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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.
~~
<-
I
0.05
I
3500 2e (degree.7) Fig. 3. X-my diffraction patterns (Cu,) of surnples n ) ZrO, , b) Zr02-770K, c) SS, rl) S5-970K rind e) S5 rrfer reriction.
3000
2500
2000
IS00
ZOO
wavenritiiber (cm ') Fig. 4. Diffuse reflectance FTIR spectra of sntnple S7, reaction of CO, nn H2 (1.3) nt 47.1 K ond 0.25 MPn.
66
Prevalent surface species Diffuse reflectance F I I R spectroscopy was used to investigate the species prevailing on the catalyst surface under CO, hydrogenation conditions at 2.5 bar. Before the FTIR measurements, the catalysts were heated in a hydrogen flow to the desired reaction temperature. The measurements were started by switching from the hydrogen flow to a defined flow of CO, and H,. Subsequently the surface reactions were studied by recording time-resolved FI'IR spectra as shown in Fig. 4. The spectra indicate that surface formate is readily formed after the reactant feed has been switched on. After 10 minutes peaks grow at 1150, 1050 and around 2900 cm-' reflecting the formation of surface-bound formaldehyde, methylate and methanol species. The peaks due to methanol are enhanced if the flow is stopped and the surface is exposed to static conditions. Methanol formation is paralled by the observation of gas phase CO originating from the reverse water gas shift reaction. It is noteworthy to mention that in all FTIR measurements performed with either CO,/H, or CO/H, no conclusive correlation between the disappearance of the prevalent formate species and the formation of methanol could be observed. On the other hand, the appearance of gas phase methanol was always associated with the observation of some CO, as well as formaldehyde and methylate surface species (ref. 20). DISCUSSION An importand aspect of the work concerns the nature of the zirconia phase produced and its influence on the catalytic behaviour. The occurence of ethanol species in the initial stage qf rraction under methanol synthesis conditions as well as the DSC and TPR data indicate the formation of anion modified oxides from acetate precursors as proposed earlier by Yurieva (ref. 23) for other oxides. We conclude from measurements with sequentially precipitated catalysts (ex zirconyl acetate/copper nitrate) and from the occurence of cupric oxide in the preparation step that oxygen ions in the zirconia are partially substituted by acetate ions. It is interesting to note that calcination of copper containing catalysts results in the formation of metastable tetragonal zirconia, while calcination of pure zirconia leads to the formation of stable monoclinic ZrO,. Although, an oxidative thermal treatment (DSC under air) shows a marked influence on the crystallization temperature of the amorphous zirconia, under reducing conditions (TF'R) the same crystallization temperature is observed for both zirconia as well as copper containing samples. These results confirm that probably copperions are responsible for the stabilizing effect onto amorphous zirconia and that the transformation of these ions into metallic copper eliminates this effect. The activity measurements (TOF values) summarized in Table 2 indicate that the intrinsic activity of the catalysts decreases with increasing copper content. An increase in CO, conversion is accompanied by a concomitant decrease in methanol selectivity. Calcination of samples S5 and H at 923 and 823 K, respectively, resulted in the transformation of the initially amorphous zirconia to crystalline tetragonal zirconia. The most striking effect of the crystallization process was the collaps of the activity and selectivity of the catalysts which was accompanied by a simultaneous decrease in the BET-surface area. It is interesting to note that
67
TABLE 2 Catalytic properties of the catalysts Catalyst Precursor"
Sl s2 s3 s4 s5 s7 S5-623 S5-923 S5N C5 H ff-823 A (b)
(a) ac = acetate, ni = nitrate. (b) zirconia was substituted by alumina. in the case of sample H the copper surface area remains constant, however the intrinsic activity decreases. As sample A (Cu/Al,O,) had about the same copper surface area as the zirconia based catalysts, we may conclude that the copper surface area alone cannot explain the catalytic behaviour of the catalysts. High activity and selectivity are related to the presence of amorphous zirconia. Owen et a1 (ref. 24) proposed that oxygen anion vacancies characteristic of the fluorit structure of zirconia could be important in the methanol synthesis reaction. The presence of anions like acetate, formate or nitrate in the precursors could result in a distortion of the oxide structure of zirconia and their removal may generate vacancies in the anion lattice which are known to stabilize the cubic structure of zirconia. The crystallization of the amorphous zirconia is likely to result in a drastic decrease of the copper/zirconia interfacial area which certainIy contributes to the loss of activity observed upon crystallization. Our investigations provide further support for the crucial role of the interfacial area in copper/zircania catalysts. Further work focusing on the structural and chemical properties of this interphase and its role in methanol synthesis is presently undertaken . As to the surface species observed by in situ FTIR measurements during methanol synthesis, the most striking result is that the surface concentration of the prevalent formate species seems not to be directly influenced by methanol formation. This behaviour will be discussed in detail elsewhere (ref. 20).
CONCLUSIONS The present studies confirm that highly active and selective Cu/ZrO, catalysts can be
68
prepared by precipitation. The catalytic behaviour of these catalysts depends strongly on the structural and chemical properties of the zirconia support. High activity and selectivity are related to the presence of amorphous zirconia which is stabilized by copper ions. Crystallization of the amorphous zirconia by calcination in air at appropriate temperatures results in the formation of metastable tetragonal zirconia and leads to a drastic decrease of the catalytic activity and selectivity. The results indicate that the structural and chemical properties of the support and particularly of its interface with the copper-species play an important role in methanol synthesis from CO,/H,. On all catalysts surface formate was found as an abundant surface species. However, the appearance of methanol is not correlated with the disappearance of formate, but with a decrease in surface formaldehyde and methylate signals. ACKNOWLEDGEMENTS This work has been supported by grants of the "Swiss National Science Foundation" (2.102086), the "Bundesamt f i r Bildung und Wissenschaft" and the "Deutsche Forschungsgemeinschaft" (SFB 213). REFERENCES 1 J.C.J. Bart and R P A . Sneeden, Catal. Today, 2 (1987) 1-124. 2 G.C. Chinchen, P.J. Denny, J.R Jennings, M.S. Spencer and KC. Waugh, Appl. Catal., 36 (1988) 1-65. 3 B. Denfie and R P A . Sneeden, Appl. Catal., 28 (1986) 235-239. 4 Y.Amenomja, AppL Catal., 30 (1987) 57-68. 5 B. Pommier and S.J. Teichner, in M.J. Philips and M. Ternan (Eds.), Proc. 9th Int. Congr. Catal., Calgary, 1988, The Chemical Institute of Canada, Ottawa, 1988, Vo1.2, pp. 610-617. 6 Y.Amenomiya, I. T. Emesh, K W Oliver and G. Pleizier, in M.J. Philips and M. Ternan (Eds.), Proc. 9th Int. Congr. Catal., C a l g q , 1988, The Chemical Institute of Canada, Ottawa, 1988, Vo1.2, pp. 634-641. 7 G.J.J. Bartley and R Burch, Appl. Catal., 43 (1988) 141-153. 8 B. Denise, 0. Cheriji, M.M. Bettahar and R.P.A. Sneeden, Appl. Catal., 48 (1989) 365372. 9 D. Gasser and A. Baiker, Appl. Catal., 48 (1989) 279-294. 10 A.Ya Rozovskii, YaB. Kagan, G.I. Lin, E.K Slivinskii, S.M. Loktev, L.G. Liberov and A.N. Bashkirov, Kinet. Catal., 17 (1976) 1132-1138 (engl). I1 G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, D.A. whan, M.S. Spencer and X C . Waugh, Am. Chem. SOC.Div. Fuel Chem., 29 (1984) 178-188. 12 G.C. Chinchen, KC. Waugh and D.A. Whan, Appl. Catal., 25 (1986) 101-107. 13 L R Jennings and M.S. Spencer, in C. Morterra, A. Zecchina and G. Costa (Eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, pp. 515-524. 14 KS.W Sing, D.H. Everett, RA.W Haul, L. Moscou, RA.Pierotti, J. Rouquh-ol and T. Siemieniewska, Pure & Appl. Chem., 57 (1985) 603-619. 15 E.P. Barrett, L.S. Joyner and P.P. Halenda, J. Am. Chem. SOC.,73 (1951) 373-380. 16 G. Halsq, J. Chem. Phys., 16 (1948) 931-93% 17 B.E. Wmen, J. AppL Phys., 12 (1941) B75. 18 D. Monti and A. Baiker, J. Catal., 83 (1983) 323-335. 19 J.K Evans, M.S. Wainwright,A.J. Bridgewater and D.J. Young, Appl. Catal., 7 (1983) 7583. 20 Ch. Schild, A. Wokaun and A. Baiker, submitted for publication. 21 P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Butggraaf and J.RH. Ross, Appl. Catal., 57 (1990) 127-148. 22 B. K Lippens and J.H. de Boer, J. CataL, 4 (1965) 319-323. 23 T.M. Yurieva, React. Kinet. Catal. Lett., 29 (1985) 49-54. 24 G. Owen, C.M. H a w k s and D. Lloyd, AppL Catal., 58 (1990) 69-81.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation ofCata2ysts V 01991 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
PREPARATION OF Ti0,-Al,O,
69
BY IMPREGNATION WITH TiC1,-CCl,
Liu Yingjun, Zhang Qinpei, Zhu Yongfa, Gui Linlin and Tang Youqi Institute of Physical Chemistry, Department of Chemistry Peking University, Beijing 100871 (China)
ABSTRACT Catalyst carrier Ti0,-AI,O, was prepared through impregnation y-Al,O, with non-aqueous solution of TiCI, (e.g. carbon tetrachloride or acetone as solvent), followed by calcination at 550'c for 24 h. Series of Ti0,-AI,O, samples with various TiO, loadings have been characterized by XRD, XPS, TEM and HEED techniques. The maximum dispersion capacities for TiO, on y-Al,O, measured by XRD and XPS are 0.12g TiO,/ g y-AI,O, and 0.1 lg TiO, / g y-Al2O3, respectively. It was also verified by TEM and HEED techniques. This value illustrates that TiO, disperses on the surface of y-Al,O, as a submonolayer and the observed monolayer coverage for TiO, on y-Al,O, is 58% as compared with a close-packed monolayer model.
INTRODUCTION With its special properties, TiO, attracts more attention recently (refs. 1-2). Especially, to be used for hydrodesulfurization (HDS) or hydrodenitrogenation (HDN) in the petroleum refining process (refs. 3-4), the character of the catalyst with TiO, carrier is superior to that with y-Al,O, carrier. However, TiO, is seldom used as a catalyst carrier in commercial process. The reason is that as compared with the widely used industrial catalyst carrier y-AI,O, it has two disadvantages, one is that its specific surface area is rather small, usually of 10 mz / g and can only reach some tens mz / g prepared in special ways, the other is its strength rather poor with its mechanical strength five times less than that of y-Al,O,. In order to improve the properties of TiO, as a carrier many authors have tried several preparing methods for modifying y-Al,O, with TiO,, such as coprecipitation (ref.5), impregnation with aqueous solution (ref.6), mixed gelatinization (ref.7) and kneading process (ref.8). This research prepared monolayer dispersion type Ti0,-AI,O, carrier with y-Al,O, as a matrix impregnated with non-aqueous solution of TiCI, (e.g. carbon tetrachloride or acetone as solvent) and characterized it with XRD, XPS, TEM and HEED techniques.
70
EXPERIMENTAL Sample preparation y-Al,O, supplied by Changling Oil Refinery of Chinese Petro-Chemical Corporation, was ground down into small particles. The 40-80 mesh of y-Al,O, particles was calcinated in a muffle furnace at 650C for 4h. After calcination the specific surface area of y-Al,O, is 205m2/ g. The samples with various titanium oxide content of Ti0,-AI,O, carrier were prepared by the following procedures: impregnation of y-Al,O, with TiCI,-CCI, (or TiCI,-CH,COCH,) solution; giving off the solvent; neutralization with 1:1 ammonia liquor; washing with dilute ammonia solution until no CI- to be detected, then calcination at 550'c for 24 h. X-ray diffraction The XRD analysis, involving qualitative and quantitative analysis, have been carried out on a home made BD-86 diffractometer, with Cu Ku radiation, Ni filter and a scintillation couter. The residual crystalline TiO, after being dispersed on y-Al,O, can be measured by the matrix-flushing method (refs. 9-10) for quantitative X-ray diffraction analysis and calculated from the equation
ZI/ I ,
=kx, /x,
(1)
where I, is the X-ray intensity of the peak (101) for TiO,; I, is the X-ray intensity of the peak (200) for the flushing agent, KCl; x l and x , are the weight fractions of TiO, to the flushing agent, KCI, respectively; the constant k is the reference intensities of TiO, to KCl. In this case, the value of k is 0.99, calculated from the Equation(1). Specific surface area Specific surface areas of y-Al,O, and Ti0,-AI,O, desorption gas chromatography method.
were determined with thermal
XPS measurements All XPS spectra were acquired by using a VG ESCA LAB 5 Spectrometer equipped with a 4025 Data System. An aluminum anode (A1 Ku 1486.6eV) operated a t 12 kV and 40 mA was used. During recording spectrum, pressure inside the sample chamber was ca. 5 x 10-*m bar. Intensities of Ti2p and A12p photoelectron lines were measured by the peak area with 4025 Data System. Sample was well spread on one side of double-sided adhesive tape and fixed on the sample stub. The extent of carbon contamination on samples controlled to be nearly the same by monitoring the CIS peak area, and then the influence of carbon contamination can be neglected.
71
u
5 -m
0.40
4 0 A
N
m
i
Crd Z L
9
o'\
0.30
4 -
d
;2 2
N
m
0.20
a
0.10
0.00 0.00
0.10
Ti0
0.40
0.30
0.20 2
0.50
loading ( g / g Y - A 1 2 0 3 )
Fig.1 Dispersion threshold of TiOz on y-Al20, by XRD quantitative extrapolationmethod (prepared with TiC1,- CCI, solution)
N 3
d
Y \
3
0.20-
.3
c
l i
0.00
0.20
0.40
0.60
T i O Z loading ( g / g f - A 1 2 0 3 )
Fig.2 Relationship between ITizp/ IAnP and TiO, loading of Ti0,-AI,O, (prepared with TiC1,- CCI, solution)
72
TEM and HEED measurements Transmission Electron Microscopy (TEM) image and High Energy Electron Diffraction (HEED) pattern were carried out with JEM-200CX Electron Microscope operated at 200 kV.
RESULTS AND DISCUSSION XRD quantitative measuremental results The intensities of TiO, (101) peak of series of Ti0,-AI,O, samples were measured by XRD quantitatively. With residual amount of TiO, crystallite in samples after calcination as ordinate and with loading of TiO, as abscissa, plot has been got in Fig.1. The plot is a broken line consisting of two straight lines and shows a threshold value. Before the threshold TiO, disperses on the y-Al,O, surface as a monolayer and no TiO, peaks were observed. After the threshold, the residual amount of TiO, is in crystalline form on y-AI,O, surface. There is a linear relationship between the residual amount of TiO, crystallite and the added loading of TiO, on y-Al,O,. As impregnation y-Al,O, with TiCI,-CCI, solution, from Fig. I , the maximum dispersion has been obtained as 0.12g TiO, / g y-Al,O,. In the case of preparing through impregnation y-Al,O, with TiCI,-CH,COCH, solution, the maximum dispersion is 0.1 Ig TiO, / g y-Al,O,. The specific surface of y-Al,O, used is 205m2/ g. In terms of surface area, the dispersion threshold for TiO, on y-Al,O, is 5.6 x 104g TiO, / m2. We can estimate the utmost monolayer capacity by employing the simple close packed monolayer model. Assuming that 02-ions from TiO, form a close-packed layer on the surface of y-Al,O, and the Ti4+ ions occupy the interstices formed by 02-ions. Taking 1.40A as the radius of 02-ion, the surface area occupied by a TiOz "molecule" on the surface of y-A1203 can be calculated as 2 . 0 4 ~ 10-'9m2, in other words, forming each m2 close-packed monolayer of TiO, requires 9.7 x 104g TiO,, i.e. the monolayer capacity of TiO, on y-Al,O, should be 9.7 x 104g TiO,/ m2. Comparing the observed dispersion threshold for TiO, on y-Al,O,as mentioned above, thus the observed monolayer coverage for TiO, on y-Al,O, is 58%. This value illustrates that TiO, disperses on the surface of y-AI,O, as a part covered monolayer or a submonolayer, rather than as a full covered monolayer. XPS measuremental results The intensities ratio ITi2, / I,,,, of XPS for series of TiO, / y-Al,O, have been determined (refs. 11-12). The relationship between ITi2,/ I,,,, and TiO, loading is shown in Fig.2. The plot consists of two straight lines with different slope, and the amount of TiO, at intersection point happened to correspond with the value of monolayer dispersion threshold by XRD as mentioned above. When the TiO, loading is less than the threshold, the monolayer coverage increases with the increase of TiO, loading. Thus the intensity I,,, increases in proportion to the TiO, loading directly and the intensity I,,,, from matrix
73
Fig.3 TEM image of Ti02-A1203 Samples (prepared with TiCI,- CCI, solution) a. y-Al,03 b. Ti02 (Anatase) c. 0.061g Ti02/ g y-Al,O, d. 0.44g TiO, / g y-Al,O,
14
Fig.4 HEED patterns of Ti0,-AI,O, Samples (prepared with TiC1,- CC1,solution) a. y-A120, b. TiO, (Anatase) c. 0.06 1g TiO, / g y-Al,O, d. 0.44g Ti02/ g 9-A120,
75
y-Al,O,
remains unchanged essentially. As a result, intensity ratios ITiZp / IAIZP increase
linearly with the increase of Ti02 1oading.When the TiO, loading is higher than the threshold, the residual crystalline TiO, appears and resides on the top layer of the sample. / I,,,, increases with the increase of TiO, loading. Since XPS is Also the intensity ratio ITiZp a surface sensitive technique, the photoelecton contribution to the peak of Ti2p depends on the inelastic mean free path. Therefore the intensity contributed by crystalline TiO, is much less than that by monolayer TiO, on the surface of y-Al,O, at the same amount. Although two lines from two sources in plot are present in linear way, the slopes of them are quite different. In Fig.2 the loading amount of TiO, at intersecting point of two straight lines corresponds to the threshold of monolayer dispersion, 0.1 l g TiO, / g y-Al,O,. The threshold a o cords with the XRD quantitative measurement by extrapolation. The result shows that both methods, XPS intensity ratio method and XRD quantitative extrapolation method, are complementary to each other in determining the dispersion and studing the surface state of TiO, / y-Al,O,. TEM and HEED measuremental results The results of XRD quantitative measurement and XPS measurement were also verified by TEM and HEED techniques. Fig.3 and Fig.4 only show the results for samples prepared with TiC14-CCI, solution omitting similar results with TiCI,-CH,COCH, solution. As the loading amount of TiO, on y-Al,O, is less than the dispersion threshold, the TEM image (see Fig.3~)and the HEED pattern (see Fig.&) show just like that of matrix y-A1,0, (see Fig.3a and Fig.4a). As the loading amount of TiO, on y-Al,O, is more than the dipsersion threshold, the TEM image (see Fig.3d) and the HEED pattern (see Fig.4d) show like that of y-Al,O, and additional crystalline TiO,.
CONCLUSIONS The results mentioned above have illustrated that as the samples of Ti0,-Al,O,carrier were prepared with impregnation of TiCI, nonaqueous solution (carbon tetrachloride or acetone as solvent), the maximum dispersion capacities for TiO, on y-Al,O, is 5.6 x 104g TiO,/ mz with coverage of 58%, and TiO, with molecule^^ disperses on y-Al,O, surface in form of a submonolayer, rather than as a full covered monolayer. It is different from the preparation with impregnation of Ti(SO,), aqueous solution. In this case strong acidity of Ti(SOJ, solution (PH 1-2) would cause y-Al,O, partially solubilizing during impregnation and then depositing with Ti4+ on y-Al,O, to form Ti0,-AI,O, mixed gelatination. Thus, the method described in this paper may be employed as a method for monolyer modification of y-Al,O, with TiO,.
76
ACKNOWLEDGMENT The authors are grateful to the National Laboratory for Structural Chemistry of Unstable and Stable Species (Beijing, China) for financial support.
REFERENCES 1 S.J. Tauster, S.C. FungandR.L. Garten, J.A.C.S., 1(1978), 170-175 2 R.T.K. Baker, E.B. Prestridge and R.L. Garten, J.Catal., 56(1979), 390-406 3 M. Takeuchi, S. Matsuda, H. Okada, H. Kawagoshi, F. Nakajima, Ger Offen 2838231 (1979) 4 H.B. Jones, and R. Smith, US Pat 4206038 (1980) 5 E. Rodenas, T. Yamaguchi, H. Hattori and K. Tanabe, J. Catal., 69(1981), 434-444 6 G.B. McVicker, and J.J. Ziemiak, J. Catal., 95(1985), 473-481 7 Zhu Yongfa, Gui Linlin and Tang Youqi, CUIHUA XUEBAO (Journal of Catalysis. China), 10(1989), 118-122 8 Liu Yingjun, Luo Shengcheng, Gui Linlin, Annual Report of National Laboratory for Structural Chemistry of Unstable and Stable Species (Beijing, China), 1987, pp.61 9 F.H. Chung, J.Appl.Crst., 7(1974), 526-531 10 Liu Yingjun, Xie Youchang, Ming Jing, Liu Jun and Tang Youqi, CUIHUA XUEBAO (Journal of Catalysis, China), 3(1982), 262-267 11 S.C. Fung, J.Catal., 58(1979), 454-469 12 Gui Linlin, Liu Yingjun Guo Qinlin, Huang Huizhong and Tang Youqi, Scientia Sinica (Series B), Chinese Ed., No.6 (1985), 509-517; English Ed., 28(1985), 1233-1242
77
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
INTERACTIONS OF THE IMPREGNATING SOLUTION WITH THE SUPPORT DURING THE PREPARATION OF Rh/Ti02 CATALYSTS
R.J. FENOGLIO, W . ALVAREZ, G . M . N U N E Z , INTEHA ( U n i v e r s i d a d N a c i o n a l d e Mar
J u a n B. J u s t o 4302, ( 7 6 0 0 ) Har d e l P l a t a , A r g e n t i n a
D . E . RESASCO d e l Plata-CONICET)
SUHHARY W e have a n a l y z e d t h e d i f f e r e n t phenomena o c c u r r i n g d u r i n g t h e impregnation of t i t a n i a w i t h aqueous s o l u t i o n s of rhodium chloride. Our r e s u l t s d e n o n s t r a t e t h a t t h e d e p o s i t i o n of t h e Rh p r e c u r s o r s on t i t a n i a , a t low pH v a l u e s ( a b o u t 3), i n v o l v e s t h r e e d i f f e r e n t p r o c e s s e s : a ) a d s o r p t i o n o f a n i o n i c species, b ) l i g a n d e x c h a n g e r e a c t i o n and c ) a c i d a t t a c k o f t h e s u p p o r t .
INTRODUCTION The i n t e r a c t i o n s
taking
place
between
the
metal
precursor
s p e c i e s and t h e s u p p o r t d u r i n g i m p r e g n a t i o n p r o c e s s e s , may p l a y decisive role in catalysts.
determining
the
resulting
properties
of
a the
I n t h e case of t h e i m p r e g n a t i o n of o x i d e s u p p o r t s w i t h
a q u e o u s s o l u t i o n s c o n t a i n i n g t h e metal p r e c u r s o r s , w e c a n i d e n t i f y
we c a n mention t h e a d s o r p t i o n o f m e t a l p r e c u r s o r species on t h e o x i d e
s e v e r a l p o s s i b l e t y p e s o f i n t e r a c t i o n s ( r e f . 1 and 2).
First,
s u p p o r t ( r e f . 3), which h a s b e e n a w i d e l y i n v e s t i g a t e d phenomenon. W e c a n a l s o c o n s i d e r t h e p o s s i b i l i t y t h a t t h e a d s o r b e d s p e c i e s may undergo a s u b s e q u e n t s u r f a c e r e a c t i o n w i t h t h e s u p p o r t ( r e f . 4 ) . F i n a l l y , when t h e i m p r e g n a t i n g s o l u t i o n is a c i d i c , t h e r e e x i s t s t h e p o s s i b i l i t y t h a t t h e s u r f a c e of t h e s u p p o r t be a t t a c k e d by t h e This acid impregnating s o l u t i o n c a u s i n g a partial d i s s o l u t i o n . a t t a c k w a s f i r s t p r o p o s e d by S a n t a c e s a r i a e t a l . ( r e f . 5 ) f o r t h e i m p r e g n a t i o n of H2PtC16 on A 1 2 0 3 . More r e c e n t l y , i t h a s a l s o been
I t h a s been s u g g e s t e d t h a t t h e acid a t t a c k m a y n o t only involve t h e partial d i s s o l u t i o n found t o o c c u r on o t h e r systems ( r e f . 6 ) . of t h e s u p p o r t , b u t a l s o
a
T h i s , i n turn,may r e s u l t
in
subsequent
p r e c u r s o r s b y s u p p o r t e d species.
a
re-deposition
partial
These
coverage
ideas
may
a s s o c i a t e d w i t h t h e w e l l known SMSI e f f e c t ( r e f .
of be 8),
7). metal immediately (ref.
the
which
also
78
i n v o l v e s a p a r t i a l c o v e r a g e o f t h e m e t a l by t h e s u p p o r t .
However,
w h i l e t h e SMSI e f f e c t r e q u i r e s a h i g h t e m p e r a t u r e r e d u c t i o n s t e p t o become e v i d e n t , an a c i d a t t a c k t o t h e s u p p o r t f o l l o w e d by r e - d e p o s i t i o n m a y have s i m i l a r e f f e c t s a f t e r low t e m p e r a t u r e reduction. In t h i s contribution we w i l l
consider
these
particular
case
of
p r e p a r e d by i m p r e g n a t i o n of T i 0 2
with
rhodium
interactions for the
of
three
types
Rh/TiOp
of
catalysts
chloride
species
from a q u e o u s s o l u t i o n s . EXPERIMENTAL To s t u d y t h e phenomena a s s o c i a t e d w i t h t h e i m p r e g n a t i o n p r o c e s s 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 Rh/Ti02 catalysts, w e have a n a l y z e d t h e c h a n g e s o c c u r r i n g 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 and t h e s u p p o r t when 60 cc o f a 3.0 x lo-’ l4 a q u e o u s s o l u t i o n of RhC13.3 H20 (from A l f a P r o d u c t s ) w a s p l a c e d i n c o n t a c t w i t h one gram o f TiOz(Degussa P25. BET 55 m 2 / g ) used a s r e c e i v e d . The rhodium c h l o r i d e complexes p r e s e n t i n s o l u t i o n , b e f o r e and a f t e r b e i n g c o n t a c t e d w i t h T i 0 2 , w e r e c h a r a c t e r i z e d by UV-visible spectroscopy. The e v o l u t i o n of p H a f t e r p l a c i n g t h e s u p p o r t i n c o n t a c t w i t h i m p r e g n a t i n g s o l u t i o n w a s c o n t i n u o u s l y m o n i t o r e d by a d i g i t a l pH-meter.
The amount of Rh d e p o s i t e d on
remaining
in
solution
during
the
the
catalyst
different
p r e p a r a t i o n were d e t e r m i n e d by a t o m i c a b s o r p t i o n . reduced
catalysts
were
further
characterized
stages
The by
and
that
of
the
fresh
and
transmision
e l e c t r o n m i c r o s c o p y (TEH) and X-ray d i f r a c t i o n ( X R D ) . RESULTS AND DISCUSSION The Rh (111) c h l o r i d e s y s t e m i n a c i d i c s o l u t i o n s can e x h i b i t a +3-nwhich m a y v a r y series of complexes o f t h e type RhC1,(HzO)6-n from RhC16 3- t o Rh(H20)6 3+ . T h e s e species c a n b e e a s i l y c h a r a c t e r i z e d by UV-visible a b s o r p t i o n s p e c t r o s c o p y . The s p e c t r a o f c h l o r o - c o m p l e x e s o f R h ( I I 1 ) e x h i b i t two m a x i m a i n t h e 300-600 nm r e g i o n a s c r i b e d t o d-d t r a n s i t i o n s and a very i n t e n s e c h a r g e t r a n s f e r band i n t h e s h o r t e r w a v e l e n g t h r e g i o n ( r e f . 9 ) . The p o s i t i o n o f t h e b a n d s d e p e n d s on t h e number o f c h l o r i d e and water l i g a n d s . When t h e number of w a t e r l i g a n d s i n c r e a s e s , t h e l i g a n d f i e l d t h e o r y p r e d i c t s t h a t t h e d-d t r a n s i t i o n m a x i m a must s h i f t t o s h o r t e r wavelengths.
I n f a c t , it is e x p e r i m e n t a l l y o b s e r v e d
that
79
t h e y s h i f t from 412-518 nn f o r t h e h e x a c h l o r o complex ( n = 6 ) t o 305-393 nm f o r t h e hexa-aquo complex (n=O). T h e r e f o r e , t h e n a t u r e of t h e species p r e s e n t i n s o l u t i o n can b e , a t least q u a l i t a t i v e l y , e s t i m a t e d from t h e s p e c t r a . F i g u r e 1 shows a b s o r p t i o n spectra of t h e aqueous s o l u t i o n of RhC13 used i n o u r i m p r e g n a t i o n p r o c e s s . The p o s i t i o n of t h e m a x i m a f o r t h e f r e s h i m p r e g n a t i n g s o l u t i o n i n d i c a t e t h a t t h e impregnating s o l u t i o n h a s a of a n i o n s w i t h 5 and 6 c h l o r i d e l i g a n d s .
high
concentration
In order t o s a t i s f y
the
m a s s and c h a r g e b a l a n c e s , t h e s e complexes must b e compensated by a
comparable number of aquo complexes, a l t h o u g h t h e low p H v a l u e of t h e s o l u t i o n i n d i c a t e s t h a t a f r a c t i o n o f t h e s e aquo complexes may have undergone h y d r o l y s i s f o r m i n g
Rh(OH)m(HgO)6-n n- complexes.
I
300
400
500
nm
600
F i g u r e 1. A b s o r p t i o n spectra of rhodium chloride supernatant solutions after different t i m e s of c o n t a c t w i t h t i t a n i a .
80
The s p e c t r a of t h e s u p e r n a t a n t s o l u t i o n s r e s u l t i n g a f t e r t h e
5
min and 24 h c o n t a c t w i t h t h e T i 0 2 are a l s o shown i n F i g . A 1. c l e a r d e c r e a s e i n t h e c o n c e n t r a t i o n o f a n i o n i c s p e c i e s compared t o t h e i n i t i a l s o l u t i o n is o b s e r v e d
in
the
supernatant
solutions,
i n d i c a t i n g t h a t a large f r a c t i o n o f t h e rhodium s p e c i e s have
been
a d s o r b e d on t h e T i 0 2 s u p p o r t .
been
This adsorption
process
has
q u a n t i f i e d by a t o m i c a b s o r p t i o n . As shown i n T a b l e 1, a f t e r 5 min c o n t a c t w i t h t h e s u p p o r t , a b o u t h a l f o f t h e Rh i n i t i a l l y p r e s e n t i n s o l u t i o n h a s been removed, s u g g e s t i n g t h a t most o f t h e a n i o n i c complexes have been a d s o r b e d on t h e TiOi(T h i s w a s indeed t h e e x p e c t e d b e h a v i o r o f t h e s y s t e m . When T i 0 2 is immersed i n an aqueous medium a t a p H v a l u e below its i s o - e l e c t r i c p o i n t , it becomes p o s i t i v e l y c h a r g e d and a b l e t o a d s o r b n e g a t i v e l y c h a r g e d species ( r e f . 3). However, as a l s o shown i n T a b l e 1, a f t e r l o n g e r o f Rh r e t a i n e d by the support I t w a s o b s e r v e d t h a t a f t e r 24 h i n c o n t a c t w i t h t h e a c i d i c s o l u t i o n a l m o s t 40 X of t h e Rh i n i t i a l l y d e p o s i t e d on t h e TiOg w a s l o s t from t h e s u r f a c e . contact
times,
the
amount
unexpectedly d e c r e a s e d .
TABLE 1
Rh c o n c e n t r a t i o n i n s o l u t i o n and on t h e contact t i m e s with TiOZ. SAMPLE
fresh solution 5 min c o n t a c t 24 h c o n t a c t
Rh mmol/l in the solution
support
after
different
w t % Rh i n t h e catalyst
3.0
-
1.3 1.9
1.1 0.7
To d e t e r m i n e t h e n a t u r e of t h e species d e p o s i t e d on t h e s u p p o r t
w e have u s e d d i f f u s e r e f l e c t a n c e s p e c t r o s c o p y . A s i l l u s t r a t e d i n F i g . 2 , t h e i n t e n s i t y of t h e band c o r r e s p o n d i n g t o t h e sample c o n t a c t e d f o r 24 h w i t h t h e i m p r e g n a t i n g s o l u t i o n is s i g n i f i c a n t l y lower t h a n t h a t of t h e 5 min c o n t a c t . This d i f f e r e n c e confirms t h e unexpected r e s u l t s o b t a i n e d by a t o m i c a b s o r p t i o n a n a l y s i s of t h e s u p e r n a t a n t s o l u t i o n f o r t h e 24 h c o n t a c t r e p o r t e d i n T a b l e 1.
81
3 80
430
480
530
580
nm
F i g u r e 2. D i f f u s e r e f l e c t a n c e s p e c t r a of t i t a n i a samples a f t e r d i f f e r e n t c o n t a c t t i m e s w i t h t h e rhodium c h l o r i d e s o l u t i o n
The a n a l y s i s of t h e band i n t e n s i t i e s d o e s n o t add much r e s u l t s d i s c u s s e d above.
But,
an
important
drawn from t h e a n a l y s i s of t h e p o s i t i o n of
o b s e r v e d t h a t when t h e c o n t a c t t i m e i n c r e a s e s
A s d e s c r i b e d above, l i g a n d s by i n d i c a t e a s u b s t i t u t i o n o f weaker
Accordingly, w e could propose t h a t , a f t e r t h e
the
can
be
maxima.
It
is
the
wavelengths t a k e s p l a c e .
to
conclusion
a
shift
this
to
lower
shift
would
stronger initial
ligands. adsorption
p r o c e s s , t h e a d s o r b e d s p e c i e s undergo a l i g a n d exchange r e a c t i o n
82
on t h e s u r f a c e by which some c h l o r o l i g a n d s ( w e a k e r ) are exchanged by s u r f a c e OH g r o u p s ( s t r o n g e r ) .
This
two-step
process
can
d e s c r i b e d i n t h e f o l l o w i n g scheme: a ) anion adsorption: Ti-OH
+
H+
+
HLx -
______
b) ligand exchange reaction: T -OHZ+ HLx + Ti-OH ______
Ti-OH2+ HLx -
Ti-OHZ+ HLX-* -0 -T i
+ HL
where L is t h e l i g a n d and H t h e c e n t r a l metal atom.
3.20
PH
3.10 0
I
I
100
2 00
time (sec)
F i g u r e 3. E v o l u t i o n o f t h e p H o f t h e 3 x lo-% rhodium c h l o r i d e s o l u t i o n a f t e r t h e a d d i t i o n of T i 0 2 .
300
be
83
T h i s scheme would p r e d i c t
that
the
pH
s o l u t i o n should v a r y d u r i n g t h e d e p o s i t i o n
of
the
process.
impregnating He
e x p e c t t h a t , i n i t i a l l y , as t h e a n i o n a d s o r p t i o n p r o c e e d s ,
should the
pH
s h o u l d i n c r e a s e b e c a u s e H+ are consumed from t h e s o l u t i o n . But, after a while, t h e l i g a n d exchange r e a c t i o n o f t h e a d s o r b e d species s h o u l d b e g i n and, c o n s e q u e n t l y , t h e p H s h o u l d d e c r e a s e as H+ are evolved from t h e s u p p o r t . H e have shown t h a t t h i s is indeed t h e case. U e have a n a l y z e d t h e e v o l u t i o n o f t h e p H o f t h e rhodium c h l o r i d e s o l u t i o n a f t e r t h e a d d i t i o n o f T i O Z . A s shown i n F i g . 3, t h e p H i n i t i a l l y i n c r e a s e d r a p i d l y from an i n i t i a l v a l u e of 3.14, r e a c h i n g a maximum a f t e r a f e w s e c o n d s , and t h e n d e c r e a s e d . By c o n t r a s t , when H C 1 s o l u t i o n s o f t h e same i n i t i a l p H were p u t i n c o n t a c t w i t h T i O Z i n s t e a d o f t h e rhodium s o l u t i o n s , no maximum w a s d e t e c t e d b u t a c o n t i n u o u s i n c r e a s e u n t i l a c o n s t a n t v a l u e w a s r e a c h e d . L i k e w i s e , when SiOz w a s used i n s t e a d o f T i O Z a l m o s t no change i n p H w a s d e t e c t e d . These observations d e m o n s t r a t e t h a t t h e a p p e a r a n c e of m a x i m a i n t h e e v o l u t i o n o f t h e p H is r e l a t e d t o p a r t i c u l a r i n t e r a c t i o n s between t h e Rh complexes and t h e T i O z s u p p o r t and n o t t o an a r t e f a c t o f t h e e x p e r i m e n t a l procedure. To e x p l a i n t h e pronounced d e c r e a s e i n t h e amount o f Rh r e t a i n e d by t h e s u p p o r t as t h e t i m e of c o n t a c t w i t h t h e s o l u t i o n i n c r e a s e s , w e have c o n s i d e r e d t h e p o s s i b i l i t y o f a n a c i d a t t a c k t o t h e support surface. W e s p e c u l a t e t h a t i f t h e s u r f a c e of t h e support is a t t a c k e d by t h e a c i d i c s o l u t i o n , a l o s s o f a d s o r b e d species can
t a k e place d u r i n g t h a t p r o c e s s . H e have a n a l y z e d t h e s u r f a c e t h e T i O Z by TEH t o i n v e s t i g a t e whether an a t t a c k is e v i d e n t .
of
As
i l l u s t r a t e d i n F i g . 4 , a T i O z sample which had been c o n t a c t e d w i t h t h e i m p r e g n a t i n g rhodium c h l o r i d e s o l u t i o n f o r 24 h e x h i b i t e d s u r f a c e f e a t u r e s t h a t d i d n o t appear e i t h e r on t h e f r e s h T i 0 2 n o r on t h e one which had o n l y been i n c o n t a c t w i t h t h e s o l u t i o n f o r 5 min. The most marked f e a t u r e o f t h e samples t h a t showed e v i d e n c e s of a t t a c k w a s t h e p r e s e n c e of o v e r l a y e r s . These o v e r l a y e r s m a y b e t h e r e s u l t of a re-deposition.
partial
dissolution
of
the
Ti02
followed
by
84
F i g u r e 4. Transmission e l e c t r o n micrograph of t h e t i t a n i a s u p p o r t c o n t a c t e d w i t h t h e rhodium c h l o r i d e s o l u t i o n f o r 24 h and r e d u c e d a t 323K W e have a l s o a n a l y z e d by XRD t h e r u t i l e / a n a t a s e
ratio
in
the
T i 0 2 s u p p o r t , which is a b o u t 1/4 i n t h e o r i g i n a l
samples.
r a t i o s were o b t a i n e d from t h e peak
corresponding
intensities
These to
t h e s p a c i n g s d = 3 . 2 5 , r u t i l e (100) and d=3.52, a n a t a s e (101). shown i n T a b l e 2 t h i s r a t i o is
significantly
increased
sample which h a s been i n c o n t a c t w i t h t h e s o l u t i o n f o r 24 h . a n a t a s e - r u t i l e t r a n s f o r m a t i o n would agree w i t h t h e
As
for
the This
dissolution
/
r e - d e p o s i t i o n h y p o t h e s i s , s i n c e r u t i l e is t h e most s t a b l e form o f T i 0 2 and i t s f o r m a t i o n c o u l d b e e f f e c t e d d u r i n g t h e r e - d e p o s i t i o n process.
85
TABLE 2 Anatase-to-rutile
t r a n s f o r m a t i o n e f f e c t e d by t h e
acid
attack
as
d e t e r m i n e d from XRD p e a k i n t e n s i t y , d = 3 . 2 5 r u t i l e (1001, 6 ~ 3 . 5 2 a n a t a s e (101).
a partial c o v e r i n g of t h e metal p r e c u r s o r s . W e have looked f o r a n o t h e r e v i d e n c e o f t h i s phenomenon by m e a s u r i n g hydrogen c h e m i s o r p t i o n c a p a c i t i e s o f a Rh/Si02 c a t a l y s t , u s e d a s r e f e r e n c e , a f t e r b e i n g i n c o n t a c t w i t h T i 0 2 i n b o t h , n e u t r a l and a c i d i c l i q u i d media. If a p a r t i a l c o v e r a g e of t h e Rh s u r f a c e by T i 0 2 species o c c u r r e d , w e s h o u l d e x p e c t a d e c r e a s e i n t h e hydrogen c h e m i s o r p t i o n capacity. A s shown i n T a b l e 3, t h e r e w a s n o much change i n t h e c h e m i s o r p t i o n v a l u e s when t h e r e f e r e n c e c a t a l y s t w a s e i t h e r c o n t a c t e d w i t h T i 0 2 i n a non a c i d i c medium o r t r e a t e d i n an a c i d i c medium w i t h o u t T i 0 2 - However, t h e r e w a s a clear d e c r e a s e when it w a s c o n t a c t e d w i t h a s l u r r y c o n t a i n i n g T i 0 2 and a HC1 s o l u t i o n ( p H = 3 . 1 4 ) . A s mentioned a b o v e , t h e r e - d e p o s i t i o n may r e s u l t i n
_ ,
TABLE 3 Hydrogen c h e m i s o r p t i o n d a t a o b t a i n e d on t h e same Rh/SiOz contacted w i t h T i O z under d i f f e r e n t c o n d i t i o n s
CONCLUSIONS W e conclude t h a t when T i 0 2 is p l a c e d i n c o n t a c t w i t h rhodium c h l o r i d e s o l u t i o n s o f low p H v a l u e s , s e v e r a l phenomena, l e a d i n g t o t h e d e p o s i t i o n of m e t a l p r e c u r s o r s on t h e T i 0 may o c c u r : 2 i ) anion adsorption.
A s the
oxide
particles
become
positively
charged i n t h e low p H s o l u t i o n , t h e a d s o r p t i o n of a n i o n i c
species
is r e a d i l y e f f e c t e d .
A f t e r t h e a d s o r p t i o n of t h e a n i o n i c by means of an exchange of c h l o r o l i g a n d s by s u r f a c e OH g r o u p s . i i ) ligand exchange reaction.
complexes, f u r t h e r a n c h o r i n g t o t h e s u p p o r t t a k e s place
iii) acid attack.
The a c i d i c s o l u t i o n a t t a c k s t h e s u r f a c e of
a partial support causing r e - d e p o s i t i o n of o x i d e species.
ACKNOWLEDGEMENTS Financial support acknowledged.
from
dissolution
CONICET
of
followed
Argentina
is
by
the the
gratefully
REFERENCES K . Foger, i n Anderson J . R., and Boudart M.(Eds.), C a t a l y s i s : S c i e n c e and Technology, S p r i n g e r , B e r l i n , H a i d e l b e r g , N.York, 1984, Ch. 4, p . 227. M . Che and C.O. B e n n e t t , Adv. C a t a l . , 36 (1989) 55 J. B r u n e l l e , i n Delnon e t a l . ( E d . ) , P r e p a r a t i o n of H e t e r o g e n - C a t a l y s t s , Vol. 11, E l s e v i e r . Amsterdam, 1979. J.C. Summers and S . A . Ausen, J. C a t a l . , 52 (1978) 455. E. S a n t a c e s a r i a , S . C a r r a , and I. Adami, I n d . Eng. Chem. Prod. R e s . & Dev., 16 (1977) 41. W.J. van den Boogert, G. van d e r L e e , H. Luo, V . Ponec, A p p l . C a t a l . (1987) Y.J. L i n , R . J . F e n o g l i o , D.E. Resasco, G.L. H a l l e r . i n Delmon e t a l . ( E d . ) , P r e p a r a t i o n of Heterogeneous C a t a l y s t s , Vol. I V , E l s e v i e r , Amsterdam, 1987, p.125. G . L . H a l l e r and D . E . Resasco, Adv. C a t a l . , 36 (1989) 173. C . K - J o r g e n s e n , Acta Chim. Scand., 10 (1956) 500.
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
IMPREGNATION OF CONTROLLED-POROSITY SILICA:
87
Cu/Si02, Co/SiO2 and Cu-Co/Si02
I N V E S T I G A T I O N OF THE PARAMETERS AFFECTING SELECTIVITY I N CO HYDROGENATION
M.A.
MARTIN LUENGO (YATES), Y. WANG and P.A.
SERMON*
S o l i d s and Surfaces Research Group, Department o f Chemistry, Brunel U n i v e r s i t y , Uxbridge, UB8 3PH, UK
SUMMARY S i l i c a spheres o f c o n t r o l l e d p o r e s i z e (83% o f pores between 6-10nm i n r a d i u s ) were impregnated w i t h aqueous n i t r a t e s o l u t i o n s t o g i v e 8%Cu/SiO , 8%Co/Si02 and 8%Cu-4%Co/SiO . Temperature-programmed r e d u c t i o n , microproge a n a l y s i s , and propane h y d r o t e n o l y s i s a l l i n d i c a t e d t h a t t h e Cu and Co i n t h e I n t h e c a t a l y s i s o f CO bimetallic catalyst i n i n t i m a t e contact. h y d r o g e n a t i o n r a t e o f CO c o n v e r s i o n i n c r e a s e d i n t h e sequence: Cu/SiO < Cu-Co/Si02,< Co/SiO w h i l e s e l e c t i v i t y t o methine i n c r e a s e d i n t h e sa&e sequence (and s e l e c t i v i t y t o methanol decreased). These s e l e c t i v i t y t r e n d s a r e t o be expected s i n c e t h e a b i l i t y o f Co t o d i s s o c i a t e CO and produce methane i s w e l l known. However, none o f these c a t a l y s t s produced s i g n i f i c a n t amounts o f e t h a n o l . T h e r e f o r e t h e s t u d y o f these i d e a l i s e d c a t a l y s t s suggests t h a t Cu-Co i n t e r a c t i o n s a r e n o t s u f f i c i e n t t o induce s e l e c t i v i t y t o e t h a n o l i n CO h y d r o g e n a t i o n a t 523K and 2MPa b u t must a l s o r e q u i r e an i n t e r a c t i o n w i t h a ZnO o r A1203 component i n t h e f u l l catalyst.
were
INTRODUCTION
A t t h e l a s t conference c o n s i d e r i n g s c i e n t i f i c bases f o r t h e p r e p a r a t i o n o f heterogeneous c a t a l y s t s some o f t h e p r e s e n t a u t h o r s d e s c r i b e d ( r e f . 1) a s t u d y o f t h e parameters a f f e c t i n g t h e d i s p e r s i o n and l o c a t i o n o f P t d u r i n g i m p r e g n a t i o n o f a s i l i c a o f c o n t r o l l e d pore s i z e w i t h solutions o f hexachloroplatinic acid. Using t h i s approach i t was p o s s i b l e t o d i f f e r e n t i a t e p o r e d i a m e t e r and s o l u t i o n t e m p e r a t u r e - v i s c o s i t y e f f e c t s i n t h e p r e p a r a t i o n o f a model mono-metallic catalyst.
T h i s approach has now been extended t o c o n s i d e r t h e importance
o f Cu-Co i n t e r a c t i o n s i n s i l i c a - s u p p o r t e d CO h y d r o g e n a t i o n c a t a l y s t s i n d e f i n i n g s e l e c t i v i t y t o ethanol. Cu-Co heterogeneous c a t a l y s t s a r e o f some c u r r e n t ' i n t e r e s t i n t h e c o n v e r s i o n
o f CO/H2 and CO/C02/H2 m i x t u r e s w i t h good s e l e c t i v i t y t o h i g h e r a l c o h o l s such as ethanol ( r e f s . 2,3).
I t seems u n c e r t a i n whether t h e Co i s e n t i r e l y reduced i n
such c a t a l y s t s ( r e f . 3 ) , b u t i t s a d d i t i o n t o t h e Cu-only c a t a l y s t i n c r e a s e s alkane f o r m a t i o n .
The p r e c i s e i n t e r a c t i o n of Cu and Co i n such c a t a l y s t s i s
c e r t a i n l y u n c l e a r and hence t h e p r e s e n t work was undertaken.
However, i t i s
88
appreciated that the present catalysts are f a r simpler than alkali-treated Cu-Co/ZnO/A1203
samples w h i c h a t 563K, 6MPa and CO/H2=0.5 gave 21-24%CO c o n v e r s i o n
w i t h 30% s e l e c t i v i t y t o e t h a n o l ( r e f . 3 ) . A g a i n s t t h i s i n t r i g u i n g b a c k g r o u n d t h e p r e s e n t r e s u l t s a r e now r e p o r t e d . EXPERIMENTAL Materials
A s i l i c a ( S h e l l I n t e r n a t i o n a l Chemical Company L t d . ) was chosen because o f i t s c o n t r o l l e d p o r o s i t y , h o m o g e n e i t y a n d p u r i t y ; i t s p r o p e r t i e s have been d e s c r i b e d p r e v i o u s l y ( r e f . 1 ) b u t i t s main c h a r a c t e r i s t i c s a r e g i v e n i n Table 1 below. TABLE 1 P r o p e r t i e s o f s i l i c a u s e d 2.5
p a r t i c l e s i z e (mm)
sN2 ( m 2 / g )
21 1
% p o r e s i n t h e r a d i u s r a n g e 6-10nm
83
I t s t o t a l s u r f a c e a r e a was e s t i m a t e d f r o m a n a l y s i s o f N assuming i t s m o l e c u l a r c r o s s - s e c t i o n a l a r e a was 0.162nm
a d s o r p t i o n d a t a a t 77K
5.
The a d s o r p t i o n
i s o t h e r m was o f t y p e I V ( r e f . 4 ) w i t h a h y s t e r e s i s l o o p t y p i c a l o f t h a t f o r a mesoporous m a t e r i a l ( i . e .
t h a t designated type H1) ( r e f . 4).
Samples o f t h i s p r e - d r i e d s u p p o r t ( 2 3 9 ) were 75cm3 aqueous s o l u t i o n o f Cu o r Co o r Cu-Co n i t r a t e s o f a s u f f i c i e n t s t r e n g t h t o g i v e t h e c a t a l y s t c o m p o s i t i o n s below: 8%Cu/silica 8%Cu-4%Co/silica 8%Co/silica These were t h e n a g i t a t e d f o r lOmin, e v a p o r a t e d t o d r y n e s s , d r i e d f o r 16h i n a i r a t 403K, and c a l c i n e d f o r 3h a t 523K i n a i r . Methods T o t a l s u r f a c e a r e a s were e s t i m a t e d f r o m N2 a d s o r p t i o n a t 77K i n a S o r p t y a p p a r a t u s ( C a r l o E r b a ) a f t e r o u t g a s s i n g a t 523K f o r 2h.
The r e d u c i b i l i t y o f t h e
samples was a s s e s s e d i n a c o n v e n t i o n a l f l o w temperature-programmed r e d u c t i o n system ( r e f .
5 ) u s i n g 5%H2/Ar f l o w i n g a t 50cm3/min o v e r 50-100mg c a t a l y s t samples.
The r a t e and s e l e c t i v i t y o f CO h y d r o g e n a t i o n was f o l l o w e d o v e r samples ( 0 . 1 9 ) o f c a t a l y s t s ( w h i c h had been p r e t r e a t e d t o 573K i n 6%H2/N2 and t h e n c o o l e d t o 523K) i n CO/H2
( = 2 ) f l o w i n g a t 20cm3/min a t 2MPa and 523K; o v e r a p e r i o d o f 3-4h
p r o d u c t s were a n a l y s e d b y gas c h r o m a t o g r a p h y u s i n g gas c h r o m a t o g r a p h s f i t t e d w i t h F I D d e t e c t o r s and Porapak Q a n d T columns.
The a c t i v i t y o f t h e c a t a l y s t s i n t h e
h y d r o g e n o l y s i s o f p r o p a n e a t 523K was a l s o d e t e r m i n e d c h r o m a t o g r a p h i c a l l y .
The
89
c a t a l y t i c c o n d i t i o n s u s e d were a s f o l l o w s : 0.29 samples p r e - r e d u c e d t o 573K i n H2 were s u b j e c t e d t o a r e a c t a n t s t r e a m (140cm3/min a t 1 0 l k P a ) c o n s i s t i n g o f p r o p a n e : N2:H2=10:30:100 mol r a t i o .
Reduced c a t a l y s t s were s e c t i o n e d and c r o s s - s e c t i o n s
a n a l y s e d i n a s c a n n i n g e l e c t r o n m i c r o s c o p e b y EDAX-microprobe methods f o r Cu and Co c o n c e n t r a t i o n p r o f i l e s . CHARACTERISATION The r e s u l t s o f BET a n a l y s i s o f N2 a d s o r p t i o n a t 7 7 K showed t h a t t h e t o t a l
2
s u r f a c e a r e a o f t h e s u p p o r t (211m / g ) d e c r e a s e d on i m p r e g n a t i o n w i t h t h e m e t a l Thus t h e C u O / s i l i c a and C u 0 - C o O x / s i l i c a samples h a d t o t a l s u r f a c e a r e a s o f o n l y 187 and 160m2 / g n i t r a t e s and c a l c i n a t i o n t o t h e s u p p o r t e d m e t a l o x i d e s .
r e s p e c t i v e l y and t h i s i s p r o b a b l y due t o p a r t i a l b l o c k i n g o f t h e s u p p o r t p o r e s by t h e metal oxide p a r t i c l e s . Cu0-CoOx/silica
r e d u c e d i n temperature-programmed r e d u c t i o n w i t h a maximum
r a t e a t 533K ( s e e F i g . 1) w h i c h was s i g n i f i c a n t l y above t h a t f o r C u O / s i l i c a o r CuO and s i g n i f i c a n t l y b e l o w t h a t f o r C o O x / s i l i c a o r COO.
T h i s suggested t h a t
t h e Cu and Co phases were i n t i m a t e l y m i x e d o n t h i s s i l i c a s u p p o r t u n d e r t h e present conditions.
I n each c a s e t h e h y d r o g e n c o n s u m p t i o n s were 34-71% above
t h e e x p e c t e d v a l u e i f CuO and COO were b e i n g r e d u c e d t o t h e z e r o - v a l e n t m e t a l s , b u t h y d r o g e n s p i l l o v e r may b e r e s p o n s i b l e f o r t h i s .
The i n t i m a t e m i x i n g o f
CuO and Coox may n o t b e s u r p r i s i n g i n t h e l i g h t o f t h e f a c t t h a t COO c a n a c c e p t up t o 25mol% CuO w i t h o u t s t r u c t u r a l change ( r e f . 6 ) .
The i n t i m a c y o f Cu and Co
phases was c o n f i r m e d b y m i c r o p r o b e a n a l y s i s o f t h e m e t a l c o n c e n t r a t i o n s i n p a r t i c l e c r o s s - s e c t i o n s ( s e e F i g . 2).
These show some p r e f e r e n c e f o r b o t h m e t a l s
a t t h e o u t e r edge o f t h e s u p p o r t p a r t i c l e s ( a s e x p e c t e d f r o m i m p r e g n a t i o n ( r e f . 1 ) b u t m o s t i m p o r t a n t l y t h a t t h e t w o m e t a l s a r e l o c a t e d i n t h e same p a r t s o f t h e s u p p o r t p a r t i c l e s ; t h i s t h e n i s c o n s i s t e n t w i t h temperature-programmed r e d u c t i o n .
F i g u r e 1 TPR o f 8%Cu-4%Co/Si02
90
Figure 2.
C o n c e n t r a t i o n p r o f i l e s o f Cu ( ) and Co ( 0 ) i n c r o s s - s e c t i o n s o f c a l c i n e d S i O ( a ) and 8%Cu-4%Co/Si02 ( b ) determined b y k c r o p r o b e a n a l y s i s o f s e c t i o n e d c a t a l y s t p a r t i c l e s whose edge i s marked by arrows .
The i d e n t i c a l depth o f p e n e t r a t i o n o f t h e two m e t a l s i n ( b ) suggest these w i l l be i n good c o n t a c t on s i l i c a impregnated i n t h e manner d e s c r i b e d .
91
Alkane h y d r o g e n o l y s i s i s a s u i t a b l e probe f o r t h e p r e s e n t model c a t a l y s t s i n t h a t z e r o - v a l e n t Cuo i s expected t o have low a c t i v i t y i n t h e r e a c t i o n , w h i l e on t h e o t h e r hand z e r o - v a l e n t Coo s h o u l d have h i g h a c t i v i t y i n t h e r e a c t i o n (ref. 7).
I f a b i m e t a l l i c phase e x i s t s 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 i n t h e
C u - C o / s i l i c a sample t h e n one would e x p e c t some i n t e r m e d i a t e a c t i v i t y , u n l e s s t h e Cu was p r e s e n t t o t h e e x t e n t t h a t Co ensembles were fragmented t o such a degree t h a t s t r u c t u r e - s e n s i t i v e h y d r o g e n o l y s i s c o u l d n o t o c c u r a t a s i g n i f i c a n t Thus t h e h y d r o g e n o l y s i s r e a c t i o n may i n d i c a t e t h e e x t e n t t o which Cu and
rate.
Table 2 g i v e s t h e s t e a d y - s t a t e
Co phases a r e i n c o n t a c t i n t h i s c a t a l y s t .
a c t i v i t i e s and s e l e c t i v i t i e s o f t h e p r e s e n t c a t a l y s t s i n propane h y d r o g e n o l y s i s T h i s shows t h a t s i l i c a - s u p p o r t e d Cuo ( l i k e S i 0 2 a l o n e ) has no a c t i v i t y
a t 523K.
i n the reaction.
As expected f o r t h e z e r o - v a l e n t s t a t e s i l i c a - s u p p o r t e d Co
does show s u b s t a n t i a l a c t i v i t y ; f u r t h e r m o r e i t shows h i g h s e l e c t i v i t y t o t o t a l h y d r o g e n o l y s i s t o methane. I f one l o o k s a t t h e d a t a f o r t h e b i m e t a l l i c c a t a l y s t , t h e n i t i s c l e a r t h a t a c t i v i t y i s v e r y l o w i n t h i s r e a c t i o n , which suggests t h a t t h e Cu and Co a r e i n i n t i m a t e c o n t a c t and t h a t t h e s u r f a c e s o f t h e b i m e t a l l i c p a r t i c l e s must be p r e f e r e n t i a l l y Cu ( w i t h few l a r g e Co ensembles). T h i s i s c o n s i s t e n t w i t h X-ray p h o t o e l e c t r o n spectroscopy evidence ( r e f . 8 ) on single crystals.
However, even w i t h t h e b i m e t a l l i c t h e r e i s some s e l e c t i v i t y t o
t o t a l h y d r o g e n o l y s i s t o methane. Thus temperature-programmed r e d u c t i o n , EDAX-microprobe a n a l y s i s and a l k a n e h y d r o g e n o l y s i s a l l i n d i c a t e t h a t i n t h e impregnated b i m e t a l l i c c a t a l y s t t h e Cu and Co phases a r e i n i n t i m a t e c o n t a c t ;
t h i s i s i n t r i g u i n g given t h e simple
p r e p a r a t i v e method used. TABLE 2
A c t i v i t i e s and s e l e c t i v i t i e s i n propane h y d r o g e n o l y s i s a t 523K
*
catalyst
% conv propane
'CH4
8%Cu/SiO 2
0
-
8%Cu-4%Co/Si02
0.53
2.51
0.25
0.66
8%Co/SiO2
9.25
2.85
0.07
11.50
* TABLE 3
'C2H6
rate 0
r a t e o f propane c o n v e r s i o n i n mmol propane/g c a t / h
A c t i v i t i e s and s e l e c t i v i t i e s i n CO h y d r o g e n a t i o n a t 523K
c a t a 1y s t
*
%CH4
%C2H6
%CH30H
%C2H50H
45.5
7.8
33.8
5.3
8%Cu-4%Co/Si02 26.2
51.3
10.8
24.0
0.0
8%Co/Si02
43.4
75.8
5.5
8.8
1.1
(SiO2)
(0.96)
8%Cu/Si02
*
rate
6.18
(40.8)
r a t e i n pmol/g cat/min
(37.7)
92
RESULTS OF CATALYSIS OF CO HYDROGENATION The r e s u l t s f o r t h e a c t i v i t i e s and s e l e c t i v i t i e s o f t h e c a t a l y s t s prepared here a r e shown i n Table 3; h e r e p a r t i c u l a r a t t e n t i o n s h o u l d be g i v e n t o t h e e f f e c t o f h a v i n g t h e z e r o - v a l e n t Cu and Co i n c o n t a c t i n t h e b i m e t a l l i c sample. These d a t a a g a i n r e l a t e t o s t e a d y - s t a t e c o n d i t i o n s a f t e r 3-4h r e a c t i o n t i m e a t 523K and 2MPa. F i r s t , t h e o v e r a l l a c t i v i t i e s o f t h e c a t a l y s t s i n terms o f t h e r a t e o f CO c o n v e r s i o n i s a l m o s t z e r o f o r S i 0 2 a l o n e and t h e n i n c r e a s e s as Co i s added t o t h e s i l i c a - s u p p o r t e d Cu and i s h i g h e s t f o r t h e C o / s i l i c a .
Therefore f o r the
b i m e t a l l i c c a t a l y s t i t would seem t h a t t h e r e r e a l l y a r e p a r t i c l e s c o n t a i n i n g b o t h m e t a l s , r a t h e r t h a n some Co p a r t i c l e s and some Cu p a r t i c l e s .
Again, t h i s
i s i n t r g u i n g f o r such a s i m p l e c a t a l y s t p r e p a r a t i v e method. Second, as t h e Co c o n t e n t i n c r e a s e s t h e s e l e c t i v i t y t o alkanes i n c r e a s e s and t h i s i s e s p e c i a l l y so f o r methane. T h i r d , as t h e Co c o n t e n t i n c r e a s e s so t h e s e l e c t i v i t y t o CH30H decreases. These t r e n d s towards methane and away from methanol a r e t o be expected i n terms o f t h e g r e a t e r a b i l i t y o f Co t o d i s s o c i a t e CO and t o f a c i l i t a t e t h e methanation reaction (ref. 9). Fourth, t h e r e i s almost no evidence f o r e t h a n o l p r o d u c t i o n f o r any o f these c a t a l y s t s ; t h i s must be i m p o r t a n t i n terms o f mechanisms and s i t e s i n t h e Cu-Co sample. D I S C U S S I O N AND CONCLUSIONS
Temperature-programmed r e d u c t i o n , EDAX-microprobe a n a l y s i s and a l k a n e h y d r o g e n o l y s i s a l l suggest t h a t i n t h e s e model impregnated heterogeneous c a t a l y s t s Cu and Co, when b o t h p r e s e n t i n t h e s i l i c a pores, e x i s t as l a r g e l y bimetallic particles.
I t may however be t h a t t h e i r c o m p o s i t i o n v a r i e s w i t h
r a d i a l p o s i t i o n i n t h e s u p p o r t p a r t i c l e s ; t h i s i s t h e s u b j e c t o f f u r t h e r study. I t i s an i m p o r t a n t f i n d i n g h e r e t h a t such b i m e t a l l i c p a r t i c l e s (which a r e
p r o b a b l y q u i t e w e l l reduced under c o n d i t i o n s o f p r e t r e a t m e n t h e r e ) a r e a c t i v e i n f a c i l i t a t i n g CO h y d r o g e n a t i o n t o e t h a n o l .
not
Alkane h y d r o g e n o l y s i h i n t s
a t Cu b e i n g p r e f e r e n t i a l l y a t t h e b i m e t a l l i c s u r f a c e s , b u t t h i s may n o t be t h e case under c o n d i t i o n s o f CO hydrogenation.
U s i n g Cu-Co/ZnO/A1203
( i n the
presence o f a l k a l i ) o r w i t h o t h e r s u p p o r t i n g o x i d e s ( r e f . 10) may produce Cuo-Coxt
p a i r s b y s t a b i l i s i n g Co i n p o s i t i v e o x i d a t i o n s t a t e s v i a i n t e r -
c a l a t i o n o f Co c a t i o n s o r f o r m a t i o n o f a s u r f a c e s p i n e l .
These would
c e r t a i n l y show d i f f e r e n t p r o p e r t i e s from t h o s e seen here and m i g h t w e l l be a c t i v e i n t h e s e l e c t i v e c o n v e r s i o n o f CO/H2 t o e t h a n o l . For t h e moment, t h e p r e s e n t s t u d y has shown t h a t Cuo-Coo on ' i n e r t '
93
silica are not in themselves and in isolation from support o r promoter effects sites of effective production of ethanol from CO/H2. This has been shown unequivocally using the model impregnation approach used here. Further work in which ZnO, A1203 and other oxides are added is in hand. It i s hoped that the study o f such model and well-defined precursors and catalysts will allow the critical sites and interactions in such catalysts to be defined. ACKNOWLEDGEMENTS The authors gratefully acknowledge SERC and Royal Society support of MAML(Y) and YW respectively during the course of this work. REFERENCES 1 M.A.Martin-Luengo, P.A.Sermon and K.S.W.Sing 'Preparation of Catalysts IV' ed. B.Delmon, P.Grange, P.A.Jacobs and G.Poncelet (1987) Elsevier p.29 2 J.G.Nunan, C.E.Bogdan, K.Klier, K.J.Smith, C.W.Young and R.G.Herman J.Cata1. 116 (1989) 195 3 P.Courty, D.Durand, E.Freund and A.Sugier J.Molec.Cata1. 17 (1982) 241; H.F.Hardman and R.Beach US patent (1978) 905,703 4 K.S.W.Sing, D.H.Everett, R.A.W.Hau1, L.Moscou, R.A.Pierotti, J.Rouquero1 and T.Siernieniewska Pure Appl .Chem. 57 (1985) 603 5 G.C.Bond and S.Namijo J.Cata1. 118 (1989) 507 6 C.Delorine Bull.Soc.Chim.Fr. Miner.Cryst. 8 1 (1958) 19 7 S.A.Goddard, M.D.Amiridis, J.E.Rekoske, N.C.Martinez and J.A.Dumesic J.Cata1. 117 (1989) 155; Z.Paa1, P.Tetenyi and M.Dobrovolszky React. Kin.Catal.Lett. 37 (1988) 163 8 D. C hadwi c k (pri va te communi cati on) 9 M.A.M.Luengo (Yates) and P.A.Sermon (unpublished results) 1 0 M.Mouaddib and V.Perrichon Proc. 9th. Intern. Cong. Catal. 2 (1988) 521; A.J.Marchi, J.di Comino and C.R.Apestiguia Proc. 9th. Intern. Cong. Catal. 2 (1988) 529; M.Mouaddib, V.Perrichon and M.Primet J.Chem.Soc.Faraday Trans. I 85 (1989) 3413
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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
95
SELECTIW HYDROGENATION OF CYCLODODECATRIENE ISOMERS TO CYCLODODECENE CATALYSED BY Cu-A1203. V. Di Castro', M. Garganoz,N. Ravasio2and M. Rossi3 Dipartimento di Chimica - Universiti3 " La Sapienza" - P.le A. Moro, 5 - 00185 Roma - Italy C.N.R. Cenm di Studio sulle Metodologie Innovative di Sintesi Organiche - Via Amendola, 173 - 70126 Bari - Italy Centro C.N.R. and Dipartimento di Chimica Inorganica e Metallorganica - Universiti di Milano Via Venezian, 21 - 20133 Milano - Italy
SUMMARY The behaviour of supported copper catalysts has been investigated in the hydrogenation of cyclododecamene to cyclododecane. The activity of these catalysts, which is influenced by the nature of oxide support and Cu(0) surface dispersion, is lower than that obtainable with conventional heterogeneous noble metal systems, whereas the selectivity is higher. In particular, Cu/A1203, prepared by chemisorption of CU(NH~)~*on alumina and preactivated in H2 atmosphere, allows high yield of cyclododecene (94%). INTRODUCTION The selective hydrogenation of mixtures of cyclododecamene (CDT) isomers, easily available through cyclotrimerization of butadiene, represents a convenient route to cis- and trans-cyclododecene (CDE). These latter compounds are of great practical importance because can be employed to prepare compounds such as 1,12-dodecandioic acid, 1,12-diaminododecane, 12-amminododecanoic acid lactam, which are monomers for polyamide manufacture, and cyclododecanecarboxilicacid, a pesticide, and its esters used as plasticizers. Highly selective catalysts are requested because the by-products, cyclododecane (CDA) and cyclododecadienes (CDD), cannot be easily separated fiom cyclododecene. Many efforts have been recently done in order to achieve this goal and different homogeneous and heterogeneouscatalysts have been tested. Best results have been obtained under homogeneous conditions by using transition metal complexes. In particular the use of Co or Ru complexes, at temperature ranging between 120 and 160°C. allows CDE yields greater than 98% (ref. 1,2). According to patent literature, heterogeneous catalysts for CDE production are based on
96
transition metals, particularly Ru, Pd, Ni, Co, dispersed on different materials such as functionalised polymers, A1203, SO2, zeolites, activated carbon, ecc. . The selectivity achieved by these catalysts is generally lower than that reported for homogeneous ones and usually CDE yields are less than 90%; only in few cases higher yields are reported, as in the case of a bimetallic Pd-Co system which produces CDE up to 95%(ref. 3). The use of copper based catalysts has been scarcely explored in this hydrogenation reaction, CDE yields never exceeding 83%(ref. 4). The high selectivity obtained during our studies on the reduction of polyenes (ref. 5-7) and alkynes (ref. 8) by using supported copper catalysts prompted us to investigate the selective hydrogenation of CDT to CDE. Preliminary results have been already claimed (ref. 9). In this paper the full investigation is reported. EXPERIMENTAL SECTION Materials
-
Cyclododecatriene (supplied by Fluka) was distilled, purified by filtration
through activated alumina and stored under nitrogen atmosphere at OOC. According to GLC analysis, the distilled product was a mixture of isomers with the following composition : trans-trans-trans-CDT = 58.7% , cis-trans-trans-CDT = 34.7% , cis-cis-trans-CDT = 2.7%, unidentified = 3.9% (probably cis-cis-cis-CDT). All oxides and salts were reagent grade products and were used without further purification. Catalysts preparation
-
Chemisorption method: all catalysts were prepared following a
procedure similar to that used by Koritala for the CuO/Si02 (ref. 10). Ammonia was added to an aqueous solution of C U ( N O ~ ) ~ . ~until H ~ Ocomplete dissolution of the initially precipitated Cu(OH), occurred. The oxide support (M&)
was added to the solution; the slurry, under
continuous stirring, was then slowly diluted with H20. The solid was separated by filtration, washed with H20, dried over night at 1lOOC and lastly calcined at 35OOC for 3 hrs. Coprecipitation method: all catalysts were obtained by addition of NaHC03 to a solution containing C U ( N O ~ ) ~ . ~ and H~O the metal nitrate corresponding to the oxide support. The solid was washed with H20, in order to eliminate Na+, dried over night at llO°C, then calcined at 35OOC for 3 hrs. Catalysts reduction - The reduction was carried out in glass reactors provided with a side arm connected to a sampler. All the catalysts were heated for 1 hr under vacuum to 27OOC before
reductive treatment. Reduction with H2 at 100 kPa was carried out either at constant temperature (270°C) or increasing the temperature, from 100 to 20O0C, at 2OC/min. Cu(0) specific area determination - The Cu(0) surface area was determined by using the
97
N20 chemisorption method (ref. 11). The measurements were carried out on samples of reduced catalysts by using a Carlo Erba Sorptomatic 1800 automatic gas burette
. N20
(75 Wa) was
introduced in the sampler and allowed to react for 0.5 hrs at 37OC. Then the gas in the sampler was analysed for N2 content. The Cu(0) specific area was computed by using a surface coverage factor ( moles of oxygen atoms per moles of surface Cu(0) atoms) 8 = 0.35 and a mean surface area for a copper site of 7.41 A2(ref. 11). Samples for ESCA determinations were stored under inert atmosphere in sealed vials and analysed with a VG ESCA 3 spectrometer employing an A1 K, source (hv = 1486.6eV) (ref. 12). Hydrogenation reactions - These were performed in glass reactors or, when H2 pressure was greater than 100 Wa, in stainless AISI 316 autoclave (60ml) . The rate of H2 uptake was measured by using a Brooks electronic mass flow meter connected by an AD interface to an IBM XT personal computer. Analysis
-
The products composition was determined by GLC with a 50 m x 0.2 mm
Carbowax 20M capillary column using a Hewlett Packard 5880A gas chromatograph equipped with a flame ionization detector. N2 from N20 chemisorption was analysed by GC with a 3 m x 2 mm stainless steel column packed with Carbosieve S using a Carlo Erba Fractovap gas chromatographprovided with a thermal conductivity detector. RESULTS AND DISCUSSION The hydrogenation of CDT catalysed by copper dispersed on different oxides, in the initial step of the reaction, follows in all cases a first order kinetic respect to CDT concentration. However, the activities of the catalysts are quite different. As shown in table 1, in fact, initial turnover number ranging from 0 to137 h-' were obtained. In all experiment the CDE produced was a mixture of trans- (65%)and cis-isomers (35%); the cislrruns ratio was independent from the catalyst, indicating that the same hydrogenation mechanism was operating for all the active systems.
The wide range of catalytic activities showed by different catalysts can be related both to a different dispersion of the active copper species on the surface of the catalytic material and to the different cooperative effect of oxide support. As reported in experimental section, all catalytic systems were prepared by reduction in
hydrogen atmosphere of CuO/Mfly
precursors; ESCA data (ref.5,6,12) showed that this
pretreatment causes the almost quantitative reduction of copper to Cu(0). This species , as suggested in our previous work, plays an essential role in promoting the hydrogenation reaction,
98
particularly with cyclic polyenes (ref.5-7). TABLE 1 Hydrogenation of Cyclododecamene catalysed by Cu/M,Oy systemsa Catal?
Pr.C
%Cu Red.temp. (OC)
Tvd (h-9
nHdnCDT
Reaction products (%moles) CDE CDD CDA
Cu/Alz03 Cu/A1203 Cu/Si02 Cu/Cr203g C Can0
I C I
7.0 9.5 7.0 37 8.4
130 9 67 16 122 77 137
2.00 1.37 1.87 2.03
94.0 45.1 83.0 90.7 93.0 93.6 91.0
3.0 42.0 12.7 5.9 3.5 3.2 4.5
3.0 1.7 2.3 3.4 3.5 3.2 4.5
40 86
1.94 2.00
90.0 90.8
7.8 4.6
2.2 4.6
Cu/MnOz Cum203
cu/Mgo
Othersh
C C I I I I I
8.7 8.7 6.3 7.4
270 100-270 100-200 270 100-200 270
<5 <5
a PH~= 100 KPa ; T = 18OOC ; CDT/Cu molar ratio = 100. All system were derived by corresponding CuO/Mfly by reduction in H2 atmosphere. Preparation method, C = coprecipitation, I = chemisorption. Turnover number at the beginning of the reaction in moles of Hz consumed per mole of copper per hr. Moles of Hz consumed per mole of CDT whcll the reaction was stopped. 2 % ca. of by-products are f m e d during the reaction. g Derived by commercial copper chromite catalyst by reduction. prepared by reduction of CuO supported on TiO,, BeO, SnOz, ZrOz,CeOz, TazOs, ZnO, CdO, Crz03,Nb0.
determined by means of NzO The values of specific Cu(0) surface area (SCU(O)), chemisorption, are reported in table 2 for different catalysts. A comparison between SCU(O)and tumover number indicates that the nature of oxide support has a predominant influence on the activity of the catalytic system. In fact, Cu/A1203 shows a higher tumover number respect to Cu/SiOz, Cu/ZnO, Cu/MgO in spite of a lower Scu(0). In particular, the best results were obtained using Cu/A1203. This catalyst, which was prepared reducing in hydrogen atmosphere a material obtained by chemisorption of Cu(NH3)4*
on alumina, showed best selectivity (CDE yield = 94% when 2 moles of Hz were consumed per mole of CDT) and high activity (Tv = 130 h-l). Also interesting for their activity and selectivity are Cu/MnOz/ and C a n 0
which
exhibited activity comparable with that of Cu/A1203 and selectivity only slightly lower. Cu/MgO, Cu/Y203and Cu/Cr203, although less active, allowed also a CDE yield 2 90%. By using Cu/Si02 we observed a gradual loss of activity which didn't allow
the reaction
to go to completion. This behaviour was probably due to the acidity of support ; the catalyst
99
surface being slowly poisoned by reaction by-product. The systems Cu/riOz, Cu/BeO, Cu/Sn02, CuiZrOz, Cu/CeOz, Cu/Ta205, C a n O , Cu/CdO, Cu/Cr,03 and Cu/NbO, prepared by the same method of Cu/AI,O,, were all almost inactive. The dependence of the activity from the nature of support indicates a strong interaction between copper and oxide support; this latter playing a role during the molecular Hz activation. This is in agreement with the literature concerning the interaction of H, with copper chromite and copper aluminate (ref. 13,14). However, for catalysts with the same support, SCU(O) strongly influences the catalytic activity: conditions that increase the Cu(0) dispersion also increase catalyst performance. Thus the higher turnover numbers observed with CuOEnO pre-reduced at low temperature (100-200 "C), respect to that of the same catalyst pre-reduced at high temperature (270 "C), is related to the change of SCU(O) . The growth of the copper particles on the surface and the consequent diminution of Cu(0) dispersion by increasing the pre-reduction temperature occur also for other supports (examples for Cu/A1203and Cu/SiO, are reported in tab. 2). TABLE 2 Cu(0) specific surface area of CUFlxOy systemsa
a All systems were derived by corresponding CuO/Mfly by reduction in H2 atmosphere. key as tab. 1. Determined by N 2 0 adsorption. Dispersion of Cu(0) species in moles of surface Cu(0)per moles of total Cu. CU/TiO2,Cu/Ce02, zr02
The dramatic loss of activity exhibited by Cu/Mn02 when pre-reduced at 270°C deserves a word of comment. As reported in a previous work (ref. 15), this behaviour is related to the
100
diffusion of the copper in the bulk of oxide support which produces a very low dispersion of Cu(0). The method employed for supporting copper also influences Cu(0) dispersion and the consequent catalyst activity. Thus, in the case of C a n 0 and Cu/Cr203, the coprecipitation method produces active catalysts whereas the chemisorption method produces almost inactive materials. On the contrary, Cu/A1203is best prepared with the chemisorption method (tab.1). The relation between preparation method and activity can be explained, once again, in term of Cu(0) dispersion (tab. 2). In our conditions, in fact, all systems with a Cu(0) dispersion below 0.02 were almost inactive regardless of the oxide support and preparation method. The relation between total copper concentration and activity was studied for the Cu/A1203 catalyst. In table 3 are reported the turnover number and SCU(O)value for a series of catalysts with copper content ranging from 3.1 to 14.5%. TABLE 3 Turnover numbersaand Cu(0) specific area for Cu/A1203bsystems with a different Cu content %
cu
3.1 4.7 6.5
8.4
9.8 14.5
SCu(0)
SCU(0)d
TvSe
Tv
141 105 95 85 65 47
139 88 93
540 670 858 900 720
108 101 110 100 65 42
53 49
595
= 300 kPa; T = 1 W C . CDT/Cu molar ratio = 100. Re-reduction temp. = 100 - 2WC. Derived by N 2 0 chemisorption. Derived by ESCA method. Initial specific turnover in moles of H2 consumed per mole of surface Cu(0) (derived by N20 chemisorption) per hr.
In order to exclude the presence of chemisorbed hydrogen (ref 14), which produces overestimated values when the Cu(0) specific area is determined by N20 chemisorption, we have also determined these values from ESCA data. In particular we have derived specific surface area of Cu(0) from Cu
2p signal intensity ratio (ref. 12). The SCU(O)
values derived by means the two different techniques show to be in agreement within a 15% (tab. 3, col. 2 and 3) and indicate that reduced catalyst doesn't contain significative amounts of
chemisorbed
hydrogen.
is observed as copper loading From the data in table 3, a progressive decrease of SCU(O) increases. Since the temperature used for pre-reduction should avoid the diffusion of copper in the bulk (ref. 16), this trend indicate an increment of average size of Cu(0) crystallite (from 2 to 7
101
nm (ref. 11)). The growth of Cu(0) particles scarcely influences the turnover number referred to the total copper (Tv) for catalysts with a copper loading less than 8.4%. This behaviour is apparently in contrast with the great variation of SCU(O), but can be explained by considering that the specific surface turnover (TvS) does not show a constant value but a maximum at c.a. 8% (tab 3 col. 4). The lowering of TvS at the extremities of the concentration range can be due to either higher effect of poisoning (low copper loading) or a less favourable surface ratio between copper and alumina (high copper loading). Finally, with the aim to optimize the performance of the Cu/A1203 systems, we have studied the influence of the operative conditions on the reaction selectivity. TABLE 4 Hydrogenation of Cyclododecatrienecatalysed by Cu/A1203 a Catal.
TPC)
C~/A120,
180 200 140 180 160 C U O / A ~ ~ O ~ ~ 180
P(kPa) CDE
Tv(h-') CDD
Reaction Products (mole%)c CDE
100
130 133 7 207 90 <5
94 92
4
4
86 91
7 4.5
7 4.5
250 300 100
3
3
a CDT/Cu molar ratio = 100. Re-reduction temp. = 27OOC; Cu% = 7.0: when 2 moles of Hz were consumed per mole of CDT. Untreated catalyst.
As shown in table 4 the catalyst was satisfactorily active at temperature higher than 14OOC, while the selectivity decreases as temperature and pressure increase. Yields up to 94% of CDE were obtained at 180°C and 100 H a . The reductive pretreatment was also essential for the catalyst activity, the untreated material being almost inert with no evidence of in situ activation. This behaviour confirms that Cu(0) is an active species in the hydrogenation of CDT. From the data above exposed we can conclude that copper carefully dispersed on alumina and other M f l ~oxides represents a effective and cheap system for selective CDE production.
102
REFERENCES
1 A. Misono and I. Ogata, Bull. Chem. Soc.Jap., 40 (1967)2718 2 D.R. Fahey, J. Org. Chem., 38(1) (1973)80 3 K.Kihara, K. Katsuragawa and K. Yoshimitsu, Jap. Pat. 7479,990- C.A. 83:27714m 4 N.V. Unilever Neth. Pat. 6,608,993- C.A. 675393511 5 C. Fragale, M. Gargano, M. Rossi, J. Am. Oil Chem. Soc.,59 (1982)465 6 C. Fragale, M. Gargano, N. Ravasio, M. Rossi, I. Santo, Inorg. Chim. Acta, 82 (1984)157 7 C. Fragale, M.Gargano, N. Ravasio and M. Rossi, Gazz. Chim. Ital., I17 (1987)43 8 M.Gargano, N.Ravasio. M. Rossi, I. Santo, La Chimica e l'Indusma, 694 (1987)1. 9 C.Fragale, M. Gargano, M. Rossi, Italian Patent 24437 A/82 10 S.Koritala, J. Am. Oil Chem. Soc.,49 (1972)83 11 T.J. Osinga, B.G. Linsen, W.P. van Beek, J. Catal., 7 (1976)277 12 V.Di Castro, C. Furlani, M. Gargano, N.Ravasio, M. Rossi., I. Electr.Spectr. (1990)in press 13 R. Bechara, G. Wrobel, M. Daage, J.P. Bonnelle. Appl. Catal., 16 (1985)15 14 L. Jalowiecki, G. Wrobel, M. Daage, J.P. Bonnelle, J. Catal., 107 (1987)375 15 V. Di Castro, C. Furlani, M. Gargano, M.Rossi, Appl. Surf. Science 28 (1987)270 16 B.R. Strohmeier, D.E. Leyden, R.S. Field, D.M. Hercules, J. Catal.,94 (1985)514.
G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catulysts V 0 1991 Elsevier Science Publishers B.V., Amsterdam - Printed i n T h e Netherlands
103
PREPARATION AND CHARACTERIZATION OF HIGHLY SELECTIVE Fe-Cu/Si02 CATALYSTS FOR PARTIAL HYDROGENATION OF ALKYNES
Y u r i k o NITTA, Yoshifumi HIRAMATSU, Y a s u a k i OKAMOTO and Toshinobu IMANAKA Department of Chemical E n g i n e e r i n g , F a c u l t y of E n g i n e e r i n g S c i e n c e , Osaka U n i v e r s i t y , Toyonaka, Osaka 560, J a p a n SUMMARY A h i g h l y s e l e c t i v e Fe-Cu/Si02 c a t a l y s t w i t h a n enhanced a c t i v i t y f o r p a r t i a l h y d r o g e n a t i o n of p h e n y l a c e t y l e n e w a s p r e p a r e d by c o p r e c i p i t a t i o n of i r o n and c o p p e r s u l f a t e s o v e r u l t r a f i n e s i l i c a g e l w i t h a n e x c e s s a m o u n t of s o d i u m The e f f e c t s o f t h e p r e c i p i t a t i o n v a r i a b l e s and carbonate a t a l o w temperature. t h e d o p i n g o f c o p p e r and s u l f a t e i o n s o n t h e r e d u c t i o n b e h a v i o r , s u r f a c e s t a t e s , and c a t a l y t i c p r o p e r t i e s of t h e p r e c u r s o r s a n d / o r r e d u c e d c a t a l y s t s were s t u d i e d by u s i n g TGA, XRD, and XPS. The p r e s e n c e of c o p p e r lowers t h e d e c o m p o s i t i o n and r e d u c t i o n t e m p e r a t u r e s of a-FeOOH t o a-Fe20g a n d Fe2+, t h u s d e c r e a s i n g t h e amount o f water produced a t h i g h e r t e m p e r a t u r e s and p r e v e n t i n g t h e s i n t e r i n g of i r o n m e t a l . The l o w p r e c i p i t a t i o n t e m p e r a t u r e and t h e u s e of metal s u l f a t e s are b o t h e f f e c t i v e t o p r o v i d e h i g h l y - d i s p e r s e d metal s p e c i e s i n t h e p r e c u r s o r , which r e s u l t i n a n i n c r e a s e i n t h e i r o n d i s p e r s i o n of t h e reduced c a t a l y s t . INTRODUCTION I r o n catalysts show s p e c i f i c a c t i v i t y and s e l e c t i v i t y f o r t h e h y d r o g e n a t i o n of c a r b o n monoxide.
For p a r t i a l hydrogenation of alkynes, analogously prepared
i r o n catalysts e x h i b i t h i g h s e l e c t i v i t y (refs. 1-5), b u t e x t r e m e l y l o w a c t i v i t y , which h i n d e r s p r a c t i c a l u s a g e of
iron-based
catalysts.
Development o f
iron
c a t a l y s t s with h i g h a c t i v i t y for a l k y n e hydrogenation is r e q u i r e d f o r i n d u s t r i a l applications.
Recently, w e have r e p o r t e d t h a t ,
c a t a l y s t s , t h e u s e o f f e r r i c s u l f a t e , non-porous
i n t h e p r e p a r a t i o n of F e / S i 0 2 u l t r a f i n e s i l i c a g e l , and a n
e x c e s s amount of Na2C03 r e s u l t s i n a n a c t i v e and s e l e c t i v e c a t a l y s t f o r t h e h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e t o s t y r e n e u n d e r m i l d r e a c t i o n c o n d i t i o n s , and t h a t t h e s e l e c t i v i t y f o r s t y r e n e i n c r e a s e s w i t h i n c r e a s i n g d i s p e r s i o n of i r o n metal ( r e f . 6 ) .
In
o r d e r t o i n c r e a s e t h e d i s p e r s i o n of i r o n ,
and
hence
to
i n c r e a s e t h e s e l e c t i v i t y as w e l l as t h e a c t i v i t y o f t h e c a t a l y s t , t h e e f f e c t of c o p r e c i p i t a t i o n of
d i f f e r e n t metals w a s examined.
The a d d i t i o n of
proper
amount of c o p p e r w a s f o u n d t o b r i n g a b o u t r e m a r k a b l e i n c r e a s e s i n t h e a c t i v i t y and s e l e c t i v i t y of
the
r e s u l t i n g c a t a l y s t s ( r e f . 7).
Moreover,
simultaneous
a d d i t i o n o f c o p p e r and s u l f a t e i o n s and a l o w p r e c i p i t a t i o n t e m p e r a t u r e a p p e a r e d e s s e n t i a l t o t h e p r e p a r a t i o n of a h i g h l y a c t i v e and s e l e c t i v e c a t a l y s t . work,
t h e e f f e c t s o f t h e p r e p a r a t i o n v a r i a b l e s and t h e d o p i n g o f
In this
c o p p e r and
s u l f a t e i o n s o n t h e p r o p e r t i e s o f F e / S i 0 2 (1:l) c a t a l y s t s were s t u d i e d i n d e t a i l by u s i n g TGA, XRD, a n d XPS.
104 EXPERIMENTAL Catalyst preparation
C a t a l y s t p r e c u r s o r s were p r e p a r e d by a p r e c i p i t a t i o n method a s f o l l o w s : s o l u t i o n o f a n e x c e s s amount of Na2C03 (1.5-1.7
m o l a r e q u i v a l e n t ) i n 2 5 m l of
d i s t i l l e d water was a d d e d t o a n a q u e o u s s u s p e n s i o n (150 m l ) c o n t a i n i n g non-porous
ultrafine silica gel
iron(lI1) sulfate,
and
(Nippon
Aerosil,
copper(l1) s u l f a t e
usually at 7540°C
Aerosil-300,
3 2 0 m'g-l),
Th e p r e c i p i t a t i o n t e m p e r a t u r e
The p r e c i p i t a t e w a s a g e d t h r e e times
an d f i l t e r e d , f o l l o w e d by w ash i n g
water a n d d r y i n g a t 110°C f o r 20 h .
2 g of
( t o t a l amount o f metal = 36 mmol)
o v e r a p e r i o d of 1-2 min u n d e r v i g o r o u s s t i r r i n g .
w a s c ha nge d i n t h e r a n g e of 8 t o 8 0 ° C .
A
f o r 15 min with hot
O t h e r d e t a i l s were i d e n t i c a l t o t h o s e
described f o r the preparation of silica-supported b a s i c n i c k e l carbonate ( r e f s . 8,9).
A f e w p r e c u r s o r s were p r e p a r e d by u s i n g i r o n ( I I 1 ) and c o p p e r ( I 1 ) n i t r a t e s
f o r c ompa ris on.
Fe-Cu/Si02
c a t a l y s t s (metal l o a d i n g = ca. 50 w t % ) were p r e p a r e d
by h e a t i n g d r i e d p r e c u r s o r s (1 g ) i n f l o w i n g h y d r o g en ( 8 1 h - l ) t e m p e r a t u r e ramp of 10°C min-' otherwise noted.
an d h o l d i n g a t
up t o 500°C a t a
temperature
for 1 h u n l e s s
C a t a l y s t s a r e d e n o t e d by t h e a t o m i c c o m p o s i t i o n a n d t h e
p r e p a r a t i o n ( p r e c i p i t a t i o n and aging) Fe-Cu(7:3)/Si02
this
temperatures
s u c h as
c a t a l y s t p r e c i p i t a t e d a t a low t e m p e r a t u r e
for
the
( u s u a l l y , 20°C)
F7C3-LH
and
s u b s e q u e n t l y a ge d a t a h i g h t e m p e r a t u r e ( u s u a l l y , 75-80%). Hydrogenation The h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e (0.6 m l ) w a s c a r r i e d o u t i n e t h a n o l ( 2 7 m l ) with a freshly-reduced
c a t a l y s t a t 60°C u n d e r a h y d r o g en p r e s s u r e of 1 MPa
by u s i n g a g l a s s a u t o c l a v e e q u i p p e d w i t h a m a g n e t i c s t i r r i n g s y s t e m . i n i t i a l h y d r o g e n a t i o n rates (Ro)
Th e
and s t y r e n e s e l e c t i v i t i e s , d e f i n e d as t h e mo l ar
p e r c e n t a g e of s t y r e n e i n a l l t h e p r o d u c t s , a t 50% c o n v e r s i o n (S50) were o b t a i n e d from t h e GLC d a t a o f t h e p r o d u c t s a t d i f f e r e n t r e a c t i o n times.
Analysis Powder X-ray
d i f f r a c t i o n (XRD)
p a t t e r n s of t h e c a t a l y s t s s e p a r a t e d from t h e
r e a c t i o n m i x t u r e w e r e m easu r ed u s i n g a Shimadzu VD-1 radiation.
T h e m e an c r y s t a l l i t e s i z e (D,)
diffractometer with C u b
of i r o n metal i n a c a t a l y s t w a s
c a l c u l a t e d f r o m t h e half-maximum b r e a d t h o f t h e (110) p eak o f a-Fe a f t e r correct i o n f o r i n s t r u m e n t a l broadening ( r e f .
10).
The c r y s t a l l i t e s i z e d i s t r i b u t i o n s
(CSD) of i r o n i n some c a t a l y s t s were o b t a i n e d a c c o r d i n g t o t h e F o u r i e r t r a n s f o r m method ( S t o k e s m e thod ) f o r X-ray l i n e p r o f i l e a n a l y s i s ( r e f . 11). T h e r m o g r a v i m e t r i c a n a l y s e s (TGA) an d d i f f e r e n t i a l t h e r m a l a n a l y s e s (DTA) of c a t a l y s t p r e c u r s o r s were c a r r i e d o u t w i t h a Shimadzu DT-30 h e a t i n g i n a stream of h y d r o g e n t o 800°C a t a otherwise noted.
rate
t h e r m a l a n a l y z e r by
of 1 0 ° C m i n - l
unless
The d e g r e e o f r e d u c t i o n o f a c a t a l y s t w a s e s t i m a t e d from t h e
105 weight l o s s i n t h e range above
1 0 0 ° C on t h e b a s i s of t h e t h e o r e t i c a l w e i g h t
l o s s f o r 100% r e d u c t i o n of t h e p r e c u r s o r . The X-ray p h o t o e l e c t r o n s p e c t r a (XPS) of some p r e c u r s o r s and f r e s h l y - r e d u c e d c a t a l y s t s were measured o n a H i t a c h i 507 p h o t o e l e c t r o n s p e c t r o m e t e r u s i n g a n A 1 The sample w a s mounted on a d o u b l e - s i d e d
anode.
adhesive tape.
Binding
e n e r g i e s (BE) were r e f e r e n c e d t o t h e S i ( 2 p ) band a t 103.1 eV due t o t h e c a t a l y s t s u p p o r t . The peak areas were u s e d f o r a q u a n t i t a t i v e s u r f a c e a n a l y s i s of
the
D e t a i l e d procedures have been described elsewhere ( r e f . 12).
sample. RESULTS
Reduction behavior
of copper-promoted
precursors
The d i f f e r e n t i a l t h e r m o g r a v i m e t r i c (DTG) p r o f i l e s i n F i g . 1 i n d i c a t e t h a t t h e r e d u c t i o n of Fe-Cu/Si02
precursors v i r t u a l l y
p r o c e e d s i n two s t e p s , v i z . ,
a
s h a r p p e a k a t a b o u t 200°C and a b r o a d p e a k a t a r o u n d 500OC. On t h e b a s i s of t h e s e p a r a t e DTG a n d DTA e x p e r i m e n t s i n H2
a n d N2
f o r Cu/Si02
and Fe/Si02
p r e c u r s o r s , i t was c o n c l u d e d t h a t t h e s h a r p DTG peak a t a r o u n d 200°C c o r r e s p o n d s t o a r a p i d w e i g h t l o s s due t o t h e r e d u c t i o n of Cu2+ t o Cuo s u p e r p o s e d by a b r o a d peak d u e t o t h e d e c o m p o s i t i o n of a-FeOOH t o a-Fe203,
and t h a t
t h e b r o a d peak a t
a r o u n d 500°C 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 i r o n o x i d e s o r s i l i c a t e t o metallic i r o n . T h i s i s c o n s i s t e n t
-
with t h e results i n literature
1 4 , 15).
(refs.
The p e r c e n t a g e
"C ( A , )
and above 3OOOC ( A , )
c u 90% Cu 70%
w e i g h t l o s s e s i n t h e r a n g e of 100-300 l i s t e d i n T a b l e 1, t o g e t h e r
c u 100%
are
with t h e
h y d r o g e n a t i o n r e s u l t s on t h e r e d u c e d catalysts.
It i s noteworthy t h a t t h e
s h a r p DTG peak due t o Cu2+ r e d u c t i o n s h i f t e d t o a lower t e m p e r a t u r e f o r
c u 20%
t h e most a c t i v e and s e l e c t i v e c o n t a i n i n g ca. 30% Cu.
catalyst
addition,
"C (A;)
In
t h e w e i g h t l o s s a b o v e 300
close t o a t h e o r e t i c a l value,
f o r t h e c o m p l e t e r e d u c t i o n of Fez+ t o whereas t h e weight l o s s e s f o r t h e catalysts were
larger
c u 0%
9.0%,
Feo (FeO - C u / S i 0 2 + F e - C u / S i 0 2 ) , other
c u 5%
f o r t h e F7C3-HH c a t a l y s t w a s
than
those e x p e c t e d , i n d i c a t i n g t h a t p a r t
of Fe3+ s p e c i e s s t i l l e x i s t s i n t h e s e
I
100
I
200
I
300
400
500
I
600
T e m p e r a t u r e / "C F i g . 1. DTG-in-H2 p r o f i l e s of Fe-Cu/Si02 p r e c u r s o r s w i t h d i f f e r e n t content of copper.
106 TABLE 1 E f f e c t s o f c o p p e r c o n t e n t on t h e r e d u c t i o n b e h a v i o r of Fe-Cu/Si02
precursorsa
and on t h e c a t a l y t i c p r o p e r t i e s a f t e r r e d u c t i o n a t 500°C f o r 1 h. Catalyst
a T e m p e r a t u r e s o f p r e c i p i t a t i o n and a g i n g are b o t h 75°C. b Peak t e m p e r a t u r e f o r r e d u c t i o n o f C u * + s p e c i e s . c P e r c e n t a g e w e i g h t l o s s e s d u r i n g 100-3OO0C( AI) and a b o v e 3OO0C(A2), and t h e p e r c e n t a g e l o s s b a s e d on t h e w e i g h t a t 3OO0C(A;). d Degree o f r e d u c t i o n measured by TGA i n H2. e Mean c r y s t a l l i t e s i z e of w F e . f S e l e c t i v i t y a t 10%c o n v e r s i o n . g Reduced at 300°C f o r 1 h i n s t e a d of 500'C. TABLE 2 E f f e c t s of p r e c i p i t a t i o n v a r i a b l e s on t h e p r o p e r t i e s of Fe-Cu(7:3)/Si02
and
F e / S i 0 2 p r e c u r s o r s and t h e r e d u c e d c a t a l y s t s . Catalyst
a R e d u c t i o n w a s c a r r i e d o u t by h e a t i n g a t a r a t e of 2.5"C/min t o 400°C and h e l d a t 400°C f o r 5 h. b R e d u c t i o n w a s c a r r i e d o u t a f t e r h e a t e d a t 300°C f o r I h i n a n N2 stream. c S e l e c t i v i t y a t 10% c o n v e r s i o n .
107 c a t a l y s t s a t 300°C.
These reduction
p r o c e s s e s were c o n f i r m e d by XPS a s s t a t e d below. lower and
The a b o v e r e s u l t s s u g g e s t t h a t t h e the
r e d u c t i o n t e m p e r a t u r e s of
Cu2+
a-FeOOH a r e , t h e m o r e a c t i v e t h e
r e d u c e d c a t a l y s t becomes.
~E f f e c t s of In
precipitation variables
t h e course of
preparation conditions,
the
s t u d i e s on
i t was found
that
t h e p r e c i p i t a t i o n t e m p e r a t u r e had a g r e a t e f f e c t on t h e c a t a l y t i c p r o p e r t i e s of r e s u l t i n g catalyst.
the
The p r o p e r t i e s o f Fe-
Cu/Si02 and F e / S i 0 2 p r e c u r s o r s p r e p a r e d a t different precipitation and/or aging t e m p e r a t u r e s are l i s t e d together
i n Table
2,
p r e p a r e d f r o m metal n i t r a t e s .
p r e c u r s o r s p r e c i p i t a t e d a t a r o u n d 2OoC had l a r g e r BET s u r f a c e a r e a s w i t h f l o s s y t e x t u r e and e x h i b i t e d h i g h e r h y d r o g e n a t i o n activities,
when r e d u c e d ,
p r e c i p i t a t e d a t 55-80°C.
catalysts,
I
.",
was
i
1 F7C3-LH
I n t h e case of t h e c o p p e r - f r e e however,
/ "C
F i g . 2. DTG p r o f i l e s of F7C3-LH and -HH p r e c u r s o r s : ( A ) a s prep a r e d , ( B ) a f t e r h e a t e d a t 300°C f o r 3 . h i n N2, and ( C ) measured i n N2.
than those
With t h e a g i n g
p r o c e s s , t h e h i g h t e m p e r a t u r e (75-80°C) preferable.
Temperature
Irrespective
of t h e s t a r t i n g s a l t , t h e Fe-Cu/Si02
500
300
100
with t h e data f o r t h e c a t a l y s t s
the low precipitation
t e m p e r a t u r e had n e g a t i v e e f f e c t s on t h e r e d u c i b i l i t y of t h e p r e c u r s o r s and on t h e catalytic properties. As c a n b e s e e n f r o m F i g .
2,
t h e DTG
peak a t a r o u n d 200°C f o r t h e most a c t i v e c a t a l y s t , F7C3-LH, lower
temperature
Dc / nm
shifted further to a than
that
f o r F7C3-HH.
The DTG p r o f i l e s measured a f t e r h e a t i n g a t
F i g . 3. CSDs of a-Fe i n F7C3-LH and F7C3-LH(N) c a t a l y s t s .
3OOOC for 3 h i n a n N2 stream, which have no s u p e r p o s i t i o n of
t h e d e c o m p o s i t i o n p r o f i l e s of
a-FeOOH,
show more c l e a r l y
t h a t t h e r e d u c t i o n of CuO i n F7C3-LH p r e c u r s o r o c c u r s a t a t e m p e r a t u r e l o w e r t h a n t h a t i n F7C3-HH
precursor.
The d i f f e r e n c e
30-50°C
i n t h e decomposition
t e m p e r a t u r e of a-FeCQH t o a-Fe203 w a s l e s s t h a n 10°C between F7C3-HH and F7C3-LH a s r e v e a l e d i n t h e p r o f i l e s measured
i n an N2
stream.
Therefore,
the
108 r e d u c i b i l i t y of Cu2+ i n t h e p r e c u r s o r seems t o h a v e a s t r i k i n g e f f e c t on t h e c a t a l y t i c p r o p e r t i e s of t h e r e d u c e d c a t a l y s t . is also s i g n i f i c a n t .
The F7C3-LH
The e f f e c t of t h e s t a r t i n g s a l t
catalyst prepared from s u l f a t e s e x h i b i t e d
a l m o s t f i v e times h i g h e r a c t i v i t y t h a n t h e c a t a l y s t
F7C3-LH(N), a l t h o u g h t h e y had similar v a l u e s o f Dc. i n F i g . 3.
c a t a l y s t s are compared
p r e p a r e d from n i t r a t e s ,
The CSDs of a-Fe
i n these
E v i d e n t l y , the p r e s e n c e of v e r y small
c r y s t a l l i t e s of i r o n e x p l a i n s t h e h i g h a c t i v i t y of t h e F7C3-LH i n c r e a s e d r e d u c t i o n t e m p e r a t u r e of Cu2'
catalyst.
The
f o r F7C3-LH(N) r e n d e r s f u r t h e r e v i d e n c e
t o t h e i m p o r t a n c e of t h e r e d u c i b i l i t y of Cu2+ i n t h e p r e c u r s o r .
XPS s t u d i e s _-_-
of v a r i o u s p r e c u r s o r s and r e d u c e d c a t a l y s t s
The XPS p a r a m e t e r s f o r v a r i o u s p r e c u r s o r s and the c a t a l y s t s r e d u c e d a t 300°C
f o r 0.5 h o r a t 500°C f o r 1 h are summarized i n T a b l e 3.
The s p e c t r a of t h e
Fe2p l e v e l f o r t h e F7C3-LH p r e c u r s o r and t h e r e d u c e d s a m p l e s are shown i n F i g . 4 .
TABLE 3 XPS b i n d i n g e n e r g i e s (BE), k i n e t i c e n e r g i e s (KE), and i n t e n s i t y r a t i o s f o r Fe-Cu/Si02 Catalyst
p r e c u r s o r s and t h e r e d u c e d c a t a l y s t s . Reduction Temp/"C Time/h
precursors with t h e copper content less t h a n 50% were v e r y c l o s e t o t h a t
f o r a-FeOOH,
w h i l e t h e BEs f o r t h e
p r e c u r s o r s c o n t a i n i n g 70 and 90% Cu were c l o s e t o t h a t f o r F e 2 ( S 0 4 ) 3 . The BE of t h e Cu(2p3/2) l e v e l and t h e k i n e t i c e n e r g y (KE) o f t h e Cu(LMM) Auger l e v e l f o r a l l p r e c u r s o r s are c o n s i s t e n t w i t h t h o s e f o r CuO ( r e f .
12).
With t h e c a t a l y s t s r e d u c e d a t
5OO0C f o r 1 h , t h e BE and KE v a l u e s i n d i c a t e t h a t i r o n e x i s t s as Fe2'
and
Feo r e g a r d l e s s o f t h e c o m p o s i t i o n ,
\\
w h i l e c o p p e r e x i s t s o n l y as Cu metal.
As
for
the
catalysts partially
r e d u c e d a t 300°C f o r 0.5 h , t h e BEs
I
750
740
i r o n e x i s t a s Fe2+ i n F7C3-LH and as
+
Fe3+ i n F7C3-HH
catalysts.
T h i s i s i n good a g r e e m e n t w i t h t h e result
of
TGA
experiments
d e s c r i b e d above.
I
720
710
700
BE/eV
of t h e Fe( 2p3/2) l e v e l i n d i c a t e t h a t Fe2'
I
730
F i g . 4. Fe2p XP s p e c t r a f o r F7C3-LH. ( A ) P r e c u r s o r , ( B ) r e d u c e d a t 300°C f o r 0.5 h , and ( C ) r e d u c e d a t 500°C f o r 1 h .
as 10
T h e Cu(LMM) KE
I
v a l u e s f o r t h e s e two c a t a l y s t s show t h a t t h e o x i d a t i o n state of c o p p e r a l s o d i f f e r s f r o m e a c h o t h e r : Cuo f o r F7C3-LH
and
catalysts. detected
Cut
f o r F7C3-HH
T h e S ( 2 p ) s p e c t r a were only
in
the
catalysts
a
N .r(
m
sa
\
N
3
V
a
c o n t a i n i n g more t h a n 60% Cu, as S042-
N .d
in the
\
p r e c u r s o r s and
as
metal
s u l f i d e s i n t h e reduced catalysts.
m
a
N
a
L
Therefore, t h e extremely l o w a c t i v i t y of t h e catalyst w i t h such a h i g h c o n t e n t of c o p p e r (see T a b l e 1) c a n be, a t least p a r t l y , a t t r i b u t e d t o t h e p o i s o n i n g e f f e c t of t h e r e m a i n i n g sulfur. In Fig.
5, t h e Fe2p/Si2p
and
Cu2p3/2/Si2p i n t e n s i t y r a t i o s f o r t h e
C u / ( F e t C u ) i n Bulk F i g . 5. F e 2 p / S i 2 p (0) and Cu2p3/,/Si2p (A) XPS i n t e n s i t y r a t i o s f o r Fe-Cu/Si02 p r e c u r s o r s p r e c i p i t a t e d a t 75OC a s a f u n c t i o n o f Cu c o n t e n t i n b u l k . ( 0 ) F e 2 p / S i 2 p a f t e r r e d u c e d a t 5OO0C f o r 1 h.
110 p r e c u r s o r s p r e c i p i t a t e d a t 75OC are p l o t t e d a g a i n s t t h e b u l k c o m p o s i t i o n .
The
F e 2 p / S i 2 p r a t i o i n c r e a s e d s i g n i f i c a n t l y w i t h i n c r e a s i n g c o n t e n t of c o p p e r i n t h e b u l k up t o 30%, which i m p l i e s t h e i n c r e a s e d d i s p e r s i o n o f i r o n i n t h e p r e s e n c e of copper.
I n t h e p r e c u r s o r o f t h e most a c t i v e F7C3-LH
catalyst, further
i n c r e a s e s i n t h e XPS r a t i o s were o b s e r v e d as c i t e d i n T a b l e 3. DISCUSSION H y d r o g e n a t i o n r e s u l t s p r e s e n t e d i n T a b l e s 1 and 2 show t h a t t h e c a t a l y s t w i t h higher a c t i v i t y e x h i b i t s higher s e l e c t i v i t y . previous r e p o r t s (refs.
6, 7 ) .
T h i s is i n conformity with our
The a c t i v i t y of t h e Fe-Cu(7:3)/Si02
p r e c i p i t a t e d a t a r o u n d 2OoC by u s i n g m e t a l s u l f a t e s , F7C3-LH,
catalyst
was three orders
h i g h e r t h a n t h a t of a n o r d i n a r y Fe/Si02 c a t a l y s t prepared from ferric n i t r a t e , FlO-HH(N).
T h e h i g h s e l e c t i v i t y of t h e F7C3-LH
c a t a l y s t w a s k e p t almost
c o n s t a n t d u r i n g t h e r e a c t i o n and w a s 99% e v e n a t 98% c o n v e r s i o n . Although t h e r e d u c t i o n e x t e n t of i r o n i s u s u a l l y i n c r e a s e d by t h e p r e s e n c e of copper ( r e f s .
