T Pt~-:~.............................................................................................................acidity: .... "................... ratios= ................. CoZSM-5 0.02 non 0.16 PtZSM-5 0.02 hydrogen 2.15 NaBH4 0.14 NaBH4+H2 0.1 Pt,CoZSM-5 0.02 0.02 hydrogen 0.27 NaBH4+H2 0.04 The PtZSM-5 and Pt,CoZSM-5 samples were divided into two parts for reduction. The first portion was suspended in water and 0.1 mol/dm 3 NaBH4 solution was added dropwise. The suspension was stirred for 8h filtered and dried. The samples obtained after such treatment are denoted as PtZSM-5(Na) and Pt,CoZSM-5(Na), respectively. The second part of the sample was reduced in hydrogen flow at 573 K. This way we received samples denoted as PtZSM-5(H2) and Pt,CoZSM-5(H2). In some experiments the PtZSM-5(Na) and Pt,CoZSM5(Na) specimens were reduced in hydrogen flow in order to check the changes in the acidities of samples. These samples marked as PtZSM-5(Na)(H2) and Pt,CoZSM-5(Na)(H2). Appropriate amounts of samples were put into the reactor followed by starting the decomposition experiments. Self-supporting wafer technique was employed for acidity and adsorption measurements. The wafers (10 mg/cm 2) were prepared from the powdered zeolites and placed into the sample holder of the i n s i t u IR cell. The pretreatment of the samples was as follows: The temperature of the wafer was slowly increased to 723 K under continuous evacuation of the cell. (When hydrogen was used for the reduction, 26.6 kPa (200 Torr) H2 was introduced in the cell and reacted with the wafer at 573 K, for 1 h, followed by degassing at the same temperature for an additional hour.) After this treatment the sample was cooled down to room temperature and the background spectrum of the zeolite was recorded by a Mattson Genesis Spectrometer. The resolution of the spectra was 1 cm -1 and 16 scans were accumulated for a spectrum.
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TEM images were made using HITACHI electron microscope in order to obtain information about the particle size distribution of platinum or bimetallic species burned in the zeolite samples upon reduction either with hydrogen or with NaBH4. The XRD patterns serving primary data on the crystallinity of the specimens were taken with a DRON 3 X-ray diffractometer. For the acidity measurements 1.33 kPa (10 Torr) pyridine was introduced into the cell and heated up to 473 K. After 1 h adsorption, the cell was evacuated for 1 h at the same temperature. After cooling the sample to room temperature spectra of the adsorbed pyridine were taken. Structural changes occurring in the zeolite framework upon reaction was investigated by IR spectroscopy (KBr pellet technique). Particularly the 800-1000 cm l range was monitored since the band generally appearing at 960 cm -~ was attributed to the structural vacancies generally caused by dealumination of the zeolite skeleton. Hydrodechlorination reactions of CC14 was carried out in the flow system applying IR spectroscopic product analysis. The catalysts placed in a glass reactor were pretreated as described above. After adjusting the desired reaction temperature (generally we started the reaction at room temperature) the carbon tetrachloride was fed using either nitrogen or nitrogen-hydrogen, or oxygen flow with a flow rate around 10 ml/min. The gas stream containing the products formed were passed through an IR gas cell and the spectra were taken at predetermined times (generally after 0.5 h reaction time). Then the temperature of the reactor was increased. Stepwise raising of the temperature was performed up to 573 K, while the products were analyzed at each step.
3. RESULTS AND DISCUSSION 3.1. TEM studies As the TEM images in Figure 1 and 2 show, the metal particles of Pt,CoZSM-5 zeolite samples are quite large. Independently of the reduction procedure (in NaBH4 solution at room temperature or in H2 stream at 573 K) their particle size distribution seems to be practically identical and homogeneous in both samples. This fact indicates that the hydrogenation capability of these samples is similar. 3.2. Infrared spectroscopy The results of acidity measurements performed with adsorption of pyridine revealed higher BrOnsted acidity of the samples reduced by hydrogen than that treated in basic NaBH4 solution. As data in Table 1 show insignificant change in Br~nsted acidity for samples first reduced by NaBH4 followed by reduction in hydrogen. This finding may play an important role when CC14 decomposition activities are compared in neutral or oxidative regime. The activity of zeolites in the conversion of chlorofluorocarbons leading to phosgene, carbon dioxide and HC1 depended on their acidities [4]. In Figure 3 IR spectra of gas phase products formed in the decomposition of carbon tetrachloride in neutral, oxidative and reductive atmosphere over PtZSM-5 zeolites reduced in hydrogen and by NaBH4 are depicted. It is seen that the products in the former two media are very similar, almost identical. Phosgene, CO2 and HC1 are the main products giving rise to bands at 1750, 2400 and 2900 cm "l. The concentration of these compounds increases with reaction temperature. This result is in accordance with those found by NMR spectroscopy [6].
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Figure 1. TEM image of Pt, CoZSM-5 sample reduced in NaBH4 solution.
Figure 2. TEM image of Pt,CoZSM-5 sample reduced in H2.
A
2500
B
1500 2500 Wavenumber (1/cm)
1500
Fig. 3. Transformation of CC14 over PtZSM-5 samples reduced in hydrogen (A) or in solution by NaBI-h (B) The reaction conditions were: temperature 523 K, time 1 h, media: oxygen (A), nitrogen (B) or hydrogen (C).
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The chlorine atoms of CC14 react with the AI of the zeolites coordinated tetrahedrally resulting in the formation of octahedrally coordinated extraframework AI. This transformation results in the formation of zeolite with vacancies in their skeleton. This has been proven by the 27A1NMR spectra [7] and the IR spectra by the band generally appearing near 930 cm l
[8].
As a consequence of these transformations, the crystal structure of the zeolite collapses for zeolites with low Si/A1 ratios, and the zeolite should be, therefore, regarded a reaction partner rather than a catalyst. Oxygen, present in the reacting mixture slows down the dealumination of zeolite framework [4]. Figure 4 shows the spectra of the products obtained over Pt,CoZSM-5 samples reduced in different ways and reacted in neutral or reductive conditions. The difference in the product distributions is clearly seen. While in neutral atmosphere phosgene, CO2 and HCI are the main products, in the experiments performed in reductive atmosphere methane, HC1 are the main products. From this follows that substantial difference should be in the mechanism
HCI
CO2
COC12
F
A b S 0
r b
CH4 ~
~
a n
co
/
i
I
i
3000
i
i
i
i
i
I
I
i
B
l
I
i
i
i
2000
i
I
i
i
9
I
I
1000
Wavenumber (1/cm) Fig. 4. Products formed in the transformation of CC14 over Pt,CoZSM-5 samples, A: Pt,Co(Na)(H2), B: Pt, Co(H2)(H2), C: Pt, Co(Na)(N2), D: Pt, Co(Hz)(N2), at 523 K after 1 h reaction time.
i
i
I
i
i
1250
IAI
i
i
1000
j
i
i
i
i
750
i
i
i
i
Wavenumber (l/cm)
Fig. 5. IR spectra of the spent catalysts, A: Na, H, B :Pt(H2)(N2),C: Pt(Na) (H2), D: Pt(H2)(H2), E: Pt(Na)(H2), F: Pt(H2)
(o~)
I,
500
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of the reaction taking place under various conditions. It is also remarkable, that the products are present in almost identical concentrations for both the hydrogen and the NaBH4 reduced samples. This result shows the negligible importance of the reduction method and acidity in the reaction performed in hydrogen. As we mentioned before, acidity played an important role at the simple decomposition reactions of chlorofluorocarbons. Here, in reductive atmosphere the importance of the acidity became insignificant. From this follows that the metal function (Pt or Pt,Co) is predominant in the hydrodechlorination reaction of carbon tetrachloride. IR spectra taken on the spent zeolite samples and depicted in Figure 5, and the XRD patterns showed unequivocally that no crystal destruction and/or significant dealumination of the zeolite framework occurred in the reactions performed in hydrogen atmosphere. No indication of the IR band attributed to framework vacancies is seen at 930 cm -l in the spectra presented. We can conclude that bimetallic zeolites prepared on the basis of high modulus materials such as ZSM-5 are promising specimens for application in hydrodechlorination reactions. The use of ZSM-5 type zeolites supplies a new, promising way for the preparation of catalysts with prolonged activity in hydrochlorination reaction, while Kim et al claimed fast deactivation of P t ~ a Y zeolite [9]. 4. CONCLUSION The completely different product distributions observed in the decomposition reaction of carbon tetrachloride in neutral, oxidative or reductive conditions suggest two different reaction mechanisms. Under neutral or oxidative regimes the carbon tetrachloride reacts with the zeolite irrespective its metal content. This reaction leads to fast dealumination of the zeolite resulting in the collapse of the crystal structure. Under reductive conditions, in hydrogen flow, hydrodechlorination takes place as the main reaction producing partially chlorinated methane derivatives. Under the experimental circumstances applied the main products were methane and HC1 being typical for the hydrodechlorination. The bimetallic ZSM-5 type zeolites proved to be promising catalysts for this transformation. REFERENCES
1. Montreal Protocol on Substances that Deplete the Ozone Layer, Final Report, United Nations Environment Programme, New York, 1987. 2. Z.C. Zhang and B.C. Beard, Appl. Catal., A General, 174 (1998) 33. 3. A. Viersma, E.J.A.X. van de Sandt, M. Makkee, C.P. Luteijn, H. van Bekkum and J.A. Moulijn, Catal. Today, 27 (1996) 257. 4. I. Hannus, Z. K6nya, P. Lentz, J. B.Nagy and I. Kiricsi, Proc. 12th Intern. Zeolite Conf., Baltimore, USA (J. Mater. Res. Soc.) Vol. IV. p. 2963 (1998). 5. A. Tam~isi, I. Kiricsi, Z. K6nya, J. Hal/tsz and L. Guczi, J. Mol. Struct., 482-483 (1999) 1. 6. I. Hannus, I.I. Ivanova, Gy. Tasi, I. Kiricsi and J. B.Nagy, Colloids Surfaces A: Physicochem. Eng. Aspects 101 (1995) 199. 7. I. Hannus, Z. K6nya, P. Lentz, J. B.Nagy and I. Kiricsi, J. Mol. Struct., 482-483 (1999) 359. 8. I. Hannus, Z. K6nya, T. Koll~r, Y. Kiyozumi, F. Mizukami, P. Lentz, J. B.Nagy and I. Kiricsi, Stud. Surf. Sci. Catal., 125 (1999) 245. 9. S.Y. Kim, H.C. Choi, O.B. Yanga, K.H. Lee, J.S. Lee and Y.G. Kim, J.Chem. Soc., Chem. Commun., 1995, 2169.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
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Biofiltration of Gasoline VOCs with Different Support Media I. Ortiz., M. Morales, C. Gobb6e, S. Revah, V.M. Guerrero 2 Shifter L. A. Garcia 4
12, R. Auria 3, G.A. P6rez4,
UAM- Iztapalapa, 09000, Mexico,, D. F 1, Mexican Petroleum Institute, Eje central lazaro Cardenas . 152 Mexico, D. F. 2; ORSTOM (Institut Francais de Recherche Scientifique pour le DOveloppement en Cooperation). Ciceron 609, Mexico, D.F. 3, University Valley Mexico 4.
BTX constitute an important part of the total VOCs in different types of commercial gasoline in Mexico. Evaporation of this Kind of compounds from storage provokes undesirable emissions to the atmosphere. Biofiltration has demonstrated be an effective control technology to eliminate these compounds. To increase the performance of biofilters it is important to understand the effects of the support on long-term operation. In this work, organic and inorganic supports were evaluated to remove VOCs from gasoline vapours in 4 liter bench-scale reactors inoculated with an adapted consortium. The supports were: peat, a mixture of vermiculite and activated carbon (V-AC), barks and porous 15 mm glass rashing rings. A mixture of benzene, toluene and xylenes (BTX) of 200 g C/m3/h was fed for more than 100 days to the 4 biofilters with an EBRT of 60 s. Removal efficiencies higher than 95% were obtained with V-AC, 85% for peat and barks and 65% for the rashing rings. In all cases, drying problems in beds were observed after several days of operation. Peat was the best material to support microbial growth without manipulation, however to restore its moisture content, it was necessary to perform mechanical mixing. In contrast, this problem was surmounted by direct water addition to the others supports. Addition of mineral media to the biofilters packed with barks and rashing rings was required until the establishment of an efficient microbial population. In steady state operation, experiment at loads from 50 to 400 g C/m3/h were carried out and the behaviour of each biofilter was studied.
1. INTRODUCTION Air pollution is a pressing problem in big cities. In Mexico City air, quality has decreased due to the emission of volatile organic compounds, nitrogen oxides and others compounds that contribute to the formation of high levels of ozone and smog. Emissions of hydrocarbons in Mexico City [1 ], derive mainly from storage evaporation (19%) and mobile
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sources (75%). The development of Air Pollution Control (APC) technologies is a response to these increased levels of pollution. The biological APC technologies have some advantages over others APC technologies [2] especially in treatment of gaseous flow with low concentration of pollutants. Biofiltration benefits from the capacity that some bacteria., fungi and yeast have to degrade a great variety of both, organic and inorganic compounds into H20, biomass and CO2. Support media has been reported [2], [3], as one of the essential factors in biofilter performance. Recently, new inert material or formulated supports have been successfully used in biofiltration. [4, 5, 6, 7 y 8]. The use of a appropriate media can help to reduce the problems associated with bed drying which has been reported to be the main cause of failure in biofilters. To increase the performance of biofilters it is important to understand the effects of the support on long- term operation. To study these effects natural and inert supports were selected and challenged against a mixture of aromatics (BTX). Inert supports used in this work were selected not only by their practical interest, but also, to look into fundamental processes that take place during the degradation. 2. EXPERIMENTAL SYSTEM Methods and materials supports. Four different packing materials were used: Peat (P), A mixture of vermiculite and activated carbon (V-AC), Pine three barks (B), porous 15 mm ceramic Siporax raching rings (GR). (Fig.l) shows a schematic representation of the experimental system used. The reactors were placed in a room with controlled temperature at 30+2~ Operating conditions were: Inlet gas concentration: 2.4 g C/m 3, EBTR: 60s, initial water content of supports: P: 65%; V-AC: 70%, B: 30%; GR: 40%, Measured parameters were: Inlet and outlet gas concentrations by FID chromatography and emitted CO2 by infrared analyser.
l
Figure 1. 1) Compressor, 2) Humidifier, 3) Water recirculation system, 4) Cyclone, 5) Chamber of solvent addition, 6) Temperature Controller, 7) Evaporator of Solvents, 8) Pump, 9) Mixing Chamber, 10) Flow distributor, 11) Sampling ports, 12) Biofilters, 13) Sampling system.
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3. RESULTS AND DISCUSSION
Biofilters were operated during for more than 100 days. (Fig. 2) shows an example of the performance of the V-AC packed biofilter. During the first 30 days of experiment, Elimination Capacity (EC) of the BTX was about 30 g C/m3h. To increase biofilter performance, nutrients were added at the 12th day. This supply of water and nutrient was correlated with an increment of CO2 production while EC remained almost constant. A lag phase of 15 days was observed before EC increased. After this period, only water additions were performed which corresponded to new EC increases. Between days 30 and 100, EC and CO2 concentration were around 125 g C/m3h and 4.5 g/m 3, respectively. This evolved CO2 corresponds to about 60% of theoretical considering total oxidation and may be explained as carbon retained for biomass, intermediates and carbonates [9]. (Fig. 3) represents EC variation for the BTX mixture (Benzene, Toluene and Xylenes). Changes of EC for each components followed the global behaviour of the gas mixture. After day 30, EC of toluene and xylenes were around 25g C/m3h and 125g C/m3h, respectively. Support:
V-AC
MMA
WA
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Time (Days) Figure 2. Evolution of EC and C02 for the V-AC biofilter, MMA-Mineral Media Addition. WA-Water Addition
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Due to the low inlet concentration of benzene (25 ppmv), EC of benzene was very low (1 g C/m3h). Toluene was the first compound to be degraded followed by xylenes. At this time para and meta isomers were completely degraded and ortho had xylenes. At this time para and meta isomers were completely degraded and ortho had an elimination of 85% and above.
Support: V-AC
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20
30
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~
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80
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Figure 3 . - Individual degradation of BTX for the V-AC Biofilter MMA-Mineral Media addition, WA: Water Additon. Higher efficiency was obtained with V-AC support for the mixture gas and each one component. Peak EC values were 260g C/m3h, 300g C/m3h, 220g C/m3h and 130g C/m3h for V-AC, peat, barks and GR, respectively. These EC values were comparable with values reported previously. [8,5,10 y 11]. Although, higher punctual EC were obtained with peat, longer periods of high EC were found in V-AC reactor, which justifies an overall better performance. The low EC in GR could be explained by the large void space with this support in the reactor. The GR has also lower biofilm superficial area, which is about half of that found with vermiculite (600m2/m3). Moreover, GR support loses water easily, so frequent addition of water, which may bring about Elimination Efficiency (%) biomass washing. Barks
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(B) permit also a direct addition of nutrients and water but several weeks were necessary to attain a proper humidity of the support due to its hydrophobic composition. In all cases, it was observed that a restoration of water cotent of bed results in increased EC. Biofilter packed with peat had similar results.
-
"
. V e II 350 O
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Support:P SupportV. CA SUpl~' B Supp~:GR
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0
I00
200
300
400
500
600
700
Load (g C/m3h) Figure 4. Integrated values until the first 92 days of BTX for the V-AC Biofilter. MMAMineral Media addition, WA-Water Addition However, humidity conditions were re-established by unpacking the bed, adding water and mixing it manually. (Fig. 4) shows variation of EC with load for the four supports. This results confirm that biofilter packed with V. CA support has the best performances to degrade the mixture of BTX. Maximum EC was about 260g C/m3h for this support.
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4. CONCLUSIONS
The main results obtained in this study were, microbial culture was appropriate to carry out degradation of the BTX mixture. Biofilter operation was strongly affected by the bed moisture content for all supports. Different strategies, depending on the support, were necessary to maintain the required operation conditions to sustain high EC. The mixture of Vermiculite and Activated Carbon (V-AC) had the best long-term results, but addition of nutrients and water was required. V-AC, GR and B required longer time for the acclimation of the population. Peat does not need additional nutrient sources as the rest of the supports. However, mixing and restoration of moisture were necessary to maintain good biofilter performance. REFERENCES
1 2 3 4 5 6 7 8 9 10 11
Riveros H. G., Tejada J., Ortiz L., Julian-Sanchez A. and Riveros-Rosas H., J. Air and Waste Manag. Assoc. 45 (1995) 973-980. Leson G. and Winer A.M., J. Air and Waste Manag. Assoc., 4 (8) (1991) 1045-1054. Bohn, H., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc. Nashville, Tennessee, U.S.A. 1996. Groenestijn Van J., Harkes M., Cox H. And Doddema H., Proc. Air Pollution Control Tech., Los Angeles Cal. U.S.A. (1996) 317-323. Lith Van C., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc., Nashville, Tennessee, U.S.A. 1996. Wang Z., Govind R. And Bishop D.F., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc., Nashville, Tennessee, U.S.A. 1996. Stewart C.W., and Kamarthi R., Proc. 90th Annual Meeting of Air and Waste Manag. Assoc, Toronto, Can. 1996. Thompson D., Sterne L., Bell J., Parker W. and Lye A., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc, Nashville, Tennessee, U.S.A. 1996. Acufia M.E., Auria R., Pineda J., Perez F., Morales and Revah S., Proc. 90th Annual Meeting of Air and Waste Manag. Assoc, Toronto, Can. 1996. Seed L. and Corsi R.L., Proc. 89th Annual Meeting of Air and Waste Manag. Assoc. Nashville, Tennessee, U.S.A. 1996. Kennes C., Cox H. H. J, Veiga M. C. and Doddema H.J., Proc. Air Pollution Control Tech., Los Angeles Cal. U.S.A.4 (1995) 2279-2284.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
A Unified View on Nitrogen Oxide Abatement Jan Paul"2
1. LuleA University of Technology, Physics, 971 87 LuleL Sweden
[email protected] 2. Hacettepe University, Chemistry, 06532 Beytepe-Ankara, Turkey
[email protected] A unified model for nitrogen oxide abatement is suggested. The model observes that nitrates are the most common surface species on oxide supported catalysts under literally all conditions below the light-off temperature. The three routes for nitrogen oxide reduction; (i) direct decomposition, (ii) selective catalytic reduction (SCR) with ammonia and (iii) hydrocarbon driven NOx reduction, merely become different ways to destabilize surface coordinated nitrates. Nitrates are decomposed by thermal excitations in direct NO decomposition and by reactions between NO 3 and NH4§ in ammonia SCR. Reactions between NO 3 and hydrocarbon derived ligands rationalize the importance of partial oxidation products for the third route. The model stresses the significance of Cu2§ n§ dimers for dual ion exchanged catalysts and lattice flexibility for direct decomposition over aluminum rich ZSM5 [TM = Transition Metal].
I. Introduction Nitrogen oxides can be reduced using different technologies. Three way conversion (TWC) of CO/NO proceeds via complete dissociation and reactions between atomic species at metal surfaces but few other routes are as well understood on a molecular level. Catalysts are often inhomogeneous and many reaction pathways have been suggested for direct NO decomposition and selective catalytic reduction (SCR) with ammonia or hydrocarbons in an oxidizing environment. The present text is an attempt to justify a 'unified' model for nitrogen oxide reduction based on our results and literature data. We discuss direct NO x decomposition and reactions with ammonia and hydrocarbons. TWC and NOx reactions with carbon supported metals are clearly excluded albeit we do acknowledge that both these paths are conceivable for hydrocarbon/NOx reactions under reducing conditions. Our model emphasizes the significance of nitrato groups at the catalyst surface, either as siteblockers or as intermediates. NO 3 is found in unidentate and bidentate coordination to single metal atoms (Fig. 1). Metal dimers and Me-O-Me pairs allow for bridge ligation. Finally, multiply coordinated or mineral nitrates with near C3v symmetry are observed but less likely to form on a surface unless strong compound formation occurs.
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2. Experimental
We use Ti doped aluminas and zeolite ZSM5, both ion exchanged with Cu 2§ or Cu 2§ in combination with ions of Ni or Pd. The materials where characterized with TGA including mass-spectrometry, temperature programmed XRD, ESCA and Infrared spectroscopy. Reactions of NO, NO + O2 or NO + 02 + CH4 where followed by in situ transmission and reflectance FTIR.
3. Results & Discussion Following exposure of a Cu-ZSM5 catalyst to NO or NO + 02 a vibrational band is observed below the light-off temperature and interpreted as a bridged nitrato group bound to Cu 2§ - O Cu 2§ dimers. This band disappears above the light-off temperature but the intensity below this temperature correlates with the catalytic activity [ 1]. These bridge coordinated nitrato groups block the active sites for NO conversion. The tentative reaction intermediate, N203, also binds in a bridge configuration to the same Cu 2§ - O - Cu 2§ dimers. A flexible lattice, such as low SiO2 : A1203 ratio ZSM5, is advantageous for direct decomposition, thermal motion in the support destabilizes ligated nitrates and opens the sites for adsorption of reaction intermediates [ 1].
NO.
Cu
N
Cu
0 - Cu
Cu
0 - Cu
Fig. 1 Unidentate NO 3 (right), bridge coordinated NO 3 (middle) and bridge N203 (right) The roles of the support in direct decomposition are twofold, to destabilize ligated nitrates by lattice vibrations at elevated temperatures and to conserve a high dispersion of metal dimers. Zeolite ZSM5 is the preferred carrier and ideally we choose a low SiO2:A1203 ratio. Ratio 48 gives maximum adsorption bond strength but higher A13§ concentrations give more flexible lattices and are thus often preferred. The significance of thermal lattice vibrations is illustrated by a comparison between hydroxyl group destabilization and NO conversion. Fig. 2 shows that water evolution from zeolite ZSM5
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with no metal ion exchange (H-ZSM5) has the same temperature dependence as the catalytic reaction over Cu-ZSM5. The common explanation is thermally activated vibrations in the lattice framework. The amplitude of these vibrations increase with the A1 content, eventually leading to dealumination unless the effect is countered and the lattice reinforced by di- or trivalent ions. 40
Cu-ZSM5-53-145 II
o~ 30 v
tO (/)
B
.,..,
G)
>
tO
0 0 Z
II
---\i\
I,.,.
2o
/ 10
OH removal Cu-ZSM5-53-O (arb. y-scale)
0
~
0
I
1O0
,
I
200
m
t
I
-
B
!
i
/"7
,
300
I
400
,
I
500
600
Temperature (C) Fig. 2 NO conversion over zeolite ZSM5 with SiO 2 "Al20 s ratio 53, ion exchanged with Cue+ to 145 %. The dotted lines shows the temperature dependence of water evolution from hydroxylgroup decomposition over Cu-ZSM5-53-O i.e. the same zeolite in its proton form.
Surface nitrates are destabilized by thermal activation but an asymmetry in the adsorption site by co-cations will translate into an asymmetry in the adsorbed N O 3, causing one N-O bond to weaken and eventual nitrate dissociation. This enables N203, the critical but short lived intermediate in direct decomposition, to form on the 'same' sites at a lower temperature for dual ion exchange zeolites than for single ion exchanged ZSM5. A destabilization of the nitrates translates into a shift to lower light-off temperature. Only small differences are observed between different co-cations which is logical given the above model. Spectroscopic data for bridge coordinated nitrates can provide information about the abundance and characteristics of the active site where N203 is formed. Other means to destabilize the surface nitrate is via ligation of a functional group derived from a reductant. This is the case for ammonia where NH4+ ideally coordinates to the same site as NO 3. Together they form a NOaNH 4 complex [2]. Ammonia is mixed with the oxygen rich exhaust over a catalyst bed of vanadia/titania in SCR units. Again abundant nitrates are descended by vibrational spectroscopy. Interaction with co-ligated NH4+ leads to N-O bond weakening in NO 3 and eventually a reaction between the two ligands [2]. Fig. 3 symbolically
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illustrates a mixed metaloxide site, without favouring a specific ligation for NO 3 and N H 4.
NH 4
(
T M n*-- 0
- T M r"§
Fig. 3 Surface bound N O 3 and N H 4 o n a mixed metaloxide site. TM ~+and TM ~+ mean different transition metal ions.
Similar to ammonia adsorption, partial dehydrogenation of a hydrocarbon leads to a surface bound species. For methane this activation occurs on a metal site. Fig. 4 illustrates a methyl species bound to a transition metal in the vicinity of a Cu 2. bound N O 3 group.
C u 2*-- 0
- T M n+
Fig. 4 NO 3 and C H 3 coordinated to different metal ions at a dual ion exchange site
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Following a familiar path this affects the nitrate which becomes partially reduced by oxygen transfer to the ligated hydrocarbon. This also explains the observed significance of partially oxidized hydrocarbons [3]. Adsorption sites capable of coordinating two ligands, NO 3 and NH4§ or NO 3 and a hydrocarbon derived species, are preferred for (ii) and (iii) and catalyst preparation should focus on this task. Slow 50/50 Cu-cocation exchange from dilute solutions at appropriate pH gives an even distribution of mixed Cu-O-Me dimers, the preferred active sites for direct decomposition [ 1]. 4. Conclusions We suggest that the key to nitrogen oxide reduction is to destabilize surface nitrates. These species readily form under oxidizing conditions albeit different coordination can give different vibrational fingerprints on different catalysts. We note that photoemission spectroscopy of the bonding orbitals provide a fingerprint less readily confused by an altered coordination. Thermal nitrate destabilization opens sites for adsorption of N203 during direct decomposition. Ammonia and hydrocarbon derived intermediates serve the same purpose for SCR reactions, both types of derivatives react with surface coordinated NO 3.
The similarity between the three ways of nitrogen oxide abatement, (i-iii) is logical when considering the similarities between the preferred catalysts. In all three cases metal ions bound to an oxidic carrier are the active material. An oxidic support does not provide any delocalized electron density for strong back-donation to ligated NO and NO will consequently not dissociate on these catalysts. Three-way-conversion (TWC) and NOx abatement over active carbon catalysts both proceeds via dissociative adsorption followed by desorption of N 2 and CO 2. TWC is active on metallic or bimetallic particles with the ability to promote dissociative adsorption. NO x abatement over carbonaceous materials are promoted by metal inclusions or metalcarbides, again with the ability to donate electrons. The latter mimics the reaction path of CO z during coal gasification. Oxide supported metal ions lacks this electron donating capacity but provide excellent supports for nitrate adsorption.
I kindly acknowledge discussions with Drs. E. Bjiirnbom, M. Kantcheva and R. Keiski REFERENCES 1. Ganemi, B. Bj6rnbom E., and Paul J., Appl.Catal.B Environmental 17 (1998) 293; Ganemi B., Bj6rnbom E., and Paul J., Micropor.Mesopor.Materials (submitted); Ganemi B., Rahkamaa K., Keiski R.L., 0hman L.O., Bj6rnbom E., Salmi T., and Paul J. (manuscript) 2. Sadykov V.A., Baron S.L., Matyshak V.A., Alikina G.M., Bunina R.V., R.V., Rozovskii A.Ya., Lunin V.V., Lumina E.V., Kharlanov A.N., Ivanova A.S., and Veniaminov S.A., Catal.Lett.37 (1996) 157; Kantcheva M., manuscript 3. Petunchi J.O., Sill G., and Hall W.K., Appl.Catal.B Environmental 2(1993)303; d.Itri J.L. and Sachtler W.M.H., Appl.Catal.B Environmental 2(1993)L7
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Measuring the adsorption of reactive probes as a tool for understanding the catalytic properties of de-NOx catalysts A. Gervasini ~ and A. Auroux b a Dipartimento di Chimica Fisica ed Elettrochimica, Universitb. di Milano, via Golgi 19, 1-2013 3 Milano, Italy b Institut de Recherches sur la Catalyse, CNRS, 2 avenue Einstein, F-69626 Villeurbanne Cedex, France The surface chemistry of three different copper based systems (Cu-ZSMS, Cu-ETS 10 and Cu-silica-alumina) prepared with different copper loadings has been studied by adsorption microcalorimetry of NO and CO. The reducibility and regeneration of the Cu2+ ions and copper oxide crystals were studied by TPR-TPO experiments. The catalytic activity for NO reduction with hydrocarbons in oxidizing atmosphere appeared to depend on the nature and dispersion of the copper species. 1. INTRODUCTION The negative impact of nitrogen oxides on human health and on the environment results in increasingly strict regulations on their emission levels. To match the severe standards, intense research in the area of catalytic reduction of NO to N2 with hydrocarbons (CrC4 species) has been developed (de-NOx catalysts) [1,2]. Copper exchanged ZSM-5 systems are widely used in the selective catalytic reduction of NOx with hydrocarbons while supported copper catalysts are active in the reduction of NOx with ammonia and with CO. The various results found in the literature imply that these reactions could proceed via different mechanisms and, probably, on different sites. The support of the active phase plays a determining role by ensuring a high dispersion of the active metal cations and by stabilizing these cations at low coordination. A lot of effort has been made to elucidate the mechanism of de-NOx reactions, mainly using kinetic evaluations and spectroscopic studies [3,4], whereas little is known about the properties of the active sites in relation with their activity. A full understanding of the NOx reduction reaction has not been yet completely achieved, in particular concerning the roles of the oxidation state of the metal, of its coordinative insaturations, and of the contribution of the support. As the adsorption of some reagent species should be among the steps that describe the catalytic reaction of NO reduction, the measurement of the adsorption properties, in terms of adsorption capacity and associated energetics, is of crucial importance to understand the catalytic performances of the active sites. In this perspective, we present a comparative investigation of various Cu based catalytic systems taking into account different host supports: an alumino-silicate, ZSM-5 (Si/Al = 15), a
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titanium silicate, ETS-10 (Si/Ti = 4.7) and a silica-alumina (Si/AI = 5.5). Our study was focused on the adsorption properties of some reactive probes (such as NO as well as CO and NH3) as determined by adsorption mierocalorimetry and on the redox properties as determined by TPRffPO cycles, in relation with the catalytic activity of the different samples in the NO reduction by C2I-I4 in oxidizing atmosphere (NO-C2H4-O2). 2. EXPERIMENTAL
The amorphous, on silica-alumina (SA), and crystalline, on ZSM-5 and ETS-10, catalysts were prepared by deposition of Cu ions from copper acetate precursor. The starting materials were the IT forms of SIO2-A1203 oxide (alumina content of 13.5 wt %, from Akzo Chemic) and of ZSM-5 zeolite (alumina content of 2.9 wt %, from ENI), and the Na§ + form of ETS10 titanium silicate (titania content of 18 wt %, from Engelhard). Conventional base exchange procedures were performed to prepare the samples with low copper amounts (3 to 11 wt %), while the more copper loaded catalysts were prepared by a two-step procedure of exchange plus impregnation. The samples were labelled as Cu-ETS-(02,03,04,1,5) and Cu-ZSM5-(02,5) depending on the amount of copper deposited. Details on their preparation and characterization can be found in [5] and [6]. The microcalorimetric studies were performed in a heat flow calorimeter (C80 from Setaram) at 150~ for NH3 adsorption and 30~ for CO and NO adsorptions. A detailed description of the technique has been reported elsewhere [7]. The microcalorimeter is linked to a volumetric adsorption line, equipped with a Barocel capacitance manometer for pressure measurements. The samples were outgassed at 400~ overnight prior to microcalorimetric measurements. The differential heats of adsorption were measured as a function of coverage by sending repeated doses of gas onto the samples until an equilibrium pressure of about 130 Pa was reached. Then the sample was evacuated for 1 h at the same temperature and a second adsorption was performed in order to allow the determination of chemisorption uptakes. Thermoprogrammed reductions (TPR) were carried out in a differential scanning calorimeter linked to gravimetry (TG-DSC 111 from Setaram) at a heating rate of 5~ Prior to TPR runs, the samples (ca. 35 mg) were pretreated at 100~ for 1 h and cooled to room temperature in a stream of helium. Then they were heated in a flow mixture of helium and hydrogen (80 vol % of H2) up to 650~ in quartz sample holders (total flow rate : 43 mL/min). The weight loss together with the heat flow signal were recorded as a function of temperature. The TPR experiments were followed by TPO runs in a flow mixture of helium and oxygen (60 vol % 02) with the same heating procedure. Catalytic activity in the NO reduction by C2H4 in oxygen-rich atmosphere (NO-C2H4-O2) was studied in a fixed-bed quartz tubular downflow reactor at atmospheric pressure. The flow of feed gases was controlled from independent mass flow controllers (Hi-Tec. from Bronkhorst). Fixed NO, C2H4 (0.4 % v/v) and oxygen (4 % v/v) concentrations in He were used. The gas flow was 5.5 L.h ~ corresponding to a space velocity of 7500 h "~ (GHSV). Temperatures in the interval 150-500~ were investigated. The reactor outflow was analyzed using a gas chromatograph equipped with a T.C.D. detector (C.P. 9000 from Chrompack) mounting a 60/80 Carboxen-1000 column (from Supelchem). The extent of NO reduction was controlled by determining the N2 production and that of C2H4 conversion by the CO2 production. Integral conversion rates of NO to N2 (TOF, in terms of mole of N2 formed per mole of Cu per second) were calculated for comparative purposes.
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Table 1 Physieo-ehemical characteristics of selected materials Sample Cu amount Exchange ETS10 Cu-ETS-02 Cu-ETS-04 Cu-ETS-1 Cu-ETS-5 ZSM5
Cu-ZSM5-02 Cu-ZSM5-5 SA Cu-SA
(wt%)
(%)
3.1 8.4 11.4 23.6
24 66 90 186 93 519 -
-
3.0 16.6 8.0
Surface Area (m2/g) 506 381 388 296 27 435 366 308 365 285
Acidity Qtmol NHdm 2) 1.68 1.15 0.87 2.22 nd 1.70 2.59 2.74 0.90 1.48
3. R E S U L T S A N D D I S C U S S I O N
A summary of the compositions, exchange levels and surface areas of the samples is presented in Table 1. Table 1 also gives the adsorbed ammonia uptakes, expressed in pmol/m2 for a given equilibrium pressure of 26.7 Pa. These values are representative of the acidity of the samples. The adsorption of NH3 on the surface of the sample is strongly influenced by the acidity of the support. The totally exchanged Cu-ETS-1 sample displays a much higher number of NH3 adsorption sites than the ETSI0 matrix. On the contrary, the low exchange ETS samples are less acidic. Both the low and over-exchanged Cu-ZSM5 samples display more NH3 adsorption sites than the corresponding ZSM5 host matrix. Thus ammonia adsorption on copper sites appears to be essentially dependent on their location in the matrix [5]. Although NO and CO adsorbates on the active surface may serve as active precursors involved in surface reactions for N2 and CO2 formation, few attempts have been made to correlate the bonding strength and concentration of adsorbates with the catalytic activity [8]. In the present work, adsorption calorimetry was used to determine the strength of active sites and reactivity of various adsorbates (CO, NO). Figs. 1 and 2 represent the differential heats of NO and CO adsorption respectively as a function of coverage for Cu-ETS, Cu-ZSM5 and Cu-SA samples. The ETS10 and ZSM5 supports do not adsorb significantly CO or NO [6]. The differential heats of NO adsorption on the Cu-ETS samples tend to increase with the copper amount for the samples prepared by single-step ion exchange, reaching a maximum for the totally exchanged Cu-ETS-1 sample, while the overexchanged Cu-ETS-5 sample displays low heats of adsorption, very close to those of the support, because of the very low surface area and partial collapse of the structure. The curves are continuously decreasing, which indicates a large heterogeneity of the systems under study. The Cu-ZSM5 samples display important NO adsorption properties compared to the host matrix and a greater homogeneity of sites, but the amount adsorbed does not vary significantly with the Cu amount. Finally, the Cu-SA sample shows the highest initial heats (ca. 130 kJ.mol1) but adsorbs slightly less NO than Cu-ETS-04 in spite of a similar copper content. The differential heats of CO adsorption (Fig. 2) show that the CO uptake on Cu-ETS samples also increases with the copper amount, reaching again a maximum for Cu-ETS-1.
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化
Q (kJ/mol)
2OO
140
CuZSM5-02
- -9- "
CuZSM5-02
"+ 9-
CuZSM5-5
--+-"
CuZSM5-5
--,V--
CuETS-02
----
Z
120 [
Q (kJ/mol)
CuETS-02
150
CuETS--04
100 - •
~"
-I-
~
CuETS-1
--E--
CuETS-04
m
CuETS-1 -5
--C .... C u E T S - 5
80~
100
6O
40
N
50
20
0 0
I
I
I
I
I
20
40
60
80
100
0
,
NO uptake (~mol/g) Fig. 1. Differential heats of NO adsorption versus coverage.
120
0
I
I
I
I
l
1
20
40
60
80
100
120
140
CO uptake (IJmol/g) Fig. 2. Differential heats of CO adsorption versus coverage.
Contrarily to the behavior with NO, the most loaded Cu-ZSM5 sample (Cu-ZSM5-5) adsorbs a much greater CO amount than the Cu-ZSM5-02 sample, and the shapes of the two curves differ by the presence of a large plateau around 140 kJ.mol 1 for Cu-ZSMS-5. CO adsorption on Cu(I) copper sites is known to be partly irreversible at room temperature, while CO stabilized on Cu(II) or Cu metal sites is almost completely depleted by briefly outgassing at room temperature [9]. So, assuming that CO adsorbs preferentially on Cu § sites, a direct correlation between the concentration of Cu § ions and the amount of CO adsorbed can be established. On the contrary, NO strongly adsorbs on Cu 2§ sites. These contrasted behaviors provide a useful tool to identify the oxidation state of matrix-encaged copper. The respective amounts of CO and NO adsorbed are similar for the Cu-ZSM5 samples, while the amount of NO adsorbed is approximately half of the amount of CO adsorbed for the Cu-ETS samples. Previous IR studies and TPR results on Cu-ZSM5 [ 10,11 ] have indicated that a significant fraction of the copper in over-exchanged Cu-ZSM5 is present in the form of dimeric Cu E+ complexes in addition to monomeric Cu 2§ ions, and that the oxygen bridged species (Cu-O-Cu) 2§ can be easily reduced to Cu § species by heating under vacuum at 400~ ; however a substantial quantity of isolated Cu 2+ ions remains. The reducibility of Cu 2+ ions located in the framework of the molecular sieve plays an important role in the catalytic NO reduction activity. The values of the peak maxima for the HE-TPR of our catalysts are shown in Table 2. The HE-TPR curves of the Cu-ETS samples have two peaks : one around 167~ and the other between 180 and 200~ The Cu-ETS-5 sample displayed a shoulder near 210~ where the higher temperature peak existed. The first and second peaks can be attributed to the reduction of CuO to Cu20 and the reduction of Cu20 to Cu ~ respectively. The Cu-ZSM5 samples also presented two peaks. The first peak was
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Table 2 Temperature of peak maxima of H2-TPR and O2-TP0 curves of selected materials Catalysts Temperature of peak maxima Temperature of peak maxima (~ (TPR) . (~ (~.TPO) Cu-ETS-04 160(s) 181 153 220 275 Cu-ETS-1 167(s) 187 167 222 265(s) Cu-ETS-5. ' 168(s) 200 210(s) 191 276 Cu-ZSM5-02 191 275 Cu-ZSM5-5 187 205(s) 212(s) 206 275 Cu-SA 197 209(s) 229 144 268 (s) " shoulder probably caused by the reduction of Cu 2+ to Cu +, while the second peak was smaller and possibly due to the reduction of CuO to Cu ~ The stability of copper in a reduced state was examined by TPO analysis. Oxidation of the Cu-ZSM5 samples began at a higher temperature than for the Cu-ETS samples. The zeolite structure strongly affects the oxidation properties of Cu ~ The Cu-ETS samples present three peaks, which may be assigned to the oxidation sequences of Cu ~ i.e. Cu ~ -~ Cu 2+ or Cu +, Cu ~ -~ Cu20 and Cu20 --~ CuO. Catalytic activity in the NO-C2H4-O2 reaction on the differently supported copper catalysts was studied in order to determine the relationships between the properties of the active copper sites and their activity. Deep differences in reactivity emerged among catalysts, reflecting the dramatic influence of the support in stabilizing Cu centers in geometrical and electronic situations suitable for activity. The two Cu-ZSM5 catalysts, as expected, had very high activity. The TOF values were in the range 55-80 and 10-20 molN2.molcd~-s1 for Cu-ZSM5-02 and Cu-ZSM5-5, respectively, at reaction temperatures in the 300-500~ interval. The decrease of TOF with increasing Cu loading suggests that only the isolated copper ions dispersed in/on the zeolite matrix are active; the over-exchanged copper, present mainly as 100 i
r
.9, ~ O z
'1
.~
20
o" rj
~,,,.~
-~....
O
*" o
c 0 '-
I
~i~ ~
3O z
i
9
~
O
c 0
~D cO
o 0
"'
100
I
200
I
300
I
400
Temperature (~
,,
..'"
8O
60 i
ol
c o_.
10
S
40
t
9
I/,g.7
#~+~,..~'/~,~:
/' ,"
20
I-~-cu'ETs'~
I
l-O-Cu- ETS-03 I
['~176176
1
500
0
100
I
I
I
I
200
300
400
500
Temperature (.~
Fig 3 Dependence of NO conversion to N2 (A) and of C2[]4 conversion to CO2 (B) on the reaction temperature for the Cu catalysts on the ETS-] 0 support
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CuO [6], was not catalytically active. The same behavior could be observed for the copper catalysts on ETS10 support. TOF values were in the range 13-23 for the least Cu-loaded catalyst (Cu-ETS-02) and regularly decreased with increasing Cu content, down to 2-5 mols2mold~-s ~ for the catalyst with high Cu content (Cu-ETS-1). It can be inferred that small copper aggregates are much more active than large ones, and therefore that the reduction of NO by hydrocarbons seems to be a "structure-sensitive" reaction. The over-exchanged catalyst (Cu-ETS-5), which has lost most of its microcrystalline character, was completely inactive toward the N2 formation, only the quantitative combustion of C2I-I4 at high temperature was observed. Moreover, the Cu-SA catalyst, which displayed scarce dispersion of the Cu ions, was ineffective for NO reduction. Presumably, these two samples (Cu-ETS-5 and Cu-SA) contain CuO crystallites which are very active for hydrocarbon combustion, therefore making C2H4 combustion the predominant reaction. The most interesting results in the NO-C~H4-O2 reaction for the catalysts of the ETS series are presented in Fig. 3 in terms of N2 and CO2 productions. The amount of N2 produced was about the same for every catalyst, and the temperature of maximum N2 production was not clearly related to the Cu content of the catalyst. A shift towards higher temperatures was observed for Cu-ETS-02 only. On the other hand, a clear dependence on Cu content was observed for the C2I-L combustion, confirming that large CuO crystallites had better combustion ability. 4. CONCLUSION The characteristics of the support and the preparation method greatly influenced the deposition of copper in the final catalyst. Copper can be stabilized in different forms: Cu § and Cu 2+ ions and crystalline aggregates of Cu20 and CuO oxides. In the NO-C2H4-O2 reaction, highly dispersed copper centers with high reduction ability are much more effective than stabilized copper centers. The study of the adsorption of different reactive probes using volumetry linked to microcalorimetry, with the help of infrared spectroscopy data from the literature, made it possible to quantify the amounts of the various copper-containing species. REFERENCES
1.
M.C. Kung, K.A. Bethke, J. Yan, J.-H. Lee, H.H. Kung, Appl. Surf. Sci., 121/122 (1997) 261. Y. Traa, B. Burger, J. Weitkamp, Micropor. Mesopor. Mater., 30 (1999) 3. 3. N.W. Cant, A. D. Cowan, Catal. Today, 35 (1997) 89. 4. A.T. Bell, Catal. Today, 38 (1997) 151. 5. A. Auroux, A. Gervasini, C. Guimon, J. Phys. Chem., (1999) in press. 6. A. Gervasini, C. Picciau, A. Auroux, Micropor. Mesopor. Mater., (1999) in press. 7. A. Auroux, Topics in Catalysis, 4 (1997) 71. 8. Z. Chajar, M. Primet, H. Praliaud, M. Chevrier, C. Gauthier, F. Mathis, Stud. Surf. Sci. Catal., 96 (1995) 591. 9. K. Hadjiivanov, L. Dimitrov, Micropor. Mesopor. Mater., 27 (1999) 49. 10. G.D. Lei, B.J. Adelman, J. Sarkany, W.M.H. Sachtler, Appl. Catal. B, Environmental, 5 (1995) 245. 11. C.Y. Lee, K.Y. Choi, B.H. Ha, Appl. Catal. B, Environmental, 5 (1994) 7. .
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Reactivity of the NOx surface species formed after co-adsorption of NO + Oz on a WO3/ZrOz catalyst: an FTIR spectroscopic study Stefan Kuba, Konstantin Hadjiivanov* and Helmut Knrzinger Institut for Physikalische Chemie, LMU, Butenandtstral3e 5-10, Haus E, M0nchen 81377, Germany The only stable surface species produced after NO + 02 coadsorption o n WO3/ZrO2 are surface bidentate nitrates. They are thermally stable at 573 K, but they easily oxidize methane at this temperature.
1. INTRODUCTION Presently, the most attractive route toward reduction of NOx-emissions from stationary sources is the selective catalytic reduction (SCR), using ammonia as a reducing agent [1-3]. Many efforts are presently made to develop the SCR of NOx by hydrocarbons (HC-SCR) [48], because the three way catalysts are not efficient for NOx control in lean burn engines, in which the burning of the fuel occurs in excess of oxygen [4]. The first reported catalyst for HC-SCR was Cu-ZSM-5 which operates with C2+ alkenes and C3+ alkanes [5,6]. Many other materials have been tested and shown promising HC-SCR activities [7-9]. A very important result in research of HC-SCR is the discovery of Li and Armor [ 10] that the reduction of nitrogen oxides can be performed with methane on Co-ZSM-5. This provides the principal possibility to replace ammonia as a reducing agent in stationary NOx sources by CH4. The following key results in the studies of CH4-SCR can be outlined: 9 Li and Armor [11] reported that, in addition to Co-ZSM-5, other Co-exchanged zeolites are active in the reaction, while oxide supported cobalt is totally inactive. 9 Adelman et al. [12] reported a comparative mechanistic study on the HC-SCR over Cu-ZSM-5 and Co-ZSM-5. These authors found that surface species characterized by a band at ca. 1526 cm1 (and assigned to nitro groups) are produced during NOx adsorption on Co-ZSM-5, but not on Cu-ZSM-5. These species, in contrast to the nitrates formed on Cu-ZSM-5, reacted easily with methane. 9 Inaba et al. [13] demonstrated that oxide-supported cobalt could also show very good properties in HC-SCR. The authors found a high activity for this reaction of Co/SiO2 prepared via impregnation of silica with cobalt acetate and very poor activity for a sample prepared via impregnation with cobalt nitrates. The authors suggested that highly dispersed
Pemlanent address: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria.
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cobalt ions are active sites for the reaction. This hypothesis was confirmed later by IR spectroscopy studies [14]. It was found that isolated cobalt ions on silica form the 1526 cm ~ species (assigned here to monodentate nitrates) atter NOx adsorption that are typical for Co-ZSM-5. These species easily oxidized methane. No such species were produced on samples prepared by impregnation with cobalt nitrate. Thus, it appears that a high dispersion of the supported phase favours SCR with hydrocarbons. In addition, IR studies of the reactivity of the surface nitrates correlates well with the activity of the samples. In this work we studied the surface NOx species produced atter NO adsorption and NO + 02 coadsorption on a WO3/ZrO2 sample as a potential candidate as a CI-h-SCR catalyst. Titania-supported vanadia and tungsta are among the commercially applied catalysts for SCR by ammonia. Although tungsta-based catalysts operate at a higher temperature than vanadiacontaining catalysts, we expected that the high surface acidity of WO3/ZrO2 might assist methane activation.
2. EXPERIMENTAL The WO3/ZrO2 sample was prepared by suspending amorphous Zr(OH)4• in an appropriate amount of aqueous solution of ammonium metatungstate. The suspension was refluxed for 16 h at 383 K and then the water was evaporated at 383 K. The solid was dried for 12 h at 383 K and finally calcined at 923 K. The nominal concentration of WO3 in the sample was 17.7 wt. %. This corresponded to a theoretical monolayer of tungsta on zirconia surface. FTIR spectroscopy studies was carried out using a Bruker IFS-66 apparatus at a spectral resolution of 2.0 cm 1 accumulating 128 scans. Self-supporting wafers (ca. 10 mg cm 2) were prepared from the sample powders and heated directly in the IR cell. The latter was connected to a vacuum/sorption apparatus with a residual pressure less than 10-3 Pa. Before the measurements all samples were activated for 1 h in a flow of oxygen at 673 K followed by 1 h evacuation at the same temperature. Raman spectroscopy was performed using a Dilor Omars 89 spectrometer equipped with a Peltier cooled CCD-camera and a Spectra Physics 2020 Ar § excitation source using the 488 nm line at a laser power of 50 mW. The spectral resolution was 5 cm-1. The specific surface area was determined by low-temperature nitrogen adsorption according to the B.E.T. method.
3. RESULTS AND DISCUSSION
3.1. Sample characterization The specific surface area of the sample was 120 m 2 g-l. Analogous zirconia samples, also calcined at 923 K are characterized by a significantly lower specific surface area indicating that the supported tungsta phase inhibits the sintering of the ZrO2. Raman spectroscopy revealed W-O-W linkages which suggested that the WOx surface species consist of small oligomeric clusters grafted onto the zirconia surface. No crystalline
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WO3 was detected. In addition, it was found that the surface WOx-species stabilized the
metastable tetragonal modification of the ZrO2 support (pure ZrO2 is stable in the monoclinic modification). The IR spectrum of the sample activated at 723 K contained, two bands in the v(OH) stretching region with maxima at 3740 and 3620 cm 1, assigned to Zr-OH and W-OH hydroxyls, respectively. In addition, an intense band at 1018 cm ~, due to v(W=O) and the respective overtone at 2025 cm "~ [ 15] are observed. Adsorption of CO at ambient temperature (1.6 kPa equilibrium pressure) on the activated sample results in the appearance of one band at 2201 cm ~ which decreases in intensity with the equilibrium pressure and shifts to 2205 cm -1 before disappearance atter evacuation. This band is typical for CO coordinated to Zr 4§ Lewis acid sites [16] and reveals the existence of bare zirconia surface on the sample. The Bronsted acidity of the sample was studied by low-temperature CO adsorption. In addition to the bands due to Zr4+-CO species, adsorption of CO at 85 K resulted in the appearance of a band at 2155 cm 1 indicating the formation of H bonded CO [17]. Simultaneously, a red shift of the W-OH band from 3620 to 3460 cm -1 was observed, i.e. Av(OH) was -160 cm ~. The value of the shift of the OH groups atter CO adsorption, compared with other oxide systems [17,18], is relatively high thus evidencing a relatively strong Bronsted acidity of the sample.
3.2. Adsorption of NO Exposure of the activated WO3/ZrO2 sample to NO (1 kPa equilibrium pressure) leads to an immediate appearance of two bands at 2286 and 2242 cm 1 in the IR spectrum (Fig. 1, spectrum a). The band at 2242 cm ~ has been observed previously after N20 adsorption on pure zirconia and assigned to the N - N stretching mode of N20 coordinated to Zr 4§ sites [19]. The band at 2286 cm ~ is probably due to N-bonded N20 on tungsta [20]. In addition, a weak band at 2130 cm ~ characterizing NO* species [21] is also visible. Two bands of very weak intensity at 1605 and 1220 cm 1 are assigned to surface bidentate nitrates [22]. Simultaneously with the formation of nitrates the band characterizing W=O bond is red shifted which indicates that at least part of the nitrates are formed with the participation of this oxo-group. Allowing the sample to stay in the NO atmosphere (Fig. 1, spectrum b) results in a strong increase in intensity of the NO + and the nitrate bands. The higher frequency component of the latter is split into two components at 1615 and 1580 cm 1. In addition, a strong band at 1935 cm ~ developed. The latter is assigned to nitrosyl species formed on sites that are affected by nearby nitrates. These results suggest that the following reactions take place on the catalyst surface: 3 NO o N20 + NO2 (1) 2 NO2 o NO + + NO3 (2)
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Fig. 1. FTIR spectra of NO (1 kPa equilibrium pressure) adsorbed on WO3/ZrO2 (a), time evolution of the spectrum (b), addition of 0.5 (c) and 1.0 kPa O2 (d), and under 30 min. dynamic vacuum at 293 (e), 423 (f), 573 (g) and 623 K (h)
3.3. Co-adsorption of NO and 02 Addition of increasing amounts of oxygen to the sample placed in a NO atmosphere (Fig. 1, spectra c, d) leads mainly to an increase in intensity of the nitrate bands, i.e. NO is oxidized to NO3. In addition, a weak band at 1748 cm l indicates formation of adsorbed N204 [23]. Evacuation at ambient temperature (Fig. 1, spectrum e) leads to the disappearance of the bands due to NO and N204 and to a slight decrease of the amounts of NO + and nitrates. When the sample is evacuated 423 K (Fig. 1, spectrum f), NO + disappears and the nitrate bands decrease in intensity. After consumption of all NO § however, further evacuation leads to no substantial changes in the spectra. This suggests recombination of NO t with surface nitrates leading to the reverse of reaction (2). The nitrates are still detectable at 673 K.
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wavenumber, cm -1 Fig. 2. FTIR spectra recorded after exposure of the WO3/ZrO2 sample to a NO + 02 mixture, followed by 30 min. evacuation at 473 K (a), after 10 min. interaction with methane (2 kPa) at 473 K (b), subsequent 30 min. evacuation at 573 K (c) and 10 min. interaction with methane (2 kPa) at the same temperature (d). 3.4. Interaction of the surface nitrates with methane It has been reported that the reaction order of HC-SCR is ca. 1 in hydrocarbons and ca. 0 in NO [24]. This suggests that the hydrocarbons react with a catalyst surface modified by NOx. For being intermediates in SCR, the surface NOx species have to be stable at temperatures at which SCR proceeds, but have to react easily with hydrocarbons. The only stable surface NOx species on our sample were the bidentate nitrates, being thus potential candidates for SCR intermediates. For this reason we studied their interaction with CI-h. Surface nitrates were produced by co-adsorption of NO and 02, followed by evacuation at 473 K (Fig. 2, spectrum a). Interaction of this nitrate-containing sample with methane at 473 K leads to a strong decrease in intensity of the nitrate bands and production of small amounts of NO § (Fig. 2, spectrum b). Simultaneously, a strong CO2 band at 2355 cm1 was detected. These results indicate that, even at that low temperature, methane is oxidized by the surface nitrates giving COz as a final product. Unfortunately, we were not able to detect the formation of nitrogen but evidently part of the nitrates was reduced to NO +. The surface nitrates lett on the surface are stable towards evacuation at 573 K (Fig. 2, spectrum c), but all of them react completely with methane at this temperature (Fig. 2, spectrum d).
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4. CONCLUSIONS: 9 The only thermally stable compounds formed upon NO + 02 co-adsorption on a WO3/ZrO2 sample are surface nitrates. 9 These nitrates easily interact with methane even at 473 K giving CO2 as a final oxidation product. The strong Bronsted acidity of the sample is probably involved in the activation of methane.
ACKNOWLEDGEMENTS: This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and the Fonds der chemischen Industrie. K. H. acknowledges a fellowship of the Alexander von Humboldt-Foundation. REFERENCES 1. F.J. Janssen, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Kn6zinger and I. Weitkamp (eds.), Willey- VCH, Weinheim, 1997, p. 1633. 2. V.I. Parvulescu, P. Grange and B. Delmon, Catal. Today, 46 (1998) 233. 3. G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal., B: Environ., 18 (1998) 1. 4. E.S.J. Lox and B.H. Engler, in "Handbook of Heterogeneous Catalysis", G. Ertl, H. Kn6zinger and I. Weitkamp (eds.), Willey- VCH, Weinheim, 1997, p. 1559. 5. M. Iwamoto and H. Hamada, Catal. Today, 17 (1991) 94. 6. W. Held, A. K6nig, T. Richter and L. Puppe, SAE Technical Paper 900496, 1990. 7. X. Feng and W. K. Hall, Catal. Lett., 41 (1996) 45. 8. L.J. Lobree, A. W. Aylor, J. A. Reimer and A. T. Bell, J. Catal., 181 (1999) 189. 9. T. Chafic, A. Ouassini and X. Verykios, J. Chim. Phys. Phys. Chim. Biol., 95 (1998) 1666. 10. Y. Li and J. N. Armor, Appl. Catal., B: Environ., 1 (1992) L31. 11. Y. Li and J. N. Armor, in "Natural Gas Convertion II", H.E. Curry-Hyde and R.F. Howe (eds.), Elsevier, Amsterdam, 1994, p. 103. 12. B. Adelman, T. Beutel, G. Lei and W.M.H. Sachtler, J. Catal., 158 (1996) 327. 13. M. Inaba, Y. Kintaichi, M. Haneda and H. Hamada, Catal. Lett., 39 (i 996) 269. 14. B. Djonev, B. Tsyntsarski, D. Klissurski and K. Hadjiivanov, J. Chem. Soc. Faraday Trans., 93 (1997) 4055. 15. G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatto and F. Bregani, Langmuir, 8 (1992) 1744. 16. V. Bolis, B. Fubini, E. Garrone, C. Morterra and P. Ugliengo, J. Chem. Soc. Faraday Trans. 1, 88 (1992) 391. 17. M. Zaki and H. KnOzinger, Mater. Chem. Phys., 17 (1987) 281. 18. A. Tsyganenko, L. Denisenko, S. Zverev and V. Filimonov, J. Catal., 94 (1985) 10. 19. T. M. Miller and V.H. Grassian, Catal. Lett., 46 (1997) 213. 20. G. Ramis, G. Busca and F. Bregani, Gezeta Chim. Ital., 122 (1992) 79. 21 K. Hadjiivanov, J. Saussey, J.L. Freysz and J.C. Lavalley, Catal. Lett., 52 (1998) 103. 22 D. Pozdnyakov and V. Filimonov, Kinet. Katal., 14 (1973) 760. 23 J. Laane and J. R. Ohlsen, Prog. Inorg. Chem., 28 (1980) 465. 24. K. Yogo and E. Kikuchi, Stud. Surf. Sci. Catal., C84 (1994) 1547.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
The mechanism of the selective NOx adsorption on 12-tungstophosphoric acid hexa-hydrate S.Hodjati, C.Petit*, V.Pitchon and A.Kiennemann LERCSI, UMR 7515 du CNRS - ECPM, 1, rue Blaise Pascal, 67070 Strasbourg, France The paper describes the study of HPW, using conditions simulating as close as possible to a real car exhaust. The amounts of NO• absorbed and desorbed prove very high with NO and NO2 in molar ratio. XRD suggests that the structure is conserved following complete saturation by NOx. The adsorption mechanism proceeds via a substitution of the water molecules by NOx to form an [H+ (NO[, NO+)] complex. 1. INTRODUCTION The elimination of NOx stemmed from Diesel exhaust is a major challenge for automotive industries in the world. Many processes have been developed with two different concepts based upon direct reduction into nitrogen on stream or selective NOx trapping followed by a reduction [1]. For this second concept, Daimler-Benz has produced a very attractive possibility called SNR (Selective NOx Recirculation) whereby NOx are recirculated to the combustion chamber where they are reduced by combustion processes upon selective adsorption [2] The key point in such a device is the efficiency of the adsorbent which should have a high NOx adsorption capacity at low temperature (100-300~ and a good ability to desorb them at temperatures relatively low (maximum 550~ to minimise the input of energy. One of their main features should be a good resistance to SO2 poisoning which so far has not been proposed in the systems existing in the literature. These systems are numerous and are mainly based on inorganic oxides [3], mixed oxides [4,5], well-defined structures such as perovskites [6], cuprates [7] or modified three-way catalysts [8]. The most efficient formulations contain barium, which easily form nitrates [9] which are displaced by SO2 to form irreversible sulphates causing a permanent deactivation [10]. Almost at the same time, appeared in the literature the results of two different teams reporting adsorption of nitrogen oxides on 12-tungstophosphoric acid (HPW), compound of the heteropolyacids family having the formula H3PW~2040, 6H20 [11, 12]. HPW is an ionic solid; its structure is formed of primary units also called Keggin anions made of 12 octahedra WO6 surrounding a central tetrahedron PO4. The Keggin's polyanions are linked together by H+(H20)2 bridges to form the so-called secondary structure. The structure was fully described in 1977 in Acta Crystallography by G.M.Brown et al. [ 13]. The mechanism of adsorption of NOx proposed by the two teams is somewhat different; For Chen and Yang, NO only interacts with the HPW to substitute water molecules at low temperature at 50-230~ and low NO concentrations. Upon rapid heating, NO is decomposed into N2. The mechanism invokes the formation of NO via a (NOH) + complex replacing the H+(H20)2 linkages in the structure based upon NO thermodesorption, XRD results and bond lengths calculations. In their case, B61anger and Moffat have observed that NO2 is adsorbed with a stoichiometry of 3 molecules of NO2 per Keggin anion. Their mechanism involves the formation HNO2+ resulting from the reaction between NO2 and surface or bulk protons. The exposure of the solid to water vapour
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at 150~ led to a partial desorption of NO2 into the form of HNO3. NO is not able to penetrate into the bulk of the structure and therefore is not adsorbed on the structure. Nevertheless, if NO2 is preadsorbed some NO adsorption is observed and such phenomenon is related to the formation of N203 as put in evidence by infrared spectroscopy. One of the main objectives consists of the development of a material able to adsorb selectively NOx issued from both Lean-Bum or Diesel engines. From this point of view, HPW seemed to be very promising but there is still a strong necessity to study its behaviour towards NOx adsorption using conditions as closed as possible from a real car exhaust, containing for example CO2, SO2 or hydrocarbons. The existing results which are reported so far are not taking into account this aspect and also seemed to lead to contradictory conclusions. Our approach was to use other experimental techniques to bring new evidence allowing the elucidation of the mechanism of both NOx adsorption and desorption on HPW. 2. EXPERIMENTAL
The catalytic material was purchased from Strem Chemicals. It was characterised by XRD before and after reaction. In some cases, 1% of platinum was added by impregnation of H2PtCI6 solution. The experimental set-up is detailed elsewhere [9]. Briefly, The experiments were conducted in a flow reactor using a series of mass flow controllers with diluted gases (NO, NO2, CO, C3H6, 02, CO2, SO2, with N2 as the diluting gas). The reactor was a quartz tube of 12 mm in diameter and a sinter, sealed up with Cajon couplings. The uptake and desorption capacity of NO/NO2 were measured according to the temperature using a furnace and a temperature controller with a mass of catalyst of 330 mg and a flow of 300 cm3.min -~. Water was added using a saturator maintained at the required temperature in order to have 5% humidity in the flow. The water was removed from the stream with a permeation tube and the NO and NO2 were specifically examined by infrared analysers (Rosemount, accuracy +/- 10 ppm). Thermogravimetric analyses were using a TG-DTA 92-10 from Setaram. Two gas compositions were used in simulation of a Diesel (NO 1000 ppm, C3H6 50 ppm, CO 300 ppm CO2 5%, O2 10% and H20 5%)or a Lean-Burn (NO 500 ppm, NO: 500ppm, CO2 5%, 02 10% and H20 5%) exhaust. The tests were made according to the following procedure: the catalyst was heated up to 80~ in air and the gas flow was switched according to the type of experiments and maintained at this temperature for 15 min. The temperature was then raised to 550~ or 170~ with a ramp rate of 4~ -~ After 15 min. at this temperature, the gas flow was switched to synthetic air (O2/N2), and the temperature cooled to 80~ The overall experiment was repeated at least twice to measure reproducibility. The adsorption capacities are expressed in mg of NOx per gram of catalyst. 3. RESULTS AND DISCUSSIONS The first test was conducted by raising the temperature up to 550~ under a Lean-Burn mixture after a dwell of 15 minutes at 80~ The operation was repeated twice. For the first run, an adsorption of NOx between 100 and 300~ is observed followed by a desorption between 300 and 550~ as described by figure 1.The amount of NOx adsorbed and then desorbed are equal and of 38 mg/g of HPW. For the two following cycles this amount drops drastically down to 4.5 mg/g. The literature reports a thermal fragility of the heteropolyacids structure [14]. Indeed, after one experience at 550~ we observe a complete loss of cristallinity by XRD and deep modifications of the infrared spectrum. This degradation of structural properties corresponds to the loss of the H+(H20)2 bridges. To understand the process of water loss we have undertaken a TGA analysis (figure 2).
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There is an overall mass loss of 12.4% spread over three zone of temperature. The first one between 25 and 75~ represents 8.6% and corresponds to physisorbed water. The second between 100 and 240~ correspond to the loss of the 6 water molecules of the structure with a percentage of 3.2% and the third one of 0.6% between 375 and 475~ is water issued from
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decomposition. At this level the Keggin units are transformed into simple oxides according to the equation: H3PW12040 ~ (1/2)P205 + 12WO3 + (3/2)H20. Therefore, the degeneracy of structural properties can be correlated with the loss of adsorption capacity after the first run. In order to evaluate the limits of the structural stability of the adsorbent as well as to define an optimal working conditions, several temperature of adsorption were tried. The best results were obtained by using 170~ as a maximum as displayed by figure 3. A very peculiar phenomenon was observed in the broad range of temperature tested. The amount of NOx adsorbed and desorbed were always different. This discrepancy is explained by the fact that on such material desorption is observed during the cooling phase. This type of behaviour is very unusual and has never been reported before.
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Fig. 3 Detailed profile of adsorption-desorption of NO and NO2 Adsorption in a Lean-Burn with a maximum temperature of 170~ and desorption in wet air. Another important point to note from figure 3 is the fact that NO and NO2 are adsorbed and desorbed in molar ratio. In this case, the ratio NO/NO2 was equal to one. It was varied between 0.2 and 5 and the same observations is available, i.e. in all cases the quantity of NO adsorbed or desorbed is equal to the one of NO2. For this reason, no adsorption was observed in the case of a NO2 free mixture, i.e. in Diesel conditions. Nevertheless, the addition of 1% of platinum provokes the oxidation of NO into NO2 between 180 and 275~ In this range of temperature, a consumption of 30 mg/g became possible. A solid containing no Pt was saturated by NO and NO2 at 170~ and then examined by infrared spectroscopy. The HPW is diluted in KBr and the spectrum recorded at 25~ is shown on figure 4b and compared with a fresh sample on figure 4a.On the reference HPW a broad band with two maxima at 1620 and 1710 cm-" are identified and correspond respectively to water and H+(H20) bridges in the secondary unit. Upon submission to NOx new bands at 1295, 1384 and 2261 cm -~ appear. According to B61anger and Moffat [15] the band at 2261 cm -~ is attributed to a pertubated NO2+ species and more particularly to HNO2 +. In the case of NO adsorption on HPW presaturated with NO2, the appearance of a new band at 1304 cm -1 is observed and is attributed to the N203 species. The absorption bands of Nujol used the IR spectra record are located in the zone between 1350 and 1450 cm -1, which possibly hide the band at 1384 cm ]. According to Yang and Chen ]~, the band at 2261 cm ~ is due to the presence of NO + species and more
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particularly to (N-O-H) + . Likewise, the same authors reckon that the bands at 1384 and 1295 cm 1 are due to the interaction of NO with KBr used to press the HPW sample [16]. Our interpretation is somehow very different. According to the fact that NO and NO2 are adsorbed simultaneously and in the same proportions on one hand and based upon literature [17, 18] on the other hand, we attribute the band at 2261 cm -~ to an NO + species in interaction with the proton of the structure, the modification of the electronic environment of NO + could explain the shift in energy observed when compared to free NO + which is normally observed~ at 2330 cm z. IR vibrations for nitro and nitrito species are reported in the 1200-1400cm- region [ 17]. We attribute the bands observed at 1295 and 1384 cm -~ to a (- O-N=O) species stabilised in the acid structure in interaction both with the proton and the terminal oxygen of the Keggin units. _
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.(2
~
400 ~ The catalytic effects on the SCR of NO and NO2 over ), -A1203 and Au/A1203 are summarized in Table 1. The conversion of NO over Au/A1203 increased with temperature, reached a maximum of-~40% at 450 oC and then declined with increasing reaction temperature. This result is in accordance with that of Ueda et. al. [2]. Note that the maximum NOx conversions is attained at almost the same temperature as that of maximum NO oxidation to NO2 (see Fig. 1). The fall in activity at T> 450 ~ is due to increased competition of NOx and 02 for C3H6. At T> 450 ~ C3H6 was mostly oxidized to CO2. Appreciable NO SCR was also noted on y-A1203 at T> 450 ~ [4]. Both materials showed a high conversion of NOx to N2 with low formation of N20. When NO was replaced with NO2, the NOx conversion over Au/A1203 increased steadily with temperature reaching conversions up to 53% at 400 oC before gradually falling off with temperature. The NOx conversion over ),-A1203 increased steadily with temperature and remained near 100% at temperatures > 400 ~ The NOx conversion in the NO2+C3H6+O2 system was higher than that of the NO+C3H6+O2 reaction because NO2 easily activates C3H6 either by oxidation or nitration. When NO2 is used as feed, the low activity of Au/AI203 in comparison to that of y -A1203 stems from the fact that the fast oxidation of C3H6 on Au/AI203 prevents adequate
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secondary interactions. In addition, in this case Au masks some of the active sites on A1203 and makes them inactive for propagation reactions.
3.2. DRIFT studies 3.2.1. NO+1/202= NO2 reaction
The results from DRIFT studies for the NO+ 1/202=NO2 and NOx SCR are presented in Figures 2 to 4. Flowing NO+O2 on A1203 and 3%Au/A1203 resulted in the formation of
Table 1. Catalytic activities and major products from the SCR of NO and NO2 with C3H6 over ~,-A1203 and 3%Au/AI203 NO2+C3H6+O2
NO+C3H6+O2 Catalyst
T
SN2
SN20
SN2
SN20
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
200
4
0.2
85
15
7
1
67
33
300
6
2
97
3
25
6
87
13
400
8
3
90
10
92
32
92
8
450
18
15
92
8
96
51
91
9
500
34
68
76
24
99
67
88
12
200
2
0.3
86
14
3
1
70
30
3 00
3
1
99
1
16
5
90
10
(~
XNOx XC3H6
X N O x XC3H6
1t-A1203
3%Au/AI203
400
22
21
89
11
53
68
94
6
450
41
94
88
12
48
98
97
3
500
33
99
72
28
40
99
98
2
Conditions: NO, NO2 =940-970 ppm, C3H6=1000 ppm, 02=5%. GHSV= 27000 h-1. Where XNOx: NOx conversion, SN2: selectivity to N2 and SN20: selectivity to N20. intense bands characteristic of NO adsorption complexes, &g., nitro and nitrate groups in the region 1630-1300 cm -1, and N204 (1749 cm -1) [5,6], as shown in Fig. 2. The nature of these species was partly confirmed by doping A1203 with NaNO2 and NaNO3, which gave strong bands at 1436, 1329, and 1252 cm-1 characteristic of the -NO2- group vibrational modes, and at 1792, 1753, 1557, 1539, 1523 cm -1 corresponding to-NO3- groups. The intensities and stability of NO adspecies depended on exposure time and temperature. The adsorbates reached a steady-state condition after 30-60 minutes. The spectral series obtained from A1203 and 3%Au/A1203 when the NO+O2 flow at room temperature in Figs. 2a and b was replaced with He flow and the temperature raised to 300-400 oC are shown in Fig. 2c-e. Purging the samples in He (with flow rate 200 cm3/min) for 30 min at 25 ~ did not cause any significant changes in the spectrum of Fig. 2 a and b. By
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raising the temperature in the range 60-100 oC, the bands in the 1550-1350 cm -1 region disappeared progressively and disappeared completely at T> 200 oC. But as seen in Fig. 2c-e, the other sorbed NOx species were thermally stable and remained adsorbed, with slight band shifts, up to 450 oC. It seems probable that, in the presence of a reductant, such as propylene, these sorbed complexes may take part in the surface NO SCR.
r ==
0
"
l_ 2400
400o
I 1800
I 1400
1000
W a v e n u m b e r , (cm" 1) Fig. 2. Reaction of 1000 ppm NO+ 5% 02 at 25oc on A1203 (a) and 3% Au/A1203 (b) followed by He purge, 200 cm3/min, at 300-400 ~ A1203" 300 ~ (d)" 3% Au/AI203:300 ~ (c) and 400 o c (e). tscan = 2h.
3.2.2.
N O x + C 3 H 6 + O 2 reaction Figure 3 and 4 show the species observed during the NO+C3H6+O2 and NO2+C3H6+O2 reactions over Au/A1203. At T< 350~ the NO+C3H6+O2 reaction on Au/A1203 produced species such as adsorbed hydrocarbons (3000-2900 cm-1), CO2 (2349 cm-1), a strong cyanide band (2195 cm'l), CO (1952 cm-1), carbonate or carboxylate (1579, 1472-1458 cm-l), and formate species (1591, 1390-1377 cm-1), the latter not shown for brevity [3]. Elevating the temperature led to the decrease in intensity of the band at 2194 cm-1, and the appearance of a weak isocyanate band at 2232 cm -1, and a -CN band at 2128 cm -1. These bands were strongest between 350-450 ~ they disappeared from the surface at T> 500 ~ When 15NO was substituted for 14NO, the 2232, 2194 and 2128 cm" 1 bands appeared at 2216 and 2159 and 2097 cm -1, indicating that these features are due to species containing nitrogen atoms. The intensity of the bands in the 3000-2900 cm- 1 region, corresponding to CH vibrations in alkyl species, increased progressively with temperature, reached their maximum at ca. 300 ~ and then declined with further increase in temperature. Only slow
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NO+C3H6+O2 3%AulAI203
~ o
~
r O
eq it) (",l
~
~lt.
400 oC the -NCO band gradually diminished whereas -CN band at 2128 cm-1 intensified. These species remained on the surface up to 500 ~ where they disappeared. Although not shown, the exposure of A1203 to a NO2+C3H6+O2 flow at 300400 oC, in addition to the bands due to hydrocarbons, CO2, CO, carbonate or carboxylate, and formate bands, a peak at 2251 cm-1, due -NCO species adsorbed at A13+ sites was noted. Based on the above and our previous results [7] and those of others [8-10], in which the reactions of isocynate and cyanide species with NOx and 02 were examined, the following reaction scheme for the NO SCR with propylene in excess oxygen on A1203 and Au/A1203 is proposed. The initiation of NO SCR on Au/y -A1203 involves the oxidation of NO to NO2 followed by coupling of the NO2 and/or NO2 adsorbed complexes (that are fairly strongly held on Au/A1203 even at elevated temperatures)with activated C3H6, possibly allyl species, on active sites on A1203 to form CnHmNxOy species, which are responsible for the propagation step. Its subsequent internal rearrangement and decomposition leads to the formation of N2 and other products. 4. C O N C L U S I O N From the above results it is concluded that: The oxidation of NO to NO2 is a prerequisite for NO SCR to proceed on Au/A1203. NO2 and its adspecies NOx-, are primary intermediates. Interaction of allyl species with NO2 and/or its adspecies lead to the formation of nitrogen-containing organic species, such as -NCO and -CN which are secondary intermediates. Due to the high conversion of NO2 to N2, A1203 and Au/A1203 can be used as secondary components in multi-bed staged systems. REFERENCES
1. M. Iwamoto and N. Mizuno, Proc. Instn. Mech. Engrs., J. Automobile Eng., 207 (1993) 23.
2. A. Ueda, T. Oshima, and M. Haruta, Appl. Catal. B., 12 (1997) 81. 3. G.R. Bamwenda, A. Ogata, A. Obuchi, J. Oi, K. Mizuno, and J. Skrzypek, Appl. Catal. B, 6 (1995) 311. 4. H. Hamada, Y. Kintaichi, M. Sasaki, T. Ito and M.Tabata, Appl. Catal., 70 (1991) L15. 5. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley, New York, 1997. 6. N.D. Parkyns, Procced. 5 th Inter. Congress Catalysis, Palm Beach, (1972) 255. 7. G.R. Bamwenda, A. Obuchi, A. Ogata and K. Mizuno, Chem. Lett., (1994) 2109. 8. A. Obuchi, C. Wogerbauer, R. Koppel and A. Baiker, Appl. Catal. B: 478 (1998) 1. 9. H. Takeda and M. Iwamoto, Catal. Lett., 38 (1996) 21. 10. A.Y. Aylor, L.J. Lobree, J. A. Reimer, and A.T. Bell, J. Catal., 170 (1997) 390.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Transient study of NO, N20, C3H6 and C3Hs interactions over a silicasupported Pt catalyst P. Denton", Y. Schuurmanb, A. Giroir-Fendler a, H. Praliaud a, M. Primet" and C. Mirodatos b aLaboratoire d'Application de h Chimie ~ rEnvironnement (LACE); UMR 5634 (CNRS-UCB Lyon 1); 43, boulevard du 11 novembre 1918; 69622 Villeurbanne Cedex; France bInstitut de Recherches sur h Catalyse; UPR CNRS 5401; 2, avenue Albert Einstein; 69626 Villeurbanne Cedex; France The generation of N2 and N20 during NO decomposition and reduction on Pt/SiO2 was investigated by means of a TAP reactor. With rich mixtures the hydrocarbon serves to create free surface sites for NO decomposition and/or to induce a direct reaction of NO with carbonaceous residues, especially for C3H6. Furthermore, results of both transient and steadystate experiments indicate that adsorbed N20 is an intermediate leading to N2. I. INTRODUCTION Selective catalytic reduction of NOx by hydrocarbons (HC) in the presence of oxygen can be performed with platinum-based catalysts, but the activity-temperature window is narrow, and the selectivity toward N20 is high, at the expense of N2 [1-3]. The kinetics and mechanism of SCR-HC over Pt have already been studied. With C3H6 the reaction seems to involve dissociation of NO on reduced Pt sites [2-4]. A direct reaction between hydrocarbonderived adsorbates and NO has, however, also been considered [5]. N2 is often considered to be formed from two dissociated NO molecules, and N20 from one N atom and molecularly adsorbed NO, but the possibility that N20 is an intermediate leading to N2 is still under debate. In this study, we used a Temporal Analysis of Products (TAP) reactor to seek further information on the possible pathways. In particular, the formation of N2 and N20 during NO decomposition or reduction on Pt/SiO2 was examined.
2. EXPERIMENTAL The support was a mesoporous silica, supplied by Grace Davison, with an average pore radius of 3 nm and a specific surface area of 596 m2 g~. The Pt/SiO2 catalyst was prepared by wet impregnation of the support by Pt(NI-I3)4(OH)2,followed by calcination under air at 773 K (heating rate 1 K min~, plateau 8 h) and reduction overnight under H2 at 573 K (heating rate 2 K min~). The Pt content was 0.90 wt. % and the dispersion was 34 % (metallic surface area of 83 m 2 g~ and homogeneously dispersed, 3 nm diameter Pt particles). A TAP experiment consists of introducing narrow pulses of gaseous reactants in a continuously-evacuated microreactor. The response of each pulse is detected by a quadrupole mass spectrometer at the reactor exit. The shape of the response reflects diffusion, adsorption,
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desorption and reaction of the reactants and products. The principle of the TAP experiment and its applications are extensively described by Gleaves et al. [6-7]. Pulses of reactant (10 ~5to 5.5"10 is molecules each, for 1.8'1017 or 9.5"10 ~6 Pt,) were introduced until steady-state coverage was reached. The internal standard was usually Ar, but Xe was used for some of the experiments with C3I-I~. Several reactions were considered" - Adsorption and dissociation of NO (or N:O) at various temperatures on a reduced (bare) or oxidized surface. NO or N20 was pulsed and the responses of inert gas, NO, N20 and N2 were measured sequentially. The fragment at m/e - 14 was used to identify Ne. - Adsorption and reactions of NO on hydrocarbon-saturated surfaces. - Adsorption of HC (C3I-I6or C3I-h) on a bare, oxidized or NO-saturated surface. Conversions, selectivities, yields (i.e., products of conversion and selectivity) and mass balances were calculated from the areas under the response signals, with the calibration factor of each species taken into account. Residence times were also considered. In steady-state conditions at 478 K, the effect of space velocity was studied with a flow containing 4000 vpm NO-2000 vpm C3I-I6-5 vol. % O2-He (total flow 4 to 14 L h~). The conversions of NO into N:, N20 and NO2 were measured. Also, N20 reduction was studied between 423 and 773 K using the following feed: 1000 vpm N20-2000 vpm C3H6-7000 vpm O~-He (total flow 10 L hq). 3. R E S U L T S A N D D I S C U S S I O N 3.1. N O a d s o r p t i o n a n d d i s s o c i a t i o n on a b a r e s u r f a c e
0.6 0.5
5
/
0.4
z
/
f
f
Oo
/
~ o.3>-
f
/
0.20.1
0.0
-
0
I
I
I
I
50
100
150
200
pulse number
Figure 1. NO pulses at 573 K on a bare surface. Yields of N2, N20 and NO and oxygen surface coverage as a function of the number of pulses.
Before each experiment, the catalyst was reduced under H2 at 573 K until the end of H2 consumption and H20 formation, then degassed, leaving Pt~ (hence, a bare surface). NO was pulsed over this bare surface at temperatures between 323 and 723 K. Initially, high yields of N2 were observed. As the number of NO pulses increased, the formation of N2 decreased steadily, while N20 was detected aRer a certain number of pulses depending on the temperature (Fig. 1). After further introduction of NO, N20 production reached a maximum and the NO signal increased. 02 was never detected, which suggests that the O adatoms (O*) resulting l~om the
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dissociation of adsorbed NO (NO*) or adsorbed N~O (N20*) remained on the surface. As well, NO2 was never observed. The broad pulse response of NO indicated a slow desorption rate well beyond the time scale of the experiment (Figure 2). The surface coverages in O and NO (0o, 0so) were calculated. The total calculated NO surface coverage was well beyond one 1.0 NO monolayer since the NO desorption was not taken into account. The N2 0.8 formation of N2 and N~O depends on =. t~ surface oxygen concentration. With increasing 0o, N~ formation 0~= 0.6 10 decreased continuously. At a) ._. intermediate oxygen coverages, N20 "i 0.4 formation increased along with the concentration of molecularly 0.2 adsorbed NO (NO*), indicating that the latter species must be present on the surface for N20 to be formed. 0.0 This N20 formation passes through 0.0 0.1 0.2 0.3 0.4 a maximum and then drops off on an time (s) oxidized surface. 0t--
I
1
1
I
}
Figure 2. NO pulses at 573 K on a bare surface. Average responses for Ar, N20, N2 and NO as a function of time.
Surprisingly, the mean residence time of N20 was shorter than that of N2 (Fig. 2), so N2* was more strongly adsorbed than N20*. According to Burch et al. [8], N* may be a relatively stable surface species. By correlating selectivity and ease of desorption, we may consider the following reaction scheme. 2 NO (g)
N20 (g)
2 NO* ~
N20* + O* --> N2* + O*
1',~
1'$
N2 (g)
1'
At this stage, we cannot yet say i f N 2 0 is necessarily an intermediate leading to N2. NO was more strongly adsorbed than N2, contrary to the observations of Lacombe et al. [5] with a polycristalline Pt sponge catalyst. This can be explained by an influence of particle size. With temperature programmed desorption experiments (TPD NO) on several Pt/SiO2 solids, we have found, in agreement with Zafiris et al. [9], that larger Pt crystallites lead to weaker Pt-NO bonds. 3.2. NO adsorption on an oxidized surface
When NO was pulsed over an oxidized catalyst, no adsorption and no dissociation were observed, in agreement with the observations ofBurch et al. [3] and in contrast with those of Eckhoffet al. [4].
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3.3. NzO adsorption and dissociation on a bare surface
Pulsing N20 at 573 or 723 K over the bare catalyst led to decomposition with the formation of N2 and O adatoms (Fig. 3), contrary to the findings of Li and Armor [10]. The following reactions are thus considered: N20 (g) + * ~ - N20* and N20* --> N2 (g) + O*. As the number of pulses increased, 0o increased and N2 yield decreased, until the surface was saturated. N20 is not dissociated on a fully oxidized surface. NO was never observed (except as a fragment of N20), so the NO* ~ N20* transformation in the above scheme is irreversible. Figure 4 shows that N2 is formed more quickly from N20 than from NO.
1.0
1.0 N~O
0.8
0.8 -4
r
o
~
0.6
~
z
x, ~"0.4
0.2
0.0
0
.
6
0.4
/'
0.2
0.0 I
I
20
40
I
I
I
60
80
100
' I
120
pulse nurrber
Figure 3. N2Opulses at 573 K on a bare surface. Yields of N2, NeO and the oxygen surface coverage as a function of the number of pulses.
0.0
0.1
"
I
I
0.2
0.3
0.4
time (s)
Figure 4. N2 response curves at 573 K. A: N2 pulse, B: N2 producthgn from N20, C: N2 production.from NO.
3.4. Discussion. Formation of N2 and N20 N 2 0 c a n be formed either by recombination of N* and NO* or by recombination of 2 NO*. We can exclude the reactions 2 N* + O* --->N20* + 2* and N* + NO (g) ~ N20*. N2 is formed either by recombination of two N adatoms (N*) resulting from the dissociation of NO* or by decomposition of N20* into N2 (g) and O*. We note that, in steady-state conditions, N20 can be reduced by C3I-I6 into N2. In the presence of 7000 vpm 02, the conversion of N20 into N2 reached 10 % at 573 K, 60 % at 673 K and 75 % at 773 K. Furthermore, during NO reduction at 478 K, N2 selectivity depends on contact time, in contrast with the results of Butch and Watling [11 ]. The overall conversion of NO decreased (from 44 to 16 %) as did that of C3H6 (from 41 to 12 %) as the space velocity increased from 6 600 to 23 000 h4. The intrinsic rates of NO reduction into N20 and of oxidation of C3I-I6 by 02 were independent of contact time, but the intrinsic rate of NO reduction into N2 decreased
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1281
as the space velocity increased. Thus, N2 selectivity, defined as N2/(N2 + N20), was equal to 33 % at 6 600 hl and 2 % at 23 000 h l. We can conclude that adsorbed N20 is a precursor for N2 formation according to the reaction N20* ~ N2 (g) + O*. This does not mean that N20* is the only possible precursor for N2. N20*, however, is probably formed by recombination of 2 NO* rather than that of N* and NO*. If the latter were true, the N20 mean residence time would be close to that of N2. 3.5. Hydrocarbon adsorption and reactions At 573 or 723 K, hydrocarbons were adsorbed on a bare surface (0HC = 1 if we consider 1 C3 for 1 Pts) and regenerated the Ptfl sites on a surface oxidized by 02 or NO. CO2 (with its CO fragment) and H20 are thus detected. Mass balance calculations indicated a PtO2 stoichiometry for HC pulses on an O2-saturated surface and for 02 pulses on an HC-saturated surface. During HC pulses on an NO-saturated surface, CO2 and H20 were detected but not N2 or N20; therefore, the surface was saturated with O* but not with NO* or N20*. 3.6. NO pulses on an HC-saturated surface and comparison with NO on a bare surface For NO pulses on an HC-saturated surface, the formation of N2 and CO2 was observed, with less CO2 than expected from the C3H6/NO and C3Hs/NO reaction stoichiometries. This suggests that part of the surface is bare, while the rest is covered in carbonaceous residues, and so two different types of reactions can occur. Reaction (1) describes the behavior of NO on a bare surface, while reactions (2) and (3) describe its behavior on surfaces saturated with C3H6 and C3I-Is, respectively. Reactions (2) and (3) may occur either by direct reaction of N- and Ccontaining intermediates or by direct decomposition of NO following the cleaning-off of adsorbed oxygen by the hydrocarbon.
(1) (2) (3)
2 N O - ~ N 2 + 2 O* 9 NO + C3H6 ~ 4.5 N2 + 3 CO2 + 3 H20 10 NO + C3I-I8 -~ 5 N2 + 3 CO2 + 4 H20
For instance, at 573 K, the percentage of N2 arising from NO decomposition is higher for C3Hs (50 %) than for C3H6 (20 %). In both cases, part of the metallic surface remains free of HC residues, but the coke deposits arising from C3H6 leave fewer l~ee sites for NO decomposition than the deposits arising from C3Hs. As the number of NO molecules was increased between 0.3 and 3.6 times the number ofPt~ (9.5"1016), N2 and N20 production (yields, and duration of N2 formation) was compared for the bare surface and surfaces saturated with C3H6 or C3I-Is. For instance, at 573 K, on a bare surface or on a surface saturated with C3H6, the initial N2 yield is strong (Fig. 5), since the surface presents free Pt~ sites able to dissociate NO or reactive carbon species able to reduce NO. When the Pt~ sites are oxidized and/or CxHy species consumed, the N2 yield decreases and becomes weaker than with C3I-h. When the surface is saturated with C3I-h, carbon deposits are less reactive and N2 formation is observed for a long time with a medium N2 yield (Fig. 5). The duration of N2 formation obeys the sequence C3I-h > C3I-I6 > bare surface. Let us recall that, in the presence of 02 and in steady-state conditions, C3I-h is far less effective than C3H6 for SCR.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1282
4. CONCLUSION 0.8
0.6
"o (D
"~, 0.4 ol
C~HB
7
0.2
Ot
0
I
I
i
f
0.9
1.8
Z7
3.6
bid irt'c~__red__(rn:xada/em) Figure 5. N2 yields at 573 K as a function of the number of NO molecules (expressed as a function of the monolayer with 1 NO per PG).
With rich mixtures, it appears that two processes occur: NO (or N20) decomposition on reduced Pt, and NO reduction involving hydrocarbon-derived adsorbates (perhaps via the cleaning-off of adsorbed oxygen), the reduction process being more important with C3H6 than with C3I-h. In the absence of hydrocarbon, N2 is formed mainly by the decomposition of adsorbed N20. This N20 originates principally from the recombination of two adsorbed NO.
REFERENCES ~
2. 3. 4. ~
6. 7. ~
9. 10. 11.
M.D. Amiridis, T. Zhang and R.J. Farrauto, Appl. Catal. B, 10 (1996) 203. R. Butch and P.J. Millington, Catal. Today, 26 (1995) 185. R. Burch, P J. Millington and A.P. Walker, Appl. Catal. B, 4 (1994) 65. S. Eckhoff, D. Hesse, J.A.A. van den Tillaart, J. Leyrer and E.S. Lox, Stud. Surf. Sci. Catal., Vol. 116, Eds. N. Kruse, A. Frennet and J.M. Bastin, Elsevier 1998, p. 223. S. Lacombe, J.H.B.J. Hoebink and G.B. Matin, Appl. Catal. B, 12 (1997) 207. J.T. Gleaves, J.R. Ebner and T.C. Kuechler, Catal. Rev. Sci. Eng., 30 (1988) 49. J.T. Gleaves, G.S. Yablonskii, P. Phanawadee and Y. Schuurman, Appl. Catal. A., 160 (1997) 55. R. Burch, A.A. Shestov and J.A. Sullivan, J. Catal., 182 (1999) 497. G.S. Zafiris and R.J. Gorte, Surf. Sci., 276 (1992) 86. Y. Li and J.N. Armor, At~I. Catal. B, (1992) L21. R. Burch and T.C. Watling, Stud. Surf. Sci. Catal., Vol. 116, Eds. N. Knase, A. Frennet and J.M. Bastin, Elsevier 1998, p. 199.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) @ 2000 Elsevier Science B.V. All rights reserved.
Electrochemical Promotion of NO reduction by C3H6 on Rh/YSZ catalystelectrodes and investigation of the origin of the promoting action using TPD and WF measurements C. Raptis, Th. Badas, D. Tsiplakides, C. Pliangos and C.G. Vayenas University of Patras, Dept. of Chemical Engineering, Patras GR 26500, GREECE The effect of Electrochemical Promotion (or Non-faradaic Electrochemical Modification of Catalytic Activity- NEMCA) was used to promote the selective catalytic reduction of NO by propylene in the presence of excess oxygen. The catalyst was in the form of a Rh polycrystalline film, interfaced with Yttria-stabilized-Zirconia (YSZ, an 02- ion conductor). It was found that dramatic changes of the catalytic activity can be obtained under NEMCA conditions, accompanied by significant alterations in the product selectivity. The origin of the electrochemical promotion is discussed in the light of temperature-programmed desorption (TPD) and Work Function (WF) measurements. I. INTRODUCTION The catalytic performance (activity, selectivity) of metals interfaced with solid electrolytes can be affected significantly upon external current or potential application. The electrochemically induced change in catalytic rate can be several orders of magnitude higher than the rate of ion transport through the solid electrolyte. This phenomenon is known as Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) [1,2] or Electrochemical Promotion [3]. It has been studied for more than 55 catalytic systems [1,2] using various metal or metal-oxide catalyst-electrode films (Pt, Pd, Rh, Ag, Ni, IrO2, RuO2, etc.) interfaced with O 2-, Na +, H +, F-ionic conductors, mixed electronic-ionic conductors (TiO2, CeO2) [4,5] or aqueous alkaline solutions (LiOH, KOH) [6] for several groups of catalytic reactions (i.e., oxidations, reductions, hydrogenations, decompositions, etc.). Work in this area has been reviewed recently [2,7]. The importance of NEMCA in catalysis has been addressed by several authors [3,8,9] and a considerable number of research groups has made an important contribution in this area [10-14]. The observed promotional phenomena have been shown to be due to the electrochemically controlled reverse spillover of ionic species migrating between the solid electrolyte support and the catalyst surface [1,2,15,16]. Consequently, an "effective" electrochemical double layer [2] is, thus, established on the catalyst surface which changes the catalyst-electrode work function, eO, by A(e~) = eAVwR (1) where VWR, is the catalyst (working) - electrode potential with respect to the reference electrode. Eq. (1) has been confirmed by Kelvin probe [ 15] and UPS [ 17] measurements. The magnitude of Electrochemical Promotion is given by the Faradaic efficiency, A, and the rate enhancement ratio, p, defined from [ 1,2,7]:
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A = Ar / (I/2F) ; p = r/r~ (2) where, r and r ~ are the electrochemically promoted and the open-circuit (unpromoted) catalytic rate values respectively, Ar=r-r~ is the electrochemically induced change in catalytic rate,/, is the applied current and F is the Faraday constant. According to this definition, a catalytic system exhibits the effect of Electrochemical Promotion, when IA]>I [1,2,7]. A and 9 values as high as 3x 105 [16] and 150 [18] have been observed, respectively. In this paper the effect of Electrochemical Promotion was used to promote the NO reduction by C3H6, in the presence of excess oxygen. The origin of Electrochemical Promotion is also discussed, using Temperature Programmed Desorption of oxygen from Pt and work function measurements on Pt films deposited on YSZ during C2H4oxidation. 2. E X P E R I M E N T A L The experimental setup used for the Electrochemical Promotion of the NO reduction by propylene consists of the flow system, the reactor and the gas analysis unit. The reactor corresponds to the "single pellet" design [1] and has been described in detail elsewhere [ 1,2,7], together with the metal film deposition technique. Rh polycrystalline films deposited onto the one side of YSZ pellets serve both as the catalyst and as the working electrode, while two Au films, deposited on the other side of the disc, serve as reference and counter electrodes. All electrodes are exposed to the same gas mixture. In all cases a series of blank experiments was carried out to confirm that the catalytic activity of the Au electrodes was negligible in comparison to that of the Rh catalyst. Reactant and product analysis was performed by means of a Perkin Elmer S-300 gas chromatograph fitted with porapak Q (C3H6, CO2, N20) and molecular sieve (O2, N2, CO) columns. Nitrogen oxides (NO, NO2) concentrations were recorded on line by means of a Teledyne mod.911/912 NO/NO• analyzer. In addition, an Anarad infrared analyzer was used to monitor the CO2 concentration in the effluent stream, while currents or potentials were applied by means of an Amel 533 Galvanostat/Potentiostat. The temperature-programmed desorption experiments were carried out in an ultrahigh vacuum chamber (base pressure 10-1~ Torr after baking) equipped with a quadrupole mass spectrometer (Balzers QMG 420) and a differentially pumped gas inlet system. The polycrystalline Pt catalyst film was deposited in the form of a half-ring on the outer surface of a tubular YSZ specimen (OD 19mm, thickness 3mm, length 30mm). Au counter and reference electrodes were deposited on the inside wall of the tubular YSZ element opposite to the catalyst film. Details on the preparation and characterization of Pt and Au electrodes are given elsewhere [2]. A series of blank experiments showed that the TPD spectra reported here can be attributed to oxygen chemisorption on the catalyst film only. Constant currents between the catalyst film and the Au-counter electrode (galvanostatic operation) or constant potentials VWR between the catalyst film and the Au-reference electrode (potentiostatic operation) were applied using an AMEL 553 galvanostatpotentiostat. The specimen was heated radiatively using an Osram Xenon lamp located inside the tubular YSZ specimen. A type-K thermocouple attached on the catalyst film was used to measure the temperature. The temperature was varied linearly at a heating rate of 1 K/s, using Eurotherm programmable temperature controller. The experimental setup for the WF measurements during C2H4oxidation on Pt was similar to the one used for the electrochemical promotion experiments described above. Reactants were certified standards of ethylene (4.5% C2H4in He) and oxygen (20% O2 in He). Typical
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flow rates were around 200 mL/min STP with a gas composition of 15% 02 and 0.2% C2H4 in He. Reaction temperature was maintained at 400~ A Kelvin probe (Besocke/Delta Phi-Electronik, Probe "S"), with a 2.5 mm diameter gold grid vibrating electrode placed -~500 ~tm from the catalyst surface, was introduced in the reactor for measuring the catalyst work function. In the Kelvin probe'S' operation, the work function signal is drawn from the vibrating gold grid, so that the Kelvin probe lock-in amplifier circuit is entirely independent of the solid electrolyte cell circuit. Details of the electrical circuit have been given previously [ 19]. A three-electrode cell was used in all experiments. The electrolyte was a thin disk (25mm diameter and lmm thick) of YSZ. On the one side of the electrolyte the Pt catalyst film/working electrode and the Au reference electrode were deposited. Gold counter electrode was deposited on the opposite side. All electrodes were exposed to the reaction gas mixture. 3. RESULTS AND DISCUSSION 3.1 Electrochemical Promotion of NO reduction by C3H6 Rh metal catalysts in the form of polycrystalline films interfaced with Yttria- stabilizedZirconia were used in this investigation, in galvanic cells of the type:
C3H6, NO, 02, products, Rh / YSZ / Au, C3H6, NO, 02, products It was found that both the catalytic activity and the selectivity of the Rh catalyst-electrode is affected dramatically upon varying its potential with respect to the Au reference electrode. The rates of CO2 and N2 production are enhanced dramatically both with positive and negative potentials or currents. The induced changes in catalytic rates are typically 3 orders of magnitude higher than the rate I/2F of O 2- supply or removal to or from the Rh catalyst electrode. Thus, the Faradaic efficiency, A, was found to take values as high as 4000 and as low as-6000. The rate enhancement ratios of the three main products (COz, N2, N20) were also found to take very high values. Positive current (or potential) application leads to an up to 150-fold enhancement in the rate of CO2 production and an up to 60-fold enhancement in the rate of N2 production [18]. The Pco2=150 value is the highest reported so far in NEMCA studies utilizing YSZ. A typical galvanostatic NEMCA experiment is shown in Figure 1, which depicts the transient effect of a constant positive current application on the rates of formation of CO2, N2 and N20, on the NO conversion and on the catalyst potential, VWR. The open-circuit (unpromoted) rates correspond to t
•
z
The observed pronounced promotional phenomena can be rationalized by taking into account the effect of changing the potential, and thus the work function [1,2], on the chemisorptive bond strength of oxygen, NO and propylene. In particular, it appears that the pronounced weakening in the Rh=O bond, induced at high potentials [1], enables NO to adsorb and dissociate even in presence of significant amounts of gaseous oxygen. This may be of significant practical importance. Also the significant weakening in the C3H6 chemisorptive bond at negative potentials, may again enhance NO adsorption and dissociation, causing the observed electrophilic behaviour.
...
80
"
100
3.2
Temperature-Programmed
Desorption and Measurements
Work
Function
The creation of two types of chemisorbed oxygen on Pt surfaces interfaced with YSZ and subject to electrochemical promotion conditions is manifested clearly by temperatureprogrammed desorption (TPD). Figure 2 shows typical oxygen thermal desorption spectra obtained a~er gaseous oxygen adsorption at Po2 =4x10-6 Torr for 1800 s (7.2 kL) at 673 K followed by Fig. 1. Transient effect of a constant applied current on the catalytic rates of CO2, N2 and N20 production, on NO conversion (XNo) and on catalyst potential (VwR).
electrochemical supply of O 2, at a rate I/2F, at 673 K for various current application times, t~, indicated on each curve. Gaseous oxygen supply (dashed line, ti=0) creates a single adsorption state (Tp=743 K), corresponding to chemisorbed atomic oxygen. Additional electrochemical oxygen supply in presence of preadsorbed oxygen creates two oxygen states. Figure 2 provides a clear explanation of the NEMCA effect when using O z- conductors. Electrochemically supplied 02- produces strongly bonded "ionic" or "backspillover" oxygen (Tp=723 to 773 K) with a concomitant pronounced lowering (up to 50 K, i.e. from 743 to 693 K) of the desorption temperature of the more weakly bound atomic oxygen obtained via gas phase adsorption. This pronounced weakening in the Pt=O bond causes the observed dramatic catalytic rate enhancement in NEMCA studies, due to the very fast oxidative action of this weakly bonded oxygen. The strongly bonded oxygen state is significantly less reactive than the atomic oxygen and acts as a sacrificial promoter. Figure 3 shows the effect of Pt catalyst overpotential on catalytic rate and catalyst work function during CaH4 oxidation on Pt deposited on YSZ. The catalytic rate increases both with
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positive and negative overpotentials. However, the rate increase is more pronounced for positive applied potentials, exhibiting an electrophobic behavior [2]. 20
2
9
,,'""
i T=400"C I Po = 15 kPa
16
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4
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0.4
600
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800
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Fig. 2: TPD spectra after gaseous oxygen adsorption at 673 K and an 02 pressure of 4x10 -6 Torr for 1800 s (7.2 kL) followed by electrochemical 02- supply (I-+15 ~tA) for various time periods.
AVwg,V Fig. 3: Effect of catalyst ohmic-drop free overpotential on reaction rate (e) and on catalyst work function changes (I).
The catalyst work function was also monitoring during the catalytic activity measurements. As shown on Figure 3 the imposed changes in catalyst overpotential, AVwR, cause almost quantitatively the same changes in catalyst work function, A(e~). Some deviations from the experimentally observed [ 1,2] and theoretically explained [2] equality (eAVwR=A(eO), dashed line) are observed for high positive overpotential. Two factors could be responsible for these deviations: (a) Under these high rate conditions, the lifetime of backspillover oxygen ions is not long enough (due to their reaction with C2H4) to establish a full electrochemical double layer which would result to an one-to-one relation between catalyst overpotential and work function changes. In more oxidizing atmospheres, or with pure oxygen mixtures, this equality was found to be exactly valid. (b) Some insulating carbonaceous species could be deposited on the Pt surface during preparation or after carbon deposition reaction from gas phase reactants. Such an insulating species could act as a means of storing electric charge on the catalyst surface and thus lead to deviations from eAVwR=A(e~) [2]. 4. CONCLUSIONS The present study shows that Electrochemical Promotion can affect dramatically the reduction of NO by propylene, in the presence of oxygen. The catalytic activity and selectivity of the Rh catalyst-electrode is promoted very significantly upon varying its potential. The rate enhancement ratios, p, of the CO2 and N2 production were found to take values as high as 150 and 60 respectively, while minor catalytic rate enhancements of the N20
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formation were observed. This behavior leads to an enhancement of the product selectivity to N2. Particularly, imposition of positive potentials causes up to 2-fold enhancement of the nitrogen selectivity in the lower temperature range of the present investigation. From the TPD experiments it was found that electrochemical 0 2- pumping to Pt catalysts in presence of preadsorbed oxygen causes backspillover of large amounts of oxygen on the catalyst surface and leads to the formation of two oxygen adsorption states, i.e. a strongly bonded anionic oxygen along with the weakly bound atomic oxygen. The two oxygen species coexist on the surface. The enhancement in catalytic rate is due to the high reactivity of the weakly bonded oxygen while the electrochemically supplied strongly bonded oxygen is A times less reactive than the weakly bonded oxygen and acts as a sacrificial promoter. The WF measurements during C2H4 oxidation have confirmed that the catalyst electrode work function is dictated by the applied potential. This electrochemically induced change in WF affects the chemisorptive bond strength of all reactants and intermediates and is thus causing NEMCA. Acknowledgment: We thank the B RITE III programme for financial support. REFERENCES:
1. C.G. Vayenas, S. Bebelis, I.V. Yentekakis and H.-G. Lintz, Catal. Today, 11 (1992) 303. 2. C.G. Vayenas, M.M. Jaksic, S. Bebelis and S.G. Neophytides, in "Modem Aspects of Electrochemistry" (J.O'M. Bockris, B.E. Conway and R.E. White eds) Vol. 29, pp. 57-202 Plenum Press, NY, (1995). 3. J. Pritchard, Nature (London), 343 (1990) 592. 4. C. Pliangos, I.V. Yentekakis, S. Ladas, and C.G. Vayenas, J. Catal., 159 (1996) 189. 5. P.D. Petrolekas, S. Balomenou and C.G. Vayenas, J. Electrochem. Soc., 145(4) (1998) 1202. 6. S. Neophytides, D. Tsiplakides, P. Stonehart, M.M. Jaksic, and C.G. Vayenas, Nature (London), 370 (1994) 45. 7. C.G. Vayenas, and I.V. Yentekakis, in "Handbook of Catalysis" (G. Ertl, H. Knotzinger, and J. Weitcamp eds.), VCH Publishers, Weinheim, pp. 1310-1338 (1997). 8. J.O'M. Bockris and Z.S. Minevski, Electrochim. Acta, 39 (1994) 1471. 9. B. Grzybowska-Swierkosz and J. Haber, in Annual Reports on the Progress of Chemistry, Vol. 91, p.395, The Royal Society of Chemistry, Cambridge (1994). 10. T.I. Politova, V.A. Sobyanin and V.D. Belyaev, React. Kinet. Catal. Lett., 41 (1990) 321. 11. L. Basini, C.A. Cavalca and G.L. Haller, J. Phys. Chem., 88 (1994) 10853. 12. I. Harkness and R.M. Lambert, J. Catal., 152 (1995) 211. 13. P.C. Chiang, D. Eng and M. Stoukides, Interface, 139 (1993) 683. 14. E. Varkaraki, J. Nicole, E. Platmer and Ch. Comninellis, J. Appl. Electrochem.,25 (1995) 978. 15. C.G. Vayenas, S. Bebelis, and S. Ladas, Nature (London), 343 (1990) 625. 16. S. Bebelis, and C.G. Vayenas, J. Catal., 118 (1989) 125. 17. W. Zipprich, H.-D. Wiemhoefer, U. Voehrer, and W. Goepel, Ber. Bunsenges. Phys. Chem., 99 (1995) 1406. 18. C. Pliangos, C. Raptis, Th. Badas and C.G. Vayenas, Solid State Ionics, submitted (1999). 19. J. Nicole, D. Tsiplakides, S. Wodiuning and Ch. Comninellis, J. Electrochemical Soc., 144(12) (1997) 312.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
The reaction of propane with mixtures of NO, N20 and 02 over platinum, palladium and rhodium catalysts Dean C. Chambers, Dennys E. Angove and Noel W. Cant Department of Chemistry, Macquarie University NSW 2109, Australia The reactions of propane with Oz, N20 and NO over Pt, Pd and Rh catalysts have been investigated under near-stoichiometric conditions. In general, Pt is the most active catalyst for propane oxidation, followed by Pd and Rh. In individual tests, the reactivity of the oxidants varies widely, the order being 0 2 > > N20 > NO for Pt, 02 ~ N20 >> NO for Rh, and N20 >> NO or 02 for Pd, where the reaction involving O2 is dependent on pretreatment and history. Interactions between oxidants alter the behaviour when all three are present. The order of reactivity changes to 02 > NO > N20 for Pt, 02 > N20 > NO for Rh, and 02 > NO > N20 for Pd. The findings imply substantial competition between oxidants, especially with Pt and Pd. 1. INTRODUCTION The selective reduction of nitric oxide by alkanes in the presence of excess oxygen over supported platinum has received considerable attention because of its possible application for pollution control on lean burn engines [1,2]. Partial reduction of NO, resulting in the formation of a large proportion of N20 rather than N2, is a known problem [1,3]. By contrast there is very little information concerning this reaction during conventional three-way catalysis with near-stoichiometric air/fuel ratios. This is somewhat surprising given that alkanes are much less readily removed than either alkenes or carbon monoxide [4,5]. As a result the final stage of pollutant removal under chemically controlled conditions is likely to involve competition for alkanes by the three oxidants O2, NO and N20, the latter having been formed largely through the prior reduction of NO by CO. The aims of the present work were to determine the intrinsic characteristics of Pt, Pd and Rh for the reaction of one alkane (propane) with these oxidants both individually and in a mixture. 2. EXPERIMENTAL The 1.1 wt% Pt/SiO2 (40% dispersion) catalyst was from the batch designated 40-SiO2-PtC1-L [6], and the 0.5 wt% Pd/SiO2 catalyst (79% dispersion) from that designated 79.1-Pd/SiO2-IV [7], in the series prepared and characterised by Burwell et al. The 0.5 wt% Rh/SiO2 catalyst was prepared here in a similar way by incipient wetness impregnation of the same silica (Davison grade 62, 180-210 ~tm, 285 m2/g) with a solution of RhC13, followed by reduction in 10% H2/He at 350~ for 2 hours. TEM analysis showed that the Rh particles approximated 3 nm diameter spheres, which corresponds to a dispersion of ca. 40%. Comparison testing was carried out in a flow system with 75 mg samples of catalyst and a total flow rate of 100 cm3/minute (GHSV = 32000 hr -~) with helium as the carrier. The propane concentration was set at 200 ppm with, in individual tests, a very slight excess of the oxidant (approximately 1000 ppm when using 02 or 2000 ppm with either NO or N20). With
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mixed oxidants the ratio was also near stoichiometric with equal amounts of O atoms available from each oxidant (approximately 333 ppm 02, 666 ppm NO and 666 ppm N20). The temperature of the sample was ramped up and down several times, usually at 2~ per minute, to test reproducibility. Hysteresis, assessed from the temperatures required for 50% conversion was no more than 10~ after the initial run, except for some Pd/SiO2 experiments as noted later. The exit gas stream was sampled by a high-speed gas chromatograph (MTI model M200) for 02, Nz and C3H8analysis. Other species (NO, N20, CO2, CO, NO2 and H20) were analysed by FTIR spectroscopy (Nicolet Magna-IR 550), using a single-pass cell with 16 cm path length, and a least-squares spectral fitting routine [8]. 3. RESULTS Figure 1 shows the conversion of propane as a function of temperature in reactions with 02, NO and N20 separately over the Pt/SiO2 catalyst. Oxidation of propane is fastest with 02 and slowest with NO, N20 being of intermediate reactivity. Reaction of NO forms N20 with a sharp peak in production (460 ppm) at 285~ followed by a steep decline during the latter stages of NO removal. The conversion versus temperature profiles are significantly different when propane is reacted with the mixed oxidant feed (Figure 2). Reaction with 02 is delayed by approximately 50~ and NO reacts ahead of N20. Formation of N20 from NO results in a substantial temperature range over which the conversion of N20 is apparently negative (i.e. more N20 in the product than the feed stream) and N20 is the sole residual oxidant when the conversion of propane is complete. Approximate kinetic orders were determined to see if the shifts in reactivity evident between Figures 1 and 2 were attributable to the threefold lower concentration of each oxidant in the mixed feed. Measurements were made at low conversion holding the concentration of one reactant at the value used in the individual experiments of Figure 1 and varying the
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300
0 -200
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Temperature/~
Fig. 1. Conversions versus temperature using individual oxidants over Pt/SiO2 (ca. 1000 ppm 02 or 2000 ppm with NO or N20).
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Fig. 2. Conversions versus temperature using mixed oxidants over Pt/SiO2 (ca. 333 ppm 02, 666 ppm NO and 666 ppm N20).
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Table 1. Approximate kinetic orders for reactions of 02, NO and N20 with propane Temp./~ Pt/SiO2 Temp./~ Rh]SiO2 Temp./~
02 C3H8 NO C3H8 N20 C3H8
195
-1.4 1.9 0 0.1 1.4 -0.8
270 245
324
0.1 0.4 0 0.5 0.3 1.5
405 350
Pd/SiOz
326
1 -0.1 -0.9 1.1 0.7 -0.4
341 188
concentration of the other reactant 50% either side of the corresponding value. The temperatures used, and the orders obtained from the slopes of log-log plots of CO2 formation versus concentration, are summarised in Table 1. The reaction of propane with oxygen is strongly negative order in 02, in which case the lower concentration of 02 in the mixed feed would be expected to reduce the temperature needed for reaction contrary to that observed. Thus competition from nitrogen oxides, not concentration differences, is the cause of the slower reaction with O2 in the mixed feed. The reactions of the two nitrogen oxides with propane show different kinetic orders, near zero order in oxidant for NO but strongly positive order in oxidant for N20. Lowering the concentration should then favour the reaction of NO over N20, which may be a partial cause for the delayed reaction of N20 in the mixed feed. However, the steepness of the NO and N20 curves in Figure 2 compared to Figure 1 implies competition between them, and from oxygen as well in the case of NO. Figure 3 shows that the activity of the Rh/SiO2 catalyst for the separate reactions of propane with each of the three oxidants is lower than that of Pt/SiO2 for the corresponding reactions (Figure 1). The order of reactivity for the oxidants, 02 > N20 > NO, is the same but the reactivity of N20 is considerably closer to that of 02. NO is surprisingly unreactive, with conversion incomplete even at 500~ The peak yield of N20 during the reaction with NO
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,
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--A~-C3H, -200 500
Fig. 3. Conversions versus temperature using individual oxidants over Rh/SiO2 (ca. 1000 ppm O 2 or 2000 ppm with NO or N20 ).
--'-- 02 - - o - - N2 0
/
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20
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'
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Fig. 4. Conversions versus temperature using mixed oxidants over Rh/SiO2 (ca. 333 ppm 02, 666 ppm NO and 666 ppm N20).
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(90 ppm) is much less than with Pt/SiO2, seemingly because the greater reactivity of N20 relative to NO on Rh/SiO2 allows reaction of N20 concurrently with its formation. Figure 4 illustrates the behaviour of Rh/SiO2 for the oxidation of propane in the mixed oxidant situation. There is a small displacement of each curve to higher temperature compared with that required for the individual reactions, the shift being greatest for N20. This must be attributed to competition effects rather than differences in oxidant concentrations, since the kinetic order in the oxidant is near zero for each reaction (Table 1). The apparent conversion of N20 is never negative reflecting its relative ease of consumption in the presence of NO. Some propane remains unconverted at 500~ and the residual oxidant is predominantly NO. The behaviour of the Pd/SiO2 catalyst for the oxidation of propane by 02 alone was strongly dependent on the pretreatment and, to some extent, on the operating history. The two extreme situations corresponding to prereduction (in 1% H2 for 30 minutes at 350~ and preoxidation (in 1% O2 under the same conditions) are shown in Figure 5. The preoxidised state is much more active. In contrast to the reaction with 02, Pd/SiO2 was quite stable for the oxidation of propane by N20 alone, or by NO alone, with behaviour as shown in Figure 5. Unlike Pt and Rh, N20 is the best individual oxidant and the separation between its removal curve and that for the corresponding reaction of NO is also the largest. Even so, reaction with NO does yield significant N20, 270 ppm at the maximum, and N20 rather than NO remains when propane conversion is complete. This indicates that NO reacts in preference to N20 when both are present. The reaction of propane with the mixed oxidants showed hysteresis on the first cycle with higher activity during the initial ascending ramp. However it was stable thereafter with behaviour as shown in Figure 6. Oxygen reacts first with behaviour intermediate between that for the two situations involving 02 in Figure 5. N20 and NO react simultaneously but only after consumption of 02 is complete. The lower reactivity of N~O in the presence of O2 is particularly noteworthy. The temperature required for 50% conversion is raised by 200~ compared with that when N20 reacts alone (Figure 5) indicating very strong inhibition by 02.
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40
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--A-NO _A_,C3H, -200 400 500
Temperature/~ Fig. 5. Conversions versus temperature using individual oxidants over Pd/SiO2 (ca. 1000 ppm 02 or 2000 ppm with NO or N20).
O NO .
