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Studies in Surface Science and Catalysis 63 PREPARATION OF CATALYSTS V Scientific Bases for the Preparation of Heterogeneous Catalysts
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 63
PREPARATION OF CATALYSTS V Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedingsof the Fifth International Symposium, Louvain-la-Neuve, September 3-6,1990
Editors G. Poncelet Catalyse et Chimie des Materiaux Divises, Groupe de Physico-Chimie Minerale et de Catalyse, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium
P.A. Jacobs Centrum voor Oppervlaktescheikundeen Colloidale Scheikunde, Katholieke Universiteit, Leuven, Heverlee, Belgium
and P. Grange and B. Delmon Catalyse et Chimie des Materiaux Divises, Groupe de Physico-Chimie Minerale et de Catalyse, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium
ELSEVIER Amsterdam - Oxford - New York - Tokyo
1991
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Q Elsevier Science Publishers B V,, 199 1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B V / Academic Publishing Division, P.O. Box 330, 1000 AH Amsterdam, The Netherlands Special regulationsfor readers in the USA - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocoples of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the pubhaher No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, inmuctions or ideas contained in the material herein. Although all advertising material is expected to conform to ethical (medical) standards, inclusion in this publication does not constitute a guarantee or endorsement of the quality or value of such product or of the claims made of it by its manufacturer. This book is printed on acid-free paper. Printed in The Nerherlands
V
CONTENTS Organizing Committee Foreword Acknowledgements Financial Support
X XI XIII XTV
Studies of unit operations in catalyst preparation Illustration of process scale-up in heterogeneous catalyst preparation I. Biay, G. Dessalces, C. Hypolite, F. Kolenda, J.P. Reymond
1
Deposition precipitation onto pre-shaped carrier bodies. Possibilities and limitations K.P. de Jong
19
Influence of the preparation procedure on the physical properties, surface acidity and dispersion of MoP/A1203 catalysts R. Prada Silvy, Y. Romero, J. Guaregua, R. Galiasso
37
Synthesis of non-stoichiometric spinel-type phases : a key tool for the preparation of tailored catalysts with specific activity M. Piemontese, F. Tnfir6, A. Vaccari, E. Foresti, M. Gazzano
49
Effect of preparation variables on catalytic behaviour of copper/zirconia catalysts for the synthesis of methanol from carbon dioxide R.A. Koeppel, A. Baiker, Ch. Schild, A. Wokaun
59
Preparation of Tia-Al203 by impregnation with TiC4-CC4 Liu Yingjun, Zhang Qinpei, Zhu Yongfa, Gui Linlin, Tang Youqi
69
Interactions of the impregnating solution with the support during the preparation of Rh/Ti@ catalysts R.J. Fenoglio, W. Alvarez, G.M. Nuiiez, D.E. Resasco
77
Impregnation of controlled-porosity silica : Cu/Si@, Co/SiO2 and Cu-Co/SiO2. Investigation of the parameters affecting selectivity in CO hydrogenation M.A. Martin Luengo-Yates, Y. Wang, P.A. Sermon
87
Selective hydrogenation of cyclododecatriene isomers to cyclododecene catalyzed by Cu-Al2O3 V. Di Castro, M. Gargano, N. Ravasio, M. Rossi
95
Preparation and characterization of highly selective Fe-Cu/Si& catalysts for partial hydrogenation of alkynes Y. Nitta, Y. Hiramatsu, Y. Okamoto, T. Imanaka
103
Some remarks on the preparation of Fe-WCa-Cr catalyst for styrene production Z. Dziewiecki, E. Ozdoba
113
Hydrogenation of 2-ethyl hexen-2-al on Ni/SiOz catalysts. Role of preparation parameters A.F. da Silva Jr, V.M.M. Salim, M. Schmal, R. Frety
123
Preparation and properties of a Wsilica and its comparison with Europt-1 S.D. Jackson, M.B.T. Keegan, G.D. McLellan, P.A. Meheux, R.B. Moyes, G. Webb, P.B. Wells, R. Whyman, J. Willis
135
VI
Factors analysis for mechanical strength in pelleting process of Fe-based high temperature shift catalyst Yongdan Li, Jiusheng Zhao, Liu Chang
145
Studies on pore size control of alumina : preparation of alumina catalyst exmudates with large unimodal pore structure by low temperature hydrothermal treatment M. Absi-Halabi, A. Stanislaus, H. Al-Zaid
155
Production of nickel-on-alumina catalysts from preshaped support bodies L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen, J.W. Geus
165
Development of a methodology for investigating the adsorption of species containing catalytically active ions on the surface of industrial carriers N. Spanos, Ch. Kordulis, A. Lycourghiotis
175
Scaling down of the calcination process for industrial catalyst manufacturing G. Groen, J. Ferment, M.J. Groeneveld, J. Decleer, A. Delva
185
Hydrothermal sintering of the active phase in alumina supported fixed bed nickel catalysts during reduction E.K. Poels, J.G. Dekker, W.A. van Leeuwen
205
Catalyst preparation via the sol-gel route The influence of silica support on polymerisation catalyst performance C.E. Marsden
215
Preparation and catalytic effects of Ce0,-MOy-A12@ (M = Ba, La, Zr and Pr) by an improved sol gel method for automotive catalysts K. Masuda, M. Kawai, K. Kuno, N. Kachi, F. Mizukami
229
Influence of preparation parameters on pore structure of silica gels prepared from tetraethoxy orthosilicate B. Handy, K.L. Walther, A. Wokaun, A. Baiker
239
Preparation of catalysts from layered structures and pillaring of clays Aspects of the synthesis of aryl sulfonic acid h4ELS@catalysts D.L. King, M.D. Cooper, W.A. Sanderson, Ch.M. Schramm, J.D. Fellmann
247
Preparation of basic silicates and their use as supports or catalysts G.A. Martin, M.C. Durupty, C. Mirodatos, N. Mouaddib, V. Pemchon
269
Soils as unusual catalysts S.A. Moya, A. Flores, M. Escudey
279
Thermal stability, acidity and cracking properties of pillared rectorite catalysts M.L. Occelli
287
Preparation and properties of large-pore RE/Al-pillared montmorillonite. A comparison of RE cations J. Sterte
30 1
Preparation of pillared montmorillonite with enriched pillars E. Kikuchi, H. Seki, T. Matsuda
311
VII
Intercalation of La203 and La2e-NiO oxidic species into montmorillonite layered structure A.K. Ladavos, P.J. Pomonis
319
Mixed Al-Fe pillared laponites :preparation, characterization and catalytic properties in syngas conversion F. Bergaya, N. Hassoun, L. Gatineau, J. Barrault
329
Zirconium pillared clays. Influence of basic polymerization of the precursor on their structure and stability E.M. Farfan-Torres, 0. Dedeycker, P. Grange
337
Control of the acidity of montmorillonites pillared by Al-hydroxy cationic species D. Tichit, Z. Mountassir, F. Figueras, A. Auroux
345
Preparation and modification of zeolite-based catalysts The chemistry of dealumination of faujasite zeolites with silicon tetrachloride J.A. Martens, P.J. Grobet, P.A. Jacobs
355
Factors affecting the formation of extra-framework species and mesopores during dealumination of zeolite Y D. Goyvaerts, J.A. Martens, P.J. Grobet, P.A. Jacobs
38 1
Treatment of galloalumino-silicate (ZSM-5 type zeolite) with KOH solution. Dispersion of aggregated zeolites into small particles J. Kanai, N. Kawata
397
Design and preparation of vanadium resistant FCC catalysts D.J. Rawlence, K. Gosling, L.H. Staal, A.P. Chapple
407
Double substitution in silicalite by direct synthesis : a new route to crystalline porous bifunctional catalysts G. Bellussi, A. Carati, M.G. Clerici, A. Esposito
42 1
Study on titanium silicalite synthesis M. Padovan, F. Genoni, G. Leofanti, G. Pemni, G. Trezza, A. Zecchina
43 1
Carbon supported catalysts Activated carbon from bituminous coal J.A. Pajares, J.J. Pis, A.B. Fuertes, J.B. Parra, M. Mahamud, A.J. PLrez
439
Carbon-supported palladium catalysts. Some aspects of preparation in connection with the adsorption properties of the supports A.S. Lisitsyn, P.A. Simonov, A.A. Ketterling, V.A. Likholobov
449
Preparation of palladium-copper catalysts of designed surface structure Zs. BodnAr, T. Mall&, S. Szab6, J. Petr6
459
Optimization and characterization of Pt-Fe alloys supported on charcoal P. Fouilloux, D. Goupil, B. Blanc, D. Richard
469
Supported metallic catalysts achieved through graphite intercalation compounds F. Beguin, A. Messaoudi, A. Chafik, J. Barrault, R. Erre
479
VIII
Prepantion of graphite-iron-potassiumcatalysts for ammonia synthesis K. Kalucki, A.W. Morawski
487
Preparation of oxidation catalysts Synthesis of V-P-0 catalysts for oxidation of Q hydrocarbons V.A. Zazhigalov, G.A. Komashko, A.I. Pyatnitskaya, V.M. Belousov, J. Stoch, J. Haber
497
Preparation of well dispersed vanadia catalysts by ultra-high intensity grinding at ambient temperature Z. Sobalik, O.B. Lapina, V.M. Mastikhin
507
Dispersion and physico-chemical characterization of iron oxide on various supports Weijie Ji, Shikong Shen, Shuben Li, Hongli Wang
517
The use of chelating agents for the preparation of iron oxide catalysts for the selective oxidation of hydrogen sulfide P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen, J.W. Geus
527
Preparation of oxidation catalysts with a controlled architecture Y.L. Xiong, L.T. Weng, B. Zhou, B. Yasse, E. Sham, L. Daza, F. Gil-Llambias, P. Ruiz, B. Delrnon
537
Structure and selectivity changes in vanadia-titania-deNOxcatalysts M. Kotter, H.-G. Lintz, T. Turek
547
Binary oxide catalysts synthesized by sequential precipitation C.S. Brooks
557
Zr@ as a support :oxidation of CO on CrO-
T. Yamaguchi, M. Tan-no, K. Tanabe
Methane oxidative coupling by definite compounds (e.g. perovskite, cubic or monoclinic structure, ...) obtained by low temperature processes J.L. Rehspringer, P. Poix, A. Kaddouri, A. Kiennemann
567
575
Novel and unusual preparation methods Preparation of strong alumina supports for fluidized bed catalysts M.N. Shepeleva, R.A. Shkrabina, Z.R. Ismagilov, V.B. Fenelonov
583
Synthesis and regeneration of Raney catalysts by mechanochemical methods A.B. Fasman, S.D. Mikhailenko, O.T. Kalinina, E.Yu. Ivanov, G.V. Golubkova
591
Controlled preparation of Raney Ni catalysts from Ni2Al3 base alloys. Structure and properties S. Hamar-Thibault, J. Gros, J.C. Joud, J. Masson, J.P. Damon, J.M. Bonnier
601
Novel type of hydromating catalysts prepared through precipitation from homogeneous solution (PFHS) method K. Somasekhara Rao, V.V.D.N. Prasad, K.V.R. Chary, P. Kanta Rao
611
IX
Preparation of manganese oxide catalysts using novel NHqMnO4 and manganese hydroxide precursors. Comparison of unsupported and alumina supported catalysts A.K.H. Nohman, D. Duprez, C. Kappenstein, S.A.A. Mansour, M.I. Zaki
617
Influence of surface OH groups and traces of water vapor during the preparation of Ti@-Si@ samples A. Muiioz-Paez, G. Munuera
627
Catalysts and preparation of new titanates R.G. Anthony, R.G. Dosch
637
New methods of synthesis of highly dispersed silver catalysts N.E. Bogdanchikova, V.V. Tretyakov
647
Preparation of high-surface-area V-Si-P oxide catalysts M. Ai
653
Preparation of fine particles of ruthenium-alumina composite by mist reduction method H. Imai, J. Sekiguchi
66 1
Designed catalysts for hydrodechlorination, reduction and reductive amination reactions J.L. Margitfalvi, S. Gobolos, E. Tilas, M. Hegediis
669
Preparation of high surface area hydrogen-molybdenum bronze catalysts C. Hoang-Van, 0. Zegaoui, B. Pommier, P. Pichat
679
New preparation of supported metals. Hydrogenation of nitriles M. Blanchard, J. Barrault, A. Derouault
687
Preparation of highly dispersed gold on titanium and magnesium oxide S. Tsubota, M. Haruta, T. Kobayashi, A. Ueda, Y. Nakahara
695
Preparation of monodisperse colloidal Pt-Re@ particles using microemulsions A. Claerbout, J. B.Nagy
705
New organometallic active sites obtained by controlled surface reaction of organometallic complexes with supported metal particles B. Didillon, A. El Mansour, J.P. Candy, J.M. Basset, F. Le Peltier, J.P. Bournonville
717
Conversion coatings on stainless steel as multipurpose catalysts L. Aries, A. Komla, J.P. Traverse
729
Author Index
74 1
Studies in Surface Science and Catalysis (other volumes in the series)
745
X ORGANIZING COMMITTEE President Prof. B. DELMON, Universitt Catholique de Louvah Executive Chairmen Dr P. GRANGE, Universitt Catholique de Louvain Prof. P.A. JACOBS, Katholieke Universiteit Leuven Dr G. PONCELET, Universitt Catholique de Louvain Dr P. RUIZ, Universitt Catholique de Louvain
SCIENTIFIC COMMITTEE
Dr D. ARNTZ, Degussa AG, Germany Dr J.L. CIHONSKI, Catalytica, U.S.A. Dr Ph. COURTY, Institut FranGais du Pttrole, France Prof. B. DELMON, Universitt Catholique de Louvain, Belgium Prof. E.G. DEROUANE, Facultts Universitaires N.-D. de la Paix, Belgium Dr T. EDMONDS, BP Research Centre, U.K. Dr J.W. GEUS, Rijksuniversiteit Utrecht, The Netherlands Dr P. GRANGE, Universitt Catholique de Louvain, Belgium Dr J. GROOTJANS, Labofina, Belgium Mr C. HAMON, Zeocat, France Dr H. HINNEKENS, Labofina, Belgium Prof. P.A. JACOBS, Katholieke Universiteit Leuven, Belgium Dr W.T. KOETSIER, Unilever Research Laboratonum, The Netherlands Dr 0. KRAUSE, Neste Oy, Finland Dr L. LEROT, Solvay & Cie, Belgium Dr G. MATHYS, Exxon Chemical International Inc., Belgium Dr T. MEURIS, Belgian Shell, Belgium Dr G. PONCELET, Universitt Catholique de Louvain, Belgium Dr L. PUPPE, Bayer AG, Germany Dr P. RUIZ, Universitk Catholique de Louvain, Belgium Dr P. SCHWARZ, Enichem SPA, Italy Dr M. TOKARZ, Eka Nobel AB, Sweden Dr D. VANDE POEL, Catalysts and Chemicals Europe, Belgium Mr A. VAN GIJSEL, UCB SA, Belgium Dr R. van HARDEVELD, DSM Research, The Netherlands Mr A. VASTEELS, Kemira SA, Belgium Dr D.E. WEBSTER, Johnson Matthey, U.K.
XI
FOREWORD The organizers are pleased to present the Proceedings of the Fifth International Symposium on the "Scientific Bases for the Preparation of Heterogeneous Catalysts". These Proceedings correspond to the fourth organized in Louvain-la-Neuve, the first having taken place in Brussels. Throughout the five symposia, held successively in 1975, 1978, 1982, 1986 and 1990, the organizers have not departed from their initial objectives, namely to bring together experts from both Industry and Universities in order to discuss the scientific problems involved in the preparation of heterogeneous catalysts, and to encourage, as much as possible, the presentation of research work on catalysts which are of real, industrial significance. Indeed, even if industrial researchers have easy access to the work carried out in university laboratories or research centers, the reverse is not always true. But this feedback is nonetheless indispensable, as the university staff is not always sufficiently aware of the needs of industry and of the problems encountered in the preparation of real catalysts, which correspond ultimately to the most challenging issues. This is one of the reasons why at least 50% of the members of the scientific committees have always come from industrial research and development organizations (at this symposium, 20 out of the 27 members came from industry). This major goal of linking Industry and Universities was partly fulfilled at the Fifth Symposium : indeed, out of the 338 participants, 182 belonged to industry. Although only 25 abstracts were submitted by industrial laboratories, the quality of the corresponding work was outstanding : 17 were selected by the Scientific committee. Another established highlight of these symposia is the reservation of a substantial part of the program to new developments in catalyst preparation, new preparation methods and new catalytic systems. Indeed, the fact that chemical reactions which were hardly conceivable a few years ago have now become possible through the development of appropriate catalytic systems proves that catalysis, like all industrial and academic activities, is in a constant state of progress. Because of the very large number of submitted abstracts (234), the unanimous wish expressed by the Scientific Committee to avoid parallel sessions, and the desire to accept the largest possible number of contributions which could be accomodated in a reasonable sized volume of the Proceedings, it was decided to organize a poster session, to suppress the half-day session devoted in previous symposia to normalization methods, and not to print the discussions. This decision allowed us to accept 70 papers, half presented orally, the other half as posters. In these Proceedings, the papers (including three extended communications) are grouped under the following headings : . Studies of unit operations in catalyst preparation (19) . Catalyst preparation via the sol-gel route ( 3 ) . Preparation of catalysts from layered structures and pillaring of clays (10) . Preparation and modification of zeolite-based catalysts (6) . Carbon supported catalysts (6) . Preparation of oxidation catalysts (9) . Novel and unusual preparation methods (17)
XI1
Finally we would like to express special thanks to 26 industrial companies for their financial support, and especially to Catalysts and Chemicals Europe who generously provided the reception on the occasion of their 25th anniversary. The financial contribution of these companies permitted us to rearrange the budget. In this way, their support allowed several participants from countries with economical difficulties to benefit from financial aid so that they could attend the Symposium and present their communication.
Prof. B. DELMON Dr P. GRANGE Prof. P.A. JACOBS Dr G. PONCELET
XI11 ACKNOWLEDGEMENTS
The Organizing Committee thanks Professor P. Macq, Rector of the Universite Catholique de Louvain, who allowed the Fifth International Symposium to be held in Louvain-la-Neuve. We also gratefully acknowledge the University Authorities for providing us with facilities, and in particular Dr L. Van Simaeys, Head of the Library of Sciences, who provided us with the lecture room where the Poster session was organized. The organizers also thank Professor V. Hanssens for his welcome address to the participants. At this Symposium, even more than in the previous ones, the members of the Scientific Committee were faced with a very difficult task in selecting the communications. They are all most sincerely thanked for the outstanding job which they accomplished. The Organizing Committee gratefully thanks the authors of the 240 submitted abstracts, those who contributed an oral or a poster presentation, as well as those whose contribution could not be selected, mainly because of the limitations of time and space. The Organizers are pleased to thank the authors of the stimulating extended communications, and in particular Dr J.P. Reymond, Dr G. Groen, Dr D.L. King and Dr K.P. de Jong for their excellent oral presentations. Sixteen people deserve special thanks for their performance as session chairmen during the symposium : Dr D. Arntz, Dr J. Cihonski, Prof. E. Derouane, Dr E.B.M. Doesburg, Prof. J. Geus, Dr C. Hamon, Mr K. Johansen, Dr G . Mathys, Prof. J. B.Nagy, Prof. J.T. Richardson, Dr D.S. Thakur, Dr D. Van de Poel, Dr D.E. Webster and Dr F. Wunde. The hostesses of the REUL (Relations Extkrieures de 1'Universitk de Louvain), and particularly Mrs F. Volon-Bex, are congratulated on their perfect achievement. We also want to extend our gratitude to Mr M. Van Windekens, of the "Service du Logement", for his dedication to the symposium, We also owe our particular thanks to the secretaries, F. Somers, M. Saenen and especially P. Theys who had the hidden part of the organization of the symposium in their charge, from its inception to its end. Finally, the Organizers want to mention in their acknowledgements all the people from the "Unit6 de Catalyse et Chimie des MatCriaux DivisCs" and the "Centrum voor Oppervlaktechemie, K.U. Leuven", who contributed to the success of the symposium, in particular : F. Bautista, N. Blangenois, R. Castillo, S. Colque, L. Daza, S. Giraldo, E. Lament, R. Maggi, H. Matralis, R. Molina, S. Moreno, E. and N. Paez, C. Papadopoulou, G. Pelgrims, E. Ponthieu, L. Portela, M. Remy, P. Ruiz, M. Ruwet, R. Sosa, M. Tielen, A. and M. Vieira Coelho, and L.T. Weng.
XIV
FINANCIAL SUPPORT
The following companies agreed to provide financial support to the Fifth Symposium. The Organizers are grateful to them for their generosity.
AKZQ Catalysts AUSIMONT CATALEZATORI BRITISH PETROLEUM International Ltd. CATALYSTS AND CHEMICALS EUROPE DEGUSSA AG DOW BENELUX B.V. DSM Research EKA NOBEL AB EXXON CHEMICALS INTERNATIONAL HALDOR TOPSOE A/S JOHNSON MATTHEY CHEMICALS Kontaktgruppe Forschungsfragen (CIBA-GEIGY AG, HOFFMANN-LA-ROCHE AG, LONZA AG, SANDOZ AG) LABOFLNA S.A. METALLURGIE HOBOKEN-OVERPELT MONSANTO EUROPE NORSK HYDRO PROCATALYSE REILLY CHEMICALS REPSOL PETROLEO TEXACO TOLSA UNION CHIMIQUE BELGE (UCB)
The Netherlands IdY U.K. Belgium Germany The Netherlands The Netherlands Sweden Belgium Denmark U.K. Switzerland
Belgium Belgium Belgium Norway France Belgium Spain U.S.A. Spain Belgium
The Organizing Committee would like to especially thank CATALYSTS and CHEMICALS EUROPE for the reception which they generously offered on the occasion of their 25th anniversary. We are also grateful to ENGELHARD-DE MEERN (The Netherlands) for supplying the conference folders.
G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
1
ILLUSTRATION OF PROCESS SCALE-UP IN HETEROGENEOUS CATALYST PREPARATION I. BIAY, G . DESSALCES, C. HYPOLITE, F. KOLENDA* and J.P. REYMOND "Labomtoire GCnie de la Fabrication des Catalyseurs Hitiroghes" UM36 CNRS-IFP ; IFP-CEDI - BP 3 - 69390 VERNAISON (France) *Institut Franqais du Pitrole, IFP-CEDI - BP 3 - 69390 VERNAISON (France) SUMMARY The command of heterogeneous catalyst preparation and process scale-up from laboratory to industrial plant requires research at a pilot scale. This paper presents the methodology followed to study catalyst preparation. The case of amorphous silica-alumina dried solids, involving a sol-gel route is here developped. Influence of operating variables on catalyst properties (i.e texture) at each stage of the process and scale effect are analyzed. Pilot plant runs point out operating problems, equipment effects on product specifications and simulate key steps in manufacturing process.
INTRODUCTION The world market for heterogeneous catalysts is expected to reach 4billion dollar global business (1). Despite a slow growth and small profit expectation, catalysts suppliers andusers arevery active in new catalyst developments. This can be explained by the strategic role a catalyst plays in production sales (2). New chemical products are requiring more elaborate catalysts. Heterogeneous catalysts can be compared to high performance materials which greatly enhance production process competitiveness andeconomics. A new catalyst can boostprocessefficiency andreduce production costsormakeitobsolete (3) In spite of numerous scientific studies (4-6), the industrial preparation of heterogeneous catalysts is regarded asempirical and still remains an "art" proceeding through a "know-how" jalously kept by the catalyst manufacturers. As aresult, it is crucial to master catalyst preparation and scale-up from the laboratory to the manufacturingplant (7). Each step of theprocess ofpreparation must be characterized not only with physical and chemical techniques but also by using chemical engineering science (8). Physical (heat, mass and momentum transfers) and chemical phenomena (reactions, kinetics) of each unit operation are pointed out and studied using chemical engineering concepts (9). These determinations are performed at the laboratory and then reproduced on a pilot plant. The size of each element of thispilotplantmust bechoseninorder to takeintoaccount limitingeffectssuchaswalleffect orsizeeffect(l0). Pilotplantpreparation using industrial equipmentis anecessaryintermediate steprequiredforasucessfull scale-up of the process. It offers the following assets.
2
I. Preparation of prototvpe catalvsts which will be as similar as possible to the industrial catalyst in terms of activity, selectivity, life time performance, shape and evaluation of the production cost. 2. Simulation of the kev sm in the manufacturingprocess and quantificationof the influenceof each step on the catalyst characteristics (porosity, surface area, chemical composition, bulk properties).
3. Establishment of extrapolationrules using chemical engineering concepts.
4.Definition of product suecificatioqchart for each manufacturingstep in order to assure catalyst performance and reproductibility.
5. Avoidanceof process scale-uu pitfalls, such as mixing problems,raw material purity and availability, fluid rheology which can make fluid transport impossible,solid handling...which cannot be clearly shown during the earIy phases of the catalyst development in the laboratory (7). Advantages and valuable information brought by pilot scale catalyst preparation are illustrated in this paper. Itrepresentspartofthe workcarried out by ourresearchp u p which associatesthe"Institut FranCais du P6trole" and the "Centre National de la Recherche Scientifique".
I - PROCESS SCHEME
In the field of heterogeneouscatalysts, silica-alumina are widely used as acid supportsor catalyst matrices. Their preparation involves a large number of unit operations, usualy present in catalyst manufacturing processes, such as precipitation or gelation, filtration, drying, ion exchange...These unit operations are gathered on figure 1. Figure 1- Example of unit operations in catalyst elaboration
Intensiveuse of fluid bed type reactors led us to study the preparationof solidsused in suchcatalytic reactors. In this paper we examine the preparationof spray-dried silica-aluminamatrices at the laboratory (several hundred of grams) and on a pilot plant (several tens of kilograms). The figure 2 shows the chosen process scheme. The first step of this industrial process is the silica hydrogel formation obtained by successive additions of reactants (batch operation) using a sol-gel technique. A dilute sodium silicate solution is partly neutralized by adding a sulfuric acid solution to form colloldal particles (sol state) which link together giving a tridimensional network (hydrogel state). Addition of aluminiumsulphatefollowedby pH adjustmentof the suspensionwith ammoniainduces alumina precipitation and its incorporationwith silica. A catalytically active phase or other solids such as clays can be added to the hydrogel. This hydrogel is then filtered, washed and repulped in Figure 2 - Catalyst elaboration process
SODIUM SILICATE
ACID
7
CLAYS
-----
SILICA HYDROGEL I
SILICA - ALUMINA HYDROGEL
I
FILTRATION ,WASHING SPRAY - DRYLNG IONIC EXCHANGES FINAL DRYING
I
5
water to make it pumpable. The suspension is spray-driedin order to produce spherical particles which are washed and dried. At the laboratory such a preparation takes place in a five liter s t i d glass vessel. This vessel is equipped with sensors (pH, temperature, stirrer torque) which allow the control of the gelation. Filtrations and washings are performed on a vacuum filter. The filter cake is repulped in water and the suspension is dried in a laboratory spray-dryer (3 kg/hr water evaporation rate). This dryer produces 20-30 micrometers diameter particles with 15-25 % moisture content. A scheme of the pilot plant unit dedicated to the preparation of the catalysts is shown on figure 3. The four first steps of the process (gelation, filtration, drying and washing) arerepresented. The final stage of the preparation which could involve drying alone or drying and calcination depending on the application, is not covered here. All the equipments are of industrial type. The size of each piece of equipment (agitators, filters, dryers, pumps ...) has been set by the need to extrapolate the manufacturing process to industrial scale. Gelation is performed in a stainless-steel reactor (1000 liters) equipped with sensors allowing us to follow the hydrogel formation (pH, temperature, stirrers rotary speed, power consumption). The hydrogel is filteredon avacuumdrumfilter. The filter cake (= 15 % solidcontent) is repulped with water and the suspension is dried in a pilot spray-dryer (100 k g h r water evaporation rate at 400°C). The spherical particles of 65 pm average diameter contain 15-25 % of residual water. Centrifugal atomization is carried out using a vane atomizer wheel. The impureties contained in the spray dried particles (sulphates) are removed by successive filtrations and washings of aqueous suspensions of these particles. These steps are simultaneously operated on a vacuum belt filter. The last step of the process consists in drying the washed particles at 350OC.
II - EXPERIMENTAL METHODS 1. Analvtical methods Sampling procedures including pretreatments have been defined in order to obtain representative samples of solids, liquids and suspensions by means of grinder, spinning rifler, sampler probe.,.. 1.1. Chemical analvsis and moisture content Inductively coupled plasma (I.C.P.) and atomic absorption quantitative chemical analysis of Na, Si, Al, S and Nare achieved by a C.N.R.S. laboratory, the "Service CentraldAnalyses (S.C.A.)". Mass balances of all catalyts preparation steps are based on these chemical analysis.
6
1.2. Textural and momholoeical determinations Pore texture of a catalyst (i.e. pore size distribution P.S.D., total pore volume and surface of the pores) governs catalytic performance such as activity and selectivity, through diffusion of reactants and products in the pore system of the solid, density, mechanical strength and thermal stability (5, 1 1, 12). 1.2.1. Thermooorometry The hydrogel formation and the spray-dryingsteps are the key operationsof the process in terms of influenceon the catalyst properties (texture and morphology). It is important to measure the influence of operatingvariables for each step on the pore textureof the catalyst. The difficulty arises from the necessity of measuring the porosityof an hydrogel (95 % water and 5 % solid) and of axemgel (drysolid particles) with the same technique. To our knowledge, thermoporometry is the only method which can apply (13,14). It is acalorimetrictechniquebased on the measurementof the temperatureof solidification of aliquidconfined or divided into a porous texture. Pore diameters ranging from 2 to 150nm (mesopores) can be measured.
1.2.2. Other textural methods The texture of solids is evaluated using well-known techniques : - structural density : helium picnometry - total pore volume of xerogels : high pressure mercury porosimeter - surface area and pore diameter distribution : nitrogen adsoption (B.E.T. and B.J.H. methods)
colloidal silica particle surface area as well as xerogel silica particle can be measured by the Sears' analytical method (15). -
Typical silica-alumina we producehavea specificarea of 250-600 m2g-l, a skeletaldensityof 2,l2,4g. cm~3,aporevolumeof0,5-0,9cm3-g~1 (poresofdiameterlessthan7,5pasdeterminedbymercury porosimetry). 1.2.3. Morphologv and size of particles Optical and scanningelectronicmiscrocopiesgive information on the morphology and the size of solid particles. The photographies of figure 4 show that silica (photo a) and silica-alumina (photo b) particles we obtain are quite spherical. Silica-aluminaparticles seem to have arougher surface than silica particles. Size distribution of powdery raw materials,suspensionsand xerogels are determined by laser diffraction (size range : 1,2-560 pn). Size measurements of powders are performed on either aqueous suspension and dry aerosols (interactions between water and dried particles can result in breakage of particles).
7
Figure 4
a) spray-dried silica
b) Spray-dried silica-alumina
Diameter of droplets generated by the cenb-ifugalatomizing device have been measured with this technique and results are discussed further in the text. 1.2.4. Other characterisations The knowledge of therheological behaviour of hydrogels is very important for pipe, pump and mixer sizing and design. It allows also a theoretical approach of internal structure of gels and interaction between gel particles. Hydrogel flow curves are established by means of a rotational rheometer. During the gelation viscosity evolution of the fluid is followed in the reactor by means of a torque sensor set up on the agitator shaft (laboratory reactor). Kinetic study of the filtration and establishment of mass balances during this unit operation allow the determination of the specific resistance, the compressibility and the filtrability coefficients of the filtration cakes (16). The optimisation of pilot filter operation can be. deduced from these determinations. Theresistance to attrition is an important characteristic of catalysts usedin fluid beds (5). This mechanical property is evaluated with an air jet Gwyn-type-apparatus (17).
8
I11 - RESULTS AND DISCUSSIONS The process involves two key steps which need to be mastered:
- the gelation step which gives to the catalyst its textural and catalytic characteristics - the spray-drying step which gives to the catalyst its morphology (shape and size of the particles).
The results presented is this paper concern these two steps. Catalytic activity of the solids will not be described here. 1. Textural studv
The influenceof operating variableson the silicaand silica-aluminatextures isquantified by means of thermoporometry. During the gelation step these variables are :
. pH of the sol-gel transition
. temperature of hydrogel formation . mixing . batch or continuous preparation . percentage of alumina . concentration of reactants . aging time of hydrogels. We also study the influence of drying on the textural characteristics of silica and silica-alumina. Figure 5 shows the differences observed between the textureof silica and silica-alumina hydrogels. Silica presents a very narrow pore size distribution (2-6 nm) with a median pore radius around 3 nm and pore volume of 0,5 cm3 g-l. The addition of alumina broadens the P.S.D. which ranges for silicaalumina from 2 to 20 nm and pore volume (1- 1,4 cm3 g-'). The figure 6 illustrates the effect of spraydrying on the texture of silica and silica-alumina ( 25 % of alumina). Drying results in an important skrinkage and noticeable reduction of pore volume and median pore diameter for the two-types of hydrogels. The effect of drying is more drastic on the silica-aluminagel. Drying tends to minimize the influenceof each operating variables of the precipitation step. Nevertheless xerogels are similar to initial hydrogels in terms of texture.
9
-
Figure 5 Comparison of pore size distribution (cumulativecurves) of silica and silica-alumina hydrogels
" .
5
0
10
I
20
15
R (nm.)
Figure 6 - Effect of drying on pore size distribution of hydrogels
The figure 7 shows the scale effect on the texture of hydrogels prepared at laboratory and on pilot plant. No differences can be observed on silica-aluminasamples (curves 3 and 4). On the contrary P.S.D. of pilot plant silica sample is broader than P.S.D. of laboratory silica sample (curves 1and 2). Differences in mixing efficiency of each reactor are probably the cause.
Figure 7 - Comparison between laboratory and pilot plant preparation
------ --
SiO2 laboratory ( I ) Si02 pilot plant ( 2 ) SiO2lAUO3 laboratory (3)
0
5
R (nm.)
15
10
20
TABLE 1 Effect of subhates on Dorous texture of silica gel filtration
gel washing
Hydrogel
X
X
Xerogels
X X
X
particle washing
X X
pore surface m2.g-1
radius nm
659
476
3,4
236 233 137
282 237 117
22 22 2,1
pore volume mm3.g-1
11
TABLE 2 Effect of subhates on vorous texture of silica-alumina gel filtration
Hydrogel Xerogel
gel washing
X
X
X
X
X
particle washing
X X
porous volume -34-1
porous surface m2.g-1
radius nm
1561
601
4.4
500
373
2s 23 2,1
267
345
239 308
Thermoporometrycan indicate optimunway and timing to realize an operationalong the manufacturing process. An example is provided by theelimination of sulphatesproduced during silicaand alumina precipitations. Table 1 and 2 show that silica and silica-aluminatextures are depending on the presence of sulphatesduring the drying. In tables 1and 2, the crosses indicate the operations (column) realizedon samples(line). In each table comparison between the hydrogel andcorrespondingxerogels shows the influence of drying. Comparison between xerogels shows the influence of the presence of sulphatesduring the drying. It has been checked that xerogelparticles washingdoes not affect theirporous texture. The texture of silica xerogels is detrimentallymodified by the presence of sulphates (18). Sulphatescristallizationanddeposition duringdryingdecreasethe pore diameter and must be avoided. Sulphatesmust be washed out before the drying of silica hydrogels in order to preserve the hydrogel texture. The same observations can be made for silica-alumina (see table 2).
2. Influence of rheolotkal behavior of eel on meparation Simulation of catalyst preparation in the laboratory and then at a pilot plant are complementary. Pitfalls along the way can lead to unsucessfullscale-up. These problemscan only be studiedat the pilot plant where all the equipment used is of an industrial conception. One of the major problems encounteredwith hydrogelsconcerns their ability to be transfered from one vessel to another. These problems are not observedin the laboratory and can result in huge operating difficultiesin industrialplants if not studied carefully. Study of the hydrogel rheology is very important for the design of agitators, pump selection and pipe sizing.
The figure 8 shows hydrogel rheograms for silica and silica-alumina prepared at the pilot plant. Hydrogels are non-newtonian plastic fluids of Bingham type and can be described by the followingrelation:
12
-
Figure 8 Rheograms of silica and silica-alumina hydrogels
-
SIUCA GEL
80 I60 240 SHEAR RATE(S-1)
320
4
zc (yield stressvalue; Pa), the minimum stress to develop in order to establish flow, is acrucial characteristic for scaling.
+, is theplastic (orBingharn) viscosity
(Pa.s) and y is the shear rate(s'*).
A slight hysteresis can be observed, showing a slight thixotropy. As a result from these measurements the minimum pressure drop,
AP
(TImi*
by unit lenght in a pipe of diameter D, needed to establish the flow can be calculated from AP
Fluids after gelation have low stress values due to a low solid content (=5 %). After filtration and repulping, zc is noticeably modified and strongly related to the solid content in the hydrogel. The Bingharn viscosity remains small (few mPa.s). The effect of the solid content in silica-alumina hydrogels is illustrated on figure 9. The value of zc increases from 2 Pa to 20 Pa if the solid content increases from 7 3 to 15%. After a certain level, an increaseof 0,l percent in the solid content can cause a drastic change of zc which must be controlled during the repulping of the filtration cakes (figure 10). Pressure drop is affected in the same way and can reach high values, as shown on figure 10, depending
13
-
Figure 9 Influence of water content of a silica-alumina hydrogei on its rheologkal properties
--------
25
84.89 % of wafer 8S.69 % of water 86.23 90ofwater 87.98 90ofwater
I
20
0
I
I
30 60 90 SHEAR RATE(S-I) Figure 10
- Repulping test
REPULPING TEST [3 RHEOLOGY MEASUREMENTS
50.0
10.0 Q
5.0
crw 4 4
I.0
9 0.5 rn 0.I
120
I50
14
on pipe diameter and pipe lay out. A bad control of the solid content can result in an inability to pump the fluid at the suction of the pump or no flow at all at the discharge side if pressure drops are excessive. Pilot plant can also avoid pitfalls in equipment selection. The two rheograms on figure 11 show how a screw type pump can affect the rheological properties of silica-alumina hydrogel. The shear stress imposed by the rotor-stator couple of the pump results in an increase of the yield value of the hydrogel. In the pump, the size of the particles which compose the hydrogel is decreased by a milling effect and the forces of cohesion are increased. This can stop the flow if the pump characteristics have been underestimated. This results also in modification of the gel structure and ability to be spray dried.
-
Figure 11 Pump effect on the rheological properties of a silica-alumina hydrogel
-
SUCTION SIDE
3. Studv of the smav-drving The hydrogel suspension is atomized using centrifugal force into a hot air stream. The drying step is rather fast (few seconds to 30 seconds). Residual moisture of the spherical particles is still high (around 20 % wt). It is necessary to master the shape, the size andresistance to attrition of spherical particles produced by the dryer. The properties of the solid are determined by the action of two phenomena: - droplet formation, which depends on gel composition, viscosity, surface tension, density and the pulverisation technique.
15
- drying phase which is the result of interactions between acontinuous phase (hot air stream) and adiscontinuous, highly dispersed phase (droplets of hydrogel). The efficiency of the drying will depend on the droplet size, hydrogel properties (texture, composition), hot air characteristics (temperature, flow rate) and air hydrodynamics in the drying chamber. It is important to understand and quantify these underlying phenomena in order to master spray drying process and its extrapolation. The centrifugal atomizing technique produces spherical particles with a size distribution between 20 to 150 microns as demonstrated on figure 4. Research is being conducted tocorrelate physico-chemical properties of the feed to droplet size distribution at the exit of the atomizing device. These experiments are conducted in a mock-up. Atomizing wheel dimension and rotational speed can be adjusted in order to have a peripheral speed of ejection varying between 50 and 100m.s-l, common valuesencountered in industrial practice. Droplet sizing is realized usinglight scattering technique. The laser beam goes through perpendicular to thedroplet umbrella created by the turbine. The figure 12 shows droplet size distribution for water and hydrogel suspension. The atomizing speed is 80 m.s-l and the feed flowrate 3 kg.hr-l (curves 1 and 2). Curve 3 represents the particle size distribution of a xerogel, which is produced from the hydrogel of curve 1dried in the laboratory spray dryer using the atomizing device described here.
-
Figure 12 Diameter distribution (cumulative curves) of droplets geuerated by atomizing vaned wheel
100 90 80
70 60 50
€3 20 10
0 5
10 50 100 DIAMETER (micron)
500
16
Droplet size distribution of atomized water and hydrogel are quite similar. These two-fluids (a newtonian one and anon-newtonianone) have same surfacetension and, from ourresults, we think that their apparentviscosities arevery similarat the high yiel stressexisting at the atomizingwheel periphery.
Comparison of curves 1 and 3 shows that drying does not affect droplet size distribution and that phenomena such as coalescenceof dropletduringdrying are not very importantfor this kind of product. Correlationscan be establishedbetween droplets and dried particle size distributions, for spray dryers of different sizes.
CONCLUSION The formulation of an industrialcatalyst dependson the choice of catalytic reaction and reactor design (19). Once the formulation has been determined, the first step of the catalyst manufacture is the choice of the type of preparation. Laboratoryexperimentsallow to specifyinfluencesof operatingvariables on the catalystcharacteristicsanddefine processoperation specifications. Pilot plantpreparation is the necessary and complementarystep, devoted to check faisability and reproductibilityof the chosen process using industrial types of equipmentsas well as pointing out scale effects and operating problems. This paper is an attempt to illustrate this methodology through the elaborationof silica-alumina microspheres usable in fluid bed reactors. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14
15
i6
S. Wilkinson and D. Hunter, Chem. Week, 144, (1989), 24-40. L.L. Hegedus (Ed.), "Catalyst Design: Progress and Perspectives", J. Wiley and Sons, NewYork, (1987), p 1-10. M.M. Van Kessel, R.H. Van Dongen and G.M.A. Chevalier, O.G.J., Feb. 16, (1987), p 55. B. Delmon and Coll. (Eds), "Preparationof Catalysts: ScientificBases for the Preparationof Heterogeneous Catalysts"; vol. I, 11, 111and IV, Elsevier, Amsterdam, 1975, 1979, 1983, 1987. J.F. Le Page and Coll. (Eds); "Catalysede Contact : Conception et Mise en Oeuvre des Catalyseurs Industriels", Editions Technip, Paris, 1978. D.L. T r i m , "Design of Industrial Catalysts", Chem. Eng. Monographs 11, Elsevier, Amsterdam, 1980. E.F. Sanders and E.J. Schlossmacher,in B.E. Leach (Ed.),"Applied Industrial Catalysis", Academic Press, London, 1,(1983),pp. 31-40. P. Trambouze, J.P. Reymond,D. Vanhove and F. Kolenda, Information Chimie n0294, (1988), pp. 275-282. J.N. Fulton, Chem. Eng., July 7, (1986), pp. 59-63. P. Trambouze, Chem. Eng. Progr., 86, (1990) pp. 23-31. K.D. Ashley and W.B. Innes, Ind. Eng. Chem., 44, (1952), pp. 2857-2863. 0. Levenspiel (Ed.), "The chemical Reactor Omnibook", OSU Book Stores Inc., Corvallis, Oregon, (1984), ch.23. M. Brun, A. Lallemand, J.F. Quinson and Ch. Eyraud, ThermochimicaActa, 21, (1977),pp. 5988. J.F. Quinson, M. Brun, in K.K. Unger and Coll. (Eds), "Characterization of Porous solids", Elsevier, Amsterdam, (1988), p 307-315. G.W. Sears Jr., Anal. Chem., 28, (1956) pp. 1981-1983 P. Rivet, "Guide de la skparation liquide-solide", SociCtC Franqaise dz Filtration, Idexpo, Ed. Cachan, (1981
17
17 J.E. Gwyn, A.I.Ch.E., 15, (1969) 35-39. 18 M.E. Winyall, in B.E. Leach (Ed.), "AppliedIndustrial Catalysis", Academic Press, London, 3, (1984), ch. 3, pp. 43-62. 19 R. Montarnal and J.F. Le Page, in B. Claude1 (Ed.), "LaCatalyse au Laboratoire et dans 1'Industrie", Masson Paris, (1967), pp. 231-287.
This Page Intentionally Left Blank
G.Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
19
DEPOSITION PRECIPITATION ONTO PRE-SHAPED CARRIER BODIES. POSSIBILITIES AND LIMITATIONS
K.P. de Jong Koninklijke/Shell-Laboratorium, Amsterdam
1003 AA
Amsterdam, The Netherlands.
(Shell Research B.V.) P.O. Box 3003,
ABSTRACT Several features of deposition precipitation onto pre-shaped carrier bodies are dealt with. Firstly, the kinetics and mass transfer effects of catalyst synthesis via deposition precipitation onto pre-shaped and powder carriers have been studied under pseudo-stationary conditions. The precipitation of manganese hydroxide onto silica by urea hydrolysis has been used as a model reaction. The overall disappearance of manganese ions from the aqueous solution could be described as a first-order process. The rate-determining step for Mn deposition is related as expected - to the urea hydrolysis. From the distribution of Mn over the silica granules after precipitation the rate constant for the surface deposition process has been determined. The latter process has a much higher rate constant than the rate-determining hydrolysis reactions. The surface reaction appears to determine the ultimate distribution by a combined process of reaction and diffusion. The consequences of this study for the viability of deposition precipitation onto pre-shaped carriers for practical application are addressed. Secondly, transient phenomena taking place initially during deposition precipitation onto large carrier particles have been considered. More specifically, the occurrence of transient pH gradients over carrier bodies directly after liquid imbibition has been utilized to control the local rate of precipitation in carrier bodies. This could be effected by applying a precipitation reaction whose rate depends on the pn. It has been found that thereby nonuniform activity distributions can be realized in a controlled manner. The method seems to be especially useful to produce egg-yolk type catalysts by the application of redox reactions for ureciuitation. Several catalvst formulations _. have been prepared via this novel synthesis method, viz. Mo/Si02, Cu/A1203 and Ag/A1203.
INTRODUCTION Catalyst synthesis by means of deposition precipitation comprises application of the active component onto an existing support via a chemical reaction. This reaction gives rise to the formation of an insoluble compound which involves the active element. The insoluble compound can be formed by an increase of the pH of the solution (hydroxide precipitation), a valence change of the element in question and the like. Under certain conditions, such as a suitable interaction between the compound and the carrier, the preparation
20 method has a number of unique features, e.g. a uniform distribution of the active component over the carrier surface even at high loadings. The deposition precipitation method has been known for quite some time; its history is summarized in Table 1. It has been extensively studied by Geus and co-workers ~51. TABLE 1 History of deposition precipitation. Year
Assignee/author
~~~
~~
Remarks
Reference
~
1943
IG Farben
Precipitation of (hydr)oxides, sulfides, selenides
1
1967
Stamicarbon
Precipitation with urea from homogeneous solution; powder carriers
2
1970
Unilever
Precipitation with urea from concentrated solutions; prevention of evaporation
3
1970
Stamicarbon
Precipitation with reduction reaction from homogeneous solutions; powder carriers
4
1983
Geus et al.
Basic studies on deposition precipitation with powder carriers
5
1988
Shell
Controlled non-uniform activity distributions 6 with large, pre-shaped carriers
The work of Geus et al. [5] on deposition precipitation has been directed towards the use of support materials consisting of small particles (powders, particle size smaller than say 1 pm). For practical applications, deposition of the active component onto powders implies that shaping of the small particles into larger bodies has to be done afterwards. The shaping procedure might invoke a number of possible disadvantages such as occlusion of the active component and a low mechanical strength. Therefore, the scope of deposition precipitation for the production of uniform metal distributions would be enhanced considerably if it were applicable with larger (pre-shaped) carrier particles. There are indications in the patent literature [ 3 ] that this is indeed possible. Recent publications [7,8],on the other hand, suggest that with medium-size silica particles (50-100 p m ) precipitation of nickel hydroxide via urea hydrolysis gives rise to significantly enhanced, but uncontrolled concentrations of the active component at the outer surface of the silica particles. In this paper we report on some work undertaken to assess the scope cf deposition precipitation onto large, pre-shaped carrier particles (about 1 mm).
21 Specifically two aspects are addressed: (i) the kinetics and mass transfer effects under pseudo stationary conditions, and (ii) the utilization of a unique feature of precipitation onto large carrier particles, viz. transient concentration gradients occurring directly after liquid imbibition. The kinetics and mass transfer effects are studied with the often considered method of precipitation of metal hydroxydes via a controlled increase of the pH of the solution by the hydrolysis of urea [5,9]. The deposition of manganese hydroxide onto silica was chosen as a model reaction because of its relative simplicity (e.g. no formation of silicates, no formation of basic carbonates). The kinetics o f the relevant reactions have been established and the mass transfer effects during precipitation assessed. Based on the quantitative data a model for the precipitation reaction is formulated. Implications for precipitation onto large carrier particles are addressed. In the course of our work on deposition precipitation onto pre-shaped carrier particles we have found that under certain circumstances the technique can also be used for realizing controlled non-uniform activity distributions over carrier bodies [ 6 ] . After careful consideration it turned out that the basic phenomenon underlying this finding is connected with transient pH gradients occurring in carrier particles directly after liquid imbibition. In case the rate of the precipitation reaction in question depends on the pH of the aqueous phase, the metal (oxide) distribution after precipitation will reflect the previous pH gradient. In this paper the principles of this new application of precipitation reactions in catalyst synthesis are elaborated. Furthermore, several examples of catalysts with their respective metal distributions are described. EXPERIMENTAL Precipitation of manganese onto silica (pseudo-stationarv) Precipitation of manganese hydroxide in the presence of silica was brought about by the hydrolysis of urea according to the overall reaction Mn2+
+
CO(NH2)2
+
3 H20
__ > Mn(OH)2
+
C02
+ 2 NH4'
To this end Mn(N03)2.6H20 was dissolved in 250 ml water; the solution was acidified with 1 ml of nitric acid ( 6 5
%)
and transferred to a double-walled,
thermostatted vessel equipped with baffles and a stirrer. Five gram of silica was added to this solution in experiment U30 and U31, while experiment U33 was carried out without silica and experiment U34 with 10 g of silica. For further details on the concentrations of reactants the reader is referred to Table 2. After heating the mixture to 90
OC
the urea was added at time zero (cf.
Figs. 1-5). At that temperature the reaction was continued for several hours while nitrogen was bubbled through (to prevent oxidation of the di-valent
22
manganese precipitate) and the pH monitored continuously. At certain intervals of time a small sample (about 2 ml) of the liquid was withdrawn, filtered, weighed and acidified. The total manganese content of these samples was determined using Atomic Absorption Spectrometry. From these data the concentration
of manganese in the solution was calculated. TABLE 2 Experimental conditions and kinetics of the precipitation of manganese. kl is the first-order rate constant of the urea decompodition. kt is the first-order rate constant of the manganese precipitation. Exp.
Carrier
Temp. Mn2+ conc. (OC) (mmol .I-')
U30 granulesa) U31 powderb) u33 ~ 3 4 granulesa)
90.5 90.5 88.5 88.5
Urea conc. (mmol.1-1)
9.1 9.1 36 36
kt *lo5 (s-1)
46 46 182 182
3.3 3.3
1.1 2.7
n.d. = not determined a) Grace Davison silica gel: 0.6-1.4 mm, pore volume 1.2 ml.g-', area (BET) 310 rn2.g-'. b) Degussa Aerosil: surface area (BET) 2 8 1 m2.g-'.
kl*
lo5
(S-5
n.d 3.8 2.7 2.7
surface
Precipitation of molybdenum onto silica spheres (transient phenomenal As has been reported before [4], molybdenum can be deposited onto silica powder via the redox reaction: 4M004~-+ 3N2Hq
+
4H20 ->
4Mo(OH)3
+
3N2
+
8OH-
A molybdate solution was prepared by dissolving the appropriate amount of (NH4)6M07024.4H20 and adding concentrated ammonia till pH=8.7. A hydrazine solution of similar pH was obtained by neutralizing N2H5QH with acetic acid. Further experimental details can be found in Table 3. The procedure of contacting the carrier with the solution is further described below. Experiment RK15 was done by first contacting the silica spheres with the Mo solution (0 OC), followed by addition of the cooled hydrazine solution. Subsequently, the mixture was rotated in a round-bottomed flask under nitrogen and heated slowly. In experiments RK29, FX33 and RK37 (Table 3) a mixed Mo/N2H4 solution was applied. This solution (0-10 OC) was circulated for several hours through a small vessel, which was maintained at higher temperatures and contained 25 g of silica spheres. Under these conditions M o was deposited exclusively in the inner part of the spheres except for catalyst RK33 where some precipitation in the liquid occurred. Catalyst RK77 (Table 3) was also prepared using a mixed solution. The reaction was observed to start at the
23
centre of the spheres and continued for 15 min at 0 OC. Subsequently, the reaction was 'quenched' by diluting the reaction mixture with water at 0 OC. TABLE 3 Synthesis conditions of Mo/Si02 catalysts. Cat.
Si02
Soln.
[Mo]
[N2H4]/[Mo]
(g)a)
(ml)
(g/l)
(mol/mol)
pH
Method
T;y?fzk;;e
(%W)
79
2.8
8.7 rotating 1 h at OC flask 0 --* 60 OC in 20 h
15.0
2.0
8.5
1300 15.4
2.0
8.7 recirculation
RK15
25
95
RK29
25
500
RK33
25
Mob)
recirculation
16
19 h at 50 OC
6.7
40 min. at 1 OC
20.6
1
6 h at 7 OC RK37
25
1300 15.4
2.0
9.0
recirculation
30 min at 2 OC
1.5
1
22 h at 30 OC 6.7
RK77 ~~~~
~
~
40
72
2.0
8.5 quench
15 min. at 0
OC
0.66
~~
a) Si02 carrier used: 1.5 mm Shell silica spheres, pore volume 0 . 9 8 ml.g-l, surface area (BET) 263 m2.g-'. b) Determined by X-ray fluorescence.
Precipitation of copDer and silver onto alumina (transient Dhenomena) Copper and silver were deposited onto alumina extrudates applying the reactions and conditions summarized in Table 4. The mixed solution of the metal salt and reducing agent was poured onto the extrudates at ambient conditions. It was observed visually that the precipitation started at, and was restricted to, the central part of the extrudates. After some time the reaction was 'quenched' by dilution with water. Catalyst characterization After drying in air the metal distribution was established using electron microprobe analysis. The scanning electron microscope used was a Jeol-35. A step size between 20 and 30 pm was applied. The metal distribution was determined by carrying out a scan along the shortest line through the centre of the catalyst body.
24
TABLE 4 Synthesis conditions of Cu/y- A1203 and Ag/y- A1203 (egg - yolk) Catal. Formulation RK129
A1203 S o h . Reactants
Cu/A1203
(g)a)
(ml)
10
64
2.7 ml N2H50H solution (80 %w)
pH
Conditions
Reaction
Metal loading')
2.4
2 h at 4Cu2++ N2Hq+2H20 1.8 %w 21 OC + 2Cu20J+N2 Cu + 8H+
1.5 ml acetic acid (99.9 %w)
11.4 g C~(N03)2.3H20 After a spontaneous reaction at 21 OC for 1.5 h and filtration the resulting clear solution was used for precipation onto alumina. RK130
Ag/A1203
41
10
5.0 g AgNO3 0.87 g formalin (37 %w '3320) nitric acid (65 %w)
0.94 3 min. 2Ag+ + CH20 + at H20+2AgJ 21 OC + CHOOH + 2H+
0.38 %w Ag
a) A1203 carrier used: AC300, 1.5 mm cylindrical extrudates, pore volume 0.68 m2.g-'. h) Determined by X-ray fluorescence. RESULTS AND DISCUSSION Kinetics and mass transfer effects under pseudo-stationarv conditions pH measurements. Figures 1 and 2 show the pH records as obtained during the four Mn precipitation experiments. I n all experiments a rapid initial increase o f the pH after addition of the urea at zero time is observed. This increase
coincides with the neutralization of the nitric acid by the ammonium hydroxide generated via the hydrolysis of urea according to the equation CO(NH2)2
+
kl 3 H20
~
> Cog + 2 NH4+ + 2 OH-
(1)
25
The rate of urea decomposition has been calculated from the increase of the pH in the range pH 2-4. The rates obtained directly from the linearized pH curves have been interpreted using the first-order kinetics for the hydrolysis reaction which are clearly indicated in the literature
[lo].
From the litera-
ture mentioned it is furthermore apparent that the rate of hydrolysis of urea is independent of pH. The first-order rate constants kl calculated and summarized in Table 2 , therefore, are valid throughout the experiments. Note that the value of kl is not affected by the presence or the absence of the carrier. The higher value of kl obtained from experiment U31 is not caused by the nature of the silica carrier but by the fact that the temperature is slightly higher in experiment U31 than in experiments U33 and U34. The qualitative features of similar pH curves have already been described by Hermans et al. [9]. In the absence of a carrier (U33 in Fig. 2), after the initial rapid increase of the pH an overshoot develops after 1.5 to 2.0 hours. This overshoot is characteristic of the precipitation process being activated due to the required formation of nuclei of the solid manganese hydroxide phase. Following the nucleation phase (pH=6.6) the pH drops, which coincides with the growth of the nuclei formed. This process of growth takes place as a pseudostationary process at virtually constant pH (pH=6.3). In the presence of a carrier no overshoot o f the pH is observed and the pH level of the pseudo-stationary precipitation phase (pH=5.8) is lowered (Fig. 2 ) . In accordance with previous studies [ 9 ] these observations are ascribed to the smooth, non-activated nucleation at the carrier and to the stabilization of the MII(OH)~ phase by interaction with silica, respectively. The pH curves obtained with silica powder on the one hand and silica granules on the other (Fig. 1) do not display any relevant differences. The apparent difference of the plateau of the records (pH=6.1 with powder and pH=6.2 with granules) is within the experimental error of the pH measurements. PH
PH
1
10
1 2
1
1 4
1
1
6
1
1
8
1
1
10
TIME, h
TIME, h
Fig. 1. Records of pH as a function of time for experiments U30 (granules) and U31 (powder).
Fig. 2. Records of pH as a function of time for experiments U33 (no carrier) and U34 (granules),
26
Removal of manganese from solution. The total concentration o f manganese species dissolved in the aqueous phase as a function of time has been established in the experiments (results in Figs. 3 and 4 ) . From Fig. 3 it is apparent that the rate of removal of Mn from the solution is very much alike for the silica powder and the silica granules. The presence or absence of silica, however, has a large impact on the kinetics of the removal of Mn (Fig. 4 ) . In the absence of silica the precipitation process is slowed down considerably. Furthermore, the shape of the plots is different. The nonlinear shape of the plots (Figs. 3 and 4 ) in the presence of silica suggests a reaction order higher than zero, whereas the linearity of the graph in the absence of silica (experiment U 3 3 , Fig. 4 ) suggests a zero-order process.
In Fig. 5 we have gathered the experimental data on the total Mn concentration in the form of a first-order plot. The experiments carried out in the presence of silica ( U 3 0 , U 3 1 and U 3 4 ) give rise to a linear relationship in Fig. 5 , which supports the idea that the overall precipitation process in the presence of silica can be described by first-order kinetics. For experiment U 3 3 (no carrier), too, a linear plot is obtained, but we hesitate to conclude
from this that the precipitation kinetics are first-order in this case. This hesitation arises from (i) the low levels of manganese 'conversion' attained in experiment U 3 3 , and (ii) the linear relationship obtained in Fig. 4 for this experiment, which suggests zero-order kinetics. From the slopes of the fitted straight lines (Fig. 5) the first-order rate constants (kt) for the overall precipitation reaction of Mn from solution in the experiments were obtained and tabulated (Table 2 ) .
0 50
-
040
-
0 0 2 03 0 L
TIME, h
TIME. h
Fig. 3 . Manganese concentration in the Fig. 4 . Manganese concentration in the liquid as a function of time for liquid as a function of time for experiments U 3 0 (granules) and U 3 1 experiments U 3 3 (no carrier) and U 3 4 (granules). (powder).
Clearly, kt is much larger in the presence than in the absence of silica. From the kt values for experiments U30 and U31, it turns out that the size of the silica particles does not affect the overall kinetics of the precipitation process. The difference with respect to kt between experiments U30 and U 3 4 is caused by the higher rate of urea hydrolysis in experiment U30, which itself is brought about by the slightly higher temperature in that experiment. The four-fold difference in initial Mn concentration (experiment U30 versus U 3 4 ) giving rise to comparable values for kt - especially when the temperature difference is taken into account - further supports the overall precipitation being adequately described as a first-order process. Since the rate of Mn deposition is increased in the presence of silica and is not affected by the silica particle size, it is tempting to conclude that the precipitation process is not influenced by mass transfer effects. In the next section it will be demonstrated, however, that these effects still play an important role. From the rate constants and concentration data gathered in Table 2 it can be easily seen that the rate of urea hydrolysis (mol/s) exceeds the rate of manganese removal (mol/s) by a factor of 5 or more. This implies that the reaction between manganese ions and urea is not a stoichiometric one and it is proposed that part of the ammonium hydroxide formed (cf. reaction (1)) will
0.8
-
Ln (C&) 7
-+ -
0.6
U30 U31
- 8 - u33
--4-u34
-
0.4
-
02-
0
-
I
I
2
I
I
4
I
I
6
I
I
8
I
I
10 TIME, h
Fig. 5 . First-order plot of the rate of removal of manganese from solution. Go manganese concentration at zero time C actual manganese concentration
28
leave the solution as ammonia while the remaining part brings about the hydrolysis reactions of the manganese species. The reactions can be written as follows:
NH~+
+
OH-
-> NH3 t
Mn2+
+
OH-
->
+
H20
(2)
M~(oH)+
(3)
Hereafter we will assume that as a first approach reactions (2) and ( 3 ) can be considered to be in equilibrium. Although the rate of hydrolysis of urea is larger than that of the removal of Mn, reaction (1) is the rate-determining step for the overall precipitation process. The ’selectivity’of the utilization of the generated ammonium hydroxide is determined by ‘equilibria‘ (2) and ( 3 ) . It is expected that higher temperatures and/or a higher pH will favour (2) over ( 3 ) . This indeed explains that for experiment U33 the rate constant kt is lower than in the other cases, seeing that, as is shown in Fig. 2, the pH at which precipitation takes place is much higher. Deuosition reaction of manganese. The final reaction which leads to deposition of the partly hydrolysed Mn species onto the silica carrier is tentatively described as
Mn(OH)+
+
SiOH
+
OH-
k4 >
SiO-Mn(0H)
+
H20
(4)
The kinetics of reaction (4) cannot be assessed from the overall precipitation reaction since - as outlined above
-
the latter is dominated by the urea hydro-
lysis in combination with the ‘selectivity’determining reactions (2) and ( 3 ) . The kinetics of the consecutive reaction ( 4 ) could be found, however, from the resulting distribution of manganese over the silica granules. An example of such a distribution as obtained from electron microprobe analysis is shown in Fig. 6. The qualitative shape of the distribution indicates that reaction (4) is influenced by both kinetic and diffusion effects. In order to further extract quantitative information from the manganese distributions we have assumed that the kinetics of reaction ( 4 ) can be approached by a first-order dependence in manganese and a zero-order dependence for the other components. With these assumptions one can make use of the Thiele concept of diffusionlimited reactions [ll].
lob
29
CONCENTRATION, C ( z / L l / C ( - 1 )
I.
0.8
1
0.6
O2 -
-
.
EXPERIMENT -MODEL
0
F 5. 6 . Distribution of manganese over a representative silica granule from
experiment U 3 4 . The drawn lines have been calculated for different values for the Thiele modulus ( @ ) . We start with the quantitative determination of k4 by defining the Thiele
modulus of reaction (4) as (5)
0 = V/S *qkq/De with
V
=
S
=
(average) surface area of the carrier particles
De
=
effective diffusivity of manganese species in the
(average) volume of the carrier particles
porous silica particles Furthermore we use the known relationship [ll] between the Thiele modulus and the concentration of manganese at the outer surface ( G o ) and that at the centre of the particles (Ci) written as Ci/Co
=
1 / (cash 0)
(6)
From an average value of Ci/C0=0.32
0 . 0 2 as determined for seven granules
obtained in experiment U 3 4 a value of +1.8 granules as cubes with a length of From an estimate of De (lo-'
follows. By approaching the silica
m we conclude that V/S=1.67*10-4
m.
m2/s) according to relationships given by Perry
[12] for the bulk diffusivity and by Satterfield [ 1 3 ] for the tortuosity of the
carrier in combination with equation (5) it follows that k4=O.12 l/s. Comparison of this value with those of the rate constants reported in Table 1 shows that the consecutive precipitation reaction ( 4 ) is much more rapid than the rate-determining urea hydrolysis.
30
Development of a semi-quantitative model. In this section we will briefly discuss a model which summarizes the above kinetic and mass-transfer effects. We distinguish three steps. viz. (i) the hydrolysis of urea, which leads to hydrolysis and precipitation of manganese, (ii) transport of (hydrolysed) manganese species from the bulk of the solution to the outer surface of the carrier particles, and (iii) reaction parallel with mass transfer inside the porous particles. These steps lead to the following expressions for the rate of removal of Mn from solution (Rt):
*
Rt
s
Rt
kl
*
=
S
kl
*
ak
(7)
C1
*
(‘B
-
‘B,i,l)
selectivity defined as the rate of Mn hydrolysis over urea hydrolysis, mol(Mn)/mol(urea).
kl kl
=
rate constant for urea hydrolysis (s-l)
- mass transfer coefficient, liquid/solid - concentration of urea, mo1/m3
c1
(m/s)
CB = concentration of Mn(OH)+ (-B) in liquid phase, mol/m3. CB,i,l= concentration of B at liquid side of interface between liquid and carrier particle, mol/m3. ak d
=
external carrier surface area, m2/m3.
=
V/S, m.
- tanh(0)/0
11
(effectiveness factor).
CB,i,s= concentration of B at solid/liquid interface at solid side, mo1/m3. and realizing that ‘B,i,s
*
=
‘B,i,l
with e
=
porosity of the carrier, m3/m3
the interfacial concentrations for B can be eliminated and equation (11) is obtained. S*Cl Rt
4-
CB
=
l/kl + l/kp*ak + l/kq*ak*d*q*r With respect to the model summarized by equation (11) it is noted that a crude
31
approximation lies in the selectivity factor
s
(equation (7)) which describes
the complex interplay between the urea hydrolysis, ammonia evaporation and manganese hydrolysis in a simplified manner. However, in the absence of more detailed data a more sophisticated description of the process cannot be validated. The advantage of equation (11) is that it presents insight into the three main resistances for manganese deposition, viz. urea hydrolysis (l/kI), transport to the outer surface of the catalyst particles (l/kl*ak), and diffusion and reaction inside the particles (l/k&*ak*d*q*e).
The first and the third
resistance follow from this work and the second one can be estimated from the literature [14]. To a first approximation, for experiment U34 the three resistances as mentioned rate as follows: 90:1:8. In other words, the largest resistance is related to the urea hydrolysis, whereas the transport to the external surface causes a negligible resistance only. From equation (11) it can be easily shown that depending on the specific value of the selectivity parameter
(s)
the overall process of Mn removal from
the solution can or cannot be described by a first-order process. In view of the importance of the surface deposition reaction for the distribution of the active components over the carrier particles, k4 has been determined for a number of catalytic components. The results shown in Table 5 indicate that the values for k4 can vary considerably; the distributions of the active component will vary correspondingly. TABLE 5 Rate constants for the surface precipitation reaction for several active components onto silica carriers. The precipitation reaction involves urea hydrolysis. Active metal Mn Rh Ni
Effects of transient concentration eradients A novel method to apply deposition precipitation which makes use of
transient concentration gradients will be illustrated by the synthesis of eggyolk type molybdenum-on-silica catalysts. Two methods have been elaborated to start and restrict the deposition of Mo at the centre of the silica spheres (cf. Table 3 ) . Note that in all experiments the volume of the solution exceeds the carrier pore volume considerably (wet 'impregnation'). The first method uses relatively dilute solutions and mild reaction conditions (pH, temperature) such that a reaction starts at the centre of the spheres and terminates spon-
32
taneously (typically after 1 h). This method was applied for experiments RK29 and RK37. The line scans for catalyst RK37 (Fig. 7 ) reveal - in accord with visual inspection of the dried catalyst
-
the Mo to be concentrated in the
centre of the spheres (a so-called egg-yolk catalyst). Note the small variation of the metal distribution between the separate spheres (Fig. 7). The second method to control the local rate of deposition was investigated in experiment RK77 (Table 3 ) , where the reaction was allowed to continue for 1 5 min at 0 OC and then quenched by diluting the reaction mixture with water, also at 0 OC. From the line scan (Fig. 8) it is apparent that again an egg-yolk catalyst was obtained, although less pronounced than with RK37 (Fig. 7 ) . In future work it might be worthwhile to consider quenching the reaction by lowering the temperature of the reaction mixture.
Fig. 7. Electron microprobe analysis of the molybdenum distribution over silica spheres of catalyst RK37 (1.5 %w Mo).
Fig. 8 . Electron microprobe analysis of the molybdenum distribution over a representative silica sphere of catalyst RK77 ( 0 . 6 6 %w Mo).
In Fig. 9 line scans of Mo/Si02 catalysts have been collected for different Mo loadings. Clearly, with increasing load the metal distribution shifts from convex to flat and, finally, to concave.
In searching an explanation for the controlled precipitation reaction at the centre of the spheres, it is important to note that the reaction between Mo0h2and N2Hh is strongly accelerated at reduced pH. This effect has been observed experimentally by us and is related to the liberation of hydroxyl ions during the reaction (cf. Experimental sectionj. Furthermore, we have observed that a more uniform Mo distribution was obtained when the reactants had contacted the carrier for an extended period under conditions which precluded any reaction. Based on the above observations, the controlled precipitation is ascribed to the existence of temporary concentration gradients inside the spheres. After imbibition with part of the solution containing the reactants, the 'acidic'
33
A ) 1 5 o/ow
Mo (RK 3 7 )
B)6.7°/owMo(RK
C ) 16 '/OW
MO (RK 15)
D) 2 1 o/ow
29)
Mo (RK 33)
Fig. 9 . Microprobe analysis of the molybdenum distribution over silica spheres of Mo/Si02 catalysts with various metal loadings. silica support will give rise to a decrease o f the pH of the penetrated solution in comparison to the pH o f the 'excess' solution outside the spheres. This will lead to concave pH profiles within the spheres, which will exist typically for 1 h". This concave pH profile results in a convex Mo distribution as the rate of precipitation increases with decreasing pH. We now propose to generalize the above described phenomena as follows:
- The combination of (i) a precipitation reaction proceeding via redox chemistry and liberating OH-(or H+) ions with (ii) the use of an acidic (or a basic) carrier may give rise to accelerated precipitation of the active component at the centre of the carrier body (egg-yolk distribution).
- The combination of (i) a precipitation reaction proceeding via redox chemistry and liberating OH-(or H+) ions with (ii) the use of a basic (or an acidic) reacting carrier may give rise to accelerated precipitation of the active component at the edge of the carrier body (egg-shell distribution). Of course, this method may be even further generalized to other types of precipitation reactions and to concentration gradients other than that of H+. In this paper, however, we will restrict ourselves to the combination of redox reactions and pH gradients.
*
Typical times to efface concentration gradients by diffusion under these conditions range from 0.5 - 1.0 h related to the Fourier process in question [17]. Adsorption phenomena may even greatly prolonge these periods (18,191.
34
We note in passing that the question whether a carrier will react basic or acidic will depend both on the nature of the carrier and on the pH of the aqueous solution. Roughly speaking, one can say that if the pH o f the solution is abovefielow the iso-electric point (IEP)'
o f the oxide in question, the
carrier will react acidic/basic. Two precipitation reactions have been designed to check the generalization concerning the egg-yolk catalysts, viz. the liquid-phase reduction of Cu2+ and of Ag+ with N2H4 and CH20, respectively (for details see Table 4 ) . Both reactions generate H+ ions and it is predicted that a basic carrier, such as y-Al2O3 at sufficiently low pH, will lead to an egg-yolk distribution. Indeed, the line scans shown in Figs. 10 and 11 prove the metals to be concentrated in the inner part of the extrudates.
Fig. 1 0 . Electron microprobe analysis of the copper distribution over an alumina extrudate of catalyst RK 129 (1.8 % m/m Cu).
Fig. 11. Electron microprobe analysis of the silver distribution over an alumina extrudate of catalyst RK 130 ( 0 . 3 8 % m/m A g ) .
The second generalization related to the egg-shell catalysts has been confirmed by repeating the copper precipitation under identical conditions with (acidic) silica spheres, Visual inspection of the precipitation process revealed the copper oxide to be concentrated at the outer surface of the spheres. Practical application of this method to produce egg-shell catalysts, however, involves the problem of precipitation in the liquid separate from the carrier. Therefore, the method seems to be especially suited for preparing egg-yolk type catalysts. GENERAL DISCUSSION AND CONCLUSIONS Kinetics and mass transfer effects (pseudo-stationarv conditions) Several conclusions for deposition precipitation onto pre-shaped carrier particles utilizing urea hydrolysis emerge from this study. Firstly, as
#
The IEP of 7 -A1203 is between 7 and 9 [ 2 0 ] while the IEP of Si02 has been reported to range from pH 0.5 to 3 . 7 [21].
35
expected, the rate-determining step is connected with the urea hydrolysis. Secondly, the ultimate distribution of metal over the carrier body is dictated by the rapid consecutive precipitation reaction (rate constant kb) at the carrier surface. An important implication is that the rate of the urea hydrolysis (and thus of the overall precipitation process) does not affect the metal distribution. Consequently, the rate may be much enhanced without affecting the distribution. Furthermore, it might be inferred that higher metal concentrations will not change the picture since we have indications that the processes are first-order in the metal concentration. In our laboratory [16] we have obtained evidence for the precipitation of nickel onto silica spheres that this is indeed the case. With respect to the viability of using deposition precipitation onto pre-shaped carriers for industrial applications the feasibility of the method is apparent from the work presented in this paper. Pore mouth plugging by deposition of the metal component will occur only with extremely rapid surface reactions. However, in general concentration gradients will result from applying this synthesis method with larger carrier particles. Whether these concentration gradients are acceptable or not depends on the catalyst and its specific application. Effects of transient concentration eradients A unique feature of deposition precipitation onto large carrier bodies has been found to be related to the occurrence of concentration gradients direct after imbibition with the reactant mixture. In general, when the iso-electric point of the carrier deviates from the pH of the solution, a pH gradient inside the carrier body will exist for some time (typically 1 h). By applying a precipitation reaction whose rates depend on the pH the
local rate
of precipitation
in the carrier body may be controlled. It has been shown that deposition precipitation via redox chemistry can be used for the production of egg-yolk type catalysts. The synthesis o f Mo/Si02, Cu/A1203 and Ag/A1203 catalysts displaying an egg-yolk type distribution has been described to illustrate this novel preparation route. ACKNOWLEDGEMENTS The skilful experimental assistance of Mr. E.J.G.M. Romers and Mr. R. van Kempen is gratefully acknowledged. Furthermore, the valuable review of this paper by Prof. Dr. J.A.R. van Veen is appreciated.
36 REFERENCES
1 German Patent No. 7 4 0 , 6 3 4 to IG Farben ( 1 9 4 3 ) . 2 Netherlands Patent Application 6 7 , 0 5 2 5 9 to Stamicarbon ( 1 9 6 7 ) . 3 US Patent 3 , 6 6 8 , 1 4 8 to Lever Brothers Company ( 1 9 7 0 ) . 4 Netherlands Patent Application 6 8 , 1 6 7 7 7 to Stamicarbon ( 1 9 7 0 ) . 5 J.W. Geus, in (B. Delmon et al., Eds.) Preparation of Catalysts 111, Elsevier, Amsterdam, 1 9 8 3 , p. 1. 6 European Patent Specification 2 5 8 , 9 4 2 ( 1 9 8 8 ) to S.1.R.M.-B.V. 7 M. Montes, Ch. Penneman de Bosscheyde, B.K. Hodnett, F. Delannay, P. Grange and B. Delmon, Appl. Catal. 12 ( 1 9 8 4 ) 3 0 9 . 8 B. Delmon, Solid State Ionics 16 ( 1 9 8 5 ) 2 4 3 . 9 L.A.M. Hermans and J.W. Geus, in (B. Delmon et al., Eds.), Preparation of Catalysts 11, Elsevier, Amsterdam, 1 9 7 9 , p. 1 1 3 . 10 R.C. Warner, J . Biological Chemistry USA 1 4 2 ( 1 9 4 2 ) 7 0 6 . 11 K.R. Westerterp, W.P.M. van Swaaij and A.A.C.M. Beenackers, Chemical Reactor Design and Operation, John Wiley, Chichester, 1 9 8 4 , p. 3 8 3 . 1 2 J .H. Perry, Chemical Engineers’ Handbook, 4th Edition, McGraw-Hill, 1 9 6 3 , pp. 1 4 - 2 3 . 1 3 C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, 1 9 8 0 , p. 3 3 6 . 14 F. Kneule, Chemie-1ng.-Techn. 2 8 ( 1 9 5 6 ) 2 2 1 . 1 5 K.P. de Jong, J.H.E. Glezer, H.P.C.E. Kuijpers, A . Knoester and C.A. Emeis, J . Catal., accepted for publication. 16 K.P. de Jong, unpublished results. 17 J. Crank, The Mathematics of Diffusion, Clarendon Press, Oxford, 1 9 7 5 , p. 8 9 . 18 P.B. Weisz, Trans. Faraday S O C . 6 3 ( 1 9 6 7 ) 1801. 1 9 M. Komiyama, Catal. Rev.-Sci. Eng. 27 ( 1 9 8 5 ) 3 4 1 . 2 0 J . P . Brunelle, Pure C Appl. Chem. 5 0 ( 1 9 7 8 ) 1211. 2 1 R.K. Iler, The Chemistry of Silica, John Wiley, New York, 1 9 7 9 , p. 188.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
37
INFLUENCE O F THE PREPARATION PROCEDURE ON THE PHYSICAL PROPERTIES, SURFACE A C I D I T Y AND D I S P E R S I O N OF MoP/A1,0,
CATALYSTS
R. PRADA S I L V Y , Y. ROMERO, J . GUAREGUA, R. GALIASSO
I N T E V E P S.A, Seccion de C a t a l i s i s ' A p l icada, Apdo. 76343, Caracas 1070-A, VENEZUELA.
SUMMARY The i n f l u e n c e o f t h e p r e p a r a t i o n procedure on t h e p h y s i c a l p r o p e r t i e s , s u r f a c e a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases i n MoP/A1,0, c a t a l y s t s i s here investigated. The r e s u l t s i n d i c a t e t h a t t h e b e h a v i o u r o f t h e molybdenum-phosphorus c a t a l y s t s depend s t r o n g l y on t h e i m p r e g n a t i o n sequence used ( c o i m p r e g n a t i o n o r d o u b l e i m p r e g n a t i o n ) . Coimpregn a t i o n procedure (Mo+P) a l l o w s t h e o b t e n t i o n o f c a t a l y s t s w i t h l a r g e r C o n c e n t r a t i o n o f a c i d s i t e s and s u r f a c e d i s p e r s i o n ! when compared t o consecuti v e i m p r e g n a t i o n procedures (P->Mo, Mo->P). T h i s sample a l s o .showed h i g h a c t i v i t y f o r t h e HDS, HDN and MHC r e a c t i o n s . The d i f f e r e n c e s i n b e h a v i o u r observed f o r t h e d i f f e r e n t samples c o u l d be m a i n l y a t t r i b u t e d t o t h e e x i s t e n c e o f d i f f e r e n t molybdenum d i s t r i b u t i o n phases, p r o b a b l y Mo-P . h e t e r o p o l compounds a r e formed f o r t h e coimpregnated sample, . w h i l e t h e f o r m a t i o n o f b u l t MOO, species c o u l d be f a v o r i z e d f o r t h e sample preimpregnated wJth phosphorus. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on alumina: i ) i t improves t h e . s u r f a c e a c i d i t y , ii) i t i n c r e a s e t h e mechanical s t r e n g t h o f t h e e x t r u d a t e s , iii) i t decreases t h e s u r f a c e a r e a o f t h e s u p p o r t . We e x p l a i n o u r r e s u l t s on t h e b a s i s o f t h e mechanism proposed f o r t h e a d s o r p t i o n o f molybdate. phosphate and phosphomolybdate compounds on alumina. INTRODUCTION Molybdenum supported alumina c a t a l y s t s promoted by m e t a l s o f t h e group
V I I I ( N i , Co) a r e w i d e l y used f o r h y d r o p r o c e s s i n g heavy o i l s o r c o a l d e r i v e d fuels.
E f f o r t s have been made t o improve c a t a l y t i c a c t i v i t y by f i n d i n g appro-
p r i a t e a d d i t i v e s (such as P,
B,
S i , T i , F , Ga, e t c . ) . , Phosphorus can be c o n s i -
dered one o f t h e most e f f e c t i v e a d d i t i v e s o f t h e molybdenum supported alumina c a t a l y s t s . I n f a c t , i t appears as a component i n a number o f f o r m u l a t i o n s o f commercial h y d r o t r e a t i n g c a t a l y s t s . The p a t e n t l i t e r a t u r e c l a i m s t h a t i t s use p r o v i d e s b e t t e r c a t a l y t i c performance, i n c r e a s i n g h y d r o d e s u l f u r i z a t i o n (HDS), h y d r o d e n i t r o g e n a t i o n (HDN), hydrodemetall i z a t i o n (HDM) , and i m p r o v i n g t h e m i l d hydrocracking
(MHC)
activity
(ref.
1-41.
phosphorus produces s e v e r a l e f f e c t s , such as:
I t has been demonstrated t h a t
i) i t p r o v i d e s a more s t a b l e
i m p r e g n a t i n g s o l u t i o n and t h u s b e t t e r d i s p e r s i o n o f t h e metal ( r e f . 1 , 4 ) , ii) i t i n h i b i t s t h e f o r m a t i o n o f Ni(Co)Al,O,-like
on
t h e support species and
enhances t h e f o r m a t i o n o f N i z t ' i o n s w h i c h a r e p r i m a r l y r e s p o n s i b l e f o r t h e f o r m a t i o n o f c a t a 1 . y t i c a l l y a c t i v e n i c k e l s u l f i d e o r NiMoS-like phase ( r e f . 3 ) ,
iii) i t a l t e r s t h e a c i d s t r e n g t h d i s t r i b u t i o n on alumina, t h e c o n c e n t r a t i o n o f medium a c i d s i t e s i n c r e a s e s p r o g r e s s i v e l y w i t h i n c r e a s i n g phosphorus l e v e l
38
5) and i v ) i t improves t h e thermal s t a b i l i t y o f gamma alumina w i t h t o s i n t e r i n g and and phase t r a n s i t i o n t o alpha alumina ( r e f . 5). The b e n e f i c a l e f f e c t s o f phosphorus has s t i m u l a t e d research on i t s i n f l u e n c e on molybdena based c a t a l y s t s . However, most o f t h e above mentioned (ref.
respect
1 i t e r a t u r e e s s e n t i a l l y focused on t h e i n f l u e n c e o f phosphorus on t h e c a t a l y t i c p r o p e r t i e s o f t h e m o d i f i e d system and i t s e f f e c t on t h e d i s p e r s i o n on the a c t i v e phase deposited on t h e alumina surface has n o t y e t been i n v e s t i g a t e d . Also,
very few works deal w i t h t h e i n f l u e n c e o f t h e sequence o f phosphorus c a t a l y s t s on t h e
i n c o r p o r a t i o n d u r i n g t h e p r e p a r a t i o n step o f the MoP/A1 ,O,
surface a c i d i t y , d i s p e r s i o n and d i s t r i b u t i o n o f t h e supported phases.
As the
phosphate i o n s s t r o n g l y i n t e r a c t w i t h alumina, competing w i t h molybdate ions, a f a c t o r o f p o s s i b l e importance i s t h e p r e p a r a t i o n procedure. Three
different
procedures
can
be
followed .to
prepare a MoP/A1 ,03
catalyst.
(1) P A1,0,
-->
MO
(2) MO + P ( 3 ) Mo --> P
->
MOP/ A1,0,
-->
This c o m u n i c a t i o n i s p a r t o f a research program aimed a t a systematic i n v e s t i g a t i o n o f t h e p r e p a r a t i o n procedure o f MoP/A1,0, catalysts.
Essentially,
we study
mild-hydrocracking
the e f f e c t o f phosphorus i n c o r p o r a t i o n
sequence on t h e s t a t e o f d i s p e r s i o n o f t h e a c t i v e phase, surface a c i d i t y and physical p r o p e r t i e s . the
following
For t h i s purpose, t h e samples were c h a r a c t e r i z e d using
physico-chemical
techniques:
BET
strength, X-ray p h o t o e l e c t r o n spectroscopy (XPS).
(SEM),
surface
area,
mechanical
scanning e l e c t r o n microscopy
surface a c i d i t y determined by p y r i d i n e adsorption.