16, 1 7 ) , t h e r e i s no s i g n i f i c a n t
difference
in
t h e percentage
r e d u c t i o n s of t h e catalysts under t h e p r e s e n t r e d u c t i o n c o n d i t i o n s , suggesting t h a t t h e d i f f e r e n c e i n a c t i v i t y is n o t due s o l e l y t o t h e d i f f e r e n c e i n t h e reduction degree of i r o n . t h e H2-reduction
It h a s b e e n r e p o r t e d t h a t t h e water produced d u r i n g
s t r o n g l y a f f e c t s t h e d i s p e r s i o n o f i r o n metal,
sintering especially at
high temperatures ( r e f s .
r e d u c t i o n c o n d i t i o n s on t h e c a t a l y t i c consistent with
these observations.
causing
The e f f e c t s of
p r o p e r t i e s , shown i n T a b l e 2 , a r e
When t h e F7C3-LH
reduced a t a lower temperature for a longer employed h e r e (50O0C, 1 h ) ,
18, 19).
precursor
was
slowly
time t h a n t h e u s u a l c o n d i t i o n s
a d e c r e a s e i n Dc and much improved a c t i v i t y w e r e
obtained, probably due t o e f f i c i e n t
e l i m i n a t i o n o f water ( r e f . 7 ) .
Our
DTG
e x p e r i m e n t s showed t h a t t h e r e d u c t i o n o f CuO and a-Fe00H i n t h e p r e c u r s o r of t h e a c t i v e catalysts c o n t a i n i n g
ca.
those i n the other catalysts.
30% Cu p r o c e e d s a t l o w e r t e m p e r a t u r e s t h a n
I n l i n e w i t h t h i s , Wielers e t a l . h a v e v e r y
r e c e n t l y r e p o r t e d t h a t c o p p e r f a c i l i t a t e s t h e r e d u c t i o n of Fe3+ t o Fe2+ as w e l l a s t h e r e d u c t i o n t o Feo ( r e f . 2 0 ) .
The i n c r e a s e i n t h e r e d u c i b i l i t y of Fe3+ i n
t h e p r e s e n c e o f c o p p e r i s c l e a r l y e v i d e n c e d by t h e XPS r e s u l t s i n T a b l e 3. A c c o r d i n g l y , one o f t h e p r o m o t i o n a l e f f e c t s of c o p p e r c a n b e e x p l a i n e d i n terms of t h e i n c r e a s e d r e d u c i b i l i t y of Fe3+:
t h e amount of water produced a t h i g h
t e m p e r a t u r e s becomes smaller i n t h e p r e s e n c e of
copper,
thus preventing t h e
s i n t e r i n g o f i r o n metal. A s f o r t h e effect of p r e c i p i t a t i o n t e m p e r a t u r e ,
it h a s been r e p o r t e d t h a t
amorphous o r smaller p a r t i c l e s of a-FeOOH are s t a b l e i n c o l l o i d a l s o l u t i o n s a t t e m p e r a t u r e s below 5 5 " C ,
w h i l e l a r g e (50-6Om) p a r t i c l e s of h e m a t i t e , cx-Fe203,
are formed a b o v e 85 OC ( r e f .
21).
It seems l i k e l y t h a t smaller p a r t i c l e s of
o x i d i z e d i r o n s p e c i e s i n t h e p r e c u r s o r are r e d u c e d more e a s i l y ,
resulting i n
111 smaller metal p a r t i c l e s .
The h i g h
BET s u r f a c e a r e a s a n d t h e
-15.0
flossy
nature of t h e precursors p r e c i p i t a t e d
a t t h e low t e m p e r a t u r e are also favorable f o r of
water
process. at
efficient
elimination
during the reduction Moreover, t h e p r e c i p i t a t i o n
lower
temperatures
cause
will
smaller e x t e n t of m e t a l - s u p p o r t
i n t e r a c t i o n and l e a d t o i n c r e a s e d r e d u c i b i l i t y o f Cu2+ a n d F e 3 + ,
as
1
e v i d e n c e d by t h e TGA and XPS r e s u l t s .
Fe2p / S i 2 p
The e x o t h e r m i c r e d u c t i o n of Cu2+ a t a lower t e m p e r a t u r e g r e a t l y f a v o r s t h e
F i g . 6. Dependence of t h e hydrogenat i o n a c t i v i t y on t h e F e 2 p / S i 2 p XPS i n t e n s i t y r a t i o s f o r Fe-Cu(7:3)/Si02 p r e c u r s o r (0)and t h e r e d u c e d c a t a l y s t (0) p r e p a r e d f r o m metal s u l f a t e s . b,& P r e p a r e d from metal n i t r a t e s .
r e d u c t i o n of Fe3+ i n a n e a r l y s t a g e
of t h e a c t i v a t i o n .
Accordingly,
it
is e v i d e n t t h a t t h e p r e c i p i t a t i o n a t the
low t e m p e r a t u r e
catalysts with
provides
the
high dispersion of
i r o n and hence h i g h a c t i v i t y and s e l e c t i v i t y .
As s h o w n i n F i g . 6 , t h e XPS
r e s u l t s support t h e i d e a t h a t t h e increase i n
a c t i v i t y is a t t r i b u t e d t o a n
improved d i s p e r s i o n o f i r o n . C o n c e r n i n g t h e e f f e c t s of catalyst,
prepared
t h e s t a r t i n g s a l t , t h e F7C3-LH(N)
from metal
nitrates,
had
relatively
p r e c u r s o r and
l a r g e XPS i n t e n s i t y
r a t i o s f o r s u r f a c e i r o n , a l t h o u g h t h e c a t a l y s t e x h i b i t e d much l o w e r a c t i v i t y than t h a t prepared from s u l f a t e s .
The CSD i n F i g . 3 showed t h e l a c k of
i r o n c r y s t a l l i t e s i n t h e F7C3-LH(N) l a r g e i r o n p a r t i c l e s i n F7C3-LH(N)
catalyst.
small
These f i n d i n g s suggest
that
e x i s t i n t h e o u t e r s u r f a c e , whereas copper
p a r t i c l e s e x i s t i n t h e i n n e r s u r f a c e i n t e r a c t i n g w i t h t h e s u p p o r t , a s is e v i d e n c e d by t h e i n c r e a s e d r e d u c t i o n t e m p e r a t u r e of Cu2+ f o r F7C3-LH(N) 2).
Thus,
provide
(Table
t h e presence of s u l f a t e i o n s i n p r e c i p i t a t i o n appears c r u c i a l
well-mixed
to
s m a l l p a r t i c l e s of i r o n and copper i n t h e p r e c u r s o r .
F u r t h e r s t u d i e s a r e n e e d e d t o make c l e a r t h e r o l e of s u l f a t e i o n s i n t h e p r e c i p i t a t i o n process. CONCLUSIONS I n t h e p r e p a r a t i o n o f h i g h l y a c t i v e a n d s e l e c t i v e Fe-Cu/Si02 c a t a l y s t f o r t h e p a r t i a l h y d r o g e n a t i o n o f p h e n y l a c e t y l e n e , c o p r e c i p i t a t i o n of 70% F e and 30% Cu, t h e u s e o f metal s u l f a t e s , a n d a r e l a t i v e l y l o w p r e c i p i t a t i o n t e m p e r a t u r e ( a r o u n d 2OoC) were f o u n d t o b e v e r y e f f e c t i v e . catalyst was
three orders higher
The a c t i v i t y of t h i s improved
than t h e o r d i n a l Fe/Si02
c a t a l y s t and
the
112 s e l e c t i v i t y r e a c h e d 99% a t 98% c o n v e r s i o n . The p r e s e n c e of c o p p e r lowers t h e d e c o m p o s i t i o n and r e d u c t i o n t e m p e r a t u r e s of a-FeOOH
toa-Fe203 and Fez+, t h u s d e c r e a s i n g t h e amount of water produced a t
h i g h e r t e m p e r a t u r e s and p r e v e n t i n g t h e s i n t e r i n g of i r o n metal.
The p r e c i p i -
t a t i o n a t a low t e m p e r a t u r e p r o v i d e s t h e p r e c u r s o r s w i t h h i g h d i s p e r s i o n o f b o t h a-FeOOH and Cu2+ s p e c i e s , which r e s u l t i n t h e c a t a l y s t s w i t h h i g h l y - d i s p e r s e d i r o n metal.
The u s e of metal s u l f a t e s a p p e a r s c r u c i a l t o p r o v i d e well-mixed
small p a r t i c l e s of i r o n a n d c o p p e r i n t h e p r e c u r s o r .
REFERENCES P a u l and G . H i l l y s , B u l l . SOC. Chim. F r . [ 5 ] , 6 (1939) 218. F. Thompson and S. B. W y a t t , J. Am. Chem. SOC., 62 (1940) 2555. Reppe, Ann., 596 (1955) 38. T a i r a , B u l l . Chem. SOC. J p n . , 35 (1962) 840. R. S. Mann a n d K. C. Khulbe, Can. J . Chem., 45 (1967) 2755. Y . Nitta, S. M a t s u g i and T. Imanaka, Chem. E x p r e s s , 4 (1989) 547. Y. N i t t a , S. M a t s u g i and T. Imanaka, Catal. L e t t . , i n p r e s s . Y . Nitta, F. S e k i n e , T. Imanaka and S. T e r a n i s h i , J . C a t a l . , 74 (1982) 382. Y. N i t t a , T. Imanaka and S. T e r a n i s h i , J. Catal., 96 (1985) 429. B. E. Warren, J. Appl. P h y s . , 12 (1941) B75. A. R . S t o k e s , P r o c . Phys. SOC. London, 61 (1948) 382. Y. Okamoto, K . F u k i n o , T. Imanaka and S. T e r a n i s h i , J. Phys. Chem., 87 (1983) 3740 and 3747. M. Rameswaran a n d C. H. Bartholomew, J. Catal., 117 (1989) 218. J . W. Geus, i n : G. P o n c e l e t , P. Grange and P . A. J a c o b s ( E d s . ) , P r e p a r a t i o n of C a t a l y s t s 111, E l s e v i e r , Amsterdam, 1983, pp.1-33. A. F. H . Wielers, A . J. H . M. Kock, C. E. C. A. Hop, J. W. Geus and A. M. van d e r K r a a n , J. C a t a l . , 117 (1989) 1. R . N. P e a s e , H. S. T a y l o r , J. Am. Chem. SOC., 43 (1921) 2179. Y. Murata a n d S. Kasaoka, Kogyo Kagaku Z a s s h i , 62 (1959) 1195. Y. M u r a t a and S. Kasaoka, Kogyo Kagaku Z a s s h i , 61 (1958) 1440. E. R u c k e n s t e i n a n d X. D. Hu, J . C a t a l . , 100 (1986) 1. A. F. H . Wielers, C. E. C. A . Hop, J. van Beijnum, A . M. v a n d e r Kraan, and J. W. Geus, J . Catal., 121 (1990) 364. J. W. G e u s , Appl. C a t a l . , 25 (1986) 313.
1 R. A. W. S.
2 3 4 5 6 7 8 9 10
11 12
13 14 15 16 17 18 19 20 21
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
SOME REMARKS ON THE PREPARATION OF
Fe-K/Ca-Cr
113
CATALYST FOR STYRENE PRODUCTION
Z.DZIEWII$KI and E. OZDOBA Institute of Chemistry, Pedagogical University, 25-020 Kielce (Poland) SUMMARY Several title catalyst specimens with an identical Fe, K, Ca, and Cr content had been prepared from the different FeOOH or Fe203 phases, and then tested at the same conditions in dehydrogenation of ethylbenzene to styrene. An influence has been discussed of the kind and the contamination degree of FeOOH or Fe 0 on the catalyst properties. The most selective but moderately active catal&$ was obtained from P-FeOOH slightly contaminated with C1 which contained traces of cc-FeOOH phase. The less selective but more active catalyst was obtained from pure K-FeOOH phase. INTRODUCTION Actually, iron oxide based catalysts are the most widespread contacts utilized in industrial dehydrogenation of ethylbenzene (EB) to styrene, and many propositions how to enhance their efficiency are still reported in the literature (refs. 1-3). A majority of the propositions has concerned, however, promoters variation rather than proper selection of the main components of the initial mixture from which the catalyst is prepared. The latter problem seems to be of importance because of a consolidating opinion that just Fe atoms are included, apart from 0 and K atoms, into such proposed active phases of the catalyst as KFe02 (ref. 4 ) , K2Fe220j4 (ref. 5), o r solid solution of potassium in y-Fe203 (ref. 6 ) . It is conceivable that at least some amount of those phases can be produced at the stage of heat treatment of the mixture from which the catalyst is obtained. The easiness of such a primary formation of the active phase will be conditioned by the kind, structure, and texture of the initial iron compounds used. It is also conceivable that the active centers are still created during the EB dehydrogenation. A feasibility of their effective formation ought to depend on the kind of iron oxide matrix adjacent to the K atoms on the catalyst surface. It is evident that a type of the matrix formed will be, in turn, conditioned by the kind of initial iron compound used. The purpose of the present work was, therefore, to elucidate which one of the numerous FeOOH or Fe203 phases could be used as the most appropriate material for the production of such a title catalyst which could exhibit high selectivity and/or satisfactory activity in the EB dehydrogenation to styrene. It seemed a l s o interesting to check to what extent a possible contamination of the initial iron oxide hydroxides could affect activity or selectivity of the catalyst prepared from the contaminated initial iron compound. Sulphates or chlorides were
114 chosen as t h e contaminants, because j u s t these substances could be predominantly incorporated i n t o FeOOH or Fe203 d u r i n g t h e i r large-scale solutions
previously
prepared
by
iron
scrap
production from the
dissolution
HC1.
i n H2S04 or
EXPERIMENTAL I n i t i a l i r o n compound p r e p a r a t i o n AnalaR reagents were used throughout a l l t h e procedures a p p l i e d . To c o n t r o l surface area and p a r t i c l e dimensions o f t h e specimen obtained, the appropriate precipitation,
ageing, and c a l c i n a t i o n c o n d i t i o n s had been chosen. Some speci-
50:-
or
C1-
mens were purposely contaminated with
i o n s . The degree o f t h e
contamination was c o n t o l l e d by a v a r i a t i o n o f t h e ageing and washing c o n d i t i o n s . The f o l l o w i n g procedures were a p p l i e d t o o b t a i n t h e FeOOH and Fe203 specimens: A solution o f
(i)a-FeOOH.
Fe(N03)3.9 H20
p e r a t u r e with t h e equivalent amount o f
3.5 M
(1 M)
was mixed a t room tern-
ammonia water, and the superna-
t a n t l i q u i d was removed a f t e r some time from above t h e t r a n s i e n t hydrated i r o n which was mixed i n t u r n w i t h approximately the same volume o f oxide s l u r r y (I), 3.5 M
KOH s o l u t i o n and heated a t p r e c i p i t a t e (11)
a-FeOOH
b u f f e r (pH.8.0) o f a-FeOOH
thoroughly,
60
2 hr.
under s t i r r i n g f o r
OC
0.01 M
was washed with
105
and d r i e d a t
s o l u t i o n o f NH4N03+NH3H20
i n air.
OC
50:-
t h a t were p a r t i a l l y contaminated with
So obtained
The other specimens i o n s were prepared ac-
c o r d i n g t o (refs. 7-8). (ii)j3-FeOOH.
0.25 mole o f
been d i s s o l v e d i n
1 dm3
of
Fe(NO3I3.9 H20
H20 ,and then
s o l u t i o n . The s o l u t i o n was b o i l e d a t about to
24 hr.
0 . 1 mole o f
and
OC
for
1 hr. and then subjected 95 'C
supernatant l i q u i d decantation, t h e obtained p r e c i p i t a t e (111)
C1-
had
1 mole o f urea was added t o t h i s
95
ageing a t the temperatures decreasing from
t h e above mentioned b u f f e r u n t i l no
NH4C1
i o n was detected with
to
20
OC.
After
was washed w i t h AgN03
solution
i n t h e f i l t r a t e . More i n t e n s e washing was r e q u i r e d , however, i f t h e intended C 1 content i n d r y
p-FeOOH
t e was d r i e d a t
60
t h a t contained more
ought n o t t o exceed
OC
under vacuum f o r
C1
were prepared from
with e i t h e r ammonium acetate ( r e f . 9)
1wt. %
24 hr.
. The obtained p r e c i p i t a The specimens o f
FeC13
P-FeOOH
s o l u t i o n by p r e c i p i t a t i o n
or pure water ( r e f . 1 0 ) used as h y d r o l y -
z i n g agents. (iii)y-FeOOH.
The procedure r e p o r t e d i n (ref. 11) was a p p l i e d t o o b t a i n
t h i s i r o n oxide hydroxide by means of t h e o x i d a t i o n w i t h bubbled a i r o f t h e suspension o f i r o n ( I 1 ) hydroxide, which had been obtained p r e v i o u s l y by the p r e c i p i t a t i o n w i t h ammonia water a t oxygen-free atmosphere from (iv)
Ge2c3.
cination o f the o f the lized
(11)
Very f i n e c r y s t a l l i n e
(I) precipitate at
precipitate at
cc-Fe203
400-550
350 0 C
FeS04
solution.
Fe203 powder was obtained by c a l -
OC
for
for
l hr. i n a i r . The c a l c i n a t i o n
2-6
w i t h a lower s p e c i f i c surface area.
hr.
yielded well crystal-
115
At first, the obtained (11) precipitate had been dehydrated at 320 OC for 1 hr. to hematite, which was subsequently reduced at 360 OC with hydrogen f o r 1 hr. to magnetite. So obtained magnetite powder was in turn reoxidized at 300 OC in air enriched with oxygen until pure maghemite phase of Fe203 was produced.
(v)
y-Fe,03.
Catalyst preparation To avoid the possible masking effects of some commonly used activators, such as Mo, V, W , and particularly Ce oxides, the relatively simple composition was chosen for the catalyst specimens investigated. In addition, each of the catalyst specimen was prepared strictly in the same manner. Before the catalyst preparation Fe content (and K content, if needed) had been determined in FeOOH o r Fe203 used actually for the catalyst preparation, and such an amount of this iron compound which contained 220 g of Fe was weighted. That amount of FeOOH or Fe203 was mixed with 28 g KOH ( o r l e s s , if FeOOH or Fe203 used had already contained some K content), 25 g CaO, and 14 g K2Cr207 . Some amount of was added to those components to obtain, by a H20 thorough kneading, a tough paste from which 4 mm diameter extrudates were obtained. The extrudates were, after drying at 105 OC and cutting into ca 6 mm length cylinders, calcined in two steps, at 350 O C and 550 OC respectively, for 4 hr. Both the cooling and the storage of so obtained specimens were accomplished at H20 and C02 free atmosphere. Some part of the cylinders were crushed and sieved to obtain the catalyst fraction with 0.5 mm mean size grains. Initial iron compounds and catalyst characterization Conventional analytical methods were applied to determine C1 ( r e f . 12) or S (ref. 13) content in the specimens obtained. BET method was used to determine specific surface areas of the specimens. X-ray powder diffraction patterns were recorded on a DRON 3 diffractometer with CuK, , FeKa , or MoKx radiation. The results were compared wlth the literature data (ref. 14) to determine phase composition of the specimens investigated. IR spectroscopy for some amorphous specimens was performed on BECKMANN IR Spectrometer. The obtained spectra were compared with those reported in the 11terature (ref. 15). Catalyst testing in ethylbenzene dehydrogenation to styrene The measurements were carried out in a conventional fixed-bed flow reactor, which had been equipped with a feed-stream overheater and with a cooler for the products. The values of catalyst bed temperature, feed-stream composition and rate, and total gas pressure were controlled by means of additional attachments.
116
Before the main test, an influence of the possible diffuse retardation on the reaction rate had been established. A microreactor housing ca 10 cm3 of the catalyst was used for a relatively quick detection if any difference appear in the catalyst activity when the catalyst grains of different dimensions had been separately investigated in the reaction. Since no substantial difference had been found for such beds of the same catalyst in the microreactor experiments, the 4 mm diameter cylinders of the catalyst were then used in the main test, which was carried out as follows: A 200 cm3 portion of the catalyst was placed in the reactor, the bed being first heated to 420 OC with a nitrogen stream and then up to 530 OC with an overheated steam passed through the bed. The catalyst temperature was then raised to 590 OC and the feed-stream was gradually enriched with ethylobenzene until its flow rate reached 7 5 cm3 EB/hr . Then, the reaction was studied under the following conditions: temperature, 590 OC, total pressure, ca 100 kPa, H20/EB molar ratio, 10, NTP space velocity, 10,000 hr-', overall reaction time 30 hr. Ouring each run, samples of the feed and product were taken at intervals and analyzed. An ELWRO N-504 gas chromatograph with a column packed with 5 % SP 1200/1.75 % Bentonite 34 on 100/120 Supelcoport was used to the feed and product analysis. Gaseous products were additionally analyzed, if needed, by means of the conventional chemical method. The results obtained at stationary state of the reaction, usually after 30 hr. , were taken for calculations. Styrene (STY) selectivity was calculated as lOO.(STY/(EB inlet - EB outlet)), styrene yield as 100.(STY/EB inlet), ethylbenzene conversion was calculated as 100.((EB inlet - EB outlet)/EB inlet). The two latter magnitudes were arbitrary assumed as a measure of the catalyst activity. RESULTS AN0 DISCUSSION
Initial iron compounds purity Of the compounds that were to be pure in intention, only cc-FeOOH, oc-Fe203, and y-Fe20j proved to be single-phase materials not contaminated with C 1 or S atoms. All the attempts to obtain non-contaminated single-phase P-FeOOH failed. This fact seems to be consistent with the supposition of many researchers that the existence of C1 ( o r theoretically F too) atoms in P-FeOOH lattice is required necessarily for the stability of this compound to be secured. There is no uniformity of views, however, to what extent the chlorine content could be lowered without a disturbance of p-FeOOH phase homogenity. Theoretically, the chlorine content could be calculated from various proposed "real" formulae, e.g. Fe403(OH)5C1 (ref. 16) or Fe8(0,0H)16C1 <2 (ref. 17), but no firm argument is so far available in favour of the definite proposed formula. Hence, we decided to apply the appropriate ageing and intense washing of (111) precipitate until the traces of other FeOOH phases would start to appear. ~
117 No references were found on the necessity of the presence of SO:ions in 7-FeOOH lattice as a prerequisite of this phase stability. Nevertheless, no y-FeOOH specimen could be prepared without 5 contamination from FeS04 solution, and an attempt to obtain chemically pure y-FeOOH from the other available iron salt solutions failed. Apparently, too strong inclusion or adsorption of the 50:- ions on y-FeOOH precipitate caused that even an intense washing of the precipitate was ineffective. Hence, we were forced to recognize the least contaminated specimens of the p-FeOOH or y-FeOOH phases as sufficiently "pure" for the aims of this work.
Relationships between the catalyst and the initial iron compound properties (i) Mutual phase composition and surface area relations All the pure FeOOH phases calcined without a presence of other substances at 550 OC were always finally transformed into cc-Fe203 after 4 hr. calcination. In spite of this, some amount of Y-Fe203 was formed apart from cx-Fe203 in some catalysts when the mentioned FeOOH phases were calcined together with the other components of the initial mixture. Table 1 illustrates which Fe203 phases appeared in the catalyst obtained from different FeOOH phases o r their mixtures. The contribution of respective Fe20j phases in the catalyst specimens was not quantitatively determined, but it might be approximately estimated.
TABLE 1 Effect of kind and contamination of FeOOH on selected properties of the catalyst FeOOH used for the catalyst preparation
Catalyst characterization
Type of the phase used
Mean surgace area, (m /g)
OL
,(amorph)
cc ,(fine cr)
cc ,(needles)
cc cc
Contamina- Mean sursace ting ele- area, (m /g) ment, (%) 240
92 48
22 19 12
45 38
11
,(plates) ,(amorph) ,(needles) y ,(fine cr) 7 ,(fine cr)
52 116 46 88 76
10 18 10 19 16
7 9 % ~+ 2 l % p 6 2 % ~+ 38%p 4 8 % ~ +. 52%p
49 49 50
13 15 11
13 p p
10
Type of the Fe 0 phase foZnd
Relatively high content of maghemite in the catalyst prepared from Y-FeOOH was confirmed in two ways. First, the line with 2.52 d spacing value, was the most intense line in XRD patterns of the catalysts prepared from T-FeDOH, similarly as it was seen in XRD pattern of phase pure y-Fe203. Secondly, the extrudates of these catalysts were easily attracted by a magnet, although a presence of no other paramagnetic materials could be detected in the extrudates. A l o wer content of maghemite must have been present in the catalysts prepared from more contaminated with C1 specimens of f3-FeOOH phase (cf. Table 1). Only two weak lines, with 1.47 and 2.95 d spacing values, could be attributed to maghemite phase in XRD patterns of those catalysts. Even fine grains of those catalysts were not attracted by a magnet. It is conceivable, that two-step mechanism of type: (j3 or y )-FeOOH -y-FegD3 oc-Fep03,with the second step inhibited e.g. by contaminants,is valid here, but more detailed study is required to confirm or deny this supposition. It is worthy noticing that the catalysts in which only cc-Fe203 was formed, had been produced from a-FeOOH or pure P-FeOOH. Calcination of the initial mixture caused expected reduction in the specific surface areas. The reduction degrees seem not to be, however, in any relation with the kind or contamination degree of the FeOOH phase used. Nevertheless, the catalysts witii higher specific surface areas could be obtained from amorphous o r fine-crystalline FeOOH specimens. (ii) Catalyst properties vs. kind and contamination dearee of FeOOH phase. Fig. 1 illustrates the values of EB conversion at 590 OC over the catalysts prepared from different pure phases of FeOOH or Fe203. As it is seen from the Figure, the catalysts prepared from iron oxide hydroxides exhibited more
-
/-. 60 o \ -
v
c .. 0 -I
ln
Ll
40
-
20
-
(u 1
C 0 0
m W
oc
Fig. 1. ( IZZZZZJ
P
;Y
cc
Y
EB conversion,(%), over the catalysts prepared either from FeOOH o r Fe20j ( EXZZ3 Kind of the phase is given under X axis
>.
119
differentiated activity than those prepared from iron oxides introduced to the initial mixture from which the catalyst was then prepared. The considerable differences observed in the catalyst specimen activity suggest that active centers of the catalyst were apparently formed from the introduced iron compound rather than from iron oxides formed ultimately in each catalyst specimen due to calcination. If the centers had been prepared from oxides formed during the calcination process, the observed differences would have been slighter, not greater than the difference in activity of the catalysts prepared directly from E-Fe 203 and y-Fep03 phases. As it can be seen from the Figure, the catalyst prepared from cc-Fe203 displays somewhat higher activity than that prepared from y-Fe203. This observation correlates with Subrt et al. opinion (ref. 18) that active phase KFe02 forms more easily from a-Fe203 than from y-Fe203. Changes in the catalysts selectivity are shown in Fig. 2. It is clearly seen from the Figure that the catalyst prepared from pure (in earlier mentioned sense) P-FeOOH phase exhibits much higher selectivity than the remainilcg catalysts investigated. On the other hand, the catalyst prepared from Y-FeOOH displays the lowest selectivity among the catalysts investigated. So considerable differencies in selectivity suggest that the kind of initial iron com-
t
A
o\-
.
v
+ x
94
i cc
P
oc
Y
Fig. 2. Selectivity, ( % I , of the catalysts prepared either from FeOOH (-1 or from FeZOj ( ) phases. Kind of the phase used is given under X axis pound used for t h e catalyst production can determine remarkably the catalyst selectivity. It is conceivable that a number of different phase features, such as e.g. FeOOH structure ability to be rearranged into somewhat differently distorted iron(II1) oxide matrices formed in the catalyst itself, or the ability of different FeOOH phases to be transformed into the iron oxides with a somewhat different surface acidity o r to induce different pore structures in the catalyst, can affect the selectivity of the catalyst.
120
Apart from the kind of the initial phase used, its contamination can exert an influence on the catalyst selectivity and activity. Fig. 3 shows the changes in selectivity o r activity values, if the same FeOOH phases, but with the different 32
42
(a)
o\*
u
(b)
A
v
-I
36
87 o\-
v
30
40
.a .,I
x
..
+J ..-I
86
95
85
94
>
.A
4 0
*x
28
38
2a, ffl
v)
26
I
1
I
I
2
3
S (a) o r C 1
36 (0)
84
I
1
I
I
2
3
33
content, (%) , in initial FeOOH
Fig. 3. Relatioship between the: (a) styrene yield, (b) selectivity of the catalysts prepared from: p-FeOOH contaminated with C1 ( -) or y-FeOOH contaminated with S (-+-m--), and the degree of contamination
contamination degree were used for a preparation of the catalysts investigated. Although both chloride and sulphate ion contaminants undoubtedly affect the catalysts activity and selectivity, scale of their influence on these features can be different and depends mainly on kind and concentration of the contaminant The 50:concentrations h i g h e r than 4 % but lesser than 10 present in T-FeOOH, exert rather a weak influence on the selectivity, and almost none on the acivity of the catalyst prepared from y-FeOOH (cf. Fig. 3). From a practical point of view there is, therefore, no need to decrease too high 50;content in the precipitated 7-FeOOH below 7 % . Theoretically, however, more thorough purification of 7-FeOOH can either substantially enhance activity of g-FeOOH based catalyst or can have no substantial effect on the activity. Since non-contaminated 7-FeOOH was so far not obtained, there was no opportunity to check which one of those two suppositions was true. For the catalysts based on contaminated P-FeOOH more substantial decrease in selectivity has been observed only when j3-FeOOH used was contaminated with more than 2 % C1 .If that oxide hydroxide was less contaminated, the lower decrease in activity, and particularly in selectivity, was observed with the C1 content increase. If the contamination degree did not exceed 0.7 % the selectivity decrease could be hardly observed. High selectivity of the catalyst based on the "pure" (i.e. least contaminated) P-FeOOH suggest that C1 incorporated into (3-FeOOH lattice cannot affect strongly the catalyst selectivity.
+
121
(iii) Properties of the catalysts prepared from mixed FeOOH phases High selectivity of the catalyst prepared from phase pure (in earlier mentioned sense) p-FeOOH, and satisfactory activity of the catalyst prepared from phase pure cc-FeOOH, prompted us to study the properties of the catalysts prepared from a mixture of the mentioned phases. Some results of such a catalysts testing are shown in Fig. 4 . Apart from experimental curves shown in the Fi-
n .\v
60
U al
4 .r(
x
50
al
c
al
FI
x
+ Lo
40 I
I
I
I,
t
Fig. 4. Activity (a) and selectivity (b) of the catalysts prepared from the mixture of cx-FeOOH and P-FeOOH pure phases. Full lines illustrate experimentally found relationship, broken lines show how these relationships would have run if additivity rule had been preserved for the features of the catalysts gure with full lines, theoretical runs, calculated under assumption that a rule of features additivity is valid for the catalysts prepared from phase mixtures, were also plotted in Fig. 4 with broken lines, It is evident from the Figure, that experimental values for selectivity are higher, if the P-FeOOH content is higher than 23 %, than the calculated values for the selectivity. It is worthy noticing that even high contribution of a-FeOOH , up t o 60 % , in the initial mixture of both considered phases practically does not decrease the high selectivity of the catalyst based on the "pure" P-FeOOH. Unfortunately, experimental values of activity of the catalysts based on mixed FeOOH phases are always lower, regerdless of p-FeOOH phase contribution, than calculated values for predicted activity. Only small changes in activity are observed (cf. Fig. 4 a> when the P-FeOOH content in the initial mixture was increased. Rather considerable decrease in activity of the catalysts based on the mixed FeOOH phases is, of course, unfavourable for an attempt to enhance low activity of the catalyst based on "pure" P-FeOOH by addition of a-FeOOH phase to the initial mixture, o r to enhance selectivity of the catalyst based on s-FeOOH by addition of the inducing selectivity P-FeOOH phase.
122
CONCLUSIONS It has been stated in this work that the kind and contamination degree of the initial iron compound used for the Fe-K/Ca-Cr catalyst preparation, which can be used in ethylbenzene dehydrogenation to styrene, exerted rather substantial influence on the catalyst activity and selectivity in the mentioned reaction. Hence, a proper selection of the kind of initial iron compound, as well as the selection of appropriate conditions for these compounds preparation can substantially help us to enhance the catalyst activity and selectivity. The most effective catalyst was prepared from pure crystalline x-FeOOH, that had been previously obtained from pure iron nitrate solution by the described in this work precipitation method. An application of the solutions obtained by iron-scrap dissolution in H2S04 or HC1 is not reccornended for FeOOH or Fe203 preparation. To obtain the high selective catalyst , the P-FeOOH phase was used as the initial iron compound. Its contamination with C1 was so lowered as it was possible by 24 hr. ageing and thorough washing of p -FeOOH precipitate. REFERENCES 1 J.L. Smith, 8.S. Masters and D.J. Smith, U.S. Patent, 4,467,046 K. Sarumaru, T. Iwakura, A. Watanabe and M. Mori, Japan Kokai, 85193,934 J. Kryska, J. Spevatek and M. Novotny, CS Patent, 230,464 (1986) 4 T. Hirano, Appl. Catal., 26 (1986) 81-90 5 M. Muhler, R . SchlOgl and G. Ertl, Surf. Interface Anal., 12 (1988) 233-238 6 J. Koppe, I. Raphtel and P. Kraak, Chem. Techn. (Leipzig), 40 (1988) 81-83 7 G. Buxbaum and F. Hund, Ger. Offen. 2,556,406 (1977) 8 A. Krause and A. Borkowska, Roczniki Chem., 29 (1955) 999-1006 9 S . Hirai, ,A. Matsumoto and K. Wakai, Japan Kokai, 78-88,698 (1978) 10 J.M. Gonzalez-Calbet, M.A. Alario-Franco and M. Gayoso-Andrade, Inorg. Nucl. Chem., 43 (1981) 257-264 11 A. Solcovd, J. Subrt, F. Hanousek, P. Holba, V. Zapletal and J. Lipka, Silikaty, 24 (1980) 133-141 12 Polish Industrial Standards, Tests PN-75/C-04617 (1975) and PN-80/C-04617.04 (1980) f o r chlorine and its compounds determination 13 Polish Industrial Standards, Test PN-74/C-04566 f o r sulphur and its compounds determination 14 ASTM 4-0755, 8-98, 13-157, 17-536, 24-72 15 T.V. Kalinskaya, L.8. Lobanova, I.V. Pologih, Zhurn. Prikl. Xhim., 40 (1982) 2463-2467 16 W. Feitknecht, R. Giovanoli, W. Michaelis and M. Mueller, Helv. Chim. Acta, 56 (1973) 2847-2856 17 P. Keller, Neues Jahrb. Miner. Abh., 113 (1970) 29-49 18 J. Subrt and J. Vins, Thermochim. Acta, 93 (1985) 489-492
123
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
HYDROGENATION OF 2 ETHYL HEXEN-2-AL
ON N i / S i O ,
CATALYSTS.
ROLE
OF
PREPARATION PARAMETERS
A.F.
DA S I L V A JUNIOR1,
'PEQ/COPPE/UFRJ, *IRC/CNRS,
V.M.M.SALIM1,
M.SCHMAL1
TELEFAX 5 5 2 1 - 2 9 0 6 6 8 6
TELEFAX
-
and R.
FRETYZ
R i o de J a n e i r o ( B r a z i l )
78894769 - V i l l e u r b a n n e
(France)
SUMMARY T h e i n f l u e n c e o f a m a c r o p o r o u s FC C e l i t e carrier and t h e p r e p a r a t i o n parameters were i n v e s t i g a t e d i n o r d e r t o o p t i m i z e presupport by p a r a t i o n and v e r i f y t h e i n t e r a c t i o n o f N i w i t h t h e a c c o m p l i s h i n g t h e m o d i f i e d p r o p e r t i e s b y TPR, q u i m i s s o r p t i o n and t h e i r e f f e c t s on t h e h y d r o g e n a t i o n o f 2 - e t h y l - h e x e n - 2 - a l . Precipl t a t i o n w i t h NaHCO,, F!H4HC0, and PIH,OH, w i t h o p t i m i z a t i o n o f a g i n g time, a 1 l o w us t o p r e p a r e c a t a l y s t s w i t h h i g h e r m e t a l l i c s u r f a c e and b e t t e r a c t i v i t y t h a n t h e impregnated and commercial N i C r , used a s r e f e r ence. The m o s t a c t i v e c a t a l y s t r e d u c e s t h e f o r m a t i o n o f heavier p r o d u c t s due b a s i c n a t u r e o f p r e c i p i t a t e d a g e n t . INTRODUCTION S i l i c a supported n i c k e l catalysts, important i n practical catalysis, t i o n s t h e y can promote.
Therefore,
z a t i o n o f such c a t a l y s t s ,
w i t h c o n t r o l l e d properties are
due t o t h e g r e a t number o f
reac
t h e p r e p a r a t i o n and characterj.
e v e n i f w e l l d o c u m e n t e d ( I - 6 ) , remains
an
a r e a where c o n s t a n t e f f o r t s a r e done. F o r example,
i n recent years,
H e r m a n s a n d Geus s h o w e d t h e i n t e r e s t
o f homogeneous p r e c i p i t a t i o n ( 6 - 7 ) ,
Montes e t a l .
( 8 - 9 ) analyzed t h e
r e l a t i o n s h i p between t h e p r e p a r a t i o n method and t h e i n t e n s i t y nickel-silica
interaction,
the m e t a l l i c area.
as w e l l
More r e c e n t l y ,
6s
the extent o f reduction
Mile
e t a1 (10) used
reveal various types o f m e t a l l i c n i c k e l
in
c a t a l y s t s a n d Wilson e t a1 ( 1 1 ) d e t a i l e d t h e
TPR
impregnated textural
of
and to
Ni/SiO,
variations
brought about by p r e c i p i t a t i o n w i t h urea o f N i c a t a l y s t s w i t h h i g h o r low N i loadings. With the aim t o hydrogenate edible o i l , loaded Ni/SiO,
catalysts,
we h a v e p r e p a r e d h i g h l y
by a d e p o s i t i o n - p r e c i p i t a t i o n
s h o w e d t h a t t h e u s e o f FC C e l i t e ,
a s i l i c a support,
method and
t u r n e d t h e ca;a-
l y s t s more e f f i c i e n t t h a n t h e ones p r e p a r e d w i t h o t h e r C e l i t e mate rials,
e i t h e r w i t h lower o r higher surface area
with high N i loading,
(12).
i t i s d i f f i c u l t to correlate the
However, variations
124
o f metal p r o p e r t i e s w i t h p o s s i b l e i n t e r a c t i o n s between t h e
nickel
and t h e s i l i c a . The p r e s e n t w o r k s t a r t e d
to verify the influence of
preparg
t i o n p a r a m e t e r s a n d t h e e f f e c t on FC C e l i t e a s s u p p o r t on l o w
loaded
N i samples.
phase
The c a t a l y s t s w e r e a l s o t e s t e d i n t h e
hydrogenation o f 2 e t h y l 2 hexenal
This r e a c t i o n h a s b e e n p r e v i o u s l y s t u d i e d , and l i q u i d phase ( 1 5 - 1 7 )
b o t h i n t h e g a s (13-14)
as t h e main products,
( H O L ) h a v e some i n d u s t r i a l
and 2 e t h y l hexanol
2 e t h y l hexanal (HAL) interest.
used d u r i n g t h e p r o d u c t i o n o f d i o c t y l p h t a l a t e , t u r i n g o f PVC softeners,
liquid
(EPA).
HOL
f o r the
is
manufac-
whereas t h e o c t a n o i c a c i d can be o b t a i n e d
b y o x i d a t i o n o f HAL. EXPERIMENTAL Preparation o f catalysts For the preparation,we support,
u s e d t h e FC C e l i t e ( M a n v i l l e ) ,
w i t h 1 w t % A1,0,.
SiO,
a
This i s a diatomaceous earth,
with
t o t a l s u r f a c e a r e a o f 4 5 mYg a n d a p o r e v o l u m e o f 3 . 7 c m 3 / g . d e d u c e d b y N,
a As
t h e p u r e FC C e l i t e w i t h o n l y 0 , 1
a d s o r p t i o n a t 77 K,
cm3/g o f mesopores volume and a n e g l i g e a b l e m i c r o p o r e s volume,
is
m a i n l y a macroporous support. Two g r o u p s o f c a t a l y s t s w e r e p r e p a r e d .
I n t h e f i r s t one,
two
s e r i e s o f c a t a l y s t s w e r e o b t a i n e d b y deposition-precipitation(Ni-D1 Ni-D2),
u s i n g Ni(NO,),.6
p r e c i p i t a t i n g agent. s o l u t i o n o f Ni(NO,),
a s s t a r t i n g N i c o m p o u n d a n d NaHCO,
H,O
( 1 7 0 cm3, 0,4
s t i r r i n g (1100 rpm).
?I) a n d s u b m i t t e d
A q u e o u s s o l u t i o n o f NaHCO,
The p r e c i p i t a t i o n o f t h e N i - D 1
to
constant
( 1 3 6 cm3, 0.8
c m 3 / m i n ) a t room temperature
PI)
(298 K).
s e r i e s was s t o p p e d f o r a N a / N i r a t i o
and t h e p r e c i p i t a t i o n o f Ni-D,
Na/Ni r a t i o o f 1.5.
as
T h e FC C e l i t e ( 3 6 g ) was s u s p e n d e d i n aqueous
was a d d e d t o t h e s u s p e n s i o n ( 1 ,7 equal t o 2,
and
series stopped f o r
a
T h e s l u r r y was t h e n m a i n t a i n e d u n d e r s t i r r i n g
i n t h e r e a c t i o n medium f o r thereafter "aging time".
different periods Then,
of
time,
called
t h e s u s p e n s i o n was f i l t e r e d ,
w a s h e d a n d d r i e d f o r 2 3 h a t 3 8 3 K,
before being submitted t o
a
c a l c i n a t i o n under a i r ,
a t 7 2 3 K,6
i n c l u d e d f o u r samples,
N i I was p r e p a r e d b y s i m p l e w e t p o i n t i m p r e g
nation;
NiDHeNH,,
NiDHoNH,
The s e c o n d g r o u p o f c a t a l y s t s
a n d NiDHoNa w e r e p r e p a r e d a s
for deposition-precipitation NH4HC0, a n d NaPCO,,
h.
water
procedure,
described
using respectively
NH,nH,
a s p r e c i p i t a t i n g compounds andar, a g i n g t i m e
3 h (optimized i n the f i r s t group o f catalysts).
of
DHe a n d DHo i n d j
125
cate t h a t the deposition p r e c i p i t a t i o n procedure i s considered h e t e r o g e n e o u s a n d homogeneous, A Ni-Cr/SiO,
catalyst,
as
respectively.
u s e d i n d u s t r i a l l y f o r EPA h y d r o g e n a t i o n ,
was u s e d a s a r e f e r e n c e c a t a l y s t . Characterization R e d u c i b i l i t y and d i s p e r s i o n t h r o u g h TPR i n s t r u m e n t s . i n (12).
catalysts
were e s t i m a t e d
The e q u i p m e n t i s s i m i l a r
of
the
t o that described
+
T h e r e d u c t i o n was c o n d u c t e d w i t h a n A r g o n
t u r e and h e a t i n g r a t e o f 8 K / m i n u p t o 8 2 3 K . was t r a p p e d
1 , 5 % H,
water
i n t o a 3 A z e o l i t e s located before the thermal
t i v i t y cell.
mix-
The e n v o l v e d
The d i s p e r s i o n o f t h e r e d u c e d N i
conduc-
was e s t i m a t e d ,
at
t h e e n d o f t h e TPR,
b y q u i c k l y c o o l i n g down o r
t h e reduced sample,
i n t h e argon-hydrogen mixture.
These h i g h tem-
p e r a t u r e v a r i a t i o n s generate e i t h e r an a d s o r p t i o n ,
or a desorption
peak t h a t , i n H,
after correction f o r flow
volume.
h e a t i n g up ( 1 1 O K / m i n )
r a t e v a r i a t i o n s , were
For the dispersion estimation,
converted
we a s s u m e d t h a t
K a n d a t r o o m t e m p e r a t u r e ( 2 9 8 K ) , hydrogen coverage was
823
t i v e l y c l o s e t o 0 and t o 1. s u r f a c e atom,
The r a t i o H / N i * ,
was t a k e n e q u a l t o 1 .
with Ni*
at
respecbeing
a
Values issued from adsorption
and d e s o r p t i o n peaks were i n good agreement. C a t a l y t i c measurements T h e h y d r o g e n a t i o n o f E P A was p e r f o r m e d i n a s e m i - b a t c h a t a t m o s p h e r i c p r e s s u r e . Sample w e i g h t c o r r e s p o n d i n g was r e d u c e d i n " s i t u " , o u t g a s s e d w i t h N, tor.
a t 7 7 3 K f o r 1 7 h w i t h a n H,
A t t h e end o f t h e r e d u c t i o n ,
12 l / h .
N,
t o 0,09
t h e reduced
reactor, g of
flow close catalyst
Ni
to was
f l o w a n d 150 m l EPA w e r e i n t r o d u c e d i n t o the r e a c
b u b b l i n g was m a i n t a i n e d d u r i n g e s t a b l i s h m e n t o f t h e r e a c t i o n
t e m p e r a t u r e . Then,
a H,
while
f l o w o f 2 0 5 l / h was a d d e d
starting
L i q u i d samples flow. n e e d e v a l v e and anawere c o l l e c t e d p e r i o d i c a l y t h r o u g h a s e a l e d t h e m e c h a n i c a l s t i r r i n g a n d s t o p p i n g t h e N,
l y z e d c h r o m a t o g r a p h i c a l l y w i t h a C a r b o w a x c o umn. RESULTS Catalysts preparation T h e f i n a l pH v a l u e s , agent,
a
t h e end o f t h e a d d i t on o f t h e p r e c i p i t a t i n g 7.0,
were r e s p e c t i v e l y 5.0,
7.0
and 8.0,
for
impregnation
a n d d e p o s i t i o n - p r e c i p i t a t i o n w i t h N a H C 0 3 , N H 4 C 0 3 a n d NH40H. T h e pH evolution,
d u r i n g p r e c i p i t a t i o n w i t h NaHCO,,
t i t r a t i o n c u r v e f o r Ni(NO,), same c u r v e f o r p u r e Ni(NO,),.
+
showed
FC C e l i t e was a l w a y s
that beneath
the the
126
Catalytic Characterization T a b l e s 1 and 2 s u m m a r i z e some c h a r a c t e r i z a t i o n p a r a m e t e r s , different catalysts, of
a n d F i g u r e s 1,
for
2 and 3 gave r e d u c t i o n profiles
typical catalysts.
TABLE 1 I n f l u e n c e o f N i c o n t e n t a n d a g i n g t i m e on r e d u c i b i l i t y a n d m e t a l l i c area f o r N i / C e l i t e catalysts. Catalyst
Ni/Na
Ni-D1-0 Ni-01-2 Ni-D1-3
0.50 0.50 0.50
Ni-D2-0 Ni-D2-3 Ni-D2-6 Ni-D2-22
0.66 0.66 0.66 0.66
A
:B
a(%)
0 2 3
8.5 10.0 10.0
0.20 0.16 0.15
55 70 84
36 42 47
0 3 6 22
5.8 6.7 7.3 8.5
0.14 0.17 0.17 0.15
59 64 60 68
23 36 31 33
Ni-4-3
i\, Ni-D2-,
e
r"
%Na
-_---.-
22
n
%Ni
A g i n g Time ( h )
~
,-.. i
U.
i.
-
o f aging time
S N ~( m z / g N i )
at
room
temperature i s given f o r Ni-D1 and N i - D Z catalysts, samples p r e c i p i t a t e d NaHCO,.
In
Figs. 1
and
by
para1l e l ,
2
show
127
The p r o p e r t i e s o f i m p r e g n a t e d a n d c o m m e r c i a l o f t h e d e p o s i t e d - p r e c i p i t a t e d ones, p r e s e n t e d i n T a b l e 2.
for
c a t a l y s t s and t h a t
aging times
3 h,
of
The c o r r e s p o n d i n g r e d u c t i o n p r o f i l e s are
is given
i n F i g u r e 3. TABLE 2 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 m e t h o d on t h e r e d u c t i b i l i t y a n d text! r a l p r o p e r t i e s o f t h e second s e t o f c a t a l y s t s . C a t a l y s t s BET ( m 2 / g )
- Influence of Preparation Methods on the TPR Profiles.
a
d) 530 180 190 130 480
128
Catalytic Activity Preliminary experiments,
using the catalyst with
the
m e t a l l i c area, were performed i n o r d e r t o minimize t h e v a r y i n g t h e gas f l o w r a t e and t h e c a t a l y s t w e i g h t . gNi/lEpA
a n d a H,
higher diffusion, u s i n g 0,6
Then,
f l o w r a t e o f 2 0 5 l / h we c o n s t r u c t e d r e a g e n t - p r o d u c t s d i s t r i b u t i o n curves, w i t h time,
for
different
the
tempera
t u r e s ( 3 9 3 , 403 and 4 1 3 K ) . The v a r i a t i o n of area
the
metallic
being l a r g e r
for set
the of
alysts,
second cat-
the
cata-
l y t i c measurements were
1imited
to
this latter
set.
F i g . 4 shows a t y p i c a l curve
-
Fig. 4 - Typical distribution of reagent products for catalyst NiDHoNa at 413 K .
-
t i o n o f E P A and t h e ucts
using
413 K. From
the
F i g u r e 5,
80
t h a t presents
70
consumption o f EPA
8 60
U
A NiDHoNH4 4 NiDHeNH4
50
the
for differents cat
A Ni I
alysts,
0 NiDHoNH4
we
lated the
L
z
f e r s t o t h e consump
c a t a l y s t N i DHoNa a t
90
=
re
formation o f prod-
€PA Consumption T 413 K
100
repre
s e n t a t i o n which
40
calcuinitial
reaction rate the
&? 30
and
corresponding
20
TON a t 4 1 3 K . These
10
v a l u e s a r e reported
0
i n T a b l e 3 together 0
1
2
3
4
Reaction Time ( h )
5
6
7
Fig. 5 - Comparison of EPA Consumption for the second set of catalyst at 413 K.
w i t h a p p a r e n t act! v a t i o n energy.
129
TABLE 3 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 m e t h o d on t h e a c t i v i t y . I n i t i a l r a t e * 102 sN i (mol/gNi*min) (m'/gNired)
Catalyst
TON(s-')
EAt(KJ/mol)
~~
N i DHoNa NiDH,NH, N i DHoNH, NiI N i C r Com.
10.1 6.8 6.2 1.2 1.0
53 38 36 13 14
1.2 1.2 1.1 0.62 0.44
69
-
52 59
The m a i n p r o d u c t s o f t h e h y d r o g e n a t i o n o f EPA a r e t h e d i a t e HAL a n d t h e f i n a l p r o d u c t HOL.
r e a d y i d e n t i f i e d b y GC-mass s p e c t r o m e t r i c a EPA ( 2 b u t h y l b u t e n - 2 - a l ,
interme-
I n a d d i t i o r ? t o i t we h a v e
c a l l e d 1SOM.EPA)
al-
p o s i t i o n isomer
of
a n d some h e a v i e r
com-
pounds l i k e 2 e t h y l h e x y l h e m i a c e t a l o f 2 e t h y l h e x a n a l and
con-
s e q u e n t l y b i s - ( 2 e t h y l h e x y l ) a c e t a l o f 2 e t h y l h e x a n a l (both c a l l e d
SOM. ACT). Table 4 presents the actual s e l e c t i v i t i e s i n standard conditions f o r a constant conversion
of 2 5 % ( 4 1 3 K ) .
pounds were d e t e c t e d o n l y f o r h i g h e r c o n v e r s i o n s .
reaction
H e a v i e r com-
I n the l a s t
o f T a b l e 4 we h a v e p r e s e n t e d t h e s e l e c t i v i t i e s o f t h e s e for
t o t a l c o n v e r s i o n o f EPA,
column
compounds
f o r t h e most a c t i v e c a t a l y s t s .
TABLE 4 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 m e t h o d i n s e l e c t i v i t y a t 413 K. Selectivity
S~~~
SHOL
Conversion Catalv s t NiI NiDH,NH, N i DHoNH NiDHoNa N i C r Corn.
'ISOM.EPA
'SOM.ACT
1.10 0.55 0.75 0.43 1.30
'HOL
100%
25% 85.6 88.7 89.0 87.5 90.0
'HAL
0.41 0.42 0.18
13.5 10.8 10.5 12.0 8.8
-
91.5 88.2 90.0
-
8.1 11.4 9.8 -
DISCUSSION D u r i n g t h e i r s t u d y on d e p o s i t i o n - p r e c i p i t a t i o n d e c o m p o s i t i o n o r c o n t r o l l e d NaOH a d d i t i o n ,
analyze t h e i n t e r a c t i o n between N i i o n s and h i g h silicas.
o f Ni,
usingurea
H e r m a n s a n s Geus surface
(6-7) area
B a s e d o n v a r i o u s m e a s u r e m e n t s and i n p a r t i c u l a r f o l l o w i n g
t h e e v o l u t i o n o f pH d u r i n g t h e p r e c i p i t a t i o n ,
they concluded
that
t h e i r e x p e r i m e n t a l c o n d i t i o n s a l l o w e d an homogeneous d e p o s i t i o n o f
130 N i precursor onto the s i l i c a support
.
I n t h e p r e s e n t case,
with a
m a c r o p o r o u s m i d d l e s u r f a c e a r e a s i l i c a a s t h e s u p p o r t a n d NaHC0,as the precipitant, ented i n (6). (II),
t h e pH v a r i a t i o n s ,
Therefore,
we t h i n k ,
a r e s i m i l a r t o t h e ones
pres
i n agreement w i t h Wilson
e t a1
t h a t o u r N i / C e l i t e c a t a l y s t s a l s o have a r a t h e r
homogeneous
d i s t r i b u t i o n o f N i onto the c a r r i e r . By i n c r e a s i n g t h e p r e c i p i t a t i o n t i m e
N i c a t a l y s t s v i a urea decomposition, t h a t the N i content increases,
during the preparation of
Richardson e t al.(18)
showed
l e a d i n g b o t h t o an increased
mean
N i p a r t i c l e s i z e and a decrease o f t h e t e x t u r a l parameters.
The
v a r i a t i o n s o f t h e t e x t u r a l p r o p e r t i e s between l o w and h i g h
loaded
N i / s i l i c a c a t a l y s t s were f u r t h e r c o n f i r m e d by W i l s o n e t a1
(11).
T e x t u r a l a n d m e t a l l i c a r e a m o d i f i c a t i o n s r e s u l t i n g f r o m an increase i n the
aging time,
Ni/Celite catalysts,
after precipitation, w i t h 50 w t % N i ( 1 9 ) .
were a l s o observed
methodology n o t o n l y changes t h e p r o p e r t i e s of m e t a l can a l s o a l t e r t h e i r a c c e s s i b i l i t y ,
on
Therefore, the p r e p a r a t i o n
by a l t e r a t i o n
particles,
of
the
but
support
texture. I n t h e p r e s e n t work,
using a constant p r e c i p i t a t i o n time,
we
re
h a v e o b s e r v e d t h a t t h e TPR p r o f i l e s a n d t h e r e f o r e t h e e x t e n t o f duction,
as w e l l as t h e m e t a l l i c a r e a a r e s l i g h t l y changed whenthe
i l i c o n t e n t o r t h e a g i n g t i m e i s changed. a g r e e m e n t w i t h known r e s u l t s ( l o ) , major r e d u c t i o n peaks, t o 773-823 N i species,
K.
way,
in
t h e TPR p r o f i l e s p r e s e n t
I n a general
two
one a r o u n d 573-643 K and t h e o t h e r
T h e f i r s t o n e seems c h a r a c t e r i s t i c o f n o n - i n t e r a c t i n g
whereas t h e second one can be c o n s i d e r e d as a
p r i n t o f the Ni-SiO,
t
i s observed a t
interaction.
Sometimes,
finger-
a l i m i t e d H,
uptake
510 K and a t t r i b u t e d t o t h e e l i m i n a t i o n o f
s t o i c h i o m e t r i c oxygen.
non
T h e n o n i n t e r a c t i n g N i seems p r e s e n t
l a r g e r q u a n t i t y when t h e p e r c e n t N i i n c r e a s e s ( F i g . the
around
aging time increases (Fig.
2).
1)
and
The c o n t r i b u t i o n o f t h i s
i n t e r a c t i n g N i t o t h e m e t a l l i c area i s however l e s s c l e a r l y l i s h e d , as i n T a b l e 1, the aging time,
non estab
the m e t a l l i c area increases s l i g h t l y
up t o 3 h,
i n when
with
b u t tends t o l e v e l o f f o r even decrease
f o r higher aging times. When c o m p a r i n g i m p r e g n a t e d w i t h p r e c i p i t a t e d c a t a l y s t s and Table 2 ) ,
i s r i c h i n non i n t e r a c t i n g N i , m e t a l l i c area. situation,
(Fig.
3
i t can be seen c l e a r l y t h a t t h e impregnated c a t a l y s t
situation leading t o a rather
The p r e c i p i t a t e d c a t a l y s t s p r e s e n t
a s m a j o r i t y o f N i i s i n t e r a c t i n g w i t h FC
an
low
opposite
Celite
and
131
generates a l a r g e r m e t a l l i c area. l y s t i s more complex: area
The c a s e o f t h e c o m m e r c i a l c a t a
the reduction i s inhibited but the metallic
lessened b e c a u s e some C r i s p r o b a b l y c o v e r i n g a f r a c t i o n o f t h e l i m i t i n g t h e r e f o r e H,
metallic particles,
adsorption.
d i f f er
The
ences o f p r o p e r t i e s between t h e v a r i o u s p r e c i p i t a t e d c a t a l y s t s a r e v e r y l i m i t e d , compared t o t h e d i f f e r e n c e s n o t e d catalysts,
as expected.
Therefore,
for
agent i s o f secundary importance i n our preparation. seems d u e t o v a r i o u s p a r a m e t e r s , precipitation,
impregnated
the nature of the precipitating This situation
among w h i c h t h e l o w temperature
the high s t i r r i n g during the preparation
and
of the
r a t h e r l o w s u r f a c e a r e a o f t h e s u p p o r t c a n b e r e t a i n e d . The b e t t e r m e t a l l i c N i a r e a o b t a i n e d f o r N i DHoNa c a n b e r e l a t e d b o t h t o l o w e r q u a n t i f y o f e a s i l y r e d u c e d N i a n d t o a m o r e homogeneous a c t i o n between N i and t h e s i l i c a ( t h e r i g h t hand s i d e o f t h e temperature Ni-SiO,
r e d u c t i o n peak i s n o t pronounced).
This
high
smaller
i n t e r a c t i o n i s t e n t a t i v e l y a t t r i b u t e d t o t h e presence
r e s i d u a l Na,
the inter
a s n o c l e a r t e x t u r a l v a r i a t i o n was o b s e r v e d .
of
Finally,
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 e t e x t u r a l p a r a m e t e r s d r o p i s more i n t e n s e f o r t h e impregnated sample t h a n f o r t h e p r e c i p i t a t e d ones. Catalytic activity The h y d r o g e n a t i o n o f EPA c a n b e r e p r e s e n t e d b y
the
following
scheme: 2 e t h y l hexen-2-a1
J
+H, ------+
2 e t h y l hexanal
Isomerization
>-
+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
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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
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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
163 REFERENCES 1.
R. J. Quan, R. A. Ware, C. W. Hung and J. Wei.
Advances in Chemical Engineering,
14(1988) 95. 2.
H. Toulhoat and J.
C. Plumall.
In "Catalysts in Petroleum Refining 1989". D. L. Trhm,
S. Akashah, M. Absi-Halabi and A. Bishara (editors), Elsevier, Amsterdam, 1990.
p.
463. 3.
C. T. Adams, A. A. Del Pagglo, H. Schaper, W. H. J. Stork and W. K.
Schiflett, Hydro-
carbon Processing, September 1989, p. 57. 4.
J. Wei.
In "Catalyst Design Progress and Perspectives", L. L. Hegedus (editor), John
Wiiey and Sons, New York, 1988, p. 245.
5.
R. L. Howell, C. W. Hung, K. R. Gibson and H. C. Chen. Oil and Gas J., 83(1985) 121.
6.
K. Onuma.
In "Preparation of Catalysts IV", B. Delmon. P. Grange, P. A. Jacobs and G.
Poncelet (editors). Elsevier, Amsterdam, 1987. p. 543. 7.
V. J. Lostaglio and J. D. Carruthers, Chem. Eng. Progr. March 1986, p. 46.
8.
D. L. Trimm and A. Stanisiaus. Appl. Catal. 21(1986) 215.
9.
T. Ono, Y. Ohguchi and 0. Togari.
In "Preparation of Catalysts HI, G. Poncelet, P.
Grange and P. A. Jacobs (editors), Elsevier, Amsterdam. 1983. p. 631. 10.
B.
C. Lippens and J. J. Steggerda.
In "Physical and Chemical Aspects o f Adsorbents
and Catalysts, Linsen (editors), Academic Press, London, 1970, p. 171. Chem. Eng. Tech. 56(1984) 455.
11.
U. Hammon and M. Kotter.
12.
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.
Froment (editors), Elsevier, Amsterdam, 1980, p. 331. 14.
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.
17.
J. Haber.
Chem. 18(1968) 301.
In "Studies in Physical and Theoretical Chemistry", P. Lacombe (editor),
Elsevier Amsterdam, 1984, p. 123. 18.
A. Kapoor, R. T. Yang and C. Wong, Catal. Rev. Sci. Eng., 31U989) 129.
19.
A. E. B. Presiand. G. L. Price and D. L. Trimm. Progr. In Surf. Scl.. X1972) 63.
20.
N. A. Gjostein, in Surface and Interfaces, T. T. Burke, N. L. Reed and V. Weiss (editors) Syracuse Univ. Press (1 967).
21.
H. Schaper, E. B. M. Doesburg and L. L. Van Reijen, Appl. Catal. 7(1983) 211.
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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 <7 to produce a silica hydrosol. Neutralisation of the soda causes the polymerisation of the silicate species to fonn particles w h i c h link to give chains and then networks resulting in the -ional hydrosel structure.
217
SODIUM SILJ.cATE
SULFINRIC ACID
High intensity mixing
Gelation
sodium sulphate renKNal
and structural rearrangement
Milling and size classification
Fig. 1
schematic diagram of the stages involvfxl in preption of ~r/si02ethylene polymerisation catalyst via gel route.
m e gel time is a function of pH and concentration of sol and is also infbmxed by temperature, additives and/or impurities. In practice this imposeS limitations on the conditions under w h i c h hcnnogeneous hydmsels can be
.
A key parameter in defining potential pore volume and hydmgel prepred strength is the Si02 concentration of the hydrosol. In general hiqh porosities
are obtained with law Si02 comzentrations but this also leads to reduced hydmsel strength. Tnerefore, althouqh hydrosols can be prepard in the range 2-20 wt% Si02, hydrosels having Si02 contents of 17% are insufficiently strong to withstand subsequent processing stages in conventional manufacture and require special proc&wes. ?he washing stage is essential for removing the large amounts of by-
prcduct scdium &phate
f m the silica to ensure high thermal stability. Upon
in the hydrogel continue to make further links resulting in a contraction of the hydrogel framework and expulsion of water (syneresis). Ihe extent of this shrinkage appears to be a fundion of the hydrcgel Si02 conOentration (Fig. 2) and influences the strength and potential pore volume of the final xercqel. standing and during washing the silica particles
218
SIO, content of hydrogel
20
after washlng 10
I
t I
I
i
I
1
I/
4
i
I Q
Potentla1
pore volume ( cmSg-
'1
S102 content of hydrosol ( a ; )
Fig. 2 Influence of hydrosel SiOz concentration on shrinkage during washing and potential pore volume of produd. xerogel prepared by dryhydrogel at this stage typically has high surface area (-800dg-l) and l m pore volume (0.4 ~m~9-l).Such Structures are unsuitable for application in polymerisation catalysts, the mall pores becaming quickly blocked with polymer causing catalyst deactivation. Hudever once a hydrogel -s is formed it can be d f i e d in the wet state to enlarye the pore size and reduce the surface area. l k i s irreversible structural rearrangement, generally referrel to as aging, is carried cut under hydrcrthermal conditions. The extent to w h i c h the structure is modified depends on the nature of the hydrogel, pH, temperature, time and presence of impurities. Fig. 3 illustrates the decrease in surface area and devel-t of pore volume as a function of aging time.
200
1
fL
,
I
01,
2
6
4
Aglng tlrne
Fig. 3
S t r u c t u t - a l changes during
8
' 0
(hrs)
aging.
The water remaining in the hydrosel at this stage is an integral part of an equilibrium structure and the ~ ~ n n in e rwhich it is removed has a powerful
219
influence on the pore volme of the final dry xemgel. '&a distinct routes are comonly used for drying : the direct removal of w a t e r a t high temperature or the displacemnt of water by a w a t e r soluble organic solvent. In the f i r s t case the final pore volume depends on the relative rate of water removal f r m the hydrosel and the rates of structural rezrangement and siloxane bond formation. Rapid remnml of water results in the hiqher pore volume (Table 1) but clearly other factors including the Si02 content of the hydmsel and the degree of aging also influence the extent of shrinkage and consequent porosity. TABLE1 Pore volume of xerogel vs. nature of hydrogel and drying methcd.
content (%)
surface area (m2Q1)
30 30 15 15
600 350 900 400
Hydrosel Si02
me
F a r e volume (cm3g-1)
Slaw
Fast
Drying
Drying
0.7 1.3
1.5 1.85 1.10
solvent
Theoretical
1.9 1.9 2.0 3.0
2.33 2.33 5.67 5.67
Drying
second rcerte involving solvent exchaqe/azeatropic distillation minimises
the degree of shrinkage by s q p r t i n g the hydrosel structure during w a t e r
removal and lcwering the surface tension of the liquid phase. .%all amounts of w a t e r remaining in the solvent can have a substantial negative influence on the final xemgel pore volume (Fig. 4 ) .
Pore volume ( cm38-
'1
L
1
0
I--_ I I -
2
4
6
Resldual water In IPA ( % )
Fig. 4
Influence of residual w a t e r during solvent exchange on xerogel pore volume.
220
A model for the develapnent of the s i l i c a gel
Fig.
5.
structure is illustrated i n
It is best appreciated by reference t o the gel manufacturirKJ
procedure outlined above.
A t the hydmgel stage the structure -rises
primary silica entities ( 2m) linked together in a
small
ional network
*
flurounded by w a t e r . Remavdl of this w a t e r t o form the xerogel causes callape to a dense and tightly packed array of s m a l l silica units. The surface area ( 800 6g-1) derives fram the external surface of these units and the measured value agrees w e l l w i t h the calculated value for 2m particles i f due allowance is made for area lost through particle contacts. The measured porosity of the xerogel (0.4 cm3Q1) is in line with that for a Landom close packed array of silica particles. Prlmarv Partlcle
Hydros ol
I
Xe ro gel
Unaged Hydrogel
(Hlgh SA, Low P V )
Structural Rearrangement
Te r t I a r y Ag g r e g ate
Secondary Partlcle (developed by clusterlng o f Prlrnary Partlcles)
Xe ro ge I
Aged hydrogel
(Low SA, Hlgh P V ) Fig. 5
W e 1 of s i l i c a gel structure.
On aging, the evidence f m surface area and pore volume s t -
and
substantiated by electron microsaopy, suggests that the primary particles marrange t o form secondary subunits w h i c h have a s i z e of about 10-20m11, and ultimately lose their identity. The surface area is now controlled by the external surface area of the SeCondaLy units and f a l l s correspdingly to values of
250-350 m2q-l.
The secondary units are also arranged in an open network with s c ~ n eloCali& clustering into tertiary aggregates. 'Ihis increased aggregation generates a r e i n f o e structure within the hydrcgel
which results in less shrinkage upon removal of water.
7%
higher pore volumes
generated as a function of the strudurdl marranganent via aging reflect this
(Fig. 3 ) .
U n d e r mnnal c i r c u i w w of gel preparation the secondary
221
particles retain their identity. However, if forcirq conditions are used for the hydrothermal aging step it is possible to llfusel'the secondary particles and the surface area reduces to 30-50 m2g-1. The porosity of the aged silica structure is thus critically dependent upon the packing and degree of aggregation of the secondary particles. ?his is obviously going to be influencd by process conditions, for example, and explains the strong dependency of porosity on drying technique. Mower the silica strength will also be dictated by the packing of these particles and hence, the structural breakdcrwn, for exan@le during polymerisation, will most easily take place via the large pores to leave tertiary aggregates of seconchy particles. CATALYST MANUFACIUFE
Catalysts are prepared from silicas with the appropriate structure by deposition of chramium, typically 1 wt%, on the surface of the silica. With low pore volume silicas (<2 cm3Q1) this can be satisfactorily accorrplished by impregrating with aqueous chromium solutions. Thus standanl comercia1 catalysts such as Crosfields EP30, suitable for the prcduction of blaw molding grades of HDPE, are prepare3 by hprqnating a law scda silica hav- a pore volume of 1.8 can3Q1 and surface area of 350 m2g-l with an aqueous solution of chromium (111) acetate. For higher porosity catalysts, (used in the IMnufacture of speciality polymers) it is necessary to use non-aqueous solvents to retain the pore volume. men d l amounts of water present at this stage cause a substantial reduction in porcsity. prior to polymerisation, the Phillips catalyst m s t be activated at high temperature to stabilise the chrcanium as a surface chrcanate, the precursor to the active site. This is achieved by trcstnientwith dry air in a fluidised bed using temperatures of 500 to 950°C and residence tines of up to 12 hours. Upon exposure to ethylene in the reactor the Cr (VI) sites are pragressively reduced to the active sites which catalyse the polymerisation. ExTULmm rnLmERIsAlTrn USING Cr/SILICA CATALYSTS 'I~JOmajor law pressure processes use Cr/silica catalysts to mufacture high density polyethylene - the Phillips slurry w s e and Unipol gas phase
--P In the Phillips slurry @mseprocess (ref. 4) a hydrocarbon diluent, ethylene, catalyst, and c a n o m if required, are charged continuously to circulating loop reactors operating at about 40 a h . The crystalline polyethylene f o m as discrete, free flawing particles in the presence of the diluent up to a critical operating temperature, w h i c h depenaS mainly on the
222
-
diluent (111'C for k c h t a n e) and on the crystallinity of the polymer. Beyond that temperatwe, particle swelling and then agglomeration with reduction in heat transfer leading to reactor fouling. Catalyst residence time in the reactor is of the order of 1-2 haxs. 'Ihe Unipol fluidisid bed gas phase process (ref. 6) utilises a continuous feed of catalyst and ethylene. fie l a t t e r acts not only as the main reactant moxaner but also as the fluidising gas and heat transfer medium for m i n g heat of reaction.
Moreover the product polyethylene also serves as the fluidisid bed mterial. Conversion per pass thrcugh the reactor is akwt 2 to 3%. The reactor pressure is n o d l y abcut 20 a h , ten@era80 to 105'C and catalyst residence time about 3 t o 5 hours. In both processes each catalyst particle creates a polymer particle many t i m e s larger than itself such that the shape and particle s i z e distribution of the polymer particles r e f l e d t h e of the catalyst. It is also faLlrd that the catalyst particle is fragmented by the polymerisation process such that the silica is distributed h a q e m m s l y -cut the polymer particle and is not easily detectable. The ultimate s i z e of the silica fragments has been estimated to be of the order of 100 w by pomsimetric charaderisation (mercwy intrusion) of the catalyst residue after polymer ashing (ref. 7) and from a conbination of electron m i v and catalyst productivity data (ref. 8 ) . T h i s value Suggests that the silica fragments produced during polymerisation are the tertiary aggregates of secondary particles. ?he s i z e of these clusters w i l l be a direct function of the way in w h i c h the secondary particles have coordinated and packed w e t h e r during the manufacture of the silica. INFLUENCE OF SILCCA STRUCXURE ON POLFR3PERTIFS (a) selectivity mlyethylenes, produced in a wide range of densities, with each density produced in a w i d e range of m e l t indices ( i n v d y related t o molecular w e i g h t ) , provide a b r a d spedrum of available polymers w i t h particular properties for specific applications (ref. 2 ) . f i e polymer properties are determined by: 1) molecularweight 2) molecular weight distribution (m) 3) degreeofchainbranching 4) distribution of chain branching ?3ese parameters can be altered to some extent by modifying the reactor conditions, i.e., ethylene concentration, tenperatum=, presence of ccplloly~ner and/or cocatalyst but the major influence is provided by the structure, ccmposition and activation of the catalyst.
223
For example, a strong correlation exists between the catalyst pore structure and the molecular wight of the polymer produced (ref. 7). ?he obsesved decrease in mlecular weight with increasing pore size is well established but is at variance with the trend expe&ed if kinetics were influenced by ethylene diffusion into the pores. A satisfactory explanation for the w e d behavicur has not yet been proposed. A trend of decreasing molecular weight with increasing catalyst activation tenperature (500 to 900°C) is also observd. ?his cannot be attributed to changes in the porous structure of the catalyst since the pore size distribution remains relatively unchanged at these temperatures in silicas containing law concentrations of soda. Instead it has been correlated with extent of dehydmxylation of the silica surface. Residual hydmxyls may coordinate to the active (=r centres thus interfering with polymerisation or alternatively, the condensation of hydroxyls may result in s t r a i n within the silica surface hence modifying the environment of the active site (ref. 9 ) . (b) Activity m to be a cxnnplex function of m e activity of the catalyst also a catalyst structure, camposition and activation temperatures. ?he structural parameters, surface area, pore volume and pore size distribution are interrelated and it is inpossible to vary any one of these in isolation. For example, although there is evidence to suggest that higher surface area, at constant pore volume, results in imreasd catalyst activity (ref. 10), the polymer produced has 1melt index reflecting the shmltaneous change in pore size distribution towards smaller pore size. camnemial catalysts typically contain 1 w t % chromium although the catalyst activity is virtually independent of chrmnium loading in the range 0.75 to 2 w t % (ref. 11). Exwss chromium leads to a loss of activity w h i c h can be attributed to formation of bulk Cr2O3 during catalyst activation resulting in loss of catalyst surface area and porosity by pore blocking. On the other hand insufficient chromium generates an hadequate number of sites to scavenge the poisons (e.g., H20, 02, CO, etc.) to which these catalysts are so sensitive. ~nfact only a small proportion of the chromium present is active in polymerisation. Determination of the active sites using l4C0 radiolabelling (ref. 8) and other techniques (ref. 7) w e s t this to be as lm as 3 to 7% of the total. INFLUENCE OF MODIFIERS Whilst a wide range of polyethylenes can be prepared using silicas of different structures this range can be substantially extend& and catalyst activity increased by the addition of modifiers. Such modifiers may be
224
inaorporated within the catalyst durhq silica gel fomtion as, for example, in the production of silica titania (ref. 12) and silica zirconia cogels (ref. 1 3 ) , or by surface inpregnation as with titanium (ref. 1 4 ) , fluoride (ref. 15) or aluminium (ref. 1 6 ) . surface modifiers alter the catalyst activity and resultant polymer characteristics either by influencing the active site directly as with titanium via the formation of titanyl ChraMte (ref. 17) or indirecuy by replacement of neigkbmrirq hydroxyl groups as with fluoride (ref. 18). Thus the broadened molecular weight distribution and himelt index observed with titanium modified catalysts can be attributed to the participation of 2 types of sites, t h e frcnn the silyl duoaMte preausors also present on unodified catalysts, wether with additional sites originating from titanyl chromate. 'Ihese latter sites produce 1molecular weight polyethylene resulting in a bimodal MWD (ref. 1 9 ) . T p R studies (ref. 20) of activated chromiq/silica catalysts lend support for the existence of two types of sites. % standard chrcanium silica catalyst shm one redudion peak with a maximum at 440°C whilst an additional peak of similar magnitude with a maxbmm at 420°C is recorded for the titanium modified catalysts. Surface modifiers can also influence the thermal stability of silica and hence the structure of the activated catalyst. For example, high pore volume silicas made by certain routes can display uncharacteristically law thermal stability even in the presence of lcw soda concentrations (200 ppn) mese can be substantially improved by hp-egnation with relatively d l amounts of titanium as sham in Table 2. Similar results are obtained with zirconium. High pore volume silicas preprd by slightly different routes have substantially higher thermal stability.
.
TABLE2
Influence of titanium hpreqnation on thermal stability of a high pore volume silica. Untreted Ti(%)
0 0.38 0.60 1.56 5.28
Ti02(%)
0 0.63 1.00 2.60 8.8
Surface area (m2g-l) 434 4 09 397 4 12 4 14
Fore volume
Calcined 870'C/2 hrs
surface
Fore
volume
(cm3Q1) 2.99 3.00 2.94 2.88 2.66
311 387 363 383 347
2.13 2.42 2.49 2.65 2.31
225
In the pruduction of cogels for application as polymerisation catalyst supports, the modifier, eg, titanium, z b n i u n or aluminium, is typically present at corcentrations of 14 w t % as oxide during hydrosol preparation. The presence of these species influences the resultant hydrogel structure and its subsequent aging kinetics. ?his is best illustrated by reference to the data in Table 3 showing the influence of different alumina contents on the unaged and aged gel structures. Clearly the presence of alumina reduces the rate at w h i c h structural modifications occur. Rdditionally, the presence of im,reasing amounts of cogelled alumina reduces the potential pore volume of a given silica system. similar trends in aging kinetics and pore volume have been abserved with titanium and zirconium but these are much less pronounced.
TABLE 3 Influence of alumina on structure and aging kinetics of silica gds. Silica alumina
Silica
cogd 1% A1203
silica alumina cogel 4% A1203
Surface Pore Surface Pore Surface Pore area volume area volume area volume (m2g-I) (cm3g-1) (m2g-l) (cm3g-l) (m2g-l) (an3g-1) ~
0
4
8
10
8.0 8.0
8.5
960
498 466 387
2.38 2.91 2.97 2.84
909 636 583
1.88 2.44 2.38
~~~
1010 865
787 553
0.82 1.84 1.95 2.12
The lower pore volumes obtained in the presence of modifiers makes it difficult to investigate the influence of modifier conoentration in a cogel (ref. 21) have atteqted to do indepndently of pore structure. Conway so despite shwing that the pore volumes and th-1 stabilities of the catalysts decrease with increasing cogel titanium content. These trends, in isolation, would be e x p e c k 3 to result in a decrease in melt index if behavicur pardlleled pure silica systems. The increased melt index obsenred with titanium loading m u s t therefore be a result of the titanium content of the
cog&. No data have been published whereby cogel and hprqnated catalysts with the same pore size distributions have been compared and hence the influence of cogelled vs imprqnated modifier cannot be absolutely differentiated. ~cwever it is not unreasonable to suggest that in addition to modifying the porous structure and thermal stability of the support, cogelation can also result in d i e modification of the active centres. Thus for titanium, the better distribution of titanium envisaged in the cogel structure vs hprqnat&
226
titanium may result in precursor sites of the type: I
-Si-O I
\
I --Ti-O
/ \0
0
I
//O
cr
Hence 3 possible types of precursor chrmate sites can be envisaged (the above together with the two already suggested for the titanium impregnated system) with the mixed site above predcaninating. sites of this type may be responsible for the narraw MWD and extremely high envirornnental stress crack resistance of polymers generated by this type of catalyst (ref. 7). coNcLus1oN
~lthoughthe cr/silica ethylene polymerisation catalysts have been used for more than 30 years, t h i s deceptively siqle system still has many of its perfomwce attrihtes poorly unfierstocd. whilst it is well established that the catalyst fractures in use and that the structure of the support influences this, no direct correlation has been established between the details of catalyst fragnr=ntation behaviour and polymerisation performance. Similarly the broad relationship between catalyst porosity and polymer structure has been clearly demonstrated lxlt again there is no detailed umkrstandbg of the mechanism whereby the pore size distrhtion of the catalyst can influence the polymerisation kinetics and hence modify the polymer MWD. Heteroatms can have two mles either pruviding different active centres and/or modifying the Ihe balance between these is not well defined. support s-. clearly the quest to develop new catalysts capable of producing polymers with superior properties continues to be a challenging task w h i c h myires the cambined expertise of the silica and polymer mnufactumrs to make progress.
-
The author wishes to thank A. L. b e l l (Cmsfield Chemicals) and D. R. Ward (UnileverFG?s%w&, Port Sunlight Laboratory) for helpful discussion in preparing this manuscript.
REFERENCES
1. 2. 3.
M. P. WlXdel, Fracturing silica-based catalysts during ethylene polymerisation, J. Polym. Sci., l%lym. Chem. Ed., 19(1981) pp 1967-1976. M. P. %Dmiel, Contmlling polymer properties with the Phillips chrcanum catalysts, M. E3lg. &em. Res., 27 (1988) pp 1559-1564. R. K. Iler, ?he Chemistry of Silica, John Wiley and Sons, New York, 1979.
227
4. 5. 6.
7. 8.
J. P. Hogan, D. D. Nortyood and C. A. Ayres, Phillips petroleum capmy loop reactor technology, J. Appl. polym. sci., Appl. polym. symp., 36 (1981) pp 49-60. S. KondD, F. Fujiwara and M. Muroya, The Effect of heat treatment of silica at high temperature, J. coll. and Interface sci., 55 (1976) pp 421430. Union Carbide corporation, Chemicals and Plastics Division, Gas-phase, high density polyethylene process, Chem. Eng., (1973) pp 72-73. M. P. &Daniel, supeortsa cbrcnnium catalysts for ethylene polymerisation, Adv. Catal., 33 (1985) pp 47-98. S. Wang, C. E. Marsden and P. J. T. Tait, Phillips-type polymerisation c a t a l p . Kinetic behaviour and active site determination, J. Mol. catal., mpress.
M. P. McDaniel and M. B. Welch, The activation of the Phillips polymerisatation catalyst 1. Influence of the hydmxyl population, J. catal., 82 (1983) pp 98-109. 10. Personal aamnunication. 11. J. P. Hogan and D. R. Witt, Supportd cbrumium catalysts for ethylene polymerisation. Award Symp. on Olefin polymerisation and Disproportionation. Joint Meet- of the ACS and Chem. SOC. Japan 9.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
Honolulu, (1979) pp 377-387. R. E. Dietz, U.S. Patent 3,887,494 (1975). R. A. Dcanbro and W. KirCh, U.S. P a a t 4,246,137 (1981). T. J. Pullukat and M. Shida, U.S. Patent 3,780,011 (1973). J. P. Hogan, U.S. Fatent 3,130, 188 (1964). S. J. Katzen and L. J. Rekers, U.S. Patent 4,100,104 (1978) T. J. Fullukat, R. E. Hoff and M. Shida, A chemical sturfy of themally activated chromic titanate on silica ethylene polymerisation catalysts, J. polym. Sci., Polym. chem. Ed., 18 (1980), pp 2857-2866. F. J. K a r o l , B. E. Wagner, I. J. mine, G. L. Goeke and A. Noshay, New catalysis and process for ethylene polymerisation catalysts, Adv. Polyolefins Proc. ACS Int. Symp. 1987 pp 337-354. M. A. Sutton, Studies of titanium mcdified chromia catalysts for olefin polymerisation. %.D. Thesis, (1981), University of Notthqham. C . E. Marsden, Unpublished d t s . S. J. Conway, J. W. Falconer, C. H. Rcchester and G. W. Dawns, a71-cwia/silica-titania cogel catalysts for ethene polymerisation, polymer characteristics, J. a m . Soc. Faraday Trans. I, 85(7)(1989), pp 18411851.
This Page Intentionally Left Blank
G.Poncelet,P.A. Jacobs,P. Grange and B. Delmon (Editors),Preparation ofCutu2ysts V 01991Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
229
Preparation and catalytic effects of CeOX-MOy-A1203(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**
*Central
Engineering
Laboratories,
Nissan
Motor
Co.,Ltd.,
1,Natsushima-cho, Yokosuga, Kanagawa 2 3 7 , Japan
* * National
Chemical
Laboratory
for
Industry,
1-7
Higashi, Tsukuba, Ibaraki 305, Japan
ABSTRACT CeOX-MOy-Al2O3 supports (M=Ba,La,Zrand Pr) were prepared by an improved sol gel method and the influence of MOy on the thermal stability of both the supports and the platinum catalysts was investigated.
Pr improves
It was found that the addition of Ba, La, Zr and the thermal stability of alumina and cerium oxide.
However, as for the thermal stability of Ba, La and
Pr
catalysts
showed
the platinum catalysts,
much higher
thermal stability
than an impregnated one, while the Zr catalyst thermal stability. The activity order was impregnated catalyst7Zr in terms of CO oxidation.
showed lower B a 7 L a 7 Pr>
INTRODUCTION Three way catalysts are generally used today for controlling exhaust emissions from automotive internal combustion engines. Cerium oxide is widely employed as an additive in three way catalysts because of its abilities to store oxygen and to improve dispersion of platinum.[l] One drawback of the three way catalyst is that it tends to give rise to thermal deactivation caused by crystallization of cerium oxide, sintering of platinum and A1203. [ 2 I It has been reported that improves the thermal stability
the addition of Ba arid La of A1203[3,41 and that the sol
230
gel method, a chemical mixing technique,is effective in obtaining However, supports composed of thermostable supports. [51 a three prepared
component by this
system(CeOX-MOy-Al2O3) have not yet been method. Moreover, Pt supported catalysts
have not been investigated sufficiently. We have newly prepared CeOX-MOy-Al2O3 supports using the sol gel method and carried out investigations on the thermal stability of both CeOX-MOy-Al2O3 supports and Pt supported catalysts. EXPERIMENTAL was developed for preparing The sol gel method that CeOX-MOy-Al2O3 supports consists of complexing ,gelation, drying and activation steps as shown in Fig 1.
Support preparation procedure by an improved sol gel method
The preparation of Ce0x-Ba0-A1203 is described below as typical example. First, 60.4 g of Al(O-SBu)3 were put in a 5 0 0 ml beaker. To this, 1.7 g of Ba(acac)2 2H20 and 25 g of hexylene glycol were added. The beaker was placed in an oil bath at 100 OC and stirred for 3 hours. Meanwhile, 1.75 g of Ce(N03) 6 H 2 0 were put in a
231
300 m l beaker. To this, 12 g of C2H50H were added and then 2.23 g
of Ba(acac):!
2 H 2 0 were added. The beaker was put into an oil bath
at 50 OC and stirred for 3 hours. Subsequently,
the temperature of the oil bath was lowered to room temperature and the contents of the beaker were filtered. The filtered solution was added to the 500 m l beaker. The beaker was put into an oil bath at 100 O C and stirred for 3 hours, after which 17 g of water were added. The beaker was maintained at 100 OC for 10 hours to obtain a gel. The gel was transferred to an eggplant type flask and dried under a reduced pressure. The dried gel was heated at 250 OC for 3 hours and calcined at 450 O C for 4 hours and at 1000 OC for 3 hours to yield a pale yellow alumina powder. The platinum catalyst was prepared by a conventional as the Pt source impregnation technique using Pt(N03)2(NH3)2 and calcining at 400 OC for 2 hours and for 0.5 hour in H2. Thermal deterioration of the supports was investigated by calcining them at 1000 OC for 3 hours and at 1000 OC for 24 hours. Thermal deterioration of the Pt supported catalysts was only examined by calcining them at 1000 OC for 4 hours. CO oxidation was carried out in a fixed-bed-type apparatus with a continuous flow system at atmospheric pressure. The thermal stability of the supports was determined by the BET method and X-ray diffraction (XRD). The crystallite size of cerium oxide was measured by XRD using the small-angle-scattering method. Lattice parameters were also measured by XRD. The supports were also analyzed by X-ray photoelectrons using VG ESCA Lab MK T I . Platinum dispersion was measured with a Quatasorb, which uses the CO adsorption method. RESULTS AND D I S C U S S I O N As it has been reported that the addition of b a r y u m i s the most effective way to inhibit the sintering of alumina, a series of CeOx-BaO-A1203 supports were synthesized by the sol gel method as shown in Fig. 1 and their 1wt% Pt catalysts were compared with the corresponding impregnated Ce0,-A1203 catalysts with respect to CO oxidation.
232
Fig.2
shows the relationship
of
temperature
the
50% CO
between the Ce content conversion
with
and
the
respect to
the
oxidation of lwt% Pt-CeOX-Ba0-A1203 catalysts and lwt% Pt-CeOX-A120S catalysts calcined at 1000 OC for 4 hours. For all Ce amounts, the ternary support catalyst showed higher activity than the corresponding binary support catalysts, the optimum concentration of the ternary catalyst was found to be 8%.
s z
v
0
cc W > z
400
0 0
s 0
In
0 0
300
0
W
cc
3 I-
2w a 5+ 200o
10
20
30
Ce CONTENT (wt%)
Fig.2
Next, 83 wt% for La, account
CO oxidation performance cerium content
as
a function of
while the contents of Ce and alumina were kept at 8 and respectively, Ba in the ternary support was substitute Zr and Pr which are known to form thermostable oxides o n of the ion radius and e1ectrj.c charge. Fig.3 shows the
233
CO oxidation reactions lwt% Pt-CeOX-La2O3-Al2O3 ,
with lwt% Pt-CeOX-BaO-Al2O3. lwt% Pt-Ce0x-ZrO2-Al2O3 and
lwt% Pt-CeOX-Pr2O3-Al2O3 catalysts than for the impregnated lwt% Pt-CeOX-Al2O3, but the lwt% Pt-Ce0x-ZrO2-Al2O3 catalyst showed lower activity than the impregnated lwt% Pt-CeOX-Al2O3.
I
""
80 -$ v
Z
omm [r
W
L O O 0
0
020n v
150
Fig.3
I
200
1
1
250 300 TEMPERATURE
Relationship between reaction temperature
CO
1
I
350 ("C)
400
conversion
450
and
From the foregoing results, two interesting phenomena can be noted. One i s that high level of activity and the optimum Ce concentration for CO oxidation are obtained with the lwt% Pt-Ce0,-Ba0-A1203 catalyst; the other is that a low
234
level
of
activity
for
CO
oxidation
is
obtained
with
the
lwt% Pt-CeOx-Zr02-A1203 catalyst. The following investigations were carried out to elucidate the reasons for differences. Fig.4 shows the XRD patterns of both CeOX-Ba0-Al2O3 and Ce0,A1203 calcined at 1000 OC for 24 hours The alumina in the ternary support keeps its 6- -structure following calcination, whereas that in the binary support does not and changes from the r - to the O( -structure as a result of calcination. The crystallite s i z e of cerium oxide of both the Ce0x-Ba0-A1203 and
.
Ce0,-A1203
were measured by
I
XRD. The
value
of Ce0,-Ba0-A1203
0
CeOx-AI203
20
Fig.4
40
60
XRD pattern of Ce0,-Ba0-Al2O3
80
CeOx-BaO-Ai203
and CeOX-Al2O3
K.
Results was 1 7 6 i, while the value of Ce0x-A1203 was 267 obtained with the BET method indicated that the surface area of Ce0,-Ba0-AL203 was 1 6 7 m 2 / g after calcination at 1000 OC for 3 hours, while the surface area of Ce0x-A1203 was 35 m2/g under the same conditions. These experimental results show that
235
Ce0x-Ba0-A1203 prepared by the sol gel method is thermostable and suitable for use as an automotive catalyst support. Fig.5
shows
supports
in
Ce
XPS
comparison
r
9 1 0
l
with
I
900
B i n d i n g
Fig.5
a series of
of
spectra
the
I
I
890
spectra
I
8 8 0
E n e r s . 7
Ce0,-Ba0-A1203
of Ce02
and
870
L e v 3
XPS spectra showing the Ce 3 d 5 1 2 binding energies for the each Ce0,-Ba0-A1203 supports
Ce2(S04)3 . The spectra of cerium oxide was characterized by two well-known peaks at 8 8 4 . 8 ev and 8 8 3 . 2 ev.[61 The 8 8 4 . 8 ev peak is attributable t o Ce3+, while the 8 8 3 . 2 ev peak is due
236
to Ce4+. A s the amount of Ce in the support increases, the 883.2 ev peak of Ce4+ increases and the 884.8 ev peak of Ce3+ decreases. At a Ce content of 8wt%, the amount of Ce4+ in CeOX-Ba0-A1203 becomes roughly equal to that of Ce3+.
Pt Fig.6 shows the relationship between the Ce content and dispersion. The maximum in the Pt dispersion curve corresponds perfectly with the minimum in the 50% CO conversion curve. It is deduced
that
in
the
lwt% Pt-CeOx-Ba0-A1203 system,
8wt%
Ce
content is the most suitable amount for promoting the redox interaction between Pt and Ce and inhibiting Pt sintering.
E
E
0.00 0
10
20
30
Ce CONTENT (%) Fig.6
Pt dispersion as a function of cerium content
The crystallite size of cerium oxide of the CeOX-ZrO2-Al2O3 and the surface area of the support was support was 111 130 m2/g. These data were roughly equal to those of
1
237
CeOx-Ba0-A1203. Accordingly, from these results it was impossible to discern why the lwt% Pt-CeOX-ZrO2-Al2O3 catalysts have low activity. Therefore, the supports were investigated further using
XRD. Table.1
shows the cerium oxide lattice constant of these supports. T h e cerium oxide lattice constants, except those of the Ce0,-Zr02-A1203, are unchanged or slightly larger than that of the original Ce02; those of CeOX-ZrO2-Al2O3 are smaller.
Table 1 Cerium lattice constant of Ce0,-MOy-A1203 support
support CeOx-Al203 CeOx-Ba0-A1203 CeOX-La203-A1203 CeOX-ZrQ-A1203
5.419 5.41 1 5.425 5.373 Ceo.75Zr0.7502: 5.349 (A) Ce l.O2(A) Ba 1.36(.&) La 1.04 (A) Zr 0.79 (A)
These results suggest that a substantial solid solution occurs between the oxides of Ce and Zr with a small radius, resulting in the formation of a complex oxide. It is therefore concluded that Ce in C C O , - Z ~ ~ ~ - A does ~ ~ Onot ~ have any effect on improving the dispersion of Pt. The physical properties of these catalysts are summarized in It is seen that the Pt dispersion of the Table 2. Pt-Ce0,-Zr02-A1203 catalyst is much lower than that of the others.
238
Table 2 Physical properties of CeOx-MOy-Al203catalyst Catalyst ~
The addition of Ba, La and Pr to a two component system (CeOx-A1203) by the sol gel method improves the thermal both the support and the platinum catalyst. stability of Although the addition of Zr to the system by the same method improves the thermal stability of the support, it docs not improve the thermal stability of the platinum catalyst. The activity
order
was
found
to
be
B a z L a 7 P r 7 Z r in
terms
of
CO oxidation. Consequently, the CeOx-Ba0-A1203 support prepared by the sol gel method is concluded to be one of the most thermostable ones for automotive catalysts. REFERENCES 1 H.S.Gandhj ,A.G.Piken,H.K.Stepien,M.Shelef,R.G.Delosh and
M.E.Heyde, SAE Paper 7 7 0 1 9 6 , 1 9 7 7 2 K.Iahara,K.Ohkubo and Y.Niura, 4th International Pacific Conference, Paper N o . 8 7 1 1 9 2 , 1 9 8 7 3 H.Ohuchi,Y.Horio and N.Yamaki, Sekiyu Gakkaishi, l O ( 1 9 7 6 ) 53 4 F.Oudet,E.Bordes,P.Courtine,G.Maxant,C.Lambert and J.P.Guerlet, Catalysis and Automotive Pollution Contorol, Elsevier, Amsterdam, 1 9 8 7 . p p . 3 1 3 - 3 2 1 5 F.Mizukami,M.Wada,S.Niwa,M.Toba and K.Shimizu, Nippon Kagaku
Kaishi, 9 ( 1 9 8 8 ) 1 5 4 2 6 G.Praline,B.E.Koel,R.L.Hance,H.I.Lee and J.M.White, J. Electron Spectrosc.Relat.Phenom., Z l ( 1 9 8 0 ) 1 7
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
239
INFLUENCE OF PREPARATION PARAMETERS ON PORE STRUCTURE OF SILICA GELS PREPARED FROM TETRAETHOXY ORTHOSILICATE B. Handyl, K. L. Walther2, A. Wokaunz, and A. Baikerl IDepartment of Industrial & Engineering Chemistry, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland 2Physical Chemistry 11, University of Bayreuth, D-8580 Bayreuth, FRG SUMMARY The influence of hydrolysis conditions (pH) and of the drying temperature on the structural properties of porous silica gels derived from tetraethoxy orthosilicate (TEOS) has been investigated using nitrogen adsorption and 29Si MAS-NMR. Acidcatalyzed hydrolysis produced silica gels containing mainly micropores. Upon drying a t higher temperature (870 K) the weakly cross-linked network collapsed due t o dehydration and further cross-linking. This is indicated by diminished pore volume and by a shift in relative number of SiO4 centers with 2- and 3-Si nearest neighbors to more 4-Si nearest neighbor centers. Additionally, the cross~ with drying, indicating a decrease in hydroxyl polarization times T s i increased content. The general isotherm shape was unchanged with drying. The highlybranched cluster aggregates formed during base-catalyzed hydrolysis of TEOS yield a dried gel with a mesoporous silica network. Cross polarization times T s i for ~ this gel are significantly shorter than for the gels prepared by acid hydrolysis, suggesting closer proximity of the Si centers to hydroxyl groups than in the latter, and/or maintenance of an abundance of hydroxyl groups despite the drying treatment. INTRODUCTION Gel-based catalysts have been commonly prepared from colloidal solutions derived from the peptidization of metal salts. Another sol-gel route involves the hydrolysis and gelation of metal alkoxides, which can be obtained in highly pure form [l-31. Sol formation proceeds via three reactions, namely, hydrolysis, polycondensation, and dissolution [4]. Initially, alkoxy groups on the parent molecule are sequentially displaced by hydroxy groups. As reaction progresses, the partially hydrolyzed species can react with each other as well as other molecules of varying degrees of hydrolysis, leading to molecular complexity. All three reactions are somewhat reversible, and the relative importance of each throughout the process is determined by solution pH, waterhlkoxide ratio, temperature, and the intrinsic reactivity of the precursor alkoxide [4-6]. For the hydrolysis and gelation of tetraethoxy orthosilicate (TEOS), under conditions of low pH and water content (i.e., pH I 2.5, H20/TEOS 541, a transparent sol of linear polymeric chains is produced, which forms a weakly cross-linked gel structure [4]. However, when hydrolysis is carried out in basic media and an excess of water, the sol consists of
240
highly-branched cluster aggregates of a colloidal nature [7]. These two condition sets represent two clearly different structural unit types. Further, intermediate sol structure forms may be realized by using conditions lying between the two conditions outlined above. Thus, control properties are available a t the solution stage that are important for catalyst design and production such as porosity, chemical composition, strength, and shape, and show promise for the development of new metal oxide catalysts. Solid state 29Si NMR spectroscopy can be useful to probe the local environment of Si in the silica matrix [5]. Magic angle spinning (MAS) and 29Si-1H cross-polarization (CP) techniques are employed to resolve the isotropic lines corresponding to different silica sites and t o increase the signal-to-noiselevel. The Si environments near surface hydroxyl groups are probed when both methods are used simultaneously since these groups are the only ligands in the silica gel network for which 1H protons are neighbors to 29% centers. EXPERIMENTAL Sample Preparation Four different types of silica were employed for study:
A
400 cm3 TEOS, 400 cm3 Ethanol, and 160 cm3 H20 (2x-distilled)were mixed and several drops 1N HC1 added to give a solution pH of 2-3. The mixture was continuously stirred for 2 hrs and the alcohol removed at 313 K under reduced pressure. The sol was diluted to 1000 cm3, allowed to stand for 24 hrs, and then dried on a rotating evaporator at 393 K.
B
Several gms of Sample A were dried in a muffle oven a t 870 K for 3 hrs.
C
20 cm3 of pH=9 ammonia water were vigorously stirred with 10 cm3 of TEOS a t 343 K for 12 hrs. The sol was dried on a rotating evaporator at 393 K, then in a muffle oven a t 870 K for 3 hrs.
D
Aerosil 200, a commercial silica gel (Degussa, Inc.). The preparation involves flame hydroIysis of dry tetrachlorosilane into non-porous, spherical particles.
Sample Analysis Surface area and pore size information were obtained from nitrogen adsorptioddesorption isotherms at 77 K, using a Micromeritics ASAP 2000 Analyzer. Prior t o measurement, all samples were degassed t o 0.1 Pa at elevated temperatures. The degassing temperature was 473 K for all samples except A, which was degassed at 393 K. BET areas were calculated assuming a crosssectional area of 0.162 nm2 for the nitrogen molecule. Mesopore size distributions were calculated using the Barrett, Joyner, and Halenda (BJH) method, assuming a cylindrical pore model [9]. Assessments of microporosity were made from t-plot
241
constructions, using the Harkins-Jura correlation [lo] for t as a function of p/po. Parameters were fitted to a low-area, non-porous silica. The NMR measurements were performed on a high resolution solid state NMR spectrometer (MSL 300, Bruker). The 29Si spectra were obtained a t 60 MHz and recorded a t room temperature. Magic-angle sample spinning was routinely carried out a t 4 kHz with Kel-f rotors. The CP/MAS spectra were obtained under Hartmann-Hahn conditions ( w1= 3 x 105 rad s-l ) in single-contact experiments by using variable contact times from 1to 50 ms, and a pulse repetition rate of 10 s. All chemical shifts are reported with respect to TMS. RESULTS Surface Area and Pore Structure Isotherms and t-plots from the four samples are shown in Figs. 1 and 3, with calculated values for surface area and pore volume displayed with each isotherm. The isotherms of acid-hydrolyzed TEOS gels A and B are nearly identical in form, being essentially Type I, with some hysteresis in the 0.35 < p/po < 0.55 regions. The t-plots show linearity for p/po < 0.3 ( t -= 0.5 nm) and a downward slope a t higher pressures. The evidence suggests that A and B consist almost entirely of micropores (dpore5 2 nm), although the intercepts obtained by extrapolation of the low-pressure data t o zero pressure show that very little pore volume is attributable to “primary micropore filling“ [lo]. The micropores could thus be very narrow slit pores with spacings of 1-2 nm that would fill completely after only several adlayers. A small percentage of slit pores sufficiently large to fill by capillary condensation would be responsible for the hysteresis region. In contrast t o A and B, the isotherms of samples C and D are of type IV and H1 hysteresis (using IUPAC convention [ll]). Analysis of the t-plots did not show evidence of microporosity in either sample, and St estimates of external surface areas are similar to the BET areas. Mesopore size distributions of C and D (Fig. 2) obtained from the desorption branches are unimodal, with most frequent pore diameters (cylindrical model) of 10 and 40 nm, respectively. I n the case of D, this is compatible with the literature describing Aerosil as aggregates of near-spherical particles of 15-30 nm diameter [12],in which pores exist as neck and interstitial spaces. A similar pore structure may hold for C as well, although the corresponding particle sizes are probably smaller than in the Aerosil.
242
-
0)
n'
I-
v)
8
v
x> 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
<: Dp(nrn) >
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
=<
two t y p e s o f e l e m e n t a r y l a y e r s have been s t u d i e d u s i n g XRD methods (6,7).
A
( s i m p l i f i e d ) schematic r e p r e s e n t a t i o n o f t h e r e c t o r i t e (and r n o n t m o r i l l o n i t e ) s t r u c t u r e i s shown i n F i g . 1.
0
T 0 T
F($
0
:.::
:-:
:.:
:.$
T 0 T
T 0 T
F i g . 1. Schematic r e p r e s e n t a t i o n o f t h e m o n t m o r i l l o n i t e (M) and r e c t o r i t e (R) s t r u c t u r e . The T-0-T 3 - l a y e r s sequence ( T = t e t r a h e d r a l , O=octahedral) i s r e p r e s e n t e d b y t r a p e z o i d s and r e c t a n g l e s . Exchangeable and non-exchangeable ( f i x e d ) charge compensating c a t i o n s a r e r e p r e s e n t e d by open and s o l i d c i r c l e s . T h i s paper d e s c r i b e s t h e physicochemical p r o p e r t i e s o f n a t u r a l r e c t o r i t e samples p i l l a r e d w i t h aluminum c h l o r h y d r o x i d e (ACH) s o l u t i o n s c o n t a i n i n g (AlI3) t h e [Al130,(OH)~,(H20)12]+7
cation.
S t a b i l i t y , a c i d i t y , c r a c k i n g a c t i v i t y and
p r o d u c t s e l e c t i v i t i e s f r o m gas o i l c o n v e r s i o n w i l l be compared t o t h o s e o f s i m i l a r l y prepared p i l l a r e d montmorillonite catalysts. EXPERIMENTAL Two r e c t o r i t e samples f r o m Garland County, Arkansas were o b t a i n e d f r o m t h e Clay Mineral Society Repository. c o n t a i n i n g 10-20% r e c t o r i t e .
The samples c o n s i s t e d o f q u a r t z aggregates
P a r t i c l e s w e i g h i n g about 259 were g e n t l y crushed,
t r a n s f e r r e d t o a 1 l i t e r p l a s t i c beaker, and t h e n d i s p e r s e d i n d i s t i l l e d w a t e r using a 3 minute u l t r a s o n i f i c a t i o n treatment.
The coarse p a r t i c l e s were
a l l o w e d t o s e t t l e f o r 10 minutes, a f t e r which t i m e t h e c l a y - s i l t s l u r r y was decanted i n t o 250 m l c e n t r i f u g e b o t t l e s . m i n u t e s a t 650 RPM u s i n g an I.E.C. clay.
The samples were c e n t r i f u g e d f o r 5
Model K c e n t r i f u g e t o s e p a r a t e t h e < 2 - m
The
289
a n o t h e r vessel by f l o c c u l a t i o n w i t h 0.5M MgC12.
A f t e r m u l t i p l e washes w i t h
d i s t i l l e d w a t e r t o remove t h e <2-um component f r o m t h e b u l k sample, t h e collected
< 2 - m c l a y was r i n s e d o f excess MgC12 b y washing t h r e e t i m e s w i t h
d i s t i l l e d w a t e r and t h r e e t i m e s w i t h a b s o l u t e methanol. sample was o b t a i n e d t h a t a f t e r d r y i n g i n a i r a t .100"C
After beneficiation, a gave an x - r a y p a t t e r n i n
e x c e l l e n t agreement w i t h JCPDS p a t t e r n No. 25-781 f o r r e c t o r i t e , F i g . 2 . amounts o f k a o l i n and q u a r t z were p r e s e n t i n t h e s e samples.
Trace
Chemical composi-
t i o n i s g i v e n i n T a b l e 1. TABLE 1 Oxide Composition of Two R e c t o r i t e s B e f o r e and A f t e r P i l l a r i n g w i t h C h l o r h y d r o l
Wt%*
Fe203
2'3
P a r e n t MgRectorites
ACHRectorites
0.80
1.34
0.69
0.98
35.5
34.4
40.5
40.6 49.9
Si02
49.4
49.5
50.6
MgO CaO
1.13
1.04
0.03
0.06
0.04
0.03
0.02
0.03
Na20
3.89
3.68
3.50
3.44
0.15
0.19
0.14
0.20
0.12
0.28
0.11
0.27
K20 Ti02
*
The d i f f e r e n c e f r o m 100% i s due t o bound w a t e r . Two r e c t o r i t e samples c o n t a i n i n g 1.34% and 0.8% i r o n as Fe203 were t h u s
o b t a i n e d and t h e n expanded by r e a c t i n g a s l u r r y c o n t a i n i n g 0.0075 g c l a y l g w a t e r w i t h an excess o f C h l o r h y d r o l ( f r o m t h e Reheis Chemical Company).
All
powder d i f f r a c t i o n measurements were o b t a i n e d w i t h a Siemens D-500 d i f f r a c t o m e t e r a t a scan o f l " / m i n u s i n g monochromatic Cu-K r a d i a t i o n . Samples were prepared as s t a n d a r d i z e d specimens by p l a c i n g 0.089 o f c l a y on a g l a s s s l i d e 27mm x 46mm i n s i z e .
A few drops o f b u t a n o l were t h e n added t o f o r m a c l a y
s l u r r y t h a t c o u l d be e v e n l y spread on t h e s l i d e .
The same specimen h o l d e r was
used t o a v o i d apparent d-spacings v a r i a t i o n r e s u l t i n g f r o m d i f f e r e n t sample thickness. Surface A c i d i t y S u r f a c e a c i d i t y was examined w i t h a N i c o l e t 170 SX s p e c t r o m e t e r .
Spectra
were a c q u i r e d w i t h 2 cm-l r e s o l u t i o n (8192 d a t a p o i n t s ) and apodised u s i n g t h e Happ-Genzel a l g o r i t h m .
S e l f - s u p p o r t i n g w a f e r s ( 4 - 8 mg/cm2 i n d e n s i t y ) were
290 p r e p a r e d by p r e s s i n g samples between 25 mm d i a m e t e r d i e f o r one m i n u t e a t
-
.6,000
P r i o r t o p y r i d i n e s o r p t i o n , t h e wafers were
7,000 pound p r e s s u r e .
mounted i n an Abspec I n s t . Corp. #2000 o p t i c a l c e l l and degassed b y h e a t i n g a t torr.
200°C f o r 2h a t
The p y r i d i n e - l o a d e d w a f e r s were t h e n h e a t e d ( i n S p e c t r a o f t h e 0-H s t r e t c h i n g
vacuo) i n t h e 200-500°C t e m p e r a t u r e range.
r e g i o n were smoothed w i t h a f i v e p o i n t s S a v i t z k y - G o l a y a l g o r i t h m and b a s e l i n e s l o p e c o r r e c t e d ; peak i n t e n s i t i e s were n o r m a l i z e d t o t h e sample d e n s i t y . RESULTS AND D I S C U S S I O N Thermai P r o p e r t i e s I r r e s p e c t i v e o f t h e i r i r o n c o n t e n t , t h e two r e c t o r i t e samples r e a c t e d w i t h C h i o r h y d r o l t o f o r m a p i l l a r e d p r o d u c t t h a t a f t e r d r y i n g i n a i r a t 10O0C/10h 0
had d(001) s p a c i n g o f 28.7 A.
C a l c i n a t i o n i n a i r a t 40OoC/10h reduced t h e
0
d(001) v a l u e t o 28.0 A p r o b a b l y due t o p a r t i a l d e h y d r o x y l a t i o n o f t h e Al13p i l l a r s , F i g . 2.