C3H 8
|9
-20 100
/
260'360'460'500
Temperature/~ Fig. 6. Conversions versus temperature using mixed oxidants over Pd/SiO2 (ca. 333 ppm 02, 666 ppm NO and 666 ppm N20).
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4. DISCUSSION
The situation with Pt/SiO2 is reasonably clear cut. The reaction orders for the C3H8/O2 reaction (Table 1) are strongly positive in propane and strongly negative in oxygen. This is the same as that reported by Yu Yao [9], who explained it in terms of a high but incomplete coverage by oxygen with competition between propane and oxygen for the remaining surface. The kinetic orders in oxidant for the other two reactions are different, near zero with NO and positive with N20. This indicates less complete oxygen coverage, probably because the dissociation rates of NO and N20 are lower than for 02, which also accounts for their slower rates of reaction (Figure 1). In the case of NO, the high yield of N20 points to a surface populated by a mixture of NO molecules and N atoms. Propane gains access at vacant sites before reacting with oxygen atoms. The situation with N20 is explainable by extension. Molecular adsorption of N20 is weak, leading to a situation in which the surface becomes dominated by partially dissociated hydrocarbon and competition occurs between C3H8and N20 for vacant sites. The reaction rate should then increase steeply with N20 concentration, and be inhibited by higher concentrations of propane, as observed. The order of reactivity when using the mixture of oxidants (Figure 2) follows similarly. The most readily dissociated one, 02, reacts first, although this seems to be hindered to some extent by competition from NO. Molecularly adsorbed NO may reduce the number of vacant sites available for propane adsorption. N20 starts to react only when all 02 is removed and when the temperature and NO concentrations are such that little molecular NO remains adsorbed. The kinetic orders for the reaction of propane with 02 over Rh/SiO2, fractional in propane and near zero in oxygen (Table 1), are also similar to those observed by Yu Yao [9]. The zero order in oxygen points to a fully oxidised surface. However, unlike Pt, the reactions with NO and with N20 are also zero order in oxidant over Rh suggesting that their dissociation rates are sufficiently fast to maintain high oxygen coverages. The rates of the 02 and NaO reactions are similar (Figure 3), as might be expected if the same steps (e.g. fragmentation of propane on nearly identical oxygen-covered surfaces) are occurring. The reaction with NO is slower, perhaps because the surface layer also contains nitrogen atoms isolated between immobile oxygen adatoms and this hinders attack by propane. The only difference with the mixture of oxidants (Figure 4) is that reaction of N20 is somewhat delayed, presumably because it is less readily dissociated than 02 on gaps created by the removal of reaction products. The situation with Pd is complicated by the possible formation of an oxidised phase which is marginally stable under reaction conditions. According to Yazawa et al. [ 10] the activity of Pd/A1203 for propane oxidation depends on the ratio of PdO to metallic Pd which is influenced both by temperature and the ratio of oxidant to reductant. Similarly, Carstens et al. [11] have shown that the presence of small areas of metallic palladium can enhance the activity of PdO for methane oxidation. The very small metal particle size of the Pd/SiO2 catalyst used here is likely to make it sensitive to transitions between metallic and oxidic forms and this is the probable reason for its variable behaviour. The kinetic orders observed here for the reaction of propane with 02 alone, near first order in oxygen and zero order in propane (Table 1), are almost the reverse of those found by Yu Yao [9], who used a strongly oxidising feed over preoxidised Pd/A1203. The present values suggest that the amount of oxygen on the metal is less than optimal. The kinetic orders for the reaction of propane with NO are quite different, near first order in propane and negative order in NO, indicating
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over-oxidation. Whilst surprising at first sight it is not unreasonable since the conversion of Pd to PdO is thermodynamically more favourable with NO than 02 due to the positive free energies of formation of nitrogen oxides in general. The reaction with N20, which occurs at a much lower temperature (Figure 5), shows different kinetics again, with fractional positive order in N20 and negative order in propane. The probable explanation is that the dissociation rate of N20 is never sufficient to produce an oxidised form of palladium. The metal surface then becomes partially covered by hydrocarbons, with the reaction proceeding when N20 dissociates on the remaining vacant sites. Increases in N20 concentration lead to an increased rate, while higher propane concentrations block more surface sites and hinder the reaction. The results for the mixed oxidants (Figure 6) are consistent with this picture. The higher dissociation rate of 02 transforms the system to a state in which N20 cannot react. The near plateau in the formation of N20 during the C3HdNO reaction indicates that N20 also has difficulty reacting in the presence of NO, as expected if the system is oxidic. In summary, the three reactions studied here show a variety of behaviours depending on the metal and oxidant. Considerable additional work with binary mixtures and Pd in particular will be required to build a complete understanding of them. Nonetheless, it is clear that Pt has by far the best performance when the oxidants are mixed. The final oxidant, N20, is largely removed below 300~ compared to 400~ with Pd and higher still with Rh. This confirms our previous conclusions with simulated exhaust mixtures that Pt is the key metal for minimising NaO emissions [5]. Loss of this activity may be a partial cause for the increases in N20 emissions seen as catalytic converters age on vehicles [12]. ACKNOWLEDGEMENTS This work was supported by a grant from the Australian Research Council. We are grateful to Professor R. Burwell for providing samples of the Pt/SiO2 and Pd/SiO2 catalysts. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
R. Burch and T.C. Watling, J. Catal. 169 (1997) 45. R. Burch and P.J. Millington, Catal. Today 26 (1995) 185. M. Inaba, Y. Kintaichi and H. Hamada, Catal. Lett. 36 (1996) 223. C. Howitt, V. Pitchon and G. Maire, J. Catal. 154 (1995) 47. N.W. Cant, D.E. Angove and D.C. Chambers, Appl. Catal. B 17 (1998) 63. T. Uchijima, J.M. Herrmann, Y. Inoue, R.L. Burwell, J.B. Butt and J.B. Cohen, J. Catal. 50 (1977) 464. B. Pitchai, S.S. Wong, N. Takahashi, J.B. Butt, R.L. Burwell and J.B. Cohen, J. Catal. 94 (1985) 478. D.W.T. Griffith, Appl. Spectrosc. 50 (1996) 59. Y-F. Yu Yao, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 293. Y. Yazawa, H. Yoshida, N. Takagi, S. Komai, A. Satsuma and T. Hattori, Appl. Catal. B 19 (1998) 261. J.N. Carstens, S.C. Su and A.T. Bell, J. Catal. 176(1998) 136. V.F. Ballantyne, P. Howes and L. Stephenson, SAE Technical Paper 940304
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Kinetics of NO reduction by CO on R h ( l l l ) : A molecular beam study F. Zaera and C. S. Gopinath Department of Chemistry, University of California, Riverside, Riverside, California 92521, USA A brief overview of results from isothermal kinetic studies on the reduction of NO by CO on rhodium (111) single-crystal surfaces is provided. These studies were performed under well-controlled ultra-high vacuum, but by using high-flux collimated molecular beams that reproduce catalytic conditions.
1.
General considerations
The experimental procedure used here for rate determinations in the NO+CO/Rh(111) system is based on exposing the clean rhodium surface, a specific premixed ratio of NO and CO, and on following the partial pressure of the different gases of interest as a function of time by mass spectrometry [ 1, 2]. As an example of the results obtained with this approach, Figure 1 shows raw kinetic data for the evolution of the partial pressures of N 2, N20 (not produced ever in these experiments), NO, CO, and CO 2 over time in the case of a 1:1 NO:CO ratio and a reaction temperature of 475 K. A series of actions are taken during these experiments, as follows: (1) At time t=l 0 s the NO+CO molecular beam is turned on with the flag in the intercepting position so the crystal is not yet exposed directly to the beam. At this point the reactants (NO and CO) are scattered throughout the vacuum chamber, and their partial pressures increase up to new steady-state values. (2) At approximately t=20 s the flag is removed from the path of the NO+CO beam in order to allow for its direct impingement on the surface. This causes both a decrease in the partial pressures of the reactants and an increase in the signals of the products during the transition from a clean surface to the steady-state. The dips in the CO and NO signals are proportional to their apparent sticking coefficient, and the CO 2 and N 2 pressures increase to reflect the rate of their formation. Notice that while a change in the CO 2 signal is seen immediately after unblocking of the beam, the signal for the formation of N 2 rises only after a delay of about 20 s from that point. (3) The system is let to evolve until a steady state is reached, which in general happens within 50 s after the unblocking of the beam. Neither the adsorption of the reactants nor the desorption of the products change with time in this steady-state condition. (4) During the steady-state regime the molecular beam is blocked and unblocked deliberately by raising and lowering the flag (at times t=70 and 90 s in this example, respectively) to check the reaction rate values. The clear increases in the partial pressure of the reactants and the accompanying drops in the partial pressure of the products are proportional to the steadystate reaction rates. (5) At about t=200 s the molecular beam is turned off. After the partial pressures of reactants return to their background levels, the crystal temperature is lowered below 250 K, and the surface is saturated with CO if titration of the O atoms left on the surface is to be performed. (6) Finally, the crystal temperature is ramped at a constant rate of 10 K/s to record the TPD traces for CO and NO (to measure the amount of any unreacted molecules), CO 2 (from CO+O recombination), N 2, and 0 2. The CO 2 (from the titration experiments) and N 2 TPD traces allow for the calculation of the coverages of O and N atoms that remain on the surface after stopping the steady-state reaction.
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II 15NO+CO/Rh(111) Kinetic Run Isothermal Molecular Beam Kinetics
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Steady-state
rates
Systematic steady-state studies were carried out as a function of surface temperature, N O + C O beam composition, and total beam flux [3]. A summary of the resulting rates is provided in Figure 2. A maximum in reaction rate is observed between 450 and 900 K, the exact temperature depending on the NO:CO beam ratio. A synergistic behavior is seen between increasing CO concentrations in the beam and higher surface temperatures. The desorption of molecular nitrogen was determined to be rate limiting for the overall NO reduction process. In particular, it was found that while the rate of CO 2 formation responds immediately to changes (blocking and unblocking) in beam flux, the nitrogen desorption traces do not (see Figure 1). 15NO + CO/Rh(111) Kinetics Steady-State Rate vs. Beam Composition and Temperature 0.02 %"
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Transient from clean R h ( l l l ) to steady-state
The transient of the NO+CO reaction from the clean Rh(111) surface to the steady state was studied as well [4]. The systematic variations observed in this transient correlate well with the overall steady-state reaction rates. Specifically, there is a time delay in the production of molecular nitrogen because of the need to build up a threshold atomic nitrogen coverage on the surface before the start of the reaction. The coverage of this nitrogen, as calculated by the time delay in the transient state, corresponds to that observed by TPD afterwards (see below). That coverage increases at a given temperature as the beam becomes richer in CO. Coverage differential parameters (aoT'aas), as defined by the difference between the coverages of the surface species (nitrogen and oxygen atoms) deposited during the initial stages of the reaction and those expected if steady-state were to be reached immediately after the start of the reaction, were found to correlate well with the behavior of the steady-state rates as a function of temperature and beam composition. The data for the case of nitrogen are shown in Figure 3.
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Initial sticking coefficients were also determined for both NO and CO in NO+CO mixtures as a function of surface temperature and beam composition [4]. The changes in S~ with temperature in particular are quite interesting, as they differ from those on clean Rh(111) [5]. While on clean rhodium the sticking coefficient of NO is temperature-independent below 800 K, in the presence of CO it increases both with NO:CO ratio and with temperature (Figure 4, left). This indicates that competition with CO alters the kinetics of NO adsorption. In general, the value of S~ in NO+CO mixtures is lower than that measured for pure NO (about 0.7), but becomes comparable for close to stoichiometric (NO:CO=1:3 to 1:7) mixtures and moderate (500-700 K) temperatures. A similar argument can be made for CO, for which the initial sticking coefficient is seen to decrease significantly above about 500 K (Figure 4, right). Ultimately, however, S~ and S~ only change by a factor of 2-3 over the most relevant temperature range, and account for a significant component of the variations of the overall NO+CO conversion rate in the steady-state regime only in the case of CO-rich beams.
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Figure 4. Initialsticking coefficients for the adsorption of NO (S~ left) and CO (S~ right) from NO:CO beams on Rh(111)surfaces as a function of surface temperature and beam composition. The total flux was kept constant in all these experiments at Fro,a~=0.50 ML/s. In general, s~ goes through a maximum at the conditions required for optimum surface stoichiometric NO and CO coverages, while S~ often starts to decrease at lower temperatures than S~ . As opposed to the weak dependence that reaction rates display on sticking coefficients, the differences in adsorption energies between NO and CO on Rh(111) are of great significance to the steady-state coverages, and as a consequence, to the steady-state rates. The adsorption of NO is clearly stronger than that of CO, as evidenced by the fact that CO is displaced by NO [4]. Assuming that optimum rates are reached with equimolar coverages of CO and NO on the surface (as inferred from steady-state measurements), the energy of adsorption of NO was estimated to be approximately 11.5+_1.0 kcal/mol higher than that of CO. This is the reason why the rate of NO+CO conversion is optimum with somewhat COrich beams. 4.
Surface coverages
The steady-state NO+CO conversion rates were found to be directly proportional to the coverage of atomic oxygen on the surface [6]. The relation between those rates and nitrogen coverage, however, proved to be much more complex. Two types of kinetically-different nitrogen atoms were identified on the surface (Figure 5). As mentioned above, the deposition of a critical coverage of strongly-bonded nitrogen is required for the start of the nitrogen recombination step to N 2. This threshold coverage is quite large at low temperatures, amounting to over half a monolayer around 400 K, but decreases abruptly above 600 K, and is quite insensitive to the ratio of NO to CO in the reaction mixture. An additional small amount of nitrogen appears to be present on the surface during catalysis but to desorb rapidly after the removal of the gas-phase reactants. The NO reduction rate displays an approximately first-order dependence on the coverage of these labile N atoms.
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recombination and decomposition to N 2 (Figure 6). NO+CO/Rh(111) Kinetics Surface Nitrogen Isotopic Exchange
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5.
Correlations
The results from the transient and steady-state measurements can be put together with the data for the surface coverages to come up with a more complete picture of the mechanism of the overall NO+CO/Rh(111) process. Figure 7 displays a selected example of this for a NO:CO beam ratio of 1:7.
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Funding for this research was provided by a grant from the National Science Foundation. 6. [1] [2] [3] [4] [5] [6]
References J. Liu, M. Xu, T. Nordmeyer and F. Zaera, J. Phys. Chem. 99 (1995) 6167. H. Ofner and F. Zaera, J. Phys. Chem. 101 (1997) 396. C.S. Gopinath and F. Zaera, J. Catal. 186 (1999) 387. C.S. Gopinath and F. Zaera, J. Phys. Chem. B (2000) in press. M. Aryafar and F. Zaera, J. Catal. 175 (1998) 316. F. Zaera and C. S. Gopinath, J. Chem. Phys. 111 (1999) 8088.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) @ 2000 Elsevier Science B.V. All rights reserved.
The surface migration of NOx adspecies as a factor determining the reactivity of supported Pt catalysts for the treatment of lean burn engine emissions G.E. Arena, I A. Bianchini, 2 G.Centi, 1'2 and F. Vazzana 1 i Dip. di Chimica Industriale, Universit& di Messina, Salita Sperone 31, 98166 Messina, Italy. Phone: +39-90-393134, fax: +39-90-391518, e-mail:
[email protected] 2 Dip. di Chimica Industriale e Materiali, University of Bologna, Italy. The dynamic of surface phenomena on a Pt(1%)/A1203 catalyst for the reduction of NO by propene/O2 has been studied by in situ DRIFT experiments and parallel transient reactivity data using the RWF (Rectangular WaveFront) method. The results indicate that NO strongly chemisorbs over Pt oxidized surface and this inhibits Pt reactivity towards hydrocarbon oxidation, but then progressively migrates over alumina surface from which can desorb as NO2. At the temperature of maximum activity in the reduction of NO, the rate of migration is slow and thus the hydrocarbon reacts with the nitrogen oxide adsorbed species over the metal surface, whereas at higher temperature the rate of migration is faster leaving free oxidized metal surface for hydrocarbon combustion.
Introduction Lean burn or diesel engines are characterized by a lower fuel consumption and C 0 2 emissions than current engines operating at a stoichiometric air/fuel ratio, but the presence of O2 in their emissions prevents the use of current "three way" type catalysts. It is thus necessary to develop catalysts able to reduce NOx to N2 in engine emissions containing oxygen. In the last years, the interest has been focused on the possibility of modifing conventional supported noble metal catalysts in order to improve their activity and temperature operative window in the reduction of NO to N2 in the presence of O2 and hydrocarbons [ 1]. This requires a better understanding of the catalytic phenomena and the dynamic of the adsorbed species [2]. The aim of this work is to contribute to the understanding of these questions using a combined approach of in situ DRIFT studies and transient reactivity investigation using rectangular modulations in the feed composition (RWF -Rectangular WaveFront method).
Experimental The Pt(1% wt.)/A1203 catalyst has been prepared using commercial "/-A1203 (RP 535) to which platinum has been added by incipient wet impregnation method using an aqueous solution of H2PtC16. After drying and calcination up to 550~ the catalyst was reduced at 400~ for 12 h with a flow of pure hydrogen and then reoxidized in mild conditions. In situ DRIFT studies have been made using an environmental diffuse reflectance chamber in series to a reactor for varying gas composition and connected to an on-line mass quadrupole detector for real-time analysis. Transient reactivity data using the RWF method have been made using a quartz fixed bed reactor. Inlet and outlet reactoi composition has been
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monitored by a mass quadrupole apparatus. Other details on the apparatus and conditions for the catalytic tests have been reported previously [3]. Results and Discussion , , o, , Steady-state activity. Figure 1 shows the cata400 . . . . . ~ + Conv. of NO lytic behavior of the Pt/AI203 sample used in - O - - Conv. of C3H6 --~ 9Conv. to NO2 this work in the steady-state conversion of NO 80 ~ Conv. to N20 in lean conditions and using propene as reductant (0.1% NO, C3H6]NO = 1, 5% O2, remain- =ing He; space-velocity 60.000 h1). In agree- .o 6o ment with literature data [4], the catalyst shows ,~ g a sharp maximum in the conversion of NO L) 40 (about 60%) centred at about 250~ (selectivity to Nz of about 50%), in correspondence of the 20 sharp increase in the conversion of propene. At higher temperatures the conversion of NO decreases and significatively increases the for0 200 300 400 mation of NO2. Reaction temperature, ~ Data reported in Figure 1 pointed out that Pt/A1203, which can be considered as a model Fig. 1 Catalytic behavior of Pt/AI203 in steadycatalyst for commercial supported noble metal state conversion of NO in the presence of three-way catalysts, is characterized from the NO and propene. presence of a sharp maximum in the conversion of NO to N2 centred at about 250~ Although this behavior can be described in kinetic terms and the model derived can correctly simulates the behavior of the performances of a light-duty diesel vehicle [5], a competition between NO and 02 chemisorption only at high temperatures must be assumed. This hypothesis do not agree with experimental evidences with labelled compounds [2] and thus a different reaction model must be considered. i,
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Transient reactivity. Transient reactivity data using the rectangular wavefront (RWF) method are summarized in Figure 2. In these experiments, in a flow of 0.1% NO in He a rectangular modulation in the oxygen concentration is made for about 3 min (RWF-O2 experiments), followed by a period of about 10 min using the first feed composition. The cycle is repeated several times, after which the NOx species remaining on the catalyst are analyzed either by thermodesorption in an inert flow or at the same temperature by sending a series of Hz-RWF modulations. Figure 2 (graphs al and a2) shows RWF-O2 experiments on Pt/AI203 catalysts at the temperature of the maximum in the activity of the catalyst in the reduction of NO (see Figure 1) during a sequence of cycles (for a better comparison, the time of the starting of the cycle was normalized to zero in all cycles). Graph al shows the conversion of NO as a function of time in the various cycles and the change of the oxygen concentration during a single RWF modulation. Graph a2 reports the corresponding conversion of NO to NO2 in the various cycles. The effect of the reaction temperature (in the 5th pulse) during these RWF-O2 tests is instead summarized in graphs b 1 and b2 of the Figure 2. No other N-contaning products were detected in these conditions. The catalyst was pretreated at 550~ in He, then at 350~ with H2 for 3 h and then reoxidized with 20% O2 in He at the same temperature to clean the surface. This cleaning procedure of the catalyst is used
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Fig. 2 RWF-O2 experiments on Pt/AI203: change of the conversion of NO, concentration of 02, and conversion of NO to NO2 during a sequence of RWF-O2 modulations at 240~ (graphs a) or during the 5thpulse at different reaction temperatures (240-350~ range; graphs b). prior of each sequence of cyclic RWF tests. In the first RWF modulation at 240~ (Graph al) there is a strong initial chemisorption of NO which does not corresponds to the formation of other N-containing species. NO2 starts to form only later and continues to grow during the 3 min of the RWF modulation. The conversion of NO reaches instead a constant value after about 1 min, although the conversion of NO is higher than the formation of NO2, indicating that a large part of NO still remains chemisorbed. Switching to the anaerobic flow after 3 min, the conversion of NO decreases to zero, apart from an initial negative conversion due to a partial desorption of the chemisorbed NO in the absence of 02 in the feed. In the consecutive RWF modulations, the behavior is quite similar, apart from a slightly lower initial chemisorption of NO and a progressive increase in the formation of NO2. The amount of NO chemisorbed during the first minute in the first modulations corresponds to a degree of Pt surface coverage of about 40% (assuming a 4 A 2 as the space occupied by a single NOx adspecies on Pt and a 100 m2/g dispersion of Pt). It is thus evident that the presence of the initial chemisorption in the consecutive pulses implies that the NOx species migrate from the Pt surface to the alumina from which can desorb as NO2. This explains the delay in the formation of NO2 and its progressive increasing formation with the number of modulations (graph a2). It was also verified that in the absence of Pt, the adsorption of NO on alumina is negligible. Increasing the reaction temperature above the maximum in the conversion of NO in the
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presence of propene/O2 (240~ the conversion of NO reaches quickly a nearly constant value (graph bl in Figure 2), as well as NO2 immediately starts to form (graph b2). Furthermore, the adsorption of NO (difference between the conversion of NO and the formation of NO2) is much lower in comparison with that observed at 240~ This change in the behavior can be reasonably interpreted as a faster process of migration of NOx species from Pt to alumina surface [3]. RWF-O2 experiments to ..-.. 100 analyze catalyst activity in pro- g pene oxidation at 240~ [temperature of the maximum in NO ~ 80 conversion to N2] are summarized in Figure 3. In these tran- " 60 sient reactivity tests, the flow is g changed from a flow on 0.1% w C3H6 in He to a flow of 0.1% ~" 40 C3H6 and 0.1% NO in He and zo back. ,o I 20
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When NO is absent in the feed, the conversion of propene quickly reaches 100%, when the 00 50 100 150 200 flow is switched to that conTime, sec taining oxygen. The trend of propene conversion follows Fig. 3 RWF-O2 experiments on Pt/AI203 at 240~ to analyze the transient reactivity of propene in the absence and presence closely the change in oxygen of NO during O2 RWF pulses. concentration. When NO is also present in the feed, a clearly distinct behavior is noted. In this case, initially the conversion of propene follows that in the absence of NO (up to about 60% of conversion), but then conversion of propene decreases following a trend close to that of the conversion of NO. These results show that catalyst reactivity is deactivated by the cofeeding of NO, due to the formation of strongly bound oxidized nitrogen oxides over platinum surface. The analysis of the thermodesorption of NO (using He as the carrier gas) after the RWF-O2 experiments indicates that different type of nitrogen oxide species remain adsorbed on the catalyst depending on the presence or not of cofeed propene. While in the absence of C3H6, a broad peak centred between 350-400~ is observed, a less intense peak centred between 250-300~ is detected when propene is cofeed together with NO. This pointed out that the nitrogen oxide species desorbing at the higher temperature is probably consumed by the reaction with propene. Temperature programmed desorpton experiments [3] suggest that these species can be related to NOx species on Pt surface. m
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In situ DRIFT studies. In situ DRIFT experiments of the formation of NOx adspecies during experiments similar to those shown in Figure 2 (graphs a) are reported in Figure 4a. In the absence of oxygen in the feed, minor amounts of NOx species form on the catalyst. The NOx species are characterized by bands at 1470 and 1330 cm "], and 1230 cm -], indicating the presence of linearly coordinated and bridging nitrito species (probably on Pt surface), respectively. A band at 1581 cm ] is also observed, assigned to bridged NO on Pt. No bands due to linearly coordinated NO, either partially negatively or positively charged, could be detected in
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0.2% NO 95 rrin 0.2% NO 200 rain 0,6 ..... 0.2% NO + 0.8%0 2 1 min 0.2% NO + 0.8% 0 2 10 min 0.2% NO + 0.8% 0 2 65 min --'--
8 r
m ..Q
,-. 0,4
O r ..Q
is exchanged with two Na § For references, all catalysts tested in this study are listed with their preparation methods and abbreviations in Table 1. The phases present in the catalyst were identified by X-ray powder difraction (XRD) using Cu-Ka radiation. The Specific surface area of the catalysts, which is also listed in Table 1, was determined by the nitrogen adsorption at -196~ after the catalyst was degassed at 300~ for 3 h in a high vacuum. To measure the catalytic activity, The calcined powder was pressed into pellets, then crushed, and sieved to 40 - 80 mesh size. The catalytic activity measurements were carried out using a fixed-bed flow system with a quartz tubular reactor (10 mm i.d.) under atmospheric pressure. The exhaust gas from the reactor was analyzed by gas chromatography using a molecular sieve 5A column for N~ and O~ and a Porapak Q column for N~O. The catalytic activity for NO decomposition was evaluated in terms of the conversion of NO to N~ (2[N~]out / [NO]in). _
Table 1 Catalysts tested in this study. Catalyst
Preparation method a~
Abbreviation
SrTio.8Fe0.203 La0.8Sro.2CoO3 MgO 10 wt% STFO/MGO 20 wt% STFO/MGO SrTio.sFeo.203 Cu/ZSM-5
SP SP Calcination Impregnation Impregnation AL Ion-exchenge
STFO-SP LSCO-SP MGO STFO10/MGO STFO20/MGO STFO-AL CZSM
a)See the text
BET area (m 2 g-~) 7.8 5.7 30.8 18.7 16.1 16.8
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3. RESULTS AND DISCUSSION
3.1 Crystalline phases of the catalysts The X-ray diffraction analyses indicated that STFO-SP and LSCO-SP were single-phase perovskites. Fig. 1 shows the XRD patterns of the MgO support (MGO), the supported catalysts (STFO 10/MGO and STFO20/MGO) and the unsupported catalyst (STFO-AL). The pattern of MGO showed five distinguishable XRD diffraction peaks at 20 = 37.0 ~ 42.9 ~ 62.3 ~ 74.7 ~ and 78.6 ~ This pattern was assigned as that of MgO having the rock salt type structure. Some peaks other than those observed in the XRD pattern of MGO appear in the XRD patterns of the supported catalysts. These peaks were commonly found in the XRD pattern of STFO-AL. The intensities of these peaks were roughly proportional to the perovskite loading. Thus, the formation of particles of the single-phase perovskite-type oxide on the MgO support was confirmed. The crystallite sizes of SrTio.sFeo.203and the MgO support calculated by using the Scherrer equation are listed in Table 2.
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20 (degrees) Fig. 1. XRD patterns (Cu-K(~) of the supported SrTi0.sFe0203 catalysts, the unsupported catalyst and the MgO support. (a) MGO, (b) STFO 10/MGO, (c) STFO20/MGO, (d) STFO-AL.
3.2 Catalytic performance in the presence of coexisting gases To study the effects of two coexisting gases, H20 vapor and 02, on the NO decomposition activity, parallel tests were carried out for the two catalysts, STFO-SP and CZSM, under comparable conditions except for the reaction temperature. The results for the effect of H20 vapor are shown in Fig. 2. The test gas of only 1% NO/He
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Table 2 The crystallite size of the MgO support and the SrTio.sFeo.203 caculated using the Scherrer equation. Catalyst
Crystallite size (nm)
MGO STFO 10/MGO STFO20/MGO STFO-AL
MgO
SrTio.sFeo._~O3
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C
Fig. 2. Effect of O~ on NO decomposition activity in 1% NO/He at 800~ for STFO-SP and 500~ for CZSM; W/F - 0.6 g s cm-3; (A) in the absence of H~O vapor, (B) with l0 vol.% of H~O, (C) in the absence of H~O vapor after exposure to H~O. was introduced for 10 h (A). When 10 vol.% of H~O was added to the test gas, the conversion over CZSM decreased significantly while STFO-SP maintained half of the initial activty over the next 5 h (B). Then, the H,O was removed from the stream. STFO-SP recovered the activity nearly to its initial level while CZSM recovered only half of the initial activity (C). These results suggest that STFO-SP is more stable against H~O vapor than CZSM in spite of the higher reaction temperature. The procedure for testing the effect of O, was the same as the test for the H,O vapor except for the step B. The results are shown in Fig. 3. After the step A, when the test gas of 1% NO/He was swiched to the mixture of NO (1%) and O~ (10%) balanced with He, STFO-SP still showed higher conversion. However, the degree of poisoning by 02 is greater for STFO-SP than the poisoning by H,O vapor. When the test gas was returned to 1% NO/He after 5 h residence at step B, both catalysts completely recovered the activity to their initial levels. It was found that the coexisting O 2 poisoned the NO decomposition activity of STFO-SP more than the coexisting H20 vapor did.
3.3 Catalytic activity of supported catalysts To improve the NO decomposition activity, especially in the presence of excess O 2, we tried to support SrTio 8Fe0203 on MgO and studied the effects of the supporting on the NO decompo-
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0
=
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>
20
~0,,,,q
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B
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Fig. 3. Effect of O, on NO decomposition activity for STFO-SP at 800~ and for CZSM at 500~ W/F = 0.6 g s cm-3; (A) 1% NO/He, (B) 1% NO + 10% O,/He, (C) 1% NO/He after (B). sition activity. Fig. 4 shows the comparison of NO decomposition activity per unit weight of SrTio.sFeo.203 between the supported and unsupported catalysts. STFO 10/MGO showed almost 100% of conversion of NO under this typical reaction condition, although it must be taken into consideration that the NO conversion of the MgO support (MGO) is 11% under the same reaction condition. The weight-normalized activity of STFO 10/MGO was about two times as high as that of STFOAL. The activity is considered to depend on the crystallite size of SrTio.sFeo203 listed in Table 2, since the activity decreased with the increase in the crystallite size. It is interesting that the NO conversion of STFO-AL was about half of that of STFO-SP in spite of larger BET area of STFOAL than STFO-SP. This suggests that the differnce in preparation method can greatly influence the NO decomposition activity. Fig. 5 shows the NO decomposition activity as a function of contact time for STFO 10/ MGO, STFO-SP, LSCO-SP and MGO. Fig. 6 shows the NO decomposition activity as a function of the coexisting O, concentration. The degree of the deactivation was found to be greatly _
100
m Z
80
0
o z
60
0
40
=
~0,,,,q
20 0
STFO 10 /MGO
STFO20 /MGO
STFO-AL
STFO-SP
MGO
Fig. 4. Effect of MgO-supporting on NO decomposition activity. W / F - 3.0 g s c m -3, 1% NO/He balance, 800~ Closed bars: W is weight of SrTio.8Fe0.~_O3,Open bar: W is weight of MGO.
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5
02 concentration (%) Fig. 6. Effects of O, concentration on NO decomposition activity. W (Perovskite)/F- 0.6 g s c m -3, 1% NO/He balance, 800~
reduced by the supporting. The conversion of the supported catalyst in the presence of 10% O~ is as high as 60%, which is the same level as that of the unsupported catalyst in the absence of O~. In this way, the impregnation of the catalysts supported on MgO was found to be effective to reduce the deactivation caused by the coexisting O~ as well as to increase the NO decomposition activity. _
4. ACKNOWLEDGMENTS This work was performed as a part of the national project "Development of Ceramic Gas Engine" supervised by the Japan Gas Association. Financial support from MITI (Ministry of International Trade and Industry, MITI) is greatly acknowledged. REFERENCES
1. J.W. Hightower and D.A Van Leirsberg, The Catalytic Chemistry of Nitroge Oxides, R.L. Klimish, J.G. Larsonv, Eds., Plenum Press, New York, 1975, p. 63. 2. M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya and S. Kagawa, J. Chem. Soc., Chem. Commun., (1986) 1272. 3. M. Iwamoto, H. Yahiro, K. Tanda, Stud. Surf. Sci. Catal., 37 (1988) 219. 4. Y. Yokoi, I. Yasuda, H. Uchida, O. Okada, Y. Nakamura and S. Kawasaki, 2 "d world Congress on Environmental Catalysis, Miami Beach, 1998. 5. Y. Teraoka, H. Fukuda and S. Kagawa, Chem. Lett., (1990) 1. 6. Y. Teraoka, S. Kagawa and T. Harada, J. Chem. Soc., Faraday. Trans., 94 (1998) 1887. 7. C.B. Martin, R.P. Kurosky, G.D. Maupin, C. Han, J.Javadapour and I.A. Aksay, Ceramic Powder Science ~ , G.L. Messing and S. Hirano Eds., American Ceramic Society, Ohio, 1987, 8. Y. Yokoi and H. Uchida, Catal. Today, 42 (1998) 167. 9. M. Iwamoto, H. Yahiro, Y. Mine and S. Kagawa, Chem. Lett., (1989) 213.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
CO and NO elimination over Pd-Cu catalysts M. Fermindez-Garcia, a'* A. Martinez-Arias a, J.A. Anderson b, J.C. Conesa a, J. Soriaa a Instituto de Cat~lisis, CSIC, Campus Cantoblanco, 28049-Madrid, Spain b Dept. of Chemistry, University of Dundee, Dundee DD 1 4HN, Scotland, UK The behaviour of a series of Palladium and Palladium-Copper catalysts supported on ceria/alumina and ceria-zirconia/alumina for the CO+NO+O2 reaction has been analysed by a combination of Electron Transmission Microscopy, Infrared and Electron Paramagnetic spectroscopies and catalytic test studies. In both systems, the catalytic behaviour is dominated by the properties of the Metal-Support Interface. While Palladium systems show a similar behaviour in both types of supports, Cu only modifies the behaviour of the ceria/aluminasupported one. 1. I N T R O D U C T I O N The strengthening of exhaust emission legislation expected for the XXI century will make necessary the improvement of the actual catalytic converter technology in automobiles. In stoichiometric conditions, low light-off catalysts, which start working earlier by decreasing the initial temperature of operation, constitute one of the most promising ways explored recently to achieve this objective [1,2]. As lead and sulphur levels have become lower in fuels, the use of palladium has increasingly attracted the attention of the catalytic community in order to develop efficient catalysts for hydrocarbon and CO oxidation at low temperatures [1,3]. However, the performance of Pd-based systems to eliminate NO requires usually improvements to become industrially applicable [1,2]. As it is well known, substitution of Rh in three-way catalysts (TWCs) is another desirable objective from an economical point of view and promotion of Pd with a second "active" metal has been revealed as a useful way to achieve this goal. Cu has been shown to enhance significantly the NO elimination activity of Pd in typical ceria/alumina three-way catalysts (TWC) [4]. Here we will extend the analysis of Pd and Pd-Cu loaded systems, comparing the chemical behaviour and catalytic performance of ceria/alumina and ceria-zirconia/alumina supported catalysts in the CO+NO+O2 reaction. 2. EXPERIMENTAL A T-A1203 powder (Condea), here designated A, was used as base support. The CA support was prepared from it by incipient wetness impregnation with aqueous cerium(III) nitrate, in an amount such as to give a final CeO2 content of 10 wt.%. The ceriazirconia/alumina (CZA) was prepared using zirconyl nitrate and cerium(III) nitrate (Ce:Zr ratio 1"1) in an inverse microemulsion (water in a heptane organic solvent, using Triton X-100 as surfactant and hexanol as cosurfactant). The amount of the alumina support required to achieve 10 wt.% of CeZrO4 was added to this microemulsion prior to mixing with a similar emulsion containing tetramethylammonium hydroxide as base. The resulting suspension (with all Ce and
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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Zr coprecipitated) was stirred for 24 h, centrifuged, decanted and rinsed with methanol. After drying at 353 K and calcination of the CA and CZA precursors in air at 773 K, all supports displayed BET surface areas similar to the parent alumina (180 m 2 g-l). A microscopy (TEM/diffraction) analysis of the CZA material shows that it contains particles of a 1:1 Ce-Zr mixed oxide phase of size around 20 A, homogeneously dispersed in the alumina support [5]. Fractions of the above support materials were (co)-impregnated with Cu(II) nitrate and/or Pd(II) nitrate and further calcined at 773 K. Metal loadings were 0.5 wt.% of Pd and 1.0 wt.% of Cu. On samples pre-calcined at 773 K, T-programmed catalytic tests at 10 K/min using a 1% CO + 0.1% NO + 0.45 % 02 (N2 balance) mixture at 30.000 h -~ were performed in a glass reactor system, coupled with a Perkin-Elmer 1725X FTIR spectrometer for on-line analysis of the gas phase composition. Characterisation of samples was carried out using a JEOL 2000 FX (0.31nm point resolution) electron microscope equipped with a LINK (AN 10000) probe for EDS analysis. DRIFTS analysis of adsorbed species present on the catalyst under reaction conditions was performed using a Perkin-Elmer 1750 FTIR fitted with an MCT detector. EPR spectra were recorded at 77 K with a Bruker ER 200 D spectrometer operating in the X-band and calibrated with a DPPH standard (g=2.0036). In this last case, samples were handled using a conventional dynamic high-vacuum line. In all cases, they were subjected to calcination pretreatment under 300 Torr of Oz at 773 K. Then they were reduced in 100 Torr of CO at 423 K followed by outgassing at the same temperature. Other sample aliquots were subjected to the same reduction treatment, aRer which they were treated with 10 Torr of NO at 423 K and thoroughly outgassed at room temperature (RT). After both treatments, EPR data were obtained following oxygen adsorption (70 ~tmologl ) plus 30 min outgassing at 77 K.
3. RESULTS 3.1. Catalytic tests The light-off behaviour of Pd and PdCu specimens is depicted in Fig. 1. For CO conversion, copper addition improves Pd performance on both types of supports, CA and CZA, although at high temperatures the CZA-supported materials show only a marginal effect. For NO, bottom of Fig. 1, only CA-containing materials present an enhancement by addition of Cu. In both mono and bimetallic systems, the CA-supported catalysts happen to be more effective than those using CZA (and these more than those on alumina) for both CO and NO elimination, except in the case of NO conversion in the high T range, in which the monometallic CZA-supported catalyst is more active than the CA-based one. Monometallic Cu catalysts gave always conversions (data not shown) much below the Pd-Cu ones.
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!
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Tem perature (K) Fig. 1. CO and NO conversion profiles for the C O + N O + O 2 reaction over Pd and Pd-Cu catalysts.
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3.2. Drifts data
The Microscopy study (electron diffraction and EDS) was not able to detect any Cu or Pdrelated signal in fresh or used samples, indicating a high initial dispersion (particles are estimated to be smaller than 10A) and the absence of strong sintering processes during reaction 9 The presence of these active metallic phases can be however detected by DRIFTS under reaction conditions. For monometallic Pd samples (Figs. 2A,B), spectra taken at low temperature show mainly (overlapping the two-lobed CO gas band) a band located at 2160 cm -l, attributable to Pd 2+ carbonyls [6]. For Pd/CA, it disappears at 363 K, indicating palladium reduction, being formed again at higher temperatures, coinciding with the onset of NO reduction. The same band is also present in sample Pd/CZA, but only at low temperatures (< 363 K). A band at 1970-1960 cm l evidences the presence of CO bridging two Pd(0) atoms [7] for both CZA-supported samples (Figs. 2B,C); the bimetallic one displaying it only at T > 425 K. For Pd/CZA, an additional contribution was also detected initially around 2090 cm -~, with a downward shit~ of 30 cm q at increasing temperatures; this band is assigned to CO ontop of Pd(0) atoms [7]. A common feature to all spectra is the presence of NCO species, giving bands at 2230 and 2255 cm l [8]. Notice that this species has been postulated as a real intermediate in NO reduction [4]. In agreement with this, Figs. 1 and 2 display a correlation between the NO light-off behaviour and the temperature range of existence and band intensity of NCO species. This species appears from 393 K for Pd-Cu/CA (spectra not shown), together with strong peaks in the 2150-2100 cm -l range which, like the similar ones observed here for Pd-Cu/CZA, are mainly due to Cu+-CO complexes [4,9]. .
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2100
2000
Wavenumber (cm "~)
1900
Fig. 2. IR spectra of A) Pd/CA, B) Pd/CZA, and C) Pd-Cu/CZA in a) O 5 flow at RT, then in a CO+NO+O 2 flow at b) 303, c) 363, d) 423, and e) 453 K.
I
I
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I
I
2300
2200
2100
2000
1900
Wavenumber (cm i) Fig. 3. IR spectra after CO adsorption at RT on used catalysts.
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After running the CO+NO+O reaction up to 523 K (and 473 K for the ceria/alumina based systems) and purging in N2 at the same temperature, CO adsorption at RT (Fig. 3) gave rise to bands at 2090 and 1970-1960 cm ~ attributable to metallic Pd in the monometallic and Pd-Cu/CZA specimens. This evidences the presence of Pd metallic particles after this treatment; these bands however seem to be absent in the CA- and A-supported bimetallic catalysts, a fact ascribed to the formation of alloyed Pd-Cu particles [4], which has been confirmed from in-situ T-programmed XANES data [12]. All Cu-containing samples yield bands in the 2150-2100 cm -~ range, characteristic, as mentioned before, of CO adsorption on Cu + and Cu 2+ centres [4,9].
3.3. EPR data The EPR results show that oxygen adsorption on ceria-containing catalysts reduced in CO leads to formation of superoxide radicals (Ce4§ -) following a process that can be envisaged as Ce3+-V~ + 02 ---->Ce4+-O2", where V ~ denotes a doubly ionised anion vacancy. Evaluation of the intensity and nature of such species gives information on the degree of reduction achieved by the sample and on the nature of the reduced centres involved in this process [4,10]. For the three samples examined, the spectra are mainly formed by similar superoxide signals in which the Ce3+-V ~ adsorption centres belong to two-dimensional entities dispersed on the alumina surface; these are characterised by presenting gz < 2.028 and gx > 2.017 (with gy = 2.012), in contrast with species formed on ceria and/or zirconia-ceria mixed oxide aggregated phases [10,11]. As shown in Table 1, more cerium ions are reduced by CO in the Pd-Cu/CZA case. However, NO interaction with the anion vacancies created by CO reduction, which leads to destruction (reoxidation) of part of them, is more efficient for Pd-Cu/CA, for which a very low intensity (disallowing quantification) is observed after NO treatment. For the Pd-Cu/CZA sample, actually, reaction with NO produces a degree of decrease in the number of O2-forming centres somewhat smaller than for Pd/CA. Thus, it would seem that the reactivity of these centres towards NO is lower on the CZA-supported specimens. Table 1. Concentration of superoxide species (btmol per gram of sample) detected after oxygen adsorption at 77 K on the samples treated in CO (+ vacuum) or in CO (+ vacuum) and subsequently in NO at 423 K; the ratio between both magnitudes is included as well. Sample
Intensity (101 [tmol.g q) [O2] co [O2] co+so
Pd-Cu/CZA 5.2 2.9 Pd-Cu/CA 1.2 n.e.* Pd/CA 2.2 0.9 n.e. not evaluated due to the small intensity of the signal
[O2"]CO+No/[O2-]CO 0.56 250~ relative to 5mo1%. The effect of high-temperature aging in automotive exhaust on some of the catalysts was simulated by heating them at 1050~ for 12h in a mixture of gas, which was alternatively oxidizing and reducing (redox aging) [2]. The evaluation of oxygen exchange properties was performed as before, using the pulsed CO/O2 method, but due to an overall reduction in oxygen storage capacities, the evaluation was commenced at 200~ rather than 50~ The results are shown in Table 5 together with additional values taken at 500~ Still, the catalyst made from pure praseodymia-ceria has generally the largest capacity, followed by the catalysts made from praseodymia-ceria materials with 5mo1% Y, Zr, and Ca (in that order), then 20mo1% Y, and finally, 10mol% Zr. In the last instance, only at 350~ does the catalyst made from the zirconium-containing praseodymia-ceria mixed oxide have a higher capacity than that made from the commercial ceria-zirconia.
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Table 5 Oxygen storage capacities of redox-aged catalysts in first (filth) pulse sequence (~tmol O/g catalyst) Evaluation Temperature (~ Pr : Ce : Zr : Y : Ca
Pd (wt%)
250
300
350
500
0.500:0.500
2.0
128(49)
279(59)
823(199)
778(669)
0.475:0.475:0.05
2.0
142(15)
166(NA)
546(123)
626(499)
2.0
169(25)
199(65)
695(144)
785(653)
2.0
51(0)
91(43)
471(50)
697(564)
1.32
73(18)
97(43)
343(104)
430(363)
2.62
77(17)
100(33)
382(104)
694(621)
2.0
85(16)
102(17)
103(57)
856(806)
0.475:0.475
:0.05
0.475:0.475
:0.05
0.450:0.450:0.100 0.400:0.400
:0.200
0.620:0.380
The specific surface areas of the various catalysts aider redox aging are shown in Table 6. Again, the positive effect of zirconium on surface area is apparent, but its magnitude at the lowest concentration, 5mo1%, is small, and it would seem to be more than offset by the larger negative effect of zirconium on oxygen storage capacity. Yttrium appears better in this regard while calcium is worse. In no case is the surface area less than 1/4 that of the catalyst made from commercial ceria-zirconia. Table 6 Specific surface areas of redox-aged catalysts Pr : Ce : Zr : Y : Ca
Pd (wt%)
BET surface area (m2/g)
0.500:0.500
2.0
2.0
0.475:0.475:0.05
2.0
2.3
2.0
2.8
2.0
1.8
1.32
2.8
2.62
2.4
0.425 : 0.425 : 0.150
1.8
5.8
0.400:0.400:0.200
1.54
7.4
2.0
7.0
0.475:0.475
:0.05
0.475:0.475
:0.05
0.450:0.450:0.100 0.400:0.400
0.620:0.380
:0.200
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4. CONCLUSIONS
In summary, a method for preparing high-surface-area praseodymia-ceria-based mixed oxides has been demonstrated, and model catalysts made from the materials produced have been shown to exhibit oxygen storage properties at low temperature that are superior to those of catalysts made from commercial state-of-the-art ceria-zirconia. Although these mixed oxides are not quite as stable as ceria-zirconia with respect to maintenance of surface area or relative oxygen storage capacity after severe high-temperature redox aging, they appear to be viable materials for use in automotive-exhaust catalysts. REFERENCES
1. M. Yu. Sinev, G. W. Graham, L. P. Haack, and M. Shelef, J. Mater. Res., 11 (1996) 1960. 2. H.-W. Jen, G. W. Graham, W. Chun, R. W. McCabe, J.-P. Cuif, S. E. Deutsch, and O. Touret, CataL Today, 50 (1999) 309. 3. C. K. Narula, L. P. Haack, W. Chun, H.-W. Jen, and G. W. Graham, J. Phys. Chem. B, 103 (1999) 3634. 4. Note: The variations in Pd loading should not affect these and subsequent comparisons since (a) PdO, which may also contribute to oxygen storage capacity, has already been substantially reduced during the lowest-temperature measurement, and it does not re-form under these conditions, and (b) we have found that oxygen storage capacities of model catalysts made from the commercial ceria-zirconia, for example, are not very sensitive to Pd dispersion for the types of loadings and aging conditions used here. 5. H. Tanaka and M. Yamamoto, SAE Paper 960794 (1996).
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Studies in Surface Science and Catalysis 130
A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors)
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9 2000 Elsevier Science B.V. All fights reserved.
Improvement of three-way catalytic performance by optimizing ceria and promoters in Pd-only catalyst prepared by sol-gel method H.-S. Soa, O-B. yanga, D. H. Kimb and S. I. Woob aSchool of Chemical Engineering and Technology, Chonbuk National University, Chonju, Chonbuk 561-756, Korea, E-mail"
[email protected] bDepartment of Chemical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon 305-701, Korea Pd-only three-way catalysts containing bulk ceria and/or stabilized ceria (CeO2-ZrO2) and promoters such as V and Zr were prepared by sol-gel method and characterized by threeway catalytic performances test, X-ray diffraction, BET surface area and pore size distribution, temperature-programmed reduction and oxygen storage capacity measurement. It was found that bulk and stabilized ceria should be physically mixed to improve the three-way catalytic performance. Optimally formulated physical mixture catalyst showed excellent activity and thermal stability even after aging at 1000~ for 50 h, which seemed to be ascribed to the significant suppression of ceria sintering by anchoring effect, and to the separation of easily agglomerating ceria from palladium by physical mixing. 1. INTRODUCTION Ceria is one of major components in current three-way catalyst (TWC). Its major roles in the catalyst are the stabilization of the active metal and the alumina support, the promotion of the water-gas shit~ reaction and the enhancement of oxygen storage capacity (OSC), thus enlarging the air/fuel window [ 1]. However, it drastically reduces the OSC and the catalytic performance by sintering at high temperature [2]. In our previous work, Pd-only three-way catalyst prepared by sol-gel method showed a good thermal stability, high low-temperature activity and strong SO2 resistance [3]. It was also reported that the optimally formulated sol-gel catalyst, Pd-V-Zr-A1203 (PVZA), was not nearly deactivated up to 800~ and kept its low-temperature activity even up to 900~ However, because of the narrow air-to-fuel (A/F) window of the PVZA catalyst, it is required
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to introduce the efficient and thermally stable CeO2-based mixed oxides to enlarge the A/F
window and to improve the three-way catalytic performance. In this study, we investigate the effect of ceria and promoters in Pd-only TWC on the three-way catalytic performance and the thermal stability. Also the catalysts were characterized with X-ray diffraction (XRD), BET surface area and pore size distribution, temperature-programmed reduction (TPR) and OSC measurement 2. EXPERIMENTAL All of the catalysts containing 1 wt% palladium prepared by sol-gel method in pH 10. Aluminum isopropoxide (All, Aldrich) was dissolved in distilled and de-ionized water by H20/All molar ratio of 100. Then NH4OH as a hydrolysis and condensation catalyst was added to the A l l solution. For the preparation of Pd(1 wt%)-V(2 wt%)-Zr(10 wt%)-A1203 catalyst which is abbreviated to PVZA in this paper and ceria incorporated PVZA (Pd(1 wt%)-V(2 wt%)-Zr(10 wt%)-Ce(29 wt%)-A1203), acetylacetonate form precursors of Zr, V and Ce in acetone were sequentially mixed with the solution and then stirred at 50~ for 1 h per promoter. Finally, palladium acetylacetonate (Pd(AcAc)2, Aldrich) as a palladium precursor in acetone was added and stirred at 50~ for 4 h. Fresh catalyst was obtained by drying at 110~ for 24 h and calcining at 500~ for 5 h after removing the solvent. Aging of the catalyst was carried out in the simulated automotive exhaust gas at 1000~ for 2 h or 50 h. Bulk ceria (b-Ce) was prepared by sol-gel method using a precursor of cerium hydroxide
(Ce(OH)4), Aldrich). Similarly, zirconia incorporated ceria compound, CeO2-ZrO2 which was denoted as a stabilized ceria (s-Ce) was prepared by the same sol-gel method except adding the zirconium acetylacetonate solution in ceria hydroxide aqueous solution with a Ce/Zr mole ratio of 2.3. PVZA, s-Ce and b-Ce were mixed at an appropriate ratio and ground in a mortar to prepare the physical mixture catalysts that are represented as x% PVZA + y% s-Ce in case of PVZA mixed with s-Ce in a weight ratio of x/y. Three-way catalytic activity was measured with simulated automotive exhaust gas containing 6000 ppm CO, 1500 ppm NO, 500 ppm C3H6, 3000 ppm H2, 6000 ppm 02, 13% H20 and N2 balance in a quartz reactor at a gas hourly space velocity of 72,000 h ~. TPR was performed at a ramping rate of 10 K/min with 5% H2 in N2. After the catalyst was reduced by Hz at 700 K for 2 h, OSC was measured by injection of 0.1 ml 02 pulses until no more adsorption of 02. XRD experiment was carried out with a scan speed of 4~
in the
target of Cu K~ (~.=1.540598 A) operated at 40 kV and 45 mA. BET specific surface area, pore size distribution and pore volume were measured by nitrogen adsorption isotherm at 77 K in a BET equipment (ASAP 2010C, Micromeritics).
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3. RESULTS AND DISCUSSION The surface areas and pore volumes of the catalysts were summarized in Table 1. Surface area of bulk ceria (b-Ce) and stabilized ceria (s-Ce) is around 30 m2/g, which is less
than those of the others as shown in Table 1. Accordingly, the surface areas of the physically mixed catalysts with ceria were drastically decreased as increasing the ratio of ceria. However, OSC of the catalyst was significantly increased as increasing the ceria contents as shown in Table 2. In the physically mixed catalysts, the amount of OSC increasing by adding of s-Ce was much higher than that by adding of b-Ce. TPR spectra of various catalysts were shown in Fig. 1. Hydrogen uptakes in TPR spectra are ascribed to the reduction of oxidized state of PdO and CeO2 to reduced state of Pd and
Ce203,respectively.
Table 1 BET surface area, pore volume and average pore radius of the catalysts Surface Pore Catalyst area volume (m2/g) (cm2/g) Pd-AI203 315.0 0.48 PVZA (fresh) 324.8 0.54 PVZA (aged, 2 h) 119.2 0.44 Pd-V-Zr-Ce-A1203 259.4 0.46 b-Ce 37.5 0.12 s-Ce 31.1 0.07 90%[70% PVZA+30% s-Ce] + 10% b-Ce (fresh) 203.0 0.36 90%[70% PVZA+30% s-Ce] + 10% b-Ce (aged, 50 h) 56.7 0.27 Table 2 Oxygen storage capacities of various catalysts Catalyst
OSC x 105 (mol-02/g-catal.)
PVZA (fresh) PVZA (aged, 50 h) Pd-V-Zr-Ce-A1203 90% PVZA + 10% s-Ce 80% PVZA + 20% s-Ce 70% PVZA + 30% s-Ce 90%[70% PVZA + 30% 90%[70% PVZA + 30% 70%[70% PVZA + 30% 50%[70% PVZA + 30%
0.11 0.06 18.40 4.22 11.05 16.57 23.82 5.65 35.27 46.44
s-Ce] s-Ce] s-Ce] s-Ce]
+ + + +
10% b-Ce (fresh) 10% b-Ce (aged, 50 h) 30% b-Ce 50% b-Ce
Average pore radius (nm) 5.00 3.34 7.36 3.56 4.62 3.51 9.56
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G
IF E
C
A
..... ,, |
O
200
i 4OO
i 6OO
| 800
T e m p e r a t u r e
i
1OOO
("C)
Fig. 1. TPR profiles of PVZA (A), 90%[70% PVZA + 30% s-Ce] + 10% b-Ce (fresh) (B), 90%[70% PVZA + 30% s-Ce] + 10% b-Ce (aged, 50 h) (C), Pd-V-Zr-Ce-A1203(fresh) (D), Pd-V-Zr-Ce-A1203(aged, 50 h) (E), b-Ce (F) and s-Ce (G). Therefore, the peak intensity of TPR spectrum is directly proportional to the amount of OSC of the catalyst. In TPR spectrum of s-Ce (Fig. 1 (G)), the strong peak around 490~ medium peak around 845~
and
result from the reduction of surface ceria and bulk ceria,
respectively [1 ]. There is most of bulk oxygen and small amount of surface oxygen in b-Ce as shown in Fig. 1 (F). Physically mixed catalyst contains both surface and bulk oxygen as shown in Fig. 1 (B). However, there is not much oxygen stored in ceria in the Pd-V-Zr-CeA1203, which shows the strong reduction peak of PdO to Pd around 140~ A little amount of oxygen was stored in PVZA catalyst. This result suggests that stabilized ceria which is homogeneous solid solution of Ce0.7Zr0.302 is essential for the enhancing the amount of surface oxygen which may be more easily used as an oxygen atom than the bulk oxygen strongly bounded in bulk ceria. Fornasiero et al. reported that the insertion of zirconia into the CeO2 lattice induced formation of defective sites also in the bulk and resulted in the enhancement of OSC [4]. XRD spectra of fresh and aged catalysts are shown in Fig. 2 and Fig. 3, respectively. In the aging of the Pd-A1203 catalyst, highly dispersed palladium and ],-A1203 was significantly sintered to large crystalline PdO and c~-A1203 and 0-A1203, respectively, resulting in the severe decreasing of surface area. However, aged PVZA was not much sintered even after aging, which might be ascribed to the role Zr as a promoter to enhance the thermal stability of Pd. Even after aging at 1000~ for 50 h, relatively high OSC was kept in the physically mixed catalyst in spite of the significant sintering and large decreasing of the
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A1
v
9
I (7-AlaO3), 9(CeO a), 0 (Zr~ a), * (PdO)
e (~,-AI=03),0 (O-AI=O3), Q (~-A1203),@ (ZrO 2), ,k (PdO),A (CeO a)
m
r r
.5 c
A
10
20
30
40
20
50
60
70
80
Fig. 2. XRD spectra of Pd-A1203(fresh) (A), PVZA(fresh) (B), Pd-V-Zr-Ce-A1203 (fresh) (C) and 90%[70% PVZA+30% s-Ce] + 10% b-Ce (fresh) (D).
9
10
2o
30
i
.
,0
i
60
,
i
,
80
2o
Fig. 3. XRD spectra of Pd-A1203 (aged) (A), PVZA (aged) (B), and 90%[70% PVZA + 30% + s-Ce] + 10% b-Ce (aged) (C).
surface area upon aging, which accounted for its highest activity before and after aging as shown in Fig. 4. This excellent activity and thermal stability of the physically mixed catalyst upon aging seems to be attributed to the significant suppression of ceria sintering by anchoring effect which results from sol-gel method by adding the zirconia, and to the separation of easily agglomerating ceria from palladium by physical mixing. Ts0 indicates the temperature at which the conversion of each reactant reaches 50%. The physically mixed catalyst showed higher activity than ceria incorporated PVZA, Pd-V-Zr-Ce-A1203 and conventional Pt-Rh/A1203 three-way catalyst. Especially, Pd-V-Zr-Ce-A1203 catalyst was drastically deactivated upon aging as shown in Fig. 4. Therefore, ceria should be physically mixed to enhance OSC and thermal stability of Pd-only three-way catalyst. Recently, extensive researches are focused on the catalysts show the high activity at low temperature because most pollutants of the automotive emission occurred during the cold-start [5]. This physical mixture of PVZA, bulk ceria and stabilized ceria showed quit high activity at low temperature because of the enough OSC contained in bulk and stabilized ceria. 4. CONCLUSIONS Physical mixing of bulk and stabilized ceria in the Pd-only TWC is essential to improve the three-way catalytic performance and the thermal stability. The physical mixture of PVZA, bulk ceria and stabilized ceria accomplished the excellent three-way catalytic
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化
450
400 350
f
" [1-'~a34
300
~
250 200
INO
I~
359
342 342 300
351 EEE3
CO
jHC
334 331 I
284
~ 25R ~
(A)
~ i
(B)
~255
(C)
265 ~
243 ~
(D) (E) Catalysts
Fig. 4. Light-off temperature (Ts0) of NO, CO and
c 3 n 6 over
(F)
I
(G)
Pd-AI203 (A), PVZA(fresh) (B),
PVZA(aged, 50 h) (C), Pd-V-Zr-Ce-Al203(fresh) (D), Pd-V-Zr-Ce-A1203(aged, 50 h) (E), 90%[70% PVZA + 30% s-Ce] + 10% b-Ce(fresh) (F) and 90%[70% PVZA + 30% s-Ce] + 10% b-Ce(aged, 50 h) (G). performance by the high activity at low temperature and by the improved thermal stability at high temperature, which seemed to be attributed to the significant suppression of ceria sintering by anchoring effect, and to the separation of easily agglomerating ceria from palladium by physical mixing. ACKNOWLEDGEMENT This research was funded by a national project granted from Ministry of Commerce, Industrial and Energy and Ministry of Science and Technology (1995 - 1998). REFERENCES 1. K.C. Taylor, Catal. Rev. Sci. Eng., 35 (1993) 457. 2. S.J. Schmieg and D.N. Belton, Appl. Catal. B, 6 (1995) 127. 3. J. Noh, O-B. Yang, D. H. Kim and S. I. Woo, Catalysis Today (1999) in press. 4. P. Fomasiero, R.D. Monte, G.R. Rao, J.Kaspar, S. Meriani, A. Trovarelli and M. Graziani, J. Catal., 151 (1995) 168. 5. J.C. Summers and W.B. Williamson, in 9Environmental Catalysis, ACS symposium series, Vol. 552, eds. J.N. Armor (American Chemical Society, 1994) p. 94.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Flue gas NOx removal by SCR with NHa on CuO/AC at low temperatures Z. P. Zhu a, Z. Y. Liu a, S. J. Liu a, H. X. Niu a, T. D. Hu b, T. Liu b, and Y. N. Xian b a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, P.R.China b State Key Laboratory of Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, 100039, P.R.China ABSTRACT NO reduction with NH3 over activated carbon supported copper oxide (CuO/AC) catalyst was studied at low temperatures (30~250~ The attention was focused on the effects of preparation parameters on catalyst structure and activity. The catalyst shows high activity for NO reduction with NH3 in the presence of 02 at temperatures above 180 ~ Some amount of Cu20 exists in the catalyst and results in a low initial activity, but it is easily oxidized into active CuO by 02 during the NO-NH3-O2 reaction. The catalytic activity is influenced by the calcination temperature and Cu loading, and the catalyst with 5wt% Cu loading and calcined at 250~ shows the highest activity. The lower activities of the catalysts with higher Cu loadings and/or calcined at higher temperatures are mainly due to aggregation of copper species. 1. INTRODUCTION Simultaneous removal of SOx and NOx from stationary sources has received more and more attention because advantages in economy and in removal efficiency in contrast with the combined removal techniques. A suitable location for NOx and SOx removal systems is at the exit of the particulate control device, where the flue gas temperature are generally around 150 ~ CuO/A1203 and activated carbon (AC) have been developed for simultaneous SOx and NOx removal using selective catalytic reduction (SCR) of NO with NH3 and the oxidation of SO2[ 1]. However, CuO/A1203 catalyst shows high activity only at temperatures of above 300 ~ while the AC only at temperatures of less than 100 ~ In contrast, at temperatures of about 150 ~ activated carbon supported copper oxide (CuO/AC) catalyst has high activity for both NO reduction with NH3 [2,3] and oxidation of SO2 [4], which makes it possible to be used in simultaneous SOx and NOx removal at low temperatures ( Ga > A1, exhibited the volcanoshape with respect to the heat of formation of the transition metal oxide ( - A HP). Hence, the effect of the specific transition metal ions on the NO reduction activity can be explained to be due to the changes in the chemical nature of the metal cation, such as redox property.
'~
tRh(1)Cu(2)
102 E 9 Z
fAg(l)
N~)
I~Ai(100) 101/ 0
,
I
,
I
,
I
I
,
100 200 300 400 500 600 _/1Hf~
Fig. 1. NO reduction rate at 623 K ( C ) ) on transition-metal aluminates as a functio~n of heat of formation of oxide (-A H~) per tool of oxygen. Metal content (tool%) are designated in parentheses.
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3.2 Effect of Cu dispersion states
Figure 2 shows the effect of Cu content and bulk structure (Cu-aluminate or A1203 supported CuO) on the NO reduction activity. Cu-aluminate showed higher activity than CuO/A1203. For Cu-aluminate, the activity (per surface area of the catalyst) increased as Cu content increased. This trend was consistent with the increase in the concentration of surface Cu 2+ species (Cu]Cu+A1 ratio) determined from XPS analysis. From these results, it is shown that Cu 2§ species atomically dispersed in the aluminate phase is responsible for this reaction. Thus, the primary role of the alumina matrix should be to disperse Cu 2§ cation in an atomic level as established in previous studies [4,11].
3.3 Effect of Cu coordination states
For Cu-aluminate samples, UV-VIS spectra showed that Cu 2§ is present in both tetrahedral and octahedral sites [8]. The fraction of octahedra and tetrahedra has been determined by the deconvolution analysis of Cu L3-edge XANES. As shown in Figure 3, the fraction of octahedra decreased as Cu content increased [8]. This may be due to the strong preference of Cu 2§ for octahedral site relative to A13§ in A1203 matrix [12]. The NO reduction activity per exposed Cu 2§ species (TOF) was estimated by using surface Cu 2§ concentration and was plotted in Figure 3. Clearly, the TOF decreased as Cu content increased, which coincides with the decrease in the fraction of octahedra. Thus, it is shown that the coordination symmetry of Cu 2§ cation, which is affected by A1203 matrix, influences the activity (factor of ~-101); Cu 2§ cation in octahedral site is responsible for the higher activity. • 10-4 "-7
E
~
30
2
.7,
20
10
10o h 9
p
O
50 ~
~D
O Z
10
.~" ( C
|
|
I
10 20 bulk Cu/mol%
~
o
I
30
Fig. 2. Rate of NO reduction at 573 K on Cu-aluminate ( O ) and CuO/AI 203 ( V ) and the surface Cu concentration of Cualuminate (+) and CuO/AI 203 ( • as a function of the bulk Cu concentration.
Z
5
9
flg ,i,,~'1
0
,
i
,
i
10 20 30 Cu content/n~l%
Fig. 3. TOF of NO reduction ( O ) at 573 K and fraction of Td ( A ) or Oh ( A ) as a function of Cu content of Cu-aluminate.
O
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3.4 Effect of Cu e l e c t r o n i c s t a t e s
Another effect of matrix on the activity of Cu 2§ cation was investigated by using various mixed oxide catalysts (1 wt% Cu in Ga203, ZrO2, A1203, Mg~204, and MgO). As shown in Figure 4, the specific rate of NO reduction was greatly dependent on the matrix (factor of "~ 102), and followed the sequence of Ga203 > ZrO2 > A1203 > MgA1204 >> MgO. It should be noted that the activity increased with a decrease of the enthalpy of formation of the oxide matrix (-A H~). Previously, Kung et al. suggested that the de-NOx properties of metal oxide catalysts are determined more by the nature of the metal ions and less by the nature of the support. However, the present result suggests that the nature of the support (oxide matrix) is an another critical factor. XRD pattern of these catalysts showed no lines due to CuO. In UV-VIS spectra of these catalysts, an absorption band due to octahedral Cu 2§ (around 750 nm [8,13]) was observed, but no peaks due to tetrahedral Cu 2§ (at around 1500 nm [8,13]) nor CuO (620 nm [13]) were observed. These results suggest t h a t dispersed Cu 2§ are in the distorted octahedral sites of mixed oxide phase. In order to discuss the effect of matrix on the electronic state of dispersed Cu 2§ Cu K-edge XANES spectra of these catalysts were measured. As shown in Figure 5, the XANES spectra of the samples did not exhibit a peak due to Cu § (8983 eV [14]) nor a peak due to CuO (8986 eV [14]). In addition, each sample exhibited characteristic XANES spectrum, which indicates that electronic state of Cu 2§ species in each sample differs through an interaction with the surrounding matrix. In addition, it appears t h a t the intensity of the peak at about 8990 eV was lower for the catalyst of higher activity. The 8990 eV peak corresponds to the electron transition from Cu ls to the unoccupied antibonding molecular orbital (MO) originating from the Cu 4p and O 2p orbitals [15]. Therefore, it is suggested that the higher electron density in this antibonding MO, or, in other word, the more labile Cu-O bond is required for the higher SCR activity. 9oev/"~ ~,_.,
Ga ~|
"~....~~.GI1203
60 ~=~
10~
E
.O" A1 40 gal
10-1 o Z
d~Mg 10 -2
300
i
I
400
i
I
500
- A He~
~
I
600 mol
,
'
2o
22
0
700
Fig. 4. NO reduction rate at 573 K ( O ) and area of 8990eV peak in Cu K-edge XANES ( ~ ) for Cu-d~spersed catalysts as a function of- A Hf of oxide per mol of oxygen.
f
8980
t
i
I
i
9000 9020 photon energy/eV
Fig. 5. Cu K-edge XANES spectra of Cu-dispersed catalysts.
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NO+C3H6+O2 (for 180 min)._ ..
NO+O2
II
Cu-Mgml204 Cu-Al203 Cu-ZrO 2
Cu_Ga203 ,
,
0
,
i
i
i
l
100 time/min
i
I
I
'
I
200
Fig. 6. Changes in the relative intensity of the acetate band normalized by intensity at the steady-state (t=0) as a function of time in a flow of NO+O 2 at 573 K.
Additional information about the surface reactivity of the Cu-dispersed catalysts was obtained by a transient in-situ IR experiment. IR result showed that the main adspecies during the reaction was the acetate for all the samples except for Cu-MgO. Recently, we have studied the mechanism of C3H6-SCR on Cualuminate catalysts and showed that the acetate adsorbed on the surface Cu-O site is a possible intermediate, which reacts with NO+O2 to produce N2 and CO2 [9]. The reactivity of the acetate on Cu-dispersed catalysts was tested by the transient response of the IR spectra. After obtaining the IR spectrum under steady-state condition, the flowing gas was switched to NO+O2. Figure 6 shows the change in the integrated intensity of the acetate band (around 1452 cm ~) in a flow of NO+O2. The acetate band decreased in NO+O2, indicating that acetate is highly reactive toward NO+O2. The initial rate of the acetate reaction followed the sequence of GaeO~ > ZrO2 > A1203 > MgAleO4, which is consistent with the order of the NO reduction activity. From these results, it is suggested that the nature of the matrix, such as reducibility, influences the activity of the Cu 2§ species; the more reducible the matrix the more labile the surface Cu-O bond, leading to the higher reactivity of the acetate intermediate toward NO+O2. 4. CONCLUSION This study demonstrated the fundamental factors affecting the activity of transition metal oxide catalysts for C3H6-SCR. The activity was determined by the nature of the transition metal element, which could influence the red-ox property of the active center. The local structure was found to be another critical parameter; the activity was dependent on the dispersion, coordination, and local
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electronic states of the metal cation. Considering the higher stability of metal oxides than zeolite matrix, the active and stable SCR catalysts will be designed by using transition metal cation with a moderate red-ox property and by controlling the above intrinsic factors through an interaction of metal cation with surrounding matrix.
ACKNOWLEDGMENT The authors thank Mr. H. Ishikawa of Toho Gas Co., Ltd. for his technical assistance on XPS measurement. K.S. acknowledges support by the Fellowship of JSPD for Japanese Junior Scientists. 5. R E F E R E N C E S
1. M. Iwamoto and H. Yahiro, Catal. Today, 22 (1994) 5. 2. K.A. Bethke, M. C. Kung, B. Yang, M. Shah, D. Alt and H. H. Kung, Catal. Today, 26 (1995) 169. 3. H. Hamada, Catal. Today, 22 (1994) 21. 4. K. Shimizu, H. Maeshima, A. Satsuma, T. Hattori, Appl. Catal. B, 18 (1998) 163. 5. K. Shimizu, A. Satsuma, T. Hattori, Appl. Catal. B, 16 (1998) 319. 6. K. Shimizu, M. Takamatsu, K. Nishi, H. Yoshida, A. Satsuma, T. Hattori, Chem. Commun., (1996) 1827. 7. K. Shimizu, M. Takamatsu, K. Nishi, H. Yoshida, A. Satsuma, T. Tanaka, S. Yoshida, T. Hattori, J. Phys. Chem. B, 103 (1999) 1542. 8. K. Shimizu, H. Maeshima, H. Yoshida, A. Satsuma, T. Hattori, Jpn. J. Appl. Phys., 38 (1999) 44. 9. K. Shimizu, H. Kawabata, H. Maeshima, A. Satsuma, T. Hattori, J. Phys. Chem. B, submitted. 10. K. Shimizu, H. Kawabata, A. Satsuma, T. Hattori, J. Phys. Chem. B, 103 (1999) 5240. 11. Z. Chajar, M. Primet, H. Praliaud, J. Catal., 180 (1998) 279. 12. Navrotsky and O.J. Kleppa, J. Inorg. Nucl. Chem., 29 (1967) 2701. 13. M. C. Marion, E. Garbowski, M. Primet, J. Chem. Soc. Faraday Trans., 86 (1990) 3027. 14. L.-S. Kau, K. O. Hodgson, E. I. Solomon, J. Am. Chem. Soc., 111 (1989) 7103. 15. N. Kosugi, H. Kondoh, H. Tajima, H. Kuroda, Chem. Phys., 135 (1989) 149.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
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Silver as a Promoter for the Catalytic Decomposition of NOx under Oxygenexcess Condition: Evidence for Oxygen Spiilover from Noble Metals to Silver W. X. Huang"
J.W. Teng a'b T.X.
Cai b
and X. H. Bao"*
aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, 116023 Dalian, China b School
of Chemical Engineering, Dalian University of Technology, Dalian 116012, China
Metallic silver as an effective additive in noble metal based catalysts enhances the migration of the tightly bound oxygen on noble metal atoms, as evidenced by TDS and photoemission electron microscopy (PEEM) observations. The actual spillover of oxygen and the unique thermal redox properties of silver result in a significant lowering of the temperature for the removal of the surface oxygen, which keeps the noble metal sites available for the adsorption of reactants, particularly in the presence of excess oxygen, and leads to an increase of the activity. For the catalytic decomposition of NO over the A1203-supported 1%Pd catalyst, the presence of 0.1% Ag as a promoter resulted in about 18% increase of the conversion of NO to N 2 at 873K.
1. Introduction The effective removal of NOx from industrial waste gases and automobile exhausts has been a major concern of environmental protection. But until now no suitable catalyst which can effectively convert NO into molecular N 2 and 02 has been found [1'2]. Studies have shown that highly active catalysts which can decompose NO, for example noble metal catalysts, usually also have strong affinities towards atomic oxygen. Adsorbed atomic oxygen does not easily desorb from these surfaces, since these oxygen atoms occupy the active centers for NO adsorption, thus impede the adsorption and decomposition of NO on the catalyst surfaces, resulting in the deactivation of the catalysts [3]. Silver is a metal which exhibits unique thermal redox properties. Gas-phase oxygen adsorbs on the silver surface and dissociates into adsorbed atomic oxygen. Upon heating, except a few percent of adsorbed surface oxygen atoms diffuses into the bulk phase of silver, most of them recombine at the silver surface as dioxygen and desorb from the surface into the gas phase at about 600K. This gives rise to a recreating of free silver surface at this temperature [46]. This implies a possibility that silver is a good catalyst for NO decomposition or is an effective additive to improve the catalytic preference of noble * To whom the correspondence should be addressed. Fax: 0086 411 4694447, Email:
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metal based catalysts in the presence of excess oxygen [7-91. In the present presentation, the catalytic chemistry of silver itself as a catalyst and as an additive of Pd-based catalysts for the catalytic decomposition of NOx in the presence of excess oxygen is reported. The variations of the surface structures during the catalytic reactions have been characterized by the application of a variety of surface techniques.
2.Experimental Supported Pd and Pd-Ag catalysts were prepared by incipient-wetness impregnation using aqueous solutions of PdC12 (First Chemicals Company of Shanghai) and AgNO3 (Chemicals Company of Beijing) at RT on 7-A1203 (from DICE B.E.T. surface area 140.2m2/g). Catalytic tests were carried out by using a quartz tubular fixed-bed reactor (6 mm i.d.) containing 0.25g catalyst, with 2g of quartz chips located prior to the catalyst bed to preheat the feed gases. After flushing with He (15ml/min) at 600 K for 30 min, a feed gas mixture of 2.0% NO, with He as the balances, was introduced into the reactor at a space velocity of 600ml/gcat.h through a mass flow controller. Products, such as N2, NO, 02, N20 and NO2 etc., were separated and analyzed on-line by a gas chromatograph (Model Shanghai 102G) equipped with a column of molecule-sieve 5A and a thermo-conductive detector (TCD). The activities were evaluated in terms of conversion to N2=2[N2]oL,t/[NO],,,* 100%. The photoemission electron microscopy (PEEM) studies were performed in a UHV chamber equipped with devices for low-energy electron diffraction, Auger electron spectroscopy, MS, PEEM and an Ag evaporator PEEM forms a magnified image of the surface due to contrast in the intensity of photoelectrons emitted from the surface irradiated by UV light f~01.It reflects the work function in the observed surface area so that areas with low work functions give rise to a high photoelectron current and yield bright regions in the PEEM image. A P t (110) crystal was polished again before being put into the UHV chamber, and the surface was cleaned in UHV by the standard procedure of repeated cycles of sputtering with Ar t, heating in oxygen and annealing. Deposition of Ag was accomplished by evaporating Ag from a resistively heated tungsten shape using a square grid as a mask, and the amount of Ag on Pt (110) was estimated from the intensity ratio of Ag to Pt.
3. Results and Discussion Metallic Pt and Pd have been found to be unique components for catalyzing the direct decomposition of NO. Fig. 1 (curve a) illustrates the variation of NO conversion to Nz with the reaction temperature over a l%Pd/A1203 catalyst. Similar to that described by other investigators N, it was found that the ignition of the NO decomposition reaction over Pd-based catalysts required higher temperatures, and there existed a pronounced dependence of the reaction activity on the reaction temperature. Moreover, the catalytic performance was restricted by the amount of excess oxygen in the system. Metallic silver has been demonstrated to exhibit only minor catalytic activity for the dissociation of nitrogen oxides due to its inertness for the adsorption of NO. However, the participation of metallic silver in
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60
50
40 0 r
.o_ C
o o
3()
20
10
e I
800
900 1000 Temperature/K
11O0
600
800 1000 Temperature/14:
Fig.1. Temperature dependence of the catalytic decomposition of NO over (a) 1Pd/A1203; (b) 1Pd-0.1%Ag/A1203 ; (c) 1Pd-0.2%Ag/A1203 ; (d) 1Pd-I%Ag/A1203 and (e) 1Pd-2%Ag/A1203 (2.0% NO in feed; space velocity, 1200 cm3g-lhour 4 ). Fig.2. Oxygen thermal desorption spectra taken after exposed 4.2% OJHe at 773K for 2 h, followed by various cooling procedures (The heating rate is 10.5K/min): (a) 1Pd /AI203 and (b) 1Pd-I%Ag/A1203, cooled down to RT in 4.2% O2/He; (c) 1Pd1%Ag/A1203, before cooled down to RT in He, swept at 773 K in He for 2h; (d) 1Pd-I%Ag/A1203, before cooled down to RT in He, swept at 773 K and cooled down to 443K in He and than held at 443K in He for 2h. the Pd-based catalysts gives rise to a distinct alteration of the catalytic behaviors. As shown in Fig.l, a small amount of Ag, 0.1% and 0.2%, respectively, promotes the catalytic activity through the lowering of the reaction temperature and enhancing the actual conversion of NO at the given temperatures. For a 30% NO conversion, the reaction temperature needed over the 1%Pd-0.1%Ag/A1203 was about 60 K lower than that for the pure Pd catalyst (1%Pd/AlzO3). The increase of the silver content in the catalyst causes an obvious restriction of the reactivity of NO dissociation. As shown in curves d and e in Fig.l, the conversions for the catalysts of Pd-Ag/A1203 with respectively 1% and 2% silver loadings were only about one fifth of the NO conversion at 1078K, which could be attributed to the covering of Ag on the Pd sites, which were the active centers for the decomposition of NO. As demonstrated, the rate-limiting step in the NO decomposition reaction over noble metal based catalysts is the efficient removal of oxygen species which were formed from the dissociation of NO or from the adsorption of excess oxygen in the gas phase [3]. In such cases, successful enhancement of the activity should be attained by the weakening of the interaction of oxygen with the noble metals such as Pd and Pt. Fig.2 presents the oxygen TDS results taken from 1%Pd/A1203 and 1%Pd-1%Ag/Al203 catalysts. Curves a and b were recorded after exposing respectively I%Pd/AI203 and I%Pd-I%Ag/A1203 to 4.2% O2/He mixture at 773K, and then cooling down to RT in the same atmosphere. As can been seen from curve a, on the A1203-supported Pd surface only a single oxygen species which is attributed to the surface
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state (%) existed. Compared with the results obtained from the Pd (110) surface [111,except for the absence of a weakly bound state oxygen (a~) which is designed as subsurface oxygen, the desorption temperature of the surface state oxygen on the A1203-supported Pd surface appeared to shift about 150K towards high temperatures. The addition of Ag did not cause significant change in the desorption behaviors of the surface state oxygen from the Pd-related sites (%), but it gave rise to a new oxygen species desrobed at a lower temperature (670K), as shown in curve b of Fig. 2. By comparison with the results of the OJAg system, the new species could reasonably be assigned to the atomically adsorbed oxygen on Ag-relating sites, and the 70K increase of the desorption temperature with respect to that of the bulk metallic silver can be ascribed to the strong interaction between Ag and the A1203 carrier. The spectral invariability of the oxygen desorbed from Pd-relating sites and the appearance of the Agrelated oxygen species reveal that the participation of Ag in the Pd/A1203 did not lead to the active formation of an Ag-Pd alloy, as it has been demonstrated that, if there is the formation of an alloy, there will be an obvious shift of the oxygen desorption peak towards lower temperatures. Instead, some isolated Ag-related sites were created, which exhibit the characteristics of metallic silver. As shown in curve c of Fig.2, when the sample of l%Pd1%Ag/A1203 was exposed to 02 at 773K and cooled down in He instead of in O2/He mixture to RT, subsequent oxygen desorption gave only one dominant peak representing the oxygen species on Pd-related sites. However, when the oxygen-saturated 1%Pd-1%Ag/A1203, before being cooled down to RT, was swept at 773 K and cooled down to 443K in He and than held at 443K in He for some time, then besides the expected Pd-relating oxygen desorption peak, a clear uptake at about 650K emerged. The appearance of the Ag-related oxygen species reveals an actual migration of the oxygen atoms, i.e., oxygen spillover, from the Pd-related centers to the Ag sites at appropriate temperatures. A direct observation of the analogous phenomenon of oxygen spillover has been carried out by using photoemission electron microscopy (PEEM) on the Ag-deposited Pt(110) surface. The use of the grid permits us to identify easily the pure Pt(110) region and the Ag-modified
140 "--:"~::3130 .__
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=,,,
a: Ag area
/
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=o z 100 133 90~ 0
20
40
60
Time/s
Fig.3. The left part shows the PEEM images of Pt(110) surface after exposed to 3.0x10 5 Pa 02 and NO mixture (O2:NO=l:l) at 400K for 600Sec (upper) and then heated to 700K in UHV (lower). The right part presents the corresponding variation of image brightness of Ag area (a) and Pt area (b) with heating time (The heating rate is about 4-6K/sec).
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region. Fig.3 presents the PEEM images taken from the Ag-modified Pt(110) after exposing to the mixture of 50%02 and 50%NO at 3.0x 103mbar of the total pressure at 400K for 10min (upper part) and heated to about 700K in UHV (lower part). In the pictures presented, (a) and (b) denote the Ag area and Pt area on the surface, respectively. Fig. 3 also presents the variations of the image brightness (photo-current) in Ag and Pd areas with the surface temperature (temperature ramp about 4-6K/sec), which were obtained by an additional computer analysis. On the clean surface (not presented), the Pt(110) regions are darker than the Ag region, which is in agreement with the fact that the work function of Pt(110) ( 5.49 eV I~21) is higher than that of Ag, being 4.2-4.7 eV depending on the configuration of the surface atoms. After exposed to the 02 and NO mixture, the Pt areas (lines) became distinctly darker, while the Ag areas (squares) remained more or less unchanged, which reflects a preferential adsorption of 02 and NO on the Pt areas. By heating the sample, the initial lowering of the brightness in both the Pt and the Ag areas reflexes the dissociation of adsorbed NO or NO2 , in which adsorption of the released atomic oxygen led to an increase of the surface work function. When rising to about 550K (22 sec), the brightness of the Pt areas began to enhance, and the Ag areas became darker correspondingly. It has been shown that the interaction of oxygen with Pt surface is very strong, and the desorption of adsorbed oxygen species occurs commonly at temperatures above 850K. On Ag-modified Pt surface, the actual variation of the work function of the oxygen-covered Pt areas and Ag-areas below 600K, as indicated by the enhancement of the PEEM brightness, reveals definitely an active migration (i.e. oxygen spillover) of the adsorbed oxygen from the Pt areas onto the Ag areas.
It has been shown that the activation energy for surface diffusion of adsorbed oxygen species over noble metal surfaces (e.g. Pd and Pt) is low, as compared with their heats of desorption [~31.Therefore, these species migrate along the surface and visit various adsorption sites during their surface residence time. Thus, although the apparent binding of oxygen with noble metal atoms are tighter than that with the silver atoms, the actual mobilization of the adsorbed oxygen species on the surfaces gives rise to the possibility for silver to trap and catch the mobile oxygen atoms, due to its higher affinity for 02 [~4].
In conclusion, on the noble metal-modified catalysts, NO molecules adsorb preferentially on metal active sites and dissociate at elevated temperatures, forming atomically adsorbed
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..~4~~'""~:~ ;7~!g~.;:@"-'~ :~: Sn-TiO2 ~ Sn-A1203 >> Sn-SiO2. 1. INTRODUCTION The selective catalytic reduction (SCR) of NO with hydrocarbons in the presence of oxygen is a very attractive process as a practical way to reach the future stricter emission standards concerning fuel and lean bum engine exhausts [1]. The NO reduction by hydrocarbons on Cu/ZSM-5 and other zeolite based catalysts has given interesting results, but their hydrothermal durability and rapid deactivation have been reported to be a problem. The alumina-supported metal oxide catalysts are more effective at higher temperatures and higher oxygen partial pressures [2-5]. The activity of oxide based catalysts can be attributed to their acidity. In this regard, the cooperation between the support and the active metal centers has to be investigated. In this work, catalysts with about 3 wt% tin oxide loading based on alumina, silica, titania, zirconia and magnesia have been investigated and compared to the support materials. The effectiveness of these catalysts for NOx reduction may depend on the state and dispersion of the tin oxide on the support, on the effect of Sn loading and on the acidity of the support. The acidity was measured by adsorption calorimetry, which is one of the best methods to feature the acidic properties of solid catalysts, providing useful information on both the concentration and strength of sites [6,7]. The changes which may occur in the support matrix due to the addition of tin have been studied.
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2. EXPERIMENTAL
Five supports with different acid-base features and presenting a surface area around 100 m2.gq were u s e d 7-A1203 oxid C from Degussa, TiO2 (anatase) DT51 from Rh6ne Poulene, SiO2 aerosil 130 from Degussa, ZrO2 137 from Degussa, MgO from Carlo Erba. The impregnation was made using solutions of tin tetrachloride pentahydrate (from Aldrich) in distilled water [8]. After drying, the samples were heated in oxygen flow for 6 h at 423 K and for 8 h at 773 K. The amounts of tin deposited were determined by ICP chemical analysis and presented as Sn weight percentages. The samples used in our experiments are listed in Table 1 together with their BET surface area (measured by nitrogen adsorption) and Sn content. Another series of samples was also prepared with a higher loading (-20 wt% Sn) for comparison [7]. Table 1 Physieoehemieal characteristics of the supports and supported SnO2 materials Samples Sn amount BET surface area V ads (wt%) (m2.gq) (lamo~m2) 7-A1203 0 110 1.71 TiO2 0 103 2.74 SiO2 0 130 0.20 ZrO2 0 90 2.60 MgO 0 120 0.23 Sn-A1203 3.1 108 2.00 Sn-TiO2 3.4 90 2.89 Sn-SiO2 3.2 112 0.54 Sn-ZrO2 3.4 78 2.99 Sn-MgO 3.7 54 1.64
V irr (lamol/m2) 0.96 1.60 0.08 1.33 0.09 1.14 1.77 0.26 1.73 0.50
Microcalorimetric studies of the adsorption of ammonia at 423 K were carried out using a heat flow microcalorimeter C80 from Setaram connected to a gas-handling and volumetric adsorption system. The differential heats of adsorption versus adsorbate coverage were obtained by measuring the heats evolved when doses of gas were admitted sequentially onto the catalyst until the surface was saturated by adsorbed species. Adsorption was emended up to an equilibrium pressure of about 130 Pa. After the adsorption was completed, pumping at the temperature of the experiment and consecutive readsorption of ammonia at the same temperature were performed in order to determine the chemisorbed amount (V~). In Table 1, V ~ and V~ are respectively the total adsorbed amount and the irreversibly adsorbed amount, expressed in lamol.m2 of sample, under an equilibrium pressure of 27 Pa. Prior to each experiment, the samples were outgassed overnight at 673 K in vacuum. Catalytic tests of NO reduction by ethylene in high oxidizing atmosphere (NO-C2I-I4-O2) were performed in a fixed-bed quartz tubular downflow reactor, introducing about 0.1 g of catalyst in powder form. The feed gases 2% NO/He, 1% C 2 ~ e , and 20% O2/He or pure 02 were special mixtures supplied by Sapio (Italy) ; they were fed from independent mass flow controllers (Bronkhorst, Hi-Tee.). For all runs the reactant gas was 5000 ppm NO, 5000 ppm C2I-h, and partial pressures of 02 from 5000 to 90,000 ppm, with balance He, at a total flowrate of 5.5 L.h "1. Space velocity corresponded to 50,000 hq (GHSV). The reaction was studied
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in the 200-500~ temperature interval, realizing ten isothermal plateaux and maintaining each temperature for 3 h. Other tests were performed under anaerobic conditions (NO-C2I-h with balance He) at higher contact times. The reactor outflow was analyzed using a gas chromatograph equipped with a T.C.D. detector (Chrompaek) mounting a 60/80 Carboxen1000 column (Supelchem) for the separation of 02, N2, N20, NO, C2H4, CO, and CO2. 3. RESULTS AND DISCUSSION Only a flight decrease of the surface area of the prepared materials was observed with respect to the support, except for the sample based on MgO. The samples were well dispersed, as the characteristic XRD lines for SnO2 were evidenced only for the highly loaded samples (20 wt% Sn). Solid state NMR spectroscopy did not show any signal specific of the presence of SnO ; only SnO2 was detected. Figs. 1a and lb represent the differential heats of NH3 adsorption versus coverage for the listed samples. A small amount of tin oxide enhances considerably the acidity when SnOz is added on SiO2 or MgO, but an increase of the loading from 3 to 20 wt% Sn has less influence [7]. However the respective numbers of strong acid sites (V~r) remain small (29 and 27 lamol.g only), but due to the large decrease in surface area the Sn-MgO sample appears to be more modified. The series of materials using alumina, titania and zireonia as supports are similar in their behavior. The presence of SnO2 on these supports produces only very small increases of the acidity, more visible at low loading than at high loading. The deposition of SnO2 on A1203 increases the number of acid sites on the support in the domain of medium and weak strength but does not affect the heats of adsorption at low coverage, and an increase of Sn loading from 3 to 20 wt% does not change significantly the acidity [7]. Q (kJ/mol)
O (kJ/mol) .......
. . . .
50
o
5
0
~
0.5
1
1.5
2
2.5
NH3 uptake (IJmol/m2)
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Figure 1a : Differential heats of NH3 adsorption versus coverage for Sn-Al203, Sn-ZrO2, Sn-SiO2 and the respective supports
0
0.5
1
1.5
2
2.5
NH3 uptake (l~mol/m2)
8
8.5
Figure lb : Differential heats of NHs adsorption versus coverage for Sn-Ti02, Sn-MgO and the respective supports.
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The effectiveness of Sn-yAI203for NO reduction under high oxygen partial pressures and in the presence of water was recently presented in the literature [2,3], while information on the influence of the acidity of the support is not yet available. The low loading of Sn (ca. 3 wt%) on the A1203, TiO2, SiO2, ZrO2 supports led to high activity in the NO-C2I~-O2 reaction, while the loading on MgO support caused activity neither towards the reduction of NO nor towards the oxidation of C2I-h. The anaerobic activity of the SnO2 surfaces (NO-C2I-h reduction in He atmosphere) was poor, while the NO conversion to N2 increased with 02 partial pressure (NO-C2I-h-O2 reaction). Starting from the stoichiometric proportion of 02 with respect to C2I-h fed (10,000 ppm of 02 for 5000 ppm C2I-h), a remarkable increase of activity was observed. This activity was maintained and slightly improved with 02 partial pressures up to 90,000 ppm 02 content. Increasing the Sn loading up to about 20%, a slight modification of the NO reduction activity was observed and a more significant increase of the C2I-h oxidation was observed. Thus, the selectivity of the catalysts with high Sn loading decreased, indicating that large Sn crystallites are more effective than small ones towards the oxidation of the hydrocarbon [9] Based on these results, we detail here the results obtained on the low loading Sn catalyst series (3 wt. %). The five different prepared Sn-based catalysts (see Table 1) were tested in the NO-C2I-h-O2 reaction as a function of temperature (200-500~ at very high space velocity (50,000 h4) in high oxidizing atmosphere (02 content 9%). Fig. 2 reports all the data collected at each reaction temperature, plotted as the conversion of NO to N2 as a function of the extent of the C2I-h conversion to CO2, considered as an index of the extent of the reaction [ 10]. CO2 is indeed the common reaction product deriving both from the reduction of NO with C2I-h and from the oxidation of C2I-h. In this representation, the most active and selective catalyst would have the N2-CO2 curve with the highest slope, giving high N2 formation and low CO2 production. On the contrary, the least active and selective catalyst would have the N2-CO2 50 I I | ~ 40 I/ Z / ~
~ z
I i w 13 Sn-ZrO2" 0 Sn-TiO2 / . ~ ~ * Sn-AI203 ~ ~-~-x-~ A Sn-Si02 / /,"[3/ 0 Sn-Mg0 / ~ ~ - - 0 "
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Figure 2: Conversion of NO to N2 as a function of CO2 produced during the NO-C2H4-O2 reaction (initial concentrations 5000:5000:90,000 ppm respectively) realized at 50,000 h"l over the SnO2-based catalysts. Each marker corresponds to different reaction temperatures in the range 200-500~
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! , | S n - Z r O 2 Sn-TiO 2 Sn-AI20 3 Sn-SiO 2 S n - M g O
i | | ! S n - Z r O 2 Sn-TiO 2 Sn-AI20 3 Sn-SiO 2 S n - M g O
Figure 3" Comparison of the yield of N2 and N20 formed from NO (A) and of the yield of CO2, CO, and CH4 formed from C2I-I403), for the SnO2 based catalysts (reaction temperature" 450~ ; GHSV" 50,000 hq', feed concentration" 5000 ppm of NO and C2H4 and 90,000 ppm of O2). curve with the lowest slope, reaching high CO2 production with low N2 formation. In any case, the conversion of NO to N2 started around 300-350~ (Fig. 2). Above this temperature, the N2-CO2 curves increase regularly with temperature, up to a more or less well-defined point, and then decrease for high temperatures (i.e., high CO2 formation). The highest N2 yield was observed over Sn-ZrO2 catalyst, for which a maximum of N2 formed (47%) was observed at 425~ (77% of C2I-I4 converted to CO2). For Sn-AI203 too, it was possible to detect a maximum of N2 formed (43%) at 475~ (48% of C2H4 converted to CO2). For Sn-SiO2 and Sn-TiO2, starting from 425~ the observed formation of N2 (29% and 34%, respectively) was maintained also for higher temperatures. Among the N-products of reaction, besides N2, N20 was detected at all reaction temperatures, in similar amounts for all catalysts (Fig. 3A); the observed amounts did not follow a regular increasing or decreasing trend with temperature. The presence of N20 could derive from the homogeneous dismutation of the unreacted NO to NO2 and N20 occurring in the reaction atmosphere. Among the C-products of reaction, CO and CH4 were detected besides CO2. Fig. 3B reports the yield of the various C-products at 450~ for the different catalysts. CH4 was only detected on Sn-ZrO2 at high temperatures (_> 450~ CO was mainly formed on Sn-TiO2 and Sn-AI203, and quite absent on Sn-ZrO2 that gave the "cleanest" reaction of NO reduction in comparison with the other Sn-based catalysts. Table 2 collects the temperatures and values of maximum conversion of NO to N2, from which an order of activity can be obtained: Sn-ZrO2 > Sn-AI203 > Sn-TiO2 > Sn-SiO2. The good activity of the SnO2 surfaces is reflected by the very high values of the integral conversion rate of NO to N2 (TOF, in terms of mole of N2 formed per mole of Sn per minute). The competitive factor (c.f. %) can be used in order to compare the selectivity of the catalysts in the NO-C2H4-O2 reaction [11]; a c.f. of 100% would mean that every molecule of hydrocarbon reacting to form CO2 has been used to reduce NO to N2. Values between 8 and 11% are typical of some of the best catalysts reported in the literature [2]. The values of c.f. determined for the SnO2-based catalysts are reported in Table 2. Sn-A1203, Sn-TiO2 and SnZrO2 catalysts have very high values of c.f., indicating a good selectivity and reflecting their ability to reduce NO in the presence of C2H4 in a high oxidizing atmosphere. As a general
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Table 2 Competitive factor (c.f. %) and integral NO conversion rates to N2 (TOF) for the Sn-based catalysts . . . . c.f(%) 102.TOF(mol Nz/mol Sn)-minq Catalyst Temperature (~ Sn-Al203 475 14.9 28.2 Sn-MgO _ Sn-SiO2 475 6.8 24.0 Sn-TiO2 450 11.0 25.8 Sn-ZrO2 425 10.3 30.5
trend, the values of c.f. decreased with increasing temperature (predominance of the C2I-I4 oxidation with respect to the NO reduction). In catalysis, the surface area, the intimate contact between the two oxide components and the size of the resulting metal oxide clusters strongly influence the rate and selectivity of chemical reactions. The notable features of the A12Oa, Z r O 2 and TiO2 supports are their ability to disperse the active metal phase, giving rise to strong metal-support interactions, especially at low loadings. The number of sites which chemisorb ammonia (V~) is always increased when tin oxide has been added on a support. The superior activity and selectivity of catalysts based on alumina, zirconia and titania, which are the most acidic supports, may be the result of a high dispersion of the active metal and the stabilization of the dispersed phase of the second component, tin oxide. The differential heat curves deduced from ammonia adsorption calorimetry indicate a wide distribution of the strength of the acid centres. However a close examination of the data sequence indicates that the surface density of the very strong acid sites (Q>140 lcJ.mol1) is almost constant, while the population of sites around 100-120 kJ.molq is the most affected. In the case of MgO, there is a possible formation of a solid solution which could be responsible of the important decrease of the surface area of the catalyst. So, it appears that depositing a small amount of tin dioxide on a support increases the number of total acid sites by creating a superficial electron deficit in comparison with the support. Moreover the acidity of the support plays a role in stabilizing the active phase in a high dispersion [3]. REFERENCES
1. T. Maunula, Y. Kintaichi, M. Inaba, M. Haneda, K. Sato, H. Hamada, Appl. Catal. B, Environmental, 15 (1998) 291. 2. M.C. Kung, P.W. Park, D.W. Kim, H.H. Kung, J. Catal., 181 (1999) 1. 3. P.W. Park, H.H. Kung, D.W. Kim, M.C. Kung, J. Catal., 184 (1999) 440. 4. T. Miyadera, K. Yoshida, Chem. Letters, (1993) 1483. 5. Y. Teraoka, T. Harada, T. Iwasaki, T. Ikeda, S. Kagawa, Chem. Letters, (1993) 773. 6. A. Auroux, Topics in Catalysis, 4 (1997) 71. 7. D. Sprinceana, M. Caldararu, N.I. Ionescu, A. Auroux, J. Therm. Anal. Cal., 56 (1999) 109. 8. B. Gergely, A. Auroux, ges. Chem. Intermed., 25 (1999) 13. 9. A. Auroux, D. Sprineeana, A. Gervasini, in preparation 10. A. Gervasini, P. Carniti, V. Ragaini, Appl. Catal. B : Environmental, (1999) in press. 11. K.A. Bethke, M.C. Kung, B. Yang, M. Shah, D. Alt, C. Li, H.H. Kung, Catal. Today, 26 (1995) 79.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Sulphated-ZrO2 prepared by impregnation with ammonium, sodium, or copper sulphate: catalytic activity for NO abatement with propene in the presence ol oxygen Valerio Indovina, Maria Cristina Campa, and Daniela Pietrogiacomi Centro di Studio "SACSO" CNR c/o Dipartimento di Chimica, Universit~ degli Studi di Roma "La Sapienza", Piazzale Aldo Moro 5, 00185 Roma, Italy. Fax: +39-6-490324. E-mail:
[email protected] 1.it Sulphated zirconia samples were prepared by impregnation of hydrous zirconia dried at 383 K or calcined at 823 K (monoclinic ZrO2) with aqueous solutions of (NH4)2SO4, Na2SO4, or CuSO4. After calcining at 823 K, samples prepared with hydrous zirconia dried at 383 K were tetragonal. At the same sulphate content, (NH4)2SOa/ZrO2 and CuSOa/ZrO2 contained sulphates of the same type. As the total sulphate content increased, IR indicated an increasing (polynuclear)/(mononuclear) sulphates ratio. (NH4)2SO4/ZrO2 and CuSO4/ZrO2 were both active for the abatement of NO with propene in the presence of oxygen. (NH4)2SOa/ZrO2 had several drawbacks: (i) CO formed in large amounts, (ii) N20 formed, (iii) the catalyst surface coked, and (iv) catalytic activity changed as a function of time on stream. CuSOa/ZrO2 catalysts had practically none of these unfavourable features. For a comparison, some catalytic experiments were also run using propane or methane as reducing agent. 1. INTRODUCTION Copper-based catalysts have been intensely studied because of their activity in the selective catalytic reduction of NOx with various hydrocarbons in the presence of 02. Two reviews have addressed the structural features, redox properties, and catalytic activity of copper exchanged in the zeolite framework or supported on oxide matrices [ 1-2]. In a recent study, using propene or ammonia as the reducing agent in the presence of oxygen, we found that the activity for NO abatement of CuOx/ZrO2 depended on copper dispersion. With propene and with ammonia, the tumover frequency (Ncu/NO molecules s" Cu-atom -') was nearly independent of the Cu-content, up to about 2.5 atoms nm L, namely below the limit evidenced by the characterisation to have good copper dispersion [3]. As a catalyst for the abatement of NO with propane in the presence of oxygen, sulphated-ZrO2 was markedly more active than pure ZrO2 [4]. With decane as the reducing agent, up to 773 K, neither sulphated-ZrO2, nor pure ZrO2 were active. To have active catalysts, copper had to be added to the sulphated-ZrO2 [5]. These facts prompted us to study sulphated-ZrO2 samples as catalysts for the abatement of NO with propene, propane or methane in the presence of oxygen. ZrO2 was sulphated to various extents by impregnation with (NH4)2SO4,Na2SO4, or CuSO4. 2. EXPERIMENTAL
2.1. Catalysts
The starting material used for the preparation of catalysts was hydrous zirconium oxide obtained from hydrolysis of zirconium oxychloride, dried at 383 K, Z(383), or calcined at 823 K, Z(823). Samples were prepared by impregnation of Z(383) or Z(823) with aqueous solutions of (NH4)2SO4, Na2SO4, or CuSO4. After impregnation, samples were dried at 383 K and calcined at 823 K. Samples are designated by A-ZSa(x, y), where A is H, Na, or Cu; Z
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stands for ZrO2 and S for sulphate; a specifies the structure after calcining at 823 K (m, monoclinic or t, tetragonal); x specifies the analytical sulphate content (molecules nm ~, alkaline extraction with NaOH 1M, ionic chromatography, Dionex 2000i), and y the analytical metal content (atoms nm 2, atomic absorption, Varian SpectrAA-30). The surface area (SA/m2g "1) of samples was measured by N2 adsorption at 77 K. Phases (m or t) were identified by means of a Philips PW 1729 diffractometer (Cuk~, Ni filtered radiation). The following samples were prepared and used as catalysts: Catalyst H-ZSm(0.2) H-ZSm(0.6) H-ZSm(1.5) H-ZSm(2.4) H-ZSt(1.5) Na-ZSm(1.9)
SA/m 2g'l 52 52 52 52 99 54
Catalyst Na-ZSt(1.9) Cu-ZSm(0.3, 0.3) Cu-ZSm(0.6, 0.7) Cu-ZSm(1.9, 2.3) Cu-ZSm(3.3, 4.2) Cu-ZSt(0.9, 1.1)
SA/m 2g'l 54 52 52 52 52 116
For a comparison, we also studied a commercial sample of sulphated zirconia (type XZO 682/1), obtained from MEL Chemicals (Manchester, England), and containing 2.3 sulphatemolecules nm -2 after calcining at 823 K (tetragonal, SA = 135 mEg'l). This sample will be hereafter referred to as H-ZStMEL. IR spectra were run at RT on an FTIR spectrometer (Perkin Elmer 2000) operated at a resolution of 4 cm "1. The powdered samples were pelletted (pressure, 15-10 ~ kg cm 2) in selfsupporting disks of ca. 15 mg cm 2, and. put in an IR cell which allowed thermal treatments in vacuum or in a controlled atmosphere. Before infrared experiments were run, samples were heated in O2 at 793 K for 0.5 h, and evacuated at the same temperature for 1 h.
2.2. Catalytic experiments
The catalytic activity was measured in a flow apparatus at atmospheric pressure. The apparatus included a feeding section where a gas stream consisting of He, NO (3% in He), 02 (10% in He), and C3H6 (1% in He), or C3H8 (1% in He), or CH4 (2% in He) was regulated by means of independent mass flow controller-meters (MKS mod. 1259, driven by a four channel unit MKS mod. 247 c) and mixed in a glass ampoule before entering the reactor. Gas mixtures were purchased from Rivoira and were used without further purification. The reactor was made of silica with an internal sintered frit of about 12 mm diameter supporting the powdered catalyst. The reactor was positioned upright in an electrical heater, with a thermocouple touching the external wall of the reactor at the middle of the catalyst bed. Temperature was maintained within + 1 K with a commercial device. A reactor bypass was provided by a fourway valve. Reactants and products were analysed by gas-chromatography (Varian mod. 6000, equipped with an Alltech CTR 1 column at 308 K). A thermal conductivity detector was used for detecting N2, N20, CO, CO2, and a flame ionisation detector for the hydrocarbons. Peak areas were evaluated by electronic integration. A fresh portion of catalyst (0.25 g) was treated in a flow of 2 % O2/He mixture, while heating the reactor from room temperature to 773 K in about 1 h and then isothermally at 773 K for 1 h. After this treatment, the reactor was bypassed and the temperature adjusted to the desired value. Catalysis was run with NO:Call6 (or C3H8):O2=4000 ppm:2000 ppm:20000 ppm, or with NO:CH4:O2=4000 ppm:4000 ppm:20000 ppm, with He as balance. After stabilisation of the reactant composition, the four-way valve was switched and the mixture allowed to flow into the reactor, thereby starting a catalysis run. The reaction temperature was changed at random without intermediate activation treatments between consecutive runs. The total flow rate was maintained at 50 cm 3 STP/min. The rate of NO abatement, RNo/molecules slnm "2, and the NO conversion were calculated l 2 from N2+N20 produced. The rate of the total hydrocarbon consumed Rcxn,/molecules s n m , and the hydrocarbon conversion were calculated from CO2+CO produced~ Percent selectivities
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(SscR=100XRNo/RCxHy; SN2=100xN2/(N2+N20); 8CO2--100xCO2/(CO2+CO)) were calculated asumxing the following overall reactions: C3H6 + 2 NO + (7+x-y)/2 O2 -* (l-x) N2 + x N20 + (3-y) CO2 + y CO + 3 H20 C3Hs + 2 NO + (8+x-y)/2 O2 ~ (l-x) N2 + x N20 + (3-y) CO2 + y CO + 4 H20 CH4 + 2 NO + (2+x-y)/2 O2 -* (l-x) N2 + x N20 + (l-y) CO2 + y CO + 2 H20
3. RESULTS AND DISCUSSION 3.1. Infrared eharaeterisation
After activation, spectra of all H-ZSm samples consisted of several bands in the 1250-900 cm l region, typical of S-O symmetric stretching modes (1200, 1060, 1034, 990, 925, and 900 cml), and a complex absorption in the 1420-1350 cm region, typical of covalent S=O asymmetric stretching modes of organic sulphates. The overall intensity of all these bands increased with the sulphate content. As the sulphate content increased, the complex-absorption maximum in the 1420-1350 cm z region shifted from 1364 cm ~ to 1400 cm", indicating an increasing (polynuclear)/(mononuclear) sulphates ratio, in agreement with previous reports [6], (Fig. 1, spectra 2-5).
~J r ,...,
O
.''''" -
,1'
.
,'
,
,
9
6. "
.-
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. "'-''"
)~ 9
0"000
i
i
600
m i
9 i
,
I
,
/
I
,
700 800 900 1000 T/K Figure 5. Molar fraction of N2 in the reaction products as a function of the reaction temperature (-~ - without binder (Si/AI = 40, 1.92 % Cu); 9 - with Nasilicate as a binder)
'O
t
0.02 eL
o. e .~~
9- n
0.00
)~
9
0.04
1
.......
i'o
9--o "go
\o 9annm-m
9
.....
do/nm
16o
Figure 6. Pore size distribution (o - without binder; 9 - with Na-silicate as a binder)
4. C O N C L U S I O N S
This study attempts to present the relation between the structure and the catalytic behaviour of Cu/ZSM-5 catalyst in NO decomposition, covering both compositional and structural aspects. The results reveal the optimal copper content at which the Cu/ZSM-5 catalyst exhibits maximum activity. Activity of Cu/ZSM-5 catalyst increases with the increase in Si/A1 ratio which leads to the conclusion that the active sites are the isolated copper species. The addition of binders does not change significantly the activity of the catalyst in spite of the changed physical properties. ACKNOWLEDGMENT
We extend our grateful thank to the Croatian Ministry of Science and Technology for their financial support to this work. REFERENCES
1. 2. 3. 4. 5. 6.
J.N. Armor, Appl. Catal. B: Environmental 1 (1992) 221. N. de Nevers, Air Pollution Control Engineering, McGraw-Hill, New York (1995) 394. J.N. Armor, Catal. Today 26 (1995) 99. V. Toma~i@, Z. Gomzi, S. ZmSevi[], Appl. Catal. B: Environmental, 18 (1998) 233. V. Toma~i[], Z. Gomzi, S. Zm[]eviD, React. Kinet. Catal. Lett. 64 (1998) 89. M. Iwamoto, H. Yahiro, K. Tanda, N. Mizuno, Y. Mine, S. Kagawa, J. Phys.Chem. 95 (1991) 3727.
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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Changes in Cation Coordination during Deactivation of C u - Z S M - 5 deNO~ Catalysts S.A. G6mez 1, A. Campero 2, A. Martinez-Hemfindez ~ and G.A. Fuentes ~ ~Area de Ingenieria Quimica, 2Area de Quimica Inorghnica, Universidad A. MetropolitanaIztapalapa. A.P. 55-534, 09340 M6xico D.F., MEXICO. e-mail:
[email protected], mx EPR analyses of Cu-ZSM-5, fresh and reaction-tested in the selective reduction of NO in the presence of 10% H20 at 673 K, evidenced the essentially equimolar presence of Cu 2+ in square planar and square pyramidal environments before permanent catalyst deactivation occurred. Once irreversible deactivation ensued, a third Cu 2+ species appeared, presumably associated with the loss in NO activity. This species, most probably located in the pockets of ZSM-5, is difficult to reduce and hence expected to be less reactive. Deactivation in our samples cannot be ascribed to dealumination of the zeolite or Cu compound formation. I. INTRODUCTION Selective catalytic reduction of NOx (SCR) has received a great deal of attention as a plausible car exhaust treatment technology in both lean-bum gasoline engines and Diesel engines. Transition metal exchanged ZSM-5 zeolites comprise a most promising alternative as catalysts, although various hurdles remain, in particular the strong sensitivity of some of these materials against water and SO2 under realistic operating conditions. Cu-ZSM-5 was the first system reported for lean burn SCR [ 1]. However, despite being the subject of numerous studies, it remains only partially understood. In particular, there are still controversial conclusions in the literature concerning the existence of various Cu n+ species in this system before and a~er deactivation [2-8]. In fact, it is not yet clear what is the relationship between those species and activity, as well as how water affects their structure and reactivity. In our opinion, a primary reason for this situation is the large variety of pretreatments used prior to analyses of Cun+ siting and the large Cu loadings used in some cases. The use of accelerated aging conditions, generally involving a temperature excursion during pretreatment or even during reaction [9-11], besides modifying Cun+ geometry or location frequently activates degradation of the zeolite framework as well as the formation of inactive Cun+ compounds [ 12, 13]. These processes may not be significant under less stringent conditions. In spite of the fact that water is present in large concentrations in the engine exhaust, its effect upon activity and upon the structure of lean burn SCR catalysts is not well understood [11, 14-19]. We have been particularly interested in studying the effect of water upon the activity and structure of Cu-ZSM-5, using it as a model system for this class of catalysts. In fact, our findings [12, 13] suggest that at low temperature (T _ 30 s'l). This oxygen is thought to bind directly with surface sites capable to adsorb NO as mononitrosyls. The number of these sites was estimated to be 50+70 % from the total amount of cobalt (depending
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10
91 0
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06 Q.
~ 0
--
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oo
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(a)
l b '2'0 '3'0 '40 time,
s
I
Ar
(b)
1
234
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o
sb ',6o'I~o 2 o o time,
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s
Fig.2. Responses of ~5N (a) and ~80 (b) isotope fraction in NO at~er 14N160to ~5N~80replacement in NO + He flow (1) mad in NO + Oz + He flows at 0.15% (2), 0.3% (3), 1.5% (4) and 3% (5) of oxygen. on the stoichiometry CoO or C0203). Thus, a part of cobalt atoms was proposed to locate in the bulk of clusters and so can't serve as adsorption sites. Oxygen label transfer from NO adsorption sites into zeolite bulk [OzEoL] proceeds considerably slower (13ZEOL= 0.02 S'l).
4.2.2. 14N160replacement with lSNl80 in NO+1602+He flow Time delays of ~SN label appearance in NO are substantially higher in the presence of oxygen (Fig.2a, curves 2-5) indicating the increase of concentration of fast replaced adsorbed species. A pronounced "slow" components on the response curves could arise from retardation of replacement of the species that are more strongly bound with the surface. In accordance with DRIFT data, we can assign the increase of concentration of fast replaced species to the tbrmation of NO2~§ species, while slow replaced species can be related to nitrite complexes. Since the time delay for ~SO emergence doesn't depend on oxygen presence in the flow (Fig.2b, curves 2-5), a direct oxygen exchange between NO28§ species and cobalt oxide clusters can be excluded. Some decrease of 180 label concentration in NO in the presence of oxygen is determined partially by label transfer into the gas phase oxygen. Mathematical simulation of the dynamics of oxygen label transfer shows that the main reason is an increase of the rate of label transfer into the catalyst bulk (13ZEOL= 0.023 s'l). NO2~§ species being directly bound with zeolite could be responsible for this process. We estimated the rates of the formation of all species depending on oxygen concentration. It was found that the rate of label transfer into 02 by the order of magnitude lower than that of NO2~ formation, i.e. their decomposition isn't accompanied by simultaneous oxygen desorption into the gas phase. Thus, NO28§ species is thought to result from interaction of preliminary adsorbed oxygen with NO. As oxygen concentration is raised, surface concentration of NO 2~+ species is increased rapidly reaching the limiting value at 1.5% of Oz (Fig.3). Since this value isn't change aRer the rise of NO concentration (from 0.6 to 1.2%), it means that equilibrium [O] + NO r [NO28+] is shitted to the fight, and almost all adsorbed oxygen is included into NO2~+. In this case, limiting coverage of NO2~+ (~2"10 ]9 molec./gc.,t) approximately corresponds to the quantity of surface active sites for oxygen adsorption, and amounts to amounts to 10% of the total quantity of cobalt atoms.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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1.0
1.0
O
8 0.5 o
L.
N9 O.5
~
E
L_
E t_
0 I-"
0.0
0.0
O ~
concentration of 0 2
Fig.3. Dependence of normalized reaction rate (o) and NO2~ concentration ( 9 ) o n
02
content.