EXPERIMENTAL Catalyst preparation Three MoP/A1,0, c a t a l y s t s having constant molybdenum and phosphorus contents (P->Mo
(10 w t % MOO,
and 4.5
w t % P,O,)
were prepared by e i t h e r consecutive
o r Mo-->P) o r simultaneous (Mo+P) impregnation o f alumina extrudates
(1/20 i n c h l e n g t h ) w i t h aqueous s o l u t i o n s o f ammonium heptamolybdate and/or orthophosphoric acid. The alumina has a s p e c i f i c surface area o f 269 m 2 / g and a pore volume of 0.69 cc/g. The impregnating s o l u t i o n s were adjusted t o pH =
1.5-2.0 by adding n i t r i c a c i d b e f o r e c o n t a c t i n g w i t h t h e support. The samples were d r i e d a t 373 K f o r 2 h and then c a l c i n e d i n two steps a t 623 K f o r 2 h and a t 7 7 3 K f o r 3 h.
39
Characterization Surface areas o f c a t a l y s t s were determined by t h e BET method from N, analyzer. An Erweka apparatus was used t o measure t h e s i z e c r u s h i n g s t r e n g t h o f c a t a l y s t extrudates.
The method determines p a r t i c l e c r u s h i n g s t r e n g t h by
measuring t h e f o r c e i n kilograms ( k ) r e q u i r e d t o crush an e x t r u d a t e o f measurA l a r g e number o f extrudates (about f o r t y o f each sample) were t e s t -
ed s i z e .
ed and t h e average value was e s t a b l i s h e d . X-ray
p h o t o e l e c t r o n spectra
(XPS)
o f c a t a l y s t s were recorded u s i n g a
Leybold Heraeus LHS-11 apparatus equipped w i t h a computer system, which a1 lowed
the determination o f
Alka l i n e ( E = 1486 eV).
peak areas.
The e x c i t a t i o n r a d i a t i o n was
the
A l l t h e samples were grounded and then pressed i n t o
Cis.
Alpp, A l Z s . M o ~ ~M , o ~ ~P2p , and P 2 s e n e r g y l e v e l s were recorded. The CIS energy l e v e l (284.5 eV) was taken as a reference. Atomic surface c o n c e n t r a t i o n o f t h e sample h o l d e r s b e f o r e t h e a n a l y s i s .
Signals corresponding t o
supported elements was evaluated form t h e peak i n t e g r a t e d areas and t h e sensit i v i t y f a c t o r s provided by t h e equipment manufacturer. The elemental p r o f i l e d i s t r i b u t i o n o f b o t h molybdenum and phosphorus, across t h e t r a n s v e r s a l s e c t i o n o f t h e alumina extrudates, was obtained u s i n g the
scanning
e l e c t r o n microscopy
technique
(SEM).
An
ISI-60 apparatus
equipped w i t h an energy d i s p e r s i v e X-ray analyzer (Kevex 5-7000) was used f o r these measurements.
C a t a l y s t extrudates were mounted on an epoxy s l i d e and
then p o l i s h e d b e f o r e scanned under t h e e l e c t r o n beam. The a c i d i t y measurements were c a r r i e d o u t i n a Cahn 1000 e l e c t r o b a l a n c e u s i n g p y r i d i n e as
probe molecule adsorbed on t h e c a t a l y s t
surface.
The
i r r e v e r s i b l y adsorbed p y r i d i n e amounts were determined a t 273 K, 473 K and 573 K.
Results a r e expressed as m o l o f p y r i d i n e i r r e v e r s i b l y adsorbed p e r
surface area o f c a t a l y s t . RESULTS Surface areas and mechanical s t r e n g t h Specific
surface areas corresponding t o MoP/A1 209 c a t a l y s t s prepared
f o l l o w i n g d i f f e r e n t procedures a r e given i n Table 1. t h e same t a b l e t h e values obtained f o r A1,0, Mo/A1,0,
samples.
I t i s also reported i n
support and f o r P/A1,0,
and
40
TABLE 1 S p e c i f i c s u r f a c e areas and mechanical s t r e n g t h corresponding t o d i f f e r e n t p r e p a r a t i o n procedures Sample
Surface Area (m'/g)
A1 Z 0 3 P / A 1 ,03 Mo/A1 ,O,
Crushing S t r e n g t h ( k g / p e s t l e )
269 241 248 214 222 234
P->Mo Mo->P MotP
For b o t h Mo/A1,0,
5.2 6.0 5.3 6.1 5.5 6.1
and P/A1,0,
samples,
t h e surface area decreased i n
8% and 11%, r e s p e c t i v e l y , a f t e r impregnation w i t h ammonium -heptamolybdate o r phosphoric acid, whereas f o r MoP/A1 ,03 c a t a l y s t s t h e l o s s
approximatively,
i n surface areas was more pronounced.
One can observe t h a t t h e s u r f a c e area
value o b t a i n e d f o r t h e sample prepared by coimpregnation i s s l i g h t l y higher than t h a t observed f o r those samples prepared by consecutive impregnation.
A l l MoP/A1,0,
c a t a l y s t s presented a pore volume i n t h e 0.52 - 0.55 c c / g range.
Concerning t h e mechanical s t r e n g t h measurements,
i t i s observed i n Table
1 t h a t phosphorus s l i g h t l y improves t h e mechanical s t r e n g t h o f alumina e x t r u dates, w h i l e molybdenum seems t o have no i n f l u e n c e on these p r o p e r t i e s .
The
p r e p a r a t i o n procedures (P->Mo) and (Mo+P) produce c a t a l y s t s w i t h simi 1 a r c r u s h i n g s t r e n g t h values. X-ray p h o t o e l e c t r o n spectroscopy (XPS) The XPS r e s u l t s obtained f o r P/A1 ,O,,
Mo/A1 ,O,
and MoP/A1 ,O,
catalysts,
a r e presented i n Table 2. The percentage o f surface d i s p e r s i o n o f both Mo and P elements ( I e / I A l x 100) i s r e p o r t e d as a f u n c t i o n o f c a t a l y s t p r e p a r a t i o n
procedure. Mo/A1,0,
When comparing t h e (Mo/A1) o r ( P / A l )
o r P/A1,0,
i n t e n s i t y r a t i o obtained f o r
s w p l e s w i t h t h a t o f MoP/A1,0,
c a t a l y s t s , we can observe
s t r i k i n g d i f f e r e n c e s i n t h e d i s p e r s i o n s t a t e o f t h e supported species.
Molyb-
denum d i s p e r s i o n o f samples v a r i e s as f o l l o w s :
while,
i n t h e case o f t h e phosphorus d i s p e r s i o n , t h e observed sequence i s as
follows: Mo+P
>
P/A1,0,
>
Mo->P
>
P->Mo
41
MO
MO
-070
0
070
Radial Position (mm) Fig. 1. Electron Microprobe p r o f i l e of Mo and P corresponding t o MoP/A1,0, c a t a l y s t s prepared following d i f f e r e n t procedures.
A t 373 K (weak acid s i t e s ) , t h e pyridine adsorbed amounts per surface area v a r i e s as follows: Mo+P > P->Mo > Mo->P = A1,0,
Whereas, f o r desorption temperatures of 473 K and 573 K (medium and strong acid s i t e s , r e s p e c t i v e l y ) , the observed sequence i s :
42
TABLE 2 XPS r e s u l t s c o r r e s p o n d i n g t o t h e c a t a l y s t s prepared u s i n g d i f f e r e n t
procedures. Sample
IMojp/ I A 1
2P
I P 2 p / I A 1 2P
__
Mo/A1 ,O,
4.5
2.7 __
P->Mo
4.1
2.3
Mo->P
4.4
2.5
Mo+P
4.8
3.3
P / A 1 *03
Scanning e l e c t r o n microscopy (SEM) F i g u r e 1 r e p r e s e n t s t h e p r o f i l e d i s t r i b u t i o n , across t h e t r a n s v e r s a l sect i o n o f t h e alumina e x t r u d a t e s , b o t h f o r phosphorus and molybdenum elements, obtained
through
procedures
(Mo+P)
SEM
and
technique. (Mo->P),
For
catalysts
b o t h Mo and
prepared
P elements
following are
the
distributed
homogeneously i n t h e s u p p o r t , whereas sample prepared a c c o r d i n g t o procedure (P->Mo),
c l e a r l y shows some h e t e r o g e n e i t i e s .
Strength o f a c i d i t y S u r f a c e a c i d i t y r e s u l t s , o b t a i n e d t h r o u g h p y r i d i n e a d s o r p t i o n , correspond i n g t o A1,0,
s u p p o r t and d i f f e r e n t MoP/A1,0,
catalysts,
Table 3 as a f u n c t i o n o f t h e d e s o r p t i o n temperature.
are presented i n
Differences i n pyridine
i r r e v e r s i b l y adsorbed amounts p e r s u r f a c e a r e a u n i t can be observed i n t h e 373 -573 K temperature range, depending on t h e p r e p a r a t i o n method used. TABLE 3 I r r e v e r s i b l y a c i d i t y o f MoP/A1,0,
prepared c a t a l y s t s ( m o l p y r i d i n e l m ' )
Sample 373
TEMPERATURE ( K ) 473
573
1.41
0.67
0.11
Mo->P
1.78 1.44
0.83 0.76
0.42 0.32
Mo+P
2.06
0.85
0.45
3'2
P->Mo
x lo3
43
DISCUSSION The above r e s u l t s show how a c i d i t y and s u r f a c e d i s p e r s i o n o f supported phases found i n MoP/A1,0,
c a t a l y s t s can be s t r o n g l y i n f l u e n c e d by t h e prepara-
Tables 2 and 3 i n d i c a t e t h a t c o i m p r e g n a t i o n o f Mo and P
t i o n procedure.
a l l o w s t h e o b t e n t i o n o f a c a t a l y s t w i t h l a r g e r c o n c e n t r a t i o n o f a c i d s i t e s and surface dispersion,
(P->Mo,
Mo->P).
t h e mechanical
i n comparison w i t h c o n s e c u t i v e i m p r e g n a t i o n procedures
Phosphorus seems t o improve t h e a c i d i t y s t r e n g t h as w e l l as p r o p e r t i e s o f alumina e x t r u d a t e s .
However,
this additive
s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f t h e support. L e t u s d i v i d e t h e d i s c u s s i o n o f o u r r e s u l t s i n two p a r t s :
we s h a l l f i r s t
d i s c u s s t h e r e s u l t s o b t a i n e d on t h e sample prepared by c o i m p r e g n a t i o n (MotP) and subsequently, examine t h e b e h a v i o u r o f those c a t a l y s t s p r e p a r e d by consec u t i v e impregnations (P->Mo and Mo->P). P r e p a r a t i o n Procedure (Mo+P) Table 2 shows t h a t c o i m p r e g n a t i o n method improves t h e s u r f a c e d i s p e r s i o n
o f b o t h molybdenum and phosphorus.
T h i s agrees w i t h r e c e n t r e s u l t s o b t a i n e d
by Atanasova e t a l . ( r e f . 6), who s t u d y i n g a s e r i e s o f NiMoP/Al,O, XPS
and c a t a l y t i c
measurements,
stated
that
c a t a l y s t by
c o i m p r e g n a t i o n method and a
s u i t a b l e alumina c a r r i e r can l e a d t o a b e t t e r d i s p e r s i o n o f a c t i v e components and consequently, t o an i n c r e a s e i n
HDS a c t i v i t y .
We s h a l l a t t e m p t t o e x p l a i n now o u r r e s u l t s supported on fundamental s t u d i e s d e a l i n g w i t h a d s o r p t i o n o f Mo and/or P
on
alumina.
L e t us r e f e r t o t h e
more i m p o r t a n t r e s u l t s o f t h e s e s t u d i e s . L i t e r a t u r e shows t h a t phosphorus i n c r e a s e s t h e s o l u b i l i t y and s t a b i l i t y o f molybdenum s o l u t i o n s
(ref.
1,4). A d s o r p t i o n s t u d i e s proposed t h a t when
alumina i s coimpregnated w i t h s o l u t i o n s c o n t a i n i n g molybdate and phosphate, t h e r e i s a c o m p e t i t i o n between b o t h i o n s f o r t h e same a d s o r p t i o n s i t e s ( b a s i c hydroxyl
groups
of
alumina),
thus,
a d s o r p t i o n o f rnolybdates (3,6-9).
the
adsorbed
phosphate
inhibits
the
However, most o f t h e s e s t u d i e s deal w i t h
molybdate and phosphate a d s o r p t i o n s e p a r a t e l y w i t h o u t c o n s i d e r i n g f o r m a t i o n o f phosphomolybdate compounds i n t h e i m p r e g n a t i n g s o l u t i o n .
the
R e c e n t l y , Cheng and L u t h r a ( r e f . 8 ) , u s i n g t h e NMR t e c h n i q u e , s t u d i e d t h e a d s o r p t i o n o f v a r i o u s phosphomolybdate compounds on alumina spheres.
Authors
observed t h a t when phosphoric a c i d i s added t o a s o l u t i o n c o n t a i n i n g amonium hep tam0 1 y b d a t e , p e t amol ybdodi phosphate compounds ( P ,Mo s o l u t i o n s c o n t a i n i n g P/Mo m o l a r r a t i o h i g h e r t h a n 0.4, remained i n f o r m o f phosphates.
1 a r e formed.
For
amounts o f phosphorus
T h i s suggests t h e e x i s t e n c e o f a chemical
e q u i l i b r i u m between b o t h phosphate and molybdate i o n s i n s o l u t i o n .
44
8 H * + 5 MOO:'
+ 2 HP0:-
=
P,Mo,O,,
6-
+
(2)
5H20
A c c o r d i n g t o t h e above e q u a t i o n , decomposition o f phosphomolybdate i n t o s i m p l e molybdate and phosphate c o u l d be f a v o u r e d by a r i s e o f s o l u t i o n pH, which would s h i f t t h e chemical e q u i l i b r i u m t o l e f t . ,
Indeed, t h i s behaviour
was observed d u r i n g phosphomolybdate a d s o r p t i o n on alumina.
The i n c r e a s e i n
pH o f t h e i m p r e g n a t i n g s o l u t i o n was a t t r i b u t e d t o w a t e r f o r m a t i o n d u r i n g i o n exchange r e a c t i o n . L u t h r a and Cheng ( r e f . 10) observed t h a t e q u i l i b r i u m b e t w e e n heptamolybdate Mo,O:i rise Mo,O,,
and molybdate MOO:-
i o n s i s a l s o a f f e c t e d by a
i n pH. 6-
+
4 H,O
I n short,
=
2-
7 MOO,
+
(3)
8 H
r e s e a r c h o f Cheng and L u t h r a c l e a r l y i l l u s t r a t e s t h a t t h e h i g h
s o l u b i l i t y and s t a b i l i t y observed when p h o s p h o r i c a c i d i s added t o molybdenum i s m a i n l y due t o t h e f o r m a t i o n o f 'phosphomolybdate compounds.
These compounds
a r e v e r y s e n s i t i v e t o changes i n t h e i m p r e g n a t i n g s o l u t i o n pH. The f a c t t h a t t h e r e a r e d i f f e r e n c e s i n a c i d s i t e s d i s t r i b u t i o n and s u r f a c e d i s p e r s i o n when u s i n g d i f f e r e n t procedures t o p r e p a r e a MoP/A1 ,O,
catalyst,
suggests t h a t n a t u r e and c o n c e n t r a t i o n o f t h e o x i d i c supported phases p r e s e n t i n these c a t a l y s t s are d i f f e r e n t . Our h y p o t h e s i s i s c o n s i s t e n t w i t h t h e r e s u l t s o b t a i n e d by o t h e r researche r s ( r e f . 3, 6, 9, 111, who combining v a r i o u s c h a r a c t e r i z a t i o n techniques, s t u d i e d t h e s t r u c t u r a l changes t h a t o c c u r r e d when phosphorus i s used as an a d d i t i v e o f Mo/A1 ,O,
catalysts.
Atanasova and Halachev ( r e f . l l ) , s t u d y i n g
t h r o u g h I R spectroscopy t h e phases p r e s e n t i n NiMoP/Al ,O, by coimpregnation, observed bands c o r r e s p o n d i n g t o AlPO, and Ni-Mo-P
heteropoly
compounds.
IR-bands
c a t a l y s t s , prepared and t o a mixed A1-Mo
c o r r e s p o n d i n g t o b u l k MOO,,
Al,(MoO,), and NiMoO, were n o t observed i n t h o s e samples. Authors observed t h a t h i g h phosphorus c o n t e n t l e a d s t o an i n c r e a s e i n degree o f molybdenum p o l y m e r i z a t i o n and t o changes i n t h e r a t i o between t h e d i f f e r e n t t y p e s o f h e t e r o p o l y compounds, Ni-Mo-P/Al-Mo loadings.
Lopez Corder0 e t a l .
r a t i o i n c r e a s e s w i t h i n c r e a s i n g phosphorus ( r e f . 9) s t u d i e d by TPR and DRS t h e s u r f a c e
d i s t r i b u t i o n o f molybdenum on two s e r i e s o f MoP/A1 ,O,
c a t a l y s t s which were
prepared u s i n g simul taneous (P+Mo) o r double i m p r e g n a t i o n (P->Mo) methods. I n c o n t r a s t w i t h t h e r e s u l t s o b t a i n e d by Atanasova and Halachev ( r e f . 111, t h e a u t h o r s observed t h e presence o f b u l k MOO,
and a l s o small c l u s t e r s o f p o l y -
molybdate m u l t i l a y e r s f o r b o t h c a t a l y s t s e r i e s . l a t t e r specieswas
The c o n c e n t r a t i o n o f t h e
more i m p o r t a n t f o r t h e preimpregnated samples.
45
C o n s i d e r i n g o u r r e s u l t s t o g e t h e r w i t h t h o s e found i n l i t e r a t u r e ,
two
p o s s i b l e e x p l a n a t i o n s can be proposed: The f i r s t e x p l a n a t i o n i s based on t h e mechanism proposed f o r molybdate o r phosphate a d s o r p t i o n on alumina ( r e f . 121, because phosphomolybdate compounds were observed t o decompose i n t o these two species d u r i n g a d s o r p t i o n . These s t u d i e s suggest t h a t phosphate as w e l l as molybdate i n t e r a c t f i r s t w i t h b a s i c h y d r o x y l groups o f alumina, g e n e r a t i n g a w a t e r molecule. Competition between phosphorus and molybdenum t a k e s p l a c e . Phosphate i s adsorbed on alumina more r a p i d l y t h a n molybdate i s adsorbed.However, t h e r a t e o f a d s o r p t i o n o f b o t h compounds depends on s e v e r a l f a c t o r s , such as: i ) n a t u r e and c o n c e n t r a t i o n o f t h e i o n s i n t h e i m p r e g n a t i n g s o l u t i o n , ii) pH o f s o l u t i o n , iii) t y p e o f alumina and i v ) a d s o r p t i o n temperature. For each exchanged h y d r o x y l group by phosphoric a c i d molecule, two new a c i d s i t e s a r e c r e a t e d . T h i s would e x p l a i n t h e i n c r e a s e s i n a c i d s i t e s c o n c e n t r a t i o n observed a f t e r 12) suggested phosphorus i n c o r p o r a t i o n i n t o alumina. Morales e t a l . ( r e f . t h a t when a l l b a s i c h y d r o x y l groups a r e t i t r a t e d , t h e a c i d h y d r o x y l groups b e g i n t o be t i t r a t e d and t h e n a monolayer o f phosphate i s formed by f u r t h e r a d d i t i o n o f p h o s p h o r i c a c i d . I n t e r a c t i o n s between n e i g h b o r i n g adsorbed phosphates c o u l d occurs l e a d i n g t o t h e f o r m a t i o n o f p o l y m e r i c phosphate c h a i n s . I n t h i s p a r t i c u l a r case, t h e a u t h o r s mentioned t h a t t h e number o f a c i d s i t e s remains almost c o n s t a n t because t h e s u b s t i t u t i o n o f two a c i d h y d r o x y l groups o f alumina would y i e l d two a c i d s i t e s a s s o c i a t e d t o phosphorus. I n t h e c o m p e t i t i v e system, we can propose t h a t b o t h molybdate and phosp h a t e i o n s c o u l d be adsorbed i n n e i g h b o r i n g s i t e s . T h i s would i m p l y t h a t adsorbed molybdates impede t h e p o l y m e r i z a t i o n o f adsorbed phosphate and phosphates would produce t h e same e f f e c t on molybdates. The l a t t e r may e x p l a i n t h e h i g h e s t s u r f a c e d i s p e r s i o n o f b o t h P and Mo observed f o r t h e coimpregnated sample. Since p o l y m e r i z a t i o n o f phosphorus as we1 1 as molybdenum was f a v o u r e d by h i g h c o n t e n t s o f t h e s e elements, we c o u l d suggest t h a t i n t e r a c t i o n s between n e i g h b o r i n g molybdate and phosphate adsorbed species may occur c a u s i n g t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds a f t e r c a l c i n a t i o n . One may s p e c u l a t e t h a t a c i d i t y produced f o r t h e l a t t e r compounds should be d i f f e r e n t f r o m t h a t produced by phosphates on alumina. The second e x p l a n a t i o n would be t o c o n s i d e r t h a t phosphomolybdate remains stable during impregnation. In t h i s case, t h e b e h a v i o u r o f these compounds towards a d s o r p t i o n would be d i f f e r e n t f r o m t h a t o f p h o s p h a t e a n d molybdate. One may s p e c u l a t e t h a t Mo-P h e t e r o p o l y compounds may be formed f r o m t h e adsorbed phosphomolybdate a f t e r c a l c i n a t i o n . T h e r e f o r e , t h e h i g h e s t d i s p e r s i o n observed f o r t h e coimpregnated sample may be a t t r i b u t e d t o t h e f o r m a t i o n o f Mo-P h e t e r o p o l y compounds, which were observed on coimpregnated samples i n a r e c e n t study ( r e f . 11). L e t us now complement o u r d i s c u s s i o n showing some i m p o r t a n t e f f e c t s
46
observed through scanning e l e c t r o n microscopy technique i n t h e a n a l y s i s o f t h e samples.
Figure 1 shows t h a t b o t h phosphorus and molybdenum a r e homogeneously
d i s t r i b u t e d i n t h e coimpregnated sample. and L u t h r a ( r e f .
8)
during adsorption o f
However, r e s u l t s obtained by Cheng
showed an i n t e r e s t i n g chromatographic e f f e c t appearing phosphomolybdate on alumina spheres,
p r e f e r e n t i a l l y l o c a t e d a t t h e edge, w h i l e molybdenum c e n t e r o f t h e spheres.
phosphorus was
was concentrated a t t h e
The l a t t e r o b s e r v a t i o n makes e v i d e n t t h e decomposition
o f phosphomolybdate d u r i n g a d s o r p t i o n and t h e c o m p e t i t i o n o f b o t h phosphate and molybdate
ions
for
the
same adsorption
sites.
I n our o p i n i o n ,
we
a t t r i b u t e t h e d i f f e r e n t behaviour observed i n b o t h s t u d i e s t o d i f f e r e n c e s i n p r e p a r a t i o n c o n d i t i o n s o f t h e samples, as w e l l as t h e type o f alumina employed. Preparation Procedures (P->Mo) and (Mo->P) As shown i n Tables 1 and 3, phosphorus seems t o improve t h e mechanical
p r o p e r t i e s as w e l l as t h e a c i d i t y s t r e n g t h o f alumina extrudates,
However,
t h i s a d d i t i v e s t r o n g l y a f f e c t s t h e s p e c i f i c surface area o f support. Our r e s u l t s a r e i n agreement w i t h those obtained by several i n v e s t i g a t o r s ( r e f . 7,9,12).
Lopez Cordero e t a l . ( r e f . 9)
suggested t h a t l o s s i n surface
area o f alumina a f t e r phosphorus i n c o r p o r a t i o n i s probably due t o a c o r r o s i v e e f f e c t o f surface caused by phosphoric a c i d molecules o r t o a pore blockage by phosphate species. t h e 30-60 explanation
A'
We have observed t h a t pores having an average diameter i n
range were t h e most a f f e c t e d by phosphorus d e p o s i t i o n .
An
o f these r e s u l t s c o u l d be t h e f a c t t h a t these pores c o u l d be
s e l e c t i v e l y plugged by polymeric phosphate adsorbed species,
However, we can
n o t d i s c a r d a p o s s i b l e c o r r o s i v e e f f e c t produced by t h e phosphoric a c i d molecules on t h e alumina surface.
An e l e c t r o n microscopy study o f t h e P/A1,0,
sample c o u l d reveal p o s s i b l e morphological changes due t o phosphorus. Table 2 i n d i c a t e s t h a t f o r t h e sample prepared f o l l o w i n g procedure ) was a f f e c t e d b y phosphorus ( P - > M o ) . t h e Mo d i s p e r s i o n ( I M o / I A 1 3P 2P i n c o r p a t i o n . The I P 2 p / I A 1 2 p i n t e n s i t y r a t i o a l s o decreased w i t h respect t o t h e value obtained f o r P/A1,0,
sample,
a f t e r molybdenum d e p o s i t i o n .
These
r e s u l t s can be explained by t h e f a c t t h a t preimpregnation o f alumina w i t h phosphorus reduces t h e number o f s i t e s a v a i l a b l e f o r molybdate adsorption. Therefore, changes i n d i s p e r s i o n and d i s t r i b u t i o n o f molybdenum species should be expected. I n t h i s case, one c o u l d suggest t h a t phosphorus promotes t h e formdtion o f b u l k MOO, species, which i s i n agreement w i t h t h e r e s u l t s o b t a i n ed by Lopez Cordero e t a l . deposited on t h e AlPO,
( r e f . 9).
monolayer,
A p a r t o f t h e l a t t e r species may be
which would e x p l a i n t h e decreasing i n
47
phosphorus
intensity
e l e c t r o n microscopy
r a t i o observed f o r study
confirms
the
this
sample.
Additionally,
heterogeneous d i s t r i b u t i o n
molybdenum species when alumina i s preimpregnated w i t h phosphorus.
the
o f the
A similar
s i t u a t i o n m i g h t be expected when t h e c a r r i e r i s preimpregnated w i t h molybdenum.
I n this
particular
case,
mechanical
strength
as w e l l
as a c i d i t y
p r o p e r t i e s s h o u l d n o t be improved by phosphorus. A c t i v i t y o f t h e (MotP) and (P->Mo) samples I n t h i s work we have e v a l u a t e d t h e c a t a l y t i c p r o p e r t i e s o f t h e molybdenum-phosphorus
catalysts.
For
this
purpose,
we
have
prepared
two
Ni-Mo-P/Al,O,
samples f o l l o w i n g t h e i m p r e g n a t i o n sequences (Mo+P->Ni) and
(P->Mo->Ni).
These samples p r e s e n t t h e same chemical
MOO,,
7 . 5 w t % P,O,
and 5.0 w t % N i O ) .
c o m p o s i t i o n (15 w t %
The c a t a l y t i c r e a c t i o n was c a r r i e d o u t
i n a h i g h p r e s s u r e f i x e d bed r e a c t o r u s i n g a vacuum g a s o i l under t y p i c a l mild-hydrocracking conditions
(T = 653K, P
= 5 MPa, LHSV = 0.65 l / h , H,/Hc
=
600). We p r e s e n t i n Table 4 t h e a c t i v i t y r e s u l t s o f t h e NiMoP/Al,O, Both samples
show comparable a c t i v i t y
i n MHC.
However,
r e a c t i o n s a r e h i g h e r f o r t h e coimpregnated sample.
catalysts.
t h e HDS and HDN
The same b e h a v i o u r was
observed by o t h e r r e s e a r c h e r s ( r e f . 13 and 1 4 ) . TABLE 4 Activity o f the
NiMoP/Al,O,
samples
% CONVERSION
HDS
HDN
MHC
(Mo+P-> N i l
82
59
13
(P-> Mo-> N i )
77
50
11
SAMPLE
I n o r d e r t o analyze t h e p o s s i b l e changes i n t h e molybdenum d i s t r i b u t i o n phases induced by phosphorus, we have c a r r i e d o u t I R measurements MoP/A1 ,O, samples b e f o r e n i c k e l i m p r e g n a t i o n . published
elsewhere
(ref.
molybdenum d i s t r i b u t i o n ,
15),
depending
confirm
the
on
both
The r e s u l t s , which w i l l be fact
modify
the
on t h e p r e p a r a t i o n procedure f o l l o w e d .
phosphorus
No
evidences about t h e presence o f phosphomolybdate compoundswere observed f o r t h e coimpregnated sample.
48
To summarize. we may conclude t h a t The d i f f e r e n c e s i n a c t i v i t y observed f o r t h e (MotP-,
N i l and (P->Mo->
N i ) samples c o u l d be associated t o changes
induced by phosphorus on t h e s u r f a c e molybdenum d i s t r i b u t i o n phases. CONCLUSIONS In
the
frame
of
the
present
work
we
conclude
that
coimpregnation
p r o c e d u r e i s t h e more a p p r o p r i a t e method o f p r e p a r i n g MoP/Al *03c a t a l y s t s . This procedure a l l o w s t h e o b t e n t i o n o f samples w i t h h i g h e r d i s p e r s i o n , surface a c i d i t y and c a t a l y t i c p r o p e r t i e s . Changes observed i n molybdenum d i s t r i b u t i o n phases caused by phosphorus depend on t h e impregnation sequence employed.
Coimpregnation procedure would
l e a d t o t h e Mo-P heteropoly compound formation,
w h i l e preimpregnation w i t h
phosphorus c o u l d induce t h e b u l k MOO, formation. I t was a l s o observed t h a t phosphorus produces t h e f o l l o w i n g e f f e c t s on
i ) i t improves t h e surface a c i d i t y , ii) i t increases t h e mechanical
alumina:
s t r e n g t h o f extrudates, iii) i t decreases t h e surface area o f support.
REFERENCES
L. Hilfman, U.S.
A. Morales, M.M. K. G i s h t i , A.
Patent 3.617.528
(1971).
Ramirez de Agudelo, Appl. Catal. 23 (1986) 23. i a n n i b e l l o , S. Marengo, G. M o r e l l i , Appl. Catal. 1 2 (1984)
381. G.A.
Mickelson, U.S.
Patent 3.749.663
S. S t a n i s l a u s , A. Asi-Habi,
(1973).
C. Dolana, Appl. Catal.,
39 (1988) 239.
P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl. Catal., 38 (1988), 235.
D. Chadwick, D.W.
Atchison, R.
C a t a l y s t s 111, (G. Poncelet, Amsterdan, p 323, 1983. W.C.
R.
B a d i l l a , L. Josefsson, P.
Grange,
P.
" I n preparation o f
Jacobs
eds).
Elsevier,
Cheng, N.P. Luthra, J. Catal., 109, (1988), 163. Lopez Cordero. N. Esquivel, J . Lazaro, J.L.G. F i e r r o , A. Lopez Agudo.,
Appl
. Catal . 48
(1989), 341.
Luthra, W.C., Cheng, J . Catal., 107, (19871, 154. 11 P. Atanasova, T. Halachev, App. Catal ., 48, (19891, 295.
10 N.P.
12 A. Morales, M.M. 261. 13 J.L.G.
Ramirez de Agudelo, F. Hernandez, Appl. Catal.,
41 (1988),
F i e r r o , A. LBpez Agudo, N. Esquivel, R. L6pez Cordero, Appl. Catal.
48 (1989), 353. 14 P. Atanasova, T. Halachev, J. U c h y t i l , M. Kraus, Appl. C a t a l . 38 (1988) 235. 15 R . Prada S i l v y , Y . Romero, M. GonzBlez, t o be published.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 01991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
49
SYNTHESIS OF NON-STOICHIOMETRIC SPINEL-TYPE PHASES: A KEY TOOL FOR THE PREPARATION OF TAILORED CATALYSTS WITH SPECIFIC ACTIVITY Michele Piemontese1,Ferruccio Trifiro'l, Angelo Vaccaril, Elisabetta Foresti and Massimo Gazzano2 'Dipartimento di Chimica Industriale e dei Materiali, Viale del Risorgimento 4,40136 BOLOGNA
pa"). Dipartimento di Chimica "G. Ciamician" ,
and CSFM (CNR) , Via Selmi 2 , 40126 BOLOGNA
(Italy). SUMMARY The preparation, stability and catalytic activity of non-stoichiometric spinel-type phases used in the synthesis of methanol were investigated as a function of the composition, heating temperature and atmosphere. It was shown that these phases formed mainly via amorphous chromates, especially for copper-rich catalysts. High activities in the synthesis of methanol were observed for zinc-rich samples (with a maximum for a catalyst in which 20% of the zinc ions were substituted by copper ions) and associated with the presence of a non-stoichiomemc spinel-type phase, stable also in the reaction conditions. On the other hand, the low activity of copper-rich catalysts was attributed to the instability of the spinel-type phase where much of the copper segregates into well crystallized metallic copper, with a further poisoning effect by zinc and cobalt. INTRODUCTION The majority of catalytic devices used in the modem chemical industry (i.e., both usual heterogeneous catalysts and materials based on applications of the catalytic properties) are based on mixed oxides (1- 3). The synthesis of specific tailor-made mixed oxides able to perform complex functions is one of the most current topics in solid state chemistry (4). Non-stoichiometric Zn/Cr and Cu/Cr mixed oxides are one of the principal examples of these unusual solids. They have applications as both solid state gas sensors ( 5 ) and catalysts for hydrogenation reactions (of CO to methanol and/or methanol-higher alcohol mixtures, and of many organic molecules) (6-12). These systems have been widely investigated over the last few years, and results obtained show that their peculiar catalytic properties may be associated with the presence of non-stoichiometric phases (with a M2+/M3+ratio higher than 0.5, M= metal), in which some of the zinc or copper ions are present in octahedral positions, i.e., with an unusual coordination. However, until now very few data have been reported regarding the changes in structure and reactivity as a function of the composition in ternary systems (for instance Cu/Zn/Cr). The aim of the research reported here was to study the changes in structure, stability and reactivity as a function of catalyst composition. In particular, attention was focused on the role of the Cu/Cu+Zn ratio and, for copper-rich catalysts, on the differences related to the partial substitution of copper by zinc or cobalt. In all samples the chromium content was 50% (as atomic ratio) in order to favour the formation of monophasic systems (13-16). EXPERIMENTAL The precursors with different atomic ratios were obtained by coprecipitation at pH= 8.0 k 0.1
50
Table 1. ComPosition of the catalyst investigated (as atom %) Sample
(Table 1). A solution of the nitrate salts of the elements was added to a continually stirred solution containing a slight excess of NaHC03 at 333K. Subsequent filtration
Cu:Zn:Co:Cr 0.0:50.0:0.0:50.0 was performed, followed by washing until the sodium 1 0 ~ 0 ~ 4 0 ~ 0 ~ o ~ o ~ 5content 0 ~ 0 was lower than 0.05% (as Na20). The precipitates Cat B were dried at 363K and heated at different temperatures Cat C 25.025.0:0.050.0 and in different atmospheres. Catalyst compositions were Cat 40’0:10.0:0*050’0 confirmed by atomic absorption using a Perkin Elmer 40.0:0.0:1~.0:50.0 mod 360 spectrophotometer. Cat E 50.0:0.0:0.0:50.0 The XRD analysis was carried out with a Philips Cat F PW1050/81 diffractometer controlled by a PW1710 unit, using Nickel-filtered CuKa radiation, h= 0.15418 nm (40kV, 40mA). The data were processed on a Olivetti M240 computer. The lattice constants were determined from diffractometric data by least squares refinements. The crystal sizes were determined by the Scherrer equation, using Warren’s correction for instrumental line broadening. Possible contributions to the line broadening due to disorder effects and/or lattice strains were not taken into account. The quantitative analysis of oxide phases in the catalysts was carried out using the method suggested by Klug and Alexander (17). The cation distribution between tetrahedral and octahedral sites in the cubic spinel-type phase was evaluated as an extension of the Bertaut method (18,19) on the basis of the I400/I440 ratio. A C.Erba Sorptomatic 1826 apparatus with N2 adsorption was used to measure the surface area. IR spectra were recorded using the KBr disk technique and a Perkin Elmer 1700 Fourier-transform spectrometer. U.v.-visible diffuse reflectance @R) spectra were recorded using a Uvikon 860 spectrophotometer, equipped with an integrator sphere. The amounts of CuO and chromates were determined after extraction with NH4OH:NH4NO3 (1:l v/w) (20) at 61511x11 and 446nm, respectively, using a Uvikon 860 spectrophotometer; in the last case, the solutions were previously buffered at pH= 5.0 f 0.1 with concentrated CH3COOH. The catalytic tests were carried out in a copper-lined piston flow reactor, operating at 6.0MPa and 500-600K, using a GHSV= 16,000h-’ and a H2:CO:C02= 65:32:3 (v/v). Before the catalytic tests, the catalysts were activated in-situ by hydrogen diluted in nitrogen; the hydrogen concentration and temperature were progressively increased during this pretreatment. Outlet gases were monitored on-line by gas-chromatography, while the liquid products were condensed under pressure in a cold trap at 253K during the time-on-stream (6h), then weighed and ‘analyzed off- line by gas-chromatography . Cat A
RESULTS AND DISCUSSION Nature of the precipitates The precipitates dried at 363K show XRD powder patterns typical of quasi-amorphous phases, identified as hydroxycarbonates on the basis of the IR spectra (21). Further information may be obtained on the basis of the values of empirical parameters A and B35, calculated from the DR spectra (22). All precipitates (Fig. 1) show similar values of the A parameter (related to the Cr-0 distance), while the B35 parameter (inversely proportional to the C3’- Cr3+ interaction) shows a minimum for the Cu/Cu+Zn= 0.5 ratio. Therefore, the same type of structure may be hypothesized for all precipitates, with an increase in surface crystallinity for Cat C .
51
I
I
I
18000
Nature of the samples heated at 653K
Figures 2a and b report the XRD powder patterns of the precipitates heated at 653K in air and in a reducing atmosphere (H2:N2= 10:90 v/v), respectively. Calcined samples (Fig. 2a) show the presence only of spinel-type phases, whose XRD patterns become more and more broad as the copper content increases. IR spectra confirm the presence, for all calcined samples, of spinel phases, and also show he presence of 0 0.5 1 .o dichromate-type phases (25), the amounts of Cu/Cu+M(IIl which increase with increasing copper content. In Fig.l.Empirica1 parameters calculated from the previous papers it was shown that nonDR spectra of precipitates dried at 363K: stoichiometric Zn/Cr spinel-type phases formed zinc (m.0); cobalt (0,0). by decomposition of amorphous chromates and that some amounts of residual Cr6' ions are present in these phases (8,15). Taking into account that copper and zinc may form mixed spinel-type phases (with cubic symmetry for high zinc contents) (20,24), we may hypothesize the formation up to a ratio Cu/Cu+Zn= 0.5 of cubic non-stoichiometric spinel-type phases, containing both elements and characterized by an I excess of bivalent ions. On the other hand, on the basis of the XRD spectra of Figure 2a, we cannot speculate about the number and/or nature of the phases present in the copper-rich catalysts. After the samples had been heated in an H m 2 atmosphere, the XRD powder patterns (Fig. 2b) again showed the presence only of spinel-type phases for Cu/Cu+Zn 20.5, while for the copper-rich samples the main phases present were Cu (Cat D and E) or CuO (Cat F). The lack of reoxidation for the metallic copper in Cat D and E, cannot be justified on the basis of differences of crystal size, but most probably can be attributed to the formation of copper-rich alloys at the surface of the particles. The presence of small amounts of zinc or cobalt does not modify the XRD powder pattern of the copper particles, but may strongly influence their physicochemical or catalytic properties (25-27). For all catalysts, the IR spectra show the presence, together with small amounts of residual carbonates, of the typical bands of spinels (even if not well resolved), wavenumbers cn except the Cu/Cr= 1.0 sample (Cat F) (Fig. 3) for which only a broad peak at 554 cm-' is present i n the low frequency Fig* 3. IR spectra Of Cat heated at 653K for 24h in air (a), N2 (b) region, attributable to the overlapping of CuO and Cr203 and H m 2 (c). absorptions (13,28).
k
\
D
El
Cat E
,
60
50
221P
40
30
60
50
2W"
40
30
60
.
,
50
.
221P
I
40
,
,
30
I
I
60
.
1
50
.
2+/"
I
40
.
I
30
. XRD powder patterns of the different catalysts heated at different temperatures and in different conditions. (a) 653K in air; (b) 653K in an H2/N2 (10:90 v/v) mixture; (c) 753K in air and (d) 853K in air. (&Tetragonal phase (ASTM 34-424); (A)ZnO (ASTM 5-664); (m) Cu (ASTM 4-836); (e)CUO(ASTM 5-661); (0)c0304 (ASTM 9-418); without symbol: cubic phase (ASTM 22-1 107 and/or 26-509) .
Fig. 2
53
Table 2 Amounts of CuO (w/w %) (a) and of Cr(h2- (w/w %) (b ) extracted for the catalysts heated in different conditions and after catalytic tests of the synthesis of methanol.
T, K 653K/air
Cat A
Cat B
Cat C
Cat D
Cat E
Cat F
a
b
a
a
a
a
a
-
6.0
b
6.0 13.5
b
13.5 26.5
b
28.5 27.5
b
20.0 13.5
b
29.5 28.0
2.5
n.d. n.d.
11.0 3.5
n.d. n.d.
n.d. n.d.
653IVI32-N~ -
0.5
10.0 0.5
24.5 1.0
40.0 2.0
41.0 2.0
48.0 1.0
853K/air
3.0
2.0
653K/N2
5.5
n.d. n.d. -
2.5
3.0
1.0
5.0
1.5
6.0
3.0
2.5
Afterreact.
-
15.0 1.5
38.5 1.0
n.d. n.d.
cuomax*
-
10.1
25.4
25.5
25.7
25.6
cuomax**
-
10.1
25.4
40.8
41.2
51.1
7.5
1.0
1.0
1.0
47.5 1.5
* calculated on the basis of a phase composition ZnCnO4 (or c O c n04) + CuCnO4 +CuO. **calculated on the basis of a phase composition CuO + ZnO (or COO) + Cr203. Further information on the effect of the heating conditions may be obtained from the values of surface area (Fig. 4) and the amounts of CuO and chromates extracted with NH40WNH4N03 solutions (Table 2). From Figure 4, it is possible to observe that the heating atmosphere has a small effect on the surface of the samples. In both cases, samples with large surface areas may be obtained, especially in the range 0s Cu/Cu+Zn 10.5. Furthermore, it is possible to observe that cobalt is a better physical promoter than zinc in both conditions. Table 2 shows that in the samples heated in a mixture of H m 2 practically all the Cu2+ ions present may be extracted. Therefore we may hypothesize that in these conditions copper gives rise to a separate phase, while the formation of spinel phases is due essentially only to zinc or cobalt, in agreement with the IR data for Cat F (Fig. 3). zoo I I I I However, it should be pointed out that copper 0 H a N 2 M Zinc AlrM 28°C containing phases were detected only for the 6 ~~rM=Coball j HUNZM=CobalI copper-rich catalysts, while for Cu/Cu+Zn ratios 150,,a 5 0.5 they escape XRD detection. Table 2 confirmes the presence in the calcined samples of increasing amounts of chromates, with a maximum for Cat F in which ca 43% of the total chromium is present as Cr6' ions. Up to a Cu/Cu+Zn ratios 0.5, the amount of CuO extracted is lower than both the theoretical value 50 and the chromate content, and does not depend directly on the latter. This is particulary true for Cat B, taking into account also the values of the 0 0.2 04 06 08 1 samples heated at 653K in N2. Therefore, two Cu / Cu + M(II) (atomic ratio) Cu2+ containing fractions are present in these spinel-type phases, which show different Fig.4. Surface area of the samples heated at 6S3K for 24h in air and H7h-7 .. - - . solubilities in the m 40H /N H 4N 03 solution, but
1
~
~~
3
~
Table 3 Crystallographic data for the samples calcined at 753 and 853 K -~~ -
~-~~_ _ ~
~
Sample T , K
Phase
a, nm
..- -
.
c,nm
c.s.,nm
cuoa
-
~
C U O ~ ZnOa
ZnOb
-~
~~
Xr
XO
xc ~~
CatA CatB CatC CatD CatE
753
cubic + ZnO
0.8356(1)
7.5
853
cubic + ZnO
0.8336(5)
13.0
753
cubic
0.8342(2)
853
cubic + CuO
0.8334(3)
753
cubic + CuO
0.8344(5)
4
25.8
0.246
0.137
0.41 1 0.448
14
25.5
0.246
0.082
7.5
-
10.1
-
15.5
0.242
0.158
0.400
12.0
2
10.1
-
15.5
0.266
0.124
0.406
6.5
3
25.4
-
0.249
0.137
0.409
853
cubic+ CuO
0.8325(1)
69.0
18
25.4
-
0.253
0.049
0.465
753
cubic + CuO
0.8323(5)
7.0
15
25.5
-
0.243
0.077
0.453
853
cubic + CuO
0.834(3)
10.0
25
25.5
-
0.242
0.005
0.502
cubic + CuO
0.833(3)
753 853
tetragonal + CuO 0.5969(4)
0.798(2)
7.O
12
25.7
-
0.23 1
0.107
0.441
28.0
27
25.7
-
n.d.
n.d.
n.d.
7.5
13
25.6
-
n.d.
n.d.
n.d.
22
25.6
-
n.d.
n.d.
n.d.
i Co,04
CatF
753
cubic > tetragonal 0.832(4)
853
tetragonal + CuO 0.6025(6)
0.780(3)
16.0
CuCr02
0.2977(6)
1.704(5)
15.0
n.d.
n.d.
n.d.
CuCrOp
0.2975(2)
1.7105(9)
22.0
n.d.
n.d.
n.d.
+ CUO CatF
753/N, 853/N, -
C.S.= crystal size of the spinel-type phase; (a) = amount detected; (b) = amount calculated on the basis of a phase composition ZnCr20, (or CoCr204)
+ CuCr,O, + CuO. xT,xc, xo = on the basis of structural formula (M2txT)
'e'rohedro'
(MZtxo,C?',c)
oclahedral
0
55
are both not detectable by XRD and IR analysis. On the other hand, the amount of CuO extracted in the copper-rich samples is mainly related to the presence of Cr6' ions (compare Cat D heated in air or N2). However, it must be noted that for all samples the amount of CuO overlaps that of chromates ( if both are expressed on an atom basis), indicating that also in these samples a consistent fraction of CuO ( more than 33% of the total CuO extracted) may be present as excess of Cu2+ ions inside a spinel-type phase. Unfortunately, no further support for this hypothesis may be obtained from XRD powder patterns. In recent papers (13,29) Bonnelle et al. have reported the formation, by heating at 643K in N2, of cubic non-stoichiometric spinel-type phases for copper chromite catalysts, with Cu/Cr ratios from 0.8 to 1.5. However it must be pointed out that these authors claimed as necessary for the stability of the spinel-type phases, the formation of consistent amounts of Cr6' ions (36% or more of the total chromium), during the controlled decomposition of the hydroxynitrate precursors. In our case, the Cu/Cr= 1.0 sample gives rise upon heating in N2, mainly to the formation of CuCro;! (Fig. 3), as c o n f i i e d also by XRD patterns, the amount of which increases with increasing temperature. This difference may be explained taking into account the different natures of the precursors; however, it must be pointed out that the amount of Cr6' ions present in Cat F calcined at 653K as well as its IR spectrum are similar to those reported in the literature (13,29). Therefore, for copper-rich compositions the formation of non-stoichiometric phases may be related mainly to a controlled oxidation of the precursor obtained by coprecipitation, while, on the contrary, this does not seem to be a key factor for the zinc-rich catalysts, in agreement with that previously reported for Zn/Cr catalysts (14). Furthermore, the data reported in this section, suggest that non-stoichiometric phases do not form by heating the precipitates in a reducing atmosphere. Thermal stabilitv of the samples.
In order to test the thermal stability of the catalysts, the precipitates were calcined at 753 and 853K for 24h. The XRD powder patterns of these samples are shown in Fig. 2c and d, and the main Surface area (rn2/g) XRD data are summarized in Table 3. A cubic spinel-type phase is the main component present in ,Ki= Zlnc 853K M= Zmc all samples calcined at 753K, also for the copperrich catalysts, suggesting that in these samples it forms via the amorphous phases discussed in the I5O previous section. Therefore a general mechanism of formation of the spinel-type phases by decomposition of chromate phases may be hypothesized. When the temperature is increased further to 853K3,the cubic phase is again detected, with the exception of Cat E and F, in which teuagonal CuCnO4 is present. With increasing calcination temperature, increased segregation of oxide phases takes place. 0 0.2 0.4 06 0.8 1 However, it must be noted in regard mainly to the Cu I Cu + M(II) (atomic ratio) CuO, that ZnO is observed only for the binary Cat A. Fig. 5 . Surface area of the samples calcined Furthermore, CuO segregation is less marked for at 753K and 853K. Cu/Cu+Zn I 0.5 ratios, while in the copper-rich
t
1
56
catalysts the values detected at 853K approach the calculated ones. The high stability towards calcination temperature of Cat B is worth noting, in which only 20%of the zinc ions are substituted by copper ions. For this catalyst ZnO is never observed, whereas 20% CuO is detected only after calcination at the highest temperature. On the other hand, the partial substitution of copper ions with zinc ions stabilizes the cubic spinel-type phase, whereas for cobalt ions this effect is less marked probably because of their tendency to segregate as c0304. In the literature it is reported that the cubic CuCr204 phase is stable only at high temperatures (30). Furthermore, it is noteworthy that no formation of CuCrO:! was observed when Cat D was heated in N2 up to 853K. The increase in stability for the ternary catalysts also is reflected by the surface areas of the samples calcined at 753K (Fig. 5). After calcination at 853K all samples show a dramatic decrease in surface area and appreciable differences are no longer detected. The collapse of the catalyst structure is also responsible for the low values of CuO extracted (Table 2), taking into account that a sample of CuO (E.Merck, Germany) calcined at 853K for 24h was fully soluble in the N H 4 0 W N 0 3 solution. As already mentioned the 14oO/I440 ratio may be assumed to be a measure of the distribution ratio between the occupancy of the tetrahedral and octahedral sites in the cubic cell of the spinel-type phase. According to Miller data of the octahedral site preference energies (31), it is assumed that the Cr3+ ions are all located in octahedral sites, whereas the Zn2+ and Cu2+ ions are present in both tetrahedral and octahedral sites, according to results previously reported for Zn/Cr catalysts (14-16). From Table 3, it is possible to observe that the tendency of the M2+ ions to be retained in the octahedral positions decreases with increasing calcination temperature and copper content, indicating that the CuO side phase detected may be mainly attributable to the segregation of Cu2+ ions present in octahedral positions of the spinel- type phase. The behaviour of Cat B is noteworthy; in this catalyst more M2+ ions tend to be retained in octahedral positions than is the case for ZdCr catalysts (16). Catalytic activitv in methanol svnthesis Methanol productivity (g/h kg Cat.) The catalytic activity in methanol synthesis of the catalysts investigated is reported in Figure 6 as a function of the Cu/Cu+Zn ratio (Cat E, containing cobalt, was practically inactive in the temperature range investigated). The progressive substitution of zinc ions with copper ions gives rise to considerable differences in the catalytic activity, as a function of the copper content. However, two general behaviours are found: 1) Up to a Cu/Cu+Zn ratio5 0.5, the presence of copper considerably increases the activity in methanol synthesis, with a very high selectivity in methanol (the main by-product being water). 2) For the highest ratio, a dramatic ' deactivation is observed, accompanied also by a 0 0.25 0.5 0.75 Cu / Cu + Zn (atomic ratio) change in selectivity, especially for Cat D, for which Fig- 6. Catalytic activity as a function of the the formation of hydrocarbons (mainly methane) is Cu/Cu+Zn ratio. also detected. On the other hand, Cat F shows the
51
formation at T2 550K of dimethylether, with a corresponding decrease in the values of productivity of methanol. However, the value of productivity in methanol for Cat F at 540K is in very good agreement with that reported by Apai er al. (32). It must be pointed out that the main increase in catalytic activity takes place for Cat B, in which only 20% of zinc ions is substituted by copper ions. The methanol productivity of this catalyst is similar to the best values reported in the literature , if based on kg of catalyst (33- 34), but clearly better if calculated on the basis of kg of copper, thus indicating the formation of very active copper-containing centers. XRD powder patterns of the catalysts after reaction show the presence until a Cu/Cu+Zn ratios 0.5 only of cubic spinel-type phases, while for copper-rich samples the main phases identified are metallic copper (Cat E) and Cu20 (Cat F). Furthermore, from Table 2, it is possible to observe that for these last catalysts practically all the theoretical CuO is extracted by NH40WNH4N03, while for Cat B and C values very similar to those of the calcined samples are obtained. In a previous paper, it was shown that there is a rough correlation between the catalytic activity in methanol synthesis and the whole chemisorption activity towards CO (35). Therefore, the low catalytic activity of copper-rich catalysts may be attributed to the segregation of much of the copper as well crystallized metallic particles (36). The further decrease in activity for Cat D and, especially, Cat E is in agreement with the hypothesis of the formation of alloys at the surface of the copper particles (26,27), taking into account the poisoning effect of small amount of cobalt (37). On the other hand, the high activity of Cat C and, especially, Cat B may be correlated to the presence of a non-stoichiometric spinel-type phase, in which some copper and zinc can be found in octahedral positions or to an interaction between highly dispersed metallic copper formed in reducing conditions and the spinel-type phase. However, it must be pointed out that this copper fraction is so dispersed and stable that it is not detected even after the catalytic tests. The role of a not detectable fraction of copper has already been reported in the literature (20, 36). Furthermore, the activity of Cat B higher than that of Cat C (notwithstanding its slightly lower CO chemisorption capacity) suggests that an important role is played by the oxide matrix, very probably in the hydrogen activation step (38,39). CONCLUSION Non-stoichiometric spinel-type phases may be obtained mainly via amorphous chromates and their stability and reactivity are strongly influenced by the composition. Very stable spinel-type phases active in the synthesis of methanol may be obtained at low copper contents, while copper-rich catalysts show a considerable tendency for segregation of metallic copper with a considerable decrease in catalytic activity . However, these last catalysts, especially those containing cobalt or zinc, may be very active in the hydrogenation of 0x0-aldehydes (12). Furtehr work will be directed towards obtaining a better understanding of the nature of the active sites, taking into consideration that the reaction conditions adopted are quite different from those reported by other authors (10,ll). REFERENCES
1 J.J. Burton and R.L. Garten (Ed.s), Advanced Materials in Catalysis, Academic, N.Y., 1977. 2 O.T. Sorensen (Ed.), Non-Stoichiometric Oxides, Academic, N.Y., 1981. 3 H. Yanagida, Angew. Chem. (Engl. Ed.) 27 (1988) 1389-1392.
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4 G. Centi, F. Trifiro’ and A. Vaccari, Chim. Ind. (Milan), 71 (1989) 57-62. 5 A. Jones, P. Mosely and B. Tofield, Chem. Brit. 8 (1987) 749-766. 6 E. Errani, F. Trifiro’, A. Vaccari, M. Richter and G. Del Piero, Catal. Lett. 3 (1989) 65-72. 7 P. Courty, D. Durand, E. Freund and A. Sugier, J. Mol. Catal. 17 (1982) 241-254. 8 A. Riva, F. Trifiro’, A. Vaccari, G. Busca, L. Mintchev, D. Sanfilippo and W. Manzatti, J. Chem. SOC.,Faraday Trans. 1 83 (1987) 2213-2225. 9 G. Fornasari, S. Gusi, F. Trifiro’ and A. Vaccari, I&EC Res. 26 (1987) 1500-1505. 10 R. Bechara, G. Wrobel, M. Daage and J.P. Bonnelle, Appl. Catal. 16 (1985) 15-27 11 L. Jalowiecki, G. Wrobel, M. Daage and J.P. Bonnelle, J. Catal. 107 (1987) 375-392. 12 G. Braca, A.M. Raspolli Galletti, F. Trifiro’ and A. Vaccari, Italian Pat. n. 21831A (1989). 13 G. Wrobel, J. Arsene, M. Lenglet, A. d’ Huysser and J.P. Bonnelle, Materials Chem. 6 (1981) 19-34. 14 G. Del Piero, F. Trifiro’ and A. Vaccari, J. Chem. Soc., Chem. Commun. (1984) 656-658. 15 G. Del Piero, M. Di Conca, F. Trifiro’ and A. Vaccari, in P. Barret and L.C. Dufour (Ed.s), Reactivity of Solids, Elsevier, Amsterdam, 1985, pp. 1029-1034. 16 M. Bertoldi, B. Fubini, E. Giamello, G. Busca, F. Trifiro’ and A. Vaccari, J. Chem. Soc., Faraday Trans. 184 (1988) 1405- 1421. 17 H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures, Wiley, N.Y., 1974, ch. 7. 18 E.F. Bertaut, C.R. Acad. Sciences 230 (1950) 213-215. 19 L. Weil, E.F. Bertaut and L. Bochirol, J. Phys. Radium 11 (1950) 208-212. 20 J. Escard, I. Mantin and R. Sibut-Pinote, Bull. Soc. Chim. France (1970) 3403-3408. 21 K. Nakamoto, Infrared and Ranian Spectra of Inorganic and Coordination Compounds, Wiley, N.Y., 1978. 22 L. Nondek, D. Mihajlova, A. Andreev, A. Palazov, M. Kraus and D. Shopov, J. Catal. 40 (1975) 46-51. 23 J.A. Campbell, Spectrochim. Acta 21 (1965) 1333-1343. 24 C. Delorme, C.R. Acad. Sciences 241 (1955) 1588-1589. 25 L. Aitchison and W.R. Barclay, Engineering non-ferrous metals and alloys, Frowde and Hodder & Stoughton, London, 1923, ch. VII. 26 T. Van Henvijnen and W.A. De Jong, J. Catal. 34 (1974) 209-214. 27 R. Cao, W.X. Pan and G.L. Griffin, Langmuir 4 (1988) 1108-1 112. 28 R.A. Nyquist and R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic, N.Y., 1971. 29 A. d’ Huysser, G. Wrobel and J.P. Bonnelle, Nouv. J. Chem. 6 (1982) 437-442. 30 V.M. Ust’yantsev and V.P. Mar’evich, Izv. Akad. Nauk. SSSR 9 (1973) 336-337. 31 A. Miller, J. Appl. Phys. Suppl. 30 (1959) 245-251. 32 J.R. Monnier, M.J. Hanrahan and G. Apai, J. Catal. 92 (1985) 119-126. 33 K. Klier, in D.D. Eley, H. Pines and P.B. Weisz (Eds), Advances in Catalysis, Academic, N.Y., 1982,31, pp. 243-312. 34 E.B.M. Doesburg, R.H. Hoppener, B. de Koning, X. Xiaoding and J.J.F. Scholten, in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds), Preparation of Catalysts IV, Elsevier, Amsterdam, 1987, pp. 767-780. 35 M. Piemontese, F. Trifiro’, A. Vaccari, B. Fubini, E. Giamello and I. Rumori, XI1 Simp. Iberoamericano de Catalisis, Rio de Janeiro, Brasil, July 27-August 3, 1990. 36 S. Gusi, F. Trifiro’, A. Vaccari and G. Del Piero, J. Catal. 94 (1985) 120-127. 37 E. Errani, G. Fornasari, T.M.G. La Torretta, F. Trifiro’ and A. Vaccari, in F. Cossio, G. del Angel, 0. Bermudez and R. Gomez (Ed.s), Act. XI Simp. Iberoamericano de Catalisis, IMP, Mexico D.F., 1988,111, pp 1239-1247. 38 G. Busca and A. Vaccari, J. Catal. 108 (1987) 491-494. 39 G. Busca, M.E. Pattuelli, F. Trifiro’ and A. Vaccari, in C. Morterra, A. Zecchina and G. Costa (Eds), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989,239-248.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
59
EFFECT O F PREPARATION VARIABLES ON CATALYTIC BEHAVIOUR OF COPPER/ZIRCONIA CATALYSTS FOR THE SYNTHESIS O F METHANOL FROM CARBON DIOXIDE R.A. KOEPPEL', A. BAIKER', Ch. SCHILD' and A. WOKAUN' 'Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH - Zentrum, CH - 8092 Zurich (Switzerland) 2Physical Chemistry 11, University of Bayreuth, D-8580 Bayreuth (FRG)
SUMMARY A series of copper-zirconia catalysts have been prepared by methods of sequential precipitation, coprecipitation and deposition precipitation. The influence of various pretreatments and of the copper/zirconla ratio on the structural and chemical properties of these samples are examined. High activity and selectivity of the catalysts is shown to be correlated to the presence of amorphous zirconia which is stabilized by copper ions. The results indicate that the structural and chemical properties of the support and particularly the interface copper/zirconia are most decisive in governing the catalytic properties of these methanol synthesis catalysts. INTRODUCTION The synthesis of methanol from syngas (CO/CO,/H,) using Cu/ZnO/Al,O, catalysts is a well established industrial process (ref. 1,2). More recently zirconia-supported copper catalysts were found to be active and selective for this reaction (refs. 3-9). Although methanol synthesis catalysts have been studied intensively for several years there is still much controversy about the nature of the active components and the reaction steps that take place on them. Many aspects of the reaction mechanism are still not fully understood and are the subject of an active debate. Several investigations (refs. 10,ll) have shown that over typical commercial catalysts practically all of the methanol is formed from CO, under industrial conditions and that support effects are minimal for these catalysts (ref. 12). Other workers reported a marked support effect for the synthesis of methanol over copper catalysts prepared by different methods (refs. 3,7,8,13) showing that the activity of supported copper catalysts depends strongly on both the choice of the support and the nature of the feedstock. The results suggest that more than one mechanism may lead to methanol. As the choice of the preparation method and also of the further thermal and chemical treatments control the behaviour of a catalytic system to a large extent, considerable differences are found in catalysts of the same nominal composition but prepared in different ways (ref. 6). The morphology of a catalytic system as well as the appearing crystallographic phases are determined by the method of preparation.
60
While the conventional methanol synthesis reaction from syngas has been studied intensively, little attention has been paid so far to the synthesis from CO, and H,. In the present work a series of Cu/ZrO, catalysts were prepared and tested for methanol synthesis from carbon dioxide and hydrogen. Special emphasis was devoted to the influence of the preparation variables on the structural, chemical and catalytic properties of the catalysts. EXPERIMENTAL Catalyst Preparation A first type of catalyst precursors was prepared by sequential precipitation at constant pH and temperature (Samples S). An aqueous solution of ZrO(CH,C00),-2H20 or ZrO(NO,), .2H20 (0.6 M) and an aqueous solution of sodium hydroxide/sodium formate (2 M each) were poured into two separate dropping funnels. The reagents were added dropwise with vigorous stirring into a Pyrex flask containing 250 ml deionised water at 363-368 K. The addition was adjusted to keep the pH constant at about 7. The precipitation was complete after 5 min and the precipitate was further aged for 15 min at the same temperature. An aqueous Cu(N0,),.3H20 or Cu(CH,COO),.H,O solution (1.5 M) was then added simultaneously with the alkaline solution under the same conditions as described above. Finally the precipitate was aged for further 30 min in the mother-liquor at 368 K and then filtered using a G-4 glass filter. The residue was washed four times by redispersing it in 200 ml deionised water. After washing of the precipitate with 200 ml methanol the voluminous gel was dried I t 333 K in a vacuum drier (the vacuum was kept at 1.25.1U Pa by a small stream of air passing through the drier) for 15 h to yield a rigid solid. This material was crushed to a grain size of 50 - 150 pm using an agate pestle and mortar. Sample A was prepared in the same way except that zirconia was substituted by alumina. Pure zirconia was prepared analogously by precipitation of zirconyl nitrate. Sample C was made by coprecipitation instead of sequential precipitation. Sample H was prepared by the method of deposition precipitation using urea. A suitable amount of amorphous zirconia was suspended in deionised water. After the addition of copper(I1)nitrat and urea the temperature was brought to 363 K under constant stirring. The reaction was accompanied by a rise of the pH to a final value of 8. The final product was treated in the same way as the sequentially precipitated catalysts. Nitrate- and acetate-precursors of copper and zirconium, respectively, were used for preparations to avoid the presence of chloride species in the final catalysts. All chemicals used were of analytical grade. The dried precursors were studied in both the freshly prepared state and after calcination in air at appropriate temperatures. Catalyst characterization The catalysts were characterized by means of nitrogen adsorption, nitrous oxide titration, X-ray diffraction (XRD), thermal analysis (TG/DSC), temperature-programmed reduction
61
(TPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Surface areas (SBm)were calculated using a value of 0.162 nm2 for the cross-sectional area of the nitrogen molecule (ref. 14). Pore size distributions were determined following the BJHmethod (ref. 15) using the equation of Halsey (ref. 16). Nitrous oxide pulse experiments were carried out using the procedure to that reported by Evans et al (ref. 19). Samples were first reduced in a flux of 75 ml min-’ 5 % H,/Ar by heating at 5 K m i d from 373 K to 523 K. Subsequently the samples were hold at this temperature for 30 min and then exposed to a flux of 75 ml m i d pure hydrogen for 1 h at the same temperature. The hydrogen was purged with 50 ml m i d He at 523 K for 5 min. After cooling to 363 K under He, pulses of nitrous oxide (0.5 cm3) were injected. Copper metal surface areas were calculated assuming 1.46.1e9 copper atoms per mz (ref. 15). Back-titration of the oxidized copper surface was realized either by pulses of CO (0.5 cm3) at 423 K or by TPRmeasurements starting at 263K. X-ray diffraction patterns were measured using a powder diffractometer (Philips PW 1700) and Cum-radiation. Mean crystallite sizes were estimated from the peak width at half maximum of the (111) reflection of Cu or CuO, respectively, using the Scherrer equation. The measured peak width was corrected for instrumental broadening using the function proposed by Warren (ref. 17). Thermogravimetric- (TG) and differential scanning calorimetric- (DSC) studies were carried out using a Mettler TA 2000C thermoanalyzer. Measurements were performed under air with a heating rate of 10 K min.’. The apparatus used for the TPR studies was described in an earlier report (ref. 18). TPR profiles were measured under the following conditions: heating rate 10 K m i d , flow rate 75 ml m i d 5 % H,/Ar. The conditions of the IR-measurements have been reported in detail elsewhere (ref. 20). Catalytic tests The apparatus used for the catalytic tests consisted of a continuous tubular fixed-bed reactor (8 mm id.) which was operated at 1.7 MPa. Details are reported in (ref. 9). The premixed gas contained 25 mol% CO, and 75 mol% H, and was fed from a high-pressure cylinder. The reaction flow rate was typically 90 ml m i d (STP). All experiments were carried out in the temperature range 433 - 533 K using 1.0 g of catalyst (50 - 150 pm sieve fraction). The prereduction of the catalysts prior to the kinetic tests was performed according to the following procedure: heating to 473 K at a heating rate of 15 K m i d in 1.25 vol% HJN, at a pressure of l@Pa. The H,-concentration was then increased stepwise (30 min per step) in the sequence 2.5/5/10/20/50 to 100%. After replacement of H, with reactant gas the temperature was brought to 533K under a pressure of 1.7 MPa. RESULTS Influence of chemical comDosition Samples with more than 20 at% copper resulted in black precipitates which is indicative
62
for the formation of cupric oxide under hydrothermal conditions. DSC curves of the decomposition behaviour of the dried precipitates are shown in Fig. 1. The endotherm events between 323 and 423 K are due to the volatilization of physisorbed and crystalline water, respectively. The corresponding TG curves showed the same characteristics as found for a zirconia gel (ref. 21). Dehydration occurs primarily between 300 and 700 K with a final weight loss of about 10% of the initial sample weight. Pure zirconia (Fig. la) exhibits an exothermic peak at about 710 K, characteristic for zirconia prepared by wet-chemical routes. The socalled glow-exotherm is commonly associated with the transition of an X-ray amorphous zirconia phase into a crystalline modification of zirconia. The presence of copper shifts the exothermal crystallization peak to 823 K for sample Sl (10 at% Cu)and to 893 K for samples S3-S7 (30-70 at% Cu, Fig. 1). The exothermic peaks at 473 K and 550 K (Fig. lc) are associated with the presence of acetate in the precursor, as emerges from a comparison with the DSC curve of the pure nitrate precursor (Fig. If). Calcination at 623 K i n air for 3 h results in an acetate free catalyst (Fig. Id). The exothermic signals appearing at 480 and 613 K after exposing this catalyst to methanol synthesis
v
I
273 373 473 573 673 773 873 973 Temperamre (K) Fig. 1. DSC curves of samples a) ZrO, , 6) SI, c) SS, d ) SS-6234 e) S5-623K after
conditions are accompanied by a small weight gain due to reoxidation of reduced copper species (Fig. le). TPR-profiles are shown in Fig. 2 for samples with various copper contents. Note that all copper of calcined samples is quantitatively reduced to cu" below 523 K in 5% H,/Ar. Increasing the calcination temperature results in a shift of the T,,-values of the reduction profiles to lower temperatures (Fig. 2d,e). The small peak at about 740 K occurring with all samples, except the one calcined at 923 K, was accompanied by a measurabIe exothermicity and is attributed to the crystallization of amorphous zirconia. Pure uncalcined zirconia support yielded also a small peak at 718 K, followed by two broad peaks at 793 and 913 K, while a calcined sample (723 K) did not show the crystallization-peak at 718 K. Uncalcined samples resulted in TPR profiles which depended on the nature of the precursor salts used for preparation. While pure nitrate precursors showed a single reduction peak at 500 K (Fig. 2f), samples prepared in the presence of acetate-ions resulted in TPR profiles consisting of three peaks at about 500,567 and 623 K (Fig. 2b,c). We conclude that the reduction peaks at 500 and 567 K are attributable to the reduction of CuO, whilst the peaks at 623 K are due to the reductive decomposition of acetate-modified zirconia. Calcination at 623 K led to the disappearence of the reduction peak at 623 K and to a shift of the peaks at 500 and 567 K to lower temperatures (Fig. 2d). Nitrogen adsorption/desorption isotherms were measured after drying/calcination as well as after methanol synthesis reaction. All the isotherms were of type IV (BDDT classification), indicating the presence of well developed mesoporous systems (ref. 14). The shape of the hysteresis loop changed from type H2 (IUPAC classification, ref. 14) for both samples dried at 393 K and calcined at 623 K, to type H1 for samples dried at 923 K. Note that all samples were degassed at 523 K. Mesopore size distributions calculated from the adsorption and desorption branch of the isotherms showed the presence of pore size maxima in the range 3 - 5 nm for all samples except for those calcined at 923 K which exhibited a shift of the maxima to 8 - 9 nm. The mesopore volume decreased simultaneously from about 0.20 cm'/g to 0.12 cm3/g. Calculated t-plots (ref. 22) indicated the presence of microporosity for uncalcined samples before and after methanol synthesis reaction. Surface areas for all samples following methanol synthesis are listed in Table 1. Influence of calcination temnerature The X-ray patterns of the uncalcined zirconia support (Fig. 3a) shows only two broad bands in the range of 20" to 40" and 40" to 75" for 26, indicating the presence of zirconia with very low degree of crystallinity. Calcination of the sample at 723 K for 3 h resulted in metastable, probably tetragonal ZrO, and some stable monoclinic ZrO,. Note that a distinction between tetragonal and cubic zirconia is not possible based on XRD data alone. However, based on thermodynamical arguments tetragonal ZrO, is more likely. Higher calcination temperatures lead to an increase of the fraction of the monoclinic phase (Fig. 3b shows a sample calcined at 770 K for 3 h in air) until an almost pure monoclinic phase is obtained after calcination at 970 K.
64
TABLEI Textural properties of the catalysts Catalyst Precursor@’ Composition Cu/Zr (at%)
from N,O from XRD 21.0 33.0 30.5 37.5 29.7 37.5 30.6 84.5 37.1 33.5 25.0 23.3 41.7
30.0 31.0 16.5
25.0 40.0 39.0
(a) ac = acetate, ni = nitrate. (b) zirconia was substituted by alumina Sample H, calcined at 823 K for 3 h, yielded XRD-patterns characteristic of crystalline CuO and tetragonal ZrO,. No indication for the presence of the monoclinic phase of zirconia could be found. The XRD-patterns of the dried sample S5 are shown in Fig. 3c and are indicative of copper oxide and amorphous zirconia. The CuO particles had a mean size of about 15 nm as estimated from the line broadening of the CuO (111) reflection. Calcination at 970 K for 3 h resulted in well crystallized tetragonal zirconia and in CuO-particles of about 29 nm mean diameter (Fig. 3d). No reflections of the monoclinic phase were found with this sample. The XRD patterns of a sample after use for methanol synthesis are shown in Fig. 3e. The occurence of reflections due to crystalline Cu,O beside of the reflections of crystalline copper particles can be explained by the exposure of the sample to air during its transfer from the reactor to the XRD-measurement. Zirconia existed as amorphous phase. Mean copper particle sizes calculated from the line broadening of the Cu (111) reflections and copper surface areas measured by N,O-titration are listed in Table 1. By using a half-sphere model the average copper particle sizes were calculated from the copper surface areas. Back titration using CO-pulses at 423 K yielded values for copper surface areas identical to those measured by N,O-pulses. No CO, could be detected in the effluent gas stream indicating that none of the adsorbed carbon monoxide was removed as carbon dioxide at 423 K in the case of zirconia containing catalysts. This contrasts the behaviour of alumina or silica supported copper catalysts where all CO evolved as CO,. Back titration using TPR resulted in reduction peaks between 333 and 373 K for all samples. A characteristic feature of all catalysts was the appearence of a broad, intense desorption peak in the temperature range 513 to 573 K.
65
Catalvtic behaviour Preliminary experiments with respect to possible influences caused by interparticle and intraparticle mass transfer limitations confirmed that such limitations could be ruled out under the conditions used in this study. The results of the CO, hydrogenation experiments over the different catalysts are summarized in Table 2 and reflect the steady-state behaviour of the catalysts at 493 K after 15 h on stream. Carbon containing products were only methanol and carbon monoxide for all catalysts. Acetate precursors yielded some ethanol in the initial stage of reaction at 533 K, probably due to the hydrogenolysis of acetate species in the acetatemodified oxides. No ethanol could be detected after calcination of these samples at 623 K. The catalytic behaviour of some catalysts was compared on the basis of uncalcined samples. Note that the decomposition of the precursor under reducing conditions resulted in improved catalytic properties compared to the oxidative decomposition (Table 2). The activities of the catalysts were also compared on the basis of turnover frequencies (TOF) which were calculated either as molecules of methanol formed or as molecules CO, reacted per copper surface atom and per second.
stnt.
h
Ih
I.
~~
-
+H*
2 e t h y l hexanol
Condensation
2 b u t h y l buten-2-a1
2 ethyl h e x i l hemiacetal o f 2 e t h y l hexanal Condensation
bis(2 ethyl hexil) acetal of 2 ethyl hexanal With the present catalysts, never observed. sumption o f EPA,
t h e u n s a t u r a t e d a c o h o l , HEOL, The c a t a l y t i c a c t i v i t y e x p r e s s e d t h r o u g h t h e and g i v e n i n T a b l e 3,
v a r i e s by a f a c t o r
of
was con
10,
t h e t h r e e p r e c i p i t a t e d c a t a l y s t s b e i n g i n one g r up a n d t h e i m p r e g n a t e d and commercial exception of
ones b e i n g i n a n o t h e r group.
t h e commercial
catalyst,
However,
with t h e
f o r which t h e m e t a l l i c
area
m e a s u r e m e n t i s p r o b a b l y n o t o p t i m i z e d , t h e TON ( s - ' ) for all cat a l y s t s i s r a t h e r s i m i l a r , as a r e t h e apparent a c t i v a t i o n energies. T h i s means t h a t ,
i n a f i r s t analysis,
t h e EPA h y d r o g e n a t i o n , i n
the
132
.
l i q u i d phase,
can be c o n s i d e r e d i n i t i a l l y as i n s e n s i t i v e
to
the
structure o f the catalysts. Concerning the s e l e c t i v i t i e s , products, studies.
HAL a n d I S 0 E P A a p p e a r a s i n i t i a l
e v e n i f I S 0 E P A was n e v e r m e n t i o n e d ,
u p t o now,
i n basic
W i t h t h e e x c e p t i o n o f t h e c o m m e r c i a l c a t a l y s t , modified
by
C r a n d l e a d i n g b o t h t o a s l i g h t l y b e t t e r s e l e c t i v i t y i n HAL a n d
l o w e r s e l e c t i v i t y i n I S 0 EPA,
a
and t h e impregnated c a t a l y s t showing
a l a r g e r p r o d u c t i o n o f HOL, a l l o u r c a t a l y s t s p r e s e n t r a t h e r c o m p a rable selectivities. i n i t i a l l y produced,
However,
l o o k i n g a t t h e l o w q u a n t i t i e s o f HOL
i t appears t h a t t h e lower
f o r t h e more a c t i v e c a t a l y s t s .
s e l e c t i v i t y i s obtained
As HOL i s t h e e n d p r o d u c t
of
the
h y d r o g e n a t i o n , o n l y t h e s u p p r e s s i o n o f HAL r e a d s o r p t i o n w o u l d a b l e t o s u p p r e s s t h e f o r m a t i o n o f HOL.
25%
A t
s t r o n g e r EPA a n d I S 0 E P A a d s o r p t i o n s a r e p r o b a b l y l i m i t i n g r a l l y t h e HAL a d s o r p t i o n .
But,
a t higher conversion,
c e n t r a t i o n s o f EPA a n d I S 0 EPA a r e v e r y l o w , have a good N i a c c e s s i b i l i t y ( i . e
be
conversion,
the
natu-
when t h e
coy!
i t seems n e c e s s a r y t o
t h e h e l p o f t h e macroporous struc
t u r e ) a n d a n e f f i c i e n t a g i t a t i o n t o l i m i t t h e HAL r e a d s o r p t i o n . fact, for
even a t 100% c o n v e r s i o n ,
t h e b e t t e r s e l e c t i v i t y i n HAL
In are
the c a t a l y s t s having the l a r g e r surface area. Increasing the conversion,
t h e HAL s e l e c t i v i t y i n c r e a s e s s l i g h t l y ,
p a s s e s t h r o u g h a maximum a n d t h e n d r o p s f o r c o n v e r s i o n s b e t w e e n 9 0 a n d 9 5 % . I n p a r a l l e l , some h e a v y p r o d u c t s , been r e c o g n i z e d ,
a r e f o r m e d . If
among w h i c h a c e t a l s h a v e
the formation o f these by-products
i s c a t a l y t i c a l o r n o t i s n o t known,
but it i s interesting to
see
t h a t p r e s e n c e o f a l a r g e r N i a r e a ( o r t h e p r e s e n c e o f Na i n t h e a l y s t ) seems t o l i m i t t h e i r f o r m a t i o n .
F u r t h e r work i s needed
cat to
t r y t o c o r r e l a t e c a t a l y s t s t r u c t u r e and f o r m u l a t i o n w i t h t h e form?
t i o n o f t h e s e h e a v y compounds. CONCLUSIONS T h e p r e p a r a t i o n o f N i c a t a l y s t s o n t h e FC C e l i t e , porous c a r r i e r , some a g i n g ,
by p r e c i p i t a t i o n a t room t e m p e r a t u r e ,
a l l o w s t h e o b t e n t i o n o f c a t a l y s t easy t o
presenting a satisfactory metallic dispersion.
a SiO,
macro
followed reduco
by and
The n a t u r e o f
the
p r e c i p i t a t i n g agent i s n o t i m p o r t a n t and t h e macroporous s t r u c t u r e o f the support i s maintained,
i n a large part.
phase hydrogenation o f 2 e t h y l 2 hexenal, s e l e c t i v e and 3 t o 4 t i m e s more a c t i v e reference catalyst.
Used i n t h e
these c a t a l y s t s are
than
a
commercial
The f o r m a t i o n o f h e a v y b y - p r o d u c t s seems
m a i n t a i n e d a t an a c c e p t a b l e l e v e l .
liquid as Ni-Cr
also
133 REFERENCES 1 2 3 4 5 6. 7 8 9 10
11
12
13 14 15 16
18
19
G.C.A. S c h u i t , L.L. van R e i j e n - Advances i n C a t a l y s i s - V o l . 10, Academic P r e s s London/New Y o r k , 1358, pp. 242-267. J.W.E. Coenen, B.G. L i n s e n - " P h y s i c a l a n d C h e m i c a l A s p e c t s o f Adsorbents a n d C a t a l y s t s , B . G . L i n s e n ( E d . ) , A c a d e m i c Press, London/New Y o r k , 1 9 7 0 , p p . 4 7 2 - 5 2 5 . J.W.E. Coenen (G. P o n c e l e t , P . G r a n g e a n d P . A . J a c o b s , Eds.), P r o c . o f t h e 3 r d I n t e r n a t i o n a l C o n g r e s s on The S c i e n t i f i c B a s e s of C a t a l y s t P r e p a r a t i o n , E l s e v i e r , A m s t e r d a n , 1 9 7 9 , p p . 89-111. J.W.E. Coenen - I n d . Eng. Chem. F u n d . , 25 ( 1 9 8 6 ) 4 3 - 5 2 . P . T u r l i e r , H. P r a l i a u d , P . M o r a l , G . A . Y a r t i n , J.A. Dalmon A p p l . C a t . 18 ( 1 9 8 5 ) 3 8 9 . Hermans, J . v a n d e r Meijden, A . J . v a n D i l l e n , J.W. Geus, L.A.M. P r o c . S i x t h I n t . C o n g r e s s on C a t a l y s i s L o n d o n , 1 9 7 6 , pp. 677685. on L.A.M. H e r m a n s , J.W. G e u s , P r o c . o f t h e 3 r d I n t . C o n g r e s s t h e S c i e n t i f i c B a s s o f C a t a l y s t P r e p a r a t i o n , E l s e v i e r , Amsterdan, 1979, pp. 113-130. M . M o n t e s , Ch. Penneman d e B o s s C h e y d e , B.K. Hodnett, F. Delannay, P . G r a n g e , B. D e l m o n , - A p p l . C a t . , 1 2 ( 1 9 8 4 ) p p . 3 0 9 - 3 3 0 . M. M o n t e s , J.B. S o u p a r t , M . S a e d e l e e r , B . K . H o d n e t t , B . Delrnon J . Chem. S . C . , Faraday T r a n s . 8 0 ( 1 ) (1980), pp. 3209-3220. B. M i l e , D. S t i r l i n g , M . A . Z a m m i t t , A . L o v e l l , M. Webb - J . C a t a l . , 114 ( 1 9 8 8 ) pp. 217-229. M.F. W i l s o n , 0. A n t i n l u o m a , J.R. B r o w n - S y m p o s i u n on t h e P r e p a r a t i o n and C h a r a c t e r i z a t i o n o f C a t a l y s t s P r e s e n t e d before the D i v i s i o n o f P e t r o l e u m C h e m i s t r y , I n c . , - A m . Chem. S O C . - L o s A n g e l s M e e t i n g - 1 9 8 8 , pp. 669-675. V.M.M. S a l i m , M . S c h m a l , R . F r e t y , M. R o d r i g u e s , M . C . S i l v e i r a 50 S e m i n a r i o de C a t a l i s e - B r a s i l ( 1 9 8 9 ) , pp. 93-101. C. P l i k l a s s o n , G . S m e d l e r , I n d . Eng. Chem. Res. 26 ( 1 9 8 8 ) 4 0 3 410. J . S m e d l e r - I n d . Eng. Chem. Res, 2 7 ( 1 9 8 8 ) , 2 0 2 3 - 2 0 3 0 . E.F. Souza A g u i a r , M . Schmal - V I I S i m p 6 s i o I b e r o - A m e r i c a n 0 d e C a t a l i s e - A r g e n t i n a , 1980. J . L . A l m e i d a , L . D i e g u e z , M. S c h m a l , I X S i m p 6 s i o I b e r o A m e r i c a no de C a t a l i s e , P o r t u g a l , 1984. e L.R.R.Araujo, L.C. D i e g u e z , R. F r e t y , M . S c h m a l , X I Simp6sio I b r o American0 de C a t a l i s e s , Mexico, 1988, pp. 1195. J . T . R i c h a r d s o n , R.J. Dubus, J.G. Crump, P . Desai, U. Osterwalder, I n t e r n a t i o n a l Congress on t h e T.S. C a l e - P r o c e d . o f t h 3 r d . Amsterdan, S c i e n t i f i c Bases o f C a t a l y s t P r e p a r a t i o n , E l s e v i e r , 1979, pp. 131-142. V.M.M.Salim, M.A. Duarte, M.C. S i l v e i r a , R. F r e t y M.Schma1 X I 1 S i m p o s i o I b e r o A m e r i c a n 0 de C a t a l i s e - B r a s i l - 1 9 9 0 . (in press.
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G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V
135
0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION AND PROPERTIES OF A PT/SILICA AND ITS COMPARISON WITH EUROPT-1 S.D. JACKSON3, M.B.T. KEEGAN', G.D. McLELLANl, P.A. MEHEUX', WEBB1, P.B. WELLS', R. WHYMAN4 AND J. WILLIS3
R.B. MOYES',
G.