These c a l c i n e d r e c t o r i t e s had BET s u r f a c e area i n t h e 160-200 200
.-
200 -
I
175
175
~.
a
,
150 125 100
D 75 50 25 ~ K
~
2
6
I0
~
14
18
L
K -
22
26
~
30
34
~
38
'
42
-
46
TWO.THETA (DEG)
F i g . 2. X-ray d i f f r a c t o g r a m s o f a sample o f n a t u r a l r e c t o r i t e b e f o r e ( A ) and a f t e r ( 6 ) b e n e f i c i a t i o n and p i l l a r i n g f o l l o w e d by c a l c i n a t i o n a t : C ) 400°C i n a i r f o r 10h; 0 ) 1000"C/lhr and E ) 1200"C/lh.
'
0
J,
& ,J/
2
4
~
6
< ~
8
,
10
1 2 1 4 1
TWO.THETA (DEG)
F i g . 3. X-ray d i f f r a c t o g r a m s o f a sample o f M g - r e c t o r i t e b e f o r e (A) and a f t e r p i l l a r i n g w i t h c h l o r hydro1 and h e a t i n g a t : B) 400°C/ 10h i n a i r ; C ) 800°C/5h i n a i r and D ) 760°C/5h w i t h steam a t 1 atm.
2 m / g range. The s u r f a c e area f r o m mercury p o r o s i m e t r y measurement was o n l y 6.2 2 m / g i n d i c a t i n g a w e l l o r d e r e d p i l l a r e d s t r u c t u r e c h a r a c t e r i z e d b y a l o n g range
stacking o f s i l i c a t e l a y e r s face-to-face.
The h i g h i n t e n s i t y and sharpness o f
t h e 001 and 002 r e f l e c t i o n s ( F i g . 2 ) suggest t h a t t h e mica- 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 a r e p r o b a b l y s t a c k e d i n a n e a r r e g u l a r manner i n a 1 : l r a t i o .
291
P i l l a r e d r e c t o r i t e s have thermal and h y d r o t h e r m a l s t a b i l i t y much s u p e r i o r t o I n fact,
t h a t o f s i m i l a r l y prepared m o n t m o r i l l o n i t e s and h e c t o r i t e c a t a l y s t s .
A C H - r e c t o r i t e s r e t a i n t h e i r p i l l a r e d s t r u c t u r e even a f t e r c a l c i n a t i o n i n a i r a t 800°C/5h o r a f t e r steam-aging w i t h 100% steam a t 760°C/5h,
F i g . 3.
High
temperature (800°C) c a l c i n a t i o n o r steaming has l i t t l e e f f e c t on t h e shape and i n t e n s i t y o f t h e c l a y 001 and 002 r e f l e c t i o n s , F i g . 3.
A t 800°C t h e d ( 0 0 1 )
0
There
v a l u e decreases t o 26.8 A owing t o t o t a l d e h y d r o x y l a t i o n o f t h e p i l l a r s . i s l i t t l e apparent r e a c t i o n between t h e AlI3-pillars 0
t h e A C H - r e c t o r i t e d(001) was 26.5 A.
and steam; a f t e r steaming
In contrast, p i l l a r e d montmorillonites,
when exposed t o steam a t 760°C, c o l l a p s e i n l e s s t h a n 2h.
The e x c e p t i o n a l
s t a b i l i t y o f A C H - r e c t o r i t e s i s p r o b a b l y due t o t h e r o b u s t m i c a - l i k e l a y e r s p r e s e n t between t h e expanded ( p i l l a r e d ) montmoril l o n i t e - l i k e l a y e r s , see F i g . 1. The t h e r m o g r a v i m e t r i c (TGA) p r o f i l e i n F i g . 4A shows t h a t a f t e r l o s i n g a b o u t 1%s u r f a c e w a t e r , t h e a i r d r i e d p a r e n t r e c t o r i t e w e i g h t remains e s s e n t i a l l y Between 400°C and 800°C, d e h y d r o x y l a t i o n induces
unchanged up t o about 400°C. an a d d i t i o n a l 5% w e i g h t loss.
I n contrast, the p i l l a r e d r e c t o r i t e s weight
decreases m o n o t o n i c a l l y w i t h temperature due t o l o s s e s o f w a t e r sorbed on t h e e x t e r n a l s u r f a c e and i n t h e microspace generated by p i l l a r i n g , F i g . 4B. d e r i v a t i v e s o f t h e TGA c u r v e suggest
Above 560°C t h e r e i s a 0.5-1.0%
between 450°C and 560°C.
The
hat p i l l a r s dehydroxylation occur mainly w e i g h t change
a t t r i b u t e d t o removal o f w a t e r r e s u l t ng f r o m t h e c r y s t a l l a t t i c dehydroxylation.
t
100
200
300
400
500
600
700
800
TEMPERATURE ("C)
F i g . 4. T h e r m o g r a v i m e t r i c curves o f a sample o f M g - r e c t o r i t e b e f o r e (A) and a f t e r (B) p i l l a r i n g w i t h chlorhydrol.
c
0
200
,
400
800 TEMPERATURE ("C)
600
1000
1
1200
F i g . 5. D i f f e r e n t i a l thermal a n a l y s i s curves o f a sample o f M g - r e c t o r i t e b e f o r e ( A ) and a f t e r p i l l a r i n g w i t h c h l o r h y d r o l .
292 The c o r r e s p o n d i n g d i f f e r e n t i a l thermal a n a l y s i s (DTA) p r o f i l e o f t h e s t a r t i n g M g - r e c t o r i t e e x h i b i t s a weak and b r o a d endotherm between 400°C and 700°C r e p r e s e n t i n g l a t t i c e d e h y d r o x y l a t i o n ,
F i g . 5A.
Near 850°C t h e r e i s t h e
b e g i n n i n g o f a second endotherm a t t r i b u t e d t o t h e c o l l a p s e o f t h e r e c t o r i t e s t r u c t u r e w i t h quartz formation.
A f t e r endotherms w i t h peak minima n e a r 100°C
and 480°C r e p r e s e n t i n g w a t e r l o s s e s due t o sorbed w a t e r and p i l l a r s dehydroxyl a t i o n , t h e DTA c u r v e o f t h e A C H - r e c t o r i t e sample remains f a i r l y f e a t u r e l e s s up t o lOOO"C,
F i g . 56.
Then, n e a r 1000°C t h e r e i s an exotherm f o l l o w e d by a sharp
endotherm r e p r e s e n t i n g t h e c o l l a p s e o f t h e A C H - r e c t o r i t e w i t h f o r m a t i o n o f m u l l i t e and q u a r t z t o g e t h e r w i t h an amorphous r e s i d u e and s m a l l amounts o f gamma-A1203, F i g . 2D.
A t 1200°C a w e l l c r y s t a l l i z e d m u l l i t e phase i s formed,
F i g . 2E. Infrared Analysis L i k e b e i d e l l i t e (8,9),
t h e p a r e n t r e c t o r i t e s e x h i b i t two bands i n t h e
OH s t r e t c h i n g r e g i o n , F i g . 6A.
The i n t e n s e band c e n t e r e d near 3654 cm-l i s
a t t r i b u t e d t o s t r e t c h i n g v i b r a t i o n s o f OH groups i n t h e c l a y o c t a h e d r a l l a y e r s ( 1 0 ) . When exposed t o p y r i d i n e , t h e r e i s no n o t i c e a b l e change i n t h i s band. However, when t h e degass-ing t e m p e r a t u r e i s p r o g r e s s i v e l y i n c r e a s e d t o 500°C f r o m 2OO"C, t h e band a t 3654 cm-l m o n o t o n i c a l l y decreased i n i n t e n s i t y owing t o dehydroxylation reactions.
The second, l e s s i n t e n s e band a t 3468 cm-l i s
a t t r i b u t e d t o OH groups a s s o c i a t e d w i t h Si-OH-A1 l i n k a g e s i n t h e c l a y tetrahedral l a y e r r e s u l t i n g from s u b s t i t u t i o n o f S i w i t h A l .
There i s a weak
s h o u l d e r on t h e l o w frequency s i d e of t h i s band p r o b a b l y generated by p e r t u r b e d
OH v i b r a t i o n s induced by t h e presence o f charge compensating c a t i o n s on t h e s i l i c a t e l a y e r s (10). A f t e r p i l l a r i n g w i t h C h l o r h y d r o l , t h e A C H - r e c t o r i t e g i v e s an I R spectrum i n t h e OH-region s i m i l a r t o t h a t o f t h e p a r e n t m a t e r i a l ; as b e f o r e t h e r e i s l i t t l e apparent i n t e r a c t i o n between t h e s e OH and p y r i d i n e , F i g . 6B.
However, a f t e r
p i l l a r i n g , d e h y d r o x y l a t i o n o f t h e exposed l a y e r s becomes more f a c i l e and a l a r g e r e d u c t i o n i n i n t e n s i t y o f t h e h i g h f r e q u e n c y band occurs when t h e degassing t e m p e r a t u r e i n c r e a s e s t o 400°C from 300"C,
F i g . 6B.
The s h o u l d e r on
t h e low f r e q u e n c y s i d e o f t h e 3470 cm-l band becomes now more pronounced.
As
b e f o r e , t h e s e h y d r o x y l s do n o t r e a c t w i t h p y r i d i n e s u g g e s t i n g t h a t t h e y a r e mostly associated w i t h the m i c a - l i k e layers present i n r e c t o r i t e .
The s h o u l d e r
a t 3430 cm-l broadens w i t h i n c r e a s i n g degassing temperature and d i s a p p e a r s a t 400°C,
F i g . 66.
A t t h i s temperature, TGA and DTA r e s u l t s i n d i c a t e t h a t most o f
t h e z e o l i t i c w a t e r i n t h e p i l l a r e d s t r u c t u r e has been removed, F i g s . 4,5. Thus, S i (1V)-0-A1 ( I V ) g r o u p i n g s a r e ( i n p a r t ) charge compensated a l s o b y r e s i d u a l mono and d i v a l e n t c a t i o n s p r e s e n t on t h e s i l i c a t e l a y e r s and t h e i n t e n s i t y o f t h e band n e a r 3470 cm-'
F i g . 6. Hydroxyl a b s o r p t i o n bands f o r a sample o f M g - r e c t o r i t e b e f o r e ( A ) and a f t e r p i l l a r i n g w i t h c h l o r h y d r o l and h e a t i n g a t : B ) 400°C/10h i n a i r , C) 800°C/5h i n a i r and D) 760°C/5h w i t h steam a t 1 atm. Samples ( a ) have been d r i e d a t 200°C and t h e n loaded w i t h p y r i d i n e and degassed a t : b ) 200°C, c ) 300"C, d ) 400°C and e ) 500°C i n vacuo f o r 2 hours a t each temperature. s t a b i l i t y o f t h e s e h y d r o x y l s has been a t t r i b u t e d t o t h e f a c t t h a t t h e charge compensating p r o t o n i s h e l d between an oxygen and an OH group on an A l ( V 1 ) i o n (10). The TGA p r o f i l e i n F i g . 4 i n d i c a t e s t h a t a t 800°C w e i g h t l o s s e s due t o
s t r u c t u r a l w a t e r removal a r e e s s e n t i a l l y completed; t h e r e s i d u a l h y d r o x y l s g i v e two weak bands a t 3648 cm-l and 3473 cm-'
c h a r a c t e r i z e d by a l a c k o f r e a c t i v i t y
w i t h p y r i d i n e and decreased r e s i s t a n c e t o d e h y d r o x y l a t i o n ,
F i g . 6C.
A f t e r steaming, t h e ease o f d e h y d r o x y l a t i o n o f t h e c l a y o c t a h e d r a l l a y e r s i s s i m i l a r t o t h a t observed a f t e r c a l c i n a t i o n a t 800°C; a t 500°C t h e h i g h 1 f r e q u e n c y band ( a t 3666 cm- ) disappears, F i g . 6D. Steaming d i d n o t change t h e
294
Y 0 z
s Y
rn
8
&%
U
8 U
,
,
I
,
18W ISSO 15W 1450 14W
1WO 1550 1500 1450 1400
WAVENUMBERS(CM - I)
F i g . 7. I R s p e c t r a and a f t e r p i l l a r i n g 800°C/5h i n a i r and vacuo a t : a ) 200"C, temperature.
o f p y r i d i n e sorbed on a sample o f M g - r e c t o r i t e b e f o r e (A) w i t h c h l o r h y d r o l and h e a t i n g a t B) 400°C/10h i n a i r , C) D) 760°C/5h w i t h steam. Samples have been degassed i n b ) 300"C, c ) 400°C and d ) 500°C f o r two hours a t each
r e a c t i v i t y o f these hydroxyls w i t h p y r i d i n e .
I t i s b e l i e v e d t h a t steaming
d i s p l a c e d charge compensating c a t i o n s f r o m A l ( 1 V ) i n t h e s i l i c a t e l a y e r w i t h f o r m a t i o n o f new S i ( 1 V ) - O H - A l ( 1 V )
groupings.
I R s p e c t r a i n t h e 1400-1600 cm-l r e g i o n , o b t a i n e d by e v a c u a t i n g t h e p y r i d i n e l o a d e d c a l c i n e d A C H - r e c t o r i t e s i n t h e 200-500°C temperature range, a r e shown i n F i g s . 7A-D. F i g . 7A.
The p a r e n t r e c t o r i t e does n o t s o r b s i g n i f i c a n t amounts o f p y r i d i n e , I n c o n t r a s t , p y r i d i n e i s r e a d i l y sorbed i n a s i m i l a r l y d r i e d ACH-
r e c t o r i t e sample g i v i n g an I R spectrum c o n t a i n i n g an i n t e n s e band a t 1452 t y p i c a l o f p y r i d i n e c o o r d i n a t e d t o Lewis ( L ) a c i d s i t e s ( 1 1 ) . c e n t e r e d n e a r 1546 cm-'
CITI-~
The weak band
has been a t t r i b u t e d t o p y r i d i n i u m i o n s f o r m a t i o n , t h a t
i s , t o t h e presence o f B r o n s t e d ( B ) a c i d s i t e s ( 1 1 ) .
The band n e a r 1490 cm-'
has been a t t r i b u t e d t o t h e presence o f b o t h B and L a c i d s i t e s ( 1 1 ) . degassing a t 300, evidence o f B r o n s t e d a c i d i t y i s l o s t .
After
I n contrast t o
s i m i l a r l y p i l l a r e d m o n t m o r i l l o n i t e s , p y r i d i n e i s e s s e n t i a l l y removed f r o m A C H - r e c t o r i t e a f t e r degassing a t 50OoC/2h, F i g . 78. I n t h e A C H - r e c t o r i t e sample c a l c i n e d i n a i r a t 800°C/5h,
a c i d i t y i s reduced
by a f a c t o r o f e i g h t and most o f t h e p y r i d i n e can be desorbed a t 300°C, 7C.
The I R s p e c t r a i n F i g u r e 7D i n d i c a t e t h a t steaming ( a t 760°C/5h)
the ACH-rectorite Bronsted type a c i d i t y .
Fig. increases
Some s u b s t i t u t i o n o f S i ( 1 V ) w i t h
A l ( 1 V ) must be p r e s e n t a l s o i n 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 and t h e r e f o r e new S i (1V)-OH-A1 ( I V ) g r o u p i n g s a r e formed when A C H - r e c t o r i t e i s steam-aged. Steaming d r a s t i c a l l y reduced Lewis t y p e a c i d i t y as w e l l as a c i d s i t e s t r e n g t h ;
295 a t 300°C most o f t h e p y r i d i n e desorbed f r o m t h e c l a y sample, F i g . 7D. summary, l i k e ACH-montmorillonites,
In
p i l l a r e d r e c t o r i t e contains both Bronsted
and Lewis a c i d s i t e s , and a c i d i t y i s m o s t l y o f t h e Lewis type.
Acid s i t e
d e n s i t y as w e l l as a c i d s i t e s t r e n g t h i n A C H - r e c t o r i t e i s l e s s t h a n t h a t observed i n s i m i l a r l y prepared p i l l a r e d m o n t m o r i l l o n i t e s . Gas O i l C r a c k i n g A f t e r separation from i t s quartz matrix, t h e b e n e f i c i a t e d r e c t o r i t e d r i e s i n t o aggregates o f f l a k e s r e s e m b l i n g mica p a r t i c l e s .
Similar materials are
obtained a l s o a f t e r d r y i n g the product o f the p i l l a r i n g reaction.
Prior to
e v a l u a t i o n f o r gas o i l c r a c k i n g a c t i v i t y , t h e p i l l a r e d r e c t o r i t e samples were crushed and s i z e d i n t o 20x60 mesh ( f l a k e - l i k e ) p a r t i c l e s and c a l c i n e d .
Typical
chemical c o m p o s i t i o n o f t h e s e c l a y c a t a l y s t s i s g i v e n i n Table I. The p a r e n t r e c t o r i t e i s e s s e n t i a l l y i n a c t i v e , T a b l e 2.
However, a f t e r
r e a c t i n g w i t h C h l o r h y d r o l and c a l c i n a t i o n i n a i r a t 40OoC/10h, a p i l l a r e d product w i t h cracking a c t i v i t y t y p i c a l o f z e o l i t i c f l u i d cracking c a t a l y s t The (FCC) and o f s i m i l a r l y p i l l a r e d m o n t m o r i l l o n i t e s i s o b t a i n e d , Tables 2,3. m i c a - l i k e p a r t i c l e s have a b u l k d e n s i t y t h a t i s l e s s t h a n 50% t h a t o f ACHb e n t o n i t e g r a n u l e s w i t h s i m i l a r s i z e , T a b l e 2.
Thus, f o r a g i v e n c a t / o i l r a t i o
l o n g e r o i l - c a t a l y s t c o n t a c t t i m e s a r e o b t a i n e d when c r a c k i n g gas o i l s a t MAT c o n d i t i o n s w i t h A C H - r e c t o r i t e s . As a r e s u l t , a mixed l a y e r c l a y such as 2 r e c t o r i t e w i t h o n l y 160-190 m / g s u r f a c e area can be as a c t i v e f o r gas o i l 2 c o n v e r s i o n as p i l l a r e d b e n t o n i t e s w i t h s u r f a c e area n e a r 300 m /g. The A C H - r e c t o r i t e p r o d u c t s e l e c t i v i t y i s t y p i c a l o f p i l l a r e d c l a y c a t a l y s t s . G r e a t e r LCGO y i e l d s a r e o b t a i n e d by c r a c k i n g more o f t h e heavy s l u r r y o i l (SO) f r a c t i o n , Table 2.
The coke make i s a l m o s t t w i c e as l a r g e as t h a t o f a
z e o l i t i c FCC ( D a v i s o n ' s GRZ-1), Lewis t y p e a c i d i t y .
which i s m a i n l y t h e r e s u l t o f t h e c l a y s t r o n g
The A C H - r e c t o r i t e tendency f o r h i g h coke make seems t o be
enhanced by t h e presence o f i r o n .
I n f a c t , t h e c l a y coke/conversion r a t i o n
i n c r e a s e s t o 0.151 f r o m 0.146 when t h e t o t a l i r o n c o n t e n t o f t h e p i l l a r e d r e c t o r i t e i n c r e a s e s t o 1.34% Fe203 f r o m 0.80% Fe203.
I r o n c a t a l y z e s secondary
c r a c k i n g r e a c t i o n s t h u s i n c r e a s i n g H2, d r y gas, C3 and C4 g e n e r a t i o n a t t h e expense o f g a s o l i n e y i e l d s , Table 2.
The e f f e c t s o f i r o n a r e d e s c r i b e d i n
d e t a i l s elsewhere (13). P i l l a r e d m o n t m o r i l l o n i t e (and h e c t o r i t e s ) have thermal s t a b i l i t y i n a i r t h a t does n o t exceed 800°C.
I n c o n t r a s t , t h e A C H - r e c t o r i t e s under s t u d y a f t e r
c a l c i n a t i o n a t 8OO0C, r e t a i n t h e i r p i l l a r e d s t r u c t u r e ( F i g . 3 ) and more t h a n 90% o f t h e i r i n i t i a l s u r f a c e area.
As a r e s u l t , a f t e r c a l c i n i n g a t 800°C/5h,
o n l y m i n o r changes i n t h e c r a c k i n g p r o p e r t i e s o f t h e two A C H - r e c t o r i t e s a r e observed, Table 2.
296
TABLE 2 M i c r o a c t i v i t y Test
(MAT) Results f o r R e c t o r i t e s and M o n t m o r i l l o n i t e s P i l l a r e d
w i t h A1?03-Clusters and Calcined i n A i r
MgR e c t o r it e I r o n (%Fe 0 ) Conversioz f V % FF) Gasoline ( V % FF) LCGO ( V % FF) SO ( V % FF) C ( V % FF) ?= ( V% FF) n-?4 (v% FF) i - C 4 ( V % FF) C- ( V % FF) CH: (Wt% FF) H (SCF/BBL) 06y Gas (Wt% FF) Coke ( W t % FF) Co ke/Converfion BET S.A. (m /g) D e n s i t y (g/cc)
T y p i c a l l y , t h e p i l l a r e d s t r u c t u r e o f m o n t m o r i l l o n i t e s can be destroyed e i t h e r by h e a t i n g a t 675°C f o r 10h o r a t 730°C f o r 2h i n presence o f steam (12). I n a d d i t i o n t o a h i g h thermal s t a b i l i t y i n a i r , t h e ACH-rectorite under study r e t a i n most o f t h e i r s u r f a c e and c r a c k i n g p r o p e r t i e s even a f t e r steamaging a t 760°C/5h w i t h 100% steam i n a f l u i d i z e d bed, Table 3.
Steaming may
have a f f e c t e d t h e i r o n d i s t r i b u t i o n ( m i g r a t i o n ) i n t h e c l a y s i l i c a t e l a y e r s . As a r e s u l t , lower l i g h t gas y i e l d s a r e obtained and t h e coke/conversion r a t i o decreases t o 0.120 from 0.146 when t h e thermal treatment i n a i r a t 800°C/5h i s replaced by steam-aging a t 76OoC/5h, Table 3.
In Table 3 i t i s shown t h a t a steam-aged (76OoC/5h) p i l l a r e d r e c t o r i t e has c r a c k i n g a c t i v i t y comparable t o t h e one o f a s i m i l a r l y steam-aged commercial FCC (Davison's GRZ-1) z e o l i t e Y (CREY).
c o n t a i n i n g an estimated 35% c a l c i n e d r a r e - e a r t h exchanged
As p r e v i o u s l y r e p o r t e d (12), a t h i g h conversion t h e two
types o f c a t a l y s t s have s i m i l a r g a s o l i n e s e l e c t i v i t y .
However, t h e ACH-
r e c t o r i t e o f f e r t h e advantage o f h i g h e r LCGO y i e l d s by c r a c k i n g more o f t h e heavy hydrocarbons i n t h e s l u r r y o i l (SO) range, Table 3.
The lower o l e f i n s
make o f t h e z e o l i t i c FCC i s a t t r i b u t e d t o i t s h i g h r a r e - e a r t h c a t i o n s content which a r e known t o f a v o r hydrogen t r a n s f e r r e a c t i o n s , Table 3. Although t h e 2 2 two c a t a l y s t s have s i m i l a r BET s u r f a c e areas (161 m / g vs. 155 m /g) a f t e r steam-aging, a t t h e same conversion l e v e l (.85%)
they e x h i b i t t o t a l l y d i f f e r e n t
297
TABLE 3 M i c r o a c t i v i t y T e s t R e s u l t s f o r P i l l a r e d R e c t o r i t e C a t a l y s t s C o n t a i n i n g 0.69% The z e o l i t i c c r a c k i n g c a t a l y s t and t h e p i l l a r e d c l a y s have been aged :E?Og'h o u r s a t 760°C w i t h 100% steam a t 1 atm.
Zeol it i c Cracking Catalyst 85.4 59.1 9.8 4.8 4.7 7.1 2.0 8.5 2.4 0.26 356 5.6 6.5 0.076 161 0.89
Conversion ( V % FF) G a s o l i n e ( V X FF) LCGO ( V % FF) SO ( V % FF) C ( V % FF) ?= ( V % FF) n-?4 (v% FF) i - C 4 ( V % FF) C i (V% FF) CH4 (Wt% FF) H (SCF/BBL) D?y Gas (Wt% FF) Coke (Wt% FF) Co ke/Convergion BET S.A. (m / g ) Density (g/cc)
Samples have been f i r s t h e a t e d i n a i r a t 700"C/lh and t h e n steam-aged i n a f l u i d i z e d bed f o r 5h w i t h 100% steam a t 1 atm a t t h e temperature indicated.
carbon s e l e c t i v i t i e s , Table 3.
The A C H - r e c t o r i t e coke/conversion r a t i o o f
0.120 w i l l have t o be decreased by about 50% b e f o r e t h e s e m a t e r i a l s can be c o n s i d e r e d f o r i n t r o d u c t i o n i n t o a f l u i d i z e d c r a c k i n g u n i t (FCCU). Another d i s t i n g u i s h i n g f e a t u r e o f A C H - r e c t o r i t e c a t a l y s t s i s t h e ease w i t h which spent m a t e r i a l s can be regenerated.
The TGA p r o f i l e i n a i r o f a spent
A C H - r e c t o r i t e ( n o t shown) shows a 2.8 w t % l o s s between 450 and 750°C which i s w e l l i n agreement w i t h a 2.6% carbon c o n t e n t d e t e r m i n e d b y chemical a n a l y s i s . The c o r r e s p o n d i n g DTA p r o f i l e ( n o t shown) i s c h a r a c t e r i z e d by a b r o a d exotherm between 450°C and 700°C w i t h peak maximum n e a r 550°C. Thus, by c a l c i n i n g i n f l o w i n g a i r a t 700"C/lh,
a c a r b o n - f r e e A C H - r e c t o r i t e w i t h BET s u r f a c e a r e a o f
153 m2/g was o b t a i n e d ; s u r f a c e p r o p e r t i e s remained e s s e n t i a l l y unchanged a f t e r a second steam-aging (76OoC/5h) t r e a t m e n t .
The r e t e n t i o n o f d-spacing and
i n t e n s i t y o f t h e 001 and 002 r e f l e c t i o n s i n d i c a t e t h a t t h e o x i d a t i v e decomposition o f carbonaceous d e p o s i t s f o l l o w e d by steaming d i d n o t a f f e c t t h e A C H - r e c t o r i t e p i l l a r e d s t r u c t u r e , F i g . 8.
The steam-aging t e m p e r a t u r e had t o
be i n c r e a s e d t o 815°C ( f r o m 760°C) i n o r d e r t o a f f e c t t h e c a t a l y s t s u r f a c e area and c r y s t a l l i n i t y , F i g . 8C.
298
0
2
6
4
8
10
12
14
16
18
TWO-THETA (DEG)
F i g . 8. X-ray d i f f r a c t o g r a m s o f A C H - r e c t o r i t e : A ) a f t e r steam-aging a t 760°C/5h. A f t e r MAT e v a l u a t i o n , t h e spent c a t a l y s t was r e g e n e r a t e d a t 700"C/lh i n a i r and steamed a t : B) 760°C and C) 815°C f o r 5h. SUMMARY N a t u r a l r e c t o r i t e s p i l l a r e d w i t h alumina c l u s t e r s have thermal as w e l l as hydrothermal s t a b i l i t y f a r s u p e r i o r t o t h a t o f s i m i l a r l y p r e p a r e d rnontmorillonite catalysts.
T h e i r s t a b i l i t y i s comparable t o t h a t o f z e o l i t e s
w i t h the Faujasite structure.
P y r i d i n e c h e m i s o r p t i o n experiments have
i n d i c a t e d t h a t these m a t e r i a l s contain both
B and L a c i d s i t e s and t h a t a t
c r a c k i n g c o n d i t i o c s a c i d i t y i s e s s e n t i a l l y o f t h e L-type.
( 760°C/5h)
Steam-aged
p i 1 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
comparable t o t h a t o f s i m i l a r l y steam-aged commercial FCC c o n t a i n i n g an e s t i m a t e d 35% CREY.
Coke s e l e c t i v i t y (as w e l l as p a r t i c l e d e n s i t y ) w i l l have
t o be improved f o r A C H - r e c t o r i t e s t o compete w i t h z e o l i t e - c o n t a i n i n g FCC. ACKNOWLEDGMENTS The many u s e f u l d i s c u s s i o n s and s u p p o r t r e c e i v e d f r o m t h e Unocal A n a l y t i c a l Department s t a f f a r e g r a t e f u l l y acknowledged.
S p e c i a l thanks a r e due t o M s . E.
R i v e t t e , D r . R. M o r r i s , and D r . P. R i t z f o r X-ray, measurements.
Thermal and Raman
A l l e x p e r i m e n t a l work was performed by M r . R. O r t i z .
299
REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13.
M. L. O c c e l l i , i n "Keynotes i n Energy Related Catalysis," S. K a l i a g u i n e Ed., E l s e v i e r , 1988, P. 101. F. Figueras, Catal. Review 30, 1988, 3, 457. J. Guan, E. Min and Z. Yu, i n U.S. Patent No. 4,757,040, 1981. R. E. G r i m , i n "Clay Mineralogy," McGraw-Hill Co., 1968. H. Kodama, Am. M i n e r a l o g i s t 51, 1966, 1035. W. F. Bradley, Am. M i n e r a l o g i s t 35, 1950, 590. M. Sato, K. Oinuma, and Kobayashi, Nature, Lond. 208, 1965. A. Schutz and G. Poncelet, NATO Workshop on Chemical Reactions i n Organic and I n o r g a n i c Systems Proc., Reidel, 1985. A. Schutz, 0. Plee, F. Borg, P. Jacobs, G. Poncelet, and J . J . F r i p i a t , Clays and Clay Min. J. 0. Russell and J. L. White, Clays and Clay Min., Proc. 1 4 t h Conf., Pergamon Press, Oxford, 181, 1966. E. P. Parry, J . Catal. 2,371, 1963. M. L. O c c e l l i , Ind. Eng. Chem. Prod. Res. Dev. 22, 1983, 553. M. L. O c c e l l i , J . M. Stencel and F. Huggins ( i n p r e p a r a t i o n ) .
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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
301
PREPARATION AND PROPERTIES OF LARGE-PORE RE/AI-PILLARED MONTMORILLONITE. A COMPARISON OF RE-CATIONS
J. STERTE Department of Engineering Chemistry I, Chalmers University of Technology, 412 96 Goteborg (Sweden)
SUMMARY RE/Al-pillared montmorillonites were prepared by cation exchange of montmorillonite with hydrothermally t r e a te d (10O-16O0C, 12-240 h) mixtures of aluminum chlorohydrate (ACH) and chlorides of La, Ce, Pr, Nd or a mixture of RE-cations. Large-pore RE/AIpillared montmorillonites, characterized by basal spacings of about 26 and surface areas of 300-550 m2/g, were formed from solutions containing La, C e o r t h e mixture of REcations, refluxed for several days or tr e a t e d in autoclaves at higher temperatures for shorter times. Pr and Nd containing solutions did not result in large-pore pillared products a f t e r t r eat m en t at t h e conditions of this investigation. The large-pore RE/Al-pillared montmorillonites were found t o be similar in elemental composition t o a conventional Alpillared montmorillonite but thermally more stable, retaining a surface a r e a of about 300 m2/g a f t e r exposure to 8OOOC for 3 h.
a
INTRODUCTION In recent y e a r s g r e a t i n t e r e s t h a s b e e n f o c u s s e d on t h e p r e p a r a t i o n and t h e characterization of different types of pillared clays and also on possible applications for these materials primarily as catalysts or adsorbents. One of t h e most interesting potential applications for pillared clays is as active components in cracking catalyst formulations designed for cracking of heavy oil fractions. The commercial use of pillared clays for this application has, however, been limited by their lack of thermal and hydrothermal stability. Recently, McCauley (ref. I) found t h a t hydrothermally stable pillared sm ect i t es can be prepared by using hydrothermally tr e a t e d pillaring solutions, containing mixtures of aluminum chlorohydrate (ACH) and a ceriurn(II1) salt in t h e preparation. Pillared smectites prepared from such solutions differ from conventional Al-pillared smectites in t ha t th e basal spacing as measured by X-ray diffraction analysis is considerably larger, i.e. 25-28
a compared with 18-20 A. This larger basal spacing is believed t o be due to
formation of a large Ce-bearing Al-polycation upon hydrothermal t r eat m en t of t h e solution. For use as heavy oil cracking catalysts, this larger spacing is another advantage over conventional Al-pillared smectites in addition t o t h e improved hydrothermal stability. McCauley found t h a t large-pore Ce/Al-pillared sm ect i t es can be prepared from solutions with Ce/AI molar ratios down to 1:52, hydrothermally t r eat ed either by refluxing for several days or by t r e a tm e n t in autoclaves at higher temperatures for shorter times.
302
NO systematic investigation on t h e dependence of properties of t h e Ce/AI- pillared products upon different synthesis parameters was, however, reported. Although t h e work of McCauley is primarily concerned with t h e preparation of Ce/Al-pillared smectites, he claims that, in addition to Ce(III), other r a r e e a r t h (RE) cations in admixture with ACH can be used for t h e preparation of large-pore pillared products. S t e r t e (ref. 2) investigated t h e effects of ti m e and temperature of hydrothermal t re a tm en t , Al-concentration, OH/Al-ratio of th e ACH, and La/AI molar ratio of t h e solution on t h e formation of large pore La/Al-pillared montmorillonite. The present paper reports on t h e preparation and characterization of pillared montmorillonites prepared by cation exchange of t h e montmorillonite with hydrothermally t re a ted solutions containing ACH in admixture with chlorides of La, Ce, Pr, Nd, or a commercial mixture of RE-cations. EXPERIMENTAL Montmorillonite A Wyoming Na+-Ca2+-montmorillonite (commercial designation, Voiclay SPV 200) was obtained from t h e American Colloid Company. Impurity quartz was removed by fractionation using conventional sedimentation techniques. The <2,um fraction, which was essentially f r e e from impurities as determined by X-ray powder diffraction (XRD) analysis, was used as starting material in all preparations. The cation exchange capacity
of t h e montrnorillonite was determined to be 89 meq/lOOg; an elemental analysis of t h e clay is given in Table 2. Pillaring agents The pillaring solutions were prepared by hydrothermal t r eat m en t of solutions containing aluminum chlorohydrate (Locron L, Hoechst, 23.4 wt% A1203, OH/A1=2.5) and rare-earth (RE) chlorides. The RE-chlorides used were: La-chloride (29.5 wt% La2O3), Cechloride (26.1 wt% CeOz), Pr-chloride (27.6 wt% Pr6O1 I), Nd-chloride (29.4 wt% Nd2O3), and a mixed RE-chloride (24.3 wt% RE-oxides; 53.4% La2O3, 20.8% CeO2, 13.9% Pr6011, and 1 1.9%Nd2O3) solution. All RE-solutions w e r e provided by Rhone-Polenc. Solutions, 2.5 M with respect to A1 and with a RE/Al molar ratio of 1:5 were used in all preparations. The hydrothermal tr e a t m e n ts were carried o u t either by ref luxing t h e solutions for 24240 h or by t r eat m en t in PTFE-coated stainless steel autoclaves at temparatures in t h e range 1200-16O0C for 12-96 h. Preparation of pillared products One gram of montmorillonite was dispersed in 200 ml of distilled water (25OC) by prolonged stirring ( 5 h) with a magnetic stirrer. The amount of pillaring solution required t o obtain an Al/montmorillonite ratio of 20 mmol Al/g montmorillonite was then added t o
303 t h e vigorously stirred dispersion. The resulting product was left in c o n t a c t with t h e solution for 1 h and then separated by centrifugation. The product was then washed by redispersing it in distilled water, and separated by centrifugation. This procedure was repeated until t h e supernatant was f r e e from chloride ions as determined by AgN03. Characterization of pillared products N2-adsorption-desorption isotherms w e r e determined using a Digisorb 2600 surfacea r e a , pore-volume analyzer (Micromeritics Instrument Corporation). The samples w e r e first outgassed at 200OC for 3 h, and t h e isotherms w e r e recorded at liquid nitrogen temperature. Surface a r e a s w e r e calculated using t h e BET equation. X-ray diffraction analyses were performed either on non-oriented or oriented mounts. The XRD patterns were obtained on a Philips powder diffractometer using Ni-filtered, fine-focus CuKr-radiation. Thermal stability was investigated by exposing s e p a r a t e samples t o temperatures in t h e range 20O0-80OOC for 3 h in air. Elemental analysis of t h e pillared samples was carried o u t by a t o m i c absorption spectroscopy (AAS) employing LiB02-fusion (ref. 3). RESULTS AND DISCUSSION Reflux experiments A series of samples was prepared from t h e different RE/Al-solutions and f r o m t h e reference ACH-solution, refluxed for 0-240 h. Fig. 1 shows t h e X-ray diffraction p a t t e r n s of samples prepared from La- and ACH-solutions refluxed f o r 48 and 120 h and from Ce-, Pr-, Nd-, and RE-solutions refluxed for 120 and 240 h. The reference sample prepared from t h e RE-free ACH-solution, refluxed f o r 48 h, shows a basal reflexion corresponding
to a basal spacing of 19.4
A. This value is within t h e range, 18-20 A, usually observed for
conventional Al-pillared smectites. Further refluxing of this solution up t o 120 h results in a n increase in crystallinity of t h e pillared product but does not significantly a f f e c t its basal spacing. The sample prepared from t h e La/Al-solution refluxed f o r 48 h shows a broad 001-reflexion corresponding t o a basal spacing of about 19.2
A. In t h e p a t t e r n
recorded for t h e sample prepared from t h e s a m e solution refluxed for 120 h, t h e major basal spacing has shifted to about 26
A. This basal spacing is similar t o those observed by
McCauley (ref. 1) for large-pore Ce/Al-pillared montmorillonite and f luorhectorite, i.e. 27.4 and 25.6
A, respectively. The 26 A peak s t a r t s to develop for samples prepared from
this solution refluxed for 72 h, grows sharper and more intensive with increasing t i m e of reflux up to about 96 h and then remains unaffected with increasing t i m e of reflux within t h e time range investigated. The Ce/Al-solution resulted in large-pore pillared products similar t o those prepared from t h e La/Al-solution but required longer t i m e s of reflux, i.e. about 240 h, in order to produce such products. Although a broadening of t h e 001-peaks is seen for t h e Pr/AI- and Nd/Al-samples prepared from solutions refluxed for 240 h, no
304
Pr/AI
----Mu-
8
5
28
5
28
5
28 5 Degrees 2 0
28
5
28
5
2
Fig. 1. X-ray diffraction p a t t e r n s of RE/Al-pillared montmorillonites prepared from refluxed solutions. major changes in t h e basal spacings of pillared products prepared from refluxed Pr/AI- or Nd/Al-solutions w e r e observed. The RE/Al-solution, containing a mixture of RE-chlorides, showed a behavior simliar to t h a t of t h e Ce/Al-solution, i.e. about 240 h of reflux was required in order to obtain a large-pore pillared product. Autoclave experiments A series of samples was prepared from t h e different RE/Al-solutions and from t h e reference ACH-solution, t r e a t e d in autoclaves at t e m p e r a t u r e s in t h e range 120-16OoC for 12-96 h. Fig. 2 shows t h e X-ray diffraction p a t t e r n s of samples prepared from La/Al-, Ce/Al-, and RE/Al-solutions t r e a t e d at 120OC for 12, 24, 48, and 96 h. For t h e samples prepared from t h e La/Al-solutions a 26
a spacing s t a r t s to develop a f t e r 24 h of treatment. Further
t r e a t m e n t of this solution, up t o 96 h, results in products with sharper and more intensive basal reflexions, indicating increased crystallinity of t h e products with increasing t i m e of hydrothermal treatment. For t h e Ce/AI- and t h e RE/Al-solutions longer t i m e s of t r e a t m e n t w e r e required in order t o obtain large-pore pillared products, which is consistent with t h e results obtained in t h e reflux experiments discussed above. All these solutions did, however, yield very crystalline products with basal spacings of 26 A a f t e r t r e a t m e n t at 120OC for 96 h. In Fig. 3, t h e X-ray diffraction p a t t e r n s of samples prepared from La/AI-, Ce/AI-, and
305
La/AI
Ce/AI
RE /A1
Degrees 28
Fig. 2. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared rnontrnorillonites prepared from solutions autoclaved a t 12OoC for 12-96 h.
La /A1
Ce/AI
REIAI
Degrees 2 8
Fig. 3. X-ray diffraction patterns of La/AI-, Ce/AI-, and RE/Al-pillared montmorillonites prepared from solutions autoclaved for 12 h at 120, 140, and 16Ooc.
306 TABLE 1 BET s u r f a c e a r e a s (m2/g) of RE/Al-pillared montmorillonites prepared from solutions hydrothermally t r e a t e d at 120-16OOC for 12-96 h.
temp.a (OC)
I
La/AI
Ce/AI
12
24
48
96
120
422
436
463
140
428
410
160
487
428
I
RE/AI
t i m e of t r e a t m e n t (h)a 12
24
48
96
493
436
443
461
389
374
429
428
412
408
507
487
I
12
24
48
96
537
430
394
434
538
407
518
422
409
393
499
386
390
468
458
442
396
~
RE/Al-solutions t r e a t e d at 120, 140 and 160OC for 12 h a r e shown. After t r e a t m e n t at 12OoC, all solutions result in products with basal spacings of about 19 8. T r e a t m e n t at 14OOC results in a large-pore pillared product from t h e La/Al-solution b u t not so for t h e Ce/Al-and RE/Al-solutions. After t r e a t m e n t at 160OC for 12 h, all t h e solutions yielded highly crystalline large-pore pillared products. These results show t h a t t h e r a t e of formation of t h e large RE/Al-polycations, believed to b e responsible for t h e 26
8 spacing
of large-pore pillared products, increases with increasing t e m p e r a t u r e of hydrothermal treatment. Hydrothermal t r e a t m e n t of t h e solution for t i m e s longer than t h a t required for t h e formation of these species does, however, result in a decline in crystallinity of t h e resulting products. Thus, solutions of La/AI, Ce/Al as well as RE/AI t r e a t e d at 16OOC for 96 h result in considerably less crystalline products in comparison with those obtained from t h e s a m e solutions t r e a t e d at this t e m p e r a t u r e for 12 h. Pr/AI- and Nd/Al-solutions t r e a t e d at 12O-16O0C for 12-96 h did not yield large-pore pillared products. All samples prepared from these solutions showed basal spacings in t h e range 19-20 8 which is within t h e range characteristic for conventional Al-pillared smectites. Autoclave t r e a t m e n t of RE-free ACH-solutions at 12O-16O0C for 12-96 h resulted in t h e formation of colloidal dispersions or gels of boehmite or pseudoboehmite. Table 1 shows t h e BET surface a r e a s of samples prepared from La/AI-, Ce/AI-, and RE/Al-solutions t r e a t e d 12O-16O0C for 12-96 h. Samples prepared from solutions t r e a t e d at 12OoC show a n increase in surface a r e a with increasing t i m e of t r e a t m e n t for all t h r e e solutions. Increasing t i m e of hydrothermal t r e a t m e n t at 14OoC appears t o result in somewhat increasing surface a r e a s for samples prepared from Ce/AI- and RE/Al-solutions and in decreasing surface a r e a s for those prepared from La/Al-solutions. For samples prepared from solutions t r e a t e d a t 16OoC t h e surface a r e a s decrease with increasing t i m e of hydrothermal t r e a t m e n t of t h e solution for all t h r e e solutions. The results of t h e surface a r e a measurements a r e in good agreement with those obtained by X-ray
307
diffraction analysis. The large-pore pillared products, showing intense and sharp basal reflexions corresponding t o a basal spacing of 26
A, generally have surface areas in t h e
vicinity of 500 m*/g.
Fig. 3 shows t h e N2-adsorption-desorption isotherms recorded for t h e starting montmorillonite, an Al-pillared montmorillonite and a large-pore Ce/Al-pillared montmorillonite prepared from a Ce/Al-solution hydrothermally treated at 12OoC for 96 h. The isotherm recorded for t h e montmorillonite is of type I1 in the classification of Brunauer, Deming, and Teller, characteristic of non-porous soiids. The isotherms recorded for t h e pillared products can both be described as composite isotherms of t h e type I1 isotherm of t h e montmorillonite and type I isotherms due t o adsorption in pores introduced by the pillaring procedure. The strong adsorption at low relative pressures, characteristic for microporous materials, is, however, less pronounced for t h e Ce/AIpillared sample. This may be taken as a further indication of larger pores in this material compared with t h e Al-pillared montmorillonite. A further discussion of t h e adsorption properties of large-pore RE/Al-pillared montmorillonite in relation to t h e nature of the pores of this material is given in (ref. 2). Elemental analysis Table 2 shows elemental analyses of t h e starting montmorillonite, of a Al-pillared montmorillonite, and of samples prepared from t h e La/AI-, Ce/AI-, Pr/AI-, Nd/Ai-, and
160
0 LO
0
05
Welafive pressure, PIP,
1.0
Fig. 4. Nitrogen adsorption-dessrpbion isotherms for staring monSrnorillonite (a), Alpillared montrnorillonite (b) and large-pore Ce/Al-pillared morrtmorillonite (c).
308 TABLE 2 Elemental analysis of Na+-Ca2+-montmorillonite, Al-pillared and RE/Al-pillared montmorillonites. metal oxide
montmo-
Al-
La/AI-
Ce/AI-
Pr/AI-
Nd/AI-
RE/AI-
(wt %)
rillonitea
m0nt.b
m0nt.c
m0nt.c
m0nt.c
m0nt.c
m0nt.c
56.7 19.6 3.84 3.02 0.8 0.39 1.94 13.1
45.6 24.4 3.27 1.99 0.2 0.40 0.24 22.4
44.3 25.4 3.20 2.24 0. I 0.30 0.25 22.2
44.6 24.6 2.89 2.00 0.0 0.36 0.40 26.4
46.2 24.4 2.89 2.12 0.0 0.36 0.40 23.4
41.5 24.1 2.59 2.11 0.1 0.18 0.13 29.6
43.0 25.7 2.69 2.47 0.0 0.19 0.18 27.1
99.4
98.5
98.0
99.8
00.3
01.3
101.2
aNa+-Ca2+-montmorillonite, Volclay SPV 200, used as starting m a t e r i a l 1 all preparations. bAi-pillared montmorillonite prepared using a RE-free ACH-solution. CAll RE/Al-pillared montmorillonites w e r e prepared from solutions t r e a t e d at 12OoC for 96 h. RE/Al-solutions t r e a t e d at 12OoC for 96 h. The analyses of t h e samples prepared from hydrothermally t r e a t e d solution differ very l i t t l e from e a c h other and a r e also similar to t h a t of t h e Al-pillared montmorillonite. This indicates a similar uptake of A1 by t h e montmorillonite from t h e different solutions and also a similar charge per A1 in t h e Alspecies taken up from t h e s e solutions. These results a r e somewhat surprising considering t h e g r e a t structural differences between t h e samples observed by X-ray diffraction and by
N2-adsorption-desorption analysis. Thermal stability Fig. 5 shows t h e X-ray powder diffraction p a t t e r n s of an Al-pillared montmorillonite and a Ce/Al-pillared sample prepared from a solution t r e a t e d at 12OoC for 96 h, both thermally t r e a t e d at 200 and 8OOOC for 3 h. The s u r f a c e a r e a s of t h e different samples a r e also given in this Figure. For t h e Al-pillared sample, t h e basal spacing decreases from about 18 8 a f t e r t r e a t m e n t at 2OOOC to about 16 8 a f t e r t r e a t m e n t at 800°C. The decrease in basal spacing is accompanied by a substantial decrease in surface area. For t h e Ce/Al-pillared montmorillonite, t h e basal spacing is approximately t h e same, about 25
8, for samples t r e a t e d at 200 and
8OOOC. After t r e a t m e n t at 800°, t h e Ce/Al-pillared
sample retains a considerable fraction of i t s original surface area, indicating a good thermal stability. I t is noticeable t h a t t h e surface a r e a of t h e Ce/Al-pillared montmorillonite t r e a t e d at 800°C is of t h e s a m e order as t h a t of t h e Al-pillared one t r e a t e d at 200oC. La/AI- and RE/Al-pillared samples responded t o t h e r m a l t r e a t m e n t in a manner similar to t h a t of Ce/Al-pillared ones while only minor improvents in t h e r m a l
309
Al.
Ce/A I
u
8
5
28 5 Degrees 2 8
2
Fig. 5. X-ray powder diffraction p a t t e r n s and surface a r e a s of thermally t r e a t e d Alpillared and large-pore Ce/Al-pillared montmorillonites. stability were observed f o r Pr/AI- and Nd/Al-pillared montmorillonites. Structure of large-pore RE/Al-pillared montmorillonite It is generally accepted t h a t t h e oligomer responible f o r t h e layer separation of conventional Al-pillared s m e c t i t e s is A11304(OH)247+ (ref. 4). The s t r u c t u r e of this cation is a c e n t r a l four-coordinated aluminum a t o m surrounded by twelve A106-octahedra joined by common edges to form a Keggin-type structure, McCauley (ref. I) found t h a t large-pore Ce/Al-pillared montmorillonites c a n be prepared from hydrothermally t r e a t e d solutions with Ce/Al-ratios down t o 1:52. This, in combination with t h e fact t h a t t h e interlayer spacing of t h e s e materials is about twice t h a t of conventional Al-pillared smectites, led him t o suggest t h a t t h e polymeric cation responsible for t h e pillaring in large-pore pillared s m e c t i t e s is built up of four A113-units, linked together by a tetrahedrally bound cerium atom. McCauley has, however, not investigated t h e s t r u c t u r e of t h e pillaring species further in order t o substantiate this suggestion and no such polymeric ion has so f a r been reported in t h e literature. Investigations of hydrothermally t r e a t e d mixtures of aluminum chlorohydrate and r a r e e a r t h salts a r e required in order t o establish t h e s t r u c t u r e of this polycation. CONCLUSION Large-pore pillared montmorillonites c a n b e prepared from hydrothermally t r e a t e d mixtures of ACH and lanthanum, cerium or RE-chlorides. The hydrothermal t r e a t m e n t can b e carried o u t e i t h e r at reflux conditions f o r several days or at higher t e m p e r a t u r e s
310
and f o r shorter durations in a n autoclave. More crystalline materials with higher surface a r e a s c a n b e prepared from autoclaved solutions compared with refluxed ones. Praseodymium and neodymium chlorides in admixture with ACH did not produce largepore pillared products a f t e r hydrothermal t r e a t m e n t at t h e conditions used in this study. The large-pore pillared montmorillonites a r e characterized by s u r f a c e a r e a s in t h e range 300-550 m2/g, basal spacings of about 2 6 A, and a good t h e r m a l stability (surface area
about 300 m2/g even a f t e r exposure t o 8OO0C f o r 3 h). Large-pore RE/Al-pillared s m e c t i t e s is a novel type of zeolite-like materials which may be of i n t e r e s t as a c t i v e components in c a t a l y s t s f o r cracking of heavy oil fractions. C a t a l y t i c cracking performance of RE/Al-montmorillonites, alone and in admixture with zeolite Y, is currently under investigation. ACKNOWLEDGMENTS The author thanks t h e Swedish Board f o r Technical Development (STU) for financial support of this project. Helpful advice from Prof. J-E. O t t e r s t e d t and experimental help from Mr. Ying Zhong-Shu a r e greatly appreciated. REFERENCES 1 J.R. McCauley, Stable intercalated clays and preparation method, Int. Pat. Appl. PCT/US88/00567, March 4, 1988. 2 J. S t e r t e , Preparation and properties of large-pore La/Al-pillared montmorillonite, submitted f o r publication. 3 J.H. Medlin, N.H. Suhr, and J.B. Bodkin, Atomic absorption analysis of silicates employing LiB02 fusion, At. Absorpt. Newsl., 8 (1969) 25-29. 4 D. Plee, F. Borg, L. Gatineau, and J.J. Fripiat, High-resolution solid-state 27Al and 29Si nuclear magnetic resonance study of pillared clays, J. Amer. Chem. Soc., 107 (1985) 2362.
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
311
PREPARATION OF PILLARED MONTMORILLONITE WITH ENRICHED PILLARS
E.
KIKUCHI,
H.
SEKI, and T. MATSUDA
Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169, Japan
SUmARY Montmorillonite intercalated with Al-polyoxycations, which will be called pillared montmorillonite, had extremely small cation exchange capacity(CEC) compared with t h e original montmorillonite. T h e CEC o f pillared montmorillonite changed a little by heat t r e a t m e n t in a stream o f nitrogen. When treated in a stream of ammonia, however, it pronouncedly increased. Thus obtained pi1 lared montmorillonite was re-intercalated with Al-polyoxycations to give pillared montmorillonite having larger number of pillars. T h e c a t a l y t i c activity o f pillared m o n t m o r i l l o n i t e for decreased with disproportionation of 1.2,4-trimethylbenzene(l,2,4-TrMB) increasing number o f pillars, which favored the formation o f 1,2,4,5tetramethylbenzene(l,2,4,5-TeMB) with the smallest molecular dimension among TeMB isomers. These results can be interpreted by increased shape selective property by enrichment o f pillars.
INTRODUCTION Intercalation of smectite clays with polyoxycations provides a new class of porous materials. Intercalated clays are called pillared clays and they have high thermal stability and large surface area. Vaughan and Lussier(ref. 1) were the first to point out the shape selective sorption property of pillared montmorillonite using various probe hydrocarbons. The shape selective catalysis in cracking of alkylbenzenes was demonstrated by Shabtai and coworkers(ref. 2). The pore opening of pillared clays, which plays an important role in shape selective catalysis, is determined both by the interlayer distance and by the density of pillar or the number o f pillars. The interlayer distance depends on the dimension of intercalating species. Considerable efforts have been undertaken to develop pillared c l a y s with d i f f e r e n t interlayer distances. Indeed, pillared clays having the interlayer distance from 0.4 to 2.0 nm were prepared using various intercalating species(refs. 3-7). In contrast, there have been few studies concerning the control of pillar density or the number of pillars. We showed in the previous works(refs. 8-10) that pillared montmorillonite was active and selective for disproportionation of 1,2.4-TrMB to 1,2,4.5-TeMB.
312
and the selectivity for formation of 1,2,4,5-TeMB was affected by the interlayer distance of pillared montmorillonite. The purpose of this work is to describe the preparation of pillared montmorillonite having a variety number of pillars and to discuss the shape selective catalysis by pillared montmorillonite.
Disproportionation of 1,2,4-TrMB was studied as a model
reaction to characterize shape selective property. EXPERIMENTAL
Preparation o f pillared montmorillonite Na-type montmorillonite with the cation exchange capacity of 1,19 meq./g was used in this study. A [Al1304(OH)24(H20)12]7t cation was prepared by addition of NaOH solution to AlC13 solution to yield OH/A1 molar ratio of 2.5. The intercalation method of Na-montmorillonite with The [A11~04(OH)24(H20)12]7+ was previously described in detail(ref. 11). intercalated montmorillonite was treated at a desired temperature in the range of 373-673 K in a stream of nitrogen or ammonia. To remove ammonia adsorbed on the acid sites, the ammonia-treated sample was treated in a stream of nitrogen at 673 K for 1 h. Characterization of pillared montmorillonite The cation exchange capacity(CEC) of pillared montmorillonite was determined a s follows. S o d i u m c a t i o n s w e r e introduced t o pillared montmorillonite by exhaustive exchange with a 1 N solution of NaCl at 323 K. The sodium exchanged pillared montmorillonite was added to a 0.1 N solution of NH4C1 and stirred for 3 h at 323 K. After filtration and washing, the amount of sodium ions replaced by ammonium ions was measured using atomic absorption spectrometer. The number o f acid sites on pillared montmorillonite was determined by means of ammonia temperature-programmed desorption(TPD). In each TPD experiment, a sample of 0.5 g was treated in vacuo at 673 K for 1 h and then cooled to 373 K. Ammonia was adsorbed at 373 K for 30 min and evacuated for 30 min. This sample was heated from 373 to 973 K at a rate of 10 K/min and the desorbed ammonia was monitored by thermal conductivity detector. As water was simultaneously desorbed with ammonia above 673 K. the ammonia TPD spectrum was obtained by point-by point subtraction o f the water desorption spectrum obtained with the sample which had not adsorbed ammonia. Apparatus and procedures Catalytic studies were performed in a continuous flow reactor with a fixed bed of catalyst. The catalyst was packed in the reactor and was treated at
313
673 K for 1 h in a nitrogen atmosphere prior to reaction. Disproportionation of 1,2,4TrMB was carried out at 473 K and atmospheric pressure. 112,4-TrMB was di 1 uted with nitrogen in a molar ratio of 1:9. Liquid
0:5
- 0.4
I
4
products were
collected in an ice trap every 10 min and were analyzed by means of gas chromatography using a flame ionization d e t e c t o r and a F F A P g l a s s capillary separation column with temperature-programmed heating from 333 to 443 K. RESULTS
0.6
AND DISCUSSION
g 0.3 E
v
W 0
0.2
0.1
0
573
673
Temperature( KI Fig. 1. E f f e c t o f heat treatment on the CEC of p i l l a r e d m o n t m o r i l l o n i t e : 0 , i n nitrogen; A , i n ammonia.
Ammonia treatment T h e density o f pillars can be controlled by t h e number o f cation exchangeable sites on montmori 1 loni te. It has been considered that heat treatment converts Al-polyoxycation of [All304(OH)24(H20)1 217+ cation to A1203, which is represented as follows:
[Al1304(OH)24(H20)12]7+
6.5 A1203
t 7
H+ + 20.5 H20
If it is the case, pillared montmorillonite after heat treatment should have almost the same size of C E C with the original montmorillonite. Hence, pillared montmorillonite with larger number of pillars will be prepared by further introduction o f Al-polyoxycations to pillared montmorillonite. Figure 1 shows the CEC o f pillared montmorillonite treated in the range from 373 to 673 K in a nitrogen atmosphere. Pillared montmorillonite treated at 3 7 3 K exhibited extremely small C E C , indicating that t h e cation exchangeable s i t e s on m o n t m o r i l l o n i t e w e r e completely occupied by [A11~04(OH)24(H20)12]7+ cations. The CEC of pillared montmorillonite had tendency to increase with the temperature of treatment, although it was smaller even after treatment at 673 K than the original montmorillonite. Pillared montmorillonite exhibited large CEC after treatment in a stream o f ammonia. When treated in ammonia at 373 K. pillared montmorillonite had small
314
F
ABLE 1 a t a l y t i c a c t i v i t i e s o f p i l l a r e d montmorillonites
PM( NH3)
Catalyst
PM
CEC(meq.9-l)
0.15
Na/PM( NH3)
Na/PM
0.61
0.15
0.61
% Conversion
32.7
36.3
17.5
0.2
Y i e l d (mol%) Xyl ene TrMB TeMB PMB+HMB
14.6 3.7 14.1 0.3
15.8 4.4 16.0 0.3
8.4 0.8 8.3 0.1
0.1
0
0.1
0
PM, P i l l a r e d m o n t m o r i l l o n i t e t r e a t e d i n n i t r o g e n : PM(NH3), P i l l a r e d P i 1 l a r e d m o n t m o r i l l o n i t e t r e a t e d i n ammonia: Na/PM. Na-exchanged p i l l a r e d montmorillonite. Reaction c o n d i t i o n s : temp., 473 K: W/F, 8000 g-cat.min/mol.
CEC compared w i t h t h e o r i g i n a l m o n t m o r i l l o n i t e . with
increasing
temperature
0.61 meq./g a t 573 K.
of
ammonia
However,
treatment,
t h e CEC increased
and
it reached
to
P i l l a r e d m o n t m o r i l l o n i t e had a s u r f a c e area o f 400 m2/g
and gave d(001) r e f l e c t i o n a t d=1.85 nm even a f t e r ammonia t r e a t m e n t a t 573 K, indicating
that
maintained.
the
layer
structure
These r e s u l t s
[A11304(OH)24(H20)12]7+
of
pillared
m o n t m o r i l l o n i t e was
indicate t h a t decomposition o f
c a t i o n s i s a c c e l e r a t e d i n t h e presence o f ammonia.
Table 1 summarizes t h e a c t i v i t y o f p i l l a r e d m o n t m o r i l l o n i t e c a t a l y s t s f o r c o n v e r s i o n o f 112.4-TrMB.
The c a t a l y t i c a c t i v i t i e s were compared u s i n g d a t a
t a k e n i n t h e i n i t i a l 10 min o f run.
Ammonia t r e a t m e n t d i d n o t a f f e c t t h e
a c t i v i t y o f p i l l a r e d m o n t m o r i l l o n i t e c a t a l y s t a t a l l i f adsorbed ammonia was subsequently changed. results
removed i n n i t r o g e n t r e a t m e n t a t 673 K,
a l t h o u g h t h e CEC was
The observed c a t a l y t i c a c t i v i t i e s were i n f a i r agreement w i t h t h e obtained
from
ammonia-TPD
experiments.
When
sodium
cations
were
i n t r o d u c e d t o t h e c a t i o n exchangeable s i t e s , p i l l a r e d m o n t m o r i l l o n i t e t r e a t e d i n ammonia a t 573 K became i n a c t i v e , s i t e s a c t e d as a c t i v e s i t e s .
i n d i c a t i n g t h a t t h e c a t i o n exchangeable
I n contrast, p i l l a r e d montmorillonite treated i n
n i t r o g e n was a c t i v e even a f t e r i n t r o d u c t i o n o f sodium c a t i o n s , c a t a l y t i c a c t i v i t y was reduced.
Thus,
although the
the type o r nature o f a c i d s i t e s i s
c o n s i d e r e d t o v a r y w i t h t h e atmosphere o f treatment. Re-intercalation Re-intercalation
o f [A11,04(OH)24(H20)12]7+
cations t o p i 1lared
m o n t m o r i l l o n i t e t r e a t e d i n ammonia was i n v e s t i g a t e d t o p r e p a r e p i l l a r e d m o n t m o r i l l o n i t e h a v i n g l a r g e number o f p i l l a r s .
F i g u r e 2 shows t h e number o f
p i l l a r s i n t r o d u c e d as a f u n c t i o n of t h e p e r i o d o f i o n e x c h a n g e a t 323 K.
[Al1304(OH)24(H20)l,J7+
cations
were h a r d l y i n t r o d u c e d
to
pillared
315
0
3
6
0
3
Ion exchange time(h)
6
9
Fig. 2. Variation in the number of pillars as a function of the period of ion exchange at 323 K: 0 , Na-montmorillonite; A , pillared montmorillonite in nitrogen; pillared montmorillonite treated in ammonia.
n,
rnontmorillonite treated at 673 K in nitrogen, due to the extremely small CEC. I n the c a s e o f pillared montmorillonite treated in ammonia at 573 K, however, re-intercalation gave pillared montmorillonite with additional number of pillars. The interlayer distance, which was
f 1.3
c
determined from d(001) reflection, did not change at all by the further introduction of [ A1 1304(OH)24( HZO)~~]~' cations. 0 0.2 0.4 0.6 As shown in Fig. 3, the number of Cation exchange capaci ty (meq,9- 1) pillars increased with the C E C of pillared montmorillonite used for Fig. 3. Relation between the CEC o f pillared montmorillonite and the total re-intercalation, indicating that number o f pillars. the polyoxycations had access to the cation exchangeable sites on pillared montmorillonite. However, the number of pillars obtained was small compared with the value expected from the charge of intercalating species and the CEC of pillared montmorillonite. If all of the cation exchangeable sites are occupied by [Al,304(OH)24(H20)12]7+ cation, the
316
Number o f p i 1 lar x lO-*O(g-l) Fig. 4. Catalytic activity and acidity of pillared montmorillonite as a function o f the number of pillars. nrimber of pillars will increase from 1.1 x
lo-'
to 1.7 x 10-20/g
intercalation of pillared montmorillonite having CEC of 0.61 meq./g.
by reThus,
diffusion of [Al,304(OH)24(H20)12]7i into the interlayer space of pillared montmori 1 loni te seems to be 1 imi ted. Figure 4 shows the catalytic activities of pillared montmorillonites as a function o f the number o f pillars. The activity of pillared montmorillonite catalyst decreased with an increase in the number of pillars. As shown in this figure, the acidity o f pillared montmorillonite increased with the number of pillars. Hence, the decrease in catalytic activity with increased number o f pillars is not caused by reduced acidity. It has been reported(refs. 12-14) that pillars as well as clay sheets are responsible f o r acidity of pillared clays. The results obtained in the present study also indicate that acid sites exist on the pillars. The pore opening and the pore volume are reduced by introduction of excess number of pillars. These results lead us to conclude that the observed change in the catalytic activity is related to the shape selective property of pillared montmorillonite: diffusion o f 1,2,4-TrMB or formation of the intermediate for disproportionation is restricted by the limited interlayer space. Disproportionation o f 1,2,4-TrMB yields all the isomers of xylene and TeMB. 112,3,5-TeMB is thermodynamically most stable among TeMB
317
Number of pillar x 10-20(g-1) Fig. 5. The selectivity of 1,2,4,5-TeMB as a function o f the number of pillars on pillared montmorillonite. isomers.
However, 1,2,4,5-TeMB has the smallest molecular dimension among
TeMB isomers. It has been reported(ref. 8) that the cross-sectional dimension of the transition state leading to 1,2,4,5-TeMB is small compared with those which lead to 1,2,3,5- and 1,2,3,4-TeMB. The 1.2.4,5-TeMB selectivity expressed by the fraction of 1,2,4,5-TeMB in TeMB isomers is thus dependent on the shape selective property o f a catalyst. The 1,2,4.5-TeMB selectivity changed with the level of 1,2.4-TrMB conversion, due to isomerization o f 1.2.4.5-TeMB to thermodynamically more stable 1,2,3.5-TeMB. Hence, the 1.2,4,5-TeMB selectivity was compared using the data taken at a common conversion level of 10%. Figure 5 shows the selectivity o f pillared montmorillonite for formation of 1,2,4,5-TeMB as a function o f the number of pillars. The 1.2,4,5-TeMB selectivity increased with increasing number of pillars. Therefore, the high selectivity of pillared montmorillonite with enriched pillars for the formation of 1.2,4,5TeMB is a result of space restriction by pillars. It is apparent from these results that the number of pillars can govern the shape selective catalysis on pillared montmorillonite.
CONCLUSION Ammonia treatment promotes decomposition o f
[ A ~ I ~ O ~ ( O H ) ~ ~ ( H ~ O )to~ ~ ] ~ +
A1203 and consequently gives pillared montmorillonite with the large number of cation exchangeable sites. Re-intercalation of thus treated pillared montmorillonite gives pillared montmorillonite with an additional number of pillars. The number of pillar increases with increasing CEC o f pillared
318 It i s
montmorillonite. [Al1304(OH),4(H20)12]7+ pillared with
these
results
t h a t
although
diffusional
limitation
appears
in
re-
The c a t a l y t i c a c t i v i t y o f p i l l a r e d m o n t m o r i l l o n i t e decreases
i n c r e a s i n g number o f p i l l a r s ,
1,2.4,5-TeMB
from
c a t i o n has access t o t h e c a t i o n exchangeable s i t e s on
montmorillonite,
intercalation.
apparent
i s enhanced.
selective catalysis
while the s e l e c t i v i t y f o r
formation
of
These r e s u l t s l e a d us t o conclude t h a t t h e shape
by p i l l a r e d m o n t m o r i l l o n i t e can
be c o n t r o l l e d
by
the
number o f p i 11ars.
REFERENCES 1 D.E.W. Vaughan and R.J. L u s s i e r , i n "Proceedings, 5 t h I n t . Conf. on Z e o l i t e , Naples, 1980". Heyden, London, 1981, p.94. 2 J. Shabtai, R. Lazar, and R.G. L u s s i e r , i n "Proceedings, 7 t h I n t . Congr. on C a t a l . I Tokyo, 1980", E l s e v i e r . Amsterdam, 1981, p.828. 3 G.W. B r i n d l e y and R.E. Sempels, Clays C l a y Miner., 12 (1977) 229. 4 S. Yamanaka and G.W. B r i n d l e y , Clays Clay Miner., 27 (1979) 119. 5 R. Burch and C.I. Warburton, J. Catal., 97 (1986) 503. 6 T.J. Pinnavaia, M. Tzou, and S.D. Landau, J. Am. Chem, SOC., 107 (1985) 4783. 7 J. S t e r t e , Clays C l a y Miner., 34 (1986) 658. 8 E. K i k u c h i , T. Matsuda, H. F u j i k i , and Y. M o r i t a . Appl. Catal., 11 (1984) 331, 9 E. K i k u c h i . T. Matsuda, J . Ueda. and Y. M o r i t a . Appl. Catal., 16 (1985) 401. 10 T. Matsuda, M. Asanuma, and E. K i k u c h i , Appl. Catal., 38 (1988) 289. 11 T. Matsuda, H. Nagashima, and E. K i k u c h i , Appl. Catal., 45 (1988) 171. 12 A.Schutz, D. Plee, F. Borg, P. Jacobs, G. Poncelet. and J.J. F r i p i a t , i n i n "Proceedings, I n t . Clay Conf., Denver, 1985". 1987, p.305. 13 M.L. O c c e l l i and R.M. Tindwa, Clays C l a y Miner., 31 (1983) 22. 14 T. M o r i and K. Suzuki, Chem. L e t t . , (1989) 2165.
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
INTERCALATION OF La203 AND La2Og-NiO MONTMORILLONITE LAYERED STRUCTURE
A.K.LADAVOS AND P.J.POMONIS Chemistry Department, University (Greece).
of
OXIDIC
Ioannina,
SPECIES
Ioannina
319
INTO
45110
SUMMARY Oxidic species La203 and La203+NiO have been intercalated into montmorillonite layered structure by carefully controlling the pH of the solution used and freeze-drying the obtained slurry. The materials thus obtained were examined by XRD at low angles and showed reflections at 28=4O and 28=8O corresponding to inter ayer distances of 25.68 and 12.88 respectively as compared to 9 . d of the original Na-montmorillonite. The thermal stability of the intercalated solids extends up to 300-350°C, temperature at which the pillared layers shrink to the d-space of the Na-montmorillonite. The specific surface area of the obtained materials dried at 2OO0C is 35m2.g-1 for the La-intercalated species and 19m2.g-l for the La-Ni-intercalated ones, as compared to 17m2.g-l for the Na-montmorillonite. The introduction of La and La-Ni into the layered smectite increase the acidity of the solid in a very impressive way as found by employing as a probe reaction the dehydration of iso-propanol. The surface titration of the solids with NH3 showed that the concentration of the acid sites is (1.3-6.1) sites/m2, depending on the intercalant and the temperature. INTRODUCTION It has been shown some time ago (refs.l.2) that layered aluminosilicates such as montmorillonite can be intercalated by means of different oxidic pillars (refs.3.4) Oxidic species which have been introduced into smectite layers include Fe203(refs.2,5) AI203(refs.2,6-8), Ti02(ref.3), ZrO2(refs.9,10), CuO(refs.l1,12) NiO(ref.4). Sn(ref.13) and a few other cations. Such materials bear some promises as industrial catalysts in petrochemical industry if they stabilize above 500°-600°C, temperature at which they usually collapse. Among the oxidic species for which, as far as we know, there are no reports for intercalation in the literature is the La2O3. This cation exchanged into zeolites is capable of water hydrolysis, the formed proton remaining quasi-free into the cage above to 5OK (ref.14). Therefore, its introduction into montmorillonite layers might result in similar interesting features
320
Apart from the introduction of La203 in pure form we also became interested in intercalated mixtures of La203 and NiO with the possible results to obtain small perovskite particles LaNiOg and/or La2Ni04 into the layers of smectite. In attempting the procedure of intercalation different authors usually follow one of two possible routes: The first one is by introducing the metallic cations into a complex, which is often polynuclear (ref.l5), and which is then intercalated into the smectite layers. It then leaves behind the oxidic pillars after thermal decomposition of its ligand. The second route is by attempting intercalation of the hydroxy-species of the metal, which results from a gradual addition of NaOH into the suspension of montmorillonite and the metal salt, often a chloride. Nevertheless, this last method is usually applied in a rather empirical way by means of a trial and error procedure (ref.4). In an attempt to rationalize this method we start from the principle that the hydroxy-species formed initially during alkalization of the solution and precipitated at large addition of alkali, appear in a gradually increasing size according to the scheme provided by the theory of crystallization (refs. 16.17): Clusterembryo - j nucleus+crystal. In order for the intercalation of small hydroxy-particles to occur we should stay at the very left extreme of this sequence of precipitation. By using this method we succeeded in intercalating hydroxy-oxidic species of the cations La alone, or La plus Ni, into montmorillonite structure. The details of intercalation as well as the acidic and catalytic properties of the obtained solids is the subject of this work. MATERIALS AND METHODS OF INTERCALATION The montmorillonite used was of Wyoming origin (Ward’s International). Such materials usually possess an exchanged capacity around 100meq/100gr (refs.4,7). The pH of its 1% suspension was equal to 9.5. The amount of smectite used for intercalation was exchanged with Na+ by letting it in equilibrium with a large excess of 0.1M NaCl for two days under continuous stirring. The excess of NaCl was then removed by successive centrifugations and washings with distilled water. Finally the traces of C1- were removed by putting the slurry in a dialysis tube until the final testing with AgN03 was negative.
32 1
The lanthanum and nickel oxidic species for intercalation were originated from La(N03)3.6H20 and Ni(NO3)2.6H20 (Merck 2.a.) in solutions of 0.1M possessing pH equal to 4.48 and 5.61 correspondingly. The equimolecular mixture of them showed a pH equal to 4.83. After several trials and in order to decide about the optimum pH for intercalation we came to the conclusion that the potentially intercalable hydroxy-species of the two cations should have dimensions of a few A, probably less than 100A. Since the conventional optical or electrophoretic methods are not able to detect such species we reach the conclusion that a way to check approximately the size of the species was electrochemically by adjusting the pH in suitable limits.
ml NaOH
QOIM
Fig.1. Titration curves for La(N03)3 0.1M ( A ) and La(N0313 O.lM-Ni(N03)20.1M ( B ) with NaOH 0.01M. The pH suitable for intercalation was chosen at the points a and b respectively. the The pH suitable for intercalation was chosen from titrating curves of La(N0313 0.1M and La(N03)3 O.lM-Ni(N03)z 0.1M with 0.01M NaOH (Fig.1). Namely the pH was chosen in the half way between the initial nitrate solutions, e.g. at no addition of NaOH, and the turning points of those curves, at which precipitation starts. The pH at those points corresponds to 5.5 for Lanthanum and 5.7 for Nickel and Lanthanum. The intercalation took place as follows: A volume, usually 100ml. of Na-montmorillonite suspension acidified with dilute HC1
322
down to the above noticed pH values, were mixed with excess solution of La(N03)3 0.2M or equimolecular mixture of La(N03)3 0.1M-Ni(N03)2 0.1M kept at the same pH values. The control of pH was made automatically by using a auto-burette-titrator of Radiometer Copenhagen. The mixture was left for equilibration for one day, centrifuged for separation of the slurry and washed by distilled water. The obtained product was then freeze-dried and examined by different techniques as described next. CHARACTERIZATION OF THE PRODUCTS XRD Studies The freeze-dried samples were characterized by XRD at low angles at different temperatures in order to investigate the d-spacing achieved by pillaring as well as the temperature of the collapse of the aluminosilicate layers. The examinations took place after heating the samples at 100, 150, 200, 25OoC and so on at atmospheric conditions. The X-rays system used was a Philips diffractometer with Fe-filtered Co Ka radiation. The samples were prepared by putting a few drops of the suspension on a glass slide and drying the sample. In this way the layered solid is settled along its z-axis. .
I
I
'
,
(B)
j
I
;oooc 150°C
3 OO°C 250°C
10" 8" 28
I 1 I
10" 8"
4"
I
, , 1
28
4O
Fig.2. XRD patterns for the products of intercalation of La-Montmorillonite ( A ) and La-Ni--montmorillonite ( B ) at different temperatures. As
it can be seen
from
Fig.2 both
La-
and
La-Ni-pillared
323
solids showed approximately similar behavior with two peaks at 28-40 and 28-80 up to 3OO0C approximately. Those values mean interlayer distances of 25.6% and 12.88, respectively and they probably correspond to first and second order reflections on the 001 and 002 crystal levels. Upon heating to 35OoC a broadening of the above peaks is apparent, due to the shrinking of the layers. The shrink becomes complete a 4OO0C for both species, with a peak at 2f3=10.5°, which corresponds to the interlayer distance d=9.88 of the parent montmorillonite.
TG Studies The wet intercalated slurries of La-montmorillonite and La-Ni-montmnrillonite as well as of the hydroxides of the pillaring agents were examined thermogravimetrically in a Chyo thermogravimetric balance model TRDA3H under N2 flow. Two typical thermographs obtained are shown in fig.3 for the La(OH)3 and La-montmorillonite species. In those diagrams we observe that the dried at room temperatures La-montmorillonite loses around 14% of its weight between 45 and 108OC which should be due to water removal trapped between the aluminosilicate layers. The pure La(OH)3 gel shows at the same region a weight loss reaching about 6 . 5 % which should be due to adsorbed water. Next it loses water in two steps, one between 298 and 324OC (6.53% weight loss) and one between 388 and 467OC (12.2% weight loss). Those two steps should correspond to successive dehydrations of La(OH)3 according to the scheme
Those dehydration reactions (1) and (2) correspond to theoretical weight losses of 4.73 and 9.46% respectively which are comparable with the experimental values. It is important to notice that at the temperature region where the first dehydroxylation is observed, e.g. around 3OO0C, the pillared material starts to shrink (see fig.2) and the shrinking is completed at 4OO0C where total dehydroxylation has taken place.
324
So it seems that the transformation of La-hydroxyor La-oxy-hydroxy-species to oxidic material has destructive effect on the pillaring.
DTA - La(OH$
/
xO 1
I
T G - La-Moni
DTA- La-Mont
600
400
200
T/OC Fig.3. TG-DTA materials.
curves
for
Surface area measurements The specific surface
the
area
La-montmorillonite
of
the
and
La(OH13
freeze-dried samples of
La-montmorillonite, La-Ni-montmorillonite as well as the original Na-montmorillonite were measured by the one-point-method in a Carlo Erba-Sorpty 1750 system by N2 adsorption at -196OC. The results are summarized in Table 1. It can be seen that the surface area of Na-montmorillonite increases substantially upon pillaring with La from 17.1 m2.g-1 to 35.4 m2.g-l. On the contrary the La-Ni-montmorillonite species showed a rather small increment of specific surface area as compared to Na-montmorillonite from 17.lm2.g-1 to 18.9 m2.g-1. Acidity measurements The surface acidity of the prepared pillared clays as well as the original Na-montmorillonite was checked by titration with NH3 at different temperatures (140-2OOoC) in a gas chromatographic column. A Varian 3700 GC equipped with a TCD and connected with a LDC Milton Roy CI-10 integrator was used for this purpose. An
325
amount
of
clay
equal about to 150 mg
was put in the GC column
and degassed at 25OoC for 20h under helium
flow
of
l2ml.min-I.
the temperature was set to 2OO0C and injections of 0.2ml of gaseous NH3 (Merk Wasserfrei, 0.2% H2O) were done into the
Next
column up to complete saturation of the sample and total elution of the injected ammonia. The sum of the irreversibly adsorbed ammonia at this particular temperature was considered as corresponding to the total (Bronsted and Lewis) acid sites of the pillared clay. Next the temperature was lowered and the experiment was repeated at 18OoC, 16OoC and 14OoC. The results are noticed in Table 1. In the same table the enthalpies of adsorption of ammonia are also noticed calculated by plotting the retention volume of it at different temperatures versus 1000/T. Perfect straight lines (correlation coefficient r=0.99-1.00) were obtained from the slops of which the values of -AHads were calculated.
TABLE 1. Specific surface areas (m2.g-l), surface NH3
adsorbed
(molecules of
acidity
per m2 at different temperatures), and enthalpy of
(kJ/mol) of NH3, degrees of conversion (x%) of iso-propanol per m2 at 18OoC and activation energies for the iso-propanol dehydration on Na-, La- and La-Ni-montmorillonite.
CATALYTIC TESTS - ISOPROPANOL DECOMPOSITION To test the catalytic activity of the prepared samples we chose to study the decomposition of iso-propanol. This is a reaction examined in details in the past (refs.20-25) and it has been shown that it is catalyzed by acid surfaces. For the catalytic tests a bench scale flow reactor similar to that described elsewhere (refs.26, 27) was used. The reactor consisted of a silica tube of lcm diameter with a perforated glass bed onto
326
which 300 mg of the catalyst was put. The system was heated with a tubular furnace within k 2 O C . Analyses of the reactants and products were carried out using a GC system described in the previous section. The column for analysis was 2m stainless steel 1/8" containing 10% Carbowax 20M on Chromosorb W-HP 80-100mesh with He as carrier gas. The same gas was also flowing through a bubble bottle (30cm3.rnin'l) containing the iso-propanol whose vapours were driven then to the reactor. Under the experimental conditions the partial pressure of iso-propanol was 0.0373 atm. Measurements were taken between 150-200°C in ten degrees intervals and the % degree of conversion x was taken as a measure of catalytic activity (Table 1). During the conversion the only products detected were H20 and C3H6. The activation energies for the conversion was calculated from the equation of the plug flow reactor Fdx=Rdm, where F-feeding rate (moles.min-l), x the % degree of conversion, R-the rate of reaction and m-the mass of the catalyst used. Considering first order kinetics, R=kPisopropano~ as usually done by different workers for this reaction (refs. 2 0 - 2 5 ) and substituting the partial pressure with the degree of conversion as done in the past for similar cases (refs. 26-27) we obtain the log forms of the corresponding Arrhenius-type plots ln[(v+n)[-ln(1-x)]-(n-11x1
=
1nB-E/RT
(3)
where B=PtmA/F, Pt=total pressure in the reactor, A-the pre-exponential term of Arrhenius equation k=Aexp(-E,/RT), v-the ratio moles of the inert gas (He): moles of the reacting gas (iso-propanol vapours) and n-the number of molecules yielded by a decomposed iso-propanol molecule. In our case n=2 and v=18.34. Plots of the left part hand of the above equation versus 1000/T results in perfect straight lines from which the activation energies of the reaction were found and listed in Table 1. In the same table the degree of conversion, calculated per m2 is given for comparison for Na-, La- and La-Ni-montmorillonite at 18OoC. DISCUSSION From the results cited in Table 1 we observe that the introduction of La into montmorillonite increases its acidity substantially. This is apparent not only in the number but also
327
in the strength of the acid sites as estimated by the enthalpy of adsorption of NH3 on its surface. One interesting result is that the simultaneous introduction of La and Ni into montmorillonite increases furthermore the number as well as the strength of the acid sites for reasons which are not apparent. On the contrary the surface area nearly doubles upon the introduction of La from 17.1 to 35.4m2.g-I but the increment is much smaller, from 17.1 to 18.9 m2.g-I upon substitution of Na with La plus Ni. It may be that this difference is due to some kind of porosity differences of the two samples but this point needs further investigation. As expected, the catalytic activity for the isopropanol decomposition as estimated by the degree of conversion per m2 runs parallel with the surface acidity. Thus the La-Ni-species are more active as compared to La-montmorillonite. Somehow strangely the activation energies of the dehydration are minimum in La-species, but this may be due to adsorption or/and compensation effects. It may be useful at this point to compare those results with similar ones obtained on Aluminum-Aluminum phosphate-M203 (M=Cr, Fe) catalyst (ref.26). In such solids the activation energy for the iso-propanol dehydration was found 2-3 times higher, the degree of conversion per m2 1-2 orders of magnitude lower, and the number of acid sites as estimated by pyridine adsorption one order of magnitude less. So it seems that the La- or La-Ni-intercalated species are very strong acid catalysts. It is exactly their strong acidity which is most certainly the reason for the early shrink of the aluminosilicate layers at 350oC. This mechanism should take place through acid attact on the sensitive aluminum sites. Methods of stabilizing such structures to higher temperatures and at the same time keeping their acidity at high or rather controllable levels would be of interest. ACKNOWLEDGEMENT The authors gratefully Aspropyrgos Refineries.
V.Movarek and M.Krauss, J.Catalysis, 87 ( 1 9 8 4 ) 452-460. V.Movarek, React.Kin.Catal.Lett.,3 0 ( 1 9 8 0 ) 71-75. G.I.Gulodets, N.V.Parlenko and A.I.Tripolskii, Kinetika i Kataliz, 2 7 ( 2 ) ( 1 9 8 6 ) 346-351. D.Petrakis, P.J.Pomonis and A.T.Sdoukos, J.Chem.Soc.Faraday Trans.1, 85 ( 1 9 8 9 ) 3173-3186. C.Kordulis, L.Vordonis, A.Lycourghiotis and P.J.Pomonis, J.Chem.Soc.FaradayTrans. I, 883 ( 1 9 8 7 ) 627-634. D.Petrakis and P.J.Pomonis (in preparation)
G. Poncelet,P.A. Jacobs,P.Grange and B.Delmon (Editors),Preparation of Catalysts V 01991Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
A1-Fe PILLARED LAPONITES :
MIXED
329
PREPARATION, CHARACTERIZATION AND
CATALYTIC PROPERTIES IN SYNGAS CONVERSION F. BERGAYA' , N HASSOUN' , L. GATINEAU' and J. BARRAULT' 'Centre de Recherche sur les Solides
a
Organisation Cristalline Imparfaite,
C.N.R.S., lB, rue de la Ferollerie, 45071 Orleans, France. 'Laboratoire de Catalyse en Chimie Organique, UA 350, C.N.R.S . ,
40, Avenue
du Recteur Pineau, 86022 Poitiers Cedex, France. ABSTRACT Mixed A1-Fe pillars were obtained by a simple method of preparation of PCL (Pillared Clay Laponite). Solutions of A1C13, FeC13 and NaOH were mixed with a clay suspension in adequate concentrations and kinetic conditions. A combination of techniques (chemical analysis, XRD, NMR and H'TPR) shows iron cations replace a few octahedral A1 in the AlI3- pillars. The mixed A1-Fe pillars present original catalytic properties with shape selectivity different from that of conventional iron catalysts.
INTRODUCTION The
synthesis of pillared clays has been extensively studied since 1974
(Ref.1) but
syntheses of mixed
present work reports mixed
pillared
clays
the pillaring of laponite
aluminium-iron pillars
in view
of
are rather scarce. The (synthetic hectorite) by
their use as catalysts in the
Fischer-Tropsch reaction.
METHODS Chemical analyses of the principal elements of the pillared and unpillared clays were determined by X-ray fluorescence, except for Li ions which were analyzed by atomic were
absorption. XRD patterns of oriented films
obtained on a Siemens diffractometer (CuKor). NMR spectra of 27Al were
obtained on a
Bruker spectrometer (model MSL) at 94 MHz, with magic angle
spinning. Temperature programmed reduction (TPR) by H2 (heating rate per
=
4°C
mn to 500°C) was performed in a glass reactor. The samples were always
predried at 130°C for 12 h and the water was trapped during the reaction.
330
PREPARATION OF Al, Fe AND MIXED A1-Fe PILLARED LAPONITES FROM AN INITIAL Na LAPONITE The laponite (a synthetic trioctahedral smectite from Laporte Industrie) was
purified by
classical
ion exchange with
1N
NaC1, and
its CEC
(determined by EDA CuC12) is 94 meq/100 g of calcined clay. Different mixtures of
an 0.1 M solution of A1C13
and of an 0.025 M
solution of FeC13 were prepared, each corresponding to one of the following values these
of the
ratio Fe/(Fe + Al) : 0 , 0 . 1 , 0.2, 0 . 3 , 0.5 and 1.0. Each of
solutions was
added slowly and at a well-controlled rate, together
+
1.21
with
a 0.12 N NaOH
clay
suspension (previously prepared by stirring 2 hours at 80°C) at 40°C
solution. [OH/(Al
Fe)
=
until the ratio of concentration reached 30 meq (A1 The
rate of
addition was
progressively from u 9 to
such that
N
the pH
to a well-stirred 2%
+
Fe) per gram of clay.
of the suspension decreased
4.
After two days ageing at room temperature, the clear supernatant was eliminated
and the remaining suspension was dyalized six times with 0.3 1
distilled water per clear supernatant of
gram of clay. After a further seven days ageing, the the dyalized suspension was
again eliminated and a
part of the remaining suspension was used to prepare oriented films for XRD while the rest was finally lyophilized to obtain powder samples. RESULTS
1. Chemical analvsis The chemical analysis of the initial laponite is given in Table 1. The formula of the initial Na laponite is :
and its molecular weight is 728.823 g. A s the clay is free of iron, all the iron of
found later corresponds to added iron. The contribution of the amount
aluminium initially present is taken into account when considering the
total aluminium present these
in the samples prepared. The chemical analyses o f
samples, as regards the major oxide SiOz, MgO, A1203 and FeZ03 are
given in Table 2.
331
TABLE 1 Chemical
analysis results and composition of the initial Na laponite based
on these data.
Oxides
S i02
Weight (%) in calcined clay 65.563
Normalized to Correspon 2 2 oxygens ding weight Oxveens Metals in gram
2.182
15.906
7.953
477.896
0.010
0.073
0.049
2.498
MgO Liz0
30.299
0.752
5.482
5.482
220.979
0.738
0.025
0.182
0.364
5.438
'3
0.356
Oxygen content, in atoms
TiOZ
0.076
0.002
Na20
2.864
0.046
K2 0
0.102
0.001
99.998
3.018
0.014
0.335
0.007
0.671
0.007
0.014
21.999
0.559 20.794 0.659 728,823
TABLE 2 Chemical analysis results normalized relatively to the major oxides. The label of
each
sample refers
to
the
composition of the mixed halides
solution used in the preparation.
S i02
nd : not determined
A12 3'
3'
68.14
31.49
0.37
59.43
27.42
13,14
55.00
24.79
11.62
8.59
51.41
22.75
9.93
15.91
47.72
20.48
7.28
24.51
43.92
19.68
0.46
35.94
38.86
16.73
nd
44,40
332
The ratio of the amounts of SiOz and MgO in the initial laponaite and in the pillared laponaites is not constant as shown in Table 3 .
TABLE 3 Relative variations between the ratio :
- SiO, in initial laponite/SiO,
RSi Rns
in pillared laponite and
= MgO in initial lamonite/Mgo in pillared laponite with the increasing amount of initial FeC13 solutions.
Lap All00
1.14b4
1 .1 4 8 ,
0 . 001,
Lap Al,, Felo
1,238,
1.270,
0.0315
Lap AlmFe2,
1.325,
1.3 8 4 ,
0.059,
Lap
1.427,
1.537,
0.109,
Lap A150 Fe50
1.551,
1.600,
0.049,
Lap Fe,M,
1.759,
1.889,
0.129,
*
Fe3o
*
Lap A15,Fe5, presents an exception for AX that we do not explain. The
be
increase in AX with increasing amount of FeC13 added initially can
explained by
the
i) causes a release of eliminated ii)
by
fact that the solution becomes more acidic, which a
small amount
of
octahedral
ions
(Mg and Li)
dyalisis, and
probably attacks
the edge of the
tetrahedral sheet with removal of
collofdal silica and destruction of a part of the layer which may therefore be different from one sample to the other. To
compare the samples among themselves and taking the remaining sheets
as reference we should take into account the collofdal silica which was not eliminated by laponites. The
dyalisis
and which
quantities
o f A1203
changed
the weight of the pillared
and Fe203 retained by
the clay and
normalized with respect to the remaining sheets are shown in Table 4 . From
these results, it is clear that iron is more retained by the clay
than aluminium. Comparing
the quantities
show a greater selectivity for iron.
introduced and
those retained,
333 TABLE 4 % of oxides r e t a i n e d by t h e remaining s h e e t s % A1203
X Fe203
14.59 14.27
10.82
13.26
21.86
10.72
37.40
0.35
57.04 82.92
2 . XRD p a t t e r n s The iron
sample (Lap
content i n
suspensions.
A
16
of t h e o r i e n t e d f i l m s show a 001 peak a t 1 7
XRD p a t t e r n s
iron-free
for
All,
and t h i s
)
t h e samples.
After heating
A
f o r the
peak i s broadened with i n c r e a s i n g
The o r g a n i z a t i o n
i s improved by ageing the
t h e samples t o 400°C, t h e 001 peak i s a t about
aluminium p i l l a r e d c l a y and a t about 2 2
A
f o r i r o n p i l l a r e d clay
proving t h e p i l l a r i n g occured.
3 . NMR s p e c t r a The pillar
NMR spectrum
species as
peak a t
A1
of t h e
i r o n - f r e e sample
confirms p i l l a r i n g
by Al13
shown by Plee e t a l , 1985 (Ref. 2 ) with t h e t e t r a h e d r a l
62.9 ppm, and a r a t i o of i n t e g r a t e d s u r f a c e s of t e t r a h e d r a l A 1
peak over o c t a h e d r a l A 1 c l o s e t o 1 2 . Luckily, that
we obtained
s p e c t r a from
two o t h e r samples i n s p i t e t h e f a c t
i r o n i s p r e s e n t (Lap A190Fe,o and Lap Al,,FeZo).
Two observations can
be made regarding t h e s e s p e c t r a . ( i ) The t e t r a h e d r a l and o c t a h e d r a l peaks p r e v i o u s l y observed with Lap A l l , are A1
always p r e s e n t ,
b u t a l i t t l e s h i f t e d . A second, very broad octahedral
peak appears. This broadness i s probably due t o t h e paramagnetic e f f e c t
of t h e i r o n i n t h e neighbouring environment. (ii)
The t o t a l amount of A 1 v i s i b l e by NMR decreases when t h e amount of Fe
increases.
Although i t
observable,
then may
could be
t h a t only t h e pure A l l ,
p i l l a r s could be
e x i s t some mixed p i l l a r s with Fe s u b s t i t u t i n g a p a r t
o f t h e A 1 atoms, t h e r e f o r e not e a s i l y d e t e c t a b l e by N M R . This concept o f an
334
isomorphic substitution suggested by
the NMR
results will
be
later
confirmed in the discussion. 4.
€i2m We
have used
this technique to characterize the state of iron at the
start of the catalytic process.
The results of the reduction give some
important information. (i)
In the
case of
samples rich in aluminium (until Lap Al,Fe,,),
there
are two species of iron, with only one species reducible by H2TPR. (ii)
In the
case of
Fe rich
samples, all the iron is reduced but at two
different rates, supporting also the idea that two species of iron exist, one more easily accessible to H2 than the other. DISCUSSION Both
and
NMR
Alr304(OH),,
(H20)12
XRD
patterns
pillar. The
confirm
the
presence
of
an
:
data from chemical analysis allow us to
calculate the charge and the pillar density. From the contribution of A1203 (14.59%) and knowing the formula weight charge/unit
cell) of
pillar (6.5 A1203 14.59 x 707.37 100 x 662.74 and
(707.37 g),
the initial laponite and
- 662.74 g) we find
the CEC (0.685
the weight of the oxides
:
- 0.16 pillar/unit cell
0.685 - 4.&+/pillar. 0.16
These
results
agree
with
previous
results
of
All,
pillared
montmorillonites obtained by Plee, 1984 (Ref.3). The H2TPR results show that we
have
two
species of Fe. From the NMR
results, we can suppose that some octahedral A1 of the pillars are replaced by Fe. Within this hypothesis, we can calculate the amount of iron necessary to compensate
for the decrease of A1 in the All, pillar for the
different samples. For the first three samples, the calculated isomorphic ratio allowsus to determine
the iron content needed to form an (Al13., Fe,)
pillar and to
deduce from the total iron retained, the iron content out of the pillar. This last amount corresponds exactly to the part of iron reduced by H2TPR, as shown in Table 5.
335 TABLE 5 Isomorphic ratio determined
(IR) of A1 by
Fe
and out
of pillars content of Fe
from chemical analysis data. Comparison with
iron content
reduced by H2TPR.
Total
IR
% A1203
(%)
%
Total Fe203
Out of pillar Fe203
% Fe203 Reduced by H2TPR
Lap A & ,Felo
14.27
2.19
10.82
10.32
Lap Al,Fe,,
13.26
9.13
21.86
19.77
21.02
Lap Al,oFe30
10.72
25.51
37.40
31.35
30.86
Lap Al,, Fego
0.35
Lap Fe,w
The
two
last
nature of
any
57.04
60.05
82.92
85.52
samples practically
consequently, no (A1Fe)13
10.45
do
not
contain any
aluminium and,
pillars, but we have no information about the
iron pillars, except
that the iron pillars are probably
reducible, and reduced later than the extra pillars iron. In summary, the
combination of all
these
techniques supports the
presence of : (i)
Al,, pillared laponite (Lap All, )
(ii) mixed (A113.xFe,)
pillared
laponites with
Fe
in isomorphic
substitution of octahedral A1 of the pillars. (iii) another kind of oxyhydroxides of
iron retained by the clay,
perhaps out of the interlamellar space. CATALYTIC PROPERTIES IN SYNGAS CONVERSION When these samples were used in the (CO, H2) reaction, we observed catalytic properties quite different from those of conventional iron catalysts for the mixed (A1 Fe) samples. 1.The activities are
significantly enhanced when the
total iron content
336 ; the fact that the initial activation phase is faster when iron
increases
content increases could be related to iron located out of the pillars. 2 . For
the
selectivities, we can consider two successive steps
(i) at the beginning of each catalytic test, the CH, selectivity is similar for all the with
increasing
(ii) After the
samples, but the selectivity of
CH,
the
iron,
(C6-Ce)
hydrocarbon increases
since the olefins decrease.
stabilization phase
(when the activityreaches a plateau),
increases but both olefins and heavier hydrocarbons decrease
considerably when the
iron content increases. This
is evidence for the
importance o f t h e h y d r o g e n a t i n g p r o p e r t i e s o f t h e i r o n l o c a t e d o u t o f t h e p i l l a r s . 3 . Surprinsingly, the hydrocarbon distribution obtained with mixed pillared
laponites does not is comparable to metal
follow a Schulz-Flory law. This selectivity deviation
the shape selectivity observed with zeolite encapsulated
clusters in the conversion of syngas or methanol into light olefins
as shown by Nazar et al, 1 9 8 3 (Ref. 4 ) . The
different behaviour
of the Lap Al,Fe,,
sample at this
stage of
stabilization, where the shape selectivity desappears, indicates that this catalyst is at
the limit of isomorphic substitution and is not entirely
representative of mixed pillared laponites, but is similar to the pillared iron catalysts which
act as
conventional
iron catalysts used at high
temperature. For these last samples, CH4 is always the major product. These new catalytic properties observed in syngas synthesis, namely high olefins
formation, shape selectivity and
stability at relatively high
temperature are probably due to the presence of mixed (Al-Fe),, pillars.
REFERENCES
1 2 3
4
D.E.W. Vaughan in R. Burch (Ed). Pillared clays : a historical perspective, Vol 2 Elsevier. Pillared clays Catalysis today, 1 9 8 8 , p.188. D. PlBe, F. Borg, L. Gatineau and 3.5. Fripiat. 3. Am. Chem. SOC. 107 ( 1 9 8 5 ) , pp. 2 3 6 2 - 2 3 6 9 . D. PlBe. ( 1 9 8 4 ) PhD thesis, Orldans, France. L.F. Nazar, G.A. Ozin, F. Hugues, 3 . Godber and D. Rancourt.Angew. Chem. Int., ( 1 9 8 3 ) , 2 2 , pp. 6 2 4 - 6 2 5 and see also Angew. Chem. Suppl. pp. 8 9 8 - 9 1 9 .
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
337
ZIRCONIUM PILLARED CLAYS. INFLUENCE OF BASIC POLYMERIZATION OF THE PRECURSOR ON THEIR STRUCTURE AND STABILITY E.M. Farfan-Torres, 0. Dedeycker, P. Grange Catalyse et Chimie des Mattriaux Divists, Universitt Catholique de Louvain, Place Croix du Sud, 2 Bte 17,1348 Louvain-la-Neuve (Belgium) ABSTRACT Increasing the pH of the pillaring solution by addition of NaOH increases the thermal stability of Zr pillared clays. The improvement may be directly correlated to the amount of ZrO2 and to the density of the pillars. The strong interaction between the pillars and the clays promotes high acidity of this microporous solid. INTRODUCTION The experimental conditions of the hydrolysis and polymerization of Zirconyl chloride largely influences the thermal stability and texture of the 3-pillared montmorillonites. Strong acidity of zirconium solution as well as high temperature of the pillaring process bring about brittleness and even partial destruction of the clay (1). An increase of the pH of the pillaring suspension modifies the hydrolysis-polymerization of the precursor as well as the cationic exchange capacity (CEC) of the clay (2,3,4). Vaughan et a1 (5) modified the pH of the ZrOC12-montmorillonite suspension adding Nap203 solution. The other papers on Zr-pillared montmorillonites paid relatively little attention to this parameter (6- 11). In this work we present the influence of the concentration of NaOH, introduced in two different ways, on the thermal stability, porosity and acidity of Zr-pillared montmorillonite. The basic solution was added either in the ZrOC12 solution before pillaring (ex-situ) or after the Zr solution was contacted with the montmorillonite suspension (in-situ). EXPERIMENTAL RESULTS Table 1 reports the way of preparation of the different samples. Sample
final pH
NaOH (0.1 M) CEC "ex-situ" "in-situ" (meq/lOOg)
EI-05
1
__
EII-01 EII-02 EII-03 EII-04
1.9 1.9 3.9 3.9
__
68.7
16.20
X
48 46.7 48.4 40.5
24.22 23.00 25.40 30.46
X X X
Zro2 (wt %)
Table 1. Zr pillared clays: pH during the pillaring process, cationic exchange capacity and concentration of ZrO2 after pillaring.
338
Montmorillonite (Weston L-Eccagum) was first exchanged with NaC1. After ageing for 30 days, acetone was added to the Na-clay in order to obtain a water-acetone ratio equal to 1/1. NaOH (0.1 M) was introduced either in the ZrOCl2 solution or in the ZrOC12-clay suspension. During the pillaring process, the suspension was stirred for 2 hrs at 4 0 O C . The clay was then washed up to constant conductivity of the solution. After freeze-drying the samples were calcined at different temperatures up to 6 0 0 O C . Four samples were prepared, modifying the pH of the pillaring solution and the way of introduction of the NaOH. In addition one Zr pillared clay was prepared without addition of base. The chemical composition of the Zr-pillared clays as well as the Na montmorillonite are reported in table 2. Silica, alumina and zirconia have been analyzed by X-Ray fluorescence; the other elements by atomic absorption spectroscopy after sulfofluorhydric dissolution of the clay (6).
I
oxide % Na+ mont.
EI-05
EII-01
EII-02
EII-03
EII-04
Si@ A1203
60.82 21.48
50.12 16.10
46.06 15.40
48.30 16.39
47.71 15.38
42.72 14.28
Fez03 K20 CaO Na20
3.66 0.83 0.11 0.05 2.74 __
2.31 0.64 0.11 0.05 0.06 16.20
2.56 0.67 0.03 0.00 0.05 24.22
2.57 0.66 0.03 0.00 0.07 23.00
2.60 0.65 0.19 0.00 0.05 25.40
2.22 0.59 1.41 0.01 0.01 30.46
H20+
10.31
14.40
9.57
8.98
8.02
8.16
w
rn
The amount of ZrQ in the clay increases with the pH of the pillaring solution. From these two figures it can be seen that the solids synthetized at pH=1.9 are thermally stable. In addition, the diffraction lines are narrower and better defined than for the sample prepared without ammonia. For higher pH, the structural characteristics of the clay are different. Two diffraction lines at 19 %, and 12.6 A are observed on the uncalcined solid EII-03. This behaviour mdicates the presence of two different basal spacings. The interlayer distance observed for the EII-04 sample is 12.6 A whatever the thermal treatment. XRD of the samples calcined up to 600°C are presented in fig. 1.
The specific surface area of the pillared montmorillonite prepared by "ex-situ"polymerisation of the Zr complex at pH=3.9, is much lower than that of the other clays. At 700°C this sample presents the same surface area as the sample prepared without NaOH.
339
10
2 10
2
10
2 10
2 10
2 28
Fig. 1. XRD spectra. Calcination temperature : (a) 25°C; (b) : 110°C; (c) : 200°C; (d) : 300°C; (c) : 400oc; (0 : 500°C; (g) : 600°C. The evolution of the basal spacing with the calcination temperature is plotted in fig. 2.
t 0
200
LOO
600
800 T I°Cl
Fig. 2. : Evolution of the basal spacing with the calcination temperature : EI-05; 0 :EII-01; R EII-02 A EII-03; A EII-04. The evolution of the specific surface area of the different samples with the calcination temperature is illustrated in fig. 3.
340
0 '
0
200
LOO
600
800
T /"C/ Fig. 3. Specific surface area.
8
: EI-05; 0 : EII-01; I : EII-02; A : EII-03; A : EII-04.
The total acidity of the solids was evaluated by Programmed Temperature Desorption (TPD) of NH3. For those experiments 0.10 g of samples seived (200<0<315 p) were treated for two hours under helium flow up to 400 or 500°C. After adsorption of NH3 for 15 mn at 100°C, the solids were flushed with helium flow for 1 h at 100°C in order to eliminate physisorbed ammonia. The heating rate during TPD experiments was 10°C min-I. The NH3 desorbed was detected by a thermal conductivity detector. The total amount of NH3 desorbed up to 400°C for the solids calcined at 400°C and 600°C is reported in table 3. Sample EI-05
mmol NH3.g-1 (up to 4OOOC) 6oooC
400°C
400
283
49
29
EII-01 EII-02 EII-03 EII-04 Mont. Na+
Table 3. TPD of NH3. All the Zr pillared montrnorillonite present much higher total acidity than the Na montmorillonite except for the solids EII-04.
341
DISCUSSION Schofield has shown that the pH of the solution controls the amount of ammonium cations fixed on montmorillonite and kaolin (13). For a determined pH, the amount of potentially fixed ions is constant, this amount increases with the pH. Electrophoretic mobility measurements of Namontmorillonite support this view. The electrophoretic mobility and the superficial charge are drastically decreased at low pH (1
342
pH = 1.9 L 00 oc __t
-
-
pH = 3.9
"in situ"
pH = 3.9
" e x situ"
LOO*C
3zzZz
x=EIZE----
Fig. 4. Schematic representation of the architecture of the Zr pillared clays.
CONCLUSIONS The increase of the pH of the pillaring solution up to 1.9 increases the amount of Zr intercalated with the layer structure. In addition, this leads to a better distribution of the pillars which induces a high thermal stability and microporous structure of the system. This improvement could be directly correlated with the density of the pillars. In addition, the number of structural defects created by the acidic solution could decrease, inducing less possible proton migration towards the octahedral layer. The strong interaction between the pillars and the silica layers improves the stability and stabilizes the 2102 amorphous phase. In this way, the acidic properties of the Zr PILCs are improved as compared with the Na montmorillonite or the bulk ZQ oxide. When the pH of the pillaring solution is adjusted at 3.9,Zr(OH)4 precipitates. The addition of NaOH to the Zr-clay suspension led to a mixed system in which bulk Z a and Zr pillars are present. When the adjustment of the pH to 3.9 is done on the ZrOC12 solution, no pillars are formed and most of the zirconium could be deposited as bulk Z e . ACKNOWLEDGEMENTS The financial support of the SPPS (Services de Programmation de la Politique Scientifique, Belgium), is greatly acknowledged. E.M. Farafan-Torres thanks the CGRI (Commissariat GCntral aux Relations Internationales de la Communautt FranGaise de Belgique) for a grant.
343
REFERENCES (1) E.M. Farfan-Torres, E. Sham, P. Grange, Clays and Clay Minerals, submitted for publication.
W. Rausch, H.D. Bale, J. Chem. Phys., 40 (1964), 3891. A. Clearfield, Inorg. Chem., 3 (1964), 146. D. Plee, F. Borg, L. Gatineau, J.J. Fripiat, J. Am. Chem. SOC.,107 (1985), 146. D.E.W. Vaughan, R.Y. Lussier, J.S. Magee, U.S. Patent (1979), 4,176,000. S . Yamanaka, G.W. Brindley, Clay and Clay Minerals, 27 (1979), 119. E. Kikuchi, R. Hamana, M. Nakano, M. Takemara, Y. Morita, J. Japan Petrol. Inst., 26 (1983), 116. (8) G.M. Muha, P.A. Vaughan, J. Chem. Phys., 33 (1960), 194. (9) R. Burch, C.I. Warburton, J. Catal., 97 (1986), 503. (10) G.J.J. Bartley, Catal. Today, 2 (1988), 233. (11) M.L. Occelli, D.H. Finseth, J. Catal., 99 (1986), 316. (12) LA. Vomovitch, J. Louvier, J. Debras-Guedon, L'analyse des Silicates, (Herman, ed.) (1962). (13) R.K. Schofield, J. Soil Sci., 1 (1949), 1. (14) K. Shibata, T. Kiyoura, J. Kitagawa, T. Sumiyoshi, K. Tanabe, Bull. Chem. SOC.Jap., 46, 1973, 2985. (2) (3) (4) (5) (6) (7)
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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
345
CONTROL OF THE ACIDITY OF MONTMORILLONITES PILLARED BY A1-HYDROXY CATIONIC SPECIES
D . TICHIT', Z . MOUNTASSIRl, F. FIGUERAS' and A.AUROUX" Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees, URA 418 du CNRS, ENSCM, 8 rue de 1'Ecole Normale, 34053 Montpellier Cedex 2, France.
" Institut de Recherches sup la Catalyse du CNRS, 2 avenue A.Einstein, F69626 Villeurbanne Cedex, France. SUMMARY The nature, number and strength of the acid sites of pillared clays have been changed by three independent methods. In the step of synthesis, the use of competitive ion exchange f o r the intercalation of the A1 hydroxy cations promotes a stronger acidity, attributed as f o r pillared beidellite, to a better retention of their individuality of the isolated All, cations. On the pillared clay , the ion exchange of the residual cations by Ce3+, La3*, Ca2*, Mg"' o r Zn2* affect differently the acidities: Ce exchange stabilizes Bronsted acidity, whereas Zn"+ suppresses i t , but the total number of acid sites is little affected. Steaming increases first Bronsted acidity through hydrolysis of the pillars, then decreases the acid strength. All these modifications also affect the thermal stability of the material. No correlation exists between the acidity of the pillared clays and their acidity, which can be then easily changed in a wide range.
INTRODUCTION Clays have long been used as catalysts due to their acidic nature after cation exchange o r acid treatments.
For montmorillonite and hectorite the
most used smectites both Lewis and Bronsted acid sites were found the origin of which are well established.
Bronsted acidity essentially results from the
dissociation of water molecules in the hydration shell of the interlayering exchangeable cations. Lewis acid sites result from the low coordination of aluminium and iron atoms at crystal edges. These clays catalyse many organic reactions
(1) and constituted also the first generation of FCC catalysts.