4.2.3. 14N160replacement with lSNlsO in NO+1602+CH4+He flow Under the reaction conditions the delay for ~SN appearance in NO is smaller compared with corresponding values observed in NO+O2+He flow (Fig.2a); "slow" components are not revealed. Since this decrease of the quantity of adsorbed species (9"10 TM mol/g.cat at Co2=3%) can not be due to consumption of mononitrosyls only (see above), then a conclusion on NO2~* and nitrite species participation in CI-I4 activation can be done. However, taking into account slow accumulation of nitrite complexes, contribution of their reaction with methane to the overall process seems to be negligible. Unlike to mononitrosyls, concentration of NO2~§ species is increased, as 02 content is raised, and correlates well with the behavior of N2 formation rate (Fig.3). Hence, NO2~§ could be considered as a main species to react with methane, and the reaction scheme may be written as follows:
1) 2 [1 + 02 +----+ 2 [O1 2) [O] + NO +---+ [NO2~+1 3) [NO2~+] + CI-I4 > [NCH20] + H20 As was shown above, concentration of adsorbed oxygen [O] is very low, and surface complexes [NCH20] seem to be oxidized via direct interaction with molecular oxygen 4) [NCH20]
+
02 d- NO -----+ C02 + HE0 + N2 + [0]
As can be seen from Fig.4, isotope fraction in N2 is initially higher than in NO (Fig.4, curves 2 and 1, respectively). Excess of isotope fraction in the product over that in labelled reagent can be observed only in a plug-flow reactor. At high rate of NO adsorption-desorption ]SN fraction in the gas phase NO and [NOx] is strongly decreased along the catalyst bed. Calculations show that in this case at almost complete ]4N replacement at the beginning of the bed (including [NOx] and [NCHzO]) a negligible outlet label content in NO can be observed. Outlet isotope fraction in N2 being equal to a half of a sum of length-averaged label fraction in NO and [NCH20] (in accordance with the reaction scheme) should be higher than ~SN fraction in the outlet NO (Fig.4, curve 1). SSITKA allows to estimate the ratio of the rate of intermediate conversion to its concentration (i.e. turnover frequency) basing on the value of the shiR of the response of isotope fraction in the reaction product relative to that in labelled reagent [9]. However, in the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1510
:~ o
o o
.fn
o
,
1.0
1.0
0.8
0.8
0.6
0.6
o.,
o.4
0.2
0.2
0.0
o
(]
0.0
5
1'0 tim e,
1'5 s
20
,
,
,
3 .-"
-=
/
O 0
5
10
15
20
tim e, s
Fig.4. Experimental (points) and calculated (lines) outlet responses of 1~1 isotope fraction in NO (o, 1) and N2 ( o, 2) as well as calculated length-averaged isotope fraction in NO (3) in NO + CH4 + 02 + He flow at 0.15% (a) and 3% (b) of oxygen. case of plug-flow reactor for such estimates we should compare isotope response in N2 with length-averaged label fraction in NO (Fig.4, curve 3). As seen from this Figure, the values of these shifts are close at the experiments withquite different oxygen concentration. It means that the rate of [NCH20] conversion into N2 isn't limited by oxygen concentration.This can be realized in the case when first nitrogen is formed and evolved into the gas phase at the reaction of this intermediate with NO, remaining C-containing fragment is oxidized to CO2. 5. CONCLUSIONS The main role of oxygen can be reduced to the formation of relatively stable oxidized sites, participating in NO2~+formation. The last species are active in the reaction with methane that was confirmed by correlation between the rate of N2 formation and NO28§ concentration depending on oxygen concentration. Step of oxygen adsorption, however, isn't included into catalytic cycle, it plays a role of buffer in steady-state conditions. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
J.N. Armor, Catal. Today, 26 (1995) 147. M.D. Amiridis, T. Zhang and R.J. Farrauto, Appl. Catal. B, 10 (1996) 203. C. Montes de Correa and A. Luz Villa de P., Catal. Lett., 53 (1998) 205. A.Yu. Stakheev, C.W. Lee, S.J. Park and P.J. Chong, Catal. Lett., 38 (1996) 271. A.D. Cowan, N.W. Cant, B.S. Haynes and P.F. Nelson, J. Catal., 176 (1998) 329. Y. Li, T.L. Slager and J.N. Armor, J. Catal., 150 (1994) 388. A.T. Bell, Catal. Today, 38 (1997) 151. L.G. Pinaeva, E.M. Sadovskaya, A.P. Suknev, V.B. Goncharov, V.A. Sadykov and B.S. Balzhinimaev, and T. Decamp and C. Mirodatos, Chem. Eng. Sci., 54 (1999) 4327. 9. E.M. Sadovskaya, D.A. Bulushev and B.S. Bal'zlfinimaev, Kin. and Catal., 40 (1999) 54.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
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Effect of Si/Al ratio of Mordenite and ZSM-5 type Zeolite Catalysts on Hydrothermal Stability for NO Reduction by Hydrocarbons Sung Yeup Chung a, Beom Seok Kim a, Suk Bong Hong b, In-Sik Nam *a and Young Gul Kim a a Research Center for Catalytic Technology, Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science and Technology (POSTECH) P.O. Box 125, Pohang 790-784, Korea b Department of Chemical Technology, Taejon National University of Technology, Taejon 300-717, Korea 1. INTRODUCTION One of the most urgent and demanding challenges in environmental catalysis is to abate NO produced on a massive scale from lean-burn gasoline and diesel engines.
The
discovery by Iwamoto et al. that copper-ion-exchanged ZSM-5 (CuZSM-5) zeolite is much more active than earlier catalysts for selective catalytic reduction of NO in an oxidizing atmosphere by hydrocarbon has led to a tremendous interest in the use of a wide variety of transition-metal ions exchanged into zeolites with different framework structures and compositions as a catalyst for this reaction system [1 ].
However, there is a serious drawback
in the practical application of transition-metal-ion-exchanged zeolites including CuZSM-5 to lean-burn engines, due to poor stability under hydrothermal conditions [1,2].
Thus,
considerable effort has been made to elucidate the origin of the deactivation of the catalyst by H20 [2,3]. Recently, it has been shown that mordenite (MOR) zeolites in both synthetic (HM and CuHM) and natural (CuNZA) forms are highly active for the reduction of NO by hydrocarbons [4].
It was also observed that in the presence of H20, the CuHM catalyst loses
significantly its deNOx activity by the competitive adsorption of NO and H20 on the catalyst surface. Unlike CuHM, however, the CuNZA catalyst showed a peculiar water tolerance for NO removal reaction [4,5].
This result was rationalized by suggesting that the hydrothermal
stability of CuNZA for NO reduction would be better than CuHM, because of its higher Si/A1 ratio.
In the present study, the hydrothermal stability of CuNZA, CuHM and CuZSM-5 with
* To whom all correspondence should be addressed. Phone: 82-562-279-2264.
Fax: 82-562-279-8299.
E-mail:
[email protected].
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different Si/A1 ratios for the NO reduction under a simulated lean NOx condition has been
systematically reported.
In addition, changes in the physicochemical properties of the
catalysts induced by hydrothermal aging are also investigated to understand the catalyst deactivation behavior. 2. E X P E R I M E N T A L
A MOR type natural zeolite (NZ), mined from Youngil, Korea [6], and a synthetic MOR (Zeolon 900Na, PQ Corp.) were employed to obtain CuNZA and CuHM, respectively. Prior to Cu 2+ ion exchange, these two MOR materials were also dealuminated by acid treatment and steaming to increase Si/A1 ratio in the zeolite.
Five CuZSM-5 catalysts with
Si/A1 ratios ranging from 14 to 95 were prepared using the corresponding HZSM-5 purchased from Tosoh Corp.
All Cu2+-exchanged zeolite catalysts were prepared by conventional
liquid-phase and solid-state ion exchange methods that are described elsewhere [4,5]. Chemical analysis for Si, A1, Cu in catalysts was performed by AA.
The degree of
dealumination of the zeolites was confirmed by 29Si and 27A1 MAS NMR.
The
physicochemical properties of zeolite catalysts employed in this study are listed in Table 1. A simulated lean-bum condition containing 1,200 ppm NO, 1,600 ppm C3H6, 3.2% 02, 3,000 ppm CO, 1,000 ppm H 2, 10% CO2, 10%
H20 and He (balance) was employed to examine the
hydrothermal stability of the catalysts.
The reaction products were analyzed by on-line gas
chromatograph (Hewlett Packard 5890 Series II).
The hydrothermally aged catalysts were
characterized by XRD, ESR, XANES and BET measurements to investigate the cause of the catalyst deactivation. Table 1. Physicochemical properties of zeolite catalysts employed in the present study Catalyst CuHM 1 CuHM2 CuHM3 CuHM4 CuHM5 CuNZA 1 CuNZA2 CuNZA3 CuNZA4 CuZSM-5-1 CuZSM-5-2 CuZSM-5-3 CuZSM-5-4 CuZSM-5-5
Cu content (wt.%) 2.02 4.20 2.55 1.03 1.73 4.37 1.84 1.75 1.64 2.49 2.90 1.43 1.20 0.44
Si/A1 6 5 12 12 22 4 10 14 19 14 26 27 34 95
Cu/A1 0.16 0.30 0.34 0.14 0.45 0.25 0.24 0.28 0.31 0.53 0.81 0.50 0.53 0.52
Surface area (m2/g) 368 434 450 179 232 128 350 344 400 330
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3. RESULTS AND DISCUSSION 3.1 Hydrothermal stability of MOR and MFI type zeolite catalysts Figure 1 shows the hydrothermal stability of CuNZA2, CuHM1 and CuZSM-5-2 catalysts for NO reduction under the lean NOx condition.
When the catalysts were aged at
800~ with 10% H20 for 6 h, the CuHM1 catalyst exhibited a deNOx efficiency of less than 5% at 450 ~
while 15 and 40% of NO conversion were still observed for CuNZA2 and
CuZSM-5-2, respectively.
Thus, the CuZSM-5-2 catalyst was found to exhibit the highest
hydrothermal stability for NO reduction among these three types of zeolite catalysts. Another important observation is that the activity maintenance of the catalysts was proportional to the Si/A1 ratio of the catalyst.
This clearly shows that the hydrothermal
stability of zeolite catalysts for NO reduction under the lean NOx condition primarily depends on the Si/A1 ratio of the catalyst, despite the differences in the zeolite structure. To further elucidate the role of Si/A1 ratio of the catalysts for the hydrothermal stability,
MOR type zeolite catalysts with different Si/A1 ratio were examined.
Figure 2
shows that deNO~ activity of CuNZA catalyst was significantly improved by dealumination. Especially, the CuNZA4 catalyst with a Si/A1 ratio of 19 still showed more than 35% of NO conversion at 550~ even after the aging at 800~ with 10% H20 for 24 h. 100 0~"
o
z q.. o to "~ o> c o O
l
100
8O
o
sO
Z
~=
t
c
40
1
604
'
i
20
300
400
500
600
700
Reaction Temperature (oC)
Figure 1. Hydrothermal stability of CuNZA2 (O,O), CuHM1 (1,[:]) and CuZSM-5-2 (A,A) catalysts. Closed and open symbols indicate fresh catalysts and hydrothermally aged ones at 800~ for 6 h with 10% H20, respectively.
O,
t
=
300
.
.
400
.
.
.
500
,
.
1
600
700
Reaction Temperature (~
Figure 2. Hydrothermal stability of the dealuminated CuNZA catalsyts 9 CuNZA2 (O), CuNZA3 ( I ) and CuNZA4 (A). These catalysts were hydrothermally aged at 800~ for 24 h with 10% HzO.
3.2 Effect of Si/Ai ratio of MOR and MFI type zeolite catalysts To understand the origin of the significant enhancement of the deNOx activity over the dealuminated CuHM catalysts, the activities of CuHM catalysts with different Si/A1 and Cu/A1 ratios have been examined and are given in Figure 3.
Note that the Si/A1 ratio of
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loo
100
. ao "~
8
8.0
i
4o
i
=o 0
30G
404)
500
84)0
0
7'00
Reaction Temperature (~
]
i
.
.
.
.
300 400 500 600 Reaction T e m p e r a t u r e (~
]
700
Figure 4. Effect of Si/A1 ratio of the CuZSM-5 catalysts on their NO removal activity : CuZSM-5-1 ( O ) , CuZSM-5-3 (ll), CuZSM-5-4 ( 9 and CuZSM-5-5 ( 9
Figure 3. Effect of Si/A1 and Cu/A1 ratms of the dealuminated CuHM catal.ysts on their NO removal actw]ty : CuHM1 (O), CuHM2 (1), CuHM3 ( 9 and CuHM4 ( 9
CuHM1 is quite similar to that of CuHM2, while these two catalysts contain Cu/A1 ratios of 0.16 and 0.30, respectively.
However, the conversion of NO was found to be not distinctive
for the catalysts, even though the reaction temperature exhibiting the maximum NO removal activity of the catalysts is slightly shifted.
For CuHM1 and CuHM4 catalysts with similar
Cu/A1 ratios, on the contrary, a significant improvement in the deNOx activity was observed after dealumination.
A similar result was also observed from the distinctive deNOx activities
of CuHM2 and CuHM3.
Therefore, the improvement in the NO removal activity of CuHM
catalysts upon dealumination cannot be attributed to the difference in Cu/AI ratio of the catalyst but the increase in Si/A1 ratio.
This speculation can be further supported by the
catalytic results obtained from CuNZA and CuZSM-5 with different Si/A1 ratios. shows NO removal activity of CuZSM-5 with similar Cu/A1 ratios.
Figure 4
There may be an
optimal S1/A1 ratio for CuZSM-5 catalyst to remove NO by hydrocarbons.
Therefore, it is
most likely that the Si/A1 ratio of the zeolite catalyst is still one of the most critical properties determining NO removal activity of the catalyst for this reaction system. 3.3 Characterization of the zeolite catalysts
When CuZSM-5-2 catalyst was hydrothermally aged at 900~
structural collapse of
the catalyst was clearly observed by a comparison of the intensity of the XRD peak of the catalyst with respect to the aging duration shown in Figure 5.
As the aging time and
temperature increase, the structure of the catalyst was progressively destroyed. intensity of the characteristic diffraction peaks of the catalyst aged at 900~ significantly reduced as depicted in Figure 5(0.
Thus, the
for 24 h was
This might well reflect not only the loss of
the deNOx activity of the catalyst but also the reduction of the catalyst surface area by hydrothermal aging.
A remarkable decrease of XRD peak intensity by hydrothermal aging
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was also observed for CuHM1 and CuNZA2 catalysts.
For the dealuminated catalysts, the
degree of structural destruction of the catalysts examined by XRD well agreed with the rate of the activity maintenance of the catalyst.
This again supports the speculation that the Si/AI
ratio of the catalyst plays an important role for the destruction of zeolite structure by hydrothermal aging as expected. (a)
~
~_t"L,.
' 10
.
L
20
L
, 30
~
(b)
r .-.,.---
~
(e) -;0
50
60
7'0
80
20 Figure 5. X-ray powder diffraction pattern of CuZSM-5-2 catalysts : (a) fresh, (b) aged at 700~ without H20 for 2 h, (c) aged at 800~ with 10% H,O for 4 h, (d) aged at 800~ with 10% H20 for 6 h,-(e) aged at 800~ with 10% H20 for 24 h and (f) aged at 900~ with 10% H20 for 24 h. The destruction of the catalyst structure upon the hydrothermal aging may alter the ionic state of Cu 2+ ions on the catalyst surface.
Figure 6. Cu 2§ ESR spectra of CuZSM5-2 catalysts : (a) fresh, (b) aged at 800~ with 10% H20 for 4 h, (c) aged at 800~ with 10% H20 for 24 h and (d) aged at 900~ with H20 for 24 h.
An alteration of the chemical environment of Cu 2+
ions on the hydrothermally aged catalyst surface was examined by ESR as shown in Figure 6. The low field hyperfine structure of CuZSM-5-2 catalyst was notably reduced when aged at 900~ with 10% H20 for 24 h.
This suggests that Cu 2§ ions on the catalyst surface were
probably transferred to copper oxide species by the sintering of the metal ions through the structural collapse of the catalyst.
Such an alteration of Cu 2§ ions was also observed for
CuHM1 and CuNZA2 catalysts after hydrothermal aging.
Therefore, it is clear that the loss
of Cu 2+ ions on the catalyst surface occurred upon hydrothermal aging and thereby caused a significant loss of NO removal activity of the catalysts. Additionally, the alteration of Cu 2+ ions to the copper oxides has been confirmed by X-ray absorption near edge structure (XANES).
As shown in Figures 7 and 8, isolated Cu 2§
ions were transformed to C u 2 0 after hydrothermal aging, as already examined by XRD and ESR studies.
Therefore, hydrothermal aging caused the structural destruction of zeolite and
the decrease of surface area, and then Cu 2+ions transformed to copper oxides that may not be active reaction sites for the present reaction system.
Based upon the peak intensity of Cu20,
the zeolite with a higher Si/A1 ratio results in less formation of Cu20 on the catalyst surface.
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A
5 ni
\
8
o
"6 activity. This might be as" 40 94 ~ O O sociated with the reducibility o and the low surface area of the z 20 92 z~ crystalline material. Redox 90 properties are one fundamental 0 130 140 150 160 170 prerequesite for the catalytic Temperature (~ effectiveness of the manganese oxide. At higher space velocities Fig. 5. NO conversion to N2 (full symbols, left) and N2 and permanent NH3 supply, the selectivity (open symbols, right) vs. reaction temperature. adsorption/activation of NH3 NH4-Y (triangles), Mn-2 (circles), Mn-5 (squares). from the gas phase is one GHSV = 48000 cma/g h, feed: 1000 ppm NO, l0 vol.-% additional reaction step. How far 02, 7 vol.-% H20, permanent supply of 1000 ppm NH3. structural NH4+ ions of the zeolite are still involved has yet to be clarified. Nevertheless, a breakthrough to lowtemperature conversion of NOx in wet feed beneath 200~ is achieved with the proposed composite catalysts. 9
!
I
9
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f
9
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This work is supported by the Senate of Berlin and the Ministry of Education and Research of the FRG (Project 4086 z O w / c / o 40122/1). Ing. R. Eckelt, Dr. H.-L. Zubowa and Ing. R. Dambowsky is thanked for assistance. M. R. and R. F. are indebted to the Verband der Chemischen Industrie (VCI) for financial support. REFERENCES 1. 2. 3. 4.
F. Pinna, Catal. Today 41 (1998) 129. M. Richter, R. Eckelt, B. Parlitz, R. Fricke, Appl. Catal. B: Environmental 15 (1998) 129. M. Richter, B. Parlitz and R. Fricke, J. Chem. Soc., Chem. Commun., 1997, 383. Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th edition, Springer Verlag 1993, Manganese C 1 (1973). 5. E. R. Stobbe, B. A. de Boer, J. W. Geus, Catal. Today, 47 (1999) 161. 6. A. K. H. Nohmann, M. I. Zaki, S. a. A. Mansour, R. B. Fahim and C. Kappenstein, Thermochimica Acta, 210 (1992) 103. 7. U. Lohse, I. Pitsch, E. Schreier, B.Parlitz, K.-H. Schnabel, Appl. Catal. A: General, 129 (1995) 189.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Catalytic properties of mesoporous molecular sieves for the reduction of NO~ A. Jentys*, W. SchieBer and H. Vinek Vienna University of Technology, Institute for Physical Chemistry Getreidemarkt 9/156, A-1060 Wien, AUSTRIA. http://www.physchem.tuwien.ac.at/catalysis/ The catalytic properties of Pt, Rh and Co supported on mesoporous molecular sieves with MCM-41 type structure for the reduction NOx with propene were investigated. Strongly acidic sites were incorporated by co-impregnation of the catalysts with H3PW12040. Pt/MCM-41 was found to be the most active catalyst, however, the selectivity to N2 formation was rather low. Rh/MCM-41 was less active, but had a significantly higher selectivity to N2 formation compared to Pt/MCM-41. Co/MCM-41 had the lowest activity among the series of catalysts studied. In the presence of water vapor a significant increase in the activity was observed for the Pt and H3PW1204o containing catalysts, while for all other catalysts studied the typical decrease in the activity was found. 1. I N T R O D U C T I O N Nitrogen oxides, formed during combustion processes in power plants, waste incinerators and diesel engines, are among the major air pollutants taking part in the formation of photochemical smog and acid rain [1]. Consequently, the catalytic reduction of NOx plays an important role in the transformation of combustion processes into an environmentally friendly technology [2]. At present, catalysts based on titanium- and vanadium-oxides, together with ammonia as reducing agent, are typically applied to remove nitrogen oxides from flue-gas streams of stationary sources [3]. For mobile sources extensive research is currently carried out to find alternative catalytic systems in order to replace NH3 with other reducing agents such as hydrocarbons [4]. Catalysts based on transition metals containing zeolites, e.g., Cu/ZSM5 [5] and Co/ZSM5 [6], were investigated in the first place, but the strong decrease in their activity when H20 and/or SO2 are present in the reactant [7], led to the development of alternative catalysts such as Fe/ZSM5 [8, 9], Pt/ZSM5 [10] and Pt group metals supported on
Si02 and A1203[I 1, 12]. * Present Address: Technische Universit~it Miinchen, Institut fiir Technische Chemie II Lichtenbergstr.4, 85747 Garching, Germany
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To combine the advantages of zeolites and oxide based systems, we studied the catalytic properties of noble and transition metals supported on mesoporous molecular sieves with MCM-41 type structure. As the acid sites of MCM-41 type materials were found to be only weakly acidic [13], the catalysts were additionally impregnated with tungstophosphoric acid, a procedure, which is known to generate strong (Bronsted) acidic sites on this type of material [14].
2. EXPERIMENTAL 2.1. M a t e r i a l s and c h a r a c t e r i z a t i o n The synthesis of siliceous MCM-41 was carried out according to ref. [15] using fumed silica, hexadecyltrimethylammoniumbromide and tetramethylammoniumhydroxide pentahydrate. The support was impregnated with aqueous solutions of PtC14, Rh(NO3)3, Co(NO3)e and H3PWleO4o. The metal loading of the catalysts used was 1.6 wt% for Pt, 1.0 wt% for Co and 1.7 wt% for Rh. The H3PW1204o loading was varied between 0 and 60wt%.(for details see refs. [16, 17]). The structure of the MCM-41 type material was verified by XRD before and after calcination and by Ne sorption, where the usual features of MCM-41 type materials, e.g., four Bragg reflexes in the XRD and a sharp step in the Neisotherm at p/p0-0.4 were observed. The d-spacing of the hexagonal unit cell, determined from the (100) reflex in the XRD, was 39 A, the BET surface area was 1006 meg -1.
2.2. Catalytic r e a c t i o n s The catalytic activity was studied in a quartz reactor (i.d. 8 mm) containing 100 mg of the catalyst. The temperature was measured with a thermocouple placed in direct contact with the catalyst bed. The composition of the reactant gas was: 1010 ppm NO (about 91 ppm NO were directly oxidized in the reaction system to NOD, 1012 ppm propene and 4.9vo1% 02 (balance helium). Up to 8 vol% water vapor could be added into the carrier gas stream using a syringe pump. The total flow of the reactants was 100 cm3/min and resulted in a space velocity of W/F = 6"10 .2 g.s.cm -3. Reactants and products were analyzed with a chemiluminescence NOx analyzer and a gas chromatograph equipped with a PoraPOLT-Q and a Molsieve 5/~ column using a TCD and a FID detector. The conversion was measured in a temperature range between 180 and 500 ~ 3. R E S U L T S AND D I S C U S S I O N The NOx and C3H6 conversions for the MCM-41 supported catalysts and for the H3PW1204o containing catalysts are shown in Figs. 1 and 2, respectively. Independently of the type of metal and the presence of H3PW12040 the conversion of NOx and propene started at the same temperature and increased
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with increasing temperature until the total conversion of propene was reached. At this temperature the maximum conversion of NO,, was obtained. A further increase of temperature led to a decrease in the NO~ conversion, while the hydrocarbon conversion level remained at 100%. 70
~. lOO
--- Pt/MCM-41 ..-v--. Rh/MCM-41
60o~ 50tO
"~ L (D > tO o x
O z
40 -
30-
80
~
60
"~
4o :~
20-
20 o
|
200
300
400
500
200
0
|
300
400
Temperature [~
Temperature [~
Figure 1: NOx and C3H6 conversion for Pt/MCM-41, Rh/MCM-41 and Co/MCM-41 70
tO
50
"~ 40 u.. to o x
O z
30 20 10
. 9100
Pt/HPW(30)/MCM-41 Rh/HPW(30)/MCM-41 Co/HPW(30)/MCM-41
6O
80
>:
60
8 "~
2o c~ 0
200
300 Temperature [~
400
500
200
300
400
Temperature [~
Figure 2: NOx and C3H6 conversion for Pt/HPW/MCM-41, Rh/HPW/MCM-41 and Co/HPW/MCM-41 (H3PWleO40 loading 30 wt%) The highest NOx conversion was observed for Pt/MCM-41 and it decreased with increasing H3PW~204o loading, while the selectivity to the nitrogen formation increased from 35% (Pt/MCM-41) to 45 % (Pt/HPW(60)/MCM-41). Also the temperature where the maximum NOx conversion was reached increased with the H3PW1204o loading of the catalysts. The activities and selectivities of all catalysts investigated are summarized in figure 3. Rh and Co supported on MCM-41 had a lower activity compared to the series of Pt containing catalysts, while for Rh/MCM-41 the highest selectivity to
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the formation of N2 (67%) was observed. The additional H3PW12040 loading of the Co and Rh containing catalysts led to a lower activity and in contrast to Pt/MCM-41 catalysts also to a lower selectivity to N2 formation. For Co containing catalysts, in contrary to the noble metal containing catalysts studied, the temperature at the maximum NOx conversion decreased after the H3PW12040 loading. 100
90
O O
Pt/MCM41 Pt/HPW(15)/MCM-41 Pt/HPW(30)/MCM-41 Pt/HPW(60)/MCM-41
1"-I m /k, A
Co/MCM-41 Co/HPW(30)/MCM-41 Rh/MCM-41 Rh/HPW(30)/MCM-41
80 70 > .
60
m
~
50
m
ffl
e,i
z
40
F-NA
9 |
30
CD
20 10 0
10
20
30
40
50
60
70
80
90
100
NOx conversion Figure3:NOx conversion and H3PW1204o containing catalysts
N2 selectivity for MCM-41
supported
and
The activity of the catalysts at 300 ~ as a function of the water vapor concentration is shown in figure 4. At this temperature, the activity and selectivity of all Pt containing MCM-41 supported catalysts in water free reaction conditions was similar. As already reported [18], a continuous decrease in the activity with increasing water vapor concentration was observed for Pt/MCM-41, leading to a 15 rel% lower conversion at a water vapor concentration of 8.1 vol%. On the contrary, H3PW1204o containing Pt/MCM-41 catalysts showed an improved activity in the presence of up to 8 vol% water vapor. Note that compared to the reaction in water free atmosphere, the NOx conversion over the Pt/HPW(60)/MCM-41 catalysts increased more than 25 rel% at the presence of 2 vol% water vapor. At higher water vapor concentrations the activity was slightly decreased, however, up to a concentration of 8 vol% H20 all Pt/HPW/MCM-41 catalysts were more active compared to the water free reaction conditions.
家 50
50
40
40.
30
30.
20
20-
Rh/MCM-41 Rh/HPW(30)/MCM-41 Co/MCM-41 Co/HPW(30)/MCM-41
化
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~c~
,_._,
c o
o~ t~
t,,..
o
tO
oX
o z
~
10
7
PtIMCM-41 Pt/H PW( 15)/MC M-41 Pt/HPW(30)/MCM-41 Pt/HPW(60)/MCM-41 ~
0
-
-
2
I
I
4
6
8
Watercontent feed [vol%]
10
0
I
I
I
I
2
4
6
8
1(
Watercontent feed [vol%]
Figure 4: Activity of the catalysts as a function of the water vapor concentration 4. C O N C L U S I O N S Pt supported on siliceous MCM-41 showed a high activity for the reduction of NOx with propene at a relatively low reaction temperature. The co-impregnation with tungstophosphoric acid led to a decrease in the activity, which resulted from a partial coverage of the Pt clusters by tungstophosphoric acid. Contrary to the results published in the literature, the Pt/HPW/MCM-41 catalysts showed a pronounced increase in the catalytic activity during the reduction of NOx with hydrocarbons in the presence of H20 vapor. The Rh/MCM-41 catalyst was less active compared to Pt/MCM-41, but the selectivity to N2 formation was significantly better. However, the decrease in activity in the presence of water vapor was more pronounced compared to Pt/MCM-41. However, Rh/MCM-41 seems to be an interesting option as the selectivity to N2 formation is significantly higher when a MCM-41 type material is used as support compared to ?-A1203. Co/MCM-41 catalysts showed a lower activity compared with both noble metal containing MCM-41 catalysts studied, which might result from an oxidation of Co during the reaction. Additionally, the activity strongly decreased when water was added into the reaction gas mixture. This clearly reflects that noble metals supported on MCM-41 type materials have a higher potential to be applied as catalysts for the reduction of NOx than non noble metals.
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Loading of the catalysts with tungstophosphoric acid led to a decreased activity, while the selectivity to N2 formation reached a similar level on all of these catalysts. Note, that this resulted in an improved selectivity for Pt/HPW/MCM-41, while for Rh/HPW/MCM-41 and Co/HPW/MCM-41 the selectivity was lower compared to the corresponding Me/MCM-41 catalysts.
ACKNOWLEDGEMENT
The work was supported by the project 7119 of the "Hochschuljubil~iumsfonds der 5sterreichischen Nationalbank ". REFERENCES
8 9 10 11 12 13 14 15 16 17 18
C.T. Bowmann, 24 th Symposium on Combustion, (1992) 859. J. N. Armor, Catal. Today, 38 (1977) 163. H. Bosch and F. Janssen, Catal. Today, 2 (1988) 369. J. N. Armor, Catal. Today, 26 (1995) 99. M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya and S. Kagawa, J. Chem. Soc., Chem. Commun., (1986) 1271. Y. Li and J. Armor, Appl. Catal. B, 1 (1992) L31. M. Iwamoto, N. Mizuno and H. Yahiro, Proc. 10th Int. Congr. on Catalysis, Budapest (1992) p. 1285. X. Feng and W. K. Hall, J. Catal., 166 (1997) 368. H.Y Chen and W. M. H. Sachtler, Catal. Lett., 50 (1998) 125. M. Iwamoto, H. Yahiro, H. K. Shin, M. Watanabe, J. Guo, M. Konno, T. Chikahisa and T. Murayama, Appl. Catal. B, 5 (1994) L1. H. Hamada, Y. Kintaichi, M. Sasaki and Y. Ito, Appl. Catal., 75 (1991) L1. R. Burch, P. J. Millington, Catal. Today, 26 (1995) 185. A. Jentys, N. H. Pham and H. Vinek, J. Chem. Soc., Faraday Trans., 92 (1996) 3287. I. V. Kozhevnikov, A. Sinnema, R. J. J. Jansen K. Pamin and H. van Bekkum, Catal. Lett., 30 (1995) 241. C. F. Cheng, D. H. Park and J. Klinowski, J. Chem. Soc., Faraday Trans. 93 (1997) 193. A. Jentys, W. Schiel3er and H. Vinek, Catal. Lett., 47 (1997) 193. A. Jentys, W. Schiel3er and H. Vinek, J. Chem. Soc., Chem. Commun., (1999) 335. W. Schiel3er, H. Vinek and A. Jentys, Catal. Lett., 56 (1998) 189.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
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Catalytic Activity of High Surface Area Mesoporous Mn-based Mixed Oxides for the Deep Oxidation of Methane and Lean-NO~ Reduction V.N. Stathopoulos a, V.C. Belessi a, C.N. Costab, S. Neophytldes, P. Falaras d, A.M. Efstathioub'*, and P.J. Pomonis a'* 9
C
aDepartment of Chemistry, University of Ioannina, Ioannina 45110, Greece 9 bDepartment of Chemistry, University of Cyprus, Nicosia 1678, Cyprus. eFORTH-ICE/HT, P.O. Box 1414, University Campus, 26500 Patras, Greece. dlnstitute of Physical Chemistry-NCSR "Demokritos", 15310 Aghia Paraskevi Attikis, Greece. 1. INTRODUCTION Catalysis has played a major role in environmental protection over the past generation. As the methodology to accomplish environmental protection has developed to include pollution prevention and green chemistry, catalysis has continued to supply the techniques, tools and methodologies which have made some of the most innovative green chemistry technologies possible (e.g., "three-way" catalytic converter). As fuel efficiency coupled with cleaner technologies are required, it is expected that catalysts will play a key role not only in the abatement of emissions but also in the front end of the process to minimize pollution at the source. The most elusive new potential application has been the NOx reduction using hydrocarbons for "lean-bum" engine technology [1, 2]. In addition, the selective reduction of NOx by hydrocarbons (HC-SCR) is becoming a strong alternative for the replacement of NH3SCR industrial process [3]. These challenges will continue well into the 21 st century. The use of catalysts for primary power generation, e.g., catalytic combustion, with virtually no generation of pollutants, is also on the front-edge as new high temperature materials [4]. The present work brings together the concept of mesoporous materials based on manganese and enriches them with other heterocations such as: A1, La, Ce, at will. The materials thus formed have been tested for important environmentally catalytic reactions: CH4/NO/O2 (lean de-NOx) and CH4/O2 (combustion). Their corresponding catalytic performances were found superior to those reported in the open literature for a large number of catalysts tested under similar experimental conditions. A large number of techniques were employed for surface and bulk characterization of the present materials. These include: X-ray photoelectron spectroscopy (XPS) for surface composition, X-ray diffraction (XRD) for crystal phases, surface area measurements (BET) and AFM/Tapping mode experiments for surface texture, and transient methods for in situ determination of adsorption capacities and interaction characteristics of the reaction molecular species with the catalyst surface. The results obtained from these techniques were correlated with the catalytic performance results obtained. Corresponding author.
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2. E X P E R I M E N T A L
The general route for obtaining large surface area, mesoporous and/or microporous manganese based oxidic systems was based on the hydrolysis of trinuclear manganese complex [Mn30(CH3COO)6(pyr)3]C104 and it is a follow-up of work published previously [5, 6]. The trinuclear manganese complex was prepared according to the original report of its synthesis [7]. The main synthesis steps are described in Fig. 1(a)-1 (c). An aquatic drop containing metal cations A n§ and NO3 is added to the acetonic solution of the trinuclear manganese complex. As the solvents mix the chemical species interact, resulting in the hydrolytic decomposition of the complex (Fig.la). The hydrolysis products are OMn4 or MnO4 tetrahedra. Cations A§ adsorb in the vicinity of Mn hydrolysis products and bond to Mn via Mn-O-A bonds (Fig.lb). Finally, as the concentration of Mn-O-A groups increases, they form larger species via Diffusion Limited Aggregation (DLA). The gel-type species upon drying ~ ~,~-~.~-form high surface area solids (Fig. 1c). The specific surface area of the solids (m2g-1) obtained after hydrolysis and drying at various temperatures, and Mn ~)H qA [ their pore size distributions were measured by multi-point Mn~"Mn Ho"Mn-oH A..o'M~. O_A ] BET method at 77K by N2 adsorption. The surface ~v,. +o.: 0%. +,+ A-Oo.~ l /tg,,M,,n - Mn Mn composition of the Mn-based samples was determined by ~Vln -" H +, ,I /O"Mn Mn/?'.lVln Mn-'~Mn..o AI means of X-ray Photoelectron Spectroscopy (SPECS LH- M~'O~ Mnx,,Mn HO Mn OH A-O ,Mn - I OH 6-A l 10). The powders were pressed firmly into a carved (b) ~. stainless steel holder so that they could be introduced into the Ultra High Vacuum chamber (8x10 -l~ mbar). Calculations of the various components composition of the A.o/Mn-o.A A-Oo_ A . solids were based on the spectra of the Mn(2p), Al(2p), La(4d), Ce(3d) and O(ls) photoelectrons. Sensitivity Mn"~'Mn,,c~ a (~'~ factors were reproduced from Wagner et al. [8]. The texture A-O" l~ln ' - ' ' " (c) o-A and morphology of the Mn-based oxides were also investigated by atomic force microscopy (AFM Nanoscope Fig. 1. Schematic representation III, Digital Instruments) in the tapping mode. Observations of the process synthesis of Mnwere performed on thin films deposited on microscope based mesoporous oxides. glass slides. X-ray diffraction patterns of the Mn-based oxides after heating at 500~ were obtained using CuKa radiation at a 2~ scanning rate in a Siemens 500 system. Details on these characterization techniques can be found elsewhere [6, 9]. Table 1 reports the main results of the physicochemical characterization of Mn-based materials based on the above mentioned techniques. In Table 1, the fractal dimension reflects the geometric complexity of the catalyst surface and it was evaluated based on a detailed fractal analysis [ 10]. Catalyst testing of the samples fired at 500~ was conducted in a conventional flow quartz microreactor. Transient experiments were performed in a flow-system which has been described elsewhere [ 11]. Continuous monitoring of the transient responses from the reactor was performed by on line mass spectrometer (MS) (Omnistar, Balzers) equipped with a fast response inlet capillary/leak valve system. The Temperature-Programmed Desorption (TPD) experiments were performed with a 0.3g sample (powder form), 30scc/min He flow and a heating rate of 30~ The analysis of the effluent of reactor in the case of CH4/O2 reaction was done by the use of a gas chromatograph (GC), while that of NO/CH4/O2 reaction by the use of a GC/MS system [11].
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Mass numbers (m/z) used were: 15, 30, 32 and 46 for CH4, NO, 02 and NO2, respectively. Catalytic activity measurements for the deep oxidation of CH4 reaction were conducted in the range 235-500~ at a flow rate of 115 scc/min (GHSV,~70000 hl), and using CH4=4.34%, 02=8.68% and He as balance. For the lean de-NOx reaction, catalytic tests were conducted in the range 200-500~ at a GHSV~12000h ~, and using CHa=6700ppm, NO=2000 ppm, 02=5% and He as balance. 3. RESULTS and DISCUSSION
3.1 Catalyst Characterization As shown in Table 1, the specific surface area and the mean pore diameter (Dp) of the Mnbased solids strongly depend on the A-cation used (A=AI, La, La+Ce). The reasons for these differences is believed to be related to the hydrolytic paths of the cations A n+, originally in the aquatic drops, as soon as they find themselves in the buffered microenvironment created by the acetic and pyridine groups. In a previous study [6] it was shown that the gradual addition of Mn3complex into aquatic solution containing a variety of cations, always leads to a final solution of pH=4.75, which corresponds to the pKa of acetic acid. The nitrate solutions of the A-cations used (A=A1, La and La+Ce) fed dropwise into Mn3/acetone solution hydrolyzing the complex. At the same time coprecipitation of the cations occurs [ 12] at a different degree leading to final products of various surface morphology and composition. For example, the A13§ ions can form oligomeric species at this pH region, among them the well-known Keggin ions [AlI3Oa(OH)24(H20)12]7+. Such species can be adsorbed on the MnOx(OH)y nano-entities resulting in an increased product surface area with resistance to sintering. More discussion on the mechanism of preparation of the present materials can be found elsewere [9]. Table 1 Ph)zsicochemical characteristics of Mn-based mixed oxides materials Property Calcination T(~ Sample Mn-A1-O Mn-La-O BET (m2/g) 711 247 300 310 166 500 3.6 4.8 Dp (rim) 300 3.8 5.1 500 Amorphous Amorphous Crystal structure/XRD 500 1.70:1 0.85:1 Mn:A in hydrolysis 0.28:0.11:0.61 0.33:0.05:0.62 Surface composition/XPS 500 9.76 13.53 Roughness/AFM (nm) 300 20 10-15 Particle size/AFM (nm) 300 2.22 2.29 Fractal dimension/AFM 300
Mn-La-Ce-O 170 122 2.6 3.5 Amorphous 0.85:0.5:0.5 0.18:0.11:0.28:0.53 -
Figure 2 presents XPS results for the Mn-La-Ce-O solid, while Table 1 shows the calculated atomic surface composition of various Mn-based mixed oxides based on the XPS results obtained. In all four samples, Mn(2p3/a) is detected at 641.6 + 0.2 eV (Fig.2). According to the literature [13], Mn(2p3/a) is observed at 642 eV for MnO2, 641 eV for Mn304, 641.4 eV for Mn203 and 641.2 eV for MnO. It is, therefore, rather dificult to conclude on the valence of manganese. However, from the surface composition it seems rather probable that the valence is
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between Mn § and Mn § The atomic surface composition calculations were carried out considering that A1 is as A1203, and La as La203. In the case of Ce, the XP spectrum (Fig.2) clearly shows the coexistence of CeO2 and Ce203 phases. The satellite peak at 916.6 eV is characteristic of CeO2, while the poor resolution of the rest spectrum indicates the coexistence of Ce203 [14]. The O(ls) spectra (not shown) exhibited one peak at 529 + 0.2 eV corresponding to the oxide peaks of all metals except A1, which is detected at 531.6 eV. 660
655
650
645
...........
640
635
Mn_(2p3/2i ' t
II N 2 Selectivity
Mn-La-Ce-O
I--'1
"LT,,'~
t~
60
d
|
920
9
,
910
9
|
900
sn)
9
( % ) 40
|
890
9
i
880
,
|
870
Binding Energy (eV) Fig. 2. X-ray photoelectron spectra for the Mn-La-Ce-O catalyst
0
200
250
300
Temperature ( o C)
350
Fig. 3. NO conversion and selectivity towards N2 formation for the CHa/NO/O2 reaction on Mn-La- O and Mn-La-Ce- O solids.
3.2 Catalyst Performance Figure 3 presents results of the CH4/NO/O2/He reaction in terms of NO conversion and N2 selectivity in the low-temperature range of 200-350~ over the Mn-La-O and Mn-La-Ce-O materials. At 250~ the NO conversion is 25%, while the N2 selectivity is 80% in the case of Mn-La-Ce-O solid. The CH4 conversion varied between 3.5 and 10% in the range 200-350 ~ on both catalysts. Addition of 4% H20 in the feed stream resulted in the remarkable 98% N2 selectivity value, while the maximum NO conversion remained at the same level, but shifted to slightly higher temperatures. This behavior remained constant even after 24h of continuous run. It is noted that a Mn-ZSM5 catalyst was reported to be active only at T>350~ for the same reaction [ 15]. To appreciate the low-temperature "lean de-NOx" catalytic performance of the MnLa-Ce-O material reported in Fig. 3, a comparison is made with one of the best metal supported catalyst (Pt/A1203) reported with potential applications in diesel and "lean-bum" engine [1]. Under C3H6/NO/O2 reaction (C3H6=2350 ppm, NO=500 ppm, O2=5%, GHSV=60000h -1) at 250~ the NE selectivity was only 38% [ 16] to be compared with the value of 80-98% observed over the present material. It is appropriate to state that CH4 activation is expected to be more difficult than that of higher hydrocarbons over metal oxide surfaces. Therefore, a higher de-NOx activity is expected by using other hydrocarbons than CH4 as reductant species [ 17]. Figure 4 presents results of the catalytic performance of the present Mn-based porous oxidic materials (MANPO) in terms of the temperature necessary for 50% conversion of CH4 (T50%). To appreciate the performance of these materials, a comparison is made with few of the best catalysts reported in the literature [18, 19] for the same reaction. It is apparent that MANPO materials are by far the best catalysts for CH4 combustion under the investigated conditions. Indeed detailed calculations show that the ratio (W/F)sarraco over the ratio (W/F)MANPOequals 20. Thus revealing more clearly the better performance of the MANPO solids.
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600
T50%
(~
" Tejuca et al. [18]
Saracco et al. [19]
MANPO
-
500 400
300 200 o
100
La
ao.6Sro.4MnO 3 Pt/AI203 ).8Sro.2MnO3
Ai-Mn-O LaMn0.sM g 0.503 LaMn0.6Mg0.403 La-Ce-Mn-O La-Mn-O LaMn0.8Mg0.203
Fig.4. Catalytic performance of Mn-based porous oxidic materials (MANPO) for CH4 combustion. For comparison, the temperature necessary for 50% conversion (T50%) of CH4 over MANPO and other materials reported [ 18, 19] is indicated. Figure 5 presents temperature programmed desorption (TPD) profiles of NO and 0 2 o v e r MnLa-O and Mn-La-Ce-O solids. After adsorption of NO at T=500~ for 15 min from a 0.5%NO/He mixture, the catalyst was cooled in NO/He to 25~ and left for 15 min. The feed was then changed to He (5 min at 25~ and the catalyst temperature was then increased to 750~ to carry out a TPD run. In the case of 02 TPD, the adsorption conditions were: T=500~ for 15 min followed by cooling of the reactor to 25~ in 5%O2/He flow. The TPD profiles of Fig. 5 show significant differences among the two samples. Table 2 Amounts (~tmol/g) of 02 and NO adsorption measured durin8 various transient experiments Experiment Catalyst Sample Mn-La-O Mn-La-Ce-O 1.TPD of 02 128.8 40.3 2.TPD of NO
27.3
10.8
3.Reaction in CH4/NO/02 at 400~
NO = 77.9 0 2 = 206.0
NO = 9.1 0 2 = 37.8
~, 6000 ,~ 500( Q 4000 3000
O2 . . . . . NO
Mn-La-O
2000 o
Ce-O
1000 0 25
125 225 325 425
525 625 725
Temperature (~ Fig.5. TPD spectra of 02 and NO over Mn-La-O and Mn-La-Ce-O catalysts.
With respect to the amounts of adsorption, the Mn-La-Ce-O sample exhibits much lower uptakes (~tmol/g) in both NO and 02 than the Mn-La-O sample. These quantities are given in Table 2. With respect to the strength of adsorption of NO and 02 with the catalyst surface, Fig. 5 clearly indicates that the binding energy of NO is much lower in the case of Mn-La-O than MnLa-Ce-O sample (TM=125~ vs. 425~ while the opposite is true for the oxygen binding energy. In both samples, desorption of oxygen starts at about 225~ where the Mn-La-O surface exhibits two distinct 02 TPD peaks (TM=405 and 610 ~ while the Mn-La-Ce-O sample exhibits a single 02 TPD peak (TM=375~ with a shoulder at the high temperature falling part of it. The amounts of chemisorbed NO and oxygen species during CH4/NO/O2 reaction at 400~ were also determined. Following reaction for 20 min, the catalyst was quickly cooled to 200~ under the reaction mixture. The feed was then switched to pure He, while the NO and 02
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desorption was measured isothermally at 200~ Subsequently, a TPD run in He flow was performed. The total amounts of NO and O2 calculated under the isothermal and TPD runs are given in Table 2. Astonishing results from this experiment is that, in the case of Mn-La-O sample, the amount of NO and 02 adsorption was significantly enhanced under reaction conditions a result opposite for the Mn-La-Ce-O sample. The transient results shown in Fig.5 and Table 2 can contribute in explaining the differences in the lean-NOx activity and selectivity of the two samples. The Mn-La-Ce-O sample is by far a better "lean-NOx" catalyst in the range 200300~ than the Mn-La-O one. At 250 ~ the Mn-La-Ce-O exhibits about twice NO conversion and N2 selectivity than the Mn-La-O. The much higher chemisorption of oxygen and NO on MnLa-O could suggest that formation of NO2 proceeds with a lower rate than in the case of Mn-LaCe-O solid, if oxidation of NO to NO/ is considered as an important step for reaction with adsorbed hydrocarbon CHx species to form N2. The XPS results may also suggest that a lower concentration of Mn with a mixed oxidation states, and the presence of Ce, are likely reasons for an enhanced activity. Worth of noting is also the TPD results of Fig.5 where in the case of MnLa-Ce-O the binding energies of NO and 02 are similar, a result opposite for the Mn-La-O sample. This could also be an intrinsic reason that largely influences the activity of the present catalysts. REFERENCES
1. A. Fritz, V. Pitchon, Appl. Catal. B: Envir. 13 (1997) 1. 2. E.S.J. Lox and B.H.Engler, "Handbook of Heterogeneous Catalysis", G. Ertl, H. Knozinger and J.Weitkamp, Eds., VCH, Weinheim, v.4, 1997. 3. V.I. P~rvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233. 4. T. Seiyama, Cat.Rev.-Sci.Eng., 34 (1992) 281. 5. C.S. Skordilis, P.J. Pomonis, J. Colloid Interface Sci. 166 (1994) 61. 6. A.D. Zarlaha, P.G. Koutsoukos, C.S. Skordilis, P.J. Pomonis, J. Colloid Interface Sci. 202 (1998) 301. 7. J.B. Vincent, H.R. Chang, K. Folting, J.C. Huffman, G. Christou and D.N. Hedrikson, J. Amer. Chem. Soc. 109 (1987) 5703. 8. C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.H. Raymond and L.H. Gale, Surf. Interf. Anal. 3 (1981) 211. 9. V.N. Stathopoulos, D.E. Petrakis, M. Hudson, P. Falaras, S.G. Neophytides and P.J. Pomonis, "Characterization of Porous Solids-V', Elsevier 1999, accepted for publication. 10. A. Provata, P. Falaras, A. Xagas, Chemical Physics Letters 297 (1998) 484. 11. C.N. Costa and A.M. Efstathiou, J. of Catalysis, submitted for publication. 12. J. Kragten, in "Atlas of Metal-Ligand Equilibria in Aqueous Solutions", Ellis and Horwood, J. Willey, New York, 1978. 13. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, "Handbook of X-ray Photoelectron Spectroscopy", Ed. G.E. Mullenberg, Minnesota (1978). 14. M. Alexandrou, R.M. Nix, Surf. Science, 321 (1994) 47. 15. A. Aylor, L. Lobree, J. Reimer, and A.T. Bell, J. Catal. 170 (1997) 390. 16. R. Burch, D. Ottery, Appl. Catal. B: Envir. 9 (1996) L 19. 17. R. Burch, P. Fomasiero, T.C. Watling, J. Catal. 176 (1998) 204. 18. L.G. Tejuca, and J.L.G. Fierro, in "Properties and Applications of Perovskite-Type Oxides", Marcel Dekker, New York, 1993. 19. G. Saracco, F. Geobaldo, G. Baldi, Appl. Catal. B: Envir. 20 (1999) 277.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Characterization of the role of Pt on CoPt and InPt Ferrierite Activity and Stability upon the SCR of NO with CH4 Laura Gutierrez, Laura Cornaglia, E. Mir6 and J. Petunchi # Instituto de Investigaciones en Cathlisis y Petroquimica - INCAPE (FIQ, UNL-CONICET) Santiago del Estero 2 8 2 9 - 3000 - Santa Fe - Argentina. Phone/Fax: 54-342-4536861. e-mail:
[email protected] The selective catalytic reduction (SCR) of NO• with CH4 in excess of oxygen was studied over a series of ferrierite-supported monometallic (Co, In or Pt) and bimetallic (Pt-Co or Pt-In) catalysts. Pt promoted the NOx to N2 conversion of both Co and In ferrierite aider the samples were reduced with 1-12at 350~ either under dry or wet reaction stream. The addition of 2% of water to the feed stream also increased the activity of the calcined PtlnFerrierite for the said reaction. Temperature Programmed Reduction and XPS results show the presence of Co ~ Pt ~ Co +2, Pt +2 and In at exchange position in the samples with the highest activity and selectivity for the SCR of NO• I. INTRODUCTION The cooperation effect of catalytic species has recently been studied for the selective catalytic reduction (SCR) of NO• in order to obtain a suitable catalyst under real reaction stream (1). In this vein Kikuchi and coworkers (2-5) performed several studies on the effect of the addition of precious metals (Pt, Rh and Ir) to In/HZSM5. They reported that such solids are highly selective for the reduction of NO with CH4 in feed streams containing up to 10% of water vapor. The role of the precious metals would be to accelerate the NO to NO2 oxidation path even in the presence of water vapor. These authors also suggested that the bifunctional catalysis of such solids is remarkably facilitated by the co-existence of the active sites in the pore of the zeolite, what they call "interpore catalysis". We also found that Pt added to CoZeolite promote the NO to N2 conversion with CH4 and increases the water resistance of such solids (6, 7). In such studies the best performance was obtained with PtCoMordenite (Co/Pt =15) after being reduced with H2 at 350~ This contribution investigates the role of the Pt-Co and Pt-In interaction on the activity and water resistance of bimetallic ferrierite and the nature of the species present, using TPR, XPS, XRD and the catalytic test.
# Our recognition and gratitude to Prof. Juan O. Petunchi for his lifelong lasting contribution to Catalysis. The authors wish to acknowledge the financial support received from CONICET, UNL (CAI+D "96 Program) and ANPCyT. They are also grateful to Claudio Maitre for his technical support and to Prof. Elsa Grimaldi for the edition of the English manuscript.
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2. E X P E R I M E N T A L
Catalyst preparation. The starting materials were K-Ferrierite (Si/AI = 9) and ml203. The Ferrierite-based samples were prepared by ion exchange and the alumina-based ones were obtained by the wet impregnation method. Further details are supplied elsewhere (6). The solids obtained were: CoFer, PtFer, PtCoFer, InFer, PtlnFer, Pt/AI203, Co/A1203, Pt/Co/AI203, In/A1203, Pt/In/Al203. Table 1 shows the prepared catalysts. Table 1 Prepared solids Catalysts
Metal content (wt %) Pt
PtFer a
0.44 0.5 0.52
CoFer a
InFer a PtCoFer ~ PtlnFer Pt/AI203
b
C0/A1203 In/A1203
Pt/In/Al203
b
1.38 1.8 -
0.52 0.49
-
-
-
2.0
2.0
b b
In -
0.5 b
Pt/Co/ml203
Co -
-
-
0.5
2.0
-
0.5
-
2.0
a Prepared by cation exchange. b Prepared by wetness impregnation.
Catalysts characterization. TPR experiments were performed with 0.100 g of pretreatment catalyst in an Okhura TP-2000S instrument with a heating rate of 10~ in a 5% flow of H2 in Ar. XPS data were obtained in a Shimadzu ESCA-750 spectrometer equipped with a Mg anode (MgK = 1253.6 eV). XRD patterns were obtained in an XD-D1 Shimadzu instrument with monochromator employing CuKa radiation and scanning rate of 1degree, min ~ . Catalytic measurements. The reaction was carried out using a fixed-bed reactor. The typical mixture was NO = C H 4 = 1000 ppm, 02 = 2%, H20 = 2% balanced to 1 atm with He. The catalytic activity and the composition of the reacting gases were analyzed with a SRI chromatograph. 3. RESULTS AND DISCUSSION The calcined Co and InFer were active for the SCR of NO• with CH4. Both samples converted c.a. 60% of NO to N2 under dry reaction stream. InFer reached such conversion at 400~ whereas CoFer did at 450~ The monometallic Co and In samples impregnated in A1203 were almost inactive up to 450~ (Fig. 1.A). The incorporation of 0.5% of Pt did not modify the activity of the oxidized monometallic Ferrierites in the 350-500~ temperature range (Fig. 1.B and Table 2) (Note the lower In loading of PtlnFer). ARer the addition of water to the reaction stream, the activity of both bimetallic solids
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was significantly affected at low temperatures (Fig. 1.C), but a dissimilar behavior of Pt-Co and Ptln samples was observed at temperatures higher than 400~ In fact, the NO to N2 conversion leveled off about 30% at 500~ on PtCoFer, whereas in the Pt-In sample, as the temperature increased, the solid recovered the activity and at 500~ it reached 85% of NO conversion. Interestingly, when water was removed, the PtlnFer presented higher activity than the fresh sample (Table 2). When the samples were reduced under flow H2 for l h at 350~ the results obtained were quite different as shown in Fig. 1.B and C. The PtlnFer presented 70% of NO conversion at 500~ under both dry and wet reaction stream, while in the PtCoFer samples, the NO conversion reached almost 100% under dry stream at 450~ However, when 2% of water was added, the maximum conversion occurred at 550~ 80% against 35% in the calcined sample and 20% in the CoFer. 00 90 A . . . . . . . . . . . 80
..............
70
.
-=o=~> 4050.ii:./.~:ii ='~~tiil _~. ~30 i 60
10
.
.
.
.
........,~IC'"U_:._::2.g- -
.** .
. . . . . . . .
......
0~
........
o
3T04~0 4T0 5"~0 5T0 3: 0 400 450 500 550 350 400 450 500 550 Temperature (~ Temperature (~ Temperature (~ Figure 1 : Catalytic behavior of solids. A- Calcined monometallic solids under dry reaction conditions: 0 Pt/ml203, A C0/A1203, V In/A1203, [ ] InFer, | CoFer, 9 PtFer. B- Bimetallic solids under dry reaction conditions: A Pt/Co/A1203, V Pt/In/A1203, F'l, l PtCoFer, ~ ,O PtlnFer,. Open symbols: calcined samples, Close symbols: reduced samples. C- Mono and Bimetallic solids under wet reaction conditions: 12], l PtCoFer, ~ O PtlnFer, ~ InFer, | CoFer, 0 Alumina-based solids. Open symbols: calcined samples, Close symbols: reduced samples.
The TON of calcined InFer and PtlnFer were comparable (Table 2), while an increase
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o f the In specific activity (TON) was observed in the reduced bimetallic samples (Table 2). The same effect appeared after the sample had been under reaction stream with 2% o f H20. These results would suggest that atter such treatments some interaction occurred between Pt and In. Table 2 Turnover Frequencies (TON) of PtCo and PtlnZeolites for NO Reduction a
Catalyst
Pretreatment
TON x 104 (s-l,) b 400 ~ 450 ~
350 ~
PtlnFer calcined 10.40 21.25 24 PtlnFer calcined c 8.03 29.75 35 PtlnFer reduced d 7.08 28.30 35 PtCoFer calcined 0.13 1.52 2.44 PtCoFer reduced d 0.89 3.04 6.30 CoFer calcined 0.86 3.53 5.0 InFer calcined 1151 26.70 26 InHZSM5 e calcined 2 86 4.62 4.62 a Reaction conditions: See Figure 1. b TON: number of NO molecules converted per second per Co or In respectively, c After water removal from the reaction stream, d Reduction at 350~ with H2, one hour. e Ref. 4. The XPS results reveal that in CoFer and PtCoFer, in both calcined or reduced state, the only species detected was Co +2 located at exchange position within the zeolite structure. In fact the B.E. o f Co2p3/2 was 783.7 eV against 781.7 in Pt/CoA1203 where the Co would form cobalt oxide species (Table 3, Figure 2.B). In calcined In/A1203 only a well defined signal of In3ds/2 of 445.1 eV was detected, while a shift to 445.4 eV was observed in Pt/In/Al203 (Table 3). These B.E. were different from the one reported by M6riaudeau et al (8) for In § in bulk In203 (B.E. 444.1 eV). In InFer a broad signal was observed, with a B.E. centered at 446.7 eV. However, this signal could include supported In species like in In/ml203 (B.E. = 445 eV) and In exchanged species with a B.E. = 446.5. These values are in agreement with the one reported for InNaY by Meriaudeau et al (8). The addition of 0.5% of Pt to InFer is likely to increase the amount of In at exchanged location. In fact, in this sample a well defined signal o f In3ds/2 with B.E. 446.5 eV was observed (Table 3). Table 3. XPS atomic ratios and binding energies of calcined catalysts
Catalysts l~t/Co/A1203 PtCoFer In/A1203 Pt/In/m1203 InFer PtlnFer
Co 2p3/2 (eV)
Co/A!
In 3d~/2 (eV)
In/A!
781.7 783.7 -
0.08 0.23 -
445.1 445.4 446.7 446.5
0.014 0.017 0.04 0.024
-
-
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The Pt4f signal partially overlaps with that of Al2p, consequently, it was not possible to accurately determine the signal corresponding to Pt in the monometallic samples with 0.5 wt% (6). The TPR profiles of InFer also suggest the presence of two In species (Fig. 2A). One of them with a maximum at 250~ is also present in In/Al203 (Fig. 2.B). This species show a low activity for the SCR of NOx (Fig. 1.A). The broad reduction peak with a maximum at 550~ could be due to In at exchange location in Ferrierite, this species being active for the said reaction in agreement with the findings reported by Kikuchi and coworkers (4) in other In zeolite systems. When 0.5% of Pt were added to the InFer, the TPR profile of the bimetallic samples showed that some reorganization of Pt and In species occurred. The lower temperature peak shifted to 150~ which would suggested that Pt promoted the reducibility the In species. The reduction profiles of Co exchanged ferrierite shows two peaks. One with a maximum centered at 300~ and the other with a maximum at 700~ The 300~ peak could be assigned to a fraction of the cobalt supported as oxide on the external surface of ferrierite. This species is also present in Co/Al203 (Fig. 2 A and B). They appear not to participate as active sites for the SCR of NOx (Fig. 1 A). The maximum at 700~ may be due to the Co +2 reduction at exchange position. The addition of 0.5% of Pt to the CoFer yields a TPR profile which does not correspond to the sum of its respective monometallic CoFer and PtFer, with maximum at 250, 400 and 520~ which suggests the Pt and Co species of different reducibility are present in ferrierite structure. Most of them could be assigned to Co § in exchanged location (Table 3). Pt promoted the reducibility of such Co species.
IB
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/ ~
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i
i
i
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:
.
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.
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Temperature (~ Figure 2: TPR Profiles. A- Ferrierite - based catalysts. B- Alumina- based catalysts.
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The TPR results (Fig. 2) of PtCoFer and PtlnFer also suggest the presence of Pt ~ Co ~ Pt +2, Co § and reduced In species (probably at exchange positions) in the samples after they have been reduced at 350~ for one hour. In no case did XRD patterns show metal or oxides particles, which suggests that the above species are highly dispersed in the zeolitic matrix. As was point out before and in agreement with our previous findings for the PtCoMordenite (7) neither Co O nor Co203 contributed to the enhancement of the bimetallic PtCoFerrierite. Co § and at exchanged position appear to play a central role as active site for the NO2 reaction with methane. The same role could be attributed to In species at exchanged position like the one species reported by Kikuchi and coworkers and Mir6 et al (9). The main role of Pt in both PtCoFer and PtlnFer would be to catalyzed the oxidation of NO to NO2.
4. CONCLUSIONS 9 Pt promotes the NO to N2 conversion of Co and InFer on the SCR with CH4 under wet reaction stream, the activity order being PtlnFer>PtCoFer>InFer>CoFer>>Pt/Co/Al203=Pt/In/A1203. 9 Pt ~ Pt 2+, Co 2+ and In in exchange positions play a role as active sites in the bimetallic Ferrierite. 9 The presence of Pt during treatment with H2 or wet stream facilitates the exchange of In in Ferrierite, probably following the mechanism proposed by Kikuchi and co-workers (4).
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
Hamada, H., Catal.Surveys from Japan 1(1997)53. M. Ogura, M. Hayashi and E. Kikuchi, Catal. Today, in press. M. Ogura, S. Hiramoto and E. Kikuchi, Chem. Lett. (1995) 1135. E. Kikuchi, M. Ogura, N. Aratmi, Y. Sugiura, S. Hiramoto and K. Yogo, Catal. Today 27 (1996) 35. M. Ogura, M. Hayashi and E. Kikuchi, Catal. Today 42 (1998) 159. Gutierrez ,L., Boix, A. and Petunchi, J., J.Catal. 179(1998)179. Gutierrez ,L., Boix, A. and Petunchi, Cat. Today, in press. P. M6riaudeau, C. Naccache, A. Thangaraj, C. Bianchi, R. Carliand S. Narayanan, J. Catal. 152(1995)313. E. Miro, L. Gutierrez, L. M. Ramallo L6pez and F. G. Requejo, Subbmited to J. of Catal. (1999)
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
化
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Cu-Cr O X I D E C A T A L Y S T S F O R C O M P L E T E AROMATIC HYDROCARBONS
OXIDATION
OF
V. G e o r g e s c u Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 77208 Bucharest, Romania
Supported mixed metal oxide systems, Cu-Cr on y - AlzO3 and ~/- Al203 + SiO2 were prepared and studied. They exhibited catalytic activity in the complete oxidation of aromatic hydrocarbons.
I. Introduction Complete oxidation of hydrocarbons or of different waste gases containing hydrocarbons on platinum catalysts is a well developed procedure [1-4]. However, for evident reasons searching for conventional efficient oxide catalysts is at present the scope of many investigations. The aim of the present paper is to check the catalytic activity and stability of a set of supported mixed oxides prepared via a complex of Cu-Cr with tartaric acid.
2. Characterization of catalysts 2.1. Samples preparation The precursor complex was obtained by precipitation at pH 7 of a mixture of Cu-Cr nitrates in aqueous solution mixed with tartaric acid, in a solution of ethanol and ammonium hydroxide 10% in volumetric ratio 1:1. After precipitation, the resulted compound was dried in vacuum at 90~ It was subsequently submitted to chemical analysis, IR spectrometry, magnetic measurements and thermal analysis. All the above methods resulted in the following formula for the precursor: [ Cr Cu4 Ta6 ]. 5H20 Ta = tartaric acid. In order to prepare the supported catalysts two procedures have been used : 9The first one consists of the binding of the precursor on the support by sueesesive impregnation of the solution of tartaric acid and of the nitrate mixtures. 9The second consits in the synthesis of the precursor, its solubilization and deposition on the support by impregnation. Two types of supports were used for impregnation, Al203 tablets (+ = 6 ram) and a mixture of AlzO3 + SiO/grains (+ =3-5 mm).The obtained metalic content of the samples is 8,5-10 %.
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The prepared catalysts were denominate: C1 and Cs for Cu-Cr / ~, - A1203, C2 and C6 for Cu-Cr/y-AI203 + SiOz. Cj ,C2 were prepared by the first procedure and C5 ,C6 by second. The supported catalysts prepared by the two above mentioned procedures, have been dried for 12 h at 90~ and calcined at 650~ for 6 h. Table 1 prezents the surface area of the samples.
Table 1. The surface area of the sample Surface area ( m 2/g ) Sample Uncalcined samples Calcined samples
"y-A1203
7 - A1203 + SiO2
C1
C2
C5
C6
161
153
160 167
124 155
140 152
123 143
2.2 Chemical and physical properties IR and electronic spectra as well as magnetic measurements have indicated that both C O O and partially HO- are coordinated at metalic ions m the precursors. An IR investigation was carried out for all the samples m the spectral range 4000 - 400 CI][1-1. The mare characteristic IR absorption bands are presented in Table 2. Table 2 The characteristics of the IR spectra Sample
v ( OH ) , v ( OH ), v tmsr~nm, V symm, r ( O H ) , y ( O H ) , v ( C-C ), v ( M e - O ) R-COOH
H20
R-OH
(C-O) (C-O)
v(C-O)
R-COOH
R-COOH
~5(OH)
R-OH
R,COOH v( CH3 )
Cu-Cr-Ta (unsupp.complex) Cu-Cr-Ta/y-AI203
3475 3100 3460 3200 Cu-Cr-Ta/y-Al203+SiO2 3480 3340
2940 2850 2960 2850 -
1628 1620 1640 1620
1375 375 375 -
1120 1062 1088 1040 1075 1045
700 680 840 710 810 720
v,m~-m Vs~m (Me-O) 590 460 610 480 630 510
A large and intense absorption band at 3400 - 3000 em 1 is typical for stretching vJ~a-ations of associated H O groups (VoH) and could be found m all the samples. The bands at 1620 - 1580 cm a as well as the sharp band at 1375 cm -1 are usualy attributed
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to asymmetric stretching vibration and symmetric vibration of the C-O bonds m carboxylic ion, respectively. These are also common to all samples and could be assigned to tartaric acid salts ( Cu- Cr ). Sharp and weak bands are found in the region 590 - 420 em-~ which are attributed to Me-O bonds. These are sometimes screened by intensive absorption bands of the supports. Finally, we think that there are credible arguments for both COO" and partially H O coordination even on alumina and alumosilica. The UV-VIS reflection spectra indicated the octahedral coordination of tartaric acid to both metalic ions [5, 6]. The 410 nm band for the Cr Cu4Ta6 5H~O complex, ascribed to the 4A2g ~ 4T lg ( F ) transition of the Crm ( d3 ) in oetahedral configuration, whereas the 590 nm peak is c,lmracteristie to an oetahedral configuration of the Cun (d9 ) ion. The transfommtion of the complex into oxides were checked by TGA analysis. Although the mass loss process stops at 550~ for all samples, the tested catalyst were calcined at 650~ m order to stabilize the oxide phase. XRD spectra of the calcined samples revealed CuO, CuCr204 and Cu2Cr204 on the copperchromium supported catalysts.
3. Experimental TPR and TPD measurements were carried out within a gas-chromatographic setup, with a thermal conductivity cell [7,8], by measuring the hydrogen consumption from a gaseous mixture of argon with 10% hydrogen and recording the hydrogen evolved in argon ( by two measuring cycles; HCR1 and HCR2 ), which was used as the carrier gas, respectively. A linear heating program of increasing the temperature in the range 20-500 ~ with a rate of 10 K min-1 was applied. The catalytic activity of the samples was measured in a flow reactor coupled with a gas-mixing system, a thermostat regulator for hydrocarbon vapour control and needle-valve controlled air flow. The analysis was carried out with an m line Hewlet Packard Gas Chromatograph model 5840A and using a Chromosorb 102 and Porapak Q packed column. The catalysts were tested in the complete oxidation of benzene, toluene, ethylbenzene and isopropylbenzene at 500, 700, 1000 p.p.m concentration of hydrocarbon in air and at 5000, 10000, 20000 h -~ space velocities.
4. Results and discussion Figs. 1 a,b,c and d shows the HCR1 and HCR2 for C1, C2, C5 and C6 supported samples respectively. For C1 and C2 samples, the HCR1 and HCR2 curves are given in Figs.1 a and b which show that the highest hydrogen consumption at lower temperatures is due to sample C1. From the same figures one can see that sample C2 exhibits the low temperature peak only in the HCR2 curve. The sample C5 ( Fig. 1c) exhibits a significant peak only at low temperatures, on the HCR1 as well as the HCR2 curves. The slight increase in the hydrogen consumption at temperatures higher than 300 ~ C can be assigned to adsorption on the support. The HCR1 curve for sample
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Fig.1 HCR1 (e) and HCR2 (=) for the samples: a- C~, b- C2, c- C5, d- C6
C6 (Fig. 1d ) does not exhibit the low temperature peak which appears only in HCR2 curve. The other samples give similar TPD curves characterized by continuous increase in evolved hydrogen at temperatures above 300 ~ C. According to the literature data [9,10], the TPR curves for the CuO/alumina catalyst exhibit a single peak located in the temperature range 280 - 367~ The temperature of the peak depends on the procedure used to prepare the sample and the experimental conditions (hydrogen concentration, flow rate and heating rate ). For the CuO/alumosilica catalysts, the TPR records exhibit either three peaks [11 ] or a peak and a shoulder [12] depending of the procedure of preparation as well as on the character of the interaction between the support and the Cu 2§ ions. The peaks are located in the temperature range 300 - 800 ~ C and have been assigned to the ~ e s
:
Cu 2§ - ~
Cu §
Cu
For the chromium oxide supported on alumina or alumosilica [13] , the TPR records exhibits two or more peaks at temperatures higher than 360 ~ C. Analysis of X-ray diffractograms of the samples showed that they contain CuO as well as CuCr204 and Cu2Cr204. The TPR curves of the supports [7] show a low hydrogen consumption ordy at high temperatures, which can be assigned to the adsorption on alumina taking
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Fig.2 Variation of concentration with temperature at 1000 p.p.m concentration of hydrocarbons in air and 20000 h4 space velocity for: a- C1, b- C2, c- Cs, d- C6, (1- benzene, 2- toluene, 3- ethylbenzene, 4- isopropylbenzene).
into account that alumosilica exhibits similar TPR curves. The low temperature peak on the TPR curve for the unsupported samples [ 8 ] can be assigned to the reduction of CuO. The high temperature peak on the HCR1 curve is probably due to the transformations : CuCr204 --, Cu2Cr204 As shown on the HCR2 curve, the sample has been restructured, not completely reduced, and, consequently, one still records two forms of hydrogen consumptiort The low-temperature form can be assigned either to hydrogen adsorption on metallic copper after the first reduction, or to the reduction of the residual forms of CuO. For the samples supported on alumina ( C1 and C5 ), their TPR curves exhibit only one lowtemperature peak which can be assigned to the reduction of CuO. The TPR curves of samples C2 and C6 supported on alumosilica show quite an unusual behaviour, ilustrated by the fact that the high temperature peak appears only after a previous
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reduction. This shows once more that the samples undergo reconstruction. These curves show only hydrogen forms which desorb at high temperatures, i.e. strongly bonded. The weakly bonded forms can be desorbed in the gas flow at room temperature. The copper-chromium catalysts have a higher activity in the complete oxidation of aromatic hydrocarbons even at temperatures below 300 ~ C, (Figs. 1. a ,b,c and d). The catalytic activity is m according to the reduction capacity at low temperature of the samples. The catalysts supported o n y-AI203 are more active than the catalysts supported on y-AI203+SiO2. With respect to the method of preparation the catalysts obtained by precursor-complex formation directly on the support are more active ( C~ and C2 ). With respect to the organic substrate, the activity of the catalysts increases in order : benzene < toluene < ethylbenzene < isopropylbenzene.
5. Conclusion
Egg-shell type catalysts were obtained by impregnating the supports with precursor coplexes. The presented data concerning the TPR and TPD of hydrogen of copper and chromium oxides unsupported and supported on alumina and alumosilica showed that the reduction is generally accompanied by surface reconstruction. In comparison with older results concerning simple oxides supported through nitrates impregnation [ 14 ] the mixed Cu-Cr catalysts obtained via precursor complexes exibited better activity and stability. References [1] J.A. Bamard and D.S. Mitchell, J. Catal., 12 ( 1968 ) 386. [2]/L Schwartz, L Holbrook and H. Wise, J. Catal., 21 ( 1971 ) 199. [3] C.I. CuUis and B.M Willott, J. Catal., 86 ( 1984 ) 187. [4] A. Inglot, React. Catal. Lett., 2 ( 1989 ) 241. [5] V. Pocol, L. Patron and P. Spacu, Rev. Roumame de Chim, 39 ( 1994 ) 1113. [6] L. Patron, V. Pocol, N. Stanica and D. Crisan, Rev. Roumaine de Chim, in press. [7] M Teodorescu, I. Sitaru, A. Banciu and E Segal, The~mochim Acta, 233 ( 1994 ) 233. [8] M Teodorescu, I. Sitaru, V. Georgescu, MI. Vass, E. Segal, Thermochim Acta, 282/ 283 ( 1996 ) 61-68. [9] S.J. Gentry, N.W. Hurst and/L Jones, J. Chem. Soc. Faraday Trans., 1 ( 1981 ) 77. [10] S. D. Robertson, J.H.de Bass, S.C.Kloet and J.W. Jenkins, J.Catal., 37(1975) 424. [ 11 ] M Shimokawabe and H. Kobayeski, Bull. Chem Soc. Jpn., 56 (1983) 133. [12] F.S.Ddk and/LVavere, J.Catal., 85 ( 1984 )380. [13] B.Parlitz, W.Hanke, R. Frike, M.Richter, W.Roost and G.Oghnann, J.Catal.,94 (1985) 24. [14] V.Georgescu and MI.Vass, Rev. Chim. 26 (1988), 341.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Ag/Lao.6Sro.4MnO3/y-Al203 catalysts for complete oxidation of methanol at low concentration H.-B. Zhang, W. Wang, Z.-T. Xiong and G.-D. Lin Department of Chemistry & State Key Lab of Physical Chemistry for the Solid Surfaces, Xiamen University, Xiamen 361005, CHINA Ag-modified Lao.6Sro.4MnO3-based catalysts with the perovskite structure were prepared, and their activity for methanol complete oxidation was evaluated. The results showed that over a 6%Ag-modified 20%Lao.6Sro.4MnO3/~,-Al203 catalyst, the "1"5oand T95 were as low as 395 and 413 K, respectively, and even at so low reaction temperature, the contents of the reaction intermediates HCHO and CO in the exit-gas were both under the detection limits. The origin of promoter action by Ag was discussed together with the results of spectroscopic characterization of the catalysts. I. INTRODUCTION Methanol or its mixture with gasoline has been suggested as alternate fuel for car operation to reduce gasoline consumption under certain circumstance [1]. However, the exhaust gas of methanol (or methanol-gasoline mixture)-fueled cars contains unburned methanol and formaldehyde [2]. These compounds may have an undesirable impact on air quality. Most of the existing catalysts for complete oxidation of methanol are precious metal (Pt, Pd, Rh etc.) and Ag catalysts supported by ~-AI203 [2,3]. In the present work, a highly active Ag/Lao.6Sro.4MnO3/~,-A1203 catalyst for complete oxidation of methanol at low concentration is reported. The results have potential significance for the catalytic removal of methanol and formaldehyde contaminants from the exhaust of methanol-fueled cars. 2. EXPERIMENTAL A perovskite-type Lao.6Sro.4MnO3 host catalyst was prepared by using a sol-gel method. Another supported host catalyst, y%(mass percentage) Lao.6Sr0.4MnO3/~-ml203, was prepared by conventional impregnating ~/-A1203 support (with a N2-BET-surf. area of 129m2g-1) with the calculated amounts of La(NO3)3, Sr(NO3)2, and Mn(NO3)3 in aqueous solution, followed by drying at 393K for 4h, and then calcining at 1073K for 4h in air. The Ag-modified Lao.6Sro.4MnO3 and y%Lao.6Sro.4MnO3/),-A1203 catalysts (marked as x%Ag/Lao.6Sro.4MnO3 and x%Ag/y%Lao.6Sro.4MnO3/),-A1203) were prepared by impregnating the corresponding host catalyst with the calculated amount of AgNO3 in aqueous solution, followed by drying at
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393K for 4h and then calcining at 773K for 4h. The ),-A1203-supported catalysts, Ag/~'-AI203, Pt/y-A1203, and Pd/y-ml203 were prepared following the procedure described above and
according to the literature [2], respectively. All these catalyst samples were pressed, crushed, and sieved to size of 70-100 mesh. The catalyst activity was evaluated in a fixed-bed continuous flow reactor-GC combination system packed with a catalyst amount of 300mg. The complete oxidation of methanol at low concentration over the catalysts was conducted at a stationary state under reaction conditions of 0.1MPa, feed-gas CH3OH/O2/N2= 0.2/1.0/98.8 (molar ratio), GHSV= 58000 h 1, and at a series of reaction temperature. The reactant and product were analyzed by an on-line GC (Model 102GD) equipped with a hydrogen flame ionization detector and 3m long GDX-103 column. The catalyst activity is expressed by the temperatures, "I'50and T95 (K), at which methanol conversion reached 50% and 95%, respectively. X-ray diffraction measurements were carried out by using a Rigaku D/Max-C X-ray Diffractometer with Cu-Ket radiation at a scanning rate of 8~ XPS spectra were taken by a VG ESCA LAB MK-2 machine with Mg-Kct radiation (10kV, 20mA, hv=1253.6 eV) and UHV (lxl07pa), calibrated internally by the carbon deposit C(ls) (B.E.) at 284.6 eV. BET-surface area of catalyst was measured by N2 adsorption using an SORPTOMATIC-1900 (CARLO ERBA) machine. Hydrogen temperature-programmed reduction (H2-TPR) was conducted on a fixed-bed continuous flow reactor-GC combination system packed with 20mg of catalyst sample at 10 K/min rate of elevating temperature; a N2-carried 5v%H2 gaseous mixture was used as reducing gas, with an on-line GC (Model 102GD) equipped with a thermal conductivity detector monitoring change of hydrogen-signal. 3. RESULTS AND DISCUSSION
3.1. Assay of catalyst activity The results of evaluation of the catalyst activity for methanol complete oxidation are shown in Table 1. It can be seen that the La0.6Sr0.4MnO3 catalyst with perovskite-phase structure displayed the activity rather higher than that of the supported noble metal catalysts, 0.1%Pd/y-ml203 and 0.1%Pt/y-A1203. 6%Ag-modification to the La0.6Sr0.aMnO3 host catalyst improved pronouncedly its catalytic performance for methanol complete oxidation, making the reaction temperatures for conversion of 50% and 95% methanol, "1"50and T95, dropped by 29 and 44 K, respectively. Over the 6%Ag/20%La0.6Sr0.4MnO3/~,-A1203 catalyst, the Ts0 and T95 lowered further down to 395 and 413 K; and even at so low reaction temperature, the contents of the reaction intermediates, HCHO and CO, in the exit gas were both under the detection limits. While on the 0.1%Pd/y-A1203 and 0.1%Pt/y-ml203 catalysts under the same reaction conditions, the content of HCHO in the exit gas attained 250 ppm and 630 ppm at their T95 temperatures, respectively. In order to directly gain information about the catalytic oxidation of CO (an intermediate of methanol complete oxidation), the oxidation activity of CO at low concentration over those catalysts was tested under reaction conditions of 0.1MPa, feed-gas CO/O2/N2=2.3/20.5/77.2 (v/v), and GHSV=23000h -1. The results are listed in Table 2 and show that the prepared
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Table 1 Results of the activity assay of the catalysts for complete oxidation of methanol in low concentration* Catalyst I"5o (K) %5 (K) Lao.6Sro.4MnO3 10% Lao.6Sro.4MnOrA/-Al203 20% Lao.6Sro.4MnOJ~-A1203 27%Lao.6Sro.4MnO3/~,-A1203 1%Ag~ao.6Sro.4MnO3 4%Ag/Lao.6Sro.4MnO3 6%Ag/Lao.6Sro.4MnO3 10%Ag/Lao.6Sro.4MnO3 6%Ag/),-AI203 6%Ag/10%Lao.6Sro.4MnO3/),-A1203 6%Ag/20%Lao.6Sro.4MnOa/),-A1203 6%Ag/27%Lao.6Sro.4MnOa/),-A1203 0.1%Pd/~,-A1203 0.1%Pt/),-AI20 ~ *Reaction condition: 0.1MPa, feed-gas 58000h -1.
437 483 478 450 428 411 408 413 408 398 395 399 460 442 CH3OH/O2/N2 =
465 531 520 507 445 432 421 421 448 415 413 423 512 468 0.2/1.0/98.8 (molar ratio), GHSV=
Table 2 Activity of complete oxidation of CO at low concentration over the x%mg/Lao.6Sro.4MnO3 catalysts and related systems** Catalyst %0 (K) T95 (K) Lao.6Sro.4MnO3 2%Ag/Lao.6Sro.4MnO3
415 368
420 388
4%Ag/Lao.6Sro.4MnO3 358 383 6%Ag/Lao.6Sro.4MnO3 353 370 10%Ag/Lao.6Sro.4MnO3 398 413 6%Ag/~/-A1203 438 470 0.1%Pd/),-AI203 476 482 0.1%Pt/),-A1203 523 531 **Reaction condition: 0.1MPa, feed-gas CO/OJN2= 2.3/20.5/77.2 (v/v), GHSV= 23000h -1.
Lao.6Sro.4MnO3 catalyst displayed higher activity for catalytic CO oxidation than that of 6%Ag/~/-AI203, 0.1%Pd/),-A1203 or 0.1%Pt/),-A1203. The modification of Ag resulted in a pronounced enhancement of oxidation activity of CO: over the 6%Ag/Lao.6Sro.4MnO3, the "1"5o and T95 came down to 353 and 370 K, respectively.
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3.2. Spectroscopic characterization of catalyst Figure 1 shows the results of XRD measurements of the x%Ag/Lao.6Sro.4MnO 3 (x=4 or 6) and related systems. Only the diffraction peaks ascribed to ABO3-type perovskite-phase were observed for the LaMnO3 and Lao.6Sro.4MnO3 systems (see Figures le and la); for the Lao.4Sro.6MnO3 system, in addition to the features of perovskite phase, quite weak peaks ascribable to the discrete SrO phase were also detected (see Figure ld). According the wellknown Scherrer's equation, it can be estimated that the particle size of the prepared Lao.6Sro.4MnO3 sample was -35 nm. Figures lb and lc were the XRD patterns of the two Agmodified Lao.6Sro.4MnO3 catalysts. Besides the features ascribed to the Lao.6Sro.4MnO3 host phase, two weak peaks were present at 20 = 38.14 ~ and 44.34 ~ which got somewhat more evident as the Ag-doping amount increased from 4% to 6% and were in agreement with the known 20 values for metallic silver, thus attributable to crystallite phase of metallic silver. On the other hand, the signals at 20 = 32.8" and 38.0 ~ which should have been the XRD features of Ag20 phase, were extremely weak and too weak and ambiguous to identify. Thus, from the XRD results, no conclusive evidence was obtained for existence of Ag20 in crystallite phase on the 4%(or 6%)Ag- modified Lao.6Sro.4MnO3 systems.
1023 ,r~
596
/ \
d 1023 ~t~ll~
S
~
~
~ ~
c
a -
23
30
40
50
20, degree
60
70
Figure 1 XRD patterns of: a) Lao.6Sro.4MnO3,b) 4%Ag /Lao.6Sro.4MnO3, c) 6%Ag/Lao.6Sro.4MnO3,
d) Lao.4Sro.6MnO3, e) LaMnO3. (* peaks due to the perovskite phase).
323
!
523
i
723
989
ii23
TEMPERATURE, T Figure 2 H2-TPR spectra of: a) Lao.6Sro.4MnO3; b) 2%Ag/Lao.6Sro.4MnO3.
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Table 3 The results of the least-square fitting of the observed O(ls)-XPS spectra Sample
02(lattice)
O22/O(a)
OH/CO32-(a)
B.E.(eV)
Content
B.E.(eV)
Content
B.E.(eV)
Content
of O(ls)
(Mol.%)
of O(ls)
(Mol.%)
of O(ls)
(Mol.%)
528.9
65.5
530.5
17.6
532.1
16.8
6%Ag/Lao.6Sro.4MnO3 528.6
51.5
530.1
33.1
532.1
15.4
Lao.6Sro.4MnO3
The XPS measurement of the Lao.6Sro.4MnO3 host catalyst showed that the surface Mnspecies on the Lao.6Sro.4MnO3 existed mainly in +3 valence-state with the B. E. of Mn3+(2p3/2) at 641.7 eV. For the 6%Ag-modified Lao.6Sr0.4MnO3 system, the XPS spectrum of Mn(2p3/2) displayed duplex peaks with the B.E. at 641.4 and 642.1 eV, indicating that there co-existed Mn 3§ and Mn 4§ at the surface of the 6%Ag/Lao.6Sro.aMnO3 [4]. On the other hand, the Ag(3d5/2)-XPS spectra of the 6%Ag-modified system displayed a main peak at 367.5 eV (B.E.) and a shoulder at 367.9 eV (B.E.), and the latter was somewhat strengthened with increasing Ag-doping amount. According to the literature [4] and the above XRD results, the observed shoulder peak at 367.9 eV was ascribable to Ag~ of metallic Ag-species at quite a low surface concentration, and the main peak at 367.5 eV was due to Ag§ This result demonstrated that most of the supported Ag20 component did not deoxidize to metallic Ag o after the calcination at 773K in the preparation procedure, and still maintained in Ag § valence-state. In addition, the XPS measurements also provided evidence of existence of oxygen-species with mixed valence-states at the surface of these systems (see Table 3): lattice O2(ls) at 528.6-528.9 eV (B.E.), adsorbed 022-/0 . (is) at 530.1-530.5eV(B.E.), and the O(ls) of oxygen-containing contaminants OH/CO3 2- at -532.1 eV (B.E.). Analysis and fitting of these O(ls)-XPS spectra revealed that the relative content (molar fraction) of the adsorbed O22-/O species in the total surface oxygen on the 6%Ag/Lao.6Sr0.4MnO 3 was almost twice as much as that on the Lao.6Sro.4MnO3. H2-temperature-programmed reduction (TPR) of the catalyst provided useful information about reducibility of the Mn and Ag components in these catalysts. On the Lao.6Sro.4MnO3 host catalyst, three peaks (623, 723, 1023 K) were observed (see Figure 2a). In view of the nonreducibility of the La 3§ and Sr2§ under the condition of H2-TPR, the observed three H2-TPR peaks should be due to the reduction of Mn 3§ species. The former two peaks may be attributed to the reduction of part of the Mn 3§ to Mn 2§ in company with the formation of a corresponding number of anionic vacancies, i.e., Lao.6Sro.4MnO3 + x H2 ---" La0.6Sr0.4MnO3_x+ x H20; and the partially reduced Lao.6Sr0.4MnO3_x intermediate with a certain number of anionic vacancies still maintained the perovskite phase structure as a whole. The 1023K peak was corresponding to the deeper reduction, which led finally to disintegrating the perovskite phase to the discrete oxide phases, La203, SrO, and MnO. On the 2%Ag/Lao.6Sro.4MnO3 (See Figure 2b), the high temperature (1023K) TPR-peak leading to the disintegration of the perovskite ohase was unchanged in the location and shane in comnarison with that of A~,-undnned
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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and area were quite apparent. It is known from the above XPS results that the supported Ag20 component at the surface of the Ag/La0.6Sr0.4MnO3 after the calcination at 773K was a kind of Ag20 species unable to pyrolyze into metallic Ag in air, and that the doping of Ag resulted in formation of a certain quantity of Mn 4§ Thus, the 403K little peak may be assigned to the reduction of the kind of Ag20 species: Ag20 + H2 ---" 2 Ag + H20, and the 523K peak was due to the reduction of the Mn 4§ to Mn 3§ and the 596K peak may originate from the reduction of part of the Mn 3+ species to Mn 2§ Therefore, it may be believed that the doping of Ag brought about an increase in the quantity of reducible Mn n§ species, simultaneously with descending obviously in their reduction temperature.
3.3. Origin of promoter action of Ag The results shown in Table 1 indicated that, on all the 7-AlzO3-supported La0.6Sr0.4MnO3 systems, conversion activity of methanol was lower than that on the unsupported single La0.6Sr0.aMnO3 system, probably due to the formation of inactive aluminates, and tended toward the latter as the loading of La0.6Sr0.4MnO3 increased and the perovskite-phase formed. This implied that not only the surface, but also the bulk perovskite-phase, both played important roles in the catalytic methanol oxidation. The use of 7-Al203 as the bottom carrier would be conducive to generating a relatively large operating surface for y-A1203-supported Ag/La0.6Sr0.4MnO3 systems, which would be in favor of improving dispersion of the Ag component on the surface. The Ag + in a proper amount doped onto the surface of La0.6Sr0.4MnO3 could partially take the place of La 3+ and Sr 2§ and occupy the A-sites in the ABO3-type lattice due to its ionic radius (0.126nm) quite close to that of La3§ and Sr2§ thus being better stabilized and favorable to inhibiting aggregation of the Agspecies at the surface. On the other hand, modification of the Ag § to the surface of La0.6Sr0.4MnO3 would bring about an increase in relative content of the surface 022-/0 . species highly reactive toward CH3OH, HCHO and CO. Besides, solution of a small amount of lowvalence metal oxides AgOx and SrO in the lattice of the trivalence-metal composite oxide LaMnO3 will also result in the formation of anionic vacancies and an increase in content of the reducible Mnn§ under the reaction temperature, which would be conducive to the adsorption-activation of oxygen on the surface of the functioning catalyst and the transport of the lattice and surface oxygen species. All these factors would be in favor of enhancing the catalyst activity for the complete oxidation of methanol. REFERENCES [1] [2] [3] [4]
L.V. MacDougall. Catal. Today, 8(1991)337 R.W. McCabe, P.J. Mitchell. Appl. Catal., 27(1986)83 H.K. Plummer, Jr.,W.L.H. Watkins, H.S. Gandhi. Appl. Catal., 29(1987)261. Handbook of X-ray photoelectron spectroscopy. Eds: C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, and G.E. Mullenberg. Published by Perkin-Elmer Co., Physical Electronics Division, Minnesota, p.74 and p.l12.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
T r e a t m e n t o f a q u e o u s s o l u t i o n s of o r g a n i c p o l l u t a n t s by h e t e r o g e n e o u s c a t a l y t i c w e t air o x i d a t i o n (CWAO). Mich~le B E S S O N ~, J e a n - C h r i s t o p h e BEZIAT ~, B e r n a r d BLANC ~, Sylvain DURECU b and Pierre GALLEZOT ~ a Institut de Recherches sur la Catalyse-CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France b TREDI, D~partement Recherche, 54505 Vandoeuvre-l~s-Nancy, France Wet air oxidation of model and industrial organic pollutants in aqueous media was carried out in batch and continuous reactors, using supported Pt and Ru catalysts. The reactivity of pollutants was studied and the effect of the temperature on the stability of these heterogeneous catalysts was investigated.
1. INTRODUCTION As a result of stricter environmental regulations, w a s t e w a t e r t r e a t m e n t has become a major issue. Wet Air Oxidation (WAO) is a liquid phase oxidation process by molecular oxygen which is efficient for the t r e a t m e n t of aqueous effluents containing high concentrations of toxic or poorly biodegradable organic materials, but the stringent conditions of temperature (200-450~ and pressure (7-25 MPa) severely affect the economy of this technology. The use of catalysts may enhance reaction rates and enable one to carry out the process under milder conditions. Soluble copper salts are generally effective for t h a t purpose [1], but h e t e r o g e n e o u s catalysts are preferred because no catalyst recovery step is required. Copper containing mixed oxides exhibit good activity, but some leaching of transition metal ions from the catalyst matrix may occur [2]. The purpose of this study was to investigate the CWAO on supported platinum and r u t h e n i u m catalysts, of model pollutants and of industrial effluents in batch and in trickle-bed reactors, with particular emphasis on the stability of these heterogeneous catalysts. Small carboxylic or dicarboxylic acids (such as formic, maleic, acetic and succinic acids) which are intermediate or stable end products of oxidation from n u m e r o u s waste streams were chosen as test r e a c t a n t s to evaluate the catalysts.
2. EXPERIMENTAL The oxidation experiments were carried out in a 250 ml Hastelloy C22 autoclave equipped with a magnetically driven impeller [3]. They were also performed in a trickle-bed reactor with cocurrent downflow circulation of air and effluent, described elsewhere [4]. The overall degradation yield was calculated according to the total organic carbon (TOC) which was determined using an automatic TOC 5050 Shimadzu analyzer. High performance liquid chromatography (HPLC) was used for analysis of organic products, using an ion-exchange column (Sarasep
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Car-H) with dilute H2SO4 solutions as eluent, and with UV and RI detectors in
series.
P l a t i n u m catalysts were p r e p a r e d by ion-exchange of the CECA 50S active charcoal (1400 m2g~) with Pt(NHs)4C12 salt in ammoniacal solution. R u t h e n i u m catalysts were provided by Engelhard (2.8%Ru~iO2, Q500-069) or p r e p a r e d by incipient-wetness impregnation of different t i t a n i a and zirconia supports by aqueous solutions of r u t h e n i u m chloride. After washing and drying, they were reduced under a flow of hydrogen (250 ml min "1) at 573K, cooled to 300K u n d e r hydrogen and finally brought into contact with air. 3. R E S U L T S AND D I S C U S S I O N
3.1. Carbon-supported platinum catalysts
Platinum catalysts supported on active charcoal can be used under very moderate conditions (air at atmospheric pressure at 326K or lower temperatures) to oxidize formic and oxalic acid into carbon dioxide and water. The catalytic oxidation of maleic acid was more difficult to achieve and an air pressure of 1.5 MPa was necessary to oxidize aqueous solutions at 403K; during this oxidation, no partial oxidation of maleic acid was detected. Acetic acid, which is known to be the most refractory product in WAO, was not oxidized in these conditions [5]. Table 1 summarizes the initial reaction rates of oxidation of the easily oxidized acids. Table 1 Oxidation rates of aqueous solutions of carboxylic acids (5g L 1) in the presence of 5%Pt/C catalyst Substrate formic acid oxalic acid maleic acid 323 503 Temperature (K) 323 0.9 0.28 Initial rate 36 (tool h 1 g~-l)
3.2. Carbon-supported ruthenium catalysts
To oxidize acetic acid [6] and C4-C 6 carboxylic diacids [7], r u t h e n i u m catalysts supported on active carbon or graphites operating at 453-473K and 5 MPa air pressure were employed. Figure 1 gives the proposed simplified p a t h w a y for wet oxidation of succinic acid and the distribution of products as a function of time. The oxidation of succinic acid occurs via three routes: the direct oxidation to CO2 was the p r i m a r y route, acrylic and acetic acids, which in t u r n were oxidized to carbon dioxide and water, were formed as reaction intermediates. The oxidation of acetic acid was slow, so t h a t all acetic acid formed from succinic acid accumulated in the solution. But it was consecutively oxidized, and at the end of the reaction, only traces could be detected and a TOC removal efficiency of more t h a n 99% was observed within 4 h. No leaching of r u t h e n i u m was observed, but a substantial consumption of the carbon support was noticed, wich makes unable the use of this type of catalyst at the operating conditions required for the refractive organic molecules.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
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化
IO0 " 0
ACR -" v
SUC----~" ACE
4
E30 E tO20
,....
0
o. 10 E CO2 . ~ -
0 o
0:
.
0
~~ .
.
III .
1
80
0
0
60
o-
40
3
20
~, 2 ti3e (h)4 ,
.
.
.
.
.
.
.
.
-~
.
.
.
.
.u m
5
.
.
.
.
6
0
v
Fig. 1. Oxidation of an aqueous solution of succinic acid (43 mmol 1~) in presence of 5% Ru/C, composition vs. time. Reaction conditions: 463K, 5 MPa total pressure. 9 succinic acid (SUC), m acetic acid (ACE), h acrylic acid (ACR), 9 TOC abatement 3.3. T i t a n i a or z i r c o n i a - s u p p o r t e d r u t h e n i u m c a t a l y s t s Titanium dioxide and zirconium dioxide were then selected as the support for ruthenium in view of their stability in acidic and oxidizing medium. A-Succinic
a c i d a n d a c e t i c a c i d d e r i v a t i v e s in b a t c h r e a c t o r
Catalytic studies were then carried out on aqueous solutions of succinic acid and other carboxylic acids with a Ru/TiO2 catalyst loaded with 2.8 wt% metal obtained from Engelhard. The results in figure 2, giving the TOC abatement vs. time, demonstrate the possibility of a complete mineralization of different carboxylic acids into CO2 and H20. The presence of a substituting group (NH2, C1 or OH) on the a-carbon of acetic acid resulted in higher oxidation rates compared to acetic acid (13 molc h 1 mol zuX). Succinic acid had an intermediate activity (43 molc h ~ mol zuX). The catalyst was recycled in five successive batch oxidations of succinic acid, without loss of activity. The absence of metal ions (Ru, Ti) within the detection level of ICP-AES (0.5 ppm and 0.1 ppm, respectively) in the effluents after reaction and the absence of particle sintering by Transmission Electron Microscopy indicate also a high stability of the catalyst under the conditions employed.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
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化
100
~ 80 e -
E 60 ~ 40
+ OA
0 20 + 0!--A+ 0 n
0
. . . .
,
. . . .
1
,
.
2
.
.
.
,
3
. . . .
,
4
time
. . . .
,
. . . .
,
5
6
(h)
. . . .
,
7
. . . .
8
Fig. 2. TOC a b a t e m e n t as a function of time. Reaction conditions: batch reactor, 150 ml of 5g 1x acid aqueous solutions, 5 MPa air, T = 190~ lg 2.8%Ru]TiO2. + acetic acid, a succinic acid,, glycine, A chloroacetic acid, x glycolic acid. B-Succinic
a c i d in t r i c k l e - b e d and support effect
reactor: stability of the catalyst
The e x p e r i m e n t s were then conducted in the micropilot trickle-bed reactor, operating under integral conditions or differentially. A set of tests of succinic acid was carried out, using the former catalyst 2.8%Ru/TiO2 compressed in grain form (0.8-2 mm), or catalysts prepared by wet impregnation of formed supports (0.9% Rufl~iO2, Degussa P25, a n a t a s e + rutile, l m m extrudates, and 1.4% Ru/ZrO2, Engelhard, 0.8-2 mm grains). Transmission Electron microscopy studies showed t h a t the particle size of these catalysts was comparable (2-5 nm). Figure 3 shows the concentration profiles of reaction products and TOC concentration as a function of the residence time. Increasing residence time were obtained by repeated recycling of the treated solution, while maintaining the liquid (60 ml h -x) and gas (51 h x) rates. 2500
40'
..-.2000,
O
E 30 E v t.O m
500 v
20
{D O
o.10 E o 0 0 0
9 am
B
.9~ , a , A
9
~1000 i
n 9
nn
F-
II
nn
U
i
9
500
9
m
,~_~ .....................
1 2 3 4 5 6 7 8 contact time (h g 1R u I1)
0
.......................................
0
1 2
3
4
5
6
7
contact time (h g 1R u I 1)
2.8% Ru / Ti02 Engelhard compressed in grains
8
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,.--,.
1557 2500
40
~.2000q
0
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v..
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mmnnm 9
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~
0.5 1 1.5 contact time (h g 1R u I 1)
o
....
0 ' . S " l
....
contact time (h g 1R u I 1)
0.9% Ru / TiO2DegussaP25 2500
v.-
0
40'
~.2000,
E
v-
E 30
c~1500 E 01000 0 500
tO
.=_9 20 if) 0
E 10
0 o
9
0 0
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a ......
1
as ~. . . . . . .
n
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contact time (h g 1R u I 1)
0
.
.
.
.
.
.
.
.
.
,
.
0.5
0
.
.
.
.
.
.
.
.
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.
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.
.
.
.
.
.
.
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contact time (h g 1
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I-1)
1.4% Ru / Zr02 Engelhard Fig. 3. Oxidation of succinic acid at 463K and 5 MPa air pressure in the tricklebed reactor: effect of the nature of the support succinic acid, m acetic acid, A acrylic acid. By comparing the experimental results of oxidation obtained, one can notice t h a t the p r e p a r a t i o n of the catalysts was crucial on the activity and the relative importance of the three routes of oxidation of succinic acid. The catalyst prepared on zirconia favored the direct oxidation of succinic acid to carbon dioxide, thus m i n i m i z i n g the formation of refractory acetic acid. As a consequence, the decrease in TOC concentration is higher (less than 7.3 mg 11 at 1.43 h gRu-111). 3.4. O x i d a t i o n
of an effluent from a bleach plant
Among the various industrial w a s t e w a t e r streams, those from the bleaching operation of kraft wood pulp deserve some attention. Wet oxidation of different effluents, generated after the chlorine dioxide stage (acidic pH) and from alkaline extraction (pH 9), was carried out at 463K and 5 MPa air in the slurry reactor. The reactivity of the wastewater was found to depend greatly on the pH of the effluent, and decreased with increasing pH. Figure 4 gives, for instance, the TOC removal versus time profiles for a concentrated basic effluent (initial TOC 8690 mg 11).
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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A rapid initial decrease in COT was observed ; as the organic compounds present initially were converted to more refractory ones, the rate at which the TOC decreases declined t h e n significantly. However, the overall TOC content could be drastically reduced by over 90%, the color removal was significant (it changed from dark brown to pale yellow, i n d i c a t i n g a successful destruction of m a j o r organic compounds responsible for coloration), a breakdown of the large molecules was observed (as shown by GPC), and the reduction of AOX (adsorbable organic halides) was effective (32.2 to 0.1 mg 11).
10000
.._, E
8oo~ (
6000 ~
0 4000 0 0 0
2000
0
.
0
.
.
,
20
. .
9
i
40
.
.
.
i
.
60
.
9
~
80 time (h)
9
9
100
9
9
9
i
120
9
. .
140
Fig. 4. TOC concentration vs. time, during oxidation of a concentrated basic effluent from pulp mill industry. Reaction conditions: batch reactor, 190~ 5MPa air, lg 2.8% Ru~iO2.
CONCLUSION The complete degradation of organic compounds in wastewaters to inorganic compounds can be achieved by heterogeneous catalytic wet air oxidation The nature of the pollutant is critical in the selection of a catalyst: Pt/C can be used in moderate conditions to oxidize easily oxidized acids, while RufriO2 or Ru/ZrO2 oxidize more refractive compounds at higher temperatures. These catalysts showed high activity. They are physically and chemically stable in hot acidic oxidizing solutions for a prolonged use. The performances of the catalysts are dependent on the support (TiO2 or ZrO2). REFERENCES
1. V.S. Mishra, V.V. Mahajani and J.B. Joshi J, Ind. Eng. Chem. Res. 34 (1995) 2. 2. S. Imamura, H. Nishimura and S. Ishida, Sekyu Gakkaishi, 30 (1987) 199. 3. J.C. B~ziat, M. Besson, P. Gallezot and S. Dur~cu, J. Catal., 182 (1999) 129. 4. J.C. B~ziat, M. Besson, P. Gallezot and S. Dur~cu, Ind. Eng. Chem. Res., 38 (1999) 1310. 5. P. Gallezot, N. Laurain and P. Isnard, Appl Catal. B, 9 (1996) L l l . 6. P. Gallezot, S. Chaumet, A. Perrard and P. Isnard, J. Catal., 168 (1997) 104. 7. J.C. Bdziat, M. Besson, P. Gallezot, S. Juif and S. Durecu, in Studies in Surf. Sci. and Catal. 110 (1997) 615
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Effect of the Support for Pt Catalysts on Soot Oxidation Akira Obuchi, Junko Oi-Uchisawa, Ryuji Enomoto, Shetian Liu, Tetsuya Nanba and Satoshi Kushiyama Atmospheric Environmental Protection Department, National Institute for Resources and Environment, 16-30nogawa, Tsukuba 305-8569, Japan
Abstract The effect of the support for Pt catalysts on the oxidation of carbon black, a model diesel-exhaust soot were examined. Among eight kinds of Pt-supported metal oxides, Pt/Ta205 showed the highest activity towards the oxidation of carbon black in a model diesel exhaust, containing 02, 1-120, NO, and SO2 in N2. Pt catalysts supported on other non-basic metal oxides such as Nb2Os, WO3, SnO2, and SiO2 showed similar high activities. The cause of the high activity for these catalysts was investigated in relation to the roles of NO2 and SO3 (or H2SO4) in this catalytic process. 1. INTRODUCTION The emission of soot from diesel engines can be reduced by placing a filter device made of heat-resistive materials in the exhaust stream. To achieve this reduction, however, the soot collected in the filter must be continuously or periodically removed from the filter by combustion. Normally, solid carbon in the soot burns only above 600 ~ which can not be realized in the exhaust stream under typical engine operating conditions. Oxidation catalysts can lower this combustion temperature. Various kinds of metal oxides [1], mixed oxides (especially those containing vanadium, molybdenum and/or copper [2], and perovskites [3]), chlorides [4], and sulfates [5] have been reported to be active catalysts for carbon oxidation. Pt catalysts exhibit the highest activity at low temperatures [6,7]. Platinum is believed to promote soot oxidation indirectly, i.e., by oxidizing the NO, normally coexisting in the exhaust gas, to NO2, which subsequently oxidizes soot to CO and CO2. Furthermore, we have recently found that SO2 and 1-120 present in the reactant in addition to NO, substantially promote carbon oxidation in the presence of Pt/SiO2 [8]. SO3 or H2SO4 produced from SO2 oxidation on Pt surface is believed to catalyze the oxidation reaction of carbon by NO2. Water is believed to promote this reaction by decomposing intermediate species through hydrolysis [9]. Here we report the effect of the Pt-catalyst support and discuss the results, based on the above reaction mechanism. 2. EXPERIMENTAL Pt (0.3 wt%) was supported on the granular metal oxides, Ta205, Nb205, WO3, SnO2, SiO2, TiO2, A1203, and ZrO2, having sizes ranging from 0.15 to 0.25 mm. In the case of WO3 and Nb2Os, the oxide granules were prepared from a commercially available oxide in the
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form of powder, by kneading with aqueous ammonia, followed by drying, calcination at 600 ~ in air for 2 h, crushing the aggregates, and sieving. Pt was supported on these oxides by the incipient-wetness method using a solution of Pt(NH3)4(OH)2, followed by drying and calcination in air at 600 ~ The prepared catalyst samples were characterized by BET surface area and Pt dispersion, i.e., (Pt exposed) / (Pt total) expressed as a percentage, determined by CO pulse adsorption at 50 ~ or Pt particle-size observation by TEM. Furthermore, temperature-programmed desorption (TPD) of CO2 was carried out to evaluate the basicity of the support materials. After calcination in air at 600 ~ the sample was exposed to 100% CO~ for 10 min at 100 ~ purged with N2 and heated at a rate of 10 ~ under a N2 flow, during which the CO2 desorption rate was continuously measured. Temperature-programmed reactions (TPR) were carried out to evaluate the catalytic performance for carbon oxidation. Commercially available carbon black (CB; Nippon Tokai Carbon 7350F, primary particle size = 28 nm, BET surface area = 80 m2/g, elemental analysis C = 97.99 wt%, H =1.12 wt%, N = 0.06 wt%) was used as a model soot. The Pt catalyst (0.5 g) and CB powder (0.005 g) were mixed together with a spatula to attain "loose" contacts between the catalyst and carbon [10]. Reactant gases containing 1000-ppm NO, 100-ppm SO2, 7% 1-120, and 10% 02 in N2, were passed through the mixture of catalyst and CB at a flow rate of 0.5 dm3/min. The reactor temperature was raised by 10 ~ from 80 to 750 ~ and the concentrations of CO2, CO, and NO2 in the product gas were continuously measured using non-dispersive IR gas analyzers and a chemiluminescence-type NOx analyzer. In addition, isothermal reaction tests were carried out at 350 ~ under the same gaseous conditions to determine if the combustion of the CB is complete at this temperature. Similarly, transient response reactions were carried out at the same temperature to investigate the effect of SO2 on the catalytic NO oxidation, under the same gaseous conditions as TPR except that no CB was mixed with the catalyst sample, and the SO2 concentration was switched between 0 and 100 ppm. 3. RESULTS AND DISCUSSION The BET specific surface area and Pt dispersion of the catalyst samples are summarized in Table 1, as well as the source of the metal oxides and the pretreatment conditions for supporting Pt. The Pt dispersions on SiO2, ZrO2, A1203, and TiO2 exceeded
Table 1 Preparationmethod and physical properties of the Pt supported catalysts Support
Manufacturer/ Code
Pt preparation BET surface Pt dispersion condition ") area / m2.g1 / % SiO2 Wako chemicals / Wako-gel 100 H/A 434 13 ZIO 2 Daiichi-Kigenso H/A 44 129 A1203 SumitomoChemicals / KHS-24 H/A 152 14 TiO2 IshiharaSangyo/ ST-B11 H/A 29 21 SnO2 Rare Metallic A 3.0 3.8 WO3 Kanto Chemicals A 4.7 --- b) Nb205 WakoChemicals A 2.7 3.3 Ta205 WakoChemicals A 2.5 3.4 ")H" reduction in 3% H, in N~ at 400 ~ for 4h. A: calcination in air at 600 ~ for lh.
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10% even after a pretreatment of calcination in air at 600 ~ for 1 h. It is not clear why the Pt dispersion exceeded 100% in the case of ZrO2. This result suggests that more than one CO molecule were adsorbed on a Pt atom, such as a 'twin CO.' No Pt particles were visible on ZrO2 using TEM at a magnification as high as 3,000,000, which implies a very high Pt dispersion. On the other hand, the Pt dispersions for SnO2, Nb2Os, and Ta205 estimated by TEM were below 4%. The low surface area of these supports may cause the relatively low Pt dispersions. Figure 1(a) shows CO2-emission curves from the TPR experiments for the Pt catalysts supported on the various oxides examined. The CO emission was negligible in all the cases. For Pt/SiO2, the initial and peak temperatures were 280 and 500 ~ respectively, the former being defined as the temperature for which the CO2 concentration exceeds 100 ppm, with a shoulder located at 350 ~ The initial temperatures for Pt/ZrO2, Pt/Al203, and Pt/TiO2 were 100 to 130 ~ higher than that for Pt/SiO2. On the other hand, Pt/Ta2Os, Pt/Nb2Os, N/WO3, and Pt/SnO2 showed higher activities than that for Pt/SiO2 near 350 ~ although the initial temperatures were almost the same. Pt/Ta205 showed a CO2-emission rate nearly twice that for Pt/SiO2 at this temperature. Figure l(b) shows NO2-emission curves obtained during the same TPR experiments. NO2 production started at temperatures as low as 200 ~ for all the catalyst samples except Pt/ZrO2 and Pt/AI203, which indicates that the catalytic activity for NO oxidation to NO2 is very high for these materials. As the temperature rose, the NO2 concentration temporarily decreased near 280 ~ which almost coincides with the initial temperature for CB oxidation. This implies that NO2 oxidizes CB while simultaneously being reduced to NO under the TPR conditions. Above 450 ~ the NO2-emission concentration reached values predicted by the 2000
' li
'
'
,
t
Support:TiO
:l~176176
,
,
,
,
[[_bsupport: ~
] /
//
//
4001
, oo
,
Ta205-~~ s.o
I
I :
1
1
,00 r SnO
lOO
0 100
200
300 400 500 Temperature / ~
600
700
100
I 200
I I I 300 400 500 Temperature / ~
I 600
700
Fig. 1 CO2 emission (a) and NO2 emission (b) curves from the TPR experiments evaluating the carbonblack oxidation activity of Pt catalysts supported on various oxides. For the experimental conditions, see the exoerimental section in the text.
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thermodynamic equilibrium among NO, NO2, and 02, thus decreasing with a rise in temperature. By contrast, in the case of Pt/ZrO2, NO2 production occurred only above 300 ~ and the concentration was also substantially lower than the equilibrium value predicted for higher temperatures. It is believed that Pt highly dispersed on ZrO2 decreases its catalytic oxidation activity, most likely by strongly interacting with the support. The behavior of Pt/TiO2 appears strange at first sight; while NO2 began to be produced at 230 ~ it was not consumed until 390 ~ Similar behavior was also observed in the case of Pt/AI203. These results suggest that factors other than NO2 production are critical in the catalytic oxidation of CB, which will be discussed later. Figure 2 shows time courses of the carbon oxidation rate at a constant temperature, 350 ~ (from 25 to 110 min), for the same Pt-supported oxides tested in the above TPR experiments. In the case of Ptffa205 and PtfNb2Os, which showed the highest performances in the TPR experiments, the CB was oxidized almost completely in 40 and 60 min at 3 50 ~ respectively. It took approximately 100 min for the CB to be removed in the Pt/SiO2 experiment. On the other hand, for Pt/Zr02 and Ptfrio2 the carbon oxidation rates were much lower, and it was necessary to raise the temperature to oxidize the carbon completely. Figure 3 shows results of transient responses of NO2 emission over different Pt catalysts at 350 ~ after the addition and subsequent cut-off of SO2. Carbon black was not mixed with the catalysts in this case. Except for Pt/ZrO2, the NO2 concentrations were 500 to 600 ppm before adding SO2. One hour after adding SO2, the NO2 concentrations produced by Pt supported on ZrO~, TiO2, A1203, SiO2, SnO2, WO3, Nb2Os, and TarO5 were higher in the stated order, which coincides with the order of the carbon oxidation rates, except for an exchange in the position of A1203 and TiO2. Following the addition of SO2, Pt/Ta~O5 showed a sudden decrease in NO2 of approximately 100 ppm, but with the cut-off of SO2, the concentration quickly returned to the original level. Similar results were obtained for Pt/Nb2Os, Pt/WO3, and Pt/SnO2. With Pt/SiO2, the response to adding SO2 was slower but the decrease became eventually more prominent; aider the SO2 cut-off, the NO2 concentration approached the original level with a slower recovery rate. Furthermore, in the case of Pt/TiO2, once SO2 was added, the NO2 concentration continuously decreased with a negligible 10001
I
t
t
I
I
!
[_
8oo/
!
!
--
"rT ~_%.
i
~o 600
------or---Pt/SiO = Pt/TiO2
!
J
I
t.,
I
/ /
I
/
"~. 400 o
/
i 1000
/ 800 ., -"1
"
""
600 400
200 0
I
~ %
200 0
20
40
60
80 Time / min
100
120
0 140
Fig. 2 Timecourses of carbon-black oxidation at 350 ~ with Pt catalysts supported on Ta:Os, Nb2Os, SiO2, TiO2, and ZrO2. The reactant-gas compositionwas the same as that shown in Fig. 1.
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化
SO2 add
I
/
~
SO2 cut
I
t
600 500
0 .~,,t rd~
Support:
-
400 300
Ta O
SiO -
eq
200
-
---,..._~---
100 I 0
~ u,.-..,u~x_,-~.i3t~(3
~ .
ZIO I 50
100
150
I 200
I 250
Time / min Fig. 3 Transient responses of the rate of NO oxidation to NO2 over different Pt catalysts at 350 ~
after the addition and subsequentcut-offof SO2 in the reactant gas. The reactant-gascompositionwas the sameas that shown in Fig. 1, exceptfor SO2. recovery following the SO2 cut-off. Similar tendency was more remarkable for Pt/ZrO2. The total sulfur amount, which passed through the catalyst samples during this experiment, corresponded to 1.43 wt% of the catalyst sample. The amounts of sulfur present in Pt/Ta2Os, Pt/Nb2Os, Pt/WO3, Pt/SnO2, Pt/SiO2, Pt/TiO2, Pt/AI203, and Pt/ZrO2 measured after the experiment were 0.03, 0.05, 0.02, 0.05, 0.02, 0.99, 1.35, and 0.45 wt%, respectively. Only the last three samples strongly adsorbed sulfur. These results indicate that Pt/Ta2Os, Pt/Nb20~, Pt/WO3, and Pt/SnO2 have a strong resistance (with Pt/SiO2 having a more limited resistance) to poisoning by sulfur (SO3 or H2SO4 derived from SO2) for NO2 production, whereas Pt/TiO2, Pt/AI203, and Pt/ZrO2 lose their catalytic activity by poisoning at this temperature. Furthermore, for the latter three catalysts, SO3 (or H2SO4), which is a catalyst to promote the oxidation of carbon by NO2, is trapped by the supports and does not reach the carbon to be oxidized. Figure 4 shows the results of CO2 TPD experiments to evaluate the basicity of the supports. The CO2 emission was observed only for ZrO2, Al~O3 and TiO2, all of which demonstrated low activity as the support for Pt catalysts. Among these, ZrO2 showed the highest total amount of desorption, reaching 0.024-m01 CO2 per mol ZrO2. By contrast, the other five oxides did not adsorb CO2 at all. A high negative correlation between the basicity and activity of the support was found. SiO2 and the metal oxides with a lower basicity have a reduced affinity toward S03 (or H2S04), and the supported Pt is poisoned to a lesser degree. As a result, the SOa (or H2S04) produced can reach the carbon surfaces more easily, thereby promoting carbon oxidation. 4. CONCLUSION Eight kinds of Pt-supported metal oxides were tested and compared for their catalytic performance in the oxidation of carbon black, mechanically mixed with the catalyst sample; the tests used a model diesel exhaust gas containing 02, 1-120,NO, and SO2. Pt/Ta205 was the
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化
2500
~
I -::1 I =
2000 ~'
I I I
Ti~ ":'"1 --'C}~ SiO2 I
1500
1000 r~
i~
!
500
100
200
300 400 Temperature
500
600
Fig. 4 CO2TPD fromPt/ZrO2,Pt/Al203, Ptfrio2, and Pt/SiO2. most active catalyst. Pt catalysts supported on other non-basic oxides, such as Nb2Os, WO3, SnO2, and SiO2, showed similar high activities. We attribute the high activity for these catalysts to their non-basicity and negligible affinity for SO3 (or H2SO4), which results in less poisoning of the supported Pt and in a smooth supply of SO3 (or H2SO4) to the carbon surface, the oxidation of which by NO2 is catalyzed by SO3 (H2SOa).