'Department of Chemistry, University of Glasgow, Glasgow, 612 8QQ, Scotland (UK 'School
of Chemistry, University of Hull, Hull, HU6 7RX (UK)
31CI Chemicals and Polymers Ltd., The Catalysis Centre, Research and Technology Dept., PO Box 1, Billingham, Cleveland, TS23 1LB (UK) 41CI Chemicals and Polymers Ltd., Research and Technology Dept., PO Box 8, The Heath, Runcorn, Cheshire, WA7 4QD (UK) SUMMARY A 0.76% Pt/silica has been prepared by conventional impregnation from aqueous solution using hexachloroplatinum( IV) ions as the Pt source. Its adsorption characteristics and reactivity are compared with those of the standard reference catalyst EUROPT-1 (a 6.3 wt% silica-supported Pt) which was prepared by ion exchange using platinum( 1I)tetrammine ions as the Pt source. Platinum dispersion by HRTEM and particle morphology by EXAFS are reported. The catalysts both showed high metal dispersion and comparable behaviour in oxygen chemisorption and butadiene hydrogenation, whereas they differed with respect to mean Pt particle size and showed different behaviours in carbon monoxide chemisorption, cyclopropane hydrogenolysis, and enantioselective methyl pyruvate hydrogenation. Clearly the choice of comparators is of importance in catalyst evaluation. INTRODUCTION The relationship between catalyst preparation and performance is complex and curious. Catalyst characterisation provides the bridge in that preparation can be directed towards the attainment of certain structural and chemical characteristics, and attempts can be made to relate performance to those same characteristics. The authors are collaborating in an extensive joint study in which the preparation, characterisation and function of a range of supported platinum catalysts is being evaluated. In part, this study will compare platinum catalysts prepared using conventional supports (e.g. silica, alumina) with those using less conventional supports (e.g. molybdena) or those prepared by less conventional methods (e.g. metal vapour deposition). The restricted object of the present paper is to compare the conventional Pt/silica prepared within this programme with the standard reference silica-supported Pt codenamed EUROPT-1 for which full preparation and characterisation details have been published (refs. 1-5).
136
NOMENCLATURE Throughout this paper the silica-supported p atinum prepared and characterised in this investigation is referred to as Pt/silica, whereas the si 1 ica-supported platinum reference catalyst is referred to as EUROPT-1. EXPERIMENTAL METHODS Catalyst preparation is described in the next Section. Conventional means were used to obtain electron microscopic images of the platinum particles in each catalyst at a magnification of ca. 450,000~. EXAFS spectra of samples at the Pt L3-edge were obtained at the Synchrotron Radiation Source at the SERC Daresbury Laboratory. Powdered samples were reduced at 523 K and examined under pure hydrogen at ambient temperature in specially constructed glass cells fitted with mylar windows. Chemisorption studies were performed in a dynamic mode using a pulse-flow microadsorption apparatus. Samples of precursor (0.3-0.5 g) were reduced in After s i t u in flowing 5% hydrogen in nitrogen by heating to 573 K at 7 K min-'. reduction the flow was changed to helium, the temperature held at 573 K until hydrogen elution ceased and then lowered to ambient. Adsorbate was injected into the helium stream ahead of the adsorbent as pulses of known size (typically 0.05 cm3 at 0.101 MPa). The amount adsorbed was determined from the difference between the peak obtained and a calibration peak. The detection limit for adsorption was 2 x 10l6 molecules. Chemisorption of [14C] carbon monoxide was investigated at ambient temperature in a static system using pressures up to 533 Pa (ref. 6 ) . FTIR spectra of adsorbed-C0 were obtained in transmission at a resolution of 2 cm-' using a Nicolet 5DXC spectrometer with TGS detector. Hydrocarbon reactions were carried out in a static reactor (200 ml) attached to a conventional grease-free high vacuum apparatus. Samples of precursor were placed in the reactor, evacuated, and reduced in pure hydrogen at 523 K for 0.5 or 1.0 h. Reactants (cyclopropane, butadiene) were admitted to the catalysts in the order (i) hydrocarbon, (ii) hydrogen; pressure fall was measured by use of a pressure transducer, and analysis was by glc. Enantioselective ester hydrogenation was conducted in the liquid phase in a stirred glass reactor (Fischer Porter). 10 ml methyl pyruvate, 20 ml ethanol, and d i h y d r o c i n c h o n i d i n e - t r e a t e d catalyst (0.1 g EUROPT-1 or 1.0 g Pt/silica) were placed in the reactor, hydrogen was admitted and maintained constant at 10 bar pressure, and the contents stirred vigorously. After the required hydrogen uptake the product was filtered, distilled, and analysed by glc and polarimetry according to procedures published elsewhere (ref. 7) to determine the optical
137
yield in methyl lactate formation. THE CATALYSTS: PREPARATION, ANALYSIS, MICROSCOPIC AND SPECTROSCOPIC PROPERTIES A stock of the precursor to Pt/silica was prepared as follows. The chosen support was M5 Cab-0-Sil silica (surface area 203 m2 g-l; zero pore volume; impurities, C1=540 f 50 ppm (by neutron activation) Na = Cu = 20 ppm (by ICP-MS)). 9,76 g chloroplatinic acid (Johnson Matthey, platinum assay 41%) was dissolved in deionised water (0.60 1) in a five litre flask. Silica (M5 Cab-OSil) was added and mixed until the suspension began to gel, at which point further deionised water ( c a . 0.50 1) was added to promote mobility. This process was repeated until 398 g of silica had been introduced; at that point the total volume of water added had become 2.5 1. The flask was then attached to a Buchi rotary evaporator and water slowly removed by maintaining the contents at 353 K under a partial pressure of dry nitrogen. After 48 h a pale yellow free-flowing powder was obtained. Its uv-visible spectrum showed bands at 456, 374 and 264 nm consistent with the presence of chloroplatinate ions (ref. 8) and a further band at 206 nm which., is assigned to an 0-ligand to Pt charge transfer (ref. 9) and which indicates the presence of hydroxochloro- or oxochloro-platinum anions, e.g. [PtC150H]2-. The Pt-content of the precursor was 0.73 wt %. The Pt(4f7/2) binding energy measured on two occasions was 72.4 and 72.7 eV. Temperature programmed reduction (TPR) of the precursor was examined over the range 180 to 773 K. With a heating rate of 10 K min-' th'e major reduction feature was an asymmetric peak having a sharp maximum in the range 400 to 410 K. Reduced Pt/silica provided values for the Pt(4f712) binding energy of 71.2 and 71.4 eV, the Pt content was 0.76 wt% and the C1 content 610 f 60 ppm. Thus, the chlorine content o f the reduced P t l s i l i c a was no greater than that o f the original support material, within the f 10% experimental error inherent in neutron activation analysis. EUROPT-1 was prepared in a 6 kg. batch by Johnson Matthey Chemicals plc (ref. 2). Sorbsil AQ U30 silica was treated with Pt(NH3)4C12 and Pt(NH3)4(0H)2 at pH 8.9, filtered, washed until free of C1-, dried at 378 K and reduced at 673 K (ref. 2). The catalyst became oxidised by air before issue (refs. 3,lO). The total surface area of EUROPT-1 is 185 m2 g-', the Pt-content is 6.3 wt% and the impurities (in ppm) are: A1 = Ca = 500; Ti = Na = 400; Mg, 200; K , 150; Fe, 90; C1, < 50; Cr < 10. The Pt(4f7/2) binding energy in the re-reduced catalyst was measured in three laboratories at 71.3, 71.4, and 71.5 eV. The catalyst may be re-reduced in pure hydrogen without sintering at temperatures up to 673 K . TPR (this work) showed that re-reduction commenced at 248 K and was complete at 425 K , the maximum in the reduction profile occurring at 340 K; this concurs
138
with a previous report (ref. 11). The platinum particle size distribution (PSD) in Pt/silica, measured by HRTEM was: ,< 1.0 nm, 60%; 1.0 - 1.6 nm, 18%; 1.6 - 2.2 nm, 14%; 2.2 - 2.8 nm 6%; 2.8 4.0 nm, 2%. It is probable that some of the smallest particles present escaped detection, and hence this distribution is to be regarded only as a guide to the very high dispersion (approaching 100%) of the platinum active phase in this catalyst. The platinum PSD in EUROPT-1 reduced below 673 K contains maximum in the distribution at 1.8 nm, 75% of the platinum particles are c 2nm in diameter, and the dispersion i s 60% (ref. 3 ) . The morphology of the platinum particles in Pt/silica and in EUROPT-1 has been studied by EXAFS spectroscopy. Catalysts were reduced in pure H2 at 573 K for 80 min (Ptlsilica) or 60 min (EUROPT-1) and spectra were taken with the catalysts in a hydrogen atmosphere. The experimental and computed spectra for Pt/silica are shown in Fig. 1, that for EUROPT-1 and a Pt foil (for reference)
-8
0
200
400
eV
600
800
Fig. 1. Pt L3-edge EXAFS spectrum of 0.76 wt% Pt/silica. Full curve experimental (unsmoothed) spectrum: dashed curve = theoretical spectrum.
=
were of comparable quality. Structural information from these spectra is shown in Table 1. The d-, JZd, /3d, and 2d spacings are observed in each spectrum together with the coordination numbers (CN) and Debye Waller factors (DWF) appropriate for each shell. The values of the coordination numbers provide
139
direct information concerning the likely morphology of the average platinum particle present in each catalyst. Fig. 2 shows three 14-atom clusters o f Pt atoms of which b and c possess coordination numbers in reasonable agreement with experiment (Table 2). Similar calculations show that an average particle in TABLE 1 Structural parameters* obtained by EXAFS spectroscopy Platinum foil
Pt/silica
EUROPT-1
Pt-Pt/nm
CN
DWF
Pt-Pt/nm
CN
DWF
Pt-Pt/nm
CN
DWF
0.277 0.392 0.481 0.547
12.0 6.1 21.9 14.0
0.010 0.014 0.016 0.012
0.276 0.391 0.477 0.543
4.4 1.6 1.3 2.1
0.012 0.011 0.009 0.010
0.276 0.391 0.477 0.543
5.5 2.1 3.8 3.9
0.013 0.018 0.017 0.015
* CN
=
Coordination number; DWF
=
Debye Waller factor
TABLE 2 Structural parameters for proposed model particles a, b, c, and d o f Fig. 2 Pt/silica Coord. Nos
EUROPT-1 Coord. Nos
P t-P t/nm
Expt
a
b
C
Expt
d
0.276 0.391 0.477 0.543
4.4 1.6 1.3 2.1
4.1 0.0 2.7 2.3
4.4 0.9 2.1 1.7
5.1 1.1 1.0 1.1
5.5 2.1 3.8 3.9
5.5 0.7 3.7 3.4
a
C
Fig. 2. Model configurations for Pt particles in Pt/silica (a.6.c) and in EUROPT-1 (d). EUROPT-1 i s larger and is reasonably described by model d of Fig. 2. This model will be discussed in greater detail elsewhere. HRTEM and EXAFS each demonstrate that the average platinum particles in Pt/silica are very small ( c a . 0.1 nm) and i n EUROPT-1 are larger ( c a . 0.2 nm). In addition the Pt particles i n each
140
catalyst appear to consist of a raft of (111)-structure, with some atoms present in a partial second layer. ADSORPTION PROPERTIES Isotherms for [14C]carbon monoxide adsorption at 298 K over the range 0 to 526 Pa were measured using reduced Pt/silica (0.268 g) and EUROPT-1 (0.150 9). The adsorption capacities of the two catalysts were similar, and each isotherm showed a primary and secondary region (ref. 12) the transition occurring at about 26 Pa. Evacuation for 0.5 h at 298 K caused the desorption of 47% (Pt/silica) or 5% (EUROPT-1) of the adsorbed-C0. Subsequent equilibration with '658 Pa [12C]carbon monoxide resulted in 90% (Pt/silica) or 58% (EUROPT-1) removal of the remaining [14C]carbon monoxide. The FTIR spectrum of CO adsorbed on Pt/silica contained one strong band at 2085 cm-l having a slight shoulder on the high frequency side, whereas that for EUROPT-1 contained three bands: 2075(s), 1849(w) and 1720(vw) cm-l attributed to linear, bridged and capped species (ref. 5). Clearly, from the exchange measurements, more CO is reversibly adsorbed on Pt/silica than on EUROPT-1, and this is consistent with the higher frequency observed for CO adsorbed in the linear form on Pt/silica i n comparison with that on EUROPT-1 which implies a stronger Pt-C bond in the latter system. Similarly, CO adsorption on Pt/silica measured by the pulse technique over the range 256 to 294 K is an activated process (to be published), whereas no such claim has yet been made for CO adsorption on EUROPT-1. In oxygen adsorption on Pt/silica at 273 K measured by the pulse technique, 5.73 x 10l8 molecules (9. cat.)-' were adsorbed at saturation which corresponds to an O:Pttotal ratio of 0.5:l.O. The corresponding stoichiometry on EUROPT-1 was 0.65:l.O (ref. 5). However, after allowance for differences in dispersion, O:PtSurf i s 0.5:l.O for Pt/silica and 0.9:l.O for EUROPT-1. Furthermore, there is evidence for bulk oxidation of EUROPT-1 by oxygen (ref. 5 ) . Thus, although comparisons must be made with caution, there is a consensus in these results that adsorption of CO and perhaps that of O2 i s stronger on EUROPT-1 than on Pt/silica. CATALYTIC PROPERTIES 1,3-Butadiene hydrogenation The kinetics and mechanism of this reaction are well understood (ref. 13); in particular a greater than expected extent of 1:4-addition is indicative of the presence of electronegative contaminants ( S , C1) at the active sites (ref. 14). Hydrogenations were conducted at 290 K over Pt/silica and EUROPT-1, each reduced at 523 K and, for reference purposes, over an evaporated Pt film at 326 K (initial pressures: C4H6, 6.6 kPa; H2, 19.7 kPa; conversion, 20%). Butene and
141
butane yields were about 67% and 33% respectively and the butene compositions were: 1-butene 76 78 75
Pt film Pt/silica EUROPT-1
t-2-butene 18 14 18
c-2-butene 6 8 7
Pt/silica and EUROPT-1 surfaces behave similarly to that of the clean evaporated film confirming the absence of C1 in the neighbourhood of the Pt sites in the supported catalysts and showing that there is no particle size effect on selectivity in this reaction. Cyclopropane hydrogenolysis The failure of butadiene hydrogenation to distinguish between Pt/silica and EUROPT-1 may be related to the very strong adsorption of this hydrocarbon on Pt. We therefore examined the conversion of cyclopropane to propane, as this cyclic hydrocarbon is among the most weakly adsorbed of those that undergo hydrogen addition. Bond has reported that orders for this reaction vary with reactant pressures such that rate passes through a maximum with increasing pressure of either reactant; moreover this behaviour conforms to expectation based on Langmuir-Hinshelwood theory (ref. 16). Pt/silica and EUROPT-1, each reduced in pure hydrogen at 523 K for 1 h were used as catalysts for this reaction at 313 K ; the variation of rate with hydrogen pressure is shown in Fig. 3. The expected maxima were observed but the behaviour o f Pt/silica conforms to expectation based on Langmuir-Hinshelwood kinetics (eq. 1) whereas that of EUROPT-1 does not. (In eqn. 1 the symbols have their usual significance, r
=
kOC
3 1
OH
=
kecOl
=
kbcPc(bH1/nPH”n)2/(1
+
bcPc + bA/nPH1/n)3
(1 1
c = cyclopropane, n is defined by the process: 2H(ads)H2(g) + n(vacant sites)). For Pt/silica the experimental points are well modelled by an equation having n = 1 (firm curve, Fig. 3a, k = 10.2, bH = 0.70, b, = 0.0.33) and n = 0.5. However, we have failed to model the behaviour of EUROPT-1 adequately. The dashed curve in Fig. 3b i s a poor fit (n = 1) and we note that only the sharp maximum is well modelled by the dotted curve (n = 2, for which there is no ready interpretation). Thus the surfaces of the small Pt particles in Pt/silica behave as an energetically homogeneous surface on the Langmuir model whereas those of the larger particles in EUROPT-1 do not. It is not clear whether failure in the latter case i s due to the larger average size of the particles or to their wider size distribution.
142
P4I T o r r
0
100
2 00
300
Pi;l Tor r
Fig. 3. Dependence o f initial rate, R/Torr min- 1 , on initial hydrogen pressure, Pi/Torr, in the hydrogenolysis of cyclopropane to propane catalysed Po = 125 Torr. [ I Torr = at 313 K by Pt/silica (a) and by EUROPT-1 (b). 133.3 Pa]. In (a) the full curve represents a vakation of rate given by eqn. 1 (see text), and the dotted and dashed curves show the corresponding theoretical variations o f surface coverages. In (b) the full curve describes the experimental variation of rate, and the dotted and dashed curves represent predicted behaviour according to eqn.1 (see text).
Asymmetric Hydrogenation
Each catalyst has been modified by cinchonidine(1) and used for the high MeCOCODEae, to MeCH(0H)COOMe according (refs. 7, 17). The reaction provides optical yields (%R %S) at 290 K and
-
the deposition on its surface of pressure hydrogenation of methyl pyruvate, to our variation of Orito's method an excess of R-(+)-lactate. Typical 25 to 50% conversion were 87% over
143
( I ) Cinchonidine. R = vinyl
Dihydrocinchonidine. R = ethyl
EUROPT-1 and 54% over Pt/silica. We have proposed that ordered adsorption of several L-shaped alkaloid molecules occurs on each platinum particle of EUROPT-1 leaving exposed shaped ensembles of platinum atoms most of which accommodate methyl pyruvate in the conformation which, on hydrogenation, gives R-(+)-methyl pyruvate (ref. 7). Pt/silica contains much smaller particles (Fig. 2) most of which may be unable to accommodate more than one alkaloid molecule; hence the steric situation is less well defined and the optical yield much lower. Indeed, it might be argued that the typical 1 nm Pt particle in Pt/silica does not contribute to the optical selectivity but catalyses the formation of racemic lactate, and that a smaller proportion of larger Pt particles produces the optical yield observed. CONCLUSIONS 1. A Pt/silica has been prepared from a C1-contain ng source in such a way that the Pt particles show no evidence of contamination by C1. 2. The platinum particles in this Pt/silica are structurally similar to, but smaller than, those in the standard reference catalyst EUROPT-1. 3 . Substances which are weakly or only moderately strongly adsorbed (cyclopropane, CO) exhibit different characteristics when adsorbed on the Pt surfaces of these two catalysts, whereas substances that are strongly adsorbed (02, butadiene) show similar or identical behaviour. 4 . Where the adsorption of a molecular template on the Pt surface is required in order to induce enantioselectivity, the optical yield i s diminished as size of the Pt particles approaches that of the template.
144
ACKNOWLEDGEMENTS We thank SERC and ICI C & P Ltd for financial support in the context of a Cooperative Award. SERC is also thanked for SRS beam time and for the award of studentships to MBTK and PAM. REFERENCES 1 2 3 4 5 6
G.C. Bond and P.B. Wells, Applied Catal., 18 (1985) 221. G.C. Bond and P.B. Wells, Applied Catal., 18 (1985) 225. J.W. Geus and P.B. Wells, Applied Catal., 18 (1985) 231. A. Frennet and P.B. Wells, Applied Catal., 18 (1985) 243. P.B. Wells, Applied Catal., 18 (1985) 259. S. Kinnaird, G. Webb and G.C. Chinchen, J. Chem. SOC. Faraday I, 8 3 (1987)
7 8 9 10 11 12. 13
I.M. Sutherland, A. Ibbotson, R.B. Moyes and P.B. Wells, J. Catal., accepted for publication. C.K. Jorgensen, Acta Chem. Scand., 10 (1956) 518. D.L. Swihart and W.R. Mason, Inorg. Chem., 9 (1970) 1749. R.W. Joyner, J. Chem. SOC. Faraday Trans. I , 76 (1980) 357. G.C. Bond and M.R. Gelsthorpe, Applied Catal., 35 (1987) 169. J.U. Reid, S.J. Thomson and G. Webb, J. Catal. 29 (1973) 421. J.J. Phillipson, P.B. Wells and G.R. Wilson, J. Chem. SOC. (A), (1969)
14
M. George, R.B. Moyes, D. Ramanarao and P.B. Wells, J. Catal., 52 (1978)
15
A.G. Burden, J. Grant, J. Martos, R.B. Moyes and P.B. Wells, Discuss. Faraday SOC., 72 (1981) 95. G.C. Bond and J. Turkevich, Trans. Faraday SOC., 50 (1954) 1335; G.C. Bond and J. Newham, Trans. Faraday SOC., 56 (1960) 1501. Y. Orito, S. Imai, and S. Niwa, Nipp. Kag. Kaishi, 8 (1979) 1118.
16 17
3399.
1351.
486.
G. Poncelet,P.A. Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
145
FACTORS ANALYSIS FOR MECHANICAL STRENGTH IN PELLETING PROCESS OF Fe-BASED HIGB TEMPERATURE SHIFT CATALYST
YONGDAN LI, JIUSHENG ZHAO and LIU CHANG Department of (China)
Chemical
Engineering,
Tianjin
University,
Tianjin
300072,
ABSTRACT The pelleting process and the factors influencing mechanical strength for Fe-based catalyst have been investigated. The horizontal crushing strength data is used in the text, their mean value and their corresponding Weibull modulus are taken as parameters for analysis. It is found that the processing precision of the pelleting machine, the pretreatment of the powder and the pelleting conditions all have strong effect on the final strength of pellets. The experimental result shows that a potentiality of increasing in overall properties exists, which means a better quality catalyst could be expected. INTRODUCTION Formation of pellets is one of the important processes in catalyst preparation, which
significantly affects the mechanical strength and the
texture, on which
the reliability of
catalyst performance in industrial
reactor and the effectiveness of pellet depend, even though it usually does not change the intrinsic catalytic activity apparently. The influence of pelleting conditions on the properties of catalyst has been studied by several authors
(ref.l,2).
pelleting pressure.
Their publications stressed mainly on
the effect of
But actually in the process of pelleting, there are
many factors which will influence the properties of the resulting pellets. In most cases these factors are interdependent on one another. Yet in general idea, they can be distinguished into three different categories, that is the processing precision of pelleting machine, the property and the treatment of
the
powder
material
pelletization. This
paper
and
effectiveness influencing both properties of
additives,
will
report
and some
the
processing
important
pressure
factors and
the mechanical strength and the
resulting pellets, and
finally
it will
also
of
their
overall
tell us
the
potentiality of getting better catalyst as a whole. SAMPLES AND EXPERIMENTAL Samples The powder material of mixed oxides was obtained from industry made
146 X
Fig.1
I
I
I
I
20
30
40
50
I
60
70
Degrees (28) The XRD diagram of the pre-pelleting mterial
by coprecipitation process. It contains about 10wt% of Cr2O3, 90wt% of Fez03 and a small amount of volatile combines. The XRD pattern (Fig.1) shows r-Fe203 phase is the most significant one. Several brands of commercial Fe-based catalyst pellets with a length of 4-7mm and a diameter of 9mm are taken for comparison. Pelletinq The pelleting was performed by using a hydraulic press with a set of specially designed die and punch, from which air can be released easily. Experiment shows that the contactsurface of both the die and the punch must be extremely smooth, or else during ejection there would be a big shearing stress "u" (refer to Fig.2) developed at the edge plane of the pellet, which in turn will induce big flaws within the pellet; at some cases there was even no complete pellet made. The precision of filling weight is also very important for strength, because under a definite operating condition the stress distribution in pelleting within the pellet is sensitive to the filling weight. The set of die and punch was carefully machined so that a variety of the strength data with a close distribution could be obtained.
The purpose
of which is to guarantee the repeatability of data for analyzing different factors involved. The process is illustrated as in Fig.2.
(a). Transfer
the material into the cylindrical cavity of the die, and rest it on the upper side of the lower punch. upper punch.
(b). Compress the powder by lowering the
(c). Eject the pellet by the upper movement of lower punch.
The pellets obtained in our laboratory under a normal processing condition with the industrial material possess a mean horizontal crushing strength (HCS)
those
of 45.8 and a Weibull modulus of 10.1, which are much higher than coming from the industry.
All the pellets in this paper made in
our laboratory have the same size, with a length of 6mm and a diameter of 9mm.
147
Fig.2
Pelleting process
Measurement The pellet strength is characterized by the HCS value, which has been shown in accordance with tensile fracture stress and proved to follow Weibull distribution (ref.3) both theoretically and practically, with the probability of failure under certain radial compression force given by
where p is the maximum stress, or in this paper, the maximum compression force, m is the Weibull modulus,
is a size factor, F(p) is the probability
of failure under p. The HCS data of solid catalyst scatters in a certain mode characterized by
the different value
of
Weibull
parameters m
and
p,
indicating the
discrepancy of preparation technology. We can see from equation (l), that higher m and lower
means less scattering of the data. In this paper, both
the mean value of HCS and the Weibull parameters of the catalyst strength were used in the analysis. The XRD pattern was taken by Rigaku 2038 X-ray diffractometer, Cu KO(, and the texture data was measured by AUTO PORE 9220 I[ porosimeter. RESULTS AND DISCUSSION Statistical result of commercial catalyst pellets Statistical results of
some properties of several brand
commercial
catalyst pellets show that the scattering of length of different samples also follows Weibull distribution reflecting the precision of the pelleting machine and bears some relationship with the scattering of HCS data, as indicated in Fig.3 and Table 1. Sample 1 and 2 are two different brands of commercial catalyst, but were pelleted by same model of pelleting machine. Sample 3 and 4 are another two brands but were pelleted by another same model of machine. We can see the Weibull modulus of strength and length given by same model machine are very close. In our
other paper, the relationship between the scattering
148 of
strength and density was
Scattering of
studied
strength, length
and
(ref.4) and density
shown the same trend.
in most
cases display
the
performance of the pelletinq machine.
h cl Q
-
0.6
6
Q
I
0 10 Fig.3
20 30 40 0 10 20 30 40 50 3.04.05.06.03.04.05.06.07.0 l(m) HCS(kg/pellet) Strength and length distribution of commercial catalysts a & b: strength distribution of sample 3 & 1 in Table 1, c & d: length distribution of sample 3 & 1.
Table 1.
Weibull parameters of length and strength
B1
mS
BS
ml
3.59
4.75 x 10-6
11.8
2
3.41
4.16 x
3
6.23
1.96
4
6.44
1.98 x
sample 1
Note:
ms
&
ps - Weibull
m1 s -
'
"the
S
1.40 x
27.8
11.5
3.25 x
43.2
2.10
10-30
24.9
46.9
2.20
10-32
31.3
10-9
34.2
parameters of strength:
- Weibull parameters of length:
most probable strength (HCS).
Factors Influencing the Mechanical Strength of Pellet For
simplicity and
convenience, two
sets
of
two
level
fractional
factorial experiments were carried out. First Set Factorial Experiment
FOUr factors, pelleting pressure,
powder size distribution, water content, and graphite amount are taken into account. Table 2 gives the factors and levels of the experiment. The results and the experimental matrix as well as the comparative effectiveness are shown in table 3. From these two tables, we can see that all the factors are sensitive. The powder
size distribution and water
content are most
effective.
The
149 pressure has a negative effect, it means as the pressure increases, the strength decreases. Notice that the change of density with strength is not in a simple manner, which shows it may be possible to get good mechanical strength with comparatively low desity after appropriate treatment.
Table 2.
Factors and levels of first set factorial experiment factors
Levels
A(kbars) pelleting pressure
B(mesh ~ 0 1 % ) powder size distribution
C(Wt%) graphite content
D(Wt%) water content
0
3
60-100, 10 < 100, 60 20-60, 30
0.5
14.5
1
5
60-100, 10 < 100, 30 20-60, 60
1
10.1
0.5
4
60-100, 10 < 100. 45 20-60, 45
0.75
11.76
Table 3.
Results and effectiveness of factors
experimental matrix
results
No.
A
-
HCS (mean) (kg/pellet)
D
B
C
0 0 1 1
0 1 0 1
0 0
0
1
1
0
0 1 0 5
0 1
0
1 2 3 4 5
0 0 0
6
1
7
1
8 9
1 0 5
1 1 0 5 A
B
C
D
Eff. for HCS Eff. for Density
-3.22 0.257
-12.3
2.07 0.013
-12.3 -0.31
0 1
0
1 0
0 5
Second Set Factorial Experiment
In this
-
2.46 2.13 2.14 2.47 2.43 2.73 2.72 2.40 2.48
58.1 47.8 43.6 48.8 31.9 57.9 42.5 29.8 45.5
1
Density (mean) (g/cm’)
AD
-11.6 0.017
BD 9.2 0
CD 0 -0.032
set, another 4 factors,
150 pelleting pressure, HNO3 concentration, r-A100H doping, and grinding time are examined. The factors and levels are shown in Table 4. Levels of E denotes same volume but different concentration of HNO3 was added into the material. F refers to doping with r-Al00II (below 300 mesh), and G stands for the time
of hand grinding of the material before pelleting.
The results and effectiveness are given in Table 5 .
FG
-12.6 0
The factors in
the second set have more striking effect on the mechanical strength, while the effect on density is still less sensitive. It is shown that by grinding and doping of r-AlOOH, the strength i s greatly enhanced, but impregnation
151 with HNO3 gives bad result. Effect of grinding shows the same effect as particle size distribution, that means smaller particle size is beneficial for strength. The highest HCS in Table 5 is 103.2, which is approximately 5 times as high as that of ordinary commercial catalyst, as its density is only 2.56, still in the range of commercial ones. The experiment shows that a great potentiallity in increasing mechanical strength is existing. Effect of Pelleting pressure Pressure on Density
Normally as pressure increases, the mean strength
of pellets increases, and the density increases too. Yet in our experiment, pellets made directly from industrial
-
50
4
c)
. i . i
40
2
\
2 -
:: X
powder
> ,’
60
pressure upper
HCS
on
limit
density of
20
pre-treatment
and
both
between
pelleting
in a
density
as
There appears strength certain
pressure.
and
range
Such
fact
incompressibility
of
mother crystals in pellets. As
for
the
gas
suggests
10
the
diffusion
limiting
water
shift reaction (ref.51, a relatively
0 1 2 3 4 5 Pelleting Pressure (kbars) Fi9.4
any
illustrated in Fi9.4.
/o-----‘
30
without
shows some limitations of pelleting
Dependence of strength and density on pelleting pressure
high
strength
and
low
density
to
a
comparatively enhance
the
effectiveness factor of the catalyst are preferred.
Table 6. Effect of pelleting pressure on the pore structure of oxidized state ~~
P
HCS
2
40.4
3 5
V
R
71.6
0.233
13
2.22
45.8
67.0
0.184
11
2.30
58.4
63.5
0.159
10
2.64
S
D
c
m 9.56 10.1 7.51
2.99 x 10-16 1.21
10-17
3.46
10-14
In the table, P(kbars): pelleting pressure, HCS (kg/pellet): mean horizontal crushing strength, S (m2/9): specific surface area, V (ml/g): porosity, R (nm): most probable pore diameter, D (g/cm’): density, m and @ : the Weibull parameters. Pressure on Pore Structure and Activity
Table 6 and 7 display the
effect of pelleting pressure on the mechanical strength and the pore structure
152 of oxidized and reduced state catalysts respectively. The reduction was performed under an optimum condition for strength developed by a set of optimization experiments Table 7.
HCS
P
S
(ref.6).
Effect of pelleting pressure on the properties of reduced state of the catalyst V
R
2
35.5
70.6
0.287
16.3
2.10
3
63.0
65.0
0.244
14.4
2.28
5
58.5
60.1
0.189
12.6
2.46
B
m
D
2.93 12.8 4.60
A
1.99 x
57.3
6.60 x
51.2
4.89
10-9
46.3
A: the apparent activity of CO conversion in a microreactor under normal high temperature shift reaction condition (ref.6). From the above two tables, we noticed that all of the parameters changed in the same trend as pelleting pressure changed, except Weibull parameters m and
p.
They give optimal values at P=3 kbars, under which the most reliable
catalyst in mechanical strength can be expected as shown in Fig.5.
Both
the HCS and the Weibull modulus of the sample pelleted at P=3 kbars is much superior to that of commerial catalysts (refer to Table 1). Meanwhile, its density and activity are quite acceptable. As a matter of fact the reliability or the probability of failure may be the most important in industrial point of view.
1.0 0.8 0.6
0.4
1.0
f
0.8
0.6
/a a'
0.4 0.2
0.2 0 10 20 3 0 40 50 60 70
40 50 60 70 8 0
20 30 40 50 60 70 80 90 100 HCS (kg/pellet)
Fig.5
HCS distribution of reduced state at different pelleting pressures a: 2 kbars, b: 3kbars. c: 5 kbars.
REFERENCES 1
J. Uchytil, M. Kraus and P. Schneider, Influence of Pelleting Conditions
on Catalyst Pore Structure and Effectiveness, Appl. Catal., 28 (1986) 13-14.
153 I. Brasoveanu, S.I. Blejoiu, A. Ssabo, P. Rotaru and I.V. Nicolescu, Structural Strains Appearing in the High Temperature Shift Conversion Fe-Cr Catalyst, Revue Roumaine de Chimie, 25(8) (1980) 1159-1169. 3 Yongdan Li, Liu Chang and Zhou Li, Measurement and Reliability Analysis of Mechanical Strength of Cylindrical Metallic Oxide Catalyst, Journal of Tianjin University, 1989 ( 3 ) 9-17. 4 Yongdan Li et al., Statistical Analysis for Mechanical Strength of Cylindrical Fe-based Catalyst. Journal of Fuel Chemistry and Technology, in press. 5 Hans Bohlbro, An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapour over Iron Oxide Based Catalysts, Second Edition, Gjellerup, Copenhagen, 1969. 6 Yongdan Li et al., Factors Analysis on Mechanical Strength in Heating and Reduction of High Temperature Shift Catalyst by Dn Saturation Optimum Experimental, C1 Chemistry and Chemical Industry, in press. 2
This Page Intentionally Left Blank
G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
STUDIES ON
155
PORE SIZE CONTROL OF ALWINA: PREPARATBN OF ALWINA CATALYST EXTRUDATES
WITH LARGE UNMODAL PORE STRUCTURE BY LOW TEMPERATURE HYDROTHEWW TREATMENT
M. Absl-Halabi, A. Stanislaus and H. Al-Zaid
Petroleum Technology Department, Petroleum, Petrochemicals and Materials Division, Kuwait Institute for Scientlflc Research, P. 0. Box 24885, 13109 Safat, Kuwait
ABSTRACT
In the present work, the application of low temperature hydrothermal treatment method for preparing y-alumina supports with large monomodal pore size distribution has been investigated. Gamma alumina in the form extrudates was subjected t o hydrothermal treatment in an autoclave for various durations in the temperature range 150-3OO0C. The effect of ammonia vapor during hydrothermal treatment was also studied. The treated catalysts were characterized for surface area, pore size distribution and mechanical strength. The samples were also examined by x-ray diffraction and transmission electron microscope. The results revealed that pores can be widened selectively with greater than 70% pore volume In the mesopore range 100-250 A' and with improved crushing strength by low temperature hydrothermal treatment (e.g. 150°C). Further shift of pore size t o any desired larger range (250-500 Ao or 500-1500A') is possible by increase of temperature and treatment time. Ammonia was found to enhance the pore enlargement. The undesirable effects on loss of surface area and mechanical strength were signlficantly low and alumina phase transition was negligible in this process.
INTRODUCTION
Alumina has been widely used as a catalyst support in many catalytic processes of industrial importance.
This is largely because it is reasonably stable, contains acidic and
basic sites and can provide, through its different phases a wide range o f surface areas and porosities which are suitable for many catalytic applications. For some applications like hydroprocessing of petroleum residues and coal derived liquids it is desirable to have catalysts with wide pores since the large complex molecules present in heavy residues must have access t o the active surface sites within the catalyst pellets through an appropriate pore network (1.2).
A greater emphasis has been placed in recent years on the development of
wide pore catalysts with small external dimensions t o overcome the problem o f diffusion limitations (3,4,5.6).
Catalysts with bimodal pore size distribution having mesopores and macro-
pores in different proportions have been recommended t o prevent rapid deactivation In residue hydroprocessing. Large pore unimodal catalysts have also been found t o be more effective in residue hydroprocessing.
156 Pore size control of supported catalysts Is effected mainly by controlling the pore size of the support.
Several methods have been described in the literature for controlling
the pore size of alumina supports (7,8.9,10).
Thermal sintering has been chosen as a meth-
od for obtaining catalyst supports with monomodal pore size distribution In the mesopore or macropore (11,12,13) range.
The use o f some inorganic salt additives and different atmos-
pheres in enhancing the rate of sintering and lowering the calcination temperature to produce large pore alumina catalyst supports has also been explored (12.14.15).
Thus, for
example, by using Moo3 doped alumina, Tischer noticed that the sintering temperature could be lowered from 1000 t o 700°C t o produce a similar pore size distribution.
Sintering In
presence of steam has also been found t o result in enlargement o f pores (7,11,13,16). However, in these studies temperatures as high as 700% have been found necessary to promote sintering and t o produce aluminas with desired large pore size. The large pore aiuminas prepared by the high temperature sintering process usually have reduced surface area and low mechanical strength. phase is converted t o other types active as the y-aiuminas.
(6. 8
&
Further, part o f the y-alumina
a) of aluminas that are catalytically not
as
These undesirable effects on surface area, mechanical strength
and alumina phase transformation may probably be minimized if selective sintering of the narrow pores could be achieved at lower temperatures. A major obJective of the present work was t o investigate the influence o f low temp-
erature hydrothermal treatment on pore size enlargement of y-alumina support.
The material
(y-alumina) In the form o f extrudates was subjected t o hydrothermal treatment in an autoclave in the presence of water vapor for various duration In the temperature range 150-300°C. gated.
The influence o f ammonia vapor during hydrothermal treatment was also investi-
The treated catalyst samples were characterized for surface area, pore volume,
pore size distribution and mechanlcai strength.
The samples were also examined by x-ray
diffraction and transmission electron microscope t o assess possible changes in the alumina phase and the extent of sintering.
The results o f the studies reveal that low temperature
hydrothermal treatment can be used for the preparation of alumina support extrudates with large unimodal pore structure by selective enlargement of pores.
EXPERIMENTAL
Alumina extrudates were prepared from Condea Chemie Pural SB boehmite gel by kneading and extrusion.
Alumina paste suitable for extrusion was prepared by peptizing and
kneading the alumina powder with the peptizing solution.
In a typical experiment, 250 g of
alumina powder and appropriate quantity o f peptizing agent were used for each batch.
The
peptizing agent(l.5X HN03) was added at a constant flow rate over 20 min duration with continuous mixing using a kneader (Linden Model D5277, Germany). The paste was extruded through 1.5 mm nozzles, then dlred at 110°C in an oven for 24 h. 370'C
The dried extrudates were calcined under programmed temperature condltions (at for 2 h, 450°C for 1 h and 550'C
for 2 h).
A slngle screw type extruder model No.
157 250 (Netzsch, Germany) was used in making the extrudates.
For hydrothermal treatment
studies y-alumina extrudates prepared from Condea Pural SB alumina by the above procedure were used as starting material.
A weighed portion o f the sample (about 10 g) was
heated In an autoclave at temperatures ranging between 150 and 300'C. reagents, namely, water and ammonium hydroxide were used in the study.
Two types of
The treatment time
and the ratio o f water t o alumina were also studied. A mercury porosimeter (Micromeritics model 9305) was used t o determine pore size A Quantasorb adsorption unit (Quantachrome Corporation, USA) was used for
distribution.
BET surface area measurements. Pharma test model PTB 300 equipment was used to meas-
ure the side crushing strength of the alumina extrudates.
X-ray diffraction patterns were
obtained using a Phillips PW 1410 x-ray spectrometer operated at 30 kV and 20 mA with C u Ka radiation.
Transmission electron micrographs were made with JEM-1200 EX microscope.
RESULTS AND DISCUSSION
Gamma alumina in the form of 1.5 mm extrudates was subjected t o hydrothermal treatment in an autoclave In the presence of water vapor, for different time periods in the temperature range 150-300°C.
The effect of water t o alumina ratio and the presence of
ammonia vapor during hydrothermal treatment in the same temperature range on the modification of pore size was also Investigated. The results are presented and discussed below.
H f e c t o f treatment t i m e .
Influence of the duration of heating on pore size
distribution was investigated at a constant temperature
.
The results obtained at a con-
stant temperature of 150°C for water t o alumina ratio 1:l (w/w) are presented in Table 1. Table 1.
Effect of Hydrothermal Treatment Time on Pore Size Distribution (Reagent: water; Temp. 150°C)
60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-100000 > 100000 Tota I
It can be noticed that the amount of pores in the 60-100 A' diameter range progressively
decreases
y-7 L;,increasing
time o f heating with a corresponding increase in the amount of
158 100-250 A' pores.
Thus, the amount o f 60-100 A' pores is reduced from 90.4 t o 36.9%
with an increase in the amount of 100-250 A' pores from 5.6 t o 59.7% when the heating time is increased from l h t o 4 h. Further increase o f heating duration t o 8 h resulted in a further increase in the amount of 100-250 A" pores (from 59.7 t o 71.5%) with a corresponding decrease in the amount o f the 60-100 A" pores.
Thermal treatment in the pres-
ence o f water at a temperature o f 150°C, thus, increases Selectively the amount of 100-250 A' pores. A similar effect o f heating t i e on selectively increasing the amount of pores o f a particular diameter was also noticed for samples heated at higher temperatures. However, the pore size range that is widened or enlarged depends t o a large extent on the temperature o f heating as shown below.
E f f e c t of temperature.
The temperature o f hydrothermal treatment was varied
between 150 and 300°C t o study its effect on pore size modification. Table 2 presents pore volume distribution data for samples heated at 150, 200 and 300°C for a fixed time of
8 h.
It Is seen that temperature has a remarkable effect in widening the pores.
Thus, a
sample heated at 150°C for 8 h contains 23.5% and 71.5% o f the pore volume in pores of diameter 60-100 Ao and 100-250 A",
respectively.
In this sample, only about 5% of the
total pore volume is contributed by pores larger than 250 Ao. Table 2.
Effect o f Temperature on Pore Size Distribution of Alumina During Hydrothermal Treatment in Presence of Water for 8 h Pore Volume
60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-1 00000 > 100000 Tota i
On increasing the temperature t o 2OO0C, the pore size distribution pattern is altered. The amount of pores in the 250-500
Ao is increased from 1.5 t o 45.9%. Substantial
increase (1.5 to 34.9%)is also noticed in the 500-1500 Ao dia. pores. Further enlargement of the pores with maximum pore volume (about 63.7%) in the 500-1500 noticed with increase o f temperature t o 300°C.
A'
dia. range is
159 The results show that pores can be widened and pore size distribution in alumina
sup-
port can be shifted from the narrow pore size range (e.g. 60-100 Ao dia.) t o the larger range (e.g. 100-250 A', 250-500 A' or 500-1500 A' dia.) by increasing the temperature of hydrothermal treatment.
E f f e c t of the Alumina: Water Ratio.
The effect o f the amount o f water
used in hydrothermal treatment studies in modifying the pore size distribution was investigated by varying the amount of water between 10 and 40 mi for a given weight (10 g) of alumina at a fixed temperature and duration. The results for the experiments conducted at a constant temperature of 150°C for a fixed duration of 1 h are shown in Table 3. The data indicate that the amount of water or in other words, the ratio between the alumina and water, used in hydrothermal treatment has no significant influence on the pore size distribution.
Similar observations were also made
for the experiments conducted at higher temperatures.
Table 3.
Influence of the Amount o f Water Used for Hydrothermal Treatment on Pore Size Distribution of Alumina at 150°C for 1 h Pore Volume
10 m i H20
-----______Pore Dia ( A " )
60-1 00 100-250 250-500 500-1 500 1500-4000 4000-1 0000 10000-1 00000 > 100000 Tota I
m i g-l
m i g-l
%
0.405
40 m i H20
___---_____
90.4
0.413
5.6
0.026
0.006 0.004
1.3 0.9
0.001
0.2
0.002 0.000 0.005 0.448
0.4 0.0 1.1 100
0.008 0.007 0.003 0.000
0.025
0.002
0.005 0.464
%
89.0 5.6 1.7 1.5 0.6 0.0 0.4 1.1 100
The results of the studies presented above clearly show that low temperature hydrothermal treatment can lead t o widening of pores.
The extent of pore enlargement is
dependent on the treatment temperature and duration, but not on the amount of water. The exact type of chemical interaction or mechanism that leads t o pore enlargement is not clearly understood. Transmission electron microscope examination of the hydrothermaliy treated samples showed a progressive increase in the alumina crystaiiite size with increasing treatment time (Fig. 1).
X-ray diffraction analysis showed progressive narrowing of y-alumina peaks indi-
cating increase of particle size.
No peaks corresponding t o other phases o f alumina were
noticed. Since porosity originates from the volume of the space between the packed alumina particles, Increase in the partlcie size may be expected to result In pore enlargement.
160
Fig. 1
. TEN 01 sluminas trailed with water a1 2 0 O O C lor 111 I hr .Ibl 2 hr and icl 8 hr
Although the exact nature of chemical interactions that has resulted in particle size growth during thermal treatment in presence of water vapor at relatively low temperatures is not clear, It would be useful t o consider the following: Sintering of alumina generally requires material transport in the solid state. bulk diffusion (17.18).
This may proceed via surface diffuslon or
The surface diffusion is very responsive t o the presence o f impuri-
ties such as adsorbed gas (19.20) and ions (14,15,21).
During the process o f heating of
y-alumina in presence of water vapor, hydroxylation and dehydroxylation of alumina are possible.
This may enhance the mobility of the oxide and hydroxyl Ions on the alumina surface
leading t o acceleration of their surface diffusion, and thus may promote the rate of particle size growth.
E f f e c t o f ammonia. The influence of ammonia on the modification of pore size distribution o f alumina during hydrothermal treatment was studied in the temperature range 150-300°C. Fig. 2 shows the effect of treatment time at a fixed temperature (150°C) on the pore size distribution of alumina.
It is seen that the pore diameter is increased progressively
when the heating duration is increased, as in the case o f hydrothermal treatment with water A comparison o f the pore size distribution data (Table 4) of hydrothermally treated
alone.
alumina in presence and absence o f ammonia indicates that ammonia has a promoting effect on pore enlargement. Ammonia is a basic gas and it may strongly enhance the rate of hydroxyiation of y-Ai203
by cleavage of the AI-0-AI
bond. Such enhanced hydroxylation during the hydroth-
ermal treatment may increase mobility of OH ions on alumina surface and enhance the rate of recrystallization and particle agglomeration and consequently lead t o pore enlargement.
161
PORE DIAMETER (A) Fig. 2. The effect of hydrothermal treatment time at 1 5OoC in presence of NH7 on the pore size distribution of alumina extrudates. Table 4.
Comparison of Pore Size Distribution Data o f Alumina Hydrothermally Treated in Presence and Absence of Ammonia at 300T Pore Volume
60-1 00 100-250 250-500
500-1 500 1500-4000 4000-1 0000 > 10000 Total
m l / g (%) 0.00 (0.0)
0.08 0.27 0.17 0.01 0.01 0.01 0.55
(14.5) (49.0) (31 .O) (1.8) (1.8) (1.8)
ml/g (%) 0.0 (0.0)
0.02 0.10 0.41 0.01 0.01 0.01 0.56
E f f e c t o f hydrothermal
(3.6) (17.8) (73.2) (1.8)
(1.8) (1.8)
ml/g
0.01 0.02 0.13 0.34 0.01 0.01 0.03 0.55
(%I
ml/g (%)
(1.8) (3.6) (23.6) (61.8) (1.8) (1.8) (5.5)
0.01 0.02 0.05 0.47 0.01 0.01 0.02 0.59
(1.7) (3.4) (8.5) (79.7) (1.7) (1.7) (3.4)
treatment on surface area and mechanlcal
strength. The surface area of hydrorthermally treated catalyst samples are plotted in Fig.
3 as a function of treatment time. increasing treatment time.
All samples show a decrease h surface area with
However, the drop in surface area Is significantly high for sam-
ples treated at higher temperatures (>150°C)for longer duration.
This is not surprising in
view of the presence of the large amount of macropores in these samples. The crushing strength of the samples show an interesting behavior (Fig. 4). For the alumina hydrothermally treated at 1 50aC, the crushing strength Increases progresslveiy with
z2
162
I -
300 .250 -E 200 M
20 r C
N
Q
w
I
4
a?
Q
w 1500
2
5 vl
E l . . , .
100-
zo
0
2
4
6
8
TIME (h)
Fig. 3. Effect of hydrothermal treatment time and temperature on surface area of alumina extrudates.
0
2
4
6
8
TIME (h) Fig. 4. Influence of hydrothermal treatment time and temperature on side crushing strength of alumina extrudates.
increasing duration of heating, although there is a considerable increase (about 65%) in the volume o f 100-250 Ao diameter pores. However, at higher temperatures, a reverse trend is noticed.
A similar effect was also noticed for samples treated in the presence of ammonia.
During hydrothermal treatment at moderate temperatures (e.g. about 150°C). rehydration of the y-alumina is possible.
This may lead to the creation of chemical functions with hydroxyl
groups which on further calcination may increase the cohesion and consequently increase the mechanical strength.
In the case of of hydrothermal treatment at higher temperatures
(e.g. 3OO0C), hydroxylation-dehydroxylation cycles leads t o larger particles that are probably loosely packed. A weak cohesion between the alumina particles can result in weak mechanical resistance. Currently, the effects o f other reagents are being investigated and further experiments t o obtain a better understanding of the mechanism through which pore widening takes place are being undertaken.
ACKNOWLEDGEMENT
The authors thank Dr. S. Mansour for the TEM work, and MS. K. assistance in catalyst characterization. for Scientific Research, Kuwait.
Al-Dolama for her
This is KlSR Publication No. 3401, Kuwait institute
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C. Lippens and J. J. Steggerda.
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and Catalysts, Linsen (editors), Academic Press, London, 1970, p. 171. Chem. Eng. Tech. 56(1984) 455.
11.
U. Hammon and M. Kotter.
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R. E. Tischer, J. Catal. 72(1981) 255.
13.
D. J. Young, P. Udaja and D. L. Trimm.
In "Catalyst Deactivation", B. Delmon and G. F.
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W. H. Gitzen, Alumina as a Ceramic materials, The American Ceramic Society, Columbus,
Ohio, 1970. 15.
P. Burtin, J. P. Brunella, M. Pijolat and M. Sousteli, Appi. Catai. 34(1987) 225.
16.
D. Aldcroft, G. C. Eye, J. G. Robinson and K. S. W. Sing, J. Appl.
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J. Haber.
Chem. 18(1968) 301.
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Elsevier Amsterdam, 1984, p. 123. 18.
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19.
A. E. B. Presiand. G. L. Price and D. L. Trimm. Progr. In Surf. Scl.. X1972) 63.
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G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
165
Production of Nickel-on-Alumina Catalysts from Preshaped Support Bodies L.M. Knijff, P.H. Bolt, R. van Yperen, A.J. van Dillen, and J.W. Geus State University of Utrecht, Department of Inorganic Chemistry, Croesestraat 77A, 3522 AD Utrecht, The Netherlands. SUMMARY To apply nickel uniformly into preshaped support bodies of a - and y-alumina, two procedures based on incipient wetness impregnation were investigated. The first one involved fixation of nickel ions by the alumina surface. Deposition-precipitation by hydrolysis of urea or nitrite within the pores of the support attached the impregnated nickel almost completely to the y-alumina; impregnation with nickel nitrate solution affixed a smaller fraction of the nickel. The a-alumina took up appreciably less nickel. According to the second procedure, a stable, high dispersion of nickel on a-alumina was obtained by precipitation of nickel magnesium oxalate within the pores. Decomposition leads to small nickel particles attached to magnesium oxide.
Introduction The most straightforward procedure to apply a catalytically active component into preshaped support bodies is incipient wetness impregnation with a solution of a precursor of the active component, followed by drying and thermal treatment. To distribute the active component uniformly over the entire support surface, the pore volume has to be filled completely by the impregnating solution. Occlusion of air and poor wetting of the support have to be avoided. Evacuation of the support bodies prior to impregnation improves the imbibition of the support considerably, especially when atmospheric pressure is readmitted after addition of the solution to the evacuated support.
To obtain a uniform distribution of the active component, two additional obstacles must be envisaged. Rapid extensive adsorption of the dissolved precursor to the support surface will cause deposition to proceed mainly at the pore mouths. Lack of interaction with the support, on the other hand, causes the distribution of the active component to be affected by migration of the solution, which may occur during the drying stage that follows impregnation. Thus the precursor is deposited mainly where the solvent evaporates. An important objective of our study was therefore to avoid the critical character of drying by immobilizing the precursor before the drying stage. Migration of the liquid phase does not disturb the distribution of the precursor when the latter has been fixed to the support. This can be achieved by (deposition-) precipitation. Geus et al. most extensively described this method for suspensions of powdered support materials [ 11. The additional requirement imposed by preshaped support bodies is that the precursor as well as the precipitant have to be distributed uniformly throughout the pore volume before the onset of precipitation. To this end we impregnated with solutions containing nitrite ions or urea, intended for in situ generation of hydroxyl ions. The reactions of urea and nitrite are, respectively: H2N(CO)NH;!
+
3 H 2 0 ->
2NH4+
+
C02(g)
+ 2OH-
3NOz- + H20 -> 2 N 0 (g) + N03- + 2OH' to beprevented: 2 N 0 + 0, -> 2 N 0 , ; 3 NO, + H20
-->
2HN0,
+ NO
166
To investigatethe feasibility of the procedure outlined above, this papex will deal with application of nickel within a - and y-alumina support bodies. With a-alumina an optimum activity per unit catalyst mass is not secured merely by a uniform distribution of the active component: the low reactivity and smoothness of the a-alumina surface causes a deposited component to be liable to sintering. The advantage of wide-porous a-alumina, providing good transport facilities, can be combined with a high catalytically active surface area by application within the alumina bodies of small clusters of a second support stabilizing a high dispersion of the active component. To this end nickel magnesium oxalate will be applied into a-alumina extrudates. Reportedly [2,3], mixed oxalates are excellent precursors for nickel catalysts. The mixed oxalate decomposes in inert atmosphere at about 350 O C to v e q fine particles consisting of metallic nickel and magnesium oxide. [4,5].However, the powder cannot readily be shaped to tablets without the (pyrophoric) nickel being oxidized. The applicability in fixed bed reactors can be improved appreciably when the mixed oxalate is incorporated into porous bodies of a support, such as a-alumina, and is decomposed in situ in the reactor. Since the oxalate precursor is insoluble, a special impregnation procedure is required. In separate experiments without a-alumina, the length of the induction period preceding precipitation of the mixed oxalate will be assessed. If the induction period is sufficiently long, the support bodies can be impregnated with a solution obtained by rapidly mixing solutions of oxalate ions, and of nickel and magnesium nitrate (co-impregnation). According to our first objective, the nickel is immobilized as the precipitating mixed oxalate before the drying stage. Alternatively, the support can be impregnated successively with the nickel-magnesiumsolution and the oxalate solution or vice versa, with an intermediate drying stage (two-step impregnation).
Experimental: Materials: Engelhard De Meern B.V. (The Netherlands) provided y-alumina supports of various shapes (specific surface areas ranging from 200 to 240 m2/g, pore volumes from 0.38 to 1.10 ml/g) produced from the same pseudo-boehmite, and a-alumina extrudates (9.1 m2/g, 0.55 ml/g). Gases. either high purity grade (quality 5.0) or purified over Linde molecular sieve 4A and reduced copper BTS-catalyst (BASF) to remove water and oxygen, respectively, were obtained from Hoekloos. -nation orocedure: Typically 1 to 5 gram of a support was impregnated to incipient wetness. In all cases, therefore, the volume of the impregnating solution was equal to the total pore volume of the support bodies. Prior to impregnation the support bodies were evacuated to a few mm Hg for at least 15 minutes to avoid occlusion of air. After the vessel had been closed, the impregnation solution was added from a syringe through a rubber septum. To allow distribution of the fluid throughout the support bodies, the vacuum was maintained for at least 10 minutes. Finally the vessel was opened to the atmosphere to apply additional force on the penetrated liquid. Impremation with urea or nitrite solutions: A slight excess of urea was added to a 0.88 M nickel nitrate solution. The solution of nickel nitrite (0.837 M) was prepared by combining solutions of barium nitrite and, in slight excess, nickel
167
sulphate and removing the barium sulphate by filtration. Alternatively, a combined solution of nickel nitrate and potassium nitrite was used. In both cases the nickel-to-nitrite ratio was determined by titration to be 0.60, theoretically sufficient to precipitate 56 % of the nickel as Ni(OH)2. After impregnation the vessel was shortly evacuated to avoid reaction of NO with 02,closed, and kept at 90 O C for either 3 or 20 hours in order to bring about hydrolysis of the urea or the nitrite. Evaporation of water was prevented. After the heating period the amount of nickel that had remained in solution was determined by crushing some of the wet impregnates, extracting nickel with distilled water, and determining the amount of nickel by atomic absorption spectrometry or complexometric titration. The remainder of the impregnates was dried in air at 120 OC. To assess the extent of fixation by the alumina without hydroxyl ions being generated, support bodies were impregnated with 0.938 M nickel nitrate solution only, and kept at 90 OC for 20 hours while evaporation of water was prevented. The nickel that was not bound by the alumina was determined as described above. Preparation of unsupported oxalates and application within a-alumina: Nickel magnesium oxalates with Ni-Mg ratios of m,3.55, 1.04,0.37, and 0 were prepared. The precipitates were centrifugated and washed with distilled water. In the application of mixed oxalates into a-alumina bodies, a Ni-Mg ratio of 6 was chosen for the nitrate solutions in order to attain a suitable nickel weight loading. Procedures, concentrations, and sample codes are to be found in Table IT. In two-step impregnations hot solutions were used in order to obtain a maximum concentration of oxalate ions. The impregnates were kept at 70 OC for about 1 hour before being dried to achieve complete reaction to nickel magnesium oxalate. Drying was performed at 100 OC in air. Besides the samples that were used in TPH (see below) the oxalates were decomposed by heating in a nitrogen flow with 5 OC/min to 400 O C , which temperature was held for 2 hours. Characterization: All samples were examined with scanning and transmission electron microscopy (SEM and E M ) . For comparison support bodies impregnated with 0.938 M nickel nitrate solution, and dried subsequently in air at 90 or 120 O C or in vacuum at 20 OC were studied. The distribution of nickel throughout the support bodies was studied with a Cambridge Stereoscan 150 S scanning electron microscope equipped with detectors for secondary and backscattered electrons and with a Link AN 10000 X-ray analysis system with energy dispersive detector. Impregnated support bodies were split, and mounted on an aluminium stub with carbon glue. A carbon layer was vapor-deposited onto the samples to provide a conducting surface. TEM samples were made by ultrasonic treatment of ground impregnates suspended in alcohol, and spreading a droplet of the suspension on a holey carbon film. The samples were examined in a Philips EM 420 "EM using an accelerating voltage of 100 kV. The reducibility of the deposited nickel species was investigated with temperature-programmed reduction (TPR) performed in 10 % H2/Ar flow with a heating rate of 5 OClmin. Except the oxalatebased catalysts, the impregnates had previously been calcined in air for two hours at 450 OC. A cold trap containing dry ice retained the water produced. The following characterization techniques were only used with the oxalate-based catalysts. Oxalate decomposition was studied with temperature-programmed heating in helium (TPH) using the
168
same apparatus as with TPR. The temperature was raised with 5 OC/min to 500 OC. Mean nickel particle sizes were determined with hydrogen chemisorption, performed in a conventional glass apparatus. The samples, typically containing 0.05 g nickel, were decomposed either in vacuum or in a flow of nitrogen. Evacuation was performed at 22 OC to a final pressure of about 5. Pa. Small doses of hydrogen were admitted to the sample at time intervals of 20 minutes unless complete uptake was attained earlier. An adsorption isotherm was measured up to a pressure of about 10 kPa. Next the samples were treated in hydrogen at 500 OC for 2 hours, the hydrogen was removed at 300 OC, evacuation was continued at 22 OC, and another chemisorption measurement was performed. The hydrogen up- take attributed to a monolayer was obtained by extrapolating the isotherm to zero hydrogen pressure. Calculation of the nickel metal surface area was based on a Ni : Had ratio of 1, and a mean surface area per nickel atom of 6.5. m2. Vibrating sample magnetomehy (VSM), a technique that was described in detail by Van Stiphout [6],was performed with the unsupported mixed oxalates, which were decomposed in a 10% H2/N2 flow at 400 OC. The magnetization of the sample was measured at 77 K as a function of the applied field strength (maximum 12 kOe). The size distribution of the nickel particles was obtained by fitting the thus measured magnetization curve with theoretical curves calculated for discrete particle sizes. Oxidation of methane, performed in an automated flow apparatus, was used as a test reaction for the stability of the nickel particles with respect to surface structure and sintering. 1.0 g of the sample A-Amox-2 (see Table II) was decomposed in a N2-flow and pretreated in a 10 % H2/He flow for one hour at 400 OC. The reaction feed, 1 mole % CH4 and 4 mole % 0, in He, passed through the catalyst bed at a space velocity of 4000 h-l. Methane conversion was measured with a Perkin Elmer 8500 gas chromatograph. As oxygen was present in excess, the nickel particles were completely oxidized. A measurement comprised a successive increase and decrease of the temperature between 350 and 750 OC in steps of 10 OC. Before each measurement the sample was reduced at 400 or at 850 OC.
Results and discussion Imuremation with urea or nitrite solutions: In Table I the extent to which nickel ions were attached to the alumina support is represented for various impregnations with and without urea or nitrite. Impregnation with nickel nitrate alone already leads to fiation of a considerable fraction of the nickel, especially at 90 OC; at 22 OC, the attachment is lower. Per unit surface area a-alumina takes up more nickel than y-alumina. The hydroxyl ions provided by hydrolizing urea or nitrite cause the fixation of nickel by y-alumina to be about complete, and by a-alumina to be much higher than with nickel nitrate alone. The distribution of nickel in the y-alumina supports was completely uniform, irrespective of the preparation technique and the drylng procedure. SEM showed a homogeneous concentration throughout the support bodies (figure 1). The degree of fiiation at the onset of drying appears to be of no importance. Probably, the pore structure of this y-alumina support prevents migration of the solution over macroscopic distances whatever the drying rate. Furthermore, it can be concluded that the produced carbon dioxide or nitric oxide can be discharged without expelling the liquid from the pore system, which would result in deposition of the active component on the outer edge of the support body.
169 TABLE I
Fixation of nickel ions in alumina supports impregnated to incipient wetness concentrations (mole/l)
support
heating time (h)
temperature fixation (OC)
(2)
?'-A1203
17
90
66
0.938
Y-A1203
20
22
35
0.938
a-Al2O3
20
90
27
90
95
65
0.938
0.88;
0.95
Y-A1203
20
0.88;
0.95
a-A1203
20
90
Y-A1203
20
90
97
"f-Al2O3
3
90
87
cc-Al2O3
19
90
72
0.837* 0.837;
1.674"
0.837"
The N02- concentration was 1.4 mole/l
On a small scale, as was revealed by E M , dried as well as calcined impregnates could not be distinguished from the fresh, unloaded alumina. Only upon reduction nickel particles developed (figure 2). Complete reduction asks for heating in hydrogen to about 800 OC or prolonged treatment at a lower temperature, as follows from TPR. Equal particle sizes were found with different reduction procedures. The size of the nickel particles ranged for some preparations from 3 to 9 nm, and for other from 5 to 18 nm. The nickel particles were evenly distributed over all clusters of alumina needles, demonstrating uniformity on a small scale as well. The interaction of dissolved nickel with the alumina surface can be assessed more in detail by TPR (figure 3). Since all samples had been calcined at 450 OC,differences must be due to different conditions during impregnation and drying. From the profiles obtained from the samples impregnated with nickel nitrate solution it appears that interaction leads to nickel species that are difficult to reduce: the sample that was dried rapidly at 22 OC, thus minimalizing interaction, exhibits reduction at temperatures much lower than the sample kept for 17 hours at 90 OC. Compared to the latter, also the samples prepared with nitrite are more readily reduced, demonstrating that generation of hydroxyl ions diminishes the extent of interaction with the alumina. The effect is more pronounced when the production of hydroxyl ions proceeds rapidly, as with KNO,-Ni(NO&. De Bokx demonstrated that the interaction between y-alumina and dissolved nickel involves for-
figure 1: Typical backscattered electron image of a cross-sectionof a nickelly-aluminacatalyst body; brightness indicates a high nickel concentration in the alumina matrix; a line profile of nickel is obtained by passing the electron beam along the projected straight line and recording the emitted Ni K u radiation.
170
mation of Feitknecht compounds ( Ni,A1y(OH-)2x-3y-z(N03-)z ) [7].The mixed compound of nickel and aluminium is converted to a species upon calcination that requires a high reduction temperature, probably nickel aluminate. With y-alumina impregnated with nickel nitrate solution and kept at 90 O C , the anchoring reaction proceeds to a considerable extent, which is, however, limited by the decrease in pH brought about by the continuing hydrolysis of nickel ions. Generation of hydroxyl ions by hydrolysis of urea or nitrite compensates for the protons released, and allows fixation to proceed to completeness. On the other hand, rapid production of hydroxyl ions leads to precipitation of Ni(OH)2 less strongly interacting with the alumina surface, resulting in an easily reducible nickel species. This effect is already apparent from the better reducibility exhibited by the sample prepared with nickel nitrite, but is obviously present in the KN02-Ni(N03), impregnate, which displayed a high rate of decomposition of nitrite. The improved reducibility is an important advantage of the method of in situ generation of hydroxyl ions when large batches of catalyst are to be reduced. The incomplete fixation of nickel to a-alumina may be due to the limited rise in pH attainable with urea (caused by the NH4+- NH3-equilibrium [8]) and the insufficient amount of nitrite present. Alternatively, instead of the concentration of hydroxyl ions, the surface area of the alumina may be the restricting factor. Possibly a fraction of the nickel hydroxide particles is not attached to the alumina surface and can be removed with distilled water. Pretreatment of the alumina or application of larger amounts of urea or nitrite may be useful to achieve complete fixation.
50nm
,-,
figure 2: TEM image of a typical nickelly-alumina catalyst after reduction
figure 3: TPR profiles of y-alumina impregnates:
_--
heated at 90 OC; Ni-nitrate .+..*..+. Ni-nitrite dried at 100 OC Ni-nitratelK-nitrite Ni-nitrate, dried subsequently at 20 O C H2 -consumptionper mmole Ni (arbitrary units)
____
Unsumorted and a m n’ckel mayesium oxalates: Results of preparations of nickel magnesium oxalates without alumina and within a-alumina extrudates are represented in Table II. To apply nickel magnesium oxalate into support bodies by coimpregnation the induction period for nucleation has to be sufficient to distribute the solution throughout the support bodies. We established the induction period in the absence of alumina under various conditions. It decreases with increasing concentration, temperature, and acidity of the solution produced by rapidly mixing a solution of magnesium and nickel nitrate with a solution supplying oxalate ions. Even when an induction pencd of half a minute is acceptable and a high Ni-Mg ratio is used, a
171
nickel weight loading of only 1.5 % can be achieved in a support having a pore volume of 0.5 ml/g. Therefore we will focuss on two-step impregnation, which allows application of solutions of a concentration correspondingto the solubility,resulting in a much higher nickel weight loading. From the data in Table II for unsupported oxalates, it is seen that with oxalic acid a considerable fraction of the magnesium is not taken up in the precipitate. Magnesium oxalate is soluble in acid solution. Ni-Mg ratios of unity and 0.25 in the solution result in ratios of 3.55 and 0.37, respectively, in the precipitate. To maintain the Ni-Mg ratio of the solution in the oxalates precipitating within the extrudates, ammonium oxalate is to be preferred over oxalic acid, in spite the higher solubility of the acid. When the a-alumina extrudates were firstly impregnated with a solution of ammonium oxalate and subsequently with the nickel magnesium nitrate solution ( A-Amox-2 ), a fairly homogeneous distribution was observed with X-ray analysis in SEM (the distribution of magnesium could not be esablished, as the energy of Mg Ka photons is too close to that of the abundant A1 Ka photons). The reverse impregnation order ( A-Ox-2 and A-Amox-1 ) led to an egg-white distribution: a relatively high amount of nickel was present in a narrow band inside the extrudates as a result of a combination of depletion and diffusion processes. Decomposition of the oxalates sets free carbon monoxide and dioxide. It turned out that the flow of gas evolved may displace the fine nickel-on-magnesiaclusters within the pores of the a-alumina. Although the uniform distribution of A-Amox-2 was not affected significantly, the aforementioned egg-white distribution became more diffuse. Decomposition of the unsupported mixed oxalates leads to severe shrinkage and a change from pale green to black. In TEM, decomposition of separate platelets of mixed oxalate by the electron beam can be observed. Platelike structures, consisting of small nickel metal and magnesium oxide particles, remain (figure 4). Mostly, the clusters of closely packed particles are irregularly shaped (figure 5a) and separate nickel particles are only discernible in a dark field image (figure 5b). It is noted that in the dark field image only a small fraction of the nickel particles shows up. In fact nickel constitutes about 38 volume percent of the specimen shown. Obviously, a relatively small amount of magnesium oxide is effective in preventing the nickel particles from sintering. Mixed oxalates that are contained in a-alumina extrudates display the same structure of packed particles as is exhibited by the unsupported samples. TABLE I1 Unsupported and a-alumina supported nickel magnesium oxalates I
* concentration after combination with Ni-Mg-nitrate solution Ox= oxalic acid; Amox= ammonium oxalate; A= a-alumina; n.m.= not measured chs= chemisorption
13.2 7.9 7.6 n.m. n.m. n.m. n.m.
M
172
figure 4: TEM micrographs of nickel magnesium oxalate platelet before (a) and afler (b) decomposition
100 nm
a
b
figure 5: TEM micrographs of nickel magnesium oxalate NUMg 1.04: bright field (a) and dark field (b)image
In figure 6 TPH profiles of unsupported oxalates with Ni-Mg ratios of 0, 0.37, 3.55, and m are shown. Pure nickel oxalate decomposed from 280 to 350 O C . The sample with a small amount of magnesium exhibited decomposition within about the same temperature range. At higher magnesium contents a shoulder at higher temperatures developed. This may be attributed to a separate phase of a higher magnesium content, but not to pure magnesium oxalate, since the latter decomposed at a higher temperature, viz., above 400 OC. TPR indicated that nickel was not completely converted to the metallic state; X-ray powder diffraction provided evidence for nickel carbide and, possibly, for nickel oxide, besides for the expected nickel metal and magnesium oxide. Nickel carbide may have originated from disproportionation of carbon monoxide to carbon dioxide and carbon at the nickel metal surface. Since the main diffraction maxima of the four components coincide, neither their relative amounts nor their particle sizes, to be obtained from line broadening, can be properly estimated. Nickel metal mean particle sizes obtained from hydrogen chemisorption and vibrating sample magnetometry are given in table 11. The values mentioned for the VSM measurements have been deduced from the particle size distributions obtained. Samples that had only been decomposed consumed an extra amount of hydrogen with increasing hydrogen pressure, probably reflecting reaction of nickel carbide (and possibly oxide). Extrapolation to zero pressure, however, led to the same cal-
173
onidizeo 2 h (50
TPH
'.
/'?
200
300
400
500
figure 6: TPH profiles of unsupported oxalates; the detector signal is normalized with respect to the amount of oxalate ions
"C
T( 3C)
? '' I I 7 3 .
o
200
400
660
800
lono
figure 7: TPR profiles of A-Amox-2; Hp consumption per mmle Ni (arbitrairy units)
culated nickel surface area as was found in the second measurement, after reduction in hydrogen. The results from the three unsupported samples demonstrate that the nickel particle size strongly depends on the Ni-Mg ratio in the original oxalate. The VSM results only qualitatively display this relation; only the value obtained for Ni/Mg 1.04 agrees well with the chemisorption measurements. Ni/Mg 0.37 and 3.55 definitely contain nickel particles exceeding the upper and lower limit, respectively, of the size range to which the VSM theory applies (2 to 15 nm) [6]. The nickel particle size measured in a-alumina impregnates was about 16.5 nm, irrespective of the impregnation procedure. Considering the high Ni-Mg ratio applied (6, and probably higher in case oxalic acid had been used as precipitant), this diameter is small compared to the unsupported oxalates (see table 11).Nevertheless, much smaller nickel particles are expected to result with a mixed oxalate of lower Ni-Mg ratio. Increasing the amount of magnesium in the impregnating solution, implying a lower attainable nickel weight loading, will therefore be more effective in obtaining a higher nickel surface area than increasing the nickel content by means of multiple impregnation steps. In figure 8 conversion plots are displayed for the oxidation of methane over A-Amox-2. It is noted that upon exposing the (completely reduced) catalyst to the reaction feed, which contains excess oxygen, the nickel is almost immediately converted to nickel oxide, the active phase in the oxidation of methane. The hysteresis between the curves at increasing and decreasing temperature indicates severe 120
figure 8: Conversion of methane by A-Amox-2 increasing temperature : decreasing temperature: - - - - - run 1 : fresh sample, reduced at 400 OC, 2 h run 2: deactivated sample, reduced at 400 OC, 2 h run 3:deactivated sample, reduced at 850 O C , 1 h
'
conversion
80 60
-
40
:
300
400
500
600
700
8OC
174
deactivation.Comparison of TPR profiles from samples of the same catalyst that were heated in air to 450 or 850 OC (figure 7)indicates that a nickel species difficult to reduce is formed at elevated tem-
perature. Figure 8 shows that reduction at 400 OC for two hours partly restores the activity. Reduction at 850 OC (one hour) brings the activity back to the original level, which implies, moreover, that the nickel particles did not suffer from sintering. Formation of mixed magnesium nickel oxide nicely explains the observations. Nickel and magnesium ions have about equal radii and may react readily to a mixed oxide of the same crystal structure. Diffusion of magnesium into the nickel oxide may lead to an inactive nickel oxide surface, probably by diminishing the amount of excess surface oxygen, which is the reactive species in the oxidation of methane. Removal of the magnesium ions from the nickel oxide can only be achieved by reducing the nickel to the metallic state. Nickel metal nucleation will be increasingly difficult for lower Ni-Mg ratios in the oxide. The reducibility of the nickel species depends on the overall Ni-Mg ratio and the degree to which interdiffusion has proceeded. The reduction procedure at 450 ‘W is therefore sufficient to reduce the nickel oxide particles containing a low concentration of magnesium ions, while nickel that has diffused into the magnesium oxide requires a more severe reduction treatment. Conclusions: Fixation of nickel ions to the inner surface of y- and a-alumina support bodies, impregnated to incipient wetness, takes place to a considerableextent, especially at elevated temperature. In y-alumina fixation proceeds to completeness upon generating hydroxyl ions by in situ hydrolysis of urea or nitrite. The gases released are discharged smoothly, without expelling liquid from the pore system. This procedure leads to an improved reducibility compared to catalysts prepared by ordinary impregthe surface area appears to restrict the amount of nickel nation and successive drying.With a - a l u m i ~ that can be anchored. In order to realize sufficient interaction with the support surface in wide porous catalyst bodies, s m a l l particles of nickel-on-magnesium oxide can be synthesized within a-alumina by decomposition of nickel magnesium oxalate. A uniform distribution of mixed oxalate in the extrudates was obtained
by two successive impregnation steps, viz., with a hot concentrated solution of ammonium oxalate, and with a solution of nickel and magnesium nitrate. Application of a low nickel-magnesium ratio strongly enhances the dispersion of the nickel particles developing upon decomposition. Thus a lower nickel weight loading may exhibit a higher specific surface area. REFERENCES 1 J.W. Geus, in: Preparation of Catalysts 111, Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon, P. Grange, P. Jacobs, and G. Poncelet, Eds., Elsevier Amsterdam, 1982 2 W. Langenbeck, H. Dreyer, D. Nehring, and J. Welker, Z.anorg.allg.Chem. 281 (1955)90-98 3 V. DaneS and P.Jir6, Coll.Czechoslov.Chem.Comm.21 (1956)765-767 4 M. Ralek and V. DaneH, Coll.Czechoslov.Chem.Comm.24 (1959)1908-1913 5 V. Ponec and V. DaneS, Coll.Czechoslov.Chem.Comm.25 (1960)820-828 6 P.C.M. van Stiphout, Ph.D. thesis, University of Utrecht, Utrecht, The Netherlands, 1987 7 P.K. de Bokx, Ph.0. thesis, University of Utrecht, Utrecht, The Netherlands, 1985 8 L.A.M. Hermans and J.W. Geus, in: Preparation of Catalysts II, Scientific Bases for the Preparation of Heterogeneous Catalysts, B. Delmon, P. Grange, P. Jacobs, and G. Poncelet, Eds., Elsevier Amsterdam, 1979
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
175
DEVELOPMENT OF A METHODOLOGY FOR INVESTIGATING THE ADSORPTION OF SPECIES CONTAINING CATALYTICALLY ACTIVE IONS ON THE SURFACE OF INDUSTRIAL CARRIERS
N. SPANOS, CH. KORDULIS and A. LYCOURGHIOTIS* Department o f Chemistry-Research I n s t i t u t e o f Chemical Engineering and High Temperature Processes, P.O.Box 1239, U n i v e r s i t y Campus GR-26110, Patras, Greece. ABSTRACT A methodology f o r e l u c i d a t i n g t h e mechanism o f a d s o r p t i o n on i n d u s t r i a l o x i d i c s u p p o r t s o f species c o n t a i n i n g c a t a l y t i c a l l y a c t i v e i o n s has been developed. This i n v o l v e s t h e c o r r e l a t i o n between t h e surface c o n c e n t r a t i o n corresponding t o monolayer and t h e s u r f a c e groups o f o x i d i c supports, t h e combined use o f p o t e n t i o m e t r i c t i t r a t i o n s and m i c r o e l e c t r o p h o r e s i s which a l lows t h e d e t e r m i n a t i o n of t h e surface and e l e c t r o k i n e t i c charge d e n s i t i e s , r e s p e c t i v e l y as w e l l as t h e mathematical a n a l y s i s o f t h e isotherms obtained. The methodology a p p l i e d t o t h e a d s o r p t i o n o f molybdates and t u n g s t a t e s on y-alumina, t o t h e adsorption o f molybdates on t i t a n i a and t o t h e adsorption o f Co2+ and N i 2 + i o n s on y-alumina l e d t o t h e f o l l o w i n g c o n c l u s i o n s : (i) Responsible f o r t h e c r e a t i o n o f a d s o r p t i o n s i t e s f o r n e g a t i v e ( p o s i t i v e ) species are, mainly, t h e protonated (deprotonated) s u r f a c e hydroxyls o f t h e o x i d i c supports. ( i i ) These species are adsorbed on e n e r g e t i c a l l y e q u i v a l e n t s i t e s o f t h e I n n e r Helmholtz Plane o f t h e double l a y e r around t h e y-Al,O, p a r t i c l e s suspended i n t h e aqueous medium. (iii)L a t e r a l i n t e r a c t i o n s a r e operational between t h e adsorbed species, t h e magnitude o f which depends on t h e nature o f t h e support and t h e species t o be adsorbed.
INTRODUCTION Although a r e l a t i v e l y l a r g e number o f supported c a t a l y s t s are prepared by a d s o r p t i o n o f a species c o n t a i n i n g t h e a c t i v e i o n on t h e s u r f a c e o f an o x i d i c s u p p o r t , e.g.
y-Al,O,,
SiO,,
TiO,,
studies dealing with catalysts
prepared by e q u i l i b r i u m adsorption f o l l o w e d by f i l t r a t i o n are r a t h e r scarce i n t h e l i t e r a t u r e [ e . g . l - 8 1 . T h i s i s p r o b a b l y t h e main reason f o r which a c l e a r methodology a l l o w i n g t h e e l u c i d a t i o n o f t h e mechanism o f a d s o r p t i o n from aqueous suspensions has n o t y e t been e s t a b l i s h e d . T h i s method should t a k e i n t o account t h e u s u a l l y i g n o r e d e x i s t e n c e o f an e l e c t r i c a l double l a y e r around t h e suspended support p a r t i c l e . Moreover, t h i s method should enable us t o i n v e s t i g a t e t h e f o l l o w i n g p o i n t s : (i) Are t h e groups responsible f o r t h e c r e a t i o n o f s o r p t i v e s i t e s , the n e u t r a l * A l l correspondence t o t h i s Author.
surface or t h e
176
charged hydroxyls? (ii)The p a r t o f t h e double l a y e r where t h e species are located, i . e . t h e surface o f t h e c a r r i e r , t h e I n n e r Helmholtz Plane ( I H P ) o r t h e d i f f u s e p a r t o f t h e double l a y e r .
i.e.
(iii)The n a t u r e o f adsorption,
whether i t i s l o c a l i z e d o r nonlocalized.
( i v ) The existence o f l a t e r a l i n -
t e r a c t i o n s between t h e adsorbed species. The e s t a b l i s h m e n t o f such a methodology i s t h e purpose o f t h i s comm u n i c a t i o n . To i l l u s t r a t e t h e proposed methodology we use s e v e r a l r e s u l t s taken from our r e c e n t adsorption s t u d i e s o f molybdates [9,10] and tungstates [ll] on y-alumina, o f molybdates on t i t a n i a [12] and o f Co2+ and N i 2 + on y-alumina [13]. EXPERIMENTAL The experimental method used t o determine t h e c o n c e n t r a t i o n o f t h e adsorbed species,
r(mo1 .m-'),
a t a given equilibrium concentration o f the
species i n t h e suspension, Cq(mol .dm?), has been described elsewhere [9,10]. P o t e n t i o m e t r i c t i t r a t i o n s have been used t o determine t h e s u r f a c e charge d e n s i t y , oo(pC.cm-2), i n t h e absence and presence o f t h e species t o be adsorbed h y d r o x y l s,
and t h e c o n c e n t r a t i o n SOH,':
of
the
surface groups
[SOH:
neutral
p r o t o n a t e d h y d r o x y l s, SO-: deprotonated h y d r o x y l s ]
d e t a i l s have been g i v e n elsewhere [14-17].Microelectrophoretic
.
Full
mobility
measurements were used t o determine t h e e l e c t r o k i n e t i c charge d e n s i t y , o,(pC.cm-'),
i n t h e absence and presence o f t h e species t o be adsorbed. F u l l
d e t a i l s are g i v e n elsewhere [18]. DESCRIPTION OF THE METHODOLOGY On t h e n a t u r e o f t h e a d s o m t i o n s i t e s . I t i s w e l l known t h a t t h e s u r f a c e o f t h e p a r t i c l e s o f a simple oxide
l i k e y-Al,O, and SiO, i s g e n e r a l l y charged i n e l e c t r o l y t e s o l u t i o n s . The w e l l e s t a b l i s h e d surface i o n i z a t i o n model [I91 describes q u i t e w e l l t h e charging mechanism. This process may be represented as:
SOH,'
K, SOH
K,
SOH
+
Hs+
SO- + Hs+
Hs+, H i : denote hydrogen ions on t h e surface o f t h e support and i n t h e b u l k s o l u t i o n , respectively.
177
I n t h e case o f TiO,,
which i s a m i x t u r e o f r u t i l e and anatase, t h e above
e q u i l i b r i a should be w r i t t e n f o r each component[20].
I t i s obvious t h a t i n o r d e r t o i n v e s t i g a t e which o f t h e s u r f a c e groups i s mainly responsible f o r t h e c r e a t i o n o f adsorption s i t e s ,
i t i s necessary
t o c o r r e l a t e t h e surface c o n c e n t r a t i o n o f t h e adsorbed species corresponding t o t h e p l a t e a u o f t h e isotherm,
rm
(see f i g u r e 8) w i t h t h e c o n c e n t r a t i o n o f
t h e d i f f e r e n t t y p e s o f groups. The l a t t e r may be r e g u l a t e d by doping t h e c a r r i e r o r by changing e i t h e r t h e pH o r t h e temperature o f t h e suspension [14-171.
WxOyz-,
I f t h e species t o be adsorbed i s n e g a t i v e l y charged, l i k e Mo,Ot-
or
i t seems reasonable t o assume t h a t t h e n e u t r a l o r t h e p o s i t i v e groups
are r e s p o n s i b l e f o r t h e c r e a t i o n o f adsorption s i t e s . T h i s may be t e s t e d by plotting
rm
t h e c o n c e n t r a t i o n o f SOH,'
o r SOH. A t y p i c a l example i s il-
l u s t r a t e d i n f i g u r e l. S i m i l a r t r e n d s were observed f o r t h e a d s o r p t i o n o f
[ll] as w e l l as f o r t h e adsorption o f t h e MoxOyz- ions on
WxOyz- i o n s on y-Al,O, TiO,,
though i n t h e l a t t e r case, uo, was used i n s t e a d o f t h e c o n c e n t r a t i o n o f
t h e p o s i t i v e groups [ 1 2 ] .
The above shows t h a t t h e p o s i t i v e groups are
r e s p o n s i b l e f o r t h e c r e a t i o n o f adsorption s i t e s f o r n e g a t i v e ions. The c o r responding p l o t s o f
rm
RlOH
t h e n e g a t i v e l y charged and t h e n e u t r a l surface
/ sites.nmF2
6.0
6.5
7.0
.05
0.55
2.05
7.5
8.0
1.55
2.