Their interest in this last field was recently renewed by the preparation of pillared interlayered clays (PILCs)
.
The swelling of the interlayer space by oxydes pillars yields a class of molecular sieves containing both Lewis and Bronsted sites after calcination up to 500°C and only of Lewis sites beyond ( 2 , 3 ) . It is generally admitted
that
the pillars are the source of the improvement of the Lewis acidity. Higher
346 acid strengths are reported f o r calcined A1
,
Ti o r Zr PILCs than for Ni o r
Cr analogs ( 4 , 5 ) . The number of acid sites generally increases with crosslinking ( 4 ) but decreases regularly with the calcination temperature of the PILCs. Lewis acid strengths as high as those in CeY zeolites were reported (2,3) for A1-PILC samples calcined at 500OC. The acidity is also dependant of the clay layer structure. The occurence of a bidimensionnal zeolite structure ( 6 ) in the case of beidellite is accompanied by an increase in the number and strength of acid sites comparatively to pillared smectites without tetrahedral substitutions.
This is due to the
existence of an acidic OH group bonding the pillar to an inverted tetrahedron of the layer and also t o the better retention of the All, individual structure due to the more localized charge of the clay layers preventing their coalescence ( 6 ) . The Bronsted acidity was shown to be mainly provided by the structural OH
groups of
the clay layers
(4).
The disapearance of
the
Bronsted acidity after calcination at relatively low temperature is in line with the more easy dehydroxylation observed for the PILC than for the parent clays. Dehydration and dehydroxylation of the pillars liberates protons. They could also enhance the Bronsted acidity. But this protons produced under calcination migrate to the negative sites of the octahedral layer of the montmorillonite. This trapping does not give active Bronsted sites and destabilizes the structure ( 7 ) . The aim of our work was to show that the acidity of the aluminium pillared montmorillonite could be modified in several ways, at first by the preparation method doing an exchange in competition of the A1,,0,0Hz4(Hz0)lz7' with NH,'
ions and then by cationic exchanges o r steaming of the well stabi-
lized structure of the calcined PILCs
.
EXPERIMENTAL Catalyst preparation The clay material used was a suspension of Volclay montmorillonite refined by CECA (Honfleur-Prance) having particles smaller than 0.5 pm in size. The A1-PILCs were prepared by adding to the clay an oligomeric cationic species All,0,0H,,(H,0)l,7+ A1C1,.6Hz0
obtained by
mixing two 0.2M solutions of
and NaOH at ambiant temperature with a ratio OH/Al=2. The
pH=4.2 of the solution was raised to pH=6 before the addition to the clay and a ratio of 5 mmole Al/g clay was used. The slurry was then aged for 3 hours a t 80°C
,
filtered, washed until chlorine free and dried at 6OoC in air. A
calcination of this intercalated clay above 5OO0C induces a stabilization of the porous network by dehydroxylation and evolution of the intercalated oligomeric cations to oxyde pillars.
347
Ionic exchange by competition In a variant of this method the exchange of the parent clay by the aluminium cations was performed in competition with NH,' NH,'/Al
ions. Solution with
molar ratio of 10 was used. No modification of the "7Al NMR spec-
trum was evidenced. The preparation is then realized as precedently described, Ionic exchange of the calcined PILCs After pillaring and calcination the C . E . C of the clay is lost due to the migration of H+ to the octahedral layer ( 8 , 9 ) . A treatment of the PILCs calcined at 5OO0C using a 4.10-3 M solution of K,CO,
at 8OoC restores the
C . E . C (10). Exchanges were further realized with 3.75.10-3 M solutions of
MgCl,.6H,O,
Ca(N0,),.4H20
,ZnCl, ,CeCl,.6H2O and LaCl,.GH,O.
After wa-
shing samples were dried at 6OoC. Steaming of the calcined PILCs Steamings at 55OOC and 65OOC under 1 atm of vapor pressure were performed during 1 7 hours on A1-PILCs samples prealably calcined a t 68OoC. These temperatures are in the range of those encountered during regeneration in catalytic craking process. Characterizations of the pillared clays Acidic properties were analyzed by several complementary methods in order to obtain informations on the nature, number and strength of the acid sites. The adsorption of pyridine, used as probe molecule, was studied by IR spectroscopy on self-supported wafers obtained by pressing the PILCs into thin films. Lewis and Bronsted sites give characteristics peaks having well established positions around 1450 and 1550 cm-I respectively. Evacuations a t increasing temperatures were useful to investigate the acid strength Adsorption isotherms of NH,
were studied by microcalorimetric method.
Samples on powder form were evacuated at 500'c. function of the quantity of NH,
.
The heat of adsorption in
gives a comparison of the respective acidity
of the PILCs. The distribution of acid strength are given plotting the differential heat of adsorption in function of the coverage. It gives the fraction of sites adsorbing NH, with a given energy. The amount of NH, adsorbed at 100°C on degassed sample, measured by thermogravimetric method give an evaluation of the number of acid sites. RESULTS Ionic exchange by competition In table 1 are reported the chemical compositions of the parent Volclay montmorillonite
,
of the PILCs prepared by a conventionnal method (Vc) o r
by ion exchange in competition (Vc.e)
.
348
TABLE 1 Chemical compositions in moles on a dry basis, of the original clay and of the pillared clays prepared from it. oxide
Volclay montmorillonite 66.6 20.6 4.6 2.9
SiO, A1,Os Fe&, MgO
vc
Ve.c
66.6 51.6
66.6 46.6 4.8
4.8
-
-
A s expected with ion exchange in competition less aluminium is introduced. With the hypothesis of A1 belonging to Al,,0,0H,,(H,0).,7'
cations in
their individual form, 327 and 274 meq/100 g clay were respectively retained. This
discrepancy
with
the
cation
exchange
capacity
of
the
clay
(80-
110 meq/100 g ) is due to the oligomerisation of the All, cations a t pH=6. IR spectroscopy characterisations of pyridine adsorption reveal no change in the nature of the acid sites between Vc and Ve.c samples. At the opposite the total number of acid sites evaluated by the calorimetric adsorption of NH,
is
slightly higher in sample Vc as evidenced on Fig.1. This agrees with the generally observed increase of acidity with the quantity of aluminium intercalated and consequently the number of pillars. The most different behaviour is observed in the acid strength distribution as shown on Fig.2. 7 -
'2 0
E
2'
Y
2wL
C
0
&
Q L
0 VI
-0 0
100
L
0
L
0
0
5
1UI 200 300 N H3 o d s o r b e d (pmoles.cjl)
Fig. 1. Differential heats of adsorption versus coverage for the adsoprtion of NH, at 15OoC on samples (a)Vc and (0) Ve.c. Acid sites of intermediate strength (60 kJ/mole ) are obtained on both samples prepared by the conventionnal o r by the competition method. Moreover in this last case there is also an intense peak characteristic of strong acid sites with heats of adsorption around 120 kJ/mole.
349
Cationic exchange of the PILCs Up to 60 meq/100g of cations could be retained by the PILC sample Vc calcined at 5OO0C (Vc-500) after the alkaline treatment
100
0
200 0
lo0
200
H e a t OF adsorption(KJ.mole' )
Fig. 2. Distribution of acid strengths f o r samples Vc and Ve.c The I R spectra after pyridine adsorption of Vc-500 and of this sample respectively exchanged with rather quantities (42-47 meq/100 g) of divalent : Caz',
Zn2' (Vc(Ca-47)-500, Vc(Zn-44)-500) and trivalent: Ce"',
(Vc(Ce-47)-500,
Vc(La-47)-500) are reported
on Fig.3.
La"'
cations
The pyridine was
desorbed by steps every 100°C up to 400OC. The nature of the acid sites is influenced by the type of cations exchanged. Both Lewis and Bronsted acidities are detected with Caz+, La"' Ce"' as in Vc-500, but only
and
Lewis acidity with Zn"'.
The number of acid sites measured by thermodesorption of NH, is in all cases in the range of 1.7-2.4 pmole NH,/m2
and not correlated to the ionic
size o r charge of the cations. Cationic exchange induces greatest change on the acid sites strength. Ca"'
restores Bronsted acid sites still observable
after pyridine desorption at 2OO0C, higher than on Vc-500 o r Vc(La-47)-500 and Vc( Ce-47) -500. Whatever cation is exchanged, the intense peak a t 1450 cm-1 reveals that the acidity is mainly of the Lewis type in accordance to what is observed on the parent non exchanged PILC Vc-500 but comparatively a general decrease of the strength is noted. This trend is much less developped with Ca"'
where Lewis sites are still observable after desorption
at 4OO0C than with La"'
and Ce"'
where a residual shoulder only remains.
The decrease of total acidity is drastic with Zn"' cations.
350
a
Quo
r
-
1500 cr
1700
1500
l’
WAVENUMBERS
WAV ENWBERS
Fig. 3. I.R. spectra of pyridine after degassing a t increasing temperatures on samples : a : Vc-500 ; b : Vc(La-47)-500 ; c : Vc(Ce-47)-500 ; d : Vc(Ca-42)-500 and e : Vc(Zn-44)-500 Acidity of steamed Al-PILCs :
IR pyridine adsorption studies show that the AL-PILC sample Vc calcined a t 68OOC (Vc-680) has a strong Lewis acidity only (Fig.4).
On this
sample steamed at 55OOC (Fig.5) a strong Bronsted acidity is observed still detectable after evacuation at 480OC. Steaming at 65OoC reduces the strength of both Bronsted and Lewis acid sites,no more detectable after degassing at 3OOOC.
351
WAVEN UM BE R
Fig. 4 . I . R . spectra of pyridine on Vc-680 after degassing at increasing temperatures
1800
1600 1400
1800 1600 1400
WAVENUMBER
Fig. 5. I . R . spectra of pyridine on Vc-680 ; a : steamed at 50OOC ; b : steamed at 65OoC after degassing at increasing temperatures.
352
DISCUSSION Ion exchange is a fast process,
which can then be limited by diffusion.
Diffusional limitations in the course of the intercalation of the oligomeric aluminium species, responsible of the inhomogeneous distribution of the pillars, are greatly reduced by competitive ion exchange with NH,'.
A first conse-
quence reported in a precedent paper (11) is an enhancement of the thermal stability of the PILC. A second one is a higher acidity induced by isolated All, pillars, which contain a tetrahedral aluminium cation. This tendency is similar to that observed in zeolites where dealumination increases the acid strength by reducing the number of neighbour A1 atoms.
I t was reported elsewhere (12) that ion exchange increases the thermal stability of the PILCs. Removal of H' from the exchange sites not occupied by the pillars reduces the reactions of hydrolysis of the framework. Ce3' and La"'
are more efficient in this purpose
than
Ca2'
and Zn2'.
The
modifications of acidity are hardly correlated to those of thermal stability. Acid strengths are reduced by cationic exchange and some changes are observed in the nature of the sites but their number is not appreciably modified. These trends are in line with the hypothesis that acidity is mainly related to the aluminium sites of the pillars. restore a Bronsted acidity cations.
But a decrease of
,
Cationic exchange is able to
resulting then from the hydrolysis the Lewis acidity also happens
of the
due to the
destabilization of the alumina pillars by formation of spinel like phases. The reactions of hydrolysis taking place during steaming liberate protons a s evidenced by the appearance of a strong Bronsted acidity not observed on the sample just calcined. An other consequence is the decrease of the thermal stability due to the probable migration of the protons into the clay layer. The decrease of the total acidity after steaming is a general phenomenon also observed on commercial FCC catalysts, which need such treatment to moderate the catalytic activity. In the case of PILCs the increase of Bronsted acidity is perfectly related to the enhancement of cracking activity observed in microactivity tests ( 1 2 ) . It is interesting to notice that all three methods reported here to modify acidity also induce great changes in the thermal stability. Competitive ion exchange, used in the synthesis of the pillared clay, leads to a n important modification of the structure of the PILC. Ion exchange and steaming are "post synthesis" treatments which permit an adaptation of the acid strength distribution of the solid f o r a given catalytic application.
353 REFERENCES J . M . Adams, Applied Clay Science, 2 (1987) 309-342. M.L. Occelli and J.E. Lester, Ind. Eng. Chem. Prod. R e s . Dev., 24 (1985) 27-32. D . Tichit, F. Fajula, F. Figueras, J . Bousquet, and C . Gueguen, in t'Catalysis by Acids and Basest' ( B .Imelik, C. Naccache, G . Coudurier, Y . Ben Taarit and J . Vedrine, Eds), Elsevier, Stud. Surf. Sci. Catal. 20 (1985) 351. H . Ming-Yuan, L. Zhunghui and M. Enze, Catalysis Today, 2 (1988) 321-338. L. Zhon Ghui and S . Guida, in "Zeolites: Synthesis, Structure, Technology and Application" ( B . Drazj, S . Hocevar, and S . Pejovnik Eds), Elsevier, Stud.Surf.Sci.Cata1. 24 (1985) 493. D . Plee, F. Borg, L. Gatineau and J. J. Fripiat, J . Amer. Chem. Soc.,lO7 (1985) 2362-2369. D . Tichit, F. Fajula, F. Figueras, B . Ducourant, G . Mascherpa, C . Gueguen and J . Bousquet, Clays Clay Miner. ,36 (1988) 369-375. J . D . Russell and R . A . Frazer, Clays Clay Miner., 13 (1971) 55. S. Yariv and L . Heller-Kallai, Clays Clay Miner., 21 (1973) 199. D.E.W. Vaughan, R. J . Lussier and J.S. Magee, U.S.Patent 4.271.043 (1981). F. Figueras, Z. Klapyta, P. Massiani, Z . Mountassir, D. Tichit, F. Fajula, C . Gueguen, J. Bousquet and A . Auroux, Clays Clay Miner., 38 (1990) 257-264. D. Tichit, F. Fajula, F. Figueras, C . Gueguen and J. Bousquet,in "Proc. 9th Int. Cong. Catal.", Calgary 1988, vol 1 Chemical Institute Canada (1988), p 112.
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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
355
THE CHEMISTRY OF THE DEALUMINATION OF FAUJASITE ZEOLITES WITH S I L I C O N TETRACHLORIDE Johan A. MARTENS, P i e t
J. GROBET and Peter A. JACOBS
Department o f Surface Chemistry, K.U. Leuven, Kardinaal M e r c i e r l a a n 92, 6-3030 Heverlee, Belgium
ABSTRACT L i t e r a t u r e d a t a on t h e dealumination w i t h Sic14 o f X and Y t y p e z e o l i t e s w i t h Na, L i , H, Ca o r La as charge compensating c a t i o n s i s reviewed. The p r o p e r t i e s o f t h e f i n a l z e o l i t e products, i n p a r t i c u l a r t h e degree o f dealumination, t h e c r y s t a l 1 i n i t y , mesoporosity, and t h e n a t u r e and content o f extra-framework aluminium and s i l i c o n species are e x p l a i n e d based on t h e chemistry i n v o l v e d i n t h e d i f f e r e n t steps o f t h e dealumination procedures. The r e a c t i v i t y towards Sic14 o f t h e framework aluminium atoms i s e x p l a i n e d based on t h e l o c a t i o n o f t h e associated charge compensating c a t i o n s over accessible and hidden s i t e s . INTRODUCTION The dealumination o f z e o l i t e Y has been s t u d i e d i n t e n s i v e l y d u r i n g t h e past mainly f o r t h e purpose o f developing new FCC c a t a l y s t s .
two decades,
The
changes i n a c i d i t y upon dealumination o f z e o l i t e Y samples has r e c e i v e d much attention
from
consequences particular,
(AIF)
of
the the
theoretical depletion
point of
of
the
view
(refs.1-11).
framework
with
The
catalytic
aluminium
and,
in
t h e changes i n product s e l e c t i v i t y w i t h t h e framework aluminium
and extra-framework aluminium (AIEF) content o f t h e z e o l i t e are extremely
important i n FCC. Reviews on t h i s s u b j e c t are a v a i l a b l e ( r e f s . 1 2 - 1 4 ) . Many dealumination reagents can be used i n c l u d i n g steam (deep bed c a l c i n a t i o n , self-steaming and steaming), aluminium sequestering agents (EDTA, ACAC, ...), mineral acids and s i l i c o n c o n t a i n i n g reagents ((NH4)2SiF6,
SiC14).
The removal o f framework aluminium d u r i n g dealumination o f t e n c o i n c i d e s w i t h a number o f secondary e f f e c t s . Hydrolyzed extra-framework A1 -0 species (AlEF) may
be formed, consists annealing
as w e l l as e x t r a - l a t t i c e Si-A1 oxide species. in of
the
generation
them w i t h
of
lattice
amorphous
defects
silica.
(hydroxyl
increased h y d r o p h o b i c i t y and speaking,
increased thermal
upon dealumination
the
nests)
Upon dealumination,
l a t t i c e c o n t r a c t s w h i l e some times mesopores are formed. Generally
Another phenomenon the
zeolite
The z e o l i t e shows an
and hydrothermal
number o f
and
the
stability.
Brensted a c i d
sites
decreases upon dealumination, w h i l e t h e average s t r e n g t h p e r s i t e increases.
356
This review focusses on the preparational aspects of dealuminated Y zeolites and will not treat the catalytic implications of the modification. The dealumination method using Sic14 was selected because with this reagent, in principle, Si atoms should be supplied rapidly and be easily substituted in the lattice, thus avoiding defect and mesopore formation. Perfect siliceous and low-alumina faujasites are expected to be formed. An attempt is made to rationalize the 1 iterature data on the dealumination methods using SiC14, to compare methods and product properties and to treat the chemistry involved in the different steps of the dealumination processes. CRYSTAL CHEMISTRY OF FAUJASITE ZEOLITES Silicon en aluminium local environments Although all tetrahedral sites (T sites) in the faujasite framework are topologically equivalent, in real zeolite Y crystals this equivalency applies only in exceptional cases, e.g. when all T sites are occupied by silicon atoms. According to the Lowenstein rule (ref.15) the first shell of T sites surrounding an aluminium atom is occupied by silicon atoms only. Differences in the local environment of aluminium atoms are, therefore, only found in the second shell of surrounding T atoms, which may contain silicon as well as aluminium atoms. A T atom in the faujasite structure has nine next nearest neighbor (NNN) T atoms. When considering the loop configuration of T sites (Fig.1) these nine NNN can be subdivided in three NNN atoms in adjacent fourrings, four NNN atoms involved in adjacent six-ring loops, and two NNN atoms in a twelve-ring loop. Minimum approach and maximum electrostatic interaction is achieved by siting the aluminium atoms diagonally across the four-rings of T sites. In principle, four types of aluminium atoms can be distinguished on this basis, having respectively 3 , 2, 1 or 0 aluminium atoms diagonally opposed to it. Aluminium atoms are expected to be more electrostatically vulnerable and more susceptible to dealumination when a higher number of diagonally opposed aluminium atoms occurs (ref.2). A further subdivision is possible according to the number of NNN aluminium atoms in the six-ring loops and the twelve-ring loop, where up to six aluminiums can be located. The latter aluminium atoms are at a longer distance from the central aluminium atom and the electrostatic interaction is weaker (Fig.1). In conclusion, although all aluminium (and silicon) atoms in zeolite Y crystals are equally accessible through the zeolite cavities, they are chemically not identical, and their reactivity towards dealumination reagents could be different. Unfortunately, no methods are actually available for determining directly the T atom environment in terms of NNN of an aluminium atom. Indeed, the different aluminium T sites are even not resolved in the 27Al resonance envelope, measured with the high-resolution solid state magic angle
357
spinning nuclear magnetic resonance (MASNMR) technique.
F i g u r e 1. S i t i n g o f t h e n i n e n e x t nearest neighbors (NNN) o f a T s i t e i n faujasite.
Figure 2 . S i t i n g o f charge-compensating cations i n faujasite.
29Si MASNMR spectra o f z e o l i t e X and Y samples e x h i b i t a s p l i t t i n g i n t o f i v e types o f s i l i c o n atoms according t o t h e number, n, o f aluminium atoms i n the
neighboring
T
sites
(ref.16).
Each
Si(nA1)
signal
represents
many
components w i t h d i f f e r e n t l o c a l environments due t o t h e many l o c a l geometries o f t h e f i r s t and second s h e l l A1 neighbors ( r e f s . 17-19). A t present, i t i s n o t p o s s i b l e t o r e s o l v e these components i n d i v i d u a l l y . Nevertheless, a 29Si MASNMR spectrum as a means t o determine t h e o v e r a l l framework composition has proven t o be very u s e f u l . From t h e 29Si MASNMR measurements, attempts have been made t o deduce complete z e o l i t e s t r u c t u r e s
(ref.20-24).
I n structures w i t h high
aluminium content, t h e aluminium d i s t r i b u t i o n i s n o t random. For most c r y s t a l compositions, however, more than one S i ,A1 o r d e r i n g scheme i s compatible w i t h t h e 29Si MASNMR r e s u l t s , e s p e c i a l l y f o r S i / A l r a t i o s r a n g i n g f r o m 2.4 t o 5. For higher Si/A1
r a t i o s t h e d i s t r i b u t i o n of
aluminium seems t o be disordered,
although considerable s c a t t e r o f t h e experimental d i s t r i b u t i o n s (ref.11).
i s observed
I t should be stressed t h a t t h e NMR technique p r o v i d e s an average
measurement and t h a t i n case o f compositional inhomogeneity, f r e q u e n t l y encountered i n dealuminated samples, t h e NMR r e s u l t s even can be misleading.
358
S i t i n q o f charqe comDensatinq c a t i o n s Aluminium atoms i n an a l u m i n o s i l i c a t e z e o l i t e i n t r o d u c e a n e t n e g a t i v e l a t t i c e charge which has t o be n e u t r a l i z e d w i t h charge compensating c a t i o n s . The c a t i o n s are d i s t r i b u t e d over t h e hexagonal supercages (Fig.2).
prisms,
s o d a l i t e cages and
F o l l o w i n g t h e c l a s s i c a l nomenclature ( r e f . 2 5 ) ,
s i t u a t e d i n t h e c e n t r e o f t h e hexagonal
Site I i s
Cations a t S i t e I are co-
prism.
o r d i n a t e d t o s i x 03 oxygens. S i t e I‘ i s l o c a t e d i n t h e s o d a l i t e cage adjacent t o t h e c e n t e r o f a s i x - r i n g belonging t o t h e hexagonal p r i s m i n c l o s e contact w i t h t h r e e 03 oxygen atoms. S i t e I 1 i s l o c a t e d i n t h e supercage adjacent t o t h e center o f a six-ring.
S i t e 11’ i s l i n k e d t o t h e same s i x - r i n g from i n s i d e t h e
s o d a l i t e cage. Cations i n s i t e I 1 and 11’ are c o - o r d i n a t e d t o 02 oxygens. S i t e s
I11 and 111‘ a r e supercage s i t e s a t t h e f o u r - r i n g s , l i n k e d t o oxygens o f type
01 and 04. Cations a t S i t e I and I ‘ are accessible o n l y through s i x - r i n g windows and can be considered t o be hidden f o r guest molecules t h a t can n o t overcome the s i x - r i n g d i f f u s i o n a l b a r r i e r . The c a t i o n s a t t h e o t h e r s i t e s a r e accessible through t h e supercages.
It w i l l be i l l u s t r a t e d t h a t t h e a c c e s s i b i l i t y o f t h e
charge compensating c a t i o n s r a t h e r than t h e NNN c o n f i g u r a t i o n s o f aluminium atoms p l a y a c r u c i a l r o l e i n t h e dealumination process w i t h SiC14. Depending on their
n a t u r e and s i t i n g ,
t h e charge-compensating
cations
may
’prevent’
or
‘ f a c i l i t a t e ’ t h e d i s l o d g i n g o f aluminium atoms from l a t t i c e p o s i t i o n s . DEALUMINATION OF FAUJASITE ZEOLITES WITH Sic14 The dealumination method using Sic14 was i n t r o d u c e d i n 1980 by Beyer and I t c o n s i s t s e s s e n t i a l l y o f passing s i l i c o n t e t r a c h l o r i d e
Belenykaja ( r e f . 2 6 ) .
vapor through a bed o f anhydrous z e o l i t e a t e l e v a t e d temperature. The r e a c t i o n w i t h Sic14 o f a z e o l i t e w i t h charge-compensating c a t i o n s o f t y p e M w i t h charge n f o r m a l l y has t h e f o l l o w i n g stoicheometry ( r e f . 2 6 ) :
Mi/,,
[A102.(Si02)x]
+
Sic14 -->
l / n MC1,
+
AlCl3
+
[(Si02)x+1]
Due t o t h e s t r o n g e x o t h e r m i c i t y o f Reaction ( l ) , t h e dealumination w i t h Sic14 vapor has t o occur under h i g h l y c o n t r o l l e d c o n d i t i o n s i n o r d e r t o o b t a i n h i g h l y c r y s t a l l i n e dealuminated Y z e o l i t e s
i n a r e p r o d u c i b l e way
(ref.27).
A f t e r c o n t a c t w i t h SiClq, t h e r e a c t o r should be f l u s h e d w i t h i n e r t gas, and t h e z e o l i t e washed w i t h deionised water. Mineral a c i d i t y develops i n t h e wash water due t o t h e h y d r o l y s i s o f AlCl3 and/or AlCl4- complexes present i n t h e z e o l i t e sample ( r e f . 26). The methods described i n l i t e r a t u r e can be subdivided i n two c a t e g o r i e s , v i z . isothermal r e a c t i o n s and temperature-programmed r e a c t i o n s , denoted f u r t h e r as methods A and B, r e s p e c t i v e l y .
359
Method A: Isothermal dealumination with Sic14 In the isothermal dealumination, zeolite Y is contacted with Sic14 vapor at a reaction temperature (TR) for a given period o f time (tR), after which the reactor is flushed at a post-treatment temperature (Tp) for a given posttreatment time (tp). Table 1 provides an overview of the literature on zeolite Y samples dealuminated by method A. The reaction between zeolite Y and Sic14 is so strongly exothermic that it is often impossible to keep the reaction temperature constant when the sample is first contacted with Sic14 (refs.2628). A pronounced temporary temperature rise, ATR, is observed in the zeolite bed (Table 1). A variety of starting materials and reaction conditions have been used, resulting in materials with a broad variation in framework A1 content.
Table 1. Dealumination of Y zeolites with Sic14 according to method A No.
a, AlF/UC in brackets; b, number of framework aluminium atoms per unit cell; c, between 60 and 120 minutes; d, after acid leaching; e, Nay, for 68% exchanged with Li+; f, containing 2% NapO; g, Nay, for 90% exchanged with NH4+ and deammoniated at 673 K .
360
Two-step dealumination mechanism. A study o f t h e r e a c t i o n o f LiY w i t h Sic14 a t a r e a c t i o n temperature o f 423 K has allowed t o d i s t i n g u i s h between two r e a c t i o n steps (ref.29). one S i
- C1
A f t e r t h e exotherm has passed through t h e z e o l i t e bed,
bond i n Sic14 i s broken, and a new S i
-
0 bond i s formed t o g e t h e r
w i t h t h e c h l o r i d e s a l t o f t h e corresponding charge-compensating c a t i o n :
c1
\ /
c1 c1 c1 c1
Si
/ \
c1
c1
\ /
Mi
\I/ Si I
->
0-
\ /
\ /
/ \
/ \
A1
Si
/ \
MC1
0.
Si
.
/
-A1
/ \
Reaction ( 2 ) was i n f e r r e d from t h e observation o f a 29Si MASNMR s i g n a l a t a chemical
s h i f t o f ca.
(ref.29).
On dehydrated Nay, contacted w i t h Sic14 a t a temperature o f 323 K , a
-45 ppm,
ascribed t o framework-bound
'SiC1-j'
species
29Si MASNMR s i g n a l a t -45 ppm i s observed next t o a resonance o f adsorbed Sic14 a t -20 ppm, and t h e s i g n a l s o f t h e framework s i l i c o n atoms r a n g i n g from -80 t o -120 ppm (Fig.3A).
zeolites,
The 'SiC13'
species are observed i n HY, LiY as w e l l as NaY
t r e a t e d w i t h Sic14 a t temperatures
below 423 K ( r e f . 3 0 ) .
A t this
stage o f t h e r e a c t i o n o f Sic14 w i t h NaY and LiY, a l l aluminium i s s t i l l present in
tetrahedral
environment
(Fig.3B,
s u b s t a n t i a l l y broadened (Fig.3B).
ref.29).
The
27Al
resonance
is
T h i s c o u l d be p a r t i a l l y due t o t h e dehydrated
s t a t e o f t h e sample, b u t a l s o t o t h e d i s t o r t i o n o f t h e aluminium tetrahedron i n t h e presence o f t h e Sic13 species. A f t e r c o n t a c t o f z e o l i t e HY w i t h Sic14 a t 423 K , t h e 27Al MASNMR resonance o f an important f r a c t i o n o f t h e aluminium i s s h i f t e d t o 35 ppm (Fig.4). A chemical s h i f t o f 35 ppm o f t h e 2 7 A l nucleus i n dealuminated (ref.31)
faujasites
has
been
ascribed
to
pentaco-ordinated
o r d i s t o r t e d t e t r a h e d r a l l y c o - o r d i n a t e d aluminium ( r e f . 3 2 ) .
aluminium In this
instance, we are i n c l i n e d t o assign t h e 35 ppm s i g n a l s t o d i s t o r t e d aluminium tetrahedra.
Indeed,
upon
hydration,
the
Sic13
species
are
e s s e n t i a l l y non-dealuminated Y z e o l i t e s are recovered ( r e f . 3 0 ) .
hydrolysed and
361
B
F i g u r e 3. 29Si MASNMR spectrum (A) and 27Al MASNMR spectrum ( B ) c o n t a c t e d w i t h S i c 1 4 vapor a t a t e m p e r a t u r e o f 323 K. ATR was 40 K.
o f Nay,
A
\\k.,
--
J/
zF ei goul ir tee , 4.c o n27Al t a c t e dMASNMR w i t h Sspectrum i c 1 4 vaporo fa t HYa t e m p e r a t u r e o f 423 K. ATR was 38 K.
1 . . . . 1 . . . . 1 . . . . 1 . . . ,
t
aa
FFM
a
Upon h e a t i n g t h e z e o l i t e t o temperatures exceeding 423 K t h e s i l i c o n atoms o f the
Sic13
species
are
inserted
i n t h e framework
after
removal
o f the
aluminium atoms ( r e f s . 2 9 , 3 0 ) : c1 c1 c1
\I/
Si
I
/ \
MC1
/ \
A1C13
/ \
MC1
/ \
AlC13 r e a c t s w i t h t h e c h l o r i d e s a l t o f t h e a l k a l i m e t a l s and forms t h e corresponding
a1 k a l i metal
tetrachloro
aluminate
complexes
( r e f .26).
aluminium atoms o f t h e s e complexes e x h i b i t a chemical s h i f t o f ca. (refs.29,33)
The
100 ppm
w i t h r e s p e c t t o aqueous sodium a l u m i n a t e . HC1 and AlC13 a r e formed
d u r i n g d e a l u m i n a t i o n o f HY z e o l i t e s ( r e f . 2 8 ) . When NaY
i s c o n t a c t e d w i t h Sic14 a t TR
=
423 K,
t h e overheating
i m p o r t a n t and R e a c t i o n s (2) and (3) o c c u r s i m u l t a n e o u s l y ( r e f . 3 0 ) .
Indeed,
is in
362
such sample a weak 29Si MASNMR signal at -45 ppm is present together with a 27Al MASNMR resonance at ca. 100 ppm (Fig.5).
A
B
Figure 5. 29Si MASNMR spectrum (A) and z7Al MASNMR spectrum (B) of Nay, contacted with Sic14 vapor at a temperature of 423 K . ATR was 67 K. Oriqin of reaction inhibition at moderate TR - temperatures. There is evidence that for TR < 523 K , a reaction front runs through the zeolite bed. Indeed, when during an experiment with NaY at 523 K , the Sic14 feed is interrupted before this front (or the reaction exotherm) has reached the exit of the reactor, a mixture of the original and the dealuminated zeolite is obtained (refs.27,28). When the reaction zone has passed once through the zeolite bed at a given TR, the zeolite has become unreactive towards Sic14 at this temperature, although the dealumination is mostly incomplete. An amount of 22 AlF/UC remains in NaY zeolite at a reaction temperature of 523 K and reaction times of 35, 90 and 240 minutes, respectively (Table 1, Nos. 3, 4 and 6). Prolongation of the contact time with Sic14 at a temperature of 523 K apparently does not further increase the degree of dealumination. A similar observation was made with LiY, for which at a TR of 523 K the degree of dealumination was not altered upon increasing the reaction time from 35 to 90 minutes (Table 1, Nos. 18 and 19). In another experiment (ref.301, LiY zeolite was contacted with Sic14 at TR = 423 K, post-treated at Tp = 763 K, and subsequently contacted a second time with Sic14 vapor at 423 K. During the second contact no exothermicitv was observed. The zeolite was purged with inert gas at a Tp of 623 K . The final zeolite Y product contained 27 aluminium atoms per unit cell, which is the same value as that obtained after the first treatment with Sic14 (Table 1, No.16). With HY zeolites 22 Al/UC are removed at a temperature of 473 K (Table 1, No. 20). The degree of dealumination seems to remain quite constant up to a TR value o f 623 K (Table 1, Nos.21 and 22).
363
The participation of the charge compensating cations in the reaction of zeolite Y with Sic14 appears in Reaction (2). The cation distributions can offer a satisfactory explanation for the observed 1 imits to the dealumination process at moderate reaction temperatures (ref.28). The charge compensating cations are distributed over cation sites in supercages, sodalite cages and hexagonal prisms (Fig.2). Given the diameter of SiC14, which is 0.687 nm (ref.38), the molecule has access to the supercages only. About 3 0 to 33 sodium cations are located in the supercages of NaY zeolites (refs.40-42). The number of aluminium atoms that can be extracted at TR temperatures below 523 K seems to be restricted to these numbers (Table 1, Nos.1-6). For Li-exchanged NaY zeolites a similar distribution of the cations over accessible and hidden positions can be expected. The upper amount of ca. 33 aluminium atoms that can be removed at 523 K (Table 1, Nos.16-19) could correspond to the number of supercage Li and Na cations. For HY zeolites the availability of hidden protons can be derived from the infrared spectra in the OH reagion. Based on these infrared spectra, the ratio of the intensities of the high frequency to the low frequency OH band and, consequently, the proportion of accessible to hidden protons is ca. 0.6 (ref.43) and the lattice charge associated with 21 aluminium atoms should be neutral ised by accessible protons. This number corresponds to the number of aluminium atoms removed from the lattice at reaction temperatures between 423 and 623 K (Table 1, Nos.20-22). At moderate reaction temperatures, the number of aluminium atoms that can be replaced with silicon seems to be limited by the number of accessible charge-compensating cations. Consequently, the preparation of samples with aluminium contents, equal to the number of hidden charge-compensating cations should be highly reproducible. Apparently, the nature and consequently the distribution of the charge-compensating cations can be handled as an instument to tune the degree of dealumination. The dealumination with Sic14 at moderate temperatures offers the advantage that, in principle, very homogeneously dealuminated samples with well-defined AlF/UC contents can be obtained, provided high overheating temperatures are avoided. Dealumination at hiqh TR - temeratures. At reaction temperatures between 573 and 773 K the behaviour of NaY is different. Under such conditions a wide variety of AlF/UC values were obtained (Table 2, Nos. 7-15 and 23). At high reaction temperatures, there does not seem to exist a limitation to the degree of dealumination. At temperatures exceeding 523 K , the less accessible sodium cations and associated framework aluminium atoms probably become reactive towards Sic14 possibly as a result of cation mobility and redistribution among hidden and accessible sites. Temperatures higher than 623 K seem to be necessary to activate aluminium atoms associated with hidden protons (Table 1, Nos.22-23).
364
There e x i s t s , (ref.27).
however,
an upper l i m i t t o t h e dealumination temperature
Beyer e t a l . r e p o r t e d t h a t t h e c o n t a c t o f dehydrated NaY w i t h Sic14
a t temperatures over 750 K produces a v i o l e n t exothermic r e a c t i o n r e s u l t i n g i n t h e f o r m a t i o n o f an amorphous product ( r e f . 2 7 ) . al.
On t h e c o n t r a r y , Klinowski e t
seemed t o succeed i n t h e p r e p a r a t i o n o f an ’ e s s e n t i a l l y
faujasite structure’ (ref.44).
However,
aluminium-free
from z e o l i t e Y by r e a c t i o n w i t h Sic14 vapor a t 833 K
i t i s n o t c l e a r whether temperature programming o r n o t was
used i n t h e l a t t e r case. Method 8: TemDerature Droqrammed dealumination w i t h Sic14 I n t h e work o f Beyer and Belenykaja ( r e f . 2 6 ) , t h e c o n t a c t o f t h e z e o l i t e w i t h Sic14 vapor was s t a r t e d a t a f i r s t r e a c t i o n temperature ( T R ~ ) , and t h e temperature was
gradually
increased
at
a
rate
(r) t o a f i n a l
reaction
temperature ( T R ~ ) . I n t h e o r i g i n a l method T R ~was 650 K, T R ~ was v a r i e d from 730 K t o 830 K and r was 4 K m i n - 1 ( r e f . 2 6 ) .
NaY z e o l i t e s were thus dealuminated t o c o n t a i n
between 4 and 9 A1 atoms p e r u n i t c e l l .
I n another study ( r e f . 2 7 ) ,
T R ~was
about 520 K. A temporary temperature r i s e between 30 and 70 K occurred d u r i n g t h e f i r s t minutes o f t h e c o n t a c t o f NaY z e o l i t e w i t h Sic14 vapor. Only when t h e r e a c t o r temperature had reached i t s o r i g i n a l value o f 520 K,
t h e z e o l i t e bed
was heated a t a r a t e o f 10 K m i n - 1 t o T R ~values between 600 K and 745 K. This way samples w i t h A1 contents between 16 and 2 A l / U C were produced (Table 2, Nos. 24-29).
I n t h e same work (ref.27)
s t r u c t u r a l damage t o t h e z e o l i t e , when t h e e x o t h e r m i c i t y i s over. earlier, with
i t was s t a t e d t h a t i n order t o avoid
t h e h e a t i n g t o T R ~should o n l y be s t a r t e d I n view o f t h e r e a c t i o n mechanism advanced
t h i s means t h a t a simultaneous a t t a c k o f aluminium atoms associated
hidden
and
accessible
cations
causes
a
too
violent
reaction
and,
consequently, l a t t i c e d e s t r u c t i o n and has t o be avoided. Table 2 shows t h a t t h e degree o f dealumination depends mainly on T R ~ . I t seems very d i f f i c u l t t o remove t h e l a s t framework aluminium atoms from NaY and consequently i t has been suggested t h a t t h e dealumination o f NaY w i t h Sic14 i s a p r o d u c t - i n h i b i t e d r e a c t i o n (refs.27,45,51). precipitation o f
sodium t e t r a c h l o r o
pores
the
terminates
progression
of
According t o t h i s hypothesis, t h e
aluminate complexes the
dealumination
inside the z e o l i t e reaction
since
it
prevents Sic14 from f u r t h e r d i f f u s i n g i n t o t h e z e o l i t e c a v i t i e s . The decomposition temperature o f NaAlC14 i n z e o l i t e Y i s estimated t o occur a t ca. 780 K ( r e f . 2 7 ) .
The use o f LiY z e o l i t e s o f f e r s t h e advantage t h a t t h e LiAlC14
complexes v o l a t i z e and/or decompose already a t a temperature o f 733 K ( r e f . 4 5 ) . Complete dealumination o f L i Y can be achieved (Table 2, No.33) under c o n d i t i o n s where 3 AlF/UC remain i n NaY (Table 2, No.29). A z e o l i t e Y product w i t h v i r t u a l l y no aluminium atoms l e f t i n t h e framework was obtained from NaY a t 833
365
K (ref.44).
Attempts to force the dealumination to completion resulted in amorphous products (ref.44).
Table 2. Zeolite Y products obtained from temperature programmed reaction with Sic14 (method B ) No. Starting material
a, no heating till end of exothermicity; b, Nay, exhanged for 9% with protons and 65% with Li+; c, Nay, exchanged for 62% with Li+; d, Nay, 62% exchanged with NH4+, and deammoniated at 650 K; e , containing 2% of Na20; f, Nay, fully exchanged with NH4+ and deammoniated. The results on HY zeolites, in which the deposition of chloro aluminium complexes doesnot occur, show that in this zeolite also the last aluminium atoms resist to dealumination (Table 2, Nos.39-42). A satisfactory explanation why in HY zeolites a few aluminium atoms per unit cell remain unreactive towards Sic14 is lacking for the moment. Dealumination of X-tvoe zeolites with Sic14 In a NaX zeolite sample with for instance 85 AlF/UC, about 40 sodium cations are located in accessible sites (ref.41). If the aluminium substitution is limited to 40 Al/UC, a zeolite with 45 AlF/UC would result. It is evident that such a faujasite-type zeolite will be destroyed by the strong acidity developed during the water washing and the hydrolysis of 40 NaAlC14 molecules per unit cell, not mentioning the hydrolysis of occluded SiC14.
366 Beyer e t a l . found t h a t NaX r e a c t s w i t h Sic14 a t a temperature o f 480 K (ref.27). cell,
The Sic14 uptake agreed w i t h a removal o f 38 aluminium atoms p e r u n i t
which corresponds r o u g h l y t o t h e number o f accessible sodium c a t i o n s .
Sulikowski e t a l . used t h e temperature programmed method t o dealuminate LiX and Lax samples w i t h Sic14 (Table 2). A t T R ~values o f 473 K and 573 K, 44 AlF/UC were found i n t h e product. This corresponds t o a removal o f 33 Al/UC.
Only a t
T R ~temperatures h i g h e r than 673 K, t h e dealumination proceeded f u r t h e r (Table
3, No.&),
as c o u l d have been predicted. Under i d e n t i c a l r e a c t i o n c o n d i t i o n s , a
Lax sample i s l e s s dealuminated compared t o a L i X sample (Table 3, Nos.46 and 47).
The amount o f accessible charge-compensation c a t i o n s i n Lax i s indeed
lower than i n LiX. I n a l l instances (Table 3), products w i t h poor c r y s t a l l i n i t y were obtained from z e o l i t e X (ref.27,
46).
Table 3. Dealumination w i t h Sic14 o f X z e o l i t e s No.
Starting m a t e r i a1
T i (
j
TR (K!
43
NaX(85)
480
-
44 45 46
LiX(79)a LiX(79)a LiX(79)a
423 423 423
473 573 673
240 240 240
473 573 673
47
LaX(79)
423
673
240
673
t.82 Tp tp (min.) (K) (min.)
AlF/UC
Ref.
47
27
120 120 120
44 44 31
46 46 46
120
44
46
The f a i l u r e t o prepare h i g h l y c r y s t a l l i n e dealuminated f a u j a s i t e s from X z e o l i t e s has l e d t o speculations
i n literature.
I n t h e work o f Beyer and
it was supposed t h a t t h e dealumination o f NaX f a i l s because t h e z e o l i t e framework i s s h i e l d e d from t h e a t t a c k o f Sic14 by t h e presence o f a h i g h c o n c e n t r a t i o n o f ' l a t t i c e ' c a t i o n s , by which probably charge-compensating c a t i o n s were meant ( r e f . 2 7 ) . Sulikowski and K l i n o w s k i suggested t h a t t h e s t r u c t u r a l damage i s t h e r e s u l t o f t h e simultaneous removal o f t h r e e aluminium Belenykaja,
atoms from t r i p l y occupied six-membered r i n g s (ref.46). of
X
zeolites
fits
perfectly
into
the
However, t h e behavior
p i c t u r e developed
higher
for
the
dealumination o f z e o l i t e Y samples w i t h SiC14. E l i m i n a t i o n o f extra-framework aluminium A f t e r t h e isomorphic s u b s t i t u t i o n (Reaction 3), under t h e form o f compensating
t h e aluminium i s present
H+ AlCl4- complexes, depending on t h e n a t u r e o f t h e charge
cations,
and t h e
temperature.
There
are,
in
principle,
two
a l t e r n a t i v e methods t o remove t h e aluminium from t h e z e o l i t e c a v i t i e s , v i z . (1)
367
decomposition of the tetrachloro-aluminium complexes (in case of LiY and Nay) and desorption of AlCl3 and HC1 (in case of HY), or (2) hydrolysis of AlC13 and the tetrachloro-aluminium complexes and leaching of the dissolved aluminium from the zeolite pores. DecomDosition of tetrachloro-aluminium ComDlexes in inert atmosohere. Thermal decomposition of the a1 kal i metal tetrachloro-aluminium complexes and subsequent desorption of AlCl3 should lead to a zeolite sample with only metal chloride salt deposited in the pores. The latter could be removed by washing with water. Starting from HY zeolite, the washing operation would even not be necessary. In practice, however, the situation is more complex. The thermostability in inert atmosphere of NaY is much reduced after the treatment with Sic14 (ref.27). Structural collapse starts already at a temperature of 770 K (ref.27). This degradation of the framework was interpreted as a reaction of the zeolite framework with AlCl3 coming from partially dissociated tetrachloro aluminate complexes (ref.27). The 27Al resonance at 100 ppm in NaY samples disappears during post-treatments at 823 K and 923 K (ref. 28) but the amount of aluminium that desorbs from the zeolite bed at these temperatures is always low (ref.28). White fumes typical of AlCl3 vapor are not observed at the outlet of the dealumination reactor (ref.29). Heating of SiClq-treated NaY samples in inert atmosphere causes the formation of mesopores (ref.28). This mesopore formation is probably due to a more gentle attack of the framework by AlCl3 and/or tetrachloro aluminate complexes and can be considered to be an onset to amorphisation. It can be concluded that thermal elimination of the dislodged aluminium does not seem to be a successful approach with Nay. The higher volatility and the lower stability of lithium compared to sodium tetrachloro-aluminium complexes is reflected in the formation o f mesopores and amorphisation (ref.28). The mesopore volume in LiY zeolites is systematically higher than in NaY zeolites, post-treated at the same temperature (Fig.6). Post-treatment temperatures of 823 K can be applied to LiY without important damage to the zeolite lattice when a TR of 423 K is used (ref.28). LiY zeolites reacted with Sic14 at 523 K and 623 K become partially amorphous during a posttreatment at 823 K , while NaY retains a better crystallinity (ref.28). The decomposition of tetrachloro aluminate complexes in inert atmosphere is harmful t o the microporosity and crystallinity of LiY zeolites. The mesopore volume of HY zeolites is situated between that o f NaY and LiY zeolites treated at the same temperatures (Fig.6). No further data are actually available on the thermostability of SiClq-treated HY zeolites.
368
d -
I
mM1
85
B
75
W
i 3 *
d
0
0
ec
65 55
45 35
o
25
20
15 600
PI
a
TR-
5 23 K
700
900
800
1000
TP (K) Figure 6. I n f l u e n c e o f t h e post-treatment temperature on t h e formation o f mesopores i n Nay, HY and LiY z e o l i t e s contacted w i t h Sic14 a t a TR o f 423 K, 523 K and 623 K. The mesopore volume i s determined according t o r e f . 2 8 . DecomDosition o f t e t r a c h l o r o - a l u m i n i u m comolexes i n t h e Dresence o f SiC14. The f i n a l temperature o f t h e Sic14 treatment used i n dealumination method-B sometimes
exceeds
the
decomposition
temperature
of
the
chloro
aluminate
complexes (Table 2 ) . I n such instances, AlC13 vapors a r e observed a t t h e o u t l e t o f t h e dealumination r e a c t o r . A ' t h i c k w h i t e fume o f A l C l 3 ' was observed when L i Y was t r e a t e d a t a T R ~value o f 733
K (ref.45).
The z e o l i t e r e t a i n e d f u l l
c r y s t a l l i n i t y . W i t h i n t h e d e t e c t i o n l i m i t s o f t h e 29Si MASNMR technique, s u b s t i t u t i o n o f s i l i c o n f o r aluminium was complete,
the
b u t even a f t e r washing a
s u b s t a n t i a l amount o f extra-framework aluminium remained i n t h e sample (Table
2, No.33). A w h i t e vapor o f AlCl3 escaped from a NaY sample d u r i n g r e a c t i o n w i t h S i c 1 4 a t 833 K ( r e f . 4 4 ) . The f i n a l product was c r y s t a l l i n e and t h e z e o l i t e framework deeply dealuminated.
The presence o f extra-framework aluminium i n t h e washed
product was detected w i t h 27Al MASNMR ( r e f . 4 4 ) . Aparently,
t h e d e s o r p t i o n o f AlC13
i s f a c i l i t a t e d and t h e d e t r i m e n t a l
e f f e c t o f AlCl3 i s n e u t r a l i s e d i n presence o f SiC14.
HY z e o l i t e samples stand very h i g h r e a c t i o n temperatures o f up t o 1023 K i n Even i n these instances, an important f r a c t i o n o f t h e dislodged aluminium i s n o t evacuated from t h e sample (Table 2, No.42).
presence o f Sic14 (Table 2 ) .
369
E l i m i n a t i o n o f dislodqed aluminium bv water washinq. I n p r e v i o u s s e c t i o n i t was explained t h a t i t i s very d i f f i c u l t , i f n o t impossible, t o desorb t h e r m a l l y a l l aluminium c h l o r i d e species from t h e z e o l i t e pores. I n t h e washing step, t h e A1 - C1 bonds are hydrolysed and s t r o n g mineral a c i d i t y develops i n t h e z e o l i t e pores.
Hydroxide complexes
of
aluminium
are
formed
and
transferred
into
s o l u t i o n , depending on t h e a c i d i t y o f t h e suspension. P e r t i n e n t d a t a from t h e l i t e r a t u r e on t h e amount o f aluminium evacuated d u r i n g t h e washing o f SiC14dealuminated samples are shown i n Fig.7.
The amount o f aluminium t h a t was
leached from t h e samples was c a l c u l a t e d from t h e o r i g i n a l and f i n a l aluminium content o f t h e samples, n e g l e c t i n g aluminium losses d u r i n g r e a c t i o n w i t h Sic14 and
eventual
post-treatments.
In
each
series
of
samples,
the
amount
of
aluminium t h a t i s removed increases w i t h i n c r e a s i n g degree o f dealumination (Fig.7).
Important
differences
are
found
between t h e
different
series o f
samples (Fig.7).
50
40
30 20 10 0 ' 0
20
10
30
40
AIP / UC Figure 7. Number o f aluminium atoms leached per u n i t c e l l versus AlF/UC. A, HY, using TR values between 473 K and 843 K ( r e f . 3 7 ) ; B, Nay, samples Nos.24-27 o f Table 2; C, Nay, samples Nos. 2, 6, 8, 11 and 15 o f Table 1; D, Nay, using TR values between 620 K and 770 K ( r e f . 3 5 ) ; E, Nay, sample No. 6 o f Table 1 and data o f r e f . 2 8 . I n t h e work o f Anderson e t a l . ( r e f . 3 3 ) t h e removal o f aluminium f r o m t h e samples
was
most
water/sample w t / w t
efficient
(Fig.7C).
The
samples
were
washed
using
a
r a t i o o f 200. Kubelkova e t a l . measured t h e pH i n t h e f i r s t
suspension and found values o f 4.8,
15, r e s p e c t i v e l y ( r e f . 3 5 ) .
2.5 and 2.8 a t AlF/UC values o f 30, 25 and
The l e a c h i n g o f aluminium was most e f f i c i e n t i n t h e
sample t h a t developed t h e s t r o n g e s t a c i d i t y (Fig.7D).
I n t h e work o f Beyer e t
370
al. (ref.28) the water/sample wt/wt ratio was 32. Although less water was used than in ref.33, lower amounts of aluminium were removed (Fig.7, B compared to C). In the work of Sohn et al. (ref.37) an amount of 2 g of NH4NaY zeolite containing 2 wt % Na2O was reacted with Sic14 at TR values from 473 K to 843 K. After reaction at a TR value below 623 K, a post-treatment at 843 K was applied. The samples were washed with 2 liters of deionised water in a Buchner funnel. In this series of zeolites the aluminium removal is very inefficient (Fig.7A). HY samples probably develop weaker acidity, due to their lower AlCl3 content, and the water washing may, therefore, be less effective. Only small amounts of aluminium were evacuated from the SiClq-dealuminated NaY samples of Goyvaerts et al. (Fig.7E). These samples were washed using a very high water/sample wt/wt ratio of 570. In that work, the zeolite powder was poored into the large water volume and pH values measured (ref.28). The pH of the wash water was typically between 2.8 and 3.2 for samples with a high degree of crystallinity, dealuminated using moderate TR and Tp values. Samples treated at high temperatures which lost partially their crystallinity, developed weaker acidity . The importance of the washing procedure can be appreciated when considering the AlEF/UC content of the different samples of Fig.7, as shown in Fig.8. The washing procedure is critical especially for samples with A1F content between 10 and 20 AlF/UC, since a large variety of AlEF/UC values can be obtained (Fig.8).
35 30 25 20
15
10 5 0 0
5
10
A1'
15 /
20
25
30
UC
Figure 8. AlEF/UC versus ALF/UC for the samples of Fig.7.
35
371
Framework dealumination d u r i n q water washinq. The a c i d i t y developed d u r i n g t h e h y d r o l y s i s o f t h e t e t r a c h l o r o aluminate complexes can be r e s p o n s i b l e f o r an a d i t i o n a l dealumination. Upon a c i d l e a c h i n g o f A1 from t h e l a t t i c e , i t can be expected t h a t Zeolite Y
a hydroxyl
n e s t c o n t a i n i n g 4 OH groups
i s formed
(ref.52).
samples deeply dealuminated w i t h Sic14 develop a sharp hydroxyl
vibration at
a s p e c i f i c wavenumber i n t h e
range
from 3730
t o 3750
cm-1
(ref.27,34,37,45,51,53). Besides t h i s assignment o f t h a t hydroxyl v i b r a t i o n a t these wavenumbers t o t h e 4 OH hydroxyl nests (ref.27,51)
i t has a l s o been
a t t r i b u t e d t o c r y s t a l t e r m i n a t i n g s i l a n o l groups o r amorphous s i l i c a (ref.54). I n h i g h l y c r y s t a l l i n e samples t h e assignment t o hydroxyl nests was confirmed by t h e observation t h a t t h e band completely disappeared upon steaming (ref.27,
51). A d d i t i o n a l support comes from t h e observation t h a t i n t e n s i t y o f t h e 3750 cm-1 band i n deeply dealuminated NaY z e o l i t e s i s h i g h e r than i n L i Y samples i n which t h e t e t r a c h l o r o - a l u m i n i u m complexes were p a r t i a l l y decomposed (ref.45).
80
B . %T + C.
%T
T
6 0 D. %T 4 0 E , abs.
A
1..
20
0 M CI
0
20
40
60
80
100
% dealumination F i g u r e 9. R e l a t i v e i n t e n s i t y o f t h e 3730-3750 cm-1 hydroxyl v i b r a t i o n i n SiC14dealuminated Y z e o l i t e s . A, HY, u s i n g TR values between 473 K and 843 K, d a t a from ref.37; B, Nay, samples Nos.24-26 o f Table 2, data from ref.27; C, Nay, Nos.6, 8 and 15 o f Table 1, d a t a from ref.53; D, Nay, Nos.7 and 13 o f Table 1, d a t a from r e f . 3 4 ; E, Nay, Nos.4 and 5 o f Table 1, d a t a from ref.28. F, Nay, using TR values between 620 K and 770 K, d a t a from r e f . 3 5 . The r e l a t i v e i n t e n s i t y o f t h e 3730-3750 cm-1 hydroxyl v i b r a t i o n i s p l o t t e d a g a i n s t t h e degree o f dealumination Fig.9.
I n t h e NaY z e o l i t e s , t h e 3730-3750
cm-1 band remains a t t h e very low i n t e n s i t y l e v e l o f t h e p a r e n t z e o l i t e up t o 50% dealumination and increases s h a r p l y a f t e r dealumination degrees h i g h e r than 75%. The d a t a obtained w i t h HY z e o l i t e s e x h i b i t more s c a t t e r .
I n these samples
372
the water washing was rather inefficient (see higher) indicating that probably only weak acidity was developed in the pores. For these samples at least part of the intensity of the 3740 cm-1 vibration should be due to SiOH groups in mesopores and/or amorphous material, in agreement with the assignment of the authors (ref .37). Framework dealumination during water washing is probably not very important. From a thermogravimetric analysis of sample No. 27 of Table 2, having a degree of dealumination of 93%, Beyer et al. derived that the dealumination during water washing accounted for only 4 AlF/UC (ref -27). Extra-framework aluminium aradients. The surface of SiClq-dealuminated zeolite crystals is sometimes enriched with aluminium, as detected with X-ray Photoelectron Spectroscopy (ref.35) and Fast Atom Bombardment Mass Spectrometry (ref.36). In the samples of Kubelkova et al. the surface Si/A1 ratio did not exceed twice the bulk ratio (ref.35). van Broekhoven et al. prepared zeolite Y samples with 27 and 17 AlF/UC in which no surface enrichment was found (ref.55). From the available data it is not possible to derive whether the accumulation of aluminium at the surface takes place during the Sic14 treatment or i s the result of migration during washing. Acid leachina of extra-framework aluminium. Hey et al. studied the leaching of aluminium from an ammonium exchanged sample with AlF/UC and AlEF/UC values of 12 and 7, respectively (ref.56). This sample was obtained by method B using a zeolite with Na21(NH4)34(AlO2)55(SiO2)137 composition as starting material. The aluminium content of the framework was monitored with the wavenumber of the internal asymmetric TO4 stretch, and the extra-framework to framework aluminium ratio with 27Al MASNMR (Fig.10). The authors concluded from their data that aluminium is extracted from extra-structural positions rather than from the zeolite framework during pretreatment at pH values o f 1.3 and above. Fig. 10 shows that the framework looses ca. 3 AlF/UC. Treatment at pH values o f 0.20 and 0.0 results in important framework dealumination (ref.56).
I
0
1
2
PH
Fi ure 10. Evolution o f A1 i!/UC and AlEF/UC with pH of aqueous suspension (data from ref.56).
373
Nature of extra-framework aluminium sDecies. AlOH groups within the extraframework species give rise to infrared bands at ca. 3700 cm-1 and ca. 3600 cm1 (ref.57). In steam-dealuminated Y zeolites, the AlOH groups giving rise to 3600 cm-1 vibrations are part of polymeric 0x0-hydroxy-aluminium species, those at 3700 cm-* belong to oligomeric species (ref.58). The intensity of the 3700 cm-1 vibration is always weak in SiClq-dealuminated samples (ref.27, 28, 34, 35, 45, 51, 53) indicating that oligomers are present in small amounts. The polymers are much more abundant judged on the intensity of the 3600 cm-1 band (ref.27,28,34,35), For two series of samples, the relative intensity of the 3600 cm-1 hydroxyl band is plotted in Fig.11 against the degree of dealumination. The 3600 cm-1 signal exhibits a maximium (Fig.11). This maximum in a series of samples washed according to the same procedure, can be explained if the formation of these polymeric species occurs in a specific range of acidities and if the generation of acidity increases with increasing degree of deal uminat ion.
I
E u
0 0 \o
m I 4
100 80
60 40
-
I
-------
i
I
: \
I
\\
1 '\
A
" ! \ 0
6
d
Q k
50
60
70
80
90
100
% dealumination
Figure 11. Relative intensity of the 3600 cm-1 hydroxyl band in SiC14dealuminated NaY zeolites A, samples Nos.24-27 of Table 2, data from ref.27; B, data from ref.35. 27Al MASNMR signals observed in washed SiClq-dealuminated zeolite Y samples are listed in Table 4. The 27Al resonance of tetrahedral AIF is in the range from 60 to 50 ppm. The z7Al chemical shift of A1F in NaX and NaY zeolites is at 62.8 ppm (ref.59). Table 4 shows that the A1F resonance is gradually shifted
374
downfield at increasing degree of dealumination. This downfield shift should be due to the changes in the distribution of the NNN environments of aluminium. The 27A1 MASNMR signals in the chemical shift range from 4 to -1 ppm are ascribed to the AlEF species. Two different species can give rise to signals in this range of chemical shifts. A sharp signal at 0 ppm in hydrated samples is due to mobile monomeric aluminium cations, whereas a broad signal at the same field is ascribed to polymeric species (ref.60). Monomeric species were found only in deeply dealuminated samples (ref.44, 46) or in a sample treated with acid solutions at a pH of 0.22 (ref.56). It is evident that deeply dealuminated samples develop the strongest acidity during water washing. Part of the monomeric aluminium cations can be extracted from the samples by extensive washing (ref.44) or exchanged with NH4+ (ref.55). For several SiC14deal uminated samples a good correspondence was found between the AlF/AlEF proportions, determined on one hand by 27Al MASNMR, and on the other hand by a combination of 29Si MASNMR and chemical analysis (ref.33, 55, 56), indicating that all the aluminium is NMR-visible. 27Al NMR-silent aluminium atoms are part of the oligomeric alumina species (ref.60). The absence of NMR-silent aluminium in SiClq-dealuminated samples is in agreement with the low intensity of the 3700 cm-1 hydroxyl vibration in these samples. As an illustration, Fig. 12 shows the 27Al MASNMR spectrum of a washed dealuminated NaY zeolite containing polymeric AIEF. 27Al MASNMR signals at ca.30 ppm due to distorted aluminium tetrahedra are found in samples that were dealuminated with Sic14 according to method A, using post-treatment temperatures of 713 K and higher (Table 4, Fig.13). It was explained higher that such a treatment is harmful to the microporosity and crystallinity of the sample.
1 , 1 4 1 1 1 . . . I . . . . 1 . . . A
1aa
FFM
a
Figure 12. 27Al MASNMR spectrum of washed SiC14-dealuminated Nay. Dealumination described in the caption of Fig.5.
I aa
PPM
0
Figure 13. 27Al MASNMR spectrum of washed SiC14-dealuminated LiY, dealuminated by method A, using TR and Tp values of 423 and 763 K, respectively.
375
Extra-framework s i l i c o n sDecies The s i l i c o n atoms i n s i l i c e o u s f a u j a s i t e s t r u c t u r e s e x h i b i t a chemical s h i f t o f ca.
-100 and -107 ppm f o r t h e S i F ( l A 1 )
and SiF(OA1)
environments,
r e s p e c t i v e l y . The presence o f amorphous s i l i c a i n t h e samples can be detected i n t h e 29Si MASNMR spectrum by broad s i g n a l s i n t h e range from -110 t o -112 ppm (refs.28,33,44).
The c o n t r i b u t i o n o f t h e S i F s i g n a l s t o t h e t o t a l resonance
envelope has been used as an NMR measurement o f t h e degree o f c r y s t a l l i n i t y (ref.28).
c r y s t a l 1 i n i t y , measured by NMR, i s always
The degree o f ’short-range’
h i g h e r than t h e degree o f ’long-range’ d i f f r a c t i o n (ref.28).
crystall inity,
of
The deconvolution
a 29Si
determined w i t h X-ray
MASNMR spectrum o f t h e
dealuminated NaY sample No.3 o f Table 1, having a NMR and XRD c r y s t a l l i n i t y o f 91% and 83%, r e s p e c t i v e l y , i s shown i n Fig.14.
I
I
-3a
,
,
,
I
-38
,
,
,
I
,
-108 F PY
Figure 14. Experimental (A) and deconvoluted 3 o f Table 1 .
[
, , , I , -11a -12a
,
(B) 29Si MASNMR spectrum o f sample
Mesopores I n theory,
t h e Sic14 technique c o u l d o f f e r t h e advantage t h a t t h e s i l i c o n
needed f o r t h e r e c o n s t i t u t i o n o f t h e framework does n o t have t o come from o t h e r p a r t s o f t h e c r y s t a l thus avoiding t h e formation o f mesopores. The existence o f mesopores can r e a d i l y be detected by t h e presence o f h y s t e r e s i s i n t h e n i t r o g e n adsorption isotherms. Adsorption isotherms w i t h n o t more h y s t e r e s i s compared t o t h e parent sample have been r e p o r t e d f o r LiY samples w i t h 24 and 27 AlF/UC (ref.28), AlF/UC
NaY samples w i t h 20, 22 and 25 AlF/UC ( r e f . 2 8 ) and w i t h 22, 24 and 33
(ref.61)
and f o r HY w i t h 29 and 34 AlF/UC
(ref.28).
I n these samples,
376
the amount of aluminium extraction from the lattice does not exceed the original number of supercage cations. These samples were obtained according to method A and have not been exposed to temperatures higher than 713 K. For deeply dealuminated samples, sorption isotherms of hydrocarbons are available (ref.26,61). Samples prepared by Method B with 6 and 8 Al/UC show a near rectilinear adsorption isotherm for hexane, butane and benzene. It was concluded that in these samples mesopores having radii in the range from 1.5 to 1.9 nm, typical of hydrothermally dealuminated samples were absent. These results do, however, not imply that the faujasite micropore system is intact (ref .61). CONCLUSIONS The reaction of Sic14 with LiY, NaY and HY zeolites at temperatures below 423 K leads to the formation of framework-bound 'SiC13' species and LiC1, NaY and HC1, respectively. The Sic13 species exhibit a 29Si MASNMR resonance at ca. -45 ppm. The 27Al MASNMR spectra of the zeolite samples at this stage of the reaction show the presence o f aluminium in distorted tetrahedral environments. Upon heating of the zeolites to temperatures exceeding 423 K, the Si atoms of the Sic13 species are inserted in the framework, while the A1 atoms are re1 eased as Al Cl3. The 1 atter i s transformed into a1 kal i metal tetrachl oroaluminate complexes in presence of alkali metal chloride salts. Charge compensating cations located in the hexagonal prisms and the sodalite cages are hidden for the Sic14 molecules. At moderate reaction temperatures, the number of A1 atoms that can be replaced with Si atoms seems to limited to the number o f charge compensating cations located in the supercages. Consequently, the preparation of samples with aluminium contents, equal to the number of hidden charge-compensating cations is highly reproducible. The accessibility of the charge compensating cations rather than the T atom environment in terms of next nearest neighbors seems to determine the reactivity of framework A1 atoms. The cation distribution can be handled as a tool to govern the degree o f dealumination as illustrated LiY, NaY and HY samples. Highly crystalline mesopore-free NaY and LiY zeolites with 22 AlF/UC and HY zeolites with 30 AlF/UC can be prepared. The A1 atoms that are associated with less accessible charge compensating Na+ cations become reactive at temperatures above 523 K, possibly as a result of increased Na+ mobility and redistribution. Temperatures higher than 623 K are necessary in order to activate A1 atoms associated with hidden protons. A simultaneous attack of aluminium atoms associated with accessible and hidden cations has to be avoided since it results in lattice destruction.
377
The dealumination of X type zeolites with Sic14 occurs via the same mechanism as explained for Y type zeolites. Thermal elimination of dislodged aluminium in NaY zeolites results in the formation of mesopores and, in the most severe conditions, in amorphisation. The higher volatility of lithium compared to sodium tetrachloro-aluminium complexes is reflected in more extensive mesopore formation in LiY compared to NaY samples. The desorption of AlCl3 is facilitated and the detrimental effect of AlCl3 neutral ised when the tetrachloro-aluminate complexes are decomposed in the presence of SiC14. However, an important fraction of the dislodged aluminium always remains in the sample. Strong mineral acidity develops during the washing step due to the hydrolysis of A1-C1 bonds. The amount of aluminium that can be leached from the sample varies largely depending on the washing conditions. Framework dealumination during water washing is not very important, unless acidified wash water is used. A hydroxyl vibration in the range from 3730 to 3750 cm-l represents the 4 OH hydroxyl nests formed upon acid leaching of A1 atoms from the lattice. Part of the intensity of this hydroxyl band may in some samples be due to SiOH groups in mesopores and/or amorphous silica. The intensity of the AlOH vibration at 3700 cm-1 is always weak, indicating that oligomeric oxohydroxy-aluminium species are not abundant in samples dealuminated with SiC14. Judged on the intensity of the 3600 cm-1 vibration, the 0x0-hydroxy-aluminium species that are present in the samples are highly polymerised. For several Sic1 4-dealuminated samples a good correspondence was found between the AlF/AlEF proportions, determined on one hand by 27Al MASNMR, and on the other hand by a combination of 29Si MASNMR and chemical analysis. This indicates that all the aluminium is NMR-visible, which is in agreement with the presence of low amounts of NMR-silent oligomeric alumina species. Distorted alumina tetrahedra are found in samples of low crystallinity. Broad 29Si MASNMR signals in the range from -110 to -112 ppm represent amorphous silica. The contribution of this signal to the total 29Si resonance envelope can be used as a ’short-range’ crystallinity measurement. The shortrange degree of cystallinity is always higher than the ‘long-range‘ crystallinity, determined with XRD. ACKNOWLEDGMENTS JAM and PJG acknowledge the Flemish National Fund for Scientific Research (NFWO) for Positions as Research Associate and Senior Research Associate, respectively. Financial Support from the Belgian Government in the context of ‘Geconcerteerde Onderzoeksakties’ and from NFWO is highly appreciated.
378
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.
379
38.L. Pauling, The Nature o f t h e Chemical Bond, C o r n e l l U n i v e r s i t y Press, Ithaca, 1939. 39.P. Fejes, I . K i r i c s i , I . Hannus, G. Schobel, i n : D. K a l l o and Kh.M. Minachev (Eds.), C a t a l y s i s on Z e o l i t e s , Akademiai Kiado, Budapest, 1988, p.205. 40.6. Eulenberger, D.P. Shoemaker and J.G. K e i l , J. Phys. Chem. 71 (1967) 1812. 41.T. Hseu, Ph.D. t h e s i s , U n i v e r s i t y o f Washington, 1972, U n i v e r s i t y M i c r o f i l m s No. 73-13835, Ann Harbor, Michigan, U.S.A. 42.W.J. M o r t i e r , E. Van den Bossche and J.B. Uytterhoeven, Z e o l i t e s 4 (1984) 41. 43.A. Corma, V. Fornes, J. Perez-Pariente, E. Sastre, J.A. Martens and P.A. Jacobs, i n : W.H. Flank and T.E. Whyte (Eds.), Perspectives i n Molecular Sieve Science, ACS Symp. Ser. 368, American Chemical Society, Washington, 1988, 555. 44.5. Klinowski, J.M. Thomas, C.A. Fyfe, G.C. Gobbi, and J.S. Hartman, Inorg. Chem. 22 (1983) 63. 45.8. Sulikowski, G. Borbely, H.K. Beyer, H.G. Karge, and I.W. Mishin, J. Phys. Chem. 93 (1989) 3240. 46.B. Sulikowski and J. Klinowski, J. Chem. SOC. Faraday Trans. 86(1) (1990) 199. 47.J.H. Lunsford, P.N. Tutunjian, P. Chu, E.B. Yeh, and D. J. Zalewski, J. Phys. Chem. 93 (1989) 2590. 48.A. Nock and R. Rudham, Z e o l i t e s 7 (1987) 481. 49.P.A. Jacobs, J.A. Martens, J. Weitkamp; and H.K. Beyer, Faraday Disc. Chem. SOC. 72 (1982) 353. 50. S. J. DeCanio, J.R. Sohn, P.O. F r i t z and J.H. Lunsford, J. Catal. 101 (1986) 132. 51. P. J. Grobet, P.A. Jacobs, H.K. Beyer, Z e o l i t e s 6 (1986) 47. 52.R.M. B a r r e r and M.B. Makki, Can. J. Chem. 42 (1964) 1481. 53 .M. W. Anderson and J. Klinowski, Z e o l i t e s 6 (1986) 457. 54. J.W. Ward i n ’ Z e o l i t e Chemistrv and C a t a l v s i s ’ . J.A. Rabo, ed.. American Chemical Society, Washington DE, American “Chemical S o c i e t y Monograph 171 (1976) 118. 55.E.H. van Broekhoven, S. Daamen, R.G. Smeink, H. Wijngaards, J. Nieman, i n Z e o l i t e s : Facts, Figures, Future, P.A. Jacobs and R.A. van Santen, eds., E l s e v i e r , Amsterdam, Oxford, New York, Tokyo, Stud. S u r f . S c i . C a t a l . 498 (1989) 1291. 56.M.J. Hey, A. Nock, R. Rudham, I . P . Appleyard, G.A.J. Haines and R.K. H a r r i s , J. Chem. SOC., Faraday Trans. I , 82 (1986) 2817. 57.P.A. Jacobs, Carboniogenic A c t i v i t y o f Z e o l i t e s , E l s e v i e r , Amsterdam, 1977. 58.R. Bertram, U. Lohse and W. Gessner, Z. anorg. a l l g . Chem. 567 (1988) 145. 59.G.Engelhardt and D. Michel, High-Resolution S o l i d - s t a t e NMR o f S i l i c a t e s and Z e o l i t e s , John Wiley & Sons, Chichester, 1987. 60.P.J. Grobet, H. Geerts, M. Tielen, J.A. Martens and P.A. Jacobs, i n Z e o l i t e s as C a t a l y s t s , Sorbents and Detergent B u i l d e r s , H.G. Karge and J. Weitkamp, Eds.,Elsevier, Stud. Surf. Sci. C a t a l . 46 (1988) 721. 61.M.W. Anderson and J. Klinowski, J. Chem. SOC. Faraday Trans I , 82 (1986) 3569.
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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
381
FACTORS AFFECTING THE FORMATION OF EXTRA-FRAMEWORK SPECIES AND MESOPORES DURING DEALUMINATION OF ZEOLITE Y
D. GOYVAERTS, J.A. MARTENS, P.J. GROBET and P.A. JACOBS Laboratorium voor Oppervlaktechemie, K.U. Leuven, Kardinaal M e r c i e r l a a n 92, 6-3030 Heverlee, Belgium
SUMMARY The dealumination o f HY, L i Y and NaY z e o l i t e s w i t h Sic14 i s i n t e r p r e t e d as a two-step process. The f i r s t step i s t h e formation o f c h l o r i d e s a l t o f t h e charge compensating c a t i o n s and o f Sic13 species probably bound t o t h e framework. T h i s r e a c t i o n takes place a t r e a c t i o n temperatures as low as 423 K. The second step i s t h e A l , S i exchange. Changes i n t h e degree o f dealumination between HY, LiY and NaY are t e n t a t i v e l y explained on t h e b a s i s o f c a t i o n d i s t r i b u t i o n s . Cloro-aluminium complexes formed from AlC13 and t h e c h l o r i d e s a l t d u r i n g t h e second step g i v e r i s e t o 27Al MASNMR resonances a t ca. 100 ppm. The micropore system can be preserved i f t h e a1 k a l i - c h l o r o - a l u m i n i u m complexes are n o t decomposed before washing. No a d d i t i o n a l dealumination was observed d u r i n g washing. The e x t r a c t e d aluminium can o n l y be removed p a r t i a l l y from t h e samples. INTRODUCTION Dealumination o f z e o l i t e Y i s an e s s e n t i a l step i n t h e p r e p a r a t i o n o f FCC c a t a l y s t s . The d e p l e t i o n o f t h e z e o l i t e framework w i t h aluminium i s accompanied by t h e
generation
of
mesopores and extra-framework
silicon
and
aluminium
species, which may p l a y important c a t a l y t i c r o l e s . Dealumination o f z e o l i t e Y w i t h Sic14 was f i r s t Belenykaya ( r e f . 1 ) .
r e p o r t e d by Beyer and
I n t h i s paper some more l i g h t i s shed on t h e chemistry o f
t h i s dealumination r e a c t i o n and on t h e mechanism o f generation o f mesopores and extra-framework s i l i c o n and aluminium species d u r i n g dealumination w i t h SiC14. EXPERIMENTAL Dealumination orocedure. NaY z e o l i t e from Ventron w i t h a Si/A1 r a t i o o f 2.4 was used as t h e s t a r t i n g m a t e r i a l . Nay, i o n exchanged f o r 68% w i t h L i + i s denoted as LiY. Z e o l i t e HY was obtained from NaY a f t e r i o n exchange w i t h NH4+ f o r 90%. The z e o l i t e powders were compressed i n t o f l a k e s , crushed and sieved. 4 g o f t h e 0.25 t o 0.50 mm f r a c t i o n was loaded i n a h o r i z o n t a l t u b u l a r q u a r t z r e a c t o r , w i t h an i n t e r n a l diameter o f 3 cm. The bed l e n g t h was ca. 1.5 cm. The temperature was monitored w i t h a thermocouple mounted i n t h e middle o f t h e bed. The dealumination procedure c o n s i s t e d o f t h r e e steps, v i z . dehydration, contact w i t h SiClq, and a post-treatment. Dehydration o f t h e z e o l i t e was done under a f l o w o f n i t r o g e n o f 60 m l per minute. The h e a t i n g r a t e was 3.3 K per minute t i l l a f i n a l temperature o f 623 K, where t h e z e o l i t e was kept f o r 2 h. Then t h e temperature was adjusted t o t h e r e a c t i o n temperature, TR, and t h e n i t r o g e n stream s a t u r a t e d w i t h Sic14 vapor. The molar N2/SiC14 r a t i o was equal t o 4 and t h e r e a c t i o n time, t R , t y p i c a l l y 35 minutes. Accordingly, ca. 25 mmol o f Sic14
382
were contacted with 4 g of zeolite. After a 20 minutes purge under nitrogen stream at TR, the temperature was increased with a rate of 3.3 K per minute to the post-treatment temperature, Tp. The duration of the post-treatment was at least 90 minutes. After cooling, 500 mg of the sample was retained for NMR measurements. The remaining sample was poored into 2 1 of deionized water. The sample was washed till the wash water was free of C1, and dried at a temperature of 423 K. The sample notation is followed by their TR and Tp temperatures (K) in brackets. MASNMR measurements. The MASNMR measurements were performed on a Bruker 400 MSl spectrometer with a magnetic field of 9.4 T. The 29Si MASNMR experiments were run at 79.5 MHz, with a pulse length of 4 p s , a pulse interval of 5 s, a spinning rate of 3 kHz and a number of 10,000 scans. For the determination of the degree of NMR crystallinity of a Sam le the sigal at -110 ppm was considered to be due to amorphous material. &A1 MASNMR was performed at 104.2 MHz, with a pulse length o f 0.6 p s , a radiofrequency field strength of 5 mT, a pulse interval of 0.1 s, usual1 a spinning frequency of 5 kHz and a number of scans of 3,000. The 29Si and &A1 spectra were deconvoluted into curves with Gaussian or Lorentz line shape using the Bruker GLINFIT program. The treatment of the sample with acetylaceton (acac) in order to visualize 27Al MASNMR-invisible aluminium was performed as described previously (ref . 2 ) . 27Al MASNMR spectra at a spinning rate of 15 kHz were recorded 24 h after the impregnation with acac, using a pulse interval of 1 s. -N2 adsomtion. Nitrogen adsorption-desorption isotherms were recorded with an ASAP 2400 instrument from Micromeritics. The samples were pretreated during 15 h at 573 K under vacuum (10 mPa). The isotherm was measured at 77 K. 160 measurements along the adsorption and desorption branch of the isotherms between P/Po values of 0.002 and 0.995 were recorded. The BET surface area was calculated from at least 10 adsorption measurements at P/P0<0.025. The volume of the mesopores with radius between 1.7 and 50 nm was calculated from the desorption branch of the isotherms. The pore diameters were calc lated using the formula of Harkins and Jura: t = [13.99/(0.034 - log P/P0]t-5 and the Kelvin equation for capillary condensation in non-intersecting cylindrical pores. Infrared soectra. Infrared spectra were recorded on a Perkin-Elmer 58808 instrument equipped with data station. Self-supporting wafers of NHq+-exchanged samples were mounted in a vacuum cell and degassed in-situ at 723 K for 3 h. The spectra were recorded at a temperature of 423 K. XRD. X-ray diffraction patterns were recorded on a Siemens instrument equipped with a Mc Brawn position sensitive detector. The intensity of the (533) reflection was used to evaluate the degree of crystallinity of dealuminated samples compared to the parent NaY zeolite. RESULTS AND DISCUSSION Influence o f the reaction time Typical temperature profiles observed during the contact of the zeolite with Sic14 are shown in Fig.1. A temperature rise occurred in all experiments. In all instances had the hot spot travelled out of the zeolite bed within 35 minutes of reaction time. The dealuminated samples had a uniform greyish colour. Prolongation of tRr to 90 minutes at T ~ = 5 2 3 K did not change the properties of the final product from NaY and LiY (Table 1). The Sic14 treatment of NaY at 523 K was interrupted after 4 minutes, the zeolite purged for 20 minutes and post-treated at 823 K. In the middle of the zeolite bed a white zone had developed. A mixture of original and dealuminated zeolite was obtained since each reflection in the XRD pattern showed a doublet.
383
AT
70
t 4 2 3 K 523K
60
~
Liy
+ 423K
t
50
t
523K
40 30
20 10 0
5
0
10
I5
20
25
30
5
0
10
t R(minutes)
AT
15
20
25
30
35
t R(minutes)
40
30
Fig.1. Overheating i n t h e m i d d l e o f t h e z e o l i t e bed d u r i n g r e a c t i o n w i t h SiC14.
20
10
0
5
0
10
15
20
25
30
35
t R(minutes) T a b l e 1. I n f l u e n c e o f d e a l u m i n a t i o n procedure on p r o p e r t i e s o f washed samples
b
Starting material
TR t~ (K) (min.)
Tp (K)
pHa
SiF/AIF
NaY NaY
523 523
35 90
713 713
3.0 3.0
7.9 7.7
91 91
83 93
24 33
LiY LiY
523 523
35 90
713 713
3.0 3.0
7.0 7.5
97 90
58
35 25
C r y s t a l l i n i t y (%) mesopore volumec,d NMR XRD (mm3 9 - 1 1
a, o f t h e wash w a t e r ; b, determined w i t h 2 9 S i NMR; c, p o r e s w i t h d i a m e t e r s f r o m 1.7 t o 50 nm; d, t h e p a r e n t NaY sample has a mesopore volume o f 35 mm3 g-1. I n f l u e n c e o f r e a c t i o n and D o s t - t r e a t m e n t t e m p e r a t u r e s 2 9 S i MASNMR s p e c t r a o f washed p r o d u c t s a r e shown i n F i g . 2 .
The resonance o f
t h e SiF(OA1) environment i s observed a t ca. -107 ppm. Resonances c o r r e s p o n d i n g t o SiF(lA1),
SiF(2A1) and SiF(3A1) o c c u r w i t h an i n c r e a s i n g chemical s h i f t o f
ca. 5 ppm f o r each e x t r a c o o r d i n a t e d aluminium. c h a r a c t e r i s t i c o f a SiNF(OA1) environment ( r e f . 2 )
A broad l i n e a t ca.
-110 ppm
i s observed i n L i Y and NaY
samples t r e a t e d a t t h e h i g h e s t Tp and/or TR t e m p e r a t u r e s . The S i F / A I F r a t i o s , d e r i v e d f r o m t h e r e l a t i v e i n t e n s i t y o f t h e S i F ( n A l ) resonances, t h e degree o f
384
c r y s t a l l i n i t y and t h e BET surface area o f t h e washed z e o l i t e products are given i n Table 2. The c r y s t a l l i n i t y and t h e BET surface area are lower when Tp i s increased a t a g i v e n TR (Table 2 ) .
Post-treatment temperatures o f 823 K can be
a p p l i e d w i t h o u t important damage t o t h e z e o l i t e l a t t i c e when o n l y a TR o f 423 K i s used. Dealuminated LiY samples w i t h h i g h c r y s t a l l i n i t y have t y p i c a l l y a S i F / A I F r a t i o between 6.0 and 7.0 (Table 2). A s l i g h t l y h i g h e r value (7.6)
i s found i n
t h e LiY(623-713) sample, which, however, t o a l a r g e e x t e n t i s amorphous (Table
2,
Fig.2).
Under comparable c o n d i t i o n s ,
HY i s l e s s dealuminated compared t o
L i Y , whereas NaY seems t o be dealuminated t o a h i g h e r e x t e n t (Table 2 ) .
A -(623-713)
(523-713)
I . I . l . L . l ,
-a0
(523-823)
-80
-100 PPM
-120
-80
-100 PPM
-120
HY
-
-80
-100
PPM
-120
I . I . I . I I I . . -a0 -100 -120 PPM
Fig.2. 29Si MAS NMR spectra o f washed samples.
-100 PPM
-120
385
NaY (523-623)
n
(523-713)
(623-713)
--(623-823)
(523-823)
-80
-1B0
-120
PPM
-80
-100
-120
PPM
-80
-126
-100
PPM
Fig. 2. Continued.
Generation o f mesooores
K on t h e
Isotherms f o r t h e adsorption and d e s o r p t i o n o f n i t r o g e n a t 77 parent NaY sample and dealuminated Nay, adsorption
isotherms
have
a
LiY and HY are g i v e n i n Fig.3.
rectangular
Langmuir shape,
The
characteristic o f
micropore adsorption. The adsorption and d e s o r p t i o n branches o f t h e isotherm on the
parent
absent,
as
NaY z e o l i t e expected.
coincide,
Hysteresis
indicating occurs
with
that
mesopores
SiClq
are
treated
virtually
samples,
the
importance o f i t depending on TR and Tp. The occurrence o f h y s t e r e s i s i n d i c a t e s t h e presence o f a mesopore system. For a l l samples t h e d e s o r p t i o n branch j o i n s t h e adsorption branch again a f t e r a sudden a r e l a t i v e pressure o f ca. 0.45.
decrease i n t h e amount adsorbed a t
T h i s has been explained as a t e n s i l e - s t r e n g t h
e f f e c t being a t t h e o r i g i n o f sudden d e s o r p t i o n o f n i t r o g e n from mesopores, connected t o t h e e x t e r i o r o f t h e adsorbent v i a openings s m a l l e r than 5 nm (ref.3).
It i s , therefore,
impossible t o determine t h e average diameter o f t h e
mesopores. Volumes o f mesopores w i t h diameters from 1.7 t o 50 nm, as d e r i v e d from t h e desorption
curves,
dealuminated HY, with SiF/AIF
are
given
in
Table
3.
The
mesopore
volume
LiY and NaY z e o l i t e s w i t h a h i g h c r y s t a l l i n i t y ,
o f 15.6,
doesnot exceed t h a t o f t h e parent NaY z e o l i t e ,
a value o f 35 mm3 g - l was determined (Table 3).
the
except NaY f o r which
For comparison, t h e mesopore
volume o f NH4Y z e o l i t e , dealuminated by s e l f - s t e a m i n g a t 973 r a t i o o f 5.5 i s 100 mm3 g-1.
of
K
and w i t h S i F / A l F
386
Table 2. I n f l u e n c e o f TR and Tp on p r o p e r t i e s o f washed products c r y s t a l l i n i t y (%) according t o XRD
H 623 713 a23 923
I
I
Li
-
90 -
Li
Na
a5 a3 41
63
45 -
53 0
6.5 7.9
4.7
7.6
8.6 ND
H
Li
Na
-
68
ao
-
64 10
5.7
6.0 7.0
75 0
-
623 713 023
I
H
Na
977
ND
=
-
n o t determined.
Table 3. Volume o f pores w i t h diameters from 1.7 t o 50 nm (mm3 9 - l )
I
Li
H
623 713 a23 923
-
Na
I
H
Li
Na
29
-
27 35
24
H
Li
Na
I
H
Li
Na
I
H
Li
Na
69
Table 4. pH o f t h e wash water
I 623 713 a23 923
H
Li
Na
I
387
Table 5.
SiF/AIF r a t i o b e f o r e washing
I
H
Li
(!i
623 713
6.5
823 923
P
o
180-
a
160-
+
140-
C
>
P.
$
I
H
Li
Na
-
6.5 6.8
7.1 8.8
5.9
No/
15.2 N i l :
Li
Na
4.6
8.4
ND
1
:-
ND
H
-
I
HY 623-713
I
200
-
-
I
140-
f 200 C
Na
180
0.1
0.4 6.5 0:s 0:7 R e l a t i v e pressure (P/Po)
012 0.3
0!8
0.9
LiY 423-823
160
--L
140
,"220
LiY 523-623
V
2 200 n I-
C m 0
E
180 200 180 160
2 200 >
180
LiY 523-713
--/
LiY 623-713
160
140
120 100
TI
0.1
I
012 0.3
014
015
I
0.6
0.7
R e l a t i v e pressure (P/Po)
Fig.3. N i t r o g e n adsorption-desorption isotherms a t 77 K .
I
0.8 0.9
388 + ads,
180
des
I
-1 ,Nay 423-823 0.1
Fig.3. Continued.
*
0.2
0.3 0.4 0.5
0.6 0.7
0.8 0.9
I
R e l a t i v e pressure (P/Po)
Extra-framework aluminium 27Al MASNMR spectra of HY, LiY and NaY samples before and after washing are shown in Fig.4A and B, respectively. Before washing, essentially two 27Al resonance lines are present in the samples. The signal at ca. 60 ppm represents AIF. The signal at ca. 100 ppm should be due to aluminium chloro complexes. The 100 ppm signal has disappeared in NaY and LiY samples which have been exposed to Tp temperatures o f 823 or 923 K . This is in agreement with observations in literature that Na(AlC14) and Li(A1Clq) complexes in zeolite Y decompose at temperatures of 780 K (ref.4) and 733 K (ref.5), respectively. z7Al MASNMR signals at ca. 100 ppm are not observed in HY samples (Fig.4A). Upon washing, the chloro aluminium complexes are hydrolysed. pH values of the wash water are given in Table 4. For samples which finally have a high degree of crystallinity and which were subjected to a post-treatment with Tp < 823 K, the pH of the wash water i s typically between 2.8 and 3.2. When Tp equals 823 K, LiY samples develop less acidity compared to Nay. Samples which have lost partially their crystallinity develop weak acidity.
389
A new signal at ca. 0 ppm appears in the spectra of washed samples (Fig.4B). This signal is due t o AlNF in octahedral environment. For T p equal t o 823 K NaY exhibits an additional 27Al resonance at ca. 35 ppm. This signal is typical o f distorted tetracoordinated A1 (ref.6) or pentacoordinated A1 (ref.7) in nonframework environments. SiF/AlF ratios o f the samples before washing determined by 29Si MASNMR are given in Table 5. The SiF/AIF ratios o f the HY, LiY and NaY samples before and after washing are very similar, indicating that no additional dealumination occurs during the washing step. Infrared spectra o f the dealuminated Nay, LiY and HY samples are shown in Fig.5. Besides the high-frequency band at ca. 3620 cm-1 and the low-frequency band at ca. 3550 cm-1 due t o bridging hydroxyl groups, hydroxyl vibrations are observed at ca. 3740, 3670 and 3600 cm-1. The 3740 cm-1 band should be due t o silanol groups. The intensity of this band increases when the sample contains a larger amount o f mesopores (Fig.6), indicating that these hydroxyls are located in the mesopores. Bands at 3600 and 3670 cm-1 are generally ascribed t o hydroxyl groups on non-framework aluminium species. It is striking that the intensity o f the 3600 and 3670 cm-1 bands is more pronounced in NaY compared t o LiY samples.
A
LiY
HY
-A A
(423-823)
(623-713)
100
FPM
NaY
0
I
I
I
I
I
1
100
I
I
PPM
I
,
,
,
I
0
(423-823 )
n
-(423-923)
100
(523-713)
0
100
0
PPM PPM Fig.4A. 27Al MASNMR spectra before washing.
1
1
1
1
1
1
100
1
1
PPM
1
1
0
1
1
1
1
390
Fig.4B. *'A1 MASNMR spectra after washing.
(623-713)
(523-713)
IIIIIIIlllllllllrllll 111111111 0
100
0
PPM
(523-623)
(523-713) (623-713) (423-823)
100
PPM
0
111111111111111111111111111111 100
PPM
0
(523-623) NaY
(523-713)
(623-713)
111111111111111 100
(423-823 )
111111111111111111111111111111 lcl0
PPM
0
100
PPM
0
PPM
0
39 1
I
0
3600
3200
l l h l 10
3600
3200
cm
I
I
I
I
- 1
Fig.5. Hydroxyl spectra of the H-form of the dealuminated samples Quantification of the amount of framework and non-framework aluminium was done using the acac method and using the absolute intensity mode. The 27Al MASNMR spectra of the acac-treated samples run at a spinning rate of 15 kHz are shown in Fig.7. The gain in resolution by the higher spinning rate can be appreciated by comparison with the spectra of Fig.4 which were run at 5 kHz. The AIF and AINF content of some of the samples are given in Table 6. The dealuminated LiY samples contain a considerable amount of AINF even when posttreated at temperatures at which the lithium-choro-aluminium complexes are decomposed. Due to changes in molecular weight of the zeolite upon the different treatments it is not straightforward to compare the aluminium content of dealuminated samples with that of the parent zeolite. From the intensity of the 27Al signal it was estimated that between 20% and 30% of the aluminium is extracted from the zeolite pores in the washing step.
392
-. i "
Fig.6. Intensity o f 3740 cm-l hydroxyl vibration against mesopore vol ume.
10
*l0-
IS
m
k
.4 .* I
10
a
.4
a
I
I
0
30
60
90
120
150
3 -1) Mesopore volume (mm g
LiY
NaY
(523-713)
AL
(523-713)
--I\
(423-823)
0 0 100 0 1°' PPM PPM PPM Fig.7. 27Al MASNMR spectra o f acac-treated samples run at a spinning rate o f 15 kHz using the absolute intensity mode. NH4Y zeolite i s used as reference. 100
Table 6. A1F and AlNF content (mmol 9-1) o f washed samples sample
Deal umination mechanism The overheating curves of Fig.1 and the experiments with shortened and prolonged tR show that a reaction zone progresses through the zeolite bed during contact with SiC14. Once the reaction zone has passed, the zeolite has become unreactive towards SiC14, although the zeolite is not yet completely dealuminated (Table 1). Apparently, the number of A1F sites that react with Sic14 at temperatures of 423 K and 523 K is limited. The dealumination reaction of NaY zeolite with Sic14 corresponds to the following stoichiometry (ref. 1): Na(A102(Si02),)
+
SiClq - - > NaAlC14 t (Si02),+1
It has been suggested that the sodium-aluminium-chloro complexes deposited in the zeolite pores protect the residual framework aluminium atoms from being further attacked by Sic14 (ref.4). However, product-inhibition can not explain the influence of the nature of the charge compensating cations on the extent of dealumination (Table 2). All the 1-atom positions in the faujasite structure are equivalent and there should be no discrimination on this basis. Since the cations take part in the reaction with Sic14 (Eq.l), the cation location could influence the rate of the isomorphic substitution reaction. The dealumination reaction could proceed via two consecutive reaction steps (ref.8). The first step could be the rupture of one Si - C1 bond and formation of a Si - 0 bond and a chloride salt of the charge compensating cation. This step is exothermic and can proceed at mild reaction temperatures (ref.8). The second step is the aluminium-silicon exchange with concomitant formation of a aluminium chloro complex. This conversion is also exothermic (ref.8) and can proceed during the posttreatment.
\ / Si
/ \
0-
\ / A1
/ \
--->
\ / Si
/ \
0
A1
/
/ \
--->
\ / Si
/ \
0
\ /
Si / \
The cations, M, are distributed over cation sites located in the supercages, the sodalite cages and the hexagonal prisms of the faujasite structure (ref.9). The Sic14 molecule has access to the supercages only. When the number of supercage cations is compared to the number of A1 atoms that were actually
394
substituted,
a s t r i k i n g correspondence i s found (Table 7).
It e x p l a i n s why
under g i v e n dealumination c o n d i t i o n s t h e degree o f dealumination f o l l o w s t h e order: NaY > LiY > HY
(3)
I n l i t e r a t u r e samples have been r e p o r t e d w i t h v e r y h i g h Si/A1 r a t i o s ( r e f s . l , 4 and 5).
Such samples can be obtained i f a f t e r t h e f i r s t step, which n e c e s s a r i l y t h e temperature i s g r a d u a l l y
has t o be performed a t low r e a c t i o n temperature,
increased i n presence o f Sic14 t o values o f 720 o r 730 K. Under such c o n d i t i o n s t h e l e s s a c c e s s i b l e c a t i o n s and associated framework aluminium atoms probably become r e a c t i v e .
Table 7. Number o f accessible c a t i o n s a and o f aluminium atoms e x t r a c t e d w i t h Sic14 from t h e l a t t i c e per u n i t c e l l NaX NaY LiY HY
40b 30c 21d
38e 30 - 34f 28 - 30f 22 - 27f
a, t o t a l c a t i o n s minus c a t i o n s i n s i t e s I and 1’; b, d a t a from ref.10; c, data from ref.11, d, c a l c u l a t e d from I R i n t e n s i t y r a t i o o f h i g h frequency and low frequency hydroxyl v i b r a t i o n s from ref.12; e, c a l c u l a t e d from Sic14 uptake a t a temperature o f 480 K; f, c a l c u l a t e d from S i F / A I F r a t i o determined w i t h z 9 S i MASNMR (Table 2).
ACKNOWLEDGMENTS
PJG and JAM acknowledge t h e Flemish National Fund f o r S c i e n t i f i c Research f o r f e l l o w s h i p s as a Senior Research Associate and Research Associate, r e s p e c t i v e l y . T h i s work has been sponsored by t h e Belgian Government i n the frame o f a concerted a c t i o n on c a t a l y s i s . We are g r a t e f u l t o D r . 0. Anton and N.V. REDCO f o r t h e n i t r o g e n adsorption measurements. REFERENCES l.H.K. Beyer and I.M. Belenykaja, Stud. S u r f . S c i . Catal. 5 (1980) 203. 2. P.J. Grobet, H. Geerts, M. Tielen, J.A. Martens and P.A. Jacobs, Stud. Surf. Sci. Catal. 46 (1989) 721. 3.S. Gregg and K. Sing, Adsorption, Surface Area and P o r o s i t y , Academic Press, London, 1982. 4. H.K. Beyer, I.M. Belenykaja, F. Hange, M. Tielen, P.J. Grobet and P.A. Jacobs, J. Chem. SOC. Faraday Trans I , 81 (1985) 2889. 5. B. Sulikowski, G. Borbely, H.K. Beyer, H.G. Karge and I . W . Mishin, J. Phys. Chem. 93 (1989) 3240. 6.A. Samoson, E. Lippmaa, 6. Engelhardt, U. Lohse and H.-G. Jerschkewitz, Chem. Phys. L e t t . 134 (1987) 589. 7. J.-P. Gilson, G.C. Edwards, A.W. Peters, K. Rajagopalan, R.F. Wormsbecher, T.G. Roberie and M.P. Shatlock, J. Chem. Commun. (1987) 91. 8. P. Fejes, I. K i r i c s i , I. Hannus, 6. Schobel, i n : D. K a l l o and Kh.M. Minachev (Eds.), C a t a l y s i s on Z e o l i t e s , Akademiai Kiado, Budapest, 1988, p.205.
395
9.W.J. Mortier, Compilation of Extra-framework Sites in Zeolites, Butterworths Scientific l t d . , Guildford, 1982. 10.G. Eulenberger, D.P. Schoemaker and J.G. Keil, J. Phys. Chem. 71 (1967)1812. ll.T. Hseu, Ph.D. thesis, University of Washington, 1972, University Microfilms No. 73-13835, Ann Harbor, Michigan, U.S.A. 12.A. Corma, V . Fornes, J. Perez-Pariente, E. Sastre, J.A. Martens and P.A. Jacobs, in: W.H. Flank and T.E. Whyte (Eds.), Perspectives in Molecular Sieve Science, ACS Symp. Ser. 368, American Chemical Society, Washington, 1988, 555.
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G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation ojCutulysts V 0 1991 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
397
TREATMENT OF GALMALUMINO-SILICATE (ZSM-5 TYPE ZEOLITE) WITH KOH SOLUTION. DISPERSION OF AGGREGATED ZEOLITES INTO SMALL PARTICLES J. KANA1 and N. KAWATA
Central Research Laboratories, Idemitsu Kosan Co., Ltd., Kamiizumi, Sodegaura, Kimitsu, Chiba 299-02, Japan
1280
As a member of Research Association for Utilization of Light Oil (RAULO), Japan. ABSTRACT We have studied about the treatment of aggregated galloaluminosilicate (ZSM-5 type zeolite) with KOH solution. The suitable KOH treatment of TPA-Si-A1-Ga is found to be a very valid preparation to disperse the aggregated zeolites into small particles and expose the external surface of unit crystals without changing other properties. Furthermore, the suitable KOH teratment of TPASi-Al-Ga increased the catalyst life in aromatization of nhexane. INTRODUCTION ZSM-5 type zeolites have attracted considerable interest f o r unusual reaction selectivities (ref.1). Moreover, the stability of these zeolites to coke formation has been also attributed to the molecular shape selectivity (refs.1-3). Dejaifve et al. (ref.4) reported that during methanol conversion coke is formed on the external surface of ZSM-5 zeolite, and proposed that coking for H-ZSM-5 zeolite could be limited by increasing particle size. On the other hand, Bibby et al. (ref.5) reported that coke is predominantly formed inside the crystals, and proposed that catalysts requiring the least frequent regeneration will have a relatively low density of active sites and small particle size. Actually, it has been reported (ref.6) that the life of catalysts containing ZSM-5 for converting hydrocarbons increased with decreasing the size of the zeolite crystals. Synthesis of smaller crystals of ZSM-5 zeolite has been attempted (ref.6) However, these attempts often result in lack of crystallinity or aggregates of small particles. For aggregates of small particles, it is difficult to distinguish which size of unit crystal or secondary aggregated zeolite represents the size
.
398
of catalyst. Furthermore, different condition of synthesis also causes the change of other properties such as distribution of A1 or morphology (refs.7,8). We have studied about the treatment of aggregated galloaluminosilicate (ZSM-5 type zeolite) with KOH solution. The present paper will show that the suitable KOH treatment can disperse the aggregated zeolites into small particles without changing other properties. Moreover, the problem of representative particle size and the relation between catalist life and particle size will be discussed. EXPERIMENTAL Catalysts Galloalumino-silicate (Si-Al-Ga) (Si02/A1203/Ga203 molar ratio = 78/1/1.3) was synthesized using tetrapropyl ammonium bromide (TPA) by the method described in a patent (ref.9). The structure of synthesized material (TPA-Si-A1-Ga) was confirmed by X ray diffraction to be that of ZSM-5 having a high crystallinity. SEM analysis of the zeolite revealed aggregates of about 1 consisting of small crystals of about 0.1 - 0.05pm (Fig.2). Three kinds of proton form of Si-Al-Ga were obtained and studied. Firstly, after calcination of TPA-Si-A1-Ga at 823 K for
rm
8h, the ammonium form of zeolite was obtained by exchanging twice with 1 M NH4N03 solution at 353 K for 3h. H-Si-A1-Ga was prepared by calcining it at 973 K for 4 h. Secondly, TPA-Si-A1-Ga were treated with 1 M KOH solution (10 g/g-zeolite) at 323, 333 and 343 K for 1 h, respectively. H-Si-A1-Ga(TPA,KOH) was prepared by treating it in the same manner as H-Si-Al-Ga. Lastly, after calcination of TPA-Si-A1-Ga at 823 K for 8h, Na-Si-Al-Ga was treated with 1 M KOH solution (10 g/g-zeolite) at 333 K for lh. H-Si-Al-Ga(Na,KOH) was prepared by exchanging twice with 1 M NH4N03 solution at 353 K for 3 h, followed by calcining at 973 K for 4 h. External surface area by benzene filling method External surface area was measured by the method of Murakami et al. (ref.lo). The zeolite sample was pretreated by calcining at 673 K in a flow of nitrogen, followed by keeping in the desiccator with benzene at room temperature for at least 10 h. The sample in a glass cell was connected to the flow-type BET apparatus, cooled to 195 K by dry ice-ethanol, and exposed to a
399
flow of N2 and He gas mixture (Nz content, 30 % ) . The sample was next chilled to 77 K to adsorb nitrogen, and then flashed by the dry ice-ethanol to desorb the nitrogen. The amount of nitrogen adsorbed was determined by measuring the amount of nitrogen desorbed. 2,l-dimethylbutane (2,Z-DMB) adsorption ability 2,2-DMB adsorption ability was measured by a pulse reactor (atmospheric pressure, 6 mm i.d. tube made of stainless steel) connected to an automated system of gas chromatographic analysis. 100 mg of 16-32 mesh particles was charged into the reactor. 2 ~ of the mixture consisting of equivalent mole of n-hexane, 3methylpentane and 2,2-DMB was injected into the reactor at room temperature, and the effluents were analyzed under the following conditions: hydrogen carrier flow rate = 22 ml min-’; and V.P.C. column = 3.5 m packing squarane. In all cases, n-hexane and 3methylpentane were completely adsorbed and only 2,2-DMB passed through the catalyst bed. The 2,2-DMB adsorption ability was defined by the next formula. 2,2-DMB adsorption ability ( % ) =
Adsorbed 2,Z-DMB moles Injected 2 ,2-DMB moles
x 100
TPD measurement TPD spectra of ammonia were measured with a conventional TPD apparatus, and desorption was detected by a thermal conductivity detector. Zeolite sample (0.1 g) was evacuated in a quartz cell at 673 K for lh, exposed to temperature for 15 min, then measurements were made from heating rate of 30 K min’l in
ammonia used as probe base at room evacuated at room temperature. TPD room temperature to 823 K with a a helium flow of 150 ml min-l.
Catalytic reactions Reactions were carried out at 773 K and 1 atm in a quartz g of 16-32 mesh tublar microflow reactor containing 0.5 particles. After heating the zeolite t o 773 K under nitrogen, nAnalysis of the reaction hexane was fed at WHSV of 2 h-’. products was carried out by on-line gas chromatography (ref.14). Conversion and yield were calculated on the carbon basis. Catalyst life was defined as the time retaining above 50 C-mol% of aromatics yield.
1
400
RESULTS Chemical analysis, BET total surface area, external surface area by benzene filling method and 2,2-DMB adsorption ability were measured about fresh catalysts of H-Si-A1-Gal H-Si-A1Ga (TPA,KOH) and H-Si-A1-Ga (Na,KOH). The results are shown in Table 1 & 2. TABLE 1 Chemical analysis of zeolites Zeolite
Si02/A120 (mo1/mo13
SiO, /Ga20 (mol/mo13
78
60
76 72 53 34
57 54 55 26
H-Si-A1-Ga H-Si-A1-Ga(TPA,KOH) 323 K 333 K 343 K H-Si-Al-Ga(Na,KOH)
TABLE 2 Physicochemical properties of Zeolite
total surface area (m2/g)
H-Si-Al-Ga H-Si-A1-Ga(TPA,KOH) 323 K 333 K 343 K H-Si-Al-Ga(Na.KOH1
zeolites external surface area (m2/g)
2,2-DMB adsorption ( % )
309
20
60
355
43 57 61 79
83 87 88 6
360
377 343
In Fig.1, aromatics yield is plotted against time on stream. The catalyst life of H-Si-Al-Ga(TPA,KOH) treated at 323 and 333 K (61 and 66 h, respectively) were longer than that of H-Si-A1Ga(52 h). On the other hand, the catalyst life of H-Si-A1Ga(TPA,KOH) treated at 343 K (35 h) and H-Si-Al-Ga(Na,KOH) ( 2 h) were shorter than that of H-Si-A1-Ga. Reactions were stopped when aromatics yield became about 4 0 Cmol%. BET total surface area and coke content were also measured about used zeolites of H-Si-Al-Ga and H-Si-A1-Ga(TPA,KOH). Coke formation rate and total surface area loss per milligram of coke are shown in Table 3. Coke formation rate and total surface area
401
Time on stream ( h) Ffg. 1. Aromatics yield as a function of time on stream. 0 , HSi-A1-Ga; H-Si-A1-Ga(TPA,KOH) treated at 323 K: m , H-Si-A1Ga(TPA,KOH) treated at 333 K; A , H-Si-A1-Ga(TPA,KOH) treated at 343 K; A , H-Si-Al-Ga(Na,KOH).
a,
TABLE 3 Aging property of zeolites ~~
Zeolite
Yea
coke formation rate (mg/s h)
H-Si-A1-Ga H-Si-A1-Ga (TPA,KOH) 323 K 333 K 343 K
total surface loss / coke (m /mg)
1.7
1.2
1.7 1.8 3.4
1.0 1.1
1.2
loss per milligram of coke were defined as the followings.
Coke formation rate =
coke content of used zeolite operation time
Total surface area loss / coke = surface area of fresh zeolite
-
that of used zeolite
coke content of used zeolite DISCUSSION
H-Si-A1-Ga(TPA, KOH) The time for filtration of TPA-Si-A1-Ga treated with KOH solution became longer than that of untreated one, indicating
402
that the KOH treatment of TPA-Si-A1-Ga dispersed,the aggregated zeolites into small particles. In order to confirm above assumption, the measurements of external surface area by benzene filling method and 2,2-DMB adsorption ability were carried out. From Table 2, it is found that both external surface area and 2,2-DMB adsorption ability of H-Si-Al-Ga(TPA,KOH) were more than those of H-Si-A1-Ga. Total surface area of H-Si-A1-Ga(TPA,KOH) were larger than that of H-Si-A1-Ga (Table 2), and all zeolites were comfinned by X ray diffraction to remain high crystallinities. Moreover, the results of SEM analysis of H-Si-Al-Ga (external surface area: 20 m2/g) and H-Si-A1-Ga(TPA,KOH) treated at 323 K (external surface area: 43 m2/g) are shown in Fig.2. SEM analysis revealed that the aggregated zeolites of about 1 P m were dispersed into small particles of about 0.3 p m by the treatment with KOH solution. These suggest that the increase of external surface is due to not noncrystallization of zeolite but dispersion of the aggregated zeolites into small particles.
Fig. 2. Scanning electron micrographs of (A) H-Si-Al-Ga and (3) H-Si-Al-Ga(TPA,KOH) treated at 323 K.
403
Let us consider the relation between the size of apparent particle by SEM analysis and the external surface area. Assuming that the size of particle = R ( p m ) and the specific gravity = a (s/cm3)I the weight of 1 particle (9) = 4/3 Xa(R/2)3 x the external surface area of 1 particle (m2) = 4 7 ~ ( R / 2 )x~ then, the external surface area ( m 2 / g )
= 6/aR.