REFERENCES
1. J.EA.Neel~ M.Makkee, H.A.Moulijn, Appl. Catal. B, 8 (1996) 57. 2. A.Bellaloui, J.Varloud, P.Meriaudiau, V. Perrichon, E.Lox, M.Chevrier, C.Gauthier, EMathis, Catal. Today 29 (1996) 421. 3. Y.Teraoka, K. Nakano, S.Kagawa, W.F.Shangguan, Appl. Catal. B, 5 (1995) L181. 4. G.Mul, F.Kapteijn, J.A.Moulijn, Appl. Catal. B 12 (1997) 33. 5. Z. Zhao, A.Obuchi, J.Uchisawa, S.Kushiyama, Stud. Surf. Sci. Catal. 121 (1999) 387. 6. P. Hawker, N. Myers, G. Huthwohl, H. T. Vogel, B. Bates, L. Magnusson and P.Bronnenberg, SAE Technical Paper Series, 970182, 1997. 7. B. J. Cooper and J. E. Thoss, SAE Paper Series No. 890404 (1989). 8. J.Oi-Uchisawa, A.Obuchi, Z.Zhao and S.Kushiyama, Appl. Catal. B 18 (1998) L183. 9. J.Oi-Uchisawa, A.Obuchi, A.Ogata, R.Enomoto, S.Kushiyama, Appl. Catal. B 21 (1999) 9. 10. J. P. A. Neefl, O. P. Pruissen, M. Makkee and J. A. Moulijn, Appl. Catal. B, 12 (1997) 21.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Dimethyl carbonate synthesis from carbon dioxide and methanol over Ni-Cu/MoSiO(VSiO) catalysts S.H. Zhong, J.W. Wang, X.F. Xiao and H.S. Li College of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China The surface structures, chemisorption properties and catalytic behaviors of Ni-Cu/MoSiO and Ni-CuNSiO catalysts have been investigated by using the techniques of XRD, TPR, IR, TPD and micro-reactor. Three types of active sites, metallic site M (Ni-Cu alloy), Lewis acid site M n§ (Mo 6§ or V 5§ and Lewis base site M=O (Mo=O or V-O), have been found on the surface of the reduced catalysts. With the cooperation effects of these active sites, CO2 can be chemisorbed on the metallic site M and Lewis acid site M n§ to form a new type of CO2 horizontal adsorption state, M--(CO)--O->M n§ and CHaOH can be dissociated on the Lewis base site M=O and Lewis acid site M n§ to form a dissociative adsorption state, Mn§ and HO--M. The selectivity of dimethyl carbonate synthesized from these adsorption species is over 85% and a surface reaction mechanism of this catalytic reaction is proposed based on the experimental results. 1. INTRODUCTION Carbon dioxide, the major man-made greenhouse gas, is mainly emitted from combustion of fossil fuels, and dimethyl carbonate (DMC) is a kind of important intermediate in organic synthesis. Using carbon dioxide and methanol as the raw materials to synthesize DMC is an important reaction in the field of synthetic chemistry, utilization of carbon resource and environment protection [ 1]. The activation of CO 2 molecule is a key problem in the course of DMC synthesis from carbon dioxide and methanol. Some exploratory researches have been done in aspect of CO2 activation on the surface sites of the transition metals, transition metal oxides or the both cooperation effects of them [2-~5]. Using ethylene oxide, carbon dioxide and methanol as the reactant materials to synthesize DMC with the liquid phase method has been reported in the literature [6]. In the present work, we have focused on the molecular design of catalysts in order to synthesize DMC directly from carbon dioxide and gaseous methanol. Our results are briefly summarized and discussed here. 2. EXPERIMENTAL
2.1. Catalyst preparation The surface complex oxide supports of MoO3-SiO 2 (MoSiO) and V205-SiO 2 (VSiO) with 50% covering degree of MoO3 or V205 onto the surface of silica carrier were prepared by the method of modifying the silica surface character in reaction with hydrochloric acid solution of MoOC14 and VOC13 [7,8]. The catalyst samples, Ni-Cu/MoSiO and Ni-CuNSiO, used in this study were prepared by incipient wetness impregnation of the MoSiO or VSiO with a mixed aqueous solution of Ni(NO3)2 and Cu(NO3)2 . After impregnation, the samples were dried overnight at 393 K, calcined at 723 K in flowing air for 5h, and reduced in flowing HE/N2 mixed gases at a suitable temperature for 2h then stored in bottles. The metal loadings of the
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catalyst samples is about 3 wt% with the Ni/Cu atomic ratio 2.
2.2. Catalyst characterization Temperature-programmed reduction (TPR) experiments of calcined NiO-CuO/MoSiO and NiO-CuONSiO samples were carded out by heating about 100 rng of the samples in a flow of 20 ml/min of a H2/N2 = 1/10 mixture and measuring the hydrogen consumption using a TCD detector. During the experiment, the temperature was raised from 300 K to 1073 K. XRD measurements of reduced Ni-Cu/MoSiO and Ni-Cu/VSiO catalysts were performed using Rigaku 2038 model diffractometer and Ni-filtered Cu Ko radiation. Lattice constants were calculated from centroid positions and particle sizes from the integral half width of the line profiles. Infrared spectra were recorded 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. Ultimate pressures of 5 • 10.6 tort were obtainable under dynamic conditions. A Hitachi 27030 spectrometer was used to measure the infrared spectra of self-supporting wafers after their drying, reduction and exposure to CO2 or CH3OH. The procedures were described in more detail elsewhere [9]. 2.3. Reaction evaluation The temperature-programmed desorption (TPD) system was similar to that described previously [10]. A 50 mg catalyst sample rested on a quartz frit in a tubular, downflow reactor (6 mm OD). The gaseous product distribution of TPD was measured by a LZL-203 quadrupole mass spectrometer located in a diffusion-pumped vacuum chamber. Before the measurements were taken, the catalyst sample was rereduced in flowing H2 at 473 K for 2 h and evacuated for 3 h at the same temperature, following cooling to 300 K. Absorbate gas (50 torr), CO2, CH3OH or CH3OH/CO2=2, flowed over the catalyst until saturation was obtained at 300 K, then evacuated the system to 104 torr. The catalyst temperature was increased linearly at a heating rate of 8 K/min to a final temperature of 673 K in a He flow rate of 20 ml/min. The catalytic reactions were performed in a stainless steel tubular fixed-bed reactor. A MRS-901 micro-reactor device controlled by a computer was employed in this work. 3. RESULTS AND DISCUSSION 3.1. Surface structures of catalysts The surface structures of the complex oxide supports MoSiO and VSiO characterized by IR technique in our previous studies [7,8] showed that they are the valence bonding type of M o O 3 and V205 combined with the SiO2 surface through oxygen bridge. The reduction character of the MoSiO and VSiO supported NiO-CuO samples measured from the TPR experiments showed that the metal oxides NiO.yCuO on the surface of MoSiO and VSiO can be all reduced at the temperature of 353 and 330 ~ respectively, and that the Mo=O and V=O in the surface of MoSiO and VSiO support can be partly reduced at the temperature of 580 and 515 ~ respectively. The metallic-phase composition of the reduced Ni-Cu/MoSiO and Ni-CuNSiO examined from the XRD measurements is a homogenous-phase Ni-Cu alloy cluster with the average particle size of 38 A and 42 A for the Ni-Cu/MoSiO and Ni-CuNSiO catalyst respectively. By combining the results mentioned above in one model, the surface constructions of the Ni-Cu/MoSiO and Ni-CuNSiO catalysts were characterized as it is shown in the Figure 1 a and b. It is clear that three types of the active sites, metallic site M (in the form of Ni-Cu
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alloy), Lewis acid site M"+(Mo 6+ or V 5+) and Lewis base site M=O (Mo=O or V=O) appear respectively on the surface of Ni-Cu/MoSiO and Ni-Cu/VSiO catalysts. The d-electron density of the Ni-Cu alloy may be changed by Cu atoms alloying with Ni atoms and the SiO2 support has an influence on the intensity of Lewis acid site M~+(Mo6+ or V 5+) and Lewis base site M=O (Mo=O or V=O). O _ O _ IMIo ~
"o"
O --O--~
o/\o\
o"
n+O / 0 / I 0, ~..A 0I x O\
,o, !"o, i"o,
a, Ni-Cu/MoSiO b, Ni-Cu~SiO Fig. 1. Surface constructions of catalysts 3.2. COs adsorption on catalysts IR spectra of CO2 adsorbed on Ni-Cu/MoSiO and Ni-Cu~SiO are shown in the Figure 2. By comparison with the IR band characters of different CO2 adsorption states occured on the transition metals or transition metal oxides [5], the IR results of CO2 adsorption on the catalysts may be identified and synthesized in the Table 1. TPD profiles of CO2 adsorbed on Ni-Cu/MoSiO and Ni-Cu/VSiO are shown in the Figure 3. Based on the analysis of CO2 adsorbing strength measured from the IR spectra, the TPD results of CO2 adsorbed on the catalysts were also synthesized in the Table 1. --'-dL-.~-.
-
~,~ J. A
~
d ~
9 ~,,,I
C
0
r
I
2500
2000
1600
Wavenum
1400 1200
1000
b e r ( c m "l)
Fig.2. IR spectra of CO2 adsorption. a Ni-Cu/MoSiO, b Ni-Cu/MoSiO - - CO2(ads), c Ni-Cu/VSiO, d Ni-Cu/VSiO -----CO2(ads)
800
40
|
I
|
I
,
I
80 120 160 Temperature (*C)
,
200
Fig.3. TPD profiles of C O 2 adsorption. a CO2 ads.--CO 2 des. on Ni-Cu/MoSiO, b CO2 ads.---CO des. on Ni-Cu/MoSiO, c CO2 ads.--CO2 des. on Ni-Cu~SiO, d CO2 ads.---CO des.on Ni-Cu/VSiO
It is clear that CO2 can be adsorbed at Lewis base site M=O (Mo=O or V=O) to form bidentate carbonate and at metallic site M (Cu or Ni in Ni-Cu alloy) to form linear and sheafing type of adsorbing species on the surface of catalysts. These common adsorption species are a reversible adsorption state and will be desorbed in the form of CO2 at the temperature range of 70-~95~ It is noteworthy that there is a new type of CO2 adsorption state, horizontal M -- (CO) -- O---~Mn§ appeared on the metallic site M(Cu or Ni in Ni-Cu alloy)
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and Lewis acid site M n+ (Mo6+and V 5+) of the catalysts. This is an irreversible adsorption state and will be desorbed in the form of CO at the temperature range of 130-140 ~ Table 1 The analysis results of IR spectra and TPD profiles of CO2 chemisorbed on catalysts Wavenumber (cml) Ni-Cu/ MoSiO
Adsorbing species
1 6 3 2 1992 1 9 7 0 1303
1594 1369
1568 1328
~
%.~o
y ~ _
Ni-Cu/ VSiO
Adsorbing species
~ ~
~i
1630 1985 1964 1300 o o 1o ~ ~o \V/
Desorption temp. ( ~
~r§
du
72(C02)
Desorption temp. (~ Wavenumber (cmt)
~ ~
1080 960
l~i
1055 960 ~
~8 +
90(C02) 1588 1560 1355 1316
135(C0) 1093 1064 945 945
o, o
%-0, ~ w
oyo
Cu
Cu
75(CO2)
94(CO2)
138(CO)
3.3. CH3OH adsorption on the catalysts IR spectra of CH3OH adsorbed on the Ni-Cu/MoSiO and Ni-CuNSiO are shown in the Figure 4. TPD profiles of CH3OH adsorbed on Ni-Cu/MoSiO and Ni-CuNSiO are shown in the Figure 5.
I I/3 r
t"q
9
I
3500
,
i
9
3000
t/W
1200
,
I
1100
1000
i
900
Wavenum ber (era "l) Fig.4. IR spectra of CH3OH adsorption. a Ni-Cu/MoSiO, b Ni-Cu/MoSiO--CH3OH (ads), c Ni-Cu/VSiO, d Ni-Cu/VSiO--CH3OH (ads)
I
200
i
I
,
250
I
,
300
Temperature (~ Fig.5. TPD profiles of CHsOH adsorption. a CH3OH ads.-CH3OH des. on Ni-Cu/MoSiO, b CH3OH ads.-CH20 des. on Ni-Cu/MoSiO, c CHaOH ads.-CH3OH des. on Ni-Cu/VSiO, d CH3OH ads.-CH20 des. on Ni-Cu~SiO
There are four absorbing bands at 3652, 2976, 2864 and 1026 cm1 and at 3658, 2976, 2864 and 1031 cm ~ appeared respectively on Ni-Cu/MoSiO and Ni-CuNSiO. By contrast with the IR spectra obtained from the gaseous CH3OH, the band at 2976 and 2864 cm1 was assigned to the stretching vibration modes of C--H bond and at 1026 and 1031 cm-~ was assigned to the stretching vibration modes of C--O bond in the oxymethyl occurred on the Lewis acid
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site Mo 6+and V 5+respectively. The band at 3652 and 3658 cm~ was assigned to the stretching vibration modes of O--H bond in the hydroxyl occurred on the Lewis base site Mo=O and V=O respectively. It turns out that the O--H bond in CH3OH interacts with the Lewis acid site M"+ and Lewis base site M=O on the surface of Ni-CtgMoSiO and Ni-Cu/VSiO to form dissociative type of adsorbing species Mn+--OCH3 and H - - O - - M . This is a kind of irreversible adsorption state and will be desorbed in the form of CH20 at the temperature range of 240---260~
3.4. Reaction performances of catalysts The TPSR results of CO2 and CH3OH chemisorbed on Ni-Cu/MoSiO and Ni-Cu~SiO are recognized as it is shown in the Table 2. It must be pointed out that the product DMC did not be detected since the limit of the LZL-203 quadrupole mass spectrometer employed in our experiments. By comparison with the TPD results of CO2 or CH3OH alone adsorbed on the catalysts, it is enough to prove that the CO2 adsorbing types of bidentate carbonate, linear and shearing are not the reaction species, that the horizontal type of CO2 adsorbing species may be able to react with the dissociative species of CH3OH to form DMC and H20 at the temperature around 126~ and that the dissociative type of CH3OH adsorbing species will be transformed into CH20 when the reaction temperature over 270~ Table 2 TPSR results of CO~+CH3OH chemisorbed on catalysts Ni-Cu/MoSiO
Temperature (~ Products
73, 90 CO2
125 H20
137 CO
278 CHzO
CH3OH
Ni-CuNSiO
Temperature (~ Products
78, 94 CO2
128 H20
140 CO
273 CH20
--CH3OH
The catalytic performance of Ni-Cu/MoSiO and Ni-CuNSiO determined from the microreactor technique showed that they are excellent catalysts for DMC synthesis from CO2 and gaseous CH3OH and the reaction products are mainly DMC, CH20, CO and H20. It has been found that the conversion rate of CH3OH and the selectivity of DMC in the reaction products depended upon the reaction temperature, reaction pressure, space velocity and feed composition of CH3OH/CO 2. A typical reaction result is shown in the table 3. Table 3 A typical reaction results for Ni-Cu/MoSiO and Ni-Cu~SiO catalysts Catalysts
P (MPa)
Temp. (~
SV (hl)
CH3OH/ CO2 (mol.)
Conv. of CH3OH(%)
Ni-CufMoSiO Ni-CuNSiO
0.1 0.1
140 140
2000 2000
2:1 2:1
16.37 14.54
Selectivity (%) DMC CH20 CO 86.52 87.84
6.45 5.73
7.03 6.43
Based on the experimental results above, a surface reaction mechanism of DMC synthesizing from CO2 and CH3OH has been set up as it is shown in the Figure 6. It will be seen from this model that the surface reactions of Mn+--OCH3 with M--CO from to DMC and M--OH with M--OH form to H20 and M=O, and that the by-products of CH20 comes from the reaction of M"+--OCH3 with M=O and the M--CO may be desorbed directly to CO. The crystal-frame oxygen M=O takes part in the courses of the surface reactions.
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o
O
Mo+
1~~n1+~ -
h M
[co +2c. o. 1
L...
l0~ l ~ ~ n + I
O I\ C O ,1 Mn+
"- ~,,ln+
" , 3
Sii~ii'............~
OH
I
(a) O O,,..oCH3 1~1 / C Mn+O
OH CH3 j. v M
(b)
IH,Ol
../
o
~
o ~
CH3 ~H3 II~n+O
" ~ ...............'S'i'i~ii'............
(d) (c) Fig.6. A reaction mechanism of DMC synthesizing from CO2 and CH3OH 4. CONCLUSIONS Ni-Cu/MoSiO and Ni-Cu~SiO are the effective catalysts for DMC synthesizing from carbon dioxide and methanol. Under a mild reaction conditions of atmospheric pressure and 140~ the conversion rate of CH3OH is about 15% and the product selectivity of DMC is over 85%. The excellent reaction behaviors of the Ni-Cu/MoSiO and Ni-Cu/VSiO catalysts come from the proper surface constructions. The interaction of the metal Ni-Cu alloy cluster with the valence bonding type of MoSiO and VSiO complex oxides can not only bring about the three types of active sites, including metallic site M (Ni-Cu alloy), Lewis acid site M "+ (Mo 6r or V 5r) and Lewis base site M=O (Mo=O or V=O) on the surface of the catalysts but also change the d-electron density of Ni-Cu alloy and the Lewis acid or Lewis base intensity of the M "r or M=O to suit the activation of CO2 and CH3OH molecules. With the cooperation effects of these active sites, CO2 can be chemisorbed on the metallic site M and Lewis acid site M ar to form a horizontal adsorption state M--(CO)--O---~M nr (Mo 6+ or Vs§ and CH3OH can be chemisorbed on the Lewis acid site M ar and Lewis base site M=O to form a dissociative adsorption state CH30--M nr and HO--M. The M=O of crystal-frame oxygen takes part in the surface reaction and the horizontal adsorbing type of CO2 and dissociative adsorbing type of CH3OH play an important role in the course of DMC synthesis. REFERENCES
1. 2. 3.
J.N. Armor. Catalysis Today, 38 (1997) 163. F. Solymosi. J. Molecular Catalysis, 65(3) (1991) 337. D. Bianchi, T. Chafik, M. Khalfallah and S.J. Teichner. Appl. Catal. A: General, 112 (1994)219. 4. F.M. Hoffmann, M.D. Weisel and J. Paul. J. Surf. Sci., 316 (1994) 277. 5. J.W. Wang and S.H. Zhong. Chinese Chemistry Progress, 10(4) (1998) 374. 6. J. Haggin. C & EN. 70 (1992) 25. 7. Y. Shao and S.H. Zhong. Chinese J. Mol. Catal., 11(5) (1997) 343. 8. Y. Shao and S.H. Zhong. Chinese J. Fuel Chem., 26(1) (1998) 71. 9. S.H. Zhong. J. Catal., 100 (1986) 270. 10. S.H. Zhong and J. Wang. Chinese J. Catal., 16(4) (1995) 263.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Meio, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
P r o m o t i o n o f Support in S y n t h e s i s of Propylene Carbonate from CO 2 and Propylene Oxide Tiansheng Zhao a, Yizhuo Han b, and Yuhan Sun b aKey Laboratory of Enert~r Sources & Chemical Engineering, Mailbox 114, Ningxia University, China 750021 bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, China 030001 The catalytic activity of potassium iodide towards the cycloaddition of carbon dioxide with propylene oxide could be remarkably promoted by its dispersion on the surface of metal oxide, particularly using amphoteric ZnO as the support. IR and XPS-AES characterization indicated that the high activity of ZnO supported potassium iodide could be closely correlated to the acidbase character of ZnO. An intermediate with Lewis acid-base pair sites was proposed for the cycloadditions.
I. INTRODUCTION The cycloaddition between CO2 and epoxides is important for CO2 activation transformation 1-2,which could be catalyzed by binary systems such as organometallic compounds and organic bases a-s. But neither organometallic halide nor organic promoter individually showed good catalytic activity 4. These also gave rise to the difficulty in both preparation and separation of the catalysts. A novel catalyst, metal oxide supported potassium iodide, was therefore developed for the cycloaddition between propylene oxide and carbon dioxide at the present work, which could lead to a heterogeneous process, and showed its high catalytic activity towards the cycloaddition. The catalytic mechanism was then proposed based on the characterization.
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2. EXPERIMENTAL 2. I. Catalyst preparation and characterization
C a t a l y s t s were p r e p a r e d by i m p r e g n a t i n g zinc oxide with a q u e o u s KI s o l u t i o n a n d t h e n dried at 60~ for 10h. Infrared s p e c t r a were r e c o r d e d at r o o m t e m p e r a t u r e on D S - 4 0 2 G s p e c t r o m e t e r . The c a t a l y s t s a m p l e s were p r e s s e d into a s e l f - s u p p o r t e d wafer with a weight of ca. 2 0 m g for the m e a s u r e m e n t . X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) a n d Auger electron s p e c t r o s c o p y (AES) were d e t e c t e d on VG ESCA LAB5-electron s p e c t r o s c o p y i n s t r u m e n t with a b a s e p r e s s u r e of 10 .9 Torr. The c a t a l y s t s a m p l e s were g r o u n d a n d m o u n t e d on the s t a n d a r d h o l d e r by m e a n s of d o u b l e - s i d e d adhesive tape. A1 Ks 1, 2 (hv = 1486.6 eV) r a d i a t i o n (9 KV a n d 18.5 mA) w a s u s e d to record the spectra. The a n a l y z e r e n e r g y w a s 100eV a n d the step 0.1 eV. The c o n t a m i n a t i o n C w a s u s e d a s the i n t e r n a l reference a n d the b i n d i n g energy of C I s w a s 284.6eV.
2.2. Activity t e s t s Activity t e s t s were carried o u t in a s t a i n l e s s steel autoclave reactor. A typical p r o c e d u r e w a s as follows: 0.1 mol of propylene oxide a n d 0.5g of c a t a l y s t were p u t into the autoclave, a n d CO2 w a s i n t r o d u c e d into the a u t o c l a v e with the initial p r e s s u r e of 5.0MPa at r o o m t e m p e r a t u r e . The autoclave w a s h e a t e d u p to the p r e s c r i b e d t e m p e r a t u r e u n d e r m a g n e t i c a l l y stirring for 2 h o u r s reaction. The p r o d u c t s were d e t e c t e d by m e a n s of GC with t h e r m a l c o n d u c t i v i t y detector.
3. RESULTS 3.1. Catalytic performance The catalytic p e r f o r m a n c e w a s d e p e n d e n t on KI c o n t e n t as s h o w n in Table 1, s u g g e s t i n g t h a t KI w a s the m a i n active c o m p o n e n t . As a c o m p a r i s o n , n e i t h e r KI n o r ZnO w a s individually active t o w a r d s the cycloaddition. The c o n v e r s i o n of PO a n d the yield of PC were lower t h a n 3.50 a n d 2.63%, respectively, implying a synergistic effect b e t w e e n KI a n d ZnO t o w a r d s the reaction. W h e n KI c o n t e n t w a s above 1.4 m m o l / g , K I / Z n O s h o w e d a high a n d stable activity (see Fig. 1). B o t h c o n v e r s i o n a n d yield of PC i n c r e a s e d with a rise in the r e a c t i o n t e m p e r a t u r e . The o p t i m a l t e m p e r a t u r e for the cycloaddition w a s 150 ~ for K I / Z n O ( see Fig. 2 ). In addition, the reaction p r e s s u r e a n d t e m p e r a t u r e were o b s e r v e d as f u n c t i o n of reaction time (see Fig. 3). After a n i n d u c i n g period of c a . . 10 min, the reaction p r e s s u r e d r a m a t i c a l l y d e c r e a s e d from 9.2 to 7.2 MPa
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in 20 min. The reaction t e m p e r a t u r e s i m u l t a n e o u s l y rose p r o m p t l y to 178 ~ After 22 min, it w e n t down to the original 150 ~ This d e m o n s t r a t e d t h a t a large a m o u n t of reaction h e a t released a n d the cycloaddtion of PO with CO2 took place very fast in the p r e s e n c e of KI/ZnO. Table
1
Catalytic p e r f o r m a n c e of ZnO s u p p o r t e d KI for cycloaddition a KI c o n t e n t 0.06 m m o l / g KI c o n t e n t 3 m m o l / g Catalysts PO conv.(%) PC yield (%) PO conv.(%) PC yield (%) KI/ZnO 47.3 41.4 95.3 94.3 ZnO b 3.50 2.63 KI c 0.70 0.53 a Reaction conditions" 150~ b No KI. c U n s u p p o r t e d , 0.166g.
1 O0
~90
. _ _ _ ~ _ _ _"---------* _ _ _ _ ~ f 100
F90 .v
o
'm 70 [-70 60 ~60-C~ o~ 50 t 50~ 9 [-40 a., 40 . . . . . . . . . . . . . . . . . . 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.~ 4.0 4.5 KI content/mmol g Fig. 1. Influence of KI c o n t e n t on the activity of KI / ZnO.
r~
.
~
7.5 t
e~.
6.5
.
.
.._..__.~"-----~,
-60"~" ~40~ -20
.
160
180
175~ 170~
--n--p / MPa 150
145 40 60 80 100 120 Time / min. Fig.3. Reaction p r e s s u r e a n d t e m p e r a t u r as function of reaction time with K I / Z n O (3 m m o l / g ) catalyst. 0
20
1 O0
180
Fig.2. Influence of reaction t e m p e r a t u r e on the activity of KI/ZnO (3 m m o l / g ) .
9
9.5 ~9.0 :~8.5
6.0
80 ~9 60 A ~ ~ 40 o o 20 O 0 80 100 120 140 Temperature/~
"-o~
10.0
#
100
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3.2 Surface structure o f the c a t a l y s t The IR s p e c t r u m of ZnO showed a b s o r b a n c e s at 530, 490 a n d 445 cm -1, respectively, which were attributed to Zn-O stretching as s h o w n in Fig.4. C o m p a r e d with t h a t of ZnO, a new adsorption peak appeared at 415 cm -~ for KI/ZnO, a n d the b a n d s at 530, 490 a n d 445 cm 1 c h a n g e d slightly. Considering t h a t KI hardly showed the IR adsorption in the 7 0 0 - 2 0 0 c m - 1 region, the new peak should be associated with the effect of KI on the Zn-O bond, a n d s u c h interaction between KI and ZnO was w e a k at room temperature.
II
o m m =N
J
700 660 560 460 3C)0' '24.0 '200 Wavenumbers/cm-1
Fig. 4. IR spectra of (a) ZnO and (b) KI/ZnO (3 m m o l / g ) at room t e m p e r a t u r e . XPS spectra and AES spectra for both KI a n d KI/ZnO are shown in Fig. 5. The strongest XP binding energy Eb(xP) of I(3ds/2) for KI and KI/ZnO was 628.5 and 633.2eV(see Fig.5a and 5b), respectively. Considering the effect of the ~9o5 9-Io 9-15 9~o 985 g;o 9;5 1o'oo'Idol charging potential on the m e a s u r e m e n t of binding energy, a modified Auger p a r a m e t e r was u s e d to characterize the chemical state of I [6]. S u c h p a r a m e t e r was defined as 0~ = Ek(AE ) + Eb(xp)=1486.6- Eb(nE) + Eb(xP). The strongest AE binding o20 o25 o30 o~5 o~.o o~,5 650 ' B i nding e n e r g y / e V energy Eb(nE)Of I (corresponding to atomic orbital layer M4NsNs, 3d3/24ds/24ds/2) for KI and KI/ZnO Fig. 5. X-ray photoelectron spectra of (a) KI and (b) KI/ZnO (3 mmol/g) and Auger electron was 989.2 a n d 991.5eV(see Fig.5c spectra for (c) KI and (d) KI/ZnO (3 mmol/g). and 5d), respectively. But the modified Auger p a r a m e t e r of I.for KI o_ c-
I.-
i
I
,
I
|
,
I
|
,
'
C
'
I
=
I
'
'
'
'
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(al) a n d for KI/ZnO (a2) was 1125.9 a n d 1128.3eV, respectively, leading to a shift of 2.4 eV. As the Auger p a r a m e t e r was very sensitive to the chemical state, this shift indicated a change of the chemical state of I, which should result from the interaction between KI and ZnO. 4. D I S C U S S I O N
Supported on the surfaces of ZnO, KI was highly active towards the cycloaddition compared with individual KI or ZnO as the catalyst (see Table 1). The comparative study (using 7-A1203, ZrO2, SiO2 and NaX as supports) showed t h a t no direct correlation could be observed between the activity and BET surface a r e a s of the supports. Thus, the e n h a n c e d activity should be related to the surface properties of the supports. It should be noted t h a t Z n O 7-9 was considered to show amphoteric character and then have the acid-base sites on the surfaces. The behavior of KI changed when supported. A weak interaction was observed for KI on the support with amphoteric character as IR a n d XPS-AES characterization illustrated. In addition, TGA of the catalysts also indicated that the interaction became stronger as the t e m p e r a t u r e increased. The dependence of the activity of KI/ZnO on the reaction t e m p e r a t u r e (see Fig. 2) revealed t h a t the importance of the interaction between KI a n d ZnO. Generally, the nucleophilicity of the anion in metal salts can be reduced because of the interaction with the cation. Under the reaction t e m p e r a t u r e the Lewis acid-base pairs of Ia-Zna+and Ka+O~- might be formed when KI was supported on ZnO. This obviously reduced the interaction between K§ a n d Iand then favored their activity towards epoxides. As a result, the interaction between KI a n d ZnO might promote the reactivity of KI. On the other h a n d , the combination of s u c h acid-base pairs would generate the effective bifunctional active sites for the cycloaddition and simultaneously led to the activation of CO9 a n d PO. Thus, the m e c h a n i s m was proposed as that in Scheme 1. ~Zn~O~Zn~
'-I
l-Zn +
"L
Q
K +0-
co
......
I -Zn +
i
K +0 -
i
"
%7
escape su a e
1
2
3
CH3
Scheme. 1. Proposed diagram of reaction mechanism.
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5. CONCLUSION
A supported KI catalyst has been successfully developed for the cycloaddition of CO2 and PO. The cycloaddition smoothly proceeded in the presence of supported KI within a short reaction time. The activity of KI could be strongly enhanced by using ZnO as both support and promoter. The suitable loading a m o u n t of KI on ZnO and the reaction t e m p e r a t u r e were needed for high catalytic activity. The amphoteric character of the supports played an important role in the reaction. The synergetic bifunctional Lewis acid-base pairs of IS-Zn~+and KS§ s- might be the active sites for activation of CO2 and PO. The a u t h o r s t h a n k the financial supports from the NSF of China and the Natural Science foundation of Shanxi province. REFERENCES
1 B. Denise, R. P. A. Sneeden, CHEMTECH, (1982) 108. 2 D. J. Darensbourg, M. W. oltcamp, Coord.Chem.Rev., 153 (1996)155. 3 R. Nomura, M. Kimura, S. Twshima, A. Ninagaa, H. Matsuda, Bull. Chem. Soc. Jpn., 55 (1982)3200. 4 A. Baba, T. Nozaki, H. Matsuda, Bull.Chem.Soc.Jpn., 60(1987)1552. 6 V. I. Bukhtiyarov, I. P. Prosvirin, R. I. Kvon, S. N. Goncharova and B. S. Bal' zhinimaev, J.Chem. Soc.,Faraday Trans., 93 (1997)2323. 7 R. P. Eischens, W. A. Pliskin, M. J. D. Low, J. Catal., 1 (1962)180. 8 A.L. Dent and R. J. Kokes, J. Phys, Chem, 74 (1970) 3653. 9 H. Nakabayashi, Chem.Lett., (1996) 945.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
A novel process for the removal of nitrates from drinking water Albin Pintar', Jurka Batista and Gorazd Ber6i6 Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, P.O. Box 3430, SI-1001 Ljubljana, Slovenia A nitrate removal process that drastically reduces salt consumption and waste discharge has been developed on a bench scale. Nitrate is removed by chloride ion exchange, and the strongbase anion resin is completely regenerated at mild reaction conditions (i.e., ambient temperature, atmospheric pressure) in a closed circuit containing a single-flow fixed-bed reactor packed with a Pd-Cu/7-A1203 catalyst. The combined treatment system avoids direct contact between the denitrification reactor and the water to be treated, and minimizes operational problems associated with each separate technique. 1. INTRODUCTION Nitrate concentrations in surface water and especially in groundwater have increased in many locations in the world. As a result, there is renewed interest in the removal of nitrates from raw water. State-of-the-art of treatment methods for the removal of excessive quantities of nitrates from drinking water is discussed in [1 ], and various treatment options are compared in terms of their effectiveness, ease of operation, and cost. Physicochemical methods allow effective removal of nitrate ions from contaminated groundwater, however, by concentrating them in a secondary waste stream. Among them, the capital and operating costs are the lowest for the ion exchange process; nevertheless, it is very difficult and costly to dispose of large quantities of spent regenerant brine in noncoastal locations where natural evaporation is impossible. The most promising techniques for nitrate removal, without any occurrence of wastewater, are biological digestion and catalytic denitrification by using noble metal catalysts [2, 3]. The main reasons for the slow transfer of biological denitrification to drinking water purification are concerns over possible bacterial contamination of treated water, the presence of residual organics in treated water, and the possible increase in chlorine demand of purified water. The reduction of aqueous nitrate solutions by using hydrogen over a solid Pd-Cu bimetallic catalyst offers an alternative process to biological treatment as a means of purifying drinking water streams. The reaction is carried out in a two- or three-phase reactor operating under mild reaction conditions (e.g., T = 278-298 K, p(H2) up to 7 bar), and obeys a consecutive reaction scheme in which nitrite appears as an intermediate, while nitrogen and ammonia are the final products. To maintain electroneutrality of the aqueous phase, consumed nitrates are replaced by hydroxide ions. Supported Pd-Cu and Pd-Sn bimetallic catalysts exhibit the highest activity for nitrate reduction and chemical resistance, but still inadequate selectivity *Corresponding author. Phone: (+386-61) 17 60 282; Fax: (+386-61) 12 59 244; E-mail:
[email protected].
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TREATED WATER
~, "
/X
7
RAW WATER
PUMP
PUMP
Fig. 1. Schematic layout of the combined ion exchange/catalytic denitrification process for nitrate removal. 1, column with the packed bed of ion exchange resin; 2, mixing point of the treated and untreated water; 3, saturator; 3a, mixing nozzle; 4, 5, pump; 6, separator. towards nitrogen production. The nitrate disappearance rate as well as nitrogen production yield decrease appreciably in the presence of hydrogencarbonates in tap water [4]. Recent results of nitrate reduction carried out in continuous-flow reactors (i.e., bubble-column fixedbed, trickle-bed) allude that by using up-to-date Pd-Cu bimetallic catalysts direct treatment of drinking water with higher amounts of hydrogencarbonates seems to be unfeasible [5]. To overcome operational problems associated with the regeneration of exhausted ion exchange resins and direct purification of drinking water by means of catalytic hydrogenation, an integrated process was invented, which efficiently combines a conventional single-bed ion exchange unit with a catalytic denitrification reactor in such a way that drawbacks of each separate technique are effectively eliminated [6]. A schematic layout of the combined process is illustrated in Figure 1. The nitrate-free effluent can be blended with a predetermined fraction of bypass raw water to produce a water stream of acceptable nitrate concentration. 2. EXPERIMENTAL The ion exchange step made use of a packed-bed of anion resin in the chloride form. In this work, a macroporous strongly basic anion exchange resin IMAC HP-555 (Rohm and Haas Co.), which is nitrate selective, was used to remove nitrate ions from groundwater. Total exchange capacity of the employed resin determined by means of both breakthrough curves (not shown here) and potentiometric titration with a A g N O 3 solution was found to be equal to 0.00286 mol/g (i.e., 0.92 eq/L). In a typical run, removal of nitrates by ion exchange was conducted in the upflow mode at T=298 K, Ptot.=l bar and OvoI.,L'-8.0 mL/min. When the resin was saturated with nitrate ions, column exhaustion was terminated. Meanwhile, H2 was introduced into the saturator (3), which operated at atmospheric pressure and was filled with 700 mL of aqueous solution of NaC1 with the initial concentration of 5.0 g/L. A centrifugal
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pump (4) recirculated the solution of chloride ions through the saturator and upflow catalytic reactor at a flowrate of 5.3 L/min. The saturator was equipped also with a heating-cooling coil (T=298 K) and mixing nozzle (3a), which provided good contacting between the gas and liquid phase. When the latter was saturated with H2, a peristaltic pump (5) started to recirculate the regenerant through the bed of ion exchange resin and saturator at a flowrate of 10.0 mL/min. The chloride ions from the regenerant substituted the nitrate ions on the ion exchange resin. Due to the continuous recirculation of aqueous solution of chloride ions through the two-phase catalytic reactor, packed with an egg-shell type Pd(1.0 wt. %)-Cu(0.3 wt. %) bimetallic catalyst, nitrate ions from the regenerant solution reacted with H2 chemisorbed on the catalyst surface and converted into nitrogen (and ammonia). The neutralization agent, i.e., the aqueous solution of HC1 (0.25 M), was applied by means of an autotitrator (Metrohm, model 751 GPD Titrino) in order to keep the pH value of regenerant constant at 5.5. The use of HCI had the advantage that no further makeup of the regenerant with NaC1 was required, since the stoichiometric required amount of chloride ions for a subsequent regeneration cycle was simultaneously introduced to the process via the neutralization of hydroxide ions produced during the liquid-phase nitrate reduction. The regenerant continued to be recirculated through the ion exchange column and the denitrification reactor, until both nitrate and nitrite ions were completely consumed. The regenerant free of these species was used in subsequent regeneration cycles. In the catalytic reactor, the catalyst layer was formed from 7-A1203 spherical particles (do=l.7 mm) on which metallic Pd and Cu phases were deposited in such a sequence that the catalyst surface was enriched with Pd clusters. Detailed preparation of the catalyst used in this process is described elsewhere [7].
3. RESULTS AND DISCUSSION It was found out in this study that regeneration of nitrate loaded IMAC HP-555 resin is possible with a diluted solution containing 2.5-10.0 g/L NaC1. Compared to the conventional regeneration procedure with 50-100 g/L NaC1, a flowrate of 2-4 BV/h and a period of approximately 30-50 min, more time and a higher flowrate are needed. However, with 5 g/L NaC1 and a flowrate of 3.0 mL/min (i.e., 25 BV/h) complete regeneration of the resin is possible in 240 min; one should note that prolonged time of regeneration does not represent any drawback for an integrated process. Although small amounts of NaC1 were consumed in performed regeneration runs, the regeneration efficiency, defined as the ratio of equivalents of nitrate removed from the resin during regeneration to the equivalents of regenerant used, is rather low and found in the range of 0.07-0.15 eq nitrate/eq chloride. Low regeneration efficiency is not a serious problem in a closed regeneration system, because the excess of NaC1 will stay in the system and is not lost in the disposed brine. In other words, the value of regeneration efficiency can be easily increased by reusing the regeneration solution. This is particularly true when it has been made free of nitrates, which can be achieved, e.g., by means of the combined process shown in Fig. 1. Of course, all advantages of the process described above come at the price of increased capital cost and process complexity. The breakthrough curves (or effluent histories) for nitrate from the IMAC HP-555 resin bed with regenerant reuse are shown in Figure 2. The breakthroughs are nearly identical and do not suggest any trend to shorter or longer nitrate removal runs. It can be thus concluded that reusing the denitrified regenerant containing 5.0 g/L NaC1 did not negatively influence the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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column performance. A major concem when a spent regenerant is reused is the accumulation of ions stripped from the resin during regeneration. These include nitrate, sulfate and hydrogencarbonate. Nitrate (and nitrite) should be eliminated by denitrification, hydrogencarbonate by neutralization with HC1 in the saturator, but sulfate can accumulate during reuse when there is a net removal of these ions during exhaustion. It was found in this study that only small amount of sulfate ions is accumulated in the regenerant solution (about 8 mg/L sulfate per regeneration cycle), which suggests that more than one-half of sulfate has been dumped from the resin during exhaustion. The results depicted in Fig. 2 demonstrate that in each run the nitrate capacity in the exhaustion mode was found to be unaffected by the amount of sulfate in the regenerant 100 ~0~ ~ 1 7 6 beforehand. After the fourth Exhaustion run #. ..I exhaustion-regeneration cycle of g resin with regenerant recycling, it "~ 80 ~_ o 1 (fresh) 9 2 was found out by means of ,a 3 ~] argentometric titration that the 9 4 IMAC HP-555 resin maintains 99 % T: 298K of its initial capacity for nitrate Ptot: 1.0 bar loading. mresin: 0.75 g IMAC HP-555 13 Experimental results of regen_ [] c(NC~)feed: 100.0 mg/L eration of ion exchange resin 1"1 (t)va.,L: 8.0 mL/min rl n saturated with nitrate and the simultaneous removal of the nitrate Z O~ ion from the regeneration solution lime,rain
J
Fig. 2. Nitrate breakthrough curves for different using catalytic hydrogenation in a two-phase fixed-bed reactor are exhaustion runs during regenerant reuse, presented in Figure 3. Temporal
course of the destructive nitrate reduction obtained in the presence of Pd(1.0 wt. %)-Cu(0.3 wt. %)h{-A1203 was followed by measuring the instantaneous concentrations of nitrate, nitrite and ammonium ions in the regenerant solution by means of flow-injection analysis (Perkin-Elmer). At the given reaction conditions, contacting time of the liquid-phase in the single-flow reactor was equal to 30 msec, and the latter operated in the kinetic regime. Although the volumetric flowrate of regenerant solution through the ion exchange column was rather low (10.0 mL/min), nitrate disappearance rate was not affected by the rate of nitrate desorption from the ion exchange resin. It is evident from Fig. 3a that by employing this process total removal of stripped nitrate ions from the regenerant is attained, which means that the latter can be used in subsequent regeneration cycles. Furthermore, one can see that the activity of the catalyst for nitrate removal decreases with a number of exhaustion-regeneration cycles. This cannot be attributed to the dissolution of Pd and Cu metallic phases, since no metal ions were detected in the regenerant solution by means of ICP-AES examination. The observed decline of the catalyst activity is due to the following reasons: (i) concentration of chloride ions in the regenerant solution increases with a number of regeneration cycles (see below); (ii) isoelectric point of the catalyst decreases during the liquid-phase nitrate reduction, which is ascribed to the consumption of weakly bound protons on the catalyst surface with hydroxide ions formed [8].
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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nitrite ions were accumulated in aqueous phase during the regeneration (Fig. 3b). This is ascribed to the low contacting time of liquid-phase in the reactor and much higher affinity of nitrate ion towards the Pd-Cu active sites on the catalyst surface in comparison to nitrite. The concentration of nitrite ion in the regenerant, after the regeneration process of saturated ion exchange resin and simultaneous destructive hydrogenation had been finished, was lower than that prescribed by the regulation of the law (i.e., 0.02 mg/L). However, it is obvious from Fig. 3b that longer reaction time is needed to achieve this goal in subsequent regeneration cycles. This is again attributed to the fact that in each reduction run the catalyst surface becomes more negatively charged, which consequently increases the repulsion between intermediate nitrites and active sites on the catalyst surface. A detailed mechanism of catalyst deactivation observed in the process of liquid-phase nitrite reduction is described in detail by Pintar et aL [8]. 101)
la
T: 298 K ..i 80 o ~ P(H2 ): 1.0 bar; F~ot.: 1.0 bar .:-. mcat : 3.0g; dp" 1.7 mm "~ o.-~T, mresin : 0.75 g IMAC HP-555 -.!. . Vsat : 700 mL; c NaCI = 5.0 g/L 60 o!.- . . -. . pH: 5.5 (const.) 0 -.~.
40 .~ Z
T.
.,,. " . o.-. '.m. o.
20
o
..
. v A. ' m ..~
A
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0
, 30(I
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'm..
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9.
9
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|
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z
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9
=
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0:
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9 .
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.
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~.
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9 O
o "'"
-
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Ir
o.. (5'"'.o. ~'" IX...V A"V' 61)0
'
-~.:o.
Regeneration step #:. .
o. ram " "A 0
.
i
Regeneration step #:. o 1
zx .. -.=..~ "*. _:. .= ~
" ".
o.
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.. l.
..
9
".
A
o9 . ,-, "III 99 " ' " .AA 60(I
Time, rain
In Fig. 3c the concentration of ammonium ions produced is shown as a function of time for various regeneration cycles. Based on these data, about 70 mol % of nitrate stripped from the ion exchange resin is transformed to ammonia. However, this value can be easily miniI ~~ mized to, e.g., 15 mol % by carrying out the 1:: 60 mmmmmm~:x .=l / 9 Regeneration step #: catalytic nitrate reduction at lower hydrogen partial pressures. As the denitrification process i m o9 2 does not take place in direct contact with the ~t oJ A 3 groundwater, there is no risk that ammonia production will affect the water quality. Data Ode , ' 0 1000 2000 300 shown in Fig. 3c confirm that no ammonia was Time, rain transferred into the gas-phase at the given Fig. 3. Temporal course of nitrate (a), nitrite reaction conditions. (b) and ammonium (c) ions for consecutive Figure 4 shows the concentration-time proregenerations of IMAC HP-555 resin by file of chloride ions in the regenerant for differmeans of catalytic denitrification. ent regeneration cycles. It is seen that the concentration of chlorides measured in the saturator increases with time, which arises from the: (i) .J / "~ 120 I. | / 90 I-
P(H2): 1.0 bar; Ptot. : 1.0 bar meat : 3.0 g; dp: 1.7 mm **" mresin: 0.75 g IMAC HP-555 TT Vsat :700mL; CNaCi =5.0g/LT* pH: 5.5 (const.) zxzxa~
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1582
favorized formation of ammonium ions, which accordingly to the reaction stoichiometry increases the consumption of HCI by a factor of two; (ii) neutralization of stripped hydrogencarbonate ions from the ion exchange resin by means of HC1; (iii) reference electrolyte (3 M KC1) I mresin:0.75g IIW~!-IP-555 I Vs~: 700 rrL outflow from the pH electrode. Additional experiments of liquid4.50[ 1(~) pi--t5.5(,c~.) 2000 300 phase nitrate reduction carried out in Tirn~ rnin a batch-recycle reactor in the Fig.4. Concentration of chloride ions (expressed presence of the same catalyst as used as NaCl) as a function of time in the reused regenein this study, show that the nitrate rant solution. (and nitrite) disappearance rate decreases by increasing the concentration of sodium chloride in the aqueous solution. Also, the inhibitive impact of produced hydroxide ions on the catalyst activity is more pronounced in the presence of chlorides. Due to the synergistic effect of these species on the reaction behavior, no kinetic analysis of data shown in Fig. 3a is possible. However, the results depicted in Fig. 4 demonstrate that no make-up of the regenerant solution is required between exhaustionregeneration cycles. In this work, no organic fouling of both the ion exchange resin and catalyst, which can be caused by humic and fulvic acids accumulated in a closed regeneration circuit, was observed. This implies that after the exhaustion-regeneration cycle has been completed, no disinfection of the unit with a solution of, e.g., peracetic acid is needed. In this respect, the denitrification process shown in Fig. 1 is advantageous over the combined ion exchange/biological denitrification techniques [9]. III
9
IV
o
$
4. CONCLUSIONS The described catalytic/physical chemical process is a very attractive technique for nitrate removal from groundwater. Compared to ion exchange brine production is very low and regeneration salt requirement is minimal. As the denitrification process does not take place in direct contact with the groundwater, there is no risk that nitrite and ammonia production will affect the water quality. Also groundwater with a high sulfate content can be treated with this technique, when a nitrate selective resin, for example IMAC HP-555, is used. REFERENCES 1. 2. 3. 4. 5. 6. 7.
A. Kapoor and T. Viraraghavan, J. Environ. Eng., 123 (1997) 371. K.D. Vorlop, T. Tacke, M. Sell and G. Strauss, German Patent No. 3830850 A1 (1988). U. Prttsse, S. Hrrold and K.D. Vorlop, Chem. Ing. Tech., 69 (1997) 93. A. Pintar, M. Setinc and J. Levec, J. Catal., 174 (1998) 72. A. Pintar and J. Batista, Catal. Today, 53 (1999) 35. A. Pintar, J. Batista, G. Berri~ and J. Levec, PCT Application No. SI99/00018 (1999). A. Pintar and J. Levec, SI Patent No. 9500357 (1998). 9
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
化
1583
Greenhouse gases and emissions control by new catalysts free of precious metals V. Gryaznov and Yu. Serov Russian University of Peoples' Friendship, 6 Miklukho-Maklay St., 117198 Moscow, Russia There were fbund the effective catalysts without precious metals for dry reforming of methane, carbon dioxide hydrogenation into ethylene and propylene and detoxication of the gas emissions of cars and industrial enterprises.
Carbon dioxide usage as chemical raw material is very important for the elimination of technogenical damages of the atmosphere. The transformation of two main greenhouse gases - CO2 and CH4 - by dry retbrming CH4 + CO2 = 2CO + 2H 2
(1)
has a limited application because of swift cooking of many proposed catalysts. The systematic study of catalytic properties of ultradispersed powders (UDP) of iron group metals performed at Russian University of Peoples' Friendship (RUPF) reveal its activity in dry reforming of methane and in carbon oxides hydrogenation with olefins C2-C 3 formation. The UDP were prepared in collaboration with Moscow Institute of Steel and Alloys (MISA) and industrial enterprise Red Star (RS) by decomposition of metal carbonyls in hydrogen plasma, electric explosion of wires, evaporation-condensation and criochemically by condensation of metal vapour in hydrocarbon matrix cooled by liquid nitrogen. The preparation method and the size distribution of UDP influence on the catalytic activity and selectivity as well as the nature of refractory oxides powders which were used for making of matrixes with metal UDP's. For example the proper choice of matrix material permits minimization of cook fbrlnation during dry reforming of methane. The most active and stable in reaction (1) without diluents at atmospheric pressure, temperature of 1013 K and space velocity 4500 h z is a system of 10 wt. per cent UDP of Fe in the refractory oxide powder. The conversion of initial substances is 94 per cent. The colour of catalyst did not change during 70 h of operation. It does mean that the cooking of catalyst is very low. Hydrogenation of CO 2 by the known catalysts gives paraffins only and the trace amount of olefins. The other result was achieved by usage of mixture of electrolitically produced the thread-shaped crystals of cobalt and iron l-10 micron length and 20-30 nm in diameter with a powder of very pure quartz. This catalyst was tested in flow reactor at atmospheric pressure and CO2/H2ratios fiom 2"1 to 1:4. The content of ethylene and propylene in products was 10
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1584
per cent at temperature of 573 K and CO2/H2 ratio 1:4 on the catalyst with 2.5 per cent Co and 7.5 per cent Fe. The next preparation of this catalyst showed that at CO2/H2 equals 2 and 613 K the content of olefins C2-C3 increased to 14 per cent. Similar data were not found in the literature. The catalytic activity of mixtures of UDP of iron group metals in the oxidation of carbon monoxide, light olefins as well as in the reduction of nitrogen oxides was determined at 373-1023 K in a flow reactor at atmospheric pressure, with space velocities from 3000 to 30000 h -~. The catalyst sample was placed into a quartz reactor equipped with a soldered quartz filter to prevent catalysts loss by being carried away. The effluent reaction products were analyzed by a gas chromatograph equipped with both thermal conductivity and flameionization detectors: Riken Keiki analyzers of CO, CH, NO,, with sensitivities of 50 (CO), 15 (CH), and 1 ppm (NO,) were also ernployed. One series of the catalysts was prepared by mixing UDP of iron, cobalt, nickel, and manganese oxides with AI_~O3and SiO2 powders. According to X-ray diffraction analysis and transmission electron microscopy, the average size of the particles is 10-12 nm. The specific surface area, as measured by the BET method is equal to 40-75 m2/g for the metallic samples under investigation and 44 m-'/g for manganese oxide. The catalysts of the second type were obtained by impregnation of A1203 supports with aqueous solutions of cobalt and manganese nitrates followed by exposure of the samples to an argon-oxygen flow at 623 K: ultrafine iron powders were produced in these systems by hydrogen-plasma treatment of the iron pentacarbonyl. The BET specific surface area of these samples ranges from 17 to 51 m-~/g. The maximum content of the metallic phase in the catalysts studied did not exceed 10 wt.%. Table 1 characterizes some of the employed catalysts. x, % CO
x, % NO~
1 O0
ioo
_
I
A 2 3
50
5O I f
/~
~/
o.I Ax III
I
473
573
673
,,
I
"i'~K 773
Fig. 1. The degree of CO oxidation on catalysts Nos. 1-3 (curves 1-3, respectively), at the space velocities in h-~: (I) 4.4x103, (II) 8x 103, (III) 14.4x 10 ~. (IV) 24x 103.
I
673
,
1
773
T,K
Fig. 2. Reduction of nitrogen oxides by carbon monoxide on catalysts Nos. 1-3 (curves 1-3, respectively).
Oxidation of CO by air on iron UDP becomes noticeable at 480 K (Fig. 1, curve 1), and conversion rises to 90% at 690 K. Nickel powder (Fig. 1, curve 2) converts CO to 96% at
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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720 K. The oxidation 99.9% of CO over the cobalt UDP occurs at 503 K (curve 3 in Fig. 1). Variation in the velocity of the initial mixture supplied to the reactor from 4.4x103 to 24x103h ~ only slightly influences both CO conversion and the temperature of complete oxidation. Fig. 2 depicts that the sequence of Fe, Ni and Co activities in reduction of nitrogen oxides by carbon monoxide was reverse. Table 1 Characterization of the catalysts used (s sp is the specific surface area; d av is the average diameter of the particles) No. 1 2 3 4 5 6 7 8
Catalyst First series 3% Fe / AI203 3%Ni / AI203 3%Co / AI203 3 % F e - 3%Co / AI_~O~ 3 % F e - 3 % C o - 3%MNO2 / AI203 3 % F e - 3%Co / SiO, 3%Fe - 3%Co - 3%MnO, / SiO, 3 % F e - 3 % C o - 3%Ni / SiO~ Second series 2 . 3 % F e - 4.5%COO / A1203 5 % F e - 5%COO / AI20~ 7.6%Fe - 5%MNO2/AI20 ~ _
_
_
s sp, m2/g
d
av,
nm
66 33 67 68 44 18 25 18
10-12 20 10-12 10-12 10-12 10-12 10-12 10-20
26 17 51
10-12 10-12 10-12
.
9 10 11
The high catalytic activity of the polymetallic systems containing manganese oxide in nitrogen oxides reduction can be explained by the fact that N20 is formed more intensely from NO in the presence of manganese oxide than in its absence (Fig. 3, curves 1 and 3), the mechanism as follows" 2NO + CO = N20 + CO2
(2)
N20 + CO = N 2 + CO 2
(3)
Over the catalysts containing no manganese oxide, the predominant reaction path leads to the formation of molecular nitrogen: 2NO + 2CO = N 2 + 2CO 2
(4)
It is well known [3-5] that at low temperatures the occurrence of this process via the stage of N20 formation is thermodynamically more favorable than direct reduction of NOx; hence, the rate for reaction (3) is considerably higher than that of reaction (4). Thus, iron-manganese catalyst No. 11 provides a higher conversion of NOx in the temperature range 550-620 K than do iron-cobalt catalyst No. 9. As for catalysts Nos. 9 and 1 l, which are the most active in
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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reduction of nitrogen oxides with carbon monoxide, it is worth noting that they performed reduction by propane at considerably higher temperature 973 K. x,% 100 -
50-
ooi AAH
J-
,
~
573 Fig. 3. Temperature dependence of the and No. 11 (II).
L
673
N20 (1,3)
T,K
773
and N: (2,4) yields over catalysts No. 9 (I)
The experiments for destroying the toxic components of engine emissions containing oxygen and water vapors were carried out with the catalysts Nos. 9 and 11 (see Table 2). Table 2 Conversion degree (z in the reactions of CO oxidation and NO, reduction Reaction
Catalyst
2CO + O~ = 2CO~ _
2NO + 2 C O - N~ + 2CO~ _
10NO+C3Hs=5N2+3CO2+4H20
9 11 9 11 9 11
(z=25% 473 463 573 483 858 793
Temperature, K cz=50% cx=75% 490 503 483 500 603 623 523 573 893 913 858 893
c~=99% 563 553 673 623 963 973
A 95% reduction of NO with CO at 623 K is obtained (Fig. 4, curve 1) in a mixture containing 0.1% NO~ and 0.6% CO. After the addition of 2.5% O~ this result still holds (curve 2), however, 16% water vapour addition decreased the extent of NOx reduction (curve 3). The comparison of curves 1-4 of Fig. 5 showes that oxygen presence in the gaseous phase increases the temperature of complete reduction of nitrogen oxides by propane approximately on 100 K. This can be attributed to blocking of the active sites of the catalyst with water formed 99% conversion of NO, is achieved over an iron-cobalt catalyst at 973 K (Fig. 6, curve 1) in a mixture containing no water vapour; this temperature increases to 1023 K _
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1587
(curve 2) in the presence of water. At the same time, 99% conversion of NOx over an ironmanganese catalyst is observed in the presence of water vapors only above 1083 K (Fig. 6, curve 4). x %NO x 100 -
..~.
A
A~2
\. 50
1
I
573
673 T,K
Fig. 4. Reduction of nitrogen oxides by carbon monoxide on an iron-manganese catalyst: in the absence of oxygen ( 1), with 2.5% 02 added (2), with 16% water vapour added (3). A 50% conversion of NO, was found on the iron-manganese catalyst even at 573 K and reached 70% at 593-623 K (Fig. 7, curve 1). The extent of propane oxidation (curve 3) rises gradually with increasing temperature and reaches 99% at 973 K. The conversion of carbon monoxide approaches 99% at 593 K and remains unchanged at higher temperatures (curve 2). As carbon monoxide is consumed, the conversion of nitrogen oxides reduces to zero at 723 K, while the interaction of NOx with propane commences above 823 K (Fig. 7, curve 1). The addition of water vapor to this reaction mixture does not affect the oxidation of either CO or propane but decreases the extent of reduction of nitrogen oxides with carbon monoxide and propane (curve 4). x % NOx 100 -
o--~x~A
.,
/A" J t~
773
A/
o ~
A7
07 1
873
/.i~ i
I"
I
973
o/
i
1
a/
9
/era /
A~.5--" ..~A-i O ~
o/
/e ~ -
,/I" ~,"
o
9
"-%~x d yo A
,o
7 y,/+ A' o i
x, % NOx 100 -
773
I1'"-
873
1
973
I
1073,rv
1073T,K
Fig. 5. Reduction of NOx with propane on iron-cobalt (1,2) and iron-manganese (3,4). Catalysts in mixtures containing no oxygen (1,3) and in the presence of 0.6% 02 (2,4).
Fig. 6. Reduction of NOx with propane on iron-cobalt (1,2) and iron-manganese (3,4) catalysts with no water (1,3) and 16% water vapour (2,4).
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1588
Since water vapour also hampers neutralization reactions of toxic components over the catalysts prepared with noble metals [6,7], the findings of this work suggest that more detailed tests of iron-manganese catalysts for purification of automobile exhausts and industrial emissions is desirable. x% 100 I 0 ~
2
O-- @-- 0------0---- @ - -
O--
o
@------ 0 A n DA
-7
50
673
773
873
973 T,K
Fig. 7. Neutralization of a mixture containing 0.13% NO, (1), 0.15% CO (2), or 0.18% C3H s (3) and 0.6% 02 on an iron-manganese catalyst. Curve 4 refers to the mixture of 0.13% NO,, 0.15% CO, 0.18% C3Hs, 0.6% 02, and 10% water vapour.
REFERENCES 1. I. Hoshiyama, J.M. Song and C.S. Kim, Trans. Jpn. Mech. Eng., 1983, v. 49B, No. 6, p. 2465. 2. I. Hoshiyama and H. Yonejima, J. Chem. Soc. Jpn., Chem. Ind. Chem., 1984, No. 6, p. 1035. 3. W.C. Hecker and A.T. Bell, J. Catal., 1984, v. 84, No. 1, p. 200. 4. M.A. Ismailov, R.B. Akhverdiev, V.S. Gadzi-Kasumov, R.G. Sarmurzina, V.I. Parchishnyi and V.A. Matyshak, Kinet. Katal., 1993, v. 34, No. 1, p. 117 [Kinet. Catal. (Engl.Transl.)]. 5. N. Mizuno, M. Tanaka and M. Misono, J. Chem. Soc., Faraday Trans., 1992, v. 88, No. 1, p. 91. 6. A. Kudo, M. Steinberg, A.J. Bard, A. Campion, M.A. Fox, T.E. Mallouk, S.E. Webber and J.M. White, J. Catal., 1990, v. 125, No. 2, p. 565. 7. R.A. Gazarov and V.P. Moiseev, Kinet. Katal., 1980, v. 7, No. 1, p. 113.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
1589
化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Study on the initial steps of the polyethylene cracking over different acid catalysts D.P. Serrano*; R. Van Grieken; J. Aguado; R.A. Garcia; C. Rojoa; F. Temprano a Department of Chemical Engineering. Faculty of Chemistry, Complutense University, 28040 Madrid, Spain. aRepsol S.A., Technology Center, C/Embajadores 183, 28045 Madrid, Spain The initial steps of the thermal and catalytic cracking of low-density polyethylene (LDPE) at different temperatures and reaction times have been studied in batch operation. The catalysts employed were silica-alumina, USY zeolite and MCM-41. The thermal degradation follows a random scission mechanism with little contribution of isomerization and aromatization reactions; cracking over SIO2-A1203 shows both products coming from random scission and isomerization processes. On the contrary, USY zeolite favours an end-chain cracking mechanism with pronounced isomerization and aromatization. LDPE degradation over MCM-41 proceeds through a random scission mechanism due to their large pores, while hydrogen transfer reactions involving high molecular weight molecules are also favoured by this catalyst. 1. INTRODUCTION Large amounts of plastic wastes are generated annually which end up in landfills or incineration plants. Plastics materials account for about 8 wt% of the municipal solid wastes (MSW), whereas this proportion increases over 20% in volume due to their low density [ 1]. Recently, feedstock recycling has shown to be an interesting alternative to transform back the plastic wastes into chemicals and fuels. Recent works on the conversion of polyolefinic plastics into fuels have revealed the advantages of catalytic processes to improve the quality of the degradation products. Mordi et al. [2] have examined the product distribution from the catalytic degradation of polypropylene over various types of zeolite catalysts under batch conditions. Uddin et al. [3] have studied the influence of silica-alumina catalysts on the polypropylene degradation. Uemichi et al. [4] have used metal-supported activated carbon for the conversion of polyethylene into aromatic hydrocarbons. Aguado et al. [5, 6] have studied the degradation of different polyolefins over a mesoporous catalyst, MCM41, showing that the use of this material leads to the formation of both gasoline and middle distillate fractions as main cracking products. Correspondence concerning this paper should be addressed to Dr. D.P. Serrano, Departamento de Ingenieria Quirnica, Facultadde Ciencias Quimicas,UniversidadComplutensede Madrid, 28040 Madrid, Espafia. e-mail:
[email protected] phone number: 34-91-394 41 85 fax number: 34-91-39441 14
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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While most of the previous works on the catalytic cracking of polyolefins deal mainly with the analysis of the volatile products coming out from the reactor, in the present research work the solid residue has been deeply characterised in order to get information on the initial stages of the degradation. In this way, a different cracking pathway has been observed when using MCM-41 as catalyst, compared with conventional acid solids such as zeolites and amorphous silica-alumina. 2. E X P E R I M E N T A L
SECTION
2.1. M a t e r i a l s
Low-density polyethylene (LDPE), supplied by Repsol S.A., was used as raw material. Commercially available catalysts, amorphous silica-alumina (Stidchemie, KA-3) and zeolite USY (Grace Davison), were employed without modification. MCM-41 with Si/Al=76 was synthesised in our laboratory according to a room temperature method [7]. 2.2. R e a c t i o n s e t u p
The catalytic degradation of polyethylene was carried out in a stirred batch reactor provided with a helicoidal stirrer (120 r.p.m.), as it is shown in figure 1. The experiments were carried out at three temperatures (380, 400 and 420~ and different reaction times (0380 minutes) under nitrogen flow, the volatile products being collected and separated into gases and liquids at 0~ Both phases were analysed by GC (Varian 3800). The liquid products and the solid residue remaining in the reactor were characterised through 1H NMR measurements (Varian 300 MHz).
I" _.,_.q
~l;~-:;~
-
CONTROLPANEL !~..,
~176 "
' ~---'-'-'-'-'-~"~-------~/ ~ - ~
/
"
' '
\ \ /
N2
\\
Fig. 1. Schematic diagram of the experimental reaction system.
/
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1591
3. RESULTS AND DISCUSSION The products obtained from the thermal and catalytic cracking of low-density polyethylene were grouped as gases (C1-C5), liquids (C6 and higher hydrocarbons retained in the cold trap) and solid (the residue inside the reactor). Table 1 shows the results obtained in both thermal and catalytic experiments. Thermal cracking at 400~ shows little activity, whereas at 4200C a decrease in the solid yield with the reaction time is observed. On the other hand, an improvement of the LDPE cracking takes place when a solid acid, as ultrastable Y zeolite or MCM-41 are used as catalysts. The latter shows higher activity than USY zeolite and amorphous SIO2-A1203. Significant differences in the product distribution are also observed, since MCM-41 and in less extension amorphous SIO2-A1203 lead mainly to liquid hydrocarbons, whereas a high proportion of gases are obtained over USY. Table 1 Results obtained in thermal and catalytic cracking. Catalyst
T (~
t
(min)
Solid Yield (%)
Liquid Yield (%)
Gas Yield ( % )
97.5
0.1
2.4
--
400
20
--
400
120
92.1
3.5
4.4
--
420
0
97.7
0.6
1.6
--
420
20
90.0
7.2
2.8
--
420
90
61.0
35.8
3.2
SIO2-A1203
380
30
98.6
--
1.4
SIO2-A1203
400
30
96.1
--
3.9
SIO2-A1203
420
30
73.0
22.1
5.0
USY
380
30
94.4
3.7
4.9
USY
400
30
87.9
6.6
5.5
USY
420
30
72.3
19.5
8.2
MCM-41
380
30
83.7
12.5
3.8
MCM-41
400
30
77.2
20.8
2.0
MCM-41
420
30
42.3
45.9
11.8
Figure 2.a shows the GC analysis of the liquid product obtained in the LDPE thermal degradation (T=420~ t=90 min), in which pairs of a-olefins/n-paraffins are clearly shown, coming directly from the cracking of C-C bonds in the polyolefinic chains. On the other hand, when MCM-41 is used as catalyst, the liquid fractions are rich in hydrocarbons within the range C6 to C~2 (Figure 2.b), with a much broader product distribution related to the extension of aromatization and isomerization reactions over this catalyst. ~H NMR of the liquid product indicates a higher degree of aromatization when MCM-41 is used as catalyst in comparison to thermal and catalytic cracking over amorphous SIO2-A1203. Likewise, the liquid fraction obtained in the LDPE degradation over USY zeolite presents a high content of aromatics and branched isomers, indicating that aromatization and isomerization reactions are also favoured by the use of this catalyst.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1592
n-
)araffin
-o tei in /, \ t 1 I
b)
i
C6C8 ClO Cl2 C14C16 C18C20 J
Cl0Cl2 C14C16C18C20 l
Jl
1
I
"~""/"~
I
2 4 6 8 10 12 4 i 6 i ' 8 20 2 4 6 8 10 12 i4 16 18 20 22 Fig. 2. GC analysis of liquid product obtained in: a) LDPE thermal degradation (T=420~ t=90 min); b) LDPE catalytic cracking over MCM-41 (T=400~ t=30 min). Figures 3 and 4 show the IH NMR spectra corresponding to the solid residue of the thermal degradation and catalytic cracking over MCM-41, respectively. They exhibit two main peaks at 0.9 and 1.3 ppm, which correspond to protons on methyl and methylene groups. The spectra also show significant differences in the area of the olefinic protons (4-6 ppm), not only in regards to the decrease in the olefinic character when MCM-41 is used as catalyst but in the different type of olefins present in the solid. Likewise, an increase in the region of aromatic protons at around 7 ppm is observed in the spectrum of the catalytically degraded solid compared to that obtained trough thermal cracking. A correlation between the olefinic protons and the cracking extension must exist, since the cleavage of the C-C bonds leads to the formation of o~-olefins and n-paraffins as primary products.
Solvent (C12CDCDC12)
B.0
7.5
7.0
6.5
6.0
/
5.5
5.0
4.5_
~'[~''''~'~'`~'~'~~'~''~'~''~'''~'~'~''~'~'~'~''~'
8
7
6
5
4
ppm
",',,~,,
3
2
1
0
Fig. 3.1H NMR spectra of the solid residue obtained by LDPE thermal cracking (T=400~ t=120 min).
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Solvent (C12CDCDC12)
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Fig. 4. 1H NMR spectra of the solid residue obtained by LDPE catalytic cracking over MCM-41 (T=400~ t=30 min). Assuming that the relative number of protons from methyl groups can be considered a measurement of the cracking extension, figure 5 shows the results from the ~H NMR analysis of the solids obtained in different cracking conditions. The residues from thermal treatment show an asymptotic trend, with a pronounced increase in the olefinic character of the solid during the initial stages of the cracking process. Similar results to those shown by thermal degradation are obtained when SIO2-A1203 or USY are used as catalysts. 0.9
9Thermal (90 and 120 min ) tx Zeolite USY (T=380,400,420"C at 30 min) 9MCM-41 (T=380,400,420"C at 30 min) o Silica -alumina (T=380,400,420"C at 30 min oThermal (0 and 5 min)
0.8 0.7
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.
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,
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,
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Fig. 5. Olefinic protons vs. methyl protons in the solid residue into the reactor. The only solid products showing a different trend are those coming from the catalytic cracking over MCM-41. In this case, these products present a clear decrease in olefinic character in regards to the ones coming from the thermal cracking or those obtained using the other types of acid catalysts in adequate combination of acid strength, high surface area and uniform pore size allows MCM-41 to promote hydrogen transfer reactions among large hydrocarbon molecules. The hydrogen generated in the aromatization and dehydrocyclization reactions of olefins is consumed by bulky radicals or carbocation intermediates, leading to a more paraffinic residue.
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Table 2 summarises the results obtained in a serie of experiments carried out over MCM-41 at 380~ at different reaction times. As the solid yield decreases, a certain increase in the aromatic character is observed, whereas the contents in olefinic protons does not seem to change significantly in the solid product. Regarding the nature of the liquid product the higher content of olefins confirms that hydrogen transfer reaction affect mainly to bulky radical and carbocations species. These results suggest that the aromatization activity of MCM-41 does not show a decline at least for the reaction times tested in this work. Table 2 Results obtained in catalytic cracking over MCM-41 at 380~ Solid Liquid Yield ~HNMR Yield ~HNMR Catalyst T (~ t (min) (%) % olef'mic % aromatic (%) % olef'lnic % aromatic MCM-41 380 30 83.7 0.106 0.012 12.5 4.25 0.16
Gas Yield (%) 3.8
MCM-41
380
60
82.1
0.094
0.033
16.6
5.04
0.16
1.2
MCM-41
380
120
67.6
0.087
0.042
24.3
4.92
0.24
8.2
4. CONCLUSIONS Thermal degradation follows mainly a random scission mechanism with little contribution of isomerization and aromatization reactions. Catalytic cracking over SIO2-A1203 shows also products coming from random scission but in this case isomerization reactions are present. On the contrary, USY zeolite promotes an end-chain cracking mechanism with pronounced isomerization and aromatization. The high pore size of MCM-41 catalyst leads to a random cracking mechanism, promoting hydrogen transfer reactions between high molecular weight molecules. The results show a relationship between the reaction mechanism and the catalyst properties. Thus, USY zeolite is a microporous catalyst which promotes endchain scission reactions, whereas LDPE cracking over mesoporous materials, MCM-41 and SiO2-AlzO3 , proceeds mainly trough a random cleavage of C-C bonds. However, the high surface area of MCM-41 and its uniformity of pore sizes favour the extension of hydrogen transfer reactions between bulky molecules. ACKNOWLEDGEMENT Financial support from the Comunidad Autrnoma de Madrid (Project n~ 07M/0420/1997) is gratefully acknowledged.
REFERENCES 1. 2. 3. 4. 5. 6.
R.J. Rowalt. Chemtech., 23, (1993), 56-60. R.C. Mordi; R. Fields; J. Dwyer. J. Chem. Soc., Chem. Commun., (1992) 374. Md A. Uddin; K. Koizumi; K. Murata; Y. Sakata. Polym Deg. Stab., 56 (1997) 37. Y. Uemichi; Y. Makino; T. Kanakuza. J. Anal. Appl. Pyrol., 14 (1989) 331. J. Aguado; D.P. Serrano; M.D. Romero; J.M. Escola. Chem Commun., (1996) 725. J. Aguado; J.L. Sotelo; D.P. Serrano; J.A. Calles; J.M. Escola. Energy Fuels, 11 (1997) 1225. 7. J. Aguado; D.P. Serrano; J.M. Escola. Microporous Mesoporous Mater., 34 (1) (2000) 43.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Catalytic conversion of tars in biomass gasification fuel gases with nickelactivated ceramic filters D.J. Draelants, H.-B. Zhao and G.V. Baron Department of Chemical Engineering, Vrije Universiteit Brussel Pleinlaan 2, B-1050 Brussel, Belgium, Phone: +32-2-6293262, Fax: +32-2-6293248 E-mail:
[email protected],
[email protected] The problem of efficient, reliable and environmentally sound tar and particle removal is common to most applications of biomass gasification technology. In this work, a novel catalytic reactor for simultaneous tar and particle removal is introduced. It consists of a ceramic candle filter, which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process with a potential reduction in investment costs. The catalytic feasibility of the concept was screened on a laboratory-scale with a sulphur- and dust-free synthetic biomass gas and with benzene and naphthalene as tar model compounds. The catalytic filter was represented by a disc-shaped ix-alumina cartridge, activated with nickel through a deposition-precipitation technique with urea. This paper gives an overview of the screening experiments that included variation of reaction temperature, flow rate and nickel loading. Above 850~ a high performance for converting benzene and naphthalene was found using gas velocities typically encountered in candle filtration. 1. INTRODUCTION Tars (benzene, naphthalene and heavy aromatics) are one of the undesirable co-products in production of fuel gas for power production by biomass gasification, because they can cause fouling of equipment and are an environmental hazard, if released [1]. Hence, in many applications the tars must be removed before the fuel gas can be utilised. Preferably, this gas cleaning is performed at temperatures close to the ones at the gasifier exit (700-900~ since this may lead to a higher energy efficiency but more importantly to simplified processes and lower cost, avoiding several high temperature heat exchangers. In addition, this cleaning needs to be performed with the smallest possible extra investment so as to make its use economically viable, especially for small-scale units. The previously proposed or existing solutions for gas cleaning, like wet scrubbing methods, do not fulfil these conditions. The use of catalysts to eliminate tars in biomass fuel gas is a good alternative to wet scrubbing because the tars can be directly converted to useful components of the fuel gas (H2 and CO), avoiding a loss of the thermal value of biomass fuel gas. Two types of catalysts (naturally available dolomites and steam reforming nickel-based catalysts) have been used, usually in a packed bed reactor at 800-900~ with the commercial nickel-based catalysts more active than calcined dolomites [2]. Nickel-based catalysts can however be deactivated by coking when the amount of the tars is high and by sulphur compounds in the fuel gas like H2S [3]. It is recognised that the catalytic bed can work under severe internal diffusion
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limitations, which prevents the efficient use of the catalyst [4]. In general, the reaction schemes of catalytic tar conversion are based on gasification of adsorbed hydrocarbon species on the catalyst surface by H20 and/or CO2 [2]. Another important fuel gas cleaning step involves the removal of the particles, causing plugging and abrasion of downstream equipment. At present, this is performed with commercially available ceramic candle filters for hot gas cleaning, which are usually made of silicon carbide or aluminium oxide. Mostly, those filters are formed of two layers, i.e., a thin filtration membrane supported on a mechanically stable large-pore support body [ 1]. In this work, a novel catalytic reactor for tar removal is studied. It consists of a classic ceramic candle filter, but which contains a nickel-based tar cracking catalyst in the support body. This concept simplifies the entire gas cleaning process by integration of the removal of particles and tars in one unit, with a potential reduction in investment costs [5]. In addition, due to the intrinsic pore structure of the filter, the gas flows now through the catalytic active pores and internal diffusion is no longer a limiting factor for the tar conversion as in the conventional packed bed reactors. Figure 1 gives a schematic representation of such a catalytic candle filter. Tar components in gaseous form flow through the catalytic filter (1 m long, 1 cm wall thickness), and are converted in the interior of the filter support body. The particles are trapped on the outside thin filtering membrane where a dust cake is formed, which should not affect the catalytic activity, because the catalytic material is situated only in the interior of the porous support structure [6]. This project focuses on the catalytic performance of the catalytic filter, since this is the intrinsic new addition to the candle filter. It involves preparation chemistry for incorporation of nickel into lab-scale filter cartridges similar to the porous support body of a candle filter, determination of their tar cracking ability and modelling of the mass transfer. At present, a preparation route to incorporate pure nickel into the preformed filter substrates has been developed and some activated filter substrates were screened with major tar model components like benzene and naphthalene in a sulphur- and dust-free biomass fuel gas. This paper gives an overview of the results of this first batch of catalyst screening experiments. Dust and tar removed fuel gas
Raw fuel gas
m
m
Fig. 1. Schematic representation of a catalytic candle filter
Pore-wall catalytically modified pore
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2. EXPERIMENTAL
2.1. Catalyst preparation The porous alumina filter substrates used (Schumacher, Germany) were disc-shaped (1 cm thickness, 3 cm diameter) and consisted of non-porous ~-A1203 grains (100-350 ~tm). They had a mean pore radius of 26 ~tm, a pore volume of 0.1 ml/g and a BET specific surface area of 0.33 m2/g and were similar to the support structure of alumina candle filters. These substrates were catalytically activated with nickel using the precipitation-deposition method with urea. The substrates are vacuum impregnated with a solution containing appropriate amounts of urea and nickel nitrate. After the excess solution is drained off, the substrates are put in a closed glass vessel and placed in an oven at 90~ during a certain period for reaction, resulting in precipitation of the nickel precursor by the slow decomposition of urea in the pores of the disc [7]. After reaction, the disc was dried at 110~ and subsequently calcined at 450~ for 4 h in an air atmosphere to decompose the precipitated nickel precursor to nickel oxide. This technique allows us to deposit up to 2 wt% of nickel by one impregnation cycle. We have demonstrated that a fairly uniform distribution of the nickel precursor throughout the substrates can be obtained and that one impregnation cycle hardly changes the porosity of the filter cartridges. More details about the catalyst preparation can be found in another publication [8]. Before being used for reaction tests, the activated substrates were reduced in 10 to 50 v% H2 in N2 overnight at 900~
2.2. Reaction set-up A laboratory-scale reaction set-up has been constructed to perform catalyst screening tests, long-term tests, deactivation studies and reaction kinetic studies. The gas mixing zone allows to feed a representative dust-free synthetic biomass fuel gas (N2, HE, CO, CO2, H20 and CH4) to the reactor, with or without addition of impurities like NH3, HES and tar (benzene and naphthalene). The inlet gas is preheated till 150~ before and after introduction of water, benzene and naphthalene to prevent condensation. The total gas flow rate can be set as such that the superficial gas velocity in the reactor is comparable to the face velocity used in candle filtration (1 - 4 cm/s). The reactor consists of a horizontal, dense alumina tube (i.d. 30 mm, length 500 mm), to minimise catalytic wall effects. The catalytic filter substrate is fixed in the middle of the tube by means of an alumina powder cement. The reactor is heated in a tube oven. A differential pressure sensor measures the pressure drop across the reactor because an increase in pressure drop can be an indication for e.g. carbon deposition. The gas leaving the reactor remains heated till 150~ to prevent condensation and finally led either into a tar sampling device or into a by-pass line. In the latter, tar and water are removed from the gas by a cold trap (0~ and sulphuric acid before the gas is led to an online gas chromatograph (Varian 3400 GC with TCD) to determine the content of the main gas components (N2, HE, CO, CO2 and CH4). The tar sampling train is composed of two washing bottles in a bath of cooling liquid at -20~ The first bottle is empty to trap most of the water and naphthalene, while the second bottle contains dichloromethane as a solvent to absorb the benzene and other tars. After sampling, the bottles are rinsed with dichloromethane and the content of the tar compounds in the solvent is off-line analysed by gas chromatography (HP 6890 GC with FID). To monitor an immediate change in behaviour of the catalytic filter substrates during the tests, the outlet gas composition was qualitatively followed by an on-line mass spectrometer (Balzers QMG 420) for N2, HE, CO, CO2, H20, CH4 and benzene.
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2.3. Reaction experiments In this study, a typical sulphur- and dust-flee biomass feed gas contained 51 v% N2, 12 v% CO, 10 v% HE, 5 v% CH4, 11 v% CO2 and 11 v% H20, 10-30 g/Nm 3 benzene and 4.5 g ~ m 3 naphthalene. The screening tests involved variation of temperature, inlet flow rate and nickel loading. The temperature was varied from 900~ till 750~ in decreasing steps of 50~ Table 1 gives an overview of the inlet flow rates used, together with their respective space velocity (at normal conditions and based on the reaction volume) and superficial gas velocity (in the reactor tube at 900~ for the two nickel loadings studied. The gas velocity is similar to the face velocity typically used in candle filtration (1-4 cm/s). A gas velocity of 6 cm/s is maybe not realistic for filtration, but was selected to test if it was still possible to get high conversions with such a low contact time. The reaction data were obtained at steady state of the reactor, based on the MS analysis.
Table 1 Flows used during the screening experiments Nickel loading of substrate
inlet flow (Nml/min)
space velocity (h -1)
Superficial velocity (cm/s)
0.5 wt% Ni
195
1655
2
585
4965
6
245
2080
2.5
395
3350
4
590
5008
6
1 wt%Ni
3. RESULTS AND DISCUSSION Figure 2 gives an overview of the conversion of the tar model compounds benzene and naphthalene for the different flow rates, temperatures and nickel loadings studied. 3.1. Effect of gas flow rate To limit the pressure drop across a candle filter, face velocities between 1 and 4 cm/s are normally used. Consequently, the contact time with the catalyst in a catalytic candle filter is imposed by the filtration step. It was not known if this limitation was compatible with the catalytic reactions that have to take place on the low surface catalyst. Our experiments show that, on the condition that the temperature is high enough (850~ the range of tested filtration gas velocities (2-4 cm/s) is adequately to obtain very high conversions of benzene and naphthalene. This indicates a high intrinsic catalytic activity for tar elimination. Mass balance calculations with the inlet and outlet gas compositions, confirmed that benzene and naphthalene were converted to gaseous components like H2 and CO. Below a certain temperature, the conversions decrease as the flow increases, which means that the reaction is then limited by contact time. Nevertheless, this decrease in conversion remains limited in comparison with the variation of the flow rate itself. As already mentioned, a velocity of 6 cm/s (590 Nml/min) is not so realistic for candle filters, but the naphthalene conversion remains very high above 850~ with this high flow rate. However, this velocity is too high to obtain full conversion of benzene (more stable molecule than naphthalene), even at 900 ~
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
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0.5 wt% nickel
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Fig. 2. Conversions of the tar model compounds benzene and naphthalene 3.2. Effect of temperature The benzene conversion is strongly affected by the temperature, and this for all tested flow rates. The experiments show that at least a temperature around 850~ is necessary to obtain a high activity for benzene conversion with typical filtration velocities. Further improving the catalyst can of course decrease this temperature. The working temperature of the catalytic filter is a very important parameter since a lower working temperature certainly improves the life time of the candle filter itself and is beneficial to oppose sintering of the nickel catalyst. It also allows operation of the gasifier at lower temperature such as with lower grade biomass. The naphthalene conversion is less affected by temperature and it seems that 800~ is sufficient for the typical filtration velocities used. 3.3. Effect of nickel loading There seems to be no important difference in activity between the two tested substrates. The conversions with the 1 wt% Ni substrate are only slightly higher compared with the 0.5 wt% Ni substrate, despite of the fact that the nickel loading was doubled. Naturally, the total nickel loading doesn't give any information about the nickel dispersion, which indicates how much of the deposited nickel atoms are really available at the surface for the catalytic reactions. In our case, increasing the loading did not seem to really improve the dispersion and hence the activity. This indicates that the original available surface for nickel deposition
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(i.e. the outer surface of the non-porous a-alumina grains) is nearly completely covered with nickel for a loading of only 0.5 wt%. The fact that a low loading of nickel is sufficient simplifies the catalytic filter preparation since multiple impregnation steps with the urea method to increase the loading do not need to be considered. The commercial nickel on (xalumina catalysts, tested for tar conversion in packed beds, had a nickel loading of about 10 20 wt% [2, 4]. In practice, they consist of porous mm-sized particles. Consequently, almost all the nickel is situated inside these particles but can only be reached by the gas by diffusion into the small pores of the particles. In practice, this leads to a low efficient use of the catalyst due to internal diffusion limitations. In a catalytic filter, all the nickel is situated on the outer surface of a-alumina grains of a few 100 ~tm diameter and the gas can instantly react with the nickel when it flows through the filter. 4. CONCLUSIONS AND FUTURE WORK In view of the application background of this project, the first catalyst screening tests have positively demonstrated the principle of activating candle filters with nickel to eliminate tars from a sulphur-free biomass fuel gas. Above 850~ a high performance for converting benzene and naphthalene was found with gas velocities typically encountered in candle filtration. These results are encouraging to further develop this system and continue more screening. However, biomass gasification gas may contain 50-200 ppmv HaS, which is a known poison for nickel catalysts. Additives like Ca and Mg may increase the sulphur resistance of the catalyst and its long-term stability. This implies an adjustment of the catalyst preparation procedure and this will be implemented in the near future in collaboration with a catalyst manufacturer. The operation time was too short to extrapolate to long term activity, but some experiments with a larger time-on-stream will be performed. In addition, future experiments may include the study of the conversion of the NOx-precursor ammonia in the biomass fuel gas, which is present in concentrations of a few thousand ppmv and can also be decomposed with nickel. REFERENCES 1. E. Kurkela, "Formation and removal of biomass-derived contaminams in fluidized-bed gasification processes", VTT Publications, Espoo, 1996. 2. P. Simell, "Catalytic hot gas cleaning of gasification gas", VTT Publications, Espoo, 1997. 3. J. Hepola and P. Simell, Applied Catalysis B: Environmental, 14 (1997) 287. 4. I. Narv~ez, J. Corella and A. Orio, Ind. Eng. Chem. Res., 36 (1997) 317. 5. G. Saracco, S. Specchia and V. Specchia, Chem. Eng. Sci., 51 (1996) 5289. 6. K. Hiibner, A. Pape and E.A. Weber, "High temperature gas cleaning", E. Schmidt et al. (Eds.), Institut Ftir Mechanische Verfahrenstechnik und Mechanik, Karlsruhe, (1996) 267. 7. L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen and J.W. Geus, Stud. Sur. Sci. Catal., 63 (1991) 165. 8. H. Zhao, D.J. Draelants, and G.V. Baron, Catalysis Today, in press.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
An e n v i r o n m e n t a l friendly c a t a l y s t for t h e high t e m p e r a t u r e shift r e a c t i o n G. C. Araujo and M. C. Rangel Instituto de Quimica, Universidade Federal da Bahia, C a m p u s UniversitY_rio de Ondina, Federa~gto. 40170-280 Salvador, Bahia, Brazil The need for environmental protection has demanded the searching nontoxic catalysts easily handled and discarded. This work deals with the use of a l u m i n u m as a substitute of chromium, in iron- and copper-based catalysts for the high temperature shift reaction. Catalysts were prepared in the commercial form (hematite) and was evaluated in more severe operational conditions t h a n the u s u a l industrial one (e.g. steam to carbon ratios, R, lower than 0.6). It was found that a l u m i n u m can replace c h r o m i u m in these catalysts leading to better catalytic properties as compared to chromium. The catalyst h a s the advantage of being non toxic and can work in low a m o u n t s of steam.
I. INTRODUCTION The production of hydrogen from the natural gas or n a p h t h a feedstock is usually increased by m e a n s of the water gas shift reaction (WGSR) [1]: CO(g) + H20(g} ~ CO2 + H20(~) AH= - 4 1 . 1 kJ.mo1-1 which is also an important stage in the commercial production of hydrogen. This reaction is a reversible and exothermic one and t h u s is favored by low temperatures and excess of steam. However, it requires high t e m p e r a t u r e s to achieve rates high enough for industrial applications. Therefore, the WGSR is often carried out in two steps, the first being performed in the range of 320-450oC (named high temperature shift, HTS). In the second stage (known as low temperature shift, LTS), carbon monoxide is removed from the feed stream in thermodynamically favorable conditions in the range of 200-250~ [ll. The HTS stage is often carried out over catalysts of chromium-doped iron oxides, available as hematite. The catalyst is reduced in situ to produce magnetite which is found to be the active phase. In a m m o n i a plants, the reduction is performed with a gaseous mixture of carbon monoxide and dioxide, hydrogen and nitrogen. This reaction is highly exothermic and should be carefully controlled to prevent damage to the reactor or to the catalyst. The production of metallic iron should be particularly avoided since it may catalyze the hydrocarbon formation [1,2]. In order to assure the magnetite stability in industrial processes, large a m o u n t s of steam are used. However, this procedure increases the operational costs and t h u s new
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systems that do not generate metallic iron, even at low contents of steam, are d e m a n d e d [3]. In spite of the chromium-doped hematite catalysts have shown high stability in performance, they have been recently modified by the addition of small a m o u n t s of copper, resulting in even more active a n d selective catalysts. However, because of environmental restrictions concerning the discarding of c h r o m i u m compounds, the search for non-toxic catalysts which could be easily handled and discarded is m u c h needed. With this goal in mind, this work deals with the replacement of c h r o m i u m by a l u m i n u m in HTS catalysts. In order to save the energy related to the steam consumption, the catalysts were also evaluated in more severe operational conditions t h a n the industrial one, e.g. lower s t e a m to carbon ratios. 2. EXPERIMENTAL
The reagents used were analytical grade. The catalysts were prepared by copreciptation techniques at room temperature, followed by heating at 500~ for 2h u n d e r nitrogen flow (100 ml. min-1). Four samples were prepared: (i) with a l u m i n u m and copper (HAC sample); (ii)with only a l u m i n u m (HA sample); (iii) with only copper (HC sample) and (iv)without any dopant (H sample). For all samples an iron to the dopant molar ratio of 10 was used. The a l u m i n u m and copper-based catalyst was prepared by adding aqueous solutions of Fe(NO3)3.9H20(1N) and AI(NO3)a.9H20(0.1N) and a concentrated (25% w/w) a q u e o u s solution of a m m o n i u m hydroxide to a beaker with water, u n d e r stirring. The final pH was adjusted to 11 and the system was kept u n d e r stirring for additional 30 min. The sol produced was centrifuged {2000 rpm, 5 min) followed by a water rinsing to remove the nitrate ions from the starting material. After a second centrifugation step, the gel was impregnated with an a q u e o u s solution of Cu(NO3)2.3H20 (0.06N) for 24h u n d e r stirring, centrifuged again and dried in an oven at 120~ The same procedure were used to prepare the other samples. For the catalysts without copper, the gel was kept in pure water for 24h, u n d e r stirring, in order to simulate the same experimental conditions used to prepare the other samples. The qualitative analysis of nitrate was performed by adding about 1 ml of concentrated sulfuric acid to 10 ml of the s o b r e n a d a n t after centrifugation. The formation of [Fe(NO)I 2§ was detected by a brown ring [4]. The absence of nitrate in the solid was confirmed by infrared spectroscopy in the range of 4000-650 cm -1 using a Shimadzu model IR-430 spectrometer and KI discs. The metal contents were determined by inductively coupled p l a s m a atomic emission by using an model Arl 3410 equipment. X-ray diffractograms were recorded at room temperature with a Shimadzu model XD3A i n s t r u m e n t using Cu Kcz radiation generated at 30 kV and 20 iliA. The surface area (BET method) was m e a s u r e d in a Micromeritics model TPD/TPO 2900 equipment on samples previously heated u n d e r nitrogen (150~ 2h). The t e m p e r a t u r e
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p r o g r a m m e d reduction (TPR)was performed in the s a m e equipment, using a 5% H2/N2 mixture. The copper area was m e a s u r e d in a CG-2001 e q u i p m e n t using the N20 pulse technique [5,6]. X-rays microanalysis were performed in a Noran microprobe coupled to a model JSM-T300 microscope operating at 20-30KV. The catalyst performance was evaluated using 0.2 cm 3 of powder within 50 and + 325 m e s h size and a fixed bed microreactor consisting of a stainless tube, providing there is no diffusion effect. All experiments were carried out u n d e r isothermal condition (370~ and at atmospheric pressure, employing a gas mixture with composition a r o u n d 10% CO, 10 % CO2, 60% H2 and 20% N2. It was used several steam to gas molar ratios: 0.2, 0.4 and 0.6. The g a s e o u s effluent was analyzed by on line gas chromatography, using a CG-35 i n s t r u m e n t . A commercial catalyst, based on chromium, copper and iron oxides, was used to compare the performance of the samples prepared in this work. In order to determine the role of copper in these catalysts, a m e c h a n i c a l mixture of the sample with a l u m i n u m (HA sample) and copper oxide was also evaluated. In this case, the catalyst tests were r u n at 200~ because of the high tendency of copper of sintering at 370oC [1]. After each experiment, the Fe(II) content in the catalysts was determined to follow the iron reduction u n d e r reaction atmosphere. Samples were dissolved in concentrated chloridric acid, u n d e r carbon dioxide a t m o s p h e r e and then titrated with p o t a s s i u m dichromate [7].
3. R E S U L T S AND D I S C U S S I O N
As shown in Figure 1, hematite was detected in the flesh catalysts while magnetite was found in the used catalysts; a l u m i n u m t e n d s to impair crystallization w h e r e a s copper tends to favor it. No other p h a s e besides hematite and magnetite was detected. The presence of either a l u m i n u m or the two d o p a n t s increased the surface a r e a of the solids (Table 1), which was not affected by doping the solid with copper alone. For the used catalysts, a l u m i n u m increased the surface area while copper decreased it. The characteristics of the spent catalysts did not show any significant dependence on the s t e a m to gas molar ratio (R) u s e d in the test reaction. The catalyst reduction strongly decreased the surface area, showing that the phase change is followed by a coalescence of particles a n d / o r porous. A l u m i n u m increased the copper areas showing t h a t this metal prevents copper sintering. The a l u m i n u m presence on the surface was confirmed by X-ray microanalysis. The TPR curves of hematite (Figure 2) showed one peak at 510oC, due to magnetite formation and another a r o u n d 770oC, attributed to the formation of metallic iron [8]. In the a l u m i n u m - d o p e d sample, the first peak was shifted to lower t e m p e r a t u r e s (400~ whereas the higher t e m p e r a t u r e peak was not affected; this behavior shows that this metal eases the formation of the active p h a s e but does not affect its stability. The copper-doped catalyst showed a TPR curve with a peak a r o u n d 210 ~ assigned to metallic
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copper [9] and other peaks a r o u n d 230 and 600oC; these TPR peaks show that copper favors the active phase formation and destruction. The a l u m i n u m and copper-doped solid showed a curve with the characteristic peak due to magnetite formation displaced to 300oC and the peak related to metallic iron at 770 oC. It m e a n s that the synergetic effect of the d o p a n t s is HACR
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Figure 1. X-ray diffractograms of (a) fresh and (b) used catalysts. H= hematite; A= a l u m i n u m and C = copper Table 1 Surface area {Sg) and copper area of the samples Sg (m2.g-1) Sg (m2.g -I) Sample (after reaction) (before reaction) R=0.6 R=0.4 R=0.2 8 9 9 27 H 24 25 28 59 HA 6 4 6 26 HC 17 22 24 86 HAC R = s t e a m / p r o c e s s gas (molar)
Copper area (fresh catalysts) - - - -
_
_
39 44
to ease magnetite formation but not to affect the production of metallic iron. The commercial catalyst showed a TPR curve with peaks a r o u n d 270, 340 and 750~ showing that c h r o m i u m is less effective in prevent the active phase formation and destruction.
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All catalysts were active to HTS reaction, as s h o w n in Table 2. A l u m i n u m leads to a slight increase in activity, which is due to a textural effect. On the other h a n d , copper increased both the activity a n d the activity per area, showing t h a t it acts as a s t r u c t u r a l promoter. The copper a r e a values, as well as the TPR results, suggest t h a t the higher activity of the copper promoted catalyst is due to an increase of the n u m b e r of active sites; it is well k n o w n t h a t copper is also active in the HTS reaction [1,2]. However, the m e c h a n i c a l mixture of the sample with a l u m i n u m {HA) a n d copper oxide (3x10 -4 mol.g-lh -~) showed a lower activity t h a n the HAC sample {7x10 -4 mol. g-~.h -I) at 200oC, suggesting t h a t there is probably an electronic effect of copper. Commercial
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Figure 2. TPR catalysts. H = hematite; A = a l u m i n u m a n d C = copper The e x p e r i m e n t s carried out at lower s t e a m to gas ratio showed that the catalysts can work at R=0.4 without any d a m a g e to both activity or selectivity to c a r b o n dioxide. At R=0.2, however, the activity a n d selectivity strongly d e c r e a s e d with the a m o u n t the steam. The analysis of the used catalysts showed t h a t this behavior was likely due to the lower Fe(II)/Fe(III) ratio, which m e a n s a lower a m o u n t of the active sites. Therefore, one can a s s u m e t h a t u n d e r lower R larger a m o u n t s of metallic iron w a s produced a n d h y d r o c a r b o n formation was catalyzed. The catalyst with both d o p a n t s showed higher activity t h a n a c h r o m i u m a n d copper-doped commercial catalyst (25x10 -4 mol.g-l.h-1). This sample p r o d u c e s the active p h a s e more easily t h a n the other catalysts, shows resistance to a f u r t h e r magnetite reduction a n d can work at severe conditions, t h a t is, low s t e a m to process gas ratio.
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Table 2 Catalytic activity (a) and selectivity selectivil (S) of the samples a/Sgxl0 s Sample axl04 Selectivity (%) (mol.g-l.h -1) (mol.m-2. h-~) R=0.6 R=0.4 R=0.2 R=0.6 R=0.4 R=0.2 R=0.6 R=0.4 R=0.2 H 7 6 5 9 7 6 50 50 20 HA 9 8 6 4 3 2 60 20 10 HC 24 23 16 40 58 27 80 60 60 HAC 34 34 17 20 15 7 70 70 80 R = s t e a m to gas molar ratio 4. CONCLUSIONS
The presence of both a l u m i n u m and copper in h e m a t i t e - b a s e d catalysts to HTS reaction favors the formation and stability of the active phase. A l u m i n u m acts as textural promoter and copper acts as a s t r u c t u r a l one. The a l u m i n u m , copper-doped hematite catalyst is more active t h a n a commercial catalyst. Therefore, a l u m i n u m is a promising dopant to replace c h r o m i u m in HTS catalysts since it shows two important features: first, it is non toxic, in comparison to the high c h r o m i u m toxicity, and second it leads to better HTS catalyts as compared to the c h r o m i u m - d o p e d ones. ACKOWLEDGEMENT
The a u t h o r s
CNPq/PADCT.
thank
the
financial
support
from CNPq,
FINEP and
REFERENCES
1. H. Bohlbro, An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapor over Iron Oxide based Catalysts, The Haldor Topsoe Laboratory, Vedback, 1960. 2. Newsome, D. S., Catal. Rev.- Sci. Eng., 21 (1980) 275. 3. M. V. Twigg, Catalyst Handbook, Wolfe Scientific Books, London, 1970. 4. F. Fergl, V. Anger and R. E. Oesper, Inorganic Analysis, Elsevier, Amsterdam, 1972. 5. C. R. F. Lund, J. J. Schorfheide and J. A. Dumesic, J. Catal., 57 (1979) 103. 6. M. J. Luys, P. H. van Oeffelt, P. Pieters and R. Terven, Catalysis Today, 10 ( 1991) 283. 7. A. I. Vogel, Quantitative Inorganic Analysis, Longman, London, 1961. 8. J. C. Gonzalez, M. G. GonzNez, M. A. Laborde and N. Moreno, Appl. Catal., 2 0 ( 1 9 8 6 ) 3 . 9. S. Pinna, T. Fantinel, G. Strukul, A. Benedetti, and N. Pernicone, Appl. Catal., 49(1997)341.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Direct catalytic conversion of chloromethane to higher hydrocarbons over various protonic and cationic zeolite catalysts as studied by in-situ FTIR and catalytic testing Denis JAUMAIN and Bao-Lian SU* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium The direct catalytic conversion of chloromethane to higher hydrocarbons over a series of zeolites such as Y, Beta and ZSM-5 in protonic and cationic form has been studied by means of in-situ FTIR and catalytic testing. This study shows that the acido-basic and structural properties of the zeolites influence the conversion rate and the selectivity of the products. Both protonic and cationic zeolites are very active for the conversion of CH3C1. Hydrocarbons such as aromatics, alkanes and alkenes are formed. The IR observations indicate a production of HC1 by elimination from CH3CI during the first step of the conversion. In protonic zeolites, the Bronsted acid sites are active sites, whereas a cooperative role of counter-ions and the framework oxygen atoms is responsible for the conversion of CH3CI in cationic zeolites. 1. INTRODUCTION In 1985, Olah and his co-workers reported an interesting three-steps route for converting natural gas to higher olefins Ill. In this new process, methane is first converted to methyl halide by a selective monohalogenation over supported acid or platinum metal catalysts. In the second step, methyl halide is transformed to methanol by a catalytic hydrolysis reaction and finally to hydrocarbons by MTG process on HZSM-5 catalyst. However, our recent studies have shown that this process can be improved by an efficient direct conversion of methyl halide into a mixture of hydrocarbons using zeolites as catalysts t2'31. This process can be finally reduced to two steps. Furthermore, this direct conversion should present an interesting alternative for the treatment of the chlorinated organic compounds, which are often the industrial hazardous solvent wastes. Their catalytic destruction or conversion to useful hydrocarbons will be an interesting matter for environment protection. It has been shown that this direct conversion can be carried out both on acid zeolites like HZSM-5 and HBeta t2-41 and on cationic zeolites such as faujasites X and Y and ZSM-5 exchanged with mono- or divalent cations [4'51. However, the initial fragment of CH3C1 molecules on zeolite catalysts is not well known. The real roles of the alkali cations and zeolite framework oxygen atoms, as basic sites, in the conversion are not clear and the reaction mechanism remains still preliminary. The aim of this work is to elucidate the mechanism of this reaction from the initial fragments of CH3CI to the formation of the C-C bonds and to study the effect of acido-basicity and structure of zeolites on the activity of catalysts and on the formation of products. A series of zeolites with different structures and Si/AI ratios such as faujasite Y (Si/A1 = 2.3), Beta (Si/A1 = 17.5) in protonic and Na form and ZSM-5 (Si/AI = 21.0) in protonic, cationic (Na, K and Cs) and as-synthesized (Na,H) form were used.
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2. EXPERIMENTAL
In-Situ FTIR. Self-supported zeolite wafers (15 mg/cm 2) were first calcined in a flow of dry oxygen at 450~ for 6 h and then in vacuum for 4 h. The spectrum of zeolite phase alone was recorded as a reference using a Perkin-Elmer Fourier Transform Spectrometer Spectrum 2000. After cooling to room temperature, adsorption of known amounts of CH3CI was conducted on zeolite wafers. The conversion of chloromethane was then performed msitu at different reaction temperatures in a range of 100-400~ during 15 min. After reaction, the samples were cooled to stop the reaction and the IR spectra were then recorded. Catalytic testing. The catalytic activity and selectivity of zeolite catalysts were evaluated by using a conventional catalytic microreactor in a flow condition under atmospheric pressure. The samples were first calcined in a flow of dry oxygen at 450 ~ for 10h and then the oxygen atmosphere is replaced by He and the temperature of reactor is adjusted to the desired temperature. The CH3CI was diluted by He. CH3CI and He flows were fixed by mass flow controllers and mixed in a mixer. The products of the conversion were analysed using a gas phase chromatograph with a FID detector and a HP PLOT Q column. 3. RESULTS AND DISCUSSION 3.1. FTIR study On both protonic and cationic zeolites, at room temperature, no conversion of chloromethane is observed. However, with increasing reaction temperature, peaks around 1387, 1465 and 2930 cm 1, assigned to -CH2- vibrations and at 1647 cm -~, corresponding to the vibrations o f - C = C - are observed from 100~ The observation of these vibrations indicates clearly the formation of C-C bond and higher hydrocarbons. It is interesting to observe a general trend on cationic zeolites, that a broad feature in the range of 3100-3600 cm -l appears due to the interaction of hydroxyls with the olefins or aromatics formed from the conversion of chloromethane. The hydroxyls on these cationic zeolites are probably generated by the interaction of framework oxygen atoms with HCI as a product of the conversion. Attributions of all the vibrations observed are given in Table 1. Adsorption and conversion of CH3C1 on HY and NaY. Figures 1Aa and 1Ba report the IR absorbance spectra of the activated zeolites HY and NaY, respectively. On these two zeolites, a small peak at 3740 cm -1 is observable and corresponds to external silanols. On HY, two other peaks at 3638 and 3535 cm ~ are detected and attributed to the bridging Si-OH-A1 present in the large cages and small cages of the framework, respectively. The introduction of chloromethane at room temperature brings the appearance of four new peaks at 2965, 2865, 1445 and 1350 cm 1 corresponding respectively to -CH3 stretchings and bendings of CH3Cl (Figures lAb and 1Bb). The broad band centered at 3314 cm -1 on the Table 1 9Wavenumbers and assignments of lR Wavenumbers (cm -1) Assignments 3000 o(C-H) of =CH2 2975 o(C-H) of =CH2 2960 o(C-H)as of-CH3 2933 o(C-H)as of-CH22872 u(C-H)s of-CH3 1627 o(C=C) ofalkenes 1610 o(C=C) of alkenes
peaks of the products formed on the zeolites Wavenumbers(cm -1) Assignments 1535 (C=C) of coke 1504 (C-C) of aromatics 1465 /5(C-H) of-CH21450 ~i(C-H)as of-CH3 1387 (CH2) deformation 1375 (CH2) deformation 1348 /5(C-H)s of-CH3
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IR spectrum of I05 , 1105 B HY is due to a ,o~ I l "o .~ shitt of the framework -~, ~_ o I ~ *~ hydroxyl of the ",/ \ o~ o~ -"3-1 .~_' lF ~ ,~ t large cage (3638 ,:Z,,~ o,~,~'~-~ t . . & , ~ II ~ ?,,il -"-'--.-~ / - ~-I cm ~) because of ~ r-5,' ~ ~ o ~ ,,,:,_~\ I h - . . . ~ ~ Io ~ .,. ,I an interaction with the adsorbed CH3CI molecules. Atter heating HY for 15 minutes at 400~ t ~ peaks in the range of 2800-3000 cm 1 (Figure 1Ac) can be observed and attributed to-CH24000 3500 3000 2500 2000 4000 3500 3000 2500 20'00 15'00 asymmetric and Wavenumbers (cm1) Wavenumbers (cm"l) symmetric Fig. 1. Adsorption and conversion of CH3CI over: A: HY, B: NaY, stretching a: Activated, b: CH3CI at RT, c: Atier heating 15 min. at 400~ d: vibrations, which After 30 min. of desorption at RT. is confirmed by the presence of the corresponding bending vibration at 1466 cm -1, and =CH2 stretching. Furthermore, the presence of several broad bands in the range of 3100-3600 cm 1 indicates that olefins seem to interact with OH sites in the zeolites. In comparison with its proton form, NaY shows almost the same behaviour (Figure 1Bc). The peaks in the range 2800-3000 cm ~ suggest the existence of important quantity of unreacted CH3CI. However, the presence of a broad band centered at 3580 cm1 and the appearence of a new peak at 3650 cm -~, corresponding to OH groups, after desorption (Figure 1Bd), imply strongly the production of HCI molecules by elimination from CH3C1 during the conversion. These HCI molecules interacting with oxygen atoms of the framework should create new OH groups, which are normally not present on a fully cation-exchanged zeolite and whose interaction with the products of the conversion gives a broad band centered at 3580 cm -1. These observations bring important informations on the first step of the conversion. The desorption of species adsorbed on HY at room temperature (Figure 1Ad) results in an important decrease of the intensity of hydrocarbons peaks, while the OH peak at 3638 cm -~ is simultaneously restored. This indicates a quite weak adsorption strength. Adsorption and conversion of CH3C1 on HBeta and NaBeta. After pretreatment, only a sharp peak at 3745 cm ~ and a small band at 3674 cm~, corresponding to external silanol and extra-framework A1-OH respectively, are present in NaBeta (Figure 2Ba). For HBeta a supplementary peak at 3612, corresponding to the bridging framework OH groups is observed. (Figure 2Aa). With introduction of CH3CI at room temperature, -CH3 stretchings and bendings peaks appear (Figure 2Ab and 2Bb). As the silanol band of NaBeta decreases in intensity upon adsorption of CH3C1, a broad band centered at 3621 cm-1 is observed and belongs to the interaction of CH3CI with silanols. For HBeta, as in the case of NaBeta, a broad band at 3602
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cm "~, due to the ,.-* tt3 e~ O O '~I"tt" ~t3 ee~ ~t~ 05 ~ 0 O en 9 ~ o O~ ~ - O~ A _ . 5 , ", ~' , o t~. . o ~ oe~n B ~,~ interaction between CH3C1 and silanols, can I Iv -~ 1~ A / ~-I~nP'-'-d /~-~ /x/v be observed. The interaction between CH3CI and the bridging framework hydroxyls gives another band at 3223 cm 4. After heating NaBeta at 400~ for 15 min., (Figure _ Iv,. [ 2Be) the peaks assigned to -CH3 4000 3500 3000 2500 2000 4000 3500 3000 2500 2000 1500 vibrations of Wavenumbers (cm-1) Wavenumbers (cm-1) chloromethane decrease and new Fig. 2. Adsorption and conversion of CH3CI over: A: HBeta, B: vibrations appear, NaBeta, a: Activated, b: CH3C1 at RT, c: Aider heating 15 min. at which are 400~ d: After 30 min. of desorption at RT. caracteristic of hydrocarbons such as aromatics (C-C 1504 cm-~), alkanes and alkenes (C=C 1627 cm -1, -CH3 2960 cm 1, -CH2- 2933, 1465, 1387 and 1375 cm-1). The OH region also presents some new bands at 3651, 3558, 3479 and 3219 cm 1. On the basis of previously reported results, they arise from the interaction of OH groups with 7t-bonds of alkenes or aromatics or with alkanes. This confirms the formation of higher hydrocarbons. The OH groups are normally not present in NaBeta, they should be generated by the presence of HCI which has been produced by dehydrohalogenation from CH3C1 during the conversion. HBeta zeolite shows an important reactivity. The peaks of the reactant disappear after heating this zeolite at 400~ for 15 min. (Figure 2Ac), and are replaced by peaks corresponding to alkanes, alkenes and aromatics. As in the case of NaBeta, broad bands at 3652, 3497 and 3203 cm -1 appear, due to the interaction of products with new OH groups formed by the presence of HC1 produced from CH3C1. Desorption of species adsorbed on these two zeolites at room temperature (Figures 2Ad and 2Bd) does not result in a significant modification, implying that the formed products adsorb strongly on HBeta and NaBeta compared to CH3C1, since a same desorption can remove all the CH3C1 from this zeolite. Adsorption on HBeta and NaBeta is also stronger than that on HY and NaY. Adsorption and conversion of CH3CI on HZSM-5 and NaZSM-5. The IR absorbance spectra of the activated HZSM-5 and NaZSM-5 are shown in figures 3Aa and 3Ba respectively. On both zeolites, a small peak at 3747 cm -1 is observable and corresponds to external silanols. On HZSM-5, a peak at 3612 cm ~ is detected and is attributed to Si-OH-AI. The small peak at 3724 cm~ and the broad band in the range of 3745-2900 cm 4 are caused by isolated internal silanols and the hydrogen bonds formed by the internal silanols respectively. The introduction of CH3CI at room temperature brings the same-CH3 stretching and bending vibration bands (Figures 3Ab and 3Bb) as in Y and Beta zeolites. The broad band at 3203 cm 1 in the 1R spectrum of HZSM-5 is the framework hydroxyl (3612 cm 1) shifted ,
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toward lower B r0.5 A 0.5 ~,~ wavenumber e,,l ~ .-. ~,I because of its t~ O ' ~ " ' t',,I e~ ~..~ ~!" interaction with , . ,o ~ ~ I1__~. ,,, . ~ J/i CH3CI molecules. On NaZSM-5, the same kind of _ I1 ~ --~ interaction t'.. ~.O e ~ ,"X~ e ~ I I [--t".. , ,., ,...4 i between CH3CI and external silanols induces a shit~ of the silanols to 3653 , ~ _lkAY cm -1 After heating for 15 minutes at 400~ (Figures 3Ac and 4000 3500 3000 2500 2000 4000 3500 3000 2500 2000 1500 3Bc), vibrations Wavenumbers (cm -~) Wavenumbers (cm-]) caracteristic of Fig. 3. Adsorption and conversion of CH3CI o v e r A: HZSM-5, B hydrocarbons (NaZSM-5, a: Activated, b: CH3C1 at RT, c: After heating 15 min. at CH2- strecthings 400~ d: After 30 min. of desorption at RT. and bendings, C=C stretchings and deformation) are observed on the two zeolites. For HZSM-5, the broad band centered at 3205 cm ~ is so important that it hides other interaction bands. This indicates that a big quantity of CH3Cl remains unreacted. For NaZSM-5, several broad bands in the range of 3200-3700 cm -1 indicates that olefins seem to interact with OH groups in the zeolites. This shows that, as for NaY and NaBeta, a dehydro-halogenation occurs on NaZSM-5 during the conversion and that new OH groups are created. The desorption of species adsorbed on HZSM-5 at room temperature (Figures 3Ad and 3Bd) results in an important decrease of the broad band, indicating the removal of unreacted CH3C1. While on both zeolites, hydrocarbons peaks are still observable, indicating that some products are strongly adsorbed on the framework, as in the case of the Beta structure.
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3.2. Catalytic testing Conversion rate and selectivity results are given in Table 2. The effect of reaction temperature on the catalytic conversion was first studied. It is found that both on protonic and cationic zeolites, the higher reaction temperature favors the conversion. At each temperature, for a given zeolite structure, protonic and cationic forms give the similar conversion rate of chloromethane. It is observed that in the studied temperature range, C2 to C6 hydrocarbons such as ethylene, propane and butane isomers are the major products both on protonic and cationic zeolites. Catalyst aging tests show that cationic zeolites and the large pore zeolites are deactivated more quickly than their protonic forms and the zeolites containing only medium pores. However, the selectivity of the products seems not to be affected at least not significantly by aging, zeolite form and reaction temperature. Y zeolite, whatever it is in protonic or Na form, gives lower conversion rate and higher deactivation. Furthermore, an important quantity of undesired methane is produced. The large pore Beta zeolite favors C4 products, principally isobutane. While the medium pore
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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ZSM-5 seems to favor lighter products such as ethylene and propane. Both protonic and Na forms show good conversion rate, but the K and Cs forms (which are more basic since the framework is more negatively charged) give lower activity. This is probably due to the size and Lewis acidity of the counter ion, which interact more weakly with the chlorine atom during the conversion. This interaction should be more efficient with proton and sodium. For all the zeolites, ethyl chloride is found, this is because of a reaction between ethylene, one of the main products, and hydrogen chloride, produced by elimination from CH3CI as observed by m-situ FTIR, or between a CH3CI and methyl group as intermediate of reaction. Table 2 : Products distribution from chloromethane conversion over Y, Beta and ZSM-5 under protonic and cationic form (400~ WHSV = 0.76 h-l). Zeolite Total Products distribution (mol. %) conversion (mol. %) CH4 C2H4 C 3 H 8 C 4 H 8 iC4H10 nC4H10 C2HsCI HY 34.7 19.7 40.8 24.8 2.1 10.2 0.8 1.6 NaY 42.0 28.6 31.8 24.6 3.0 9.2 0.5 2.3 HBeta 45.4 15.9 31.6 16.5 1.7 29.1 3.7 1.5 NaBeta 54.4 4.3 24.1 26.5 2.8 36.4 3.6 2.3 HZSM-5 49.3 5.1 39.0 37.3 4.6 6.1 3.7 4.2 Na, HZSM-5 56.2 4.7 35.2 47.0 6.0 1.4 0.9 4.8 NaZSM-5 55.8 3.2 38.2 40.7 5.0 4.6 3.6 4.7 KZSM-5 26.6 6.7 13.2 61.7 13.4 0 0 5.0 CsZSM-5 20.6 2.3 5.7 3.4 88.6 0 0 0 4. CONCLUSION The present work shows that both protonic and cationic zeolites could be the efficient catalysts for the direct conversion of chloromethane to higher hydrocarbons such as ethylene, propane, butane isomers and aromatics. However, zeolites exchanged with Na cations are more active than the corresponding protonic form. The acido-basic and structural properties of zeolites infuence strongly the conversion and aging of catalysts. On the basis of the above results, a reaction mechanism implying an initial fragment of chloromethane by a dehydrohalogenation both on protonic and cationic zeolites is proposed. In protonic zeolites, the Bronsted acid sites are active sites, whereas a cooperative role of counter-ions and the framework oxygen atoms is responsible for the conversion of chloromethane in cationic zeolites. REFERENCES [ 1] G. A. Olah, B. Gupta, M. Farina, J. D. Felberg, J. Am. Chem. Soc., 107 (1985), 7097 [2] B. L. Su, D. Jaumain, K. Ngalula, M. Briend, Proceedings of 12th IZC, MRS (1999) 2681 [3] X. R. Xia, Y. L. Bi, T. H. Wu, Catalysis Lett., 33 (1995), 75 [4] B. L. Su and D. Jaumain, Book of extended abstracts, International Symposium on Zeolites and Microporous Crystals, ZMPC'97, p 198 [5] D. K. Murray, J. W. Chang, J. F. Haw, J. Am. Chem. Soc., 115 (1993) 4732
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Catalytic conversion of 2-chloropropane in oxidizing conditions: a FT-IR and flow reactor study. C. Pistarino a, F. Brichese a, E. Finocchio a, G. Romezzano a, R. Di Felice a, M. Baldi b and G. Busca a a Dipartimento di Ingegneria Chimica e di Processo, Universit/l di Genova, P.le J.F. Kennedy, I-16129 Genova, Italy. Fax: +39-010-3536028, e-mail:
[email protected] b Dipartimento di Ingegneria Idraulica e Ambientale, Universit~ di Pavia, via Ferrata 1, 1-27100 Pavia, Italy. Fax +39-382-505589. The conversion of 2-chloropropane (2CP) in the presence of oxygen has been investigated over a number of oxide catalysts. Mn oxide and Mn containing oxides are deactivated as combustion catalysts by chlorine, but are active in converting 2CP into propene and HC1. VWTi SCR catalysts are also very active in converting 2CP into propene, and, if vanadium content is relatively high, they also catalyse the burning of the resulting propene into CO and CO2. Acid catalysts such as alumina, silica-alumina and ZSM5 zeolite also catalyse the dehydrochlorination of CP to propene. However, HZSM5 is rapidly deactivated by coking. On silica alumina, dehydrochlorination occurs selectively at low temperature. FT-IR data and preliminary kinetic studies allowed to propose a reaction mechanism where an irreversible and fast conversion of 2CP into 2-propoxides is a key reaction step. 1. INTRODUCTION Chlorinated hydrocarbons have a large number of industrial applications as chemical intermediates, solvents, monomers, pest control agents. Problems arise from their disposal and their abatement from waste gases: catalytic oxidation can be a good solution for their treatment [ 1]. Noble metal based catalysts are known to be very active in the catalytic incineration of chlorinated volatile organic compounds [2], but they can also be easily deactivated by chlorine species. Alumina-chromia based catalysts, very active in catalytic incineration [3], represent themselves not environmentally friendly materials due to their chromium content. In the present paper we summarize some results of a study undertaken to test the activity of pure and supported metal oxide based catalysts in the conversion of chlorinated hydrocarbons in oxidizing atmospheres. 2-Chloropropane (2CP) has been chosen as a simple partially chlorinated model reactant. FT-IR surface studies, GC-MS analysis of reaction by-products and preliminary kinetic experiments were used to determine the reaction path.