5
A~oH; / sites.nm-2 F i g . 1. S a t u r a t i o n surface Mo(V1) c o n c e n t r a t i o n obtained a t v a r i o u s temperat u r e s (ref.10) as a f u n c t i o n o f the c o n c e n t r a t i o n o f t h e protonated (curve a) and n e u t r a l (curve b) surface hydroxyls r e g u l a t e d by v a r y i n g t h e temperat u r e of t h e impregnating suspension o f t h e y-Al,O, (ref.16).
178 AlOH
-2
/ sites.nm
n
2
4
0
2
4
6
6
8
8
-2 A ~ O - / sites-nm
Fig. 2. Saturation surface Ni2' concentration obtained for the system Ni2'/yAl,O,-F at room temperature (ref.13) as a function of the concentration o f the negative (curve a) and neutral (curve b) surface hydroxyls regulated by varying the F- content (ref.15)
I
SOLID
SOLTIT ION
Shear
I Fiq. 3 . Structure of the solid-solution interface according to the "triple layer model". up, ud and uek refer to the total charge from the surface o f the support t o the IHP, to the OHP, and t o the shear plane,respectively,
179
groups f o r t h e Ni2' ions, shown i n f i g u r e 2, show t h a t t h e n e g a t i v e groups are mainly responsible f o r the c r e a t i o n o f adsorption s i t e s f o r p o s i t i v e ions [13]. S i m i l a r r e s u l t s were obtained f o r t h e Co2+ ions. P a r t o f t h e double l a v e r where t h e adsorbed soecies are l o c a t e d - Q u a l i t a t i v e aDDroach. Adopting t h e " t r i p l e l a y e r model" f o r t h e double l a y e r ( f i g . 3 ) t h r e e p o s s i b i l i t i e s do e x i s t : t h e adsorbates may be l o c a t e d on t h e surface, on t h e
I H P o r i n t h e d i f f u s e p a r t o f t h e double l a y e r . I n t h e f i r s t case adsorption o f negative ions i s expected t o cause a decrease i n t h e surface charge den-
s i t y , whereas i n t h e t h i r d case t h i s type o f adsorption
would r e q u i r e p o s i -
t i v e e l e c t r o k i n e t i c charge d e n s i t y . F i g u r e s 4 and 5, and s i m i l a r ones observed f o r t h e WxOyZ-/y-A1203 and MoxOyZ-/TiO, systems, show t h a t l o c a t i o n o f t h e adsorbates on t h e surface o r i n t h e d i f f u s e p a r t o f t h e double l a y e r i s precluded. Therefore, t h e o n l y p o s s i b i l i t y i s t h e a d s o r p t i o n a t t h e IHP. I n f a c t , i n t h a t case t h e negative i o n s are expected t o promote t h e appearence o f a d d i t i o n a l SOH,'
groups on t h e surface by forming i o n p a i r s [14,
15, 171
and t h e r e f o r e t o i n c r e a s e t h e p o s i t i v e s u r f a c e charge d e n s i t y ( F i g .
4).
Moreover, t h e presence o f negative ions a t t h e I H P i s i n agreement w i t h t h e negative e l e c t r o k i n e t i c charge d e n s i t y i n t h e pH range s t u d i e d ( F i g . 5). Based on r e s u l t s i l l u s t r a t e d i n f i g u r e s 6 and 7 and f o l l o w i n g t h e above reasoning we may conclude t h a t t h e p o s i t i v e i o n s are a l s o l o c a t e d a t t h e I H P
600
N I
300
E
Y
4
\
0"
-300
-600 3
4
5
6
7
8
9
PH
Fig. 4. Surface charge d e n s i t y o f y-A1203 as a f u n c t i o n o f pH o f t h e suspens i o n a t 25OC. ( a ) i n t h e presence o f MoxOyZ- i o n s (ammonium heptamolybdate s o l u t i o n , C0=l*10-3 mol Mo(VI)/dm3, i o n i c strength, I = O . 1 mol/dm3 NH,N03), (b) ions (0.1 mol/dm3 NH,NO, s o l u t i o n ) . i n t h e absence o f MoxO:'
180
0.5 \
x
0"
0.0
- 0.5 - 1.0
Fig. 5. Electrokinetic charge density of y-A1 0 as a function of pH of the suspension at 25OC. (a) in the presence of Moxb:- ions (ammonium heptamolybdate solution, C0=l*10-3 mol Mo(VI)/dm3, ionic strength, I=O.Ol mol/dm3 NH,NO,), (b) in the absence of MoxOt- ions (0.01 mol/dm3 NH,NO, solution). of the double layer. Useful information concerning the mechanism of adsorption may also be drawn from the form of the isotherms obtained. For the systems already mentioned at various temperatures [lo] as well as for the systems
N
I
6
3.5
4.5
5.5
6.5
7.5
PH
Fig. 6. Surface charge density of y-Al,O, as a function of pH of the suspensolution, sion at 25OC. (a) in the presence of Co2+ ions ( Co(N0,),.6H20 C0=l*10-3 mol Co2+/dm3, ionic strength, I=O.1 mol/dm3 NH,NO, ), (b) in the absence of Co2+ ions (0.1 mol/dm3 NH,NO, solution) .
181
N I
5
U
5
0.0
\
\
2
0"
'0.5
-1.0
I
4
5
6
7
8
9
10
1
PH
Fig. 7 . Electrokinetic charge density of y-A1 0 as a function of pH of the solution, suspension at 25°C. (a) in the presence of Co2' ions ( Co(N03),.6H,0 C0=l*10-4 mol Co2+/dm3, ionic strength, I=O.Ol mol/dm3 NH,NO,), (b) in the absence of Co2+ ions (0.01 mol/dm3 NH,NO, solution). Moody-Al,O,-Na [lo], Co2+/y-Al,0,-F [13] and Ni2+/y-A1203-F [13] at room temperature, the isotherms may be classified as S and I type suggesting localized, Langmuir type, adsorption at the IHP with strong and weak lateral interactions, respectively [21,22]. Typical examples of the $ type isotherms obtained are illustrated in figure 8. Part of the double laver where the adsorbed sDecies are located-Ouantitative Amroach. The next step is to analyse the isotherms obtained on the basis o f the following assumptions: (i) More than one kind of ions (e.g. MOO:-, Mo,O,"-), are specifically adsorbed at the IHP, as it has been inferred above. (ii) The adsorbed ions are located on energetically equivalent sites as suggested from the Langmuirian shape of the isotherms. (iii) One specifically adsorbed ion, i , replaces one water molecule from the IHP [9]. Assuming no lateral interactions between the adsorbed species we may derive [9] the "Stern-Langmuir" equation
where rm represents the saturation surface concentration of the adsorbed species (maximum in the and S type isotherms). The constant K is given by eqn (2).
182
N
'E
10
-
4
5
\
-
L
0.0 1
n nn L."V
0. 3
0.02
c,,
/ mol.dm
-3
F i g . 8. Surface c o n c e n t r a t i o n o f M o ( V 1 ) as a f u n c t i o n o f t h e e q u i l i b r i u m
Mo(VI) c o n c e n t r a t i o n a t v a r i o u s temperatures o f t h e impregnating suspension o f t h e y-Al,O,. pH=5, 1 4 . 1 M NHN , O., 13: 20°C, 0 : 3OoC, A : 45OC. K
=
Ii[(ai/55.5)exp(-AGaads,i/RT)],
where ai and
AGOads,,
(2)
represent a c o e f f i c e n t (independent from t h e Cq
b u t de-
pH and t h e n a t u r e o f t h e species i) and t h e
pendent on t h e temperature,
standard f r e e energy o f adsorption f o r t h e i o n i, r e s p e c t i v e l y . Assuming l a t e r a l
i n t e r a c t i o n s between t h e adsorbed species we may
d e r i v e [ 9 ] t h e "Stern-Langmui r-Fowl e r " equation
where
E
i s t h e energy o f t h e l a t e r a l i n t e r a c t i o n s ,
i s given by eqn (4):
I
K
=
Ii[(ai/55.5)exp(-ZiFUlg
(4)
/RT-AGocsJRT)],
where Z i and Wg r e p r e s e n t t h e charge o f t h e ith k i n d o f t h e i o n s t o be adsorbed and t h e p o t e n t i a l
at
IHP,
respectively.
Equations
(1) and ( 3 )
describe a l s o t h e adsorption o f one k i n d o f species b u t i n t h i s case simpler expressions f o r t h e values o f K and
are a v a i l a b l e [ 9 ] .
It was found t h a t i n a l l cases studied, eqn(3) described b e t t e r t h e experimental r e s u l t s as compared t o eqn(1). This i n d i c a t e s t h a t l a t e r a l i n t e r a c t i o n s e x i s t between t h e adsorbed species. However, t h e magnitude o f these i n t e r a c t i o n s depends on t h e k i n d o f t h e support and t h e ions adsorbed (Table 1). With regard t o t h e support, i t may be observed t h a t t h e "support-
183
adsorbed species interactions", estimated by the value of t, are stronger in the case of TiO,. This justifies the relatively weaker lateral interactions observed with this carrier in the case o f adsorption of the molybdates. Concerning the adsorbate phase, it may be suggested that the lateral interactions between the Co2+ or Ni2' ions are negligible, in comparison with those observed for MoxO:- and WxO:in which an S type of isotherm was obtained. TABLE 1 Values of the lateral interactions energy (E) and the adsorption constant (K) determined for different catalytic systems at temperature 25T. No
REFERENCES 1. L.Wang and W.K.Hal1, J.Catal., 77(1982)232. 2. S.Kasztelan, J.Grimblot, J.P.Bonnelle, E.Payen, H.Toulhoat and Y.Jacquin, Applied Catalysis, 7(1983) 91. 3. C.V.Caceres, L.G.Fierro, A.L.Agudo, M.N.Blanko and H.J.Thomas, J.Catal., 95(1985)501. 4. J.A.R. Van Veen, H.De Wit, C.A. Emein and P.A.J.M. Hendriks, J.Catal., 107(1987)579. 5. P.A.J.M.Hendriks and J.A.R. Van Veen, Polyhedron, 5(1985)75. 6. D.S.Kim, Y.Kurusu, I.E.Wachs, F.D. Hardcastle and K.Segawa, J.Catal., 120(1989)325. 7. K.Y.S. Ng and E.Gulari, J.Catal., 92(1985) 340. 8. J.P.Brunelle, Pure Appl.Chem., 50(1978)1211. 9. N.Spanos, L.Vordonis, Ch.Kordulis and A. Lycourghiotis, J.Catal., in press. lO.N.Spanos, L.Vordonis, Ch.Kordulis, P.Koutsoukos and A.Lycourgiotis, J.Cata1, in press. 11.L.Karakonstadi s, Ch .Kordul is and A. Lycourghiot i s, in preparation. lE.N.Spanos, Ch.Kordulis and A.Lycourghiotis, in preparation. lS.N.Spanos, L.Vordonis, Ch.Kordu1 is, P.Koutsoukos and A. Lycourghiotis, in preparation. 14.L.Vordonis, P.G.Koutsoukos and A.Lycourghiotis, J.Catal., 98(1986)296. 15. L. Vordoni s , P.G.Koutsoukos and A. Lycourghiot i s , J. Catal ., 101 ( 1986) 186. 16.K.Akratopoulou, L.Vordonis and A.Lycourghiotis, J.Chem.Soc. Faraday Trans I . 82(1986)3697.
184
17.K.Akratopoulou, L.Vordonis and A.Lycourghiotis, J.Catal., 109(1988)41. 18.P.H.Wiersema, A.L.Loeb and J.Th.G.Overbeek, J. Colloid Interface Sci., 22(1966)78. 19.C.P.Huang and W.J.Stumm, J.Colloid Interf.Sci., 43(1973)409. 20.K.Ch.Akratopoulou, Ch.Kordulis and A.Lycourghiotis, submitted. El.J.Lyklema, in "Adsorption from solution at the solid/liquid Interface" G.P.Parfitt and C.H.Rochester eds, Acad.Press, London, 1983 ch.5. EE.C.H.Giles, D.Smith and A.Huitson, J.Colloid Interface Sci., 47(1974)755.
G . Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
185
SCALING DOWN OF THE CALCINATION PROCESS FOR INDUSTRIAL CATALYST MANUFACTURING G. GROEN' , J . FERMENT',
M.J . GROENEVELD' , J . DECLEER2 and A. DELVA2
lKONINKLIJKE/SHELL-LABORATORIUM, AMSTERDAM (Shell Research B.V. ) Badhuisweg 3 , 1031 CM Amsterdam, The Netherlands 2SHELL GHENT - CATALYST PRODUCT TECHNOLOGY, Passagierstraat 100, 9000 Gent, Belgium
SUMMARY A computer model for the calculation of the temperature/time profile and the composition of the gas atmosphere in rotary kilns is described. The model is applied for scaling down the continuous commercial calcination of catalyst materials to batch calcination in laboratory rotary kilns as used in catalyst development work. The results of the model are compared with measurements carried out in the rotary kiln of the catalyst plant at Ghent, which produces y-alumina extrudates used as carrier for heterogeneous catalysts. The plug flow transport model assumed for the solids is confirmed by residence time distribution measurements. The measured temperature profiles are in agreement with the calculated profiles after adjustment of the kinetic rate constants. INTRODUCTION Industrial catalyst manufacturing involves several process steps such as preparation and mixing of solutions or suspensions, crystallization, filtration, washing, drying, mixing and kneading o f powders, shaping, drying, calcination and impregnation. Catalyst recipes to be developed in the laboratory must be properly translatable to an industrial processing scheme, taking into account the equipment and instrumentation of an existing catalyst plant. Hence the translation problem is merely that o f scaling down rather than that of scaling up: in view of the possible commercialization of a catalyst recipe each process step in the laboratory needs to be carried out in a way representative of the commercial production. Tests on small-scale process steps are representative if the process conditions and the scale applied are such that the target product properties are similar to those to be obtained in the commercial process step. Representative equipment and methods should already be used in the generally small-scale experiments in the initial phase of the laboratory recipe development, but often a larger scale is necessary. Muffle furnaces might be used in the laboratory, but it should be realized that these furnaces are usually not representative of commercial calcination in rotary kilns. Computer models simulating the performance of commercial equipment are therefore applied to an increasing extent to assist in scaling. This paper will discuss a computer model for the calcination of catalyst materials in rotary kilns. The pur-
186
pose of such a model is, among other things, to calculate the temperature/time profile and gas composition in a commercial rotary kiln in order to be able to apply the same conditions (preferably batchwise) in a laboratory rotary kiln at the smallest conceivable scale (e.g. 50 ml catalyst). The model can also be used for translation from one type or size of commercial rotary kiln to another and for optimization of existing calcination practices. Also in the manufacturing of special catalysts (e.g. based on zeolites), calcination might be the critical factor for the performance of the final catalyst. Representative calcination experiments will therefore remain essential for successful commercialization. The scaling down of the calcination process for industrial catalyst manufacturing requires knowledge of both the processing characteristics of the commercial rotary kiln and, for each different catalyst material, the physical and chemical processes taking place during the calcination. In this paper the elements of the model will be described in more detail and the problems of its validation discussed. It should be realized that the model is still in the development phase. Therefore, the most important heat and mass transfer phenomena occurring in a rotary kiln must be described properly first. A description of the development of important catalytic properties such as surface chemistry, crystallinity, pore structure and metal dispersion is still beyond the scope of the present model. DESCRIPTION OF ROTARY KILNS Calcination can be carried out in various types of process equipment such as fixed beds, moving beds, fluid beds, tunnel kilns, moving belts and rotary kilns. Because of their versatility rotary kilns are widely used in the catalyst manufacturing industry both for calcination of zeolite powders and for calcination of shaped carriers and catalysts. Other types of equipment (e.g. moving belts) are used less frequently. Only the use of rotary kilns will be discussed. Various types of rotary kilns are found in commercial catalyst plants:
(1) Directly gas-fired rotary kilns (Fig. 1); (2) Indirectly gas-fired rotary kilns e.g. for the production of powders or in cases where combustion flue gases are harmful to the catalyst material ; ( 3 ) Electrically heated rotary kilns for small-scale production or for use
in the laboratory; Rotary kilns are often equipped with internals such as longitudinal strips on the wall to prevent slipping of the solids bed over the tube wall and with solids flow restrictions such as dams and slotted diaphragms (Fig. 2). Flights for raining down of particles through the gas phase, as used in rotary driers
187 are not commonly used for calcination of catalyst materials. Commercial-scale rotary kilns are as a rule continuously operated. Rotary kilns in the laboratory might be operated batchwise or continuously and are usually indirectly heated.
EXTRUDATES F R O M S I E V E S
-
SOLIDS FEED L I N E
-
PROPANE PRIMARY AIR SECONDARY AIR
#
EXTRUDATES TO HOPPER
Fig. 1. Rotary kiln o f the alumina catalyst plant at Ghent.
SOLIDS FLOW
TYPE
I
Fig. 2. Slotted diaphragms used in rotary kilns
TYPE
II
188 DESCRIPTION OF COMPUTER MODEL The following elements are essential in any computer model of rotary kiln calcination:
(1) A description of the transport of solids and gas through the rotary kiln;
(2) A description of the heat-transfer processes taking place between solids, gas and walls, including heat losses from the outside walls of the kiln to the surroundings; ( 3 ) A description of the physical and chemical conversions with their heat
effects, including mass transfer between the solids and bulk gas phases. The first two items are common elements for rotary kiln models and are discussed in many publications. A brief summary will be given and a few areas where uncertainties still persist will be indicated. The third item varies with the material to be calcined. A few general outlines will be given here. More details will be presented in subsequent sections. Gas and solids transuort Gas and solids are both assumed to flow in plug flow from one end of the kiln to the other, either cocurrently or countercurrently. Hence, neither of the two phases is assumed to mix in axial direction. For the gas phase this is justified because the gas flow in commercial roc:ry kilns is usually turbulent and the length-to-diameter ratio of rotary kilns is usually large (i.e. larger than 10). For these reasons the bulk gas phase is assumed to be completely mixed in a cross section of the kiln (except for thin film layers adjacent to the tube wall and the solids bed surface). The transport mechanism of the solids depends on the surface roughness o f the inside wall, the inside diameter of the kiln, the solids properties and the operating conditions of the kiln. Henein et al. [l] distinguish slipping, slumping, rolling, cascading, cataracting and centrifuging beds. The latter two are not relevant for catalyst calcination. Existence regions have been experimentally determined as a function of fill percentage (bed depth) and the Froude number (rotation speed) and have been founded with theoretical arguments [l]. The present model assumes a rolling bed, which has been confirmed for the rotary kiln of the catalyst plant at Ghent. The rolling bed is characterized by two regions (Fig. 3 ) : a thin layer of sliding or rolling particles at the bed surface, and the bulk part of the bed, in which the particles are stagnant with respect to the rotating tube wall. Differences in axial velocity between particles travelling only in the middle of the bed (i.e. small particles) and particles travelling only on the periphery of the bed (i.e. large particles) are averaged out for particles following random paths in the solids bed (see right-hand side of Fig. 3 ) . Plug flow is therefore justified for
189
non-segregating beds of particles and this has also been experimentally confirmed, as shown in a subsequent section.
SOL FE
I t
CROSS SECTION OF T H E KILN
CROSS SECTION A-A (VIEW OF BED SURFACE) INSIDE THE RED -SLIDING/ROLLING OVER THE BED SURFACE
Fig. 3 . Particle trajectories in a rolling bed. In a cross section of the kiln, the rolling type of solids bed is assumed to be well mixed for heating up calculations and for kinetic calculations of relatively slow chemical or physical conversions in spite of the absence of relative motion in the stagnant part of the bed. This is justified because the Fourier time of a solids bed in commercial kilns is in the order of hours, while the cyclus time of a particle is in the order of seconds. Hence, bed heating occurs mainly by the continuous replacement of particles in the bed rather than by conduction. For a rolling bed, Saeman [2] has derived the following formula for the rate of volumetric solids transport:
dh dz is the slope of the bed surface with respect to the kiln axis, which makes
where qh
=
Eq. (1) an ordinary differential equation with as boundary condition:
h = hL at z = L
(3)
where hL is slightly larger than zero for rotary kilns without a flow constriction at the solids discharge end of the kiln or slightly larger than the dam height for kilns with a dam at the solids discharge end. Eq. (1) can easily be solved numerically taking into account possible bed volume changes as a consequence of chemical or physical conversions. For kilns with dams at several intermediate locations, the integration can be carried out section by section in
190
upstream direction. For kilns with slotted diaphragms the solution procedure involves the evaluation of the solids flow characteristics of the slotted diaphragms as a function of the operating conditions: qSD
VSDN ( a + f(Ah/h,,Fr,B-d))
=
(4)
where the first term in braces a represents the "pumping" action of the slotted diaphragm and the second term the "levelling out" action as a function of the bed depth difference over the diaphragm, the Froude number and the dynamic angle of repose of the material corrected for the slope of the kiln. The solids flow characteristics have to be determined experimentally by testing of a (preferably full-scale) model of the slotted diaphragm or have to be derived from the overall solids hold-up characteristics of the kiln under various conditions. For rotary kilns without solids flow constrictions and with a large length-to-bed depth ratio (shallow beds) Eq. (1) can be considerably simplified. Saeman [ 2 ] has derived for that case the following equation:
He validated his model with experimental data of Sullivan et al.[3]. Description of heat transfer The heat transfer in the kiln is described in terms of enthalpy flows of the solids and gas phases and in terms of heat losses to the environment:
a'_d _ dz - 'loss 'st
=
'loss
'gs =
+
' g u
"us
- 'us
-
' s b
where f
=
+1 for flow in the positive z direction and
E
=
-1 for flow in the negative z direction,
T,
=
Ts for gas species produced in the solids bed and
T,
=
T
g
for gas species consumed by the s o l i d s bed.
g and 0, are the enthalpy flows of the gas and the solids, respectively and is the heat lost to the surroundings of the kiln. The heat fluxes on the right-
P,
hand sides of Eqs. (6)-(10) are schematically shown in Fig. 4 , being a cross section of a directly fired rotary kiln. The present model considers only heat transfer in the radial direction. Axial radiative exchange is neglected, which
191 is justified only for directly fired rotary kilns with a separate combustor. Figure 4 also shows that, for heat-transfer calculations, infinitesimally thin boundary layers in the solids bed are assumed, one adjacent to the solids bed surface and one adjacent to the wall.
loss 0
Fig. 4 . Temperatures and heat flows in a kiln cross section. pgu and
(pgs
are the net heat fluxes flowing from the gas to the uncovered
wall (that part of the wall in contact with the gas phase) and the solids bed surface, respectively. Both fluxes consist of a convective and a radiative part. pgu is the net radiative heat flux flowing from the uncovered wall to the solids bed surface. The radiative exchange in the gas space is evaluated with the methods described by Frisch and Jeschar [ 4 ] . The gas emissivity is calculated with the method outlined by Leckner [ 5 ] . For the convective heat transfer from the gas phase to the solids bed and to the uncovered wall, the correlation for turbulent pipe flow as recommended by the VDI [ 6 ] is used. However, the heat transfer to the solids bed is multiplied by an enhancement factor based on the observations of Tscheng and Watkinson [ 7 ] , who report the coefficient for gas-to-bed heat transfer to be a factor of 10 higher than for the gas-to-wall heat transfer for gas flow in the laminar/turbulent transition region (Re
=
2000 - 8 0 0 0 ) . Reich and Beer [ 8 ] report that rotation of the tube suppresses turbulent motion and therefore reduces the heat transfer coefficient. Their experiments show that this effect is significant only at very high rotational speeds (i.e. centrifuging beds). psb is the net heat flux flowing from the covered wall to the solids bed. The heat transfer coefficient is calculated
192
according to the procedure described by Schlunder [ 9 ] and Martin [lo] and includes a contribution by radiation not present in the model of Lybaert [ll]. The effect of heat capacity of the thick refractory wall on the heat transfer of directly fired rotary kilns is taken into account with the semi-empirical expressions derived by Vaillant [12] for the evaluation of the temperatures of the covered and uncovered wall together with a heat balance over the wall for evaluation of the heat loss, 'ploss, to the environment. The convective and radiative heat losses from the outside kiln shell have been described by Kuhle [13]. DescriDtion of mass transfer Exchange of gas species occurs over the solids bedbulk gas interface as a consequence of chemical and physical conversions in the solids bed. This exchange consists of two parts:
(1) a convective part due to a net production or net consumption of gas species by the solids bed;
(2) a mass transfer part due to concentration differences between the gas in the solids bed and in the bulk gas. The mass transfer on the gas side of the interface is supposed to be determined by a (thin) layer adjacent to the interface and is described with a mass transfer coefficient, k. The calculations are simplified by defining an imaginary thin layer on the solids side of the interface, in which all diffusive resistance is concentrated. Consequently the concentrations inside the solids bed will be assumed constant. The mass transfer model is schematically depicted in Fig. 5 for the case of drying. The overall mass transfer rate for species i is described by: bnet,i = Xs,i
4~
+
(11)
kov,i b (Cs,i - cg,i)
where 4 is taken positive in the direction from solids bed to bulk gas phase, b is the surface area of the solids bed per unit length of the kiln, and xi and
ci are the mol fraction and the concentration of species i in the gas phase, respectively. Eq. (11) adds two unknowns to the mathematical model and hence we need an additional equation - which might be obtained by equating &,et,i to zero for a gas species not involved in chemical or physical conversions
-
a
thermodynamic equilibrium equation, or a kinetic rate equation. An expression for the overall mass transfer coefficient in rotary kilns has not yet been published. Eq. (11) is merely used to check the maximum mass transfer rate over the solids bed/bulk gas interface, which might be limiting the rate of a chemical or physical conversion in the solids bed. In that case kov,i equals ki, which can be obtained from the analogy with convective heat transfer.
193
INTERFACE
c
BULK G A S PHASE -I
Fig. 5 . Schematic concentration profiles during drying in a rotary kiln CALCINATION OF PSEUDOBOEHMITE EXTRUDATES IN A DIRECTLY-GAS-FIRED ROTARY KILN Introduction The catalyst plant of Shell Ghent produces a variety of heterogeneous catalysts, many of which are supported on y-alumina extrudates of various qualities. The extrudates are manufactured by kneading and peptization of pseudoboehmite powder with water and (in)organic aids to a paste, followed by extrusion, drying, classification, longsbreaking and finally calcination. The rotary kiln in the alumina plant at Ghent (Fig. 1) was used for validation of the model. The measurements were carried out during normal commercial production of
1.5 nun y-alumina trilobes@ as a catalyst carrier. A mixture of two different commercial pseudoboehmites, PURAL SB - a high-density pseudoboehmite (700 k g / m 3 )
from Condea Chemie GmbH -, and VERSAL 250, a low-density pseudo-
boehmite (200 kg/m3) from Kaiser Chemicals - was used as starting material and organics were used as feeding aids. Five runs were carried out with different throughputs and kiln rotation speeds. Residence times and temperature and conversion profiles were measured for each run. The results will be presented after discussion of the dehydration of pseudoboehmite. Thermal dehvdration of pseudoboehmite Pseudoboehmite is a poorly crystallized form of boehmite, Al203.1 H20. It usually consists of agglomerates of platelets of very small size (nanometers)
194 but a fibrillar form has also been reported [14].The amount of structural water varies from the stoichiometric amount of 1 mol H 2 0 per mol of A1203 to as much as 3.5 for almost amorphous boehmite. Reviews of pseudoboehmite and other alumina-related compounds have been given by Lippens and Steggerda [15] and by Misra [16]. The differential thermal gravimetry (DTG) curves of PURAL SB and VERSAL 250 are given in Fig. 6 . PURAL SB has been described earlier by Decleer [17]. The first endothermic peak is due to the desorption of water and the second due to the conversion of pseudoboehmite to 7-alumina. This conversion takes place over a broad temperature range, dependent on the crystallinity of the pseudoboehmite. For nearly amorphous pseudoboehmite the conversion takes place at a temperature as low as 300
"C
[18], while on the other hand the conversion of
well crystallized boehmite takes place in a narrow temperature range between 450
and 580
"C
[18]. Hence, each different type of pseudoboehmite requires a
new evaluation of its thermal behaviour.
1 dw , -Wo d t
I
200
I
400
I
600
1
I
1000 TEMPERATURE, C
800
Fig. 6. DTG curves of two commercial pseudoboehmites (4 "C/min in air) Considering the DTG analysis a kinetic model on the thermal dehydration o f dried pseudoboehmite extrudates should include the following elements:
195
(1) Evaporation of physically adsorbed water left after removal of the bulk amount of water in the drying step. For the removal of the residual amount of moisture the proper adsorption isotherm for multilayer and possibly multicomponent ad/desorption should be taken into account; (2) Thermal decomposition of the pseudoboehmite including changes in particle diameter, porosity and surface area. The decomposition should distinguish between the loss of stoichiometric and the loss of excess water; (3) Thermal dehydroxylation of the alumina surface taking into account any physical adsorption or chemisorption equilibrium;
(4) Thermal decomposition of additives; (5) Mass- and heat-transfer resistances within the extrudates; (6) The thermochemical data of all species involved and the adsorption and chemisorption heats of water. Some kinetic work has been carried out on the decomposition of (pseudo)boehmite. Callister et al. [19] reported on the effect of the water pressure. Tsuchida et al. [20] reported on the effect of crystallite size and confirmed the effect of water pressure as determined by Callister et al. [19]. A consistent kinetic model for the decomposition of pseudoboehmite, which is applicable over the entire temperature range of interest in calcination (20 - 800 "C) and which takes into account the effects of particle size and water pressure is not available in the literature. Therefore the model of Leyko et al. [21] has been fitted to the data of Fig. 6 as a first approximation: y
=
WlXl
+ w2x2 + w3x1x2x3 + 12.6057)(1-~1)~'~
dxl/dt
=
exp(-56,5E6/R gT
dx2/dt
=
exp(-E2/R gT
dx3/dt
=
exp(-31.OE6/R T - 3.0943)(1-x3)'.' g
+
ln(kr,2)) ( 1 - ~ 2 ) O . ~ / x 2 ~ . ~
(12) (13) (14) (15)
with the constants as given in Table 1. The exponent of 0.14 in Eq. (14) of the original model of Leyko et al. [ 2 1 ] has been replaced by 0.6 to obtain a better fit with our pseudoboehmites. The constants for w1 given in Table 1 apply to the pseudoboehmite powders as received. In the dried extrudates, an initial XI was used corresponding to the measured loss-on-ignition (LOI) of the dried extrudates. It is assumed that the model derived from powder data is also applicable to the decomposition of pseudoboehmite in extrudates. The thermal decomposition of the organic peptization and feeding aids must also be considered since even the addition of as little as 1 %w organic acid
may cause a potential adiabatic temperature rise in the order of 200 "C upon complete combustion inside the solids bed phase. However, prior to this the
196 organics may partly evaporate or decompose incompletely to gaseous products. It is also possible that during evaporation, organic decomposition products ignite upon their release from the solids bed such that diffusion flames can be observed just beyond the bed surface. For the calculations presented here, it was assumed that part of the organics had been evaporated during drying and that the remainder (1.5 %w of dry product) could only be removed by thermal oxidative decomposition. TABLE 1 Constants fitted to the kinetic model of Leyko et al. [21], Eqs.(12)-(15). Pseudoboehmite PUPAL SB VERSAL 250
w1
W2
w3
0.056 0.096
0.158 0.148
0.024 0.044
E2 130.OE6 100.OE6
ln(kr,2) 14.8057 9.8057
Most thermochemical data in the present model have been taken from Barin et al. [ 2 2 ] . The data for 7-alumina have been taken from the JANAF tables [ 2 3 ] . No data are available for pseudoboehmite [24].As an approximation, the thermochemical data of liquid water were added to the data of Haas et al. [ 2 5 ] for crystalline boehmite in proportion to the molar amount of "excess" water present in pseudoboehmite. Experimental The residence time distributions were measured by pulse injection of a sample of calcined extrudates labelled with technetium-99m. The movement of the injected sample was followed with one scintillation detector in the solids feed line and three around the chute at the solids discharge end of the kiln (Fig. 1). The amount of injected radio-active material was too small for its passage through the kiln to be followed with detectors along the outside wall of the kiln, as has been done for measurement of the solids transport in rotary kilns used for production of clinker [26,27]. The temperatures and conversions were measured either before or after the radio-active tracer experiments. Intermediate samples were taken from nozzles at one quarter, at half and at three quarters of the kiln length. The latter two nozzles were also used for measurement of the gas and solids temperatures with a specially constructed thermowell, which could be quickly inserted or removed during rotation of the kiln. Samples were also taken from the solids feed and from the cooler. Samples were analysed for LO1 and boehmite conversion using X-ray diffractometry.
197
The solids outlet temperature was measured with a fixed thermocouple. Temperatures of the outside shell wall (the skin) were measured with a contact thermometer and with colour chalk. A shielded velocity thermocouple (suction pyrometer) was used for measurement of gas temperatures at the cold and hot ends of the kiln. Gas samples were taken from the cold (gas outlet) end of the kiln using a long sampling tube to minimize inclusion of false air from the kiln seals at the cold end. The sampled gas was analysed on line for oxygen to determine the total amount of excess combustion and false air sucked in from the hot end of the kiln. Results of residence time distribution measurements One example of a measured residence time distribution is given in Fig. 7. The measured distributions were interpreted with a model of n ideal mixers in series [ 2 8 ] . The results are given in Table 2, together with the theoretical predictions using the rolling-bed model of Saeman [ 2 ] . The theoretical particle velocity has been calculated with E q . ( 5 ) . The agreement between theoretical and measured bed velocities is considered good for the deep bed runs. The larger discrepancies with the shallow bed depths can be explained by the presence of the longitudinal strips. Particles lifted by the strips are withheld longer from axial movement than particles in the bed.
L
..
~.
A .
Figure 7. Residence time distribution measured for run no. 1. Table 2 also shows that the number of ideal mixers is so large that solids mixing in axial direction can be neglected. It appears that the number o f mixers is of the same order of magnitude as the average number of times a particle r o l l s down the surface of the solids bed.
198 This supports the assumption of the rolling-bed model that mixing occurs only in the rolling layer. TABLE 2 Comparison between measured and theoretical solids transport parameters Run #
1 2 3
4 5
Kiln Bed Average axial particle speed, height, velocity, m/h rpm cm measured theoretical 0.845 0.5 1.5 1.5 0.5
15.5 14.4 7.8 9.7 17.6
10.00 5.76 14.19 15.17 6.20
9.78
5.81 17.37 17.37 5.81
Number of mixers
Number of particle falls
#
#
1416 473 312 648 922
563 587 851 748 519
ComDarison of calculated and measured temperature and conversion Drofiles Calculated and measured temperature and conversion profiles of one run are compared in Fig. 8. The measured solids inlet and outlet temperatures of the rotary kiln were used as boundary conditions of Eq. (6) and ( 7 ) , since these values are more accurate than the measured gas inlet and outlet temperatures. Fig. 8a compares the results using the kinetic decomposition model derived from the thermal analysis of the pseudoboehmite powder, neglecting the organics content of the extrudates and ignoring enhanced convective heat and mass transfer at the solids bedbulk gas interface. The calculated solids temperatures at half and at three quarters of the kiln length appear to be about 9 0 and 65
'C
lower than the measured values, respectively. The measured gas temperature at the solids inlet is about 1 5 "C lower than the calculated value. The measured gas temperatures at half and three quarters of the kiln length are 4 0 and 30 " C lower than the calculated values, respectively, but are possibly too low due to radiation losses from the unshielded thermocouple tips. The measured gas temperature at the solids outlet side of the kiln is about 50 "C higher than the calculated value, which is ascribed to the fact that hot gases from the combustor and false air from the seals and the cooler have not yet been completely mixed at the solids outlet end o f the kiln. Measured and calculated skin temperatures agree within a few degrees. The small discontinuity in the calculated skin temperature profile is due to the use of a better fire-resistant but less insulating brick lining in the hotter part of the kiln. The conversion of the pseudoboehmite starts earlier according to the simulation than has been measured, but completion of the conversion is predicted correctly. To improve the agreement of measured and calculated profiles, the convective heat and mass transfer coefficients were enhanced by a factor of 10, in accor-
dance with the experimental data of Tscheng et al. [ 7 ] . The physically adsorbed water was replaced (at constant LOI) by acetic acid as a model compound for oxidation in the temperature range of 300-450
"C
as reported by Abrams [ 1 4 ] .To
simulate a possible retarding effect of steam on the decomposition of pseudoboehmite [19,20] and a possible retarding effect of pressure flow limitation inside the extrudates, we lowered the reaction rate at low temperatures by increasing the activation energy to 390 MJ/kmol and correspondingly increasing the pre-exponential constant of Eq. (14) (i.e. to log(kr,2)
=
57.7422) such
that the measured conversion profile was matched. The results of the improved simulation are given in Fig. 8b, showing that the agreement between measured and calculated solids temperatures had significantly improved. Better agreement was also obtained for the skin temperatures, while the agreement o f the gas temperatures had improved for the intermediate kiln locations but had deteriorated on the inlet and outlet sides of the kiln. The oxygen consumption by the oxidation of the acetic acid was maximal at a solids temperature of about 440 " C , while the maximal mass transport over the gas film at this temperature was a factor of 10 higher. Hence, no oxygen mass transfer limitation occurred for the kinetics and mass transfer enhancement assumed. FURTHER DEVELOPMENT OF THE MODEL From Fig. 8 it is clear that the rotary kiln model cannot be firmly validated with the measurements in the catalyst plant at Ghent due to the absence of a consistent kinetic model and due to the lack of reliable thermochemical data on the conversion of pseudoboehmite to y-alumina. If the measured decomposition rate of pseudoboehmite is fitted by adjustment of the rate constants, then an acceptable agreement with the solids temperature profile is also obtained. Uncertainties exist on the enhancement of the coefficients for convective heat and mass transfer between the turbulent flowing gas and the solids bed. The convective heat and mass transfer is also enhanced by the presence of longitudinal strips on the inside wall of the kiln from which particles are falling through the gas phase from a maximum height corresponding with the angle of repose. Corrections can be made for this with the methods described for rotary driers [30,31], which are designed for the purpose of raining the solid particles through the gas phase. These corrections have not been made in the present model. More work is also required on the oxidative decomposition of organic additives present in the solids and their possible combustion at the surface of the solids bed. Residual organic species present in the calcined product might ultimately have a negative effect on the catalytic performance.
200 TEMPERATURE, OC
700
CONVERSION, ‘10
/7-1’:
.. .
RELATIVE DISTANCE FROM SOLIDS INLET
a. Before parameter adjustment.
800
TEMPERATURE,
OC
p-E
CONVERSION, %
20 I0 0 0
I /2 3/4 I RELATIVE DISTANCE FROM SOLIDS INLET
1/4
b. After parameter adjustment.
Fig. 8. Comparison between calculated and measured temperature and conversion profiles in rotary kiln of alumina plant at Ghent. Legend: solids temperatures, -------- &------- gas temperatures, ---+--skin temperatures, 3 -pseudoboehmite conversion.
*--.--.--.
201 The model is now being used for scaiing down the calcination of various types of catalysts requiring different kinetic models. Comparisons are being made between products obtained at different scales of rotary kilns and guidelines are being established for carrying out a representative calcination at the smallest conceivable scale. CONCLUDING REMARKS (1) A computer model has been developed for the calculation of temperature and conversion profiles in rotary kilns.
(2) Residence time measurements confirm a plug-flow-type solids transport, which is consistent with the rolling-bed model of solids transport. (3) The average residence time in the rotary kiln of the catalyst plant at Ghent is satisfactorily predicted by Saeman's simplified solids transport equation for shallow beds (Eq. (5)).
(4) The temperature profiles in the rotary kiln of the catalyst plant can be predicted reasonably well provided that a good kinetic model is available. ACKNOWLEDGEMENT Dr. R. Jacobs (Rijksuniversiteit Gent) is acknowledged for making available the university facilities for preparing the Tn-99m labelled samples. NOMENCLATURE b c D Deff E Fr g
H h k
F N q R
Rg r0 S
T t
' S D W X
Y
Z
U
Chord in kiln cross section corresponding to solids bed surface, m Concentration, kmol/m3 Inside diameter of rotary kiln, m Effective diffusion coefficient. . m2/s , Activation energy J/kmol Froude number = N2D/g Gravity constant = 9.80665 m/s2 Enthalpy , J/kmol Bed height, m Mass transfer coefficient, kmol/(m2.s) Reaction rate constant Kiln length, m Kiln rotation speed, '-s Volumetric solids bed transport, m3/s Kiln radius, m Gas constant = 8314.3 J/(kmol.K) Distance between kiln axes and solids bed surface centre line, m Slope of the kiln with respect to the horizontal, rad Temperature, K Time, s Volume of solids bed sliced out per revolution by slotted diaphragm, m3 Weight fraction of mass intake Mol fraction in E q . (ll), otherwise conversion Weight loss as fraction of mass intake Distance from solids inlet, m Constant in Eq. ( 4 )
202
6 8 f @
4
3 'p
Layer thickness characteristic for diffusion resistance, m Central bed angle, rad +1 (or -1) for flow in positive (or negative) direction Enthalpy flow, Watt Gas flow crossing the solids bed bulk gas interface, kmol/(m.s) Slope of the bed surface with the kiln axes, rad Net heat exchange per unit kiln length, W/m
Subscripts a b g i L ov S
SD T U
X
air bottom of solids bed gas index of gas species solids outlet side of kiln overall solids Slotted Diaphragm Total of gas species uncovered kiln wall either g or s
REFERENCES
1 H. Henein, J.K. Brimacombe and A.P. Watkinson, Met. Trans., B 14B (1983) 6 , pp. 191-205 and pp. 207-220. 2 W.C. Saeman, Chem. Eng. Progr., 47 (1951) 10, p. 508. 3 J.D. Sullivan, C.G. Maier and O.C. Ralston, U.S. Bureau of Mines, Technical Paper 384 ( 1 9 2 7 ) . 4 V. Frisch and R. Jeschar, Zement-Kalk-Gips, 36 (1983) 10, p. 549 (in German). 5 B. Leckner, Combustion and Flame, 1 9 ( 1 9 7 2 ) , p. 3 3 . 6 VDI Warme Atlas, 4th edition 1 9 8 4 , VDI-Verlag GmbH, Dusseldorf. 7 S.H. Tscheng and A . P . Watkinson, Can. J. Chem. Eng., 57 (1979) 8 , p. 433. 8 G. Reich and H. Beer, Int. J. Heat Mass Transfer, 32 (1989) 3 , p. 551. 9 E.U. Schlunder, Chem. Ing. Technik, 53 (1981) 1 2 , p. 925 (in German). 10 H. Martin, Chem. Ing. Technik, 52 (1980) 3 , p. 199 (in German). 11 P. Lybaert, Int. J. Heat Mass Transfer, 30 (1987) 8 , p. 1663. 1 2 A. Vaillant, Ph.D. Thesis Columbia University, 1965, University Microfilms International, Ann Arbor, Michigan, U.S.A., 1979. 1 3 W. Kuhle, Zement-Kalk-Gips, (1970) 6 , p. 263 (in German). 1 4 L . Abrams and M.J.D. Low, I&EC Product and Development, 8 (1969) 1, pp. 38-48. 1 5 B.C. Lippens and J.J. Steggerda, in B.G. Linsen (Ed.), Physical and Chemical Aspects of Adsorbants and Catalysts, Academic Press, New York, 1 9 7 0 , pp. 171-211. 16 C. Misra, Industrial Alumina Chemicals, ACS Monograph 1 8 4 , American Chemical Society, Washington D.C., 1986. 1 7 J.G.M. Decleer, Bull. SOC. Chim. Belg., 98 (1989) 7 , p. 449. 1 8 R.C. MacKenzie and G. Berggren, in R.C. MacKenzie (Ed.), Differential Thermal Analysis, V o l . 1 , Academic Press, London, 1 9 7 0 , pp. 279-302. 1 9 W.D. Callister, Jr., I.B. Cutler and R.S. Gordon, J. h e r . Ceramic SOC., 49 (1966) 8 , pp. 419-422. 20 T. Tsuchida, R. Furuichi and T. Ishii, Thermochim. Acta, 39 ( 1 9 8 0 ) , pp. 103-115. 21 J . Leyko, M. Maciejewski and R. Szuniewicz, J. Therm. Anal., 1 7 ( 1 9 7 9 ) , pp. 275-286. 22 I. Barin, 0 . Knacke and 0 . Kubaschewski, Thermochemical Properties of Inorganic Substances, Springer Verlag, Berlin, 1973. Supplement, 1977.
203
23 JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data, 14 Suppl. (1985), pp. 156-159. 24 S.C. Carniglia, J. Amer. Ceramic SOC., 66 (1983) 7, p . 495. 25 J.L. Haas Jr., G.R. Robinson Jr. and B.S. Hemingway, J. Phys. Chem. Ref. Data, 10 (1981) 3, pp. 575-665. 26 K. Akerman, Chem. Ing. Techn, 43 (1971) 22, p. 1204. 27 H. Costa and K. Petermann, Silikattechn., 10 (1959) 4, p. 209 and 10 (1959) 5, p. 253. 28 0. Levenspiel, Chemical Reaction Engineering, 2nd ed., John Wiley and Sons, Inc., New York, 1972, Chapter 9. 29 R.S.C. Rogers and R.P. Gardner, Powder Technology, 23 (1979), p . 159. 30 F.A. Kamke and J.B. Wilson, AIChEJ, 32 (1986) 2, p. 269. 31 H. Hirosue, Powder Technology, 59 (1989), p. 125.
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G . Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparationof Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
205
HYDROTHERMAL SINTERING OF THE ACTIVE PHASE IN ALUMINA SUPPORTED FIXED BED NICKEL CATALYSTS DURING REDUCTION E.K. POELS, J.G. DEKKER and W.A. VAN LEEUWEN Unilever Research Laboratory, Vlaardingen, The Netherlands
SUMMARY Sintering of the active phase of alumina supported fixed bed nickel catalysts due to the hydrothermal conditions present during reduction is investigated. Three mechanisms are proposed which could lead to lowering of the degree of metal dispersion on the support: a) Sintering of the support; b) Thermal sintering of nickel metal particles and c) Sintering of the Ni2‘ precursor prior to formation of metallic nickel. Three nickel catalysts have been prepared in order to test the relative contribution of the proposed mechanisms: Two catalysts by impregnation with nickel nitrate solution of a gamma alumina support susceptible to sintering and of a sinter stable alpha alumina support. The third catalyst was prepared via a novel route developed in our laboratory not using nitrate. The results obtained after various treatments show that sintering of the nickel metal crystallites and of the support play only a minor role. The predominant mechanism is hydrothermal sintering of the Ni2+ species present on the support prior to their reduction. The novel catalyst proved far more stable than the other two. This could partly be ascribed to the fact that nitric oxide vapours evolved with the nitrate prepared catalysts enhanced Ni” sintering considerably.
INTRODUCTION A key step in the manufacturing route of nickel catalysts is the reduction by hydrogen at high temperature to produce small, stable crystallites of metallic nickel. I t is well known in the literature that sintering of this active phase can occur as a consequence of the hydrothermal conditions present during reduction, leading to lowering of catalyst quality [l]. The purpose of this study has been to identify the main mechanism underlying the sintering phenomenon and to compare the behaviour of catalysts prepared by a new method developed in our laboratory with those obtained by more conventional procedures. Three mechanisms can be envisaged which could lead to lowering of the degree of metal dispersion on the support:
1. Sintering of the support. I t is possible that the support used, shows surface area loss
due
to the hydrothermal treatment during
reduction, thus
influencing metal support interaction and perhaps surface mobility of the active phase. 2.
Sintering of the metal crystallites. Thermal sintering of nickel particles at high temperatures in dry hydrogen due to atomic migration or Ostwald
ripening [2] and in a steam atmosphere (due to film formation) [ 3 ] have both 3.
been reported in the literature. Sintering of the N i Z + precursor prior to formation of metallic nickel. During calcination and reduction hydrothermal treatment of the NiZf species present occurs, perhaps with similar effects on the dispersion of such species
as
described above. The emphasis in the literature is clearly on the second mechanism which is widely studied for noble and base metal catalysts. In this paper the relative contribution of the above mechanisms during reduction of alumina supported fixed bed nickel catalysts at realistic conditions is investigated. For this purpose three nickel catalysts have been prepared containing similar nickel loadings: a. A catalyst prepared by impregnating gamma alumina with aqueous nickel
nitrate solution in order to test the effect of support sintering on the metal dispersion. b. A catalyst prepared by impregnation of alpha alumina with aqueous nickel nitrate solution in order to test the effect of precursor and/or metal crystallite sintering with minimal support sintering. c. A catalyst prepared via a novel impregnation method developed in our laboratory not involving application of nickel salts derived from strong mineral acids on a special wide pore support. (In many petroleum processing applications these now commercialised catalysts show superior performance to currently available products). The three catalysts were subjected to various reduction and pretreatment conditions and subsequently their BET surface area, metal surface area, nickel crystallite size and degree of reduction were compared. EXPERIMENTAL Two catalysts were prepared by impregnation of the alumina support in question with an aqueous nickel nitrate solution. The concentration of this solution was such as to result in a metal loading of approximately 11 wt.% (prior to calcination or reduction) after saturation of the pore volume of the support by submersion and subsequent filtration. A commercial gamma-aluminawas used for one catalyst; an alpha-aluminasupport was prepared from this carrier by calcination at 1110°C for 2 hours. The third catalyst was prepared following a commercially applied preparation method involving impregnation of a special wide pore alumina support with a solution not containing nickel salts derived from strong mineral acid. This procedure was carried out such as to obtain the same nickel loading as described above. The catalysts were dried at 120°C for 16 hours.
207
TABLE 1 Analysis data of the catalysts studied. code
alumina reduct. Ni cont. S(BET) support method (wt.X ) (m2/g)
1037 1034 1045 1074 1075 1076
gamma alpha special gamma alpha special
stdd stdd stdd super super super
10.7 10.2 11.6 11.8 10.1 12.0
Ni surf.area Ni Diam. Deg.of red. (m2/gNitOt) (m) (X)
228 64 97
126 160 158 158 193 193
2.5 2.2 2.2 1.9 2.0 1.9
74 80
79 69 90 86
The BET surface area of the catalyst samples was determined using nitrogen physisorption. Nickel surface area determination by hydrogen chemisorption; calculation of metal crystallite sizes and measurement of degree of reduction were conducted according to reference [4]. In some instances catalyst samples were passivated after reductive treatment in all-glass flow equipment using nitrogen containing ca.1 v01.Z
0, and subsequently transferred into
the
chemisorption cell. In other cases the treatment could be carried out directly in the H, chemisorption apparatus. Sample sizes for chemisorption measurements and treatments were approximately three grammes. Nickel contents were determined using x-ray fluorescence spectroscopy. The analysis data of the catalysts thus obtained are presented in table 1. Ni dispersion retention
(X)
100
v. cat.
60
Ni(N03)2/alphaA1203 40
(N03)2/gammaA1203
a
b
C
d
procedure Fig. 1. Reiention of nickel surface area for the three catalysts tested after: a ) Standard reduction; b) standard reduction followed by l h H , extra at reduction temp.; c) lh H/H,O extra at reduction temperature; d) heating up in N 2 / H 2 0 to reduction temperature prior to standard reduction.
208
Chemisorption results are given after a "standard" reduction procedure as well as after a "super" reduction involving extremely efficient removal of the water generated. Water contents of moist treating gases were always the saturation level at ambient temperature (ca. 2.4 vol.%) unless otherwise stated. RESULTS AND DISCUSSION
In a first set of experiments the three catalysts were reduced using a "standard" set of conditions and also with prolonged exposure to hydrogen or moist hydrogen at the reduction temperature after the standard reduction. A third reduction was preceded by a treatment in moist nitrogen up to the reduction temperature (see figure 1). From this plot it is clear that the nickel surface area loss upon prolonged treatment in dry hydrogen at reduction temperature is quite small. A flow of 2 . 4 vol.% H,O
in H, after standard reduction did not
result in extreme nickel sintering either. A more substantial loss of nickel dispersion was observed upon pretreatment in moist nitrogen at elevated temperature. From nitrogen physisorptionmeasurements of the nitrate prepared gamma alumina based catalyst it is clear that only very limited BET surface area l o s s occurred upon the treatments carried out (figure 2). As a result one could already cautiously rule out sintering of the support as a decisive cause BET surface area retention (%)
110,
100 90 80 70
60 5n --
std
std(duplo)
H2+H20
support
procedure
sup+H
sup+H/HSO
Fig. 2. Retention of BET surface area of gamma alumina supported nickel catalyst and the bare carrier after: standard reduction; standard reduction (duplo) ; reduction followed by lh treatment in moist hydrogen at reduction temperature; the bare support; the support treated under reduction conditions; the support reduced and treated for lh in moist hydrogen, respectively.
209
for hydrothermal sintering during reduction. Metal crystallite growth is also less likely on the basis of the minimal effects of prolonged H,
and H,/H,O
treatments. In order to test the importance of the remaining mechanism: NiZr precursor sintering the unreduced alpha alumina supported and novel catalyst were subjected to increasingly severe calcination treatments. In figure 3 it can be seen that calcination in a rotary calciner at 250°C already results in some loss of metal dispersion in the conventionally prepared catalyst. Calcination at 350°C of a monolayer of catalyst yields a worse nickel surface area and, finally, calcination of a bed of catalyst a few centimeters deep in a narrow necked ErlenMeyer flask (i.e. removal of moisture is very restricted) really destroys catalyst quality. The novel catalyst is in all cases much more sinter stable. The extent of sintering of the support was checked by treating the alpha alumina support in the same way as described above and subsequently measuring BET surface areas of both the maltreated bare support samples and the catalysts based on this carrier. Indeed modest support sintering had occurred in all cases as expected (figure 4 ) . What surface area loss is apparent in the catalyst samples in comparison with the support must probably be ascribed to a decrease in the contribution of the active phase to the BET area, as in maltreated gamma alumina supported catalysts (i.e. a much less sinter stable carrier) surface area loss was negligable (see figure 2 ) . In order to check whether at standard reduction conditions water removal is Ni dispersion retention (%)
100 80 60 40
20
0
none
rotary
monol.
limited N2/H20
procedure
Fig. 3 . Retention of nickel surface area of standard reduced alpha alumina supported catalyst and the novel catalyst after: just reduction; rotary calcination; calcination of a monolayer on gauze; calcination in an Erlen-Meyerflask; heating up in moist N, prior to reducticn, respectively.
210
BET surface area retention (%)
/!
I
110
100 90
80
port
70 60I
std
calc.
H2/H20
procedure
Fig. 4 . Retention of BET surface area of the alpha alumina supported catalyst and the bare carrier after: standard reduction conditions: reduction followed by If. extra treatment in moist hydrogen at reduction temperature; calcination i n an Erlen-Meyer flask prior to reduction, respectively. inefficient and therefore in itself causes sintering thus rendering these presintered catalysts insensitive to
subsequent maltreatment, the
following
experiments were conducted. A so-called "super" reduction was performed i .e. water removal was enhanced by applying low heating rate and very high hydrogen flow rate. The water content of moist gases (when applicable) was doubled compared to the standard reduction experiments by saturating the gas at 3 5 ° C (i.e. ca.5.0 vol.% H,O)
instead of ambient temperature in order to further
enhance the effects (from this moment on all moist treating gases were saturated with water at 35°C). All post- and pre-treatments of the catalysts were extended to two hours for the same reason. During the standard reduction experiments after moist nitrogen treatment at reduction temperature, the switch to hydrogen flow was made at this elevated temperature. It may be assumed that at this temperature where reduction rate is high a sudden large quantity of water is generated by this switch. To avoid this shock treatment of the catalyst the reactor was cooled to ambient after moist nitrogen treatment prior to the super reduction. The results, summarised in figure 5 are quite similar to those of the standard reduction experiments (figure 1) although initial nickel surface areas are higher than upon standard reduction (see table 1) illustrative of the effect we
se'i
out
to study. Therefore, the above derived conclusions still hold. Another conclusion to be derived from
211
Ni dispersion retention (%)
L
40
dev. cat.
20
03)2/alphaA1203 std
std+H2 H2/H20N2/H20
procedure
Fig. 5. Retention of nickel surface area of all three catalysts studied after "super" reduction and various treatments. For treating conditions see figure 1 and the text. the standard and super reduction experiments is that the order of initial metal dispersion is: novel
f:
novel
n
alpha
n
gamma (see table 1). And the order of sinter stability is
gamma > alpha (see figures 1 and 5 ) . The increased sinter stability of
the gamma alumina catalyst compared to the alpha alumina case is probably due to a better metal support interaction of the first catalyst as is reflected by the degrees of reduction of the catalysts (table 1). In figure 6 it can be seen that the nickel surface area loss of the catalysr s upon pretreatment in moist nitrogen for the alpha alumina Supported sample is totally due to an increase in crystallite size. For the gamma alumina prepared catalyst a combination of crystallite sintering and decrease in degree of reduction, probably caused by surface spinel formation, is apparent. With the novel catalyst reasoning along the same lines, only a slight surface NiA1,0, spinel formation is underlying the minor surface area loss observed. Quite another question is why the nickel nitrate based catalysts are
so
much
less sinter stable than their newly developed counterpart. The influence of NO,
containing fumes generated by nitrate decomposition during heating was studied in the following way. 11 Grammes of the novel catalyst were placed in a narrow glass tube on top of the same amount of the gamma alumina supported nickel nitrate containing catalyst separated by a layer of quartz wool. The tube was then calcined at 350°C for 2 hours, It was established by thermo-
212
degr. of red. (%/
[ryst. size (nm)
100
5
90
4
80
3
70
2
60
I' std
I
I
etd+H2
H2/H20
'
50 N2/H20
procedure
Fig. 6. Nickel crystallite size and degree of reduction of all three catalysts studied after "super" reduction and the same treatments as in figure 5. + = Ni(NO,),/gamma A1,O3; * = Ni(N0,)Jalpha Al,O,; 0 = novel catalyst. gravimetric analysis that at this temperature the nitrate fully decomposed. The same experiment was repeated with two layers of the development catalyst i.e. not creating any nitrogen oxides. The results are given in figure 7. The sintering without the NO, is not extreme considering the limited exhaust of vapours in the test tube (compare e.g. the surface area loss for this catalyst upon calcination in figure 3 ; there the effect of calcination with hindered gas exhaust was 89 % dispersion retention). The sintering upon exposure to nitrogen oxide vapours in combination with moisture is clearly more pronounced confirming the negativz effect of these oxides on catalyst quality. In order to check this effect on the nitrate prepared alpha alumina supported catalyst two samples of this catalyst were calcined for two hours at 3 5 0 ° C in an excessively high nitrogen flow (ca. 1000 ml/min) . The nickel surface areas of the samples after "super" reduction and after a moist nitrogen treatment at elevated temperature followed by similar reduction (in the same cell) are given in figure 8 . For comparison the metal surface area of an uncalcined sample treated in the
same way as the latter of the above described samples is included in the plot. It may be assumed that the two calcined catalysts were substantially free of nitrates when reduced, whereas during heat treatment of the uncalcined sample NO, must have been present. From the plot it can, again, be concluded that exposure of Ni2+ to moisture is causing a decrease in the reduced metal dispersion. Furthermore, although the precalcination in high nitrogen flow will limit the
213 water generated during reduction thus restricting sintering somewhat, the results seem to confirm that the combination of moisture with NO, fumes is even worse.
In order to prove that gaseous decomposition products are not the cause of sintering of the novel catalysts as well, a commercially calcined catalyst from Crosfield was obtained containing 15 wt.% nickel (trade name: HTC 4 0 0 ) . Upon standard reduction the nickel surface area proved 170 m2/gNitOt (i.e. a better dispersion than the lab prepared catalysts after standard reduction at higher nickel loading!). Upon treatment inwet nitrogen at elevated temperature; cooling to ambient and finally standard reduction, the surface area retention was 71%, proving that NiO is also susceptible to hydrothermal sintering. For the alpha alumina catalyst (figure 8) when precalcined in a very high flow of dry nitrogen followed by treatment with moist nitrogen and intermediate cooling, upon subsequent reduction a metal surface area retention of circa 73% was observed. The same treatment applied to the novel
catalyst resulted in a
nickel surface area retention of 96%. Although it may be concluded from the above that NO, fumes are responsible for considerable enhancement of the hydrothermal sintering of the Ni2+ precursor salt and their absence is a major cause for the increased sinter stability of the newly developed catalyst, it is clear from this experiment, that this effect cannot entirely explain the stability of the nickel dispersion of the novel catalyst Ni dispersion retention (%)
7
ref
-NOx
+NOx
treatment Fig. 7. Retention of nickel surface area after: "super" reduction; calcination of the novel catalyst in a test tube prior to reduction and; calcination of the novel catalyst in a tube on top of a nitrate prepared catalyst prior to reduction, respectively.
214 Ni dispersion retention (%) I
'O 80 0I
0
L
calc/super
calc/N+H20
nocalc/N+HPO
treatment
Fig. 8 . Retention of nickel surface area of the "super" reduced alpha alumina supported .. catalyst after the following pretreatments: calcination in very high nitrogen flow; calcination in nitrogen flow followed by treatment in moist nitrogen and cooling to ambient; treatment in moist nitrogen and cooling to ambient without preceding calcination. CONCLUSIONS
1. The major mechanism leading to poor metal dispersion of alumina supported nickel catalysts is hydrothermal sintering of NiZ+ precursors prior to reduction. Sintering o f the support or metal crystallites once formed are relatively unimportant.
2. Nitrogen oxide vapours produced during high temperature treatment o f nickel nitrate prepared catalysts in combination with moisture greatly enhance sintering of the active nickel species. 3.
Improved NiZ+ stability can be achieved using new catalyst preparation procedures.
4.
This study has provided valuable information in identifying a key aspect in
the reduction step which must be controlled during manufacture in order to obtain optimum quality catalysts. REFERENCES 1. G . C . Chinchen, in J . R . Jennings (Ed.), Critical Reports on Applied Chemistry, Vol. 12, Selected Developments in Catalysts, Blackwell Scient. P u b l . , London, 1 9 8 5 , p.2. 2. K.-T. Kim and S . - K . Ihm, J . Catal., 96 ( 1 9 8 5 ) 12. 3. E. Ruckenstein and X.D. Hu, J. Catal., 100 ( 1 9 8 6 ) 1. 4 . a) J.W.E. Coenen, Ph.D. thesis, Technical University Delft, 1 9 5 8 . b) R.Z.C. vanMeerten, A.H.G.M. Beaumont, P.F.M.T. van Nisselrooij andJ.W.E. Coenen, Surf. Sci., 135 ( 1 9 8 3 ) 5 6 5 .
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
C. E. Marsden, Crosfield chemicals, P. 0. Box 26, Warr-n,
215
Erqlanfi.
SOMMARY silica is an inprtant support for ethylene polymerisation catalysts. It plays an active role in the polymerisation precess and its structure and camposition influence the catalyst activity, polymer morphology and properties. PKC&UES for making a wide range of silica structures are illustrated and key steps in their preparation highlightel. Ihe particular influence of support structure on polymer properties is described, together with the role of heteroatanr; in modifying the silica structure and polymer properties.
mrucrIoN In excess of 4 million tonnes of high density polyethylene are prcduad annually using Phillips type cr/SiOa catalysts in slurry and gas phase processes. In cmplete contrast to m t other supported catalyst systems, the catalyst in olefin polymerisation must fragment during the reaction (ref. 1). Failure to do so results in the pores b s a u *n g blocked with polymer and the reaction ceasing at very law produdivity. Since the catalyst is not rammed f m the final polymer during manufacture, failure to fragment properly can also lead to wfiesirably large pieces of material in the polymer merm it unsuitable for virtually all applications. H a w e v a whilst it is essential that the catalyst particles break up during polymerisation, they must also be sufficiently strong to resist attrition during prolorqed activation treabnent at high temperature in a fluidid bed reactor. A delicate balance of properties is therefore required in the silica support. In addition to the mecharu'cal strength requirements hwever the role of the silica suppo~3in influercw polymer characteristics is of key importance. It is clear that silica does more than act as an inert support and that aspeds of its structure, in particular porosity, influenoe the activity of the catalyst and control polymer structure in ternrs of molecular weight, molecular weight distrhkion and chain branching (ref. 2). These polymer strucharacteristics in turn dictate the application properties of the polymers, eg, melt flaw, melt elasticity, enviromental stress crack resistance, etc, and hence d e f h its suitability for specific uses.
216 SIILCA
m-
synthetic m@ous silicas may be arbitrarily divided into gels and precipitates, although in many cases it is not easy to classify the material on the basis of their intrinsic properties and division m r e typically reflects the methcd of preparation. Kitherto silica gels have been regarded as rather hard, dense materials with a well-defined pore structure, whereas precipitates were typically loose, voluminous materials with less easily defined structural characteristics. However, silica technolcgy has advanced to the point where both types of product can now be prepared f m routes previously considered to be either precipitate or gel manufacturing methcds. Both gels and precipitates are carmmercially manufacture3 by neutralisation of scdium silicate with acid. The technology for manufacturing silicas is dictated by the polymerisation rate/@ curve (ref. 3 ) . The rate passes through a maxinnrm at pH7 and control is difficult under these conditions. In practice therefore cammercial silicas tend to be prepared at either low or high pi. Gels are conventionally prepared at lcw pH, high concentration and law temperature, whenxis precipitates are typically produced at high pi, lcwer concentrations, higher temperatures and often in the presence of added electrolyte. Generally gel mufacture is a semi-continuow operation whereas precipitates are prepared in batch stirred tanks. The silicas used in polymrisation catalysts tend to be derived fmm the gel route where the specific particle size and strength requirements are more easily obtained. The silicas needed for gas and slurry phase reactors typically have particle sizes of 20-9Op and 50-18Op respectively. precipitated silica products are generally less than 30p. Additionally the silicas used must have a high dqree of t h d stability to withstand the high pretreatmnt temperatures required during activation of the catalyst (ref. 4). Amorphous silicas are metastable with respect to crystallisation and structural collapse, a process catalysed by alkali m e t a l ions (ref. 5). !the requisite law soda levels are more difficult to achieve with precipitated silicas as a consequence of the high preparation pH. A wide range of silica structures can be prepared via the gel route with surface areas in region 50-1000 m2g-l and pore volumes 0.4-3 ~3n~g-l.The main stages of gel preparation are illustrated in Fig 1. All the preparation steps influence the structure of the final silica prcduct. sodium silicate and acid are mixed at pH v)
200
I
n
S(BET) = 770 Vpore = 0.43
0)
n' I-
v)
-0
200 -
0.5
0.0
400
m
>
.
'
.
.
I
.
.
*
0.5
J
I
I)
-
.
1.o
0.5
0.0
Nitrogen isotherms of Samples A-D. %BET) in m2/g and Vpore, defined as the total pore volume a t p/po=O.98, in cdg.
0
20
40
60
80
100
FIG
1.o
200 0 0.0
FIG.
V-
600 -
v)
v
S(BET) = 530 Vpore = 0.31
' D 1000 . S(BET) = 197 800 - Vpore = 1.4
h
8
2oo[ 100
1.o
0.5
0.0
300
Pore Size Distributions for Samples C and D. Distributions were derived from the desorption branches of hysteresis.
1.o
243
500
P a !cn
300
V V v
-
I
'
-
.
0
400
0.5
1
1.o
0
.
I
.
B
-
-
100 -
2
m
*
200
v)
-CJ
h Icn
.
400 -
h
h
"
0
0
0.0-
/+. "
"
.
I
"
"
1
.
&) = 265
0 0
v v)
-0
2
FIG. 3
t-plots of Samples A - D. S(t)values in m2/g. The corresponding isotherms are plotted in Figs. l.A-D.
NMR Spectroscopy The observed MAS spectra were composed of three lines which can be assigned to three different structural units Q2, Q3, and Q4, corresponding to Si centers bridged via -0-linkages t o two, three, and four Si nearest neighbors [8]. Typical isomer shift values were -91.4 ppm for Q2, -100.7 ppm for Q3, and -110.0 ppm for Q4. Typical linewidths were obtained as 350 Hz for Q2,480 Hz for Q 3 , and 600 Hz for Q4, respectively. The NMR data are tabulated in Table 1. When the CP-MAS technique is used, the relative strength of the Q2 and Q3 signals is enhanced, due t o the presence of protons near these Si centers. Since the protons exist in hydroxyl groups on or near the surface, the method provides some information of Si sites in this region. In order t o study the dynamics of the magnetization transfer from the protons to the 2% nuclei, the CP-MAS experiments have been performed with variable Hartmann-Hahn contact times. The theory and assumptions of the model 29Si
244
employed are described in detail in Ref [13]. Usually the cross polarization dynamics is described within the framework of spin thermodynamics [141, which predicts a n exponential rise in S-spin magnetization in the rotating frame with contact time. The measured time evolution of the 29Si magnetization was characterized by a n oscillatory contribution, with amplitudes that varied from site to site [13]. The oscillations observed in the experiments were analyzed in the framework of the model developed by Miiller et al. [ E l , and a combined model formed to provide a description of the time dependence of the magnetization M(t) [13]. Omitting oscillatory contributions in the present discussion for brevity, the cross polarization dynamics is described by the equation:
-
where M, = M( t+ -, TIp+ ) is the maximum magnetization achievable in the absence of spin-lattice relaxation, h = TIs/rlp , T i p = spin-lattice relaxation time, TIS = spin-difision time from proton I-spin to 29Si S-spin (=TSiH in Table 1). Where no decrease in magnetization was observed, TI,, was set to infinity. Since the spin diffusion times can be related to the distance between the coupled spins [141, i.e., T 1 s - 1 rIS-6, ~ then these constants are useful for inferring distance relationships between the 29Si centers and 1H centers in the silica structure. The parameter values are summarized in Table 1, showing M, values for the three Q n structural units in samples A-D. Sample B is the only investigated system where a long time decrease of the 29Si magnetization could be observed. The values of the time constant T i p are finite for all sites in this sample.
DISCUSSION The formation of clear, transparent gels as seen with A and B indicates the presence of weakly cross-linked polymeric chains, since they do not scatter light. The similarity in form of the isotherms for samples A and B would indicate similar structure. Thus, the primary difference between the two is the reduced surface area and pore volume of B. The base-catalyzed sample C exhibited features of a colloidal gel. Under basic conditions, gelation (network-forming) reactions are favored over the hydrolysis reactions and the resulting sol species are more clusterlike 141. The packing of the separate colloidal entities is more open than the interwoven, weakly cross-linked strands of the structure from acid hydrolysis. The cluster-cluster contact is also more mechanically rigid. Upon solvent removal, the capillary forces from the receding liquid collapse the weak structure. Drying of the acid-hydrolyzed gel A a t 870 K leads to further dehydration, manifested by condensations between neighboring hydroxyl groups, and forming more siloxane bridges. This is quite clear from the Mo data (Table l), which show decreases in the relative amounts of Q2 and Q3 units and increases in the Q4 units,
245
TABLE 1 Parameters characterizing the 29Si NMR spectra of the investigated silica gels. Sample
A acid-hydrolyzed TEOS,dried 390 K
B
Structural Mo [%I Mo [%I TsiH type MAS CP/MAS [ms] 7.3 36.0 56.7
7.9 52.4 39.7
5.0 6.8 15.0
2.6 18.0 79.4
5.2 38.8 56.0
8.3 8.8 16.4
Q3 Q4
0.7 14.0 85.3
6.2 35.0 58.8
0.8 1.0 12.2
Q2 Q3 134
3.9 17.0 79.0
9.9 39.7 50.4
0.5 0.6 1.2
Q2
Q3 Q4
acid-hydrolyzed TEOS, dried 870 K
Q2
base-hydrolyzed TEOS, dried 870 K D Aerosil200, dried 393 K
Q2
C
Q3 Q4
i.e., increased cross-linking with drying temperature. The fraction of cross-linked Si centers is even higher in the base-hydrolyzed gel, dried at 870 K , although a low temperature dried sample is not available for comparison. The CP-MAS technique sheds light on the nature of the hydroxyl groups. A t short contact times, Si atoms with directly bound hydroxyl groups (Q2 and Q3) have the largest intensity , whereas a t longer contact times, the Q4 signal also increases as Si nuclei which are removed a t least four bonds from the nearest hydroxyl group become polarized. Thus, the values of TSiH increase from Q2 to Q4 . In the acidhydrolyzed gels, the loss of pore volume is reflected in a significant increase of T s i ~ for all three Si structural units. In contrast to the acid-hydrolyzed gels, the basehydrolyzed gel is characterized by very short TsiH constants. The constants are even shorter than those of the 393 K-dried acid gel, indicative of a large proton reservoir. Even though the degree of cross-linking, as reflected by the number of Q4 sites, is significantly higher than in the acid-catalyzed gels, the surface hydroxyl groups appear to be sufficiently close to promote fast cross polarization. The large difference between the TsiH values for Q4 (12 ms) on the one hand, and Q2 and Q3 (-1 ms) on the other hand is indicative of the differences between sites in the interior (Q4) and on the surface (Q2, Q3) of the compact aggregates formed by basic hydrolysis. 29Si parameter values for the Aerosil sample D indicate the high degree of cross-linking (high percentage of Q4), as expected of the high temperature flame hydrolysis that produces the non-microporous “bulk silica” spheres. With very
246
short cross-relaxation times, it resembles the base-hydrolyzed gel more closely than the acid-hydrolyzed gels. CONCLUSIONS Silica gels of different pore structure were prepared by the sol-gel route, using tetraethoxy orthosilicate as a precursor. From 2% solid-state MAS NMR measurements, the relative abundance of the different Si sites (Q2, Q3, and Q4) has been determined. The degree of dehydration and cross-linking in the gels increases when the drying temperature is raised from 390 K to 870 K. At 870 K, the fraction of cross-linked Si centers is higher in the base-hydrolyzed than in the acid-hydrolyzed silica gel. In CP-MAS measurements, large differences in cross-polarization times TsiH exist between the acid and base-hydrolyzed gels. In a n acidic medium, many of the surface hydroxyl groups are protonated and are desorbed as water upon drying a t 390 K. This leads to long TsjH times. Loss of water is greater at 870 K, reflected in even longer TSiH times. In contrast, hydrolysis in a basic medium results in stable clusters which apparently retain their surface hydroxyl groups upon drying at 870 K. ACKNOWLEDGEMENTS Financial support of this work by ETH and the Deutsche Forschungsgemeinschaft (SFB 213) is kindly acknowledged. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14.
15.
D. Ulrich, J. Noncrystalline Solids, lOO(1-3) (1988) 174 P.A. Haas, Chem. Engr. Prog., April 1989 B.E. Yoldas, J. Mater. Sci., 14 (1979) 1843 C.J. Brinker, J. Noncrystalline Solids, lOO(1-3) (1988) 31 M. Guglielmi and G. Carturan, J. Noncrystalline Solids, lOO(1-3) (1988) 16 C.J. Brinker and G.W. Scherer, in L.L. Hench and D.R. Ulrich (Eds.) Ultrastructure Processing of Ceramics, Glasses, and Composites, Wiley, New York, 1984, Chapter 5 Stober, W., A. Fink and E. Bohn, J Coll. and Int. Sci., 26 (1986) 62 G. Engelhardt, High Resolution Solid State NMR of Silicates and Zeolites, Wiley, New York, 1987, p. 76 E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Am. Chem. SOC.,73 (1951) 373 P.J.M. Carrott and K.S.W. Sing, in K.K. Unger et al (Eds.) Characterization of Porous Solids, Elsevier, Amsterdam, 1988, pp. 77-87 K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure & Appl. Chem., Vol. 57, No.4, 1985, pp. 603-619 E. Wagner and H. Briinner, Angew. Chem., 72( 19-20) (1960) 744 K.L. Walther, A. Wokaun, and A. Baiker, submitted for publication M. Mehring, High Resolution NMR Spectroscopy in Solids, Springer, 1976, p. 138 L. Miiller, A. Kumar, T. Baumann, and R.R. Ernst, Phys. Rev. Lett., 32 (1974) 2402
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
241
ASPECTS OF THE SYNTHESIS OF ARYL SULFONIC ACID M E L P CATALYSTS
DAVID L. KING, MICHAEL D. COOPER, WILLIAM A. SANDERSON, CHARLES M. SCHRAMM, and ERE D. FELLMANN Catalytica, Inc., 430 Ferguson Drive, Mountain View, California 94043 (USA) ABSTRACT Tetravalent metal phosphonates (MELSB) are layered compounds that can be considered organichnorganic polymers. The organic functionality provided by the phosphonate largely determines the surface chemistry of these materials. Choosing phosphonates with an arylsulfonic acid function produces materials that are effective as acid catalysts. This paper describes synthetic procedures used to prepare arylsulfonic acid MELS and methods to increase and stabilize their surface area, porosity, and swelling properties. Acid catalysis by MELS is exemplified by the synthesis of methyl tertiary butyl ether. INTRODUCTION Tetravalent metal phosphonates, or MELS (for Molecularly Engineered Layered Structures), provide a novel class of materials that combine many of the properties of inorganic metal oxides with the organic functionality more commonly found in functionalized polymeric resins. Early development work on these materials was carried out by Alberti and co-workers [ref. 11 and Dines et al. [ref. 21. Synthesis and characterization of related zirconium phosphates that also contain phosphonate groups as pillars have been described by Clearfield [ref. 31. There is a substantial patent estate for tetravalent metal phosphonates, and exclusive rights to this estate are owned by Catalytica [ref. 41. Despite the significant work reported in the literature for these materials, there are few examples of the synthesis of zirconium phosphonates containing pendant acid groups and their utilization as ion exchangers or as solid catalysts [refs. 5-81. The ability to incorporate specific pendant functional groups into MELS provides opportunities for the development of several classes of novel solid acid catalysts. This paper provides an introduction to MELS materials, a description of synthetic methods used to produce solid MELS catalysts containing pendant sulfonic acid groups, and a discussion of some of the physical and catalytic properties of these materials.
248
STRUCTURAL CONSIDERATIONS The structure of alpha zirconium bis(monohydrogen orthophosphate), Zr(03POH)2.H20, has been determined by Troup and Clearfield from single crystal work [ref. 91. This structure forms the basis for the structures of zirconium and other metal phosphonates. As shown in fig. 1, zirconium phosphate is a two-dimensional layered structure, with an octahedral coordination of oxygen surrounding the metal ion and tetrahedral coordination surrounding phosphorus. Each Zr is connected to a neighboring Zr through an 0 - P - 0 linkage; the phosphorus is connected through three oxide linkages to three different Zr ions, with the remaining phosphorus bond projecting perpendicular to the plane of the layer. It is this last bond, which is P-OH in zirconium phosphate and P-R in zirconium phosphonate, that leads to the unusual and varied surface properties of the MELS compounds. OH
OH
I
OH
I
H@
OH
Ii
7.5
I
Fig. 1 . Structure of alpha zirconium phosphate (from ref. 2). (a) Side view; (b) top view. The structure shown in fig. 1 is idealized for a crystalline material. The ordered stacking of these layers occurs with a fixed interlayer spacing responsible for the characteristic large d-spaced line in the X-ray diffraction pattern. The d-spacing (7.5 A for a-zirconium phosphate) varies with the nature of the material, with smallest spacing achieved with a P-H (phosphite) group (5.7 A) and extending to greater than 35 8,for long-chain alkyl groups (>CI2). The d-spacing is consistent with that predicted by molecular models.
249
Materials with a less well-defined X-ray pattern are frequently formed during synthesis. These materials are semicrystalline and are likely to have similar structure but comprise much smaller crystallites that may stack irregularly. An example of diffraction patterns for crystalline and semicrystalline zirconium phenylphosphonate is shown in fig. 2. A key parameter in MELS is the lateral spacing within the layer between adjacent P groups, since this defines the density of groups on the surface. For zirconium phosphate, the minimum
distance is 5.3 A, leading to an effective cross section of 24 A2 [ref. 101. This value probably holds true for zirconium phosphonate as well. The spacing is less for titanium phosphonates and greater for MELS derived from heavier metals such as thorium or uranium. In some cases where the phosphonate group is bulky, this difference in lateral P-to-P spacing may have a significant
effect on whether a MELS can be formed. It may also explain the differences in rate of formation and ultimate crystallinity that can be obtained with certain bulky phosphonates and differing metals; heavy metals such as Th typically form more crystalline samples [refs. 11, 121. This difference in spacing can also serve to explain the benefits of adding a phosphate group or a second (less bulky) phosphonate group to obtain a more structurally stable material, described later. The P-to-P distance also defines the interlamellar void space created by materials in which the layers are linked by pillaring groups that spread apart adjacent layers to a fixed distance [ref. 131. SYNTHESIS OF MELS Svnthesis from Precursors The synthesis of metal phosphonates is typically achieved via a precipitation reaction, where two soluble precursors are mixed, typically in aqueous solution, to produce an insoluble product. Any tetravalent metal ion can be utilized that can accommodate an octahedral coordination environment, such as Zr, Ti, Sn, Ce, Th, U. Most reported work employs zirconium as the metal, due to its availability, easy formation of products, and moderate cost. Other metals may be chosen for specific properties; i.e., to provide larger intralayer spacing. For synthesis of zirconium phosphonates, typical precursors such as zirconium oxychloride, ZrOCI2.8H2Oor zirconium sulfate, Zr(SO&. 4H20 can be used. The source of the zirconium precursor can influence the time and conditions required to synthesize a material having the required z r ( 0 ~ P Rstoichiometry. )~ This is because the different ligating ability of zirconium by chloride, sulfate, and other anions influences the competition by the phosphate or phosphonate [-P(O)(OH)2] group for the metal ion. The phosphorus source is typically a phosphonic acid, delineated generically by (HO)zP(O)R, although analogs such as phosphonate diesters (R'OhP(0)R and dihalides X2P(O)R also may be utilized. Using diphosphonic acids (HO)2P(O)R'P(O)(OH)2 in the precipitation reaction results in pillared materials where the layers are attached at fixed distances [refs. 3, 11-13].
250 PS
4100.0 2690.01200.09870.08460.07050.05640.0-
4230.02820.0
-
1410.0-
20
20
Fig. 2. X-ray diffraction patterns for (a) crystalline and (b) semicrystalline zirconium phenylphosphonate.
251
Crystalline Phases The initially precipitated semicrystalline product is generally refluxed for a period of several hours, followed by filtration or cenmfugation and washing with water to remove impurities such as residual chloride or sulfate. This material is typically amorphous or poorly crystalline as determined by X-ray diffraction. In the case of zirconium phosphate, more crystalline material can be prepared by reflux in excess phosphoric acid [ref. 141, or by using fluoride in the synthesis mixture [ref. 151. For MELS, using excess phosphonic acid does not appear to have similar effect in enhancing crystallinity, and the HF addition method generally is employed to provide a more crystalline material. It is thought that complexation of the metal ion by fluoride is responsible for a slow release of the metal ion to solution, enhancing formation of crystalline materials. Exceptions to this method of producing crystalline samples occur when the final solid is substantially water soluble, for example with sulfonic acid-based materials, described later. Such materials have defied attempts to prepare them in crystalline form. Syntheses Involving More Than One Functional Group One of the attractive features of the metal phosphonates is that they can be tailored for a specific application through modification of the organic functionality of the layer. More than one functional group can be added during the synthesis step to provide greater flexibility of surface properties such as hydrophobicity or hydrophilicity or to provide for more than one chemical function at the surface. MELS containing more than one functional group are typically prepared by simultaneous addition of two (or more) phosphonic acids to the initial synthesis mixture [ref. 161. Solids formed typically contain both functional groups in approximately the concentration given in the original synthesis mixture provided it is stoichiomemc (P/zr = 2), although quantification by methods such as NMR provides a useful confirmation. With such materials, the distribution of the two functions at the molecular level has not been established, but a random distribution seems likely. Attempts by Alberti [ref. 171 to produce crystalline solids containing two or more functional groups without phase segregation have met with some success. Supporting evidence comes predominantly from X-ray diffraction,which reveals that the composite solid contains neither of the single component phases. Alberti suggests that crystalline materials comprising two different functional groups may alternate between being enriched in one function on one side of the layer and enriched in the other function on the opposite side of the layer. Creating layered crystalline materials having a uniform spacing or regular distribution of dissimilar functional groups across the surface has remained an elusive goal. Actual concentrations of the two components within the final solid may differ substantially in the final crystalline product compared to the precursor mixture, if fluoride is used to enhance crystallinity and the solution is not taken to dryness. At high concentrations of both components, phase segregation may occur; a single phase appears to most readily f o m when one of the components is present in significantly higher concentration.