TABLE 4 Relation between the size of particle (R) and external surface area Size of particle (R)
( P I 1.0 0.3
0.1
the calculated
calculated exte nal surface area (m5/g) 3.4 11 34
Assuming that a = 1.78 (g/cm3), the relation between the size of particle (R) and the calculated external surface area is shown in Table 4. The calculated external surface area of the particle size of 1.0 and 0 . 3 are ~ 3.4 and 11 m2/g, respectively. These values are much smaller than the actual external surface area of Si-Al-Ga having the same apparent particle size of the secondary aggregates (20 and 43 m2/g, respectively). On the other hand, the calculated external surface area of the unit crystal (whose size is 0.05-0.1 ,um) is between 34 and 67 m2/g, which is much larger than the actual external surface area of H-Si-A1-Ga (20 m2/g), and is equal to those of H-Si-A1-Ga(TPA,KOH) (from 43 to 61
-
m2/s) From above results, it is suggested that unit crystals in aggregated as-synthesized Si-Al-Ga are not completely separated each other, and the considerable parts of external surface are useless. Si02/A1203 ratio of H-Si-A1-Ga(TPA,KOH) were smaller than that of H-Si-Al-Ga (Table l), indicating that some SiO, were dissolved from catalyst by the treatment with KOH solution. Then It is inferred that the KOH treatment can seperate unit crystals by dissolving some SiO, on their external surface and expose the external surface. From Fig.1, it is found that the KOH treatment at 323 and 333 K
404
increased the catalyst life. We reported (ref.12) that the change of acid density of zeolite or the amount of non-framework Ga species affects the catalyst life in aromatization of n-hexane over galloalumino-silicate. However, TPD spectra of ammonia of HSi-A1-Ga(TPA,KOH) were compared with that of H-Si-A1-Gal and no change was observed in their TPD spectra. Moreover, the coke formation rate of H-Si-A1-Ga(TPA,KOH) treated at 323 and 333 K were the same with that of H-Si-A1-Ga (Table 3). Then it is suggested that the increase of catalyst life by the KOH treatment is not due to either the change of acid density of zeolite or the amount of non-framework Ga species. The reasons for the increase of catalyst life by the KOH treatment at 323 and 333 K are considered as following: (1) If the effectiveness factor of aggregated catalyst (H-SiA1-Ga) is below 1.0 under aging, the dispersion of aggregates into small particles would increase the effectiveness factor of catalyst under aging and the catalyst life. (2) From Table 2, the increase of total surface area seems to be due to the increase of external surface. Furthermore, the ratio of increase of catalyst life is almost equal to the ratio of increase of total surface area. If the external surface has the same aging property with the internal surface, the catalyst life would depend on the total surface area when the effectiveness factor of catalyst is 1.0. The reason for the increase of catalyst life is not clear from the results of this experiment. However, whichever the reason, it is suggested that the KOH suitable treatment of aggregated zeolites will have more effect on the catalyst life in reactions which the effectiveness factor of catalyst is very small. On the other hand, as shown in Table 3, the rate of coke formation of H-Si-A1-Ga(TPA,KOH) treated at 343 K was faster than that of H-Si-A1-Ga, and consequently the catalyst life became shorter (Fig.1). Since Si02/A1203 ratio of H-Si-A1-Ga(TPA,KOH) decreased with the treated temperature (Table l), it is suggested that the excess treatment with KOH solution increased the acid density of external surface and consequently the coke formation rate on them. H-Si-A1-Ga [Na,KOH) As shown in Table 2, total surface area and external surface area of H-Si-Al-Ga (Na,KOH) were larger than those of H-Si-A1-Ga.
405
Furthermore, SEM analysis also revealed that the aggregated zeolites were dispersed into small particles. However, Si02/A1203 ratio of H-Si-Al-Ga(Na,KOH) was below half of that of H-Si-A1-Ga (Table 1), and 2,2-DMB adsorption ability of H-Si-Al-Ga(Na,KOH) was much smaller than that of H-Si-Al-Ga (Table 2). The low 2,2-DMB adsorption ability seems to depend on more hydrophilic property of external surface of H-Si-A1Ga (Na,KOH) than that of H-Si-Al-Ga. Furthermore, TPD spectrum of ammonia of H-Si-A1-Ga (Na,KOH) was considerably different from that of H-Si-A1-Ga (Fig.3), and the catalyst life of H-Si-A1Ga(Na,KOH) was much shorter than that of H-Si-Al-Ga (Fig.1).
373
473
573
673
773
Tempemture ( K I
.
Fig. 3. TPD spectra of ammonia of (-) H-Si-Al-Ga (Na,KOH)
H-Si-A1-Ga and ( - - - - - - 1
From above results, it is suggested that the dissolution of SiOz from external surface of H-Si-Al-Ga(Na,KOH) is more severe than that of H-Si-A1-Ga(TPA,KOH), and considerable SiO, was also dissolved from internal surface of H-Si-Al-Ga (Na,KOH) . The reason for the decrease of catalyst life of H-Si-Al-Ga(Na,KOH) would be due to the change of acidity of both internal and external surface. This showsthat the existence of TPA in the pore of zeolite is inevitable for the suitable and selective KOH treatment of external surface of zeolite. CONCLUSION (1) It is suggested that unit crystals of aggregated Si-Al-Ga are not completely seperated each other, and the considerable
406
parts of external surface are useless. (2) The suitable treatment of TPA-Si-A1-Ga with KOH solution is found to be a very valid preparation to disperse the aggregated Si-Al-Ga into small particles and expose the external surface of unit crystals without changing other properties. (3) The suitable KOH teratment of TPA-Si-A1-Ga increased the catalyst life, This is suggested to be due to the dispersion of the aggregated zeolites into small particles or the increase of external surface area. REFERENCES 1 P. Weisz, in T. Seiyama and K. Tanabe (Editors), Proceedings of the 7th International Congress on Catalysis, Tokyo, 1980, P l 2 L.D. Rollman and D.E. Walsh, J. Catal., 56 (1979) 139 3 L . D . Rollman and D.E. Walsh, J. Catal., 56 (1979) 195 4 P. Dejaifve, A. Auroux, P.C. Gravelle, J.C. Vedrine, Z. Gabelica and E.G. Derouane, 3. Catal., 70 (1981) 123 5 D.M. Bibby and C.G. Pope! J. Catal., 116 (1989) 407 6 C.J. Plank, E.J. Rosinski and A.B. Schwartz, U.S. Patent 3 ,926,782
E.G. Derouane, S. Detremmerie, 2. Gabelica and N. Blom, Appl. Catal., 1 (1981) 201 8 E.G. Derouane, J.P. Gilson, Z. Gabelica, C.M. Desfuquoit and J. Verbist, J. Catal., 71 (1981) 447 9 C.J. Frank, G.B. Patent 1,402,981 10 M. Inomata, M. Yamada, S. Okada, M. Niwa and Y. Murakami, J. Catal. , 100 (1986) 264 11 J. Kanai and N. Kawata, 3. Catal., 114 (1988) 284 12 J. Kanai and N. Kawata, Appl. Catal., in press 7
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
407
D. J. RAw"X1, K. a & , L. H. S l Y d a n d A . P. cHAppIE2 h f i e l d Catalysts, WarringtOn (U.K.) 2~nilever-, port sunlight ~aboratory,~ u a r r y 1l0ad ~ a s t ,wington, W k r a l , Me.rseyside L63 3JW (U.K.)
SUMMARY
The addition of primary distillation residues to the Fee feedstock leads toinmeas& vanadium loadings on the catalyst and major problem with catalyst deactivation. The deactivation process is believed to involve the formation of low r,eltingpoint V205-Na2O-Al203 phases which accelerate the normal zeolite dealurnination process. Intamqting this precess by pruvidhg a suitable trap for the acidic V205 preserves catalyst activity without introducing undesirable properties. The efficiency of a strontium titanate trap is demonstrated in laboratory tests and a camnercial trial. m
m
C
N
Transition metal contarmnan ' ts in the form of pozphyrin ccanplexes are present to varying extents in the majority of crude oils. primary refinery ' tstothe distillation processes consign a large proportion of these contarmnan distillation residues and yield gas oil distillates w h i c h are low in metals. Such gas oils have traditionally been the principal feed to fluid catalytic cracking (m) units, and although the metal contaminants slowly accumulate on the catalyst particles throughcplt their life, the equilibrium levels remain relatively low (<2000ppn total metals) and do not cause serious problens. The increasing m e to maximize the yields of valuable products f m each barrel of clude oil, however, means that it is rmw CQRKX~to blend 10-25% of distillation residues into the gas oil feed to the m unit. The result is that (up to 8000 ppn) are retained significantly higher levels of metal conon the catalyst. one of the major contaminants is nickel. In the m unit this ads predcaninanty as a PakFerfUl aehydrogeMtion catalyst and leads to excessive yields of coke and hydrogen. ?hese products can impose severe constraints on the feed rate to the unit leading, ultimately, to the loss of valuable prcducts. 'Ihe effects of nickel can be reduced, haever, either by chemical passivation (ref. 1) or by lcwering the effective concentration through catalyst wiWdrawal. Iron and capper contaminants act in a similar manner to nickel but are present at significantly lower amxntrations (ref. 2).
408
?he other important metal conbminant is vanadium. Although it exhibits distinct dehydrqenation activity, the major effect of vanadium is to cause marked and irreversible deactivation of the catalyst. ?his paper is concerned with the mecham'sm of this deactivation pmcess and shews that, frm a lawwledge of the chemical and physical properties of the vanadium species, it is possible to design efficient trapping agents w h i c h can be incorporated into the catalyst particle. T h e effectiveness of this apprcad~is demonstrated t h r c q h laboratory tests and confirmed by the results of full-scale trials.
-
Four series of catalysts were employed in this study. An w i m e n t d l rare-earth exchanged Y-zeolite (REHY) catalyst was used for the work on the mechanisn of vaMdium attack. Tim severities of anm~niumion-excharqe gave catalysts with 0.16 and 0.55 wt% Na20. A further series of experimental RElIy catalysts was prepared for vanadium trap studies. lmis consisted of a control (no trap) and. catalysts containing 0.485 moles of CaTiO3, SrTiO3 and &Ti03 per kg of dry catalyst. Finally, Cmsfield SLS-80 and SIS-SOW were used for detailed laboratory tests and SLS-100 and SLS-1OOW for cawmercial trials. vanadium m mtion Experimntal catalysts used for mechanistic studies were irqxqnated with 0, 1000, 3000 and 5000 ppn vanadium by weiqht by applying aqueous oxalic acid solutions of d u m vanadate using incipient wetness techniques. Catalysts used for vandim trap studies were inprqnated with Xylem solutions of vanadyl naphthenate according to the commonly used Mitchell method (ref. 3). Catalysts w e r e dried at 100°C and heated to 540°C for 3 hours prior to steam deactivation. catalyst Deactivation Experimental catdlysts were deactivated by heating in 100%steam for 5 hours at 788°C in a fluid bed reactor. Detailed procedures and a description of the equipment have been given elsewhere (ref. 4). Catalysts involved in trap studies w e r e Steam deactivated at 760°C using similar procedures. catalvst Evaluation Deactivated catalysts w e r e tested according to the microactivity (W) test procedure describe2 previously (ref. 4). X-Rav Diffraction x-my pmder diffraction patterns w e r e used to assess the relative zeolite crystallhities of catalysts according to ASIM D-3906. crystdllographic unit cell size s t were carried out using RSIM D3942. Surface Area
surface area was derived fmm nitrugen adsorpticol/desorptionkothems generated with a MiQomeritics AsAp 2400 instrument. Totdl surface areas
409
were calculated fram the adsorption isotherms at p/po <0.07. Micropore areas were obtained by difference from the BET area and the non-micrq0rcu.s area calculated fran the de Boer t-plot (ref. 5). Mesopore areas w e r e derived f m the cumulative surface area obtained fram the pore size distributions according to the Barrett, Jayner and Halenda (B.J.H.) theory (ref. 6). Fcc cATAz;ysIs The majority of FCC catalysts are mcro-spheroidal c a r p s i t e s of zeolite Y and clay bound together w i t h eirher s i l i c a or alumina sols.
Tn addition,
proprietary silica-alumina species can be added to modify activity and selectivity. The zeolite c m p m t is the dcaninant same of catalytic activity and considerable effort is devoted to its prooessing to provide activity and to h p m e both thermal and hydrutherml stability. Important steps in the stabilization include the reduction in s c d i u m content and the partial dealumination of the zeolite framework. In the latter context, specialized processes are used to bring about the controlled removal of the m t wlnerable aluminium atarrrs fram the framemrk and their substitution by silicon. The extent of dealumination can be monitored through measuzpment of the Unit cell size. 'Ihis relates directly to framework ccanposition and decreases in magnitude as the aluminium atarrrs are replaced by silicon. In the Fcc Unit the catalyst cycles between the cracking zone ( 500"C, 2-8 seconds residence time, reducing atmosphere) and the regenerator ( 700"C, 15-30 minutes residence time, partial steam and oxidizing atmosphere). As with most advances in catalyst technology, the inproved stability of current catalysts is exploited, not solely to increase the life of the catalyst, but also to allow the use of higher regenerator tenperatures for the more rapid and cuqlete removal of coke. under the repeated expcsme to these severe conditions, the zeolite ccanponent of the catalyst underyoes further, progressive, degradation w h i c h leads ultimately to ccanplete loss of catalytic activity. A study of equilibrium catalyst particles, separated according to their working age, showed a pmgressive decrease in zeolite unit cell size (ref. 7) thus indicating the key involvement of frammork dealumination in the lass of structure and activity. A PROPOSED MECHANISM FOR VANADIUM-IMXTCED F O 1 S O " G An accelerated test for the vanadium tolerance of cracking catalysts involves hpreqnation with a vanadium ccanpound follawed by air calcination and steam deactivation. ExarmM ' tion of experimentdl catalysts after irpregnation with 0, 1000, 3000 and 5000 p ~ m nvanadium sham loss of micro and meso-pore surface areas, crystallinity and catalytic activity in direct proportion to the
410
level of added vanadium. Bese results are shown graphically i n Figum 1 and Mate that the zeolite acanponent of the catalyst is the min focus of attack by vanadium w i t h only minor, but definite, Qmage being suffered by the matrix. Additional results shown in Fig. 1 co~lcernthe h f l m of the level of residual sodium i n the catalyst. it is clear that the sinailtaneous presence of vanadium and high levels of sodium grsatly increse the destmction of the zeolite. 'Ihe effect on the nntrix is less dxrious in these catalysts.
2oo
r
\
40
0.16% No20
h
3 30 \ N
E "
. . I
XI
< v)
50
-
0
0.55% No20
2000
4000
6000
0
(bl
0.55% N a 2 0
a
I
< 20
v)
0
VANADIUM (ppm)
I
I
2000
4000
I
.
6000
VANAD I UM (ppn) (d)
(C)
0
2000
4000
VANADIUM ( p p d Fig. 1.
6000
0
2000
4000
6000
VANADIUM (ppd
effect of vanadium and sodium on the Fhysical and catalytic
prvperties of stearn cbxtivated m m catalysts, (a) micmpore surface area, (b) mescpore surface area, (c) relative zeolite crystallinity, and (d) MAT conversion.
411
studies of the distribution of metals in sectioned equilibrium catalyst particles have been carried out using imging secondary ion mass spedrcanetry
(ref. 8 ) . These have shown that nickel accumulates a t the catalyst surface with minor penetration only behg evident in the oldest particles. In contrast, vanadium spreads thrcughout the catalyst particle and, i n addition, can m e between particles (ref. 7, 8 ) . The diffuse distribution of vanadium accOuntS for the extent of its destructive attack on the mysical and catalytic properties of the catalyst and, in turn, demands that a valid explanation of ' of its mobility. vanadium poisoning must include an urdThe vanadium (IV) cmplexes w h i c h are i n i t i a l l y depcsited on the catalyst
particles are unlikely to retain their identity for long. Organic fragments w i l l be remrved by thermal and catalytic processes within the riser and any r e m a w parts of the porphyrin skeleton w i l l be destroyed in the oxidizing atmosphere of the regenerator. 'Ihe most likely vanadium species to result is an oxide. A recent study using x-ray akorbance spectroscapy (ref. 9) has sham that vanadium remains as V(IV) in the reducing atmosphere of the riser and is not reduced to V ( I I 1 ) . Ixlring the regeneration cycle, haever, vanadium is oxidized t o the V(V) state. An examination of the melting points of the cannon vanadium oxides - V2O3 (1970'C), VO2 (1967°C) and V2O5 (680'C) indicates that V2O5 w i l l be a liquid m e r regenerator conditions and suggests that it is a t t h i s stage in the FCC cycle that vanadium is most likely t o diffuse thrmgh the catalyst. An approximate idea of the rate of diffusion was dtained by heatintimate mixhrres of V2O5 and catalyst particles in a i r a t 700°C for various t i m e s . Follawhg rapid cooling, the particles w e r e embedded and sectioned, and the vanadium distribution mapped ushg EDX techniques. F'rom Fig. 2 it is clear that penetration of vanadium is rapid and virtually wmplete within 15 minutes - less than the time spent in the regenerator in a single FCC cycle.
(a)
(b)
(c)
Fig. 2. Vanadium distribution maps of cross-sections of-6Op catalyst particles after heating w i t h V2O5 a t 700°C for (a) 5 m b , (b) 10 m b and (c) 15 m i n .
412
In the presence of soda, V2O5 forms mixed oxide phases (ref. 10) with melting This extends dawIlwards the tenpxature range for vanadium mability and such systems are likely to be responsible for the enhanced zeolite destruction found when vanadium and sodium are bath present. s accclIIlpanied The loss of zeolite structure in the presence of vanadium i by a reduction in the average crystallcgramc Unit cell size as shown in Fig. 3 . ?his presents a direct indication that vanadium functions as a poison by accderat- the dealmination process in the zeolite. Furthrmore, a similar process occurring in the silica-alumina matrix cmpmenb is likely to account for the observed changes in the catalyst matrix. such enhanced dealumination can also be b t abcut by subjecting a catalyst to hiteqeratums and/or pmlomgd steam treatment (ref. 4 ) w h i c h suggests that the principal action of vanadium is to accelerate the destructive changes that take place within the catalyst in normal use. points down to 525'C.
No20
24.26 CI
u)
' -I -J
c
t
24.24
c . (
z 3
0
2000
4000
6000
VANADIUM (ppm) The effect of vanadium and scdium on the u n i t cell size of the Fig. 3 . zeolite ccgnponent of steam deactivated exprimental catalysts. Further examination of binary and ternary mixed oxide phase diagram (ref. 11, 12) shm thatV205 and the V205-Na20 system will dissolve alumina at temperatures greater than 610°C. 'Ihe solubility of alumina haeases with increasing tenperatum of the melt and at 700°C in the v205-Al203-Na20 system up to 1 mole percent A1203 will be in solution. I f such a system is cooled, alumina will be deposited until the new equilibriun cgnposition is reached for the 1temperature. With the temperature cycling that occurs as the catalyst moves frcnn the regenerator to the riser and back to the regenerator again, ample opportunity exists for the dissolution of alumina, its transport, and its deposition in other regions of the catalyst structure. The formation
413 of solid AlVO4
- the only stoichianetric
A1203 (ref. 13)
-
is a possibility.
c m p m d to form between V2O5 and
This a u l d inhibit the movement of
vanadium, however, its melting point of 695'C suggests that it would not be very effective, especially i n units processing high residue-containing feeds where regenerator
temperatures tend to be significantly higher.
TIE a l u m i n a dissolved i n the m e l t can, in principle, originate fmw various saun=es, e.g., z e o l i t e framework alminium, extra-framswork aluminium,
and aluminium fmm the catalyst matrix. me intrinsic instability of the zeolite Y framework a t high temperatures, however, provides a pawerful driving force for the net flux of aluminium f m framework to mn-framework positions and offers a ready explanation for the preferential attack on the zeolite. me 'a, based on accelerated dealmination, is sham schematically proposed-
in Fig.
4.
Si
Si
Al
*
Na20- V 0
+
- - - - - - - zeolite framework
+
H P
liq
I ,610'C
Na20
\Ti 0
- -
-
?( /
OH HO
- - -
0
-
dealuminated zeolite
- A1203- V 20
-AI$i
+
-
liq
+
non-framework aluminium species
Nag
- v205
I iq
prcp?osed mecharu 'sm for zeolite and m a t r i x destruction by vanadim shming the involvement of an Na20-V205 m e l t in the dealumination process.
Fig. 4.
VANADIUM TRAPPING SYSTENS
Maintenance of catalytic activity is fundamental to the FCc process. In the presence of hi@ levels of vanadium the additional activity loss is nonmlly countered by increasing the active zeolite content of the catalyst inventory either by increasing the catalyst make-up rate or by incorporating
higher levels of zeolite into the catalyst.
A
more satisfactory approach wuld
be to innrbilize the vanadium species within the catalyst particle and thereby
inhibit its destructive action.
For such a trap t o be viable, a number of
criteria have t o be met : the trap must be catalytically inert, it m u s t be stable throughout the catalyst preparation processes, it shall be capable of reading rapidly arid irreversibly with vanadium species t o form high melting
414
ccsrpxxmds, and it nust be cost-effective in caparison w i t h the traditional replacanent and high level zeolite d e s . Furthermore, the hap-vanadium -1ex nust be stable d e r the thermal., hydrourermal and redox conditions prevailing i n the Fa3 unit. A S b r t U'K J point for vanadium trap design is a recognition that V205 is an acidic oxide and hence w i l l react, a t suitable tenperatures, with basic oxides t o meet the above criteria, the choice to form mixed oxide systems. In 0 of basic oxide is limited. For exanple, rare earth oxides react w i t h V2O5 to form d c s w i t h n-elting points in excess of 1500°C. Unfortunately, rare earth oxides exhibit undesirable catalytic activity. Further examples are
given by the alkaline earth oxides whi&
react to form stable vanadates k u t are
themselves i n c m p t i b l e with the acidic conditions experienced during conventional catalyst manufacture. The acid-base approach is still viable,
hmever, i f the trap is f i r s t neutralized with an acidic oxide which is a In use, such a system would react with V2O5 by ' vanadate and liberate the weakly acidic displaament t o form the
weaker acid than V2O5.
oxide. It isessentl'al that this weakly acidic oxide has no undesirable catalytic properties. A nunber of ccpnbinations of basic oxide (e.g. W i n e earths, rare earths) and weakly acidic oxides (e.g. Si02, TiO2, ZrO2, Sn02, C02) can be envisaged as potential vanadium traps. when dl1 the desirable p r q e r t i e s are taken into account, however, the list of practical traps becapnes much shorter. For exanple, scane combinations require very high temperatures for their
synthesis, others give extremely hard materials which are difficult to process and scane are insufficiently stable in the catalyst slurry. Frcnn a rnrmber of trials, calcium, s t r o n t i u m and barium titanates have the best ccsnbination of properties. ccanparative vanadium trapping performance has been tested and the results Shawn in Table 1 indicate that strontium titanate (ref. 14) gives the greatest activity retention. TABIE 1 Effect of traps on the retention of catalyst activity in the presence of 5000 ppn vanadium
63 80 90 85
a: activity retention
=
MAT amversion w i t h 5000 rxpn V MAT conversion with no added V
x
100
415
strontium has dso been sham to be superior for vanadium trapping to either calcium or barium when t e s t e d as carbonates in catalysts prepared with a neutral silica-alumina gel binder system (ref. 15). VANADIUM ' I " CATALySrs A key advantage of strontium titanate is that its preferred form is a highly crystalline, acid stable, perovskite structure which is prepared conveniently by calcination of the relevant oxides or oxide precwsors. Wing manufacture for this purpose, a slight excess of titania ensures the absence of free strontia in the pruduct and also directs the reaction t o w a r d the ABo3 perovskite structure in preference to the less stable, strontia-rich phases, e.g. Sr3Ti207. crystalline strontium titmate has no pore structure and its effectiveness as a vanadium trap is reliant upon maximizing its external surface area. ?his is achieved by reducing the average particle size of the trap, by milling and classification, to levels ccanparable with other catalyst ingredients. Thereafter, the desired level of trap is incorporated into a particular catalyst formulation through displacement of an equal weight of clay. Addition of strontium titanate to Fa3 catalysts has no adverse effects on the physical properties - attrition resistance is not affect& and density is marginally inrreased (ref. 16). Furthermore, the results of the MAT evaluation of amnercial catalysts with and without trap, given in Table 2, indicate no detrimental effect either to catalytic activity or to key prcduct selectivities. MANUFACIURE OF
TABLE2 Effect of SrTiO3 trap on catalyst performance mtalysta
Conversion (wt%) Gasoline (wt%) Gasmline/cOnversion specific cokeb
SIS-80
SIS-8otTRAp
67.6 49.7
68.3 50.0 0.733 1.23
0.736
1.21
a: catalysts deactivated at 760'C/5 hoUrs/lOO% steam b: defined as catalytic coke/khetic conversion - see ref. 4 TRAPPX EFFI(IIENcy
Trapping efficiency is assessed by the retention of catalytic performance and by the degree of passivation of the vanadium dehydrogenation activity. In
both cases the magnitude of the effect is assessed against vanadium
416
concentration. Fig. 5a shws that the addition of strontiumtitanate to an SIS-80 catalyst to form SIS-8OW clearly has a pronounced and beneficial effect on activity retention. 'Ihe parallel retention of surface area is Shawn in Fig. 5b w h i c h demonstrates that the trap is effective in pr0teCtb-q the zeolite and suggests that surface area can be used as an alternative methcd for assessing vanadium tolerance.
R +J E
h
z
z
3
El I-
2 u)
z
m
50 > z W
W IW
'
e <
0
u I-
< 40 2 :
VI
.
0
2000 4000 VANADIUM (ppd
6000
0
2000
4000
6000
VANADIUM (ppd
Fig. 5. 7% effect of vanadium on the properties of stem deactiMted carnnercial catalysts with and without a vanadium trap, (a) MAT conversion, and (b) surface area retention. key property of the trap is that the trapvanadium species must be catalytically inert. 'Ihis is demonstmted in Figs. 6a and 6b which shaw a dramatic reauction in MAT coke and hydrosen yields. %ese yields are related to d&ydroqenation activity and provide clear evidence for the passintion of vanadium catalytic activity. 'Ihe trap is least effective at law levels of vanadium as this catalyst itself has scnne inherent vanadium tolerance. At levels of 2000 plan vanadium or greater, however, the trap is clearly beneficial and its comtration in the catalyst formulation can be varied to meet individual requhmmts. Fig. 7 s h c m that a law level of incorporation is effective up to 3,000 plan whilst the indications are that higher concentrations can tolerate over 5000 plan. Another
-
EYAIIJATION
'Ihe incorporation of a vanadium trap will increase catalyst costs, and to be viable, this cost has to be offset by reduced catalyst addition ana/or inprove3 selectivity. In turn, the corcentration of the trap in the catalyst
417
7 7 0.5 6 w x
ii 4J
5 -
0
t
U
2
U
z W
4 -
In
0. 4
0.3
0
u
L
L
c)
rl
W
(b)
0.2
3 -
I
0
SLS-80 + TRAP
0. 1
2-
2000
4000
6000
VANAD I UM (ppn) Fig. 6. 'Ihe effect of vanadium on the selectivities of steam deadivated cammercial catalysts with and without a vanadium trap, (a) MAT specific coke yields and (b) MAT hydrcgen yield.
7l
P
6
SLS-80
w Y
-
/
0 U
U
%
SLS-80 WITH
INCREASING
TRAP LEVEL
U W
a v)
0
2000
4000
6000
VANADIUM (ppm) Fig. 7. The effect of vanadium on the M?!T specific coke yields of steam deactivated carmnercial catalysts containing increasing levels of vanadium trap. will be dictated by the level of vanadium in the feedstock to be processed and the acceptable catalyst make-up rate. In a trial recently c o n d ~ ~ c tin d a European refinery, Crosfield SLS-1OOW replad SIS-100 i n order to aope with an increased level of residue in the feed whilst maintaining, or even rducing the catalyst addition rate. At the erd of the six mnth trial, vanadium levels
418
had increased by 33% to 4000 ppn yet the catalyst addition rate had decreased & selectivity frum 3.1 to 2.0 t o m per day. In addition, the b q m ~ ~ coke allcued the overall level of conversion to incrsase by 5 w t % of f e d . As can in conversion be seen f m Table 3, the greatest praportion of the
came fran a greater utilization of feed (HCO). TABLE 3 mmnercial t r i a l of vanadium trap catalyst.
U n i t Y i e l d s (wt%)
-myst SIS-100 sLs-1oovr
Conversion
62.5
67.5
Coke
5.5 16.5 21.0 44.5 12.5
5.4 12.5 20.0 47.0 15.1
3.1
2.0
HCO L1=0
Gasoline
Gas+LpG
catdlyst addition rate (W) axJcwSI0N
The destructive effect of vanadium on FCC catalysts can be inhibited by of a vanadium trawing system. Suitable trap can be designed using the principles of acid-base chemistry, but consideration must also be given to the stability of the trap during catalyst manufacture and the tthat the trap has no undesirable catalytic effects. Laboratory tests show that strorrtium titanate confers vanadium tolerance to a catalyst, markedly reducby zeolite destruction, preserving catalytic activity, and reducing coke and hydrcqen formation. These results agree w i t h the conclusions of a successful ccarmercial t r i a l . L ~ addition e
R T lm E N m 1.
2. 3.
4. 5. 6.
G. H. Ble and D. L. McKay, Passivate M e t a l s in FCC Feeds, H y d m c a h n
mpcesS, 56(9) (1977) 97-102. G. A. Mills, Aging of Cracking Catalysts. Less of Selectivity, I M . Elq. chem., 42 (1950) 182-187. B. R. Mitchell, Metal Contamination of Cracking Catalysts. I. Synthetic &tals DepositiononFreshCatalysts, Ind. Erg. chem. Prod. Res. Dev., 19 (1980) 209-213. D. J. Rawlence and K. Gosling, Fcc Catalyst 0 EvdluatiOn, -1. C d t a l . , 43 (1988) 213-237. B. C. Lippens and J. H. de Boer, Studies on -re Systenrs in Catalysts, V. The t M e w ,J. Catalysis, 4 (1965) 319-323. E. P. l3arrett, L. G. Joyner, and P. P. Halenda, The Detennination of pore Volume and Distributions in ponxls substances. I. ccarprtations frcin Nitrogen Isathernrs, J. Amer. Cbem. Soc., 73 (1951) 373-380.
419 7. 8.
J. L. Palmer and E. B. Cornelius, Separating Equilibrium (3racking Catalyst into Activity Graded Fractions, -1. Catal. , 35 (1987) 217-235. E. L. Kugler and D. P. Leta, Nickel and Vanadium on Equilibrium (3racking catalysts by Imaging Seconlary Ion Mass spectrC4netryf J. Catal., 109 (1988) 387-395.
9.
10.
11. 12. 13. 14. 15. 16.
.
G. L Wollery, A. A. Chin, G. W. Kirker and A. €Iuss, X-Ray Absorption Study of Vanadium in Fluid cracking catdlysts, in: M. L. ocoelli (IM.), Fluid Catalytic Cracking, ACS Symposium Ser. 375, ACS, Washington, 1988, pp 215-228. phase Diagram for Cerarm 'sts, G. smith (Ed.), Fig. 5075, The American Cexamic Society, Inc., 1981. Ref. 10, Fig. 5192. Ref. 10, Fig. 5332. Fhase Diagram for Ceranu'sts, M. K. Reser (Ed.), Fig. 320, The American Ceramic Society, Irc., 1964. A. P. Chapple, Eur. Pat. -1. EP204543, Dec. 1986. E. L. Kugler, Eur. Pat. -1. EP209240, Jan. 1987. Crosfield Technical publication: Vanadium Trap (VT) Catalysts, Crosfield Catalysts, W a r r i n g t o n (U.K.), 1990.
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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
42 1
DOUBLE SUBSTITUTION IN SILICALITE BY DIRECT SYNTHESIS: A NEW ROUTE TO CRYSTALLINE POROUS BIPUNCTION~CATALYSTS
G. Bellussi1, A. Caratil, M.G. Clerici', A. Esposito2 'Eniricerche S.p.A., San Donato M.se (MI) - Italy Enichemsynthesis R&D, San Donato M.se (MI) - Italy
Abstract We have synthesized silicalites containing both Ti and a trivalent element (Al, Ga, Fe) in lattice position. Syntheses are similar to that of TS-1. As we add increasing amounts of a trivalent metal nitrate to the reaction mixture, we obtain silicalites containing both titanium and the other metal. Titanium content decreases with increasing content of the trivalent metal. Physico-chemical analyses (e.g., XRD, FT-IR, EPR, MAS-NMR, exchange capacity) and oxidizing and acidic catalytic activities confirmed isomorphous substitution of both elements.
Introduction Most of the interesting catalytic properties of zeolites are related to the presence of strong acid sites. Many processes have been developed taking advantage of the zeolites acidity and shape selectivity. At the end of the seventies, we have prepared a silicalite with titanium in lattice positions (ref.s 1,2). The discovery of TS-1 and its peculiar properties in oxidative processes with hydrogen peroxide started a completely new field of shape selective reactions (ref. 3 ) . Several patents claimed also the synthesis of zeolites containing both titanium and a trivalent element (ref.s 4 , s ) . However only in few of them proofs about the presence of both elements in lattice position are given (ref. 5 ) . The presence of Ti and A1 (or Ga, or Fe) in lattice position can give rise to catalysts possessing a good activity in both oxidizing and acid catalyzed reactions. In this paper we report the results of our studies about the synthesis and characterization of aluminum, gallium and iron containing titanium-silicalite.
422
Experimental Zeolite synthesis TS-1 is synthesized as described in example n. 1 of US Pat. n. 4 410 501. A solution of 1.5 g tetraethyltitanate (TET) in 45 g of tetraethylsilicate (TES) was poured in a 400 ml pyrex glass vessel containing 100 g of a 2 0 % aqueous solution of tetrapropylammonium hydroxide (TPA-OH). We kept the resulting mixture at 60 OC for 3 hours. Occasionally we added distilled water to replace the water lost by evaporation. Molar ratios in + the final reaction mixture were: SiO /TiO2=32.I, TPA /Si02=0.46, 2 H20/Si02=35. We heated the mixture in a 260 r n l stainless steel autoclave in an oven at 175 OC, under autogeneous pressure, without stirring, for 24 hours. We separated the crystalline product from the liquid by filtration. We washed than several times with water, dried 2 hours at 100 OC and finally we calcined it 5 hours at 550 OC in air. We prepared several TPA-OH solutions containing known amounts of + cations in the reaction NaOH to verify the influence of Na mixture. In the synthesis of the Al(Ga, Fe)-TS-1 samples, we dissolved the required amount of trivalent metal nitrate in 20 g of ethyl alcohol and added to the TES before the TET addition. The Si02/M203 molar ratio in the reaction mixture was varied from 100 to 1000. A reference ZSM-5 sample was prepared as the A1-TS-1 except that TET was not added to the reaction mixture. The final product (used as standard for ZSM-5 type catalysts) had a molar ratio Si02/A1203=164. We used the following reagents: TES (Dynasil 40 from Dynamit Nobel), TET (purum from Fluka), 20 % w aqueous TPA-OH (from Enichemsynthesis, free from alkaline impurities) , NaOH (RPE-ACS drops from C. Erba), A1 ( NO3 ) 9H20 (RPE-ACS from C. Erba), Ga(N03)3.8H20 (puriss. from Fluka), Fe(N03)3-9H20 (RPE-ACS from C. Erba). X-Ray analyses were made by step scanning procedure on a Philips powder diffractometer equipped with a pulse height analyzer. The CuK& radiation (A =1.54178 A ) was used. FT-IR spectra were collected on a Perkin-Elmer 1730 spectrometer using the KBr wafer tecnique. MAS-NMR analysis was performed on a Bruker CXP-300 spectrometer (7.0 magn. field). Samples were put in a Delrin-made sample-holder and rotated at 4 KHz. EPR measurements were carried out on a Varian E-109 spectrometer operating at the X band (9.5 GHz) with 100 KHz power modulation,
-
423
using the strong pitch Varian (9=2.002, spin conc.=3xl0 15 spin/cm) as a reference. Catalytic activity test We evaluated both acid and oxidative (with H202) catalytic activities. The catalyst tested in these experiments had the following molar composition: Si02/Ti02= 43 for TS-1; Si02/A1203= 164 for the standard ZSM-5; Si02/Ti02= 45, Si02/A1203= 150 for A1-TS-1; Si02/Ti02= 43, Si02/Ga203= 294 for Ga-TS-1; SiO,/TiO,= 42, Si02/Fe203= 362 for Fe-TS-1. All samples were well crystallized. Epoxidation of 1-butene with H2g2 A solution was prepared by dissolving 8 g of 1-butene in 100 g of methanol. The latter was previously distilled and stored on 4A molecular sieves. In a typical run, 25.5 g of this solution stored at -20 OC was quickly transferred in a 150 ml glass reactor, weighed and then kept at 5f0.1 OC. When we reached 5 OC, we added benzene (0.177 g, as g.1.c. internal standard) and the required amount of 33% hydrogen peroxide. We removed an aliquot of solution and titrated it iodometrically to determine the hydrogen peroxide concentration. The reaction started when we added the catalyst (0.74 wt % ) to the stirred solution. Aliquots were removed at time intervals and analyzed by gas-cromatography and iodometric titration. G.1.c. analyses were performed on a Hewlett-Packard HP 5880 gascromatograph using a FID and a glass column (2.4mx4mm) containing Porapak PS as the stationary phase. Selectivities and yields are based on hydrogen peroxide. Hydroxilation of phenols A 250 ml flask containing 112.0 g of phenol, 20.8 g of acetone, 27.2 g of water and 5.6 g of catalyst was heated to reflux temperature ( - 98'C ) under stirring. Then we added 16 g of H202 (60 % w/w) dropwise in 4 5 minutes. After 1 hour from the last addition all the hydrogen peroxide was converted. The analyses were performed by using a FID and a glass column containing SE-52. We weighed the tars after removing volatile materials on a BUCHI-GKR-50 evaporator. Oligomerization of 1-octene We placed 6 ml of 1-octene, freshly distilled and stored under nitrogen and 0.4 g of catalyst in a 35 ml glass pressure vessel. The slurry was heated at 165 OC under stirring for 2.5 hours. After cooling, the mixture was filtered and the solution was
424
analyzed by Porapak PS.
gas-cromatography on
a
glass
column
containing
Results and discussion In general alkali cations favor the insertion of aluminum in the zeolite framework. They are more suitable than TPA' ions because they are less inhibited by steric hindrance. Unfortunately, alkaline cations can interfere with the synthesis of Al( Ga, Fe )-TS-l since it has been reported that they prevent the framework incorporation of titanium in the TS-1 (ref. 6 ) . 0 2 4 6 8 10 In our work we observed the (a)ppm Na x same effect. By using the inFig.1 Effect of the presence of tensity of the TS-1 IR band Na on the Ti insertAon in the Silicalite lattice. ( measured on at 970 cm-' as a probe test the TPA-OH solution) for the presence of framework titanium (ref. 21, we observed that adding increasing amount of NaOH to the reaction mixture, the 970 cm-' band intensity decreases (Figure 1) even if titanium is still present. The intensity of the 970 cm-l IR band is measured relative to the intensity of the band due to Si-0-Si symmetrical stretching at + 800 cm-'. At higher Na content more titanium is present in the solid. At the same time XRD shows the presence of anatase. The zeolites synthesized in the presence of sodium have less oxidation activity than TS-1 made without Na. Potassium has a similar effect. Sodium in the reaction mixture during the synthesis of TS-1 or other titanium containing silicalites (Al-TS-1, Ga-TS-1, Fe-TS-1) have a small influence on the amount of titanium in the final product. However, it reduces the intensity of the 970 cm-I IR band. Preliminary characterization results of extraframework titanium indicate that before calcination, extralattice Ti is in the form of amorphous titanium-silicate. After calcination at 5 5 0 ° C amorphous Ti is partially or totally converted to anatase. This suggests that the presence of sodium in the reaction mixture at the synthesis conditions described above promotes the
425
SiO /Ti02
A 970” ‘800
1
LIt
140-
0.80.4L L
0
100-
I
40
A
O
60.
100 200 300 400 500 SiO, /A1
*O
Fig. 2 A1 content vs. framework Ti (represented by the reiftive intensity of the 970 IR band) in A1-TS-1. cm
0
500 SiO f M 2 0
iooo
Fig. 3 Ti content vs. trivalent element in Al(Ga, Fe)-TS-1.
formation of insoluble titanium-silicate species which reduce the amount of Ti available for the formation of the TS-1 crystals. The absence of sodium is a critical condition for the synthesis of TS-1 type materials. When we add A1(N03)3 to the reaction mixture we make silicalites which contain both Ti and Al. The maximum amount of each element depends on the concentration of the other. As the amount of A1 increases above a certain critical level the amount of Ti starts to decrease (Figure 2). Above a Si02/A1203 molar ratio of 150 the Si02/Ti02 becomes about 4 5 . This is the lowest Si02/Ti02 ratio we ever found in TS-1 (ref.2). For Si02/A1203 molar ratios below 1 5 0 the Si02/Ti02 is higher than 4 5 . The intensity of the 970 cm-l IR band is proportional to the titanium content. It is similar to that found in TS-1 for Si02/A1203 ratios above 150. Ga and Fe behave in a similar way except that the critical SiO2/Ga203 and Si02/Fe203 ratios are higher (-300, see Figure 3). The only crystalline phase observed during these experiments was a silicalite type with orthorhombic symmetry. The 27A1-MAS-NMR of A1-TS-1 after calcination in air shows a peak at - 5 4 ppm. This suggests the presence of aluminum in tetrahedral coordination. In the case of Ga-TS-1, the pattern of 71Ga was very broad. We could not tell from this whether Ga is in tetrahedral or octahedral configuration.
426
The EPR spectrum of Fe-TS-1 after calcination shows a clear signal at g=4.3 indicating the presence of tetracoordinated iron atoms in the lattice (ref.7). The exchange capacity has been determined at room temperature. We treated the silicalite samples with 0.1 N CsCl aqueous solutions. The exchange capacity increased with increasing trivalent element concentration (Figure 4). The differences among the three curves observed mainly in the left side of the diagram are probably due to the presence of different amounts of extraframework metal oxides. The exchange capacity and the presence of a
10.
5
0 0
’
1c 10 500Si0 2 f M 2 0 3
Fig.4 Exchange capacity vs. trivalent element content in Al(Ga, Fe)-TS-1.
IR band at 970 cm-l, indicate that both Ti and the trivalent TABLE 1: Oligomerization of 1-octene element are in the silicalite frameCatalyst C16H3 2 C24H48 work. The amount of (%I (%I trivalent element and titanium that 100.0 TS-1 can be inserted into 49.0 Ref. ZSM-5 47.0 3.9 the silicalite lat67.0 1.3 A1-TS-1 31.0 tice is limited. Ga-TS-1 59.3 1.5 38.5 The relative abounFe-TS- 1 89.7 5 10.0 0.2 dance of the two atomic species in the silicalite is related to the concentration of the two elements in the liquid phase in which the crystals are dispersed. Catalytic activity experiments further confirm that both Ti and Al(Ga,Fe) are in the lattice. Unlike ZSM-5 which has only acid activity and TS-1 which has only oxidation activity, A1-TS-1, Ga-TS-1, Fe-TS-1 are all active in acid catalyzed reactions and in oxidations in the presence of H202. Table 1 shows 1-octene oligomerization results. TS-1 is not active because it has no strong Bronsted acid sites. The other samples are active.
F
strong
427
TABLE 2: Epoxidation of 1-butene with H,LO,L Xun 3.
initial concentration of hydrogen peroxide. 1,2-dihydroxy-butane and its monomethylethers-polyethers have not been calculated. The activity of A1-TS-1 and Ga-TS-1 is comparable with that of ZSM-5. The activity of Fe-TS-1 is much lower than that of A1 and Ga-TS-1. This difference may be due to the number and/or the strength of the acid sites created TABLE 3: Hydroxylation of phenol by lattice iron a (Ref.8). tars Catalyst Run O/P H2°2 Table 2 shows the n. ratio Yield % I results obtained in TS-1 the epoxidation of 1 1.20 0.80 80.8 1-butene with H202 Ref.ZSM-5 2 < 1 A1-TS-1 at 5 OC. Only sam3 1.18 0.91 81.0 1.38 1.02 4 77.5 ples containing Ti Ga-TS-1 Fe-TS-1 have significant o5 < 10 3.7 xidation activity. a H 0 Yield = (moles of diphenols produced/ Acidity affects mgl& of H202 converted) x 100. oxidation activity and selectivity. Glycol selectivity increases with decreasing the H202 conversion rate (Figure 5). The reason of the H202 conversion decrease may be that the slow diffusion of glycols from the Al(Ga)-TS-l channels hinders the diffusion of H202 and the olefins. ZSM-5 has also very low phenol hydroxylation activity (Table 3 ) . A1-TS-1 and Ga-TS-1 acid sites do not affect catalytic activity. The behaviour of these solids is comparable with that of TS-1. 0
428
The Fe-TS-1 is an exception. It makes high molecular weight products instead of diphenols and has an high activity toward the H202 decomposition to H20 and 02. A small amount of non framework iron may be responsible for this.
.04 0
0
20
40
60min. 0
40
80 120 160
12.5
2 5 37.5 50
.04 0 I f
0 H,O,
40
80
120 160min. 0
0 Epoxide
AGlycol
Fig. 5 Catalytic activity of TS-1 and Al(Ga, Fe)-TS-1 in the 1-butene oxidation.
Conclusions We have synthesized silicalites containing both titanium and a trivalent element in the framework. Acidic form of Al(Ga, Fe)-TS-1 may catalyze both acidic and oxidation reactions. The presence of two different sites may modify selectivities. For example, in olefin oxidation acidity increases glycol selectivity. Ion exchange capacities of A1-TS-1, Ga-TS-1 and Fe-TS-1 increase with increasing lattice trivalent element concentration. This provide the possibility to introduce a transition element into a non-framework position by ion exchange. The product will contain two (or in the case of Fe-TS-1, three) transition elements. The presence of different sites inside the zeolitic channels make this class of catalysts suitable to investigate about the synergism among them. The direct synthesis of bifunctional zeolitic catalysts creates new opportunities for the application of shape selective materials in heterogeneous catalysis.
429
Aknowledgements The authors are indebted to Dr. R. Millini, Dr. A. Gervasini, Dr. L. Montanari for providing X-Ray diffraction, EPR, MAS-NMR respectively and to EN1 Companies: Eniricerche S.p.A., Enichemsynthesis S.p.A. and Snamprogetti S.p.A. for the permission to publish these data. References M. Taramasso, G. Perego and B. Notari, US Pat.n.4410501 (1983). a) G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo andA.. Esposito, Stud. on Surf..Sci. and Catal.., 2 (1986) - ~- . 1 2 9 . b) G. Bellussi, G. Perego, A. Esposito, C. Corno and F. Buonomo, Proc. of Fifth Italian Nat. Cong. on Catalysis, Universita degli Sudi di Cagliari Ed., (1986) p. 423. U. Romano, A. Esposito, F. Maspero and C. Neri, Proc. of Int. Symp. on "New development in Selective Oxidations", G. Centi and F. Trifir6 Eds, Rimini 1989, Preprints B1. a) H. Baltes, H. Litterer, I.E. Leupold and F. Wunder, EP Appl.n.77522 (1982). b) B.M.T. Lok, B.K. Marcus and E.M. Flanigen, EP Appl.n.179876 and EP Appl.n.181884 (1985). a) G. Bellussi, A. Giusti, A. Esposito and F. Buonomo, EP Appl.n.226257 (1988). b) G. Bellussi, M.G. Clerici, A. Giusti and F. Buonomo, EP Appl.n.226258 (1988). c) G. Bellussi, M.G. Clerici, A. Carati and A. Esposito, EP Appl.n.266825 (1988). J.El Hage-qi Asswad, J.B. Nagy, Z. Gabelica and E.G. Derouane, 8 Int. Zeolite Conf., Recent Research Reports, J.C. Jansen, L. Moscou, M.F.M. Post Eds, Amsterdam 1989, p. 475. a) B.D. McNicol and G.T. Pott, J. Catal., 25 (1972) p.223. b) E.G. Derouane, M. Mestalagh and L.J. Vielvoye, J. Catal., 33 (1974) p.169. C.T.W. Chu and C.D. Chang, J. Phys. Chem., 89 (1985) p.1569.
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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
431
S T U D Y ON T I TAN1 UM S I L I CALI TE SYNTHESI S
Padovan(a) F. Genoni(a) G. L e o f a n t i ( a ) , G. Petrini(a), T r e z z a ( b ) : A. Z e c c h i n a ( c ) : (a)Montedipe S . r . l . Unith d i R i c e r c a d i B o l l a t e , Via S. P i e t r o 50 2 0 0 2 1 B o l l a t e (Milano), I t a l y . Montedipe S. r. 1. Centro Ricerche d i Marghera, Via d e l l a C h i m i c a 5, 3 0 1 7 5 P o r t o Marghera (Venezia), I t a l y . D i p a r t i m e n t o d i Chimica I n o r g a n i c a Chimica F i s i c a e d e i Mater i a l i , V i a P. G i u r i a 7, 1 0 1 2 5 Torino, I t a l y .
M. G.
SUMMARY The s y n t h e s i s of T i s i l i c a l i t e w a s s t u d i e d i n a s e a l e d s i l i c a t u b e v i a a d r y i m p r e g n a t i o n of a preformed m i c r o s p h e r o i d a l porous s i l i c a w i t h a s o l u t i o n c o n t a i n i n g TPAOH and a s o l u b l e a l k y l t i t a n a t e . I n few hours t h e c r y s t a l l i z a t i o n proceeds towards t h e f o r mation of t h e z e o l i t e having s u b s t a n t i a l l y a l l T i i n c l u d e d i n t h e framework. E x t r a s t r u c t u r a l T i appears o n l y i n l o n g t i m e s y n t h e s i z e d products. INTRODUCTION
During t h e l a s t p e r i o d c o n s i d e r a b l e a t t e n t i o n has been g i v e n t o i n v e s t i g a t i o n s on T i s i l i c a l i t e ( T S ) , a zeolite exhibiting v e r y v a l u a b l e c a t a l y t i c p r o p e r t i e s f o r a v a r i e t y of r e a c t i o n s of i n d u s t r i a l i n t e r e s t , i n p a r t i c u l a r f o r cyclohexanone ammoximation, phenol h y d r o x y l a t i o n and o l e p h i n s e p o x i d a t i o n (refs. 1, 2 ) . D e s p i t e t h a t , t h e s y n t h e s i s of TS, l i k e fundamental a s p e c t s of i t s c h a r a c t e r i s t i c s and i t s c a t a l y t i c behaviour, has n o t been s t u d i e d . I n t h e p r e s e n t work, t h e c r y s t a l l i z a t i o n of t h e z e o l i t e s t r u c t u r e and t h e i n s e r t i o n of titanium i n t o t h e framework were i n v e s t ig a t e d . EXPERIMENTAL
Sample p r e p a r a t i o n TS samples a t v a r i o u s s y n t h e s i s t i m e were o b t a i n e d a c c o r d i n g Dried m i c r o s p h e r o i d a l s i l i c a t o t h e method d e s c r i b e d i n r e f . 3. (Grace S G 360) w a s impregnated up t o i n c i p i e n t wetness w i t h an aqueous s o l u t i o n c o n t a i n i n g b o t h t h e t i t a n i u m s o u r c e and t e t r a propilammonium hydroxide (TPAOH). The s o l u t i o n w a s p r e p a r e d i n a s t i r r e d v e s s e l under i n e r t atmosphere by adding T i i s o p r o p o x i d e
432 t o i s o p r o p y l a l c o h o l f o l l o w e d by TPAOH aqueous s o l u t i o n , and subs e q u e n t h e a t i n g a t 353K t o remove t h e a l c o h o l . A f t e r i m p r e g n a t i o n t h e m i x t u r e was d i v i d e d
i n t o eight parts:
k e p t a t 448K i n s e a l e d g l a s s 1 5 and 20 hours.
The p r o d u c t s w e r e
5,
10,
7,
t h e n f i l t e r e d , washed
w a t e r and d r i e d f o r 1 6 hours a t 353K. was t h e n c a l c i n e d a t 823K f o r
were
t h e f i r s t seven
t u b e s r e s p e c t i v e l y 1, 2,
A f r a c t i o n of e a c h
with sample
These samples w e r e
10 hours.
la-
b e l l e d a s TS f o l l o w e d by a number r e p r e s e n t i n g t h e s y n t h e s i s t i m e (TS1, TS2, TS5, e t c . ) .
The e i g t h p a r t
of t h e s t a r t i n g
mixture
was s u b m i t t e d t o t h e same t r e a t m e n t b u t t h e s y n t h e s i s t e m p e r a t u r e k e p t a t 353K o n l y f o r 1 hour.
The p r o d u c t was l a b e l l e d a s TSO.
Chemical a n a l y s i s The T i c o n t e n t of v a r i o u s samples was d e t e r m i n e d by XRF. A1 c o n t e n t of s t a r t i n g s i l i c a , a s measured by AAS,
The less
resulted
t h a n 200 kg/g. XRD -
The X-ray
powder d i f f r a c t i o n
patterns were
recorded with
P h i l i p s d i f f r a c t o m e t e r equipped w i t h
a proportional counter,
using a Ni-filtered
The
CuK
a
radiation.
w i t h o u t any p r e v i o u s p r e t r e a t m e n t .
samples w e r e
by
examined
The c r y s t a l l i n i t y d e g r e e
d e t e r m i n e d by a p r o c e d u r e developed i n our l a b o r a t o r i e s
a
was
(ref. 4).
The method was based on t h e comparison between t h e i n t e g r a t e d i n t e n s i t i e s o f two d i f f e r e n t s p e c t r a l r a n g e s , s p e c i f i c a l l y a f f e c t e d
by t h e c r y s t a l l i n e and respectively.
by t h e amorphous
f r a c t i o n s of t h e
solid
I n t h i s manner t h e n e c e s s i t y of e x t e r n a l s t a n d a r d s ,
having known c r y s t a l l i n i t y , can be avoided.
The c r y s t a l l i t e s i z e
was d e t e r m i n e d from t h e h a l f width of t h e peaks by u s i n g d o u b l e t and Sherrer equation a f t e r correcting f o r t h e Kal,a2
the the
instrumental broadening ( r e f . 5 ) . TGA ( t h e r m o q r a v i m e t r i c a n a l y s i s 1 on 393K d r i e d samples TGA was c a r r i e d o u t u s i n g a Mettler TG 3000 equipment.
About 2 0 mg of e a c h sample w e r e h e a t e d a t t h e r a t e of 4 K min-l i n a H e flow of 50 cm3 min-l from r . t . t o 1073K. TGA on samples w i t h p r e a d s o r b e d m-xylene
B e f o r e t h e m-xylene a d s o r p t i o n each sample was i n s e r t e d i n t o a h o l d e r c o n n e c t e d t o a vacuum l i n e ( u l t i m a t e vacuum h e a t e d a t 573K f o r 2 hours.
mbar) and
The h o l d e r was t h e n c o o l e d t o
r. t . ,
433
i s o l a t e d from t h e vacuum
system and f i n a l l y
a s l i g h t excess
of
l i q u i d m-xylene w a s i n j e c t e d i n t o it. A f t e r 10 m i n . t h e w e t sample a t r.t. i n H e t i m e l o n g enough t o remove t h e most l i q u i d e x c e s s . F i n a l l y t h e sample was h e a t e d a t 4 K min-' r a t e i n a H e f l o w up t o 673K, r e c o r d i n g t h e weight loss ( r e f . 6 ) .
was removed, p u t i n t o flow (50 cm3
N2
t h e thermobalance and k e p t
m i n - l ) for a
adsorption
N2 a d s o r p t i o n i s o t h e r m s a t 7 7 K
were measured u s i n g a C.
Erba
SORPTOMATIC 1900 on p r e v i o s l y o u t g a s s e d samples (573K, 10 h o u r s , f i n a l vacuum
mbar). The
as method ( r e f . 6 )
h y d r o x y l a t e d s i l i c a of r e f . l a y e r region
of t h e
s t r a i g h t l i n e was
micropore volume was d e t e r m i n e d
by u s i n g a s r e f e r e n c e
t h e d a t a of
by
nonporous
6 and e x t r a p o l a t i n g t h e l i n e a r m u l t i -
plot t o
as=O.
t h e n used
s l o p e of
The
t o calculate
the
obtained
the external
surface
a r e a of t h e c r y s t a l s o r , m o r e e x a c l y , o f t h e primary p a r t i c l e s i z e . The above methods have been r e p o r t e d w i t h more d e t a i l s i n r e f .
UV-Vis
6.
DRS ( d i f f u s e r e f l e c t a n c e s p e c t r o m e t r y )
Diffuse reflectance spectra
w e r e obtained equipped w i t h a reference.
with a
over t h e
r a n g e 12500-50000
Perkin-Elmer LAMBDA
diffuse reflectance
mbar) and i t s i n s e r t i o n
MgO a s
(optical path 1
a cm)
vacuum system ( u l t i m a t e vacuum
i n an e l e c t r i c
furnace. I n t h i s
manner
each sample was o u t g a s s e d a t 393K b e f o r e t h e measure i n o r d e r e l i m i n a t e t h e adsorbed water.
-1
spectrophotometer
attachment using
A s p e c i a l l y designed quartz c e l l
allowed t h e connection t o a
15
cm
to
The Kubelka-Munk f u n c t i o n was used
t o express t h e experimental d a t a ( r e f . 8 ) . I R - DRS ( d i f f u s e r e f l e c t a n c e s p e c t r o m e t r y )
D i f f u s e r e f l e c t a n c e s p e c t r a w e r e measured between 4 0 0 and 4000 cm - 1 u s i n g a Perkin-Elmer F T I R 1640 spectrophotometer equipped w i t h a d i f f u s e r e f l e c t a n c e attachement. samples w e r e i n t i m a t e l y mixed w i t h i n t h e c a s e of UV-Vis
Before e a c h measure
K B r and f i n e l y ground.
the Like
s p e c t r a t h e r e s u l t s w e r e e x p r e s s e d i n terms
of Kubelka-Munk f u n c t i o n . DISCUSSION The whole s e t of r e s u l t s of t h e s y n t h e s i s p r o c e s s
ii,
allowed t o s e p a r a t e t h e four parts:
examination
i ) the i n i t i a l
of TPAOH w i t h S i a n d / o r T i compounds(TS0 s a m p l e ) , i i ) t h e
reaction disso-
434 l u t i o n of S i and T i compounds and t h e s t a r t i n g z e o l i t e (TS1 s a m p l e ) , i i i ) t h e c r y s t a l l i z a t i o n the zeolite structure
of t h e major f r a c t i o n
(TS2-TS10 s a m p l e s ) and
t h e c r y s t a l l i z a t i o n and
i v ) the final
formation
t h e completion
p r e c i p i t a t i o n of T i
of of
from
t h e s o l u t i o n (TS15-TS20 s a m p l e s ) . TSO sample
The s t a r t i n g m a t e r i a l looked l i k e a d r y powder w i t h t h e
whole
l i q u i d e n t r a p p e d i n i t s p o r e s , so t h a t p r a c t i c a l l y a l l t h e c o u l d be r e c o v e r e d from t h e t u b e (see Fig. s t e p of t h e s y n t h e s i s t i t a n i a had a l r e a d y
t h e r e a c t i o n of TPAOH taken place
In this
la).
solid initial
w i t h s i l i c a and
so o n l y a
f r a c t i o n of
or
organic
b a s e c o u l d be removed by washing. According t o t h a t , w e l l e v i d e n t bands i n t h e 2800-3100 and 1300-1500 ranges ( r e f . 9), b e l o n g i n g t o templating agent, w e r e detected i n t h e
I R spectra,
as w e l l a s
a
s t r o n g DTG peak a t 461K c o r r e s p o n d i n g t o a 1 4 . 0 % weight l o s s (see Fig.
2). A t t h i s s t a g e t h e s o l i d was c o m p l e t e l y amorphous a s dem-
o n s t r a t e d by:
60
i ) t h e absence of
Solid
any d e t e c t a b l e peak i n XRD
yield
P ?
4 6 1 ~6 3 8 ~
t
nm 0
firistallite
0
.
:: .
-TS1
*
.*-.
i
.
TS 2C
....._,.... .*’
I
size
DTG
.: .. :.:
400
400
pat-
-1
Wt. loss
461 K
..)..“ .- ....--.-........ *’ - \
m2g1 1 0
\‘External
5
10
7 surface area
0
15
20
-
.P Oh
“C .*,
638 K
*..
I
I
I
L
Synthesis t i m e ( h )
Fig. 1. Y i e l d and c h a r a c t e r i s t i c t r e n d s of s y n t h e s i s m a t e r i a l s .
Fig. 2. Peaks of TPA decomposit i o n (DTG c u r v e s ) and TG t r e n d s versus synthesis t i m e .
435
t e r n ( s e e Fig.
band i n
IR
s p e c t r a ( r e l a t e d t o f i v e membered r i n g s s y s t e m c h a r a c t e r i s t i c
l c ) , ii)
of
z e o l i t e framework r e f . 670K ( s e e
9,
t h e a b s e n c e of t h e Fig.
550 cm-'
3 ) and i i i ) t h e a b s e n c e o f t h e DTG
s t a t e of T i was reflectance s p e c t r a , where a broad band was o b s e r v e d a t a b o u t 4 3 5 0 0 cm-' ( s e e Fig. 4). A comparison w i t h t h e known s p e c t r a ( r e f s . 10, 11) of TS, c o p r e c i p i t a t e d T i 0 2 / S i 0 2 , and T i O Z a l l o w e d a few c o n s i d e r a t i o n s t o be made. The framework t e t r a h e d r a l T i i n TS i s a s s o c i a t e d w i t h a s t r o n g band a t 48000 cm-' h a v i n g a d i s t i n c t l i g a n d
peak a t
Fig.
2).
As
f a r as
the
concerned, i n f o r m a t i o n c o u l d be g a i n e d from t h e U V - V i s
t o m e t a l c h a r g e t r a n s f e r ( c . t. ) c h a r a c t e r . The amorphous T i 0 2 / S i 0 2 c o p r e c i p i t a t e s a r e c h a r a c t e r i z e d by a frequency v a r i e s with t h e Ti/Si r a t i o . t h e peak
approaches t h e
atoms become more and creasing t h e Ti/Si
whose
l o w Ti/Si
ratios
A t very
f r e q u e n c y found
more i s o l a t e d and
r a t i o the Ti
b r i d g e s ) g i v i n g T i 0 2 c l u s t e r s of undergoes a
peak i n t h e U V - V i s ,
gradual red
i n TS
because t h e
t e t r a h e d r a l and by
atoms a g g l o m e r a t e ( v i a
oxygen-
i n c r e a s i n g s i z e : t h e c. t.
s h i f t towards
the
values
Ti
inband
typical
of
a
U
3
-
600 R e la t i v e
.5 0
cm
intensity
n I
5
970/550 I 10 15 20 Synthesis time(h)
Fig. 3. I R peaks and r e l a t i v e i n t e n s i t y versus synthesis time.
1 DO0
I
30000
1
45000 c m-1
Fig. 4. D R S U V - V i s c h a r a c t e r i z a tion a t increasing synthesis time.
436
anatase crystals cm
-1
(where
c h a r a c t e r i z e d by
Ti is
i n octahedral
could conclude t h a t a t t h i s
an absorption position).
On
band a t
21500
t h i s basis
s t a g e of t h e p r e p a r a t i o n v e r y
we
small
c l u s t e r s of o c t a h e d r a l T i O Z g r a f t e d on s i l i c a w e r e p r e s e n t . TS1 sample During t h e f i r s t hour under hydrothermal c o n d i t i o n s a d i s s o l u t i o n of S i and T i compounds t o o k p l a c e : p l e r e p r e s e n t e d 90% o f t h e t o t a l
obtainable s o l i d
This sample c o n t a i n e d o n l y 1%T i (see Fig.
partial
c a l c i n e d TS1
sam-
(Si02+Ti02).
l b ) s o meaning t h a t
a
T i r i c h p h a s e had t o be p r e s e n t i n s o l u t i o n . A t t h i s s t a g e a p a r -
t i a l c r y s t a l l i z a t i o n had a l r e a d y t a k e n p 1 a c e : t h e z e o l i t e c o n t e n t s of TS1 sample w a s a b o u t from XRD p a t t e r n ,
10%.
The above c o n c l u s i o n was
I R band a t 550 cm-'
obtained
on 823K c a l c i n e d sample and
from TGA on 353K d r i e d s a m p l e . I n p a r t i c u l a r t h e i n s p e c t i o n of DTG c u r v e p o i n t e d o u t t h e p a r t i a l s u b s t i t u t i o n of t h e 461K peak
with
a peak h a v i n g a maximum a t 638K c h a r a c t e r i s t i c of b o t h s i l i c a l i t e and TS ( s e e Fig.
2 ref.
2).
The agreement among t h e d a t a from t h e
various techniques i n d i c a t e d t h a t g r e e w e l l formed
a l s o a t low c r y s t a l l i n i t y
c r y s t a l l i t e s predominated on
w e l l d e t e c t a b l e by XRD.
de-
small nuclei
not
The r e l a t i v e l y l o w c r y s t a l l i z a t i o n d e g r e e
and c o n s e q u e n t l y t h e l a r g e amount of t h e amorphous f r a c t i o n
made
d i f f i c u l t t o i n v e s t i g a t e t h e i n s e r t i o n of T i i n t h e growing frame work. A s a m a t t e r o f f a c t t h e U V - V i s spectrum was dominated by l a r g e band a t 38000 cm-'
and t h e 48000 cm-'
band of framework
a Ti
c o u l d n o t be d e t e c t e d . A s i m i l a r c o n c l u s i o n c o u l d be d e r i v e d a l s o from t h e I R s p e c t r a . I n f a c t
t h e i n t e n s i t y of t h e 9 7 0 cm-I
band
( a s s i g n e d t o a s t r e t c h i n g mode of a [ S i 0 4 ] s t r u c t u r e bonded t o
a
framework T i , r e f . 1 0 ) does n o t s u b s t a n t i a l l y d i f f e r from t h a t 02 s e r v e d on TSO sample. F i n a l l y T G c u r v e s of TS1 and TSO samples c o n t a i n i n g p r e a d s o r b e d m-xyl ene w e r e a l s o s u b s t a n t i a l l y i ndi s ti ng u i s h a b l e : i n p a r t i c u l a r t h e weight l o s s i n t h e t e m p e r a t u r e r a n g e t y p i c a l of TS ( r e f . 6 ) was n o t d e t e c t e d on TSO and TS1. The whole o b s e r v a t i o n s p r e v i o u s l y made s u g g e s t , a l t h o u g h t h e s t a t e of T i i s u n c e r t a i n , t h a t d u r i n g t h e f i r s t hours of hydrothermal c o n d i t i o n s a f o r m a t i o n of s i l i c a l i t e n u c l e i
has a l r e a d y t a k e n p l a c e . I t
most n o t i c e a b l e t h a t a s i m i l a r b e h a v i o u r was found f o r t h e
is
ZSM-5
s y n t h e s i s by i n c i p i e n t wetness i m p r e g n a t i o n of s i l i c a ( r e f . 1 2 ) . TS2-TS 10 samples On i n c r e s i n g t h e s y n t h e s i s t i m e t h e y i e l d of s o l i d r e c o v e r e d
437
a t t h e end of each experiment i n c r e a s e d . A t t h e same t i m e c r y s t a l l i z a t i o n proceeded,
r e a c h i n g a b o u t 60% a t 5 hours and a b o u t
95%
a f t e r 10 hours. The measure of t h e z e o l i t e c o n t e n t by X R D , I R -1 b a n d ) , TGA (638K DTG p e a k ) and N2 a d s o r p t i o n (see F i g .
(550
cm
lc)
gave s i m i l a r r e s u l t s p o i n t i n g o u t t h e r e g u l a r development of
the
framework. The o b s e r v e d i n c r e a s e of c r y s t a l l i t e s i z e (see Fig. I d ) s u g g e s t e d a p r o c e s s of c r y s t a l growth r a t h e r t h a n a f o r m a t i o n
of
new n u c l e i . A f t e r a minimum a t a b o u t 3 hours hydrothermal t r e a t i n g t h e T i c o n t e n t showed a t r e n d towards g r a d u a l i n c r e a s e . I n a p a r a l l e l way t h e band a t 48000 cm-'
c h a r a c t e r i s t i c of framework
p r o g r e s s i v e l y s u b s t i t u t e d t h e a b o u t 40000 cm-I one. The
Ti,
building
up of TS w a s a l s o s u p p o r t e d by t h e p a r a l l e l i n t e n s i t y i n c r e a s e of t h e 970 cm-l
band i n
t h e I R spectrum
m-xylene a d s o r p t i o n ( s e e Fig. 5 ) .
and by
t h e increment
of
These o b s e r v a t i o n s a l l o w e d us t o
suppose a mechanism i n which, a f t e r a n i n i t i a l r a p i d
dissolution
of t h e s u r f a c e T i compounds and a f o r m a t i o n of s i l i c a l i t e n u c l e i , T i w a s p r o g r e s s i v e l y r e c o v e r e d from t h e s o l u t i o n and i n c o r p o r a t e d
i n t h e growing z e o l i t e c r y s t a l s . T h e growth of t h e c r y s t a l s i n t h e 5-10 hours of hydrothermal t r e a t i n g i n t e r v a l o c c u r r e d a t a p p r o x i mately c o n s t a n t T i
concentration. I n f a c t ,
a t l e a s t within
e r r o r s of t h e method, t h e r a t i o between -1 s i t i e s of 970 cm band ( r e l a - r t e d t o s t r u c t u r a l T i c o n t e n t ) 'Y
the inten-
experimental
and 550
cm
-1
band
(related
to
z e o l i t e content) was constant. I t
i s most
r.
a
noticeable t h a t . during
DTG
t h e s a m e i n t e r v a l a l s o t h e a d s o r 200ption capacity
of m-xylene
per
u n i t channel volume remained
COG
s t a n t . The above r e s u l t s l e t
us
t o c o n c l u d e t h a t d u r i n g t h e main to
10
hours)
the
changes
in
mother s o l u t i o n due t o t h e z e o l L t e formation to
not
were s m a l l
induce
enough
the
.....TSO
I
.-....,............... \...
0-
-
-TS20
**
1
I
I
1
7 1
Am
El
.04
composition
changes i n t h e growing c r y s t a l s . TS15 TS20 samples As r e p o r t e d above t h e sample
TSlO had 9 6 % c r y s t a l l i n i t y .
.03
5
10
'5
-
20
Synthesis t i m e hh)
Fig. 5. DTG c u r v e s and m-xylene adsorption trend.
438
F u r t h e r i n c r e a s e i n s y n t h e s i s t i m e d i d n o t change t h e z e o l i t e con t e n t of t h e s o l i d w h i l e i t s y i e l d i n c r e a s e d a p p r o a c h i n g 100%. Dug i n g t h i s s t a g e t h e amorphous
s i l i c a has a l r e a d y been
consumed and t h e c r y s t a l l i z a t i o n c o u l d the solution.
completely
o c c u r o n l y a t expense
of
The T i c o n t e n t of t h e s o l i d f u r t h e r i n c r e a s e d from
TSlO t o TS20. N e v e r t h e l e s s t h e normalized i n t e n s i t i e s of t h e
970
a d s o r p t i o n c a p a c i t y of t h e s o l i d
did
cm
-1 band and t h e m-xylene
n o t change s u g g e s t i n g t h e framework t i t a n i u m was a b o u t
constant.
The seeming
could
be
d a t a . I n f a c t an a b s o r p t i o n
in
c o n t r a d i c t i o n between
e x p l a i n e d on t h e b a s i s of W-Vis t h e 35000-37000 c m - l under d i s c u s s i o n . A s
the
above r e s u l t s
r a n g e developed d u r i n g t h e s y n t h e s i s reported i n
Ref. 6
period
t h i s a b s o r p t i o n can
be
r e l a t e d t o t h e f o r m a t i o n of t i t a n i a o r T i r i c h s i l i c a - t i t a n i a soli d s o l u t i o n , meaning t h a t i n t h e l a s t mother s o l u t i o n t h e remaini n g T i was i n c o r p o r a t e d i n extra-framework o c t a h e d r a l p o s i t i o n s . CONCLUSIONS I n c o n c l u s i o n o u r method allowed t o e a s i l y o b t a i n p u r e and w e l l c r y s t a l l i z e d T i s i l i c a l i t e samples by u s i n g t h e a p p r o p r i a t e s y n t h e s i s t i m e i n o r d e r t o a v o i d a r e s i d u a l amorphous c o n t e n t o r extra-framework T i . F u r t h e r work i s s t i l l i n p r o g r e s s i n o r d e r t o s y n t h e s i s tempei n v e s t i g a t e t h e e f f e c t of T i c o n c e n t r a t i o n , r a t u r e , TPAOH c o n c e n t r a t i o n and s i l i c a i m p u r i t i e s . REFERENCES P. R o f f i a , G. L e o f a n t i , A. Cesana, M. Mantegazza, M. Padovan, G. P e t r i n i , S. T o n t i . P. G e r v a s u t t i . N e w Developments i n S e l e c t i v e O x i d a t i o n , E l s e v i e r , Amsterdam, 1990, pp 43-52. U. Romano, M. C l e r i c i , A. E s p o s i t o , F. Maspero, C. Neri, i b i d . , pp 33-42. I t . Pat. 21511A/85. G. Carazzolo, F. G a t t i , A. Ponzoni, M. Solari, Montedipe I n t e r n a l Rept., 1982 H. P. Klug, L. E. Alexander, X Ray D i f f r a c t i o n Procedure f o r p o l y c r i s t a l l i n e and amorphous M a t e r i a l s , 2nd edn. , John Wiley & Sons L t d . , C h i c h e s t e r , 1974. G. L e o f a n t i , F. Genoni, M. Padovan, G. P e t r i n i , G. Trezza, A. Zecchina, Proc 2nd. IUPAC-Symposium on C h a r a c t e r i z a t i o n of Porous S o l i d s , A l i c a n t e , Spain, May 6-9, 1990, i n p r e s s . S. J. Gregg, K. S. W. Sing, Adsorption, S u r f a c e Area and Poros i t y . , 2nd. e d n . , Academic P r e s s , London, 1982. R. A. Schoonheydt i n F. Dalannay (Ed. ), C h a r a c t e r i z a t i o n of h e t e r o g e n e o u s Cat., M. Dekker I n c . , New York, 1984, pp. 125-160. K. F. M. G. J. S c h o l l e , W. S. Veeman. P. Frenken, G. P. M. van d e r Velden., Appl. C a t a l . , l ’ l , 1985, pp. 233-259. 1 0 ) M.R. Boccutx, K.M. Rao, A. Zecchina, G. L e o f a n t i . , G . P e t r i n i . Surfaces, Elsevier, Amsterdam, S t r u c t u r e and R e a c t i v i t y of 1989, pp 133-144. 1 1 ) G. P e t r i n i e t a l . , i n p r e p a r a t i o n . 1 2 ) M. Padovan, G. L e o f a n t i , M. S o l a r i , E. M o r e t t i . , Z e o l i t e s , 4 , 1984, pp 295-289.
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
439
A C T I V A T E D CARBON FROM B I T U M I N O U S COAL
J..4.
Fajares, J.J. Pe'rez I n s t i t u t o Nacional SPA IN
A. J.
P i s , -4.B.
del
Fuertes,
Carbo'n,
J.B.
C.S.I.C.,
M.
Parra,
Aptdo
73,
Mahamud a n d
33080 Oviedo,
ABSTRACT
The effect, o n the preparation of activated carbons, of coal preosidation, particle size and activating flow rate was studied. .4s starting material A high bituminous coal was used. Data on the progressive evolution of texture in oxidized coal samples, and in subsequent chars and activated material are presented. T h e flow rate of the activating agent ( C O , ) has a big influence on the development of the porous network o f the activated carbons. INTRODUCTION
Activated carbon has traditionally been used as an adsorbent and catalyst
(ref.
1,2),
especially
in
some
gas-phase
osidation
reactions at low temperature. Its use in simultaneous SO, and NO, removal, H,S elimination from industrial gaseous effluents, and also it,s role as activator agent of chlorination and dehydrochlorination
is wellknown (ref. 1 - 4 ) . Activated carbon, as a catalyst support is in increasing demand nowadays (ref 2 - 5 1 . factors
like
Especially
shape, particle
notable is its ability to control size
(powdered and
granular) and
mechani-cal properties (soft, hard carbon). It also presents a good potential on pore size distribution and subsequent specific surface area
(30 - 3000 m'g-'),
together
with
a
hydro(phi1)phobicity
character and/or surface concentration o f acid/basic centers. A l l these aspects make
activated carbon a versatile material, with a
range of properties adequate
f o r supporting all types and amounts
ofmetallicand osidic active elements. Examples of this is its use as
support
of
zinc
acetate
in
the
industrial
preparation
of
vynil-acetate polymer ( ref. 3 , 6 ) , its strong performance as support of
noble
preparation
metals,
mainly
processes
for
hydrogenation
(ref. 5 , 7 , 8 ) .
Also
in
notable
fine is
chemical its large
surface area, which can give a high metal dispersion with relatively weak metal/support interaction (ref. 3 ) , and the possibility of an
440 easy r e c o v e r y o f
t h e a c t i v e component
just
by b u r n i n g .
A l l these
f e a t u r e s d e t e r m i n e i t s i n t r o d u c t . i o n as a c a t a l y t s s u p p o r t e v e n i n such
f i e l d s a s SO,
traditional
oxidation
Granular shell,
( G A C ) are
activated carbons
peat,
and
bituminous
coals.
The
(ref.
8 ) , hydrocracking
10).
( r e f . 9 ) and h y d r o d e s u l p h u r i s a t i o n ( r e f .
made m a i n l y
use
of
an
from coconut
abundant,
cheap
s o u r c e , i m p l i e s t , h a t c a r b o n e x - m i n e r a l c o a l s r e p r e s e n t more t h a n 6 0 % o f GAC p r o d u c t . i o n
(ref.
11). D e t a i l s of
t h e manufacturing process
haire u s u a l l y b e e n kept. a s a n i n d u s t r i a l s e c r e t . H o w e v e r , v a r i a b l e s l i k e coal and
rank,
yield in
particle
the
size,
oxidation,
activation step,
must
flow
rate o f
be taken
into
reactants account
in
t a i l o r i n g an a r t i v a t e d carbon f o r a s p e c i f i c application. In
this
paper,
some
of
these
are
variables
studied
for
the
p r e p a r a t i o n o f a c t i v a t e d c a r b o n s by a c t i v a t i o n o f a b i t u m i n o u s c o a l S p e c i a l e m p h a s i s h a s b e e n p u t m a i n l y o n t h e e f f e c t of c o a l
w i t h CO,. oxidat.ion
and
r'esults,
that
rate
fl.ow
the
were
of
the
restricted
a p p r o s i m a t e l y 52% b i i r n - o f f ,
activating
to
agent.
materials
From
the
activated
to
g e n e r a l l i n e s may b e t r a c e d for d e s i g n
o f a c t i v a t e d c a r b o n s wit.h a p a r t i c u l a r p o r e s i z e d i s t r i b u t i o n . D a t a on
the
progressive
evolut,ion
of
texture
of
oxidized
coals,
and
s u b s e q u e n t c h a r s a n d a c t i v a t , e d p r o d u c t s a r e also p r e s e n t e d . EXPER I MENTAL A High B i t u m i n o u s C o a l from t h e Ma L u i s a m i n e ,
from t h e C e n t r a l
A s t u r i a n B a s i n h a s b e e n u s e d . The most i m p o r t a n t c h a r a c t e r i s t i c s o f t.he s t a r t i n g m a t e r i a l a r e g i v e n i n T a b l e 1 . The c o a l was g r o u n d a n d two
size
t1.00-3.00 T.4RI.E
fractions
were
used:
tO.125-0.425
mm
(P
series)
and
mrn ( G s e r i e s ) .
1
C h x r a c t r r i s t i c s of t h e c o a l used Prosimate a n a l y s i s ( % w t ) Yoisture
Ash
1.35
3.80
V.M. (dry) (daf)
37.16
Arnu t e s t
Tr,K 622 F.S.I.
Ts,K 692
U l t i m a t e Analysis
(%wt, daf)
C
H
N
S
0 (diff.)
86.70
5.04
1.27
0.52
6.47
Maceral C o m p o s i t i o n
Tc,K 738
b,% 179
Vit.
65.2
: Free Swelling Index
Exi. 10.6
Semif. 7.4
F.S.I. Fus. 13.6
8
441
The oxidation of coal was carried out in an oven with forced circulation
.
For series P oxidation was carried out at 4 7 3 K for
different periods of time between 0 and 24 hours. F o r series G runs were carried out at 5 1 3 K and oxidation times were between 21 and 72 hours. The
pyrolysis
performed
of
fresh
coal
and
oxidized
coal
under nitrogen at 1 1 2 3 Ii with a heating
samples
was
rate of about
60 K min-' and 5 min of soaking time. The activation was carried out with CO, in a vertical reactor
quartz
(I.D. 20 m m ) , at 1 1 2 3 K and two different CO, flow rates:
7 and 500 mL min". conditions,
at
Gasification was carried
102.7
(770
kPa
mm
Hg)
out
under
isobaric
untilapproximately
52%
burn-off. Textural
properties
were
obtained
from
measurement
of
true
(helium) and apparent (mercury) densities, total open pore v o l u m e s and
pore
volume
For
distributions.
determination of
the
helium
densities, a Micromeritics Autopycnometer 1 3 2 0 uas used. Apparent densities were determined
in a Carlo Erba Macropore Unit 1 2 0 . The
pore volume distributions were evaluated with a mercury porosimeter, Carlo Erba 2 0 0 0 . Specific surface areas were determined by physical adsorption in a Oninisorb 3 6 0 and a Sorptomatic Carlo Erba 1 9 0 0 . N , at 7 7 K and CO, at 2 7 3 K were used. We assumed for a
a
cross-section
molecule of N~ of 0.162 nm2 and of 0 . 1 8 7 nm2 for a molecule
of CO,. 411 textural properties
are expressed
on
a dry ash free
basis (daf). RESULTS AND DISCUSSION Oxidation Preoxidation
of
bituminous
coal
is
a
crucial
step
in
the
preparation of activated carbons. Oxidation produces a decrease i n the caking properties, or even its total destruction (ref. 1 2 ) . I n fact, an important transformation
in the chemical composition and
in the porous structure of the coals (ref. 1 3 ) was produced. Some of the more important characteristics of oxidized coals are given in Table 2. The caking properties of samples decrease as a result of air oxidation, so a drastic reduction in free swelling index ( F S I ) is observed from 8 in fresh coal to 1 and 0 in oxidized samples. Likewise, an important decrease in carbon content, and a parallel increase in volatile matter and oxygen content,, mairil>- 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 .
c:alculated from mercury porosimetry and helium densities, of chars obtained f r o m oxidized coal and activated at the above mentioned CO, f l o w rat-es. In general, an important enhancement in pore volume can h?
observed when coal. preoxidation is increased and when low f l o w
445
rates (high CO concentrations) are used. When low flow rates of CO, were used the increase of pore volume is especially noticeable for the pore volume contribution of pores with a radius smaller than 3.7 nm. These results are in agreement with our previous results (ref. 19) and with the studies of Rand and Marsh (ref. 18), who observed that a greater micropore volume is developed by gasification when a lower flow rate i s used, presumably because o f the inhibiting effect of CO in the outermost section o f the particle.
As can be seen in Figure 2, low flow rates of
C 0 2 give activated
carbons with a bigger development of mesopores.
This is o f great
importance in view o f the eventual use of these materials (ref. 20).
In fact, a well-balanced
pore
size distribution is essential
in
processes in which both surface area and the penetration o f reactive gases into the inner porosity of the particle are important. Textural
parameters
obtained
in
the
analysis
of
adsorption
isotherms of N, at 77 K and CO, at 273 K is shown in Table 4. in the N,
the volume adsorbed surface
area;
W,,
isotherm, and S,,
the
volume
is
the BET especific
isotherm; S,,,,
micropore
\;ads
obtained
from
the
CO,
the corresponding equivalent surface area of K O .
TABLE 1 Textural properties of the active carbons obtained Active Carbon
In general, adsorption capacit3 increased with coal oridatj.ori. I n fact, activated carboils prepared
from the most oxidized coals
and activated at lower flow rates, exhibit the highest adsorpt-ion capacity. Powdered materials, activated with high flow rates of CO,, give
the
largest
differences
between
SB,,r
and
S D R . These
biq
446 differences micropores
suggest
that
gasifying
of the particles
gas
gets
into
the
small
in the activation steps. In powdered
materials, when low flow rates of CO, are used, the values of S,,, are twice as big as those obtained at high flow rates. The difference between S,,,
an S,,,
that is practically zero in
activated carbons obtained from fresh coal (P.OS), increases with raw coal oxidation. An enlargement in the diameter of micropores is observed when low flow rates o f CO, are used. These results agree with these of Marsh
(ref. 21) who maintains that the increase in
( S B E T - S D Rinvolves ) a growth in the diameter of micropores.
The
results obtained
with granular materials
(G series) are
sigpificantly different. The volume of gas adsorbed in the isotherms of N,
and CO, is higher than the corresponding volume of series P.
Moreover in all the samples the difference ( S , , ,
- S D R )is positive,
and approximately constant in the samples activated at low flow rates. In the samples G.24s, G . 4 8 ~ and G . 7 2 ~this difference is indicative of condensation of N, carbons, and
in the micropores
of activated
suggests the existence of supermicropores i n
these
samples. The volume of micropores, W,,
is practically
CO, activation flow rate. However, the ,,S,
independent of the
values are always higher
in the samples activated with a small flow rate of CO,, as can be seen in Table 4. These results suggest that, the higher the coal preosidat,ion and the lower the CO, activation flow rate, the bigger the development of porous texture. The right choice of coal oxidation and activating gas flow rate, can produce activated carbons with a suitable textural development. CONCLUSIONS
The flow rate the development
of the activating agent has a big influence on of
the porous network of the activated carbons.
High CO, flow rates give rise to materials with a wide pore size distribut,ion, while low CO, flow rates gives materials with a higher pore volume, a better developed microporosity and a corresponding higher surface area. Raw coal preoxidation always has a beneficial effect
on
the
textural
development
of
the
activated
carbons
obtained, both in powdered and granular materials. Activated carbons with a suitable textural
development can be obtained if the right
choice o f raw coal osidation and activating gas flow rate ratio is made.
447
ACKNOWLEDGEMENTS The a u t h o r s t h a n k F u n d a c i 6 n p a r a e l F o m e n t 0 d e l a I n v e s t i g a c i o n e n A s t u r i a s (FICYT) f o r
f i n a n c i n g t h i s work.
t o e x p r e s s t h e i r t h a n k s t o M.E.C.
f or F.P.I.
A.J.P.
and M . M .
wish
grants.
REFERENCES
10 11
12
13
14
15
M . Smisek and S. C e r n y , A c t i v e Carbon, E l s e v i e r , Amsterdam, (1970). R . C . B a n s a l , J . B . Donnet and F. S t o e c k l i , A c t i v e Carbon, H a r c e l Dekker, New York, ( 1 9 8 8 ) . H . J u n t g e n , A c t i v a t e d c a r b o n as c a t a l y s t s u p p o r t . I\ r e v i e w o f new r e s e a r c h r e s u l t s , F u e l , 6 5 ( 1 9 8 6 ) 1 4 3 6 - 1 4 4 6 . D . L . T r i m m , i n C . K e i n b a l l a n d D . A . Dowden ( E d s . 1 , C a r b o n a s a c a t a l y s t and r e a c t i o n s o f c a r b o n , C a t a l y s t , 4 , p . 210, London, 1981. i n : B . Mc E n a n e y a n d T . J . Mays ( E d s . 1 , JOP P. Ehrburger, P u b l i s h i n g l t d . , B r i s t o l , 1988, p. 361. G . H . v a n d e r B e r g a n d B . H . R i j n t e n , i n €3. D e l m o n , P . G r a n g e , P . J a c o b s a n d G . P o n c e l e t ( E d s . ) , P r e p a r a t i o n o f C a t a l y s t s 11, E l s e v i e r , .Amsterdam, 1 9 7 9 , p . 2 6 5 . L . D a z a , T . G o n z A l e z .4yUSO, S . M e n d i o r o z a n d J . A . P a j a r e s , Decomposition o f p r e c u r s o r s on p r e p a r a t i o n of R h / a c t i v e carboii c a t a l y t s . Appl. C a t a l y s i s , 1 3 ( 1 9 8 5 ) 295-304. T . Mallat, J . P e t r o , S . Mendioroz and ,J.A. P a j a r e s , Appl. C a t , a l y s i s , 3 3 , ( 1 9 8 7 1 , 245. V.H.J. de B e e r , F . J : D e r b y s h i r e , C . K . l a R o o t , R . P r i n s , . A . W . S c a r o n i and J . M . S o l a r , H y d r o d e s u l p h u r i z a t i o n a c t i v i t y and coking p r o p e r t y of carbon and alumina s u p p o r t e d c a t a l y s t s . F u e l , 63 ( 1 9 8 4 ) 1095-1100. J . C . P u c h e t , E.M. van O e r s , V . Y . S . d e B e e r and R . P r i n s , J . C a t a l y s i s , 80 ( 1 9 8 3 ) 386. R . E . K i r k and D.F. Othmer, E n c y c l o p e d i a of Chemical 'Technology, The I n t e r s c i e n c e E n c y c l o p e d i a , N e w Y o r k , 3rd. E d i t i o n , 1 9 8 1 , \ o l . 4 , p.561. J . J . P i s , A . C a g i g a s , P. Sim6n a n d J . J . L o r e n z a n a , E f f e c t o f aerial o x i d a t i o n o f c o k i n g c o a l s on t h e t e c h n o l o g i c a l p r o p e r t i e s of the r e s u l t i n g c o k e s . Fuel P r o c e s s i n g T e c h n o l . , 20 ( 1 9 8 8 ) 307-316. M.M. L u d v i g , G.1,. Gard and P.H. E m m e t t , U s e of cotitr'olled o x i d a t i o n t o i n c r e a s e t h e s u r f a c e area o f c o a l . A p p l i c a t i o n t o a b i t u m i n o u s a n d a s e m i - a n t h r a c i t e c o a l . F u e l , 62 ( 1 9 8 3 ) 1 3 9 3 - 1 3 9 6 . D . J . Maloney an d R . G . J e n k i n s , I n f l u e n c e o f c o a l p r e o s i d a t i o n a n d t h e r e l a t i o n b e t w e e n c h a r s t r u c t u r e arld g a s i f i c a t i o n p o t e n t i a l . F u e l , 64 ( 1 9 8 5 ) 1 4 1 5 - 1 4 2 2 . J . J . P i s , J . A . P a j a r e s , A . B . F u e r t e s , M . f.fahamiid, J . R . P a r r a , A.J. P6rez and B. R u i z , I n f l u e n c e of c o a l o x i d a t i o n on t h e preparation of act,ive carbon precursors. Carbone 90, P a r i s France (accepted)
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448
1 6 O . P . M a h a j a n , M . Komatsu a n d P . L . W a l k e r , Jr., L o w - t e m p e r a t u r e a i r o x i d a t i o n o f c a k i n g c o a l s . 1. E f f e c t on s u b s e q u e n t r e a c t i v i t y o f c h a r s p r o d u c e d . F u e l , 59 ( 1 9 8 0 ) 3 - 1 0 . 1 7 J.A. P a j a r e s , J.J. P i s , A.B. F u e r t e s , A.J. P B r e z , M . Hahamud a n d J.B. P a r r a , I n f l u e n c e o f c o a l p r e o x i d a t i o n a n d r e a c t i v e g a s f l o w r a t e o n t e x t u r a l p r o p e r t i e s o f a c t i v e c a r b o n s . COPS T I , A l i c a n t e , S p a i n , may 1 9 9 0 ( a c c e p t e d ) . 1 8 B . Rand a n d H . M a r s h , The p r o c e s s o f a c t i v a t i o n o f c a r b o n s b y g a s i f i c a t i o n w i t h C0,-I11 Uniformity of g a s i f i c a t i o n . Carbon, 9 ( 1 9 7 1 ) 79-85. 1 9 J . J . P i s , A . B . F u e r t e s , A . J . P B r e z , J.J. L o r e n z a n a , S . M e n d i o r o z a n d J.A. P a j a r e s , M o d i f i c a t i o n o f t e x t u r a l p r o p e r t i e s o f S p a n i s h c o a l - d e r i v e d c h a r s by a c t i v a t i o n w i t h c a r b o n d i o x i d e . F u e l P r o c e s s i n g T e c h n o l . , 24 ( 1 9 9 0 ) 3 0 5 - 3 1 0 . 20 T . Wignians, I n d u s t r i a l a s p e c t s o f p r o d u c t i o n a n d u s e o f a c t i v a t e d c a r b o n s . C a r b o n , 27 ( 1 9 8 9 ) 1 3 - 2 2 . 2 1 H . Marsh, A d s o r p t i o n methods t o s t u d y m i c r o p o r o s i t y i n c o a l s and c a r b o n s - a c r i t i q u e . C a r b o n , 25 ( 1 9 8 7 ) 4 9 - 5 8 .
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
449
CARBON-SUPPORTED PALLADIUM CATALYSTS. SOME ASPECTS OF PREPARATION I N CONNECTION WITH THE ADSORPTION PROPERTIES OF THE SUPPORTS
A.S. LISITSYN, P.A. SIMONOV, A.A. KEPTERLING and V.A. LIKHOLOBOV Institute of Catalysis, Novosibirsk 630090 (USSR) Abstract Adsorption of metal complexes on carbon supports can be accompanied by ligand-exchange involving intrinsic n-fragments of carbon framework. The size of metal crystallites derived from carbon-adsorbed species does not correlate with the proportion of metal accessible to carbon monoxide and reagents of catalytic reaction: this suggests blocking a part of the metallic surface by walls of the support pores. Appropriate choice of the support, metal precursor and procedures at the deposition step makes it possible to eliminate or, at least. diminish such a detrimental effect and to control the size. shape, sites of location. distribution along the catalyst grain and, thus, catalytic properties of final metal crystallites. A strightforward use of carbon-adsorbed complexes as heterogenized catalysts is also discussed.
IN!PRODUCTION Despite the wide application which supported metal catalysts have found in industry and fundamental researches, knowledge on their genesis is still rather limited and is especially poor f o r the catalysts based on carbon supports. In part it is caused by a secretive character of most works in the field of catalyst manufacture. In addition, carbon supports prepared from natural polymeric substances, such as coal or wood, were mainly used so far, with the texture, ash content and kind of admixtures being variable from batch to batch; this makes comparison and interpretation of published results difficult. This paper deals with properties of palladium catalysts on carbon supports and the results presented below concern processes taking part at the first stage of the catalyst preparation,namely, deposition of metal precursor onto support. When starting the work, we addressed question on C ) how strong the state of supported precursor can be reflected in catalytic properties of final samples, i t ) what factors are more and what less important in this respect and C C C ) how can one operate by these factors to achieve desirable catalytic properties.
450 0.,1 ,
. 0,7
,
P/Ps
no. Support 1 2 3 4 5 6
7
5
lo' t , A
*
PN
Eponitll3H
PME800
5550 5270 PM105 5450
Surface, sq.m./g t -me thod
BET
770 850* 700 550* 270 110* 450
185 450 1 90 450
a single point determination
F i g . 1 . Nitrogen adsorption data (78 K) and representative t-plots for carbon supports used: (1.2) active carbon, (3.6) carbon black, (4.5.7) Sibunit: t statistical thickness of liquid nitrogen film on non-porous sorbents at relative presure p/p S-
EXPERIMENTAL
Commercial active carbons and carbon blacks and samples of recently developed Sibunit carbon [I3 were tried as supports; their characteristics are indicated in Fig. 1. Unlike carbon blacks and, especially, active carbons which displayed a high proportion of micropores (t-plots [ 2 I give large positive intemepts on the adsorption axis), the Sibunit carbons used possessed well-developed mesoporous structure with a small (5270) and negligible (S450, S550) volume of micropores (the t-plots pass through the origin and specific surfaces derived from their slope appeared equal to BET values). Before use, supports were purified by treating with aqueous solutions of HC1 and H F . Adsorption measurements were performed using a W VIS spectrometer providing sufficient time for equilibrium to be reached (20 hs as a rule) [3,41. Following drying in vacuum or in air, the samples were reduced (if necessary) in a flow of hydrogen (2000 h-', 25OoC, 2 h; as tested by XPS [5], these conditions are suffioient for metallic palladium to be formed in all cases) and flushed with nitrogen before contact with air. Measurements with CO chemisorption were performed by a pulse technique (H2 flow, -8OOC). Catalytic testings i n hydrogenation and vinyl-exchange reactions were carried out in absence of external diffusional limitations; technique has been described elsewhere [4,51.
451
Fig. 2. ( 8 ) Adsorption i s o t h e r m s for benzene, naphthalene and phenanthrene ( 1 - 3 ) . respectively, from methanol solution. (b) The same for H PdC14 from aqueous solution ( 1 1 and (CH CN) PdCl from 2 ( 2 ) . ( C ) Effect of addition of HC1 ( 1 ) : &C1 T2) acetonitrile and Na CO (3) on adsorption of palladium species from solution of H PdC1; s0.025 M, 0.5 mg-atom Pd/g sup. to be a maximum of possibge coverages ; in the cross-hatched region palladium oxide precipitates rapidly upon adding solution of sodium carbonate in absence of carbon support). ,3270 (b1.c) and ,9450 (a.b2) as sorbents. ambient temperature.
RESULTS AND DISCUSSION 1. AdBorption studies Adsorption measurements with organic complexes of palladium and detailed investigation of adsorption of %PdC14 from aqueous solutions have been performed earlier [3,4,63. Results of complementary study are presented in Fig. 2. It is comonly accepted that the adsorption on carbon materials results from either van der Vaals forces or reactions with the surface oxygen groups (ion/ligand exchange and hydrolysis). Not discussing this in detail, it is only expedient to point out the strong dispersive interaction which arises between substances of aromatic nature and aromatic fragments of the carbon support. Anchoring of corresponding complexes proved to be possible thereby even on a graphite basal plane ( [ 7 1 and references cited therein). The ligands carrying functional groups (for complexing metal ions) which a r e attached to condensed rings might be appropriate f o r anchoring on carbon supports by such a means. As follows from Fig. 2a, the strength of adsorption to be enhanced as the number of fused benzene rings in the aromatic %nchort*increases. Adsorption of palladium chlorides represents a more complex case. It has been found [3,61 that up till a half of a monolayer
452
coverage, palladium species is absorbed from solutions of €$PdC14 or corresponding salts almost completely. The process was accompanied by releasing chloride ions (the Cl/Pd ratio in the surface complex was caclulated to become equal two) and there were virtually no changes in the pH value for the solution. The adsorbed species could be removed from the support (with exception of ca. 10 $% which remained irreversibly adsorbed) when treating the samples with concentrated solution of HC1; at the same time, treatment with solutions of HN03 or HC104 (poor complexing agents towards palladium) appeared ineffective. The number of adsorption sites for both irreversible. strong and weak adsorption (the second part of monolayer) appeared proportional to specific surface of the supports. The adsorption constants for the sites of strong adsorption have also been determined and proved to be somewhat higher in the case of micropous carbons. The same was true for preliminary oxidized supports, but the number of the strong adsorption sites in this case decreased. All this evidences that adsorption of palladium chlorides may be considered as a coordination process which involves n-fragments of the carbon framework. The exact nature of such fragments could alter from support to support thus explaining difference in the strength of adsorption on different carbons. In particularly, the surface oxygen groups in preoxidized supports are not excluded to enhance stability of the surface complexes through increasing the back donation of electrons from palladium to olefinic moiety due to decrease in electron density on carbon matrix. If one considers adsorption of ( CH3CN),PdC12 from acetonitrile solution (Fig. 2b, curve 2), the much lower coverage attained in this case is obviously explained by the large ooncentration of free ligand; adsorption perhaps takes place on diene-like fragments only. It could also be expected that conversion of [PdCl4I2- into [Pd(OH)4]2- species occurring in alkaline solutions would lead to decrease of adsorption because [Pd(OH)412- is a poorer electrophile to form complexes with olefinic moieties. This did hold true (Fig. 2c, case ( 3 ) at sufficiently large quantity of additive) but one can reach the opposite effect if a d d m alkali in a small surplus only to that for a simple neutralization of H2PdC14. This phenomenon reflects formation of polynuclear Pd(I1) oxy/hydroxy species (colloids) under such conditions.
453
RATE W
0.8
0.4
1
4
.0.3 .0.2 -0.1
b
1 1
2
3
4
5
K ~ = lo3 I
2
4
6
Pd, w k %
Fig. 3. (a.b) Mean diameters of palladium crystallites as determined by small-angle X-ray scattering method (a). fraction of palladium exposed (a) and turn over frequencies in cyclohexene hydrogenation (b) f o r catalysts on different supports (as designated in 318. 1 ; precursor in the state of strong adsorption, 1 pg-atom Pd/m : KQvalue for the constant of strong adsorption on the surface sites: CO/Pdsurf was assumed equal unity). (C) Total rate of cyclohexene hydrggenation on catalysts with progressively increased P d loading (cm H2 / m g cat/min. for conditions see Table 1 ; for clarity. data on catalysts 1 and 3 are omitted). Catalyst preparation: incipient wetness impregnationowith solution of H2PdC14. aging (20 drying in vacuum (50-70 C ) and reduction in hydrogen ( 2 5 0 C )
8).
.
2. Properties of reduced catalysts Data on the samples which were dried and reduced under similar conditions are collected in Fig. 3 and Table 1. SAXS data in Fig. 3a indicate tendency for the size of palladium crystallites to increase as the constant for strong adsorption of precursor decreases. Such a trend did find confirmation upon studying the samples with electron microscopy. The increase in metal particle size proved to be not accompanied, however, by decrease in fraction of the palladium exposed. Moreover, it was the medium-sized palladium particles on Sibunit carbons which displayed highest proportion of metal accessible to carbon monoxide (Fig. 3a). Being determined on the basis of the CO chemisorption data, specific activities of the samples in cyclohexene hydrogenation (reaction known to be structure-insensitive on metal catalysts [ a ] ) did fall within a rather narrow interval (Fig. 3b). In view of these data it seems logical to suppose a partial blockade of the surface of small metal particles by walls of the support pores. Suggestion may also be made that the relationship between metal
454
particle size and Ks values reflects mainly dependence of the size on the roughness of the support surface (with what Ks values correlate) rather than on the strength of the precursor bonding to the surface. At least, the latter provides little influence on Catalytic properties of the samples. One can see in Fig. 3c that the increase in palladium loading on a support (when the sites of the irreversible, strong and weak adsorption are sequentially occupied) does not lead t o appreciable changes in activity per gram of palladium basis (the curves seem to be linear and pass through the origin or nearby). The blocking effect provides an easy explanation for recent data [51 on properties of the PWC catalysts which were prepared from different precursors. Palladium crystallites with a broad distribution in size (2-20 nm) were seen by electron microscopy after reduction of supported palladium chloride in alkaline liquid phase and medium-sized palladium particles (1-4 nm) upon thermal decomposition of Pd(0) complex with dibenzylidenacetone. Nevertheless, both catalysts appeared approximately twice as active as the catalyst which was prepared via reduction of adsorbed palladium chloride in hydrogen and contained palladium particles of 1-2 nm in size. Because those were Sibunit-based catalysts in Ref. 5, the results show possibility for the blocking effect to be displayed in the case of mesoporous supports as well. The different character of interaction of palladium complex with aromatic ligands with the carbon surface, in comparison with that for palladium chloride, presumably plays a role in determining properties of the Pd(dba)2-derived catalyst. Richard et al. have reported that reduction of related platinum complex Pt (dba)2 resulted in plate-like particles on basal planes of graphite microcrystallites but usual platinum particles on the edge-planes were seen after reduction of supported platinum chloride [91. Specific features of palladium catalysts which pass alkaline treatment (procedure employed often in the catalyst preparation [lo]) may be derived from the data Fn Table 1. Due to adsorption on carbon support, the hydroxy-species are stabilized against agglomeration (which occurs in solutions) and, so, still small particles of metallic palladium can be produced. TEM examination showed that Pd crystallites in catalysts 1 and 2 are indistinguishable in size (1-2 nm; 3-8 nm as predominant size in sample 3.1). Moreover, fraction of the palladium exposed proved to be substantially higher in catalysts 2 than in Catalysts 1 . And
455 Table 1 . Catalytic properties of €$PdCl -derived catalysts (1 .O wt.8 of Pd on 5450 carbon) in respec4 to the mode of precursor deposition. ~
No.
Proceduresa)
co -
Pd 'loo
c-hexeneC) TON R
nitrobenzenedl W TON R 27 25 38
W
1 - 1 adsorption 1.2 i. w. impregnation
45 45
24 22
3.8 3.6
2.1 adsorption. then treatment with alkali 2.2 as (2.1) but i.w.impr.
65
29
3.3
75
33
3.3
4
45
19
3.1
1.7
50
22
3.3
3
45
3.1 conversion into colloidal Pd oxide. then adsorption 3.2 as (3.1) but i.w.impr.
~
Activity datab)
3.5 3.5 3.5
4.5
4.3 4.3
2.5 2 2.5
42
4.2
2.5
40
6.6
1.2
6.7
2
a Adsorption via adding dropwise solution of H2PdC14 (1.1, 2.1 ) or colloidal species (3.1) to a stirred slurry of carbon support ( 1 g ) in 10 ml of water. Sodium carbonate as alkali agent (6/1 and 2/1 in cases 2 and 3. respectively). Following deposition step, washing with water (10 ml), dryoing in a i r (ambient temperature) and reduction in hydrogen (250 C 1 . 3 bVelocity of hydrogen consumption w (cm /me; cat/min)u100 ( 2 1 0 and turn over numbers TON ( m o l %/&atom Pd exposed/s) for preliminary milled catalysts (5-7 pm as a mean grain size.COULTER TAII countings); R ratio of activities f o r the milled and original not ground catalysts (70-100 wJ. Cethanol solution (0.5 MI. 0 C. 1 bar. dmethanol solution (0.2 M), 30° C. 1 b a r .
even catalyst 3.1 with larger Pd crystallites is not unfavourable in this respect as compared with catalysts 1 . So, the blocking effect is probably eliminated as the precursor is converted into hydroxy species. S e c o n d l y , these results show a small size of metal crystallites to be not necessary accompanied by a pronounced blocking effect. Note also that reduction of palladium acetate on a preliminary oxidized Sibunit carbon allowed the very small Pd particles t o be obtained [Ill, with catalytic activity of the sample in cyclohexene hydrogenation close to that for catalysts 2 in Table 1. There are probably a number of factors which determine whether there will be or not and in what extent the blocking effect displayed. Supposedly, among them are morphology and chemical state of the surface, chemical composition and dimentions of the metal precursor, capability for preliminary adsorbed species and metal atom/clusters formed to moving around the surface: so,
456
conditions of drying, reduction or other steps at catalyst preparation oan also provide their influence. When colloidal palladium oxide is prepared in advance (methods 3 in Table I), the enhanced capability of such species to adsorbing provides their deposition onto the exterior surface of catalyst grains. Diffusion limitations for rapid catalytic processes diminish thereby (cf. the R factors for different catalysts in Table 1 ). Naturally, the active component is forced to penetrate more towards the grain core if incipient wetness impregnation is used instead of adsorption (method 3.2). 3. Carbon-adsorbed complexes as heterogenized catalysts. Surprisingly, but despite great interest to immobilized metal complexes, as the systems combining merits of both homogeneous and heterogeneous catalysts, there were few detailed studies on carbon-anchored catalysts. Meanwhile, the latters seem to be most promising for bringing in commercial practice, the goal which in the case of supported complexes has so far been attained in very limited cases [71. In Fig. 4 one can find the selected data on properties of carbon-adsorbed complexes in the vinyl-exchange reaction, which allow discussing the factors one should take into account when preparing and using such catalysts. Fig. 4 (a,b) shows that the catalyst preparation might be reduced to the simplest of all possible ways, namely, tn s t t u deposition of active component onto carbon support. Interestingly, even the complex nearly insoluble in the reaction mixture can be supported by such a technique (IC;PdCl4 as example; by itself, it displays no activity in trans-vinylation but its rapid transfer onto the support added results in the same reaction rate as on the complex supported in advance, see curve 2 in Fig. 4b). In view of the known sensitivity of trans-vinylation to steric factor, it seems somewhat surprising that the anchoring of Li2PdC14 leads to diminishing the activity by a factor of two only (Fig. 4a). Perhaps , those changes in chemical composition of the chloride complexes which took part upon anchoring provide promotion effect which compensates in part the detrimental effect of proximity of the active center to the support. When the chlorides were deposited from acetonitrile, their fixation supposedly took place on somewhat different surface sites, may be less hindered, which explains the enhanced catalytic activity in this case (cf. data in Fig. 4c (curve 2) and 4a,4b). Such a
457
50
100
min
50
100
min
10
30
win
Fig. A. Trans-vinylation on carbon-anchored palladium chlorides (on the ordinate axis conversion of vinyl acetate into vinyl propionate: initial propionic acid/vinyl acetate ratio 10 v/v; static reactor. 50 OC. S450 as support. content of supported palformal concentration of ladium l wt.% (a.b.cl) and 0.5 wt.% ( ~ 2 1 , palladium In the reaction mixture ca. 6 (a.b,cl) and 2 mg-atom/l). Catalysts: ( a ) Li2PdClA. as homogeneous ( 1 ) and C n eCtu supported ( 2 . successive runs 1. (b) K2PdC1 , as adsorbed in advance from aqueous solution (1) and as loaAed as solid ( 2 ) ; in case (2) the arrow indicates moment when carbon support was added. ( C ) sPdC14/C. promotion with lithium propionate (0.4 mol/l) ( 1 ) and (CH3CN)2PdC12 as anchored from acetonitrile (21.
trivial method as promotion with carboxylates may also be applied (Fig. 4c, curve 1). The catalysts with adsorbed palladium chlorides could be reused several times and elemental analysis showed that deactivation, if observed, should be assigned to other side-processes (e.g., reduction of Pd(I1) species) rather than palladium leaching. As far as complexes with aromatic ligands are concerned, which were anchored due to dispersive forces, carboxylic acid competed rather effectively with them f o r the adsorption sites; complexes which are insoluble, by themselves, should be used in this case to prevent leaching of palladium. It appeared also preferable to use complexes in which the central atom was brought out of the plane of aromatic anchor [41, but pronounoed decrease in activity has been observed (an order of magnitude) f o r phenanthroline complex phen.Pd(OAc)2 upon anchoring. There is still uncertainty, however, in the site of looation of active Pd species in the former case, whether it is bound to the aromatic ligand or to carbon framework of the support.