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2. EXPERIMENTAL
The catalysts studied are reported in table I. Manganese, vanadium and tungsten based catalysts were prepared by conventional coprecipitation-impregnation procedures. SCR and acid catalysts are commercial samples. Table I. Characteristic of catalyst. Catalysts ~r1304
Composition -
Surface (m2/g)
area
24
Hausmannite
Mn-W-A1203
Mn304-WO3/A1203 =
144
MnWO4 on y-A1203
WO3-A1203 CuC12-A1203 A1203 Mn-A1203 H-ZSM-5
10.5: 30.5:59 (wt/wt) 26.5% WO3, 73.5% A1203 9% Cu/A1203 (wt/wt) Atomic ratio Mn:A1=1:1. Si:AI=30:1
147 117 225 106 400
amorphous 7-A1203 7-A1203 (from STREM) ct-Mn3Oa/~-Mn203/7-A1203 ZSM-5 zeolite (from Zeolyst)
SIO2-A1203
87% SIO2, 13% A1203
330
SCR1
VEOflWO3/TiO2
70
Amorphous silica-allumina (from STREM) Commercial SCR catalyst
SCR2
0.51/9.93/89.56 V2Os/W'O3/TiO2 3.7/9.3/87
47
Commercial SCR catalyst
2CP / oxygen / helium gaseous mixtures were taken from commercial cylinders from SIAD; liquid 2-chloropropane was from Aldrich. Catalytic tests were carried out at atmospheric pressure in a continuous flow fixed bed tubular glass reactor. 0.1 to 0.5 g of catalyst were loaded in form of fine powder (particle size 60 + 70 mesh) mechanically mixed with 0.4 g of inert, low surface area material (quartz). The total gas flow varied from 300 to 370 ml / min and the feed composition presented always a large excess of oxygen and a 2CP content from 0.05 % to 3.5 % in He. The reactants and the reaction products were analysed using two on-line gas chromatographs (HP 5890). Permanent gases (02, CO and CO2) were separated using a Carbosieve S-II (Supelco) packed column connected to a TCD detector. Other reaction products, as well as the organic reactant and by-products, were analysed employing a wide-bore VOCOL column (Supelco), connected to a FID detector and a GC-MS instrument equipped with a HP-VOC capillary column. CI ions together with hypochlorite ions have been quantified by mean of ionic chromatography, after adsorption of the effluent from the reactor in NaOH solution. This analysis showed that C1/was never produced in detectable amounts. The IR spectra were recorded by a Nicolet Magna 750 FT-IR instrument. The adsorption and oxidation experiments were performed using pressed disks of the pure powders activated by outgassing at 300-1070 K.
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3. RESULTS
3.1. Transition metal oxide oxidation catalysts Catalysts very active in the total oxidation of hydrocarbons and oxygenates [4], such as pure and alumina supported Mn oxide, do not give rise to oxidative conversion of 2CP to COx, due to the almost irreversible poisoning of the catalyst surface by HC1 and / or by the chlorinated hydrocarbon itself. However, the conversion of 2CP can be complete, producing
2CP with s t e a m
over
2CP with s t e a m
Mn304
100
/
80 60 40 20 0 300
over
0,5 % V205 --r O2(C) t -m-- MCP(C) i -o--C3H6(S) - e - CO2(S) .-,a- CO(S)
100
400
N
8(1
-I-- C3H6IS) ~ N co2 (S) 40
I
/
I
f [
0
500
600
700
T (K)
Fig. 1.2CP conversion and product selectivities over Mn304 catalyst
800
300
400
500
600
700
800
T (K)
Fig 2. 2CP conversion and product selectivities over 0.5 % V 2 0 5 SCR catalyst
exclusively propene. In presence of steam (Fig. 1), propene selectivity is a little lower (95 %), with additional production of COE. The supported Mn oxide and the MnWAI catalyst show the same dehydrochlorination activity, the latter at even lower temperature (500 K). For these oxidation catalysts the GC-MS analysis shows the presence of several different chlorinated organic by-products in traces in the effluent from the reactor. The alumina-supported copper chloride catalyst totally converts 2CP near 550 K with non negligible selectivity to CO2 (near 20%). Very high selectivity to propene is observed at low temperatures but it decreases at higher temperatures, due to the formation of highly chlorinated hydrocarbons. The GC-MS analysis shows the presence of several chlorided hydrocarbons (monochloropropenes, dichloropropanes, dichloropropenes and halogenated C 1).
3.2. SCR-type catalysts The conversion of 2CP has been studied over two V205-WO3-TiO2commercial SCR catalysts [5]. These materials differ for the amount of vanadium oxide component and for their application. The sample with 0.5 % V205 is applied to the treatment of waste gases of power plants through the SCR process, while the sample with 3.7 % V205 is applied to the treatment of waste gases from urban solid waste incinerators. In Fig. 2 is shown the behavior of the 0.5 % V2Os-WO3-TiO2 in presence of steam. At very low temperature, namely 430 K, 2CP is already totally converted, propene being the only product. However, by increasing temperature, propene bums to a mixture of CO and CO2 and oxygen is consumed in parallel. A similar behavior is observed even without steam and for the sample with the higher V content, but oxidation is even faster in this case. This shows that the V2Os-WO3-TiO2 SCR catalysts are very active in the dehydrochlorination of 2CP to propene + HC1, but they still
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display oxidation catalytic activity, allowing the burning of propene. They are consequently not deactivated (at a laboratory measurement scale) by chlorine.
3.3. Acid catalysts The tested acid catalysts (SIO2/A1203, WO3/A1203, 7-A1203 and ZSM5) show high catalytic activity for 2CP dehydrochlorination to propene. Pure Al203 and silica-alumina (Fig. 3) convert chloropropane to propene with selectivity near 100% at temperatures near 400 K and almost no chlorinated by-products. Over WO3/A1203 catalyst (Fig. 4) higher hydrocarbons are mostly formed as byproducts. 2CP overSiO2-A1203
2CP over W-Al2Oa
100
8 ~9
613
m
40
.~ 20 0 3oo
400
100
/
1
5OO
- - - " -
- '-;- - - ~
..--n
8o
l+2cPcc) l ..... c 3 ~ s )
60
700
800
T (K)
Fig. 3.2CP conversion and product selectivities over SIO2/A1203 catalyst.
I+ 2CP (C) I - m - C3H6(S)
4O
0 600
..................F ..................... ' - - - .
I
r
:300
400
500
600
700
800
T (K)
Fig. 4. 2CP conversion and product selectivities over WO3/A1203 catalyst.
Over ZSM5 zeolite 2CP is almost totally converted above 650 K. Propene is the predominant product (selectivity near 60%) but a number of heavy byproducts (mainly nonhalided hydrocarbons) are formed. An analysis of the catalyst IR spectrum shows the deep coking of the sample, with the formation of strong bands in the region 1600-1300 cm -1. 3.4. 1-chloropropane versus 2-chloropropane conversion. Over several of the cited catalysts, the conversion of the isomer 1-chloropopane has also been investigated. In all cases the reaction products from the two isomers are the same and the selectivities are very similar. On the other hand, the conversion of 1-chloropropane is always slower than that of 2CP. This can be interpreted below on the basis of the kinetic and mechanistic studies. 3.5. Kinetic consideration and FT-IR mechanistic studies. In spite of the oxidizing atmosphere we used for our experiments, the dehydrochlorination elimination reaction appears to be the predominant one in all cases, at least at low temperature. The analysis of the thermodynamics of the dehydrochlorination reaction indicates that, in our experimental conditions, the equilibrium is almost totally shifted towards propene + HC1. Preliminary kinetic evaluations have been performed on the silica-alumina catalyst, that appeared to be the most active and the most selective to propene in the dehydrochlorination step. Experiments performed by varying the 2CP concentration in the range 0.1% - 3.5 % at constant contact time (x = 1,07 + 0.02 goat sec / mol2cp) do not evidence significant changes in the reaction rate at a given temperature. This supports that the 2CP reaction order is zero in our conditions. Activation energy has also been evaluated by the help of the Arrhenius plots of different experiments and its average value was found to be 14.5 kcal/mol (60.6 kJ/mol).
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FT-IR studies have been performed on Mn oxide, on alumina and on silica-alumina. In all cases it has been shown that molecular adsorption of the reactant occurs reversibly. However, a nucleophilic substitution by oxide ions over the chlorine atom is evident giving rise to 2propoxy groups. These species are strongly adsorbed, but evolve giving rise to gas-phase propene. Independently, HC1 desorption is also found, thus closing a catalytic cycle. 3, 0] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2,52 2,0-:
1.5-
i8 :~
1,0! o, 5: o,o"
~is _o.~!
i
C
i
"
'
i
b
2, 2, 1500
1400
1300
1200
..................................................................................... W.a.venum...b.er.s...!..cm.:.l.!........................................................................ Fig. 5. FT-IR spectra of the surface species arising from: 2CP adsorption over A1203 in the presence of the gas (a), after outgassing at room temperature (b), IPA adsorption over the same surface (c). In Fig. 5 spectra of the adsorbed species over alumina surface are reported: bands due to isopropoxy species are detectable in the region 1500-1350 and 1100-1200 cm -1, due to CH bending modes and CC/CO stretching modes respectively. Isopropoxy groups arise both from 2CP and isopropanol (IPA) adsorption already at room temperature. The kinetics of the reaction can be rationalized in terms of the following mechanism:
H3C\
H/C\c1
H
H3% FH3 D
0
HoC\~I
,C--CH2 HC1
H3% / C H 3 H/C\o -
[ el
_
HO cr
J
o
The irreversible fast conversion of adsorbed 2CP into 2-propoxides results in the saturation of all active sites constituted by a Lewis acidic center (allowing the coordination of the chlorine atom of 2CP) and a nucleophilic oxygen atom (responsible for the nucleophilic substitution). The slow step consists in the evolution of propene from 2-propoxy-groups, or, alternatively, in the desorption of HC1. The saturation of the active sites justifies the zero order measured for the reaction.
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4. CONCLUSION
Among the studied acidic catalysts, silica-alumina shows the best dehydrochlorination activity around 400 K, and the best selectivity, propene being the only relevant reaction product. In our condition the reaction is zero order with respect to 2CP. On the other hand, the conversion of 2CP is faster than that of its isomer 1-chloropropane. FT.-IR studies show that the molecular adsorption of 2CP (and in general of chlorocarbons) is relatively weak and reversible. However, a nucleophilic substitution occurs giving rise to strongly adsorbed alkoxide species (2-propoxides from 2CP). This species is, on silica alumina, stable up to 400 K where it disappears giving rise to gaseous propene. Thus, the zero order kinetics and the faster conversion of 2CP with respect to 1CP agree assuming that the rate determining step is the decomposition of the strongly adsorbed alkoxides with an E1 mechanism, i.e. through a carbenium ion species. The average activation energy of 2-CP measured over silica-alumina, 14.5 kcal/mol (60.6 kJ/mol), appears to be consistent with this picture and confirms that our experiments have been performed in a chemical kinetic regime. Pure and alumina supported Mn and Mn/W oxides are very likely poisoned by HC1 and still display good dehydrochlorination activity in spite of a loss in their oxidative activity. SCR type catalysts are also active in 2CP dehydrochlorination but they still show oxidation activity (depending on the V205 content) giving rise to COx at high reaction temperatures. REFERENCES
[1] J. N. Armor (ed.), Environmental Catalysis, ACS Symposium Series, Washington, DC 1994 [2] Y. Wang, H. Shaw and R. J. Farrauto in Catalytic Control of Air Pollution Mobile and Stationary Sources, R. G. Silver et al. (eds.), ACS Symposium Series 495, Washington, DC 1992, p.125 [3] S. K. Agarwal, J. J. Spivey, G. B. Howe, J. B. Butt and E. Marchand, Catalyst Deactivation 199 i, C. H. Bartholomew and J.B.Butt (eds.), Elsevier, Amsterdam, 1991 [4] M. Baldi, E. Finocchio, F. Milella and G. Busca, Appl. Catal. B: Environmental, 16 (1998) 43-51 [5] E. Finocchio, M. Baldi, G. Busca, C. Pistarino, G. Romezzano, F. Bregani and G.P. Toledo, Catal. Today, in press
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Catalytic Degradation of Polychlorinated Biphenyls at Low Temperature Dong-Keun Lee, Eun-Suk Byun, In-Cheol Cho t and Sung-Woo Kim'* Department of Chemical Engineering/Environmental Protection, Research Institute of Environmental Protection, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyongnam 660-701, Korea The chemical degradation of PCBs in water was carried out at 90~ in the presence of methanol, 4,4'-Dipyridyl, Fe 3§ and H202. The PCB mixtures could successfully be transformed into CO2 and CI, and the degradation reaction was suggested to occur with hydroxyl radical(" OH) . The hydroxyl radical was believed to be formed from the reaction of H202 with Fe 2§ which had been reduced from Fe 3§ by the charge transfer between methanol and 4,4'-Dipyridyl. The hydroxyl radical-induced PCBs degradation was composed of two sequential steps, that is, the dechlorination and the oxidation of the dechlorinated intermediates into carbon dioxide. The produced dechlorinated intermediates were proposed to be oxidized completely and very quickly. 1. INTRODUCTION Polychlorinated biphenyls(PCBs) are a family of compounds produced commercially by the direct chlorination of biphenyl, with 209 different PCB congeners possible(I). Because of their excellent flame resistance, electrical properties, and chemical stability, PCBs have been used worldwide as heat-transfer fluids, hydraulic fluids, solvent extenders, plasticizers, flame retardants, organic diluents, and dielectric fluids(l). Such an extensive application of these chemically and thermally stable compounds has resulted in widespread contamination(2,3). Due to potential adverse human health and environmental effects, the use of PCBs has ceased, and remediation methodology for existing PCB contamination has become of substantial interest(4,5). PCBs are principally being destroyed by incineration(6), which has become the most widely used technique for their removal(7). Incineration, however, often produces more toxic compounds if it is not carefully controlled. Erickson et al.(8), for example, indicated that
*Present address : Kyong Sang Nam-Do Provincial Government Institute of Health & Environment, Kyongnam, Korea. "Present address : Sam Hyeob Resource Development Co~, Ltd., Kyongnam, Korea. * This project was supported by Clean Technology Center at Seoul National University.
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polychlorinated dibenzofurans(PCDFs) and polychlorinated dibenzodioxins(PCDDs) were both observed in the combustion of PCBs. About 1% of the PCBs in the system was converted into PCDFs. Especially when PCBs are present at low concentration in aqueous media, incineration becomes inadequate for the treatment of PCBs. Other methods for the destruction of PCBs that have been proposed include wet air oxidation(9), biodegradation( 10-12), reaction with superoxide(13), photodechlorination(14), sodium metal-promoted dehalogenation(15), electrolytic reduction(16, 17), and dechlorination with zero-valent iron(18-20). In this study a chemical degradation of PCBs in water was carried out at low temperature(90 ~ in the presence of methanol, 4,4'-Dipyridyl, Fe 3+ and H202. Charge transfer between methanol and 4,4'-Dipyridyl was supposed to transform 02 into 02-which in turn would reduce Fe 3+ to Fe 2+. Hydrogen peroxide was then going to react with Fe 2§to produce hydroxyl radical(" OH) . The produced hydroxyl radical will participate in the oxidation of PCBs into CO2 and CI. 2. EXPERIMENTAL
The PCB mixtures(Aroclor 1242 and 1248) were purchased from Supelco. 4,4'Dipyridyl(98%), hydrogen peroxide(30wt.%), Fe2(SO4)3"5H20(97%), methanol(THM grade) and hexane(+99%) were supplied from Aldrich. The degradation of PCB mixtures was performed in a batch reactor. Twenty milligrams of PCBs, dissolved in 2mL methanol, were added to the reactor containing lmL 10mM 4,4'Dipyridyl in methanol solution. 950mL distilled deionized water was then introduced into the reactor. The mixture solution was stirred mechanically with a glassed blade and warmed to 90~ Additional 47mL distilled deionized water solution containing lmL H20 z and lmL 100mM Fez(SO4)3"5H20 was successively added at once to the mixture while being stirred. 10mL aliquots for different reaction times were withdrawn from the reactor. The samples were twice extracted with 10mL hexane. One ~ of the hexane layer was injected into GC(HP 5890) equipped with ECD detector and into GC/MS(HP 5972). The aqua layer was analyzed for chloride ion concentration with ion chromatography(Dionex 300 series) after diluting with deionized water. Carbon dioxide gas produced in the degradation reaction was introduced into 0.1N Ba(OH)2 solution with 50mL/min suction speed and its concentration was determined by an aqueous pH titration with 0.1N HC1 GC conditions for PCBs and other hexane extracts were as follows: column, HP-5 capillary column(30m • 0.20mm i.d.) with 0.32 llm film thickness; column temperature, 150~ for 1min isothermal, 3 ~ to 220 ~ 2 ~ 250 ~ 250 ~ isothermal for 10 min; carrier gas, nitrogen, 0.9mL/min. GC/MS conditions were almost the same as those of GC except the column and carrier gas. Instead of HP-5 and nitrogen, HP-1 and helium were used as the column and carrier gas. Ion chromatographic conditions for chloride ion were as follows: column, Dionex ionpec AS 14(13 tan, 25cm • 0.40cm i.d.) equipped with a 4cm • 0.40cm i.d. precolumn having AG 14(13/an); eluent, 3.5mM Na2CO3-1.0mM NaHCO3, flow-rate 1.3mL/min.
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3. RESULTS AND DISCUSSION
Figure 1 and 2 show the gas chromatograms of Aroclor 1242 and 1248 solutions before and after reaction at 90 ~ Aroclor 1242 and 1248 PCB mixtures were composed mainly of di-, tri-, tetra- and penta chlorinated congeners, and essemially all the PCB congeners disappeared after 1 h reaction. The result in Figure 1 is stmamarized in Figure 3 which shows the reaction occurring with Aroclor 1242 at different reaction times. The relative peak area recorded on the Y-axis is the ratio of the GC peak area of Aroclor 1242 before reaction and the peak area during reaction. Most of the dechlorination reactions took place within 20min. The total concentration of chloride ion produced after reaction for l h was 8.3 mg/L which is nearly the same as the theoretical value of 8.4 mg/L for the complete dechlorination of Aroclor 1242. When Aroclor 1248 was used, similar results were obtained. Since Aroclor 1242 and 1248 are the mixtures of congeners having differem number of chlorine atoms, the dechlorination rates of the congeners might be different to each other. The differem dechlorination rates will inevitably result in the differem composition of congeners. In Table 1 are listed the composition of congeners in Aroclor 1242 and 1248 solutions before and during reaction at 90 ~ The fraction of higher chlorine-comaining congeners increased with reaction time, which indicates that the lower chlorine-containing PCBs were more
I
a
Figure 1. Gas chromatograms of Aroclor 1242 solution. (a) before reaction. (b) reacted for 5min at 90 ~
Figure 2. Gas chromatograms of Aroclor 1248 solution. (a) before reaction (b) reacted for 5min at 90 ~
(c) reacted for 20min at 90 ~
(c) reacted for 20min at 90 ~
(d) reacted for l h at 90 ~
(d) reacted for l h at 90 ~
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efficiently dechlorinated. This result is consistent with that of Matsunaga et al.(13) who carried out superoxide radical-induced degradation of PCBs. Since an electron capture detector (ECD) of GC is not so effective for the detection of other products, for example hydrocarbons, except halogenated compounds, the solutions of Aroclor 1242 and 1248 were analyzed again with GC/MS. As shown in Figure 4 and Figure 5 for Aroclor 1242 and 1248 solution, no detectable compounds other than the PCB congeners were observable. The production of carbon dioxide gas a~er the reaction of Aroclor 1242 and 1248 solution for 1 h was recognized by the formation of the BaCO3 precipitate, and the amounts of the produced CO2 was about 46% of the theoretically produced CO: for the complete oxidation of PCBs. Although the reason why the concentration of carbon dioxide had not been coincided with the complete oxidation of PCBs is not clear, part of the produced CO2 might be dissolved in the aqueous solution of the reactor. The absence of dechlorinated products such as biphenyl and aliphatic compounds, and the production of chloride ion and carbon dioxide suggest tha~ the produced dechlorinated intermediates had been oxidized to carbon dioxide completely and very quickly. The dechlorination of PCBs and the oxidation of dechlorinated intermediates are believed to proceed under the action of hydroxyl radicals(" OH) which were produced via the steps described as follows : Methanol(MeOH) ~-
+
4,4'-Dipyridyl(DIP)
DIP"
+
02
DIP
- [ - O 2"
02
+
Fe 3+ ~
02
+
Fe 2+
+
H20 2 *~- Fe 3+ +
(MeOH) + +
DIP.
Fe2+ OH-
+
"OH
1.0
10
m 0.8
8~
L_ W
0.6
I ~ Chlorideion Aroc10r1242
ID
ne O.2
E. 0 0 c
Q.
~9 0.4
~,,
4.~ .e0
2"~ 0
0.0
v
0
20
time(rain)
40
60
Figure 3. Changes in Aroclor 1242 and chloride concentration ion with reaction time.
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Table 1. Product distribution of congeners in Aroclor 1242 and 1248 solution before and after reaction at 90"C PCBs
Reaction time(min) 0
1 Aroclor 1242
Aroclor 1248
3 5 10 20
C12 16 13 10
8 6 6 0 0 0 0 0
0 3 5 10 20
C12 = dichlorinated biphenyls, C14 = tetrachlorinated biphenyls,
Product distribution (wt%) C13 C14 41 41 40 41 38 48 35 53 33 57 31 58 37 55 33 58 30 61 29 61 27 62
C15 3 3 4
4 5 5 8 9 9 10 11
C13 = trichlorinated biphenyls C15 = pentachlorinated biphenyls
I
,
9
.t.l
.
1 .
.
.
t
. . . . .
a
_ . . . . . .
.,,,,4
t_
"o
time(min)
Ll, Llt..... time (min)
Figure 4. GC/MS Chromatograms of Aroclor 1242 solution. (a) before reaction. (b) reacted for 5min at 90 ~
Figure 5. GC/MS Chromatograms of Aroclor 1248 solution. (a) before reaction. (b) reacted for 5min at 90 ~
(c) reacted for 20min at 90 ~
(c) reacted for 20min at 90 ~
(d) reacted for lh at 90 ~
(d) reacted for l h at 90 ~
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4. CONCLUSION
The hydroxyl radical-induced degradation of the PCB mixtures could successfully be carried out at 90*(2. Most of the PCBs were dechlorinated and oxidized into CO2 and CI within 20 min. The dechlorinated intermediates were completely oxidized and their oxidation rates were very high. Hydroxyl radical was proposed to be formed from the reaction of H202 with Fe 2+ which had been reduced from Fe 3+ by the charge transfer between methanol and 4,4'-Dipyridyl. REFERENCES
1. Hutzinger, O., Safe, S. and Zitko, U, The Chemistry of PCBs, CRC Press, Cleveland, OH, 1974. 2. Jensen, S., New Sci., 32(1996)612. 3. Buckley, E.H., Science, 216(1982)520. 4. Waid, J.S., PCBs and the Environment, CRC Press, Boca Raton, FL, 1986 5. D'ltri, F.M. and Kamrin, M.A., PCBs : Human and Environmental hazards, Butterworth Publishers, Boston, 1983. 6. Wentz, C.A., Hazardous Waste Management; Clark, B.J., Morriss, J.M., Eds.l; McGrawHill Chemical Engineering Series; McGraw-Hill, New York, 1989. 7. Hawarl, J., Demeter, A. and Samson, R., Environ. Sci. Technol., 26(1992) 2002. 8. Erickson, M.D., Swanson, S.E., Flora Jr., J.D., and Hinshaw, G.D., Environ. Sci. Technol., 23(1989)462. 9. Randall, T.L. and Knopp, P.V., J. Water Pollut. Control Fed., 52(1980)2117. 10. Thomas, D.R., Carswell, K.S. and Georgiou, G., Biotechnol. Bioeng., 40(1992)1395. 11. Williams, W.A. and May, R.J., Environ. Sci. Technol., 31(1997) 3491. 12. Commandeur, L.C.M., van Eyseren, H.E., Opmeer, M.R., Govers, H.A.J. and Parsons, J.R., Environ. Sci. Technol., 29(1995) 3038. 13. Matsunaga, K., Imanaka, M., Kenmotsu, K., Oda, J., Hino, S., Kadota, M., Fujiwara, H. and Mori, T., Bull. Environ. Contam. Toxicol., 46(1991) 292. 14. Yao, Y., Kakimoto, K., Ogawa, H.I., Kato, Y., Hanada, Y., Shinohara, R. and Yoshino, E., Bull. Environ. Contam. Toxicol., 59(1997) 238. 15. Weitzman, L., Treatment and Destruction of Polychlorinated Biphenyls and Polychlorinated Biphenyl-Contaminated Materials: in Exner, J.H.(ed.), Detoxification of Hazardous Waste, Ann Arbor Science, Ann Arbor, 1982. 16. Zhang, S. and Rusling, J.F., Environ. Sci. Technol., 27(1993)1375. 17. Sugimoto, H., Matsumoto, S. and Sawyer, D.T., Environ. Sci. Technol., 22(1988) 1182. 18. Liu, Y., Schwartz, J. and Cavallaro, C.L., Environ. Sci. Technol., 29(1995) 836. 19. Wang, C-. B. and Zhang, W-. X., Environ. Sci. Technol., 31 (1997) 2154. 20. Grittini, C., Malcomson, M., Fernando, Q. and Korte, N., Environ. Sci. Technol., 29(1995) 2898.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Microwave Effects in Exhaust Catalysis Michael Turner a, Robert L. Laurence a, K. Sigfrid Yngvesson b and W. Curtis Conner a aDept. Chemical and bElectrical Engineering University of Massachusetts Amherst, Massachusetts 01003 USA* Many attempts have been made to employ electromagnetic energy as a selective coreagent in catalytic reactions. Photocatalysis employs visible light for a variety of oxidation reactions on a very limited series of catalysts, those employing TiO2. In contrast, attempts to excite specific atom-atom bonds of adsorbed species in the infrared by vibrational resonance have not proved to break the specific bonds selectively. The differences are significant. Many recent studies have suggested that microwave energy can be employed in catalysis and that the results differ from "conventional" heating of the systems studied. Although "photocatalysis" is well accepted, "microwave-catalysis" has not here-to-fore been accepted as an approach to change the selectivity or efficiency of catalysis in the presence of microwave energy. We have studied the influence of microwave energy on sorption and catalysis, particularly on automotive exhaust catalysis. The development of a new generation of automotive exhaust catalysts faces several significant challenges, which might be overcome by the use of microwave energy. The first challenge is to shorten the time required for catalyst "light-off," since a disproportionate fraction of the pollutants are produced as the catalyst is heated up to operating temperature. The second challenge is the influences of sulfur contaminants that impede the catalytic activity, particularly during light,off. Based on preliminary experiments, we propose that indeed microwave energy can induce catalyst light-off more efficiently than conventional heating and can reverse the poisoning by SO2 for a commercial three-way catalyst. 1. BACKGROUND Numerous studies have demonstrated that the sulfur present in gasoline decreases the effectiveness of catalyst for the control of contaminants (hydrocarbons, CO and NOX) in auto exhaust (1-4). The current "state-of-the-art" catalysts used in the control of vehicle emission will need to be modified or replaced to meet the future requirements. It is generally believed that oxides of sulfur are the dominant forms of sulfur present in auto exhaust. These species will adsorb on the catalyst surface to form sulfates and sulfides by reaction with surface species. Sulfur is perceived to react with the noble metals to form sulfides and with the ceria to form sulfates. Ideally, microwave energy alone might decompose the *This research was sponsoredby the CoordinatingResearch Council, CRC, and the National Science Foundation division of CRP, engineering.
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sulfides and sulfates on the surface. These are the most polarizable bonds in the entire system and would be most receptive to absorb microwave energy.
1.1 Sulfur Poisoning of Exhaust Catalysts Although there are several mechanisms by which sulfur (and other "poisons") may inhibit catalytic activity(5), the effect of sulfur containing contaminants on auto exhaust catalysts is reversible (1). The reversibility also suggests that the influence of sulfur involves reaction between the sulfur containing compounds present in automotive exhaust and the catalyst surface.., possibly involving other species present in the exhaust. All proposed influences of sulfur on automotive exhaust catalyst involve surface reactions between sulfur oxides (primarily $02) and the active components. Sulfur is perceived to react with noble metals to form sulfides. These metal sulfides are not (equally) catalytically active for the required reaction of the CO, NOx and unburned hydrocarbons present in auto exhaust. The mechanism by which metal sulfides are formed from SO2 and a surface metal (Pt is employed as an example) in the presence of a reductant (atomic hydrogen, as Hs, is employed for illustration) is shown below: Pt + SO2 + 4Hs-> PtS + 2 H20 (1) Similar mechanisms can be written for the formation of PdS or RhS and/or involving CO or other reducing species. The point is that the active metals can form sulfides which are less active (or are probably totally inactive). Ceria is a crucial component in the most active, state-of-the-art, automotive catalysts. Ceria (as CeO2 or Ce203 ... or mildly defected forms) is believed to be required to store and release oxygen during catalytic cycles. SO2 (or SO3 formed by in situ oxidation of SO/) is commonly understood to react with ceria to form cerium sulfate or oxysulfate on the surface. This is illustrated below for the formation of cerium sulfate from SO2 and ceria, as CeO2 ; although, similar mechanisms exist for the formation of sulfates and sulfites from Ce203 or defected ceria, viz.: (n+m)CeO2 + (2n+m)SO2 + (2n/x+m/x)O x -> nCe(SO4)2 + mCe0SO 4
(2)
The formation of cerium sulfates (or sulfites) impedes their ability to promote the catalytic activity. Indeed, the formation of cerium sulfates and sulfites are commonly believed to be the primary mechanism by which overall catalytic activity is reduced due to exposure to sulfur (as SOx). Several investigators(I,2) have also suggested that SO2 (or SO3 formed by in situ oxidation of SO2) can react with the alumina support to form aluminum sulfate or oxysulfates, e.g.: A1203 + x SO 2 -I-Ox-> A1203_x (SO4)x
(3)
These mechanisms [1-3] may not be precisely those which exist on the surface of active exhaust catalyst to result in loss in activity; however, it is certain that the oxides of sulfur present in exhaust will react with the surface of the catalyst forming sulfides, sulfates and sulfites.
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1.2 Microwave Radiation in Sulfur Catalysis Most recently, Deng and Lin have showed that microwave radiation can be employed to prepare Cu dispersed on dealuminated Y zeolite (6) for SO2 removal from a process effluent and its eventual decomposition. U. S. Patent 4,322,394 (1982) describes a process of rapid regeneration of non-carbon adsorbents by the use of microwave heating. Their examples involve the separation of CO2 and H2S (from >20% to 98%) when they are first adsorbed onto activated (pyrolitic) carbon and exposed to microwave radiation (2.45 GHz). The products are elemental sulfur, N2 and 02.
1.3 Regeneration of Catalytic Activity by Microwave Energy Regeneration of catalytic activity due to the "poisoning" would involve reactions by which the sulfur would be removed from the noble metals (decomposing the sulfides) and/or from the ceria (or alumina) decomposing the sulfates or sulfites. Obviously these surface species are stable under normal (periodic) operating conditions; otherwise, there would be no long term loss of activity. Conventional surface chemical regeneration involves higher temperatures and specific treatments with oxidizing/reducing gases. The higher temperatures are required to provide the conditions for favorable reaction kinetics and equilibria for the decomposition of the surface species which have poisoned the reaction. The whole system [bulk as well as surface] must be heated to the temperature required for the decomposition of the compounds responsible for poisoning (loss of catalytic activity). The catalyst must then be exposed to highly reducing or oxidizing environments; often in a specific sequence. We have shown that microwave energy can be absorbed selectively on the surface of oxide supported catalysts (9, 10). The result is a rapid, efficient, selective heating of the surface. As a consequence, desorption can be induced without the necessity of heating the whole system to the same temperature. Potentially, catalytic reactions at the surface can be induced by less energy while the bulk is still at a lower temperature, such as during light-off. 2. EXPERIMENTAL We have studied the influence of microwave energy on a commercial, state-of-theart, three-way catalyst (Englehard Corp.). Continuous microwave energy at different power levels and 2.45 GHz was employed in a flow system with a mass spectrometer to monitor the effluent (10). A fiber optic probe was employed to measure the temperature in the monolith and in the gas phase before and after the bed. The feed contained 650ppm C3H8
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and 6000ppm CO with oxygen above and below stoichiometric. 802 was introduced at -~200ppm. The space velocity of these studies was 20,000. A tube furnace upstream from the catalyst allowed us to raise the temperature of the gas entering the 5cm long x 18mm diameter monolith. The monolith was enclosed in a glass tube, which passed through a microwave waveguide perpendicular to the short axis. Metal chokes (short grounded metal tubes) enclosed the tubular glass reactor and prevented the leaking of microwave radiation outside the waveguide. The electric field inside the waveguide is in the TEl0 mode at 2.45GHz. This system allowed us to Study the light-off of the catalyst as the temperature was increased with and without the presence of microwave energy exposed to the catalyst. Further, the influence of SO2 on the catalytic activity can be measured with and without exposure to microwave energy. 3. RESULTS AND DISCUSSION Two types of experiments were performed. In the first, the light-off of commercial auto-exhaust catalysts was studied by conventional heating compared to heating of the catalyst augmented by microwave radiation. In the second set of experiments, conventional heating was employed to initiate activity. Subsequently, sulfur dioxide was introduced to suppress the catalytic activity. Finally, microwave radiation was employed to determine its influence on the dynamic poisoning.
3.1 Light-off Augmented by Microwave Energy In all cases studied, the time required to initiate catalytic activity was significantly decreased when microwave energy was employed during the initial light-off of the catalyst. Further, the reaction appeared to light-off at a lower temperature (measured in the effluent gas) than measured for conventional increased heating of the inlet reactant stream. We do not believe that this means that microwave radiation is specifically promoting the catalytic reaction(s), but that the surface (the active phase) is able to more effectively absorb microwave energy than the bulk of the catalyst. There was no way to monitor the "effective" surface temperature which, we propose, was higher than for conventional heating alone. We propose that the selective microwave heating of the surface results in a non-isothermal system. The catalytic activity reflects the surface temperature (not measured) while the transfer of heat to the support and gas phase was too slow to achieve thermal equilibrium. Thus, the exposure of microwave energy to a system that comprises a strongly absorbing (surface) on a weakly absorbing (A1203) support can result is a significant temperature difference. Thus, the catalytic activity will not be characteristic of the "bulk" or gas-phase temperatures. Further, less energy (and time) will be required to initiate the catalytic activity since it is not necessary to heat the whole system to the effective reaction temperature.
3.2 Reversal of Sulfur Poisoning by Microwave Energy As the temperature of inlet gas was increased by conventional means, the conversion of CO (m/e = 28) and the increase in CO2(m/e = 44) was evident as the outlet temperature increased above 135~ When SO2 was introduced, the conversion decreased significantly as the CO increased in the effluent while the CO2 decreased. The SO2 poisoned the CO
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oxidation at light-off and the influence increased with time of exposure to CO. When 300W of microwave energy was then turned on, the CO conversion increased to 80+% of that without SO2 present and the conversion remained stable. The microwave energy was able to reverse the influence of the SO2 to poison the catalysis. However, only a fraction of the microwave energy was adsorbed in this experiment, most of the energy passed through the wave guide in this configuration. Again, however, there is a problem in measuring and understanding the influence of surface temperatures in these experiments. The initial experiments (above) suggest that we cannot measure the "effective" temperature directly. 4. CONCLUSIONS
We conclude (propose) that microwave energy is selectively absorbed by the surface/adsorbed-layer of the three-way catalyst. The catalyst lights off at lower temperatures than if the whole catalyst were heated by conventional means. The "effective" temperature is very local and the rate of heat transfer is not as rapid as desorption/reaction. Thus, the "effective" surface temperature is higher than the support or gas-phase temperatures. We cannot measure the local "effective" temperature. These studies give considerable insight into the potential influence of microwave energy on heterogeneous catalytic reactions, a "microwave effect". Specifically, the influence will be greatest if the energy can be directed to the surface, i.e., the bulk is transparent to microwave energy. The concentration of energy due to microwave energy on the surface will depend on the nature of the surface (hydroxyls, etc.), active sites (metals), and the specific adsorbing (reacting or desorbing) species. It is also important that the reaction, or desorption, can occur prior to thermal equilibration; otherwise, there will be little difference for microwave induced sorption/reaction compared to conventional heating. These preliminary experiments suggest that microwave energy can be employed in auto exhaust to decrease the time required for light-off and, thus, can significantly decrease the overall emissions. Further, microwave radiation may be effective in reversing the poisoning of catalytic activity due to SO2 in the exhaust. More studies are needed to document the effects and to optimize the use of microwave energy. Microwave energy can be controlled more precisely than conventional methods of heating. The heating can be instantaneously increased, decreased or pulsed. This control permits unique possibilities to optimize its use for auto exhaust control. Further, we are faced with an interesting challenge! How can we separate the influence of temperature on the catalytic activity when we conclude that the system is not isothermal, i.e., we infer that exposure of a catalytic system to microwave radiation can result in significant temperature differences within the system. REFERENCES
1. CRC Auto/Oil Symposium, 11-12 September 1997, Dearbom, MI. 2. R. Heck and R. J. Ferrauto, Catalytic Air Pollution Control, Van Rostram Reinhold, pbs., New York, 1995 3. Gandhi, H. and Shelef, M., "Effects of Sulfur on Noble Metal Automotive Catalysts", Applied Catal. 77, 175-86 (1991)
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4. Ferrauto, R. and Wedding, B. "Poisoning by Sulfur Oxides of Some Base Metal Auto Exhaust Catalysts", J. Catal. 33,249-55 (1974) 5. W. Conner, J.R. Kittrell and J.W. Eldridge, "Deactivation of Stationary Source Air Emission Control Catalysts" A Chapter in Catalysis volume 9, J. J. Spivey, editor, The Royal Society pbs., Ch. 3, p. 126-182 (1992) 6. Deng, S., Lin, Y., Chem. Eng. Sci., 52 1563 (1997) 7. Wan, J. K. S., and Kock, T. A., in Microwave-Induced Reactions Workshop Pacific Grove, California, 1993), p. A-3. 8. Cha, C. Y., Res. Chem. Intermed, 20(1), 13, (1994). 9. W. Conner, M. Turner, K. S. Yngvesson and R. L. Laurence "The Influence of Microwave Radiation on Sorption in Zeolites" International Congress on Zeolites, Baltimore, M.R.S.. Pbs., M. Treacy, ed. V 1 p 97-102 (1999) 10. M. Turner, R. L. Laurence, K. Sigfrid Yngvesson and Wm. Curtis Conner, "The Influence of Microwave Energy on Zeolite Sorption: Desorption and Competitive Sorption" in press AIChE Journal 1999
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
A new way to Ti-containing zeolite beta catalysts for selective oxidation Francesco Di Renzo l, Sylvie Gomez l, R~mi Teissier 2 and Francois Fajula l* ILaboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS, 8, rue de l'Ecole Normale, 34296 Montpellier Cedex 5. France 2Elf-Atochem, Centre de Recherche Rh6ne-Alpes, BP 20, 69310 Pierre-B6nite. France Ti-containing zeolite beta has been prepared by a simple and rapid post-synthesis treatment consisting of reacting the crystals in a concentrated solution of perchloric or nitric acid in the presence of a soluble source of titanium. Simultaneous dealumination and Ti TM incorporation into the framework leads to catalysts that demonstrate good activity and selectivity for the epoxidation of olefins and oxidation of thioethers into sulfones using hydrogen peroxide as the oxidant. 1. INTRODUCTION
Ti-containing zeolite beta (Ti-BEA) catalysts have demonstrated interesting properties for the selective oxidation by hydrogen peroxide of substrates that cannot enter the restricted pores of the MFI structure of TS-1. Several synthesis procedures and post-synthesis treatments (1- 5) have been proposed, with mixed success, to incoporate isolated tetrahedrally coordinated Ti in the framework of purely silicic zeolite beta. Most methods are affected by the presence of small amounts of trivalent elements in the lattice or by the formation of dense Ti-containing phases which negatively affect selectivity in oxidation reactions. We report here on an effective and very simple post-synthesis method of Ti incorporation in zeolite beta based on the attack of a parent zeolite by concentrated acid solution containing a dissolved Ti source. 2. EXPERIMENTAL
Ti-containing zeolites beta (BEA) have been synthesized either by direct synthesis, by incorporating Ti(C4H90)4 as a titanium source to a crystallization medium containing small amounts of aluminium or boron (1,2) or by an original post-synthesis procedure allowing simultaneous dealumination of the framework and insertion of Ti atoms in the lattice (6). The latter procedure consisted in refluxing the zeolite (as-made or in a decationized form) in a solution of concentrated acid (4 - 16 mol/L) in the presence of a soluble source of Ti. Several mineral acids (orthophosphoric, sulphuric, hydrochloric, nitric and perchloric) and Ti sources (Ti(CaH90)4, TiF4, TiO2) were investigated with the results described below. The final materials were recovered by filtration, oven dried at 80~ and characterized by XRD, UV-Vis, FTIR, HR NMR spectroscopies, elemental analysis and nitrogen adsorption.
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Catalytic performances of selected representative samples were evaluated at 80~ for the epoxidation of olefins (1-octene, cyclohexene), and at 30~ for the oxidation of sulfides into the corresponding sulfones. In this latter case the conditions were set in order to obtain quantitative efficiency of the oxidant and the results were compared to those obtained on a TS-1 catalyst (Ti/Si = 0.01). For the epoxidation reaction, results were expressed on the basis of the efficiency of hydrogen peroxide towards decomposition and oxygen incorporation into primary (epoxide) and secondary ( diol, other heavies) products. The epoxidation reactions were carried out in bis(2-methoxyethyl)ether for comparison with the test conditions of industrial catalysts non described in this study. 3. RESULTS AND DISCUSSION 3.1. Incorporation of Ti into the framework of zeofite beta The nature of both the Ti source and the acid appeared critical for Ti incorporation. Regarding the former, TiO2 (the less soluble source of titanium in all the media investigated) led to materials exhibiting two signals in the UV-Vis spectra, at 48 000 and 32 000 cm ~, characteristic of framework Ti TM and of titanium in TiO2, respectively (7). Ti(CaH90)4 and TiF4 proved very efficient for Ti incorporation but the final amount of Ti recovered in the solid, as well as the state of it, depended on the type of acid used. Figure 1 shows as an example the UV-Vis spectra of samples of zeolite treated in the presence of Ti(CaH90)4 (Ti/Si = 0.011) by 14 M solutions of sulphuric, hydrochloric and nitric acids. Although Ti in all samples was incorporated mainly as framework Ti TM, it is clear that sulphuric acid and hydrochloric acids led to the incorporation of various amounts of Ti w (band at 36000 cm~). By contrast, the concentration of the acid did not appear as a critical parameter for Ti incorporation, but for molarities lower than 10 M, the extraction of aluminium was less effective (8). When considering the whole data, a clear relationship could be set between the yield of titanium incorporation (expressed as the ratio between the amount of Ti in the solid and the amount of Ti in the synthesis batch) and the average electronegativity of the acid anion, as illustrated in Figure 2.
.5
9
2.0 !
9
9
9
Figure 1. UV-Vis diffuse reflectance spectra of zeolite beta after treatment by Ti(CaH90)4 in (dotted line) sulphuric acid, (dashed line) hydrochloric acid, and (solid line) nitric acid.
!
\
1.5 9
p
\
1.0 0.5 ....
500010 o4
\\ \ \\ \ , ,~'~,__, 4.0x 10 04 3.0x 10.04 cm-I
"2.0x 10 04
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1.00
A
化
Figure 2: Yield of titanium incorporation v s the average electronegativity of the acid anion. (Solid line for visual aid)
0.75 o,~
E ._= 0.50 t~
[..,
o,..d
0.25 -
0"0~. 0
~2 2.4 2.6 2.8 Anion electronegativity
In other words, in the presence of less electron-donor acid anions (perchlorate and nitrate) with high standard reduction potential, the zeolite lattice competes successfully for the complexation of the transition-metal cation and its incorporation as Ti TM in framework position. Further details on the incorporation of Ti in zeolite beta, mordenite and faujasite are reported elsewhere (9). 3.2. Olef'm epoxidation Cyclohexene and 1-octene epoxidation has been investigated over the series of catalysts prepared according to the procedures listed in Table 1 and the results are summarized in Table 2. Table 1 Preparation procedures and properties of the epoxidation catalysts Catalyst BEA 1
Preparation procedures
Si/A1
Si/B
Ti/Si
AI-BEA synthesized in the presence of tetraethylammonium (TEA) and Na cations
19
Ti-BEA 2
Synthesized according to ref 2
170
Ti-B-BEA 3
Synthesized according to ref 1
170
11.5
0.017
Ti- BEA 4
Ti-B-BEA 3 treated with 12 M HNO3
> 1000
>100
0.01
Ti-BEA-5
As made BEA-1 (5 g) treated with a 10 M solution of HCIO4 (500 mL) containing 0.175 mL of Ti(OBu)4)
Ti-BEA 6 Ti-BEA 7
As made BEA-1 (5 g) treated with a 16 M solution of HNO3 (500 mL) containing 0.25 mL of Ti(OBu)4) Sample Ti-BEA 5 calcined in flowing dry air at 500~
0.007
> 1000
0.01
> 1000
0.01
> 1000
0.01
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The results of the epoxidation tests are in line with what is known on the dramatic influence of acidity (resulting from the presence of trivalent elements or silanol groups) on the selectivity of the reaction. Actually, consecutive reactions leading to diol and heavy products and/or H202 decomposition could not be totally suppressed under the conditions of this study, particularly when using 1-octene as olefin. For instance, the extensive removal of aluminium and boron from sample Ti-B-BEA 3 allowed to reduce the formation of oligomers and to increase the epoxide yield but resulted in a significant increase of the rate of oxidant decomposition (Ti-BEA 4). Similarly, partial healing of the silanol groups (8) by calcination at 550~ (Ti-BEA 5 vs Ti-BEA 7) hardly affected the product distribution. Good results (High epoxide yield, no heavies and marginal H202 decomposition) were nevertheless obtained for the epoxidation of cyclohexene using Ti-BEA 6. Table 2 Oxidation of cyclohexene and 1-octene Catalyst
H202 Conversion
(%)
H202 efficiency (%) towards Decomposition Epoxide Diol
Others
Ti-BEA 2*
80
10
33
34
23
Ti-B-BEA 3*
92
13
5
22
60
Ti-BEA 4*
92
43
43
9
4
Ti-BEA 5*
100
0
48
20
32
Ti-BEA 6*
84
14
74
14
0
Ti-BEA 7*
98
0
40
28
30
Ti-BEA 5**
90
36
13
7
44
Reaction conditions: Stainless steel stirred autoclave, T = 80~ 2.6 g catalyst, olefin (*cyclohexene, **l-octene) = 0.5 mol., H202 = 0.026 mol., solvent - Bis(2methoxyethyl)ether (26 g). Data after 3 h reaction. 3.3. Oxidation of sulfides to sulfones The results of the low temperature oxidation of thioethers into sulfones using a variety of substrates on Ti-beta (Ti-BEA 6) and TS-1 are reported in Table 3. In this reaction, in the experimental conditions used, the H202 efficiency and selectivity to sulfone are higher than 97%, therefore catalyst performances can be directly compared from the conversion level achieved under standard conditions. Ti-beta and TS-1 feature similar activity for the oxidation of small substrates such as diethylsulfide using methanol as the solvent, suggesting that the efficiency of the active sites is the same for the two types of catalysts. By contrast, in the presence of t-BuOH as the solvent or when bulkier thioethers are used, the conversion levels achieved with Ti-BEA catalysts are definitely higher than with TS-1. Actually, TS-1 proved inactive for the conversion of diphenylsulfide due to restricted transition state shape selectivity and diffusion limitations in the narrow pores of the structure. Moreover, the reactivity order found over
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zeolite beta agrees with the nucleophilicity of the S atom, alkyl sulfides being more easily oxidized than allyl or aryl sulfides by an electrophilic oxidant like H202 (10, 11). Table 3" Oxidation of thioethers. Influence of the substrate Substrate
Solvent
Conversion (%) Ti-BEA TS-1
Et2S
MeOH
95
87
PraS
t-BuOH
92
-
AllyI2S
t-BuOH
80
-
Bu2S
MeOH
86
67
PhMeS
t-BuOH
70
-
Ph2S
MeOH
38
0
t-BuOH
18
5
Reaction conditions: T=30~ R2S = 2 mmol., catalyst.Data after 2 h reaction
H202 =
2 mmol., 10 mL solvent, 40 mg
The influence of the solvent on catalytic activity was investigated in more detail for the oxidation of diethylsulfide. Figure 3 compares the results obtained over TS-1 and Ti-BEA and in the absence of catalyst when using six different solvents. The results show that the order of reactivity of TS-1 parallels the one measured in the absence of any catalyst, with methanol as the best solvent. The conversions achieved with Ti-BEA where in all cases higher than with TS-1, with the three protic solvents and acetonitrile leading to very similar activities, larger than those reached with THF and acetone. The relative reactivity order in the various solvents - which follows the solvent polarity - has been explained (12, 13) by assuming a heterolytic mechanism for the cleavage of the peroxide bond leading to transfer of oxygen to the organic substrate, where the partially or totally ionic intermediates are stabilized by the solvent. 100 O
1= C
o
90
60
8
40
II
E =
BTS-1
70
m=
E |
IlWithout
80
Figure 3 9Oxydation of Et2 S at 30 ~ C. Influence of the solvent
I I T i -BEA
so 3o 20 10 0
THF
Me2CO
tBuOH
MeCN
EtOH
MeOH
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4. CONCLUSIONS
Ti-containing zeolite beta with promising catalytic properties in selective oxidation reactions using hydrogen peroxide can be prepared in an efficient way by contacting parent crystals with a concentrated acid solution containing a dissolved source of Ti. The insertion of tetrahedral Ti in the lattice of the zeolite is governed by the redox and complexation properties of the acid anion. The various acids investigated can be ranked in the following sequence of increasing effectiveness for titanium incorporation: orthophosphoric acid < sulphuric acid < hydrochloric acid < nitric acid < perchloric acid. This very simple strategy of preparation of oxidation catalysts has been also successfully applied to mordenite and faujasite zeolites (6, 9). REFERENCES .
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. Derewinski et al., Stud. Surf. Sci. Catal., 69 (1991) 127. M.A. Camblor et al., J. Chem. Soc., Chem. Commun., 589 (1992) J.C. van der Waal., Stud. Surf. Sci. Catal., 105 (1997) 1093. B. Kraushaar et al., Catal. Let., 1 (1988) 81. J. Sudhakar et al., Stud. Surf. Sci. Catal., 94 (1995) 309. F. Di Renzo et al., French Pat. Appl. 95 09436 (1995) F. Geobaldo et al, Catal. Let., 16 (1992) 109. E. Bourgeat-Lami et al., Microporous Mater., 1 (1993) 237. F. Di Renzo et al., submitted to Microporous and Mesoporous Mater. V. Hulea et al., Stud. Surf. Sci. Catal., 108 (1997) 361. C. Walling, Acc. Chem. Res., 8 (1975) 125. V. Hulea et al., J. Mol. Catal., A: Chemical, 113 (1996) 499. P. Moreau et al., Appl. Catal., A: General, 155 (1977) 253.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 92000 Elsevier Science B.V. All rights reserved.
Synthesis of Organically Modified Titania-Silica Aerogels: Application for Epoxidation of Cyclohexenol A. Gisler, C.A. M(~ller, M. Schneider, T. Mallat and A. Balker Laboratory for Technical Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Z0rich Fax: +41 1 632 11 63; e-mail:
[email protected] Mesoporous titania-silica mixed oxides with covalently bound amino and acetoxy groups were prepared from trimethoxysilane which carries the functional group, tetramethoxysilane and titanbisacetylacetonatdiisopropoxide using a sol-gel process and ensuing low temperature supercritical extraction with CO 2. The aerogels containing 10 wt% TiO 2 were characterized by thermal analysis, N2 and NH 3 adsorption, infrared spectroscopy, NMR, X-ray photoelectron spectroscopy and transmission electron microscopy. The amorphous mesoporous structure of the aerogels depended markedly on the nature of the modifier. Titanium dispersion within the silica matrix was slightly lower for the modified materials, as compared to unmodified titania-silica. 29Si CP-MAS NMR measurements indicated a more cross-linked network for the unmodified catalyst. Modifying groups located at the surface replaced part of the silanol groups present in the unmodified aerogel, resulting in a decreased adsorption of ammonia. The materials were tested in the epoxidation of 2-cyclohexen-l-ol. Organic modification of the titania-silica aerogel led to enhanced epoxide yield. 1. INTRODUCTION
Recently, titania-silica aerogels [1] were shown to be highly active and selective for the epoxidation of a variety of bulky cyclic alkenes, alkenones and alkenols [2, 3]. These sol-gel derived materials require alkylhydroperoxides as oxidant; the use of aqueous H20 2 deactivates the hydrophilic materials. Klein and Maier [4], and Kochkar and Figueras [5] successfully reduced the hydrophilicity of sol-gel titania-silica by organic modification of the silicon matrix (surface methyl and phenyl groups). However, the catalytic activities and selectivities observed with aqueous H20 2 as oxidant were low compared to the reaction with t-butylhydroperoxide (TBHP) in organic medium. We applied a different strategy, various polar organic groups were built into the titania-silica matrix with the aim of improving the activity and selectivity of mixed oxides in demanding epoxidation reactions, using TBHP as oxidant. Several trialkoxysilane precursors RSi(OMe)3 were synthesized by varying the modifying group R in order to study its influence on the aerogel structure and the catalytic behavior in liquid phase oxidation reactions. Esters and amines were used as functional groups because of their ability to coordinate to Ti. The precursors and aerogels derived from them are listed in Table 1.
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Table 1: Aerogels and corresponding modifiers Aerogel a
Modifier "
EDAP-A
---si~ ~ ," v
PDAP-A
- "'" --.SigN ,,
H
v
v
~
H ~......
-NH2
"'~:Si~N~NH2 "'-.
AcOB-A
/N~
Modifier content in aerogel [%]
*" "~ ~,
H
.
- ~ s , ~
NH2
5
5
10 OAc
DAcOB-A
.... si
10 ~"'"'"~OAc
OAc a
acronyms are composed of abbreviation of modifiers and -A, which stands for aerogel.
2. EXPERIMENTAL 2.1. Catalyst Synthesis The syntheses of all precursors were carried out under an argon atmosphere using Schlenk-tube techniques. All compounds were used as received unless otherwise stated. 2.1.1. Amine precursors 30.5 ml (450 mrnol) ethylenediamine (Fluka, >99.5%) and 16.5 rnl (90 rnmol) (3-chloropropyl)trimethoxysilane (Aldrich 97%) were heated in a 100 ml round bottom flask and refluxed at 100~ The mixture was stirred at this temperature for 2 h and then cooled to room temperature. Two layers formed, the lower layer containing the ammonium hydrochloric salt and propylenediamine was removed and the upper layer with the desired product was distilled under reduced pressure. Distillation (81~ 0.5Torr) yielded 14.8 g (66.5 retool) (ethylenediaminopropyl)trimethoxysilane (EDAP). (Propylenediaminopropyl)trimethoxysilane (PDAP) was synthesized as described above. Distillation (90~ Torrlyielded 14.2 g (60.4 mrnol) of the desired product. According to the I H-, 13C-and 29cSi-NMR spectra, two isomers of PDAP could be detected (Table 1). The ratio of these two isomers was 1 2.7, 9 according to the 1HNMR spectra.
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2.1.2. Acetoxy precursors
Previous experiments showed that hydrosilylation does not take place, if an unprotected alcohol group is present, but a Si-O bond is formed instead. Therefore the alcohol modifiers had to be protected by acylation. Acy/ation of 3-buten-2-ol: 25.24 g (30.2 ml; 0.35 mol) 3-buten-2-ol (Fluka, >97%) were dissolved in 50 ml CH2CI 2 (dist. over Call2) and degassed. Then 890 mg (7.28 mmol) DMAP (Fluka, >98%) and 42.5g (0.42 mol) Et3N (Fluka, >99.5%) were added. The reaction mixture was cooled to 0~ and subsequently 43 g (0.42 mol) acetic anhydride (pract., distilled) was added dropwise. The solution was warmed to room temperature and the reaction was monitored by GC. After 16 h no reactant could be detected anymore and the solvent was removed. The residue was worked up by distillation and yielded 36.52 g (0.32 tool) of 3-buten-2-yl acetate. Hydrosilylation of 3-buten-2-yl acetate: In a 50 ml Schlenk-tube 205 mg (5.10 .4 mol) H2PtCI 6 (Fluka, -38% Pt) were degassed and the catalyst was covered with 11.4 g (0.1 mol) 3-buten-2-yl acetat. The reaction tube was protected against light by an aluminum foil. The reaction mixture was degassed and cooled to -78~ in a dry ice/ i-PrOH bath. 14.6 g (0.12 mol) HSi(OMe)3 (Fluka, pract -95%) was added dropwise with a syringe. The mixture was slowly allowed to reach room temperature. The reaction was controlled by IR-spectroscopy. As soon as the H-Si bond could not be detected anymore, the reacion mixture was distilled under reduced pressure. Distillation (60~ Torr) yielded 14.9 g (63 retool) of (acetoxybutyl)trimethoxysilane (AcOB). 3-Buten-1,2-diol (Fluka, >99%) was acylated according to the procedure described above. Hydrosilylation of the acylated precursor was performed as described above, yielding (diacetoxybutyl)trimethoxysilane (DAcOB).
2.1.3. Aerogel synthesis and characterization
The aerogels were prepared according to procedures previously published [1, 6]. Sol-gel processes were carried out in a glass reactor at room temperature under an Ar atmosphere. The total volume of the liquid was ca. 170 ml and the corresponding molar ratios water : silicon alkoxide : acid :THA were 5 : 1 : 0.1 : 0.15. Prehydrolysi$ of the precursors in i-PrOH with aqueous HNO 3 hydrolysant under vigorous stirring (1000 rpm) lasted 6 h. The prehydrolysis was necessary to compensate for the different sol-gel reactivity of the precursors [6, 7]. Subsequently, tetramethoxysilane (TMOS; Fluka, puriss.) and titanbisacetylacetonatdiisopropoxide (TIBADIP, 75% in i-PrOH; Aldrich puriss.) in i-PrOH were added. The titania content of all aerogels was 10 wt% TiO 2 for a theoretical catalyst TiO2-SiO2. After 24 h, trihexylamine (THA, Fluka >97%) in i-PrOH was added and the stirring speed reduced (500 rpm). Gelation to an opaque monolithic body occurred within 1 h. Different preparation conditions for the amine-modified aerogels had to be chosen because the amine precursors themselves act as base catalyst. A solution of TMOS and TIBADIP in i-PrOH was mixed with the acidic hydrolysant. After 6 h, THA and the amine modifier in i-PrOH were added and gelation occurred immediately. All gels were aged for 7 days. Semicontinuous extraction with supercritical CO 2 was carried out at 40~ and 230 bar. A glass liner was used to prevent contamination originating from the steel autoclave. The as-prepared aerogel clumps were ground in a mortar and calcined in a tubular reactor with upward flow at 100~ All samples were heated at a rate of 10~ min -1 in an air flow of 5 I min 1 and kept at the final temperature for 1 h. The calcination temperatures were chosen on the basis of thermal analytical investigations. The
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composition of the samples with regard to Si, Ti and Fe was determined by inductively coupled plasma atomic emission spectroscopy (ICPAES). The Si to Ti ratio was nominal and the Fe content was below 0.01% (detection limit). The aerogels were characterized by thermal analysis, N2 and NH 3 adsorption, IR, 29Si- and 13C-NMR, XPS, and HRTEM as described elsewhere [6].
2.2. Catalytic Tests 2-Cyclohexen-l-ol (Fluka, ca 97%) and tert-butylhydroperoxide (TBHP, Fluka, ca. 5.5 M solution in nonane, stored over molecular sieve 4 A) were used as received. Toluene (Riedel-de Hahn, >99.7%) was distilled over sodium and stored over molecular sieve 4 A (Chemie Uetikon). In a 25 ml round bottom flask with reflux condensor and thermometer, 70 mg of the catalyst was heated to 100~ under a He stream for 2 h. After cooling to room temperature, a solution of 2 ml toluene, 20 mmol olefin, 0.4 g internal standard and 5 mmol TBHP was added and the mixture was stirred. The reaction was held under a He atmosphere to prevent contamination with oxygen or moisture. Samples were taken via syringe, filtered (Machery-Nagel 0.2 ram) and analyzed using a HP 6890 gas chromatograph (cool on-column injection, HP-FFAP column). Yield, olefin conversion and olefin and peroxide selectivity are defined as follows: Yield = [Epoxide] [TBHP] 0
Olefin conv. 9XOlefi n =
[Epoxide] Peroxide sel." Speroxide = [TBHP] 0 - [TBHP] [ Olefin ]0 - [ Olefin ] [TBHP] 0
[Epoxide ] Olefin sel.: SOlefin = [Olefin]0 [Olefin]
3. RESULTS 3.1. Structural properties For each modifier, the sol-gel process had to be adjusted to ensure the desired properties: i) well dispersed Ti in the silica matrix, ii) mesoporous structure providing access for bulky reactants to the active sites, and iii) covalently anchored organic functional groups stable under the conditions of catalyst preparation and epoxidation reaction. Table 2 compares some structural properties of the synthesized aerogels. Table 2" Structural properties of unmodified and organically modified aerogels Aerogel
SBET [m2g-1]
Vpa [cm3g -1]
dmaxb [nm]
Q4/Q3 c
unmodified
813
2.3
62
3.0
EDAP-A
372
1.58
80
1.67
PDAP-A
436
1.38
32
1.41
AcOB-A
172
0.52
2.4, 67
0.79
DAcOB-A
251
2.02
62
1.83
_
..,
a Total volume of the pores with a diameter between 1.7 and 300 nm b graphically determined maximum of the pore size distribution c Qn stands for Si(OSi)n(OX)4_ n determined from 29Si CP-MAS NMR (X stands for H or Ti)
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All modified aerogels showed significantly lower surface areas and pore volumes than the unmodified titania-silica aerogel. AcOB-A had the lowest BET-surface area and pore volume and exhibited a bimodal pore size distribution. The pore size distribution of all other aerogels were monomodal and indicated mesoporous structure. All modified aerogels showed considerably lower degree of crosslinking, expressed as Q4/Q3, than the original unmodified aerogel. Generally, the textural properties (SBET, Vp and dmax) depended significantliy on the modified silicon precursor used. The thermal stability of the catalysts was limited by the decomposition behavior of the organic groups. Despite this restriction, the precursor structures of the organic modifications were preserved in the aerogel. Crosslinking in the bulk structure was elucidated using 29Si-NMR, confirming the expected aerogel-type structure. Preservation of the structure of the covalently bound modifiying groups after synthesis was confirmed by 130NMR, infrared spectroscopy and thermal analysis combined with mass spectrometry. Transmission electron microscopy corroborated the amorphous structure of the aerogels.
3.2. Catalytic properties
The aerogels were tested in the epoxidation of cyclohexenol with TBHP. Epoxidation activity and selectivity varied depending on the structure of the modifying group. Table 3 illustrates the performance of aerogels in the epoxidation of cyclohexenol. All catalysts showed preferential formation of the cis-epoxide. The fraction of cis-epoxide was 72 % for the unmodified aerogel, 74 % for the amine modified aerogels and reached 79 % for DAcOB-A.
Table 3: Epoxidation of 2-cyclohexene-l-ol (90-~ toluene, TBHP, 2h) Aerogel
Initial activity
Yield120 min [mmolglmin-1] [%]
XOlefin [%]
Solefin [%]
Speroxide [%]
unmodified
4.9
63
88
72
74
EDAP-A
4.3
69
89
77
74
PDAP-A
5.8
77
93
83
81
AcOB-A
5.0
72
=100
72
73
DAcOB-A
8.7
75
=100
76
72
Comparison of the two aerogels modified by bidentate amino groups, EDAP and PDAP with the unmodified aerogel shows an increase in olefin selectivity and yield. The best catalytic performance was achieved with PDAP-A, which afforded highest yield, olefin and peroxide selecitvities. Acetoxy modified aerogels (AcOB-A and DAcOB-A) also showed improved yield. Olefin selectivity was slightly improved with DAcOB-A, whereas peroxide selectivity remained almost unchanged. Highest initial activity was observed with DAcOB-A. Generally catalytic tests were carried out with an excess of olefin corresponding to an olefin : peroxide ratio of 4: 1. In order to elucidate the influence of the olefin : peroxide ratio on olefin selectivity, reactions were also investigated with a ratio of 1:1 with unmodified aerogel and PDAP-A. Application of a ratio of 1:1 resulted in a selectivity decrease of SOlefin from 72 to 57 % with the unmodified catalyst, and 83 to 78 % for
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PDAP-A. In contrast, the peroxide selectivity Speroxide decreased more strongly for PDAP-A (81 to 73 %) than for the unmodified aerogel (74 to 70 %).
4. DISCUSSION
The modified aerogels showed considerably improved olefin selectivity. An exception is AcOB-A for which no significant improve in SOlefin could be observed. A possible explanation for the better performance of organically modified titania-silica aerogels is the interaction of the amino- and acetoxy-alkyl groups with the Lewis-acidic Ti active sites and the Br6nsted acid Si-OH functions. In the former case the amino and acetoxy groups act as electron donor ligand of Ti, which can modify the acidity and thus the activity of the Ti-peroxo complex. On the other hand, the H-bonding interaction with the surface silanol groups can block these acidic sites and prevent undesired acid-catalyzed side reactions of the reactant and/or product epoxide [3]. The lack of influence on the peroxide selectivity observed for most modifiers suggests that the modifying group does not influence peroxide decomposition or peroxide consumption by undesired oxidation reactions. Interestingly acetoxy groups did not hydrolize during sol-gel process as could be expected based on the acidic (basic) reaction conditions. 130 NMR and DRIFT measurements of the aerogels showed preservation of the acetoxy structure. Concerning the influence of the textural properties on the catalytic performance, there is no clear effect discernible. It seems that the catalytic properties of the mesoporous aerogels were not significantly affected by the textural properties. This is reflected by the fact that activity changed only moderately for aerogels with significantly different textural properties. 5. CONCLUSIONS
Amino and acetoxy groups were incorporated into the matrix of titania-silica aerogels using correspondig modified trimethoxysilane precursors, tetramethoxysilane and titanbisacetylacetonatdiisopropoxide in a sol-gel process. Supercritical extraction of the sol-gel products afforded amorphous mesoporous aerogels which were tested in the epoxidation of cyclohexenol with TBHP. All modified aerogels showed improved epoxide yields compared to the unmodified aerogel. The improved yield is mainly due to higher olefin selecitivity, whereas peroxide selectivity was only enhanced for the aerogel derived from (propylenediaminopropyl)trimethoxysilan. REFERENCES
1. Dutoit, D. C. M., Schneider, M., and Baiker, A., J. CataL 153, 165 (1995). 2. Hutter, R., Mallat, T., and Baiker, A., J. CataL 157, 665 (1995). 3. Dusi, M., Mallat, T., and Baiker, A., J. Mol. Catal. A: Chem., 138, 15 (1999). 4. Klein, S., and Maier, W. F., Angew. Chem. 108, 2376 (1996). 5. Kochkar, H., and Figueras, F., J. CataL 171,420 (1997). 6. M(~ller, C.A., Maciejewski, M., Mallat T., and Baiker A., J. CataL 184, 280, (1999) 7. Brinker, C. J., and Scherer, G.W., "Sol-Gel Science,". Academic Press, Boston, 1990. 8. Notari, B., Adv. CataL 41, 107 (1996).
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Characterization and Catalytic Properties of Titanium Pillared Clays in the Epoxidation of Allylic Alcohols L. Khalfallah-Boudali, A. Ghorbel, F. Figueras* and C. Pinel* Laboratoire de Chimie des Mat6riaux et Catalyse, D6partemem de Chimie, Facult6 des sciences, Tunis, 1060 Tunisia and *Institut de Recherches sur la Catalyse. 2, Avenue Albert Einstein. 69626 Villeurbanne C6dex, France Abstract: The conditions of preparation of montmorillonite pillared by titanium polycations have been obtained. This Ti-PILC shows a surface area of 350 m2/g, micropore volume of 0.16 mL/g, basal spacing of 2.4 nm, is stable up to 773 K and then the characteristics of a true PILC. It has been used in the gas phase for the conversion of isopropanol at 373 K and exhibits essentially an acid behaviour with a significant selectivity to ether, attributed to a high Lewis acidity. In the liquid phase, it is a good catalyst for the epoxidation of allylic alcohols with t-butyl hydroperoxide in presence of (+) diethyl tartrate. The yields to the corresponding epoxides are comparable to those reported in the literature. The determination of the enantiomeric excess in the product show some participation of asymetric catalysis.
1. INTRODUCTION Redox clays, i.e. clays pillared by transition metal cations (PILC) have recently been pointed out as potential catalyst for liquid phase oxidation (1). Cu-A1 pillared clays can be used for the complete oxidation of phenols (2) and several reports claim the asymmetric epoxidation of allylic alcohols on clays pillared by V (3), Cr (4) or Ti (5). This reaction has been reported to be catalysed by Ti-alkoxydes in the homogeneous phase (6) and the Ti clay had been prepared by deposition of this compound on K10 (5). In the usual preparation of PILC a calcination is required to anchor the pillars to the clay sheet (7), but calcination of Ti-K10 destroyed the catalytic properties, therefore the experiments were performed on a non calcined clay. In non calcined clays the bond between the pillars and the surface is loose and the possibility exists of partial dissolution of the Ti-alkoxyde and therefore of an homogeneous catalysis. These previous results have not been confirmed or contradicted, probably because of the difficulty of preparation of these Ti-PILC. Indeed the difficulty resides in the necessity to introduce a bulky cation, about 1.6 nm in size into the interlayer space of the clay. Sterte (8) used a pillaring solution comisfing of TiCI4/HCI and reached a surface area in the range 200-350 n~/g with an average size of the pores 2V/S = 1.9 nm. Using Ti-isopropoxide as precursor, Yamanaka et al (9) obtained interlayer spacings of 1,35-1,7 nm with surface areas 300 m2/g. Bemier et al (10) investigated the effect of the temperature of intercalation and observed the larger pore size working
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at 298 K. In that case the pore size was 1,4 nm and the surface area stable up to 673 K was 349
m2/g.
We described in an earlier report the preparation and some characterizations of Ti-PILC obtained from pillaring solutions consisting of TiCI4 hydrolysed in aqueous solutions of hydrochloric or sulfuric acids (11). The nature of the acid sites of the solid changed significantly with the type of preparation: the sulfated solid showed relatively strong Bronsted acidity, whereas the Ti-PILC prepared from HC1 solutions was essentially a Lewis acid. Solids can be valuable epoxidation catalysts, and we present here an exploratory study of the epoxidation of aUylic alcohols on well characterised Ti-PILC. 2. EXPERIMENTAL METHODS
2.1. Catalyst preparation The preparation of the pillared clay consists in an ion exchange between a pillaring solution containing TiCI4 and the clay. The original material is a Volclay montmorillonite from CECA (Honfleur, France) of chemical composition SiO2 : 58.5%, TiO2 : 0.3%, A1203 : 20.4%, Fe203 : 3.4%, MgO 1.9%, K20 : 0.2%, Na20 3.1% and H20 : 12.1%. The pillaring solution is obtained by a slow dissolution of TiCI~ into a 6 M HCI solution under vigourous stirring. Final concentrations of 0.82 M in titanium and 0.2, 0.5 or 1 M in HC1 were reached by adding water. IT/Ti values of 0.12, 0.24, 0.6 and 1.2 were thus obtained. The fresh pillaring solution was then added dropwise to 500 cm3 of a suspension containing 2 g of clay, in such quantities that a final Ti/clay ratio of 6, 10, 20 and 40 mmol/g were obtained. The kinetics of intercalation was followed by sampling the solid and analysing by X Ray diffraction. After intercalation the solid was separated by filtration and washed extensively with distilled water, dried at room temperature, then calcined in a flow of air with temperature prograrnmation (1 K/min) to avoid self steaming. The samples are referenced A-C1-B in which A is the IY/Ti ratio and B the Ti/clay ratio. 2.2. Characterizations. The chemical composition was determined by UV spectroscopy analysis after dissolution of the sample in 6N sulfuric acid at 353 K, and addition of 5 mL 1-[O2 per mg of Ti (12). The basal spacings were measured by X Ray diffraction using a CGR diffractometer and CuK~ radiation: oriented samples were used before drying and powder samples for calcined solids. BET surface areas and porosities were obtained from the isotherms of N2 adsorption determined on a Micromeritics ASAP 2000 instrument. According to Plee et al. (13) the interlamellar porosity can be approximated by the micropore volume ~fn obtained by application of the Dubinin equation. Since the original clay shows no microporosity, the micropore volumes measure the extent of pilla-
r~ag.
The catalytic properties were determined for the conversion of isopropanol in the gas phase and for the liquid phase epoxidation of (E)-2-hexen-l-ol and of (E)-3-phenyl-2-propenol by tertbutyl hydroperoxide (TBHP), in presence of (+) diethyltartrate (DET). In that case a 250 mL three neck glass flask equipped with a condensor, mechanical stirring and septum for sampling was used as batch reactor. The reactor was maintained under a flow of nitrogen (containing ppm amounts of water) or helium (perfectly dry). The solvent was anhydrous CI-I2CI2, and the temperature was varied from 253 K to 298 K. The traces of water contained in TBHP were
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eliminated by azeotropic distillation in toluene. The reactants (Fluka) were also distilled before use. For the reactions at 253 K, the reaction medium composed ofDET, dichloromethane and 60 mg of the Ti-PILC (0.4 mmol Ti) was cooled to the reaction temperature. When the temperature was stabilised, TBHP in toluene was added dropwise, then the allylic alcohol in dichloromethane. The reaction temperature was controlled between 253 and 258 K. At the end of the reaction the catalyst was filtered, and the solvents were evaporated. The catalyst used in these experiments was first calcined in air at 673 K , reached with a ramp of 1 K/min and temperature at 673K maintained for 3h. The products were separated by column liquid chromatography using a mixtm'e ethylacetate/hexane (2:8) from Prolabo as eluent. They were analysed and quantified by IH NMR. The enantiomeric excess was measured on a Perkin Elmer 141 apparatus. It has to be pointed out that this procedure permits to measure only the desired products of the reaction. 3. RESULTS and DISCUSSION
3.1. Preparation of the PILC The amount of TiO2 retained by the clay as a function of the experimental conditions is reported in Table 1. Changing the acidity of the pillaring solution (H+/Ti) has a strong impact on the amount of Ti fixed by the clay : a decrease is noticed when the acidity increases, at a constant ratio Ti/clay = 10. This suggests that the charge on the polycations decreases by hydrolysis at low acidity, therefore a larger amount of Ti is required to neutralise the charge of the lattice. The increase of the Ti concentration of the pillaring solution (ratio Ti/clay) does not increase the amount of Ti on the PILC as expected. Indeed in several cases a decrease is observed. Cation exchange is fast and therefore controlled by diffusion, and not by equilibrium. Table 1. Amount of TiO 2 retained by the clay using different conditions of intercalation. H+/Ti Ti/clay wt% TiO2 BET surface area Micropore m2/g volume (mIJg) 0.12 10 54.2 268 0.038 0.12 20 37.4 328 0.125 0.12 40 34.4 362 0.13 0.24 10 39.3 316 0.166 0.6 10 20.5 240 0.045 1.2 10 20.6 192 0.023 1.2 20 14.5 135 0.02 1.2 40 13.6 130 0.01 A further evidence can be found in the changes of the X R spectrum of the pillared clay reported earlier (11) as a function of the number of washings. The XRD pattem contains two lines, one at 1.48 nm attributed to the fraction of the clay non intercalated and one at 2.62 nm corresponding to a pillared material. Extensive washing of the material leads to the development of the line at low angle, with a concomittent decrease of the line at higher angle. This behaviour evidences the diffusion of the Ti species within the particles, and the better quality of the resulting PILC. Since the line at 1.48 nm can contain the 001 reflexion of the non pillared clay and the 002 reflexion of the Ti-pillared clay, it is difficult from XRD alone to conclude on the quality of pillaring.
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The micropore volume can give a good idea of the quality of the product since microporosity is due to pillaring. It can be concluded from the results of Table 1 that surface area and micropore volume go through a maximum at about 40 % TiO2. At this maximum the micropore volume is close to that reported by Plee et al. (13) for a well intercalated beidellite, and suggests then a good quality for the Ti-PILC. A flaker evidence can be found in the results of the study of the thermal stability reported in Table 2. Thermal treatment induces first a shift of the basal spacing from 2.63 to 2.37 ran corresponding to dehydroxylation of the Ti polycation, but after dehydroxylation the solid is stable since the lattice spacing, surface and micropore volume show little change. Table 2. Stability of the Ti-clay (0.24-C1-10) to calcination evaluated from different criteria Calcination temperature D001 Surface area Micropore vol (K) (rim) (m2/g) (cm3/g) 373 2.63 1.59 316 0.166 473 2.36 305 573 2.37 1.26 295 0.142 773 2.30 278 0.12 In the case of a poor intercalation, only the outer fraction of the particles are exchanged and the chore is exchanged by the more mobile protons, present in the acidic pillaring solution. Upon calcination, dehydroxylation of the pillars occurs, and the H-form of the clay hydrolysed by the water produced by this process becomes amorphous. This sintering modifies the surface area of the PILC, and the changes of surface area and porosity are then related to the quality of pillaring. The good stability of the sample referenced 0.24-C1-10 is then consistent with its high surface area and micropore volume. In conclusion of this preparation, the 0.24-C1-10 Ti-PILC shows all characteristics of a homogenously distributed Ti pillars, and was choosen as standard catalyst for the catalytic reactions.
3.2. Catalysis of organic reactions. 3.2.1. Conversion of isopropanol. This reaction was used to estimate the acidity of the solid. The reaction was investigated at low conversion between 338 and 673 K, in a flow reactor using He (70 mL/min) saturated with the partial pressure of isopropanol at 298 K (about 6000 Pa) and 0.1 g of catalyst. The products are propene, di-isopropyl ether and acetone. The catalytic properties measured at 373 K are reported in Table 3. Table 3. Conversion of isopropanol on Ti-PILC at 373 K Sample %TIO2 Activity 10.8 mol.g~.s ~ propene 0.24-C1-10 39.3 3.1 55 0.6-C1-6 18.7 36 85 0.12-C1-20 14.5 5.9 33
Selectivity to acetone 8 1.5 46
ether 37 13.5 20
The standard catalyst showed the lower acidity as evaluated from the rate and a high selectivity for di-isopropyl ether. This high selectivity for the ether suggests a predominance of Lewis acidity (14),
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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which is in good agreement with the previous observation of this acidity by FTIR study of pyridine adsorption. A high Lewis acidity is considered to be a good situation for epoxidation (15). 3.2.2 Epoxidation of allylic alcohols. Two reactants were tried : hexene-ol and 6mmmic alcohol (scheme 1). The first series of experiments was carried out with (E)-2-hexen-l-ol, using nitrogen as flush gas, the results are summarised on Table 4. The non calcined clay was inactive for epoxidation at 298 K, or produced at 253 K the hexenoic acid corresponding to the substrate. This is a major difference with earlier results of Choudary et al. (5). Moreover the reaction of Sharpless (6) has been described with Ti-alkoxydes and not chlorides, and is not likely to proceed here. At low temperature the substrate is therefore adsorbed by the alcohol function, and this favours the parallel oxidation of the alcohol to the acid. The shift to the epoxide at higher temperature may simply be due to a higher activation energy for epoxidation. O
R/~''x'j'"
OH 3?~2"/I/3~
~6~
R~
~
OH
O
H
R = C3H7or Ph Scheme ]. Reactions of aHylicalcohols onthe Ti-PILC. Table 4. Results of the reaction of (E)-hex-2-en-1-ol with TBHP in presence of (+) diethyl tartrate on a Ti-PILC. Reaction time (h) Reaction temp (K) Catalyst Product Yield (%) pretreatment 4 253-258 non calcined acid 30 4 253-258 calcined 673 K acid 45 4 273 calcined 673 K acid 26 6 298 calcined 673 K epoxyde 48 23 298 calcined 673 K epoxyde 37 18 298 non calcined none 0 Reaction conditions : Tgtartrate =0.52, alcohol/tartrate=l 6.66 and alcohol/TBHP =- 0.118. The epoxidation of cinnamyl alcohol was investigated in different experimental conditions at 253 K in a nitrogen atmosphere. In the conditions of Gao et al. (6a). or those of Choudary (5) the Ti-PILC was inactive, but a set of conditions could be found in which the uncalcined clay and the PILC were active. However as illustrated in Table 5 the product is then not the epoxide but the aldehyde (E)-3-phenyl-2-propenol or (E) -3 phenyl-2-propenoic acid obtained by oxidation of the reactant. In the same conditions but in helium atmosphere the selectivity to the epoxide becomes reasonably good, and reaches values comparable to those reported in homogeneous phase. It must be pointed out that no Ti trace could be analysed in the solution after filtration of the solid, therefore homogeneous catalysis is discarded.
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An attempt was made to measure the enantioselectivity of the epoxide by measuring the optical rotation as reported by Gao et al. (6). The enantiomeric excess measured by this method is about 15%. However the product was not perfectly pure and probably contained some DET, with an opposite polarisation, therefore the measured enantiomeric excess is probably underestimated. In conclusion, the epoxidation of aUylic alcohols can be obtained on Ti-PILC with reasonable selectivities. The success depends on the preparation of the sample but also on the reaction conditions and particularly on the amount of water present in the medium. Table 5. Reaction of (E) 3-phenylprop-2-en-1-ol with TBHP in presence of DET Reaction time (h) Temperature (K) Solid Product Yield (%) Operation with nitrogen flush 4 253-258 uncalcined aldehyde 26 4 253-258 calcined acid 40 4 298 calcined epoxide 15 20 298 uncalcined none 0 20 298 calcined epoxide 29 dry helium as flush 20 298 calcined epoxide 72 23 298 calcined epoxide 75
REFERENCES
1) R.A. Sheldon and J. Dakka, Catal Today, 19 (1994) 215-246. 2) J. Barrault, C. Bouchoule, K. Echachoui, N. Frini-Srasra, M. Trabelsi, F. Bergaya, Applied Catal B 15 (1998) 269-274 3) B.M. Choudary, V. L. K. Valli and A. Durga Prasad, J. Chem. Soc. Chem. Commun (1990) 721. 4) B.M. Choudary, V.L.K. Valli and A. Durga Prasad, Tetrahedron Lett. 31 (1990) 5785. 5) B.M. Choudary, V.L.K. Valli and A. Durga Prasad, J. Chem. Soc. Chem. Commun (1990) 1186. 6) a) Y. Gao, R. M Hanson, H. Masamune and K. B. Sharpless, J. Amer Chem Soc 109 (1987) 1186. b) Y. Gao, R.M. Hanson, J.M. Klunder, S.Y. Ko, H. Masamune and B.K. Sharpless, J. Amer. Chem. Soc. 109 (1987) 5675. 7) F. Figueras. Catal.Rev.Sci-Eng., 3__0,457-499 (1988) 8) J. Sterte, Clays & Clay Minerals, 34 (1986) 658. 9) S. Yamanaka, T. Nishimara and M. Hattori, Materials Chmistry and Physics 17 (1987) 87. 10) A. Bemier, L.F. Admiai and P. Grange, Applied Cata. 77 (1991) 268. 11) L. Khalfallah Boudali, A. Ghorbel, D. Tichit, B. Chiche, R. Dutartre and F. Figueras, Microporous Materials 2 (1994) 525. 12) G. Chariot -
~ - ~ - -A,, 9
20
cis-cyclooctene
9 cyclohexene
-
9 1-octene 0
1
/
/
\
\ \
\,g/
~
L
I
I
I
t
I
t
t
2
3
4
5
6
7
8
9
0
Run
Fig. 4. Recycling of PI-DAT.Mo catalyst in the epoxidation of various alkenes at 80~
period. One of the authors has already reported on the possible structures of polymer-supported Mo complexes [6,13]. Figure 3 shows the effect of solvent on the epoxidation reaction. Since the level of imbibition into the catalyst for all the liquids involved was very similar (TBHP, cyclohexene, toluene, chlorobenzene, 1,2-dichloroethane ), the observed effects are unlikely to be associated with differences in the swelling characteristics of the polyimide. Rather they are more likely to be associated with the polarity of the solvent, but the rate was reduced in donor polar solvents, t-BuOH and ethanol. The donor solvents are very strongly associated with the catalytic species. The results are consistent with homogeneous and polymer-supported complex catalysts reported before. High activity and selectivity in the epoxidation of various olefins using the supported Mo complex catalysts were observed under favorable conditions (Figure 4). The prolonged activity of polymer-supported heterogenized catalysts is probably the most important factor in their performance. The deactivation of the catalyst by either degradation of the polymer support itself or by leaching of active species from the supported catalyst is unfavorable. Figure 5 shows the yield of cyclohexene oxide after 120min. The polyimide-supported Mo catalysts were recovered at the end of each run and used repeatedly under identical
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conditions. The catalyst shows substantial retention of activity over 10 recycles unlike an earlier polybenzimidazolesupported Mo complex, where the latter displayed rapid deactivation on recycling. The presently reported retention of activity is most encouraging, and suggests that catalysts based on functional polyimide particulates might form the basis of a range of stable polymer-supported metal complex catalysts, where the support is readily synthesized and is highly cost-effective. Application on the both a laboratory and a technical scale also seems feasible.
,IL
100
A
A
~ 80 .-rx o ~, t-x
A
A
~
\\
60
\I(
\
J
\ / A"
~-6 >" 40 t.) ,.. o _~ 7- 2o o
9
C P I - D A T . M o 70~
9
C P I - D A T . M o 60~
9
PI-DAT.Mo
60~
0 I
I
I
I
I
I
I
I
]
1
2
3
4
5
6
7
8
9
10
Recycle number
Fig. 5. Recycling of polyimide-supported Mo catalysts in the epoxidation of cyclohexene by TBHP at 120min. REFERENCES
1 F.R. Hartley, Supported Metal Complexes, Reidel, Dordrecht, 1986. 2. P. Hodge and D.C. Sherrington (eds.), Polymer-supported Reactions in Organic Synthesis, Wiley-Interscience, Chichester, 1980. 3. Y.I. Yermakov, B.N. Kuznetsov and V.A. Zakharov, Catalysis by Supported Complex, Elsevier, Amsterdam, 1981. 4. D.C. Sherrington, Pure Appl. Chem., 60 (1988) 401. 5. M.M. Miller, D.C. Sherrington and S. Simpson, J. Chem. Soc. Perkin Trans. 2 (1994) 2091. 6. M.M. Miller and D.C. Sherrington, J. Catal. 152 (1995) 368, 377. 7. D. Wilson, H.D. Stenzenberger and P.M. Hergenrother (eds.), Polyimides, Chapman and Hall, New York, 1990. 8. L.H. Lee (ed.), Adhesives, Sealants and Coatings for Space and Harsh Environments, Polymer Science and Techology, Plenum, New York, 1988. 9. T. Brock and D.C. Sherrington, J. Mater. Chem., 1 (1991) 151. 10. T. Brock, D.C. Sherrington, and J. Swindell, J. Mater. Chem., 4 (1994) 229. 11. J.H. Ahn and D.C. Sherrington, J. Chem. Soc. Chem. Commun., (1996) 643. 12. J.H. Ahn and D.C. Sherrington, Macromol., 29 (1996) 4164. 13. S. Leinonen, D.C. Sherrington, A. Sneddon, D. McLoughlin, J. Corker, C. Canevali, F. Morazzoni, J. Reedijk, and S.B.D. Spratt, J. Catal. 183 (1999) 251.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
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Epoxidation of Soybean Oil Catalysed by CH3ReOafI-1202 H. Sales, a'b R. Cesquini, a S. Sato, b D. Mandelli c and U. Schuchardt a a Instituto de Quimica, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970, Campinas, SP - Brazil. b Henkel S/A Indfstrias Qufmicas, P.O. Box 050, 12300-970, Jacaref, S P - Brazil c Instituto de Ci~mcias Biol6gicas e Qufmica, Pontiffcia Universidade Cat61ica de Campinas, P.O. Box 111 l, 13020-904, Campinas, SP - Brazil.
The epoxidation of soybean oil with hydrogen peroxide catalysed by CH3ReO3 was investigated in order to evaluate the effect of nitrogen-containing bases (pyridine, 2-picoline, 3-picoline, 4-picoline, pyrazole and bipyridine N,N'-dioxide) on the activity and selectivity of the catalytic system. 1. INTRODUCTION The oxidation of hydrocarbons with hydrogen peroxide or dioxygen catalysed by transition metal complexes is an important industrial process which produces compounds with a higher added value. Epoxidised oils, especially soybean oil, are produced On an industrial scale (--104 ton/year) and are used as stabilisers and plasticisers in the production of polyvinyl chloride [1]. These componds can be prepared in the presence of CH3ReO3 as a catalyst. This alkylrhenium complex reacts with H202 forming the bis-peroxo species CH3ReO(O2)2H20, which is active in the epoxidation of olefins [2]. The current industrial process for the production of epoxidised olefins is via percarboxylic acid, which is a slow reaction, producing acids as side products. The epoxidation catalysed by CH3ReO3 (A) shows a much higher reaction rate and can be carried out at room temperature, reducing the amount of side products. (A) reacts with two H202 molecules to form (C), which is the catalytically active intermediate for the epoxidation of alkenes, as shown in Figure 1 [2]. (C) transfers one oxygen atom to the alkene, forming the epoxide and regenerating (B).
~H3 Re
+H202
-H O
-~
O~
?
+H202
-H O OH 2
=-
[ / Re OH2
(A) (B) (C) Fig. 1. Formation of peroxo and bis-peroxo species from CH3ReO3 and H202 [2].
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A serious problem in these processes is the acid catalysed ring-opening reaction of the formed epoxide [3]. Recently, Sharpless and co-workers have reported that the addition of pyridine derivatives in a biphasic system (solvent/water) suppresses the ring-opening reaction, with no effect on the reaction rate [4]. Pyridine stabilizes the CH3ReO3/H202 system and increases the generation of the peroxorhenium species. The base coordinated to the bisperoxorhenium complex seems to be also responsible for the acceleration of the epoxidation reaction. The basicity of pyridine also lowers the activity of hydronium ions in the reaction medium, reducing the rate of epoxide ring opening [5,6]. A large excess of pyridine is required to maintain the catalytic activity due to the rapid oxidation of pyridine to pyridine Noxide, in the presence of CH3ReO3. Pyridine N-oxide also forms a highly selective, but less active co-catalyst with CH3ReO3/H202. The presence of the aqueous phase ensures high epoxidation activities, due to the rapid and efficient removal of the pyridine N-oxide [6] from the organic media, avoiding its contact with the catalyst. 2. EXPERIMENTAL 2.1. Materials Aqueous hydrogen peroxide (30 wt.%), dichloromethane, and the nitrogen-containing bases (pyridine, 2-picoline, 3-picoline, 4-picoline, and pyrazole) were used without further purification. The bipyridine N,N'-dioxide was prepared by a procedure described in the literature [7]. Soybean oil was obtained from Henkel S.A Ind~strias Qufmicas. CH3ReO3 was prepared as described in the literature [3]. 2.2. Epoxidation reactions In the epoxidation reactions, soybean oil (5.0 g, 24,4 mmol of double bonds), CH3ReO3 (25 mg, 0.1 mmol), nitrogen-containing base (2.4 mmol), H202 30% (3.3 g, 29.1 mmol) and hexadecane (0.65 g, 2.9 mmol, internal standard) were added to 10 mL of CH2C12. The mixture was kept at 25~ under magnetic stirring for 7 h. The reaction was quenched by adding a catalytic amount of MnO2 to decompose any remaining H202 and a stoichiometric amount of anhydrous MgSO4 to remove the formed water. The suspension was then filtered and submitted to transesterification with methanol, following a procedure described in the literature [8]. The methyl esters obtained were quantified by gas chromatography. 2.3. Product characterisation The yields of the epoxidation products were determined by GC, using a Hewlett-Packard gas chromatograph (Hewlett-Packard 5890 II) equipped with a split/splitless injector, flame ionization detector and fitted with a Ultra II HP-5 column (Hewlett-Packard). External standard calculations were applied to obtain the response factor of the epoxidized soybean oil methyl ester products. The structures of the products were determined by GC/mass spectrometry (HewlettPackard 5970) and by comparison of their retention times with those of authentic samples, which were obtained in a large scale epoxidation experiment via peracids, and isolated using column chromatography. These authentic samples were analysed by ~H NMR and ~3C NMR, IR and GC-MS. The ~3C and ~H NMR spectra were recorded on a Bruker AM 300 spectrometer at 300 MHz, using CDC13 as the solvent. Infrared spectra were obtained on a Perkin-Elmer 1600 FTIR M-80 Specord spectrometer.
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3. RESULTSANDDISCUSSION The typical fatty acid composition of soybean oil is shown in Figure 2.
~
O
H
PalmiticAcid 11%
~
O
H
StearicAcid
4%
~
O
H
OleicAcid 25%
~
O
H
LinoleicAcid 51%
~
O
H
Linolenic Acid 9%
Fig. 2: Typical fatty acid components of soybean oil The principal products obtained in the epoxidation of soybean oil using CH3ReO3/H202 (after transesterification) are shown in Figure 3.
(l)
~~~~~~~~o"
~
o
"
(2)
(3) ~ ~ ~ . v ~ V ~ ~ o . .
~ ~ v ~ ~ ~ ~ o . .
(4)
(5) ~
~
(6)
O
"
O
OH
" .
(9) ~
O
"
~
O
OH
"
(10)
OH Fig. 3: Principal products obtained in the epoxidation of soybean oil. Oleic, linoleic and linolenic esters present in the soybean oil are thus converted to mono-, di-, tri-epoxides and their respective diols. A typical chromatogram obtained for the epoxidised soybean oil after transesterification with methanol is shown in Figure 4. The epoxidation of olefins with CH3ReO3 is a stereospecific (cis) addition. Since the starting olefin (unsaturated vegetable oil) is a cis isomer, the resulting epoxide will be cis as well. The bis-peroxo complex can be coordinated on both sides of the double bond. Thus, oleic esters give two enantiomeric epoxystearates, which appear as a single peak, correponding to the racemic mixture (5). Linoleate gives two (9) diasteroisomeric racemates.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1664
The diasteroisomeric racemates are not necessarily formed in equal amounts [9]. The racemic mixtures from the linolenate epoxide (12), were not detected by gas chromatography. These epoxides react, giving oligomers detected by GPC (gel permeation chromatography).
solvent standard
(1)
(3,4)
(7)jJ5)
(2)
10
20
30
40 time (min)
Fig. 4. Chromatogram of methyl esters of epoxidised soybean oil.
3.1 Epoxidation of soybean oil in the presence of pyridine. The results for the epoxidation reaction of soybean oil with CH3ReO3/H202 in the absence and presence of pyridine are shown in figure 5 and 6.
&,
100
0
~
0
9
.0
X--
X
80
~
60
~e~
D - - Select 7,8 ~a~
40
Conv Select 5
x ~ Select 9
20
~o~ 0
1
2
3
4
5 6 time (h)
Select 6
7
Fig. 5: Epoxidation of soybean oil without pyridine. CH3ReO3/H202/double bonds/ hexadecane = 1/291/244/46 The addition of pyridine causes an increase in the reaction rate. The reaction is practically finished after 60min, while it takes 300min in the absence of pyridine. The monoepoxide intermediates (7) and (8) react faster, forming the diepoxides (9) in a shorter reaction time. In the absence of base, 0.3 mol of the diol (6) was produced, and no diol (6) was produced in the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1665
presence of pyridine. This Lewis base accelerates the generation of the bis-peroxorhenium species, which coordinates to the olefin. The acceleration of the epoxidation reaction is explanied by its higher concentration with the presence of pyridine [5].
/
100"
"~ 80
o---o
o-
-o
9
- - o - - Conv x - - o - - Select 7,8
60.~ , / x - - - - - - - - - x ~ X 40.x
~
j z x ~l z x \_ _ ~~_ z x l_ _ _~_ _ _x_ _ _ _ z
zx~ Select 5 ~ x - - Select 9
20.
0, 0
~o~ 1
2
3
4
5 6 time (h)
Fig. 6: Epoxidation of soybean oil bonds/hexadecane/pyridine = 1/291/244/46/24
with
Select 6
7 pyridine.
CH3ReO3/H202/double
3.2 Epoxidation of soybean oil in the presence of other bases.
We have compared the effect of pyridine, 4-, 3- and 2-picoline, pyrazol and bipyridine N,N'-dioxide in the epoxidation of soybean oil with CH3ReO3/H202. The results are shown in Figure 7. 100 v
80 60 40
20
, ~ 5 r162
II Select. mono-epox (%) 17 Select. di-epox (%)
Fig.7. Effect of the addition of bases in the epoxidation of soybean oil after 60 min.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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The addition of bases normally increased both conversion and selectivity for the di-epoxide (9). This effect was more pronounced for pyridine, showing that the intrinsic basicity of the base was not proportional to the yield of the products, since 3- and 4-picoline have a higher pKb. The bases also prevented the formation of the diol (6). The addition of 2-picoline reduced the activity of the catalyst, giving a 5% yield to the mono-epoxide after 60 min, which is probably due to the steric hindrance of the active site caused by the methyl group at the ortho position. A similar result was described by Sharpless in the epoxidation of cyclooctene [4]. The reactions carried out with bipyridine dioxide and pyrazole showed low conversions (approx. 40 and 18%, respectively). According to the literature [7,10], these bases increase the activity of the catalyst and the selectivity for the epoxide in the epoxidation of t~-olefins and styrene, respectively, but were rather inactive when soybean oil was used as the substrate. 4. CONCLUSIONS We have found that CH3ReO3/H202 is an active catalytic system for the epoxidation of soybean oil. The activity of this system can be increased by the addition of nitrogencontaining bases, which also prevent the formation of diols, thus increasing the yield of the desired epoxides. Pyridine proved to be the most efficient co-catalyst, whereas typically efficient bases such as pyrazol and bipyridine N,N'-dioxide are not active. 2-picoline poisons the catalyst due to steric hindrance of the active site. ACKNOWLEDGEMENTS This research was supported by Henkel S.A. Indtistrias Qufmicas, FAPESP and CNPq. The authors are grateful to Dr. Ricardo Sercheli and Ir. Michiel van Vliet for helpful discussions. REFERENCES:
1. F.D. Gustone, J. Am. Oil Chem. Soc., 70 (1993) 1139. 2. A.M. al-Ajlouni and J.H. Espenson, J. Org. Chem., 61 (1996) 3969. 3. W.A. Herrmann, R.W. Fischer, M.U. Rauch and W. Scherer, J. Mol. Catal., 86 (1994) 243. 4. J. Rudolp, K.R. Reddy, J.P. Chiang and K.B. Sharpless, J. Am. Chem. Soc., 119 (1997) 6189. 5. W. Wang and J.H. Espenson, J. Am. Chem. Soc., 120 (1998) 11335. 6. W.A. Herrmann, H. Ding, R.M. Kratzer, F.E. K0hn, J.J. Haider and R.W. Fischer, J. Organomet. Chem. 549 (1997) 319. 7. M. Nakajima, Y. Sasaki, H. Iwamoto and S. Hashimoto, Tetrahedron Lett., 39 (1998) 87. 8. U. Schuchardt and O.C. Lopes, J. Am. Oil Chem. Soc., 65 (1988) 1940. 9. L.T. Han, J. Am. Oil Chem. Soc., 71 (1994) 669. 10. W.A. Herrmann, R.M. Jratzer, H. Ding, W.R. Thiel and H. Glas, J. Org. Chem., 555 (1998) 293.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Epoxidation of limonene with layered-double hydroxides as catalysts M. A. Aramendia, V. Borau, C. Jim6nez,* J. M. Luque, J. M. Marinas, F. J. Romero, J. R. Ruiz and F. J. Urbano
Departamento de Quimica Orgdnica, Facultad de Ciencias, Universidad de C6rdoba, Avda San Alberto Magno s/n, E-14004 C6rdoba, Spain This paper reports and discusses the results obtained in the epoxidation of limonene to limonene oxide by using H202 over different layered double hydroxides (LDHs). The solid obtained by calcining Mg/AI LDH at 773 K was found to be the most active among the Mg/M(III) LDHs tested (M = AI, Ga, In). The results are compared with those obtained by using conventional MgO and A1203 catalysts. I. INTRODUCTION Hydrotalcite, [Mg6A12(OH)i6]CO3-4H20, is an anionic clay whose name is used to designate a large number of isomorphic compounds and polytypes known collectively as Alayered double hydroxides (LDHs). The structure of an LDH is similar to that of brucite, Mg(OH)2, with Mg 2+ cations occupying the centres of octahedra joined by their edges to form infinitely superimposed layers linked by hydrogen bonds between hydroxyl groups at octahedral vertices [1]. In hydrotalcite and LDHs, some Mg2+ ions are substituted by tervalent ions; the latter introduce a charge deficiency in the layers that is neutralized by anions in the interlayer region, which also contains crystallization water. Alkenes were initially epoxidized with percarboxylic acids [2] since the use of metals in the presence of hydrogen peroxide led to slow reactions subject to the competition of other processes. Improved results have been obtained in recent years by using molecular sieves containing titanium
[3-5].
2. EXPERIMENTAL 2.1. Preparation of catalysts The three LDHs tested were synthesized following the traditional precipitation procedure at a constant pH, which involves mixing two solutions of the metal nitrates and pouring the resulting solution over one of Na2CO3 at pH 10 and 333 IC The pH was kept constant during precipitation by adding appropriate volumes of 1 M NaOH. Once synthesized, the LDHs were exchanged with carbonate in order to remove nitrate ions from the interlayers. The resulting solids were named HT-Mg/AI, HT-Mg/Ga and HT-Mg/In. All were calcined in a nitrogen atmosphere to obtain the catalysts called HT-Mg/AI-773, HT-Mg/Ga-773 and HT-Mg/In-773, respectively. An MgO-based solid obtained by calcination of commercially available Mg(OH)2 at 873 K and an also commercially available, A1203 solid, were also used for comparison.
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2.2. Characterization of catalysts The elemental composition of the samples was determined on a Perkin-Elmer 1000 ICP spectrophotometer under standard conditions. X-ray diffraction patterns were recorded on a Siemens D-500 ditfractometer using CuK~ radiation. The specific surface areas of the solids were determined by the BET method, which was implemented on a Micromefitics ASAP 2000 analyser. 27A1and 7~Ga MAS NMR spectra were recorded at 104.26 and 121.98 MHz, respectively, on a Bruker ACP-400 spectrometer. Finally, solid basicity was measured by CO2 chemisorption.
2.3. Oxidation reactions Oxidation reactions were conducted in a 250 mL flask containing the catalyst (0.450 g), limonene, the nitrile and hydrogen peroxide. Methanol (70 mL) was used as solvem. The reaction was conducted at 333 K and monitored by gas chromatography, using a 30 m-0.25 mm i.d. DB-1 column. Reaction products were identified by mass spectrometry.
3. RESULTS AND DISCUSSION 3.1. Characterization of catalysts The elemental analysis of the carbonate-exchanged LDHs provided Mg/M(III) ratios consistent with the theoretical predictions. Table 1 gives the formulae of the three exchanged LDHs. The XRD analysis of the solids revealed a high crystallinity in all. As can be seen from the XRD patterns of Fig. l, the solids had typical structures, with stacked layers similar to those previously found by Reichle et al. [6]. Calcination at 773 K of the three LDHs yielded a mixture of magnesium oxides and the corresponding tervalent metal where MgO exhibited a periclase-like structure and the M203 oxides were amorphous (Fig. 1). Table 1 also summarizes the textural properties and basicity of the three LDHs and their calcination products, together with those of the pure magnesium and aluminium oxides studied.
1 g,,,~.-_-
~
,
'
HT-Mg/A1 E ~
'..,..
'~.,j
-w,.~-~,w....,. ~ ]
: r,o
.
.,~,4
HT-Mg/Ga
.,
i'
10
~, ~ x . . . . . , ~
!~
I!
20
k HT-Mg)Ga-773
~ - - - ~ ~
HT-Mg/In 1
,~
30
,
"I
40
:
i
50
60
2 Theta (~
!HT-Mg/In-773 70
80
10
20
30
40
50
60
70
80
2 Theta (o)
Figure 1. XRD patterns for the LDHs and their calcination products. The 27A1MAS NMR and 71Ga MAS NMR spectra for solids HT-Mg/AI and HT-Mg/Ga, and those of their calcination products at 773 K, reveal that the tervalent metal adopts octahedral
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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coordination in the LDH, whereas both metals can be in octahedral and tetrahedral coordination upon calcination at 773 K. These results are consistent with previous findings [7,8].
Table 1. Chemical formulae and surface properties of the catalysts. Catalyst Formula SBET(m2"g-~) HT-Mg/AI Mg075A10.25(OH)2(CO3)0.125"0.70H20 56.2 HT-Mg/Ga Mg078Ga0.23(OH)2(CO3)0120-0.62H20 51.7 HT-Mg/In Mg075In025(OH)2(CO3)0125"0.53H20 98.5 HT-Mg/AI-773 143.5 HT-Mg/Ga-773 116.9 HT-Mgfln-773 120.1 MgO 116.0 A1203 59.3
Ftb (ILtmolCO2"g-')
330 182 193 257 43
3.2. Catalytic activity The epoxidation oflimonene under the reaction conditions used is a complex process as regards both the reactants, the catalysts and the resulting products. It is thus crucial to determine the role played by each reactant as a function of its nature and concentration. This entails conducting a series of kinetic tests ensuring a constant behaviour in the reactants with time. The overall process can be fitted to a general kinetic equation such as v = k[limonene][H202][nitrile] which contains a concentration term for each reactant. This involves the unsurmountable difficulty of mixing arising from the low aqueous solubility of the organic products over some concentrations ranges. We those chose to conduct our experiments under reaction conditions where the orders with respect to H202 and the nitrile used would be zero, so the overall kinetic equation would simplify to v-- k'[limonene] and the results obtained would only be a consequence of changes in the limonene concentration. Consequently. the experiments described below were carried out in excess H202 and nitrile relative to the stoichiometric amounts. Figures 2 and 3 show the results of two experiments where the conversion obtained by adding successive portions of H202 or benzonitfile, respectively, was monitored at regular intervals. From the graphs it follows that, when these reactants are at low concentrations, the conversion responds to different orders in such concentrations: however, a portion exists in both curves where the apparent reaction rate, as determined from the slope, does not change upon addition of either reactant, which suggests that both possess a zeroth kinetic order. Accordingly, in subsequent experiments we ensured that the H20 and nitrile concentrations would never fall below such levels. Double bonds can be epoxidized in the absence of acid-base catalysis. In order to check the effect of the blank reaction on our results, we conducted a series of experiments where one of the reactants was excluded from the start. As can be seen from Fig. 4, the reaction can develop in the absence of the catalyst and nitrile; however, both are required in order to achieve the conversion and selectivity levels reported in this paper. The limonene/benzonitrile/H202 mole proportion was 1:5.5" 19.6. Hydrogen peroxide was used in a large excess and was only partly employed in the epoxide formation-it also disappeared through formation of the diepoxide and the direct hydroxylation oflimonene. Figure 5 shows the variation of the limonene and H202 concentrations.
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as determined by iodometry, with time. Alter 5 h of reaction, 85% (in mol) of the initial amount oflimonene was converted and so was 22% of the initial H202 used. However, the H:O2 continued to deeongmse up to 50% (of the initial amount) at 2 h. The yield ofthe process, measured as mol 1-1202 used per mol oflimonene converted, was greater at short reaction times. As the epoxidation rate started to decrease, the presence of byproducts (diepoxides, diglyeols) became significant. In order to boost the selectivity towards the epoxide one must therefore have the reaction complete within as short a time as possible by favouring any factor contributing to a faster reaction. Tests involving limonene oxide as the starting substrate conducted under the previous reaction conditions revealed that the 1,2-glycol was not formed through opening of the oxirane ring but via a direct transformation of the double bond. /// // 2.5~
I
O
I
2.0-
~9
;/
1.O
o.o
~
'-
II I I I
E
I II
illll 14.7 mmol 24.5 mmol 34.3 mmol
2.8 mmol 5.6 mmol 11.3 mmol 22.6 mmol 45.3 mmol 90.6 mmol 121.3 mmol
I I I
i
,,ii !li
I 0
3
6
9
12 15 18 21 24 27
t (h) Fig. 2. Variation of limonene conversion with the addition of H202.
0
2
4
6
8
21
24
27
30
t (h) Fig. 3. Variation of limonene conversion With the addition ofbenzonitrile
Other nitriles tested provided poor yields. As can be seen from Table 2, if the nitrile used contained acid ot protons (e.g. in acetonitrile and butyronitrile), the reaction hardly developed. These substrates reportedly form carbanions very easily and such species may be retained on the hydrotalcite surface, thereby hindering the reaction. When the nitrile contains no acid protons, the reaction is efficiently accelerated by the catalyst. It was outside the scope of this work to elucidate the role of the nitrile in the process, which is currently being investigated. However, we have found that it becomes benzamide during the process. Table 2. Influence of the particular nitrile used on limonene conversion. Nitrile Reaction time (h) Conversion (%) Acetonitrile 8 30.9 Butyronitfile 8 22.2 Benzonitrile 8 86.3 The next step involved examining the influence of the catalyst. For this purpose, the different LDHs and their calcination products were tested under the previously established reaction conditions. As can be seen from Fig. 6, the LDHs exhibited only slight differences in activity. By contrast, the activity of the calcined LDHs decreased with increasing ionic radius of the tervalent
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metal. Table 3 gives the activity and selectivity data extracted from the graphs. The selectivity for the epoxide also appears to be related to the nature of the tervalent metal in the LDH. Like the conversion, it decreases ~om A13+to In 3§ In all respects, solid HT-Mg/AI is the best catalyst for the process, even in relation to the conventional catalysts tested (Fig. 6). 25 20 =
o 9~
15
A
,
~, 10
I
o
o
[]
Blank without peroxide Blank without catalyst Blank without nitrile
5 0 0
2
4 6 8 t(h) Figure 4. Variation oflimonene conversion with time in various blank tests. 60 i
50
_
j]
40 I,
9
H202
l
o
Limonene I
_
I, / !oi/
.
.
.
.
.
.
.
.
.
.
.
6.2 (, 00% 1 ~ . ~._~_~.-.~:_~.~c3:--0--~_; . . . .
0
5
10
....
.
15
~.,
[
20
25
t (h) Figure 5. Variation of limonene conversion and the H202 concentration with time. Table 3. Activit), and selectivity achieved in the epoxidation of limonene with various catalysts. Catalyst Conversion (%) Selectivity 1,2-epoxy + 1,2-glycol Others HT-Mg/AI 86.3 88.1 11.9 HT-Mg/Ga 86.0 88.8 11.2 HT-Mg/In 80.0 82.7 17.3 HT-Mg/AI-773 92.1 89.6 10.4 HT-Mg/Ga-773 78.0 77.3 22.7 HT-Mg/In-773 50.0 57.3 42.7 MgO 85.3 79.2 20.8 AbO3 39.3 96.0 4.0
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100
~
80-
=o
60 4020 0
v
0
2
4
6
80
t (h) 9 A 9 9
HT-Mg/AI HT-Mg/Ga HT-Mg/In AI203 MgO
I
1
2
4 t (h)
9 A 9 o
HT-Mg/AI-773 HT-Mg/Ga-773 HT-Mg/In-773 AI203 MgO
Figure 6. Influence of the catalyst on limonene conversion 4. A C K N O W L E D G E M E N T S
The authors wish to express their gratitude to Spain's DGESIC (Project PB97-0446) and the Consejeria de Educaci6n y Ciencia of the Junta de Andalucia for funding this work. The Nuclear Magnetic Resonance and Mass Spectrometry Services of the University of C6rdoba are also gratefully acknowledged. REFERENCES ~
2. 3. .
5. .
7. 8.
S. Miyata, Clays Clay Miner., 23 (1975) 369. D. Swern, in R. Adams (ed.), Organic Reactions, Wiley, Mew York, 1953, p. 378. A. Corma, J. L. Jordd, M. T. Navarro and F. Rey, J. Chem. Soc., Chem, Commun., (1998) 175. E. Jorda, A. Tuel, R. Teissier and J. Kervennal, J. Catal., 175 (1998) 93. V. Hulea, E. Dumitrin, F. Patcas, R. Ropot, P. Graffm and P. Moreau, Appl. Catal. A: General, 170 (1998) 169. W. T. Reichle, S. Y. Kang and D. S. Everhardt, J. Catal., 101 (1986) 352. A. Corma, V. Fom6s and F. Rey, J. Catal., 148 (1994) 205. M. A. Aramendia, Y. Avil6s, J. A. Benitez, V. Borau, C. Jim~nez, J. M. Marinas, J. R. Ruiz and F. J. Urbano, Microp. Mesop. Mater., 29 (1999) 319.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Epoxidation of Electron-Deficient Alkenes using Heterogeneous Basic Catalysts J. M. Fraile, a* J. I. Garcfa, a D. Marco, a J. A. Mayoral, a E. Sfinchez, a A. Monz6n, b E. Romeo b a Dep. Qufmica Orgfinica, Fac. Ciencias, Inst. Ciencia de Materiales de Arag6n, Univ. Zaragoza-C.S.I.C., E-50009 Zaragoza, Spain. b Dep. Ingenierfa Qufmica y Tecnologfa del Medio Ambiente, Fac. Ciencias, Univ. Zaragoza, E-50009 Zaragoza, Spain. 1. INTRODUCTION The development of heterogeneous catalysts to promote selective organic reactions is a field of growing interest. Whereas heterogeneous acid catalysis has been applied to a good number of processes, basic catalysis is far less developed. Oxidation reactions are among the most interesting processes from the industrial point of view, and epoxidation is one of the most important oxidation reactions, given the usefulness of epoxides as synthetic intermediates. The epoxidation of electron-deficient alkenes can be carried out with hydroperoxides in the presence of a base. Our group described for the first time the use of hydrotalcites as basic solids in the epoxidation of electron-deficient alkenes with hydrogen peroxide [ 1]. This system has the advantage of using one of the oxidants of choice in industry, but it suffers from the drawback of a number of side reactions. Another heterogeneous system uses KF/alumina as the base and tert-butyl hydroperoxide (TBHP) as the oxidant [2]. However, this method suffered from problems associated with solubility, but these were overcome by the use of toluene [3]. The present communication describes our most recent results in this type of epoxidation of alkenes using both systems. Two relevant aspects are studied; firstly the influence of the hydrotalcite composition on the catalytic activity and the product selectivity and, secondly, the use of this methodology in the diastereoselective epoxidation of chiral alkenes. 2. EXPERIMENTAL
2.1. Preparation of the hydrotalcite catalysts The catalysts were prepared by coprecipitation of a 1 M aqueous solution of a mixture of the metallic nitrates (Mg and A1) with a 2.4 M aqueous solution of KOH and K2CO3 (KOH/K2CO3 ratio = 16.42). The pH and temperature of the slurry were kept at 10.2_+0.2 and 60 ~ respectively, during coprecipitation. The hydrated precursors were separated by filtration, washed with hot bidistilled water and dried overnight at 70 ~ The corresponding * Corresponding author. FAX: +34 976 762077. E-mail:
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mixed oxides were obtained by calcination of the dried precursors in a flow of N2 at 500 ~ for 14h.
2.2. Characterization of the catalysts The specific surface area was measured by adsorption of NE at 77 K with a Micromeritics Pulse Chemisorb 2700. The XRD patterns were recorded within the range of 5 to 80 ~ (20) using a Rygaku/Max system diffractometer. All the precursors showed the expected structure of hydrotalcite. The number of basic sites was measured by adsorption of phenol. The hydrotalcite (10 mg) was added to a 0.016 M solution of phenol (3 mL) in cyclohexene, containing decane as an internal standard. The mixture was stirred at room temperature for 24 h and the solution analysed by gas chromatography. The amount of phenol adsorbed was calculated by difference. 2.3. Epoxidation with hydrogen peroxide and hydrotalcite To a solution of the alkene (1 mmol) in methanol (1 mL) was added hydrotalcite (150 mg) and 30% hydrogen peroxide (0.35 mL). The reaction mixture was stirred at room temperature for the appropriate time (1 or 24 h), filtered and the catalyst washed with methanol (5 mL). 100 mg of ethylene glycol dimethyl ether as an internal standard was added and the solution was analysed by gas chromatography. The ratio epoxide/dioxolane was determined by 1HNMR spectroscopy. 2.4. Epoxidation of chiral alkenes with hydrogen peroxide/hydrotalcite The hydrotalcite (285 rag) was added to a solution of the alkene (1.94 retool) in methanol (25 mL). 30% hydrogen peroxide (0.68 mL, 6.6 mmol) was added and the mixture was stirred at room temperature for 24 h. The catalyst was filtered off and washed with methanol. The solvent was evaporated under reduced pressure and the crude product was analysed by ~HM R spectroscopy. 2.5. Epoxidation of chiral alkenes with TBHP and KF/alumina To a solution of alkene (0.569 mmol) in anhydrous toluene (10 mL) was added KF/alumina (95 rag, Fluka, -5.5 mmol F/g) and TBHP (0.57 mL, 3 M in isooctane, 1.71 mmol). The mixture was stirred at room temperature under an Ar atmosphere for 24 h. The catalyst was filtered off and washed with toluene. The solvent was evaporated under reduced pressure and the crude product analysed by 1H-NMR spectroscopy. 3. RESULTS AND DISCUSSION 3.1. Effect of Mg/AI ratio Our preliminary results with acyclic unsaturated ketones, such as mesityl oxide (1) (Scheme 1), showed that the system hydrotalcite/H202 led not only to the epoxide (2) but also to the corresponding 3-hydroxy-l,2-dioxolane (3).