252
Characterization of Short -Ranee Order in MELS by Solid State NMR Techniaues
Good quality X-ray diffraction data for solid metal phosphonates is typically obtained from crystalline samples; this provides incentive for preparation of such materials. However, many practically useful materials are X-ray amorphous. For such materials, solid state NMR is a very useful technique, because it provides information on short-range rather than long-range order. Combining the techniques of cross polarization (CP) and magic angle spinning (MAS) allows us to obtain spectra with excellent resolution of MELS materials. Both 3lP and l3C NMR are useful for characterizing MELS materials. These techniques include the ability to: 1. describe the bonding between phosphorus and the metal ion. The chemical shift of the phosphorus resonance allows one to distinguish between the free acid, a phosphonate salt, or a MEiLS. We have observed that the isotropic chemical shift of the phosphorus is approximately 30 ppm upfield in a MELS relative to the free acid. The sign of the phosphorus chemical shift anisotropy reverses, changing from negative to positive when going from the free acid to a MELS. The static (no MAS) 31PNMR spectra of the phosphate, hexyl, and phenyl MELS are shown in fig. 3 and clearly demonstrate the dramatic change in the spectrum associated with MELS formation. Additionally, the relaxation behavior and cross polarization efficiencies of these forms are quite different and can be used to further characterize these materials. 2. detect phosphorus-containing impurities in the structure. A phosphorus impurity in MELS may occur, for example, if the original phosphonate source is impure or when subsequent chemistry is carried out on the solid metal phosphonate. A common impurity is phosphate. 3 . detect and quantitate the concentrations of different phosphorus groups when more than one group is present or has been used in the synthesis. For accurate quantitation, cross polarization is not used, since it may result in incorrect intensities being measured. For quantitative measurements, spectra are obtained with MAS using only single pulse excitation. We have found that the concentration of phosphonate groups in the final material is not always the same as the concentration of the initial solution precursors. 4 . utilize carbon NMR to verify the organic composition of the material. In the case of pendant groups such as aryl rings, carbon NMR can sometimes distinguish between ortho, meta, or para substitution and indicate mono, di- or m-substitution on the ring. In the case of the “one-pot’’ materials described later, NMR analysis gives unique evidence for the presence of both aryl and alkyl groups in the solid, leading to the elucidation of the structure.
253
MELS
Acid
A
Phosphate
I . d L I - L J _ I L _ L L L I.L_._I-dLIL-I-LILl-#L
B
80
80
40
20
0
-20
-40
-60
-00
M 80
80
40
20.
0
-20
-40
-60
-00
PrtA
Hexyl
-l--L-l-I-l-l-l-.L.l-l-uL-
80
80
40
20
0
PPh4
-20
-40
-60
-80
_1I.ILL_LLLI__(_I-LI_LI__I__IL
80
60
40
20
0
-20
-40
-60
-00
PPhl
C
Fig. 3. Static 3 l P NMR spectra comparing free phosphonic acid and corresponding zirconium phosphonate: (a) phosphate, P-OH; (b) hexyl, P - C ~ H I ~(c) ; phenyl, P-C6H5.
254
SYNTHESIS OF ACIDIC MELS AND PRECURSORS Variation in Aciditv through Choice of Functional Group A wide range of acidic materials can be synthesized by the choice of the phosphonate pendant R group. The inorganic "end member" of a series of acidic materials is zirconium phosphatean ion exchanger [ref. 181 and weak acid catalyst. The acidity of this group has been demonstrated by catalysis of reactions such as isomerization of cyclopropane and linear butenes [ref. 191, dehydration of cyclohexanol [ref. 201, and dehydration and decomposition of methanol and ethanol [ref. 211. Organic functions expand the range of acidity that can be incorporated into the structure from weakly acidic carboxylic acids to strongly acidic sulfonic acids to very strongly acidic perfluoroalkylsulfonic acids. Materials of each type have been prepared at Catalytica and evaluated for catalytic activity. Substantial and ongoing work has been done to prepare MELS acid catalysts containing arylsulfonic groups, some of which is described below. Synthesis of SulfoDhenvlDhosDhonicAcid Zirconium sulfophenylphosphonate can be prepared by the reaction of a water soluble zirconium salt with sulfophenylphosphonic acid. This phosphonic acid has been described in patent literature [ref. 221, but there is no evidence of its actual synthesis. We found that sulfophenylphosphonic acid can be synthesized by the reaction of phenylphosphonic dichloride with ClS@H or SO3. The reaction of phenylphosphonic dichloride with excess ClS@H proceeds smoothly to m-sulfophenylphosphonic dichloride at 150 OC and to the phosphonic acid on subsequent hydrolysis. Purification of the acid requires removal of excess sulfate by barium precipitation, followed by ion exchange to remove excess barium. For a sulfate-free synthesis, we tried the direct sulfonation of phenylphosphonic acid with liquid S@. The sulfonation proceeds readily at 125 'C, with excess SO3 relative to stoichiometry. We found that 1:l ratios of S@:phenylphosphonic acid are insufficient for complete sulfonation, even under forcing conditions, due to the competitive formation of mixed anhydrides of sulfophenylphosphonic acid and SO3, depicted in fig. 4. This competitive formation results in a consumption of greater than 1 mole of S@ per mole of phenylphosphonic acid. These anhydrides are thermally stable but may be converted to sulfophenylphosphonic acid by hydrolysis; however, this method also requires sulfate removal from the final product. 0
0
0
II/
0
0
OH
Fig. 4. Picture of possible mixed anhydrides formed by SO3 treatment of phenylphosphonic acid.
255
The 31P and l3C NMR spectra of sulfophenylphosphonic acid is shown in fig. 5(a). The phosphonate 3lP resonance is typically downfield by 18 ppm relative to phosphate. The carbon distribution pattern is consistent with meta bonding of the sulfonic acid group relative to the phosphorus-bound carbon.
A
B
Fig. 5. (a) 31Pand 13C NMR spectra of sulfophenylphosphonic acid (in dimethylsulfoxide solvent); (b) 3lP and l3C NMR spectra of disulfophenylphosphonic acid (in dimethylsulfoxide solvent).
256
Svnthesis of DisulfoohenvlphosDhonicAcid and Related Acids Under more forcing sulfonation conditions, disulfophenyl phosphonic acid can be prepared from phenylphosphonic acid [ref. 231. Preferred preparation conditions use 70% oleum as the reagent at 250 'C. The 31P and 13C NMR spectrum of disulfophenylphosphonic acid is shown in fig. 5(b). Based on the l3C NMR spectrum, the substitution pattern on the aromatic ring is 1,3,5. The titration curve for this acid is shown in fig. 6, which depicts the anticipated 3:l strongacidweak-acid distribution. Biphenyl p,p'-diphosphonic acid can also be sulfonated under forcing conditions to introduce a maximum of one SO3H group per aromatic ring. Double sulfonation on each ring may be inhibited by steric effects between groups on adjacent rings.
Base Equivalents (arbitrary units)
Fig. 6. Titration curve for disulfophenylphosphonic acid. Svnthesis of MELS Containing Aromatic Sulfonic Acid Groups (i) Svnthesis from solution Drecursors. The reaction of sulfophenylphosphonic acid with a water soluble zirconium salt such as zirconium oxychloride produces zirconium sulfophenylphosphonate [ref. 71. The material is sufficiently hydrophilic that a solid does not precipitate from aqueous solution. Evaporation of the solution to dryness produces a glassy solid which is shown by NMR to be MELS. The glassy solid has a very low surface area and is not useful practically as a catalyst unless it is allowed to swell in the solvent medium. This occurs readily in polar media
251
but finds practical limitations in fixed-bed or other reactor configurations where a dimensionally stable solid is necessary. Addition of a second phosphonate or a phosphate function to the solution containing the metal and sulfophenylphosphonic acid allows the preparation of an acidic solid that is recoverable and filterable. The mechanical integrity and surface area increases with the amount of the second function, at the expense of acid titer. (ii) Svnthesis bv sulfonation of zirconium phenvlphosphonatc. Zirconium sulfophenylphosphonate can also be prepared by direct sulfonation of zirconium phenylphosphonate [refs. 6,7]. Typical sulfonating agents include SO3, CIS03H, and oleum. Reaction of phenyl MELS in excess oleum (typically containing 1%24%SO3) at 60-70 OC results in complete monosulfonation of the aromatic rings. Under synthesis conditions the solid zirconium sulfophenylphosphonate MELS appear to dissolve. However, quenching the reaction mixture with water produces a recoverable solid. Upon further washing to remove entrained sulfuric acid and purify the product, loss of solid product is noted, indicating some level of solubility of the sulfonated material in water. By methods analogous to those described for the preparation of zirconium sulfophenylphosphonate from solution precursors, a mixed material can be prepared that maintains its mechanical integrity. This can be accomplished by sulfonation of a material containing two functions (e.g., phenyl and alkyl), or by addition of more zirconium and phosphonic (or phosphoric) acid to the synthesis mixture following the sulfonation and quenching steps. In general, the use of a second phosphonate moiety provides reduced solubility of the sulfonic acid species, at a loss of acid titer. Disulfophenylphosphonate may be used to increase the titer.
CHARACTERIZATIONOF ACIDIC MELS Titration of the phosphonic acid is generally a useful procedure for determining purity of the phosphonic acid and the resulting MELS. Sulfophenylphosphonic acid titrates as two strong acid equivalents and a single weak acid equivalent, producing a 2:1 titration curve, or a 3: 1 curve in the case of disulfophenylphosphonic acid, as demonstrated in fig. 6. Impurities (e.g.. presence of phosphate or sulfate) produce departures from this curve. A titration of pure zirconium sulfophenylphosphonate produces a curve having a single break, with an equivalence point in good agreement with the theoretical value of 3.6 meq/g. If the titration is continued beyond the neutralization of the sulfonic acid sites, a flattening of the titration curve occurs, a result of decomposition of the MELS in alkaline solutions. Thermal stability of the sulfonic acid MELS can be obtained from thermogravimemc analysis under either air or nitrogen. A comparison of TGA curves under air for zirconium phenylphosphonate and zirconium sulfophenylphosphonate is provided in fig. 7. Phenyl MELS has high thermal stability, which increases further with increasing crystallinity of the sample. The
258
thermal stability of the sulfonic acid containing MELS is significantly lower. The decrease in thermal stability is due to the decomposition of the sulfonic acid moiety; nevertheless, the thermal stability of the sulfophenyl MELS significantly exceeds the thermal stability of arylsulfonic acid ion exchange resins.
Fig. 7. Thermogravimetric analysis in air of (a) phenyl and (b) sulfophenyl MELS. Hydrothermal stability of the arylsulfonic acid MELS is of practical commercial interest. A significant drawback of the arylsulfonic acid ion exchange resins as catalysts is their relatively poor hydrothermal stability, which restricts the useful operating range of these materials for reactions in polar media (for example, olefin hydration reactions [ref. 243). A comparison of the hydrothermal stability of MELS and an ion exchange resin is provided in fig. 8. Samples of an arylsulfonic acid MELS and Amberlyst 15 were steamed at 200 O C for increasing amounts of time. Desulfonation of both materials was followed by integration of the resonances of the sulfonated and nonsulfonated aromatic carbons in their 13CCPMAS NMR spectra. The hydrothermal stability of the arylsulfonic acid MELS compound is clearly demonstrated. No desulfonation of the MELS compound is observed over a 30-hour time span, while the resin is more than 30% desulfonated. This greater stability is probably due to the inductive effects provided by the presence of the phosphorus group bound to the aromatic ring, and the meta positioning of the sulfonic acid group relative to the phosphonate group. The net result is a reduced tendency toward desulfonation in the presence of steam.
259
110 100
.-
9
.Y
C 2
80
$
s
70
-
60
-t-
Sulfophenyl MELS
-M-
Arnberlyst 15
50
0
5
10
15
20
25
30
35
Hours at 200% (Steam) Fig. 8. Hydrothermal stability comparison of sulfophenyl MELS and Amberlyst 15 (see text).
ONE-POT PREPARATION OF HIGH SURFACE AREA, POROUS MATERIALS CONTAINING SULFONIC ACID GROUPS One-Pot Svnthesis of MELS Containing Aromatic Functionality The synthesis of aryl sulfonic acid MELS derives ultimately from the precursor phenylphosphonic acid, which is combined with the metal ion either before or after sulfonation-to make the solid acid. Preparation of a pillared material containing sulfonic acid groups provides threedimensional stability to the solid and reduces swelling in polar media. Preparation of this latter material requires a diphosphonic acid precursor, either aryl or alkyl. Aryl pillars are desirable due to their greater rigidity, thermal stability, and their potential to be sulfonated, thus adding to the acid titer. Phenylphosphonic acid is commercially available, albeit expense. (Po1y)aryldiphosphonic acids are not readily available from commercial sources. Consequently, developing preparation methods that allow lower cost production of phenylphosphonic and related arylphosphonic and diphosphonic acids is desirable. Our approach was first to focus upon the synthesis of phenylphosphonic acid. This material is prepared from the reaction of benzene with PCl3 using a Friedel Crafts catalyst, typically AlCl3, followed by an oxidation and hydrolysis step. The synthetic sequence is shown in fig. 9(a). Although none of the steps require complicated or expensive reagents, the product phenylphosphonic acid is expensive in part due to the cost of purification and separation of the
260
AlC13 or its hydrolyzed product from the synthesis mixture. We postulated that known Friedel Crafts catalysts of alternate metal chlorides could be used in place of AlC13, preferably metal
halides that could be converted into MELS . Examples of such Friedel Crafts catalysts include ZrC4 and Tick. Subsequent oxidation and hydrolysis steps could then follow to produce Zr or Ti phenylphosphonate, thereby eliminating the need to remove the catalyst from the mixture [ref. 251. The general scheme is depicted in fig. 9(b).
AICI,
-0 1 1 1
PCI,
I
I
+
HCI
+
HCI
Oxidation
Hydrolysis
AIC13
removal
.
ZrCI,
1. Oxidation
2. Hydrolysis
Fig. 9. (a) Synthesis of phenylphosphonic acid; (b) one-pot synthesis of zirconium phenylphosphonate.
261
Several approaches based on this synthetic theme have been pursued. The reaction of PCl3 with ZrC4 in excess benzene proceeds at 85 'C. Reaction progress is evidenced by evolving HCI. The product can be oxidized to the phosphonate precursor by various means; chlorosulfonic acid proved to be a convenient reagent for the oxidation. Subsequent hydrolysis of the rather glassy resulting solid produced zirconium phenylphosphonate. Equivalence of this material with the conventionally prepared analog was verified by l3C and 3lP MAS NMR. Use of T i c 4 as the catalyst for reaction of PCl3 with benzene, on the other hand, showed no evidence of reaction (no HC1 evolution), indicating that T i c 4 is insufficiently active as a Friedel Crafts catalyst for this reaction. Solvent-mediated One-Pot Svnthesis of Pillared MELS We then explored using a solvent to make the ZrC4-phenylphosphonate oxidation product more tractable, since hydrolysis of the glassy material resulting from the oxidation step was difficult and time consuming and would not be practical in commercial synthesis. The 1,2dichloroethane was found to be a useful solvent to dissolve the oxidized product. Hydrolysis then proceeded rapidly, necessitating the use of ice or water-ice mixtures to control temperature. We next explored using 1,2-dichloroethane solvent throughout the reaction; i.e., replacing excess PC13 or benzene previously used as solvent medium. We expected this to provide a more physically manageable reaction mixture and to allow the subsequent hydrolysis to proceed as readily as before. The reaction of benzene, PCl3. and ZrC4 proceeded readily in the presence of dichloroethane, but yields of product solids were greater than 100%. Characterization of the final material by l3C CPh4AS NMR produced an unexpected result. The spectrum of the product is shown in fig. 10(b); it is distinctly different from the spectrum of phenylphosphonate MELS shown in fig. lO(a). The resonance at 142 ppm indicates that the aromatic ring is disubstituted, and that alkyl (at ca. 30 ppm) and aryl carbon groups were observed. Elemental analysis of the final product verified the absence of chlorine, suggesting that the aliphatic structure was not due to entrained solvent. Thus, the alkyl groups must have derived from the solvent dichloroethane that participated in the reaction sequence and ended up as an allcyl bridge between aryl rings from adjacent layers. The NMR data are indicative of predominantly para substitution on the aromatic ring. Thus, during the PC13-benzene reaction, an additional reaction has occurred: the Friedel Crafts alkylation by the solvent to produce a pillared bibenzyl group. Analysis of the material by XRD showed the presence of a weak, high d-space line consistent with the spacing between the layers from an axyl-ethylene-arylgrouping. Extension of this approach to other chlorinated solvents is possible and leads to a range of pillared materials where the interlayer spacings can be quite large. As an example, the 13C CPMAS NMR spectrum of the 10-carbon bridged, biphenyl-pillared MELS is shown in fig. 10(c). Again, XRD confirms a large d-spacing for this material. A xylyl-bridged, biphenyl-pillared MELS was similarly prepared from the corresponding dichloroxylene solvent. The simplified formulas for these three materials synthesized by the solvent-mediated one-pot method are shown in fig. 11.
262
A
Fig, 10. l3C NMR of one-pot phenyl MEiLS prepared using various solvents during the Friedel Crafts synthesis: (a) benzene; (b) 1,2-dichloroethane; (c) 1,lO-dichlorodecane.
263
ethylene-bridged
B
Fig. 11. Formulas for pillared one-pot materials prepared using chlorinated solvents during the Friedel Crafts synthesis: (a) dichloroethane; (b) 1,lo-dichlorodecane; (c) 1,4-di(chloromethyI) benzene.
u
The surface area and pore volumes of fully pillared one-pot materials are relatively low,
typically I 2 5 m2/g and 0.1 cm3/g, respectively, somewhat dependent on the particle size of the sample. The low pore volume is suggestive of a material that has low porosity due to the bulk and density of the pillaring groups. The interlamellar regions may not be readily accessible to solvent or reagent molecules. This was c o n f i i e d by attempts at sulfonation of a fully pillared, bibenzyl one-pot material to generate an acid catalyst, which proved to be quite difficult using either C1S03H or SO3 as the sulfonating agent. Only about 50% of the aromatic rings were sulfonated, even at forcing conditions. As a practical approach to the problem of access to the interior layers, we utilized excess pCl3 in the initial synthesis step (Friedel Crafts reaction of PC13 with benzene in chlorinated solvent) with the idea that upon final hydrolysis the product would contain both aryl-akylene-aryl pillars and smaller P-OH groups. This preliminary step would create interlamellar voids that would allow both better access of sulfonating reagents to the aryl pillaring groups during catalyst synthesis and also good access of chemical reagents during utilization of the material as a catalyst. The excess PCl3 approach proved successful. Both functional groups were present in the final material, as demonstrated by 3lP MAS NMR characterization. The material comprised P-OH and P-phenyl (as bibenzyl pillars) groups in a ratio of 3:1, and had a surface area of 150 m2/g and pore volume of 0.45 cm3/g, indicative of a more open structure. Complete monosulfonation of the
TABLE 1 Comparative performance of arylsulfonic acid-based catalysts for MTBE synthesis.
2.08
Acid Titer (mwg) WHSV (MeOH) Methanol Conversion (%) Productivity, g MTBWg cat-h Activity, m o l e MTBWmeq-s (x102)
N Q,
&
2.30
2.29
3.0
100
33
100
33
100
33
100
33
52
72
21
65
4
12
30
76
143
65
58
59
11
10.9
83
69
21
9.9
7.9
8.1
1.5
1.5
8.7
7.3
265
aromatic rings in the structure was accomplished under standard sulfonation conditions by using SO3 in dichloroethane as the sulfonating agent. CATALYTIC PERFORMANCE OF ARYLSULFONIC ACID MELS We evaluated the catalytic performance of the arylsulfonic acid MELS in a number of reactions, including isomerization of butenes, MTBE synthesis, methanol dehydration, aromatic alkyl'ation, and MTBE cracking. An example of its utilization as a catalyst for MTBE synthesis follows. The synthesis of methyl tertiary butyl ether from methanol and isobutylene is a convenient reaction to study since catalysts are readily tested in a fmed bed at moderate temperatures and pressures [ref. 261. Since sulfonated ion exchange resins are the catalysts of choice for this reaction, use of this reaction provides a convenient comparison of the efficiency of the MELS catalysts relative to ion exchange resins [refs. 7,271. We chose three mixed MELS containing arylsulfonic acid pendant groups to examine the effect of differing second functionalities (-H, -OH, -CH3) on the catalytic properties of the final catalyst. All were prepared by the direct sulfonation of phenyl MELS with oleum, followed by addition of zirconium and the second phosphorus (R') group (H3P03. H3P04, and CH3H2PO3, respectively) to the quenched mixture in proportions to maintain the 2 1 phosphorus:zirconium stoichiometry and approximate 1:l ratios of arylsulfonic: R' functionality. The mixture was refluxed, and the solid was recovered, washed free of residual sulfuric acid, and dried. Prior to using these materials as catalysts for MTBE synthesis, the materials were subjected to extraction with hot methanol at 62 OC for 48 h to remove any residual soluble (phosphonic) acid species. Amberlyst 15 was pretreated by ion exchange with 1N HC1 followed by washing with distilled water and drying at 110 "C. The MTBE synthesis reaction was carried out in a fixed bed reactor at 60 OC, 110 psig, and a methano1:isobutylene molar feed ratio of 1.21. A small amount (1 mole %) of n-heptane was added to the feed as an internal standard. Analysis of the reactor effluent was carried out by gas chromatography. Under the reaction conditions, MTBE is virtually the only product observed; C4 olefin dimers are observed only in trace amounts. Due to difficulties in reliably quantitating methanol, formation of MTBE was used in the quantitation of catalyst activity and methanol conversion. Table 1 provides a comparison of the activity of the three MELS sulfonic acid catalysts and Amberlyst 15 sulfonic acid resin at two different space velocities. The activity observed with Amberlyst is in good agreement with published literature [ref. 281. It is clear that the activity of the sulfophenyl MELS containing the phosphate second function is more active than the catalyst containing the phosphite second group, and both are substantially more active than the catalyst containing the methyl group. Thus, the second functional group can affect overall catalytic performance. We believe that the differences in activity experienced between the different catalysts reflects both variations in the hydrophilicity and in the swelling properties of the catalyst. These
266
materials are typically low surface area, gellular solids in the dry state. Their ability to swell allows access of reactants to the internal sulfonic acid sites. The catalyst productivities for the phosphate-based MELS and the Amberlyst 15 on a pergram catalyst basis are comparable at the lower space velocity, but activity and productivity is clearly greater for the phosphate-containingh4ELS at higher space velocity where methanol conversion is lower and the system is farther from thermodynamicequilibrium. The differences may reflect effects of a MTBE-rich versus methanol-richreaction medium, the swelling nature of MELS versus the more rigid macroreticular resin, and differences in activity. It is informative to compare the activity of MELS with Amberlyst on a per-acid-site basis, also shown in Table 1. Assuming comparable site accessibilitybetween the phosphate-containingMELS and Amberlyst 15, the sulfonic acid sites of the phosphate/sulfophenylphosphonate MELS catalyst demonstrate greater turnover rates than the acid sites of Amberlyst 15. REFERENCES 1. 2. 3. 4.
G. Alberti, U. Constatino, S. Allulli, and N. Tomassini, J. Inorg. Nucl. Chem., 40 (1978) 1113 M. B. Dines and P. DiGiacomo, Inorg. Chem., 20 (1981) 92; M. B. Dines and P. C. Griffith, J. Phys. Chem., 86 (1982) 571. B. Z. Wan, R. G. Anthony, G. Z. Peng, and A. Clearfield, J. Catal., lOl(1) (1986) 19. Representative patents include: P. M. DiGiacomo and M. B. Dines, US.Patent 4,298,723 (1981); P. M. DiGiacomo and M. B. Dines, U.S. Patent 4,232,146 (1980); V. E. Parziale, M. B. Dines and P. M. DiGiacomo, U.S. Patent 4,373,079 (1983); all assigned to Occidental Research Corporation. P. M. DiGiacomo and M. B. Dines, Polyhedron, 1 (1982) 61. C. Y. Yang and A. Clearfield, Reactive polymers, ion exchangers, sorbents, 5(1) (1987) 13. D. L. King, M. D. Cooper, M. A. Faber, U.S. Patent 4,868,343 (1989) assigned to Catalytica, Inc. D. L. King and P. H. Kilner, Zeolites (Japan), 6 (1989) 19. J. M. Troup and A. Clearfield, Inorg. Chem., 16 (1977) 3311. A. Clearfield and D. S. Thakur, Appl. Catal., 26 (1986) 1. M. B. Dines and P. C. Griffith, Inorg. Chem., 22 (1983) 567. M. B. Dines and P. C. Griffith, Polyhedron, 2 (1983) 607. M. B. Dines, R. E. Cooksey, P. C. Griffith, and R. H. Lane, Inorg. Chem., 22 (1983) 1003. A. Clearfield and J. A. Stynes, J. Inorg. Nucl. Chem., 26 (1964) 117. G. Alberti, U. Constatino, and M. L. Giovagnotti, J. Inorg. Nucl. Chem., 41 (1979) 634; G. Alberti and E. Torracca, J. Inorg. Nucl. Chem., 30 (1968) 317. M. B. Dines, P. M. DiGiacomo, K. P. Callahan, P. C. Griffith, R. H. Lane, and R. E. Cooksey, Catalytically modified surfaces in catalysis and electrocatalysis,in: J. S. Miller, (Ed.), ACS Symposium Series 192, Washington, D.C., 1982, 225. G.Alberti, U. Constatino, J. Kornyei, and M. L. Giovagnotti, Reactive Polymers, 4 (1985) 1. Alberti, G., Accts. Chem. Res., 11 (1978) 163. K. Segawa, Y. Kurusu, Y. Nakajima, and M. Kinoshita, J. Catal., 94 (1985) 491. T. N. Frianeza and A. Clearfield, J. Catal., 85 (1984) 398. S. Cheng, G.Z. Peng, and A. Clearfield, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 219. A. C. Mc Kinnis, U.S. Patent 2,776,985 (1957), assigned to Union Oil Co., California. W. A. Sanderson, PCT Patent Application WO 89/11485 (1989), assigned to Catalytica, Inc. R. L. Albright and I. J. Jakovac, Catalysis by Functionalized Porous Organic Polymers, Rohm and Haas Product Literature, 1985. W. A. Sanderson and D. L. King, patent pending.
267
26. F. Colombo, L. Con, L. Dalloro, and P. Delogu, Ind. Eng. Chem. Fundam., 22 (1983) 219. 27. P. M. Lang, F. Martinola, and S . Oeckl, Hydrocarbon Proc., Dec. (1985) 5 1 28. F. Ancilloti, M. M. Massi, and E. Pescarollo, J. Catal., 46 (1977) 49.
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G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION OF CATALYSTS
BASIC
G.A.MARTIN, M.C. V. PERRICHON
SILICATES AND
DURUPTY,
C.
THEIR USE
MIRODATOS,
N.
AS
269
SUPPORTS OR
MOUADDIB*
and
Institut de Recherches sur la Catalyse, C.N.R.S., Conventionng l'Universit6 Claude Bernard LYON I, 2 avenue Albert Einstein, 69626 - VILLEURBANNE C&dex - France ABSTRACT The utilization in catalysis of non-swelling basic Silicates such as chrysotile and talc are briefly reviewed. Their preparations by hydrothermal synthesis for new applications in catalysis are described. They lead to original Co-Cu catalysts for alcohols synthesis from syngas and to Li/MgO-Si02 systems active in the oxidative coupling o f methane. INTRODUCTION Basic silicates belong to the family of clay minerals which exist in the nature under numerous varieties (1). Their formation under hydrothermal conditions has long been studied by geologists and soil scientists. Concerning catalysis, there is now a renewed interest in the modified clay minerals and particularly in the pillared interlayer clays for oil cracking (refs. 2-6), but only a limited number of studies are available concerning the possible use of basic silicates for catalytic applications (ref. 7). Their structure derives from that of brucite Mg(OH)2 where OH groups are replaced by silica tetrahedras according to the following formulas :
brucite
chrysotile talc lizardite By isornorphous substitution, the Mg2+ ion can be replaced by other divalent ions such as Ni2+ and Co2+ or by monovalent cations such as L i ' . In this latter case, the excess of charge must be compensated by an additional exchangeable cation located between the layers. Talc and chrysotile of cobalt or nickel were prepared in our laboratory as precursors of Ni and Co/SiO2 catalysts
leading to metal particles with, in some cases, interesting preferential crystallographic orientations (refs. 8-9). It has to be noted that most of the catalytic studies deal with Ni or Co chrysotile (refs. 10-11). The possibility of introducing two divalent cations could be envisaged in some particular cases to obtain precursors of alloys or bimetallic systems. For example, it could be interesting to prepare cobalt-copper catalysts for which the association of cobalt and copper is involved in the selective synthesis of C2+ alcohols from CO,H2 (refs. 12-13). Some of these silicates are present in the nature. However they often contain impurities which make them unsuitable for catalytic purposes, so that the synthetic way appears desirable to prepare these compounds. This paper gives the results we obtained on the preparation of magnesium basic silicates supports and their use in the CO hydrogenation or oxidative coupling of methane after deposition of cobalt-copper or lithium phases respectively. Attempts to synthesize cobalt-copper chrysotile in which the Co and Cu ions would replace part of the Mg ions are also reported. EXPERIMENTAL METHODS Hydrothermal synthesis were performed in a passivated stainless steel autoclave (PROLABO). In a typical preparation of a Mg chrysotile, a stoichiometric mixture of 3.17 g Si02 (Aerosil 200 Degussa) and 4.56 g Mg(OH)2 in suspension in 60 ml H20 was stirred vigorously with a mixer during 5 mn. The autoclave charged with the slurry was sealed and heated at 623 K for 120 h. After cooling, the product was washed and dried 24 h at 373 K. Mg(OH)2 was prepared by precipitation of MgC12 solution (0.25 N) by KOH (0.5 N). The precipitate was washed to eliminate chlorine ions and driedovernight at 353 K under vacuum. Solid products were examined by X-ray powder diffraction using the Cu K Q radiation. BET areas were determined by N2 volumetric adsorption at 77 K. Chemical analyses were performed by atomic absorption spectroscopy. PREPARATION OF MAGNESIUM TALC AND CHRYSOTILE SUPPORTS A first set of experiments was performed to synthesize the Mg talc by heating under hydrothermal conditions a mixture of Si02 and Mg(OH)2 in a ratio Si/Mg=2. An excess of silica compared to the normal stoichiometry of the talc (1.33) was used as a silica source in order to increase the rate of talc formation (ref. 14). Sampling of the products were effected after 24 h at temperatures
271
ranging between 313 and 623 K. They were studied by X-ray diffraction and their specific area was determined after 5 h desorption under vacuum at 423 K. The spectrum of the initial mixture shown in Fig. 1 contains the four main lines of brucite, the silica being amorphous. At temperatures in excess of 373 K, the Mg(OH)2 pattern disappears and broad lines are detected corresponding to a new phase. Increasing the temperature up to 623 K confirms the formation of magnesium talc (Fig. 1).
I I
41
2 00
I
200
I
I
400
I
600
RK)
Fig. 1. X-ray diagrams showing the formation of Mg talc as a function of the temperature of the hydrothermal treatment. Fig. 2. Evolution of the BET surface after the hydrothermal treatments at different temperatures. Fig. 2 shows the evolution of the BET area as a function of the preparation temperature. Between 295 and 353 K, there is an increase from 150 to 260 m2.g-I, phenomenon which can be attributed to hydration effects (ref. 15). A net gap is observed between 353 and 373 K, from 260 to 435 m2.g-l which confirms the brucite transformation in talc. For higher preparation temperatures, the specific area decreases continuously, inagreement with the better crystallization observed on the diagram. After one week at 623 K, the specific area is 132 m2. A new set of experiments between 295 and 623 K, with a ratio Si/Mg=l, shows also evidence of the disappearance of the brucite X-ray lines between 353 and 373 K. A mixture of phases was
272
obtained, mainly talc and a small fraction of chrysotile. However, even after 8 days at 623 K, the sample remained poorly crystallized. From several experiments at 623 K, it results that Mg chrysotile could be obtained as a single phase only when starting from the stoichiometric composition. Moreover, a vigourous mixing of the precursors for at least 5 mn with an homogenizer was necessary for the synthesis. Otherwise, even with a stoichiometric mixture, the synthesis resulted in a polyphasic system with chrysotile, talc and brucite. Thus it is possible to prepare either talc or chrysotile at temperatures as low as 373 K provided that the starting materials are in a stoichiometric ratio and well mixed. The specific areas are higher than 300 m2g-l. After the hydrothermal treatment at 623 K, the corresponding BET areas are between 90 and 110 m2g-l. PREPARATION OF Co-Cu/CHRYSOTILE CATALYSTS FOR THE ALCOHOLS SYNTHESIS FROM CO/H2 In the case of Co-Cu bimetallic catalysts which are known to be selective for the hydrogenation of CO into higher alcohols, it is thought that a key factor for the selectivity is the proximity of the cobalt and copper atoms. Such a mixture at the surface scale is made difficult by the poor bulk miscibility of the two constituants ( 10% Cu maximum in cobalt and 0.2% Co in copper). In with homogeneous order to realize such a mixed system composition of equivalent amounts of Co and Cu, we have investigated the possibilities of using the peculiar properties of the basic silicates. Preparation Two types of preparation were tried, exchange and hydrothermal synthesis. The cationic exchanges were realized under N2 flow on 2 g of Mg chrysotile of 113 m2g-l in suspension in 100 ml H20. The cobalt and copper were used as nitrate salts. The conditions and the results are summarized in Table 1. EC refers to co-exchange whereas ES corresponds to successive exchanges starting with cobalt first. In all cases, the resulting Co concentration was low (0.1-0.3%) and that of copper was one order of magnitude higher. In spite of this relatively low copper content, it was possible to observe by X-ray analysis, the presence of a well defined phase, the gerhardite Cu2(0H)3N03 together with the initial Mg chrysotile. The easy formation of this phase can be taken as an evidence of the great affinj.ty of the surface towards copper,
273
which may explain that the exchange occurs selectively with copper rather than with cobalt. TABLE 1 Cobalt and copper exchanges on Mg chrysotile Catalyst EC1 EC2a ESlb ES2
Exchange temp. : 323 K. Theoritical Cu and Co contents : 5% a Theoritical Cu and Co contents : 2.5 % Exchange temperature : 298 K.
For the preparation of a mixed MgCoCu chrysotile, several attempts were made starting from different precursors salts, but always with the same ratio (Cu+Co+Mg)/Si=1.5 which corresponds to the stoichiometry of the chrysotile. Four samples were prepared as follows : - HS1 : starting from Co and Cu hydroxides HS2 : as above, but with addition of Mg(OH)2 HS3 : from CuC12-CoC12 and Mg(OH)2 with a low Cu percentage HS4 : from the solution of the chlorides salts, precipitated insitu by K2CO3 on silica before the hydrothermal treatment. The formation of chrysotile was never observed. However, evidence of Mg or Co talc could be detected with the characteristic line at 0.95 nm. In the absence of magnesium, a badly crystallized Co talc was obtained together with CuO. In the presence of Mg, the Mg talc structure seemed to be favoured but the degree of crystallization remained low. Consequently, the introduction of copper in the chrysotile stucture appears very difficult. This fact has already been pointed out by Wey et al. (ref. 16) and explained by a Jahn-Teller effect, which makes the structure distorted and creates an unstability for the whole crystal.
-
Catalytic behaviour in the CO/H:! reaction All the cobalt-copper catalysts were tested under 10 bar after a reduction at 573 K by H2 at atmospheric pressure. Table 2 gives some significant results corresponding to the activity after stabilization at 523 K, i.e. practically after 5 h on stream. For a better comparison, the rates are expressed for 1 g cobalt. For the catalysts prepared by exchange, the main common feature is the high methanol selectivity which can be attributed to the high coppsr surface oonoentration. The activities are low.
214
TABLE 2 Catalytic properties of CoCu catalysts in the CO/H2 reaction. T=523 K ; P=10 bar : CO/H2=0.5 ; D=1.8 1.h-1 ; m=100 mg. Catalyst
Selectivity %
Activity
.
mmole h-l.g - k o EC2 ES2 HS1 HS2 HS3 HS4
0.7 0.3 4 25 ia 3
Hydrocarbon C1-C6 ia 19 38 78
78 26
Alcohols Cl-C5
C2+0H
a2 a1 62 22 22 74
3 24 9 11
a
a
Concerning the hydrothermal method, the activities are much higher and the best data for the higher alcohols selectivity are obtained with the hydroxides as precursors without Mg(OH)2 addition. PREPARATION O F Li/MAGNESIUM BASIC SILICATES CATALYSTS FOR METHANE OXIDATIVE DIMERIZATION The objective was to improve the design of catalytic phases active and stable for the oxidative coupling of methane into C2 hydrocarbons. Among the numerous and various formulas tested up to now, the most selective and productive catalysts are generally basic oxides promoted with alkali compounds, such as Li/MgO catalyst (ref.17). Such catalysts are however rather unstable in the severe conditions of the reaction (T>900K, reaction mixtures with H20, CO, C02, 02 and hydrocarbons), mainly due to i) the loss of alkali (vaporization, reaction with the tubular quartz reactor) and ii) the loss of surface area (sintering of the oxides) ( ref. 18 ) Attempts have been made in this laboratory to stabilize the reference catalyst Li/MgO by means of hydrothermal synthesis, on the basis that adding silica to the magnesia structure could induce beneficial effects on Li content and surface area.
.
Preparation of Li promoted magnesium silicates A series of magnesium silicates has been prepared according to the general recipe of hydrothermal synthesis, with 3Si/Mg ratios variing from 0 (brucite) to 4 (talc) with intermediate ratios corresponding to mixtures of brucite, chrysotile and talc structure as reported in table 3 The various Mg silicates were then impregnated with lithium carbonate aiming at a constant atomic ratio Li/Mg (around 0.6).. An other sample of chrysotile (3Si/Mg=2) has been alkalized at a lower content (Li/Mg+Si= 0.1)
.
275
in order to test the stability of the Li content in comparison with Li/silica and Li/magnesia loaded with similar content of Li. Fig.3 reports the changes in alkali content as a function of the 02-treatment temperature. The alkali appears to be very stable on silica and chrysotile while a major loss of Li is observed on magnesia for T>900K. The different behavior between silica and magnesia supports has been explained in (ref.19) by considering that a stabilizing lithium silicate interface was formed between lithium and silica phases while no equivalent compound could be formed with the magnesia support. The stabilization of lithium which is observed on chrysotile (Fig.3, curve a) indicates therefore that the addition of silica to magnesia via the hydrothermal synthesis could allow interface Li silicate to develop, preventing the loss of alkali at high temperature. 0.15
it 2m
chrysotile f
0.05
0
:M 0
I
I
500
700
I
900
I
1100
1 K)
Fig. 3. Changes in Li content vs temperature of treatment (flowing 02 for 15h) f o r a : Li/chrysotile ; b : Li/SiO2 : c : Li/MgO. In Fig.4 are depicted the changes in morphology which are observed by TEM on Li/chrysotile before and after activation and catalytic test. Additional informations on surface composition were provided by STEk analysis. Initially (Fig.4, a), the catalyst is formed with two distinct phases: well crystallized chrysotile-type flakes and Li2CO3 crystals. After calcination at 723K (Fig.4, b), the chrysotile phase displays the same overall structure, now covered with lithium carbonate but with a heterogeneous composition indicating a local demixion of the Mg and Si phases. This effect could correspond to the formation of some Li/Si compound as postulated above. Finally, after catalytic test (Fig.4, c), the chrysotile structure is collapsed and replaced by a clustering of large partlcules ( 5 0 to 200 nm),
276
mainly MgO coated with Li2CO3 (from XPS measurements) and very large rafts of segregated silica. This picture is close to what is observed with the reference Li/MgO sample, but the initial insertion of silica tends however to stabilize both the lithium content and the surface area (Table 3).
Fig. 4 . Electron micrographs of Li/chrysotile. a : initial : b : after calcination at 723 K catalytic test ar 1023 K.
:
c : after
Catalytic behavior of Li promoted magnesium silicates Table 3 gives some data concerning the catalytic activity and selectivity in methane oxidative coupling obtained with the Li/magnesium silicate series ( for more details see ref.20). It is mainly observed that: - for small amounts of silica added to the reference MgO , leading to mixture of magnesia and chrysotile, beneficial effects
277
such as surface stabilization and prevention of alkali loss induce an increase in the CH4 conversion with a high C2 selectivity. - at higher silica contents, the above positive effects are counterbalanced by the development of a silica and Li silicate surface, unfavorable towards selectivity and activity. As a matter of fact, acidic surfaces are unable to activate methane in these catalytic conditions and the preferential reaction of lithium with silica is likely to hinder the formation of the Li/MgO interface necessary for methane coupling. TABLE 3 Characterization data and catalytic properties of Li/MgO-SiO2 Initial Atomic ratio structure 3Si/Mg Li/Mg (XRD)~ magnesia magnesia+ chrysotile chrysotile talc silica
BET area CH4 couplingC before after activity C2 select. catalytic test (mmole (%) (m .g-l) h-l.g-l)
0
0.63
50
tl
3
73
1.2 2.3 4.0
0.63 0.59 0.62 b
110 130 150 180
3 6 12 80
15 20 6 tl
70 46 18 8
aSuperimposed with the Li2CO3 structure bLi/Si = 0.11 CReaction carried out at 1023 K; PCH4'7.8 flow rate = 3.6 1.h-l.
kPa; P02=4.6 kPa, total
In conclusion, although the initial complex structures of magnesium silicates are destroyed in the severe reaction conditions of methane coupling, they may induce positive effects on catalytic performances, at least for reduced Si/Mg ratios. CONCLUSION The possible uses in catalysis of metal hydroxide silicates as supports or precursors of active phases were evaluated in two reactions, the synthesis of higher alcohols from syngas and the oxidative coupling of methane. Although the initial structure of the silicate is often destroyed during the activation step and converted into a mixture of phases, it may induce positive effects on the physicochemical and catalytic properties. Particularly, magnesium basic silicates show a better thermal stability compared to MgO and favour the stabilization of lithium during the CH4 oxidative coupling. Due to the uniformity of the surface hydroxyls groups, the exchange methods should be limited to monometallic exchanges. Finally, the preparation of basic silicates homogeneous in composition remains an open and promising domain.
278
ACKNOWLEDGEMENTS We wish to thank Mrs M.T. Gimenez for the X-Ray diffraction measurements and I. Mutin for the electron microscopy and STEM studies. Part of this work was supported by GDF. REFERENCES 1 F. Liebau, Structural Chemistry of Silicates, Springer-Verlag eds., Berlin, (1988) 213-231. 2 T.J. Pinnavaia, Science, 220 (1983) 365-371. 3 V.N. Parulekar and J.W. Hightower, Appl. Catal., 35 (1987) 249262. 4 S. Yamanaka and M. Hattori, Catalysis Today, 2 (1988) 261-270. 5 C.I. Warburton, Catalysis Today, 2 (1988) 271-280. 6 F. Figueras, Catal.Rev. Sci.Eng. 30 (1988) 457-499. 7 H.E. Swift in: J.J. Burton and R . L . Garten (Eds), Advanced Materials in Catalysis, Academic Press, London, 1977 p. 230. 8 G. Dalmai-Imelik, C . Leclercq and A. Maubert-Muguet, J. Solid State Chem., 16 (1976) 129-139. 9 J.A. Dalmon and G.A. Martin, C.R. Acad. Sci. Paris, 267C (1968) 610. 10 Y. Ono, N. Kikuchi and H. Watanabe, in: B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds), Preparation of Catalysts IV, Elsevier Science Publishers B.V., Amsterdam, 1987 pp. 519-528. 11 L. Bruce, H. McArthur and T.W. Turney, Proc.of the 12th Aust. Chem. Eng. Conf., Melbourne, Australia, (1984) pp. 649-654. 12 R.M. Baillard-Letournel, A.J. Gomez-Cobo, C . Mirodatos, M. Primet and J.A. Dalmon, Catal. Letters, 2 (1989) 149. 13 N. Mouaddib, V. Perrichon and M. Primet, J. Chem. SOC., Faraday Trans. I, 85 (1989) 3413-3424. 14 H. Muraishi and S . Kitihara, Proc. Int. Symp. on Hydrothermal reactions, (1982) pp. 377-392. 15 C. Sudhakar and M.A. Vannice, Appl. Catal. 14 (1985) 47-63. 16 R. Wey, B. Siffert and A. Wolf, Bull.Gr.Fr. des Argiles, 20 (1968) 79-92. 17 T. Ito, J.X. Wang, C.H. Lin and J.H. Lunsford, J.Am.Chem.Soc., 107 (1985) 5062. 18 C. Mirodatos, V. Perrichon, M.C. Durupty and P. Moral, in : B. Delmon and G.F. Froment (Eds), Catalyst Deactivation, Elsevier Science Publishers, Amsterdam, 1987 pp. 183-195. 19 V. Perrichon and M.C. Durupty, Appl. Catal., 42 (1988) 217. 20 G.A. Martin, P. Turlier, V. Ducarme, C. Mirodatos and M. Pinabiau, Catal. Today, 6 (1990) 373
G.Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 01991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
279
SOILS AS UNUSUAL CATALYSTS SERGIO A. MOYA, ANSELMO FLORES and MAURICIO ESCUDEY Departamento de Quimica, Facultad de Ciencia, Universidad de Santiago de Chile Casilla 5659, Correo 2, Santiago(Chi1e) ABSTRACT Samples from the B horizons of three profiles of Chilean soils were used as catalysts for the water gas shift reaction. The system produced in alkaline media develops a catalytic activity which depends on the soil mineralogy, organic matter and iron oxide contents. The yield obtained for WGSR using the soils as catalyst is comparable to those produced by prepared catalysts supporting Fe203 on y-Al203. INTRODUCTION During the last decades literature shows some attempts to apply clay fractions as catalysts for several reactions: cracking of petroleum products; formation of amino-acids and their polymerization into peptides; polymerization of benzene; polymerization of styrene ( 1 - 4 ) . However, there is no information about the possibility of using soils directly as catalysts. The iron oxides or iron derivatives with different mineralogy are common and significant components of many soils and are usually dispersed throughout the external soil matrix as a coating of the aluminosilicate core. Thus the iron oxides may have an important influence on the properties of the soil as potential catalysts for many reactions. In this way we have made pioneer works searching into this possibility. Recently we reported the use of volcanic-ash-derived-soil as iron oxide supported catalyts ( 5 ) . Continuing our studies, we report
here the application of three Chilean soils as
catalysts for the water gas shift reaction (WGSR), taking into account that the characteristic of Chilean soils derived from volcanic material is their large specific surface area and high iron oxides content ( 6 ) . The used soils were characterized in terms of their chemical properties, X-ray powder diffraction, isoelectric points and MBssbauer spectroscopy which was used to identify and characterize Fe oxides in the soils. The influence of factors such as soil pretreatment, heating and pH in the catalytic run were also considered. EXPERIMENTAL Samples of the B horizons of Chilean soils (Collipulli, Osorno and San Patricio) derived from volcanic ashes ( 7 ) were used. Collipulli: A Xeric P a lehumult with a clay fraction dominated by halloysite (> 50%); minor
280 components ( >5%) a r e c h l o r i t e ; g i b b s i t e , g o e t h i t e , p l a g i o c l a s e , q u a r t z and a-cristobalite.
Osorno: A T y p i c D y s t r a n d e p t w i t h a c l a y f r a c t i o n dominated by
a l l o p h a n e ( >50%); minor components ( >5%) a r e f e r r i h y d r i t e , o r g a n o - a l l o p h a n i c complexes, h a l l o y s i t e , g i b b s i t e and a - c r i s t o b a l i t e .
San P a t r i c i o : A
Hydric
Dystrandept w i t h a c l a y f r a c t i o n dominated by a l l o p h a n e ( > 5 0 % ) ; minor components ( > 5 % ) are
a-cristobalite,
g o e t h i t e , p l a g i o c l a s e and h a l l o y s i t e .
Chemical a n a l y s i s w a s c a r r i e d o u t by atomic a b s o r p t i o n s p e c t r o s c o p y a f t e r d i s s o l u t i o n i n T e f l o n bombs (8) and o r g a n i c c a r b o n w a s determined by a d r y combustion method. X-ray d i f f r a c t i o n s were c a r r i e d o u t on powdered samples i n a P h i l i p s Nor e l c o i n s t r u m e n t w i t h Cu& r a d i a t i o n and a carbon c r y s t a l monochromator.
The
s p e c i f i c s u r f a c e area w a s o b t a i n e d by a g r a v i m e t r i c method based on t h e r e t e n t i o n of e t h y l e n e g l y c o l monoethyl e t h e r (EGME)
(9).
The Melssbauer s p e c t r a were o b t a i n e d i n a c o n v e n t i o n a l A u s t i n s p e c t r o m e t e r w i t h c o n s t a n t a c c e l e r a t i o n a t room t e m p e r a t u r e . The s o u r c e w a s 2.13 m C i 5 7 C 0 i n a Pd m a t r i x . The r e p o r t e d isomer s h i f t v a l u e s are r e l a t e d t o n a t u r a l i r o n ( 5 7 F e o ) . The d a t a were a d j u s t e d w i t h a n i t e r a t i v e l e a s t - s q u a r e s program which u s e s a Marquandt a l g o r i t h m de-veloped
i n t h e Mtlssbauer S p e c t r o s c o p y L a b o r a t o r y
of t h e P h y s i c s Department of t h e U n i v e r s i t y of S a n t i a g o . E l e c t r o p h o r e t i c m o b i l i t i e s w e r e measured w i t h a Zeta Meter (ZM 77) a p p a r a t u s f i t t e d w i t h a n a u t o m a t i c sample t r a n s f e r system. Samples of a b o u t 5
-3
mg were suspended i n 100 m l of 10
M i o n i c strength solutions fixed with KC1.
The m o b i l i t i e s were averaged and t h e z e t a p o t e n t i a l (ZP) w a s c a l c u l a t e d u s i n g t h e Helmholtz-Smoluchowski e q u a t i o n (10). A computer program i n BASIC language w a s employed t o o b t a i n t h e i . e . p .
I n a l l e x p e r i m e n t s doubly d i s t i l l e d w a t e r
was used. P r i o r t o u s e diglyme and t h e o t h e r chemicals were p u r i f i e d a c c o r d i n g t o p r o c e d u r e s a l r e a d y r e p o r t e d i n t h e l i t e r a t u r e (11). The WGSR w a s c a r r i e d o u t under r e l a t i v e l y m i l d c o n d i t i o n s ( l O O ° C ,
Q
1 atm
CO (Matheson, 99.99%)). A Perkin
- E l m e r model 8500 g a s chromatograph provided w i t h a GP-100
p r i n t e r w a s used f o r a n a l y s i s of g a s m i x t u r e s . A c a r b o s i e v e S-I1 column
( 3 mX2.4 mm, Supelco) w a s used t o a n a l y z e H2, CO and C02, employing H e as c a r r i e r g a s . The a b s o l u t e y i e l d of hydrogen was determined by c a l i b r a t i o n of t h e GC u s i n g known volumes of hydrogen.
281 TABLE 1
Chemical a n a l y s i s ( w t % ) , o r g a n i c c a r b o n c o n t e n t ( w t % ) , i s o e l e c t r i c p o i n t (IEP i n pH u n i t s ) and s u r f a c e area (m'g-l)
a s a f u n c t i o n of h e a t i n g f o r s o i l
samples. Temperature of h e a t i n g
Fe203
A1203
SiO2
Organic Carbon
IEP
Surface Area
46.7 50.0 51.0
0.6 0.2 0.1
2.8 2.9 3.0
155 135 94
43.5 45.4 51.6
3.7 0.6 0.1
6.7 6.9 6.7
142 98 63
35.6 51.6 53.6
13.2 1.6 0.2
3.2 6.1 6.7
118 100 43
CoLlipulli 124°C 350°C 600°C
13.8 14.1 14.3
124°C 350°C 600°C
9.5 11.3 12.7
124'C 350°C 600°C
7.3 10.0 10.5
19.8 21.0 23.6 Osorno 17.5 19.8 21.3 San P a t r i c i o 12.1 18.2 20.1
The a n a l y s i s of t h e g a s samples and t h e t y p e of r e a c t o r used were s i m i l a r t o those already described (12). T y p i c a l Fez03 s u p p o r t e d c a t a l y s t s were p r e p a r e d by a w e t i m p r e g n a t i o n p r o c e d u r e ( e x c e s s of s o l u t i o n ) of a S t r e a m
y-Al203 ( S BET
=
188 m2 g-l and
s i e v e d t o 1 mm) u s i n g a s o l u t i o n of Fe(N03)3*9H20. S o l u t i o n s of 11 w t % (g of Fez03 p e r 100 g of d r i e d y - ~ l 2 0 3 ) were p r e p a r e d . The impregnated samples were d r i e d a t 100°C and 27 kN m-2 and f i n a l l y c a l c i n e d a t t h e t e m p e r a t u r e of t h e s o i l samples. A l l t h e s e p r e p a r e d c a t a l y s t s were used t o c a t a l y z e t h e WGSR. RESULTS AND DISCUSSION Osorno and San P a t r i c i o s o i l s a r e Andepts w i t h a h i g h o r g a n i c matter c o n t e n t dominated by v a r i a b l e s u r f a c e c h a r g e i n o r g a n i c components (Table 1 ) . On t h e o t h e r hand, C o l l i p u l l i s o i l i s an U l t i s o l w i t h low o r g a n i c m a t t e r c o n t e n t , dominated by c r y s t a l l i n e c l a y
m i n e r a l s w i t h l i t t l e o r no v a r i a b l e
surface charge. Andepts samples mineralogy i s dominated by low c r y s t a l l i n i t y compound and h e a t i n g (600'C) (Figure 1).
was n o t observed t o have any e f f e c t on t h e c r y s t a l l i n i t y
282
G
I t
t v)
z w
t
z
-I I
H P
h-
H
G
600
H P
G
G
-
H G
1
35
500
H P,
“2‘
010
H P
-
b-c
350
H P
c-c
290 220
i - 1
“Lc
124 ~
$0
~
2’s
7
-
20 5; TWO THETA (DEGREES)
Fig.1. X-ray diffraction for as a function of heating temperature for San Patricio sample. The heating temperatures are shown at the right ( “ C ) . The small peaks are attributed to halloysite (H), goethite (G), plagioclase (P) and a-cristobalite (a-C) above shown. The isoelectric point shows the influence of dominant surface sites. The organic matter has active sites with low pKa values, consequently soils with high organic matter content show low IEP value. In well crystallized aluminosilicates the structural charge is more important than the pH dependent surface charge and soils dominated by those compounds will show low IEP values. Poorly crystallized aluminosilicates and iron oxides show variable surface charge, active surface sites dominated by A1-OH and Fe-OH and consequently high IEP values (between 8 to 9). The water lost due to heating, results in a decrease of its IEP values. After the above discussion, the IEP of soils depends on the organic matter content, the mineralogy and the soil hydration. Non-allophanic soils (Collipulli) have a low IEP value due to the presence of more stable crystalline aluminosilicates and iron oxides which are more important than their low organic matter content (Collipulli soil has 0.6% of organic carbon), and little or no change of the IEP is observed with heating (Table 1). In allophanic soils (Osorno, San Patricio), the IEP depends on organic matter content; as organic matter content increases the IEP decreases. A s result of heating from 1 2 4 O C to 6 0 O D C , two different types o f reactions occur, the gradual destruction of organic matter (dehydration, dehydrogenation, decarboxylation and oxidation reactions; Table 1, Figure 11, and the dehydration and dehydroxylation of inorganic compounds. In allophanic soils
283
with high organic matter content the IEP increases as result of exposure of A1-OH and Fe-OH active surface sites; conversely, in allophanic soils with low organic matter content a decrease of IEP is observed as a result of dehydration and dehydroxylation of A1-OH and Fe-OH active surface sites. In both cases, due to similar mineralogy of allophanic soils, a IEP about 6 to 7 is obtained. After heating (600°C) an IEP of 6.7 was obtained for Osorno and San Patricio soil samples (Table 1 ) . The Mtlssbauer spectra at room temperatures for the studied soils showed appreciable amounts of quadrupole doublet together with a six-line envelope. As expected from the volcanic origen of the soils, the presence of magnetite . i s supported by the broad peaks each one comprising two unresolved peaks of the 12 peaks of the magnetite spectrum ( 1 3 ) .
The unresolved peaks also show
the presence of hematite and probably goethite or ferrihydrite in small amounts ( 1 4 ) , if heating is not higher than 350°C because at this temperature important changes on ferrihydrite crystallinity occur, as observed in differential scanning calorimety (15).
COLLIPULLI
1
.rl,,r.%rl
. 4lOy
0
1
2
3
4 22 TI ME (h)
26
26
Fig.2. Hydrogen production as a function of the reaction time of the WGSR and temperature of the catalyst preparation (Collipulli).
284
F i g . 3 . Hydrogen p r o d u c t i o n as a f u n c t i o n of t h e r e a c t i o n time of t h e WGSR and t e m p e r a t u r e of t h e c a t a l y s t p r e p a r a t i o n (Osorno).
H, Produced (uMole)
I
SAN PATRlClO
TIME (h) F i g . 4. Hydrogen p r o d u c t i o n as a f u n c t i o n of t h e r e a c t i o n time of t h e WGSR and t e m p e r a t u r e of t h e c a t a l y s t p r e p a r a t i o n (San P a t r i c i o ) . The s o i l s samples, s i e v e d a t 1 mm, were h e a t e d a t t h e f o l l o w i n g t e m p e r a t u r e s , 124"C, 220"C, 29O"C, 350°C, 410"C, 500°C and 600°C t o g r a d u a l l y d e s t r o y t h e o r g a n i c m a t t e r and a f t e r t h i s t r e a t m e n t t h e y w e r e used d i r e c t l y as c a t a l y s t s . S o i l samples h e a t e d a t 124°C showed p r a c t i c a l l y no o r g a n i c m a t t e r d e s t r u c t i o n . Conversely, i n t h o s e s o i l samples h e a t e d a t 600°C a l m o s t a l l t h e o r g a n i c matter w a s d e s t r o y e d ( T a b l e 1 ) . Thus, t h e r e i s a r e l a t i o n s h i p between
285
the heating temperature, the remaining organic matter content, the I E P (Table 1) and the catalytic activity observed (Figs. 2 - 4 ) . heating, a shifting of the IEP
As a result of
is observed depending on organic matter
content, its destruction, and reaction of inorganic components (as explained above).
As the heating temperature increases, the catalytic activity increases
too, due to the organic matter destruction. A maximum in catalytic activity is observed at about 500°C for Collipulli and Osorno soils and at about 410°C for San Patricio soil where the best ratio between organic matter destruction and inorganic components dehydroxylation is reached (5). If the heating temperature is increased, the reactions of dehydroxylation and crystallization of inorganic components are more important than the organic matter destruction and a significant decreases of surface area is observed (Table l ) , consequently in all samples, a lower catalytic activity is observed. The WGSR was performed in basic media and mild conditions (KOH, 100°C and
0.9 atm CO). Good correlation was obtained for the CO consumed in the reaction and the H2 produced. However, C 0 2 was always detected in lower quantities which can be attributed to adsorption occurring on the catalyst or to some reactions with aqueous hydroxide to produce probably carbonate or formate. The catalytic systems studied showed an increasing catalytic activity during the first hour of reaction. After that time the activity as can be seen in Figs. 2 and 3 decreased significantly for Collipulli and Osorno soils. However, San Patricio soil maintained a high level of hydrogen production for a l o n g time. As shown in Fig. 4 there is a limited temperature to obtain the highest hydrogen production and when the heating temperature of a soil preparation rises the hydrogen production decreases rapidly. Control experiments indicate no reaction in the absence of the catalyst. The activity of the catalyst decreases in neutral media and no activity at all is shown in the absence of KOH, which is a clear indication that OH- play a key role in the mechanism of the WGSR, probably in the interaction of CO, metal surface and OH- to generate C 0 2 . In order to compare the soil samples used as catalysts, specific Fez03 supported catalysts on A1203 were prepared at the same temperature of the soils. When the WGSR is carried out u s i n g these catalysts a similar pattern to that of the soil samples was observed. Under the experimental conditions used, the yields obtained in the WGSR catalyzed by the soil samples were comparable to those produced by the supported catalysts prepared. The differences found in catalytic activities can be explained by the different forms that Fe takes in the samples after the heating. Thus the Mussbauer spectra show magnetite as the dominant component in Collipulli soil and hematite as the dominant component in Osorno and San Patricio soils. However, for the San Patricio soil, the Mussbauer spectra shows a doublet
286
which indicates the presence of a component with a higher content of Fe(II1) which would be responsible for the greater catalytic activity shown by this soil. ACKNOWLEDGMENTS The authors express their gratitude to Direcci6n de Investigaciones Cientificas y Tecnol6gicas of the Universidad de Santiago de Chile and to Fondo Nacional de Investigaciones Cientrficas y Tecnol6gicas for financial support (grants 0899-90 and 0039-89). REFERENCES
1
2
3
4 5 6 7 8 9 10 11
12 13 14 15
T.H. Milliken, G.A. Mills and A.G. Oblad, Trans. Faraday SOC. 8 (1950) 279. D.H. Solomon, B.C. Loft and J.D. Swift, Clay Miner. 7 (1968) 399. F. Stoessel, J.L. Guth and R. Wey, Clay Miner., 12 (1976) 255. D. Njopwuo, G. Roques and R. Wandji, Clay Miner., 22 (1987) 145. M. Escudey and S.A. Moya, Colloids and Surfaces, 37 (1989) 141. M. Escudey and G.G. Galindo, Colloid Interface Sci., 93 (1983) 78. A. Mella and A. Kuhne, In J . Tosso (Ed.), Suelos Volc6nicos de Chile, INIA, Santiago, 1988 p. 548. B. Bernas, Anal. Chem., 40 (1968) 1682. M.D. Heilman, D.L. Carter and C.L. Gonzzlez, 100 (1965) 409. R.J. Hunter, Zeta Potential in Colloid Science: Principles and Applications, Academic Press, London, (1981) p. 59. Ch. Urgermann, V. Landis, S.A. Moya, H. Cohen, H. Walker, R.G. Pearson, R.G. Rinker and P.C. Ford, J . Am. Chem. SOC., 1 0 1 (1979) 5922. S . A . Moya, A. Mansilla and F.J. Gil, Bull. SOC. Chim. Bel., 97 (1988) 9. H.P. Weber and S.S. Hafner., 2. Krist., B-133 (1971) 327. J.M. Bigham, D.C. Golden, L.H. Bowen, S.W. Buol and S. B. Weed, Soil Sci. SOC. Am. J., 42 (1978) 816. M. Escudey, Unpublished results.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
281
THERMAL STABILITY, ACIDITY AND CRACKING PROPERTIES OF PILLARED RECTORITE CATALYSTS
M A R I O L. OCCELLI Unocal C o r p o r a t i o n , Science & Technology D i v i s i o n P. 0. Box 76, Brea, C a l i f o r n i a 92621
ABSTRACT X-ray d i f f r a c t i o n (XRD), p y r i d i n e c h e m i s o r p t i o n , and m i c r o a c t i v i t y t e s t (MAT) r e s u l t s have been used t o c h a r a c t e r i z e a sample o f n a t u r a l r e c t o r i t e p i l l a r e d w i t h alumina c l u s t e r s . A f t e r r e a c t i o n w i t h c h l o r h y d r o l , a p i l l a r e d p r o d u c t was o b t a i n e d t h a t a f t e r d r y i n g a t 10O0C/10h had d(001) s p a c i n g o f 28.7 A. The p i l l a r e d r e c t o r i t e r e t a i n e d i t s s t r u c t u r e even a f t e r c a l c i n i n g i n a i r a t 800°C/5h o r a f t e r steam a g i n g a t 760°C/5h w i t h steam a t 1 atm. Thus, p i l l a r e d r e c t o r i t e s have thermal and hydrothermal s t a b i l i t y comparable t o t h a t o f zeolites w i t h the Faujasite structure. Steam-aged p i l l a r e d r e c t o r i t e s , a t MAT c o n d i t i o n s , have c r a c k i n g a c t i v i t y s i m i l a r t o t h a t o f a commercial f l u i d i z e d c r a c k i n g c a t a l y s t (FCC) and can be r e g e n e r a t e d w i t h ease. However, t h e i r coke and l i g h t gas (hc, CH ) s e l e c t i v i t y w i l l have t o be d r a s t i c a l l y improved b e f o r e t h e s e c l a y s can 6ompe?e w i t h z e o l i t e s i n cracking c a t a l y s t preparation. INTRODUCTION Although p i l l a r e d clays could generate low cost f l u i d i z e d cracking c a t a l y s t s (FCC) w i t h unique s e l e c t i v i t y p r o p e r t i e s , t h e y have n o t y e t been accepted by the petroleum industry.
In f a c t , r e f i n e r s ( t o d a t e ) have been r e l u c t a n t t o
f i e l d t e s t t h e s e new c a t a l y s t s because, i n a d d i t i o n t o a h i g h tendency f o r coke g e n e r a t i o n , t h e y e x h i b i t hydrothermal s t a b i l i t y i n f e r i o r t o t h a t o f t h o s e z e o l i t e s used i n hydrocarbon c o n v e r s i o n processes.
The physicochemical
p r o p e r t i e s o f p i l l a r e d c l a y s have been reviewed elsewhere (1,2). R e c e n t l y , J. Guan and co-workers ( 3 ) have r e p o r t e d t h a t by p i l l a r i n g a sample o f n a t u r a l r e c t o r i t e w i t h p o l y o x o c a t i o n o f aluminum o r z i r c o n i u m , i t i s p o s s i b l e t o o b t a i n p i l l a r e d c l a y w i t h hydrothermal s t a b i l i t y t y p i c a l of z e o l i t e s and z e o l i t e - c o n t a i n i n g f l u i d c r a c k i n g c a t a l y s t s ( 3 ) .
R e c t o r i t e i s an
i n t e r s t r a t i f i e d l a y e r e d s i l i c a t e m i n e r a l c o n s i s t i n g o f a r e g u l a r s t a c k i n g of m i c a - l i k e and m o n t m o r i l l o n i t e - l i k e l a y e r s ( 4 ) .
The n a t u r e o f t h e l a y e r s as
w e l l as t h e i r s t a c k i n g sequence i s d i f f i c u l t t o p r e d i c t because o f t h e v a r i a t i o n found w i t h i n and between samples.
Kodama ( 5 ) has r e p o r t e d t h a t t h e
m i c a - l i k e l a y e r s a r e s i m i l a r t o p a r a g o n i t e whereas t h e m o n t m o r i l l o n i t e - l i k e l a y e r s can have a b e i d e l l i t e c h a r a c t e r .
Probable s t a c k i n g sequences o f t h e s e
288
= - in this last element, is observed. Micropore surface areas f o r the fresh and preoxidized coals are
442
presented
i n Table
2.
P r e o s i d a t i o n had
l i t t l e e f f e c t o n t h e CO,
s u r f a c e a r e a s o f c o a l s a m p l e s . T h e e n h a n c e m e n t o f s u r f a c e area d u e
is o f
t o preoxidation
as t h o s e o b t a i n e d
t h e same o r d e r
by o t h e r
a u t h o r s ( r e f . 1 4 ) when u s i n g s i m i l a r c o n d i t i o n s o f o x i d a t i o n .
TABLE 2 Analyses of t h e o x i d i z e d coals. Coal sample
Oxidation conditions
PO P6 P18 G24 G4 8 G72
fresh 473 K - 6 h 4 7 3 K - 18 h 543 K - 24 h 543 K - 48 h 5 4 3 K - 72 h
FSI
V.M.
(%I
8 1 0 1 0 0
37.2 32.6 33.1 37.9 40.8 42.3
C
H
N
(%)
(%)
S (%)
O
(%)
(%)
co,
86.7 81.7 78.9 76.3 70.2 69.6
5.0 4.1 3.6 3.1 2.0 1.3
1.3 1.7 1.7 1.7 1.9 2.1
0.5 0.4 0.4 0.4 0.4 0.4
6.5 11.9 15.2 18.2 25.4 26.6
146 159 180 168 217 211
A l l r e s u l t s are e x p r e s s e d on a d . a . f .
SDR
basis.
Pyrolysis C h a r s o b t a i n e d b y p y r o l y s i s f r o m o x i d i z e d c o a l show a n i m p o r t a n t e n h a n c e m e n t i n s u r f a c e a r e a as c a n b e s e e n i n F i g u r e 1 . The d r a s t i c r e d u c t i o n i n p l a s t i c p r o p e r t i e s o f b i t u m i n o u s c o a l w h i c h o c c u r s as a r e s u l t o f o s i d a t i v e t r e a t m e n t , seems to b e t h e p r i n c i p a l c a u s e o f
t h i s i n c r e a s e . In f a c t , a s a c o n s e q u e n c e o f t h e p r e v i o u s d e s t r u c t i o n
of
caking
during
the
properties, pyrolysis
a more o p e n p o r o u s step
(ref.
structure was
15). Coal
produced
oxidation affects
the
t e x t u r a l p r o p e r t i e s o f t h e s u b s e q u e n t c h a r s as shown i n T a b l e 3 . The i n c r e a s e i n CO, authors (ref.
s u r f a c e a r e a i s b i g g e r t h a n t h a t o b t a i n e d by o t h e r s
16).
TABLE 3
T e s t u r a l p r o p e r t i e s of t h e c h a r s o b t a i n e d Raw coal sample
S area (3g-l)
Porosity
P o r e v o l u m e ( cm3g-')
(%)
Total F.0 P. 6
P. 18 G.24 G.48 G.72
196 692 616 426 518 514
16.5 28.5 29.6 26.7 32.1 33.5
111 215 229 201 252 269
radius (nml
>50
43 37 34 38 33 31
3.7/50
c3.7
1 1 5 0 1 1
53 59 61 62 66 68
I n b o t h series, powdered and g r a n u l a r materials, p r e o x i d a t i o n o f c o a l d e t e r m i n e s a bi.g
i n c r e a s e i n t h e CO,
s u r f a c e area of t h e c h a r s
443 o b t a i n e d , t h i s i n c r e a s e b e i n g more n o t a b l e i n t h e c a s e o f p o w d e r e d
m a t e r i a l , as c a n b e s e e n i n T a b l e 3 . V a l u e s o f N, s u r f a c e a r e a ( r e f .
1 7 ) a r e v e r y much l o w e r t h a n t h o s e d e t e r m i n e d f r o m CO, a d s o r p t i o n , c o n f i r m i n g t h e r e l a t i v e i m p o r t a n c e of t h e m i c r o p o r e network i n t h e e v o l u t i o n of t h e material.
_____ Activation During
the
char
formation
process,
in
the
pyrolysis
step,
a
Later, t h i s s t r u c t u r e l e a d s t o
primary p o r e s t r u c t u r e is developed.
t h e development o f porosj-ty d u r i n g g a s i f i c a t i o n i n t h e a c t i v a t i o n step.
This
increase
in
porosity
and
initial
pore
s t r u c t u r e are
s t r o n g l y i n f l u e n c e d by p r e v i o u s t r e a t m e n t o f t h e c a k i n g c o a l s , e . g .
air oxidation. F i g u r e 1 shows t h e e v o l u t i o n o f and
activated
different
carbons
degrees.
obtained
Also
the
t h e CO,
from
s u r f a c e area o f
coal
evolution
of
samples the
chars
oxidized
to
area
of
surface
o x i d i z e d c o a l s w i t h t h e t i m e o f o x i d a t i o n are p r e s e n t e d .
A s can be
s e e n , c o a l o x i d a t i o n p r o d u c e s a b i g i n c r e a s e i n t h e s u r f a c e area o f chars obtained
by p y r o l y s i s and
in
t h a t of
t h e activated carbons
o b t a i n e d from c h a r g a s i f i c a t i o n . During g a s i f i c a t i o n a p r o g r e s s i v e e n l a r g e m e n t o f t h e p o r e s , p r e v i o u s l y formed i n t h e p y r o l y s i s s t e p , i s p r o d u c e d . The e v o l u t i o n o f p o r o s i t y i n c h a r s h a s a b i g e f f e c t o n
t h e t e x t u r a l p r o p e r t i e s o f t h e a c t i v a t e d m a t e r i a l s . I n f a c t , as c a n be s e e n i n F i g u r e 1 , t h e g r e a t e r t h e s u r f a c e area o f t h e c h a r s , t h e g r e a t e r t h e s u r f a c e area o f t h e a c t i v a t e d c a r b o n s .
In t h e p r e p a r a t i o n o f
activated carbons of
t h e c o n t r o l of o p e r a t i o n a l parameters
g r e a t importance is
for tailoring their texture
f o r s p e c i f i c a p p l i c a t i o n s . Flow r a t e o f o x i d i z i n g g a s a n d p a r t i c l e size,
together
with coal
preoxidation,
a r e two o f
t h e parameters
t h a t c a n b e u s e d i n t h i s way. In
t h e c a r b o n - CO,
react,ion,
the
inhibitory
e f f e c t of
t h e CO
produced c a n g i v e rise t o non-uniform g a s i f i c a t i o n , o f t h e p a r t i c l e . Rand a n d M a r s h ( r e f . 1 8 ) s u g g e s t e d t h a t a n i n c r e a s e i n CO c o n c e n t r a tion
i n reactant
uniformity
of
results
in
gasification.
In
concentrations in
gas
enhancement of
order
to
create
the
degree of
different
CO
r e a c t i o n area,
a series of
i n w h i c h two d i f f e r e n t CO,
flow rates,
t h e v i c i n i t y of
experiments w e r e performed,
an
7 a n d 500 m L m i n - l , w e r e u s e d .
the
444
I
4
A
1.000
\
-€
800
a W
01
a
600 0
" 1
0
0
0
COAL
O-0
V
I
I
I
I
72
48
24
0
T I M E O F C O A L OXIDATION, h
Figure 1. Variation of CO, surface area of coal, chars and activated carbons, with the time of coal oxidation. Coal particle size: tl.OO - 3.00 mm. Temperature of oxidation: 5 4 3 K .
COAL PREOXIDATION h
x x x
n
X I
+---I
K
____
W
a W
500 mL C 0 2 / r n i n
xx xx xx xx xx x x x x x
l
.
*
I
$aiilRp >50 nm n3.75 n o t t o p e r t u r b its composition. The product of
The
t h e r e a c t i o n w a s f i l t e r e d , dried i n vacuum and heated a t 350OC. solid
obtained
was
pressed,
ground
and
0.25-0.50 mm being taken f o r c a t a l y t i c s t u d i e s .
sieved,
the
fraction
498
In o r d e r t o s t u d y t h e influence of t h e shape-forming
s t e p , two
methods of shaping w e r e u s e d pressing of t h e powder with a r o t a r y 4.8 mm
p r e s s PTM-41 or extruding of t h e w e t pulp. Tablets with 5 =
and L = 5-6 nun or r i n g s with t h e o u t e r R = 4.8 nun and inner R = 1.2 nun w e r e obtained.
The
activation
extruding.
was
step
were
They
studied
activated
by
with
heating
tablets
in
obtained
by
stream of
the
gas
m i x t u r e s of d i f f e r e n t compositions in t h e conditions n e a r t o t h o s e used in
the
catalytic
experiments
and
then
rapidly
cooled
to
room
t e m p e r a t u r e i n helium.
C a t a l y t i c properties
w e r e determined in a q u a r t z
of t h e Temkina-Kulkova type, and t h e v a r i a t i o n s of in t h e 121.
of
course
The
the
reacting
w e r e followed in t h e
reaction
mixture
flow r e a c t o r
these properties
contained
1.45-1.50
pulse r e a c t o r
vol.%
C4H10
of
in
air.
analysis
Structural
way
the
Brucker
described IFS-113V
in
was
C31.
carried
FT-IR
spectrometer
out
by
with
the
X-ray
were
spectra
diffraction
recorded
resolution
of
4
in
with
the
cm-'.
The
samples w e r e pressed with K B r i n t o t h i n w a f e r s .
Details
of
the
e l e c t r o n microscope described Erba out
in
2000 with
experimental
131. The
pore
porosimeter. the
help
procedure
used
in
scanning
< S E M > and photoelectron spectroscopy CXPS> are
of
structure
The
a
determined
was
thermogravimetric
Paulik-Paulik-Erdey
with
D
Q-1500
a
was
analysis
Carlo
carried
derivatograph
in t h e helium atmosphere.
RESULTS AND DISCUSSION P r e c u r s o r Eynthesis After
mixing
of all components
in
the
autoclave,
temperature
w a s r i s e d t o 104OC, a t which t h e s y n t h e s i s w a s c a r r i e d o u t . Results
of
s t r u c t u r a l a n a l y s i s revealed t h a t after 3 hours of t h e s y n t h e s i s
a
set
of
lines
< m o s t intense
0.438,
0.340
characteristic
and
0.288
nm
also p r e s e n t . Since t h e s e of
V205
C61 t h e y
may
VOHP04.0.5H20 lines in of
the
that bands
synthesis
even C420,
for
lines are given in
after
490,
be
with
. H o w e v e r ,
other
lines
intesity
to the
Results beside
690,
VOHP04.0.5H20
are n e a r
assigned
respect
645,
the
values
15 hours,
527,
Table
930,
to
V205.
ratio the
The
standard,
of
the
strong
1:0.85:0.63
reference
FT-IR
with
analysis
VOHPO4.0.5H2:
1105 and
were values
i n t e n s i t i e s of increase
1130 cm
at
the time
shows
absorption
>
also
weak
499
bands a t 505, 995 and 1025 cm-' to
V205
171.
As
indicated
are p r e s e n t . They can b e assigned
by
the
given
data
in
Table
2,
the
catalysts obtained f r o m t h i s m a s s are n o t s e l e c t i v e i n t h e n-butane oxidation.
TABLE 1
X - r a y phase c o n t r o l o f t h e c a t a l y s t s y n t h e s i s
d,
-
__
0.570 0.451 0.367 0.328 0.293 The
Time of s y n t h e s i s , h o u r s 6
3
intensity
I/Io, -65 40
0.570 0.451 0.367 0.328 0.292
33 40 75
of
I/Io, -70 42 35
d , nm
X
0.293
line
15 d , nm
X
I/Io,
X
-
50 79
characterictic
0.571 0.450 0.367 0.328 0.293
36
for
VOHP04.1/2H20
75
44
49 84
the
phase w a s t a k e n as 100%. In
order
to confirm t h i s conclusion t h e
w e r e s e t up. The sample of
V205
was devided
following into
experiments
three
different
granulometric fractions (wgt %> : A: or
coke deposite. The X P S study revealed both t h e NO
polyvinyl
t h e t a b l e t s . U r e a and urotropin (samples
increase in porosity of 14 and
CPEO)
polyethylenoxide
of
presence
a
of
film
of
NHx
or
presence of
groups and significantly higher c o n t e n t of s u r f a c e carbon.
C a t a l v s t activation The effect of t h e the
phase
properties
has
already
of
composition
composition
of been
the
activating
catalyst
considered in
and
our
m i x t u r e s on
gas
their
earlier
catalytic paper
C121.
It w a s shown t h a t t h e presence of a reducing a g e n t (including t h e reaction
product)
leeds
to
formation
of
a
metallic
phase
of
a
promotor which in t u r n is adversly a f f e c t i n g t h e process of n-C4H10 partial
oxidation. A
reduction of
change of t h e vanadium C+4> 5).
An
excess
of
oxidant
the
promotor
takes
place
without
valency, as i t w a s proved by XPS (Table during
the
activation
leeds
to
of vanadium also without change of t h e promotor valency
x
f3
0m
v1
Fig.3. R a t e of butane oxidation (1, and s e l e c t i v i t y t o m a l e i c anhydride C 2 > as a function of WO>2P207 concentration
oxidation
505
Better
were
results
the
mixtures;
resulting
obtained
catalyst
by
activation
phosphate. The d a t a i n Table 6 show a n effect, of the
activating
VOHP04.1/2H20 present the
in
n-butane
(pulse
significant
this
the
on
the
phase
oxidation,
technique,
selectivity
reactive
and
promotor
Promotor
content in
C4HI0
formation
CVO>2P207
phosphate
is
t h e selectivity pulse)
growth
and
the
corresponding
maleic
of
between anhydride content.
2F’207
to
the
from
already
Fig.3 shows t h e dependence
2-nd
is c l e a r l y
content
of
rate
precursor.
in t h e initial catalyst.
rate of
formation A
mixture
with
2P207 f o r m a t i o n
P 0
Butane
v0l.X
6 hrs
2.1 1.7
55
after a c t i v a t i o n , % 2 2 7 24 h r s 48 h r s 96 h r s
1.5
1.1
A
possible
experiments
on
were
58
of
oxidation the
lower
the
vanadium
performed
in
This
than
0.5
condition
vol.% and
of
valency the
d a t a i n Table 7 i t is s e e n t h a t lower
catalyst
study
should
100 93
92
88 90 70
82
48
w a s c o n t r o l l e d by
reactor
concentrations
a6 ao
73 72 74
53 50 30
95 83
zone
an
C4HI0
determines t h e ultimate
in
effect
during
the
differential
be
150 h r s
the of
fixed-bed
low
butane
activation.
The
reactor. From
the
c o n c e n t r a t i o n should n o t b e in
the
range
conversion
of
0.7-0.55
a hydrocarbon
during t h e a c t i v a t i o n by t h e r e a c t i v e mixture.
TABLE 7 E f f e c t of t h e n - b u t a n e c o n t e n t on t h e s u r f a c e vanadium s t a t e
Binding e n e r g i e s of V2p3/2 ~
photoelectrons i n e V
~~
Butane i n a i r , Vol.%
Time of t h e a c t i v a t i o n , h o u r s
24
72
96
1.5
517.7 517.7
0.3
517.7
517.6 517.7 517.8 517.8 517.9
597.6 517.6 519.0 517.7, 519.0 517.7, 519.0
0.7 0.5
0.15
517.7
517.8
v01.Z.
506
Careful consideration of the
opportunity
highly-effective
to
select
catalysts
each s t a g e of t h e s y n t h e s i s creates conditions
of
the
preparation
for t h e oxidation of n-butane
anhydride. REFERENCES
I 2 3
4
V.A.Zazhigalov, and V.M.Belousov in C a t a l y s i s and P r o g r e s s in C h e m i c a l E n g i n e e r i n g CRuss), N o v o s i b i r s k , 1984, p 218. V.A.Zazhigalov, Yu.P.Zaitzev, V.M.Belousov, M. W o l f and N. W u s t n e c k , React.Kinet.Catal.Letters, 24 (1984) 375. V.A.Zazhigalov, V.M.Belousov, G.A.Komashko, A . I . P y a t n i t z k a y a , Yu.N.Merkureva, A Z P o n y a k e v i c h , J.Stoch and J . H a b e r , Proc.9t.h Catal., Calgary, vo1.4 (1988>, p.1546. 1nternat.Congr.on J.W. Johnson, D.C.Johnaon, A. J.Jacobson and J.F.Brody, J.Am.Chem. SOC., 106 (1984) 8123.
V.A.Zazhigalov, V . M . B e l o u s o v , H.Ludwig, G.A.Koma9hko and A.I. P y a t n i t z k a y a , Ukr.Khim.Zhurn., 54 (1988) 35. 6 A.S.T.M. 1967. 7 L. A b e l l o , E.Husson, Y R e p e l i n and Q. L u c a z e a u , J.SoLid State 8. H.S.Horowitz, C.M.Biackst.one, A . W . S l e i g h t and G . T e u f e r , A p p L C a t a l y s i s , 38 (1988) 193. V.M.Belousov, A.I.Pyatnitzkaya G.A.Komashko, 9. V.A.Zazhigalov, A.V.Chkarin and L.S.Khuzhakaeva, Zh.Prikl.Khim., 61 C 1 9 8 8 > 101. 10 M.I.Temkin, Kinet.Katal., 16 C1975> 504. 11. V.A.Zazhigalov, V.M.Belousov, N.D.Konovalova, Yu.N.Merkurieva, A . I . P y a t n i t z k a y a and G.A.Komashko, React.Kinet.Catal.Letters, 38 C19893 147. 12 V.A.Zazhigalov, V.M.Belousov, A . I . P y a t n i t z k a y a G.A.Komashko, Yu.N.Merkureva and J.Stoch, i n G C e n t i and F . T r i f i r o (Ed.> New D e v e l o p m e n t s in Selective O x i d a t i o n , Univ.Bologna, Bologna, 1989, P r e p r . C 8. 5
of
to m a l e i c
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATI ON
VANADI A
WELL DISPERSED
OF
CAraYsTs
BY
507
ULTRA-HI
GH
I NTENSI yry GRI NDI NG AT AMBI ENT TEMPERATURE 2. S O B A L f K ' ,
O.B.
'Institute
LAF'INA
2
and V . M .
Inorganic
of
MASTIKHIN
Chemistry,
2
Czechoslovak
Academy
of
M a j a k o v s k g h o 2 4 , 160 00 P r a g u e C C z e c h o s l o v a k i a 3 .