458
CONCLUSION The results of the present study show that it would be mistake t o consider carbon supports as an inert matrix and much attention should be payed to both physical and chemical characteristics of these supports. In particular, the very rich chemistry of the carbon surface provides conditions for strong anchoring of metal complexes by such a simple technique as adsorption and the heterogenized species might be used in liquid-phase catalytic . processes. If these species are taken as a precursor for metal crystallites, the state and properties of final metal particles can be influenced to a great extent by the nature of initial complex and technique of its deposition
.
ACKNOWLEDGEMENTS The authors are very thankful to Drs V.N.Kolomiichuk and A.L. Chuvilin f o r SAXS and TE?d data, Prof. V.B.Fenelonov and Mrs L.G. Okkel f o r nitrogen adsorption measurements, Mrs V.P.Mel’nikova and N.I.Gergert for experimental assistance and Dr G.V.Plaks3n for a gift of Sibunit carbons.
REFERENCES 1 2 3
4
5 6
7 8 9 10
11
Yu.1. Yemakov, V.F. Surovikin, G.V. Plaksin, V.A. Semikolenov, V.A. Likholobov, A.L. Chuvilin and S.V. Bogdanov, React.Rlnet. Catal. Lett., 33 (1987) 435-440. s.J. G r e g and K.S.W. S i n g , Admrptton, Surpace Area and Porosity, 2nd edn., Academic Press, London, 1982. P.A. Simonov, V.A. Semikolenov, V.A. Likholobov, A.I. Boronin SSSR, S e r . Khtm., and Yu.1. Yermakov, Izu. Ak.ud. (1988) 2719-2724. A.A. Ketterl A.S. Lisitsgn, V.A. Likholobov, A.A. Gall and A.S. Trachum tnet. KataZ.. in press. A.S. Lisitsyn, S.V. Gurevich, A.L. Chuvilin, A.I. Boronin, V.I. Bukhtiyarov and V.A. Likholobov, React. litnet. CataZ. Lett., 38 (1989) 109-114. Yu.A. Ryndin, O.S. Alekseev, P.A. Simonov and V.A. Likholobov, J . YOZ. cataz., 55 (igag) 109-125. V.A. Likholobov and A.S. Lisitsyn, J . YendeZeev Soc. (USSR), 34 (1989) 340-348E.E. G O ~ Oand Y. Boudart, J . Cata‘l.. 52 (1979) 462-714. D. Richard, P. Gallezot, D. Neibecker and I. Tkatchenko, CataZ. Today, 6 (1989) 171-179. A.B. Stiles, CataZyst Mmj’acture. Larboratory and CommerctaZ Preparat tons, Marcel Dekker, New York, 1 983. S.V. Gurevich, P.A. Simonov, A.S. Lisitsgn, V.A. Likholobov, E.Y. Moroz, A.L. Chuvilin and V.N. Kolomiichuk, React. H i n e t . CataZ. Lett., 41 (1990) 211-216.
,?’
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
459
PREPARATION OF PALLADIUM-COPPER CATALYSTS OF OESIGNEO SURFACE STRUCTURE ZS. BOONARl, T. WLLAT',
S . SZAB6'
and J. P E T R d
'Department of Organic Chemical Technology, Technical University of Budapest, H-1521 Budapest, Hungary 'Central Research I n s t i t u t e f o r Chemistry, Hungarian Academy of S c i e n c e s , H-1525 Budapest, Hungary SUMMARY T h e 2 p e p a r a t i o n of Pd+Cu c a t a l y s t s has been s t u d i e d by c o n s e c u t i v e r e d u c t i o n of Cu o n t o Pd or Pd/C. I n g e n e r a l , bulk Cu d e p o s i t i o n is k i n e t i c a l l y mre favourable, although adsorbed Cu (submonolayer) is mre s t a b l e . Various methods f o r t h e e l i m i n a t i o n of bulk metal d e p o s i t i o n , a r e discussed. Hydrogenation i n formic a c i d or ionadsorption followed by hydrogenation a r e found t o be s u i t a b l e methods f o r p r a c t i c a l a p p l i c a t i o n . Pd/C c a t a l y s t s modified by bulk or adsorbed Cu, have different selectivities in the partial reduction of 4chlormitrobenzene.
INTRODUCTION Bimetallic c a t a l y s t s a r e g e n e r a l l y prepared by s i n u l t a n e o u s or consecutive r e d u c t i o n of corresponding p r e c u r s o r s . I n t h e c a s e of sirmltaneous r e d u c t i o n t h e composition of t h e c a t a l y t i c a l l y a c t i v e s u r f a c e is d i f f i c u l t t o d e s i g n owing t h e phenomenon of s e g r e g a t i o n ( r e f s . 1 - 3 ) .
-
although
not
to
By t h e method of c o n s e c u t i v e r e d u c t i o n
a s widely a p p l i e d - s u r f a c e s t r u c t u r e of
the
monometallic
c a t a l y s t can be modified a s r e q u i r e d , by metal d e p o s i t i o n o n t o t h e s u r f a c e . T h i s may o f f e r an economical method f o r preparing small amounts of c a t a l y s t s i n
fine
chemical industry. According t o e a r l i e r i n v e s t i g a t i o n s (refs.4-5) i n t h e c a s e of a Pd+Cu c a t a l y s t prepared by s i r m l t a n e o u s r e d u c t i o n , f o u r d i f f e r e n t , Cu-containing phases may form: (i) bulk Cu, ( i i ) adsorbed Cu, ( i i i ) d i s o r d e r e d a l l o y and ( i v ) ordered a l l o y (PdCuj) phases. I n t h e absence of C1- o r Pd2+ i o n , no a l l o y phase formation occurs during c o n s e c u t i v e r e d u c t i o n ( r e f . 6 ) . I n t h e l a t t e r c a s e bulk metal and adsorbed metal formations occur s i n u l t a n e o u s l y . T h i s phenomenon was observed e.g. i n t h e c a s e of Cu d e p o s i t i o n by hydrogen r e d u c t i o n from an aqueous s o l u t i o n of /CU(NH~)&/(OH)~ o n t o a Pd/C c a t a l y s t (Fig. 1). The d o u b l e t i n t h e range of 0.3-0,4 V p o i n t s t o bulk Cu d e p o s i t i o n on various c r y s t a l f a c e s of Pd ( r e f . 7 1 , whereas the peak around 0.55 V is due t o i o n i z a t i o n of t h e adsorbed Cu (refs.6-7). I n t e r a c t i o n between Cu atoms is c h a r a c t e r i s t i c of " t h r e e dimensional" bulk C u , whereas t h e c h a r a c t e r i s t i c f e a t u r e of "two-dimensional" adsorbed Cu is t h e
460
Fig.1. Potentiodynamic curve o f a carbon-supported Pd+Cu c a t a l y s t prepared by consecutive r e d u c t i o n (ref. 4 )
i n t e r a c t i o n between Pd and Cu atoms (Fig. 2). I n the case o f
bulk
deposition,
coverage o f the Pd c a t a l y s t i s uncertain, i t i s g r e a t l y dependent on t h e e x p e r i mental parameters. With adsorbed metal deposition, coverage i s unambiguously due t o quantitative conditions. this
From c a t a l y t i c aspects l a t t e r s t r u c t u r e seems t o be more favourable. I n paper,
through
t h e example o f Pd+Cu c a t a l y s t s , we wish t o present
methods
which b u l k w t a l d e p o s i t i o n can be avoided, and b i m e t a l l i c c a t a l y s t s with designed
uniform
bimetallic method
s u r f a c e s t r u c t u r e can be a t t a i n e d . The s u r f a c e
c a t a l y s t s has been s t u d i e d by an electrochemical
and
catalytic
4-chloronitrobenzene
Fig.2.
properties
were analyzed by
of
structure
polarization
selective
by
well-
(EP)
reduction
of
(CNB).
a. 9 b. , Bulk (a) and adsorbed (b) metal on t h e surface of t h e c a t a l y s t : b a s i c m e t a l (Pd) metal (Cu) deposited by consecutive r e d u c t i o n
EXPERIMENTAL A n a l y t i c a l grade reagents and d i s t i l l e d water ( t r i p l e d i s t i l l e d f o r chemical p o l a r i z a t i o n ) were used. A f t e r hydrogen reduction, a l l
electro-
catalysts
were
washed with water i n hydrogen atmosphere and d r i e d i n a i r . Pd c a t a l y s t
A: Palladium was prepared from PdCl2, v i a Pd(OH)2 by hydrogen r e d u c t i o n ( r e f ,8).
461
Pd/C c a t a l y s t s
El: The 10 w t % pd on a c t i v a t e d carbon was a commercial product ("Selcat Finomvegyszer
Szovetkezet, Hungary). I t was used a f t e r p u r i f i c a t i o n
Q",
by
dilute
aqueous H2S04 s o l u t i o n i n hydrogen atmosphere.
C:
The c a t a l y s t contained 11 w t % Pd on a c t i v a t e d carbon ("Carbo C
BET
Extra",
surface
area: 930 m2g-l). 50 g carbon was impregnated with a s o l u t i o n o f 9.2 g 3 i n 20 cm 17 w t % HC1 and d r i e d a t 105OC. The powder was added t o a mixed 3 s o l u t i o n o f 40 g NaHC03 i n 500 cm water a t 7OoC. A f t e r 2 h t h e s l u r r y was
PdC12
hydrogenated a t 3OoC f o r 1 h. The d i s p e r s i o n o f Pd was 0.16 by TEM measurement. Pd+Cu c a t a l y s t f o r EP
0: 53
0.5 g Pd(A) was prehydrogenated i n 20 cm3 water f o r 0.5 h. A s o l u t i o n o f 3 CuS04.5H20 i n 20 cm 85 w t % HCOOH was added a f t e r decantation and
mg
hydrogenation continued f o r 0.5 h.
E: 0.5 g Pd/C (El) was prehydrogenated i n 20 crn3 water. A f t e r 0.5 h a s o l u t i o n o f
212 mg
citric
acid
and
79 mg CuS04.5H20 i n 20
cm3
water
was
added,
and
hydrogenation continued f o r 2 h.
F: A s l u r r y o f 0.49 g Pd (A) and 0.45 g CuS04.5H20 i n 20 cm3 water was mixed f o r 1.5 h. The c a t a l y s t was f i l t e r e d o f f and washed Cu2+-free 3 Then the c a t a l y s t was hydrogenated i n 50 cm water f o r lh. air
with
in
water.
(Pd+Cu)/C c a t a l y s t s f o r hydrogenation o f CNB I n a l l s e r i e s Pd/C and
60
(C) c a t a l y s t was used and Cu content o f the c a t a l y s t s was 30
a t % Cu/Pds. The 0 a t % Cu/Pds c a t a l y s t s were
prepared
similarly,
but
w i t h o u t Cu, t o check the e f f e c t o f organic a d d i t i v e s .
G:
2.5
g
solution
3 water, a t 25OC
Pd/C was prehydrogenated i n 20 cm
of of
1 h.
for
0.11 g c i t r i c a c i d and an appropriate amount o f CuS04
in
10
A 3 cm
water was i n j e c t e d and hydrogenation continued f o r l h . H: 2.5 g Pd/C was prehydrogenated i n 20 cm3 water a t 25OC f o r 1 h. 40 cm3 85 w t %
HCOOH c o n t a i n i n g an a p p r o p r i a t e amount o f CuS04 was added a f t e r decantation hydrogenation continued a t O°C f o r 3 h. I: 2.5 g Pd/C i n 30 cm3 water c o n t a i n i n g an appropriate amount o f
CuS04
and were
mixed f o r 1 h i n a i r , then 1 h i n hydrogen. E l e c t r o n microscopy (TEM) Oispersion
of t h e Pd/C (El) c a t a l y s t was measured by means o f a PHILIPS TEM
microscope.
505
About 1000 p a r t i c l e s were counted and sized. The surface mean 3 2 di /gni.di) was transformed t o approximate d i s p e r s i o n (0) by diameter (d=gni the
.
following
equation
(ref .9):
0=2.5.a-d-1,
where
"a"
is
the
lattice
constant o f Pd. Electrochemical p o l a r i z a t i o n (EP) The P t sheet bottomed c e l l and p o l a r i z a t i o n method have been d i s c r i b e d ( r e f .lo). Potentiodynamic electrolyte,
in
polarization
was
carried
out
in
0.5
M
H2S04
supporting
n i t r o g e n atmosphere. Anodic sweep commenced from 0.03
V
with
462 1 mvs-l sweep r a t e , up t o 1.0 V. Hydrogenation of 4-chloronitrobenzene (CNB)
3
mg c a t a l y s t was prehydrogenated i n 10 em e t h y l acetate f o r 0.5 h. 9.5 3 rnm1 CNB d i s s o l v e d i n 20 cm e t h y l acetate was i n j e c t e d and hydrogen consumption 30-100
was measured (24OC, 1 bar). The products r e r e analyzed by GLC. RESULTS AND DISCUSSION Preparation o f Pd+Cu . c a t a l y s t s au
Over
of
field
past 25 years, metal adsorption has been a popular
the
electrocatalysis
elaborated
for
procedures.
In
(refs.11-13). metal
Evidently,
deposition
involve
therefore, mainly
Since
special only
stress
procedures
on
methods
the
the
methods
the
for
suitable
applicable i n the
in
electrochemical
the f o l l o w i n g , we wish t o g i v e a b r i e f summary o f with
possibilities, realization.
adsorbed
topic
liquid
various
industrial
phase
are
of
p r a c t i c a l s i g n i f i c a n c e , these methods w i l l be described below. In
the d e p o s i t i o n o f adsorbed metal, the phenomenon
deposition"
from
positive other
can
be u t i l i z e d . By t h i s theory, d i f f e r e n t
termed
"underpotential
metals
t h e i r r e v e r s i b l e N e r s t p o t e n t i a l w i l l discharge
at
potentials
and
adsorb
metal surfaces. The adsorbed metal can be e a s i l y separated from t h e
on
bulk
metal by an EP method (Fig. 1). Thermodynamically, adsorbed between
metal
is
the
b a s i s o f t h e p r e p a r a t i o n of a c a t a l y s t
t h a t the e l e c t r o d e p o t e n t i a l o f t h e
catalyst
the p o t e n t i a l s of b u l k and adsorbed m e t a l (Fig. 3). (E.g.
atmosphere,
t h e e l e c t r o d e p o t e n t i a l o f Pd can be d e f i n e d as a
mdified should in
by
fall
hydrogen
reversible
H/H+
electrode).
E cat E 'Hebulk /Men*
Fig.3.
'Mead
Inen+
E l e c t r o d e p o t e n t i a l s i n the deposktion o f adsorbed metal
I. The simplest way f o r t h , i s i s t o c o n t r o l t h e p o t e n t i a l of t h e Pd c a t a l y s t by
a
potentiostat,
a
method g e n e r a l l y
applied
in
electrochemistry,
not
p r a c t i c a l , however, for the p r e p a r a t i o n of l a r g e r amounts of c a t a l y s t s .
11. Another s o l u t i o n may be t h e adjustment of t h e p o t e n t i a l by redox Systems. The
cheapest
and
p u r e s t reducing agent i s hydrogen. The
potential
of
a
Pd
463 c a t a l y s t (as a H/H+ electrode) and of a metal/metal i o n system t o be reduced can be determined by the f o l l o w i n g equations:
E
Mebulk
The
electrode
calculated
from
or
(ref.111,
potential
aqueous
the
derived
of
an adsorbed
(2)
metal/metal
work f u n c t i o n d i f f e r e n c i e s o f base from
p o l a r i z a t i o n curves. On the
ion
system
and
can
adsorbed
basis
of
be
metals
the
above
by a p p r o p r i a t e choice of the metal i o n c o n c e n t r a t i o n and pH values,
equations, the
potential
= EoMe + (RT/nF) I n a E
conditions
acidic
negative
to
given i n
F i g . 3 can be a t t a i n e d . I n
medium, a number o f metals with standard
this
electrode
hydrogen (Table 1.) are capable o f a d s o r p t i o n on Pd
metal d e p o s i t i o n , e.g.
way,
in
potentials
without
bulk
Pb or Cd (ref.15).
TABLE 1 Standard e l e c t r o d e p o t e n t i a l s o f Me/&"+
systems ( r e f . 1 4 )
E0
System
Pb/Pb2+
-0.13
H/H+
pH = 0
0.00
Cd/Cd2+
-0.40
pH = 7
-0.41
TUT~+
-0.34
pH = 14
-0.82
System
/v/
Sn/Sn2+
sn / s n +
In
the
+
E0
/v/
-0.14
cu/cu2+
0.34
0.15
cu /cu+
0.52
cu+/cu2+
0.16
case o f Cu, however, which i s a metal with
an
electrode
potential
p o s i t i v e t o hydrogen, b u l k metal d e p o s i t i o n i n aqueous medium cannot be avoided. By
changing t h e pH value, Cu2+ concentration, temperature, and H2 pressure,
no
r e a l i s t i c values p r e v e n t i n g b u l k metal d e p o s i t i o n can be a t t a i n e d .
1x1. A much mre f e a s i b l e s o l u t i o n f o r adsorbed Cu d e p o s i t i o n i s t o c a r r y o u t reduction
i n a s o l v e n t o t h e r than water. I n f o r m i c a c i d t h e standard
P o t e n t i a l of t h e ( r e f .16).
cU/cU2+
electrode
system r e l a t e d t o the hydrogen e l e c t r o d e i s Eo= -0.14
I n the potentiodynamic curve o f c a t a l y s t
hydrogen r e d u c t i o n (Fig.4),
the maximum (0.4-0.6
0 prepared i n f o r m i c a c i d by
V ) d e r i v e d from t h e
ionization
of
adsorbed Cu can o n l y be observed. As c o u l d be expected, t h e r e i s no b u l k
,on
the
surface.
The
two
v
inseparable peaks o f adsorbed
Cu
a d s o r p t i o n on d i f f e r e n t c r y s t a l faces. I n the range o f 0-0.25
are
V, the
due
to
Cu Cu
ionization
464
peak
of d i s s o l v e d hydrogen and t h e adsorbed hydrogen, discerned a s a s h o u l d e r , appear, whereas above 0.7 V a wave c h a r a c t e r i s t i c of oxygen a d s o r p t i o n may be observed ( r e f . 1 7 ) . I.mA/
Fig.4. Potentiodynamic c u r v e of c a t a l y s t D reduced i n formic a c i d by H2 (m=2 mg, v = l mvs-l)
0
0,2
0,4
0,6
0,8
E,V
IV. I n aqueous s o l u t i o n a t room temperature under atmospheric p r e s s u r e , p o t e n t i a l of Pd c a t a l y s t is a f u n c t i o n of pH, with a maxirmm value of 0 V.
the With
s t r o n g complexing a g e n t s t h e e l e c t r o d e p o t e n t i a l of Cu can be s h i f t e d towards a negative d i r e c t i o n , t h u s t h e r e l a t i o n s h i p of t h e e l e c t r o d e p o t e n t i a l s d e p i c t e d
e t r i e d o u t a p p l i c a t i o n of Cu i n Fig. 3 may be a t t a i n e d . W
2+ complexes with EDTA,
/)-alanine, c i t r i c a c i d and KCN. Bulk Cu depositon could not be avoided, however, i n any of t h e s e c a s e s . An i n t e r e s t i n g phenomenon was observed ( F i g . 5) i n t h e c a s e of c a t a l y s t E prepared i n t h e presence of c i t r i c a c i d by hydrogen V). A possible reduction, where only bulk Cu formation occurred (0.25-0.35 explanation f o r t h e formation of bulk Cu may be t h a t a f t e r d e p o s i t i o n of t h e Cu atoms on t h e s u r f a c e , t h e rest of t h e atoms d e p o s i t onto t h e Cu atoms owing t o 1,mA
1.6 1,4
12 1.0.
48 0,6 '0,4
02
Fig.5. Potentiodynamic c u r v e of carbon-supported Pd+Cu c a t a l y s t E reduced i n t h e ppesence of c i t r i c acid by H~ (m.2 mg, v = l mV5-l)
465
t h e s t r o n g a d s o r p t i o n of c i t r i c a c i d on Pd. The c o m p e t i t i v e a d s o r p t i o n of c i t r i c a c i d and o t h e r a d d i t i v e s is u t i l i z e d a l s o i n t h e p r e p a r a t i o n of "yolk"-type A1203 supported P t c a t a l y s t s ( r e f .18).
"egg s h e l l " o r
A s p e c i a l c a s e of r e d u c t i o n i n H2 atmosphere is t h e metal a d s o r p t i o n v i a i o n i z a t i o n of preadsorbed hydrogen ( r e f . 1 3 ) . The method may be i n t e r p r e t e d by t h e r e d u c t i o n of metal i o n s by m a n s of a c a l c u l a t e d amount of reducing a g e n t
V.
hydrogen). This is a method f o r t h e p r e p a r a t i o n of Pt+Xad c a t a l y s t s , where x= Cu,Ag,Bi,Au (ref.13). Unfortunately, i n t h e c a s e of Pd c a t a l y s t s , which c o n t a i n high amounts of d i s s o l v e d hydrogen, t h i s method is not a p p l i c a b l e . VI. According t o t h e r e l a t i o n s h i p shown i n Fig.3 , t h e p o t e n t i a l of t h e (adsorbed
c a t a l y s t can be a d j u s t e d not only with hydrogen b u t a l s o by means of redox systems. The e l e c t r o d e p o t e n t i a l s of a number of o r g a n i c redox systems have been determined ( r e f s . 14,16). The use of d i f f e r e n t s u b s t i t u t e d quinone-hydroquinone t y p e systems is e s p e c i a l l y favoured. Unfortunately, however, by a p p l i c a t i o n of t h e s e systems the s u r f a c e of t h e c a t a l y s t becomes contaminated, which r e n d e r s EP i n v e s t i g a t i o n s impossible. A s p e c i a l c a s e is when t h e adsorbable metal ion of v a r i a b l e valence is a t t h e same t i m e t h e redox agent. In t h i s way, through d i s p r o p o r t i o n a t i o n of Sn2+ a c a t a l y s t modified by adsorbed Sn can be obtained ( r e f . 1 3 ) . The procedure is r a t h e r simple: t h e c a t a l y s t ( p o s s i b l y oxidized on t h e s u r f a c e ) is shaken t o g e t h e r with an aqueous s o l u t i o n of Sn2+ s a l t . I n t h i s c a s e t h e p o t e n t i a l of t h e P t c a t a l y s t is determined by t h e Sn2+/Sn4+ system, whose e l e c t r o d e p o t e n t i a l is p o s i t i v e t o t h e Snbulk/Sn 2+ system, b u t n e g a t i v e t o t h e Snad/Sn 2+ system (Table 1.). With Cu+ a s p r e c u r s o r t h i s method is n o t a p p l i c a b l e , owing t o t h e bulk d e p o s i t i o n of Cu. VII. I n c o n t r a s t t o t h i s , t h e i o n a d s o r p t i o n method, where t h e s t r o n g l y adsorbed metal ion is reduced (by hydrogen), i s n o t based on e l e c t r o c h e m i c a l p r i n c i p l e s . T h i s procedure can be a p p l i e d , however, o n l y i f t h e coverage of Pd by t h e s t r o n g l y adsorbed metal i o n s is below monolayer, and t h e e x c e s s metal i o n s can be e a s i l y removed (e.g. washed o u t ) . The p r e c u r s o r is, t h u s , a metal i o n bound t o Pd by s t r o n g adsorption. A s had been v e r i f i e d e a r l i e r ( r e f . l 9 ) , Pd poisoned by aqueous l e a d a c e t a t e , termed as t h e L i n d l a r c a t a l y s t ( r e f . 2 0 ) , is a c t u a l l y Pd modified by Pdad. C a r e l e s s p r e p a r a t i o n (e.g. incomplete removal of e x c e s s Pb2+) may l e a d t o bulk l e a d d e p o s i t i o n , which decreases a c t i v i t y and s e l e c t i v i t y a s well. Fig.6. shows t h e EP curve of a Pd+Cu c a t a l y s t prepared by t h e method of ion a d s o r p t i o n . On t h e s u r f a c e of Pd only adsorbed Cu (0.45-0.65 V) and hydrogen adsorbed on free Pd atoms (0.15-0.3 V) can be d e t e c t e d . With t h e assumption of Cuad/Pdskl and Had/PdsGl can be determined from t h e a r e a s below ratios, t h e curve ( r e f s . 2 1 , 2 2 ) . Preliminary design of t h i s v a l u e (here: 0.12) is n o t p o s s i b l e as it is dependent on t h e parameters of e x c e s s Cu2+ removal (washing).
oCu
466 1,mAI
Fig.6. Potentiodynamic curve of Pd+Cu c a t a l y s t F prepared by hydrogen reduction a f t e r i o n adsorption (m=2 mg, v = l mvs -1 ) 0
the
0.4
0,2
0,6
0,8
E,V
U n c e r t a i n t i e s a r i s i n g from c a t a l y s t washing may be avoided by adsorption i n presence of a c a l c u l a t e d amount of metal ions and r e d u c t i o n , o m i t t i n g t h e
washing s t e p ( s e e c a t a l y s t I). The b e n e f i t of t h e method is t h a t by a r e l a t i v e l y simple
and
cheap
procedure,
a surface free
from
catalyst
poisons
can
be
prepared
by
attained. P a r t i a l r e d u c t i o n o f 4-chloronitrobenzene (CNB) Pd
c a t a l y s t s m d i f i e d by adsorbed Cu a s d e s c r i b e d above can be
hydrogen followed
r e d u c t i o n i n formic a c i d medium (H) ( r e f .23) by i o n adsorption, by hydrogenation ( I ) . In t h e following, c a t a l y t i c p r o p e r t i e s of t h e s e
and t h o s e containing only bulk Cu prepared i n c i t r i c a c i d medium (G) compared. As a test r e a c t i o n , we have chosen t h e well-known procedure of
catalysts are
p a r t i a l r e d u c t i o n of CN8 ( r e f .24). I n i n d u s t r i a l procedures, reduction i s o e n e r a l l v c a r r i e d out in a c i d i c mrlitrm. n r hv r e o l e c i n n Prl hv P t . which is wore
___
exoensive htit a s hinher s_e-l_e c_t-i v i t-v,. - - r - - - h -I n s t u d y i n g t h e above r e e c t i o n , we have found
that aniline
(A)
is
c o n t i n u a l l y formed on both Pd/C and (Pd+Cu)/C c a t a l y s t s , and r e d u c t i o n of t h e n i t r o group and hydro-dehalogenation t a k e p l a c e simultaneously. I n t h e following, measurable a t 100 % conversion of E, w i l l be given t h e amount of by-product t o determine t h e s e l e c t i v i t y of t h e c a t a l y s t . The d a t a of Pd/C (C) and (Pd+Cu)/C (G,H,I) c a t a l y s t s a r e given i n Table 2. It has been found t h a t the s e l e c t i v i t y of 0 a t % Cu/Pds c a t a l y s t s , prepared a s r e f e r e n c e m a t e r i a l t o c l a r i f y t h e e f f e c t of a d d i t i v e s , d i f f e r s from t h e
of Pd/C (C) c a t a l y s t when an a c i d i c " a d d i t i v e " ( c i t r i c a c i d formic a c i d ) was a p p l i e d . The i n h i b i t i n g e f f e c t of v a r i o u s acids hydrodehalogenation h a s been described ( r e f . 2 4 ) , and we have a l s o found t h a t selectivity
a d d i t i o n of a c e t i c a c i d t o the e t h y l a c e t a t e s o l u t i o n , t h e r a t e of c a n be reduced.
A
or on by
formation
467
TABLE 2. S e l e c t i v e r e d u c t i o n of CNB w i t h carbon supported Pd+Cu c a t a l y s t s A Catalysts cu cu -
structure
C
-
0
-
G
bulk bulk
30
-
I
adsorbed adsorbed adsorbed adsorbed
H
PdS at % 0
60 0 30 60
0 30
60
, 7.2 3.1 3.5
2.8 6.8
5.5 2.2 3.8 2.8
0.1
Modification of t h e s u r f a c e by bulk Cu d e p o s i t i o n caused p r a c t i c a l l y change i n t h e s e l e c t i v i t y . T h i s is due t o t h e f a c t t h a t even a t a r a t i o Of
no
a t % Cu/Pds,
CU
a small p r o p o r t i o n of s u r f a c e Pd atcuns a r e covered by
inactive
(see Fig.2). I t s h l o u l d a l s o be taken i n t o c o n s i d e r a t i o n t h a t t h e d i s c h a r g e of hydrogen
may be l o c a l l y separated ( r e f . 2 5 ) . Thus
ionization
Cubulk
Cu2+
and
pay
also
cu
d e p o s i t onto carbon support of good conductivity. (One can conclude t o @ t h e amount of adsorbed hydrogen. ) I n t h e c a s e of c a t a l y s t s modified by adsorbed C u , t h e r a t i o of Cu/PdS also
Pd
coverage.
In
both series of c a t a l y s t s (I and H),
the
60
from means
of
amount
A
d e c r e a s e s with t h e i n c r e a s e of t h e amount of i n a c t i v e Cu. As a r e s u l t of t h e j o i n t e f f e c t of Cuad and a c i d ( s e r i e s H), 4 - c h l o r a a n i l i n e (CA)can be prepared i n 99.9 % p u r i t y . (Reaction r a t e is t h e n approximately f o u r times a s low a s t h e r a t e a t t a i n e d with reference catalyst In
our
o p i n i o n , t h e i n c r e a s e i n s e l e c t i v i t y caused by adsorbed
explained
by
geometric
e f f e c t s . By coverage of t h e
active
Pd
Cu
a
c e r t a i n l i m i t - w i l l l e a d t o s i g n i f i c a n t change
in
the
may
surface
Cu, t h e number and s i z e of a c t i v e s i t e "ensembles" d e c r e a s e ,
inactive beyond
C.) be with
which
-
selectivity
(ref .26). On t h i s b a s i s , reducing t h e amount of &would be p o s s i b l e a l s o by bulk d e p o s i t i o n a t higher Cu/Pds r a t i o . This cannot be r e a l i z e d , however, owing t h e u n c e r t a i n t y of t h e geometry of Cu d e p o s i t s (and t h u s , Pd coverage) which dependent on t h e k i n e t i c f a c t o r s of deposition.
Cu to
is
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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
OPTIMIZATION AND CHARACTERIZATION OF Pt-Fe ALLOYS SUPPORTED CHARCOAL
ON
P. FOUILLOUXI, D. GOUPIL”, B. BLANC2 and D. RICHARD” IUnite Mixte CNRS-Rh6ne Poulenc, 24 Avenue Jean Jaures, 69151 Decines-Charpieu CBdex BP166 (France) 21nstitut de Recherche sur la Catalyse, 2 Avenue Einstein, 69626 Villeurbanne Cedex (France)
SUMMARY The optimization of Pt-Fe catalysts supported on charcoal was made by means of an empirical method. The carbon support was washed, oxydized in a liquid phase and thermally desorbed before its impregnation. The metals precursors were reduced by hydrogen. The reduced metals were characterized by electron microscopy, magnetization measurements and X Ray diffraction. The two metals are alloyed under the form of finely divided particles. The charcoal supported Pt-Fe catalysts are very active and selective in hydrogenation of cinnamaldehyde to cinnamyl alcohol. INTRODUCTION The platinum-iron alloys described in this work were prepared for the selective hydrogenation of alpha-beta unsaturated aldehydes on the background of Adam*s work published on this topic (ref. 1). This author showed that an addition of FeC1, to an hydrogenation solution containing cinnamaldehyde was able to produce cinnamyl alcohol with a good selectivity. More recently (ref. 2), the addition of FeCl,, SnC1, and GeC1, to platinum catalysts has also proved to be effective in this hydrogenation. Under the reaction conditions, we don’t know whether the cationic iron, added as a selectivity promoter, will be reduced or not to the metallic state in the presence of platinum particles and hydrogen pressure. This observation led us to experiment the properties of a bimetallic Pt-Fe under the form of an alloy if possible (ref. 3 ) . Charcoal was chosen as a support because of its good resistance to corrosion. Platinum-iron catalysts supported on carbon were first prepared by Bartholomew and Boudart (ref. 4). For this purpose, they oxidised the carbon surface by partial burning and then the carbon was impregnated by the metals salts. This gaseous phase oxidation is not easy to extrapolate to a larger scale and we describe here a safer method to preDare well dispersed Pt-Fe alloys supported on charcoal.
470
PREPARATION OF THE Pt-Fe/C CATALYSTS .Cinnamaldehydehvdroaenation as a auide for catalvsts Drenaration During the hydrogenation of cinnamaldehyde, hydrogen can be added to carbon-carbon double bond or to the carbonyl group: Cinnamaldehyde A
CHO
Cinnamyl alcohol A
The reaction vessel was a stainless steel autoclave with a PTFE coating and a magnetically coupled stirrer. The solvent - a mixture of 50ml isopropanol, lOml water and 2.5ml of 0.1M NaOAc was injected in the autoclave with 0.69 catalyst and preheated at the reaction temperature (60'C). Cinnamaldehyde was introduced into the catalyst suspension after the temperature had reached the desired value. Liquid samples were periodically withdrawn and analysed by gas chromatography in the aim to determine the initial hydrogenation rate V, and the selectivity in cinnamyl alcohol (ref. 3 ) . Pretreatment of the charcoaL A commercial activated carbon black (505 from Carbonisation et Charbons Actifs) with a high specific surface area was chosen. Originally it contains 11.5wt% ashes with a non negligible amount of iron (ref. 5). For this reason, we washed the crude charcoal with hydrochloric acid which is a good complexing agent for iron ions. Washing with HNO, gives similar results (Table I) but nitrous evolution is produced and we preferred HC1 washing. The second stage of the support pretreatment was an oxidation with sodium hypochlorite following a well known process (7). The charcoal is added slowly to a concentrated solution of hypochlorite. The oxidation reaction is very exothermic and the slurry must be cooled in ice. This oxidation produces superficial groups on the carbon surface which prevent the metallic precursor
471
salts from migrating during hydrogen reduction. This favours the formation of a more divided supported metal (ref. 4). TABLE I Composition (wt%) of the charcoal before and after washing.
Element
Non washed charcoal
Charcoal washed HCL 2N
Charcoal washed HNO, 2N
Ashes Fe P Ca K sio,
11.5 0.121 2.18 1.36
2.35
2.2
3.15
0.02 0.074 0.096 0.465 1.484
0.054 0.032
0.1
1.008
5.852
-
Charcoal washed NaClO
0.056
-
ImDreanation and reduction of the Pt-Fe/ catalvsts The oxidized charcoal is filtered, dried at 120'C in an oven and finally desorbed at 430'C in flowing nitrogen. The pretreated TABLE I1 Optimization of the preparation parameters on a 30at% Pt-Fe/C.
Drying of impregnated
Reduction
V,.103(mole.mn-1.g-1)
charcoal
Undr.ied lh He 2h He 2H He 15h He lh He lh He lh He 15h He 15h He 15h He
Figure 1. Surface distribution of pure platinum particles.
5.0
2.5
0 0
25
50
75
Particles diameter (A)
Figure 2. Surface distribution of Pt-Fe/C with 45.5at% Fe.
473
carbon powder
is then put into a solution of H,PtCl,
and
Fe(NO,):,, n H,O in a mixture of benzene and ethanol. The liquid of the slurry is removed under vacuum in a rotatory evaporator. We determined the best conditions for the catalyst preparation on the background of activity measurements (Table 11). A mid iron content (30at%) in the Pt-Fe catalyst was taken to exemplify the influence of preparation parameters. Before reduction, the powder must be dried at low temperature under neutral gas flow to remove the remainder of the solvent and the catalytic activities of table I1 show that the best conditions are a heating temperature of 120'C during 15 hours. The most effective catalyst was obtained after a reduction in hydrogen at 430'C during 6 hours. This last process was followed for the preparation of all the platinum-iron catalysts. We prepared catalysts of a total metal loading of 5% and with iron to total metal percentages varying by steps of 10at% from 10 to 70at% . The most interesting compositions for the activity and the selectivity in hydrogenation of cingamaldehyde lying between 10 and 50 at%, we devoted our whole attention to this composition range. CHARACTERIZATION OF THE Pt-Fe/C CATALYSTS Metallic disDersion of the catalvsts The high surface area of the charcoal used in this work prevented us from using gas chemisorption to determine metallic dispersion of our catalysts. We used an electron microscope (Jeol JEM 100 CX) to investigate the geometric appearance of the particles. At first sight, the micrographs show that in the pure platinum catalyst the metal crystallites form two populations whereas in the bimetallic preparations the size is much more uniform. A statistical determination of particle size was made from electron micrographs. The results of these measurements are plotted on figure 1 for the monometallic supported platinum powder and on figure 2 for a bimetallic containing 45.5at% iron. The size distribution diagramms confirm that the pure platinum particles have a large binodal size distribution around 90 and 200A. The particles diameter for the bimetallic is narrower and lies about 25A. A third determination for a powder containing 18.7at% iron showed that the distribution presents a unique maximum at 45A. Thus the presence of iron in the bimetallic catalyst plays a
474
promoting role on the metallic dispersion and on the homogeneity of the particle size. Overall and local comvosition of the catalvsts Our catalysts were analysed by chemical means which give the overall composition. The sample were dissolved in aqua regia and the support was eliminated by concentrated nitric and perchloric acids. The elements were then titrated in solution by classical means. The local analysis is much more interesting to see whether the distribution of the elements in the catalyst grain is homogeneous or not. We performed these measures by means of X-Ray emission spectroscopy in a STEM apparatus (Vacuum Generator HB5) where the excited area has a diameter of 10 angstroms. It is possible to analyse the tiny metal particles individually. TABLE I11 Overall composition of two Pt-Fe catalysts and composition of metallic particles of different size (compositions in at%).
Analyzed area or particle Catalyst I overall 20A part. 30A part. 60A part. Catalyst11 overall 20A part. 30A part. 50A part.
Chemical analysis
X Ray emission analysis
Pt
Fe
Pt
Fe
81.3
18.7
78 77 84 82
22 16 18
52
48
54.5
45.5
53 54 57
23
47
46 43
The results of table I11 allow us to compare the data obtained by chemical analysis to those given by X Ray emisssion for global compositions. The agreement between the two methods is satisfying. The physical method is able to go further in the knowledge of the samples: it detects a uniform composition for a given bimetallic catalyst - within the precision limits whatever the composition and the metal particle size. This allows to rule out a surface enrichment in one of the two metals because it should involve a parallel enrichment for the small particles.
476
-. P”
200
.
z 3
After ref.ll ref.12 ( 0 ) After ref.13 (0)Our results
( 0 ) After
3
-
a,
m s-. e F
.3 0
100
-
5
U
m
v)
50
-
o s do’ 0
.i’ 3’
/.
I=
.3 0
150
./,r
(m,.)
/d
oo
0
;--ge,3
Fe (at%)
,
20
40
60
80
100
Figure 5. Saturation magnetization of the reduced catalyst as a function of its composition.
Fe composition (at%) Figure 6. curie temperature of the reduced catalysts in function of their composition.
477
Crvstal structure of Pt-Fe particles X Ray diffraction was performed on three samples by DebyeScherrer method in a controlled atmosphere camera. The powder was prereduced in situ under hydrogen before each experiment. The diffraction pattern presented a background due to the charcoal and additional enlarged lines produced by the metallic particles. None of them were those of pure platinum or pure iron. The diffraction diagram was in agreement with a cfc structure. The calculated lattice parameters were plotted in figure 3 and compared with those of bulk alloys (ref. 8) of the same composition. The agreement was considered as satisfying. This gives us a first proof that the supported metal should be under the form of an alloy. The electron beam wasfocussedon individual particles of the bimetallic catalysts. We obtained diffraction patterns of monocrystals which is a proof that each metallic particle is a monocrystal but we were not able to have a sufficient precision on the lattice spacings to make a difference between pure platinum and bimetallic particles. Maanetic measurements The first study of Pt-Fe/C by magnetic measurements was made by Bartholomew and Boudart (ref.9). In a first stage we used saturation magnetization measurements after heating the supported catalysts precursors after heating under hydrogen to precise the reduction conditions. The results are plotted on figure 4: the sample shows a diamagnetic behaviour of the carbon support before hydrogen reduction (negative slope of magnetization curve). Heating the sample by steps produces an increase in magnetization up to 400'C where the sample is completely reduced. A further exposition to air lowers the magnetisation but rereduction is performed at low temperature. This phenomenon was also observed by other authors with Pt-Fe/SiO, (ref.9-10). The magnetization data obtained on reduced samples proved to be very similar to those of bulk alloys (figure 5 ) . The second type of magnetic investigation was made by Curie temperature determinations in a thermomagnetic balance (ref.11). The Curie temperatures of the catalysts were also very near to those of bulk alloys (figure 6). DISCUSSION-CONCLUSION The Pt-Fe/C catalysts prepared in this work were optimized due to a test system. An iron content of 20at% gives catalyst activities 100 times higher than pure platinum and selectivities
478
of 90% in our test reaction (ref.3). The preparation method is very convenient and easy to perform with a very simple equipment. characterization of the most effective bimetallics by various physical methods leads us to the conclusion that platinum is alloyed to iron to form monocrystalline particles. Iron plays the role of a texture promoter. Platinum promotes the reduction of iron to metallic state during the initial preparation and after subsequent exposition to air so that the alloy could be formed by simple action of hydrogen at low temperature before hydrogenation reactions. REFERENCES 1 W.F. Tuley and R. Adams, J.A.C.S.,45(1925)3061. 2 S . Galvagno, A. Donato, G. Neri, R. Pietropaolo and D. Pietropaolo, J. Mol. Catal., 49(1989)223-32. D. Goupil, P. Fouilloux and R. Maurel, React. Kinet. Catal. 3 Lett., 35(1987)185-193. 4 C.H. Bartholomew and M. Boudart, J. of Cata1.,25(1972)173. 5 D. Goupil, These de Doctorat Universite de Lyon, "9086, (1986). 6 D. Richard and P. Gallezot,in B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet (Eds.), Preparation of catalysts, Elsevier, Amsterdam, 1987, pp.71-81. 7 J.B. DONNET, F. HUEBER, C. REITZER, J. ODOUX and G. RIESS, Bull. SOC. Chim. France, (1962)1727-35. W.B. Pearson, Handbook of spacings and Structures of metals 8 and alloys, Pergamon Press, (1964)651. 9 C.H. Bartholomew and M. Boudart, J. of Cata1.,29(1973)278 10 L. Guczi, Catal. Rew. Sci. Eng., 23(1981)329. M. Fallot, Ann. Phys., 10(1938)291. 11 G.E. Bacon and J. Crangle, Proc. Roy. SOC., A272(1963)387. 12 J. Crangle and W.R. Scott, J. of Appl. Phys.,36(1965)921. 13 A. Kussmann and G. Rittberg, Z. Meta11.,41(1950)470. 14 V. Perrichon, J.P. Candy and P. Fouilloux,,Progress in vacuum 15 microbalance techniques, Heyden and Sons, London, 3(1975)18. ~~
~
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
479
SUPPORTED METALLIC CATALYSTS ACHIEVED THROUGH GRAPHITE INTERCALATION COMPOUNDS
F. BEGUINl, A. MESSAOUDIl, A. CHAFIK2, J. BARRAULT2 and R. ERRE' 'C.R.S.O.C.I. - C.N.R.S., 1B rue de la FCrollerie, 45071 OrlCans CCdex 02, France. 2Catalyse en Chimie Organique, UnkersitC de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers CCdex, France. SUMMARY New preparations of supported metallic catalysts (Fe, Co, Ni) using a graphite intercalation compound as precursor were described. The graphite-MClx derivatives were reduced by otassium and metallic particles were obtained within a graphite matrix. The reduction a!ot MCl, salt or the decomposition of a metallocene lead to the formation of clusters at the edge of the graphite flakes. Only graphite samples with low specific area were used to obtain the intercalated precursors . Consequently, these materials do not present a very high activity in the hydrogenation of carbon monoxide. However, a marked selectivity to alkenes formation was observed with the graphite supports of the largest specific area. INTRODUCTION The research and the preparation of heterogeneous catalysts were largely developed during the last years. We present here the results of studies for the valorization of C1 molecules (carbon monoxide, methane...) and the development of new processes in fine chemistry. Initially, catalysts were not specifically designed with respect to the complex reactions to which they were applied or as regards high selectivity. Recently, significant advances were obtained in preparing largely multifunctional catalysts, e.g. useful in the selective conversion of syngas to light olefins or alcohols or in hydrofunctionalization reactions, for which several kinds of reactional centers are necessary in the catalyst. For applications in fine chemistry processes, one must develop new selective catalysts taking into account the particular conditions of their use : presence of solvent, of heteroatoms (0,N, S, halogen ...) in the reagents and consequently a possible formation of products (H20, NH3, H2S, HX...) which are well known to modify the classical catalysts. For these reasons,we were interested in a new synthesis of metals dispersed on carbon. Indeed, this support allows us to obtain high-performance catalysts selective either in gas or liquid phases. For the preparation of such catalysts one must take into account the classical concepts of heterogeneous catalysis : geometric and electronic effects, SMSI, effect of a promoting agent ... Inhomogeneous M catalysts are generally obtained by impregnation of a support with a metallic salt dissolved in aqueous solution, then reduced by hydrogen. With
480
graphite as support, a homogeneous dispersion of the metal could be obtained from its intercalation compound, as stated by Volpin (ref.1) to occur with the reduction of the binary graphite MC?, compounds (where M is a transition metal). Braga (ref.2) later claimed that he obtained intercalated metals by reduction of a metallic halide MCl, with the binary Kc8. More recently, Inagaki (ref.3) repeated Braga's work with CoC12 and found that the reaction gives a ferromagnetic ternary graphite-Co-THF compound. However, some authors claimed that whatever the process, a part of the metal is included and not intercalated (ref.4,S). In this paper, we corroborate that, starting either from donor of from acceptor GIC, reduction only yields metallic particles, even under mild conditions. Accordingly, processes based on the chemical reduction of acceptor graphite-MCl, compounds or on the reaction of KC8 with metallic halides or metallocenes constitue a good method for the synthesis of metallic catalysts supported on graphite. These new compounds were tested in the hydrogenation of carbon monoxide. RESULTS - DISCUSSION Reaction of graphite MC1,- (M = Ni,Co.Fe) with metallic potassium Second stage G-MCl, binaries were prepared from the direct reaction of a metallic chloride (FeC13, CoC12, NiC12) with natural graphite under a pressure of chlorine (ref. 6). The G-MCl, compound was then allowed to react with potassium under vacuum at 3 0 0 ° C for about two days. According to some authors, the alkali metal can intercalate the second stage acceptor compound to give a heterostructure (ref.7, 8). We have however found that MC!, and K, located in adjacent domains, are very reactive even at this relatively low temperature, and that the following reaction occurs spontaneously :
The X-ray diffraction diagrams mainly show two sets of lines due to KCl and metallic clusters. After washing with a water/ethanol mixture (l/l), the lines of KC1 are still present, indicating that the products of the reaction are included in the graphite matrix. X.P.S. analysis shows that the main contribution of the 2p core level of the M element is due to a metallic state M" (80%). Compared to the chemical analysis (Table l),the C/M ratio obtained by X.P.S. shows a very weak concentration of the element M at the surface of graphite except with the G - FeC13 sample in which a part of the iron has migrated to the surface during the reduction process. In all cases, chemical analysis and X.P.S. reveal a K/C1 ratio greater that one due to the intercalation of excess potassium in the graphite freed by the reaction. Transmission electron microscopy shows a rather narrow and homogeneous distribution of particles sizes (about 3 0 nm). After exposure to air, the metal in only slightly oxidized, indicating that the particles are protected by the lattice or/and covered by the KC1 produced during the reaction.
481
TABLE 1 Atomic ratios given by chemical analysis (C.A.) and X.P.S. on the products found after reduction of G-CoC12, G-NiC12, G-FeC13 by potassium at 300" C. Sample
C.A.
G-CoC12 G-NiC12 G-FeC13
3.99 5.09 5.33
c/c1 X.P.S. 3.3 1.6 1.1
K/C1 C.A. X.P.S. 1.3 1.4 1.2
1.9 1.4 1.1
C/M C.A. X.P.S.
8.1 10.6 12.2
44.8 120.0 10.5
For catalytic applications, an exfoliation of the host lattice would be essential. To explain this reaction, we propose the following mechanism : the first step is the formation of a biintercalation compound with distinct islands of K and MCl,, since, in the pleated layered structure, the reagents occupy all the interlayered spaces of the host lattice ; then, even at low temperature, the reagents can react together to give metal clusters.
Reduction of metallic halides bv K Q The KC8 binary is prepared by the direct reaction of potassium on graphite under vacuum (ref. 9). Due to the electronic transfer to the graphene plane, this compound is a strong reducing agent. If it is allowed to react with a metallic halide MClX (M = Fe,Co,Ni) dissolved in tetrahydrofuran (THF), the reaction occurs spontaneously at room temperature and leads to the formation of metal M" dispersed on graphite :
Three phases were identified by X-ray diffraction : graphite, KCI and the M" metallic species. The X.P.S. spectra of the reaction product show that the 2p core level of M may be attributed essentially to a metallic state (70 to 80%). However X.P.S. analysis reveals a strong concentration of the elements which constitue the KX salt on the surface. In Table 2, the atomic ratios given by chemical analysis are compared to the quantitative results deduced from X.P.S. spectra. The C/K ratio obtained by X.P.S. is always lower than the one given by chemical analysis. The K/X ratio is close to 1, confirming the existence of KCl. The KCl can be completely eliminated by washing with a carefully degased water/ethanol (1/1) mixture, but this treatment which can be used with Ni or Co, is not applicable to the iron derivatives as the metal reacts to give an oxihydroxide FeOOH which transforms to Fe2O3 under vacuum. All three M species are oxidized after exposure to air, proving that the metallic clusters are easily attacked by any reagent. Moreover transmission electronic micrographs show that the metallic particles are localized at the edge of the graphite flakes.
482
TABLE 2 Atomic ratios given by chemical analysis (C.A.) and X.P.S. for the products formed by reduction of some salts in THF solution by Kc8.
Salt
C/K C.A. X.P.S.
coc12 NiBr2 FeC12 FeC13
7.9 8.5 9.6 18.6
1.9 1.5 1.0
1.0
K/X C.A. X.P.S.
C/M C.A. X.P.S.
1.0 0.8 1.1 0.8
14.0 12.1 15.8 36.4
1.1 1.3 1.1 1.0
15.9 8.0 16.0 12.0
DecomDosition of a metallocene bv KC8 The main disadvantage of the previous reaction (2) is the simultaneous formation of KCl which partly inhibits the properties of the metallic element. We have found that under appropriate conditions, the reaction of KC8 with metallocenes (Fe(C5Hg)z and Ni (CgH5)2) dissolved in dimethoxyethane (DME) gives soluble by-products, permitting the preparation of "fine M / C by the following reaction :
The results of chemical analyses are given in Table 3. For the reaction with Fe(CgHg)2, chemical analysis shows an important concentration of residual potassium. In the case of nickelocene, only traces of potassium were detected and two phases were observed by X-ray diffraction : pure metallic nickel Ni" and graphite. TABLE 3 Chemical analysis of the products obtained by the decomposition of metallocenes in DME by KC8. Global Formula
Metallocene
TC
C
K
M
H
Ni(C5H5)2
20 60
69.9 63.2
1.1 1.5
15.2 19.4
1.2 1.7
87.4 C22.5Ko.iNilH4.5 85.8 C25.9K0.1NiiHg.i
Fe(C5H5)2
60
71.0
9.1
10.5
0.5
91.1 C31Ki.2FeiH2.7
X
483
The intensity and the binding energy of the 2p core level of Ni confirm the metallic state. With ferrocene, reaction (3) is not complete because this molecule is stabler than nikelocene : X-ray diffraction clearly proves this results, with small peaks due to metallic iron and others attributed to a high stage intercalation compound. Due to its aromatic character, ferrocene probably first intercalates and reacts in the interlayer space. We tested this hypothesis by studying the direct reaction of KC8 with Fe(CgHg)2, with no solvent. Chemical analysis gives a global formula C28.3K2.3FeH10, equivalent to C~~.~K~J(F~C~O Thus, H ~ Oapproximately ). two KCs molecules react with one Fe(CgHg)2. X.P.S. analysis shows that the iron particles are essentially in the metallic state Fe" : the total decomposition of ferrocene by KCg would occur along the following reaction path :
and the 001X-ray lines with an identity period I, = 12.3 A could correspond to a second stage K2(CgHg)2C16 ternary phase with iron supported by the graphite matrix. At present, we are however unable to explain the exact nature of the intercalated compound found which may contain a polymerization product of the cyclopentadienyl groups.
Hvdrogenation of carbon monoxide For better appreciation of the performances, the new catalysts were tested in the hydrogenation of carbon monoxide. In view of the results presented above, and taking into account the size and distribution of the metallic particles as determined by X-ray diffraction and Transmission Electron Microscopy (ref. lo), only the Co products prepared according to reaction (2) were studied. In fact, the best results were obtained with cobalt using the impregnation method (ref. 11). The distribution of the metal and the occlusion and migration of superficial species, as well as their reduction will greatly depend on the nature of the carbon used for the preparation of KCg. To prepare the supported cobalt, three different carbon materials were chosen : - Ceylon graphite (O.3m2g-') - Graphitized carbon black (LeCarbone Lorraine, Srn2.g-l) - Lonza graphite (reference HSAG, 300m2.gb1) The results are given in table 4. The nature of the carbonaceous support and probably its specific area have a determining effect on the catalytic properties of these solids. In the same experimental conditions, the catalyst from Lonza graphite is 60 times more active than the catalyst from Ceylon graphite. Moreover, the selectivity of Co/Lonza graphite is very different from the other solids as it gives a large amount of hydrocarbons and particularly olefins.
484
TABLE 4 Influence of the carbonaceous support on the properties of the Co/C catalysts used in the hydrogenation of carbon monoxide. P = 1bar, H2/Co = 1, Total flow 3.6 1.h-I. The solids are reduced by hydrogen at 400°C before the reaction. Support Reaction temperature
(“C) Activity ( x l d ) mole h-l g-lCat Selectivity (%)
This behaviour is in good agreement with the structural observations on Co/Lonza graphite indicating well distributed metallic particles localized at the edge of the graphite flakes (ref. 10). With the other supports and owing to the lower specific area, the number of reactional centers at the edge of the flakes is smaller and the particle size larger than in the case of Lorna graphite. A comparison with the selectivity of a catalyst prepared by impregnation of a Lonza graphite with cobalt nitrate and reduced by hydrogen is given in table 5, its total activity being 8.6 x l o 3 mole.h-l.G-l cat.
TABLE 5 Properties of a Co catalyst prepared by impregnation of Lorna graphite with cobalt nitrate and reduced by hydrogen at 410°C. Co 5% / Lonza graphite HSAG 300 (3O0m2.g-l) Temperature of the reaction 240°C. Selectivity (%>
co2 8.5
CHq 53
c2 5
c3 5.6
c4
5.5
c5-c9 21.5
485
Taking into account the differences in the temperatures of reaction, it appears that the impregnated catalyst is 20 to 50 times more active than the solid prepared from KC8. Moreover, our catalyst gives a larger amount of superior hydrocarbons. This behaviour could be due to the presence of potassium as many studies show that an alkali element favors chain growth (ref. 12). Nevertheless this catalyst is particularly selective for alkenes formation (= 30%).
CONCLUSION The three reactions studied in this paper always lead to metallic clusters supported by the graphite matrix in the conditions of our experiments. Depending on the initial binary, the particles are either at the surface or included.Best activity was observed with the catalysts prepared from KC8. However, for the materials prepared from Graphite-MCI,, improved activity could be obtained after previous exfoliation of the graphitic matrix. The relatively low activity of supported cobalt obtained from an intercalation compound could be due to the low specific area of the support or the presence of KCl. Some trials on washed samples or on specimens prepared as in reaction (3) could give more information. The most striking fact with our products a particular selectivity to alkene formation with a support of relatively high specific area. REFERENCES 1
2
3 4 5 6 7 8 9 10 11 12
M.E. Volpin, Yu.N. Novikov, N.D. Lapkina, V.I.Kasatochkin, Yu.T. Struchkov, M.E. Kazakov, R.A. Stukan, V.A.Povitskij, Yu.S. Karimov and A.V. Zvarikina, J. Amer. Chem. SOC.,97 : 12 (1975) 3366. P. Braga, A. Ri amonti, D. Savoia, C. Trombini and A. Umani-Ronchi, J.C.S. Chem. Comm., 8978) 927. M. Inagaki, Y . Shiwashi and Y. Maeda, J. Chim. Phys., 81 (1984) 847. G. Bewer, N. Wichmann and H.P. Boehm, Mat. Sci. and Eng., 31 (1977) 73-76. H. Schafer-Stahl, J.C.S. Dalton (1981) 328. S. Flandrois, J.M. Masson, J.C. Rouillon, J. Gaultier and C. Hauw, Synth. Met., 3 (1981) 1. G. Furdin, L. Hachim, D. GuCrard, A. HCrold, C.R. Acad. Sci., 3 0 1 (1985) 579. R. Erre, F. Bkguin, D. GuCrard, S. Flandrois, Proc. 4th International Carbon Conference, Baden-Baden (1986 516. A. HCrold, Bull. SOC.Chim. Fr., t1955) 999. F. BCguin, A. Messaoudi and R. Erre, submitted to Carbon and A. Messaoudi, Ph. D. Thesis, OrlCans, France (1989). A. Chafik, Ph. D. Thesis, Poitiers, France (1988). J. Abbot, N.J. Clark and B.G. Baker, Appl. Cat., 26 (1986) 141.
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G . Poncelet,P.A. Jacobs,P.Grange and B. Delmon (Editors),Preparation of Catalysts V 01991 Elsevier Science PublishersB.V., Amsterdam -Printed in The Netherlands
487
PREPARATION OF GRAPHITE-IRON-POTASSIUM CATALYSTS FOR AMMONIA SYNTHESIS K. KALUCKI and A.W.MORAWSK1
Department of Foundations of Chemical Technology, Institute of Chemical Technology, Technical University of Szczecin, ul. Pulaskiego 10, 70-322 Szczecin (Poland) SUMMARY The preparation of graphite-Fe-K catalysts for ammonia synthesis was described. The process was realized in steps: 1) intercalation of iron (111) chloride into graphite; 2 ) reduction of obtained FeC13-GIC; 3) removal of a-iron and 4) intercalation of potassium into Fe-graphite precursor. The influence of both iron and potassium on activity in ammonia synthesis was studied. Preparation o f catalyst in a large scale has been performed. The preparation method for these catalystsis proposed.
INTRODUCTION Trying to prepare very active catalysts, many attempts have been devoted to the systems derived from so-called graphite intercalation compounds (GICs) (refs. 1-4). Graphite possesses a layered structure, consisting of planes of sp2-hybridized benzenoid carbon. The weak 1-bonds between the carbon planes can easily be broken to allow insertion of guest atomic or molecular layers between them without disrupting the layered topology o f graphite. In chemical sense, the intercalation of graphite is analogous to the formation of an "infinite" ion of aromatic molecules. The distance between the carbon layers is increased by the thickness of the intercalated layer. The catalytic formation of ammonia from hydrogen and nitrogen over alkali metal-graphite-transition metal chloride complexes has been discovered by Ichikawa et al. (ref.5). Several studies on the formation of ammonia on GICs catalysts have been carried out in recent years (refs. 6-10). The above mentioned studies proved that the FeCl GlCs are good precursors 3to highly active catalysts made by double step reduction followed by HC1 treatment. In spite of considerable amount of work performed in this field, the identity of the catalytically active species has not yet been established. The created iron is rather Fe(0) produced by reduction of Fe(I1) with potassium and located in the graphite matrix. Such active iron is not readily accessible to the HC1 (ref. 10).
488
In the present work we report experimental data from studies on the influence of both the iron and the potassium contents in catalyst on activity in ammonia synthesis. We also performed the preparation of catalyst in large laboratory scale (ca. 100 g). As a consequence of this research we have proposed the technology of pre-
paration of graphite-fe-K catalyst for ammonia synthesis. EXPERIMENTAL Catalyst preparation The FeC13-GICs samples were prepared according to the commonly used method. Powdered natural Sri Lanka graphite with a particle size of 1-20 microns in thickness and 30-100 microns in width was used in this study (Fig.1.). The mixture of powdered graphite and pure anhydrous ferric chloride (Riedel) was heated at a temperature of 300 OC for 24 h. The intercalation reaction was carried out with different amounts o t FeC13, controlled
by
means
of
the ratio of solid graphite to solid FeC13 in
starting mixture (ref.11).
Fig.1. Photomicrograph of natural Sri Lanka graphite. The products of intercalation were freed from excess metal chloride by washing with an aqueous solution of HCl(l:l), filtering, washing with distilled water, and drying overnight at 110 OC. The reduced samples of precursors were obtained by polythermal reduction of the GICs with a mixture of nitrogen and hydrogen (1:3) under atmospheric pressure at temperatures from 150 OC to 300 OC with a
489
heating step of 25 OC per 24 h, and from 300 OC to 625 OC with a heating step of 50 OC per 24 h followed by further reduction at 625 OC for 5 days with a space velocity of gases of ca. 1000 h-l (ref.12). The a-iron which partially appeared during the reduction process was removed by an aqueous solution of HCl(1:l). Sa_mpl_es_characteriEatio n Both the precursors and the catalysts have been characterized by X-ray fluorescence spectroscopy (XRFS), Mu), and scanning electron microscopy (SEM) techniques. The following apparatus was used for the measurements: X-ray fluorescence spectrometer VRA-30 (GDR) for XRFS, Universal Roentgen-Diffractometer HZG-4 (GDR) for XRD, vakia) for SEM.
and BS 300 scanning microscope (Czechoslo-
Catalyst-activities
Both the activation of Fe-graphite precursors and the activity measurements were performed in flow reactor (Fig.2).
Fig. 2.
Presssure reactor for both
activation and activity measurements.
- thermocouples; 2 - inlet gas; - outlet gas; 4 - thermocouple wall 5 - precurosr of catalyst; 6 - potassium; 8 - position regulator; 9 - outlet to 1,7,
3
vacuum.
8
The metallic potassium was introduced into the reduced precursors of GICs by vapour deposition at a temperature reaction of 350 O C under a pressure
490
of about 6 Pa. The potassium introduction process was controlled by the time of reaction. The activity
of samples
was
studied
at 10 MPa with space velocity
(s.v.) of 30000 h-'. RESULTS AND DISCUSSION The results of both composition and activity of used precursors of catalysts are summarized in Table 1. TABLE 1 Composition and activity of used precursors of catalysts. t=350 OC; S.V. 30000 h-l; p = 10 MPa. Sample
Denotation
Cl/Fe
C/Fe
K/Fe
X NH3
-
-
FeC13-GIC
A B C
2.99 2.98 2.95
53.15 17.96 14.15
FeC13-GIC (reduced)
Ax BR CR
0.37 0.32 0.30
53.15 18.97 14.54
FeC13-GIC (reduced, HC1 treated)
ART BRT CRT
0.70 0.73 0.73
80.28 43.15 33.20
K-graphite
-
-
-
-
-
-
-
0.03 0.04 0.04
3.99
0.25
The polythermal reduction of FeCl3-G1C in a stream of 3H2+N2 which was proposed by the authors of this work, leads to formation of non-intercalated iron and the various phases of intercalated iron presumably with mixing of the stages of iron-GICs (ref.12). The free graphite phase was also present during each treatment (intercalation, reduction and HC1 treatment of reduced samples) with intensity depending on iron concentration. The method of reduction prevented evaporation of FeC13-GIC but did not prevent partial de-intercalation of FeCl during reduction. The de-intercalated FeC13 was 3 reduced to the a-Pe phase (Fig.3). The amount of a-Fe that collects on the surface of the carbon is dependent on the total iron concentration. The a-Pe consists of metal clusters which can be removed from the carbon surface by acid treatment (compare Fig. 3 and Fig.4 ) . The reduction of FeCl3-GIC was incomplete and ca. 0.7 - 0.3 chlorine atom per atom of iron remained (Tab. 1). The remaining iron-graphite precursors contain a phase probably with a mixture of stages that are temperature resistant up to 625 OC and are stable in aqueous HC1. Both a-iron supported on particles of graphite and iron encapsulated in graphite were practically inactive in ammonia synthesis reaction at
491
temperature
of
350
OC
(ref.13 and Tab. 1) even at presence of KON
( r e f . 13).
Fig. 3. Photomicrograph of FeCI3-
Fig.4. Photomicrograph of FeC13-
-GIC
-GIC
(1 stage ) reduced.
(1 stage) reduced and HC1 treated.
The iron encapsulated in graphite can be
a good precursor to produce
highly active catalyst at mild conditions after activation with metallic
potassium. The effect of composition on ammonia yield of studied catalysts is presented in Table 2. The influence of both the iron and the potassium contents on activity in ammonia synthesis was calculated using multiple regression. The response function is given as equation (1): y = 0.07502 x1 t 0.329993 x2 t 0.00378 x12
-
0.021718 x22 t 0 . 0 2 8 8 4 ~ ~ ~ ~
where :
- represents yield of ammonia v01.-X at 35OoC, p=10 MPa,s.v. 30000 h-l x1- represents content wt.-X of iron in Fe-graphite precursor y
492
x2- represents molar ratio potassium per iron in catalyst The
data computed
according to
the relationship (1) are presented in
Fig. 5. TABLE 2 Composition and activity of studied catalysts. 350
OC,
s.v.30000 h-l,10 MPa
Sample of catalyst ~
Cl/Fe
C/Fe
C/K
~
K/Fe
X NH3
~
ARTK ARTK ARTK ARTK
(11.07 X K) (14.85 X K) (28.0 X K) (37.8 X K)
0.66 0.68 0.65 0.68
80.53 80.44 80.45 80.51
24.10 17.20 7.74 4.95
3.33 4.60 10.38 16.20
1.53 1.70 1.70 1.65
BRTK BRTK BRTK BRTK BRTK
(
1.69 X (13.53 X (17.39 X (25.51 % (38.4 X
0.69 0.65 0.68 0.69 0.66
43.29 43.22 43.27 43.19 43.59
164.3 18.02 13.41 8.23 4.11
0.26 2.39 3.23 5.24 10.59
0.68 2.19 2.50 2.89 3.08
0.77 0.75 0.74 0.75
32.9 33.03 32.99 34.65
557.6 11.38 5.70 4.41
0.06 2.90 5.76 7.85
0.55 3.23 4.17 4.22
K) K)
K) K) K)
~
CRTK ( 0.48 X K) CRTK (19.16 X K) CRTK (32.1 X K) CRTK (39.0 % K)
Fig.5.
~~
Response plot of predicted activities of catalysts. t = 350 OC; p = 10 m a ; S.V. 30000 h-
.
graphite-Fe-K
As it is shown in Fig. 5 the maximum of activity was reached at molar ratio K/Fe ca. 5 - 6. The further loading of potassium to Fe-graphite precursor did not influence the activity.
493
CL
4
m
20°
30'
28
10° rl
0
B
0
v)
m
E 0 0
goo
loo
20°
30'
80°
70'
60'
50'
40'
30'
2e
20°
Pig.6. Diffractograms of samples in each step preparation. a ) FeC13-GIC, MoK, radiation b) reduced FeC13-GIC, MoKa radiation c j passivated graphite-Fe-potassiu catalyst, CoKa radiation
28
494
In Figures 6a,6b,6c are given the diffractograms of samples at each step of catalyst preparation,in large laboratory scale (ca. 100 8).
The starting intercalation compounds (Fig.6)
forms mixture of FeC13-GIC
1 stage with dl = 936 pm and small amount o f
FeC12-GIC 1 stage with
dl = 960 pm and free graphite. The total molar ratio C/Fe was 8.13. After reduction such FeC13-GIC (Fig. 6b) the dominant phases were Fe-GlC 1 stage with d1 = 581 p and graphite (d=335 pm). The total molar ratios were: C/Fe=8.07; Cl/Fe=0.88.
Pig. 7 . Photomicrograph of passivated graphite-Fe-K catalyst. The graphite-Pe-K catalyst (Fig. 6c) forms mixture of KC1, a-Fe. graphite and new periodic phase with d = 1224 p. The sandwiched structure 1 of catalyst is very clearlydemonstrated in Fig. 7. ‘ h e described catalyst exhibits higher activity as compared to typical iron industrial catalysts (Fig. 8 ) particulary at lower temperatures. Proposed technology of preparation Based on the experimental work outlined above and our earlier works, an integrated preparation process is proposed. Procedure f o r preparation of paphite-Fe-K catalyst is illustrated by diagram given in Fig. 9.
495
Fig. 8 .
Ln(k)
versus temperature of
graphite-Fe-K catalyst ( x ) and iron industrial catalyst ( 0 ) . Reaction rate constant “k” in kgNH3MF’aoa5/(kgkat. h) 1.3
1.5 1.7 1000/T
S.V.
1.9
.
100000 h-l, p = 10 MPa.
intercalation of FeC13 into graphite
3 reduction of FeCl -graphite
I
I
1
removal of a-iron
I intercalation of potassium into Fe-graphite
1
final catalyst
Fig. 9. Proposed technology of preparation of graphite-Fe-K catalyst.
CONCLUSIONS A novel graphite-Pe-potassium catalyst system can be made by intercalation of graphite. The iron encapsulated in graphite i s a good precursor to produce highly
496
active
catalysts
after activation
with
metallic potassium. The metallic
potassium is capable of getting into the graphite matrix of Fe-graphite presursor and of creating a donor type compound. The presence of sandwiched phase has been found in catalyst. The described graphite-Fe-K catalyst exhibits higher activity as compared to typical iron industrial catalyst. As a consequence of our work,a method for preparing these catalysts is proposed.
REFERENCES 1 L.B.Ebert, J. Mol. Catal., 15 (1982) 275-296. 2 M.A.M.Boersma, Catalytic properties of graphite intercalation compounds, in: J.J.Burton and R.L.Garten, Advanced Materials in Catalysis, Academic Press, New York, San Francisco, London 1977, pp. 67-99. 3 W.Setton, Synth. Metals 23 (1988) 467-473. 4 K.Aika, T.Yamaguchi and T.Onishi, Appl. Catal., 23 (1986) 129-137. 5 M.Ichikawa, T.Kondo, K.Kawase, M.Sudo, T.Onishi and K.Tamaru, J.C.S.Chem.Comm., No 3 (1972) 176-177. 6 K.Kalucki, W.Arabczyk and A.W.Morawski, Stud.Surf.Sci.Catal., 7 (1981) 1496-1497. 7 Ju.N.Novikov and M.E.Volpin, Physica B+C, 105 (1981) 471-477. 8 K.Kalucki and A.W.Morawski, Graphite intercalation compounds of iron as catalysts for low temperature ammonia synthesis, in: Proc. Inter. Carbon Conference, Baden-Baden, FRG, June 30 - July 4, 1986, Arbeitskreis Kohlenstoff Der Deutchen Keramischen Gesellschaft E.V., 1986, Qp. 531-532. 9 K.Kalucki and A.W.Morawski, lron intercalated in graphite as catalysts f o r ammonia synthesis, in: Proc.Inter. Conference on Carbon, Newcastle upon Tyne, England, Sept. 18-23, 1988, Publlished by IOP Publishing Ltd, 1988, pp. 215-217. 10 A.W.Morawski, K.Kalucki, A.Pron, Z.Kucharski, M.tukasiak and J.Suwalski, Reactivityof Solids, 7 (1989) 199-205. 11 K.Kalucki and A.W.Morawski, Reactivity of Solids, 4 (1987) 269-273. 12 K.Kalucki and A.W.Morawski, Reactivity of Solids, 6 (1988) 29 - 38. 13 K.Kafucki and A.W.Morawski, Synth. Metals, 34 (1990) 713-718.
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
497
SYNTHESIS OF V-P-0 CATALYSTS FOR OXIDATION OF C4 HYDROCARBONS V.A. ZAZHIGALOV, G.A. KOMASHKO, A.I. PYATNITSKAYA, V.M. BELOUSOV, J.ST0CH'
and J.HABER'
L.V.Pisarzhevsky I n s t i t u t e o f Physical Chemistry of Academy of Sciences, Kiev-28 CUkraina, USSR>
the
' I n s t i t u t e of Catalysis and S u r f a c e Chemistry, Polish Sciences, 30-239
Ukrainian
Academy
of
SUMMARY
Based on t h e application o f s e v e r a l techniques, t h e effect of p r e p a r a t i o n conditions on t h e a c t i v i t y and s e l e c t i v i t y of promoted V-P-0 c a t a l y s t s in t h e n-butane oxidation t o maleic anhydride is described. The t y p e of solvent used, t h e t i m e of synthesis, conditions of shape-forming operation and composition of t h e g a s mixture f o r a c t i v a t i o n are considered. INTRODUCTION In t h e s y n t h e s i s of a V-P-0
c a t a l y s t , t h r e e basic s t e p s may be
distinguished:
-
p r e p a r a t i o n of t h e p r e c u r s o r and its s e p a r a t i o n ; shape-forming operation; a c t i v a t i o n of t h e p r e c u r s o r including its t r a n s f o r m a t i o n i n t o t h e
a c t i v e component.
Each of t h e m may influence t h e c a t a l y t i c p r o p e r t i e s of t h e f i n a l In t h e p r e s e n t paper t h e importance of p a r a m e t e r s of
catalyst.
t h e s e s t e p s in t h e s y n t h e s i s of metal-promoted discussed
for
their
properties
in
partial
V-P-0
c a t a l y s t s is
oxidation
of
C4
hydrocarbons. EXPERIMENTAL T h e V-P-0
V205
catalyst w e r e
synthesized by t h e reaction b e t w e e n
and H3P04 dissolved in butanol E l l ,
a 500 dm3 autoclave
which w a s c a r r i e d o u t
in
equipped with a s t i r r e r . The p r o g r e s s in
the
reaction w a s followed by e x t r a c t i n g samples of t h e r e a c t i n g mixture 3 s m a l l enough (200 c m > 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 <1:15> 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
to
14,51
appeared
I>. 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
Binding energy (BE> V2p<3/2> 517.7 e V c o r r e s p o n d s t o V. The second value for t h e 01s peak with higher BE is due t o s u r f a c e groups containing H or C . The p r e c u r s o r c o n t e n t 0.1 at%. "Conversion
of n-butane
Shape-forming gives
a
product
<
60 %. A
-
air.
by e x t r u s i o n is t h e most
with
b r i n g s an i n c r e a s e of
larger
proportion
of
prospective transport
method. I t
pores
which
t h e s e l e c t i v i t y . The catalyst formed as r i n g s
504
case shows
also in t h i s
better
catalytic
properties,
c a t a l y s t h a s lower c o n t e n t of p o r e s with diameter
>
though
such
50 nm, because
of s m a l l e r addition of w a t e r during t h e forming operation. An
addition
does n o t
improve
the
15) increase
however
the
significantly
catalytic
unsatisfactory.
This
nitrogen-containing
properties
and
s
alcohol
with a n exception of
the
content
properties be
may
groups
of
caused
(effect
of
NH3
of
small
No
transport
pores,,
samples
remain
the by
the
like
C111> 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
contained
increase
of
visible
TABLE 6
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 rate of CVO>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
<
region C
350 nml.
does not p r a c t i c a l l y
The r e f l e c t a n c e s p e c t r u m of t h e p u r e V20s changed by t h e UHIG C s e e Fig. former
octahedraly
23 t h u s i n d i c a t i n g no change of
coordination
of
the
400
300
ion
V5+
during
the the
treatment .
a
700
500
600
200
wavelength (nm)
Fig.2. R e f l e c t a n c e W M S s p e c t r a of V m i x t u r e s C 7% w t % of V 0 > 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
'
(
measured H2 consumption
- theoretical H2 consumption )
The catalyst was crushed and a sieve fraction (0.4-0.63 mm) was taken. A 10 vol % HgAr flow (50 m1/,,,in )was passed through a sample of 100 mg. The temperature was raised with 5 '/mi". The hydrogen consumption was measured with a hot-wire detector. Selective oxidation of hvdroeen sulfide: The catalytic properties of the different catalysts in the selective oxidation of H2S were tested in a continuous microflow apparatus. Care was taken to assure plug flow and isothermal conditions. The feed consisted of 1.0 vol% H2S, 5.0 vol% 0 2 , and 30 vol% H20 in He. The effluent was analysed with a gas chromatograph. In order to obtain a stable activity, the catalyst remained for 12 hours in the reactor under reaction conditions. This procedure converted the iron oxide into iron(I1) sulfate. Subsequently the temperature of the microflow reactor was varied stepwise from 453 K (18OOC) to 593 K (320°C) and back to 453 K with a step size of 10 K. The thus obtained data provide information about the conversion and selectivity at various temperatures. The selectivity is defined by equation ( 2 ) .
531
The selectivities (table III) were taken at low conversions. Re-exponential factors (b) of reaction (3) were calculated assuming first order kinetics { 3 } . The activation energy used (85 kT/mole) had previously been determined by measuring different iron oxide catalysts.
RESULTS and DISCUSSION Figure 1 shows transmission electron micrographs of catalysts prepared using different compounds. Table 111 contains the particle diameters of the different catalysts. It is apparent that preparation with chelated iron compounds leads to small iron oxide particles (2 to 5 nm). Zron citrate impregnation resulted in small iron oxide particles uniformly distributed over the silica. The catalysts prepared by impregnation with iron gluconate contained small iron oxide particles not intimately contacting the support. The results of impregnation with iron EDTA depended on the pH value of the impregnating solution. Higher pH values (7.1, 8.5, and 10.0) provided small iron oxide particles well dispersed over the silica. Lower pH values (5.3) led to larger particles (>lonm) situated in between the silica spheres of the support. Impregnation with iron nitrate and iron sulfate, gave rise to large clusters (size > 200 nm) present locally within the silica spheres. Impregnation with a solution of iron chloride resulted in even larger clusters of iron oxide particles, vu., about 1 Frn in size.
-
50 nm Figure 1 Transmission electron micrographs of catalysts prepared with different precursors
a) NH4Fe citrate,
b) NH4FeEDTA (pH S.3),
c) NH4FeEDTA @H 8.S),
d) Fe gluconate,
d) Fe(N03)3,
e) FeC13,
Magnification 1350000~.
532
Using back-scattered electrons SEM can provide information about the distribution of the iron species deposited within the extrudates on a larger scale. Figure 2 shows back-scattered electron images of the differently prepared catalysts. It can be seen that impregnation with iron citrate and iron gluconate leads to a very uniform distribution of iron within the extrudates. With catalysts prepared with iron EDTA of a pH of 5.3, the distribution is less homogeneous. Featherlike regions can be seen of a higher concentration of iron. Impregnation with an iron nitrate solution leads to iron-rich areas of about 200 nm, and with an iron chloride solution of about 1 pm, which agrees nicely with the observations in the TEM.
5 wm
Figure 2 Back-scatterscanning electron micrographsof catalysts prepared with different precursors a) W F e citrate,
b) W F e E D T A (pH 5.3),
d) Fe gluconate,
d) Fef.N03)3,
c) W F e E D T A @H 8.5), e) FeCl3, Magnification 1 5 0 0 ~ .
It is interesting that the distribution of the iron species within the extrudates can be determined very easily within the light microscope. Since LM does not call for expensive equipment and can be fairly easily performed, light-microscopical characterization of catalysts is very attractive. As indicated in table 11, the catalysts showed a remarkable difference in color after the thermal pretreatment. Catalysts prepared with iron chloride as precursor showed an inhomogeneously distributed dark red purpleblue color, while catalysts made from chelated iron compounds were pale yellow-red. Differences in optical behaviour were also observed after impregnating with the immersion oil and examining the extrudates within the light microscope. Catalyst extrudates prepared from citrate or gluconate became transparent / opalescent red, whereas extrudates prepared from iron nitrate were intransparent redorange. At higher magnifications clusters were seen in the extrudates prepared with iron chloride.
533
The large differences between the four catalysts prepared with iron EDTA solutions of different pH values were striking. The catalyst prepared with a basic iron EDTA solution (pH 8.5) was homogeneously colored as the catalysts prepared with the other chelating agents, whereas the catalyst prepared with iron EDTA solutions of a lower pH (pH 5.3,7.1)showed an eggshell distribution with featherlike structures "entering" the extrudate (figure 3). The dependence of the color and transparency of the catalysts on the particle size of the iron oxide can be attributed to Rayleigh scattering. Relatively large particles have a high scattering intensity and show a low transparency, whereas small particles show a high transparency. The results obtained by LM agree with those obtained by "EM and SEM. It can therefore be concluded that LM provides reliable information about the particle size and the distribution of the material within a silica support. TABLE II Summary of the Light Microscopy results PH colour catalyst Precursor NH4FeEDTAl S H 2 0
homogeneity
without oil
with oil
5.3
red
mans. red linmans. yellow
eggshell
7.1
red & yellow
trans. redhtrans. yellow
eggshell
8.5
yellow-red
trans.red
homogeneous
10.0
yellow-red
intrans. yellow
homogeneous
NH4Fe citrate
4.3
yellow-red
trans. red
homogeneous
Fe gluconate
7.0
yellow-red
trans. red
homogeneous
FeC13.6H20
1.1
red & purple-blue intrans. red
Fe(N03)3,9H20
1.0
strong red
slightly trans. red
homogeneous
Fe2(S04)35H20
1.3
white
trans. white
homogeneous
Figure 3
inhomogeneous
0.2m Light microscope pictures of a catalyst extrudate prepared with NH4FeEDTA a) pH
=
5.3, b) pH
= 7.1, c)
pH = 8.5
XRD also showed large differences between the different catalysts (figure 4). Catalysts prepared from organic chelating compounds were found to contain mainly maghemite (y-FezO3) while catalysts prepared from nitrate and chloride only contain hematite (a-Fe2O3). This was confirmed by the magnetic properties of the catalysts. During thermal decomposition in air the iron moieties are
534
reduced to magnetite (Fe304) by the organic chelating agents [8,9]. After the reducing species are decomposed, oxygen oxidizes the magnetite to maghemite (y-Fe203). The catalyst prepared with iron(II1)sulfate only contains dehydrated iron(III)sulfate. Apparently the temperature used was not high enough to decompose the iron sulfate. XRD also exhibited large differences in the shape and the height of the diffraction maxima indicating a wide variation in crystallinity and particle size of the catalysts. Peaks due to large particles are higher and narrower than those due to small particles. The calculated particle sizes (table 111), however, only give information about the weight-mean particle size, and are thus dominated by the largest particles. Fmm the XRD patterns, it can be concluded that there is a large influence of the pH of the iron EDTA solution on the particle size. When impregnating with solutions of a low pH (5.3,7.1)more relatively large maghemite particles were formed. When solutions of a higher pH are used, the amount of large particles decreases.
Fg(S04)3
FeC13
FeWW3 W F e E D T A (5.3) W F e E D T A (7.1) W F e E D T A (8.5) NmFeEDTA (10.0)
Fe gluconate NHqFe citrate
I 90°
I
I
I
I
I
600 45O 300 15O e 28 Figure 4 XRD patterns of catalysts prepared with different precmars.(Ka1+=1.9373 A) 75O
DRIFTS proved to be an appropriatetechnique to determine the bare part of the silica surface. It is found that the silica is most completely covered by the iron oxide, when iron citrate is used as a precursor (table DI).Very distinct is the difference between catalysts prepared from the chelating iron compounds and from the simple iron salts. Also the inffuence of the pH of the Fe EDTA solution on the coverage of the silica can be seen.
535
The fraction of the iron oxide intimately contacting the silica obtained with TF'R is given in table III. Very interesting is high extent of reduction of the catalysts prepared with iron gluconate and iron EDTA of a pH of 5.3. The small fraction of iron oxide strongly interacting with the silica was also evident from TEM mimgraphs. TABLE III Summary of the results of E M , XRD, DRIFTS ,TF'R and activity measurements Precursor pH TEM XRD*) DRIFTS TPR Activity Selectivity diameter (nm) WFeEDTA.lSH20
coverage (%) F (%)
k,(xlO*s-l)
%
5.3
5-25
23 a)
28
19
10.0
97.6
7.1
2-25
23 a)
29
62
9.1
97.6
8.5
2- 16
16 a)
33
51
10.0
97.7
10.0
5-20
21 a)
35
38
10.0
97.3
4.3
2-5
5.5 a)
41
82
13.8
97.8
Fe gluconate
7.0
2-6
6.6 a)
32
26
11.2
96.8
FeC13.6H20
1.1
10-200
19
29
0.60
73.1
FeW03)39H20
1.o
10-20
7
14
Fe2(S04)35HzO
1.3
10-35
2.90 1.38
95.7
w
e cifrate
sob) 16b) 34c)
-
-
92.7
*)XRDpeaks used for calculation: a) (311) y-Fe203, b) (104) a-Fe203, c) (113) Fe2(SO4)3. The above data show a very good agreement between the results of the different characterization techniques. Generally the selective oxidation of hydrogen sulfide reflects nicely the structure of the catalysts as evident from the characterization. Catalysts prepared from chelated iron compounds exhibited a high activity and selectivity. Catalysts prepared from simple salts, on the other hand, exhibited a low activity. Interesting is the high activity of the catalyst prepared from iron EDTA of pH 5.3. Although most of the active material was less well dispersed, the activity still was high. The activity apparently was determined by the relative small fraction of well dispersed iron oxide particles. The high activity of the catalyst prepared from iron gluconate is also noteworthy. However, the activity of this catalyst continuously decreased pointing to sintering of the iron sulfate not interacting with the silica. In summary it can be concluded that catalysts containing small particles of iron oxide highly dispersed on the silica surface show the best performance in the selective oxidation of H2S. Impregnation with solutions of iron chelates, especially of iron citrate, provides the best results. With iron EDTA solutions, different particle sizes and distributions of the precursor after drying were found with solutions of a different pH. The effect of the pH can be attributed to a different crystallization of the precursor during drying. At low pH values iron EDTA easily crystallizes as m e E D T A , where at high pH values badly crystallizing anions are present in the solution. With iron EDTA at low pH values the solubility of NH$eEDTA is low, therefore saturation and primary crystallization already takes place at the outer surface of the catalyst. The narrow pores within the small crystallites of the initially crystallized precursor cause migration of the remaining solution to the external edge of the extrudates. Subsequent drying leads to an eggshell distribution. Initial nucleation of small crystallites of the precursor can also proceed in the exterior macropores of the extrudates.
536
Migration of the liquid to the initial clusters of crystallites leads to the featherlike structures observed in the extrudates impregnated with iron EDTA (pH 5.3) (figure 3). When the solubility is high (e.g. with iron nitrate and iron chloride), the evaporation zone moves back into the extrudates. Eventually the solution breaks up into micro domains [10,11]. When the crystallization takes place in these micro domains inhomogeneously distributed clusters result as shown in figure 1 and 2. When crystallization does not proceed the precursor can coat the surface of the carrier without causing redistribution of the impregnated solution. This results in a higher dispersion.
CONCLUSIONS The results demonstrate that precursors, which do not easily crystallize, such as ammonium iron EDTA, ammonium iron citrate, and iron gluconate provide the best results. During drying, crystallization does not proceed readily; the precursor can fairly completely cover the surface of the carrier with an amorphous layer. During calcination highly dispersed iron(1II) oxide results. When the precursor crystallizes to small crystallites, micropores are generated that withdraw the solution from the pores of the support. With wide-porous supports nucleation of small crystallites can easily lead to an inhomogeneous distribution of the precursor. Therefore impregnation with solutions of chelated iron compounds leads to highly dispersed active components and thus to highly active catalysts. These results could not be obtained by impregnation with simple iron salts.
ACKNOWLEDGEMENT The authors wish to thank R. Zefrin for the SEM. Also VEG-GASINSTITUUT is greatly acknowledged for the financial support.
REFERENCES 1. C.N. Satterfield, Mass transfer in heterogeneous catalysis, M.1.T Press, Cambridge, (1970). 2. J.A. Lagas, J. Borsboom, P.H. Berben, "SUPERCLAUS - The answer to Claus plant limitations", 38th Canadian chemical engineering conference, Edmonton, Canada, (1988). 3. G.R. Meima, B.G. Dekker, A.J. v. Dillen, J.W. Geus e.a., in Preparation of Catalysts IV, B. Delmon, P. Grange, P.A. Jacobs, and G. Poncelet (Eds.), Elsevier, Amsterdam, 83, (1987). 4. A.Q.M. Boon, Ph.D.Thesis in preparation, University of Utrecht, (1990). 5. P.H. Berben, P.J. van den Brink, M.J. Kappers and J.W. Geus. Preprints of the IUPACSymposium on Characterisation of Porous Solids, Dechema, 224, (1987). 6. E.Vogt, Ph.D.Thesis, University of Utrecht, (1988). 7. W.J.J van der Wal, Ph.D.Thesis, University of Utrecht, (1987). 8. M.Booy, T.W. Swaddle. Can. J. Chem., 56, 402, (1978). 9. M.A. Blesa, E. MatijeviC, Advances in Colloid Interface Science, 29, 173-221, (1989). 10. T.M.Shaw, Phys. Rev. Lett., 59 (15), 1671-74, (1987). 11. V.B. Fenelonov, A.V. Neimark, L.I. Kheifez, in Preparation of Catalysis 11, B. Delmon, P. Grange, P.A. Jacobs, and G. Poncelet (Eds.), Elsevier, Amsterdam, 233, (1978).
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
537
PREPARATION OF OXIDATION CATALYSTS WITH A CONTROLLED ARCHITECTURE Y.L. XIONG', L.T. WENG, B. ZHOU2, B. YASSE, E. SHAM, L. DAZA3, F. GILLLAMBIAS4, P. RUIZ and B. DELMON Unit6 de Catalyse et Chimie des MatCriaux DivisCs, UniversitC Catholique de Louvain, 1 Place Croix du Sud, 1348 Louvain-la-Neuve, Belgium on leave from Xiamen University, Xiamen, China Institute of Chemical Physics, Academy of Sciences, Dalian, China on leave from Instituto de Catalisis y Petroleoquimica, CSIC, Spain Departamento de Quimica, Facultad de Ciencias, USACH, Santiago, Chile
2 presently: Dalian
ABSTRACT' We present examples of a method for improving the activity of two-phase catalysts where a remote control operates. The method is based on the combination of special impregnation methods with the detachment and fragmentation of the impregnated precursor when it transforms to the supported oxide. MoO3, Sn02, and Fe2(Mo04)3 were impregnated by quantities of Sb204 equal to those necessary for forming one or a few monolayers. Solutions of antimony chlorides in CHC13, or antimony butoxide in isobutanol were used. The well dispersed precursor fragmented to small crystallites during low temperature calcination or during catalytic work. This method produced catalysts which exhibited activities and/or selectivities superior to those of mechanical mixtures of separately prepared phases in spite of the fact that the quantity of activating oxide or promoter used (Sb2O4) was 6 to 75 times less than in the mechanical mixture. INTRODUCTION This work deals with methods for preparing catalysts made of two or several phases. In all catalysts composed of two (A and B) or several phases, the size of the particles of A and B, the number and the nature of the A-B contacts are critical. Making such catalysts is a challenge. A quite common type of catalysts n u d e of two phases corresponds to supported catalysts, where the active phase is dispersed on the surface of a carrier. In that case, the carrier may be inert. The problem of catalyst preparation can become more complicated when both phases play an active role, namely there is a cmmeration between phases A and B. This is the case we shall examine here. A well-known cooperation effect is due to bifunctional catalysis: in catalytic reforming, both Pt and alumina carry out part of the catalytic work. Several other types of cooperation have been mentioned in the last years, e.g. between catalysts within zeolitic structures and another component (1-3) or between separate oxide phases in oxidation or related reactions (4-12). Explanations for these cooperations often rest on spill over processes (13-15). The present article will focus on catalysts composed of two (or several) phases active in selective oxidation or oxidative dehydrogenation. Oxidation catalysts are often much more active and selective when they contain several phases. We have shown that a very frequent explanation for that is the occurrence of a "remote
538
control". One phase is able to activate molecular oxygen forming a mobile oxygen species (spillover). These spill-over oxygen flow onto the surface of other oxide phases, where they create (or regenerate) active sites able to produce the selective products (14,15). A schematic representation of this mechanism is given in Fig. 1. Our results rest, on the one hand, on the study of a model reaction (oxygen aided dehydration of N-ethyl formamide (5,6,14,15,16)) which allowed the identification of catalytic sites created through the remote control (namely : Bronsted acidic sites), and, on the other hand, on the oxidation of isobutene to methacrolein and the oxidative dehydrogenation of 1-butene to butadiene. In these studies, the catalysts have been prepared by mechanically mixing oxides prepared separately (about 40 different mixtures were studied). When a remote control operates, spill-over oxygen must flow easily from one phase to the other. The number of contacts between the small domains of different chemical composition should thus be large and the quality of the contacts excellent for permitting an easy "jump" of spill over species from one phase to the other. The formation of the spill-over species depends on the surface area developed by the phase dissociating oxygen (e.g. Sb2O4). The number of sites to create on the catalytic phase (e.g. MoO3) depends on the surface area of that last phase. A second very important parameter is thus the phase A/phase B surface area ratio. In general, conventional methods cannot allow to achieve these goals. Mixing already formed solids is not very efficient, because the starting solids may have formed aggregates which cannot be easily dispersed: grinding or the use of ultra sound very rarely permit to reduce the size of the aggregates to below 0.5pm. Mixing intimately already formed gels turns out to be nearly as difficult, although technological developments may lead to better results in the near future. In principle, a general method could be used. This would be to start from finely dispersed, non-aggregated precipitates and to mix them energetically at a pH such that the surfaces of each of the phases to be mixed possess opposite electric charges. A modification of the method would be to use micromicelles instead of precipitated particles. Unfortunately, very little has been done in this direction. Another method for obtaining finely interdispersed phases can be employed when the phases to mix exhibit little chemical affinity for each other. The method consists in starting from a
homogeneous precursor (e.g. salts of organic acids) incorporating all the elements and letting the to the desired phases during a controlled decomposition to oxides. This can be achieved, for example, for producing a finely interdispersed mixture of BiP04 and MoO3, which have no affinity for each other (17). It is easy to prepare the bismuth salt of phosphomolybdic acid. Upon decomposition, this gives the desired mixture (18). The catalyst is extremely active. Still another method rests on the fact that some oxides may have (at least under certain oxidoreduction conditions) very little affinity with the support of other oxides (19). In principle, one can select a precursor of the oxide to be impregnated, and adapt the impregnation conditions so that the precursor spreads perfectly all around the surface of the other. One can speculate that, if decomposition is made carefully, the continuous layer of precursor will fragment to very tiny oxide particles, thus giving a highly dispersed phase in good contact with the other oxide.
539
J\uy ISOBUTENE
€IN
Phase A Holacular
02 Uokular 02
ISOBUTENE ME THACROLEIN
-
Figure 1. Schematic representation of Remote Control Mechanism. Selective active sites ("imgated' by spill over oxygen) non selective sites ("non irrigated) Os.o Spill over oxygen This is the strategy that we have adopted in the present work. The objective is to show that active and selective catalysts for the oxidation of isobutene to methacrolein can be produced by this type of preparation method. Three oxide phases have been chosen as supports: Sn02, Moo3 and Fe2 (Mo04)3. These phases are very active in the oxidation of isobutene, but not very selective in the formation of methacrolein. These three phases have a very low (if any) chemical affinity for antimony oxide. This last oxide (Sb2O4) is very active for the dissociation of oxygen. Important synergetic effects have been observed in biphasic catalysts formed by mixtures of the above-mentioned oxides, prepared separately, with Sb2O4. A remote control operates in these systems (14,16,20,22). The phases selected as "supports" have been impregnated with solutions of antimony salts in amounts theoretically necessary to form coverages of 0.5 to a few monolayers of Sb204 over their surface. EXPERIMENTAL, 1. Preparation of catalysts a) Preparation of supports i) Sn@ was prepared by precipitation of an aqueous solution of SnC12.2H20 with NH3 at pH=7.5 followed by drying at 110°C for 16h and calcination at 600°C for 8h.
540
ii) Moo3 was obtained by thermal decomposition of (NH&M07024 for 20h.
. 4H20 in air at 500°C
iii) Fe2(Mo04)3 was prepared by mixing a solution of Fe(N03)3 in aqueous citric acid with a solution of (N@)6Mo7024 . 4H20 in water at pH 1-1.5, followed by evaporation and drying at 110°C and calcination at 5OOoC/20h. b) Impregnation of supports The quantity necessary for forming a monolayer of S h O 4 over the supports was estimated from the size of the unit cell of Sb2O4 (0.16 nm2, calculated from its structure (21)) and the surface area of the supports: Sn02 = 9.0 m2 , g-1; Moo3 = 2.0 m2 . g-l and Fe2(Mo04)3 = 2.5 m2 . g-l. The calculated amount of Sb2O4 to form a monolayer represents 4% wt for Sn02, 0.64%wt for Moo3 and 1 % wt for Fe2(Mo04)3. Two types of solution of antimony salts were used: in one case a chloride solution in CHC13 and in the other an organic solution of the alcoxide. Additional solvent was used in order to homogenize the impregnated ions over all the surface. In the present case, the detachment of the impregnated layer of precursor for forming dispersed oxide crystallites on the surface of the support is made thanks to calcination. The details for each of the system are as follows: i) Procedure for impregnation of S n q and MoO3. A solution containing Sb+3 and Sb+5 (Sb+3/Sb+5= 1/1) was prepared from SbCl3 and SbC15. CHC13 was used as solvent. The support powder was immersed in the necessary amount of this solution in a rotavapor, with the addition of 250 ml CHC13. Evaporation was done under reduced presbure. The solvent was removed slowly. The powder so obtained was washed with an ammonia solution in order to eliminate C1- and finally dried at 1 10°C overnight. Impregnated SnO2 was not calcined. Impregnated Moo3 was calcined at 45OOC for lh. Samples with quantities of Sb+3 and Sb+s necessary to form 0.25, 1 and 2 monolayers were prepared for Sn02. They are denoted as 1/4Sb/Sn02, 1Sb/Sn02 and 2Sb/Sn02. Samples of Moo3 containing the amounts of Sb+3 and Sb+s necessary to form 0.5, 1 and 4 monolayers were prepared and denoted as l/2Sb/MoO3, lSb/MoO3 and 4sb/Moo3. ii) Procedure for impregnation of Fe2(Mo04)3. The impregnating solution was composed of antimony butoxide (Sb(OCqHg)3) in isobutanol. This solution was added in the powder support. Water was added dropwise under agitation.at anibiant temperature. A gel was formed. The solvent was evaporated slowly in a rotavapor. The catalyst obtained was dried overnight at 110°C and calcinated at 400°C for 2h. Samples with quantities of antimony necessary to form 1, 3 and 6 monolayers of Sb204 were prepared. They are denoted 1Sb/Fe2(Mo04)3, 3Sb/Fez(Mo04)3,6Sb/Fe2(Mo04)3. 2. Characterization techniques Samples were characterized before and after BET surface area measurements, XRD, XPS, ISS, electron microscopy , Mossbauer spectroscopy, ESR and electrophoretic migration. The measurement procedures are described elsewhere (16,20,22).
541
3. Catalyric activity Isobutene oxidation was carried out in a continuous flow, fixed bed reactor (diameter 8 mm). 800 mg of catalysts (particle diameter = 500-800 pm) were used. The temperature of reaction was
400°C and the composition of the reactant gases: isobutylene/Ofl2 = 1/2/7 with a total gas flow of 30 mllmin. RESULTS More details concerning the characterization and the catalytic activity results are given in references 16,20 and 22.
1. Surface area When the oxides supports are impregnated, the BET surface area increases slightly. After the catalytic reaction, the surface area increases subtantially. (The same effect occurs after a simple calcination in the case of the SnO2 and Fe2(Mo04)3 samples). As an example, BET surface areas of pure and impregnated supports are presented in Table 1. Table 1. BET surface area (m2 g-') for pure and impregnated supports Sample Ma3 1/2Sb/MoO3 1Sb/Mo@ 4sb/Moo3 Sn02 1/4Sb/SnO2 1Sb/SnO2 2 s b/Sn02 FeAMo04h 1Sb/FeAMo04)3 3Sb/Fe2(MoO4)3 6sb/FeAMo04)3 (in addition calcined at 5W0/2h I 1S b/FedMo04)3
2. X ray diffraction When the amount of Sb ions is low, only lines characteristics of the support were observed. When the Sb ions amount was higher, lines characteristics of Sbz04 were detected. N o changes were observed after reaction. No new peak appeared. 3. XPS and ISS The binding energies observed in XPS correspond to the values characteristic of pure Sn02, Sb2O4, Moo3 and Fe2(Mo04)3. These values do not change after reaction. For impregnated supports, the intensity of the Sb XPS signals increases when the amount of Sb deposited increases. For all supports, the Sb signals decrease after reaction, in particular when the Sb content is low. In some cases, calcination alone brings about a decrease of Sb signal. The changes of the ISS signals indicate the same evolution as those of XPS.
542 4. Electron microscopy and analytical electron microscopy
When the concentration of Sb is low, only the signals of the metal supports are observed. AS the concentration of Sb increases, isolated particles of antimony are observed.
5. Electrophoretic migration measurements The zero points of charge (ZPC) are characteristics of S b O 4 oxides on the surface of the Sn02 or Fe2(MoO4)3 (23).
6. Mossbauer spectroscopy The spectra are characteristic of Sn02 and Fez(Mo04)3. No changes are observed with the impregnation of antimony ions over the supports or the catalytic reaction. 7. ESR A very weak ESR signal corresponding to the dissolution of Sb+5 in SnOZ is observed. This signal decreases after reaction. A summary of the characterization results is presented in Table 2. Table 2 Summary of the characterization results of the impregnated oxides supports using different physico-chemical techniques. A minus sign means that the contaminating layer decreases or disappears and the tendency to form two separate phases predominates. A zero "0" sign means that the technique was not used. 'I-"
Methods
XRD XPS BE intensity ISS
AEM
TEM SEM Mossbauer Zeta potential ESR
8. Catalytic activity An example of typical results obtained is presented in Table 3. Results obtained for pure supports are indicated in parenthesis. The catalytic properties of pure oxide supports (indicated in parentheses) are greatly improved by the addition of Sb ions. Compared with pure support, the methacrolein yield and selectivity increase when Sb ions are deposited on its surface. This effect is more pronounced as the amount deposited increases. At the same time, the total conversion decreases.
The deposition of small amounts of Sb ions on the surface of the supports modifies considerably their catalytic properties. When compared with pure supports, the yield and the selectivity in methacrolein are significantly increased. In the case of Sn@, this effect is dramatic. It suffices that an amount of Sb ions equivalent to that necessary to form one monolayer be present for observing an increase of 700% in the yield. The selectivity is increased from 3 to 25%. (The two effects together bring about a diminution of the conversion to half the value observed for pure S n e ) . In order to emphasize the effect of using the impregnation method, the results concerning catalytic activity observed for mechanical mixtures (50%of each oxide, prepared separately. (16,20, 22)) and the corresponding supports impregnated with a quantity of Sb2O4 equal to that necessary to form one monolayer, are compared in Table 4. The impregnated catalysts present, in spite of the small Sb content, activities comparable to those observed in the mechanical mixtures. The selectivity andlor the yield in methacrolein are better for the impregnated supports. The effect is particularly important in the case of iron molybdate. The surface area developped by antimony oxide over the surface of the support can be calculated by the difference, after reaction, between the surface area of the impregnated support and that of the pure support. For example, in the case of 1Sb/Sn02, the antimony oxide develops a total
Table 4. Comparison of the catalytic activity between mechanical mixtures and catalysts impregnated by a quantity of Sb2O4 equal to that necessary to form one monlayer (composition in wt%). SAMPLE M 0 0 3 + S ~ 0 4(50% of Sb204) lSb/MoOg (0.64 wt % of Sb204)
Sn02+Sb204 (50% of Sb2O4) 1Sb/SnO2 (4 wt % Sb2O4) Fe2(Mo04)3+Sb04 (50% of SbO4) . l S b/Fe2(MoO& ( 1 wt % Sb204)
SBETm2.g-1 2.0 2.0 9.2 9.8 2.8 2.6
C(%) 26.8 23.3 20.0 26.0
Y(%) 7.5 7.0 5.0 7.0
S(%) 28.0 30.0 25.0 27.0
10.3 15.2
3.0 10.4
29.0 68.0
544
surface area of 0.8 m2, Table 1. On the contrary, the total surface area of Sb204 forming the mechanical mixtures (50%in weight) is 1 m2, Table 4 (surface area of separately prepared Sb204 is 2 m2/g). This means that only 4 wt % of S k O 4 in the impregnated catalysts develop a surface area similar to that of Sb2O4 in mechanical mixtures. In other words, in the impregnated catalysts, the specific surface area by gram of Sb2O4 is 20 m2/g, which is 10 times greater than the specific area of SbzO4 in the mechanical mixtures. From Table 4,we conclude that only 4 wt% of Sb2O4 in the impregnated catalyst activates, at least, twice the number of active centers on SnO2 supports compared with the SnO;? activated in mechanical mixtures. Similar or more significant results can be obtained for other amounts of antimony impregnated or other oxide supports.
I
Mechanical mixture
Impregnated support
Figure 2. Comparison of the architecture of mechanical mixtures and impregnated catalysts. A: active support (MoO3, SnO2 or Fe2(MoO4)3) B: SkO4 selective sites. Irrigated and protected by oxygen spill over The impregnation method improves significantly the selectivity and the yield in methacrolein, as we have discussed above. This means that this method not only allows the formation of a supported phase with a high specific surface area, but at the same time, that it makes that the small crystallites formed on the surface of the support be highly dispersed, thus increasing the number and the quality of the contacts between the two phases. This particular architecture of the catalysts allows the small crystallites of antimony to irrigate with great efficiency the active supports with spill-over oxygen, thus improving their catalytic properties, and consequently a higher surface area of the support is activated (or protected) by the Sb2O4. Fig. 2 shows the architectures of both oxides in mechanical mixtures and in the impregnated catalysts. Our approach to maximizing the interaction by remote control between two oxide phases has thus been to use two phenomena, the perfect spreading of the precursor of the activating (controlling) oxide on the surface of the potentially active phase (the support), and the detachment and fragmentation of the thus formed "envelope" to produce tiny crystallites in contact with the support. It is necessary to obtain two opposite effects with the same partners (Sb on the one hand and the potentially active oxide on the other hand), first a very good wetting, and in the second stage, a detachment. It is clear that the second stage depends on the particular couple of oxides investigated: they must "dislike" each other, namely be unable to form mixed oxides or solid solutions (at least
545
dissolve in each other only in very small proportion, as is the case with the Sb204-Sn@ system) and possess no mutual surface affinity. But the first stage, namely impregnation with the precursor, can be controlled by carefully selecting the impregnating salt or compound (e.g. alcoxy) and using adequate solvents, including "unconventional" organic solvents. Our work shows that, by the impregnation method, active and selective catalysts can be obtained for the oxidation of isobutene to methacrolein. The optimization of the properties of the active phase of the supports (textural: specific surface area, chemical: dopes, etc.) and of the supported phases (surface area, ability for oxygen activation, etc.) coupled with an adequate strategy in the impregnation procedure, can allow to optimize the cooperative effects between the phases to get very perfomant catalysts. In the work presented here, the potentially active phase is selected as a support, and the activating phase is dispersed on it. The other situation is also possible, namely to use the activating phase as a support, and to disperse the potentially active phase on it (19). We have achieved this in several cases (16b, 22, 20). ACKNOWLEGMENTS The financial support of the Service de Programmation de la Politique Scientifique SPPS (P.R.), the Commission of the European Communities (L.D., Y.L.X.), the Universitk Catholique de Louvain (L.T.W., B.Z.) are gratefully acknowledged. We also acknowledge the contribution of Dr. Jean Ladrikre for Mossbauer Spectroscopy (UnitC de Chimie Inorganique et NuclCaire, U.C.L.) and of Dr. Patrick Bertrand for ISS (UnitC de Physico-Chimie et de Physique des MatCriaux, U.C.L.). We are indebted to Mr. M. Genet for his constructive discussion and comments on the XPS measurements. REFERENCES N.S. Gnep, M.L. Martin de Armando, M. Guisnet, in G.M. Pajonk, S.J. Teichner, J.E. Germain, (eds), Spill-over of Adsorbed Species, Elsevier, Amsterdam, 1983, p. 309. 2 K.H. Steinberg, V. Mroczek, F. Roessner, in K.H. Steinberg (ed), 2nd Conference on Spillover, K. Marx Universitat, Leipzig, 1989, p. 150. 3 R. Le Van Mao, L. Dufresne, Appl. Catal., 52( 1989), 1. 4 L.T. Weng, Y.L. Xiong, P. Ruiz and B. Delmon. Tokyo Conference Catal. Sci. Technol., Tokyo, 1990. 5 B. Zhou, S. Ceckiewicz, B. Delmon, J. Phys. Chem., 91(1987), 5061. 6 S. Ceckiewicz, B. Delmon, J. Catal., 108(1987), 294. 7 L.T. Weng, B. Zhou, B. Yasse, B. Doumain, P. Ruiz and B. Delmon, in M.J. Philips and M. Ternan (eds), Proc. 9th Inter. Congress Catal., vol. 4, 1609, Calgary, Canada, Chemical Institute of Canada, Ottawa (1988). 8 F.Y. Qiu, L.T. Weng, E. Sham, P. Ruiz and B. Delmon, in K.H. Steinberg (ed.), Proc. 2nd Internatioal Conf. Spillover, Leipzig, 1989, p. 136. 9 F.Y. Qiu, L.T. Weng, E. Sham, P. Ruiz and B. Delmon, Appl. Catal., 51(1989), 235. 10 a) L.T. Weng, E. Sham, B. Doumain, P. Ruiz and B. Delmon, Proc. New Devel. in Selective Oxidation, I. World Conference and 2nd European Workshop, Rimini, Sept. 18-22 (1989), n°F 19. b) L.T. Weng, P. Patrono, E. Sham, P. Ruiz and B. Delmon, op.cit., nOL3. 1
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11 U. Ozkan and G.L. Schrader, J. Catal. 95(1985), 120; 3. Catal. 95(1985), 137. 12 U. Ozkan, E. Moctezuma and S.A. Driscoll, Appl. Catal., 58(1990), 305. 13 K. Becker, K.H. Steinberg, H. Spindler, in K.H. Steinberg (ed), 2nd Conference on Spillover, K. Marx Universitat, Leipzig, 1989, p. 204. 14 P. Ruiz and B. Delmon, Catal. Today, 3(1988), 199. 15 B. Delmon and P. Ruiz, React. Kinet. Catal. Lett., 35(1987), n"1-2,303. 16 a) P. Ruiz, B. Zhou, M. Remy, T. Machej, F. Aoun, B. Doumain and B. Delmon, Catal. Today, 1(1987), 181. b) B. Zhou, E. Sham, P. Bertrand, T. Machej, P. Ruiz and B. Delmon, submitted for publication. c) B. Zhou, PhD Thesis, Universite Catholique de Louvain (1988). 17 J.M.D. Tascbn, P. Grange, B. Delmon, J. Catal., 97(1986), 287; J.M.D. Tascon, P. Bertrand, M. Genet, B. Delmon, J. Catal., 97(1986), 300; J.M.D. Tascon, M.M. Mestdagh, B. Delmon, J. Catal., 97(1986), 312. 18 M.V.E. Rodriguez, B. Delmon, J.P. Damon, in T. Seiyama, K. Tanabe (eds), Proc. 7th Intern. Confer. on Catalysis, Kodansha and Elsevier, Tokyo and Amsterdam, 1981, 1141. 19 B. Delmon, J. Mol. Catal., 1990, in press. 20 Y. Xiong, L. Daza, P. Bemand, P. Ruiz and B. Delmon, submitted for publication. 21 P.S. Gopalakrishonan and H. Manohar, Cryst. Struct. Comm., 4(1975), 203. 22 L.T. Weng, P. Ruiz and B. Delmon, submitted for publication. 23 F. Gil-Llambias, Y.L. Xiong, L.T. Weng, B. Zhou, P. Ruiz and B. Delmon, submitted for publication.