0 1
2
Scheme I
0--0 3
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Recently it has been described that an increase in basicity, by anion exchange with the t e r t butoxy anion, increases the activity of the hydrotalcite [4], although no mention of selectivity problems was made. Thus, we decided to study the effect of the basicity of the hydrotalcite on this reaction by changing the Mg/A1 ratio, both on the catalytic activity and the selectivity epoxide/dioxolane. To ensure the reproducibility of the results a precise control of the pH in the synthesis of the hydrotalcite is necessary, so an automatic pH controller system was used. The effect on the epoxidation of another ketone that does not have problems of selectivity, such as isophorone (4) (Scheme 2), was also studied for comparison. In both cases, conversion after 1 h was taken as indicative of the catalytic activity and conversion after 24 h was also measured. O
~
O
Scheme 2 Most of the basicity measurements on calcined hydrotalcites have previously been carried out by microcalorimetry with CO2 [5,6]. However, we decided to try a new method that is closer to the experimental conditions of the reactions. This new method involves adsorption in the liquid phase at room temperature. The probe molecule chosen was phenol and it was analysed by gas chromatography in the liquid phase after adsorption. The amount of adsorbed phenol was then calculated by difference. The values obtained in this way lead to the conclusion that the solid with Mg/A1 = 3 shows the highest number of basic sites (Figure 1), which contrasts with results previously described [5,6], in which magnesium oxide is the solid with the greater number of basic sites. This may be due to the different acidity of the probe molecule used (CO2 vs. phenol). phenol (mmol/g)"
S (m2/g)
2.5
8O
240 220
2.0
60
200 180
1.5
160
.,40
w_. o
20
140
1.0 0.0
0.1
AI/Mg+AI
0.2
0.3
120
Figure 1. Surface area (filled symbols) and amount of phenol adsorbed (open symbols) on hydrotalcites of different Mg/AI ratio.
o.o
AI/Mg+AI
0.3
o.,
Fig 2. Conversion after 1 h of isophorone (open symbols) and mesityl oxide (filled symbols) with H202 and hydrotalcites of different Mg/AI ratio.
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The most relevant results of reactivity are gathered in Figure 2, which shows the conversion of alkene after 1 h plotted against the A1/Mg+A1 ratio of the catalyst. As can be seen, there is a maximum for the catalytic activity and with both ketones, isophorone and mesityl oxide, the hydrotalcite showing the highest catalytic activity has a ratio Mg/AI = 3. This fact seems to indicate that the number of basic sites able to abstract a proton from hydrogen peroxide should be higher in the hydrotalcite with Mg/A1 = 3. This result is in agreement with the previous measurements of the catalyst basicity, which are shown in Figure 1. On the other hand, the values of surface area (Figure 1) do not show any relationship with the behaviour in catalytic activity. The formation of dioxolane (Table 1) is also influenced by the basicity of the solid. The higher basicity of the catalyst, the higher amount of the dioxolane obtained at short reaction times. According to the reaction mechanism (Scheme 3), the formation of dioxolane should be a reversible reaction, whereas the formation of the epoxide should be irreversible. So, at long reaction times the preferent obtention of epoxide is always observed. Consequently, the use of solids of high basicity and short reaction times are necessary for the efficient production of dioxolane. Table 1 Epoxide/dioxolane ratio in the reaction of mesityl oxide with H202/hydrotalcite. A1/Mg+A1 epoxide/dioxolane (1 h) epoxide/dioxolane (24 h) 0.00 51/49 76/24 0.14 28/72 95/5 0.25 8/92 95/5 0.33 32/68 64/36
'
O
i
O
O--O
ASo. O--O
Scheme 3
Finally, we considered it interesting to test the reusability of these catalysts. The solid with Mg/A1 = 3, filtered off and washed after 1 h reaction with isophorone, was dried and reused without further activation. The catalyst maintained its activity during at least two recycling experiments.
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3.2. Epoxidation of chiral alkenes Another objective of this work was the application of heterogeneous systems to the epoxidation of chiral alkenes. With this aim in mind, two substrates with interest from a synthetic point of view were selected, a ketone (6) and a cyanoester (7) (Scheme 4), which are precursors of sugars and polyhydroxyamino acids, respectively. In fact, only two related examples have been described previously in the literature and these involve homogeneous systems and an unsaturated sulfone [7] or a ketone [8] as substrates. With alkenes 6 and 7 we decided to compare the system HEO2/hydrotalcite (Mg/A1 = 3) with KF-alumina/TBHP [2,3].
••'0
Y
0~~~~/x
oxidant ~ ' ~ 0 base" O ~
Y x S
X•O
y
R ....
6: X = COCH3,Y = H 7: X = COOCH3,Y = CN Scheme 4 As can be seen in Table 2, both systems are efficient in the epoxidation reaction. However, the use of hydrogen peroxide leads to some by-products, such as the Michael addition of methoxide to the ketone (6) or the partial hydrolysis of the nitrile group to the amide in the cyanoester (7) (Scheme 5). With both substrates, which bear the same chiral auxiliary, the diastereoselectivity is nearly the same, with up to 50% de obtained with the system KFalumina/TBHP. The use of hydrogen peroxide leads to lower diastereoselectivities, due probably to the smaller size of the peroxide anion, which attacks the double bond. However, solvation effects cannot be ruled out given that in the two heterogeneous systems different solvents are used.
Table 2 Epoxidation of chiral alkenes with heterogeneous systems. % yielda % d.e. a alkene oxidant base % conv. a H202 hydrotalcite 70 60 b 10 6 6 TBHP KF/alumina 100 100 40 7 H202 hydrotalcite 100 70 c 36 7 TBHP KF/alumina 100 100 48 a Determined by 1H-NMR spectroscopy, b 10% yield of several by-products, c 30% yield of nitrile hydrolysis.
The hydrolysis reaction can be used in a preparative way with total selectivity, given that the H202/hydrotalcite system led to a quantitative conversion of the nitrile group in the molecule bearing ketal, epoxide and ester groups (Scheme 5) without problems of hydrolysis of any of them.
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H202
COOCH3
hydrotalcite
~ " ~ .O
CONH 2
O~COOCH
3
Scheme5 4. CONCLUSIONS The catalytic activity of the hydrotalcites in the epoxidation of electron-deficient alkenes depends on the basicity of the solid, which can be controlled by the Mg/A1 ratio. The highest catalytic activity corresponds to a hydrotalcite with a ratio Mg/A1 = 3. The solid with this Mg/AI ratio is also the catalyst with the highest number of basic sites, as measured by phenol adsorption in liquid phase. The epoxide/dioxolane selectivity also increases with the basicity of the solid and the reaction time, showing the reversibility of the reaction of dioxolane formation. The hydrotalcite is recoverable and reusable at least twice without loss of catalytic activity. The heterogeneous systems hydrotalcite/H202 and KF-alumina/TBHP are also active in the epoxidation of chiral alkenes derived from D-glyceraldehyde, which are precursors of interesting products from the point of view of biological activity. Up to 50% de is obtained with the system KF-alumina/TBHP, and this is the most efficient in terms of yield, selectivity to epoxidation and diastereoselectivity. The size of the oxidant (TBHP>H202) and solvation effects (toluene v s . aqueous methanol) may account for this higher efficiency.
Acknowledgement:This
work was made possible by the generous financial support of the
C.I.C.Y.T. (Project MAT96-1053).
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
C. Cativiela, F. Figueras, J. M. Fraile, J. I. Garcfa, J. A. Mayoral, Tetrahedron Lett. 36 (1995) 4125. V.K. Yadav, K. K. Kapoor, Tetrahedron Lett. 35 (1994) 9481. J.M. Fraile, J. I. Garcfa, J. A. Mayoral, F. Figueras, Tetrahedron Lett. 37 (1996) 5995. B.M. Choudary, M. L. Kantam, B. Bharathi, C. V. Reddy, Synlett (1998) 1203. J. Shen, J. M. Kobe, Y. Chen, J. A. Dumesic, Langmuir 10 (1994) 3902. J.I. Di Cosimo, V. K. Dfez, M. Xu, E. Iglesia, C. R. Apestegui'a, J. Catal. 178 (1998) 499. A.D. Briggs, R. F. W. Jackson, P. A. Brown, J. Chem. Soc., Perkin Trans. 1 (1998) 4097. P.C. Ray, S. M. Roberts, Tetrahedron Lett. 40 (1999) 1779.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
H y d r o x y l a t i o n o f benzene to phenol with nitrous oxide on Fe-silicalites R. Monaci a, E. Rombi "~,M.G. Cutrufelio a, V. Solinas a and G. Berlier l', G. Spotob "Dipartimento di Scienze Chimiche, Universi~ di Cagliari, via Ospedale 72, 09124 Cagliari, Italy bDipartimento di Chimica IFM, Universit~i di Torino, via P. Giuria 7, 10125 Torino, Italy The title reaction was studied, at 623 K and atmospheric pressure, over Fe-si|icalites with different Fe203 content. The best results were obtained for the sample with a Fe203 content of 0.82 wt %. FTIR spectroscopy was used to investigate the nature of the extraframework iron species in Fe-silicalites using NO as probe molecule. 1. INTRODUCTION Phenol is an important intermediate for the production of petrochemicals, agrochemicals and plastic products. More than 90 % of the phenol produced world-wide is obtained by the multi-step process of cumene oxidation. An economical single-step process by means of the direct hydroxylation of benzene to phenol has been studied for some time. Conventional partial oxidation methods with molecular oxygen as oxidant have not given satisfying results, as the reaction mainly leads to the destruction of the aromatic ring [1]. Better results have been obtained in the hydroxylation of benzene with an oxygen/hydrogen mixture, using silicasupported precious metals as catalysts [2]. More promising results seem to derive from the use of alternative oxidants as nitrous oxide. In the first studies on the benzene oxidation by N20, silica-supported vanadium, molybdenum and tungsten oxides had been used as catalysts [3]. More recently, zeolite catalysts as Fe,Na-ZMS-5 and Fe-silicalites structurally similar to ZSM-5 zeolites have been used, providing very' interesting results [4,5]. The iron species located in the so called a-sites, which are particularly suitable for the N20 decomposition, seem to play a key-role on the catalytic performance of these materials, as shown by Panov and co-workers [6,7]. According to them, the N20 decomposition occurs on extraframework iron microclusters and leads to surface reactive oxygen species (a-oxygen), which are responsible for the oxidation of benzene to phenol [8]. The aim of this work is to investigate the influence of the iron content on the catalytic activity of Fe-silicalites and give useful additional information on the surface properties of these materials, by studying the adsorption of probe molecules by FT1R spectroscopy.
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2. EXPERIMENTAL
2.1. Materials Benzene was a > 99 % pure Aldrich reactant; nitrous oxide was a 99.998 % pure SIAD product. 2.2. Catalysts Four different samples of Fe-silicalite, synthesised as reported elsewhere [9], were used. The iron content of the catalysts, expressed as percent by weight of Fe203, is reported in Table 1. 2.3. Apparatus and procedure The experimental runs were performed at atmospheric pressure in a continuous fixed-bed quartz-glass microreactor, with the following operative conditions: temperature, 623 K; space velocity. (WHSV), 4 h-l; R (N20 moles / benzene moles), 5. Hourly collected products samples were dissolved in acetone and analysed by a Carlo Erba 4160 gas chromatograph fitted with a fused silica capillary column (50 m Petrocol DH 50.2, 0.25 mm I.D., 0.25 lure film thickness; Supelco) and a flame ionisation detector. Products were identified by gas chromatography / mass spectroscopy (GC,qVIS) analysis. Phenol was the main product; only at higher temperatures (> 673 K) small amounts of byproducts, formed by further oxidation of phenol, were detected (mainly benzoquinone and benzofuran). The complete oxidation of benzene to carbon dioxide was the main undesired reaction. The results have been expressed in terms of benzene conversion (moles of benzene reacted / moles of benzene feeded) and of selectivity to phenol (moles of phenol formed / moles of benzene reacted). 2.4. IR measurements The IR experiments were carried out on a Bruker IFS 66 FTIR instrument equipped with a cryogenic MCT detector and running at 2 cm -1 resolution. The Fe-silicalite samples were in form of self-supporting pellets suitable for measurements in transmission mode. 3. RESULTS AND DISCUSSION Activity comparison runs were performed on fresh catalyst samples at the previously mentioned standard conditions. To point out the influence of the iron content of the catalysts on their activity, Figure 1 shows the selectivity towards phenol for three Fe-silicalites at conversion values of 10 mol % (3 h on-stream) and 4 mol % (7 h on-stream), for different weight percentages of Fe203. The Fe-Si-1 sample, containing 0.06 % of Fe203, turned out to be inactive. As it can be seen from the figure, selectivity increases with the Fe203 percentage, reaching the highest value for an Fe203 content of 0.82 % (Fe-Si-3). Higher Fe203 contents mainly favour the complete oxidation at the expense of the phenol fbrmation. Figure 2 shows the change in conversion and selectivity to phenol with time-on-stream for the Fe-Si-3 sample. Conversion decreases during the run, whereas selectivity increases. The same trend was shown by all the samples.
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Table 1. Iron content of Fe-silicalite catalysts. Catalyst
Fe-Si- 1
Fe-Si-2
Fe-Si-3
Fe-Si-4
FezO3 (wt %)
0.06
0.36
0.82
1.70
100-
o
L%_, /
80-
E 60-
~
I
J /
/l .-ill ~Jm
~
40r~
I
20O0.36
/Am IWT
0.82
1.70
F e 2 0 3 ( w t o4)
Figure 1. Selectivity to phenol at 10 % (m) and 4 % (i--1) conversion.
100 80
Z
60
o
E
40 20
~.......................i't!!!i...... ,::. I
1
t
|
2
I
i
3
I
|
4
I
|
5
I
!
6
l
I
7
'";~;':'
8
t-o-s (h) Figure 2. Conversion (ii!ii~)and selectivity (A) vs. time-on-stream; Fe-Si-3 catalyst, T - 623 K, WHSV = 4 h -1, RN20/C6H6 = 5.
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As reported by other authors [ 10], the benzene oxidation by N20 is accompanied by coke fbrmation and subsequent catalyst deactivation. Removing coke from the catalysts, by burning it out in air flow, resulted in the recovery of their catalytic activity. The decrease in the conversion value during the run and the complete deactivation of the catalyst within 10 hours on-stream can therefore be ascribed to the production of phenol and polyphenols, which are coke precursors. A possible interpretation of the selectivity increase with time-on-stream could be the presence, on the surface, of catalytic sites which can activate N20 in different ways and then produce phenol or favour the total oxidation. The nature of the extraframework iron species was studied by FTIR spectroscopy, using NO as probe molecule, on the most selective catalyst (Fe-Si-3). The IR spectrum of the sample (previously outgassed at 773 K), in equilibrium with 10 torr of NO at room temperature, is shown in Figure 3 (dotted line). In the same figure, the effect of the gradual decrease in the equilibrium pressure of NO is also reported (full and dashed lines). Examining the figure, it can be said that: i) at the maximum NO coverage, the spectrum is dominated by a strong absorption at 1808 cm ~ superimposed on a second band centred at 1839 cm -~. Weaker bands appear at 1914 and 1765 cm-1; ii) reducing the NO pressure, the bands at 1914 and 1808 cm- are progressively erosed and in the end completely disappear. This phenomenon is 1.5
1.5-
~ ,
t2
~ ~
! !
1.2-
0.9
D.9-
=o
r-
tO
2Mo6++2V4+ isobutyric acid ~ methacrylic acid In the reactions of selective oxidation the important role belongs to the absorbed oxygen stability of its linkage with catalyst and the amount of reaction capable oxygen. To
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make clear this problem the thermodesorption of oxygen from the surface of various HPA was investigated. For this purpose, the adsorption of oxygen was carried out at 300-450~ for 20-30 min. Aiter this, the system was cooled until the room temperature and then it was heated with the linear velocity of 15~ until 500~ On the spectra of thermodesorption of oxygen from the catalysts HsPWIoV2040/SiO2 three peaks with the maximum of temperature 120-160, 200-290 and 440-450~ are observed (Table 4). The displacement of W-atoms by V-ones leads to a slight removal of the maximum of temperature to the side of low temperature, which can testify the weak linkage of oxygen with lhe catalyst. The amounts of adsorbed oxygen corresponding to these forms also differ. According to the data presented in the Table 4, it follows that under the substitution of W-atoms for V-ones the amount of low temperature oxygen decreases but, on contrary, the volume of high temperature oxygen increases. Table 4 Thermodesorption of oxygen from HPA Sample
H3PWlzO40/SiO2 HsPW10V2040/SiO2
The temperature of maximum, T~ 160 290 440 120 200 440
The amount of desorbed oxygen, mmol/g 1.04 0.630 0.554 0.545 0.668 0.682
Therefore, the catalytic activity of HPA in selective oxidation of isobutyric aldehyde into a isobutyric acid also increases. REFERENCES
1. J.V.Kojevnikov, Russian Chemical Review, No. 9 (1987) 1417. 2. E.A.Nikytina, Heteropoly Compounds, Mir, Moscow, 1961. 3. S.M.Zulfugarova, M.K.Munshieva, D.B.Tagiev, P.S.Mamedova, The Method of the Obtaining of C6-, C8-olefins Dimers, Author Certificate (USSR) No. 1723792 (1991). 4. D.B.Tagiev, S.M.Zulfugarova, M.K.Munshieva, P.S.Mamedova, The Catalyst for Oligomerization of C6-olefins, Author Certificate (USSR) No. 1743035 (1992). 5. S.M.Zulfugarcwa, D.B.Tagiev, M.K.Munshieva, P.S.Mamedova, The Method of Oligomerization of C6-olefins, Author Certificate (USSR) No. 1823415 (1992). 6. J.B.Mollat, G.B.Garvey, NATO Adv. Res. Workshop, Chattily, London, (1990) 193.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G, Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Modifications of vanadium phosphorus oxides by aluminium phosphate for n-butane oxidation to maleic anhydride Steve Holmes a'b, Louisa Sartoni a'c, Andy Burrows d, Vincent Martin a Graham J. Hutchings c, Chris Kielyb'd, Jean-Claude Volta a a. Institut de Recherches sur la Catalyse, 2 Avenue A. Einstein, 69626, Villeurbanne C6dex, France. b. Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool, Liverpool, Merseyside L693BX, United Kingdom. c. Department of Chemistry, Cardiff University, Cardiff, PO Box 912, Cardiff CF10 3TB,United Kingdom. d. Department of Engineering, University of Liverpool, Liverpool, Merseyside L693BX, United Kingdom Aluminium phosphate associated with vanadium phosphorus oxide is demonstrated in improving both the catalytic performance for the production of maleic anhydride from n-butane and the time of activation of the catalyst as compared to conventional v a n a d i u m p h o s p h o r u s oxide catalysts. A l u m i n i u m phosphate is prepared at a fixed pH and then added during the preparation of the VOHPO 4, 0.5 H20 precursor. The morphology of the VPO precursor is influenced by the nature of the A1PO material. The VPO-A1PO catalyst is then activated in situ under an-butane/air atmosphere. During the activation, ( V O ) 2 P 2 0 7 is formed and the original amorphous A1PO material crystallizes. No new ternary VA1PO phase is observed. The improvement in the catalytic properties of the mixed VPO-A1PO oxide system cannot solely be explained in terms of a doped V(A1)PO catalyst (A1/V= 1%) prepared independently, for which the catalytic performances are lower. 1. INTRODUCTION Few examples exist in the open literature on the possibility of supporting vanadium-phosphorus oxides (VPO) catalysts [1-7]. If some of these supports, such as alumina, silica or titania are considered favourably for mild oxidation of olefins, the catalytic performances for n-butane oxidation to maleic anhydride are usually poor. Aluminium phosphate has been reported as a support material for VPO catalysts for selective oxidation of 1-butene to maleic anhydride [4]. In this communication, we show that there is a significant benefit in using a l u m i n i u m phosphate associated with VPO to improve both the catalytic performances for
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the production of maleic anhydride from n-butane and the time of activation of the catalyst. The interaction between VPO and A1PO is studied. 2. EXPERIMENTAL
2.1. Catalysts preparation A1PO materials were first prepared in aqueous media by precipitation at constant pH (ranging from 4 to 9) of aluminium nitrate by ammonium hydroxide in a phosphoric acid solution (P/A1 = 1.6). The materials were then calcined under air at 500~ Two A1PO materials precipitated at pH 6 and pH 9 (500~ calcined) and called A1PO-6 and A1PO-9 were respectively chosen for a series of further VPO-A1PO preparations. For the synthesis of the VPO-A1PO precursors, the V/A1 ratio was chosen on the basis of a theoretical monolayer of the VOHPO 4. 0.5 H20 precursor on the A1PO material, taking into account the BET area of the support. The incorporation of the VOHPO 4. 0.5 H20 precursor on the A1PO's was effected during the preparation of this precursor. Thus, V20 s (11.Sg) was first refluxed with isobutanol (250 ml) for 16 hours. The mixture was then cooled at room temperature and A1PO was then added with phosphoric acid (16.49 g, 85%) at a suitable V/A1 ratio and the mixture was refluxed for a further 16 hours. The light blue suspension was then separated from the organic solution by filtration and washed with isobutanol (200 ml) and ethanol (150 ml, 100%). The resulting solid was refluxed in water ( 9 ml H20/ g solid), filtered hot, and dried in air (110~ 16 hours). 2.2. Characterisation techniques The atomic composition of the materials was determined by atomic absorption spectroscopy. Powder X-ray diffraction analyses were performed using a Siemens goniometer equipped with a quartz front monochromator (Cu K~ radiation). UV-visible spectra were recorded on a Perkin-Elmer Lambda 9 spectrometer equipped with an integration sphere . An Hitachi $800 scanning electron microscope was employed to obtain topographical information from the precursors and catalysts. TEM observations were made in a JEOL 2000 FX electron microscope equipped with a LINK system EDX detector. The 31p and 27A1 measurements were performed on a Bruker MSL 300 NMR spectrometer. Conventional spectra were obtained at 121.5 MHz using a 90~ sequence. Raman spectra of the precursors and catalysts were recorded on a Dilor Omars 89 spectrophotometer equipped with an intensified photodiode array detector. The 514.5 nm emission line from an Ar § ion laser was used for excitation. 2.3. Catalytic measurements The oxidation of n-butane was performed at 400~ at atmospheric pressure under oxidizing conditions ((nCa = 2%, 02 = 18%, He = 80%) - GSHV = 2000h 1. Analysis of reactants and the products of reaction was performed by on-line gas chromatography.
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3. RESULTS A N D DISCUSSION
After calcination at 500~ the A1PO solids exhibited a BET area ranging from 65 m2/g (pH = 6) to 91 m2/g (pH = 9). They are amorphous and mesoporous (pore size 6.7-15 nm) with a type IV isotherm. Calcination at 1000~ gave a well crystallized A1PO4 tridymite structure. Figure 1 shows the XRD spectra of the VPO-A1PO precursors prepared from A1PO-6 and A1PO-9 materials. The peaks present are both characteristic of only the VOHPO4. 0.5 H20 hemihydrate and of the A1PO4 tridymite phases. It is interesting to note that crystallized A1PO4 (trid.) is observed when the initially A1PO amorphous materials have been added during the synthesis of the VPO precursor. The VOHPO 4. 0.5 H20 diffraction spectrum is typical of the so called ~ V P D - morphology (broad (001) and slight intense (220) line) which is known to be obtained by reduction of VOPO4,2 H20 by isobutanol [8]. Intensity
(a.u.)
~ ~"
,.q "-'al
10
15
2~)
A
~
i
i ~
iI1~
30
o VOHPO4. 0.5 H20 4, ALP04(trid.)
VPO-AIPO-9 ,'~ ,~,
3~5
4~) 20( ~)
Fig. 1 : XRD spectra of the VPO-A1PO-6 and VPO-A1PO-9 precursors The SEM examination of both VPO-A1PO precursors reveals the existence of two morphologies : the first is characteristic of , O
(13
~ 30
20 ""--c I
I
300 600 W.F-1 / g.h.mol-I
I
900 0
I
20
I
I
40 60 Conversion / %
0
80
Fig. 4. Dependence of the conversion on W/F (A) and selectivity to MA as a function of the conversion (B). (m): C-4 (Oc), (n); C-4 (Bu), (0); 17wt%VP-Oc/SiO 2, (V); 13wt%VP-Oc/SiO2(w), (O); 17wt%VP-Bu/SiO 2, (ix); 15wt%VP-AA/SiO2, (A); 29wt%VP-AA/SiO 2. XRD of 17wt%VP-Oc/SiO 2 and 13wt%VP-Oc/SiO2(w) showed t h a t weak peaks due to VOHPO40.5H20 were detected before the reaction. After the reaction, broad and weak peaks due to (VO)2P207 appeared for these VP-Oc/SiO2, while these peaks were not clear for other VP-SiO 2 composites. It is likely t h a t the thin layered (VO)2P207 is the active surface phase of the SiO2-composites for the MA
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formation. The lower selectivities of these composites than those of C-4 are probably due to the presence of V 5§ species and low crystallinity of VP species. Benziger et al. inferred that VP oxides prepared from the compounds of VOHPO40.5H20 with amines showed an activity with the selectivity for MA (19 50%), while the activity was low [18]. Kung et al. [19] reported t h a t VPoxide/SiO 2 (P/V = 1) obtained from NH4VO 3 and NH4H2PO 4 gave 20 - 30% selectivity to MA. Furthermore, Herron et al. claimed [20] that VP oxides/SiO 2 prepared from a complex like (VO)3(C2H50)2PO2)6CH3CN , was highly active, while the selectivity was 30%. In conclusion, our novel method utilizing intercalationexfoliation is useful for the catalyst preparation of selective oxidation of n-butane and will be applicable to other types of catalyst. This work was partly supported by NEDO. REFERENCES
1. T. Shimoda, T. Okuhara, and M. Misono, Bull. Chem. Soc. Jpn., 58 (1985) 2163. 2. T.P. Moser and G. L. Schrader,. Catal., 92 (1985) 216. 3. G. Busca, F. Cavani, G. Centi, F. Trifiro, J. Catal., 99 (1986) 400. 4. T. Okuhara, I~ Inumaru, and M. Misono, ACS Symposium Series 523 (1993) 156; tL Inumaru, T. Okuhara, and M. Misono, Chem. Lett., (1992) 1955. 5. E. Bordes, Catal. Today, 16 (1993) 27. 6. M. Pepera, J. L. Callahan, M. J. Desmond, E. C. Milberger, P. R. Blum, N. J. Bremer, J. Am. Chem. Soc., 107 (1985) 4883 7. G. Koyano, T. Okuhara, and M. Misono, J. Am. Chem. Soc., 120 (1998) 767. 8. P. Joensen, R. F. Frindt, and S. R. Morris, Mater. Res. Bull., 21 (1876) 457. 9. E. R. Kleinfeld and G. S. Furgson, Science, 265 (1994) 370. 10. S. W. Kelle, H.-N. Kim, and T. E. Mallouk, J. Am. Chem. Soc., 116 (1994) 8817. 11. R. Abe, tL Shinohara, A. Tanaka, M. Hara, J. N. Kondo, and tL Domen, Chem. Mater., 9 (1997) 2179. 12. T. Sasaki, S. Nakato, S. Yamauchi, and M. Watanabe, Chem. Mater., 9 (1997) 602. 13. G. Ladwig, Z. Anorg. Allg. Chem., 338 (1965) 266. 14. H. Igarashi, tL Tsuji, T. Okuhara, and M. Misono, J. Phys. Chem., 97 (1993) 7065. 15. T. Nakato, Y. Furumi, and T. Okuhara, Chem. Lett., (1998) 611. 16. E. Bordes, P. Courtine, and G. Pannetier, Ann. Chim., 8 (1973) 105. 17. H. Nakajima and G. Matsubayashi, J. Mater. Chem., 5 (1995) 105. 18. J. B. Benziger, V. Guliants, and S. Sunderesan, Catal Today, 33 (1997) 49. 19. tL E. Birkeland, S. M. Babitz, G. tL Bethke, H. Kung, and G. W. Coulson, S. R. Bare, J. Phys. Chem. B, 101 (1997) 6895. 20. N. Herron, D. L. Thorn, R. L. Harlow, and G. W. Coulston, J. Am. Chem. Soc., 119 (1997) 7149.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Molecular Structure-Reactivity Relationships in n-Butane Oxidation over Bulk VPO and Supported Vanadia Catalysts: Lessons for Molecular Engineering of New Selective Catalysts for Alkane Oxidation V.V. Guliants a*, J. B. Benzigerb, S. Sundaresanb, and I. E. Wachs c
'Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221-0171 bDepartment of Chemical Engineering, Princeton University, Princeton, NJ 08544 CDepartment of Chemical Engineering, Zettlemoyer Center for Surface Studies, Lehigh University, Bethlehem, PA 18015 The present study addresses the nature of the promoter effect in the bulk VPO and supported vanadia catalysts. No correlation was found between the electronegativity of the promoter or oxide support cation and the catalytic properties of these two catalytic systems. The enhancement of surface acidiw had a beneficial effect on both the rate of n-butane oxidation and selectivity to maleic anhydride over the bulk VPO and supported vanadia catalysts. These findings suggested that the activation of n-butane on both the bulk and supported vanadia catalysts may require both a redox and an acid site. The results of the present study further demonstrate that the supported vanadia catalysts represent a suitable model for bulk VPO catalysts. 1.
INTRODUCTION
1,
The partial oxidation of n-butane to maleic anhydride over the bulk VPO catalysts is the only known commercial process for an alkane oxidation. Vanadyl(IV) pyrophosphate, (VO)2P207, displaying preferential exposure of the (100) crystal planes is critical for active and selective VPO catalysts [1, 2]. The catalytic activity of the bulk VPO catalysts is confined to a very thin surface region of the (100) planes of (VO)2P207 [3, 4], suggesting similarities between the bulk VPO and supported catalysts. The vanadyl dimers present in the hypothetical surface (100) planes of (VO)2P207 have been suggested as the active sites for n-butane oxidation to maleic anhydride [ 1, 5-7]. Supported vanadium(V) oxide catalysts have recently attracted attention as promising model catalysts for selective oxidation of n-butane [8]. Unlike the bulk VPO catalysts, the model supported vanadia catalysts possessed the surface molecular structures that could be reliably established by a variety of spectroscopic techniques [9]. The recent study [10] suggested the critical involvement of the bridging V-O-support bond, and particularly V-O-P bond in n-butane oxidation. Moreover, this oxidation reaction was more efficient when multiple surface vanadia sites were present at high coverages, which is similar to the proposed models of n-butane oxidation over the bulk VPO catalysts [ 1, 5-7]. These findings indicate that as a model system, the supported vanadia catalysts can provide insights into the mechanism of n-butane oxidation over bulk VPO catalysts.
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The purpose of this paper is to study the promoter effect and further elucidate the structure-reactivity relationships for n-butane oxidation to maleic anhydride on well-defined promoted VPO and supported vanadia catalysts. 2. 2.1.
EXPERIMENTAL Synthesis The model bulk organic VPO precursor, VOHPO4x0.5H20, was prepared by a twostep method [11] and transformed thermally into (VO)2P207 in 1.2 vol. % n-butane in air at 673K. Prior to this transformation, ca. 0.25 wt.% of promoter elements (Si, Ti, Zr, and V alkoxides, Aldrich, Inc.) dissolved in anhydrous ethanol were introduced via incipient wetness impregnation of VOHPO4x0.5H20. The details of this preparation method may be found elsewhere [ 10]. A reference unpromoted VOHPO4x0.5H20 precursor was prepared by a similar impregnation method using anhydrous ethanol. The supported vanadia catalysts were prepared by the incipient-wetness impregnation of the metal oxide supports (SiO2, TiO2, ZrO2, Nb2Os, and A1203) with a vanadium isopropoxide solution in methanol (Alfa, 95-99% purity) [10]. 2.2.
Characterization The powder X-ray diffraction, Raman and BET procedures have been previously described [2]. The XPS analysis was performed using a Model DS800 XPS surface analysis system (Kratos Analytical Plc.). During kinetic tests, ca. 1 g of promoted VOHPO4x0.5H20 or supported vanadia catalyst was placed into a U-tube Pyrex glass reactor inside an aluminum split block. The kinetic study was conducted in 1.2% n-butane in air at 653K and 494K for the promoted VPO and supported vanadia catalysts, respectively. The details of the kinetic tests and product analysis may be found elsewhere [2]. 3.
RESULTS The XRD patterns and Raman spectra of the bulk model organic VPO precursors as well as the flesh and equilibrated catalysts showed the presence of only VOHPO4x0.5H20 and (VO)2P207, respectively, and are not shown here. The pyrophosphate Raman band was observed at 924 cm ~, suggesting the presence of some VOPO4 phase [2]. The intensity ratio of the "interlayer" to in-plane reflection of (VO)2P2Ov, I200/I042,which is frequently used as an indicator of the crystal morphology and disorder [2] showed little variation among the promoted catalysts. The (VO)2P207 phase in these catalysts possessed a thin platelet morphology that can be observed in the SEM pictures (Figure 2 in [5]). These platelets preferentially exposed the (100) planes, which have been proposed to contain the active and selective sites for n-butane oxidation according to several recent models [ 1, 5, 12]. Based on the SEM observations, the surface (100) planes accounted for nearly 90% of the total surface area of these catalysts. The surface areas of the VPO catalysts were low (ca. 4.5 m2/g), reflecting the large size of the platelet crystals. The promoter surface coverage was calculated based on several assumptions. It was assumed that the promoters were completely localized at the surface and formed a square close-packed lattice of the surface metal oxide phase. Significant surface enrichment in the promoter elements was indeed confirmed by the XPS surface measurements of the Si- and Nb-promoted VPO catalysts (Table 1). The Sipromoted catalyst showed higher surface enrichment in the promoter element than the Nbpromoted catalyst despite the lower silica content. The promoter surface coverage was estimated from the knowledge of the quantity of the promoter applied, the surface area of the promoted VPO catalysts and the promoter metal-oxygen bond distance. The average metaloxygen bond distances were taken from the published crystal structure data [13] for the corresponding metal oxides. The estimated promoter surface coverages (Table 1) are very
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close to 0.25 monolayer with the exception of the Si-promoted catalyst which had a slightly lower coverage (19=0.14).
Table 1 The effect of promoters on n-butane oxidation to maleic anhydride (MA) on bulk VPO catalvsts at 653K in 1.2 vol.% n-butane in air. Promoter (M)
Wt. (g)
Flow (cc/min)
Conv(C4) (mol.%)
s(MA) (mol.%)
C'4 TOF (10"5/s)
MATOF (105/s)
19
R
none EtOH Si Ti Zr
0.29 0.34 0.49 0.36 0.36
15.5 14.4 15.7 16.1 18.2
20 21 23 13 20
35 37 47 16 11
53 40 39 36 62
19 15 18 6 7
0 0 0.14 0.26 0.25
29 nc nc
V Nb
0.35 0.35
12.3 25.0
19 17
36 53
43 80
16 43
0.26 0.26
nc 19
Note: Wt. is the catalyst weight; 19, the promoter surface coverage, R, the ratio of the promoter concentration in the 2-4 nm surface region (XPS) to its total concentration; nc, not collected. The results of n-butane oxidation over a number of promoted VPO catalysts are summarized in Table 1. The model organic VPO catalysts displayed lower selectivity to maleic anhydride as compared to the conventional organic VPO catalysts [2]. The presence of some VOPO 4 phase suggested by the Raman band shift of pyrophosphate [2] may be responsible for the inferior catalytic performance of the model organic system. The time required to reach the steady state did not appreciably vary among the promoted VPO catalysts of this study and was ca. 240h under catalytic reaction conditions. 70
9 none A D
2 50
a Si
D DD
9
DD
ITi
9 D
+Nb
30
oZr
~'~ 20
DV
e ~ ell
~
10
OO
r.~ 0
i
0
5
1
10
i
15
i
9
20
25
n-Butane Conversion (rml.%) Figure 1. Catalytic performance of the bulk VPO catalysts in oxidation of n-butane to maleic anhydride in 1.2 vol.% n-butane in air at 653K.
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The reaction rates were calculated assuming a pseudo-first order reaction [ 1]. Maleic anhydride and carbon oxides were the main oxidation products detected. The thin platelet morphology of the bulk VPO system was particularly suitable for studying fundamental structure-catalytic property relationships of this system. In the present study, the relationship between the catalytic activity and the number of the surface vanadium ions present in the crystallographic (100) planes of vanadyl pyrophosphate (5.034x10tS/m 2 [14]) corrected for the promoter surface coverage was investigated. Both the catalytic activity and selectivity to maleic anhydride were significantly affected by the presence of the promoters (Table 1 and Figure 1). Only the Zr- and Nb-promoted bulk VPO catalysts possessed n-butane oxidation activity superior to that of the unpromoted catalysts, and only the Si- and Nb-promoted catalysts were more selective to maleic anhydride. 4.
DISCUSSION The promoter elements were introduced in the present bulk VPO catalysts by the impregnation of the VOHPO4x0.5H20 precursor at a level that was too low for the promoters to have a structural effect. Therefore, according to a classification proposed by Hutchings [15], these promoted bulk VPO catalysts belong to the Type 2 systems. Recent studies suggested that the bulk VPO promoters present at a low level may function as selective poisons which block unselective surface sites present in the surface (001) planes of (VO)2P207 [16, 17]. Other promoter effects have been also suggested: the enhancement of oxidation of V~Vpo phases into VvoPo4 in fresh catalysts, which accelerates the attainment of the steady state and optimizes the surface Vs+/V4§ distribution [18], formation of ((VO)xMI-x)2P207 solid solutions, which display improved catalytic properties [15], and intercalation and cleavage of the hydrogen phosphate layers in the catalyst precursor structure, VOHPO4x0.5H20 which leads to preferential exposure of the (100) planes of (VO)2P207 in equilibrated catalysts [19]. Promotion by poisoning unselective surface sites may be discerned by observing a decrease in the rate of n-butane oxidation upon addition of an otherwise catalytically inactive promoter as the selectivity to maleic anhydride is improved at fixed catalytic reaction conditions. Examination of the kinetic data in Table 1 suggests that the promoter effect by poisoning the unselective sites may only play a role in the case of the bulk VPO catalyst promoted with silica, which by itself is inert in this hydrocarbon oxidation. The catalytic performance data for the other promoted bulk VPO catalysts shown in Table 1 do not support this mechanism of promoter action. Formation of oxidized phases in the fresh or equilibrated bulk VPO catalysts was detected by neither XRD nor Raman. Moreover, the attainment of the steady was not affected by the presence of promoters in the bulk VPO catalysts. Therefore, this mechanism of promoter action does not appear to be important for the promoted bulk VPO catalysts of this study. The intercalation of the promoters into the layered structure of the VOHPOax0.5H20 precursor and the cleavage of its (010) planes should result in an increase of the surface area and preferential exposure of the (100) planes of (VO)2P207[19]. However, the surface area and relative exposure of the (100) planes of (VO)2P207in the catalysts of the present study remained essentially unchanged. Furthermore, the promoter elements in the most active and selective Nb- and Si-promoted catalysts were concentrated in the surface region. The Si-promoted catalyst was characterized by a much higher RM ratio than the Nb-promoted catalyst despite the lower promoter content, which probably indicates higher solubility of the Nb promoter in the VPO matrix. Therefore, it appears that the promoter elements may partially form a solid solution. However, this process is primarily limited to the surface region and affects the catalytic properties of the bulk VPO catalysts.
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Lessons for Molecular Design of New Oxidation Catalysts Previously, it was concluded that the bridging V-O-support bonds were the kinetically critical functionalities in the selective oxidation of butane to maleic anhydride. The catalytic activity of the supported vanadia catalysts was found to be a very strong function of the specific metal oxide support (Table 1 in [10]). However, the catalytic activity of the supported vanadia catalysts did not correlate with the Sanderson or Pauling electronegativity of the metal cation of the oxide support [10]. Similar to the supported vanadia system, the catalytic activity of the promoted bulk VPO system did not correlate with the abovementioned electronegativity scales. The bulk VPO catalyst treated with the acidic Nb promoter was the most active, which suggests that the surface acidity plays an important role in n-butane oxidation (see below). The treatment of the unpromoted bulk VPO precursor with anhydrous alcohol had a somewhat negative effect on the catalytic activity of the unpromoted bulk VPO catalyst (Table 1) and no effect on its surface area. The surface areas of all equilibrated catalysts of the present study remained relatively constant at ca. 4.5 m2/g within the accuracy of the BET method. Similar alcohol treatment in an earlier study [19] resulted in a significant improvement in the surthce area and catalytic properties of the unpromoted bulk VPO system. It is possible that such treatment in the earlier study [19] facilitated removal of some inactive surface components, such a s VO(H2PO4) 2 or excess orthophosphoric acid. The maleic anhydride selectivity trends revealed that the electronegativity properties [20] of the promoter or bridging V-O-support bond were not related to selectivity, since the selectivity trends were Nb>Si=unpromoted>V>Ti>Zr and AI>Nb>Ti>Zr [10] for the promoted bulk VPO and supported vanadia catalysts, respectively. However, these selectivity trends parallel the strength of the Lewis acidity of the oxide supports and promoter cations, since alumina possesses the strongest Lewis acid sites followed by niobia [21 ]. The other supports and promoter cations of this study possessed only weak Lewis acidity. The silica overlayers were inert in n-butane oxidation, and the improvement of selectivity to maleic anhydride observed in this case was possibly due to selective blockage of surface sites responsible for the total oxidation of n-butane. The V, Ti, and Zr-promoted bulk VPO catalysts possessed the catalytic activity similar to the unpromoted bulk VPO system. However, these promoter cations were less selective in n-butane oxidation to maleic anhydride. These observations indicated the importance of surface acidity for high activity and selectivity of the bulk VPO and supported vanadia catalysts for selective oxidation of nbutane. Similarly, Zazhigalov et al. [22] observed a correlation between the selectivity to maleic anhydride and the surface acidity of the promoted bulk VPO catalysts. According to Zazhigalov et aL [22], moderate surface acidity facilitates desorption of maleic anhydride and prevents its complete oxidation to carbon oxides. In fact, the acidic promoters, such as Nb, employed in this study had a beneficial effect on the selectivity to maleic anhydride over bulk VPO and supported vanadia catalysts, suggesting that these promoters play a crucial role in controlling further kinetic steps of n-butane oxidation. According to several recent models of the active surface sites [ 1, 5-7], pairs of active surface vanadium sites present in the (100) plane of vanadyl pyrophosphate were required for selective oxidation of n-butane on bulk VPO catalysts. The multiple surface vanadia sites present at ca. 0.75 monolayer coverage of vanadia on TiO, were indeed more efficient at oxidizing n-butane to maleic anhydride than the isolated surface vanadia sites present at lower surface coverages (Figure 11 in [10]). Further enhancement in catalytic activity and selectivity to maleic anhydride in the model vanadia/TiO2 system was observed when an acidic metal oxide, Nb2Os, was present at the surface of the V2Os/TiO 2 catalyst, (| Or=0.17 in Table 3 [10]). These findings suggest that the efficiency for maleic anhydride formation on supported vanadia catalysts might be related to the presence of two adjacent surface vanadia sites or a combination of a surface vanadium oxide redox and surface acid
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sites. Similar to the Nb-promoted supported vanadia system, the Nb-promoted bulk VPO catalyst (| displayed the highest activity and selectivity among the bulk VPO catalysts (Table 1). The observed similarities in the catalytic behavior between the supported vanadia and promoted bulk VPO catalysts suggest that selective oxidation of n-butane on the bulk VPO catalysts may also require a Lewis or Bronsted acid site in combination with a surface vanadium redox site [ 10, 23]. 5.
ACKNOWLEDGEMENT This work was supported by the AMOCO Chemical Corporation (Princeton), National Science Foundation Grant CTS-9100130 (Princeton), and the Division of Basic Energy Sciences, Department of Energy under Grant DEFG02-93ER14350 (Lehigh). REFERENCES [1] G. Centi, F. Trifirb, J. R. Ebner, and V. M. Franchetti, Chem. Rev. 88 (1988) 55. [2] V.V. Guliants, J. B. Benziger, S. Sundaresan, I. E. Wachs, J. M. Jehng, and J. E. Roberts, Catal. Today 28 (1996) 275. [3] M.A. Pepera, J. L. Callahan, M. J. Desmond, E. C. Milberger, P. R. Blum, and N. J. Bremer, J. Am. Chem. Soc. 107 (1985) 4883. [4] M. Abon, K. E. Brrr, and P. Delich~re, Catal. Today 33 (1997) 15. [5] V.V. Guliants, J. B. Benziger, S. Sundaresan, N. Yao, and I. E. Wachs, Catal Lett. 32 (1995) 379. [6] B. Schiott, K. A. J~rgensen, and R. Hoffmann, J. Phys. Chem. 95 (1991) 2297. [7] P.A. Agaskar, L. DeCaul, and R. K. Grasselli, Catal. Lett. 23 (1994) 339. [8] V.V. Guliants, Catal. Today 51 (1999) 255. [9] I.E. Wachs and K. Segawa, Characterization of Catalytic Materials, I.E. Wachs (Ed.), Butterworths, Stoneham, MA, 1992, p. 69. [ 10] I.E. Wachs, J.M. Jehng, G. Deo, B. M. Weckhuysen, V. V. Guliants, J. B. Benziger, and S. Sundaresan, J. Catal. 170 (1997) 75. [11] H. Igarashi, K. Tsuji, T. Okuhara, and M. Misono, J. Phys. Chem. 97 (1993) 7065. [ 12] G.J. Hutchings, C. J. Kiely, M. T. Sananes-Schulz, A. Burrows, and J. C. Volta, Catal. Today 40 (1998) 273. [ 13] Metal oxide structure files, Cerius 2 version 3.8 molecular modeling package (1998), Molecular Simulations Inc., 9685 Scranton Road, San Diego, CA, 92121. [ 14] Yu. E. Gorbunova and S: A. Linde, Dokl. Akad. Nauk SSSR 245 (1979) 584. [ 15] G.J. Hutchings, Appl. Catal. 72 (1991) 1, and references therein. [ 16] I. Matsuura and M. Yamazaki, in: New Developments in Selective Oxidation, eds. G. Centi and F. Trifirb (Elsevier, Amsterdam, 1990), 563. [ 17] T. Okuhara, K. Inumaru, and M. Misono, in: Catalytic Selective Oxidation, eds. J. Hightower and S. T. Oyama (ACS, Washington, DC, 1993), 156. [18] J.C. Volta, Catal. Today 32 (1996) 29. [19] D. Ye, A. Satsuma, A. Hattori, T. Hattori, and Y. Murakami, Catal. Today 16 (1993) 113. [20] R.T. Sanderson, Polar Covalence (Academic Press, New York, 1983); L. Pauling, The Chemical Bond (Cornell Univ. Press, Ithaca, NY, 1967); R. G. Pearson, Inorg. Chem. 27 (1988) 734. [21] J. Datka, A. M. Turek, J. M. Jehng, and I. E. Wachs, J. Catal 135 (1992) 186. [22] V.A. Zazhigalov, J. Haber, J. Stoch, I. V. Bacherikova, G. A. Komashko, A. I. Pyatnitskaya, Appl. Catal. A 134 (1996) 225. [23] F. Cavani and F. Trifirb, in: 3rd World Congress on Oxidation Catalysis, eds. R. K. Grasselli, S. T. Oyama, A. M. Gaffney, and J. E. Lyons (Elsevier Science B.V., Amsterdam, 1997), 19.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiomz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
The n a t u r e of the Cobalt Salt affects the Catalytic P r o p e r t i e s of P r o m o t e d VPO L a u r a Cornaglia, Carlos Carrara, J u a n Petunchi and E d u a r d o Lombardo Instituto de Investigaciones en Cathlisis y P e t r o q u i m i c a - INCAPE (FIQ,UNLCONICET), Santiago del Estero 2829 - 3000 S a n t a Fe - Argentina. FAX # 54-342-4536861. E-marl :
[email protected] Cobalt-impregnated VPO catalysts containing 2 and 4% of the metal by weight were p r e p a r e d using two different cobalt salts. The catalytic tests showed t h a t cobalt impregnation significantly increased the overall activity. The use of cobalt acetylacetonate led to a more selective high loading catalyst. To investigate the origin of the cobalt effect, the solids were characterized using XRD, R a m a n Spectroscopy, FTIR and XPS. No structural effects were detected through XRD. After several h u n d r e d hours on stream, the only phase detected in all cases was V(IV) vanadyl pyrophosphate. The surface oxidation state of v a n a d i u m was V(IV). The Co 2p XPspectrum has an intense shoulder at 788 eV, indicating t h a t Co(II) species are present. 1.INTRODUCTION The VPO system exhibits an unique ability to activate and selectively oxidize alkanes. Vanadyl pyrophosphate is used commercially to catalyze the selective oxidation of n - b u t a n e to maleic anhydride. It is generally agreed upon t h a t the best catalyst precursor is VOHPO4.0.5H20 which is converted to (VO)2P207 [1] during activation. Among them, several VOPO4 phases have been claimed as necessary for the catalytic act [2]. However, these phases have never been detected in the so-called equihbrated catalysts t h a t have been on stream for over several h u n d r e d hours [3]. A variety of cations have been added to v a n a d i u m p h o s p h a t e catalysts to improve activity and selectivity. It has been claimed t h a t the addition of the first row transition metal produces an improvement in maleic anhydride yield [4,5 ]. Several papers [4-9] refer to the effect of cobalt used as a promoter. The aim of our work is to study the effect of the Co salt impregnation on precursors prepared by the conventional organic method. Our recognition and gratitude to Prof. Juan Petunchi for his lifelong lasting contribution to Catalysis. The financial support was received from ANPCyT(PICT N~ 14-00000-00719) and UNL (CAI+D 96 Program). The authors are grateful to the Japan International Cooperation Agency (JICA) for the donation of the major instruments used in this study. Thanks are given to Prof. Elsa Grimaldi for the edition of the English manuscript
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2. E X P E R I M E N T A L
Catalysts were p r e p a r e d by refluxing v a n a d i u m pentoxide in a mixture of isobutanol and benzyl alcohol during three hours. Orthophosphoric acid (100%) was then added and refluxed for two hours to obtain the desired P/V ratio in the precursor. The suspension was filtered and the solid dried at 390 K for 24 hs. The promoter was i m p r e g n a t e d on the precursor using isobutanol solutions of two different cobalt salts: Co acetate (VPCox-I1) and acetylacetonate (VPCox-I2), where x indicates the Co w%, I1 or I2 the i m p r e g n a t i o n salt. The catalysts were activated in two steps: 1) The precursor was h e a t e d up to 603K while a flowing mixture of 0.75% n - b u t a n e in air, with a gas hourly space velocity (GHSV) of 900 h 1. 2) The concentration of n-butane was increased to 1.5%. The t e m p e r a t u r e was increased with the limitation t h a t conversion never exceeded 80% and then the GHSV was raised to 2500 h -1. These conditions were m a i n t a i n e d for at least 500 h until stable conversion and selectivity values were obtained (equilibrated catalysts). The catalysts were characterized t h r o u g h Xray Diffraction and Raman, FT Infrared and X-ray Photoelectron Spectroscopies. 3.RESULTS AND DISCUSSION Figures 1.a and u n p r o m o t e d solids.
1.b show the
catalytic results of
promoted and
50 a
70
40
60
30
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20
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30 640 TEMPERATURE/K
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.
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.
--A--VPCo4-I 1 --O--VPCo4-I2
0640
750 TEMPERATURE/K
Figure 1. Catalytic behavior of equilibrated catalysts. Reaction c o n d i t i o n s 1.5% n-butane/air, G H S V - 2500 h 1.
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50
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1500
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W A V E N U M B E R / c m -1 Figure 2. FTIR spectra of VPO, VPCo4-I2 and VPCo4-I 1 precursors The Co-promoted catalysts present a higher yield of maleic anhydride. The impregnated solids present a significantly higher conversion in the whole temperature range, with the exception of the VPCol.5-I. This solid shows the lowest conversion of all the catalysts. The VPCo4I-2 presents a better selectivity at temperatures higher than 663 K. At these temperatures the yield is comparable to industrial catalysts. To investigate how cobalt affects the structure and surface of the VPO catalysts several techniques were used. The FTIR spectra of the precursors are shown in Figure 2. All of them show the characteristic vibrations of the VOHPO4.0.5H20. The C=C vibrations in the plane of the aromatic ring are also observed. These bands are due to benzyl alcohol and benzaldehyde retained in the solid after drying. In the VPCo4-I1 spectrum, bands at 1564 cm -1 and 1430 cm-1 are observed, while in the VPCo4-I2 spectrum, bands at 1523 cm 1 and 1430 cm-1 are detected. They are coincident with the most intense ones of cobalt acetate and cobalt acetylacetonate, respectively. This indicates that even after drying, the cobalt salts are present in the precursor. However, after activating the solids, these bands are not observed and the presence of cobalt oxides is not detected, either. Zazhigalov et al. [8] observed the appearance of Co2P207 for the catalysts with C o N > 0.15. In our case (CoN = 0.13), neither XRD nor FTIR indicate the presence of Co-containing phases. After several h u n d r e d hours on stream, vanadyl pyrophosphate was the only crystalline phase detected. No structural modifications (Table 1) were observed on impregnated solids through XRD. The Raman spectra show that none of the equilibrated catalysts contain VOPO4 p h a s e s . However, Volta and coworkers [7]
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sustain that Co and Fe dopants can modify the VOPO4/(VO)gPgO7 dispersion. They concluded that the activity for maleic anhydride formation is a function of the concentration of the V(V) phases present. In our catalysts, cobalt impregnation produces an increase of the precursor surface P/V ratio. The P/V surface ratio increases upon equilibration in the unpromoted VPO, whereas this ratio decreases in the Co containing catalysts.
Table 1 XRD and XPS data of precursors and equilibrated catalysts. Solid
VPO VPCo 1.5I 1 VPCo4-I 1 VPCo2-I2 VPCo4-I2 a) b) c) d)
FWH1Vro (001) 1.0 1.0 1.14 1.2 1.2
Precursor . P/V)~,: CoN)~ 2.15 3.80 3.70 3.40 3.40
Equilibrated catalyst FWHM d PN)~ c CoN)~ c
0.36 0.60 0.38 0.70
(200) 1.0 1.0 1.1 1.0 1.0
2.8 2.6 2.6 2.7 2.5
0.31 0.26 0.25 0.32
Bulk P/V ratio = 1.26 VOHPO4.1/2H~O (20=15.5~ FWHM in 20 Surface atomic ratios measured by XPS (VO)~P207 (20= 23.2o). FWHM in 20
The surface oxidation states of vanadium were studied by XPS through the analysis of the separation of the signals corresponding to O ls and V 2p.~/2 [10]. This method avoids the use of a reference as we concluded in a previous study [10]. The results are shown in Table 2. This value is equal to 14.2 + 0.2 eV for promoted and unpromoted catalysts. Coulston et al. [12] have reported a correlationship between the A value and the average vanadium oxidation state. These calculated values are around 4. In all the cases studied, the curve fitting leads to a single, well-defined binding energy for V 2p3/2 (517.7+0.1 eV) assigned to V(I\/) in agreement with the values reported by Lbpez Granados et al. [3] for unpromoted catalysts. Both methods support the overwhelming presence of V(IV) on the surface of the VPO equilibrated catalysts. The addition of cobalt does not produce modifications on the vanadium surface oxidation states. Binding energies for cobalt were also measured by XPS (Table 2) for the high loading catalysts. For the VPCo4-I2 and VPCo4-I1 catalysts, the Co 2p3/2 binding energies were different. The value of 783.6 eV indicates the presence of CoO on the surface is unlike. The Co 2p3/2 B.E. is 780.1 eV for CoO. For Co promoted vanadyl pyrophosphate the binding energy is much higher. The Co 2p spectrum has an intense shoulder at 788 eV. In addition, the 2p1/~-2p~2 spinorbit splitting of 15.7 eV for this catalyst indicates that Co(II) species are
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present. This shift might be related to strong interactions between the metal and other atoms of the pyrophosphate matrix and/or to a highly dispersed cobalt. The different B.E. between the two impregnated catalysts could be related to a different surface Co(II) species. Binding energies similar to those were reported [11] for the ion exchanged form of cobalt in zeolites. In these solids, the higher than normal binding energy is suggestive of cobalt in a highly oxidize environment. Zazhigalov et al. [8] reported a similarly high binding energy of 783.0 eV for Co-promoted VPO catalysts. They compared their results with the value of 782.8 found in cobalt molybdate or 782.2 and 783.0 for COC12 and CoF2 and concluded t h a t the metal ligand bonds are polarized in these reference compounds and a similar structure may be expected for cobalt phosphates. They sustained that the high binding energy values of 782.7-783 eV observed in their catalysts can be considered as an additional indication of the formation of cobalt phosphate (detected by XRD). On the other hand, they suggested the existence of strong interactions between cobalt atoms and phosphate groups with a shift of bonding electrons towards the phosphate anions. In our catalysts the C o N nominal ratio is lower than 0.13, and no cobalt phosphates were detected by XRD, FTIR and Raman. In agreement with our results, Hutchings and Higgins [5] have recently reported that the XRD patterns do not even hint the formation of solid solutions at low promoter ratios, M/V < 0.12. Hutchings [4] propose that the promoters at high promoter/vanadium atomic ratios, act as phosphorus scavengers by either formation of metal phosphates or by formation of sohd solutions. Table 2. The surface oxidation states of the equilibrated catalysts Solids
V 2p:~/2"~." A[O Is -V 2p3/2] Vox b Co 2p3/2 c B.E.(eV) FWHM (eV) B.E.(eV) VPO 517.9 2.3 14.3 4.09 -VPCo4-I1 517.8 2.2 14.4 4.03 782.6 VPCo4-I2 517.8 2.3 14.3 4.09 783.4 a Binding energies determined by curve fitting b Using the following correlation : Vox = 13.82 -0.68(A[O ls -V 2p3/2]), from Coulston et a1.[12] c O ls Binding energy equal to 532.2 eV was used as reference. The Co addition significantly increases the overall activity. This seems to correlate with the presence of cobalt on the equilibrated catalyst surface. Using cobalt acetylacetonate for the precursor impregnation produces a solid with higher selectivities and performances comparable to commercial catalysts. In this complex structure the promoter might be incorporated in the crystal lattice of (VO)2P2OT, as reported by Takita et al.[6]. They studied the incorporation of
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promoter elements employing metal acetylacetonates added during precursor synthesis. Incorporation of the promoter elements into the crystal lattice of (VO)2P207 brings about the shift in the IR absorption bands of the V=O stretching mode. In the case of the catalyst prepared in an aqueous medium, in which Zn is located at the surface of vanadyl pyrophosphate, no shifts in wavenumber were observed. In our catalysts the V=O vibration was not modified by the presence of cobalt. 4.CONCLUSIONS The addition of Co significantly increases the overall activity in all the temperature range, while slightly decreasing the maleic anhydride selectivity at low reaction temperatures. The impregnated VPCo4-I2 catalyst has shown the highest selectivity and yield. This seems to be due to a different surface COOI) species present in this equilibrated catalyst. REFERENCES 1. G.Centi, F. Trifir5, J. Ebner and V. Franchetti, Chem. Rev. 88 (1988) 55. 2. F. Ben Abdelouahab, Olier, R. , Ziyad, M. and J. C. Volta, J. Catal. 134 (1992) 151. 3. M. L6pez Granados, J.L.Garcia Fierro, F. Cavani, A. Colombo, F. Giuntoli and F. Trifir5, Catal. Today, 40 (1998) 251. 4. G.Hutchings, Appl.Catal., 72 (1991) 1. 5. G. Hutchings and R. Higgins, J. Catal., 162 (1996) 153. 6. Y. Takita, K. Tanaka, S. Ichimaru, Y. Mizihara, Y.Abe and Y. Ishihara, Appl. Catal. A, 103 (1993) 281. 7. G.Hutchings, C. Kelly, M.T. Sanan6s-Schulz, A. Burrows and J.C. Volta, Catal. Today, 40 (1998) 273. 8. V.A. Zazhigalov, J. Haber, J. Stoch, A. Pyatnitzkaya, G.A. Komashko and V.M. Belousov, Appl. Catal. A, 96 (1993) 135. 9. L. Cornaglia, C. Carrara, J. Petunchi and E. Lombardo, Appl. Catal.A, 183 (1999) 177. 10.L. Cornaglia and E.A. Lombardo, Appl. Catal. A, 127 (1995) 125. 11.J. Stencel, V. Rao, J. Diehl, K. Rhee, A. Dhere and R. DeAngelis, J. Catal, 84 (1983) 109. 12.G.W. Coulston, E.A. Thompson and N. Herron, J. Catal., 163 (1996) 122.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Oxidation of toluene to benzaldehyde over VSbl.xTixO4 catalyst. Kinetics studies. A. Barbaro, S. Larrondo and N. Amadeo a a Labomtorio de Procesos Cataliticos. Depto.de Ing. Quimica, Facultad de Ingenieria, UBA. Pabell6n de Industrias, Ciudad Universitaria, 1428 Buenos Aires, Argentina. In this work a kinetic study of the vapor phase oxidation of toluene by air using titanium doped vanadium antimonate as catalytic system, is reported. The catalytic tests were performed in an integral fixed-bed reactor operating at atmospheric pressure. From the results obtained in the introductory kinetic tests, a kinetic scheme that considers the partial oxidation of toluene to benzaldehyde, a consecutive oxidation of benzaldehyde to carbon oxides and a parallel oxidation of toluene to carbon oxides, is proposed. The corresponding kinetic parameters are estimated using non-linear regression. There is a good fitting between the regression curves and the experimental data. The oxidation of toluene takes place on a partially reduced catalyst surface. The overall rate depends on the partial pressures of the reactants. The three steps of the kinetic mechanism have similar activation energies indicating that the same intermediate is involved. In the range of the operating conditions, there is no rate-controlling step. 1. INTRODUCTION The processes of catalytic partial oxidation of aromatic hydrocarbons in vapor phase are of great importance in chemical technology[1 ]. The partial oxidation of hydrocarbon has been subject of many research works. This is because oxidation catalysis is complex and involves many parallel and consecutive reactions. There have been few comprehensive studies of the kinetics of selective oxidation reactions. In particular, for toluene oxidation, kinetic equations are usually of the power rate law type and Mars-van Krevelen redox model type [2-6]. In many of these works [4-6], low hydrocarbon pressures and high oxygen pressures were used. In consequence, the rate expressions are nearly first order in the hydrocarbon pressure and zero order in oxygen pressure. The data available in the literature about the vapor phase oxidation of toluene show that it is common to obtain selectivities to carbon oxides of 40% and higher, even at low toluene conversion levels [7-8]. This would indicate the presence of a route for direct oxidation of toluene to carbon oxide. However, there are few works [2] considering this direct oxidation of toluene by a parallel route, in the kinetic studies. In a previous work it was found that the vanadium antimonate doped with titanium, with nominal composition VSb0.8Ti0.204 [9], presents good catalytic performance in the toluene oxidation reaction. The products obtained with this catalytic system were benzaldehyde and carbon oxides. In the present work a kinetic study of vapor phase oxidation of toluene by air, using a titanium doped vanadium antimonate as catalytic system, is reported. The aim is to
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propose a kinetic scheme suitable to identify the main reaction steps as well as to determine a kinetic expression that provides a good fitting with the experimental data. Further studies will optimize the operation conditions in order to obtain better selectivites to benzaldehyde. 2. EXPERIMENTAL
2.1. Catalyst Preparation The catalysts were prepared by solid state reaction of mechanical mixtures of pure vanadium (V) oxide, antimony 0ID oxide and titanium (IV) oxide. The amount of each oxide in the mixtures, was the necessary for obtaining a solid catalyst with a nominal composition of VSb0.sTi0.204. The samples were put in quartz crucibles and heated in air by a conventional furnace with a temperature calcination programme, according to the method published elsewhere [9]. X-Ray powder diffraction (XRD) technique was used to check that the structure of the phase obtained was similar to that previously reported as vanadium antimonate with 20% of titanium doping [ 10].
2.2. Catalytic tests The catalytic oxidation of toluene was studied in an integral fixed-bed reactor operated at atmospheric pressure. The reactor was made of a Pyrex glass tube of 13-mm inner diameter. Toluene was fed by means of a carrier air stream flowing through a saturator. The feed toluene/air molar ratio was controlled by adjusting both the saturator temperature and the input air flow rate. The reaction temperature was measured with a sliding thermocouple placed inside the bed. Since the oxidation reactions are highly exothermal, the catalyst bed was diluted (1:10) with glass particles, of the same diameter range, in order to avoid adverse thermal effects.The composition of the feed gas and the effluent were analysed by on-line gas chromatography. The reactor was operated in steady-state conditions. The catalytic tests were performed under the following conditions: catalyst mass: 100-400 mg; total feed rate: 200-600 ml.min 1, toluene molar fraction: 0.001 to 0.01; oxygen molar fraction: 0.04 to 0.2 ; temperature range: 673-723K; particle diameter 95% of phenol conversion for copper catalysts and >80% for nickel ones). However, nickel catalysts showed lower amounts of intermediate oxidation products such as quinones and carboxylic acids. Phenol conversion decreased continously over time for these samples mainly because of the continuous loss of the CuO and NiO phases by elution during the reaction. On the other hand, the spinel phases (obtained for the samples calcined at 973K) showed higher conversions (between 40-75%) and were stable at these reaction conditions. They did not show any loss in activity after a continuous working run of 15 days using a trickle-bed reactor. However, when the experiments were performed in an autoclave a loss of activity was observed for the copper spinel phase (probably due to polymer formation) but not for the CuNi-spinel catalyst.
1. Introduction Hydrotalcite-like materials (HT) belonging to the class of anionic clays have a brucite-like Mg(OH)2 network in which Mg 2+ is isomorphously substituted by a trivalent element M 3+. The structure of the hydrotalcite is very similar to that of brucite. In brucite, each magnesium cation is octahedrally surrounded by hydroxyls [1,2]. The resulting octahedron shares edges to form infinite sheets with no net charge. When Mg 2+ ions are replaced by a trivalent ion (the radius of which, like A13+, is not too different), a positive charge is generated in the brucite sheet. The positively charged Mg-A1 double hydroxide sheets (or layers) are charge-balanced by the carbonate anions residing in the interlayer sections of the clay structure. In the free space of this interlayer, the crystallization water also finds a place [ 1-3]. Anionic clays based on hydrotalcite-like compounds (HT) can be used as catalysts, catalyst supports, ion exchangers, stabilisers, adsorbents etc., mainly because of their variable chemical
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compositions. Recently, they have been among the most widely investigated catalyst precursors [1-3]. Furthermore, all the divalent metals from Mg 2+ to Mn 2+ form hydrotalcitelike compounds, except Cu 2+, which forms HT only when other bivalent cations such as Zn, Cr, Co, Mg, Mn, etc. are present [ 1]. Reichle [4] reports the formation of CuAICO3- HT when the gel obtained by using aqueous bicarbonate solutions as a precipitant is crystallized at relatively high temperatures. However, CuA1-HT are always mixed with other phases, such as malachite or gerhardtite, due to the Jahn-Teller effect of the Cu 2+ ion [ 1]. Supported metal oxide catalysts are also widely used in oxidation processes. The oxidation of dilute aqueous solutions for organic pollutants using air or oxygen over a solid catalyst is a useful and inexpensive process in which the organic compounds are oxidised to carbon dioxide and water. Batch and semi-batch approaches have previously been studied, and recently continuous processes in a trickle-bed reactor have been reported [5-7]. However, the copper and nickel catalysts that have been used up to now for long run-times (e.g. more than 10 days) undergo a considerable loss in activity due to the strong oxidation conditions of the processes, and the solubilisation of the active phases and/or the formation of polymers [5]. Mixed oxides crystallizing in the spinel form are potential catalysts for the oxidation of phenol in aqueous solutions because they are stable in these reaction conditions and their activities can be related to the BET area of the spinel form [5]. This paper studies the catalytic behaviour of several copper-nickel-aluminium mixed-oxide samples with high BET areas, using hydrotalcite-like phases as precursors for the oxidation of phenol aqueous solutions in a trickle-bed and in a semi-batch reactor for comparison. We also study the nature and characteristics of these copper, nickel and copper-nickel- hydrotalcite-like compounds and the chemical changes which take place before the spinel phases are formed.
2. Experimental section 2.1 Sample preparation Seven samples of copper-nickel-aluminium hydrotalcite-like compounds were synthesized with different Cu/Ni/A1 ratios (see Table 1). They were obtained by coprecipitation from two aqueous solutions at a constant pH between 8 and 8.5 __ 0.2, one of which contained appropriate amounts of Cu(NO3)2 x 6H20, Ni(NO3)2 x 6H20 and Al(NO3)3 x 9H20 while the other contained an aqueous solution with triethylamine (1M). The addition was completed in 3h under vigorous stirring. During the coprecipitation process, a constant flow of CO2 was bubbled through the glass reaction vessel. This led to carbonate anions forming in the interlayer region of the solids. The precipitated gel was filtered, washed several times with warm distilled water and then dried in a vacuum at room temperature for 48 h. The calcination process of the samples was performed with a heating rate of 1K/min in air and the final calcination temperature was maintained for 16 h. The copper-nickel-aluminium samples obtained in this way are written as HT[n:I:m](K) where [n:l:m] are the Cu~i/A1 atomic ratios and K the calcination temperature in degrees K. The Cu/A1 contents in the coprecipitates were determined using a JEOL 2000FXII equipped with a LINK probe for EDS analysis and atomic absorption spectroscopy (Hitachi Z-8200). The results obtained from these techniques are very similar and give the following Cu/Ni/A1 atomic ratios: 0.9/0/2 for HT[1:0:2], 5.8/0/2 for HT[6:0:2], 0/0.97/2 for HT[0:1:2], 0/5.9/2 for HT[0:6:2], 0.24/0.77/2 for HT[0.25:0.75:2], 0.49/0.5/2 for HT[0.5:0.5/2] and 0.71/0.24/2 for HT[0.75:0.25:2].These values were similar to the expected values.
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2.2 Sample characterization BET surface areas were calculated from the nitrogen adsorption isotherms at 77 K with a Micromeritics ASAP 2000 surface analyser. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained with a Siemens D5000 diffractometer. The structural evolution during thermal treatment in air was monitored in situ with a high-temperature chamber (HTK Anton Paar) attached to the XRD. The FT-IR spectra were recorded on a Nicolet 5ZDX Spectrometer in the 4000-400 cm -1 wave number range using pressed KBr pellets. Thermogravimetric analyses (TG) were carried out on a Perkin-Elmer TGA 7 microbalance with an accuracy of 1 mg and equipped with a 272-1273 K programmable temperature furnace. The nature of the gases evolved during the thermal decomposition process was monitored with a QTMD FISONS mass spectrometer. 2.3 Catalytic measurements The catalytic behaviour of the samples for the oxidation of an aqueous solution of 5g/L in phenol using air as oxidant was tested using a trickle-bed and a semi-batch reactor for comparison. The reaction conditions were the following: the temperature was 413 K, the air flow-rate 8.3 NL/h and the partial oxygen pressure was maintained at 0.9 MPa. For the continuous reactor, the liquid flow-rate was 35.4 ml/h working at 0.41 h of inverse WHSV (weight hourly space velocity), the particle diameter of the catalysts was 25-50 mesh and 14 g of the catalyst was loaded into the reactor; for the semi-batch reactor (a 1000 ml autoclave), 125 ml of the aqueous solution of phenol and 2 g of catalyst were loaded into the autoclave. Phenol conversion and product distribution were analysed by HPLC (Beckman System Gold) and TOC (Shimadzu 5050). 3. Results and discussion Table 1 shows the results of the BET areas and XRD phases detected for the catalysts. The composition of the samples and the calcination temperatures have a considerable influence on the surface areas. When both the copper or nickel concentration of the sample and the calcination temperature increase, the BET area values decrease. Samples HT[6:0:2] and HT[0:6:2], which have the highest amounts of copper or nickel respectively, have the lowest BET area values. All the samples show a maximum BET area at temperatures around 623 K (which corresponds to the disappearance of the HT-XRD pattern). When the calcination temperature increases to 973 K, the BET area decreases. The BET area values detected for the samples which have a divalent/trivalent ratio of 0.5 were higher because of the formation of more amorphous phases such as hydroxides-carbonates and alumina hydrate (gibbsite) instead of the hydrotalcite phase during the coprecipitation process. The pure and mainly pure hydrotalcite-like phase is only detected for the HT[0:6:2] and HT[6:0:2] samples, respectively, at 373 K. This HT phase is also detected for the other samples at 373 K but is poorly crystallized and is always accompanied with other side phases such as malachite and gibbsite (probably also amorphous hydrotalcite-like phases). The yields and crystallinity of the hydrotalcite-like phase are therefore higher when the copper or nickel content of the sample increases.This is because a divalent/trivalent molar ratio equal to or larger than 2 is needed to prepare crystallographically pure hydrotalcites [6]. However the HT[6:0:2] sample also shows traces of malachite phase. This can be attributed to the lower stability of the copper hydrotalcite phase due to the Jahn Teller effect in the Cu 2+ ion which improves the formation of the more stable malachite phase. A pure hydrotalcite-like phase is only
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obtained for the sample HT[0:6:2]. The samples HT[6:0:2] and HT[0:6:2] calcined at 623 K yield incipient CuO and NiO phases, respectively, with crystallite sizes lower than 9 nm. The calcination of the other samples at this temperature (except for HT[1:0:2]) yields incipient NiO and spinel phases with particle sizes lower than 4 nm. Practically pure spinel phases are detected, at calcination temperatures around 973 K, for these samples (traces of NiO phase are also detected) with particle sizes lower than 7 nm. The copper spinel phase is detected for the sample HT[I:0:2] at temperatures around 973 K. On the other hand, nickel and/or coppernickel spinel begins to form at lower calcination temperatures (623 K). Table 1 Some Characteristics of the Cu-Ni-Based Catalysts BET HT samples Calcination XRD Area (m2/g) Cu/Ni/Al Temperatures (K) Phases (b) [ 1:0:2] [6:0:2] [0:1:2] [0:6:2] [0.25:0.75:2] [0.5:0.5:2] [0.75:0.25:2]
373 623 973 373 623 973 373 623 973 373 623 973 373 623 973 373 623 973 373 623 973
HT,Ma,Gi CuSp HT,Ma T CuSp, T HT, Gi Bu, NiSp NiSp, Bu HT Bu Bu, NiSp HT, Gi Bu, CuNiSp CuNiSp HT, Gi T, CuNiSp CuNiSp HT, Gi CuNiSp CuNiSp
180 220 145 45 55 50 90 190 80 19 115 85 40 167 102 42 155 87 37 163 98
XRD Phases (d)
T,M, B CuSp T,M,B,HT T,M,CuSp Bu,NiSp,B Bu,NiSp Bu,HT,B Bu,NiSp Bu,T,CuNiSp,B CuNiSP Bu,T,CuNiSp, CuNiSp T,CuNiSp,B CuNiSp
XRD Phases (a) B CuSp B CuSp NiSp,B NiSp HT,B NiSp CuNiSp,B CuNiSp CuNiSp,B CuNiSp CuNiSp,B CuNiSp
HT = Hydrotalcite-like phase, Ma=Malachite, Gi=Gibbsite, CuSp=CuA1204, NiSp = N i A I 2 0 4 , CuNiSp = Copper-Nickel Spinel phase, B=bohemite, Bu = Bunsenite (NiO), T = Yenorite (CuO), (b) (d) and (a) = Before, during and after reaction. The FT-IR and TG equipped with a mass detector show carbonate (mainly) and nitrate (traces) as counteranions interacting in the interlayer region. Loosely bound carbonate and nitrate anions and one strongly bound type of carbonate (mainly for copper samples) were found. The hydrotalcite-like samples calcined at 373 K show practically no conversion for the catalytic oxidation of phenol in aqueous solutions under the conditions given in the experimental section. Figure 1(A,B) shows the conversion during a 30-day run for the samples calcined at 623 K.
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Figure 1 (A,B,C,D) Evolution of the phenol conversion for several samples calcined at 623 K (A,B) and at 973 K (C,D) using trickle bed (A,B,C) and semi-batch (D) reactors. Symbols for Figure 1 A,B,C : ~' HT[6:0:2], $ HT[I:0:2], o HT[0,75:0,25:2], 9 HT[0,5:0,5:2], o HT[0,25:0,75:2], x HT[0:6:2], + HT[0:I:2] . Symbols for Figure 1 D : + HT[I:0:2] (run 1), 9 HT[I:0:2] (run 2), o HT[I:0:2] (run 3), [] HT[0,5:0,5:2] (run 1), 9 HT[0,5:0,5:2] (run 2), x HT[0,5:0,5:2] (run 3).
As can be seen, all the catalysts show a more or less constant decrease in phenol conversion. Therefore, the reaction products for all the catalysts are CO2 (>70%), diphenols, quinones and organic acids (oxalic, formic, acetic, succinic, etc.). However, it should be pointed out, that pure nickel catalysts (HT[0:6:2] and HT[0:1:2]) show lower amounts of intermediate products than pure copper catalysts (HT[6:0:2] and HT[I:0:2]). These nickel catalysts mainly convert phenol into CO2 (>90% instead of >70% for copper ones) but their initial conversions are always lower (see Fig. 1 A). There are several reasons for the activity loss observed for these nickel and copper catalysts (see Fig.1 A,B). For example the solubilisation of the active phase (i.e. mainly CuO and/or NiO), formation of new crystalline phases such as copper oxalate and boehmite, and the regeneration of the hydrotalcite phase (mainly for the HT[0:6:2] sample)
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because of the reaction conditions (water, higher temperature reactions and formation of CO2) (see Table 1). It should also be pointed out, that the spinel phases (see Fig. 1 C) (obtained for the samples calcined at higher temperatures and subjected to HC1 treatment until no elution of copper or nickel oxide was detected) showed higher conversions (>40%) and were stable in these reaction conditions. They showed no loss in activity after a continuous working run of 15 days using the trickle-bed reactor. The pure copper spinel phase is more active than the pure nickel spinel phase (65% and 40% conversion respectively). However, the activity of the copper-nickel spinel phase of the sample HT[0.5:0.5:2](973K) is the highest (around 75% conversion). It seems that there is a synergic effect between copper and nickel. Copper shows higher conversion and nickel shows lower intermediate oxidation products. It seems that nickel is less efficient at cleaving the phenol ring (which is the limiting step of the oxidation process of phenol), but surprisingly is more active at transforming the quinones and carboxylic acids to CO2 than the copper catalysts. The conjunction of these two effects may explain the higher activity and selectivity towards CO2 (>90%) detected for the HT[0.5:0.5:2](973K) catalyst. It should also be pointed out that polymerisation products do not form when this trickle-bed reactor is used. However, when the experiments were performed in the autoclave there was a loss of activity for the copper spinel phase for runs 1-3 (due to polymer formation) but not for the Cu-Ni-spinel phase (see Fig.lD). The polymers are formed by two reactions in the liquid phase: the addition of glyoxal to phenol or the polymerisation of glyoxal [5]. So, the Cu-Nispinel phase also prevents polymers from forming during phenol oxidation, even in an autoclave reactor, because the conversion to intermediate products is lower. This means that the catalyst potentially has a longer life. REFERENCES
1. F. Cavani, F. Trifiro, and A. Vaccari, Catal. Today, 11 (1991) 173. 2. F.Trifiro and A Vaccari, Comprehensive Supram.Chem.,7 (1996) 25. 3. M. J. Climent, A. Corma, S. Iborra and J. Prima, J. Catal., 151 (1995) 60. 4. W.T. Reichle, J. Catal. 94 (1985) 547. 5. A. Alejandre, F. Medina, P. Salagre, A. Fabregat, and J. E. Sueiras, Appl. Catal. B: Environ. 18 (1998) 307-315. 6. A. Pintar, and J. Levec, J. Catal. 135 (1992) 345. 7. J.Beziat, M. Besson, P. Gallezot and S. Durecu, Ind. Eng. Chem. Res. 38 (1999) 1310.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Influence of temperature and catalyst loading on the aqueous-phase catalytic oxidation of phenol A. Santos, P. Yustos, B. Durbfin and F. Garcia-Ochoa Departamento Ingenieria Quimica. Facultad. CC. Quimicas. Universidad Complutense. 28040- Madrid. Spain. ABSTRACT The oxidation of phenol over a copper oxide-copper chromite catalyst has been studied in a basket stirred tank reactor in the temperature range 127-180~ with a catalyst loading range of 0-120 g/L and using an initial pH of 3.5 and 10. The phenol conversion and its mineralization to carbon dioxide have been measured. A higher oxidation rate of phenol was found when the initial pH was acid. At the lower temperatures studied (127 and 140~ an increase of both the phenol oxidation rate and the CO2/phenol conversion ratio was observed when the catalyst loading was increased. At the highest temperatures used (160 and 180~ the homogeneous mechanism is the main contribution to the phenol oxidation rate. 1. INTRODUCTION The problem of the pollution reduction of the aqueous streams containing phenolic compounds is of great importance in wastewaters from many industries (agroalimentary processes, pharmaceutical, fine chemical, petrochemical, etc). These effluents can not be treated through conventional processes of biological oxidation, because of their little biodegradability. The process currently used is the wet oxidation or supercritical oxidation, that requires high pressures and temperatures, and consequently high cost (1). An interesting alternative is the heterogeneous catalytic oxidation, because the catalyst permits a significant reduction of the temperature and pressure improving the economy of the process (2-4). Studies carried out about this topic show that the reaction involves a free radical mechanism, with an induction period before the steady-state regime is achieved, considering both homogeneous and heterogeneous reaction contributions. The influence of variables such as pH, catalyst loading and temperature on the induction period and steady-state rate are not yet clarified and results obtained from different authors often disagree (5-7). Most of the studies have been carried out in slurry reactors, with a maximum catalyst loading of 4-5 g/L. The few papers where the reaction is carried out in fixed bed (usually as trickle bed reactors) did not study the effect of different catalyst weights in the reactor. Hence the catalyst loading influence on the phenol conversion and its mineralization to carbon dioxide requires further studies. Furthermore, most of the papers do not consider the conversion to CO2 but only the conversion of phenol. However the phenol oxidation does not produce directly carbon dioxide but many intermediates have been detected in the oxidation route, being the main
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oxidation intermediates the dihidroxyphenols (cathecol and hydroquinone), the benzoquinones, the C-4 and C-2 dicarboxilic acids, the acetic acid, etc. The best catalyst should be as less selective as possible and promote the oxidation of these intermediates to carbon dioxide.
2. EXPERIMENTAL 2.1. Catalyst A commercial catalyst from Engelhard (Cu-0203) with copper as active component has been employed. The catalyst properties are summarized in Table 1. Leaches of metallic ions Cu and Cr have been measured after the catalyst has been in contact with an acid aqueous phenolic solution at 140~ and 16 atm of pressure during 10 hours. An almost negligible amount of those ions in solution was found, as can be seen in Table 1. Table 1. Properties of the Catalyst Engelhard Cu-0203
Chemical Composition ~
Compounds Copper oxide 67-77 Copper chromite 20-30 Synthetic graphite 1-3
~
Metals 60% Cu 10% Cr
Physical Properties
Metal leaches after 10 h (mg ion/g cat) T=140~ P=I 6 atm.
Sg m2/g
V cm3P/g
dp mm
Cu
Cr
10
0.10
3.175 (1/8")
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9.3.10 -4 9.10 -4 % Cro
2.2. Experimental Set-Up and Procedure A basket stirred tank reactor (BSTR) from Autoclave Engineers (500 ml Spectrum Reactor) was employed to carry out the experimental runs. A volume of 250 ml of the aqueous solution was introduced in the reactor after setting the pH to an initial value (pHo). The catalyst was boiled twice in distilled water before its use and placed in the basket of the reactor, and the rotating speed of the stirrer was fixed at 700 rpm. The reactor was pressurized at 16 atm. Steady state conditions for temperature and concentration of dissolved oxygen were achieved by means of a continuous flow of oxygen (0.2 L lnin -R at NTP conditions). Then, the volume of a loop containing a concentrated solution of phenol was injected being the initial concentration of phenol in the reactor liquid phase of 1200 ppm. Experimental runs were carried out at different temperatures (127 to 180~ catalyst loadings (0 to 120 g/L) and initial pH (3.5 and 10). Gas and liquid samples were taken periodically from the reactor and analyzed. Residual phenol concentration and some intermediates in the samples from the reactor were determined by HPLC with a Diode Array detector (HP G1315A). Other reaction intermediates were identified and quantified by means of a GC/MS technique (Hewlett Packard model 6890). Main intermediates identified at acid pH were 1,2 benzenediol, 1,4 benzenediol and 2,5-cyclohexadiene-l-4dione. Also maleic and oxalic acids were detected. Total organic carbon in the liquid phase values (TOC) have been
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obtained in a SGE analyzer. Copper and chromium leaches have been determined by means of a fotometer (Aqualytic AL282). The pH values of the liquid samples were also measured with time.
3. RESULTS and DISCUSSION 3.1. Influence of the pH on phenol and TOC conversion
The results obtained at 140~ and using a catalyst loading of 0 and 60 g/L for both initial pH 3.5 and 10 are shown in Figures 1a and lb. As can be seen, the phenol conversion is much slower at basic pH, probably due to the free radical mechanism proposed for this reaction (5). Regarding to the mineralization of the phenol to CO2 similar curves with and without catalyst are obtained at each pH. However at basic pH the mineralization achieved for a given phenol conversion is slightly higher than the obtained at acid pH.
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Figures 2a,b and 3a,b show the results obtained for the phenol and the TOC conversion with time at the two extremes catalyst loading employed. As can be seen, for both catalyst loadings in the reactor, 0 and 120 g/L, the phenol conversion is highly affected by the temperature until 160~ then the increase of this variable does not produce an increase of the phenol conversion rate. On the other hand an increase of the temperature increases the mineralization grade if no catalyst is added. When the reaction is carried out at 120 g/L of catalyst loading no effect of the temperature on the mineralization achieved was noticed.
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Figure 3. Effect of the temperature on the mineralization of phenol, pHo=3,5, P=16 atm. a) Ccat-0 g/L b) Ccat=120 g/L
3.3. Effect of the catalyst loading The effect of the catalyst concentration on the phenol conversion and its mineralization to CO2 at the four temperatures studied is shown in Figures 4a,b,c and 5a,b,c. As can be seen in Figure 4c at temperatures higher than 160~ the addition of the catalyst does not improve the phenol conversion. Thus, the main contribution to the phenol oxidation rate is due to the homogeneous reaction. At 180~ a similar behavior for the mineralization was found with and without catalyst. On the contrary, at 160~ the addition of the catalyst yields a higher mineralization of the intermediates produced from the oxidation of phenol. At the lower temperatures used, 127 and 140~ a significant positive influence of the addition of catalyst on both phenol and TOC conversion was found, as can be seen in Figures 4a,b and 5a,b. At these temperatures no differences in the grade of mineralization of phenol were found for a catalyst loading over 30 g/L.
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Figure 5. Effect of the catalyst loading on the mineralization, pHo=3,5, P=I 6 atm
a) 127~ b) 140~ c) 160, 180~
a) 127~ b) 140~ c) 160, 180~
In Figures 6a,b,c the pH change with time is shown at the two extremes catalyst loading used (0 and 120 g/L) for the temperatures studied. It can be seen that the catalyst loading has a significant influence on the pH change. When the catalyst was added the pH decreased initially with the increase of the phenol conversion, achieving a minimum, and then increased. This can be explained considering that the acid compounds (mainly C-2 and C-4 dicarboxilic acids) are the last intermediates of the oxidation route before the mineralization is achieved. The catalyst produces a higher oxidation rate of these acid compounds. The higher
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oxidation of the acid compounds achieved with the catalyst can explain the differences in the mineralization grade observed in Figures 5a,b,c. When the temperature increases the results obtained with and without catalyst for the oxidation of those acid compounds are closer. For example, negligible differences in pH change with time were found with and without catalyst at 180~ and consequently the same TOC abatement vs. phenol conversion was also observed. )H
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b) 140~ c)160, 180~
ACKNOWLEDGEMENTS This work has been supported by CAM under contract no. 07M/0035/1998. The authors wish to thank Engelhardt who kindly supplied the catalyst. REFERENCES
1. T.D. Thornton and P.E. Savage, J. Supercritic. Fluids 3(1990) 240. 2. A. Pintar and J. Levec, Catal. Today. 24 (1995) 51. 3. Z. Ding, S.N. Aki and M.A. Abraham, Env. Sci. Tech. 29 (1995) 2748. 4. S.H. Lin and S.J. Ho, Appl. Catal. B: Env. 9 (1996) 133. 5. A. Sadana and J.R. Katzer, J. Catal. 35 (1974) 140. 6. H. Otha, S. Goto and H. Teshima, Ind. Eng. Chem. Fundam. 19 (1980) 180. 7. A. Pintar and J. Levec,, Ind. Eng. Chem. Res. 33 (1994) 3070
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Giyoxal Synthesis by Vapour- Phase Ethylene Glycol Oxidation on a Silver and Copper Catalysts O.V. Vodyankina a, L.N. Kurina a, A.I. Boronin b and A.N. Salanov b aTomsk State University, Tomsk, 634050, Lenin str. 36, Russia bBoreskov Institute of Catalysis SB RAS, Novosibirsk, 630090, Lavrent'ev pr. 5, Russia The process of the ethylene glycol oxidation with using a number of different catalytic systems has been studied. Efficiency of supported Ag catalysts is associated with the stabilization of ion Ag +~ state under the influence of oxygen of the support lattice. It is shown that the application of the double- layer Cu - Ag catalytic system increases the selectivity of the ethylene glycol oxidation into glyoxal. 1. INTRODUCTION Glyoxal widely using in industry is prepared by both nitric acid oxidation of acetaldehyde and catalytic oxidation of ethylene glycol. The latter process is more perspective method because this process is realized in gas phase, and it is characterized by higher productivity. Ethylene glycol oxidation is realized at 773-923 K on Ag and Cu catalysts generally. In Ref. [1,2] a SiC supported silver catalysts were prepared. Without phosphorous promoters the efficiency of supported Ag catalysts is essentially lower in comparison with double - layers C u - Ag catalysts [3]. The reasons are the subject of investigation in the present work. 2. EXPERIMENTAL The samples of metal catalyst were prepared according to Ref. [4,5]. Supported silver catalysts were.prepared by chemical reduction method. Catalytic activity has been examined in quartz fixed bed reactor (0.82 sm2). The catalyst mass was 1.9 g. The analysis of the liquid products has been performed according to Ref. [6]. The interaction of ethylene glycol on the clean and oxidized surface of the catalysts has been studied by thermal programmed reaction method (TPR). Adsorption of ethylene glycol took place at T = 473 K and 573 K. Adsorption of O2 was carried out at T=473 K. The coke formation has been studied in a flowing reactor, which makes it possible to observe in the regime in situ the growth of the coke deposits (CD). The X-ray photoelectron spectra (XPS) were recorded with a VG ESCALAB electron spectrometer using AII~ radiation. Experimental resolution characterized by full width at half maximum (FWHM) of Ag3ds/2 line is 1.3 eV if pass energy of analyzer equals to 20 eV. Spectra were calibrated against Eb(Ag3d5/2) = 368.1 eV and Eb(Cu2p3/2) =932.7 eV [7]. Investigations by Scanning Electron Microscopy (SEM) have been performed with aid of microscope BS-350 (TESLA) equipped with electron gun of 16 kV. 3. RESULTS AND DISCUSSION Table 1 gives the experimental data on different catalyst systems. It is known that an active component in the supported Ag catalysts does not wholly cover support surface [8], but it is situated as crystallites with dimensions of 50 - 90 nm. Thus, a part of the support surface is opened.
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Table 1 Catalytic activity of different system in the ethylene glycol oxidation process Simple Composition T, K Molar Conver- Glyoxal Concentration of acid of support ratio of sion, % yield, % centers, (mol/m 2) x 103 SiO~/AbO~ O~/EG Before After Ag el. crystals 823 1.0 80.8 56.0 28.28 19.04 Double Cu-Ag 853 1.0 97.2 78.4 10% A g / 1 ~ - A1203 853 0.9 87.5 35.3 1.52 0.32 10% Ag / 2 80/20 853 0.9 94.3 38.1 10% A g / 3 40/60 853 0.9 96.1 55.0 4.90 5.98 40% Ag / 4 70/30 853 0.9 90.7 42.5 3.46 8.26 10% A g / 5 c~- A1203 853 1.1 93.1 28.2 1.57 0.20 10% Ag / 6 SiO2 853 1.1 81.3 23.6 -
In the previous work [3] it was shown that on clean surface of carriers the unselective oxidation of ethylene glycol (EG) occurred. Besides during the time catalytic systems prepared on the base of ~ - A1203 (sample 1,5) and SiO2 (sample 6) decrease their catalytic activity. The reason of catalyst activity decrease may be associated with agglomeration of Ag on the support surface under the action of high temperature reaction media. One can see from the data of Table 1 that both activity and selectivity of supported Ag systems depend strongly on SiO2 containing in supports. It may be associated with stabilization of Ag in high - dispersity state on surface of alumina - silicate supports as it was shown in Ref [9]. H i g h - dispersity Ag state may appear as the result of both the formation of chemical surface compound of Ag with support and the Ag +5 state formation under the influence of oxygen of support lattice. Taking into account the silver ions are stabilized by oxygen atoms of the support lattice, the electron density of such oxygen ions effects on the polarization Ag 6+-- 05.- Me bond. On the surface of supports prepared on the base of individual oxides (1,5,6 samples) such stabilization is practically absent and Ag is able to agglomerate leading to the decrease of catalytic activity. To estimate the degree of the ligand influence of oxygen of the support lattice on the Ag +5 state, we used method of potentiometric titration of the supported Ag catalysts by potassium ethylate (CzHsOK) in the media of non - proton organic solvent [10]. These data are presented in Table 1. Four types of acid centers, which are different in strength, have been found for the catalyst surface in agreement with the experimental potentiometric curve. At first the strongest acidic centers are titrated by CzHsOK then weaker centers interact with C2HsOK [10]. One can see that the surface of massive Ag crystals is characterized by the highest acid properties. Thus acid properties of supported Ag systems depend on the distribution of active component along support surface. Treatment of catalysts by the reaction mixture leads to both the increase of acid center concentration on surface of alumina- silicate Ag supports and simultaneously the acid center concentration decrease on the Ag catalysts prepared on the base of individual ~ - A1203. This may be associated with the formation of high - despersity Ag particles such as isolated Ag ions and two - dimensional clusters characterizing by highest effective charge on the surface of alumina- silica supports. Under action of the reaction mixture amount of high - dispersity Ag state increase in the result of partial decomposition of more large clusters. The similar correlation is observed at the investigation of catalytic activity of these systems in the process of ethylene glycol oxidation.
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To elucidate details of the ethylene glycol oxidation process into glyoxal on 493 ~ 683 the catalyst surface, we used methods of TPR. Ethylene glycol is reversibly adsorbed on the clean Ag (Fig.la). According to calculated kinetic parameters for this TPD spectrum (activation energy = (CHO)A2~r H2 b ~ / ~ ('(HO),) 59.6 + 0.8 kJ/mol and desorption order = 0.9) the multilayer of ethylene glycol was H2O453 I-t20~ / ~ ~ :02 formed at such conditions. Similar conclusion was made at the investigation of the ethylene glycol interaction on the clean Ag (110) surface in Ref. [11]. The 4m -9" ~ CO.> interaction of ethylene glycol with oxidized surface of the silver catalyst depends on both O-Ag bond strength and the ~,,~ ) 373 573 773 T,K 473 673 883 adsorption species of alcohol. The reaction of ethylene glycol dehydrogenation to Fig. 1. Tile etlvlene glycol reaction on file clean (a) glyoxal occurs on the oxygen - containing ,'uld o~gen - doped catalyst smthce (b,c) under folcenters of Ag surface at Tads = 473 K lowilN conditions: (a) T~.~1~EG=473 K, (b) Tads 02, Tads EG=zI73 K, (c) Tads 09=573 K, Tads EG=d73K (Fig. lb). While the temperature of oxygen adsorption increases until 573 - 673K the glyoxal formation by the oxidation way takes place (Fig 1c). It is reliably evidenced by the absence of hydrogen in the desorption products. Another part of ethylene glycol is adsorbed on the oxidized Ag surface where the process of the EG decomposition leads to the formation of secondary products: formaldehyde and CO2 (Fig. 1b,c). In contrast to the silver catalyst the chemisorption of EG on the clean Cu surface is characterized by the process of ethylene glycol dehydrogenation (Fig. 1a). At low coverage of oxygen the formation of glyoxal occurs along the oxidative route predominantly (Fig.lb). While the adsorption temperature increases up to 573 - 6 7 3 K the reaction transition from the copper catalyst surface to the reactor volume is observed. Thus, the application of the double layer Cu - Ag catalytic system increases the selectivity of the ethylene glycol oxidation to glyoxal although the basic process is realized inside the silver catalyst layer. The efficiency of Cu layer grows up at the decrease of oxygen concentration and temperature. While using of double - layer Cu - Ag system as a catalyst in the selective ethylene glycol oxidation process the intense coke formation has been observed in the Cu layer [3]. To study the catalyst morphology, nature and states of carbon in the coke deposits, we used a number of physical methods such as SEM, TGA and XPS together with catalytic methods. In our previous work [12,13] it is shown that the quantity of the coke deposits on Cu is significantly higher then on Ag. This fact may be associated with the difference of the coke formation mechanisms on the surface of Ag and Cu catalysts. According to TPR data glyoxal formed on the unoxidized Cu surface unlike Ag where this process is realized only on o x y g e n containing active centers. Therefore the process of carbon deposit (CD) formation at the oxygen absence conditions goes on the clean Cu surface because glyoxal is the basic coke formation agent. TGA analysis of CD has shown that on Cu surface the formation of two types of carbon deposits burning out at 723 and 653 K is possible in dependence on the composition of the reaction mixture. TG analysis made for Ag samples shows burning of CD at temperature of 663 K only. Thus, in contrast to Ag on the Cu catalyst surface two types of CD are formed.
Ag
-t
I
Cu
!
t~,
..... i
I
I
i .............
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Cls
In Fig.2 C ls spectra of the silver and copper samples investigated after the action of the reaction mixture are presented. The treatment of Cu catalyst with pure EG produces C 1s spectrum (curve a) where the main peak with Eb(C l s) = 284.2 eV together with [ ' k 286.8 the satellite line (Eb = 290.5) can be reliably associated with a graphite - like structures. The variation of the reaction mixture composition leads to the essential changes in C ls spectra where the 285.1 chemical shift of main component 0.5+0.9 eV is observed (Fig.2, b,c). We propose that the basic components of Cls spectra with Eb = 285.1+284.7 eV correspond to oxygen- carbon structures such as C O - C which is formed by the cyclization of ethylene glycol or glyoxal on the oxidized catalyst surface. c) te) Thus, XPS spectra demonstrate that carbon deposits contain not only carbon atoms, but they consist of oxycarbon structures on the surface. In the result of the catalysis action the surface morphology of the Ag, Cu samples changes ......... " ........ dramatically. Initially the Ag, Cu samples represent 280 285 290 295 the entire openwork object plaited from the threads with the diameter of 20 - 30 ~tm for Ag and 2 0 0 - 300 ~tm for Cu. At first, the formation of holes with a Fig.2. C ls spectra obtained after the mean size around 0.1 ~tm is observed on the Ag action of reaction mixture on copper surface in the result of the reaction mixture action at (curves a,b) and silver (curves c) 823 K for l h (Fig.3 a,b). The similar phenomenon is revealed in Ref [ 14] catalysts: (a) O2/EG = 0.0, (b) O2/EG in the course of methanol oxidation on Ag foil. = 0.3, Y = 873 K, EG/H20 = 0.6; (c) Authors concluded that the formation of holes O2/EG - 1.0, EG/H20 = 0.4, T resulted from the reaction of b u l k - dissolved 773 K. hydrogen and oxygen. However, it is noteworthy that the treatments of Ag samples by gaseous mixtures (H2 + 02 +N2, H2 + N2 etc.) at such conditions did not lead to the morphological changing of surface (not shown here). Comparison with XPS data allows us to suppose that holes serve as storage of carbon containing depositions. According to SEM data the holes penetrate to Ag crystal from beginning to end (Fig.3c). Besides the process of the hole formation is only realized when the reaction conditions are favor for selective oxidation of ethylene glycol into glyoxal. This is allowed to suppose that the formation of holes is realized with the participation of glyoxal. Probably the process of coke formation is realized in the bulk of Ag crystal in the direction from volume to surface. At the elevated temperature the increase of mobility of Ag surface layers leads to the appearance of the coke on surface, where it weakly bum out forming channels in the presence of oxygen of gas phase. Taking into account the morphology of observed channels such coke deposits must have filament structure. In contrast to Ag the Cu surface is covered the shallow cracks (Fig.3d). According to XPS data this effect is associated with the formation of the surface layer of copper oxide (I). The increase of the treatment time of catalyst surface by the reaction mixture leads to the formation of thick films of the carbon-containing deposits. The thickness of the film on Ag catalyst is 0.5 ~tm while it is about 3 ~tm on the Cu. On the Ag surface the deposited carbon
a)
/~84.8
b)
]~286.9
_._J
~
e
Binding Energy,eV
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
Fig.3. SEM images of the surface of silver (a,b,c,e) and copper (d,f) catalysts after the action of reaction mixture: (a) T = 823 K, (b) T = 973 K, (c) T = 873 K, (e) T = 898 K, O2/EG = 1.0, EG/H20 = 0.4; (d) T = 773 K, (f) T = 873 K, O2/EG = 0.5, EG/H20 = 0.6.
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film looks as a compact crust (fig.3 e). Close inspection of this crust shows that it consists of carbon filaments tightly plaited. The morphology of carbon deposits on Cu is different one. The deposited film on the Cu catalysts consists of carbon filaments tight-fitting to each other, which are perpendicular to the surface (Fig. 3 f). Thus, the growth of the oxycarbon structures occurs as filaments on the oxidized catalyst surface. According to catalytic data presented earlier in [12,13] the oxygen - content increase up to 0.6 - 0.9 in the reaction mixture is accompanied by the glyoxal yield growth on both Ag and Cu catalysts. This phenomenon could not take place because of self- regeneration process only, but one can assume that the oxycarbon layers are likely a reservoir of an additional condensed glyoxal. 4. CONCLUSION The data presented in this work show that efficiency of supported Ag catalysts is associated with the stabilization of active component in the h i g h - dispersity Ag +~ state under the influence of oxygen of the support lattice. However, uncovered by Ag lots of catalyst surface is answerable for unselective ethylene glycol oxidation. The application of the doublelayer Cu-Ag catalytic system increases the selectivity of the ethylene glycol oxidation into glyoxal. One of the main reasons is the growth of efficiency of Cu layer located in the field of reactor where more low oxygen concentration and temperature in comparison with Ag layer take place. One can assume that the coke deposition on the oxidized metal catalyst surface exert favorable influence on selectivity of the major process. The coke deposits are the oxycarbon structures which grow as filaments. It is proposed the destruction of oxycarbon layer under the oxygen action leads to the glyoxal appearance in the reaction products. REFERENCES 1. P. Gallezot, S. Tretjak, Y.Christidis et al, J. Catal. 142 (1993) 729. 2. J. Deng, J. Wong and X.Xu, Catal. Lett. 36 (1996) 207. 3. O.V. Vodyankina, L.N. Kurina, L.A. Petrov et al., Chem. Ind. 12 (1997) 802. 4. L.A. Petrov, I.G. Rozanov, A.P. Ckhramov et al., EMRS Fall Meeting, 4 European East West Conference, St.-Petersburg (1993), A4. 5. Antsiferov V.N., Ovchinnikova V.I., Makarov A.M. Open cell foam structures, catalysts supported there by and method of producing the same, US Patent WO 95/11752 (1995). 6. O.V. Vodyankina and S.I. Galanov, Ind. Lab. 8 (1995) 13. 7. D. Briggs, M.P. Seah (Eds.), Practical surface analysis by Auger and X- Ray photoelectron spectr., John Wiley&Sons Ltd., Chichester-New-York-Brisbane-TorontoSingapore, 1983. 8. S.M. Brailovskii, O.N. Temkin and I.V. Trophimova, Probl. Kinet.Catal. 19 (1985) 146. 9. A.N. Pestryakov, A.A. Davidov and L.N. Kurina, J. Phys. Chem. 62 (1988) 1813. 10. V.N. Belousova, O.G. Kuznetsova and L.N. Kurina, J. Appl. Chem. (1984) 921. 11. A.J. Capote, R.J. Madix, J. Amer. Chem. Soc., 3 (1989) 3570. 12. L.N. Kurina, L.A. Azarenko and O.V. Vodyankina, React. Kinet. Catal. Lett. 63 (1998) 355. 13. L.A. Arkatova, L.N. Kurina and O.V. Vodyankina, J. Appl. Chem. 72 (1999) 534 14. A. Nagy, G. Mestl, T. Ruhle et al, J. Catal. 179 (1998) 548.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
P r o d u c t i o n of A l k e n e s t h r o u g h O x i d a t i v e C r a c k i n g of n - B u t a n e o v e r OCM C a t a l y s t s M. N akamura, S. Takenaka, I. Yamanaka and K. Otsuka* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Oxidation of n-butane was carried out over rare-earth metal oxide catalysts. Oxidative pyrolysis of n-butane proceeded over these catalysts to form ethene and propene. Modification of the catalysts by Li + brought about the suppression of total oxidation and the improvement of selectivity to alkenes. Alkene yield was the highest over Li+-added-Y203 catalyst, and its catalytic activity was very stable. XRD patterns and O K-edge XANES spectra of the catalysts suggested that solid solutions were not formed between Y203 and Li*, but Y20~ was covered with a thin layer of Li2CO3.
1. INTRODUCTION The utilization of light alkanes to produce more valuable chemicals is always attractive. One of the promising routes is the conversion to unsaturated hydrocarbons. The production of alkenes from alkanes by catalytic oxidation is preferable because the process can overcome the thermodynamic limitation in pure dehydrogenation processes as well as the deactivation of catalysts due to coking. Many efforts have been made toward the development of active catalysts for the oxidative dehydrogenation of light alkanes. Most of the catalysts reported so far contain vanadium or molybdenum oxides as active species. In oxidation of hydrocarbons over these types of catalysts, the lattice oxygen species in the catalysts activate the light alkanes to form products. For example, vanadium-magnesium mixed oxide catalyst (VMgO) was reported to be active for the oxidative dehydrogenation of n-butane to butenes and butadienes [1]. However, this catalyst enhanced the deep oxidation of n-butane considerably because of strong oxidizing activity of vanadium oxide. Oxidative coupling of methane (OCM) is believed to proceed in the gas-phase via methyl radicals which have been generated on the catalyst surface [2]. We have already reported that basic metal oxides, such as rare-earth metal oxides, are effective catalysts for OCM, that is, the successive oxidations of the formed methyl radicals and ethene are suppressed due to a basic property of the catalysts [3]. Therefore, we expected that the basic metal oxide catalysts efficiently generate alkyl radicals from light alkanes, and the catalysts do not oxidize the formed alkyl radical species and alkenes deeply into COx. In addition, in the oxidative coupling of methane over basic metal oxide catalysts, oxygen species adsorbed on the catalysts are believed to activate methane, generating methyl radicals [2,4]. The adsorbed oxygen should exhibit catalytic performance
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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different from the lattice oxygen in vanadium or molybdenum oxide catalysts mentioned above. In this study, we carried out oxidation of n-butane over rare-earth metal oxide catalysts. In addition, we investigated the effect of the addition of Li § to the catalysts on the catalytic performance. 2. E X P E R I M E N T A L
Catalysts Li*-added metal oxide catalysts were prepared by impregnating the metal oxides with aqueous solutions of Li2CO3, and drying up the impregnated samples at 353 K. The samples were calcined at 1023 K for 5 h under an air stream. The amounts of added Li § were adjusted to be a molar ratio Li*/X = 0.4 (X; metal ions in metal oxides). V205(20 wt%)/MgO and MOO3(36 wt%)/MgO were prepared by impregnation of aqueous solutions of NH4VO3 and (NH4)6Mo7024"4H20 into MgO, respectively. The samples were dried at 363 K for 12 h and calcined at 1023 K for 5 h in an air stream. The catalysts were pressed into pellets. The pellets were crushed and sieved to 20/45 mesh size. Reactions
The oxidation of n-butane was performed in a conventional gas flowing system with a fixed-bed reactor at an atmospheric pressure. The cylindrical reactor made from alumina had an i.d. of 5.5 mm and a length of 360 mm. In order to minimize the contribution from any gas-phase reaction, quartz sands filled the space above and below the catalyst bed in the reactor. Catalyst (0.2g) was introduced in the reactor and was heated to 1023 K in a flow of oxygen, prior to the reaction. The temperature profile was measured using a ChromelAlumel thermocouple, which was placed in an axial thermowell and centered in the catalyst bed. n-Butane and oxygen were fed with a helium carrier over the catalyst, and gaseous products were analyzed on-line by gas chromatographs.
Characterization of Catalysts X-ray absorption experiments were carried out on beamline BL-8B1 at UVSOR, Institute for Molecular Science, Okazaki, Japan. O K-edge XANES spectra of catalysts were recorded at room t e m p e r a t u r e in the total electron yield mode. The sample was put on Cu-Be dinode which is attached to the first position of the electron multiplier. X-ray diffraction patterns (XRD) were collected using a Rigaku RINT 2500V diffractometer. 3. R E S U L T S A N D D I S C U S S I O N
The oxidation of n-butane was performed over various rare-earth metal oxide catalysts to investigate the catalytic performance of basic metal oxides. In addition, non-catalytic experiments were carried out in order to examine the contribution of the gas-phase reaction. In this case, quartz sands were packed in the catalyst bed. The results were shown in Table 1. For the reactions over all rare-earth metal oxide catalysts, the conversions of n-butane and oxygen were much higher t h a n those over quartz sands (in the absence of catalysts), suggesting t h a t rare-earth metal oxides catalyze the oxidation of n-butane. For the reaction over these catalysts, ethene, propene, CO, and CO2 were formed mainly, and little amounts of methane, ethane, and propane were observed. The results indicated that the oxidative pyrolysis of n-butane took place selectively
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Table 1
1783 Oxidation of n-butane over rare-earth metal oxide catalysts a)
catalyst
temp. b) conv. / % selectivity / % /K C4Hlo 02 C2H4 C3H6 C4Hs C~H~ Y203 973 65.9 83.4 36 26 4 tr 1023 80.6 91.7 33 21 3 tr Sm203 973 66.5 92.8 35 23 4 tr 1023 74.1 97.4 36 23 4 tr Gd~O3 973 66.3 85.1 34 24 4 tr 1023 77.7 92.7 34 22 4 tr La203 973 66.1 91.5 34 20 3 tr 1023 73.2 96.6 34 19 4 tr CeO2 973 38.8 98.2 29 12 2 tr 1023 54.3 98.4 34 15 1 tr quartz 973 10.1 3.0 34 45 6 tr 1023 42.3 18.6 37 41 5 tr a) P(n-CtHlo)=P(O2)=8.4 kPa, P(He)=84.5 kPa, flow rate=180 ml-min. b) reaction temperature, c) total yield of alkenes.
CO 14 12 12 12 12 13 10 10 5 6 1 3
CO2 11 10 14 13 12 12 15 14 38 25 tr tr
O.y.c~/% 42.9 46.3 41.3 45.9 41.3 46.0 37.9 41.7 16.7 27.5 8.6 35.0
over rare-earth metal oxide catalysts. The yield of alkenes was the highest over Y203 catalysts among all the rare-earth metal oxide catalysts. Over CeO2 catalysts, the conversion of oxygen was high at relatively low temperatures (N923 K), and total oxidation to CO2 was appreciable. In order to improve selectivities to ethene and ethane in oxidative coupling of methane, alkali ions were often added to metal oxide catalysts, for examples Li § added MgO [5] and Sm203 [6]. Therefore, we tried to apply this modification of the catalysts to n-butane oxidation. The results of the oxidation over Li+-added catalysts are shown in Table 2. The addition of Li § into any of the rare-earth metal oxide catalysts resulted in the decreases in conversions of n-butane and oxygen. Over CeO2 catalyst, the conversion of oxygen was significantly suppressed by addition of Li § On the other hand, selectivity to CO2 became low, and those to alkenes, especially propene, became high by Li*-addition, t h a t is, oxidative pyrolysis occurred selectively. Although alkene yields decreased slightly by addition of Li § for most of the catalysts due to the effects of Li § mentioned above, the alkene yield over Y203 catalyst increased by Li+-addition, exceeding 50 %. The highest alkene yield of 58 % was obtained with this catalyst when the gas-catalyst contact time was increased by decreasing the gas-flow rate to 60 ml.min 1. There have been few reports on the oxidation of light alkanes to alkenes over basic metal oxide catalysts. As described above, Li+-modified rare earth matal oxides, in particular Li§ Y208, are concluded to be effective catalysts for the oxidative pyrolysis of n-butane. For comparison, the results of the oxidation over V2Os/MgO and MoO3/MgO catalysts are shown in Table 2. Selectivities to butenes and butadiene were significantly high for both catalysts, which is in striking contrast to the results of the rare-earth matal oxide catalysts both with and without Li § It is accepted t h a t in oxidation of hydrocarbons over the catalysts containing vanadium or molybdenum oxides as active centers, the lattice oxygen species in the catalysts activate hydrocarbons and the removed lattice oxygen species are reproduced by gaseous oxygen. On the other hand, it is reported t h a t in the oxidation over basic metal oxide catalysts, such as Sm203, molecular oxygen species adsorbed on the
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Table 2
Oxidation of n - b u t a n e over r a r e - e a r t h m e t a l oxide c a t a l y s t s a)
catalyst
temp. b) conv./ % selectivity/% C2H4 C3H6 C4Ha C4H6 /K C4Hlo 02 Li§ 973 51.2 38.3 39 33 5 tr 1023 70.7 78.5 40 26 6 tr Li+-Sm~O3 973 26.1 18.4 37 36 6 tr 1023 57.1 41.3 38 31 5 tr Li+-Gd903 973 29.9 18.8 37 36 3 tr 1023 57.4 42.7 40 33 3 tr Li+-La~Oa 973 24.1 13.7 36 37 6 tr 1023 53.0 30.8 38 33 5 tr Li+-CeO~ 973 16.4 10.0 37 40 5 tr 1023 43.8 23.2 39 36 5 tr V~.Or/MgO 753 14.4 45.4 tr tr 38 21 773 32.8 97.5 1 tr 24 34 MoOJMgO 973 30.6 43.8 15 8 27 37 1023 51.6 60.7 30 20 12 21 a) P(n-C4Hlo)=P(O~)=8.4 kPa, P(He)=84.5 kPa, flow rate=180 ml-min. b) reaction temperature, c) total yield of alkenes.
O.Y.C~/% CO 2 2 2 2 1 2 2 3 1 2 11 13 6 9
CO2 9 13 6 6 6 6 4 4 3 3 29 23 4 4
39.2 51.5 20.7 42.3 24.0 44.6 19.1 40.4 13.5 35.0 8.5 19.4 26.6 42.8
c a t a l y s t s a c t i v a t e hydrocarbons [4]. The selectivities to a l k e n e s described above m i g h t be d e t e r m i n e d by t h e k i n d s of active oxygen species on t h e catalysts. It w a s r e p o r t e d t h a t in oxidative ~ 100 coupling of m e t h a n e over Li+-added MgO ~ 80 conv. catalyst, t h e conversion of m e t h a n e and t h e -~-yield of e t h e n e decreased w i t h the t i m e on ~ ~ s t r e a m [7]. The cause of this deactivation ~ .~ 60 ~ 40 ~---'-------'............................... -alkeneyield .... w a s a t t r i b u t e d to t h e loss of Li § from t h e 8~ .~ c a t a l y s t s d u r i n g t h e oxidation. The loss of ~-~ 20 ............................................................................... Li + m a y occur in our case. Therefore, we aa i n v e s t i g a t e d t h e stability of the Li§ ~ o , ,,, , , , Y2Oa catalyst. F i g u r e 1 shows t h e r e s u l t s ~ 3 .....0....._.........~ ............ 4 0 C2H4_._ of t i m e - c o u r s e of t h e n - b u t a n e oxidation over Li+-added Y 2 O a catalyst. The conversion of n - b u t a n e , yield of alkenes a n d selectivities to all products were C3H6 ] i~ 20 I -.............................................................................. a l m o s t c o n s t a n t d u r i n g t h e t e s t e d period, C0 2 l i n d i c a t i n g t h a t Li§ Y2Oa c a t a l y s t w a s 10 "-AA---~t---~,--at----,t- ~t---A--at- i---~:---A ^ C v e r y stable. In oxidative coupling of m e t h a n e over Li+-added MgO catalyst, it 0 2 4 6 8 10 12 w a s i n d i c a t e d t h a t t h e introduction of CO2 time on stream / h into a m i x t u r e of feed gases p r e v e n t e d t h e Figure 1. Time course of the oxidation d e a c t i v a t i o n of t h e c a t a l y s t s [7]. The of n-butane over Li+-added Y203 catalyst. cofeed of CO2 b r o u g h t about t h e formation Reaction conditions: catalyst 0.2 g, of Li2COa on t h e catalysts. The Li2COa, flow rate 180 ml.min-1 t h e source of t h e catalytic active sites for P(n-C4H10) = P(O2) = 8.4 kPa, t h e oxidative coupling of m e t h a n e , would P(He) = 84.5 kPa, temperature 1023K be stabilized in t h e presence of a high
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partial pressure of CO2. In oxidation 35 of n-butane over Li+-added Y~O8 30 catalyst in this work, a relatively ...................1 o I.....................t high partial pressure of CO2 was ~" 25 present due to a high conversion of - ............................i g .~ ..................... 1 n-butane, compared to the i~ 20 oxidation of methane. The CO~ .....................1 15 : ............................1 " formed during the oxidation of n- 0 10 butane may stabilize the catalytic activity of the Li§ Y208 catalyst. In an attempt to obtain 0 1 2 3 4 5 6 7 evidence for the reaction pathway W/F / 10-3g rain ml-1 in the oxidation over Li§ Y203 catalyst, the effect of varying Figure 2. Effect of W/F on the oxidation the residence time (W/F; W: of n-C4Hlo over Li+-added Y203 catalyst. catalyst weight, F: flow rate of Reaction conditions: catalyst 0.2 g, e m p e r a t u r e 893 K, flow rate 25 to 240 ml.min-1, feedstock) on product distributions tP(n-C4Hlo) = P(O2) = 8.4 kPa, P(He) = 84.5 kPa was studied. Figure 2 shows the change in selectivities with W/F. In this reaction, flow rates of feedstock were changed from 25 to 240 ml.min 1, with a constant weight (0.2 g) of the catalyst. In spite of change of W/F, the selectivites to propene and ethene were kept at constant values, although the conversion of n-butane increased from 7 to 28 %. The selectivity to CO decreased and that to CO2 increased slightly with the W/F, but the sum of these carbon oxides was constant. The results indicate that the successive oxidation of the alkenes does not occur in the oxidation over Li§ Y208 catalyst. We consider that the oxidation of alkenes was suppressed due to a basic property of the Li§ Y208 catalyst. Oxidation to alkenes and that to COx may proceed on different active sites and/or by different active oxygen species on the catalyst. The results of the n-butane oxidation indicated the improvement of catalytic performance of Y203 by addition of Li § The addition of Li § to Y208 is expected to form mixed compounds or to cause the structural change of Y203. Therefore, we measured XRD patterns of Y20~ and Li § added Y203 samples. Figure 3 shows the Li § added Y203 XRD patterns. By addition of Li § into Y203, the peak positions of Y208 did not change, although small peaks due to LiYO2 were observed at 20 = 21.8, 41.0, 54.4 ~ in o the XRD patterns of Li+-added Y208. The formation of LiYO2 may cause the improvement of catalytic performance by Li § addition. To clarify the assumption, I I I I I I we carried out oxidation of n-butane over 20 25 30 35 40 45 50 LiYO2 catalysts. However, the catalytic 2e activity was much lower than that over Figure 3. ~]~D patterns of Y203 Li+-added Y 2 O s catalyst, although and Li § added Y203 . selectivities to alkenes were almost the
!!