Sciences,
'Institute
Siberian Branch
Catalysis,
of
of
t h e USSR
A c a d e m y of
S c i e n c e s , N o v o s i b i r s k 630 090 C U . S. S. R . 3
SUMMARY V a n a d i u m oxide s u p p o r t e d c a t a l y s t s have been p r e p a r e d by t h e u l t r a - h i g h i n t e n s i t y g r i n d i n g of m i x t u r e s of V 2 0 5 w i t h oxidic supports at t h e a m b i e n t t e m p e r a t u r e . TPR, UV/VIS diffusion reflectance, ' H NMR and 'lV NMR m e a s u r e m e n t s f o r t h e V 0 - A 1 0 a n d V O5 - T i 0 2 ' b o t h a n a t a s e a n d 2 5 . 2 5 r u t i l e s h o w t h a t t h e v a n a d i a l a y e r on prepared m a t e r i a l i s w e l l spread and t h a t there is a significant interaction w i t h the
tge
support p r e p a r e d by t h i s m e t h o d have c a t a l y t i c T h e V 0 -Ti02 s a m p l e s activity N O -NH3 r e a c t i o n c o m p a r a b l e t o t h e c a t a l y s t s p r e p a r e d by i m p r e g n a t i o n .
is
I N T R O D U C T I ON V a n a d i um dioxide
oxide
have
sel e c t i v e reduction
oxidation of
catalysts
received
NO
such catalysts
X
by
much
of
or gani c
ammonia.
have been
on
supported attention
a1umi na
both
compounds
Various
suggested i n
as and
methods the
for
or
ti t a n i u m
catalysts
for
cataly t i c
for
preparation
literature,
one
of
of the
l a t e s t r e v i e w s o n t h e s u b j e c t w a s g i v e n by B o s c h and J a n s s e n C r e f . 11.
The i n t e r a c t i o n
b e t w e e n t h e vanadia and t h e s u p p o r t
is m o s t l y
regarded a p r e r e q u i s i t e i n t h e r e a c t i o n a c t i v i t y .
As
reported
CUHIG-treatment3
previously
of
t e m p e r a t u r e w a s f'ound c h a n g e of
deeply
treatment
with
A1203
or
TiO,
t o c a u s e a vanadia-support
t h i s work
the
intensity
ultra--high
i r a n a d i u m C V> e n v i r o n m e n t s C r e f .
T h e a i m of more
Vg05
the
at
ambient
i n t e r a c t i o n and a
23.
w a s t o e x a m i n e by d i f f e r e n t
vanadia-support
grinding
interaction
during
techniques the
UHIG
i n o r d e r t o o b t a i n m o r e c o m p l e t e p i c t u r e of t h e process
a n d t o d e t e r m i n e c a t a l y t i c a c t i v i t y of t h e r e s u l t i n g m a t e r i a l .
508 EX PER1 MENTAL
Mater i a1s
2
3 m /g
The T i 0 2 C r u t i l e ;
3 a n d V20s
p . a . q u a l i t y , y A1203 CGOST 8136-85; 2 81 m /g3 w a s p r e p a r e d b y p r e c i p i t a t i o n of
NH OH.
4
w e r e c o m m e r c i a l products 2 220 m /g3. T i 0 2 C a n a t a s e ;
f r o m TiC14 s o l u t i o n
with
a n d c a l c i n a t i o n a t 5OO0C f o r 4 h o u r s .
drying at llO°C
Methods. Sampl es p r e p a r a t i o n Four methods a r e u s e d t o p r e p a r e t h e v a n a d i a c a t a l y s t s . Ci3 UH I G t r e a t m e n t
A m e c h a n i c a l m i x t u r e of c a t a l y s t components w a s t r e a t e d f o r u p to
20
min
metallic
in
a
mill
spheres
CO. 6
the
material
diameter3
the m i l l
r o u n d t h e c e n t r e of planet-like
where cm
in
disintegrated which
by
rotated
a n d a t t h e s a m e t i m e revolved
r o u n d i t s own a x i s .
movement
was
containers
in a
I n t h i s arrangement
the
s p h e r e s a c q u i r e d a n a c c e l e r a t i o n of a b o u t 20 g. C i i 3 C a l c i n a t i o n of s i m p l e
mixtures
A m e c h a n i c a l m i x t u r e of
components w a s c a l c i n e d for
catalyst
1 4 h a t 500OC. C i ii3 W e t impregnation
or
A1203
Ti02
was
impregnated
by
c x a l s + . e . The s a m p l e s w e r e d r i e d a t l l O ° C
solution
a
of
vanadyl
f o r 2 h and t h e n c a l c i n e d
i n air f o r 3 h a t 500OC. Civl G r a f t i n g Samples w e r e p r e p a r e d N2
carrier
gas
with
the
b y i n t e r a c t i o n of support
samples
V0Cl3
calcined
vapour
at
at dry
20OOC.
The
h y d r o l y s i s w a s t h e n c a r r i e d o u t a t t h e s a m e t e m p e r a t u r e i n wet a i r . C a t a lvst characterization
The EET s u r f a c e measurements
of
t h e c a t a l y s t s w e r e performed
by t h e D i g i s o r b 2600 a p p a r a t u s , m e r c u r y p o r o s i m e t r y b y means of t h e Auto-Pore The
9200 C M i c r o m e r i t i c s , USA3 a p p a r a t u s .
solid-state
proton
NMR
spectra
with
MAS
s p i n n i n g method> t e c h n i q u e h a v e b e e n r e c o r d e d on a spectrometer.
magic-angle
The e x p e r i m e n t a l p a r a m e t e r s a n d t h e d e t a i l s of vacuum
p r e t r e a t m e n t of Diffuse
C
Bruker CXP 300
s a m p l e s h a v e been
reflectance
described previously Cref.
measurements
performed
were
by
33.
UV/VIS
s p e c t r o m e t e r Shimadzu MPS 2000 w i t h d i f f u s e r e f l e c t a n c e a t t a c h m e n t i n t h e w a v e l e n g t h r a n g e of 250-700 nm u s i n g MgO as t h e r e f e r e n c e . The
'lV
NMR
spectra
were
obtained
s p e c t r o m e t e r as d e s c r i b e d p r e v i o u s l y C r e f .
on
23.
a
Bruker
MSL-400
509
Ctemperature
TPR performed
a
in
programmed
conventional
reduction3
flow
experiments
apparatus
with
were
thermal
-1
and a f l o w c o n d u c t i v i t y d e t e c t o r u s i n g a h e a t i n g r a t e of 20 K min -1 . The amount of s a m p l e u s e d w a s of 80 m l min of 1 0 % H2 i n N chosen
so
that
the
amount
of
e x p e r i m e n t s w a s a b o u t 5 mg.
vanadium
in
the
A l l measurements
reactor
most
in
w e r e done on samples
0
a f t e r a t least 2 h o u r s of c a l c i n a t i o n a t 500 C i n a i r .
tests w e r e
Catalytic reactor. of NH3,
3% of
gram of
sample.
NO/NO
carried
out
in
an
integral
isothermal
The f l o w r a t e of t h e r e a c t i o n m i x t u r e CO.40% of NO, 02, n i t r o g e n as t h e c a r r i e r
g a s 3 w a s 30 N 1 h
0.40%
-1
per
Reagent and r e a c t i o n p r o d u c t s w e r e a n a l y z e d u s i n g
chemi 1umi n e s c e n c e a n a l y z e r 951A C B e c k man3.
X
RESULTS AND D I S C U S S I O N C a t a1ysts Char a c t er i z a t i on
Specific surface The u n t r e a t e d p u r e s u p p o r t s or m i x t u r e s w i t h V205 influenced
by
calcination
at
500°C
with
exception
a r e n o t much of
mi x t u r es where s o m e si n t e r i ng w a s o b s e r v e d . Marked d e c r e a s e of
was
alumina
caused
by
t h e s p e c i f i c s u r f a c e of the
UHIG
treatment.
The
V205 -Ti02
b o t h a n a t a s e and decrease is
even
h i g h e r i n case of V 0 -Ti02 C a n a t a s e 3 m i x t u r e s . 2 5 Tab. 1 S p e c i f i c s u r f a c e of v a n a d i a s a m p l e s
v205v2°5 w t%
-
2. s 5.0 7.5
V 0 - Ti0 2 5 2
Al2O3
2 -1 S. m . g
'2OS
21 8
-
[a1 Cbl
a
wt%
[a1 [bl 1.5 3.0 7.0 12.0
46
A1203;
UHIG-treated
a
A1203.
2
m .g
-1
81 69 27
-
112 142 155 103
25.0
S,
31
34 12
anatase;
UHIG t r e a t e d anatase.
The p o r o s i t y measurements p r o v e d t h a t p r e f e r e n t i a l l y
small
diameters
calcination
of
o r i g i n a l alumina e x h i b i t s l i n e s w i t h a chemical s h i f t
of
are
filled
or
sinter
during
p o r e s of
UHIG-pretreated V 0 0 i O 2 samples. 2 5
' H NHR measurements The
6 = -0.6 ppm,
which b e l o n g s t o t h e b a s i c OH-groups
and l i n e s with
510 chemical
shift
of
OH-groups
.
C s e e Fig.
1a.d.
3.0
about
The s p e c t r u m
ppm
to
belonging
more
the
acidic
of o r i g i n a l
Ti0 contains l i n e s with 2 6 = 1 . 5 . 3 . 6 and 6.7 b e l o n g i n g t o d i f f e r e n t OH bands on t h e s u r f a c e p r o t o n s a t t h e alumina and 20 -1 g , respectively.
The whole q u a n t i t y of
a n a t a s e w a s found a b o u t
g-land
I d 0
3.0
3.6
I
1
40
I
I
I
20
I
I
-
I
I
I
I
I
20
40
-20
0
0
PPD
Fig.1. 'H N M R MAS s p e c t r a of A l UHI G t r e a t e d vanadi a m i x t u r e s C 7 a n a t a s e Cd>.
%
2%of
A t samples w i t h 7 w t X of i n t e n s i t y decreased C s e e Fig.
V205
Ca>
,
I
I
I
-20
PPm
Ti0 C c >
and
the
2 V 0 3 w i t h alumina C b> or 2 5
on b o t h s u p p o r t s t h e s p e c t r u m the m o s t
Ib.d>,
prominent
decrease
w a s i n d i c a t e d f o r m o r e b a s i c OH-groups on alumina C 6 = -0.6 p p d a n a t a s e Cb 1 . 5 and 3.6 p p d this
band
even
fully
.
or
Gn alumina sample w i t h 25 wt% of V205
disappeared.
The
other
OH
bands
on
both
s u p p o r t a r e less i n f l u e n c e d by t h e p r o c e s s .
Diffuse ref lectcrnce spectra The p o s i t i o n of
V5+
ions Cdo>
the
environment
octahedral
t h e high i n t e n s i t y charge-transfer
i s s t r o n g l y i n f l u e n c e d by t h e number
of
the
central
ions
Cref.
band
of
ligands i n ions
in
c o o r d i n a t i o n g i v e s CT band a t 400 t o 480 nm r e g i o n .
By
41.
The
of
V5'
511 d e c r e a s i n g t h e c o o r d i n a t i o n number of t h e c e n t r a l i o n and forming a t e t r a h e d r a l c o o r d i n a t i o n t h i s band s h i f t s towards t h e h i g h e r e n e r g y
b e f o r e C cl and a f t e r U H I G Lregt%e:g?al. S p e c t r a of p u r e V20S2b%ore C d3 and a f t e r 20 min of UHIG t r e a t m e n t Cbl.
The central
decrease ion
of
the
coordination
and f o r m a t i o n p r o b a b l y of
a
number
V5+
s p e c i e s d u r i n g t h e WIG t r e a t m e n t of
or
TiOa w a s i n d i c a t e d by a marked s h i f t of
t h e h i g h e r e n e r g y r e g i o n , C s e e Fig.
of
the
tetrahedraly
vanadium
coordinated
mixtures with A l 0 2 3 t h e a b s o r p t i o n towards
V205
2a> where r e s u l t s on V205-A1
mixtures are presented .
0
2 3
5fv NMR spec t r a The assignment of
t h e s i g n a l s w e r e made on t h e b a s i s
of
the
512 d a t a o b t a i n e d i n p r e v i o u s p a p e r s C r e f . 5.61. w i t h a n axial ppm,
vanadia environment e x h i b i t s a l i n e
with an octahedral
Ve05
a n i s o t r o p y of
6,, = - 1270 p p d
t h e chemical
small
with
quadrupole effects C s e e Fig.
s h i f t tensor
peaks
-5 00
-1500
-1000
Cdl
-310
=
first
the
order
3a3.
I
0
to
due
.
0
I
.
-500
l
.
1
-1500
-1000
PPm
.
PPm
F i g . 3. 51V NMR s p e c t r a of V 0 C a > . a n d i t s m i x t u r e s w i t h a l u m i n a C b l C 5 w t % of $ Q I2,’and T i s C c 3 C a n a t a s e . 3 wt% of V 205> a f t e r c a l c i n a t i o n a t 520°C i n a i r . 5 1 NMR ~ s p e c t r a of v - anatase mixtures c 5 wt% v205> d u r i n g i n c r e a s i n g t i m e of &?G t r e a t m e n t Ce-93. P u r e V205 a f t e r W I G t r e a t m e n t Cd3. After
calcination
at
mixtures
about,
u n r e a c t e d CFig. can
be
3 b.c1.
attributed
of
510 OC
untreated for
1 4 hours
pure
V205
to partial
C s e e
or of
V205
The l o w i n t e n s i t y l i n e a t a b o u t formation
d i s t o r t e d t e t r a h e d r a l environment. the
V 0 -A1203 2 5 t h e bulk
Fig.
3d3
During
only
the
of
a
the local
d i s t o r t e d w h i l e t h e g e n e r a l crystal s t r u c t u r e of
remains
-570 ppm. V
in
treatment
of
environment
is
species
UHIG V
V 0 -Ti.O 2 5 2
Ve05
with
is r e t a i n e d .
Much m o r e p r o f o u n d i n t e r a c t i o n c a n be i n d i c a t e d f r o m t h e i r
NMR
s p e c t r a d u r i n g t h e t r e a t m e n t of m i x t u r e s of v a n a d i a w i t h t h e o x i d i c
513 supports C s e e Fig. the signal
I t results
3e-gl.
-310 ppm from Ve05
at
s p e c i e s w i t h NMR s i g n a l
in
g r a d u a l d i s a p p e a r a n c e of
and p a r a l l e l
a chemical
having
f o r m a t i o n of
a new
in
t h e r a n g e of
-500 t o -700 ppm.
Most p r o b a b l y
all
shift
this
new
to
attributed havi ng
lines
environment
with
CRef.
These
oxygen
atoms
new
forms
prevai1
e v i d e n t 1y prepared
by
all
three
Fig.
43.
be
atoms
t e t r ahedr a1
a
2.3.
can
vanadi um
UHIG
ramp1 es
in
treatment
supports
used
on
Csee
I t s h o u l d b e n o t e d t h a t 51V
NMR
spectra
treated
at
of
the
procedure a r e s i m i l a r o b t a i ned
for
calcination
2.5.63 I
~
+
L
t o those
prepared
conventional
3
UHIG
vanadi a suppor t e d
catalysts
-
samples
after
5OO0C
by
impregnationprocedure
Cref.
.
A
- 5 0 0 -1000 -1500 PPm
0
Fig. 4 5 1 ~NMR s p e c t r a of Ca3, and i t s m i x t u r e s w i t h and a1 umina C b3 C 2 5 w t % of V Cc3 C 1 2 wt% of V r u t i l e Cd3 C 7 . 5 w t % of V 0 3 , a f t e r 20 min of UHIG t r e a t m e n t .
g 2 , Iga?ase
$2,
2 5
T e m p e r a t u r e progrcunmed r e d u c t i o n
The
reduction
successive s t e p s , V 0
t o VsO13
peak
shifts
2 5
supposed
of
the
the f i r s t
Cref.
bulk
VzOs
peak
c o r r e s p o n d s t o t h e r e d u c t i o n of
this
unsupported bulk
in
a
number
of
7 > . For s u p p o r t e d samples p o s i t i o n of t h e f i r s t
markedly t o l o w e r
that
procceds
V205
technique and V20s
temperature Cref. could
83.
discriminate
i n a d i s t i n c t s t a t e of
I t s h o u l d be between
the
interaction
with t h e support. On c a t a l y s t s p r e p a r e d by i m p r e g n a t i o n or g r a f t i n g t h e v a n a d i a -
s u p p o r t i n t e r a c t i o n i s r e f l e c t e d by a d e c r e a s e of t h e f i r s t maximum
for a b o u t 100°C
C s e e Fig.
t h e position
5a,b,g3
if
of
compared
514 w i t h t h e bulk V20s. a l o n e h a s p r a c t i c a l l y no e f f e c t
The UHIG t r e a t m e n t of t h e V20s on t h e p o s i t i o n calcination
of
of t h e f i r s t maximum C s e e F i g . untreated
Nevertheless m o s t
5d.e).
of
S j 3 . A s a r e s u l t of with
V205
M203
TiO,
or
r e d u c t i o n peak a t lower t e m p e r a t u r e emerged C s e e
Canatase> a small Fig.
mixtures of
t h e reduction still
proceeds
at
temper a t ur es c h a r a c t e r i sti c of unsuppor t e d V205.
400
LOO
600
200
800
400
600
800
200
400
600
tO,C
tO,C
800
tO,C
Fig.5. TPR p r o f i l e s f o r V 0 -Al f 3 m i x t u r e s w i t h C 5 w t % of VzOEsl b e f o r e Cdl and a f t e r UHIS ?re, ment Cc>; sample p r e p a r e d by impregnation C 9 . 3 wt% of V20s3 C b3 or by g r a f t i n g C7.0 w t % of VeOsl Ca3 . TPR p r o f i l e s f o r C& -Ti0 Canatase3 m i x t u r e s C 5 w t % WIG Ereatment C f l . ; C g > sample b e f o r e C e > and a f t e r p r e p a r e d by i m p r e g n a t i o n C 5 . 7 A % of V 0 >.
'fa%>
2 5
TPR p r o f i l e s f o r V
- r u t i l e m i x t u r e s C 7 . 5 w t % of V 0 3 b e f o r e Ci3 and a f t e r WIG2 3 r e a t m e n t C h> ; p u r e V20s b e f o r e CkT 2nd a f t e r UHI G t r e a t m e n t C j l . No
d e c r e a s e of
the
r u t i l e mixtures C s e e Fig.
reduction
temperature was
indicated
With all s u p p o r t s s t u d i e d t h e U H I G t r e a t m e n t r e s u l t e d characteristic t y p i c a l of
shift
of
for
5il. the
first
TPR peak
to
lower
i n the
temperature
d i s t i n c t v a n a d i a i n t e r a c t i o n w i t h t h e s u p p o r t Csee F i g .
5 c,f,hl.
A c t i v i t y at NO-NH3
Reported
in
reaction Fig.
6
are
catalytic
activities
of
anatase
or
515 r u t i l e s u p p o r t e d samples i n s e l e c t i v e c a t a l y t i c r e d u c t i o n of ammonia. The
NO by
samples a r e
a c t i v i t i e s o b t a i n e d f o r t h e WHIG-treated
comparable a t t h e whole t e m p e r a t u r e r e g i o n t o c a t a l y s t s p r e p a r e d b y impregnation.
The
untreated
samples
at
have
low
temperatures
i nf er i o r a c t i v i t y .
I
I
300
250
I
I
I
I
I
200
I
L
I
I
I
400
350
t,
450
OC
F i g . 6 C a t a l y t i c tests on u n t r e a t e d m i x t u r e s of anatase with 5 w t % C 0 3 o r 12 w t % of V 0 C 0 3 a f t e r c a l c i n a t i o n and s a m p l e s 5 p r e p a r e d by t h e WIG t r e a f m e n t of anatase w i t h 5 w t % C 0 3 or r u t i l e w i t h 1 2 wt% of V205 Ca3. Model f o r t h e c a t a l y s t p r e p a r e d by t h e U H I G t r e a t m e n t This w o r k vanadium
shows t h a t
oxide
it
by t h e UHIG
is
t o prepare w e l l
possible
treatment
of
Ti02 and A1203 a t ambient t e m p e r a t u r e s .
m i x t u r e s of
spread with
Vz05
The t r e a t m e n t c a n provoke
i n some cases s i n t e r i n g of t h e s u r f a c e a t s u b s e q u e n t h e a t i n g of t h e mixture.
W e b e l i e v e t h a t t h e c o n c e p t of
processes
by
energy
intensive
understanding t h e r e s u l t s C s e e r e f . mechanical elevation
deformation at
the
and
contact
s t i m u l a t i o n of
grinding
of
to the effect
of
useful
short-time
direct
impact
m a t e r i a l a g a i n s t t h e s p h e r e s a c c e l e r a t e d d i f f u s i o n of way s i m i l a r
solid-state
be
in
113. I t s h o u l d b e supposed t h a t
consequent
points
would
prolonged
heating
temperature of
the
solid
vanadia i n a
Cref.91.
but
the
516 e x t e n t of
t h e process is higher.
I n analogy t o impregnated o x i d i c systems C r e f . are involved
b a s i c OH-groups Results
of
consistent
reflectance
UV/VIS
with
i n t h e p r o c e s s of
the
concept
of
and
of
environment
o x i d e l a y e r w i t h predominantly t e t r a h e d r a l
spreading.
experiments
transformation
with square-pyramidal
c r y s t a l l i n e V205
NMR
51V
101, m a i n l y t h e
vanadia
the
are
original
i n t o a f o r m of
coordination during t h e
The i n t e r a c t i o n of v a n a d i a w i t h t h e s u p p o r t a t t h e
UHIG treatment.
r e s u l t i n g o x i d e l a y e r b r i n g s a b o u t a d e c r e a s e of t h e t e m p e r a t u r e of r e d u c t i o n s i m i l a r l y as i n t h e i m p r e g n a t e d or g r a f t e d c a t a l y s t s . I n comparison t o h i g h - t e m p e r a t u r e of
broad c o n c e n t r a t i o n r a n g e . by
the
WHIG
activity i n other for
spreading,
where o n l y p a r t
i s i n v o l v e d , t h e WHIG t r e a t m e n t p r o d u c e d v a n a d i a l a y e r i n a
V205
treatment NO-NH3
methods.
which
It
The v a n a d i a - s u p p o r t
resulted
reaction is
in
the
comparable
remarkable t h a t
interaction
V 0 /Ti02 2 5
to
catalysts
beyond
caused
catalysts
with
prepared
by
t h e a c t i v e supports
h i g h t e m p e r a t u r e s p r e a d i n g s h o u l d b e supposed C r e f .
91
r a t h e r good a c t i v i t y w a s f o u n d a l s o f o r r u t i l e s u p p o r t e d s a m p l e s . ACKNOWLEDGEMENT The
Nosov,
authors
M r . B. P.
and
Novosibirsk)
are
thankful
to
Zolotovskij
M s . 0. N .
Novgorodova,
CInstitute
of
M r . A. V.
Catalysis,
f o r providing t h e c a t a l y s t s preparation.
REFERENCES B o s c h , F. J a n s s e n , C a t a l . Today 2 C 1 9 8 8 1 369. Sobalik 0. B. L a p i n a , 0. N. Novgorodova a n d V. M. M a s t i k h i n , Appl . C a t a l . , 63 C 1 9 9 0 2 191. 3 K.I. Zamaraev a n d V . M . M a s t i k h i n , C o l l o i d s a n d S u r f a c e , 12 C 19842 4 0 1 . 4 G. L i s c h k e , W. Hanke, H.-G. J e r s c h k e w i t z a n d G. Ohlmann, J . C a t a l . 91 CIS853 5 4 . 5 0. B. L a p i n a , A. V. Simakov, V. M. M a s t i k h i n , S. A. Veniaminov a n d A . A. S h u b i n , J . M o l . C a t a l . , 50 C 1 9 8 9 1 55. 6 H. E c k e r t a n d I . E . Wachs, J . P h y s . Chem., 93 C 1 9 8 9 1 6796. 7 H. B o r c h , B. J . K i p , J . G. v a n Ommen a n d P. J . Gelling’s, J . Chem. Soc., F a r a d a y T r a n s . 1 , 80 C19841 2479. 8 H . Bosch a n d P. J . S i n o t , J . Chem. Soc. , F a r a d a y T r a n s 1 , 8 5 , 1 4 2 5 C1989I. 9 J . H a b e r , T. Machej a n d 1. C z e p p e , S u r f a c e . S c i . , 151 C19853 301. 10 B.M. Reddy, V . M . Mastikhin. i n M . J . P h i l i p s a n d M. Ternana CEds. I , P r o c . 9 t h I n t . Congr. C a t a l . , C a l g a r y , 1988, v o l . 1 , p. 82. 11 K. T k d t o v b , M e c h a n i c a l A c t i v a t i o n of M i n e r a l s , D e v e l o p m e n t s i n Mineral p r o c e s s i n g , V o l . 11, D. W . F e u r s t e n a u CEd. I , E l s e v i e r , Amsterdam, 1989. 1
2
H. ‘7.
I
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
517
DISPERSION AND PHYSICO-CHEMICAL CHARACTERIZATION OF IRON OXIDE ON VARIOUS SUPPORTS Weijie Ji, Shikong Shen, Shuben Li and Hongli Wang Lanzhou Institute of Chemical Physics, 730000, PR China ABSTRACT A somewhat slow but controllable impregnation method with the dipping of support into an excess of aqueous solution of (NH4)-j[Fe(C204)3].xH20 is adopted to prepare well-dispersed or monolayer-type femc oxide on various supports. Other methods with the same and other conditions are also used to make a comparison among them. The influences on the adsorption process are investigated and the suitable conditions are determined for the different supported systems. Extensive characterization has been done on the physico-chemical properties of these systems. The catalytic performances for different test reactions are also carried out on the various supported systems with distinctly different dispersions. INTRODUCTION Much attention has been devoted in recent literature to the phenomenon of interaction between oxides (refs. 1-3). The supported oxides in the form of three-dimensional crystallites whose properties are similar to bulk crystals, do not interact saongly with the support. However, when the oxide is dispersed in monolayer on the oxidic supports, the interaction between them becomes strong and the properties of such monolayer of the oxide differ from those of bulk oxide. Growth of three dimensional crystallites occurs only after a substantial fraction of the surface is covered by the monolayer and this is often the case for the oxides of 0,Mo, W, V, Re and Ni. Extensive studies have been devoted to these systems. For other systems, however, the state of dispersion is complicated, depending on the nature of the support, the preparative method and the conditions used in such process. In this paper, we tried to prepare well-dispersed iron oxide supported on various supports by different methods and investigate the physico-chemical properties of these iron oxides. EXPERIMENTAL PreDaration of catalvsts The supports employed in this study are y-Al2O3 (225 m2/g), MgO (23 m2/g), Zr02 (16 m2/g), Ti02 (43 m2/g) and Z n O Ti02 (anatase) was prepared by hydrolysis of T i c k . The other chemicals used are commercial products of analytical grade. Three methods were used to prepare the supported oxides. (I) Method of dipping into an excess of aqueous solution of (NH4)3[Fe(C204)3].xH20. An aqueous solution of the chosen compound with the proper concentration is added to the support particles under continuous stimng at room temperature or at 323 K. The dipping process is controlled by the contact time, pH of the aqueous solution and temperature.
* Supported by Chinese National Natural Scientific Fundation.
518
(11) Incipient wetness impregnation or dipping in an excess of aqueous solution of iron nitrate. The incipient wetness method was performed with a series of aqueous solutions of iron nitrate. The supported samples were also prepared by the dipping method into an excess of aqueous solution of low concentration. (111) Batch adsorption at high temperature (ref. 4). After adsorption, the particles were carefully washed and the remaining solution was removed by filtration. After impregnation or adsorption, all the samples were dried and calcined in air at 373 K for 2 h, and 773 K for 2.5 h, respectively. Characterization The loadings of Fe were determined by atomic absorption spectroscopy or chemical analysis. BET surface areas were measured by N2 adsorption at 77 K. XRD measurements were operated on a Rigaku/D/MAX-RB X-ray diffractometer with Cu Ka. ESR spectra were recorded at room temperature on a E-115 spectrometer operated at band frequency with 100 KHz field modulation, IR absorption spectra were recorded on diffuse reflectance attachment of a Nicolet IODX FTIR spectrometer. Laser-Raman spectra were taken on SPEX 1403, and 5145 A emission lines were used for excitation. UV-Vis diffuse reflectance spectra were recorded in the wavelength range 250-850 nm (Shimadzu UV-365) using MgO (S) as a reference. XPS results were obtained on Perkin-Elmer PHI-550 with Mg cathode (320mw). The intensities of the peaks were referred to the area of the peaks including the satellite peaks for Fe2p levels. TPR experiments were carried out at the temperatures in the range of RT to 973 K. Before reduction, the samples were pretreated in oxygen flow at 773 K for 0.5 h. The test reactions were pulse reactions of CO oxidation and oxidative dehydrogenation of butene to butadiene with or without gaseous oxygen supply. Continuous flow reaction of CO hydrogenation was also used to test the catalytic activity for those samples with different dispersions. The syngas was a 2.8:l HdCO mixture and a flow of 50 mVmin was used. RESULTS AND DISCUSSION Preparation of catalysts Van Ommen and coworkers (4) have developed a method for the preparation of monolayertype supported samples of femc oxide. However, this method has some limitations especially when using basic oxide supports. In the present approach, we med to use a new method, which requires only simple chemicals and operating conditions. When an oxide particle is brought in contact with an aqueous solution, surface polarization will occur. The sign and extent of surface charging will be determined by the isoelecmc point of the oxide (IEPS) and the pH of the aqueous solution. If the Fe3+ complex with negative charge is to be adsorbed in a monolayer, a positively polarized support surface is obviously required. With this consideration, (NH&[Fe(C204)3] .xH2O was chosen as raw material. Another reason for this choice is that the decomposition product in air of this compound is only ferric oxide. Because all the supports used in the experiment whose IEPS is above 5 and pH of this complex salt
519
:(d)r dds
~ = 2 34mnio:/L t=2h
0.3 0.2
* €1
,
.
0 4
-(b')
2
3
c=11.7 m m o 1/ 1
0.3 t (h)
t=2h
0.4 0.3
c(mriol/l)
support surfaces will be positively charged under these conditions and adsorption of the
complex with negative charge through electrostatic forces is expected to occur. On the other hand, the pH has an additional
effect determining the stability of a particular complex. At high pH, the hydrolysis becomes a noticeable side reaction. Fig. l(a) illustrates the influence of the pH of the aqueous solution o n the adsorbed amount of Fe and the dispersion.
1,' 1 8 . '
1a t i o n of a d s o r hed amount Fe. I g d l u m i n d , d t RT.
on, e n t
amount is quite small. This is against the prediction of the electrical double layer model. It should be noted that foreign ions can modify the IEPS and lead to competititve adsorption. So it is believed that the competitive adsorption of C2O4' is more favorable (
of
under this low pH condition. Adsorbed amount of Fe is increased with increasing the pH of solution. However, above p H 4 , the disintegration of the complex by the formation of a surface complex accompanied by substitution of C2O4= ligands or by hydrolysis on the camer surface becomes significant. Successive depositions on the covered surface rather than the free A1203 surface certainly make it difficult to obtain a "monolayer" type dispersion. Heating the solution may be helpful for the diffusion of the complex along the pore. In Fig. 1, the influences on the adsorbed amount of Fe by the concentration of solution and contact time are also indicated. As there is a relationship between pH and concentration of the solution, the pH will decrease and change in a narrow range during the dipping process when the concentration is relatively high. The adsorption of the complex in solution is in turn controlled by dipping. When the contact time was prolonged, the Fe loading increased slowly and continuously at the inital stage. Again, this increase is connected with the change of pH in solution. As the contact time changes from 0.5 h to 5 h, the final pH of the solution varies from 4 to 6. Therefore, the good dispersion of femc oxide cannot be obtained if the contact time is longer than one hour when the initial pH of the solution is relatively high and further increased during the dipping process. From our experiments, the suitable preparation conditions may be summarized as follows : (1) For y-Al2O3, the pH should be controlled in the range of 2.0-3.0, the dipping time is about 1 h and the concentration of the solution can be varied in a relatively wide range. (2) For Ti02 and ZQ2, because of their low values of IEPS, the pH of the solution is better adjusted at around 1.5 with the solution of H2C204.2H20. The solution temperature is maintained
520
at 323-328 K to accelerate the adsorption process and the contact time is prolonged to three hours. (3) For MgO and ZnO carriers, because of their basicity, especially for MgO, the pH of the solution should be below 2 so as to avoid the rapid disintegration of the complex. Moreover, a vigourous stirring is necessary. It seems that the most important factor in this case is the contact time. Usually, it must not be longer than two minutes. The third method is satisfactory for preparing monolayer-type materials except for the MgO and ZnO supports. The second method, however, is not suitable to get the uniformly dispersed samples in general. Characterization X-ray diffraction (XRD) shows no iron-containing crystalline phase for the samples prepared by the first and third methods. For the samples prepared by the second method, when the Fe loading is small, no XRD pattern can be attributed to the presence of iron-containing phase. However, as the Fe loading is increased above 3 wt%, the XRD pattern of a-Fe2O3 crystallite begins to appear. This loading is much lower than the threshold reported by Xie et al. (ref. 5). It is perhaps caused by the different preparation conditions used. Disappearance of the XRD patterns is probably caused by several reasons. So, we cannot be sure as to what the actual structure of the dispersed femc ions is, if judged only by the XRD results. The ESR results of the iron-containing specimens on the different supports give rise to rather different spectra, mainly depending on the location of the ferric ions and the interaction between them. As shown in Fig. 2, the iron-containing specimens on y-alumina, anatase, and ZrOz prepared by the first and third methods generate ESR lines at g14.28 and g2=2.02.3. The first signal is typical of high spin ferric ions in sites with rhombic symmetry. This signal is assigned to femc ions in rhombically distorted sites which may be tetrahedral or octahedral and "isolated" (ref. 6). The presence and the amount of this kind of ferric ions strongly depend on the nature of the support. In fact, on MgO F i g . 2. K S 4 s p p c t r a o f v a r i o u s well-dispeised s u p p o r t e d s a m p l e s . and ZnO, whatever the preparative methods used, this signal is always very weak indicating that the Fe3+ ions do not exist in the "isolated" state. The signal at g=2.0-2.3 for the different kinds of supported samples varies noticeably; the shape. and linewidth seem to be practically dependent on the preparative method and Fe content. This signal is assigned to the Fe3+ ions in the
521
nearest neighboring sites with strong coupling or only in the neighboring sites with a weak coupling. If the concentration of Fe on MgO is very low, the signal centered at g=2 almost disappears. For the femc ions on ZrO2, when the concentration of Fe is in the submonolayer range, an intense signal at g=4.28 and a very weak and broad signal centered at g=2 can be observed. As the concentration is increased near to full monolayer coverage, the intensity of the weak signal increases while the intensity of the signal at g=4.28 decreases significantly. l.FA(11)2.1 2. FA(I1I)l
5
3. FA(I)1.7
4 . F Z r (III)0.4
A few UV-Vis diffuse reflectance spectra are shown in Fig. 3. In the range 250-550 nm, the absorption bands are described as charge transfer bands (CT bands) (ref. 7). This process is rather bandto-band transition. The CT bands arising from the well-dispersed ferric ions shift of about 60 nm to the high wave numbers as compared with the crystallites of iron oxide. This shift is probably caused by the following reasons. The large distortions in symmetry at the surface increase the energy separation of the d orbitals in the cation. The ionic surfaces could reduce the ionic charges of the surface Fe3+ ions, and make the energy of the surface orbitals shift downward and enhance the covalence of the cation-anion bond. In addition to the situations mentioned above, two other causes may be responsible for the blue shift of the CT bands. One is the substitution of surface ions by Fe3+ ions and localization in these relatively constricted sites. The other
is the occupancy of the femc ions in smaller interstices such as the tetrahedral holes on 3 no 500 700 /np F i g . 3 . I J V - V i s DPS o f s a ~ p l e s the support surfaces. For ferric ions on s i i p p o r t e d o n d i f f e r e n t c r l ~I i e r s . alumina, both processes could happen but substitution process may be not so important on Ti02 and ZrO2 because of difference in valence. For femc ions on MgO, they may be restricted to the surface or form a surface chemical compound. Because all the octahedral holes are already occupied by the Mg2+ ions, the substitution could take place only in the octahedral sites and the occupancy in the tetrahedral holes. It is to be pointed out that the structural environment of the femc ions in the samples prepared by dipping into an excess of aqueous solution of iron nitrate always shows the characteristics of the "cluster" or microcrystalline MgFe204, which evidently differs from the behaviour of the ferric ions in the samples prepared by
522
the other two methods. The ferric ions have penetrated into MgO to a deeper level due to this preparation method. For ZrO2 support, there are roomier interstitial sites on it; the occupancy of these holes seems to be reasonable and can result in the red shift of the bands. This is the case for the sample with the near full monolayer coverage. When the Fe loading is in the submonolayer range, a large part of ferric ions become "isolated", and the CT bands shift to the short wavelengths. When the supported samples contain microcrystalline iron oxide, the spectra change noticeably. The positions of bands or edges appear in the long wavelengths, showing apparently the bulk characteristics. FTIR-DRS shows no characteristic infrared absorption of crystalline iron oxide in the supported samples prepared by the first and third methods below the monolayer coverage. An attempt was also made to study the Laser-Raman spectra of those samples of well-disersed ferric ions in the range 100-1OOO cm-1 with or without grinding the samples into fine powder. Generally, the Raman spectra are ill-defined because of broadening, and the lines are very weak. For the samples prepared by method (l), only an intense phosphorescence peak arising from Fe can be observed as the samples were ground into powder. Almost the same results can be obtained without grinding the samples by using back-scattering. For the samples prepared by method (3), very broad and weak lines centered at around 800 cm-1 can be found using 90°-scattering and are probably due to the surface femc ions. The line at about 192 cm-l is almost certainly due to the presence of a two dimensional phase of Fe2O3, which exists in Zr02-supported sample with nearly full monolayer coverage. XPS measurements are illustrated in ( k'e / A 1) Fig. 4. In order to get more accurate 0.2 results the peak area sensitivity factors are
0.1
used instead of peak height sensitivity factors. The results indicate that there is no threshold existing in the sample series prepared by incipient wetness method with
aqueous solution of nitrate. The dispersion state of femc ions on alumina is complicated 2 4 6 8 1 0 1 2 with increasing the Fe loading. The (Fe/Al), x I V 2 formation of "clusters" or crystallites of iron Fig. 4 . P h o t o e l e c t r o n i c r-esponse oxide with a broad size distribution and the (Fe/hl),ps v s . t h e molar r a t i o (Fc/Al), i n t h e s t i p p l e ser-ies solubility of ferric ions into the support pr-pared hy nethod(I1). lattice produce a changeable X P S response with increasing Fe loadings. In a word, a nonlinear relationship suggests a nonunifonnity in alumina coverage by iron species on alumina. TPR results of various supported samples indicated that the Fe3+ ions which are welldispersed at the support surfaces (on alumina, MgO, Ti@ and ZnO) cannot be reduced beyond the Fe2+ state. The stability is thought to be caused by the formation of surface compounds. The femc ions at the surface of MgO and ZnO are reduced with difficulty to Fe2+as compared with the other 0.0
523
supported systems. In the samples supported on Z d 2 , small amount of well-dispersed femc ions can be reduced to Fe through the formation of an intermediate surface compound and the reduction of the femc ions is relatively easier. CATALYTIC PERFORMANCES 1. Pulse reacb'on of CO oxidation
CO oxidation with the alumina-supported catalysts with different loadings and dispersions indicates that the catalytic activity seems to be independent of the dispersion of the femc ions on the support at high reaction temperature 693 K and with the supply of 0 2 . In this case, 0 2 must be highly activated and the density of this active oxygen is higher at the catalyst surfaces, and the adsorbed CO is rapidly attacked. When the temperature is decreased to 623 K or when no 0 2 is supplied at 693 K, the activity is obviously connected to the loading and dispersion. The activation of 0 2 by monolayer-dispersed Fe3+ ions becomes weaker as compared with that by crystallites of iron oxide. On the other hand, surface oxygens coordinated to "monolayer"-type Fe3+are relatively inactive and not easily removed by CO. The amount of this kind of surface oxygens is much smaller on the monolayer materials.
2. Pulse reaction of OXD of butene to butadiene The results are shown in Fig. 5. Generally, the monolayer catalysts have a certain activity either with or without the supply of oxygen. It thus reveals that only one atomic layer of femc ions is needed for this reaction. However, the oxidation-reduction properties of the surface femc ions strongly determine their activity. For instance, the MgO- and ZnO-supported samples have a low activity due to the difficult reduction of femc ions. The relatively superior performance of the
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5 . P r o d u c t d i s t r i h u t i o n o f 2-C4€18 p u l s o r . o a c t i o n o n t h e v a r i o u . ; s u p p o r t e d s a m p l e s w i t h d i f f e r e n t d i s p e r s i o n . (a). 1 - C / , 1 1 ~ ~ , ( h ) . r e s i d . , ( c ) . C O X a n d ( d ) . (:4Fiy (T=643 Y ) . Fig.
524
monolayer catalyst on y-alumina as compared with the other supported systems may arise from the structural factor. Because y-alumina has a spinel structure as that of y-Fe2O3, localization of ferric ions on the surface interstices makes the surface properties much similar to those of y-Fe2O3 rather than those of a-Fe203. The amount of COXproduct formed in OXD reaction is small on monolayer materials as compared with that for crystallites of Fe2O3 in high loading supported samples. The results indicate that the metal-oxygen bond strength in these samples may be responsible for the low combustion activity. Because of the presence of "isolated' Fe3+ ions, it can be expected that the density of lattice oxygens coordinated with this kind of Fe3+ ions is low on the surface. Therefore, it is not favorable for the combustion which requires a relatively high density of lattice oxygens in local environment. For the high loading samples supported on y-alumina, high yield of butadiene was obtained as reported earlier (ref. 8). The reason for this stimulation of the formation of butadiene is not yet clear. When an excess of these active lattice oxygen atoms which could result in combustion was removed in the first pulse, it helped the formation of butadiene in the second pulse on high loading sample. However, regeneration of surface active sites in the supported crystallites of iron oxide by diffusion of lattice oxygen is not very rapid but faster than that in monolayer catalysts. 3. CO hvdroeenation Dwyer and coworkers (ref. 9) have found that the catalytic activity of an iron foil is increased ten times when the foil is preoxidized in dry oxygen before exposure to a CO/H2 mixture. Reymond et al. (ref. 10) have reported that unsupported a-Fe2O3 is more active in the CO+H2 conversion than a prereduced oxide. However, our results do not agree with these observations. Because all the catalysts are pretreated in 0 2 for 0.5 h and then in He for 0.5 h at 560 K, respectively, before introduction of the syngas, an oxidized state of the catalysts can be expected. Surprisingly, no CHq is produced for either monolayer materials on various supports or supported crystallites of ferric oxide. This inactivity must be connected with the fact that the active sitedphases are absent on the catalyst surfaces under the reaction conditions. Of course, the low H2/CO ratio used in our experiment may cause a deposition of inactive carbon on the surfaces, which is at least a part of the reason for the inactivity. Temperature programmed reaction of the syngas indicates that no C& was observed until the temperature was raised to about 823 K for the supported crystallites of ferric oxide. In this case a part of ferric ions must be reduced to Fe which is responsible for CH4 production. Under the same conditions, no C& product could be found even when the temperature was raised to 873 K for the monolayer catalysts. Because the femc ions in the monolayer samples resist the reduction of Fe3+ beyond the Fez+ state, this further suggests that the well-dispersed Fe3+ ions including Fe2+ ions are not active. The superior catalytic activity of the unreduced a-Fe2O3 catalyst is due to the formation of very small crystallites of a-Fe and X-iron carbide which did not form in most cases in our experiments (ref. 11).
525
Conclusion Different methods were used to prepare well-dispersed or monolayer type materials on various carriers. A desirable dispersion can be obtained by the first method developed in our experiments as long as certain conditions are carefully chosen and controlled for the different systems. 'The location, coordination and M - 0 bond strength of those surface-dispersed ferric ions are strongly determined by the nature of the supports and the preparative methods. Significant changes in oxidation-reduction properties and catalytic performances for the most well-dispersed or monolayer dispersed samples suggest that there is a strong interaction between the support and supported component. REFERENCES 1 G.C. Bond, Famday Discuss. Chem. SOC.,87 (1989). 2 J. Haber, Pure Appl. Chem., 56 (1984) 1163.
3 4
5 6 7 8
Z. Iwasawa, Adv. Catal., 35 (1987) 187. J.G. Van Ommen, H. Bosch, P.J. Gellings and J.R.H. Ross, in Preparation of Catalysts IV (B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, eds.) Elsevier, Amsterdam, 1987, p. 151. Xie Youchang, Xu Xianping, Zhao Biying, Tand Youqi and Wu Gongbao, Proc. 4th Nat. congr. Catal., Teijing, China, 1988, Vol. 2 (1988) 1-E-29. (a) D. Cordischi, M.L. Jacono, G. Minelli and P. Porta, J. Catal., 102 (1986) 1. (b) G.T. Pott and B.D. Mchicol, Discuss. Faraday SOC.,87 (1971) 121. K. Klier, Catal. Rev., 1 (1968) 207. E. Rodenas, T. Lizuka, H. Katsumata and K. Tanabe, React. Kinet. Catal. Lett., 19 (1982)
341. 9 D.J. Dwyer and G.A. Somorjai, J. Catal., 52 (1978) 291. 10 J.P. Reymond, P. Meriaudeau and S.J. Teichner, J. Catal., 75 (1982) 32. 11 R.A. Dictor and A.T. Bell, J. Catal., 97 (1986) 121.
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G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation ofCata2ysts V 01991Elsevier Science Publishers B.V.. Amsterdam -Printed in The Netherlands
527
The use of chelating agents for the preparation of iron oxide catalysts for the selective oxidation of hydrogen sulfide. P.J. van den Brink, A. Scholten, A. van Wageningen, M.D.A. Lamers, A.J. van Dillen, J.W. Geus Deparmenr of Inorganic Chemistry, University of Utrecht, Croesestraat 77A, 3522 AD Utrecht. The Netherlands.
Iron oxide-on-silica catalysts have been prepared for the selective oxidation of hydrogen sulfide to elemental sulfur. Preshaped silica extrudates (Aerosil 0x50) were impregnated with aqueous solutions containing different precursors. The precursors used are ammonium iron(II1) EDTA, ammonium iron(II1) citrate, iron(1II) gluconate, iron(1II) chloride, iron(III) nitrate, and iron(II1) sulfate. After drying and heating in air the resulting iron oxide on silica catalysts were characterized using TEM, SEM, Light Microscopy, XRD, DRETS, and Temperature-Programmed Reduction. Moreover, the catalytic properties in the selective oxidation of H2S were tested. It is found that catalysts prepared from precursors that do not easily crystallize, such as the above mentioned chelated iron compounds, contain high quantities of small, highly active iron oxide particles (2-5 nm). However, catalysts prepared with precursors that crystallize readily, like most of 0 Catalysts containing small iron the simple iron salts, contain large iron oxide particles ( ~ 2 nm). oxide particles exhibit higher activities and selectivities. INTRODUCTION Oil refineries and natural gas plants often produce large amounts of hydrogen sulfide. The process utilized most for converting the hydrogen sulfide into elemental sulfur is the Claus process. In this process one third of the hydrogen sulfide is first combusted with molecular oxygen to sulfur dioxide: H2S
+ SO2 + H2O
+
In the subsequent thermal and catalytic stages of the Claus process, the remaining part of the H2S reacts with the S@ to elemental sulfur: 2 H2S
+
S@
3
1S , + 2 H20
Due to the unfavorable equilibrium, high levels of H2S conversion are possible only by removing the sulfur. A Claus plant therefore consists of a burner and two or three additional catalytic converters with intermediate sulfur condensers. Still 3 to 5 % of the H2S has finally not reacted to sulfur. At the University of Utrecht a new catalytic process has been developed to oxidize these low concentrations of H2S selectively to sulfur without establishing the equilibrium of reaction (2). The newly developed process involves direct oxidation of hydrogen sulfide to elemental sulfur [l]:
In order to obtain high sulfur yields in excess of oxygen, the following three reactions leading to SO2 and, hence, affecting the selectivity adversely, have to be inhibited or at least minimized:
528
I) (sequential) oxidation of elemental sulfur in excess of oxygen:
IS"
+
02
+ so2
(4)
11) direct (parallel) oxidation of H2S according to reaction (1); In) establishment of the equilibrium of reaction (2).Because Claw tail gas contains large amounts of water vapour (up to 30%), the equilibriuminvolves appreciableconcentrations of H2S and SO2 (2). By using an iron oxide (Fe2O3) precursor a high selectivity in the oxidation of H2S can be obtained. Under reaction conditions the iron oxide reacts to iron(I1)sulfate (FeS04). On iron(II)sulfate reaction (3) proceeds much faster than reactions (l), ( 2 ) and (4).To minimize sequential oxidation and establishment of the equilibrium (3), the transport within the catalyst bodies must proceed fast, which calls for a low Thiele-modulus [2]. Therefore a catalyst of a high porosity and wide pores and, thus, a low surface area is required. With this low surface area a high activity and stability, required with industrial catalysts, can only be achieved provided the iron sulfate and, thus, the iron oxide is highly dispersed on a support. The interaction with the support prevents sintering. A suitable support is silica, which has a low activity for the reactions (l), (2) and (4). Production of supported catalysts from pre-shaped bodies of the support is technically attractive. However, to achieve a uniform distribution of the active component throughout the support by impregnation of pre-shaped supports of a low specific surface area having wide pores is difficult. Crystallization of the active precursor during drying of the impregnated support will lead to small particles enclosing narrow pores. Because capillary pressures in wide pores are substantially less than in narrow pores, transport of the impregnated liquid to narrow pores within clusters of small crystallites of the precursor proceeds during drying. This will result in clusters of small particles of the active precursors not uniformly distributed within the bodies of the support. Clusters of small active particles are liable to sintering and thus to deactivation and the narrow pores within the clusters can lead to a drop in selectivity. The crystallization of the dissolved precursor and thus the formation of clusters of small crystallites strongly depends on the nature of the dissolved compound. The choice of the proper precursor will therefore be essential to obtain a highly dispersed and active catalyst. It has been established that with supports containing wide pores high dispersion of the active material can be obtained when the carrier is impregnated with a complex of the cation of the active compound with a chelating agent [3,4]. Using an iron EDTA complex [ 5 ] , iron oxide on alumina catalysts for the selective oxidation of hydrogen sulfide have already been prepared. In this paper the effect of the nature of the dissolved precursor on the final distribution of the active component over silica extrudates will be investigated extensively. The precursors investigated can roughly be divided into two classes ,viz., iron chelates and simple iron salts. Table I surveys the precursors investigated. With iron EDTA solutions the effect of the pH of the impregnated solution (5.3 ,7.1, 8.5 and 10.0) was studied. At low pH values the FeEDTA- anion predominates, while at higher pH levels FeEDTAOH2- or FeEDTA(OH)$ is mainly present. The catalysts were characterizedusing Light Microscopy (LM),Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Diffuse Reflection Infra Red Fourier Transform Spectroscopy (DRIFTS), and Temperature-Programmed Reduction (TPR) The effects of the preparation on the catalytic properties in the H2S oxidatiotl were also assessed.
529
EXPERIMENTAL Preparation: Silica extrudates (about 1/2" x 1/12") were made by extrusion, drying and heating of a aqueous paste of Aerosil OX50 (Degussa) powder. This powder consists of relative large spheres (diameter 20 - 80 nm) produced by flame hydrolysis. The pore volume of the extrudates was 0.8 ml/g (E = a%), the average pore radius 35 nm, and the specific surface area (BET) 44 m2/g. Incipient wetness impregnation of the silica extrudates with aqueous solutions of the iron containing precursors resulted in a number of catalysts. The precursors were ammonium iron(I1I) EDTA, iron(II1) citrate, iron(II1) gluconate, ammonium iron(II1) citrate, iron(II1) chloride, iron(II1) nitrate, and iron(II1) sulfate. The concentration of the solutions of the precursors in distillated water was adjusted to result with the known pore volume (0.8 ml/g) in the desired loading of 5 wt% Fe2O3. The pH value of the different ammonium iron EDTA solutions was adjusted with ammonia. The extrudates were evacuated for at least 15 minutes before being impregnated. Subsequently the solution was admitted to the evacuated support. All impregnates were dried for two hours at room temperature in a stream of air. Subsequently the impregnated supports were kept in air successively for at least three hours at 60 O C , for at least three hours at 12OoC,and for three hours at SOO'C. TABLE I Survey of the precursors used for the impregnation Ikon chelate: 1 ammonium iron(II1) EDTAa)
I
NH&EDTA.lSH20
PH
colour solution
5.3 7.1
green blood red
8.5
blood red
10.0
blood red
ammonium iron(II1)citrate
W F e ciuate b)
4.3
yellow-green
Iiron(11I)gluconate.
Fe ghlCOMte c,
7.0
deep brown
iron@) chloride
FeC13.6H20
1.1
orange-yellow
iron(III) nitrate
Fe(NO3)3.9H20
1.o
orange-yellow
Iron salt:
iron(III) sulfate Fe2(S04)3'5H20 a) ethylene dkmine tetra acetate b) Fe content 15.2%
1.3 c) Fe content 10%
pale yellow
Light Microscopy: The extrudates were sliced radially or longitudinal and smoothed to a thickness of 0.5 mm. The samples were examined with a Leitz light microscope.. The extrudates were soaked in
an immersion oil (Leitz, n=1.52) to make the silica (n about 1.5) transparent. Both bright field (transmitted-light), and dark field (incident-light, illumination angle 60') illumination was used. Transmission Electron MicroscoDv; The samples of the catalysts were ground in a mortar and ultrasonically dispersed into ethanol. The suspension was brought onto a holey carbon film supported by a copper grid. The samples were examined in a Philips EM 420 transmission electron microscope. The accelerating voltage was 120kV. Scanning Electron Microscopv: The catalyst extrudates were broken and examined in a Jeol JSM840A scanning electron microscope at an accelerating voltage of 25 kV. To image the iron containing species more clearly, a back-scattered electron detector was used.
530
x-rav Diffraction; The extrudates were ground and pressed into a sample holder. The samples were examined in a Philips diffractometer o Ti order. Assuming the oxygen lablity near the rare earth cation
facilitate the formation of
0 - , 02-and 02*-
species, the
energy of the rare earth bond has been computed using the metal-oxygen distances determined by P. Poix ( 9 ) and the partial charges of ions are determined by M.Henry'smethod. Thus the energy of the bond is computed by conventionnal formulas:
E
-
p
*
q'
- cation-oxygen invariant length q - partial charge of the cation
/ r
r
q'
selectivity
conversion
C H (%) ~
- partial charge of the anion
31.9
C2H4 13
40.4 28.4 27.9
10.2
32.5
30.7
(aC2H6
rield C2(%
surf ce area
??
be for($ Ig&fter 7.2
9
7
8
31.2
17.5
19.7
2.3
1.6
4.5
5.5
2.8
5
4.3
7.5
4.9
10.5
9.3
12.1
13.9
2.5
1.4
30
2.9
3.3
1.9
7.0
6.9
29.8
14.5
9.2
7
2.7
2.55
35.7
25.2
16.3
14.8
5.4
4.9
27.7
4.2
4.5
2.4
6.9
6.5
Table 3: Catalytic results of rare-earth pyrochlores compounds. If we take in account these results, for a close rare earth-oxygen energy and an increasing Sn-0, Zr-0 or Ti-0 bond energy, we notice that the increase of bond energy follow the decrease of C2 yield (table 4 ) . Thus,we computed the energy data
for the previous catalytic systems (table 5).
These results show clearly that catalytic conversion and selectivity are also related to rare earth bond energy but also to its direct oxygen environment. conversion(%)
yield
bond
nergy B-0
Ln-0 0.1401
0.1428
CH4 31.9
c2 % 7
40.4
19.7
0.1352
0.0637
28.4
2.8
0.1400
0.1537
27.9
4.9
0.1294
0.1427
32.5
13.9
0.1248
0.0643
30
1.9
0.1293
0.1536
29.8
7
0.1540
0.1427
35.7
14.8
0.1488
0.0647
27.7
2.4
0.1539
0.1536
Table 4: Relationship between bond energy
snd catalytic properties
581
catalyst
oxygen
conversion
coordination
CH4
%
selectivity
bond energq
LaLi02
7
17.7
c2 % 42.9
0.1212
LaNa02
7
30.3
32.4
0.1229
La203 Lay03 bix
6-8
27.
23.5
0.1375
6-8
28.6
14.3
0.1378
La2Zr207
8
31.5
11.4
0.1386
NdLiO2
7
30.4
38
0.1231
NdNaO2
7
36.8
33.9
0.1380
6-8
28.6
31
0.1398
8
33.3
3.6
0.1402
7
31.9
28.7
0.1229
Nd203 Nd2Zr207 SmLi02 SmNa02 Sm203 Sm2Zr2O7
7
30.4
31.6
0.1199
6-8
25.5
25.1
0.1402
8
31.9
7.03
0.1401
Table 5 : Relationship between rare earth bond energy and catalytic properties. Conclusion. As a conclusion the methane oxidative coupling performances of some definite compounds obtained by low temperature processes are studied. The results show that: -the nature of the starting salts highly affects the selectivity -a 12 fold oxygen environment of the cation decreases drastically the selectivity (perovskite structure) -for the oxygen environment of 6 to 9,a relation can be found
between
coulombian bond energy of the cations and a high C2 hydrocarbon selectivity. This could be interpreted as an enhanced mobility of oxygens in the neighbourhood of the rare earth cation. References. /1/ T. Ito, J.X. Wang, C.H. Lin and J.H. Lunsford, J.A.C.S. 107 (1985)
5062. /2/G.E. Keller and M.M. Bahsin, J. Catal, 73 9 (1982)
582
/3/ G.J. Hutchings, M.S. Scurell, J.R. Woodhouse,J. Chem. SOC. rev.
18(1987) 251. / 4 / N. Yamagata, K. Tamaka, S. Susaki and S. Okazoti, Chem. lett (1987) 81.
/5/ A. Kaddouri, R. Kieffer, A. Kiennemann, J.L. Rehspringer, A p p l . catal 51 (1989) L1. /6/ Zhen Kaiji, Liujian and Bi Yingi, Cata. lett. 1 (1989) 299.
/7/ P. Poix, C.R. Acad. Sciences, 270 1852-1853 (1970). /8/
M. Henry, thesis universite P. et M. Curie, Paris (1988).
G.Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
583
PREPARATION OF STRONG ALUMINA SUPPORTS FOR FLUIDIZED BED CATALYSTS M.N. Shepeleva, R.A. Shkrabina, Z.R. Ismagilov and V.B. Fenelonov Institute of Catalysis, Siberian Branch of the USSR Academy of Sciences, Pr. Akademika Lavrentieva, 5, Novosibirsk 630090 (U.S.S.R.)
SUMMARY Physico-chemical processes o c c u r i n g i n alumina g r a n u l e s moulded by t h e Hydrocarbon-Ammonia G r a n u l a t i o n Method have been i n v e s t i g a t e d ; o p t i m a l r e a l i s a t i o n c o n d i t i o n s o f main t e c h n o l o g i c a l s t e p s have been e s t a b l i s h e d . As a r e s u l t , h i g h l y s t r o n g s p h e r i c alumina g r a n u l e s w i t h developed s p e c i f i c a r e a and p o r o s i t y , a p p l i c a b l e as a s u p p o r t f o r f l u i d i z e d bed c a t a l y s t , have been o b t a i n e d . INTRODUCTION Many modern technological processes are supplied with energy by combustion of organic fuels. Economically effective and ecologically clean installations with
fluidized bed of the catalyst for flameless combustion were
being deve-
loped within the last years. Highly effective apparatuses of this type are the Catalytic Heat Generators (CHG) developed by the Institute of Catalysis (Siberian Branch of the USSR Ac. Sci.)
[1,2]
.
Catalysts in the course of work in CHG are subjected to at least three kinds of influence: chemical, thermal and mechanical. These factors are interconnected, they
complete and magnify the action of each other, destroying catalyst
granules. The structural-mechanical properties of the supported catalysts are known to be determined at great extent by the properties of a support. The object of this work is the investigation of production conditions of spheric
J'
-alumina,whichpossesses ahigh mechanical strength
as
well as a
highly developed surface area and porosity. It is known, that the preparation conditions and granulation method influence
the product characteristics. Recently the Hydrocarbon-Ammonia Granula-
tion Method (HAG method)
possessing high productivity and facility of techno-
logical parameters regulation has received wide distribution. The distinctive feature of the method is the alumina chemical treatment at several stages, which allows to alter the texture of the initial substance in the necessary direction. In this respect
the HAG method has advantages in comparison with the
widely used method of mechanical moulding.
584
EXPERIMENTAL METHODS Argon thermal desorption was used for the determination of the specific area
A. The pore volume V and the pore size (r, nm) distribution were determined by P
mercury porometer "Porosizer-2300" from the "Micromeretiks" company (USA). Searching for a test of the granules mechanical properties
shows the following pe-
culiarities. Granules work in the CHG fluidized bed is complicated by chemical and thermal factors; granules are subjected not only to the surface friction, but at great extent to impact loadings, and gradual increase of the internal structural macro- and microtensions. The crushing test comprising granules press between two parallel plates has given satisfactory correlation with real speed of granules destruction in the CHG. Therefore, the sample strength was characteminimal S . and maximal S values of the individual rized by the average S av' min max granules crushing pressure S. (MPa) in a series of 30 granules. Smin and Smax were calculated from 5 minimal and maximal values of Si. The operating time of a catalyst support has been established to be more than 0.5 year for samples with S 3 25 MPa and S m i n 3 7 MPa. av
RESULTS AND DISCUSSION Raw Material Preparation Pseudoboehmite aluminium hydroxides are usually used as a raw material in the HAG method. We have previously investigated the aluminium hydroxides obtained
by gibbsite dissolution in the alkaly and deposition at pH 8.5-9.0 by nitric It was shown in [5-71 that the conditions of aluminium hydroxide acid [5J
.
synthesis determine the morphology and the structural type of the particles-", as well as the nature of binding between primary particles. A s it was shown by physico-chemical methods, the size of particles obtained from hydroxides synthesized at T 6 4OoC does not exceed 10 nm and the size of the secondary crumbly enough aggregates can exceed 100 nm. The bonds between the particles in such aggregates are mostly of Van-der-Vaals nature. Therefore, the acid treatment at the initial stage of peptization leads to the formation of a disordered system of fine needles and fibres. The dispersion of such mass in ammonia solution leads to rapid coagulation, fine particles of aluminium hydroxide (-
3-4 nm)
being densely packed. Granules of aluminium hydroxide formed in these conditions have fine porous monodisperse
A Z= 250 m2/g, Vp
*
=
structure. After calcination, the alumina with 3 0.3-0.4 cm / g , Sav 3 25 MPa is obtained.
Preparation of these hydroxides is connected,however, with certain difficul-
'According to Rebinder [ 8 ] , structures are divided into two main types: coagulative, in which ion-solvate shell on the particle contact places is preserved and crystallizative with point or phase contact between primary particles.
585
ties, for
example, at the stages of washing off alkaline metal
ions and fil-
tration. That is the reason that monodisperse hydroxides are not widely used. Hydroxides obtained by precipitation at T f 4OoC o r mixtures of precipitates obtained at high and low temperatures are used in many researches
[9,10]
.
These hydroxides have contacts between the primary particles of both types; an extent of aggregates packing changes at the next technological stages is determined by their number ratio. In the systems with phase contacts
between the
particles, the peptization does not lead to the aggregates destruction. Macropores preservation between the remained aggregates leads to the formation of lowstrength alumina granules. Because of this, from each hydroxide obtained as in
[9,lO]
, one can prepare alumina granules with strength not exceeding a certain
limiting value, unless special technological methods (e.g.,
high temperature
calcination, additive incorporation, etc.) are used. A s is shown in [5] , for the usually applied aluminium hydroxides, the values of Sav of the obtained granules do not exceed 12 MPa. Application of these alumina granules in CHG is not effective, therefore we tried to change the structural type of hydroxide in order to strengthen the final alumina granules. Mechanical activation is known to be one of the ways to increase the solid reactivity. The object of o u r investigation was aluminium hydroxide containing equal mixture of deposits obtained by interaction
between the sodium aluminate and nit-
ric acid at pH 8.7 and temperatures of 20 and 100°C. The phase composition of this hydroxide corresponds to the pseudoboehmite with the range of coherent dissipation 12 nm. Specific area and total pore volume of the sample dried at llO°C are 230 m2 /g and 0.27 cm3 / g respectively, 12% of total pore volume is the volume of macropores (r > l o 0 nm). The radius distributions of pore volume of aluminium hydroxide before
and
after the treatment in various mills are shown in Fig. 1. It is seen that the treatment in a disk mill does not allow to destroy the secondary aggregates of hydroxide. Large pores are also preserved in the final alumina. The macroporosity of alumina could be removed by grinding intensification, which also increases Sav and Smin substantially, rises the bulk density
and slightly decreasesthe
surface area A ( s e e Table in Fig. 1 ) . We have given in [11] the results of physico-chemical investigation of aluminium hydroxide grinding products. It was shown that the main result of grinding is connected not only with the destruction of the initial aggregates of aluminium hydroxyde, but also with an exchange of strong phase contacts by weak coagulative contacts. This does not practically change the structure of the primary partjkles. Peptization Stage Liquid mass capable t o flow freely from the moulding device spinnerets, is
586
0.4
~~
No1 2
0.3
3 4
Q,
\
d, 9 mkm -
>I00
25 10
-_s,_arPa--av
min
4
2 2 12 17
5 31 35
A, g/ om”
A,
18 /g
0.69 0.70
250 270 240 220
0.84 0.84
I+)
E0
>”
0.2
0.I
Fig. 1. Pore volume radius distribution for initial (1) and grinded (2-4) aluminium hydroxides. Mills: 2 - disk; 3 - ball; 4 - jet. The most abundant particle size, d mkm: 2 - over 100; 3 - 25; 4 - 10.
P’
obtained at this stage by acid treatment of aluminium hydroxide. It should be noted, that basic aluminium salts show thixotropic properties and, therefore, it is necessary to adjust the mass preparation conditions to establish thixotropic setting time
(s) long enough for a free mass flow along the pipelines.
The mass rheological characteristics and properties of the final alumina granules are strongly dependent on the mass preparation conditions, particularly, on the mass maturation time
(m).
During the mass maturation the he-
terogeneous system obtained as a result of the component mixing, becomes homogeneous due to the precipitate swelling. ‘t‘ (m) is influenced by the temperature in reactor
- plastificator.
It was shown, that at T
=
+
20-25OC L (m) makes
up 1.0-1.5 days. The temperature rise leads to a substantial speeding up of the mass maturation ( such masses
(m)
=
4-10 hrs). However, the alumina granules obtained from
possess not only the increased strength (Sav = 30-40 MPa) but also
fine porosity, which complicates the drying and calcination stages and decrease s the granules water stability. r-
L
If M(a)
(m) also depends on the amount of acid added, M(a),
g-m/g-m of alumina. /
equals to 0.06-0.08and the solid phase concentration is 25-30%, L (m)
makes up 1.0-1.5 days as necessary. The decrease of M(a) leads to the increase of
(m) and vice versa. the time of mass thixotropic setting. A s it
Besides, M(a) influences was shown in our experiments,
5
(s) should be within the range of 15-60 min.
587
We have established the dependence of the optimal value of M(a)
lOO°C
.
leads to the decrease of S The strongav est granules are obtained while drying in air. This widely recommended method cannot be applied to continuous technology. It was found that lowering of the initial temperature and its gradual increase lead
to the increase of S and av
'mine strengthens the granules due to the The thermal treatment at T >200°C transfer of coagulative structure to the crystallization one. The calcination of granules with bidisperse
or wide porous structure does not present
great
difficulties. Calcination of fine porous samples is more complicated. It was shown that the reduction of temperature rise and granule bed height
as well as
the increase of calcination time and temperature (up to 75OoC) facilitate the slow moisture removal, granules shrinking, fine pore sintering. As a result, simultaneous rise of V
P
and Sav is observed, the specific area slightly drops.
CONCLUSIONS The investigation of the physico-chemical processes taking place at the main
590
stages of alumina moulding by hydrocarbon-ammonia method enables us to develop the scientific background of preparation of granules with different characteristics, to establish the optimal conditions for each stage, to improve the process apparatus. It was shown that the preparation of strong alumina
gra-
nules requires the directed conducting of all the technological steps. Direct interrelation between the properties of the initial aluminium hydroxides and structural-mechanical properties of the final alumina granules was established. Preparation of fibrous pseudoboehmite in mild precipitation conditions allows to prepare from it the fine porous strong alumina granules. Formation of pseudoboehmite in the form of well crystallized needles and plates unable to react with acid-peptizator requires the introduction of the intensive grinding into the technological process. The investigation of peptization process shows that this stage breaks ground granules. So-
for textural and mechanical characteristics of the final alumina
lid phase concentration, nature and amount of acid-peptizator determine the rheological properties of mass and extent of dispersion of secondary aggregates of aluminium hydroxide. The properties of substance-coagulant and the residential time of granules in ammonia solution influence the completion of coagulative hardening. The conditions of granule thermal treatment allow to increase the amount of contacts between particles and aggregates in granule, strengthen these contacts due to transformation of coagulative type into the phase one. Alumina
granules prepared by the developed method posess the specific area
and porosity necessary for the incorporation of the required amount of the active component and can be applied as a support for fluidized bed catalysts. REFERENCES 1
G.K. Boreskov, E.A. Levitskii and Z.R. Ismagilov. Zh. Vsesouznogo Khim. Obshchestva, 29 (1984) 379-385. 2 Z.R. Ismagilov, in: D.N. Saraf and D. Kunzry (eds.) Proc. Intern. Conf. on Advances in Chem. Eng., Kanpur, January 4-6, 1989, Tata McGrow Hill Publ. Co. Ltd, New Delhi, 1989, pp. 310-315. 3 USA Patent 2805206 (1953). 4 Ya.R. Katsobashvili and N.S. Kurkova, Zh. Priklad. Khim. 39 (1966) 24242429. 5 M.N. Shepeleva, V.B. Fenelonov, R.A. Shkrabina and E.M. Moroz, Kinet. Katal. 27 (1986) 1202-1207. 6 M.N. Shepeleva, R.A. Shkrabina, L.G. Okkel, V.I. Zaikovskii, V.B. Fenelonov and Z.R. Ismagilov, Kinet. Katal. 29 (1988) 195-200. 7 Z.R. Ismagilov, M.N. Shepeleva, R.A. Shkrabina and V.B. Fenelonov, Appl. Catal (in press). 8 P.A. Rebinder, Physical and Chemical Mechanisms of Dispersed Structures, Nauka, Moskva, 1966, p . 3 . 9 M.D. Efros, A.V. Tabulina and N.V. Ermolenko, Izv. Akad. Nauk BSSR, 1 (1971) 9-13. 10 E.A. Vlasov, I.A. Rizak and E.A. Levitskii, Kinet. Katal., 5 (1972) 13111314. 11 M.N. Shepeleva, Z.R. Ismagilov, R.A. Shkrabina, E.M. Moroz, V.B. Fenelonov and V.I. Zaikovskii, Kinet. Katal.(in press).
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Prepamtion of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
591
SYNTHESIS AND REGENERATION OF RANEY CATALYSTS BY MECHANOCHEMICAL METHODS
A. B. FASMAN 1, G.
S. D
v .GOLUBKOVA~
.MIKHAIL E N K O ~, 0.T .KALININA'
.
, E ~ u I.V A N O V ~ ,
Institute of Organic Ca tal ysys and Electrochemistry, 142, K.Marx st., 480100,Alma-Ata,USSR 21nstitute of Solid State Chemistry, 18, Derzhavina st. 630091.Novosibirsk, U S S R
I
SUMMARY The effect of mechanical alloying (MA) on the structure of Raney catalysts and their activity and selectivity in liquid-phase hydrogenation reactions has been studied. The data illustrating a possibility of the method developed by the authors f o r mechanochemica1 (MC) regeneration of Raney catalysts irreversibly deactivated in hydrogenation process are given. INTRODUCTION Raney catalysts are widely used in industry due to their high activity, technological ability and relatively low cost (ref.1).
As
a rule they are made by leaching a non-noble component from pyrometallurgical alloys (PM). The MC method for synthesis of alloys from initial component powders traditional PM ones.
With lower
one stage to produce alloy a wider concentration range
has a number of advantages over the energy
expenses it allows within
powders that form skeleton catalysts in due to a higher efficiency while reac-
ting with alkali (ref.2). At the same time the conditions of MA become the parameters influencing the properties of catalysts. The object of the present work is to study the effect of the MA conditions on the structure of alloys and Raney Ni-catalysts made from them as well as on adsorption properties, activity and selectivity of catalysts in liquid-phase hydrogenation reactions. EFFECT OF PREPARATION CONDITION ON THE FORMATION MECHANISM OF M A ALLOYS AND THEIR STRUCTURE BEFORE AND AFTER LEACHING
MA elloy structure Initial alloys were made in a planetarium-type ball mill and a attritor as in (refs.3,4). Use was made of commercial carbonyl nickel and aluminium powders. The phase composition was analysed by the X-ray diffraction method using CuK,,~emission. It had been found before (ref.4) that in the attritor
with an
uncooled
case the MA
592
was characterized by a latent period during which local heating could result in A1 melting. Then the exothermic effect initiates a reaction which proceeds very fast. The composition of its products differ but little from the equilibrium one. At the same time the mechanism of MA alloy formation in a cooled planetarium mill is close to a diffusion type, whereas the phase composition is far from an equilibrium one. Table 1 gives phase composition of alloys produced in these mills. TABLE 1 Effect of preparation conditions on the MA alloy phase composition Charge composition
Duration MA, min
Phase composition
Planetarium ball mill Before annealing After
17 A183
30
Al+NiA13
i25A1?5
30
NiA13+Ni2A13
Ni32A168
5 30
Ni+Al Ni2A13
i35A '65
5 30
NiZA13 +Ni+Al NiAl
i42A158
20
NiAl
i52
5 20
NiAl+Ni+Al NiAl
A1+NiAlj NiAl3 Ni2A13 +NiA13 Ni2A13
NiAl NiAl
Attri tor Al+NiA13 Ni2A13 +A1 Ni+A1 N i2A13+N iA 13 Ni+A1 Ni2A13 NigAlg +NiA1 Ni+A1 NiAl
Of importance is, probably, the fact that the reaction proceeds in
an open apparatus (an attritor) and it has outside characteristics it is accompanied by a puff, and the activation was ceased right after its proceeding. A planetarium mill is a close-type apparatus so the control like that is impossible, that is why structural changes can occur after the reaction. Structures Ni2A13 and NiAl are close and the former can be produced from the latter by replacing one third of nickel atoms for definitely-ordered vacancies. Their disordering that take place with MA leads to formation of solid solutions on the NiAl basis. T h u s the NiAl homogeneity range is widened from equilibrium 45-60% to metastable 35-60% Ni. This supposition was verified by means of the X-ray method for radial distribution of atoms (ref.5). The MA Ni35Aluproduced in a planetarium mill during 60 min has a diffraction pattern that corresponds to the BCC-structure of NiA1.
593 The positions of coordination maxima on the radial distribution curve (r.d.c-1 correspond to BCC-structure of NiAl too, but there
ore deviations from theoretical values as to the distribution of their areas. Table 2 gives the relationships betwen the areas of experimental r.d.c. maxima P e and theoretical P t for a model relating to a non-distorted structure NiAl (Ni/A1=50/50) and for a model, describing a solid solution on the NiAl-basis (Ni/A1=35/65). It is seen that the second model fully corresponds to experiment. Hence MA alloys are, indeed, able to produce solid solutions on the NiAl-basis losing up to 40% of nickel atoms. TABLE 2 Structural parameters of MA Ni35A165 as compared to NiAl
structure
models
Rt, 2.50 4.08 5.78 6.29 7.08
Re, 2.50 4.05 5.72 6.32 7.00
P e / P t (Ni/Al=50/50)
Ni/A1=35/65)
P,/Pt(
1.018 1.125 1.089 1.019 0.979
0.549 0.422 0.471 0.448 0.368
Structure of Raney nickel catalysts from MA alloys Raney catalysts from Ni-A1 MA alloys possess structural peculiarities. Leaching of A1 from N i s A k 5 -Ni50A15~,as a rule does not lead to any changes of diffraction pattern though from 60 to 20% A1 is removed. It should be noted that the PM NiAl does not react with alkali. Evidently the defect structure of MA NiAl facilitates A1 extraction. From Table 3 it is seen that leaching does not result in changes of MA alloy srtuctural parameters (a borderline composition is taken as an example, Ni35A16s). TABLE 3 Structural parameters of MA Ni35A165 before and after leaching ~
~
Sample Initial alloy Catalyst
0
Lattice parameter.A 2.860-0.001 2.860-0.002
0
Particle size,A 120 110
C . 103 7.94 8.77
On the diffractogram one can see only NiAl lines and a very weak diffused maximum that canbe attributedto FCC-Ni. However, calculations of r.d.c. show that other components make contributions too. Taking account of the fact that the leaching degree was, In this
594
case, 55% by technique (ref.6).
a difference
curve
([cat.]-0.45*
*[alloy]) was drawn. The calculation results correlate tern of FCC-Ni on whose background there are maxima of Table 4 shows P e and P t of r.d.c. peaks for models supposition of 100 and 40% content of Ni in a leached second
model well agrees with experiment, though one
to the patNiO. built up in sample. The
can notice a
greater lowering of coordination numbers with an increase of R. It is, evidently. due to a high dispersity of the nickel. When use is made of the regularities established in (ref.7) then from the slope of the P,/P+=f(R)
0
the size of Ni particles as C=C< bond along the carbon chain affects but little the isomerization ability of Raney catalysts. 1n.this case the coefficient of isomerization for all samples differed little indeed.A decrease
of the migration
coefficient is
due to an increase of the
598 TABLE 5 The
activity (W ml H2/min*g) and migration (F,)
(Fc) coefficients N i content i n alloy.% 25
W 584 814 290
35
40
MA ( 5 - 8 J A ) Fm F;
bound
isomerisation
0.27
0.44
0.30
PM ( 5 - 8 f i )
W
0.63 0.65 0.67
hydrogen.
Fm
113 176
0.61
248
0.65
-
-
a rise
of
verified by
TPD-curves which with PM catalysts (5-8&) range of low temperatures. Thus, a high-adsorption their increased selectivity
W
0.25
and
This is
PM ( 4 0 - 6 0 s ) Fm Fi
FL
0.36
-
surface adsorption heterogeneity strongly
and
of MA and PM catalysts in hexen-1 hydrogenation.
202 114
the
0.77 0.78
0.58
0.72 0.72 0.73
fraction of
comparison of the
have a broad arm in
the
potential of MA catalysts influences in hydrogenation of different comp-
ounds. In their turn, specific adsorption properties of M A catalysts are due to peculiarities of their structure,which can be imagined as a sinter of high-dispersion microcrystals of an active phase that are fixed on particles of an unleached initial aluminide. REGENERATION OF SPENT CATALYSTS VIA MA The MA method can be used for making new Raney catalysts
from
spent and deactivated ones in industrial processes. Utilization and regeneration of the latter is
an important
problem in economy and
ecology which has not been settled as yet.
There are big difficul-
ties
in
associated
easy
oxidizability
the remelting of powders wich is leading
with
their
to big losses (up to 40%) of metallic
nickel (ref.12). Experiments on MA alloy preparation using
deactivated Raney Ni
and A1 powder as initial components have shown that in this case leachable alloys are formed. Table 6 presents the activity of skeleton catalysts from such alloys in hydrogenation of some of unsaturated compounds. Completely deactivated catalysts that were used for hydrogenation of some of organic compounds, were taken as Ni components in MA. A3 seen from Table 6 the regenerated catalysts are, as a rule, more active than b o t h the PM Ni-Raney and the MA ones for making of which commercial nickel was used. The nature of this effect requires further investigations; however, it is pos-
sible
to assume
that in
the
burn-out of organic residues on the
599
TABLE 6 Activity (ml H2/min-g) of Raney Nickel from MA and PM Ni35A165 in the hydrogenation of different compounds ( t=4OoC) Reaction Potassium maleate Phenylacetylene Nitrobenzene Hexen-1 ( t=20°C)
PM catalyst (5-~JA)
70 140 100 200
MA catalyst (commercial Ni) 100 480 160
aio
MA catalyst (spent Raney Ni) 160 700 220 1000
surface of catalysts they start interacting with it, modifying them in a specific way. Thus, MA can become a promising efficient method for utilization of production waste in catalytic processes and at the same time a a way to increase their activity. CONCLUSION Thus, mechanical alloying can be considered as a promising alternative for the pyrometallurgical method of producing initial alloys for Raney catalysts. The MA performance conditions made it possible to influence the structure of initial alloys and, mainly to obtain nonequilibrium solid solutions on the NiAl basis the leaching of which as compared to PM ones yields more active and selective catalysts in a number of processes. Besides, MA can be the basis of a few-operation technology for regeneration of Raney-Ni deactivated in industrial processes. The leaching of MA alloys produced from spent Raney catalysts yields catalysts that are superior in activity to those from PM and even analogous MA alloys on the basis of commercial Ni powder. ACKNOWLEDGMENTS The authors are very thankful to Dr.E.V.Leongard for carrying out experiments on the temperature-programmed desorption of hydrogen, to A.K.Dzhunusov. who took part in investigation of samples by the r.d.a. method and to Professor E.M.Moroz for useful discussion of the results of the r.d.a. experiments. REFERENCES 1
E-1-Gildebrandtand A.B.Fasman, Skeleton catalysts chemistry, Nauka. Alma-Ata, 1982 (in Russian).
in organic
E-Ivanov, T-Grigorieva, G.Golubkova, V-Boldyrev, A.B.Fasman, S.D.Mikhailenko. 0.T.Kalinina. Raney nickel catalysts from mechanical Ni-A1 alloys, Materials Letters 7(1-2) (1988) 55-56 E.Ivanov, T-Grigorieva, G.Golubkova, V-Boldyrev, A.B.Fasman, S.D.Mikhailenko, O.T.Kalinina, Synthesis of Ni aluminides by chanical alloying, Materials Letters 7(1-2) (1988) 51-54 S.D.Mikhailenko, B.F.Petrov, 0.T.Kalinina. A.B.Fasman, Nickel aluminide mechanochemical synthesis mechanism, Powder metallurgy,lO (1989) 44-48 (in Russian). K.G.Rikhter, X-ray analysis of amorphous catalysts by r.d.a. method, in: Rentgenografiya katalizatorov, Nauka, Novosibirsk, 1977,pp.5-40. E.M.Moroz, Development of X-ray methods for the investigation of fine-dispersive systems, Doctor's thesis, Novosibirsk. 1989. V.N.Kolomiichuk, On the correctness of quantitative charcteriscurves, in: tics of catalyst structure obtained from r.d.a. Rentgenografya katalizatorov, Nauka. Novosibirsk, 1977, pp.67-70 E.Yu.Ivanov. T.F.Grigorieva, G.V.Golubkova, V.V.Boldyrev,A.B.Fasman, S.D.Mikhailenko, O.T.Kalinina, Mechanochemical synthesis of nickel aluminide, Izvestiya SO AN SSSR (ser khim), 19(6) (1988) 80-83. V.I.Vorobieva, V.M.Safronov. G.A.Pushkarieva, A.B.Fasman, Phenylacetylene hydrogenations on the Raney Ni from Ni-A1 alloys of different composition and dispersion, Vestnik AN KazSSR, 4(1987) 54-58. 10 A,B.Fasman, T.A.Khodareva, S.D.Mikhailenko, E.V.Leongard, A-1-Lyashenko, The effect of preparation condition on structure All Union and properties of modified Raney Ni catalysts,Proc.II Seminar on Scientific basis of catalyst preparation, Minsk, September 25-28, 1989, Nauka, Minsk, 1989, p.295. 11 T.A.Khodareva. E.V.Leongard, S.D.Mikhailenko, Raney Ni transformation under the influence of thermal treatment in redox media, in: Science and technology problems of catalysis, Nauka. Novosibirsk,1989, p.101 (in Russian). 12 A.I.Kryagova,
A new merhod for spent Raney N i catalyst regeneration, Trudy LVMI, 5 (1956) 85-90.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
601
CONTROLLED PREPARATION OF RANEY Ni CATALYSTS FROM Ni,A13 BASE ALLOYS
.
-
STRUCTURE AND PROPERTIES.
.
S HAMAR-THIBAULT’, J. GROSI, J. C JOUD1, J. MASSONZ, J.P. DAMON2 and J.M. BONNIER’ 1.N.P.Grenoble - L.T.P.C.M. (CNRS-UA29), BP 75, 38402 Saint Martin d’H8res-Cedex FRANCE. Universitg Joseph Fourier, L.E.D.S.S. (CNRS-UA332), BP 5 3 X , 38041 Saint Martin d’H8res-Cedex FRANCE. SUMMARY Physicochemical properties (metallic surface area, total surface area) of Raney Nickel catalysts prepared from well defined precursor alloys were related to the metallurgical structures of these alloys. The microstructure of the catalysts was correlated with the physicochemical characteristics and their activities f o r hydrogenation of acetophenone in the liquid phase. INTRODUCTION Raney Nickel catalysts are extensively used industrially and in laboratories in hydrogenation, hydrogenolysis and other reactions. NiR were prepared by removing A1 from Al-rich A1-Ni alloys in alkali solutions. The residue consists of small Ni particles connected in a porous agglomerate with small amounts of A1 in a metallic state and also in an oxidized state as alumina (1). Although Raney catalysts have been used for a long time, the knowledge of the influence of the metallurgical structure of the precursor alloy upon catalytic properties is limited. Moreover, catalytic properties can be strongly modified by metallic addition (2-4). This work is part of a systematic study on the influence of metallurgical parameters on the catalytic properties of doped and undoped Raney Nickel catalysts. Different types of structures were obtained with different solidificationtechniques.Besides c o n v e n t i o n a l solidification, rapid quenching from high temperature was used in order to obtain supersaturation of the dopant in the phases. In order to be able to precise the influence of the microstructures of the precursor alloys, these alloys were well characterized before they were turned into catalysts.