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
STRUCTURE AND CATALYSTS
SELECTIVITY
CHANGES IN
M. KOTTER, H . 4 . LINTZ and T.TUREK Institut fur Chemische Verfahrenstechnik Karlsruhe, F.R.G.
547
VANADIA-TITANIA-DENOX
der Universitat Karlsruhe (TH), D-7500
SUMMARY The rapid evaporation of thin liquid films of precursor solutions containing vanadium and titanium followed by calcination is an adequate method to prepare active and selective catalysts for the SCR process. Catalysts useful for operation at temperatures above 350 OC can be obtained by modifying this preparation method. Addition of sulfuric acid to the precursor solution leads to high surface area catalysts with a low level of vanadia species weakly interacting with the titania carrier, if the vanadia loading does not exceed 10 wt%. Catalytic tests demonstrate that sulfate stabilization increases NO conversion while NzO formation is suppressed. The addition of tungsten is equally beneficial for high temperature operation of the catalyst. Quantitative information about activity and selectivity of the developed catalysts is obtained by rate measurements in a gradientless system. INTRODUCTION A new NO, abatement strategy proposes the use of the Ljungstrom heat exchanger of the power plant as a chemical reactor [I]. In that case the total gas flow, the residence time and the temperature profile inside the heat exchanger/reactor are given a priori and the catalyst has to be adapted t o those prefixed conditions. We need special highly active catalysts for a broad range of temperatures ( 200 to 400 "C). The usual catalysts in SCR processes consist of mixed transition metal olddes. Normally titania is taken as support, among the additional compounds W 0 3 , MOOS and especially VzO5 seem to be the most frequently used [2]. The nature of the active vanadia species and its interaction with the underlying titania have been discussed controversially. However, there is now general agreement that thin layers of vanadium oxide on anatase are required as active and selective catalysts in several important industrial processes [3-71. Based on these findings a new preparation method had been introduced leading to highly active and selective catalysts [8]. It is the aim of the present study to extend the operational range of those catalysts to temperatures above 400 "C. This requires both sufficient stability of the carrier and a strong vanadia-titania interaction t o resist in that temperature range.
548
EXPERIMENTAL Preparation of catalvsts All preparations start with an aqueous solution of titanium oxychloride (Kronos Titan; 43 wt% calculated as TiC14). T o obtain vanadia-titania catalysts ammonium vanadate is dissolved in that standard solution. After 1 h stirring the dark brown solution shows a pH of about 6, indicating the presence of decavanadate ions [Q]. In the case of quaternary oxides an aqueous solution of ammonium metatungstate is added. Stabilized titanium dioxide carriers or catalysts may be obtained by addition of concentrated sulfuric acid t o the corresponding solutions. Thin layers of the as-prepared solutions are dried and calcined in a preheated oven for 1 h in the temperature range of 250 to 450 OC. The samples reach the calcination temperature within 5 min at 450 OC and 20 min at 250 OC. Characterization of catalvsts The total vanadium content of the catalysts is analyzed manganometrically after dissolution in hot concentrated sulfuric acid. The quantity of vanadia which is weakly interacting with the titania carrier is determined by treatment with 0.3 m ammonia solution followed by manganometric titration [lo]. In order to determine the sulfate content of the stabilized catalysts powdered samples are suspended in water. The acidic suspension is titrated with sodium hydroxide solution up to pH 7. The SO3 content is calculated based on the following stoichiometry:
+
T i ( S 0 4 ) ~ 4 NaOH
-
Ti(0H)d
+ 2 NazSO4
(1)
Surface areas are determined by nitrogen adsorption using a standard volumetric BET apparatus. X-ray diffraction (XRD) is performed by use of a Seifert diffractometer with C u K a radiation. Porosity is measured with a Carlo-Erba mercury porosimeter. Egg shell type catalysts are used in the catalytic test measurements. Thin layers of the active compounds have been fixed on nonporous steatite pellets (2 - 3 mm diameter) using a colloidal silica solution (DuPont Ludox A S 4 0 ) as a binder. The tests are performed in an integral flow reactor using a standard gas mixture containing 4% 02, 1000 ppm NO and 1200 ppm NH3 in nitrogen. The gas stream passes over 5 g catalyst with 3 wt% of the active compound. The space velocity, calculated with respect to the total catalyst volume including the inert carrier, is maintained at 29000 h-l. Rate measurements Rate measurements are carried out in a gradientless recirculation system at a flow rate of 33.3 cc/min (STP) and a reflux ratio of 14. In that case the powdered catalyst is fixed on
549
both sides of ceramic platelets to simulate the flow in the Ljungstrom heat exchanger. The reacting system is unambiguously described by three linearly independent equations:
NH3 NH3 NH,
+
+ +
NO
NO
+ +
+
0.25 0 2 1.25 02
0.75
0 2
-
--+
-t
Nz NO NzO
+ + +
1.5HzO 1.5HzO 1.5HzO
The corresponding rates r
m,i
L.$ m
=
are related to total catalyst weight. The extent of reaction ti is determined by mass balancing the open system in steady state. The concentrations of ammonia, nitric oxide, nitrous oxide and for verification nitrogen dioxide are measured by non-dispersive infrared spectroscopy. Oxygen is determined by use of a magnetic device. RESULTS AND DISCUSSION Figure 1 illustrates the effect of different oven temperatures on the preparation of a 20 wt % V Z Oon ~ Ti02 catalyst. . 0.5
I
I
1
I
I
-0
250 300 350 400 450
T/ "C FIG.l BET surface area ( 0 ) and fraction of soluble vanadia different oven temperatures
(0)after
1 h calcining at
550
The quality of the catalysts is controlled by measuring its surface area and the amount of vanadia species weakly interacting with the underlying anatase and therefore being soluble in ammonia solution. At temperatures less than about 350 OC high surface areas correspond to low amounts of soluble vanadia, the strongly interacting species are favoured. This is consistent with geometrical considerations [ll]estimating the vanadia content necessary to form an ideal monolayer of VzOs on anatase. The results indicate a strong correlation between the surface area and the different vanadia species. Operation of the as-prepared catalysts at temperatures above 350 OC is not possible. Changes of structure and morphology occur, causing a severe damage of activity and selectivity. A rapid sintering of the anatase carrier is leading to the formation of soluble, crystalline vanadia species the appearance of which is strongly promoting the formation of NzO. This has been reported elsewhere in detail [8]. Those results show that the lack of temperature stability is mainly due to the sintering of the anatase carrier which is additionally enhanced by the presence of vanadia in excess of 10 wt%. In all cases examination by XRD merely indicates the presence of anatase, no traces of rutile or crystalline V205have been observed.
Stabilized Carriers and Catalvsts Stabilization of titania and of catalysts with vanadia contents less than 10 wt % is achieved by adding sulfuric acid to the solutions. Evaporation and calcination at 350 OC forms Ti02 (anatase), vanadium and vanadium-tungsten containing catalysts with lower BET surface area (40 - 65 ma/g) than the values reported above. However, the calcination at 450 O C , originally proposed as a test for thermal stability and normally accompanied by loss of surface area due to the sintering of the anatase carrier, now increases the surface area and forms stable carriers and catalysts with surface areas in the range of 80 to 90 m2/g. In Table 1 surface areas and the fractions of soluble vanadia are given for three different catalysts. The results obtained with a sulphur-free sample are compared to those measured with two stabilized catalysts the nominal sulphur to titanium ratio of which is equal to nS/nTi = 0.28. The 9 wt % vanadia reference sample shows a drastic decrease in surface area and the corresponding increase of the fraction of soluble vanadia. In the case of the stabilized samples a growth of surface area is observed but the content of soluble vanadia is not changed in a significant way.
551
TABLE 1 Characteristics of fresh (1 h at 350 "C) and calcined catalysts
fresh
115
64
54
24 h at 450 'C
54
80
93
fresh
0.25 0.60
0.43 0.36
0.32 0.37
m2
'BET/,
mv,sol mv
24 h a t 450 "C
The evolution of surface area with calcination time (Fig. 2) shows quite similar patterns for the stabilized pure titania and the vanadia titania sample. There is no interference of the vanadia loading with the thermal processes occurring.
' 11."7...1
I/ 0 9 wt% v20,
10
L ns/nTi = 0.28
Ti02
0
weight loss
20
FIG.2 Evolution of surface area with calcination time.
P O a content
0
10
20
FIG.3 Weight loss and corresponding SOX contents as a function of calcination time at 450 OC.
Our interpretation of the observed phenomena in the case of sulphur containing catalysts is based on the findings reported in Figure 3. We suppose the formation of a titanium sulfate intermediate during evaporation and calcination at 350 "C, even if the real nature of the species could not be confirmed. The only species identified by XRD throughout
552
the thermal treatment has been the anatase form of titania. Nevertheless the existence of different Ti (IV) sulfate species is known from the literature [12]. The sulfate intermediate is decomposed in course of the calcination. This leads to an increase in surface area accompanied by a significant weight loss and a simultaneous decrease of the SO3 content determined by titration as shown in Fig. 3. The structure changes are completed after about 10 h. The remaining catalyst has a high surface area and contains still about 2 wt % SOB. The measured pore volumes for the 9 wt % catalyst are shown in Figure 4. Calcination causes only a slight increase of mean pore diameter but significant growth of the total pore volume. Very similar results are obtained in the case of pure titania and of tungsten containing catalysts.
0.2
1
10
1000
100
pore radius
/
10000
nm
FIG.4 Pore volumes for 9 wt% Vz05 catalyst (ns/nTi=0.28) before calcining at 450 OC (0).
(0)
and after 24 h
Catalytic Tests In Figs. 5 and 6 the formation of NzO as a sensitive measure of catalyst damage [8] is reported for several samples. Fig. 5 shows the strong negative effect of calcining at 450 OC for a 20 wt% VzO5; catalyst (nS/nTi = 0).
553
300
500 20 wt% V& 400
g 200-
E
2 300
a
\ 0,
0"
\
9 200
100-
0
-
0
I
100 I
FIG.5 NzO formation for samples calcined at 450 0C ( 0 1 h; o 24 h)
(0)
)(.
(0)
('1
0
I
FIG.6 NzO formation for catalysts 24 h calcined at 450 OC
LEGEND (FIGS. 6 and 7)
2
20 wt% V 2 0 5 9 wt%Vz05
C
9 wt% V z O 5
8 wt% V z O 5
16 wt% XOQ
} nS/nTi = 0
1
$!
0.5
8
z"
nS/nTi = 0.28 -
0
I
200
24 h at 450 "C 250
300
T
/
350 400 "C
FIG.7 NO conversion for samples 24 h calcined at 450 OC
554
Crystalline vanadia species are formed which promote the NzO formation. Figs. 6 and 7 demonstrate how the catalytic behaviour of catalysts calcined 24 h at 450 OC is improved at temperatures above 300 OC by (a) reducing the vanadia loading and (b) stabilization with sulfuric acid. The stabilized samples exhibit the lowest level of NzO formation while NO conversion (Fig.7) is significantly higher than with the 9 wt% and 20 wt% catalysts prepared without addition of sulfuric acid. RATE MEASUREMENTS The tests at constant space velocity allow a fast catalytic screening but give no detailed information about the selectivity behaviour because the catalysts are operated at different conversions. Rate measurements using a gradientless recirculation system provide the only means to quantify activity and selectivity of catalysts properly. Stationary values of the three reaction rates to rm,3are measured isosystatically as a function of either the NO- or the NH3-concentration. Typical results are shown in Fig. 8. The catalyst chosen is a 20 wt% vanadia titania ternary oxide after calcination at 450 OC for 1 h to give detectable levels of the side reactions r,,Z and rm,3. Nevertheless the rate of the main reaction remains two orders of magnitude higher, demonstrating the high selectivity even in the case of a thermally deteriorated catalyst.
lo4
10"
1o
N H ~concentration / rnol*P FIG.8 Reaction rates for a 20 wt% catalyst I h calcined at 450 "C
-~
555
Its concentration dependence can be quantified as follows
with
for the example shown. The influence of the oxygen concentration has not been investigated as it is known to be negligible under technically relevant conditions [13]. The values of the parameters indicated above are not free from the influence of inner transport phenomena. Variation of the thickness of the catalyst layer (lcat) has shown, that this remains true down to a thickness of only 40 p ,thus demonstrating the high intrinsic activity of the catalyst. CONCLUSIONS The results show that rapid drying followed by calcination of the precursor solutions gives an adequate method to prepare active and selective catalysts for the SCR process. The catalytic layer in the Ljungstrom reactor should be structured with respect to the temperature profile. In the low temperature domain (below 35OoC) a ternary oxide with a high vanadia content (20 wt%) gives the best performance. T o be useful in the high temperature region the vanadia content has to be reduced and the carrier must be stabilized by addition of sulfuric acid to the precursor solutions. The catalytic tests clearly show that sulfate stabilization causes a significant increase in the NO conversion accompanied by the suppression of the NzO formation. The performance of the high temperature catalyst may be improved further by addition of tungsten oxide. Rate measurements in a gradientless recirculation system emphasize the high activities of the catalysts obtained in that way. The use of unusual characteristic lengths of the catalyst layer is necessary to prevent the interference of inner pore diffusion with the chemical kinetics. ACKNOWLEDGEMENTS This work was financially supported by P.E.F. (Projekt Europiiisches Forschungszentrum fur Massnahmen zur Luftreinhaltung), project 87/002/3. Aid through the Fonds der Chemie is equally acknowledged.
556
LITERATURE M. Kotter, H.-G. Lintz, Entropie, 137 (1987), 109. H. Bosch, F.J.J.G. Janssen, Catal. Today., 2 (1988), 369. G.C. Bond, J.Sarkany, G.D. Parfitt, J. Catal., 57 (1979), 476. R.Y. Saleh, I.E. Wachs, S.S. Chan, C.C. Chersich, J.Catal., 98 (1986), 102. H. Bosch, F.J.J.G. Janssen, F.M.G. van den Kerkhof, J. Odenziel, J.G. van Ommen, J.R.H. Ross, Appl. Catal., 25 (1986), 239. A. Baiker, P. Dollenmeier, M. Glinski, A. Reller, Appl. Catal., 35 (1987), 351. F. Cavani, E. Foresti, F. Trifiro, G. Busca, J. Catal., 106 (1987), 251. M. Kotter, H.-G. Lintz, T. Turek, D.L. Trimm, Appl. Catal., 52 (1989), 225. R.J.H. Clark, Vanadium, in J.C. Bailar, A.F. Trotman-Dickenson (eds.) Comprehensive Inorganic Chemistry, Vol. 3, Pergamon Press, Oxford 1973, p. 520. S. Yoshida, T. Iguchi, S. Ishida and K. Tarama, Bull. Chem. SOC.Jpn., 45 (1972), 376. F. Roozeboom, M. C. Mittelmeijer-Hazeleger , J.A.Mouli jn, J .Medema, V.H.J. de Beer, P.J. Gellings, J.Phys.Chem., 84 (1980), 2783. Gmelin, Handbook of Inorganic Chemistry, 8th edn., Titanium, Vol.1, Verlag Chemie GmbH, Weinheim, 1951, pp.345-351. . M. Inomata, A. Miyamoto, Y.Murakami, J . Catal., 62(1980), 140.
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors),Preparation of Catalysts V
BINARY OXIDE CATALYSTS SYNTHESIZED BY SEQUENTIAL PRECIPITATION C . S . BROOKS R e c y c l e M e t a l s , 4 1 Baldwin Lane, G l a s t o n h u r y , CT 06033, U.S.A.
SUMMARY S e q u e n t i a l p r e c i p i t a t i o n of mixed h y d r o x i d e s o f t h e Cu/Al, Cu/Cr, Ni/Fe and Cu/Fe have been conducted i n t h e a c i d w i t h aluminum, chrominum o r f e r r i c h y d r o x i d e as t h e s o r p t i o n / p r e c i p i t a t i o n of copper o r n i c k e l as t h e second demonstrated t h a t a f t e r a i r c a l c i n e ( 2 5 0 - 3 5 6 C ) enhanced l y t i c a c t i v i t y f o r t h e room t e m p e r a t u r e decomposition of and o x i d a t i o n of henzaldehyde a r e provided by t h e b i n a r y Cu/Fe and Ni/Fe.
b i n a r y systems, N i / A l , p r e s e n c e of o x a l i c f i r s t s t a g e and ads t a g e . It h a s been d i s p e r s i o n and c a t a hydrogen p e r o x i d e o x i d e s , n o t a b l y Cu/Al,
INTRODUCTION I n t h i s i n v e s t i g a t i o n a n e v a l u a t i o n w a s conducted of how a d s o r p t i o n - c o p r e c i p i t a t i o n can h e used t o b e s t a d v a n t a g e w i t h mixed metal h y d r o x i d e s t o r e a l i z e The b i n a r y systems c o n s i d e r e d c o n s i s t e d of
h i g h d i s p e r s i o n and homogeneity. combinations such as Cu/A1, N i / A l ,
Cu/Fe, Ni/Fe and CulCr.
The h y d r o x i d e of
Al, C r o r Fe i s p r e c i p i t a t e d i n a m o d e r a t e l y a c i d pH r a n g e t o become t h e supp o r t f o r t h e second m e t a l , Cu o r N i , t o b e adsorbed-ion exchanged.
The f r e s h l y
p r e c i p i t a t e d A1,Cror Fe h y d r o x i d e i s a n e s p e c i a l l y a c t i v e s u b s t r a t e f o r i o n exchange.
F u r t h e r examination w a s made of modifying t h e a d s o r p t i o n - c o p r e c i p i t a -
t i o n p r o c e s s by u s i n g a n i o n i c a g e n t s such a s a c e t i c a c i d , c i t r i c a c i d , e t h y l e n e diamine t e t r a a c e t i c a c i d (EDTA) and o x a l i c a c i d complexed w i t h t h e Cu and N i cations. The h y d r o x i d e c o p r e c i p i t a t e s were a i r c a l c i n e d a t 250 o r 350'C.
For s e l e c t -
ed s y s t e m s BET s u r f a c e a r e a s and c a r b o n monoxide c h e m i s o r p t i o n measurements
were made.
Xray d i f f r a c t i o n measurements w e r e made on s e l e c t e d samples t o
characterize crystallinity.
C a t a l y t i c performance t e s t s were conducted a t
2 0 " C f o r hydrogen p e r o x i d e decomposition and benzaldehyde o x i d a t i o n by hydrogen p e r o x i d e . S e v e r a l i n v e s t i g a t i o n (1-5)
of t h e p r e p a r a t i o n of s u p p o r t e d c a t a l y s t s have
used s p e c i a l a d s o r p t i o n c o n d i t i o n s f o r a n i o n i c and c a t i o n i c P t s p e c i e s i n v o l v i n g t h e i o n exchange c h a r a c t e r i s t i c s o f t h e alumina o r s i l i c a s u p p o r t and c o n d u c t i n g a d s o r p t i o n on t h e a c i d pH s i d e of t h e z e r o p o i n t o f change (ZPC). The n o v e l a s p e c t of t h e p r e s e n t approach i s t h e u s e of a f r e s h l y p r e c i p i t a t e d hydroxide such as aluminurn,chromium o r f e r r i c h y d r o x i d e i n s t e a d o f a c a l c i n e d , s t a b i l i z e d oxide as t h e support. P r i o r a t t e n t i o n h a s been g i v e n t o t h e a d s o r p t i o n o f m e t a l c a t i o n - a n i o n i c complexes on m e t a l o x i d e s and h y d r o x i d e s b u t r a r e l y f o r t h e purpose of s y n t h e -
558 sizing coprecipitated c a t a l y s t precursors.
A number of s t u d i e s ( 4 , 5 - 1 3 )
h a v e involved t h e u s e of metal o r g a n i c complexes such as c i t r a t e s , o x a l a t e s , e t c . which a r e s u b s e q u e n t l y decomposed t o p r o v i d e enhanced d i s p e r s i o n and homogeneity f o r t h e o x i d e c a t a l y s t p r e c u r s o r .
However, t h e p r e s e n t u s e of
anionic-metal complexes f o r s e q u e n t i a l p r e c i p i t a t i o n i s c o n s i d e r e d n o v e l . EXPERIMENTAL PROCEDURES Binary systems s y n t h e s i z e d c o n s i s t e d of Cu/Fe, Ni/Fe, Cu/A1 and N i / A l and Cu/Cr f o r 4-10 w t p e r c e n t Cu o r N i i n t h e c a l c i n e d mixed o x i d e .
Anionic
complexing a g e n t s a c e t i c , c i t r i c and o x a l i c a c i d s and EDTA were used i n molar r a t i o s of 1:l w i t h t h e i n i t i a l copper o r n i c k e l .
Two s t a g e p r e c i p i t a t i o n s
w e r e used s t a r t i n g w i t h a n i n i t i a l f o r m a t i o n of aluminum, chromium
hydroxide
or ferric
by a d d i t i o n of NaOH t o a n aqueous s o l u t i o n of A 1 n i t r a t e , C r n i t r a t e
o r Fe c h l o r i d e .
In t h e second s t a g e aqueous s o l u t i o n s of Cu s u l f a t e o r N i
n i t r a t e were mixed w i t h t h e i n i t i a l p r e c i p i t a t e w i t h o r w i t h o u t the p r e s e n c e of a 1:l mole r a t i o
of s e l e c t e d a n i o n i c complexing a g e n t s t o complete t h e p r e c i p -
A second mode o f c o p r e c i p i t a t i o n used w a s t o p r e a d s o r b o x a l i c a c i d
itation.
on t h e i n i t i a l l y p r e c i p i t a t e d A 1 , C r o r F e hydroxide. The c o p r e c i p i t a t e s were s e p a r a t e d by f i l t r a t i o n , a i r d r i e d and c a l c i n e d a t 250 o r
tests.
350" C i n p r e p a r a t i o n f o r c h a r a c t e r i z a t i o n and c a t a l y s t performance
The m e t a l c o n t e n t s of t h e c a l c i n e d c o p r e c i p i t a t e s w e r e c a l c u l a t e d from
t h e r e s i d u a l m e t a l c o n t e n t s of t h e e q u i l i b r i u m f i l t r a t e s as determined by a t o m i c a b s o r p t i o n (Griswold and F u s s Environmental L a b o r a t o r i e s , Manchester, CT). BET n i t r o g e n s u r f a c e areas w e r e measured by S t r u c t u r e P r o b e , F a i r f i e l d , CT
.
A d d i t i o n a l BET s u r f a c e s and CO c h e m i s o r p t i o n measurements w e r e made by Porous Materials, Inc., Ithaca, W .
Xray d i f f r a c t i o n a n a l y s e s were made f o r s e l e c t e d
samples t o c h a r a c t e r i z e t h e c r y s t a l l i n i t y of t h e c a l c i n e d c o p r e c i p i t a t e s . C a t a l y t i c performance t e s t s a t 2 O o C c o n s i s t e d hydrogen p e r o x i d e decomp o s i t i o n and t h e o x i d a t i o n of benzaldehyde t o b e n z o i c a c i d b y p a s s i n g h y d r o g e n p e r o x i d e (3% a q . s o l n . )
through a bed of c a l c i n e d c o p r e c i p i t a t e ( 0 . 1 t o 0.2 gm.)
s u p p o r t e d on a c o a r s e g l a s s f r i t and measuring
t h e r a t e o f oxygen e v o l u t i o n .
The benzaldehyde o x i d a t i o n w a s conducted by v i g o u o u s l y mixing 1 0 m l of a x y l e n e s o l u t i o n of b e n z a l d e h y d e ( 2 0 w t X ) w i t h 5 0 m l o f hydrogen p e r o x i d e (3%) i n the p r e s e n c e of 0.1-0.2
gm. of c a l c i n e d c o p r e c i p i t a t e w i t h N a l a u r y l s u l f a t e
(50 ppm) s u f a c t a n t p r e s e n t t o f a c i l i t a t e phase mixing.
Benzoic a c i d p r o d u c t i o n
w a s e a t a b l i s h e d by t i t r a t i o n w i t h NaOH a f t e r 30-60 m i n r e a c t i o n t i m e .
Ben-
zaldehyde /H202 o x i d a t i o n tests w e r e a l s o conducted by p a s s i n g 1 0 m l of s o l u t i o n of 6 w t % benzaldehyde i n aqueous methanol (24 v o l . p e r c e n t ) t h r o u g h a bed of
0.lg
of c a l c i n e d c o p r e c i p i t a t e s u p p o r t e d on a g l a s s f r i t and conducting
a NaOH t i t r a t i o n of the f i l t r a t e .
559 RESULTS AND DISCUSSION
C o p r e c i p i t a t e s w i t h Aluminum Hydroxide One series of N i / A 1 b i n a r y h y d r o x i d e c o p r e c i p i t a t e s w a s p r e p a r e d w i t h a n i n i t i a l atomic r a t i o of 1: 1 N i / A l M t h n i c k e l e q u i l i b r a t e d w i t h a n i o n i c a g e n t s a c e t i c a c i d o r c i t r i c a c i d o r EDTA i n a m o l e c u l a r r a t i o 1:l and mixed w i t h t h e i n i t i a l l y p r e c i p i t a t e d A 1 hydroxide.
I n t h i s s y s t e m s e q u e s t r a t i o n of t h e N i
i n s o l u t i o n o c c u r r e d u n t i l a pH of 10-12 w a s a t t a i n e d p r e c l u d i n g a s t a g e d cop r e c i p i t a t i o n i n a n a c i d regime.
A second series o f N i / A l b i n a r y h y d r o x i d e
c o p r e c i p i t a t e s u s i n g a lower i n i t i a l a t o m i c r a t i o
of 0 . 5 f o r N i / A l w a s pre-
pared i n t h e p r e s e n c e of a 1:l m o l e c u l a r r a t i o of c i t r i c a c i d o r o x a l i c a c i d . (Table 1.). I n t h i s c a s e N i l o a d i n g s i n t h e r a n g e of 4.0-4.3
w t . % were obtained
a t pH v a l u e s of 10.0 and 7.5 r e s p e c t i v e l y b u t no improvement i n t h e s t a t e of d i s p e r s i o n as i n d i c a t e d by t h e BET a r e a s of t h e p r e c i p i t a t e s c a l c i n e d a t 35OoC was o b t a i n e d . A series of s e q u e n t i a l c o p r e c i p i t a t i o n s was
a n i n i t i a l atomic r a t i o
conducted a l s o w i t h C u / A 1 i n
of 1:l and w i t h a m o l e c u l a r r a t i o o f 1 : l w i t h c i t r i c
a c i d o r o x a l i c a c i d ( T a b l e 1). In t h e s e systems Cu l o a d i n g s i n t h e r a n g e of
3.9-5.8 wt.% were o b t a i n e d a t pH v a l u e s of 4.0 and 7 . 4 r e s p e c t i v e l y .
.
In this
sequence t h e p r e s e n c e of c i t r i c a c i d r e s u l t e d i n a d e c r e a s e d s t a t e of d i s p e r s i o n as i n d i c a t e d by t h e BET a r e a of t h e c o p r e c i p i t a t e c a l c i n e d a t 350°C when com-
pared w i t h t h e r e f e r e n c e s y s t e m w i t h no a n i o n i c p r e s e n t .
On t h e o t h e r hand t h e
system p r e p a r e d w i t h t h e o x a l i c a c i d p r e s e n t p r e a d s o r b e d a n t h e i n i t i a l l y prec i p i t a t e d Al h y d r o x i d e p r o v i d e d a n o r d e r of magnitude i n c r e a s e i n t h e d i s p e r s i o n a s i n d i c a t e d by t h e BET area of t h e c a l c i n e d o x i d e . TABLE 1 B i n a r y N i / A 1 , Cu/A1 c a t a l y s t p r e c u r s o r s
pH FOR
BINARY
PRECIPITATION
ANIONIC
N i /Al NVAI N i/A I
12.0 10.0 7.5
C I TRlC ACID
C u/A1 Cu/Al Cu/AI
8.0 4.0 7.4
2 nd METAL W T O/e
BET AREA m 2/g
OXALIC ACID
4.4(Ni) 4.3 ( N i l 4.0(Ni)
4.8 4.3 2.9
None C I T R I C ACID OXALIC ACID
5.6(Cu) 3.9(Cu) 5.8 (Cu)
11.0 1.4 112.0
None
560 C o p r e c i p i t a t e s w i t h F e r r i c Hydroxide A series of N i / F e b i n a r y h y d r o x i d e c o p r e c i p i t a t e s w e r e p r e p a r e d i n a s i m i -
l a r manner, i n t h e p r e s e n c e of the a n i o n i c a g e n t s a c e t i c a c i d o r c i t r i c a c i d o r EDTA i n a m o l e c u l a r r a t i o of I:1 w i t h t h e N i p r i o r t o mixing w i t h t h e i n i t i a l l y p r e c i p i t a t e s Fe hydroxide. r a n g e 5.5-6.0
P a r t i a l c o p r e c i p i t a t i o n was o b t a i n e d i n t h e pH
i n t h e p r e s e n c e of a c e t i c a c i d o r EDTA b u t s e q u e s t r a t i o n of t h e N i
occurred with t h e c i t r i c acid.
E f f i c i e n t c o p r e c i p i t a t i o n was o b t a i n e d o n l y i n
t h e pH r a n g e 10-13 f o r a l l t h r e e a n i o n i c s p r e c l u d i n g e f f i c i e n t s t a g e d p r e c i p i t a t i o n i n a n a c i d regime.
The same r e s u l t s were o b t a i n e d f o r a series of Cu/Fe
b i n a r y h y d r o x i d e c o p r e c i p i t a t e s p r e p a r e d i n t h e p r e s e n c e of t h e s e t h r e e a n i o n i c complexing a g e n t s .
A second series of b i n a r y h y d r o x i d e c o p r e c i p i t a t e s u s i n g a
l o w e r i n i t i a l a t o m i c r a t i o of 0.5 f o r Ni/Fe was p r e p a r e d i n t h e p r e s e n c e of a
1:l m o l e c u l a r r a t i o o f o x a l i c a c i d t o N i ( T a b l e 2 ) . i n c r e a s e d from 10.5 t o 2 1 . 0 w t . % .
The r e s u l t a n t N i l o a d i n g
The BET area of t h i s c o p r e c i p i t a t e c a l c i n e d
a t 350° C i n c r e a s e d from 35 t o 7 4 sq.m.per gm. i n d i c a t i n g a d i s t i n c t improve-
ment i n t h e d i s p e r s i o n f o r t h e o x i d e p r e p a r e d i n t h e p r e s e n c e o f o x a l i c a c i d . A s i m i l a r sequence of b i n a r y h y d r o x i d e c o p r e c i p i t a t e s u s i n g a n i n i t i a l
a t o m i c r a t i o of 0.5 f o r Cu/Fe w a s p r e p a r e d i n t h e p r e s e n c e of a 1:1 m o l e c u l a r ratio
of o x a l i c a c i d t o Cu ( T a b l e 2 ) .
I n t h i s case t h e Cu l o a d i n g i n c r e a s e d
from 1 . 9 t o 23.0 w t . % , a f a c t o r g r e a t e r t h a n 10.
For t h i s s y s t e m t h e BET a r e a
i n c r e a s e d from 25 t o 78 sq.m.per gm f o r t h e c a l c i n e d p r e c i p i t a t e i n d i c a t i n g a n even g r e a t e r improvement i n d i s p e r s i o n . TABLE 2
Binary N i / F e , Cu/Fe c a t a l y s t p r e c u r s o r s
B I N ARY
pH FOR PREClPlTATION
2 nd
AN IONIC
METAL WT
O/o
BET AREA
m 2/g
N i /Fe
6.0
Ni/ Fe
4.0
None OXALIC ACID
lO.S(Ni) 21 .O(Ni)
35 74
Cu/Fe Cu/Fe
4.0 6.0
None OXALIC ACID
I . 9 (CU) 23.0(Cu)
78
25
561 C o p r e c i p i t a t e s w i t h Preadsorbed O x a l i c Acid Several a d d i t i o n a l c o p r e c i p i t a t e s w e r e prepared with N i / A l ,
Cu/A1 and
Cu/Cr i n which s e q u e n t i a l p r e c i p i t a t i o n w a s conducted f o r atomic r a t i o s of 0.1-0.2
w i t h o x a l i c p r e s e n t p r i o r t o a d d i t i o n of t h e N i o r Cu t o t h e A 1 o r C r
hydroxide.
The r e s u l t s f o r t h e s e c a t a l y s t s a r e summarized i n T a b l e s 3
CATALYTIC PERFORMANCE The c a l c i n e d c o p r e c i p i t a t e s (250 " C ) w e r e e v a l u a t e d f o r c a t a l y t i c performance a t 20" C f o r two r e a c t i o n s , hydrogen p e r o x i d e decomposition and benzaldehyde o x i d a t i o n t o b e n z o i c a c i d by hydrogen p e r o x i d e ( F i g s . 1-3).
The
Cu/A1 c a t a l y s t w i t h 3 . 9 w t . % Cu l o a d i n g p r e p a r e d i n t h e p r e s e n c e of c i t r i c a c i d and c a l c i n e d a t 250'C
provided a n i n c r e a s e i n t h e hydrogen p e r o x i d e decomposi-
t i o n k i n e t i c s g r e a t e r t h a n a n o r d e r of magnitude and a n i n c r e a s e of o v e r two o r d e r s o f magnitude f o r t h e system p r e p a r e d i n t h e p r e s e n c e of o x a l i c a c i d (Fig. 1 ) .
R IC
mmoles 0 2 g cat.min.
I .c
I NO ANIONIC
CITRIC ACID
OXALIC ACID
FIG. 1 Hydrogen p e r o x i d e decomposition a t 2 0 ° C o v e r Cu/A1 B i n a r y oxide c a t a l y s t s
562 T h i s l a t t e r system shows a n improvement i n c a t a l y t i c performance c o r r e s p o n d i n g t o t h e i n c r e a s e i n t h e BET s u r f a c e area of a n o r d e r of magnitude.
The i n c r e a s e
i n t h e hydrogen p e r o x i d e decomposition k i n e t i c s r u n s c o u n t e r t o t h e d e c r e a s e i n t h e BET s u r f a c e area. The i n c r e a s e of a n o r d e r of magnitude f o r t h e hydrogen p e r o x i d e decomposit i o n k i n e t i c s f o r t h e b i n a r y Ni/Fe and Cu/Fe c a t a l y s t s c o r r e l a t e s w e l l w i t h t h e d o u b l i n g of t h e BET s u r f a c e areas ( F i g . 2 ) .
Ni
I
NO ANIONIC FIG. 2
OXALIC ACID
Hydrogen p e r o x i d e decomposition a t 20°C o v e r Ni/Fe and Cu/Fe binary metal oxide c a t a l y s t s
The r a t e s f o r benzaldehyde o x i d a t i o n t o b e n z o i c a c i d a t 20'
C are an o r d e r
of magnitude lower t h a n t h e hydrogen p e r o x i d e decomposition r a t e s on t h e s e same c a t a l y s t s f o r N i / A 1 and Cu/A1 c a l c i n e d p r e c i p i t a t e s and provided o n l y modest i n c r e a s e s f o r t h e systems p r e p a r e d i n t h e p r e s e n c e of o x a l i c
acid (Fig. 3 ) .
563
1.0 mmoles Benzoic Acid g cat. min.
cu
R-
o+--c"
.01
NO ANIONIC
FIG. 3
OXALIC ACID
O x i d a t i o n of Benzaldehyde a t 20'C and Cu/A1 b i n a r y o x i d e c a t a l y s t s
t o benzoic acid over N i / A 1
A d d i t i o n a l c a t a l y t i c a c t i v i t y t e s t s and b a s e a d s o r p t i o n c a p a c i t i e s were determined f o r N i / A l ,
Cu/Al and Cu/Cr b i n a r y c o p r e c i p i t a t e s w i t h o u t and w i t h
preadsorbed o x a l i c a c i d ( s t o i c h i o m e t r i c w i t h t h e i n i t i a l N i o r Cu added i n t h e second s t a g e ( T a b l e 3 ) .
The Cu/A1 c o p r e c i p i t a t e s w i t h preadsorbed o x a l i c a c i d
provided t h e most s i g n i f i c a n t improvement f o r H202 decomposition a t room temperature.
A l l t h e c o p r e c i p i t a t e s , w i t h o u t o r w i t h preadsorbed o x a l i c a c i d
f e l l w i t h i n a r e l a t i v e l y narrow r a n g e of 0.41-1.18
maq. a c i d p e r gram of c a t a -
l y s t p e r minute f o r benzaldehyde o x i d a t i o n a t room t e m p e r a t u r e .
These r e a c t i o n
r a t e s ( T a b l e 3 ) determined by p a s s i n g t h e benzaldehyde r e a c t i o n s o l u t i o n through t h e c a t a l y s t bed are a n o r d e r o f magnitude h i g h e r t h a n o b t a i n e d f o r t h e o x i d a t i o n rates o b t a i n e d w i t h t h e c a t a l y s t suspended i n a s t i r r e d r e a c t i o n mixt u r e ( F i g . 3 ) and a r e c o n s i d e r e d t h e more a c c u r a t e measure of c a t a l y t i c a c t i v ity.
564 TABLE 3
C a t a l y t i c a c t i v i t y f o r hydrogen p e r o x i d e decomposition and benzaldehyde o x i d a t i o n and b a s e a d s o r p t i o n c a p a c i t y
beta1
WALYST
Ni/Al-18
Ni/Cu wt.%
A 4.4Ni
N i /Al-2 4
8.3Ni
2Preadsorw O x a l i c Acid
3 ~ 2 ~ 2 Decoripn.
0 zmles
2
gcat.min.
‘Benzaldehyde Oxidation nxq acid qcat.min.
.
5Base Pdsorp NH40I-’meq/qcat
m t a l Qlemisorb,
7-
No
0.024
0.41
0.4
0i1.0
Yes
0.048
1.01
1.9
0
1.2
Cu/Al-l
A 5.6Cu
No
0.0048
0.63
1.7
00.8
Cu/Al-3
A 5.8Cu
Yes
1.24
1.18
2.2
0
CWAl-6
A
5.6Cu
Yes
1.99
0.85
1.6
o 1.1
Cu/Cr--18
A
5.7a
No
0.54
0.67
0.3
-
Cu/Cr-20
A
9.6Cu
Yes
0.96
0.48
1.8
-
IWE3:
1.1
1 ) Based on w e i g h t after 1 hr. a i r calcine a t 250 OC ( A ) or 35OOC ( A ) . 2 ) 1:l stoickiometric w i t h N i o r Cu added t o Al or C r hydroxide. 3) Based on 0 evolution f r o m 3 w t . % aq H 0 passed through 0 . l g of catalysz supported on porous glas; 4 ) Based on meg. of N / 1 0 LhOH titer for benzaldehyde 6 w t . % i n H202 (3 s t . % ) and. CH30H (24 v o l . % ) solution passed through 0 . l g of catalyst supported on porous glass frit. 5 ) Based on meq. of N / 1 0 €?a titer f r o m 5 m l of N/10 W40H solution pass& through 0.lg catalyst supprted on porous glass frit after i n i t i a l drying 1 hr. 15OOC ( 0 ) and f o l l o w i n q i n i t i a l NH40H exposure and. drying 1 hr. 15OOC. 6) C r ( V I ) appeared in sscond NH40H f i l t r a t e .
Lit.
Base a d s o r p t i o n c a p a c i t i e s determined by p a s s i n g t h e N/lO NH OH t h r o u g h 4 t h e c a t a l y s t bed i n d i c a t e s i g n i f i c a n t enhancement i n a l l c a s e s where t h e CO-
p r e c i p i t a t e s w e r e p r e p a r e d i n t h e p r e s e n c e of preadsorbed o x a l i c a c i d . Approxi m a t e l y o n e h a l f t o two t h i r d s of t h e b a s e a d s o r p t i o n c a p a c i t i e s f o r adsorbed NH40Hare a t t r i b u t e d t o c h e m i s o r p t i o n ( n o t desorbed a f t e r 1 h r . 1 5 0 ° C h e a t i n g ) f o r t h e Cu/A1 system w i t h o u t and w i t h o x a l i c a c i d and f o r t h e N i / A l It i s r e c o g n i z e d t h a t NH OH s o l u t i o n a d s o r p t i o n , 4 a s d i s t i n g u i s h e d from v a p o r phase NH a d s o r p t i o n , may have some s e r i o u s l i m i t a 3
w i t h preadsorbed o x a l i c a c i d .
565 t i o n s f o r c h a r a c t e r i z a t i o n of t h e b a s e a d s o r p t i o n s i t e s o n t h e s e o x i d e s ( 1 4 ) . The chemisorbed b a s e a d s o r p t i o n c a p a c i t i e s a r e c o n s i d e r e d t o h a v e more l e g i t imacy b u t no c o r r e l a t i o n s are o f f e r e d pending more t h o r o u g h c h a r a c t e r i z a t i o n and o p t i m i z a t i o n o f t h e c a t a l y s t c o m p o s i t i o n s . CONCLUSION It h a s been demonstrated t h a t s e q u e n t i a l p r e c i p i t a t i o n i n a moderately a c i d pH r a n g e f o r t h e b i n a r y m e t a l s y s t e m s Cu/Al, Cu/Cr, Cu/Fe and N i / F e i n t h e p r e s e n c e of o x a l i c a c i d w i t h aluminum o r f e r r i c h y d r o x i d e a s t h e f i r s t s t a g e and adsorption/precipitation o f c o p p e r o r n i c k e l a s t h e second s t a g e p r o v i d e s metal o x i d e s ( a f t e r 250-350' C a i r c a l c i n e ) w i t h c o n s i d e r a b l e enhancement i n d i s p e r s i o n and i n c a t a l y t i c a c t i v i t y , n o t a b l e f o r Cu/Al,
for the
room t e m p e r a t u r e d e c o m p o s i t i o n o f hydrogen p e r o x i d e and b e n z a l d e h y d e oxidat i o n by hydrogen p e r o x i d e . REFERENCES J . P . B r u n e l l e , P r e p a r a t i o n of C a t a l y s t s I1 P r o c e e d . 2nd I n t . Symp. Louvainla-Neuve S e p t . 4-7, 1978, Eds. B. Delmon, P. Grange, P. J a c o b s , G . P o n c e l o t , E l s e v i e r , Amsterdam ( 1 9 7 9 ) , p.211 2 A.T. B e l l , S u p p o r t s and Metal-Support I n t e r a c t i o n s i n C a t a l y s t D e s i g n C h a p t e r 4 i n C a t a l y s t D e s i g n - P r o g r e s s and P e r s p e c r i v e s , L.L. Hegedus Ed., J o h n Wiley & S o n s , NY (1987) 3 M. Che and L . B o n n e v i o t , The Change of P r o p e r t i e s of T r a n s i t i o n Metal I o n s and t h e R o l e o f t h e S u p p o r t as a F u n c t i o n of C a t a l y s t P r e p a r a t i o n , P. 147 i n S u c c e s s f u l D e s i g n of C a t a l y s t s , T . I n u d , Ed., E l s e v i e r , Amsterdam (1989) 4 G . H . Van d e n Berg, H. Th. R i j n t e n , P r e p a r a t i o n o f C a t a l y s t s I1 P r o c e e d . 2nd I n t . Symp. Louvain-la-Neuve S e p t . 4 - 7 , 1978, Eds. B. Delmon, P . Grange, P . J a c o b s , G. P o n c e l o t , E l s e v i e r , Amsterdam ( 1 9 7 9 ) , p.265 5 E. M a t i j e v i c , P r e p a r a t i o n of C a t a l y s t s I1 P r o c e e d . 2nd I n t . Symp. Louvainla-Neuve S e p t . 4-7, 1 9 7 8 , Eds. B. Delmon, P. Grange, P. J a c o b s , G . P o n c e l o t , E l s e v i e r , Amsterdam (1979), p.555 6 V . N i k o l a i e n k o . V. Busacek, B.L. Danes, J. C a t a l y s i s 2 , (1962), p.127 7 J O S . A. Van D i l l e n , J . W . Geus, Leo A.M. Hermans, J a n Van Der M e i j d e n , P r o c e e d . 6 t h I n t . Congress o n C a t a l y s i s , Eds., G.C. Bond, P.B. Wells and F.C. Tompkins, The Chem. SOC., London (1977) p.677 8 Ph. C o u r t y and Ch. M a r c i l l y , P r e p a r a t i o n o f C a t a l y s t s 111 P r o c e e d . 3rd I n t . Symp., Louvain-la-Neuve, S e p t . 6-9, 1 9 8 2 , E d s . , G . P o n c e l o t , P. Grange and P. A . J a c o b s , E l s e v i e r , Amsterdam (1983), p.485 9 J . W . Geus, P r e p a r a t i o n of C a t a l y s t s 111 P r o c e e d . 3 r d I n t . Symp., Louvainla-Neuve, S e p t . 6-9, 1982, E d s . , G . P o n c e l o t , P. Grange and P.A. J a c o b s , E l s e v i e r , Amsterdam (1983), p . 1 1 0 Ch. S i v a r a s and P. K a n t a r a o , Applied C a t a l y s i s , 4 5 , ( 1 9 8 8 ) , p.103 11 Y. T e r a o k a , Hua-Min-Zhang, N. Yamazoe, P r o c e e d . 9 t h I n t . C o n g r e s s on C a t a l y s t s , J u n e 26-July 1, C a l g a r y , Canada, Eds., M . J . P h i l l i p s and M. T e r n a n , Chemical I n s t . of Canada, O t t a w a ( 1 9 8 8 ) , p.1984 12 M.F. Wilson, 0 . Antinluoma, J . R . Brown, Am. Chem. SOC. P e t . Div. R e p r i n t from Symp. o n 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 of C a t a l y s t s , S e p t . 25-30, N a t l . Mtg., L o s A n g e l e s , CA, Vol. 3 3 ( 4 ) , ( 1 9 8 8 ) , p.669 1 3 J . M . Jehng and I . E . Wachs, Am. Chem. S O C . , P e t . Div. R e p r i n t from Symp. on New C a t a l y t i c Materials and T e c h n i q u e s , S e p t . 10-15, N a t l . Mtg. M i a m i , F1, V o l . 3 4 ( 3 ) , (1989), p.546 14 K. Tanabe, S o l i d A c i d s and B a s e s , C h a p t e r 3 , Academic P r e s s , New York,(1970)
1
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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
567
ZrO2 AS A SUPPORT : OXIDATION OF CO ON CrOx/ZrOZ
T. YAMAGUCHI, M. TAN-NO and K. TANABE Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060 (Japan)
SUMMARY
Supported CrOx catalyst was prepared from Cr(C0) , (NH4)2Cr04 and Cr(N0 )3 by depositing o n t o ZrO and SiO2. Amount of frOx deposited was O.? 1.8 mmol/g (0.7 - 13.4 w t 8 a s Crp03). Catalytic activity was evaluated by the oxidation of CO. Catalytic activity of Zr02-supported CrOx was 20 times higher than that of Si02-supported one at the lower loading range. Physicochemical properties of resulting samples were examined by means of XRD. ESR, and XANES. I n conclusion, CrOx dispersed on SiO is i n the form of tetrahedral coordination with three dimensional cryst5 growth and hence lower number of catalytically active sites, while CrOx is finely dispersed or two dimensionally dispersed on ZrOz support with a square pyramidal coordination and hence the higher effective surface area.
INTRODUCTION Structure, electronic states, and binding states of supported metal oxides are considered to be i n different states compared with bulk metal oxides because of their interaction with supports. These factors may affect strongly the catalytic performances. Supported Cr oxides are being widely used for the polymerization of ethylene, the hydrogenation of alkene, oxidation reactions and so on, and are one of the important catalysts in industry. Cr/Si02 is one o f the typical catalyst i n supported Cr catalysts family, and the many works have been reported o n the nature of active site for the polymerization of ethylene (refs.1-3). Cr/A1203 as well as Cr/Si02 is also investigated in the sight of alcohol decomposition (refs. 4,5). Most of works on the supported Cr catalysts i s concentrated to use SiO2 or A1203. and only a little work was found for the use of other supports such as Ti02 and Zr02 (refs. 6 , 7 ) . Zirconium dioxide shows specific catalytic actions for the cleavage of a CH bond (ref.8) and the hydrogenation of buta-1,3-diene by a molecular hydrogen and hydrogen donor molecules such as cyclohexadiene (refs.9-11) and high selectivities for the formation of 1-olefins from secondary alcohols (ref.12) Recently decomposition of and of isobutane in CO + HZ reaction (ref.13). triethylamine to yield acetonitrile, in which both dealkylation by acidic sites and dehydrogenation by basic sites were involved, was reported (ref.14). These characteristic behaviors of Zr02 are considered due to the acid-base
568
bifunctional catalysis. Z i r c o n i u m d i o x i d e i s n o t o n l y an i n t e r e s t i n g o x i d e c a t a l y s t , becoming even more i m p o r t a n t as a c a t a l y s t support.
For i n s t a n c e ,
but i s t h e Rh
supported on ZrO2 e x h i b i t s h i g h e r c a t a l y t i c a c t i v i t y f o r t h e h y d r o g e n a t i o n o f
CO and C02 compared w i t h t h a t supported on A1203, SiOz, e t c . ( r e f s . l 5 , 1 6 ) . partlcular,
the
Rh/Zr02
catalyst
h y d r o g e n a t i o n o f C02 ( r e f . 1 5 ) .
shows
the
highest
In
activity for
the
Such a s p e c i f i c c a t a l y t i c p r o p e r t i e s has a l s o
been r e p o r t e d o v e r Rh c a r b o n y l c l u s t e r s impregnated o n t o Z r - c o n t a i n i n g
silica
( r e f s . 17,18). T h i s paper d e a l s w i t h t h e p r e p a r a t i o n and t h e c h a r a c t e r i z a t i o n o f C r o x i d e supported on Zr02 b y t h e o x i d a t i o n r e a c t i o n o f CO on t h e one hand and t h e s t r u c t u r a l i n v e s t i g a t i o n by XRD,
XANES, and ESR on t h e o t h e r .
EXPERIMENTAL Supported CrOx c a t a l y s t s were prepared f r o m (NH4)2Cr04 by d e p o s i t i n g on t o Zr02 and S i O z and c a l c i n i n g a t 773 K f o r 3 h. C r ( C 0 ) G used as a s t a r t i n g m a t e r i a l .
and Cr(N03)3 were a l s o
Amount o f CrOx d e p o s i t e d was 0.1 -1.8
mmol/g
- 13.4 w t % as Ct-203). C a t a l y t i c a c t i v i t y was e v a l u a t e d by t h e o x i d a t i o n o f CO ( 4 5 T o r r ) w i t h 10 ( o r 90) T o r r o f 02 a t 473 K and by 60 min r e a c t i o n by (0.7
using a closed r e c i r c u l a t i o n reactor. Physicochemical p r o p e r t i e s o f t h e samples were examined by means o f XRD.
ESR.
and X A N E S .
Rigaku-Denki Amounts of
D-9C
A q u a n t i t a t i v e a n a l y s i s o f Cr2O3 p h a s e was p e r f o r m e d on X-ray
d i f f r a c t o m e t e r u s i n g CaF2 as an i n t e r n a l
C r loaded were measured on P h i l l i p s PW-1404
spectrometer.
X-ray
standard.
fluorescence
ESR spectrum was o b t a i n e d by u s i n g V a r i a n E-3 Spectrometer a t
room t e m p e r a t u r e o r l i q u i d n i t r o g e n temperature. X-ray
a b s o r p t i o n experiments
i n t h e t r a n s m i s s i o n mode were c a r r i e d o u t on EXAFS f a c i l i t i e s
installed a t
BLlOB a t t h e Photon F a c t o r y i n Tsukuba, Japan. RESULTS AND DISCUSSION S t a r t i n q m a t e r i a l and c a t a l y t i c performance E f f e c t o f s t a r t i n g m a t e r i a l on t h e CO o x i d a t i o n was e x a m i n e d b y u s i n g Cr(C0)6, state,
(NH4)2Cr04 and Cr(N03)3.
Table 1 summarizes t h e r e s u l t s . I n o x i d i z e d
t h e r e i s l i t t l e d i f f e r e n c e i n c a t a l y t i c a c t i v i t y even though t h e
d i f f e r e n t s t a r t i n g m a t e r i a l s were employed. I n p a r t i a l l y reduced s t a t e s , t h e r e i s a s l i g h t d i f f e r e n c e when S i 0 2 was used as a support.
Si02 i s r e l a t i v e l y
i n e r t s u p p o r t and t h e i n t e r a c t i o n w i t h CrOx may be weak. R e v e r s i b i l i t y between C r 6 + and C r 3 '
may be more p r o n o u n c e d i n C r ( N 0 3 ) 3 - d e r i v e d
(NH4)2CrO4-derived catalysts.
one.
catalyst than
No d i f f e r e n c e was f o u n d on t h e Z r 0 2 - s u p p o r t e d
Thus i t can be concluded t h a t t h e s t a t e o f C r O x d e p o s i t e d on Zr02
569 TABLE 1 E f f e c t o f s t a r t i n g m a t e r i a l and s u p p o r t on CO o x i d a t i o n
C r loaded
catalyst
COP y i e l d/pmo 1 a )
s t a r t i n g material
02 oxidizedb)CO reducedb)
/mmol g-1
a Y i e l d a f t e r 60 min r e a c t i o n a t 473 K. O x i d a t i o n o r r e d u c t i o n a t 773 K. is
almost
independent
of
the
starting
materials i n t h i s
range.
Hereafter
s t a r t i n g m a t e r i a l was f i x e d t o use (NHq)$r04. E f f e c t o f r e d u c t i o n on CO o x i d a t i o n An e f f e c t o f r e d u c t i o n on C O o x i d a t i o n was examined b y u s i n g t h e sample w i t h 0.2 mmol-Cr/g i s shown i n F i g .
1.
CO r e d u c t i o n
a t 573 K r e s u l t e d i n t h e i n c r e a s e i n t h e c a t a l y t i c regardless
a c t i v i t y
the catalysts;
c o p r e c i p i t a t e d Cr203-Zr02 c a t a l y s t showed an i n t e r m e d i a t e a c t i v i t y w i t h a s i m i l a r enhancement by t h e reduction.
0.4
and
a: oxdn. ( S O O T ) b: redn. (3OO'C) c : redn. (5OO'C)
x
a 0.3
-E E
.T3 W
0.2
5
0.1
A f u r t h e r increase i n
c a t a l y t i c a c t i v i t y was observed by t h e 773 K r e d u c t i o n . The c a t a l y t i c reduced C r O x / Z r 0 2
a c t i v i t y of s t i l l
higher
CrOx/Si02.
than
t h a t
An enhancement
c a t a l y t i c
a c t i v i t y
is o f
i n the by the
r e d u c t i o n may be o b t a i n e d by t h e increase
i n the
number
o f
Fig. 1. E f f e c t o f CO r e d u c t i o n on catalytic activity. ( a ) 0 o x i d a t i o n a t 773 K. ( b ) C 6 r e d u c t i o n a t 573 K. ( c ) CO r e d u c t i o n a t 773 K. C r c o n t e n t = 0.2 mrnol/g.
570
catalytically active sites or by the reduction to lower oxidation states. Effect of loading amount on catalytic activity S i n c e t h e s u r f a c e area o f supports is limited, amount of deposited material also affects catalytic performances as well as support itself and starting materials. Figure 2 compares the changes of catalytic activities 0 0.5 10 1.5 . 2.o obtained over (NH4)pCrOq-derived C r content I rnrnolg-1 Cr oxide supported on SiO2 and ZrOE when the amount o f Cr loaded Fig. 2. Catalytic activity vs. was changed. It is interesting to Cr content in CO oxidation. note that the catalytic activity o f supported CrOx varies with the amount of Cr loaded on both CrOx/Zr02 and CrOx/SiOZ but in a different manner. On CrOx/ZrOp, the catalytic activity first increases steeply until the amount o f Cr loaded reaches t o 0.5 mmol/g and then the slope becomes lower. On the
other hand, oxidation activity on CrOx/Si02 was very low at low Cr content but became high at higher Cr content. Catalytic activity o f Zr02supported CrOx was 20 times higher < I / than that of Si02-supported one at CrlSiO7 the lower loading range. Even at the highest amount of loading, the catalytic activity o f CrOx/Zr02 was still 7 times higher than that of CrOx/Si02. The activity per a
/
Cr atom on CrOx/Si02 was 0.3 below 0.5 mmol-Cr/g and this increased to 0.6 above this amount, while that o f CrOx/Zr02 was 7 . 5 below 0.5 mmol-Cr/g and decreased to that of CrOx/Si02. Reaction kinetics also depends o n C r c o n t e n t and support. O n CrOx/SiOZ. zero-th o r d e r with
0
C r content I mrnoig-1
Fig. 3. Change of XRD intensity vs. Cr content.
571
respect to oxygen was found, while on CrOx/Zr02, a half order and zero-th order kinetics were observed below and above 0.5 mmol-Cr/g, respectively. These findings suggest that the state of Cr species varies with not only the amount of Cr dispersed, but also the support.
Crystallization and loadinq amount Crystal structure and its development upon the amount of loading were investigated by XRD. When Si02 was used as a support no diffraction line was observed below 0.5 mmol-Cr/g. Diffractions from Cr2O3 starts to appear at 0.5 mmol-Cr/g and the intensity was increased by the increase of Cr content. No phase such as CrpO5 and Cr20 other than Cr2O3 was found. On a Zr02 support, no phase other than tetragonal and monoclinic Zr02 was found below 0.5 mmol-Cr/g. A Cr2O3 phase develops above this amount. Normalized intensities by using CaF2 as an internal standard were measured and plotted against Cr contents. Figure 3 illustrates the change of the normalized intensities obtained for Zr02 and SiO2 supports. This clearly indicates that the development of Cr203 phase is more pronounced on a SiO2 support than on Zr02. Thus a likely conclusion is that a raft-like phase of Cr2O3 grows preferably on a Zr02 support while a three-dimensional crystals tend to grow on SiO2. If we consider the dispersion simply based on the surface area (SiOz :
299 m2/g, Zr02
an i l l dispersed state may be expected on Zr02. But it was not true. Aggregation took place more easily on a S i O 2 surface than on Zr02. A s p e c i f i c i n t e r a c t i o n may be expected on Zr02 surface as in the case of perovskite (ref.19). In Fig.4, catalytic activities are plotted against XRD intensities of CrOx/ZrOp and of CrOx/Si02. On CrOx/ZrOZ, a sharp increase in catalytic activity was found at the ill-crystallized or well-dispersed phase , while the activity of CrOx/Si02 was kept low ;
-0-
50 m2/g),
CrlZr02
--C C r l S i 0 2
0
0
0.5
.o
1
XRD intensity / a.u
Fig. 4. Catalytic activity vs. extent of crystallization.
572
even at highly crystallized state. Thls suggests that a lower catalytic activity over the CrOx/Si02 catalyst could be postulated by the lower surface area available.
A ~,=1986,,
36G
-4k
(a)
g,=1 978
State at lower amount of loading ESR spectroscopic examination may be helpful t o u n d e r s t a n d valence states and coordination of metal cations microscopically. Electronic state o f Cr cations, especially at the lower amount of loading was examined by means o f ESR s p e c t r o s c o p y . F i g u r e 5 i l l u s t r a t e s t h e ESR s p e c t r a obtained a t room temperature for oxidized Cr0x/SiO2(Fig.5a) and CrOx/ZrOz(Fig.5b).
Cr5+
exhibited
Fig. 5. ESR spectra of CrOx/SiOz(a) and CrOx/ZrOp(b) in oxidized state. Cr content = 0.2 mmol/g.
on CrOx/Si02 (g,=1.986, g,,=1.905, iili=i45G). A H at 77 K was 173G and the intensity ratio was 20, though Curie's law predicts this ratio should be 4. Adsorption o f water resulted in the g,,=l.958, AH=36G); no temperature dependency was narrowing in AH (g,=1.978, observed in A H . Thus it was concluded that the Cr species on Siop was tetrahedrally coordinated Cr5+. On Zr02 support, Cr species also exhibited as Cr5+ ( g = 1 . 9 7 1 , AH=49G); no temperature dependency was found. Adsorption of water reduces line width to 39G but there was no temperature dependency. T h i s clearly indicates that Cr species on Zr02 is in the form of Cr5+ in a square pyramidal coordination. CO reduction of CrOx/Si02 and CrOx/Zr02 at 773 K resulted in a complete removal o f Cr5+ species. Though Cr3+ was found (g=1.97, A H = 8206) on CrOx/Si02, no signal was obtained on CrOx/Zr02 after reduction. Cr species can be reduced regardless the coordination structures, but square pyramidally coordinated Cr species are reduced more easily and deeply. Temperatureprogrammed-reduction of CrOx/Si02 and CrOx/Zr02 also supported this observation. Thus CrOx on SiO2 was in the form o f tetrahedral coordination while that on Zr02 was in a well-dispersed square pyramidal coordination. Cr K-edge XANES of CrOx/Si02 (0.2 mmol/g) clearly shows the pre-edge peak of 1s-3d transition, which indicates the existence o f tetrahedrally
573
coordinated Cr6+. This observation supports that Cr ions on Si02 was
i n the form of tetrahedral coordination. In conclusion, Cr2O3 crystals grow on both ZrO2 and SiO2 when the amount of Cr loaded was increased. However, there is a difference in crystal growth on these supports as illustrated in F i g . 6 . CrOx is finely dispersed or two dimensionally dispersed on Zr02 support with a square pyramidal coordination at lower Cr content and grows t o Cr2O3 at higher Cr content, while CrOx dispersed on Si02 is i n the form of tetrahedral coordination with three dimensional crystal growth regardless t h e amount o f Cr loaded. It has been Fig. 6. Model of crystal growth of reported that terminal oxygens CrOx on Zro2 and si02bound to tetrahedrally coordinated Cr are less active (ref. 20). Thus, the lower activity toward CO oxidation on CrOx/Si02 can be interpreted i n terms of the lower surface area and the tetrahedral coordination of Cr ions. The higher activity on CrOx/Zr02. on the other hand, is based on the higher effective surface area and the square pyramidal coordination.
3. D.L.Myers and J.H.Lunsford. J . Catal., 92 (1985) 260. 4. E.M.Ezzo, N.A.Yousef and H.S.Maznar, Surf. Technol., 14 (1981) 65. 5. M.Richter, E.Alsdorf, R.Fricke, K.Jancke and G.Ohlmann. Appl. Catal., 24 (1986) 117. 6. F.D.Hardcastle and 1.E.Wachs. J. Mol. Catal., 46 (1988) 173. 7. A.Cimino, D.Cordischi, S.D.Rossi, G.Ferraris, D.Gazzoli, V. Indovina, G.Minelli, M.Occhiuzzi and M.Valigi. Proc. 9th Intern. Congr. Catal., 3 (1988) 1465. 8. T.Yamaguchi, Y.Nakano. T.Iizuka and K.Tanabe, Chem. Lett., (1976) 1053. 9. T.Yamaguchi and J.W.Hightower. 3. Am. Chem. SOC., 99 (1977) 4201. 10. Y.Nakano. T.Yamaguchi and K.Tanabe. J. Catal., 80 (1983) 307. 11. H.Shima and T.Yamaguchi, 3 . Catal., 90 (1984) 160.
574 12. T.Yamaguchi, H.Sasaki and K.Tanabe, Chem. L e t t . , (1973) 1017. 13. K.Maruya, A, Inaba. T.Maehashi, K.Domen and T.Onishi. J. Chem. SOC. Chem. Commun., (1985) 487 : K.Maruya, T.Maehashi. T.Haraoka, S.Narui. K.Domen and T.Onishi, i b i d . , (1985) 1494. 14. B.-Q.Xu, T.Yamaguchi and K.Tanabe. Chem. L e t t . , (1987) 1053; B.-Q.Xu. T.Yamaguchi and K.Tanabe, i b i d . . (1988) 281: B.-Q.Xu, T.Yamaguchi and T.Yamaguchi and K.Tanabe, Mat. K.Tanabe, i b i d . , (1989) 149 : 6.-Q.Xu, T.Yamaguchi and K.Tanabe. Appl. Chem. Phys., 19 (1988) 291 ; 6.-Q.Xu, Catal.. i n p r i n t . 15. T . I i z u k a , Y.Tanaka and K.Tanabe. J. Mol. Catal., 17 (1982) 381. 16. T.Iizuka, Y.Tanaka and K.Tanabe, 3. Catal., 76 (1982) 1. 17. M.Ichikawa, M.Sekizawa, K.Shikakura and M.Kawai. J . Mol. Catal., 11 (1981) 167. 18. T.M.Salama and T.Yamaguchi. Proc. I n t e r n . Symp. Acid-Base Catal.. Hokkaido U n i v e r s i t y , Sapporo, Kodansha S c i e n t i f i c , Tokyo, 1989. 19. N.Mizuno, H . F u j i i and M.Misono, "Shokubai" ( C a t a l y s t ) , 30 (1988) 392. 20. Y.Iwasawa. Y.Sasaki and S.Ogasawara, J. Mol. Catal., 16 (1982) 27.
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
Methane oxidative coupling by definite compounds( e.g. perovskite, cubic or monoclinic structure,. . . ) obtained by low temperature processes
Abstract. We investigate the catalytic properties of several rare earth definite compounds ( e.g. perovskite, cubic or monoclinic structure,...). We notice that the nature of the rare earth oxygen environment is in relation with the selectivity. Coulombian energy computations show clearly these relationships.
Introduction. Since the last ten years we observe an increase of researches concerning the production of chemicals and liquid transportation fuels using basically methane rather than syngas. The conversion of natural gas into methanol, ethylene or ethane is one of the solutions retained. The coupling oxidative reaction of methane into C2 hydrocarbon seems to be the most promising way. Since the first works of Lunsford (l), Keller and Bahsin (2),a large scale catalyst screening test and evaluation exercises have been carried out. But the diversity of the experimental works does'nt permit to draw any broad conclusions concerning catalyst activity and selectivity. Four catalytic systems can be considered: oxides from IIA group, rare earth oxides, transition metal oxides, oxides from IIIA,IVA and VA ( 3 ) . All these oxides are in fact doped by alcaline using the impregnation process. It has been recently discovered a new group formed by definite compounds from the former oxides. We can notice that the greater activity of lithium oxide deposited on magnesium oxide in comparison with sodium oxide on MgO has been assumed by the fact that Li+ and Mg"
have close ionic distances and thus lithium
oxide can easily form s o l i d solutions with MgO ( 4 ) .
575
The studies of reaction mechanisms have shown clearly the presence of surface species such as 0 - ,02- or 02*- as mediators of the creation of methyls radicals and the possibility to have sites or species which are specific for selective or non selective oxidation. For all these facts we try to bind the reactivity and the selectivity of several rare earthdefinite compounds with their crystallographic structure and peculiarlywith the bond energy of the rare earth-oxygen bond. We have prepared and tested rather different compounds such as: cubic perovskite, pyrochlore and garnet compounds
.
Experimental A.Catalytic compound preparations. i) LnLiO2 (Ln= La, Nd, Sm) The catalyst are prepared from aqueous solution of rare earth nitrates, lithium carbonate or lithium hydroxide. The solids are obtained by evaporationtodrynessat110"C-120°Cofthesolution
or suspension in which
the rare earth is precipited as oxalate by oxalic acid (pH = 2) or as hydroxide by ammonia (pH 9 ) . After heat-treatment at 7 5 0 ° C for 24h we
-
obtain the definite compounds asshownin figure 1.
I A \
I\
Figure 1: X-ray pattern of LaLi02 structure. ii) Lay03
We disolve log La(N03)3,6H20 and 8g of Y(N03)3,4H$
in distilled water at
room temperature. We introduce 75 ml of aqueous solution of ammonia. The
577 precipitation occurs and the white suspension is filtered and washed with pure water. A 15 hours drying treatment is carried out and the resulting solid is heat-treated at 680°C for two hours under atmospheric conditions. X-Rays analysis (figure 2) shows the presence of bixbyite structure. A heat-treatment at 1000°C brings
the compound to the perovskite structure.
( 1 1 LaY03 b i x b y i t e ( 2 ) LaY03 p e r o v s k i t e
20
2s"'
I
Figure 2: X-ray pattern of Lay03
iii) Pyrochlore compounds, A2B2O7. For these preparation we use a sol-gel process. 5g of La203 are dissolved in propionic acid and we introduce 12,88g of zirconium propylate. A clear solution is obtained after stiring. The solution is brought at acid boiling point and the exces of organic acid is removed. We obtain a transluscent solid which is
calcined
at 725°C for two hours. X-rays analysis (figure
3 ) on the final compounds show the presence o f pyrochlore type compounds.
1
-
Figure 3 : X-ray pattern of La2Zr207 pyrochlore compound.
578
B. Catalytic activity. The activity and the selectivity of the various samples are determined in a fixed bed quarz reactor (6.6 mm I.D.) under the following conditions: inlet temperature
- 600-750°C;feed gas pressures: 0.133 atm CH4, 0.0665 atm 02
and 0.8 atm He; gas flow: 4.5 l/h catalyst(STP); weight of catalyst- 0.67g; ratio CH4/O2=2 ( 2CH4
+
02
--> C2H4 + 2H20).
Methane conversion is calculated as: moles of transformed CHq/rnoles of initial CH4
*loo.
Selectivity in product i is defined as moles of
transformed CH4 in product i / moles of transformed CH4
product i: conversion * selectivity
*
*
100; yield in
100.
Results and discussion. i ) LnLiO2
compounds.
Table 1 shows the influence of the preparation method on C2 selectivity thought X-ray analysis show the same crystallographic structures before and after reaction. But we notice a relationship between specific area and selectivity in C2 for SmLi02aswellas f o r NdLi02. By FT-IR we see a very weak carbonate band. Catalyst preparation
surface area (m2/d
C2 selectivity%
Li20
3.6
4.4
Li2CO3
4.2
14
nitrate + Li2CO3
4.8
24.6
Samarium hydroxyde " I'
oxalate
+
+
+ LiOH + Li2CO3 nitrate + Li2CO3 oxalate + LiOH
oxalate
Neodymium oxalate
5.2
28.7
0.6
14.2
4.0
27.1
5.75
38
Table 1: Relationship between catalyst preparation, surface area and selectivity.
id Lay03 compounds. This compound can take two crystallographic structures, bixbyite and perovskite. In the bixbyite structure the lathanum and the yttrium anions are both placed in a mean eight-fold oxygen environment and in perovskite structure the lanthanum cation is strictly placed in a 12-fold oxygen environment and thus yttrium in a 6-fold environment. For a low temperature process we reach firstly the bixbyite structure and the perovskite
579 structure appears for a heat-treatment at 1000°C. But this fact leads also to specific surface changes as we can see in table 2. catalyst
conversion
selectivity
%
%CH4 %02
C2H4
C2H6
C2
26.9
12.3
11.2
23.5 6 1 . 2
8
6.3
14.3
75.7 1 0 . 0
0.001
0.4
0.4
84.8
14.7
6.3
4.9
5.7
12.3
18.0
75.3
6.6
6.8
3.9
24.8
97.8
99.2
C02
CO
before after
15.3
22 23.6
14 11.2
Table 2 : Relation ships between structure and catalytic datas for Lay03 compounds, In the case Lay03 with perovskite structure,the C2 selectivity is close or equal to zero for a CH4 conversion rate similar to the one obtained with bixbyite structure catalyst. In comparison the hexagonal oxides, La203 and Y2O3, with a same oxygen environment for the cationas in the bixbiyte structure, have close catalytic capabilities. A s a general rule,the perovskites are well known for their total oxidative capabilities into CO and C02 for CH4 gas ( 6 ) . A s an example,the SmAlO3 perovskite ( conversion 2 1 . 3 8 , selectivity 6 . 6 % ) is in agreement with last results.
These results show that the metal-oxygen distance and thus the metal-oxygen bond energy can be the driving data of the oxidative coupling of methane. iii)pyrochlore compounds. Three kinds of pyrochlore have been prepared and tested: Ln2Zr207, Ln2Sn207, Ln2Ti207. In the pyrochlore structure both fold oxygen environment.
cations have a six-
Table 3 gives the catalytic conversion and
reactivity for samarium, gadolinium and europium pyrochlores. We notice that the C2 yield results of these three pyrochlores follow the
decreasing S o Zr> 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)
<
z
at which opt (m) are within the required limits on the aluminium hydroxide pro-
(s) and perties and mass preparation conditions.
The formation of the stable disperse
system from the structurated precipita-
te or gel is known to be caused by the formation of the double electric layer on particles surface.
Let us consider now the peptizator distribution in
the bulk of aluminium hydroxide. The concentration of peptizator added after mixing with aluminium hydroxide is:
3 V are mass (kg) and volume (m ) of the peptizator, respectively; pep' Pep Wo is moisture content evaluated by drying at l l O ° C (kg H20 / kg A1203); m is mass concentration of A1 0 in hydroxide (kg); is liquid phase density (kg/ 2 3 3 m ). The mass balance equation calculated on the basis of A1203 is the following:
where m
2 is specific sorption of substance - peptizator (kg/m ) ; A is speci2 fic area of A 1 0 (m / g ) ; C is equilibrium peptizator concentration in the in2 3 P 3 termicellar liquid of hydroxide (kg/m ). In the left part of eq. (2) the first component expresses the liquid consumption for sorption and chemical interaction with aluminium hydroxide particles, the second component - for creation of equilibrium concentration of pepti-
where
d
zator in intermicellar liquid. The value of M(a ) can be expressed as follows opt
where M and MA1203 are molecular masses of peptizator and A1203, respectiPep vely The formula ( 3 ) can be simplified by taking into consideration the following: (1) while using the nitric acid, the ratio MAl / Mpep is equal to 1.619; (2) A values of oxides and hydroxides of pseudoboeh&& structure (A ) are close h enough; (3) the value of V /m is small. The formula (3) after simplification Pep looks like:
.
588
A
1;
”
E
4
M (a,opt)
0.9
L)
1
7 0.84
24
- 250 I
I
M (ai
0.08
0.06
A
Fig. 2. Dependence of the average strength Sav, the bulk density and the specific area A of alumina granules on the M(a) value for the plastificated mass with alumina concentration 28%
According to the experimental data, 3 kg/m
.
= (4
‘I 1)
kg/m2; C P
=
(14 + - 2)
S o , if the aluminium hydroxide is treated to M(a) = M(a ) , the time needed opt for mass maturation and the time of mass thixotropic setting are achieved,
which establishes high mechanical properties of alumina
granules. As it can be
seen from Fig. 2, the acid treatment of hydroxide up to M(a the alumina
granules characteristics.
opt
) d o e s not change
Sphere Formation and Coagulation The preparation
liquid
of spheric alumina
granules occurs in the column with two
layers: the upper layer is hydrocarbon, the lower one is a coagulant
SO-
lution. In the upper layer of hydrophobic liquid the mass drop is subjected to the surface tension forces which tighten it into the sphere. The granules hardening proceeds in the course of a coagulant diffusion into the volume of sphere and structure formation. The ammonia solution is usually used as a coagulant. For high strength alumina granules preparation it is necessary to establish the complete interaction between mass and coagulant. It was shown that while mould0.1 ing of masses with alumina concentration being less than 25% and M(a)&
589
granules hardening is completed in 30-40 s in 16-19% ammonia solution. If M(a)
is increased, it is necessary to increase the contact time up to 60-80 s.
and to rise the concentration of the ammonia solution. Granules Thermal Treatment The granules drying was carried out by several ways: in air, in a drying box, in aggregates with moving ribbon and heaters over it. The experimental results are presented in Table 1 .
TABLE 1 The influence of drying conditions on alumina Drving of Granules <
-
T (OC)
Method ~~
z (h)
Alumina A(mL/g)
granules characteristics Characteristics
Vp(cm3/g)
Sav(MPa) Smin(MPa)
Smax(MPa)
~~~
in air 20 in a drying box 110 under heat200 ers the same 40-200
48
240
0.31
41.8
20.7
59.2
4
240
0.27
23.5
6.8
38.9
0.5 0.5
2 40 240
0.25 0.28
22.8 31.3
6.0 12.8
39.3 45.3
It is seen that the drying mode does not practically affect the specific area A and pore volume V
but it influences strongly the granule strength. This signiP' ficant strength change at the unchanged porosity could be probably explained
with an increase of residual microtensions with the rise of drying speed. So rapid drying under heaters at T >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 <50 A was determined.
TABLE 4 Structural parameters of Ni-Raney according to difference r.d.c. Re. 2.50 4.30 5.56 6.60 7.47
h
P t (Ni-100%)
88 176 176 353 353
P t (Ni-40%) 35.25 70.50 70.50 141.00 141.00
Pe
33.20 61.80 60 88 114.40 108.90
-
Thus, simulation of catalyst structure made it possible to establish that it is a mixture of an unleached NiAl (or a solid solution on its basis) of high-dispersion nickel and small amounts of NiO. ADSORPTION PROPERTIES, ACTIVITY AND SELECTIVITY OF MA CATALYSTS Effect of initial MA alloy structure on catalyst activity MA catalysts were studied in a model reaction of phenylacetylene hydrogenation. The charge content and the activation duration were varied in MA. Since in storage the MA alloys can relax their active state that is expressed in a lowering of their accumulated energy, studied was also the effect of MA alloy aging on the properties of catalysts. Hydrogenation was carried out in a reactor with intensive stirring in 96% ethanol at 4OoC. Catalyst activity was determined by the rate of reaction with adsorption of 25-75% of required hydrogen volume. Alloys with 25,30,35 and 40 % Ni were studied. The MA duration was 10,20,40 min. Determined was the activity of catalysts made from fresh alloys and those stored for 6 months. Fig.1 shows the dependence of catalyst activity on the factors enumerated. One must note that catalysts from MA alloys at t=40 min are more stable and 15-30% more active than PM ones (when comparison is made between catalysts of similar dispersion and in the same
595
"4
t=10 min
600
F
E \
d
-200
€
3
t=40 min
./
---
-
c
l-4
7:
t=20 min
-
40
30
20
20
40
30
20
40
30
Ni content in initial alloy, 8
Fig. 1. The relation between the catalyst activity in phenylacetylene hydrogenation and MA duration and charge composition. 0 , e - hydrogenation of >C=C< bond A,A- hydrogenation of -C=C- bond -fresh MA alloy - 6-month stored MA alloy
--
conditions).
MA
alloys
The with
Ni*A&j&atalyst To explain structure
least stable in storage were catalysts made from a
high
A1 content and t=10 min.
was the most stable one. the observed facts it is
both of the initial
With all t the
necessary to consider the
and leached MA alloys. It was shown
previously that their phase content is determined not only charge content but by the time of MA (ref.8). activity
with
an increase
of
by
t from 10 to 20 min is due to more
complete reaction of MA and transfer of the rest of non-reacted into
leachable compound.
the
The general growth of
This is,
partly,
Ni
an explanation of the
growth of catalyst activity with an increase of t up to 40 min. But in this
case
more important
is the growth of the degree of alloy
nonequilibrium at the expense of lattice distortions.
No other fac-
tors can explain, for example, the activity growth of catalyst from MA
NiaA160 the phase
change.
content
of
which
after t=20 min does not
These defects are likely to affect the catalyst structure.
The long life-time of defects is seen from the fact that even after 6 month storage the activity of t=40 min catalysts is higher than that of catalysts made of fresh alloys with t=20 min.
596
1
100
I
I
300
I
I
500
Fig. 2. DSC-curves of MA N i X A k 1- fresh ( 4 . 4 8 kJ/mol) 2- 6-month stored (1.95 kJ/mol)
I
T,OC
The decrease of catalyst activity with alloy storage is well correlated with the results of differential scanning calorimetry. From Fig.2 it follows that the DSC-curves recorded on DSC-111 for fresh and aged alloys obey the same rules. However, in the first case the store of excess energy is much higher. Apparently, a higher activity of MA catalysts as compared to PM ones can be explained by a high degree o f disordering. With all t the catalysts from Ni35AlG5 were most active. As to PM catalysts their activity is increased up to 75% A1 (ref.9). As shown above the composition NisA165 is related by a solid solution on the NiAl-basis with a maximum possible deficit of Ni atoms. Alloys with a higher Ni content are close to equilibrium NiA1, whereas with the smaller one they have other phases (NiA13 , NizA13). Apparently, the highest activity of these catalysts is due to the leaching the largest quantity of fact that as a result of MA N&Ak most dispersive Ni is formed, which is fixed on unleached NiAl particles. The thing is that richer nickel alloys are leached to a less extent and contain less Ni skeleton phase. Whereas Ni2A13 and NiA13 leaching yields are not so fine particles of an active metal. Selectivity of MA catalysts and their adsomtion vroverties MA catalysts are more often of a higher selectivity then PM ones. In hydrogenation of phenylacetylene into styrene and of styrene into ethylbenzene,the relationship of rates varies from 1.8-2.0 for t=10 min to 2.3-3.5 for alloys with t=20 min and thus selectivity reaches 90-93%. With PM catalysts from alloys of the same dispersivity (5-8&) selectivity is 8045%. Catalyst selectivity is closely connected with adsorption properties which were studied in this work by the method of hydrogen TPD. Fig.3 presents TPD-curves of catalysts from fresh and 6-month stois the red MA alloys. The surface of a catalyst from fresh Niq~Al~jo most energy-homogeneous. It adsorbs, mainly, weakly bound hydrogen
597
139
I
100
300
11 II
I /
100
100
300
Fig. 3 . TPD-curves of MA catalysts.
--
fresh alloy 6-month stored alloy
---
that is unable to displace styrene from the surface, reason of its lowest selectivity -75%. (S=93%) is the one from fresh MA
300T,"G
which
is the
The most selective catalyst
Ni35A165 , the TPD curve of which
is moredisplaced to the high-tempereture range. that the arm in the low-temperature range
It is
belongs
of interest
only to
NiA13
catalyst, that contains nickel in a less dispersive state than that made from WiA1. Its selectivity S=87%. Fig.3 shows that storage, practically, does not change the TPDcurve of a NiqoAl60 catalyst. Its selectivity does not change either. It is of interest to compare the selectivity of catalysts MA alloys after 6-month storage in another model reaction hexenehydrogenation.
same
from
that of
Table 5 presents results for MA catalysts
for the sake of comparison for PM ones of the and of coarser
-
and
dispersiveness
dispersion. This comparison allows to find out whe-
ther the high activity of MA catalysts is due to their high dispersiveness or it is aconsequence of their microstructure.
It follows
from the table the MA catalyst activity is 2-3 times higher, which favours the second supposition. Their selectivity is also much higher
than that of coarse-dispersion PM catalysts and differs but
little from highly dispersed
ones. The relatively low activity
of
the latter can be due to partial oxidation of alloys dissipated
in
an air separator. It is known that even introduction of oxide-forming additions or redox treatment (refs.lO,ll),suppressing
migration
of the >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.
.
-
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
611
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 .
612
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
<4
oxides:
The e f f e c t o f l a c t i c a c i d added i n an aqueous
medium i s v e r y c l e a r , w h i l e t h a t o f e t h y l e n e g l y c o l i s small.
(3) 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 (<
ca. 1 nm) are observed in the smallest particle.
This
the size
suggests that
hydrogenolysis process in a particle is greatly influenced by the size
of
particle. In the medium-sized particle which is the representative particle
the the in
the powder, the size of ruthenium clusters is less than about 2 nm. Both of the ruthenium clusters and alumina are amorphous because
FigI.6. TEM micrograph.
no
electron diffraction
667 pattern was observed. Amorphous ruthenium clusters in amorphous alumina may prepared because the fine droplets were rapidly dried and then rapidly
be
reduced
by hydrogen. The adsorption of carbon monoxide on supported ruthenium has been extensively studied by IR spectroscopy (ref. 4 ) . General
agreement
the presence of three IR bands. The LF band at 1990-2030 cm-l
is
exists
on
assigned
to
the vibration of carbon monoxide linearly bonded on ruthenium crystallites. The bands at 2080 and 2140 cm-l correspond to the vibrations of a multicarbonyl. In a recent investigation (ref. 5 ) , this species was shown
to
be
a
tricarbonyl
associated with Ru2+ cations bonded directly to the support. Figure
7
shows
the
IR
spectrum of
carbon monoxide
adsorbed
on
ruthenium-alumina composite. Three absorption bands are observed at 2040, and 2140 cm-l, although the bands assigned to the vibrations are small. Comparison with the result on the impregnated
of
multicarbonyl
Ru/A1203
the same temperature (ref. 4) reveals that the LF band on
the
reduced
present
shifts to high frequency side. No clear interpretation can be made at
v
but the amorphous nature of the present
sample may
because the
surface
electronic
structure
and
be
one
density
of
the 2080
of
at
sample
present,
the
reasons
adsorbed
carbon
monoxide may be different.
a
x
"
2
0
0
30 60 90 1 2 0 1 5 0 1 8 0 Time
(min)
Fig. 8. Catalytic activity for
Fig. 7. IR spectrum of adsorbed
benzene hydroganation.
carbon monoxide.
Reduction temperature = 673 K , Reaction temperature
=
423 I:.
(a) Obtained by direct synthesis, (b) Wixed oxide sample.
668 Catalytic activity Catalytic activities for benzene hydrogenation were measured at 423 reaction product was cyclohexane, and no other
product
reaction condition studied. The catalytic activity
of
was the
detected mixed
K.
The
in
the
oxide,
BET
surface area of which was 4 . 1 m2/g, was also measured after reduction at 673 for 16 h. The activities of both samples decrease a little with as shown in Fig. 8 . The catalyst obtained by the
direct
much higher (about 8 times) activity than that prepared
reaction time
synthesis by
(a) shows
reduction of
mixed oxide (b). REFERENCES 1
J. G. Highfield, A. Bossi and F. S. Stone, Proc. 3rd Intern. Symp. on
Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, 1982, B5. 2
H. Imai and F. Orito, Nippon Kagaku Kaishi, ( 1 9 8 4 ) 851-855. Hannan, J. Phys. Chem., 64 ( 1 9 6 0 ) 1526-1530.
3
J.
B. Peri and R.
4
F.
Solymosi and J. Rasko, J. Catal., 1 1 5 ( 1 9 8 9 ) 107-119;
5
G. H. Yokomizo, C. Louis and A. T. Bell, J. Catal., 120 ( 1 9 8 9 ) 1-14.
B.
K
and references there in.
the
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
669
DESIGNED CATALYSTS FOR HYDRODECHLORINATION, REDUCTION AND REDUCTIVE
A M IMAT I0N RE ACT I0N S J.L.Margitfalvi, S.GbbGlBs, E.Tdlas and M.Hegediis Central Research Institute f o r Chemistry o f the Hungarian Academy of Sciences, 1525 Budapest, P O B 17, Hungary SUMMARY Palladium containing alumina supported catalysts were designed f o r hydrodechlorination o f chlorobenezne and f o r conversion of 4-chloro-Z-nitroaniline, in which both hydrodechlorination and r e duction steps are involved. Best results were obtained on catalysts containing palladium in ionic form. F o r reductive amination o f acetone a skeletal nickel catalyst and its tin modified version was designed. On thesecatalysts the ratio of primary to secondary amines could be controlled and the formation o f isopropyl alcohol was strongly suppressed. INTRODUCTION There are different approaches i n catalyst design[l]. In our approach the design of an active and selective catalyst f o r a given process is based on (i) the primary knowledge of the reaction network and the reaction mechanism and (ii) the use of different types of Controlled Surface Reactions (CSRs) t o introduce and stabilize either the active component or the added modifier of the given catalyst i n the required form and environment. In this work results obtained in :wo case studies will b e given and discussed: design of catalysts (i) f o r hydrodechlorination o f chlorobenzene and conversion o f 4-chloro-2-nitroaniline (CNA) to orthophenylendiamine (OPDA) and f o r (ii) reductive amination o f acetone. CATALYST DESIGN o f catalyst for hydrodehalogenGion and f o r reduction of the nitro group. The dehalogenation of aryl halides can be carried out i n stoichiometric or catalytic reactions i n the presence of bases. Reducing agents as LiA1H4 o r NaBH4 are used i n the stoichiometric reactions 1 2 1 . The reaction can be carried out in the presence of homogeneous and heterogeneous catalyst under condition o f transfer hydrogenation
Oesign
670 [3,4] or i n
the presence o f gas phase hydrogen [ 5 , 6 1 .
The d e s i g n o f c a t a l y s t s f o r h y d r o d e h a l o g e n a t i o n i s b a s e d on t h e m e c h a n i s m o f o x i d a t i v e a d d i t i o n o f a r y l h a l i d e s t o d8 o r d” s i t i o n metals
+
ArX
-
(reaction
M+”Lm
tran-
(111. +n+2 Ar-K LmX
(1)
I t has a l s o been proposed t h a t r e a c t i o n (11 i s t h e r a t e l i m i t i n g s t e p i n h y d r o d e h a l o g e n a t i o n o f a r y l h a l i d e s i n t h e p r e s e n c e o f homogeneous a n d m e t a l l o c o m p l e x c a t a l y s t s [ 7 , 8 ] .
B a s e d o n t h i s know-
l e d g e i t has been s u g g e s t e d t h a t t h e i n t r o d u c t i o n o f p a l l a d i u m i n t o the support not i n m e t a l l i c but i n i o n i c form should increase the rate
o f h y d r o d e h a l o g e n a t i o n p r o v i d e d t h e mode o f s t a b i l i z a t i o n o f
t h e i o n i c f o r m o f p a l l a d i u m can be found. I n the preparation o f hydrodechlorination catalyst the following
-
surface reactions are involved: 3-OH
+
C4HgLi
n ( 1 ) +PdC12
___b
a(-O)nPdC12-n
+
H2
3 O L i
(I)
+
g(-O)nPdC12-n (-OInPd
‘qH’10
(2)
nLiCl
(3)
(2-nlHC1
(4)
+
+
R e a c t i o n (21 and ( 3 ) has been w i d e l y used f o r t h e p r e p a r a t i o n o f s i l i c a supported metallocomplex c a t a l y s t s [9,10].Characteristic f e a t u r e o f t h e c a t a l y s t s p r e p a r e d i n t h i s way i s t h e p r e s e n c e o f a n c h o r e d i o n i c p a l l a d i u m on t h e a l u m i n a s u p p o r t . Design o f c a t a l y s t f o r r e d u c t i v e amination o f acetone. Reactions involved i n r e d u c t i v e amination o f acetone are given i n Scheme I. CH3COCH3
+ NH3, +H2
CH3CHNH2CH3
+(CH31 ZC0,+H2 -HZO
(CH3CHCH3) 2NH
CH3CHOHCH3 Scheme 1 .
F o r r e d u c t i v e a m i n a t i o n o f a c e t o n e w i t h ammonia t w o t y p e s of c a t a l y s t s were d e s i g n e d :
(i)
s k e l e t a l N i c a t a l y s t prepared form a
N i - A 1 a l l o y and ( i ) i t s t i n m o d i f i e d v e r s i o n s .
Both types o f cata-
l y s t s were used i n a c o n t i n u o u s f l o w gas phase r e a c t o r . q u i r e m e n t s f o r t h e s e c a t a l y s t s were as f o l l o w s : mechanical s t a b i l i t y ,
The
re-
h i g h t h e r m a l and
high rates f o r the formation o f both primary
and secondary amines and s u p p r e s s i o n o f t h e f o r m a t i o n o f i s o p r o p y l alcohol
f r o m acetone.
Upon p r e p a r i n g t h e s k e l e t a l N i c a t a l y s t s t h e h i g h a c t i v i t y t o wards t h e f o r m a t i o n o f b o t h p r i m a r y a n d s e c o n d a r y a m i n e s r e q u i r e s optimalization
o f t h e l e a c h i n g p r o c e s s and u s i n g a t h e r m a l t r e -
671 a t m e n t p r o c e d u r e a b o v e 200OC. I t h a s b e e n d e m o n s t r a t e d e a r l i e r t h a t a c i d - b a s i s c a t a l y s t s can b e i n v o l v e d i n t h e f o r m a t i o n o f s e c o n d a r y amines [10,11].
Based on t h e above knowledge t h e p r e p a r a t i o n o f t h e
s k e l e t a l c a t a l y s t was aimed t o c r e a t e n o t o n l y h i g h l y a c t i v e m e t a l l i c s i t e s b u t t o f o r m t h e p r e c u r s o r o f s i t e s r e q u i r e d for t h e c o n d e n s a t i o n s t e p ( s e e Scheme I . ) . The f o r m a t i o n o f i s o p r o p y l a l c o h o l c o u l d be s t r o n g l y d e c r e a s e d by s e l e c t i v e p o i s o n i n g o f s i t e s r e s p o n s i b l e f o r t h e h y d r o g e n a t i o n of t h e carbonyl group.
The s e l e c t i v i t y o f t h e s k e l e t a l n i c k e l c a t a -
l y s t t o w a r d s t h e f o r m a t i o n o f i s o p r o p y l a l c o h o l was c o n t r o l l e d v i a s e l e c t i v e p o i s o n i n g o f t h e n i c k e l s i t e s b y t i n u s i n g CSRs b e t w e e n a d s o r b e d h y d r o g e n on t h e n i c k e l s i t e s a n d t i n a l k y l s w i t h g e n e r a l
E l n . D e t a i l s on s u r f a c e c h e m i s t r y i n v o l v e d i n (4-n) t h e a b o v e CSRs h a s b e e n d i s c u s s e d f o r P t / A 1 2 0 3 [ 1 2 ] . f o r m u l a o f SnR
EXPERIMENTAL
Catalyst preparation [i)
Solvents
used were c a r e f u l l y d r i e d and d e o x y g e n a t e d . T h e a l u m i n a s u p p o r t was t r e a t e d i n v a c u u m a t S X I O - ~ b a r i n t h e t e m p e r a t u r e r a n g e o f 150-
SOOOC. R e a c t i o n s (2) a n d ( 3 ) w e r e c a r r i e d o u t i n n - h e x a n e a n d a c e t o n e , r e s p e c t i v e l y . A f t e r c o m p l e t i o n o f r e a c t i o n (2) a n d ( 3 ) a w a s h i n g p r o c e d u r e w a s u s e d t o r e m o v e u n r e a c t e d bu:y?lithiurn(Euiij PdC12,
respectively.
or
T h e f i n a l s t e p o f t h e c a t a l y s t p r e p a r a t i o n was
a h e a t t r e a t m e n t i n n i t r o g e n or h y d r o g e n a t m o s p h e r e i n t h e t e m p r e r a t u r e r a n g e o f 15O-20O0C.
F u r t h e r d e t a i l s on c a t a l y s t p r e p a r a t i o n
w i l l be g i v e n i n R e s u l t s and D i s c u s s i o n . [Ti) Preparation of c a t a l y s t s f o r reductive amination.
Granular
s k e l e t a l n i c k e l c a t a l y s t w i t h p a r t i c l e s i z e o f 3 - 5 mm was p r e p a r e d by l e a c h i n g a N i - A 1
a l l o y c o n t a i n i n g 50 w t % n i c k e l .
Half of t h e
amount o f a l u m i n a was l e a c h e d o u t w i t h 3 w t % NaOH-water
a t 50'C
solution
f o r 12 h o u r s . A f t e r l e a c h i n g t h e c a t a l y s t was w a s h e d w i t h
d i s t i l l e d w a t e r a n d w a s k e p t u n d e r a n a q u e o u s s o l u t i o n h a v i n g pH=9. P r i o r t o t h e m o d i f i c a t i o n w i t h t i n t h e c a t a l y s t was d r i e d i n flowing n i t r o g e n a t 12OoC f o r 4 hours.
After drying t h e catalyst
w a s t r e a t e d i n h y d r o g e n a t 200 or 300'C
f o r 2 h o u r s f o l l o w e d by
c o o l i n g t o room t e m p e r a t u r e i n h y d r o g e n . catalyst
The modification of t h e
w i t h t i n a l k y l c o m p o u n d s was c a r r i e d o u t a t 5OoC u s i n g
20 g o f g r a n u l a r s a m p l e a n d 100 c m 3 o f b e n z e n e s o l v e n t . Decomposit i o n o f s u r f a c e complex formed i n t h e r e a c t i o n o f t i n a l k y l s w i t h h y d r o g e n a d s o r b e d on nickel was p e r f o r m e d i n h y d r o g e n
using a heating
672
a n d a f i n a l t e m p e r a t u r e o f 25OoC. F u r t h e r d e t a i l s
r a t e o f 2'C/rnin
on t h e p r e p a r a t i o n w i l l b e g i v e n i n R e s u l t s a n d D i s c u s s i o n . Catalyst characterization
T h e p h a s e c o m p o s i t i o n o f s k e l e t a l n i c k e l c a t a l y s t s was s t u d i e d by u s i n g a P h i l l i p s 1700 p o w d e r d i f f r a c t o m e t e r e q u i p p e d g r a p h i t e m o n o c h r o m a t o r a n d CuK, ed a t 2OoC u s i n g a JEOL JES-FE3X
with a
r a d i a t i o n . E 5 R s p e c t r a were r e c o r d s p e c t r o m e t e r . XPS measurements
were t a k e n b y u s i n g a V G ESCA 3 s p e c t r o m e t e r w i t h a n a l u m i n i u m Ka r a d i a t i o n s o u r c e . A l l b i n d i n g e n e r g i e s were r e f e r r e d t o t h e A12p
l i n e ( B t = 7 4 . 7 eV). Catalytic reactions The h y d r o d e c h l o r i n a t i o n o f c h l o r o b e n z e n e , and c o n v e r s i o n 4 - c h l o ro-2-nitro-aniline
t o o r t h o p h e n y l e n e d i a m i n e was c a r r i e d o u t u n d e r
d i f f e r e n t r e a c t i o n c o n d i t i o n s u s i n g s t i r r e d t a n k a n d t r i c k l e bed r e a c t o r s i n t h e p r e s s u r e r a n g e o f 1-70 b a r . The r e d u c t i v e a m i n a t i o n o f a c e t o n e was s t u d i e d i n a c o n t i n u o u s f l o w g a s p h a s e r e a c t o r a t 20-50 b a r a n d 160-2OO0C.
Both i n t h e hydrodehalogenation and reduc-
t i v e a m i n a t i o n t h e r e a c t i o n p r o d u c t s were a n a l y s e d by g a s chrornatography. RESULTS AND
DISCUSSION
Preparation of c a t a l y s t s f o r hydrodehalogenation ( i ) S t u d y o f s u r f a c e r e a c t i o n s ( 2 ) and ( 3 ) . E x p e r i m e n t a l v a r i a b -
l e s u s e d i n c a t a l y s t p r e p a r a t i o n were a s f o l l o w s : t e m p e r a t u r e o f dehydroxylation , amount o f E u L i u s e d ,
temperature and d u r a t i o n o f
r e a c t i o n s ( 2 ) a n d (31, mode o f r e m o v a l o f B u L i , f i n a l treatment.
condition of t h e
P r e f e r e n c e w a s g i v e n for e x p e r i m e n t a l c o n d i t i o n s
r e s u l t i n g i n h i g h p a l l a d i u m l o a d w i t h a v o i d i n g r e d u c t i o n o f PdC12 t o metallic palladium.
Conditions o f t h e p r e p a r a t i o n and properties
o f c a t a l y s t s p r e p a r e d a r e s u m m a r i z e d i n T a b l e 1.
I n r e a c t i o n ( 2 ) e x c e s s E u L i was u s e d . Washing and e x t r a c t i o n w i t h n - h e x a n e a p p e a r e d t o b e t h e m o s t e f f e c t i v e mode f o r t h e r e m o v a l o f u n r e a c t e d BuLi a d s o r b e d o n t o t h e A 1 2 0 3 .
Decomposition of t h e
u n r e a c t e d E u L i by h e a t t r e a t m e n t r e s u l t e d i n r e d u c t i o n o f P d C 1 2 t o
m e t a l l i c p a l l a d i u m c o m p a r e c a t a l y s t s N o 2 a n d 4 . U n d e r 50°C t h e r a t e of
s u r f a c e r e a c t i o n ( 3 ) was v e r y low. A t h i g h e r t e m p e r a t u r e r e a c t i o n ( 3 ) r e d u c t i o n o f PdClZ
a n d upon i n c r e a s i n g t h e d u r a t i o n of was o b s e r v e d
( see
No 3 a n d 7 ) . T h e f o r m a t i o n o f m e t a l l i c p a l l a -
673 Table 1 Conditions o f the preparation o f i o n i c palladium catalysts.
Temperature o f dehydroxylation G i v e n i n mmol/g A f t e r r e a c t i o n 1 2 ) t r e a t m e n t a t 15OoC f o r 1 h o u r a t I x I O - ~ b a r A f t e r r e a c t i o n ( 3 ) t r e a t m e n t a t 4OO0C f o r 2 h o u r s i n H2 P a r t i c l e size t 0 . 0 4 5 mm, i n o t h e r s a m p l e s : 0 . 3 1 - 0 . 6 3 mm. d i u m was a l s o o b s e r v e d i n t h e p r e s e n c e o f s m a l l amount
o f water
i n t r o d u c e d i n t o t h e a c e t o n e t o i n c r e a s e t h e s o l u b i l i t y o f PdC12. ( i i j 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 b y ESR a n d XPS. ESR s i g n a l w i t h g = 2 . 0 0 4
A narrow
was d e t e c t e d i n c a t a l y s t s c o n t a i n i n g
T h i s s i g n a l was v e r y s t a b l e n o c h a n g e s i n t h e
i o n i c Fd.
g v a l i i e was
o b s e r v e d a f t e r h e a t i n g i n n i t r o g e n o r h y d r o g e n a t 200°C.
No E S R
s i g n a l was d e t e c t e d on c a t a l y s t s p r e p a r e d b y c o n v e n t i o n a l t e c h n i q u e
o r on l i t h i a t e d a l u m i n a . The o b s e r v e d ESR s i g n a l was a t t r i b u t e d t o a f r e e e l e c t r o n o r i g i n a t e d f r o m e l e c t r o n i c i n t e r a c t i o n between i o n i c p a l l a d i u m and t h e a l u m i n a s u p p o r t Table
1.
[ 1 3 ] . XPS r e s u l t s a r e g i v e n i n
The b i n d i n g e n e r g i e s a r o u n d 3 3 5 . 0
s i g n e d t o m e t a l l i c and i o n i c p a l l a d i u m ,
and 336.7
respectively
eV w e r e as-
[141. Catalyst
c o n t a i n i n g i o n i c p a l l a d i u m h a d a r e l a t i v e l y b r o a d p e a k w i t h FWHM around 4.0
eV,
whereas sample c o n t a i n i n g m e t a l l i c p a l l a d i u m had a
n a r r o w p e a k w i t h FWHM a r o u n d 3 . 0
eV.
XPS measurements s t r o n g l y
d i c a t e c t h a t a n c h o r e d i o n i c p a l l a d i u m i s s t a b l e u p t o 200'C hydrogsn atmosphere,
however,
h e a t i n g a t 400'C
in-
even i n
r e s u l t e d i n reduc-
t i o n o f t h e i o n i c s p e c i e s t o m e t a l l i c one. Preparation o f catalysts f o r reductive amination ( i1 P r e p a r a t i o n a n d
c h a r a c t e r iz a t i o n o f s k e 1e t a 1 n ic -k e 1c a t a l y s t s .
674
I n t h e l e a c h i n g p r o c e s s d i l u t e d NaOH was u s e d . O n l y h a l f o f t h e a l u m i n i u m i n t h e a l l o y was l e a c h e d o u t , I n t h i s way t h e h i g h m e c h a n i c a l s t a b i l i t y o f t h e a l l o y c o u l d b e m a i n t a i n e d . The c o n d i t i o n o f l e a c h i n g was f a v o u r a b l e f o r t h e f o r m a t i o n o f oxygen c o n t a i n i n g s u r f a c e s p e c i e s of aluminium. The A 1 and N i c o n t e n t o f t h e c a t a l y s t s was 22 a n d 5 4 w % , r e s p e c t i v e l y , X R O m e a s u r e m e n t s p e r f o r m e d on t h e thermally t r e a t e d s k e l e t a l nickel c a t a l y s t indicated t h e presence of m e t a l l i c N i ,
A13Ni2,
AlNi,
A 1 ( O H 1 3 a n d AlO(OH) p h a s e s .
NiO,
( i i ) Modification of t h e s k e l e t a l n i c k e l c a t a l y s t w i t h t i n .
Sur-
f a c e r e a c t i o n b e t w e e n h y d r o g e n a d s o r b e d on n i c k e l and t i n a l k y l compounds h a v e b e e n u s e d for t h e m o d i f i c a t i o n o f s k e l e t a l n i c k e l by tin.
S u r f a c e r e a c t i o n s i n v o l v e d i n t h e m o d i f i c a t i o n can be w r i t t e n
a s follows: xNiHa
+
SnRnC14-n
Nix-SnRn-xC14-n
Nix-SnRn-xC14-n
& A
Nix-Sn
+
(n-x)RH
+
xRH
(5)
+ (4-n)HC1
(6)
(I)
R e s u l t s o b t a i n e d upon s t u d y i n g s u r f a c e r e a c t i o n ( 5 ) a n d (6) are s u m m a r i z e d i n T a b l e 2. Table 2 Study o f s u r f a c e r e a c t i o n s involved i n t h e modification o f s k e l e t a l r l i r l - e l c a t a l y s t by t i n . a
T i n precursor compound
1.
I n i t i a l concentration mnal.dm
0.6 3.0 9.1 3.0 3.0 3.0 3.0 3.0
2. 3.
4. 5.
6. 7. 8.
-3
Surface r e a c t i o n (5) r a t e o f t i n anchoring number of R -3 . - 1 -7 reacted mo1.dm m i n x10 (XI
2.96 3.00 3.33 2.33 3.76 3.91 3.23 1.09
0.17 0.53 1.85 0.26 0.47 7.67 0.61
29.79
a
S t a n d a r d e x p e r i m e n t a l c o n d i t i o n s : t e m p e r a t u r e o f H2 t r e a t m e n t : 30OoC; c o o l i n g i n H 2 ; s o l v e n t u s e d i n r e a c t i o n (5): b e n z e n e A f t e r H 2 t r e a t m e n t c o o l i n g i n N, C H, t r e a t m e n t a t 200OC d Reaction (51 i n n-hexane b
Reaction detected. ded
(5) was v e r y s e l e c t i v e , o n l y s a t u r a t e d h y d r o c a r b o n s w e r e The i n i t i a l r a t e o f s u r f a c e r e a c t i o n 1 5 ) s t r o n g l y d e p e n -
on t h e i n i t i a l
c o n c e n t r a t i o n o f t h e t h i s p r e c u r s o r compound.
S i g n i f i c a n t i n c r e a s e i n t h e i n i t i a l r a t e was o b s e r v e d w h e n r e a c t i o n
675 ( 5 1 was c a r r i e d o u t i n n-hexane a n d i n s t e a d o f t i n t e t r a a l k y l s compound w i t h g e n e r a l f o r m u l a o f S n R Z C I Z was
used as t i n precursor.
t e r i s t i c feature o f skeletal nickel is, i e r findings
Charac-
that contrary t o our earl-
1 1 2 1 , m o r e t h a n one a l k y l g r o u p h a s b e e n l o s t i n r e a c -
tion (5). Catalytic reactions ( i ) Hydrodechlorination
o f chlorobenzene.
Hydrodechlorination
o f c h l o r o b e n z e n e has been used as a t e s t r e a c t i o n t o compare t h e hydrodechlorination a c t i v i t y o f t h e c a t a l y s t s prepared. t a i n e d upon s t u d y i n g two c a t a l y s t s : 3.
Results ob-
PdM a n d N O 1 a r e g i v e n i n T a b l e
T h e s e d a t a r e v e a l s t h a t i n t h e t e m p e r a t u r e r a n g e o f 2O-7O0C
ca-
t a l y s t p r e p a r e d by a n c h o r i n g ( c a t a l y s t N O 1 1 has h i g h e r hydrodechlor i n a t i o n a c t i v i t y t h a n c a t a l y s t c o n t a i n i n g m e t a l l i c Pd (PdM). Table 3 Hydrodechlorination
o f chlorobenzene i n s t i r r e d tank r e a c t o r a Temperature
Catalyst
mrno1.s
(OC)
PdM
20
NO1
20
__-__
of catalyst:
0.3
g,
-1
-1
. g Pd
1.93 5.33 4.53 12.90
70 70
PdM NO1
'Amount
I n i t i a l rate
3 mrnol c h l o r o b e n z e n e i n 2 0 cm3 e t h a n o l
( i i l Conversion of 4-chloro -2- _ n i t r o a n i l i n e t o o r t h_ ophenylenediT y p i c a l k i n e t i c c u r v e s o f t h e f o r m a t i o n o f OPOA a n d CPDA a r e
amine.
shown i n F i g . of
1. U n d e r g i v e n e x p e r i m e n t a l c o n d i t i o n t h e f o r m a t i o n
o r t h o n i t r o a n i l i n e (ONA) was n e g l i g i b l e .