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same levels as those over Li*-added Y208 catalyst. Therefore, we consider t h a t LiYO2 was not the active species for the oxidation. The XRD patterns provide the physical information about bulk structures of the ~ e r samples, while O K-edge XANES spectra measured in total electron yield mode give the information of the surface of the samples. o Figure 4 shows O K-edge XANES spectra of +-added Y203 Y203 and Li§ Y203 catalysts, as well as Li2CO 3 and the physical mixture of Li2CO3 and Y203 (Lily = 0.4). In the XANES spectrum of Y203, two peaks were observed at ca. 534 and 538 eV. The first peak is I I I I assigned to the transition from O ls to the 530 540 550 560 5 2 0 hybridized orbital of the oxygen 2p and photon energy/eV yttrium 4d (t2g), and the other to eg [8]. By the addition of Li + to Y203, the feature of the Figure 4. O K-edge XANES spectra XANES spectra changed. The XANES of the catalysts and reference samples. spectrum of Li+-added Y203 was almost compatible with t h a t of Li2CO 3. The XANES spectrum of the physical mixture of Y203 and Li2CO3 at a molar ratio Li/Y = 0.4 was different from t h a t of Li§ Y203, and was similar to that of Y203. These observations suggest t h a t the surface of Li§ Y203 was covered with Li2CO3. For the XRD profile of the Li§ Y203 catalyst, no peak due to Li2CO3 was found. Hence, Li2CO3 would be present on Y203 surface as a thin layer. The attainment of the higher yield of alkenes m a y be ascribed to the formation of Li2CO3 layer which enhances the formation of active oxygen species but reduces the deep oxidation of alkenes. 4. C O N C L U S I O N Rare-earth metal oxides, especially Y203, are effective catalysts for oxidative pyrolysis of n-butane to form ethene and propene. Modification of the metal oxide catalysts by Li § brought about the increase in selectivities to alkenes. The alkene yield attained to 58 % in the oxidation over Li*-added Y203 catalyst, and the activity of the catalyst was very stable. For the Li§ Y208 catalyst, the surface of Y203 was covered with a thin layer of Li2CO3. REFERENCES
1. H. H. Kung, Adv. Catal., 40 (1994) 1. 2. J. H. Lunsford, Stud. Surf. Sci. Catal., 81 (1994) 1. 3. K. Otsuka, K. Jinno, and A. Morikawa, J. Catal., 100 (1986) 353. 4. K. Otsuka, and K. Jinno, Inorg. Chim. Acta, 121 (1986) 237. 5. T. Ito, J. X. Wang, C. H. Lin, and J. H. Lunsford, J. Am. Chem. Soc., 107 (1985) 5062. 6. K. Otsuka, Q. Liu, and A. Morikawa, J. Chem. Soc., Chem. Commun., 586 (1986). 7. E. E. Wolf (eds.), Methane Conversion by Oxidative Processes, Van Nostrand Reinhold, New York, 1992. 8. J. G. Chen, NEXAFS Investigations of Transition Metal Oxides, Nitrides, Carbides, Sulfides and Other Interstitial Compounds, Elsevier, 1997.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Selective oxidation of monosaeeharides using Pt- containing catalysts E. Sulman a, N. Lakina a, M. Sulman a, T. Ankudinova a, V. Matveeva a, A. Sidorov a, S. Sidorov b aBiotechnology and Chemistry Department, Tver Technical University, A. Nikitina str., 22, Tver, 170026, Russia bA.N. Nesmeyanov Institute of Organoelement Compounds RAS, Vavilova str., 28, Moscow, 117813 Russia L-sorbose oxidation selectively to 2-keto-L-gulonic acid has been studied using Ptpolymer catalyst derived from hypercrosslinked polystyrene containing Pt nanoparticles (HPS). Varying the reaction conditions, such as sorbose and soda concentration, catalyst amount, temperature, oxygen feed rate, and stirring rate, the optimal conditions for achieving the selectivity of 98% have been found. Studying the reaction kinetics allowed to choose the mathematical model which describes well the experimental data. Kinetic experiments at various reaction temperatures permitted to obtain apparent activation energy value of 36 kJ/mol.
1. INTRODUCTION In recent years the problem of selectivity has become one of the most important problems of catalysis. With decreasing selectivity, side products are formed, thus the quality of the whole product deteriorates. Nowadays the reactions of selective oxidation of monosaccharides underlie the methods for the production of biologically active compounds. Thus, D-glucose oxidation to D-gluconic acid is used in the production of calcium gluconate which is both a medicine and an intermediate in riboflavin (vitamin B2) synthesis. The main intermediate in the production of ascorbic acid (vitamin C) is 2-keto-L-gulonic acid obtained by L-sorbose oxidation [ 1, 2]. Literature analysis on direct catalytic oxidation of L-sorbose to 2-keto-L-gulonic acid [3-6] shows that oxidation of monosaccharides with oxygen is recommended to be conducted on Pt, Pd, Os catalysts, better applied on activated charcoal at normal or increased temperatures (50-70~ and pressure, at neutral or alkaline pH. In acid solutions the process doesn't take place. Strongly alkaline medium causes by-products formation. Optimal pH values are within the limits of 8-10. As alkalizing reactants, alkaline salts of weak organic or mineral acids were suggested. Earlier we studied selective heterogeneous catalytic oxidation of D-glucose to Dgluconic acid (Scheme 1) [7]. There have been used platinum- and palladium-containing catalysts (including modified) supported on A1203, SiO2, carbon material "Sibunit". The regularities of D-glucose oxidation have been found. The reaction mechanisms have been detailed. The kinetic models with adequate description of the experiments have been also developed.
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化
H~C--O H~C HO~C
I I
H--C
f H--C I H2C
NaO~C
~O
r I [o], Na~_ H O - - C ~ H
OH
H~C
H
Cat
OH
-
I H~C
OH
H~C
OH
I I H2C
OH
OH
OH
D-glucose
OH
sodium D-gluconate Scheme 1
As for solving the selectivity problem in direct L-sorbose oxidation (Scheme 2), most of authors search for selective modified catalytic systems on the basis of Pt and Pd [8-11]. Under optimal conditions when Pt/C (5% Pt) catalyst was used, 28-37% yield of 2-keto-Lgulonic acid was achieved at 100% L-sorbose conversion [10]. So, such a catalyst did not provide high selectivity in L-sorbose oxidation. Lately attention was focused on synthesis and studying of catalytic properties of colloidal metal particles of nanometer size (nanoparticles), incorporated into polymer matrix. It provides a stabilization of small particles with huge surface areas and high reactivity [12]. In the present paper we report on catalytic properties of new Pt catalyst derived from hypercrosslinked polystyrene (HPS) [13] containing Pt nanoparticles. The kinetics of Lsorbose selective oxidation is discussed. CH2OH
I H0--C I HO--C--H I H~C--OH I HO~C~H I H2C
H
COOH
C=O
I HO--C ~ I HO--C~H O
L-sorbose ( B 1 )
O2~ I Cat~ H ~ C - - - O H
I I H 2 C ~
COOH
I HO~C I HO--C~H O
HO~C~H
L-gulosone
o2
]
Cat ~ H ~ C - - O H
I I H2C
HO~C~H
I I C
C--O HO O
H
O2~ I Cat~ H - - C - - O H
I I H2C - -
HO--C~H
OH
2-keto-L-gulonic acid (B2) Scheme 2
2. EXPERIMENTAL TECHNIQUE 2.1. Catalyst preparation
Catalyst was prepared at the A.N. Nesmeyanov Institute of Organoelement Compounds by impregnation of HPS with a solution of H2PtC16 in THF [13]. After drying procedure (3 days in vacuum of 1.5 Mbar), HPS-Pt contained 3-6 wt.% Pt.
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2.2. L-sorbose oxidation technique The experiments have been conducted in a specially constructed apparatus [ 14] which allows to carry out a batch process varying such parameters as sorbose and soda (NaHCO3) concentration, catalyst amount, temperature, oxygen feed rate, and stirring rate. The catalyst and L-sorbose solution at predetermined concentration were placed in thermostated set-up supplied with magnetic stir bar and reflux condenser. Oxygen-containing gas feed was controlled by rotameter. Equimolar quantity of alkalizing agent (sodium hydrogencarbonate, NaHCO3) was fed to the set-up once or gradually in certain periods to maintain constant pH. High rate stirring allows to carry out the process without diffusion limitations. During the reaction, probes of the reaction mixture were taken for analysis at certain periods of time. After the end of the experiment the catalyst was separated by filtration. The filtrate was analyzed for the presence of unreacted L-sorbose and 2-keto-L-gulonic acid formed. The determination of residual monosaccharide was carried out by gas chromatography (GC) using "Chrom-5" chromatograph. It is supplied with flame-ionoization detector and glass column filled with 5% SE-30 on Chromaton N-AW at a constant temperature. The quantity of the product, 2-keto-L-gulonic acid, was determined by Heinz classical iodometric method [ 15]. 3. RESULTS AND DISCUSSION The kinetics of the selective L-sorbose oxidation on the HPS-Pt catalytic system has been studied. The experiments have been conducted while varying the ratio between the initial L-sorbose concentration (Co) and catalyst (HPS-Pt) concentration (CD, q - C0/Co, from 0.77 g/g to 2.05 g/g. The experimental conditions have been the following: T was 70 ~ oxygen-containing gas flow rate was 20.10 -6 m3/s; stirring rate was 1000 rpm. From the dependencies obtained in the plot of "sorbose conversion (X1) vs. reaction time" (Fig. 1.). It follows that the higher the q value, the longer the reaction time.
1.0{ 0.9O.80.7-
X 0.6
-
0.5
-
0.4
-
0 , 0
9 o v x7
q=0.77 q=0.88 q=l.02 q=2.05 I
I
I
I
I
3000
6000
9000
12000
15000
T, S
Fig. 1. Sorbose conversion (X1) vs. reaction time
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The primary analysis of the experimental data has allowed to reveal the dependence of the time of 30% L-sorbose conversion on q. This dependence is shown by the expression: (n=0.5)
1:0.3 "~ qn
According to GC analysis on catalysate, the process was established to occur selectively and only a few by-products were formed. So the following process scheme has been suggested: B1 k, > 9 2 , where B1 is L-sorbose and B2 is 2-keto-L-gulonic acid. For the reduction of the experimental data belonging to different Co and Cr values to a family of curves, an independent variable has been introduced: O = r / q " , where r is the time and n - 0.5. The X : - 0 dependence obtained, where X2 is the current concentration of B2, is presented in Fig. 2.
0.6
0.5-
o/r
0.4r
,/
0.3
0.2-
I I
4000
q = 0.77 q = 0.88
v
q = 1.02
Calculation
I
0.1-
0
9
9 o
r
v
v
T
o I
6000
i
i
i
8000
10000
12000
|
~/q0.5
14000
16000
Fig. 2.2-keto-L-gulonic acid yield vs. |
The application of the explicit integral method has permitted to develop the mathematical model for L-sorbose oxidation on the HPS-Pt catalyst describing the experiments: W -- - k . X 1 - ~
where W is the reaction rate, k is the rate constant.
(1)
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The comparison of experimental and computed data (Fig. 2.) shows that this model agrees well with the experimental values. From Fig. 2 one can see an induction period. The existence of the induction period is apparently connected with unsteady state of the catalyst surface in the oxygen flow and with a formation of the catalyst active sites. Such a phenomenon takes place in many catalytic reactions [ 16]. To obtain supplementary information about the regularities of L-sorbose oxidation on the HPS-Pt catalytic system, the influence of the temperature on this reaction has been studied. L-sorbose aqueous solution with 0.119 mol/L concentration and q equal to 0.88 g/g has been oxidized. Temperature has been varied from 60 to 80 ~ The dependences obtained while analyzing experimental data reveal that temperature increase from 60 to 80 ~ results in increase of L-sorbose oxidation rate. At the same time, selectivity decreases at a temperature higher than 70 ~ It can be caused either by 2-keto-Lgulonic acid overoxidation products (because of reaction rate increase) or by the decomposition of L-sorbose which is a thermolabile compound. On the basis of the experimental results at various temperatures and using model (1), k parameter has been computed. Arrhenius dependence (Ink vs. l/T) for the apparent activation energy determination has been plotted. The Eapp. value equal to 36 kJ/mol was found to be higher for HPS-Pt than for Pt/SiO2 catalyst (Eapp. = 28 kJ/mol) described in [ 14]. On the other hand, the relative oxidation rate for HPS-Pt was 1.2-10-3 mol B2/gPt.mol B l-S which is twice as high compared to the relative rate for Pt/SiO2 (0.6-10 -3) [14]. In the case of polymer catalyst, the number of active sites taking part in the reaction is supposed to increase. To determine oxidation optimal conditions, the following parameters have been varied: catalyst amount (Cc) from 20 to 75 g/L, sorbose concentration (Co) from 0.22 to 0.44 mol/L, NaHCO3 (alkalizing reactant) concentration (Calk.react) from 0.22 to 0.44 mol//L; reaction temperature (T) from 60 to 80 ~ oxygen feed rate (V0) from 6-10-6 to 14.10 -6 m3/s; stirring rate from 200 to 1000 rpm. Maximum selectivity (98%) has been achieved under the following conditions" Cc = 63 g/L, Co = 0.36 mol/L, Calk.react. = 0.36 mol/L, T = 70 ~ V0 = 14.10 -6 m3/s, stirring rate equal to 1000 rpm. Reaction was carried out as a batch process with continuous alkaline agent feed. 4. CONCLUSION
The catalyst derived from Pt-containing hypercrosslinked polystyrene (HPS-Pt) has showed high catalytic activity and selectivity in the reaction of L-sorbose direct oxidation. Varying of the parameters of L-sorbose oxidation has allowed to find optimal conditions of Lsorbose selective oxidation with selectivity of 98%. The kinetics of L-sorbose oxidation was studied. The kinetic data obtained have allowed to chose the mathematical model which describes well the experimental results. In the future this model will be used for the computation of the industrial reactor for 2-keto-L-gulonic acid synthesis. ACKNOWLEDGMENTS We thank Russian Foundation for Basic Research (Grant No. 98-03-33372) and Federal Program "Integration" (Educational Research Center on Chemistry and Physics of Polymers) for financial support of this research.
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REFERENCES
1. L. Shnaidman, Vitamins productions, Russia, Moscow, 1973. 2. V. Berezovsky, Chemistry of vitamins, Russia, Moscow, 1973. 3. E. Merck, Verfahren zur Herstellung von 2-keto-L-gulonsaure, Germany Patent No. 692 897 (1965). 4. O. Dalmler and K. Heyns, Process for the Production of Ascorbic Acid from Sorbose, US Patent No. 2 189 778 (1956). 5. O. Dalmler and K. Heyns, Process for the ProduDtion of Keto Culonic Acid from Sorbose, US Patent No. 2 190 377 (1954). 6. E.Merck, Procede pour la preparation de 1 acide 2-ceto-l-gulonigue, France Patent No. 829 236 (1968). 7. E.M. Sulman, O.B. Sannikov, A.I. Sidorov, M.V. Avtushenko, A.T. Kirsanov, Method for calcium gluconate synthesis, Russian Patent No. 2 118 955 (1996). 8. C. Bronnimann, T. Mallat and A. Baiker, J. Chem. S o c . - Chem. Comm., 13 (1995) 1377-1378. 9. C. Bronnimann, and Z. Bodnar, and R. Aeschimann et al, Joumal of Catal., 161, 2 (1996) 720-729. 10. T. Mallat, and C. Bronniman, and A. Baiker, Applied Catal. A, 149, 1 (1997) 103-112. 11. T. Mallat, C. Bronniman, A. Baiker, J.Molec. Catal. -A, 117, 1-3 (1997) 425-438. 12. H. Hirae, Macromol. Chem. Suppl., 14 (1985) 55. 13. L. Bronstein, S. Sidorov, A. Gourkova, P. Valetsky, J. Hartmann, M. Breulmann, H. C61fen, M. Antonietti, Inorg. Chim. Acta, 280, 1-2 (1998) 348-354. 14. M.V. Avtushenko, Catalytic oxidation of monosaccharides - Thesis, Tver, Russia, 1996. 15. K. Heyns, Analyt. Chem., 558 (1947) 171-192. 16. E.M. Sulman, Russian Chemical Reviews, 63, 11 (1994) 923-936.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Synthesis of New Neo-carboxylic Acids via Rearrangement and Oxidation of 1,3-Dioxanes + Jutta Fischer and Wolfgang F. H61derich* Department of Chemical Technology and Heterogeneous Catalysis University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany I. ABSTRACT Several methods for the preparation of new interesting and valuable compounds 3-alkyloxypivalic acids starting from substituted 1,3-dioxanes were examined. A two-step environmentally benign method could be established using O2 as oxidant in the presence of heterogeneous catalysts. The scope of the method was tested by use of several 1,3-dioxanes and the isolated new pivalic acid derivatives were characterised. 2. INTRODUCTION The isomerisation of 1,3-dioxanes to 3-alkoxypropanals in the presence of various heterogeneous catalysts has been discussed in detail [ 1,2]. Catalysts include zeolites [3], metal oxide impregnated silica [4], acid phophates having a zeolite structure [5] and acid treated metal oxides [6]. Highest activity and selectivity, however, are displayed by pentasil zeolites [7,8]. By employing 5,5-dimethyl substituted 1,3-dioxanes in the isomerisation reaction, pivalic aldehyde derivatives are formed. The corresponding 3-alkoxypropanols or 3-alkoxypropyl amines can be obtained by introducing hydrogen or a combination of H2 and ammonia into the reaction system using metal modified catalysts [9,10]. These neo-compounds can find use in organic synthesis for the preparation of biologically active chemicals. Also, the phthalates of the alcohols can be used as effective plasticisers for PVC. Therefore, it is surprising that the synthesis of 3-alkyloxypivalic acids which could be valuable building units for organic synthesis or lubricants has not been studied so far. Only 3-benzyloxypivalic acid has been mentioned in literature, synthesised from pivalolactone or 2-1ithio-2-methyl-propionic acid as starting materials [11,12] which are expensive and thus unsuitable for synthesis in commercial scale. Also, oxidation of 3-benzyloxypivaldehyde could be achieved with potassium permanganate or with oxygen catalysed by cobalt or manganese salts [ 13]. These oxidations can be environmentally hazardous because of the high amounts of inorganic salts formed and the difficult separation of homogeneous catalysts from the reaction mixture. Thus, it was our aim to find an easy, environmentally benign and generally applicable synthesis route for various 3-alkoxypivalic acids. + This work carried out within the Sonderforschungsbereich SFB 442 was sponsored by the Deutsche Forschungsgemeinschaft to whom we extend our thanks. * to whom correspondence should be addressed
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3. EXPERIMENTAL
3.1 Reaction conditions Batch experiments were carried out in a 70ml steel autoclave tested for pressures up to 100 bar equipped with a glass coated magnetic stirrer and two high pressure valves. Starting materials (0,1g catalyst, lg 1,3-dioxane, 2ml benzene) were given into the autoclave; after sealing the autoclave, oxygen was added while stirring. The autoclave was heated using a heating band and a Eurotherm temperature controller. In case of two-step reactions, oxygen was added after cooling following the isomerisation step. Samples were prepared for GC by adding CH2N2 to form the ester of the acids. Continuous gas phase experiments were carried out in a tube reactor under atmospheric pressure. 2g of catalyst pellets were placed in a fixed bed. A solution of lg 1,3-dioxane in 5g of toluene was introduced into the evaporating zone where the carrier gas entered the reactor. The reactor was placed in a furnace where isothermic reaction conditions were maintained. The reaction mixture was collected in a double-walled glass flask equipped with a reflux condenser. In the two-step reaction, oxygen was introduced via a tube led through the condenser into the flask which was heated using its double wall. Samples were modified by CH2N2 for GC-Analysis. 3.2 Starting materials and catalysts 1,3-dioxanes were prepared by reaction of neopentylglycol with the appropriate aldehydes according to known procedure [14] catalysed by Amberlyst 15~ and purified by destillation under reduced pressure. Starting materials as well as solvents were obtained from Fluka and used without further purification. BMFI-catalysts were kindly provided by BASF company and A1MFI-catalysts from Degussa company. Silica-alumina was bought from Aldrich company. Synthesis of V-MCM41 [ 15] as well as impregnation and other treatments were carried out in our laboratories. For metal oxide impregnation of the catalysts, 10g of zeolite powder were stirred in 50ml of water and the appropriate amount of metal salts (0,159g silver nitrate; 0,263g ammoniumtetravanadate, 1,923g copper nitrate trihydrate) were added. The mixtures were stirred for 30min at room temperature and dried at 60~ under reduced pressure. The materials were calcined at 550~ to form the oxides. 3.3 Reaction products Yields were determined by GC-analysis. Reaction products were characterised by and after isolation by IR- and NMR-spectroscopy. Isolation of the products was achieved by selective extraction. Washing of the mixture with sodium bicarbonate solution isolated acidic components. The aqueous was washed with diethylether and acidified with HC1. The separating solid or extracted with diethylether, which was dried with Na2SO4 and evaporated.
GC-MS reaction solution oil was
4. RESULTS AND DISCUSSION
4.1 Isomerisation of 1,3-dioxanes in oxygen atmosphere In analogy to the "one-pot"-synthesis of neoalcohols and neoamines, our first strategy was introducing oxygen into the reaction system to allow oxidation of the formed aldehyde to the corresponding acidsimultaneously with the rearrangement. This was tried both in batch reactors and continuous gas phase reaction. As a test molecule 2-phenyl-5,5-dimethyl-l,3dioxane was used.
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Conversion according to (1) gives the known 3-benzyloxypivalic acid as a product. O .~
0
1
OH
(l)
2
Problems were anticipated since the isomerisation runs effectively only at temperatures higher than 250~ a temperature favouring decarbonylation of radical species formed during the oxidation of the aldehyde. Furthermore, an ether function in the isomerisation product offers an point of attack for both oxygen and the acid catalyst. In batch reactions, to avoid decarbonylation the reaction was performed under pressure; in gas phase reactions, short contact times at the catalyst were used to suppress consecutive reactions. However, both ways were unsuccessful for the preparation of component 2. In both reaction types, benzoic acid 3 was the main product (selectivity up to 60%) due to cleavage of dioxane I and products to benzaldehyde 4 and subsequent oxidation. Only trace amounts of 2 and no aldehyde intermediate (component 5) could be detected. As by-products, mainly benzoates are formed from the fragments of product degradation. Consecutive reactions are obviously dominating under batch conditions Therefore, the reaction was transferred to the gas phase. A boron containing pentasil zeolite (catalyst A) known as the best catalyst for the rearrangement of the 1,3-dioxanes to the neo-aldehydes under these conditions was impregnated with metal oxides to promote oxidation, such as V205 (B), CuO (C) and Ag20 (D). Additionally, a vanadium containing MCM-41-material was used (E)to reach a more selective oxidation by isolated V-species and to reduce diffusion constraints through its larger pore size, avoiding consecutive reactions. Table 1 Comparison of different catalysts in the isomerisation of 2-phenyl-5,5-dimethyl-1,3-dioxane under oxidising atmosphere; 290~ 15 1/h 02, WHSV = lh -1, TOS = 6h catalyst
conversion 1/ %
selectivity 5/ %
A B D E
69 54 51 100
23 46 65 6
selectivity 2/ % 0 1 2 0
selectivity 4/ % 6 17 10 69
selectivity 3/ % 4 4 6 17
Table 1 shows a low selectivity to component 5 obtained by catalyst A, not to be expected from published results. Presence of oxygen has to be a limiting factor. The metal oxide impregnated catalysts B and D give higher selectivities, but only very low amounts of the corresponding acid 2 are formed. These catalysts are less active due to blocking of some acidic centres by metal oxide. Vanadium seems to favour the formation of benzaldehyde, either because its oxophilic character impedes resorption, allowing consecutive reactions, or by facilitating an electron shift in the starting material by its ease in changing oxidation states. The results obtained with Cu-BMFI (C) are not shown because of complete blocking of the reactor after one hour time on stream (TOS) due to coke formation.
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4.2 Two-step reaction for the synthesis of 3-benzyloxypivalic acid For the aforementioned reasons, 2 had to be synthesised in two steps according to equation (2), separating the high temperature isomerisation from the low temperature oxidation. 0 cat,
1
0
0~, > ~...0~.~
5
OH
(2)
2
In batch isomerisations, catalyst A was studied by variation of several reaction parameters. It was determined that selectivity to the aldehyde drops slowly after 16 hours, while formation of benzaldehyde increases. Furthermore, the combined influence of oxidation and isomerisation catalyst on the oxidation was examined. While the aldehyde is nearly completely oxidised without a catalyst, Ag20 slightly increases conversion. The presence of catalyst A does not impede oxidation. Combining these results, the yield of 2 was optimised up to a selectivity of 48% (table 2). An aluminum containing catalyst (F) led to the formation of higher amounts of benzaldehyde since its stronger acid centres slow down desorption of the formed aldehyde. Catalyst C is less active in the isomerisation because active centres are blocked by CuO. Table 2 Comparison of different MFI-based catalysts in the two-step batch reaction synthesis of 3-benzyloxypivalic acid; isomerisation: 300~ 16h; oxidation: 50~ 20 bar 02, 6h; m (dioxane)/m (catalyst) - 10 for both reactions catalyst F A C
conversion 1/ % 97 70 61
selectivity 5/ % 2 0 0
selectivity 2/ % 27 48 26
selectivity 4/ % 18 8 10
selectivity 3/ % 4 3 5
By selective extraction (section 3.3) all acidic components are isolated. This leads generally to contamination of isolated 2 with benzoic acid formed from benzaldehyde present from the isomerisation step. To avoid this, a combination of gas phase isomerisation and liquid phase oxidation in a semi-continuous reaction mode was developed (section 3.1). As in the case of the batch reactions, reaction parameters in both reaction steps were varied. Optimal isomerisation conditions using catalyst A were found to be about 300~ and a WHSV of lh -1 which is in accordance with data given in the literature[7]. Yields of 5 are about 65%. For the oxidation step, the optimum of temperature and the necessity of a catalyst were the main topics of interest. It was found that a temperature of 80~ offers a compromise between high conversion of the aldehyde and acceptable selectivities to component 2. Several silver and silver oxide containing materials were found to slightly increase conversion (table 3). More important, however, is the enhancement of selectivity using highly dispersed silver on alumina or charcoal, giving about 90% yield of 2 in the oxidation step. Thus, a combination of optimised conditions for both steps yields more than 50% of 2.
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Table 3 Comparison of oxidation catalysts in the semi-continuous two-step synthesis of component (2); 80~ 02 20 l/h catalyst without Ag20 5% mg on A1203 50% Ag20 on A1203 15% Ag on charcoal - _ -. . . . . . . . . . . . . . . . . . : _ - . : . . . . . . . . . . : _ - _ - _ - _ - : _ - _ - . : . : . : : . : . : . : . : _
loading (g 1/g metal _c__0mp0nent)
.:_ .:.:_ _ .: . . . . . . . . . . . . . . . .:.:.:.:.:_-. -_ . . . .
conversion of 5 / %
selectivity to 2/%
90 95 91 96 91
76 65 89 80 92
1200 3000 1200 3000 .:.:.:.:.:_-_-.:.:.:.:.:.:.:.:.:.:.:.:.:_-.:_
_-_ _ .:_. -_ . . . . . . . . . . . . . . .
-_ . . . . . . . .
_-_ _ _ .:.:_-.:.:.:.:.:_
_ .:.:.:.:_-.:.:.:.:_-.:.:.:.:_-
. . . . . . . . . . . . . . . . . . . . . . . . .
_ -. . . . .
_-.:.:.:.:.:.:.:_-.:.:_-_-
. . . . . . . . . . . . . . . . . . . . . . . . . . .
In transferring the reaction to a pilot plant, also the duration of the activity of catalyst A was tested. During 50 hours time on stream (TOS) catalyst activity decreased. Deactivation set in rapidly but proceeded more slowly after a TOS of 20 hours, being stable at 45% during the last 25 hours. Selectivity is very high at 90% and does not change significantly. In the oxidation, a scale up leads to lower conversion of the aldehyde. Reason for this is the relatively lower amount of oxygen brought into the reaction mixture since the reflux condenser could not accommodate a higher gas stream. On the other hand, a higher selectivity to 2 is observed, this way keeping the over all yield (see table 4). Table 4 Scale up of the oxidation; 80~
02 20 l/h, m(dioxane) 9m(catalyst) = 1200, catalyst Ag20
scale
conversion of 5/%
selectivity to 2/%
laboratory (70ml reaction volume)
95
65
scale up (11 reaction volume)
86
84
4.3 Synthesis of new 3-alkoxypivalic acids To examine the scope of the developed reaction route, several 1,3-dioxanes were converted to acids under conditions optimised for I (figure 1):
0 H3C
0
CH3
3-propyloxypivalic a c i d 6
H3C~O.'~~. H3C/ H3C
H3C
0
CH3
~ H3C CH3
CH3
3-butyloxypivalic acid 7
3-isobutyloxypivalic acid 8
OH CH3
O.'~~_..OH H3C CH3
0
3-(2-ethylbutyloxy)-pivalic acid 9
CH3
[ ~
0
3-(2-methylphenylmethoxy)-pivalic acid 10
Fig. 1 New 3-alkoxy pivalic acids prepared by two-step synthesis from 1,3-dioxanes
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Table 5 Conversion of different 1,3-dioxanes to the corresponding 3-alkoxypivalic acids; 300~ carrier gas N2 12 l/h, WHSV = lh "1, TOS = 6h, atmospheric pressure
acid 6 7 8
9 10
e-onve-rsion....... -seieciivity to 6 -- i (JJ/-eonv~e:rsi-0-n~ ili-e" 1,3-dioxane/% % oxidation/% 64 55 82 66 53 88 45 62 86 27 48 85 50 42 61
sdeetMiy t0 6 :-~i0~:~tile .... oxidation/% 89 67 62 66 87
All corresponding acids 6 to 10 could be isolated by the process described in section 3.3 and were characterised by MS, IR and 5. CONCLUSIONS An easy and generally applicable method for the synthesis of 3-alkoxypivalic acids has been developed and five new substances of this class were isolated and characterised. A discontinuous liquid phase oxidation reaction was the best solution for the production of the acids. Good yields of 3-benzyloxypivalic acid can be achieved. Yields up to 40% over both steps could be obtained and the acid could be isolated in 98% purity. In a scale up of the reaction both the isomerisation and the oxidation step gave satisfying results. Employing several 1,3-dioxanes as starting materials, the broad scope of the developed method could be illustrated. This way, a wide variety of pivalic acid derivatives containing different ether groups could be obtained. REFERENCES
[11 [21 [31 [4] [5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15]
C.S. Rondestvedt, G.J. Mantell, d. Am. Chem. Soc., 1960, 82, 6419. C.S. Rondestvedt, J. Am. Chem. Soc., 1962, 84, 3319. W.F. H61derich, F. Merger, R. Fischer, EP 0 199 210 B1, (16. 10. 1991), BASF AG, Ludwigshafen. H.-J. Arpe, DE 29 22 698, (11. 12. 1980), Hoechst AG, Frankfurt. W.F. H61derich, F. Merger, EP 0 291 807 B1, (15. 01. 1992), BASF AG. W.F. H61derich, F. Merger, H. Lermer, EP 0 291 806 B1, (26. 08. 1992), BASF AG. M.E. Paczkowski, W.F. H61derich, Stud. Surf. Sci. Catal., 1994, 83, 399. W.F. H61derich, F. Merger, R. Fischer, EP 0 199 210, (29. 10. 1986), BASF AG, Ludwigshafen. W.F. H61derich, M.E. Paczkowski, D. Heinz, DE 196 21704, (30.05. 1996), Hoechst AGRuhrchemie. W.F. H61derich, M.E. Paczkowski, D. Heinz, Th. Kaiser, DE 196 21703, (30.05. 1996), Hoechst AG-Ruhrchemie. U. Luhmann, W. Liittke, Chem. Ber., 1972, 105, 1350. Y-I. Matsushita, Chem. Lett., 1983, 1397. A. Abiko, J.C. Roberts, T. Takemasa, S. Masamune, Tetrahedron Lett., 1986, 27(38), 4537. F.A.J. Meskens, Synthesis, 1981, 501. A.B.J. Amold, J.P.M. Niederer, T.E.W. Niegen, W.F. H61derich, Microporous Mesopourous Mater, 1999, 28, 353.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Low Temperature Radical Oxidation of Butane over in-situ Prepared Silica Species A. Satsuma ~*, N. Sugiyama l, Y. Kamiya 1, T. Kamatani I and T. Hattori 2 ~Department of Applied Chemistry, Nagoya University, Nagoya 464-8603, Japan. 2Research Center for Advanced Waste and Emission Management, Nagoya University, Nagoya 464-8603, Japan.
Silica species prepared by in-situ method using silicon alkoxide as a precursor were found to catalyze the radical oxidation of n-butane at low temperatures around 650 K. The silica species play a role of an initiator, and the reaction propagates in gas-phase. The in-situ hydrolysis of silicon alkoxide also resulted in the formation of active silica species. It was indicated that the active sites were generated from surface silanols on silica surface. 1. INTRODUCTION Silica itself is generally thought to be inactive for catalytic reactions. Although there are some reports about the catalysis of silica [1-5], the catalytic activity of silica is very low and the reaction conditions are much limited in these cases. For example, partial oxidation of methane over silica is achieved only at high temperatures around 8 0 0 - 1000 K [1,2]. It is also reported that the evacuation of silica powder at high temperatures generates catalytic activities for dehydrogenation [4] and photocatalytic oxidation [5]. The requirement of such high temperatures may suggest the difficulty in the activation of silica surface. Previously, we have found that silica species prepared by in-situ method are significantly active for partial oxidation and oxidative dehydrogenation of butane at lower temperatures around 650 K [6]. The introduction of tetraethylorthosilicate (TEOS) vapor into nbutane/Oz/N2 mixture gas (in-situ preparation) resulted in the formation of active silica species and the conversion of butane. The activity of the in-situ prepared silica was comparable to that of (VO)2P207 which is known to be efficient catalyst for the industrial production of maleic anhydride [7]. Although the radical oxidation was suggested, the reaction mechanism of silica species is still unclear. In the present work, roles and active structure of silica species are investigated, and the reaction mechanism over in-situ prepared silica species is discussed. 2. E X P E R I M E N T A L To identify the active silica species, the preparation of silica species and the butane oxidation were performed separately. The continuous flow reaction was carried out in a
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conventional flow apparatus using an empty Pyrex-tube reactor at atmospheric pressure. A mixture of n-butane/O2/N2 (2%/18%/balance) was used as a feed gas at a flow rate of 64.2 cm 3 min ~. The pulse reaction was carried out in the same flow apparatus equipped with a sampling column for the injection of n-butane/O2/N2 mixture (2%/18%/balance, 0.80 cm 3) into flowing He as a carrier gas. The Pyrex-glass reactor was mainly made of capillary glass tube (I.D. 1 mm, O.D. 6 mm) and partly large space for reaction (I.D. 9.6 mm, O.D. 12 mm, and length 100 mm). Silica species were prepared by the following two methods: in-situ preparation and in-situ hydration. For in-situ preparation, tetraethylorthosilicate (TEOS, 0.037%)was introduced in the stream of n-butane/O2/N2 mixture at 643 K for 1 h. For in-situ hydration, 0.25 cm 3 of TEOS was injected in flowing 10%H20/N2 at 643 K. SiO2 (JRC-SIO-8, 303 m2g-1) was supplied from the Committee on Reference Catalyst of the Catalyst Society of Japan. 3. RESULTS AND DISCUSSSION 3.1 B u t a n e O x i d a t i o n o v e r in-situ P r e p a r e d Silica Species Fig.1 shows the conversion of butane and oxygen in the empty reactor in the absence and presence of TEOS. Although the butane conversion was negligibly small in the absence of TEOS, the conversion steeply increased after introduction of TEOS. When TEOS vapor was removed from the feed stream, the butane conversion remained unchanged for longer than 1000 min. It should be mentioned that color of the reactor turned to white after the reaction, suggesting the deposition of silica on wall of the reactor. Based on the assumption that all the loaded TEOS molecules contributed to the butane conversion, turnover number was above 80. Therefore, the oxidation is not a homogeneous reaction in gas-phase but a heterogeneous catalytic reaction over the solid silica species derived from TEOS. 100
,
100
I
',with', ',TEOS',
~. 8O
without TEOS
~ 80 o
I I I
o 60
~ 60
9 ~-,,i
Butane
t:x:rT-o----
o
~ 40
=o 40 9
20 t
m 20
Oxygen
,, I
0
|
500 1000 Time / min
1500
Fig. 1 Conversion of butane ( O ) and oxygen (O) in an empty reactor at 643 K in the absence and presence of TEOS.
600
650
700
Temperature / K Fig. 2 Conversion of butane in an empty reactor ( A ) , over in-situ prepared silica species ( O , O ) , and over SiO2 (E-l).
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Fig.2 shows the temperature dependence of butane conversion. The silica species were prepared in flowing TEOS/butane/O2/N2 at 643 K for 1 h in the empty reactor. Then, the butane oxidation was carried out in the absence of TEOS. The temperature dependence of butane conversion over the in-situ prepared silica was quite unique and entirely different from conventional catalytic reactions. The butane conversion increased steeply from 603 K, reached to the maximum at 643 K, and gradually decreased at higher temperatures. This profile was reproducible on the increase (open symbols) and decrease (closed symbols) in the reaction temperature. The steep increase in the conversion between 603 K and 623 K corresponds to the activation energy of 550-700 kJ mo1-1 which is extremely higher than those in usual catalytic oxidation reactions. Above 643 K, negative temperature coefficient of reaction rate was observed. The butane conversion over SiO2 gradually increased with reaction temperature, but the activity was negligibly smaller than that of the in-situ prepared silica. Table 1 shows the product distribution on the in-situ prepared silica. Butene and acetaldehyde were the main products in the organic molecules. The selectivity to butene increased with the increase in the reaction temperature, while the selectivity to acetaldehyde decreased from 30.0 to 18.4 %. As for the oxygenated products, alcohol, aldehyde, ketone and furan were observed. Maleic anhydride was not produced at all, though this is the main product of butane oxidation over (VO)2P207 [7]. The product distribution was also different from that over SiO2 at 793 K, at which the conversion was comparable to that over the in-situ prepared silica species. Table 1 also shows the product distribution in gas-phase radical oxidation reported by Euker and Leinroth [8]. The product distribution in our experiment was quite similar to that of gas-phase radical oxidation, indicating the butane oxidation over the in-situ prepared silica species proceeds as a radical oxidation in gas-phase. Fig. 3 shows the effect of space volume in the reactor. In this series of experiments, the thicker reactor (I.D. 13.6 mm, O.D. 16 mm, and length 100 mm) was used, controlling the space volume by insertion of glass rod (O.D. 3.0 - 8.0 mm). The butane conversion significantly decreased with the reduction of the space volume. The strong dependence on the
Table 1 Conversion and selectivity in butane oxidation. Catalyst/Condition In-situ prepared silica
a)
SiO2 a) (JRC-SIO-8) 683 793
gas-phase t,j oxidation 598
Temperature / K 623 643 Conversion/% butane 41.7 57.8 25.5 26.5 16.9 Selectivityc / % butene ) 15.9 14.0 36.9 13.1 26.7 C2-C3 olefins 5.4 6.5 8.4 13.3 6.6 acetaldehyde 30.0 20.6 18.4 3.2 14.9 C3 oxygenated d) 6.0 3.9 4.4 0.1 2.1 C4 oxygenated e) 2.1 1.3 2.0 0.9 1.2 furan + dihydrofuran 4.0 3.3 3.1 8.2 7.5 C1-C3 paraffins 0.1 0.2 0.1 0.2 0.9 others 7.2 7.7 8.6 0.4 16.5 CO + CO2 29.3 42.7 18.1 60.5 23.7 a) This work, n-butane/O2 = 2%/18% in Pyrex glass tube, b) reported by Euker et al., nbutane/O2 = 7% /14% in stainless tube, c) 1-butene, cis-2-butene, and trans-2-butene, d) propionaldehyde, acetone, and propanol, e) methyl ethyl ketone, butanol, and crotonaldehyde.
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space volume suggests that the oxidation involves chain propagating steps in gas-phase and the butane oxidation terminates on the l~176 / wall of the reactor. 14.5 cm 3 The negative temperature coefficients of ~-~ 8o reaction rate (Figs. 2 and 3) can be observed in .2 13.8 cm 3 free radical gas-phase oxidation of ~ 6o hydrocarbons at low temperatures, which is o= called "cool flame" [9,10]. The reaction starts ,~ 4 0 from the abstraction of hydrogen from hydrocarbons followed by formation of m 20 peroxide radical [11]. At lower temperatures, isomerization of peroxide radical to a chain 0 600 650 700 branching step containing the formation of Temperature / K aldehydes and ketones is predominant, while olefins are formed through decomposition of Fig. 3 Effect of space volume on peroxide radical as a non-chain branching step at higher temperatures. Thus, these different butane oxidation in silica coated empty reaction routes result in the negative reactor at 643 K in the absence of TEOS. temperature coefficient of reaction rate [9,10]. Based on this reaction mechanism, acetaldehyde is the main product of the chain branching reaction. As shown in Table 1, the selectivity to acetaldehyde decreased with the increase in the temperature, which is in accordance with the reaction mechanism of cool flame phenomena. From the profiles of the catalytic activity and the selectivity of the in-situ prepared silica species, it can be concluded that in-situ prepared silica species act as the initiator for the gasphase radical oxidation, and they act as the sold catalysts on the wall of the reactor.
//
3.2 Structure of Active Silica Species Table 2 shows the effect of the feed gas in the preparation of silica species. The active silica species was generated only in the presence of butane and oxygen. As for the preparation temperature, the active silica was only generated at 603 - 643 K, at which butane converted in the presence of TEOS. These results suggest that the reaction of butane and oxygen is essential for the generation of the active silica species, i.e., any products may contribute to the generation of the active silica species. Furthermore, the active silica species were also generated by the in-situ hydrolysis of TEOS in flowing 10%HaO/N2. The activity and selectivity were the same as that of the in-situ prepared silica species. Fig. 4 shows the initial activity of the silica species generated by the in-situ hydrolysis of TEOS, as investigated by pulse reaction with butane/O2 mixture. Since no induction period was observed in the initial stage of the reaction, there is no contribution of any carbonaceous species to the active species. Thus, the active species should be directly generated from highly hydrated silica surface. The presence of radicals on the silica species was confirmed by introduction of NO as a radical scavenger. The addition of 8.7 x 10.5 mol of NO to the in-situ hydrated silica species suppressed the butane conversion from 30.7 % to 10.4 %. Figs. 5 and 6 show the effect of drying and water vapor at 643 K on the in-situ hydrated
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Table 2 Effect of preparation conditions and catalytic activity of silica species for butane oxidation. Preparation Reaction a) Atmosphere Temperature Conversion /K /% TEOS 643 0.0 butane, TEOS 643 0.0 0 2 , TEOS 643 0.4 b u t a n e , 0 2 , TEOS m 573 0.0 butane, 02, TEOS m 603 9.7 butane, O2, TEOS m 643 57.8
.............
N;6;-
i
iS-g
............
.....................
"i~ 40 O
O
O O o
20
,l,..a
10 0
........
a) Flow reaction at 643 K, n-butane/O2/N2 = 2% /18%/balance, b) TEOS/n-butane/O2/N2= 0.046% /2%/18%/balance, c) TEOS (0.25 cm 3) was fed as a pulse in the stream of 10%H20/N2.
,
I
1
,
I
2
,
3
Amount of feed gas/cm Fig.
4
Initial
activity
of in-situ
hydrated silica species as a function of pulse amount of the feed gas.
silica species. As shown in Fig. 5, both the treatments deactivated the silica species. After the water vapor treatment, the butene conversion immediately recovered in the stream of butane/O2~a mixture at 643 K (Fig. 6). On the contrary to this, the activity slowly recovered after the drying in He. These series of experiments indicate that the highly hydrated surface is required for the butane oxidation, i.e., the active silica species should be generated from silanol species on the silica surface. 50 -~ 40 O
O
..,,~
.,.-i r~
30( ' O o
O o
20
;::I
mlO
m lOO 200 300 Time / min
400
Fig. 5 Butane conversion in pulse reaction over in-situ hydrated silica species as a function of the treatment time in flowing He (O) and 10% H20/N2 (O) at 643 K.
0
100 200 300 400 500 600 Time / min
Fig. 6 Butane conversion in flow reaction over in-situ hydrated silica species after the treatment in drying for 2 h (O) and in water vapor for 12 h (O) as a function of time.
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The activation of ordinary silica is well investigated by Low et al. [ 12]. They reported that ordinary silica can be activated by a procedure consisting of methylation, pyrolysis of the methyl layer, and high temperature degassing. The reactive silica can adsorb oxygen to form radical site, which is active for ring disruption of cyclopropane, benzene and toluene. Although the preparation conditions and the reactivity of the in-situ prepared silica specis are different, it can be expected that the active silica species were generated in the similar way: The elimination of OH or H from surface silanols may produce radical sites for the initiation of the catalytic oxidation and dehydrogenation. 4. CONCLUSION Based on the above results, the catalysis of in-situ prepared silica species can be proposed as shown in Fig. 7. The butane oxidation proceeds over the silica species as follows: (1) The active sites are directly generated from surface silanol species, (2) the reaction is initiated by radical sites derived from silanol species, (3) high butane conversion is achieved by the role of radical species in gas phase.
products
Si(O02H5)4 04Hlo+O2 { ,.,o~ ~, ~k, H20 H ~ ' ' ~ ~ hase~*~ 04H1~
_.
Fig. 7
tiat, on
9termination
wallof reactor
Schematic model of butane oxidation over in-situ prepared silica species.
REFERENCES 1. 2. 3. 4. 5.
S. Kasztelan and J. B. Moffat, J. Chem. Soc., Chem. Commun., (1987) 1663. G.N. Kastanas, G. A. Tsigdios, and J. Schwank, Appl. Catal., 44 (1988) 33. J.N. Armor and P. M. Zambri, J. Catal., 73 (1982) 57. Y. Matsumura, K. Hashimoto, and S. Yoshida, J. Catal., 117 (1989) 135. H. Yoshida, T. Tanaka, M. Yamamoto, T. Funabiki, and S. Yoshida, Chem. Commun., (1996) 2125. 6. A. Satsuma, N. Sugiyama, Y. Kamiya, and T. Hattori, Chem. Lett., (1997)1051. 7. F. Cavani and F. Trifiro in Catalysis vol. 11, Royal Society of Chemistry, Cambridge, (1994), p.246, and references there in. 8. C.A. Euker, Jr. and J. P. Leinroth, Combust. Flame, 15 (1970) 275. 9. N.N. Semenov, Some Problems in Chemical Kinetics and Reactivity, Chap.7, Princeton Univ. Press, Princeton, New Jersey, 1958. 10. I. Glassman, Combustion (Third Edition), Chap.2, Academic Press, San Diego (1996). 11. M. Carlier and L-R. Sochet, Combust. Flame, 25 (1975) 309. 12. M. J. D. Low, E. McNelis, and H. Mark, J. Catal., 100 (1986) 328.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
M e c h a n o c h e m i c a l preparation of V-Ti-O catalysts for o-xylene low temperature oxidation V.A. Zazhigalov l*, J. Haber l#, J. Stoch l#, A.I. Kharlamov 2, A. Marino 3 L. Depero 3 and I.V. Bacherikova I*, 1) Ukrainian-Polish Laboratory of Catalysis: *Institute of Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Gen.Naumova 13, Kyiv, 252680, Ukraine #Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek l, Krakow 30-239, Poland 2) Institute for Materials Science Problems, National Academy of Sciences of Ukraine,, ul.Kzhyzhanovskogo 3, Kyiv, 250680, Ukraine 3) Structural Chemistry Laboratory, Dip. di Chimica e Fisica per l'Ingegneria e per i Materiali, University of Brescia, 9 Via Valotti, 25123 Brescia, Italy
Abstract Catalysts were prepared by mechanical treatment of the powder mixture of V205 and TiO2 in air, water and ethanol as dispersing medium. When prepared in alcohol they showed already at 573 K 89% conversion of o-xytene with 87% selectivity to phthalic anhydride. This may be related to the increased exposure of (010) plane of V205 and its decoration with Ti ions, as revealed by XRD, XPS and Raman spectroscopic analysis. 1. INTRODUCTION The V-Ti-O oxide system is a catalyst of many processes, including industrial o-xylene oxidation and NOx reduction [ 1,2]. They are usually prepared by one of following methods: i) impregnation of TiO2 with different vanadium compounds, ii) vanadium chemical bonding with functional groups on TiO2, and iii) V and Ti co-precipitation from compounds of Me(OR)n type [2,3], iv) from the powder mixture of vanadia and titania oxides by grinding them in an agate mortar [4]. In this paper a possibility of the catalyst synthesis by a mechanochemical treatment of the V205 and TiO2 powder mixture is considered. 2. EXPERIMENTAL Initial reagents were TiO2 powder (Aldrich, p.a.), V 2 0 5 - p (powder, REACHIM, p.a.) and V205 - m melted in air and then crushed. Powders were thoroughly mixed in an agate
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mortar and then ground for different periods in a planetary mill (3000-rpm). Ethanol or water were used as dispersing medium. Mass of mixed oxides was 25 g, mass of balls 450 g. After the treatment was completed, catalysts were filtered and dried at 393 K. Specific surface area was determined by thermal desorption of argon (GASCHROM-1). A phase composition was found with a Philips MPD 1880 diffractometer (CuKcx excitation, graphite monochromator). Relative line intensity and width (FWHM) were determined with Automated Powder Diffraction Program (version 3.5, Philips). Micro Raman spectra were obtained with Dilor Labram spectrograph (HeNe l a s e r - 632.8 nm). The spectrograph was linked to a microscope for selection of proper analysis spot (resolution better than 1 ~tm). Surface composition was determined with XPS using VG ESCA-3 spectrometer. The spectra were calibrated using C ls line at 284.8 eV; the spectra handling were described elsewhere [5]. Catalytic properties of the samples (fraction 0.25 - 0 . 5 0 mm) were studied in a flow system with steel microreactor loaded with 0.5 cm 3 of catalyst. The contact time was modified by controlling flow rate, of the feed mixture in the range 12-65 cm3/min. Reactions of oxylene oxidation (0.9 vol. % in air) and n-pentane oxidation (1.6 vol. % in air) were selected as test reactions. Outlet gases were analyzed in the on-line system of two chromatographs (Chrom-5) equipped with: a) 2.5 m column filled with 4.3% F-50 on Chromosorb G (temp. program 373-598 K) for analysis (DTP) of products of partial oxidation of o-xylene, b) 2 m column filled with silicagel KSK-2.5 (temp. program 323-393 K) for analysis of CO2 and hydrocarbons, and c) 2 m column filled with molecular sieves CaA (temp. 273 K) for analysis of 02 and CO. Chromatograms were recorded and then analyzed on a PC 486 DX 100 computer.
3. RESULTS AND DISCUSSION The XRD spectra of V205 - p and V 2 0 5 - m revealed the most intense reflection at 2 0 = 20.30 corresponding to the (010) plane. However, while in the first sample the reflection intensity ratio (010)/(110) was near 3/1, in the second one it was about 10 times higher. This indicates that V205 - m was in the form of platelet-like crystals developed along the (010) plane. The XRD spectrum of TiO2 was typical for anatase with strongest reflection at 2| = 25.30 corresponding to the (101) plane. In the case of V2Os/TiO2 mixtures and after their mechanochemical treatment the XRD spectra were a superposition of diffractograms of simple oxides. Simultaneously this treatment changed the intensities of all reflections, initially typical for simple oxide, leaving their FWHM unchanged. For V2Os-m/TiO2 an increase in time of milling did not change the reflection intensity ratio (010)/(101) though it was remarkable larger comparing to V2Os-p/TiO2. This is connected with differences in intensity of the (010) reflections in initial forms of vanadium oxide. In the mixture of V2Os-p/TiO2, after 10 min of the treatment in alcohol, the intensity ratio of (010)/(101) reflections (informing about Ti/V ratio) decreased from about 12 to 6. However, when time of the treatment increased from 10 to 30 min, the ratio rose from 5.9 to 8.3. In water this ratio already after 10 min of the treatment decreased to 10 and remained almost constant after further exposition. Somewhat lower intensity of the (010) reflection after the treatment in water can be caused by partia dissolution of V205 observed already earlier during its mechanochemical treatment [6].
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5000
400
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I
500
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9 ----"-'----~
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Wavenumber (cm-l)
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F-t
200
o-
r
500
B10
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Wavenumber (cm-l)
500
1000
B30
1ti00
Wavenumber (cm-l) t
Figure 1. Micro-Raman spectra of V205-TiO2 mixture after 10 and 30 min of the mechanochemical treatment in water for different spots A and B. The Raman spectrum of TiO2 presented typical set of bands at 145 (very strong- v.s.), 400, 520 and 645 (v.s.) cm "1. Bands at 155 (v.s.), 205, 295 (v.s.), 307, 405,487, 540, 705 and 1000 (v.s.) cm -l were characteristics of both V205 samples. The spectra of the V2Os/TiO2 mixture before and after mechanochemical treatment were the result of the superposition of those of initial oxides. However, micro-Raman spectra taken for two different points (A and B in Fig.l) of the near-surface region showed that, in the case of the VEOs-p/TiO2 after 10 (Fig.l, A10, B10) and 20 min of the treatment in water, differences in intensity of absorption bands 145/155 and 645/1000 cm 1 in points A and B were reflecting heterogeneity of the sample. After 30 min (Fig. 1, A30, B30) almost equal ratios of the intensity band in these points were observed, indicating homogeneity of the V2Os-p/TiO2 system. On the treatment in
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alcohol, already after 20 min the system was nearly homogeneous, while after 30 min it was fully homogeneous. The treatment of the V2Os-rn/TiO2 system in ethanol after 20 min also gave a uniform component distribution. As follows from XPS data (Table 1), binding energies of electrons in the V2Os/TiO2 system was not much influenced by the mechanochemical treatment being close to that already published [7]. This shows that the oxidation state of elements did not change under the treatment. However, the atomic ratio of surface elements was substantially changed. Treatment in ethanol is increasing the Ti/V ratio, more significant in V2Os-m, while in water this ratio decreased (Table 1). Table 1 Surface properties of the V-Ti-O composition after the mechanochemical treatment Treatment XPS binding energy, eV Composition Medium Time. Ti 2p V 2p O 1s V/Ti V/Ti (XRD) V205 (XRD) min (XPS) (010)/(101) (010)/(11 O) 2
3
4
5
6
7
8
Ethanol Ethanol Ethanol
10 20 30
459.2 459.1 459.1 459.3
517.7 517.6 517.6 517.6
530.8 531.3 531.0 530.9
0.77 0.50 0.56 0.48
0.08 0.17 0.16 0.12
3.1 5.3 4.7 4.5
Water Water Water #
10 20 30
459.2 459.4 459.3
517.6 517.9 517.8
531.0 531.4 531.1
1.10 1.10 0.91
517.4 517.6
531.0 530.9
0.38 0.43
0.10 0.08 0.09 4.76 0.30 0.29
3.0 3.0 3.2 45.5 9.0 25.0
l
#
Ethanol ^ Ethanol ^
10 459.1 20 459.2 ^V205-m. # untreated mixture
The data obtained, including those concerning mechanochemical modification of V205 [6,8] allowed us to assume the following picture of changes accompanying the mechanochemical treatment of the V2Os/TiO2 mixture. As already established the mechanochemical treatment of V205 in ethanol produces anisotropic crystal deformation [6,8] resulting in an increase of the (010) plane exposure. Such change in the contribution of (010) crystal plane was observed in the sample V205-p after 10 min of the treatment (Table 1, Column 8) since the (010)/(110) ratio increased from 3.1 to 5.3. But further exposition unexpectedly brought small reversion of the process, i.e. slight decrease in the vanadium XRD (010)/(110) ratio (Table 1, column 8). Consequently, the XRD (010)/(101) ratio initially increased by the factor 2, mostly as the result of strong development of the (010) vanadium plane. On further treatment this plane intensity ratio was consecutively decreasing from 0.17 to 0.12, partly reflecting increase of the relative vanadium (010) plane content and partly by loss of the vanadium visibility by XRD in highly dispersed oxide. Changes in XRD data (table 1, col. 7 and 8) suggest that comparing TiO2 and V205 oxides, the last one is more susceptible to destruction and is easily dispersed. In conclusion, our results show that prolonged mechanochemical treatment of the V2Os-p/TiO2 composition in ethanol produces a mixture of larger TiO2 crystals and much
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smaller V205 particles. Since the XPS V/Ti ratio remained small, there is no reason to suppose coating of TiO2 with V205, but rather agglomeration of dispersed vanadia should be assumed. Different picture is observed in V2Os-m. Very high XRD (010)/(110) ratio in the starting powder (Table 1, col.8) points to strong preference for the (010) plane. During the treatment these large, initially fiat crystals are crushed vertically to this plane, which reduces its relative exposure, but after some time, gliding of layers along the (010) plane begins to be dominant. Considering the treatment of the V2Os-p/TiO2 mixture in alcohol, the most striking feature is the interruption of evolution of vanadium (010) plane exposure with time of treatment after its initial growth, as opposed to pure V205. With exactly the same treatment, the only reason for the effect was the presence of titanium atoms. On the other hand each inorganic compound dissolves to some extent in every solvent. These trace amounts of both substances are available during the treatment. A hypothesis may be advanced that single titanium atoms or oxygen-titanium polyhedra become incorporated to the V2Os-p lattice, decorating edges or staking-faults and in consequence hindering the gliding of vanadia (010) planes. The surface enrichment in titanium could be detected by XPS which shows the decrease of the surface concentration V/Ti ratio could not be detected by any XRD data because of XRD insensitivity in such systems. When V2Os-m was used, the anisotropic deformation was less extended because of its higher hardness. This is evidenced by the absence of changes in the XPS and XRD composition during the treatment (Table 1, col. 6 and 7). In the presence of water a chaotic destruction of crystals [8] and partial dissolution [6] proceed in V205. As the result, the lowest values of the (010)/(101) reflection intensity ratio and the V/Ti ratio at the surface have been observed.
Table 2. Catalytic properties of the V-Ti-O composition after the mechanochemical treatment Treatment n-Pentane oxidation. T = 573 K o-Xylene oxidation. T = 533 K Time Conversion% Conversion SMA.* SPH. * SMA. SPH. % Medium mol. % mol. % mol. % mol. % # 18 12 3 38 12 48 Ethanol 10 21 34 3 73 61 Ethanol 20 72 8 70 Ethanol 30 23 37 2 83 9 79 Water 10 35 36 12 59 21 48 Water 20 67 16 52 Water 30 54 45 10 69 14 56 Ethanol ^ 10 86 6 85 Ethanol ^ 20 89 3 87 * SMAand SPH- selectivity in maleic and phthalic anhydride formation, respectively, ^V2Osm. # untreated mixture Mechanochemical modification of V-Ti-O samples leads to the change of their catalytic properties (Table 2). The treatment in ethanol did not influence much n-pentane conversion, while the conversion of o-xylene significantly increased. After the treatment in water
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conversion of both hydrocarbons also increased, although the activation energy remained constant and amounted to 126 kJ (30 kcal)/mole for o-xylene and 142 kJ (34 kcal)/mole for npentane. In n-pentane oxidation selectivity towards maleic anhydride increased independently of the environment of treatment, while that towards phthalic anhydride increased only when water was used. On the contrary, in o-xylene oxidation an increase in both activity and selectivity to phthalic anhydride was very pronounced, independently of the nature of dispersing medium. Moreover, high yield of phthalic anhydride was observed already at relatively low temperature of 533 K. The treatment did not change significantly the selectivity to maleic anhydride. This study showed that catalytic properties could be significantly influenced by modification of the catalyst accompanying the mechanochemical treatment. It seems that the mechanism is not directly related to the chemical action, but the chemical action controls the physical process, which produces the more effective product. Concluding, the mechanochemical modification of V-Ti-O compositions enhances their catalytic ability to selective oxidation of n-pentane and o-xylene.
Acknowledgment This work was supported by the Polish State Committee for Scientific Research, Grant 3T09A016 16.
4. REFERENCES V.Nikolov, D.Klissurski, A.Anastasov. Catal. Rev.-Sci.Eng., 33 (1991) 319. C.R.Dias, M.F.Portela. Catal. Rev.-Sci.Eng., 39 (1997) 169. B.Grzybowska-Swierkosz. Appl. Catal., A. 157 (1997) 263. G.Centi, E.Giamello, D.Pinelli, F.Trifiro. J. Catal., 130 (1991) 220. V.A.Zazhigalov, J.Haber, J.Stoch, A.I.Pyatnitskaya, G.A.Komashko, V.M.Belousov, AppL CataL, A, 96 (1993) 135. 6. V.A.Zazhigalov, J.Haber, J.Stoch, A.I.Kharlamov, L.V.Bogutskaya, I.V.Bacherikova, A.Kowal, SolidState Ionics., 101/103 (1997) 1257. 7. B.M.Reddy, B.Chowdhury, I.Ganesh, E.P.Reddy, T.C.Rojas, A.Fernandez. J.Phys. Chem., B., 102 (1998) 10176. 8. V.A.Zazhigalov, A.I.Kharlamov, I.V.Bacherikova, G.A.Komashko, S.V.Khalameyda, L.V.Bogutzka, O.G.Byl, J.Stoch, A.Kowal, Teoret. Experim. Khim., 34 (1998) 180.
1. 2. 3. 4. 5.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
A Common Concept Accounting for Selectivity in Mild and Total Oxidation Reactions: the Optical Basicity of Catalysts. P. Moriceau, B. Taouk, E. Bordes and P. Courtine D6partement de G6nie Chimique, ESA 8067, Universit6 de Technologie, B.P. 20529, 60205 Compi6gne, France. Correlations are settled between the "optical basicity", A, which is defined as the electron donating (accepting) power of the lattice oxygen (cation) of the solid, and its equivalent for the gaseous phase: the ionization energy of the molecules. The optical basicity of any catalytic oxide is easily calculated and depends on the cation(s) valence and coordination. The difference between the ionization energies of reactant and of product, AI, accounts for the selectivity which is driven by the catalyst. Linear relationships AI = aA + b are drawn by gathering reaction-to-product/selective catalyst couples. The a and b constants depend on the nature of the reactant (paraffin, olefin, alcohol) and on the type of selective reaction (mild oxidation/oxidative dehydrogenation, ammoxidation, total oxidation). Some theoretical considerations are proposed to justify the linearity of the correlations, which permit to select a priori a selective catalyst for a given reaction.
1. INTRODUCTION Several properties of oxide catalysts able to selectively oxidize organic compounds have been used in the past to set up correlations with catalytic properties. In our research group, we are mostly working on the role of structural properties and solid state reactivity in selective oxidation, because the way the oxygens of the solid participate to the reaction is strongly related to them [1, 2]. At least two steps depend on the characteristics (strength, energy, spatial display) of the metal-oxygen bonds: the activation of the C-H bond of the reactant by surface nucleophilic 02- specie and the insertion of oxygen into the intermediate [3-6]. The selectivity of the process depends mostly on this second step, the catalyst oxygen being found in the oxygenated product (case of mild oxidation, MOx), and/or in water (case of oxidative dehydrogenation, ODh). At least two types of nucleophilic oxygens (linked to the oxidizing cations) are therefore needed in mild oxidation reactions [3]. We have searched for a parameter which could account for the specific action of each solid catalyst in a given reaction. The aim of this paper is to show that all reaction/catalyst couples can be related to a common parameter ruling selectivity, the 'optical basicity' of the solid. The optical basicity, A, of an oxide represents the electron donating power of the lattice oxygen (or the electron acceptor power of the associated cation). It is widely used
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in inorganic chemistry and allows to foresee the progress of reactions [7], including acidbase and redox systems. The optical basicity can be calculated for any formula and varies with the valence, coordination and spin state of the cation [7-9]. We have shown recently [10] that the optical basicity of an oxide which is selective in a given reaction is related to the difference between the ionization energies of the organic molecules of the reactant and of the product. This difference, AI, is assumed to represent selectivity, its absolute value accounting for the electron exchange during the considered reaction which is ensured by the selective catalyst. The former study in which linear relationships were drawn for mild oxidation of saturated and unsaturated hydrocarbons [10] is completed in the present paper by other types of reactions involving oxygen.
2. RESULTS AND DISCUSSION 2.1. Optical basicity Experimental values of the optical basicity of ionic oxides were first obtained by Duffy [7] from the energy of ultraviolet absorption bands. The adaptation of this concept to more covalent oxides was done by Lebouteiller and Courtine who provided data of optical basicity A of transition metal cations and oxides [9, 10]. The calculation of A for a mixte oxide Axa+Byb+On2- is simply made by using the equation A = 1/2n (aXAA + byAB), where a and b, AA and AB are the valence and optical basicity of A and B, respectively. The optical basicity ranges from 0.30 (CO2, acidic) to 1.7 (Cs20, basic). Going from acidic to basic catalysts, the trend is as follows (A in parentheses): H4PMollVO40 (0.48) < (VO)2P207 (0.49) < WO3 (0.51) < MoO3 (0.52) = Fe2P207 < V205 (0.63) < ~-CoMoO4 (0.64) < V6013 (0.68) < Mg2V207 (0.72) < Fe203 (0.77) < Bi2MoO6 (0.86) < NiO (0.91) < Cu20 (0.98) < Ag20 (1.25), and for some common supports: SiO2 (0.48) < A1203 (0.60) < ZrO2 (0.71) < TiO2 (0.75) < MgO (0.78) < SnO2 (0.87). An acidity/basicity scale of oxidic compounds is therefore obtained, in which any combination of cations can find its place. For example, the optical basicity of PMol2Valr0.02Cu0.1Co0.aK0.3On which is claimed in a patent [11] to be selective in the oxidation of isobutane to methacrylic acid is A = 0.55. This example gives the opportunity to point out a drawback of optical basicity which is a theoretical parameter [7, 9]. To calculate the oxygen stoichiometry (n = 44) and A, the valence and coordination of cations have been supposed to be the highest. This is only an assumption because their actual values in operating conditions are not known. In cases where A varies strongly with valence and/or coordination, like for 6-coordinated molybdenum (Mo6+: A = 0.52; MoS+: A = 1.17), the A value may be far from reality [10].
2.2. Correlations between A (catalyst) and AI (gaseous or fiquid molecules). Each selective catalyst is responsible for the fact that a given reactant molecule is transformed into a definite product because of its electron donating-accepting capacity. For example, (VO)2P207 is used to oxidize n-butane to maleic anhydride [12] (AI = I IRIpI - 0.27 eV) but not to butadiene (AI = 1.46 eV). It is not selective in the oxidation of,
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e.g., propane to acrolein (AI = 0.85 eV). When supported on TiO2, V205 is selective in the oxidation of o-xylene to phthalic anhydride (AI = 1.44) [13], not in the oxidation of toluene to benzaldehyde (AI = 0.67 eV) [14]. Various reactions of MOx and ODh have been considered and the corresponding reactant-product/catalyst couples characterized by their {AI,A} values [10]. By plotting AI against A, two straight lines were obtained which gathered saturated (mild oxidation of 'paraffin', line MOP) on the one side and unsaturated (mild 'olefin' oxidation, line MOO) hydrocarbons on the other (Fig.). The less demanding the reaction, the more numerous the catalysts, so that for a same AI several values of A may appear. For example, in the ODh of but-l-ene to butadiene (AI = 0.51 eV), A of Bi2MoO6, FeSbO4, USb3Ol0 [15] and Sb2Oa/SnO2 [16] have been considered. This is true also for ODh of C2-C4 alkanes. When only the alkyl group is modified (e.g., toluene-benzaldehyde, o-xylene-phthalic anhydride, etc.), the {AI,A} of unsaturated hydrocarbons fit the line MOP obtained for paraffin activation. The thermodynamic method we use, which consists in looking at the initial and final states and not in considering the mechanism, is the explanation. For the latter reason, ethylbenzene-styrene or methane-ethylene fits MOP line, although a carbon layer or radicals are supposed to be involved in the reaction, respectively. 5
4
3
2
MOO
~
llr/ll
MOA
AP "%4IN, hiP r
0.4
I
I
I
I
0.6
0.8
1
1.2
A
Figure. Correlations between [A(I)[ and A for combustion of paraffinic bonds (full squares, line CP) and olefins (open squares, line CO), mild oxidation of alcohols (open lozenges, line MOA), of paraffins (full triangles, line MOP) and of olefins (open triangles, line MOO), and ammoxidation of paraffins (full circles, line AP).
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Following the same methodology, we have examined other types of reaction, like ammoxidation of hydrocarbons, mild oxidation of alcohols and total oxidation. In the latter case, the correction of AI by the np/n R ratio has been systematically applied (n R and
np are carbon number in reactant and product, respectively). Linear relationships AI = aA + b have again been obtained (Fig.). The range of AI and A, the a and b coefficients and reliability factors R 2 are gathered in the following Table. Table. Characteristics of linear relationships between AI and A obtained for various reactions Reactions AI range A range AI = aA + b Reliability (eV) a b factor R 2 'Alkane' Oxidation 0.18-2.00 0.48-0.91 4.09 - 1.73 0.94 'Olefin' Oxidation 0.06-1.22 0.48-1.25 - 1.51 + 1.90 0.93 Alkane Ammoxidation 0.11-0.34 0.49-0.93 0.58 - 0.15 0.81 Alcohol Oxidation 0.02-0.60 0.53-1.03 1.11 - 0.56 0.95 Alkane Combustion 1.26-4.90 0.87-1.07 19.39 -15.72 0.96 Olefin Combustion 3.20-4.54 0.85-0.93 18.94 - 12.95 0.89 During the ammoxidation of hydrocarbons in which NH3 is added to 02, the surface lattice oxygens are known to be responsible for selectivity [17]. The ammoxidation of propane to acrylonitrile and toluene to benzonitrile with various multicomponent catalysts based on molybdenum or vanadium (Fig., line/kiP) has been mainly considered here. The mild oxidation of Cl to C6 alcohols to aldehydes or ketones is presented (Fig., line MOA). For example, Mn304 is selective in MOx of propanol to acetone (AI = 0.91 eV) as well as of allylic alcohol to acrolein [ 18], which has the same value of AI. Finally, selective combustion reactions have been considered. Because of the large contribution of CO2 in the calculation of AI, the slopes of the combustion of paraffins C1 to Ca (line CP) and of olefins C2 to C4 (line CO) are very steep. Real selective catalysts for combustion are not so numerous and the range of A is small (Table). Perovskites like LaCoO3 (A = 0.88) are known as good catalysts for the total oxidation of isobutene. Another very good catalyst is MnO2 which has the same value, A = 0.88.
2.3. Attempts to justify linear correlations between AI and A. In a study of the ionization of aromatics condensed molecules owing to the action of various cation-exchanged faujasites, Richardson [19] used the charge transfer theory to express the number of cation radicals Ni + obtained by ionization of N molecules i as a function of their ionization energy Ii and of the electron affinity Ac of the catalyst cation: l n ~ , 9-A~+W-IikT
(k is Boltzman constant and T is temperature)
(1)
(W is the dissociation energy of the excited state of the charge transfer complex). In selective oxidation the redox mechanism is widely used. During the first step, the
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reactant R adsorbed on the oxidized form KO of the catalyst is transformed into the product P, which desorbs from the reduced form K of the catalyst. Eq. (1) becomes: lnN~ AKo+WR--IR and NR kT -
I-N~-AK+Wp-IP .U~aakT
(2)
These equations may be rewritten as a function of I to make AI appearing: AI =IR-IP =--AWR-P -AAKo-K + k
NR -In NP ln~-~-R
=--(~KO-K +AWR-0 +kl~ lnN--~++R +InNP / L NP NR J
(3)
where AAK-KO = AKo-AK and AWR_p = WR-Wp respectively. If the ratio Np/NR is considered to be constant at the steady state, eq. (3) is reduced to: AI =IR--IP =C lnN--~++d NP
(4)
where d =-AAKo-K-AWR-P +kT lnNP is constant for the considered reaction/catalyst NR couple and c = kT is little varying (T = 623-923 K). This equation already shows that a relationship exists between AI and a characteristic of the catalyst, which is the electron affinity of the free ion in the present case. On the other hand, according to Duffy [7], there is a linear relationship between A and the ratio of concentrations of the oxidized and reduced cations in the case of (ideal) MZ§ redox systems, which can be applied to K/KO: In-[Mz+[M(Z")--kpA+q
becomes ln[~K~]=~A+I3
(5)
This type of relationship is valid provided that the reduced and oxidized forms belong to the same phase, which can be assumed to be the case on the surface of a catalyst. It is not yet possible to find an equation by combining eqs. (4) and (5) but we think that, by this approach, the empirical relationships AI = aA + b should be validated in the near future. 3. CONCLUSION
The above results have been obtained by using a thermodynamical method in which only the initial and final states of the reaction are considered, in something similar to "standard" conditions since temperature is not taken into account. As a consequence, nothing is said about the mechanism (radicals, carbanions, etc.), the activity or the number and display of active sites on the surface. However, the method is powerful and allows to correlate the Lewis-type basicity/acidity of the reaction (by means of AI) with acidity/basicity (by means of A) of the selective catalyst. Linear relationships AI = aA + b can be drawn by plotting AI against A, each one characterizing a type of reactant (paraffin-type, unsaturated hydrocarbons, alcohols) involved in a type of reaction (mild
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oxidation and oxidative dehydrogenation, ammoxidation, total oxidation). The slope a may be positive or negative, and steep or not, depending on the relative Lewis acidity of the reactant and of the product, and on the extent of oxidation: total combustion is steeper than mild oxidation. The physical meaning of the {AI,A} correlations by use of the charge transfer theory is in progress. As a consequence, it becomes possible to use these correlation lines to search for new catalytic formula that would be selective in a chosen reaction. The main drawback of the method is that assumptions have to be made on the coordination, valence and spin state of the cation(s) in real operating conditions.
Acknowledgement C. Pham and Elf-Atochem are thanked for helpful discussions and funding.
REFERENCES P. Courtine, in: Solid State Chemistry in Catalysis, Eds. R.K. Grasselli and J.F. Brazdil, ACS Symp. Series, 279 (1985) 37. E. Bordes, in: Elementary Reaction Steps in Heterogeneous Catalysis, ed. R.W. Joyner and R.A. van Santen, Kluwer Academic Publ., 1993, 137. V. D. Sokolovskii, Catal. Rev.-Sci. Eng., 32 (1990) 1. J. Haber in: Solid State Chemistry in Catalysis, Eds. R.K. Grasselli and J.F. 4. Brazdil, ACS Symp. Series, 279 (1985) 3. M. Che and A.J. Tench, Adv. Catal., 32 (1983) 1. J. Haber and M. Witko, J. Molec. Catal., 9 (1980) 399. J. A. Duffy, Geochim. Cosmochim. Acta, 57 (1993) 3961. J. A. Duffy, J. Non Cryst. Solids, 86 (1886) 149. A. Lebouteiller and P. Courtine, J. Solid State Chem., 137 (1998) 94; A. Lebouteiller, Ph.D. thesis, Compi6gne 1997. 10. P. Moriceau, A. Lebouteiller, E. Bordes and P. Courtine, Phys. Chem. Chem. Phys., 1 (1999) 5735. 11. Mitsubishi Rayon Co. Ltd, T. Kuroda, O. Motomu, JP 04,128,247, 1992. 12. F. Cavani, F. Trifir6, Appl. Catal. A General, 88 (1992) 115. 13. M. Gasior, I. Gasior and B. Grzybowska, Appl. Catal., 10 (1984) 87; B. Grzybowska, Catal. Today, 1 (1987) 341. 14. J.E. Germain, R. Laugier, Bull. Soc. Chim. Fr., (1972) 2541. 15. B.C. Gates, J.R. Katzer and G.C.A. Schuit, in: Chemistry in Catalytic Processes, McGraw-Hill, 1979, 366. 16. F. Sala and F. Trifir6, J. Catal., 41 (1976) 1. 17. R. K. Grasselli and J.D. Burrington, Adv. Catal., 30 (1981) 133. 18. B. Grzybowska, J. Haber and J. Janas, J. Catal., 49 (1977) 150. 19. J. T. Richardson, J. Catal., 9 (1967) 172. .
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Copper species in CuCIz/3c-AI203 catalyst for ethylene oxychlorination M. Garilli ~, D. Carmello a, B. Cremaschi a, G. Leofanti at, M. Padovan a*, A. Zecchina b, G. Spoto b, S. Bordiga b, and C. Lamberti b ~EVC Italia, Inovyl Technology Centre, Via della Chimica 5, 1-30175 Porto Marghera, Italy bDipartimento di Chimica IFM, Via P. Giuria 7, 1-10125 Torino, Italy CuCI2 on 7-alumina, the base catalyst for oxychlorination reactions, has been investigated by several techniques as a function of Cu loading, sample aging and heating. The results point out the formation of a surface Cu aluminate species and a highly dispersed CuCI2. The latter slowly hydrolyzes under aging, but CuCI2 is restored under heating and is the active species in oxychlorination as demonstrated by activity tests. 1. INTRODUCTION Nowadays almost all the total world production of vinyl chloride is based on cracking of 1,2-dichloroethane, in its turn produced by catalytic oxychlorination of ethylene with hydrogen chloride and oxygen: C2I-I4 + 2HC1 + 89O2 ---) C2H4C12 + H20, [ 1]. The reaction is performed at 490-530 K and 4-6 atm, using both air and oxygen in fluid or fixed bed reactors. Commercial catalysts are produced by impregnation of alumina with CuCI2 (4-8 wt% Cu) and, preferably, also with other chlorides (mainly alkaline or alkaline earth ones) in a variable concentration. The other chlorides are added in order to improve the catalytic performances making the catalyst more suitable for the use in industrial reactors. In spite of several papers published since 1973 the problem to identify the species present on the catalyst is not fully resolved, even in the case of the basic catalyst [2-7], i.e. alumina supported CuCI2 without additives. The aim of our contribution is threefold: i) the identification of the species present at different Cu content, ii) the determination of their concentration and iii) the investigation about the possibility of their transformation during catalyst life.
2. EXPERIMENTAL This study has been performed on a set of seven CuClz/alumina samples (having a Cu content of 0.035, 0.25, 0.5, 1.4, 2.3, 4.6 and 9.0 wt% respectively) prepared by impregnation of 7-allumina (Condea Puralox SCCa 30/170, surface area: 168 mZg-1, pore volume: {).50 cm3g -~) with an aqueous solution of CuClz'H20 following the incipient wetness method. Samples were examined in different conditions: i) immediately after impregnation of alumina, ii) after 1 day and 6 months aging at RT, iii) under heating up to typical oxychlorination reaction temperature (500-550 K. To fulfil this task, different complementary techniques such as XRD (Siemens D500), UV-Vis-NIR DRS (Perkin-Elmer Lambda 9), EPR (Varian E 109), EXAFS (EXAFS1, beamline at LURE DCI with a Si(331) monochromator), solubility test in ethyl alcohol and activity test in ethylene oxychlorination (pulse method) have been adopted. Present address: Via Firenze 43, 20010 Canegrate (Milano), Italy * Present address: Villa Mirabello 1, 20000, Milano, Italy
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3. RESULT AND DISCUSSION
3.1. Fresh catalyst: low Cu loaded samples As far as the fresh catalyst is concerned, the main results can be summarized as follows. At low Cu content the formation of an insoluble surface aluminate species takes place, where isolated Cu n ions occupy octahedral vacancies of alumina surface. The chlorine released by CuC12 during its interaction with alumina reacts with the support giving rise to >AI-C1 species as documented by the constant CI/Cu ratio = 2 as found by elemental analysis. The formation of surface aluminate has been evidenced by UV-Vis, EXAFS, EPR and solubility test, where the experimental features are very similar to those observed on chlorine free samples prepared from nitrates, for which the formation of the surface aluminate is well documented (see [8-11 ] and refs. therein). In particular, UV-Vis spectra are characterized by a low intensity d-d 2Eg --> 2T2gtransition at about 13000 cm l, typical of an octahedral Cu n species containing chemically equivalent ligand, and by a single CT band in the 40000-43000 cm -~ range (Fig. 1). These features are not affected by drastic thermal treatments (up to 923 K), able to remove all the chlorine from the sample. EXAFS analysis (performed on the 1.4, wt% sample) indicates that Cu n ions are surrounded by five oxygen ligands at about 1.92 A and discards the presence of C1 in its first coordination shell (see section 3.2). No evidence of a significant Cu-Cu second shell is observed, indicating that we are dealing with a dispersed phase. EPR spectra exhibits an axial (gxx = gyy ~- g• and gzz = g//) symmetry (Fig. 2). The spectrum of the 0.035 wt% loaded sample is typical of isolated Cu H species, where the hyperfine splitting into quartets, due to the interaction between the unpaired electron and the copper nucleus (3/2 nuclear spin), is clearly visible. By moving to the 1.4 wt% sample, a progressive loss of the resolution occurs, particularly relevant in the perpendicular component. This is the clear manifestation of spin-spin relaxation phenomena. The data extracted from the solubility test (vide infra Fig. 4) indicate that the maximum achievable concentration of surface aluminate is 1.6 wt% Cu, corresponding to 0.95 wt% Cu/100 m 2 and to 1.06 wt% CI/100 m 2. These values represent the maximum capacity of the alumina surface to accommodate Cu and C1 ions deriving from copper chloride complexes or, in other words, its saturation point. 3.2. Fresh catalyst: high Cu loaded samples At higher Cu concentrations excess copper chloride precipitates directly from solution during the drying in an amorphous phase, overlapping progressively the surface aluminate as pointed out by solubility test, UV-Vis and EPR. The first argument in favor of this hypothesis is the presence, at high loading, of a consistent fraction of soluble Cu. In fact, copper chloride is the only Cu compounds, among chlorides, oxychlorides, hydroxychlorides and oxides, having an appreciable solubility. Moreover, the soluble fraction increases linearly with Cu concentration (vide infra Fig. 4, A data): this is the expected behavior if, after completion of surface aluminate, the excess CuCI2 precipitates from solution inside the pores during the drying process, without interaction with the support surface. As far as UV-Vis is concerned, samples with Cu concentration higher than 2.3 wt% show the development of a second CT band in the 28000-31000 cm -1 range, progressively overlapping the band in the 40000-43000 cm -1 range present in the low copper concentration samples (Fig. 1). The consistent difference (about 9000-11000 cm -~) between the CT of the two species (surface aluminate and supported CuC12.H20) clearly indicates that a partial substitution of oxygen ligands occurs by passing from low to high concentration species. This model agrees with the Jorgensen empirical equation [12] (E(CT) = 30000 [Zopt(ligand ) - Z opt(metal)I, in cm -1) since the difference in optical electronegativity between chlorine (Zopt = 3.0) and alternative ligands (O, Zopt3.2-3.3"
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OH-, ~opt 3.5; HzO, ~opt3.5) gives a difference between the corresponding CT band maxima of 6000-15000 cm -1. A further proof of the presence of CuClz'H20 is given by the abrupt change in intensity of the d-d transition band observed at copper loading _=_ 2.3 wt% and by the comparison with the spectrum of unsupported CuCIz'H20 (Fig. 1): note that the high intensity of the d-d band of CuC12"HzO is due to the presence of two chemically different ligands (C1 and O) in the coordination sphere of Cu n. In fact, the spectra of 9.0 wt% loaded sample and of hydrated CuCI/ are very similar in the whole range, although the former is less resolved, probably because of the disorder induced by the support.
~~'l
I
'
.
9"%~
10
.. j
.
ee
: :
Z'~176 I
o t
~
......
'
Cu9.0
~
,
9
...
Cu4.6
...
9
Cu2.3 ~
Cu1.4 CuO.5
Cu0.25 Cu0.035
0
-
20000
Wavenumber
40000
9
( c m -~)
Fig.1. Full lines: UV-Vis DRS spectra of 1 hour aged samples. From top to bottom: 9.0, 4.6, 2.3, 1.4, 0.5 and 0.25 Cu wt% loaded samples. Dotted line correspond to the spectrum of CuC12.H20
!
2500
9
3000
,
!
3500
B (Gauss)
9
40(30
,
Fig. 2. 77 K EPR spectra of all samples. The amplitude of the EPR signal of all samples has been multiplied by different factors for graphical reasons. Before cooling, samples were evacuated at RT up to 10-3 Torr.
The presence of a second C u II species is also supported by EPR data, as shown in Fig. 2 where, by moving to high loaded samples, any vestige of the hyperfine structure is progressively lost, being totally absent at 9.0 wt%. This sample shows a broad EPR spectrum, similar to that obtained on copper chloride treated under similar conditions (not reported), and typical of an isotropic g tensor (gxx = gyy gzz- giso) [3-5]. It is worth recalling that XRD gives no useful information for the identification of the structure of such species since the XRD pattems of all samples are identical to that of bare alumina. This implies that the species present at high concentration must be in an amorphous state, or in form of nanoclusters having a size falling in the undetectable region of the diffraction technique (less than about 20-30/k). =
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3.3. Aged catalyst
The behavior of the sample upon aging depends on Cu concentration. At copper content lower than 0.95 wt% Cu / 100 m2 support, the Cu surface aluminate, is stable: no changes have been observed in UV-Vis, EPR spectra and in results of solubility tests. Over the alumina adsorption capacity, Cu chloride slowly hydrolyses under aging, forming various insoluble copper hydroxochlorides, depending mainly on drying conditions. In any case the final product of hydrolysis is crystalline paratacamite, as evidenced by UV-Vis, XRD, EPR, EXAFS and solubility test. The most relevant modifications in UV-Vis spectra induced by aging can be summarized as follows: i) the intensity of the d - d transition band undergoes a strong decrease; ii) the component at 40000-43000 cm -~ in the CT region, clearly visible in the as prepared sample, disappears upon aging. The resulting spectrum is very similar to that of paratacamite (spectra not reported for brevity). This evolution is not associated with loss of chlorine (as pointed out by the elemental analysis). Also XRD shows the progressive development of paratacamite diffraction lines. An evolution with time has also been observed by EPR: a progressive gain in resolution is observed upon aging. This reflects the evolution from amorphous, highly dispersed, CuClz'2H20 to crystalline paratacamite, i.e. the evolution from a strong heterogeneous Cu lI environment to a much more homogeneous one (spectra not reported). The upper curve reported in Fig. 3 is the radial distribution obtained from the EXAFS data of CuClz'2HzO model compound. This spectrum is dominated by two peaks (at 1.49 and 1.94 A without phase correction) due to two oxygens at 1.95 /~ and two chlorines at 2.29 /~ and will be used to evaluate, in a qualitatively way, the average fraction of oxygen and chlorine atoms in the first coordination shells of copper in the different samples (middle spectra in Fig. 3). For the 1.4 wt% Cu loaded sample (dotted line), quantitative EXAFS data analysis gives rise to N=4.8(_+0.5) and r(CuO)=1.92(_+0.02)/k, and is interpreted on the basis of a dispersed surface aluminate phase. On the contrary, 9.0 wt% sample (dashed line) exhibits a o " much broader first shell peak, clearly shifted at 9.0 wt% A k o higher R values (maximum at 1.63 A and shoulder at 1.85 A) indicating a coexistence of both Cu-O and Cu-C1 contributions from different scattering atoms located at different distances in three different phases (surface aluminate, CuC12"H20 and paratacamite). For this reason, quantitative EXAFS ' ) RiA) results can not be safely extracted. The 2.4 wt% sample (full line) is in an intermediate situation, Fig.3 k3-weighted, phase uncorrected being more close to the 1.4 wt% one. The above FT of the EXAFS functions for 6-m. picture is further supported by the results obtained aged samples and CuC122H20 from solubility test, which allows to measure quantitatively the copper chloride content of each sample. Fig. 4 gives an overall view of the behavior of the different samples under aging. In the graph the copper solubility is reported vs. the total Cu concentration. Full line represents the reference, i.e. the value corresponding to a hypothetical total solubility of the copper as CuCI2. Dashed, dotted and dot-dashed lines represent the solubility data of 1 hour, 1 day and 6 months aged samples respectively. At each abscissa value, the difference between
CuCI2"2~20
;
I
O
9
heated / / ........1.4wtVo ~_ / \ ___2.3wt~176
--aT
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化
the measured line and the reference line represents the value corresponding to the insoluble fraction. In freshly prepared samples the insoluble Cu (0.95 wt% Cu/100 m1) is due to the surface aluminate species, hosted in octahedral sites of y-alumina surface. At each abscissa value, the difference between the solubility measured at two aging times (1 h-1 day; 1 day-6 months) gives the amount of paratacamite formed during the corresponding period. After 1 day aging, copper chloride is almost absent in the 2.3 wt% sample, but it is still present on
8
,~-_ 8-
A 1 hour aging /
:
'6d~Yonging
...'~
/
~6
_._= 4 oor)
O. 0
0
2
4
6
8
Cu concentration (%)
Fig.4. Dependence of copper solubility as a function of both Cu content and aging. To verify the linearity of the reported trend for 6-months aged samples, a 6.8 wt% Cu loaded sample has been added.
9
m
0 2-
m
9
9
m
m
_-,ooO _oooOO'" ooooO -
[]
] mo~ ~176176 OUU'v" 0
m mm
m m
.
-
2
mmm mmm
9
-o 6-~ ~ > r-o4-
v
0
o
10
Pulses
20
Fig.5. Ethylene conversion to 1,2dichloroethane versus the number of pulses on 9.0, 4.6 and 1.4 wt% samples: m, 9 and A symbols respectively. Symbols and O refers to 9.0 and 4.6 wt% loaded samples normalized by "active" Cu concentration, see text.
samples with higher Cu content. In the tbllowing months, the formation of paratacamite continues: however, even after 6 months, the copper chloride is not completely transformed into the hydroxochloride. This suggests that, once the basic sites of alumina are consumed by HC1 (formed in the chloride hydrolysis), the reaction stops or, at least, becomes much slower. 3.4. Heated catalyst
Samples with low Cu content i.e. samples containing only surface Cu aluminate, are not affected by thermal treatments, even at temperatures as high as 923 K. On the contrary, in high loaded samples the alumina releases partially the fixed HC1 under heating and the reverse transformation of paratacamite into copper chloride can take place as evidenced by XRD, UV-Vis, EXAFS and solubility test. Temperature dependent XRD measurements indicate that paratacamite starts to disappear at a temperature comprised between 383 K and 438 K and that it approaches the completion at 493 K. UV-Vis DRS spectra (not reported), collected on high Cu loaded samples heated at 523 K and re-exposed to air for 2 hours to rehydrate the chloride are fully comparable with those of the fresh samples (see Fig. 1). Thermal activation implies a loss of the resolution of the hyperfine signal of EPR spectra, progressively appeared upon aging up to 6-mounths. This experimental evidence reflects the evolution from a Cu n environment with long range ordering (crystalline paratacamite) to an highly heterogeneous one (amorphous, highly dispersed, anhydrous CuClz). EXAFS data of
I
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the 9.0 wt% sample aged and heated (Fig.3 bottom curve) indicates that heating removes nearly all the oxygen from the first coordination shell of Cu (only the small fraction of surface aluminate being not affected) and gives rise to a quantitative data analysis fully compatible with the local structure of anhydrous CuC12:4 chlorine atoms at 2.26 A.
3.5. Working catalyst From what learnt in the previous sections, in the oxychlorination reaction environment (ethylene, 02 and HCI) paratacamite can not be present and only surface aluminate and copper chloride or products arising from the interaction of these compounds with reactants or reaction products are present on the catalyst. Activity tests have evidenced that only CuCIz is the active phase and that the reaction takes place by C1 donation from CuClz to ethylene with formation of CuCI, and dichloroethane. In order to explore the reaction mechanism, we have performed depletion tests, where ethylene is sent on the sample in absence of HC1 reactant. In such a way, the only chlorine source available for the C2H4 conversion into CzH4Clz, is on the catalyst itself. The total ethylene conversion obtained on samples with 9.0, 4.6, 1.4 wt% Cu are reported in Fig. 5 (solid symbols) as a function of the number of pulses. We observe that only catalysts containing CuCI2 (9.0 and 4.6 wt% Cu) are active. The increment of conversion measured by moving from 4.6 to 9.0 wt% Cu catalyst is more than directly proportional to the increment of total copper. This fact is not surprising since we have demonstrated that the fraction of Cu forming the surface aluminate (1.6 wt% Cu for our support having 168 m2 g-L) is inactive: this means that the fraction of active copper species in samples 4.6 to 9.0 wt% Cu loaded is only 3.1 and 7.5 wt% Cu respectively. By re-plotting the activity curves of these samples renormalized by factors 1/3.1 and 1/7.5, we see that they overlaps rather well, see Fig. 5 open symbols, giving a further proofs of the validity of our model. We are strongly indebted to G.L. Marra (EniChem, Istituto G. Donegani) for XRPD measurements, to G. Vlaic (Sincrotrone Trieste and Dipartimento di Chimica Universit?~ di Trieste) and F. Villain (LURE) for their relevant and friendly support during EXAFS measurements, to G. Turnes Palomino and E. Giamello (Dipartimento di Chimica IFM Universit~t di Torino) for EPR measurements and related enlightening discussion and to B. Cremaschi (EVC, Porto Marghera) for her analytical support.
REFERENCES 1. M. Garilli, T.L. Fatutto, F. Piga La Chimica e l'Industria, 80 (1998) 333. 2. A. Arcoya, A. Cortes and X.L. Seoane, Can. J. Chem. Eng., 60 (1982) 55, 3. P. Avila,, J. Blanco, J.L. Garcia-Fierro, S. Mendioroz, and J. Soria, Stud. Surf. Sci. Catal., 7B, 1031 (1981). 4. A. Baiker, D.Monti, and A. Wokaun, Appl. Catal., 23 (1986) 425. 5. L. Garcia, D. E. Resasco, Appl. Catal., 46 (1989) 251. 6. G. Zipelli, J.C. Bart, G. Petrini, S. Galvagno, and C. Cimino, Z. Anorg. Allg. Chem., 502 (1983) 199. 7. J. Rouco; Appl. Catal. A 117 (1994) 139. 8. P.A. Berger, and J.F. Rooth, J. Phys. Chem., 71, (1967) 4307. 9. B.R. Strohmeier, D.E. Leyden, R.S. Scott Field and D.M. Hercules, J. Catal., 94(1985)514. 10. M.C. Marion, E. Garbowski, and M. Primet, J. Chem. Soc. Farad. Trans., 86, (1990) 3027. 11. R.M. Friedman, G. J. Freeman, and F.W. Lytle, J. Catal., 55, (1978) 10. 12. C.K. Jorgensen, Progr. Inorg. Chem., 12 (1979)101.
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Studies in Surface Science and Catalysis 130 A. Corrna, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
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A study of the main path and of side-reactions oxychlorination over CuCl2=Al203 based catalysts.
upon
ethylene
A. Marsella l, D. Carmello l, E. Finocchio 2, B. Cremaschi l, G. Leofanti 3, M. Padovan 4 and G.Busca2 EVC Italia, Inovyl Technology Centre, Via della Chimica 5, 30175 Porto Marghera, Venezia (Italy) Fax -39-041-2912685; e-mail:
[email protected] Dipartimento di Ingegneria Chimica e di Processo, Universit/t di Genova, P.le J.F. Kennedy, I-16129 Genova, Italy. Fax: 39-010-3536028, e-mail:
[email protected] 3 present address: Via Firenze 43- 20010 Canegrate (MI) (Italy) 4
present address: Via Villa Mirabello 1-20125 Milano (Italy)
CuC12/A1203 based ethylene oxychlorination catalysts have been characterized by using IR spectroscopy of the surface hydroxy-groups and of adsorbed pyridine and CO2. The ethylene oxychlorination reaction to 1,2-dichloroethane and the dehydrochlorination of ethylchloride (chosen as a probe molecule) have been investigated by using both pulse reactor measurements and in-situ IR experiments in static conditions. Experiments performed over doped alumina and doped CuC12/A1203 confirmed that the exposed support surface can be responsible of the dehydrochlorination of 1,2-dichloroethane (EDC) to vinyl chloride monomer (VCM), with a consequent loss in selectivity in the oxychlorination reactor. Doping alumina with MgC12 and in particular with KC1 limits the activity of the bare support in this side-reaction. 1. INTRODUCTION. The heterogeneously catalyzed gas-phase oxychlorination of ethylene to 1,2-dichloroethane (ethylene dichloride, EDC): H2C=CH2 + 2 HC1 + 8902 ---> C1-CH2-CH2-C1+ H20 followed by the dehydrochlorination of EDC represents the main way for the production of vinyl chloride monomer (VCM) and of its polymer polyvinyl chloride (PVC). The catalysts currently used are based on CuC12 supported on alumina and doped with other metal chlorides. Several studies have been devoted to the characterization of the industrial and model catalysts, mostly regarding the characterization of the Cu centers. This study summarizes the results of i) the characterization of the overall surface of both undoped and doped CuC12/alumina catalysts by IR spectroscopy of adsorbed probe molecules; and ii) reactivity in oxychlorination and dehydrochlorination (the most important side reaction) by in situ IR spectroscopy and pulse microreactor experiments.
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2. E X P E R I M E N T A L
The samples under study are summarized in Table I. Table I. Samples studied. Sample Composition Impregnating salt notation A 7-A1203 " -1CuC1A 1% Cu / y-A1203 CuC12.2 H20 5CuC1A 5 % Cu / y-A1203 " 9CuC1A 9 % Cu / y-AI/O3 " 5CuKC1A 5 % Cu - 2.8 % K / y- CUC12.2H20 + KC1 A1203 5CuMgC1A 5 % Cu - 1.7 % Mg / CuC12.2H20+ y-A1203 MgC12.6H20 KC1A 2,8 % K/y-A1203 KC1 MgC1A 1,7 % Mg / y-A1203 MgC12.6H20 9 at 503 K
Surf. area Ethylene EtC1 mZ/g Conv. Conv. * 174 0.09 20.8 188 9.19 8.2 154 41.24 7.6 117 51.56 5.1 130 1 165
5.1
140 160
4.6 5.6
The preparation procedure has been described in Ref. 1,2. FT-IR spectra have been recorded by a Nicolet Magna 750 instrument, using conventional IR cells with NaC1 windows, connected to an evacuation / gas manipulation apparatus. The sample powders were pressed into self-supporting disks and outgassed before the interaction experiments. Catalytic reactivity tests on ethylene conversion and ethylchloride have been carried out on a pulse microreactor system as reported in Ref. 1,2.. In the case of dehydrochlorination reaction, the reactants were injected in the microreactor with the following sequence: ethylchloride, HC1 and oxygen at 483 and 503 K. The injection of HC1 and oxygen is required in order to avoid catalyst deactivation (reduction) by the oxychlorination of ethylene produced upon ethylchloride dehydrochlorination. 3.RESULTS 3.1. Ch ar acte riz ation.
The surface of the catalysts has been characterized by pyridine and COz [1] adsorption experiments and by t h e i R analysis of the OH groups (fig. 1) in order to obtain complete informations on acidic and nucleophilic sites and on the coverage degree of the support surface. The analysis of the OH stretching region shows for the catalysts at quite high CuC12 loading (5% and 9% Cu content) the presence of paratacamite-like particles, characterized by two IR main bands at 3455 and 3360 cm l. These bands disappear in the range 523-623 K, temperature at which paratacamite particles decompose, leaving a disordered bulk-like copper chloride species. Impregnation with CuC12 even at 9% (amount sufficient to complete the
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monolayer) still leaves traces of free alumina OHs. On 5CuKC1A and 5CuMgC1A doped catalysts small but detectable amount of paratacamite particles are still detectable at 523 K and a band due to free OHs of uncovered alumina (3730 cm 1) persists, together with a very strong, broad band centerd at 3500 cm l and due to H-bonded OHs. Doping seems to increase the thermal stability of paratacamite-like structure. The obtained results show that the support surface is still exposed in part on the active oxychlorination catalyst, also in the case of the 9CuC1A sample, nominally containing a "full monolayer loading" of CuC12. Pyridine adsorption on the three CuC1A catalysts surface (spectra not reported for sake of brevity) shows that Cu chloride species have medium-strong Lewis acidity. On the sample at highest CuC12 loading, a band centered at 1608 cm 1 has been identified, due to pyridine strongly interacting with Cu centers. C02 adsorption also points out the presence of some "free" alumina surface nucleophilic centers even in the 9CuC1A sample. A study of the effect of pretreatment at high temperature with HC1 shows that HC1 itself is nearly totally dissociated on the catalyst surfaces, and does not give rise to significant amounts of Bronsted acid sites. We can also note that the nature of Lewis centers exposed (A13§ and Cu 2§ coordinatively unsaturated) is not substantially changed. Impregnation with CuC12 and doping with MgC12 and KC1 kills most of alumina nucleophilic sites (exposed oxide anions).
1.8 Ab so rb an ce
1.6 1.4 1.2 1.0
_b
4000
3500 3000 Wavenumbers (cm-1)
2500
Fig. 1. FT-IR spectra of OHs groups: bulk paratacamite (a), 5CuC1A outgassed at 473 K (b) and 523 K (c), 5CuMgC1A out. at 523 K (d), MgC1A out. at 523 K (e), A1203 out. at 523 K(f).
3.2. Study of the oxychlorination activity. The oxychlorination reaction was studied by "in-situ" IR in static conditions [ 1]. The disapperance of the reactants ethylene and HC1 and the appearance of EDC was monitored in the gas phase while, simultaneously, the surface species were also detected. On the pure alumina support there is no evidence of ethylene adsorbed species at room temperature while on copper containing catalysts the formation of IR bands at 1544, 1424 and 1275 cm 1 assigned to copper-ethylene complexes is observed. Acetate and formate species coordinated on Cu centers are observed at 473-523 K, likely involved in total combustion of ethylene.
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We could also observe the oxychlorination process in the IR cells starting from the 9CuC1A catalyst pretreated with HC1 and ethylene. At 573 K gaseous 1,2 dichloroethane, characterized by IR bands at 1235 and 725 cm l , is observed in the gas phase. This study allowed us to propose a mechanism for the activation and oxychlorination of ethylene over the CuC1JA1203 samples, that implies a role of a Cu-ethylene complex as an intermediate in the selective way and of formate species as intermediates in the competitive combustion of ethylene. Pulse oxychlorination catalytic studies show that alumina is essentially inert towards ethylene conversion and that the sample with very low copper loading 1CuC1A is already significantly active although not very selective to EDC because of the formation of byreaction products such as VCM, 1.1.2 trichloroethane, trichloroacetaldehyde, trichloromethane. By increasing Cu loading, the ethylene conversion increases almost proportionally, while the selectivity increases more.
3.3. Study of the main side-reaction: dehydrochlorination. The dehydrochlorination activity of the catalysts and of the bare support has been investigated by both FT-IR and pulse reactor experiments using EtC1 as probe molecule (table I) [21. At room temperature IR studies point out the presence of adsorbed EtC1 over pure alumina surface (fig. 2). The IR spectrum shows bands at 1451, 1382, 1395 cm -l (CH bending modes), 1303, 1286 cm -I (CH2 wagging mode), 1250 cm -I (CH2 twisting mode)and 1075 (CH3 rocking mode) with the corresponding CH stretching bands in the high frequency region. After heating, a complex absorption in the region 1200-1000 cm -~ grows progressively up to 523 K. These bands, associated to the CH3 deformation modes at 1447 and 1388 cm t, can be attributed to the C-C and C-O stretchings of alkoxy groups, they disappear at 573 K, just when we start to detect ethylene in the gas phase. In the same temperature range the bands of adsorbed EtC1 have also fully disappeared. The spectra recorded after treating at temperatures higher than 573 K show bands due to a new strongly adsorbed species: a couple of weak bands centered at 1570 and 1472 cm -~ is formed, due to a strongly adsorbed species. They are assigned, in agreement with results of acetic acid adsorption experiments, to the asymmetric and symmetric -COO stretching of acetate species. In the gas phase features due to ethylene and HC1 are observed starting from 573 K. Diethylether is also detected at lower temperatures as by-product (band at 1146 cm -~ in the gas phase), while CO and CO2 start to be formed in traces at the highest temperatures. These data show that alumina is highly active in the dehydrochlorination of ethyIchloride to ethylene + HC1. IR studies show that two main pathways for the production of ethylene over Cu-free alumina are possible, depending on the chlorination of the catalyst surface. The first reaction path (scheme I) requires the formation of an alkoxy species at the surface involving a coordinatively unsaturated A1 cation coupled with two exposed oxide anions. A nucleophilic substitution occurs on ethylchloride to give ethoxy species. The desorption of ethylene and HC1 restores the active site. A by reaction occurring during this process is the nucleophylic substitution of ethoxy groups on ethylchloride giving rise to diethylether.
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H3C
H2C-CH 2
H--CH 2
\
H2C\ O=
Cl
/ I
~
V AI3+
O=
O-
H2C l
250 ~ O
Q T 3+ AI
CI"
"OH
J
CI" AI3+
Scheme I. Dehydrochlorination activity on pure alumina. It seems evident that HC1 inhibits the mechanism because it interacts with the Lewis acid sites of alumina. On the other hand, this inhibition by HC1 is only partial and reversible. On chlorinated alumina (alumina pre-treated with HC1) another "concerted" elimination mechanism is proposed without the intermediacy of the alkoxide. The interaction of ethyl chloride with 9CuC1A sample shows that up to 373 K the ethyl chloride adsorption is mostly non reactive, while alkoxides become predominant at 473 K; starting from this temperatures we observe the growth of a couple of intense bands near 1590 and 1440 cm -l, assigned to acetate species. In the presence of CuC12 alkoxy ions are formed more slowly than on the pure alumina, and can also be oxidized extensively to acetates by the copper ions, that are consequently reduced. The behavior of the MgC12- (fig. 3) and KCl-doped CuC12-A1203 catalysts is, qualitatively, very similar to the one observed on the corresponding supports, MgC12- and KCl-doped alumina. The only evident difference is that IR bands of acetate species are stronger than bands found in the spectra of all CuC12-A1203 catalysts. This provides evidence that the main dehydrochlorination activity occurs on the support, but oxidizing reactivity typicall occurs on cupric centers.
,, ;:'~
,
0.2
0,4
"t/tl _
18oo
~
nc~,l ~---,-_0-"'~"~,,,..._
~~.___S
=
. . . . . .
~ 4o s
aA~ n~.R~L~n~D[]o~
,
iii ill
;J___;~.,,~L:'.,_JU,_.,
! , ~
,
ii ~
;'J~._..J k~' :~j~
d
AAAAA/k/kAA/k
0 0
~
~
~
~
5
10
15
20
Time ( h )
25
Fig.3 Catalytic stability of Li/SiO 2, Li/TiO z and Li/SZ catalysts. Li/TiO~ and LilSZ--650~ LilSiO=--600~
10
20
:30
40
50
60
70
80
2O
Fig.4 X R D p a t t e r n s of various I.i/oxide catalysts before and and after reactio~ (a) fresh catalysts, (b)used cata_lysts.
The XRD pattems of the fresh and reacted LiC1/oxide catalysts prepared in this investigation are presented in Figure 4. Monoclinic and tetragonal ZrO2 are the major phases in LiC1/ZrO2 and LiC1/SZ catalysts. A small amount of Li2ZrO3 was also found in two catalysts; however, the
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intensities of Li2ZrO3 peaks are weaker on LiC1/ZrO2. For LiC1/AI203, Li20 is present apart from the major crystallites 7-A1203. Surprisingly, the peaks for LiCl were not identified in the above LiCl-doped oxide catalysts. This is probably due to the lower amount in amorphous phase or well dispersion of LiCl in catalysts. TiO2 and LiCl coexist in LiC1/TiO2, while four phases, quartz, LiESiO3, LiESiO5 and LiCl, are present in LiC1/SiO2. The XRD patterns of the used catalysts, LiC1/TiO2 and LiC1/SiO2, display different profiles from those of the fresh catalysts, indicating that a phase transformation occurred on these catalysts during the reaction. However, there is no significant phase change on the LiC1/SZ. From Fig.4, it is seen that the intensities of the XRD diffraction peaks of the used LiC1/SiO2 are weaker, suggesting the occurrence of decrystallization. No LiC1 peaks can be found, indicating the loss of LiC1. For LiC1/TiO2, a larger amount of Li2TiO3 is formed after reaction and no LiC1 was detected. It has been found that Li2TiO3 exhibits a much lower activity in the oxidative dehydrogenation of ethane. Therefore, the phase transformation and leaching of LiC1 will account for the deactivation of the LiCl-based catalysts. Figure 5 shows the TPR profiles of the LiCl/oxide catalysts. It is seen that these . . . . . . . uc~s~o~ catalysts demonstrate a different reduction behavior. There is a weak reduction peak LiCl/TiO2 II LiCI/AI203 I between 400-500 ~ in the TPR profile of ---LiCllZrO2 I LiCI/A1203. One weak reduction peak = uc~/sz t~ I appears at 500-630 ~ in the TPR profile of z" .o / LiCI/ZrO2. The reduction of LiC1/TiO2 E J happens at high temperatures around 500 ~ ~, t- = / / while a strong reduction occurs after 600 ~ 8 _ ~ _ .....on LiC1/SZ. For LiCI/SiO2, there are two ~ . . . . . ~ _- -. -~ . ~ ~ ---- ~ ~ weak peaks at lower temperatures and a ~ . . . . . . . . . . . _. . . . . . . strong peak at high temperatures in the TPR .... profile centered at 250, 450 and 660 ~ . . . . . . . . respectively. These results indicate that ~oo zoo 300 400 soo soo 700 LiC1/SiO2 and LiC1/SZ can produce m o r e Temperature(~ oxygen species, which will promote the Fig.5 TPR patterns of various Liloxide catalysts. activation of ethane. It is believed that catalytic activity in the oxidative dehydrogenation of ethane has a close relationship with the acid-base and redox properties of the catalysts. If the surface basicity of a catalyst was very large, strong CO; adsorption would occur on the catalyst surface, resulting in low catalytic activity. For LiCl-promoted catalysts, the acidity/basicity will depend on the support. It is know that SZ and SiO2 are acidic supports, TiO2 and ZrO2 show weak acidity while A1203 is a basic oxide [1]. Therefore, it may be deduced that lower acidity of LiC1/A1203 is attributed to its high basicity while high activity of SiO2 and SZ supported catalysts promotes the contact of ethane and the catalyst surface. The TPR results also demonstrate that LiC1/SiO2 and LiC1/SZ show high reducibility, which may also account for their higher activity. In addition, CI ions in these catalysts also play an important role. CI ions can prevent the active sites from CO2 poisoning and lower the reactivity of oxygen species to ethylene oxidation [10,11]. Chlorine radicals are also thought to favor the homogeneous decomposition of ethyl radicals to ethylene. All these will contribute to the increase of ethylene selectivity over the LiC1/oxide catalysts. Due to the strong interaction of C l ions and basic metal oxides, few chlorine radicals will be produced at high temperatures, resulting in lower activity and ethylene selectivity on the
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LiCI/AI203 and LiCI/ZrO2 catalysts.
4.
CONCLUSIONS
Metal oxides such as A1203, SiO2, TiO2, ZrO2 and sulfated ZrO2 exhibit a moderate activity in the oxidative dehydrogenation of ethane. Addition of LiCl in those oxides promotes the ethane conversion and ethylene selectivity. However, LiCl-based catalysts show a varying catalytic behavior depending on their bulk and surface properties. Among the catalysts prepared, LiCI/SZ was found to exhibit high conversion and a longer period time of stability. The deactivation of catalysts can be attributed to the phase transformation and loss of LiC1. REFERENCES
1. 2. 3. 4. 5. 6. 7.
T. Blasco and J.M. Lopez-Nieto, Appl. Catal., 157 (1997), 117. F. Cavani and F. Trifiro, Catal. Today, 24 (1995), 307. E.A. Mamedov and V. Cortes-Corberan, Appl. Catal., 127 (1995), 1. D. Wang, M.P. Rosynek, and J.H. Lunsford, J. Catal., 151 (1995), 155. D. Wang, M.P. Rosynek, and J.H. Lunsford, Chem. Eng. Technol., 18 (1995), 118. K. Otsuka, M. Hatano, and T. Komotsu, Catal. Today, 4 (1989), 409. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki, Chem. Commun., 1999, 103. 8. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki, Catal. Lett., 59 (1999), 173. 9. L. Ji, J. liu, X. Chen and M. Li, Catal. Lett., 39 (1996), 247. 10. S.J. Conway and J.H. Lunsford, J. Catal., 131 (1991), 513. 11. R. Burch, E.M. Crabb, G.D. Squire and S.C. Tsang, Catal. Lett., 2 (1989), 249.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Oxidative Dehydrogenation of Ethane to Ethylene over NiO/AI203 Catalyst Xinjie Zhang, Gang Yu, Yunqing Gong, De'en Jiang and Youchang Xie* Institute of Physical Chemistry, Peking University, Beijing 100871, P.R.China Oxidative dehydrogenation of ethane to ethylene was studied over NiO/AI203 catalysts. The effects of various A1203 supports, NiO loading, calcination temperature of the supports and of the catalysts, as well as some dopants were investigated. It was found that P205 is the best dopant. An optimum NiO-P2Os/AI203 catalyst can give ethane conversion of 58.3% and selectivity to ethylene of 72.0% with yield to ethylene of 42.0% at reaction temperature of 450~ 1. INTRODUCTION The thermal pyrolysis of ethane to ethylene has been one of the most important industrial processes for a long time, but it is costly in energy consumption and investment, because it is an endothermic process carried out at high temperature and special alloy reactor is used. The catalytic oxidative dehydrogenation of ethane (ODE) to ethylene has received more and more attention since 70's, because it is an exothermic reaction and can be carried out at low temperature with high conversion and selectivity if a good catalyst is found. For the last two decades, a variety of catalysts such as MoVNbSbCaOx[1-5], Li/MgO[6-8], La/CaOx[9,10], have been examined for ODE. Among them, MoVNbSbCaOx catalyst developed by Union Carbide Corporation is the only type of catalyst that can be operated at low temperature with good conversion and selectivity. Recently, Martin et al.[ 11 ] reported another new type of low temperature catalyst, NiO/SiO2, for ODE. Li et al.[12] improved this catalyst by using A1203 as support, but the highest yield to ethylene over their catalysts is only about 25.3%, much lower than the yield of 50% given by Union Carbide catalyst. The present study is to look for a better NiO/A1203 catalyst by screening various A1203 supports and studying the effects of NiO loading, calcination temperature of the supports and of the catalysts, as well as some additives on the catalytic properties of the catalysts. It was found that a good catalyst can be obtained if suitable amount of NiO is supported on a suitable A1203, and the catalyst is calcined at a suitable temperature with P205 as a promoter. 2. EXPERIMENTAL
2.1. Catalyst preparation In general, the catalysts were prepared by the impregnation of A1203 powders in aqueous solution of Ni(NO3)E'6H20, followed by evaporation to dryness at 140~ ovemight. The resulting solids were then calcined at 450~ in air for 5 hours unless stated otherwise. If an additive was used, it was dissolved in the Ni(NOa)2"6H20 aqueous solution for impregnation. * Corresponding author. E-mail"
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The catalysts were crushed and sieved to 40-80 mesh for the evaluation of catalytic property.
2.2. Catalytic property measurement The catalytic tests were carried out with a fixed-bed flow glass microreactor, using 0.5g catalyst and operating under atmospheric pressure. The reaction temperature ranged from 350~ to 450~ The feed consisted of 10(vol)% ethane, 10(vol)% 02 and 80(vol)% N2 with a space velocity of 338 ml C2H6/g cat. h (GHSV at STP). The products were analyzed on an on-line gas chromatography with a 4m Porapak Q column and a hydrogen flame ionization detector. A methanator fitted with Ni catalyst was equipped to the GC for the analysis of carbon monoxide and carbon dioxide. 2.3. Catalyst characterization X-ray diffraction analysis was performed with a BD-86 X-ray diffractometer using Cu Kct radiation at 40 kV and 20 mA. The BET surface areas of the samples were measured by using nitrogen adsorption at 77K. H2-TPR was carried out by using 5 vol% H2 in Ar as reducing gas with a flow rate of 20 ml/min and a heating rate of 10~ 3. RESULTS AND DISCUSSION 3.1. Effect of various Ai203 supports Several A1203 supports made from different precursor aluminum salts were used to prepare NiO/AI203 catalysts. Since these supports have different surface areas, we prepared these catalysts with NiO loading on the base of the surface area of the supports. The catalysts, C1 to C4, have NiO loading of 0.24g/100m 2 surface of A1203. Their catalytic properties are reported in Table 1. Table 1 Catal ic ro erties of the catal sts usin different A1203 su _ys
Cat.*
.......................... .3__5..0.............................................
S% C% .............................. Y%
C2H4 CO2
.40.0
orts
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.................................................
S% C% .............................. Y%
C2H4 CO2
.4_50 .........................
S% C% .............................. Y%
C2H4 CO2
C1 12.0 79.9 20.1 9.6 31.8 75.2 24.8 23.9 57.2 67.1 32.9 38.4 C2 9.2 75.2 24.8 6.9 26.6 71.6 28.4 19.0 52.2 67.6 32.4 35.3 C3 14.7 53.4 46.6 7.8 36.8 52.0 48.0 1 9 . 1 52.7 53.4 46.6 24.1 C4 10.4 76.9 23.1 8.0 29.2 69.6 30.4 20.3 55.6 65.3 34.7 36.3 C%: ethane conversion, S%: selectivity, Y%: yield to ethylene * The A1203 supports used in C1, C2, C3 and C4 were prepared by hydrolysis of aluminum alkoxide, precipitation of aluminum sulphate by ammonia solution, aluminum nitrate by ammonia solution and sodium aluminate by aluminum sulphate, respectively. .............................................