602
EXPERIMENTAL PreDaration of the alloys and catalysts. Conventional solidification, annealing at high temperature and rapid quenching from the melt ( / A ) were used to obtain precursor alloys exhibiting well defined microstructures. The rapid quenching was performed under helium atmosphere. The temperature of the melt before ejection was kept at about 156OOC. The influence of the ejection pressure and the rolling velocity on the microstructure have been reported previously (5,6). Undoped and doped Ni,.,M,Al, base alloys were tested (M = Cr, Cu) Ni catalysts were prepared from powdered alloys by treating twice 2 hours in a boiling 6N sodium hydroxide solution. The samples were then washed by NaOH solutions of decreasing concentrations and carefully washed in water and in an appropriate solvent before use.
.
Characterization techniaueg. The bulk composition of the catalysts was determined by chemical analysis. All compositions were expressed as atomic ratios. The total surface area was determined by adsorption of N2 at 77K and the nickel surface area by reactive adsorption of 3methylthiophene in the liquid phase as described previously (7). Transmission electron microscopy observations (TEM-JEMZOOCX), of Ni catalysts were performed on samples prepared by two different ways: the first as just described, the second from bulk alloys. For the first samples, a suspension of Ni catalyst in alcohol was deposited on a copper microgrid and dried in the specimen introduction chamber to avoid any contact with air. Samples prepared from bulk samples were first electrolytically thinned with an acid solution at room temperature then observed directly after Al-leaching. Al-leaching was sufficient to obtain thin observable regions. Quantitative microanalyses were obtained on a STEM-VG.HB5 which allows a high resolution in EDX and EELS microanalyses (lateral resolution of 1.5 nm at the sample level). Some X P S and Auger examinations were equally performed on the precursor alloys and on the catalysts. Catalytic tests. The catalysts were tested in the hydrogenation of acetophenone with reference to a Ni-A1 (50.50wt%) catalyst. Hydrogenation in
603
cyclohexane solution at 353K was carried out in a 250ml static reactor under a constant hydrogen pressure (0.9MPa) with a constant initial acetophenone concentration (0.3mol.l-1) and at a stirring speed of 1800rpm so that the diffusional limitation did not affect the reaction. RESULTS Microstructure of mecursor allovs. Raney Ni catalysts were usually prepared from Ni-A1 50.50wt% alloys. According to the A1-Ni binary phase diagram, these alloys contain the different binary phases formed during solidification (Ni2A13,NiA1, and the eutectic Al/Al,Ni). The solidification paths for Cu and Cr as-cast Ni2A1, alloys were different but the solidification began with the Ni2A1, phase. For Cu as-cast alloys, the solidification path was similar to the undoped Ni$13 alloy and ended with the NiA1, phase in agreement with the A1-Ni phase diagram (8). In the case of Cr-doped alloys, only 0.8at%Cr was solubilized in the primary phase, therefore the solidification ended with a Cr-rich binary compound with a composition around A18(CrNi)5. After annealing at high temperature, Cu-doped Ni,Al, was homogeneous but for the Cr-doped alloy, a Cr-A1rich phase remained. The composition of this phase was analysed after 17 days at 950°C. It corresponds to Cr,~l,,Ni5. It had only be possible to solubilize 1.5at% Cr in the Ni$l, primary phase. In p-crystallized alloys, a typical microstructure was observed. Grains showed a dendritic structure with a central part constituted of the NiAl phase surrounded by large domains of the Ni2A1, phase. The composition of the interdendritic groove depended on the dopant (5). In the case of Cr-doped 1.1 alloys, only Cr segregation was detected at fine-scale observations. Phvsicochemical characteristics of Ranev Nickel catalysts. Table 1 summarizes physicochemical characteristics of doped and undoped catalysts with different microstructures. Total and metallic surface areas of catalysts prepared from commercial or undoped Ni,Al, as-cast alloys were almost the same. A slight decrease was observed with the Cu-doped alloy. On the contrary, total and metallic surface area increased in the presence of Cr.
604
Catalysts prepared from p alloys had smaller surface areas than those prepared from as-cast alloys; this was observed for both doped and undoped alloys. The reduction of surface area was the most significant in the case of the Cr-doped p alloys, wherethe metallic surface area was only 45% of that of the catalyst issued from the Cr-doped as-cast alloy. For Cu and undoped alloys, the metallic surface area remained about 80%. No significant influence on surface areas was observed by annealing the precursor alloys. TABLE 1 Physicochemical characteristics of the catalysts.
'I-----Alloys
Catalysts
Surface m2g-1 total meta 11ic
I
80
60
10
Ni2A13
80 66
64 53
26 32
69
59
49 39 54
48 62
I
120 72 120
33
I
Nil. gcUO. lA13 as cast fl
annealed
F
annealed
75
In the precursor alloy, M/Ni
75 73
=
1
~
Ni-A1 50.50
L 1 I
~
Composition % at Al/Ni M/Ni
I
53
4.9 5.1 4.9
48 75 63
5.4
3 4
5at%.
Using Ni,Al, as precursor alloy favored A1 retention. In the presence of a dopant, A1 retention increased; moreover this phenomena was enhanced by the microstructure. Al/Ni ratio was respectively 0.48 to 0.75 in catalysts prepared from Cr-doped ascast and L,I alloys. The dopant in the catalyst remained at the same level as in the precursor alloy. During alkali leaching, dopant loss was small: 7 for Cu and 15% for Cr. Auger and XPS analyses indicated that Ni and A1 were only in the metallic state in all types of catalysts ( 9 - 1 1 ) . These results contrasted with the work of Okamoto et a1 (12) who mentioned the presence of both metallic and oxidic Al. Cu remained in a metallic state in agreement with previous results (13). The surface and the bulk composition were the same (Cu/Ni surface ratios were 3.5 and
605
4.5at% in catalysts prepared from as-cast and p alloys respectively. On the contrary large oxidized Cr1Irsegregation was observed depending on the microstructure of the precursor alloy. In catalysts prepared from as-cast and p Cr-doped alloy, the surface ratio Cr/Ni were respectively around 0.70 and 0.50. A1 concentration was smaller on the surface than in the bulk. Microstructure of Ranev Nickel catalysts. By leaching out the alloys, the catalysts obtained were formed of different typesof agglomerates, the composition of which depended on the dopant and the microstructure of the precursor alloy. Inside each agglomerate, the composition was generally homogeneous. In commercial Raney Nickel catalysts, the observations showed that the Al/Ni ratio in the majority of Ni agglomerates varied from 0.07 to 0.12 whichconcords with the chemical analyses. However, some Ni agglomerates had a much higher (0.20) or much lower (0.03) Al/Ni ratio.These results indicated that the composition of the Ni agglomerates corresponds to precursor phases in agreement with the known leachability of the binary phases where NiA1, retained after leaching about 5at% of A1 and Ni,Al, over 23at%.
Fig. 1. Bright field image of a Ni-agglomerate prepared from ascast Cr-doped alloys: selected diffraction area and EDX spectrum. When the doped as-cast alloys were turned into Ni catalysts, the residual A 1 content depended also on the nature of the
606
different phases present in the precursor alloys. EDX microanalyses performed on the agglomerate showed that the ratios Al/Ni ( 0 . 2 2 ) and Cr/Ni (0.08) were in good agreement with the chemical analyses (fig.1). These values indicated that a large number of the agglomerates were formed from leaching out the primary Ni,Al, phase. However, some Cr-rich agglomerates were also observed in these catalysts. They were formed from the Cr-A1 rich phase previously m e n t i o n e d in this Cr doped as-cast alloy. Catalysts prepared from /I-alloys appeared as Ni crystallites supported on a NiAl core not completely leached. This result is in agreement with the known leachability of the different A1-Ni binary phases. The leachability decreases from NiA1, to Ni,A13 and is very slow f o r NiA1. The NiAl core not completely leached could explain the large A1 amount in Ni catalysts prepared from palloys. As shown on catalyst obtained directly after leaching a bulk sample (fig.2), the A1 level was low on the external part of the leached zone and high in the center.
Fig. 2. Bright field image of bulk p Cr-doped alloys after leaching. EDX spectra showing the inhomogeneity of A1 composition. In these catalysts, all prepared from the Ni2A1, base alloys, the edges of the Ni-agglomerates were thin enough for 200kV e1ect:rons to transmit as shown in fig.1. Intensity enhancements on ring patterns indicated the presence o f a mosaic of Ni crystallites with preferential orientations due to the orientation relationships between Ni2A1,-NiA1 and NiAl-Ni cells (14). Catalvtic wroperties of catalysts Drewared from Ni,Al, allovs. As with other types of Ni catalysts (15,16), hydrogenation of acetophenone (AC) led to phenyl 1-ethanol (PE). Secondary
607
reactions such as hydrogenation of the aromatic ring and hydrogenolysis of the hydroxy group gave by-products: methylcyclohexylketone (MCC), 1-cyclohexylethanol (CE) and ethylbenzene (EB). Table 2 gives the initial rates for acetophenone hydrogenation on the different catalyst. The selectivity is illustred by the maximum of MCC and PE.
Alloys Ni2A13
Catalysts
g-l "AC m-2
%
MCCmax
%%ax
Ni2-3 pNi2-3
5.6 4.4
8.6 8.3
6.5 5.2
79 84
Nil. gcUO. lA13 as cast p hom.
Ni-Cu pNi-Cu hNi-Cu
3.7 3.7 3.2
7.0 9.5 5.9
5.5 5.1 4.0
79 81 82.5
Nil.gCrO. lA13 as cast P
Ni-Cr pNi-Cr
4.8 4.6
13.9
6.8
0.8 1.5
83 90
P
vOAC
: initial hydrogenation rate of acetophenone, expressed in mmol .mix'. g-1 and mmol.min-~.m-~,i10+2. %MCC,,, and %PE,,, : maximum yield in MCC and PE.
Catalysts either doped or undoped had the same degree of activity expressed per m2 of Ni. Nevertheless, Cu slightly reduced the activity without modifing the selectivity (79% in PE). The addition of Cr did not really alter the activity but improved the selectivity (from 79 to 83% in PE) by reducting notably the ring hydrogenation: the maximum percentage in MCC decreasedfrom 6.5 to 0.8. We have shown in previous papers that Cr can increase the acetophenone hydrogenation activity and also the PE selectivity, these improvements being in relation to the amount of Cr added to Ni (7,9). The Cr content of the catalyst presented in this work (Cr/Ni = 5at%) corresponded to an effect only on the selectivity. We noted different catalytic properties according to the precursor used, as-cast or p-crystallized alloys, these effects being enhanced by the presence of a dopant. By p-crystallization of the undoped alloy, the activity (mz) of the resulting catalysts, remained unchanged but the selectivity in PE increased from 79 to
608
84%. The same behaviour was observed in the presence of Cu. In the case of the Cr-doped catalyst, besides a similar effect of the precursor alloy microstructure on the selectivity (an increase in PE from 83 to go%), the catalyst was twice as active per m2 of Ni surface as the catalyst obtained from the as-cast alloy. We only tested the annealed Nil,gCuo.lA13 alloy. Indeed, only this alloy became homogeneous by annealing as-cast alloy at high temperature; the Cr-doped alloy remained heterogeneous as shown previously. The homogeneisation of the precursor alloy gave a catalyst with only a slightly improved selectivity in PE. DISCUSSION As previously reported, physicochemical characteristics of catalysts prepared from undoped or Cu-doped alloys were rather similar. For both samples, p-crystallizationcauses a similar effect: reduction of total and metallic surface areas (15-20%) and an increase of the A 1 content ( 2 0 - 3 0 % ) . From Auger and XPS analyses, Cu was found in a metallic state and seemed substituted at random in the Ni lattice. The same effect on catalytic properties by p-crystallization of the precursor alloy, observed with the undoped and Cu-doped catalysts, could be explained by the similarity of the microstructure of both types of samples. In Cr-doped catalysts prepared from as-cast alloys, Cr was oxidized and moreover segregated to the surface. The best selectivity in PE of these samples was recently attributed to the surface oxidized chromium (10-15). But an other point must be mentioned. In the three cases (undoped, Cu and Cr-doped catalysts), a similar behaviour was observed concerning the influence of the microstructure. The yield in PE (%PE,,,) was always higher in catalysts prepared from p-alloys than from as-cast alloys. The increase of selectivity observed in p-systems seems due to a significant decrease in by-product formation (essentially EB) as seen in fig.3 which illustrates results obtained on Cr-doped as-cast and p-precursor alloys. In p-crystallized alloys, the grains appeared with a dendritic shape: a NiAl core surrounded with large domains of the Ni@, phase, the interdendritic groove being constitued of Al-rich phases. The p-crystallization stabilized an architecture with internal phases poorer in A1 than the external phases. After
609
alkali leaching, these different phases gave Raney catalysts with different residual A 1 content. So, ,u-crystallization produced catalysts with increasing A1 contents from the core outwards. The A 1 content at the surface is then probably different between catalysts issued from as-cast or p-crystallized alloys and this difference can explain the variation in selectivity.
Fig.3. Hydrogenation of acetophenone: scheme of reaction and products distribution as a function of time. With a dopant, the respective size of the different zones in the ,u-crystallized alloys changes as well as the distribution of the dopant. It is not the case for Cu, so the difference in selectivity between the samples issued from the as-cast and the pcrystallized alloys was in the same order as the one obtained with the undoped catalysts. In the case of Cr-doped p-alloys, chromium was segregated in the interdendritic zone and no Cr-rich phases such as A1,(CrNi)5 were observed as in the as-cast alloy. The pcrystallization of Cr-doped alloy modified both the A 1 and the Cr distribution. Therefore, it is not surprising that not only the activity, but also the selectivity were modified. Another hypothesis could explain the modification in selectivity occurringwith catalysts originating from p crystallized alloys: the sensitivity of hydrogenolysis to the structure of the catalysts. As shown in fig.3, the increase in PE selectivity between catalysts issued from as-cast and ,u-crystallized alloys originates principally from a decrease of the hydrogenolysis. We have shown that alkali-leaching is not anarchic; the crystallographic orientation of Ni-crystallites is direcly related to the
610
orientation of the precursor phase (6). Then Raney Nickel catalyst keeps the memory of the metallurgical and crystallographic structures of the precursor alloy. As hydrogenolysis is a demanding reaction, sensitive to the structure of the catalyst, we can presume that this reaction will be dependent on the precursor. ACKNOWLEDGEMENT XPS analyses were performed in the "Laboratoire de Catalyse" of Louvain la Neuve. This study was supported by the CNRS (Chemical ATP 904332) and conducted in a Stimulation Plane of the CEE (Codest Program). REFERENCE 1 P. FOUILL0UX.- Appl. Catal., 8, (1983) 1-42. 2 M.S. WAINWRIGHT and R.B. ANDERSON J. Catal., 64 (1980) 124-131. 3 J.M. BONNIER, J.P. DAMON and J. MASSON Appl. Catal., 42, (1988) 285-297 4 S.R. MONTGOMERY Catalysis of Organic Reactions, W.R. Moser ed, M. Dekker, New York (1981) 383. 5 J. GROS, S. HAMAR-THIBAULT and J.C. JOUD J. Mat. Science, 24, (1989) 2987-2998. 6 J. GROS, S. HAMAR-THIBAULT and J.C. JOUD Surface and Interface Analysis, 11, (1988) 611-616. 7 S. SANE, J.M. BONNIER, J.P.DAMON and J. MASSON.Appl. Catal., 9, (1984) 69-83.
-
.
-
-
8
9
10 11 12 13 14 15 16
M. HANSEN and A. ANDERKO - Constitution of binary Alloys. New York, Mac Graw Hill (1958). C KORDULIS, B. DOUMAIN, J. DAMON, J. MASSON, J.L. DALLON and F. DELANNAY - Bull. SOC. Chim. Belge, 94, (1987) 371-377. J.M. BONNIER, J.P. DAMON, B. DELMON, B. DOUMAIN and J. MASSON J. Chem. Phys., 84, (1987) 889-894. F. DELANNAY, J.P. DAMON, J. MASSON and B. DELMON Appl. Catal., 4, (1982) 169-180 Y. OKAMOTO, Y. NITA, I. IMANAKA and S. TERANISHI J.C.S. Faraday I, 76, (1980) 998-1012. V. BIRKENSTOCK, R. HOLM, B. REINFANDT and S. STORP J. Catal., 93, (1985) 55-67. F. DELANNAY - Reactivity of Solids, 2, (1986) 235-243. T. KOSCIELSKI, J.M. BONNIER, J.P. DAMON and J. MASSON Appl. Catal., 43, (1989) 91-99. J.M. BONNIER, J. COURT, P. WIERZCHOWSKI and S. HAMAR-THIBAULT- Appl. Catal., 53 (1989) 217-231.
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G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in T h e Netherlands
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NOVEL TYPE O F HYDROTREATING CATALYSTS P R E P A R E D THROUGH PRECIPITATION FROM HOMOGENEOUS SOLUTION (PFHS) METHOD KAZA SOMASEKHARA RAO'I,
V.V.D.N.
PRASADI, K.V.R. CHARY' a n d P. KANTA
RAO~ 'Chemistry D e p a r t m e n t , Andhra University, P.G. Extension C e n t r e , Nuzvid - 521 201, A.P., India 2Catalysis Section, Indian I n s t i t u t e of Chemical Technology, Hyderabad 500 007, A.P., India SUMMARY y -Alumina supported unpromoted c a t a l y s t molybdenum sulphide and promoted c a t a l y s t s cobalt sulphide - molybdenum sulphide; nickel sulphide - molybdenum sulphide w e r e prepared by P r e c i p i t a t i o n F r o m Homogeneous Solution (PFHS) t e c h nique using t h i o a c e t a m i d e hydrolysis in a single step. Oxygen chemisorption studies, hydrodesulphurisation (HDS) and hydrogenation (HYD) s t u d i e s w e r e m a d e f o r t h e s e catalysts. These catalysts do not need pre-sulphidation prior to HDS reaction. INTRODUCTION Willard and Tang (ref. 1) utilised t h e t e c h n i q u e f o r t h e precipitation of basic aluminium sulphate by t h e controlled hydrolysis of u r e a t o yield a m m o n i a and called it a s Precipitation F r o m Homogeneous Solution (PFHS) which is t h e basis for d e v e lopment.
Since t h e n a l a r g e number of m e t h o d s w e r e developed and t h e y w e r e
reviewed (refs. 2-4).
Anion r e l e a s e t e c h n i q u e involves t h e r e l e a s e of anion in solution
so a s t o p r e c i p i t a t e m e t a l ions present through controlled hydrolysis.
Thioacetamide
hydrolysis w a s used t o p r e c i p i t a t e molybdenum sulphide (ref. 5) and nickel s u l phide (ref. 6 ) .
R e c e n t l y PFHS m e t h o d h a s been identified a s a good m e a n s of
making b e t t e r c a t a l y s t s (refs. 7-10). H y d r o t r e a t i n g of petroleum c r u d e s and c o a l derived liquids is a n industrial c a t a l y t i c process.
T h e commonly employed c a t a l y s t s during hydrodesulphurisat ion
(HDS) a r e MoS2 or WS2 promoted with cobalt or nickel on a high s u r f a c e a r e a
g a m m a alumina. support cobalt
Usually t h e s e c a t a l y s t s a r e prepared by t h e impregnation of alumina
using aqueous solution containing a m m o n i u m molybdate and n i t r a t e s of or
molybdenum followed by calcination a t
higher t e m p e r a t u r e (500°C).
Single s t e p sulphide c a t a l y s t p r e p a r a t i o n method is not r e p o r t e d so f a r . In present
work, w e report
new t y p e of hydrot r e a t ing catalysts consisting in
MoS2 promoted with C o o r Ni prepared by PFHS method.
The a c t i v i t i e s of t h e
c a t a l y s t s w e r e e v a l u a t e d f o r hydrodesulphurisat ion (HDS) of t h i o p h e n e and hydrogena t i o n (HYD) of cyclohexene.
A comparison of t h e p e r f o r m a n c e of t h e s e c a t a l y s t s
has been m a d e with c o m m e r c i a l h y d r o t r e a t i n g c a t a l y s t s .
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REAGENTS AND APPARATUS Molybdenum t r i o x i d e (Moo3), cobalt n i t r a t e ( C o ( N 0 ) .6H20), nickel n i t r a t e 3 2 (Ni(N03)2 .6H20, t h i o a c e t a m i d e (CH3CSNH2), u r e a (NH2CONH2), (all f r o m LOBA Chemie), n i t r i c acid (BDH), cyclohexene (Fluka), t h i o p h e n e (Kodak) w e r e a l l of 1 analytical reagent grade. y-A1203 (Harshaw, S.A. 234 m2g-’, P.V., 0.65 ml g - ). A conventional high vacuum glass s y s t e m w a s used t o m e a s u r e t h e BET s u r f a c e 2 a r e a s by nitrogen (0.162 nm ) absorbed a t -196°C. X-ray d i f f r a c t o g r a m s w e r e recorded o n a Philips pW 1051 d i f f r a c t o m e t e r . EXPERIMENTAL P r e p a r a t i o n of Molybdenum Sulphide An aqueous solution of IOOml containing lOml of Moo3 (O.lM), I g urea, 0.75ml c o n c e n t r a t e d n i t r i c a c i d and 3 0 m l of t h i o a c e t a m i d e (0.135M) placed in a 250ml conical flask.
T h e flask w a s c o v e r e d with rubber c o r k and t h e c o n t e n t s in t h e
flask w e r e h e a t e d o n a w a t e r b a t h (90-95°C) for about 3 hours by i n t e r m i t t e n t stirring.
A f t e r t h e precipitation w a s c o m p l e t e (pH 2.0 t o 3.0) it w a s f i l t e r e d ,
washed with distilled w a t e r and dried a t 110°C f o r I hour. P r e p a r a t i o n of Cobalt Sulphide An aqueous solution of lOOml containing lOml of cobalt n i t r a t e (O.Iml), 5 g urea, I m l n i t r i c a c i d (0.75N) a n d 3 0 m l of t h i o a c e t a m i d e solution (0.133M) placed in a 250ml conical flask.
T h e flask w a s c o v e r e d with rubber c o r k and t h e c o n t e n t s I in t h e flask w e r e h e a t e d on a w a t e r b a t h (90-95OC) for about 2- hours by i n t e r m i t t e n t 2 stirring. A f t e r t h e precipitation w a s c o m p l e t e pH (7.5 t o 8.5), it w a s f i l t e r e d ,
washed with distilled w a t e r and dried a t 110°C f o r 1 hour. P r e p a r a t i o n of Nickel Sulphide An aqueous solution of
IOOml containing lOml of nickel n i t r a t e (O.IM),
5g
urea, l m l n i t r i c a c i d (0.75N) and 5 0 m l of t h i o a c e t a m i d e solution (0.133M) placed in a 250ml conical flask. T h e flask w a s c o v e r e d with rubber c o r k and t h e c o n t e n t s in flask w e r e h e a t e d on a w a t e r b a t h for about 2-1 hours. A f t e r t h e precipitation w a s 2 c o m p l e t e (pH 7-8). It w a s f i l t e r e d , washed with distilled w a t e r and dried a t 110°C f o r about 1 hour. P r e p a r a t i o n of Y-Al2O3 supported Molybdenum Sulphide Suspend 4.7, 2.3,
1.5, 1.1, 0.86,
0.70, 0.54 g r a m s of Y-A1203
in a n aqueous
solution of IOOml in a 250ml conical flask containing lOml Moo3 (0.lM) solution, I g urea, 0.75ml conc. n i t r i c acid and 3 0 m l of t h i o a c e t a m i d e (0.133M) f o r obtaining
2,4,6,8,10,12,15
p e r c e n t a g e s of Mo/Y-A1203 c a t a l y s t s respectively.
The contents
of t h e flask w e r e h e a t e d t o 90-95°C f o r about 3 hours with i n t e r m i t t e n t stirring.
T h e resultant solids w e r e f i l t e r e d , washed and dried a t llO°C.
613 P r e p a r a t i o n of Co-Mo/y-AlzOs C a t a l y s t s Suspend 1.905, 1.41, 1.12 g of 8% MoS2/y-A1203 in a n aqueous solution of IOOml in a 250ml conical flask containing lOml cobalt n i t r a t e solution ( O . l M ) ,
5 g urea,
I m l , 0.75 N H N 0 3 and 3 0 m l t h i o a c e t a m i d e (0.133M) f o r obtaining 3 , 4 , 5 p e r c e n t a g e s of CoS2 on 8% MoS2/y-Al2O3
c a t a l y s t s respectively.
The c o n t e n t s of t h e flask
w e r e h e a t e d t o 90-95°C f o r about 3 hours with i n t e r m i t t e n t stirring.
T h e resultant
solids w e r e f i l t e r e d , washed and dried a t 110°C. P r e p a r a t i o n of Ni-Mo/y-AlzOs C a t a l y s t s Suspend 1.89, 1.41, 1.12g of 8% MoS /y-A1 0 in aqueous solution of IOOml 2 2 3 in a 250ml conical flask containing lOml of nickel n i t r a t e (O.IM) solution, 5 g urea, I m l , 0.75 n i t r i c a c i d and 3 0 m l t h i o a c e t a m i d e (0.133M) for obtaining 3,4,5 p e r c e n t a g e s of NiS2 on 8% MoS2/y-Al2O3
c a t a l y s t s respectively.
The c o n t e n t s of t h e
flask w e r e h e a t e d t o 90-95°C for about 3 hours with i n t e r m i t t e n t stirring.
The
resulting solids w e r e f i l t e r e d , washed and dried a t 110°C f o r o n e hour. ACTIVITY MEASUREMENTS
A d i f f e r e n t i a l flow m i c r o r e a c t o r , o p e r a t i n g under normal a t mospheric pressure and interfaced t o a gas chromatograph
by a six-way gas-sampling valve, w a s used
t o m e a s u r e t h e a c t i v i t i e s of t h e c a t a l y s t .
In a t y p i c a l experiment ca 0.3g of c a t a -
lyst s a m p l e was s e c u r e d b e t w e e n t w o plugs of pyrex glass wool inside t h e glass r e a c t o r (pyrex glass t u b e , 0.5cm
i.d.).
The r e a c t i o n t e m p e r a t u r e w a s adjusted
t o 400°C for t h i o p h e n e HDS and f o r cyclohexene HYU.
The c a t a l y s t was c o n t a c t e d
with t h e r e a c t i o n mixture, which consisted of a s t r e a m of hydrogen s a t u r a t e d with t h i o p h e n e o r cyclohexene a t 25°C.
The partial pressures of t h i o p h e n e a n d c y c l o -
h e x e n e w e r e 80.0 and 85.0 Torr respectively.
All r a t e s w e r e measured under s t e a d y -
s t a t e conditions. ANALYSIS The HDS product of t h i o p h e n e w a s b u t a n e and w a s analysed by gas c h r o m a t o graphy with t h e help of a 2 m stainless-steel column packed with 10% OV-17, maint a i n e d a t 100°C.
Cyclohexane w a s t h e only product found for t h e HYD of cyclo-
hexene under t h e e x p e r i m e n t a l conditions and was analysed by 20% PEG-1500 (2m column maintained a t 90°C).
A c a r r i e r g a s (nitrogen) flow of 40 C m 3 min-l and
a n FID w e r e used in both cases. CHEMISORPTION MEASUREMENTS A conventional high-vacuum s y s t e m was used.
0.5g
In a t y p i c a l experiment ca.
of c a t a l y s t w a s placed in t h e c a t a l y s t c h a m b e r and s y s t e m w a s e v a c u a t e d
a t 400°C for 2 hours a t l o 4 Torr.
T h e c a t a l y s t c h a m b e r w a s t h e n cooled t o -78°C
by dry-ice + a c e t o n e b a t h and t h e e v a c u a t i o n w a s continued a t t h i s t e m p e r a t u r e for 15 min.
Oxygen f r o m a reservoir, c o n n e c t e d t o a high vacuum manifold,
614 w a s allowed t o e n t e r t h e c a t a l y s t c h a m b e r with known d e a d space.
An initial
quick fall in t h e p r e s s u r e w a s followed by a levelling off within ca. 10 min. and t h e equilibrium pressure was noted.
This process w a s r e p e a t e d with d i f f e r e n t
initial pressures and t h e first adsorption isotherm, r e p r e s e n t i n g both t h e chemisorbed and physisorbed oxygen, w a s generated. After this t h e catalyst was evacuated I a t -78°C f o r 17 hour a t Torr t o r e m o v e t h e physisorbed oxygen a n d t h e second isotherm r e p r e s e n t i n g only t h e physisorbed oxygen, w a s g e n e r a t e d in a n identical manner.
F r o m t h e s e t w o linear and parallel i s o t h e r m s t h e amount of chemisorbed
oxygen w a s d e t e r m i n e d by t h e method dof P a r e k h a n d Weller (ref. 11). A f t e r t h e chernisorption experiment t h e BET s u r f a c e a r e a of t h e c a t a l y s t w a s d e t e r m i n e d a t -196°C. RESULTS AND DISCUSSIONS P r e c i p i t a t e s of molybdenum sulphide, cobalt sulphide, nickel sulphide f o r m e d w e r e found t o b e quantitative. Oxygen Chemisorpt ion Oxygen chemisorption e x p e r i m e n t s w e r e c a r r i e d out a t
-78°C for d i f f e r e n t
compositions of unpromoted MoS2/y-A1203 and p r o m o t e d Co-Mo/y-Al2O3, Ni-Mo/AI2O3 c a t a l y s t s and t h e r e s u l t s w e r e given in T a b l e 1.
BET s u r f a c e a r e a results w e r e
also r e p o r t e d . TABLE 1 Composition, oxygen u p t a k e and BET s u r f a c e a r e a s of various c a t a l y s t s Catalyst
a T h e b a l a n c e w a s y-Al2O3. The BET s u r f a c e a r e a of t h e alumina (Harshaw A l - I I I - 6 1 E , 2 -I g ) which w a s used t o p r e p a r e c a t a l y s t s .
S.A. 234 m
615
F r o m t h e r e s u l t s of t h e t a b l e , it w a s observed t h a t oxygen chemisorption increases linearly in t h e c a s e of unpromoted c a t a l y s t a s a function of M o loading u p t o 8% (w/w) a n d t h e n d e c r e a s e s with higher M o c o n t e n t .
This is probably d u e
t o t h e i n c r e a s e in s i z e of t h e individual MoS crystallites. This 8% level corresponds 2 t o a t t a i n m e n t of a monolayer of MoS2 o n t h e alumina surface. T h e dispersion of MoS2 (O/Mox100) i s found t o b e 0.036, X-ray d i f f r a c t i o n (XRD) r e s u l t s indicates that
n o X R D peaks corresponding t o MoS2 w e r e observed f o r high Mo-loading.
This, in turn, indicates t h a t molybdenum sulphide is present in highly dispersed and amorphous s t a t e o n t h e s u r f a c e of
y-A1203.
In t h e c a s e of promoted c a t a l y s t s
t h e oxygen u p t a k e values w e r e higher for 8:5 Mo-Co/y-A1 0 and 8:5 Mo-Ni/y2 3 A1203. BET s u r f a c e a r e a of t h e unpromoted Mo c a t a l y s t s d e c r e a s e d with increasing
Mo loading and i n t h e promoted c a t a l y s t s d e c r e a s e d with increasing promoted loadings. HDS and HYD A c t i v i t i e s HDS a c t i v i t y of t h e c a t a l y s t s r e p o r t e d a s t h e steadystate
r a t e of HDS of
thiophene and HYD a c t i v i t y of cyclohexene a r e r e p o r t e d in Table-2. TABLE 2 HDS and HYD a c t i v i t i e s of various c a t a l y s t s a t 400°C Catalyst
Compositions of 1-13 c a t a l y s t s a r e given in t h e Table-I. T h e results of T a b l e 2 show t h a t t h e HDS a c t i v i t y is maximum f o r unpromoted
8% Mo loading o n y-A1 0 and 8:5 Mo-Co and 8:5 Mo-Ni c a t a l y s t s . These results 2 3 with t h e oxygen chemisorpt ion values. HDS a c t i v i t y oi t h e c a t a l y s t s
correlate
prepared by PFHS w a s higher when c o m p a r e d t o c o m m e r c i a l c a t a l y s t s .
It is a l s o
of i n t e r e s t t o n o t e t h a t HDS a c t i v i t i e s of cobalt and nickel p r o m o t e d c a t a l y s t s a p p e a r t o b e higher than t h o s e of u n p r o m o t e d catalysts. T h e r e f o r e it a p p e a r s t h a t
616
t h e role of promoter is mainly t o increase t h e intrinsic activity of t h e HDS sites and not t o increase t h e number of a c t i v e sites responsible f o r HDS of thiophene. It is generally accepted t h a t co-ordinatively unnaturated Mo ions (CUS) on sulphided
c a t a l y s t s a r e t h e a c t i v e sites for hydrodesulphurisation and hydrogenolysis react ions and t h a t t h e s e a r e located on MoS2 a s a patchy - monolayer on t h e surface of alumina support (refs. 12, 13). HYD activity decreases for unpromoted and promoted catalysts.
HYD activity appears t o b e a function of only t h e extensive property
(i.e. t h e no. of sites) and only d u e to dispersion e f f e c t . Conclusion The c a t a l y s t s prepared by PFHS method a r e not required t o sulphide prior t o HDS reaction.
These sulphide catalysts c a n be prepared in a single step.
c a t a l y s t s have higher HDS
These
activity t h a n t h e c a t a l y s t s prepared by other met hods.
Thus PFHS method is found t o b e a novel method for preparing highly a c t i v e hydrot r e a t i n g catalysts. ACKNOWLEDGEMENT The authors thank t h e Council of Scientific and Industrial Research, New Delhi for t h e i r financial support t o KSR and for awarding fellowship t o V.V.D.N. Prasad. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13.
H.H. Willard and N.K. Tang, J. Amer. Chem. SOC., 59 (1937) 1190. P.F.S. Cartweight, E.J. Newrnan and D.W. Wilson, Analyst., Rev., Vol. 92, No. 1100 (1967) pp. 663. Kaza Somasekhara Rao, U. lvturalikrishna and V.G. Vaidya, Quarterly Chemistry Rev., Vol. I , No. 2 (1985) 134-150. Kaza Somasekhara Rao, Acta Ciencia Indica, Vol. XIlc, No. 3 (1986) 122. F. Burriel - Marti and A.M. Vidan, Anal. Chim. Acta. 26 (1962) 163. D.H. Klein, D.G. P e t e r s and E.H. Swift, Talanta, 12 (1965) 357. J.A. Van Dillen, J.W. Gevs., L.A.M. Hermans and J. Vander Meijden, in Proc. 6th Int. Congr. Catal., Edn., G.C. Bond, P.B. Wells and F.C. Tompkins, The Chemical Society, London, 1976, (1977), p. 677. H. Sehapper, E.B.M. Duisburg, J.M.C. Quantel and L.L. Van Reijen, in Preparation of Catalysts 111, Eds. G. Poncelet, P. Grange and P. Jacobe, Elsevier Amsterdam, 1983 p. 301. Ch. Sivaraj, B. Prabhakara Reddy, 8. Rama Rao and P. Kanta Rao, Applied Catal., 24 (1986) 25. Vemulapalli Prasad, Komanduri Chary, Kaza Somasekhara Rao and Panja Kanta Rao, J. Chem. SOC., CHEM COMMUN, 22 (1989) 1747. B.S. Parekh and S.W. Weller, J. Catal., 47 (1977) 100. N.K. Nag, J. Catal., 92 (1985) 432. W.S. Millman and W.K. Hall, J. Catal., 59 (1979) 311.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
617
PREPARATION OF MANGANESE OXIDE CATALYSTS USING NOVEL NH4MnO4 AND MANGANESE HYDROXIDE PRECURSORS. COMPARISON OF UNSUPPORTED AND ALUMINA SUPPORTED CATALYSTS A.K.H.
N O H M A N ~ ~D. ~ ,DUPREZ~,c. KAPPENSTEIN~, S.A.A. MANSOUR' AND M.I. ZAKI'
l c h e m i s t r y Department, F a c u l t y o f Science, M i n i a U n i v e r s i t y , El-Minia, EGYPT 2Catalyse en Chimie Organique, Faculte des Sciences de P o i t i e r s , FRANCE.
SUHMARY
Unsupported and a1 umina supported manganese oxide c a t a l y s t s were prepared using manganese n i t r a t e , manganese hydroxide and ammonium permanganate. They were b u l k and surface c h a r a c t e r i z e d by thermal analysis, X-ray d i f f r a c t i o n , d i f f u s e r e f l e c t a n c e , I R and photoelectron spectroscopy, SBET and TPR. Moreover H202 decomposition and CO o x i d a t i o n were used as t e s t r e a c t i o n s . The most a c t i v e supported c a t a l y s t s are t h e manganese hydroxide coated samples which show a rMn 03 phase. For t h e ammonium permanganate-based c a t a l y s t s a s t r o n g i n t e r a c t i o n w i t h t h e c a r r i e r was evidenced.
INTRODUCTION
Studies concerning supported manganese oxides are relatively scarce, despite their potential activity in oxidation reactions. For example manganese oxide catalysts are very active for CO oxidation, particularly when they are promoted with CuO or COO (1). They are also used for methanol oxidation and ethylene hydrogenation (2). These catalysts were first prepared and investigated by Selwood et a1 ( 3 ) by impregnating manganese (11) nitrate onto high surface area alumina and then thermally decomposed. Thereafter, these catalysts were characterized by several techniques (4-9). Baltanas et a1 (2) prepared these catalysts by impregnating manganese nitrate onto alumina and in situ precipitation of manganese hydroxide by ammonia solution. The aim of the present study was to prepare various series of bulk and alumina-supported manganese oxide catalysts. Novel NH4Mn04 and manganese hydroxide precursors as well as the conventional manganese nitrate were used in hope of defining the impacts of the precursors and the support on physico-chemical characteristics and activity of the final catalysts. KMn04 was previously used by Cavallaro et a1 ( 8 ) but with little information. We have taken NH4Mn04 for our investigation to avoid
618
more complications arising from the presence of potassium. By adopting a coating procedure in case of manganese hydroxide, we aimed to have surface layers of manganese oxide on alumina which may be more easily detectable than in the case of the two previous precursors. EXPWIWENTAt
Materials High surface area r-alumina (214 m2 g-l) was obtained by slow gel formation between ammonia and aqueous aluminum nitrate solutions, followed by decantation, drying (12OOC; 4 days) and calcination (450'C: air, 5 h; oxygen, 2 h). Three precursors were used to prepare the various series of unsupported and supported manganese oxide catalysts: (i) manganese (11) nitrate ( W ) ,(ii) ammonium permanganate NH4Mn04 ( m synthesized ) by a metathetical reaction between NH4C1 and KMn04 (lo), and (iii) manganese hydroxide coat (*A) obtained by slow addition of Mn(N03)2 6H2O solution to aqueous ammonia followed by filtration and drying Torr, 25'C). The unsupported catalysts were obtained by calcination of the and &G ! precursors at 150', 300' and 6OO0C for 5 h in air. The products thus obtained are designated by formula like Hn2[1501 or MnC13001. In the case of precursor, crystals of NH4Mn04 were first slowly decomposed at 12OOC for 2 h in air prior to calcination. The product is then calcined as above: the calcination products are denoted like Mn7f1501. In case of manganous nitrate and ammonium permanganate, the supported catalysts were obtained by impregnation from aqueous solutions of various concentrations. The loading level for the cationic adsorption of Mn2+ remains low (0.13, 0.3 and 0.6 wt%-Mn) whereas for MnO4- the impregnation leads to higher values (0.5, 0.9 and 1.7 wt%-Mn). For the manganese hydroxide precipitate, a coating procedure was carried out by formation of the precipitate in presence of the carrier, to provide the loading levels of 0 . 4 , 4.1 and 6.8 wt%-Mn. All these samples were subsequently filtered, dried (25'C and l o q 2 torr) and then calcined (150, 300 or 600-C). They are denoted like v, where x gives the Mn loading.
619
Characterization techniaues The various samples of unsupported and supported manganese oxide catalysts were subjected to a range of physical and chemical characterization methods, so as to examine their surface as well as bulk properties, and hence the effect of the preparation variables. For the bulk properties the following techniques were used : thermogravimetric and differential thermal analysis (TGA and DTA), Shimadzu apparatus type DT-30 H, heating rate 10'C min-l, reference a-A1 203; X-ray diffraction (XRD), Siemens D 500 diffractometer with microcomputer attachment, Cu Ka radiation (1.5418 A ) ; infrared spectroscopy (IR), Perkin-Elmer recording spectrophotometer (Model 580 B), KBr pellets : and temperature programmed reduction (TPR) in H2: pulses of H2 (0.285 cm3) being injected every other minute from ambient temperature to 500'C (4 C min-l ) On the other hand, the following methods were employed as surface characterizing techniques: - surface area measurements (BET) by low temperature nitrogen adsorption method; - diffuse reflectance spectroscopy (DRS), Beckmann 5240 spectrometer equipped with an integrating sphere and coupled to an HP 9816 microcomputer; dehydrated BaS04 was used as a standard for all the spectral regions (250 - 2500 nm); - X-ray photoelectron spectroscopy (XPS), Riber spectrometer , A1 Ka source (1488.6 eV), reference C l s at 285 eV.. Moreover, and in order to reveal the effect of preparation variables on the redox activity of these catalysts, two model reactions were studied: (i) H202 decomposition in aqueous solution and (ii) CO oxidation in transient flow, carried out with the same chromatographic apparatus as for TPR measurements. The latter technique leads to the determination of the oxygen storage capacity (OSC) of the catalyst: pulses of CO were injected every other minute at 3OO0C on a sample predosed with O2 pulses at 300'C .
.
RESULTS AND DISCUSSION
Bulk characterization A part of the catalysts used are listed in Table 1. TGA and DTA results of precursor indicated that this material commences decomposition at 80-C to give Mn02 which leads to
620
a-Mn2O3 upon calcination at 600'C, in agreement with previous results (11). XRD data and IR findings confirmed these results (table 1). For the supported catalysts, the thermal behavior was not the same, indicating a probable interaction of manganese nitrate with r-A1203 (6) due to the very low load of the samples. No detectable thermal events have been evidenced, indicating that the surface species do not change upon heating, also reflected by the pale brown color exhibited by all the samples. However TPR profiles are similar for calcination temperature 150 and 30OOC but different for 6OO0C. (Fig.1). The first peak of the TPR curves (= 350'C) in the case of the samples calcined at 150 and 300'C can be attributed to the reduction of adsorbed nitrate ions. It was shown previously that NO3- ions adsorbed on A1203 reduced quantitatively into N2 during TPR, thus requiring 5H/N03- for their reduction to be completed (12). However even at 15OOC the content of residual nitrate is low, typically of the order of 30% of the initial loading associated with Mn. At higher calcination temperature, these ions are decomposed. TABLE 1: crystalline phases, surface area, oxygen storage capacity
and kinetic rate (9-1 catalyst)
.
constant
for
the
decomposition
of
H20i
ISC, 300'C lcrnol 0 9-3 Mn2(150)
NS
(300) NS (600) NS
0.6Mn2(150) (300) (600)
MnC(150)
S
S S
NS
( 3 0 0 ) NS ( 6 0 0 ) NS S S
S
NS NS NS S S S
€5-Mn02 + few a-Mn2Og R-Mn02 + few a-Mn203 a-Mn203 Only r-A1203j Only r-A1203, Only r-A1203;
10 11 12
4.3
3.2
2.4
139 149
0.03:
0.10 0.13
180
r-Mn203 22 r - ~ n ~ + o~ r ~ -1 ? ~ 0 ~ 28 a-Mn2O.3 24
178 86
r-Mn203 + F - A ~ ~ o ~174 1833 r-Mn2O3 + ~ ~ 1 ~ 0 1883 r - ~ n ~+ o~ ~ ~ 1 ~ 0
115 83 39
17.6
--
11.0
a-Mn203 +few MnOl.88 MnOle88 + a-Mn203 MnOl 88 + a-Mn20 more * crystallize2 Only I'-Al2O3{ Only r-A1203, Only r-A1203;
a) NS: non supported:
S:
supported.
60
131
82
149 130 147
79
907 480
-25
0
10.7 11.3 10.2 18.3 10.8
24.5 22.2
10.2
2.1
0.52
621
UNSUPPORTED
200 I
I
400 I
I
600 T("C) I
I
-
Fig.1: TPR p r o f i l e s . Surface area i n mmol H 4 - l
C L1
t 1
C
t 1
0
li
._ s 0
w
F i g . 2 : thermal a n a l y s i s o f some sarrlpl e s .
622
For the unsupported samples, the thermal analysis curves (Fig.2) and the XRD indicate that this precursor is most probably changed from the hydrated r-Mn203 to the a-Mn2O3 form above 40OOC. The presence of nitrate ions up to 300'C was evidenced by IR bands at 1385 cm-I (Fig.3) and by the different exothermic peaks of the DTA curve (Fig.2). This is in agreement with the TPR profiles (Fig.1) showing a first reduction peak between 300 and 340'C which disappears for MnCf600). sample. TPR profiles for Mn reduction (>400'C) for MnCf1501 and MnC16001 display a small difference which can be associated with the change from the r to the Q form of Mn2O3. This behavior was modified on coating the carrier, since up to 600'C the surface species of the supported samples remains r-Mn203 (Table 1 and Fig.4). The TPR curves show that the content of NO3is higher for 6.8HnCf150). than for 6.8MnCf6001 whereas the profiles at higher reduction temperature remain the same. Accordingly, the bulk phases of manganese oxide in these supported catalysts are detectable and the calcination temperature does not markedly affect either the crystalline or the chemical nature of these species, whereas the unsupported catalyst calcined at 600'C was markedly affected. Hence the interaction with the support may play an important role. In case of &Q unsupported and supported samples, bulk phases of manganese oxides were expected to be different owing to the different mode of decomposition and subsequent calcination. This was proved to be the case through the different results obtained for these catalysts. No bulk phases of manganese oxides were detectable for the supported catalysts by XRD, due to the low Mn loading. The possibility of strong interaction with the support is evident for the catalysts calcined at 600'C (cf. TPR profiles in fig.1). Surface characterization Surface characteristics of these catalysts are reflected on their SBET, DRS and XPS results. The surface areas of and unsupported samples are lower than the corresponding knx catalysts. This may be attributed to the differences in the porosity despite the similarity of the chemical nature of these different samples, as pointed out in their bulk characterization. On the other hand, the drop of surface area of all the supported catalysts relative to the support (214 m2 g-1), is most likely due
623
1
1050 650 75c F ig . 3 : IR-spectra of MnC f o r d i f f e r e n t c a l c i n a t i o n temperatures. 1450
DIF-KUB
.F
.5 0.6 :ln2(600)
.4
.2
461
0 700
10
\
1000
0. 9 Mn7
5
3 ' : 'r v )3 .
5=a
*a !=
I--!
r e f . : ?-?In 0 2 3
Fig.4: X-ray data f o r alumina, coated sample and d i f f e r e n c e s p ect r u n . The 1 ines correspond t o t h e reference compounds Y-Al 203 and Y-Mn203.
0 Fig.5: OR d i f f e r e n c e spe c tra o f some samples obtained by substra c tion of t h e spectrum of Y - A ~2 3 3 .
624
to the formation of a manganese oxide phase for 6.8MnC samples or to a blockage of the micropores for 0.9Mn7 and 0.6Mn2 samples (13). DRS results can give information on surface species of Mn present in supported samples. The difference spectra (Fig.5) of 0.9Mn7 samples show that the surface species at 600'C are different from those formed at lower calcination temperatures, in accordance with the TPR curves of the corresponding samples (Fig.1). Moreover the surface species at 6OO0C are comparable to those of 0.6Mn21600LI displaying the same band position ( = 460 nm). Thus the surface species are probably the same as in the case of 0.9Mn7f6001 sample,the activities becoming similar for the two catalysts. From the XPS data given in table 2, the variations of the Mn2p3/2 binding energy can be associated with the oxidation state of manganese (9,,14,15). Thus for the mechanical mixture Mn3O4 + A1203, this value (640.4 eV) corresponds to the presence of Mn(I1) and Mn(II1). In the case of 4.1NnC and 0.5Mn7 samples the oxidation state of Mn is higher probably between I11 and IV, and decreases slightly after calcination for 4.lMnC. The Mn/A1 ratio for the mechanical mixture is in agreement with the value calculated from the composition of the mixture (0.042). For 4.lMnC samples this ratio is higher than the calculated Mn/A1 ratio (0.04), reflecting the partial coating of the alumina surface, and for 0.5Mn7 the values correspond to a good dispersion of manganese on the surface of the carrier (calculated Mn/A1 ratio : 0.0047). TABLE 2: XPS data, surface area ratio and kinetic rate constant for the decomposition of H707. I
I
Binding Energy IIEbll eV fO. 2 Samples
Mn
0
1 1 1 1 3P
2P1/2 2P3/2
Is
Area ratio
C, 30°C
;-lg-1 O/Al
Mn3O4 + A1203 651.9 640.4 48.5 531.2 (5 wt%-Mn)
1.76
4.1MnC(RT) 4.1MnC(600)
653.3 641.7 48.7 531.3 652.7 641.2 48.5 531.2
1.79 1.76
0.069 0.060
0.12 0.10
8.1
653.7 642.0 48.7 531.3 653.6 641.9 48.5 531.2
1.77
1.80
0.024 0.025
0.044
0.043
7.0 0.15
0.5Mn7(RT) 0.5Mn7(600)
I
t
I
I
6.2
625
Activity The rate constant values K~~~~ obtained at 3Q°C for the catalyzed decomposition of H202 as well as the values of OSC at 3QO'C are reported in table 1. These values are clearly correlated despite the fact that one of them is performed in aqueous solution, whereas the other is carried out in the gas phase Concerning the supported catalysts, 6.8MnC are the most active samples whereas the activity of the 0.6Mn2 samples for both reaction remains very low. The catalytic activities cannot be correlated with the values of SBETl except for the supported 0.6Mn2 serie. As a rule, when the atomic surface ratio Mn/O increases, the activity of the corresponding catalysts increases (compare 4.1MnC and 0.5Mn7 series, Table 2 ) . Accordingly one may conclude that a samples contain more surface manganese species with more surface active oxygen as indicated from OSC, which can initiate the decomposition of H202. For both and XNnC series the rise of the calcination temperature results in a decrease of the catalytic activity, this being more pronounced €or the former. This can be attributed to the loss of surface hydroxyl groups and probably of surface active oxygen upon calcination although the manganese oxide phase remains the same (6.8MnC series). Similar effects were already stated on Mn02 (16). On the contrary, for 0.6Nn2 supported samples, with lower values of the kinetic rate constant, the catalytic activity increases with the calcination temperature This can be correlated with the increase of the surface area and a possible explanation is the migration of manganese leading to a better dispersion. In the case of the unsupported samples the series displays the highest activity in correlation with higher surface areas. The variation in the activity of these catalysts reflects the role of the stoichiometry and crystalline modification of the manganese oxides.
.
CONCLUSION
The most active unsupported catalysts for both model reactions are the permanganate-based samples m ,after calcination at 300 or 60Q'C; these samples display the highest surface area and correspond to the highest oxidation number of manganese. On the prepared with the contrary, the supported catalyst samples m , same precursor, exhibit a drastic drop in the catalytic activity
626
despite an equally high surface area. The TPR measurements showed these supported samples to be difficult to reduce after calcination at 6 0 0 ° C , thus suggesting a strong interaction with the support. For the supported samples the use of the coating technique, with the hydroxide precursor, leads to the most active catalysts and manganese oxide phases were XRD detectable. Moreover, for this precursor, supported 6.8MnC and unsupported samples show comparable activities, in relation with the dispersion effect of the support. In the case of the supported catalysts prepared with manganese (11) nitrate, higher loadings of manganese are recommended, in order to verify the influence of the calcination temperature; this needs to change the impregnation procedure which presently limits the loading level. ACKNOhlLEDGJiXENT
We thank very g r a t e f u l l y P r o f . J.F. Hemidy (Univ. o f Caen) and P r o f . G. Perot (Univ. o f P o i t i e r s ) f o r DRS data, XPS measurements and valuable discussions. A.K.H. Nohman thanks a p p r e c i a t e l y t h e Egyptian Government f o r t h e g r a n t given t o him.
REFWENCES
1 W.B. Innes, i n : P.H. Emmett (Ed.), Catalysts, Vol. 2, Reinhold, New-York, 1955, Ch.1. 2 M.A. Baltanas, A.B. S t i l e s and J.R. Katzer, Appl. Catal., 28 (1986) 13-33. 3 P.W. Selwood, T.E. Moore, M . E l l i s , J. Amer. Chem. SOC., 73 (1949) 693. 4 G.T. P o t t and B.D. McNicol, Discuss. Faraday SOC., 52 (1971) 121-131. 5 M. Lo Jacono, M. S c h i a v e l l o and G. Mercati, Gazz. Chim. I t a l . , 105 (1975) 1165-1176. 6 L. Burlamacchi and P.L. V i l l a , React. K i n e t . Catal. L e t t . , 3 (1975) 199-204. 7 M. Lo Jacono and M. S c h i a v e l l o i n : B. Delmon, P.A. Jacobs and G. Poncelet (Ed.), Preparation o f C a t a l y s t s I, E l s e v i e r , Amsterdam, 1976, pp. 474-487. 8 S. Cavallaro,.N. Bertuccio, P. Antonucci, N. Giordano and G.C. Bart, J. Catal., 73 (1982) 337-348. 9 B.R. Strohmeier and D.M. Hercules, J. Phys. Chem., 88 (1984) 4922. 10 L.L. Bircumshaw and F.M. Taylor, J. Chem. SOC., (1950) 3674. 11 R.D.W. Kemmit, in: J.C. B a i l a r (Ed.), Comprehensive I n o r g a n i c Chemistry, Vol. 3, Pergamon Press, New York, 1973, Ch.37, p.771.
12 0. Duprez and S . Kacimi, personal communication. 13 D. D o l l i m o r e and J. Pearce, Powder Technology, 25 (1980) 71-78. 14 C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder; "Handbook o f X-Ray Photoelectron Spectroscopy", Eds. G.E. Mullenberg, 1979.. 15 M. Lenglet, A. D'Huysser, J. Kasperek, 3. P. Bonnelle and J. Durr, Mat. Res. B u l l . , 20 (1985) 745-757. 16 S.B. Kanungo, J. C a t a l . , 58 (1979) 419-435.
G. Poncelet,P.A.Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 0 1991 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
INFLUENCE OF SURFACE OH GROUPS AND TRACES DURING THE PREPARATION OF Ti02-Si02 SAMPLES
OF
627
WATER VAPOR
A. MUBOZ-PAEZ and G. MUNUERA
Dept. of Inorganic Chemistry and Instituto de Ciencia de Materiales (UNSE-CSIC) P.0.Box 1115, 41071 Sevilla SPAIN SUMMARY
TiOz-SiOz samples have been prepared by impregnation of silica support with n-hexane solutions of titanium alcoholate controlling the hydration/ hydroxylation degree of the silica surface. Once calcined, the samples were characterized by IR, XRD, SEM/EDAX and XAS. The mechanism proposed for the decomposition of the alcoholate involves reaction with adsorbed molecular water in a first step, followed by anchoring by reaction with acid OH- groups. The amorphoustitaniumoxide obtained after calcination shows a layered open structure of clusters formed by a few octahedra sharing edges and corners. INTRODUCTION Titania has been widely used as support in metal catalyst due to its ability to modify the catalytic properties of the metal (ref. 1). As a consequence, the study of the interactions taking place at the metal-titania interface has attracted the interest of several research groups (ref. 2). Nevertheless it is very difficult to obtain high surface area titania (>lo0 m2/g), and while studying the metal-titania interactions it is difficult to get information from the support because the bulk properties of the Ti02 mask those of the surface, the unique part of the titaniaaffectedby the metal. To overcome both problems, inert oxides like silica, have been used as support to obtain high surface area dispersed titania, by grafting to the SiO, support (refs. 3,4)throughthe impregnation from n-hexane solutions of Ti alcoxides, that by hydrolysis and calcination would produce the final coated Ti02-Si02 powders. EXPERIMENTAL Preparation af catalysts The surface oxide was prepared by impregnation of silica Aerosil-200 (SBE,=200 m2/g), with a n-hexane solution of a Ti-alcoholate (tetraisopropyltitanate, Ti(OPr’)4 from Tilcom, 16.9% Ti). The TiOz percentage (by weight) used has been c.a. 12%. This value correspondsroughly to the amount required to form a monolayer of titania on this type of silica (14.7%, 5.5 Ti/nm2) and
628
is close to the amount needed to allow the grafting of each Ti atom to one hydroxyl group of the silica surface (13 %, 5 OH/nm2) (ref. 5). In the standard procedure, the desired amount o f Ti(OPri)4 was dissolved in dried n-hexane (25 ml/g of silica in Methods 1-3 and 6 ml/g of silica in Method 4) and the solution was allowed to react with the surface of the silica for several hours. After that, the solvent was removed at room temperature by flowing Nz, and the sample heated in N 2 up to 673K. Subsequently the solids were calcined i n air at 873K. Four differents methods were used as follows: Method 1. Reaction under Nz at 300K for 20h of undried SiOz. Method 2. Reaction under Nz at 300K for 20h of Si02 dried at 388K for 2h. Method 3 . Reaction under Nz at 350K for 5h of SiOz dried at 440K for 2h, cooling down to room temperature, filtering and washing with n-hexane and subsequently with water. Method 4. Incipient impregnation at 300K in the air of undried silica with a n-hexane solution of Ti Tests were made along the preparation (Methods 1 to 3) by gas chromatography to detect i-PrOH in the liquid phase as well as the presence of Ti(OPr’)4 by hydrolysis in aliquots of the liquid phase. Characterization of solids IR spectra were carried out at 300K on a wafer of the sample mounted in a cell that allows in situ thermal treatments under controlled atmospheres up to 773K, using a Perkin-Elmer 684 spectrometer fitted to a 3600 data station. X-ray diffractograms were recorded in a Phillips 1730 diffractometer and scanning electron micrographs using an IS1 microscope model SS-40, with an energy dispersive X-ray analyzer (EDAX) KEVEX, model 8000 fitted to it. XAS experiments were performed on the EXAFS station 8.1 in the Synchrotron Radiation Source at Daresbury Laboratories with ring energies of 2 GeV and ring currents of 250 mA. The EXAFS spectrum was recorded at 140K in an “in situ” cel1,wherethe sample was placed after being pressed with BN into a wafer with an absorbance ( p x ) of 2.5 at the Titanium K-edge assuring an optimum signal to noise ratio. Data analysis was carried out by fitting in kand R-space using the phase and amplitude corrected Fourier transforms to identify the different contributions (ref. 6). Phase shift functions and backscattering amplitudes were obtained from reference compounds. RESULTS Assuming that the two reactions taking place during the decomposition of the alcoholate to TiOZ are grafting through OH- groups on the surface of the silica and hydrolysis of the Ti-alcoholate by water to produce colloidal
629
particles (refs. 3 , 4 ) , the only competitor to decompose the alcoholate would be the water vapor from moisture. Thus, we have used several preparation methods in which moisture was carefully avoided. Therefore, in the first case (Method l ) , the grafting would involve only OH- groups and/or water molecules adsorbed on the silica surface. Taking into account that water physisorbed on the surface of the silica could produce mainly ungraphted titania, we have carried out a second preparation method (Method 2) in which adsorbed water was avoided by submitting the silica to a previous outgassing treatment at 388K that would remove at least physisorbed water. A new method (Method 3 ) was designed in which all molecular water was removed and, considering that the hydrolysis process could be very slow at room temperature in such extremely dry conditions, the reaction temperature was raised up to the boiling point of the n-hexane. In this case, after five hours of reaction the liquid phase, still containing Ti(OPr’)4, was filtered off and the sample was thoroughly washed with n-hexane, to remove the unreacted alcoholate, and then with water to get a complete hydrolysis of the grafted alcoholate. During the washing with water, formation of a thin, opaque white layer was clearly observed which, unlike the transparent silica, remained stuck on the surface o f the filter. This white coating, presumably TiOz, once dried, calcined and weighed,turn out to be c.a. 50% of the total amount of titanium oxide that should be formed by decomposition of all the alcoholate employed in this preparation. Control test during the preparation in Methods 1 and 2 showed a complete hydrolysis of Ti(OPri)4 at the end of the reaction time (20h), while in Method 3 the liquid still contained the alcoholate in spite of the more drastic thermal conditions used in this case. Finally, a fourth type of preparation was carried out consisting in the well known incipient impregnation method, using the n-hexane solution of Ti (OPr i , and the si 1 ica support without any drying pretreatment. In principle, the degree of success of the anchoring process in our preparations could be followed by checking the changes in the concentration o f one of the reactants (i.e.surface OH-/HzO at the silica support) using I R spectroscopy, since silica aerosil shows a characteristic sharp band at 3750 -1 cm due to basic free hydroxyls together with a broader band due to more acidic OH- groups at 3680 cm-’ (ref. 7). So, their reaction can be followed by changes in their intensities as previously observed in similar preparations (ref. 8). Thus, figure 1 shows IR spectra o f the SiOp support and of a sample prepared by method 1 containing only 1% Ti02 before submitting the samples to any thermal or outgassing treatment. The unique change observed after the addition of the alcoholate is the decrease of the intensity in the range
630 100.
b
a
Fig. 1. I R spectra in the OH stretching a sample region of l%TiOz-SiOz (a) and of si 1 ica support (b) registered in the atmosphere (solid line) and after outgassing at 673K for 2h.
%A
50 I
\
I
\
I
\
I
I 0
.
I
4000
'
1% Ti0 S i O
\,
2
\\,
cm
.% ' -1
2
I
3000
4000
cm
--I
3000
cm-', where the bands due to the more acidic OH- groups and/or molecular water appear, what suggests that these species are those mainly IR involved in the interaction of the alcoholate with the SiOz support. The spectra in the same figure, recorded after outgassing at 673K to remove the water readsorbed upon exposure to air, clearly show the decrease in the intensity of the band at 3680 cm-' assigned to the more acidic OH- groups of the silica, thus suggesting their participation in the decomposition of the alcoholate. Figure 2 shows I R spectra in the range 2900-3400 cm-' of the samples prepared with c.a. 12% Ti02 by the four methods (spectra have been normalized using u s i - o at 1830 cm-' from the bulk of the silica, to make them comparable). Except for the sample prepared by method 3 , in all other cases the intensity o f the band at 3750 cm -1 remains nearly unchanged with respect to that of the silica support pretreated under similar conditions, while changes in intensity and/or position are observed in the band at 3680 cm-' in all the preparation methods. The increase in intensity o f the IR bands in the 0-H stretching region in sample prepared by Method 3 is probably related with the final washing with water used in this method. oxide phases XRD was used to check the crystallinity of the titanium formed after calcination by decomposition of the hydrolyzed alcoholate. Only small shoulders appear in sample 1, 2 and 3 in the position o f the most 3700-3550
63 1
h
8 b
0
m
I Ti0 S i O -4 2
0
-c---c--t-cm
+
-1
Fig. 2 . IR spectra in the range 3900-3400 cm-' of the silica support and the samples 12%TiOz-SiO2 prepared by the four methods outgassed at 673K for 2h. intense peak of anatase, while no peaks were visible at the positions of the most intense diffraction lines of anatase, rutile or brookite in sample 4 . Nevertheless, when the alcoholate was hydrolyzed with water in the absence of silica and calcined under similar conditions, strong peaks appear in the positions of the most intense diffractions of anatase, thus indicating that the hydrolysis o f the pure alcoholate produces crystalline phases. Analysis o f the samples using SEM/EDAX was carried out to examine thehomogeneityof the titanium distribution on the TiOz-Si02 samples, and the homogeneity in grain shape and size. Thus, in sample prepared by Method 2, (using dried Si02) the grains have angular shapes and the local concentration of Ti changes drastically when going from one grain to another. The changes are less drastic, although still remarkable in sample 1, that shows round grains. In sample 3 the particle size was bigger than in the other cases, and the existence of different types of particles (opaque and transparent) could be seen without the aid of the microscope. The most homogeneous sample, considering grain shape and size, as well as titanium dispersion was sample 4 , that has a very homogeneous spongy appearance with constant concentration of T i in all the grains. From the previous results, we deduced that method 4 is the best one, so this sample was studied by XAS to get a deeper insight into the structure around Ti ions. The XANES region of this sample has been plotted in figure 3 , where the corresponding spectra o f anatase and rutile, measured as a
632 C
D 0.1
h
' " 0 Lr
-0.1
J I
-20
I
I
0
1
E(eV) 20
I
40
Fig. 3 . Ti k-edge XANES spectra of TiOz rutile (a), TiOz anatase (b) 12%TiOz-SiOz prepared by method 4. Fig. 4. Ti k-edge, Fourier transforp of the EXAFS syectrum of sample lZ%TiOz-SiOz prepared by method 4. (k , Ak=3.12-11.00 A- ) . Arrows indicate the ranges for Fourier filtering used during the data analysis. reference, have been included as well. In addition to the round shape of features C and D, typical of amorphous compounds (ref. 9), it has to be pointed out the appearance of the triplet A,,A2,A3 characterisitic of octahedral symmetry (ref. 10) that indicates that the absorbing atom is six fold coordinated. Nevertheless, there is a remarkable change in the intensity ratio between peaks AZ and AB, that is close to 1 in anatase or rutile and close to 0.5 in sample 4 where it shows a shape similar to the spectra of uncalcined TiOz colloids prepared from hydrolyzed Ti(OPr')4 (ref. 11). A similar shape has been observed in the spectra of titania-silica glasses prepared by gelation in air of Ti and Si alcoxides by Emili et a1 (ref. 9 ) , who have assigned it to the existence of Ti ions in tetrahedral environment. The Fourier Transform o f the EXAFS signal yields the radial distribution function shown in figure 4, where we can see an intense peak at around 1.7 A due to backscattering from the first shell of oxygen atoms. For higher distances there is a drop in intensity that, in principle, could be assigned to the lack of higher coordination shells. Nevertheless no good fit could be obtained with only one or two shells. So, we have performed the data analysis shell by shell, doing inverse Fourier Transforms of increasing ranges, shown
633
TABLE 1
1
TiOz-Si02-4 Shel 1
N R(A) -
I
Anatase ~
Ao2(AZ)
5.8
1.93
0.011
1.0 6.7
3.09 3.78
-0.005 0.03
7.2
4.37
0.06
3.3
5.34
0.00
Shel 1 Ti-Ol Ti-Ti Ti-02 Ti-Ti2 Ti-03
Ti-Ti3
NxR(A) 4x1.93 2x1.98 4x3.04 8x3.86
number octahedr lS'
4~3.78 8x4.25 8x4.27 4x4.75 ax4. 85
qrd
by the arrows in figure 4. We started the analysis considering the basic octahedra of anatase Ti06 RTi-O= 1.95 A (fit range 0.16- 2.3 A). Afterwards, we expanded the range up to 3 A, and included a shell Ti-Ti. When the fitting range was expanded to 4.1 A , two new Ti-0 bonds were required to reach a good fit. Finally, to reach the final values the range for the Fourier filtering was 0.16-5.4 A requiring the inclusion of a new Ti-0 bond at 5.3 A . The parameters of this fit are summarized in Table 1, that includes the number of neighboring atoms, N, the absorbing atom-neighbor distance, R, the Debye-Waller factor, Ao', related to static and thermal disorder, as well as the structural parameters of crystalline anatase appearing in a cluster of 4 octahedra (ref. 12). A plot of the raw data and the best fit in k and R space for the wider range has been included in Fig 5. The first peak in the Fourier transform may be attributed to the six Ti-Ol bonds of the basic octahedron, as already predicted from the XANES data. The distance is the same that the short bond of the distorted octahedra in anatase. The peaks between 3 and 6 A are a complex result of the overlap of four different features. The first one, Ti-Ti at 3.09 A, is very similar to the distance observed in anatase between two octahedra sharing edges (3.04 A), while the next one, Ti-02 at 3.78 A, is very close to the distance of the oxygen atoms in the second octahedron (3.86 A). The shell Ti-03 would correspond to oxygen atoms in a third octahedron in an anatase-like structure. The shell Ti-04 has no correspondance in a cluster of anatase structure including four octahedra. In relation with the similarities with the anatase strucutre in the other four shell, it has to be pointed out the low coordination number of the Ti-Ti bond at 3.09 A , as well as the lack
634
Fig. 5 . Ti k-edge EXAFS spectrum and Fourier transform (kl, Ak=3.5-10.5 A-') of the raw data (solid line) and best fit (dotted line) of sample 12%TiOz-Sioz prepared by method 4. of Ti-Ti bonds for higher shells. Both facts indicate that only small clusters of TiOs octahedra are present on the SiOz support. DISCUSSION Formation in our conditions of colloidal particles of Ti02 grafted to the high surface area SiOz can be assumed t o occur according to one of the two following schemes: Scheme 1
Ti (OR)
+
(-Si-O)n-Ti(OR)4-n
-
Si-OHb __ > (-Si-O)n-Ti(OR)4-n+
+
(4-n)HzO
n ROH
>(-Si-O)n-Ti(OH)4-n+(4-n)ROH
(1) (2)
Scheme 2
Ti (OR) -
Si-OHa
+ +
4 HZOads
Ti(OH)4
------> Ti (OH)4 + > Si-O-Ti(OH)3
4
ROH
(3)
+ H20
(4)
where -Si-OHa and -Si-OHb stand for basic and acid OH- groups at the SiOz surface, Ti (OR)4 for Ti(OPri)4 monomers and HZOads for physisorbed/chemisorbed water. In the first case, grafting should involve in a first step the more basic OH- groups of the silica through a hydrophylic attack, and in a second step hydrolysis by reaction with adsorbed water or moisture. According to scheme 2, hydrolysis of the alcoholate by adsorbed water at the SiOz support is postulated, leading t o T i hydroxide colloidal particles in a first step, which must be followed by anchoring to the SiOz surface through reaction with more acidic OH- groups, a process that should be enhanced by the final thermal treatment during the calcination used in the preparation of the samples.
635
IR data in figures1 and 2 suggest that Scheme 2 (hydrolysis by adsorbed water followed by grafting) is the most likely in the conditions used in our preparative work, since the band at 3750 cm-’, due to more basic OH- groups, is not modified during the whole process. Moreover, changes in the band at 3680 cm-’ due to more acidic hydroxyls, can be explained by assuming that the grafting involves this type of hydroxyls of the silica surface. It is worth noting that preparation by Method 3 , where adsorbed water and probably part of the acidic OH- groups have been removed from the S i 0 2 support before reaction, only allows ca. 50% reaction of the Ti-(OPr1)4 in spite of the presence of all the basic OH- groups. This fact again excludes these In fact, hydrolysis of the alcoholate groups from the process (reaction (1)). remaining at the SiO surface in this case only occurs by washing with water 2 what probably also produces breaking of siloxane bridges at the Si02 surface, (partially dehydroxylated) as detected by the much larger intensities of the IR bands for this sample in figure 2. If we assume Scheme 2 , grafted colloidal titania particles, similar to those obtained from simple hydrolysis of Ti-(OPr’)4 with water, should be obtained and therefore their structure should not be very different from that recently proposed by Leaustic et a1 (ref.11). In fact, the XANES spectrum of our sample is very similar to the spectrum recorded by these authors for such colloidal particles. However, there are big differences in the EXAFS region that can be explained by the smaller size of the titania particles obtained in our system. Moreover, after heating at 373K, these authors obtain crystalline anatase, as previously did Kozlowski et al.(ref. 12) and Reichmann et a1 (ref. 3 ) during the preparation of similar systems, while the crystalline structure of anatase could not be detected by XRD in our samples, even after calcination at 873K thus implying that the layered open structure, remains stabilized on the surface of the silica. It is not surprising that the best preparation method for this type of ultradispersed Ti02-SiOz systems was the incipient impregnation, since in this conditions the lack o f an excess of solvent will probably prevent the growth of the original nuclei t o bigger colloidal particles. Additionally, this method has the advantage that it is the easiest and provides homogeneous and well dispersed amorphous samples. The analysis of the XAS spectrum of this sample is far from easy. Thus, although the XANES region of titanium oxides (anatase and rutile) has been the object o f several experimental and theoretical studies (refs. 9-14) the definitive explanation of all the features appearing in this region has not been given yet. Nevertheless, by comparing it with the spectra of previously studied compounds, we can use this region of the spectrum as a finger print.
636
Thus, from the comparison with the Ti k-edge XANES spectra of several alcoholates previously measured (ref. 1 4 ) , we can discard the presence of tetrahedral or square planar geometry around the Ti centers, as well as the long range order typical of crystalline structures, like anatase, rutile or brookite (refs. 12,13), confirming in this way the conclusions reached by XRD. The EXAFS results point to the existence of a phase similar to anatase but, since the distances Ti-Ti2 and Ti-Ti3 are missing, the coordination numbers for Ti-Til, Ti-02 and Ti-03 are very small, and there is a new distance Ti-04 above 5A, it seems that the new structure is more open and has grown in two dimensions. The parameters obtained are compatible with a structure similar to the Ti02-B, proposed by Brohan et al. (ref.15) and more recently by Reichmann and Bell (ref.16) as a precursor o f anatase in the decomposition of TiC14. In conclusion, incipient impregnation of SiOz with a n-hexane solution of Ti(OPr')4 leads to TiOz coated material with an extremely high dispersion where very small clusters of Ti06 octahedra (probably 3-4 octahedra sharing edges and corners) are formed. The process involves hydrolysis by physisorbed/chemisorbed water followed by anchoring during calcination. ACKNOWLEDGEMENTS. The authors wish to thank Prof. D.C.Koningsberger for the use o f his EXAFS analysis programs, CICYT and Junta de Andalucia for financial support, and the staff in the SRS (Daresbury lab., SERC) for help during the XAS measurements. REFERENCES i
G.C Bond and R.Burch, Catalysis (Specialist Periodical Report).Chem.Soc.,
6 (1983) 27-60.
2 K.Foger. Catalysis,Science and Technol. 6 (1984) 227-305. 3 M.G.Reichmann and A.T.Bel1, Appl.Catal., 32 (1987) 315-326. 4 C.Morrison and J.Kiwi, J.Chem.Soc.,Faraday Trans.1, 85(5) (1989) 1043-1048. 5 J.B.Peri and A.Hensley, J.Phys.Chem., 72 (1968) 2926 6 J.B.A.D van Zon, D.C.Koningsberger, H.F.J. van't Blik, and D.E.Sayers, J.Chem.Phys., 82 (1985) 5742-5754. 7 J.B.Peri, Catalysis,Science and Technol., 5 (1984) 171-220. 8 E.T.C. Vogt, M.de Boer, A.J. van Dillen, and J.W.Geus, Appl. Catal., 40 (1988) 255-275.
M.G.Reichmann and A.T.Bel1, Langmuir, 3 (1987) 111-116.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V 01991Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
637
CATALYSTS AND PREPARATION OF NEW TlTANATES R. G. ANTHONY' and R.G. DOSCIf 'Department of Chemical Engineering,Texas A&M University, College Station, TX 77843-3122 (USA) 'Sandia National Laboratories, Div. 6211, Albuquerque, NM 87185 (U.S.A.)
SUMMARY A series of new crystalline titanates ((3) are shown to have considerable potential as catalysts supports. For Pd supported catalysts, the catalytic activity for pyrene hydrogenation was substaniatially different depending on the type of CT,and one was substantially more active than Pd on hydrous titanium oxide (HTO). For 1-hexenehydrogenation the activities of the new CTs were approximately the same as for the hydrous metal oxide supports. Stereochemical effects, such as shape selective catalysis, appears to be Occurring when pyrene is hydrogenated. INTRODUCTION Hydrous titanium oxides (HTO) have been shown to be excellent supports for Co, Mo, Ni, Pd, or vanadia for hydrogenation and oxidation reactions (ref. 1, 2,3,4,5). Specifically, Dosch et al. (ref. 1, 2, 3) have used ion exchanged techniques to prepare Co-HTO, Mo/Ni-HTO, and Pd-HTO to produce Fischer-Tropsch catalysts and coal liquefaction catalysts. Gruber (ref. 5) has used HTO as a support for vanadia in the selective catalytic reduction of NO with ammonia in the presence of oxygen. A unique feature has been the high activities and surface areas which can be obtained by using the HTO based catalysts. To increase the activity of metals or metal oxides supported on hydrous titanium oxide and to increase our ability to tailordesign catalysts, the synthesis of new crystalline titanates was initiated. The basis for synthesis of the new crystalline titanates was to modify the procedures developed at Sandia for preparing the hydrous metal oxides, and to utilize techniques for synthesis of zeolites and pillaring of layered materials (ref. 6-13). After several attempts in which anatase titania was synthesized, new crystalline titanates with d-spacings of 1.0, 1.17, and 1.6 nm were synthesized.
This paper reports on the
chemicals used in the synthesis, propeaies of the titanates and catalytic activities of the titanates when used as supports for Pd for the hydrogenation of pyrene and of 1-hexene.
638
EXPERTMENTAL Praaration The chemicals used in the preparation of the titanates were tetraisopropyl titanate, an aqueous solution of NaOH,
a solution of tetramethylammonium hydroxide in methanol,
Al(NO3),*9&O, tetrapropylammonium chloride, and tetrapentyl ammonium chloride. These chemicals were mixed in an appropriate manner to produce in most cases a white precipitate.
In some cases, depending on the solution composition, a crystalline titanate formed at room temperature. The solution (slurry) mixture was divided and charged to 3/4 inch Swagelok tees, which were placed in an oven with the temperature set below 180 T. Each tee was removed at a
specified time and rapidly cooled to room temperature. The contents were fiitered in a Buchner funnel, washed with acetone, and air dried. Samples were ion exchanged with HCl and then Pd2+to prepare catalysts for evaluation of the activity for hydrogenation of pyrene and 1-hexene. The new titanates were also pillared by
ion exchange with an aluminum solution prepared from "microdry." The pH of the solution corresponded to the expected formation of the Al,," Keggin ion. The catalysts were characterize by using XRD,Raman, FT-IR,BET for surface areas and pore
size distributions, TGA, DSC, and AA.
RESULTS AND DISCUSSION Five new types of crystalline titanates were prepared (XRD patterns are shown in Figure 1.). A room temperature titanate (Type 1) was unstable except in the
mother liquor or in
isopropanol, but it had a f i i t reflection at a d-spacing of approximately 1.0 nm. The XRD pattern suggests a poorly crystalline material. However, these crystals were easily seen in an optical microscope, and they had a needle type of morphology. The next two types (Types 2 & 3) had d-spacings of approximately 1.0 nm for the f i t reflection, however the remaining
portions of the XRD pattems had slight differences. Type 2 contains aluminum, whereas, Type
3 contains only titanium, sodium, organic cations, and oxygen. Even though hydrothermal synthesis was used in the preparation of both types of titanates, Type 3 was prepared from the amorphous hydrous titanium oxide. The next group of titanates were classified as Types 4a and 4b because of the differences in surface areas, reactivity, and synthesis conditions. The d-spacing
of the first reflection of this group was 1.17 nm and the rest of the patterns were essentially identical. The Type 5 of titanate had a d-spacing of 1.6 nm,but the material showed a mixture of anatase and the new layered titanate.
639
Figure 1. Comparison of X-Ray Diffraction Patterns of New Crystalline Titanates with Each Other and With Anatase riania
640
Surface Areas and Pore Size Distributions Surface areas and pore size distributions of selected samples (Figure 2) were determined by
BET using a Micromeritics Digisorb 2600. The samples were degassed at 150 "C prior to the nitrogen sorption experiments. Typical sorption curyes for layered materials were obtained. Type 2 titanate had a pore volume of 0.79 cc/g, pore sizes up to 50 nm, and were bimodal with peaks at ca. 5nm and 10 nm. Examination of the cumulative pore volume plots (not shown) and comparing the result with the total pore volume suggested possibility of pores less
than 1.5 nm. The surface area of Type 2 titames was almost twice the surface areas of the Type 2,3, and 4 titanates. The Type 4 titanates (d-spacing=1.17 nm) had surface areas of 94 to 133 mz/g and with bimodal distributions (Figure 2). The pore sizes are in the range of 1.8 to 5.0 nm with peaks at 2 and 4 nm. Pore volumes of these samples are low being 0.069 and 0.056 cc/g. We interpret these low pore volumes to indicate that the space between the layers are fiied. Elemental analysis and a carbon balance indicated a mixture of tetramethylammonium and tetrapropylammonium
cations occupy the space between the layers with 25% tetrapropyl
ammonium and 75% tetramethyl ammonium ions. Ion Exchange and Catalytic Activity The Type 1 titanates were equilibrated in solutions containing a two-fold or more excess of H', N i o , or V(V) ions based on the ion exchange capacity. These materials had surface areas of 377, 373, and 232 m2/g, respectively, and pore volumes of 0.52, 0.42, and 0.43 g/cc, respectively, after outgassing at 300 "C. Under similar conditions sodium hydrous titanium oxide has a surface area of 41 mz/g and pore volume of 0.14 cc/g. In addition, the pore size distributions for these exchanged samples were bimodal and trirnodal. Whereas, bdrous sodium titanium oxide (HTO) is unimodal.
The Type 1 crystals were used to prepare a Ni-Mo catalyst
by ion exchanging with ammonium heptamolybdate at a pH of 3, rinsed, fidtered, and dried, reslurried in deionized water and ion exchanged with Ni(NO,), at a pH of 6, rinsed with acetone, dried, and then acidified with HC1 to a pH of 3 for removal of the sodium ion. The catalyst was amorphous after calcining at 300 "C, but the catalytic activity as measured by the hydrogenation of pyrene was sisnifcantly greater than catalysts prepared from the amorphous HTO. Types 1, 2, 3, and 4 were ion exchanged with a PdCL, solution to obtain a Pd loading of approximately 1%. As prepared (Ap),the final pH of the solution was greater than 10. After fiitering and drying, the catalysts were acidified (AC) with sulfuric acid to a pH of 3.5 to 4. Pd was also loaded onto anatase, amorphous titanium oxides, N%3T, Na,,,,T, and an amorphous titanium-silicon oxide, Ng,T-Si, and catalytic activity was tested for comparison with the
641
DIFFERENTIAL PORE VOLUME PLOT (DESORPTION)
PORE DIAMETER, A DIFFERENTIAL PORE VOLUME PLOT (DESORPTION)
PORE DIAMETER, A
PORE DIAMETER, A
Figure 2. Pore Size Distributions for Types 2, 3, 3-Al-Pillared (After Calcining), and Type 4 Crystalline Titanates
642
activity obtained with the new CTs. The activities for pyrene hydrogenation is measured by
zero-order rate constants k,with the units of mg pyrene hydrogenated/(sec-g Pd). For 1-hexene hydrogenation, a first order rate constant was used. 'Ihe results of these test are reported in Table 1.
The CT Catalysts, except for IJT TP4a&b AC, had activities greater than the Pd-HTO catalysts, i.e. Nao,T AC and N%,T AC. Type 2 acidified (Cr TP2 AC) had an activity more than twice that of the Nao,T AC and almost twice the activity of the N%,,,TAC. The reason for the significant increase in activity of the IJT TP2 AC (Type 2) is unknown, but it could be due
to the ordering introduced by crystallization of the support. It might also be due to the fact that an aluminum cation was used in the synthesis of the titanate. Table 1 Evaluation of catalvtic activity: Test reactions-Hydrogenation of pyrene and 1- hexene: Reaction conditions-100"C. charge pressure- 100 psig @ 22°C. Sample
Pd in cr. Wt.%
k' (pyrene)
d (1-hexene)
CTTP1 AC? CTTP2AC CTTP3 AC CTTP4aAC CTTP4b AC CTTP4bAp" Anatase AC N%,T AC N%.3T AC N%,TSi(Pd#3)
0.74 0.78 1.36 0.87 0.56 0.55 0.83 0.63 0.53
380 610 348 82 24 15 170 199 329 577
--
ID.
0.55
1Nn s)
59
---
34 46
_-
56 --
--
1) Units are mg of pyrene hydrogenated&econd gram of Pd). 2) AC refers to acidified after preparation. Non acidified catalysts were significantlyless active than the acidifkd catalysts. fl refers to crystalline titanate. Tp1 is Type 1 titanate. 3) First order rate constant for hexene hydrogenation, l/(second gram of Pd). 4) AP refers to as prepared prior to acidification.