This fact
indicated that
n o t t h e r e d u c t i o n b u t t h e h y d r o d e c h l o r i n a t i o n s t e p is t h e r a t e lim i t i n g one i n t h e f o r m a t i o n o f OPDA.
Upon i n c r e a s i n g t h e p a l l a d i u m
content o f the catalyst a strong increase i n the i n i t i a l r a t e o f f o r m a t i o n of OPDA was o b s e r v e d .
I t can a l s o b e s e e n t h a t t h e l o w e r
t h e i n i t i a l r a t e o f t h e f o r m a t i o n o f CPDA t h e h i g h e r is t h e t o t a l y i e l d o f OPDA. Upon i n t r o d u c t i o n s m a l l amount o f m e t a l l i c p a l l a d i u m i n t o t h i s type o f catalyst significant n a t i o n a c t i v i t y was o b s e r v e d .
decrease i n t h e i n i t i a l h y d r o d e c h l o r i The r e s u l t s a r e shown i n F i g .
2.
these c a t a l y s t s t h e i n t r o d u c t i o n o f m e t a l l i c p a l l a d i u m i n 0.05 0.1
w % was c a r r i e d o u t
p r i o r t o the l i t h i a t i o n step
I n and
[reaction (211.
The c o n v e r s i o n o f CNA was a l s o i n v e s t i g a t e d i n a t r i c k l e
bed r e a c t o r .
~
676
aJ
40t
a
0
a U
0
30
60 90 time, rnin
120
0
30
60 90 time, min
120
F i g . 1 . I n f l u e n c e o f t h e p a l l a d i u m c o n t e n t o n t h e OPDA a n d C P D A y i e l d s . S t i r r e d t a n k r e a c t o r ; NH3-H20 (50-50%), 3 0 0 cm3; T : 95OC; : 3 0 b a r ; C N A : 30 g ; a m o u n t o f c a t a l y s t : 3 . 2 g ; p a l l a d i u m c o n -
( w % ) : 0-0.24,
20
x - 0.35;
40 60 time, rnin
a - 0 . 4 9 i c a t a l y s t s No8,9
and ?O, r e s p e c t i v e l y ) , .
80
F i g . 2 . I n f l u e n c e o f t h e p r e s e n c e of m e t a l l i c p a l l a d i u m on t h e f o r m a t i o n of OPDA f r o m CNA. S t i r r e d t a n k r e a c t o r ; i - p r o p a n o l - w a t e r ( 9 0 : l O l 100 c m 3 , T : 6 O o C ; P : 1 b a r ; C N A : 5 g , a m o u n t o f c a t a l y s t : 0 . 4 g ; a m o u n t o f Pd a n c h o r e d : 0 . 9 5 w % ; m e t a l l i c p a l l a d i u m c o n t e n t ( w % l : rn - 0 . 0 ; 0 - 0 . 0 5 ; n - 0 . 1 0 . F i g . 3. T e m p e r a t u r e dependence o f t h e p r o d u c t f o r m a t i o n from CNA i n t r i c k l e bed r e a c t o r . P : 2 b a r ; a t h a n o l ( 5 % C N A ) ; l i q u i d f l o w r a t e : 0.74 c r n 3 / m i n ; g a s f l o w r a t e : 1 1 0 c m 3 / m i n ; a m o u n t o f c a t a l y s t : 5 g ; c a t a l y s t : No1 ( s e e T a b l e 2). T h e r e q u i r e m e n t f o r t h i s s t u d y was t o o b t a i n h i g h O P D A s e l e c t i v i t i e s a t c o m p l e t e c o n v e r s i o n of of
CNA.
e x p e r i m e n t s are s h o w n i n F i g .
Results obtained i n t h i s series
3 . Under r e l a t i v e l y low t e m p e r a t u r e
a n d l o w h y d r o g e n p r e s s u r e h i g h c o n v e r s i o n o f C N A w a s o b t a i n e d . Upon increasing t h e reaction temperature t h e s e l e c t i v i t y of t h e
OPDA
677 Table 4 R e d u c t i v e a m i n a t i o n o f a c e t o n e on t i n m o d i f i e d n i c k e l c a t a l y s t s . a Sn C1 Conversion wt% wt%
Catalysts
I
s
e 1 e c t i v i t i e s,b % IPA OIPA IPPA IPAL
Ni
0
0
99.3
83.7
8.6
Ni-SnEtqC
0.082
0
99.2
65.2
Ni-SnBu - I d
0.025
0
98.3
74.7
Ni-SnRuR-2'
0.076
0
98.5
0.5
7.3
24.2
1.7
6.0
20.6
0.6
4.0
70.9
20.7
2.9
5.5
Ni-SnEt,CIZC
0.12
0.07
38.2
12.6
12.6
1.4
Mi-SnBz:ClZd"
0.30
0.17
95.3
87.0
0.5
4.2
4.0 (1
a amount of c a t a l y s t = 20 g , P = 0.5 MPa, WHSV = 0.8 h - l , m o l a r r a t i o H 2 : N H 3 : A C = 2 : 4 : 1 , r e a c t i o n t e m p e r a t u r e 200OC; b a s e d o n t h e c o n t e n t o f (CH3-CH-Ct3 m o i e t i e s i n t h e p r o d u c t s ; a n d d p r i o r t o m o d i f i c a t i o n t h e c a t a l y s t p r e t r e a t m e n t i n H, w a s c a r r i e d o u t a t 300 and 2 0 0 ° C , r e s p E c t i v e l y ; i n s t e a d o f b e n z e n e a c e t o n e was u s e d a s a o l v e n t i n r e a c t i o n 1 1 ) . formation increased w i t h p a r a l l e l decrease CPDA.
t r a c e amount. 95
o f t h e s e l e c t i v i t y of
U n d e r g i v e n e x p e r i m e n t a l c o n d i t i o n O N A was d e t e c t e d o n l y i n
% .Upon
A t 9O0C t h e s e l e c t i v i t y o f t h e OPOA f o r m a t i o n was
further increase o f the reaction temperature the selec-
t i v i t y o f OPOA s t r o n g l y d e c r e a s e d d u e t o t h e f o r m a t i o n o f d i f f e r e n t condensation products.
A l l of t h e s e r e s u l t s s t r o n g l y i n c i c a t e t h a t upon i n t r o d u c i n g i o n i c p a l l a d i u m i n t o alumina it i s p o s s i b l e t o o b t a i n a c a t a l y s t ,
in
which t h e hydrodechlorination a c t i v i t y is s i g n i f i c a n t l y h i g h e r than
t h e a c t i v i t y f o r reduction of t h e n i t r o group. (iii)Reductive amination of acetone.
Results obtained i n reduc-
t i v e amination of acetone a r e given i n Table 4. C h a r a c t e r i s t i c f e a t u r e o f reductive amination r e a c t i o n s is t h e strong c o n t r o l
of t h e
p r o d u c t d i s t r i b u t i o n by t h e r m o d y n a m i c s . R e s u l t s g i v e n i n T a b l e 4 . r e v e a l s t h e s e l e c t i v i t y c o n t r o l by t i n m o d i f i c a t i o n . T h e i n t r o d u c t i o n o f v e r y s m a l l amount o f t i n i n t o t h e s k e l e t a l n i c k e l c a t a l y s t f r o m t i n t e t r a a l k y l s r e s u l t e d i n a s t r o n g d e c r e a s e i n t h e IPA/OIPA ratio
,IPA and OIPA:
isopropylarnine and diisopropylamine,
respect-
i v e l y ] . The s e l e c t i v i t y o f t h e i s o p r o p a n o l ( I P A L : f o r m a t i o n showed a l s o a s m a l l d e c r e a s e . However, no c o r r e l a t i o n h a s been found b e t w e e n t h e t i n c o n t e n t a n d t h e IPA/DIPA r a t i o . cursor
Upon u s i n g t i n p r e -
c o m p o u n d w i t h g e n e r a l f o r m u l a o f SnR2C12 t h e I P A / O I P A
ratio
was o n l y s l i g h t l y a l t e r e d b u t s i g n i f i c a n t d e c r e a s e w a s o b t a i n e d i n t h e s e l e c t i v i t y o f t h e i s o p r o p a n o l . C a t a l y s t m o d i f i e d w i t h SnBz2C12
678
(B
=
benzyll has the lowest selectivity f o r the formation of IPAL.
On this catalyst sites responsible f o r hydrogenation both of the
carbonyl group and the double bound of the i s o p r o p y l i d e n e - i s o p r o p y l amine (IPPAI are strongly poisoned. These results strongly indicate that upon using CSRs f o r selective poisoning of nickel b y tin the selectivity of the reductive amination o f acetone can be controlled and undesired side reaction can be suppressed. CONCLUSIONS Results obtained in this study reveals that catalyst design based on (il t h e primary knowledge of the reaction network and the mechanism of reactions involved in it and (ii) the application of Controlled Surface Reactions in catalyst preparation and rnodification can be used t o obtain highly active and selective catalysts for different organic reactions taking place in the presence of hydrogen. REFERENCES
1 2 3
4 5 6 7 8 9
L.L.Hegedis (Editor), Catalyst Design,Progress and Perspectives, John Wiley and Sons, New York, 1987. A.R.Pinder. Synthesis 1980, 425. G.Bringer and I.J.Nestrick, Chemical Reviews, 7 4 (19741 567. T.Okamoto and S.Oka, Bull.Soc.Chim.Jpn. 5 4 (19811 1265. P.Dini, J.C.J.Bart and N.Giordano, J.Chem.Soc. Perkin II., 1975, 1479. B . C o q , G.Ferrat and F.Figueras, J.Catal., 101 (1986) 434 J.K.Stille, K.S.Y,Lau, J.Am.Chem.Soc., 98 (1976) 5841. C.Z.Sharf, A.S.Gurovets, I.B.Slinjakova, L.P.Finn, L.H.F eidlin and V.N.Krutii, 1zv.Akad.Nauk SSSR, Ser.Khirn., 1980, 114 Yu.I.Yermakov and V.A.Likholobov, Kinetika i Kataliz., 2 (19801 1208.
10 A.Le Bris, G.Lefebvre p.
1360, 1584.
and F.Coussernant, Bull.Soc.Chim..
964,
I 1 V .V.Antonova, T. I.Ovchinnikova, B. F .Ustavshikov and V . K . Promonenkov, Zh.Org.Khi’m., 16 (19801 547. 12 J.Margitfalvi, E.Tdlas and S . G B b E l E s , Catal.Today. 6 (1989) 73. 13 P.A.Berger and J.F.Roth, J.Cata1. 4 ( 1 9 6 5 1 717. 14 Y.Shen, S.Wang and K.Huang, Appl.Catal., 57 (1990) 55.
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
679
PREPARATION OF HIGH SURFACE AREA HYDROGEN MOLYBDENUM BRONZE CATALYSTS
C. HOANG-VAN*, 0. ZEGAOUI, B. POMMIER and P. PICHAT
URA au CNRS Photocatalyse, Catalyse et Environnement, Ecole Centrale de Lyon, B.P. 163 69131 Ecully Cedex (France)
SUMMARY Highly divided hydrogen molybdenum bronzes HxMo03 (0< x 4 2) have been prepared from ultra fine orthorhombic MOO3 powders obtained in a flame reactor. The so-called hydrogen spillover process has been used for the preparation of Pt/HxMoO3 bronzes or that of neat H1.6Mo03 by momentarily contacting MoO3 with a Pt/Al2O3 catalyst in the presence of H2. Neat HxMo03 bronzes (with x 4 0.9) could also be obtained by use of an alcohol as a source of hydrogen atoms, either in the dark or under UV-illumination. The high surface area hydrogen-molybdenum bronzes thus obtained are potential catalysts for several types of reactions.
INTRODUCIION Hydrogen bronzes are insertion compounds of atomic hydrogen in oxides (or chalcogenides) in which there is no formal chemical bond between the anion of the host lamce and the inserted element (ref. 1). These compounds have been used as catalysts for alkene hydrogenations and practical applications in that direction have been suggested (ref. 2). Any process in which atomic hydrogen is generated, e.g. nascent hydrogen from Zn and HCl, electrochemical reduction of H+, hydrogen plasma, can lead to the formation of a hydrogen bronze. For catalytic purposes, the so-called hydrogen spillover process is the preferred route (refs 3 and 4). In that case, small particles of Pt or Pd (metallic activators) are dispersed on the oxide surface by impregnation and hydrogen bronzes are formed when molecular hydrogen is brought into contact with the solid. The presence of Pt or Pd particles does not allow one to easily discriminate between the catalytic activity of the bronzes and that of the metal. Recently, however, one of us (ref. 5 ) has succeeded in preparing the bronze H1.jMOO3 without metal particles deposited on its surface and has definitely shown that H1.WoO3 is a catalyst for the hydrogenation of ethylene by molecular hydrogen in the absence of any metallic activator. In this paper we describe methods for the preparation of finely divided bronzes HxMo03, either neat or coated with Pt particles. Unlike most of the previous-works in which hydrogen molybdenum bronzes were elaborated from single crystals or from powders of Mo03 of low surface area (a few rn2g1), the high division state of the materials used in this study makes the bronzes particularly suitable for catalytic applications.
680
EXPERIMENTAL 1m ‘on t lof M a Ultra fine MOO3 particles were prepared in a flame reactor which has already been described (ref. 6). It should just be recalled that the oxide aerosol is generated from the vapor of a volatile metallic compound (Mo02C12 or Moc15) which is injected into the burner fed with hydrogen and oxygen and then decomposed in the flame either by hydrolysis or by oxidation. The oxide issued from the flame reactor was treated at 673 K in air for 24 h (standard treatment) in order to eliminate residual chlorine and to transform all the M a into the orthorhombic phase (seebelow).
2 - Preparation of bronzes Pt/HxMo03 bronzes were prepared by impregnation of MoO3, previously submitted to the standard treatment, with a solution of H2PtC4j (analytical grade) whose concentration was adjusted in order to obtain Pt contents in the range 0.1 - 1 wt 76. Evaporation of the solvent was carried out at 343 K under continuous stirring and the powder was then dried at 373 K overnight. To achieve the decompsition of H2PtC16, the impregnated oxide was heated under vacuum at 473 K for 2 h. This procedure is similar to that used by Marcq et al. (ref. 2). The temperature was then adjusted to that of the formation of the bronze and the powder was exposed to H2 in a volumetric apparatus until saturation of Mo03 with inserted hydrogen.
Neat H x M o 0 3 bronzes The bronze H1.6Mo03 was obtained by contacting M o 0 3 with Pt/Al2O3 in a hydrogen atmosphere at 433 K for 24 h. The Pt/A1203 was then removed from the reactor by a windlass device (ref. 7). The bronze H0.34Mo03 was prepared in a dynamic differential reactor by exposing Mo03 to a flow of allylic alcohol (20 Torr) and H2 (740 Torr) at 323 K for 20 h. This bronze could also be obtained at room temperature in a static photoreactor by UV-illuminating MoO3 suspensions in a liquid alcohol (such as methanol or 2-propanol). Addition of Ti@ to the MoO3 suspensions allowed the formation of H0.9M0@.
3 - Other exuerimental techniaues Surface areas were measured in a dynamic chromatography system (ref. 8) using N2 at 77 K. X-ray diffraction patterns were obtained with a Siemens diffractometer (Kristalloflex D500) using C u G radiation filtered through nickel. Preparation of Pt/HxMoOg bronzes and hydrogen absorption measurements were performed in a conventional Pyrex glass volumetric apparatus.
681
RESULTS AND DISCUSSION 1 -preDar;ltl‘onof M a The surface area and morphology of MoO3 are controlled by the temperature of the flame, the concentration of MoCl5 injected as a vapor into the burner and the residence time of this vapor in the
flame.This is illustrated by the results reported in Table 1 and by electron micrographs of Fig. 1 and L.
TABLE 1 Surface areas of MoO3 samples prepared in a flame reactor
Ma-A Ma-B MoO3-C Ma-D
1200 1900
2400 1900
3600 1140 1140 1140
82 50 49 17
20 24 20 161
20 40 34 16
(a) :Temperature of the flame. [ 0 2 m 2 ] = 1 to 1.5 (b) : Total flow rate (c) :MoCl5 mass velocity (d) : after a standard treatment at 673 K for 24 h.
Fig. 1 - Electron micrograph of MoO3-B
Fig. 2 - Electron micrograph of MoO3-D
682
For low residence time and small concentration of MoCl5 vapor in a cold flame (T = 1200 K), the sample obtained (Mo03-A) exhibits the largest surface area (line 1. Table 1). Samples prepared in hot flames (T 3 1900 K) present surface areas which strongly depend on the mass velocity of MoCl5 (c~mpareM a - D with M a - B and M a - C ) . The micrographs of M a - B and M a - D samples are presented on Fig. 1 and 2, respectively. Figure 1 shows particles of different sizes. Some of them are elongated plaques of 50 to 300 nm in length, whereas the others are very small (< 10 nm in diameter). By contrast, the micrograph of the low surface area sample (fig. 2) exhibits large particles (ca. 100 nm in diamter) in various shapes (squares. rectangles, ovals) and some very small particles (< 10 nm in diameter). When molybdenum chloride enters the flame, it reacts with water or oxygen leading to the formation of very small initial particles of M a by nucleation from the vapor phase. Following the nucleation, the growth process takes place in the hot zone either by the diffusion of the condensing species to the particle surface and condensation on this surface or by the collision between initial particles and coalescence of those particles. In both cases, the size of the oxide increases with the partial pressure of the chloride and the residence time of the "active species" in the flame which are respectively controlled by the mass velocity of MoCl5 and by the total flow rate of gases. The residence time is also affected by the temperature of the flame which determines the reaction hot zone. Therefore, high surface area MOO3 powders are preferentially obtained with a cold flame, at low MoC15 mass velocity and at high total flow rate. However, the surface area of samples issued from the flame reactor greatly decreases as a result of the standard treatment. The larger the initial surface area, the more important its decrease (Table 1, columns 5 and 6). This is particularly obvious for the Mo03-A sample prepared in a cold flame since its surface area is reduced more than fourfold. The samples directly issued from the flame reactor are composed of two crystalline phases : the orthorhombic phase and a metastable polymorphic h-Mo03 phase described by Kihlborg (ref. 9). This metastable phase is completely transformed into the orthorhombic phase as a result of the standard treatment. In this work, hydrogen bronzes were prepared from MOO3 having retained a large surface area (2 30 m2g-1) after the standard treatment.
2 - & p g a a'on of bronzes PtiHXMo03 The insertion of hydrogen within finely divided M a 3 coated with 0.1 to 1 wt Z Pt leads to the formation of bronzes HxMo03 whose composition depends upon the reduction temperature and duration. The bronzes thus formed have been characterized by volumetric measurements and by XRD analyses. The results obtained for a 0.2 7% Pt-Mo03 catalyst are summarized in table 2.
683
TABLE 2 Formation of bronzes 0.2 8 €'t/I-IxM@
30 120
263 298 298 323 433 473 573
400 240
Ho.34Md3 Ho.9Md3 m.9M003 +H l . m d 3 H0.9Md3 + H l . m d 3 H0.9Md3 + H1.6Md3 amorphous amorphous
1.44
1.60 1.67 1.86 2.10
100 150 800
* : x values determinedvolumetrically at equilibrium compositions.Initial H2 pressure = 760 Tom.
l
>
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+ - <
80.
.-- - .-
+-
5
Co-Si0,-Al,O,
co-c
Co-AI,O,
Co- ZnO
15
10
rt
20 T i m (hn)
Fig. 2
691 3 / E f f e c t o f metal
The a c t i v i t i e s a n d s e l e c t i v i t i e s of t h r e e metals a r e r e p o r t e d i n t h e
1 a n d 2.
tables
T h e s e c d t d l y s t s dre less d c t i v e t h d n t h e c o n v e n t i o n d l one
b u t t h e o r d e r o f r e a c t i v i t y is t h e same.
I t is w o r t h m e n t i o n i n g t h a t t h e s e
dnd c o b d l t c a t a l y s t s a r e more s e l e c t i v e t o w d r d s t h e p r o d u c t i o n o f
nickel
secondary dmines t h d n t h e c o n v e n t i o n d l n i c k e l and c o b d l t c d t d l y s t s prepdred by i m p r e g n d t i o n . T h i s means t h a t t h e c o n d e n s a t i o n o f t h e p r i m d r y m i n e w i t h t h e i m i n e a n d t h e s u b s e q u e n t h y d r o g e n d t i o n o f t h e d d d u c t dre f d s t e r t h a n t h e r e d u c t i o n of t h e n i t r i l e .
In
compdrison
with
nickel
and
cobalt,
copper
shows
a
considerdbly
lower c d t d l y t i c a c t i v i t y i n t h e hydrogendtion o f n i t r i l e s and t h e d i f f e r e n c e s
a r e much more i m p o r t a n t t h a n w i t h c o n v e n t i o n d l c d t d l y s t s ( N i / C o / C u 10/1
(4,5).
Moreover,
even
with copper
cdtdlysts,
the
tertidry
drnine i s
n o t formed. TABLE 1 E f f e c t o f t h e m e t a l on t h e h y d r o g e n a t i o n o f l d u r o n i t r i l e pH2
z
T = 12OoC,
1 atm.
TABLE 2 I n i t i d l h y d r o g e n a t i o n r a t e o f l d u r o n i t r i l e on metal - Al2o3 c a t d l y s t s -1 4 x 1 0 ) T = 120°C, pH2 = 1 d t m . (mole h-’ . g ~~
Ni
co
cu
100
50
1
; 20/
692 DISCUSSION O f RESULTS As
far
ds t h e p r e p d r d t i o n o f
s u r f d c e r e a c t i o n s may o c c u r ,
t h e c d t d l y s t i s concerned t h e f o l l o w i n g
by d n d l o g y w i t h t h o s e which hdve been s t u d i e d
when t h e r e d u c t i o n i s c a r r i e d o u t i n homogeneous phdse. The f i r s t s t e p i s t h e r e a c t i o n o f t h e d l k y l a l u m i n i u m w i t h t h e s u r f d c e
ZnO,
-OH groups o f t h e s o l i d s u p p o r t (A1203, Si02-A1203,
C)
:
Et
-OHsur f This
is
.+
'
A1Et3 ->-O-Al 250C(surf.)'
+
EtH
Et
w e l l known p r o c e s s which i s used f o r t h e t i t r d t i o n o f s u r -
d
f d c e h y d r o x y groups o f s o l i d s . The second s t e p i s t h e r e d u c t i o n o f C o ( d c a c ) Z by t h e g r d f t e d r e d u c i n g dgent.
A1
i s supposed t h d t
It
dtom.
from
prelimindry
this
r e a c t i o n occurs
(3,6)
report
is
it
The
200°C
third
with
15h,
the
step
syngds
wdtrr
is
dfter
this
used
is
(CO,
syngds
the
produces
for
petredtment
the
hydrolysis o f is
this
of
the
reduction
w i t h the formdtion
C2,
(C,,, the
of
the
pretredtment, surfdce
cdtdlyst
which
....)
C3
supported metdl,
d
thdt
together
(dctivation)
During t h i s
hydrocarbons
pretredtment
vicinity
e t h d n e dnd e t h y l e n e .
O f
ZH2).
the
known
gives r i s e t o very small p d r t i c l e s o f cobalt, o f Al(dCdC)3 dnd t h e d i s c h d r g e
in
&.
and
Al(dCdC)3.
surrounded
d t
l d s t s dbout This
The c d t d l y s t by
dlumind
ds
i t dppedrs from X-ray s p e c t r a . ( 7 ) AS
lysts of
fdr
dS
rdflk
C d t d l y t i C p r o p e r t i e s d r e concerned,
among
primdry
those
dmines
most
from
higher
i s d e p o s i t e d on
d f f e c t e d dnd The
results
metdl
there
is
presented
dnCh0ring
d
fdtty
dcids
n i c k e l and C o b a l t
dnd used
via
the
support,
o n l y t h e degre o f
no S U b S t d n t i d l m O d i f i C d t i O n o f in
ledd to
dnd c o n s e q u e n t l y
described
in
the
Cdtd-
production
hydrogendtion o f
the
I t wds a l s o dssumed i n p r e v i o u s papers t h d t when
corresponding n i t r i l e s . nickel
often
this
pdper
importdnt
i n dctivity
show
chdnges
thdt
both
nickel dispersion i s the s e l e c t i v i t y the
in nitrile,
and s e l e c t i v i t y .
I f the
support
and
(5). the
hydrogen d c t i v d t i o n r e s u l t s c o n f i r m thdt
t h e seconddry m i n e s e l e c t i v i t y deCredSeS when c h a n g i n g t h e m e t d l ( c u > 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 . .
>
~
<
RCH2NH2
R-CHNH2
I
HN-CH2R
V
RCHZNHCHZR
the
incredse o f
seconddry
<
dmine
HZ
>
RCH=NCH R 2
s e l e c t i v i t y shows t h d t
the r d t e o f redc-
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 d r y m i n e ( s t e p 8 ) is h i g h e r than t h e h y d r o genation
rate of
the
imine
to
primdry
dmine
which
i s quite
unusual
for
n i c k e l dnd c o b d l t c a t a l y s t s . The dhSenCe o f t e r t i a r y m i n e s which are formed i n t h e r e a c t i o n between p r i m d r y i m i n e and secondary m i n e is a l s o unexpected. CH-R
RCH = NH + (RCH2)2NH
7 RCH
I
- N
'
(E)
CH2R
NH2
I
(RCH2)3N
<
HZ
[RCH=CHN(CH2R)2]
and we d r e now s t u d i y n g t h i s p e c u l i a r p r o p e r t y .
REFERENCES
7
M. Blanchdrd, D. Vdnhove, F. P e t i t dnd A . M o r t r e u x , 3. Chem. SOC. Chem. Comm., 1980, 908. D. Vdnhove, M. Bldnchdrd, F. P e t i t dnd A . M o r t r e u x , Nouv. J o u r n d l Chimie, 1981, 5-4, 205. c. Bechardergue-Lahiche, 5 . M d i l l e , P. Cdnesson, M. B l d n c h d r d and D. VdnhOVe, P r e p a r a t i o n o f c a t a l y s t s 8. Delmon e t d l E d i t o r E l s e v i e r , Amsterddm, 1987, 31, 725. 3 . Pdsek, N. K O S t O V d dnd B. Dvordk, C o l l e c t . Czech. Chem. Comm. 1981, 46, 1011. 3. V o l f and 3. Pdsek, C d t a l y t i c h y d r o g e n d t i o n , L. Cerveny E d i t o r , E l s e v i e r , Amsterdam, 1988, 27, 105. 3. Goma, C. Kdppenstein, B u l l . SOC. Chim. F r . , 1988, 621. H. Derule, Ph. D. Thesis, P o i t i e r s (1989)
8 9
H.
1 2
3 4
5 6
10
g,
1923, 3 6 , 1988. Greenfield. I n d . Eng. Chem., Prod. Res. Develop., 1967, A . B d i k e r , 3 . K i j e n s k i , C d t a l . Rev. S c i . Eng. 1985, 27-4, 653.
3. Brdun, G. B l e s s i n g and F. L o h e l , Chem. Ber.,
6,
142.
This Page Intentionally Left Blank
695
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
PREPARATION OF HIGHLY DISPERSED GOLD ON TITANIUM AND MAGNESIUM OXIDE Susumu TSUBOTA, Masatake HARUTA, Tetsuhiko KOBAYASHI, Atsushi UEDA, and Yoshiko N A K A H A R A Government Industrial Research Institute of Osaka Midorigaoka I , IKEDA 563, Japan
ABSTRACT
Gold c o u l d be highly d i s p e r s e d o n t i t a n i u m o x i d e and m a g n e s i u m o x i d e i n t h e i r a q u e o u s d i s p e r s i o n c o n t a i n i n g Mg citrate. The mean diameter of gold particles are smaller than 5nm. These gold catalysts are active for the oxidation of CO O n m a g n e s i a s u p p o r t , Mg e v e n at a t e m p e r a t u r e b e l o w 0°C. c i t r a t e a c t s n o t a s a r e d u c i n g a g e n t but a s a s t i c k i n g a g e n t w h i c h b l o c k s t h e c o a g u l a t i o n of g o l d p a r t i c l e s . On titania s u p p o r t d i s p e r s e d i n n e u t r a l s o l u t i o n M g 2 + i o n s i n s t e a d of citrate ions are mainly adsorbed. It is likely that Mg2+ ion suppresses the transformation of amorphous titania to anatase during calcination and prevent gold particles from coagulation caused by earthquake effect.
INTRODUCTION Gold has been regarded as catalytically far less active than platinum-group metals.
This is because of its chemically inert
character and of low dispersion in supported catalysts. recently
reported
that
through
smaller than 10 nm can be highly Ni01-3),
and Be(OH)24).
coprecipitation dispersed
gold
These gold catalysts are active in the
coprecipitation is valid mentioned
particles
c0304, ' a - F e 2 0 3 ,
on
o x i d a t i o n of C O a t a t e m p e r a t u r e a s l o w a s - 7 O O C . oxides as
We have
However,
only for a selected group of metal
above, because
support metal hydroxide and
gold
the
precipitation
hydroxides and
their
rates of affinity
might determine in the dispersion of gold. This paper deals with the methods for supporting gold in a highly dispersed state on pre-formed
T i 0 2 and M g O p o w d e r , o n
which ultrafine gold particles have been
d i f f i c u l t to be
supported by the conventional methods. EXPERIMENTAL Preparation of gold catalysts The following materials were used for catalyst supports; magnesia
(Ube
Industries,Ltd.;
crystalline
small
particles
696 p r e p a r e d by v a p o r m e t h o d ; Co.;
BET=140m2/g),
amorphous d r i e d a t 120°C;
TI04;
anatase;
dispersed magnesia
settled
t o 7.0
for
2
hrs
( c i t r a t e s o f Mg,
6.OmolIAu
for
distilled
water
after
Na,
o r NH4,
and
then
T h e pH o f
aqueous of
a
titania
was
adjusted,
d i s p e r s i o n s were
variety
of
reagents
for magnesia, were
precursors
filtered.
w a s
w h i l e t h e pH f o r
o r HCHO; 2 . 5 m o l / A u These
supports
intentionally The
addition
the
titania).
not
(JRC-
titania-B
these
HAuC14.
9.6.
around
of
( I d e m i t s u Kosan
and
w i t h Na2CO3,
was
which
at
Each
s o l u t i o n of
was adjusted
dispersion,
naturally stirred
BET=40m2/g).
i n an aqueous
dispersion
titania-A
BET=l10m2/g),
was
The c a k e
washed
and with
vacuum
dried
and c a l c i n e d i n a i r f o r 5 h r s a t 400°C and 250°C f o r Ti02 and
MgO,
respectively.
The
gold
o b t a i n e d were 1 a t . % ( A u / T i )
content
of
these
i n A u l t l t a n i a and
catalysts
thus
2 a t . Z (Au/Mg)
in
Aulmagnesia. Catalytic A c t i v i t y measurements The
a c t i v i t i e s of
the
o x i d a t i o n o f CO o r H2.
gold
c a t a l y s t s were
E x p e r i m e n t s were
measured
in
the
c a r r i e d o u t i n a small
f i x e d bed r e a c t o r w i t h 0.10g of c a t a l y s t s t h a t had p a s s e d t h r o u g h
70 a n d 1 2 0 m e s h s i e v e s .
A standard gas of
1.0 vol.%
H2 o r
CO
h s l a n c e d w i t h a i r t o 1 atm was p a s s e d t h r o u g h t h e c a t a l y s t b e d a t a flow rate of determined
33mlIrnin.
through
Y a n a g i m o t o Co.
Ltd.)
gas of
The
conversion of
chromatographic
C O a n d H2 w a s
analyses
(G-2800,
e f f l u e n t from t h e reactor.
C h a r a c t e r i z a t i o n of Catalysts The s t r u c t u r e s of Hitachi
H-9000
diffraction
the
c a t a l y s t s were
20-SXC
Infrared
spectrometer.
For
by
pressed i n t o a
t h i n wafer.
Co.Ltd.).
X-ray
w i t h a SSX-100
a
X-ray system
IR analysis,
each
sample
10 w t . % f o r t i t a n i a ) , a n d
D i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) t h e r m a l a n a l y z e r ( S e i k o D e n s h i Kogyo
photoelectron
spectrometer
at
s p e c t r a were t a k e n w i t h a
the
was m i x e d w i t h K B r ( 2 w t . % f o r m a g n e s i a ; was m a d e b y u s i n g a S S C - 5 2 0 0
observed using
300 kV. u s i n g a Rad-B
operated
(XRD) a n a l y s i s w a s made
(Rigaku Denki Co.Ltd.). Nicolet
gold
e l e c t r o n microscope
spectroscopy
(XPS)
was m e a s u r e d
(Surface Science Laboratories,
Inc.).
RESULTS Gold s u p p o r t e d on magnesia Table
1 shows
the
catalytic
prepared with d i f f e r e n t additives.
activities
of
Au/magnesia
It w a s found t h a t c a t a l y t i c
697 activities were enhanced by the addition o f Mg citrate.
When Mg
c i t r a t e w a s a d d e d i n t o t h e s u s p e n s i o n b e f o r e t h e a d d i t i o n of HAuC14, the activity enhancement could not be observed.
The use
of Na citrate or HCHO caused lower catalytic activity.
The pH
of
the suspension during
the
preparation, usually
increased to 11 when Na citrate was added. to the suspension produced
a purple
9.6,
was
The addition of HCHO
color, which indicated the
reduction of Au3+ to colloidal gold. Figure 1 shows the XRD
patterns of Au/magnesia
w h e r e t h e p r e s e n c e of Mg(OH)2,
starting material, MgO, changed to Mg(OH)2 aqueous suspension.
catalysts,
not MgO, are evidenced. by
The
hydration in the
From the width of the XRD peak of Au(200),
the particle size of gold is calculated as about 14nm for Au/MgO prepared without additives, and this value i s i n good agreement On
with 10 n m determined by TEM observations. i n the catalyst prepared with
the other hand,
the addition of Mg citrate, gold
particles smaller than 3 nm are observed by TEM. very
Although such
small particles of gold did not show the diffraction peak
in X R D , t h e p r e s e n c e o f m e t a l l i c g o l d was binding energy
of 84.2
confirmed
e V f o r t h e XPS p e a k
of
by
the
Au4f5/2.
The
catalyst prepared with the addition of HCHO contained only large gold particles (more than 20nm, by TEM observation). Figure 2
shows the IR spectra of the precursor of
Au/magnesia before calcination.
W i t h o u t Mg c i t r a t e , t h e IR
absorptions of surface H 2 0 and MgC03 are observed at 1638cm-1 and 1 4 4 9 c m V 1 , respectively.
In the case of the precursor
prepared
w i t h Mg c i t r a t e , o t h e r a b s o r p t i o n s a r e d e t e c t e d a t 1 5 9 5 c m - l , 1 4 2 3 ~ m - l ~1 2 6 3 ~ m - ~ 1083cm-l, , and
1061~m-~.
These absorption
bands coincide with those obtained for pure Mg citrate powder. TABLE 1 Catalytic activity of Au/magnesia prepared with various additives. Additives
Cat a 1 y tic activity T1/2[H2],'C
CO conv.,% none Mg ct. Na ct.
HCHO
10 100 5 0
CO conv.:CO conversion at -7OOC T1/2:temperature for 50% conversion ct.:citrate
>200
67
>200 >200
698
20
Fig. 1. XRD patterns of Au/magnesia. (a)prepared with Mg citrate; (b)prepared without Mg citrate.
0 2000
I
1800
I
1600
I
1400
I
1200
1000
WAVENUMBER (cm”) Fig. 2. IR spectra of Admagnesia before calcination. (a)prepared with Mg citrate; (b)prepared without Mg citrate. (resolution 4cm-1; accumulation lootimes)
699
Gold supported on titania In Table 2, the effect of the addition of Mg citrate on the catalytic activity i s compared o n the two different types of T i 0 2 While the catalytic activity o f
supports.
( a m o r p h o u s ) i s e n h a n c e d by u s e o f Mg (anatase) s h o w s
Au/titania-A
citrate,
Au/titania-B
a high catalytic activity regardless of the
addition o f Mg citrate. Figure 3 shows
TEM p h o t o g r a p h s
When Mg citrate i s added
citrate.
Au/titania catalysts.
in the dispersion, the gold
are highly dispersed on titania-A gold is about 4nm),
of
particles
(the average particle size
of
and gold particles become larger without Mg
I n the case of titania-B, h o w e v e r , the small gold
particles are highly dispersed even when Mg citrate was not used. T h e IR a b s o r p t i o n s p e c t r a o f t h e p r e c u r s o r o f prepared with Mg citrate band
are
at 1 4 0 0 ~ m - on ~ titania-A
citrate species.
shown in Fig. 4. might
Au/titania
The adsorption
correspond to the adsorbed
Compared with the case of Au/Mg(OH)2,
however,
t h e a m o u n t of c i t r a t e s p e c i e s i s much l e s s o n t h e t i t a n i a support.
TABLE
2
Catalytic activity of Au/titania prepared
with and without
Mg citrate, (Comparison of two different titania supports).
Titania Support
Catalytic activity Addition of Mg citrate none , c
Titania-A(amorphous) Titania-B(anatase)
25
35
35
T1/2 :temperature for 50% conversion
TABLE 3
Catalytic activity with additives. Additives none
HCHO
Na ct. NH4 ct. Mg ct. Mg(NOgj2
of
Au/titania-A
prepared
Catalytic activity T1/2 [COl,°C T1/2[H2I1'C
35 83 23 5
ct.:citrate T1/2: temperature for 50% conversion
139 165
93
85 25 35
139
34
700
Fig. 3. TEM photograph of Au/titania prepared with or without Mg citrate. (a)with Mg citrate; (b)without Mg citrate, on titania-A (amorphous): (c)with Mg citrate; (d)without Mg citrate, on titania-B (anatase).
701
2000
I
I
I
I
1800
1600
1400
1200
1C 0
WAVENUMBER (cm-1)
Fig. 4 . IR spectra of A d t i t a n i a prepared with Mg citrate before calcination. (a)on titania-A; (b)on titania-B. (resolution 4cm-I; accumulation 100times)
1 -
anatase (101)
-KO.,
AuiTi (Blank) h,,[CO]
35 "C
2%
!
I
AuiTi Mg-at T,,[CO] < 0°C
i
I
20
2 8 (4 (c
5.0m
AuiTi Na-cit Tvz[CO] 23pC
2.5K
20
F i g . 5. XRD patterns of Au/titania-A prepared with various additives. (a)without any reagent(b1ank); (b)with Mg citrate; (c)with Na citrate, (d)with Mg(N03)~.
702 Table 3 s h o w s the prepared
c a t a l y t i c a c t i v i t i e s of
with a variety
enhancement
of
the catalytic activity
through the addition o f addition. activity.
of additives.
Au/titania-A
The appreciable is
observed
not
only
Mg c i t r a t e b u t a l s o t h r o u g h Mg(N03)z
Other citrates bring about a slight increase in Similarly to the case o f
c a u s e s t h e r e d u c t i o n of Au3+
the magnesia support, HCHO
in the suspension of
titania-A
giving a poor catalytic activity. Figure 5 Au/titania.
shows the XRD patterns
The amorphous titania-A
of
f o u r k i n d s of
i s transformed
into anatase
by calcination, and Mg2+ seems t o suppress this crystallization. The catalytic activity tends to become low with an increase in crystallinity of the support. Figure 6 shows DTA curves for t h e precursors o f Au/titania-A before calcination. 460°C
There is an exothermic
in each signal.
These
peak
transformation from amorphous titania t o anatase. that t h e a d d i t i o n of Mg2+
at around
peaks corresponds
to
the
It i s clear
shifts the temperature
for the
crystallization toward higher temperature.
DISCUSSION It has been demonstrated that Mg citrate plays a n important role
in the preparation of highly dispersed
Mg(0H)z
gold catalysts with
and T i 0 2 as supports.
Fig. 6. DTA curves f o r Au/titania-A before calcination. (a)without any reagent(b1ank); (b)with Na citrate; (c)with Mg(N03)2, (d)with Mg citrate. (heating rate : 5'C/min in air).
703
However,
it has appeared t h a t
t h e r o l e o f Mg c i t r a t e i s d i f f e r e n t
b e t w e e n T i 0 2 a n d Mg(OH)2. Gold s u p p o r t e d on m a g n e s i a The
pH
the
of
p r e p a r a t i o n .
suspension of
A t
a
s u c h
s u f f i c i e n t l y hydrolyzed
pH
magnesia r e g i o n ,
i n t o Au(OH)3
gold s p e c i e s i n aqueous
solutions5).
Au
might
the
be
deposited
on
of
zero charge
(PZC)
positively charged
surface
9.6
AuC14-
judging
of
a d d i t i v e s are introduced
is
from
of
of
Mg(OH)2
the
s h o u l d
be
the
Then t h e
before
Since
appears a t
stability
hydroxide
Mg(OH)2
i n t o the suspension.
during
pH
=
the
of
the
point
126),
t h e
surface is suitable for the adsorption
of
a n i o n s such as c i t r a t e i o n , as o b s e r v e d i n t h e I R spectrum. The r e d u c t i o n of
Au3+
p a r t i c l e s l a r g e and lowered related
considered coagulation
to
the
high
of
gold
A
i l l u s t r a t e d i n Fig. pH
activity.
The a d s o r b e d
t o act as a s t i c k i n g reagent which
c a l c i n a t i o n . The
the catalytic activity
of
the
species
i n
s p e c u l a t e d
the
suspension
suspension
c l o s e t o t h e PZC o f Mg(OH)2. the
catalytic
1 and
b e h a v i o r
o f
citrate
ion
can block and/or
c i t r a t e
is the
during i o n
i s
7. containing
Na
citrate
activity
might
not
be
observed
the in
p r e p a r e d w i t h Na c i t r a t e . 4H2C00CC0H)COf' dH2COO-
was
11 a n d
Since the effective adsorption
c i t r a t e i o n i s n o t e x p e c t e d a t s u c h a pH r e g i o n , of
(Tables
t h e r e d u c i n g power o f c i t r a t e i o n seems n o t t o
3), and t h e r e f o r e , be
by HCHO i n t h e s u s p e n s i o n m a d e t h e Au
'
MgZt
Mg citrate
F i g . 7. S p e c u l a t e d b e h a v i o r of Mg c i t r a t e i n a q u e o u s d i s p e r s i o n .
of
enhancement Au/Mg(OH)2
704
Gold supported on titania S i n c e t h e pH o f t h e s u s p e n s i o n o f t i t a n i a i s a d j u s t e d t o
7.0, Au species to be deposited on the support might be Au(OH)3 a s in the case of Au/Mg(OH)2.
However, since the surface of
titania in the suspension w a s proved to be negatively charged, a s w a s reasonable from the PZC of T i 0 2 at such as Mg2+
pH
=
4 -66),
cations
s h o u l d be m o r e e a s i l y a d s o r b e d o n t i t a n i a t h a n
citrate anion. F i g u r e s 4 a n d 5 s h o w t h a t t h e a d d i t i o n o f Mg c i t r a t e or M g ( N 0 3 ) ~ enhances the catalytic activity of Au/titania-A suppresses the crystal growth o f anatase.
On the other hand, when
anatase support, titania-B, w a s used a s a starting support, appreciable effect
of Mg
citrate
and
addition
was
no
observed.
The liability of the surface of the carrier will accelerate the coagulation of supported species due to the so called "earthquake effect".
The presence of Mg2+ prevents amorphous titania-A from
crystallization and therefore from the "earthquake effect". On the other hand, since anatase support has already a crystalline structure,
there i s no appreciable effect of Mg citrate
addition on the dispersion of gold.
A small amount of citrate species is detectable on titaniaA in Fig. 6. T h e citrates other than Mg salt might have also a certain
effect
Au/titania-A.
o n the
increase
in
catalytic
activity
of
I t i s probable that citrate ions in Au/titania-A
play similar role to that in Au/Mg(OH)2. Acknowledgment We would
l i k e to
t h a n k Mr.
M. G e n e t ( U C L , B e l g i u m ) f o r
the analyses of X-ray photoelectron spectroscopy.
REFERENCES 1) M. Haruta, N. Y a m a d a , T. Kobayashi, and S.Iijima, J.Catal., 1 1 5 , 301-309 (1989). 2 ) M. H a r u t a , H . K a g e y a m a , N. K a m i j o , T. K o b a y a s h i , a n d F. Delannay, Stud. Surf. Sci. Catal. 4 4 , 33-42 (1988). 3) M . H a r u t a , T. K o b a y a s h i , S. I i j i m a , a n d F. D e l l a n n a y , Proc. 9 t h Intern. Congr. Catal., 3 , 1206-1213 (1988). 4 ) M. Haruta, K.Saika, T. Kobayashi, S. Tsubota, and Y. Nakahara, Chem. Express, 3 , 159-162 (1988). 5) R . J . P u d d e p h a t t , T h e C h e m i s t r y o f G o l d , E l s e v i e r , Amsterdam, 1978, p.91. 6) G . A. Parks, Chem. Rev. 6 5 , 177-198 (1965).
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
705
PREPARATION OF MONODISPERSE COLLOIDAL R-Re02 PARTICLES USING MICROEMULSIONS A. CLAERBOUT and J. B.NAGY Laboratoire de Catalyse, Facultes Universitaires Notre-Dame de la Paix, 61, rue de Bruxelles, B-5000 Namur (Belgium) ABSTRACT Two microemulsions composed of Cetyltrimethylammonium bromideHexanol-Water or Pentaethyleneglycoldodecylether-Hexane-Water were used to obtain monodisperse Pt, Re02 and Pt-Re02 particles by reducing K2PtC14, H2PtC16 and NaReOq with hydrazine. The aggregation of the particles is governed by the surface charge of the particles and by the adsorption of the surfactant molecules on the monodisperse particles. The dependance of the particle size on the precursor concentration is explained by the specific location of the precursor salt in the inner water cores of the microemulsion. INTRODUCTION Monodisperse particles present the advantage of uniform active site distribution and can be considered as models for heterogeneous catalytic reactions. Monodisperse metals, metal oxides or metal borides can now be easily obtained using microemulsions, vesicles, polymers or normal micelles (refs. 1-4). Microemulsions were used to obtain monodisperse particles of platinum (refs. 5-7), palladium (refs. 5,6), rhodium (refs. 5,6), iridium (ref. 5) and gold (ref. 8) by reducing the precursor metal ions with hydrogen, hydrazine, sodium borohydride or solvated electrons. Monodisperse nickel boride (refs. 1,9-12), cobalt boride (refs. 1,10,13-17), nickel-cobalt boride (refs. 1 ,lO,lS-17), and mixtures of iron boride and iron oxides (refs. 1,18) were prepared by sodium borohydride reduction of the precursor metal ions. Iron oxides (ref. 19), magnetite (ref. 20), calcium carbonate (ref. 21) and silver chloride (ref. 22) were obtained by precipitation reactions. On the other hand, bimetallic catalysts (ref. 23) including Pt-Re pair become important in catalysis, because the activity and the selectivity of the two metals are strongly influenced by their dispersion in the alloyed particles. More recently, dispersed metals deposited on dispersed oxides have been reported to possess a rather high activity (ref. 24).
706
In the present paper, we report the formation of monodisperse Pt, Re02 and Pt-Re02 particles using microemulsions. The precursor ions are dissolved in most of the cases in the dispersed water phase ( inner water cores ), the organic medium forming the continuous phase. The so-obtained catalysts can be used for hydrogenation and/or CO oxidation reactions.
EXPERIMENTAL Materials The commercial products PEGDE (Fluka, > 98 %), CTAB (Serva, 99 Yo), nhexane (Merck,P.A), hydrazine hydrate (Fluka, > 99 Yo),sodium perrhenate (Alfa, 99,95 Yo), potassium tetrachloroplatinate (Alfa, 99,9 O/.) and hexachloroplatinic acid (Alfa,P.A) were used without further purification.
Preparation of the particles The monodisperse Pt particles were prepared by reducing with hydrazine at room temperature K2PtC14 or H2PtC16 dissolved in two microemulsions : cetyltrimethylammonium bromide (CTAB) 30 wt Yo Hexanol 50 % - Water 20 Yo and pentaethyleneglycol dodecylether (PEGDE) 9.5 % - Hexane 90 % - Water 0.5 %. The monodisperse Re02 particles were prepared only in the second microemulsion by reducing NaRe04 with hydrazine. The Pt-Re02 systems were obtained from a constant metal ion concentration (0.1 molal) varying the [Pt]/([Pt]+[Re]) ratio from 0 to 1. Fig.1 illustrates the preparation scheme of the catalysts (ref. 9). The synthesis was carried out in a glove box under argon atmosphere to prevent oxidation of the particles.
x CTAB
y Hexenol
z n,o
moles of N m hms
Fig. 1. Experimental particles.
procedure
for the preparation of monodisperse
707
Electron microscopy The average size of the particles was measured using a Philips EM 301 electron microscope in the transmission mode. For these measurements the particles were dispersed in n-butanol using ultrasound and deposited on grids covered with Formvar. RESULTS AND DISCUSSION Monodisperse platinum particles from CTAB-HexanolWater microemulsion using H2PtC16 and K2PtC14 The monodisperse Pt particles prepared from H2PtC16 dissolved in the CTAB-Hexanol-Water microemulsion have an average size of 4 0 f 5 A and their size is not dependent on the H2PtC16 concentration (ref. 7). The aqueous solution of hydrazine containing a ten-fold molar excess of hydrazine with respect to H2PtC16 had an initial pH = 10. The metal particle precursor, in this case, is dissolved in both the dispersed inner water core and the continuous organic (or hexanol) phases. It has been supposed, that the nucleation occurs in both phases, and hence, the particle size is only dependent on their stabilization by the adsorbed surfactant molecules (refs. 1,7). This is verified in the concentration range of H2PtC16 from 5 x 10-3 to 5 x 10-2 molal with respect to water. If K2PtC14 is used, instead, as particle precursor (for the same hydrazine to K2PtCI4 excess), a complex behaviour is observed as a function of the pH. In the low pH region (1 < pH c 4), no Pt particles could be obtained. At 5 c pH c 8 , dispersed Pt particles are formed but the reduction could not be carried out until completion during 24 h. For the high pH region (pH > 9) complete reduction of the Pt-salt occurs, but the so-obtained particles are highly aggregated. It is clear that the surface charge does influence the aggregation of the metal particles. In addition, the adsorption of the surfactant molecules, also pH dependent, can also greatly influence the particle aggregation (refs. 25, 26). Monodisperse platinum particles from PEGDE-HexaneWater microemulsion In order to avoid the latter phenomenon, a neutral surfactant PEGDE is used to form the microemulsion of composition PEGDE 9.5 % - Hexane 90 Yo Water 0.5 Yo. Only K2PtC14 as precursor salt is tested, however, because it is insoluble in the organic medium. Its concentration was varied from 1 x 1 0 - 3 to 3 x 10-1 molal with respect to water.
708
Fig. 2 illustrates the Pt particles size obtained at high and low initial K2PtC14 concentrations. The particles are not aggregated and their size is quite uniform.
Fig. 2. TEM photographs of monodisperse Pt particles prepared from PEGDE 9.5% - Hexane 90% - Water 0.5% containing 0.1 (A) or 001 (B) molal K2PtC14 Table 1 and Fig. 3 show the variation of the size as a function of K2PtC14 concentration. The standard deviation is small in all cases studied. The particle size increases monotonously with increasing K2 P t C 1 4 concentration and approaches a plateau at high concentrations. This behaviour seems to be different from those previously observed for the P t particles formation using H2PtC16 dissolved in CTAB-Hexanol-Water and for the Ni2B particles obtained from the same microemulsion. In the first case, a constant particle size is obtained irrespective of the initial H2PtC16 concentration (refs. 1,7), while in the second case a minimum was observed in the curve particle size of Ni2B as a function of NiC12 concentration (refs. 1,9,10).
Fig. 3. Variation of the Pt particles size as a function of the initial K2PtC14 concentration with respect to water The observed minimum in the curve was adequately explained, provided a critical number of Ni(ll) ions is assumed for the formation of one nucleus. This number was determined to be equal to 2 for the formation of Ni2B and C02B (refs. 1,9,10,15,16). The number of nuclei (Nn) formed by inner water core (NM) is determined as follows Wt Nn =-W
where Wt is the total weight of the catalysts prepared per kilogram of microemulsion and w is the weight of one particle. Knowing the volumic mass and the size of the particles, w is easily computed (refs. 1,10,17). Table 1 shows the different values obtained for different Pt concentrations. The average number of inner water core per kilogram of microemulsion is computed from the total volume of water (Vt) and the size of the inner water core (rM) (refs. 1,10,17,27) : NM =
Vt 4/3
n: ( r M ) 3
710
In the present case, the literature value of ca. 60 8, is taken as the mean radius of the inner water core, a value which was obtained from light diffusion measurements (refs. 28, 29). Finally from the total K2PtC14 - ) easily concentration, the number of Pt(ll) per water core (n P t C l ~ l ~ is calculated (Table 1). As the distribution of the PtC142-ions in the microemulsion follows the Poisson statistics, the probability to have k Pt atoms per water core (pk) is given by :
where k is one integer and h = n PtC142m
Table 2 shows the comptuded values for z p k . The initial value of k is k= 1 equal to 1 because it is necessary to have,at least, one PtCl42-ion in one water core to obtain the formation of one surfactant stabilized Pt atom, which is considered as the nucleus of the Pt particle. Indeed, it can be shown, that :
m
where F is a scaling factor and more
x p k gives the probability to have one or k= 1 PtC142- ions per inner water core. 00
Figure 4 shows the variation of concentration.
Nn Z p k and of - as a function of K2PtC14 hnn k= 1
711
0.9
0.1
0.2
0.4
0.3
KptClponcentrntlon (rnolallwster) m
-
Nn x p k and as a function of K2PtC14 concentration. NM k= 1 For low initial K2PtC14 concentration (up to 0.01 molal with respect
Fig.4. Variation of
Nn to water),increases as well as NM This behaviour was already
m
C p k as a function of Pt concentration.
k=l
found in
the case of the Ni2B and Co2B Nn particles (refs. 1,10,17). However, for higher Pt-concentrations, the NM ratio decreases, leading to larger particles. Note, that this ratio is between 10-3 and 10-2, showing that every hundred or every thousand of inner water cores leads to the formation of Pt particles. This is also expressed by the scaling factor F, where a maximum variations of ten fold is observed. If however, a critical number of initial PtC142- ions higher than 1 is supposed, the variation of the so-computed F becomes larger. This analysis reinforces the hypothesis, that one surfactant-stabilized Pt atom is able to initiate the final Pt particle. For higher initial K2PtC14 concentration , the number of nuclei per inner water core decreases. This behaviour was not observed previously for Ni2B and C02B particles. Nn It is not clear, at present, why this reduction of occurs in the NM PEGDE-Hexane-Water microemulsion. A more systematic study is necessary to shed some light on the influence of the nature of the surfactant molecules, the mobility of the interface and the influence of hydrazine concentrations.
-
712
Monodisperse Re02 particles The monodisperse Re02 particles were obtained by reducing NaReOq with hydrazine in the system PEGDE-Hexane-Water. The presence of Re02 is confirmed by XPS experiments. Fig. 5 shows the monodisperse Re02 particles for two different initial Na Re04 concentrations.
Fig. 5. TEM photographs of monodisperse Re02 particles from PEGDE 9.5%Hexane 90%-Water 0.5% containing 0.3 (A) or 0.005 (B) molal of NaReO4 Table 2 and Fig. 6 illustrate the variation of the particle size as a function of NaReOq concentrations.
[NaReOq]
molal/H20
0.01
0.05 0.1
0.3 1 .o
d
0
(A)
(A)
18 27 31
2 3
4 5
42
55
a.PEGDE 9,5 wt % - n-Hexane 90 %
6
-
H20 0.5 %.
-
60 70
5 50
k
Z
i m
c
713
40 30
E $! 20 a 10
0
0.1
0.2 0.3 0.4 0.5 0.6 0.7 [NaReOgl M / H20
Fig. 6. Variation of the Re02 particles size as a function of initial NaReOq concentration Once again the size of the monodisperse particles approaches a plateau
for high NaReOq concentrations and this behaviour is quite similar to that
of the Pt particles. However, a similar quantitative analysis for the Re02particles could not be carried out because NaReOq is only partially reduced in our experimental conditions (refs. 30,31).
Monodisperse P t - R e 0 2 particles Monodisperse Pt-Re02particles were prepared from the PEGDE-Hexane -Water rnicroernulsion using a total ion concentration [K2PtC14]+[NaRe04] = 0.10 molal with respect to water. The monodispersity of the particles is illustrated in Fig. 7.
Fig.7. TEM photographs of monodisperse Pt-Re02 particles prepared from PEGDE 9.5 wt % - n-Hexane 90 % - H 2 0 0,5% containing [K2PtC14] + [NaRe04] = 0.10 rnolal with respect to water
714
Table 3 and Fig. 8 show the variation of the particle size as a function of the mole fraction x of KzPtCI4.
TABLE 3: Variation of the monodisperse Pt-Re02 particles size as a function of the mole fraction (x) of K2PtC14alb
mole fraction x of K2PtC14
0 0.16 0.33 0.5 0.66 0.8
-
-
Pt
d (A) Re@
fc)
(C)
-- 30 29 28 .., 27 -- 22 20
30 35 50 70 80
Pt-Re02
31+3 25+3 24+3 27f3 25+2 38+4
a. PEGDE 9.5 wt % - n-Hexane 90 % - H20 0.5%
+ [NaRe04] = 0.10 molal with respect to water. Hypothetical particle size estimated in the case where the system would contain pure Pt or R e 0 2 particles.
b. [K2PtC14] C.
50-
0
0.2 0.4 Molar
0.6
ratio
0.8
1
(x)
Fig. 8. Variation of the Pt-Re02 particles size as a function of mole fraction x of KzPtC14 ([K2PtC14] + [NaReOs] = 0.10 molal with respect to water) It is surprising, that up to x = 0.7, the diameter of the particles remains quasi constant and is close to that of the pure Re02 particles. For higher initial K2PtC14, the diameter of the particles increases monotonously to reach that of the pure Pt particles. The quasi constancy of the particle diameter for low K2PtC14 concentration suggests, that in that
715
region of concentration, the Pt is dispersed on the Re02 particles. Indeed, the slight decrease of the size could be due to the decrease of the particle size of the Re02 particles as it can be seen on the Fig. 5. For high K2PtC14 content, the reverse situation could occur, i.e. the dispersion of Re02 particles on the larger Pt particles. This hypothesis will be later checked by STEM measurements. All these results are different from those one could expect on the basis of a mechanical mixture. Indeed, in that case a bimodal distribution is expected at least for x 2 0.5, based on the different size of the separate Pt and R e 0 2 particles. Table 3 also includes the hypothetical separate particles estimated from Figs 4 and 5. This comparison makes clear, that the presence of Re02 induces a higher dispersion of the Pt-Re02 particles for x > 0.7. Presently, experiments are carried out to deposit these particles on a support and their stabilisation is systematically studied to prevent them from sintering. CONCLUSIONS Monodisperse Pt, Re02 and Pt-Re02 particles are easily prepared by reducing with hydrazine the precusor salts dissolved in the inner water cores of PEGDE -Hexane - Water microemulsions. The Pt and Re02 particles size increases with increasing precursor salt concentration and approaches a plateau at high concentrations. NaReOq system, the At high initial Pt concentration in the K2PtC14 presence of Re02 deposited on the Pt particles seems to impede the increase of Pt - particles size.
-
REFERENCES 1 . J. B.Nagy, E.G. Derouane, N. Lufimpadio, I. Ravet and J.P. Verfaillie in K.L. Mittal (Ed), Surfactants in Solution, Vol 10, Plenum, New-York, 1989, pp. 1-43. 2 . J.H. Fendler, Chem.Rev., 87 (1987) 877-899. 3 . M. Haruta and B. Delmon, J.Chem.Phys., 83 (1986) 859-868. 4 . T. Sugimoto, Adv.Colloid Interface Sci., 28 (1987) 65-108. 5 . M. Boutonnet, J. Kizling, P. Stenius and G. Maire, Colloids and Surfaces3 (1982) 209-225. 6 . M. Boutonnet, J. Kizling, V. Mintsa-Eya , A. Choplin, R. Touroude, G. Maire and P. Stenius, J.Catal., 103 (1987) 95-104. 7 . A. Whatelet, Memoire de Licence, Facultes Universitaires, Namur, Belgium, 1984. 8 . K. Kurihara, J. Kizling, P. Stenius and J.H. Fendler, J.Am.Chem.Soc., 105 (1983) 2574-2579. 9 . J.B. Nagy, A. Gourgue and E.G. Derouane, Stud.Surf.Sci.Catal., 16 (1983) 193-202.
716
10. J.B. Nagy, Colloids and Surfaces,35 (1989) 201-220. 11. D. Rosier, J.L. Dallons, G. Jannes and J.P. Puttemans, Acta Chim. Hung.124 (1987) 57-64. 12. G. Jannes, J.P. Puttemans and P. Vanderwegen, Catalysis Today, 5 (1989) 265-272. 13. I. Ravet, N.B. Lufimpadio, A. Gourgue and J.B. Nagy, Acta Chim.Hung.,llS (1985) 155-166. 14. I. Ravet, A. Gourgue,Z. Gabelica and J.B. Nagy, Proc.8th Int.Congress on Catalysis, Berlin West, July 2-6, 1984, Vol IV, Verlag Chemie, Weinheim-Basel, 1984, pp. 871-878. 15; I. Ravet, J.B. Nagy and E.G. Derouane, Stud.Surf.Sci.Catal., 31 (1987) 505-51 7 . 16. I. Ravet, A. Gourgue, and J.B. Nagy in K.L. Mittal and P. Bothorel (Ed), Surfactants in Solution, Vo1.5, Plenum, New-York, 1987, pp. 697-712. 17. J.B. Nagy, I. Bodart-Ravet and E.G. Derouane, Faraday Discuss. ChemSoc., 87 (1989) 189-198. 18. N.B. Lufimpadio, J.B. Nagy and E.G Derouane, in K.L. Mittal and B. Lindman (Eds), Surfactants in Solution, Vol. 3, Plenum, New-York, 1983, pp.1483-1493 19. V.R. Palkar, M.S. Multani and P. Ayyub, in K.L. Mittal (Ed.), Surfactants in Solution, Vol 10, Plenum, New-York, 1989, pp. 293-295. 20. M. Gobe, K. Kon-no,K. Kandori and A. Kitahara, J.Colloid Interface Sci., 93 (1983) 253-263. 21. K. Kandori, K. Kon-no, A. Kitahara, M. Fujiwara and T. Tamaru, in K.L. Mittal (Ed), Surfactants in Solution, Vol.10, Plenum, New-York, 1989, pp. 253-262. 22. R. Leung, M.J. Hou,C. Manohar,D.O. Shah and P.W. Chun, in D.O.Shah (Ed.),Macro- and Microemulsions, ACS Symposium Series 272, American Chemical Society, Washington D.C.,1985, pp. 325-344. 23. H. Charcosset, Int. Chem.lng., 23 (1983) 187-212. 24. A. Baiker, Faraday Discuss. Chem. SOC.,87 (1989) 239-251. 25. J. Kiwi, K. Kalyasundaram and M. Gratzel, Stuctrure and Bonding , Springer, Berlin, 1982, pp. 39-1 25 26. I. Bodart-Ravet, Ph.D Thesis, Namur, 1988. 27. J.B. Nagy, I. Bodart - Ravet, E.G.Derouane, A. Gourgue and J.P. Verfaillie, Colloids Surfaces, 36 (1989) 229-261. 28. S. Friberg and I. Lapczynska, Progr. Colloid and Polymer Sci., 56 (1976) 16-20. 29. S . Friberg, I. Lapczynska and G. Gillberg, J. Colloid Interface Sci, 56 (1976),19-32. 30. P. Dormont, Memoire de licence, Namur 1990. 31. A. Claerbout, Ph. D. thesis, Namur, in preparation.
G . Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 1991Elsevier Science Publishers R.V., Amsterdam - Printed in The Netherlands
717
NEW ORGANOMETALLIC ACTIVE S I T E S OBTAINED BY CONTROLLED SURFACE REACTION OF ORGANOMETALLIC COMPLEXES WITH SUPPORTED METAL PARTICLES B.
DIDILLON (11,
A.
EL MANSOUR
(11,
J.P.
CANDY
( 1 1 , ,J.M.
BASSET
(l),
F. LE PELTIER (21, and J.P. BOURNONVILLE ( 2 )
( 1 ) I R C , 2 avenue A l b e r t E i n s t e i n , 69626 Villeurbanne, FRANCE ( 2 ) IFP, BP 311, 92506 Rueil-Malmaison cPdex, FRANCE
ABSTRACT The c o n t r o l l e d s u r f a c e r e a c t i o n o f an o r g a n o m e t a l l i c compound, such as t e t r a b u t y l t i n , w i t h hydrogen covered rhodium p a r t i c l e s supported on s i l i c a leads t o a v e r y w e l l defined s u p e r f i c i a l o r g a n o m e t a l l i c species. These s u p e r f i c i a l organometall i c species are c h a r a c t e r i z e d by r h o d i u m - t i n bonds and c o n t a i n s b u t y l r a d i c a l s s t i l l l i n k e d t o t i n atoms. As t h e temperature o f t h e hydrogen thermal treatment i s increased, b u t y l r a d i c a l s a r e p r o g r e s s i v e l y eliminated leading a t l a s t t o the formation o f bulky rhodium-tin a l l o y particles. The presence o f t h e s e b u t y l r a d i c a l s a t t h e s u r f a c e o f t h e m e t a l l i c p a r t i c l e s induces an i n c r e a s e n o t o n l y i n s e l e c t i v i t y b u t a l s o i n a c t i v i t y r e g a r d i n g t h e hydrogenation o f t h e carbonyl f u n c t i o n o f an unsaturated aldehyde such as c i t r a l .
INTRODUCTION Supported group V I I I metals c a t a l y s t s r e a c t i o n s b u t they
a r e n ' t enough
are a c t i v e t o promote numerous
selective,
mainly
when
polyfunctionnal
s u b s t r a t e s have t o be transformed ( 1 ) . The c o n t r o l l e d a d d i t i o n o f a m e t a l l i c promotor,
v i a the reaction of
organometallic t i n compounds w i t h a group V I I I metal supported on s i l i c a can l e a d t o a new g e n e r a t i o n o f a c t i v i t y and s e l e c t i v i t y ( 2 - 4 ) . (5,6)
and
more
recently
bimetallic
catalysts,
For example,
Ni-Sn/Si02
and
which
exhibit
unusual
b i m e t a l l i c c a t a l y s t s Rh-Sn/Si02 Ru-Sn/Si02
(7-9)
present
s e l e c t i v i t y and a c t i v i t y i n t h e hydrogenation o f e t h y l - a c e t a t e t o ethanol.
high
I n t e r e s t i n g l y , t i n a d d i t i o n n o t o n l y suppresses t h e m u l t i p l e hydrogenolysis of C-C
and C-0 bonds,
b u t a l s o enhances t h e r a t e o f alcohol
formation.
This
improvement i n s e l e c t i v i t y can be i n t e r p r e t e d by t h e s u p e r f i c i a l d i l u t i o n of group V I I I metal by i n a c t i v e metal atoms o f t i n ( 1 0 ) . Otherwise, f o r t h e increase i n a c t i v i t y , s i t e has been considered,
t o account
the modification o f the nature o f the active
b u t n o t p r e c i s e l y described ( 1 1 ) . Furthermore,
if
t h e o v e r a l l composition o f t h e m e t a l l i c p a r t i c l e s i s mastered and known, t h e misunderstanding o f t h e s u p e r f i c i a l s t r u c t u r e and o f t h e nature o f t h e a c t i v e phase hinders t o e t a b l i s h accurate r e l a t i o n s w i t h t h e c a t a l y t i c p r o p e r t i e s . In
this
paper
we
intend
to
show
how
the
reaction
between
an
organometallic complex ( t e t r a b u t y l t i n ) and t h e s u r f a c e o f supported m e t a l l i c p a r t i c l e s (Rh/Si02)
proceeds,
as w e l l
as
t h e e v o l u t i o n of
t h e precursor
species i n t h e course o f thermal a c t i v a t i o n . A l l t h e successive stages o f t h e genesis o f t h e a c t i v e phase have been studied.
This knowledge allowed t o
s t a b i l i z e very w e l l d e f i n e d s u p e r f i c i a l organometallic complexes RhSn(n-C4H9)x which have been c h a r a c t e r i z e d by temperature programmed r e a c t i o n (T.P.R.
1,
i n f r a - r e d spectroscopy and e l e c t r o n microscopy. The hydrogenation o f unsatured aldehydes has been used as t e s t r e a c t i o n .
EXPERIMENTAL S i l i c a (Degussa A e r o s i l , Conventional microscope. monometallic
200 m2/g) was used as t h e support m a t e r i a l .
e l e c t r o n microscopy
(CTEM) was performed on a Jeol
100 C X
It was used t o determine t h e p a r t i c l e s s i z e o f b o t h supported
and
bimetallic
catalysts.
Scanning
Transmission
Electron
Microscope (STEM) HB 5 f r o m Vacuum Generator was used t o c h a r a c t e r i z e t h e b i m e t a l l i c c a t a l y s t s . I n f r a r e d spectra were obtained w i t h a N i c o l e t 10 MX-1 F o u r i e r t r a n s f o r m instrument. I t was used t o c h a r a c t e r i z e t h e a l k y l groups e v o l u t i o n on t h e surface d u r i n g r e a c t i o n between Rh/Si02 and Sn(n-C4H9)4. Preparation o f t h e monometallic c a t a l y s t s Rhodium supported on s i l i c a i s prepared by c a t i o n i c exchange between (RhC1(NH3)5)2C ions and surface ( $ S i - O ) -
(NHq)'
groups i n amnonia s o l u t i o n a t
pH 10. The surface complex obtained by t h i s route,
(sSiO-)2(RhC1(NH3)5)2+ i s
decomposed by c a l c i n a t i o n a t 573 K i n f l o w i n g d r y a i r and then reduced i n f l o w i n g hydrogen a t 573 K. The treatment w i t h d r y a i r a t 300 K g i v e s c a t a l y s t A which contains 1 w t % o f rhodium.
719
Preparation o f t h e b i m e t a l l i c species I n t e r a c t i o n between Sn(n-CqHg)4 and Rh/Si02 c a t a l y s t was performed i n a c l o s e d vessel. A g i v e n amount ( t y p i c a l y 0,3 g ) of o x i d i z e d m o n o m e t a l l i c sample A Rh203/Si02 i s reduced under hydrogen a t 623 K ( c a t a l y s t 6 ) and i s o l a t e d under
20
KPa
hydrogen
at
room
t e t r a b u t y l t i n Sn(n-C4H9)4 (Sn/Rh and t h e vessel
=
temperature.
The
requested
amount
of
1 ) i s t h e n i n t r o d u c e d w i t h o u t any s o l v e n t
i s heated by i n c r e m e n t s of
temperature, m a i n t a i n e d f o r a p e r i o d of
50 K up t o
30 inn,
573 K.
For
each
quantitative analysis o f the
gas phase i s c a r r i e d o u t by v o l u m e t r y and mass s p e c t r o m e t r y . The butane amount e v o l v e d g i v e s access by d i f f e r e n c e t o t h e number of b u t y l groups s t a y i n g on t h e s u r f a c e . A b l a n k experiment c a r r i e d o u t on s i l i c a i n d i c a t e s t h a t i n s i m i l a r experimental
conditions
no
reaction
occurs
between
Sn(n-C4H9)4
and
the
surface. Catalytic tests Hydrogenation o f c i t r a l i s performed i n a u t o c l a v e a t 340K i n l i q u i d phase under hydrogen.
The c a t a l y s t
i s i n t r o d u c e d i n t h e a u t o c l a v e under argon,
w i t h o u t c o n t a c t w i t h a i r . The argon i s removed by f l o w i n g hydrogen,
then a
s o l u t i o n o f 0.9 m l o f c i t r a l and 0 . 4 m l o f t e t r a d e c a n e i n 10 m l o f n-heptane i s i n t r o d u c e d under hydrogen. The hydrogen p r e s s u r e i s r a i s e d t o 7.6 MPa. The k i n e t i c s o f t h e r e a c t i o n i s f o l l o w e d by chromatographic a n a l y s i s o f t h e l i q u i d phase.
RESULTS Characterization o f the catalysts The m e t a l l i c phase of
t h e monometallic c a t a l y s t p r e c u r s o r ( c a t a l y s t A )
has been c h a r a c t e r i z e d . The average p a r t i c l e s i z e and t h e d i s t r i b u t i o n o f t h e p a r t i c l e s i z e has been determined by E l e c t r o n Microscopy ( C T E M ) . As r e p o r t e d i n the figure
lA,
t h e d i s t r i b u t i o n o f t h e p a r t i c l e s i z e i s narrow,
i n the
range of 1-2 nm, wi t h an average p a r t i c l e s i z e c l o s e t o 1,5 nm. T h i s v a l u e i s i n good agreement w i t h t h e c h e m i s o r p t i o n r e s u l t s a l r e a d y p u b l i s h e d ( 1 2 ) .
720 150
100
140
a80
120
YI
m
z 2 60
100 80
cn
z-
UJ
50
i=
40
z20
20 0
U 1 1 8 2 2 5 3 3 5 4 4 . 6
PARTICLES SIZE (am)
Figure 1 : Particles
size
distribution
of
catalysts
A
(Rh/Si02)
(A)
and
C2 (RhSn(n-C4Hg)2/Si02) ( 2 ) . The
temperature
controlled
reaction o f
tetrabutyl
tin
with
rhodium
p a r t i c l e s , a l l o w e d us t o f o l l o w t h e e v o l u t i o n o f t h e n a t u r e o f t h e s u p e r f i c i a l o r g a n o b i m e t a l l i c complex as shown i n f i g u r e 2 . C,/Sa c
E
5 4-
H 3a
2rn
m a
10
373
473
573
TEMPERATURE (I0
F i g u r e 2 : Butane e v o l u t i o n d u r i n g temperature c o n t r o l l e d i n t e r a c t i o n between Sn ( n-C4Hg)
and Rh-H/Si 02.
Below 323 K no r e a c t i o n occurs
between
tetrabutyl
tin
and
hydrogen
adsorbed on rhodium p a r t i c l e s . A t 323 K t e t r a b u t y l t i n begins t o r e a c t w i t h adsorbed hydrogen. T h i s r e a c t i o n l e a d s t o t h e f o r m a t i o n o f r h o d i u m - t i n bond and t o t h e evolvement o f one m o l e c u l e o f butane. A t t h i s stage 3 b u t y l groups remain l i n k e d t o t h e t i n atom ( c a t a l y s t Cl). at
373 K
( c a t a l y s t C2).
The f o r m u l a o f
A second b u t y l group i s removed
the
superficial
s p e c i e s o f t h e c a t a l y s t C2 can be d e s c r i b e d as f o l l o w i n g : Si02- RhS
-
Sn (nC4HgI2
organobimetallic
721
The removal o f t h e two remaining groups, a f t e r h e a t i n g up t o 473
K, lead
t o t h e f o r m a t i o n o f a rhodium t i n b i m e t a l l i c p a r t i c l e s ( c a t a l y s t D ) . Different
samples
(A,C2,
D)
have been i s o l a t e d
and c h a r a c t e r i z e d
in f r a - r e d spectroscopy and e l e c t r o n microscopy (CTEM and STEM)
by
(figures 3
and 4 ) .
I
3200
2800
2400
2000
1800
1600
1400
1200
WAVENUMBER (em-')
F i g u r e 3 : I n f r a r e d s p e c t r a o f Sn(n-C4H9I4 (A), Rh-H/Si02 (13). RhSn(n-C4H9)2/Si02 (C), RhSn/Si02 ( D ) . The f i g u r e 3 presents t h e i n f r a - r e d s p e c t r a o f :
-
t e t r a b u t y l t i n ( f i g u r e 3A)
-
Rh-H/Si02
c a t a l y s t A ( f i g u r e 36)
Rh-Sn(nC4H9)2/Si02
- Rh-Sn/Si02
-
-
c a t a l y s t C2 ( f i g u r e 3C)
c a t a l y s t D ( f i g u r e 3D).
On t h e spectrum o f t h e pure t e t r a b u t y l t i n , V(C-H) and
8
2800-3000 cm-'
t h e t y p i c a l wavenumbers o f
(C-H) band o f b u t y l groups are e a s i l y i d e n t i f i e d i n t h e range and i n t h e range 1200-1600 cm-'.
While no such bands a r e
detected on t h e s p e c t r a o f c a t a l y s t s A and D,
t h e y are detected on t h e
spectrum o f c a t a l y s t C2. These observations c o n f i r m t h a t t h e rhodium p a r t i c l e s are covered by d i b u t y l t i n fragment a f t e r r e a c t i o n o f t e t r a b u t y l rhodium p a r t i c l e s a t 373 K under hydrogen.
t i n with
722
The c a t a l y s t C2 has a l s o been c h a r a c t e r i z e d by e l e c t r o n microscopy (CTEM, STEM).
The t i n anchoring on rhodium p a r t i c l e s broadens t h e p a r t i c l e s i z e
d i s t r i b u t i o n and s h i f t s t h e mean p a r t i c l e s i z e towards higher p a r t i c l e s i z e i n comparison w i t h t h e monometallic rhodium c a t a l y s t
: from 1-5 nm t o 2.0
nm
( f i g u r e 1B). Moreover t h e shape o f t h e p a r t i c l e s have changed from s p h e r i c a l (monometallic
rhodium
particles)
to
flatter
and
bordered-less-contrasted
p a r t i c l e s (Rh-Sn(nC4H9)2/Si02). The STEM a n a l y s i s ( f i g u r e 4) i n d i c a t e d t h a t t i n i s never alone on t h e c a r r i e r : i t i s always associated w i t h t h e rhodium. The s i g n a l s corresponding
kcV
F i g u r e 4 : STEM a n a l y s i s o f c a t a l y s t C2 RhSn(n-C4H9)2/Si02. Catalytic properties A l l t h e c a t a l y s t s have been t e s t e d i n t h e s e l e c t i v e hydrogenation o f
c i t r a l . This molecule s u i t s very w e l l t o t h e study o f t h e i n f l u e n c e o f t h e nature and t h e s t r u c t u r e o f t h e a c t i v e phase on i t s c a t a l y t i c p r o p e r t i e s , because i t i n c l u d e s t h r e e k i n d s o f unsaturations : ( 1 ) an aldehydic f u n c t i o n ,
(2) a conjugated o l e f i n i c bond and ( 3 ) an i s o l a t e d o l e f i n i c bond. Moreover, rhodium
or
platinum
supported
on
silica
are
not
selective
for
the
hydrogenation o f c i t r a l t o d i o l e f i n i c a l c o h o l s ( g e r a n i o l and n e r o l ) (13, 14). The o v e r a l l r e a c t i o n p a t h f o r r e d u c t i o n o f c i t r a l t r a n s i s represented i n f i g u r e 5. Depending on t h e s e l e c t i v i t y o f t h e f i r s t hydrogenation step, t h r e e
723
d i f f e r e n t products could be obtained : geraniol, t h e dimetyl-3,7 and t h e c i t r o n e l 1a1
.
trans DIMETHYL-3.7 OCTENE-2 AL
DIMETHYL-3,7 OCTANAL
GERANIOL
/
trans DIMETHY L - 3,7 OCTENE-2 OL
DIMETHYL 3,7 OCTANoL
C
A+C CITRONELLAL
b0
A
A+B
CITRAL TRANS
octene 2 a1
CITRONELLOL
F i g u r e 5 : R e a c t i o n scheme o f c i t r a l ( t r a n s ) c a t a l y t i c h y d r o g e n a t i o n . I n t h e f i g u r e s 6A, 68 and 6C, t h e v a r i a t i o n s o f t h e c o n c e n t r a t i o n s o f t h e d i f f e r e n t p r o d u c t s , r e s u l t i n g f r o m t h e c i t r a l hydrogenation, proceeds, a r e r e p o r t e d f o r t h e c a t a l y s t A,
D and C2.
as t h e r e a c t i o n
724
0.300.25
A
-
0,05 5
15
10
TIME (hours) Figure 6 : Evolution o f t h e products concentration d u r i n g c i t r a l hydrogenation c a t a l y s e d by Rh/Si02 ( A ) ,
RhSn/Si02 ( B ) and RhSn(n-C4H9)2/Si02
p r e s s u r e = 7.6 MPa, T = 340
K, R h / C i t r a l
=
0,005.
(C). Hydrogen
725
C a t a l y s t A (Rh/SiOp). The c o n j u g a t e d o l e f i n i c bond i s hydrogenated t o g i v e t h e c i t r o n e l l a l (dimethyl-3,7,
octene
6-all.
Then,
the
isolated
olefinic
bond
of
the
c i t r o n e l l a l i s p a r t i a l l y and s l o w l y hydrogenated t o g i v e t h e s a t u r e d aldehyde (dimethyl-3,7
octanal).
The f o r m a t i o n
o f t h e satured alcohol
i s very low
whatever t h e c o n v e r s i o n of t h e d i o l e f i n i c aldehyde. C a t a l y s t 0 (RhSn/Si02). A t first,
c o n j u g a t e d o l e f i n i c band and c a r b o n y l group a r e hydrogenated i n
p a r a l l e l t o g i v e c i t r o n e l l a l and d i o l e f i n i c a l c o h o l ,
which appear as p r i m a r y
p r o d u c t s . Then, t h e y a r e hydrogenated m a i n l y i n t o o l e f i n i c a l c o h o l c i t r o n e l l o l (dimethyl-3,7
octene-6
011,
t h e satured alcohol
(dimethyl-3,7,
octanol
b e i n g d e t e c t e d i n a s m a l l amount.
When t h e c i t r a l
citronellol
01) h y d r o g e n a t i o n b e g i n s l e a d i n g t o
(dimethyl-3,7
octene-6
i s f u l l y consumed,
1) the an
increase i n t h e r a t e o f formation o f t h e saturated alcohol. Lastly,
o n l y two
p r o d u c t s a r e d e t e c t e d a f t e r 20 h o u r s o f r e a c t i o n : t h e d i m e t h y l - 3 , 7
octanol 1
(80 % ) and t h e c i t r o n e l l o l (20 % ) . C a t a l y s t C2 (RhSn(n-C4H9)2/Si02). I n t h i s case, t h e a l d e h y d i c f u n c t i o n h y d r o g e n a t i o n i s v e r y s e l e c t i v e even a t v e r y h i g h conversion. A v e r y s m a l l amount o f c i t r o n e l l a l i s d e t e c t e d i n t h e
No p r o d u c t
e a r l y t i m e o f r e a c t i o n and d i s a p p e a r s as t h e r e a c t i o n proceeds.
f r o m c i t r o n e l l a l h y d r o g e n a t i o n has been d e t e c t e d owing t o t h e accuracy o f t h e analysis .
DISCUSSION The c o n t r o l l e d m o d i f i c a t i o n o f t h e m o n o m e t a l l i c supported rhodium phase has a s t r o n g i n f l u e n c e on t h e c a t a l y t i c p r o p e r t i e s , a c t i v i t y and s e l e c t i v i t y :
- Rhodium a l o n e a f f e c t s m a i n l y t h e o l e f i n i c bonds and a t
first
the
c o n j u g a t e d one.
-
The
rhodium
tin
alloy
formation
increases
the
rate
of
citral
t r a n s f o r m a t i o n . The two c o n j u g a t e d u n s a t u r a t e d carbon-carbon and carbon-oxygen bonds
affected
:
hydrogenated.
are
firstly When
one
of
the
carbon-oxygen
bonds
has
been
the
carbon-carbon
conjugated
hydrogenated,
one
unsatured the
being
more
fastly
carbon-carbon
remaining
one
is
and
easily
726
hydrogenated. Then, t h e l a s t carbon-carbon o l e f i n i c bond i s hydrogenated.
group
The s u p e r f i c i a l leading
to
organobimetallic
selectivity
as
high
complex a f f e c t s as
96
%
for
only the geraniol
carbonyl
and
nerol
p r o d u c t i o n , when t h e c i t r a l i s f u l l y converted. This
controlled modification o f
the
superficial
composition
of
the
m e t a l l i c a c t i v e phase a l l o w s t o master t h e s e l e c t i v i t y i n t h e h y d r o g e n a t i o n of m u l t i f u n c t i o n n a l compounds. I n t h e case o f c i t r a l , h i g h s e l e c t i v i t i e s c o u l d be reached i n t h e p r o d u c t i o n o f :
-
C i t r o n e l l a 1 when supported rhodium a l o n e i s used as c a t a l y s t .
- C i t r o n e l l o l (3,7 d i m e t h y l octene-6 01) when supported rhodium t i n a l l o y i s used as c a t a l y s t .
-
G e r a n i o l and n e r o l when t h e s u p e r f i c i a l o r g a n o b i m e t a l l i c complex i s t h e
a c t i v e species. Moreover t h e s t a b i l i t y o f t h e s u p e r f i c i a l o r g a n o b i m e t a l l i c complex has been checked.
A f t e r reaction,
t h e two b u t y l groups
are s t i l l
present
: a
thermal t r e a t m e n t o f t h e used c a t a l y s t , under f l o w i n g hydrogen, up t o 523 K l e a d s t o t h e removal o f two b u t y l groups. The presence o f t i n e i t h e r i n t h e rhodium t i n a l l o y o r i n t h e s u p e r f i c i a l organobimetallic
complex a l l o w s t h e
carbonyl
function
hydrogenation.
Tin,
which can be c o n s i d e r e d as e l e c t r o p h i l i c , c o u l d induce a s p e c i f i c a d s o r p t i o n o f t h e u n s a t u r a t e d aldehyde b y i t s a l d e h y d i c group f o l l o w i n g scheme f o r t h e o r g a n o b i m e t a l l i c s p e c i e s :
SCHEME
[15),
as shown i n t h e
This e l ectroph l e e f f e c t o f t i n i s w e l l known i n c o o r d i n a t i o n chem s t r y (16). The d i f f e r e n t c a t a l y t i c behaviour between rhodium t i n a l l o y and rhodium t i n (n-C4Hg)2 complex can be i n t e r p r e t e d by a s p e c i f i c poisoning e f f e c t of t i n according t o t h e n a t u r e o f t h e s u p e r f i c i a l s t r u c t u r e . The t i n p o i s o n i n g e f f e c t on t h e hydrogenation o f unsaturated carbon-carbon bonds has been w i d e l y proved (17). I n t h e case o f rhodium t i n a l l o y , t h e t i n d i f f u s i o n
inside the metallic
p a r t i c l e restores
which
superficial
rhodium atoms
ensembles
are
able
to
hydrogenate carbon-carbon o l e f i n i c bonds. I n t h e case o f t h e o r g a n o b i m e t a l l i c s u p e r f i c i a l complex, t h e remaining b u t y l groups s t a b i l i z e t h e t i n atoms on t h e m e t a l l i c p a r t i c l e s surface. Then, t i n fragments,
t h e rhodium p a r t i c l e s coverage by d i b u t y l
i n h i b i t s f u l l y t h e hydrogenation o f unsaturated carbon-carbon
bonds. Moreover,
i t i s p o s s i b l e t h a t b u t y l groups c o u l d a c t as an "organic
molecular sieve", t h u s c o e r c i n g t h e molecular d i f f u s i o n o f t h e reagents t o t h e active sites.
CONCLUSION The knowledge o f
a l l t h e stages o f t h e m o d i f i c a t i o n o f
a
supported
m e t a l l i c phase leads t o t h e genesis o f p e r f e c t l y d e f i n e d s u p e r f i c i a l complex. As
a function
of
the
thermal
a c t i v a t i o n procedure,
the
nature o f
this
s u p e r f i c i a l complex i s v a r y i n g . This e v o l u t i o n o f t h e s u p e r f i c i a l n a t u r e and s t r u c t u r e o f t h e a c t i v e phase s t r o n g l y affects t h e c a t a l y t i c p r o p e r t i e s . Moreover,
as f a r as we know,
i t i s the f i r s t time that not only the
n a t u r e o f a supported o r g a n o b i m e t a l l i c a c t i v e species has been c h a r a c t e r i z e d but, above a l l , i t s presence has been c o r r e l a t e d w i t h a s t r o n g increase i n t h e selectivity for
a given
reaction.
p o s s i b i l i t i e s o f t h e "tailor-made"
Finally,
these
results
illustrate
the
supported m e t a l l i c c a t a l y s i s .
REFERENCES 1 2 3
G. CORDIER, Y. COLLEUILLE and P. FOUILLOUX. "Catalyse par l e s metaux", CNRS Ed. 1984, P a r i s . Y . I . YERMAKOV, B.N. KUZNETSOV and V.A. ZAKHAROV. " C a t a l y t i c hydrogenation, s t u d i e s i n s u r f a c e E l s e v i e r , Amsterdam, 27, 459, 1986. J . MARGITFALVI, S . S Z m O and F. NAGY. " C a t a l y t i c hydrogenation, s t u d i e s i n s u r f a c e E l s e v i e r , Amsterdam, 27, 373, 1986.
sc ence
and
catalysis",
sc ence
and
catalysis",
4
5 6 7 8 9 10 11 12 13 14 15 16 17
US Patent 4.380.673. US Patent 4.456.775. US Patent 4.504.593. US Patent 4.628.130. Ch. TRAVERS, t h e s i s ENSPM, P a r i s 1982. Ch. TRAVERS, J.P. BOURNONVILLE and G. MARTINO. "Proc. o f 8 t h I n t e r n a t i o n a l Congress on C a t a l y s i s " , B e r l i n , West Germany, j u l y 2-6, 1984, Verlag Chemie Ed. I V , 891-902. O.A. FERRETTI, t h e s i s ENSPM, P a r i s 1986. O.A. FERRETTI, J.P. BOURNONVILLE, J.P. CANDY and G. MARTINO. To be submitted. P. LOUESSARD, t h e s i s , Lyon, 1988. A. EL MANSOUR, J.P. CANDY, J.P. BOURNONVILLE, O.A. FERRETTI and J.M. BASSET. Angew. Chem. I n t . Ed. Engl. 28 ( 3 1 , 347, 1989. J.P. CANDY, O.A. FERRETTI, G. MABILON, J.P. BOURNONVILLE, A. EL MANSOUR, J.M. BASSET and G. MARTINO. J. Catal., 210, 1988. J.P. CANDY, O.A. FERRETTI, G. MABILON, J.P. BOURNONVILLE, A. EL MANSOUR, J.M. BASSET and G. MARTINO. J. Catal., 201, 1988. P.N. RYLANDER. " C a t a l y t i c hydrogenation i n organic synthesis", Academic Press, New-York 72, 1980. S. GALVAGNO, Z. POLTAREWSKI, A. DONATO, G. N E R I and R. PIETROPAOLO. J. Chem. SOC. Chem. Comm., 1729, 1986. Z. POLTAREWSKI, S. GALVAGNO, R. PIETROPAOLO and P. SAITI. J. Catal., 190, 1986. F. CORREA, R. NAKAMURA, R.E. STIMSON, R.L. BURWELL Jr and D.F. SCHRIVER. J. Am. Chem. SOC., 102, 5112, 1980. J . BARBIER, i n " D e a c t i v a t i o n and Poisoning o f C a t a l y s t s " . M. DEKKER, New-York, 20, 109, 1985.
112, 112,
102,
G . Poncelet,P.A. Jacobs,P. Grange and B. Delmon (Editors),Preparation of Catalysts V 1991 Elsevier Science PublishersB.V., Amsterdam - Printed in The Netherlands
729
CONVERSION COATINGS ON STAINLESS STEEL AS MULTIPURPOSE CATALYSTS
L. ARIES, A. KOMLA
and J.P. TRAVERSE Laboratoire de Recherche sur 1’Energie Universitk Paul Sabatier 31062 TOULOUSE Cedex - FRANCE -
ABSTRACT Through a chemical treatmeht of iron-chromium based alloys, supported catalysts can be prepared in one main step from the substrate which furnishes constitutive elements of the coating. The conversion coating is a microporous physically and chemically heterogenous medium with a fractal structure. Variations of the compositional and textural properties are studied against nature of the substrate, duration of the chemical treatment and cgFditions of thermal or chemical post-oxidation treatment.
I. INTRODUCTION
An original method for catalyst preparation has been developed. Applications have been found in hydrogenation, coal The hydro-liquefaction and automobile emission control ( 1 ) , ( 2 ) . process involves either anodic oxidation or chemical treatment of iron-chromium based alloys. Supported catalyst can be prepared is one main step from the substrate which furnishes constitutive elements of the coating. Powder catalyst can be obtained from such a coating by separating it from substrate. The paper focusses on preparational aspects. Our purpose is to describe and discuss more particularly the chemical process. 11. BASIS OF WORKING PROCESS AND EXPERIMENTAL
Austenitic and ferritic steel sheets or turnings were used. The metal substrate can also be in the form of a conventional ca) The compositions are given talyst (rings, beads, foam, etc in Table I. In the chemical treatment the surfaces were prepared by simple dippin2 of the steel into a bath. One of the main conditions of the treatment is the fitting of .the electrode potential of the sample to the value of the natural corrosion potential of the steel ( E o c ) in the active state (Fis.1). This potential must be lower than the primary passive potential of the steel ( E p ) . It is then necessary to control the surface potential during the treatment having previously determined the electrochemical characte-
...
730
ristics of the interface metal solution, by means of polarization curves.
TABLE I. Chemical composition of stainless steels (weight per cent)
4 [dc"l
ctiity
:
passivity
...-...
eoc
;
Er,
Ep
0
500
Fig.1. Typical anodic polarization curve of stainless steels in sulfuric acid solutions. EOC : natural corrosion potential, Ep : passivity potential, E r : rupture potential. F o r some alloys, this condition of potential is naturally fulfilled for the treatment baths used. Generally, the potential can be adjusted to the required value by cathodic activation of the surface in the treatment bath with the help of a current generator and counter electrode playing the part of anode. The operation time was in the region of one minute. The exact time depends on the initial oxidation state of the surface to be treated. The coatings were prepared in an acid bath with suitable additives, particularly substances containing chalcogenides. Sulphur seems to give the best results, and it is preferable to put sodium sulphide or sodiumthiosulphate in the bath. It is possible to use very different acids such as sulphuric acid, nitric acid and hydrochloric acid. It can be profitable to add a corrosion inhibitor specific to the alloy and the treatment'bath to further control the thickness of the coating. This effect can be correlated with the electro-chemical behaviour of the steel in the treatment bath. The plotted polarization curves of the steel show that the addi-
731
tion of propargyl alcohol to the bath brings a s large decrease of current density in the active domain. This influence of propargyl alcohol on the anodic behaviour of the steel is characteristic of a corrosion inhibitor effect.' The presence of propargyl alcehel reduces the aggressiveness of the bath and leads to a decreasi in the coat thickness. The compositions of baths used in this part of the study are given in table 11. Table 11. Typical elaboration conditions Steel
Sulfuric acid vol %
Austenitic
1
Sulfured species moI 1-1
Propargyl alcohol moI 1-1
NazS203.5H20 4 1cd
1
Na2S203.5H20 4 10-3
5
Na2S.9H20 1.25 103
8.5
After the preparation of the conversion coating, the samples were washed with water. They were then dried in an oven at SO'C or dried in ambient air for about 10 minutes. After rinsing, in some cases, the coatings were subjected to chemical oxidation treatment in an aqueous bath or to heat oxidation treatment in air. The bath temperature was generally in the range 4 5 to 60°C. The most easily changeableparameter is the duration of treatment, which was therefore used to modify the characteristics of the coatings. It was varied from a few minutes to up to an hour. Surface characterisation was achieved with different methods : microscopy (SEM and high voltage microscopy), secondary ion mass spectroscopy (SIMS), electron spectroscopy for chemical analysis (X.P.S.). Textural properties were analysed with the followin$ methods : B.E.T., microscopy, impedance electrode mesurements, voltametry
.
111. RESULTS AND DISCUSSION 111.1 GENERAL CHARACTERISTICS OF THE CATALYST The conversion coating prepared by the process described is a microporous medium of thickness between 100 nm and few p m . It is composed of three types of particles : metal crystallites of about 100 nm diameter, crystallites of metal compounds, mainly oxides, of about 100 nm diameter and microparticles in a size range of a few nm to 50 nm. It is, in fact, a heterogenous, porous medium with a random texture. Its characteristics seem to
732
lead to a fractal type structure. The range of internal similarity can, in our coatings, cover a scale of characteristic lengths from a nanometer to several tens of micrometers. Chemically, the microporous material is composed of a mixtuand in less proportion of sulphides of the main re of oxides elements present in thevsubstrate and of the alloy itself. Minority elements are also present as dopants. There is a composition gradient from the support up to the surface where the metallic element are entirely in the combined state. In general by adjusting the nature of the substrate alloy and the conditions of treatment it is possible to modify the characteristics of the catalysts. The conversion coating can be subjected to oxidation treatments which modify its composition. Heat treatment was performed at temperatures between 150 and 600'C. Oxidation of the layer was also carried out by sAbjecting it to the action of an oxygenated aqueous bath. The chemical modifications are, in this case, restricted to the layer itself.
-
111.2
-
CHEMICAL COMPOSITION : INFLUENCE OF THE PREPARATION CONDITIONS
111.2.1. Influence of the nature of the substrate. We present the results for thin films prepared from two typical substrates : ferritic and austenitic steel (see table I). The different analyses of the conversion coatings reveal their complex nature. On the one hand we can identifly strata which have a difference in the cohesion and in chemical compositions ; on the other hand there are numerous chemical compounds present in various crystallization states. 111.2.1.1. Thin films on ferritic steel Fig.2 shows a tentative phase representation of the typical coating drawn from all the analytical techniques used ( 3 1 . The width of the domain of given phases, at a given deph, is proportional to the ratio of the number of metal atoms present in these phases to the total number (only for the main compounds). This scheme allows the relative importance of components to be shown at various depths. There are five domains, and it is possible to distinguish 3 zones according to the depth : the superficial film (A) , the external ( B ) and the internal ( C ) zones which together form the deep zone. The thickness of the superficial film may be estimated at 20 nm or thereabout. Its adhesion to the coating is quite weak. The thickness of the deep zone is in the region of 135 nm. This zone is quite adherent. 111.2.1.2. Thin film on austenitic steel The distribution profiles of the elements obtained by SIYS shows that the treatment leads to an enrichment of the layer in nickel. From XPS analysis, the chromium included in the compounds is shokn to be in the form of oxides or hydroxide and the nickel
733
in the form of sulphate, *stalphide and hydroxide. SIMS analysis in the absence of oxygen indicates that the layer is composed of several sublayers corresponding to different degrees of oxidstion of the metallic elements. In the surface layer, iron, chrdmium and nickel are present in their highest state of oxidation. In the first sub-layer C.he oxides contain Fez+ and the levels of Crs+ and Ni2+ are lower than at the surface. In the second sublayer the metallic phase becomes increasingly preponderant.
S P U TC'
,'
COATING Ill
STEEL
ATOMIC PROPORTION
Fiq.2. Representation of the composition of the typical selective coating : the scheme gives the atomic proportion against the depth. Atomic proportion is the ratio between the number of metal atoms in the different compounds to the total number of metal atoms. (A):the superficial film, (B):the external zone (deep zone), (C):the internal Bone (deep zone), 1:domain of Fe3+ and Cr3+ oxide and hydroxide, 1I:domain of C r 3 + substituted magnetite, 1II:domain of metallic iron and chromium (alloy), 1V:domain of metal sulphate(s), V : domain of metal sulphide(s). 111.2.2. Influence of the duration of treatment The variation of treatment time leads, of course, to variation of the thickness of the layer. A more detailed study of the growth of these layers has already been made (4). For example, the distribution profiles (SIMS) of the various components which make up the conversion coating after various treatment times are shown in fiq 3 with the coating steel interface at the origin. The ionic intensity of sulphur and oxygen always steadily decreases from the coating surface to metal substrate. However the sulphur concentration reaches a steady minimum much more quickly than oxygen at any time. The chemical treatment of 26CNb17 steel which contains little nickel, leads to enrichment in this element. The proportion of iron and chromium metal in the whole of the conversion coating decreases throughout treatment, indicating an increase in the all o y oxidation. In the course of treatment the coating becomes ri-
734
'=ig.3.
and rnin. S-
SINS intensity vs. sputtering time for Fe' , C r ' , X i + , 0- , for the preparation times of 2min, 5min, l0min and 20
C-
During the treatment the deep zone rapidly thickens, whereas the surface zone remains nearly constant at about 20 nm. A s regards the deep zone the distribution of the elements at a given distance from the substrate seems to be independent of the treatment duration. The very thin surface layer composition, including the coating solution interface, depends on the duration of the treatment. The oxygen ionic intensity is, at the beginning of treatment, particulary high whereas that of sulphur varies little. 111.2.3.
Influence of oxidation treatment
Thermal oxidation : ferritic steels The thermal oxidation of conversion coatings on austenitic and ferritic steels has been the object of several publications ( 5 ) . We shall give a brief outline here of the main results ob111.2.3.1.
735
tained with ferritic steels. The modifications brought about are different according to wether the oxidation is carried out in air o r under a low oxygen pressure (p02=2.10-2Pa). Up to 4 0 0 to 5 0 0 ' C in air, oxidation is limited to the conversion coating itself. It mainly affects the major component i.e substituted magnetite. The oxidation of substituted magnetite during drying leads to the formation of the substituted aFezO3 phase which remains a minor phase. From 15O'C the substituted magnetite becomes transformed according to the reaction : 2Fe2+(Fe3+z-yCr3+y)04t 1/202
->
aFez03 t 2 ( F e i - ~ C r) 2~0 3
The phase (Fei-xCrx)203, identified at the surface, is in fact doped by the minority elements from the metal substrate (e.g. the ions Nb5+and Si4+). It presents interestina semiconducting properties (6). 9 Above 500'C, oxygen diffuses into the substrate and the appearance of phases such as FeCrz04 is noted. Thermal oxidation not only brings about the major che'mical modifications mentioned but also minor chemical modifications which have important effects on the catalytic activity : possible elimination of certain sulphur-containing compounds, enrichement of the oxidized compounds in elements already present in the substrate. Also,'we note crystallisation of the amorphous phases and a modification of particle size. 111.2.3.2. Chemical oxidation : austenitic steels Coatings with characteristics close to those given above, are oxidized in aqueous medium containing Hz02. The oxidation is restricted to the coating. After treatment a 2-layer organisation remains but the total disappearance of iron in the surface layer was noted. Throughout the coating there is a strong decrease in the proportion of sulphides (and sulphates at the surface) as well in that of elements in the metallic state. Ni (OH12 on the other hand increases. The thickness of the coating is hardly modified. It retains good adherence and presents good physicochemica1 stability. 111.3. TEXTURE :
IYFLUENCE OF THE PREPARATIOB CONDITIONS
111.3.1. Influence of the type of substrate Here, we compare the thin coatings corresponding to the composition analyses given in section 111.2.1. The coatings o n austenitic steel were thicker than on ferritic steel(about 2 5 % thicker). They were also rougher (Ra=O.45pm) than on ferritic steel (Ra=O.Zym) (steel Z6CBb17). Unlike on ferritic steels, the distribution of the asperities is close to being Gaussian (Fiz 4). Observations with scanning electron microscope show that in all cases the same type of irrezularity can be observed at different scales. The coatings therefore present a fractal nature. The diagrams of electrochemical impedance of the coatinzs
736
present a capacitive ar2 characteristic of the charge transfer process at the electrode solution interface and at the very high frequencies, a domain which we attribute to a process of diffusion into the pores (fig. 5). L ~m Z
i
Fig.4. steel.
Surface
profile
of a t y p i c a l c o a t i n g o n Z 8 C 1 7 s t a i n l e s s
h\J0 3,955 H
9960 Hz
0
15
20'
25
Fig.5. Impedance d i a g r a m o f a t y p i c a l c o a t i n g on Z 8 C 1 7 s t a i n l e s s s t e e l . E l e c t r o l y t e NazSOI 0 . 1 ?I a t 2 0 ° C . E = - 1 . 2 V / e c s . F r e q u e n c y i s i n Hz The c a p a c i t i v e a r c i s n o t c e n t r e d o n t h e r e a l a x i s : t h e a n of t h e arc around i t s h i g h frequency l i m i t g l e of r o t a t i o n 8, d o e s n o t d e p e n d o n t h e a p p l i e d e l e c t r o d e p o t e n t i a l . The h i g h f r e which g i v e s t h e r e s i s t a n c e p e r u n i t area o f the quency l i m i t , e l e c t r o l y t e , R E , i s i n d e p e n d e n t o f t h e i m p o s e d p o t e n t i a l : RE = 5 +- 1 c m - 2 . T h i s f r e q u e n c y d i s p e r s i o n , a ( a = 1 - 2 8 / ~ ) ,known s i n c e
737 the works of Cole and Cole on dielectrics ( 7 ) indicates the texcomplexity of this type of coating (physical and/or chemical heterogeneity). It is shown that the transfer arc, whzch is centered in the case of a flat smooth interface, is subjected, through creation of porosity and/or roughness, to rotation around its high-frequency limits. This difference from a smooth interface is due to the distribution of the response time constant of the system i.e. the'distribution of the current according to a scale law : the electrochemical impedance follows a relationship of the type Z = (jw)-.. To interpret any correlation which may exist between the particular physicochemical texture of certain interfaces and the dispersion factor, a, various authors have introduced a nondimensional parameter, df,which is prepresentative of the difference from the ideal s'tuation of a perfectly smooth and homogeneous surface. F o r an interface presenting internal similarity df should be identified to its fractal dimension. Several largely debated relationships have been proposed to determine df from the angle of rotation, 8 , o r from the dispersion parameter, a , of the capacitive arc. In the present case, we evaluated the complex texture of this type of material by the value of df obtained by the relationship proposed by Le Mehautd et al. (8) for sinkered powder electrodes (eg. sintered nickel) : df = ltl/a. The conversion coatings studied present fractal dimensions which, although close, remain distinct : df = 2 . 2 6 to 2 . 2 7 2 0 . 0 2 for ferritic steel coatings and df 2 . 2 0 ?: 0 . 0 1 5 for austenitic steel coatings. F o r coatings on both types of steel the shape of the diagram at very high frequency indicated the existence of a process of diffusion into the pores of the coating but the curves have a completely different appearance suggesting the existence of cylindrical pores for ferritic coatings and spherical pores for austenitic steels. The geometrical characteristics of the pores can be calculated by means of simplification hypotheses from the characteristic frequencies. The radius of the cylindrical pores (ferritic steels) is from about 10 to 2 0 nm and that of the spherical pores (austenitic steel) about 5 nm. Overall porosity is difficult to evaluate (thin coatins on relatively thick substrate). Indirect measurements suggest values Measurement of the specific area by the of between 25 and 50% BET method is problematic for the same reasons. Values of about 8 0 0 m 2 / m 2 were obtained for coatings on alloy ZaC17 compared to 2 6 0 m z / m 2 on the austenitic steel Z3CN1810. -. tural
.
111.3.2. Influence of the duration of treatment
As seen before, the thickness of the coating increases v e r y rapidly during the first minutes of treatment, then it becomes steady and proportional to time. The various parameters described above are also modified with time of treatment. It is particularly interesting to observe the variation of the fractal dimension (fig 6 ) At the start of treatment the value of df is the same for the three types of steel studied ; this value is characteristic
738 o f t h e o r i g i n a l steel sur'face and o f i t s o x i d e l a y e r t h a t is f o r med n a t u r a l l y i n a i r . For v e r y s h o r t t r e a t m e n t t i m e s d i decre-ases a n d t h e s u r f a c e o f t h e material t e n d s t o w a r d s a n " i d e a l " s t a t e w i t h a s m o o t h , homogeneous a s p e c t (df = 2 ) . A s t h e t r e a t m e n t t i m e becomes t i m e g r e a t e r t h a 9 4 m i n u t e s , di i n c r e a s e s and t e n d s t o w a r d s a maximum v a l u e w i c h i s d e p e n d e n t o n t h e n a t u r e o f t h e s t a r t i n g mater ia l : 2 . 2 6 f o r f e r r i t i c s t e e l s a n d 2 . 1 9 f o r a u s t e n i t i c steel.
x
0
5
10
15
20
25
F i 9 . 6 . V a r i a t i o n o f df a g a i n s t t r e a t m e n t t i m e . ( ~ ) : Z 8 C 1 7 , (+):Z6CNb17, (*):Z3CN18-10 The
specific
surface
30
35
a r e a o f t h e c o a t i n g on f e r r i t i c s t e e l
w a s s t u d i e d a g a i n s t t r e a t m e n t t i m e by c y c l i c v o l t a m e t r y . The res u l t s were c o n f i r m e d by BET measurement. There w a s a r a p i d in-
c r e a s e i n t h e s p e c i f i c s u r f a c e a r e a from t h e f i r s t m i n u t e s o f ( > 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.
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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
Occelli M.L.
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.
Salim V.M.M. Sanderson W.A. Schild Ch. Schmal M. Scholten A. Schramm Ch.M. Seki H. Sekiguchi J.
744
Tanabe K. Tang Youqi Tan-no M. Tichit D. Traverse J.P. Tretyakov V.V. Trezza G. Trifiro F. Tsubota S . Turek T.
567 69 567 345 729 647 43 1 49 695 547
Ueda A.
695
Vaccari A. van den Brink P.J. van Dillen A.J. van Leeuwen W.A. van Wageningen A. van Yperen R. Walther K.L. Wang Hongli Wang Y. Webb G.
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.
537
Yamaguchi T. Yasse B.
567 537
Zaki M.I. Zazhigalov V.A. Zecchina A. Zegaoui 0. Zhang Qinpei
617 497 43 1 679 69
Zhao Jiusheng Zhou B. Zhu Yongfa
145 537 69
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