,
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Table 1 shows that the catalyst C 1, which was made with an A1203 support prepared by hydrolysis of aluminum alkoxide, is the best one. It gives good ethane conversion and good selectivity to ethylene for the whole reaction temperature, and the highest yield is obtained at 450~ Therefore, this A1203 support was used to prepare the catalysts in the following experiments.
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3.2. Effect of NiO loading The effect of NiO loading on the catalytic with the A1203 support calcined at 900~ for 5 Fig. 1. In Fig. 1, the NiO loading is counted on the base of 100m 2 surface of the A1203 supports. As the NiO loading increases from 0.03 to 0.21g NiO/100m 2 surface of A1203, the ethane conversion increases, and then the ethane conversion decreases slightly when the NiO loading exceeds 0.2 lg NiO/100m 2 surface of A1203. In Fig. 1, the selectivity to ethylene decreases gradually as the NiO loading increases. The highest yield to ethylene is obtained over the catalysts with NiO loading of 0.21-0.27g NiO/100m 2 surface of A1203.
property was tested over a series of catalysts hours. Their catalytic properties are shown in 0
E 80
O~o~,..
~-0--0--0~0~0-- 0 - 0 ~ 0 _ 0
tD
N 60
o ~~--0 01,.o~" /U~/ll 9_ _ 9~ U ~ m m ~l.....i ~ ll~ll--ll--n--ll --
_ 40 O
,
0.0
i
0.!
,
l
0.2
,
t
0.3
,
i
0.4
NiO/AI20 3 (g/100m 2 surface)
Fig. 1. Ethane conversion(o), selectivity (o) and yield to ethylene(,,) as a function of NiO loading. Reaction temperature: 450~
3.3. Effect of calcination temperature of the supports A1203 was calcined at 550~ 800~ 900~ 1000~ 1100~ and 1200~ for 5 hours to obtain supports with different surface areas and surface properties. The surface areas of the supports decrease as the calcination temperature increases, as is shown in Table 2. Table 2 Surface areas of A1203 calcined at different temperatures ........................ .Sup.p_o_a_.s_ ............................. . % . 0 . ........... ._s__8._0.0........... _ _s..9__0_9_......... _.S..l__0._0_0_ .......... __S_l_l_0__0_ .......... . % 0 0 _ .....
Calcination temp_.erature.(~
550
According to the results of 3.2, we chose 0.24g NiO/100m 2 surface of A1203 as a suitable NiO loading to prepare the catalysts with the A1203 supports obtained at different calcination temperatures. The ethane conversion, selectivity and yield to ethylene over the catalysts at the reaction temperature of 450~ are shown in Fig. 2. Fig. 2 shows that the calcination temperature of the supports has significant effect on the catalytic properties of the catalysts. Though the amount of NiO on the catalysts decreases as the calcination temperature of their A1203 supports raises, the ethane conversion and yield to ethylene
800
o-e, 0
900
80
1100
1200
~/0 ~0-.-.-----0~ " / 0 - - - - - - - _ 0
60
o___________o
N
4o
~
20
o
1000
O~
~ o k ,
~
"------------"/'----"~'~~'~o
500 600 700 800 900 1000"1100 1200 1300 Calcination temperature of the support(~
Fig. 2. Ethane conversion (o), selectivity (o) and yield (m) to ethylene as a function of calcination temperature of supports. Reaction temperature" 450~
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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increase slightly rather than decrease over these catalysts as the calcination temperature of supports increases from 550~ to 900~ Fig. 2 also shows that as the calcination temperature of the supports increases from 550~ to 1100~ the selectivity to ethylene over the respective catalysts increases. When the calcination temperature raises to 1200~ the support converts to t~-A1203, and the selectivity to ethylene decreases abruptly. The data in Table 1 and Fig. 2 reveal that the best catalyst can be obtained by using support prepared by hydrolysis of aluminum alkoxide and calcined at 900~ 3.4. Effect of calcination temperature of the catalysts
A series of catalysts with various NiO loading were prepared with the A1203 supports calcined at 900~ for 5 hours, and the catalysts were calcined at different temperatures. The catalysts are designated as NiOxTy, where x represents NiO loading (g/100m 2 surface of A1203), and y represents the calcination temperature (~ of the catalysts. The catalytic properties of the NiO0.24Ty catalysts are shown in Table 3. Table 3 Effe ct 0fca!cinatir0 n temperature 0fthecatalysts on the catal~ic ~pr0Pe~ies
...........
.......... _T(!.c)_ ............................ 3_5.0 ......................................... _40.0......................................... _45_0.....................
S% S% C% .......................... Y% C% .......................... Y% C% C2H4 CO2 C2H4 CO2 NiO0.24T400 23.4 66.9 33.1 15.7 44.7 66.1 33.9 29.5 . NiO0.24T500 15.2 73.8 26.2 11.2 39.7 69.7 30.3 27.7 59.5 NiO0.24T600 4.8 81.0 19.0 3.9 14.9 78.0 22.0 11.6 33.4 NiO0.24T700 . . . . 4.0 83.0 17.0 3.3 11.0 C%: ethane conversion, S%: selectivity, Y%: yield to ethylene Cat.
S% ......................... Y%
C2H4 CO2
.
. . 64.5 35.5 38.4 70.7 29.3 23.6 81.3 18.7 8.9
In Table 3, as the calcination temperature of the catalysts increases, the ethane conversion decreases for all the reaction temperature, while the selectivity to ethylene increases. The other catalysts demonstrate similar trend as the calcination temperature of the catalysts increases. The catalysts were characterized by XRD and H2-TPR to investigate the change of the catalytic property. XRD patterns of NiO0.12Ty and NiO0.30Ty are shown in Figs. 3 and 4.
9NiO
10
20
30
40
50
60
70
2 Theta/~ Fig. 3. XRD patterns of NiO0.12Ty.
10
20
9
30
9
40 50 2 Theta/~
60
Fig. 4. XRD patterns of NiO0.30Ty.
70
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The XRD pattems of NiO0.12Ty catalysts in Fig. 3 have no obvious difference with that of the A1203 support, and there is no evident crystalline NiO on these catalysts. The result indicates that NiO is highly dispersed on the NiO0.12Ty catalysts. In Fig. 4, for the catalysts (NiO0.30Ty) with higher NiO loading, it is observed that there is crystalline NiO on the catalysts. Fig. 4 also shows that almost all the crystalline NiO on the NiO0.30Ty catalysts does not change greatly as the calcination temperatures of the catalysts increase. According to the results of Xie et al. tl31, not all NiO is crystalline NiO on the catalysts if the NiO loading exceeds 0.12g NiO/100m 2 surface of A1203, so there exists both highly dispersed NiO and crystalline NiO on the NiO0.24Ty catalysts. Fig. 5 shows the H2-TPR curves of the NiO0.30Ty catalysts. There are two kinds of reduction peaks. The peaks at low temperature ( 300~176 ) are due to the reduction of crystalline NiO, and the peaks at high 571 temperature ( 400~176 ) are due to the ~NiO0.30T70( reduction of highly dispersed NiO[13]. Though 547 both the low temperature peaks and the high .~~NiO0.30T60( temperature peaks shift to higher temperature as the calcination temperature of the catalysts ----~---~NiO0.30T50( increases, the high temperature peaks shift more than the low temperature peaks. The TPR peaks ~ N i O 0 .i3 0 T!4 0 ( shift to higher temperature maybe ascribes to that 100 200 300 400 500 600 700 800 900 some highly dispersed NiO has formed spinel NiA1204 with A1203 owing to the calcination at Temperature (~ higher temperature. This is consistent with the Fig. 5. H2-TPR curves of NiO0.30Ty. activity decline of the catalysts calcined at higher temperature. *
3.5. Effect of doping oxides Several oxides such as WO3, MOO3, B203, P205, were used to dope NiO/A1203 catalysts, and their effects on the catalytic properties were tested. The data show that P205 is the best dopant. The catalytic property of a NiO-P2Os/A1203 catalyst (0.24g NiO/100m 2 surface of A1203 and Ni:P=I 0:1 (mol)) is shown in Fig. 6, which shows that the dopant P205 can make the selectivity to ethylene increase about 5%, while the ethane conversion changes a little. The P205 doped catalyst gives ethane conversion of 58.3% and selectivity to ethylene of 72.0% with yield to ethylene of 42.0% at 450~
9
,
9
*
9
*
9
,
9
,
9
,
,
,
,
~ 80 _o_
coov Oov** 9
3
~ 60 --o-- Sel. overNiO/AI203 --ZX--Conv. overNiO-P~Os/Ai20 ~ ~ / . ~ _ o --V-- Sel. overNiO-P2Os/Al20.a/~~" 40 =~ 20 O w 340
,
i
,
360
!
380
,
i
400
,
I
420
,
i
440
,
460
Reaction temperature (~ Fig. 6. Ethane conversion and selectivity to ethylene as a function of reaction temperature over the NiO/A1203 catalysts with or without P205 dopant.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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4. CONCLUSION This work investigated oxidative dehydrogenation of ethane to ethylene over NiO/AI203 catalysts. The effects of various A1203 supports, NiO loading, calcination temperature of the supports and of the catalysts, as well as some dopants were studied. The A1203 prepared by the hydrolysis of aluminum alkoxide and calcined at 900~ is an appropriate support. The appropriate NiO loading on the support is 0.21-~0.27g NiO/100m 2 surface of A1203. The appropriate calcination temperature of the catalyst is 450-500~ P205 is a good promoter for the NiO/A1203 catalyst. It can increase selectivity to ethylene, while ethane conversion does not decrease. For an optimum NiO-P2Os/AI203 catalyst, ethane conversion is 58.3% with a selectivity to ethylene of 72.0%, i.e., yield to ethylene of 42.0% is obtained at the reaction temperature of 450~ The NiO-P2Os/A1203 catalyst is a better low temperature catalyst for ODE. ACKNOWLEDGEMENT The authors acknowledge the National Science Foundation of China for financial support (Project No: 29733080 ). REFERENCES
1. E.M. Thorsteinson, T. P. Wilson, F. G. Young and P. H. Kasai, J. Catal., 52(1978)116. 2. F.G. Young and E. M. Thorsteinson, Low temperature oxydehydrogenation of ethane to ethylene, US Patent No. 4250346(1981). 3. J.H. McCain, Process for oxydehydrogenation of ethane to ethylene, US Patent No. 4524236 (1985). 4. J.H. McCain, Process for oxydehydrogenation of ethane to ethylene, US Patent No. 4568790(1986). 5. R.M. Manyik, J. L. Brockwell and J. E. Kendall, Process for oxydehydrogenation of ethane to ethylene, US Patent No. 4899003(1990). 6. E. Morales and J. H. Lunsford, J. Catal., 118(1989)255. 7. S.J. Conway and J. H. Lunsford, J.Catal, 131 (1991)513. 8. S.J. Conway, D. J. Wang and J. H. Lunsford, Appl. Catal. A, 79(1991)L 1. 9. X. Zhou, Z. Chao, J. Luo, H. Wan and K. Ksai, Appl. Catal., A, 133(1995)263. 10. L. Ji, J. Liu, X. Chen and M. Li, Catal. Lett., 39(1996)247. 11. V. Ducarme and G. A. Martin, Catal. Lett., 23(1994)97. 12. T. Chen, W. Li, C. Yu, in: The Development of Catalysis--Theses of the Eighth Catalysis in China (H. Zhang, X. Cai and D. Liao, eds., Xiamen University plb.)(1996) p.273. 13. Y. Xie, Y. Tang, Adv. Catal., 37(1990)1.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendiorozand J.L.G. Fierro (Editors) 9 2000 ElsevierScience B.V. All rights reserved.
化
1841
Oxidative dehydrogenation heteropolycompounds
of ethane
over
catalysts
prepared
via
N. Haddad a, C. Rabia a, M.M. Bettahar b. A. Barama a aLaboratoire de Chimie du Gaz Naturel, Institut de Chimie, USTHB, BP32 E1 Alia Bab Ezzouar, Alger, Algerie bLaboratoire de catalyse h6t6rog6ne, Universit6 de Nancy 1-Henri Poincarr6 54506 Nancyles-Vandoeuvres-Cedex. Catalytic performance of supported catalysts prepared via polyoxometalate compounds H3PMol2040 (PMoI2), n3PW12040 (PWI2), n4PMollVO40 (PMollV) and (NH4)6PMotlNiO40 (PMollNi) were studied in the selective oxidation of ethane at atmospheric pressure, T - 400-650~ and C2H6/O2 -2-10 ratio. The catalysts have been characterized by their BET area and XRD and IR spectras. The main result is the obtention of high selectivity (75%-100%) of ethene at substancial conversion (up to 60%). The activity depended on the reaction conditions, the nature of the metal atom and support. The support played an important role in the stability of the catalysts. 1. INTRODUCTION The production of olefins is gaining more and more interest, especially because of their use as an original material for manufacture of various valuable products. Ethane is an available natural resource, also its oxidative dehydrogenation would be a promising route for ethene production and, further, that of valuable chemicals such as styrene, ethyl benzene, ethanol, acetaldehyde, vinyl acetate, etc. In this way many catalytic formulations have been tested in the selective oxidation of ethane into ethene [ 1-4]. In the present work, we report the results obtained during this reaction at atmospheric pressure on catalysts prepared from heteropolyacids (HPA): H 3 P M o 1 2 0 4 0 ( P M o I 2 ) , H 3 P W I 2 0 4 0 ( P W l 2 ) , H 4 P M o l I V O 4 0 ( P M o l l V ) and (NH4)6PMollNiO40 (PMollNi). The catalysts have been characterized by their BET area, IR spectras and XRD. 2. EXPERIMENTAL The catalysts were prepared by conventional impregnation of the active phase on A1203, Sm203, SiO2, Dy203 or La203 with an aqueous solution of HPA. They were heated in air at 400-650~ for 16h. The specific areas, XRD and IR spectra were performed on a BET Coultronics 2100D, Phillips 1710 diffractometer and Phillips 9800 FTIR respectively.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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The catalytic performances were carded out in a fixed bed quartz tubular reactor under the following experimental conditions: P=I bar, T=450-650~ C2HdO2 = 2-10 and Wcat-0.5g. The reactants and products were analyzed on line using Delsi 121ML FID and TCD GC equipped with porapak (C1-C2 analysis) and carbosieve SII (C1 , C O , CO2 , air analysis) columns respectively. The activity of catalysts has been evaluated at steady state.
3. RESULTS AND DISCUSSION 3.1. Characteristics of the catalysts
The specific surface areas depended on the nature of the support that of the active phase (Table 1). High BET area was obtained with the SiO2 support (237 m2/g), the BET area decreased after reaction as probably an effect of the sintering of the active phase. It is woth to note that, in the used catalyst, the specific area was divided by 2 for the unreducible supports (A1203 and SiO2) but to a lesser extent for the reducible supports ( Sm203 La203 and Dy203). In the case of the PMol2/A1203 catalyst, the BET area (3.7 m2/g) increased by substituting one Mo atom by one V atom (PMol iV/A1203 ) (21.3 m2/g). Table 1 Variation of specific areas of catalysts with PMollV loading , nature of support and precursors formula: Tc = 600~ Tr = 650~ Catalyst 15% PMol IV/support 15%HPA/(x-AI203 (x-AI203 SiO2 Sm203 La203 Dy203 PMoI2 PWI2 PMollVPMollNi S (m2/g) fresh 21.2 237 9.7 11.3 6.3 3.7 4.4 21.3 6.5 S (m2/g) used ll.3 113 7.5 9.4 5.9 . . . . The XRD spectra of the PMollV based catalysts showed sharp bands of well crystallized solids. As expected, the HPA phase decomposed during the heating step [5]. Indeed, besides the support phase spectrum, the MOO3, V204, V205 and P205 crystalline phases were evidenced. For the fresh catalysts, the V205 oxide was observed on the unsupported PMollV catalyst and in the presence of all the supports except La203. Whereas the V/O4 oxide appeared only in the presence of A1203 and La203 supports. The V204 oxide was also observed on the used PMoIIV/AI/O3 catalyst (figure 1 and 2) J~
[counts] 1200
lV~,
i
lO00 J
@ V2Os ]
800
4OO 20O -
: +
2. 0. 4. 0. .
.'1
Fig. 1 9XRD spectra of the fresh 15% PMol!V/AI203
60
J" [)~AI20, [
/|
goo !
+
600
0
1200
AI20.~ | p2os ]
~
1000
Ii
~ I
4-
600' 400 ++
200 ....
+-_
[20]
80
t I +_
_
'
A.+I
,
20
,
40
60
Fig.2 9XRD spectra of the used 15% PMol IV/AI203
[201
80
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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The FTIR spectras of PMol2 and PMoltM (M = V, Ni) supported on alumina showed the disappearance of the bands attributed to the Keggin structure (in the 1200-300 cm "l) after heating at 600 ~ thus confirming the decomposition of the HPA phase (Figures 3,4). 0'
80
j _jA
8 70 -~ 60
50"
379
3O 40 96;1 1100
782~
900 .... 700_ 500_ Wavenumber [ em -l]
300
16 4000
3000
2000 Wavenumber[ em"l]
1000 600
Fig.3: FTIR Spectra of PMol~V before heating Fig. 4: FTIR Spectra of PMollV ,Tc=600~ 3.2. Catalytic activity
All catalytic tests were carded out at 650~ over catalysts heated at 600~ These conditions drive to the better performances. The products of the reaction were: C2I-I4, CH4, CO, CO2 and traces of C2H5OH. For the supported catalyst the steady state was reached after about 2 h and the catalytic activity remained constant over 3 days. For the unsupported catalyst a slight and progressive decrease in activity was observed and no well defined steadystate was observed within 6h-48h. Ethane conversion and products selectivities depended on the nature of the metal (Mo, W, V, Ni) atom and the nature of the support. The main results of steady-state activity are reported in Tables (2-4) and Figure (5). 3.2.1. Effect of the reaction conditions
The reaction temperature increased the conversion of C2H6 and 02 with relatively high selectivity to ethene as illustrated in Figure 5 for the 15% PMollV/Sm203 catalyst. For example, when the reaction temperature was increased from 450~ to 650~ conversion of both reactants increased from 9.1 to 60.6% and 2.3 to 100% respectively, whereas the selectivity to ethene decreased from 100% to only 75.9%. Cracking (methane) and combustion (carbon oxides) products were favored at high reaction temperature but their selectivity remained low (5.1-13.8 % and 13.4-10.2% respectively) in the studied range of temperature (Figure 5).
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
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化
* ConvC2H6
mConv 02
9
9 SCO + C02 mSCH4 Selectivity (%) Conversion (%) 1201 ........................................................................................................................................................................................................................... 120 i
100!
A
9
-
-
_-
100
80
80
60!
60
40
40
~
20
20 ~
0
400
450
.
.
.
.
500 550 Temoerature *r
600
650
Fig.5 9Variation of the conversion and selectivities as a function of the reaction temperature for 15% PMOllV/Sm203 catalyst. C2H6/O2 = 5; Tc = 600~
The effect of C2HffO2 ratio on the performances of 15 % PMol IV/AI203 catalyst was investigated at 650~ The results obtained (Table 2) showed that, when the C2H6/O2 ratio increases from 2 to 10, the conversion of C2H6 passed through a minimum (47.8%) for C2H6/O2 = 5 whereas O2 was totally consumed. In parallel, carbon deposition also passed through a minimum (5.0%). This minimum in activity should be attributed to changes in the surface redox potential with change in the chemical composition of the reactive atmosphere. As to the selectivities, that of C2H4 increased from 59.4 to 82.2 % at the expense, notably, of that of the combustion by-products which decreased from 23.8% to 5.4% [6]. The selectivity to methane was relatively not changed (12.3%-16.7%). Table 2 Effect of the C2H6/O2 ratio on the ODE over 15% PMollV&t-A1203 : Tcal = 600~ Tr = 650~ Rapport Conversion (%) Selectivity (%) % Coke C2H6 02 C2H4 CH4 CO + CO2 2 77.8 100 59.4 16.7 23.8 5.7 5 47.8 100 78.9 13.7 7.4 5.0 10 59.4 100 82.2 12.3 5.4 17.2 3.2.2. Effect of the nature of the support
The catalytic activity results obtained on PMolIV unsupported and supported catalysts at 650~ with a C2H6/O2 ratio of 5 are reported in Table 3. No important changes were observed in conversions nor in selectivities by supporting the active phase nor varying the nature of the support. Thus the conversion of ethane and selectivity of ethene remained
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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around 50% and 80% respectively. However, coke deposition was the highest (15.0%) for the unsupported catalyst and high carbon formation (12.8%) was also observed with the lanthanum oxide Support. Then, in the present conditions, the redox properties of a multiphasic mixture of metal oxides seemed to be the driving force of the reaction course. Further investigations are needed for a better understanding of the exact nature of this driving force. The main effect of the support, as reported above, was to stabilize the active phase by a better dispersion. Table 3 Effect of the nature of the support on the ODE over 15% PMollV/support: Teal = 600~ Tr = 650~ C2H6/O2=5 (Feed composition: 50% of C2tt6,10% of 02 and 40% of N2) catalyst Conversion (%) Selectivity (%) (yield %) %Coke C2H6 02 C2H4 CH4 CO + CO2 PMollV 50.0 100 78.3 (27.4) 13.8 (4.8) 7.8 (2.7) 15.0 PMoIIV/A1203 47.5 100 78.9 (33.7) 13.7 (5.8) 7.4 (3.1) 5.0 PMoI~V/SiO2 46.9 100 78.6 (29.5) 11.7 (4.4) 9.6 (3.6) 9.3 PMollV/Sm203 60.6 100 75.9 (39.7) 13.8 (7.2) 10.2 (5.3) 8.2 PMo~V/La203 51.0 100 82.3 (31.6) 10.1(3.8) 7.5 (2.8) 12.8 PMoIIV/Dy203 53.9 100 79.9 (35.3) 10.9 (4.8) 9.1 (4.0) 9.3
3.2.3. Influence of the nature and composition of the metal phase
The influence of the nature of the metal on the C2H6 conversion and products distribution in C2H6 + 02 reaction was studied in the same conditions as above and the obtained results are reported in Table 4. It can be seen that the catalysts prepared via the heteropolymolybdic acid were more active than the catalyst prepared via the heteropolytungstic acid (47.8-61.2% of conversion against 33.5%). However, they yielded more coke (up to 11.2% against 3.7%). The observed difference can be attributed to the Mo redox and acid-base properties in agreement with the literature data [7]. Although the vanadium was more oxidative element than molybdenum, the replacement of one Mo atom by one V atom resulted by notable decrease in C2H6 conversion (57.5 to 47.8%) in good accordance with literature [8]. However, no change in ethene selectivity has been observed (about 80%), although improvement was expected [8]. In contrast, when a molybdenum atom was substituted by a nickel atom, both the C2H6 conversion and the C2H4 yield were improved (respectively 61.2% and 39% against 57.5% and 36%) (Table 4). A synergetic effect between MoO3 and NiMoO4 phases (both present in PMol iNi catalyst) should be the starting point of this improvement[9]. Table 4 Effect of the metal composition on the ODE over 15% PMI2-xM'x/AI203: (M: Mo, W and M':V; Ni, x=0; 1): Teal = 600~ Tr = 650~ C2H6/O2 = 5 Catalyseur % Conversion Selectivity (%) (yield %) % Coke C2H4 CH4 CO + CO2 C2H6 02 PMOl2 57.5 100 77.5 (36.0) 13.2 (6.1) 9.2 (4.2) 11.2 PWI2 33.5 100 72.5 (21.5) 13.5 (4.0) 13.6 (4.1) 3.6 PMollV 47.8 100 78.9 (33.7) 13.7 (5.8) 7.4 (3.1) 5.0 PMollNi 61.2 100 75.6 (38.9) 14.8 (7.6) 9.5 (4.8) 9.7
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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4. CONCLUSIONS
This study shows that the catalysts prepared via heteropolycompounds are suitable for ODE. The Selectivity towards C2H4, exceeding 82%, can be obtained at high conversion of ethane. The stability of the active phase is highly sensitive to the nature of the support. The coke deposition decreased in the presence of support, as probably, a consequence of a decrease of acidic surface properties. The catalytic behavior of these solids depends on the reaction temperature conditions, the nature of support and of the metal atom. The ODE was favored with molybdenum based catalyst. The presence of vanadium in the catalytic formula decrease the conversion whereas that of nickel enhanced it. REFERENCES
1- K. Ruth, R. Burch and R. Kiffer J. Catal. 175 (1998) 27. 2- J. Bandira, Y. Ben Tgmrit, Appl. Catal. A. : General 152 (1997) 5. 3- J.Z. Luo, X.P. Zhou, Z.S. Chao, H.L. Wan Appl. Catal. A: General 159 (1997) 9. 4- K. Ruth, R. Kiffer, and R. Burch, J. Catal. 175 (1998) 16. 5- C. Rocchiccioli-Deltcheff, A. Aouissi, S. Launay, M. Foumier, Journal of molecular catalysis A: Chemical 114 (1996) 331-334 6- D. W. Flick and M. C. Huff, Catalysis Letters 47 (1997) 91-97 7- T. Okuhara, N. Mizuno and M. Misono, Advance in catalysis, 41 (1996) 113. 8- B. Grzybowska-Swierkosz, F. Triffiro and J.C. Vedrine eds., Appl.Catal. A General, 157 (1997)1. 9- O.Lezla, E.Bordes, P.Courtine and G.Hecquett, J of catalysis, 170, (1997) 346-356.
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化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
The Importance of Nonstoichiometric Oxygen in NiO for the Catalytic Oxidative Dehydrogenation of Ethane Tong Chen v, Wenzhao Li*, Chunying Yu, Rongchao Jin and Henyong Xu Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian, 116023, China, Fax: +86-411-4691570, E-mail:
[email protected] Abstract The catalytic behavior ofNiO, pretreated in oxygen at 773 K and 973 K, respectively, for the oxidative dehydrogenation of ethane (ODHE) was investigated. Nonstoichiometric oxygen in NiO and its role in the selective ODHE reaction was quantitatively and qualitatively characterized by O2-TPD-MS, TGA, in situ magnetic measurements and pulse experiments. The catalytic behavior depends markedly on the amount of nonstoichiometric oxygen in NiO, which can be controlled by the temperature of its pretreatment in an oxygen atmosphere. With decreasing amounts of the nonstoichiometric oxygen species in NiO, the ODHE activity is lower. It is suggested that only the p-type semiconductor NiO containing nonstoichiometric oxygen rather than the intrinsic NiO is the active phase for ODHE to ethylene. The ODHE selectivity seems to be related to the ease of the Ni ~2+5)+-- Ni 2+ (0~ 2oi r O
o
r
r
r
#
C
80
= .= 40 t r
0 i
.........
0
2
4
, r
c~nvemionlCsFePMo12
I
conversion/CsFePMo11V
"
=
r
~ .....
+
6
8 time(h;O
I
--I--selectiv~lCsFePMo12~
C
selectivity/CsFePMo11~
Fig.5: Cyclohexane conversion and selectivity of benzene as a function of time reaction over Cs2.sFeo.08PMol2 and Cs2.sFeo.08PMol~V catalysts. Reaction conditions: T=400~ N2/O2=1.
,
20"
o
i i
L
,
r A
---
i
1O0 200 conversion -~ benzene selectivity !
r
300 time(mn)
Fig.6: Cyclohexene conversion and selectivity of benzene as a function of time reaction over Cs2.sFeo.08PMol2 catalyst. Reaction conditions: T=400~ N2/O2=1.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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4. CONCLUSION
In conclusion, the HPA exhibit high catalytic activities for dehydrogenation of cyclohexane. On these catalysts, the product distribution depends on operating conditions and the composition of the catalyst, which suggest that the nature of counter-ion and oxometal affects the properties of the catalyst. Heteropolyacid and heteropoysalts catalysts show interesting differences, which could be related to the different redox and acid-basic properties. The higher benzene selectivities on HPA can be the result of the simultanous presence of several metals (FeMo, FeMoV or MoNi ..... ) in the catalyst at different oxidation degrees. REFERENCES
1. J. B. Moffat, Appl. Catal. A : General 146 (1996) 65-86. 2. N. Mizuno, M. Tateiski, M. Iwamoto, App. Catal.; A. General 128 (1995) L165-L170. 3.W. Li and W. Ueda., 3rd World. Cong. on Catal., Elsevier (1997) 433. 4. W. Ueda, Y. Suzuki, W. Lee and S. Imaoka; 11th. Cong. on Catal., Stud. Surf. Sci.Catal., Elsevier, Vol. 101 (1996), 1065. 5. H. D. Lanh, NG. Khoai, H.S. Thoang and J. Volter, J. Catal. 129, 58-66 (1991). 6. J. Fung and I. Wang, J. Catal. 164, 166-172 (1996). 7. R. C. Chang and I. Wang, J. Catal. 107, 195-200 (1996). 8. W. Russel,W. Maatman, P. Mahassy, P. Hoekstra and C.Addink, J. Catal., 23, 105-117 (1971). 9. M.C. Kung and H.H. Kung, J. Catal. 128, 287-291 (1991) 10. R. Maggior, N. Giordano, C. Crisafulli, F. Caastelli, L. Solarino and J.C.J.Bart, J.Catal. 60, 193-203 (1979) 11. G.A. Tsigdinos and C. Hallada, Inorg. Chem., 7, (1968) 437 12. C. Rabia, M.M. Bettahar, S. Launay, G. Herve, M. Foumier, J. Chem. Phys., 92, 14421456 (1995) 13. S. Hocine, C. Rabia, M.M. Bettahar, M. Foumier, submitted for publication. 14. C. Rocchiccioli-Deltchef, R. Thouvenot, R. Franck, Spectrochim. Acta., 32A, 587, (1976). 15. C. Rocchiccioli-Deltchef, R. Thouvenot, J. Chem. Reasearch, 546, (1977). 16. M. Foumier, C. Feumi-Jantou, C. Rabia, G. Herv6, S. Launy, J. Mater. Chem., 2, 971, (1992).
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
T r a n s i e n t r e s p o n s e s t u d i e s of i s o b u t a n 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 over m o l y b d e n u m c a t a l y s t s N. V. Nekrasova), N. A. Gaidai a), Yu. A. Agafonova), S. L. Kipermana), V. Cortes Corberfin h) and M. F. Portela c) a) N. D. Zelinsky Institute of Organic Chemistry, R.A.S., 47 Leninskii Prospekt, 117913 Moscow, Russia b) Instituto. de Catfilisis y Petroleoquimica, CSIC, Campus UAM Cantoblanco, 28049 Madrid, Spain c) GRECAT, Instituto Superior Tecnico, Av, Rovisco Pais, 1096 Lisbon, Portugal Kinetics and mechanism of isobutane oxidative dehydrogenation were studied over cobalt and nickel molybdate catalysts. The data obtained in nonstationary and stationary regimes showed that kinetics and mechanism are the same over both catalysts. Isobutene and carbon oxides are primary reaction products. Lattice oxygen takes part in dehydrogenation reactions. Carbon oxides formation proceed by the interaction with adsorbed oxygen. Cobalt molybdate is more active and selective catalyst for isobutane oxidative dehydrogenation. It was shown t h a t nickel molybdate catalyst is stable only at high oxygen concentration while cobalt molybdate catalyst can work at lower oxygen concentrations. I. I N T R O D U C T I O N Many studies are carried out in the last years on the light alkane oxidative dehydrogenation (ODH) as ODH is an attractive way for olefin production owing to the absence of thermodynamic limitations. Most of the studies are devoted to developing new catalysts and much less attention was paid to the kinetics and, especially, the dynamics of the process. It is known that the study of the dynamic response can get valuable information about reaction mechanisms [1-3]. The works devoted to studies of isobutane ODH mechanism are practically absent. TAP reactor was used for investigation of propane ODH over V-Mg-O catalyst [4]. It was shown t h a t partial and total oxidation of propane occur on the same surface site but involve different forms of reactive oxygen: nucleophilic lattice oxygen takes part in the propane partial oxidation to propene, while adsorbed electrophilic oxygen is involved in the total oxidation process of propane. The transient response method was used in the study of the ammoxidation of propene and propane over an Sb-V-oxide [5]. It was shown there t h a t ammoxidation of
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1902
propane to acrylonitrile occurs t h r o u g h the O D H of p r o p a n e a n d the reaction scheme w a s proposed for this process. However, the t r a n s i e n t r e s p o n s e s were obtained only for the r e p l a c e m e n t of inert gas by reaction m i x t u r e s . In this s t u d y we have i n v e s t i g a t e d t r a n s i e n t processes d u r i n g the a t t a i n m e n t of a s t e a d y s t a t e u n d e r different conditions to obtain the detailed i n f o r m a t i o n for m e c h a n i s m of isobutane ODH over m o l y b d a t e c a t a l y s t s (C00.95M004 a n d aNiMoO~) in a wide i n t e r v a l of process p a r a m e t e r s . Besides, the reaction kinetics of this process was studied u n d e r s t a t i o n a r y conditions over these c a t a l y s t s . 2. E X P E R I M E N T A L
The e x p e r i m e n t s u n d e r n o n s t a t i o n a r y conditions were carried out in special g r a d i e n t l e s s reactors of small volume, connected to a time-of-flight massspectrometer. T r a n s i e n t response m e t h o d [1-3] w a s applied in this case. The relaxation curves describing a t r a n s i t i o n of the s y s t e m to a new s t e a d y s t a t e were obtained after an j u m p w i s e change in the corresponding r e a c t a n t concentrations. A vertical line m e a n s a change of reaction conditions; the r e l a x a t i o n curve is r e l a t e d to the last change of these ones. Since the composition of the reaction m i x t u r e s leaving the reactor was a n a l y z e d every 1 s, the response curves for the time scale indicated in the figures i l l u s t r a t e n e a r l y continuous processes. Initial m i x t u r e s of specified composition such as (O~+C4Hlo+N2), (N2+C4H10), (O~+Ne) were p r e p a r e d by mixing the flows of the corresponding gases, purified and dried prior to the experiments. The residence time, defined as the ratio of the volume of the r e a c t i n g s y s t e m to the flow rate, did not exceed 2 s. It w a s considered at the t r e a t m e n t of the e x p e r i m e n t a l data. For all e x p e r i m e n t s the r e l a x a t i o n time w a s lower t h a n the t u r n o v e r time. This allows to characterize the observed r e l a x a t i o n s as intrinsic ones (i.e., those d e t e r m i n e d only by the reaction m e c h a n i s m ) [6, 7]. The m/e ratios employed were as follows: 15 (methane), 18 (water), 32 (oxygen), 43 (isobutane), 44 (carbon dioxide), 56 (isobutene). Kinetic e x p e r i m e n t s u n d e r s t e a d y - s t a t e conditions were carried out in g r a d i e n t l e s s units at a t m o s p h e r i c pressure. B l a n k r u n s (with a n d w i t h o u t quartz) showed t h a t an exit of the reaction into the volume as well as the homogeneous reaction can be neglected u n d e r e x p e r i m e n t a l conditions. The m a i n reaction products were isobutene, carbon oxides, low hydrocarbons a n d w a t e r . The i n t e r v a l of process p a r a m e t e r s was c h a n g e d in the following ranges: t e m p e r a t u r e 470 - 560 ~ P,:so~,,t,,,: = 0.08 - 0.50, Po.~ = 0.032 - 0.18 a t m . Total conversion (x) and isobutene selectivity varied in the i n t e r v a l s 0.02 - 0.26 a n d 0.70 - 0.95 on CoMoO4. Analogous p a r a m e t e r s on NiMoO4 were 0.05 - 0.20 and 0.50 - 0.80, respectively. 3. R E S U L T S A N D D I S C U S S I O N F i g . l a i l l u s t r a t e s the response w h e n the reaction m i x t u r e w a s i n t r o d u c e d onto CoMoOl a n d NiMoO~l c a t a l y s t s p r e v i o u s l y t r e a t e d by air. The r e l a x a t i o n
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
1903 0,3
化
0,5
a
0,4
-~
0,3
:~ g
0,2
~
0,2
0,1
0,1 0,0
JF
l
T
,
0
10
20
30
'
40 0
5
1
0
0
10
I
20
S
0
10
20
II
Fig.1. The change of isobutene concentration with the time (s) in the responses" (O2+N2)/(O2+CnHlo) (a) and (O2+C4Hlo)/(O2+N2) (10) over: I) CoMoO_l, 490 ~
Poe / Pb aH,o
0.2; II) NiMoO4, 520 oC
No2 " / Pb 4H,0
= 1.0.
curve has a delay, which can be attributed to some time necessary for isobutane adsorption, and a maximum. This m a x i m u m in the concentration of isobutene at the initial stage of the process is due to the participation of the highest amounts of lattice oxygen at the beginning of the process. Thus, this type of oxygen takes part in isobutene formation. The form of relaxation curves in this case is the same on both catalysts, but for the reverse response (O2+C..,Hlo)/(O2+N2) (see Ib, IIb) the relaxation curve has a m a x i m u m over NiMoOl at a variance of the response over CoMoO4. This can be connected with a higher heterogeneity of Ni-catalysts as compared with Cocatalysts. This heterogeneity of NiMoO4 is further d e m o n s t r a t e d in the response (N2+C4H10)/(O2+C4Hlo) (curves not shown). The change of isobutene concentration is monotonous in the response (N2+C~,H10)/(O2+C4Hlo) over CoMoO4 while it shows a m a x i m u m over NiMoO4. These two relaxation curves have no delay in these responses as in this case no time is needed for the isobutane adsorption. The relaxation curve of isobutene formation observed in the response O2/N2/(O2+C4Hlo) over both catalysts is monotonous after the intermediate blowing off by nitrogen during 1 minute. It means t h a t concentration of reactive lattice oxygen is decreased during the nitrogen t r e a t m e n t owing to a decrease of adsorbed oxygen concentration. The form of relaxation curves in the tests where the introduction of one reactant is shifted from one to the other (e.g. in the response (O2+N2)/(N2+C4Hlo) or the reverse one) confirms the participation of lattice and adsorbed oxygen in the dehydrogenation reaction and CO2 formation, respectively. The relaxation curves of CO2 formation in the responses (O2+N2)/(O2+C4Hlo)
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
1904
2
0,8
化
a
1,5
0,6
a
b
5 a/ 1
C~ o 0,4
r
0,2
0,5
Y
0
35
s I
II
Fig.2. The change of carbon dioxide concentration with time (s) in the responses: (O2+N2)/(O2+C~H10) (a) and (O2+C4Hlo)/(O2+N2) (b) over: "("H,o - 0.2; II) NiMoO4, 520 oC, " " -1.0. I) CoMoO,, 512 oC, P ~
po/PcH,o
over CoMoO.~ and NiMoO ~are p r e s e n t e d in Fig.2. The characteristic curves of the CO2 concentration in the responses (O2+N2)/(O2+C4Hlo) had a m a x i m u m over CoMoO~ and reached the s t a t i o n a r y level monotonously over NiMoO4. The absence of a m a x i m u m over Ni-catalyst is connected with the excess of oxygen in the reaction mixture (a fivefold higher concentration t h a n in the case of Cocatalyst). In the reverse responses (O2+C4Hlo)/(O2+N2) both curves have a m a x i m u m which is much higher in the case of NiMoO~. It can be explained by hydrocarbon reduction and coke formation on catalyst surface, m u c h higher on Ni-catalysts. The oxygen introduction results in coke burning. This m a x i m u m became much lower with a higher oxygen concentration in the reaction m i x t u r e (P"02/ P"isobutane : 2 . 0 ) . Therefore, an excess of oxygen is n e c e s s a r y for keeping stable the performance of Ni-catalysts in ODH of isobutane. Similar results were obtained in [8] for ODH of propane over NiMoO4 The concentration of CO2 in the response (O2+C.IH10)/(O2+N2) (b) is higher t h a n the one in s t a t i o n a r y state (a). It m e a n s t h a t the oxygen concentration in the reaction m i x t u r e (O2+C4H10) is mostly consumed in filling up the oxygen vacancies or in w a t e r formation but not in coke burning. Therefore the filling up the oxygen vacancies proceeds more quickly t h a n coke burning. A complex s t r a t e g y [9-12] to obtain justified models u n d e r s t a t i o n a r y conditions and to prove their adequacy was used in this investigation. We applied the methods of previous analysis to t r e a t the kinetic d a t a obtained: the dependence of reaction rates on dilution [13], a change of process selectivity in dependence on operation conditions [14]. It was shown t h a t overall order in the n u m e r a t o r in kinetic equation for carbon dioxide formation is higher t h a n the power of the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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denominator and the order of the n u m e r a t o r in the kinetic equation for light hydrocarbon formation is equal to the power of the denominator. The following kinetic equations describing different routes of the process over both the catalysts under investigation were found: 1) isobutane formation:
Pc4H,oPo2 r, - k, 1-"o.~ + k, t"-'C4H,o
(1)
2) carbon dioxide formation: 0.5
(k,, PC4H,o +k',, PC4H~+k'i, P c o ) Po~ 1" tl =
NO.5
02 + k~
PC4H8
(2)
3) carbon monoxide formation:
r., - k,.
PC4H~P"o~
o~ Po2 +k2 PC4H~
(3)
4) cracking products formation:
r , . - k , . D..~
PC4H,o
~0~ +k~ PC4H~
(4)
The kinetic data are in accordance with the reaction mechanism proposed in the nonstationary investigation: isobutene formation is characterized by redox mechanism (isobutane reduces the catalyst surface which can be oxidized by oxygen), COz is formed from isobutane, isobutene and CO, formation of CO proceeds mainly from isobutene and cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. The observed rate of isobutene formation (rj) taking into account its transformation into carbon oxides can be described by the following equation: F/- r / -
[(k'.+k,.)Pc4HJPo~ _o
5
Po~ +k2 PC~H~
(5)
The equations (1)-(5) described all the transformations observed in this system. The comparison of isobutene formation rates over the two catalysts shows that it is 1.5 - 2.0 times higher over CoMoO4. This catalyst is more selective for isobutene formation and it needs lower oxygen concentration to keep up a stable performance t h a n NiMoO4, which needs a large excess of air to be stable.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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4. C O N C L U S I O N
The kinetic data of processes taking place under stationary and unstationary conditions over CoMoO4 and NiMoO4 show that the kinetics and the mechanism for ODH of isobutane are the same over both catalysts. The following mechanism was proposed: isobutene formation proceeded according to the redox mechanism, CO2 is formed from isobutane, isobutene and CO; formation of CO proceeds mainly from isobutene; cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. However, both catalysts differ in their catalytic performance and stability. CoMoO~ is a more active and selective catalyst for ODH of isobutane than NiMoO4. Furthermore, NiMoO.l needs a large excess of oxygen to keep its catalytic activity stable, while CoMoO4 can operate at much lower oxygen-tohydrocarbon ratios (air/isobutane ratio = 2) with no change in stability. As a consequence, productivity of CoMoO~ catalyst can be larger than that of NiMoO4 at equal total space velocity. ACKNOWLEDGEMENTS The authors acknowledge INTAS (N 96-1117) for financial support. REFERENCES
1. C. O. Bennet, Cat. Rev.- Sci. Eng., 13 (1976) 121. 2. H. Kobayashi and M. Kobayashi, Cat. Rev. - Sci. Eng., 10 (1974) 139. 3. M. Kobayashi, Chem. Eng. Sci., 37 (1982) 393. 4. A. Pantazidis, S. A. Bucholz, H. W. Zanthoff, Y. Schuurman and C.Mirodatos, Catal. Today. 40 (1997) 207. 5. R. Nilsson and A. Anderson, Ind. Eng. Chem. Res., 36 (1997) 5209. 6. M. I. Temkin, Kinet. Katal., 17 (1976) 1095. 7. F. S. Shub, A. G. Ziskin, M. G. Slin'ko and, M. I.Temkin, Kinet. Katal., 20 (1979) 334. 8. R. Rosso, A. Kaddouri, R. Anouchinskym, C. Mazzocchia, P. Gronch and P. Centola, J. Mol. Catal., 135 (1998)181. 9. S.L. Kiperman, Uspekhi Khim. (russ.), 47 (1978) 3. 10. S. L. Kiperman, D. M. Shopov, A. Andreev, N. E. Zlotina and B. S. Gudkov, Izv. Khim. Bull. of Chemistry, Bulg. Akad. Sci, 4 (1971) 237. 11. S. L. Kiperman, Chem. Eng .Commun., 100 (1991) 3. 12. S. L. Kiperman, Kinet. Catal. (English Edn.), 36 (1995) 7. 13. N. I. Koltsov and S. L. Kiperman, I. Res. Inst. Catalysis, Hokkaido Univ., 26 (1978) 85. 14. N. I. Koltsov and S. L. Kiperman, Teor. Exp. Khim. (russ.), 13 (1977) 630.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Olefins formation by oxidative dehydrogenation of propane over monoliths at short contact times V. A. Sadykov, S. N. Pavlova, N. F. Saputina, I. A. Zolotarskii, N. A. Pakhomov, E. M. Moroz, V. A. Kuzmin, A. V. Kalinkin, A. N. Salanov, I. G. Danilova, E. A. Paukshtis The Boreskov Institute of Catalysis, Siberian Branch of the RAS, Lavrentieva, 5, Novosibirsk, RUSSIA; 630090, Fax: 007(3832343766), E-mail:
[email protected] A specially designed tubular microreactor allowing to rapidly quench reaction products was used to test performance of Pt supported onto corundum micromonoliths in the reaction of propane oxidative dehydrogenation at short contact times. Promoters known as dehydrogenation catalysts (tin, zinc aluminate spinel, transition metal pyrophosphates) were used and reaction mixture composition was tuned to increase propylene yield. FTIRS data on propane interaction with a model Pt/SiO2 system helped to analyze the possible routes and limits of propylene yield improvement. 1. INTRODUCTION Last years, autothermal oxidative dehydrogenation of paraffins over Pt-containing foam monoliths at temperatures in the range of 900 -1000 ~ was shown to be quite efficient in olefins production at short contact times [1, 2]. The most important side reactions leading to decrease of higher olefins yield are cracking and complete combustion. In this work, to change catalytic properties of platinum in propane oxidative dehydrogenation, its combination with non-metallic dehydrogenating promoters (transition metal pyrophosphates, SnO2 and zinc aluminate) as well as lower operating temperatures were used. To reduce probability of propylene secondary reactions (oxidation, cracking, steam reforming), straight channel thin wall corundum and zinc aluminate monolithic supports were used. A special reactor design was elaborated to ensure a rapid quenching of the reaction products thus minimizing secondary gas-phase reactions. For tuning the feed mixture composition, propane/oxygen ratio was varied and water and hydrogen were added. To elucidate the roles of the active component and support in propane transformation, FTIRS studies of propane adsorption on the model Pt/SiO2 system have been carried out. 2. EXPERIMENTAL As supports, proprietary corundum monoliths (surface area - of 5-10 m2/g) annealed at 1300 ~ and bulk zinc aluminate monolithic supports (surface area 15-50 m2/g ) o f - 16 mm diameter with wall thickness 0.25 mm and channel sizes ca 1 mm were used. To suppress the inherent acidity of alumina and decrease microporosity, a mullite surface layer was formed using impregnation of support with silica sol followed by drying and calcination at 1300 C.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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Pyrophosphates sols were prepared by mixing Co or Mn nitrate and acid ammonium phosphate solutions. Corundum micromonoliths were impregnated by sols, purged by air to remove their excess and calcined at 700 ~ thus ensuring pyrophosphate content-~ 5 wt. %. Pt was supported onto monolithic supports via incipient wetness impregnation with aqueous solutions of H2PtC16. Sn was added either to a-A1203 by impregnation with solution of [Pt(SnCI)3C12]2 complex in 2M HC1 [3] or to Pt-containing catalysts calcined at 700 ~ by impregnation with SnC12 solutions. After impregnation, all samples were dried and calcined at 700 ~ Pt and SnC12 concentration in the solution were chosen in such a way as to ensure 0.52.5 wt. % Pt loading and a required Pt: Sn ratio. Usually, samples of micromonoliths with lengths in the range of 2-20 mm were used. Model sample 2 wt. % Pt/SiO2 was prepared using incipient wetness impregnation of high purity silica support (specific surface 300 m2/g) by H2PtC16 solution followed by drying and air calcination at 500 C for 4 h.. The phase composition of catalysts was studied by XRD (URD -63 diffractometer, Cu I ~ radiation). Pt particles sizes were estimated from the diffraction lines broadening using the Scherrer equation. The surface composition of catalysts was characterized by XPS. Photoelectron spectra were recorded for samples dusted onto an adhesive tape without any pretreatment using a VG ESCA-3 spectrometer and A1 Kot radiation. The surface texture of catalysts was studied by SEM (a BS-350 "Tesla" microscope) A specially designed quartz tubular reactor allowing to independently preheat a feeding mixture flow up to desired temperature (usually ca 300 ~ and control the monolith temperature was used for catalysts testing. Pieces of monolithic catalysts with diameters ca 16 mm wrapped in silica-alumina fiber cloth for insulation and preventing reactant bypass were placed into the reactor. A piece of corundum monolith was usually situated up-stream of the catalyst. Catalysts were pretreated in H2 at 700~ for 2 h. To take probes of the gas phase immediately after catalyst, a tube sampler kept at constant temperature to prevent water condensation and stop any homogeneous reactions was used. The catalyst temperature was measured by a thermocouple inserted into the plugged up monolith channel. Inlet and outlet compositions were analyzed by GC, conversion and selectivities were calculated on the carbon atoms basis. Within the usual limits of GC uncertainty (+ 10%), the carbon balance was closed. In all experiments, 02 conversion was shown to be complete. Propane adsorption on Pt/SiO2 was studied using wafers with the densities-10 mg/cm 2 and an IR cell allowing thermal treatments in the controlled atmosphere. The samples were pretreated in 02 (100 Torr) for l h at 773 K, then cooled 298 K and evacuated up to 10-3 Torr. Propane (20 Torr) was added at 298 K followed by sample heating up to 400 ~ IR spectra were recorded using an IFS-113v spectrometer. 3. RESULTS AND DISCUSSION
3.1. Samples texture, phase composition and Pt dispersion Some data on samples composition and properties are presented in Table 1. In some samples, metallic Pt phase was not registered by XRD either due to its low content or small particle sizes. XPS is not able to detect Pt due to superimposition of its characteristic 3d5/2 line with the support A1 2p line. For P-3 sample prepared using Pt-tin chloride complex, too low Sn content as judged by XPS can be explained by a partial sublimation of SnC14 during air calcination [3]. As a result, after reaction only Pt phase represented by rather big particles was
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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Table 1. Samples composition Sample
Support
Pt, wt.%
P-1 P-2
ot-A1203 oc-A1203 a
3 2
P-3 P-4 P-5
Phase composition c
d
Pt, S n O 2
Pt3Sn+ PtSn;SnO2 Pt Pt3Sn
Pt particle size, A
S~A1 ratio, c/d
200
0.1/0.29
c~-A1203b 2 Pt 360 0.043/0.017 ZnA1204 b 0.5 150 0.56/9 Co2P207/ 0.5 ot-A1203 P-6 MnzP207/ 0.5 ot-A1203 a-Atomic ratio Pt/Sn-1:7, tin was added after Pt supporting; b- Atomic ratio Pt/Sn-1:3, Pt-Sn complex was used; c-before reactions; d- after reaction. observed, and aggregation of tin oxide particles was suggested by a decline of the Sn/A1 ratio. Such a sublimation appears to be suppressed when more basic support is used (sample P-4) as revealed by higher Sn/A1 ratio and formation of Pt-Sn alloy after reaction. Fig. 1 shows typical SEM images for P-2 sample in the initial (oxidized) state and after prolonged working in propane-rich feeds.
,~~ ~ a
b
c
d
i
Fig. 1. SEM images of P-2 sample in the initial (oxidized) state (a, b) and after prolonged working in rich feeds (c, d). a, c-general view of the monolith wall texture; b, d- active component fixed on corundum particle (b) and graphite-like platelet (d).
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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Two features are to be stressed: (i) after reaction, the active component particles appear not to be blocked by carbon; in fact, they are now supported on carbon (presumably, graphite) platelets of--- 10 mkm thickness not contacting with the initial support; (ii) in reaction media, recrystallization of the active component proceeds forming rather bulky polycrystalline dendrites of Pt-Sn alloy (see XRD data). Some increase of Sn/AI ratio after reaction (Table 1) can be explained by a screening effect of the carbon layer on AI signal. Hence, carbon build-up certainly proceeds on support, while particles of the active component appear to be clean, presumably, due to their efficiency in carbon gasification by CO2 and H20 even in the absence of oxygen in the gas phase.
3.2. Effect of platinum modification by tin dioxide and support As follows from Table 2, at 1"1 reaction mixture composition, SnO2 has a promoting effect as regards propylene selectivity. This result agrees with the data of Schmidt et al for the Table 2. Effect of tin on performance of Pt supported catalysts Sample
Tcat~
Contact time, sec
Selectivity, %
C3H8conversion, %
P-1 835 0.03 P-2 850 0.1 P-3 840 0.1 P-4* 880 0.1 *SupPort specific surface 50 m2/g
63 47 62 33
CH4 18 11.7 14.5 5.2
C2H4 34 23.5 29.5 12.4
C3H6 17.5 26.4 20.6 25.3
CO 21 2.5 15.2 10
CO2 2.7 28.5 11.5 45
reaction of ethane oxidative dehydrogenation [4]. As judged by decreased methane selectivity, tin promotion helps to decrease propylene cracking. However, promotion by tin has a drawback in decreasing ethylene yield mainly due to a higher carbon dioxide selectivity, especially for P-2 catalyst with a higher tin content and P4 catalyst supported onto ZnA1204 (Table 1, 2). Hence, SnOx addition increases surface oxidation potential. In the case of P-4 sample, very low methane selectivity suggests basic support helps to suppress propylene cracking catalyzed by the surface acidic centers [5]. The highest yield of CO2 for this sample can be explained by too high specific area of this support. As the result, some intra-pore oxidation of olefins can occur.
3.3. Effect of feed composition A substantial gain in propylene selectivity was achieved by tuning propane/oxygen ratio (Fig. 2). For 3"1 mixture, CO2 formation was suppressed, while no channels plugging or performance deterioration with time were observed. Simultaneous increase of both reagents concentration provided their ratio being kept constant was found to increase further propane conversion and ethylene selectivity, while propylene selectivity improved only slightly. At a constant feed mixture preheat, water addition up to 40% was found to decrease the monolith temperature by about 100 ~ without changing propane conversion.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1911
.. 7O I o~. 60~9 501 40 ~ 30 o 20 ~ 10-
o 0
0
"=4
.7
110
1"5 210 215 310 propane/oxygen ratio
Thus, for P-2 catalyst, at 21,000 GHSV and propane/O2 1"1 mixture, H20 increased propylene selectivity from 20 to 30%, while ethylene selectivity remained unchanged. Concomitant decline of methane and carbon dioxide selectivities (from 6.8 to 4.6% and from 53 to 45%, resp) was observed evidencing suppression of cracking and deep oxidation reactions as the reason for improving propylene yield. Hydrogen addition was found to be also beneficial for olefins production. Thus, on P-2 catalyst at 850 ~ addition up to 10% H2 to feed containing 21% C3H8 and 18 % 02
Fig. 2. C3H8 conversion (1), C3H6 (2), C2H4(3) in N2, increased propane conversion from 45 and CO2 (4) selectivities vs. C3H8/O2 ratio, to 60%, integral olefin selectivity was raised 700 ~ 22 vol. % C3H8. P-2 sample, up to 60% at the expense of carbon oxides selectivity decline from 30 to 20%.. 3.4. Feed tuning to maximize propylene selectivity
For the most promising systems, to maximize propylene selectivity, water and hydrogen were added simultaneously to propane-rich feed. Table 3 shows the results of these experiments. Table 3. Performance of catalysts in propane-rich mixtures (20 vol. %) and H20 Catalyst
H20, vol. %
GHSV, 103h-1
Tcat ~
(C3H8/O2-3)in the presence of H2
Propane cony.,%
P-2 34 660 32 P-4* 38 26 598 20 P-5 40 14 560 18 P-6 38 14 560 13 * Specific surface of zinc aluminate support 16 m2/g
Selectivities, %
CH4 5.4 5.6 2 3
C2H4 18 20 18 19
C3H6 53 72 69 71
CO/CO2 1.7/22 2.5/0 11/0 7/0
As follows from these results, very high integral olefins selectivity (ca 90%) was obtained. In the presence of water, CO2 was absent in the products thus suggesting secondary reactions of olefins oxidation can be efficiently suppressed. Moreover, low methane selectivity indicates suppression of propylene cracking. Nevertheless, ethylene selectivity remains appreciable suggesting its formation in part via the primary reaction of propane activation on Pt. Meanwhile, CO selectivity can be assigned to gasification of the surface carbon by water.
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3.5. IR studies of propane transformation on model Pt/SiO2 catalyst
Propane adsorption at 298 K on oxidized sample was accompanied by appearance of bands at 2960, 2925, 2850, and 2800 cm l with simultaneous decrease of band at 3735 cm 1 corresponding to terminal Si-OH groups. It suggests appearance of Si-O-CnHn+I alkoxy groups due to interaction between terminal hydroxyls and propyl radical formed via homolyric activation of propane on oxidized Pt. Bands assigned to surface formates (1580 cm ~ COO asymmetric stretching; 1370 cm 1, COO symmetric stretching) [6] were also observed. After sample heating to 200-250 ~ C2H4 (v (-C-H) bands at 3025 and 3100 cm !) and C3H6 (v (=C-H) bands at 3010 and 3085 cm l ) [7] were detected in the gas phase. Intensity of bands at 1508, 1540, 1615 cm l (8(HOH) or v(C-C)), 1730 cm ~ (v(C-O) in adsorbed aldehyde or acetone) was increased. At 400 ~ acetone or acetaldehyde (band 1710 cm -1, v(C-O) ) also appears in the gasphase along with propylene and ethylene, while surface coverage by these species increases. These results suggest that both propylene, ethylene and formate species are the primary products of propane transformation on oxidized platinum. Hence, in all conditions, some yield of ethylene is to be observed determined by probability of [3 C-C scission in propyl radical [1, 2]. Carbon oxides seem to be secondary products of formate transformation in the presence of gas-phase oxygen, and their formation can be suppressed by retarding activation of molecular oxygen. Without oxygen, oxygenates such as acetone and aldehydes are formed, and, along with alkyl radicals, they may be important for catalyst-supported homogeneous process. If formation of alkoxy-groups effects the yields of products including coke, the acid-base properties of support can be of a great importance in propane oxidative dehydrogenation. 4. CONCLUSIONS For the reactions of propane oxidative dehydrogenation at short contact times on Pt supported monolithic catalysts, propylene yield was shown to be improved due to quenching of the reaction products, Pt modification by typical dehydrogenation compounds and feed tuning. FTIRS study of propane activation on supported Pt helps to understand these data. Acknowledgments. The financial support of the Engelhard Corporation is gratefully acknowledged. Authors are indebted to R. J. Farrauto for valuable discussion.
REFERENCES 1. 2. 3. 4. 5.
M. Huff. and L.D. Schmidt, J. Catal., 149 (1994) 127. L.D. Schmidt and C. T. Goralski. Studies Surf. Sci. Catal., 110 (1997) 491. N.A. Pakhomov and R. A.Buyanov, Kinetika i kataliz, 22 (1981) 488. C. Yokoyama, S. S. Bharadway, and L. D. Schmidt. Catal. Lett., 38 (1996) 181 B.C. Gates, J. R. Katzer, and G. C. A. Schuit, Chemistry of Catalytic Processes, McGrawHill Book Company, 1979. 6. A.A. Davydov. Infrared Spectroscopy of Adsorbed Species on Surface of Transition Metal Oxides, Wiley, Chichester, 1990. 7. L.M. Sverdlov, M. A. Kovner, E. P. Kraynov, Vibrational Spectra of Multiatomic Molecules, Nauka, Moscow, 1970.
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1913
化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Experimental and Theoretical Investigation on the Roles of Heterogeneous and Homogeneous Phases in the Oxidative Dehydrogenation of Light Paraffins in Novel Short Contact Time Reactors A. Beretta, E. Ranzi, P. Forzatti Dipartimento di Chimica Industriale e Ingegneria Chimica, Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano - Italy -*e-mail:
[email protected] The role of a Pt/AI203 catalyst in the oxidative dehydrogenation of light paraffins was investigated by means of a novel annular reactor which allows to realize very high space velocities and controlled temperature conditions. While a number of previous papers claimed that Pt is extremely active in the selective oxidation to olefins, the experiments in the annular reactor showed that the present noble-metal catalyst is highly active in the non selective oxidation to COx, HE and H20 only. Gas-phase experiments and simulations of a homogeneous reactor showed that the formation of olefins at high temperature and short contact times can be largely explained by the gas-phase oxidative pyrolysis. However, autothermal experiments demonstrated that the catalytic phase can be exploited in an adiabatic reactor for igniting and thermally supporting the pyrolytic process; the combined catalytic-homogeneous reactor performed as a homogeneous reactor, but the presence of the catalyst allowed to produce high yields to oleflns at much lower reactor volume and lower pre-heat temperatures. 1. INTRODUCTION Oxidative dehydrogenation of light paraffins ranks among the novel routes for the chemical conversion of natural gas to valuable chemicals, as it represents a potential altemative to the traditional endothermic processes for the production of short-chain olefins. Very high selectivities to olefms were reported by Schmidt and co-workers [1, 2] in the selective oxidation of light alkanes over Pt-coated foam monoliths, at extremely short contact times (1-10 ms). In spite of the high reaction temperatures related to the adiabatic operation of the foam monolith (700-1000~ possible contributions from homogeneous reactions were ruled out and a thoroughly heterogeneous mechanism was proposed for the synthesis of propylene and ethylene. In this work, a study of ethane oxidative dehydrogenation over a structured Pt/y-Al203 catalyst is addressed and a comparison with previous results of propane oxidative dehydrogenation [3, 4] is made. Catalytic tests were performed at short contact time under both isothermal and adiabatic conditions. Gas-phase experiments were performed as well. The results were compared with the predictions of a detailed kinetic scheme of hydrocarbon oxidative pyrolysis [5] about the expected performance of a purely homogeneous reactor. New
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1914
pieces of evidence were thus obtained on the roles that catalytic phase and gas phase play in the formation of olefins at high temperatures and short contact times.
2. EXPERIMENTAL AND MODELING Catalyst - The catalyst used in the present study was a commercial 5% Pt/y-A1203 system (Engelhard, ESCAT 24). In the annular reactor, the catalyst was tested in the form short (1-5 cm) and thin (50-100 ~tm) layers, deposited onto a mullite tube. Annular reactor - The annular reactor consisted in a 60 cm long quartz tube (Figure 1) wherein the catalyst-coated ceramic tube was inserted. The mullite tubes had a diameter of 4.75 nun, while a 9 mm ID quartz tube and a 7 mm ID quartz tube were used in the catalytic and homogeneous experiments, respectively. Special Cajon fittings were used to keep inner (ceramic) and outer (quartz) tube in coaxial position and to connect the resulting annular chamber to the inlet and Quartz tube outlet lines. The axial Gas Flow ~, temperature profile of the ] Cat. [ Mullite tube reactor was measured from inside the same mullite tube, which was exploited as themocouple-well. Figure 1 - Schematic picture of the annular reactor Millisecond-contact time adiabatic reactor - The adiabatic tests of oxidative dehydrogenation of propane and ethane were instead realized by depositing the catalyst onto Fecralloy fibers and by packing the coated metallic elements in a 7 ID mm quartz tube. Guardbeds of ceramic particles were placed upstream and downstream from the catalytic portion in order to guarantee the insulation along the axial direction. Additional inert material was wrapped around the quartz tube to prevent heat dispersion in the radial direction. The axial temperature profile was measured by sliding a thin thermocouple along a stainless-steel well, placed inside the reactor. Homogeneous reaction scheme - The detailed reaction scheme herein used for the simulation of ethane (and propane) partial oxidation has been deeply described and discussed elsewhere [5]. The model involves more than 150 species in about 3000 elementary and lumped reactions and it has been widely validated for partial oxidation and combustion processes. It consists in a hierarchical and modular structure, which allows easy extension to analyze the effect of new species and new conditions.
3. ACTIVITY TESTS IN THE ANNULAR REACTOR In a previous work, some of the authors have investigated the role of Pt/A1203 in the oxidation of propane [3]. By varying the fiu~ace temperature from 200~ to 700-800~ and operating at short contact time with respect to the catalyst phase, it was found that from low to medium temperature (200-500~ only products of deep oxidation were formed. At higher temperatures, olefins were formed with selectivities as high as 60%. However, comparison with blank experiments and with the predictions of the detailed kinetic model showed that the production of olefins observed in the presence of the catalyst could be well explained by the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1915
single contribution of radical reactions in the empty volume of the reactor. Additional experiments were then performed under operating conditions which minimised the extent of homogeneous reactions; upon variation of the catalyst load, the production of COx, H20 and H2 increased accordingly, while the small residual formation of olefins due to gas-phase reactions decreased upon addition of increasing amounts of catalysts. The whole bulk of data seemed to suggest that the Pt catalyst was uniquely active in the total oxidation of propane to CO2 and H20, and (above 450~ in reforming routes responsible for the formation of CO and H2; on the opposite, no evidence of any activity of Pt/AI203 in the selective oxidation to propylene could be found. An analogous investigation has been herein addressed with regard to the oxidative dehydrogenation of ethane. Schmidt and co-workers reported in fact very high selectivities to ethylene by using Pt-coated A1203 foam monoliths under adiabatic conditions at 5 ms contact time [1 ]. The authors claimed that the formation of olefins had a purely heterogeneous genesis. Holmen et al. [6] on the opposite found that the oxidative dehydrogenation of ethane in the presence of Pt/Rh gauzes mainly proceeded in the gas-phase while the gauze was effective in igniting the homogeneous reactions. The Pt/A1203 catalyst was tested in the annular reactor under reference operating conditions with a high value of gas hourly space velocity GHSV = lxl06 L(NTP)/kgcat/h, and molar feed composition ethane/oxygen/nitrogen = l/l/4. The effect of temperature was investigated by spanning the oven T from 200~ to over 750~ Already at 200~ conversion of both reactants was observed with prevailing consumption of oxygen (Figure 2). The values of conversion kept almost constant up to 500~ above this T, a sharp increase of ethane conversion was observed; conversion of both reactants was complete at the highest temperature investigated. 100 e--(-
2
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' 600
'
760
'
800
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Figure 2 - ODH of ethane in annular reactor. GHSV =106 NL/kgcat/h. C2/O2/N2=1/1/4. At low oven temperature, CO2 and water were the only reaction products observed in the outlet stream. As shown in Figure 3, the selectivity of CO2 decreased at increasing temperature in favor of the formation of CO. Production of water decreased as well while the production of H2 progressively enhanced. At 550-600~ the temperature at which ethane conversion had the
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1916
100
....
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sharp increase, ethylene was formed. Its selectivity increased with T up to a maximum value at 650~ Methane was also formed above 600~
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Interesting pieces of evidence were provided by the temperature profiles of the annular reactor, 0 which changed significantly at 1 O0 200 300 400 5C)0 600 700 800 increasing oven T. Figure 4 Oven Temperature, *C compares the axial reactor profiles Figure 3 - Product distribution. Conditions as Fig. 2 with the longitudinal oven Tprofiles in the extreme cases of low and high heating T, in correspondence with the unique formation of combustion products, and the formation of ethylene, respectively. It is evident that while the low T formation of COx was characterised by the presence of an important hotspot in correspondence with the catalyst layer (located at the length 40-41.3 cm of the muUite tube), the high-T formation of olefins was characterised by the onset of a hot-spot directly at the entry region of the reactor well upstream from the catalyst bed, while the catalyst temperature did not provide evidence of any exothermic reaction. 9
20
methane 9 ethylene
-.
..
~,~ /!,
IJ
"
450 400
9 Treactor I 700" 9 Toven
~~
350 i
300
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9
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150 100
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,
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.
6'0
,
70
550
2o
.
3'0
,
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,
s'o
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7o
Figure 4 - Axial temperature profiles (T in ~ of the annular reactor and of the heating wall. Axial coordinate = length of the mullite tube. Catalyst layer is located between 40 and 41.3 cm. This was interpreted as an indication of the possible onset of gas-phase reactions at the high temperatures. In order to better quantify the role of the homogeneous oxidative pyrolysis of ethane, experiments were performed in the empty reactor, in the absence of catalyst. 4. R O L E OF T H E H O M O G E N E O U S
P H A S E : TESTS IN THE EMPTY REACTOR
AND MODEL SIMULATIONS The single effects of reaction temperature, contact time, feed composition were investigated. The experimental results were compared with the simulations of a purely homogeneous isothermal plug flow reactor, operating within the same conditions as those investigated experimentally.
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I,
0.9
.
1917 T=627~ C T=667"C
~
Ethylene
o 0.8 0.7 0.6
9
A
0.5) 0.4
L
0
10
20
30
40
50
60
70
80
T =
~o~
T = 670 *C 9 T = 725 =C 0 T = 650-725"C
t
9
O
90
Figure 5 - Calculated (left) and experimental (right) dependence of ethylene selectivity vs. % ethane conversion. Feed composition: ethane and air with C2/O2 = 1/1. Experiments and theory showed that gas-phase reactions were active above 600~ and were very selective towards olefins (with over 60% C-selectivity to ethylene at more than 70% ethane conversion), which are in fact primary products of alkane oxidation processes. In general, it was found that for a given feed composition, the product distribution of the gas-phase reactions mainly depends on the degree of alkane decomposition (either ethane or propane), despite of the operating conditions at which the reaction takes place. This is shown for instance in Figure 5 which illustrates, respectively, the calculated and experimental dependence of ethylene selectivity on ethane conversion in ethane oxidative dehydrogenation; experiments and calculations refer to different combinations of temperature and contact time. This unique dependence reminds of the characteristic behaviour of an intermediate species which is involved in consecutive reactions, wherein 0.9 the steps of formation and consumption have 0.8 comparable activation energies; ethylene is indeed 0.7 an intermediate species of the gas-phase oxidation 0.6 0.5 of ethane and the metathesis reactions through 0.4 \ I which the pyrolysis process proceeds have typically 0.3 small activation energies. 0.2 As a consequence, high selectivities to ethylene 0.1 ~ T=1027~ "[ = 0-10 ms J can be guaranteed up to 50-60% conversion of 00 10 20 30 40 50 60 70 80 90 100 ethane operating both at 600~ and contact times Figure 6 - Ethylene selectivity vs. ethane of 1 second, and 1000~ and contact times in the conversion. C2/O2/N2= 1/1/4 order of few milliseconds, as shown in Figure 6. The results of the model simulations and of the gas-phase tests were compared with those of the catalytic tests; additional experiments were then performed in order to study the sensitivity of the product distribution upon variation of the catalyst load under operating conditions which guaranteed a negligible contribution from gas-phase reactions. It was concluded that likewise the results of the previous work with propane/air mixtures [3, 4], the Pt/A1203 catalyst is much active in the total oxidation of ethane already at low temperature; the combustion reaction rate is so high that the process underwent mass transfer limitations in the structured annular reactor. Over 400-450~ the catalyst was also active in the production of syn-gas, likely due to steam or dry reforming reactions of the alkane.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1918
5. OXIDATIVE DEHYDROGENATION OF LIGHT ALKANES IN ADIABATIC REACTOR: A SYNERGYSM BETWEEN CATALYST PHASE AND GAS PHASE
It was thought that the Pt-catalyst, given its high combustion activity, could be exploited in an adiabatic reactor for igniting the desired gas-phase reactions, highly selective towards olefins. The expected performance of a purely homogeneous reactor was first theoretically analysed. Model predictions showed that autothermal operation is expected to guarantee the realisation of C-selectivities to olefins up to 55-60%, in correspondence with high alkane conversions (>60%). Autothermal experiments were then realised at millisecond contact times by using the Pt-coated Fecralloy fibrous support. The effects of alkane/O2 feed ratio, N2 content, flow rate, reactor design were investigated. Indeed, in the case of fuel-rich propane/air mixtures (C3/O2 > 2) nearly 50% total yield to ethylene+propylene was achieved [4]. In the case of ethane/air mixtures (C2/O2 = 2), over 63% selectivity to ethylene was observed at 71% ethane conversion. The lab-scale adiabatic reactor operated at 5 ms contact time and was preheated at 250~ Simulations showed that the same performance could be obtained from a purely homogeneous reactor at longer contact time (20 ms) or higher inlet temperature (about 600~ The presence of the Pt-catalyst thus accelerated ignition. It is finally observed that the catalyst might have an important role in minimizing the formation of coke; homogeneous tests indicated that this is highly favored under the present high temperatures and low oxygen contents, however stable activity of the Pt-containing autothermal reactor was observed along several days of operation. Preliminary results showed that ignition of the homogeneous reactions could be realized by using combustion catalysts other than the Pt/A1203. When using a BaMnAlllOl9 catalyst, however, coke formation occurred and the catalyst needed a daily reactivation. Further work is presently on-going in order to verify the possible relationship between the absence of coke and the activity of the Ptcatalyst in steam reforming reactions, which are not active over the BaMnAl~ ~O!9 catalyst. The results herein presented support the hypothesis according to which at high T and short contact times the oxidative dehydrogenation of light alkanes to olefins is mainly governed by gas phase reactions. In an adiabatic reactor, a highly active combustion catalyst such as the present Pt/A1203 system can be exploited for igniting the homogeneous process in the surrounding gas volume. Theoretical calculations and experimental demonstration of this concept showed that yields as high as 50% of short chain olefins can be realised without external heat input by exploiting the synergism between heterogeneous and homogeneous phases. This work was finanr supported by ENI and SNAMPROGETTI [ 1] M. Huff, L.D. Schmidt, J. Phys. Chem., 97 (1993) 11815. [2] M. Huff, L.D. Schmidt, J. Catal., 149 (1994) 127. [3] A Beretta, L. Piovesan, P. Forzatti, J. Catalysis 184 (1999) 455. [4] A. Beretta, P. Forzatti, E. Ranzi, J. Catalysis 184 (1999) 469. [5] E. Ranzi, P. Gaffuri, T. Faravelli, A. D'Anna, A. Ciajolo, Combust. Flame 108 (1997) 24. [6] R. Lodeng, O. A. Lindvag, S. Kvisle, H. Reiner-Nielsen, A. Holmen, Appl. Catal. A:General, 187 (1999) 25-31.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个
1919
化
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Composite nickel-oxide surfaces, modified by palladiun~ R.G.Baisheva, Z.K.Kanaeva, K.A. Zhubanov, Zh.K.Kairbekov Chemical Department AI-Farabi Kazakh State National University 95 Vinogradova St., Almaty, Kazakhstan 480012 The following is the study of influence of composite nickel-oxide (SiO2, TiO2, MgO, V205, and Fe203) electrodes' surface modification by palladium on their electrocatalytic activity and area of real nickel surface in the process of nitrobenzene electroreduction. Electrocatalytic activity of studied electrodes was estimated by the scale of nitrobenzene electroreduction stationary currents in the area of hydrogen evolution under-0.2B potential (reversible hydrogen electrode- r.h.e.). Stationary currents of nitrobenzene electroreduction on composite electrodes modified by palladium have grown 4-5 times. 1. INTRODUCTION Since recently, composite electrodes became widely used for conducting electrochemical reactions. Depending on nature of combining components, it is possible to obtain cheap composite electrocatalysts with sufficiently high electrochemical characteristics. Such variously composed electrodes have high and low overtension of oxygen and hydrogen evolution, which allows for conducting electrocatalytic synthesis of organic compounds with high speed and selectability. Presently such works are not many, which is why the study of electrocatalytic proiperties of composite electrodes under reduction processes of aromatic nitrocompounds is remarkably important for solving a number of electrocatalysis" fundamental problems. Application of metallic and non-metallic sub-mono-layers to the electrodes" surfaces influences the processes of sorbtion and ionization of adsorbed hydrogen [1], the catalytic properties of electrodes [2], whereas under different conditions the efficiency of ad-atoms activity appears to be different [3]. Data represented in Article [3] show, in one example, the role of both macro- (form and character of crystals) and micro-structure (surface energy spectrum) of surface in electrocatalytic processes. 2. EXPERIMENTAL Composite electrodes were prepared the following way. Nickel from chloridesulphate solution was precipitated on steel net with area of 4.5 cm 3. Then, fine-
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1920
disperse powders of oxides (SiO2, TiO2, MgO, V205I and Fe203) were introduced into electrolyte, and repeated electrolytic coating was conducted under intensive mixing at 50~ For electrochemical research were used potentiostat P-5848 and 3-section electrochemical cell with separated cathode and anode spaces. Oxide-mercury electrode served as a reference electrode. Measurement of nickel surface was conducted by galvanostatic charge curve in the area of 0.03-0.18B potentials using method [4] in 1N KON. Nitrobenzene electroreduction stationary currents were measured in ethanol-alkaline solutions. Potentials are showed relatively to reversible hydrogen electrode in the same solution. Contents and structure of composite electrodes' surfaces were studied by X-ray Spectroscopy Microanalysis (RSMA) method using Jeol electronic-probe micro-analyser (Japan). Previously developed method of estimating nickel surfaces for pressed nickel powder was used for estimating real nickel surface of composite nickel electrodes [3,4]. As real surface, accordingly to its definition, is understood the surface on which occur reversible processes of adsorbed hydrogen oxidation and adsorptions of weakly-bound oxygen. [4] shows that on nickel electrode in alkaline solution filling by adsorbed oxygen is quite insignificant up to 0.17B potential. Correspondingly, whole amount of electricity, spent while taking measurements galvanostatic curve from 0.0 to 0.2B, goes to oxidation of adsorbed hydrogen: H,,as + O H - = H 2 0 + e ;
active surface of nickel electrode is calculated by formula: Sa = C/Cs, where C = d Q / d t , under Er=0.1B Cs=1120 m k f / c m 2 [7]. For standardizing nickel electrode surface, special pre-treatment was employed: holding electrode under E=-0.2B for restoring electrode surface, and under E = 0.1-0.2B for removing hydrogen dissolved in the metallic body. Modification of composite electrodes by palladium was conducted by short-time sinking of the electrodes into 1% solution of PdC12, after which they were washed in distilled water and placed in electrochemical research cell. Before each experiment, the electrodes were activated by cathode polarization during 15 min by 50 mA current in 0.1N KONsolution. 3. RESULTS AND DISCUSSION Analysis of surface sectors' contents was conducted by RSMA. In all cases, galvanic nickel layers with 92-97% concentration of nickel and 0.1-3% of correspondent oxide's disperse phase in some areas were observable. Modification of surface by palladium leads to appearance of palladium centers of variable concentrations from 0.25% to 35%. Size of surface agglomerates for all systems changes within the limits of (0.4-1.5mkm), when modified by palladium, grew 1.5-3.0 times. Process of nitrobenzene electroreduction led to formation of surface layer with smaller particles (0.15-0.70 mkm) and to its enrichment in palladium (23-44% Pd). Modification of composite electrodes' surfaces
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1921
by palladium leads to decline in nickel content, in case of NiTiO2 - up to 3% of nickel in some areas. Table I shows data for estimations of real nickel surfaces on freshly prepared composite nickel-oxide electrodes and its change during subsequent reduction of increasing weighted portions of nitrobenzene. Presence of oxide's disperse phase promotes formation of galvanic layer with sufficiently large nickel surface, while its area depends on the nature of oxide: as appears from Table 2, it varies between 0,79 m 2 (Ni+MgO) and 0.03 m 2 (Ni+Fe203). Besides, subsequent reduction of nitrobenzene portions on the same electrode in all cases leads to decrease in nickel surface after the first weighted portion, and its stabilization in the subsequent cycles of reduction, i.e. non-reversible poisoning of surface by nitrobenzene and products of its reduction does not occur. Table 1. Changes in real nickel surface of composite nickel-oxide electrodes in the process of electroreduction of subsequent weighted portions of nitrobenzene
electrode Ni+MgO (Ni+MgO) Pd Ni+V205 (Ni+V2Os) Pd Ni+TiO2 (Ni+TiO2) Pd Ni+SiO2 large cells (Ni+SiO2) Pd Ni+SiO2 small cells (Ni+SiO2) Pd Ni+Fe203 (Ni+Fe203) Pd
Sreal,M 2 concentration of nitrobenzene, C'10 -2 mol/l 1.0 2.5 5.0 7.5 10 0.79 0.74 0.76 0.68 0.55 0.52 0.90 0.74 0.60 0.60 0.58 0.51 0.23 0.09 0.08 0.08 0.08 0.07 0.10 0.09 0.06 0.06 0.05 0.05 0.05 0.03 0.03 0.03 0.03 0.03 0.05 0.03 0.03 0.03 0.03 0.02 0.42 0.29 0.32 0.27 0.27 0.29 0.42 0.25 0.26 0.18 0.18 0.22 0.20 0.21 0.20 0.19 0.18 0.16 0.25 0.23 0.22 0.20 0.20 0.18 0.03 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01
Modification of composite electrodes by palladium did not affect the form of charge curve and the area of modified electrodes' nickel surface practically matches that of non-modified electrodes. For systems (Ni+MgO)Pd and (Ni+V2Os) Pd there appear variation in surface areas of freshly prepared samples, compared to non-modified ones, but after the first cycle of electroreduction it disappears. Electroprecipitation of composite coating Ni+SiO2 on steel net with large and small cells leads to formation of different nickel surfaces: 0.42 and 0.20-0.25 m 2. Therefore, under modification the nickel coating with ingrained particles of disperse phase of oxide persists and individual palladium centres appear. Real nickel surface of nickel-oxide composite electrodes remarkably decreases after reduction of the first weighted portion of nitrobenzene and subsequently stabilizes.
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After determining real nickel surface area, the potential equal to 0B was applied to composite electrode, chain was disunited and corresponding weighted portion of nitrobenzene was introduced, after which change in potential was registered over time until establishing stationary value. Table 2. Influence of modification by palladium on values of anode shifts of nickeloxide composite electrodes' potentials under nitrobenzene adsorption from solutions of it's different concentrations.
Composite electrode Ni+SiO2 large cells (Ni+SiO2) Pd Ni+SiO2 small cells (Ni+SiO2) Pd Ni+TiO2 (Ni+TiO2)Pd Ni+V2Os (Ni+V2Os) Pd Ni+MgO (Ni+MgO) Pd Ni+Fe203 (Ni+Fe203) Pd
1.0 0.55 0.49 0.34 0.33 0.30 0.41 0.25 0.62 0.29 0.37 0.25 0.52
2.5 0.55 0.55 0.35 0.37 0.52 0.41 0.30 0.67 0.39 0.43 0.28 0.57
E,B Cmtro~e*lO -2 mol/1 5.0 7.5 0.58 0.60 0.57 0.62 0.37 0.37 0.41 0.41 0.52 0.53 0.43 0.44 0.30 0.31 0.69 0.72 0.43 0.45 0.49 0.51 0.32 0.33 0.61 0.63
10 0.64 0.65 0.39 0.44 0.55 0.46 0.37 0.72 0.46 0.67 0.40 0.65
The value of anode shift increases with rising concentration of nitrobenzene and depends on nature of electrode (Table 2). Surface palladium modification of composite electrodes Ni+MgO, Ni+V2Os and Ni+Fe203 has increased value of electrode potential's shift, whereas with Ni+TiO2 it insignificantly declined. Stationary value of anode potential's shift of composite palladium-modified Ni+SiO2 electrode on large-cell net remained unchanged and increased on small-cell net. After determining stationary values of potential's anode shift under adsorption of nitrobenzene, currents of its reduction were measured, by shifting step by step electrode's potential from initially established at 0.05B toward negative direction. Electroreduction of nitrobenzene is observable already at positive potentials and significantly increases in the area of hydrogen evolution. The Table 3 shows values of specific stationary nitrobenzene electroreduction currents of palladium-modified composite nickel-oxide electrodes under E=-0.2B in comparison to correspondent values for non-modified electrodes. As demonstrated by the Table3, palladium modification of surface leads to increase in electroreduction stationary currents on studied composites for all concentrations of nitrobenzene. Remarkably, specific currents increased 3-5 times, which is observable under 0.1 mol/1 nitrobenzene concentration, compared to nonmodified electrodes. Size of steel nets' cells (large and small) influences values of
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nitrobenzene electroreduction currents on composite Ni+SiO2 (for instance, 95.1 and 35.4 m A / m 2, and for modified - 270.2 and 143.1 mA/m2). Table 3. Specific stationary electroreduction currents under various nitrobenzene concentration on nickel-oxide composite electrodes modified by palladium.
J, mA/m2 Composite electrodes Ni+SiO2 large-cell net (Ni+SiO2)Pd Ni+SiO2 small-cell net
(Ni+SiO2)Pd
Ni+TiO2 (Ni+TiO2)Pd Ni+V205 (Ni+V2Os)Pd Ni+MgO (Ni+MgO)Pd Ni+Fe203 (Ni+Fe203)Pd
C nbl0-2mol/1 1.0 34,77
2.5 37,3
5.0 61,3
7.5 84,3
10 95,1
89,14 0,7
119,04 14,2
200,7 17,7
236,2 26,4
270,2 35,4
18,0 9,3 75,0 20,2 32,2 22,2 24,3 17,4 109,68
34,3 47,7 540,0 24,8 168,5 23,1 26,0 54,64 135,0
73,0 73,1 620,0 28,8 212,7 25,1 36,8 67,5 165,0
102,0 123,1 1006,7 38,7 225,7 31,6 54,9 84,1 247,5
143,1 349,6 1940,0 57,9 300,7 35,9 135,0 135,0 337,5
Reduction of nitroaromatic compounds with purpose of obtaining amines is important process in chemical industry. Due to their high reaction capacity amines are used for synthesis of almost all synthetic paints, as well as in production of medicines, photochemicals, antiseptics, etc. Preparatory electrolysis of o-nitrophenol nitrobenzene was conducted on freshly prepared composite nickel-oxide and palladium-modified electrodes in galvanostatic regime inside stationary cell. As demonstrated by the Table 4, output of correspondent amines depends on the nature of oxide, used in composite coating. Maximum amines output is observed on Ni+MgO and Ni+V205, and minimum- on Ni+Fe203 and Ni+SiO2 composite electrodes. In the same order decreases basic capacity of oxides, which apparently influences the degree of electronic interaction between metals and oxides.
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Table 4.Electrosynthesis of amines on composite nickel-oxide electrodes. (I=0.2 A).
Electrodes
Nitrocompound
Ni+SiO2
Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol Nitrobenzene o-nitrophenol
(Ni+SiO2) Pd Ni+TiO2 (Ni+TiO2) Pd Ni+V205 (Ni+V2Os) Pd Ni+MgO (Ni+MgO) Pd (Ni+Fe203) Pd
Amines output by matter, % 81.1 74.0 97.5 85.0 83.5 69.1 97.2 83.7 85.0 82.0 98.0 95.6 87.0 78.3 98.5 94.5 85.8 82.7
Amines output by current, % 79.4 72.0 95.6 83.0 81.9 67.2 95.3 80.2 83.3 79.3 96.0 90.0 85.3 75.5 97.2 90.3 84.1 79.5
4.CONCLUSION Therefore, it follows from the present paper's results that presence of oxide's disperse phase promotes formation of electrodes with remarkably large active surface, size and electrocatalytic activity of which is strongly influenced by the nature of oxide. By the order of increasing activity, under nitrobenzene electroreduction, the nickel-oxide composite electrodes produce the following sequence: Ni-MgO < Ni-V205 < N i - S i 0 2 < Ni-Fe203 < N i - T i 0 2 . Modification of nickel-oxide composite electrodes' surfaces by palladium leads to formation of surface agglomerates of various size, depending on the nature of oxide, to appearance of palladium centres, which significantly increases electrocatalytic activity of modified composites in nitrobenzene electroreduction reaction. Electrochemical reduction of aromatic nitrocompounds has shown, that developed composite electrodes can be recommended for synthesis of amines. REFERENCES
1. M.I.Zhirnova, O.A.Petrii. Elektrokhimiya, 14 (1978) 975. 2. R.R.Adzic, M.D.Spasojevic, A.R.Despic. J.Electroanalyt. Chem., 92 (1978) 31. 3. O.A.Petrii, C.Ya.Vasina, M.I.Zhirnova. Elektrokhimiya, 16 (1980) 348. 4. O.S.Abramzon, S.D.Chernyshev, A.G.Pshenichnikov, Elektroknimiya, 12 (1976) 1667.
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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Photocatalyst-Coated Acrylic Waveguides for Oxidation of Organic Compounds Lawrence W. Miller, M. Isabel Tejedor-Tejedor, Montserrat Perez Moya, Ramona Johnson, Marc A. Anderson Water Chemistry Program, University of Wisconsin, 660 N. Park Street, Madison, WI 53706, USA Acrylic (polymethyl methacrylate) sheets were coated with a thin film of porous, titanium dioxide (TiO2) semiconducting photocatalyst. The sheets were first passivated with a thin film of silica to prevent them from being oxidized by the TiO2 semiconductor photocatalyst. The coated acrylic structures act as waveguides that propagate ultraviolet (UV) light in an Attenuated Total Reflection (ATR) mode. A recirculating reactor was used to evaluate the acrylic waveguides for their ability to photocatalytically oxidize formic acid in water. The TiO2-coated waveguides utilize activating ultraviolet light four times more efficiently for the photocatal~ic oxidation of formic acid then TiO2 films illuminated directly with diffuse UV light.
I. Introduction Photocatalytic oxidation of organic contaminants on the surface of titanium dioxide (TiO2) is an attractive process for remediating liquid or gas-phase waste streams. Organic compounds can be oxidized to carbon dioxide and water in the presence of TiO2, ultraviolet (UV) light (wavelength of ca. 380 nm or less), and an electron acceptor such as oxygen. This process occurs at room temperature. While the potential of TiO2 photocatalysis for environmental remediation has generated considerable research interest (1), development of commercially viable systems has been hindered in large part by the expense of UV light generation and capture. Conventional heterogeneous photocatalytic reactors are difficult to scale to the dimensions necessary for commercial applications because light is severely attenuated within the reactor through absorption or reflection by the catalyst, the reactant, reactor walls, and catalyst supports. Scaling-up of conventional photoreactors is also difficult and expensive because light intensity diminishes with the distance from the diffuse source. Since the rate of a heterogeneous photocatalytic reaction depends on the light intensity, a scaled-up photoreactor practically requires many light sources closely spaced throughout the reactor volume. The use of waveguides to support and illuminate photocatalysts was initially proposed more than twenty years ago as a way to overcome the light distribution limitations inherent in conventional photoreactor designs (2). In a waveguide photoreactor, light enters the catalyst-coated waveguides (e.g. transparent optical fibers or planar structures) near the source where it is most intense, and it is distributed to the catalyst via successive internal
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reflections at the waveguide surface. Waveguide photoreactors would have several advantages in comparison to conventional photoreactor designs (e.g. packed bed or slurry reactors) in which the catalyst is directly illuminated by sunlight or artificial light. By capturing light from a single source and propagating it internally to the photocatalyst, waveguide reactors should increase the amount of fixed, illuminated catalyst per reactor volume. Furthermore, a waveguide photoreactor would more evenly distribute light from a diffuse source throughout the reactor volume. Some researchers have coated silica optical fibers with TiO2 in an effort to realize the advantages of waveguide supported photocatalysis for environmental remediation (3,4). While these efforts yielded photoreactors that successfully oxidized organic compounds, the TiO2-coated fibers that were developed did not propagate light effectively. Light was refracted out of the fibers at the TiO2-coated surface. At the Water Chemistry Program of the University of Wisconsin, we developed planar silica waveguides coated with TiO2 that propagate UV light in an attenuated total reflection (ATR) mode (5). In an ATR mode, light propagates internally via successive total reflections at the waveguide boundaries. The TiO2 coating absorbs a small portion of the light at each reflection. These systems do not lose light through refraction at the waveguide surface. We used these TiO2-coated waveguides to photocatalytically oxidize formic acid in water. A recent paper by Miller, et al. explains the optics of a low-refractive index substrate coated with a higher refractive index TiO2 film (6). While fused silica is an attractive waveguide medium because of its transparency in the UV spectrum, there are several practical limitations to using silica in commercial reactors. It is expensive, heavy (ca. 4 g/cm3), fragile, and difficult to machine. For this reason, we developed planar waveguides made from acrylic (polymethyl methacrylate) sheet. Acrylic is an attractive waveguide substrate because it is lighter than glass, easily machined, thermoformable and moldable. It is can be formulated to be reasonably transparent in the near UV spectrum (transmittance = ca. 70% cm l at wavelength - 360 nm), and an industry exists for the manufacture of UV-stable and transparent cell-cast acrylic sheet. Here we describe a process for coating acrylic sheet with a mesoporous TiOz film. The acrylic waveguides are passivated with a microporous silica film, and the TiO2 is deposited on top of the silica. The silica film protects the acrylic from photooxidation by the TiO2 semiconductor photocatalyst. We evaluate the waveguides' photocatalytic performance, and propose a reactor design that demonstrates the advantages of TiO2-coated acrylic waveguides for the scale-up of TiO2-based photocatalytic processes.
2. Experimental Section Acrylic sheet (thickness = ca. 0.32 cm) was obtained from Cyro Industries (cat. no. AE-OP4). All chemicals were obtained from Aldrich Chemical Co. (Milwaukee, WI). Laboratory grade water (18 M.Q cm -1) was generated with a Barnstead system. The sol-gel synthetic method was used to prepare a colloidal silica sol. Tetra-ethyl orthosilicate was hydrolyzed in water and KOH (7). The resulting sol is composed of silica particles of ca. 3 nm in diameter and has a pH of ca. 9.5. The particles have a zeta potential of ca. 4.0. The pH of the silica sol was adjusted to ca. 3.5 by addition of 1 wt. % nitric acid in water. Acrylic waveguides were prepared by cutting acrylic sheet to the desired dimensions. The waveguide edges were sanded with successively finer grained sandpaper (finished with
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1000 grit paper). The edges were then polished with 45 ~tm diamond slurry on a polishing wheel. To prepare the acrylic waveguides for coating with silica and then TiO2, they were first saponified in 5M NaOH for 1 hr., rinsed in water and air-dried. This step was necessary to make the acrylic surface hydrophilic. A microporous silica film was deposited onto the saponified acrylic waveguides by dipping them into the silica sol and withdrawing at a controlled rate of 2.5 cm/min followed by drying in air. The thickness of the silica film was adjusted by repeating the coating process. For the waveguides used in this study, three layers of silica were applied. A mesoporous TiO2 film was then deposited onto the silica-passivated acrylic by dipping the waveguides into a colloidal TiO2 sol of pH = ca. 3.5 (8). The withdrawal rate was ca. 8 cm/min. TiO2 films prepared from this method have a high surface area (> 150 m 2 g-~) and a pore radius of ca. 1.5 nm to 4 nm (8). These films have been shown to be effective photocatalysts (9). Six layers of TiO2 were coated onto the waveguides. After coating, the edges were repolished to remove any TiO2 or silica. Profilometry measurements (Tencoro model 2000) made on equivalently coated glass plates were used to estimate the thickness of the silica and TiO2 films. The films were inspected visually and with an optical microscope to determine their uniformity. SEM micrographs were obtained with a LEO model 952 scanning electron microscope. An internal reflection photoreactor described in a recent paper was used to evaluate the TiO2-coated acrylic waveguides for degradation of formic acid in water (5). A schematic of the reactor is shown in Figure 1. With this reactor, a rectangular acrylic waveguide measuring 5 cm x 2 cm x 0.3 cm was held in a flow-through liquid cell. A solution of 0.001 M formic acid in water was recirculated from an oxygenated reservoir across the TiO2-coated face of the waveguide at a rate of 4 mL/min. The geometric surface area of the TiO2 film exposed to the reactant was ca. 7.1 cm. Solution volume measured 10 mL. A fluorescent bulb (Sylvania, model F8T5350blb) positioned ca. 0.5 cm from the polished waveguide edge provided UV illumination. UV light intensity (all wavelenghs < 400 nm) incident on the polished waveguide edge was measured with a radiometer (International Light Corp.). Formic acid concentration was measured using a Total Organic Carbon (TOC) analyzer (Shimadzu, model TOC5000). Measurements were made at the beginning of each experiment and after 4 hours of reactor operation.
ReactantS" 7
Jo Ool
/~ ~
['~flow-thru~
i lcel!
Acrylic Waveguide
i.
I:::t I1 I
i
iI
'] .~~
Fluorescent 0 2 UV source Figure 1: Schematic of recirculating photoreactor
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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The reactor was modified to allow light to shine on the directly onto the surface of the TiO2-coated acrylic waveguides. This modification allowed comparisons between reaction rates for TiO2 films illuminated by internally reflected light and for films illuminated directly. Light entered this modified flow-through reactor through the acrylic support and illuminated the TiO2 film that contacted the formic acid solution. Lamp position was adjusted to make the intensity of the light striking the TiO2 film roughly equivalent to that striking the waveguide edge in the above-described series of experiments. TiO2 film thickness was the same in both sets of experiments and the same geometric surface area of TiO2 was allowed to contact the reactant. The reactant solution was 15 mL of 0.001 M formic acid in water. Experiment duration was 25 minutes. In both sets of experiments, the reactor was operated without illumination, and no degradation was observed. Flow rates were varied with no apparent change in reaction rates. For both sets of experiments (internally propagated light and direct illumination of the TiO2), three samples were evaluated in the reactor, and the experiment was repeated three times for each sample. The change in concentration with time was used to calculate an absolute rate of reaction adjusted for geometric TiO2 surface area for each sample (reported in units of mol s-~ cm-2).
3. Results and Discussion
Profilometry performed on equivalently coated glass plates showed a thickness of 50-100 nm for the silica films and 350-400 nm for the TiO2 films. SEM analysis of the coated acrylic waveguides showed similar film thicknesses. A representative SEM micrograph is shown in Figure 2.
Figure 2" SEM Micrograph of TiO2/silica film on acrylic
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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Uponvisual examination, the TiO2 and silica films have few observable defects. The coated waveguides exhibit birefringence to visible light, indicating that the films are transparent with boundaries parallel to the acrylic surface. Gentle abrasion of the waveguides when they were immersed in water did not remove the films, indicating reasonable adhesion. However, exposure of the waveguides to methanol, acetone or other solvents that attack acrylic removed the films. No film delamination was observed during operation of the test reactor. The highest quality silica films were obtained when the colloidal silica sol used to coat the waveguides was adjusted to pH = 3.5. At this pH, the surface charge of the silica particles is neutral or slightly negative (7). If one assumes that the saponified surface of the acrylic has a pKa value similar to that of acrylic acid (pKa = ca. 4.2), then the surface of the acrylic is protonated at this pH. We suspect that these conditions allow for favorable film deposition and adhesion. When the recirculating photoreactor was operated in internal reflection mode, the concentration of the 0.001 M formic acid solution was reduced by 36 + 4% in 4 hours of reaction time. Light intensity striking the waveguide edge measured 12 + 1 mW/cm 2. The reaction rate normalized to the geometric surface area of TiO: film exposed to the reactant was calculated using the following relation: r =
CoxXxV txA
(1)
where r is the area-adjusted reaction rate (mol s~ cm-2), Co is the initial concentration of the reactant solution (mol/L), X is the fractional conversion of the reactant, V is the reactant solution volume (L), t is the reaction time (seconds), and A is the geometric surface area of TiO2 film exposed to the reactant (cm2). The calculated rate of formic acid oxidation for the TiO2/silica-coated acrylic waveguides was 3.5 + 0.4 x 10-~1 mol s~ cm 2. For the TiO2/silica-coated acrylic illuminated directly with the same UV light intensity, the overall formic acid degradation measured 43 + 2% in 25 minutes of reaction time. This was equivalent to a rote of 6.0 + 0.3 x 10-~~mol s ~ cm 2. Under the conditions of the experiment, it appears that the TiO2 coating is ca. 17 times more effective when illuminated directly rather than by light propagated internally through the waveguide support. However, this apparent advantage is negated when one considers that a much greater geometric surface area of TiO2 could potentially be illuminated in a reactor consisting of an array of TiO2-coated acrylic waveguides. This difference is illustrated in Figure 3. An annular reactor design, as specified in Figure 3 Design A, wherein the TiO2 film is illuminated directly by a cylindrical UV source (e.g. a fluorescent UV bulb) would have ca. 63 cm 2 of illuminated catalyst. However, an array of coated waveguides as configured in Design B would have an illuminated, geometric catalyst surface area of ca. 4400 cm ~. Thus a far greater amount of TiO2 catalyst can be illuminated in the waveguide reactor design than in the annular design. In fact, the waveguide design should be ca. 4 times more effective at degrading formic acid under identical conditions of illumination.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
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E O
..
E co rt~
E --I
t_
200
500
800
Wavelength / nm
60
-.c-
40
O
20
Powdered Ti02
2
20
1 O0
Film Thickness /nm
Fig. 2. UV-Vis absorption spectra of TiO2/PVG photocatalysts having
Fig. 3. The photocatalytic reactivity of TiO2/PVG photo-catalysts having
various film thicknesses prepared by the ICB method. The film thickness; (a) 50, (b) 100, (c) 300 (nm).
various film thickness and powdered TiO2 for the photocatalytic degradation of 2-propanol diluted in water.
3.2 C l u s t e r e d TiO2 e m b e d d e d o n A C F
Characterizations of TiO2/ACF photocatalysts prepared by the I CB method by the SEM technique showed the titanium oxides to be deposited on
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the ACF as small clusters. The migration of titanium ion clusters was not easy on ACF with the numerous micropores, resulting in the formation of titanium oxide clusters and not thin films. XRD and XAFS measurements of the TiO2/ACF photocatalysts suggest crystalline anatase as the main structure present. Figure 4 shows the reaction time profiles of the liquid-phase photocatalytic reaction on the TiO2/ACF photocatalysts. In the initial stage of the reaction under dark conditions, the adsorption of 2-propanol onto the photocatalysts can be observed. When the UV light is turned on, the 2propanol decomposes into acetone and CO2. These TiO2/ACF photocatalysts exhibited a higher and more effective photocatalytic reactivity than the catalysts prepared on ACF by the impregnation method using an aqueous solution of ( N H 4 ) 2 T i O ( C 2 0 4 ) 2 . These results indicate that with the ICB method it is possible to prepare highly crystalline anatase titanium oxide photocatalysts at low calcination temperatures without damaging the microporous structure and adsorption properties of the ACF supports. The adsorption ability of substrates kept the original levels while the amount of deposition TiO2 was less than 2 wt%. With increase in the amount of deposited TiO2 over 2 wt%, the adsorption ability of substrates decreased gradually.
o~
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-
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1200
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, 0.2 8 o.15 0
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,
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O
E
0.1
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,
450 550 650 Temperature (K)
Fig. 3" TPD after 20 min toluene PCO on TiO2
Benzene
0.05 0 .,. ~ ~ ~ ~ ~ C 300
~ ~ ~ ,
400 500 600 Temperature (K)
700
Fig. 4: TPH after 20 min toluene PCO on TiO2 and interrupted TPD
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9
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,
.
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400 500 600 700 Temperature (K) Fig. 5: TPH after 20 min toluene PCO on Pt/TiOz and interrupted TPD
0
A
9
5
10 15 20 25 Time (min) Fig. 6" CO2 and benzene formation during 20 min benzaldehyde PCO on Pt/TiO2
times higher than the formation rate for the unhumidified toluene flow. A small amount of benzaldehyde and benzene also formed during the PCO. Atter 19 h of PCO, toluene uptake and CO2 formation were still measureable (Fig. 8). Approximately 16 ME of toluene or 3.8 ME ofbenzaldehyde were consumed during 19 h of PCO. After 42 h or PCO, toluene uptake was still barely measureable, but CO2 production was negligible indicating that the catalyst does eventually deactivate even with water present in the flow gas. The TPH spectra obtained following PCO of a continuous flow of unhumidified toluene and interrupted TPD to 348 K is shown in Fig. 9. While 25 l.tmol toluene/g catalyst desorbed during interrupted TPD following transient PCO on Pt/TiO2, only ---3 ~tmol unreacted toluene/g catalyst desorbed following continuous flow PCO. Thus, the catalyst was covered with intermediates during continuous flow PCO, and little unreacted toluene was on the surface. The TPH spectra following PCO of both continuous flow humidified and unhumidified toluene were similar with only the amounts of the desorbing species differing. Unlike the TPH spectra following transient PCO of toluene on Pt/TiO2, little MCH desorbed ( TiO2 (e'ob, h+vb)
(1)
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1978
02(ads) +
e"
02(.d~)
2H20
+
O'2(ads)
-+ +
e
~
(2) H202
+
2OH"
(3)
H202 -k e" ~
OH + OH-
(4)
OH'(~u~0 + h+r
--+
(5)
••--
+'OH
OH ~---
~2HO
+ other products ---~ CO2 + H20
(6)
CH3
(ads)
HO-~CH3 4.
+ other products ---~ CO2 + H20
(7)
CONCLUSIONS
The results indicate that the photocatalytic degradation rate of toluene in aqueous suspensions of polycrystalline TiO2 was enhanced by the presence of a surfactant (CI4DMAO) added to the suspension. The role of surfactant is not linked to a direct interaction between the toluene and surfactant molecules; it seems likely that the surfactant positively affects the adsorption properties of the TiO2 surface. Differently from the gas-solid systems for which benzaldehyde was the main intermediate product, in the studied liquid-solid system p-cresol was found to be the main intermediate product. Work is in progress in order to confirm and explain the findings of this investigation. REFERENCES 1. M. Schiavello (ed.), Photocatalysis and Environment. Trends and Applications, Kluwer, Dordrecht, 1988. 2. N. Serpone and E. Pelizzetti (eds.), Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. 3. D.F. Ollis in E. Pelizzetti and N. Serpone (eds.), Homogeneous and Heterogeneous Photocatalysis, Kluwer, Dordrecht, 1986, p. 651. 4. V. Augugliaro, L. Palmisano, A. Sclafani, C. Minero and E. Pelizzetti, Toxicol. Environ. Chem. 16 (1988) 89. 5. V. Augugliaro, A. Di Paola, V. Loddo, G. Marci, L. Palmisano and M. Schiavello, Catalysts and Catalysis 41 (1999) 73. 6. M. Fujihira, Y. Satoh and T. Osa, Nature 293 (1981) 206. 7. M. Fujihira, Y. Satoh and T. Osa, J. Electroanal. Chem. 126 (1981) 1053. 8. A. Navio, M. Garcia G6mez, M. A. Pradera Adrian and J. Fuentes Mota, J. Mol. Catal. 104 (1996) 329. 9. T. Ibusuki and K. Takeuchi, Atmos. Environ. 20 (1986) 1711. 10. V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese, G. Martra, L. Palmisano and M. Schiavello, Appl. Cat. B: Environ. 20 (1999) 15, and references therein.
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1979
Photooxidation of Benzene to Phenol by Ru Complex Occluded in Mesoporous FSM-16
Keiko Fu]ishima, Atsushi Fukuoka, and Masaru Ichikawa Catalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan
[Ru(bpy)3]Cl2 occluded
in mesoporous FSM-16
shows a high activity in the
photooxidation of benzene to phenol using hydrogen peroxide as an oxidant. based on Ru and the selectivity to phenol was 98%.
The TON was 430
The catalytic oxidation of benzene may
proceed by a photo-excited state of the Ru complex.
1. Introduction Selective oxidation of benzene to phenol is currently one of the challenging goals in mimicking enzymes in heterogeneous and homogeneous catalyses [1 ].
In the case of the Fenton's
reagent method, which is widely studied in the direct oxidation of benzene, the reaction scheme is generally proposed as follows; hydroxyl radical produced from H202by Fe2§ reacts with benzene to form an intermediate hydroxycyclohexadienyl radical.
The hydroxycyclohexadienyl radical reacts
with Fe 3§ to form phenol and regenerated Fe 2+. B iphenyl is usually produced as a byproduct according
to
the
dimerization
0f
two
~--N
N--~
hydroxycyclohexadienyl radical [2]. We have reported the preparation of hybrid
composites
of
metalloporphyrins
and
fullerenes in mesoporous FSM-16 and their activity in photooxidation of propylene [3].
It was also
OR N
Fig. 1. Structure of [Ru(bpy)3]~+.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1980
found that the fullerenes occluded in FSM-16 catalyzed the photooxidation of cyclohexene and
benzene with 02 [4]. [Ru(bpy)3]Cl2 (Fig. 1) has three excited states: metal-ligand charge transfer (MLCT), ligand charge (LC) transfer and metal charge transfer (MC). luminescence and bimolecular quenching. release ofbpy ligand.
MLCT and LC excited states exhibit
On the other hand, MC excited state results in the
For most Ru( II ) polypyridine complexes, the energy position of the MLCT
is lower than that of the MC, and [Ru(bpy)3]Cl 2 shows strong luminescence and quenching [5]. Here we report that
[Ru(bpy)3]Cl2 (bpy = 2,2'-bipyridyne),
which has unique
photochemical and photophysical properties of excited state, occluded in the mesopores of FSM-16 shows a high catalytic activity in liquid phase photooxidation of benzene to phenol.
2. Experimental FSM-16 (channel diameter 2.7 nm) was prepared according to the literature method [6], and evacuated at 623 K for 2 h.
[Ru(bpy)3]Cl2 (Ardrich Chem. Co.) was occluded in FSM-I 6 by
the impregnation from a methanol/water solution (Ru complex 1 wt%) (Fig. 2). grade)
was
occluded
in FSM-16
in a sealed
glass tube under
C6o (Hoechst gold
vacuum
at
773
K.
[Ru(bpy)a]CI:4FSM-16 and C6o/FSM-16 were washed with appropriate solvents to remove weakly adsorbed [Ru(bpy)3]C12 or C60, and the amount of occluded species were determined by UV-vis. spectroscopy.
Photooxidation of benzene was performed in a quartz vessel for photochemistry,
and a high-pressure mercury lamp (USHIO UM-102 100 W, 300-600 nm) was used as a light source.
!Evacuation,623 K, 2 h ,:
[Ru(bpy)3]Cl2(aq.)= :~ ~
........ St!.m'ng.un.dcrN2a..trn0sphc~, 4 h ....... Vacuumed300 K, overnight ,"-
[Ru(bpy)3]Cl~/FSM-16
.~..
Wash ~m br [Ru(bpyh]CI2/C6o/FSM- 16
Fig. 2. Preparation of [RuCopy)a]Cl2/Ceo/FSM-16.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1981
A mixture of benzene (Kanto Chem. Ltd.) and aq. H202 (Mitsubishi Gas Chemical, 20 - 30 %) was
stirred with a catalyst under irradiation.
Products were analyzed by GC, GC-MS (Shimazu, DB-
WAX 25 m), and HPLC (Nihon Millipore Ltd., 5~t C~8 100 A).
Glass filter (Toshiba UV-39D and
UV-D33S) were used in experiments for wavelength dependence.
3. Results and Discussion 3.1. Catalytic activities in photooxidation of benzene [Ru(bpy)3]Cl2 was supported in the mesopores of FSM-16 by the impregnation and C6o by the CVD method.
After washing out the weakly adsorbed [Ru(bpy)3]Cl 2 and/or C6o, the loading
determined by UV-vis. were as follows: [Ru(bpy)3]CIJFS-16, 0.1% for Ru metal; C6o/FSM-16, 316%; [Ru(bpy)3]CI2/C6o/FSM-16, 0.1% for Ru and 1.2 % for C6o. Table 1 summarizes the catalytic results of photooxidation of benzene by Ru complexes using H202 as an oxidant (Eq. 1). +
H202
OH + 0
0 +
(1)
In a mixture of benzene and aq. H202, [Ru(bpy)3]Cl2 itself shows a turnover number of 170 for the formation of phenol in 24 h under irradiation (Table 1, Run 1), which was larger than that by that of 83 for RuCI 3. The oxidation of benzene by [Ru(bpy)3]Cl 2 resulted in a high selectivity of phenol; quinone and biphenyl were formed as minor products and their TONs were much lower and catecol, hydroquinone and resorcinol were not detected.
It is interesting to note
that the occlusion of [Ru(bpy)3]Cl2 in FSM-16 gave a higher TON (430) for phenol production (Run 2).
No reaction was observed in flowing 02 (1 atm) as oxidant for [Ru(bpy)3]Cl 2 or Table 1. Catalytic activities for benzene oxidation, a) 9 CatalFsts [Ru(bpy)3]Cl2 [Ru(bpy)3]CI2/FSM-I 6 RuCI3 [Ru(bpy)3]Cl2/C60 [Ru(bpy)3]CI2/C60/FSM-16
phenol 170 430 83 53 86
TON(/Ru) quinone biphenyl 3 0.8 6 0.8 8 0 2 0.1 10 0.1
a) Conditions; Benzene 840 mmol, H202 710 retool, Ru 5.7x10 ~ retool, Coo 1.0xl0 2 mmol, irradiated 300-600 nm, 24 h, room temperature.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1982
350
[Ru(bpy)3]Cl2/FSM- 16 catalysts. Although
the
oxidation
larklightdark li~lt
.~/,//
by
[Ru(bpy)3]CI~FSM-16 slightly proceeded in dark (TON of phenol: 35 in 24 h) (Fig. 3, line A), the
25O 2OO
irradiation of light greatly enhanced the reaction (Fig. 3, line B).
It suggests that this reaction is
not an autooxidation reaction.
The original
orange color of [Ru(bpy)3]Cl2 solution gradually turned to pale yellow during the irradiation, and the
visible
t
spectroscopy
showed
the
A:Un~ 0
0
5
tO
15 20 Tmle (h)
25
30
Fig. 3. Dependence of benzene oxidation to phenol on irradiation by lRu(bpy)a]CI~/FSM-16. ~
photosubstitution of the bipyridyl ligand from Ru
O.
center (Fig. 4) [7]. The
-"
/.-.
0.6~
dependence
wavelength was studied.
of
TONs
on
35
"
=
;
/
/
Oh
-
3h
'~
;-m\
6h
In the case of
irradiation at 300-400 nm in the MC excited range [5], the TON of phenol was almost the same as whole wavelength irradiation (300-600 0
nm), and the TONs of quinone and biphenyl were decreased.
On the other hand, at 400-600
nm of the MLCT excited range [5], the TONs were decreased.
In all cases except under the
dark, the color of catalyst changed and it suggests the degradation of
[Ru(bpy)3]Cl2.
"-. . . . . . .
300
450
500
Wavel e n g t h (rim)
550
600
Table 2. Wavelength dependence for the benzene oxidation by [Ru(bpy)a]C12.
These data indicate that [Ru(bpy)n] 2+ (n--l-3) species formed from the MC excited species are
300-400 a)
involved in the catalytic photooxidation of
400-600 a) dark . .
Furthermore, the
,
400
Fig. 4. Spectral change of the visible spectra of [Ru(bpy)a]Clg. in H,O~ during the irradiation.
W. L. ( n m ) 300-600
benzene by [Ru(bpy)3]Cl 2.
x...........
350
TON (/Ru) quinone biphenyl 4.0 0.4 100 2.3 ND 41 2.0 0.1 15 0.4 O.1 .
phenol 96
.
initial activity under irradiation was higher than that at 30 h in Fig. 3, which suggests that [Ru(bpy)3] 2+ species is more active than the degradated species.
When a mixture of [Ru(bpy)3]Cl2 and C60 was used as catalyst in the reaction with
H202,
the TON for phenol was lower than that by [Ru(bpy)3]Cl 2, but the decomposition of [Ru(bpy)3]Cl2
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1983
was effectively inhibited. catalyzed
the
C60 also
photooxidation
[Ru(bpy)3]2+* '.
~[Ru(bpy)n]2**/
H202
of
benzene with aq. H202 to produce phenol with a TON of 3.
J
The
/
occlusion of both [Ru(bpy)3] C12 and C60 in FSM-16 increased the TON for
[Ru(bpY)nl2+
',t.
[Ru(bpy)n] 3*
/
0
phenol compared to the non-occluded [Ru(bpy)3]Cl2
and
C60, and
the
formation of quinone was slightly improved.
Furthermore,
in
the
Fig. 5. Proposed reaction cycle for the photooxidation of benzene by [Ru(bpy)3]Cl~.
reaction using 02 as oxidant, [Ru(bpy)3]CI2/C6o/FSM-16 catalyzed the phenol formation with a TON of 2-3.
3.2 Discussion of the reaction mechanism.
Although the mechanism of the photooxidation of benzene is not yet clear at this moment, the excited state of [Ru(bpy)3]Cl2 may promote both the formation of OH radical from H202 and the attack of the OH radical to benzene as proposed for the Fenton reagent [2].
From the experiments
of the wavelength dependence, we propose that the MC excited state of Ru complex and its derivative may be an active species to produce hydroxyl radical from the hydrogen peroxide and hydroxycyclohexadienyl radical in the catalytic oxidation of benzene to phenol. hydroxycyclohexadienyl radical by R u 3+ results in the formation of phenol.
The oxidation of
The dimerization of
the radical gives biphenyl, but in our case the dimerization path is minor because only a trace amount of biphenyl was formed as shown in Table 1.
The Ru(lll) is a strong oxidant, and
reduction to phenol is faster than dimerization to biphenyl. In the reaction by [Ru(bpy)3]Cl2/C60, C60 seems to work as a quencher of the excited [Ru(bpy)3]Cl2 and prevents the excited Ru complex from oxidizing benzene and degradating.
4. Conclusion
[Ru(bpy)3]Cl 2
catalytically oxidizes benzene to phenol by H202, and the TON and
selectivity of phenol are high.
The excited state Ru complex drastically acceralates the reaction.
家 学 ww •家 w — .ch — em 为 j. 化 cn 学 找 个 化
1984
C6o may prevent [Ru(bpy)3]Cl 2 from the photo-oxidation by quenching from the excited Ru states.
References 1. Catalytic Oxidation -Principles and Applications, R. A. Sheldon and R. A. van Santen (eds.), World Scientific Publishing Co., Singapore, 1995; Catal. Today, 41, No. 4 (1998). 2. J. R. L. Smith et al., J. Chem. Soc., 2897(1963), A. Kunai et al., J. Am. Chem. Soc., 108 (1986) 6012. 3. J. Tachibana, M. Chiba, M. Ichikawa, T. Imamura, and Y. Sasaki, Supramol. Sci., 5 (1998) 281. 4. A. Fukuoka, K. Fujishima, M. Chiba, J. Tachibana, and M. Ichikawa, Shokubai (Catalyst), 40 (1998) 490, and to be published. 5. A. Juris and V. Balzani, Coord. Chem. Rev., 84, (1988) 85. 6. S. Inagaki, Y. Fukushima, and K. Kuroda, J. Chem. Soc., Chem., Commun., (1993) 680. 7. B. Durham, J. V. Caspar, J. K. Nagle and T. J. Meyer, J. Am. Chem. Soc., 104, (1982) 4803.