Of particular interest is that the Pd#3, an amorphous titanium silicon oxide, had an activity
within 10%of the C T "2
AC. If use of procedures similar to those used to make the new
crystalline titanates resulted in the synthesis of a crystalline Na,,sTSi material, the potential exists for preparing catalysts with activities significantly greater than the HTOSi supported catalysts. Thermopravimetric and Differential Scanning Calorimetry Studies TGA and DSC experiments were conducted on the Type 4 titanates. Heating rates for the
643
TGA studies were 5 T/min and for the the DSC experiments the heating rates were 10 "/min. The first weight loss, approximately 5 to 896, occurred below 100 "C, and is probably due to loss of water. Very little weight loss occurs up to 200 "C and then a fairly rapid loss of weight occurred up to a temperature of 400 "C. The TGA in air and nitrogen differ slightly but show the same general trend. Total weight loss is approximately 18 to 20% in nitrogen and air, respectively.
The DSC's were conducted at 10 "C/min in the presence of nitrogen and air. A strong exotherm occurred in air over the temperahue range of 240 to 360 with the peak at 320 "C, which was probably due to the combustion of the organic template.
A second exotherm
occurred with a peak at 450 C, which was probably due to a phase transformation. In nitrogen no reactions appear to have occurred except for a peak at 400 "C which is probably due to a phase transformation. Surprisingly, no endotherm occurred due to the pyrolysis of the organic template.
Infrared and Raman Smctra Infrared and Raman Spectra were obtained on selected samples of these new titanates. However, additional work is required to interpret these spectra. Bridged and non bridged oxygens are evident in the Raman spectra (ref. 2, 3).
The IR spectra are diffuse reflectance
spectra and illustrate the incorporation of the tetraalkylammonia cation. Somewhat surprising was that some spectra did not have bands in the 3900-4500 crri' which would be typical of the quaternary ammonium. Also, the spectra substantiate the differences in the five types of new titanates. Figure 3 illustrates typical spectra obtained in these studies.
PilliXhlg Types 2,3,4and 5 titanates were ion exchanged with a solution of alumjnum ions produced by dissolution of "microdry" aluminum hydroxy chloride. The solution should have contained cations of (AlI3O,(OH),
* 12 &O)'*, the aluminum Keggin ion.
The exchange was conducted
for 2 h and the pH controlled to 4.9. The amount of aluminum exchanged into the crystalline titanates is given in Table 2. Table 2 Extent of ion exchange Type Wt.6 Al
2 2.1
3 16.8
4 5 4.8 17.4
XRD patterns were obtained on the resulting samples before and after calcining at 300 "C for 1 hour. The XRD patterns (Figure 4)for Types 2 and 3, show that the layered structure is
I RAMAN SPECTRA OF NEW CRYSTALLINETITANATES
Figure
3.
Raman and I R Spectra of New C r y s t a l l i n e Titmates
645
P-Type 2-Al-Pillared Before m 0 Ca lc in ing
*N
-
Type 2-Al-Pillared A f t e r
Two-Theta (degrees)
Figure
4. X-Ray Diffraction Patterns for Aluminum Pillared Crystalline Titanate Before and After Calcining at 300°C for 1 h o u r .
646
retained after exchange and heating. The d-spacings after heating were 0.99 and 0.94. Hence, a slight shrinkage in the d-spacing occurred, and the loss in crystallinity may be due to pillars being in an amorphous state. Anatase was formed from Types 4 and 5 titanates, after calcining. Prior to the incorporation of the aluminum Keggin ion, the
crystalline titanates became
amorphous between 200 and 300 "C, and on further heating anatase was formed. Figure 2 shows the pore size distributions for Type 3 titanate as prepared and after ion exchange with the aluminum Keggin ion and then calcined for 1 hour at 300 "C.
The sorption curves (not
shown) for Type 3-Al-pillared titanate showed the characteristics of layered and possibly pillared materials. There is, however, a loss of surface area and pore volume, apparently due to heating at 300 T. CONCLUSIONS The results presented above clearly indicate the potential of these new titanates as catalysts
supports, and the need for further work on the synthesis
and physical and catalytic
characterization. Acknowledgment The majority of the work reported above was conducted at the Sandia National Laboratories while Professor Anthony was on "an academic study leave from Texas A&M University" i.e. a sabbatical. REFERENCES 1 R. G. Dosch, H. P. Stephens, and F. V. Stohl,U.S.Patent No. 4,511,455 (April 16, 1985). 2 R. G. Dosch, H. P. Stephens, F. V. Stohl, B. C. Bunker, and C. H. F. Peden, Hydrous Metal OxideSupporkd Catalysts: Part I. A Review of Preparation Chemistry and Physical and Chemical Properties, SAND89-2399, Sandia National Laboratories, 1990. 3 R. G. Dosch, H. P. Stephens, and F. V. Stohl, Hydrous Metal Oxide-Supported Catalysts: Part II.A Review of Catalytic Properties and Applications, SAND89-2400, Sandia National Laboratories, 1990. 4 H. P. Stephens, R. G. Dosch and F. V. Stohl, Ind. & Engr. Chem. Prod. Res.& Dev., 24 (1985) 15-19. 5 Gruber, K. A., "The Selective Catalytic Reduction of Nitric Oxide With Ammonia in the Presence of Oxygen", M. S. Thesis, Chem. Eng. Dept., Texas A&M University, College Station, TX, (August 1989) Thesis Advisor: R. G. Anthony. 6 A. Clearfiild, Chem. Rev., 88 (1988) 125-148. 7 A. Clearfiild and A. Lehto, J. of Solid State Chemistry, 73 (1988) 98-106. 8 J. Lehto, Sodium Titanate for Solidification of Radioactive Wastes- Prepaiation, Structure and Ion Exchange Properties, Academic Dissertation, Report Series in Radio chemistry, (5/1987), University of Helsinki, Finland. 9 J. Lehto and A. Clearfield, A., J. Radioanal. Nucl. Chem., Letter, 118 No.1 (1987) 1-13. 10 J.M. Adams, Awl. Clay Sci., 2 (1987) 309-342. 11 F. Figueras, Catal. Rev.Sci. Eng., 30(3) (1988) 457499 . 12 T. J. Pinnavaia, Science, 220-No. 1595 (1983) 365-371. 13 D. E. W. Vaughan, Catalysis Today, 2 (1988) 187-198.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
647
NEW METHODS OF SYNTHESIS OF HIGHLY DISPERSED SILVER CATALYSTS N.E. Bogdanchikova and V.V. Tretyakov Institute of Catalysis, 630090 Novosibirsk (USSR)
ABSTRACT
Original methods of synthesis of highly dispersed silver catalysts (based upon the application of strong reducing properties of electrons solvated in soldium-ammonia solutions, the adsorptioncontact method of drying and a weak solubility in nitric acid of the Si02-supported small silver clusters) allowed us to synthesize Si02-supported silver particles of sizes less than 6 nm. It makes possible some unusual catalytic and other physico-chemical properties of these particles to be discovered. INTRODUCTION It is well known that metal particles less than 6 nm in size may essentially differ from the bulk metal in adsorption, catalytic and other physico-chemical properties; structural sensitivity of catalytic reactions are exhibited in this range of particle sizes. For the investigation of size effects in catalysis, the most suitable support for metal particles is Si@, since its interaction with them is negligible as compared with other supports. It is widely believed that regardless of the preparation method, the most probable minimum size of Ag particles supported on Si02 is 6 nm, which is due to a weak interaction of Ag with the support [l]. In fact, one cannot find in literature any description of Ag samples supported on pure Si02 with reliably established size of Ag particles less than 6 nm. In this work we have succeeded in synthesizing such supported silver samples using the following new methods.
METHODS Ag dispersion was determined by various methods : size of coherent scattering region (average size d,) was obtained by X-ray method; surface average size ds - by adsorption method; size distribution of Af particles was obtained by the method of small angle X-ray scattering (SAXS) and TEM. The two last methods were applied to determine the most probable size of Ag particles d,. The specific surface S,, of silver blacks was determined by the BET method through the adsorption of N2. The specific surface of supported Ag catalysts was calculated from the data on chemisorption of 0 2 . Size distribution of Ag particles was obtained by transmission electron microscope JEM-100 CX and the SAXS method (KRM-1 apparatus). An average value of the regions of coherent X-ray scattering was determined through the widening of X-ray lines registered with a DRON type apparatus. Spectra of diffused reflection were recorded by the Shimadzu UV-300 spectrometer.
648
RESULTS AND DISCUSSION Pure Si02 (the moulded aerosil of the "A-175'' brand, specific surface volume 1 cm3/g, dominant radius of pores > 4 nm) was chosen as support.
-
- 235 m2/g, pore
Method 1 For the preparation of the silver samples, hydrogen, borohydride, formaldehyde, hydrazine are usually used as reducers. One of the conditions preventing silver crystallites from growing, is a high rate of reduction of Ag+ cations. As a rule, the higher the rate of reduction, the higher the value of the redox potential of the reducer. Therefore it was of interest for the preparation of highly dispersed Ag particles to use one of the strongest reducers - solutions of alkaline and alkaline earth metals in liquid ammonia, where the electron solvation was performed by ammonia [2] Na + (m + n) NH3 -+ Na+ (NH3)m + e (NH3),,. To prepare the silver catalysts, we used the strong reducing properties of the electrons solvated in ammonia [3] Ag+ (NH3)m + e (NH3)n -+ Ago (NH3)m+n. We flowed up the AgN03 solution in liquid ammonia to that of metal sodium (at 239.5 K). Darkening of the solution was therewith observed as a result of formation of the highly dispersed metallic silver. The obained compound was divided into two parts, one of which was thoroughly mixed with Si02. Both portions were kept in the air up to the complete evaporation of ammonia. The next step was washing of the obtained samples from the sodium cations with distilled water : the sample supported on SiOz was washed on buchner funnel, and the non-supported silver was washed by centrifugation. Resulting from centrifugation, silver blacks settled at the bottom, while - 4% of the whole silver was still in solution in the form of colloidal particles (Table 1, sample 1). Silver black (sample 2) and supported silver (sample 4) were dried in the air at room temperature. Silver black was also prepared by application of some other succession of the solutions being flowed to : the sodium solution in liquid ammonia was flowed to the solution of AgN03 in liquid ammonia. Flakes of silver black, settling at the bottom of a glass, were therewith formed. Formation of the colloidal particles was not observed in this case. Washing out the sodium ions was performed by decantation. Afterwards, the sample was dried in air at room temperature (sample 3). The second supported catalyst was prepared in conditions favouring the formation of ultrafine Ag particles. For this, the aerosil was suspended in the AgN03 solution in liquid ammonia. This mixture was flowed to the sodium solution in liquid ammonia. Inasmuch as in the water solutions there is adsorption of the ammonia complex of silver nitrate on Si@ [4], it might have been expected that in the liquid ammonia there would also be an adsorption. This is the circumstance hindering growth of particles on the support surface. Sample 5 obtained by such a way, was washed by decantation and dried in air at room temperature. Next, two cycles of treatment for the sample 5 were performed with 02 and H2 at 473 K, pressure of gases < 800 Pa, the duration of the treatment being - 5 days (sample 6). While preparing the samples, the initial concentration of the AgN03 and
649
TABLE 1 : Characteristics of catalysts* Ag (wt.%l
Sample
1 Colloid s i l v e r solutinn 2 Sill-er black 3 Silver black 4 A:/SiO? 5 Ag/SiO; 6 As/Si@; 7 Bg/SiO, 8 .Ag/SiOi; 9 Ag/SiO;
-
99.8 99.8 13.0
i.4 1.4 2.4
2.0 2.1
.Average s i z e o f A g c r y s t a l l i t e s ( n m ) absorb. S-rav TEY d a t a data data dc d,. s i z e ranse
75.2 248.4 44.0
-
3.7 3.5 4.0
-
-
2-50
50
-
15 am.
2-20
20
14 am.
am. 9.0
-
0.5-6
1-60
1-6 1-6
1-10
s.41s**
d_ 12
data dn
-
-
3.3
3 1 7 3.0
0.8
-
2.0 5.0
-
1.0
-
-
-
*
am. amorphous; d , d , d - s u r f a c e ai’eraqe s i z e , volume o n e a n d t h e m o s t p r o b g b l e v s i z e P o f Aq p a r t i c l e s c o r r e s p o n d i n g l r . * * P a r t i c l e s i z e s w e r e a l e r a g e d i n t h e r a n g e 0 . 5 - 3 0 nm.
Fig. 1 .
TEM photographs of silver samples : a - 1 (Ag colloid), b - 8 (AdSi02).
sodium in liquid ammonia were in the range from 0.01 to 0.90 M. The characteristics of the catalysts calculated from the data of the different physico-chemical methods are given in Table 1. X-ray data conform to the adsorption data for sample 2 (particle sizes
650
are 50 and 75 nm, respectively) and represent for sample 3, the average size one order less, which is obviously stipulated by the fact that silver particles (- 250 nm in size) of sample 3 are agglomerates of microcrystals of a lesser size (- 20 nm). Grinding this sample in a mortar, unlike sample 2, led to the increase of S,, by the factor of 1.5, while the size obtained by the X-ray method was the same. These microcrystals in the agglomerates are possibly less stably connected and for this reason, the agglomerates may be destroyed upon grinding. The microphotograph of silver colloid made approximately a month after it was prepared is given in Fig. la. The size distributions of the colloid silver particles (sample 1) and of the freshly prepared supported silver particles (sample 4)are different. First, smaller particles are formed (dp = 3 nm), which are "conserved" on a support; the colloid particles which are enough stable in time, are coarser (dp = 12 nm). The projection of colloid particles on a surface is chiefly hexagonal. The most possible size of the colloid silver particles has not practically changed after two years of ageing. The role of a stabilizer in this colloidal solution may be possibly performed by Na+. The TEM patterns of the freshly prepared specimen point to particles I 1 nm in size to be present, which is c o n f i i e d by the SAXS method. The second examination of the sample kept in air did not allow to determine these particles on a support, which may be connected with a decrease of contrast of representation due to oxidation of silver particles in air. This is proved by the data of diffuse reflectance electron spectroscopy. The data of Table 1 indicate that the synthesis of Ag samples from silver cation reduction with electrons solvated in liquid ammonia allows to obtain stable colloidal solution of highly dispersed silver particles of 2-50 nm in size (dp = 3 nm), silver blacks with specific surface of 2.3 and 7.6 m2/g, and supported silver samples with a great contribution of particles < 6 nm in size. Method 2
In this case, we used the traditional method of impregnation, carried out in conditions leading to the formation of highly dispersed Ag particles on the support surface : (1) samples were prepared with a low content of Ag (-2 wt.%); (2) Ag was supported by adsorption on Si02 surface of the ammonia complex of the diluted silver nitrate solutions. In this case, the formation of the supported particles at the later stages of the sample preparation was mainly performed from the adsorbed silver complex. Conhibution of this complex being in volume of support pores was practically excluded. ( 3 ) samples with supported silver complex were dried by the method of sublimation or by the adsorption-contact method which preserved the uniformity of adsorbed silver complex distribution on the support surface. This contributed to the obtention of a more homogeneous distribution of metal particles after subsequent reduction. The application of the adsorption-contact drying method for the preparation of the supported metal catalysts has not been found in literature. For the drying by sublimation, a wet sample was introduced in an ampoule and immediately frozen in liquid nitrogen. Evacuation under vaccum was carried out by keeping the sample temperature lower than 268 K (sample 7). Sample 8 was dried by the adsorption-contact method developed in the Institute of Catalysis (Novosibirsk, USSR). This method is based on the contact of dehydrated desiccant with the grains of the catalyst impregnated with the solution of the active
651
component. For the security of transfer of the solvent (water) through the gas phase, a definite amount of desiccant was taken. It was calculated supposing a monolayer coverage of adsorbed water on its surface. y-Al2O3 (Ssp 255 m2/g, dominant pore radius < 4 nm) was used as a
-
desiccant. For the calculation it was assumed that 8 Fmol/m2 or more of water was needed to form a monolayer. The mixture of the desiccant and the wet sample was thoroughly shaken in a flask for a few minutes. The sample dried in this way was separated from the wet desiccant by means of a sieve. As we used the dilute solution of the ammonia complex, only a small part of it (I3 wt. % of the complex kept by the support in an adsorbed state) was transferred to the desiccant. Sample 9 was dried in a cabinet drier for 6 h at 385 K. Drying the samples in a cabinet drier, in contrast with the adsorption-contact method and that of sublimation, is probably favourable to the formation of larger aggregates from the complex salt molecules adsorbed on support surface. It is accompanied by the partial decomposition of the salt into silver oxide or even (according to X-ray data), into metallic Ag. This is indicated by electron spectra of diffused reflection of the samples dried by these methods. The dried samples were reduced by H2 in a flowing-circulating installation for 6 h at 473 K with freezing of water in getters cooled by liquid nitrogen. From the data given in Table 1 and Fig. 1b, the application of the adsorption-contact drying and the method of solvent sublimation allow to obtain the sample where the particle size of Ag is dominantly 2-3 nm. A narrower particle size distribution was therefrom reached, as compared with the analogous samples of silver catalysts prepared according to the usual method of drying (sample 9). TEM data indicate that increasing the Ag content up to 10 wt. % leads to the increase of the part of coarser metal particles (more than 10 nm in size), but the change of the position of the maximum of the silver particle distribution is negligible. The use of the adsorption-contact drying and the sublimation method leads to the obtention of Ag particles on a support with a practically equal particle size distribution. However, the adsorptioncontact drying has some advantages, as it is less time-consuming (usually a few minutes) than drying by sublimation (pumping out lasts for about 20 h) and does not require special equipment (vacuum system). Method 3 This method is based on the isolation of "pure" small Ag clusters which form a rather strong bond with Si02. It was found that these clusters were obtained by heating in a water bath the Si02supported samples with the most probable Ag particle size of 3 nm (samples 7 and 9 obtained by Method 2). The heating was done in a 13% HNO3 solution during 12 h. Further, the samples were washed from HNO3 with distilled water and dried in air under an IR lamp for about 30 minutes. The analogous acid treatment of Si02-supported samples containing large Ag crystallites, leads to complete removal of Ag from the support surface. Inasmuch as Ag content in the samples treated with acid was low (less than 0.1 wt.%), the application of the usual methods for studying the structure of supported metals (X-ray, TEM, etc.. .)
652
was impeded. Therefore, for the investigation of the properties of these clusters, the method of diffuse reflectance electron spectroscopy was used and found to be highly sensitive. Silver supported samples 7 and 9, not treated with HNO3 solutions, were dark brown, which was observed as structureless absorption in the whole region of the spectrum. The influence of HNO3 solution resulted in a decrease of the intensity of the sample colouring, and two absorption bands (a.b.) could be observed in the spectrum : at 320 and 400-440 nm. As the time of treatment was increased, the relative intensity of 320 nm a.b. was also increased. Increasing the treatment time up to 13 h led to the disappearance of the a.b. at 400 nm, and only the a.b. at 320 nm was observed. The absorption band at 400 nm is attributed to the surface plasma resonance of conduction electrons in the small metal Ag particles. The change of such a resonance with decreasing size of Ag particles in a photo-sensitive glass obtained in [5] was analogous to that observed in our work : increasing the treatment time of the sample with acid led to an increase of the relative intensity of the 320 nm a.b. and a decrease of the 400 nm a.b.. For Ag particles of 2.3 nm, only the 400 nm a.b. was recorded [5],but for 1 nm particles, two a.b. at 320 and 400 nm were observed. This allows to suppose that in our case the a.b. of 320 nm refers to small Ag clusters, the size of which is not more than 1 nm. Recent investigations of highly dispersed silver supported on aluminosilicate catalysts made it possible to assign the 320 nm a.b. observed to Ag clusters 1 nm in size.
-
It was found that the samples obtained by Method 1 containing Ag particles of about 3 nm in size, exhibited noticeable activity towards ethylene chemisorption and homoexchange of ethylene, different from pure bulk Ag catalysts. At the same time they were less active than large Ag crystals towards processes occurring with participation of oxygen (02 adsorption, interaction of adsorbed oxygen with H2, C02 adsorption on oxidized Ag surface, complete and selective catalytic oxidation of ethylene) [6]. X-ray photoelectron and Auger electron spectroscopy data suggested that the observed size effect was due to changes in electronic properties of metal silver [7]. The Ag samples obtained by Method 1 and Method 3 containing Ag particles of 1 nm in size, possessed unusual optical properties and abnormal features with respect to redox reactions [8]. Thus, using the original methods, Si@-supported silver particles of less than 6 nm in size were synthesized. This made it possible to discover unusual catalytic and some other physicochemical properties of these particles. REFERENCES 1 2 3 4
K.P. Jong and J.W. Geus, Appl. Catal., 4 (1) (1982) 41-51. G. Thompson, Electrons in Liquid Ammonia, Mir, Moscow, 1979. G.W. Watt, J. Chem. Education, 34 (11) (1957) 538-541. M. Jarjoui, B. Maraweck, P.C. Gravelle and S.J. Teichner, J. Chim. Phys., 75 (11-12) (1978)
5 6
L. Genzel, T.P. Martin and U. Kreibig, Z. Physik 8 , 21 (4)(1975) 339-346. N.E. Bogdanchikova, D.A. Bulushev, Yu.D. Pankratyev, E.A. Paukshtis and A.V. Khasin, Kinet. Katal., 31 (1) (1990) 151-157. N.E. Bogdanchikova, A.I. Boronin, V.I. Buktiarov, V.I. Zaikovskii, S.V. Bogdanov and A.V. Khasin, Kinet. Katal., 31 (1) (1990) 145-150. N.E. Bogdanchikova, M.N. Dulin, A.A. Davydov and V.F. Anufrienko, React. Kinet. Catal. Lett., 41 (1) (1990) 73-78.
7 8
1060-1068.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
PREPARATION OF HIGH-SURFACE-AREA
V-Si-P
653
O X I D E CATALYSTS
M. A i Research l a b o r a t o r y o f Resources U t i l i z a t i o n , Tokyo I n s t i t u t e o f Technology, 4259 Nagatsuta. Midori-ku, Yokohama 227 (Japan)
ABSTRACT The e f f e c t s o f t h e c o m p o s i t i o n and t h e methods o f p r e p a r i n g V-Si-P t e r n a r y o x i d e s on t h e i r c a t a l y t i c performance i n t h e vapor-phase a l d o l condensation o f p r o p i o n i c a c i d w i t h formaldehyde t o f o r m m e t h a c r y l i c a c i d were s t u d i e d . The presence o f b o t h vanadyl pyrophosphate and l a r g e s u r f a c e area was found t o be r e q u i r e d t o achieve a good c a t a l y t i c performance. Phosphorus serves t o form and s t a b i l i z e vanadyl pyrophosphate which i s b e l i e v e d t o be a c t i v e s i t e s and s i l i c o n serves t o produce a l a r g e s u r f a c e area and t o m o d i f y t h e vanadyl pyophosphate. The presence o f l a c t i c a c i d i s i n d i s p e n s a b l e t o produce a l a r g e s u r f a c e area when t h e S i / V atomic r a t i o i s i n t h e range o f 1 t o 4.
INTRODUCTION V-P b i n a r y o x i d e c o n s i s t i n g o f vanadyl pyrophosphate, (VO)2P207,
i s a unique
c a t a l y s t possessing an e x c e l l e n t s e l e c t i v i t y i n o x i d a t i o n o f butene and n-but a n e t o m a l e i c anhydride.
F u r t h e r , t h i s o x i d e i s known t o be e f f e c t i v e a l s o
as a c a t a l y s t f o r a vapor-phase
a l d o l condensation o f a c e t i c a c i d and p r o p i o n i c
-
a c i d w i t h formaldehyde (HCHO) t o form a c r y l i c a c i d and m e t h a c r y l i c acid, t i v e l y [l-31.
CH3COOH CH3CH2COOH
+ HCHO + HCHO
CH2=CHCOOH
+
CH2=C(CH3)COOH
respec
H20
+
H20
I t was found t h a t t h e c o m b i n a t i o n of t i t a n i u m phosphate, which has a s m a l l excess o f phosphorus w i t h r e s p e c t t o s t o i c h i o m e t r i c t i t a n i u m pyrophosphate, TiP207, w i t h (VO)2P207
b r i n g s about an enhanced c a t a l y t i c performance i n t h e
r e a c t i o n o f a c e t i c a c i d and r e l a t e d compounds w i t h HCHO [4-71.
However, t h e
combination o f t i t a n i u m phosphate does n o t improve t h e performance i n t h e r e a c t i o n o f p r o p i o n i c a c i d and r e l a t e d compound w i t h HCHO 14.81. More r e c e n t l y , i t has a l s o found t h a t V-Si-P
t e r n a r y oxides e x h i b i t t h e
most p r o m i s i n g c a t a l y t i c performance i n t h e r e a c t i o n o f p r o p i o n i c a c i d and r e l a t e d compounds w i t h HCHO [8,9]. The o b j e c t of t h e p r e s e n t s t u d y i s t o o b t a i n high-surface-area
V-Si-P
oxide
c a t a l y s t s w i t h a h i g h s e l e c t i v i t y i n t h e f o r m a t i o n o f m e t h a c r y l i c a c i d and methacrylates.
The s t r e s s i s p l a c e d on t h e e f f e c t o f t h e V-Si-P
c o m p o s i t i o n on
t h e a c t i v i t y and s e l e c t i v i t y and t h e e f f e c t o f l a c t i c a c i d used i n p r e p a r i n g
654 catalysts,
s i n c e i t has been r e p o r t e d t h a t homogeneous m i x t u r e o f metal o x i d e s
(amorphous) w i t h a l a r g e s u r f a c e area can be o b t a i n e d b y u s i n g a hydroxy carbox y l i c a c i d as a complex-making agent [ l O , l l ] . EXPERIMENTAL Catalysts As t h e sources o f vanadium,
s i l i c o n , and phosphorus, NH4V03,
s i l i c a "Snowtex 0" (Nissan Chem. Ind.) used.
colloidal
c o n t a i n i n g 20% Si02. and 85% H3P04 were
Unless i n d i c a t e d o t h e r w i s e , NH4V03 (20 t o 60 g ) was d i s s o l v e d i n a h o t 2+
w a t e r c o n t a i n i n g about 20 m l o f l a c t i c a c i d , y i e l d i n g a b l u e s o l u t i o n o f VO
.
I t was t h e n mixed w i t h t h e r e q u i r e d amounts o f 85%H3P04 and t h e c o l l o i d a l
silica.
Excess w a t e r was evaporated w i t h s t i r r i n g i n a h o t a i r c u r r e n t .
The
o b t a i n e d cake was d r i e d i n an oven g r a d u a l l y h e a t i n g f r o m 50 t o 200°C f o r 6 h. The r e s u l t i n g s o l i d was ground and s i e v e d t o g e t a 8- t o 20-mesh s i z e p o r t i o n . It was c a l c i n e d f i n a l l y a t 450°C f o r 6 h i n a stream o f a i r .
Procedures f o r t h e a l d o l condensation The r e a c t i o n o f p r o p i o n i c a c i d and HCHO was c a r r i e d o u t w i t h a continuousf l o w system.
The r e a c t o r was made o f a s t e e l t u b e (50 cm X 1.8 cm I.D.)
t e d v e r t i c a l l y and immersed i n a l e a d bath.
moun-
N i t r o g e n was f e d i n f r o m t h e t o p
o f t h e r e a c t o r a t a f i x e d r a t e o f 140 ml/min ( a t 20°C) as t h e c a r r i e r o r t h e diluent,
and a m i x t u r e o f t r i o x a n e [(HCH0)3]
and p r o p i o n i c a c i d was i n t r o d u c e d
i n t o t h e p r e h e a t i n g s e c t i o n o f t h e r e a c t o r by means o f an i n j e c t i o n s y r i n g e pump.
The f e e d r a t e s o f p r o p i o n i c acid, HCHO, and n i t r o g e n were 33.6.
and 350 mmol/h,
respectively.
d e s c r i b e d p r e v i o u s l y [3.4.9].
16.8,
The o t h e r procedures were t h e same as t h o s e The y i e l d (mol-%) was d e f i n e d as 100 t i m e s
(moles o f m e t h a c r y l i c a c i d ) / ( m o l e s o f HCHO fed). Characterization o f catalysts The s u r f a c e areas o f t h e c a t a l y s t s were measured by t h e BET method u s i n g n i t r o g e n as adsorbate a t -196°C.
The average o x i d a t i o n numbers o f vanadium
i o n s i n t h e c a t a l y s t s were determined by t h e redox t i t r a t i o n method d e s r i b e d p r e v i o u s l y [12-141. RESULTS AND DISCUSSION E f f e c t o f t h e V-Si-P
composition
The e f f e c t s o f t h e c o m p o s i t i o n o f t h e V-Si-P
t e r n a r y o x i d e s was s t u d i e d by
changing b o t h t h e s i l i c o n and phosphorus contents; l/x/y,
where x and y were changed.
The vapor-phase
V/Si/P
atomic r a t i o =
a l d o l condensation o f
p r o p i o n i c a c i d w i t h HCHO was conducted o v e r 20 g p o r t i o n s of seven s e r i e s o f
655 uv
b
Catalyst V-Si-P = I - X - Y
3
2
1
F i g . 1. E f f e c t o f t h e c o m p o s i t i o n o f t h e V-Si-P maximum y i e l d o f m e t h a c r y l i c a c i d . c a t a l y s t s a t temperatures from 270 t o 330°C.
5
4
Y
t e r n a r y o x i d e c a t a l y s t s on t h e
The y i e l d o f m e t h a c r y l i c a c i d
i n c r e a s e d as t h e temperature was r a i s e d , passed t h r o u g h a broad maximum, and t h e n decreased,
The maximum y i e l d s a r e shown i n F i g . 1 as a f u n c t i o n o f t h e
phosphorus c o n t e n t , y.
There e x i s t s an o p t i m a l c o n t e n t o f phosphorus which
i n c r e a s e s as t h e s i l i c o n c o n t e n t increases; with the V/Si/P
1/1/2.1.
atomic r a t i o o f 1 / x / [ 2
1/2/2.7,
1/4/2.4,
1/8/2.8,
+
t h e highest y i e l d s are obtained
(0.1-0.2)x],
1/16/3.3,
1/32/3.8,
f o r example, V / S i / P and 1/50/4.5
=
oxides.
P o s s i b l y , a p a r t of phosphorus i s i n t e r a c t e d w i t h s i l i c a , as a r e s u l t s , a s m a l l excess o f phosphorus w i t h r e s p e c t t o s t o i c h i o m e t r i c (VO)2P207 i s r e q u i r e d t o s t a b i l i z e t h e (VO)2P207
species.
The one-pass y i e l d o f m e t h a c r y l i c
a c i d reached 55 mol-% on HCHO b a s i s a t t h e p r o p i o n i c acid/HCHO m o l a r r a t i o o f 2 The s p e c i f i c s u r f a c e areas o f t h e seven s e r i e s o f c a t a l y s t s were measured by t h e BET method.
The r e s u l t s a r e shown i n Fig. 2.
The s u r f a c e a r e a
decreases as t h e phosphorus c o n t e n t increases, w h i l e i t i n c r e a s e s markedly as t h e s i l i c o n c o n t e n t increases. F i g u r e 3 shows t h e average o x i d a t i o n numbers o f vanadium i o n s i n t h e V-Si-P oxide catalysts. increased.
The o x i d a t i o n number decreased as t h e phosphorus c o n t e n t
It should be n o t e d t h a t a good performance i n t h e a l d o l condensa-
t i o n i s achieved w i t h t h e c a t a l y s t i n which t h e o x i d a t i o n number o f vanadium i o n s i s around 4.0,
regardless o f t h e content o f s i l i c o n .
These f i n d i n g s
suggest t h a t t h e a c t i v e s i t e s i s a s c r i b e d t o (VO)2P2O7 s i m i l a r t o t h e case of
656
160
Catalyst
-
V- Si- P = 1 - X - Y
140
UI
“120
E
v
‘ . ,
100
X=l6
80 60 I T J
Y F i g . 2.
E f f e c t o f t h e c o m p o s i t i o n o f t h e V-Si-P
o x i d a t i o n o f n-butane t o m a l e i c a n h y d r i d e [15,16],
o x i d e s on t h e s u r f a c e area. and t h a t t h e presence o f an
excess o f phosphorus i s r e q u i r e d t o s t a b i l i z e t h e (VO)2P207 species. E f f e c t o f t h e l a c t i c a c i d used i n p r e p a r i n q t h e c a t a l y s t s The e f f e c t s o f t h e methods o f p r e p a r i n g V-Si-P studied:
t e r n a r y o x i d e c a t a l y s t s were
t h e V/Si/P c o m p o s i t i o n s were chosen so as t o g e t a good c a t a l y t i c
performance b a s i n g on t h e r e s u l t s o b t a i n e d i n t h e p r e c e d i n g s e c t i o n (Fig. Series A catalysts:
1).
The c a t a l y s t s were prepared i n t h e presence o f l a c t i c
a c i d and t h e procedures were d e s c r i b e d i n t h e Experimental s e c t i o n . Series B catalysts:
The c a t a l y s t s were prepared i n t h e presence o f o x a l i c
NH4V03 was added t o a h o t w a t e r c o n t a i n i n g o x a l i c a c i d i n amounts s u f f i 2+ The o t h e r procec i e n t t o d i s s o l v e t h e NH4V03, y i e l d i n g a s o l u t i o n o f VO acid.
.
dures were t h e same as t h o s e f o r t h e S e r i e s A c a t a l y s t s . Series C catalysts: glycol.
The c a t a l y s t s were prepared i n t h e presence o f e t h y l e n e
NH4V03 was d i s s o l v e d i n a h o t w a t e r c o n t a i n i n g e t h y l e n e g l y c o l [5-71.
The o t h e r procedures were t h e same as t h o s e f o r t h e S e r i e s B c a t a l y s t s . Series N catalysts:
The c a t a l y s t s were prepared i n t h e absence o f an
657
4.81 4.6 > *4.4 0
4.2
I C C
t
catalyst: v - s i - P = I - X -
Y
3.8
0
3.4'
'
I
I
1
I
2
I
I
I
3
I
4 Y
F i g . 3.
E f f e c t o f t h e c o m p o s i t i o n o f t h e V-Si-P
I
L
5
o x i d e s on t h e average o x i d a -
t i o n numbers o f vanadium i o n s .
NH VO
organic solvent.
was d i s s o l v e d i n a warm w a t e r c o n t a i n i n g t h e r e q u i r e d 4 3 The o t h e r Then, i t was mixed w i t h t h e c o l l o i d a l s i l i c a .
amount of H3P04.
procedures were t h e same as t h o s e f o r t h e S e r i e s A c a t a l y s t s . Series S catalysts:
The c a t a l y s t s were prepared i n a non-aqueous medium;
i n i s o b u t y l a l c o h o l - b e n z y l a l c o h o l medium, a c c o r d i n g t o t h e method o f Katsumoto and Marquis [15].
As an index of t h e a c t i v i t y f o r t h e a l d o l condensation, t h e y i e l d s (mol-%) o f m e t h a c r y l i c a c i d o b t a i n e d under t h e c o n d i t i o n s d e s c r i b e d i n t h e Experimental s e c t i o n were measured f o r f o u r d i f f e r e n t amounts o f each c a t a l y s t .
The s u r f a c e
areas and t h e average o x i d a t i o n numbers o f vanadium i o n s i n t h e f r e s h c a t a l y s t s were a l s o determined.
The r e s u l t s a r e g i v e n i n Table 1.
The r e s u l t s may be summarized as f o l l o w s .
( 1 ) V-P b i n a r y o x i d e s w i t h o u t s i l i c a : The s u r f a c e areas o f t h e o x i d e s pre2 pared i n an aqueous medium a r e i n t h e range o f 2 t o 4 m /g, w h i l e t h o s e prepared i n an i s o b u t y l a l c o h o l - b e n z y l a l c o h o l r e a c h
20 t o 30 m2/g.
The a d d i t i o n
o f l a c t i c a c i d and e t h y l e n e g l y c o l i n an aqueous medium does n o t serve f o r i n c r e a s i n g t h e s u r f a c e area.
(2) 1 ,< S i / V
8 oxides:
The s u r f a c e areas o f t h e o x i d e s a r e h i g h r e g a r d l e s s o f
t h e a d d i t i o n of l a c t i c a c i d i n an aqueous medium.
( 4 ) The s u r f a c e areas o f t h e o x i d e s i n c r e a s e as t h e c o n t e n t o f s i l i c o n i n creases.
658
TABLE 1 Comparison o f t h e V-Si-P Catalyst V/Si/P Method
t e r n a r y o x i d e c a t a l y s t s prepared by d i f f e r e n t methods
Surface area (m2/g>
ratio
A B
c
l/Oll.l
N S
11112.1
11812.8
113213.6
10.4 10.0 8.2 16.4 40.8
5
1.2
0.6
33.8
18.2
11.4
40.2
20.0
13.2
21.3
12.0
10.4 2.4
4.1 3.9
A B
34.0 2.3 2.2 5.1
3.9 3.9 3.8 4.0
53.2 20.6 10.0 30.1
42.0 17.4
A
36.0 8.6
3.9 3.1
54.7 11.7
40.8 9.5
19.3
14.4
A B
51.4 40.0 19.8
4.0 3.8 4.0
52.2 51.2 21.4
44.2 41.4 21.0
21.3 20.2
14.8 15.4
A
86. 73.
3.9 3.9
53.5 52.2
50.5 49.5
27.1 30.5
19.6 20.0
A
132. 116.
4.1 4.2
53.5 52.2
48.8 47.2
30.0 32.2
20.2 23.0
N
111613.2
20
4.1 4.4 4.1 4.4 4.0
A
N
1/4/2.4
Y i e l d (mol-%) o f MAA" Amount o f c a t a l y s t used (9)
52.0 10.6
c
11212.2
2.5 3.0 2.2 3.9 23.0
Oxidation number o f v ions
*
One-pass y i e l d o f m e t h a c r y l i c a c i d m o l a r r a t i o o f 2 and 320°C.
on HCHO b a s i s a t t h e p r o p i o n i c acid/HCHO
(5)
C a t a l y t i c a c t i v i t y i n c r e a s e s as t h e s u r f a c e area o f c a t a l y s t increases.
However,
i t l e v e l s o f f a t S i / V atomic r a t i o = 16, s u g g e s t i n g t h a t t h e s u r f a c e
area measured does n o t r e p r e s e n t t h e amount o f a c t i v e s i t e s when t h e s i l i c o n c o n t e n t i s h i g h ; S i / V 3 16.
( 6 ) The average o x i d a t i o n numbers o f vanadium i o n s i n t h e f r e s h c a t a l y s t s a r e about 4.0, r e g a r d l e s s o f t h e d i f f e r e n c e i n t h e method o f p r e p a r i n g c a t a l y s t . ( 7 ) The maximum y i e l d s o f m e t h a c r y l i c acid, t h a t i s . t h e y i e l d s w i t h 20 g p o r t i o n s o f c a t a l y s t s , a r e c o n s t a n t a t about 53 t o 54 mol-%, o v e r t h e V-Si-P t e r n a r y o x i d e c a t a l y s t s w i t h a l a r g e s u r f a c e area. Discussion As i s seen i n Figs. w i t h t h e V-Si-P
1 - 3, t h e maximum y i e l d s of m e t h a c r y l i c a c i d o b t a i n e d
t e r n a r y o x i d e c a t a l y s t s a r e t h e same l e v e l : 52 t o 55 mol-%, and
t h e maximum y i e l d s a r e o b t a i n e d w i t h t h e c a t a l y s t s possessing t h e average o x i d a t i o n numbers o f vanadium i o n s o f about 4.0.
suggesting t h a t t h e a c t i v e s i t e s
659
a r e r e l a t e d t o (VO)2P207
V-Si-P
species,
However, t h e y i e l d s o b t a i n e d w i t h t h e
t e r n a r y o x i d e s a r e c l e a r l y h i g h e r t h a n t h o s e o b t a i n e d w i t h t h e V-P
b i n a r y o x i d e s c o n s i s t i n g o f (VO)2P207-
T h i s l e a d s us t o c o n s i d e r t h a t t h e
(VO)2P207 m o d i f i e d by s i l i c o n phosphate i s more s u i t a b l e t h a n p u r e (VO) P 0 2 2 7 Therefore, t h e
as a c t i v e s i t e s f o r t h e r e a c t i o n o f p r o p i o n i c a c i d w i t h HCHO.
a c i d and base p r o p e r t i e s were checked i n d i r e c t l y by t h e c a t a l y t i c a c t i v i t i e s f o r d e h y d r a t i o n o f 2-propanol
and dehydrogenation of acetaldehyde,
respectively
However, a c l e a r d i f f e r e n c e i n t h e c a t a l y t i c a c t i v i t y f o r t h e s e r e a c t i o n s was n o t observed between t h e V-Si-P
and V-P o x i d e s .
P o s s i b l y , t h e y i e l d may be
governed by a more s u b t l e d i s t i n c t i o n i n t h e acid-base p r o p e r t i e s . As has a l r e a d y been known, i t i s hard t o p r e p a r e V-P b i n a r y o x i d e s w i t h a h i g h surface area i n an aqueous medium, even i n t h e presence o f an o r g a n i c compound such as l a c t i c a c i d and o x a l i c a c i d . of V-Si-P
t e r n a r y oxides,
aqueous medium.
i t i s p o s s i b l e t o produce a l a r g e s u r f a c e area i n an
When t h e c o n t e n t o f s i l i c o n i s n o t high: 1 4 S i / V , C 4 , t h e
presence o f l a c t i c a c i d i s i n d i s p e n s a b l e . not effective.
w h i l e o x a l i c a c i d i s u n s t a b l e and i s decomposed
d u r i n g t h e d r y i n g of c a t a l y s t p r e c u r s o r s .
Si/Va 8,
O x a l i c a c i d and e t h y l e n e g l y c o l a r e
P o s s i b l y , t h e presence o f c a r b o x y l i c group i s r e q u i r e d as a
complex-making agent [10.11].
acid.
On t h e o t h e r hand, i n t h e case
When t h e c o n t e n t o f s i l i c o n i s high,
o x i d e s w i t h a h i g h surface a r e a can be o b t a i n e d w i t h o u t u s i n g l a c t i c
Vanadium phosphate and s i l i c o n phosphate can i n t e r a c t s u f f i c i e n t l y w i t h
each o t h e r even i n t h e absence o f a complex-making agent. I n c o n c l u s i o n , we would l i k e t o c o n s i d e r t h a t phosphorus serves t o f o r m and s t a b i l i z e (VO)2P207 species,
and t h a t s i l i c o n serves t o m o d i f y t h e (VO)2P207
and a l s o t o enhance l a r g e l y t h e s u r f a c e area.
The presence o f l a c t i c a c i d as
a complex-making agent i s i n d i s p e n s a b l e t o g e t a l a r g e s u r f a c e area when t h e c o n t e n t o f s i l i c o n i s n o t high. REFERENCES 1 2 3 4 5 6 7
8
9 10 11 12 13
R.A. Schneider (Chevron Res. Co.). U.S. P a t e n t 4 165 438, 1979. M. A i , J. Catal.. 107 (1987) 201-208. M. A i . Appl. Catal., 36 (1987) 221-230. M. A i . i n : M.J. P h i l l i p s and M. Ternan (Eds.), Proc. I n t e r n . Congr. Catal.. Calgary, 1988, Chem. I n s t . Canada, Ottawa, 1984, pp. 1562-1569. M. Ai. J. C a t a l . . 113 (1988) 562-566. M. A i . Appl. C a t a l . , 48 (1989) 51-61. M. A i . Appl. Catal., 54 (1989) 29-36. M. A i , Appl. C a t a l . , i n press. M. A i . B u l l . Chem. SOC. Jpn., 63 (1990) 199-202. P. C o u r t y and B. Delmon. C. R. Acad. Sc. P a r i s , S e r i e s C. 268 (1969) 1874-1 875. P. Courty, H.A.C. M a r c i l l y , and B. Delmon, Powder Technology, 7 (1973) 21-38. M. Nakamura, K. Kawai, and Y. F u j i w a r a , J. Catal., 34 (1979) 345-355. B.K. Hodnett, P. Permanne. and B. Delmon, Appl. Catal.. 6 (1983) 231-244.
660
14 15 16
M. Niwa and Y. Murakami, J. Catal.. 76 (1982) 9-16. K. Katsumoto and D.M. Marquis (Chevron Res. Co.). U.S.
1979. E. Bordes and
P. Coutine, J. Catal., 57 (1979) 236-252.
Patent 4 132 670,
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
661
PREPARATION OF FINE PARTICLES OF RUTHENIUM-ALUMINA COMPOSITE BY MIST REDUCTION METHOD
H. IMAI and J. SEKIGUCHI Research Laboratory of Engineering Materials, Tokyo Institute of Technology, Midori-ku, Yokohama, 227 (Japan)
SUMMARY Fine particles of ruthenium-alumina composite (Ru = 2.3 wt8) were prepared by reduction of a mist of a mixed solution sf ruthenium chloride and aluminum nitrate. A mist of the mixed solution was treated in a hydrogen stream through three furnaces, successively. Temperatures of the furnaces were adjusted for evaporation of water, hydrogenolysis of the mixed metal salts, and reduction of the particles, respectively. Amorphous ruthenium clusters dispersed in an amorphous alumina particle were prepared by this method. The particles were porous and spherical with a narrow particle size distribution (average diameter=0.64 am). The diameter of the ruthenium clusters was less than 2 nm. The IR band of linearly adsorbed carbon monoxide shifted to high frequency side compared to that on an impregnated catalyst. The catalyst obtained by the direct synthesis showed much higher activity for benzene hydrogenation than that prepared by reduction of the mixed oxide. INTRODUCTION Supported metal catalysts have been prepared
usually
by
the
impregnation
method. More finely dispersed metal catalysts were reported to be
prepared
by
superficial reduction of dilute mixed metal oxide solid solutions (ref. 1). The dispersion of
metal
may
be
improved
by
direct
synthesis
composite, because the chance of the aggregation of metal atoms
of
metal-oxide
is
diminished
in the direct synthesis. Moreover, clusters with a different structure obtained by the direct synthesis because the conditions
of
cluster
may
be
formation
are different. In this paper, we report a direct preparation
method
composite by reduction of the mist of a mixed solution
ruthenium-alumina
of
of
ruthenium
chloride
and aluminum nitrate. The essential features of the method are
as
mist of the mixed solution
hydrogen
is
generated
into
a
stream
of
follows.
supersonic atomizer, and treated successively through three furnaces.
by
A a
Tempera-
tures of the furnaces are adjusted for evaporation of water, hydrogenolysis
of
the mixed metal salts, and further reduction of the particles, respectively. This method has the common advantages of solution methods for preparation of fine particles, i.e., better homogeneity and better
purity
of
the
material,
lower temperature of preparation, precise control of the composition and so on.
662 Moreover, this method can control more precisely the structure and the
texture
without coalescence of particles, because the temperature of individual process is controlled independently (ref. 2 ) . METHODS Materials Ruthenium (111) chloride (99.9%), aluminum (S
nitrate
grade) were obtained from Wako Pure Chemical.
(S
grade)
Hydrogen
was
and
benzene
obtained
from
Nippon Oxygen Co. and used through a dry ice trap. Carbon monoxide (99.5%) was obtained from Takachiho
Chemical,
and
purified
by
distillation
at
liquid
nitrogen temperature. Preparation of composite particles Figure 1 shows the schematic diagram of the apparatus. A
mist
of
a
mixed
solution (5wt8) of RuC13 and Al(NO3I3 was generated into a stream of
hydrogen
by a supersonic atomizer ( 6 ) , the diameter of the droplet being ca. 5
w. The
mist was treated successively through three furnaces (7-9). Temperatures of the furnaces were adjusted for evaporation of water ( 4 4 3 K), hydrogenolysis of mixed metal salts (573 K), and further
reduction
of
the
particles
respectively. The flow rate was controlled at 1 l/min by a control valve It should be noted that
the
retention
time
of
temperature portion (*5 K ) of the furnaces is ca. collected by a glass filter (10) at
393 K.
A
the 10
gas
mist S.
in
The
handling
the
(13).
constant
particles valve
the
(773 K),
were
(1) was
provided to replace hydrogen in the apparatus with nitrogen when the filter
is
r
0
n 17
Fig. 1. Schematic diagram of apparatus. 1: Gas handling valve, 2: Safety valve, 3: Reservoir, 4-5: Valve, 6: Supersonic atomizer, 7-9: Furnace, 10: Filter, 11: Tail gas treatment System, 12: Flow meter, 13: Control valve, 14: Pump.
663 exchanged. The exit gas from the filter was passed through a tail gas treatment system (11) to remove water, nitrogen dioxide etc. before entering into a flow meter (12). The powders were pressed into tablets,
crushed
and
sized
(32-60
mesh) for use in the measurements.
A reference catalyst was prepared by reduction (at 673 K)
of
a
aluminum mixed oxide which had been prepared in the same way in the
ruthenium stream of
air with the same raw materials. Physico-chemical characterization Temperature-programmed reduction (TPR) measurement volumetric method with a
constant pressure
gas
was
carried
circulation
200 ml. Liquid nitrogen traps were placed before and after a
quartz
tube to remove the water formed. After evacuation of sample at min, the temperature of the sample was decreased to
room
out
system
by
a
of
ca.
measuring
513 K
for
30
temperature, and
constant pressure of hydrogen was admitted to the system. Then the
a
temperature
was increased at a rate of 5.3 K/min. Infrared (IR) spectrum of adsorbed carbon monoxide was recorded with a JASCO FT/IR-3 Fourier-transform IR spectrometer. The construction of
the
vacuum
cell used for the measurements was similar to that reported by Peri and (ref. 3). The
sample was
pressed
into
a
thin
self-supporting wafer
pretreated in the cell. After reduction with hydrogen at 673 K
for
IR
Hannan
2
and
h,
the
sample was evacuated at the same temperature for 30 min and the temperature of the sample was lowered to room temperature for adsorption of
carbon
monoxide.
The adsorption was carried out at a constant pressure of 40 Tor+ for 15 min room temperature. Carbon monoxide in
the
gas
phase
was
evacuated
temperature for 5 min before IR measurement. The spectrum taken
at
before
at room
carbon
monoxide adsorption was used as the background spectrum. Specific surface areas were measured by the BET method
with
adsorption
nitrogen at 77 K. A JEOL JSM-T2OO scanning electron microscope JEM-2000EX transmission electron microscope (TEM) and X-ray diffractometer with
nickel
filtered CuKd
a
(SEM), a
Rigaku
radiation
Denki
were
of
JEOL
powder
used
for
characterization of the samples. Catalytic activity The catalytic activity for benzene hydrogenation was
measured
method at 423 K. A given amount of the sample was packed in After pretreatment with hydrogen at 673 K for 16
h,
the
a
by
Pyrex
temperature
a
flow
reactor.
of
the
sample was lowered to the reaction temperature in the hydrogen stream, and
the
hydrogen gas containing 5.25 Torr of
the
packed bed at
a
rate
of
30
benzene
ml/min.
The
vapor
was
products
flowed
were
through
analyzed
chromatography with a 2-m ethylene glycol adipate/Chromosorb W column.
by
gas
664
Pig. 2. SEM micrograph of ruthenium-alumina composite powder.
20
w
10
0
0 Diameter
(pm)
~ ~ i g3.. Particle size distribution.
665 RESULTS AND DISCUSSION Characterization of particles The particles of ruthenium-alumina
composite are
spherical
Fig. 2. The particle size distribution is narrow with an
as
shown
average diameter of
0.64 wn, as shown in Fig. 3. A cumulative surface area of 1.9 m 2 / g is from the distribution curve. Comparison between the m2/g) surface areas suggests that
the
obtained
cumulative and
particles are
in
porous.
(5.4
BET
The
ruthenium
content of the sample was determined to be 2.33 wt8 by a chemical analysis. Result of TPR measurement is shown in Fig. 4, together with the result
with
the mixed oxide. With the mixed oxide sample ( b ) which had been prepared in air stream, consumption of hydrogen started at about 400 K and completed
at
about
5 7 0 K. This indicates that the reduction of ruthenium oxide takes place in this
temperature range. With the ruthenium-alumina
composite sample
(a), on
other hand, consumption of hydrogen started at about 5 2 0 K , and the hydrogen consumed below 5 7 0 K is much smaller than that
of
the
the
amount
mixed
of
oxide.
This indicates that most of the ruthenium ions were reduced in this sample. Figure 5 shows the XRD spectra of the composite particles. The XRD
of the original
sample
(a) shows
no
crystalline peak;
peaks
spectrum
of
neither
ruthenium metal nor ruthenium dioxide were detected. However, crystalline peaks of ruthenium metal, ruthenium dioxide and d-alumina appeared after
the
sainple
was heated in the stream of helium up to 1273 K at a rate of 5 K/min (b). This
81
,............
... ___., _: .*
* *
I
b
....**
0
*
a
,
500
700
Temperature
(K)
0
Fig. 4. TPR measurements. Pi12 = 2 0 1 Torr,
Fig. 5 . XRD saectra.
Rate of temperature increase
(a) Original sample,
=
(a)
5 . 3 K/min,
Obtained by direct synthesis,
(b) Mixed oxide sample.
(b) Treated in helium up to 1 2 7 3
a: Ru,
0:
Xu02,
0 :K-AlzOj
:.
666 indicates that the original sample was amorphous, and
that
the
size
the
of
ruthenium clusters increased by sintering during the high temperature treatment. Figure 6 shows the TEM micrograph of 3 particles, the diameters of which are 125, 580 and 900 nm, respectively. Various sizes of spots are observed
in
the
particles. Spots larger than 10 nm may be ascribed to the porous
structure of
the particles, but smaller spots may be
of
caused
by
aggregation
ruthenium
metal atoms because the BET surface area is only about 3 times as large as cumulative surface area. The micrograph shows that the size
of
the
ruthenium
clusters depends on the size of the particle; ruthenium clusters of small (
c
ib
In z w I-
z
10
20
30
40
50
60
TWO-THETA (DEGREES)
Fig. 3 - X-ray diffractograms of 0.2 % Pt/HxMoOg reduced at 323 K (A), 433 K (B), 473 K (C) and 573 K @). Reduction duration for each sample is indicated in Table 2. In diffractogramsA and B, the main peaks are those of H0.9M003 ; the other peaks (indicated by arrows) belong to the H 1 .6M003 phase.
684
The X-ray diffractogramsof samplesreduced at 323 K (A) and at 433 K (B) show the main peaks of the H0.9Mo03 phase together with some other peaks that may be indexed into the Hl.gMo03 phase. For those samples, hydrogen contents determined by volumetric measurementscorrespond to x values ranging from 1.44 to 1.67 (Table 2, column 3). Therefore, discrepancies exist between volumetric measurements and XRD analyses for bronzes reduced at temperatures in the range 298 433 K. This can be explained by the unstability of H1.6MoO3 bronze with respect to oxidation at ambient temperature whereas for low hydrogen contents ( x d 0.9) the process is very slow (ref. 10). A partial transformation of H1@lm into m.9MoO3 is very likely to occur because of exposure to air during the transfer to the X-ray diffiactometer. This is corroborated by the color change in samples from bordeaux-red to dark blue. Reduction at temperatures above ca. 433 K allows the formation of large hydrogen content bronzes (x = 1.86 - 2.10, Table 2, column 3) that are amorphous with respect to XRD (fig. 3, spectres C and D). The surface area of crystalline bronzes is about 30 m2g-1 whereas the amorphous sample reduced at 573 K (last line) exhibits a surface area of 20 m2g-l . Electron micrographs of the bronzes Pt/HxMo03 show the presence of small Pt particles of 1 to 3 nm in diameter, homogeneously dispersed on the support.
3 - F'reparan'onof neat HrMo03 We have used three methods to prepare molybdenum bronzes without deposited metal. In one of the methods, MoO3 is contacted with Pt/Al2O3 and exposed to H2 at 433 K for 24 h in a "reactor with an elevator" already described (ref. 7). The Pt/Al2O3 is then removed from the reactor by a windlass device (ref. 7). The bronze H1.6Mo03 thus obtained has about the same surface area as that of the MOO3 host sample. In an other method, finely divided MoO3 is placed in a dynamic differential reactor under a flow of allylic alcohol (20 Torr) and H2 (740 Torr) at 323 K for 20 h ; H0.34Mo03 is formed. If the carrier gas in the flow is N2 instead of H2, the same bronze is produced at ca. 373 K only. In both cases, the bronze obtained has almost the same surface area as that of the starting oxide. Thus, MoOg in a fmely divided state is capable of extracting hydrogen atoms from the allylic alcohol molecule at low temperatures. As yet, we have no clear explanation for the beneficial effect of H2 in this process. Another possibility of preparing neat HxMoOg bronzes at low temperature results from the photosentive properties of highly divided MoO3. Indeed, upon UV-illumination at room temperature a suspensionof MoOg in methanol or in 2-propanol turned rapidly to a deep dark blue color. The Xray diffractograms of samples show the presence of m.34MoO3. By use of a mixture of MoO3 and T i 0 2 instead of MoO3 alone, the bronze H0.9M003 is formed as indicated by the X-ray diffractogram of the centrifugated solid and by its deep slate-blue color. The bronzes formed under UV-illumination retain the high division state of the starting materials.
685
4 - Potential catalytic applications
The bronzes Pt/HxMo03 may be considered to some extent as bimetallic catalysts (ref. 2). The Pt metal particles on the surface may be expected to interact with the modified oxide and, under reaction conditions, fast spillover/reverx spillover of hydrogen between metal and support is expected. Those remarkable characteristics suggest the possibility of unusual interesting catalytic applications, particularly for very finely divided bronzes. Indeed, current investigations in our laboratory show that they are good and stable catalysts for the hydrogenation of various compounds (ethylene, acetylene, allylic alcohol, carbon monoxide, etc.). In the case of neat HxMo03 bronzes, one of us has shown, for the first time, that Hi.jMoO3 can catalyse the hydrogenation of ethylene by molecular hydrogen without the presence of metallic particles (ref. 5). The bronze H1.6Mo03 is therefore capable of activating molecular hydrogen without modifications of its structure under the reaction conditions. The initial rate of hydrogenation of ethylene at 433 K on H1.6Mo03 was close to that previously observed on 0.5 % Pt/H1.6MoO3 (ref. 2). The bronze H09Mo03 was observed to be active in the isomerization of methylcyclopropane at 353 K (ref. 11). Finally, preliminary experiments show that cinnamaldehyde can be photocatalytically hydrogenated into cinnamyl alcohol by a mechanism of hydrogen transfer from an alcohol, via a bronze HxMo03, to cinnamaldehyde. Under the same conditions, Pt/Ti02, used as a reference bifunctional photocatalyst, leads to the saturation of the C=C bond.
CONCLUSION Various methods of synthetizing finely divided bronzes H x M a with or without the presence of metallic activator particles are available. The starting molybdenum trioxide can be prepared in a flame reactor, under conditions where nucleation and growth processes of the oxide particles in the hot zone of the flame are optimized in order to obtain high surface areas (230 m2g-I) and a stable structure (orthothombic phase) after a standard dechlorination treatment of Mo03. The hydrogen spillover process can be used in the prepration of either Pt/HxMo03 or H1.@4o03 (by momentarily contacting Mo03 with PdAl2O3 in the presence of H2 in this latter case). Neat HxMoOg (with x 6 0.9) bronzes can be obtained by using an alcohol as a hydrogen source either in the dark (allylic alcohol, H2, 323 K or N2, 373 K) or under UV-illumination (methanol or 2propanol, room temperature). In this latter case, a mixture of M d 3 and Ti@ allows the formation of Hr3.9Mo03 instead of HO.34M003, which indicates interparticle hydrogen atom transfer. The finely divided hydrogen molybdenum bronzes thus prepared are potential catalysts for several types of reactions.
686
REFERENCES 1 D. Tinet, H. Estrade-Szwarckopf and J.J. Fripiat, Bul. SOC. Fr. Phys., 42 (1981) 28. 2 J.P. Marcq, X.Wispenninckx, G. Poncelet, D. Keravis and J.J. Fripiat, J. Catal., 73 (1982) 309 and references cited therein. P.A. Sermon and G.C. Bond, Catal. Rev., 8 (1973) 211. 3 4 S.D. Jackson, B.J. Brandreth and D. Winstanley, Appl. Catal.,27 (1986) 325. 5 R. Benali, C. Hoang-Van and P. Vergnon, Bull. SOC. Chim. Fr., (1985) 417. 6 M. Formenti, F. Juillet, P. Meriaudeau, S.J. Teichner and P. Vergnon, J. Colloid Interf. Sci., 39 (1972) 79. 7 D. Maret, G.M. Pajonk and S.J. Teichner, in G.M. Pajonk, S.J. Teichner and J.E. Germain (Eds), Proc. Int. Symp. Spillover of Adsorbed Species, Lyon-Villeurbanne, September 1216, 1983, Elsevier, Amsterdam. 1983, p. 215. 8 B. P o d e r , F. Juillet and S.J. Teichner, Bull. SOC. Chim. Fr., (1972) 1268. 9 L. Kihlborg, Acta. Chem. Scand., 13 (1959) 954. 10 J.J. Birtill and P.G. Dickds, f.?h€H !&ate Chem., 29 (1979) 367. 11 Inpreparation.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
687
NEW PREPARATION OF SUPPORTED METALS. HYDROGENATION OF NITRILES M. BLANCHARD, 3. BARRAULT and A. DEROUAULT L d b o r a t o i r e de Catalyse en Chimie Organique URA CNRS 350 40, Avenue du Recteur Pineau 86022 POITIERS Ckdex (France)
SUMMARY An a l k y l aluminium i s g r a f t e d on a s o l i d by r e a c t i o n w i t h the hydroxy surface groups. By a d d i t i o n o f a s o l u t i o n o f a metal s a l t , a surface react i o n occurs which produces the metal and an aluminium s a l t . This aluminium s a l t i s hydrolysed t o alumina. The f i n a l s t a t e i s a supported metal surrounded by surface dlumina. N i c k e l , c o b a l t , copper c a t a l y s t s supported on oxides o r a c t i v a t e d carbon are prepared f o l l o w i n g t h i s procedure and are used f o r the l i q u i d phase hydrogenation o f l o n g c h a i n n i t r i l e s . Their a c t i v i t i e s and select i v i t i e s towards the formation o f amines (primary, secondary, t e r t i a r y ) dre compared w i t h t h e one obtained w i t h conventional c a t a l y s t s . INTRODUCTION During led
to
an e a r l i e r
prepare h i g h l y
investigation
i n t o CO hydrogenation
d i v i d e d metals
i n dn organic
(1,2)
solvent
we were
i n order
to
o b t a i n a s l u r r y which had t o be s t a b l e w i t h o u t any annealing o f t h e m e t a l particles.
For t h i s purpose a m e t a l l i c s a l t i s reduced a t room temperature an
aluminium d k y l
i n homogeneous
phase,
and the
slurry
h e a t i n g t h i s s o l u t i o n up t o 2OO0C under a stream o f water
which
forms,
i n situ,
by
the
reduction o f
the
by
i s formed by
synthesis
gas
: the
cdrbon monoxide i s
used f o r the h y d r o l y s i s o f the aluminium s a l t i n t o dlumina ( 3 ) . We
wish
supported i s their one
is
to
report
catalysts. ability
the
dispersion,
One
a similar
of
the
procedure
advdntages
of
can be used
these
of
changing
the
support
metal-support
interactions
nitriles
chosen
and
and the
to
supported
t o s e t t l e very e a s i l y from the l i q u i d phase,
possibility the
that
therefore acid-base
prepare
catalysts
and another the metal properties
o f the c a t a l y s t . Long
chain
were
r e d u c t i o n leads t o primary,
as
model
compounds
because
secondary or t e r t i a r y f a t t y amines.
their
I t i s thus
i n t e r e s t i n g t o study the s e l e c t i v i t y o f the c a t a l y s t s towards t h e production o f one c l a s s o f these compounds.
EXPERIMENTAL 11 C a t a l y s t p r e p a r a t i o n In
d
GFSC 59,
t y p i c a l experiment,
d
sample o f powdered alumind (Rhone Poulenc
p a r t i c l e s average didmeter 0.1
temperdture d u r i n g 7 hours.
mm)
i s d r i e d under vacuum
I t i s then trdnsferred,
i n t h e r e a c t i o n f l a s k dnd 120 m l o f d r y benzene d r e added. are
introduced
tion dS
with
the
the
slowly
while
surfdce
OH groups
dischdrge
O f
the
Stops,
gdS
suspension
i s
room
dt
under dn A r atmosphere
cooled
occurs
immedidtety
(ethdne,
ethylene)
0°C.
dt
and d
2.5
m l o f A1Et3
The
redc-
sooti
ds
Solution
O f
dry
CO
( a c a c ) 2 i n benzene ( l g Co and 100 m l benzene) i s added t o t h e s l u r r y which t u r n s t o b l a c k immediately.
This
reaction i s
followed
by
the d d d i t i o n o f
2.59 o f b u t a d i e n e i n 10 m i o f benzene. The s l u r r y i s t h e n t r d n s f e r r e d i n t o t h e h y d r o g e n a t i o n r e a c t o r by medns of
dn
dir-tight
syringe.
During
all
these
operations
the
apparatus
are
m d i n t d i n e d under an dtmosphere o f i n e r t gas ( A r ) . This
gds
i s t h e n r e p l d c e d by
stream o f
d
synthesis
gas
60 m l o f dodecane a r e added and t h e benzene is d i s t i l l e d . i s progressively
i n c r e a s e d (12OC per h o u r ) up t o 190°C and as soon
p r o d u c t s o f t h e r e d u c t i o n o f CO appear the synthesis
(CO:H2=1:2),
The temperature the
ds
i n t h e gas phase ( m a i n l y CH4)
gas i s r e p l d c e d by p u r e hydrogen dnd t h e s l u r r y
i s kept
d t
t h i s temperature d u r i n g 15 hours. A f t e r t h i s pretreatment,
t h e temperdture i s b r o u g h t down t o t h e d e s i r e d
experimentdl value. The sdme procedure
Cu)
dispersed
supports
:
on
the
i s used f o r
same
Silicd-Alumina
alumina
the prepdrdtion o f dnd
(KET3EN LA
of
3P),
cobalt zinc
other metals
(Ni,
d i s p e r s e d on d i f f e r e n t
oxide
(CRAM)
dnd
cdrbon
(LONZA-HSAG3OO). 21 C d t d l y t i c h y d r o g e n a t i o n
All
t h e r e a c t i o n s were C a r r i e d o u t
dtmospheric p r e s s u r e w i t h
d
r a n g i n g f r o m 50°C t o 120°C i n dn Activity
in
d
250 m l s t a t i c r e d c t o r under
c o n t i n u o u s f l o w o f hydrogen and a t temperatures apparatus d e s c r i b e d p r e v i o u s l y ( 3 ) .
dnd s e l e c t i v i t y v a l u e s
were o b t a i n e d by gds phase chromdto-
graphy a n a l y s i s o f t h e s o l u t i o n on a Cp S i l 5 c a p i l l a r y column.
689 RESULTS The in
catalytic
hydrogenation of
l i q u i d phase on v a r i o u s
long
catalysts
chdin
i n order
nitriles
was
to
their
check
dnd t h e i r s e l e c t i v i t y towards t h e p r o d u c t i o n o f m i n e s .
out
activity
Besides t h e fundd-
m e n t a l aspect o f t h i s r e d c t i o n i t i s w o r t h w h i l e t o o b t a i n t i v e f o r one c l a s s o f m i n e ( p r i m a r y ,
cdrried
d
c d t d l y s t selec-
seconddry o r t e r t i a r y ) because these
compounds a r e i n v o l v e d i n t h e p r e p a r a t i o n o f v a l u d b l e p r o d u c t s .
11 E f f e c t o f t h e method o f p r e p d r d t i o n .
2 the
I n the figure
r e s u l t s o f t h e r e d u c t i o n o f CI1HZ3CN a r e r e p o r t e d
f o r two c o b d l t c a t a l y s t s a t 12Ooc. One i s prepared by a c o n v e n t i o n a l method which i n v o l v e s t h e i m p r e g n a t i o n o f cobdlt n i t r a t e
followed
by
d
an alumina s u p p o r t
with a solution o f
r e d u c t i o n w i t h hydrogen a t 400OC.
The o t h e r
i s p r e p d r e d as p r e v i o u s l y d e s c r i b e d .
80-
60..
40.-
20-
5
5
10
15
Time (hrs)
Fig. 1 Our
catalyst
i s less active f o r
b u t i t i s more s e l e c t i v e
for
the production o f t h e primary m i n e ,
t h e p r o d u c t i o n o f t h e seconddry m i n e .
f i r s t r e s u l t shows t h a t t h e c o n v e n t i o n a l c a t a l y s t i s the
other
type
of
cobalt
catalyst
is
less
d
This
good r e d u c i n g dgent ;
active
for
the r e d u c t i o n
690
of t h e -CZN
t r i p l e bond b u t more e f f i c i e n t f o r t h e r e a c t i o n o f t h e i m i n e
w i t h t h e p r i m a r y m i n e R-CH2NH2
R-CH=NH
(R-CH2)2NH.
Morever
the
tertiary
which p r o d u c e s t h e s e c o n d d r y amine
amine i s n e v e r o b s e r v e d w i t h t h i s l a s t
CdtdlySt.
2/ E f f e c t of t h e s u p p o r t . The method p r e v i o u s l y d e s c r i b e d f o r t h e s u r f a c e r e d u c t i o n o f C o ( a c a c ) 2 was u s e d f o r t h e p r e p a r a t i o n o f v a r i o u s s u p p o r t e d c a t a l y s t s .
In the figure 2,
t h e i r s e l e c t i v i t y towards t h e production of t h e secondary m i n e is p l o t t e d v s t h e time o f r e a c t i o n .
These a r e p r a c t i c a l l y t h e same a t 12OoC, e x c e p t
f o r t h e one which i s p r e p d r e d on ZnO. I t i s i n t e r e s t i n g t o n o t e t h a t t h i s
i s a l s o t h e less a c i d i c
support
one and
t h e r e f o r e is not t h e b e s t
for
t h e r e a c t i o n between t h e i m i n e and t h e p r i m a r y dmine.
Y
1
% (R-CH,),NH (R-CH,),NH %
loo~L
- - = r/ec+ - N i >
Co),
nevertheless
nickel catalysts
dn
(see
important tdble
R NH s e l e c t i v i t y 2
1).Then
i t appears
i s rdpidly
obtained w i t h
t h d t hydrogendtion proper-
t i e s o f m e t d l s a r e m o d i f i e d by a l u m i n a s p e c i e s formed d u r i n g syngds d c t i v d t i o n step. the
The p o s i t i o n dnd t h e l o c d l d e n s i t y o f such s p e c i e s depend upon
initial
hydroxyl
groups
strength
dnd
repdrtition.
According
pdthwdy proposed i n i t i a l l y b y BRAUN ( 8 ) dnd s t u d i e d more r e c e n t l y by GREENFIELD
(Y), BAIKER ( 1 0 ) .
to
the
RCN
HZ
H2
RCH = NH
f
RCH = NH + RCH2NH2 . .
>
~
30 t r e a t m e n t which l e v e l l e d o u t f o r l o n g e r t r e a t m e n t t i m e s min).
iII.3.3. INFLUENCE OF OXIDATIOK TREXTYENTS 111.3.3.1. H e a t o x i d a t i o n It s h o u l d f i r s t be n o t e d t h a t t h e d r y i n g c o n d i t i o n s modify t h e parameters s t u d i e d . D r y i n g i n a m b i e n t a i r l e a d s t o q u i t e d i f f e r e n c e s i n c e r t a i n parameters compared t o t h o s e o b t a i n e d a f t e r in air. A l s o , o x i d a t i o n induced v a r i a t i o n s of drying a t 90' t h i c k n e s s , which, from a c e r t a i n t e m p e r a t u r e , a l s o d e p e n d e d on the treatment t i m e ( 9 ) . oxidation treatWithin t h e l i m i t s of t h e conditions used, ments d i d n o t erase t h e f r a c t a l c h a r a c t e r i s t i c o f t h e m a t e r i a l . The f r a c t a l d i m e n s i o n , however, - d e t e r m i n e d from t h e impedance
739 diagrams - was modified. 'Table I11 gives the values obtained for the thermal treatment in air at various times. TABLE 111. Influence of the heat treatment at 400'C in air on the fractal dimensions of the conversion coatings. 2.20 f 0.015
2233
* 0.015
Concerning the porosity, the impedance diagrams show a sphericalization of pore shape : the linear very high frequency range - when it exits - tends towards a pseudo arc. In general, heat treatment in air brings about moreclosed pore shapes. The geometrical characteristics were evaluated in -certain cases. Concerning the specific surface area measurements carried out by the BET method on conversion layers oxidized by heat at 4OO'C in the air show that lower specific surface areas are obtained. For example, for coatings on austenic steel it is about half : 170 m 2 / m z for oxidized layers against 360 m 2 / m 2 f o r refere-cce layers. 111.3.3.2 Chemical oxidation : layers on austenitic steel The thickness of the layers does not vary appreciably. This is not true for the fractal dimension which vary appreciably with the duration of treatment. The appearance of the very high frequency impedance diagrams shows that the treatment greatly modifies the porosity of the coating. It seems that the pore shape changes f r o m closed to more open ( cylindrical pores) during the fi,rst minutes ot the treatment but drying in the oven then causes a reverse effect. The specific surface area is seen to be much greater for the oxidized layers compared to the original coatinz. References 1 L.ARIES and J.P.TRAVERSE, Fr. Pat. n"86.18124 (1986) pat. n'88.08102 (1988) 2 L.ARIES and J.P.TRAVERSE, ~ r . PCT/FR 11-89 00295 3 L.ARIES, P.FORT, J.A.FLORES and J.P.TRAVERSE S o l . Energy Materials, 1 4 , (1986), 143-159 4 L.ARIES, D.FRAYSSE, R.CALSOU and J.P. TRAVERSE Thin Solid Fi., 151, (1987), 413-128 5 L.ARIES, R.CALSOC', J.A. FLORES and J.P. TRAVERSE J.X?icrosc.Spectrosc. Electron. 11,(1989), 41-53 ( 6 ) L.ARIES, J.ROY and J.P.TRAVERSE Interfinish PIRIS 1988. Proc. vol I1 713-720 ( 7 ) R.S.COLE, R.H.COLE, J.Chem. Phy, 9 , 1941, 311-351 ( 8 ) X.LE YEHAUTE, G.CREPY, Solid State Ionics, 9-10,17,1983 (9) L.I\RIES,Y.EL BAKKOURI, J.ROY and J.P. TRAVERSE, R CALSOU and R.SEMPERE. Thin Solid Fi., to be published.
This Page Intentionally Left Blank
741
AUTHOR INDEX Absi-Halabi M. Ai M. Alvarez W. Al-Zaid H. Anthony R.G. Aries L. Auroux A. Baiker A. Barrault J. Basset J.M. Beguin F. Bellussi G.
155 653
77
155 637 729 345 59,239 329,479, 687 717 479 42 1
Daza L. Decleer J.
537 185
Dedeycker 0. de Jong K.P. Dekker J.G. Delmon B.
337 19 205 537
Delva A. Derouault A. Dessalces G. Di Castro V. Didillon B.
185 687
Dosch R.G. Duprez D.
95 717 637 617
Durupty M.C. Dziewiecki Z.
269
El Mansour A: Erre R. Escudey M. Esposito A.
717 479 279 42 1 337 591
7 17
Farfan-Torres E.M. Fasman A.B. Fellmann J.D.
557
Fenelonov V.B.
Belousov V.M.
497
Bergaya F. Biay I. Blanc B. Blanchard M. B.Nagy J. Bodnir Zs. Bogdanchikova N.E. Bolt P.H. Bonnier J.M. Bournonville J.P. Brooks C.S.
329 1 469 687 705 4.59 647 165 60 1
Fenoglio R.J. Candy J.P. Carati A. Chafik A. Chang Liu Chapple A.P. Chary K.V.R. Claerbout A.
1
Clerici M.G. Cooper M.D.
717 42 1 479 145 407 61 1 705 42 1 247
Damon J.P. da Silva Jr A.F.
601 123
113
247 503 77 185 345
Ferment J. Figueras F. Flores A. Foresti E. Fouilloux P. Frety R. Fuertes A.B.
49 469 123 439
Galiasso R. Gargano M. Gatineau L. Gazzano M.
37 95 329 49
279
742
Genoni F. Geus J.W. Gil-Llarnbias F. Gobolos S. Golubkova G.V. Gosling K. Goupil D. Goyvaerts D. Grange P. Grobet P.J. Groen G. Groeneveld M.J. Gros J. Guaregua J. Gui Linlin Haber J. Hamar-Thibault S. Handy B. Haruta M. Hassoun N. Hegediis M. Hiramatsu Y. Hoang-Van C. Hypolite C. lmai H. Imanaka T. Ismagilov Z.R. Ivanov E.Yu. Jackson S.D. Jacobs P.A. Ji Weijie Joud J.C. Kachi N. Kaddouri A. Kalinina O.T. Kalucki K.
Ladavos A.K. Lamers M.D.A. Lapina O.B. Leofanti G. Le Peltier F. Likholobov V.A. Lintz H.-G. Li Shuben Li Yongdan Lisitsyn A.S. Liu Yingjun Lycourghiotis A.
319 527 507 43 1 717 449 547 5 17 145 449 69 175
Mahamud M. Mallk T. Mans0urS.A.A. Margitfalvi J.L. Marsden C.E. Martens J.A. Martin G.A.
439 459 617 669 215 355, 381 269
Kanai J. Kanta Rao P. Kappenstein C. Kawai M. Kawata N. Keegan M.B.T. Ketterling A.A. Kiennemann A. Kikuchi E. King D.L. Knijff L.M. Kobayashi T. Koeppel R.A. Kolenda F.
743
Martin Luengo-Yates M.A. Masson J. Mastikhin V.M. Masuda K. Matsuda T. McLellan G.D. Meheux P.A. Messaoudi A. Mikhailenko S.D. Mirodatos C. Mizukami F. Morawski A.W. Mouaddib N. Mountassir Z . Moya S.A. Moyes R.B. Muiioz-Paez A. Munuera G. Nakahara Y. Nitta Y. Nohman A.K.H. Nuiiez G.M.
Padovan M. Pajares J.A. Pama J.B. PCrez A.J. Perrichon V. Petrini G. Petr6 J. Pichat P. Piemontese M. Pis J.J. Poels E.K. Poix P.
43 1 439 439 439 269 43 1 459 679 49 439 205 575
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Pommier B. Pomonis P.J. Prada Silvy R. Prasad V.V.D.N. Pyatnitskaya A.I. Ravasio N. Rawlence D.J. Rehspringer J.L. Resasco D.E. Reymond J.P. Richard D. Romero Y. Rossi M. Ruiz P.
679 319 37 61 1 497 95 407 575 77 1 469 37 95 537
Sermon P.A. Sham E. Shen Shikong Shepeleva M.N. Shkrabina R.A. Simonov P.A. Sobalik 2. Somasekhara Rao K. Spanos N. Staal L.H. Stanislaus A. Sterte J. Stoch J. Szab6 S.
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567 69 567 345 729 647 43 1 49 695 547
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49 527 165,527 205 527 165
Wells P.B. Weng L.T. Whyman R. Willis J. Wokaun A.
239 517 87 135 135 537 135 135 59,239
Xiong Y.L.
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Yamaguchi T. Yasse B.
567 537
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745
STUDIES IN SURFACE SCIENCE AND CATALYSIS Advisory Editors: B. Delmon, Universitb Catholique de Louvain, Louvain-la-Neuve,Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1 Preparation of Catalysts I . Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 1417, 1975 edited by 6. Delmon. P.A. Jacobs and G. Poncelet Volume 2 The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Volume 3 Preparation of Catalysts II. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, P. Grange, P. Jacobs and G. Poncelet Volume 4 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci6t6 de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Volume 5 Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9- 1 1,1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Volume 6 Catalyst Deactivation. Proceedings of an InternationalSymposium, Antwerp, October 13- 75,1980 edited by B. Delmon and G.F. Froment Volume 7 New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July 4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Volume 8 Catalysis by Supported Complexes by Yu.1. Yermakov, B.N. Kuznetsovand V.A. Zakharov Volume 9 Physics of Solid Surfaces. Proceedings of a Symposium, Bechyiie, September 29October 3, 1980 edited by M. UzniEka Volume 10 Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an InternationalSvmDosium. Aix-en-Provence. SeDtember 2 1-23, 198 1 edited by J. Rouqueroland K.S.W. Sing Volume 1 Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16. 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Volume 2 Metal Microstructures in Zeolites. Preparation - Properties - Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. J i r G and G. Schulz-Ekloff Volume 13 Adsorption on Metal Surfaces.An Integrated Approach edited by J. Benard Volume 14 Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz
746 Volume 15 Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Volume 16 Preparation of Catalysts 111. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third InternationalSymposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Volume 17 Spillover of Adsorbed Species. Proceedings of an International Symposium, LyonVilleurbanne, September 12- 16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Volume 18 Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P. Jirir, V.B. Kazansky and G. Schulz-Ekloff Volume 19 Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Volume 20 Catalysis by Acids and Bases. Proceedings of an InternationalSymposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Volume 2 1 Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Volume 22 Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Volume 23 Physics of Solid Surfaces 1984 edited by J. Koukal Volume 24 Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoroi-Portorose, September 3-8, 1984 edited by B. Drfaj, S. Hotevar and S. Pejovnik Volume 25 Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K.Soga Volume 26 Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15- 19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Volume 27 Catalytic Hydrogenation edited by L. Cervenq Volume 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakarni, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Knozinger Volume 3 0 Catalvsis and Automotive Pollution Control. Proceedinas of the First International Symposium, Brussels, September 8-1 1,1986 edited by A. Crucq and A. Frennet Volume 3 1 Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth InternationalSymposium, Louvain-la-Neuve, September 1 4 , 1 9 8 6 edited by B. Delmon. P. Grange, P.A. Jacobs and G. Poncelet Volume 3 2 Thin Metal Films and Gas Chemisorption edited by P. Wissmann Volume 33 Synthesis of High-silica Aluminosilicate Zeolites by P.A. Jacobs and J.A. Martens Volume 3 4 Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment
.,
Volume 35 Keynotes in Energy-Related Catalysis edited by S. Kaliaguine Volume 36 Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Volume 37 Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Volume 38 Catalysis 1987. Proceedings of the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Volume 39 Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29, 1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Volume 40 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-1 1, 1987 edited by J. Koukal Volume 4 1 Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15- 17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule. D. Duprez, C. Montassier and G. Perot Volume 42 Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel. revised and edited by 2. Pa61 Volume 43 Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Volume 4 4 Successful Design of Catalysts. Future Requirements and Development. Proceedings of the Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. lnui Volume 45 Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Volume 46 Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wurzburg, September 48,1988 edited by H.G. Karge and J. Weitkamp Volume 47 Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Volume 48 Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13- 16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Volume 49 Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference. Amsterdam, July 10- 14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Volume 50 Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AlChE Meeting, Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G. Anthony Volume 5 1 N e w Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Volume 52 Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-1 9, 1989 edited by J. Klinowski and P.J. Barrie Volume 53 Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm. S. Akashah, M. Absi-Halabi and A. Bishara
748 Volume 54 Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura Volume 55 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F. Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin PolymerizationCatalysts, Tokyo, October 23-25, 1989 edited by T. Kelli and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 578 Spectroscopic Analysis of HeterogeneousCatalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction t o Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-5, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 6 0 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Volume 6 1 Natural Gas Conversion. Proceedings of the Natural Gas Conversion Symposium, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPS II),Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso,J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of Catalysts V. Proceedings of the Fifth International Symposium on the Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-laNeuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon
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Preparationof Catalysts I, II, 111 and IV Scientific Bases for the Preparationof Heterogeneous Catalysts Preparation of Catalysts I Proceedingsof the International Symposium, Brussels, Belgium, October 14-17,1975 editedby 8. Delmon, P.A. Jacobs andG. Poncelet Studies in Surface Science and Catalysis, Vol. 1 1976 3rd repr. 1987 xvi + 706 pages ISBN 0-444-41428-2 "...very useful and full of latest information on preparation of Catalysts. Technical Books Review
Preparation of Catalysts II Proceedings of the 2nd International Symposium, Louvain-la-Neuve, September 4-7, 1978 editedby B. Delmon, P. Grange, P. Jacobs andG. Poncelet Studies in Surface Science and Catalysis, Vol. 3 1979 2nd repr. 1987 iv + 762 pages ISBN 0-444-41733-8
Preparation of Catalysts 111 Proceedingsof the 3rd InternationalSymposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Studies in Surface Science and Catalysis, Vol. 16 1983 xvi + 854 pages 0-444-42184-X "...essential reading for anyone concerned with the preparation or investigationof catalysts. It is well up to the high standard set by earlier volumes in this series and is likely to be a useful source of information for many readers. " Applied Catalysis
Preparation of Catalysts IV Proceedingsof the 4th International Symposium, Louvain-la-Neuve, September 1-4,1986 edited by 8. Delmon, P.Grange, PA. Jacobs and G. Poncelet Studies in Surface Science and Catalysis, Vol. 31 1987 xviii + 868 pages ISBN 0-444-41428-2
